Low-frequency Raman modes in Cs- and Rb-doped single wall carbon nanotubes

Single wall carbon nanotubes (SWNT) doped at saturation by alkali-metals are investigated by Raman scattering. The Raman profiles in the frequency range of the tangential modes (TM) are close for Cs and Rb doping and only weakly dependent on the laser energy. This loss of resonance is assigned to a loss of Van Hove singularities in the electronic density of states. The most striking intrinsic features of the doped compounds are two low-frequency peaks assigned to modes involving both radial motions of tubes and alkali-atoms vibrations.