Thermodynamic properties of the layered superconductor 2H-NbSe₂

Abstract

The specific heat in the normal and superconducting states of single crystals of the transition metal dichalcogenide superconductor 2H-NbSe ₂ was measured from 1.7 to 9 K by use of a thermal relaxation method. The thermodynamic critical field and the specific heat behavior are different from those predicted by the BCS theory. The anisotropy and strong coupling effects in the thermodynamic properties are made evident through comparison with the Clem-Sheahen-Gubser relations, which take account of these effects near T _c and 0 K. A simple semiempirical model is proposed to explain the temperature dependences of the thermodynamic critical field and the superconducting electronic specific heat. Under the assumption of a spheroidal Fermi surface based on the tight-binding approximation, the model takes into account effects of the anisotropy and the strong coupling interaction in the expression for the energy gapΔ _k (t)=Δ ₀ (t)(1+ε ₂ cos ₂ ϑ). The strong coupling parameter 2Δ ₀ (0)/K _B T _c and the energy gap anisotropy parameter ε ₂ are the only two adjustable parameters. The thermodynamic behavior deduced from the model gives good agreement with experimental results for values of 2Δ ₀ (0)/k _B T _c⋍5.0 and ε ₂ ≃-−0.6.