  Current-voltage characteristics of suspended single-wall carbon nanotube
quantum dots show a series of steps equally spaced in voltage. The energy scale
of this harmonic, low-energy excitation spectrum is consistent with that of the
longitudinal low-k phonon mode (stretching mode) in the nanotube. Agreement is
found with a Franck-Condon-based model in which the phonon-assisted tunneling
process is modeled as a coupling of electronic levels to underdamped quantum
harmonic oscillators. Comparison with this model indicates a rather strong
electron-phonon coupling factor of order unity.
