Unconventional ferroelectricity in moiré heterostructures

The freedom of stacking different layers of 2D van der Waals (vdW) materials with an arbitrary twist angle opens up tremendous opportunities in studying emergent properties that are not accessible in the natural crystal form. In particular, the recently discovered magic-angle twisted bilayer graphene shows that the twist angle and moiré potential can substantially enhance electron correlations for spontaneously broken symmetry states and turn semi-metallic graphene into insulator, superconductor, and quantum anomalous Hall insulator at will. With the right system and careful design, interaction-driven broken symmetry states can have many other manifestations that are not only fundamentally intriguing but also invaluable to new device applications. Here, we report on our observation of switchable ferroelectricity in graphene-based moiré vdW heterostructures. Our systematic transport measurements reveal rich and striking response as a function of the displacement field and electron filling, not previously observed in any 2D systems, and beyond the framework of conventional ferroelectrics. Our study may pave the way for a new class of 2D multiferroics, which has a potential implementation in many important applications, such as memristive devices for neuromorphic computing.