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

摘要

The heat capacity of a 2H-NbS2 single crystal has been measured by a highlysensitive ac technique down to 0.6 K and in magnetic fields up to 14 T. At verylow temperatures data show excitations over an energy gap (2DS/kBTc \approx2.1) much smaller than the BCS value. The overall temperature dependence of theelectronic specific heat Ce can be explained either by the existence of astrongly anisotropic single-energy gap or within a two-gap scenario with thelarge gap about twice bigger than the small one. The field dependence of theSommerfeld coefficient shows a strong curvature for both principal-fieldorientations, 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 dataobtained by scanning-tunneling spectroscopy. We conclude that the two-gapscenario better describes the gap structure of NbS2 than the anisotropic s-wavemodel.