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

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 ($θ'$) and stackings between TBG and BN. We find that the slightly misaligned configuration with $θ' = 0.54^\circ$ and the AA/AA stacking has the globally lowest total energy due to the constructive interference of the moiré 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 $θ'$ close to those of the $1 \times 1$ supercells, the broadened flat bands can still be resolved from the spectral functions. The $θ' = 0.54^\circ$ is also identified as a critical angle for the evolution of the electronic structure with $θ'$, at which the energy range of the mini-bands around $E_F$ begins to become narrower with increasing $θ'$ and their gaps from the dispersive bands become wider. The discovered stablest TBG/BN with a finite $θ'$ of about $0.54^\circ$ and its gapped flat bands agree with recent experimental observations.