Andreev Scattering and the Kondo Effect

TL;DR

This paper investigates how an infinite-U Anderson impurity interacts with normal and superconducting metals, revealing modifications in the Kondo resonance and conductance due to superconductivity, with implications for quantum dots and point contacts.

Contribution

It introduces a generalized noncrossing approximation incorporating multiple Andreev reflections to analyze impurity spectral functions and conductance in hybrid normal-superconductor systems.

Findings

Kondo resonance develops structure at superconducting gap energies

Superconductivity alters the magnitude of the Kondo resonance at the Fermi level

Zero-bias conductance shows observable changes due to superconductivity

Abstract

We examine the properties of an infinite-$U$ Anderson impurity coupled to both normal and superconducting metals. Both the cases of a quantum dot and a quantum point contact containing an impurity are considered; for the latter, we study both one and two-channel impurities. Using a generalization of the noncrossing approximation which incorporates multiple Andreev reflection, we compute the impurity spectral function and the linear-response conductance of these devices. We find generically that the Kondo resonance develops structure at energies corresponding to the superconducting gap, and that the magnitude of the resonance at the Fermi energy is altered. This leads to observable changes in the zero-bias conductance as compared to the case with no superconductivity.