Specific heat measurements of a superconducting NbS2 single crystal in an external magnetic field:
Energy gap structure
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
(2ΔS/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 γ shows a strong curvature for both principal-field orientations, parallel H||c 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 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.
Figure 1: Open circles: electronic specific heat of NbS2 in zero magnetic field extended down to 0.6 K. Dashed line: BCS single-gap weak-coupling case. Solid line: two-gap model with 2ΔS/kBTc=2.1, 2ΔL/kBTc=4.6 and respective relative contributions γS /γn=0.4, γL /γn=0.6. The anisotropic-gap model with anisotropy parameter α =0.5 and 2ΔS/kBTc=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 single-gap weak-coupling limit.
Figure 2: (a) Open circles: normalized Sommerfeld coefficient γ as a function of magnetic field perpendicular to the ab planes of NbS2. 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/γn for both orientations of the magnetic field in NbS2 and MgB2, respectively.
Figure 3: Anisotropy of NbS2 (full circles) compared to MgB2 (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=Hc2ab/Hc2c.