Local Polar Order Research Articles

Collective dynamics and pair-distribution function of active Brownian ellipsoids in two spatial dimensions

While the collective dynamics of spherical active Brownian particles is relatively well understood by now, the much more complex dynamics of nonspherical active particles still raises interesting open questions. Previous work has shown that the dynamics of rod-like or ellipsoidal active particles can differ significantly from that of spherical ones. Here, we obtain the full state diagram of active Brownian ellipsoids in two spatial dimensions without hydrodynamic interactions depending on the Péclet number and packing density via computer simulations. The system is found to exhibit a rich state behavior that includes cluster formation, local polar order, polar flocks, and disordered states. Moreover, we obtain numerical results and an analytical representation for the pair-distribution function of active ellipsoids. This function provides useful quantitative insights into the collective behavior of active particles with lower symmetry and has potential applications in the development of predictive theoretical models.

Mapping Polar Distortions using Nanobeam Electron Diffraction and a Cepstral Approach.

Measuring local polar ordering is key to understanding ferroelectricity in thin films, especially for systems with small domains or significant disorder. Scanning nanobeam electron diffraction (NBED) provides an effective local probe of lattice parameters, local fields, polarization directions, and charge densities, which can be analyzed using a relatively low beam dose over large fields of view. However, quantitatively extracting the magnitudes and directions of polarization vectors from NBED remains challenging. Here, we use a cepstral approach, similar to a pair distribution function, to determine local polar displacements that drive ferroelectricity from NBED patterns. Because polar distortions generate asymmetry in the diffraction pattern intensity, we can efficiently recover the underlying displacements from the imaginary part of the cepstrum transform. We investigate the limits of this technique using analytical and simulated data and give experimental examples, achieving the order of 1.1 pm precision and mapping of polar displacements with nanometer resolution.

Deforming polar active matter in a scalar field gradient

Active matter with local polar or nematic order is subject to the well-known Simha-Ramaswamy instability. It is so far unclear how, despite this instability, biological tissues can undergo robust active anisotropic deformation during animal morphogenesis. Here we ask under which conditions protein concentration gradients (e.g. morphogen gradients), which are known to control large-scale coordination among cells, can stabilize such deformations. To this end, we study a hydrodynamic model of an active polar material. To account for the effect of the protein gradient, the polar field is coupled to the boundary-provided gradient of a scalar field that also advects with material flows. Focusing on the large system size limit, we show in particular: (a) the system can be stable for an effectively extensile coupling between scalar field gradient and active stresses, i.e. gradient-extensile coupling, while it is always unstable for a gradient-contractile coupling. Intriguingly, there are many systems in the biological literature that are gradient-extensile, while we could not find any that are clearly gradient-contractile. (b) Stability is strongly affected by the way polarity magnitude is controlled. Taken together, our findings, if experimentally confirmed, suggest new developmental principles that are directly rooted in active matter physics.

Analytical and numerical modeling of optical second harmonic generation in anisotropic crystals using ♯SHAARP package

Electric-dipole optical second harmonic generation (SHG) is a second-order nonlinear process that is widely used as a sensitive probe to detect broken inversion symmetry and local polar order. Analytical modeling of the SHG polarimetry of a nonlinear optical material is essential to extract its point group symmetry and the absolute nonlinear susceptibilities. Current literature on SHG analysis involves numerous approximations and a wide range of (in)accuracies. We have developed an open-source package called the Second Harmonic Analysis of Anisotropic Rotational Polarimetry (♯SHAARP.si) which derives analytical and numerical solutions of reflection SHG polarimetry from a single interface (.si) for bulk homogeneous crystals with arbitrary symmetry group, arbitrary crystal orientation, complex and anisotropic linear dielectric tensor with frequency dispersion, a general SHG tensor and arbitrary light polarization. ♯SHAARP.si enables accurate modeling of polarimetry measurements in reflection geometry from highly absorbing crystals or wedge-shaped transparent crystals. The package is extendable to multiple interfaces.

Spontaneous helielectric nematic liquid crystals: Electric analog to helimagnets

Recently, a type of ferroelectric nematic fluid has been discovered in liquid crystals in which the molecular polar nature at molecule level is amplified to macroscopic scales through a ferroelectric packing of rod-shaped molecules. Here, we report on the experimental proof of a polar chiral liquid matter state, dubbed helielectric nematic, stabilized by the local polar ordering coupled to the chiral helicity. This helielectric structure carries the polar vector rotating helically, analogous to the magnetic counterpart of helimagnet. The helielectric state can be retained down to room temperature and demonstrates gigantic dielectric and nonlinear optical responses. This matter state opens a new chapter for developing the diverse polar liquid crystal devices.

Role of locally polar regions in the superconductivity of SrTiO3

A possible cooperation between ferroelectric and superconducting order parameters in SrTiO${}_{3}$ has long been suspected. Here, the authors find that the superconducting critical temperature of SrTiO${}_{3}$ correlates with the length scale of polar order. The findings point to spin-orbit coupling as an important ingredient in the superconductivity of SrTiO${}_{3}$. The ability to control the size of the polar domains in thin films opens up new opportunities to control the superconductivity in SrTiO${}_{3}$ and other superconductors that exhibit local polar order

A New Class of Chiral Nematic Phase with Helical Polar Order (Adv. Mater. 35/2021)

Liquid Crystals A new class of cholesteric phase with local polar order is realized by Fumito Araoka and Hiroya Nishikawa in article number 2101305. In general, the periodicity of the conventional cholesteric liquid crystal corresponds to the half-pitch of the helix, having head-and-tail symmetry in the local nematic director. On the other hand, the newly found polar cholesteric liquid crystal is characterized with a full-pitch periodicity of the helix due to the polar degeneracy in the local nematic director. This delivers some unprecedented properties, such as fast electro-optic response based on the photonic band modulated with polar helix deformation.

Fano resonance and relaxor behavior in Pr doped SrTiO3: A Raman spectroscopic investigation

The phenomenon of relaxor ferroelectrics is studied through detailed Raman spectroscopy on polycrystalline Sr1-xPrxTiO3 (x = 0.01–0.17) samples. The polar mode was observed up to very high temperature ~1000 K indicating presence of dipoles well above the characteristic relaxor temperatures and they develop a short range correlation at 505 K leading to formation of PNRs. Below 505 K, the TO2 polar mode anomalously soften along with Fano resonance and changes in the octahedral tilt angle. This establishes growth of local polar order associated with local structural phase transition at T ~505 K. The local TiO6 octahedral tilt scales with the intensity of the polar mode and hence can be used as an order parameter for relaxor transition. It is shown that the paraelectric to relaxor ferroelectric phase transition in this compound is random polarizability instability driven phenomenon which has been correlated with the local octahedral tilt angle.

Learning to flock through reinforcement.

Flocks of birds, schools of fish, and insect swarms are examples of the coordinated motion of a group that arises spontaneously from the action of many individuals. Here, we study flocking behavior from the viewpoint of multiagent reinforcement learning. In this setting, a learning agent tries to keep contact with the group using as sensory input the velocity of its neighbors. This goal is pursued by each learning individual by exerting a limited control on its own direction of motion. By means of standard reinforcement learning algorithms we show that (i) a learning agent exposed to a group of teachers, i.e., hard-wired flocking agents, learns to follow them, and (ii) in the absence of teachers, a group of independently learning agents evolves towards a state where each agent knows how to flock. In both scenarios, the emergent policy (or navigation strategy) corresponds to the polar velocity alignment mechanism of the well-known Vicsek model. These results (a) show that such a velocity alignment may have naturally evolved as an adaptive behavior that aims at minimizing the rate of neighbor loss, and (b) prove that this alignment does not only favor (local) polar order, but it corresponds to the best policy or strategy to keep group cohesion when the sensory input is limited to the velocity of neighboring agents. In short, to stay together, steer together.

Epitaxial Strain Control of Relaxor Ferroelectric Phase Evolution.

Understanding and ultimately controlling the large electromechanical effects in relaxor ferroelectrics requires intimate knowledge of how the local-polar order evolves under applied stimuli. Here, the biaxial-strain-induced evolution of and correlations between polar structures and properties in epitaxial films of the prototypical relaxor ferroelectric 0.68PbMg1/3 Nb2/3 O3 -0.32PbTiO3 are investigated. X-ray diffuse-scattering studies reveal an evolution from a butterfly- to disc-shaped pattern and an increase in the correlation-length from ≈8 to ≈25 nm with increasing compressive strain. Molecular-dynamics simulations reveal the origin of the changes in the diffuse-scattering patterns and that strain induces polarization rotation and the merging of the polar order. As the magnitude of the strain is increased, relaxor behavior is gradually suppressed but is not fully quenched. Analysis of the dynamic evolution of dipole alignment in the simulations reveals that, while, for most unit-cell chemistries and configurations, strain drives a tendency toward more ferroelectric-like order, there are certain unit cells that become more disordered under strain, resulting in stronger competition between ordered and disordered regions and enhanced overall susceptibilities. Ultimately, this implies that deterministic creation of specific local chemical configurations could be an effective way to enhance relaxor performance.

Ergodicity of fine‐grained canonical relaxor ferroelectric (Bi0.5Na0.5)1‐xBaxTiO3 films

AbstractThe drastic reduction in dimensions in thin films, together with the low crystallization temperatures used, normally results in a large reduction in the grain size. It has been reported that relaxor ferroelectric states are stabilized at room temperature for fine‐grained ceramics and films that behave as normal ferroelectrics for large grains. In this work, the effects of the grain size reduction on the relaxor characteristics are analyzed for a composition that is already a canonical relaxor with a nonergodic state at room temperature: (Bi0.5Na0.5)1‐xBaxTiO3 (BNBT). The comparison of the local polar ordering within BNBT grains studied with piezoresponse force microscopy on large‐grained ceramics and fine‐grained thin films shows that the development of stable long‐range ferroelectric order with the application of an electric field is hampered due to the small grain size of the grains. The ergodic character of the high‐temperature phase is thus stabilized at room temperature, following a similar mechanism as the one discussed for other noncanonical relaxors.

Thermooptical and Dielectric Studies of a Calcium-Induced Ferroelectric Phase in a SrTiO3 Incipient Ferroelectric

We have examined temperature changes of the light refraction, birefringence, dielectric permittivity, and dielectric hysteresis loops in Sr1 – xCaxTiO3 single crystals with x = 0.014 (SCT-1.4). The dielectric properties of Sr1 – xCaxTiO3 with x = 0.007 (SCT-0.7) have been studied. We have performed ab initio calculations of equilibrium structures and total energies for three low-temperature phases of SrTiO3 and CaTiO3, based on which we have determined an expected symmetry of the ground state of their solid solution and spontaneous polarization directions in a calcium-induced ferroelectric phase in Sr1 – xCaxTiO3. In SCT-1.4, we have separated a spontaneous contribution to the light refraction, which arises due to the spontaneous electrooptical effect caused by the spontaneous polarization and its fluctuations. From the spontaneous contribution to the light refraction, based on a previously developed our phenological approach, we have quantitatively determined for the first time the values and temperature dependences of root-mean-square fluctuations of the order parameter—the polarization Psh = 〈P fl 2 〉1/2(short-range, local polar order) in the ferroelectric phase. From optical and dielectric measurements in SCT-1.4, the average value of spontaneous polarization Ps (the contribution from the long-range order) has been determined. We have estimated the values of Psh and Ps, which characterize the short- and long-range orders in the ferroelectric phase of SCT-0.7. Separate determination of the values and temperature dependences of Ps and Psh (which considerably exceeds the value of Ps in the ordered phase) has allowed us to reveal on a quantitative level new particular features of the formation of the induced polar phase in Sr1 – xCaxTiO3.

Local Structure and Polar Order in Liquid N-Methyl-2-pyrrolidone (NMP).

N-Methyl-2-pyrrolidone (NMP) is an exceptional solvent, widely used in industry and for nanomaterials processing. Yet despite its ubiquity, its liquid structure, which ultimately dictates its solvation properties, is not fully known. Here, neutron scattering is used to determine NMP's structure in unprecedented detail. Two dominant nearest-neighbor arrangements are found, where rings are parallel or perpendicular. However, compared with related solvents, NMP has a relatively large population of parallel approaches, similar only to benzene, despite its nonaromaticity and the presence of the normally structure-reducing methyl group. This arrangement is underpinned by NMP's dipole moment, which has a profound effect on its structure: nearest-neighbor molecules arrange in an antiparallel but offset fashion. This polar-induced order extends beyond the first solvation shell, resulting in ordered trimers that reach the nanometer range. The degree of order and balance of interactions rationalize NMP's high boiling point and versatile capabilities to solvate both charged and uncharged species.

Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound.

Here, we report the observation of ferroelectric nematic and ferrielectric smectic mesophases in an achiral bent-core azo compound consisting of nonsymmetrical molecules with a lateral fluoro substitution on one of the wings. These mesophases are enantiotropic in nature with fairly low transition temperatures and wide mesophase ranges. The liquid crystalline properties of this compound are investigated using polarizing optical microscope, differential scanning calorimeter, X-ray diffraction, and electro-optical studies. As revealed by X-ray diffraction measurements, the nematic mesophase is composed of skewed cybotactic clusters and, in the smectic mesophase, the molecules are tilted with respect to the layer normal. The polar order in these mesophases was confirmed by the electro-optical switching and dielectric spectroscopy measurements. The dielectric study in the nematic mesophase shows a single relaxation process at low frequency ( f < 1 kHz) measured in the range 10 Hz to 5 MHz, which is attributed to the collective motion of the molecules within cybotactic clusters. The formation of local polar order in these clusters leads to a ferroelectric-like polar switching in the nematic mesophase. Of particular interest is the fact that the smectic phase exhibits a field induced ferrielectric state, which can be exploited for designing of the potential optical devices due to multistate switching.

Dynamically generated patterns in dense suspensions of active filaments.

We use Langevin dynamics simulations to study dynamical behavior of a dense planar layer of active semiflexible filaments. Using the strength of active force and the thermal persistence length as parameters, we map a detailed phase diagram and identify several nonequilibrium phases in this system. In addition to a slowly flowing melt phase, we observe that, for sufficiently high activity, collective flow accompanied by signatures of local polar and nematic order appears in the system. This state is also characterized by strong density fluctuations. Furthermore, we identify an activity-driven crossover from this state of coherently flowing bundles of filaments to a phase with no global flow, formed by individual filaments coiled into rotating spirals. This suggests a mechanism where the system responds to activity by changing the shape of active agents, an effect with no analog in systems of active particles without internal degrees of freedom.

Термооптические и диэлектрические исследования сегнетоэлектрической фазы, индуцированной кальцием, в виртуальном сегнетоэлектрике SrTiO-=SUB=-3-=/SUB=-

AbstractWe have examined temperature changes of the light refraction, birefringence, dielectric permittivity, and dielectric hysteresis loops in Sr_1 – x Ca_ x TiO_3 single crystals with x = 0.014 (SCT-1.4). The dielectric properties of Sr_1 – x Ca_ x TiO_3 with x = 0.007 (SCT-0.7) have been studied. We have performed ab initio calculations of equilibrium structures and total energies for three low-temperature phases of SrTiO_3 and CaTiO_3, based on which we have determined an expected symmetry of the ground state of their solid solution and spontaneous polarization directions in a calcium-induced ferroelectric phase in Sr_1 – x Ca_ x TiO_3. In SCT-1.4, we have separated a spontaneous contribution to the light refraction, which arises due to the spontaneous electrooptical effect caused by the spontaneous polarization and its fluctuations. From the spontaneous contribution to the light refraction, based on a previously developed our phenological approach, we have quantitatively determined for the first time the values and temperature dependences of root-mean-square fluctuations of the order parameter—the polarization P _sh = 〈 P _fl ^2 〉^1/2(short-range, local polar order) in the ferroelectric phase. From optical and dielectric measurements in SCT-1.4, the average value of spontaneous polarization P _ s (the contribution from the long-range order) has been determined. We have estimated the values of P _sh and P _ s , which characterize the short- and long-range orders in the ferroelectric phase of SCT-0.7. Separate determination of the values and temperature dependences of P _ s and P _sh (which considerably exceeds the value of P _ s in the ordered phase) has allowed us to reveal on a quantitative level new particular features of the formation of the induced polar phase in Sr_1 – x Ca_ x TiO_3.

Polar ordering and structural distortion in electronic domain-wall properties of BiFeO3

Bandgaps, band edges, and structural properties of domain walls (DWs) in multiferroic BiFeO3 (BFO) are studied using density functional theory. The ferroelectric DWs (namely, the 71°, 109°, and 180° DWs) in rhombohedral BFO are considered, with a view of critically examining the relationships between local bandgap, polar ordering, and octahedral distortion reported for this material. Our study reveals that the domain-wall electronic properties of BFO sensitively depend on the polarization behavior and structural distortion at the wall center. The value of the bandgap at the wall decreases with the weakening of ferroelectricity at the wall center. Compared to all the wall types, DWs with the paraelectric-like phase at the wall center exhibit a significantly reduced bandgap. The bandgap reduction is also found to be closely related to two key parameters of the octahedral distortion at the wall: Fe-O bonds and O-Fe-O angles. The reduction in the Fe-O bond deviation and the straightening in the O-Fe-O angles at the wall appear with a marked bandgap reduction in the BFO DWs. Such a structural change at BFO DWs is reminiscent of a cubic structure, which is consistent with the analysis of paraelectric-like behavior at DWs. However, our analysis reveals that the straightening of Fe-O-Fe does not have any obvious correlation with bandgap reduction as suggested in previous works. The influence of the octahedral distortion at DWs is further clarified by investigating the bandgap and structural properties of various BFO bulk phases including Pnma, Pna21, tetragonal (P4mm), and Amm2. Our result establishes important correlations between electronic properties, polar ordering, and structural distortion in BFO DWs. This study also suggests that photo nanocatalysts employing BFO DWs may be a promising strategy to enhance photocatalytic activity.

Polar interactions between bent–core molecules as a stabilising factor for inhomogeneous nematic phases with spontaneous bend deformations

ABSTRACTIt is generally accepted that the transition into the twist–bend nematic phase (NTB) is driven by an elastic instability related to the reduction of the bend elastic constant. Here we use a molecular–statistical theory to show that sufficiently strong polar interactions between bent-shaped molecules may lead to experimentally observed reduction of the bend elastic constant in the nematic phase even if electrostatic dipole–dipole interactions are not taken into account. We propose a simple model of bent–core particles and derive explicit analytical expressions which enable one to understand how polar molecular shape affects the elastic constants, and, in particular, the important role of the bend angle. Numerical graphs showing temperature variations of all elastic constants are also presented including the variation of the bend and splay elastic constants before and after the renormalisation determined by local polar order of molecular steric dipoles and the corresponding polar correction to the one-particle distribution function.

Contribution of spontaneous polarization and its fluctuations to refraction of light in ferroelectrics

The expressions for the spontaneous polar contribution delta n (i) (s) to the principal values of the refractive index due to the quadratic electro-optic effect in ferroelectrics have been considered within the phenomenological approach taking into account the polarization fluctuations. A method has been proposed for calculating the magnitude and temperature dependence of the root-mean-square fluctuations of the polarization (short-range local polar order) P (sh) = aEuroP (fl) (2) aEuro parts per thousand(1/2) below the ferroelectric transition temperature T (c) from temperature changes in the spontaneous polar contribution delta n (i) (s) (T) if the average spontaneous polarization P (s) = aEuroPaEuro parts per thousand characterizing the long-range order is determined from independent measurements (for example, from dielectric hysteresis loops). For the case of isotropic fluctuations, the proposed method has made it possible to calculate P (sh) and P (s) only from refractometric measurements. It has been shown that, upon interferometric measurements, the method developed in this work allows calculating P (sh) and P (s) directly from the measured temperature and electric-field changes in the relative optical path (the specific optical retardation) of the light.

Interfacial Organization in Dipolar Dye-Based Organic Nanoparticles Probed by Second-Harmonic Scattering

We have undertaken polarization-resolved SHS experiments to investigate a series of two fluorescent organic nanoparticles (FONs) made from dipolar dyes bearing a triphenylamine-donating end-group and a slightly hydrophilic acceptor end-group. The FONs, which show very large negative surface potentials, responsible for their good colloidal stability in water, are very bright SHS emitters in water, attractive for single-particle tracking and bioimaging. With the polarization-resolved SHS technique, we come up with a new and noninvasive method to gain insight into the origin of the surface potential of the FONs, as well as into their evolution upon change of ionic strength by following their multipolar response when adding salt. In pure water, the polarization-resolved SHS spectroscopy reveals the presence of a region of local polar order close to the surface (i.e., orientation of the dipolar dyes), leading to a strong dipolar contribution. Quite interestingly, when adding KCl salt, the initial two-lobe SHS ...