Anomalous Gate-tunable Capacitance in Graphene Moiré Heterostructures

Abstract

Moiré ferroelectricity in graphene/hexagonal boron nitride (hBN) heterostructures driven by charge ordering instead of traditional lattice displacement has drawn considerable attention because of its fascinating properties and promising applications in neuromorphic computing and so on. Combining the interface engineered ferroelectricity and strong correlations in moiré heterostructures could enable the realization of novel quantum states such as ferroelectric superconductivity and multiferroicity. Here we study the electronic transport properties of twisted double bilayer graphene (TDBLG), aligned with one of the neighbouring hBN. We observe a strong gating hysteresis and the electronic ratchet effect. We find that the top gate is anomalously screened, while the back gate is anomalously doubly efficient in injecting charges into graphene, that is, the effective back gate capacitance is two times larger than its geometry capacitance. This unexpected gate-tunable capacitance causes a dramatic change in electric fields between forward and backward scans. The anomalous change in capacitance could be explained with a spontaneous electric polarization located between top hBN and graphene. Our work provides more insights into the mysterious ferroelectricity in graphene/hBN moiré heterostructures and paves the way to the understanding of the underlying mechanism.