Visualizing delocalized correlated electronic states in twisted double bilayer graphene

Canxun Zhang,Michael F Crommie,Xuehao Wu,Kenji Watanabe,Tiancong Zhu,Michael P Zaletel,Birui Yang,Shaowei Li,Takashi Taniguchi,Hongyuan Li,Charlotte Herbig,Stefano Cabrini,Alex Zettl,Salman Kahn,Feng Wang

doi:10.1038/s41467-021-22711-1

Abstract

The discovery of interaction-driven insulating and superconducting phases in moiré 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 moiré 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 moiré systems.

Highlights

The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems
Experimental signatures of interaction-driven electronic states in moiré van der Waals (vdW) stacks were first observed in magic-angle twisted bilayer graphene, where the coexistence of insulating and superconducting phases resembles the phase diagram of high temperature cuprate superconductors[3,4,5,6,7]
We observe that the low-energy electronic structure of twisted double bilayer graphene (tDBLG) is dominated by two narrow moiré mini-bands that we refer to as the conduction flat band (CFB) and the valence flat band (VFB), each of which accommodates four electrons per moiré unit cell due to spin and valley degeneracies

Summary

Introduction

The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. In contrast to the electronic structure of tBLG1,2,21–25, we find that flat band wavefunctions in tDBLG are delocalized in real space and that the correlation-induced LDOS reduction is present everywhere in the moiré unit cell. The CFB+ and CFB– peaks and the dip feature at the Fermi level persist throughout the entire moiré unit cell and do not appear to depend strongly on local stacking order.

Results

Conclusion