Josephson current through a quantum dot coupled to a molecular magnet

TL;DR

This paper theoretically investigates how a magnetic molecular magnet coupled to a quantum dot influences the Josephson current in a superconducting junction, revealing tunable 0-$$ transitions and current enhancements.

Contribution

It introduces a model for a magnetically tunable quantum dot Josephson junction, analyzing the effects of exchange interaction and magnetic field on Andreev states and current-phase relations.

Findings

Transitions from 0 to  states can be triggered by tuning the energy level, magnetic field, or exchange interaction.

The critical current sharply increases with the energy level, magnetic field, or exchange interaction.

Magnetic field induces spin precession, significantly modifying the Josephson current.

Abstract

Josephson currents are carried by sharp Andreev states within the superconducting energy gap. We theoretically study the electronic transport of a magnetically tunable nanoscale junction consisting of a quantum dot connected to two superconducting leads and coupled to the spin of a molecular magnet. The exchange interaction between the molecular magnet and the quantum dot modifies the Andreev states due to a spin-dependent renormalization of the quantum dot's energy level and the induction of spin-flips. A magnetic field applied to the central region of the quantum dot and the molecular magnet further tunes the Josephson current and starts a precession of the molecular magnet's spin. We use a non-equilibrium Green's function approach to evaluate the transport properties of the junction. Our calculations reveal that the energy level of the dot, the magnetic field and the exchange interaction between the molecular magnet and the electrons occupying the energy level of the quantum dot can trigger transitions from a 0 to a $\pi$ state of the Josephson junction. The redistribution of the occupied states induced by the magnetic field strongly modifies the current-phase relation. The critical current exhibits a sharp increase as a function of either the energy level of the dot, the magnetic field or the exchange interaction.