Evidence for Time-Reversal Symmetry Breaking in the Noncentrosymmetric Superconductor LaNiC2

Muon spin relaxation experiments on the noncentrosymmetric intermetallic superconductor LaNiC₂ 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 them nonunitary. They include the intriguing possibility of triplet pairing with the full point group symmetry of the crystal, which is possible only in a noncentrosymmetric superconductor.