Josephson Effect through an isotropic magnetic molecule

TL;DR

This paper studies how an isotropic magnetic molecule affects the Josephson current in a superconducting junction, revealing complex phase transitions influenced by exchange coupling and Kondo effects.

Contribution

It introduces a detailed numerical analysis of the interplay between exchange coupling, Kondo correlation, and superconductivity in molecular Josephson junctions.

Findings

Exchange coupling suppresses Kondo correlation and induces 0-$ppa$ phase transitions.

Antiferromagnetic coupling causes reentrant $ppa$ states and restores 0 state at large coupling.

Asymmetric supercurrent dependence on exchange coupling can help determine its sign experimentally.

Abstract

We investigate the Josephson effect through a molecular quantum dot magnet connected to superconducting leads. The molecule contains a magnetic atom, whose spin is assumed to be isotropic. It is coupled to the electron spin on the dot via exchange coupling. Using the numerical renormalization group method we calculate the Andreev levels and the supercurrent and examine intertwined effect of the exchange coupling, Kondo correlation, and superconductivity on the current. Exchange coupling typically suppresses the Kondo correlation so that the system undergoes a phase transition from 0 to $\pi$ state as the modulus of exchange coupling increases. Antiferromagnetic coupling is found to drive exotic transitions: the reentrance to the $\pi$ state for a small superconducting gap and the restoration of 0 state for large antiferromagnetic exchange coupling. We suggest that the asymmetric dependence of supercurrent on the exchange coupling could be used as to detect its sign in experiments.