Fractional Chern insulators in magic-angle twisted bilayer graphene

Yonglong Xie (), Andrew T. Pierce, Jeong Min Park, Daniel E. Parker, Eslam Khalaf, Patrick Ledwith, Yuan Cao, Seung Hwan Lee, Shaowen Chen, Patrick R. Forrester, Kenji Watanabe, Takashi Taniguchi, Ashvin Vishwanath, Pablo Jarillo-Herrero () and Amir Yacoby ()
Additional contact information
Yonglong Xie: Harvard University
Andrew T. Pierce: Harvard University
Jeong Min Park: Massachusetts Institute of Technology
Daniel E. Parker: Harvard University
Eslam Khalaf: Harvard University
Patrick Ledwith: Harvard University
Yuan Cao: Massachusetts Institute of Technology
Seung Hwan Lee: Harvard University
Shaowen Chen: Harvard University
Patrick R. Forrester: Harvard University
Kenji Watanabe: National Institute for Material Science
Takashi Taniguchi: National Institute for Material Science
Ashvin Vishwanath: Harvard University
Pablo Jarillo-Herrero: Massachusetts Institute of Technology
Amir Yacoby: Harvard University

Nature, 2021, vol. 600, issue 7889, 439-443

Abstract: Abstract Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue towards manipulating non-Abelian excitations. Early theoretical studies1–7 have predicted their existence in systems with flat Chern bands and highlighted the critical role of a particular quantum geometry. However, FCI states have been observed only in Bernal-stacked bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN)8, in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field. By contrast, magic-angle twisted BLG9–12 supports flat Chern bands at zero magnetic field13–17, and therefore offers a promising route towards stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in magic-angle twisted BLG enabled by high-resolution local compressibility measurements. The first of these states emerge at 5 T, and their appearance is accompanied by the simultaneous disappearance of nearby topologically trivial charge density wave states. We demonstrate that, unlike the case of the BLG/hBN platform, the principal role of the weak magnetic field is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum geometry favourable for the emergence of FCIs. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in flat moiré Chern bands.

Date: 2021
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DOI: 10.1038/s41586-021-04002-3

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