title: Van Hove singularities and excitonic effects in the optical conductivity of twisted bilayer graphene.
abstract: We report a systematic study of the optical conductivity of twisted bilayer graphene (tBLG) across a large energy range (1.2-5.6 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.
authors.affiliation: School of Applied and Engineering Physics, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Robin W
authors.initials: RW
authors.lastname: Havener
authors.forename: Yufeng
authors.initials: Y
authors.lastname: Liang
authors.forename: Lola
authors.initials: L
authors.lastname: Brown
authors.forename: Li
authors.initials: L
authors.lastname: Yang
authors.forename: Jiwoong
authors.initials: J
authors.lastname: Park