Effects of screening on the rate of energy loss in degenerate surface layers of compound semiconductors at low lattice temperatures

The energy loss rate of an electron in degenerate surface layers of a compound semiconductor for inelastic interaction with deformation and piezoelectric acoustic phonons is calculated with due account of the screening of the perturbing potential under the condition of low lattice temperature when the approximations of the well known traditional theory is not valid. The numerical results obtained for GaAs and CdS exhibits interesting features, significantly different from what follows if one either makes the traditional approximation of negligible phonon energy or disregards the screening of the scattering potential.     

Energy loss to intravalley acoustic modes in nano-dimensional wire structures at low temperatures

The theory of rate of loss of energy of non-equilibrium electrons due to inelastic interaction with the intravalley acoustic phonons in a nano-dimensional semiconductor wire has been developed under the condition of low lattice temperature, when the approximations of the well known traditional theory are not valid. Numerical results are obtained for narrow-channel GaAs-GaAlAs wires structures. On comparison with other available results it is revealed that the finite energy of the intravalley acoustic phonons and, the use of the full form of the phonon distribution without truncation to the equipartition law, produce significant changes in the energy loss characteristics at low temperatures.     

Fundamental mode microbending loss in graded-index and W fibres

The microbending loss in single-mode or quasi-single-mode fibres is approximately determined from a calculation of the fundamental mode field in a fibre with constant curvature. The result of the microbending loss essentially depends on the spot size of the fundamental mode, only. The approximations are verified by studying a statistical model and by comparing the approximations with exact calculations in the case of W fibres.     

Hybrid density-functional theory and the insulating gap of UO2

We report the first calculations carried out with a periodic boundary condition code capable of examining hybrid density-functional theory (DFT) for f-element solids. We apply it to the electronic structure of the traditional Mott insulator UO2, and find that it correctly yields an antiferromagnetic insulator as opposed to the ferromagnetic metal predicted by the local spin density and generalized gradient approximations. The gap, density of states, and optimum lattice constant are all in good agreement with experiment. We stress that this results from the functional and the variational principle alone. We compare our results with the more traditional approximations.     

Bend loss calculation in single-mode graded-index fibers using variational fields

Calculation of bend loss for single-mode graded-index fibers utilizes the fundamental modal field. Using some of the single and two parameter scalar variational approximations available in the literature for this mode, the bend loss has been computed. The exact results are obtained using a well known numerical method. The accuracies of the results obtained from the use of these variational fields have been compared.     

Polarization behaviour in lossy non-linear birefringent optical fibres

We examine the non-linear polarization properties of birefringent optical fibres when the initial polarization state is linear. In particular, we look at non-linear interactions in lossy fibres, where the loss may be in one or both of the orthogonal polarization axes. Exact and useful simple approximate solutions are presented for the case of zero loss, and various approximations to the field and polarization evolution are given for cases of non-zero loss.     

Piezoelectric scattering limited mobility characteristics of a degenerate surface layer in compound semiconductors at low lattice temperatures

The theory is developed for piezoelectric scattering rate of carriers in a degenerate surface layer under the condition of low temperature when the approximations of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics for the surface layers under similar condition of low temperature. The results for the surface layers in GaAs and ZnO show that when one takes into account either the degeneracy of the carrier ensemble or the finite energy of the phonons or both, the energy dependence of the scattering rates changes significantly from what follows for a non-degenerate ensemble or from the traditional theory, where one makes use of the high-temperature approximation and thus assumes equipartition law for the phonon distribution, and neglects the phonon energy in the energy balance equation of the electron–phonon system. It is observed that the zero-field mobility characteristics that follow from these scattering rates are interesting in that they are quite different from what turns out either for a non-degenerate ensemble or in the high-temperature approximations.     

多电荷碳离子的电荷损失截面

 研究了多电荷碳离子分别与中性氢原子或氦原子碰撞的电荷损失截面,同时计算了多电荷碳离子的电势函数和电子动量分布,其中电荷数为5的碳离子的截面数据与Shirai等人的计算结果符合得相当好。所导出的计算公式和编制的计算程序可以推广计算任何一个多电荷离子A^q+与H或He碰撞的电荷损失截面,因而具有一定的普适性。     

New mean-field theories for the liquid–vapour transition of charged hard spheres

The phase behaviour of the primitive model of electrolytes is studied in the framework of the various mean-field approximations obtained recently by means of methods pertaining to statistical field theory (J.-M. Caillol, J. statist. Phys., 115, 1461 (2004); extended version: arXiv:cond-mat/0305465). The role of the regularization of the Coulomb potential at short distances is discussed in detail and the link to more traditional approximations of the theory of liquids is discussed. The values computed for the critical temperatures, chemical potentials, and densities are compared with available Monte Carlo data and other theoretical predictions.     

A conservative level set method suitable for variable-order approximations and unstructured meshes

This paper presents a formulation for free-surface computations capable of handling complex phenomena, such as wave breaking, without excessive mass loss or smearing of the interface. The formulation is suitable for discretizations using finite elements of any topology and order, or other approaches such as isogeometric and finite volume methods. Furthermore, the approach builds on standard level set tools and can therefore be used to augment existing implementations of level set methods with discrete conservation properties. Implementations of the method are tested on several difficult two- and three-dimensional problems, including two incompressible air/water flow problems with available experimental results. Linear and quadratic approximations on unstructured tetrahedral and trilinear approximations on hexahedral meshes were tested. Global conservation and agreement with experiments as well as computations by other researchers are obtained.     

The behavior of higher order mode shape derivatives in damaged,beam-like structures

This work considers the effect that damage has upon the higher order derivatives of mode shapes of structures having primarily beam-like vibration. Via numerical investigations, the sensitivity of various damage related parameters in inducing changes in these higher order modal derivatives is determined, leading to a more complete understanding of what factors make the most contribution to significant changes in these derivatives. It is concluded that higher order mode shape derivatives (e.g., modal curvature, third derivative, and fourth derivative) are better indicators of damage than the mode shapes. Three distinct types of response for the damage-induced higher order derivative discontinuities are identified as three key parameters (the mass loss, stiffness loss, and damage radius scale) vary. From this, formal approximations are obtained for the expected forms of the higher order derivative discontinuities based upon the underlying behavior predicted by a simple relation among these three parameters. These approximations are checked with numerical simulations, and an excellent level of agreement is observed under appropriate conditions. Finally, the potential of these higher order derivative changes for indicating the presence and location of damage in a global setting is examined.     

Free of speckle ultrasound images of small tissue structures

By using the Born approximation deconvolved inverse scattering method instead of the traditional pulse-echo method for analyzing ultrasound pulse reflections from plastic phantoms and soft tissue specimens, improvement in image resolution is shown to be possible provided these targets are fair approximations to layered media. These images are free of speckle and are more vivid than the usual pulse-echo images.     

Collective properties of the final state in processes involving the production of hadron jets

Experimental results obtained by studying soft hadron jets in pion-proton and pion-nucleus reactions at intermediate energies with the aid of traditional collective variables are presented. Analytic approximations that describe, at a qualitative level, the dependences of collective parameters on the energy and multiplicity are proposed. Estimates obtained for strong coupling constant by studying collective variables are in reasonable agreement with its world-average value and with the results extracted by using different methods. The behavior of traditional collective variables as functions of the multiplicity in various interactions makes it possible to obtain a universal estimate for the lower boundary of the region of experimental manifestations of jets in multiparticle-production processes.     

Simulation of the range of keV-energy electrons in a dielectric target upon spatial charge accumulation

A method is proposed for calculating the relative range of electrons in the material with low conductivity, which surface is coated by a conductive film. The method takes into account the average retarding electric field of spatial charge created in the material. The calculations were carried out using the classical generalized Widdington’s law of slowing down within the model of single-collision loss. Due to the approximations used, the calculations have a qualitative character.     

Steady oscillatory states of a finite Josephson junction

Under the assumption that solutions have traveling-wave form, time-periodic solutions are found for the Josephson phase equation for a finite-length tunnel junction with uniform current feed and linear loss term. Exact current-voltage characteristics are found and compared with simple approximations. The complete current-velocity and mean-width-velocity curves for isolated fluxons are found. Comparison with characteristics for a finite junction shows that end effects obtained from analysis of a circuit model of the junction shows that end effects introduce lower- and upper-current thresholds.     

Energy loss straggling of fast charged particles

Energy loss straggling of fast charged particles colliding with atoms have been considered in the eikonal approximation. The result is represented in the form of the Fano formula with a nonperturbative correction. The known nonperturbative Titeica formula (which is transformed to the Fano formula when perturbation theory is applicable) is obtained only under certain approximations in eikonal calculations. It has been shown that straggling calculated with allowance for nonperturbative effects at large charges of the projectile can be significantly different (by an order of magnitude) from the results obtained by Titeica, Fano, and Bohr. Energy loss straggling of fast highly charged ions on hydrogen and copper atoms have been calculated. The latter results are compared to experimental data.     

Relativistic effects in electromagnetic meson-exchange currents for one-particle emission reactions

Following recent studies of non-relativistic reductions of the single-nucleon electromagnetic current operator, here we extend the treatment to include meson-exchange current operators. We focus on one-particle emission electronuclear reactions. In contrast to the traditional scheme where approximations are made for the transferred momentum, transferred energy and momenta of the initial-state struck nucleons, we treat the problem exactly for the transferred energy and momentum, thus obtaining new current operators which retain important aspects of relativity not taken into account in the traditional non-relativistic reductions. We calculate the matrix elements of our current operators between the Fermi sphere and a particle-hole state for several choices of kinematics. We present a comparison between our results using approximate current operators and those obtained using the fully relativistic operators, as well as with results obtained using the traditional non-relativistic current operators.     

A retrospective view on “3-dimensional radiation in absorbing, emitting and scattering media using discrete-ordinates approximation” by J.S. Truelove

Discrete-ordinates (DO) approximations to the radiative transfer equation in three-dimensional enclosures have extensively been used during the last three decades. The 1988 paper by Truelove [1] is one of the pioneering works in this field wherein traditional DO formulations were adapted to radiative transfer problems, and has impacted both the science and the technology related to large-scale combustion chambers since it was published. The following is a short introduction to this seminal JQSRT paper.