Tunable fractional Chern insulators in rhombohedral graphene superlattices

Fractional Chern insulators showing transport effects with fractionally quantized Hall plateaus under zero magnetic field provide opportunities to engineer topological electronics. By construction of a topological flat band with moiré engineering, intrinsic fractional Chern insulators have been observed in twisted MoTe₂ and rhombohedral pentalayer graphene superlattices. Here we demonstrate moiré superlattices consisting of rhombohedral hexalayer graphene and hexagonal boron nitride that exhibit both integer and fractional quantum anomalous Hall effects. By tuning electrical and magnetic fields at 0 < ν < 1 (v, moiré filling factor), we have observed phase transitions showing a sign reversal of the Hall resistivity at finite magnetic fields. The fractional Chern insulator state at ν = 2/3 survives in the phase transitions, exhibiting a quantized Hall resistivity across both phases. Finally we have theoretically demonstrated the crucial role of the moiré potential in the formation of flat Chern bands. Our work enriches the family of fractional Chern insulators and can advance the exploration of quasi-particles with fractional charge and non-Abelian anyons.