Visualizing delocalized correlated electronic states in twisted double bilayer graphene

Canxun Zhang, Tiancong Zhu, Salman Kahn, Shaowei Li, Birui Yang, Charlotte Herbig, Xuehao Wu, Hongyuan Li, Kenji Watanabe, Takashi Taniguchi, Stefano Cabrini, Alex Zettl, Michael P. Zaletel (), Feng Wang () and Michael F. Crommie ()
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Canxun Zhang: University of California
Tiancong Zhu: University of California
Salman Kahn: University of California
Shaowei Li: University of California
Birui Yang: University of California
Charlotte Herbig: University of California
Xuehao Wu: University of California
Hongyuan Li: University of California
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Stefano Cabrini: Molecular Foundry, Lawrence Berkeley National Laboratory
Alex Zettl: University of California
Michael P. Zaletel: University of California
Feng Wang: University of California
Michael F. Crommie: University of California

Nature Communications, 2021, vol. 12, issue 1, 1-8

Abstract: 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.

Date: 2021
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DOI: 10.1038/s41467-021-22711-1

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