Specific heat measurements of a superconducting NbS2 single crystal in an external magnetic field: Energy gap structure
Résumé
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 2 S /kBTc 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 H c and perpendicular H c 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.
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