Observation of a pronounced Hebel-Slichter peak in the spin-lattice relaxation rate and implications for gap and pairing symmetry in LaNiGa$_2$

TL;DR

This study observes a Hebel-Slichter peak in LaNiGa$_2$'s superconducting state, challenging previous triplet pairing models and supporting a conventional two-gap BCS-like pairing scenario.

Contribution

It provides experimental evidence favoring a two-gap BCS-like pairing over non-unitary triplet pairing in LaNiGa$_2$, impacting understanding of its superconducting symmetry.

Findings

Pronounced Hebel-Slichter peak observed in NQR data.

Data best fit by a two-band singlet BCS-like pairing with equal gaps.

Non-unitary triplet pairing models are inconsistent with the data.

Abstract

We report a pronounced Hebel-Slichter coherence peak in the zero field nuclear quadrupolar resonance (NQR) spin-lattice relaxation rate of the topological crystalline superconductor LaNiGa$_2$ in the superconducting state. Previously, a two-band internally antisymmetric non-unitary triplet pairing (INT) state was proposed for this system, with equal spin-pairing and two distinct gaps associated with different spins. A detailed examination of the temperature dependence of the NQR data shows that the data best fit an INT model if the two gaps are equal and the model is unitary. Even a tiny non-unitarity with two unequal gaps causes the coherence peak to diminish rapidly and deviate from the data. On the other hand, the data are well-fit by a two-band singlet BCS-like pairing with two distinct gaps consistent with previous measurements. This raises doubts on the identification of non-unitary triplet-pairing with time-reversal symmetry breaking in this material.