Chiral response of twisted bilayer graphene

TL;DR

This paper develops a minimal theoretical model for chiral two-dimensional materials like doped twisted bilayer graphene, revealing unique electromagnetic responses, plasmonic modes, and optical activity enhancements due to doping and magnetic fields.

Contribution

It introduces a new effective theory capturing chiral electromagnetic phenomena in twisted bilayer graphene without a topological gap.

Findings

Large paramagnetic response at neutrality point

Substantial longitudinal Hall response upon doping

Chiral plasmonic modes with magnetic moments

Abstract

We present an effective (minimal) theory for chiral two-dimensional materials. These materials possess an electro-magnetic coupling without exhibiting a topological gap. As an example, we study the response of doped twisted bilayers, unveiling unusual phenomena in the zero frequency limit. An in-plane magnetic field induces a huge paramagnetic response at the neutrality point and, upon doping, also gives rise to a substantial longitudinal Hall response. The system also accommodates nontrivial longitudinal plasmonic modes which are associated with a longitudinal magnetic moment, thus endowing them with a chiral character. Finally, we note that the optical activity can be considerably enhanced upon doping.