Optical phonon anomaly in Bernal stacked bilayer graphene with ultrahigh carrier densities

Electron-phonon coupling (EPC) in Bernal stacked bilayer graphene (BLG) at different doping levels is studied by first-principles calculations. The phonons considered are long-wavelength high-energy symmetric and antisymmetric optical modes. Both are shown to have distinct EPC-induced phonon linewidths and frequency shifts as a function of the Fermi level E-F. We find that the antisymmetric mode has a strong coupling with the lowest two conduction bands when the Fermi level E-F is nearly 0.5 eV above the neutrality point, giving rise to a giant linewidth (more than 100 cm(-1)) and a significant frequency softening (similar to 60 cm(-1)). Our ab initio calculations show that the origin of the dramatic change arises from the unusual band structure in BLG. The results highlight the band structure effects on the EPC in BLG in the high-carrier-density regime.