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.