Moir\'{e} superlattice effects and band structure evolution in near-30-degree twisted bilayer graphene

TL;DR

This study investigates how moiré superlattice effects influence the electronic band structure in twisted bilayer graphene with large twist angles, revealing new minibands, Dirac points, and mechanisms of electron scattering.

Contribution

It demonstrates that moiré effects persist at large twist angles in bilayer graphene and identifies the origin of minigaps and van Hove singularities in this regime.

Findings

Observation of dramatic valence band topology changes

Detection of moiré minibands and secondary Dirac points

Identification of electron scattering mechanisms at large twist angles

Abstract

In stacks of two-dimensional crystals, mismatch of their lattice constants and misalignment of crystallographic axes lead to formation of moir\'{e} patterns. We show that moir\'{e} superlattice effects persist in twisted bilayer graphene (tBLG) with large twists and short moir\'{e} periods. Using angle-resolved photoemission, we observe dramatic changes in valence band topology across large regions of the Brillouin zone, including the vicinity of the saddle point at $M$ and across 3 eV from the Dirac points. In this energy range, we resolve several moir\'{e} minibands and detect signatures of secondary Dirac points in the reconstructed dispersions. For twists $\theta>21.8^{\circ}$, the low-energy minigaps are not due to cone anti-crossing as is the case at smaller twist angles but rather due to moir\'{e} scattering of electrons in one graphene layer on the potential of the other which generates intervalley coupling. Our work demonstrates robustness of mechanisms which enable engineering of electronic dispersions of stacks of two-dimensional crystals by tuning the interface twist angles. It also shows that large-angle tBLG hosts electronic minigaps and van Hove singularities of different origin which, given recent progress in extreme doping of graphene, could be explored experimentally.