Evidence for superconductivity with broken time-reversal symmetry in locally noncentrosymmetric SrPtAs

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation ($\ensuremath{\mu}$SR) measurements. Zero-field $\ensuremath{\mu}$SR 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 $\ensuremath{\mu}$SR 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.