Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from nanoARPES

TL;DR

This study demonstrates the electrostatic tuning of van Hove singularities in twisted bilayer graphene using nanoARPES, revealing how doping can control electronic properties and induce correlated phases.

Contribution

It provides the first direct observation of electrically tunable van Hove singularities in twisted bilayer graphene over a wide energy range.

Findings

Van Hove singularities can be shifted by doping in twisted bilayer graphene.

The energy range of tunability extends to 0.4 eV.

Doping enables placement of singularities at the Fermi energy to induce correlated phases.

Abstract

The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue towards engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. While the twist angle-dependence of these properties has been explored, direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines a functional twisted bilayer graphene device using in-operando angle-resolved photoemission with a nano-focused light spot. A twist angle of 12.2$^{\circ}$ is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene.