Role of spin orbital coupling in unconventional superconductivity*

We have studied the superconducting properties of the ternary noncentrosymmetric superconductors TaXSi (X = Re, Ru), with the help of muon spin rotation/relaxation ($mu$SR) and density functional theory calculations. Our transverse-field $mu$SR measurements reveal isotropic s-wave superconductivity in TaReSi and multi-gap superconductivity with gap nodes in TaRuSi. Zero-field $mu$SR measurements, highly sensitive to very small magnetic fields find no evidence for spontaneous fields in the superconducting state of TaReSi, whereas we observe spontaneous fields which onset with superconductivity indicating broken time reversal symmetry (TRS) superconductivity in TaRuSi. Broken TRS in weakly coupled TaRuSi can be attributed to a non-unitary triplet pairing state, while in TaReSi, this state is suppressed due to strong anti-symmetric spin orbital coupling. Our results in TaXSi demonstrate that the strength of spin orbit coupling can be responsible for stabilizing unconventional superconductivity.