Single feed low profile omnidirectional antenna with slant 45 degrees linear polarization

It is shown that a side-fed bifilar helix antenna with a single feed, can generate a slant 451 linearly polarized onmidirectional toroidal pattern. The antenna has a low profile and does not require a ground plane. The bifilar helix antenna provides slant 45 degrees polarization over a solid angle of almost 4 pi steradians as compared to a crossed dipole which generates a tilted 45 degrees linearly, polarized pattern only over a solid angle of 1.14 pi steradians. The computed results are validated by experimental data.

Stability of 71 degrees stripe domains in epitaxial BiFeO3 films upon repeated electrical switching

The 71 degrees stripe domain patterns of epitaxial BiFeO3 thin films are frequently being explored to achieve new functional properties, dissimilar from the BiFeO3 bulk properties. We show that in-plane switching and out-of-plane switching of these domains behave very differently. In the in-plane configuration the domains are very stable, whereas in the out-of-plane configuration the domains change their size and patterns, depending on the applied switching voltage frequency.

Ordered 180 degrees ferroelectric domains in epitaxial submicron structures

Large range ordered La(0.7)Sr(0.3)MnO(3) and SrRuO(3) epitaxial dots were fabricated by pulsed laser deposition using stencil masks and were embedded in ferroelectric PbTiO(3) epitaxial films. PbTiO(3) films grown on top of La(0.7)Sr(0.3)MnO(3) dots form arrays of 180 degrees domains that are switchable and have good ferroelectric properties. PbTiO(3) films made on top of SrRuO(3) dots have a monodomain polarization state. These observations point out the importance of the electronic properties of the bottom electrode in the selection of a preferential polarization state in epitaxial ferroelectric films and propose a route of fabricating large arrays of switchable 180 degrees ferroelectric domains. (C) 2011 American Institute of Physics. [doi:10.1063/1.3630232]

Evidence of plasma polarization shift of Ti He-alpha resonance line in high density laser produced plasmas

A spectroscopic study of the He-alpha (1s(2) S-1(0) - 1s2p P-1(1)) line emission (4749.73 eV) from high density plasma was conducted. The plasma was produced by irradiating Ti targets with intense (I approximate to 1x10(19) W/cm(2)), 400nm wavelength high contrast, short (45fs) p-polarized laser pulses at an angle of 45 degrees. A line shift up to 3.4 +/- 1.0 eV (1.9 +/- 0.55 m angstrom) was observed in the He-alpha line. The line width of the resonance line at FWHM was measured to be 12.1 +/- 0.6 eV (6.7 +/- 0.35 m angstrom). For comparison, we looked into the emission of the same spectral line from plasma produced by irradiating the same target with laser pulses of reduced intensities (approximate to 10(17) W/cm(2)): we observed a spectral shift of only 1.8 +/- 1.0 eV (0.9 +/- 0.55m angstrom) and the line-width measures up to 5.8 +/- 0.25 eV (2.7 +/- 0.35 m angstrom). These data provide evidence of plasma polarization shift of the Ti He-alpha line.

X-ray Polarization Spectroscopy from Ultra-Intense Interactions

Detailed knowledge of fast electron energy transport following the interaction of ultrashort intense laser pulses is a key subject for fast ignition. This is a problem relevant to many areas of laser-plasma physics with particular importance to fast ignition and X-ray secondary source development, necessary for the development of large-scale facilities such as HiPER and ELI. Operating two orthogonal crystal spectrometers set at Bragg angles close to 45 degrees determines the X-ray s- and p-polarization ratio. From this ratio, it is possible to infer the velocity distribution function of the fast electron beam within the dense plasma. We report on results of polarization measurements at high density for sulphur and nickel buried layer targets in the high intensity range of 10(19) - 10(21) Wcm(-2). We observe at 45 degrees the Ly-alpha doublet using two sets of orthogonal highly-orientated pyrolytic graphite (HOPG) crystals set in 1(st) order for sulphur and 3(rd) order for nickel.

Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs: polarization signatures

The violent merger of two carbon-oxygen white dwarfs has been proposed as a viable progenitor for some Type Ia supernovae. However, it has been argued that the strong ejecta asymmetries produced by this model might be inconsistent with the low degree of polarization typically observed in Type Ia supernova explosions. Here, we test this claim by carrying out a spectropolarimetric analysis for the model proposed by Pakmor et al. for an explosion triggered during the merger of a 1.1 and 0.9 M⊙ carbon-oxygen white dwarf binary system. Owing to the asymmetries of the ejecta, the polarization signal varies significantly with viewing angle. We find that polarization levels for observers in the equatorial plane are modest (≲1 per cent) and show clear evidence for a dominant axis, as a consequence of the ejecta symmetry about the orbital plane. In contrast, orientations out of the plane are associated with higher degrees of polarization and departures from a dominant axis. While the particular model studied here gives a good match to highly polarized events such as SN 2004dt, it has difficulties in reproducing the low polarization levels commonly observed in normal Type Ia supernovae. Specifically, we find that significant asymmetries in the element distribution result in a wealth of strong polarization features that are not observed in the majority of currently available spectropolarimetric data of Type Ia supernovae. Future studies will map out the parameter space of the merger scenario to investigate if alternative models can provide better agreement with observations.

Predicting polarization signatures for double-detonation and delayed-detonation models of Type Ia supernovae

Calculations of synthetic spectropolarimetry are one means to test multidimensional explosion models for Type Ia supernovae. In a recent paper, we demonstrated that the violent merger of a 1.1 and 0.9 M⊙ white dwarf binary system is too asymmetric to explain the low polarization levels commonly observed in normal Type Ia supernovae. Here, we present polarization simulations for two alternative scenarios: the sub-Chandrasekhar mass double-detonation and the Chandrasekhar mass delayed-detonation model. Specifically, we study a 2D double-detonation model and a 3D delayed-detonation model, and calculate polarization spectra for multiple observer orientations in both cases. We find modest polarization levels (<1 per cent) for both explosion models. Polarization in the continuum peaks at ∼0.1–0.3 per cent and decreases after maximum light, in excellent agreement with spectropolarimetric data of normal Type Ia supernovae. Higher degrees of polarization are found across individual spectral lines. In particular, the synthetic Si II λ6355 profiles are polarized at levels that match remarkably well the values observed in normal Type Ia supernovae, while the low degrees of polarization predicted across the O I λ7774 region are consistent with the non-detection of this feature in current data. We conclude that our models can reproduce many of the characteristics of both flux and polarization spectra for well-studied Type Ia supernovae, such as SN 2001el and SN 2012fr. However, the two models considered here cannot account for the unusually high level of polarization observed in extreme cases such as SN 2004dt.

A dielectric continuum molecular dynamics method

We introduce a novel method to simulate hydrated macromolecules with a dielectric continuum representation of the surrounding solvent. In our approach, the interaction between the solvent and the molecular degrees of freedom is described by means of a polarization density free energy functional which is minimum at electrostatic equilibrium. After a pseudospectral expansion of the polarization and a discretization of the functional, we construct the equations of motion for the system based on a Car-Parrinello technique. In the limit of the adiabatic evolution of the polarization field variables, our method provides the solution of the dielectric continuum problem "on the fly," while the molecular coordinates are propagated. In this first study, we show how our dielectric continuum molecular dynamics method can be successfully applied to hydrated biomolecules, with low cost compared to free energy simulations with explicit solvent. To our knowledge, this is the first time that stable and conservative molecular dynamic simulations of solutes can be performed for a dielectric continuum model of the solvent. (C) 2001 American Institute of Physics.

Enhancement of dielectric constant and associated coupling of polarization behavior in thin film relaxor superlattices

Thin film capacitor structures in which the dielectric is composed of superlattices of the relaxors [0.2Pb(Zn1/3Nb2/3)O- 3-0.8BaTiO(3)] and Pb(Mg1/3Nb2/3)O-3 have been fabricated by pulsed laser deposition. Superlattice wavelength (Lambda) was varied between similar to3 and similar to 600 nm, and dielectric properties were investigated as a function of Lambda. Progressive enhancement of the dielectric constant was observed on decreasing Lambda, and, in contrast to previous work, this was not associated with the onset of Maxwell-Wagner behavior. Polarization measurements as a function of temperature suggested that the observed enhancement in dielectric constant was associated with the onset of a coupled response. The superlattice wavelength (Lambda =20 nm) at which coupled functional behavior became apparent is comparable to that found in literature for the onset of coupled structural behavior (between Lambda =5 nm and Lambda =10 nm). (C) 2001 American Institute of Physics.

Scaling of domain periodicity with thickness measured in BaTiO3 single crystal lamellae and comparison with other ferroics

The focused ion beam microscope has been used to cut parallel-sided {100}-oriented thin lamellae of single crystal barium titanate with controlled thicknesses, ranging from 530 nm to 70 nm. Scanning transmission electron microscopy has been used to examine domain configurations. In all cases, stripe domains were observed with {011}-type domain walls in perovskite unit-cell axes, suggesting 90 degrees domains with polarization in the plane of the lamellae. The domain widths were found to vary as the square root of the lamellar thickness, consistent with Kittel's law, and its later development by Mitsui and Furuichi and by Roytburd. An investigation into the manner in which domain period adapts to thickness gradient was undertaken on both wedge-shaped lamellae and lamellae with discrete terraces. It was found that when the thickness gradient was perpendicular to the domain walls, a continuous change in domain periodicity occurred, but if the thickness gradient was parallel to the domain walls, periodicity changes were accommodated through discrete domain bifurcation. Data were then compared with other work in literature, on both ferroelectric and ferromagnetic systems, from which conclusions on the widespread applicability of Kittel's law in ferroics were made.

Effect of Target Polarization in Electron-Ion Recombination

We present results of a study of the effect of target polarization on electron-ion recombination, and show that coherent radiation by the target electrons gives a large contribution to the recombination rate. It significantly modifies the nonresonant photorecombination background. A procedure has been devised whereby this contribution can be evaluated together with the conventional radiative recombination, independently of the dielectronic recombination component. Numerical results are presented for Zn2+, Cd2+, Sn4+, and Xe8+, showing up to an order-of-magnitude enhancement.

The role of target polarization in electron-ion recombination

We investigate the role of dynamic polarization of the target electrons in the process of recombination of electrons with multicharged ions (polarizational recombination). Numerical calculations carried out for a number of Ni- and Ne-like ions demonstrate that the inclusion of polarizational recombination leads to a noticeable increase (up to 30%) in the cross sections for incident electron energies outside the regions of dielectronic resonances. We also present a critical analysis of theoretical approaches used by other authors to describe the phenomenon of polarizational recombination.