Optical-absorption spectra in armchair tubes

The optical-absorption spectra of armchair tubes have been studied by using the extended Su-Schrieffer-Heeger (SSH) model with the Coulomb interaction included. It is found that the line shape of the absorption spectra of armchair tubes is anisotropic, not depending on whether the Coulomb interactions are present or not; with the increase of the carbon atom number, the position of the first peak shifts to lower energies and the oscillator strengths of the lower-energy peaks are relatively larger than those of the higher-energy ones; because of the Coulomb interactions, overall peak positions are shifted to higher energies and the oscillator strengths of higher-energy peaks have relatively larger weight than those in the free-electron case.