Two-gap time reversal symmetry breaking superconductivity in noncentrosymmetric LaNiC₂

We report a mu SR investigation of a noncentrosymmetric superconductor (LaNiC2) in single-crystal form. Compared to previous mu SR studies of noncentrosymmetric superconducting polycrystalline and powder samples, the unambiguous orientation of single crystals enables a simultaneous determination of the absolute value of the magnetic penetration depth and the vortex core size from measurements that probe the magnetic field distribution in the vortex state. The magnetic field dependence of these quantities unambiguously demonstrates the presence of two nodeless superconducting energy gaps. In addition, we detect weak internal magnetic fields in the superconducting phase, confirming earlier mu SR evidence for a time-reversal symmetry-breaking superconducting state. Our results suggest that Cooper pairing in LaNiC2 is characterized by an interorbital equal-spin pairing model introduced to unify the pairing states of LaNiC2 and the centrosymmetric superconductor LaNiGa2.