Hierarchical ferroelectric and ferrotoroidic polarizations coexistent in nano-metamaterials

Design of electromagnetic metasurface using two dimensional crystal nets

Metasurfaces are of great interest as they exhibit unique electromagnetic properties. Currently, metasurface design focuses on generating new meta-atoms and their combinations. Here a topological database, reticular chemistry structure resource (RCSR), is introduced to bring a new dimension and more possibilities for metasurface design. RCSR has over 200 two-dimensional crystal nets, among which 72 are identified as suitable for metasurface design. Using a simple metallic cross as the metaatom, 72 metasurfaces are constructed from the atom positions and lattice vectors of the crystal nets templates. The transmission curves of all the metasurfaces are calculated using the finite-difference time-domain method. The calculated transmission curves have good diversity, showing that the crystal nets approach is a new engineering dimension for metasurface design. Three clusters are found for the calculated curves using the K-means algorithm and principal component analysis. The structure-property relationship between metasurface topology and transmission curve is investigated, but no simple descriptor has been found, indicating that further work is still needed. The crystal net design approach developed in this work can be extended to three-dimensional design and other types of metamaterials like mechanical materials.

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.

Organometallic complexes of carbon nanotori

New organometallic complexes of carbon nanotori were designed and theoretically described by means of density functional theory. After a systematic structural search, it was found that energetically favorable complexes were formed by the metal atoms Cr and Ni, both located at the center of a nanotorus with diameter around 5 Å and 120 carbon atoms. The nature of the metal-nanotorus interaction shows a partial polar-covalent character, different from those found in other well-known organometallic compounds. Interactions were studied through molecular orbitals and thermodynamic stability. Ten bonds are set up between the metal atom and nanotorus, confirmed by electron density topology analysis, showing ten bond critical points among the metal atoms and the surrounding carbon atoms. The response of the induced electron current caused by a magnetic field perpendicular to the nanotorus was analyzed to explain the electron delocalization and aromaticity of the complexes. Only in the case of the chromium complex, the electron density is fully delocalized on the whole complex. According to a geometry-based index of aromaticity, interaction with the metal atom only changes the aromatic character of the carbon rings slightly. Also, induced currents were used to elucidate the presence of a ferrotoroidal behavior. The isolated nanotorus and its compound with a single Ni atom have well-defined ferrotoroidal behavior because they present broken symmetries and could help to design a topological insulator. Meanwhile, the nanotorus with a Cr atom at the center lacks ferrotoroidal behavior as a consequence of the absence of magnetic vortices. Graphical abstract Organometallic complex of carbon nanotorus with chromium and induced currents on it by applying an external magnetic field.

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.

Polar Superhelices in Ferroelectric Chiral Nanosprings

Topological objects of nontrivial spin or dipolar field textures, such as skyrmions, merons, and vortices, interacting with applied external fields in ferroic materials are of great scientific interest as an intriguing playground of unique physical phenomena and novel technological paradigms. The quest for new topological configurations of such swirling field textures has primarily been done for magnets with Dzyaloshinskii-Moriya interactions, while the absence of such intrinsic chiral interactions among electric dipoles left ferroelectrics aside in this quest. Here, we demonstrate that a helical polarization coiled into another helix, namely a polar superhelix, can be extrinsically stabilized in ferroelectric nanosprings. The interplay between dipolar interactions confined in the chiral geometry and the complex strain field of mixed bending and twisting induces the superhelical configuration of electric polarization. The geometrical structure of the polar superhelix gives rise to electric chiralities at two different length scales and the coexistence of three order parameters, i.e., polarization, toroidization, and hypertoroidization, both of which can be manipulated by homogeneous electric and/or mechanical fields. Our work therefore provides a new geometrical configuration of swirling dipolar fields, which offers the possibility of multiple order-parameters, and electromechanically controllable dipolar chiralities and associated electro-optical responses.