Influence of covalence on the critical temperature in the spinel superconductors

Abstract

Influence of covalence on the critical temperature in spinel superconductors of the type AB2X4 (where A = Li, Cu; B = Ti, V, Rh; × = O, S, Se) is considered. For this purpose the vacancy model is used and an effective number of valence electrons per molecule is proposed. Calculations of the vacancy model parameters, i.e. the ion packing coefficients, the fulfilment coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing anion radius, leading to a decrease of the critical temperature. The occurrence of the superconducting state in the spinel compounds with a large covalence is still possible if they possess a large vacancy concentration, i.e. a large value of the vacancy parameter. It was observed that the critical temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the superconducting process. These effects are explained with the aid of a charge transfer mechanism using the Kondo-like X-X hopping terms.