Polar Superhelices in Ferroelectric Chiral Nanosprings

Mechanical Writing of Polar Skyrmionic Topological States via Extrinsic Dzyaloshinskii-Moriya-like Flexoelectricity in Ferroelectric Thin Films

Exploring complex topological structures in condensed matter has shown promising applications in nanotechnology. Although polar topologies such as chiral vortices and skyrmions have been observed in ferroelectric heterostructures, their existence in simple systems has posed challenges due to the absence of intrinsic noncollinear interaction (like Dzyaloshinskii-Moriya interaction in ferromagnetics). Here, we demonstrate that a nanoindentation mechanically switches local polarizations to stable polar topologies, including skyrmions, within a room-temperature PbTiO3 thin film via the flexoelectric effect as a noncollinear (Dzyaloshinskii-Moriya-like) driving force using phase-field simulations. In addition, by moving the indenter, the continuous polarization switching leads to the "writing" of arbitrary polar patterns (such as donut-like skyrmionium). Furthermore, the written topologies can be "erased" by applying a voltage with the same conducted indenter. Therefore, this study shows the writing and erasing process of room-temperature polar topologies in a ferroelectric thin film, which significantly advances their potential applications.

Emergence of non-trivial polar topologies hidden in singular stress field in SrTiO3: topological strain-field engineering

Discovery of non-trivial topological structures in condensed matters holds promise in novel technological paradigms. In contrast to ferromagnetics, where a variety of topological structures such as vortex, meron, and skyrmion have been discovered, only few topological structures can exist in ferroelectrics due to the lack of non-collinear interaction like the Dzyaloshinskii-Moriya interaction in ferromagnetics. Here, we demonstrate that polarization structures with a wide range of topological numbers (winding numbernfrom -3 to +1) can be mechanically excited and designed by the mode-I singular stress field formed near the crack-tip in incipient ferroelectric SrTiO₃. Our phase-field simulations based on Ginzburg-Landau theory successfully reveals that the near-tip polar topology is driven by the flexoelectric coupling with intense strain gradient at the tip, while a variety of the far-field topological structures is triggered by a collaboration between the electrostrictive and flexoelectric effects. The strain (gradient) field analysis further shows that the unexpected topological characters are implied in the singular stress field, which develops a variety of polar topologies near the crack tip. Therefore, our work provides a novel insight into the unusual interplay between mechanical- and ferroelectric-topologies, i.e. 'topological strain-field engineering', which paves the way to the mechanical design of functional topologies in the matter.

Topological ferroelectric nanostructures induced by mechanical strain in strontium titanate

Ferroelectric materials exhibit novel topological polarization configurations due to geometric confinements originating from the material shapes and interfaces at the nanoscale. In this study, we demonstrate that those nontrivial topological ferroelectric nanostructures can be tailored in paraelectric nanoporous materials by mechanical loads using phase-field modeling. That is, in nanoporous strontium titanate, periodically-arrayed ferroelectric nanostructures in the shape of networks are formed due to strain concentrations by mechanical loads, and topological polarization configurations, such as hierarchical vortices, woven fabrics and nested structures of spiral like Hopf fibration, are stabilized in the structures strongly affected by the pore arrangements. Our work indicates that various ferroelectric nanostructures with novel shapes and topologies can be designed by controlling the pore arrangements and strain conditions in nanoporous SrTiO₃, and thus provides a new pathway to realize novel topological ferroelectric nanostructures, which are essential for future nanodevices.

Periodically-arrayed ferroelectric nanostructures induced by dislocation structures in strontium titanate

A dislocation induces ferroelectricity around it in incipient ferroelectric SrTiO3 due to some reasons such as electro-mechanical coupling and it being a one-dimensional ferroelectric nanostructure. Furthermore, this microstructure is arrayed periodically in the material and dislocation structures such as a dislocation wall are formed. Due to these facts, periodically-arrayed ferroelectric nanostructures, which show various intriguing polarization configurations and functionalities depending on the internal periodic structure, may be fabricated by dislocations. The phase-field simulation exhibits that a ferroelectric nano-region induced by the strain concentration and incidental electric field around a dislocation connects with each other in a dislocation wall. As a result, a periodic ferroelectric nano-region, which is a periodically-arrayed ferroelectric nanostructure embedded in paraelectric matrices, is formed. Our findings provide a new pathway for the fabrication of novel functional nanodevices in ferroelectric systems.

Self-ordering of nontrivial topological polarization structures in nanoporous ferroelectrics

Topological field structures, such as skyrmions, merons, and vortices, are important features found in ordered systems with spontaneously broken symmetry. A plethora of topological field structures have been discovered in magnetic and ordered soft matter systems due to the presence of inherent chiral interactions, and this has provided a fruitful platform for unearthing additional groundbreaking functionalities. However, despite being one of the most important classes of ordered systems, ferroelectrics scarcely form topological polarization structures due to their lack of intrinsic chiral interactions. In the present study, we demonstrate using multiphysics phase-field modelling based on the Ginzburg-Landau theory that a rich assortment of nontrivial topological polarization structures, including hedgehogs, antivortices, multidirectional vortices, and vortex arrays, can be spontaneously formed in three-dimensional nanoporous ferroelectric structures. We realize that confining ferroelectrics to trivial geometries that are incompatible with the orientation symmetry may impose extrinsic frustration to the polarization field through the enhancement of depolarization fields at free porous surfaces. This frustration gives rise to symmetry breaking, resulting in the formation of nontrivial topological polarization structures as the ground state. We further topologically characterize the local accommodation of polarization structures by viewing them in a new perspective, in which polarization ordering can be mapped on the order parameter space, according to the topological theory of defects and homotopy theory. The results indicate that the nanoporous structures contain composite topological objects composed of two or more elementary topological polarization structures. The present study therefore offers a playground for exploring novel physical phenomena in ferroelectric systems as well as a novel nanoelectronics characterization platform for future topology-based nanotechnologies.