Unusual upper critical fields of the topological nodal-line semimetal candidate SnNbSe.

We report superconductivity in SnNbSe, a topological nodal-line semimetal candidate with a noncentrosymmetric crystal structure. The superconducting transition temperatureof this compound is extremely sensitive to Sn concentrationand Se deficiency, 5.0 K for SnNbSeand 8.6 K for SnNbSeand SnNbSe. In all samples, the temperature dependence of the upper critical field() differs from the prediction of the Werthamer-Helfand-Hohenberg theory. While the zero-temperature value of the in-plane upper critical field of SnNbSewith the higheris lower than the BCS Pauli paramagnetic limit, that of the lowersample exceedsby a factor of ∼2. Our observations suggest that a possible odd-parity contribution dominates the superconducting gap function of SnNbSe, and it can be fine-tuned by the Sn concentration and Se deficiency.