Anomalously enhanced superconductivity and<i>ab initio</i>lattice dynamics in transition metal carbides and nitrides

Abstract

The lattice dynamics for the monocarbides and mononitrides of the early transition metals has been studied using first-principles density functional perturbation theory. It is shown that the superconductivity of transition metal carbides is directly related to anomalies in the phonon spectra which in their turn are connected to the number of valence electrons. The calculated electron-phonon interaction constants are in excellent agreement with experimental data. Superconductivity is considerably enhanced for substitutional $M{\mathrm{C}}_{x}{\mathrm{N}}_{1\ensuremath{-}x}$ alloys. We also predict that perfect VC in the sodium chloride structure $(B1)$ is a superconductor with a transition temperature ${T}_{c}=11.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The experimental failure to sythesize some transition metal carbides and nitrides within the $B1$ structure is connected to their dynamical instability.