Josephson versus Kondo coupling in a quantum dot connected to two superconductors

TL;DR

This paper investigates how Kondo correlations and superconducting pairing compete in a quantum dot junction, revealing a transition to a π-like state with non-sinusoidal phase dependence as the superconducting gap increases.

Contribution

It introduces a Gutzwiller-like variational approach to analyze Josephson conduction in quantum dots with dominant Kondo correlations, focusing on the regime without Andreev bound states.

Findings

Josephson current increases with superconducting gap.

Phase dependence becomes strongly non-sinusoidal at larger gaps.

The junction exhibits π-like behavior due to antiferromagnetic correlations.

Abstract

We apply a Gutzwiller-like variational technique to study Josephson conduction across a quantum dot with an odd number of electrons connected to two superconducting leads. Our method projects out all states on the dot but the Kondo singlet and is valid when Kondo correlations are dominant and no Andreev bound states localized at the dot are available for Kondo screening. In these conditions superconducting pairing is a competing effect and the junction is $\pi -$like, to optimize antiferromagnetic correlations on the dot. As the superconducting gap increases, the Josephson current also increases, but its phase dependence becomes strongly non sinusoidal.