Strong interplay between polar and structural topologies.

Singular stress field induced two-dimensional topological polar structures in SrTiO3: Topological number manipulation via loading modes

As analogies to magnetic skyrmions, polar skyrmions in ferroelectric superlattices and multilayers have garnered widespread attention for their non-trivial topology and novel properties like negative capacitance and nonlinear optical effect. So far, they have only been theoretically predicted to be able to assemble ordered hexagonal skyrmion lattices (SkLs) in ferroelectric thin films. Here, based on phase-field simulations, we report the critical roles of flexoelectricity playing in the stabilization and transformation of polar SkLs. Different polar SkL patterns can emerge in the ferroelectric thin films, including tetragonal-SkL, and hexagonal-SkLs with diverse orientations, as summarized by phase diagrams. These emergent SkL states are attributed to the material anisotropy modified by the flexoelectric effect. Interestingly, we further found that the hexagonal-SkLs can be rotated by applying strain gradient or in-plane electric field to the films. Moreover, a nonreciprocal bending response of tetragonal-SkL is also induced by the flexoelectric effect. Our results provide useful guidelines for the implementation of polar skyrmion lattices in experiments.

Ferroelectric (FE) ultrathin films tend to favor a homogeneous polar state at strong interfacial bound charge screening conditions. Reducing screening parameter (β) may induce topological textures such as vortices, labyrinth stripes, and skyrmion bubbles (sk-bs). Here, we exploreβ- and temperature-induced phase transitions of polarization reversal in FE PbZr0.2Ti0.8O3(PZT) thin films using phase-field simulations under a 1% compressive strain. The results unveil a phase diagram comprising polarization reversal, hysteresis loop and topological structures. At room temperature, a phase transition from FE to antiferroelectric-like (AFE*) state occurs atβ∼0.53. At increasing temperatures, the FE and AFE* phases convert to a paraelectric state and form a tricritical point atβ∼0.78 andT∼ 770 K. Notably, at the phase boundary, a coexistence region of uniform polarization and isolated sk-bs emerges in a narrow screening range, which can be modulated by an applied electric field. Moreover, simulations of thermal effects on the FE to AFE* phase transition via the indirect method reveal a large electrocaloric effect around room temperature at low electric fields. Our findings uncover rich phenomena and elucidate underlying mechanisms in FE thin films, which hold promise for advanced device applications.

Classification and role of modulators on crystal engineering of metal organic frameworks (MOFs)

Topological domain structures have drawn great attention as they have potential applications in future electronic devices. As an important concept linking the quantum and classical magnetism, a magnetic Bloch point, predicted in 1960s but not observed directly so far, is a singular point around which magnetization vectors orient to nearly all directions. Here we show polar Bloch points in tensile-strained ultrathin ferroelectric PbTiO3 films, which are alternatively visualized by phase-field simulations and aberration-corrected scanning transmission electron microscopic imaging. The phase-field simulations indicate local steady-state negative capacitance around the Bloch points. The observation of polar Bloch points and their emergent properties consequently implies novel applications in future integrated circuits and low power electronic devices.

Recently, the observation of ferroelectric vortex arrays has triggered the investigation of topological domain structures and their characteristics. Vortices are typical topological domain structures with chirality in nanoscale ferroelectric materials. The chirality of a single vortex in a nanodot can be easily manipulated, but the collective control of a vortex array is exceptionally difficult and has not yet been realized. This Letter proposes an effective scheme for the collective control of a vortex array and investigates it via phase-field simulations. The results indicate that the collective control of a vortex array with bidirectional switching can be realized by introducing a bending film with periodic large curvatures under alternative electric fields. Furthermore, a general rule for determining the electrically controllable chirality of ferroelectric vortices is proposed. This Letter demonstrates the feasibility of the collective control of vortex arrays and provides insights for developing ferroelectric nanoelectronic devices based on vortex arrays.

Many organic conducting materials are represented by the charge transfer (CT) complexes of tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), which show high conductivity at room temperature. CT complexes of TTF and TCNQ each form columnar structures in crystals that enable the application of CT complexes in the formation of one-dimensional nanostructures such as nanowires and nanorods. Crown ethers units can also be used to support the preparation of self-assembled one-dimensional structures. In this study, we reported that the structures and electrical properties composed of TTF derivative (TTF-ER) and CT complexes composed of (TTF-ER)(TCNQ), (TTF-ER)(F2TCNQ) and (TTF-ER)(F4TCNQ). TTF-ER and the CT complexes organized one-dimensional structures on solid substrate, and in particular, TTF-ER formed high oriented nano-branched structures. In UV–vis and IR spectra, the CT complexes has conductivity similar to that of semiconductors, the electrical conductivities of (TTF-ER)(F2TCNQ) and (TTF-ER)(F4TCNQ) at room temperature were found to be 1.2 × 10−5 and 1.4 × 10−5 S cm−1, respectively.

As a geometric property that cannot overlap with its mirror image, chirality is becoming more and more common in electronic structures. The recent discovery of chiral topological polar structures in artificial ferroelectric superlattices is not only of scientific importance but also holds promise for application in tunable frequency oscillators. Although the tubular chiral polarization vortex has been observed in PbTiO3/SrTiO3 superlattices, the conditions for its stability remain elusive. Here, we construct a temperature-strain phase diagram for the (PbTiO3)10/(SrTiO3)10 superlattices by using phase field simulation in which four domain states exist at different temperatures and misfit strains. It is found that the chiral polarization vortex appears only in a small range of misfit strain, elucidating the absence of chirality in most ferroelectric superlattices. The chiral polarization vortex is an intermediate state between the ordinary achiral polarization vortex and the mixed domain structure. In addition, by changing strain or temperature, chiral vortices can be reversibly transformed into achiral vortices, demonstrating its tunability. The present work provides theoretical guidance for creating the chiral vortex in ferroelectric superlattices.

Phase-field simulations of thermomechanical behavior of MnNi shape memory alloys using finite element method

Morphology of the constituent phase in a microstructure influences the mechanical properties of the materials similar to the chemical composition, crystal structure and volume fraction of the phases. Often to enhance the mechanical properties, heat treatment techniques that exclusively facilitate the morphological evolution without any phase transformation are adopted. In the present work, the mechanism and the kinetics of this morphological transformation governed by the inherent curvature-difference introduced by the shape, referred to shape instabilities, are analysed through phase-field simulations. By monitoring the temporal evolution of the elliptical plate, a finite three-dimensional structure associated with two-phase titanium alloys, it is observed that the transformation is dictated by the recession of the edges, or termination migration. It is identified that the elliptical structure transforms to a rod before assuming an ellipsoidal shape at the midpoint and ultimately, evolves into a spheroid. The results of this work are compared with the existing analytical approach and the deviations introduced by the geometrical approximations of the theoretical study are discussed. Furthermore, for the first time, it is shown that the hitherto assumed monotonic decrease in the driving force is interrupted by a series of ‘aberrations’ predominantly in the first half of the transformation, which intensifies with increase in aspect ratio. An analytical prediction which includes this non-monotonic evolution of the curvature is presented from the outcomes of the simulation.

Two new coordination polymers with the same topological structure, [Zn(L)]_2n·n(H₂O) (<b>1</b>) and [Co(L)]_2n·n(H₂O) (<b>2</b>) (H₂L = 5-(pyridin-2-ylmethoxy)-isophthalic acid), have been hydrothermally synthesized by reactions of metal salts and H₂L. Single crystal X-ray analyses reveal that <b>1</b> and <b>2</b> are isostructural and crystallize in orthorhombic, chiral <i>P</i>2₁2₁2 space group. These compounds feature 2D undulated layer structures with (3,6)-connected <b>kgd</b> topology, which further extended into a 3D suparmolecular framework <i>via</i> intermolecular hydrogen bonds. In addition, the luminescent properties of <b>1</b> and <b>2</b> and the magnetic property of <b>2</b> were also investigated.

Chirality exists universally in nature and is closely related to various exotic physical phenomena. In nanoscale functional materials, various chiral topological structures exist fruitfully together, forming an exotic chiral topological zoo. Chirality has always been challenging to control. Ferroelectric vortices, as well-known in-plane chiral topological domain structures, have been extensively studied experimentally and theoretically in recent years. Compared with ferromagnetic vortices, ferroelectric vortices are two-dimensional, and their chirality only includes circulation but no polarity, which has led to the longstanding misconception that polar vortices, like Bloch-type merons, do not exist in ferroelectric materials. In this paper, we conduct a series of phase-field simulations using ferroelectric epitaxial nanodisks to propose an effective scheme for constructing a three-dimensional polar vortex domain structure with central polarity and a series of strategies for manipulating the chirality of polar vortices (including circulation and polarity), providing inspiration for the design of multistate information memories. The results show that misfit strain, the aspect ratio of a nanodisk, and the mole fraction of Zr and Ti atoms in $\mathrm{PbZ}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{T}{\mathrm{i}}_{x}{\mathrm{O}}_{3}$ are decisive factors to polar vortex generation. Furthermore, applying either an axial electric field or in-plane torque, the polarity of a polar vortex can be changed while keeping its circulation unchanged; the combined application of the axial electric field and in-plane torque realizes alternating changes in the polarity and circulation of a polar vortex. In this paper, we enrich the understanding of ferroelectric topological physics, which is conducive to facilitating the development of ferroelectric nanoelectronic devices.

Voltage controlled 180° magnetization reversal has been achieved in BiFeO3-based multiferroic heterostructures, which is promising for the future development of low-power spintronic devices. However, all existing reports involve the use of an in-plane voltage that is unfavorable for practical device applications. Here, we investigate, using phase-field simulations, the out-of-plane (i.e., perpendicular to heterostructures) voltage controlled magnetism in heterostructures consisting of CoFe nanodots and (110) BiFeO3 thin film or island. It is predicted that the in-plane component of the canted magnetic moment at the CoFe/BiFeO3 interface can be reversed repeatedly by applying a perpendicular voltage across the bottom (110) BiFeO3 thin film, which further leads to an in-plane magnetization reversal in the overlaying CoFe nanodot. The non-volatility of such perpendicular voltage controlled magnetization reversal can be achieved by etching the continuous BiFeO3 film into isolated nanoislands with the same in-plane sizes as the CoFe nanodot. The findings would provide general guidelines for future experimental and engineering efforts on developing the electric-field controlled spintronic devices with BiFeO3-based multiferroic heterostructures.

Screening of the alkali-metal ion containing materials from the Inorganic Crystal Structure Database (ICSD) for high ionic conductivity pathways using the bond valence method

Experimental observation methods for understanding industrially important solid-state sintering are essential for the development of new materials and devices. To experimentally characterize solid-state sintering, the limitations posed by the complexity of target materials, experimental equipment, and observation conditions must be overcome. Therefore, hybrid techniques for predicting sintering behavior based on experimental datasets and physics-based simulation models are highly sought after. Herein, we propose a new technique for combining a physics-based model and experimental observation results from solid-state sintering using a nonsequential Bayesian data assimilation (DA) method. The proposed technique assimilates experimental data—obtained using in situ electron tomography/scanning transmission electron microscopy—into the corresponding phase-field (PF) model to enable high-fidelity PF simulations by estimating multiple material parameters included in the PF model. This study demonstrates the inverse estimation of seven parameters, including temperature-dependent diffusion coefficients, from the time-series information on the morphology of sintered nanoparticles observed in situ. The estimated parameters provide the high-fidelity PF simulation to capture the observed solid-state sintering of copper nanoparticles. Thus, this study contributes to the construction of digital twins for solid-state sintering based on DA-integrated PF simulations and in situ observation datasets and deepens our understanding of the sintering process.