Moiré Capacitor Effect Stabilizes Fractional Chern Insulators in Rhombohedral Graphene 

The parent band of rhombohedral graphene can host both chiral superconductors or fractional Chern insulators (FCIs) depending on whether or not it is aligned with a boron nitride (hBN) substrate. Yet it has remained unclear how this nominally weak coupling so strongly alters the phase diagram. We propose and derive an electrostatic mechanism — the moiré capacitor effect — to explain the transformative effect of hBN. We explain its common origin with the physics of Fermi velocity renormalization in monolayer graphene and discuss its effect on the phase diagram, from charge neutrality to the formation of the parent Chern band. By developing a generalized stability criterion for the formation of FCIs away from the Landau level limit, we explain the appearance of the 2/3 FCI state in both the moiré-proximal and moiré-distant regimes. Our results are substantiated with comprehensive multi-band exact diagonalization calculations.