High temperature SP-band superconductors

Abstract

The superconductivity, low temperature heat capacity and magnetic susceptibility of a number of face-centered cubic nitrides and carbides of niobium have been studied. In agreement with earlier work it is found that the closer the approach to stoichiometry the higher the transition temperature. The values of the electronic heat capacity coefficient and magnetic susceptibility are unusually low and are accounted for by a proposed model in which the carriers are predominantly in sp-bands. Insofar as its superconducting properties are concerned the most stoichiometric nitride, ${NbN}_{0.91}$, may be considered as a sort of "stiff lead". A superconducting transition temperature of $\mathit{18}\ifmmode^\circ\else\textdegree\fi{}K$ is found by extrapolation to hypothetical stoichiometric $NbN$ which reinforces the empirical suggestion that $\mathit{18}\ifmmode^\circ\else\textdegree\fi{}K$ may be an upper limit for the occurrence of superconductivity.