Visualizing delocalized correlated electronic states in twisted double bilayer graphene

TL;DR

This study uses scanning tunneling microscopy and spectroscopy to reveal delocalized correlated electronic states in twisted double bilayer graphene, showing energy splitting and symmetry breaking driven by electron interactions.

Contribution

It provides the first direct visualization of delocalized correlated states in twisted double bilayer graphene and links these to exchange-driven symmetry breaking.

Findings

Observation of van Hove singularity splitting at half-filling

Detection of spatially delocalized correlated states

Self-consistent Hartree-Fock suggests exchange-driven symmetry breaking

Abstract

The discovery of interaction-driven insulating and superconducting phases in moir\'e van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moir\'e unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moir\'e systems.