Tunable Topological Phase Transitions in Rhombohedral Pentalayer Graphene

In two-dimensional electron systems, a combination of non-trivial band topology and strong electronic interactions can lead to correlated topological phases beyond the single-particle picture. Rhombohedral pentalayer graphene is a promising candidate for realizing such phases. When aligned to a layer of hexagonal boron nitride (hBN) on one side to form a moiré superlattice, the graphene develops narrow topological minibands. Recent experiments have demonstrated the fractional quantum anomalous Hall effect in this system when the conduction electrons are pushed away from the hBN interface. We explore this system in the opposite moiré-proximal limit, where the superlattice potential is considerably stronger, via planar capacitance measurements. Here, we report a variety of correlated phases at fractional fillings of the first moiré band: generalized Wigner crystals, fractional Chern insulators, and symmetry-broken Chern insulators that have both charge density wave order and non-trivial topology. The phase diagram is highly sensitive to both displacement and magnetic fields, which tune the band dispersion and redistribute the electronic wavefunctions over the moiré Brillouin zone. Our work establishes the moiré-proximal limit of rhombohedral pentalayer graphene as a highly-tunable platform for the study of topological phase transitions.