Unusual upper critical fields of the topological nodal-line semimetal candidate SnxNbSe2-δ.

We report superconductivity in SnxNbSe2-δ, a topological nodal-line semimetal candidate with a noncentrosymmetric crystal structure. The superconducting transition temperatureTcof this compound is extremely sensitive to Sn concentrationxand Se deficiencyδ, 5.0 K for Sn0.13NbSe1.70and 8.6 K for Sn0.14NbSe1.71and Sn0.15NbSe1.69. In all samples, the temperature dependence of the upper critical fieldHc2(T) differs from the prediction of the Werthamer-Helfand-Hohenberg theory. While the zero-temperature value of the in-plane upper critical field of SnxNbSe2-δwith the higherTcis lower than the BCS Pauli paramagnetic limitHP, that of the lowerTcsample exceedsHPby a factor of ∼2. Our observations suggest that a possible odd-parity contribution dominates the superconducting gap function of SnxNbSe2-δ, and it can be fine-tuned by the Sn concentration and Se deficiency.