Gapless plasmons in carbon nanotubes and their interactions with phonons

Carbon nanotubes possess a very rich spectrum of plasmon modes. For the single wall, metallic carbon nanotubes, in addition to the high frequency plasma modes in the UV range and the depolarization shifted van Hove plasma resonances, there are also low frequency, gapless plasmon modes in the far-infrared frequency range which couple to phonon excitations. We show that defects as well as formation of nanotube bundles can dramatically enhance the optical signatures of this coupling. These results explain the recent Raman-scattering experiments on single nanotubes and bundles. We also propose an experimental test for the idea of the momentum quenching as the main mechanism controlling the observed signatures of the electron-phonon coupling.