Quasi-one-dimensional metallic conduction channels in exotic ferroelectric topological defects

On-demand nanoengineering of in-plane ferroelectric topologies

Hierarchical assemblies of ferroelectric nanodomains, so-called super-domains, can exhibit exotic morphologies that lead to distinct behaviours. Controlling these super-domains reliably is critical for realizing states with desired functional properties. Here we reveal the super-switching mechanism by using a biased atomic force microscopy tip, that is, the switching of the in-plane super-domains, of a model ferroelectric Pb0.6Sr0.4TiO3. We demonstrate that the writing process is dominated by a super-domain nucleation and stabilization process. A complex scanning-probe trajectory enables on-demand formation of intricate centre-divergent, centre-convergent and flux-closure polar structures. Correlative piezoresponse force microscopy and optical spectroscopy confirm the topological nature and tunability of the emergent structures. The precise and versatile nanolithography in a ferroic material and the stability of the generated structures, also validated by phase-field modelling, suggests potential for reliable multi-state nanodevice architectures and, thereby, an alternative route for the creation of tunable topological structures for applications in neuromorphic circuits.

Nanoconfinement Effect in Water Processable Discrete Molecular Complex-Based Hybrid Piezo- and Thermo-Electric Nanogenerator

Water-processable hybrid piezo- and thermo-electric materials have an increasing range of applications. We use the nanoconfinement effect of ferroelectric discrete molecular complex [Cu(l-phe)(bpy)(H2O)]PF6·H2O (1) in a nonpolar polymer 1D-nanofiber to envision the high-performance flexible hybrid piezo- and thermo-electric nanogenerator (TEG). The 1D-nanoconfined crystallization of 1 enhances piezoelectric throughput with a high degree of mechano-sensitivity, i.e., 710 mV/N up to 3 N of applied force with 10,000 cycles of unaffected mechanical endurance. Thermoelectric properties analysis shows a noticeable improvement in Seebeck coefficient (∼4 fold) and power factor (∼6 fold) as compared to its film counterpart, which is attributed to the enhanced density of states near the Fermi edges as evidenced by ultraviolet photoelectric spectroscopy and density functional based theoretical calculations. We report an aqueous processable hybrid TEG that provides an impressive magnitude of Seebeck coefficient (∼793 μV/K) and power factor (∼35 mWm-1K-2) in comparison to a similar class of materials.

Electric-field-induced multiferroic topological solitons

Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets.

Mechanical manipulation for ordered topological defects

Randomly distributed topological defects created during the spontaneous symmetry breaking are the fingerprints to trace the evolution of symmetry, range of interaction, and order parameters in condensed matter systems. However, the effective mean to manipulate topological defects into ordered form is elusive due to the topological protection. Here, we establish a strategy to effectively align the topological domain networks in hexagonal manganites through a mechanical approach. It is found that the nanoindentation strain gives rise to a threefold Magnus-type force distribution, leading to a sixfold symmetric domain pattern by driving the vortex and antivortex in opposite directions. On the basis of this rationale, sizeable mono-chirality topological stripe is readily achieved by expanding the nanoindentation to scratch, directly transferring the randomly distributed topological defects into an ordered form. This discovery provides a mechanical strategy to manipulate topological protected domains not only on ferroelectrics but also on ferromagnets/antiferromagnets and ferroelastics.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Nanoscale multistate resistive switching in WO3 through scanning probe induced proton evolution

Multistate resistive switching device emerges as a promising electronic unit for energy-efficient neuromorphic computing. Electric-field induced topotactic phase transition with ionic evolution represents an important pathway for this purpose, which, however, faces significant challenges in device scaling. This work demonstrates a convenient scanning-probe-induced proton evolution within WO₃, driving a reversible insulator-to-metal transition (IMT) at nanoscale. Specifically, the Pt-coated scanning probe serves as an efficient hydrogen catalysis probe, leading to a hydrogen spillover across the nano junction between the probe and sample surface. A positively biased voltage drives protons into the sample, while a negative voltage extracts protons out, giving rise to a reversible manipulation on hydrogenation-induced electron doping, accompanied by a dramatic resistive switching. The precise control of the scanning probe offers the opportunity to manipulate the local conductivity at nanoscale, which is further visualized through a printed portrait encoded by local conductivity. Notably, multistate resistive switching is successfully demonstrated via successive set and reset processes. Our work highlights the probe-induced hydrogen evolution as a new direction to engineer memristor at nanoscale.

Absence of critical thickness for polar skyrmions with breaking the Kittel's law

The period of polar domain (d) in ferroics was commonly believed to scale with corresponding film thicknesses (h), following the classical Kittel's law of d ∝ [Formula: see text]. Here, we have not only observed that this relationship fails in the case of polar skyrmions, where the period shrinks nearly to a constant value, or even experiences a slight increase, but also discovered that skyrmions have further persisted in [(PbTiO3)2/(SrTiO3)2]10 ultrathin superlattices. Both experimental and theoretical results indicate that the skyrmion periods (d) and PbTiO3 layer thicknesses in superlattice (h) obey the hyperbolic function of d = Ah + [Formula: see text] other than previous believed, simple square root law. Phase-field analysis indicates that the relationship originates from the different energy competitions of the superlattices with PbTiO3 layer thicknesses. This work exemplified the critical size problems faced by nanoscale ferroelectric device designing in the post-Moore era.

A self-assembly growth strategy for a highly ordered ferroelectric nanoisland array

Ferroelectric nanoislands have attracted intensive research interest due to their size effect induced exotic physical properties and potential applications in non-volatile ferroelectric memories. However, the self-assembly growth of highly ordered ferroelectric nanoisland arrays is still a challenge. Here, by patterning a LaAlO₃ substrate with etched nanocavities to provide preferential nucleation sites, highly ordered self-assembled BiFeO₃ nanoisland arrays with robust ferroelectric topological quad-domain configurations were achieved. From the thermodynamic and kinetic perspectives, three factors are critical for achieving highly ordered self-assembled nanoisland arrays, that is, preferential nucleation sites, an appropriate relationship between the surface energy and the interface energy, and the growth rate difference of films. This approach can also be employed for the self-assembly growth of nanoisland arrays in other ferroelectric materials, which facilitates the design of ferroelectric nanostructure-based nanodevices.

Nonvolatile ferroelectric domain wall memory integrated on silicon

Ferroelectric domain wall memories have been proposed as a promising candidate for nonvolatile memories, given their intriguing advantages including low energy consumption and high-density integration. Perovskite oxides possess superior ferroelectric prosperities but perovskite-based domain wall memory integrated on silicon has rarely been reported due to the technical challenges in the sample preparation. Here, we demonstrate a domain wall memory prototype utilizing freestanding BaTiO₃ membranes transferred onto silicon. While as-grown BaTiO₃ films on (001) SrTiO₃ substrate are purely c-axis polarized, we find they exhibit distinct in-plane multidomain structures after released from the substrate and integrated onto silicon due to the collective effects from depolarizing field and strain relaxation. Based on the strong in-plane ferroelectricity, conductive domain walls with reading currents up to nanoampere are observed and can be both created and erased artificially, highlighting the great potential of the integration of perovskite oxides with silicon for ferroelectric domain wall memories.

Integrating ferroelectric perovskite oxides on Si is highly desired for electronic applications but challenging. Here, the authors show emergent in-plane ferroelectricity and promising nonvolatile memories based on resistive domain wall in BaTiO₃/Si.

Exploring far-from-equilibrium ultrafast polarization control in ferroelectric oxides with excited-state neural network quantum molecular dynamics

Ferroelectric materials exhibit a rich range of complex polar topologies, but their study under far-from-equilibrium optical excitation has been largely unexplored because of the difficulty in modeling the multiple spatiotemporal scales involved quantum-mechanically. To study optical excitation at spatiotemporal scales where these topologies emerge, we have performed multiscale excited-state neural network quantum molecular dynamics simulations that integrate quantum-mechanical description of electronic excitation and billion-atom machine learning molecular dynamics to describe ultrafast polarization control in an archetypal ferroelectric oxide, lead titanate. Far-from-equilibrium quantum simulations reveal a marked photo-induced change in the electronic energy landscape and resulting cross-over from ferroelectric to octahedral tilting topological dynamics within picoseconds. The coupling and frustration of these dynamics, in turn, create topological defects in the form of polar strings. The demonstrated nexus of multiscale quantum simulation and machine learning will boost not only the emerging field of ferroelectric topotronics but also broader optoelectronic applications.

Nonvolatile Ferroelectric-Domain-Wall Memory Embedded in a Complex Topological Domain Structure

The discovery and precise manipulation of atomic-size conductive ferroelectric domain walls offers new opportunities for a wide range of prospective electronic devices, and the emerging field of walltronics. Herein, a highly stable and fatigue-resistant nonvolatile memory device is demonstrated, which is based on deterministic creation and erasure of conductive domain walls that are geometrically confined in a topological domain structure. By introducing a pair of delicately designed coaxial electrodes onto the epitaxial BiFeO₃  film, a center-type quadrant topological domain with conductive charged domain walls can be easily created. More importantly, reversible switching of the quadrant domain between the convergent state with highly conductive confined walls and the divergent state with insulating confined walls can be realized, resulting in an apparent resistance change with a large on/off ratio of >10⁴  and a technically preferred readout current (up to 40 nA). Owing to restrictions from the clamped quadrant ferroelastic domain, the device exhibits excellent restoration repeatability over 10⁸  cycles and a long retention of over 12 days (>10⁶  s). These results provide a new pathway toward high-performance ferroelectric-domain-wall memory, which may spur extensive interest in exploring the immense potential in the emerging field of walltronics.

Thickness-Dependent Polar Domain Evolution in Strained, Ultrathin PbTiO3 Films

Ferroelectric ultrathin films have great potential in electronic devices and device miniaturization with the innovation of technology. In the process of product commercialization, understanding the domain evolution and topological properties of ferroelectrics is a prerequisite for high-density storage devices. In this work, a series of ultrathin PbTiO₃ (PTO) films with varying thicknesses were deposited on cubic KTaO₃ substrates by pulsed laser deposition and were researched by Cs-corrected scanning transmission electron microscopy (STEM), reciprocal space mapping (RSM), and piezoresponse force microscopy (PFM). RSM experiments indicate the existence of a/c domains and show that the lattice constant varies continuously, which is further confirmed by atomic-scale STEM imaging. Diffraction contrast analysis clarifies that with the decrease in PTO film thickness, the critical thickness for the formation of a/c domains could be missing. When the thickness of PTO films is less than 6 nm, the domain configurations in the ultrathin PTO films are the coexistence of a/c domains and bowl-like topological structures, where the latter ones were identified as convergent and divergent types of meron. In addition, abundant 90° charged domain walls in these ultrathin PTO films were identified. PFM studies reveal clear ferroelectric properties for these ultrathin PTO films. These results may shed light on further understanding the domain evolution and topological properties in ultrathin ferroelectric PTO films.

Periodic Polarization Waves in a Strained, Highly Polar Ultrathin SrTiO3

SrTiO₃ is generally paraelectric with centrosymmetric structure exhibiting unique quantum fluctuation related ferroelectricity. Here we reveal highly polar and periodic polarization waves in SrTiO₃ at room temperature, which is stabilized by periodic tensile strains in a sandwiched PbTiO₃/SrTiO₃/PbTiO₃ structure. Scanning transmission electron microscopy reveals that periodic a/c domain structures in PbTiO₃ layers exert unique periodic tensile strains in the ultrathin SrTiO₃ layer and consequently make the highly polar and periodic states of SrTiO₃. The as-received polar SrTiO₃ layer features peak polar ion displacement of ∼0.01 nm and peak tetragonality of ∼1.07. These peak values are larger than previous results, which are comparable to that of bulk ferroelectric PbTiO₃. Our results suggest that it is possible to integrate large and periodic strain state in oxide films with exotic properties, which in turn could be useful in optical applications and information addressing when used as memory unit.

Investigation of corner states in second-order photonic topological insulator

Recently, higher-order topological insulators have been investigated as a novel topological phase of matter that obey an extended topological bulk-boundary correspondence principle. In this paper, we study the influence of BNN interaction on photonic higher-order corner states. We find both next-nearest-neighbor (NNN) hopping and perfect electric conductor (PEC) boundaries can solely result in two kinds of corner states which are quite different from the traditional "zero-energy" state. To demonstrate this intuitively, we design a novel all-dielectric structure that can effectively shield the influence of NNN couplings while remain the effect of PEC boundaries, so that we can distinguish the contributions from NNN hopping and PEC boundaries. In addition, we also investigate the total contribution on corner states when NNN couplings and PEC boundaries coexist, and some interesting features are revealed. These findings may expand our understanding of the high-order corner modes in a more general framework.

Atomic mapping of periodic dipole waves in ferroelectric oxide

A dipole wave is composed of head-to-tail connected electric dipoles in the form of sine function. Potential applications in information carrying, transporting, and processing are expected, and logic circuits based on nonlinear wave interaction are promising for dipole waves. Although similar spin waves are well known in ferromagnetic materials for their roles in some physical essence, electric dipole wave behavior and even its existence in ferroelectric materials are still elusive. Here, we observe the atomic morphology of large-scale dipole waves in PbTiO3/SrTiO3 superlattice mediated by tensile epitaxial strains on scandate substrates. The dipole waves can be expressed in the formula of y = Asin (2πx/L) + y 0, where the wave amplitude (A) and wavelength (L) correspond to 1.5 and 6.6 nm, respectively. This study suggests that by engineering strain at the nanoscale, it should be possible to fabricate unknown polar textures, which could facilitate the development of nanoscale ferroelectric devices.