Neutral magic-angle bilayer graphene: Condon instability and chiral resonances
We discuss the full optical response of twisted bilayer graphene at the neutrality point close to the magic angle within the continuum model (CM). Firstly, we identify three different channels consistent with the underlying $D_3$ symmetry, yielding the total, magnetic, and chiral response. Secondly, we numerically calculate the full optical response in the immediate vicinity of the magic angle $\theta_m$ which provides a direct mapping of the CM onto an effective two-band model. We, further, show that the ground-state of the CM in the immediate vicinity of $\theta_m$ is unstable towards transverse current fluctuations, a so-called Condon instability. Thirdly, 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 we denominate as {\it chiral resonances}. Finally, we discuss symmetry relations for the optical response and their consequences for the chiral response.