On magic angles and band flattening in twisted bilayer graphene

Abstract

When two graphene layers are rotated from AA or AB configuration by a small angle, the band structure changes dramatically. Numerical calculations have shown that, at certain discrete angles called magic angles, the low energy bands become flat leading to localization of electrons. The origin of this strange behavior, however, is not well understood. Here, I propose a theory that offers an understanding of the phenomenon, focusing on the first magic angle. It is shown that coupling between the layers, in addition to renormalizing the Dirac velocity, introduces higher order momentum terms in the energy dispersion that are not all of the same sign. Partial cancellation among these terms leads to the flatness of the low energy bands. Also, while there is modulation of electron density in real space, there is no localization---the modulation arises due to the superposition of plane wave states with different momenta in the two layers. In addition, it is conjectured that there is an underlying geometric reason for the appearance of more than one magic angle which can be exploited to predict higher magic angles approximately without computing the band structure.