Optical Phonon Anomaly in Bilayer Graphene with Ultrahigh Carrier Densities

TL;DR

This study uses first-principles calculations to analyze how electron-phonon coupling in bilayer graphene varies with doping levels, revealing a significant anomaly in optical phonons at high carrier densities due to unique band structure effects.

Contribution

It provides the first detailed ab initio analysis of EPC-induced phonon anomalies in bilayer graphene at ultrahigh carrier densities, emphasizing the role of band structure.

Findings

Strong EPC-induced linewidth and frequency shift for the antisymmetric mode at high doping.

Giant linewidth (>100 cm$^{-1}$) and softening (~60 cm$^{-1}$) near E_F = 0.5 eV.

Band structure effects cause dramatic EPC changes at high carrier densities.

Abstract

Electron-phonon coupling (EPC) in bilayer graphene (BLG) at different doping levels is studied by first-principles calculations. The phonons considered are long-wavelength high-energy symmetric (S) and antisymmetric (AS) 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 AS 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 ($\sim$ 60 cm$^{-1}$). Our \emph{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.