Moiré-less Correlations in ABCA Graphene

Proceedings Of The National Academy Of Sciences Of The United States Of America 118 (4), (2021)

Moiré-less Correlations in ABCA Graphene

Alexander Kerelsky, Carmen Rubio-Verdú, Lede Xian, Dante M. Kennes, Dorri Halbertal, Nathan Finney, Larry Song, Simon Turkel, Lei Wang, K. Watanabe, T. Taniguchi, James Hone, Cory Dean, Dmitri Basov, Angel Rubio, Abhay N. Pasupathy

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform towards 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 but without the need of having a moir´e 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 (micronscale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp flat band of 3-5 meV half-width. We demonstrate that when this flat band 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 indicate that the 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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This work was supported by Programmable Quantum Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science,Basic Energy Sciences (BES), under award DESC0019443.STM equipment support was provided by the Air Force Office of Scientific Research via grant FA9550-16-1-0601 and by the Office of Naval Research via grant N00014-17-1-2967. DMK acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy - Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1 - 390534769. Gef¨ordert durch die Deutsche Forschungsgemeinschaft (DFG) im Rahmen der Exzellenzstrategie des Bundes und der L¨ander - Exzellenzcluster Materie und Licht f¨ur Quanteninformation (ML4Q) EXC 2004/1 - 390534769.LX and AR acknowledge the support by the European Research Council (ERC-2015-AdG694097), cluster of Excellence AIM, SFB925 and Grupos Consolidados (IT1249-19). The Flatiron Institute is a division of the Simons Foundation. We acknowledge support from the Max PlanckNew York City Center for Non-Equilibrium Quantum Phenomena.

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