  The tangential G-band in the Raman spectra of a metallic single-wall carbon
nanotube shows two peaks: a higher frequency component having the Lorentzian
shape and a lower-frequency component of lower intensity with a
Breit-Wigner-Fano (BWF)-type lineshape. This interesting feature has been
analyzed on the basis of phonon-plasmon coupling in a nanotube. It is shown
that while the gapless semi-acoustic plasmon cannot account for the observed
spectra as claimed by other investigators, the low-lying optical plasmon
corresponding to the tangential motion of the electrons on the nanotube surface
can explain the observed features. In particular, this theory can explain
occurrence of both the Lorentzian and BWF lineshapes in the G-band Raman
spectra of metallic single-wall carbon nanotubes. Furthermore, the theory shows
that the BWF peak moves to higher frequency, has a lower intensity and a lower
half width at higher diameters of the nanotube. All these features are in
agreement with experimental observations.
