HYBRIDIZATION EFFECTS AND METALLICITY IN SMALL RADIUS CARBON NANOTUBES

Hybridization of the sigma and pi* states of the graphene network is shown to be as important as band-folding effects in determining the me tallicity of small radius carbon nanotubes. Using detailed plane-wave ab initio pseudopotential local density functional (LDA) calculations, we find that the electronic properties of small tubes are significant ly altered from those obtained in previous tight-binding calculations. Strongly modified low-lying conduction band states are introduced int o the band gap of insulating tubes because of strong sigma-pi* hybrid ization. As a result, the LDA gaps of some tubes are lowered by more t han 50%, and a tube previously predicted to be semiconducting is shown to be metallic.