Displacement field-controlled fractional Chern insulators and charge density waves in a graphene/hBN moiré superlattice

Rhombohedral multilayer graphene, with its flat electronic bands and concentrated Berry curvature, is a promising material for the realization of correlated topological phases of matter. When aligned to an adjacent hexagonal boron nitride (hBN) layer, the graphene develops narrow minibands with non-trivial topology. By tuning an externally-applied electric displacement field, the conduction electrons can either be pushed towards or away from the moiré superlattice. Motivated by the recent observation of the fractional quantum anomalous Hall effect (FQAHE) in the moiré-distant case, we study the opposite moiré-proximal case, where the superlattice potential is considerably stronger. We explore the physics within the moiré conduction bands through capacitance measurements that allow us to determine the inverse electronic compressibility and extract energy gaps of incompressible states. We observe integer and fractional Chern insulator states at superlattice filling factors v = 1, 2/3, and 1/3 with Streda slopes of -1, -2/3, and -1/3, respectively. Remarkably, the v = 1/3 state persists down to a magnetic field of 0.2 T. In addition, we also observe numerous trivial and topological charge density waves. We map out a phase diagram that is highly sensitive to both displacement and magnetic fields, which tune the system between various ground states by modifying the band dispersion and the structure of the electronic wavefunctions. This work demonstrates displacement field control of topological phase transitions in the moiré-proximal limit of rhombohedral pentalayer graphene, creating a highly-tunable platform for studying the interplay between intrinsic band topology and strong lattice effects.