Superconductivity and Anomalous Phonon Dispersion in TaC
The family of rocksalt-structure transition metal carbides and nitrides contains materials whose physical properties (such as hardness, melting temperature, and superconducting transition temperature T c ) tend toward extremes.1 Values of T c greater than 10°K and less than 0.1°K occur in closely related materials such as TaC and HfC (which are both good metals). Recently Smith and Glaser2 and Smith3 have reported inelastic neutron scattering studies of the phonon dispersion in several compounds. Their data clearly indicate that high values of T c are associated with phonon anomalies which are absent from the spectra of low-T c materials. In particular, TaC has sharply defined softening of the LA mode at large q in every direction which was measured, and similar behavior of the TA modes in many directions, while HfC has a nearly identical dispersion relation except for the lack of these anomalies.
KeywordsMode Softening Superconducting Transition Temperature Phonon Dispersion Inelastic Neutron Usual Theory
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