Electrons and Phonons in Low-Angle Twisted Bilayer Graphene

A low twist angle between the two stacked crystal networks in bilayer graphene enables self-organized lattice reconstruction with the formation of a periodic domain. This superlattice modulates the vibrational and electronic structures, imposing new rules for electron-phonon coupling and the eventual observation of strong correlation and superconductivity. In this talk we show direct optical images of the crystal superlattice in reconstructed twisted bilayer graphene, generated by the inelastic scattering of light in a nano-Raman spectroscope, working in the tip-enhanced Raman Spectroscopy (TERS) domain. The observation of the crystallographic structure with visible light is made possible due to lattice dynamics localization, the images resembling spectral variations caused by the presence of strain solitons and topological points. The results are rationalized by a nearly-free-phonon model and electronic calculations that highlight the relevance of solitons and topological points, particularly pronounced for structures with small twist angles, and the effect of electron-phonon interaction. The latest is also studied with date dependent doping selecting the available Raman scattering paths. The results shed linght in the physics of strongly correlated systems in twisted bilayer graphene.