  Exciton effects are studied in single-wall boron-nitride (BN) nanotubes.
Linear absorption spectra are calculated with changing the chiral index of the
zigzag nanotubes. We consider the extended Hubbard model with atomic energies
at the boron and nitrogen sites. Exciton effects are calculated using the
configuration interaction technique. The Coulomb interaction dependence of the
band gap, the lowest exciton energy, and the binding energy of the exciton are
discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite
interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the
exciton is 0.50 eV for these parameters. This energy agrees well with that of
other theoretical investigations. We find that the energy gap and the binding
energy are almost independent of the geometries of the nanotubes. This novel
property is in contrast with that of the carbon nanotubes which show metallic
and semiconducting properties depending on the chiral index.
