Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 118, Issue 4, Pages -Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.2017366118
Keywords
scanning tunneling microscopy; scanning tunneling spectroscopy; graphene; electron correlations; topology
Categories
Funding
- Programmable Quantum Materials, an Energy Frontier Research Center - US Department of Energy, Office of Science, Basic Energy Sciences [DE-SC0019443]
- Air Force Office of Scientific Research [FA9550-16-1-0601]
- Office of Naval Research [N00014-17-1-2967]
- European Union [844271]
- European Research Council [ERC-2015-AdG694097]
- Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) [EXC 2056 -390715994, RTG 2247, SFB925, IT1249-19]
- Max Planck-New York City Center for Non-Equilibrium Quantum Phenomena
- DFG under Germany's Excellence Strategy - Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) [EXC 2004/1 -390534769]
- DFG [SPP 2244]
- Simons Foundation [579913]
- Marie Curie Actions (MSCA) [844271] Funding Source: Marie Curie Actions (MSCA)
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This study demonstrates the formation of emergent correlated phases in multilayer rhombohedral graphene without the need for twisted van der Waals layers. The study shows that two layers of bilayer graphene twisted by a tiny angle can host large regions of uniform rhombohedral four-layer graphene with a sharp van Hove singularity. Furthermore, the study suggests that the broken symmetry state in ABCA graphene could be attributed to a charge-transfer excitonic insulator or a ferrimagnet.
Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene-a simple material that also exhibits a flat electronic band edge but without the need of having a moire superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3-5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.
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