Coulomb interactions in carbon nanotubes

The effect of electron-electron interactions on the electronic properties of nonchiral single-wall carbon nanotubes is investigated by an extended Hubbard model resolved within the generalized unrestricted Hartree-Fock approximation. On-site U and nearest-neighbor u Coulomb interactions are considered in tubules with different geometries at half-filling. A phase diagram is obtained in the coordinates U and $u.$ For the electron-electron interaction strength estimated to hold for graphite, carbon nanotubes would lie close to the boundary region between metallic and insulator density wave states. Therefore, any small external perturbation can substantially modify their electronic properties.