Optical activity and transport in twisted bilayer graphene: Spatial dispersion effects

This study investigates the mechanisms driving optical activity and quantum transport in twisted bilayer graphene systems. We demonstrate that optical activity results from spatial dispersion, making it inadequate to consider the system purely two-dimensional. Therefore, we utilize the transfer matrix method to describe the propagation of electromagnetic waves through multilayer systems, which is compatible with our treatment of spatial dispersion. Using an effective continuum model, we analyze the formation of electron states in the bilayer systems and the correlation of transverse and longitudinal motions of electrons in the two graphene layers. The chiral structure of the atomic lattices results in a finite drag component of the optical conductivity tensor due to incomplete cancellation of these correlations. We show the decisive role of this drag component for optical activity by analyzing the circular dichroism spectrum of the system. Additionally, we calculate the dc conductivity, which shows that the twisted bilayer graphene supports a quantum conductivity value proportional to ${e}^{2}/h$ at the intrinsic Fermi energy.