Pseudo-magnetic fields, particle-hole asymmetry, and microscopic effective continuum Hamitonians of twisted bilayer graphene

TL;DR

This paper develops effective continuum models for twisted bilayer graphene at 1.05° using two microscopic approaches, accurately reproduces energy spectra, and uncovers particle-hole asymmetry near the Gamma point not seen in previous models.

Contribution

It introduces a validated continuum theory for twisted bilayer graphene based on microscopic models, revealing particle-hole asymmetry overlooked in earlier models.

Findings

Continuum models match tight-binding spectra with second-order gradient expansion.

Identifies significant particle-hole symmetry breaking near the Gamma point.

Shows differences between ab-initio and existing continuum models.

Abstract

Using the method developed in the companion paper, we construct the effective continuum theories for two different microscopic tight binding models of the twisted bilayer graphene at the twist angle of $1.05^\circ$, one Slater-Koster based and the other ab-initio Wannier based. The energy spectra obtained from the continuum theory -- either for rigid twist or including lattice relaxation -- are found to be in nearly perfect agreement with the spectra from the tight binding models when the gradient expansion is carried out to second order, demonstrating the validity of the method. We also analyze the properties of the Bloch states of the resulting narrow bands, finding non-negligible particle-hole symmetry breaking near the $\Gamma$ point in our continuum theory constructed for the ab-initio based microscopic model due to a term in the continuum theory that was previously overlooked. This reveals the difference with all existing continuum models where the particle-hole symmetry of the narrow band Hilbert space is nearly perfect.