  Raman spectra and electrical conductance of individual, pristine, suspended,
metallic single-walled carbon nanotubes are measured under applied gate
potentials. The G- band is observed to downshift with small applied gate
voltages, with the minima occurring at EF = +/- 1/2 Ephonon, contrary to
adiabatic predictions. A subsequent upshift in the Raman frequency at higher
gate voltages results in a 'W'-shaped Raman shift profile that agrees well with
a non-adiabatic phonon renormalization model. This behavior constitutes the
first experimental confirmation of the theoretically predicted breakdown of the
Born-Oppenheimer approximation in individual single walled carbon nanotubes.

Mesoscale and Nanoscale Physics