Unusual upper critical fields of the topological nodal-line semimetal candidate Sn x NbSe2−δ

Abstract

We report superconductivity in Sn x NbSe2−δ , a topological nodal-line semimetal candidate with a noncentrosymmetric crystal structure. The superconducting transition temperature T c of this compound is extremely sensitive to Sn concentration x and Se deficiency δ, 5.0 K for Sn0.13NbSe1.70 and 8.6 K for Sn0.14NbSe1.71 and Sn0.15NbSe1.69. In all samples, the temperature dependence of the upper critical field H c2(T) differs from the prediction of the Werthamer–Helfand–Hohenberg theory. While the zero-temperature value of the in-plane upper critical field of Sn x NbSe2−δ with the higher T c is lower than the BCS Pauli paramagnetic limit H P, that of the lower T c sample exceeds H P by a factor of ∼2. Our observations suggest that a possible odd-parity contribution dominates the superconducting gap function of Sn x NbSe2−δ , and it can be fine-tuned by the Sn concentration and Se deficiency.