Atomic engineering of interfacial polarization switching in van der Waals multilayers

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here, we show that introducing interlayer rotations in multilayer vdW heterostructures modulates both the spatial ordering and switching dynamics of polar domains, engendering unique tunability that is unparalleled in conventional bulk ferroelectrics or polar bilayers. Using operando transmission electron microscopy we show how changing the relative rotations of three WSe2 layers produces structural polytypes with distinct arrangements of polar domains, leading to either a global or localized switching response. Introducing uniaxial strain generates structural anisotropy that yields a range of switching behaviors, coercivities, and even tunable biased responses. We also provide evidence of physical coupling between the two interfaces of the trilayer, a key consideration for controlling switching dynamics in polar multilayer structures more broadly.Comment: 22 pages, 5 figure

Local atomic stacking and symmetry in twisted graphene trilayers

Moir\'e superlattices formed from twisting trilayers of graphene are an ideal model for studying electronic correlation, and offer several advantages over bilayer analogues, including more robust and tunable superconductivity and a wide range of twist angles associated with flat band formation. Atomic reconstruction, which strongly impacts the electronic structure of twisted graphene structures, has been suggested to play a major role in the relative versatility of superconductivity in trilayers. Here, we exploit an inteferometric 4D-STEM approach to image a wide range of trilayer graphene structures. Our results unveil a considerably different model for moir\'e lattice relaxation in trilayers than that proposed from previous measurements, informing a thorough understanding of how reconstruction modulates the atomic stacking symmetries crucial for establishing superconductivity and other correlated phases in twisted graphene trilayers.Comment: 18 pages, 5 figure

Local atomic stacking and symmetry in twisted graphene trilayers

Moiré superlattices formed by twisting trilayers of graphene are a useful model for studying correlated electron behaviour and offer several advantages over their formative bilayer analogues, including a more diverse collection of correlated phases and more robust superconductivity. Spontaneous structural relaxation alters the behaviour of moiré superlattices considerably and has been suggested to play an important role in the relative stability of superconductivity in trilayers. Here we use an interferometric four-dimensional scanning transmission electron microscopy approach to directly probe the local graphene layer alignment over a wide range of trilayer graphene structures. Our results inform a thorough understanding of how reconstruction modulates the local lattice symmetries crucial for establishing correlated phases in twisted graphene trilayers, evincing a relaxed structure that is markedly different from that proposed previously