Lattice model for the Coulomb interacting chiral limit of the magic angle twisted bilayer graphene: symmetries, obstructions and excitations

TL;DR

This paper develops a localized Wannier state framework for chiral twisted bilayer graphene, analyzing symmetries, topological obstructions, and excitation spectra, and explores how Coulomb interaction range affects electronic properties.

Contribution

It introduces a simple method for constructing valley-polarized Wannier states that preserve symmetries and analyzes excitations and Coulomb effects in the chiral limit.

Findings

Wannier states can be centered on honeycomb lattice sites with preserved symmetries.

Single particle and hole excitations are dominated by on-site and nearest neighbor hopping.

The Coulomb interaction range influences the gap size and effective mass.

Abstract

We revisit the localized Wannier state description of the twisted bilayer graphene, focusing on the chiral limit. We provide a simple method for constructing such 2D exponentially localized -- yet valley polarized -- Wannier states, centered on the sites of the honeycomb lattice, paying particular attention to maintaining all the unobstructed symmetries. This includes the unitary particle-hole symmetry, and the combination of $C_2\mathcal{T}$ and the chiral particle-hole symmetry. The $C_2\mathcal{T}$ symmetry alone remains topologically obstructed and is not represented in a simple site-to-site fashion. We also analyze the gap and the dispersion of single particle and single hole excitations above a strong coupling ground state at integer fillings, which we find to be dominated by the on-site and the nearest neighbor terms of a triangular lattice hopping model, with a minimum at the center of the moire Brillouin zone. Finally, we use the insight gained from this real space description to understand the dependence of the gap and the effective mass on the range of the screened Coulomb interaction.