Time-reversal-symmetry breaking and unconventional pairing in the noncentrosymmetric superconductor La7Rh3

Noncentrosymmetric superconductors have sparked significant research interests due to their exciting properties, such as the admixture of spin-singlet and spin-triplet pairing. Here we report on the muon spin rotation and relaxation and thermodynamic measurements on the noncentrosymmetric superconductor ${\mathrm{La}}_{7}{\mathrm{Rh}}_{3}$, which show an isotropic superconducting gap but also spontaneous time-reversal-symmetry breaking occurring at the onset of superconductivity. We show that our results pose severe constraints on any microscopic theory of superconductivity in this system. A symmetry analysis identifies ground states compatible with time-reversal-symmetry breaking, and the resulting gap functions are discussed. Furthermore, general energetic considerations indicate the relevance of electron-electron interactions for the pairing mechanism, in accordance with hints of spin fluctuations revealed in susceptibility measurements.