Gap Energy of Superconducting Niobium Carbide

The first estimate of the superconducting energy gap in a transition-metal carbide was recently made from thermal conductivity data on Nb${\mathrm{C}}_{0.96}$ The quantity $\frac{2\epsilon \mathrm{}\left(0\right)\mathrm{}}{k{T}_c}$ was treated as an adjustable parameter in fitting the data to Bardeen-Rickayzen-Tewordt (BRT) theory, and a good fit was found for $2\epsilon \mathrm{}\left(0\right)k{T}_c=4.0\mathrm{}$. The resulting value of the gap energy has now been verified by tunneling measurements using both probe and thin-film techniques. The best value is $2\epsilon \mathrm{}\left(0\right)=3.2\mathrm{}\pm 0.1$ meV. The result supports the applicability of BRT theory to the transitionmetal carbides having low vacancy concentrations and demonstrates that under favorable circumstances, which may be unique to nonstoichiometric compounds, thermal conductivity data can be used to evaluate the gap energy of a superconductor.