Visualizing delocalized correlated electronic states in twisted double bilayer graphene

• Published in: Nature communications Vol. 12; no. 1; p. 2516
• Main Authors: Zhang, Canxun, Zhu, Tiancong, Kahn, Salman, Li, Shaowei, Yang, Birui, Herbig, Charlotte, Wu, Xuehao, Li, Hongyuan, Watanabe, Kenji, Taniguchi, Takashi, Cabrini, Stefano, Zettl, Alex, Zaletel, Michael P., Wang, Feng, Crommie, Michael F.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 04.05.2021; Nature Publishing Group; Nature Portfolio
• Subjects: 147/138; 639/766/119/995; 639/925/918/1052; Bilayers; Broken symmetry; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Conductance; Conduction; Conduction bands; Electric fields; Electron states; Graphene; Heterostructures; Humanities and Social Sciences; Microscopy; multidisciplinary; Resistance; Scanning tunneling microscopy; Science; Science (multidisciplinary); Spectroscopy; Spectrum analysis; Superconductivity; Unit cell
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-021-22711-1

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. Twisted double bilayer graphene is a novel van der Waals system that hosts an electric-field-tunable correlated state at half-filling. Here the authors reveal the delocalized nature of this state by scanning tunnelling microscopy and spectroscopy, suggesting an underlying mechanism of symmetry breaking driven by non-local exchange.