Fractional Chern insulators in magic-angle twisted bilayer graphene

TL;DR

This paper reports the experimental observation of fractional Chern insulators in magic-angle twisted bilayer graphene at low magnetic fields, demonstrating their potential for zero-field realization and manipulation of non-abelian excitations.

Contribution

It provides the first experimental evidence of FCIs in MATBG at low magnetic fields, highlighting the role of native flat Chern bands and quantum band geometry.

Findings

Eight FCI states observed at low magnetic fields

FCIs emerge at 5 T with disappearance of charge density wave states

Magnetic field redistributes Berry curvature, enabling FCIs

Abstract

Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue toward manipulating non-abelian excitations. Early theoretical studies have predicted their existence in systems with energetically flat Chern bands and highlighted the critical role of a particular quantum band geometry. Thus far, however, FCI states have only been observed in Bernal-stacked bilayer graphene aligned with hexagonal boron nitride (BLG/hBN), in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field and limiting its potential for applications. By contrast, magic angle twisted bilayer graphene (MATBG) supports flat Chern bands at zero magnetic field, and therefore offers a promising route toward stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in MATBG 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. Unlike the BLG/hBN platform, we demonstrate that the principal role of the weak magnetic field here is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum band geometry favorable 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 moir\'e systems with native flat Chern bands.