Model of convergent extension in animal morphogenesis

We argue that energy minimization can explain the pattern of cell movements in the morphogenetic process known as convergent extension provided that the cell-cell adhesive energy has a certain type of anisotropy, which we describe. This single simple property suffices to cause the cell elongation, cell alignment, and lengthening of a cellular array that characterize convergent extension. We show that the final aspect ratio of the array of cells depends on the anisotropy and is independent of the initial configuration and of the degree of cell elongation.     

Origin of magnetic anisotropy of Gd metal

Using first-principles theory, we have calculated the energy of Gd as a function of spin direction, theta, between the c and a axes and found good agreement with experiment for both the total magnetic anisotropy energy and its angular dependence. The calculated low temperature direction of the magnetic moment lies at an angle of 20 degrees to the c axis. The calculated magnetic anisotropy energy of Gd metal is due to a unique mechanism involving a contribution of 7.5 microeV from the classical dipole-dipole interaction between spins plus a contribution of 16 microeV due to the spin-orbit interaction of the conduction electrons. The 4f spin polarizes the conduction electrons via exchange interaction, which transfers the magnetic anisotropy of the conduction electrons to the 4f spin.     

A study of the effective magnetic anisotropy and the corresponding spin configurations in two dimensional ferromagnetic/antiferromagnetic system

The spin configurations of two dimensional ferromagnetic/antiferromagnetic system were investigated using model calculations and Monte-Carlo simulation methods. The lowest energy state was obtained under various coupling conditions to investigate the role of interfacial interaction on anisotropy. We found that the total ferromagnetic layer anisotropy is contributed not only from its own crystalline anisotropy but also from the antiferromagnetic layer spin flop effect. The overall ferromagnetic layer effective anisotropy is calculated as a function of the exchange energy of antiferromagnetic layer and the interfacial interaction energy. If the effective anisotropy from the spin flop effect is comparable with the crystalline anisotropy, the asymmetric spin configuration is generated. In this configuration, the magnetization direction of the ferromagnetic layer is neither perpendicular nor parallel to the antiferromagnetic spin direction. Temperature effect on the perpendicular-to-collinear coupling transition was also investigated using Monte-Carlo simulation, and the relationship between the effective anisotropy and the temperature was obtained.     

Computer simulations of the Stranski-Krastanov growth of heteroepitaxial films with elastic anisotropy

A three-dimensional finite element method is developed to simulate the surface morphological evolution during the Stranski-Krastanov heteroepitaxial growth. In the formulation, the surface evolves through surface diffusion driven by the gradient of the surface chemical potential, which includes the elastic strain energy, elastic anisotropy and surface energy. Surface condensation rate is assumed to depend on the difference between the surface chemical potential and the chemical potential of the vapor phase. Our simulations reveal that the self-assembly of quantum dots are strongly dependent on the variation of growth rate and elastic anisotropy strength. With appropriate choice of growth rate and elastic anisotropy strength, a relatively more uniform and regular quantum dot array can be obtained.     

Calculation of anisotropic strain-engineered patterns of monolayer islands

The self-organization of monolayer epitaxy islands in presence of anisotropies in surface stress, applied stress, and lattice mismatch between the film and substrate materials is investigated. The fundamental nature of island interactions is addressed in the context of a model wherein the system free energy consists of the excess energy and strain energy of atomic surface steps. It is shown the anisotropy can change the character of island interactions. An energy-reducing kinetic relation is adopted to evolve an initially random morphology towards a generally metastable minimum energy state. It is found the self-organization of islands into a regular array requires both the repulsion between islands and tendency for islands to aligned in a particular direction. Small anisotropies provide the required repulsion but not the tendency for islands to align and large anisotropies provide the necessary alignment but cause islands to attract. Modest levels of anisotropy provide the most favorable conditions of self-organization.     

Main results of the study of ultrahigh-energy cosmic rays in the Yakutsk Extensive Air Shower Array

This paper describes the current state of the Yakutsk Extensive Air Shower Array (EASA) and the prospects for its modernization. The main results of the study of the energy spectrum, mass composition, and anisotropy of the ultra-high energy cosmic rays (UHECRs) that were obtained recently by the array are given.     

Hysteresis in small arrays of interacting magnetic nanoparticles

The out-of-plane hysteresis loops of small arrays of magnetic nanoparticles, under the influence of an external field applied perpendicular to the array and the dipolar interaction are investigated. The particles are assumed to have a perpendicular anisotropy energy that tends to align the magnetic moments to be perpendicular to the array. The magnetization is found to exhibit a plateaux-and-jumps structure as the external field is swept up and down. These jumps are associated with jumps in the energy of the system, and correspond to transition from one configuration of the moment orientation to another. The energy of different configurations of the magnetic moments for a 3×3 array in the limit of weak dipolar interaction is analyzed, as a means to understand the hysteresis loop. These jumps are more pronounced in arrays of smaller sizes and when the dipolar interaction is weak. The configuration of magnetic moments at zero external field as the field is swept up and down is found to be highly sensitive to the dipolar interaction.     

Three-and two-dimensional calculations for the interface anisotropy dependence of magnetic properties of exchange-spring Nd2Fe(14)B/α-Fe multilayers with out-of-plane easy axes

Hysteresis loops,energy products and magnetic moment distributions of perpendicularly oriented Nd2Fe(14)B/α-Fe exchange-spring multilayers are studied systematically based on both three-dimensional(3D)and one-dimensional(1D)micromagnetic methods,focused on the influence of the interface anisotropy.The calculated results are carefully compared with each other.The interface anisotropy effect is very palpable on the nucleation,pinning and coercive fields when the soft layer is very thin.However,as the soft layer thickness increases,the pinning and coercive fields are almost unchanged with the increment of interface anisotropy though the nucleation field still monotonically rises.Negative interface anisotropy decreases the maximum energy products and increases slightly the angles between the magnetization and applied field.The magnetic moment distributions in the thickness direction at various applied fields demonstrate a progress of three-step magnetic reversal,i.e.,nucleation,evolution and irreversible motion of the domain wall.The above results calculated by two models are in good agreement with each other.Moreover,the in-plane magnetic moment orientations based on two models are different.The 3D calculation shows a progress of generation and disappearance of vortex state,however,the magnetization orientations within the film plane calculated by the 1D model are coherent.Simulation results suggest that negative interface anisotropy is necessarily avoided experimentally.     

Modernization of the Carpet-2 array of the Baksan Neutrino Observatory

The state of the art and the project of modernization of the extensive-air-shower array Carpet-2 of the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences are described. The modernized array will allow the performance of detailed study of variations in the cosmic ray intensity, the energy spectra and composition of primary cosmic rays in the energy range 10¹³–10¹⁶ eV, and the anisotropy of primary cosmic rays with energies above 10¹³ eV.     

Anisotropic p−ƒ mixing mechanism explaining anomalous magnetic properties in Ce monopnictides

Anomalously small crystalline field splitting in the paramagnetic region and extremely strong magnetic anisotropy in the ordered phases in CeSb and CeBi are explained based on the anisotropic mixing mechanism between the 4? states and the valence bands. In the paramagnetic region, the mixing gives the effective crystalline field splitting which is estimated to cancel the splitting of the point charge model in good agreement with experiment. The anisotropy energy of CeSb calculated by using the realistic valence bands is consistent with the strong anisotropy observed experimentally.     

Analysis of anisotropy of cosmic rays with the energy of about 1017 eV by Yakutsk EAS array data

A harmonic analysis of the directions of arrival of cosmic-ray particles with an energy of about 10¹⁷ eV in the vicinity of the registration threshold of the Yakutsk extensive air showers (EAS) array is given. A method for determining the contribution of inhomogeneous observation conditions and seasonal variations of the frequency of extensive air showers to the observed anisotropy is suggested. Taking into account these factors results in a considerable decrease of the amplitude characterizing the degree of anisotropy of cosmic-ray primaries. The amplitude of the first harmonic with respect to the right ascension is (0.45 ± 0.55)%, which shows that no probably significant anisotropy of the primary radiation is observed at 10¹⁷ eV.     

Nonlinear regimes of ultrashort pulse propagation in an array of anisotropic tunneling states

Dynamics of an ultrashort electromagnetic pulse in a system with an array of anisotropic tunneling states spanned by the pulse spectrum are analyzed. A system of nonlinear wave equations is derived for the ordinary and extraordinary components of the pulse propagating at an arbitrary angle to the anisotropy axis. Different regimes of ultrashort pulse propagation parallel and perpendicular to the anisotropy axis are examined. Ultrashort-pulse propagation regimes analogous to self-induced transparency and extraordinary transparency are identified. The properties of rational soliton-like pulses having no quasi-monochromatic analogs are analyzed. A longitudinal electric field component is generated in each regime, whereas off-resonance quasi-monochromatic pulses propagating under similar conditions (parallel and perpendicular to the anisotropy axis) have no longitudinal components. Stability of the solutions obtained and the effect of diffraction on ultrashort pulse dynamics are analyzed. The values of pulse parameters for which defocusing dominates over self-focusing are calculated.     

Structural, electronic, and magnetic properties of bcc iron surfaces

Trends in atomic multilayer relaxations, surface energy, electronic work function, and magnetic structure of several low-Miller-index surfaces of iron are investigated employing density functional theory total energy calculations. The calculated topmost layer relaxations reproduce well the experimental contractions and their variation with the surface crystallographic orientation, and surface roughness. The multilayer relaxation sequences correlate with the reduced coordination in surface layers and can be explained in terms of a simple electrostatic picture. The surface energies scale almost linearly with the surface roughness. They agree well with the experimental surface tensions and show a small anisotropy in agreement with predictions based on measurements for other metals. The equilibrium shape of a bcc Fe crystal is determined and discussed. The work function anisotropy is calculated and rationalized in terms of changes in the valence charge distribution. Significantly increased local magnetic moments of atoms in the surface region are determined. The correlation between the anisotropy of the surface magnetic moments and atomic coordination in the outermost layers is demonstrated to follow a simple rule.     

Direction of the anisotropy vector of TeV cosmic rays

The preliminary results of measuring the direction of the stellar anisotropy vector of cosmic rays with the Carpet array at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) are presented. The direction of the anisotropy vector is determined by analyzing the distribution of time delays of the signal from distant detectors of the array with respect to its central part. It is shown that the anisotropy vector has the direction α₀ (R.A.) ≈ 1.5 h, δ₀ ≈ 62° in the equatorial coordinate system. At such a direction, the CR anisotropy measured with the Baksan Underground Scintillation Telescope and the Andyrchi array is ≈0.2%.     

Phase diagram of a two-dimensional square lattice of magnetic particles with perpendicular anisotropy

The ground state of an array of small single-domain magnetic particles having perpendicular anisotropy and forming a square two-dimensional lattice is studied in the presence of a magnetic field. The stability of some basic states with respect to nonuniform perturbations is analyzed in a linear approximation, and analytical model calculations and numerical simulation are used for an analysis. The entire set of states at various anisotropy constants and magnetic fields is considered when a field is normal to the array plane. Two main classes of states are possible for an infinite system, namely, collinear and noncollinear states. For collinear states, the magnetic moments of all particles are normal to the array plane. At a sufficiently high anisotropy, a wide class of collinear states exists. At low fields, a staggered antiferromagnetic order of magnetic moments takes place. An increase in the magnetic field causes an unsaturated state, and this state transforms into a saturated (ferromagnetic) state with a parallel orientation of the magnetic moments of all particles at a sufficiently high field. At a lower anisotropy, the ground state of the system is represented by noncollinear states, which include a complex four-sublattice structure for the components of the magnetic moments in the array plane and a nonzero projection of the magnetic moments of the particles onto the field direction. A phase diagram is plotted for the states of an array of anisotropic magnetic particles in the anisotropy constant-magnetic field coordinates. For a finite array of particles, sample boundaries are shown to play a significant role, which is particularly important for noncollinear states. As a result of the effect of the boundaries at a moderate field or anisotropy, substantially heterogeneous noncollinear states with a heterogeneity size comparable with the sample size can appear in the system.     

On the temperature dependence of the surface energy of actinide crystal faces

The temperature coefficient of the surface energy has been calculated and the influence of the temperature on the anisotropy of the surface energy has been demonstrated for crystal faces of the polymorphic phases of actinides.     

Density of states,charge densities and autocorrelation function of the electronic ground state in orthorhombic sulfur

The molecular orbitals of the S₈ molecule have been calculated selfconsistently without any fitting parameters using the LCAO-MO-method in the STO-4G basis. The ground state charge density, energy spectrum and autocorrelation function of the one-electron density matrix (Fourier transformed Compton profile) are presented and compared with experimental data obtained from the orthorhombic crystal. While the energy spectrum agrees with photoemission data the autocorrelation function shows a strong anisotropy between directions parallel and perpendicular to the plane of the ring. This anisotropy is interpreted in terms of intra- and intermolecular interactions.     

Diverse features of magnetization curves of uniaxial crystals:A simulation study

We present a simulation of the magnetization curves, energy, probability, and torque landscapes of uniaxial systems with up to five anisotropy constants. The total energy used in the simulation is the sum of the anisotropy and Zeeman energies. The exchange interaction is not considered in the present work in which we treat single-domain-particle systems within a classical mechanics-based model. Diverse features of the calculated magnetization curves are highlighted for the studied systems. These diverse features are strongly dependent on the sign and magnitude of the simulation parameters.The model is versatile enough to handle both hypothetical and real material systems, e.g. HoFe_(11)Ti and Y_2Co_(17).     

Electronic-spin generation in multi-photon processes: Cubic anisotropy of pumping

We study the spin generation in multi-photon absorption processes in a bulk semiconductor, pumped it by a circularly polarized intense light by varying the angle of incidence. The generated spin polarization is calculated using the eight-band Kane model in the limit of large spin–orbit splitting and on the basis of the multi-photon photo-generation rate of the conduction electron spin density. It is found that the spin polarization strongly depends on the pumping photon energy outside the band edge. Cubic anisotropy in crystal pumping is also calculated. The results show that due to this anisotropy the spin generation differs by ∼8%, in consistence with earlier results obtained by others.     

磁性薄膜原子层数对极化方向的影响

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