Electronic structure of the carbon vacancy in NbC

A tight-binding calculation for the carbon vacancy in NbC has been performed. The metalmetalloid overlaps are taken into account in the perfect-crystal Hamiltonian, so that changes of these overlaps due to the vacancy can be considered in the defect calculation. As a consequence the vacancy perturbation extends to the first neighbors and a resonant peak associated with the vacancy appears in the conduction-band minimum, in agreement with recent photoemission experiments. The Green's-function method has been used for this calculation. A more qualitative discussion is given for a high vacancy concentration, from which it is suggested that the peak still exists for interacting vacancies and that NbC is a system with competitive first-neighbors (metal-metalloid) and second-neighbors (metal-metal) interactions. This affords a rather natural explanation for the high vacancy concentration.