  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.
