Radiation-stimulated diffusion in solids

Irradiation of solids produces a microscopic nonequilibrium state in which the vibrational energy distribution function of the atoms deviates from the thermodynamically equilibrium function. Expressions are obtained for the nonequilibrium distribution function and for the frequencies of activational transitions of atoms out of a potential well. It is shown that the radiation stimulation of diffusion processes involves a deviation of the temperature dependences of the frequencies of transitions of the atoms out of positions of equilibrium from the Arrhenius law. Under subthreshold irradiation conditions the rate of diffusion processes is higher for atoms whose vibrations thermalize over long times and depends linearly on the irradiation intensity. Under above-threshold irradiation conditions the characteristics of cascade regions in solids — their sizes and the vibrational excitation energy of the atoms — can be determined by comparing the computed and experimental temperature dependences of the diffusion coefficient. Zh. Tekh. Fiz. 68, 67–72 (August 1998)     

Ultrasonic wave propagation in concentrated slurries – The modelling problem

The suspended particle size distribution in slurries can, in principle, be estimated from measured ultrasonic wave attenuation across a frequency band in the 10s of MHz range. The procedure requires a computational model of wave propagation which incorporates scattering phenomena. These models fail at high particle concentrations due to hydrodynamic effects which they do not incorporate. This work seeks an effective viscosity and density for the medium surrounding the particles, which would enable the scattering model predictions to match experimental data for high solids loading. It is found that the required viscosity model has unphysical characteristics leading to the conclusion that a simple effective medium modification to the ECAH/LB is not possible. The paper confirms the successful results which can be obtained using core–shell scattering models, for smaller particles than had previously been studied, and outlines modifications to these which would permit rapid computation of sufficient stability to support fast particle sizing procedures.     

利用Taylor展开法研究Lamb波在功能梯度材料中的传播特性

超声技术可用于对功能梯度材料(FGMs)的性质进行评估. 由于FGMs性质的非均匀性,采用分布函数来描述FGMs弹性常数和密度沿厚度方向的变化趋势,并提出利用Taylor展开的方法来解决分布函数为任意函数时的FGMs中Lamb波的传播问题. 利用本征函数展开法得到了铁基氧化铝FGMs中Lamb波的相速度色散曲线,讨论了材料性质分布对铁基氧化铝FGMs中Lamb波传播特性的影响. 为FGMs性质(沿板厚方向变化)的反演提供了理论依据. 关键词： 功能梯度材料 Lamb波 Legendre多项式 分布函数     

非球面检测中最佳入射球面波和最佳参考球面波的确定

为了能够对非球面光学元件面型进行高精度的干涉检测,提出了一种确定最佳入射球面波和最佳参考球面波的新方法。该方法通过计算分析入射球面波与非球面反射波干涉条纹密度,确定最佳入射球面波的波源位置;通过计算分析在干涉图记录平面CCD上干涉条纹的密度,确定非球面检测时参考球面波波源的最佳位置。应用该理论与方法,不仅可明确非球面检测时CCD等光路元件选型的具体策略,而且可用于指导非球面检测调试过程,并能够通过对干涉图的深入分析,获得更多被测非球面的信息。     

Momentum transfer in crystal lattices with vibrating atoms

A momentum transfer equation previously used to describe non-elastic deformation in crystalline solids represented by point masses at fixed lattice positions is extended to take into account the existence of intrinsic (e.g. thermal) small amplitude vibrations of the masses about their mean positions in a lattice. Use of the time-dependent Schroedinger equation to describe momentum transfer and deformation is also discussed in terms of this vibrating point-mass lattice model. The result is that a modified and identical differential equation for momentum transfer is obtained from each approach; some solutions to this equation are presented. The previous particle momentum wave frequency dependence on wave vector and resulting applications to non-elastic deformation are unchanged, but these particle momentum waves can now be considered as modulating the usual high-frequency waves associated with the elastic modes of a crystalline solid.     

Wave motion of incompressible turbulent fluid

Evolution of small disturbances in a fully developed incompressible turbulent flow is considered on the base of the transport equation for the single-point probability density function (PDF) of velocity fluctuations. It is shown that at high frequencies this equation is similar to the Vlasov equation for charged plasma in a self-consistent electromagnetic field having longitudinal wave solutions for turbulent stresses similar to Langmuir waves. It is found that the longitudinal waves of turbulent stresses have a constant phase velocity and can be damped, neutral, or growing waves, depending on the type of undisturbed probability density function of velocity fluctuations. The obtained result differs from the previously published solutions to this problem using the statistical moments closures according to which the wave disturbances should be neutral or damped. The possibilities of experimental observation of longitudinal waves of turbulent stresses are analyzed.     

Ground-state properties of a one-dimensional system of hard rods

A quantum Monte Carlo simulation of a system of bosonic hard rods in one dimension is presented and discussed. The calculation is exact since the analytical form of the wave function is known and is in excellent agreement with predictions obtained from asymptotic expansions valid at large distances. The analysis of the static structure factor and the pair distribution function indicates that a solidlike and a gaslike phases exist at high and low densities, respectively. The one-body density matrix decays following a power law at large distances and produces a divergence in the low density momentum distribution at k=0 which can be identified as a quasicondensate.     

Wigner distribution of a transducer beam pattern within a multiple scattering formalism for heterogeneous solids

Diffuse ultrasonic backscatter measurements have been especially useful for extracting microstructural information and for detecting flaws in materials. Accurate interpretation of experimental data requires robust scattering models. Quantitative ultrasonic scattering models include components of transducer beam patterns as well as microstructural scattering information. Here, the Wigner distribution is used in conjunction with the stochastic wave equation to model this scattering problem. The Wigner distribution represents a distribution in space and time of spectral energy density as a function of wave vector and frequency. The scattered response is derived within the context of the Wigner distribution of the beam pattern of a Gaussian transducer. The source and receiver distributions are included in the analysis in a rigorous fashion. The resulting scattered response is then simplified in the single-scattering limit typical of many diffuse backscatter experiments. Such experiments, usually done using a modified pulse-echo technique, utilize the variance of the signals in space as the primary measure of microstructure. The derivation presented forms a rigorous foundation for the multiple scattering process associated with ultrasonic experiments in heterogeneous media. These results are anticipated to be relevant to ultrasonic nondestructive evaluation of polycrystalline and other heterogeneous solids.     

一维有限超晶格的电子态与透射问题的转移矩阵方法研究

采用转移矩阵方法,研究了一维有限超晶格的电子态与透射问题.计算了一维有限超晶格含单个缺陷层或少量缺陷层的透射谱和波函数,以及当电子被束缚在一维有限超晶格中电子的本征值和相应的定态本征函数.给出的方法对于研究电子通过任意排列的一维有限超晶格的输运具有普适性.     

多个子玻色-爱因斯坦凝聚气体膨胀叠加形成的量子涡旋现象研究

基于二维模型,研究了多个子玻色-爱因斯坦凝聚气体在谐振势阱内膨胀叠加形成的量子涡旋现象.运用传播子方法,分析了对称分布的三个子玻色-爱因斯坦凝聚气体膨胀叠加形成宏观量子涡旋的物理过程,得到量子涡旋随时间演化的规律;发现涡旋核分布在谐振势阱内出现振荡;涡旋与反涡旋随时间演化而相互转变,并对这些现象进行了物理分析.     

Matter Density and Relativistic Models of Wave Function Collapse

Mathematical models for the stochastic evolution of wave functions that combine the unitary evolution according to the Schrödinger equation and the collapse postulate of quantum theory are well understood for non-relativistic quantum mechanics. Recently, there has been progress in making these models relativistic. But even with a fully relativistic law for the wave function evolution, a problem with relativity remains: Different Lorentz frames may yield conflicting values for the matter density at a space-time point. We propose here a relativistic law for the matter density function. According to our proposal, the matter density function at a space-time point x is obtained from the wave function ψ on the past light cone of x by setting the i-th particle position in |ψ|² equal to x, integrating over the other particle positions, and averaging over i. We show that the predictions that follow from this proposal agree with all known experimental facts.     

共轭对称延拓傅里叶计算全息

提出一种计算全息算法,将物光波进行共轭对称延拓,经快速傅里叶变换直接生成实函数,其中包含物光波的幅度和相位信息,可编码得到灰度全息图.利用此全息图可清晰地再现原物光波.通过共轭延拓生成计算全息图的方法与以往基于干涉的计算全息不同之处在于,这里并不需要模拟物光和参考光的干涉,计算效率很高.利用生成的全息图在数字再现和光电再现实验中均获得了良好的效果.理论推导和实验结果都验证了算法的有效性. 关键词： 计算全息 共轭对称 数字再现 光电再现     

Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields

Few-electron systems confined in two-dimensional parabolic quantum dots at high magnetic fields are studied by the Hartree-Fock (HF) and exact diagonalization methods. A generalized multicenter Gaussian basis is proposed in the HF method. A comparison of the HF and exact results allows us to discuss the relevance of the symmetry of the charge density distribution for the accuracy of the HF method. It is shown that the energy estimates obtained with the broken-symmetry HF wave functions become exact in the infinite magnetic-field limit. In this limit the charge density of the broken-symmetry solution can be identified with the classical charge distribution.Received: 24 October 2003, Published online: 6 January 2004PACS: 73.20.Qt Electron solids - 73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems     

Dynamics of electron beams in plasmas

The characteristic features of the relaxation of the energy and momentum distribution functions of the electrons in a plasma produced by a low-voltage beam discharge in helium are investigated. It is established that, contrary to widely held opinion, the energy of an intense electron beam may relax due to the wave excitation. The critical currents corresponding to a jumplike transition from one relaxation mechanism to another are measured. The density of metastable helium atoms is determined from the comparative analysis of theoretical and experimental results on the structure of the energy spectrum of the electrons of an intense beam. An intense electron beam is found to become more isotropic in the course of its interaction with Langmuir waves in a collisionless plasma. The cross section for quasi-elastic collisions between the electrons and Langmuir plasmons is estimated. The wave nature of the beam-plasma mechanism for the relaxation of the anisotropic electron energy distribution function is demonstrated, and the mechanism itself is shown to come into play when the discharge current exceeds a certain critical level. The experimental threshold criterion for the energy relaxation of an intense monoenergetic beam is obtained for the first time. It is shown that the relaxation occurs in two stages: the isotropization stage, in which the beam energy decreases insignificantly, is followed by the stage in which the beam relaxes to a state with a plateau-like energy distribution function. The threshold criterion for the relaxation of the anisotropic electron energy distribution function is universal in character regardless of the cause of anisotropy.     

Density of states of a one-dimensional disordered photonic crystal

An analytic theory of the density of states in one-dimensional disordered photonic crystals is proposed. It is shown that the problem of the density of optical modes can be reduced in the small dielectric contrast approximation to solving a generalized Fokker-Planck equation for the distribution function of the logarithmic derivative of the electric field (the wave phase). The exact analytic solution and density-of-states asymptotics deep in the band gap of the photonic crystal and close to the band gap edge are derived. The results obtained agree well with the empirical relations derived earlier from numerical experiments.     

群墩上的随机波浪力

本文用建立传递函数的方法,通过入射波浪谱(方向谱)得出群墩上作用力谱及特征值。入射波浪频谱采用JONSWAP谱,方向分布函数为平方余弦函数。计算了2至4墩各种排列情况下群墩力系数。可供工程设计使用。对单墩单一方向频谱进行计算的实验资料的对比,符合良好。     

Elastic scattering of electrons by spherical nuclei

The differential effective scattering cross sections of electrons by spherical nuclei are obtained using a proposed density distribution. The analysis is based on the method of the distorted wave high energy approximation. From the comparison of the obtained results with the corresponding experimental data, the parameters of the density distributions of protons, neutrons and nucleons are determined, for which the best agreement is achieved.     

Photodetachment microscopy of

Photodetachment of ions is performed in the presence of a static electric field. A high spatial resolution electron detector is used to map out the ejected electron's wave function. Propagation over half a metre does not alter the stationary structure the electron wave possesses in the direction orthogonal to the detachment motion. Nodes and antinodes of electron current density are observed, that can be interpreted as direct imaging of the internal atomic wave function. Because detachment of a negative ion can be represented accurately in the free-electron approximation, the obtained images can also be viewed as direct experimental representations of the Green function of the uniform acceleration problem. Agreement of the measured distributions with the exact expression of this wave-function appears quite satisfactory. Received: 5 June 1998 / Accepted: 16 October 1998