Terahertz circular dichroism in commensurate twisted bilayer graphene
Spenser Talkington, E. J. Melé
Abstract
We report calculations of terahertz ellipticities in large-angle, $21.{79}^{\ensuremath{\circ}}$ and $38.{21}^{\ensuremath{\circ}}$, commensurate twisted bilayer graphene and predict values as high as 1.5 mdeg in the terahertz region for this nonmagnetic material. This terahertz circular dichroism is surprisingly large and has a magnitude comparable to that of chiral materials in the visible region. At low frequencies, the dichroic response arises from the strong interlayer hybridization, which allows us to probe the symmetry and strength of these couplings. Crucially, lateral interlayer translation tunes this response, in striking contrast to the near invariance of small twist angle bilayer to interlayer translation. We examine the magnitude and phase of the interlayer coupling for all structures containing fewer than 400 atoms per unit cell. Finally, we find that the dichroism can be manipulated by applying an electric field or with doping.