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Published January 28, 2021 | Submitted + Supplemental Material
Journal Article Open

Correlation-driven topological phases in magic-angle twisted bilayer graphene

Abstract

Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron–electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when ±1, ±2 and ±3 electrons occupy each moiré unit cell, and lead to the formation of various correlated phases. Although some phases have been shown to have a non-zero Chern number, the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations. These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron–hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases.

Additional Information

© 2021 Nature Publishing Group. Received 21 August 2020; Accepted 13 November 2020; Published 18 January 2021. We acknowledge discussions with A. Young, G. Refael and S. Mashhadi. The device nanofabrication was performed at the Kavli Nanoscience Institute (KNI) at Caltech. This work was supported by NSF through grants DMR-2005129 and DMR-1723367 and by the Army Research Office under grant award W911NF-17-1-0323. Part of the initial STM characterization was supported by CAREER DMR-1753306. Nanofabrication performed by Y.Z. was supported by the DOE-QIS programme (DE-SC0019166). J.A. and S.N.-P. also acknowledge the support of IQIM (an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation through grant GBMF1250). Y.P. acknowledges support from a startup fund from California State University, Northridge. A.T., C.L. and J.A. are grateful for support from the Walter Burke Institute for Theoretical Physics at Caltech and the Gordon and Betty Moore Foundation's EPiQS Initiative, grant GBMF8682. Y.C. and H.K. acknowledge support from the Kwanjeong fellowship. Data availability: The data that support the findings of this study are available from the corresponding authors on reasonable request. These authors contributed equally: Youngjoon Choi, Hyunjin Kim. The authors declare no competing interests. Peer review information: Nature thanks Vincent Renard, Yayu Wang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Attached Files

Submitted - 2008.11746.pdf

Supplemental Material - 41586_2020_3159_Fig10_ESM.webp

Supplemental Material - 41586_2020_3159_Fig11_ESM.webp

Supplemental Material - 41586_2020_3159_Fig12_ESM.webp

Supplemental Material - 41586_2020_3159_Fig13_ESM.webp

Supplemental Material - 41586_2020_3159_Fig14_ESM.webp

Supplemental Material - 41586_2020_3159_Fig5_ESM.webp

Supplemental Material - 41586_2020_3159_Fig6_ESM.webp

Supplemental Material - 41586_2020_3159_Fig7_ESM.webp

Supplemental Material - 41586_2020_3159_Fig8_ESM.webp

Supplemental Material - 41586_2020_3159_Fig9_ESM.webp

Supplemental Material - 41586_2020_3159_MOESM1_ESM.pdf

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Additional details

Created:
August 22, 2023
Modified:
October 20, 2023