We show that electron-phonon coupling (EPC) is the major source of broadening for the Raman $G$ and $G^{-}$ peaks in graphite and metallic nanotubes. This allows us to directly measure the optical-phonon EPCs from the $G$ and $G^{-}$ linewidths. The experimental EPCs compare extremely well with those from the density functional theory. We show that the EPC explains the difference in the Raman spectra of metallic and semiconducting nanotubes and their dependence on tube diameter. We dismiss the common assignment of the $G^{-}$ peak in metallic nanotubes to a resonance between phonons and plasmons and we attribute it to a resonance between phonons and electron-hole pairs. For metallic tubes, we assign the $G^{+}$ and $G^{-}$ peaks to TO (circumferential) and LO (axial) modes, the opposite of what is commonly done in literature.

• Received 29 August 2005

DOI:https://doi.org/10.1103/PhysRevB.73.155426