Atomic structure of CaF2/MnF2-Si(1 1 1) superlattices from X-ray diffraction

X-ray reflectivity and non-specular crystal truncation rod scans have been used to determine the three-dimensional atomic structure of the buried CaF₂-Si(1 1 1) interface and ultrathin films of MnF₂ and CaF₂ within a superlattice. We show that ultrathin films of MnF₂, below a critical thickness of approximately four monolayers, are crystalline, pseudomorphic, and adopt the fluorite structure of CaF₂. High temperature deposition of the CaF₂ buffer layer produces a fully reacted, CaF₂-Si(1 1 1) type-B interface. The mature, “long” interface is shown to consist of a partially occupied layer of CaF bonded to the Si substrate, followed by a distorted CaF layer. Our atomistic, semi-kinematical scattering method extends the slab reflectivity method by providing in-plane structural information.     

Stabilization of the pentagonal surface of the icosahedral AlPdMn quasicrystal by controlled Si absorption

The Debye-temperature of the pentagonal surface of the icosahedral AlPdMn quasicrystal (QC) is measured by means of low-energy electron diffraction after the absorption of different amounts of Si. We observe an increase of the surface Debye-temperature from 300±7 K for the freshly prepared surface to 330±7 K after the absorption of 60-Å Si. Because the quasicrystalline order persists at the surface in spite of the diffusion of Si into the substrate, we suggest that the diffusion is dominated by a vacancy-mediated process.     

Quantification of the push-pull character of the isophorone chromophore as a measure of molecular hyperpolarizability for NLO applications

The push-pull character of a series of para-phenyl substituted isophorone chromophores has been quantified by the ¹³C chemical shift difference of the three conjugated partial CC double bonds and the quotient of the occupations of both the bonding and anti-bonding orbitals of these CC double bonds as well. The correlations of the two push-pull quantifying parameters, and to the corresponding bond lengths, strongly recommend /π_CC as the general parameter to estimate charge alternation and as a very useful indication of the molecular hyperpolarizabilities for NLO application of the compounds studied.     

The free energy of mixing for n-variant martensitic phase transformations using quasi-convex analysis

The construction of effective models for materials that undergo martensitic phase transformations requires usable and accurate functional representations for the free energy density. The general representation of this energy is known to be highly non-convex; it even lacks the property of quasi-convexity. A quasi-convex relaxation, however, does permit one to make certain estimates and powerful conclusions regarding phase transformation. The general expression for the relaxed free energy is however not known in the n-variant case. Analytic solutions are known only for up to 3 variants, whereas cases of practical interests involve 7-13 variants. In this study we examine the n-variant case utilizing relaxation theory and produce a seemingly obvious but very powerful observation regarding a lower bound to the quasi-convex relaxation that makes practical evolutionary computations possible. We also examine in detail the 4-variant case where we explicitly show the relation between three different forms of the free energy of mixing: upper bound by lamination, the Reuß lower bound, and a lower estimate of the -measure bound. A discussion of the bounds and their utility is provided; sample computations are presented for illustrative purposes.     

Oblique stagnation-point flow of an incompressible visco-elastic fluid towards a stretching surface

An analysis is made of the steady two-dimensional stagnation-point flow of an incompressible viscoelastic fluid over a flat deformable surface when the surface is stretched in its own plane with a velocity cx, where x is the distance from the stagnation-point and c is a positive constant. It is shown that for a viscoelastic fluid of short memory (obeying Walters’ B model), a boundary layer is formed when the stretching velocity of the surface is less than ax, where ax+2by is the inviscid free-stream velocity and y is the distance normal to the plate, a and b being constants and the velocity at a point increases with increase in the elasticity of the fluid. On the other hand an inverted boundary layer is formed when the surface stretching velocity exceeds ax and the velocity decreases with increase in the elasticity of the fluid. A novel result of the analysis is that the flow near the stretching surface is that corresponding to an inviscid stagnation-point flow when a=c. Temperature distribution in the boundary layer is found in three cases, namely: (i) the sheet with constant surface temperature (CST); (ii) the sheet with variable surface temperature (VST) and (iii) the sheet with prescribed quadratic power law surface heat flux (PHF) for various values of non-dimensional parameters. It is found that in all the three cases when a/c>1, temperature at a point decreases with increase in the elasticity of the fluid and when a/c<1, temperature at a point increases with increase in the elasticity of the fluid. Further temperature at a point decreases with increase in the radiation parameter and wall temperature parameter.     

Non-periodic finite-element formulation of orbital-free density functional theory

We propose an approach to perform orbital-free density functional theory calculations in a non-periodic setting using the finite-element method. We consider this a step towards constructing a seamless multi-scale approach for studying defects like vacancies, dislocations and cracks that require quantum mechanical resolution at the core and are sensitive to long range continuum stresses. In this paper, we describe a local real-space variational formulation for orbital-free density functional theory, including the electrostatic terms and prove existence results. We prove the convergence of the finite-element approximation including numerical quadratures for our variational formulation. Finally, we demonstrate our method using examples.     

C语言指针运用探讨

分析 C语言指针读解、误用问题 ,以实例探讨 C语言指针运用     

一种并行融合部分求值的别名分析方法

通过并行融合部分求值和别名分析技术, 提出一种新的别名分析方法, 并给出形式化描述. 实践表明, 该方法在具有部分输入的情形下不仅优于传统的别名分析方法, 而且优于串行融合部分求值的别名分析方法.     

应用双层指针缩短切换时延及优化路由的分布式移动性管理方案

根据锚点位置固定有否,移动IP协议分为锚点固定的移动性管理协议和分布式移动性管理协议两类,后者允许移动节点在会话开始时动态地选择较近的接入路由器作为锚点,从而可以缩短切换时延并实现近似最优的路由.然而,对于给定的会话,锚点的位置是固定的,在会话持续时间内进行移动节点切换时,切换操作与锚点固定的移动性管理协议相同,存在切换时延长和路由欠优问题.为解决该问题,提出了应用双层指针的分布式移动性管理方案.使用"低层指针"在具有移动性管理功能的接入路由器间建立指向关系以缩短切换时延;使用"高层指针"定期向锚点或对端节点注册优化路由并动态更换锚点;推导了所提方案和已有方案的切换时延公式.数值分析和仿真结果表明,所提方案可以缩短分布式移动性管理的切换时延.