Spontaneous vortex state and $\varphi$-junction in a superconducting bijunction with a localized spin

TL;DR

This paper investigates a superconducting bijunction with a magnetic quantum dot, revealing a spontaneous vortex state and a tunable phase $\

Contribution

It introduces a novel model of a frustrated bijunction with a magnetic dot, demonstrating spontaneous vortex formation and tunable phase states in superconducting circuits.

Findings

Spontaneous vortex state stabilized by frustration.

Tunable $\\varphi$-junction with arbitrary equilibrium phase.

Generation of noninteger spontaneous flux in multi-loop setups.

Abstract

A Josephson bijunction made of three superconductors connected by a quantum dot is considered in the regime where the dot carries a magnetic moment. In the range of parameters where such a dot, if inserted in a two-terminal Josephson junction, creates a $\pi$-shift of the phase, the bijunction forming a triangular unit is frustrated. This frustration is studied both within a phenomenological and a microscopic model. Frustration stabilizes a phase vortex centered on the dot, with two degenerate states carrying opposite vorticities, independently of the direction of the magnetic moment. Embedding the bijunction in a superconducting loop allows to create a tunable "$\varphi$"-junction whose equilibrium phase can take any value. For large enough inductance, it generates noninteger spontaneous flux. Multi-loop configurations are also studied.