Observation of Fractional Chern Insulators in a van der Waals Heterostructure

Chern bands are 2D bands which exhibit a quantized Hall conductivity when fully filled. Landau levels are a special case of Chern bands with a Chern number, C = 1. The Hofstadter butterfly, a fractal energy spectrum which forms in electronic systems with a lattice and a strong magnetic field, is also tunable Chern band structure where C can take on any integer value depending on magnetic field and electron density. An outstanding question is whether topological order driven by electron interactions (e.g. the paradigmatic fractional quantum Hall insulator) can exist at fractional filling of a non-Landau level Chern band, i.e. a fractional Chern insulator. We measured the magnetocapacitance of a graphite/hexagonal BN encapsulated bilayer graphene device with a moiré potential between the bilayer and hBN dielectric. We find a Hofstadter butterfly made up of many single-particle Chern bands which are tuned by electron density, magnetic, and electric fields. At fractional filling of some C =-1 and +2 bands, we observe fractional Chern insulators consistent with Laughlin-like states. Our observations open up the experimental study of new topological quantum phase transitions and the realization of lattice-defect based states which are inaccessible in traditional quantum Hall systems.