Longitudinal optical phonons in metallic and semiconducting carbon nanotubes.

We analyze the high-energy Raman modes, G + and G - , in a pair of one metallic and one semiconducting nanotubes. By combining Rayleigh scattering with Raman resonance profiles of the radial breathing mode and the high-energy modes, we show that the observed G - and G + peaks can originate from longitudinal optical phonons of different tubes. The G - peak is the longitudinal mode of the metallic tube; it is broadened and downshifted due to strong electron-phonon coupling in the metallic nanotube. The G + peak is due to the longitudinal mode in the semiconducting tube. This result resolves an ongoing debate in the literature.