Study of some surface properties using a simple gradient expansion of the kinetic energy density functional

Summary Reasonable surface energies and very accurate surface dipole barrier and work function for a semi-infinite jellium are calculated from a simple model of the density. The model was proposed by Smith and contains one universal parameter. In the Kohn-Sham energy functional, the kinetic energy is approximated by its simple gradient expansion including a fourth-order term. The exchange and correlation are treated within the local-density approximation. Using the simple gradient, the energy is minimized with respect to the universal parameter of the density. The parameter which minimizes the energy is used to calculate the surface dipole barrier and the work function. A comparison was made with the Lang and Kohn results and with the Ma and Sahni results.     

HT-7等离子体密度反馈控制系统的研究

介绍了HT－7等离子体密度反馈控制系统,分析了影响等离子体密度控制的因素,改进了密度反馈控制方式,使控制系统密度反馈更为完整、规范,实现了控制系统从单纯的反馈控制向多功能转化。给出了在HT－7装置上的实验结果,并提出了进一步改进的措施。     

Density of phonon states in amorphous germanium and silicon

The density of phonon states in amorphous germanium and silicon is calculated by statistically averaging the crystalline phonon density of states according to the radial distribution function. A simple rigid ion model is used to calculate the density of phonon states at various lattice spacings. The appropriate model parameters are obtained from the pressure dependent elastic constants and the Raman frequency. The calculated results compare favorably to experimental data obtained by infrared and Raman scattering and the results of other theoretical calculations.     

Diffusion modeling of an HF argon plasma discharge in a magneticfield

Using the basic moment equations (number density, flow density, and energy density) for singly ionized argon and for electrons, a simple diffusion equation (essentially one proposed by Lieberman and Lichtenberg, 1994) is adopted (by comparison with experiment), together with an integrated power balance equation. The effects of scaling magnetic field and radius are then discussed first for simple cylinders, for which there are known analytic diffusion solutions. For specific cases having the more complicated geometry of two abutting coaxial cylinders with considerably different radii, the numerical results are well approximated and delimited by considering only the wider cylinder for low magnetic field, and at high magnetic field, where end effects dominate, using the total length and the smallest diameter of the system     

Effect of correlations on electron momentum density in liquid metals

A scheme to calculate the electron momentum density in simple liquid metals, with the effect ofboth electron correlations and ionic potentials included, is given. This scheme is applied to the case of liquid aluminium. The results are substantially different from calculations considering only the ion potentials, and also from the results for a homogeneous electron gas of corresponding density.     

低温低密度等离子体密度及分布函数测量

一、引言 近来,尽管等离子体诊断技术有了很大发展,古老的朗缪尔探针由于结构简单,使用方便且有较好的时间空间分辨,至今仍作为基本诊断工具而广泛使用。但是,简单的朗缪尔探针理论只适用于中等密度(L》r》λD,L是平均自由程,r是探针半径,λD是德拜长度),对于低密度等离子体(r≤λD),朗缪尔探针理论复杂得失去了实用价值。本文将给出两个适合于低密度情况下的简单公式,并介绍一种实验中所用的静电能量分析器。     

A rigorous proof of the existence of band tails in disordered magnetic insulators

From the knowledge of the first moments of the density of states and, using a Lagrangian formalism, exact upper and lower bounds to the density of states of a simple hole in a magnetic insulator are calculated within the Hubbard model. These bounds provide a rigorous proof of the existence of band tails in the case of an antiferromagnetic spin arrangement in a simple cubic lattice. When the spin arrangement is random, the results suggest very strongly the existence of band tails.     

X射线激光在等离子体中传播过程的简化模拟研究

对于电子密度为线性分布的等离子体,在几何光学近似下,通过简化模型研究了X射线激光的传播和增益过程,并给出了计算X射线激光的偏转角、发散角和最大光强的简单方法。其结果与理论模拟和实验结果都符合得比较好。     

The coupling constant averaged exchange–correlation energy density

ABSTRACT

The exchange–correlation energy, central to density-functional theory, may be represented in terms of the coupling constant averaged (CCA) exchange–correlation energy density. We present an approach to calculate the CCA energy density using accurate ab initio methods and its application to simple atomic systems. This function provides a link between intrinsically non-local, many-body electronic structure methods and simple local and semi-local density-functional approximations (DFAs). The CCA energy density is resolved into separate exchange and correlation terms and the features of each compared with those of quantities commonly used to construct DFAs. In particular, the more complex structure of the correlation energy density is found to exhibit features that align well with those present in the Laplacian of the density, suggesting its role as a key variable to be used in the construction of improved semi-local correlation functionals. The accurate results presented in this work are also compared with those provided by the Laplacian-dependent Becke–Roussel model for the exchange energy.     

平面、薄膜及凹槽靶激光等离子体的参数分布特性与其靶结构关系的实验研究

本文报道最近在中国科学院上海精密光学机械研究所六路高功率激光装置上对平面、薄膜及凹槽靶激光等离子体的实验结果。诊断结果表明,激光等离子体各参数的空间分布与其靶结构有着十分密切的关系。讨论了各种靶激光等离子体的电子温度、密度及离化态分布的特点并与理论计算的结果进行了初步的比较。     

Density-dependent potential for multi-neutron halo nuclei

We apply a simple density-dependent potential model to the three-body calculation of the ground-state structure of drip-line nuclei with a weakly bound core. The hyperspherical harmonics method is used to solve the Faddeev equations. There are no undetermined potential parameters in this calculation. We find that for the halo nuclei with a weakly-bound core, the calculated properties of the ground-state structure are in better agreement with experimental data than the results calculated from the standard Woods-Saxon and Gauss type potentials. We also successfully reproduce the experimental cross sections by using the density calculated from this method. This may be explained by the fact that the simple Fermi or Gaussian function can not exactly describe the density distribution of the drip-line nuclei.     

Investigation of cosmic rays above the knee by means of muon bundles detected in a wide range of zenith angles

The results of investigation of muon bundles at the Earth’s surface in a wide range of zenith angles on the basis of the new phenomenological characteristic of extensive air showers—spectra of local muon density—have been investigated. Features of the local-density spectra are considered within a simple analytical model. The expected distributions of muon density in a wide range of zenith angles and primary energies, calculated on the basis of the CORSIKA code, are compared with the experimental data.     

Electronic properties of one-dimensional quasiperiodic lattices: Green's function renormalization group approach

Using the inflation-deflation symmetry, we have developed a new real-space decimation approach to study the electronic properties of one-dimensional quasiperiodic lattices. The key result is the construction of a compact renormalization group that allows simple calculation of the average Green's function and the average density of states to any degree. The Fibonacci and the generalized Fibonacci lattices are used to demonstrate the method. Numerical results for the average density of states of these lattices show a good agreement with the results obtained by other methods. This confirms the validity and the efficiency of the approach.     

The space-time-averaging procedure and modeling of the RF discharge

An approach to modeling RF discharges and the ensuing analysis of fast electron and ion motions for the case of electrode sheaths in the high-pressure RF discharge is discussed. Time-averaging over fast electron motions with the applied voltage frequency gives analytic expressions for the average electric field and average ionization density. The resulting relatively simple equations for the ion density profile describe drift, diffusion, ionization, and recombination processes. The simple scaling rules, the approximate expressions for the density profile in various regions, the sheath length, the ion density at the plasma-sheath boundary, and the dimensionless criteria for various discharge regimes can be deduced. For the non-self-sustained discharge, it is demonstrated that the ion drag towards the electrode and the diffusion results in significant lowering of the ion density in the sheath compared with the positive column at not too high a pressure. The analytic transition criterion from α to γ forms of the self-sustained discharge is obtained. The numerical solution of the averaged ion equations yields the results which nearly coincide with the results of full-scale modeling     

Employing oxygen-plasma posttreatment to improve the laser-induced damage threshold of ZrO2 films prepared by the electron-beam evaporation method

ZrO2 films are deposited by the electron-beam evaporation method. Parts of the prepared samples are post-treated with oxygen plasma at the environment temperature. The laser-induced damage threshold (LIDT) of the films increases from 15.9 to 23.1 J/cm2 after treatment with oxygen plasma. Compared with that of the as-grown samples, significant reduction of the average microdefect density and absorption are found after oxygen-plasma posttreatment. These results indicate that the oxygen-plasma posttreatment technique is an effective and simple method for reducing the microdefect density and absorption to improve the LIDT.     

A pocket model for the surface tension of compressed nuclei

The surface structure of compressed semi-infinite isoscalar nuclear matter is studied starting from a simple energy density functional and adding constraints equivalent to some external pressure. For a wide class of compression modes we derive analytical expressions for the surface tension σ as a function of the (de)compressed bulk density n_c. The σ(n₀) = 0 theorem is verified for all modes. The model is tested with numerical calculations using realistic Skyrme-ETF energy density functionals including spin-orbit and effective mass contributions. They confirm the simple analytical model results almost quantitatively. The mode which leads to an antiscaled density (increasing surface diffuseness with increasing bulk density) leads to minimum values for σ around n₀. The implications of the static model for the compression modes in finite nuclei are discussed.     

Pair effects in the impurity band density of states

Results of a simple two center approximation for impurity bands are presented. For low concentrations the density of states deviates strongly from the single site CPA and agrees well with numerical “exact” results.     

Cellular theory of ordered AB alloys

The heat and volume of formation of ordered simple metal AB alloys are calculated using the density functional formalism. A new method is proposed in which the alloy is partitioned into cells defined by surfaces on which the gradient of the electron density vanishes. The total alloy energy is formulated in terms of cellular and Madelung contributions and is minimized subject to the constraint of equal A and B cell boundary densities. Quantitative results for LiMg are presented.     

Eigenvalue density of linear stochastic dynamical systems: A random matrix approach

Eigenvalue problems play an important role in the dynamic analysis of engineering systems modeled using the theory of linear structural mechanics. When uncertainties are considered, the eigenvalue problem becomes a random eigenvalue problem. In this paper the density of the eigenvalues of a discretized continuous system with uncertainty is discussed by considering the model where the system matrices are the Wishart random matrices. An analytical expression involving the Stieltjes transform is derived for the density of the eigenvalues when the dimension of the corresponding random matrix becomes asymptotically large. The mean matrices and the dispersion parameters associated with the mass and stiffness matrices are necessary to obtain the density of the eigenvalues in the frameworks of the proposed approach. The applicability of a simple eigenvalue density function, known as the Marenko–Pastur (MP) density, is investigated. The analytical results are demonstrated by numerical examples involving a plate and the tail boom of a helicopter with uncertain properties. The new results are validated using an experiment on a vibrating plate with randomly attached spring–mass oscillators where 100 nominally identical samples are physically created and individually tested within a laboratory framework.     

Trap statistics and the gap density of states in amorphous materials

We present a one-electron theory of non-equilibrium trap occupancy in amorphous materials, with simple results for electrons in states above the equilibrium Fermi level. The density of these states can then be obtained by analysis of transient photoconductivity. Specific values are given for a-Si: H samples for energies in the range 0.55–0.40 eV below the conduction band mobility edge. We conclude that the state density there can be low, as suggested by the results of deep level transient spectroscopy.