Unravelling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene

TL;DR

This study combines experimental and theoretical methods to demonstrate that van Hove singularities in twisted bilayer graphene are intrinsic, robust, and present across a wide range of twist angles, revealing their fundamental electronic nature.

Contribution

It provides conclusive evidence of the ubiquity and robustness of van Hove singularities in twisted bilayer graphene across various angles, using combined microscopy, spectroscopy, and modeling.

Findings

Van Hove singularities are present from 1° to 10° twist angles.

The energy separation of vHs reveals the Fermi velocity and interlayer coupling strength.

vHs survive in the presence of a third graphene layer and are unaffected by interlayer distance variations.

Abstract

Extensive scanning tunnelling microscopy and spectroscopy experiments complemented by first principles and parameterized tight binding calculations provide a clear answer to the existence, origin and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1{\deg} to 10{\deg}) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the origin of the related moir\'e corrugation detected in the STM images.