Aligned Configuration Research Articles

POLARIZATION EVOLUTION OF EARLY OPTICAL AFTERGLOWS OF GAMMA-RAY BURSTS

ABSTRACT The central engine and jet composition of gamma-ray bursts (GRBs) remain mysterious. Here we suggest that observations on the polarization evolution of early optical afterglows may shed light on these questions. We first study the dynamics of a reverse shock and a forward shock that are generated during the interaction of a relativistic jet and its ambient medium. The jet is likely magnetized with a globally large-scale magnetic field from the central engine. The existence of the reverse shock requires that the magnetization degree of the jet should not be high (σ ≤ 1), so that the jet is mainly composed of baryons and leptons. We then calculate the light curves and polarization evolution of early optical afterglows and find that when the polarization position angle changes by 90° during the early afterglow, the polarization degree is zero for a toroidal magnetic field but is very likely to be nonzero for an aligned magnetic field. This result would be expected to provide a probe for the central engine of GRBs because an aligned field configuration could originate from a magnetar central engine and a toroidal field configuration could be produced from a black hole via the Blandford–Znajek mechanism. Finally, for such two kinds of magnetic field configurations, we fit the observed data of the early optical afterglow of GRB 120308A equally well.

Shell deposition-induced vertically aligned core-shell arrays of W18O49/V2O5 nanorods

A novel heterostructure array of W18O49/V2O5 core-shell nanorods with high alignment and uniform shell layer is prepared with the induction of shell layer deposition to roughly aligned W18O49 nanorods. The roughly aligned W18O49 nanorods with diameter of about 20nm are first grown via in-situ thermal oxidation of sputtered W film. After uniform sputtering-deposition and annealing of polycrystalline shell layer of V2O5, the formed W18O49/V2O5 core-shell nanorods array exhibits an interesting improvement in alignment. A highly ordered array of heteronanorods is achieved when the V2O5-shell is about 20nm in thickness. The possible mechanism of alignment due to shell deposition is suggested. This kind of core-shell heteronanorods with highly aligned geometrical configuration and uniform coating will be important for applications of one-dimensional (1D) heterostructures in nanodevices.

Staggered Transverse Tripoles With Quadripolar Lateral Anodes Using Percutaneous and Surgical Leads in Spinal Cord Stimulation

In spinal cord stimulation for low-back pain, the use of electrode arrays with both low-power requirements and selective activation of target dorsal column (DC) fibers is desired. The aligned transverse tripolar lead configuration offers the best DC selectivity. Electrode alignment of the same configuration using 3 parallel percutaneous leads is possible, but compromised by longitudinal migration, resulting in loss of DC selectivity. This loss might be repaired by using the adjacent anodal contacts on the lateral leads. To investigate if stimulation using adjacent anodal contacts on the lateral percutaneous leads of a staggered transverse tripole can restore DC selectivity. Staggered transverse tripoles with quadripolar lateral anodes were modeled on the low-thoracic vertebral region (T10-T12) of the spinal cord using (a) percutaneous lead with staggered quadripolar lateral anodal configuration (PERC QD) and (b) laminotomy lead with staggered quadripolar lateral anodal configuration (LAM QD), of the same contact dimensions. The commercially available LAM 565 surgical lead with 16 widely spaced contacts was also modeled. For comparison with PERC QD, staggered transverse tripoles with dual lateral anodes were modeled by using percutaneous lead with staggered dual lateral anodal configuration (PERC ST). The PERC QD improved the depth of DC penetration and enabled selective recruitment of DCs in comparison with PERC ST. Mediolateral selectivity of DCs could not be achieved with the LAM 565. Stimulation using PERC QD improves anodal shielding of dorsal roots and restores DC selectivity. Based on our modeling study, we hypothesize that, in clinical practice, LAM QD can provide an improved performance compared with the PERC QD. Our model also predicts that the same configuration realized on the commercial LAM 565 surgical lead with widely spaced contacts cannot selectively stimulate DCs essential in treating low-back pain.

Stretched Contact Printing of One-Dimensional Nanostructures for Hybrid Inorganic–Organic Field Effect Transistors

We demonstrate a stretched contact printing technique to assemble one-dimensional nanostructures with controlled density and orientation from either dry or wet sources. The random, chaotically arranged nanostructures can gradually transform to a highly aligned configuration. Our results show that up to 90% of the printed nanowires are aligned within ±15° of the primary stretching direction. This approach is easily applicable to a variety of nanowires and nanotubes on different substrates, and we demonstrate various field effect transistors with nanowire and hybrid nanowire–polymer networks. The hybrid inorganic–organic transistors based on a parallel aligned nanowire network and a semiconducting polymer revealed a significant enhancement in transistor mobility, a 10-fold reduction in subthreshold slope (∼0.26 V decade–1), and superior air stability compared to a pristine polymer host.

Interaction of toroidal focal conic defects with shear flow

Toroidal focal conic defects form in smectic liquid crystals due to antagonistic surface anchoring conditions. The resulting layer structure, defect size, and the formation of ordered patterns of defects have been studied in detail. Here, we investigate the effect of shear flow on toroidal focal conic defects. The defects are formed and subjected to a steady linear Couette flow in a microscale shear cell. In situ visualization of the evolution of the polarization texture of individual defects reveals the impact of three distinct flow regimes on the layer structure. At low Ericksen numbers, individual defects exhibit an early time elastic regime in which the optical intensity of the layer structure does not change in the presence of the applied shear flow. At increasing applied strains, the defect layer structure deforms, resulting in a monotonically increasing optical intensity within its original footprint. At even larger applied strains, satellite defects are emitted from the anchored defect. Cessation of shear flow at moderate strains, prior to the formation of satellite defects, leads to an exponential decay of the polarization intensity of the defect, suggesting that relaxation of the layer structure occurs with a time constant that is consistent with that predicted by a permeation flow mechanism. This study establishes the relevant length and time scales involved in the interaction of shear stresses and elastic stresses in smectic liquid crystalline samples whose structure deviates significantly from an idealized aligned lamellar configuration.

Synthesis and characterization of multi-lamellar mesostructured hydroxyapatites using a series of fatty acids

A family of lamellar mesostructured hydroxyapatites are synthesized using five terminal linear aliphatic carboxylic acids (capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid with the general formula of Cn−1H2n−1CO2H, n = 10, 12, 14, 16, 18). The study of X-ray diffraction shows that the hydroxyapatite samples exhibit similar multi-lamellar mesostructure and their interlamellar spacing is proportional to the carbon numbers of carboxylic acids. The transmission electron microscopic images on the products prove the parallel aligned configuration. Fourier transform infrared spectra displays the characteristic components of hydroxyapatite in as-prepared samples. Thermogravimetric analysis, elemental analysis (C, H), and inductively coupled plasma analysis were further used to confirm the organic and inorganic composition of the products.

Electrode alignment of transverse tripoles using a percutaneous triple-lead approach in spinal cord stimulation

The aim of this modeling study is to determine the influence of electrode alignment of transverse tripoles on the paresthesia coverage of the pain area in spinal cord stimulation, using a percutaneous triple-lead approach. Transverse tripoles, comprising a central cathode and two lateral anodes, were modeled on the low-thoracic vertebral region (T10–T12) using percutaneous triple-lead configurations, with the center lead on the spinal cord midline. The triple leads were oriented both aligned and staggered. In the staggered configuration, the anodes were offset either caudally (caudally staggered) or rostrally (rostrally staggered) with respect to the midline cathode. The transverse tripolar field steering with the aligned and staggered configurations enabled the estimation of dorsal column fiber thresholds (IDC) and dorsal root fiber thresholds (IDR) at various anodal current ratios. IDC and IDR were considerably higher for the aligned transverse tripoles as compared to the staggered transverse tripoles. The aligned transverse tripoles facilitated deeper penetration into the medial dorsal columns (DCs). The staggered transverse tripoles always enabled broad and bilateral DC activation, at the expense of mediolateral steerability. The largest DC recruited area was obtained with the rostrally staggered transverse tripole. Transverse tripolar geometries, using percutaneous leads, allow for selective targeting of either medial or lateral DC fibers, if and only if the transverse tripole is aligned. Steering of anodal currents between the lateral leads of the staggered transverse tripoles cannot target medially confined populations of DC fibers in the spinal cord. An aligned transverse tripolar configuration is strongly recommended, because of its ability to provide more post-operative flexibility than other configurations.

Analysis of a Class of Symmetric Equilibrium Configurations for a Territorial Model

Motivated by an animal territoriality model, we consider a centroidal Voronoi tessellation algorithm from a dynamical systems perspective. In doing so, we discuss the stability of an aligned equilibrium configuration for a rectangular domain that exhibits interesting symmetry properties. We also demonstrate the procedure for performing a center manifold reduction on the system to extract a set of coordinates which capture the long term dynamics when the system is close to a bifurcation. Bifurcations of the system restricted to the center manifold are then classified and compared to numerical results. Although we analyze a specific set-up, these methods can in principle be applied to any bifurcation point of any equilibrium for any domain. AMS subject classifications: 37N25, 37G10

Magnetic oscillations of critical current in intrinsic Josephson-junction stacks

A key phenomenon related to the Josephson effect is oscillations of different properties of superconducting tunneling junctions with magnetic field. We consider magnetic oscillations of the critical current in stacks of intrinsic Josephson junctions, which are realized in mesas fabricated from layered high-temperature superconductors. The oscillation behavior is very different from the case of a single junction. Depending on the stack lateral size, oscillations may have either the period of half flux quantum per junction (wide-stack regime) or one flux quantum per junction (narrow-stack regime). We study in detail the crossover between these two regimes. Typical size separating the regimes is proportional to magnetic field meaning that the crossover can be driven by the magnetic field. In the narrow-stack regime the lattice structure experiences periodic series of phase transitions between aligned rectangular configuration and triangular configuration. Triangular configurations in this regime is realized only in narrow regions near magnetic-field values corresponding to integer number of flux quanta per junction.

Co-rotating stationary states and vertical alignment of geostrophic vortices with thin cores

We investigate the evolution of nearby like-sign vortices whose centres are at different vertical levels in a stably stratified rotating fluid. We employ two differently singularized representations of the potential vorticity distribution in the quasi-geostrophic equations (QG), in order to elucidate the pair-interaction behaviour previously seen in non-singular QG numerical solutions. The first is an analytically tractable conservative (Hamiltonian) elliptical-moment model (EM) for thin-core vortices, which exhibits a regime of very strong horizontal elongation of a vortex in response to the strain induced by its partner. We interpret this as an early evolutionary stage towards the irreversible dissipative merger and alignment interactions. This interpretation is strengthened by weakly dissipative numerical solutions of a thin-core contour-dynamics model (CD), which exhibit even further progress towards the completion of these vortex interactions in the same regime.In the EM model we classify the co-rotating stationary states which exist always for vertically offset thin-core vortices. However, the mutual strain field among the vortices cannot be balanced by co-rotation in a weakly elongated stationary state for a certain class of neighbouring, but substantially non-aligned, vortex configurations, and our interpretive assumption is that such configurations will rapidly evolve in non-singular QG solutions towards a more aligned configuration through significantly non-conservative reorganizations of the potential vorticity field. Both the EM and CD models show qualitatively similar regime boundaries between evolutions with weakly and strongly deformed vortices. In particular, there is a fairly close correspondence between the occurrence of strong vortex elongation in the EM solutions and significant filamentation and splitting in the CD solutions.

Harmonics of Thomson Backscattered X-Rays

A new concept of harmonic generation of Thomson backscattered X-rays by the angular alignment between the axes of the electron beam and incident laser beam has been proposed. Various properties of the backscattered X-ray harmonic radiation are examined, including the linewidth, radiation pattern on the observation plane, and the behavior of the radiation spectra at ultrahigh laser intensities. It is shown by the simulation results that when the electron beam is deliberately aligned at a suitable small angle in the polarization plane of the linearly polarized laser, both even and odd harmonics along the electron beam axis are generated, which is different from the perfectly aligned case where only odd harmonic radiations peak on the electron-laser axis. Moreover the high harmonics on the angularly aligned electron-laser configuration may have several times higher intensities compared with the axial intensity of the fundamental harmonic in a perfectly aligned case.

Non-collective degrees of freedom in 192Pb

Two positive-parity and two negative-parity groups of states of non-collective character with excitation energies ranging from 3 to 7 MeV and spins as high as 21 ħ have been established in 192Pb. The main features of each group are described phenomenologically as four-quasiparticle configurations, arising from coupling of the 2 + 1, 4 + 1 and 6 + 1 states to a specific aligned neutron two-quasiparticle configuration. Supporting evidence for this interpretation and the importance of proton excitations in this singly closed-shell nucleus is obtained from the predictions of the quasiparticle multi-step shell model (QMSM). In addition, a four-quasiparticle structure of predominant neutron character is proposed on the basis of comparisons with the experimental data on 194Pb and with the results of the QMSM calculations.

Crystal structures of stage- n iodine-intercalated compounds IBi 2 nSr 2 nCa nCu 2 nO x

The crystal structure of stage-3 iodine-intercalated superconducting IBi 6Sr 6Ca 3Cu 6O x has been determined by transmission electron microscopy to belong to the space group Pma2 with lattice parameters a=5.4 A ̊ , b=5.4 A ̊ and c=49.4 A ̊ . Ioidine atoms intercalated between every three BiO bilayers expand the distance between the BiO layers by 3.6Å and alter the atomic stacking across BiO layers from the staggered configuration characteristic of host superconducting Bi 2Sr 2CaCu 2O x to an aligned configuration characteristic of stage-1 iodine-intercalated superconducting IBi 2Sr 2CaCu 2O x . Higher-stage intercalation has also been observed as stacking faults which predominantly contain both stage-2 and stage-3 phases. The space groups and c-axis dimensions of the higher-stage phases have been deduced to be Pma2 with c=3.6+15.3n A ̊ when stage number n is odd, and Bbmb with c=2(3.6+15.3n) A ̊ when n is even.

Off-axis orientation of the electronic transition moment for a linear conjugated polyene

IN many applications of conjugated polyenes for nonlinear optoelectronics and as probes of biophysical systems, the orientation of the electronic transition dipole moment relative to the long axis of the chains is an important quantity. Simple models predict that the transition moment lies closely along the chain axis1,2 or at an angle of about 30° to this axis3, but the difficulty of preparing perfectly oriented samples has made these predictions hard to test. Here we report the results of polarized single crystal spectroscopy of a linear conjugated tetraene in the highly aligned configuration made possible by incorporating these molecules as guests in the channels of urea crystals. The angular dependence of the absorption spectrum indicates that the transition moment lies at an angle of 15° to the chain axis. Molecular-orbital calculations can reproduce this value when they include the effects of electron correlation.

Magnetic impulses and associated optical signatures in the dayside aurora

We present five magnetic impulse events observed at South Pole and in the magnetically conjugate area at Iqaluit. The magnetic signatures recorded during these events appear to be similar to those discussed by several authors as possible ionospheric signatures of sporadic dayside reconnection via the flux transfer process. Coordinated ground‐based image‐intensified all sky camera data were acquired at South Pole during the events. The optical data show that prior to the onset of each of the events the dayside cusp aurora is several degrees poleward of the station, signifying event initiation on closed field lines. Overhead aurora appears at the onset of the magnetic event and precipitation seems to expand over a large region equatorward of the quiescent cusp aurora. As the event decays the aurora dies away and the quiescent dayside cusp aurora, in four out of five cases, returns poleward of the station. This morphology is inconsistent with the model in which the ground based signature of an FTE is caused by a field aligned current configuration moving over a ground station in the antisunward direction. The observations show temporal events which begin at or inside the boundary of closed field lines. These are consistent with sudden precipitation activity on flux tubes which were previously equatorward of the cusp/cleft boundary and which became temporarily disconnected (non‐dipolar).

Properties of magnetic superlattices with antiferromagnetic interfacial coupling: Magnetization, susceptibility, and compensation points.

Magnetic superlattices composed of ferromagnetic films (Fe and Gd) which couple antiferromagnetically at the interfaces are studied. In a magnetic field a variety of different spin configurations are possible. We show how these phases are reflected in magnetization and susceptibility measurements. In particular, a twisted spin configuration is characterized by a much larger susceptibility than an aligned spin configuration. We show that small changes in the layering pattern can lead to large changes in /ital M/(/ital T/) and static susceptibility. In contrast, changes in the value of the interface exchange parameter cause large changes only in the weak-coupling limit.

Evidence for a four-quasiparticle isomer in 84Kr

A 12 + isomer has been found in 84Kr at 5373 keV with a half-life of T 1 2 =45(5) ns . For the new isomer the aligned 4qp configuration v( g 9 2 ) 8 + −2π ( p 3 2 , f 5 2 ) 4 + is strongly suggested on the ba of the measured g-factor of g=+0.14(3) and from the sum of the corresponding 2qp energies.

Transition from Prolate to Oblate to Triaxial Shapes in 158Yb

The decay of the entry states in 158Yb populated in the reactions of 149 MeV 20Ne with 144Nd and 146Nd has been investigated with a 4π multidetector system gated by a Ge counter. The average excitation energy, the γ-ray spectra and the angular distributions as a function of multiplicity show several changes in the γ-ray decay. These changes suggest a transition from prolate to particle aligned oblate configuration at low spin. At I ∼ 38-48 collective transitions with dipole and quadrupole component possibly built on high K single particle states are observed. Furthermore, above I ∼ 48 the dipole component disappears suggesting a further change toward more triaxial shape.

Rotational band crossings in 232Th

The alignment of i 13 2 protons and j 15 2 neutrons in rotating actinide nuclei gives rise to quasiparticle bands that cross the ground-state configuration. Coulomb excitation of 232Th provides evidence for both crossings. In the yrast sequence we observe transitions to the aligned proton and the aligned neutron configuration, in the octupole sequence we observe transitions to the aligned proton configuration.

Excitations in a spin-flop Heisenberg magnet

Dynamic spin correlations in the spin-flop phase of a Heisenberg antiferromagnet are studied in terms of the quantities that determine the inelastic scattering of neutrons from single spin-wave states. A complete expression for the cross-section is given using linear spin-wave theory. The non-linear features of the spin dynamics which are intrinsic to the spin-flop state are studied by perturbation theory. Two limiting cases are considered, and these correspond to states where the spins are close to an antiferromagnetic configuration (weak field limit) and the almost completely field aligned configuration. Analytic expressions are given for the appropriate collisional self-energies, in the limit of low temperatures, for selected wavevectors in one and three dimensional systems.