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

TL;DR

This study reveals evidence of non-unitary triplet-pairing superconductivity with broken time-reversal symmetry in TaRuSi, contrasting with conventional s-wave superconductivity in TaReSi, through muon spin rotation and density functional theory analysis.

Contribution

It demonstrates the presence of non-unitary triplet pairing and broken TRS in TaRuSi, a novel finding in noncentrosymmetric superconductors, supported by experimental and theoretical evidence.

Findings

TaReSi exhibits isotropic s-wave superconductivity.

TaRuSi shows multi-gap superconductivity with broken TRS.

Weak spin-orbit coupling in TaRuSi correlates with non-unitary triplet pairing.

Abstract

We have studied the superconducting properties of the isostructural 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 indicate isotropic s-wave superconductivity in TaReSi and multi-gap superconductivity in TaRuSi. Zero-field $\mu$SR measurements, highly sensitive to very small magnetic fields, and no evidence for 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 time-reversal symmetry (TRS) in TaRuSi to a non-unitary 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.