Two gap time reversal symmetry breaking superconductivity in non-centrosymmetric LaNiC2

Non-centrosymmetric superconductors lack an inversion center in their crystal structure, one of the typical symmetries for the formation of Cooper pairs. The absence of inversion symmetry strongly influences the allowed Cooper pairing states. Here, we report a muon spin rotation/relaxation ($\mu$SR) study of LaNiC$_2$ single crystals. Earlier zero-field $\mu$SR measurements on a polycrystalline LaNiC$_2$ sample revealed the onset of spontaneous magnetic fields at T$_c$. This finding signifies the breaking of time-reversal symmetry (TRS) in the superconducting phase, which is compatible with a non-unitary triplet-pairing state. However, thermodynamic measurements of LaNiC$_2$ have provided evidence for conventional BCS s-wave pairing, point nodes in the gap function and even two-gap superconductivity. Consequently, the nature of the superconducting energy-gap structure in this compound has remained unsettled.

Our detailed μSR investigation of LaNiC$_2$ was carried out on a sample in single crystal form. From measurements that probe the magnetic field distribution in the vortex state, we have simultaneously determined the behaviors of the absolute value of the magnetic penetration depth and the vortex core size. The magnetic field dependence of these two quantities unambiguously demonstrates the presence of two nodeless superconducting energy gaps. In addition, we have confirmed broken TRS in the superconducting phase of LaNiC$_2$. These two results suggest that Cooper pairing in LaNiC$_2$ is characterized by an interorbital equal-spin pairing model introduced to unify the pairing states of LaNiC$_2$ and the centrosymmetric superconductor LaNiGa$_2$.