Evidence for nonunitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi

We have studied the superconducting properties of the isostructural ternary noncentrosymmetric superconductors $\mathrm{Ta}X\mathrm{Si}$ $(X=\mathrm{Re}, \mathrm{Ru})$ with the help of muon spin rotation/relaxation $(\ensuremath{\mu}\mathrm{SR})$ and density functional theory calculations. Our transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements indicate isotropic $s$-wave superconductivity in TaReSi and multigap superconductivity in TaRuSi. Zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements, highly sensitive to very small magnetic fields, find no evidence of spontaneous fields in the superconducting state of TaReSi, whereas we observe small spontaneous fields that onset with superconductivity indicating broken time-reversal symmetry (TRS) superconductivity in TaRuSi. Using density functional theory calculations, we find that spin-orbit coupling is relatively weak in TaRuSi and strong in TaReSi. Using symmetry analysis, we attribute the broken TRS in TaRuSi to a nonunitary triplet pairing state. Such a state is not allowed in the presence of strong spin-orbit coupling: our finding of no evidence for broken TRS in TaReSi is consistent with this expectation.