Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like superconductivity in La${}_7$Pd${}_3$

Time-reversal symmetry breaking (TRSB) with a Bardeen-Cooper-Schrieffer (BCS) -like superconductivity occurs in a small, but growing number of noncentrosymmetric (NCS) materials, although the mechanism is poorly understood. We present heat capacity, magnetization, resistivity, and muon spin resonance/relaxation ($\mu$SR) measurements on polycrystalline samples of NCS La${}_7$Pd${}_3$. Transverse-field $\mu$SR and heat capacity data show La${}_7$Pd${}_3$ is a type-II superconductor with a BCS-like gap structure, while zero-field $\mu$SR results provide evidence of TRSB. We discuss the implications of these results for both the La${}_7{X}_3$ (where $X=$ Ni, Pd, Rh, Ir) group of superconductors and other CS and NCS superconductors for which TRSB has been observed.