  Large variations of up to two orders of magnitude are observed in the Raman
intensity of pristine, suspended quasi-metallic single-walled carbon nanotubes
in response to applied gate potentials. No change in the resonance condition is
observed, and all Raman bands exhibit the same changes in intensity, regardless
of phonon energy or laser excitation energy. The effect is not observed in
semiconducting nanotubes. The electronic energy gaps correlate with the drop in
the Raman intensity, and the recently observed Mott insulating behavior is
suggested as a possible explanation for this effect.
