  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.
