Evidence for time-reversal symmetry breaking in the non-centrosymmetric superconductor LaNiC$_2$

TL;DR

This study provides evidence that the non-centrosymmetric superconductor LaNiC₂ exhibits time-reversal symmetry breaking in its superconducting state, indicating unconventional pairing mechanisms.

Contribution

The paper reports muon spin relaxation experiments showing spontaneous magnetic fields in LaNiC₂, revealing time-reversal symmetry breaking and constraining possible pairing symmetries.

Findings

Superconductivity onset coincides with spontaneous magnetic fields.

Time-reversal symmetry is broken in the superconducting state.

Possible pairing states include non-unitary triplet states.

Abstract

The results from muon spin relaxation experiments on the non-centrosymmetric intermetallic superconductor LaNiC$_2$ are reported. We find that the onset of superconductivity coincides with the appearance of spontaneous magnetic fields, implying that in the superconducting state time reversal symmetry is broken. An analysis of the possible pairing symmetries suggests only four triplet states compatible with this observation, all of which are non-unitary. They include the intriguing possibility of triplet pairing with the full point group symmetry of the crystal, which is only possible in a non-centrosymmetric superconductor.