bluuebunny/crossref_metadata_2025 huggingface dataset:
A source with ion extraction from the plasma volume
Authors: Makov B. N.
Published: July 2002

The physical basis of an effective new variant of ion source with extraction of ions from the plasma volume, operating in a longitudinal magnetic field developed at the Kurchatov Institute, is described. A thin layer with an electric field is produced in the plasma column of the source without violation of plasma quasineutrality, in order to transport all types of charged particles across the magnetic field toward the extraction slit. This allows high plasma flows to be transported toward the extraction slit with low potential difference ΔU across the layer, and the ion current density in the extracted ion beam can be increased by up to 10–15 times. The induced plasma flow also increases significantly the content of multicharged ions in the beam. A strong dependence of the ion beam current intensity on the potential across the layer provides a means for pulsed modulating or stabilizing the current intensity at the target by special programming or feedback in an inertia-free manner, without changes in the discharge conditions. The ion source operates with various species—gas or sputter materials. An ion current density of 1.5 A/cm2 was measured in our experiments at ΔU=10 V. The source can be used for accelerators, in semiconductor technology, and for other applications.
Modeling of incident particle energy distribution in plasma immersion ion implantation
Authors: Tian X. B. , Kwok D. T. K. , Chu Paul. K.
Published: July 2002

Plasma immersion ion implantation is an effective surface modification technique. Unlike conventional beam-line ion implantation, it features ion acceleration/implantation through a plasma sheath in a pulsed mode and non-line-of-sight operation. Consequently, the shape of the sample voltage pulse, especially the finite rise time due to capacitance effects of the hardware, has a large influence on the energy spectra of the incident ions. In this article, we present a simple and effective analytical model to predict and calculate the energy distribution of the incident ions. The validity of the model is corroborated experimentally. Our results indicate that the ion energy distribution is determined by the ratio of the total pulse duration to the sample voltage rise time but independent of the plasma composition, ion species, and implantation voltage, subsequently leading to the simple analytical expressions. The ion energy spectrum has basically two superimposed components, a high-energy one for the majority of the ions implanted during the plateau region of the voltage pulse as well as a low-energy one encompassing ions implanted during the finite rise time of the voltage pulses. The lowest-energy component is attributed to a small initial expanding sheath obeying the Child-Langmuir law. Our model can also deal with broadening of the energy spectra due to molecular ions such as N2+ or O2+, in which case each implanted atom only carries a fraction (in this case, half) of the total acceleration energy.
Spectral diffusion in polyethylene: Single-molecule studies performed between 30 mK and 1.8 K
Authors: Donley Elizabeth A. , Plakhotnik Taras
Published: July 2002

Linewidth distributions for single terrylene molecules in polyethylene have been measured in the temperature range from 30 mK to 1.83 K. The temperature dependence of the average linewidth is linear over the full temperature range. Linewidth distributions were simulated using the tunneling two-level system model and compared to the data in order to extract the lifetime-limited linewidth distributions and the distributions of coupling strengths between the probe molecules and the two-level systems. Differences of the average lifetime and coupling strengths were observed for samples produced with different sample-preparation techniques.
Ultrathin silicon nitride gate dielectrics prepared by catalytic chemical vapor deposition at low temperatures
Authors: Sato Hidekazu , Izumi Akira , Matsumura Hideki
Published: July 2002

The feasibility of using ultrathin silicon nitride (SiNx) films, prepared by catalytic chemical vapor deposition (Cat-CVD) method, as an ultrathin gate dielectric is reported. The effects of postdeposition treatments carried out using hydrogen (H2)-decomposed species or NH3-decomposed species formed by catalytic cracking of H2 and NH3 are also studied. A small hysteresis loop is seen in the C–V curve of as-deposited Cat-CVD SiNx films. The leakage current in the case of these films with equivalent oxide thickness (EOT) of 3 nm is slightly larger than that in the conventional thermal SiO2 films of similar EOT. However, it is found that the properties of Cat-CVD SiNx films are markedly improved by the postdeposition H2 or NH3 treatments, that is, the hysteresis loop disappears and the leakage current decreases by three orders of magnitude.
Ab initio study of the role of vibronic coupling in the ultraviolet valence/Rydberg spectrum of formaldehyde: Handling of vibronic interaction between three electronic states
Authors: Perić Miljenko , Grein Friedrich , Hachey Michel R. J.
Published: July 2002

Two ab initio approaches are developed to handle the vibronic coupling between three molecular electronic states. Both of them are based on representation of the vibronic Hamiltonian and wave functions by Fourier series in the internal coordinate in question. The first approach employs adiabatic potential surfaces, and the second one their diabatic counterparts. Transformation of adiabatic potential curves into their diabatic counterparts is carried out employing the relationship between corresponding electric transition moments. Both approaches are applied to calculate the vibronic structure of the formaldehyde spectrum in the energy region between 8 and 9.5 eV. The structure of this spectrum is determined by presence and mutual interaction along the C–O stretching coordinate of a valence-type state [1(π,π*)] and several species of Rydberg character.
A laser photodetachment technique for the measurement of H− in a high frequency traveling wave discharge
Authors: Dias F. M. , Tatarova E. , Crespo H. , Ferreira C. M.
Published: July 2002

A probe based laser photodetachment system has been developed to measure the H− density in a high frequency hydrogen discharge sustained by a traveling surface wave. Problems associated with application of this technique in high frequency discharges in general are pointed out. The influence of the time varying voltage across the plasma probe sheath on the probe measurement has been minimized by applying a passive method of compensation. An emissive probe, placed inside the launcher, has been used as a reference electrode in order to minimize the discharge disturbance and to reduce the additional resistance in the measuring system. A filtering technique has been incorporated in the detection system to separate the photodetachment signal from the signal at the discharge stimulating frequency. The big heating of the probe, set at electron saturation condition, has been avoided by using a convenient time sequence of the detection system operation. The photodetachment H− density measurements are confirmed by applying a high sensitivity second derivative probe technique as a second reference method. Axial variation of H− density, as measured, correlates with the axial distribution of hydrogen atoms. Experimentally obtained radial H− density profile confirms the well known fact that the negative ions are confined in the existing radial potential well.
Resolution improvements for hemispherical energy analyzers
Authors: Sagara T. , Boesten L. , Nishida S. , Okada K.
Published: July 2002

An electron tracing code in spherical coordinates was used to treat irregular, nonspherical boundaries in the “Herzog regions” at the exit and entrance planes of hemispherical energy analyzers. Some of the classical designs, i.e., the sudden cutoff “ideal field” approximation, real apertures in the 180° plane, the Herzog correction, the Jost correction, and shortened spheres were compared and checked for improvements. Ratios of gap to average radius from 0.2 to 0.55 are covered. Focusing, maximum trace width, base resolution, and the banana-shaped asymmetries from the circular reference path are investigated. Nearly ideal focusing and beam width can be restored in all devices by simply rotating the entrance beams through a small negative angle. Design equations are provided.
Novel techniques for the efficient reduction of data generated by charge-coupled device detectors
Authors: Welch S. J.
Published: July 2002

With the increasingly widespread employment of charge-coupled devices as detectors, there is much interest in methods of processing the data which they produce. Taking the reflection grating spectrometer instrument on the European Space Agency’s recently launched X-ray MultiMirror Mission spacecraft as an example, some novel techniques for removing unwanted data are explored. Entirely independent of compression techniques which may be employed elsewhere in the same system, deletion of unwanted data as close as possible to the source can be desirable to save subsequent processor time, or communications bandwidth—both quantities that are typically in short supply in the spacecraft case. This article sets out the general case for choosing to remove unwanted pixel data close to the source, considers the implications of this on the overall quantity of data, and then examines some methods for reducing the data volume.
Time response analysis of ZnSe-based Schottky barrier photodetectors
Authors: Monroy E. , Vigué F. , Calle F. , Izpura J. I. , Muñoz E. , Faurie J.-P.
Published: July 2002

We report on the characterization of ZnSe- and ZnMgBeSe-based Schottky barrier photodetectors grown on semi-insulating GaAs(001) by molecular-beam epitaxy. The spectral response of the devices shows a very sharp cutoff at variable wavelength, determined by the alloy composition, with a large stop-band rejection. Short-wavelength responsivities of 0.10 A/W and detectivities as high as 1.4×1012 cm Hz1/2 W−1 at −3.5 V bias have been achieved. Their time response behavior has been analyzed in detail. When light is switched off, the devices show photocurrent decays in the microsecond range, consisting of two exponential components with very different time constants. The slower component becomes dominant for high load and reverse bias. This behavior is related to the strong frequency dependence of the device capacitance.
Stokes shift in InGaN epitaxial layers
Authors: Zheng Ruisheng , Taguchi Tsunemasa
Published: July 2002

By analyzing the features of quasi-low-dimensional structures, we recognize that the Stokes shift is a characteristic of quantum-wire and quantum-disk systems. Including the smearing effect of the broad distribution of sizes of the nanostructures into consideration, we found that the Stokes shift is proportional to the light-emission peak energy of the systems. We propose that the Stokes shift observed in the optical spectra of InGaN epitaxial layers might originate from the self-formed quantum-wire and/or quantum-disk structures in the epitaxial layers.
Implantation of size-selected Si clusters into graphite
Authors: Neuendorf R. , Palmer R. E. , Smith R.
Published: July 2002

Molecular dynamics simulations have been performed to explore the implantation of silicon clusters into a graphite substrate to well-defined depths. The cluster sizes range from N=20 up to N=200 atoms per cluster, deposited with kinetic energies from E=500 eV up to E=5000 eV per cluster. We find that the clusters remain intact as coherent, amorphous structures after implantation. The implantation depth is well defined and scales with the kinetic energy of the clusters and the inverse of the cross-sectional area. This indicates a constant decelerating force, associated with the lateral displacement of carbon atoms as the cluster “drills a hole” in the substrate. The main dissipation channels for the energetic silicon clusters are the creation of phonons in the graphite substrate and the breaking of C–C bonds.
Thermostatic control of a sample tube in a nuclear magnetic resonance spin-echo spectrometer
Authors: Anisimov A. V. , Dautova N. R.
Published: July 2002

The equipment is based on the Peltier effect and is designed for thermostatic control of a sample for a nuclear magnetic resonance (NMR) spin-echo spectrometer with a pulsed magnetic field gradient for both relaxation and diffusion measurements within a temperature range −20 °C to +70 °C. A special thermostatically controlled nozzle on the NMR probe enables simultaneous thermostatic control of several samples at a given temperature and to transfer the samples in turn into the NMR probe measuring circuit without contact with the ambient atmosphere. The equipment consists of three independent units connected to the NMR probe and fits modern small-sized NMR spin-echo spectrometers, making NMR experiments significantly cheaper and more efficient.
One-dimensional crystal with a complex periodic potential
Authors: Boyd John K.
Published: July 2002

A one-dimensional crystal model is constructed with a complex periodic potential. A wave function solution for the crystal model is derived without relying on Bloch functions. The new wave function solution of this model is shown to correspond to the solution for the probability amplitude of a two-level system. The energy discriminant is evaluated using an analytic formula derived from the probability amplitude solution, and based on an expansion parameter related to the energy and potential amplitude. From the wave function energy discriminant the crystal band structure is derived and related to standard energy bands and gaps. It is also shown that several of the properties of the two-level system apply to the one-dimensional crystal model. The two-level system solution which evolves in time is shown to manifest as a spatial configuration of the one-dimensional crystal model. The sensitivity of the wave function probability density is interpreted in the context of the new solution. The spatial configuration of the wave function, and the appearance of a long wavelength in the wave function probability density is explained in terms of the properties of Bessel functions.
Portable fluorescence photometer for monitoring free calcium
Authors: Struckmeier Jens , Klopp Erk , Born Matthias , Hofmann Martin , Tenbosch Jochen , Jones David B.
Published: July 2002

We introduce a compact and portable photometric system for measurements of the calcium dynamics in cells. The photometer is designed for applications in centrifuges or in zero gravity environment and thus extremely compact and reliable. It operates with the calcium-sensitive dye Indo-1. The excitation wavelength of 345 nm is generated by frequency doubling of a laser diode. Two compact photomultiplier tubes detect the fluorescent emission. The electronics provide the sensitivity of photon counting combined with simultaneous measurement of the temperature, of air pressure, and of gravitational force. Internal data storage during the experiment is possible. A newly developed cell chamber stabilizes the cell temperature to 37.0±0.1 °C and includes a perfusion system to supply the cells with medium. The system has a modular setup providing the possibility of changing the light source and detectors for investigation of ions other than calcium. Measurements of the intracellular calcium concentration are based on a comprehensive calibration of our system. First experiments show that the calcium dynamics of osteosarcoma cells stimulated by parathyroid hormone is observable.
X-ray parabolic collimator with depth-graded multilayer mirror
Authors: Protopopov V. V. , Shishkov V. A. , Kalnov V. A.
Published: July 2002

A very reliable and easily adjustable x-ray parabolic collimator with uniform intensity distribution in its output cross section is designed using the technology of depth-graded multilayer mirrors. The W/C multilayer mirror structure was specially designed so as to compensate for the decrease in output intensity at lower grazing angles. The advantages of depth-graded multilayer mirrors with respect to laterally graded ones are inexpensive and versatile technology, the absence of any need of precise axial adjustment along the parabolic profile, and the possibility of creating any desired reflection curve shape. With a focal length of 1015 mm and a source projection of 0.1 mm, the total angular divergence of the output beam was measured to be 25 angular seconds at Cu Kα radiation, i.e., only 5 arcsec larger than the theoretical limit. The total output flux at both the Cu Kα1 and Cu Kα2 lines is a factor of 50 larger than that delivered by a conventional Si(111) monochromator.
Characteristics and design of metal vacuum arc plasma source power supply for pulsed-mode plasma immersion ion implantation
Authors: Wang L. P. , Gan K. Y. , Tian X. B. , Tang B. Y. , Chu P. K.
Published: July 2002

Metal vacuum arc plasma sources enhance the capability of plasma immersion ion implantation (PIII) by providing a convenient and efficient means by which to introduce metallic ions into the plasma for metallic ion implantation and/or thin film deposition. The power supply of a metal vacuum arc plasma source is usually based on the artificial transformation line design, but it has several drawbacks. For instance, the pulse width cannot be adjusted conveniently and the pulsing frequency cannot exceed a predefined value. These restrictions make process optimization and synchronization with the sample high voltage modulator complicated in pulsed-mode PIII operation. In this work, we experimentally investigate the voltage–current characteristics of our metal vacuum arc plasma source. Our results suggest two different power supply designs. By adopting the design incorporating a gradual voltage–current decline, we successfully construct a simple and reliable power supply that works in a stable manner for a prolonged period of time.
Ultrathin high-quality tantalum pentoxide films grown by photoinduced chemical vapor deposition
Authors: Zhang Jun-Ying , Boyd Ian W.
Published: July 2002

Ultrathin tantalum pentoxide films with thicknesses between 4 and 9 nm, equivalent to SiO2 thicknesses between 1 and 3 nm, as required for the next generation ultralarge-scale-integrated gate insulators, have been grown by the promising technique of ultraviolet-assisted injection liquid source chemical vapor deposition (CVD) at low temperature (350 °C). Optical band gap energies of 4.14±0.08 eV, fixed oxide charge content in the range of 1×1011–8×1010 cm−2, and leakage current densities as low as 5.0×10−8 A/cm2 at an electric field of 1 MV/cm were readily achievable in the as-deposited films. These properties are superior to those for layers grown by any other technique and annealed at significantly higher temperatures of 700–800 °C.
Aggregation kinetics in two dimensions: Real experiments and computer simulations
Authors: Vincze Attila , Agod Attila , Kertész János , Zrı́nyi Miklós , Hórvölgyi Zoltán
Published: July 2002

The aggregation of silanized glass spheres (75±5 μm diam) was studied experimentally at liquid–air (water–air, aqueous surfactant solution–air, and aqueous glycerol solution–air) interfaces from a kinetic point of view. The number, the size, and the polydispersity of clusters was investigated as a function of time. Particles having water contact angles of ≈30° (lower hydrophobic sample) and ≈82° (higher hydrophobic sample) were prepared and used in the aggregation experiments. In the early stage of aggregation the kinetics was found to be of the second order. The experiments revealed that the increasing particle hydrophobicity increased the rate constants in every case, which could be attributed to the increasing particle–particle attractions and the decreasing hydrodynamic resistance of particles (clusters) to motion. Moreover, the lower hydrophobicity of particles manifested itself in a more important polydispersity of clusters and an unexpected cross-over during the growth. The cluster–cluster aggregation was succeeded by a particle- (large) cluster aggregation after the first (initial) part of the growth. An off-lattice computer simulation of cluster-cluster aggregation, based on molecular dynamics, was also developed for the better understanding of the interfacial aggregation. The simulations supported well the conclusions derived from the real experiments, and gave an indispensable possibility for the study of the effect of single parameters on the complex phenomenon.
Nonlocal theory of drift type waves in a collisional dusty plasma
Authors: Chakraborty M. , Sen S. , Fukuyama A.
Published: July 2002

A nonlocal theory of low-frequency drift-type waves has been developed in a magnetized collisional dusty plasma in a sheared slab geometry. It is shown that the well-known stability of the drift waves in a sheared slab geometry does not hold in the presence of dust particles and a new unstable mode exists for typical laboratory plasma parameters.
Magnetoresistance of ferromagnetic single-electron transistors
Authors: Karlsson C. , Wang X. H.
Published: July 2002

We investigate the magnetoresistance of the single-electron transistor made of ferromagnetic leads and island. By lowering the temperature, the Coulomb blockade of the single electron tunneling is enhanced, which in turn influences the magnetoresistance of the device. The corresponding magnetoresistance ratio as a function of the temperature is studied by taking into account both the spin-dependent tunneling processes and the Coulomb blockade effect. Finally, we determine the parameter range, in which the ferromagnetic single-electron transistor can be used as a device with the magnetoresistance ratio enhanced by the Coulomb blockade effect.
Closed constraint algebras and path integrals for loop group actions
Authors: Alekseev Anton , Schomerus Volker , Strobl Thomas
Published: July 2002

In this paper we study systems with a closed algebra of second class constraints. We describe a construction of the reduced theory that resembles the conventional treatment of first class constraints. It suggests, in particular, to compute the symplectic form on the reduced space by a fiber integral of the symplectic form on the original space. This approach is then applied to a class of systems with loop group symmetry. The chiral anomaly of the loop group action spoils the first class character of the constraints but not their closure. Proceeding along the general lines described above, we obtain a 2-form from a fiber (path) integral. This form is not closed as a relict of the anomaly. Examples of such reduced spaces are provided by D-branes on group manifolds with WZW action.
Kinetics of condensation of gases: A new approach
Authors: Schmelzer Jürn W. P. , Schmelzer Jürn
Published: July 2002

A recently developed new approach for the determination of the work of critical cluster formation in nucleation is applied here to the description of the kinetics of condensation of gases. This method is a generalization of the classical Gibbs’ approach retaining its advantages and avoiding its shortcomings. For an illustration, the method is developed here for the case of condensation in a one-component van der Waals gas. The surface tension between liquid and gas is described by a modification of Macleods equation. However, any other relationships specifying the state of the system under consideration, which may be considered eventually as more appropriate, can be employed as well. For relatively small supersaturations, the classical Gibbs’ results (employing the capillarity approximation) are retained as a special case. However, similarly to the van der Waals–Cahn and Hilliard and more recent methods of density functional calculations in the determination of the work of critical cluster formation, for initial states, approaching the spinodal curve, the work of critical cluster formation is shown to tend to zero. In the intermediate range of supersaturations, it leads to smaller values of the work of critical cluster formation as predicted by the classical theory. The method can be extended also straightforwardly to the description of condensation in multicomponent gases as well as to bubble formation in liquids.
Surface relief gratings generated by pulsed holography: A simple way to polymer nanostructures without isomerizing side-chains
Authors: Baldus O. , Leopold A. , Hagen R. , Bieringer T. , Zilker S. J.
Published: July 2002

Photoaddressable polymers based on azobenzene side-chains have recently drawn a lot of industrial attention due to their possible application as data storage media. From the scientific point of view an additional effect, the occurrence of light induced surface relief gratings, is nowadays extensively studied. This article describes the behavior of side-chain and guest–host polymers (with and without azobenzene groups) in holographic experiments with pulse lasers. We observe surface relief gratings also for polymers, which only contain an absorbing, but nonisomerizing chromophore. Relief formation in pulsed holographic experiments is shown to arise from thermal effects, but not from trans–cis isomerization.
Effect of ions and radicals on formation of silicon nitride gate dielectric films using plasma chemical vapor deposition
Authors: Ohta Hiroyuki , Nagashima Atsushi , Hori Masaru , Goto Toshio
Published: July 2002

We have clarified effects of ions and radicals on the film property of ultrathin silicon nitride (SiNx) films of 5 nm in thickness formed on Si substrates at 300 °C in electron cyclotron resonance plasma-enhanced chemical vapor deposition (PECVD) employing ammonia and silane (NH3/SiH4), and nitrogen and silane (N2/SiH4) gases. In situ Fourier transform infrared reflection absorption spectroscopy and in situ x-ray photoelectron spectroscopy confirmed that in N2/SiH4 plasma, Si-N bonds in the film were increased by eliminating charged species, and thus, radicals promoted the formation of the SiNx film of high Si-N bond density. On the other hand, Si-N bonds in the film were decreased by eliminating charged species, and eventually, ions played an important role in forming the film of high Si-N bond density in NH3/SiH4 plasma. The excellent hysteresis of 0.04 V was successfully achieved with the NH3/SiH4 plasma. Furthermore, the quadrupole mass spectroscopy suggests that NH4+ charged species make a significant contribution to the formation of ultrathin SiNx films with high quality. These results provide insights into important species to be controlled in the PECVD for low temperature formation of the SiNx gate dielectric films in ultralarge scale integrated circuits.
Electron and ion stimulated desorption of positive ions from SF6 adsorbed on a Ta surface
Authors: Souda Ryutaro
Published: July 2002

The mechanism of secondary ion emission has been investigated by electron and He+ ion irradiation of the SF6 layer formed on a polycrystalline Ta substrate. Quite similar results are obtained between electron and He+ excitations; the fragment ions (SFx+, x=0–5) are emitted intensively from multilayers but they are absent when adsorbed directly on Ta due to delocalization of the valence hole(s). The F+ yield, though very small from the physisorbed SF6 layer, is increased by more than ten times after heating or irradiation with the electron or He+ beam, indicating that F+ arises from the chemisorbed F adatoms much more intensively than from the physisorbed SF6 molecule. This is because the F+ ion is not initiated by the valence holes state but arises from the screened F 2s core–hole state followed by the intra-atomic Auger decay after breakage of the chemisorptive bond, thereby avoiding efficient resonance neutralization on the metal surface.
Evidence for multiple chemical ordering in AlGaN grown by metalorganic chemical vapor deposition
Authors: Ruterana P. , De Saint Jores G. , Laügt M. , Omnes F. , Bellet-Amalric E.
Published: July 2002

In contrast to the GaN/InN system where there is a misfit of about 10%, the misfit between AlN and GaN is only 2.5%, and one would expect the growth of AlGaN to be more stable. In this work, it is shown that the growth of AlGaN can be complicated. Even in the low Al composition range, 10%–15%, many types of chemical ordering take place: AlN/GaN (1:1), Al0.25Ga0.75N(3:1) only recently noticed in InGaN, and we show evidence for an additional type of chemical ordering which corresponds to Al0.16Ga0.84N(10:2). The three types were found to coexist in the same layers, meaning that the growth may lead to nonhomogeneous composition in the AlGaN layer.
Diffusion of single quantum well Si1−xGex/Si layers under vacancy supersaturation
Authors: Griglione Michelle , Anderson Timothy J. , Law Mark E. , Jones Kevin S. , van den Bogaard Alex , Puga-Lambers Margarida
Published: July 2002

The interdiffusion of a Si/Si0.85Ge0.15/Si single quantum well heterostructure subjected to thermal annealing in a nitriding ambient was investigated as a function of anneal temperature and time. Nitridation of the silicon surface alters equilibrium vacancy point defect populations throughout the structure, which allows the determination of the point defect species important in interdiffusion. Diffusion coefficients of Ge after nitridation of ∼1×10−14 cm2/s for 1100 °C and ∼1×10−13 cm2/s for 1200 °C were extracted. The extent of diffusion in a nitriding ambient was much less than in an inert ambient, which indicates minimal vacancy contribution to interdiffusion. This is in contrast to results from previously published studies performed in an oxidizing ambient.
Photoluminescence of Yb3+-doped CuInS2 crystals in magnetic fields
Authors: Tsujii N. , Imanaka Y. , Takamasu T. , Kitazawa H. , Kido G.
Published: July 2002

Single crystals of Yb3+-doped CuInS2 were isolated by an In-flux technique. The Yb3+ concentration is estimated to be 4.5×1019 cm−3 in terms of the Curie–Weiss behavior of magnetic susceptibilities. The intra-4f-shell transitions of Yb3+ were resolved at low temperatures by photoluminescence peaks at 1.2–1.25 eV. The Yb-related peak intensities are weak in comparison with those arising from defect and impurity centers in the host. The magnetic field dependence of photoluminescence spectra shows interesting differences among the Yb-related peaks, with only part of them being Zeeman split. An energy level scheme on the basis of a single type of Yb3+ center in a tetragonal crystal field is proposed to explain the temperature- and magnetic-field dependence of the optical spectra.
Noisy chemical equilibria and flow reactor’s yield
Authors: Fuliński A.
Published: July 2002

Discussed is the role of the environmental fluctuations and/or extrinsic oscillatory fields in the chemical equilibria. It is well known that the simple relation between thermodynamic equilibrium constant, kinetic rate constants, and equilibrium concentrations of reagents holds for ideal systems only, and breaks down for nonideal ones. We show that when fluctuations and/or oscillating fields play an essential role in the chemical kinetics, this simple relation breaks down even for ideal systems. Uni- and bimolecular reactions with mass-action kinetics (ideal systems) are considered in detail, for time-dependent periodic (sinusoidal or square-wave) perturbations and random dichotomous ones. It is shown that such perturbations (of zero mean) of the kinetic reaction rate parameters kj, although they leave unchanged the thermodynamic equilibrium constant Keq, at the same time may change considerably the “kinetic” one Kkin defined as the ratio of mean (averaged over oscillating perturbations) equilibrium concentrations (raised to powers equal to their stoichiometric coefficients) of products and substrates. Equilibrium concentrations in a closed reactor, and the reaction yield (output concentrations) in a flow reactor are closely related to each other. Stationary-in-the-mean perturbed yield is being calculated and shown to be changed by time-dependent changes of either reaction rate coefficients or input and output of reagents. The values of kinetic parameters may be altered in some situations by changes of physical parameters such as light intensity or electrode potential. The input and output of the flow reactor is easily controlled. This creates the possibility of pushing the chemical reactions in a desired direction, which can be of importance both in theory and in applications, and also enables the experimental verification of our results, especially in the flow-reactor conditions.
Instability in Si(110) etched with tetramethyl ammonium hydroxide
Authors: Moktadir Zakaria , Sato Kazuo
Published: July 2002

Experimental data for the morphological evolution of etched Si(110) is presented. The observed results are interpreted using a continuum equation. The results reveal the presence of unstable etching which leads to the formation of elliptical hollows on the surface. The early stage of the formation of hollows can be explained by a linear theory similar to that which describes the early stage of spinodal decomposition. This instability is caused by anisotropic surface tension.
Auroral acceleration in astrophysical plasmas
Authors: Haerendel Gerhard
Published: July 2002

The essentials of the auroral acceleration process are summarized, and a simple set of relations is derived which enables the analysis of parallel potential drops and related parameters in relevant astrophysical systems. The main condition for the occurrence of such potential drops is the existence of intense field-aligned currents driven by forces in the outer, weak magnetic fields of an astrophysical object. In the strong fields near the object the currents may reach a critical limit where the mirror force or current-driven anomalous resistivity require the setup of parallel potential drops for the maintenance of current continuity. Analysis of three candidate systems, solar flares, cataclysmic variables and accreting neutron stars, reveals the possibility of fast and efficient production of highly relativistic electrons manifesting themselves by gamma ray emission.
Tuning the scattering length on the ground triplet state of Cs2
Authors: Kokoouline V. , Vala J. , Kosloff R.
Published: July 2002

Two schemes for tuning the scattering length on the ground triplet state of Cs2 are developed. The absolute value of the triplet scattering length of 133Cs2 is determined from the experimental data [A. Fioretti, D. Comparat, C. Drag, C. Amiot, O. Dulieu, F. Masnou-Seeuws, and P. Pillet, Eur. Phys. J. D 5, 389 (1999)], we demonstrate that the large scattering length can be made small and positive by coupling the 3Σu+(6S+6S) potential to the 3Πg state by strong off-resonant radiation. A weaker laser field coupling the 3Σu+(6S+6S) continuum to the lowest bound level of the excited 3Σg+(6S+6P) state also leads to a small positive scattering length. The scattering length of the 135Cs isotope is found to be positive. The method used solves the Schrödinger equation for two electronic states coupled by an electromagnetic field with no approximations employed. The scattering length is determined from the calculated continuum wave functions at low energies.
One-, two-, and three-dimensional modeling of the different phases of wire array Z-pinch evolution
Authors: Chittenden J. P. , Lebedev S. V. , Bland S. N. , Beg F. N. , Haines M. G.
Published: July 2002

A series of one-, two-, and three-dimensional (1-D, 2-D, and 3-D) resistive magnetohydrodynamic models are used to build up a composite model of the different phases of wire array Z-pinch implosions. 1-D(r) and 2-D(r,z) “cold-start” simulations of single wire experiments are used to illustrate some of the important processes in the plasma formation phase of wire arrays. Detailed comparison of the simulation results with data from single wire experiments provides an excellent method of code verification. 2-D simulations in the r–θ or x–y plane show how the combination of the core–corona structure of the wire plasmas and the magnetic field topology result in the formation of radial plasma streams and a precursor plasma on axis well before the implosion phase commences. The same 2-D(x–y) model is also used to illustrate how the implosion trajectories of nested wire arrays are controlled by the levels of momentum, energy, and magnetic flux which are transferred during their collision. Preliminary results showing the evolution of a single wire in the array in 3-D are presented. These results suggest that the dynamics and structure of imploding wire arrays at Imperial College can potentially be explained in terms of the current breaking through the wire cores rather than in terms of the Rayleigh–Taylor instability.
Mapping the optical intensity distribution in photonic crystals using a near-field scanning optical microscope
Authors: Campillo A. L. , Hsu J. W. P. , White C. A. , Rosenberg A.
Published: July 2002

A near-field optical microscope (NSOM) is used in collection mode to map the optical intensity distribution inside channel glass photonic crystal samples. The intensity distribution at wavelengths spanning the fundamental transmission minimum (i.e., the fundamental photonic band gap) was studied. The dependence of the NSOM signal on tip-sample separation allows us to confirm that the NSOM is probing the evanescent tail of the light confined to the crystal. At a given wavelength, NSOM images of low-index-contrast unetched samples, consisting of glass cylinders in a matrix of a different glass material, show bands of high and low optical intensity. The spacings of these bands are comparable to the cylinder separation, but display a nontrivial wavelength dependence. We show that these results can be explained by interference between incoming and reflected light inside the photonic crystal. Based on this, we can then determine dispersion relations of the photonic crystal modes directly from the NSOM images. Our experimental results are consistent with the calculated dispersion relations of the lowest order photonic crystal modes with no adjustable parameters. In addition we show an example where the NSOM images reflect the variation in coupling due to sample features rather than the guided mode structures. Based on these results, we discuss possible improvements in photonic crystal design for NSOM studies.
Optical properties of Bi12SiO20 single crystals doped with 4d and 5d transition elements
Authors: Marinova V. , Veleva M. , Petrova D. , Kourmoulis I. M. , Papazoglou D. G. , Apostolidis A. G. , Vanidhis E. D. , Deliolanis N. C.
Published: July 2002

Bi 12 SiO 20 single crystals doped with Ru, Rh, Pd, Re, Os, and Ir transition elements were obtained. Optical absorption and optical activity were measured in the visible spectral range. Strong changes were observed in transmission spectra when doping with various elements. Doping with Os and Pd causes strong bleaching to the crystals, changes the transmission curve shoulder, and decreases the absorption coefficient. Transmission spectra of Ru, Re, and Ir doped crystals are shifted to the red region and the absorption coefficient is considerably increased. Optical rotatory power remains unchanged by doping with Rh, Pd, Re, and Ir. It is increased negligibly by Pd doping, considerably increased by Os doping, and strongly decreased by Ru doping. The influence of doping elements on optical rotatory power is discussed on the basis of the two elementary structural unit cells (BiOn polyhedra and SiO4 tetrahedra) and of the transition energy of electrons located in their vicinity.
Ion dynamics and oscillation frequencies in a linear combined trap
Authors: Nakamura T. , Ohtani S. , Wada M. , Okada K. , Katayama I. , Schuessler H. A.
Published: July 2002

Ion traps have been pivotal in opening new frontiers for the precision spectroscopy of stable ions. We report on the demonstration of an additional ion trap: the linear combined trap. This device is particularly well suited for trapping ions with unstable nuclei, due to its large range of stability parameters that facilitates external injection online to an accelerator. The motion of an ion in such a linear combined trap is investigated theoretically and experimentally. In the trap ions oscillate harmonically in the radial direction and move nearly harmonically between fixed boundaries along the longitudinal axis. The presence of a homogeneous magnetic field and the applied dc and rf electric fields, lead to a set of coupled Mathieu equations. Their approximate solutions exhibit motional frequencies, which are combinations of shifted macromotion frequencies and the cyclotron frequency. The dependence of these motional frequencies on the applied trapping fields was studied in detail. For the measurements we used small clouds of laser cooled B9e+ ions and crystallized B9e+ ions. The observed oscillation frequencies are compared to both the results of zeroth order analytical solutions and to numerical simulations in which the frequency spectrum was obtained from the ion orbits by using the fast fourier transform formalism. The various motional resonances were experimentally recorded by applying a weak dipole excitation field to one of the trap electrodes, and by simultaneously observing at resonance the changes in the fluorescence intensity. Depending on the detuning of the cooling laser the ions gain energy during the motional excitation process in such a way that they are shifted further in or out of optical resonance by the Doppler effect. This leads to either positive or negative ion motion signals at the various ion oscillation frequencies.
Pulsed laser deposition of skutterudite thin films
Authors: Caylor J. C. , Stacy A. M. , Gronsky R. , Sands T.
Published: July 2002

Skutterudite (CoSb3 and IrSb3) thin films of high phase purity and crystallinity have been prepared by pulsed laser deposition. It is found that while the crystallinity of the films increases with increasing temperature, the phase purity decreases due to antimony volatility. The skutterudite phase can be stabilized to higher temperature by addition of excess antimony to the pulsed laser deposition target. Target-to-substrate distance must be optimized to obtain stoichiometric skutterudite films with low stress. A decrease in the flux of energetic particles and changes in particle stoichiometry with increasing distance caused by plume tilting results in lower film stress, but the diantimonide phase is nucleated if the distance is too large. It is possible to further reduce compressive stress by growing films in an inert background gas to decrease the flux of energetic particles. Overall, the highest quality films are grown from targets with excess antimony at a temperature of 270 °C with a target-to-substrate distance of 7 cm and a background argon gas pressure of 10 mTorr. The resistivity, mobility, and carrier concentration of these films are indicative of a degenerate p-type semiconductor with hole concentrations in the range of previously measured bulk values.
Simulation of electron diffraction patterns from III–V alloys with CuPt ordering: Effect of clusters and antiphase boundaries
Authors: Dobročka E. , Vávra I. , Wallenberg L. R.
Published: July 2002

A method for the simulation of electron diffraction patterns from partially ordered semiconductor compounds with CuPt-type ordering is proposed. The simulation procedure is based on the generation of two-dimensional model structures containing different types of imperfections. The defects are randomly dispersed in the structure and they are characterized by statistical parameters (density, average size, shape, etc.). Performing the intensity calculations for electron diffraction, the influence of the selected types of defects on the diffuse scattering in the transmission electron microscopy (TEM) diffraction patterns can be analyzed and the correlation between the statistical parameters describing the defects and the fine structure of the diffuse scattering can be established. By means of this method the influence of the plate-like clusters and of the antiphase boundaries (APBs) on the TEM diffraction pattern is analyzed. It is shown that the slope of the diffuse streaks around the ordering diffraction spots is determined by the mean orientation of the cluster boundaries irrespective of the type of clusters. The role of the fine structure of the inclined APBs in the formation of the diffraction patterns is established.
Cathodoluminescence study of InxGa1−xN quantum wells
Authors: Oh Eunsoon , Lee Myoung Hee , Kim Kwang Joo , Ryu M. Y. , Song J. H. , Park S. W. , Yu P. W. , Park H. , Park Y.
Published: July 2002

We studied the cathodoluminescence spectra of InxGa1−xN, focusing on the spatial variation of the spectra. Strong inhomogeneity of cathodoluminescence spectra was observed on 22 nm thick InxGa1−xN layers, where the peak energy varied up to 400 meV. In a double quantum well with a well width of 10 Å, the luminescence peak in a broad area spectrum was at 3.18 eV, but on some areas the peak was at 3.07 eV, the size of the area being about 1 μm. The variation of the cathodoluminescence spectra clearly indicates the presence of in-plane potential fluctuation in some InxGa1−xN samples, although the cathodoluminescence spectra of most of our quantum wells optimized for devices were found to be uniform at least within the spatial resolution limit.
Isomorphic surface acoustic waves on multilayer structures
Authors: Hunt William D.
Published: July 2002

There has been growing interest in recent years over the investigation of bulk acoustic waves (BAWs) which propagate along certain directions in anisotropic crystals with a minimum of diffraction. One application of these BAWs is for multichannel acousto-optic devices. The fact that the beams propagate with the minimum diffraction implies that the channels in such a device can be closely packed. Since surface acoustic waves (SAWs) are constrained to be within roughly one acoustic wavelength from the surface, the possibility exists to deposit thin films of isotropic or anisotropic material on the substrate and embue the aggregate multilayer structure with properties not present in the beginning substrate material. The characteristic investigated in this article is the velocity anisotropy which, as is known, predominates SAW diffraction. Specifically, we present a method whereby self-collimating SAWs can be generated on surfaces even though the substrate material itself does not exhibit this behavior. We discuss the particular case of a ZnO layer on (001)-cut 〈110〉-propagating GaAs for which a fair amount of slowness surface data exists. Finally, using angular spectrum of plane waves diffraction theory, we present data which substantiate the claim that self-collimating can more accurately be viewed as isomorphic because the SAW beam profile can propagate without changing its shape.
High-temperature goniometer for thin film growth and ion scattering studies
Authors: Lippmaa M. , Furumochi T. , Ohashi S. , Kawasaki M. , Koinuma H. , Satoh T. , Ishida T. , Nagasawa H.
Published: July 2002

We have developed a laser-heated six-axis goniometer for oxide thin film growth and ion scattering studies. The goniometer requires only a single CF152 flange for mounting in a vacuum chamber and includes three positioning and three rotational degrees of freedom. All translation and rotation axes are decoupled. A sample with a maximum size of 10 mm×10 mm can be heated to 1200 °C in a pure oxygen environment. The heating source is a 300 W continuous wave Nd:YAG laser. Light from the laser is brought to the sample stage with a combination of flexible and rigid optical fibers. The goniometer includes a motorized manipulator for two ablation targets, making it possible to grow thin films by pulsed laser ablation. Film growth and surface structure can be monitored by reflection high-energy electron diffraction and coaxial impact collision ion scattering spectroscopy. Samples can also be transferred from the goniometer to a room-temperature scanning tunneling microscope inside the vacuum chamber.