Correlation-driven quantum ratchet in a layer-contrasting moiré structure

In electronic systems, electrons can come in two distinct flavors, itinerant and localized. Itinerant electrons in a dispersive band can be essential to phenomena, such as Dirac fermions and superconductivity. Meanwhile, localized electrons in a flat band tend to enhance electron correlation. Here, we discover that a layer-contrasting moiré potential can continuously convert the electron flavor between localized and itinerant in a unidirectional fashion, realizing a novel quantum ratchet. Specifically, by aligning a Bernal bilayer graphene (BLG) with two hexagonal boron nitride (BN) at vastly different angles, we create a strongly asymmetric moiré potential landscape. Experimental observation reveals a spontaneous dichotomy of localized and itinerant electrons. The enhanced Coulomb correlation in the localized system induces a continuous unidirectional charge conversion between the two systems. Its irreversible nature powers a highly gate-tunable hysteretic response. Theoretical investigation suggests many-body effects are important in mediating the ratcheting charge conversion and stabilizing a remnant polarization. Our study demonstrates engineered moiré landscape as a novel and generic approach to designing and studying the interplay of electrons with distinct natures.