Phonon softening in individual metallic carbon nanotubes due to the Kohn anomaly

We have studied the line shape and frequency of the G band Raman modes in individual metallic single walled carbon nanotubes (M-SWNTs) as a function of Fermi level (epsilon(F)) position, by tuning a polymer electrolyte gate. Our study focuses on the data from M-SWNTs where explicit assignment of the G(-) and G(+) peaks can be made. The frequency and line shape of the G(-) peak in the Raman spectrum of M-SWNTs is very sensitive to the position of the Fermi level. Within +/- h omega/2 (where h omega is the phonon energy) around the band crossing point, the G(-) mode is softened and broadened. In contrast, as the Fermi level is tuned away from the band crossing point, a semiconductinglike G band line shape is recovered both in terms of frequency and linewidth. Our results confirm the predicted softening of the A-symmetry LO phonon mode frequency due to a Kohn anomaly in M-SWNTs.