Specific heat of an anisotropic superconductor:Nb3X4withX=S, Se, and Te

Abstract

The specific heats of ${\mathrm{Nb}}_3$ ${\mathrm{S}}_4$, ${\mathrm{Nb}}_3$ ${\mathrm{Se}}_4$, and ${\mathrm{Nb}}_3$ ${\mathrm{Te}}_4$ were measured in the temperature range from 0.5 to 30 K. The magnitude of the jump at the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ was 1.11±0.02γ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{Nb}}_3$ ${\mathrm{S}}_4$ and 0.66±0.01γ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{Nb}}_3$ ${\mathrm{Se}}_4$. The superconducting property for both compounds is discussed on the basis of the weak-coupling BCS theory as extended by Clem to include the effects of energy-gap anisotropy. We estimated the mean-squared anisotropy 〈${\mathit{a}}^2$〉=0.07±0.02 and the reduced-energy gap 2${\mathrm{\varepsilon }}_0$/(${\mathit{k}}_{\mathit{BT}}$)=3.1±0.1 for ${\mathrm{Nb}}_3$ ${\mathrm{S}}_4$, and 〈${\mathit{a}}^2$〉=0.14±0.03 and 2${\mathrm{\varepsilon }}_0$/(${\mathit{k}}_{\mathit{BT}}$)=2.8±0.1 for ${\mathrm{Nb}}_3$ ${\mathrm{Se}}_4$. For ${\mathrm{Nb}}_3$ ${\mathrm{Te}}_4$, although it becomes superconducting at about 2 K in the electrical-resistivity measurement, an excess of specific heat due to the superconducting transition was not found down to about 0.5 K. We also discuss the superconducting property of ${\mathrm{Nb}}_3$ ${\mathrm{Te}}_4$. © 1996 The American Physical Society.