Nonflat bands and chiral symmetry in magic-angle twisted bilayer graphene

Abstract: In this work, we study an interacting tight-binding model of magic-angle twisted bilayer graphene (MATBG), with a twist angle of $1.05^\circ$. We derive effective theories based on a mean-field normal state at charge neutrality, thereby including the renormalizations coming from integrating out high-energy modes. In these theories, the flat bands display a sizable increase of the bandwidth, suggesting the renormalization of the magic angle. Additionally, the corresponding wavefunctions flow towards the limit of perfect particle-hole symmetry and sublattice polarization (the 'chiral' limit). We further represent the flat bands in the 'vortex Chern' basis and discuss the implications on the dynamics, regarding the 'flat' and 'chiral' symmetries of MATBG, as manifested in the symmetry-broken states at neutrality.