Showing papers in "Physica Scripta in 1996"

The tanh method: I. Exact solutions of nonlinear evolution and wave equations

Abstract: A systemized version of the tanh method is used to solve particular evolution and wave equations. If one deals with conservative systems, one seeks travelling wave solutions in the form of a finite series in tanh. If present, boundary conditions are implemented in this expansion. The associated velocity can then be determined a priori, provided the solution vanishes at infinity. Hence, exact closed form solutions can be obtained easily in various cases.

The tanh method: II. Perturbation technique for conservative systems

Abstract: With the aid of the tanh method, nonlinear wave equations are solved in a perturbative way. First, the KdVBurgers equation is investigated in the limit of weak dispersion. As a result, a general shock wave profile, with a perturbative solitary-wave contribution superposed, emerges. For a particular choice of the parameters, a comparison with the exact solution is made. Further, the MKdVBurgers is investigated in the same limit and similar results are obtained.

Precision lifetime measurements on alkali atoms and on helium by beam-gas-laser spectroscopy

Abstract: The results of a new series of precision lifetime measurements on alkali atoms by means of beam–gas–laser spectroscopy are reported and discussed. The lifetimes of the first excited states in lithium, sodium, potassium and rubidium were measured with uncertainties ranging from ±0.14% to ±0.25%. The results are discussed in comparison with other recent experimental and theoretical work. The longstanding discrepancy between ab initio line strength calculations and the measurements of Gaupp et al. [Phys. Rev. A26, 3351 (1982)] for the resonance transitions in lithium and sodium is resolved by the new measurements in favour of the theoretical calculations. Finally the role of the beam divergence in lifetime measurements on fast particle beams is discussed and a method to correct for this effect is presented. The importance of the beam divergence effect is illustrated by additional recent lifetime measurements on helium and on lithium.

Generalized Lorenz equations for acoustic-gravity waves in the atmosphere

Abstract: It is shown that the low-frequency, short-wavelength acoustic-gravity perturbations in the atmosphere can be described by a system of four generalized Lorenz equations. These coupled equations reduce to the three usual Lorenz equations when the rotation of the earth is not considered.

Ion-acoustic solitons in a magnetized plasma with nonthermal electrons

Abstract: Obliquely propagating ion-acoustic solitons in a magnetized plasma consisting of warm adiabatic ions and nonthermal electrons have been investigated. The reductive perturbation method has been employed to derive the Korteweg-de Vries equation which admits a solitary wave solution for small amplitude limit. The highly nonlinear situation has also been studied by the numerical solution of the full nonlinear system of equations. The presence of nonthermal electrons changes the nature of ion-acoustic solitons. In the small amplitude limit the soliton may change from compressive to rarefactive, while the full nonlinear equations may allow rarefactive and compressive solitary waves to coexist. The effects of external magnetic field, obliqueness and ion temperature on the amplitude and width of the compressive and rarefactive solitons are discussed.

Absolute electron-scattering total cross section measurements for noble gas atoms and diatomic molecules

Abstract: Absolute electron-scattering total cross sections for noble gas atoms (He, Ne, Ar, Kr, and Xe) and H,, N, , CO, NO, and 0, molecules have been measured at impact energies between 0.5 and 250eV by the linear transmission method with the same experimental set-up for all the investigated targets. Generally, our total cross sections have been found to be in good agreement with other data with respect to shape. Some systematic discrepancies in magnitude still exist between results of different groups, especially at the lowest applied energies and in the energy range where the cross sections peak. At these energies, new measurements should be carried out before establishing a reference set of data.

Theory of nonlinear plasma surface waves

Abstract: Nonlinear wave phenomena in uniform or slightly nonuniform plasmas are now comparatively well known. In the present review paper we shall therefore, as a complement, focus our attention on nonlinear effects which are specific to bounded plasmas, and show that plasma surface solitary waves can appear in the vicinity of the interface between the plasma and the bounding medium.

The bias-dependence change of barrier height of Schottky diodes under forward bias by including the series resistance effect

Abstract: Schottky barrier height shifts depending on the interfacial layer as well as a change of the interface state charge with the forward bias while considering the presence of bulk (semiconductor) series resistance are discussed both theoretically and experimentally. It has been concluded that the barrier height shift or increase in Schottky diodes is mainly due to the potential change across the interfacial layer and the occupation of the interface states as a result of the applied forward voltage. One assumes that the barrier height is controlled by the density distribution of the interface states in equilibrium with the semiconductor and the applied voltage. In nonideal Schottky diodes, the values of the voltage drops across the interfacial layer, the depletion layer and the bulk resistance are given in terms of the bias dependent ideality factor, n, different from those in literature. These values are determined by a formula obtained for Vi and Vs by means of change of the interface charge with bias.

Estimation of inaccuracies in oscillator strength calculations

Abstract: In recent years, the quality and resolution of observational data, notably from the Hubble Space Telescope, has so improved that the accuracy of atomic data is frequently the limiting factor in progress. It is therefore imperative not merely that atomic data of improved accuracy are determined, but also that a realistic assessment of that accuracy is provided. Calculations of oscillator strengths are necessarily approximate. We discuss various indicators of accuracy, how semi-empirical corrections can be made to improve ab initio results, and how this data can be used to assess the level of accuracy of calculations for specific transitions. By way of illustration, these ideas are applied to set estimates of accuracy of a recent calculation of the lifetime of the 2s2p3 5S2o level of N II.

Nonlinear development of the weibel instability and magnetic field generation in collisionless plasmas

Abstract: The Weibel instability is expected to be one of the mechanisms at play in the generation of quasi-static magnetic fields by high-intensity laser pulses propagating with relativistic amplitudes in underdense plasmas. This instability can be excited in a plasma with two electron populations: the background cold electrons and the high energy electrons accelerated by the laser radiation. Here we investigate the nonlinear stage of the evolution of this instability both analytically and numerically. We consider both the non-relativistic and the relativistic regimes within the frame-work of a cold two-electron fluid plasma description, where ions are assumed to be at rest and to provide a uniform neutralizing background.

Nanofabricated Three Dimensional Photonic Crystals Operating at Optical Wavelengths

Abstract: We describe the nanofabrication of the first three-dimensional (3-d) photonic crystals with a forbidden photonic bandgap lying in the near infrared region of the electromagnetic spectrum, 1.1 μm < λ < 1.5 μm, just beyond the electronic band-edge of GaAs. We fabricated these structures by chemically assisted ion beam etching through a triangular hole array mask, defined by electron beam lithography on GaAs. The 3-d forbidden photonic bandgap was spectrally tuned by 2-d lithographic control of the 3-d spatial periodicity. Optical transmission spectra were generally in good agreement with microwave frequency transmission on centimeter scale models. Nevertheless, we find that the mid-gap optical reflectivity is surprisingly sensitive to structural errors in the photonic crystal, degrading the optical rejection from an expected 95% to an observed 80%. We suggest that mid-gap attenuation in the most vulnerable k-space directions, rather than overall thickness, is the relevant Figure-of-Merit for nano-scale photonic crystals.

Possibilities for exciton condensation in semiconductor quantum-well structures

Abstract: Recent progress in understanding the behavior of condensed excitonic systems is reviewed. The Bose-Einstein condensed excitonic insulator is studied numerically within the mean-field pairing approximation. In particular, we examine in some detail a system of spatially separated, two-dimensional (2D) electrons and holes, as a candidate system for study in semiconductor heterostructures. Based on these calculations, we suggest that it may be possible to create a droplet of high density exciton condensate fluid in semiconductor quantum-well structures. In heterostructures, the recombination of electrons and holes is dipole-allowed, which presents both difficulties and opportunities. The excitonic insulator state should be superradiant, which provides a direct measure of the coherence of the ground state. Additionally there is a gap between the absorption and emission spectrum, and collective modes lying within this quasiparticle gap. Coupling of the condensate to radiative modes of an optical cavity may offer an opportunity to manipulate the condensate and its optical response.

Radiation damage of graphite - degradation of material parameters and defect structures

Abstract: Based on our previous works, we propose a new model for damage structure of graphite based on sp2 to sp3 transition of carbon atoms that can explain the change of the materials' parameters observed both in damaging and annealing processes. In early stage of the irradiation, defects are produced in a basal plane and/or in-between the basal planes (in-plane or two dimensional defects) accompanying sp2 to sp3 transition of some carbon atoms and losing their graphitic bonding. They play a critically important role on the reduction of thermal conductivity, increase of electrical resistivity, increase of hydrogen retention and so on. They also cause a dimensional change through the increase of lattice spacing between the basal planes. Such in-plane defects are rather easily annealed out and the materials' parameters for lightly irradiated graphite recover their initial values by annealing far below the graphitization temperature. However, there is no way to avoid their production under neutron irradiation, particularly at low temperatures. After high dose irradiation, the defects grow into three dimensional structures probably including the formation of sp3 clusters. Accordingly, the basal planes lose their ordering resulting in turbulence and bending, and remaining open spaces. They play an important role on the volume expansion, which cannot be caused totally by the lattice expansion. Once graphite loses its 2 dimensional structure, the recovery to the initial structure is very difficult.

Inner-shell excitation energy and autoionization rates for Li-, Be-, B-like ions with Z = 6–54

Abstract: Absolute energies of the autoionization states 1s2s22pn [LSJ] with n = 0–2, 1s2s2pn [LSJ] with n = 1–3 and 1s2pn [LSJ] with n = 2–4 and non-autoionizing states 1s22s22pn [LSJ] with n = 0–1, 1s22s2pn [LSJ] with n = 0–2 and 1s22pn [LSJ] with n = 1–3 were calculated for ions with nuclear charge Z = 6–54 by using the 1/Z perturbation theory method. Relativistic corrections were taken into account in the frame of the Breit operators. The autoionization rates for the above-mentioned autoionization states for ions with Z = 6–54 were computed as well. These data are very important for modelling the satellite spectra produced by high-temperature plasma with thermal and non-thermal electrons.

The possible physical mechanism of initiation and growth of lightning

Abstract: According to insight on the liquid drop instability as for the surface electric discharge it is shown that the initiation of lightning discharge can be related to instability of negatively charged water drops in an external electric field. In the total intracloud field (quasiconstant and chaotically varied in time) the large charged drops and the watered ice crystals (R > 30 μm) can be unstable and start to emit highly-dispersed (r ≤ 0.01R) heavily charged drops, unstable by the Rayleigh criterium of stability. These drops break down in a period of time of the order of tens of intrinsic oscillation periods ~10−6 s, emitting ~100 more smaller drops. The field strength of intrinsic charges near the drop surface is sufficiently large for effective autoelectron emission (108–109 V/cm). Due to the total electric field near the surface of the droplets the emitted electrons initiate electron avalanches whose characteristic linear sizes are determined by the dimensions of the space region where the coefficient of electron multiplication exceeds unity. The lifetime of one avalanche is ~10−7 s. The same is the time interval between the consistent emission of two electrons by a drop. A transit of the series of consistent avalanches following the same track leads to the development of a plasma frame in the avalanche track. The overlap of the frames of adjacent avalanches in the vicinity of a group of closely located drops gives rise to formation of a plasma region (PR). The PR polarization in an intracloud quasiconstant electric field E0 and its growth along E0 (which is controlled by electric charges arriving into the PR from the surrounding space and originates from the charged drops instability as well as development of the ionization-avalanche processes near the PR tops) enables one to explain the phenomenon of discharge channel branching and propose a physically clear mechanism of collection of negative electric charges from separate drops carried out by lightning. A stepped leader is treated as a result of the ion-sound waves drive in the discharge channel plasma.

Investigations of the interactions of highest charge state ions with surfaces

Abstract: Experimental results from a series of investigations, the aim of which is to provide an exploratory overview of the wide range of phenomenon associated with the interaction of slow very highly charged ions with solids and solid surfaces, are reported. Incident charge state dependencies of secondary electron, x-ray and secondary ion emission are measured. In addition, microscopic studies are performed using an Atomic Force Microscope to investigate highly charged ion induced surface defects. The ions, used for these studies, range from O7+ to Th80+ and U82+ with low incident kinetic energies (~2 eV/amu to 3 keV/amu). As an overall result, enhancements for the low energy secondary electron, sputter ion and photon emission are observed with increasing charge states. X-ray emission spectra are found to be consistent with the formation of so called hollow atoms above and at the surface as part of the neutralization processes. Small angle scattering behaviour, under specular reflection conditions, indicate very fast decay processes for the hollow atom, outside the solid surface. The microscopic studies reveal interesting nano-size surface defects caused by single highly charged ion incidence.

The Alfvén resonance in a magnetized dusty plasma

Abstract: Wave propagation in a dusty magnetized plasma at frequencies below and of the order of the ion-cyclotron frequency, and with a non-zero electron temperature is considered The general dispersion relation for waves propagating at an arbitrary angle with respect to the external magnetic field and for finite ion-cyclotron frequency is derived and discussed Modification of the Alfven resonance absorption mechanism due to the presence of a finite proportion of the negative charge of the plasma on stationary dust grains is investigated

Experimental results on flow separation and transitions in the circular hydraulic jump

Abstract: We describe experimental results on the circular hydraulic jump in a viscous fluid (ethylene-glycol mixed with water) in a setup, where the depth of the fluid far away from the jet is controlled On increasing the depth, we find a transition from a state (type 1) with separation occurring only on the bottom to a state (type 2) with a "roller", ie separation occurring also on the free surface, as in a broken wave The system is laminar both before and after the transition and can thus be very accurately controlled We present data for the height profile of the fluid layer, for the surface velocity and for the location of the separation points Further, we show direct numerical simulations of the Navier-Stokes equations confirming our interpretation of the transition

Processes of the nuclear isomer 229mTh(3/2+, 3.5 ± 1.0 eV) resonant excitation by optical photons

Abstract: The excitation processes of the low energy isomeric level in nucleus 229Th by optical photons are investigated for different values of relations between energies of nuclear transition and photons. The role of the atomic shell, which "works" as electron bridge, is considered. The cross-sections of the resonance nuclear excitation by laser beam (the narrow line) and by special lamp (the wide line) are calculated. The possibilities of experimental researches are discussed.

Construction of energy matrix for complex atoms in space of (nd + n's)N + 2 + Σi,j ndN + 2 − wi − wjniliwinjljwj (where wi + wj ≤ 2) configurations

Abstract: A method of parametrization of interactions in the fine-structure of 3d-atoms is extended to the model space of configurations (nd + n's)N + 2 + Σi,j ndN + 2 − wi − wjniliwinjljwj. It accounts for the first- as well as the second-order effects of the perturbation theory. The close analogy to the parametrization of the hyperfine structure interactions is shortly discussed, this can serve as a sensitive test for the quality of fine structure analysis.

Semiconductor device physics with conjugated polymers

Abstract: Processible conjugated polymers can be used to fabricate a range of thinfilm diodes which can be designed to show good characteristics both as electroluminescent diodes and also as photoconductive diodes. We consider here the present understanding of the operation of a range of semiconductor devices. Field-effect transistors are readily fabricated, but are limited by low carrier mobilities. However, these can be improved by ordering the polymer films. For light-emitting diodes we highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus bring about electron-hole capture to generate excitons at this interface. Photon absorption produces excitons which are considered to be bound at room temperature. Charge generation requires ionisation of these excitons, which can be achieved efficiently at heterojunctions between layers with different electronegativities, and we discuss how this can be achieved at the interfaces between interpenetrating networks of electron- and hole-accepting polymers.

Inelastic processes in 1-1000 keV/u collisions of Beq+ (q = 2–4) ions with atomic and molecular hydrogen

Abstract: Owing to the use of beryllium in fusion reactors and the consequent need to model and diagnose plasmas containing this species of impurity, cross sections are presented here for inelastic collisions of 1-1000 keV/u Beq+ (q = 2-4) with H and H2. In particular, the classical trajectory Monte Carlo technique is used to compute total cross sections for (i) ionization, (ii) state-selective excitation, and (iii) state-selective charge transfer.

Isoelectronic comparison of the Al-like 3s23p 2P–3s3p2 4P transitions in the ions P III-Mo XXX

Abstract: New observations of the 3s23p 2P–3s3p2 4P intercombination transitions in Al-like ions have been made for Cl V from spark spectra recorded at Lund and for Kr XXIV and Mo XXX from spectra obtained at the JET tokamak. The new results have been combined with other identifications of these transitions along the sequence and empirically systematized and compared with theoretical calculations. A set of smoothed and interpolated values for the excitation energies of the 3s3p2 4P levels in P III-Mo XXX is presented.

Standard and nonstandard Turing patterns and waves in the CIMA reaction

Abstract: We describe experimental evidence of stable triangular and hexagon-band mixed mode nonstandard patterns, in a three-dimensional chemical reaction-diffusion system with steep gradients of chemical constraints. These gradients confine the structures in a more or less thick stratum of the system. At onset, patterns develop in monolayers which approximate two-dimensional systems; but beyond onset, three-dimensional aspects have to be considered. We show that the nonstandard pattern symmetries result from the coupling of standard hexagonal and striped pattern modes which develop at adjacent positions, due to the differences in parameter values along the direction of the gradients. We evidence a Turing-Hopf codimension-2 point and show that some mixed mode chaotic dynamics, reminiscent of spatio-temporal intermittency combining the Turing and the Hopf modes, are also a consequence of the three-dimensional aspect of the structure. The relations between these observations and the theoretical studies performed in genuine two-dimensional systems are still open to discussion.

Pattern formation in drying water films

Abstract: A thick water film is deposited on a mica substrate, which it wets, from pure water vapour at a low temperature. The vapour pressure is reduced so that the water evaporates. Despite the fact that under equilibrium conditions the film wets the substrate, when it reaches a thickness of about 500 A, dry patches nucleate in the film and the water recedes to the wet parts. During the recession, a thick rim of water is created, which becomes hydrodynamically unstable and produces a beautiful reproducible pattern of water drops. Experiments have been done showing both homogeneous and heterogeneous nucleation. The basic physics of the de-wetting, the nucleation and the origin of the pattern formation are discussed. The effect is not restricted to water on mica, but has been observed with other fluids on wetted substrates.

On the occurrence of decoherence in nonrelativistic quantum mechanics

Abstract: We investigate the conditions which can lead to the occurrence of "decoherence" (i.e., of "environment-induced superselection"), in a quantum system interacting with another, infinite quantum system (an apparatus, environment, etc.), in the context of the nonrelativistic quantum mechanics. We are particularly concerned with: a kind of interaction, its relative strength, initial state of the apparatus (environment), and compatibility of the self-Hamiltonian and the interaction Hamiltonian of the composite system, "system plus apparatus", including time dependent interactions. As necessary conditions concerning occurrence of decoherence appear the two requirements: (i) That the interaction Hamiltonian is of the separable kind, and (ii) That the complete Hamiltonian, Ĥu satisfies commutator relation [Ĥu(t), Ĥu(t')] = 0 (t and t' arbitrary instants). Since non-separable interaction cannot produce the same, we conclude that occurrence of decoherence is neither a rule (as it is sometimes assumed in the context of the "environment theory"), nor an exceptional consequence of the dynamics of a system interacting with its environment (as it is sometimes claimed in the criticism of the environment theory). Exhaustiveness and generality of our analysis extend applicability of related results (which are independent on a particular model of (infinite) environment), at least as a qualitative test and/or quantitative estimation of the results of the calculations obtained by the use of the different methods (semiclassical methods, path-integration technique, etc.) in quantum many-particle physics.

The use of complete sets of orthogonal operators in spectroscopic studies

Abstract: Complete sets of orthogonal operators are used to calculate eigenvalues and eigenvector compositions in complex spectra. The latter are used to transform the LS-transition matrix into realistic intermediate coupling transition probabilities. Calculated transition probabilities for some close lying levels in Ni V and Fe III illustrate the power of the complete orthogonal operator approach.

The history and future of semiconductor heterostructures from the point of view of a Russian scientist

Abstract: The early history of semiconductor heterostructures and their applications in different electronic devices is described. The article also contains a short historical review of the physics, technology of preparation and applications of quantum wells and superlattices. Recent progress in quantum wires and especially quantum dots structures and future trends and perspectives of these new types of heterostructures are discussed.

Static exchange and quantum defect analysis of x-ray absorption spectra of carbonyl compounds

Abstract: We have performed quantum chemical calculations of carbon and oxygen x-ray absorption spectra for a series of carbonyl containing molecules. Systematic variations in energy positions and intensities of the spectra have been investigated. Based on the good reproduction of experimental spectra, quantum defect and population analyses were utilized for detailed assignments. In some cases new assignments are proposed.

Electron beam effects on the spectroscopy of satellite lines in Aluminum X-pinch experiments

Abstract: Aluminum wire X-pinch experiments performed at the Cornell University XP pulsed power generator show detailed high resolution spectra for satellite lines of Li-like, Be-like, B-like and C-like ions. These lines, which correspond to transitions originating from autoionizing levels, are observed in the direction of the anode with respect to the hot X-pinch cross point. The intensities of such satellites are much smaller or absent in the direction of the cathode. These transitions are caused by collisions of ions with energetic electrons (5-15 keV) which are created by inductance between the hot spot and the anode. A collisional-radiative model was constructed using a non-Maxwellian electron energy distribution consisting of a thermal Maxwellian part plus a Gaussian part to represent the high energy electron beam. The shapes of the observed satellite structures are consistent with the calculated spectrum for electron temperatures between 30-100 eV, and beam densities of about 10−7 times the plasma electron density.