where the Born-Oppenheimer approximation is not valid?

[Farzaneh]

Graphene is a semiconductor with a zero energy gap that turns into a metal if subjected to a voltage Vg. Graphene electrons have two-dimensional massless scattering near the Fermi energy, described by the Dirac cone. It has been observed that the change in Vg leads to the stiffening of the optical oscillatory mode E2g in the center of the first Brillouin zone, which cannot be described by the Born-Oppenheimer approximation.

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