Time-reversal-symmetry breaking and unconventional pairing in the noncentrosymmetric superconductor La$_{7}$Rh$_{3}$ probed by $\mu$SR

Noncentrosymmetric superconductors are expected to exhibit an unconventional superconducting ground-state, which consists of an admixture of spin-singlet and spin-triplet Cooper pairs. Nevertheless, unconventional nature of superconducting ground-state has yet to be conclusively verified in any noncentrosymmetric superconductors to this date. Here we report that the superconducting ground state of La$_{7}$Rh$_{3}$ is gapped and spontaneously breaks the time reversal symmetry at the onset of superconductivity. The analysis of the pairing symmetries identifies the ground states compatible with time reversal symmetry breaking. Theoretical results suggest that superconductivity is driven by electron-electron interactions, confirming the unconventional nature of superconductivity.