We present an effective (minimal) theory for chiral two-dimensional materials. These materials possess an electromagnetic 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 that 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 and our general approach would enable systematic exploration of 2D material heterostructures with optical activity.
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We acknowledge interesting discussions with Luis Brey. This work has been supported by Spain’s MINECO under Grants No. MDM-2014-0377, No. FIS2017-82260-P, No. FIS2015-64886-C5-5-P, and No. FIS2014-57432-P, and by the Comunidad de Madrid under Grant No. S2013/MIT-3007 MAD2D-CM. T. L. acknowledges support by the National Science Foundation, NSF/EFRI Grant No. EFRI-1741660.