Neutral Magic-Angle Bilayer Graphene: Condon Instability and Chiral Resonances.

The full optical response of twisted bilayer graphene at the neutrality point close to the magic angle within the continuum model (CM) is discussed. First, three different channels consistent with the underlying symmetry are identified, yielding the total, magnetic, and chiral response. Second, the full optical response in the immediate vicinity of the magic angle is numerically calculated, which provides a direct mapping of the CM onto an effective two-band model. It is, further, shown that the ground state of the CM in the immediate vicinity of is unstable toward transverse current fluctuations, a so-called Condon instability. Third, due to the large counterflow, the acoustic plasmonic excitations with typical wave numbers have larger energies than the optical ones and their energy density may be largely enhanced at certain frequencies which are denominated as chiral resonances. Finally, symmetry relations for the optical response and their consequences for the chiral response are discussed.