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.
Highlights
By contrast, moiré superlattices with native topological bands[13,14,15,16,17] provide a promising avenue to search for Fractional Chern insulators (FCIs) at zero magnetic field
Features with integer t ≠ 0 and integer s correspond to integer quantum Hall states or Chern insulators (ChIs), some of which have been identified as translation symmetry (TS)-broken states resulting from unit-cell doubling
To demonstrate that our system provides the topological bands and strong correlations essential for the realization of FCIs, we focus on the range of filling factors near ν = 3, as in this density range the band structure can be best approximated by isolated Chern bands
Summary
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. Recent analytical considerations[30] and numerical calculations[31,32,33] have predicted FCI ground states in MATBG aligned with hBN These works show the close competition between FCIs and other correlated phases such as charge density waves (CDWs), and highlight the importance of Berry curvature distribution homogeneity and the quantum metric in stabilizing FCIs in MATBG. The FCIs observed beyond this range, where the parent Chern states possibly reacquire their multicomponent character, are more complex, probably owing to the interplay between multiple degrees of freedom, and demonstrate the potential of MATBG for exploring novel emergent topological order
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