Polar Alignment Research Articles

High‐Pressure Deformation of Iron–Nickel–Silicon Alloys and Implications for Earth’s Inner Core

AbstractEarth’s inner core exhibits strong seismic anisotropy, often attributed to the alignment of hexagonal close‐packed iron (hcp‐Fe) alloy crystallites with the Earth’s poles. How this alignment developed depends on material properties of the alloy and is important to our understanding of the core’s crystallization history and active geodynamical forcing. Previous studies suggested that hcp‐Fe is weak under deep Earth conditions but did not investigate the effects of the lighter elements known to be part of the inner core alloy. Here, we present results from radial X‐ray diffraction experiments in a diamond anvil cell that constrain the strength and deformation properties of iron‐nickel‐silicon (Fe–Ni–Si) alloys up to 60 GPa. We also show the results of laser heating to 1650 K to evaluate the effect of temperature. Observed alloy textures suggest different relative activities of the various hcp deformation mechanisms compared to pure Fe, but these textures could still account for the theorized polar alignment. Fe–Ni–Si alloys are mechanically stronger than Fe and Fe–Ni; extrapolated to inner core conditions, Si‐bearing alloys may be more than an order of magnitude stronger. This enhanced strength proportionally reduces the effectivity of dislocation creep as a deformation mechanism, which may suggest that texture developed during crystallization rather than as the result of postsolidification plastic flow.

Interstellar Extinction, Polarization, and Grain Alignment in the Sh 2-185 (IC 59 and IC 63) Region

Abstract Optical and infrared continuum polarization from the interstellar medium is driven by radiative processes aligning the grains with the magnetic field. While a quantitative, predictive theory of radiative alignment torques (RATs) exists and has been extensively tested, several parameters of the theory remain to be fully constrained. In a recent paper, Medan & Andersson showed that the polarization efficiency (and therefore grain alignment efficiency) at different locations in the wall of the Local Bubble (LB) could be modeled as proportional to the integrated light intensity from the surrounding stars and OB associations. Here we probe that relationship at high radiation field intensities by studying the extinction and polarization in the two reflection nebulae IC 59 and IC 63 in the Sh 2-185 H ii region, illuminated by the B0 IV star γ Cassiopeia. We combine archival visual polarimetry with new seven-band photometry in the Vilnius system, to derive the polarization efficiency from the material. We find that the same linear relationship seen in the LB wall also applies to the Sh 2-185 region, strengthening the conclusion from the earlier study.

Polarization reconfigurability enabled metamaterial inspired dielectric resonator based Fabry‐Perot resonator cavity antenna with high gain and bandwidth

A novel metamaterial inspired small array Fabry-Perot Resonator cavity antenna is designed over a wide-band dielectric resonator antenna to enhance gain and bandwidth operating at C-band applications. Further, the cavity resonator is modified with the application of 12 no. of RF p-i-n diodes between the upper layer of the posterior unit-cells to achieve the polarization reconfigurability (PR) keeping the bandwidth, gain and efficiency intact. The overall dimension of the proposed antenna is 0.8 λ × 0.8 λ × 0.56 λ. With the PR architecture, the antenna imparts 4.8 GHz bandwidth operating between 4 and 8.8 GHz with an average simulated efficiency of 72% and an average gain of 20 dBi throughout the operating band. The specific alignment of the diode polarity enables the model to obtain left-handed circular polarization and/or right-handed circular polarization.

Active particles with polar alignment in ring-shaped confinement.

We study steady-state properties of active, nonchiral and chiral Brownian particles with polar alignment and steric interactions confined within a ring-shaped confinement (annulus) in two dimensions. Exploring possible interplays between polar interparticle alignment, geometric confinement and the surface curvature, being incorporated here on minimal levels, we report a surface-population reversal effect, whereby active particles migrate from the outer concave boundary of the annulus to accumulate on its inner convex boundary. This contrasts the conventional picture, implying stronger accumulation of active particles on concave boundaries relative to the convex ones. The population reversal is caused by both particle alignment and surface curvature, disappearing when either of these factors is absent. We explore the ensuing consequences for the chirality-induced current and swim pressure of active particles and analyze possible roles of system parameters, such as the mean number density of particles and particle self-propulsion, chirality, and alignment strengths.

An Argument Communication Model of Polarization and Ideological Alignment

This multi-level model of opinion formation considers that attitudes on different issues are usually not independent. In the model, agents exchange beliefs regarding a series of facts. A cognitive structure of evaluative associations links different (partially overlapping) sets of facts on different political issues and determines agents’ attitudinal positions in a way borrowed from expectancy value theory. If agents preferentially interact with other agents who hold similar attitudes on one or several issues, this leads to biased argument pools and increasing polarization in the sense that groups of agents selectively believe in distinct subsets of facts. Besides the emergence of a bi-modal distribution of opinions on single issues as most previous opinion polarization models address, our model also accounts for the alignment of attitudes across several issues along ideological dimensions.

A 26–32-GHz Dual-Polarization Receiver Front-End With Rapid-Response Mixed-Signal Polarization Alignment for Ultrareliable Low-Latency Communications

A 26–32-GHz Dual-Polarization Receiver Front-End With Rapid-Response Mixed-Signal Polarization Alignment for Ultrareliable Low-Latency Communications

Improved quark coalescence model for spin alignment and polarization of hadrons

We propose an improved quark coalescence model for spin alignment of vector mesons and polarization of baryons by spin density matrix with phase space dependence. The spin density matrix is defined through Wigner functions. Within the model we propose an understanding of spin alignments of vector mesons $\phi$ and $K^{*0}$ (including $\bar{K}^{*0}$) in the static limit: a large positive deviation of $\rho_{00}$ for $\phi$ mesons from 1/3 may come from the electric part of the vector $\phi$ field, while a negative deviation of $\rho_{00}$ for $K^{*0}$ may come from the electric part of vorticity tensor fields. Such a negative contribution to $\rho_{00}$ for $K^{*0}$ mesons, in comparison with the same contribution to $\rho_{00}$ for $\phi$ mesons which is less important, is amplified by a factor of the mass ratio of strange to light quark times the ratio of $\left\langle \mathbf{p}_{b}^{2}\right\rangle $ on the wave function of $K^{*0}$ to $\phi$ ($\mathbf{p}_{b}$ is the relative momentum of two constituent quarks of $K^{*0}$ and $\phi$). These results should be tested by a detailed and comprehensive simulation of vorticity tensor fields and vector meson fields in heavy ion collisions.

Reconfigurable emergent patterns in active chiral fluids

Active fluids comprised of autonomous spinning units injecting energy and angular momentum at the microscopic level represent a promising platform for active materials design. The complexity of the accessible dynamic states is expected to dramatically increase in the case of chiral active units. Here, we use shape anisotropy of colloidal particles to introduce chiral rollers with activity-controlled curvatures of their trajectories and spontaneous handedness of their motion. By controlling activity through variations of the energizing electric field, we reveal emergent dynamic phases, ranging from a gas of spinners to aster-like vortices and rotating flocks, with either polar or nematic alignment of the particles. We demonstrate control and reversibility of these dynamic states by activity. Our findings provide insights into the onset of spatial and temporal coherence in a broad class of active chiral systems, both living and synthetic, and hint at design pathways for active materials based on self-organization and reconfigurability.

Modelling collective cell motion: are on- and off-lattice models equivalent?

Biological processes, such as embryonic development, wound repair and cancer invasion, or bacterial swarming and fruiting body formation, involve collective motion of cells as a coordinated group. Collective cell motion of eukaryotic cells often includes interactions that result in polar alignment of cell velocities, while bacterial patterns typically show features of apolar velocity alignment. For analysing the population-scale effects of these different alignment mechanisms, various on- and off-lattice agent-based models have been introduced. However, discriminating model-specific artefacts from general features of collective cell motion is challenging. In this work, we focus on equivalence criteria at the population level to compare on- and off-lattice models. In particular, we define prototypic off- and on-lattice models of polar and apolar alignment, and show how to obtain an on-lattice from an off-lattice model of velocity alignment. By characterizing the behaviour and dynamical description of collective migration models at the macroscopic level, we suggest the type of phase transitions and possible patterns in the approximative macroscopic partial differential equation descriptions as informative equivalence criteria between on- and off-lattice models. This article is part of the theme issue 'Multi-scale analysis and modelling of collective migration in biological systems'.

Polarization and Localization of Single-Photon Emitters in Hexagonal Boron Nitride Wrinkles.

Color centers in two-dimensional hexagonal boron nitride (h-BN) have recently emerged as stable and bright single-photon emitters (SPEs) operating at room temperature. In this study, we combine theory and experiment to show that vacancy-based SPEs selectively form at nanoscale wrinkles in h-BN with its optical dipole preferentially aligned to the wrinkle direction. By using density functional theory calculations, we find that the wrinkle's curvature plays a crucial role in localizing vacancy-based SPE candidates and aligning the defect's symmetry plane to the wrinkle direction. By performing optical measurements on SPEs created in h-BN single-crystal flakes, we experimentally confirm the wrinkle-induced generation of SPEs and their polarization alignment to the wrinkle direction. Our results not only provide a new route to controlling the atomic position and the optical property of the SPEs but also revealed the possible crystallographic origin of the SPEs in h-BN, greatly enhancing their potential for use in solid-state quantum photonics and quantum information processing.

A mechanochemical model for rho GTPase mediated cell polarization

Directed motility of eukaryotic cells requires the polarization of the actomyosin cytoskeleton. In many cell types the polar alignment of the actomyosin cytoskeleton occurs in response to a front-rear symmetry break of active Rho GTPase. Experimental evidence in neutrophils indicates that membrane tension plays an important role in the confinement of active Rac to the front domain. We suggest a mechanochemical model for polarization, including Rho GTPase mediated actomyosin cytoskeleton dynamics and changes in membrane tension as an upstream controller of Rho GTP that reflects this observation. The model comprises the Rho GTPases Rac and RhoA which can become activated in response to external signals. The active states regulate the actomyosin mechanics. The model cell is considered as a thin, effectively two dimensional, sheet adhering to a flat substrate. Morphological changes of the actomyosin cytoskeleton induce changes in membrane tension. We numerically show that the model exhibits key features of neutrophil polarization. The model accounts for a simple mechanochemical circuit with the ability to generate robust polarity patterns, wherein cell mechanics serve as a long range signal transmitter.

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.

Alignment of Polarization against an Electric Field in van der Waals Ferroelectrics

Going against the grain: Normally, the electric polarization of a ferroelectric compound switches to align itself with a sufficiently strong applied field. Here the authors report polarization switching in ferroelectric van der Waals (vdW) crystals that is based on ion migration across the vdW gap. This ionic mechanism is complementary to but fundamentally different from the polarization rotation underpinning the usual picture of ferroelectric switching. These findings change the way we think about materials featuring both ionic and dipolar properties and enable fresh functionality in ultrathin vdW structures, to advance device physics and nanotechnology.

Revealing photoinduced bulk polarization and spin-orbit coupling effects in high-efficiency 2D/3D Pb–Sn alloyed perovskite solar cells

This paper reports that growth-orientated grains by introducing bulky organic ligands can essentially leads to enhanced spin-orbit coupling (SOC) and out-of-plane photoinduced bulk polarization, which are accountable for the enhanced photovoltaic performance in 2D/3D Pb–Sn alloyed perovskite solar cells (PSCs). Specifically, the enhanced SOC in the 2D/3D perovskites is evidenced by our photoexcitation-polarization dependent photocurrent studies where the photocurrent is modulated by switching photoexcitation between linear and circular polarizations, and confirmed by magnetic field effects of photocurrent. The enhanced SOC can improve the spin conversion from optically generated bright states (spin-allowed to recombine) to dark states (spin-forbidden to recombine), increasing the possibilities for charge dissociation towards better photovoltaic actions. On the other hand, the out-of-plane orientated grains in 2D/3D perovskite with reduced traps are favorable to align optical transition dipoles. This provides the precondition to realize photoinduced bulk polarizations, which is first suggested by our photoinduced impedance measurements and further verified through photoexcitation polarization-direction dependent photoluminescence (PL) characteristics. It is noted that the photoinduced bulk polarization in 2D/3D perovskite can enhance charge dissociation via dipole-dipole interaction between excitons, towards enhancing photovoltaic actions. Additionally, the carrier lifetime in 2D/3D perovskite is increased due to the decreased nonradiative loss from reduced defects and the lowered Coulomb scattering from orbital polarization alignment. Therefore, our work reveals that photoinduced bulk polarization and enhanced SOC can be realized to enhance photovoltaic performance with the 2D/3D design in Pb–Sn alloyed solar cell. • Incorporating 2D ligands promotes grain orientation on out-of-plane direction. • Grain orientation increases photoinduced bulk polarization and SOC. • Stronger SOC converts spin-allowed to spin-forbidden states for dissociation. • Photoinduced bulk polarization facilitates charge dissociation. • Increased spin-forbidden states and dissociation generate better PV actions.

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics

We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.

Ultra-high-resolution SD-OCM imaging with a compact polarization-aligned 840 nm broadband combined-SLED source.

We analyze the influence of intrinsic polarization alignment on image quality and axial resolution employing a broadband 840 nm light source with an optical bandwidth of 160 nm and an output power of 12 mW tailored for spectral-domain optical coherence microscopy (SD-OCM) applications. Three superluminescent diodes (SLEDs) are integrated into a 14-pin butterfly module using a free-space micro-optical bench architecture, maintaining a constant polarization state across the full spectral output. We demonstrate superior imaging performance in comparison to traditionally coupled-SLED broadband light sources in a teleost model organism in-vivo.

Rectification and separation of mixtures of active and passive particles driven by temperature difference.

Transport and separation of binary mixtures of active and passive particles are investigated in the presence of temperature differences. It is found that temperature differences can strongly affect the rectification and separation of the mixtures. For active particles, there exists an optimal temperature difference at which the rectified efficiency is maximal. Passive particles are not propelled and move by collisions with active particles, so the response to temperature differences is more complicated. By changing the system parameters, active particles can change their directions, while passive particles always move in the same direction. The simulation results show that the separation of mixtures is sensitive to the system parameters, such as the angular velocity, the temperature difference, and the polar alignment. The mixed particles can be completely separated under certain conditions.

Theoretical Model and Outstanding Performance from Constructive Piezoelectric and Triboelectric Mechanism in Electrospun PVDF Fiber Film

AbstractFlexible materials with high electromechanical coupling performance are highly demanded for wide applications for electromechanical sensors and transducers, including mechanical energy harvesters. Here, outstanding electromechanical performance is obtained in electrospun‐aligned polyvinylidene fluoride (PVDF) fiber film. A theoretical model is developed from systematic theoretical analyses to clarify the underlying constructive piezoelectric‐triboelectric mechanism in the polarized PVDF fiber films that explains the experimental observations well. The electrospinning process induces polarization alignment and thus tunes the electron affinity for PVDF fibers with different polarization terminals, which results in the constructive piezoelectric and triboelectric responses in the obtained PVDF fiber films. Extremely large effective piezoelectric performance properties are achieved in the direct piezoelectric measurements, reaching the maximum effective piezoelectric strain and voltage coefficients of −1065 pm V−1 and −9178 V mm N−1, respectively, at 100 Hz. In the converse piezoelectric measurements without a significant contribution from reversible triboelectric effect, the maximum effective piezoelectric strain and voltage coefficients are −166 pm V−1 and −1499 V mm N−1, respectively. The theoretical analyses and experimental results show the great potential of the electrospun aligned polar PVDF fiber material for various electromechanical device applications, particularly for mechanical energy harvesting.

Ionization induced dynamic alignment of water

Two-body dissociation resulting from strong-field double ionization of water is investigated. Two distinct features are seen in the alignment of the fragment momenta with respect to the laser polarization. One feature shows alignment of the H-OH axis with the laser polarization, while the other indicates polarization alignment normal to the H-OH axis. By analyzing kinematic differences between the OH+/D+ and OD+/H+ channels of HOD, these two alignment features are shown to result from dissociation from different states in the dication. Only dissociation from one of these states has an alignment dependence consistent with predictions of sequential strong-field tunneling ionization models. The alignment dependence of dissociation from the other state can only be explained by dynamic alignment launched by the unbending of the molecule during ionization.

Polar coupling enabled nonlinear optical filtering at MoS2/ferroelectric heterointerfaces

Complex oxide heterointerfaces and van der Waals heterostructures present two versatile but intrinsically different platforms for exploring emergent quantum phenomena and designing new functionalities. The rich opportunity offered by the synergy between these two classes of materials, however, is yet to be charted. Here, we report an unconventional nonlinear optical filtering effect resulting from the interfacial polar alignment between monolayer MoS2 and a neighboring ferroelectric oxide thin film. The second harmonic generation response at the heterointerface is either substantially enhanced or almost entirely quenched by an underlying ferroelectric domain wall depending on its chirality, and can be further tailored by the polar domains. Unlike the extensively studied coupling mechanisms driven by charge, spin, and lattice, the interfacial tailoring effect is solely mediated by the polar symmetry, as well explained via our density functional theory calculations, pointing to a new material strategy for the functional design of nanoscale reconfigurable optical applications.