Optical Absorption in Twisted Bilayer Graphene

TL;DR

This paper investigates how the optical absorption spectra of twisted bilayer graphene vary with different stacking geometries and rotation angles, providing a spectroscopic method to identify the twist angle.

Contribution

It introduces a theoretical analysis of optical absorption in twisted bilayer graphene using both tight-binding and continuum models, revealing systematic spectral shifts with rotation angle.

Findings

Absorption spectra evolve continuously with rotation angle.

Spectral peaks relate to van Hove singularities and shift systematically.

Effective continuum model accurately reproduces tight-binding results for small angles.

Abstract

We theoretically study the optical absorption property of twisted bilayer graphenes with various stacking geometries, and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers. We find that the absorption spectrum almost continuously evolves in changing the rotation angle, regardless of the lattice commensurability. The spectrum is characterized by series of peaks associated with the van Hove singularity, and the peak energies systematically shift with the rotation angle. We calculate the optical absorption in two different frameworks; the tight-binding model and the effective continuum model based on the Dirac equation. For small rotation angles less than $10^\circ$, the effective model well reproduces the low-energy band structure and the optical conductivity of the tight-binding model, and also explains the optical selection rule analytically in terms of the symmetry of the effective Hamiltonian.