Neutral Magic‐Angle Bilayer Graphene: Condon Instability and Chiral Resonances

Abstract

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 D 3 symmetry are identified, yielding the total, magnetic, and chiral response. Second, the full optical response in the immediate vicinity of the magic angle θ normalm 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 θ normalm 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.

Chirality Probe of Twisted Bilayer Graphene in the Linear Transport Regime

“…Thus, TBG is intrinsically chiral which has been experimentally demonstrated by observing optical dichroism without breaking time-reversal symmetry . The effect becomes largest for frequencies which induce transitions between states that are maximally delocalized between the two layers such that the misalignment between layers is most effective, , but also in the dc-limit, the chirality of TBG is manifested by intrinsic magnetic–electric coupling. − In the dc-limit with broken time reversal symmetry, an electrical magnetochiral anisotropy is anticipated to emerge . Nevertheless, in conventional transport experiments where the net current in voltage leads is maintained at zero, , the influence of chirality has not been a significant factor thus far.…”