Nodal superconductivity with spin-triplet component in a noncentrosymmetric weakly-correlated metal

Abstract

The most compelling evidence for spin-triplet superconductivity has emerged from strongly correlated electron systems, yet whether a substantial spin-triplet component can be realized without strong electronic coupling, by virtue of antisymmetric spin-orbit coupling (ASOC), remains unresolved. We address this question in the weakly-correlated noncentrosymmetric superconductor Nb$_{18}$Re$_{82}$ using low-temperature scanning tunneling spectroscopy on single crystals with different crystallographic orientations. The tunneling spectra exhibit orientation-dependent variations. A symmetry-constrained analysis shows that understanding the complete spectroscopic dataset requires an superconducting order parameter combining a nodal spin-singlet component with a spin-triplet contribution reaching up to half of the singlet amplitude. These results resolve the debated pairing symmetry of Nb$_{18}$Re$_{82}$ and demonstrate that ASOC alone can generate substantial parity mixing, suggesting that triplet superconductivity may be more widespread than previously recognized.