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.

• Received 26 September 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.075501