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

TL;DR

This paper investigates the effects of interactions on the electronic structure of magic-angle twisted bilayer graphene, revealing renormalized flat bands, increased bandwidth, and emergent chiral symmetry properties.

Contribution

It introduces effective theories that incorporate interaction effects, showing how flat bands evolve and how chiral symmetry emerges in MATBG.

Findings

Flat bands exhibit increased bandwidth due to interactions.

Wavefunctions tend toward particle-hole symmetry and sublattice polarization.

Theoretical framework connects flat band properties with chiral symmetry.

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.