Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment

TL;DR

This paper compares theoretical predictions and experimental measurements of symmetry and electronic properties in rotated graphene layers, revealing discrepancies that challenge existing models.

Contribution

It provides direct experimental evidence that non-Bernal rotations preserve graphene symmetry, contradicting many theoretical predictions.

Findings

No Van Hove singularity detected

Fermi velocity remains largely unchanged

Weak interlayer coupling observed

Abstract

Graphene stacked in a Bernal configuration (60 degrees relative rotations between sheets) differs electronically from isolated graphene due to the broken symmetry introduced by interlayer bonds forming between only one of the two graphene unit cell atoms. A variety of experiments have shown that non-Bernal rotations restore this broken symmetry; consequently, these stacking varieties have been the subject of intensive theoretical interest. Most theories predict substantial changes in the band structure ranging from the development of a Van Hove singularity and an angle dependent electron localization that causes the Fermi velocity to go to zero as the relative rotation angle between sheets goes to zero. In this work we show by direct measurement that non-Bernal rotations preserve the graphene symmetry with only a small perturbation due to weak effective interlayer coupling. We detect neither a Van Hove singularity nor any significant change in the Fermi velocity. These results suggest significant problems in our current theoretical understanding of the origins of the band structure of this material.