Time-Reversal-Symmetry-Broken State in the BCS Formalism for a Multi-Band Superconductor

TL;DR

This paper explores how frustration in three-band superconductors with repulsive interactions can lead to a degenerate, time-reversal-symmetry-broken state, revealing a new phase transition within the BCS framework.

Contribution

It demonstrates the existence of a TRSB state in multi-band superconductors and characterizes the phase transition between TRSB and conventional BCS states.

Findings

TRSB state arises from complex gap functions due to frustration

A phase transition occurs between TRSB and conventional BCS states

The transition depends on interband coupling strength

Abstract

In three-band BCS superconductors with repulsive interband interactions, frustration between the bands can lead to an inherently complex gap function, arising out of a phase difference between the bands in the range 0 and {\pi}. Since the complex conjugate of this state is also a solution, the ground state is degenerate, and there appears a time-reversal-symmetry-broken state. In this paper we investigate the existence of this state as a function of interband coupling strength and show how a new phase transition appears between the TRSB and conventional BCS states.