Polarization Quantities Research Articles

A statistical analysis of dust polarization properties in ALMA observations of Class 0 protostellar cores

Context. Recent observational progress has challenged the dust grain-alignment theories used to explain the polarized dust emission routinely observed in star-forming cores. Aims. In an effort to improve our understanding of the dust grain alignment mechanism(s), we have gathered a dozen ALMA maps of (sub)millimeter-wavelength polarized dust emission from Class 0 protostars and carried out a comprehensive statistical analysis of dust polarization quantities. Methods. We analyze the statistical properties of the polarization fraction Pfrac and the dispersion of polarization position angles S. More specifically, we investigate the relationship between S and Pfrac as well as the evolution of the product S × Pfrac as a function of the column density of the gas in the protostellar envelopes. We compare the observed trends with those found in polarization observations of dust in the interstellar medium and in synthetic observations of non-ideal magneto-hydrodynamic (MHD) simulations of protostellar cores. Results. We find a significant S ∝ Pfrac−0.79 correlation in the polarized dust emission from protostellar envelopes seen with ALMA; the power-law index significantly differs from the one observed by Planck in star-forming clouds. The product S × Pfrac, which is sensitive to the dust grain alignment efficiency, is approximately constant across three orders of magnitude in envelope column density (from NH2 = 1022 cm−2 to NH2 = 1025 cm−2), with a mean value of 0.36−0.17+0.10. This suggests that the grain alignment mechanism producing the bulk of the polarized dust emission in star-forming cores may not systematically depend on the local conditions such as the local gas density. However, in the lowest-luminosity sources in our sample, we find a hint of less efficient dust grain alignment with increasing column density. Our observations and their comparison with synthetic observations of MHD models suggest that the total intensity versus the polarized dust are distributed at different intrinsic spatial scales, which can affect the statistics from the ALMA observations, for example, by producing artificially high Pfrac. Finally, synthetic observations of MHD models implementing radiative alignment torques (RATs) show that the statistical estimator S × Pfrac is sensitive to the strength of the radiation field in the core. Moreover, we find that the simulations with a uniform perfect alignment (PA) of dust grains yield, on average, much higher S × Pfrac values than those implementing RATs; the ALMA values lie among those predicted by PA, and they are significantly higher than the ones obtained with RATs, especially at large column densities. Conclusions. Ultimately, our results suggest that dust alignment mechanism(s) are efficient at producing dust polarized emission in the various local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. Further studies will be needed to understand how more efficient grain alignment via, for example, different irradiation conditions, dust grain characteristics, or additional grain alignment mechanisms can reproduce the observations.

Lyman-αpolarization intensity mapping

We present a formalism that incorporates hydrogen Lyman-alpha ($\mathrm{Ly}\ensuremath{\alpha}$) polarization arising from the scattering of radiation in galaxy halos into the intensity mapping approach. Using the halo model, and $\mathrm{Ly}\ensuremath{\alpha}$ emission profiles based on simulations and observations, we calculate auto and cross power spectra at redshifts $3\ensuremath{\le}z\ensuremath{\le}13$ for the $\mathrm{Ly}\ensuremath{\alpha}$ total intensity, $I$, polarized intensity, $\mathcal{P}$, degree of polarization, $\mathrm{\ensuremath{\Pi}}=\mathcal{P}/I$, and two new quantities, the astrophysical $E$ and $B$ modes of $\mathrm{Ly}\ensuremath{\alpha}$ polarization. The one-halo terms of the $\mathrm{\ensuremath{\Pi}}$ power spectra show a turnover that signals the average extent of the polarization signal, and thus the extent of the scattering medium. The position of this feature depends on redshift, as well as on the specific emission profile shape and extent, in our formalism. Therefore, the comparison of various $\mathrm{Ly}\ensuremath{\alpha}$ polarization quantities and redshifts can break degeneracies between competing effects, and it can reveal the true shape of the emission profiles, which, in turn, are associated to the physical properties of the cool gas in galaxy halos. Furthermore, measurements of $\mathrm{Ly}\ensuremath{\alpha}$ $E$ and $B$ modes may be used as probes of galaxy evolution, because they are related to the average degree of anisotropy in the emission and in the halo gas distribution across redshifts. The detection of the polarization signal at $z\ensuremath{\sim}3--5$ requires improvements in the sensitivity of current ground-based experiments by a factor of $\ensuremath{\sim}10$, and of $\ensuremath{\sim}100$ for space-based instruments targeting the redshifts $z\ensuremath{\sim}9--10$, the exact values depending on the specific redshift and experiment. Interloper contamination in polarization is expected to be small, because the interlopers need to also be polarized. Overall, $\mathrm{Ly}\ensuremath{\alpha}$ polarization boosts the amount of physical information retrievable on galaxies and their surroundings, most of it not achievable with total emission alone.

A numerical study on interface shearing of granular Cosserat materials

This article presents the interface shearing behaviour of a granular soil layer in contact with a bounding structure under motion. For this purpose, an elasto-plastic Cosserat continuum model (EPCCM) and finite element method (FEM) in the updated Lagrange frame are employed. Plane quasi-static shearing of a granular soil layer, bounded by upper and lower rigid boundaries with different surface roughness conditions, is simulated under free dilatancy and constant vertical pressure. Herein, the additional Cosserat kinematical boundary constraints along the bounding surfaces allow a more detailed description of the surface roughness. Specific emphasis is given to the effects of surface roughness as well as the boundary conditions of the entire system on the shear behaviour of layer. According to the obtained results, the shear band location is different in an infinite or finite shear layer depending on the adopted boundary constraints. Further numerical simulations of plane interface shearing are also carried out here using discrete element method (DEM), complementary to Cosserat FE analyses, to better understand the micro-mechanics of granular media near interfaces. FE results are compared with those of experiments as well as DEM simulations and reasonable agreements are seen. The evolution and distribution of state variables and polar quantities in DEM simulations are in proper accordance with the predictions of Cosserat FE model.

Numerical simulation of the effect of grain fragmentation on the evolution of microstructure quantities

In this paper the effect of grain fragmentation of a cohesionless granular material on the change of microstructure quantities is investigated using a micropolar continuum model. To this end the change of the grain size distribution is related in a simplified manner to a reduction of the mean grain diameter, which enters the constitutive equation as the internal length. The additional densification of the grain skeleton is modelled by a reduction of the incremental stiffness, which is related to the so-called solid hardness defined in the sense of a continuum description. The reduction of the mean grain diameter and the solid hardness caused by grain fragmentation is described by corresponding evolution equations, which depend on an increasing mean pressure and an increasing deviatoric stress. The focus of the numerical investigations is on the evolution of microstructure quantities like the void ratio, mean grain diameter and couple stresses under monotonic compression. It is shown that for an initially inhomogeneous distribution of the void ratio polar quantities like microrotations and couple stresses can develop even outside of shear bands. With increasing compression the polar quantities can increase, while the range of fluctuation of the void ratio decreases.

B23-O-08Microstructures in improper ferroelectric compounds revealed by electron microscopy

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [1]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [2]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr)3Ti2O7, whichresults from the combination of two or more non-ferroelectric structural order parameters [3]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl2O4 with the hexagonal structure [4]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2a ×2a×c. In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported. [1] A. P. Levanyuk et al., Sov. Phys. Usp., 17 (1974) 199-214. [2] T. Choi, et al., Nature Mater. 9 (2010) 253-258. [3] Y. S. Oh et al., Nature Mater. 14 (2015) 407-413. [4] S. Mori et al., Ferroelectrics 464 (2014) 116-121. Fig.1 Microstructure in the ferroelectric phase of BaAl2O4. B23-O-07 doi:10.1093/jmicro/dfv165

B23-O-07Multivariate Statistical Analysis of EMCD Spectra measured at the Fe/MgO Interface

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [1]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [2]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr)3Ti2O7, whichresults from the combination of two or more non-ferroelectric structural order parameters [3]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl2O4 with the hexagonal structure [4]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2a ×2a×c. In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported. [1] A. P. Levanyuk et al., Sov. Phys. Usp., 17 (1974) 199-214. [2] T. Choi, et al., Nature Mater. 9 (2010) 253-258. [3] Y. S. Oh et al., Nature Mater. 14 (2015) 407-413. [4] S. Mori et al., Ferroelectrics 464 (2014) 116-121. Fig.1 Microstructure in the ferroelectric phase of BaAl2O4. B23-O-07 doi:10.1093/jmicro/dfv165

Size Effect on the Deformation Characteristic and Shear Band of Granular Material Based on Hypoplastity

The deformation characteristic of granular material is important. It is important factor to analysis the bearing capacity and the thickness of granular material footings. There are many factors influence the deformation of granular material, such as material characteristic, content of water, degree of compaction, test method, etc. For analyzing size effect on the deformation characteristic of granular material, hypoplastic constitutive model considering void ratio is introduced. The influence of an initial void ratio, pressure and a mean grain diameter on the deformation is investigated by polar quantities: rotations, curvatures, couple stresses and a characteristic length. Thus, the influence of density and pressure on the initiation of shear bands can be investigated with a single set of constitutive constants.

On energy formulations of electrostatics for continuum media

In this paper we present a unified energy formulation of electrostatics for continuum media including conductors, dielectrics, ferroelectrics, etc. The effects of boundary devices, external charges and polarizations are taken into account in formulating the free energy. We further explore the relations between various energy formulations including Landau–Ginzburg–Devonshire's formulation, Toupin's formulation, Ericksen's formulation, formulations with respect to a change of comparison or background medium, a formulation without introducing the quantity of polarization, and the Hashin–Shtrikman variational principle. Finally, we apply the formulation to calculate the effective free energy of a polarizable body in the proximity of a conductor, and to estimate the effective properties of nonlinear dielectric composites. We expect that this formulation and clarification will provide a solid ground for addressing electro-elastics of deformable bodies.

The influence of variable quantity of polarization on the current intensity of the electron emission from La-doped Pb(Zr, Sn, Ti)O3 ferroelectric cathode

Ferroelectric cathodes exhibit huge potentials in high-power microwave tube electron beam source, panel display, and the propeller space navigation, due to their superior properties. The material properties of the ferroelectric cathode have been proved to have a significant influence on electron emission, which is indicated in recent research work. In the course of electron emission, the variation of polarization can be caused by non-shielded surface charge which is induced by high trigger voltage. A certain relationship may be found between polarization variation and current intensity of electron emission. To study the relationship between current intensity of electron emission and polarization variation in ferroelectric cathodes, the samples of lanthanum-doped lead zirconate stannate titanate ferroelectric and antiferroelectric ceramics are prepared by the method of solid state calcinations, and the polarization variations of the material under different voltages are measured in the positive half cycle test of hysteresis loop. The curve of the electron emission current intensity versus the trigger voltage is measured, and then the relationship between electron emission current intensity and polarization variation is investigated. The results show that the electron emission current intensities of the two samples are both directly proportional to the polarization variation.

FE simulation of shear localization along granular soil–structure interfaces using micro-polar elasto-plasticity

The paper presents FE simulation of shear localization along the interface between cohesionless granular soil layer and a rough surface of bounding structure under large shearing movement. The distribution and evolution of static and kinematic quantities are mainly focused regarding infinite layer of micro-polar material under constant vertical pressure and free dilatancy, particularly with advanced large movement of bounding structure. The FE results demonstrate that the displacement magnitude of bounding structure has significant effect on the distribution and evolution of state variables and polar quantities in the granular soil layer.

Modeling shear localization along granular soil–structure interfaces using elasto-plastic Cosserat continuum

The current study presents finite element simulations of shear localization along the interface between cohesionless granular soil and bounding structure under large shearing movement. Micro-polar (Cosserat) continuum approach is applied in the framework of elasto-plasticity in order to overcome the numerical problems of localization modeling seen in the conventional continuum mechanics. The effects of different micro-polar kinematic boundary conditions, along the interface, on the evolution and location of shear band are shown by the numerical results. Furthermore, shear band thickness is also investigated for its dependence on the initial void ratio, vertical pressure and mean grain size. Here, the distribution and evolution of static and kinematic quantities are the main focuses regarding infinite layer of micro-polar material during plane shearing, especially with advanced large movement of bounding structure. The influence of such movement has not been investigated yet in the literature. Based on the results obtained from this study, shear localization appears parallel to the direction of shearing. It occurs either in the middle of granular layer or near boundaries, regarding the assumed micro-polar kinematic boundary conditions at the bottom and top surfaces of granular soil layer. Narrower shear band is observed in lower rotation resistance of soil particles along the interface. It is emphasized that the displacement magnitude of bounding structure has significant effect on the distribution and evolution of state variables and polar quantities in the granular soil layer. However, continuous displacement has no meaningful effect on the thickness of shear band. Here, smooth distributions of void ratio and shear stress components are obtained within the shear band, what the other previous numerical investigations did not receive. Despite indirect linking of Lade’s model to the critical state soil mechanics, state variables tend towards asymptotical stationary condition in large shear deformation.

Polymeric ferroelectric and oxide semiconductor-based fully transparent memristor cell

Employment of the memristor for the next-generation large-area electronics can be expected to release various devices with interesting functions. In this article, for the first time we proposed a fully transparent memristor cell, which was composed of one-memory thin-film transistor (TFT) and one-switch TFT using an oxide semiconducting active channel. A poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] was used as a gate insulator for the memory TFT and the channel conductance of the TFT was modulated by changing the quantity and direction of ferroelectric polarization. The fabrication procedures were designed so that the proposed transparent memristor cell could be implemented at process temperature below 200°C. It was successfully conformed that the fabricated memristor cell exhibited good TFT behaviors and modulated output characteristics programmed into the memory TFT.

AN ARBITRARY CONFORMAL ARRAY PATTERN SYNTHESIS METHOD THAT INCLUDE MUTUAL COUPLING AND PLATFORM EFFECTS

The research in the paper puts forward a novel method for synthesizing conformal arrays and optimizing low cross-polarizations including the efiects of elements mutual coupling and mounted platform. Starting from the far-fleld superposition principle, an e-cient approach with active element pattern technique is proposed to calculate the far-fleld patterns. A coordinate transform is used to create polarization quantities, and a general process for the element polarized pattern transformation is proposed. The two numerical examples are proposed, and the desired sidelobe levels and low cross- polarizations are optimized. Numerical results indicate the proposed method is valid.

Estimating effective elasticity tensors from Christoffel equations

We consider the problem of obtaining the orientation and elasticity parameters of an effective tensor of particular symmetry that corresponds to measurable traveltime and polarization quantities. These quantities — the wavefront-slowness and polarization vectors — are used in the Christoffel equation, a characteristic equation of the elastodynamic equation that brings seismic concepts to our formulation and relates experimental data to the elasticity tensor. To obtain an effective tensor of particular symmetry, we do not assume its orientation; thus, the regression using the residuals of the Christoffel equation results in a nonlinear optimization problem. We find the absolute extremum and, to avoid numerical instability of a global search, obtain an accurate initial guess using the tensor of given symmetry closest to the generally anisotropic tensor obtained from data by linear regression. The issue is twofold. First, finding the closest tensor of particular symmetry without assuming its orientation is challenging. Second, the closest tensor is not the effective tensor in the sense of regression because the process of finding it carries neither seismic concepts nor statistical information; rather, it relies on an abstract norm in the space of elasticity tensors. To include seismic concepts and statistical information, we distinguish between the closest tensor of particular symmetry and the effective one; the former is the initial guess to search for the latter.

Increasing the pump-up rate to polarize 3He gas using spin-exchange optical pumping method

In recent years, polarized 3 He gas has increasingly been used as neutron polarizers and polarization analyzers. Two of the leading methods to polarize the 3 He gas are the spin-exchange optical pumping (SEOP) method and the meta-stable exchange optical pumping (MEOP) method. At present, the SEOP setup is comparatively compact due to the fact that it does not require the sophisticated compressor system used in the MEOP method. The temperature and the laser power available determine the speed, at which the SEOP method polarizes the 3 He gas. For the quantity of gas typically used in neutron scattering work, this speed is independent of the quantity of the gas required, whereas the polarizing time using the MEOP method is proportional to the quantity of gas required. Currently, using the SEOP method to polarize several bar-liters of 3 He to 70% polarization would require 20−40 h. This is an order of magnitude longer than the MEOP method for the same quantity of gas and polarization. It would therefore be advantageous to speed up the SEOP process. In this article, we analyze the requirements for temperature, laser power, and the type of alkali used in order to shorten the time required to polarize 3 He gas using the SEOP method.

FE Analysis of Shearing of Granular Bodies in a Direct Shear Box

ABSTRACT A plane strain analysis of a deformation and stress field in cohesionless granular bodies during shearing in a direct shear tester was performed with a finite element method on the basis of a hypoplastic constitutive law enhanced by polar quantities: rotations, curvatures, couple stresses, and a mean grain diameter used as characteristic length. The constitutive law takes into account the effect of pressure, void ratio, direction of deformation rate, mean grain diameter, and grain roughness on the material behavior. The FE calculations were carried out with a different initial void ratio, vertical load, mean grain diameter, and specimen length. Attention was focused on the size effect caused by the size of microstructure related to the specimen dimensions and the effect of side boundaries on the shear zone formation. The FE results show that the thickness of the shear zone increases with increasing initial void ratio, pressure level, mean grain diameter, and specimen length. Due to the effect of boundary conditions, the thickness changes along a horizontal midsection (it is widest in the mid-region).

Fe-simulations of a direct and a true simple shear test within a polar hypoplasticity

Two different plane strain shear testers used to measure properties of granular materials are compared: a direct shear tester and a true simple shear tester. The evolution of deformations and stresses within cohesionless granular bodies during shearing is investigated with a finite element method on the basis of a hypoplastic constitutive model extended by polar quantities: rotations, curvatures and couple stresses. A mean grain diameter is used as a characteristic length. The constitutive model takes into account the effect of pressure, void ratio, direction of deformation rate and mean grain diameter. The attention is focused on the effect of boundary conditions on the shear zone formation.

FE-Simulations of a Direct Wall Shear Box Test

Paper deals with numerical simulations of a direct wall shear box test in conditions of plane strain. The behaviour of dry sand during shearing along a very rough wall is numerically modelled with a finite element method using a hypoplastic constitutive relation within a polar (Cosserat) continuum. The constitutive relation is obtained through extension by polar quantities, viz. rotations, curvatures and couple stresses, using the mean grain diameter as a characteristic length. It is characterised by simplicity and a very wide range of applications. The material constants can be found by means of standard element tests and simple index tests. They are correlated with grain properties. The FE-calculations investigate the influence of the initial void ratio, layer height and mean grain diameter on the shear resistance. The attention is laid on the effect of boundary conditions on the shear zone thickness. The FE-results are compared both with corresponding experimental results of laboratory model tests carried out with a parallel-guided direct shear box.

Effect of cyclic shearing on shear localisation in granular bodies

Cyclic shearing of an infinite narrow layer of dry and cohesionless sand between two very rough boundaries under constant vertical pressure is numerically modelled with the finite element method using a polar hypoplastic constitutive relation. The constitutive relation was obtained through an extension of a non-polar model by polar quantities, viz. particle rotations, curvatures, couple stresses using the mean grain diameter as a characteristic length. The proposed model captures the essential mechanical features of granular bodies in a wide range of densities and pressures with a single set of constants. The material constants can be easily determined from granulometric properties and laboratory tests. The attention of numerical simulations is laid on the influence of number of cycles on the thickness of an induced shear zone for both an initially dense and loose granular specimen. In addition, the effect of a stochastic distribution of the initial void ratio on shear localisation is demonstrated.

Nuclear spin polarization transfer across an organic-semiconductor interface

Motivated by Tycko’s proposal to harness optically pumped nuclear spin polarization for the enhancement of nuclear magnetic resonance (NMR) signals from biological macromolecules, we investigate the transfer of thermal nuclear spin polarization between H1 or F19 in an organic overlayer and P31 at the surface of micron-sized InP particles by Hartmann–Hahn cross polarization. Comparison with analytic and numerical models indicates that the total quantity of polarization transferred across the semiconductor-organic interface is limited by the relatively short room-temperature H1 T1ρ (11 ms) and the slow diffusion of nuclear spin polarization in the semiconductor. Models and spin-counting experiments indicate that we are able to transfer approximately 20% of the total nuclear spin polarization originating in the organic overlayer to the semiconductor, supporting the feasibility of transferred optically pumped NMR.