Seminars in quantum nanophysics

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Department of quantum mesoscopics: seminar, Thursday, June 23, 2022, ITF, 2:00 pm
Sergei Popruzhenko (Institut obshchei fiziki im. A.M. Prokhorova RAN)
Obratnyi effekt Faradeya v elektromagnitnoi volne ekstremal'noi intensivnosti

Rassmotren protsess vozbuzhdeniya sil'nogo kvazistaticheskogo magnitnogo polya pri vzaimodeistvii tsirkulyarno polyarizovannoi volny vysokoi intensivnosti (10^23 Vt/sm^2 i vyshe) s plazmennym sloem. Pokazano, chto v besstolknovitel'nom rezhime vzaimodeistviya prodol'noe magnitnoe pole voznikaet za schet sily radiatsionnogo treniya. Eta spetsificheskaya realizatsiya obratnogo effekta Faradeya mozhet byt' ispol'zovana dlya poiska makroskopicheskikh proyavlenii radiatsionnogo treniya v eksperimentakh, provedenie kotorykh planiruetsya na sverkhmoshchnykh lazernykh ustanovkakh.

[Unusual day and time!] [video]
Landau ITP colloquium, Friday, June 10, 2022, Landau ITP, 11:30 am
Karen Michaeli (Weizmann Institute of Science)
Chirality and spin in organic molecules

In the late 19th century, Louis Pasteur discovered that biology shows a preference for molecules with a specific handedness. Ever since, researchers have been trying to understand the origin of life’s homochirality and its implications. For example, electron transport in organisms, an essential part of basic biological processes such as respiration and photosynthesis, is realized via insulating helical molecules. Recent studies further found that the transmission probability of electrons through such molecules is strongly spin-dependent, with the preferred spin direction set by the chirality. This direct connection between spin and chirality raises numerous fundamental questions. It thus opens a new arena of research at the interface between biology and quantum physics. In my talk, I will review the most important experimental findings and describe a theoretical model that explains the origin of spin-dependent transport: The helical geometry induces robust spin filtering accompanied by, and intimately related to, strongly enhanced overall transmission through chiral molecules. In particular, I will focus on spin-selectivity in the chiral system with strong electron-phonon interactions. Finally, I will present some ideas for integrating organic molecules into solid-state devices for various applications.

Video

Department of quantum mesoscopics: seminar, Friday, May 27, 2022, ITF, 3:00 pm
Pavel Maksimov (LTF im. N.N.Bogolyubova, OIYaI, Dubna)
Frustrirovannyi magnetizm na primere modeli Kitaeva

Frustratsiya v magnitnykh sistemakh - eto situatsiya, kogda ne vse magnitnye vzaimodeistviya mogut byt' minimizirovany klassicheski odnovremenno, i iz-za etogo razlichnye sostoyaniya mogut byt' polnost'yu vyrozhdeny. Takoe vyrozhdenie usilivaet kvantovye popravki i magnon-magnonnoe vzaimodeistvie, kotorye mogut polnost'yu razrushit' uporyadochennoe sostoyanie. Frustratsiya v dannykh sistemakh mozhet byt' vyzvana kak geometriei reshetki, tak i anizotropnymi vzaimodeistviyami. My pokazhem svoistva takikh sistem detal'no na primere predlozhennoi Alekseem Kitaevym modeli anizotropnykh vzaimodeistvii na geksagonal'noi reshetke, kotoraya otkryla put' k tochno reshaemym spinovozhidostnym fazam.

[video]
Scientific Council of the Landau Institute, Friday, May 13, 2022, Landau ITP, 11:30 am
A. A. Mazanik, Ya. V. Fominov
Peculiarities of the density of states in SN bilayers

We study the density of states (DoS) ν(E) in a normal-metallic (N) film contacted by a bulk superconductor (S). We assume that the system is diffusive and the SN interface is transparent. In the limit of thin N layer (compared to the coherence length), we analytically find three different types of the DoS peculiarity at energy equal to the bulk superconducting order parameter Δ0. (i) In the absence of the inverse proximity effect, the peculiarity has the check-mark form with ν(Δ0)=0 as long as the thickness of the N layer is smaller than a critical value. (ii) When the inverse proximity effect comes into play, the check-mark is immediately elevated so that ν(Δ0)>0. (iii) Upon further increasing of the inverse proximity effect, ν(E) gradually evolves to the vertical peculiarity (with an infinite-derivative inflection point at E0). This crossover is controlled by a materials-matching parameter which depends on the relative degree of disorder in the S and N materials.
arXiv:2205.06171

Department of quantum mesoscopics: seminar, Friday, April 22, 2022, ITF, 3:00 pm
M. E. Ismagambetov, P. M. Ostrovskii
Low-energy transport in a weakly disordered Weyl semimetal

Weyl semimetal is a solid material with isolated touching points between conduction and valence bands in its Brillouin zone---Weyl points. Low energy excitations near these points exhibit a linear dispersion and act as relativistic massless particles. Weyl points are stable topological objects robust with respect to most perturbations. We study effects of weak disorder on the spectral and transport properties of Weyl semimetals in the limit of low energies. We use a model of Gaussian white-noise potential and apply dimensional regularization scheme near three dimensions to treat divergent terms in the perturbation theory. In the framework of self-consistent Born approximation, we find closed expressions for the average density of states and conductivity. Both quantities are analytic functions in the limit of zero energy. We also include interference terms beyond the self-consistent Born approximation up to the third order in disorder strength. These interference corrections are stronger than the mean-field result and non-analytic as functions of energy. Our main result is the dependence of conductivity on the electron concentration $\sigma = \sigma_0 + A n^{1/3} + (B/\sigma_0) n^{2/3} \ln|n|$ with $A \approx -0.142$ and $B \approx 0.019$.

Landau ITP colloquium, Friday, April 15, 2022, Landau ITP, 11:30 am
Oleg Tret'yakov (The University of New South Wales, Sydney, Australia)
ZAROZhDENIE I DINAMIKA MAGNITNYKh SOLITONOV V TOPOLOGIChESKIKh MATERIALAKh

Skirmiony predstavlyayut soboi topologicheski zashchishchennye spinovye tekstury, kotorye mogut sluzhit' budushchimi elementami spintronnoi pamyati i ustroistv logiki. Poetomu zarozhdenie skirmionov v topologicheskikh sistemakh, takikh kak topologicheskii izolyator/ferromagnetik, yavlyaetsya interesnym voprosom, osobenno tak kak v etikh sistemakh pokazano chto skirmiony dvigayutsya bystree. Ya opishu zarozhdenie skirmionov, vyzvannoe spin-perenosnymi uglovymi momentami, kharakternymi dlya granitsy topologicheskogo izolyatora s ferromagnitnym izolyatorom. Ispol'zuya mikromagnitnoe modelirovanie i energetiku, my opisyvaem svoistva nukleatsii skirmionov na poverkhnosti topologicheskikh izolyatorov, takie kak vremya zarozhdeniya, kriticheskoe elektricheskoe pole i chisla skirmionov [1]. Ya pokazhu chto vremya zarozhdeniya obratno proportsional'no prilozhennomu elektricheskomu polyu, i takzhe opishu zavisimosti kriticheskogo polya nukleatsii ot zatukhaniya Gil'berta i temperatury. Krome togo, ya obsuzhu vliyanie vzaimodeistviya Dzyaloshinskogo-Morii i prodemonstriruyu chto temperaturnaya zavisimost' mozhet byt' ob'yasnena umen'sheniem shcheli magnonnogo vozbuzhdeniya. Vo vtoroi chasti ya opishu topologicheskie solitony v sistemakh s vnutriploskostnoi magnitnoi anizotropiei. Eti solitony nazyvayutsya bimeronami i yavlyayutsya analogami skirmionov (imeyut tot zhe topologicheskii zaryad). Snachala ya opishu ikh stabil'nost', staticheskie i dinamicheskie svoistva v ferromagnetikakh [2]. Zatem ya pereidu k antiferromagnetikam, gde ya pokazhu, chto po analogii so skirmionami eti topologicheskie solitony ne obladayut skirmionnym effektom Kholla, i sredi prochikh interesnykh svoistv demonstriruyut khaotichnoe povedenie pri vozbuzhdenii peremennymi tokami v sistemakh s nizkim zatukhaniem
1. D. Kurebayashi, O.A. Tretiakov, arXiv:2112.12967 (2021).
2. B. Gobel, I. Mertig, O.A. Tretiakov, Phys. Rep. 895, 1 (2021).

Landau ITP colloquium, Friday, April 1, 2022, Landau ITP, 10:00 am
Dmitri K. Efetov (LMU Munich, Germany)
Plethora of Many-Body Ground States in Magic Angle Twisted Bilayer Graphene

Twist-angle engineering of 2D materials has led to the recent discoveries of novel many-body ground states in moiré systems such as correlated insulators, unconventional superconductivity, strange metals, orbital magnetism and topologically nontrivial phases. These systems are clean and tuneable, where all phases can coexist in a single device, which opens up enormous possibilities to address key questions about the nature of correlation induced superconductivity and topology, and allows to create entirely novel quantum phases with enhanced interactions. In this talk we will introduce some of the main concepts underlying these systems, concentrating on magic angle twisted bilayer graphene (MATBG) and show how symmetry-broken states emerge at all integer electron fillings [1]. We further will discuss recent experiments including screened interactions [2], Chern insulators [4], magnetic Josephson junctions [4], quantum criticality [5], re-entrant correlated insulators at high magnetic fields [6] and discuss some of the avenues for novel quantum sensing applications [7].

[1] Nature, 574, 653 (2019).
[2] Nature, 583, 375–378 (2020).
[3] Nature Physics, 17, 710 (2021).
[4] arXiv:2110.01067 (2021).
[5] arXiv:2108.07753 (2021).
[6] arXiv:2201.09260 (2021).
[7] arXiv:2111.08735 (2021).

Department of quantum mesoscopics: seminar, Friday, March 25, 2022, ITF, 3:00 pm
I. V. Bobkova
Magnon-Cooparons in magnet-superconductor hybrids

Generation and detection of spinfull Cooper pairs in conventional superconductors has been intensely pursued by designing increasingly complex magnet-superconductor hybrids. We demonstrate theoretically that magnons with nonzero wavenumbers universally induce a cloud of spinfull triplet Cooper pairs around them in an adjacent conventional superconductor. The resulting composite quasiparticle, termed magnon-cooparon, consists of a spin flip in the magnet screened by a cloud of the spinfull superfluid condensate. Thus, it inherits a large effective mass, which can be measured experimentally. Furthermore, we demonstrate that two magnetic wires deposited on a superconductor serve as a controllable magnonic directional coupler mediated by the nonlocal and composite nature of magnon-cooparons. Our analysis predicts a quasiparticle that enables generation, control, and use of spinfull triplet Cooper pairs in the simplest magnet-superconductor heterostructures.

Department of quantum mesoscopics: seminar, Friday, March 18, 2022, onlain, 3:00 pm
Kh.Nazaryan, L.Levitov
Ustoichivost' vikhrei v potokakh elektronov

Vikhri, kotorye poyavlyayutsya v potokakh elektronov, vyzyvayut interes, tak kak yavlyayutsya neoproverzhimym effektom elektronnoi gidrodinamiki. Odnako do sikh po otsutstvovala obshchaya skhema, pozvolyayushchaya svyazat' zavikhrennost', nablyudaemuyu na makrourovne, s mikroskopicheskimi mekhanizmami vzaimodeistviya i rasseyaniya. V nashei rabote my pokazyvaem, chto zavikhrennost' ustoichivym obrazom voznikaet iz-za nelokal'nosti provodimosti \sigma(k), i eto delaet zavikhrennost' svoistvom, kotoroe prisutstvuet v razlichnymi rezhimakh dvizheniya. Pri etom ballisticheskii i gidrodinamicheskii techeniya v realistichnoi geometrii dayut zavikhrennosti, blizkie po absolyutnomu znacheniyu v obeikh rezhimakh. Tem ne menee, ballisticheskie vikhri na neskol'ko poryadkov bolee ustoichivy k prisutstvii protsessov, relaksiruyushchikh impul's, chem gidrodinamicheskie vikhri. Poetomu to, kak legko vikhri gasyatsya besporyadkom i rasseyaniem fononov, mozhet sluzhit' diagnostikoi mikroskopicheskogo proiskhozhdeniya zavikhrennosti v potokakh elektronov.

Scientific Council of the Landau Institute, Friday, March 4, 2022, Landau ITP, 11:30 am
Yan Fedorov
Izuchenie topologicheskoi trivializatsii landshaftov v sluchainykh sistemakh vysokoi razmernosti

Ya rasskazhu o trekh nedavnikh rabotakh (arXiv:2112.11250, arXiv:2112.13446 i arXiv:2202.03815), v kotorykh issledovalis' raznye modeli sluchainykh landshaftov v prostranstve vysokoi razmernosti s tsel'yu proyasneniya universal'nykh aspektov yavleniya topologicheskoi trivializatsii.

Department of quantum mesoscopics: seminar, Friday, February 25, 2022, ITF, 3:00 pm
A.B. Van'kov, I.V. Kukushkin (IFTT)
Kulonovskie korrelyatsii kak pervoprichina ferromagnitnogo perekhoda v rezhime kvantovogo effekta Kholla s \nu=2

Poyavivshiisya v poslednyuyu dekadu novyi tip geterostruktur na osnove ZnO/MgZnO pozvolyaet zaglyanut' v fiziku dvumernykh elektronnykh sistem (DES) sushchestvenno dal'she, chem eto bylo vozmozhno v traditsionnykh poluprovodnikakh – kremnii i arsenide galliya. Prichinoi tomu yavlyayutsya sil'nye mezhchastichnye korrelyatsii v sochetanii s malym neodnorodnym ushireniem energeticheskikh urovnei. V etikh dvumernykh sistemakh sravnitel'no legko dostigaetsya parametr Vignera-Zeitsa r_s~5–12. Pri takom nevedomom ranee sochetanii parametrov «ne rabotayut» prezhnie teoreticheskie modeli, desyatiletiyami obkatannye na dvumernom elektronnom gaze. Kachestvennuyu transformatsiyu preterpevayut dazhe «khorosho izuchennye» odnochastichnye yavleniya, takie kak tselochislennyi kvantovyi effekt Kholla (KEKh) – potomu kak vsledstvie kulonovskogo vzaimodeistviya apriori zaputannoi stanovitsya sama sistema urovnei Landau. V prezhnikh magnitotransportnykh issledovaniyakh pokazano, chto samo yavlenie KEKh v geterostrukturakh ZnO ostaetsya, odnako osnovnoe sostoyanie sistemy sil'no modifitsirovano.

V dannoi rabote obnaruzhen ferromagnitnyi perekhod v rezhime tselochislennogo KEKh s chetnymi faktorami zapolneniya v sistemakh na osnove ZnO. Metodami fotolyuminestsentsii i neuprugogo rasseyaniya sveta udalos' okharakterizovat' fazovyi sostav DES, nablyudat' diapazony parametrov sistemy, otvechayushchie ferro- i paramagnitnomu uporyadocheniyu [1]. Postroena fazovaya diagramma dlya sostoyaniya KEKh \nu=2 pri izmenenii parametrov elektronnoi kontsentratsii i ugla naklona magnitnogo polya. Iz fazovoi diagrammy izvlecheny perenormirovannye fermi-zhidkostnye parametry dlya effektivnoi massy i spinovoi vospriimchivosti DES. Pokazano, chto imenno vozrastayushchaya rol' kulonovskikh korrelyatsii pri umen'shenii kontsentratsii DES yavlyaetsya pervoprichinoi dannogo fazovogo prevrashcheniya.

Pomimo fenomenologii ferromagnitnogo perekhoda v nastoyashchei rabote proveden analiz mikroskopicheskogo mekhanizma etogo yavleniya. Metodom neuprugogo rasseyaniya sveta proslezheno povedenie nizkoenergeticheskikh kollektivnykh vozbuzhdenii pri izmenenii kontsentratsii DES i pokazano, chto ferromagnitnyi perekhod nastupaet vsledstvie smyagcheniya tsiklotronnogo spin-flip vozbuzhdeniya (CSFE), i posleduyushchego lavinoobraznogo perekhoda elektronov mezhdu sosednimi urovnyami Landau s protivopolozhnymi spinovymi proektsiyami [2]. Analogichno, v terminakh kollektivnykh spinovykh vozbuzhdenii ob'yasnyaetsya i obratnyi perekhod ferromagnetik-paramagnetik [3]. V oboikh sluchayakh pokazana klyuchevaya rol' nalichiya magnitorotonnogo minimuma v dispersii spinovykh vozbuzhdenii.

Eksperimental'nye dannye po energii vovlechennykh kollektivnykh vozbuzhdenii khorosho soglasuyutsya s raschetom, provedennym metodami tochnoi diagonalizatsii i ekranirovannogo priblizheniya Khartri-Foka [2,3].

[1] A.B.Vankov, B.D.Kaysin, I.V.Kukushkin, Phys.Rev.B 96, 235401 (2017).
[2] A.B.Vankov, B.D.Kaysin, I.V.Kukushkin, Phys.Rev.B 98 121412(R) (2018).
[3] A.B.Vankov, I.V.Kukushkin, Phys.Rev.B 104, 165144 (2021).

Scientific Council of the Landau Institute, Friday, February 25, 2022, Landau ITP, 11:30 am
A.V. Lunkin
Effekt babochki v sisteme kvantovykh tochek v modeli Sachdeva-Ie-Kitaeva

V rabote predlozhen sposob vychisleniya anomal'no uporyadochennogo vo vremeni korrelyatora v obobshchenii modeli Sachdeva-Ie-Kitaeva (SYK) s nenulevoi prostranstvennoi razmernost'yu. Nash rezul'tat primenim ne tol'ko na malykh vremenakh, kogda khaotichnye svoistva sistemy razvity slabo, no i na bol'shikh – poryadka vremeni Erenfesta. My pokazali, chto informatsiya o prilozhennom vozmushchenii, kotoruyu opisyvaet dannyi korrelyator, rasprostranyaetsya ballisticheski v vide fronta. Nami vpervye byla vychislena skorost' fronta dlya modelei dannogo tipa.

Department of quantum mesoscopics: seminar, Friday, February 4, 2022, onlain, 3:00 pm
P. S. Alekseev, A. P. Alekseeva (FTI im. A.F. Ioffe)
Vyazkouprugii rezonans i poperechnye magnitozvukovye volny v vyazkoi dvumernoi elektronnoi zhidkosti

V ochen' chistykh obraztsakh normal'nykh provodnikov elektrony provodimosti pri nizkikh temperaturakh mogut formirovat' vyazkuyu elektronnuyu zhidkost'. V rabote izuchayutsya vysokochastotnye techeniya takoi zhidkosti v magnitnom pole. Pokazano, chto ee koeffitsienty sdvigovoi vyazkosti imeyut rezonans pri chastote techeniya, blizkoi k udvoennoi tsiklotronnoi chastote elektronov. Takoi rezonans obuslovlen sobstvennoi dinamikoi tenzora vyazkikh napryazhenii elektricheski zaryazhennoi zhidkosti v magnitnom pole i, pri otsutstvii rezonansov na drugikh garmonikakh tsiklotronnoi chastoty, yavlyaetsya priznakom realizatsii gidrodinamicheskogo vysokochastotnogo transporta. Pokazano, chto v ochen' chistykh i dostatochno shirokikh obraztsakh predskazannyi vyazkouprugii rezonans proyavlyaetsya v koeffitsiente zatukhaniya magnitoplazmonov. Izucheny takzhe poperechnye sdvigovye magnitozvukovye volny, kotorye mogut vozbuzhdat'sya v elektronnoi zhidkosti pri dostatochno bol'shoi sile mezhchastichnogo vzaimodeistviya. V ramkakh gidrodinamicheskogo priblizheniya rasschitan ikh zakon dispersii i ikh vklad v lineinyi otklik zhidkosti na radiochastotnoe elektricheskoe pole. Chastota poperechnykh voln v predele malykh volnovykh vektorov ravna udvoennoi tsiklotronnoi chastote. Eto privodit, dlya techenii v dostatochno uzkikh obraztsakh, k proyavleniyu vyazkouprugogo rezonansa za schet komponenty techeniya, sformirovannoi magnitozvukovymi volnami. Dlya mikroskopicheskogo obosnovaniya ispol'zovannogo gidrodinamicheskogo opisaniya bylo pokazano, chto kineticheskoe uravnenie dlya vysokochastotnoi dinamiki kvazichastits elektronnoi fermi-zhidkosti v predele bol'shoi velichiny mezhchastichnogo vzaimodeistviya svoditsya k gidrodinamicheskim uravneniyam Nav'e-Stoksa s perenormirovannymi koeffitsientami. Tsiklotronnaya chastota, vkhodyashchaya v vysokochastotnuyu vyazkost' v etikh uravneniyakh, perenormiruetsya mezhchastichnym vzaimodeistviem i stanovitsya otlichnoi ot neperenormiruemoi "obychnoi" tsiklotronnoi chastoty, opredelyayushchei tsiklotronnyi rezonans. Dany argumenty v pol'zu togo, chto vozbuzhdenie izuchennykh poperechnykh magnitozvukovykh voln ob'yasnyaet anomal'nyi pik v fotosoprotivlenii naibolee kachestvennykh vysokopodvizhnykh kvantovykh yam GaAs, v kotorykh, po-vidimomu, realizuetsya gidrodinamicheskii rezhim elektronnogo transporta.

Department of quantum mesoscopics: seminar, Friday, January 28, 2022, online, 4:00 pm
Romain Vasseur (University of Massachusetts)
Measurement-induced criticality

An open quantum system is continuously "monitored" by its environment, so its dynamics consists of two competing processes: unitary evolution, which generates entanglement and generically leads to chaotic dynamics, and non-unitary operations resulting from measurements and noisy couplings to the environment, that tend to irreversibly destroy quantum information by revealing it. A minimal model that captures these competing processes consists of a quantum circuit made up of random unitary gates interlaced with local projective measurements. Remarkably, this minimal model undergoes a dynamical phase transition as the rate of measurements is increased. In this talk, I will introduce an exact replica statistical mechanics approach to this problem, and show that the transition is described by a CFT with central charge c=0. I will also present numerical results on the spectrum of this theory using a transfer matrix approach that indicate multifractal scaling of correlation functions at the critical point, reflected in a continuous spectrum of scaling dimensions.

[Unusual time: 16:00] [video]
Scientific Council of the Landau Institute, Friday, January 28, 2022, Landau ITP, 11:30 am
A.V. Lunkin, N/V/ Feigel'man
High-frequency transport and zero-sound in an array of SYK quantum dots

We study an array of strongly correlated quantum dots of complex SYK type and account for the effects of quadratic terms added to the SYK Hamiltonian; both local terms and inter-dot tunneling are considered in the non-Fermi-liquid temperature range T≫TFL. Electric σ(ω,p) and thermal κ(ω,p) conductivities are calculated as functions of frequency and momentum, for arbitrary values of the particle-hole asymmetry parameter E. At low-frequencies ω≪T we find the Lorentz ratio L=κ(0,0)/σ(0,0) to be non-universal and temperature-dependent. At ω≫T the conductivity σ(ω,p) contains a pole with nearly linear dispersion ω≈splnωT reminiscent of the "zero-sound", known for Fermi-liquids.

Department of quantum mesoscopics: seminar, Friday, January 14, 2022, ITF, 4:00 pm
Leonid Kulik (IFTT)
Neravnovesnyi ansambl' neitral'nykh vozbuzhdenii v Laflinovskom sostoyanii 1/3

Dvumernye elektronnye sistemy v kvantuyushchem magnitnom pole predstavlyayut isklyuchitel'nyi interes s tochki zreniya vovlecheniya kvazichastits s ne bozevskoi i ne fermievskoi statistikoi (enionnoi), v oblast' prikladnykh fizicheskikh zadach. Drobnye sostoyaniya DKEKh yavlyayutsya edinstvennymi na segodnyashnii den' eksperimental'no realizovannymi kandidatami na rol' sistemy kvazichastits s enionnoi statistikoi. Dlya vyyavleniya statisticheskikh i termodinamicheskikh svoistv enionnykh sistem eksperimental'no sformirovan makroskopicheskii ansambl' neitral'nykh vozbuzhdenii v DKEKh 1/3. Okazalos', chto vozbuzhdeniya imeyut sverkhdlinnye vremena zhizni i proyavlyayut bozevskie svoistva.

(Neobychnoe vremya: 16.00!)

Department of quantum mesoscopics: seminar, Friday, January 14, 2022, onlain, 3:00 pm
Vadim Shestakov (IF im. L.V. Kirenskogo)
Temperaturnaya zavisimost' londonovskoi glubiny proniknoveniya magnitnogo polya pri nalichii perekhoda mezhdu sverkhprovodyashchimi sostoyaniyami s++- i s±-tipa

Odnim iz glavnykh kandidatov na rol' vzaimodeistviya, privodyashchego k sverkhprovodimosti v pniktidakh i khal'kogenidakh zheleza vystupayut spinovye fluktuatsii. Oni privodyat k vozniknoveniyu sverkhprovodyashchego parametra poryadka s±-tipa. Dannoe neobychnoe sverkhprovodyashchee sostoyanie obladaet odnim zamechatel'nym svoistvom: ono mozhet pereiti v s++-sostoyanie pri uvelichenii nemagnitnogo besporyadka. Obnaruzhenie takogo perekhoda podtverdit sushchestvovanie s±-sostoyaniya v chistykh sverkhprovodnikakh na osnove zheleza i, sootvetstvenno, podtverdit spin-fluktuatsionnyi mekhanizm sverkhprovodimosti. V dannoi rabote issledovana temperaturnaya zavisimost' londonovskoi glubiny proniknoveniya v ramkakh podkhoda Eliashberga dlya dvukhzonnoi modeli s++- i s±-sverkhprovodnikov s nemagnitnymi primesyami i opredelen nabor kharakternykh osobennostei, ukazyvayushchikh na nalichie perekhoda s±→s++, a takzhe provedeno kriticheskoe sravnenie s imeyushchimisya eksperimental'nymi dannymi.

Department of quantum mesoscopics: seminar, Friday, December 17, 2021, ITF, 3:00 pm
Aleksey V. Lunkin and Mikhail V. Feigel'man
High-frequency transport and zero-sound in an array of SYK quantum dots

We study an array of strongly correlated quantum dots of complex SYK type and account for the effects of quadratic terms added to the SYK Hamiltonian; both local terms and inter-dot tunneling are considered in the non-Fermi-liquid temperature range T >> T_FL Electric σ(ω, p) and thermal κ(ω, p) conductivities are calculated as functions of frequency and momentum, for arbitrary values of the particle-hole asymmetry parameter. At low frequencies ω << T we find the Lorentz ratio L = κ(0, 0)/σ(0, 0) to be non-universal and temperature-dependent. At ω >> T the conductivity σ(ω, p) contains a pole with nearly linear dispersion ω ≈ sp ln (ω/T) reminiscent of the "zero-sound", known for Fermi-liquids.

Department of quantum mesoscopics: seminar, Friday, December 10, 2021, onlain, 4:00 pm
Claudio Chamon (Boston University, USA)
Superconducting Circuit Realization of Combinatorial Gauge Symmetry

We propose a superconducting quantum circuit based on a general symmetry principle—combinatorial gauge symmetry—designed to emulate topologically ordered quantum liquids and serve as a foundation for the construction of topological qubits. The proposed circuit exhibits rich features: in the classical limit of large capacitances its ground state consists of two superimposed loop structures; one is a crystal of small loops containing disordered U(1) degrees of freedom, and the other is a gas of loops of all sizes associated with Z2 topological order. We show that these classical results carry over to the quantum case, where phase fluctuations arise from the presence of finite capacitances, yielding Z2 quantum topological order. A key feature of the exact gauge symmetry is that amplitudes connecting different Z2 loop states arise from paths having zero classical energy cost. As a result, these amplitudes are controlled by dimensional confinement rather than tunneling through energy barriers. We argue that this effect may lead to larger energy gaps than previous proposals that are limited by such barriers, potentially making it more likely for a topological phase to be experimentally observable. Finally, we discuss how our superconducting circuit realization of combinatorial gauge symmetry can be implemented in practice.

PRX QUANTUM 2, 030341 (2021)

(Neobychnoe vremya: 16.00! Translyatsiya: Zoom)

[video]
Department of quantum mesoscopics: seminar, Friday, December 3, 2021, ITF, 3:00 pm
Michael Rampp (KIT, Germany)
Topologically enabled superconductivity

Majorana zero modes are a much sought-after consequence of one-dimensional topological superconductivity. Here we show that, in turn, zero modes accompanying dynamical instanton events strongly enhance - in some cases even enable - superconductivity. We find that the dynamics of a one-dimensional topological triplet superconductor is governed by a theta-term in the action. For isotropic triplets, this term enables algebraic charge-2e superconductivity, which is destroyed by fluctuations in non-topological superconductors. For anisotropic triplets, zero modes suppress quantum phase slips and stabilize superconductivity over a large region of the phase diagram. We present predictions of correlation functions and thermodynamics for states of topologically enhanced superconductivity.

(Translyatsiya: Zoom)

Department of quantum mesoscopics: seminar, Friday, November 26, 2021, ITF, 3:00 pm
N. Kishmar i M. Feigel'man
Teoriya dzhozefsonovskogo toka cherez izolyator Andersona bol'shoi tolshchiny

Vychislen srednii dzhozefsonovskii tok I_c*\sin\phi cherez izolyator Andersona bol'shoi tolshchiny (mnogo bol'she dliny lokalizatsii \xi). Pokazano, chto amplituda toka ubyvaet priblizitel'no eksponentsial'no s tolshchinoi bar'era, odnako effektivnaya dlina ubyvaniya vo mnogo raz prevyshaet dlinu lokalizatsii elektronnykh sostoyanii v izolyatore. Tem samym, ob'yasnen davno izvestnyi eksperimental'nyi rezul'tat, izvestnyi pod nazvaniem "long range Josephson effect". Razvitaya teoriya pokazyvaet chto kachestvenno etot effekt svyazan s a) sil'nymi fluktuatsiyami volnovykh funktsii lokalizovannykh sostoyanii, i b) mottovskimi rezonansami na parakh lokalizovannykh sostoyanii. Pokazano, chto pri tolshchine izolyatora L bol'she "mottovskoi dliny" L_M = \xi \ln(\delta_\xi/\Delta) izmenyaetsya znak amplitudy toka I_c, to est' realizuetsya sverkhprovodyashchii "pi-kontakt".

(Translyatsiya: Zoom)

[video]
Department of quantum mesoscopics: seminar, Friday, November 12, 2021, ITF, 3:00 pm
Aleksei Dmitriev
(aspirant Laboratorii iskustvennykh kvantovykh sistem MFTI)

Doklad po materialam kandidatskoi dissertatsii
"Issledovanie nelineinykh kvantovo-opticheskikh effektov pri rasseyanii sveta na sverkhprovodnikovom kubite v odnomernom prostranstve"

(Translyatsiya: Zoom)

Landau ITP colloquium, Friday, November 12, 2021, Landau ITP, 11:30 am
John Chalker (Physics Department, University of Oxford)
Many-body delocalisation as symmetry breaking

I will start this talk with an overview of recent work on quantum dynamics in many body systems far from equilibrium. This work has led to an understanding from the perspective of quantum information, of how a such systems may approach an equilibrium state at long times, in which initial conditions are effectively forgotten. It has also led to an appreciation that there are classes of system in which an equilibrium state is not reached at long times. One of these classes is made up of many-body localised systems, in which randomness in the Hamiltonian is responsible for a long-lived memory of the initial state of a system. By varying the strength of randomness, a transition can be induced between localised and ergodic phases, which seems very different from conventional symmetry-breaking phase transitions.
In the second part of the talk I will discuss minimal models for quantum chaos and many-body localisation. The models are Floquet quantum circuits for lattice spin systems, in which time evolution is generated by unitary gates that couple neighbouring sites. In particular, I will examine the circumstances in which a version of the so-called diagonal approximation (originally developed for the semiclassical limit in low-dimensional chaotic systems) can be applied to these systems. Within this framework I will show that the many-body delocalisation transition can be seen as a form of symmetry breaking transition, having many of the features generally associated with conventional phase transitions in classical statistical mechanical models.

Joint work with Sam Garratt: Phys. Rev. X 11, 021051 (2021) and Phys. Rev. Lett. 127, 026802 (2021)

Department of quantum mesoscopics: seminar, Friday, November 5, 2021, ITF, 11:00 am
Gleb Fedorov
(aspirant Laboratorii iskustvennykh kvantovykh sistem MFTI)

Doklad po materialam dissertatsii
"Modelirovanie kvantovogo vzaimodeistviya izlucheniya i veshchestva s ispol'zovaniem massivov sverkhprovodnikovykh iskusstvennykh
atomov"

(Neobychnoe vremya: 11.00! Translyatsiya: Zoom)

[video]
Landau ITP colloquium, Friday, October 29, 2021, Landau ITP, 3:30 pm
Alexander Abanov (SUNY Stony Brook)
Limit shape phase transitions. A merger of Arctic circles

A limit shape phenomenon in statistical mechanics is the appearance of a most probable macroscopic state. This state is usually characterized by a well-defined boundary separating frozen and liquid spatial regions. The earliest studies related to this phenomenon in the context of crystal shapes are in works by Pokrovsky and Talapov [1]. We will review a few examples of the models leading to the appearance of limit shapes. Then we consider a class of topological phase transitions in the limit shape problem of statistical mechanics. The problem considered is generally known as the Arctic circle problem. The considered transition can be visualized as the merging of two melted regions (Arctic circles). We establish the mapping, which identifies the transition as the transition known in lattice QCD and random matrix problems [2,3]. The transition is a continuous phase transition of the third order. We identify universal features of the limiting shape close to the transition using the hydrodynamic description.
[1] V. L. Pokrovsky and A. L. Talapov, Phys. Rev. Lett. 42, 65 (1979). "Ground State, Spectrum, and Phase Diagram of Two-Dimensional Incommensurate Crystals."
[2] D. J. Gross and E. Witten, Phys. Rev. D, 21 (2): 446, (1980). "Possible third-order phase transition in the large-n lattice gauge theory"; S. R. Wadia, Phys. Lett. B 93, 403 (1980). "$N=\infty$ phase transition in a class of exactly soluble model lattice gauge theories."
[3] M. R. Douglas and V. A. Kazakov. Phys. Lett. B, 319 (1-3): 219–230, 1993. "Large n phase transition in continuum QCD2."

Video

Department of quantum mesoscopics: seminar, Friday, October 29, 2021, onlain, 11:30 am
K. S. Tikhonov
Mnogochastichnaya lokalizatsiya: obzor razlichnykh rezul'tatov

(Neobychnoe vremya: 11.30! Translyatsiya: Zoom)

In the first part, I will briefly summarize our current understanding of the relation between Anderson localization on random regular graphs and many-body localization (MBL) [1]. In the second part, I will review several recent preprints, which question observability of genuine MBL transition in state-of-the-art numerical simulations [2,3,4].

[1] Tikhonov, Mirlin; Annals of Physics, 168525
[2] Morningstar, Colmenarez, Khemani, Luitz, Huse; arXiv:2107.05642
[3] Sels; arXiv:2108.10796
[4] Sierant, Zakrzewski; arXiv:2109.13608

Department of quantum mesoscopics: seminar, Friday, October 22, 2021, ITF, 3:00 pm
V. S. Khrapai (IFTT)
Razdelenie tepla i zaryada v trekhterminal'noi NSN strukture iz nanoprovoloki InAs

(Ceminar v normal'nom rezhime, s translyatsiei: Zoom)

V samoe poslednee vremya nablyudaetsya interes k nelokal'nomu kvazichastichnomu otkliku v strukturakh s maioranovskimi nanoprovolokami [1]. Predpolagaetsya, chto issledovanie nelokal'noi provodimosti pomozhet prolit' svet na proiskhozhdenie podshchelevogo transporta i nadezhno ustanovit' tochku topologicheskogo fazovogo perekhoda. Ya rasskazhu ob eksperimente [2], postavlennom v trivial'noi faze i demonstriruyushchem vtorichnost' nelokal'noi provodimosti po sravneniyu s nelokal'nym teplovym otklikom. V osnove eksperimenta lezhit sochetanie nelokal'nogo transporta s nelokal'nym shumovym otklikom, pozvolivshee prodemonstrirovat' effektivnoe razdelenie tepla i zaryada na sverkhprovodyashchem kontakte. V rezul'tate takogo razdeleniya srednii zaryad proshedshei kvazichastitsy okazyvaetsya blizok k nulyu, tak chto nelokal'nyi kvazichastichnyi transport yavlyaetsya zaryadovo-neitral'nym. Krome togo, nablyudenie predskazannogo nedavno Andreevskogo vypryamleniya [3] demonstriruet, chto etot effekt ne yavlyaetsya unikal'nym svoistvom topolicheskogo fazovogo perekhoda.

[1] D. Puglia et al., Phys. Rev. B 103, 235201 (2021) https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.235201

[2] A.O. Denisov et al., Semicond. Sci. Tech., 36, 09LT04 (2021) https://iopscience.iop.org/article/10.1088/1361-6641/ac187b/meta

[3] T.Ö. Rosdahl et al., Phys. Rev. B 97, 045421 (2018) https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.045421

Department of quantum mesoscopics: seminar, Friday, October 15, 2021, online, 4:00 pm
Anton Andreev
Electronic pumping of heat in absence of charge pumping

A mechanism of electron-mediated pumping of heat in the absence of net charge transfer is proposed. It may be realized in charge-neutral electron systems, such as graphene, coupled to an external electric potential. The flow of heat in this pumping cycle is not accompanied by a buildup of voltage along the system, which offers advantages over traditional thermoelectric cooling setups. Efficiency of heat pumping and magnitude of heat flux are studied in the hydrodynamic regime for weak disorder. In a pristine system, even for an infinitesimal pumping potential the heat flux remains finite. In particular, for a potential in the form of a traveling wave moving with velocity c the pumping is perfect; the entire heat content of the electron liquid is advected with velocity c. For a general pumping cycle the heat flux is determined by the cycle geometry and disorder strength.

Landau ITP colloquium, Friday, October 1, 2021, Landau ITP, 11:30 am
Jon Keating (Oxford)
The Riemann Hypothesis and Quantum Physics

The Riemann Hypothesis concerns the positions of the zeros of the Riemann zeta-function. It has been one of the central unsolved problems in mathematics for over 150 years. Surprisingly, there appear to be deep connections between the theory of the Riemann zeta-function and quantum physics, and these have stimulated close collaborations between the two areas. In particular, analogies with quantum chaos and random matrix theory have been the focus of considerable attention. In this colloquium I will review some of these ideas and their implications.

Video

Scientific Council of the Landau Institute, Friday, September 3, 2021, Landau ITP, 11:30 am
Burmistrov I.S.
Anomalous elasticity of anisotropic flexible two-dimensional materials

We develop the theory of anomalous elasticity in two-dimensional flexible materials with orthorhombic crystal symmetry. We show that in the universal region, where characteristic length scales are larger than the rather small Ginzburg scale ~10 nm, these materials possess flat phases with anisotropic bending rigidity and Young's modulus. Remarkably, the problem has continuous hidden symmetry, which leads to formation of the line of fixed points. We demonstrate that due to this symmetry the power law scaling with momentum is controlled by the single universal exponent (the same along the whole line). We demonstrate that these anisotropic flat phases are uniquely labeled by the ratio of absolute Poisson's ratios. We apply our theory to monolayer black phosphorus (phosphorene).

Scientific Council of the Landau Institute, Friday, June 18, 2021, Landau ITP, 12:30 pm
A.S. Osin, Ya.V. Fominov
Superconducting phases and the second Josephson harmonic in tunnel junctions between diffusive superconductors

We consider a planar SIS-type Josephson junction between diffusive superconductors (S) through an insulating tunnel interface (I). We construct fully self-consistent perturbation theory with respect to the interface conductance. As a result, we find correction to the first Josephson harmonic and calculate the second Josephson harmonic. At arbitrary temperatures, we correct previous results for the nonsinusoidal current-phase relation in Josephson tunnel junctions, which were obtained with the help of conjectured form of solution. Our perturbation theory also describes the difference between the phases of the order parameter and of the anomalous Green functions. The talk is based on the paper [1].

[1] A.S. Osin and Ya.V. Fominov, arXiv:2105.05786

Video

Scientific Council of the Landau Institute, Friday, June 18, 2021, Landau ITP, 11:30 am
Ivan M. Khaymovich (MPIKS, Dresden)
Relation between multifractality and entanglement for nonergodic extended states

The multifractality provides a way of ergodicity breaking in term of chaotization and equipartitioning over degrees of freedom. On the other hand, in quantum information theory it is the entanglement entropy which represents the main measure of ergodicity and thermalization. In this talk I will represent an exact relation between the above measures, showing that the fractal dimension of the non-ergodic wave function puts an upper bound on its entanglement entropy [A]. I will also provide a couple of explicit examples demonstrating that the entanglement entropy may reach its ergodic (Page) value when the wave function is still highly non-ergodic and occupies a zero fraction of the total Hilbert space. If time permits I will briefly discuss some other possible deviations from ergodicity relevant for the chaotic many-body systems [B-E].

[A] G. De Tomasi, I. M. K., “Multifractality meets entanglement: relation for non-ergodic extended states”, Phys. Rev. Lett. 124, 200602 (2020) [arXiv:2001.03173]
[B] I. M. K., M. Haque, and P. McClarty, “Eigenstate Thermalization, Random Matrix Theory and Behemoths”, Phys. Rev. Lett. 122, 070601 (2019) [arXiv:1806.09631].
[C] M. Haque, P. A. McClarty, I. M. K. , “Entanglement of mid-spectrum eigenstates of chaotic many-body systems—deviation from random ensembles.” [arXiv:2008.12782].
[D] A. Bäcker, I. M. K., M. Haque,, “Multifractal dimensions for chaotic quantum maps and many-body systems”, Phys. Rev. E 100, 032117 (2019) [arxiv:1905.03099].
[E] G. De Tomasi, I. M. K. , "Ergodic Entanglement of many-body multifractal states in quadratic Hamiltonians", in preparation

Video

Scientific Council of the Landau Institute, Friday, June 11, 2021, Landau ITP, 11:30 am
E.S. Andriyahina, I.S. Burmistrov
Interaction of a Neel-type skyrmion and a superconducting vortex

Superconductor-ferromagnet heterostructures hosting vortices and skyrmions are new area of an interplay between superconductivity and magnetism. We study [1] an interaction of a Neel-type skyrmion and a Pearl vortex in thin heterostructures due to stray fields. Surprisingly, we find that it can be energetically favorable for the Pearl vortex to be situated at some nonzero distance from the center of the Neel-type skyrmion. The presence of a vortex-antivortex pair is found to result in increase of the skyrmion radius. Our theory predicts that a spontaneous generation of a vortex-anti-vortex pair is possible under some conditions in the presence of a Neel-type skyrmion that is in agreement with recent experiment [2].
[1] E.S. Andriyakhina. and I.S. Burmistrov “Interaction of a Néel–type skyrmion and a superconducting vortex” // arXiv: https://arxiv.org/abs/2102.05434.
[2] A.P. Petrović et al. “Skyrmion-(Anti)vortex coupling in a chiral magnet-superconductor heterostructure” // Phys.Rev. Lett.126, 117205 (2021).

Video

Scientific Council of the Landau Institute, Friday, June 11, 2021, Landau ITP, 11:30 am
U.E. Khodaeva, M.A. Skvortsov
Vortex structure in a clean superconductor in the vicinity of a planar defect

We investigate the structure of the quasiparticle states localized in a superconducting vortex core in the vicinity of a planar defect. It is shown that even a highly transparent defect leads to a significant modification of the excitation spectrum, with the opening of a minigap at the Fermi energy. The magnitude of the minigap exceeds the mean level spacing for a clean vortex already for small values of the reflection coefficient off the defect. It is maximal for the vortex sitting right at the defect, decreases with increasing the distance from the defect and closes at some point. We then generalize the problem for various configurations of several linear defects (periodic structures, two crossing lines, stars). Though the minigap remains, a strong commensurability effect is observed. For two crossing linear defects, the magnitude of the minigap strongly depends on how close the intersection angle is to a rational number.

Video

Department of quantum mesoscopics: seminar, Friday, June 4, 2021, ITF, 4:15 pm
A.S. Osin and Ya.V. Fominov
Superconducting phases and the second Josephson harmonic in tunnel junctions between diffusive superconductors

(Ceminar v normal'nom rezhime, s translyatsiei: Zoom)

We consider a planar SIS-type Josephson junction between diffusive superconductors (S) through an insulating tunnel interface (I). We construct fully self-consistent perturbation theory with respect to the interface conductance. As a result, we find correction to the first Josephson harmonic and calculate the second Josephson harmonic. At arbitrary temperatures, we correct previous results for the nonsinusoidal current-phase relation in Josephson tunnel junctions, which were obtained with the help of conjectured form of solution. Our perturbation theory also describes the difference between the phases of the order parameter and of the anomalous Green functions.

[1] A.S. Osin and Ya.V. Fominov, arXiv:2105.05786

Department of quantum mesoscopics: seminar, Friday, June 4, 2021, onlain, 3:00 pm
Ivan Khaimovich
Localization beyond convergence of the locator expansion or Correlation-induced localization

Recently, the standard picture of ALT in long-range systems [1, 2] has been argued from the following perspective. There have been reported [3-6] several counterintuitive examples of single-particle systems with long-range hopping, where almost all states are (at least power-law) localized even in a nominally ergodic regime, where the standard locator expansion breaks down [1, 2]. Some of these "new" models demonstrate critical behavior [3, 4] for disorder strengths below the ones at the ALT [1, 2]. In the other ones [6, 7] wave-function spatial decay rates obey a "mysterious" duality [6] mapping different powers of power-law bending [2]. The seminal paper on second-type models [7] has been unnoticed until recently.
What is the striking difference between the standard long-range models [1-2] and the new ones [3-8]? The systems [3-8] belong to a new universality class where correlations of hopping terms play a significant role in the localization properties. In my talk I address this intriguing question. I present a general approach applicable to all such models and uncover the role of correlations and the origin of the duality [A]. This method is based on the localization-delocalization principles both in the coordinate and in the momentum space. I will show that the so-called Mott's criterion (which is shown to be the sufficient criterion of ergodic delocalization) and the Anderson criterion (which is shown to be the sufficient criterion of localization) are not complimentary. There is a gap between their regions of validity where the non-ergodic extended phases and the novel correlation-induced localization may occur. To illustrate the above mentioned principles I will consider the models either with infinitely-long range hopping [3-5] or with power-law decaying ones [1-2, 6-8, A-F] and focus mostly on the latter class. The well-known example of the latter class is the power-law random banding matrices (PLRBM).
Furthermore, we extend the above mentioned models by a new class that bridge between the models with uncorrelated hopping and those with fully correlated ones. These are the models with translation-invariant (TI) long-range hopping terms which allow one to apply the above mentioned principles both in the coordinate and momentum spaces completing their phase diagrams. A remarkable feature of these random TI models is the presence of the Poisson spectral statistics within the delocalized phase. This goes against the common wisdom that the Poisson statistics signals of localization. The reason for such a behavior is that the Poisson spectral statistics emerges in the parameter region where the states in the coordinate space are, indeed, extended and weakly ergodic but those in the momentum space are localized. The common wisdom assumes by default that the states in momentum space are always chaotically extended. The TI models introduced in this paper constitute a class of models where this assumption fails. If time permits, I will mention some of the recent results on the topic for higher-dimensions, anisotropic models, and the effects of partial correlations in TI models.

Literature:
[1] L. S. Levitov, Europhy. Lett. 9, 83 (1989); Phys. Rev. Lett. 64, 547 (1990).
[2] A. D. Mirlin, Y. V. Fyodorov, F.-M. Dittes, J. Quezada, and T. H. Seligman, Phys. Rev. E 54, 3221 (1996).
[3] H. K. Owusu, K. Wagh, and E. A. Yuzbashyan, J. Phys. A: Math. Theor. 42, 035206 (2009).
[4] A. Ossipov, J. Phys. A 46, 105001 (2013).
[5] G. L. Celardo, R. Kaiser, and F. Borgonovi, Phys. Rev. B 94, 144206 (2016).
[6] X. Deng, V. E. Kravtsov, G. V. Shlyapnikov and L. Santos, Phys. Rev. Lett. 120, 110602 (2018).
[7] A. L. Burin and L. A. Maksimov, JETP Lett. 50, 338 (1989).
[8] J. T. Cantin, T. Xu, and R. V. Krems, Phys. Rev. B 98, 014204 (2018).
[A] P. Nosov, I. M. K., V. E. Kravtsov, “Correlation-induced localization” Phys. Rev. B 99, 104203 (2019) [arXiv:1810.01492]
[B] P. A. Nosov, I. M. K., “Robustness of delocalization to the inclusion of soft constraints in long-range random models”, Phys. Rev. B 99, 224208 (2019) [arXiv:1904.11509].
[C] A. Kutlin, I. M. K., “Renormalization to localization without a small parameter”, SciPost Phys. 8, 049 (2020) [arXiv:2001.06493]
[D] X. Deng, A. L. Burin, I. M. K., “Anisotropy-driven localization transition in quantum dipoles” [arXiv:2002.00013]
[E] A. G. Kutlin, I. M. K., "Multifractality emergence in energy stratified random models", in preparation
[F] W. C. Tang, I. M. K., “Non-ergodic delocalized phase with Poisson level statistics”, in preparation

Scientific Council of the Landau Institute, Friday, June 4, 2021, Landau ITP, 11:30 am
Vladimir Kravtsov
Dynamical phases in Rosenzweig-Porter model with a ’multifractal’ distribution of hopping

We consider a Rosenzweig-Porter (RP) random matrix model with broad distribution of off-diagonal matrix elements which emerge as a model equivalent to the Anderson model on random regular graph. In this work we study the survival probability of a quantum particle with wave function initially localized on one site described by such type of model and show that the strongly non-Gaussian, broad distribution of hopping leads to the stretch-exponential decay of survival probability with time. We show that the ergodic phase with the stretch-exponential behavior of survival probability emerges in this model exactly where on a finite Bethe lattice there is a multifractal phase and relate the stretch exponent $\kappa$ with the kernel \epsilon_{\beta} of the transfer-matrix equation on a Bethe lattice.
We also consider the extension of this RP model relaxing the symmetry inherited from the Bethe lattice and find lines of phase transitions from the exponential to the stretch exponential behavior of survival probability.

Department of quantum mesoscopics: seminar, Friday, May 28, 2021, ITF, 4:15 pm
A. V. Lunkin and M. V. Feigel’man
Non-equilibrium Sachdev-Ye-Kitaev model with quadratic perturbation

(Seminar v normal'nom rezhime, s translyatsiei: Zoom)

We consider a non-equilibrium generalization of the mixed SYK4 +SYK2 model and calculate the energy dissipation rate W(ω) that results due to periodic modulation of random quadratic matrix elements with a frequency ω. We find that W(ω) possesses a peak at ω close to the polaron energy spliting E1 found recently in [1], demonstrating physical significance of the energy E1. Next, we study the effect of energy pumping with a finite amplitude A1 at the resonance frequency ω = E1 and calculate, in presence of pumping, non-equilibrium boson distribution function and dissipation rate due to low-frequency parametric modulation. At sufficiently strong pumping, low-frequency dissipation rate recovers its large value, pertinent to the pure SYK4 limit.

Department of quantum mesoscopics: seminar, Friday, May 28, 2021, onlain, 3:00 pm
Milan P. Allan (TU Delft)
Direct evidence for Cooper pairing without a spectral gap in a disordered superconductors above Tc

(Nazhmite dlya podklyucheniya: Zoom)

Doklad po rabote arXiv:2101.08535

Seminar po fizike tverdogo tela IFM RAN (N.Novgorod), Tuesday, May 25, 2021, onlain, 4:00 pm
Sergey Frolov (University of Pittsburgh)
Tekushchee sostoyanie poiska fermionov Maiorany v nanoprovodakh

Seminar budet v Zume. Dostup - tol'ko po predvaritel'noi registratsii. Chtoby poluchit' dostup, napishite zaranee sekretaryu seminara Aleksandru Kopasovu kopasov@ipmras.ru.

Department of quantum mesoscopics: seminar, Friday, May 21, 2021, ITF, 3:00 pm
V. Zyuzin
Neustoichivost' nakachennykh dirakovskikh magnonnykh par

(Ceminar v normal'nom rezhime, s translyatsiei: Zoom)

V rabote byla teoreticheski issledovana nakachka magnonnykh par v Dirakovskie tochki spektra magnonov ferromagnetika na reshetke tipa pchelinye soty. V chastnosti byl rassmotren tak nazyvaemyi protsess Sula vtorogo roda, v kotorom, blagodarya vzaimodeistviyu mezhdu magnonami, dva kvanta elektromagnitnogo polya nakachki pogloshchayutsya sistemoi, v rezul'tate chego sozdaetsya para magnonov. Byl izuchen bozonnyi analog kuperovskoi lestnitsy protsessov pererasseyaniya takoi pary magnonov, i bylo pokazano, chto pri opredelennykh usloviyakh pary dirakovskikh magnonov yavlyayutsya naibolee neustoichivymi v sisteme, a chastota polya nakachki, sootvetstvuyushchaya energii dirakovskikh tochek, - rezonansnoi. Nami vydvinuto predpolozhenie, chto v rezul'tate takoi neustoichivosti ferromagnetik perekhodit v novoe sostoyanie s nulevoi namagnichennost'yu.

V.A Zyuzin, "Pumped Dirac magnons paired state”, arXiv:2012.12051

Landau ITP colloquium, Friday, May 21, 2021, Landau ITP, 11:30 am
Xiao-Li Huang and Yuli V. Nazarov (TU Delft)
Topology protection–unprotection transition: Example from multiterminal superconducting nanostructures

We show theoretically that in the superconducting nanostructures the gapped states of different topology are not always protected by separating gapless states. Depending on the structure design parameters, they can be either protected or not, with a protection–unprotection transition separating these two distinct situations. We build up a general theoretical description of the transition vicinity in the spirit of Landau theory. We speculate that similar protection–unprotection transitions may also occur for other realizations of topological protection in condensed matter systems.

Video

Department of quantum mesoscopics: seminar, Friday, May 14, 2021, ITF, 3:00 pm
Ul'yana Khodaeva
Struktura vikhrya v chistom sverkhprovodnike vblizi ploskogo defekta

(Ceminar v normal'nom rezhime, s translyatsiei: Zoom)

Rassmatrivaetsya zadacha o strukture sostoyanii, lokalizovannykh v kore vikhrya, raspolozhennogo vblizi granitsy mezhdu dvumya chistymi sverkhprovodnikami. Granitsa predpolagaetsya dostatochno prozrachnoi, s koeffitsientom otrazheniya R<<1, chto predstavlyaet soboi predel obratnyi standartnomu priblizheniyu tunnel'nogo gamil'toniana, primenimomu dlya plokho prozrachnykh granits. Pokazano, chto nalichie granitsy privodit k obrazovaniyu shcheli v spektre lokalizovannykh v kore vikhrya nizkoenergeticheskikh sostoyanii , kotoraya imeet poryadok velichiny E_g ~ Δ sqrt(R), chto uzhe pri ochen' malykh koeffitsientakh otrazheniya prevyshaet srednee rasstoyanie w_0 ~ Δ^2/E_F mezhdu urovnyami kiral'noi vetvi spektra. Shchel' maksimal'na, kogda vikhr' raspolozhen pryamo na defekte, umen'shaetsya po mere uvelicheniya rasstoyaniya b do defekta i zakryvaetsya pri k_F b ~ E_g/w_0. Issledovano takzhe obobshchenie zadachi na sluchai razlichnykh konfiguratsii s neskol'kimi granitsami (periodicheskie struktury, peresechenie dvukh lineinykh granits, «zvezda» s neskol'kimi luchami). Vo vsekh sluchayakh shchel' v spektre sokhranyaetsya, odnako v sluchae peresekayushchikhsya granits voznikaet sil'no vyrazhennyi effekt soizmerimosti, kogda velichina shcheli sushchestvenno zavisit ot togo, naskol'ko khorosho ugol peresecheniya (v edinitsakh polnoi okruzhnosti) approksimiruetsya ratsional'noi drob'yu.

Scientific Council of the Landau Institute, Friday, May 14, 2021, Landau ITP, 11:30 am
Alexander Shnirman (Karlsruhe Institute of Technology)
Microwave response of a chiral Majorana interferometer

We consider an interferometer based on artificially induced topological superconductivity and chiral 1D Majorana fermions. The (non-topological) superconducting island inducing the superconducting correlations in the topological substrate is assumed to be floating. This allows probing the physics of interfering Majorana modes via microwave response, i.e., the frequency dependent impedance between the island and the earth. Namely, charging and discharging of the island is controlled by the time-delayed interference of chiral Majorana excitations in both normal and Andreev channels. We argue that microwave measurements provide a direct way to observe the physics of 1D chiral Majorana modes.

Scientific Council of the Landau Institute, Friday, May 14, 2021, Landau ITP, 11:30 am
A.S. Ioselevich, N.S. Peshcherenko
Two-impurity scattering in quasi-one-dimensional systems

In a quasi-one-dimensional system (a tube) with low concentration of defects $n$ the resistivity $\rho$ has peaks (van-Hove singularities) as a function of Fermi-energy. We show that due to non-Born scattering effects a deep narrow gap should appear just in the center of each peak. The resistivity at the bottom of a gap ($\rho_{\min}\propto n^2$) is dominated by scattering at rare ``twin-pairs'' of close defects, while scattering at solitary defects is suppressed. This effect is characteristic for multi-channel systems, it can not be observed in strictly one-dimensional one.

Department of quantum mesoscopics: seminar, Friday, April 30, 2021, onlain, 5:30 pm
V.A.Zakharov i I.S.Burmistrov
Ostatochnaya ob'emnaya vyazkost' dvumernogo elektronnogo gaza s besporyadkom

(Nazhmite dlya podklyucheniya: Zoom)

Otlichnaya ot nulya ob'emnaya vyazkost' signaliziruet o narushenii masshtabnoi invariantnosti. My pokazali, chto besporyadok v dvumernom nevzaimodeistvuyushchem elektronnom gaze v perpendikulyarnom magnitnom pole privodit k nenulevoi ob'emnoi vyazkosti. Polucheno analiticheskoe vyrazhenie dlya usrednennoi po besporyadku ob'emnoi vyazkosti v bornovskom priblizhenii. Eta ostatochnaya ob'emnaya vyazkost' daet nizhnyuyu otsenku dlya ob'emnoi vyazkosti v dvumernykh elektronnykh sistemakh.

Department of quantum mesoscopics: seminar, Friday, April 30, 2021, onlain, 4:30 pm
E.S.Andriyakhina i I.S.Burmistrov
Vzaimodeistvie neelevskogo skirmiona i pirlovskogo vikhrya

(Nazhmite dlya podklyucheniya: Zoom)

Poslednee vremya nablyudaetsya bol'shoi eksperimental'nyi i teoreticheskii interes k geterostukturam, sostoyashchim iz ferromagnetika s magnitnymi skirmionami i sverkhprovodnika vtorogo roda. Obuslovleno eto perspektivoi sozdaniya ustoichivogo ob'ekta iz skirmiona i vikhrya s posleduyushchei vozmozhnost'yu manipulirovat' etoi sistemoi. V nashei rabote [1] izuchaetsya vzaimodeistvie neelevskogo skirmiona i pirlovskogo vikhrya za schet magnitnogo polya razmagnichivaniya. Nam udalos' pokazat', chto pri nekotorykh obstoyatel'stvakh pirlovskii vikhr' stremitsya raspolozhit'sya na nenulevom rasstoyanii ot tsentra skirmiona i chto etot effekt chuvstvitelen k kiral'nosti ferromagnetika. Pri etom kharakternyi razmer skirmiona uvelichivaetsya blagodarya vzaimodeistviyu s vikhrem. Takzhe nasha teoriya predskazyvaet vozmozhnost' spontannogo rozhdeniya pary vikhr'-antivikhr' v prisutstvii skirmiona, chto nakhoditsya v kachestvennom soglasii s rezul'tatami nedavnego eksperimenta [2].

[1] E.S. Andriyakhina. and I.S. Burmistrov “Interaction of a Néel–type skyrmion and a superconducting vortex” // arXiv: https://arxiv.org/abs/2102.05434.
[2] A.P. Petrović et al. “Skyrmion-(Anti)vortex coupling in a chiral magnet-superconductor heterostructure” // Phys.Rev. Lett.126, 117205 (2021).

Scientific Council of the Landau Institute, Friday, April 30, 2021, Landau ITP, 11:30 am
Kh. Nazaryan, M.V. Feigelman
Conductivity and thermoelectric coefficients of doped SrTiO3 at high temperatures

We developed a theory of electric and thermoelectric conductivity of lightly doped SrTiO3 in the non-degenerate region k_B T ≥ E_F , assuming that the major source of electron scattering is their interaction with soft transverse optical phonons present due to proximity to ferroelectric transition. We have used kinetic equation approach within relation-time approximation and we have determined energy-dependent transport relaxation time τ (E) by the iterative procedure. Using electron effective mass m and electron-transverse phonon coupling constant λ as two fitting parameters, we are able to describe quantitatively a large set of the measured temperature dependences of resistivity R(T ) and Seebeck coefficient S(T ) for a broad range of electron densities studied experimentally in recent paper by K.Behnia and his colleagues. In addition, we calculated Nernst ratio in the linear approximation over weak magnetic field in the same temperature range.

Department of quantum mesoscopics: seminar, Friday, April 23, 2021, online, 5:00 pm
Lara Faoro and Lev Ioffe
Critical behavior of surface code statistical models

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With the advent of large arrays of qubits coupled by high fidelity gates the possibility of forming a logical qubit from a large number of faulty physical ones becomes a reality. The problem of finding the probability of logical error, P(L), in a large system of size L ≫ 1 can be mapped into the statistical model, with P(L) that decreases to zero exponentially in the correctable phase. This decrease is sensitive to the probabilities of individual qubit errors, p, measurement errors, q, and error correlations, p_corr. Using a combination of numerical simulations and statistical physics arguments, we identify the regime in the parameter (p , q, p_corr) space in which error correction is possible and we find the logical error dependence on the system size. We argue that for surface code statistical models describing optimal error decoding at the Nishimori line, the critical exponent of the correlation length can be found exactly thereby allowing one to determine P(L) with limited numerical work. As the transition line is approached, we show that the dynamics in the models slows down dramatically due to Griffiths phase physics in these disordered systems, especially at large q. This result as well as the effect of p_corr show that at q ∼ p the error correction is much more difficult than for q ≪ p.

Scientific Council of the Landau Institute, Friday, April 23, 2021, Landau ITP, 11:30 am
P.D. Grigor'ev
Otchet sektora elektronnykh i opticheskikh svoistv tverdykh tel
Department of quantum mesoscopics: seminar, Friday, April 9, 2021, ITF, 3:00 pm
Anton Khvalyuk, Konstantin Kechedzhi
Analiticheskaya teoriya k nekotorym ne-integriruemym diskretnym kvantovym skhemam konechnogo razmera v prisutstvii shuma

(Ceminar v normal'nom rezhime, s translyatsiei: Zoom)

V etom doklade ya predstavlyu perturbativnoe rassmotrenie, razrabotannoe dlya spetsial'nogo tipa kvantovykh skhem v prisutstvii shuma. Poslednie yavlyayutsya ob'ektom prakticheskogo interesa, poskol'ku oni dopuskayut estestvennuyu realizatsiyu na sushchestvuyushchikh kvantovykh ustroistvakh posredstvom posledovatel'nostei mikrovolnovykh impul'sov v sisteme sverkhprovodyashchikh kubitov. My rassmatrivaem odnomernye periodicheskie (Floke-) diskretnye skhemy, sostavlennye iz tak nazyvaemykh «dual-unitary» dvukhkubitnykh geitov, kotorye mogut byt' grubo opredeleny kak unitarnye kak vo vremennom, tak i v prostranstvennom napravleniyakh. Khotya nekotoroe ponimanie unitarnoi dinamiki v dannom tipe skhem sushchestvuet v literature, ochen' malo izvestno o vliyanii slabykh vozmushchenii, narushayushchikh svoistvo «dual-unitary», ne govorya uzhe pro effekt ot vneshnego shuma v sisteme. My predlagaem prostuyu model' s odnokubitnym shumom i provodim kak chislennyi, tak i analiticheskii analiz modeli, v osnovnom rassmatrivaya povedenie parnogo korrelyatora dvukh operatorov kak funktsii koordinat i vremeni. Poluchennye dannye predostavlyayut nekotoroe kachestvennoe ponimanie opisaniya dannykh neintegriruemykh sistem. Vazhno otmetit', chto razrabotannyi podkhod yavlyaetsya osnovoi dlya bolee detal'nogo izucheniya predlagaemoi modeli v razlichnykh rezhimakh. Nakonets, nashi rezul'taty mogut byt' ispol'zovany kak sposob kharakterizovat' lokal'nyi shum v fizicheskoi sisteme iz mnogikh kubitov.

Department of quantum mesoscopics: seminar, Friday, April 2, 2021, onlain, 5:00 pm
Kostya Kechedzhi (Google AI Quantum, Venice, California, USA)
Quantum circuit kinetics of programmable quantum computers

(Nazhmite dlya podklyucheniya: Zoom)

Recent demonstration of the first beyond classical quantum computation with a programmable quantum computer [1] opens the path to discovery of new quantum physics phenomena using these hardware systems. Gate model quantum computers used in [1] realize complex multi-qubit evolution in terms of discrete gates, elementary one and two qubit unitary operations, practically realized by a local time-dependent Hamiltonian. In this talk we present a theory and experimental data describing quantum circuit kinetics in such systems, which answers the following question: how does a system initialized in a product state generate highly entangled states and in the case of a non-integrable system approach universal random matrix statistics? We characterize circuit kinetics using time-dependent evolution of out-of-time-order correlators (OTOC) and their ensemble variance. We demonstrate that dynamics of an ensemble average OTOC can be mapped onto a classical population dynamics process, an analog of hydrodynamic description. OTOC variance on the other hand is subject to a “sign problem” and therefore evades efficient classical description. We use ensemble average OTOC to verify hardware output whereas OTOC variance provides a signature of operator entanglement generated in the system [2].

[1] Arute et. al. Nature, Vol 574, 505 (2019)
[2] Mi et. al. arXiv:2101.08870

Department of quantum mesoscopics: seminar, Friday, March 26, 2021, onlain, 4:30 pm
Boris Spivak
Propagation of nonlinear waves in disordered media

(Nazhmite dlya podklyucheniya: Zoom)

We study the propagation of stationary waves in disordered nonlinear media described by the nonlinear Schrödinger equation and show that, for given boundary conditions and a given coherent wave incident on the sample, the number of solutions of the equation increases exponentially with sample size. We also discuss the ballistic case, the sensitivity of the solutions to the change of external parameters, the similarity of this problem to the problem of spin glasses, and time-dependent solutions.

Department of quantum mesoscopics: seminar, Friday, March 26, 2021, ITF, 3:00 pm
Viktor Galitskii
Many-Body Level Statistics of Single-Particle Quantum Chaos

(Obychnyi doklad s onlain translyatsiei: Zoom)

We consider a noninteracting many-fermion system populating levels of a unitary random matrix ensemble —a generic model of single-particle quantum chaos. We study the corresponding many-particle level statistics by calculating the spectral form factor analytically using algebraic methods of random matrix theory, and match it with an exact numerical simulation. Despite the integrability of the theory, the many-body spectral rigidity is found to have a surprisingly rich landscape. In particular, we find a residual repulsion of distant many-body levels stemming from single-particle chaos, together with islands of level attraction. These results are encoded in an exponential ramp in the spectral form factor, which we show to be a universal feature of nonergodic many-fermion systems embedded in a chaotic medium.

Scientific Council of the Landau Institute, Friday, March 12, 2021, Landau ITP, 11:30 am
A. V. Khvalyuk, M. V. Feigel'man
Order parameter distribution in strongly disordered superconductors: analytical theory

We analyze the spatial distribution of the order parameter \Delta(r) in strongly disordered superconductors close to Superconductor-Insulator Transition. The analysis is based on a model of a superconductor on a locally loopless lattice with a pseudogap. We derive and solve a set of equations for the local distribution function P(\Delta) at zero temperature. The results are applicable both in the region of relatively small disorder corresponding to a Gaussian profile of P(\Delta) and in the region of strong fluctuation. The analytical results are in excellent agreement with the direct numerical solution of the self-consistency equations.

Scientific Council of the Landau Institute, Friday, March 12, 2021, Landau ITP, 11:30 am
K.S. Tikhonov, A.S. Ioselevich and M.V. Feigel'man
Tail states and unusual localization transition in low-dimensional Anderson model with power-law hopping

We study determininstic power-law quantum hopping model local Gaussian disorder in low dimensions d = 1, 2 under the condition d < β < 3d/2. We demonstrate unusual combination of exponentially decreasing density of the ”tail states” and localization-delocalization transition (as function of disorder strength w) pertinent to a small (vanishing in thermodynamic limit) fraction of eigenstates. In a broad range of parameters density of states ν(E) decays into the tail region E < 0 as simple exponential. We develop simple analytic theory which describes E0 dependence on power-law exponent β, dimensionality d and disorder strength W , and compare its predictions with exact diagonalization results. At low energies within the bare ”conduction band”, all eigenstates are localized due to strong quantum interference at d = 1, 2; however localization length grows fast with energy decrease, contrary to the case of usual Schrodinger equation with local disorder.

Scientific Council of the Landau Institute, Friday, March 5, 2021, Landau ITP, 11:30 am
Pavel Ostrovskii
Anizotropiya spinovogo perenosa namagnichennosti i relaksatsii Gil'berta za schet vzaimodeistviya Rashby

Postroena mikroskopicheskaya teoriya protsessov perenosa namagnichennosti, zatukhaniya Gil'berta i effektivnoi perenormirovki spina v dvumernom elektronnom gaze so spin-orbital'nym vzaimodeistviem Rashby v gryaznom predele. Pokazano, chto eti yavleniya imeyut sil'no vyrazhennuyu anizotropiyu. V sluchae dvukh chastichno zapolnennykh spinovykh podzon ustanovleno neozhidannoe sootnoshenie mezhdu tremya perechislennymi velichinami. Pri otsutstvii vneshnego magnitnogo polya i drugikh sil, deistvuyushchikh na namagnichennost', eto sootnoshenie predskazyvaet dvizhenie magnitnoi tekstury pod deistviem elektricheskogo toka s klassicheskoi skorost'yu dreifa elektronov provodimosti. Takzhe rassmotrena dinamika namagnichennosti pod deistviem vneshnego elektricheskogo polya (effekt Edel'shteina).
I.A. Ado, P.M. Ostrovsky, and M. Titov, Phys. Rev. B 101, 085405 (2020).

Scientific Council of the Landau Institute, Friday, March 5, 2021, Landau ITP, 11:30 am
Pavel Ostrovskii
Rasseyanie medlennykh chastits s anizotropnym spektrom

Predlozhena prostaya kvantovaya model' rasseyaniya na tochechnykh potentsial'nykh primesyakh pri proizvol'nom zakone dispersii. Eta model' obobshchaet standartnoe priblizhenie medlennykh chastits v kvantovoi mekhanike na anizotropnyi sluchai i pozvolyaet polnost'yu opisat' energeticheskuyu zavisimost' secheniya rasseyaniya pri pomoshchi odnogo fenomenologicheskogo parametra, analogichnogo faze rasseyaniya v s-kanale. Odnim iz neposredstvennykh sledstvii postroennoi teorii okazyvaetsya nezavisimost' vremeni svobodnogo probega nositelei v poluprovodnikakh ot ikh polozheniya na fermi-poverkhnosti.

Department of quantum mesoscopics: seminar, Friday, February 26, 2021, onlain, 3:00 pm
Khachatur Nazaryan i Mikhail Feigel'man
Conductivity and thermoelectric coefficients of doped SrTiO3 at high temperatures

(Nazhmite dlya podklyucheniya: Zoom)

We developed a theory of electric and thermoelectric conductivity of lightly doped SrTiO3 in the non-degenerate region k_B T ≥ E_F , assuming that the major source of electron scattering is their interaction with soft transverse optical phonons present due to proximity to ferroelectric transition. We have used kinetic equation approach within relation-time approximation and we have determined energy-dependent transport relaxation time τ(E) by the iterative procedure. Using electron effective mass m and electron-transverse phonon coupling constant λ as two fitting parameters, we are able to describe quantitatively a large set of the measured temperature dependences of resistivity R(T) and Seebeck coefficient S(T) for a broad range of electron densities studied experimentally in recent paper by K.Behnia and his colleagues. In addition, we calculated Nernst ratio in the linear approximation over weak magnetic field in the same temperature range.

Scientific Council of the Landau Institute, Friday, February 26, 2021, Landau ITP, 11:30 am
I.S. Burmistrov
Multifractally-enhanced superconductivity in thin films

The multifractal superconducting state originates from the interplay of Anderson localization and interaction effects. In this article we overview the recent theory of the superconductivity enhancement by multifractality and extend it to describe the spectral properties of superconductors on the scales of the order of the superconducting gap. Specifically, using the approach based on renormalization group within the nonlinear sigma model, we develop the theory of a multifractal superconducting state in thin films. We derive a modified Usadel equation that incorporates the interplay of disorder and interactions at energy scales larger than the spectral gap and study the effect of such an interplay on the low-energy physics. We determine the spectral gap at zero temperature which occurs to be proportional to the multifracally enhanced superconducting transition temperature. The modified Usadel equation results in the disorder-averaged density of states that, near the spectral gap, resembles the one obtained in the model of a spatially random superconducting order parameter. We reveal strong mesoscopic fluctuations of the local density of states in the superconducting state. Such strong mesoscopic fluctuations imply that the interval of energies in which the superconducting gap establishes is parametrically large in systems with multifractally-enhanced superconductivity.

Department of quantum mesoscopics: seminar, Friday, February 5, 2021, onlain, 3:00 pm
Konstantin Tikhonov
Obobshchayushchaya sposobnost' neirosetevykh variatsionnykh anzatsev dlya frustrirovannykh magnitnykh sistem (podrobnoe izlozhenie raboty, kratko predstavlennoi na US ITF 4 dekabrya 2020)

(Nazhmite dlya podklyucheniya: Zoom)

Sostoyaniya, opisyvaemye neironnymi setyami, (NQS) privlekayut bol'shoe vnimanie v svyazi s ikh potentsialom stat' ochen' vyrazitel'nym variatsionnym priblizheniem dlya opisaniya kvantovykh mnogochastichnykh sistem. My issleduem faktory, kotorye opredelyayut primenimost' NQS k frustrirovannym magnetikam, obuchaya set' na osnovnykh sostoyaniyakh neskol'kikh Gamil'tonianov s ispol'zovaniem nebol'shogo podmnozhestva znachenii tochno naidennoi volnovoi funktsii. My pokazyvaem, chto sposobnost' seti k obobshcheniyu, -- sposobnost' vosstanovit' osnovnoe sostoyanie po nebol'shomu chislu amplitud i pravil'no predskazat' volnovuyu funktsiyu v ostal'noi chasti Gil'bertova prostranstva, rezko padaet s perekhodom vo frustrirovannyi rezhim. My demonstriruem, chto v nastoyashchii moment, dlya ispol'zovaniya NQS v izuchenii realistichnykh modelei, sushchestvenna imenno obobshchayushchaya sposobnost', a ne vyrazitel'nost' takogo anzatsa. Po rabote: T. Westerhout, N. Astrakhantsev, K. S. Tikhonov, M. I. Katsnelson, A. A. Bagrov, “Generalization properties of neural network approximations to frustrated magnet ground states”, Nature Communications 11, 1593 (2020).

Department of quantum mesoscopics: seminar, Friday, January 29, 2021, onlain, 3:00 pm
Egor Kiselev (Karlsruhe Institute of Technology)
Dirac liquids: phase space dynamics, non-local transport properties and collective excitations

(Nazhmite dlya podklyucheniya: Zoom)

It is shown that hydrodynamic collision processes in undoped graphene can be described in terms of a Fokker-Planck equation with a fractional derivative, corresponding to Lévy flights in momentum space. Electron-electron collisions give rise to frequent small-angle scattering processes that are interrupted by rare large-angle events. The latter give rise to a superdiffusive phase-space dynamics whose importance for relaxation processes, charge and heat transport, as well as collective modes will be discussed. I will also comment on implications for experiments with surface acoustic waves in graphene-piezoelectric devices.

Scientific Council of the Landau Institute, Friday, January 29, 2021, Landau ITP, 11:30 am
M.V. Feigel'man
Otchet sektora kvantovoi mezoskopiki
Department of quantum mesoscopics: seminar, Friday, January 15, 2021, onlain, 3:00 pm
Vladimir Zyuzin
Nechetnoe po chastote sparivanie v neuporyadochennykh provodnikakh

(Nazhmite dlya podklyucheniya: Zoom)

V etoi rabote my izuchili nechetnoe po chastote sparivanie, sparivanie Berezinskogo, v neuporyadochennykh provodnikakh. V pervom primere nami byl rassmotren sluchai neuporyadochennoi sistemy vblizi ee perekhoda v izolyator. Tam sovmestnoe deistvie kulonovskogo vzaimodeistviya i rasseyaniya na primesyakh mozhet byt' istochnikom tripletnogo sparivaniya nechetnogo po chastote. V drugom rassmotrennom nami sluchae takoe sparivanie mozhet proiskhodit' vblizi sverkhprovodyashchego perekhoda. V svyazi s etim my rassmotreli sushchestvuyushchie eksperimenty i sdelali predpolozhenie, chto nechetnoe po chastote sparivanie privodit k dielektricheskomu sostoyaniyu. Takoe sostoyanie mozhet byt' rassmotreno kak nezavisimaya faza, sushchestvuyushchaya pered sverkhprovodyashchim perekhodom.

Po rabote Vladimir A. Zyuzin, Alexander M. Finkel’stein, "Odd-frequency spin-triplet instability in disordered electron liquid", arXiv:1912.04258

Department of quantum mesoscopics: seminar, Friday, December 18, 2020, onlain, 4:00 pm
Leonid Glazman (Yale University)
Critical fluorescence of a transmon at the Schmid transition

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We investigate inelastic microwave photon scattering by a transmon qubit embedded in a high-impedance circuit. The transmon undergoes a charge-localization (Schmid) transition upon the impedance reaching the critical value. Due to the unique transmon level structure, the fluorescence spectrum carries a signature of the transition point. At higher circuit impedance, quasi-elastic photon scattering may account for the main part of the inelastic scattering cross-section; we find its dependence on the qubit and circuit parameters. Our theory of total inelastic cross-section explains the measured quality factors of devices consisting of a transmon in a galvanic connection with a high-impedance Josephson junction array.

Department of quantum mesoscopics: seminar, Friday, December 11, 2020, onlain, 4:00 pm
Vladislav Kurilovich (Yale University)
Quantum-critical dynamics of a Josephson junction at the topological transition

(Nazhmite dlya podklyucheniya: Zoom)

Superconducting quantum wires with broken time-reversal and spin-rotation symmetries are emerging as a building block of a topological qubit. By varying the applied external magnetic field or chemical potential such wires may be tuned through the quantum critical point into the topological phase. We investigate signatures of this quantum phase transition in the dynamic response of a Josephson junction. To this end, we compute the finite-frequency admittance Y(ω) of the junction at the topological transition and in its vicinity. At the critical point, the dependence of the admittance on frequency and temperature is universal and determined by the symmetries of the system. We find that, despite the closing of the spectral gap at the transition, the dissipative response may remain weak: ReY(ω) ~ max(ω, T)². This unusual power-law behavior is markedly different from the electromagnetic response of a normal metal. Away from the critical point, a gap opens in the spectrum of the system. The dependence of the admittance on frequency, temperature, and spectral gap has a scaling form. We apply our universal theory to find the scaling functions for different combinations of symmetries.

Landau ITP colloquium, Friday, December 4, 2020, Landau ITP, 4:00 pm
Katharina Franke (Free University Berlin, Germany)
Photon-assisted tunneling at the atomic scale: Probing resonant Andreev reflections from Yu-Shiba-Rusinov states

Exchange coupling of magnetic adsorbates to a superconducting substrate leads to Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap. These can be probed by scanning tunneling spectroscopy as a pair of resonances at positive and negative bias voltage and over a wide range of tunnel conductances. At low tunneling rates, the current is carried by single-electron processes, where each excitation is sufficiently quickly followed by a relaxation into the energetic continuum. Upon increasing the junction conductance, the relaxation rates suppress single-electron tunneling and resonant Andreev processes start to dominate the transport process. The cross-over of these processes is expressed in the variation of the ratio of YSR peak height at positive and negative bias voltage [1]. Here, we investigate these transport processes by photon-assisted tunneling. While applying highfrequency radiation to the tunneling junction, we record the differential conductance spectra in the low and high-conductance regime. At low conductance, the YSR states exhibit symmetrically spaced sidebands with their spacing directly evidencing single-electron tunneling. Surprisingly, at large junction conductance, the spacing remains the same while the patterns become asymmetric. We show that this asymmetry is direct evidence of a resonant Andreev reflection with tunneling threshold conditions imposed on its electron and hole component [2]. We suggest that photon-assisted tunneling can be a powerful tool for the determination of the nature of the charge carriers in a single tunneling event.
References
[1] M. Ruby, F. Pientka, Y. Peng, F. von Oppen, B.W. Heinrich, and K.J. Franke, "Tunneling Processes into Localized Subgap States in Superconductors", Phys. Rev. Lett. 115, 087001 (2015).
[2] O. Peters, N. Bogdanoff, S. Acero González, L. Melischek, J. Rika Simon, Gaël Reecht, C.B. Winkelmann, Felix von Oppen & K.J. Franke, "Resonant Andreev reflections probed by photon-assisted tunnelling at the atomic scale", Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-0972-z; arXiv:2001.09534

Video

Scientific Council of the Landau Institute, Friday, December 4, 2020, Landau ITP, 11:30 am
M.V. Feigel'man
Quantum breakdown of superconductivity in low-dimensional materials

A brief overview of the main results of the experimental and theoretical works of the last ~ 10 years on quantum phase superconductor-insulator and superconductor-metal transitions will be done. In addition, it is supposed to provide some list of the main unsolved problems in this area.
B. Sacépé, M. Feigel'man, T.M. Klapwijk, Quantum breakdown of superconductivity in low-dimensional materials, Nature Physics 16, 734-746 (2020); arXiv:2007.15870

Scientific Council of the Landau Institute, Friday, December 4, 2020, Landau ITP, 11:30 am
K.S. Tikhonov
Obobshchayushchaya sposobnost' neirosetevykh variatsionnykh anzatsev dlya frustrirovannykh magnitnykh sistem

Sostoyaniya, opisyvaemye neironnymi setyami, (NQS) privlekayut bol'shoe vnimanie v svyazi s ikh potentsialom stat' ochen' vyrazitel'nym variatsionnym priblizheniem dlya opisaniya kvantovykh mnogochastichnykh sistem. My issleduem faktory, kotorye opredelyayut primenimost' NQS k frustrirovannym magnetikam, obuchaya set' na osnovnykh sostoyaniyakh neskol'kikh Gamil'tonianov s ispol'zovaniem nebol'shogo podmnozhestva znachenii tochno naidennoi volnovoi funktsii. My pokazyvaem, chto sposobnost' seti k obobshcheniyu, -- sposobnost' vosstanovit' osnovnoe sostoyanie po nebol'shomu chislu amplitud i pravil'no predskazat' volnovuyu funktsiyu v ostal'noi chasti Gil'bertova prostranstva, rezko padaet s perekhodom vo frustrirovannyi rezhim. My demonstriruem, chto v nastoyashchii moment, dlya ispol'zovaniya NQS v izuchenii realistichnykh modelei, sushchestvenna imenno obobshchayushchaya sposobnost', a ne vyrazitel'nost' takogo anzatsa.
Po rabote: T. Westerhout, N. Astrakhantsev, K. S. Tikhonov, M. I. Katsnelson, A. A. Bagrov, “Generalization properties of neural network approximations to frustrated magnet ground states”, Nature Communications, volume 11, Article number: 1593 (2020)

Department of quantum mesoscopics: seminar, Friday, November 27, 2020, onlain, 3:00 pm
K.S. Tikhonov
Doklad po zadacham v rabote

(Nazhmite dlya podklyucheniya: Zoom)

Ya rasskazhu o zadachakh, nad kotorymi seichas rabotayu. Rabota nad chast'yu etikh zadach nakhoditsya na stadii podgotovki stat'i k publikatsii, nad nekotorymi – na etape formulirovki voprosa. Primernyi spisok:

  1. Neravnovesnyi instanton Korshunova v real'nom vremeni (Ya. Rodionov, A. Dotdaev)
  2. Elektronnaya defazirovka na poverkhnosti topologicheskogo izolyatora (I. Gornyi)
  3. Eksponentsial'nye khvosty v lokalizatsii so stepennymi pryzhkami (M. Feigel'man, A. Ioselevich)
  4. Eilerovskaya neustoichivost' s besporyadkom (S. Evseev, I. Burmistrov, V. Kachorovskii)
  5. Korrelyatsii v rasseyannoi na dvukhurovnevoi sisteme volne (N. Masnev, Yu. Makhlin)
  6. Sostoyaniya neironnykh setei dlya kvantovykh magnetikov (A. Bagrov, N. Astrakhantsev, T. Westerhout)

Scientific Council of the Landau Institute, Friday, November 27, 2020, Landau ITP, 11:30 am
A. S. Ioselevich, N. S. Peshcherenko
Nebornovskie effekty v rasseyanii elektronov v provodyashchei poloske s nizkoi kontsentratsiei primesei

Plotnost' sostoyanii i soprotivlenie chistykh kvaziodnomernykh sistem obladayut osobennostyami Van Khova pri prokhozhdenii urovnya Fermi cherez dno podzony poperechnogo kvantovaniya. Odnako, uchet rasseyaniya na primesyakh dolzhen zamyvat' eti osobennosti. Kak bylo pokazano v nashei predydushchei rabote [Phys. Rev. B 99, 035414 (2019)] dlya sluchaya chistykh provodyashchikh trubok, kharakter zamytiya osobennostei sushchestvenno zavisit ot kontsentratsii primesei $n$. Dlya $n>n_c$ osobennosti prosto razmyvayutsya, v to vremya kak dlya $n < n_c$ iznachal'naya osobennost' asimmetrichno rasshcheplyaetsya v 2 pika dlya prityagivayushchikh primesei. Zdes' $n_c$ — kriticheskaya kontsentratsiya primesei. V dannoi rabote my obobshchaem eti rezul'taty dlya sluchaya «polosok» — kvaziodnomernykh struktur v dvumernykh provodnikakh. V etom sluchae pri $n < n_c$ takzhe imeet mesto asimmetrichnoe rasshcheplenie osobennosti v 2 pika. Odnako, v otlichii ot sluchaya trubki, amplitudy rasseyaniya na primesyakh zavisyat ot ikh pozitsii, i eti piki okazyvayutsya neodnorodno ushirennymi. Naibolee sil'no ushiryaetsya levyi maksimum, voznikayushchii (tol'ko dlya sluchaya prityagivayushchikh primesei) vsledstvie rasseyaniya na kvazistatsionarnykh urovnyakh. Dannye urovni, v svoyu ochered', obrazuyut otnositel'no shirokuyu primesnuyu zonu so slaboi osobennost'yu, pokhozhei na osobennost' Van Khova, na nizhnei granitse. Razlichnye uchastki zavisimosti soprotivleniya opredelyayutsya vkladami razlichnykh grupp primesei. Tak, vblizi minimuma soprotivleniya sil'nee vsego rasseivayut, chto udivitel'no, samye «slabye» primesi, raspolozhennye vblizi uzlov volnovoi funktsii. Oblast' kvazi-osobennosti Van Khova vblizi levogo maksimuma opredelyaetsya rasseyaniem na «sil'nykh» primesyakh, raspolozhennykh vblizi puchnostei volnovoi funktsii.

Landau ITP colloquium, Friday, November 20, 2020, Landau ITP, 4:00 pm
Andrei Bernevig (Princeton University, USA)
Mòire Samples: The twisted bilayer graphene scenario

We present a full theory of the interacting insulating phases of twisted bilayer graphene around the first magic angle where the bandwidth of the “active” bands becomes very small. We show that the single particle Hamiltonian is fully anomalous: it contains stable topology for every set of bands. Furthermore, we analyze the Coulomb interaction and obtain exact insulating ground states as well as the full excitation spectrum in certain limits.

Video

Scientific Council of the Landau Institute, Friday, November 20, 2020, Landau ITP, 11:30 am
A.Ya. Maltsev
Reconstructions of the electron dynamics in magnetic field and the geometry of complex Fermi surfaces

We consider the semiclassical dynamics of electrons on complex Fermi surfaces in the presence of strong magnetic fields. The reconstructions of the general topological structure of such dynamics are accompanied by the appearance of closed extremal trajectories of a special form, closely related to geometry and topology of the Fermi surface. The study of oscillation phenomena on such trajectories allows, in particular, to propose a relatively simple method for refining the parameters of the dispersion relation in metals with complex Fermi surfaces.

Manchester Condensed Matter Theory Seminar, Wednesday, November 18, 2020, online, 7:00 pm
Ignat Fialkovsky, UFABC
Casimir-Polder force and torque for anisotropic molecules, with possible implications on separation membranes

(Link: Zoom)

We derive the Casimir-Polder force and torque for an anisotropic molecule close to a conducting plane with a general tensorial conductivity. The results are then applied to study carbon dioxide CO2 molecule behaviour close to a Hall plane and to a free standing graphene. The Casimir-Polder torque acting on this linear molecule is found to favor the orientation of its main axis perpendicular to the surface, thus potentially facilitating penetration of the molecule through a membrane.

Department of quantum mesoscopics: seminar, Friday, November 13, 2020, onlain, 3:00 pm
Nicolas Roch, Benjamin Sacepe, Thibault Charpentier (Neel Institute, Grenoble)
The fate of the superfluid density near the SIT in amorphous superconductors

(Nazhmite dlya podklyucheniya: Zoom)

Superconducting films of amorphous Indium Oxide (a:InO) thin films undergo a transition to insulation upon increasing disorder. This quantum phase transition is driven by the localization of preformation of Cooper pairs, which has been evidenced through various unusual spectroscopic properties. The continuous decrease of the critical temperature as the critical disorder approaches indicates a similarly continuous suppression of the superfluid density. In this talk I discuss the fate of the superfluid density in the vicinity of the transition to insulation in a:InO films. To access the superfluid density we conducted a systematic study of the whole plasmon dispersion spectrum of high quality factor microwave resonators made of a:InO, combined with DC resistivity measurements, as a function of disorder. Our main observations are a breakdown of the Mattis-Bardeen theory and a record kinetic inductance of 17nH per square for critical, superconducting samples. Surprisingly, we find that the superfluid density remains finite at the critical disorder. Besides providing new insights to understand the SIT in a:InO, these results may find applications in superconducting quantum circuits where very large inductances are highly desirable.

Scientific Council of the Landau Institute, Friday, November 6, 2020, Landau ITP, 11:30 am
D.S. Antonenko, M.A. Skvortsov
Superconductivity Suppression in Disordered Films: Interplay of Two-dimensional Diffusion and Three-dimensional Ballistics

Suppression of the critical temperature in homogeneously disordered superconducting films is a consequence of the disorder-induced enhancement of Coulomb repulsion. We demonstrate that for the majority of thin films studied now this effect cannot be completely explained in the assumption of two-dimensional diffusive nature of electrons motion. The main contribution to the $T_c$ suppression arises from the correction to the electron-electron interaction constant coming from small scales of the order of the Fermi wavelength that leads to the critical temperature shift $\delta T_c/T_{c0} \sim - 1/k_Fl$, where $k_F$ is the Fermi momentum and $l$ is the mean free path. Thus almost for all superconducting films that follow the fermionic scenario of $T_c$ suppression with decreasing the film thickness, this effect is caused by the proximity to the three-dimensional Anderson localization threshold and is controlled by the parameter $k_Fl$ rather than the sheet resistance of the film.
D.S. Antonenko, M.A Skvortsov,. JETP Lett. (2020). https://doi.org/10.1134/S0021364020190017

Department of quantum mesoscopics: seminar, Friday, October 30, 2020, onlain, 3:00 pm
Leonid Golub, Mikhail Glazov (FTI im.A.F.Ioffe)
Dolinnyi effekt Kholla v usloviyakh effekta uvlecheniya

(Nazhmite dlya podklyucheniya: Zoom)

Dolinnyi effekt Kholla zaklyuchaetsya v poyavlenii dolinnogo potoka chastits – protivopolozhno napravlennykh potokov v dvukh dolinakh zony Brillyuena. Dolinnyi potok perpendikulyaren iskhodnomu potoku chastits po analogii s tem, kak v spinovom effekte Kholla spinovyi tok poperechen elektricheskomu. Iskhodnyi potok chastits mozhet byt' zadan vneshnim polem, a mozhet byt' obuslovlen uvlecheniem nositelei zaryada ili eksitonov fononami ili fotonami. Tsel' doklada – izlozhit' razvituyu v rabotakh [1,2] teoriyu dolinnogo effekta Kholla v dvumernykh kristallakh v usloviyakh effekta uvlecheniya.

Pokazano, chto imeetsya tri vklada v dolinnyi tok: (i) ot asimmetrichnogo (skew) rasseyaniya (vklyuchaya kogerentnoe rasseyanie na parakh primesyakh i dvukhfononnye protsessy asimmetrichnogo rasseyaniya) i anomal'nye vklady, obuslovlennye (ii) sdvigami volnovykh paketov pri rasseyanii i (iii) ikh anomal'noi skorost'yu. Prodemonstrirovana chastichnaya kompensatsiya anomal'nykh vkladov i ukazana ee fizicheskaya priroda. Vypolneno sopostavlenie dolinnykh tokov elektronov i eksitonov v usloviyakh effekta uvlecheniya s rezul'tatami dlya vneshnego staticheskogo polya i pri nalichii gradienta temperatury chastits (dolinnyi effekt Nernsta) i prodemonstrirovana chuvstvitel'nost' effekta k prirode tyanushchei sily. Dlya eksitonov takzhe provedeno obobshchenie dreifovo-diffuzionnoi modeli, uchityvayushchei dolinnye stepeni svobody, i rasschitany prostranstvennye profili eksitonnoi plotnosti i dolinnoi polyarizatsii.

[1] Valley Hall effect caused by the phonon and photon drag, M. M. Glazov and L. E. Golub, Phys. Rev. B 102, 155302 (1-16) (2020).
[2] Skew Scattering and Side Jump Drive Exciton Valley Hall Effect in Two-Dimensional Crystals, M. M. Glazov and L. E. Golub, Phys. Rev. Lett. 125, 157403 (1-7) (2020).

Scientific Council of the Landau Institute, Friday, October 23, 2020, Landau ITP, 11:30 am
I.S. Burmistrov
The effect of superconducting fluctuations on the ac conductivity of a 2D electron system in the diffusive regime

We report a complete analytical expression for the one-loop correction to the ac conductivity $\sigma(\omega)$ of a disordered two-dimensional electron system in the diffusive regime. The obtained expression includes the weak localization and Altshuler–Aronov corrections as well as the corrections due to superconducting fluctuations above superconducting transition temperature. The derived expression has no divergency in the static limit, $\omega\to 0$ , in agreement with general expectations for the normal state conductivity of a disordered electron system. The results are published in I.S. Burmistrov, Ann. Phys. 418, 168201 (2020).

Department of quantum mesoscopics: seminar, Friday, October 16, 2020, ITF, 3:00 pm
M.A. Skvortsov
Doklad po zadacham v rabote

  1. Perevernutyi mayatnik i smezhnye zadachi (N. Stepanov)
  2. Kvantovyi transport v maioranovskikh provodakh (D. Antonenko, P. Ostrovskii)
  3. Vikhr' u ploskoi stenki (U. Khodaeva)
  4. Kogerentnoe proskal'zyvanie fazy (A. Bun'kov, O. Astaf'ev)
  5. Fluktuatsii v odnofotonnykh detektorakh (A. Semenov)
  6. Neodnorodnyi sverkhtekuchii tok (M. Feigel'man, D. Fazlizhanova)
Seminar ochnyi, no s online translyatsiei dlya interesuyushchikhsya, no otsutstvuyushchikh: link na translyatsiyu

Scientific Council of the Landau Institute, Friday, October 16, 2020, Landau ITP, 11:30 am
Pavel Nosov, Igor Burmistrov, Srinivas Raghu
Interaction-induced metallicity in a two-dimensional disordered non-Fermi liquid

The interplay of interactions and disorder in two-dimensional (2D) electron systems has actively been studied for decades. The paradigmatic approach involves starting with a clean Fermi liquid and perturbing the system with both disorder and interactions. We instead start with a clean non-Fermi liquid near a 2D ferromagnetic quantum critical point and consider the effects of disorder. In contrast with the disordered Fermi liquid, we find that our model does not suffer from runaway flows to strong coupling and the system has a marginally stable fixed point with perfect conduction.

Scientific Council of the Landau Institute, Friday, October 16, 2020, Landau ITP, 11:30 am
M.A. Skvortsov and M.V. Feigel'man
Spectroscopic evidence for strong correlations between local resistance and superconducting gap in ultrathin NbN films

Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap Δ and of the critical temperature Tc, the ratio Δ/kBTc remaining roughly constant. In addition, in most films, an emergent granularity develops with increasing disorder and results in the formation of inhomogeneous superconducting puddles. These gap inhomogeneities are usually accompanied by the development of bosonic features: a pseudogap develops above the critical temperature Tc and the energy gap Δ starts decoupling from Tc. Thus the mechanism(s) driving the appearance of these gap inhomogeneities could result from a complicated interplay between fermionic and bosonic effects. By studying the local electronic properties of an NbN film with scanning tunneling spectroscopy (STS), we show that the inhomogeneous spatial distribution of Δ is locally strongly correlated to a large depletion in the local density of states (LDOS) around the Fermi level, associated to the Altshuler-Aronov effect induced by strong electronic interactions. By modeling quantitatively the measured LDOS suppression, we show that the latter can be interpreted as local variations of the film resistivity. This local change in resistivity leads to a local variation of Δ through a local Finkelstein mechanism. Our analysis furnishes a purely fermionic scenario explaining quantitatively the emergent superconducting inhomogeneities, while the precise origin of the latter remained unclear up to now.

Scientific Council of the Landau Institute, Friday, October 9, 2020, Landau ITP, 11:30 am
Konstantin Tikhonov
Critical behavior at the localization transition on random regular graphs

We study numerically the critical behavior at the localization transition in the Anderson model on infinite Bethe lattice and on random regular graphs. As a first step, we carry out a precise determination of the critical disorder. After this, we determine the dependence of the correlation volume on disorder on the delocalized side of the transition by means of population dynamics. The asymptotic critical behavior is found to be in agreement with analytical prediction based on the stability analysis of the symmetry-broken solution near criticality.

Scientific Council of the Landau Institute, Friday, October 9, 2020, Landau ITP, 11:30 am
M. Smith, A.V. Andreev, M.V. Feigel'man, and B.Z.Spivak
Conductivity of superconductors in the flux flow regime

We develop a theory of conductivity of type-II superconductors in the flux flow regime taking into account random spatial fluctuations of the system parameters, such as the gap magnitude Δ(r) and the diffusion coefficient D(r). We find a contribution to the conductivity that is proportional to the inelastic relaxation time τin, which is much longer than the elastic relaxation time. This new contribution is due to Debye-type relaxation, and it can be much larger than the conventional flux flow conductivity due to Bardeen and Stephen. The new contribution is expected to dominate in clean superconductors at low temperatures and in magnetic fields much smaller than Hc2.

Department of quantum mesoscopics: seminar, Friday, October 2, 2020, ITF, 3:00 pm
I.S. Burmistrov
Doklad po zadacham v rabote

  1. Metallicheskoe povedenie, indutsirovannoe vzaimodeistviem, v dvumernoi neuporyadochennoi ne-Fermi zhidkosti (sovmestno s P. Nosov i S. Raghu)
  2. Skirmiony i vikhri v geterostrukturakh sverkhprovodnik-kiral'nyi ferromagnetik (sovmestno s E. Andriyakhina)
  3. Magnitnye primesi v neuporyadochennykh sverkhprovodnikakh (sovmestno s A. Lyublinskaya, S. Babkin, B. Pashinskii)
  4. Mezoskopicheskie fluktuatsii plotnosti sostoyanii v neuporyadochennykh sverkhprovodnikakh (sovmestno s F. Evers, M. Stosiek)
  5. Vyazkost' i svyazannye s nei transportnye koeffitsienty v dvumernom neuporyadochennom elektronom gaze v magnitnom pole (sovmestno s V. Zakharov, M. Goldstein)
  6. Fluktuatsii izgiba palochki s uchetom besporyadka (sovmestno s S. Evseev, K. Tikhonov, V. Kachorovskii)

Seminar ochnyi, no s online translyatsiei dlya interesuyushchikhsya, no otsutstvuyushchikh: link na translyatsiyu

Scientific Council of the Landau Institute, Friday, October 2, 2020, Landau ITP, 11:30 am
Yu.N. Ovchinnikov
Singulyarnoe osnovnoe sostoyanie v mnogozonnykh neodnorodnykh sverkhprovodnikakh

V mnogozonnykh sverkhprovodnikakh sushchestvuyut sostoyaniya s narushennoi simmetriei otnositel'no obrashcheniya vremeni. My rassmotreli sistemy takogo tipa i pokazali, chto funktsional Ginzburga-Landau imeet osnovnoe sostoyanie bez ``vnutrennego'' magnitnogo polya -- s nulevym znacheniem plotnosti toka. Osnovnoe sostoyanie bylo issledovano v teorii vozmushchenii po slaboi neodnorodnosti. Bylo pokazano, chto v otsutstvie vneshnego magnitnogo polya neodnorodnosti ne privodyat k vozniknoveniyu spontannogo vnutrennego magnitnogo polya. Rasshirenie funktsional'nogo prostranstva s vklyucheniem singulyarnykh reshenii imeet bol'shoe znachenie dlya ponimaniya eksperimental'noi situatsii v mnogozonnykh sverkhprovodnikakh.
doklad po stat'e: Yu.N. Ovchinnikov, D.V. Efremov, Singular ground state of multiband inhomogeneous superconductors, Phys. Rev. B 99, 224508 (2019)

Department of quantum mesoscopics: seminar, Friday, September 25, 2020, ITF, 3:00 pm
M.V. Feigel'man
Doklad M.Feigel'mana s obzorom raznykh reshaemykh seichas s razlichnymi kollegami zadach

Fail s prezentatsiei: zdes'

Scientific Council of the Landau Institute, Friday, September 25, 2020, Landau ITP, 11:30 am
Yu.N. Ovchinnikov
Zeros of Riemann’s Zeta Functions in the Line z=1/2+it0

Investigation of Josephson effect, current flow in narrow superconducting stripes, dynamical states in superconductors lead to the necessity to deal with an important phenomenon: phase slip events. The study of the distribution of zeros for Riemann's Zeta function also requires an analisis of the same phenomenon.
It was found that, in addition to trivial zeros in points ($ z = -2N, N = 1, 2, ... $, natural numbers), the Riemann’s zeta function $\zeta(z)$ has zeros only on the line {$z = 1/2 + i t_0$, $t_0$ is real}. All zeros are numerated, and for each number, N, the positions of the non-overlap intervals with one zero inside are found. The simple equation for the determination of centers of intervals is obtained. The analytical function $\eta(z)$), leading to the possibility fix the zeros of the zeta function $\zeta(z)$, was estimated. To perform the analysis, the well-known phenomenon, phase-slip events, is used. This phenomenon is the key ingredient for the investigation of dynamical processes in solid-state physics, for example, if we are trying to solve the TDGLE (time-dependent Ginzburg-Landau equation).
J. Supercond. Novel Magn., 32(11), 3363-3368 (2019)

Scientific Council of the Landau Institute, Friday, September 25, 2020, Landau ITP, 11:30 am
A. V. Lunkin, A. Yu. Kitaev, and M. V. Feigel'man
Perturbed Sachdev-Ye-Kitaev model: a polaron in the hyperbolic plane.

We study the SYK model with a weak quadratic perturbation, beyond the simplest perturbative limit considered previously. For intermediate values of the perturbation strength, fluctuations of the Schwarzian mode are suppressed, and the mean-field solution remains valid beyond the expected interval. Out-of-time-order correlation function displays at short time intervals exponential growth with maximal Lyapunov exponent 2\piT, but its prefactor scales as T at low temperatures

Scientific Council of the Landau Institute, Friday, September 18, 2020, Landau ITP, 11:30 am
Yu.N. Ovchinnikov
Superconductors with Spin-Orbit Interaction in Low-Temperature Region

Issledovana prostranstvennaya i temperaturnaya zavisimost' parametra poryadka v tonkikh sverkhprovodyashchikh plenkakh v prodol'nom magnitnom pole i sil'nom spin-orbital'nom vzaimodeistvii obshchego vida v okrestnosti perekhoda v prostranstve: magnitnoe pole, spin-orbita, temperatura. Vblizi kriticheskoi poverkhnosti parametr poryadka est' neyavnaya funktsiya otnosheniya dvukh malykh parametrov: temperatury i samogo parametra poryadka. [1]
Issledovano vozmozhnoe obrazovanie poverkhnostnogo sostoyaniya v izolyatore s malym zazorom. Bylo podschitano, chto otvet zavisit ot real'nykh granichnykh uslovii na poverkhnosti. «Estestvennoe» granichnoe uslovie dlya obraztsa konechnogo razmera privodit k sokhraneniyu shcheli v spektre vozbuzhdeniya. [2]
[1] Yu.N. Ovchinnikov, Singular Temperature Dependence of the Equation of State of Superconductors with Spin-Orbit Interaction in Low-Temperature Region – II, J. Supercond. Novel Magn., 31(12), 3855–3866 (2018)
[2] Yu.N. Ovchinnikov, Topological Insulator: Surface localized States, J. Supercond. Novel Magn., 32 (5), 1327-1331 (2019)

Scientific Council of the Landau Institute, Friday, September 4, 2020, Landau ITP, 11:30 am
Nikolai A. Stepanov, Mikhail A. Skvortsov
Lyapunov exponent for Whitney's problem with random drive

We consider the statistical properties of a non-falling trajectory in the Whitney problem of an inverted pendulum excited by an external force. In the case when the external force is white noise, we recently found the instantaneous distribution function of the pendulum angle and velocity over an infinite time interval using a transfer-matrix analysis of the supersymmetric field theory. Here, we generalize our approach to the case of finite time intervals and multipoint correlation functions. Using the developed formalism, we calculate the Lyapunov exponent, which determines the decay rate of correlations on a non-falling trajectory.

Scientific Council of the Landau Institute, Friday, June 19, 2020, Landau ITP, 11:30 am
I.V. Poboiko, M.V. Feigel'man
Dvumernoe kulonovskoe steklo kak model' pinninga vikhrei v sverkhprovodyashchikh plenkakh

Predlozhena stekol'naya model' pinninga vikhrei v tonkikh sverkhprovodyashchikh plenkakh s sil'nym besporyadkom v oblasti magnitnykh polei B « H$_{c2}$ i pri nizkikh temperaturakh. Ispol'zuetsya metod, podobnyi razvitomu v rabote Myullera i Ioffe (2004) dlya "kulonovskogo stekla" v modeli Efrosa-Shklovskogo dlya kulonovskoi shcheli. Stekol'noe sostoyanie opisyvaetsya v ramkakh priblizheniya "1-RSB", kotoroe okazyvaetsya ochen' khoroshim priblizheniem chislenno (khotya i yavlyaetsya, strogo govorya, neustoichivym), sootvetstvuyushchii parametr malosti imeet poryadok 10$^{-5}$. Raspredelenie effektivnykh lokal'nykh energii vikhrei P(h) yavlyaetsya, s toi zhe tochnost'yu, shchelevym, chto obespechivaet nalichie effektivnoi sverkhprovodyashchei plotnosti (konechnoi kineticheskoi induktivnosti) plenki v sil'nom magnitnom pole.

Scientific Council of the Landau Institute, Friday, June 5, 2020, Landau ITP, 11:30 am
Nikolay Stepanov, Mikhail Skvortsov
Inverted pendulum driven by a horizontal random force: statistics of the non-falling trajectory and supersymmetry

We study stochastic dynamics of an inverted pendulum subject to a random force in the horizontal direction. Considered at the entire time axis, the problem admits a unique solution which always remains in the upper half plane. We develop a new technique for treating statistical properties of this unique non-falling trajectory. In our approach based on the supersymmetric formalism of Parisi and Sourlas, statistics of the non-falling trajectory is expressed in terms of the zero mode of a corresponding transfer-matrix Hamiltonian. The emerging mathematical structure is similar to that of the Fokker-Planck equation, but it is rather written for the «square root» of the distribution function.
We derive the specific boundary conditions that correspond to the non-falling trajectory. Our results for the distribution function of the angle and its velocity at the non-falling trajectory are in perfect agreement with direct numerical simulations of the stochastic pendulum equation. In the limit of very strong noise, an exact analytical solution is obtained.

Department of quantum mesoscopics: seminar, Friday, May 29, 2020, webinar, 3:00 pm
Vladimir Kravtsov
Fragile ergodic phases in logarithmically-normal Rosenzweig-Porter model

Webinar link: http://bbb2.itp.ac.ru/b/qme-erf-krk
Gaussian Rosenzweig-Porter (GRP) random matrix ensemble is the only one in which the robust multifractal phase have a status of a mathematical theorem. In this paper we suggest an extension of the GRP model, the LN-RP model, by adopting a logarithmically-normal (LN) distribution of off-diagonal matrix elements. We show that large matrix elements from the tail of this distribution give rise to a peculiar weakly-ergodic phase that replaces both the multifractal and the fully-ergodic phases present in GRP ensemble. A new phase is characterized by the broken basis-rotation symmetry which the fully-ergodic phase respects. Thus in addition to the localization and ergodic transitions in LN-RP model there exists also the FWT transition between the two ergodic phases. We formulate the criteria of the localization, ergodic and FWT transitions and obtain the phase diagram of the model. We also suggest a new criteria of stability of the non-ergodic phases and prove that the Anderson transition in LN-RP model is discontinuous, in contrast to its GRP counterpart.

Department of quantum mesoscopics: seminar, Friday, May 22, 2020, webinar, 3:00 pm
A.V. Lunkin, A.Yu. Kitaev, M.V. Feigel'man
Sachdev-Ye-Kitaev model with perturbations: polaron in hyperbolic plane

Webinar link: http://bbb2.itp.ac.ru/b/qme-erf-krk
We study SYK_4 model with weak SYK_2 random terms of typical magnitude Γ beyond the simplest perturbative limit considered previously. In the intermediate range J/N << Γ << J/N^{1/2} of the perturbation strength Γ, fluctuations of the Schwarzian soft mode are suppressed and conformal mean-field solution for the Majorana Green function G(τ) is valid much beyond usual timescale t_∗ ∼ N/J characteristic for pure SYK_4 model. However, high momenta of the probability distribution of the Green function are shown to grow as ln([G(τ)]^p) ∝ p^2 at p >> 1, indicating log-normal shape of the distribution P[G(τ)] . Short-time decay of the out-of-time-order correlation function is characterized by the universal exponent 2πT/\hbar, while the pre-factor is much smaller than in pure SYK_4 model, and shows unusual temperature dependence.

Landau ITP colloquium, Friday, May 15, 2020, Landau ITP, 11:30 am
Piet Brouwer (Free University Berlin, Germany)
Higher-order topological insulators and superconductors

Topological insulators combine an insulating bulk with gapless states at their surfaces. This talk introduces "higher-order topological insulators", which are crystalline insulators with a gapped bulk and gapped crystalline boundaries, but topologically protected gapless states at corners or at edges of the crystal. I'll show that such "higher-order boundary states" are a generic boundary manifestation of the nontrivial bulk topology, if the bulk topology relies on a crystalline symmetry for its protection.

Department of quantum mesoscopics: seminar, Friday, May 8, 2020, webinar, 3:00 pm
A. V. Andreev, Steven A. Kivelson, B. Spivak
Hydrodynamic description of transport in strongly correlated electron systems

Webinar link: http://bbb2.itp.ac.ru/b/qme-erf-krk
We develop a hydrodynamic description of the resistivity and magnetoresistance of an electron liquid in a smooth disorder potential. This approach is valid when the electron-electron scattering length is sufficiently short. In a broad range of temperatures, the dissipation is dominated by heat fluxes in the electron fluid, and the resistivity is inversely proportional to the thermal conductivity, κ. This is in striking contrast to the Stokes flow, in which the resistance is independent of κ and proportional to the fluid viscosity. We also identify a new hydrodynamic mechanism of spin magneto-resistance.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.106.256804

Department of quantum mesoscopics: seminar, Friday, May 1, 2020, webinar, 3:00 pm
Zvi Ovadyahu (Hebrew University of Jerusalem)
Long-range influence of manipulating disordered-insulators locally

Webinar: http://bbb2.itp.ac.ru/b/qme-erf-krk
Localization of wavefunctions is arguably the most familiar effect of disorder in quantum systems. It has been recently argued [V. Khemani, R. Nandkishore, and S. L. Sondhi, Nature Physics, 11, 560 (2015)] that, contrary to naive expectation, manipulation of a localized-site in the disordered medium may produce a disturbance over a length-scale much larger than the localization-length ξ. Here we report on the observation of this nonlocal phenomenon in electronic transport experiment. Being a wave property, visibility of this effect hinges upon quantum-coherence, and its spatial-scale may be ultimately limited by the phase-coherent length of the disordered insulator. Evidence for quantum coherence in the Anderson-insulating phase may be obtained from magneto-resistance measurements which however are useful mainly in thin-films. The technique used in this work offers an empirical method to measure this fundamental aspect of Anderson-insulators even in relatively thick samples.

In addition, it is recommended by the author to read the following two papers before the seminar:
[1] I. M. Lifshitz, & V. Y. Kirpichenkov, Tunnel transparency of disordered systems, Zh. Eksp. Teor. Fiz. 77, 989 (1979) [Sov. Phys. JETP 50, 499 (1979)].
[2] J. B. Pendry, Quasi-extended electron states in strongly disordered systems, J. Phys. C: Solid State Phys. 20, 733 (1987).

Scientific Council of the Landau Institute, Friday, May 1, 2020, Landau ITP, 11:30 am
K.S. Tikhonov, M.V. Feigel'man
Strange metal state near quantum superconductor-metal transition in thin films

We develop a theory of quantum T=0 phase transition (q-SMT) between metal and superconducting ground states in a two-dimensional metal with frozen-in spatial fluctuations of the Cooper attraction constant. When strength of these fluctuations exceeds some critical magnitude, usual mean-field-like scenario of the q-SMT breaks down due to spontaneous formation of local droplets of superconducting phase. The density of these droplets grows exponentially with the increase of average attraction constant. Interaction between the droplet's order parameters is due to proximity effect via normal metal and scales with distance as inverse power of distance, with power exponent slightly larger than 2. We treat this interaction by means of strong-disorder real-space renormalization group and find the RG flow formally similar to the Berezinski-Kosterlitz-Thouless RG for 2D XY model. Line of fixed points of this RG corresponds to a Griffiths phase of a metal with large fractal clusters of strongly coupled superconducting islands. Superconducting side of the transition is characterized by a non-monotonic variation of physical properties with logarithm of the temperature T, which results of a very weak T-dependence in a broad temperature range.

Department of quantum mesoscopics: seminar, Friday, April 24, 2020, webinar, 4:00 pm
Grigory Tarnopolsky (Harvard University)
Fractional Chern Insulator States in Twisted Bilayer Graphene

Webinar: http://bbb2.itp.ac.ru/b/qme-erf-krk
Recent experiments on twisted bilayer graphene (TBG) near the magic angle have observed spontaneous integer quantum Hall states in the presence of an aligned hexagonal boron nitride (hBN) substrate. These states arise from valley ferromagnetism, and the complete filling of Chern bands. A natural question is whether fractional filling of the same bands would lead to fractional quantum Hall states, i.e. to fractional Chern insulators (FCIs). Using a variant of the continuum model for TBG with zero hopping between AA Moire cites we construct a Laughlin wave function on a real-space torus in full analogy with the Haldane and Rezayi construction for two-dimensional electrons at the lowest Landau-level on a torus.

Scientific Council of the Landau Institute, Friday, April 24, 2020, Landau ITP, 11:30 am
I.S. Burmistrov
Disorder-induced rippled phases and multicriticality in free-standing graphene

One of the most exciting phenomena observed in crystalline disordered membranes, including a suspended graphene, is rippling, i.e. a formation of static flexural deformations. Despite an active research, it still remains unclear whether the rippled phase exists in the thermodynamic limit, or it is destroyed by thermal fluctuations. We demonstrate that a sufficiently strong short-range disorder stabilizes ripples, whereas in the case of a weak disorder the thermal flexural fluctuations dominate in the thermodynamic limit. The phase diagram of the disordered suspended graphene contains two separatrices: the crumpling transition line dividing the flat and crumpled phases and the rippling transition line demarking the rippled and clean phases. At the intersection of the separatrices there is the unstable, multicritical point which splits up all four phases. Most remarkably, rippled and clean flat phases are described by a single stable fixed point which belongs to the rippling transition line.

Landau ITP colloquium, Friday, April 17, 2020, Landau ITP, 11:30 am
Kamran Behnia (CNRS/ESPCI, PSL Research University, Paris, France)
Charge and entropy transport in strontium titanate

The ferroelectric instability in pristine strontium titanate is aborted by quantum fluctuations. Therefore, the static electric permittivity saturates to an extremely large value at low temperature and the effective Bohr radius approaches a micron. In this context, removing a tiny fraction of oxygen atoms turns the system to a dilute metal with a sharp Fermi surface and a superconducting instability. The focus of this talk will be charge and entropy transport by electrons in this dilute metal and their general implications.
The temperature dependence of the resistivity of this dilute metal at low-temperature implies that electron-electron scattering can generate a T-square resistivity even in absence of Umklapp scattering with a single Fermi pocket. The magnitude of resistivity at a low temperature would imply a mean-free-path too short to be physically plausible. Combined with the temperature dependence of the Seebeck coefficient, this implies that non-degenerate electrons become heavier with warming. We will see that these observations cannot be explained by available polaronic theories. There is a whole family of dilute metals emerging from doped quantum paraelectrics with similar properties.

Department of quantum mesoscopics: seminar, Friday, April 10, 2020, webinar, 4:00 pm
Elio Koenig (Rutgers University)
Soluble limit and criticality of fermions in Z2 gauge theories

Webinar: http://bbb2.itp.ac.ru/b/qme-erf-krk
Quantum information theory and strongly correlated electron systems share a common theme of macroscopic quantum entanglement. In both topological error correction codes and theories of quantum materials (spin liquid, heavy fermion and high-Tc systems) entanglement is implemented by means of an emergent gauge symmetry. Inspired by these connections, here we introduce a simple model for fermions moving in the deconfined phase of a Z2 gauge theory, by coupling Kitaev's toric code to mobile fermions. This permits us to exactly solve the ground state of this system and map out its phase diagram. By changing the sign of the plaquette term in the toric code, we are able to tune the groundstate between an orthogonal metal and an orthogonal semimetal, in which the single particle correlators of the original Fermi operators are gapped, despite the existence of gapless collective modes. The small-to-large Fermi surface transition between these two states occurs in a stepwise fashion with multiple intermediate phases. We are able to access the physics beyond the integrable point using a novel diagrammatic perturbation expansion, which allows us to examine various instabilities of the deconfined phase and to derive the Ising (XY) critical theory at the transition between deconfined and confined metal (semimetal). Finally, the connection to quantum information science allows to discuss an analogue quantum emulator (an array of Majorana Cooper pair boxes) for fermions in Z2 gauge theories. Analytical results for this minimal model pave the way for a better understanding of quantum materials with itinerant fermions and the connection to the toric code opens the potential for simulation using upcoming quantum information techniques.

Scientific Council of the Landau Institute, Friday, March 6, 2020, Landau ITP, 11:30 am
Pavel Ostrovsky
Spin-torque resonance due to diffusive dynamics at a surface of a topological insulator

We investigate spin-orbit torques on magnetization in an insulating ferromagnetic layer that is brought into close proximity to a topological insulator (TI). In addition to the well-known fieldlike spin-orbit torque, we identify an anisotropic anti-damping-like spin-orbit torque that originates in a diffusive motion of conduction electrons. This diffusive torque is vanishing in the limit of zero momentum (i.e., for a spatially homogeneous electric field or current), but it may, nevertheless, have a strong impact on spin-torque resonance at finite frequency provided the external field is neither parallel nor perpendicular to the TI surface. The required electric-field configuration can be created by a grated top gate.

Scientific Council of the Landau Institute, Friday, March 6, 2020, Landau ITP, 11:30 am
Pavel Ostrovsky
Robust weak antilocalization due to spin-orbital entanglement in Dirac material Sr3SnO

The presence of both inversion (P) and time-reversal (T) symmetries in solids leads to a well-known double degeneracy of the electronic bands (Kramers degeneracy). When the degeneracy is lifted, spin textures can be directly observed in momentum space, as in topological insulators or in strong Rashba materials. The existence of spin textures with Kramers degeneracy, however, is very difficult to observe directly. Here, we use quantum interference measurements combined with first-principle band structure calculations to provide evidence for the existence of hidden entanglement between spin and momentum in the antiperovskite-type 3D Dirac material Sr3SnO. We find robust weak antilocalization (WAL) independent of the position of EF. The observed WAL signal at low doping is fitted using a single interference channel, which implies that the different Dirac valleys are mixed by disorder. Notably, this mixing does not suppress WAL, suggesting contrasting interference physics compared to graphene. We identify scattering among axially spin-momentum locked states as a key process that leads to a spin-orbital entanglement, giving rise to robust WAL. Our work sheds light on the subtle role of spin and pseudospin, when both could contribute to the same quantum effect.

Scientific Council of the Landau Institute, Friday, February 28, 2020, Landau ITP, 11:30 am
I.S. Burmistrov
Unrestricted electron bunching at the helical edge

A quantum magnetic impurity of spin $S$ at the edge of a two-dimensional time reversal invariant topological insulator may give rise to backscattering. We study here the shot noise associated with the backscattering current for arbitrary $S$. Our full analytical solution reveals that for $S>½$ the Fano factor may be arbitrarily large, reflecting bunching of large batches of electrons. By contrast, we rigorously prove that for $S=½$ the Fano factor is bounded between 1 and 2, generalizing earlier studies. Based on paper P.D. Kurilovich, V.D. Kurilovich, I.S. Burmistrov, Y. Gefen, M. Goldstein "Unrestricted electron bunching at the helical edge", Phys. Rev. Lett. 123, 056803 (2019).

Scientific Council of the Landau Institute, Friday, February 21, 2020, Landau ITP, 11:30 am
D.S. Antonenko, P.M. Ostrovskii, M.A. Skvortsov
Mezoskopicheskie fluktuatsii kondaktansa v sverkhprovodyashchikh topologicheskikh provolokakh

V rabote izuchaetsya kvazichastichnyi transport v neuporyadochennykh topologicheskikh sverkhprovodyashchikh provolokakh klassa simmetrii D, kotorye mogut soderzhat' paru kraevykh Maioranovskikh sostoyanii. Rassmotren kriticheskii rezhim, realizuyushchiisya na granitse mezhdu topologicheskoi i trivial'noi fazami, pri kotorom zakryvaetsya ob'emnaya shchel', a para Maioranovskikh sostoyanii ischezaet. Ranee bylo pokazano, chto v kriticheskom rezhime srednii kondaktans na bol'shikh dlinakh L vedet sebya kak 1/\sqrt{L}. My vychislyaem dispersiyu kondaktansa var g s pomoshch'yu nelineinoi supersimmetrichnoi sigma-modeli s dvumya replikami, chto svoditsya k fur'e analizu na supermnogoobrazii sigma-modeli. S pomoshch'yu parametrizatsii Ivasavy postroeny sobstvennye funktsii sootvetstvuyushchego operatora Laplasa-Bel'trami. Poluchena yavnaya formula dlya var g pri proizvol'nykh L, opisyvayushchaya krossover ot rezhima drudevskoi provodimosti na malykh dlinakh k rezhimu shirokogo raspredeleniya kondaktansa pri bol'shikh dlinakh. Takzhe issledovan sluchai disbalansa levykh/pravykh kanalov v provoloke, opisyvaemyi sigma-model'yu s topologicheskim chlenom Vessa-Zumino-Vittena.

Department of quantum mesoscopics: seminar, Friday, February 14, 2020, ITF, 3:30 pm
M.Feigel'man i K.Tikhonov
Strange metal state near quantum superconductor-metal transition in thin films

We develop a theory of quantum T = 0 phase transition (q-SMT) between metal and superconducting ground states in a two-dimensional metal with frozen-in spatial fluctuations δλ(r) of the Cooper attraction constant. When strength of fluctuations δλ(r) exceeds some critical magnitude, usual mean-field-like scenario of the q-SMT breaks down due to spontaneous formation of local droplets of superconducting phase. The density of these droplets grows exponentially with the increase of average attraction constant λ. Interaction between the droplet's order parameters is due to proximity effect via normal metal and scales with distance ∝ 1/rβ, with 2 < β ≤ 3. We account for this interaction by means of a real-space strong-disorder renormalization group (RG). Near the q-SMT the RG flow is, formally, a dual equivalent of the Kosterlitz-Thouless RG. The corresponding line of fixed points describes a Griffiths phase of a metal with large fractal clusters of superconducting islands. Superconducting side is described by a runaway of RG trajectories into the strong-coupling region. Close to the transition point on the SC side, 0 < −δ << 1, RG trajectories possess an extremum as function of the RG parameter ∼ |δ| 1/2 ln(1/T). It results in a very wide temperature range where physical properties are nearly T-independent. This observation may be relevant to the understanding of a ”strange metal” state frequently observed near q-SMT.

Scientific Council of the Landau Institute, Friday, February 14, 2020, Landau ITP, 11:30 am
D. Vodolazov, V. Plastovetz
Paramagnetic Meissner, vortex and 'onion' ground states in Fulde-Ferrell finite-size superconductor

We theoretically find that finite size Fulde-Ferrell (FF) superconductor (which is characterized by spatially nonuniform ground state with Delta(r) ~ exp(-i q_{FF} r), where Delta is a superconducting order parameter) has paramagnetic Meissner, vortex and 'onion' ground states with spatially nonuniform |Delta|. These states are realized due to boundary effect when the lateral size of superconductor L ~ 1/q_{FF}. We argue, that predicted states could be observed in thin disc/square made from superconductor-ferromagnet-normal metal trilayer with L ~ 150-600 nm.

Scientific Council of the Landau Institute, Friday, February 14, 2020, Landau ITP, 11:30 am
V.V. Ryazanov, T.E. Golikova (ISSP RAS)
Non-equilibrium effects in Josephson SNS junctions

In this talk we review recent experiments on planar submicron superconductor/normal metal/superconductor junctions which have been performed in the Laboratory of superconductivity, ISSP recently. We investigated how quasiparticles and spins injected into leads and barrier of SNS junctions affect the critical current, phase difference inversion and non-local effects.

Scientific Council of the Landau Institute, Friday, February 14, 2020, Landau ITP, 11:30 am
L.Ya. Vinnikov (ISSP RAS)
Coexistence of superconductivity and ferromagnetism, structure of magnetic flux in bulk single crystals of magnetic superconductors

With the help of the magnetic decoration method, we investigate the structure of the magnetic flux in single-crystalline magnetic superconductors based on EuFeAs doped by phosphorus. In a single crystal of ferromagnetic superconductor EuFe2(As0:79P0:21)2 with superconducting transition temperature (Tsc=22K) by decoration technique we observe vortex structure in domains of spontaneous magnetic flux (vortex domains) near the temperature of ferromagnetic transition (TSurie= Tc =18K). In a narrow temperature interval (~1K) around Tc =18K we also observe Meissner domains, detected previously by the magnetic-force microscopy method. When temperature is lowered, we observe a domain structure (vortex domains) with changing direction of the magnetic flux.

Scientific Council of the Landau Institute, Friday, February 14, 2020, Landau ITP, 11:30 am
M.A. Skvortsov
Inhomogeneous states in disordered superconductors

An increase in disorder in a superconductor leads to the smearing of the coherence peak and appearance of localized quasiparticle states under the average gap. At a phenomenological level, the smeared density of states was calculated by Larkin and Ovchinnikov in 1971. I will overview further developments in this filed, both from a theoretical and experimental point of view.

Department of quantum mesoscopics: seminar, Friday, February 7, 2020, ITF, 3:00 pm
A.N. Poddubnyi (FTI im.A.F.Ioffe)
Vzaimodeistvie dvukh fotonov s massivom atomov v volnovode

Rassmatrivaetsya teoreticheskaya zadacha o rasprostranenii pary fotonov v odnomernom volnovode, v kotoryi pomeshchen periodicheskii massiv dvukhurovnevykh atomov. Otlichiem ot klassicheskoi modeli Dikke yavlyaetsya uchet konechnogo nenulevogo rasstoyaniya mezhdu atomami. Blagodarya fotonnoi blokade - nevozmozhnosti pogloshcheniya dvukh fotonov odnim atomom - mezhdu fotonami voznikaet effektivnoe ottalkivanie. Budet pokazano, chto eto ottalkivanie privodit k obrazovaniyu dvukhfotonnykh sostoyanii, kogda odin iz fotonov lokalizovan, a vtoroi obrazuet stoyachuyu volnu. Takie dvukhchastichnye mody mozhno rassmatrivat' kak analog indutsirovannykh vzaimodeistviem opticheskikh reshetok.

Scientific Council of the Landau Institute, Friday, January 31, 2020, Landau ITP, 11:30 am
Ya.V. Fominov, A.A. Mazanik, M.V. Razumovskiy
Surface density of states in superconductors with inhomogeneous pairing constant

We consider a superconductor with surface suppression of the BCS pairing constant $\lambda(x)$. We analytically find the gap in the surface density of states (DOS), behavior of the DOS $\nu(E)$ above the gap, a ``vertical'' peculiarity of the DOS around an energy equal to the bulk order parameter $\Delta_0$, and a perturbative correction to the DOS at higher energies. The surface gap in the DOS is parametrically different from the surface value of the order parameter due to a difference between the spatial scale $r_c$, at which $\lambda(x)$ is suppressed, and the coherence length. The vertical peculiarity implies an infinite-derivative inflection point of the DOS curve at $E=\Delta_0$ with square-root behavior as $E$ deviates from $\Delta_0$. The coefficients of this dependence are different at $E < \Delta_0$ and $E > \Delta_0$, so the peculiarity is asymmetric. The talk is based on the paper [Ya.V. Fominov, A.A. Mazanik, M.V. Razumovskiy, Phys. Rev. B 100, 224513 (2019)].

Scientific Council of the Landau Institute, Friday, January 31, 2020, Landau ITP, 11:30 am
Igor Lukyanchuk
Ferroelectric as topological material: Hopf fibrations, multilevel logic, negative capacitance and THz vibrations

Formation of unusual textures of polarization is imminent for nano-scale ferroelectric samples, films, rods, and granules, where the depolarization surface effects play the crucial role. The topologically protected stability of such textures and security of information storage is coming from polarization vorticity, provided by condition of absence of the energetically-unfavorable depolarization charge. The endurance of ferroelectric formations with respect to high-energy irradiation makes them ideal for the aerospace industry, and the periodic domain walls structures can be used as a platform for terahertz radiation generators and detection devices. Polarization domains that alternate the surface charge distribution can be formed in ferroelectric thin films as an effective mechanism to confine the depolarization field to the near-surface layer and diminish the depolarization energy. However their existence have long been considered as barely possible until the direct theoretical predictions [1-3] and experimental evidences [4-6] in thin oxide-based superlattices. Very recently we have demonstrated that the effective capacitance of ferroelectric layers with domains is negative [7]. This effect is explained by the opposite orientation of the depolarizing field with respect to the field-induced averaged polarization. This phenomenon is currently considered as the platform for realization of the dissipation-free high performance nano-circuits [8]. Moreover, in sub-THz region the resonance plasmonic effect can be induced by oscillating domain walls [9] and can be suitable for design of the ultra-small low-energy THz chips. Multi-vortex [10] and skyrmion [11] states can be formed inside ferroelectric cylindrical nano-dots and nanorods to reduce the depolarization energy. We study the stability of such states and demonstrate that the topological class of the most stable topological excitations can be driven by the geometrical and electrical parameters of the system, external field and temperature. We target the multi-domain and topological excitations in FE nanodots as a platform for IT-secured multivalued logic units, breaking ground for neuromorphic computing [12,13].

[1] A. M. Bratkovsky and A. P. Levanyuk, Phys. Rev. Lett. 84, 3177 (2000).
[2] V. A. Stephanovich, I. A. Luk'yanchuk, and M. G. Karkut, Phys. Rev. Lett., 94, 047601 (2005)
[3] I. Luk'yanchuk, L. Lahoche, and A. Sene, Phys. Rev. Lett., 102, 147601 (2009)
[4] S. K. Streiffer, J. A. Eastman, D. D. Fong et al., Phys. Rev. Lett. 89, 067601 (2002);
[5] S. O. Hruszkewycz, M. J. Highland, et al., Phys. Rev. Lett.110, 177601 (2013).
[6] Yadav, A. K., Nelson,et al.. Nature, 530 (7589), 198. (2016)
[7] P. Zubko, M. Hadjimichael, S. Fernandez-Pena, A. Sené, I. Luk’yanchuk, J.-M. Triscone & J. Íñiguez, Nature, 534, 524 (2016)
[8] Khan, A. I., Chatterjee, K., Wang B. et al. Nature Materials 14, 182–186 (2015).
[9] I. Luk'yanchuk, A.Pakhomov, A.Sené, A. Sidorkin, V. Vinokur, arXiv:1410.3124
[10] G. Pascoli L. Lahoche, I. Luk'yanchuk, Integrated Ferroelectrics, 99, 60 (2008)
[11] L Baudry, A Sené, IA Luk'yanchuk, L Lahoche, and JF Scott, Phys. Rev. B 90, 024102 (2014)
[12] P.-W. Martelli, S. M. Mefire, I. Luk'yanchuk, Europhys. Lett. 111, 50001 (2015)
[13] Baudry, L., Lukyanchuk, I. & Vinokur, V. M. Sci. Rep. 7: 42196 (2017)

Scientific Council of the Landau Institute, Friday, January 17, 2020, Landau ITP, 11:30 am
L.Yu. Barash
Analiz ustoichivosti algoritma kvantovogo adiabaticheskogo nestrukturirovannogo poiska

Algoritm kvantovogo adiabaticheskogo nestrukturirovannogo poiska - odin iz nemnogikh algoritmov kvantovoi adiabaticheskoi optimizatsii, dlya kotorykh dokazany teoremy o tom, chto oni rabotayut znachitel'no bystree, chem ikh klassicheskie analogi. V svyazi s tem, chto dlya adiabaticheskikh kvantovykh vychislenii v nastoyashchee vremya otsutstvuyut teoremy ob otkazoustoichivosti, garantiruyushchie ustoichivost' takikh algoritmov k oshibkam, izuchenie vliyaniya vozmozhnykh defektov pri ikh vypolnenii na rezul'tat vychislenii imeet nauchnoe i prakticheskoe znachenie. My izuchaem ustoichivost' algoritma k sleduyushchim trem tipam defektov: netochnost' v vypolnenii interpolyatsionnogo grafika, nalichie gaussovogo shuma v gamil'toniane, vzaimodeistvie s teplovym okruzheniem. Pokazano, chto kvadratichnoe uskorenie algoritma nestrukturirovannogo poiska, voobshche govorya, ne yavlyaetsya ustoichivym pri nalichii lyubogo iz vysheperechislennykh defektov. V nekotorykh sluchayakh my nakhodim, chto ustoichivost' algoritma privodit k nerealistichnym trebovaniyam dlya zavisimosti sily istochnikov shuma ot razmera sistemy, neobkhodimoi dlya sokhraneniya kvadratichnogo uskoreniya.
[1] M. Slutskii, T. Albash, L. Barash, I. Hen, New Journal of Physics, 21, 113025 (2019), https://dx.doi.org/10.1088/1367-2630/ab51f9

Scientific Council of the Landau Institute, Friday, January 10, 2020, Landau ITP, 11:30 am
L.Yu. Barash
Vychislenie plotnosti sostoyanii frustrirovannykh spinovykh sistem

Vychislenie plotnosti sostoyanii sistem s sil'no peresechennymi landshaftami svobodnoi energii yavlyaetsya izvestnoi slozhnoi i aktual'noi zadachei vo mnogikh oblastyakh fiziki, ot spinovykh stekol do biopolimerov. Takzhe, chislennoe opredelenie plotnosti sostoyanii nedavno stalo instrumentom dlya testirovaniya korrektnosti raboty ustroistv dlya kvantovogo otzhiga, pozvolyayushchikh delat' vyborku. Ispol'zovanie standartnykh podkhodov mozhet privodit' k lozhnoi skhodimosti vychislenii k metastabil'nym minimumam, i nalichie etoi problemy chrezvychaino slozhno obnaruzhit' v protsesse vychislenii. My predlagaem metod vyborki, osnovannyi na algoritme otzhiga populyatsii, usovershenstvovannom pri pomoshchi mul'ti-gistogrammnogo analiza. My issleduem proizvoditel'nost' predlozhennogo metoda dlya spinovykh stekol i demonstriruem vozmozhnost' v ryade sluchaev preodolet' problemu oshibochnogo opredeleniya plotnosti sostoyanii, prisushchuyu drugim metodam entropiinoi vyborki. Eto privodit v ryade sluchaev k znachitel'nomu preimushchestvu nashego metoda, pozvolyayushchemu poluchit' vernyi otvet za znachitel'no men'shee vremya, po sravneniyu s drugimi metodami, chto mozhet okazat'sya poleznym dlya obnaruzheniya novoi fiziki pri pomoshchi komp'yuternogo modelirovaniya. Novyi metod pozvolyaet izbezhat' ryada nedostatkov, prisushchikh izvestnym drugim metodam, i on mozhet byt' primenen k shirokomu klassu sistem bez dopolnitel'nykh razbienii na energeticheskie okna i sshivok. Sravnitel'noe testirovanie effektivnosti metodov opredeleniya plotnosti sostoyanii bylo proizvedeno pri pomoshchi primeneniya neskol'kikh skhem, pozvolyayushchikh tochno nakhodit' stepeni vyrozhdeniya energeticheskikh urovnei dlya testiruemykh obraztsov modeli Edvardsa-Andersona.
[1] L. Barash, J. Marshall, M. Weigel, I. Hen, New Journal of Physics, 21, 073065 (2019).

Scientific Council of the Landau Institute, Friday, December 27, 2019, Landau ITP, 11:30 am
Mirlin A.D.
Disorder and interaction in chiral chains: Majoranas versus complex fermions.

We study the low-energy physics of a chain of Majorana fermions in the presence of interaction and disorder, emphasizing the difference between Majoranas and conventional (complex) fermions. While in the noninteracting limit both models are equivalent (in particular, belong to the same symmetry class BDI and flow towards the same infinite-randomness critical fixed point), their behavior differs drastically once interaction is added. Our density-matrix renormalization group calculations show that the complex-fermion chain remains at the noninteracting fixed point. On the other hand, the Majorana fermion chain experiences a spontaneous symmetry breaking and localizes for repulsive interaction. To explain the instability of the critical Majorana chain with respect to a combined effect of interaction and disorder, we consider interaction as perturbation to the infinite-randomness fixed point and explore correlations of wave functions that enter interaction matrix elements. Our numerical and analytical results exhibit a rich structure of critical eigenstate correlations. This allows us to identify a relevant interaction operator that drives the Majorana chain away from the infinite-randomness fixed point. For the case of complex fermions, the interaction is irrelevant.

Landau ITP colloquium, Friday, December 20, 2019, Landau ITP, 3:00 pm
Vadim Smelyanskiy (Google, Los Angeles)
Non-ergodic delocalized states for efficient population transfer within a narrow band of the energy landscape

We will review the advances and challenges in the field of quantum combinatorial optimization and closely related problem of low-energy eigenstates and coherent dynamics in transverse field quantum spin glass models. We will discuss the role of collective spin tunneling that gives rise to bands of delocalized non-ergodic quantum states providing the coherent pathway for the population transfer (PT) algorithm: the quantum evolution under a constant transverse field that starts at a low-energy spin configuration and ends up in a superposition of spin configurations inside a narrow energy window. We study the transverse field induced quantum dynamics of the following spin model: zero energy of all spin configurations except for a small fraction of spin configuration that form a narrow band at large negative energy. We use the cavity method for heavy-tailed random matrices to obtain the statistical properties of the low-energy eigenstates in an explicit analytical form. In a broad interval of transverse fields, they are non-ergodic, albeit extended giving rise to a qualitatively new type of quantum dynamics. For large transverse fields »1 the typical runtime of PT algorithm $\sim \sqrt{2^n / \Omega e^r}$ scales with n and Ω as that of the Grover’s quantum search, except for the small correction to the exponent θ ≈ 1/(2). The model we consider is non-integrable. As a result, our PT protocol does not require any fine-tuning of and may be initialized in a computational basis state. We argue that our approach can be applied to study PT protocol in other optimization problems with the potential quantum advantage over classical algorithms.

Landau ITP colloquium, Friday, December 20, 2019, Landau ITP, 11:30 am
Joerg Schmalian (Karlsruhe Institute of Technology)
Ordered fluctuations: about vestigial order in quantum materials

A hallmark of the phase diagrams of quantum materials is the existence of multiple electronic ordered states. In many cases those are not independent, competing phases, but instead display a complex intertwinement. In this talk, we focus on a realization of intertwined orders with a fluctuation-driven vestigial phase characterized by a composite order parameter. In other words, we are investigating the condensation of fluctuations.
We demonstrate that this concept naturally explains the nematic state in iron-based superconductors and nematic superconductivity in doped topological insulators. In addition we propose a natural mechanism for charge 4e superconductivity with half flux quanta. We present a formalism that provides a framework to understand the complexity of quantum materials based on symmetry, largely without resorting to microscopic models.

Department of quantum mesoscopics: seminar, Friday, December 13, 2019, ITF, 3:30 pm
2. Anton Khvalyuk
Obzor osnovnykh algoritmov kvantovykh simulyatsii

Iz-za apparatnykh ogranichenii sushchestvuyushchie kvantovye protsessory sposobny proizvodit' lish' opredelennyi nabor unitarnykh operatsii nad odnim ili dvumya kvantovymi bitami v techenie konechnogo promezhutka vremeni, posle kotorogo ustroistvo teryaet kogerentnost'. Chtoby ispol'zovat' podobnye ustroistva dlya simulyatsii proizvol'nogo vida gamil'tonianov, neobkhodimo stroit' priblizheniya, pozvolyayushchie s pomoshch'yu dostupnykh operatsii vosproizvesti proizvol'nyi operator evolyutsii i pri etom ispol'zuyushchie kak mozhno men'she obrashchenii k kvantovomu ustroistvu.

Izvestno neskol'ko printsipial'no razlichnykh algoritmov postroeniya podobnykh priblizhenii, i v dannom obzore my rassmotrim naibolee aktivno izuchaemye:

  • metod Suzuki-Trotter, osnovannyi na khoroshem priblizhenii eksponenty ot summy ermitovykh operatorov cherez eksponenty ot kazhdogo iz chlenov summy,
  • pryamoe razlozhenie eksponenty operatora evolyutsii v ryad Teilora po vremeni evolyutsii,
  • metod Quantum Signal Processing, osnovannyi na effektivnom metode simulyatsii polinomov ot zadannogo ermitovogo operatora, chto zatem ispol'zuetsya dlya polinomial'noi approksimatsii eksponenty ot operatora evolyutsii.

My razberem detali raboty kazhdogo iz predstavlennykh algoritmov, obsudim sushchestvuyushchie otsenki dlya ikh vremeni raboty i chisla ispol'zuemykh elementarnykh operatsii.

Department of quantum mesoscopics: seminar, Friday, December 13, 2019, ITF, 2:30 pm
1. Ivan Tsitsilin (MISiS, RKTs)
Eksperimenty so sverkhprovodyashchimi kubitami

Razvitie tekhnologii proizvodstva i izmereniya sverkhprovodyashchikh kubitov v Rossii uzhe pozvolyaet sozdavat' mnogokubitnye svyazannye sistemy, iz kotorykh mozhno delat' razlichnye analogovye i tsifrovye simulyatory. Poluchennyi v sozdanii dvukhkubitnykh geitov opyt pokazal nam napravlenie dlya dal'neishego razvitiya v etoi sfere: kakie tipy geitov neobkhodimo ispol'zovat' i uluchshat', kak vyklyuchat' ostatochnoe ZZ vzaimodeistvie vo vremya prostaivaniya kubitov. Ya predstavlyu nashi rezul'taty i rasskazhu o potentsial'nykh sposobakh prodvizheniya v etikh oblastyakh.

Scientific Council of the Landau Institute, Friday, December 13, 2019, Landau ITP, 11:30 am
P.G. Grinevich, P.M. Santini
Effect of small dissipation on the NLS anomalous waves recurrence.

We provide analytic formulas decribing the effect of small loss/gain on the recurrence of anomalous waves in the focusing Nonlinear Schrodinger equation. We show that very small loss or gain essentially affects the statistics and the character of the recurrence. In particular, our formulas explains the results of numerical simulations from the paper by O. Kimmoun, H.C. Hsu, H. Branger, M.S. Li, Y.Y. Chen, C. Kharif, M. Onorato, E.J.R. Kelleher, B. Kibler, N. Akhmediev, A. Chabchoub (2016).

Department of quantum mesoscopics: seminar, Friday, November 29, 2019, ITF, 3:00 pm
V.Kuznetsov (eksperimental'naya chast') i S.Dikman (teoreticheskaya chast')
Sovmestnyi doklad po rabote S.Dikmana, L.Kulika i V.Kuznetsova (IFTT)
"Coherence-decoherence transition in a spin-magnetoexcitonic ensemble in a quantum Hall system"

In the talk, we focus on the physics of magneto-excitons in a quantum Hall insulator at the filling factor of 2. The simplest realizations are magneto-excitons formed by an electron promoted from the occupied zeroth Landau level to the empty first Landau level and by the vacancy under the Fermi level. There are two magneto-excitons: a spin-singlet with total spin S = 0; and a spin-triplet (CSFE) with total spin S = 1. The first one is magneto-plasmon which decays with the emission of a photon. In contrast, the CSFE is not radiatively active owing to electron spin conservation and its energy is lower than the cyclotron energy. Consequently, spin-triplet magneto-excitons exhibit relaxation time reaching up to 900 us. We also discovered condensation of excitons by measurement of exciton lifetime in a temperature range of 0.5–1.5 K. Here we report on spin exciton transfer at distance exceeding 200 μm by pump-probe photo-luminescence measurements and photo-resonant reflection. These effects exhibit threshold both in excitation power and temperature.

The experimental studies on CSFE ensembles that have to obey the Bose-Einstein statistics signal the emergence of an excitonic coherent phase. In the present paper, the theory is developed of the weakly interacting Bose gas of spin-cyclotron excitations in terms of a virial correction to the single magnetoexciton energy. The condition for a coherent-incoherent phase transition is discussed. It is expected to be strongly related to the studied long-distant inter-excitonic correlations. The results obtained theoretically are discussed in terms of their agreement with specific experimental data.

Scientific Council of the Landau Institute, Friday, November 29, 2019, Landau ITP, 11:30 am
Baerbel Rethfeld (Technische Universitaet Kaiserslautern, Germany)
Relaxation dynamics of nonequilibrium electrons in laser-excited solids

When an ultrashort laser pulse of visible light is absorbed by a solid, mainly the electrons in the material are excited. In metals, free electrons in the conduction band can directly absorb photons. In semiconductors and dielectrics, on the other hand, a band gap has to be overcome first, as almost no free electrons are present at room temperature in the unexcited material. Due to this excitation, the electronic system, or the so-called electron-hole plasma, is in a nonequilibrium state. A sequence of different relaxation processes transfers the material into a new equilibrium. Depending on the interaction associated with the particular relaxation process, it occurs on a characteristic timescale. On the basis of complete Boltzmann-type collision integrals, we calculate the transient distribution functions of electrons and phonons in different materials. We consider electron-electron interaction, different ionization processes, as well as electron-phonon coupling. By that we trace the relaxation cascade of nonequilibrium electrons after ultrafast heating. Distinct material properties enter through the density of states of the electrons in the conduction band. We study in particular noble metals, dielectrics and ferromagnets. In noble metals and ferromagnets, d-electrons play an important role, whereas in dielectrics two separated bands govern the dynamics and the ionization state may differ from. We show, that the electron distributions deviate from Fermi distributions for timescales up to a few picoseconds. While the initial thermalization within one band has an intrinsic timescale of typically only a few tens of femtoseconds, nonequilibrium occupations of the different bands as well as continous electron-phonon coupling can drive the conduction electrons out of equilibrium for much longer times [1, 2]. We present in detail the mutual influence of different interaction and relaxation processes.
[1] N. Brouwer and B. Rethfeld, Phys. Rev. B 95, 245139 (2017). [2] S.T. Weber and B. Rethfeld, Phys. Rev. B 99, 174314 (2019).

Scientific Council of the Landau Institute, Friday, November 15, 2019, Landau ITP, 11:30 am
A.Ya. Maltsev
Geometry of level lines of quasiperiodic functions and related problems

The report will provide an overview of the results so far obtained in the problem of describing the geometry of level lines of quasiperiodic functions on a plane and problems associated with it. In particular, we will consider cases of quasiperiodic functions on a plane with different numbers of quasiperiods, as well as features of the behavior of trajectories of dynamical systems associated with such functions (and also some of their generalizations). As can be shown, in many interesting cases, the trajectories of such systems can be represented by a finite number of different types corresponding to various nontrivial sets in the parameter space of such systems. As an example of dividing the parameter space into such sets, one can address to the division of the angular diagrams of the conductivity of metals in strong magnetic fields into a finite number of complexity classes.

Scientific Council of the Landau Institute, Friday, November 8, 2019, Landau ITP, 11:30 am
P.D. Grigoriev, R. Ramazashvili, M. V. Kartsovnik
Zeeman spin-orbit coupling and magnetic quantum oscillations in antiferromagnetic conductors

Using the symmetry arguments we show that in many metals with antiferromagnetic ordering the effective g-factor of charge carries, measured from magnetic quantum oscillations, is exactly zero. The experimental study of this effect is performed in several compounds and compared with the proposed theory. We find that the Néel state of the layered organic conductor κ-(BETS)2FeBr4 shows no spin modulation of the Shubnikov-de Haas oscillations, contrary to the paramagnetic state of the same material. This is evidence of spin degeneracy of Landau levels -- a direct manifestation of the generic Zeeman spin-orbit coupling, predicted for antiferromagnetic conductors. Likewise, we find no spin modulation in the angle dependence of the slow Shubnikov-de Haas oscillations in the optimally electron-doped cuprate Nd2−xCexCuO4. This points to the presence of Néel order in this superconductor even at optimal doping.

Scientific Council of the Landau Institute, Friday, November 8, 2019, Landau ITP, 11:30 am
P.D. Grigoriev, P.A. Vorobyev, A.A. Sinchenko
Competition of band anticrossing and charge-density wave

We calculate the electron susceptibility of rare-earth tritelluride compounds RTe3 as a function of temperature, wave vector, and electron-dispersion parameters. Comparison of the results obtained with the available experimental data on the transition temperature and on the wave vector of a charge-density wave in these compounds allowed us to make predictions about the evolution of electron-dispersion parameters with the variation of the atomic number of rare-earth elements (R). Our measurements of the Hall coefficient in RTe3 compounds reveal a strong hysteresis between cooling and warming in the low temperature range where a second unidirectional charge density wave (CDW) occurs. We propose that this effect may result from the interplay between two instabilities: band crossing of the Te px and py orbitals at the Fermi level and CDW, which have a close energy gain and compete. Calculation of the electron susceptibility at the CDW wave vector with and without band anticrossing reconstruction of the electron spectrum yields a satisfactory estimation of the temperature range of the hysteresis in the Hall effect measurements.
[1] P.A. Vorobyev, P.D. Grigoriev, K.K. Kesharpu and V.V. Khovaylo, Materials 12, 2264 (2019).
[2] P.D. Grigoriev, A.A. Sinchenko, P.A. Vorobyev, A. Hadj-Azzem, P. Lejay, A. Bosak, P. Monceau, Phys. Rev. B 100, 081109(R) (2019).

Department of quantum mesoscopics: seminar, Friday, November 1, 2019, ITF, 3:00 pm
Vasilii Vadimov (IFM, Nizhnii Novgorod)
Mody Khiggsa v gibridnykh sistemakh sverkhprovodnik-normal'nyi metall

Effekt blizosti v gibridnykh sistemakh sverkhprovodnik-normal'nyi metall okazyvaet sushchestvennoe vliyanie na kogerentnye kolebaniya sverkhprovodyashchego parametra poryadka Δ, izvestnye kak mody Khiggsa. Standartnaya moda Khiggsa s chastotoi 2Δ ispytyvaet zatukhanie iz-za utechki kvazichastits iz sverkhprovodnika v normal'nyi metall. Pomimo etogo iz-za kogerentnogo tunnelirovaniya elektronov mezhdu sverkhprovodnikom i normal'nym metallom poyavlyayutsya dve dopolnitel'nye khiggsovskie mody s chastotami, opredelyaemymi pervichnoi shchel'yu Δ i navedennoi shchel'yu v normal'nom metalle. Eti tri mody opredelyayut asimptoticheskoe povedenie sverkhprovodyashchego parametra poryadka, vozmushchennogo elektromagnitnym impul'som libo rezkim izmeneniem parametrov sistemy. Issledovanie mod Khiggsa v gibridnykh strukturakh vazhno dlya ponimaniya dinamicheskikh svoistv sverkhprovodyashchikh priborov, osnovannykh na effekte blizosti.

Scientific Council of the Landau Institute, Friday, October 25, 2019, Landau ITP, 3:00 pm
Yan V. Fyodorov (Dept. of Mathematics, King’s College London)
Zeroes of S-matrix entries and random 'anti-lasing'

Motivated by recent experimental interest in 'random anti-lasing' (e.g. K. Pichler et al. Nature 567, 351 (2019)) I consider manifestations of zeroes of scattering matrices in wave-chaotic cavities. In particular, I will introduce the notion of Reflection Time Difference playing the same role for the S-matrix zeroes as the Wigner time delay plays for the S-matrix poles, and a possibility of its experimental measurement. I will then discuss how statistics of complex zeroes of scattering matrix entries can be described in the framework of RMT-based model of resonance scattering.
Presentation will be based on papers arXiv:1908.06920 and J. Phys. A 50, 30LT01 (2017).

Landau ITP colloquium, Friday, October 25, 2019, Landau ITP, 11:30 am
Yuval Gefen (Weizmann Institute of Science, Israel)
Physics at the Edge of a QHE setting

The structure of the edge of a QHE edge is constrained, but not dictated, by the topology of the bulk. Particularly interesting is the class of hole-conjugate fractional QH states. For such states the edge hosts counter-propagating modes that are responsible for quantized electrical and thermal conductance. In the coherent quantum limit renormalized edge modes emerge, which may involve neutral modes with non-trivial exchange statistics. I will discuss the behavior of topology-influenced transport coefficient both in the coherent and the fully equilibrated regimes, topological classification of non-equilibrium shot noise, and the relation to recent experiments.

Scientific Council of the Landau Institute, Friday, October 11, 2019, Landau ITP, 11:30 am
Ya.V. Fominov
Konferentsiya NANO-2019: Limits of Nanoscience and Nanotechnologies

Otchet ob uchastii v konferentsii "NANO-2019: Limits of Nanoscience and Nanotechnologies" (Kishinev, Moldaviya).

Scientific Council of the Landau Institute, Friday, October 11, 2019, Landau ITP, 11:30 am
Ya.V. Fominov
Sverkhprovodyashchie tripletnye spinovye klapany tipa FFS so splavom Geislera v kachestve odnogo iz ferromagnitnykh sloev

Provedeno teoreticheskoe soprovozhdenie novoi serii eksperimentov po izucheniyu effekta sverkhprovodyashchego tripletnogo spinovogo klapana v sisteme tipa FFS (gde F — ferromagnetik, S — sverkhprovodnik). Effekt sostoit v tom, chto kriticheskaya temperatura Tc sistemy zavisit ot vzaimnoi orientatsii namagnichennostei dvukh ferromagnetikov. V eksperimente v kachestve srednego ili vneshnego F sloya byl vzyat splav Geislera Co2Cr1-xFexAl, magnitnye svoistva kotorogo zaviseli ot uslovii prigotovleniya. Dlya ob'yasneniya poluchennykh rezul'tatov byla sformulirovana teoriya, uchityvayushchaya proizvol'nye obmennye polya F sloev i proizvol'nye parametry granits v ramkakh kvaziklassiki v diffuznom predele.
Doklad osnovan na sleduyushchikh rabotakh:
[1] A.A. Kamashev, N.N. Garif’yanov, A.A. Validov, J. Schumann, V. Kataev, B. Büchner, Ya.V. Fominov, I.A. Garifullin, "Superconducting switching due to a triplet component in the Pb/Cu/Ni/Cu/ Co2Cr1-xFexAl y spin-valve structure", Beilstein J. Nanotechnol. 10, 1458 (2019).
[2] A.A. Kamashev, N.N. Garif’yanov, A.A. Validov, J. Schumann, V. Kataev, B. Büchner, Ya.V. Fominov, I.A. Garifullin, "Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers", napravleno v Phys. Rev. B.
[3] A.A. Kamashev. N.N. Garif'yanov, A.A. Validov, I. Shumann, V. Kataev, B. Byukhner, Ya.V. Fominov, I.A. Garifullin, Pis'ma v ZhETF 110, 325 (2019).

Scientific Council of the Landau Institute, Friday, October 4, 2019, Landau ITP, 11:30 am
I.S. Burmistrov
Absolute Poisson's ratio and the bending rigidity exponent of a crystalline two-dimensional membrane

We compute the absolute Poisson's ratio $\nu$ and the bending rigidity exponent $\eta$ of a free-standing two-dimensional crystalline membrane embedded into a space of large dimensionality $d = 2 + d_c$, $d_c \gg 1$. We demonstrate that, in the regime of anomalous Hooke's law, the absolute Poisson's ratio approaches material independent value determined solely by the spatial dimensionality $d_c$: $\nu = -1 +2/d_c-a/d_c^2+\dots$ where $a\approx 1.76\pm 0.02$. Also, we find the following expression for the exponent of the bending rigidity: $\eta = 2/d_c+(73-68\zeta(3))/(27 d_c^2)+\dots$. These results cannot be captured by self-consistent screening approximation.

Scientific Council of the Landau Institute, Friday, October 4, 2019, Landau ITP, 11:30 am
I.S. Burmistrov
The effect of anomalous elasticity on the bubbles in van der Waals heterostructures

It is shown that the anomalous elasticity of membranes affects the profile and thermodynamics of a bubble in van der Waals heterostructures. Our theory generalizes the non-linear plate theory as well as membrane theory of the pressurised blister test to incorporate the power-law scale dependence of the bending rigidity and Young's modulus of a two-dimensional crystalline membrane. This scale dependence caused by long-ranged interaction of relevant thermal fluctuations (flexural phonons), is responsible for the anomalous Hooke's law observed recently in graphene. It is shown that this anomalous elasticity affects dependence of the maximal height of the bubble on its radius and temperature. We identify the characteristic temperature above which the anomalous elasticity is important. It is suggested that for graphene-based van der Waals heterostructures the predicted anomalous regime is experimentally accessible at the room temperature.

Scientific Council of the Landau Institute, Friday, September 20, 2019, Landau ITP, 11:30 am
Boris Luk'yanchuk (MGU & Nanyang Technological University, Singapore)
High order Fano-resonances and extreme effects in field localization

The weakly dissipating dielectric spheres (glass, quartz, etc.) permit to realize high order Fano resonances for internal Mie modes. These resonances for specific values of the size parameter yield field-intensity enhancement factors on the order of 104–107, which can be directly obtained from analytical calculations. These “super-resonances” provides magnetic nanojets with giant magnetic fields, which is attractive for many applications.

Landau ITP colloquium, Friday, September 6, 2019, Landau ITP, 11:30 am
Sergej Flach (Institute for Basic Science, Republic of Korea)
Dynamical Glass - en route from KAM and FPUT to MBL

Classical many body interacting systems are typically chaotic (nonzero Lyapunov exponents) and their microcanonical dynamics ensures that time averages and phase space averages are identical (ergodic hypothesis). In proximity to an integrable limit the long- or short-range properties of the network of nonintegrable action space perturbations define the finite time relaxation properties of the system towards Gibbs equilibrium. I will touch upon few analytical results including the KAM theorem, and review a number of computational studies which originate from the pioneering work of Enrico Fermi, John Pasta, Stanislaw Ulam and Mary Tsingou. I will then focus on short range networks which lead to a dynamical glass (DG), using a classical Josephson junction chain in the limit of large energy densities or small Josephson energies. Close to these limits the Josephson coupling between the superconducting grains induces a short-range nonintegrable network in the corresponding action space. I will introduce a set of quantitative measures which lead to the Lyapunov time TΛ, the ergodization time TE, and to a diffusion constant D. In the DG the system fragments into large patches of nonresonant ’integrable’ grains of size l separated by triplets of resonant chaotic patches, all surviving over large times. TE sets the time scale for chaotic dynamics in the triplets. Contrary, TE ≈ l2/D is the much larger time scale of slow diffusion of chaotic triplets. The DG is a generic feature of weakly non-integrable systems with a short range coupling network in action space, and expected to be related to nonergodic quantum metallic states of quantum many-body systems in proximity to a many-body localization phase.

Landau ITP colloquium, Friday, June 21, 2019, Landau ITP, 11:30 am
Anton Andreev
Two-fluid phenomena in one-dimensional quantum liquids

One-dimensional quantum liquids are commonly treated using the Luttinger liquid theory, which neglects elementary excitations with high energies. In this approximation, in addition to the number of particles, energy, and momentum the liquid possesses another conserved quantity - J, which may be interpreted as the difference between the numbers of right- and left- moving particles. Beyond the Luttinger liquid approximation J is no longer conserved, but its relaxation time is exponentially long at low temperatures. I will show that as a result, one-dimensional quantum liquids exhibit two-fluid behavior. In particular in a wide frequency interval they support two sound modes that are similar to the first and second sound in superfluid He.

Scientific Council of the Landau Institute, Friday, June 7, 2019, Landau ITP, 11:30 am
Lev Spodyneiko
Termal'nyi effekt Kholla kak topologicheskii invaraint

We show that derivatives of thermal Hall conductance of a 2d lattice quantum system with respect to parameters of the Hamiltonian are well-defined bulk quantities provided correlators of local observables are short-range. This is despite the fact that thermal Hall conductance itself has no meaning as a bulk transport coefficient. We use this to define a relative topological invariant for gapped 2d lattice quantum systems at zero temperature. Up to a numerical factor, it can be identified with the difference of chiral central charges for the corresponding edge modes. This establishes bulk-boundary correspondence for the chiral central charge. We also show that for Local Commuting Projector Hamiltonians relative thermal Hall conductance vanishes identically, while for free fermionic systems it is related to the electric Hall conductance via the Wiedemann-Franz law.

Scientific Council of the Landau Institute, Friday, June 7, 2019, Landau ITP, 11:30 am
I.S. Burmistrov
Dissipative and Hall viscosity of a disordered 2D electron gas

Hydrodynamic charge transport is at the center of recent research efforts. Of particular interest is the nondissipative Hall viscosity, which conveys topological information in clean gapped systems. The prevalence of disorder in the real world calls for a study of its effect on viscosity. Here we address this question, both analytically and numerically, in the context of a disordered noninteracting 2D electrons. Analytically, we employ the self-consistent Born approximation, explicitly taking into account the modification of the single-particle density of states and the elastic transport time due to the Landau quantization. The reported results interpolate smoothly between the limiting cases of weak (strong) magnetic field and strong (weak) disorder. In the regime of weak magnetic field our results describes the quantum (Shubnikov-de Haas type) oscillations of the dissipative and Hall viscosity. For strong magnetic fields we characterize the effects of the disorder-induced broadening of the Landau levels on the viscosity coefficients. This is supplemented by numerical calculations for a few filled Landau levels. Our results show that the Hall viscosity is surprisingly robust to disorder.

Shmidtovskii seminar po sverkhprovodimosti, Wednesday, June 5, 2019, MISiS, B-734, 3:00 pm
Prof. A. I. Buzdin (University Bordeaux, France)
Chirality-controlled spontaneous currents in spin-orbit coupled superconducting rings

At a superconductor interface with a ferromagnetic insulator (FI), the FI acts to induce a local exchange field within the S layer, which in the presence of spin-orbit interaction promotes a phase- modulated superconducting state. Here we demonstrate that within a thin superconducting loop that is partially proximitized by a FI, spontaneous currents form with a magnetization-orientation-dependent chirality with sizable shifts in LittleParks oscillations. Furthermore, the critical temperature of the loop is also magnetization-orientation-dependent and conversely, the superconducting transition itself may influence the magnetization direction. More generally, the superconducting region above the FI serves as a “phase battery” and so offers a new device concept for superconducting spintronics.

Venue: NUST MISIS, conference hall № B-734 (Main Building, 7th floor)

Entrance for seminar participants who are not students, graduate students or employee of MISIS, will be provided upon presentation of the passport upon prior registration for the seminar. Registration is opened till 14:00 June, 5.

To register https://smm.misis.ru/en/event/xxi-shmidt-seminar-on-superconductivity/

Landau ITP colloquium, Friday, May 24, 2019, Landau ITP, 11:30 am
D. Christian Glattli (Nanoelectronics Group, Service de Physique de l’Etat Condensé, CEA Saclay, France)
Minimal excitations states: From time resolved single particle fermionic states for Electron Quantum Optics to Digital communication and music.

In the 90’s, an impressive series of works by theoreticians from the Landau Institute on electrons shot noise in quantum conductors [1] and on the statistics of transfer of electrons [2] has leaded to the emergence of the beautiful concept of minimal excitation states [3-5]. These minimal excitation states can be generated by applying voltage pulses on the contact of a conductor to inject short single electron pulses. These states show minimal noise and provide a convenient and clean single electron source for electron optics whose aim is to perform quantum optics tasks with electrons instead of photons. The minimal excitations states, now called levitons, have been produced in recent experiments [6] and have triggered a large number of theoretical works. They have enabled Hong Ou Mandel like experiments [6] with electrons and single electron quantum Tomography [7]. Extension to fractionally charged anyons is possible.
At the root of the minimal excitation property is a specific single side band modulation of the electron wave by the Lorentzian voltage pulse. This property can be applied to classical electromagnetic or acoustic waves for applications in digital communication [8] or in music sound synthesis.
[1] G. B. Lesovik, JETP Letters, 49 (9), 592-594 (1989).
[2] L.S. Levitov, G.B. Lesovik, Charge-transport statistics in quantum conductors, JETP Lett., 55 (9), 555-559 (1992).
[3] A. Ivanov, H.W. Lee, L.S. Levitov, Coherent states of alternating current, Phys. Rev. B 56(11), 6839-6850 (1997); cond-mat/9501040
[4] L.S. Levitov, H. Lee, G.B. Lesovik, Electron Counting Statistics and Coherent States of Electric Current, J. Math. Phys., 37(10), 4845-4866 (1996); cond-mat/9607137.
[5] J. Keeling, I. Klich, and L. S. Levitov, Minimal Excitation States of Electrons in One-Dimensional Wires, Phys. Rev. Lett. 97, 116403 (2006).
[6] Minimal-excitation states for electron quantum optics using levitons, J. Dubois, T. Jullien, F. Portier, P. Roche, A. Cavanna, Y. Jin, W. Wegscheider, P. Roulleau & D. C. Glattli, Nature, 502, 659–663 (2013).
[7] Quantum tomography of an electron, T. Jullien, P. Roulleau, B. Roche, A. Cavanna, Y. Jin & D. C. Glattli, Nature, 514, 603–607 (2014).
[8] Power Spectrum Density of Single Side Band CPM Using Lorenztian Frequency Pulses, Haïfa Farès, D. Christian Glattli, Yves Louet, Jacques Palicot, Preden Roulleau, and Christophe Moy, IEEE Wireless Communications Letters, 6 (6), 786-789, (2017).

Theory seminar, Thursday, May 23, 2019, Kapitza IPP, 11:30 am
D. Christian Glattli (Nanoelectronics Group, Service de Physique de l’Etat Condensé, CEA Saclay, France)
A Josephson relation for e/3 and e/5 fractionally charged anyons

Anyons occur in two-dimensional electron systems as excitations with fractional charge in the topologically ordered states of the fractional quantum Hall effect (FQHE). Their dynamics are of utmost importance for topological quantum phases and possible decoherence-free quantum information approaches, but observing these dynamics experimentally is challenging. Here, we report on a dynamical property of anyons: the long-predicted [1] Josephson relation fJ = e*V/h for charges e* = e/3 and e/5, where e is the charge of the electron and h is Planck’s constant [2].

The relation manifests itself as marked signatures in the dependence of photo-assisted shot noise (PASN) [3-4] on voltage V when irradiating contacts at microwaves frequency fJ [4]. The validation of FQHE PASN models indicates a path toward realizing time-resolved anyon sources based on levitons. The method may be of interest to provide a demonstration of anyonic statistics, a pre-requisite for topological quantum computing.

[1] X. G. Wen, Edge transport properties of the fractional quantum Hall states and weak-impurity scattering of a one-dimensional charge-density wave, Phys. Rev. B 44, 5708–5719 (1991). [2] M. Kapfer, P. Roulleau, M. Santin, I. Farrer, D. A. Ritchie, and D. C. Glattli, A Josephson relation for fractionally charged anyons, Science 363, 846–849 (2019). [3] G. B. Lesovik and L. S. Levitov, Noise in an ac biased junction: Nonstationary Aharonov-Bohm effect, Phys. Rev. Lett. 72, 538–541 (1994). [4] C. de C. Chamon, D. E. Freed, and X. G. Wen, Tunneling and quantum noise in one-dimensional Luttinger liquids, Phys. Rev. B 51, 2363–2379 (1995).

Department of quantum mesoscopics: seminar, Friday, May 17, 2019, ITF, 3:00 pm
Anton Khvalyuk
Simple model for describing trotterization effects in quantum simulations

Kvantovye vychisleniya v poslednee vremya yavlyayutsya aktivno razvivayushcheisya oblast'yu prikladnoi fiziki i teorii informatsii. Odnim iz mnogoobeshchayushchikh primenenii vozmozhnostei segodnyashnikh kvantovykh komp'yuterov yavlyayutsya kvantovyi simulyatsii fizicheskikh sistem na sistemakh kubitov. Odnako, sushchestvennymi problemami na puti k fizicheski korrektnym simulyatsiyam yavlyayutsya trotterovskaya oshibka diskretizatsii i razlichnye effekty shuma. V etom doklade budet predlozhena prostaya tochno reshaemaya model' sistemy spinov, pozvolyayushchaya izuchit' osnovnye effekty trotterovskoi oshibki i dayushchaya vozmozhnost' dlya izucheniya effektov shuma.

Landau ITP colloquium, Friday, April 26, 2019, Landau ITP, 11:30 am
Boris Narozhny (KIT)
Hydrodynamic approach to electronic transport

The last few years have seen an explosion of interest in hydrodynamic effects in interacting electron systems in ultra-pure materials. One such material, graphene, is not only an excellent platform for the experimental realization of the hydrodynamic flow of electrons, but also allows for a controlled derivation of the hydrodynamic equations on the basis of kinetic theory. The resulting hydrodynamic theory of electronic transport in graphene yields quantitative predictions for experimentally relevant quantities, e.g. viscosity, electrical conductivity, etc. In this talk I will review recent theoretical advances in the field, compare the hydrodynamic theory of charge carriers in graphene with relativistic hydrodynamics and recent experiments, and discuss applications of hydrodynamic approach to novel materials beyond graphene.

Video (v svyazi s oshibkoi video zapisano ne do kontsa)

Scientific Council of the Landau Institute, Friday, April 19, 2019, Landau ITP, 11:30 am
S. I. Matveenko
Realizatsiya fermionnykh modelei v sistemakh kholodnykh atomov

Pokazano, chto topologicheskoe sverkhtekuchee sostoyanie s p-sparivaniem (p_x + i p_y) mozhet byt' realizovano v dvumernoi opticheskoi reshetke fermionnykh polyarnykh molekul. Dlya sistem s odnosloinoi i dvukhsloinoi geometriei vychislennye temperatury fazovykh perekhodov v sverkhtekuchee sostoyanie okazalis' eksperimental'no dostizhimymi, poryadka desyatkov nanokel'vinov. [A.K. Fedorov, S.I. Matveenko, V. I. Yudson, G.V. Shlyapnikov , "Novel p-wave superfluids of fermionic polar molecules", Sci. Rep. 6, 27448; (2016).]
Issledovana chislenno i analiticheski fazovaya diagramma dvusloinoi sistemy fermionnykh polyarnykh molekul s disbalansom molekulyarnykh plotnostei v sloyakh. Dlya disbalansa (effektivnogo magnitnogo polya), prevyshayushchego kriticheskoe znachenie, sistema perekhodit pri dostatochno nizkikh temperaturakh iz sostoyaniya odnorodnoi sverkhtekuchei zhidkosti v sostoyaniya Larkina-Ovchinnikova-Ful'de-Ferrella s periodicheskoi strukturoi parametra poryadka. V otlichie ot sluchaya kontaktnogo vzaimodeistviya s singletnym sparivaniem, dipol'-dipol'noe vzaimodeistvie generiruet nenulevuyu tripletnuyu sostavlyayushchuyu parametra poryadka. [Hao Lee, S. I. Matveenko, Daw-Wei Wang, G. V. Shlyapnikov, "Fulde-Ferrell-Larkin-Ovchinnikov state in bilayer dipolar systems", Phys.Rev. A, 96, 061602(R) (2017)].
Issledovana odnomernoi model' dvukhkomponentnogo fermionnogo gaza s sil'nym kontaktnym ottalkivaniem chastits s protivopolozhnym spinom i nelokal'nym prityazheniem fermionov s odinakovym spinom. Model' mozhet byt' eksperimental'no realizovana v sisteme kholodnykh atomov. Pri etom parametry vzaimodeistviya mogut izmenyat'sya v shirokom diapazone s pomoshch'yu vneshnego magnitnogo polya. Ispol'zuya metody bozonizatsii i renormgruppy, naideno osnovnoe sostoyanie sistemy v zavisimosti ot parametrov vzaimodeistviya. Pokazano, chto pri kriticheskom znachenii nelokal'nogo vzaimodeistviya proiskhodit fazovyi perekhod pervogo roda v sostoyanie s nenulevoi namagnichennost'yu. [D. V. Kurlov, S. I. Matveenko, V. Gritsev, G. V. Shlyapnikov, "One-dimensional two-component fermions with contact even-wave repulsion and SU(2) breaking near-resonant odd-wave attraction", Phys. Rev. A, (prinyata v pechat')].

Landau ITP colloquium, Friday, April 12, 2019, Landau ITP, 11:30 am
Yan V. Fyodorov (King's College London)
On reconstructing nonlinearly encrypted signals corrupted by noise.

I consider the problem of reconstructing a source vector from its encrypted image corrupted by an additive Gaussian noise. Assuming encryption to be given by a random Gaussian mapping, the reconstruction problem in the framework of the Least Square Scheme turns out to be equivalent to finding the configuration of minimal energy in a certain version of spherical spin glass model. As a measure of the quality of the signal reconstruction one can use the mean overlap between the original signal and its recovered image. Thi overlap is analysed in the framework of Parisi scheme of Replica Symmetry Breaking. If the mapping is quadratic, there exists a threshold in the noise-to-signal ratio beyond which the reconstruction is impossible. The behaviour close to the threshold is controlled by the replica symmetry breaking mechanism and is characterized by a nontrivial exponent 3/4.

Video

Theory seminar, Thursday, April 11, 2019, Kapitza IPP, 11:30 am
Yan Fyodorov (King's College London)
On energy landscape of elastic manifolds pinned by random potentials

We consider an elastic manifold of internal dimension d and length L pinned in a N dimensional random potential and confined by an additional parabolic potential of curvature μ. First we consider N = d = 1 case of a directed polymer and show how counting stationary points of its energy provides an upper bound on the strength of the depinning force. Then we consider a mean-filed limit: first N→∞ at fixed Ld, and then L→∞, and find the mean spectral density ρ(λ) of the Hessian matrix K at the absolute minimum of the energy functional. We show that for a confinement curvature μ exceeding a critical value, the so-called "Larkin mass", the system is replica-symmetric and the Hessian spectrum is always gapped (from zero). The gap vanishes quadratically at approaching the Larkin mass. For smaller curvatures the replica symmetry breaking (RSB) occurs and the Hessian spectrum is either gapped or extends down to zero, depending on whether RSB is 1-step or full. In the 1-RSB case the gap vanishes in all d as fourth power of the distance to the transition. In the full RSB case the gap is identically zero.

The presentation will be based on Y.V. Fyodorov et al., Ann. Phys. 397, 1–64 (2018) and Y.V. Fyodorov and P. Le Doussal, arXiv:1903.07159.

Department of quantum mesoscopics: seminar, Friday, April 5, 2019, ITF, 3:00 pm
Tim Ludwig (Karlsruhe Institute of Technology)
Noise of charge current generated by driven magnet

We determine the noise of charge current that is generated by a precessing magnetization of a small itinerant ferromagnet, which is driven by ferromagnetic resonance (FMR) and tunnel-coupled to two normal metal leads. Even when the leads are in equilibrium with each other, i.e. neither voltage nor thermal bias is applied, the electron system is driven out of equilibrium by the precessing magnetization. In turn, a non-equilibrium electron distribution develops in the small ferromagnet. This distribution is governed by the dynamics of the magnetization and determines the (zero-frequency) noise of charge current. We show that this noise can be used to gain information about the magnetization dynamics.

Landau ITP colloquium, Friday, April 5, 2019, Landau ITP, 11:30 am
Vladimir Kravtsov
Correlation-induced localization

Conventional Anderson localization is due to destructive interference of matter waves described by local random Hamiltonians. Correlations in random diagonal elements of such a Hamiltonian are known to favor delocalization. Recently systems with non-local Hamiltonians become experimentally accessible. We consider two families of such random matrix Hamiltonians with correlations in the long-range hopping terms and demonstrate that localization is enhanced and the wave function ergodicity is progressively degrading as the correlations become stronger.

We review the localization/delocalization criteria of Mott and Anderson and show that the former is the sufficient criterion of weak ergodicity and the latter is the sufficient criterion of localization. The fact that these two criteria are not complimentary is the reason why the non-ergodic extended (multifractal) states may exist when neither the Mott, nor the Anderson criterion is fulfilled.
We suggest a new class of random matrix models (Toeplitz RMT) with translation-invariant hopping integrals and identify the character of eigenfunction and eigenvalue statistics in them. We formulate the principles of level statistics if the type of eigenfunction statistics is known both in the coordinate and in the momentum basis and demonstrate that for the Toeplitz RMT the ergodic delocalization in the coordinate space may coexist with the Poisson level statistics.
Finally, we suggest a matrix-inversion trick that allows to identify uniquely the type of eigenfunction statistics and prove the absence of delocalized states in the bulk of spectrum of long-range Hamiltonians with deterministic (fully correlated) hopping.

Video

Scientific Council of the Landau Institute, Friday, March 22, 2019, Landau ITP, 11:30 am
A.Ya. Maltsev
Obshchie osobennosti uglovykh diagramm provodimosti metallov v sil'nykh magnitnykh polyakh i slozhnost' uglovykh diagrammakh magnitoprovodimosti v sil'nykh magnitnykh polyakh

My rassmotrim samye obshchie osobennosti uglovykh diagramm provodimosti v normal'nykh metallakh so slozhnymi poverkhnostyami Fermi v prisutstvii sil'nykh magnitnykh polei. Opisanie takikh osobennostei budet osnovano na topologicheskom opisanii dinamicheskoi sistemy, voznikayushchei dlya evolyutsii elektronnykh sostoyanii na poverkhnosti Fermi v prisutstvii vneshnego magnitnogo polya. My rassmotrim uglovye diagrammy provodimosti dlya normal'nykh (monokristallicheskikh) metallov so slozhnymi poverkhnostyami Fermi v prisutstvii sil'nykh magnitnykh polei. Povedenie provodimosti v etom sluchae sil'no zavisit ot napravleniya magnitnogo polya i ustoichivye netrivial'nye rezhimy takogo povedeniya sootvetstvuyut pri etom spetsial'nym zonam ustoichivosti na uglovoi diagramme, otvechayushchim opredelennym (topologicheskim) svoistvam tenzora provodimosti. Kak my pokazhem, v obshchem sluchae mozhno razdelit' takie diagrammy na dva obshchikh tipa, prostye (tip A) i slozhnye (tip B). Nas budut interesovat' pri etom diagrammy vtorogo tipa, obladayushchie ryadom spetsificheskikh osobennostei (beskonechnoe chislo zon ustoichivosti, nalichie khaoticheskikh rezhimov i t.p.), kotorye my rassmotrim bolee podrobno. Po rezul'tatam rabot: ZhETF, t. 151, vyp. 5, 944-973 (2017); ZhETF, t. 152, vyp. 5(11), 1053-1064 (2017) , ZhETF, 154(6), 1183-1210 (2018)

Scientific Council of the Landau Institute, Friday, March 22, 2019, Landau ITP, 11:30 am
A.Ya. Maltsev, S.P. Novikov
Ergodic properties of dynamic systems on two-dimensional surfaces and electron transport phenomena in normal metals

V doklade obsuzhdaetsya svyaz' spetsial'nykh kharakteristik dinamicheskikh sistem na poverkhnostyakh (indeksov Zoricha - Kontsevicha - Forni) s povedeniem provodimosti v metallakh v sil'nykh magnitnykh polyakh. Dannoe rassmotrenie yavlyaetsya vazhnym v sluchae vozniknoveniya naibolee slozhnykh (khaoticheskikh) elektronnykh traektorii na poverkhnosti Fermi, vozmozhnom dlya dostatochno slozhnykh poverkhnostei i spetsial'nykh napravlenii magnitnogo polya. Kak mozhno pokazat', upomyanutye kharakteristiki imeyut neposredstvennoe otnoshenie k povedeniyu magnitoprovodimosti, takim obrazom, mozhno v printsipe govorit' o vozmozhnosti ikh eksperimental'nogo opredeleniya v sootvetstvuyushchikh sluchayakh. Po rezul'tatam raboty: Trudy MIAN, tom 302, (2018) str. 296–315

Department of quantum mesoscopics: seminar, Friday, March 15, 2019, ITF, 3:00 pm
A.V. Lunkin
Geometricheskii podkhod k resheniyu SYK

(po motivam izucheniya stat'i A.Yu. Kitaeva)

Scientific Council of the Landau Institute, Friday, March 15, 2019, Landau ITP, 11:30 am
A.V. Lunkin, K.S. Tikhonov, M.V. Feigel'man
SYK model with quadratic perturbations: the route to a non-Fermi-liquid

Model' SYK (Sachedev-Ye-Kitaev) opisyvaet sistemu sluchaino vzaimodeistvuyushchikh maioranovskikh fermionov bez kvadratichnykh chlenov v gamil'toniane. V predele bol'shogo chisla fermionov (N) i nizkikh temperaturakh (T « J, gde J - kharakternyi masshtab vzaimodeistviya) funktsiya Grina modeli, v sedlovom priblizhenii, imeet ne fermi-zhidkostnoe povedenie G(t) ~ t^(-1/2). Odnako, sedlovye uravneniya obladayut vysokoi simmetriei i dopuskayut zamenu t na proizvol'nuyu monotonnuyu funktsiyu f(t). Takoe reshenie imeet tol'ko SL(2,R) simmetriyu. Ponizhenie simmetrii ot polnoi gruppy reparametrizatsii do SL(2,R) privodit k sushchestovaniyu myagkoi mody, kotoraya stanovitsya sushchestvenna pri NT « J . Funktsiya Grina na samykh bol'shikh vremenakh menyaet svoyu asimptotiku na G(t) ~ t^(-3/2).
V nashei rabote my issleduem vliyanie kvadratichnogo vozmushcheniya na eto povedenie. Naivnoe rassmotrenie sedlovykh uravnenie pokazyvaet, chto na bol'shikh vremenakh fermi-zhidkostnoe povedenie dolzhno vosstanavlivat'sya. Odnako, voznikaet vopros: chto, proizoidet esli eti vremena budut stol' bol'shimi, chto nuzhno uchityvat' fluktuatsii myagkoi mody? Rassmatrivaya vtoroi poryadok teorii vozmushcheniya my pokazyvaem, chto sushchestvuet nenulevoi interval amplitud vozmushchenii, kogda povedenie funktsii Grina ne menyaetsya, sokhranyaya asimptotiku G(t) ~ t^(-3/2). Eto pozvolyaet nadeyat'sya na ispol'zovanie modeli SYK dlya postroeniya kontroliruemoi teorii ne-fermizhidkostnogo povedeniya sil'no vzaimodeistvuyushchikh fermionov.

Department of quantum mesoscopics: seminar, Friday, March 1, 2019, ITF, 3:00 pm
A.V. Poshakinskii, A.N. Poddubnyi (FTI im. A.F. Ioffe)
Rasseyanie sveta na drozhashchikh telakh s rezonansnoi dielektricheskoi pronitsaemost'yu

Postroena teoriya, opisyvayushchaya vzaimodeistvie elektromagnitnogo polya s drozhashchei dispergiruyushchei sredoi. V pervom poryadke po mekhanicheskomu smeshcheniyu vychislena popravka k polyarizuemosti sredy, obuslovlennaya drozhaniem. Pokazano, chto dvizhenie v prostranstve elektricheskogo dipolya privodit k poyavleniyu magnitnogo dipol'nogo i elektricheskogo kvadrupol'nogo momentov. Teoriya primenena dlya opisaniya neuprugogo rasseyaniya sveta na drozhashchikh chastitsakh i membranakh. Ustanovleno, chto kharakter interferentsii elektromagnitnykh voln, izluchennykh razlichnymi mul'tipolyami, opredelyaetsya chastotnoi zavisimost'yu dielektricheskoi pronitsaemosti. V sluchae, kogda ona imeet rezonans, vblizi nego svet rasseivaetsya preimushchestvenno vpered, a vdali ot nego — nazad. Takzhe opisana konversiya polyarizatsii sveta v protsesse neuprugogo rasseyaniya.

A. V. Poshakinskiy and A. N. Poddubny, Phys. Rev. X 9, 011008 (2019).

Scientific Council of the Landau Institute, Friday, March 1, 2019, Landau ITP, 11:30 am
L. Faoro, M. Feigel'man, L. Ioffe
Non-ergodic phase of the Quantum Random Energy Model

Pokazano, chto fazovaya diagramma odnoi iz prosteishikh modelei kvantovogo spinovogo stekla – kvantovogo obobshcheniya modeli Derridy sluchainykh urovnei energii – obladaet neergodicheskoi delokalizovannoi fazoi s vremenem relaksatsii, eksponentsial'no dlinnym po chislu spinov n. Kak vremya relaksatsii, tak i ob'em Gil'bertova prostranstva, zanimaemyi mnogochastichnoi volnovoi funktsiei, yavlyayutsya stepennymi funktsiyami polnoi razmernosti Gil'bertova prostranstva N = 2^n. Oba eti pokazateli stepeni naideny v ramkakh priblizhennoi analiticheskoi teorii, poluchennye dlya nikh otvety neplokho soglasuyutsya s rezul'tatami pryamoi chislennoi diagonalizatsii. Poluchennye rezul'taty mogut byt' polezny dlya fizicheskoi realizatsii kvantovogo simulyatora dlya resheniya zadach algoritmicheskoi optimizatsii tipa number partitioning.

Landau ITP colloquium, Friday, February 22, 2019, Landau ITP, 11:30 am
Ferdinand Evers (Regensburg University)
Recent theoretical developments in the integer quantum Hall effects

The quantum Hall effects belong to the most striking phenomena in condensed matter physics. Despite of intensive theoretical efforts over the last three decades, important aspects of the quantum Hall transitions are still not fully understood. In particular, there is still no consensus concerning the critical field theory and the corresponding scaling properties of the observables near and at the plateau transition.
Notwithstanding this status, the last ten years have seen considerable progress in understanding basic properties of scaling near localization-delocalization transitions that have implications also for the quantum Hall transition. These concern, e.g., the important topic of corrections to scaling, the wavefunction statistics and higher-order multifractality. A brief review of these developments will be offered in the first part of the talk.
The second part of the talk will be devoted to the spin quantum Hall effect, which is a variant of the integer quantum Hall effect (class A) taking place in the Altland-Zirnbauer class C. For this transition several critical exponents are known analytically and can serve as reference points. Therefore, this transition provides an ideal testbed for the qualitative predictions made by analytical theories. Analytical predictions will be confronted with most recent numerical results.

Video

Department of quantum mesoscopics: seminar, Friday, February 15, 2019, ITF, 3:00 pm
Mariya Kostenko (SPbGU)
Anomal'nyi skeiling v turbulentnosti: renormgruppovoi podkhod

V doklade budet obsuzhdat'sya zadacha ob anomal'nom skeilinge v turbulentnosti. Eto yavlenie proyavlyaetsya v singulyarnom (vozmozhno stepennom) povedenii razlichnykh statisticheskikh velichin, zavisyashchikh ot integral'nogo masshtaba turbulentnosti s beskonechnym kolichestvom nezavisimykh anomal'nykh pokazatelei. I eksperimenty, i chislennoe modelirovanie pokazyvayut, chto otkloneniya ot klassicheskoi teorii Kolmogorova bolee yarko vyrazheny dlya passivnogo perenosa skalyarnykh polei (naprimer, polya temperatury ili plotnosti primesi), chem dlya samogo polya skorosti. Samyi znachitel'nyi progress byl dostignut dlya modeli Kreichnana, gde pole skorostei perenosa yavlyaetsya gaussovym, ne korrelirovannym po vremeni. V protsesse doklada budet podrobno obsuzhdat'sya metod renormgruppy v primenenii k etoi modeli, a takzhe k bolee slozhnym i realistichnym modelyam perenosa.

Department of quantum mesoscopics: seminar, Friday, February 8, 2019, ITF, 3:00 pm
V.V. Marinyuk, D.B. Rogozkin (MIFI)
Mezoskopicheskie fluktuatsii kondaktansa na subdiffuzionnykh masshtabakh

V ramkakh diagrammnoi tekhniki vyvoditsya analiticheskoe vyrazhenie dlya dispersii kondaktansa neuporyadochennogo obraztsa cherez propagatory, podchinyayushchiesya transportnomu uravneniyu. Vychisleniya osnovany na sootnosheniyakh dlya vershiny Khikami bez ispol'zovaniya diffuzionnogo (gradientnogo) razlozheniya i bornovskogo priblizheniya dlya odnotsentrovogo rasseyaniya. Obsuzhdayutsya prichiny usileniya fluktuatsii pri perekhode ot kvaziballisticheskogo k diffuzionnomu rezhimu transporta.

Scientific Council of the Landau Institute, Friday, February 1, 2019, Landau ITP, 11:30 am
A.Ya. Maltsev
On complex angular diagrams of magnetic conductivity in strong magnetic fields

We consider angular conductivity diagrams for normal (single-crystal) metals with complex Fermi surfaces in the presence of strong magnetic fields. The behavior of conductivity in this case strongly depends on the direction of the magnetic field and the stable nontrivial regimes of this behavior correspond to special zones of stability on the angular diagram corresponding to certain (topological) properties of the conductivity tensor. As we show, in the general case such diagrams can be divided into two general types, simple (type A) and complex (type B). We will be interested in the diagrams of the second type, which have a number of specific features (an infinite number of stability zones, the presence of chaotic regimes, etc.), which we will consider in more detail.

Scientific Council of the Landau Institute, Friday, February 1, 2019, Landau ITP, 11:30 am
A.Ya. Mal'tsev, S.P. Novikov
Ergodicheskie svoistva dinamicheskikh sistem na dvumernykh poverkhnostyakh i elektronnye transportnye yavleniya v normal'nykh metallakh

V doklade obsuzhdaetsya svyaz' spetsial'nykh kharakteristik dinamicheskikh sistem na poverkhnostyakh (indeksov Zoricha - Kontsevicha - Forni) s povedeniem provodimosti v metallakh v sil'nykh magnitnykh polyakh. Dannoe rassmotrenie yavlyaetsya vazhnym v sluchae vozniknoveniya naibolee slozhnykh (khaoticheskikh) elektronnykh traektorii na poverkhnosti Fermi, vozmozhnom dlya dostatochno slozhnykh poverkhnostei i spetsial'nykh napravlenii magnitnogo polya. Kak mozhno pokazat', upomyanutye kharakteristiki imeyut neposredstvennoe otnoshenie k povedeniyu magnitoprovodimosti, takim obrazom, mozhno v printsipe govorit' o vozmozhnosti ikh eksperimental'nogo opredeleniya v sootvetstvuyushchikh sluchayakh.

Scientific Council of the Landau Institute, Friday, February 1, 2019, Landau ITP, 11:30 am
A.Ya. Mal'tsev
Obshchie osobennosti uglovykh diagramm provodimosti metallov v sil'nykh magnitnykh polyakh.

My rassmotrim samye obshchie osobennosti uglovykh diagramm provodimosti v normal'nykh metallakh so slozhnymi poverkhnostyami Fermi v prisutstvii sil'nykh magnitnykh polei. Opisanie takikh osobennostei budet osnovano na topologicheskom opisanii dinamicheskoi sistemy, voznikayushchei dlya evolyutsii elektronnykh sostoyanii na poverkhnosti Fermi v prisutstvii vneshnego magnitnogo polya.
Po rezul'tatam rabot: ZhETF, t. 151, vyp. 5, 944-973 (2017); ZhETF, t. 152, vyp. 5(11), 1053-1064 (2017)

Department of quantum mesoscopics: seminar, Friday, January 18, 2019, ITF, 3:00 pm
N.E. Nefedkin, A.A. Zyablovskii, E.S. Andrianov, A.A. Pukhov, A.P. Vinogradov (VNII Avtomatiki im. N.L. Dukhova)
Kooperatsiya mod v dvumernykh plazmonnykh lazerakh s raspredelennoi obratnoi svyaz'yu

Issledovana dinamika plazmonnogo lazera s raspredelennoi obratnoi svyaz'yu (DFB lazera) na osnove dvumernoi perforirovannoi metallicheskoi plenki. Dlya takoi struktury mozhno vydelit' dva kharakternykh tipa mod. K pervomu iz nikh otnosyatsya mody, chastota kotorykh nakhoditsya v razreshennoi zone, imeyushchie bol'shie izluchatel'nye poteri i vysokii porog generatsii (svetlye mody). Ko vtoromu tipu otnosyatsya mody, chastota kotorykh nakhoditsya na krayu razreshennoi zony, imeyushchie nizkie izluchatel'nye poteri i nizkii porog generatsii (temnye mody). Pokazano, chto kogda pyatno nakachki men'she razmera poverkhnosti DFB lazera, nelineinoe vzaimodeistvie mezhdu modami periodicheskoi plazmonnoi struktury cherez aktivnuyu sredu privodit k novomu effektu, a imenno k kooperatsii mod [1]. Kooperatsiya mod proyavlyaetsya kak generatsiya svetlykh mod s vysokimi poteryami vmesto generatsii temnykh mod s nizkimi poteryami. Pokazano, chto etot effekt imeet mesto v dvumernykh DFB lazerakh i ne proyavlyaetsya v odnomernykh DFB lazerakh. Vzaimodeistvie svetlykh mod voznikaet iz-za neortogonal'nosti lazernykh mod v oblasti nakachki, gde svetlye mody stanovyatsya sinkhronizirovannymi po faze. Kak sledstvie, voznikaet ikh konstruktivnaya interferentsiya. Prodemonstrirovano, chto vsledstvie kooperatsii mod imeet mesto ushirenie diagrammy napravlennosti vyshe poroga generatsii, chto nablyudalos' v nedavnikh eksperimentakh [2].

[1] N.E. Nefedkin, A.A. Zyablovsky, E.S. Andrianov, A.A. Pukhov, and A. P. Vinogradov, ACS Photonics 2018 5(8): p. 3031-3039.
[2] Tenner, V.T., M.J.A. de Dood, and M.P. van Exter, Measurement of the phase and intensity profile of surface plasmon laser emission. ACS Photonics, 2016. 3: p. 942−946.

Scientific Council of the Landau Institute, Friday, January 18, 2019, Landau ITP, 11:30 am
P.D. Grigoriev, A.D. Grigoriev, A.M. Dyugaev
Inelastic neutron scattering as a confirmation of a new type of gapped surface excitations in liquid helium

We analyze the experimental data on inelastic neutron scattering by a thin ~5-atomic-layer film of liquid helium at three different temperatures: T=0.4K, 0.98K and 1.3K. The neutron scattering intensity plots, in addition to the previously know dispersion of phonons and ripplons, suggest a branch of gapped surface excitations with activation energy ~4.5K and the dispersion similar to that expected for surfons -- the bound quantum states of helium atoms above liquid helium surface, proposed and investigated theoretically. These data, probably, provide the first direct experimental confirmation of surfons. Before these surface excitations received only indirect experimental substantiation, based on the temperature dependence of surface tension coefficient and on their interaction with surface electrons. The existence of surfons as an additional type of surface excitations, although being debated yet, is very important for various physical properties of He surface. We also analyze previous numerical results on excitations in liquid helium and argue that surface excitations similar to surfons have been previously obtained by numerical calculations and called resonance interface states.

Scientific Council of the Landau Institute, Friday, January 18, 2019, Landau ITP, 11:30 am
P.D. Grigoriev
Linear magnetoresistance in the charge density wave state of quasi-two-dimensional rare-earth tritellurides

The magnetoresistance of a TbTe3 two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.
[1] A.V. Frolov, A.P. Orlov, P.D. Grigoriev, V.N. Zverev, A.A. Sinchenko, P. Monceau, Magnetoresistance of a Two-Dimensional TbTe3 Conductor in the Sliding Charge-Density Wave Regime, JETP Lett., 107(8), 488-492 (2018)

Scientific Council of the Landau Institute, Friday, December 28, 2018, Landau ITP, 11:30 am
Pavel Ostrovskii
Elektronnye svoistva neuporyadochenogo grafena

Doklad po predstavlyaemoi k zashchite doktorskoi dissertatsii.

Department of quantum mesoscopics: seminar, Friday, December 21, 2018, ITF, 3:00 pm
A.S. Petrov, D.A. Svintsov (Laboratoriya optoelektroniki dvumernykh materialov MFTI)
Teoriya vozmushchenii dlya gidrodinamicheskikh plazmennykh effektov v dvumernykh elektronnykh sistemakh

V doklade budet predstavlen odin iz metodov dlya opisaniya gidrodinamicheskikh plazmennykh effektov v dvumernykh elektronnykh sistemakh (DES). A imenno, budet primenen operatornyi podkhod k uravneniyam elektronnoi gidrodinamiki i postroen analog kvantovo-mekhanicheskoi teorii vozmushchenii. Razrabotannaya teoriya pozvolit poluchit' vyrazhenie dlya pervoi popravki k sobstvennoi chastote plazmonov v prisutstvie slabogo dreifa nositelei, magnitnogo polya, vyazkosti i rasseyaniya na primesyakh. Na model'nykh primerakh budet vyyavlen kharakter vliyaniya etikh vozmushchenii na spektr i ustoichivost' plazmennykh voln v razlichnykh strukturakh. Tak, budut opredeleny: (1) obshchie svoistva tokovykh neustoichivostei v ogranichennykh DES, (2) vliyanie dreifa nositelei na spektr plazmonov v periodicheskikh DES (usilennyi effekt Doplera), i (3) mekhanizmy vozbuzhdeniya kraevykh mod postoyannym tokom.

Scientific Council of the Landau Institute, Friday, December 21, 2018, Landau ITP, 11:30 am
I.S. Burmistrov
Quantum corrections to conductivity of disordered electrons due to inelastic scattering off magnetic impurities

We study the quantum corrections to the conductivity of the two-dimensional disordered interacting electron system in the diffusive regime due to inelastic scattering off rare magnetic impurities. We focus on the case of very different g factors for electrons and magnetic impurities. Within the Born approximation for the inelastic scattering off magnetic impurities we find additional temperature-dependent corrections to the conductivity of the Altshuler-Aronov type.
The talk is based on I. S. Burmistrov and E. V. Repin, Phys. Rev. B 98, 045414 (2018)

Scientific Council of the Landau Institute, Friday, December 21, 2018, Landau ITP, 11:30 am
I.S. Burmistrov
Magnetism of Bi2Se3 thin films with Eu-rich flat inclusions

I report about theoretical support of experimental data on the measurement of the magnetic properties of thin films of bismuth selenide doped with europium atoms, which form flat inclusions. The magnitudes of the various mechanisms of magnetic ordering are theoretically estimated. The estimates obtained are in satisfactory agreement with the experimental data.
Report is based on the paper: L.N. Oveshnikov, Ya.I. Rodionov, K.I. Kugel, I.A. Karateev, A.L. Vasiliev, Yu.G. Selivanov, E.G. Chizhevskii, I.S. Burmistrov and B.A. Aronzon, "Magnetism of Bi2Se3 Thin Films with Eu-rich flat inclusions", J. Phys .: Condens. Matter 30, 445801 (2018)

Department of quantum mesoscopics: seminar, Friday, December 14, 2018, ITF, 3:00 pm
Dmitrii Smirnov (Fiziko-tekhnicheskii institut im. A.F. Ioffe)
Spektroskopiya spinovykh i zaryadovykh fluktuatsii v odinochnoi kvantovoi tochke

Razvita teoriya opticheskoi spektroskopii neravnovesnykh spinovykh i zaryadovykh fluktuatsii v kvantovoi tochke, pomeshchennoi v mikrorezonator. Pokazano, chto v rezhime sil'noi svyazi fotonnoi mody mikrorezonatora s trionnym rezonansom kvantovoi tochki fluktuatsii koeffitsienta prokhozhdeniya sveta cherez mikrorezonator soderzhat informatsiyu kak o spinovoi dinamike sistemy, tak i o statistike fotonov v mikrorezonatore. V rezhime slaboi svyazi vozmozhno formirovanie "odetykh fotonami" sostoyanii, chto pozvolyaet opredelyat' parametry spinovoi spinovoi dinamiki ne tol'ko dlya osnovnogo, no i dlya vozbuzhdennogo sostoyaniya kvantovoi tochki po fluktuatsiyam opticheskikh signalov Faradeya ili Kerra. Prilozhenie k sisteme prodol'nogo magnitnogo polya privodit k vozmozhnosti detektirovaniya zaryadovykh fluktuatsii v kvantovoi tochke.

Scientific Council of the Landau Institute, Friday, December 14, 2018, Landau ITP, 11:30 am
I.S. Burmistrov
Probing spin susceptibility of a correlated two-dimensional electron system by transport and magnetization measurements

I report theoretical support of the data on measuring the spin susceptibility at different temperatures and electron concentrations in a two-dimensional electron system based on a silicon field-effect transistor in the group of V.M. Pudalov (Lebedev Institute).
The short talk is based on the work of V. M. Pudalov, A. Yu. Kuntsevich, M.E. Gershenson, I.S. Burmistrov, and M. Reznikov, Phys. Rev. B 98, 155109 (2018).

Scientific Council of the Landau Institute, Friday, December 14, 2018, Landau ITP, 11:30 am
I.S. Burmistrov
A thermally driven spin-transfer-torque system far from equilibrium: enhancement of the thermoelectric current via pumping current

We consider a small itinerant ferromagnet exposed to an external magnetic field and strongly driven by a thermally induced spin current. For this model, we derive the quasi-classical equations of motion for the magnetization where the effects of a dynamical non-equilibrium distribution function are taken into account self-consistently. We obtain the Landau-Lifshitz-Gilbert equation supplemented by a spin-transfer torque term of Slonczewski form. We identify a regime of persistent precessions in which we find an enhancement of the thermoelectric current by the pumping current.
The talk is based on T. Ludwig, I.S. Burmistrov, Y. Gefen, A. Shnirman, "A thermally driven spin-transfer-torque system far from equilibrium: enhancement of the thermoelectric current via pumping current", arxiv:1808.01192

Shmidtovskii seminar po sverkhprovodimosti, Thursday, December 13, 2018, MISiS, auditoriya B-734, 3:00 pm
A. Buzdin (University Bordeaux, France and University of Cambridge, United Kingdom)
Electromagnetic Aharonov-Bohm like proximity effect in planar superconductor-ferromagnet structures

The spread of the Cooper pairs into the ferromagnet in proximity coupled superconductor – ferromagnet (SF) structures is shown to cause a strong inverse electrodynamic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, we found a very generic orbital mechanism of the magnetic moment transfer from a ferromagnet to a superconductor which is unavoidable in S/F hybrids. It is related with the fact that the common superconducting wave function in S and F (near the interface) does not permit to exclude the vector-potential of the magnetization by gauge transformation. From the experimental point of view, this phenomenon reminds the Aharonov-Bohm effect since the current inside the attached superconductor is induced by the ferromagnetic layer which cannot create the magnetic field in the outside in the absence of such superconducting environment. At the same time, the true physical key point is that the wave function penetrating the ferromagnet is responsible for this effect. Let us stress that the characteristic length of the above proposed inverse electrodynamic effect is of the order of the London penetration depth.

Department of quantum mesoscopics: seminar, Friday, December 7, 2018, ITF, 3:00 pm
W. Belzig (Univ. of Konstanz)
Topological effects in multi-terminal Josephson proximity systems

This will include e.g. works like
[1] R. Klees, G. Rastelli, J.C. Cuevas, and W. Belzig, "Quantum Geometry of Josephson Matter" (submitted to Science Advances, not on arxiv yet).
[2] Tomohiro Yokoyama, Johannes Reutlinger, Wolfgang Belzig, and Yuli V. Nazarov, "Order, disorder and tunable gaps in the spectrum of Andreev bound states in a multiterminal superconducting device", Phys. Rev. B 95, 045411 (2017) [arXiv:1609.05455].
[3] J. Reutlinger, L. Glazman, Yu. V. Nazarov, and W. Belzig, ""Smile" gap in the density of states of a cavity between superconductors", Phys. Rev. Lett. 112, 067001 (2014) [arXiv:1308.2529].

Presentations: 1.QuantumGeomJJ.pdf, 2.SmileTopo.pdf

Landau ITP colloquium, Friday, December 7, 2018, Landau ITP, 11:30 am
Wolfgang Belzig
Noisy quantum measurements: just a nuisance or fundamental physics?

Weak, almost non-invasive quantum measurements differ from the standard text book example of strongly invasive, projective measurements, since they leave the measured system basically unchanged. This opens the path to measure e.g. non-commuting observables and at the same time poses several open questions: Which order of operators is measured? Can quantum tests like Bell or Leggett-Garg be reformulated? What time scales are involved in the measurement process? We will address some basic properties of weak measurements leading to surprises like apparent spontaneous time-reversal symmetry breaking or the possibility of engineered detectors to tailor the measured quantum correlations.

Theory seminar, Thursday, December 6, 2018, Kapitza IPP, 11:30 am
Wolfgang Belzig (University of Konstanz, Germany), Akashdeep Kamra (NTNU Trondheim, Norway)
Unconventional magnons and their impact on spin pumping transport

Exotic quasiparticles have been observed in complex spin systems exhibiting spin ice rules, skyrmions and so on. Here, we discuss the emergence of novel quasiparticles, mediated by magnetic dipolar interactions that have been hiding in simpler spin systems with uniformly ordered ground states. Amongst other properties, these quasiparticles exhibit a spin ranging from zero to above 1ℏ [1]. Of particular interest is our finding that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin-1 magnons [2]. These unusual properties originate from a competition between quantum mechanical squeezing (increasing the spin) and hybridization (decreasing the spin).
We then present a theoretical study of spin transport across a ferrimagnet/non-magnetic conductor interface, when a magnetic eigenmode is driven into a coherent state. In the simple case of ferromagnets with non-integer “effective spin” above 1ℏ, we show that spin-current noise measurement can reveal this fundamental quantum phenomenon [1]. This is in full analogy to the effective charge known e.g. in the fractional quantum Hall regime, which has been experimentally determined via shot noise measurements.
Furthermore, we extend our model to continuously encompasses systems from ferromagnets to antiferromagnets [3] and include novel dissipation terms [4], thereby allowing analytical results for the full range of materials within a unified description. We also allow arbitrary interfaces (disordered and asymmetric). The obtained spin current expression includes intra- as well as cross-sublattice terms. We find that the cross-sublattice terms, disregarded in previous studies, play an important role and result in qualitative changes to our understanding of spin pumping in antiferromagnets. The dc current is found to be sensitive to the asymmetry in interfacial coupling between the two sublattice magnetizations and the mobile electrons, especially for antiferromagnets.
References:
[1] A. Kamra and W. Belzig, Super-Poissonian shot noise of squeezed-magnon mediated spin transport, Phys. Rev. Lett. 116, 146601 (2016).
[2] A. Kamra, U. Agrawal, and W. Belzig, Noninteger-spin magnonic excitations in untextured magnets, Phys. Rev. B 96, 020411(R) (2017).
[3] A. Kamra and W. Belzig, Spin pumping and shot noise in ferrimagnets: bridging ferro- and antiferromagnets, Phys. Rev. Lett. 119, 197201 (2017).
[4] A. Kamra, R. E. Troncoso, W. Belzig, and A. Brataas, Gilbert damping phenomenology for two-sublattice magnets, arXiv:1808.04385 (to appear in Phys. Rev. B).

Landau ITP colloquium, Friday, November 30, 2018, Landau ITP, 11:30 am
Andrei Turlapov, IPF RAN, g. Nizhnii Novgorod
Eksperimenty s fermi- i boze-gazami

Sposoby dostizheniya temperatur urovnya nanokel'vinov v atomnykh gazakh stanut pervoi temoi doklada. Dalee budut privedeny primery interesnykh eksperimentov s kvantovymi gazami. V to zhe vremya rech' poidet o slozhnostyakh, kotorye vstrechayutsya v eksperimentakh i o vozmozhnykh, eshche ne realizovannykh, putyakh ikh preodoleniya. V zaklyuchenii podrobno budet obsuzhdat'sya neopublikovannyi eksperiment po interferentsii tsepochki boze-kondensatov, v kotorom voznikaet prostranstvenno periodicheskaya interferentsionnaya kartina, nesmotrya na to, chto otnositel'nye fazy kondensatov sluchainy.
Video

Scientific Council of the Landau Institute, Friday, November 23, 2018, Landau ITP, 11:30 am
P. A. Ioselevich, P. M. Ostrovsky, Ya. V. Fominov
Mesoscopic supercurrent fluctuations in diffusive magnetic Josephson junctions

We study the supercurrent in quasi-one-dimensional Josephson junctions with a weak link involving magnetism, either via magnetic impurities or via ferromagnetism. In the case of weak links longer than the magnetic pair-breaking length, the Josephson effect is dominated by mesoscopic fluctuations. We establish the supercurrent-phase relation (CPR) along with statistics of its sample-dependent properties in junctions with transparent contacts between leads and link. High transparency gives rise to the inverse proximity effect, while the direct proximity effect is suppressed by magnetism in the link. We find that all harmonics are present in the CPR. Each harmonic has its own sample-dependent amplitude and phase shift with no correlation between different harmonics. Depending on the type of magnetic weak link, the system can realize a \varphi_0 or \varphi junction in the fluctuational regime. Full supercurrent statistics is obtained at arbitrary relation between temperature, superconducting gap, and the Thouless energy of the weak link.

Scientific Council of the Landau Institute, Friday, November 23, 2018, Landau ITP, 11:30 am
Konstantin Tikhonov
Statistics of eigenstates near the localization transition on random regular graphs

Dynamical and spatial correlations of eigenfunctions as well as energy level correlations in the Anderson model on random regular graphs (RRG) are studied. We consider the critical point of the Anderson transition and the delocalized phase. In the delocalized phase near the transition point, the observables show a broad critical regime for system sizes below the correlation volume and then cross over to the ergodic behavior. Eigenstate correlations allow us to visualize the correlation length that controls the finite-size scaling near the transition. The critical-to-ergodic crossover is very peculiar, since the critical point is similar to the localized phase, whereas the ergodic regime is characterized by very fast diffusion which is similar to the ballistic transport. In particular, the return probability crosses over from a logarithmically slow variation with time in the critical regime to an exponentially fast decay in the ergodic regime. We find a perfect agreement between results of exact diagonalization and those resulting from the solution of the self-consistency equation obtained within the saddle-point analysis of the effective supersymmetric action. We show that the RRG model can be viewed as an intricate limit of the Anderson model in spatial dimensions.

Theory seminar, Thursday, November 22, 2018, Kapitza IPP, 11:30 am
Vladimir Krasnov (Stockholm University, Sweden)
Josephson and non-Josephson emission from Bi2Sr2CaCu2O8+δ mesa structures

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. When voltage is applied to such mesas, they can generate various types of waves (photons, phonons, polaritons, magnons, plasmons) in a variety of different ways. In this talk I will overview both Josephson and non-Josephson mechanisms of emission, including the standard ac-Josephson effect for emission of electromagnetic waves [1], segnetoelectric effect for emission of phonons and polaritons [2] and non-equilibrium quasiparticle relaxation and recombination for emission of any type of bosons, having strong electron-boson interaction and participating in pairing [3]. Those effects are important both for applied research, e.g., creation of tunable, compact, continuous wave and monochromatic THz source with a frequency span in the whole THz gap region and beyond 0.1-15 THz [1,2], and for fundamental understanding of the mechanism of pairing in high-temperature superconductors [3].
References:
[1] E. A. Borodianskyi and V.M. Krasnov, Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures, Nature Commun. 8, 1742 (2017).
[2] S. O. Katterwe, H. Motzkau, A. Rydh, and V. M. Krasnov, Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions, Phys. Rev. B 83, 100510(R) (2011).
[3] V.M. Krasnov, S.O. Katterwe, & A. Rydh, Signatures of the electronic nature of pairing in high-Tc superconductors obtained by non-equilibrium boson spectroscopy, Nature Commun. 4, 2970 (2013).

Scientific Council of the Landau Institute, Friday, November 16, 2018, Landau ITP, 11:30 am
Ya.V. Fominov
Poverkhnostnyi impedans na diffuznoi granitse kiral'nogo p-volnovogo sverkhprovodnika

Vychislena lokal'naya kompleksnaya provodimost' i obuslovlennyi eyu poverkhnostnyi impedans na diffuznoi granitse kiral'nogo p-volnovogo sverkhprovodnika. Imenno kiral'noe p-volnovoe sostoyanie schitaetsya naibolee veroyatnym v sverkhprovodyashchem rutenate strontsiya Sr2RuO4. Ono anizotropnoe i pri etom polnost'yu shchelevoe (modul' parametra poryadka fiksirovan, a faza zavisit ot napravleniya). My rassmatrivaem otklik na vneshnee elektromagnitnoe pole kak na podshchelevykh, tak i na nadshchelevykh chastotakh. Izucheny anomal'nye osobennosti poverkhnostnogo impedansa, svyazannye s generatsiei vblizi granitsy nechetnykh po chastote sverkhprovodyashchikh korrelyatsii (sostoyanie tipa sverkhprovodimosti Berezinskogo). Teoreticheskie rezul'taty sopostavleny s izmereniyami poverkhnostnogo impedansa Sr2RuO4, provedennymi v IFTT. Nablyudaetsya kachestvennoe soglasie teorii i eksperimenta.
Doklad osnovan na rabote S. V. Bakurskiy, Ya. V. Fominov, A. F. Shevchun, Y. Asano, Y. Tanaka, M. Yu. Kupriyanov, A. A. Golubov, M. R. Trunin, H. Kashiwaya, S. Kashiwaya, and Y. Maeno, "Local impedance on a rough surface of a chiral p-wave superconductor", Phys. Rev. B 98, 134508 (2018); https://arxiv.org/pdf/1807.11735.pdf

Scientific Council of the Landau Institute, Friday, November 16, 2018, Landau ITP, 11:30 am
Ya.V. Fominov
Sverkhprovodyashchii spinovyi klapan v sistemakh so splavom Geislera

Provedeno teoreticheskoe soprovozhdenie eksperimentov po izmereniyu effekta sverkhprovodyashchego spinovogo klapana v sisteme tipa FFS (gde F — ferromagnetik, S — sverkhprovodnik). Effekt sostoit v tom, chto kriticheskaya temperatura Tc sistemy zavisit ot vzaimnoi orientatsii namagnichennostei dvukh ferromagnetikov. V eksperimente v kachestve srednego F sloya byl vzyat splav Geislera Co2Cr1-xFexAl, i eto pozvolilo usilit' effekt (raznost' Tc pri parallel'noi i antiparallel'noi orientatsii) po sravneniyu s ranee issledovannymi sistemami s zhelezom. Znak effekta zavisit ot tolshchiny sloya. Teoriya ob'yasnyaet poluchennye rezul'taty. Usilenie effekta okazyvaetsya svyazano s umen'shennoi velichinoi obmennogo polya v splave Geislera.
Doklad osnovan na sleduyushchikh rabotakh:
[1] A. Kamashev, A. Validov, N. Garif’yanov, Ya. Fominov, P. Leksin, J. Schumann, J. Thomas, V. Kataev, B. Büchner, I. Garifullin, "Isolation of proximity-induced triplet pairing channel in a superconductor/ferromagnet spin valve", EPJ Web of Conferences 185, 08001 (2018).
[2] A.A. Kamashev, A.A. Validov, J. Schumann, V. Kataev, B. Büchner, Ya.V. Fominov, I.A. Garifullin, "Increasing the performance of a superconducting spin valve using a Heusler alloy", Beilstein J. Nanotechnol. 9, 1764 (2018).

Shmidtovskii seminar po sverkhprovodimosti, Thursday, November 15, 2018, MISiS, auditoriya B-607, 5:00 pm
Tim Duty (University of New South Wales, Australia), Karin Cedergren, Sergey Kafanov, Roger Ackroyd and Jared H. Cole
Magnetic-field-induced parity effect in insulating Josephson junction chains

We report the experimental manifestation of even-odd parity effects in the transport characteristics of insulating Josephson junction chains, which occur as the superconducting gap is suppressed by applied magnetic fields at millikelvin temperatures. The primary signature is a non-monotonic dependence of the critical voltage, Vc, for the onset of charge transport through the chain, with the parity crossover indicated by a maximum of Vc at the parity field B*, We also observe a distinctive change in the transport characteristics across the parity transition, indicative of Cooper-pair dominated transport below B*, giving way to single-electron dominated transport above B*, For fields applied in the plane of the superconducting aluminum films, the parity effect is found to occur at the field, B*||, such that the superconducting gap equals the single-electron charging energy, Δ(B*||)=EC. On the contrary, the parity effect for perpendicularly applied fields can occur at relatively lower fields, B*⊥≃ 2Φ0/AI, depending only on island area, AI. In this case, the parity effect occurs in sync with formation of the single-vortex state of the islands in the chain. Our results suggest a novel explanation for the insulating peak observed in disordered superconducting films and one-dimensional strips patterned from such films, which occurs at a finite magnetic field.

Scientific Council of the Landau Institute, Friday, November 2, 2018, Landau ITP, 11:30 am
Victor Dotsenko
Two-temperature statistics of free energies in (1+1) directed polymers

The joint statistical properties of two free energies computed at two different temperatures in the same sample of (1+1) directed polymers is studied in terms of the replica technique. The scaling dependence of the free energies differenceon the two temperatures $T_{1}$ and $T_{2}$ is derived. In particular, it is shown that if the two temperatures $T_{1} < T_{2}$ are close to each other the typical value of the fluctuating part of the free energies difference is proportional to $(1 - T_{1}/T_{2})^{1/3}$. It is also shown that the left tail asymptotics of this free energy difference probability distribution function coincides with the corresponding tail of the Tracy-Widom distribution.
Europhysics Letters, 116, 40004 (2016); arXiv:1703.04305

Scientific Council of the Landau Institute, Friday, October 26, 2018, Landau ITP, 11:30 am
A. S. Ioselevich, N. S. Peshcherenko
Non-Born effects in scattering of electrons in clean quasi-one-dimensional conductors

Quasi-one-dimensional systems demonstrate Van Hove singularities in the density of states $\nu_F$ and the resistivity $\rho$, occurring when the Fermi level $E$ crosses a bottom $E_N$ of some subband of transverse quantization. We demonstrate that the character of smearing of the singularities crucially depends on the concentration of impurities. There is a crossover concentration $n_c\propto |\lambda|$, $\lambda\ll 1$ being the dimensionless amplitude of scattering. For $n\gg n_c$ the singularities are simply rounded at $\varepsilon\equiv E-E_N\sim \tau^{-1}$ – the Born scattering rate. For $n\ll n_c$ the non-Born effects in scattering become essential, despite $\lambda\ll 1$. The peak of the resistivity is split: for $\varepsilon>0$ there is a broad maximum at $\varepsilon\propto \lambda^2$. For $\varepsilon\lt 0$ there is a deep minimum at $|\varepsilon|\propto n^2\ll \lambda^2$. The behaviour of $\rho$ below the minimum depends on the sign of $\lambda$. In case of repulsion $\rho$ monotonically grows with $|\varepsilon|$ and saturates for $|\varepsilon| \gg \lambda^2$. In case of attraction $\rho$ has sharp maximum at $|\varepsilon| \propto \lambda^2$. The latter feature is due to resonant scattering by quasistationary bound states that inevitably arise just below the bottom of each subband for any attracting impurity.

Scientific Council of the Landau Institute, Friday, October 26, 2018, Landau ITP, 11:30 am
M. V. Feigel'man, V. E. Kravtsov
Electron-phonon cooling power in Anderson insulators

A theory for electron-phonon energy exchange in Anderson insulators with long localization length is developed. The major contribution to the cooling power as a function of electron temperature is shown to be directly related to the correlation function of the local density of electron states, which is enhanced near the localization transition by multi-fractality and by the presence of Mott's resonant pairs of states. The theory we develop explains huge enhancement of the cooling power observed in insulating Indium Oxide films as compared to predictions of standard theory for disordered metals

Theory seminar, Thursday, October 25, 2018, Kapitza IPP, 11:30 am
Viktor Dotsenko (Universite Jussieu, Paris)
Velocity distribution functions and intermittency in one-dimensional randomly forced Burgers turbulence

The problem of one-dimensional randomly forced Burgers turbulence is considered in terms of (1+1) directed polymers. In the limit of strong turbulence (which corresponds to the zero temperature limit for the directed polymer system) using the replica technique a general explicit expression for the joint distribution function of two velocities separated by a finite distance is derived. In particular, it is shown that at length scales much smaller than the injection length of the Burgers random force the moments of the velocity increment exhibit typical strong intermittency behavior. Literature: J.Stat.Mech., 083302 (2018); arXiv:1804.08294.

Landau ITP colloquium, Friday, October 19, 2018, Landau ITP, 11:30 am
Gregory Falkovich (Weizmann Institute of Science, Israel)
Wonders of viscous electronics

Quantum-critical strongly correlated systems feature universal collision-dominated collective transport. Viscous electronics is an emerging field dealing with systems in which strongly interacting electrons flow like a fluid. Such flows have some remarkable properties never seen before. I shall describe recent theoretical and experimental works devoted, in particular, to a striking macroscopic DC transport behavior: viscous friction can drive electric current against an applied field, resulting in a negative resistance, recently measured experimentally in graphene. I shall also describe conductance exceeding the fundamental quantum-ballistic limit, field-theoretical anomalies and other wonders of viscous electronics. Strongly interacting electron-hole plasma in high-mobility graphene affords a unique link between quantum-critical electron transport and the wealth of fluid mechanics phenomena.

Scientific Council of the Landau Institute, Friday, September 28, 2018, Landau ITP, 3:00 pm
Werner Krauth, Ecole normale supérieure, Paris (France)
Irreversible Markov chains: From the TASEP to all-atom Coulomb computations

The Markov chain Monte Carlo method traditionally consists in exploring large configuration spaces using a reversible random walk where moves are accepted or rejected based on an energy criterion. In this talk, I will present recent progress on irreversible Markov chains that challenge this picture. In one-dimensional particle systems, the new algorithms are related to the TASEP (totally asymmetric simple exclusion model). We can rigorously prove that they mix on much shorter time scales than the reversible Metropolis algorithms. I will then show how these algorithms sample the Boltzmann distribution (and thus explore configuration space) without computing the energy. In long-range interacting systems, where the computation of the energy is time-consuming, this provides a key advantage for the new method. For locally charge-neutral systems in three dimensions, we obtain a highly efficient algorithm, of N log N complexity in the number N of particles. I discuss the main paradox of this method: How is it possible to sample the Boltzmann distribution without computing the energy, and then review some recent successes as well as prospects and challenges for irreversible Markov chains in statistical physics.
References:
S. C. Kapfer, W. Krauth, Physical Review Letters 119, 240603 (2017)
Z. Lei, W. Krauth, arXiv:1806.06786 (2018)
M. F. Faulkner, L. Qin, A. C. Maggs, W. Krauth, arXiv:1804.05795 (2018)

Scientific Council of the Landau Institute, Friday, September 28, 2018, Landau ITP, 11:30 am
Victor Yakovenko (University of Maryland)
Superconductivity that breaks time-reversal symmetry and its experimental manifestations

Since 2006, it has been found experimentally that superconductivity spontaneously breaks time-reversal symmetry (TRS) in certain materials, such as Sr2RuO4, UPt3, URu2Si2, and Bi/Ni bilayers. In the latter case, we argue that the superconducting order parameter has the winding number of +-2 around the Fermi surface, thus making Bi/Ni bilayers a rare example of intrinsic 2D topological superconductivity [1]. The experimental evidence for TRS breaking comes from the polar Kerr effect, which is rotation of polarization of normally incident light upon reflection from the sample. Theoretical studies indicate that this effect is possible only if a superconductor has more than one band. To clarify these conditions, we study a model of chiral TRS-breaking superconductivity on the honeycomb lattice [2]. We consider superconducting pairing on the neighboring sites belonging to different sublattices. The matrix of this superconducting pairing is non-unitary and does not commute with the normal-state Hamiltonian. We find that the latter condition is necessary for experimental manifestations of the TRS breaking. We show that such superconducting pairing generates persistent loop currents around each lattice site and opens a topological mass gap at the Dirac points with the corresponding chiral edge states, as in Haldane's model of the quantum anomalous Hall effect. We calculate the intrinsic ac Hall conductivity in the absence of an external magnetic field, which determines the polar Kerr effect, and show that it is proportional to the loop-current order parameter.
[1] X. Gong, M. Kargarian, A. Stern, D. Yue, H. Zhou, X. Jin, V. M. Galitski, V. M. Yakovenko, and J. Xia, Science Advances 3, e1602579 (2017), arXiv:1609.08538
[2] P. M. R. Brydon, D. S. L. Abergel, D. F. Agterberg, and V. M. Yakovenko, arXiv:1802.02280

Landau ITP colloquium, Friday, September 28, 2018, Landau ITP, 11:30 am
Victor Yakovenko (University of Maryland)
Superconductivity that breaks time-reversal symmetry and its experimental manifestations

Since 2006, it has been found experimentally that superconductivity spontaneously breaks time-reversal symmetry (TRS) in certain materials, such as Sr2RuO4, UPt3, URu2Si2, and Bi/Ni bilayers. In the latter case, we argue that the superconducting order parameter has the winding number of +-2 around the Fermi surface, thus making Bi/Ni bilayers a rare example of intrinsic 2D topological superconductivity [1]. The experimental evidence for TRS breaking comes from the polar Kerr effect, which is rotation of polarization of normally incident light upon reflection from the sample. Theoretical studies indicate that this effect is possible only if a superconductor has more than one band. To clarify these conditions, we study a model of chiral TRS-breaking superconductivity on the honeycomb lattice [2]. We consider superconducting pairing on the neighboring sites belonging to different sublattices. The matrix of this superconducting pairing is non-unitary and does not commute with the normal-state Hamiltonian. We find that the latter condition is necessary for experimental manifestations of the TRS breaking. We show that such superconducting pairing generates persistent loop currents around each lattice site and opens a topological mass gap at the Dirac points with the corresponding chiral edge states, as in Haldane's model of the quantum anomalous Hall effect. We calculate the intrinsic ac Hall conductivity in the absence of an external magnetic field, which determines the polar Kerr effect, and show that it is proportional to the loop-current order parameter.
References:

  • [1] X. Gong, M. Kargarian, A. Stern, D. Yue, H. Zhou, X. Jin, V. M. Galitski, V. M. Yakovenko, and J. Xia, Science Advances 3, e1602579 (2017), arXiv:1609.08538
  • [2] P. M. R. Brydon, D. S. L. Abergel, D. F. Agterberg, and V. M. Yakovenko, arXiv:1802.02280

Theory seminar, Thursday, September 13, 2018, Kapitza IPP, 11:30 am
Alexey Kitaev (Caltech)
Statistical mechanics of a two-dimensional black hole

Rassmatrivaemaya zadacha voznikla pri izuchenii modeli SYK, kotoraya sostoit iz bol'shogo chisla vzaimodeistvuyushchikh fermionov (vzaimodeistvie «vse so vsemi», t.e. effektivnaya razmernost' sistemy — 0+1). Pri nizkoi temperature vazhnuyu rol' igraet myagkaya moda s deistviem, vyrazhayushchimsya cherez shvartsian. Eto deistvie ekvivalento dvumernoi gravitatsii s dilatonom v opredelennom predele. Pervaya chast' doklada posvyashchena obzoru upomyanutykh rezul'tatov. Zatem shvartsiannaya moda (ili dvumernaya gravitatsiya) budet rassmotrena kak otdel'naya kvantovaya sistema. Statsumma v etoi modeli imeet neobychnoe vyrazhenie: Z = tr(e-βHP), gde P — opredelennyi operator, kommutiruyushchii s gamil'tonianom.

Video

Theory seminar, Thursday, September 6, 2018, Kapitza IPP, 11:30 am
Denis Ullmo (LPTMS, CNRS-Universite Paris-Sud)
Post-Ehrenfest many-body quantum interferences in ultracold atoms far-out-of-equilibrium

Recent experimental progress with ultracold atomic gases has made it possible to investigate in exquisite detail the far out-of-equilibrium many-body quantum dynamics of isolated systems. This dynamics necessarily generates interferences beyond an Ehrenfest time scale, where quantum and classical expectation values diverge. Theoretically speaking, the heavily-relied-upon truncated Wigner approximation leaves out these interferences.

In this talk, I will present a semiclassical theory which bridges classical and quantum concepts in many-body bosonic systems and properly incorporates such missing quantum effects. For mesoscopically populated Bose-Hubbard systems, this theory captures post-Ehrenfest quantum interference phenomena very accurately.

Theory seminar, Thursday, July 5, 2018, Kapitza IPP, 5:10 pm
Yigal Meir (Department of Physics, Ben Gurion University of the Negev, Israel)
Spontaneous Time-Reversal Breaking and Edge Reconstruction in Quantum Hall and Topological Insulators

Topological states, such as the quantum Hall state or the quantum spin Hall state, are usually bulk insulators, with helical edge states that may carry current. The edge structure of such two dimensional systems is usually studied with sharp boundary conditions in spite of the fact that the confining potential in physical systems is expected to be smooth. It is shown that such a smooth confining potential may lead to edge reconstruction and formation of additional edge states. This is demonstrated explicitly for the case of integer and fractional quantum Hall systems, explaining several recent experimental puzzles. Moreover, the effect is also manifested in two-dimensional topological insulators (TIs), which are predicted to support helical edge modes that come in counter-propagating pairs, due to the time-reversal symmetry (TRS). The TRS protection of these edge states led to various suggested applications of TIs, ranging from spintronics to quantum computation. Here edge reconstruction leads to spontaneous TRS breaking, a finite Hall resistance at zero magnetic field and possible spin current. Such spontaneous TRS breaking may have important implications on transport properties and possible applications.

Theory seminar, Thursday, July 5, 2018, Kapitza IPP, 4:00 pm
Yuval Oreg (Weizmann Institute of Science, Israel)
Spin liquids from Majorana Zero Modes in a network of Cooper Boxes

Spin liquid phases are insulating states of matter with unique properties. In certain cases the phase hosts edge modes, end modes, and emergent non-abelian quasiparticles. The latter is a key element in several suggestions for topological quantum computation. In this talk, I`ll describe a proposal to construct a platform for creating effective spin models using semiconductor nanowires. The wires are tuned to the topological regime; with Majorana zero modes on each end. We group them into three-wires building blocks called hexons, each containing six Majorana zero modes. In the presence of a strong charging energy, the hexon becomes a Cooper box that is equivalent to two spin-1/2 degrees of freedom. This structure enables a flexible control (using local gates only) of the couplings between the Majorana zero modes. This tuning of the Hamiltonian governing the low energy effective spins, provides us with a path of simulating interacting spin-models in one- and two-dimensions. I will describe several examples including realizations of different phases of 1/2 Heisenberg spin chains, topological spin phases on a two dimensional Fisher lattice and their experimental signature.

Scientific Council of the Landau Institute, Friday, June 22, 2018, Landau ITP, 11:30 am
L. Glazman (Yale University)
Quantum Many-Body Physics of Qubits

The ongoing development of superconducting qubits has brought some basic questions of many-body physics to the research forefront, and in some cases helped solving them. I will address two effects in quantum condensed matter highlighted by the development of a fluxonium qubit. The first one is the so-called cosine-phi problem stemming from the seminal paper of Brian Josephson: the predicted there phase dependence of the dissipative current across a Josephson junction was observed in a fluxonium, after nearly 50 years of unsuccessful attempts by other techniques. The second one is the dynamics of a weakly-pinned charge density wave: we predict that the dynamics may be revealed in measurements of microwave reflection off a superinductor, which is a key element of the fluxonium.

Theory seminar, Thursday, June 7, 2018, Kapitza IPP, 11:30 am
A.Yu. Kuntsevich (FIAN)
Elektronnye svoistva dopirovannykh khal'kogenidov vismuta

Khal'kogenidy vismuta yavlyayutsya topologicheskimi izolyatorami s naibol'shimi shirinami zapreshchennoi zony. Pri dopirovanii Bi2Se3 med'yu, strontsiem ili niobiem Bi2Se3 stanovitsya sverkhprovodyashchim (predpolozhitel'no, nematicheskim topologicheskim sverkhprovodnikom).

V doklade budet dan obzor imeyushchikhsya na segodnyashnii den' dannykh o nematicheskoi sverkhprovodimosti dannogo materiala, v tom chisle i nashi rezul'taty o korrelyatsii nematichnosti so strukturnymi iskazheniyami [arXiv:1801.09287].

Takzhe ya rasskazhu o chasto obsuzhdaemom v literature izmerenii fazy Berri iz magnitoostsillyatsii. Nami privedeny prostye makroskopicheskie argumenty [Phys. Rev. B 97, 195431 (2018)], pochemu v real'nykh topologicheskikh izolyatorakh eta faza mozhet sil'no otlichat'sya ot teoreticheski ozhidaemogo znacheniya.

Theory seminar, Thursday, May 31, 2018, Kapitza IPP, 11:30 am
Baruch Meerson (Hebrew University of Jerusalem)
Large deviations of surface height in the Kardar-Parisi-Zhang equation

The Kardar-Parisi-Zhang (KPZ) equation describes an important universality class of nonequilibrium stochastic growth. There has been a surge of recent interest in the one-point probability distribution P(H,t) of height H of the evolving interface at time t in one dimension. I will show how one can use the optimal fluctuation method (OFM) to evaluate P(H,t) for different initial conditions and in different dimensions.

In one dimension the central part of the short-time height distribution is Gaussian, but the tails are non-Gaussian and strongly asymmetric. One interesting initial condition is an ensemble of Brownian interfaces, where we found a singularity of the large deviation function of the height at a critical value of |H|. This singularity results from a breakdown of mirror symmetry of the optimal path of the system, and it has the character of a second-order phase transition. At d>2 the OFM is valid, in the weak-coupling regime, at all times. Here the long-time height distribution P(H) is time-independent, and we use the OFM to determine the Gaussian body and strongly asymmetric non-Gaussian tails of P(H).

Theory seminar, Thursday, May 17, 2018, Kapitza IPP, 11:30 am
D.A. Bandurin (School of Physics & Astronomy, University of Manchester, UK)
Electron hydrodynamics in graphene: introduction and status

Transport in systems with many particles experiencing frequent mutual collisions (such as gases or liquids) has been studied for more than two centuries and is accurately described by the theory of hydrodynamics. It has been argued theoretically for a long time that the collective behavior of charge carriers in solids can also be treated by the hydrodynamic approach. However, despite many attempts, very little evidence of hydrodynamic electron transport has been found so far. Graphene encapsulated between hexagonal boron nitride (hBN) offers an ideal platform to study electron hydrodynamics as it hosts an ultra-clean electronic system with the electron-electron mean free path being the shortest lengths scale in the problem. In the first part of my talk we will discuss why electron hydrodynamics has not been observed before and how it manifests itself in electron transport. Furthermore, it will be shown that electrons in graphene can behave as a very viscous fluid (more viscous than honey) forming vortices of applied electron current [1]. In the second part, we will discuss the measurements of the viscosity of an electron fluid by its superballistic flow through graphene point contacts [2]. Then we will talk about the behavior of electron fluids in the presence of magnetic field where I will report the experimental measurements of the Hall viscosity in two dimensions [3]. This dissipationless transport coefficient has been widely discussed in theoretical literature on fluid mechanics, plasma physics and condensed matter physics, yet, until now, any experimental evidence has been lacking, making the phenomenon truly a unicorn. Last but not least, we will discuss how electron hydrodynamics can be used for the development of resonant terahertz photodetectors.

References: [1] D.A. Bandurin, A. Principi, G.H. Auton, E. Khestanova, K.S. Novoselov, I.V Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini, Science 351, 1055 (2016). [2] R.K. Kumar, D.A. Bandurin, F.M.D. Pellegrino, Y. Cao, A. Principi, H. Guo, G.H. Auton, M. Ben Shalom, L.A. Ponomarenko, G. Falkovich, I.V. Grigorieva, L.S. Levitov, M. Polini, and A.K. Geim, Nat. Phys. 13, 1182 (2017). [3] A.I. Berdyugin, S.G. Xu, F.M.D. Pellegrino, R. Krishna Kumar, A. Principi, I. Torre, M. Ben Shalom, T. Taniguchi, K. Watanabe, I.V. Grigorieva, M. Polini, A.K. Geim, and D.A. Bandurin, to appear on arxiv soon.

(seminar provoditsya sovmestno s laboratoriei FKS VShE)

Scientific Council of the Landau Institute, Friday, April 27, 2018, Landau ITP, 11:30 am
M.V. Feigel'man, D.A. Ivanov, E. Cuevas
Dielectric response of Anderson and pseudogapped insulators

Using a combination of analytic and numerical methods, we study the polarizability of a (non-interacting) Anderson insulator in one, two, and three dimensions and demonstrate that, in a wide range of parameters, it scales proportionally to the square of the localization length, contrary to earlier claims based on the effective-medium approximation. We further analyze the effect of electron-electron interactions on the dielectric constant in quasi-1D, quasi-2D and 3D materials with large localization length, including both Coulomb repulsion and phonon-mediated attraction. The phonon-mediated attraction (in the pseudogapped state on the insulating side of the Superconductor-Insulator Transition) produces a correction to the dielectric constant, which may be detected from a linear response of a dielectric constant to an external magnetic field.
M.V. Feigel’man, D.A. Ivanov, E. Cuevas, Dielectric response of Anderson and pseudogapped insulators, New J. Phys. 20, 053045 (2018); arXiv:1711.05972

Seminar na Fizfake MGU, Tuesday, April 17, 2018, Fizfak MGU, aud. 3-32b, 5:00 pm
Tuğrul Hakioğlu (Department of Physics and Energy Institute, İstanbul Technical University, Turkey)
Understanding the Skyrmion Anomalies in Strong Topological Insulators: An Interaction Approach

Interactions between the surface and the bulk in a topological insulator (TI) cause a finite quasiparticle lifetime of the topological surface states (TSS) as shown by the recent experiments for Bi2X3 (X=Se,Te). Previously, hexagonally warped topological band has been detected and other anomalies related to warping have been reported. The most remarkable among which is the 6-fold periodic spin canting anomaly (SCA) of the in-plane spin around the spin-1/2 vortex.

This talk is focused on understanding this SCA from an interaction point of view between the TSS and the bulk. We first device a general scheme based on interactions and the spin-off-diagonal component of the self energy. It is shown that, the spin-off diagonal interaction channel is topology preserving. Then the effective interaction strength, strong spin-orbit coupling and the Fermi surface warping is shown to cooperate in giving rise to the periodic spin canting anomaly. Our result points at the presence of the topology friendly interaction channels in strong TIs.

KAK POPAST' NA SEMINAR: Na fizfake MGU propusknoi rezhim, zhelatel'no imet' s soboi pasport. Organizatory vstretyat uchastnikov. Telefon organizatora: +7-911-703-2266 (Mikhail Evgen'evich Zhuravlev).

Scientific Council of the Landau Institute, Friday, April 13, 2018, Landau ITP, 11:30 am
Pavel Grigor'ev
Magnetic oscillations of in-plane conductivity in quasi-two-dimensional metals

We develop the theory of transverse magnetoresistance in layered quasi-two-dimensional metals. Using the Kubo formula and harmonic expansion we calculate intralayer conductivity in a magnetic field perpendicular to conducting layers. The analytical expressions for the amplitudes and phases of magnetic quantum oscillations (MQO) and of the so-called slow oscillations (SlO) are derived and applied to analyze their behavior as a function of several parameters: magnetic field strength, interlayer transfer integral and the Landau-level width. Both the MQO and SlO of intralayer and interlayer conductivity have approximately opposite phase in weak magnetic field and the same phase in strong field. The amplitude of SlO of intralayer conductivity changes sign at $\omega_c\tau\approx\sqrt{3}$. There are several other qualitative differences between magnetic oscillations of in-plane and out-of-plane conductivity. The results obtained are useful to analyze experimental data on magnetoresistance oscillations in various strongly anisotropic quasi-2D metals.

Scientific Council of the Landau Institute, Friday, March 30, 2018, Landau ITP, 11:30 am
Alex Kamenev (University of Minnesota)
Dynamic phase transition in rare events statistics of 1D KPZ problem

I will review the concept of non-equilibrium phase transitions in rare events statistics as well as a recent dramatic progress in studies of 1D KPZ. The focus of my talk is on the reflection symmetry breaking phase transition recently found stationary KPZ problem: https://arxiv.org/abs/1606.08738

Scientific Council of the Landau Institute, Friday, March 23, 2018, Landau ITP, 11:30 am
Markus Garst (TU Dresden)
Chiral magnetic crystals

The weak Dzyaloshinskii-Moriya interaction in chiral cubic magnets like MnSi, FeGe or Cu2OSeO3 twists the magnetization on long length scales resulting in spatially periodic magnetic textures — magnetic crystals. There exist especially magnetic crystals with a one- and two-dimensional periodicity corresponding to the magnetic helix and the topologically non-trivial skyrmion lattice, respectively. In this talk, we provide an overview of their properties. In particular, we discuss the crystallization process of these magnetic crystals that is characterized by strongly correlated chiral paramagnons that drive the transition first-order [1,2]. This fluctuation-induced first-order transition is well described by a theory put forward by Brazovskii. We will introduce the magnon band structure and their non-reciprocal properties in the presence of a magnetic field [3,4]. For the skyrmion lattice, this band structure is topological and characterized by finite Chern numbers that can be attributed to the formation of magnon Landau levels due to an emergent orbital magnetic field [5,6,7]. Finally, we will discuss domain walls of helimagnets that share similarities with grain boundaries consisting of disclination and dislocation defects of the helimagnetic order [8].

References: [1] M. Janoschek et al. Phys. Rev. B 87, 134407 (2013). [2] A. Bauer, M. Garst and C. Pfleiderer, Phys. Rev. Lett. 110, 177207 (2013). [3] M. Kugler et al. Phys. Rev. Lett. 115, 097203 (2015) [4] T. Weber et al. arXiv:1708.02098 [5] C. Schütte and M. Garst, Phys. Rev. B 90, 094423 (2014). [6] T. Schwarze, J. Waizner, M. Garst, A. Bauer, I. Stasinopoulos, H. Berger, C. Pfleiderer, and D. Grundler, Nat. Mater. 14, 478 (2015). [7] M. Garst J. Waizner, and D. Grundler, J. Phys. D: Appl. Phys. 50, 293002 (2017) [8] P. Schoenherr et al. Nat. Phys. in press, arXiv:1704.06288

Theory seminar, Thursday, March 22, 2018, Kapitza IPP, 11:00 am
V.E. Kravtsov (Abdus Salam ICTP, Trieste)
Duality in Power-Law Localization in Disordered One-Dimensional Systems

The transport of excitations between pinned particles in many physical systems may be mapped to single-particle models with power-law hopping, 1/rα. For randomly spaced particles, these models present an effective peculiar disorder that leads to surprising localization properties. We show that in one-dimensional systems almost all eigenstates (except for a few states close to the ground state) are power-law localized for any value of α > 0. Moreover, we show that our model is an example of a new universality class of models with power-law hopping, characterized by a duality between systems with long-range hops (α < 1) and short-range hops (α > 1), in which the wave function amplitude falls off algebraically with the same power γ from the localization center.

The paper has just been published: X. Deng, V. E. Kravtsov, G. V. Shlyapnikov, and L. Santos, Phys. Rev. Lett. 120, 110602 (2018).

Scientific Council of the Landau Institute, Friday, January 26, 2018, Landau ITP, 11:30 am
I.S. Burmistrov
Differential Poisson's ratio of a crystalline two-dimensional membrane

We compute analytically the differential Poisson's ratio of a suspended two-dimensional crystalline membrane embedded into a space of large dimensionality $d \gg 1$. We demonstrate that, in the regime of anomalous Hooke's law, the differential Poisson's ratio approaches a universal value determined solely by the spatial dimensionality $d_c$, with a power-law expansion $\nu = -1/3 + 0.016/d_c + O(1/d_c^2)$, where $d_c=d-2$. Thus, the value $-1/3$ predicted in previous literature holds only in the limit $d_c\to \infty$.

Theory seminar, Thursday, January 25, 2018, Kapitza IPP, 11:30 am
Vladimir Manucharyan (University of Maryland)
Quantum simulation of Luttinger liquid physics in Josephson transmission lines

Here we present the first quantum simulator for an impurity scattering in interacting 1D wires. The simulator consists of a transmission line made out of more than 30,000 Josephson junctions serving as a high-impedance media for microwave photons and a small phase slip Josephson junction playing the role of a back-scattering impurity. The system can be described by a boundary sine-Gordon model where the interaction strength is defined as g = Z/Rq with Z being the transmission line impedance and Rq = 6.5 kOhm the resistance quantum. By measuring scattering amplitudes and a spectrum of inelastically scattered microwave photons we can find the first and higher order correlation functions related to an AC conductance of the impurity. The controllability of the transmission line parameters and the finite size of the system allow us to fabricate lines with impedances exceeding Rq while keeping the phase slip rate of the line’s junctions very low. It gives us the unique opportunity to test Luttinger liquid physics at both sides of the critical point g = 1. A similar experimental setup can be used to simulate a Kondo impurity.

Scientific Council of the Landau Institute, Friday, January 19, 2018, Landau ITP, 11:30 am
I.S. Burmistrov
Mesoscopic Stoner instability: Suppression by tunneling to a reservoir

We derive the generalized Ambegaokar-Eckern-Schon action which governs the dynamics of the charge and spin degrees of freedom for the quantum dot described by the universal Hamiltonian and tunnel coupled to a reservoir. Contrary to previous works, we derive this dissipative action without the following assumptions (i) the absolute value of the spin is not a ected by the tunneling coupling to a reservoir and (ii) the spin rotates slowly such that the adiabatic approximation holds. We use the derived dissipative action for analysis of stability of the mesoscopic Stoner phenomenon with respect to the electron tunneling to a reservoir. We nd that at nite temperature the electron tunneling suppresses the mesoscopic Stoner instability at tunneling conductance which depends on temperature. At zero temperature we predict the existence of the quantum phase transition between the mesoscopic Stoner phase and the paramagnetic phase.

Scientific Council of the Landau Institute, Friday, December 15, 2017, Landau ITP, 11:30 am
A.M. Dyugaev
Poverkhnostnye mikrochastitsy v zhidkom gelii. Kvantovyi zakon Arkhimeda

Issledovany otkloneniya ot zakona Arkhimeda dlya sfericheskikh chastits radiusa R0, vypolnennykh iz molekulyarnogo vodoroda u poverkhnosti zhidkogo He4. Klassicheskii zakon Arkhimeda imeet mesto, esli R0 bol'she kapillyarnoi dliny geliya Lk ≅ 500 µm. Pri etom velichina vozvysheniya chastitsy nad zhidkost'yu h+ ~ R0. V oblasti 30 < R0 < 500 µm sila Arkhimeda podavlyaetsya siloi poverkhnostnogo natyazheniya i h+~ R03⁄Lk2. Pri R0 < 30 µm chastitsa nakhoditsya pod poverkhnost'yu zhidkosti. Zdes' sila Arkhimeda konkuriruet s siloi Kazimira, kotoraya ottalkivaet chastitsu ot poverkhnosti vglub' zhidkosti. Rasstoyanie chastitsy do poverkhnosti h- ~ Rs5⁄3⁄R02⁄3, esli R0 > Rs. Zdes' Rs – masshtab, nabrannyi, v osnovnom, iz mirovykh postoyannykh, Rs ≈ (ℏc/ϱg)1⁄5 ≈ 1µm. (ℏ - postoyannaya Planka, c - skorost' sveta, g – uskorenie svobodnogo padeniya, ϱHe – plotnost' geliya). Dlya ochen' malen'kikh chastits ( R0 < Rs) rasstoyanie do poverkhnosti zhidkosti h- ne zavisit ot ikh razmera h- = Rs.

Scientific Council of the Landau Institute, Friday, December 15, 2017, Landau ITP, 11:30 am
P.D. Grigor'ev
Medlennye ostsillyatsii magnitosoprotivleniya v vysokotemperaturnykh sverkhprovodnikakh YBCO: ikh chastota daet ploshchad' karmanov poverkhnosti Fermi ili velichinu mezhsloevogo pereskoka elektronov

Nablyudaemye magnitnykh kvantovye ostsillyatsii (MKO) v vysokotemperaturnykh sverkhprovodnikakh YBCO (YBa2Cu3O6 +x i YBa2Cu4O8) imeyut dovol'no neobychnyi sostav garmonik. Preobrazovanie Fur'e etikh MKO soderzhit tri blizkikh ravnoudalennykh pika pri dostatochno nizkikh chastotakh ~530T i 530+-90T, chto sootvetstvuet okolo 2% zony Brillyuena, prichem tsentral'nyi pik imeet amplitudu v ~2 raza vyshe, chem dva pika s kazhdoi iz storon. Obshcheprinyatoe ob'yasnenie takikh kvantovykh ostsillyatsii predpolagaet perestroiku iskhodnoi bol'shoi poverkhnosti Fermi iz-za fluktuiruyushchei volny zaryadovoi plotnosti (VZP). Odnako takoe ob'yasnenie imeet ryad sushchestvennykh nedostatkov. Naprimer, ono predpolagaet takzhe nabor drugikh blizkikh chastot MKO, kotorye ne nablyudayutsya eksperimental'no. Krome etogo, v etoi modeli nablyudaemye chastoty MKO dolzhny sil'no zaviset' ot urovnya dopirovaniya, chto takzhe ne nablyudaetsya v eksperimente. My predlagaem al'ternativnuyu interpretatsiyu nablyudaemykh magnitnykh ostsillyatsii v YBCO, kotoraya dovol'no estestvennym obrazom ob'yasnyaet nablyudaemuyu kartinu bez dopolnitel'nykh podgonochnykh parametrov. Eta predlagaemaya model' MKO uchityvaet dvukhsloinuyu kristallicheskuyu strukturu YBCO i vklyuchaet sootvetstvuyushchee rasshcheplenie elektronnogo spektra i dispersiyu elektronov perpendikulyarno provodyashchim sloyam. Pri etom osnovnaya nablyudaemaya chastota ~530T sootvetstvuet velichine bisloinogo rasshchepleniya, a ne malomu karmanu poverkhnosti Fermi (PF), vyzvannomu ee perestroikoi iz-za VZP. Takie "medlennye" magnitnye ostsillyatsii poyavlyayutsya dazhe bez perestroiki PF i pochti ne zavisyat ot urovnya dopirovaniya. Krome togo, v otlichie ot obychnykh MKO, eti "medlennye" ostsillyatsii ne podavlyayutsya dlinnovolnovoi prostranstvennoi neodnorodnost'yu obraztsa, tipichnoi dlya VTSP kristallov i privodyashchei k razmytiyu urovnya Fermi vdol' obraztsa. Poetomu oni legche nablyudayutsya v eksperimente. Predlozhennaya interpretatsiya pozvolyaet razreshit' «protivorechie» dannykh ARPES i MKO, a takzhe ob'yasnyaet nekotorye drugie osobennosti nablyudaemykh MKO v YBCO, kotorye dovol'no slozhno ob'yasnit' stsenariem perestroiki PF vyzvannoi VZP.

Scientific Council of the Landau Institute, Friday, November 17, 2017, Landau ITP, 11:30 am
P. D. Kurilovich, V. D. Kurilovich, I. S. Burmistrov, M. Goldstein
Helical edge transport in the presence of anisotropic magnetic impurity

We consider the effects of electron scattering on a quantum magnetic impurity on the current-voltage characteristics of the helical edge of a two-dimensional topological insulator. We compute the backscattering contribution to the current along the edge for a general form of the exchange interaction matrix and arbitrary value of the magnetic impurity spin. We find that the differential conductance might be a non-monotonous function of the voltage with several extrema. Effects of magnetic anisotropy for the impurity are considered.

Theory seminar, Thursday, November 16, 2017, Kapitza IPP, 11:00 am
M.M. Glazov (Fiziko-tekhnicheskii institut im. A.F. Ioffe RAN)
Spinovye fluktuatsii elektronov v poluprovodnikakh

V poslednie desyat' let voznik interes k izucheniyu fluktuatsii spinov elektronov i elektron-dyrochnykh kompleksov v poluprovodnikakh i poluprovodnikovykh nanosistemakh. V doklade predstavlena teoriya spinovykh fluktuatsii elektronov, lokalizovannykh v poluprovodnikovykh kvantovykh tochkakh, ili svyazannykh na donorakh v ob'emnom poluprovodnike, uchityvayushchaya sverkhtonkoe vzaimodeistvie spinov nositelei zaryada so spinami yader osnovnoi reshetki. Pokazano, chto spektr spinovykh shumov neset informatsiyu kak o funktsii raspredeleniya yadernykh polei, tak i o pryzhkakh elektronov mezhdu tsentrami lokalizatsii. Analiziruyutsya proyavleniya v spektre shumov obmennogo vzaimodeistviya mezhdu elektronami, lokalizovannymi na sosednikh tsentrakh. Rezul'taty teorii sopostavleny s dannymi eksperimentov.

Literatura: [1] M. M. Glazov and E. L. Ivchenko, Spin noise in quantum dot ensembles, Phys. Rev. B 86, 115308 (2012). [2] D.S. Smirnov, M.M. Glazov, E.L. Ivchenko, Vliyanie obmennogo vzaimodeistviya na spinovye fluktuatsii lokalizovannykh elektronov, FTT 54, 254 (2014). [3] M. M. Glazov, Spin noise of localized electrons: Interplay of hopping and hyperfine interaction, Phys. Rev. B 91, 195301 (2015). [4] A. V. Shumilin, E. Ya. Sherman, and M. M. Glazov, Spin dynamics of hopping electrons in quantum wires: Algebraic decay and noise, Phys. Rev. B 94, 125305 (2016). [5] Ph. Glasenapp, D. S. Smirnov, A. Greilich, J. Hackmann, M. M. Glazov, F. B. Anders, and M. Bayer, Spin noise of electrons and holes in (In,Ga)As quantum dots: Experiment and theory, Phys. Rev. B 93, 205429 (2016).

Scientific Council of the Landau Institute, Friday, October 27, 2017, Landau ITP, 11:30 am
P.L. Stroganov, Ya.V. Fominov
Cooper pair splitting in ballistic ferromagnetic SQUIDs

We consider ballistic SQUIDs with spin filtering inside half-metallic ferromagnetic arms. A singlet Cooper pair cannot pass through an arm in this case, so the Josephson current is entirely due to the Cooper pair splitting, with two electrons going to different interferometer arms. In order to elucidate the mechanisms of Josephson transport due to split Cooper pairs, we assume the arms to be single-channel wires in the short-junction limit. Different geometries of the system (determined by the length of the arms and the phases acquired by quasiparticles during splitting between the arms) lead to qualitatively different behavior of the SQUID characteristics (the Andreev levels, the current-phase relation, and the critical Josephson current) as a function of two control parameters, the external magnetic flux and misorientation of the two spin filters. The current-phase relation can change its amplitude and shape, in particular, turning to a pi-junction form or acquiring additional zero crossings. The critical current can become a nonmonotonic function of the misorientation of the spin filters and the magnetic flux (on half of period). Periodicity with respect to the magnetic flux is doubled, in comparison to conventional SQUIDs.

Scientific Council of the Landau Institute, Friday, October 20, 2017, Landau ITP, 11:30 am
L. Ioffe. M. Feigel'man
Mikrovolnovye svoistva sverkhprovodnikov vblizi perekhoda v izolyator

Predlozhena osnova teorii kollektivnykh mod v sil'no neuporyadochennykh sverkhprovodnikakh, nakhodyashchikhsya vblizi kvantovogo perekhoda v dielektricheskoe sostoyanie. Pokazano, chto s priblizheniem parametrov sverkhprovodnika k kvantovoi kriticheskoi tochke, v spektre vozbuzhdenii s neizbezhnost'yu voznikaet nenulevaya plotnost' nizkolezhashchikh (s energiyami sushchestvenno nizhe sverkhprovodyashchei shcheli) sostoyanii. Pri eshche bol'shem besporyadke eti podshchelevye sostoyaniya okazyvayutsya delokalizovannymi, chto dolzhno privodit' k zametnoi dissipatsii v mikrovolnovom otklike.

Scientific Council of the Landau Institute, Friday, October 6, 2017, Landau ITP, 11:30 am
Igor Poboiko and Mikhail Feigel'man
Paraconductivity of pseudogapped superconductors

We calculate Aslamazov-Larkin paraconductity for a model of strongly disordered superconductors (dimensions d=2,3) with a large pseudogap whose magnitude strongly exceeds transition temperature Tc. We show that, within Gaussian approximation over Cooper-pair fluctuations, paraconductivity is just twice larger that the classical AL result at the same ε = (T-Tc)/Tc. Upon decreasing ε, Gaussian approximation is violated due to local fluctuations of pairing fields that become relevant at ε < ε1 « 1. Characteristic scale ε1 is much larger than the width ε2 of the thermodynamical critical region, that is determined via the Ginzburg criterion, ε2 ≈ ε1d. We argue that in the intermediate region ε2 < ε < ε1 paraconductivity follows the same AL power law, albeit with another (yet unknown) numerical prefactor. At further decrease of the temperature, all kinds of fluctuational corrections become strong at ε < ε2; in particular, conductivity occurs to be strongly inhomogenuous in real space.

Scientific Council of the Landau Institute, Friday, September 22, 2017, Landau ITP, 11:30 am
A.V. Lunkin, K.S. Tikhonov, M.V. Feigel'man
Perturbed Kitaev model: excitation spectrum and long-ranged spin correlations

Dlya modeli Kitaeva na shestiugol'noi reshetki s razlichnymi tipami vozmushchenii byl razrabotan metod podscheta korrelyatsionnoi funktsii spin-spin v infrakrasnom predele. Bylo pokazano, chto zadacha o podschete dannoi funktsii mozhet byt' svedena k zadache o nakhozhdenii korrelyatsionnoi funktsii kvadratichnykh operatorov, sostoyashchikh iz svobodnykh Maioranovskikh fermionov. Pri etom vklad vysshikh poryadkov teorii vozmushchenii dlya iskhodnoi zadachi mozhet byt' uchten kak kvadratichnoe vozmushchenie dlya etikh fermionov. V kachestve prilozhenii rassmotreny sleduyushchie vozmushcheniya: magnitnoe pole, DM vzaimodeistvie cherez odnogo.

Theory seminar, Thursday, September 14, 2017, Kapitza IPP, 11:30 am
Sam Carr (University of Kent, England)
Interaction induced topological phases in one dimension

We consider a one-dimensional single channel quantum wire with a spin gap but gapless charge excitations. We show that the spin gap can be generated in two different ways, one of which has non-trivial topological properties. This topology manifests itself in two ways - firstly in the existence of gapless single-particle edge states, and secondly in an insensitivity of the wire to weak impurities reminiscent of the helical edge states of two dimensional topological insulators. We will demonstrate a number of ways such a phase of matter can be engineered, including spin-orbit quantum wires or two coupled helical edge states. If time permits, we will also touch on recent work concerning strong impurities in such systems.

References: [1] Kairanis and Carr, PRB 92, 035139 (2015). [2] Santos, Gutman and Carr, PRB 93, 235436 (2016). [3] Kainaris, Santos, Gutman and Carr, Forschr. Phys. 1600054 (2017). [4] Kainaris, Carr and Mirlin, coming soon.

Scientific Council of the Landau Institute, Friday, June 23, 2017, Landau ITP, 11:30 am
Ivan Protopopov
Transport in a disordered ν=2/3 fractional quantum Hall junction

Electric and thermal transport properties of a ν=2/3 fractional quantum Hall junction are analyzed. We investigate the evolution of the electric and thermal two-terminal conductances, G and G_Q, with system size L and temperature T. This is done both for the case of strong interaction between the 1 and 1/3 modes (when the low-temperature physics of the interacting segment of the device is controlled by the vicinity of the strong-disorder Kane-Fisher-Polchinski fixed point) and for relatively weak interaction, for which the disorder is irrelevant at T=0 in the renormalization-group sense. The transport properties in both cases are similar in several respects. In particular, G(L) is close to 4/3 (in units of e2/h) and G_Q to 2 (in units of πT/6ℏ) for small L, independently of the interaction strength. For large L the system is in an incoherent regime, with G given by 2/3 and GQ showing the Ohmic scaling, G_Q\sim 1/L, again for any interaction strength. The hallmark of the strong-disorder fixed point is the emergence of an intermediate range of L, in which the electric conductance shows strong mesoscopic fluctuations and the thermal conductance is G_Q=1. The analysis is extended also to a device with floating 1/3 mode, as studied in a recent experiment [A. Grivnin et al, Phys. Rev. Lett. 113, 266803 (2014)].

Referativnyi seminar, Friday, June 16, 2017, ITF, 3:00 pm
Leon Ogorodnikov
Coherent quantum phase slip

Referativnyi doklad po rabote http://www.nature.com/nature/journal/v484/n7394/full/nature10930.html

Department of quantum mesoscopics: seminar, Friday, June 9, 2017, ITF, 3:00 pm
V. V. Enaldiev (IRE RAS)
Resonance Absorption of Terahertz Radiation in Nanoperforated Graphene

Recent measurements of the conductivity of nanoperforated graphene are interpreted in terms of edges states existing near the edge of each nanohole. The perimetric quantization of edge states should result in the formation of a quasi-equidistant ladder of quasistationary energy levels. Dirac fermions filling this ladder rotate about each nanohole in the direction determined by the valley index. We show that the irradiation of this system by circularly polarized terahertz radiation leads to a resonance in absorption in one of the valleys. The magnitude of absorption at the resonance frequency can be controlled by means of gate voltage.

Referativnyi seminar, Friday, June 2, 2017, ITF, 3:00 pm
Daniil Antonenko
arXiv 1703.01304: Investigating anisotropic quantum Hall states with bi-metric geometry
Theory seminar, Thursday, May 25, 2017, Kapitza IPP, 11:30 am
K.E. Nagaev (IRE, teoriya), V.S. Khrapai (IFTT, eksperiment)
Vliyanie elektron-elektronnogo rasseyaniya na provodimost' mnogomodovykh ballisticheskikh kontaktov i kanalov

Privodyatsya teoreticheskie i eksperimental'nye rezul'taty po elektroprovodnosti shirokikh ballisticheskikh kontaktov v dvumernom elektronnom gaze pri nalichii elektron-elektronnogo rasseyaniya za predelami gidrodinamicheskogo priblizheniya. Obsuzhdayutsya lineinaya zavisimost' popravki k elektroprovodnosti ot temperatury i polozhitel'noe magnetosoprotivlenie v slabykh polyakh. Rassmatrivaetsya takzhe protivopolozhnyi sluchai dlinnogo kanala s gladkimi stenkami.

Seminar of Condensed-matter physics laboratory, HSE, Wednesday, May 24, 2017, MLFKS VShE, 3:00 pm
Ilya A. Gruzberg (Ohio State University)
Low-frequency conductivity of disordered wires: integrability and instantons

Generic states of non-interacting electrons in disordered wires are localized, and the DC conductivity of a wire vanishes at zero temperature. However, the AC conductivity is non-vanishing, and its general asymptotic form at low frequency was obtained by Mott who used intuitive qualitative arguments. Then this formula was rigorously derived by Berezinsky for a strictly one-dimensional (1D) disordered system. Later Hayn and John have re-derived the Mott-Berezinsky formula applying instanton techniques to localized states in the Lifshits tails (at large negative energies). We revisit the instanton approach and apply it to the statistics of wave functions and AC transport, calculating exactly the integral over gaussian fluctuations around the exact two-instanton solution. We demonstrate that quite generically the contribution of zero modes to the fluctuation determinant exactly cancels the Jacobian that appears when the collective variable are introduced. Thus, we derive the correlations between eigenfunctions at different energies beyond the leading order in small energy difference. This allows us to calculate the leading corrections to the Mott-Berezinsky law. We also extend this approach to quasi-1D wires.

Scientific Council of the Landau Institute, Friday, May 19, 2017, Landau ITP, 11:30 am
Igor Lukyanchuk
Domains, switching and negative capacitance in nano-scale ferroelectrics

Formation of unusual textures of polarization is imminent for nano-scale ferroelectric samples, films, rods, and granules, where the depolarization surface effects play the crucial role. We consider the unconventional topological polarization textures in several nano-scale systems, in which they were either already directly observed or can be yet discovered. Polarization domains that alternate the surface charge distribution, first proposed by Landau (1935) in contents of ferromagnetism can be formed in ferroelectric thin films as an effective mechanism to confine the depolarization field to the near-surface layer and diminish the depolarization energy. Very recently we have demonstrated that the few-nanometer thick ferroelectric/paraelectric superlattices with periodic domain structures exhibit the striking feature. The effective capacitance of ferroelectric layers is negative. This effect is explained by the opposite orientation of the depolarizing field with respect to the field-induced averaged polarization. Moreover, in sub-THz region the real part of the dielectric constant becomes positive, passing through zero at frequency ~0.3-3THz, inducing the resonance effect, suitable for the design of the ultra-small low-energy THz chips. Multi-vortex and skyrmion states are formed inside ferroelectric cylindrical nano-dots and nanorods to reduce the depolarization energy. We study the stability of such states and calculate the phase diagram of the system. We demonstrate that the topological class of the most stable topological excitations can be driven by the geometrical and electrical parameters of the system, external field, and temperature. We target the multi-domain and topological excitations in FE nanodots as a platform for multivalued logic units, for neuromorphic computing.

Seminar of Condensed-matter physics laboratory, HSE, Thursday, May 18, 2017, MLFKS VShE, 11:30 am
Z.D.Kvon (Institut fiziki poluprovodnikov im. A.V.Rzhanova SO RAN)
Topologicheskie izolyatory na osnove HgTe

V doklade dan obzor eksperimental'nykh issledovanii dvumernykh i trekhmernykh topologicheskikh izolyatorov (TI) na osnove HgTe kvantovykh yam i plenok. On sostoit iz dvukh chastei: pervaya posvyashchena dvumernomu TI na osnove HgTe yam s inversnym spektrom, a vtoraya trekhmernomu TI na osnove napryazhennoi plenki HgTe. V pervoi chasti osoboe vnimanie udeleno opisaniyu eksperimentov, imeyushchikh klyuchevoe znachenie dlya detektirovaniya kraevykh tokovykh sostoyanii. Podrobno opisyvaetsya polevaya tranzistornaya struktura, pozvolyayushchaya upravlyat' polozheniem urovnya Fermi kak dvumernogo, tak i trekhmernogo topologicheskogo izolyatora. Privodyatsya naibolee interesnye eksperimental'nye rezul'taty, poluchennye s ee pomoshch'yu. Esli govorit' o svoistvakh dvumernogo TI, to k nim mozhno otnesti nablyudenie nelokal'nogo ballisticheskogo i diffuzionnogo transporta v dvumernykh TI, magnitnyi proboi dvumernogo TI i anomal'nuyu temperaturnuyu zavisimost' soprotivleniya kraevykh kanalov. Esli zhe govorit' o trekhmernom TI, to eto dostizhenie rekordno vysokoi podvizhnosti poverkhnostnykh dvumernykh dirakovskikh fermionov i opredelenie blagodarya etomu vsekh ego osnovnykh parametrov (ob'emnoi shcheli, kontsentratsii dirakovskikh fermionov na obeikh ego poverkhnostyakh), a takzhe poluchenie informatsii o faze shubnikovskikh ostsillyatsii, svidetel'stvuyushchei o zhestkoi svyazi ikh spina i impul'sa. V zaklyuchenie obsuzhdayutsya perspektivy novykh eksperimentov.

Scientific Council of the Landau Institute, Friday, April 28, 2017, Landau ITP, 11:30 am
P.G. Grigor'ev
Anizotropiya elektronnykh transportnykh i sverkhprovodyashchikh svoistv v FeSe

Naideno i kolichestvenno opisano obshchee svoistvo: esli neodnorodnaya sverkhprovodimost' v anizotropnom provodnike poyavlyaetsya v vide izolirovannykh sverkhprovodyashchikh ostrovkov, ona umen'shaet elektricheskoe soprotivlenie anizotropno s maksimal'nym effektom vdol' osi s naimen'shei provodimost'yu. Eto svoistvo daet prostoi i ochen' obshchii instrument dlya izucheniya neodnorodnoi (ostrovkovoi) sverkhprovodimosti v razlichnykh anizotropnykh soedineniyakh i pozvolyaet otsenit' velichinu i temperaturnuyu zavisimost' ob'emnoi doli sverkhprovodyashchei fazy. Yavlenie nablyudaetsya v razlichnykh vysokotemperaturnykh sverkhprovodnikakh i v organicheskikh metallakh.

C pomoshch'yu opisannogo svoistva i izmerenii elektronnoi provodimosti i diamagnetizma vpervye pokazano poyavlenie neodnorodnoi sverkhprovodimosti v ob'emnom soedineniya FeSe pri atmosfernom davlenii i pri temperature v 5 raz vyshe, chem izvestnaya ranee temperatura sverkhprovodyashchego perekhoda Tc=8K, sootvetstvuyushchaya nulevomu soprotivleniyu. Eto otkrytie pomogaet ponyat' mnogochislennye neobychnye sverkhprovodyashchie svoistva FeSe, takie kak pyatikratnoe povyshenie Tc pri uvelichenii davleniya do neskol'kikh kilobar. Obsuzhdaetsya vozmozhnost' primeneniya etogo svoistva dlya obnaruzheniya prostranstvenno neodnorodnoi sverkhprovodimosti v drugikh anizotropnykh soedineniyakh, takikh kak kupraty i organicheskie metally.

Theory seminar, Thursday, April 27, 2017, Kapitza IPP, 11:30 am
L. Levitov (MIT)
Together we can: transport properties of electron viscous flows

Strongly interacting electrons can move in a neatly coordinated way, reminiscent of the movement of viscous fluids. In viscous electron flows interactions give rise to a collective behavior, facilitating transport and allowing conductance to exceed the fundamental Sharvin-Landauer ballistic limit Gball. This talk will describe a theory of the ballistic-to-viscous crossover in a constriction exhibiting the ballistic transport at T = 0 but governed by electron hydrodynamics at elevated temperatures. An approach based on quasi-hydrodynamic variables predicts an additive relation G = Gball + Gvis, where the viscous contribution Gvis dominates over Gball in the hydrodynamic limit. We will also discuss recent measurements of electron transport through graphene constrictions, finding that conductance below 150 K increases with increasing temperature. The measurements help to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, this work shows that viscous effects can facilitate high-mobility transport at elevated temperatures, a potentially useful behavior for designing graphene-based devices.

Seminar of Condensed-matter physics laboratory, HSE, Tuesday, April 4, 2017, MLFKS, 3:00 pm
Zheng Vitto Han (Shenyang National Laboratory for Materials Science, China)
Two-dimensional artificial nano-hybrids and their quantum properties

When different materials are interfaced/jointed with each other, the resulted hybrid systems often manifest fascinating physical properties that do not exist in nature. Among the many artificial hybrids (sometimes addressed as meta-materials or hetero-structures), nano-hybrid made with low-dimensional materials and other functional materials is attracting tremendous attentions in recent years. In this talk, we will mainly go through a couple of examples of our recent progresses on the graphene/superconducting-islands, and the 2D materials/h-Boron-Nitride nano-hybrid systems. In the former, the superconducting behavior is successfully coupled to the gate-tunable feature of graphene, leading to a metal-superconducting quantum phase transition at the ground state. While in the later, ultra-flat and ultra-clean interface allow ballistic electronic transport, opening new path to realize the paradigm of electron optics. Our recent experimental progresses on MoS2/h-BN hetero-structures will also be discussed. We expect that artificial nano-hybrids and their quantum properties can be expanded into many research areas that are important for both fundamental studies and future applications.

Seminar of Condensed-matter physics laboratory, HSE, Tuesday, March 28, 2017, CMP Lab, 3:00 pm
Oded Agam (Hebrew University of Jerusalem, Israel)
Instability of the Abrikosov Lattice due to Nonanalytic Core Reconstruction of Vortices in Bosonic Superfluids

The impact of the nonanalytic reconstruction of vortex cores on static vortex structures inweakly coupled superfluids is considered. It is shown that, in rotating two-dimensional systems, the Abrikosov vortex lattice is unstable to vortex core deformation: Each zero of the wave function becomes a cut of finite length. The directors characterizing the orientations of the cuts are themselves ordered in superstructures due either to surface effects or to interaction with shear deformations of the lattice (spiral structure). Similar instability may also be observable in clean superconducting films.

Seminar of Condensed-matter physics laboratory, HSE, Tuesday, March 21, 2017, MLFKS, 3:00 pm
D. Basko (CNRS, Grenoble)
Exciton-phonon scattering and radiative decay of excitons in two-dimensional semiconductors

We study exciton radiative decay in a two-dimensional material, taking into account large thermal population in the nonradiative states, from which excitons are scattered into the radiative states by acoustic phonons. We find an analytical solution of the kinetic equation for the nonequilibrium distribution function of excitons in the radiative states. Our estimates for bright excitons in transition-metal dichalcogenides indicate a strong depletion of radiative state population due to insufficient exciton-phonon scattering rate at low temperatures.

Scientific Council of the Landau Institute, Friday, February 10, 2017, Landau ITP, 11:30 am
I.S Burmistrov
Entanglement entropy and particle number cumulants of disordered fermions

We study the entanglement entropy and particle number cumulants for a system of disordered noninteracting fermions in d dimensions. We show, both analytically and numerically, that for a weak disorder the entanglement entropy and the second cumulant (particle number variance) are proportional to each other with a universal coefficient. The corresponding expressions are analogous to those in the clean case but with a logarithmic factor regularized by the mean free path rather than by the system size. We also determine the scaling of higher cumulants by analytical (weak disorder) and numerical means. Finally, we predict that the particle number variance and the entanglement entropy are nonanalytic functions of disorder at the Anderson transition.

Theory seminar, Thursday, January 12, 2017, Kapitza IPP, 11:30 am
Kirill Shulga (Russian Quantum Center and MISiS)
Indutsirovannaya magnitnym polem prozrachnost' kvantovogo metamateriala, sostoyashchego iz zerkal'nykh potokovykh kubitov

V svoem doklade ya rasskazhu ob obnaruzhennom nami effekte, voznikayushchem v kvantovom sverkhprovodyashchem metamateriale v predele sil'noi svyazi. Metamaterial sostoit iz tsepochki meta-atomov (kubitov), kazhdyi iz kotorykh soderzhit dva sverkhprovodyashchikh kol'tsa, soedinennykh tunnel'nym perekhodom. Elektromagnitnye svoistva takogo materiala mogut byt' opisany kvantovoi induktivnost'yu kazhdogo atoma, kotoroi mozhno upravlyat' v shirokom chastotnom diapazone pri pomoshchi izmeneniya vneshnego magnitnogo polya. Rezkoe podavlenie prokhozhdeniya mikrovolnovogo izlucheniya cherez takoi metamaterial chereduetsya s ego rezonansnym usileniem, voznikayushchim iz-za rasshchepleniya mezhdu vyrozhdennymi potokovymi sostoyaniyami. Osnovnoe sostoyanie sistemy mozhet byt' pereklyucheno v sostoyanie superpozitsii dvukh potokovykh sostoyanii, prilozheniem k obraztsu magnitnogo polya opredelennoi velichiny. Budet predstavleno neskol'ko kvantovykh modelei povedeniya takoi sistemy, kotorye mogut opisat' poluchennye eksperimental'nye dannye.

Theory seminar, Thursday, December 29, 2016, Kapitza IPP, 11:30 am
V.E. Kravtsov (Abdus Salam ICTP, Trieste)
Replica approach with one step replica symmetry breaking in the problem of Anderson localization on the Bethe lattice

We formulate general criteria for localized, extended ergodic and extended non-ergodic (multifractal) phases in disordered quantum systems and apply these criteria to the problem of Anderson localization on Bethe lattice, random regular graphs and generalized Rosenzweig-Porter random matrix ensemble. We focus on the replica symmetry breaking approach to the problem and show how this approach provides a natural classification of phases and phase transitions.

Scientific Council of the Landau Institute, Friday, December 23, 2016, Landau ITP, 11:30 am
N.M. Shchelkachev
Vzaimovliyanie kulonovskoi blokady, ferroelektrichestva i elektronnogo transporta v mezoskopicheskikh nanostrukturakh

We investigate the interplay of ferroelectricity and quantum electron transport at the nanoscale in the regime of Coulomb blockade. Ferroelectric polarization in this case is no longer the external parameter but should be self-consistently calculated along with electron hopping probabilities leading to new physical transport phenomena.

Theory seminar, Thursday, December 15, 2016, Kapitza IPP, 11:30 am
Pavel Aseev (University of Basel)
Finite-temperature conductance of strongly interacting quantum wire with a helical nuclear spin order

We study the temperature dependence of the electrical conductance of a clean strongly interacting quantum wire in the presence of a helical nuclear spin order. The nuclear spin helix opens a temperature-dependent partial gap in the electron spectrum. Using a bosonization framework we describe the gapped electron modes by sine-Gordon-like kinks. We predict an internal resistivity caused by an Ohmic-like friction these kinks experience via interacting with gapless excitations. As a result, the conductance rises from G=e^2/h at temperatures below the critical temperature when nuclear spins are fully polarized to G=2e^2/h at higher temperatures when the order is destroyed, featuring a relatively wide plateau in the intermediate regime. The theoretical results are compared with the experimental data for GaAs quantum wires obtained recently by Scheller et al. [Phys. Rev. Lett. 112, 066801 (2014)]. (arXiv:1611.10238)

Theory seminar, Thursday, December 8, 2016, Kapitza IPP, 11:30 am
Yan Fyodorov (King's College London)
O podschete chisla tochek ravnovesiya v neuporyadochennykh sistemakh

Ya planiruyu obsudit' kratko dve raboty, svyazannye obshchei ideologiei o podschete chisla tochek ravnovesiya v neuporyadochennykh sistemakh. V pervoi chasti, sleduya rabote ArXiv:1610.04831, ya opishu, kak takaya zadacha reshaetsya dlya nerelaksatsionnoi dinamiki v modeli tipa spinovogo stekla na sfere bol'shoi razmernosti, i obsuzhu fazovyi perekhod v rezhim eksponentsial'no bol'shogo chisla tochek ravnovesiya. Vo vtoroi chasti (osnovannoi na eshche ne zavershennoi rabote s P. Le Doussal i C. Texier) ya kratko obrisuyu podschet chisla tochek ravnovesiya dlya napravlennogo polimera v sluchainom potentsiale, gde zadacha svoditsya k izucheniyu fluktuatsii pokazatelya Lyapunova v odnomernoi lokalizatsii v rezhime lifshitsevskogo khvosta.

Theory seminar, Thursday, December 1, 2016, Kapitza IPP, 11:30 am
S.V. Zaitsev-Zotov (Kotel’nikov IRE RAS, Moscow)
Experimental search for one-dimensional edge states at surface steps of the topological insulator Bi2Se3

Results of detailed study of the topological insulator Bi2Se3 surface state energy structure in the vicinity of surface steps by scanning tunneling microscopy (STM) and spectroscopy (STS) methods are described. Increase of the chemical potential in the vicinity of the step edge is observed. The value of the increase is found to correlate with the step height and is caused by redistribution of electron wave functions between outer and inner edges of surface steps, as it is known for usual metals, as well as by presence of dangling bonds on the step. Smaller value of the shift and its larger characteristic length reflect specifics of the helical surface states. This increase is accompanied by enhancement of the relative value of the differential tunneling conductance, dI/dV, at the Dirac point and thereby produces an illusion of appearance of edge states. We show that the enhancement is reproduced in the framework of the tunneling model, which takes into account the dependence of the tunneling gap transparency on the voltage. References: [1] N.I. Fedotov, S.V. Zaitsev-Zotov, JETP Letters, 104, #11 (2016) (in press); arXiv:1609.08294. [2] N.I. Fedotov, S.V. Zaitsev-Zotov, arXiv:1609.08911.

Scientific Council of the Landau Institute, Friday, November 25, 2016, Landau ITP, 11:30 am
Oleg Tretiakov (Tohoku University, Japan)
Skyrmion Dynamics in Ferromagnets and Antiferromagnets

Manipulating small spin textures that can serve as bits of information by electric and spin currents is one of the main challenges in the field of spintronics. Ferromagnetic skyrmions recently attracted a lot of attention because they are small in size and are better than domain walls at avoiding pinning sites while moved by electric current. Nevertheless, ferromagnetic skyrmions also have certain disadvantages, such as the presence of stray fields and transverse dynamics, making them harder to employ in spintronic devices. To avoid these unwanted effects, we propose a novel topological object: the antiferromagnetic (AFM) skyrmion and explore its properties using analytical theory based on generalized Thiele equation and micromagnetic simulations. This topological texture has no stray fields and we show that its dynamics are faster compared to its ferromagnetic analogue. We obtain the range of stability and the dependence of AFM skyrmion radius on the strength of Dzyaloshinskii-Moriya interaction coming from relativistic spin-orbit effects. Moreover, we study the temperature effects on the stability and mobility of AFM skyrmions. We find that the thermal properties, e.g. such as the antiferromagnetic skyrmion radius and diffusion constant, are rather different from those for ferromagnetic skyrmions. More importantly, we show that due to unusual topology the AFM skyrmions do not have a velocity component transverse to the current (no topological Hall effect), and thus may be interesting candidates for spintronic memory and logic applications.

Scientific Council of the Landau Institute, Friday, October 14, 2016, Landau ITP, 11:30 am
Burmistrov I.S.
Two-instanton approximation to the Coulomb blockade problem

We develop the two-instanton approximation to the current-voltage characteristic of a single electron transistor within the Ambegaokar-Eckern-Sch\"on model. We determine the temperature and gate voltage dependence of the Coulomb blockade oscillations of the conductance and the effective charge. We find that a small (in comparison with the charging energy) bias voltage leads to significant suppression of the Coulomb blockade oscillations and to appearance of the bias-dependent phase shift.

Theory seminar, Thursday, October 13, 2016, Kapitza IPP, 11:30 am
S.A. Tarasenko (FTI im. A.F. Ioffe RAN, Sankt-Peterburg)
Drozhanie elektronov i spin-gal'vanicheskii effekt v poluprovodnikakh

Pokazano, chto elektrony provodimosti v poluprovodnikovykh strukturakh so spin-orbital'nym vzaimodeistviem vo vneshnem magnitnom pole ispytyvayut drozhanie. Drozhashchee dvizhenie (Zitterbewegung) voznikaet v otsutstvie peremennoi sily, ego priroda analogichna prirode Zitterbewegung svobodnykh dirakovskikh elektronov. V doklade obsuzhdayutsya rezul'taty pervogo eksperimental'nogo i teoreticheskogo issledovaniya kogerentnogo drozhaniya elektronov, indutsirovannogo opticheskoi orientatsiei ansamblya elektronov v odinakovoe spinovoe sostoyanie.

Scientific Council of the Landau Institute, Friday, September 30, 2016, Landau ITP, 11:30 am
Burmistrov I.S.
Mesoscopic fluctuations of the single-particle Green's function at Anderson transitions with Coulomb interaction

Using the two-loop analysis and the background field method we demonstrate that the local pure scaling operators without derivatives in the Finkel'stein nonlinear sigma model can be constructed by straightforward generalization of the corresponding operators for the noninteracting case. These pure scaling operators demonstrate multifractal behavior and describe mesoscopic fluctuations of the single-particle Green's function. We determine anomalous dimensions of all such pure scaling operators in the interacting theory within the two-loop approximation.

Scientific Council of the Landau Institute, Friday, September 23, 2016, Landau ITP, 11:30 am
Gordei Lesovik

V doklade budet rasskazano o rezul'tatakh, poluchennykh v nedavnikh rabotakh: 1) N-teorema v kvantovoi fizike [G.B. Lesovik, A.V. Lebedev, I.A. Sadovskyy, M.V. Suslov, V.M. Vinokur, H-theorem in quantum physics, Sci. Rep. (Nature Publishing Group) 6, 32815 (2016)]. 2)Trading coherence and entropy by a quantum Maxwell demon [A. V. Lebedev, D. Oehri, G. B. Lesovik, G. Blatter (PRA 2016)]. 3) Phase Estimation Algorithms for Quantum Metrology with a Single Qubit [S. Danilin, A. Vepsalainen, A. V. Lebedev, G. B. Lesovik, G. Blatter, G. S. Paraoanu, in preparation)].

Theory seminar, Thursday, September 22, 2016, Kapitza IPP, 11:30 am
Moshe Goldstein (Tel-Aviv University)
Dissipation-induced topological insulators: A recipe

It has recently been realized that driven-dissipative dynamics, which usually tends to destroy subtle quantum interference and correlation effects, could actually be used as a resource. By proper engineering of the reservoirs and their couplings, one may drive a system towards a desired quantum-correlated steady state, even in the absence of internal Hamiltonian dynamics. An intriguing class of quantum phases is characterized by topology, including the quantum Hall effect and topological insulators and superconductors. Which of these noninteracting topological states can be achieved as the result of purely dissipative Lindblad-type dynamics? Recent studies have only provided partial answers to this question. In this talk, I will present a general recipe for the creation, classification, and detection of states of the integer quantum Hall and 2D topological insulator type as the outcomes of coupling a system to reservoirs, and show how the recipe can be realized with ultracold atoms and other quantum simulators. The mixed states so created can be made arbitrarily close to pure states, and the construction may be generalized to other topological phases.

Scientific Council of the Landau Institute, Friday, September 16, 2016, Landau ITP, 11:30 am
Yuval Gefen (Weizmann Institute of Science)
Physics at the Edge: A New Paradigm for Shot Noise

Questions on the nature of edge reconstruction and ‘where does the current flow’ in the quantum Hall effect (QHE) have been debated for years. Moreover, the recent observation of proliferation of ‘upstream’ neutral modes in the fractional QHE raised doubts about the present models of edge channels. I will focus on the hole-conjugate state \nu=2/3 , and present a new picture of edge reconstruction. For example, while the hitherto accepted model for \nu=2/3 consists of a single downstream charge channel with conductance 2/3 and an upstream neutral mode, it has been recently predicted theoretically and found experimentally that the current is carried by two separate downstream edge channels, each with conductance 1/3 accompanied by upstream neutral mode(s). We find that if the two downstream channels are not equilibrated, inter-mode equilibration (via particle exchange) is accompanied by an excitation of upstream neutral modes. In turn, the counter-propagating neutral modes, moving in close proximity to the charge modes, fragment into propagating charges, inducing thus downstream current fluctuations with zero net current – a novel mechanism for non-equilibrium noise. The latter is shot noise with quantized Fano factors, which does not involve beam partitioning.

Theory seminar, Thursday, September 15, 2016, Kapitza IPP, 11:30 am
Anton Andreev (University of Washington, Seattle,USA)
Spiraling energy dispersion of arc states in Weyl semimetals

Weyl semimetals are recently discovered materials in which the valence and conduction bands touch at isolated points (Weyl nodes) in the Brillouin zone. This gives rise to unusual electronic properties of these materials. In particular, Weyl semimetals host peculiar surface electron states whose Fermi lines are shaped as open arcs. I will show that static electric fields that are necessarily present near the crystal surface result in a spiraling structure of Fermi arcs. The winding angle of the spiral is controlled by the chirality of the Weyl node and the magnitude of the surface potential. I will also discuss magnetoresistance of a pn-junction in a Weyl semimetal.

Scientific Council of the Landau Institute, Friday, September 9, 2016, Landau ITP, 11:30 am
Dimitri M. Gangardt (University of Birmingham)
Dynamic correlations in 1D superfluids: the particle-wave duality

While phonons are routinely used to describe low-energy properties of quantum superfluids, repulsive bosons in one dimension are special: their low energy excitations allow for an alternative decription in terms of fermionic quasiparticles. In my talk I will present the quasiclassical theory of these excitations which allows calculation of the dynamical structure factor in a generic 1D Bose liquid. In this theory the main role is played by singular kink-like configurations of the bosonic field. I will demonstrate that it is the kinetics of these quasiparticles rather than hydrodynamics which provides an effective description of dynamics of 1D interacting bosons.

Theory seminar, Thursday, June 9, 2016, Kapitza IPP, 11:30 am
Ya.M. Bel'tyukov (FTI im. A.F. Ioffe RAN, Sankt-Peterburg)
Random matrix approach to the jamming transition

Amorphous materials as diverse as foams, emulsions, colloidal suspensions and granular media can jam into a solid-like disordered state. In this state each particle has a certain equilibrium position and can vibrate around it.

In this talk, I will show that the dynamical matrix M describing harmonic oscillations in such media can be represented in the form M = AAT. The rows of the matrix A correspond to the degrees of freedom of individual granules and its columns correspond to elastic contacts between granules. This representation allows to apply the random matrix theory and estimate the vibrational density of states. The resulting vibrational density of states is approximately constant over a wide frequency range which is determined mostly by the ratio of the number of degrees of freedom and the total number of elastic contacts in the system. The results are in a good agreement with numerical experiments performed by various authors.

Department of quantum mesoscopics: seminar, Friday, June 3, 2016, ITF, 4:00 pm
A. P. Dmitriev (FTI im. A. F. Ioffe)
Elektron v pole izgibnykh kolebanii membrany: kvantovoe vremya, magneto-ostsillyatsii i razrushenie kogerentnosti

Izucheno dvizhenie elektrona v membrane pod vliyaniem ravnovesnykh izgibnykh kolebanii, korrelyator kotorykh ubyvaet s rasstoyaniem po zakonu r^{-2\eta}. Podrobno rassmotren sluchai \eta<1/2, kogda dazhe pri skol' ugodno slabom vzaimodeistvii teoriya vozmushchenii neprimenima. Pokazano, chto v etom sluchae obratnoe kvantovoe vremya proportsional'no g^{1/(1-\eta)} T^{(2-\eta)/(2-2\eta)}, gde g - konstanta elektron-fononnogo vzaimodeistviya, a T - temperatura. Razvityi metod prilozhen k vychisleniyu plotnosti sostoyanii elektrona v perpendikulyarnom k membrane magnitnom pole. Pokazano, v chastnosti, chto v rezhime slaboushirennykh besporyadkom urovnei Landau oni imeyut gaussovu formu s shirinoi, zavisyashchei ot magnitnogo polya po zakonu B^\eta. Krome togo, vychisleno vremya sboya fazy volnovoi funktsii elektrona za schet ego vzaimodeistviya s izgibnymi fononami v sluchae \eta<1/2. Pokazano, chto imeetsya neskol'ko temperaturnykh intervalov, v kotorykh vremya sboya fazy vyrazhaetsya razlichnymi stepennymi funktsiyami ot konstanty elektron-fononnogo vzaimodeistviya, temperatury i energii elektrona.

Department of quantum mesoscopics: seminar, Friday, June 3, 2016, ITF, 2:30 pm
A. V. Semenov (MGPU)
Coherent excited states in superconductors due to a microwave field

We describe theoretically the depairing effect of a microwave field on diffusive s-wave superconductors. The ground state of the superconductor is altered qualitatively in analogy to the depairing due to a dc current. In contrast to dc-depairing the density of states acquires, for microwaves with frequency \omega_0, steps at multiples of the photon energy \Delta \pm n\hbar\omega_0 and shows an exponential-like tail in the subgap regime. We show that this ac-depairing explains the measured frequency shift of a superconducting resonator at high microwave power and low temperatures.

Scientific Council of the Landau Institute, Friday, June 3, 2016, Landau ITP, 11:30 am
A. Shtyk, G. Goldstein, K. Shamon
Elektrony na obez'yan'em sedle: chetvernaya tochka Lifshitsa

My rassmatrivaem 2D vzaimodeistvuyushchie elektrony s dispersiei tipa "obez'yan'e sedlo" (OS), $\propto p_x^3-3p_xp_y^2$. Podobnaya dispersiya estestvenno voznikaet pri sliyanii trekh obychnykh sedel (osobennostei van Khova) v elementarnoi katastrofe tipa ellipticheskoi ombiliki. My pokazyvaem, chto takaya dispersiya voznikaet v dvukhsloinom grafene v poperechnom elektricheskom pole, prichem s tochki zreniya topologii poverkhnosti Fermi podobnaya singulyarnost' yavlyaetsya chetvernoi tochkoi Lifshitsa. Tochka Lifshitsa etogo tipa obladaet kharakternym povedeniem urovnei Landau $E_m\propto (Bm)^{3/2}$ i svyazannykh ostsillyatsii v termodinamicheskikh i transportnykh svoistvakh, takikh kak ostsillyatsii de-Gaaza-van-Al'fena i Shubnikova-de-Gaaza, chei period utraivaetsya pri peresechenii singulyarnosti. My demonstriruem, chto dlya izolirovannogo OS teoriya, opisyvayushchaya nevzaimodeistvuyushchie elektrony, neustoichiva po otnosheniyu k renormgruppovomu potoku, i v sisteme libo voznikaet sverkhprovodyashchaya neustoichivost', libo ne-Fermi-zhidkostnoe povedenie. V dvukhsloinom grafene, v kotorom imeetsya dva OS okolo $K$ i $K^\prime$ tochek, voznikaet konkurentsiya neustoichivostei, a imenno ferromagnetizma, voln spinovoi i zaryadovoi plotnostei, i nematichnosti.

Theory seminar, Thursday, June 2, 2016, Kapitza IPP, 11:30 am
A.V. Rozhkov (MFTI; ITPE RAN; VNIIA im. Dukhova)
Majorana fermion from Landau quantization in 2D topological superconductors

We study the generation of Majorana fermions in a two-dimensional topological superconductor placed in a transverse magnetic field B. A topological insulator/superconductor heterostructure and a two-dimensional p-wave superconductor are discussed. It is demonstrated that in these systems a single vortex creates two Majorana fermions, one hosted at the vortex core. The wave function of the second Majorana state is localized in the superconductor volume along a circle of radius rB−1 centered at the vortex core. In the case of many vortices, the sensitivity of r to the magnetic field B may be used to control the coupling between the Majorana fermions. The latter property could be an asset for quantum computations.

Theory seminar, Thursday, May 12, 2016, Kapitza IPP, 11:30 am
Pavel Frantsuzov (Novosibirsk)
Dolgovremennye fluktuatsii izlucheniya nanorazmernykh chastits

Odinochnye nanorazmernye chastitsy, takie kak kolloidnye poluprovodnikovye nanokristally, nanosterzhni (nanorods), nanoprovoloki (nanowires) demonstriruyut fluktuatsii intensivnosti fluorestsentsii (mertsanie) na shirochaishem spektre vremennykh masshtabov, ot dolei millisekund do chasov. V doklade budut obsuzhdat'sya sushchestvuyushchie teoreticheskie mekhanizmy i modeli mertsaniya nanochastits, vklyuchaya model' Efrosa/Rozena [Al.L. Efros & M. Rosen, Phys. Rev. Lett. 78, 1110 (1997)] i Multiple recombination center model [P.A. Frantsuzov, S. Volkan-Kacso & B. Janko, Phys. Rev. Lett. 103, 207402 (2009)].

Department of quantum mesoscopics: seminar, Friday, April 22, 2016, ITF, 3:00 pm
D. M. Basko (CNRS, Grenoble)
O roli effekta Landau-Zinera v rabote kvantovogo turniketa

Kvantovyi turniket predstavlyaet soboi nanoelektronnoe ustroistvo, kontroliruemym obrazom perenosyashchee elektrony po odnomu mezhdu dvumya sverkhprovodyashchimi elektrodami putem podachi peremennogo napryazheniya na metallicheskuyu nanochastitsu, raspolozhennuyu mezhdu dvumya elektrodami. Okazyvaetsya, chto kogerentnaya svyaz' mezhdu elektronnymi sostoyaniyami v nanochastitse i kvazichastichnymi sostoyaniyami v sverkhprovodnikakh nakladyvaet fundamental'nye ogranicheniya na rabotu ustroistva.

Scientific Council of the Landau Institute, Friday, April 22, 2016, Landau ITP, 11:30 am
Teun Klapwijk (TU Delft)
Josephson-effect in 3DTI and 2DTI based on HgCdTe heterostructures

I will report on experimental results obtained at Molenkamp’s group in Würzburg on ‘missing odd’ Shapiro steps and on emission of Josephson radiation. It is known that as a consequence of the Josephson equations, a strong relation exists between the voltage V measured across a Josephson junction and the characteristic frequency fJ =2eV/h of the current flowing in it. By applying microwave radiation steps occur at integer values of the voltage. Alternatively, ’listening’ to the rf signal radiated by the circulating supercurrent is a passive way of probing its properties. The data provide compelling evidence for the presence of a substantial 4\pi Josephson-current as would be expected from zero-enegry states. The results are available on the arxiv at: arXiv:1603.09611, arXiv:1601.08055, arXiv:1503.05591

Work done by: J.Wiedenmann, E.Bocquillon, R. Deacon, T.M.Klapwijk and various co-authors.
The research is supported by a 2014 Alexander von Humboldt prize.

Theory seminar, Thursday, April 21, 2016, Kapitza IPP, 11:30 am
D. Basko (LPMMC, CNRS, Grenoble, France)
Protsessy perevorota spina i radiatsionnoe zatukhanie temnykh eksitonov v dikhalkogenidakh perekhodnykh metallov

Dvumernye monosloi soedinenii MoS2, MoSe2, WS2, WSe2 yavlyayutsya pryamoshchelevymi poluprovodnikami s opticheskoi (eksitonnoi) shchel'yu v vidimom diapazone. Shchel' nakhoditsya v dvukh elektronnykh dolinakh, svyazannykh simmetriei obrashcheniya vremeni. Iz-za sil'nogo spin-orbital'nogo vzaimodeistviya zony rasshchepleny po spinu. Znaki rasshchepleniya v raznykh dolinakh protivopolozhny. Iz 16 vozmozhnykh eksitonnykh sostoyanii s raznymi kombinatsiyami spinovykh i dolinnykh indeksov elektrona i dyrki, tol'ko 4 mogut annigilirovat' bez perevorota spina i/ili doliny. Ostal'nye sostoyaniya nazyvayutsya temnymi. V dannoi rabote rassmatrivayutsya temnye eksitony, raspad kotorykh razreshen po doline, no zapreshchen po spinu.

Department of quantum mesoscopics: seminar, Friday, April 15, 2016, ITF, 3:00 pm
M. V. Feigel'man
Quantum tunneling vs. thermal effects in experiments on adiabatic quantum computing

Referativnyi doklad po rabote D. M. Silevitch, T. F. Rosenbaum and G. Aeppli, Eur. Phys. J. Special Topics 224, 25 (2015)

Traditional simulated annealing uses thermal fluctuations for convergence in optimization problems. Quantum tunneling provides a different mechanism for moving between states, with the potential for reduced time scales and different outcomes. Thermal and quan- tum annealing are compared in two concentration regimes of a model disordered magnet, where the effects of quantum mechanics can be tuned both by varying an applied magnetic field and by controlling the strength of thermal coupling between the magnet and an external heat bath. The results indicate that quantum annealing hastens convergence to the final state, and that the quantum character of the final state can be engineered thermodynamically.

Scientific Council of the Landau Institute, Friday, April 15, 2016, Landau ITP, 11:30 am
Suslov M.V.
Doklad po kandidatskoi dissertatsii (ruk. Lesovik G.B.)

Polnaya statistika perenosa kvantovykh chastits, kvantovaya metrologiya i sozdanie zaputannykh sostoyanii" V dissertatsii s pomoshch'yu formalizma pervichnogo kvantovaniya avtorom polucheny novye vyrazheniya dlya kharakteristicheskoi funktsii polnoi statistiki perenosa, uchityvayushchie energeticheskuyu zavisimost', zavisimost' ot vremeni, a takzhe obmennye effekty, obuslovlennye konechnym perekrytiem volnovykh paketov. V sluchae rasseyaniya dvukh fermionov poluchena subbinominal'naya statistika dlya nezaputannykh vkhodyashchikh sostoyanii, v to vremya kak zaputannye sostoyaniya mogut porozhdat' superbinominal'nyi shum. Opisan sluchai s postoyannym napryazheniem, gde uchityvaetsya zavisimost' rasseyaniya ot energii i konechnykh vremen izmereniya. Predlozhena skhema, soderzhashchaya uslovnye izmereniya, v kotoroi neskol'ko kubitov sluzhat detektorom v zadache o polnoi statistike perenosa. Sformulirovana i reshena zadacha scheta v terminakh problemy razlichimosti kvantovykh sostoyanii pri odnokratnom izmerenii, dayushchaya konstruktivnuyu skhemu pribora dlya kvantovogo algoritma scheta. Issledovany razlichnye vozmozhnosti pribornoi realizatsii etogo algoritma. Rassmotreny vozmozhnosti sozdaniya zaputannykh sostoyanii putem chastichnogo opredeleniya chisla proshedshikh chastits. Predlozhen novyi sposob izmereniya napryazheniya pri pomoshchi odnogo ili neskol'kikh zaryadovykh kubitov.

Scientific Council of the Landau Institute, Friday, April 15, 2016, Landau ITP, 11:30 am
Lesovik G.B.
Kommentarii o razvitii tematiki dissertatsii.
Department of quantum mesoscopics: seminar, Friday, April 8, 2016, ITF, 3:00 pm
K. S. Tikhonov
What is the computational value of finite range tunneling?

Referativnyi doklad po rabote V. S. Denchev et al (Google Inc.), arxiv.org/1512.02206

Quantum annealing (QA) has been proposed as a quantum enhanced optimization heuristic exploiting tunneling. Here, we demonstrate how finite range tunneling can provide considerable computational advantage. For a crafted problem designed to have tall and narrow energy barriers separating local minima, the D-Wave 2X quantum annealer achieves significant runtime advantages relative to Simulated Annealing (SA). For instances with 945 variables, this results in a time-to-99%-success-probability that is ∼108 times faster than SA running on a single processor core. We also compared physical QA with Quantum Monte Carlo (QMC), an algorithm that emulates quantum tunneling on classical processors. We observe a substantial constant overhead against physical QA: D-Wave 2X again runs up to ∼108 times faster than an optimized implementation of QMC on a single core. We note that there exist heuristic classical algorithms that can solve most instances of Chimera structured problems in a timescale comparable to the D-Wave 2X. However, we believe that such solvers will become ineffective for the next generation of annealers currently being designed. To investigate whether finite range tunneling will also confer an advantage for problems of practical interest, we conduct numerical studies on binary optimization problems that cannot yet be represented on quantum hardware. For random instances of the number partitioning problem, we find numerically that QMC, as well as other algorithms designed to simulate QA, scale better than SA. We discuss the implications of these findings for the design of next generation quantum annealers.

Scientific Council of the Landau Institute, Friday, April 8, 2016, Landau ITP, 11:30 am
Dmitry Bagrets (Institute for Theoretical Physics, University of Cologne, Germany)
Low-energy field theory of disordered Weyl metals.

In my talk I discuss the transport properties of disordered Weyl semimetals. In these systems, mechanisms of topological origin lead to the protection against Anderson localization, and at the same time to different types of transverse electromagnetic response -- the anomalous Hall effect (AQHE), and chiral magnetic effect (CME). I will demonstrate how an interplay of symmetry breaking and the chiral anomaly leads to the low-energy field theory containing two types of topological terms --- the Pruisken term describing AQHE and a variant of the non-Abelian Chern-Simons term responsible for CME. I will then discuss the signatures of CME in the magnetoconductance and shot noise assuming a quasi-one-dimensional geometry with a disordered Weyl semimetal nanowire being placed in the contact with two normal leads. An application of the magnetic field B along the nanowire creates the chiral 1d channels propagating parallel to B. They survive disorder averaging and the amount of them equals to the number of flux quanta piercing the nanowire. As the result, the magnetoconductance shows the crossover from quadratic to linear in B behavior while the Fano factor F is exponentially suppressed at high B as compared to F=1/3 at low B.
References: Dmitry Bagrets and Alexander Altland, PRL 114, 257201 (2015); PRB 93, 075113 (2016)

Shmidtovskii seminar po sverkhprovodimosti, Thursday, April 7, 2016, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Nikolaos Lazaridis (Crete University, Greece)
Quantum coherence in a qubit chain induced by self-induced transparent electromagnetic pulses

Superconducting quantum bits (qubits) are at the heart of quantum information processing schemes since they satisfy the requirements for being the building blocks of viable quantum computers. Here it is shown that quantum coherence, in the form of population inversion pulses, is induced by self-induced transparent pulses propagating in a quantum metamaterial comprising superconducting charge qubits. The experimental confirmation of that effect may open a new pathway to potentially powerful quantum computing.

Department of quantum mesoscopics: seminar, Friday, March 25, 2016, ITF, 3:00 pm
Lara Faoro (U Paris 6 & 7)
Out-of-time-order correlators in solid state physics

Out-of-time-order correlators introduced recently in the context of quantum gravity describe the delocalization of information in quantum systems. In classical chaotic systems, these correlators are known to grow exponentially with the Lyupanov exponent. I discuss the behavior of these correlators in canonical examples of solid state quantum chaotic systems: disordered electrons with and without phonon interaction.

Department of quantum mesoscopics: seminar, Friday, March 11, 2016, ITF, 3:00 pm
D. Shapiro, A. Shnirman and A. Mirlin
Current-phase relation and h/e-periodic critical current of a chiral Josephson contact between 1D Majorana modes

We explore a long Josephson contact transporting Cooper pairs between 1D charge-neutral chiral Majorana modes in the leads via charged Dirac chiral modes in the normal region. We investigate the regimes of transparent and tunnel junctions implemented in 3D topological insulator/superconductor/magnet hybrid structures. The setup acts as a SQUID controlled by the magnetic flux enclosed by the chiral loop of the normal region. This chirality leads to the fractional h/e-periodic pattern of critical current. The current-phase relation can have sawtooth-like shape with spikes at unusual even phases of 2\pi n.

Scientific Council of the Landau Institute, Friday, March 4, 2016, Landau ITP, 3:00 pm
Alessandro Silva (SISSA, Trieste)
Dynamical phase transitions and statistics of excitations

Dynamical phase transitions can occur in isolated quantum systems that are brought out of equilibrium by either a sudden or a gradual change of their parameters. Theoretical examples rage from the behaviour of the O(N) model in the large N limit as well as spin-model with long range interactions, both showing dynamical criticality in their prethermal steady-states. In this talk I will start by discussing the characterization of such dynamical phase transitions based on the statistics of produced excitations. I will focus both on the role of fluctuations as well as on the difference between sudden and gradual changes of the parameters. Finally, I will discuss a second type type of dynamical criticality discussed in the literature, related to the emergence of zeroes in the Loschmidt amplitude, and show that this phenomenon is much less generic and robust than standard dynamical criticality.

Scientific Council of the Landau Institute, Friday, March 4, 2016, Landau ITP, 11:30 am
V.Yu. Kachorovskii (FTI im. A.F. Ioffe RAN), I.V. Gornyi, A.D. Mirlin
Anomal'nye uprugie svoistva dvumernykh neuporyadochennykh kristallov: primenenie k grafenu

Grafen predstavlyaet soboi znamenityi primer dvumernoi kristallicheskoi membrany. Teplovye fluktuatsii membrany stremyatsya razrushit' dal'nii poryadok v sisteme. Konkuriruyushchim effektom, blagodarya kotoromu membrana ostaetsya termodinamicheski stabil'noi, yavlyaetsya sil'noe angarmonicheskoe vzaimodeistvie mezhdu izgibnymi i prodol'nymi kolebatel'nymi modami. Angarmonicheskie effekty demonstriruyut kriticheskoe povedenie na masshtabakh bol'shikh, chem dlina Ginzburga. V chastnosti, izgibnaya zhestkost' chistoi izolirovannoi membrany vozrastaet stepennym obrazom po mere uvelicheniya razmera sistemy: $\varkappa \propto L^{\eta}.$ V rabote izuchaetsya vliyanie besporyadka na kriticheskoe povedenie membrany. Pokazano, chto sluchainye fluktuatsii krivizny membrany, vyzvannye staticheskim besporyadkom, radikal'no menyayut kharakter skeilinga izgibnoi zhestkosti. Vyvedeny uravneniya renormgruppy (RG), opisyvayushchie izmenenie $\varkappa$ i effektivnoi sily besporyadka $b$ po mere uvelicheniya razmera sistemy. Pokazano, chto obe velichiny nemonotonno zavisyat ot $L.$ Analiz uravnenii RG pozvolyaet postroit' kriticheskuyu krivuyu $b(\varkappa),$ razdelyayushchuyu ploskuyu i «skomkannuyu» (crumpled) fazy membrany. Geometriya membrany v okrestnosti kriticheskoi krivoi kharakterizuetsya (v opredelennom intervale masshtabov) fraktal'noi razmernost'yu. Takzhe podrobno izucheno povedenie $\varkappa$ i $b$ v ploskoi faze. Pokazano, chto gluboko v ploskoi faze besporyadok ubyvaet stepennym obrazom na masshtabakh bol'shikh, chem dlina Ginzburga, kotoraya, takim obrazom, zadaet kharakternyi masshtab ripplov (ripples) – staticheskikh izgibnykh deformatsii, eksperimental'no nablyudaemykh v izolirovannom grafene. V predele $L \to \infty $ ripply kharakterizuyutsya anomal'noi eksponentoi $2\eta$ po kontrastu s dinamicheskimi fluktuatsiyami, kotorye spadayut stepennym obrazom s pokazatelem $\eta$. Esli nachal'nyi besporyadok dostatochno velik, to sushchestvuet promezhutochnyi interval masshtabov, gde ripply ubyvayut sushchestvenno medlennee, s pokazatelem $\eta/4.$. Rezul'taty raschetov sravnivayutsya s imeyushchimisya eksperimental'nymi izmereniya parametrov ripplov v grafene. V rabote takzhe izuchaetsya povedenie membrany pod deistviem vneshnego natyazheniya. Pokazano, chto lineinyi zakon Guka narushaetsya, a imenno rastyazhenie lista grafena okazyvaetsya nelineinoi funktsiei prilozhennogo malogo natyazheniya: $\Delta L \propto \sigma^{\eta/(2-\eta)}$ i $\Delta L \propto \sigma^{\eta/(8-\eta)},$ sootvetstvenno, dlya chistoi i razuporyadochennoi membrany.

Scientific Council of the Landau Institute, Friday, February 26, 2016, Landau ITP, 11:30 am
Ya.V. Fominov, M.A. Skvortsov
Podshchelevye sostoyaniya v neuporyadochennykh sverkhprovodnikakh s sil'nymi magnitnymi primesyami

Issledovana plotnost' sostoyanii v diffuznykh sverkhprovodnikakh s magnitnymi primesyami, podchinyayushchimisya statistike Puassona. Takim obrazom mogut byt' opisany primesi proizvol'noi «sily», a izvestnyi sluchai bornovskikh magnitnykh primesei s gaussovoi statistikoi yavlyaetsya predel'nym sluchaem nashego rassmotreniya. Teoriya srednego polya predskazyvaet netrivial'nuyu strukturu plotnosti sostoyanii, soderzhashchuyu kontinuum kvazichastichnykh sostoyanii, a takzhe (v nekotorom diapazone parametrov) primesnuyu zonu. V priblizhenii srednego polya vse kraya spektra yavlyayutsya zhestkimi i oboznachayut chetkie granitsy mezhdu oblastyami konechnoi i nulevoi plotnosti sostoyanii. Rassmatrivaya instantony v tekhnike replichnoi sigma-modeli, my vychislyaem srednyuyu plotnost' sostoyanii za ramkami srednepolevogo priblizheniya. Takim obrazom, my nakhodim podshchelevye sostoyaniya («khvosty»), kotorye razmyvayut kraya spektra za schet fluktuatsii potentsial'nogo (obychnye primesi) i nepotentsial'nogo (magnitnye primesi) besporyadka. Neodnorodnost' kontsentratsii magnitnykh primesei privodit k poyavleniyu podshchelevykh sostoyanii za schet dvukh mekhanizmov: fluktuatsii para-razrushayushchego parametra (magnitnyi analog mezoskopicheskikh fluktuatsii) i fluktuatsii parametra poryadka (opredelyaemogo samosoglasovanno). V predel'nykh sluchayakh vosproizvodyatsya ranee poluchennye rezul'taty dlya podshchelevoi plotnosti sostoyanii v gryaznykh sverkhprovodnikakh s magnitnymi primesyami.

Theory seminar, Thursday, February 4, 2016, Kapitza IPP, 11:30 am
Serguei Brazovskii
Electronic phase transitions induced by electric or optical impacts

Controlled transformations of electronic states or even of whole phases are achievable today by impacts of very strong electric fields and/or the ultrafast optical pumping. The experimental success is coming from ferroelectrically and ionically enhanced field effect in high-Tc superconductors, induced metallization in oxides of transition metals and in organic materials, field-effect superconductivity in MBE monolayers. The techniques of the femto-second optical pumping span from the purely optical setups to the newer time-resolved photoemission spectroscopy and to the latest time-sliced diffraction. The tested electronic phases include: superconductivity, charge density waves, charge ordering, ferroelectricity, magnetic phases, Peierls and Mott insulators. A super goal is to attend “hidden” states which are inaccessible and even unknown under equilibrium conditions. Such a bistable switching has been achieved in a “polaronic Wigner-crystalline Mott insulator” 1T-TaS2. After the experimental review, some time will be devoted to a phenomenological theory (collaboration with N. Kirova). Thus, the modeling of the quasi-condensate of excitons interacting with the order parameter recovers the dynamical realization of the “excitonic insulator” state and spacio-temporal patterns with self-focusing, domains segregation, and local dynamical phase transitions.

Scientific Council of the Landau Institute, Friday, January 29, 2016, Landau ITP, 11:30 am
Serguei Brazovskii (LPTMS, CNRS & University Paris-Sud & NUST MISiS)
Collective conductivity, criticality and reconstruction in systems with an infinite permittivity

This talk will be devoted to conducting systems with unhindered polar deformations: ferroelectric semiconductors near the critical temperature and charge density waves (CDW) subjected to pinning and junctions constraint. For ferroelectrics, epitomized by Mott insulators in organic stack materials, we shall find access to the critical dynamics, to the creep of domain walls, and to the polarization dumping by solitons. For CDWs, we shall model the reconstruction of their current carrying states in junctions channels via dynamical and static vortex structures. For completeness, we shall recall also old stories of frequency and temperature dependence of the permittivity of CDWs under pinning and Coulomb interactions.
The presented results are based on collaborations with P. Monceau and F. Nad in experiment and with N. Kirova and A. Larkin in theory.

Scientific Council of the Landau Institute, Friday, January 22, 2016, Landau ITP, 11:30 am
Yu.N. Ovchinnikov, I.M. Sigal
Instability of threeangular Abrikosov lattice at the Ginzburg-Landau parameter value κ, close to one

V rabote issleduetsya myagkaya poperechnaya moda besshchelevykh vozbuzhdenii, svyazannaya s deformatsiei treugol'noi reshetki Abrikosova s odnim kvantom potoka v elementarnoi yacheike pri proizvol'nom znachenii parametra Ginzburga-Landau kappa. Pokazano. chto v uzkoi oblasti znachenii kappa, blizkikh k edinitse, reshetka Abrikosova s uglom F=\pi/3 mezhdu vektorami elementarnoi yacheiki neustoichiva. Spektr vozbuzhdenii rassmatrivaemoi mody pri malykh znacheniyakh impul'sa k (v k^2 priblizhenii) izotropen pri k, lezhashchem v ploskosti, perpendikulyarnoi magnitnomu polyu. Naiden interval neustoichivosti po velichine kappa.

Scientific Council of the Landau Institute, Friday, December 25, 2015, Landau ITP, 1:00 pm
Pavel Ioselevich
Rasshcheplenie kuperovskikh par v ferromagnitnykh skvidakh

We study Josephson junctions between superconductors connected by two parallel ferromagnetic arms. For fully polarised ferromagnets, supercurrent through such a SQUID only flows via Cooper pair splitting between the differently polarised arms. The average current is suppressed, but mesoscopic fluctuations lead to a significant typical current. We calculate this current for the SFS SQUID, as well as for a SQUID with metallic arms with magnetic impurities. The latter shows an h/e periodicity in flux. The current in both systems is of fluctuational origin and is stronger for materials with low conductivity and a low superconducting gap.

Scientific Council of the Landau Institute, Friday, December 18, 2015, Landau ITP, 11:30 am
P. M. Ostrovsky
Anomalous Hall effect in weakly disordered ferromagnets

Anomalous Hall effect arises in systems with both spin-orbit coupling and magnetization. Generally, there are three mechanisms contributing to anomalous Hall conductivity: intrinsic, side jump, and skew scattering. The standard diagrammatic approach to the anomalous Hall effect is limited to computation of ladder diagrams. We demonstrate that this approach is insufficient. An important additional contribution comes from diagrams with a single pair of intersecting disorder lines. This contribution constitutes an inherent part of skew scattering on pairs of closely located defects and essentially modifies previously obtained results for anomalous Hall conductivity. We argue that this statement is general and applies to all models of anomalous Hall effect. We illustrate it by an explicit calculation for two-dimensional massive Dirac fermions with weak disorder. In this case, inclusion of the diagrams with crossed impurity lines reverses the sign of the skew scattering term and strongly suppresses the total Hall conductivity at high electron concentrations. The same mechanism for ordiary electrons with spin-orbit coupling (Bychkov-Rashba model) produces an opposite effect increasing the Hall conductivity. In the conduction band, skew scattering with crossed impurity lines is the only source of the anomalous Hall effect.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, December 17, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Arkadii Fedorov (Universitet Kvinslenda, Avstraliya)
Issledovanie kontekstual'nosti so sverkhprovodyashchimi kvantovymi tsepochkami

Kontekstual'nost' yavlyaetsya odnim iz fundamental'nykh svoistv, otlichayushchikh kvantovuyu mekhaniku ot klassicheskoi teorii. Predpolagalos', chto imenno blagodarya etomu "volshebnomu" parametru proiskhodit eksponentsial'noe uskorenie v kvantovykh komp'yuterakh. My predostavim pervye eksperimental'nye dokazatel'stva etogo resursa dlya trekhurovnevoi kvantovoi sistemy, postroennoi na sverkhprovodyashchikh konturakh - odnoi iz osnovnykh skhem dlya kvantovykh vychislenii. Svyazav dispersionnye sdvigi sverkhprovodyashchego kutrita s SVCh-rezonatorom, my poluchili sil'nye proektsionnye izmereniya s dvoinymi rezul'tatami. V sochetanii s vysokoi kontrastnost'yu schityvaniya, osushchestvlyaemoi blagodarya parametricheskomu usilitelyu, eto pozvolyaet nam ob'edinit' oba izmereniya i sovmestit' obkhody tsiklov, kotorye prisutstvovali v predydushchikh eksperimental'nykh ispytaniyakh.

Theory seminar, Thursday, December 10, 2015, Kapitza IPP, 11:30 am
Serguei Brazovskii
Recent identifications of microscopic solitons in quasi 1D electronic systems and generalisations to higher dimensions

We review a progress in experiments and theory, elucidating the role of microscopic solitons in quasi-1D electronic systems with a symmetry breaking. The new interest rises from studies of the «electronic ferroelectricity» in organic conductors, and from nano-scale tunneling experiments in Charge Density Wave (CDW) materials. Individual solitons have been visually captured in recent STM experiments. On this basis we extrapolate to a picture of combined topological excitations in general strongly correlated systems: from doped antiferromagnets to strong coupling and spin-polarized superconductors. At more macroscopic scales, we recover the electronic vortices generated in mesa-junctions, and domain walls evolving in femtosecond pump-probe experiments.

Seminar "Coherent and self-organization phenomena in metamaterials", Wednesday, December 9, 2015, MISiS, 11:00 am
George P. Tsironis (NUST «MISIS»; University of Crete, Greese)
Coherent and self-organization phenomena in metamaterials

09 December 2015 the seminar "Coherent and self-organization phenomena in metamaterials" will take place in NUST «MISIS» (auditorium G-410, Leninsky pr., 6, The Mining College building, Metro station: Oktyabr`skaya kol`tsevaya)

The shedule:

11:00 «Chimeras in SQUID metamaterials», George P. Tsironis (NUST «MISIS»; University of Crete, Greese)

11:45 «Control of power in parity-time symmetric lattices», George P. Tsironis (NUST «MISIS»; University of Crete, Greese)

12:30 «Qubit lattice coherence induced by electromagnetic pulses in superconducting metamaterials», Zoran Ivic (Institute of Nuclear Sciences "Vinca", Serbia)

13:15 «Imaging coherent response of a superconducting metasurface», Alexander S. Averkin (NUST «MISIS»)

Everyone is welcome - especially young scientists interested in new experience in the field of micro- and nano-technology of superconductors.

Skip for seminar participants who are not students, graduate students or employees of NUST "MISIS", will be carried out upon presentation of passport subject to prior registration for the seminar

Contact: Andrew Maliszewski, tel. +7 916 135 03 25, vabanque@mail.ru

Scientific Council of the Landau Institute, Friday, December 4, 2015, Landau ITP, 11:30 am
Serguei Brazovskii (LPTMS, CNRS & University Paris-Sud & NUST MISiS)
Dynamical phase transitions in electronic systems induced by ultra-fast optical pumping.

I shall report on several studies of phase transformations in cooperative electronic systems achieved by means of a femto-second optical pumping. 1. Experiments on charge density waves recovered coherent unharmonic undulations of the order parameter, critical slowing down of the collective mode, and evolution of the particle-hole gap. The numerical modeling reproduced the dynamical phase transition, and the waves emitted by “earthquakes” from in depth annihilation events of topological defects.*) 2. The bistable switching to a “hidden” state has been achieved in a “polaronic Wigner-crystalline Mott insulator” 1T-TaS2. The theory focuses upon evolution of electrons and holes as mobile charge carriers, and the crystallized electrons modifiable by intrinsic defects.*) 3. The special case of resonance optical pumping to excitons is realized in systems with a neutral-ionic ferroelectric transition. The modeling of the quantum-coherent quasi-condensate of excitons interacting with the order parameter recovers the dynamical realization of the “excitonic insulator” state and spacio-temporal patterns with self-focusing, domains segregation, and local dynamical phase transitions.**) ----------------------------------------------------------------------------------------------- *)After collaboration with D. Mihailovic group at the Jozef Stefan Institute, Ljubljana, Slovenia. **) After collaboration with N. Kirova, LPS, University Paris-Sud, Orsay, France.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, November 26, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Prof. Yukio Tanaka (Nagoya University, Japan)
Odd-frequency pairing and Andreev bound states

Superconductivity is a phenomenon where the macroscopic quantum coherence appears due to the pairing of electrons. The symmetry properties of the pairing, i.e., the parity and spin-singlet/spin-triplet, determine the physical properties of the superconducting state. Odd-frequency pairing originally discussed by Berezinskii as a bulk state. Although the bulk odd-frequency superconductor has not discovered experimentally, the importance of odd-frequency pairing amplitude has been recognized in ferromagnet / superconductor junctions and non-uniform systems. We have clarified that odd-frequency pair amplitude arises in the spatially non-uniform situation quite ubiquitously where bulk superconductor has a conventional even-frequency symmetry. Especially, it has been revealed that when the Andreev bound state (ABS) appears at the surface/interface of the sample, odd-frequency pairing is enhanced. It has been revealed that there are many exotic properties relevant to odd-frequency pairing like anomalous proximity effect in spin-triplet superconductor junctions. This anomalous proximity effect becomes prominent if we consider disordered normal (N) nano wire attached to a topologically nontrivial superconducting (S) one. The transport properties in superconducting nano-wire junctions show universal behaviors irrespective of the degree of disorder: the quantized zero-bias differential conductance at in NS junctions. The odd-frequency pairs exist wherever the Majorana fermions stay. We further discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs in several topological superconducting systems.

Theory seminar, Thursday, November 26, 2015, Kapitza IPP, 11:30 am
A.V. Samokhvalov (IFM RAN, Nizhnii Novgorod)
Elektrodinamika sloistykh gibridnykh struktur sverkhprovodnik/ferromagnetik/sverkhprovodnik v okrestnosti 0-π perekhoda

Gibridnye struktury sverkhprovodnik/ferromagnetik/sverkhprovodnik (SFS) s effektom blizosti yavlyayutsya udobnym ob'ektom dlya izucheniya fazovykh perekhodov, vyzvannykh vzaimodeistviem dvukh konkuriruyushchikh parametrov poryadka: sverkhprovodyashchego i ferromagnitnogo. Neobychnoe povedenie sloistykh SF struktur, kak pravilo, svyazano s interesnym yavleniem π-sverkhprovodimosti pri kotorom v gibridnoi strukture formiruetsya ravnovesnoe sostoyanie s netrivial'noi raznost'yu faz π mezhdu sosednimi sverkhprovodnikami. Effekt blizosti na SF granitsakh privodit k chastichnomu ili polnomu podavleniyu sverkhprovodimosti v S sloyakh, a velichina effekta zavisit ot struktury parnoi volnovoi funktsii, t.e. ot tipa sostoyaniya (0 ili π), kotoroe ustanavlivaetsya v SFS strukture. V doklade obsuzhdayutsya prichiny rezkogo uvelicheniya effektivnoi glubiny proniknoveniya magnitnogo polya λ(T) pri ponizhenii temperatury T, kotoroe nablyudalos' v diffuznykh SFS strukturakh Nb/PdNi/Nb s tonkimi (poryadka sverkhprovodyashchei dliny kogerentnosti) S sloyami [1]. Predlozhena model', ob'yasnyayushchaya podobnuyu anomaliyu proyavleniem fazovogo perekhoda mezhdu 0 i π sverkhprovodyashchimi sostoyaniyami SFS struktury. Osnovyvayas' na mikroskopicheskikh uravneniyakh Uzadelya, polucheno razlozhenie Ginzburga—Landau, koeffitsienty kotorogo zavisyat ot raspredeleniya parnoi volnovoi funktsii v ferromagnetike i razlichny dlya 0 i π sostoyanii SFS struktury. Vypolneny raschety svobodnoi energii 0 i π sostoyanii, temperatury 0−π perekhoda, i privedena fazovaya diagramma SFS struktury na ploskosti parametrov (temperatura — tolshchina F sloya). Perekhod mezhdu 0 i π sostoyaniyami pri izmenenii temperatury proiskhodit po tipu fazovogo perekhoda pervogo roda, soprovozhdaetsya skachkoobraznym izmeneniem amplitudy volnovoi funktsii v S sloyakh i, kak sledstvie, rezkim izmeneniem ekraniruyushchikh svoistv SFS struktury. Priveden obobshchennyi funktsional Ginzburga—Landau, spravedlivyi dlya proizvol'noi raznosti faz sverkhprovodyashchikh parametrov poryadka S sloev, i vypolneny raschety tok-fazovoi zavisimosti SFS dzhozefsonovskogo perekhoda s tonkimi sverkhprovodyashchimi elektrodami v okrestnosti perekhoda mezhdu 0 i π sostoyaniyami [2]. Literatura: [1] N. Pompeo et al., Phys. Rev. B 90, 064510 (2014). [2] A. V. Samokhvalov and A. I. Buzdin, Phys. Rev. B 92, 054511 (2015).

Scientific Council of the Landau Institute, Friday, November 13, 2015, Landau ITP, 11:30 am
I.S. Burmistrov
Charge relaxation resistance in the cotunneling regime of multi-channel Coulomb blockade: Violation of Korringa-Shiba relation

We study the low frequency admittance of a small metallic island coupled to a gate electrode and to a massive reservoir via a multi channel tunnel junction. The ac current is caused by a slowly oscillating gate voltage. We focus on the regime of inelastic cotunneling in which the dissipation of energy (the real part of the admittance) is determined by two-electron tunneling with creation of electron-hole pairs on the island. We demonstrate that at finite temperatures but low frequencies the energy dissipation is ohmic whereas at zero temperature it is super-ohmic. We find that (i) the charge relaxation resistance (extracted from the real part of the admittance) is strongly temperature dependent, (ii) the imaginary and real parts of the admittance do not satisfy the Korringa-Shiba relation. At zero temperature the charge relaxation resistance vanishes in agreement with the recent zero temperature analysis [M. Filippone and C. Mora, Phys. Rev. B86, 125311 (2012) and P. Dutt, T. L. Schmidt, C. Mora, and K. Le Hur, Phys. Rev. B 87, 155134 (2013)].

Theory seminar, Thursday, November 12, 2015, Kapitza IPP, 11:30 am
Georg Schwiete (SPICE and Johannes Gutenberg Universität Mainz, Germany)
Thermal transport in the disordered electron liquid

In this talk, I will present a theoretical study of thermal transport in the disordered two-dimensional electron liquid. At temperatures smaller than the impurity scattering rate, in the diffusive regime, thermal conductivity acquires non-analytic quantum corrections. Our approach to this problem is based on an analysis of the heat density-heat density correlation function. To this end, Luttinger’s gravitational potentials are introduced in the action as sources that couple to the heat density. In a two-stage procedure, a renormalization group calculation based on the Keldysh non-linear sigma model in the presence of Luttinger’s gravitational potentials is supplemented with a perturbative study of scattering processes induced by the Coulomb interaction in the sub-temperature energy range. These scattering processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. As an application, I intend to discuss thermal transport on the metallic side of the metal-insulator transition in Si MOSFETs. References: G. Schwiete and A.M. Finkel’stein, PRB 90, 060201 (2014); PRB 90, 155441 (2014); arXiv:1509.02519; arXiv:1510.06529.

Department of quantum mesoscopics: seminar, Friday, November 6, 2015, ITF, 3:00 pm
M. V. Feigel'man
Level repulsion exponent β for Many-Body Localization Transitions and for Anderson Localization Transitions via Dyson Brownian Motion

Referativnyi doklad po rabote Cecile Monthus, arxiv:1510.08322.

Scientific Council of the Landau Institute, Friday, November 6, 2015, Landau ITP, 11:30 am
Konstantin Tikhonov
Chislennoe issledovanie statistiki urovnei i volnovykh funktsii na sluchainykh regulyarnykh grafakh s besporyadkom

Interes k zadache ob odnochastichnoi lokalizatsii na sluchainom regulyarnom grafe (RRG) v poslednee vremya vozros v svyazi s problemoi mnogochastichnoi lokalizatsii (MBL). Kachestvenno, MBL mozhno ponimat' kak Andersonovskuyu lokalizatsiyu v prostranstve Sleterovskikh determinantov, kotorye igrayut rol' uzlov v neoporyadochennoi pryzhkovoi modeli. Tak kak struktura Fokovskogo prostranstva ierarkhichna, sootvetstvuyushchaya reshetka napominaet sluchainyi regulyarnyi graf. V etoi rabote my izuchaem Andersonovskii perekhod v ansamble RRG s besporyadkom s pomoshch'yu tochnoi chislennoi diagonalizatsii. My pokazyvaem, chto poluchennaya kartina perekhoda na RRG sovmestima s sushchestvuyushchei teoriei, s usloviem pravil'noi interpretatsii poluchennykh rezul'tatov i ucheta effektov konechnosti razmera grafa. Nash analiz snimaet protivorechie mezhdu imeyushchimisya teoreticheskimi rezul'tatami i chislennymi issledovaniyami drugikh avtorov.

Scientific Council of the Landau Institute, Friday, October 23, 2015, Landau ITP, 11:30 am
Pavel Vigman
Geometricheskii transport v kvantovom effekte Kholla

Khorosho izvestno chto provodimost' Kholla kvantuetsya s vysokoi tochnost'yu. Sushchestvuyut li drugie transportnye koeffitsienty, kvantuyushchiesya s takoi zhe tochnost'yu? Okazyvaetsya, chto takikh koeffitsientov kak minimum dva. Oni svyazany s transportom, vyzvannym izmeneniem geometricheskoi formy elektronnoi zhidkosti.

Po rabote: S. Klevtsov, P. Wiegmann: Phys. Rev. Lett. 115: 086801 (2015)

Department of quantum mesoscopics: seminar, Friday, October 16, 2015, ITF, 3:00 pm
Sergei Mironov (MFTI)
Dzhozefsonovskii transport cherez nanoprovoloki: mnogokanal'naya interferentsiya i magnitnye ostsillyatsii

V rabote teoreticheski pokazano, chto kriticheskii tok dzhozefsonovskogo kontakta, soderzhashchego nanoprovoloku s dvumya provodyashchimi kanalami, ispytyvaet mnogoperiodicheskie magnitnye ostsillyatsii. Mnogoperiodichnost' voznikaet iz-za kvantovomekhanicheskoi interferentsii mezhdu kanalami, na kotoruyu sushchestvennoe vliyanie okazyvayut sil'nye spin-orbital'noe i zeemanovskoe vzaimodeistviya. Rassmotrennaya minimal'naya dvukhkanal'naya model' pozvolyaet ob'yasnit' slozhnye interferentsionnye yavleniya, obnaruzhennye v eksperimentakh po izmereniyu dzhozefsonovskogo transporta cherez nanoprovoloki vismuta.

MIPT Interdisciplinary center for basic research: colloquium, Wednesday, October 14, 2015, MFTI, 113GK, 5:05 pm
Sergei Mironov
Prostranstvenno neodnorodnye sostoyaniya v gibridnykh sistemakh sverkhprovodnik-ferromagnetik

Khorosho izvestno, chto sverkhprovodimost' i magnetizm - antagonisticheskie yavleniya, kotorye ne mogut odnovremenno realizovyvat'sya v odnom materiale. Odnako, ikh sosushchestvovanie stanovitsya vozmozhnym v mnogosloinykh gibridnykh sistemakh, v kotorykh sverkhprovodyashchie i ferromagnitnye oblasti prostranstvenno razdeleny. Osnovnaya osobennost' takikh sistem - vozniknovenie neodnorodnykh sverkhprovodyashchikh sostoyanii s ekzoticheskimi magnitnymi i transportnymi svoistvami, kotorye mogut byt' polozheny v osnovu novogo klassa ustroistv vychislitel'noi mikroelektroniki. Seminar budet posvyashchen obzoru osnovnykh svoistv takikh neodnorodnykh sostoyanii i printsipov raboty ustroistv na ikh osnove.

Scientific Council of the Landau Institute, Friday, October 9, 2015, Landau ITP, 11:30 am
V.V. Sivak, A.S. Ioselevich
“Polzuchaya” provodimost', relaksatsiya i fluktuatsii v sisteme podvizhnykh metallicheskikh granul

My rassmatrivaem nestatsionarnuyu sistemu provodnikov, soprotivleniya mezhdu kotorymi ispytyvayut sil'nye fluktuatsii vo vremeni. Transport mezhdu paroi sosednikh provodnikov opredelyaetsya temi momentami, kogda sootvetstvuyushchee soprotivlenie anomal'no malo (provodniki “kasayutsya drug druga”). V takie momenty voznikaet “proboi” i zaryad legko peretekaet mezhdu paroi soprikosnuvshikhsya provodnikov. Protsess perenosa zaryada za schet proboev, voznikayushchikh v sluchainye momenty v sluchainykh mestakh, my nazvali polzuchei provodimost'yu. My rassmatrivaem kak trekhmernye, tak i nizkorazmernye massivy provodnikov i issleduem provodimost' sistemy, rastekanie zaryada, pervonachal'no sosredotochennogo na odnom provodnike, a takzhe fluktuatsii zaryadov i elektricheskikh polei i ikh prostranstvenno-vremennuyu korrelyatsiyu. Obsuzhdayutsya vozmozhnye realizatsii modeli v kolloidnykh rastvorakh i sistemakh metallicheskikh sharikov, soedinennykh deformiruemymi polimernymi molekulami.

Scientific Council of the Landau Institute, Friday, October 9, 2015, Landau ITP, 11:30 am
Igor Poboiko, Mikhail Feigel'man
Thermal transport in disordered one-dimensional spin chains

We study one-dimensional anisotropic XY-Heisenberg spin-1/2 chain with weak random fields hziSzi by means of Jordan-Wigner transformation to spinless Luttinger liquid with disorder and bosonization technique. First we investigate phase diagram of the system in terms of dimensionless disorder γ=⟨h2⟩/J2≪1 and anisotropy parameter Δ=Jz/Jxy and find the range of these parameters where disorder is irrelevant in the infrared limit and spin-spin correlations are described by power laws. Then we use the diagram technique in terms of plasmon excitations to study low-temperature behavior of heat conductivity κ and spin conductivity σ in this power-law phase. The obtained Lorentz number L≡κ/σT differs from the value derived earlier by means of memory function method. We argue also that in the studied region inelastic scattering is strong enough to suppress quantum interference in the low-temperature limit.

Department of quantum mesoscopics: seminar, Friday, October 2, 2015, ITF, 3:00 pm
K. Tikhonov
Kagome chiral spin liquid as a gauged U(1) symmetry protected topological phase

Referativnyi doklad po rabote Y.-C. He, S. Bhattacharjee, F. Pollmann, R. Moessner, arXiv:1509.03070

Department of quantum mesoscopics: seminar, Friday, September 25, 2015, ITF, 3:00 pm
I. Burmistrov
Tunneling into the localized phase near Anderson transitions
Scientific Council of the Landau Institute, Friday, September 25, 2015, Landau ITP, 11:30 am
M. V. Feigel'man
Sverkhtekuchaya plotnost' sil'no neuporyadochennykh sverkhprovodnikov i ikh anomal'noe povedenie pri nizkikh temperaturakh i sil'nykh magnitnykh polyakh.

Doklad budet sostoyat' iz dvukh chastei. V pervoi iz nikh budet pokazano, na osnove sovmestnoi s L.B.Ioffe raboty [1], chto sverkhprovodniki s psevdoshchel'yu obladayut anomal'noi zavisimost'yu sverkhtekuchei plotnosti \rho_s ot velichiny sverkhprovodyashchei shcheli \Delta: \rho_s ~ \Delta^2. Pokhozhee povedenie bylo nedavno nablyudeno [2]. Vo vtoroi chasti doklada ya pokazhu, kak mozhno ob'yasnit' shiroko izvestnoe dlya sil'no gryaznykh sverkhprovodnikov anomal'noe povedenie verkhnego kriticheskogo polya H_c2(T) pri nizkikh temperaturakh, ispol'zuya nedavnii eksperimental'nyi rezul'tat [3] o nalichii perekhoda Berezinskogo-Kosterlitsa-Taulesa v sluchae sverkhprovodyashchei plenki v sil'nom magnitnom pole i rezul'taty izmereniya kriticheskogo toka kak funktsii magnitnogo polya v predele ochen' nizkikh temperaturakh [4].
[1] M. V. Feigel'man, L. B. Ioffe arXiv:1509.05138, Phys Rev B, accepted.
[2] D. Sherman et al, Nature Phys. (2015)
[3] S. Misra, L. Urban, M. Kim, G. Sambandamurthy, and A. Yazdani, Phys. Rev. Lett. 110, 037002 (2013)
[4] J.Seidemann et al, unpublished (Master diploma thesis, Grenoble 2013).

Department of quantum mesoscopics: seminar, Friday, September 18, 2015, ITF, 3:00 pm
E. Repin
Topologicheskii Kondo izolyator

Referativnyi doklad pro vozmozhnuyu realizatsiyu topologicheskogo izolyatora v sil'nokorrelirovannoi elektronnoi sisteme. Teoreticheskoe predskazanie i eksperimental'nye svidetel'stva v materiale SmB_6.

[1] M. Dzero, K. Sun, V. Galitski, P. Coleman, PRL 104, 106408 (2010).
[2] M. Ye, J. W. Allen, K. Sun, http://arxiv.org/abs/1307.7191
[3] S. Wolgast, Ç. Kurdak, K. Sun, J. W. Allen, D.-J. Kim, Z. Fisk, PRB 88, 180405(R) (2013).

Scientific Council of the Landau Institute, Friday, September 18, 2015, Landau ITP, 11:30 am
Daniil Antonenko
Kvantovyi raspad sverkhtokovogo sostoyaniya i sobstvennaya emkost' Dzhozefsonovskikh perekhodov za ramkami tunnel'nogo priblizheniya

A nondissipative supercurrent state of a Josephson junction is metastable with respect to the formation of a finite-resistance state. This transition is driven by fluctuations, thermal at high temperatures and quantum at low temperatures. We evaluate the life time of such a state due to quantum fluctuations in the limit when the supercurrent is approaching the critical current. The decay probability is determined by the instanton action for the superconducting phase difference across the junction. At low temperatures, dynamics of the phase is massive and is determined by the effective capacitance, which is a sum of the geometric and intrinsic capacitance of the junction. We model the central part of the Josephson junction either by an arbitrary short mesoscopic conductor described by the set of its transmission coefficients, or by a diffusive wire of an arbitrary length. The intrinsic capacitance can generally be estimated as $C_* \sim G/E_g$, where $G$ is the normal-state conductance of the junction and $E_g$ is the proximity minigap in its normal part. The obtained capacitance is sufficiently large to qualitatively explain hysteretic behavior of the current-voltage characteristic even in the absence of overheating.

Scientific Council of the Landau Institute, Friday, September 18, 2015, Landau ITP, 11:30 am
S. Autti, V.V. Dmitriev, V.B. Eltsov, J. Makinen, G.E. Volovik, A.N. Yudin, V.V. Zavjalov
Observation of half-quantum vortices in superfluid 3He

Recently in Kapitza Institute a new phase of the superfluid 3He was reported to exist in the aerogel-like nafen structure - the so-called polar phase. As distinct from the other phases - the chiral superfluid 3He-A with Weyl nodes and the fully gapped topological 3He-B with Dirac nodes on the surface - the polar phase has Dirac nodal line in bulk and dispersionless band of Andreev-Majorana fermions on the surface. Being the spin-triplet superfluid with equal-spin pairing the polar phase can support an exotic object - the half-quantum vortex (HQV). Originally the HQVs have been predicted to exist in the Weyl superfluid 3He-A in 1976, but still they have not been observed there: unfavorable combination of spin-orbit and magnetic anisotropy effects on the orientation of the order parameter did not allow to stabilize these vortices in bulk 3He-A under rotation. In nafen, the nearly parallel aerogel strands pin the orbital part of the order parameter of the polar phase. As a result, if the magnetic field is absent or is oriented parallel to nafen strands, the half-quantum vortices win over the conventional single-quantum vortices.
If HQVs are formed in rotation and then the field is tilted with respect to the strands, one expects that the spin-orbit interaction would induce the formation of the topological solitons between the neighboring HQVs providing the peculiar satellite peak in the NMR spectrum. We used this theoretical prediction to stabilize and identify the HQVs in the Helsinki rotated cryostat. For that we cool down a sample of nafen through transition temperature to the polar phase in rotation up to 2.75 rad/s without magnetic field. Then the field is switched on, and in the tilted field we observe a satellite peak in the NMR spectrum. The dependence of the satellite on the rotation velocity, temperature and the field orientation is in agreement with the predictions. If cool-down occurs with the field applied in the transverse direction, no satellite eak is observed. This demonstrates that the single-quantum vortices are formed instead, which have no NMR signatures. in such arrangement these vortices win over the HQVs as expected.

Theory seminar, Thursday, September 10, 2015, Kapitza IPP, 11:30 am
D.S. Shapiro (Vserossiiskii NII avtomatiki im. N.L. Dukhova; IRE im. V.A. Kotel'nikova RAN), A.A. Zhukov, V.V. Pogosov, Yu.E. Lozovik
Dynamical Lamb effect in a tunable superconducting qubit-cavity system

Dinamicheskii effekt Lemba yavlyaetsya odnim iz vozmozhnykh mekhanizmov parametricheskogo vozbuzhdeniya atoma v rezonatore [A.M. Fedotov, N.B. Narozhny, and Yu.E. Lozovik, PRA 64, 053807 (2001)]. Effekt voznikaet v rezul'tate modulyatsii lembovskogo sdviga atoma za schet, naprimer, neadiabaticheskogo izmeneniya chastoty fotonnoi mody. Tem ne menee, v sisteme s perestraivaemym rezonatorom etot mekhanizm vozbuzhdeniya slozhno izolirovat' ot drugikh kanalov, svyazannykh s generatsiei i pogloshcheniem kazimirovskikh fotonov. V doklade predlozhena al'ternativnaya realizatsiya dinamicheskogo effekta Lemba, kotoraya vozmozhna v sovremennykh sverkhprovodnikovykh sistemakh kubit-rezonator v rezhime perestraivaemoi energii svyazi. Pokazano, chto imenno za schet neadiabaticheskogo pereklyucheniya energii vzaimodeistviya mozhno modulirovat' lembovskii sdvig i vozbuzhdat' kubit v otsutstvie fotonnoi nakachki. Takoi podkhod pozvolyaet izbezhat' rozhdeniya kazimirovskikh fotonov, chto yavlyaetsya klyuchevym obstoyatel'stvom dlya realizatsii dinamicheskogo effekta Lemba. Dinamika sistemy opisyvaetsya v ramkakh nestatsionarnoi modeli Rabi dlya edinichnogo i periodicheskogo pereklyuchenii energii svyazi kubita i rezonatora. [D.S. Shapiro, A.A. Zhukov, W.V. Pogosov, Yu.E. Lozovik, "Dynamical Lamb effect in a tunable superconducting qubit-cavity system", Phys. Rev. A 91, 063814 (2015); arXiv:1503.01666]

Shmidtovskii seminar po sverkhprovodimosti, Thursday, July 23, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Prof. doktor Mikhail Zigel' (Tekhnologicheskii Institut Karlsrue, Germaniya)
Odnofotonnye i teragertsovye detektory na osnove sverkhprovodyashchikh nanoprovolok

Odnofotonnye detektory na osnove sverkhprovodyashchikh nanoprovolok (Superconducting Nanowire Single-Photon Detectors (SNSPD)) issleduyutsya nauchnym soobshchestvom uzhe v techenii nekotorogo vremeni, chto privelo k sushchestvennomu progressu v ponimanii ikh raznoobraznykh opticheskikh, elektricheskikh i drugikh fizicheskikh svoistv. Spektral'naya polosa propuskaniya SNSPD ogranichennaya dlinoi volny otsechki, kotoraya opredelyaetsya parametrami sverkhprovodyashchei plenki, ispol'zuemoi dlya detektora, i geometriei detektora. Na segodnyashnii den' sushchestvuet mnozhestvo razlichnykh prilozhenii, gde SNSPD detektory, integrirovannye v izmeritel'nye kriogennye sistemy, pozvolyayut provodit' eksperimenty v oblasti kvantovoi kriptografii, izlucheniya kvantovykh tochek ili spektroskopii. Prichinu shirokogo ispol'zovaniya SNSPD detektorov dlya takikh zadach mozhno legko ob'yasnit', esli prinyat' vo vnimanie nekotorye poleznye svoistva SNSPD detektorov, a imenno nizkuyu temnovuyu skorost' scheta, vysokuyu skorost' detektirovaniya i vysokuyu tochnost' sinkhronizatsii v pikosekundnom diapazone.

V nashi dni SNSPD sistemy osnovany, glavnym obrazom, na odnopiksel'nykh detektorakh. Mnogopiksel'nye sistemy dolzhny rasshirit' potentsial'nye oblasti primeneniya detektorov. Po etoi prichine, neobkhodimy novye kontseptsii dlya organizatsii schityvaniya dannykh s mnogopiksel'nykh SNSPD sistem. My daem obzor vozmozhnykh kontseptsii, uslovii raboty i sposobov optimizatsii SNSPD detektorov dlya konkretnykh prilozhenii, rabotayushchikh v odnofotonnom rezhime.

Osnovyvayas' na tekh zhe sverkhprovodyashchikh sverkhtonkikh plenkakh mogut byt' realizovany Bolometry na goryachikh elektronakh (Hot-Electron Bolometers (HEBs)) dlya neposredstvennogo detektirovaniya i ponizhayushchego preobrazovaniya teragertsovogo izlucheniya. My obsudim HEB-smesiteli na osnove ul'tratonkikh plenok NbN dlya geterodinnogo detektirovaniya TGts signalov. Krome togo, my razrabotali detektor neposredstvennogo TGts-detektirovaniya na osnove tonkoplenochnykh nanostruktur iz vysokotemperaturnogo sverkhprovodnika YBa2Cu3O7-kh. Eti detektory imeyut predel'noe vremennoe razreshenie okolo 10-20 ps, pozvolyaya provodit' izmereniya pikosekundnykh impul'sov, ispuskaemykh istochnikom sinkhrotronnogo izlucheniya.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, July 16, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
prof., doktor Rudol'f Gross (Walther-Meissner-Institut, Technische Universität München, Germany)
Sverkhprovodyashchie gibridnye kvantovye sistemy
Shmidtovskii seminar po sverkhprovodimosti, Thursday, July 2, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Val'ter Pogosov (Vserossiiskii nauchno-issledovatel'skii institut avtomatiki im. N. L. Dukhova)
Dinamicheskii effekt Lemba v svyazannoi sisteme sverkhprovodnikovogo kubita i SVCh-rezonatora

Nekotoroe vremya nazad bylo rassmotreno povedenie edinichnogo atoma, pomeshchennogo v polost' s izmenyayushchimisya vo vremeni parametrami - [A. M. Fedotov, N. B. Narozhny, Yu. E. Lozovik, Phys. Rev. A 64, 053807 (2001)]. Bylo pokazano, chto v takoi sisteme realizuetsya neskol'ko mekhanizmov parametricheskogo vozbuzhdeniya atoma. Odin iz nikh svyazan s vozbuzhdeniem za schet neadiabaticheskoi modulyatsii lembovskogo sdviga v osnovnom sostoyanii atoma. Eto yavlenie bylo nazvano dinamicheskim effektom Lemba (po analogii s dinamicheskim effektom Kazimira). V sluchae real'nogo atoma, pomeshchennogo v polost' s izmenyayushchimisya vo vremeni parametrami, dinamicheskii effekt Lemba okazyvaetsya dovol'no slozhno izolirovat' ot inykh mekhanizmov vozbuzhdeniya atoma. V dannoi rabote pokazano, chto dlya realizatsii dinamicheskogo effekta Lemba perspektivno ispol'zovat' iskusstvennyi «atom»-kubit, svyazannyi s SVCh-rezonatorom. Kharakternuyu energiyu vzaimodeistviya mezhdu kubitom i rezonatorom mozhno menyat' vo vremeni, chto vpolne osushchestvimo s tochki zreniya sovremennogo eksperimenta. Za schet neadiabaticheskogo izmeneniya energii vzaimodeistviya mozhno dobit'sya vozbuzhdeniya kubita. Dannyi podkhod pozvolyaet izbezhat' rozhdeniya kazimirovskikh fotonov, chto yavlyaetsya klyuchevym obstoyatel'stvom dlya realizatsii dinamicheskogo effekta Lemba.

[D. S. Shapiro, A. A. Zhukov, W. V. Pogosov, Yu. E. Lozovik, "Dynamical Lamb effect in a tunable superconducting qubit-cavity system", Phys. Rev. A 91, 063814 (2015); arXiv:1503.01666]

Shmidtovskii seminar po sverkhprovodimosti, Thursday, June 11, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Aleksandr Kirichenko (HYPRES, SShA)
Sverkhprovodnikovyi komp'yuter: status i perspektivy

S uvelicheniem vychislitel'nykh moshchnostei, ostro vstal vopros potreblyaemoi energii. Razrabotka sverkhprovodnikovogo komp'yutera vo mnogikh laboratoriyakh rassmatrivaetsya kak reshenie problemy nizkoenergeticheskikh vychislenii. Nekotorye vidy adiabaticheskikh vychislenii na sverkhprovodnikakh (naprimer QFP) priblizhayutsya k teoreticheskomu predelu energozatrat. Odnokvantovaya logika, khotya i ustupaet im v energovydelenii, yavlyaetsya naibolee perspektivnoi na segodnyashnii den'.

V doklade budet priveden obzor tekushchego sostoyaniya del v razrabotke SFQ protsessorov, a takzhe budut zatronuty nekotorye drugie perspektivnye napravleniya v vychislitel'noi elektronike.

Scientific Council of the Landau Institute, Friday, May 22, 2015, Landau ITP, 11:30 am
S.V. Iordanskii
Novyi kvantovyi mekhanizm provodimosti v dvukhsloinykh geterostrukturakh
Shmidtovskii seminar po sverkhprovodimosti, Thursday, May 21, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
N.V. Klenov (MGU im. Lomonosova, Fizicheskii fakul'tet)
Vol't-potokovye preobrazovaniya v dzhozefsonovskikh strukturakh na osnove sverkhprovodyashchikh kvantovykh interferometrov

Sverkhprovodyashchie kvantovye interferometry (SKVIDy) shiroko izvestny svoei sposobnost'yu prevrashchat' prilozhennyi magnitnyi signal v otklik napryazheniya. V dannom doklade my predstavlyaem obzor ryada eksperimental'nykh i teoreticheskikh rabot, napravlennykh na sozdanie na osnove SKVIDov struktur so spetsificheskim vidom vol't-potokovoi kharakteristiki (primerom mogut posluzhit' struktury s vysokoi lineinost'yu preobrazovaniya magnitnogo potoka v napryazhenie). Budut razobrany fizicheskie osnovy sozdaniya sverkhprovodyashchikh kvantovykh interferentsionnykh fil'trov (SKIFov), differentsial'nykh dzhozefsonovskikh struktur, bi-SKVIDov i bi-SKVID SKIFov.

Theory seminar, Thursday, May 21, 2015, Kapitza IPP, 11:30 am
Yu.A. Kosevich (Institut khimicheskoj fiziki im. N.N. Semenova RAN)
Interferentsiya fononov v atomarnykh metazerkalakh, «fononnykh kondensatorakh» i v teploperenose cherez granitsy razdela kristallov

Seminar budet posvyashchen opisaniyu novykh nizkorazmernykh metamaterialov i transportnykh yavlenii, v kotorykh vazhnuyu rol' igraet interferentsiya fononov na atomarnykh defektakh reshetki kristalla. Budet pokazano, chto vozmozhno rezonansnoe polnoe otrazhenie ili polnoe pogloshchenie fonona dvumernym defektom reshetki, effektivnaya tolshchina kotorogo znachitel'no men'she dliny volny fonona. Chastoty takikh rezonansov opredelyayutsya dinamicheskimi svoistvami defektnoi ploskosti. Yavlenie polnogo otrazheniya (antirezonansa prokhozhdeniya) vozmozhno, v chastnosti, na monoatomnoi kristallicheskoi ploskosti, chastichno zapolnennoi atomarnymi defektami, i na dvumernom defekte reshetki, v kotorom sushchestvenny vzaimodeistviya kak blizhaishikh, tak i ne blizhaishikh sosedei. Analiz etikh yavlenii pokazyvaet, chto opredelyayushchuyu rol' v nikh, v otlichie ot rezonansa Fano, igraet destruktivnaya interferentsiya fononov, rasprostranyayushchikhsya cherez dvumernyi defekt po dvum putyam: po silovym svyazyam atomov matritsy i defektov, ili po svyazyam blizhaishikh i ne blizhaishikh sosedei. Pokazano, chto sluchainoe raspredelenie defektov v kristallicheskoi ploskosti i angarmonizm silovykh svyazei ne razrushayut interferentsionnyi antirezonans. Shirinu antirezonansnogo pika otrazheniya mozhno svyazat' s dlinoi kogerentnosti fononnogo volnovogo paketa, kak i v sluchae destruktivnoi interferentsii fotonnykh volnovykh paketov v effekte Khonga-U-Mandela. Pokazano, chto destruktivnaya interferentsiya fononov ponizhaet teplovoi kondaktans granitsy razdela kristallov, chastichno zapolnennoi rezonansnymi atomarnymi defektami. Na osnove atomarnykh metazerkal mozhet byt' postroen ul'tra kompaktnyi «fononnyi kondensator» dlya khraneniya i izlucheniya kogerentnykh teragertsovykh fononov. Vse osnovnye predstavlennye vyvody podtverzhdayutsya analiticheskimi raschetami ekvivalentnykh kvaziodnomernykh reshetok i molekulyarno-dinamicheskim modelirovaniem realistichnykh trekhmernykh reshetok. [Literatura: 1. Yu.A. Kosevich «Capillary phenomena and macroscopic dynamics of complex two-dimensional defects in crystals», Progr. Surf. Sci. 55, 1 (1997). 2. Yu.A. Kosevich «Mnogokanal'noe rasprostranenie i rasseyanie fononov i fotonov v nizkorazmernykh nanostrukturakh», UFN 178(8), 884 (2008). 3. H. Han, L. Potyomina, A.A. Darinskii, S. Volz, and Yu.A. Kosevich «Phonon interference and thermal conductance reduction in atomic-scale metamaterials», Phys. Rev. B 89(R), 180301 (2014). 4. H. Han, B. Li, S. Volz, and Yu.A. Kosevich «Ultracompact interference phonon nanocapacitor for storage and lasing of coherent terahertz lattice waves», Phys. Rev. Lett. 114, 145501 (2015).]

Department of quantum mesoscopics: seminar, Friday, May 8, 2015, ITF, 3:00 pm
E.S. Tikhonov (IFTT RAN, dokladchik), D.V. Shovkun, D.Kh. Kvon, N.N. Mikhailov, S.A. Dvoretskii, V.S. Khrapai
Drobovoi shum kraevogo transporta v kvantovoi yame HgTe s invertirovannoi zonnoi strukturoi

My izuchaem tokovyi shum v CdHgTe/HgTe/CdHgTe kvantovoi yame s invertirovannoi zonnoi strukturoi v rezhime neuporyadochennogo kraevogo transporta. V soglasii s predydushchimi eksperimentami soprotivlenie kraya sushchestvenno prevoskhodit h/e2 i slabo zavisit ot temperatury. Drobovoi shum v rezhime kraevogo transporta znachitel'no podavlen po sravneniyu s puassonovskim znacheniem i kharakterizuetsya faktorom Fano v diapazone 0.1 < F < 0.3 dlya razlichnykh obraztsov i znachenii zatvornogo napryazheniya. S uchetom togo, chto dlina kraya men'she ballisticheskoi dliny sboya fazy, eti nablyudeniya isklyuchayut vozmozhnost' odnomernogo gelikal'nogo kraevogo transporta v nashikh obraztsakh. Rezul'taty eksperimenta kachestvenno mozhno ob'yasnit' v modeli neuporyadochennogo mnogokanal'nogo kvazi-odnomernogo transporta vdol' kraya.

Scientific Council of the Landau Institute, Friday, May 8, 2015, Landau ITP, 11:30 am
T.T. Heikkila and G.E. Volovik
Topology of Dirac lines and nexus in graphite

We consider the Z_2 topology of the Dirac lines - lines of band intersection - in graphite. Four lines (three with topological charge N_1=1 each and one with N_1=-1) merge near the H-point and annihilate due to summation law 1+1=0. The merging point is similar to the real-space nexus, the analog of Dirac monopole at which the Z_2 strings terminate.

Theory seminar, Thursday, May 7, 2015, Kapitza IPP, 11:30 am
D. Basko (LPMMC, CNRS, Grenoble, France)
O roli effekta Landau-Zinera v rabote kvantovogo turniketa

Kvantovyi turniket predstavlyaet soboi nanoelektronnoe ustroistvo, kontroliruemym obrazom perenosyashchee elektrony po odnomu mezhdu dvumya sverkhprovodyashchimi elektrodami putem podachi peremennogo napryazheniya na metallicheskuyu nanochastitsu, raspolozhennuyu mezhdu dvumya elektrodami. Okazyvaetsya, chto kogerentnaya svyaz' mezhdu elektronnymi sostoyaniyami v nanochastitse i kvazichastichnymi sostoyaniyami v sverkhprovodnikakh nakladyvaet fundamental'nye ogranicheniya na rabotu ustroistva.

Scientific Council of the Landau Institute, Friday, April 24, 2015, Landau ITP, 11:30 am
A. S. Vasenko, Denis M. Basko, F. W. J. Hekking (University Grenoble Alpes, CNRS, LPMMC)
Giant mesoscopic fluctuations of the elastic cotunneling thermopower of a single-electron transistor

We study the thermoelectric transport of a small metallic island weakly coupled to two electrodes by tunnel junctions. In the Coulomb blockade regime, in the case when the ground state of the system corresponds to an even number of electrons on the island, the main mechanism of electron transport at the lowest temperatures is elastic cotunneling. In this regime, the transport coefficients strongly depend on the realization of the random impurity potential or the shape of the island. Using random-matrix theory, we calculate the thermopower and the thermoelectric kinetic coefficient and study the statistics of their mesoscopic fluctuations in the elastic cotunneling regime. The fluctuations of the thermopower turn out to be much larger than the average value.
[Phys. Rev. B 91, 085310 (2015)]

Shmidtovskii seminar po sverkhprovodimosti, Thursday, April 23, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Yu.V. Maslennikov (Institut radiotekhniki i elektroniki Rossiiskoi akademii nauk (IRE RAN); OOO «NPO KRIOTON»)
MAGNITOMETRIChESKIE SISTEMY NA OSNOVE SKVIDOV DLYa BIOMEDITsINSKIKh PRIMENENII

V vystuplenii budut rassmotreny voprosy razrabotki i sozdaniya osnovnykh elementov SKVID-sistem dlya biomagnitnykh issledovanii: SKVID-datchikov, stekloplastikovykh nemagnitnykh kriostatov, izmeritel'nykh zondov i elektroniki, v tom chisle dlya SKVID-sistem na osnove vysokotemperaturnykh sverkhprovodnikov. Rassmotreny voprosy razrabotki i sozdaniya magnitometricheskikh SKVID-sistem dlya primenenii v magnitokardiografii i magnitoentsefalografii. Privedeny rezul'taty ikh prakticheskogo ispol'zovaniya v klinicheskikh usloviyakh.

MIPT Interdisciplinary center for basic research: colloquium, Wednesday, April 22, 2015, MFTI, 113 GK, 5:05 pm
D. Basko (CNRS, Grenoble, France)
Mnogochastichnaya lokalizatsiya: ot kvantovykh sistem k klassicheskim

When single-particle excitations of a disordered quantum many-body system are all localized in the coordinate space, a sufficiently weak local interaction does not destroy localization at sufficiently low temperatures. Namely, all many-particle excitations remain localized in the Fock space, and there is no relaxation or transport in the system, which is known as many-body localization. It occurs because the local interaction effectively couples a finite number of discrete energy levels, corresponding to localized excitations. When the interaction is weak, these localized excitations are only weakly mixed.

In contrast to a quantum system, where a continuous energy spectrum can be obtained only in the limit of the infinite system size, and is thus subject to localization in the Fock space, in a classical system (e.g., in the disordered nonlinear Schroedinger chain, representing the classical limit of the Bose-Hubbard model), motion can have continuous spectrum already for a finite number of degrees of freedom, and this leads to chaotic dynamics. Because of this, in a disordered system with an extensive energy (with however small density), the probability to be on a chaotic trajectory tends to unity with increasing system size. This produces finite transport coefficients at arbitrarily weak nonlinearity.

Department of quantum mesoscopics: seminar, Friday, April 17, 2015, ITF, 3:00 pm
Igor' Zagorodnev (MFTI/IRE RAN)
Rezonansnoe rasseyanie elektronov v grafene na kruglom nanootverstii

Vychisleno transportnoe sechenie rasseyaniya elektronov v grafene na kruglom otverstii (ili defekte). Predpolagaetsya, chto na krayu otverstiya mogut sushchestvovat' kvazistatsionarnye kraevye (tammovskie) sostoyaniya, kotorye privodyat k pikam v sechenii rasseyaniya, kotorye mogli nablyudat'sya v transportnykh eksperimentakh [Yu.I. Latyshev i dr., Pis'ma v ZhETF 98, 242 (2013), Yu.I. Latyshev et al., Scientific Reports 4, 7578 (2014)]. Obsuzhdaetsya uchet neodnorodnosti kraya, a takzhe zaryazhennosti nanootverstiya (defekta), chto privodit k razmytiyu i smeshcheniyu pikov.

Kurs lektsii D. Basko, Tuesday, April 14, 2015, Konferents-zal IFP im. P.L.Kapitsy, 10:00 am
D. Basko (LPMMC, CNRS, Grenoble, France)
Elektrony v nizkorazmernykh neuporyadochennykh sistemakh

Dlya starshekursnikov i aspirantov, spetsializiruyushchikhsya v oblasti kvantovoi teorii kondensirovannogo sostoyaniya

Programma kursa: 1. Andersonovskaya lokalizatsiya v trekh izmereniyakh, lokatornoe razlozhenie. 2. Lokalizatsiya v odnomernoi tsepochke, uravnenie Fokkera-Planka dlya koeffitsienta prokhozhdeniya. 3. Lokalizatsiya v tolstykh kvaziodnomernykh provolokakh (kriterii Taulessa), dinamicheskaya lokalizatsiya v kvantovoi tochke pod nakachkoi i otobrazhenie na kvaziodnomernuyu provoloku. 4. Slabolokalizatsionnaya popravka k provodimosti v dvukh izmereniyakh. 5. Elektron-elektronnoe vzaimodeistvie v kvantovykh tochkakh, universal'nyi gamil'tonian, kulonovskaya blokada, skorostnye uravneniya. 6. Elektron-elektronnye stolknoveniya v nizkorazmernykh sistemakh, mnogochastichnaya lokalizatsiya. 7. Kotunnelirovanie cherez kvantovuyu tochku v rezhime kulonovskoi blokady. 8. Effekt Kondo v kvantovoi tochke v rezhime kulonovskoi blokady.

Vremya: po vtornikam (10-12) i chetvergam (16-18)

Pervaya lektsiya - vo vtornik 14 aprelya 2015 g.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, April 9, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Aleksei Vladimirovich Samokhvalov (Institut fiziki mikrostruktur RAN, g. Nizhnii Novgorod)
Vikhrevye sostoyaniya v neodnosvyaznykh sverkhprovodnikakh i gibridnykh strukturakh

Privedeny rezul'taty teoreticheskikh issledovanii elektronnoi struktury i osobennostei individual'nogo pinninga vikhrya Abrikosova, zakhvachennogo neprovodyashchim tsilindricheskim defektom (polost'yu) s kharakternym razmerom men'she ili poryadka sverkhprovodyashchei dliny kogerentnosti. Predlozhena mikroskopicheskaya model' depinninga (sryva) vikhrya Abrikosova pod vliyaniem vneshnego transportnogo toka, kotoraya uchityvaet izmenenie topologii kvaziklassicheskikh orbit s energiei Fermi i perestroiku spektra vozbuzhdenii v vikhre, zakhvachennom na protyazhennom defekte. Izucheno formirovanie neodnorodnykh i vikhrevykh sostoyanii v gibridnykh strukturakh sverkhprovodnik–ferromagnetik s neodnosvyaznoi geometriei (polyi sverkhprovodyashchii tsilindr s ferromagnitnoi serdtsevinoi), v kotorykh orbital'nyi i obmennyi mekhanizmy, podavlyayushchie sverkhprovodimost', konkuriruyut drug s drugom, a effekt blizosti mozhet stimulirovat' perekhody mezhdu sverkhprovodyashchimi sostoyaniyami s razlichnoi zavikhrennost'yu.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, March 26, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
M.V. Fistul' (Rurskii Universitet, Bokhum Germaniya & Kafedra TFiKT, laboratoriya sverkhprovodyashchikh metamaterialov, NITU MISiS)
Sverkhprovodyashchie kvantovye metamaterialy

Sverkhprovodyashchie kvantovye metamaterialy - iskusstvenno sozdannye novye materialy, sostoyashchie iz bol'shogo chisla sil'no vzaimodeistvuyushchikh kubitov. Elektrodinamicheskie svoistva takikh materialov opredelyayutsya zakonami kvantovoi mekhaniki na makroskopicheskom urovne.

V doklade budet predstavlen obzor nedavnikh eksperimental'nykh i teoreticheskikh issledovanii prokhozhdeniya voln v sverkhprovodyashchikh kvantovykh metamaterialakh. Budut obsuzhdat'sya razlichnye kollektivnye kvantovye effekty v takikh sistemakh, osoboe vnimanie budet udeleno vozmozhnosti sinkhronizatsii kvantovykh dinamicheskikh ostsillyatsii otdel'nykh kubitov.

Scientific Council of the Landau Institute, Friday, March 13, 2015, Landau ITP, 11:30 am
P.D. Grigor'ev
Novyi mekhanizm rasseyaniya elektronov v magnitnom pole v sostoyanii s volnoi plotnosti v usloviyakh neideal'nogo nestinga

V sostoyanii s volnoi zaryadovoi ili spinovoi plotnosti v usloviyakh neideal'nogo nestinga chast' elektronnykh sostoyanii ostaetsya na urovne Fermi, chto sokhranyaet metallicheskuyu provodimost' obraztsa. Odnako poverkhnost' Fermi elektronov provodimosti v takom sostoyanii sil'no otlichaetsya ot iskhodnoi, potomu chto rasseyanie na vektor volny plotnosti privodit k perestroike poverkhnosti Fermi. Obychno eto soprovozhdaetsya vozniknoveniem novykh chastot magnitnykh kvantovykh ostsillyatsii, sootvetstvuyushchikh perestroennoi poverkhnosti Fermi. Odnako, v sil'nom magnitnom pole voznikaet magnitnyi proboi, kotoryi mozhet vosstanovit' dvizhenie elektronov po traektoriyam, sootvetstvuyushchim iskhodnoi poverkhnosti Fermi. Eto proiskhodit iz-za togo, chto rasstoyanie po energii mezhdu starymi i novymi elektronnymi traektoriyami okazyvaetsya malym, poryadka potentsiala rasseyaniya na volne plotnosti, to est' poryadka energeticheskoi shcheli volny plotnosti. Pri etom veroyatnost' elektronov dvigat'sya vdol' staroi ili novoi poverkhnosti Fermi eksponentsial'no zavisit ot velichiny etoi shcheli. Poetomu prostranstvennye fluktuatsii parametra poryadka volny plotnosti privodyat k prostranstvennym fluktuatsiyam veroyatnosti magnitnogo proboya, i v itoge k dopolnitel'nomu rasseyaniyu elektronov. Pri vysokoi kontsentratsii neodnorodnostei volny plotnosti etot novyi mekhanizm rasseyaniya elektronov provodimosti mozhet okazat'sya dostatochno sil'nym: sil'nee, chem rasseyanie na primesyakh. Etot mekhanizm mozhet privodit' k neobychnym effektam, takim kak sil'noe nemonotonnoe magnitosoprotivlenie, ili inversiya fazy magnitnykh kvantovykh ostsillyatsii. Imenno eto nablyudaetsya v razlichnykh organicheskikh metallakh, naprimer, v a-(BEDT-TTF)2KHg(SCN)4. Obsuzhdaetsya takzhe vozmozhnost' ispol'zovaniya etogo mekhanizma rasseyaniya dlya izmereniya fluktuatsii parametra poryadka volny plotnosti v vysokotemperaturnykh sverkhprovodnikakh.

Scientific Council of the Landau Institute, Friday, March 6, 2015, Landau ITP, 11:30 am
A.B. Shabat
Raznostnoe uravnenie Shredingera i kvazisimmetricheskie mnogochleny

Na model'nom uravnenii Shredingera s del'taobraznym potentsialom obsuzhdayutsya sushchestvennye otlichiya obratnoi zadachi rasseyaniya na vsei osi ot sluchaya poluosi. Ispol'zuetsya pri etom ekvivalentnost' rassmatrivaemoi model'noi zadachi diskretnoi tsepochke uravnenii s vzaimodeistviem blizhaishikh sosedei dostatochno obshchego vida. Ustanavlivayutsya takzhe yavnye formuly dlya funktsii Grina uravnenii Shredingera s del'taobraznymi potentsialami i issleduyutsya svoistva etikh funktsii Grina.

MIPT Interdisciplinary center for basic research: colloquium, Wednesday, March 4, 2015, MFTI, GK 113, 5:05 pm
M.E. Gershenzon (Rutgers University, USA)
Macroscopic Quantum Effects in Superconducting Circuits and Quantum Computing
Shmidtovskii seminar po sverkhprovodimosti, Thursday, February 26, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Valerii Pavlovich Koshelets (Institut radiotekhniki i elektroniki RAN (IRE RAN), Moskva, Rossiya)
Issledovanie TGts spektrov izlucheniya Bi2Sr2CaCu2O8 meza-struktur s
pomoshch'yu sverkhprovodnikovogo integral'nogo priemnika

V doklade predstavlena istoriya sozdaniya sverkhprovodnikovogo integral'nogo priemnika i dan kratkii obzor ego primenenii. Odnim iz takikh prilozhenii stalo issledovanie spektrov izlucheniya Bi2Sr2CaCu2O8 meza-struktur v teragertsovom diapazone. Privedeny rezul'taty izmerenii, obsuzhdayutsya perspektivy sozdaniya kompaktnykh istochnikov teragertsovogo izlucheniya.

Theory seminar, Thursday, February 26, 2015, Kapitza IPP, 11:30 am
V.A. Khodel (Kurchatov Institute & Washington University)
Occurrence of flat bands in strongly correlated Fermi systems and high-Tc superconductivity of electron-doped compounds

We consider a class of strongly correlated Fermi systems that possess interaction-induced flat bands, pinned to the Fermi surface. We demonstrate that in such systems, the fundamental Landau equation, connecting the single-particle spectrum to the quasiparticle momentum distribution, fails. We propose a method, allowing to rectify drawbacks of Landau theory and, with the aid of the Pitaevskii identity, generalize equations obtained to apply the method to electron systems of solids. The emergent non-Fermi-liquid behavior, derived from the theory constructed, is compared with relevant experimental data on two-dimensional liquid He-3, heavy-fermion metals and electron-doped high-Tc compounds.

Scientific Council of the Landau Institute, Friday, February 20, 2015, Landau ITP, 11:30 am
S.V. Iordanskii
Vozniknovenie bol'shikh tokov v dvukhsloinykh geterostrukturakh
Theory seminar, Thursday, February 19, 2015, Kapitza IPP, 11:30 am
L. Levitov (MIT)
Topological Valley Currents in Gapped Dirac Materials

Gapped 2D Dirac materials, in which inversion symmetry is broken by a gap-opening perturbation, feature a unique valley transport regime. The system ground state hosts dissipationless persistent valley currents existing even when topologically protected edge modes are absent or when they are localized due to edge roughness. Topological valley currents in such materials are dominated by bulk currents produced by electronic states just beneath the gap rather than by edge modes. Dissipationless currents induced by an external bias are characterized by a quantized half-integer valley Hall conductivity. The under-gap currents dominate magnetization and the charge Hall effect in a light-induced valley-polarized state.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, February 12, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Sergei Vital'evich Shitov (IRE RAN & MISiS)
Sverkhprovodyashchie aktivnye metamaterialy dlya polucheniya izobrazheniya v teragertsovom diapazone

Razvitie metodov polucheniya izobrazheniya na SVCh i v oblasti dal'nego IK izlucheniya yavlyaetsya bystro rastushchei oblast'yu, ogranicheniem kotoroi na segodnya yavlyaetsya effektivnost' ustroistv schityvaniya. Predlozheno usovershenstvovat' sistemu schityvaniya s razdeleniem chastot dlya sverkhprovodyashchikh TES detektorov, otkazavshis' ot nabora SKVID-usilitelei v pol'zu odnogo shirokopolosnogo okhlazhdaemogo usilitelei SVCh na poluprovodnikakh, kotoryi mozhet obsluzhit' do 10 tysyach i bolee pikselei. Predlozheno nagruzit' mostikom TES vysokodobrotnyi SVCh rezonator, slabo svyazannyi s liniei peredachi, i registrirovat' otklik na chastote rezonansa v vide izmeneniya urovnya prokhozhdeniya, vyzvannogo razogrevom takogo mostika, analogichno MKID detektoram. Dannaya kontseptsiya byla proverena na mostikakh-prototipakh iz niobiya v diapazone temperatur 1-5 K, i bylo polucheno NEP = 2×10^{‑14} Vt/Gts^{0.5} pri kriticheskoi temperature plenok Tc=5 K v diapazone 600-700 GGts. Byla skonstruirovana metapoverkhnost' (matritsa), pozvolyayushchaya registrirovat' slabye signaly, a takzhe razrabotan printsip uvelicheniya razmernosti takoi matritsy.

S tsel'yu dal'neishego integrirovaniya tsepei priema (matritsy detektorov) i tsepei obrabotki signala (generatora nakachki i usilitelya), razrabatyvayutsya dva novykh aktivnykh metamateriala: dlya generatsii i usileniya signalov gigagertsovogo diapazona. Oba pribora osnovany na dvumernykh reshetkakh dzhozefsonovskikh kontaktov s parallel'no-posledovatel'nym vklyucheniem po postoyannomu toku, podstraivaemykh s pomoshch'yu magnitnogo polya. Sverkhprovodyashchii generator predstavlyaet soboi skhemu iz neshuntirovannykh tunnel'nykh dzhozefsonovskikh perekhodov, a usilitel', naoborot, iz shuntirovannykh kontaktov, imeyushchikh bezgisterezisnuyu VAKh. Dvukhmernye reshetki dopuskayut neogranichennoe uvelichenie sistemy, chto pozvolyaet uvelichit' diapazon moshchnosti signala, a takzhe optimizirovat' impedans. Detal'noe elektromagnitnoe modelirovanie pokazyvaet, chto generator sposoben rabotat' na chastotakh do 0,7 TGts s perestroikoi chastoty magnitnym polem okolo 2%, a usilitel' - s shumami u kvantovogo predela v diapazone chastot 8-12 GGts s usileniem okolo 10 dB.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, January 29, 2015, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Kupriyanov Mikhail Yur'evich (NIIYaF MGU)
Sverkhprovodnikovye spinovye ventili (Superconducting spin valves)

Rassmotreny osnovnye osobennosti effekta blizosti i effekta Dzhozefsona v strukturakh, sostoyashchikh iz sverkhprovodyashchikh, normal'nykh i ferromagnitnykh materialov. Dan kratkii obzor sovremennogo sostoyaniya issledovanii v oblasti razrabotki sverkhprovodnikovykh spinovykh ventilei. Obsuzhdeny vozmozhnye trudnosti prakticheskoi realizatsii takikh ustroistv.

Theory seminar, Thursday, January 29, 2015, Kapitza IPP, 11:30 am
Vadim Khrapai (IFTT RAN)
Energeticheskii transport v sisteme vstrechnykh kraevykh kanalov i model' Lattinzhera

Budut predstavleny rezul'taty nedavnego eksperimenta [1] po izucheniyu neravnovesnykh potokov energii mezhdu vstrechnymi kraevymi kanalami v rezhime kvantovogo effekta Kholla. Sistema iz dvukh takikh kanalov, razvyazannykh gal'vanicheski, no vzaimodeistvuyushchikh posredstvom kulonovskikh sil, yavlyaetsya pryamym analogom besspinovoi zhidkosti Lattinzhera [2] i ideal'no podkhodit dlya izucheniya energeticheskogo transporta. Original'nyi bolometricheskii podkhod k izmereniyu izbytochnogo potoka energii P v kraevom kanale pozvolil issledovat' zavisimosti P ot tyanushchego napryazheniya i protyazhennosti otrezka lattinzherovskoi zhidkosti. Pokazano, chto protsess peredachi energii kolichestvenno soglasuetsya s kontseptsiei uprugogo rasseyaniya kollektivnykh vozbuzhdenii plotnosti na granichnykh tochkakh lattinzherovskogo otrezka, pri etom imeetsya vozmozhnost' nepreryvnogo kontrolya parametra vzaimodeistviya i veroyatnosti rasseyaniya. Literatura: [1] M.G. Prokudina, S. Ludwig, V. Pellegrini, L. Sorba, G. Biasiol, V.S. Khrapai, Tunable non-equilibrium Luttinger liquid based on counter-propagating edge channels, Phys. Rev. Lett. 112, 216402 (2014). [2] Yuval Oreg and Alexander M. Finkel'stein, Interedge Interaction in the Quantum Hall Effect, Phys. Rev. Lett. 74, 3668 (1995).

Seminar v ITF, Friday, January 9, 2015, ITF, 11:00 am
Y. Asano (Hokkaido University, Japan)
Odd-frequency Cooper Pairs

The concept of the odd-frequency Cooper pairs was first proposed by Berezinskii in 1974 to describe superfluidity of 3He [1]. Although the odd-frequency superconductivity has never been confirmed in any materials, the odd-frequency pairs appear as the subdominant pairing correlation in various proximity structures such as superconductor / ferromagnet (SF) junctions [2] and normal-metal / superconductor (NS) junctions with unconventional pairing symmetry [3]. In the presentation, we explain what the odd-frequency pairs are, how they appear in such proximity structure, how to detect them, and how they are different from usual even-frequency pairs in the bulk. We first discuss the enhancement of the quasiparticle density of states at the zero-energy [2,4-5] and its influence on the low-energy electric transport [6]. Next we show the paramagnetic property of odd-frequency pairs [7-10] which can be confirmed in the surface impedance measurement at low temperature [11]. We also discuss a relation to the Majorana physics [12-13]. Finally we explain why such unusual pairs appear in inhomogeneous superconducting systems [14].

[1] V. L. Berezinskii, JETP Lett. 20, 287 (1974).
[2] F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Phys. Rev. Lett. 86, 4096 (2001) [3] Y. Tanaka and A. A. Golubov, Phys. Rev. Lett. 98, 037003 (2007).
[4] Y. Asano, Y. Tanaka, and A. A. Golubov, Phys. Rev. Lett.98, 107002 (2007).
[5] T. Kontos, et. al. Phys. Rev. Let. 86, 304 (2001).
[6] Y. Asano, et. al., Phys. Rev. Lett. 99, 067005 (2007).
[7] H. Walter, et. al., Phys. Rev. Lett. 80, 3598 (1998).
[8] S. Higashitani, J. phys. soc. Jpn., 66, 2556 (1997).
[9] S.-I. Suzuki and Y. Asano, Phys. Rev. B 89, 184508 (2014).
[10] Y. Tanaka, et. al., Phys. Rev. B 72, 140503(R) (2005).
[11] Y. Asano, et. al,, Phys. Rev. Lett. 107, 087001 (2011).
[12] Y. Asano and Y. Tanaka, Phys. Rev. B 87, 104513 (2013).
[13] S. Ikegaya, Y. Asano, and Y. Tanaka, arXiv: 1411.3438
[14] Y. Asano, Y. V. Fominov, Y. Tanaka, Phys. Rev. B 90, 094512 (2014).

Department of quantum mesoscopics: seminar, Friday, January 9, 2015, ITF, 11:00 am
Yasuhiro Asano (Hokkaido University, Japan)
Odd-frequency Cooper Pairs

The concept of the odd-frequency Cooper pairs was first proposed by Berezinskii in 1974 to describe superfluidity of 3He [1]. Although the odd-frequency superconductivity has never been confirmed in any materials, the odd-frequency pairs appear as the subdominant pairing correlation in various proximity structures such as superconductor / ferromagnet (SF) junctions [2] and normal-metal / superconductor (NS) junctions with unconventional pairing symmetry [3]. In the presentation, we explain what the odd-frequency pairs are, how they appear in such proximity structure, how to detect them, and how they are different from usual even-frequency pairs in the bulk. We first discuss the enhancement of the quasiparticle density of states at the zero-energy [2,4-5] and its influence on the low-energy electric transport [6]. Next we show the paramagnetic property of odd-frequency pairs [7-10] which can be confirmed in the surface impedance measurement at low temperature [11]. We also discuss a relation to the Majorana physics [12-13]. Finally we explain why such unusual pairs appear in inhomogeneous superconducting systems [14].

[1] V. L. Berezinskii, JETP Lett. 20, 287 (1974). [2] F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Phys. Rev. Lett. 86, 4096 (2001) [3] Y. Tanaka and A. A. Golubov, Phys. Rev. Lett. 98, 037003 (2007). [4] Y. Asano, Y. Tanaka, and A. A. Golubov, Phys. Rev. Lett.98, 107002 (2007). [5] T. Kontos, et. al. Phys. Rev. Let. 86, 304 (2001). [6] Y. Asano, et. al., Phys. Rev. Lett. 99, 067005 (2007). [7] H. Walter, et. al., Phys. Rev. Lett. 80, 3598 (1998). [8] S. Higashitani, J. phys. soc. Jpn., 66, 2556 (1997). [9] S.-I. Suzuki and Y. Asano, Phys. Rev. B 89, 184508 (2014). [10] Y. Tanaka, et. al., Phys. Rev. B 72, 140503(R) (2005). [11] Y. Asano, et. al,, Phys. Rev. Lett. 107, 087001 (2011). [12] Y. Asano and Y. Tanaka, Phys. Rev. B 87, 104513 (2013). [13] S. Ikegaya, Y. Asano, and Y. Tanaka, arXiv: 1411.3438 [14] Y. Asano, Y. V. Fominov, Y. Tanaka, Phys. Rev. B 90, 094512 (2014).

V.V.Shmidt superconductivity seminar, Thursday, December 25, 2014, MISIS, main building B-636, 3:00 pm
A.V.Ustinov (zaveduyushchii kafedroi fiziki Tekhnologicheskogo Instituta Karlsrue, Germaniya; zaveduyushchii laboratoriei «Svekhprovodyashchie metamaterialy» NITU «MISiS»)
Kvantovoe tunnelirovanie i termicheskaya aktivatsiya v SKVIDe

Izvestno, chto klassicheskii ob'ekt, nakhodyashchiisya v metastabil'noi potentsial'noi yame, ne mozhet pokinut' ee pri nulevoi temperature. Kvantovyi zhe ob'ekt mozhet tunnelirovat' iz takoi yamy dazhe pri nulevoi temperature. Voznikaet vopros, chto zhe proizoidet, esli kvantovaya stepen' svobody mozhet sochetat'sya s klassicheskoi stepen'yu svobody? Dannyi vopros byl teoreticheski rassmotren v rabote [1] dlya sluchaya slaboi svyazi mezhdu dvumya stepenyami svobody. Avtory obnaruzhili, chto tunnelirovanie kvantovomekhanicheskoi stepeni svobody menyaet potentsial klassicheskoi peremennoi, chto pozvolyaet takomu ob'ektu pokinut' metastabil'nuyu yamu. Etot effekt napominaet istoriyu o Barone Myunkhgauzene, kotoryi utverzhdal, chto vytashchil sebya i svoyu loshad' iz tryasiny, poetomu takoe teoreticheskoe predskazanie povedeniya bylo nazvano «effektom Myunkhgauzena» [1]. My popytalis' realizovat' predlozhennuyu sistemu shuntirovaniem odnogo iz dvukh perekhodov PT-SKVIDa dopolnitel'noi emkost'yu. V nashikh opytakh byla obnaruzhena svyaz' mezhdu kvantovymi i klassicheskimi protsessami pokidaniya yamy, chto svyazano s napravleniem dvizheniya v prostranstve energii. Bylo obnaruzheno, chto pri izmenenii vneshnego magnitnogo potoka makroskopicheskoe kvantovoe tunnelirovanie periodicheski smenyaetsya termoaktivirovannym vykhodom [2], chto yavlyaetsya priznakom «effekta Myunkhgauzena».

[1] A. U. Thomann, V. B. Geshkenbein and G. Blatter, Phys. Rev. B 79, 184515 (2009). [2] S. Butz, A.K. Feofanov, K.G. Fedorov, H. Rotzinger, A.U. Thomann, B. Mackrodt, R. Dolata, V.B. Geshkenbein, G. Blatter, and A.V. Ustinov, arXiv: 1404.3602 (2014), to appear in Phys. Rev. Lett. in December 2014.

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Na seminar priglashayutsya vse zhelayushchie. Propusk slushatelei seminara, ne yavlyayushchikhsya studentami, aspirantami ili sotrudnikami NITU «MISiS», budet osushchestvlyat'sya pri pred'yavlenii pasporta pri uslovii predvaritel'noi zapisi na seminar po ssylke http://smm.misis.ru/events/seminars/ (do 14:00 dnya seminara).

Seminar 25 dekabrya 2014 g. (15:00) sostoitsya v Auditorii – B-636 (glavnyi korpus) NITU «MISiS», 6 etazh.

Department of quantum mesoscopics: seminar, Friday, December 19, 2014, ITF, 3:00 pm
E. J. Koenig (Karlsruhe)
Half-integer quantum Hall effect of disordered Dirac fermions at a topological insulator surface

The unconventional (half-integer) quantum Hall effect for a single species of Dirac fermions is analyzed. In the introduction I will discuss experimental measurements of the half-integer Hall conductance g_{xy} of topological insulator surface states and unveil its field theoretical origin. For the calculation of the conductivity tensor, the following two step procedure is applied. In the first step, semiclassical transport coefficients are derived in the case of both orbital and Zeeman coupling to an external magnetic field. In this context, I plan to give a pedagogic review of the modified Boltzmann transport theory for the anomalous Hall effect. The semiclassical conductivity tensor serves as input parameters for the second step: It consists in the renormalization group treatment of the appropriately modified non-linear sigma model and thus accounts for localization effects. The resulting phase diagram (levitation scenario) will also be discussed in this context. If time permits, I will further explain how to reconcile Laughlin's flux insertion argument with half-integer Hall conductance and/or present the calculation of the current density beyond linear response in the limit of smooth scalar potential.

Scientific Council of the Landau Institute, Friday, December 19, 2014, Landau ITP, 11:30 am
Azat Sharafutdinov
Spin susceptibility and tunneling density of states in quantum dots

Detal'no issledovany staticheskie prodol'nye i dinamicheskie poperechnye spinovye vospriimchivosti v kvantovykh tochek i nanochastitsakh v ramkakh gamil'toniana, kotoryi yavlyaetsya rasshireniem universal'nogo gamil'toniana na sluchai odnoosnogo anizotropnogo obmena. Dlya predel'nykh sluchaev obmennogo vzaimodeistviya tipa Izinga i Geizenberga, izucheno kak fluktuatsii odnochastichnykh urovnei energii vliyayut na stounerovskuyu neustoichivost' v kvantovykh tochkakh. Okazyvaetsya, chto v rezhime sil'nykh fluktuatsii zadacha svoditsya k zadache o statistike ekstremumov nekotorogo gaussovskogo protsessa. Dokazano, chto, nesmotrya na sil'nye fluktuatsii odnochastichnykh urovnei energii, spinovaya vospriimchivost' i vse ee momenty raskhodyatsya odnovremenno v tochke, kotoraya opredelyaetsya s pomoshch'yu standartnogo kriteriya neustoichivosti Stounera s uchastiem tol'ko srednego rasstoyanie mezhdu odnochastichnymi urovnyami energii. Takzhe issledovano povedenie tunnel'noi plotnosti sostoyanii v sluchae anizotropnogo obmena. Pokazano, chto predskazannoe v rabote Y. Gefen, M. Kiselev, PRL96, 066805 (2006) nemonotonnoe (s dvumya maksimumami i minimumom) povedenie tunnel'noi plotnosti sostoyanii, kak funktsii energii, na samom dele ne sushchestvuet. Podrobno, nashi rezul'taty predstavleny v sleduyushchikh rabotakh: 1. D.S. Lyubshin, A.U. Sharafutdinov, I.S. Burmistrov, «Statistics of spin fluctuations in quantum dots with Ising exchange», Phys. Rev. B 89, 201304(R ) (2014). 2. A.U. Sharafutdinov, D.S. Lyubshin, I.S. Burmistrov, «Spin fluctuations in quantum dots», Phys. Rev. B 90, 195308 (2014). 3. A.U. Sharafutdinov, I.S. Burmistrov, «Tunneling density of states in quantum dots with anisotropic exchange», gotovitsya k otpravke v pechat'.

Theory seminar, Thursday, December 18, 2014, Kapitza IPP, 11:30 am
G.B. Lesovik
Shumy i statistika perenosa elektronov v kvantovykh provodnikakh pri nalichii peremennykh polei i vozmozhnosti sozdaniya odnochastichnykh izolirovannykh vozbuzhdenii

V pervoi chasti doklada budet sdelan nebol'shoi obzor teoreticheskikh rabot po shumam i statistike perenosa v tselom. Zatem obsudim teoreticheskie raboty, v kotorykh byli opisany vozmozhnosti sozdaniya odnochastichnykh izolirovannykh vozbuzhdenii (govorya bolee tochno, vozbuzhdeniya neobychnoi pary elektron-dyrka pri pomoshchi impul'sa vektornogo potentsiala lorentsevskoi formy), ne zaputannykh za fermi more. Vo vtoroi chasti budet obsuzhdat'sya eksperiment gruppy Glattli (opublikovano v 2013 g., Nature), v kotorom anonsirovano nablyudenie sootvetstvuyushchikh vozbuzhdenii. Budut izlozheny argumenty v pol'zu togo, chto v etom eksperimente usloviya dlya sozdaniya sootvetstvuyushchikh vozbuzhdenii (i ikh registratsii) ne vypolnyayutsya. Tem ne menee v eksperimente opredelenno nablyudalos' drugoe yavlenie, svyazannoe s katastrofoi ortogonal'nosti dlya fermi morya, pri opredelennykh formakh impul'sa.

Department of quantum mesoscopics: seminar, Friday, December 12, 2014, ITF, 3:00 pm
I. V. Zagorodnev (IRE RAN)
O vozmozhnykh spektrakh poverkhnostnykh sostoyanii v 2D i 3D topologicheskikh izolyatorakh

Obychno schitaetsya, chto v topologicheskikh izolyatorakh (TI) spektr poverkhnostnykh sostoyanii imeet besshchelevoi lineinyi (ul'trarelyativistskii) spektr v tsentre zony Brillyuena. Takoi spektr deistvitel'no poluchaetsya v prosteishikh modelyakh pri ispol'zovanii “otkrytykh granichnykh uslovii” (open boundary conditions) [X.-L. Qi, S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011)], t.e. granichnykh uslovii zanulyayushchikh volnovuyu funktsiyu na granitse. Odnako, izvestno, chto effektivnaya volnovaya funktsiya, opisyvayushchaya elektrony v kristalle, ne obyazana zanulyat'sya na granitse [E.L. Ivchenko “Optical Spectrosopy of Semiconductor Nanostructires”, Alpha Science, Harrow, UK (2005) i ssylki v nei]. Teoriya TI, a tochnee “printsip sootvetstviya granitsa-ob'em” (bulk boundary correspondence) predskazyvaet nalichie kraevykh sostoyanii v ob'emnoi shcheli, no ne sam spektr, kotoryi dolzhen zaviset' ot svoistv i mikrostruktury poverkhnosti, chto ne uchityvaetsya otkrytymi granichnymi usloviyami. V nashei rabote arXiv:1407.0945 byli proanalizirovany vozmozhnye granichnye usloviya v 2D TI na osnove kvantovykh yam CdHgTe i v 3D TI Bi2Se3. Pokazano, chto poverkhnostnye sostoyaniya mogut imet' sushchestvenno ne lineinyi spektr ili vovse otsutstvovat' v tsentre zony Brillyuena.

Scientific Council of the Landau Institute, Friday, December 12, 2014, Landau ITP, 11:30 am
Ya.V. Fominov
Dva tipa nechetnoi po chastote sverkhprovodimosti

Printsip Pauli privodit k antisimmetrii anomal'noi funktsii Grina, opisyvayushchei sverkhprovodimost', pri perestanovke koordinat elektronov. Polnyi nabor koordinat vklyuchaet v sebya, pomimo prostranstvennykh koordinat i proektsii spinov, takzhe i vremya. V matsubarovskom predstavlenii funktsiya Grina, v printsipe, mozhet byt' nechetnoi po chastote, chto bylo otmecheno Berezinskim. Zatem teoreticheski bylo pokazano, chto nechetnye po chastote sverkhprovodyashchie korrelyatsii mogut voznikat' v neodnorodnykh kontaktakh sverkhprovodnikov i ferromagnetikov kak subdominantnye komponenty vblizi granitsy. Neobychnoi osobennost'yu takogo sostoyaniya yavlyaetsya "paramagnitnyi" (protivopolozhnyi obychnomu) znak svyazi mezhdu vektornym potentsialom i sverkhtokom (anomal'nyi effekt Meissnera). V ob'emnom sverkhprovodnike takoe sostoyanie bylo by neustoichivym. Nedavno neskol'kimi gruppami byla predlozhena vozmozhnost' realizatsii ob'emnogo nechetnogo po chastote sverkhprovodyashchego sostoyaniya s obychnym diamagnitnym meissnerovskim otklikom. V doklade budut obsuzhdeny fizicheskie sledstviya takogo predpolozheniya.

Shmidtovskii seminar po sverkhprovodimosti, Thursday, December 11, 2014, MISiS, auditoriya B-636 (glavnyi korpus),, 3:00 pm
Kuz'min Leonid Sergeevich (Chalmers University of Technology, Sweden; NSTU, N. Novgorod; SINP, MSU, Moscow)
Sverkhprovodyashchii Bolometr na Kholodnykh Elektronakh: Fizika i Primeneniya

Predlozhena novaya kontseptsiya Bolometra na Kholodnykh Elektronakh (BKhE) dlya sverkhchuvstvitel'nogo priema radioastronomicheskikh signalov. V etom bolometre realizovan effekt pryamogo elektronnogo okhlazhdeniya absorbera, chto pozvolyaet uluchshit' shumovye svoistva i rasshirit' dinamicheskii diapazon bolometra po sravneniyu s bolometrami na krayu sverkhprovodyashchego perekhoda. BKhE okazalsya nechuvstvitel'nym k kosmicheskim lucham iz-za malosti ob'ema nanoabsorbera i printsipial'noi razvyazki elektronnoi i fononnoi sistem. V rezul'tate interes k BKhE rezko vozros dlya primenenii v kosmicheskikh i ballonnykh proektakh.

Vtoroi original'noi kontseptsiei, predlozhennoi dlya realizatsii v evropeiskikh kosmicheskikh proektakh, yavlyaetsya Rezonansnyi Bolometr na Kholodnykh Elektronakh (RBKhE), ispol'zuyushchii vnutrennii nanofil'tr iz kineticheskoi induktivnosti sverkhprovodyashchei poloski i emkosti tunnel'nogo perekhoda. Stimuliruyushchei prichinoi dlya izobreteniya RBKhE yavilsya zapros Evropeiskogo Kosmicheskogo Agenstva na sovmeshchenie v odnom piksele dvukh i bolee chastot dlya umen'sheniya razmera fokal'noi ploskosti i sravneniya signalov raznykh chastot.

Budet obsuzhdeno primenenie etikh kontseptsii v ballonnykh i kosmicheskikh proektakh.

Na seminar priglashayutsya vse zhelayushchie. Propusk slushatelei seminara, ne yavlyayushchikhsya studentami, aspirantami ili sotrudnikami NITU «MISiS», budet osushchestvlyat'sya pri pred'yavlenii pasporta pri uslovii predvaritel'noi zapisi na seminar po ssylke http://smm.misis.ru/events/seminars/ (do 14:00 dnya seminara).

Seminar 11 dekabrya 2014 g. (15:00) sostoitsya v Auditorii – B-636 (glavnyi korpus) NITU «MISiS», 6 etazh.

MIPT Interdisciplinary center for basic research: colloquium, Wednesday, December 10, 2014, MFTI, GK 113, 5:00 pm
Leonid Levitov (MIT)
Atomnyi kollaps v grafene

Budet rassmotreno odno iz interesnykh yavlenii - atomnyi kollaps, predstavlyayushchii soboi spontannoe rozhdenie elektron-pozitronnykh par v sil'nykh elektricheskikh polyakh vblizi sverkhtyazhelykh atomnykh yader. Dokladchik podrobno rasskazhet o simmetriinykh aspektakh kollapsa, svyazannykh s anomal'nym narusheniem masshtabnoi invariantnosti, a takzhe o pervom nablyudenie etogo yavleniya v 2013 godu.

Podrobnaya informatsiya

Ob'yavlenie o doklade na saite FOPF

Department of quantum mesoscopics: seminar, Friday, December 5, 2014, ITF, 3:00 pm
V. V. Sivak, A. S. Ioselevich
"Polzuchaya" provodimost' i relaksatsiya v sisteme metallicheskikh granul

My rassmatrivaem sistemu provodnikov, pomeshchennykh v dielektricheskuyu matritsu. Matritsa ne predpolagaetsya sovershenno zhestkoi (naprimer, eto mozhet byt' zhidkost'), poetomu tolshchiny dielektricheskikh zazorov mezhdu provodnikami fluktuiruyut vo vremeni i inogda provodniki mogut soprikasat'sya. V takie momenty voznikaet "proboi" i zaryad legko peretekaet mezhdu paroi soprikosnuvshikhsya provodnikov. Protsess perenosa zaryada za schet proboev, voznikayushchikh v sluchainye momenty v sluchainykh mestakh, my nazvali polzuchei provodimost'yu. My rassmatrivaem kak trekhmernye, tak i nizkorazmernye massivy provodnikov i issleduem rastekanie zaryada, pervonachal'no sosredotochennogo na odnom provodnike, provodimost' na postoyannom toke, fluktuatsii zaryadov na provodnikakh i ikh prostranstvenno-vremennuyu korrelyatsiyu.

Scientific Council of the Landau Institute, Friday, December 5, 2014, Landau ITP, 11:30 am
Boris Fain, Skolkovskii Institut Nauki i Tekhnologii
Obrashchaya khaos vspyat'

Ponyatie "khaos" chasto upominaetsya v kontekste osnov statisticheskoi fiziki. V to zhe vremya, do sikh por ne do kontsa yasno, chto nazyvat' khaosom v mnogochastichnykh kvantovykh sistemakh. Sushchestvuet rasprostranennoe mnenie, chto povedenie kvantovykh sistem yavlyaetsya klassicheski khaotichnym v makroskopicheskom predele na fizicheski nablyudaemykh (t.e. ne slishkom dlinnykh) vremenakh. V etom doklade budet pokazano, chto, dazhe v makroskopicheskom predele i na fizicheski nablyudaemykh vremenakh, tipichnye neintegriruemye sistemy vzaimodeistvushchikh kvantovykh spinov ne obladayut opredelyayushchim svoistvom klassicheskogo khaosa, a imenno, eksponentsial'moi chuvstvitel'nost'yu k malym vozmushcheniyam. Etot rezul'tat poluchen putem sravneniya povedeniya sistem klassicheskikh i kvantovykh spinov v otvet na nepolnoe obrashchenie spinovoi dinamiki vo vremeni. Takoe obrashchenie vo vremeni izvestno v statisticheskoi fizike kak "Loshmidtovskoe ekho" i realizuemo metodami yadernogo magnitnogo rezonansa kak "magicheskoe ekho". V poslednem sluchae, otsutstvie eksponentsial'noi chuvstvitel'nosti k malym vozmushcheniyam dolzhno byt' eksperimental'no nablyudaemo. [B. V. Fine, T. A. Elsayed, C. M. Kropf, and A. S. de Wijn, Phys. Rev. E 89, 012923 (2014)]

Department of quantum mesoscopics: seminar, Friday, November 28, 2014, ITF, 3:00 pm
Lev Ioffe
Nonergodicity and many body localization in Josephson junction chains
Scientific Council of the Landau Institute, Friday, November 28, 2014, Landau ITP, 12:00 pm
K.S. Tikhonov, M.V. Feigel’man
Admittance of a long diffusive SNS junction

Issledovany dinamicheskie svoistva SNS perekhoda s zadannoi raznost'yu faz sverkhprovodnikov na dvukh beregakh pod deistviem slabogo vysokochastotnogo vneshnego napryazheniya. Ispol'zovan glavnym obrazom analiticheskii metod, osnovannyi na reshenii nestatsionarnogo uravneniya Uzadelya, okonchatel'nye vychisleniya provedeny chislenno. Vychislena veshchestvennaya i mnimaya chasti lineinoi vysokochastotnoi provodimosti Y(ω) kak funktsii staticheskoi raznosti faz φ pri proizvol'nom sootnoshenii mezhdu chastotoi ω, temperaturoi T, energiei Taulesa ETh, no predpolagaya, chto vse eti velichiny maly po sravneniyu so shchel'yu Δ v spektre sverkhprovodnika. Poluchennye rezul'taty nakhodyatsya v khoroshem soglasii s eksperimentom.
[Napravleno v Phys. Rev. B., oktyabr' (2014)]

V.V.Shmidt superconductivity seminar, Thursday, November 27, 2014, MISIS, 3:00 pm
A.A.Golubov, MIPT and University of Twente (the Netherlands)
Fazo-chuvstvitel'nye eksperimenty po opredeleniyu simmetrii parametra poryadka v sverkhprovodyashchikh ferropniktidakh

Sformulirovana teoreticheskaya model' dlya opisaniya kontaktov mezhdu obychnymi nizkotemperaturnymi sverkhprovodnikami i sverkhprovodyashchimi materialami na osnove ferropniktidov, v kotorykh predskazana vozmozhnaya realizatsiya nefononnogo mekhanizma sverkhprovodyashchego sparivaniya. Obsuzhdaetsya rol' primesnogo rasseyaniya, v chastnosti, vozmozhnost' fazovogo perekhoda mezhdu razlichnymi simmetriinymi sostoyaniyami parametra poryadka v zavisimosti ot sily rasseyaniya. Obsuzhdayutsya vozmozhnye fazo-chuvstvitel'nye eksperimenty po opredeleniyu simmetrii parametra poryadka, kotorye pozvolyat opredelit' mekhanizm sverkhprovodyashchego sparivaniya s dannykh materialakh. Eksperimenty osnovany na ispol'zovanii Dzhozefsonovskikh kontaktov mezhdu obychnymi sverkhprovodnikami i sverkhprovodyashchimi materialami na osnove ferropniktidov. Sformulirovany usloviya na parametry , kotorye pozvolyat proizvesti takie eksperimenty.

----------------------- Na seminar priglashayutsya vse zhelayushchie. Propusk slushatelei seminara, ne yavlyayushchikhsya studentami, aspirantami ili sotrudnikami NITU «MISiS», budet osushchestvlyat'sya pri pred'yavlenii pasporta pri uslovii predvaritel'noi zapisi na seminar po ssylke http://smm.misis.ru/events/seminars/ (do 14:00 dnya seminara).

Seminar 27 noyabrya 2014 g. (15:00) sostoitsya v Auditorii – B-636 (glavnyi korpus) NITU «MISiS», 6 etazh.

Department of quantum mesoscopics: seminar, Friday, November 21, 2014, Landau ITP, 3:00 pm
Daniil Antonenko
Kvantovyi effekt Kholla v grafene

Obzor rabot

Nature 505, 528 (2014) Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state, A F Young, J D Sanchez-Yamagishi, B Hunt, S H Choi, K Watanabe, T Taniguchi, R C Ashoori and P Jarlillo-Herrero,

Nat Phys 8, 550 (2012) Spin and valley quantum Hall ferromagnetism in graphene, A F Young, C R Dean, L Wang, H Ren, P Cadden-Zimansky, K Watanabe, T Taniguchi, J Hone, K L Shepard and P Kim

Department of quantum mesoscopics: seminar, Thursday, November 20, 2014, Kaptiza Institute, 11:30 am
K. S. Tikhonov
Obzor nedavnikh rabot po mnogochastichnoi lokalizatsii
Department of quantum mesoscopics: seminar, Friday, November 14, 2014, Landau ITP, 3:00 pm
Eslam Khalaf (MPI FKF Stuttgart)
Localization at the edge of a 2D topological insulator

We study transport via edge modes of a 2D topological insulator. Topological protection prevents complete localization of the edge states; however, quantum interference effects are still relevant for the transport properties at finite length scales. We mainly focus on the two most experimentally relevant cases: (i) a junction between two quantum Hall insulators with different filling factors and hence an imbalance in the number of right- and left-propagating modes (symmetry class A) and (ii) a relatively thick HgTe quantum well in the insulating state with an arbitrary number of edge modes (symmetry class AII). We derive the distribution of transmission probabilities as a function of the distance between leads. This allows us to demonstrate topological effects in the average conductance and the shot noise of the setup. We also consider mesoscopic fluctuations and compute the variance of conductance. This quantity is strongly influenced by topology in the quantum Hall case. All the calculations are carried out assuming localization effects are weak, i.e., in the short length limit. Technically, this amounts to studying 1D non-linear sigma model with a proper topological term and source fields on the semiclassical level. Remarkably, the semiclassical limit of the 1D sigma model can be exactly mapped onto a fully quantum 0D sigma model of a different symmetry class. This allows us to identify the distribution of transmission probabilities with the spectrum of a certain random matrix.

Scientific Council of the Landau Institute, Friday, November 14, 2014, Landau ITP, 11:30 am
P. M. Ostrovsky
Density of states in a two-dimensional chiral metal with vacancies

We study quantum interference effects in a two-dimensional chiral metal (bipartite lattice) with vacancies. We demonstrate that randomly distributed vacancies constitute a peculiar type of chiral disorder leading to strong modifications of critical properties at zero energy as compared to conventional chiral metals. In particular, the average density of states diverges as ρ ~ E-1 |lnE|-3/2 and the correlation length Lc ~ |lnE| in the limit E→0. When the average density of vacancies is different in the two sublattices, a finite concentration of zero modes emerges and a gap in the quasiclassical density of states opens around zero energy. Interference effects smear this gap resulting in exponentially small tails at low energies.

V.V.Shmidt superconductivity seminar, Thursday, November 13, 2014, MISIS, 3:00 pm
E.V.Il'ichev, group head at Quantum detection laboratory, Institute of Photonic technology , Jena, Germany
Superconducting detectors

V pervoi chasti doklada budut predstavleny printsipy raboty i osnovnye kharakteristiki sverkhprovodyashchikh detektorov, takikh kak bolometry i SKVIDy. Budut obsuzhdat'sya nekotorye napravleniya, v kotorykh sverkhprovodyashchie datchiki nashli shirokoe primenenie.

Vtoraya chast' doklada budet posvyashchena original'nym rezul'tatam, poluchennym gruppoi, kotoroi rukovodit avtor. Oni osnovany na uzkom raspredelenii pereklyuchenii neshuntirovannogo dzhozefsonovskogo tunnel'nogo kontakta iz sostoyaniya s nulevym napryazheniem v sostoyanie s konechnym napryazheniem pri temperaturakh nizhe 1 K. Takoe povedenie kontakta mozhet byt' ispol'zovano dlya realizatsii sverkhchuvstvitel'nykh detektorov v gigagertsovom diapazone. Budut predstavleny teoreticheskie otsenki i pervye eksperimental'nye dannye.

Theory seminar, Thursday, October 30, 2014, Kapitza IPP, 11:30 am
Andrei Golov (School of Physics and Astronomy, The University of Manchester, UK)
Dissipation of turbulence in superfluid 4He in the limit of zero temperature

We will review Manchester experiments on quantum turbulence in superfluid 4He generated by various means. The main focus is on the limit of zero temperatures, in which the turbulence is fully represented by tangled quantized vortex lines. In this regime, the dynamics of the vortex lines span length scales from the size of container to nearly atomic scale, at which energy is transmitted to phonons. A quantum cascade of energy, involving individual quantized vortex lines (and hence, having no analogs in classical turbulence), is necessary for the energy of large-scale flow to be transferred to the dissipative short scales. The processes maintaining the cascade are believed to be: hydrodynamic interactions of vortex lines, reconnections of vortex lines, interaction of excitations such as small vortex loops and Kelvin waves on vortex lines. The dissipative length scale can be increased (i.e. the quantum cascade curtailed) by increasing temperature above 0.4 K due to the scattering of thermal excitations by vortex lines. Three different means of generating turbulence will be discussed (each having an analog in classical turbulence, thus allowing comparison with classical results): (a) With the towed grid, we produce turbulence that is nearly homogeneous and isotropic. Studying the free decay allows the rate of dissipation to be quantified [6]. (b) An unsteady rotation of a square-shaped container allows to create anisotropic turbulence — that decays more slowly [1,5,6]. In the presence of the steady background rotation, the following phenomena are observed: steady polarization of vortex lines, non-zero threshold for turbulence onset, resonances of inertial waves [4]. (c) With an immersed jet of variable duration and intensity (created by a current of injected electrons), a cross-over from ultraquantum («non-structured» or «Vinen») turbulence to quasiclassical («structured» or «Kolmogorov») turbulence is observed [2,3]. (References: [1] P. M. Walmsley, A. I. Golov, H. E. Hall, A. A. Levchenko and W. F. Vinen, Dissipation of quantum turbulence in the zero-temperature limit, Phys. Rev. Lett. 99, 265302 (2007). [2] P. M. Walmsley and A. I. Golov, Quantum and quasiclassical types of superfluid turbulence, Phys. Rev. Lett. 100, 245301 (2008). [3] A. I. Golov, P. M. Walmsley, P. Tompsett, Charged Tangles of Quantized Vortices in Superfluid 4He, J. Low Temp. Phys. 161, 509-525 (2010). [4] P. M. Walmsley and A. I. Golov, Rotating quantum turbulence in superfluid 4He in the T=0 limit, Phys. Rev. B 86, 060518(RC) (2012). [5] Paul Walmsley, Dmitry Zmeev, Fatemeh Pakpour, and Andrei Golov, Dynamics of quantum turbulence of different spectra, PNAS 111 (Supplement 1) 4691-4698 (2014). [6] D. E. Zmeev, P. M. Walmsley, A. I. Golov, P. V. E. McClintock, S. N. Fisher, and W. F. Vinen, Turbulence in superfluid 4He generated by various means (in preparation).)

Department of quantum mesoscopics: seminar, Friday, October 24, 2014, Landau ITP, 3:00 pm
Tat'yana Golikova (IFTT)
Effekt blizosti i kogerentnye yavleniya v gibridnykh strukturakh sverkhprovodnik-normal'nyi metall-ferromagnetik

Rabota posvyashchena izgotovleniyu submikronnykh planarnykh S-N-S dzhozefsonovskikh kontaktov, planarnykh gibridnykh struktur na osnove sverkhprovodnika, normal'nogo metalla i ferromagnetika, eksperimental'nomu issledovaniyu osobennostei effekta blizosti i sverkhprovodyashchego transporta cherez takie struktury, realizatsii S-(N/F)-S - dzhozefsonovskogo kontakta s ferromagnitnym monodomennym podsloem (F), issledovaniyu vliyaniya ferromagnitnogo podsloya na parnyi i kvazichastichnyi transport cherez dzhozefsonovskii S-(N/F)-S kontakt, izucheniyu nelokal'nykh neravnovesnykh effektov v mnogoterminal'nykh SNS strukturakh, svyazannykh s inzhektsiei kvazichastits v odin iz beregov dzhozefsonovskogo S-N-S kontakta.

Theory seminar, Thursday, October 23, 2014, Kapitza IPP, 11:30 am
V.M. Pudalov (LPI)
Electron-electron interaction effects in monotonic and oscillatory magnetotransport in 2D electron systems

Studying the oscillatory magnetorestance in crossed fields, we found that the product (m*TD) that determines damping of quantum oscillations, to the first approximation is equal in the majority and minority subbands even though the spin polarization degree amounts to about 66%. This result confirms theory predictions that the interaction takes place at high energies > EF rather than within narrow strip of energies EF ± T. To the next approximation, we revealed a difference in the damping factor of the two spin subbands, which causes skewness of the oscillation lineshape. The difference, quantified with the skew factor can be as high as 20%. The skew factor decreases as B or temperature grow, or B decreases; for low electron densities and high in-plane field the skew factor even changes sign. In contrast to the conventional theory, the product (m*TD) varies with perpendicular field and (nonmonotonically) with temperature. These dependencies explain notable scattering of the m*(n) values experimentally obtained under assumption of the T-independent (m*TD).

The monotonic magnetoconductance in the in-plane field δσ(B,T) was found to scales in a sharp contrast to the theory predictions: whereas below a density dependent temperature T<T* it scales as theory predicted, (B2/T), at higher temperatures T>T*, it scales as (B2/T2). The latter dependence, hence, mimics the behavior anticipated for the low-temperature diffusive regime of interaction. These functional dependencies are at odd with interaction quantum corrections. The crossover temperature T*(n) correlates well with the inflection point at the strong σ(T) dependence in zero field, inherent for high mobility samples. Our data thus point at the existence of an energy scale T* beyond the EF. The results also call in question the previous attempts to exploit the parallel field MR as a tool to determine Fermi liquid coupling constants, F0a and γ2, and to plot the two-parameter phase diagrams for interacting and disordered 2D electron systems.

Department of quantum mesoscopics: seminar, Friday, October 10, 2014, Landau ITP, 3:00 pm
Igor' Gornyi
TBA
Scientific Council of the Landau Institute, Friday, October 10, 2014, Landau ITP, 11:30 am
Alexander Shnirman (KIT), Yuval Gefen, Arijit Saha, Igor S. Burmistrov, Mikhail N. Kiselev, Alexander Altland
Ambegaokar-Eckern-Schön theory for a collective spin: geometric Langevin noise

We consider small ferromagnetic particles or quantum dots close to Stoner instability, and investigate the dynamics of the total magnetization in the presence of tunneling coupling to the metallic leads. We generalize the Ambegaokar-Eckern-Sch\"on effective action and the corresponding semiclassical equations of motion from the U(1) case of the charge degree of freedom to the SU(2) case of the magnetization. The Langevin forces (torques) in these equations are strongly influenced by the geometric phase. As a first but nontrivial application we predict low temperature quantum diffusion of the magnetization on the Bloch sphere, which is governed by the geometric phase. We propose a protocol for experimental observation of this phenomenon.

Theory seminar, Thursday, October 9, 2014, Kapitza IPP, 11:30 am
Leonid Pryadko (Kaliforniiskii Universitet, Riversaid)
Kvantovye nizkoplotnostnye kody i nelokal'nye spinovye modeli

Lokal'nost' — odin iz osnovopolagayushchikh printsipov postroeniya fizicheskikh modelei. Tak, v modeli Izinga, kazhdyi chlen v gamil'toniane mozhet sootvetstvovat' svyazi mezhdu blizhaishimi sosedyami na zadannoi reshetke. Chto poluchitsya esli uslovie lokal'nosti opustit'? Ya otvechu na etot vopros na primere neuporyadochennykh spinovykh modelei opisyvayushchikh protseduru dekodirovaniya kvantovykh nizkoplotnostnykh kodov. Takim obrazom mogut byt' polucheny kak traditsionnye lokal'nye spinovye modeli (model' Izinga, kalibrovochnaya model' Z2), tak i netrivial'nye nelokal'nye modeli kotorye ne byli izucheny ranee. Uporyadochennaya faza v takikh modelyakh sootvetstvuet nalichiyu glubokogo minimuma svobodnoi energii; “post-topologicheskimi” vozbuzhdeniyami yavlyayutsya protyazhennye defekty obobshchayushchie ponyatie domennoi stenki. Izvestno neskol'ko konstruktsii kvantovykh nizkoplotnostnykh kodov s konechnoi skorost'yu. V sootvetstvuyushchikh spinovykh modelyakh chislo razlichnykh tipov defektov eksponentsial'no veliko. V takikh modelyakh vozmozhen chisto entropiinyi fazovyi perekhod, kogda analog poverkhnostnogo natyazheniya dlya defektov kazhdogo tipa ostaetsya konechnym.

[A. A. Kovalev and L. P. Pryadko, “Spin glass reflection of the decoding transition for quantum error correcting codes”, arXiv:1311.7688]

MIPT Interdisciplinary center for basic research: colloquium, Wednesday, October 8, 2014, MIPT, GK 113, 5:00 pm
A.D.Mirlin (Karlsruhe Institute of Technology, Germany)
Anderson localization: multifractality, symmetry, topology and interaction

V doklade budet predstavlen obzor sovremennogo sostoyaniya issledovanii v oblasti andersonovskoi lokalizatsii v neuporyadochennykh sistemakh. V chastnosti, budut obsuzhdeny: mul'tifraktal'nost' volnovykh funktsii v okrestnosti perekhodov Andersona ili mezhdu razlichnymi fazami topologicheskikh izolyatorov, klassy simmetrii neuporyadochennykh sistem i rol' simmetrii v lokalizatsii, effekty topologii, topologicheskie izolyatory i sverkhprovodniki, rol' mezhelektronnogo vzaimodeistviya.

Department of quantum mesoscopics: seminar, Friday, September 26, 2014, Landau ITP, 3:00 pm
S. V. Zaitsev-Zotov (IRE RAN)
Vliyanie strukturnykh defektov i atomnykh stupenei na lokal'nuyu plotnost' sostoyanii atomno-chistoi poverkhnosti topologicheskogo izolyatora Bi2Se3

Predstavleny rezul'taty issledovaniya atomno-chistoi poverkhnosti topologicheskogo izolyatora Bi2Se3 metodami sverkhvysokovakuumnoi nizkotemperaturnoi skaniruyushchei tunnel'noi mikroskopii (STM) i spektroskopii (STS). Obnaruzheny defekty glubina zaleganiya i ozhidaemyi zaryad kotorykh sootvetstvuet r-tipu provodimosti ob'ema kristalla. Znak termo-eds takzhe sootvetstvuet r-tipu provodimosti. Rezul'taty issledovanii metodami tunnel'noi spektroskopii pokazyvayut, chto uroven' khimicheskogo potentsiala nakhoditsya vnutri zapreshchennoi zony ob'ema. Obnaruzheno izmenenie lokal'noi plotnosti sostoyanii vblizi kraya pyatisloinoi stupeni, kotoroe sootvetstvuet sdvigu polozheniya vershiny dirakovskogo konusa otnositel'no ob'emnoi zony ili zhe takzhe sdvigu khimicheskogo potentsiala vblizi kraya stupeni.

po rabote A.Yu. Dmitriev et al., http://arxiv.org/abs/1408.4991

Scientific Council of the Landau Institute, Friday, September 26, 2014, Landau ITP, 11:30 am
Maksym Serbyn, Z. Papic, Dmitry Abanin
Quantum quenches in the many-body localized phase

Many-body localized (MBL) systems are characterized by the absence of transport and thermalization, and therefore cannot be described by conventional statistical mechanics. In this talk I will discuss the behaviour of local observables in an isolated MBL system following a quantum quench. For the case of a global quench, the local observables reach stationary, highly non-thermal values at long times as a result of slow dephasing characteristic of the MBL phase. These stationary values retain the local memory of the initial state due to the existence of local integrals of motion in the MBL phase. The temporal fluctuations around stationary values exhibit universal power-law decay in time, with an exponent set by the localization length and the diagonal entropy of the initial state. Such a power-law decay holds for any local observable and is related to the logarithmic in time growth of entanglement in the MBL phase. This behaviour distinguishes the MBL phase from both the Anderson insulator (where no stationary state is reached), and from the ergodic phase (where relaxation is expected to be exponential). Quench protocols considered in this talk can be naturally implemented in systems of ultra cold atoms in disordered optical lattices, and the behaviour of local observables provides a direct experimental signature of many-body localization. [arXiv:1408.4105; arXiv:1403.0693]

Theory seminar, Thursday, September 25, 2014, Kapitza IPP, 11:30 am
V.E. Kravtsov (Abdus Salam ICTP, Trieste)
Anderson localization models on Bethe Lattice and Random Regular Graphs: is the extended state always ergodic?
MIPT Interdisciplinary center for basic research: colloquium, Wednesday, September 24, 2014, MIPT, GK 113, 5:00 pm
V.E.Kravtsov (ICTP, Trieste, Italy)
Statistical properties of wave functions: disorder, fractality, localization
Department of quantum mesoscopics: seminar, Friday, September 19, 2014, Landau ITP, 3:00 pm
M. V. Feigel'man
Raznoe
Theory seminar, Thursday, September 18, 2014, Kapitza IPP, 11:30 am
A.Yu. Kuntsevich (LPI)
Entropy measurements in 2D systems

In this talk I am going to discuss the experimental technique to measure entropy-per-electron in gated 2D systems, the we recently developed. We applied this technique to study magnetooscillations in Si and GaAs-based 2D systems and zero-perpendicular field effects. The latter originate from: (i) non-degeneracy of the Fermi system and (ii) electron-electron correlations. In particular, we explore enhanced effective mass in non-degenerate strongly correlated plasma regime EF < T < U (where U is the Coulomb energy e2 / ‹ r ›).

Department of quantum mesoscopics: seminar, Friday, September 12, 2014, Landau ITP, 3:00 pm
Artem Sboychakov (ITAE RAS, Moscow)
AA-stacked bilayer graphene in an applied electric field: Tunable
antiferromagnetism and coexisting exciton order parameter

We study electronic properties of AA-stacked bilayer graphene. In the single-particle approximation such a system has one electron band and one hole band crossing the Fermi level. If the bilayer is undoped, the Fermi surfaces of these bands coincide. Such a band structure is unstable with respect to a set of spontaneous symmetry violations. Specifically, strong on-site Coulomb repulsion stabilizes antiferromagnetic order. At small doping and low temperatures, the homogeneous phase is unstable and experiences phase separation into an undoped antiferromagnetic insulator and a metal. The metallic phase can be either antiferromagnetic (commensurate or incommensurate) or paramagnetic depending on the system parameters. We derive the phase diagram of the system on the doping-temperature plane and find that under certain conditions, the transition from the paramagnetic to the antiferromagnetic phase may demonstrate reentrance. The application of the transverse voltage induces the exciton order parameter coexisting with antiferromagnetism. The value of this second order parameter is proportional to the biased voltage and the value of the nearest- neighbor inter-plane Coulomb repulsion.

Scientific Council of the Landau Institute, Friday, September 5, 2014, Landau ITP, 11:30 am
M.V. Feigelman and A.S. Ioselevich
Coulomb blockade for tunneling through a “long island”

We consider a Coulomb blockade effects for tunnelling through a piece of wire with large resistance $R\gg 1$. This system can not be treated as a zero-dimensional one, as the dynamics of internal inhomogeneous degrees of freedom is crucial. At moderately high temperatures the linear conductance $G$ of the system is suppressed due to the one-dimensional Coulomb zero bias anomaly effect. At low $T$, besides the standard activational factor, there is an additional $T$-independent (though also exponentially strong!) suppression of $G$. It arises due to the tunnelling evolution of the charge in the wire to the equivipotential distribution. In the intermediate range of $T$ the $G(T)$ dependence is a power law, as in the phenomenological environmental theory. The effective ``environmental resistance'', entering the power exponent, is found explicitly. It depends on the length of the wire and on the positions of the contacts.

Department of quantum mesoscopics: seminar, Monday, July 28, 2014, Kapitza Institute, 10:00 am
E. Sukhorukov (University of Geneve)
Fermi Edge Singularity in Chiral One-dimensional Systems far from Equilibrium

We study the effects of strong coupling of a localized state charge to one-dimensional electronic channels out of equilibrium. While the state of this charge and the coupling strength determine the scattering phase shifts in the channels, the non-equilibrium partitioning noise induces the tunneling transitions to the localized state. The strong coupling leads to a non-perturbative backaction effect which is manifested in the orthogonality catastrophe and the Fermi edge singularity in the transition rates. The non-Gaussian component of noise brakes the charge symmetry, shifting the position of the tunneling resonance depending on the sign of the dissipative current and the transparency of the noise emitting partitioning quantum point contact.

Department of quantum mesoscopics: seminar, Friday, July 4, 2014, ITF, 11:00 am
Y. Tanaka (Nagoya University, Japan)
Theory of topological superconductivity
Scientific Council of the Landau Institute, Friday, June 20, 2014, Landau ITP, 11:30 am
Ivan Protopopov
KINETICS IN DISPERSIVE LUTTINGER LIQUIDS: BOSE-FERMI DUALITY

We show that nonequilibrium phenomena in dispersive Luttinger liquids posses remarkable weak-strong coupling duality between bosonic and fermionic descriptions. The duality manifests itself both in the collisionless dynamics of a density pulse (via crossover from essentially free-fermion evolution to hydrodynamics) and in the relaxation of fermionic and bosonic excitations.

Scientific Council of the Landau Institute, Friday, May 30, 2014, Landau ITP, 11:30 am
Konstantin Tikhonov
Defazirovka elektronov v grafene za schet vzaimodeistviya s izgibnymi fononami.

My issleduem defazirovku elektronov v grafene za schet vzaimodeistviya s izgibnymi fononami. Okazyvaetsya, chto sootvetstvuyushchii vklad mozhet byt' poryadka elektron-elektronnogo vklada dazhe pri nizkikh temperaturakh. Vychislenie provoditsya v shirokom diapazone temperatur, prodemonstrirovano, chto obsuzhdaemyi vklad menyaet temperaturnoe povedenie s T2 na T c rostom temperatury i rastet s elektronnoi plotnost'yu, v otlichie ot vklada elektron-elektronnykh vzaimodeistvii, chto delaet printsipial'no vozmozhnym ego obnaruzhenie v eksperimentakh po slaboi lokalizatsii v grafene.

Scientific Council of the Landau Institute, Friday, April 25, 2014, Landau ITP, 11:30 am
Denis Basko (CNRS, Grenobl')
Ob umnozhenii nositelei v grafene

Izuchaetsya relaksatsiya odnogo vozbuzhdennogo elektrona v zone provodimosti grafena pri nulevoi temperature i urovne Fermi v dirakovskoi tochke posredstvom ispuskaniya elektronno-dyrochnykh par za schet kulonovskogo vzaimodeistviya. Vazhnuyu rol' v etom protsesse igraet otklonenie spektra ot konicheskoi formy (trigonal'naya gofrirovka). V rezul'tate, relaksatsiya okazyvaetsya zapreshchennoi dlya nekotorykh napravlenii nachal'nogo impul'sa elektrona i razreshennoi dlya drugikh (dlya etogo sluchaya soschitano vremya relaksatsii).

Theory seminar, Thursday, April 24, 2014, Kapitza IPP, 11:30 am
V.Ya. Pokrovsky (IRE RAS)
Unikal'nye elektromekhanicheskie svoistva kvaziodnomernykh provodnikov s volnoi zaryadovoi plotnosti

Rech' poidet o kvaziodnomernykh provodnikakh, v kotorykh nizhe temperatury paierlsovskogo perekhoda voznikaet volna zaryadovoi plotnosti (VZP). Bol'shinstvo takikh soedinenii predstavlyayut soboi nitevidnye kristally. Budet rasskazano o sleduyushchikh effektakh: snizhenii uprugikh modulei pri sryve VZP s primesei, izmenenii dliny obraztsov pod deistviem elektricheskogo polya ili termotsiklirovaniya, neodnorodnoi deformatsii obraztsov v elektricheskom pole. Naibolee podrobno budet rasskazano o deformatsii krucheniya, v chastnosti, o krutil'nykh kolebaniyakh. Vse perechislennye effekty mozhno kachestvenno opisat' na yazyke uprugogo vzaimodeistviya VZP s reshetkoi kristalla. Odnako est' osnovaniya polagat', chto takoe opisanie — chrezmerno uproshchennoe. [Literatura: V.Ya. Pokrovskii, S.G. Zybtsev, M.V. Nikitin, I.G. Gorlova, V.F. Nasretdinova, S.V. Zaitsev-Zotov. «Vysokochastotnye, „kvantovye“ i elektromekhanicheskie effekty v kvaziodnomernykh kristallakh s volnoi zaryadovoi plotnosti», UFN, 183, 33 (2013).]

Department of quantum mesoscopics: seminar, Friday, April 18, 2014, Landau ITP, 3:00 pm
V. V. Pogosov (ITPE)
Veroyatnostnyi podkhod k resheniyu uravnenii Richardsona-Godena (anzatsa Bete)

Predlozhen novyi pokhod k resheniyu uravnenii Richardsona-Godena, kotorye opredelyayut sobstvennye sostoyaniya gamil'toniana teorii Bardina-Kupera-Shriffera (BKSh). Vmesto neposredstvennogo resheniya etikh uravnenii mozhno pereiti k veroyatnostnomu predstavleniyu i zatem rekonstruirovat' polozheniya kornei etikh uravnenii na kompleksnoi ploskosti, ispol'zuya metody integrirovaniya. Pri etom voznikayut integraly sel'bergovskogo tipa, figuriruyushchie v konformnoi teorii polya i teorii sluchainykh matrits. V rabote provedeny analiticheskie raschety dlya nizkoenergeticheskoi chasti spektra sistemy v termodinamicheskom predele, dlya kotorogo podtverzhdeno soglasie so srednepolevymi predskazaniyami BKSh. Takzhe predstavleny nekotorye rezul'taty dlya sistem proizvol'nogo razmera, dlya chego byla privlechena ideya o simmetrii mezhdu parami elektronov i parami dyrok. Veroyatnostnyi podkhod mozhet byt' polezen i dlya resheniya drugikh tipov uravnenii Bete.

Theory seminar, Thursday, April 17, 2014, Kapitza IPP, 11:30 am
Yu.I. Latyshev (IRE im. V.A. Kotel'nikova RAN)
Kvantovaya interferentsiya dirakovskikh fermionov v nanoperforirovannom grafene

Rassmotreny nedavnie eksperimenty po effektu Aaronova-Boma na provodyashchikh kraevykh sostoyaniyakh, lokalizovannykh vokrug nanootverstii v nanoperforirovannovannykh grafenovykh strukturakh [1], a takzhe eksperimenty po nablyudeniyu orbital'nogo kvantovaniya kraevykh dirakovskikh fermionov pri ikh dvizhenii vokrug nanootverstiya v nulevom magnitnom pole [2]. Rezul'taty eksperimentov oboikh tipov soglasuyutsya s teoriei kraevykh sostoyanii tammovskogo tipa v grafene. Literatura: [1] Yu.I. Latyshev et al., cond-mat arXiv : 1310.0991. [2] Yu.I. Latyshev, A.P. Orlov, A.V. Frolov et al., JETP Lett., 98, 214 (2013).

Department of quantum mesoscopics: seminar, Friday, April 11, 2014, Landau ITP, 3:00 pm
E. V. Devyatov (IFTT)
Dinamika namagnichennosti, vyzvannaya protekayushchim tokom, na granitse dvumernoi elektronnoi sistemy s sil'nym spin-orbital'nym vzaimodeistviem

My eksperimental'no issledovali transport cherez interfeis mezhdu ferromagnetikom (permalloi) i kraem dvumernoi elektronnoi sistemy s sil'nym spin-orbital'nym vzaimodeistviem tipa Rashby. My obnaruzhili sil'no nelineinye transportnye kharakteristiki v oblasti malykh tokov pri millikel'vinovykh temperaturakh. Obnaruzhennaya nelineinost' polnost'yu podavlyaetsya pri dostizhenii nekotorykh znachenii temperatury, magnitnogo polya ili toka cherez interfeis. My interpretiruem dannoe povedenie kak rezul'tat spinovoi akkumulyatsii vblizi interfeisa i ee absorbtsii, vyzvannoi protekayushchim tokom (magnetization torque).

Department of quantum mesoscopics: seminar, Friday, April 4, 2014, Landau ITP, 3:00 pm
Dmitry Bagrets (Institute for Theoretical Physics, University of Cologne)
Quantum criticality in quasi one-dimensional topological nanowires

Using the combination of analytical and numerical techniques we develop the theory of quantum criticality in disordered topological multi-channel nanowires of symmetry classes AIII and BDI [1]. The minimal representative phase portrait is characterized by two parameters $(\mu, w)$, where $\mu$ is the external control parameter (e.g. the chemical potential) and $w$ is the disorder strength. With the increase of $w$ at a fixed value of $\mu$ the wires generically undergo a sequence of topological phase transitions before the asymptotic state of a trivial Anderson insulator is reached. The universal description of the systems is given in terms of two parameters $(g,\chi)$, where $g$ is conductance and $\chi$ is a topological angle. Contour lines of half-integer valued $\chi$ in $(\mu, w)$-parameter space define phase boundaries between distinct topological sectors. Upon increasing the length of the wire, the pair $(g, \chi)$ exhibits flow similar to the celebrated two parameter flow describing the class A quantum Hall insulator. However, unlike the quantum Hall system, an exact analytical description of the entire phase diagram can be given. We corroborate the quantitative validity of our theory by comparison to numerical transfer matrix computations.

[1] A. Altland, D. Bagrets, L. Fritz, A. Kamenev, H. Schmiedt, arXiv:1402.1738

Theory seminar, Thursday, April 3, 2014, Kapitza IPP, 11:30 am
L.B. Ioffe
Fazovaya diagramma tsepochki bezdissipativnykh dzhozefsonovskikh kontaktov
Department of quantum mesoscopics: seminar, Friday, March 28, 2014, Landau ITP, 2:00 pm
P. Grigor'ev
Magnitosoprotivlenie sloistykh metallov

Budet dana klassifikatsiya razlichnykh predel'nykh sluchaev, svyazannykh s sootnosheniyami mezhdu integralom mezhsloevogo pereskoka, rasstoyaniem, i shirinoi urovnei Landau. V kazhdom predel'nom sluchae budut ukazany metod vychisleniya mezhsloevoi provodimosti v sil'no anizotropnykh kvazidvumernykh metallakh, oblast' primenimosti etogo metoda, i glavnye rezul'taty etikh raschetov.

[*] P. D. Grigoriev, "Longitudinal interlayer magnetoresistance in quasi-two-dimensional metals", Phys. Rev. B 88, 054415 (2013).

a takzhe
[1] P. D. Grigoriev, «Theory of the Shubnikov-de Haas effect in quasi-two-dimensional metals», Phys. Rev. B 67, 144401 (2003).
[2] P. D. Grigoriev, “Monotonic growth of interlayer magnetoresistance in strong magnetic field in very anisotropic layered metals”, JETP Lett. 94, 47 (2011).
[3] P. D. Grigoriev, “Weakly incoherent regime of interlayer conductivity in magnetic field”, Phys. Rev. B 83, 245129 (2011).
[4] P. D. Grigoriev, M. V. Kartsovnik, W. Biberacher, «Magnetic-field-induced dimensional crossover in the organic metal α-(BEDT-TTF)2KHg(SCN)4», Phys. Rev. B 86, 165125 (2012).
[5] A. D. Grigoriev and P.D. Grigoriev, Crossover from the weak to strong-field behavior of the longitudinal interlayer magnetoresistance in quasi-two-dimensional conductors, Fizika nizkikh temperatur 40(4), 472 (2014).

Scientific Council of the Landau Institute, Friday, March 28, 2014, Landau ITP, 11:30 am
G.E. Volovik
Flat-band and room-temperature superconductivity
Theory seminar, Thursday, March 27, 2014, Kapitza IPP, 11:30 am
Andrey Turlapov (IPF RAN, Nizhnii Novgorod)
Perekhod mezhdu fermi- i boze-statistikoi v 2-mernoi sisteme

V edinom eksperimente proiskhodit perestroika sistemy mezhdu bozonnym i fermionnym sostoyaniem. Eksperiment vypolnen s gazom fermi-atomov litiya-6 s s-vzaimodeistviem pri temperature ~ 10 nK. Pri roste vzaimodeistviya atomy ob'edinyayutsya v 2-atomnye molekuly bozony. 2-mernost' dostignuta plotnym uderzhaniem vdol' odnogo iz napravlenii. Chistota ul'trakholodnoi atomnoi sistemy daet vozmozhnost' sravnivat' dannye s teoreticheskimi modelyami, postroennymi iz pervykh printsipov, bez svobodnykh parametrov. V chastnosti, naidennoe nami otlichie davleniya slabo vzaimodeistvuyushchego fermi-gaza ot predskazaniya dlya odnorodnoi fermi-zhidkosti mozhet stat' stimulom dlya razvitiya teorii 2-mernykh mezoskopicheskikh sistem. Izmereniya v sil'no vzaimodeistvuyushchem gaze, mezhdu fermi- i boze-rezhimami, pokazyvayut neobkhodimost' ucheta 3-mernogo kharaktera vzaimodeistvii pri modelirovanii gaza s 2-mernoi kinematikoi.

Scientific Council of the Landau Institute, Friday, March 21, 2014, Landau ITP, 11:30 am
Ya.V. Fominov
Otchet o XVIII Mezhdunarodnom simpoziume "Nanofizika i nanoelektronika", 10-14 marta 2014 g., Nizhnii Novgorod
Scientific Council of the Landau Institute, Friday, March 7, 2014, Landau ITP, 11:30 am
S.V. Iordanskii
Provodimost' napolovinu zapolnennogo urovnya Landau
Theory seminar, Monday, February 24, 2014, Kapitza IPP, 10:00 am
Boris Narozhny (Karlsruhe Institute of Technology, Germany)
Spinful fermionic ladders at incommensurate filling: Phase diagram, local perturbations, and ionic potentials

We study the effect of external potential on transport properties of the fermionic two-leg ladder model. The response of the system to a local perturbation is strongly dependent on the ground state properties of the system and especially on the dominant correlations. We categorize all phases and transitions in the model (for incommensurate filling) and introduce ’’hopping-driven transitions’’ that the system undergoes as the inter-chain hopping is increased from zero. We also describe the response of the system to an ionic potential. The physics of this effect is similar to that of the single impurity, except that the ionic potential can affect the bulk properties of the system and in particular induce true long range order.

Theory seminar, Thursday, February 13, 2014, Kapitza IPP, 3:00 pm
M.A. Zudov (School of Physics and Astronomy, University of Minnesota, USA)
Nonequilibrium transport in high Landau levels of 2D systems: recent developments

Over the last decade, magneto transport in very high Landau levels of high-mobility 2D electron systems (2DES) hosted in GaAs/AlGaAs quantum wells revealed a variety of new intriguing phenomena. Two prime examples of these phenomena are microwave-induced resistance oscillations (MIRO) and associated zero-resistance states which emerge when a high-mobility 2DES is irradiated by microwave radiation. Another prominent effect is Hall field-induced resistance oscillations (HIRO) which appear in differential resistivity when a system is driven by a dc field. Both MIRO and HIRO originate from inter-Landau level transitions owing to photon absorption and/or impurity scattering. In this talk I will discuss our recent experimental studies on i) Shubnikov-de Haas oscillations in conventional high-mobility 2D electron gas in GaAs/Al0.24Ga0.76As quantum wells irradiated by sub-terahertz (up to 0.4 THz) radiation, ii) MIRO and HIRO in AlxGa1-xAs/Al0.24Ga0.76As quantum wells with x up to 0.0078, and, finally, iii) observation of MIRO in a moderate-mobility 2D hole gas hosted in a pure Ga/Si0.2Ge0.8 quantum well.

Scientific Council of the Landau Institute, Friday, February 7, 2014, Landau ITP, 11:30 am
Karen Michaeli (Weizmann Inst)
Novel phases in topological superconducting quantum dots

Recent progress in realizing topological superconductors has paved the road to study new physical phenomena resulting from the non-abelian statistics of the Majorana modes they host. A particularly interesting situation arises when Majorana bound states in a closed topological superconducting dot are coupled to external normal leads. In this talk , we will show that interactions with the quantum dot drive the lead electrons into a non-Fermi liquid phase, which can be understood by mapping the problem to a variant of a Kondo system. Interestingly, the non-Fermi liquid states in these systems are more robust than in the conventional two channel Kondo problem. This is because realizations with different numbers of metallic leads are connected to each other by a line of fixed points. We will conclude with a discussion of the experimental consequences of our theory.

Theory seminar, Thursday, February 6, 2014, Kapitza IPP, 11:30 am
Karen Michaeli (Weizmann Institute of Science)
Probing interactions with thermal transport

Thermal and thermoelectric conductivities are ideal probes of interaction effects in correlated electron systems. This is because, in contrast to an electric current, a heat current can be transmitted also by neutral quasiparticles. For instance, energy can be carried by excitations that mediate interactions between other quasiparticles. In my talk I will present two examples of the dramatic effect of interactions on thermal and thermoelectric transport phenomena. The first is the Nernst effect in the vicinity of the superconducting phase transition. I will discuss the anomalous behavior of the Nernst effect near the magnetic-field-induced quantum critical phase transition. The second example is thermal conductivity in spin liquids. Spin liquids can form in the vicinity of the Mott metal-insulator transition when the charge is gapped while the spin degrees of freedom strongly fluctuate. These low energy excitations, dubbed spinons, can conduct heat. The spinons also exhibit a magnetic interaction that leads to non-Fermi liquid behavior. I will show that even in the absence of disorder this strong interaction provides an efficient relaxation mechanism for the heat current.

Department of quantum mesoscopics: seminar, Friday, January 31, 2014, Landau ITP, 3:00 pm
S. I. Dorozhkin (IFTT)
Flips of electric field in microwave-induced states with spontaneously broken symmetry

We present new results on the bistability effect recently observed [Dorozhkin et al., Nature Physics 7, 336 (2011)] in two-dimensional electron systems subjected to the microwave radiation. The effect appears as switching between two states with different electric potential distribution across a sample. Our results here imply that the main difference between these two states is an opposite direction of a long lengthscale electric field which arises spontaneously under conditions when the observable magnetoresistance tends to zero. Although a possibility of the field reversal is an inherent property of states with spontaneously broken symmetry, so far such effect was never observed under stationary external conditions.

Scientific Council of the Landau Institute, Friday, January 31, 2014, Landau ITP, 11:30 am
Igor Lukyanchuk
Static and dynamic properties of polarization domains in ferroelectric nanostructures

Developing of novel technologies of functionalization of nano-size ferroelectric samples emerged the growing interest in exploration of such naturally-formed structures as polarization domains. I will present two recent results concerning the dynamic and static properties of domain self-organization.
The first part of my talk will concern the possible application of the domain structures in emerging technologies of Terahertz-detecting devices. We have studied the dynamical permittivity in ferroelectric nanometricaly-thin films with periodic domain structure, sandwiched between two paraelectric layers. The resulting frequency-dependent permittivity demonstrates collective resonance mode in sub-Terahertz frequency region and Debye-like relaxation behavior at low frequencies.
In the second part of my talk I will consider the study of domain-shape instabilities, recently observed in the ferroelectric polymer PVDF-TrFE films having the lower orthorhombic crystallographic symmetry and demonstrating the hexagonal domain faceting. This effect can arise from purely electrostatic depolarizing forces. We show that in contrast to magnetic bubble shape domains where such type of deformation instability has a predominantly elliptical character, the emergence of more symmetrical circular harmonics is favored in ferroelectrics with high dielectric constant.