Spontaneous currents in a superconductor with s+is symmetry

TL;DR

This paper investigates whether spontaneous currents can occur in an s+is superconductor that breaks time-reversal symmetry, finding that currents only appear under external strain that breaks lattice rotational symmetry.

Contribution

It demonstrates that TRS breaking alone does not produce spontaneous currents, but external strain can induce supercurrents in an s+is superconductor.

Findings

No spontaneous currents from TRS breaking alone.

Supercurrents emerge under external strain that breaks C4 symmetry.

Implications for detecting TRS breaking via muSR experiments.

Abstract

We analyze $s + is$ state proposed as a candidate superconducting state for strongly hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Such a state breaks time-reversal symmetry (TRS) but does not break any other discrete symmetry. We address the issue whether TRS breaking alone can generate spontaneous currents near impurity sites, which could be detected in, e.g., muSR experiments. We argue that there are no spontaneous currents if only TRS is broken. However, supercurrents do emerge if the system is put under external strain and C4 lattice rotation symmetry is externally broken.