Electronic ratchet effect in a moiré system*

Electronic ferroelectricity represents a new paradigm where spontaneous symmetry breaking is driven by electronic correlations, in contrast to traditional lattice-driven ferroelectricity, which leads to the formation of electric dipoles. Despite the potential application advantages arising from its electronic nature, switchable electronic ferroelectricity remains exceedingly rare. In this talk I will discuss our discovery of a novel electronic ratchet effect in a layer-contrasting graphene-boron nitride moiré heterostructure, which could serve as new evidence of switchable electronic ferroelectricity. Our engineered layer-asymmetric moiré potential landscapes result in layer-polarized localized and itinerant electronic subsystems. At particular fillings of the localized subsystem, we find a non-volatile ratcheting injection of itinerant carriers, which leads to a highly unusual response tuned by gate voltages. Importantly, the memory states can be stabilized in a quasi-continuous fashion, which exhibits behavior markedly distinct from known ferroelectrics. Our experimental observations, simulations, and theoretical analysis suggest that dipolar excitons could be the driving force and elementary ferroelectric units in our system. This indicates a new type of electronic ferroelectricity where the formation of dipolar excitons with aligned moments generates a macroscopic polarization and leads to an electronically-driven ferroelectric response.