Lattice orientations of driven vortex matter in amorphous MoGe films

We have investigated the lattice orientation of driven vortex matter in amorphous MoGe films. Mode locking experiments in the flux flow state reveal that in addition to the theoretically predicted lattice orientation parallel to the flow direction also the perpendicular orientation occurs. Mapping out the orientations in a phase diagram, the perpendicular orientation is found to dominate the phase diagram covering a wide field and temperature range. Scanning tunneling microscopy images of the vortex lattice frozen from the flux flow state confirm the switching between parallel and perpendicular orientations in the phase diagram. The effect is possibly caused by the influence of the sample edge.     

The mica slit-pore as a tool to control the orientation and distortion of simple liquid monolayers

Grand canonical ensemble Monte Carlo computer simulations have been used to study mono-layer octamethylcyclotetrasiloxane (OMCTS) and cyclohexane films confined between mica-like surfaces to determine the effect of the mica surfaces on the orientation and distortion of the films at different surface alignments. The film molecules are packed as a highly ordered lattice. The orientation of the lattice is fixed relative to the mica surfaces and depends on the size of the film molecule. Registry shifts distort the film lattice by effectively stretching it along a particular direction that depends on the size of the film molecule. For a particular registry, OMCTS and cyclohexane monolayers are stretched in perpendicular directions. Coupling between the monolayers and the mica surfaces generates a nonzero shear stress when the surfaces are out of alignment, but the film does not become disordered or melt. It is possible that precisely controlled solid surfaces could be used to create packed arrays of film molecules with desired orientation and degree of distortion that may be useful in nanotechnological applications.     

Error analysis and correction for Lattice Boltzmann simulated flow conductance in capillaries of different shapes and alignments

We study the relative error in conductance calculations, for simulated flow of a single component single phase fluid through a capillary in three dimensions, by the Lattice Boltzmann (LB) method with bounce-back boundary conditions. The relative error with respect to analytical results for capillary cross-sections of circular, triangular and square shapes are calculated as a function of the cross-section diameter, a, and for different alignment of the cross-section relative to the underlying lattice grid. It is shown, when the shapes are not aligned perfectly to the lattice, that the relative error decreases systematically with the size, a, as ～1/a when a is evaluated by mapping the computed cross-sectional area, in terms of the enclosed number of grid points, to the respective geometrical shapes concerned. For perfectly aligned geometries, viz. the square capillary aligned to the LB lattice grid or rotated with its side along the diagonal of the LB grid, the relative error decreases as ～1/a². A simple method is suggested to locate the boundary wall depending on its orientation relative to the grid, such that the exact conductance of the new shape matches the LB computed conductance.     

On crystal shear,lattice rotation and constraint stress in (1 1 0) channel die compression: rate-independent and viscoplastic analyses and predictions compared

Rate-independent crystal plasticity theory and a classic viscoplastic power-law are investigated, contrasted and compared for finite deformation analysis of fcc crystals in channel die compression, including full consideration of lattice straining. Both experiment-based anisotropic and isotropic (Taylor) hardenings are evaluated in rate-independent theory; and an unlimited range of power-law exponent n is considered in viscoplasticity. The focus is on predictions of lateral constraint stress, lattice rotation and crystal shear, and their comparison with experiment. General elastic-plastic equations (for both theories) are given for the range of unstable lattice orientations in (1?1?0) compression (‘range I’) and evaluated before and after a finite rotation of the lattice about the load axis. Equations also are given and evaluated for the ‘Brass’ orientation. It is shown that the theories can be in close agreement at the onset of finite deformation in range I, but that viscoplasticity gives results (for any n) after finite rotation that are in sharp contrast to rate-independent theory. The latter’s predictions for crystal shear and lattice rotation are in good to very good agreement with finite deformation experiments on aluminium and copper. The inclusion of lattice elasticity is found to have a negligible effect in range I. In contrast, for finite deformation in the stable Brass orientation, elastic-viscoplastic theory can be made to agree very closely with rate-independent theory and with experiment.     

Three-dimensional lattice Boltzmann method for simulating blood flow in aortic arch

The three-dimensional （3D） lattice Boltzmann models, 3DQ15, 3DQ19 and 3DQ27, under different wall boundary conditions and lattice resolutions have been investigated by simulating Poiseuille flow in a circular cylinder for a wide range of Reynolds numbers. The 3DQ19 model with improved Fillippova and Hanel （FH） curved boundary condition represents a good compromise between computational efficiency and reliability. Blood flow in an aortic arch is then simulated as a typical haemodynamic application. Axial and secondary fluid velocity and effective wall shear stress profiles in a 180° bend are obtained, and the results also demonstrate that the lattice Boltzmann method is suitable for simulating the flow in 3D large-curved vessels.     

相同尺度下气泡与复杂壁面的耦合特性研究

采用格子Boltzmann方法(LBM)建立了气液固三相耦合的动力学模型,研究了相同尺度下上浮气泡与复杂壁面的相互耦合作用.首先,基于黏性流体理论,通过构建一组格子Boltzmann(LB)方程来描述气液两相的运动,并以LB离散体积力的形式计入了黏性力、表面张力和重力.同时,采用LBM中的Half-way反弹模型与有限差分格式相结合的方式进行固壁边界的处理.然后,利用本文建立的模型,对不同特征尺寸比条件下,气泡与考虑边缘效应的平面固壁和曲面固壁的耦合特性进行了研究.研究发现固壁边界条件以及特征尺寸比对气泡的运动和拓扑结构的变化都具有明显的非线性影响.最后,研究了流体属性对气泡与复杂壁面耦合规律的影响.     

Disorientation angles and compatible walls for phase transition hexagonal to monoclinic phase

Orientation of compatible domain walls and magnitude of disorientation angle of a ferroelastic domain twin resulting from phase transition hexagonal to monoclinic phases is expressed in crystallographic unit-cell parameters of the low-symmetry phase. These two characteristics, the orientation of the compatible wall and the disorientation angle, depend on the spontaneous strain in two single-domain states R ₁, R ₂ from which the domain twin is formed. They have been determined for all classes of the compatible domain walls as a function of the strain-tensor components [A. Authier, International Tables for Crystallography, in Physical Properties of Crystals, Chapter 3.4, Vol. D, A. Authier, ed., Kluwer Academic Publishers, Dordrecht, 2003, pp. 449–505]. If relative changes of crystal lattice are small, then the second rank symmetrical strain tensor u can be calculated from the crystallographic unit-cell parameters before and after the deformation [J.L. Schlenker, G.V. Gibbs, and M.B. Boisen Jr, Strain-tensor components expressed in terms of lattice parameters, Acta. Cryst. A 34 (1978), pp. 52–54; L. Jian and C.M. Wayman, Domain boundary and domain switching in a ceramic rare-earth Orthoniobate LaNbO_{4 ,} J. Am. Ceram. Soc. 79 (1996), pp. 1642–1648]. An alternative approach expresses the disorientation angle and orientation of the compatible domain wall [J. P?ívratská, Disorientation angle expressed in terms of lattice parameters, Ferroelectrics 291 (2003), pp. 197–204] in terms of the crystallographic unit-cell parameters of the low-symmetry phase.     

Domain wall oscillations in magnetic garnet films

Domain wall oscillations in magnetic garnet films have been observed in the stripe lattice and in the bubble lattice for applied in-plane magnetic fields approaching the saturation limit of the film. Observations are reported in (111) and (001) oriented films. An effective domain wall mass model is developed which allows the variation of the azimuthal angle of the spins in a moving wall when an in-plane field is applied in the plane of the wall. The new model gives results which are in much better agreement with the experimental results than previous models. Reasonable agreement is also observed between theory and experiment when the cubic anisotropy is included. Experimental evidence of the Hubert wall structure and its change for in-plane fields less than 8M is also reported.     

Atomically sharp magnetic domain wall in thin film Fe(110): a first principles noncollinear magnetism study

Magnetic domain wall structures in an Fe (110) monolayer are determined by the highly precise first principles full-potential linearized augmented plane-wave method including intra-atomic noncollinear magnetism. The self-consistent results demonstrate that the magnetization changes from one orientation to the opposite (180 degrees ) orientation within an 8 A width without any abrupt rotation. This narrow domain wall is found to arise from band effects. Our results are consistent with and support domain walls having a 6 A width recently observed in spin-polarized scanning tunneling microscopy experiments.     

Simulation study for the orientation of the driven vortex lattice in an amorphous superconductor

We investigate the orientation of the vortex lattice driven by an applied current by means of numerical simulations based on the time-dependent Ginzburg–Landau (TDGL) theory. A lattice order is restored by a current driving of vortices under the influence of random vortex pinnings. The orientation of the moving vortex lattice is different between the presence and the absence of vortex pinnings. We show results of TDGL simulations for these phenomena.     

A histogram-based segmentation method for characterization of self-assembled hexagonal lattices

Lattice characterization techniques are often used to quantify the effects of different anodization conditions on nano-porous anodized aluminum oxides. In this work, we develop a comprehensive hexagonal lattice characterization method to evaluate the amount of ordering of the lattice and localize the domains of the image and report their characteristics. A robust preprocessing is proposed to find pores’ centroids. Different domains of SEM images usually have different orientations. Pores orientation distribution is analyzed using angle-histogram. The valleys of angle-histogram are employed as thresholds to separate different dominant orientations. We show that using orientation as a distinguishing feature of different domains, significantly improves the robustness of the algorithm against tolerance parameters. Some new parameters are introduced to exactly characterize each of the domains and the whole lattice.     

Light diffraction features in an ordered monolayer of spheres

The structure and optical diffraction properties of monolayers of monodisperse spheres crystallized on transparent dielectric substrates are studied. Two types of diffraction phenomena are considered: surface light diffraction on the lattice of spheres and waveguide resonances in the monolayer plane. For experimental study of these phenomena, optical retroreflection and transmission spectra are measured as functions of the light incidence angle and azimuthal orientation of the incidence plane. The monolayer structures determined by scanning electron microscopy and light diffraction methods are in quantitative agreement. It is concluded that one-dimensional Fraunhofer diffraction is applicable to describe surface diffraction in the hexagonal lattice of spheres. In the case of oblique light incidence, anisotropy of diffraction and transmission spectra depending on the light incidence plane orientation with respect to the sphere lattice and linear polarization of incident light is detected. Waveguide resonances of the planar two-dimensional photonic crystal are approximated within the light diffraction model in the “empty” hexagonal lattice. The best approximation of the waveguide resonance dispersion is achieved using the effective refractive index, depending on the wavelength. Surface diffraction suppression by waveguide resonances of the photonic crystal is demonstrated. Surface diffraction orders are identified as diffraction at singular points of the Brillouin zone of the planar twodimensional photonic crystal.     

Modeling of surfaces of constant force above a lattice of close-packed atoms in the repulsive mode

Surfaces of constant force (force contours) are calculated for the scanning of an AFM tip over a lattice of close-packed atoms in the repulsive mode. It is shown that discontinuities are observed on the force contours in the regions between the atoms of the surface lattice for sufficiently small initial scanning heights of a tip with a single atom at its end. A cluster model of the tip end, which ensures continuity of the scanning at arbitrary initial heights, is constructed. The dependence of the AFM images on the orientation of the cluster on the tip end relative to the crystallographic axes of the surface is investigated for both an unperturbed lattice of close-packed atoms and a lattice containing point defects. The diagnostic possibilities of the findings are discussed. Zh. Tekh. Fiz. 67, 77–85 (June 1997)     

Effects of draw-down ratio and annealing treatment on structure formation in extruded strands of a thermotropic liquid crystalline copolyester

The molecular orientation, thermal behavior, and crystal lattice structure in extruded strands of a thermotropic liquid crystalline polymer (LCP) were studied with wide-angle x-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The purpose of this work is to elucidate the effects of draw-down ratio and annealing treatment on the structure development in the LCP strands. The crystal orientation function markedly increased with increasing draw-down ratio, but the increase of orientation function saturated at higher draw-down ratio. Annealing treatment below 250°C slightly increased the degree of orientation, whereas the molecular orientation was relaxed by annealing at 270°C. In addition, the thermal properties and crystal lattice structure were sensitive to the annealing treatment. The change of DSC curves with annealing temperature suggested that the initial crystalline texture of as-extruded samples was reorganized into a more ordered structure by the annealing treatment. Draw-down ratio had some effects on the thermal properties. The molecular orientation facilitated the crystallization during annealing.     

Plasmon resonances in a two-dimensional lattice of metal particles in a dielectric layer: Structural and polarization properties

The results of experimental and theoretical investigation of planar two-dimensional (2D) samples of plasmon structures are presented. The samples represent a 2D lattice of gold nanoparticles embedded in a thin dielectric layer and are studied by atomic force microscopy (AFM) and optical methods. Absorption bands associated with the excitation of various surface plasmon resonances (SPR) are interpreted. It is found that the choice of the mutual orientation of the polarization plane and the edge of the unit cell of the 2D lattice determines the spectral position of the lattice surface plasmon resonance (LSPR) related to the lattice period. It is shown that the interaction of p- and s-polarized light with a 2D lattice of nanoparticles is described by the dipole–dipole interaction between nanoparticles embedded in a medium with effective permittivity. Analysis of the spectra of ellipsometric parameters allows one to determine the amplitude and phase anisotropy of transmission, which is a consequence of the imperfection of the 2D lattice of samples.     

Observation of vortex pinning in Bose-Einstein condensates

We report the observation of vortex pinning in rotating gaseous Bose-Einstein condensates. Vortices are pinned to columnar pinning sites created by a corotating optical lattice superimposed on the rotating Bose-Einstein condensates. We study the effects of two types of optical lattice: triangular and square. In both geometries we see an orientation locking between the vortex and the optical lattices. At sufficient intensity the square optical lattice induces a structural crossover in the vortex lattice.     

Change of crystal dimensions due to the alignment of anisotropic elastic dipoles

A method is proposed to determine the elastic dipole tensor of a lattice defect. The length change of a crystal was measured, when the defect orientation in the lattice is changed.     

石榴石薄膜中的条畴铁磁共振

本文详细报道了条畴铁磁共振(FMR)特性与微波磁场和条畴间夹角的依赖关系。实验结果表明,由Smit和Beljers等人所提出来的关于磁畴共振的理论需要进一步改进和完善。 关键词：     

Photonic band structure of 3D ordered silica matrices

The symmetry and orientation of a photonic lattice of 3D ordered porous silica matrices (synthetic opals) are determined by analysis of transmission electron micrographs. By optical transmission measurements the photonic band edges are mapped out at pointsLandKof the Brillouin zone. It is shown that modulating the refractive index of opal pores by filling them with liquids results in variation of the light attenuation length within the forbidden gap. To increase the refractive index contrast of the lattice, the pores of opals were filled with CdS. A dramatic decrease in attenuation length is observed for opal/CdS. The parameters of a photonic lattice with a ‘pseudo-gap’, that is, with a prominent depletion of the photonic density of states, are estimated.     

用格子Boltzmann方法模拟壁面微结构对管流特性的影响

采用格子Boltzmann方法和Shan-Chen多相流模型,模拟液滴在常力驱动下在微管道内的流动,研究微孔道壁面润湿性和几何结构对降压增注效果的影响.阐明具有一定粗糙度的疏水表面阻力减小的原因,研究表明,壁面润湿性和粗糙度对管流特性有显著的影响,同时也表明,用Lattice Boltzmann方法模拟,具有相当精确的可预测性,在石油储层微观渗流减阻机制研究方面有很好的应用前景,其可为研究纳米降压增注效果提供有用工具.