Session W41: The Emergence of vdW Synthetic Ferroelectrics: Fundamentals and Applications

Topological Domain Anti-ferroelectricity in Twisted Bilayer Transition Metal Dichalcogenides

The twisted interface of van der Waals (vdW) materials provides a new platform where exotic new material properties emerge. One of the most recent discoveries is the appearance of ferroelectric domains in twisted stacked vdW layers with engineered non-centrosymmetry. While various local probes and global transport measurements reveal the ferroelectric domains with alternating dipole moments formed in the moiré lattice, the nature of the domain dynamics in relation to the ferroelectric hysteresis and its connection to the atomic and moiré structure remains a mystery. In this talk, we discuss a framework for understanding the polar domain dynamics in the moiré superlattice. We show the moiré domains with alternating dipole moments as an unconventional polar state of a domain antiferroelectric (DAF) with the characteristic topology of a domain wall network (DWN) configuration. Monitoring the structural change under the applied electric field by an operando TEM method shows that the DAF responds systematically to the applied electric field to exhibit finite polarization. However, the complete transition to the ferroelectric phase is forbidden due to the topological protection provided by the DWN configurations. While the domain walls can be pinned by the underlying disorder, generating Barkhausen noise in the polarization hysteresis loop, the overall domain dynamics associated with the hysteresis is determined by the topology of the DWNs. At the small twist angle where the DWN disappears, we observe full ferroelectric response, demonstrating the design principle of engineering the response of ferroelectricity and DAF in a moiré grating by tuning the topological configurations of the DWN via twist angle control. Our discoveries of the connection between moiré domain structures and their polar domain dynamics under applied electric field will provide a new insight into engineering the structural response of polar moiré systems in an electrical device operating state. Since the unconventional polar moiré platform is fabricated by stacking atomically thin vdW layers, understanding its dynamics is crucial for the realization of next-generation information storage devices at the ultimate thickness level.

    

Ferroelectric switching at symmetry-broken interfaces by local control of dislocation networks

Recently discovered interfacial ferroelectricity in bilayers of transition metal dichalcogenide films provide an opportunity to combine the potential of semiconducting ferroelectrics with the design flexibility of two-dimensional material devices. In this talk, I will discuss scanning tunneling microscopy experiments revealing ferroelectric domains in a marginally twisted WS₂ bilayer.

We show that by tuning the electric field under a scanning tunneling miscroscpe, one can achieve local control of the ferroelectric domains at room temperature. We discuss their reversible evolution using a string-like model of the domain wall network (DWN). We identify two characteristic regimes of DWN evolution: (i) elastic bending of partial screw dislocations separating smaller domains with twin stackings due to mutual sliding of monolayers at domain boundaries and (ii) merging of primary domain walls into perfect screw dislocations, which become the seeds for the recovery of the initial domain structure upon reversing electric field.   

Out-of-equilibrium layer electric phases in van der Waals multilayers

Dielectric screening in insulators often counteracts an applied displacement field, reducing the total electric field that electrons experience. This ubiquitous screening property is intimately tied to the equilibrium distribution of electrons. In this talk, we discuss how this behavior can be transformed when materials are pushed out-of-equilibrium. In particular, we will discuss how electrons pushed far out-of-equilibrium in layered materials (e.g., van der Waals heterostructures) can naturally display anti-screening behavior: i.e. the layer electric polarization is opposite to the total electric field sustained across layers. When pushed to the extreme, such layer polarizations can become self-induced, exhibiting multiple self-consistent phases of out-of-plane electric dipole (and concomittant with an energy gap) for a single applied displacement field. Strikingly multiple steady states persist even for zero applied displacement field. These out-of-equilibrium layer electric phases can provide on-demand means of controlling electric polarizations in layered materials.   

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.