In the search of topological superconductors, nailing down the Fermiology of the normal state is as crucial a prerequisite as unraveling the superconducting pairing symmetry. In particular, the number of time-reversal-invariant momenta (TRIM) in the Brillouin zone enclosed by Fermi surfaces is closely linked to the topological class of time-reversal-invariant systems, and can experimentally be investigated. We report here a detailed study of de Haas van Alphen quantum oscillations in single crystals of the topological semimetal CaSn3 with torque magnetometry in high magnetic fields up to 35 T. In conjunction with density functional theory based calculations, the observed quantum oscillations frequencies indicate that the Fermi surfaces of CaSn3 enclose an odd number of TRIM, satisfying one of the proposed criteria to realize topological superconductivity. Nonzero Berry phases extracted from the magnetic oscillations also support the nontrivial topological nature of CaSn3.

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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.

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We report the superconductivity of the topological nodal-line semimetal candidate Sn x NbSe2-δ with a noncentrosymmetric crystal structure. The superconducting transition temperature T c of Sn x NbSe2-δ drastically varies with the Sn concentration x and the Se deficiency δ, and reaches 12 K, relatively higher than those of known topological superconductors. The upper critical field of this compound shows unusual temperature dependence, inconsistent with the WHH theory for conventional type-II superconductors. In a low-Tc sample, the zero-temperature limit of the upper critical field parallel to the ab plane exceeds the Pauli paramagnetic limit estimated from the simple BCS weak coupling model by a factor of ∼ 2, suggestive of unusual superconductivity stabilized in Sn x NbSe2-δ . Together with the robust superconductivity against disorder, these observations indicate that Sn x NbSe2-δ is a promising candidate to explore topological superconductivity.

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