Fermi liquid approach for superconducting Kondo problems

TL;DR

This paper develops a Fermi liquid theory for superconducting Kondo problems with large Kondo temperature, analyzing Andreev spectra and current-phase relations, revealing a persistent 4π periodicity distinct from topological effects.

Contribution

It introduces a Fermi liquid framework for superconducting Kondo systems applicable at high Kondo temperatures, providing new insights into Andreev spectra and Josephson effects.

Findings

Discovery of 4π periodic Andreev spectrum in particle-hole symmetric Kondo limit

Persistence of 4π periodicity under small voltage bias

Distinction of 4π effect from topological Majorana junctions

Abstract

We present a Fermi liquid approach to superconducting Kondo problems applicable when the Kondo temperature is large compared to the superconducting gap. To illustrate the theory, we study the current-phase relation and the Andreev level spectrum for an Anderson impurity between two $s$-wave superconductors. In the particle-hole symmetric Kondo limit, we find a $4\pi$ periodic Andreev spectrum. The $4\pi$ periodicity persists under a small voltage bias which however causes an asymmetric distortion of Andreev levels. The latter distinguishes the present $4\pi$ effect from the one in topological Majorana junctions.