Axis Dispersion Research Articles

Group velocity of light in biaxial crystals.

Based on the wave vector surface, we have derived the analytical expressions for the group velocity of light traveling in an optically biaxial crystal in an arbitrary direction. Our formulas are in terms of either the wave vector direction or the ray direction. An axis dispersion vector is introduced to specify the rotation of the dielectric frame in monoclinic and triclinic crystals, which is found to be responsible for the asymmetry of the group velocity surfaces of these two crystal systems when compared to their phase and ray velocity surfaces. Our formulas confirm that the group velocity of light in a nondissipative medium equals the ratio between the Poynting vector and the total energy density of the electromagnetic field. The group index of light is found to be decomposable into three parts: the ray index, the group index due to principal refractive index dispersion, and that due to axis dispersion. Numerical calculations are performed for the group velocity of light in ${\rm Sn}_2{\rm P}_2{\rm S}_6$ crystal at 550 nm wavelength. Our results show that the dispersion of the principal refractive indices has a considerable contribution to the group index.

Dispersion of knee helical axes during walking in young and elderly healthy subjects

Knee joint rotation center displacement can be estimated in vivo through the analysis of helical axis (HAs) dispersion. HAs can be analyzed during walking, providing information on joint stability. The study aim was to describe knee HAs dispersion during walking in dominant and non-dominant legs of young and elderly healthy subjects.Twenty young (YG: age 23.3 ± 2.4 years) and twenty elderly (EG: age 69.3 ± 4.6 years) healthy subjects were asked to walk on a treadmill at a self-selected speed with reflective markers placed bilaterally on thighs and shanks to detect HAs dispersion and knee kinematics with an optoelectronic system. HAs dispersion was described during the following four phases of gait cycle: (1) flexion from 95% of the previous gait cycle to 10% of the subsequent gait cycle, (2) extension from 10% to 40%, (3) flexion from 40% to 75% and (4) extension from 75% to 95% of the gait cycle. Mean Distance (MD) and Mean Angle (MA) were used as HAs dispersion indexes during each gait phase.Participants showed greater MD and MA in sagittal and frontal planes during the first and second phases. EG revealed higher MD (p = 0.001) and MA (p < 0.001) during the first phase and higher MA (p = 0.001) during the fourth phase in both dominant and non-dominant legs on the sagittal plane.HAs dispersion could be related to the amount of forces acting on knee (first two phases) and knee degeneration (elderly). These results may be used as reference data in further studies on HAs dispersion in presence of knee pathologies or after knee surgery or rehabilitation.

Magnetic anisotropies in oblique columnar growth of FeCoB films

We report on the evolution of anisotropies of FeCoB thin films deposited by magnetron sputtering with different oblique angles increasing from α = 0° to α = 75°. The oblique incidence deposition using the Fe43Co43B14 target leads, under chosen conditions, to columnar growth without bundling. This columnar structure results in a correlated increase in the anisotropy field and of the resonance frequency values, observed with an increasing oblique incidence angle from α = 0° to α = 45°. However, these values decrease from α = 45° to α = 75°. To get a better understanding of the reversal magnetization process, in plane angular distribution of different magnetic parameters was carried out, and the role of easy axis dispersion was especially investigated. The optimized conditions of deposition, leading to controlled high resonance frequencies, could be useful in the design of FeCoB-based microwave devices.

Micromagnetic model of exchange bias: effects of structure and AF easy axis dispersion for IrMn/CoFe bilayers

A micromagnetic model of an exchange bias bilayer is used to examine the impact of the physical structure and the easy axis dispersion of the antiferromagnetic (AF) layer on the exchange bias field (HEB) in an IrMn/CoFe system. Because of the different timescales, the magnetization dynamics of the IrMn and CoFe layers are modelled using respectively a kinetic Monte Carlo (kMC) approach and Landau–Lifshitz–Gilbert (LLG) equation. The easy axis dispersion is modelled using a Gaussian distribution. The calculations show that HEB increases with increasing IrMn thickness and grain size, in agreement with experimental work. Moreover, the model allows the visualization of the switching process at the micromagnetic level to reveal the reversal mechanism. We find that the effect of AF easy axis distribution not only strongly affects the reduction of HEB but also drives non-coherent behaviour in the reversal mechanism. This confirms that the easy axis distribution is an important factor with strong impact on the magnetic properties and exchange bias field of an exchange bias system.

The propagation of pulses of a special shape in an inhomogeneous anisotropic medium with dispersion and torsion of the optical axis

An oblique incidence of polarized light pulses with different envelope shapes is considered: triangular, Gaussian, rectangular and in the form of giant laser pulses onto a flat inhomogeneous anisotropic layer with torsion of the optical axis and optical dispersion. The wavelength of an optical pulse carrier is close to the resonance wavelength of an inhomogeneous anisotropic layer. The problem of the propagation of s- and p-polarization waves through a layer, the reflection of impulses of cross-polarized components, the dependence of modulation coefficients during medium torsion and frequency detuning is considered.

Helical axis analysis to quantify humeral kinematics during shoulder rotation

ABSTRACT Information pertaining to the helical axis during humeral kinematics during shoulder rotation may be of benefit to better understand conditions such as shoulder instability. The aim of this study is to quantify the behavior of humeral rotations using helical axis (HA) parameters in three different conditions. A total of 19 people without shoulder symptoms participated in the experiment. Shoulder kinematics was measured with an optoelectric motion capture system. The subjects performed three different full range rotations of the shoulder. The shoulder movements were analyzed with the HA technique. Four parameters were extracted from the HA of the shoulder during three different full-range rotations: range of movement (RoM), mean angle (MA), axis dispersion (MDD), and distance of their center from the shoulder (D). No significant differences were observed in the RoM for each condition between left and right side. The MA of the axis was significantly lower on the right side compared to the left in each of the three conditions. The MDD was also lower for the right side compared to the left side in each of the three conditions.The four parameters proposed for the analysis of shoulder kinematics showed to be promising indicators of shoulder instability.

A phenomenological approach to study the effect of uniaxial anisotropy on the magnetization of ferromagnetic nanoparticles

We study the effect of the uniaxial anisotropy in a system of ideal, noninteracting ferromagnetic nanoparticles by means of a thermodynamical model. We show that the effect of the anisotropy can be easily assimilated in a temperature shift Ta∗, in analogy to what was proposed by Allia et al. (2001) in the case of interacting nanomagnets. The phenomenological anisotropic Ta∗ parameter can be negative, indicating an antiferromagnetic-like behavior, or positive, indicating a ferromagnetic-like character as seen in the inverse susceptibility behavior as a function of temperature. The study is done considering an easy axis distribution to take into account the anisotropy axis dispersion in real samples (texture). In the case of a volumetric uniform distribution of anisotropy axes, the net effect makes Ta∗ to vanish, and the magnetic susceptibility behaves like a conventional superparamagnetic system, whereas in the others a finite value is obtained for Ta∗. When magnetic moment distribution is considered, the effect is to enhance the Ta∗ parameter, when the dispersion of the magnetic moments becomes wider.

Modelling of the flow of ellipsoidal particles in a horizontal rotating drum based on DEM simulation

The granular flow involving ellipsoidal particles in a horizontal rotating drum is investigated by DEM, in which ellipsoidal particle is modeled by super-ellipsoids and periodic boundary condition is adopted. Recurring to the abundant information obtained from the DEM simulation, a comparably synthetical investigation is performed for the better understanding of the fundamental of the granular flow involving ellipsoidal particles in a horizontal rotating drum. All cases utilized in this work are characterized by the continuous flow, namely in rolling/cascading regime. How the aspect ratio and the rotation speed of the drum influence the transverse mixing is surveyed. And some attempt has been made to elucidate the above influence on the particle mixing from the perspective of particle diffusion and particle convection. A brief description about the influence of the aspect ratio and rotation speed of the drum on the axis dispersion is also provided. The Euler angles, including the precession angle, nutation angle and spin angle, are utilized to study the distribution profiles of the orientation for ellipsoidal particles. Simultaneously, some simple methods have been devised for different investigation purposes involving particle orientation.

Model of advanced recording media: The angular dependence of the coercivity including the effect of exchange interaction

We use a micromagnetic model based on the kinetic Monte-Carlo approach to investigate theoretically the magnetic properties of advanced recording media. The model is employed to examine the impact of the magnetostatic and exchange interaction between grains of realistic perpendicular recording media on the angular-dependent coercivity since the exchange field between grains is an important factor in recording performance. The micromagnetic model allows to take the easy axis distribution and the exchange interaction between grains into account. The results confirm the importance of exchange interaction since the variation of coercivity with angle between the applied field and the orientation of easy axis which is perpendicular to the film plane, (θ) is seen to broaden with decreasing exchange field. We show that a two-stage fitting procedure involving the separate determination of the exchange field and easy axis dispersion provides a useful tool for the characterization of media for perpendicular recording and heat assisted recording. We find excellent agreement between previous experimental results and the simulations including exchange interactions leading to estimate of the exchange coupling and easy axis dispersion.

Use of the alpha shape to quantify finite helical axis dispersion during simulated spine movements

In biomechanical studies examining joint kinematics the most common measurement is range of motion (ROM), yet other techniques, such as the finite helical axis (FHA), may elucidate the changes in the 3D motion pathology more effectively. One of the deficiencies with the FHA technique is in quantifying the axes generated throughout a motion sequence. This study attempted to solve this issue via a computational geometric technique known as the alpha shape, which bounds a set of point data within a closed boundary similar to a convex hull. The purpose of this study was to use the alpha shape as an additional tool to visualize and quantify FHA dispersion between intact and injured cadaveric spine movements and compare these changes to the gold-standard ROM measurements. Flexion–extension, axial rotation, and lateral bending were simulated with five C5–C6 motion segments using a spinal loading simulator and Optotrak motion tracking system. Specimens were first tested intact followed by a simulated injury model. ROM and the FHAs were calculated post-hoc, with alpha shapes and convex hulls generated from the anatomic planar intercept points of the FHAs. While both ROM and the boundary shape areas increased with injury (p<0.05), no consistent geometric trends in the alpha shape growth were identified. The alpha shape area was sensitive to the alpha value chosen and values examined below 2.5 created more than one closed boundary. Ultimately, the alpha shape presents as a useful technique to quantify sequences of joint kinematics described by scatter plots such as FHA intercept data.

Collisional family structure within the Nysa–Polana complex

The Nysa–Polana complex is a group of low-inclination asteroid families in the inner main belt, bounded in semimajor axis by the Mars-crossing region and the Jupiter 3:1 mean-motion resonance. This group is important as the most likely source region for the target of the OSIRIS-REx mission, (101955) Bennu; however, family membership in the region is complicated by the presence of several dynamically overlapping families with a range of surface reflectance properties.The large S-type structure in the region appears to be associated with the parent body (135) Hertha, and displays an (eP,aP) correlation consistent with a collision event near true anomaly of ∼180° with ejecta velocity vej∼285m/s. The ejecta distribution from a collision with these orbital properties is predicted to have a maximum semimajor axis dispersion of δaej=0.005±0.008AU, which constitutes only a small fraction (7%) of the observed semimajor axis dispersion, the rest of which is attributed to the Yarkovsky effect. The age of the family is inferred from the Yarkovsky dispersion to be 300-50+60My.Objects in a smaller cluster that overlaps the large Hertha family in proper orbital element space have reflectance properties more consistent with the X-type (135) Hertha than the surrounding S-type family. These objects form a distinct Yarkovsky “V” signature in (aP,H) space, consistent with a more recent collision, which appears to also be dynamically connected to (135) Hertha. Production of two families with different reflectance properties from a single parent could result from the partial differentiation of the parent, shock darkening effects, or other causes.The Nysa–Polana complex also contains a low-albedo family associated with (142) Polana (called “New Polana” by Walsh et al. (Walsh, K.J. et al. [2013]. Icarus 225, 283–297)), and two other low-albedo families associated with (495) Eulalia. The second Eulalia family may be a high-aP, low-eP, low-iP component of the first Eulalia family-forming collision, possibly explained by an anisotropic ejection field.

Effect of couple stress and magnetic field on the unsteady convective diffusion in blood flow

The effect of magnetic field on unsteady convective diffusion in a couple stress fluid (blood) is studied using a time dependent dispersion model. This model is used to calculate the mean concentration distribution of a solute, bounded by the porous layer and is expressed as a function of dimensionless axial distance and time. The magnetic field, arising as a body couple in the governing equations is shown to increase the axis dispersion coefficient. This is useful to the control of haemolysis caused by artificial organs implanted or extracorporeal. Dispersion coefficient and mean concentration are computed for different values of Hartmann number (M), Couple Stress Parameter (a) and Porous Parameter (σ).

Properties of Ba-hexaferrite thin films with an interfacial layer deposited at various substrate temperatures

• An interfacial layer of 20-nmthick BaM is introduced between the film and substrate. • The interfacial layers deposited at various substrate temperatures ( T s ) were studied. • The texture and perpendicular c -axis orientation are greatly improved when T s = 300 °C. M-type Ba-hexaferrite (BaM) thin films with a 20 nm thick BaM interfacial layer were deposited on (0 0 1) sapphire substrates by RF magnetron sputtering. Effects of the interfacial layers deposited at different substrate temperatures ( T s ) on the surface morphologies, crystallographic and magnetic properties of the films were investigated. Experimental results indicated that the interfacial layer deposited at T s of 300 °C greatly enhanced the growth of c -axis perpendicularly oriented columnar-type grains and improved the crystallinity of hexagonal BaM phase. BaM thin film prepared on this interfacial layer exhibited an excellent perpendicular c -axis orientation with c -axis dispersion angle (Δ θ c ) as small as 0.28°. However, in the BaM thin films with an interfacial layer deposited at T s either lower or higher than 300 °C, acicular-type grains keeping their c -axis in-plane and/or randomly oriented were grown, thus the crystallographic characteristics and perpendicular c -axis orientations deteriorated.

Crystal structures and magnetic properties of epitaxial Co–W perpendicular films

According to previous reports, Co–W alloy films with the hexagonal close-packed (hcp) exhibit high magnetic anisotropy and magnetically induced phase separation due to the miscibility gap formed along the magnetic phase transition, both of which are favorable for high density magnetic recording media. In the present study, the crystal structure, magnetic anisotropy energy (MAE), and their correlation are systematically investigated for Co100−xWx films (x=5, 10, 15) epitaxially grown on MgO (111) single crystals covered with Ru buffer layers by DC magnetron sputtering. The MAE strongly depends on the films thickness as well as the W content x and exceeds that of pure hcp-Co. Comparing with the elaborately analyzed crystal structures, such as lattice parameters (a, c), crystal axis ratios c/a, crystal axis dispersion, volume ratio of the fcc phase to the hcp phase, and deformation/growth stacking fault densities, the MAE is found to be closely correlated with the crystal axis ratio c/a, being qualitatively consistent with the classic single-ion anisotropy model and the recent first principles calculations.

Temperature Dependences of Magnetoimpedance of Nanocrystalline Fe-Based Ribbons

The influence of a magnetic field or the mechanical stresses on the impedance of the Fe73.5Si16.5B6Nb3Cu1 and Fe73.5Si13.5B9Nb3Cu1 ribbons was investigated in the (297 divided by 433) K temperature range. It is shown that impedance changes, which are observed under the application of mechanical stresses and temperature, are conditioned by the structural features and magnetic properties of the alloys. The temperature increase leads to a decrease of the saturation magnetostriction constant, an increase of the anisotropy axis dispersion and a decrease of the anisotropy effective value. It was determined that the maximum sensitivity of impedance to the mechanical stresses reaches the value of about 2%/MPa for the Fe73.5Si16.5B6Nb3Cu1 and Fe73.5Si13.5B9Nb3Cu1 samples.

Reversal of primary and pseudo-primary T wave abnormalities by ventricular pacing. A novel manifestation of cardiac memory

"Cardiac memory" refers to abnormal T waves (TW) appearing after transient periods of altered ventricular depolarization. The aim of the study was to test the hypothesis that in the presence of abnormal TW, short periods of tailored ventricular pacing (VP) can be followed by normalization of ventricular repolarization. Ten patients with normal TW (control group) and 18 patients with abnormal TW (study group) underwent 15 min of VP at a cycle length of 500 ms. In the control group, VP was performed from the right ventricular apex, and in the study group from right or left ventricular sites that resulted in paced QRS complexes of opposite polarity to that of the abnormal TW. Before and after VP, atrial pacing was maintained at a stable cycle length. Simultaneous 12-lead electrocardiography (ECG) was recorded before, during, and following VP to assess changes in TW polarity, amplitude, electrical axis, QTc interval, and QTc interval dispersion. As expected, VP was followed by memory-induced changes in TW in eight of ten patients in the control group. Mean T wave axis shifted from +60 degrees + or - 21.2 degrees to +23.5 degrees + or - 50.7 degrees (p = 0.01) in the frontal plane. In the study group, complete or partial normalization of TW occurred in 17 of 18 patients. Mean T wave axis shifted from -23.7 degrees + or - 22.9 degrees to +19.7 degrees + or - 34.7 degrees (p < 0.0002) in the frontal plane when paced from right ventricular outflow tract. The QTc interval shortened after VP both in the control group (424 + or - 25 vs. 399 + or - 27 ms; p = 0.007) and in the study group (446 + or - 26 vs. 421 + or - 22 ms; p < 0.0002). No significant changes were found in QTc interval dispersion. Transient changes in the sequence of ventricular activation may either induce or normalize abnormal TW. The background of preceding ventricular depolarization needs to be taken into account before determining the clinical significance of a given pattern of ventricular repolarization.

Long-lived planetesimal discs

We investigate the survival of planetesimal discs over Gyr timescales, using a unified approach that is applicable to all Keplerian discs of solid bodies -- dust grains, asteroids, planets, etc. Planetesimal discs can be characterized locally by four parameters: surface density, semi-major axis, planetesimal size and planetesimal radial velocity dispersion. Any planetesimal disc must have survived all dynamical processes, including gravitational instability, dynamical chaos, gravitational scattering, physical collisions, and radiation forces, that would lead to significant evolution over its lifetime. These processes lead to a rich set of constraints that strongly restrict the possible properties of long-lived discs. Within this framework, we also discuss the detection of planetesimal discs using radial velocity measurements, transits, microlensing, and the infrared emission from the planetesimals themselves or from dust generated by planetesimal collisions.

Impact of dynamical screening on the phonon dynamics of metallicLa2CuO4

It is shown within a quantitative calculation that poor dynamical screening of the long-ranged Coulomb interaction due to the slow charge dynamics around the $c$ axis leads in metallic ${\text{La}}_{2}{\text{CuO}}_{4}$ to low-energy electronic collective excitations in a small region around this axis, strongly mixing with certain interlayer phonons. The manifestation of such a phonon-plasmon scenario in layered systems based on a nonadiabatic charge response recently proposed on a qualitative level is quantitatively investigated by a realistic calculation of the frequency and wave-vector-dependent irreducible polarization part of the density response function. Our calculation corrects for a generic deficit of LDA calculations which as a rule predict a too large electronic $c$-axis dispersion insufficient to describe the $c$-axis charge response in general and in particular the phonons along this axis in the cuprates. As shown in this work, the latter are highly sensitive with respect to the interlayer coupling and thus a very accurate electronic dispersion along the $c$ axis is crucial. Linear response theory is used to calculate the coupled mode dispersion in the main symmetry directions of the Brillouin zone and the charge-density redistributions excited by certain strongly coupling phononlike and plasmonlike modes. The corresponding mode induced orbital averaged changes in the self-consistent potential felt by the electrons are assessed. Our analysis should be representative for the optimally to overdoped state of the cuprates where experimental evidence of a coherent three-dimensional Fermi surface and a coherent $c$-axis charge transport is given. It is demonstrated that modes from the outside of a small region around the $c$ axis can reliably be calculated within the adiabatic limit. On the other hand, modes inside this sector have to be determined nonadiabatically. In particular, the relevance of a strongly coupling nonadiabatic phononlike apex oxygen $Z$-point breathing mode ${\text{O}}_{z}^{Z}$ at about 40 meV is emphasized whose mode energy decreases with less doping.

Spectropolarimetric method for optic axis, retardation, and birefringence dispersion measurement

A new method and algorithm for measuring optical linear birefringence is proposed. The method allows the measurement of both the principal axis orientation angle and the retardation simultaneously by a three-step measurement. The average absolute error of the retardation of an achromatic quarter-wave plate (QWP) is found to be better than 10 -4 parts of the wavelength over the whole spectrum, and its principal axis system orientation is determined with accuracy better than 0.18 deg. In comparison to other methods, the current technique holds several advantages: wavelength independence, low cost, compact setup, ease of alignment, use of a simple algorithm, no polarization reflectance dependence, and possesses high accuracy. The method was applied also to the measurement of an arbitrary retardation of a sapphire plate and to the assessment of the dynamic retardation of a liquid crystal device.

Self-Masking of Spontaneous Symmetry Breaking in Layer Materials

We study d-wave Fermi-surface deformations (dFSD), the so-called Pomeranchuk instability, on bilayer and infinite-layer square lattices. Since the order parameter of the dFSD has Ising symmetry, there are two stacking patterns along the c axis: (+, +) and (+, -). We find that, as long as the c axis dispersion is finite at the saddle points of the in-plane band dispersion, the (+, -) stacking is usually favored independently on the details of interlayer coupling, yielding no macroscopic anisotropy. The dFSD provides unique spontaneous symmetry breaking that is self-masked in layer materials.