Controllable pi junction in a Josephson quantum-dot device with molecular spin

TL;DR

This paper demonstrates a controllable $$ junction in a molecular spin Josephson device, where the $$ phase can be reversed by tuning system parameters, enabling potential applications in quantum switching and qubits.

Contribution

It introduces a model for a molecular spin Josephson junction with controllable $$ phase behavior based on system parameter tuning.

Findings

The system exhibits $$ junction behavior at low temperatures.

The $$ shift can be reversed by changing temperature or quantum dot level.

The free energy landscape shows bistability, suitable for qubit implementation.

Abstract

We consider a model for a single molecule with a large frozen spin sandwiched in between two BCS superconductors at equilibrium, and show that this system has a $\pi$ junction behavior at low temperature. The $\pi$ shift can be reversed by varying the other parameters of the system, e.g., temperature or the position of the quantum dot level, implying a controllable $\pi$ junction with novel application as a Josephson current switch. We show that the mechanism leading to the $\pi$ shift can be explained simply in terms of the contributions of the Andreev bound states and of the continuum of states above the superconducting gap. The free energy for certain configuration of parameters shows a bistable nature, which is a necessary pre-condition for achievement of a qubit.