Quantum phase transition in a double quantum dot Josephson junction driven by electron-electron interactions

TL;DR

This study models a double quantum dot Josephson junction to explore complex phase transitions driven by electron interactions and magnetic fields, revealing controllable quantum states and non-local magnetization effects.

Contribution

It introduces a surrogate BCS model with exact diagonalization to analyze multiple controllable phase transitions in a hybrid quantum dot system.

Findings

Multiple phase transitions can be controlled by tuning quantum dot interactions.

Magnetic fields induce reversible ferromagnetic-antiferromagnetic transitions.

Non-local magnetization phenomena emerge under weak magnetic fields.

Abstract

In this work, we employ a surrogate BCS model with discrete energy levels to investigate a hybrid system comprising two quantum dots (QD1 and QD2), where QD1 is tunnel-coupled to two superconducting leads. Through exact diagonalization of this system, we obtain numerically exact solutions that enable rigorous computation of key physical quantities. Our analysis reveals a rich phase diagram featuring multiple controllable phase transitions mediated by quantum dot interactions. Specifically, the system first undergoes an initial phase transition when tuning QD2's interaction strength while maintaining QD1 in the non-interacting regime. Subsequent adjustment of QD1's interaction induces a secondary phase transition, followed by a third transition arising from inter-dot coupling modulation. Furthermore, we demonstrate that parallel magnetic field application can drive reversible ferromagnetic-antiferromagnetic phase transitions under specific parameter conditions. Finally, we report the emergence of non-local magnetization phenomena when subjecting QD1 to weak magnetic fields. And our results demonstrate that the orientation of nonlocal magnetization can be precisely manipulated through systematic adjustment of the on-site interaction strength $U_2$ in QD2.