Evidence of electron-electron interactions around Van Hove singularities of a graphene Moir\'e superlattice

TL;DR

This study provides experimental evidence of electron-electron interactions near Van Hove singularities in twisted bilayer graphene, achieved by tuning low-energy VHSs close to the Fermi level using a small twist angle.

Contribution

It demonstrates a method to access and visualize electron-electron interactions around VHSs in graphene via controlled twisting, overcoming previous energy distance limitations.

Findings

Observation of VHS splitting in twisted bilayer graphene.

Visualization of symmetry breaking around VHSs.

Evidence of strong electron-electron interactions near VHSs.

Abstract

A variety of new and interesting correlated states have been predicted in graphene monolayer doped to Van Hove singularities (VHSs) of its density-of-state (DOS). However, tuning the Fermi energy to reach a VHS of graphene by either gating or chemical doping is prohibitively difficult, owning to their large energy distance (3 eV). Therefore, these correlated states, which arise from effects of strong electron-electron interactions at the VHSs, have remained experimentally elusive. Here, we report experimental evidences of electron-electron interactions around the VHSs of a twisted bilayer graphene (TBG) through scanning tunneling microscopy measurements. By introducing a small twisted angle between two adjacent graphene sheets, we are able to generate low-energy VHSs arbitrarily approaching the Fermi energy. The split of the VHSs are observed and the symmetry breaking of electronic states around the VHSs are directly visualized. These results experimentally demonstrate the important effects of electron-electron interactions on electronic properties around the VHSs of the TBG, therefore providing motivation for further theoretical and experimental studies in graphene systems with considering many-body interactions.