Misalignment instability in magic-angle twisted bilayer graphene on hexagonal boron nitride

TL;DR

This study investigates the stability and electronic properties of magic-angle twisted bilayer graphene on hexagonal boron nitride, revealing a critical misalignment angle of about 0.54° where the structure is most stable and exhibits gapped flat bands.

Contribution

It provides the first detailed analysis of the stability and electronic structure of TBG/BN heterostructures at various twist angles, identifying a specific angle with enhanced stability and characteristic electronic features.

Findings

The most stable configuration occurs at a twist angle of approximately 0.54°.

Gaps open at the Fermi level in certain stacking configurations.

The electronic structure shows a critical angle where flat bands narrow and gaps widen.

Abstract

We study the stability and electronic structure of magic-angle twisted bilayer graphene on the hexagonal boron nitride (TBG/BN). Full relaxation has been performed for commensurate supercells of the heterostructures with different twist angles ($\theta'$) and stackings between TBG and BN. We find that the slightly misaligned configuration with $\theta' = 0.54^\circ$ and the AA/AA stacking has the globally lowest total energy due to the constructive interference of the moir\'{e} interlayer potentials and thus the greatly enhanced relaxation in its $1 \times 1$ commensurate supercell. Gaps are opened at the Fermi level ($E_F$) for small supercells with the stackings that enable strong breaking of the $C_2$ symmetry in the atomic structure of TBG. For large supercells with $\theta'$ close to those of the $1 \times 1$ supercells, the broadened flat bands can still be resolved from the spectral functions. The $\theta' = 0.54^\circ$ is also identified as a critical angle for the evolution of the electronic structure with $\theta'$, at which the energy range of the mini-bands around $E_F$ begins to become narrower with increasing $\theta'$ and their gaps from the dispersive bands become wider. The discovered stablest TBG/BN with a finite $\theta'$ of about $0.54^\circ$ and its gapped flat bands agree with recent experimental observations.