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 stoichio-metric nitride, N6iV0 91, may be considered as a sort of ”stiff lead”. A supercon-ducting transition temperature of 18°K is found by extrapolation to hypothetical stoichiometric NbN which reinforces the empirical suggestion that 18°K may be an upper limit for the occurrence of superconductivity