An Investigation into the Heat Capacities of the Superconducting Spinels

Abstract

A calorimeter, using a silicon wafer as sample holder, temperature sensor and sample heater, and an adiabatic calorimeter with a radiation shield have been designed and constructed for measuring the heat capacities of two superconducting spinel compounds, LiTi204 and CuRh2Se4, over the temperature ranges 1 - 30°K and 30 - 300°K, respectively. These data have been analysed at low temperatures to determine coefficients which describe the electronic and lattice contributions for both compounds and at high temperatures to provide an estimate of their anharmonicity. In both cases, the thermal behaviour is very satisfactorily described by the theories which have been developed for strong-coupled superconductors. Some reasons for the superior superconducting qualities of LiTi204 compared with CuRh2Se4 are given. In particular, it is found that the major contribution to the improvement in the electron-phonon coupling comes from changes in the McMillan-Hopfield parameter in agreement with the recent suggestions of Allen and Dynes. Finally, evidence is presented which indicates that anharmonicity may be an important factor for superconductivity among the materials which crystallize with spinel structure. The heat capacity results thus confirm the conclusions which had been drawn previously from X-ray and Mossbauer investigations on the spinel series CuRh2-xSnxSe4.