Unusual upper critical fields of the topological nodal-line semimetal candidate SnNbSe_2-

We report superconductivity in SnNbSe_2-, a topological nodal-line semimetal candidate with a noncentrosymmetric crystal structure. The superconducting transition temperatureT_cof this compound is extremely sensitive to Sn concentrationxand Se deficiencyδ, 5.0 K for Sn_0.13NbSe_1.70and 8.6 K for Sn_0.14NbSe_1.71and Sn_0.15NbSe_1.69. In all samples, the temperature dependence of the upper critical fieldH(T) differs from the prediction of the Werthamer-Helfand-Hohenberg theory. While the zero-temperature value of the in-plane upper critical field of SnNbSe_2-with the higherT_cis lower than the BCS Pauli paramagnetic limitH_P, that of the lowerT_csample exceedsH_Pby a factor of ∼2. Our observations suggest that a possible odd-parity contribution dominates the superconducting gap function of SnNbSe_2-, and it can be fine-tuned by the Sn concentration and Se deficiency.