Resonant Raman scattering and the zone-folded electronic structure in single-wall nanotubes

Resonant Raman scattering from optical phonons in single-wall nanotubes of mean diameter 1.1 nm has been measured for incident laser beams with wavelength between 450 and 800 nm. The scattering intensities exhibit anomalies in the vicinity of 690 nm, there being a minimum in the longitudinal components and a maximum in the transverse. Our optical-transmission measurement also exhibits a dip in the same photon energy range. The anomalies are caused by the incident laser beam resonant with the zone-folded electronic structures in nanotubes. These results provide direct experimental evidence of the critical point in the density of states produced by the cylindrical symmetry of nanotubes.