Exploring adiabatic/non-adiabatic phase transitions in suspended metallic carbon nanotubes

We investigate the non-adiabatic Kohn anomaly in suspended pristine metallic single-walled carbon nanotubes by studying the dependence of the Raman $G$ band and $2D$ band frequency on Fermi energy. We find that by varying temperature, metallic nanotubes can switch between a regime in which the non-adiabatic Kohn anomaly is clearly observed, to a regime where the non-adiabatic Kohn anomaly is absent. Furthermore, we find that the non-adiabatic Kohn anomaly is always accompanied by a dramatic gate-induced modulation of the $G$ band Raman intensity. By establishing a quantitative correlation between the strength of the non-adiabatic Kohn anomaly and the modulation of Raman intensity, we determine that the underlying mechanism that leads to both these effects is the same.