Time-reversal symmetry breaking and multigap superconductivity in the noncentrosymmetric superconductor, La₇Ni₃

Unconventional superconductors are the most studied classes of materials and are still an active area of research in condensed matter physics. Non-centrosymmetric superconductors (NCS) are the one particular class that exhibit unconventional superconducting features and the spontaneous breaking of an additional symmetry known as the time-reversal symmetry (TRS). In NCS, the antisymmetric spin-orbit coupling (ASOC) leads to the admixture of spin-singlet and triplet components and is also proposed to be related to the broken TRS. However, the recent observation on these systems opens up questions on the correlation of the broken TRS with ASOC and requires further research to conclude the mechanism inducing unconventional ground state.

In this regard, we have thoroughly investigated NCS, La₇Ni₃ belonging to the La₇X₃ family where the frequent occurrence of TRS breaking has been observed employing macroscopic and microscopic (muon spin rotation/relaxation) measurements. Our results establish La₇Ni₃ as the first member of the Th₇Fe₃ family displaying both time-reversal-symmetry-breaking and multigap superconductivity. Moreover, our observations hint towards the role of elemental "La" in inducing the TRS breaking. Theoretical analysis also suggests two possible TRS broken pairing states.