Polarization and extra-atomic relaxation on the surface and in the bulk of solids: influence of the molecular shape, polarizability and complex nature of the adsorbate

The influence of the molecular cavity shape and of the angular dependence of the dielectric constants in a low symmetry crystal on the polarization energy E_pol, induced by electron holes in the bulk and on the surface of the molecular solids, is investigated. The analytical expressions are obtained and evaluated for SF₆, SO₂, CS₂ and seven aromatic organic compounds. The theory is in good agreement with the experimental data. The deviation of the molecular cavity shape from a sphere has a marked influence on the calculated extra-atomic relaxation energy in large molecules. The E_pol value depends on the hole position. In SF₆ this value is larger for F atom photoionization than for S atom photoionization. The angular dependence of the dielectric constants has a smaller effect on the E_pol value than does that of the cavity shape. The E_pol value on the surface of a single crystal depends on the crystallographic plane. The polarization energy for CO molecules on the top of Xe layers on Al(111) is calculated in dependence on the number of Xe layers. The calculated extra-atomic relaxation energy is in good agreement with experiment.     

FORCE schemes on unstructured meshes I: Conservative hyperbolic systems

This paper is about the construction of numerical fluxes of the centred type for one-step schemes in conservative form for solving general systems of conservation laws in multiple space dimensions on structured and unstructured meshes. The work is a multi-dimensional extension of the one-dimensional FORCE flux and is closely related to the work of Nessyahu–Tadmor and Arminjon. The resulting basic flux is first-order accurate and monotone; it is then extended to arbitrary order of accuracy in space and time on unstructured meshes in the framework of finite volume and discontinuous Galerkin methods. The performance of the schemes is assessed on a suite of test problems for the multi-dimensional Euler and Magnetohydrodynamics equations on unstructured meshes.     

A few points on iron-based superconductors

We highlight a few points on iron-based superconductors in this paper. Experimental evidence for intrinsic inhomogeneity and phase separation in Fe-based superconductors is reviewed first. The non-BCS feature and other similarities between the Fe-based superconductors and the cuprates are also presented. The pressure effect on the FeSe-based superconductors observed recently is then discussed in terms of the interplay between superconducting transition temperature and structural characters. It is suggested that there is room for exploring new superconductors with higher transition temperature and that the improvement on the sample quality is necessary for further investigation on the superconducting mechanism.     

Conductance noise investigations with four arbitrarily shaped and placed electrodes

A general relation is derived for the spectral noise density of the voltage between two arbitrarily shaped and placed sensor electrodes on a conductor, when a constant current or voltage is applied to another pair of arbitrarily shaped and placed driver electrodes. The general relation is based on the sensitivity calculation in linear electrical networks. The relation is elaborated for conductivity fluctuations due to 1/f noise by using empirical 1/f noise relations. The influence of spot radii of the sensor electrodes on the noise is demonstrated. The theoretical results for 2 and 3-dimensional conductors are in agreement with our experimental results for carbon resistance sheets and on silicon and germanium in the resistivity range of 1Ωcm to 400Ωcm. The possibility of using the four-point probe for measuring the fluctuations in the resistivity has been demonstrated.     

基于超图的超网络相继故障分析

分析了快递超网络和电子元件超网络的相继故障扩散方式, 结合超图理论提出了2-section 图分析法和线图分析法, 并仿真分析了无标度超网络耦合映像格子的相继故障进程. 结果表明: 无标度超网络对外部攻击表现出了既鲁棒又脆弱的特性. 针对相继故障的不同扩散方式, 无标度超网络的相继故障行为表现出不同的特点. 超网络的相继故障行为和超网络的超度以及超边度分布有密切的联系, 也和超网络中超边的个数有关. 通过和同规模的Barabasi-Albert (BA)无标度网络对比, 在同一种攻击方式下同规模的无标度超网络都比BA 无标度网络表现出了更强的鲁棒性. 另外, 基于超边扩散的相继故障进程比基于节点扩散的相继故障进程更加缓慢.     

Shape effect of elongated grains on ultrasonic attenuation in polycrystalline materials

Longitudinal and transverse wave attenuation coefficients are obtained in a simple integral form for ultrasonic waves in cubic polycrystalline materials with elongated grains. Dependences of attenuation on frequency and grain shape are described in detail. The explicit analytical solutions for ellipsoidal grains in the Rayleigh and stochastic frequency limits are given for a wave propagating in an arbitrary direction relative to ellipsoid axes. The attenuation exhibits classic frequency dependence in those frequency limits. However, the dependence on the grain shape in the stochastic limits is unexpected: it is independent of the cross-section of the ellipsoidal grains and depends only on the grain dimension in the propagation direction. In the Rayleigh region attenuation is proportional to effective volume of the ellipsoidal grain and is independent of its shape. A complex behavior of attenuation on the grain shape/size and frequency is exhibited in the transition region. The results obtained reduce to the classic dependences of attenuation on parameters for polycrystals with equiaxed grains.     

The theory of SERS on semiconductor and dielectric substrates

It is demonstrated that the reason for SERS on dielectric and semiconductor substrates is enhancement of the electric field in the regions of the tops of surface roughness with a very small radius or a very large curvature. The enhancement depends on the dielectric constant of the substrate and is stronger for a larger dielectric constant. It is indicated that the enhancement on dielectrics and semiconductors is weaker than on metals with the same modulus of the dielectric constant. The result obtained is confirmed by experimental data on the enhancement coefficients obtained for various semiconductor and dielectric substrates.     

A new approach for the estimation of the axial velocity using ultrasound

The most used estimation method for calculating the blood velocity in commercial scanners is the autocorrelation approach. The calculation of the mean velocity used in this method depends on the center frequency of the interacting ultrasound pulse which downshifts as a function of depth, introducing a bias. A new velocity estimator for the mean axial velocity is presented. The estimation principle is based on the 2D Fourier transform and the Radon transform. The input data are a sequence of RF data forming a 2D data input, one column for each pulse emission. A 2D segment is selected for a specific depth. This data segment is first transformed by a 2D Fourier transform, and the result is then transformed by a Radon transform. The center of gravity for the angles of the lines intersecting the origin of the R-theta coordinate system in the Radon domain gives the mean axial velocity for the data segment. The benefit of this method is an estimate of the mean axial velocity which is independent of the center frequency of the propagating ultrasound pulse. The estimate will only depend on f(s) and f(pr f). Results of the estimation method is presented based on both simple generated RF harmonic data for different signal/noise ratios and simulated acoustic RF responses from a 3D measurement situation with an array transducer and a tube with plug flow. The new method shows improvement with a factor of 1.5-4 on the standard deviation on the estimated mean velocity for the simulated case.     

Cl+HD反应产物的极化与态分布

利用准经典轨线理论 ,在BW 2和G3两个势能面上 ,研究了Cl +HD反应的动力学 .计算结果表明 ,产物的转动取向对势能面及反应体系的质量因子非常敏感 .在BW 2势能面上 ,计算的两个产物的转动取向强于在G3势能面上计算的结果 ,而无论是在BW 2势能面上还是在G3势能面上 ,DCl产物的取向都强于HCl产物的取向 .计算结果还表明 ,在不同的势能面上反应物的转动激发对反应的影响有着显著的不同 .在BW 2势能面上 ,反应物的初始转动激发有利于Cl+HD反应的进行 ;而在G3势能面上 ,反应物的初始转动激发消弱了反应的反应性     

Interpolating statistics and the two-skyrmion system

The effect of the Chern-Simons term on the dynamics of CP¹ skyrmions in 2 + 1 dimensions is investigated. The skyrmions have an internal degree of freedom, corresponding to the twisting of a band, which is of importance for the spin and statistics properties. The dynamics is described in terms of a model of particles moving on a Kähler manifold and provides a convenient geometric setting for studying the braid group corresponding to particle exchanges. The internal degree of freedom is studied in detail for the two-skyrmion system and, in a simple case of well-separated skyrmions, the dynamics is disentangled and the dependence of the second virial coefficient on the interpolating statistics parameter is evaluated.     

多脉冲激光辐照下介质损伤理论研究

针对纳秒脉冲和飞秒脉冲不同的损伤机制,分别建立了两种多脉冲激光损伤模型。脉宽小于10 ps时,损伤是由于等离子体形成造成介质发生烧蚀所致,对此建立了基于电子密度演化方程的介质击穿模型;脉宽大于100 ps时,损伤是由于热沉积造成介质发生熔融所致,对此建立了基于傅里叶热传导方程的介质热损伤模型。通过计算两种模型下激光参数和材料参数对多脉冲损伤的影响,发现由于损伤机理不同,不同参数对单脉冲损伤阈值和多脉冲损伤阈值的影响趋势不完全一致,敏感程度也不同。通过计算得到了与实验结果一致的多脉冲损伤阈值与脉冲数间关系,使定量预估多脉冲损伤阈值和元件使用寿命成为可能。     

Comparison of algorithms for the symbolic computation of the NP spin coefficients and curvature components

Algorithms for the symbolic computation of the NP spin coefficients and curvature components for a given null coframe based on the structural equations of Cartan and the complex vectorial formalism of Debever are described. The efficiency of the algorithms is compared theoretically and also empirically in a number of test cases using implementations in the computer algebra system Maple. The test results confirm the theoretical superiority of the algorithm based on Debever's formalism over the one based directly on Cartan's first structural equations for the computation of the spin coefficients both with respect to execution time and storage requirements. The algorithm for the computation of the curvature components based on Debever's formalism is generally superior to the one based on Cartan's second structural equations but the advantage is not as marked as for the spin coefficients.     

Optical transitions on a type II semiconductor interface in the empirical tight-binding theory

A tight-binding theory is elaborated for multilayer semiconductor heterostructures of type II in which the states of electrons and holes are dimensionally quantized in adjacent layers and overlap in a narrow region near the interface. The major effort is focused on the calculation of linear photoluminescence polarization induced by the anisotropy of chemical bonds on the ideal interface under the radiation along the axis of growth. An expression for the matrix element of the optical transition on the type-II interface under arbitrary polarization of the emitted photon is obtained. The treatment is based on the sp 3 tight-binding model. The effect of the interface tight-binding parameters considered as free ones on the linear photoluminescence polarization is analyzed. The theory allows for the giant linear photoluminescence polarization discovered in the ZnSe/BeTe heterostructure; it also predicts that the polarization plane usually coincides with the plane containing the chemical bonds at the heterojunction.     

A model for the Auger electron spectroscopy of systems exhibiting layer growth,and its application to the deposition of silver on nickel

The dependence of Auger intensities on deposition time for deposits which grow in layer-by-layer fashion is analysed in terms of a simple model. It is shown that it is possible in principle to calibrate the Auger signals as a function of thickness absolutely with no prior assumption of values for sticking coefficients. The extent to which theory and experimental data for the deposition of silver on nickel are in agreement is examined, and values deduced for sticking coefficients and escape depths of Auger electrons in silver. It is also found that silver grows epitaxially on nickel at room temperature, and that silver/nickel bicrystals do not alloy after heat treatment.     

基于聚类NASVD的CE2-GRS月表放射性元素Th计数率分布

月表Th 元素分布特征对于分析月表岩石成因及化学特征等提供了重要依据。为了减小嫦娥二号伽玛谱(CE2-GRS) 噪声对获取Th 元素分布特征的影响,提出了一种基于噪声调整的奇异值分解(NASVD) 去噪算法。通过伽玛谱预处理、去噪、本底扣除及净峰面积求解等步骤,获得月表放射性元素Th计数率全月分布图。通过与国内外其他方法所获得的Th 元素计数率分布图对比有较高的一致性。与传统伽玛谱去噪算法比较,聚类NASVD算法能有效地去除统计涨落噪声影响,提取出嫦娥二号伽玛谱中的特征峰信息。The distribution of thorium on the lunar surface provides the important evidence for lunar evolution history. It is difficult to obtain the distribution of thorium on the lunar surface from CE2-GRS because of noise in the spectrum. The method for smoothing the spectrum is proposed which is based on cluster NASVD. The counting rate map of thorium on the lunar surface is achieved from CE2-GRS. Counting rate map of thorium gamma-rays shows a surface thorium distribution that is in general agreement with other measurement from LP-GRS and SLENE GRS which have better accuracy. It is more effectively to reduce the noise and get the weak information of characteristic peak used the method of cluster NASVD than other traditional methods for smoothing gamma-ray spectrum.     

Optical tweezers technique for the study of red blood cells deformation ability

The presented here technique is developed for in vitro estimation of the influence of external conditions on the deformation ability of human red blood cells. The method is based on the use of single-beam laser trap for capturing of erythrocytes fixed to the bottom of the liquid cell by the adhesion force. The approach is suitable for evaluation of the properties of individual cells as well as for real-time acquisition of the data on ensemble of erythrocytes large enough for subsequent statistical analysis. It is shown that both a change in the saline solution concentration and presence of certain stabilizing agents lead to measurable shift of the maximum of the cells distribution by their deformability.     

Resonant electron-positron pair photoproduction on a nucleus in a pulsed light field

The resonant photoproduction of an electron-positron pair on a nucleus in the field of a pulsed light wave is studied theoretically. The approximation where the electromagnetic pulse duration is much longer than the characteristic time of wave oscillations is considered. The interaction of the electron and positron with the Coulomb potential of the nucleus is considered in the Born approximation. An analytical expression for the resonant differential cross section is derived for the range of moderately strong external fields. This cross section contains a resonant peak whose height and width are determined by the external pulsed wave characteristics. The resonant cross section for pair photoproduction on a nucleus in a pulsed laser field can exceed the corresponding cross section for pair photoproduction on a nucleus in the absence of an external field by an order of magnitude.     

Compact fourth-order finite volume method for numerical solutions of Navier–Stokes equations on staggered grids

The development of a compact fourth-order finite volume method for solutions of the Navier–Stokes equations on staggered grids is presented. A special attention is given to the conservation laws on momentum control volumes. A higher-order divergence-free interpolation for convective velocities is developed which ensures a perfect conservation of mass and momentum on momentum control volumes. Three forms of the nonlinear correction for staggered grids are proposed and studied. The accuracy of each approximation is assessed comparatively in Fourier space. The importance of higher-order approximations of pressure is discussed and numerically demonstrated. Fourth-order accuracy of the complete scheme is illustrated by the doubly-periodic shear layer and the instability of plane-channel flow. The efficiency of the scheme is demonstrated by a grid dependency study of turbulent channel flows by means of direct numerical simulations. The proposed scheme is highly accurate and efficient. At the same level of accuracy, the fourth-order scheme can be ten times faster than the second-order counterpart. This gain in efficiency can be spent on a higher resolution for more accurate solutions at a lower cost.