Evidence for Time-Reversal-Symmetry-Broken Superconductivity in Locally Noncentrosymmetric SrPtAs

TL;DR

This study provides evidence that superconductivity in SrPtAs breaks time-reversal symmetry, likely due to a chiral d-wave order parameter, as shown by muSR measurements indicating spontaneous magnetic fields and a nodeless gap.

Contribution

First experimental evidence of time-reversal symmetry breaking in SrPtAs superconductivity using muSR techniques, supporting a chiral d-wave pairing state.

Findings

Spontaneous static magnetic fields appear at superconducting transition.

Superfluid density indicates a nodeless superconducting gap.

Chiral d-wave pairing is most consistent with data.

Abstract

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (muSR) measurements. Zero-field muSR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field muSR is nearly flat approaching T = 0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d + id (chiral d-wave) order parameter is most consistent with our experimental data.