Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene

TL;DR

This study investigates the optical conductivity of twisted bilayer graphene over a broad energy range, revealing multiple van Hove singularities and significant excitonic effects, with first-principles calculations supporting experimental observations.

Contribution

It provides the first comprehensive analysis of high-energy optical features in tBLG, emphasizing the importance of excitonic effects and electron-hole interactions in modeling its spectra.

Findings

Identification of multiple van Hove singularities in tBLG

Evidence of excitonic effects influencing optical spectra

Necessity of including electron-hole interactions in calculations

Abstract

We report a systematic study of the optical conductivity of twisted bilayer graphene (tBLG) across a large energy range (1.2 eV to 5.6 eV) for various twist angles, combined with first-principles calculations. At previously unexplored high energies, our data show signatures of multiple van Hove singularities (vHSs) in the tBLG bands, as well as the nonlinearity of the single layer graphene bands and their electron-hole asymmetry. Our data also suggest that excitonic effects play a vital role in the optical spectra of tBLG. Including electron-hole interactions in first-principles calculations is essential to reproduce the shape of the conductivity spectra, and we find evidence of coherent interactions between the states associated with the multiple vHSs in tBLG.