Pauli-limit violation and nodeless two-gap superconductivity in noncentrosymmetric h-NbS single crystals

Abstract

Noncentrosymmetric superconductors, characterized by broken inversion symmetry, represent a distinctive subclass of unconventional superconductors. Here, we report the growth of single crystals of a noncentrosymmetric superconductor h-NbS, and systematical characterization of crystal structure and superconducting properties. Different from o-NbS, h-NbS crystallizes in a NiAs-type hexagonal superstructure with site-selective Nb vacancies. First-principles calculations reveal that the Nb−4d states dominate at the Fermi level (EF), with the density of states near EF manifesting significant sensitivity to the concentration of the Nb vacancies. Nodeless moderately coupled two-gap superconductivity with a critical temperature of 2.6 K is confirmed in this strong type-II superconductor. The anisotropic parameters based on upper and lower critical fields have small values of ∼1.4 and ∼1.3, respectively, which demonstrate that the Cooper pairs show almost isotropic resilience to magnetic field. We surprisingly note that the upper critical field along the ab plane surpasses the Pauli limit, largely arising from the two-dimensional Fermi-surface sheet around the K and H points of the Brillouin zone, which is related to the breaking of inversion symmetry. In contrast to quasi-one-dimensional and quasi-two-dimensional Nb-chalcogenide superconductors, the three-dimensional h-NbS featuring Pauli-limit violation provides a unique material for investigating unconventional superconductivity.

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