Electronic structure of the carbon vacancy in NbC

The electronic structure of an isolated carbon vacancy in the B1-structure NbC is studied with use of the muffin-tin Green's-function method. Both the vacancy region and the neighboring shell of Nb atoms are allowed to readjust self-consistently to the absence of the carbon atom. The change in the electronic density of states is dominated by a Γ₁ (s-like) symmetry vacancy-induced peak 2.6 eV below the Fermi level EF arising from a symmetric combination of surrounding Nb d states. A smaller, broader Γ₁₅ symmetry peak, also composed of Nb states, occurs around 1.75 eV below EF. The present results suggest that the peak at 1.9 eV below EF seen in the x-ray photoemission spectrum of NbC0.85 by H\"ochst et al., although semiquantitatively well described by the present results, is significantly influenced by vacancy-vacancy interactions and/or short-range ordering of vacancies.