Институт теоретической физики им. Л.Д. Ландау

Российской академии наук

Семинар, посвященный 80-летию Анатолия Ивановича Ларкина.

8 ноября (ИФП, Москва)

12.00-13.00 Александр Финкельштейн (Weizmann Institute and TA&MU)
Fluctuation effects in disordered superconducting films above Tc
I will report on the fluctuation corrections in homogeneously disordered superconducting films. We find large contributions to the Nernst Effect and Hall conductivity, which experimentally can be tracked to temperatures well above Tc and magnetic fields well above the upper critical field, Hc2(0). These contributions arise from the superconducting fluctuations, and there is a quantitative agreement between our theoretical analysis and recent data obtained for the Nernst Effect (K. Behnia, France) and the Hall conductivity (Kapitulnik, Stanford). I will also report about our recent calculation of the fluctuation corrections, which is based on the Usadel equation in the real-time formulation. We adjust this approach to derive analytic expressions for the corrections to the longitudinal conductivity in the entire metallic part of the temperature-magnetic field phase diagram. This method allows us to obtain fluctuation corrections in a compact and effective way, establishing a direct connection between phenomenological and microscopic calculations.

13.00 - 14.00 Андрей Варламов (University Roma 2 )
Pseudogap and zero-bias anomaly due to fluctuation suppression of quasiparticle tunneling

14.00 - 15.00

15.00 - 16.00 Константин Матвеев (Argonne National Lab)
Conductance of uniform quantum wires
I will discuss corrections to the conductance of long uniform quantum wires caused by interactions between electrons. I will show that relaxation processes not captured by the Luttinger liquid theory lead to equilibration of the excitations with the electric current and give rise to a temperature-dependent correction to the conductance.

16.00 - 17.00 Юрий Овчинников (ИТФ им. Л.Д. Ландау)
Воспоминания ученика

17.00 - 18.00 Михаил Шифман, teleconf talk (Univ. of Minnesota)
Non-Abelian vortices: are they relevant in condensed matter physics?
Vortices with nontrivially interacting gapless excitations localized on them were found in high-energy physics in connection with confining strings in super-Yang-Mills theory. Later they were identified and discussed in high-density QCD.

The question I pose is whether similar vortices can show up in condensed matter physics.

9 ноября (ИФТТ, Черноголовка)

11.30 - 12.30 Лев Иоффе (Rutgers Univ. and Universite Paris VI)
Universal magnetoresistance in hopping regime
I argue that quantum hops over impurities with positive scattering amplitudes are equivalent to a problem of directed polymer in random potential characterized by anomalous scaling exponents. Small magnetic field leads to the interference between hoping paths and leads to the decrease in the localization length proportional to B0.8. The presence of negative scattering amplitude might lead to a random sign of the propagating wave function. This change appears as a phase transition if the scattering amplitudes are limited in magnitude. In the sign disordered phase the correction to the localization length changes the sign but retains its anomalous power law dependence at smallest fields.

12. 30 - 13. 30 Вадим Гешкенбейн (ETH Zurich)
Strong pinning, dynamic aspects
We determine the current-voltage characteristic of type II superconductors in the presence of strong pinning centers. The small pin-density limit generically provides us with a nearly linear characteristic shifted by the critical force density which approaches the flux-flow regime only at large drives. This reminds the Coulomb's Law of dry friction: Kinetic friction is independent of the sliding velocity. Furthermore, we find that the dynamical response of the vortex solid to pinning is crucially altered near the Labusch point, where strong pinning crosses over to the weak collective pinning behavior. Whereas strong pins induce a hysteretic and hence discontinuous response at small velocities generating a jump in the current-voltage characteristic, the latter evolves smoothly past the critical current in the regime near the Labusch point.

13.30 - 14.30

14.30 - 15.30 Михаил Скворцов (ИТФ им. Ландау)
Subgap states in disordered superconductors

We revise the problem of the density of states in disordered superconductors. Randomness of local sample characteristics translates to the quenched spatial inhomogeneity of the spectral gap, smearing the BCS coherence peak. We show that various microscopic models of potential and magnetic disorder can be generally reduced to the Abrikosov-Gor'kov model of paramagnetic impurities with some long-range fluctuating field.

The resulting form of the density of states is generally described by two parameters: the width measuring the broadening of the BCS peak, and the energy scale which controls the exponential decay of the density of the subgap states.

We the refine the existing instanton approaches for determination of tail width and show that they appear as limiting cases of a unified theory of optimal fluctuations in a nonlinear system. Applications to experimentally relevant types of disorder including universal mesoscopic disorder are discussed.

15.30 - 16.30 Константин Ханин (University of Toronto)
The Intermediate Disorder Regime for Directed Polymers in Dimension 1+1

16.30 - 17.00 Давид Хмельницкий, teleconf talk (Cambridge University)
Bi-critical point in a compressible solid
This work is a variation on the theme pioneered by Larkin and Pikin (LP) in 1969. A bi-critical point is the crossing point of two lines of second order phase transitions. According to LP, for the case of a usual second order phase transition, the compressibility leads to transformation of the smooth line of the free energy into a self-crossing line and, therefore, to transformation of the second order transition into that of the first order. The bi-critical point gives example of similar catastrophe in multidimensional space and could be considered by the same means. The analysis results in construction of the possible phase diagrams.

17.00 - 18.00 Леонид Глазман, teleconf talk (Yale University)
Interference of Quantum Phase Slips in a Long Chain of Josephson Junctions
Quantum phase slip is an event of tunneling of a superconducting order parameter between configurations which are not connected by a classically-allowed trajectory. Until recently, coherent phase slips were clearly observed only in qubits consisting of a few Josephson junctions. Similar observations in multi-junction arrays or nanowires were hampered by extrinsic effects. These difficulties were circumvented in a recent spectroscopic experiments with a circuit dubbed Fluxonium. It comprises a loop made of large number of "strong" Josephson junctions closed by a single "weak" one. Observation of the rare phase slips in an array of N=43 "strong" junctions was achieved due to the interference of these slips with the ones occurring in "weak" junction. This talk presents the Fluxonium theory, along with the experimental evidence of the Aharonov-Casher effect in the interference of phase slips.