Specific-heat measurements of superconducting NbS2 single crystal in an external magnetic field: Study on the energy gap structure

TL;DR

This study investigates the energy gap structure of superconducting NbS2 single crystals using specific heat measurements under magnetic fields, revealing evidence for a two-gap superconducting scenario rather than a simple anisotropic s-wave model.

Contribution

The paper provides detailed specific heat data of NbS2 under magnetic fields, supporting a two-gap superconductivity model over anisotropic single-gap explanations.

Findings

Energy gap (2DS/kBTc) is approximately 2.1, smaller than BCS value.

Data favor a two-gap superconducting scenario over anisotropic s-wave.

Superconducting anisotropy varies with magnetic field orientation.

Abstract

The heat capacity of a 2H-NbS2 single crystal has been measured by a highly sensitive ac technique down to 0.6 K and in magnetic fields up to 14 T. At very low temperatures data show excitations over an energy gap (2DS/kBTc \approx 2.1) much smaller than the BCS value. The overall temperature dependence of the electronic specific heat Ce can be explained either by the existence of a strongly anisotropic single-energy gap or within a two-gap scenario with the large gap about twice bigger than the small one. The field dependence of the Sommerfeld coefficient shows a strong curvature for both principal-field orientations, parallel and perpendicular to the c axis of the crystal, resulting in a magnetic field dependence of the superconducting anisotropy. These features are discussed in comparison to the case of MgB2 and to the data obtained by scanning-tunneling spectroscopy. We conclude that the two-gap scenario better describes the gap structure of NbS2 than the anisotropic s-wave model.