Electronic excitations and deexcitations in narrow-gap carbon nanotubes

The band structures play an important role in excitations and deexcitations in narrow-gap carbon nanotubes. The temperature-induced free carriers exist in the low energy states and cause the low-frequency intraband and interband e-h excitations. Such kinds of excitations could be the effective decay channels for the excited states of the lowest conduction and valence bands. The Coulomb decay rates are sensitive to the changes in radius, wavevector, temperature and deexcitation mechanisms. Those with interband decay increase as the radius increases, while the others with intraband decay exhibit the opposite behavior except at low temperature. The decay rates are dominated by the intraband deexcitations. For the conduction-band states, the decay rates quickly rise and then decline with increasing temperature, and a linear increase appears as the temperature further increases. The wavevector dependence, on the other hand, is weak. As to the valence-band states, the decay rates drop rapidly when the initial states deviate from the band-edge state, and they increase as the temperature becomes higher. Moreover, the decay rates of narrow-gap carbon nanotubes contrast sharply with those of metallic and moderate-gap ones. For example, they are the largest at room temperature. The femtosecond time-resolved spectroscopies could be utilized to verify the predicted results.