Phonon Spectra and Superconductivity in Some Transition- and Actinide-Metal Carbides

The phonon dispersion curves have been measured by inelastic neutron scattering for the carbides of the transition metals, Ta, Hf, Nb, V, and Zr in which unusual features in some of the modes are observed for those compounds which become superconducting, ($\mathrm{TaC}$ , $\mathrm{NbC}$ ), compared to those that do not. Studies immediately above and below the transition temperature in $\mathrm{TaC}$ did not reveal any changes in phonon frequency or width. No unusual features are observed in the phonon spectra of the actinide UC, a non‐superconductor. Although ${\mathrm{NbC}}_{\text{.76}}$ (not superconducting above 1.2°K) does not show the changes in some of the modes that are seen in ${\mathrm{NbC}}_{\text{.98}}$ some of the phonons are extremely broad and asymmetric. This is possibly related to the highly defect nature of the crystal. These results seem to corroborate McMillan's suggestion for the transition metals, i.e., that the electron‐phonon coupling constant, and hence the transition temperature, is very sensitive to the phonon frequencies in the strong‐coupled superconductors.