We study the superconducting diode effect (SDE) in a diffusive superconductor – normal metal (SN) bilayer subjected to an in-plane magnetic field. The supercurrent flows along the layers, perpendicular to the field. The SDE, manifested as an asymmetry in the critical (depairing) currents and kinetic inductance for opposite current directions, arises from an orbital mechanism due to the inhomogeneous distribution of the Meissner currents caused by a spatially varying superfluid density. Recently, Levichev et al. [Phys. Rev. B 108, 094517 (2023)] demonstrated the realization of this effect in such a structure, supporting numerical calculations for an ideal interface with an experiment. In this work, we investigate the influence of a nonideal interface with finite resistance on the SDE. Employing an analytical approach, we focus on limiting cases corresponding to weak intralayer inhomogeneities. We find that the strength of the SDE depends nonmonotonically on the interface resistance when the bilayer thickness is small compared to the coherence length. Remarkably, a nonideal interface can enhance the SDE compared to the ideal case.

The talk is based on preprint arXiv:2604.09504

We study the behavior of hydrodynamic modes in dissipative fermion systems with a conserved number of particles. The model under consideration describes a class of fermion systems with a symmetric two-band spectrum, a conserved number of particles, and energy and momentum dissipation due to interaction with a boson bath. The evolution of the density matrix of such a system can be described using the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) equation or, equivalently, in terms of Keldysh field theory. In previous papers on this topic, we studied the system's dynamics using diagrammatic techniques within the Keldysh functional integral formalism. It turned out that the fermion dynamics are described by a system of two coupled equations for the densities in the upper and lower bands, which have the form of the Fisher-Kolmogorov-Petrovskii-Piskunov (FKPP) equation. In this work, we continue the theme using field theory methods: in the Keldysh action, we can clearly distinguish hydrodynamic and massive modes that describe the dynamics of deviations from the saddle-point approximation. Averaging over the massive modes reduces the hydrodynamic action to the familiar FKPP equations and allows us to use the renormalization group method to determine whether previously obtained results are tied to a small dissipation coefficient, as well as whether the model exhibits a phase transition with respect to this parameter. This work was carried out within the framework of state assignment FFWR-2024-0017.

Dirty superconducting films with magnetic impurities can exhibit nontrivial behavior in a magnetic field that polarizes the impurity spins. As predicted by Kharitonov and Feigelman (KF) [JETP Lett. 82, 421 (2005)], this polarization reduces the exchange scattering rate. Consequently, a parallel magnetic field can enhance the critical temperature $T_c$ when magnetic-field pair breaking is weak, as realized for strong spin-orbit scattering and small film thickness. Recently, Llanos et al. [Nat. Phys. (2026)] observed a pronounced enhancement of $T_c$ consistent with the KF theory. The same experiment also reported an enhancement of the perpendicular upper critical field $H_{c2}^{\perp}$ and a suppression of the London penetration depth $\lambda_L$ by a parallel magnetic field. These quantities were not considered in the original KF theory. To address this gap, we develop a theoretical framework based on Gor'kov's diagrammatic technique for dirty superconductors. We extend the KF theory in two experimentally relevant directions: (i) to arbitrary temperatures $T < T_c$ and several superconducting observables, and (ii) to arbitrary magnetic-field orientations. As a result, we demonstrate theoretically the suppression of $\lambda_L$ and the enhancement of $H_{c2}^{\perp}$ by a parallel magnetic field, in agreement with experiment.

The talk is based on preprint arXiv:2605.17408

We study thermal phase slips in an infinite 2D superconducting film within the Usadel model. Recently, the free-energy instanton was obtained in the Ginzburg–Landau region in the vicinity of the critical current. The proximity to the critical current allows one to reduce the Ginzburg–Landau equations to the exactly integrable Boussinesq equation [1]. Using the scaling of the instanton parameters obtained in [1], we perform a gradient expansion of the Usadel equations near the critical current. Keeping only the leading terms in the free energy functional, we transform it into the Boussinesq form and find the activation barrier at arbitrary temperatures using the known instanton profile. In the 1D geometry, our approach yields the asymptotics of the barrier near the critical current, consistent with numerical results from [2].

[1] M. A. Skvortsov and A. V. Polkin, arXiv:2506.18130.
[2] A. V. Semenov, P. A. Krutitskii and I. A. Devyatov, Jetp Lett. 92, 762 (2010)

Within the quantum field-theoretical approach describing the evolution of a quadratic Liouvillian in the basis of Keldysh contour coherent states, we investigate the spectral and transport properties of a dissipative superconducting system coupled to normal Fermi reservoirs. We derive a generalization of the Meir-Wingreen formula and Onsager matrix for a superconducting system subject to an arbitrary number of fermionic baths. Following Kirchhoff's rule, we obtain an expression describing the dissipation-induced loss current and formulate modified quantum kinetic equations. For wide-band contacts locally coupled to individual sites, we find that each contact reduces the degeneracy multiplicity of the non-equilibrium steady state by one. These results are numerically verified through several cases of the extended Kitaev model at symmetric points with a single contact. Furthermore, in the linear response regime at low temperatures, we demonstrate that (non-)degenerate non-equilibrium steady states correspond to (non-)quantized conductance peaks. Revisiting a paradigmatic problem of resonant transport in the Majorana mode of the Kitaev model we demonstrate that the dissipation accounts for the zero-bias peak suppression and its asymmetry.

The search for conditions supporting degenerate steady states in nonequilibrium topological superconductors is important for advancing dissipative quantum engineering, a field that has attracted significant research attention over the past decade. In this study, we address this problem by investigating topological superconductors hosting unpaired Majorana modes under the influence of environmental dissipative fields. Within the Gorini-Kossakowski-Sudarshan-Lindblad framework and the third quantization formalism, we establish a correspondence between equilibrium Majorana zero modes and non-equilibrium kinetic zero modes. We further derive a simple algebraic relation between the numbers of these excitations expressed in terms of hybridization between the single-particle wavefunctions and linear dissipative fields. Based on these findings, we propose a practical recipes how to stabilize degenerate steady states in topological superconductors through controlled dissipation engineering. To demonstrate their applicability, we implement our general framework in the BDI-class Kitaev chain with long-range hopping and pairing terms — a system known to host a robust edge-localized Majorana modes.

We study the spatial dependence of pair correlation functions of velocity field components in a rotating turbulent fluid on a background of a coherent geostrophic vortex. The statistics of the turbulent pulsations are determined by their dynamics, which is the dynamics of inertial waves affected by the differential rotation in the vortex and a weak viscous damping. We are interested in distances which are larger than the scale of the wave forcing but smaller than the radius of the coherent vortex. We establish the anisotropy of the velocity field correlation function at the distances. All the diagonal elements of the correlation function decay logarithmically in the streamwise direction and power-like in radial direction and the direction along the rotation axis. This laws are independent of the details of the forcing correlation function that indicate “coherency” of the flow. On the contrary, the cross-correlation function of the radial-azimuth velocity components, which turns into the Reynolds stress for zero distance, demonstrates strong dependence on the forcing correlation function and decays quickly at distances larger than the forcing scale.

Leon L. Ogorodnikov, Sergey S. Vergeles. “Long-range spatial velocity statistics in a rotating coherent turbulent vortex”, Physical Review Fluids, vol. 10, p. 124702 (2025)

Through a massive computation we reached the fourth superharmonic instability branch of the Stokes' wave. Using the obtained results we checked phenomenological formulae for the dependence of the instability growth rates corresponding to different branches of instability on the nonlinearity parameter (steepness, defined as the wave swing to wavelength ratio) in the vicinity of the new instability branch appearance and far from it. It is demonstrated, that the formulae, the one obtained as a least squares fit using the information from the first three branches of instability and a phenomenological asymptotics, work for the fourth branch and previously reported branches as well. Range of applicability of the relations was corrected. Growth rates for all four instability branches are reported.