Superconductivity

Specific heat measurements of a superconducting NbS_{2} single crystal in an external
magnetic field:
Energy gap structure The heat capacity of a 2HNbS_{2} 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
(2Δ_{S}/k_{B}T_{c}≈2.1) much smaller than the BCS value. The overall temperature dependence of the electronic specific heat C_{e} can be explained either by the existence of a strongly anisotropic singleenergy gap or within a twogap scenario with the large gap about twice bigger than the small one. The field dependence of the Sommerfeld coefficient γ shows a strong curvature for both principalfield orientations, parallel Hc and perpendicular H⊥ 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 MgB_{2} and to the data obtained by scanningtunneling spectroscopy. We conclude that the twogap scenario better describes the gap structure of NbS_{2} than the anisotropic swave model.
Figure 1: Open circles: electronic specific heat of NbS_{2} in zero magnetic field extended down to 0.6 K. Dashed line: BCS singlegap weakcoupling case. Solid line: twogap model with 2Δ_{S}/k_{B}T_{c}=2.1, 2Δ_{L}/k_{B}T_{c}=4.6 and respective relative contributions γ_{S} /γ_{n}=0.4, γ_{L} /γ_{n}=0.6. The anisotropicgap model with anisotropy parameter α =0.5 and 2Δ_{S}/k_{B}T_{c}=3.6 follows essentially the same line. Inset: exponential dependence of the specific heat, the full line represents the best fit of the exponential decay, the dashed line is the behavior expected for a BCS singlegap weakcoupling limit. Figure 2: (a) Open circles: normalized Sommerfeld coefficient γ as a function of magnetic field perpendicular to the ab planes of NbS_{2}. Line: model accounting for highly anisotropic gap with α=0.5. Inset is the derivative of the corresponding curves from the main panel: open circles–of the measured data, line–of the model. (b) and (c) γ/γ_{n} for both orientations of the magnetic field in NbS_{2} and MgB_{2}, respectively. Figure 3: Anisotropy of NbS_{2} (full circles) compared to MgB_{2} (open circles): (a) field dependence of effective anisotropy defined as the ratio of the fields applied in both principal orientations that correspond to the same γ value in Figs. 2(b) and 2(c), (b) temperature dependence of anisotropy Γ=H_{c2}^{ab}/H_{c2}^{c}. 