Clustering of exponentially separating trajectories

It might be expected that trajectories of a dynamical system which has no negative Lyapunov exponent (implying exponential growth of small separations) will not cluster together. However, clustering can occur such that the density ρ(Δx) of trajectories within distance |Δx| of a reference trajectory has a power-law divergence, so that ρ(Δx) ∼ |Δx| ^−β when |Δx| is sufficiently small, for some 0 < β < 1. We demonstrate this effect using a random map in one dimension. We find no evidence for this effect in the chaotic logistic map, and argue that the effect is harder to observe in deterministic maps.     

Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120° to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.     

高温高密度氦的压缩特性及其结构的分子动力学模拟

用分子动力学方法和不同参数的指数 6势函数计算了T =30 4K的高密度氦的等温压缩线和能量分布 .给出了能精确描述高密度氦原子间相互作用的指数 6势函数优化参数 .并用优化的势函数计算了高密度氦T =30 0K和T =2 98K的等温压缩线 ,计算结果和实验值非常吻合 .进一步用优化的势函数模拟了高温高密度氦的状态方程及其结构 ,发现当把 ρ限定为 1 .6 0g /cm³ 时 ,其径向分布函数的第 2个峰将在 2 0 0 0～ 30 40K区间消失 ,表明此时发生了固 液相变过程 .     

Modelling of magnetic satellite intensity in the neutron diffraction of an incommensurate helical structure: NiBr2 and Ni0.91Fe0.09Br2

We describe the application of a neutron multidetector diffractometer (D16, I.L.L.) to the study of the incommensurate helical magnetic structure of pure and Fe²⁺-doped single crystals of NiBr₂. Use of the multidetector permits detailed mapping of satellite intensity around 003_M arising from three magnetic domains, from which the magnitude and direction of the helix propagation vector τ are deduced. τ moves from [110] (0.0138(5) Å^?1) in NiBr₂ to [100] (0.0151(5) Å^?1) in Ni_0.91 Fe_0.09Br₂. The measured intensity profiles in (2θ, ω, v space are fitted to model profiles constructed by superimposing profiles measured at 003_M in the collinear phase at 28 K on a ring with radius τ, and optimizing the weighting of intensity around the ring.     

多尺度B样条小波边缘检测算子

基于B样条理论提出了一类新的多尺度小波变换,通过其零交叉或模极值能有效地表示和检测信号或图象的边缘,对任意n次B样条,导出了相应的分解和重建的快速算法,对应的用于分解和重建的滤波器的时域和频域响应也被精确地给出.从时频局部化的角度对不同次数的B样条作了分析,认为3次B样条小波在边缘提取等实际应用中是渐近最优的,结果也为B样条小波在立体视觉匹配、滤噪等方面的进一步应用提供了基础.     

Femtosecond laser-based fabrication of a new model material to study fracture

The ductile fracture process consists of the nucleation, growth and coalescence of voids in a material. Predictive models of ductility require a complete understanding of the coalescence event. However, coalescence occurs over very small strains and is therefore difficult to observe experimentally. We have addressed this by developing a new class of model material. It consists of femtosecond laser drilled holes and diffusion bonded metallic sheets, which can be mechanically tested in situ either by scanning electron microscopy (SEM) or by X-raycomputed tomography (XRCT). The fabrication steps are presented and the model material is characterized by optical and electron microscopy, nanoindentation and tomography. The heat affected zone around the laser holes is found to be harder than the unaffected material and consists of nano-scale grains. Finally we show that the coalescence event is well captured using both SEM and XRCT. The fabrication method is adaptable to a wide range of materials and enables one to produce 2D and 3D arrays of holes or cracks with controlled size, volume fraction and distribution. PACS  62.20.Mk; 62.25.+g; 79.20.Ds     

Synthesis of rhodium(II) pyrazolate complexes: Crystal structure of tetra-μ-3,5-dimethylpyrazolato dirhodium(II) bis-acetonitrile, (Rh-Rh)

The reaction of Rh₂(O₂CMe)₄ with the sodium salt of 3,5-dimethylpyrazole (3,5-Me₂pzH) in acetonitrile gives Rh₂(3,5-Me₂pz)₄ · 2MeCN. This yellow diamagnetic compound on heating gives Rh(3,5-Me₂pz)₄ which in turn forms adducts with different unidentate ligands, L, to give Rh₂(3,5-Me₂pz)₄ · 2L. The binuclear tetra bridged structure has been established for the acetonitrile complex by X-ray diffraction. The Rh-Rh distance is 2.353(3) Å and the Rh-N (acetonitrile) distance is 2.202(5) Å. Some unsubstituted pyrazolates have been made.     

Isomer-specific separation and quantitation of tetrachlorodibenzo-p-dioxins by HRGC and HRGC/MS

A method is described for the determination of 22 tetrachlorodibenzo-p-dioxin isomers at the low part-per-trillion (ppt) level. High resolution, narrow bore, open tubular columns (OTCs) with 100 μm i.d. can achieve better separations than presently used 320 or 250 μm i.d. columns in about half the total time. Relative retention times and response factors for all 22 TCDD isomers are presented for the electron capture detection and for the molecular ion mass of individual isomers under electron impact ionization with selected ion monitoring (SIM).