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

Youngjoon Choi, Hyunjin Kim, Yang Peng, Alex Thomson, Cyprian Lewandowski, Robert Polski, Yiran Zhang, Harpreet Singh Arora, Kenji Watanabe, Takashi Taniguchi, Jason Alicea and Stevan Nadj-Perge ()
Additional contact information
Youngjoon Choi: T. J. Watson Laboratory of Applied Physics, California Institute of Technology
Hyunjin Kim: T. J. Watson Laboratory of Applied Physics, California Institute of Technology
Yang Peng: California State University
Alex Thomson: Institute for Quantum Information and Matter, California Institute of Technology
Cyprian Lewandowski: Institute for Quantum Information and Matter, California Institute of Technology
Robert Polski: T. J. Watson Laboratory of Applied Physics, California Institute of Technology
Yiran Zhang: T. J. Watson Laboratory of Applied Physics, California Institute of Technology
Harpreet Singh Arora: T. J. Watson Laboratory of Applied Physics, California Institute of Technology
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Jason Alicea: Institute for Quantum Information and Matter, California Institute of Technology
Stevan Nadj-Perge: T. J. Watson Laboratory of Applied Physics, California Institute of Technology

Nature, 2021, vol. 589, issue 7843, 536-541

Abstract: 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 phases1–4. Although some phases have been shown to have a non-zero Chern number5,6, 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 correlations7,8. 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.

Date: 2021
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DOI: 10.1038/s41586-020-03159-7

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