Intralayer and interlayer electron-phonon processes in twisted bilayer graphene investigated by resonance Raman spectroscopy

In this presentation, I will discuss the use of Raman spectroscopy to study electron-phonon

interactions in twisted bilayer graphene structures. Raman spectroscopy is a fundamental tool to study twisted bilayer graphene (TBG) since the

Raman response is hugely enhanced when the photons are in resonance with optical transitions of

TBGs. Moreover, new peaks appear in the Raman spectra and are due to phonons within the

interior of the Brillouin zone of graphene that are activated by the Moire superlattice by either the

intralayer or the interlayer electron-phonon processes. I will present multiple-excitation

Raman results in many TBG samples with different twisting angles beyond the magic angle and using several different

laser excitation energies in the NIR, visible and UV ranges. The experimental results reveal two different resonance mechanisms enhance the new Raman peaks, involving intralayer and interlayer electron-phonon interaction. The behavior of phonons both from the acoustic and optic branches are explained by

theoretical calculations of the double-resonance (DR) Raman process in graphene, by imposing

the momentum conservation rules for the intralayer and the interlayer electron-phonon processes.