Low Temperature Anharmonicity in the Superconducting Spinels

Abstract

Strong comparisons between the properties of the high transition temperature superconductors in the β- tungsten and NbN groups and the superconducting spinels are described within the framework of present theories of high Tc superconductivity. In particular, emphasis is laid upon the known presence of lattice instabilities in both groups of compounds. X-Ray diffractometric measurements of the crystallographic parameters of the superconducting spinels have been made and the  superconducting transition temperatures determined using the Meissner Effect. Mössbauer spectrometry experiments have been performed which establish the presence of low temperature anharmonicity within the superconducting spinels by measurement of the recoil-free fraction. Coincident total shift measurements have enabled the present theory of the band structure to be tested. By making use of recent theories employing Mössbaver effect measurements, the validity of McMillan's theory applied to the superconducting spinels has been investigated and shown to be reasonable, within the approximations of the theory.