bluuebunny/crossref_metadata_2025 huggingface dataset:
Long-time stability of multimacroion domains in polyelectrolyte solutions
Authors: Sedlák Marián
Published: July 2002

Long-time stability of multimacroion domains in polyelectrolyte solutions was investigated over time intervals ranging up to 37 months. Periodic static and dynamic light scattering measurements were performed on salt-free solutions of sodium poly(styrenesulfonate) and ionized samples of poly(acrylic acid) and poly(methacrylic acid). Extremely slow kinetic changes were observed. While the size of domains did not change over time, a slow decrease of the domain density (the difference between polyion concentrations inside and outside the domain) was observed. This decrease was reflected in the decrease of domain scattering, the increase of scattering by “free” polyions due to their release from domains and in the increase of domain diffusion coefficient due to increased drainability. The shape of time dependencies was always such that a stronger dependence was observed in the beginning with leveling off at longer times. Quantitatively, however, relatively large differences were observed from sample to sample. This was interpreted as due to different initial conditions given by different arrangement of polyions in domains according to the bulk structure and solution preparation. Results show that domains cannot be characterized as equilibrium dynamic fluctuations, but rather as long-lived associates of macroions. Performed scattering experiments on domains enabled for the first time to discriminate between tightly packed structures and loose structures with solvent inside. The infinite (or at least extremely long) life-times of loose domains suggest thermodynamic attraction between polyions.
The slowest soap-film tunnel in the Southwest
Authors: Georgiev D. , Vorobieff P.
Published: July 2002

Gravity-driven soap-film channels offer a convenient way to study two-dimensional hydrodynamics in the laboratory. With recently developed quantitative soap-film diagnostic techniques, velocity and thickness field information can be acquired to provide a complete description of the flow. We present a study of soap-film flow in a state-of-the-art tilted soap-film tunnel which can sustain a mean freestream velocity about 0.5 m/s with velocity fluctuations on the order of 1%. Our investigation concentrates on the evolution of the velocity and thickness profiles with downstream distance. We observe a hydrodynamically fully developed flow within a substantial range of distances and compare our results with the analytical solution based on the assumptions of linear air drag and constant film thickness. We find air drag to be the dominant dissipative mechanism in the flow, although the role of internal viscous dissipation is also apparent. Direct measurements of the air drag coefficient are in good agreement with the values inferred from the analytical solution.
Development of half-metallic ultrathin Fe3O4 films for spin-transport devices
Authors: Soeya Susumu , Hayakawa Jun , Takahashi Hiromasa , Ito Kenchi , Yamamoto Chisato , Kida Ayumu , Asano Hidefumi , Matsui Masaaki
Published: July 2002

We attempted to fabricate a high-quality Fe3O4 film while satisfying both low-thermal preparation (≦573 K) and film thinness (≦500 Å). X-ray diffractometry showed that our prepared Fe3O4 film was epitaxially grown onto a MgO (100) substrate. The saturation magnetization, resistivity, and Verwey point were, respectively, ∼438 emu/cm3, ∼10 000 μ Ω cm, and ∼110 K. These values were comparable to those of the Fe3O4 bulk. Our experimental results suggested that a high-quality Fe3O4 film could be obtained even under the crucial conditions of the deposition temperature being low (∼523 K) and the film being ultrathinned (∼100 Å).
Brownian dynamics simulations of salt-free polyelectrolyte solutions
Authors: Chang Rakwoo , Yethiraj Arun
Published: July 2002

The static and dynamic properties of dilute and semidilute salt-free polyelectrolyte solutions are studied using Brownian dynamics simulations. The polymer molecules and counterions are modeled as charged bead–spring chains and charged spheres, respectively. Results are presented for the static properties, polymer rotational dynamics, self and collective diffusion coefficients, intermediate scattering functions, and dynamic structure factors of polyelectrolytes and counterions. The simulations reveal a strong dynamical coupling between counterions and polyions at low concentrations; this coupling becomes weaker as the concentration is increased. The polyion self-diffusion coefficient shows a nonmonotonic concentration dependence, but the counterion self diffusion coefficient decreases monotonically as the concentration is increased. In semidilute solutions, the collective density relaxation dynamics are slowed down at wave vectors where there is a peak in the corresponding static structure factor. This can be qualitatively, but not quantitatively understood via the concept of de Gennes narrowing. A comparison with relevant experiments and theories is also presented.
Shear viscosity of strongly coupled Yukawa systems
Authors: Saigo T. , Hamaguchi S.
Published: July 2002

With the use of equilibrium molecular dynamics (MD) simulations, shear viscosity of the Yukawa system is evaluated under strongly coupled conditions. In the limit of weak screening, it is confirmed that the obtained Yukawa shear viscosity approaches the previously known shear viscosity of the one-component plasma. It is shown that Yukawa shear viscosities with appropriate normalization follow a simple temperature scaling formula. Yukawa shear viscosities obtained from the present MD simulations are significantly larger than those obtained previously based on a different numerical method. It is argued that the new simulations provide more plausible values for Yukawa shear viscosities than the previously known results.
N-type organic thin-film transistor with high field-effect mobility based on a N,N′-dialkyl-3,4,9,10-perylene tetracarboxylic diimide derivative
Authors: Malenfant Patrick R. L. , Dimitrakopoulos Christos D. , Gelorme Jeffrey D. , Kosbar Laura L. , Graham Teresita O. , Curioni Alessandro , Andreoni Wanda
Published: July 2002

N , N ′ -dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8H) thin films have been implemented into organic thin-film field-effect transistors. Mobilities up to 0.6 cm2 V−1 s−1 and current on/off ratios >105 were obtained. Linear regime mobilities were typically half of those measured in the saturation regime. X-ray studies in reflection mode suggest a spacing of ∼20 Å for thin evaporated films of PTCDI-C8H, which is consistent with the value of ∼21±2 Å obtained from our simulations when an interdigitated packing structure is assumed.
Improved efficiency of light-emitting diodes based on polyfluorene blends upon insertion of a poly(p-phenylene vinylene) electron- confinement layer
Authors: Morgado J. , Friend R. H. , Cacialli F.
Published: July 2002

We report the improvement of the electroluminescence efficiency of light-emitting diodes (LEDs) based on polyfluorene blends, upon insertion of a thin film of poly(p-phenylene vinylene), PPV, between a hole-injection layer of poly(3,4-ethylene dioxythiophene), doped with polystyrene sulfonic acid, and the polyfluorenes emissive layer. For LEDs using a blend of poly(9,9′-dioctylfluorene), with 5 wt % of the green emitter poly(9,9′-dioctylfluorene-altbenzothiadiazole), and calcium cathodes, the efficiency increases from 2.1 to 4.1 cd/A upon insertion of such a PPV layer. We propose that such an improvement is mainly due to the electron-blocking effect of the PPV layer, leading to improved charge carriers balance within the emissive layer.
The virtual origin as a first-order correction for the far-field description of laminar jets
Authors: Revuelta Antonio , Sánchez Antonio L. , Liñán Amable
Published: July 2002

The far-field velocity and composition fields of a submerged laminar jet are known to approach a self-similar solution corresponding to the flow induced by a point source of momentum and scalar. Previous efforts to improve this far-field description have introduced a virtual origin for the streamwise coordinate to remedy the singular behavior of the self-similar solution near the jet origin. The purpose of this note is to show, by means of a perturbative analysis of the point-source solution, that this virtual origin is in fact the first-order correction to the leading-order description. The perturbative analysis, which uses the ratio x of the streamwise distance to the length of jet development as an asymptotically large quantity, also indicates that the displaced point source provides the description in the far field with small relative errors of order x−3 for the round jet and of order x−10/3 for the plane jet. The values of the virtual origin are obtained by numerical integration of the boundary-layer equations in the region of jet development, giving values that depend on the shape of the jet velocity profile at the exit.
Solution of phase space diffusion equations using interacting trajectory ensembles
Authors: Donoso Arnaldo , Martens Craig C.
Published: July 2002

In this paper, we present a new method for simulating the evolution of the phase space distribution function describing a system coupled to a Markovian thermal bath. The approach is based on the propagation of ensembles of trajectories. Instead of incorporating environmental perturbations as stochastic forces, however, the present method includes these effects by additional deterministic interactions between the ensemble members. The general formalism is developed and tested on model systems describing one-dimensional diffusion, relaxation of a coherently excited harmonic oscillator coupled to a thermal bath, and activated barrier crossing in a bistable potential. Excellent agreement with exact results or approximate theories is obtained in all cases. The method provides an entirely deterministic trajectory-based approach to the solution of condensed phase dynamics and chemical reactions.
Scanning Kelvin probe microscopy of surface electronic structure in GaN grown by hydride vapor phase epitaxy
Authors: Simpkins B. S. , Schaadt D. M. , Yu E. T. , Molnar R. J.
Published: July 2002

Scanning Kelvin probe microscopy is used to image surface potential variations in GaN (0001) grown by hydride vapor phase epitaxy. The influence of finite probe tip size on these measurements is analyzed, suggesting that significant differences between measured and actual surface potential variations may exist. Experimentally, localized regions in which the surface work function increases by ∼0.1–0.2 V are observed, indicating a shift in the Fermi level toward the valence band; these are attributed to the presence of negatively charged threading dislocations. The magnitudes of the observed variations in surface potential are comparable to those reported in the literature, and compare favorably with those predicted on the basis of a model in which the dislocation is represented as a filled line of acceptor states and the interaction between the sample and a probe tip of finite size is considered. In this model, the finite size of the probe tip is found to exert a substantial influence on the degree to which the full variation in surface potential is observed in scanning Kelvin probe measurements.
Formation and disappearance of a nanoscale silver cluster realized by solid electrochemical reaction
Authors: Terabe K. , Nakayama T. , Hasegawa T. , Aono M.
Published: July 2002

We have developed a nanostructuring method using the solid electrochemical reaction induced by a scanning tunneling microscope (STM). This method has some distinctive features that have not previously been obtained by conventional nanostructuring STM methods. The formation and disappearance of the nanostructure are reversible, and the rates can be controlled using STM. These features are realized via a local oxidation/reduction reaction of mobile metal ions in an ionic/electronic mixed conductor. In this study, a crystal of silver sulfide (Ag2S), a mixed conductor, was used as the material for the STM tip. A nanoscale Ag cluster was formed at the apex of the Ag2S tip when a negative bias voltage was applied to the sample. The Ag ions in the Ag2S tip are reduced to Ag atoms by the tunneling electrons from the sample, and the Ag cluster is formed by the precipitation of the Ag atoms at the apex of the tip. The Ag cluster shrank gradually and disappeared when the polarity of the sample bias voltage was switched to positive. Ag atoms in the Ag cluster are oxidized to Ag ions, and the Ag ions redissolve into the Ag2S tip. The formation and disappearance rates of the cluster were controlled by regulating the tunneling current.
Electron conduction through quasi-one-dimensional indium wires on silicon
Authors: Uchihashi Takashi , Ramsperger Urs
Published: July 2002

Electron conduction through quasi-one-dimensional (1D) indium atomic wires on silicon (the Si(111)-4×1-In reconstruction) is clarified with the help of local structural analysis using scanning tunneling microscopy. The reconstruction has a conductance per square as high as 100 μS, with global conduction despite numerous surface steps. A complete growth of indium wires up to both the surface steps and the lithographically printed electrodes is essential for the macroscopic transport. The system exhibits a metal–insulator transition at 130 K, consistent with a recent ultraviolet photoemission study [H. W. Yeom, S. Takeda, E. Rotenberg, I. Matsuda, K. Horikoshi, J. Schaefer, C. M. Lee, S. D. Kevan, T. Ohta, T. Nagao, and S. Hasegawa, Phys. Rev. Lett. 82, 4898 (1999)].
Jupiter’s Great Red Spot and zonal winds as a self-consistent, one-layer, quasigeostrophic flow
Authors: Marcus Philip S. , Lee Changhoon
Published: July 2002

We present the point of view that both the vortices and the east–west zonal winds of Jupiter are confined to the planet’s shallow weather layer and that their dynamics is completely described by the weakly dissipated, weakly forced quasigeostrophic (QG) equation. The weather layer is the region just below the tropopause and contains the visible clouds. The forcing mimics the overshoot of fluid from an underlying convection zone. The late-time solutions of the weakly forced and dissipated QG equations appear to be a small subset of the unforced and undissipated equations and are robust attractors. We illustrate QG vortex dynamics and attempt to explain the important features of Jupiter’s Great Red Spot and other vortices: their shapes, locations with respect to the extrema of the east–west winds, stagnation points, numbers as a function of latitude, mergers, break-ups, cloud morphologies, internal distributions of vorticity, and signs of rotation with respect to both the planet’s rotation and the shear of their surrounding east–west winds. Initial-value calculations in which the weather layer starts at rest produce oscillatory east–west winds. Like the Jovian winds, the winds are east–west asymmetric and have Kármán vortex streets located only at the west-going jets. From numerical calculations we present an empirically derived energy criterion that determines whether QG vortices survive in oscillatory zonal flows with nonzero potential vorticity gradients. We show that a recent proof that claims that all QG vortices decay when embedded in oscillatory zonal flows is too restrictive in its assumptions. We show that the asymmetries in the cloud morphologies and numbers of cyclones and anticyclones can be accounted for by a QG model of the Jovian atmosphere, and we compare the QG model with competing models.
Diffusion in Hamiltonian systems
Authors: Kozlov V. V. , Moshchevitin N. G.
Published: July 2002

The study is reported of a diffusion in a model of degenerate Hamiltonian systems. The Hamiltonian under consideration is the sum of a linear function of action variables and a periodic function of angle variables. Under certain choices of these functions the diffusion of action variables exists. In the case of two degrees of freedom during the process of diffusion, the vector of the action variables returns many times near its initial value. In the case of three degrees of freedom the choice of Hamiltonian allows one to obtain a diffusion rate faster than any prescribed one.
A MATHCAD-based method for enhanced system resolution studies
Authors: Kennedy Jack
Published: July 2002

A method is presented using the mathcad commercial software for investigating the resolution of an optical or microwave system. The method allows parametric variation of both the sizes of objects being observed and the objective of the system. The integrations required are performed automatically and the results are plotted using one set of instructions typed directly into the mathcad display window. This allows the user to quickly begin studying the effect of varying the dimensions without dwelling on programming issues that would otherwise be encountered. The method demonstrates the power and flexibility of this type of software for many basic problems that may have previously been neglected because they require computational processing to solve.
Power series solution of linear and nonlinear partial differential equations via symbolic programming
Authors: Pickett Robert C. , Lindgren Gordon E. , Anderson Ronald K.
Published: July 2002

A method is described for obtaining the exact power series solutions, to arbitrary order, of several classes of partial differential equations. Several examples pertinent to mathematical physics are incorporated.
Visualizing atomic orbitals
Authors: Wang Jing B. , Abbott Paul C. , Williams Jim F.
Published: July 2002

The Mathematica software package provides a range of tools for working with atomic and molecular systems. The symbolic tools include orthogonal polynomials and Clebsch–Gordan coefficients, while the graphical capabilities cover polar plots, spherical plots, density plots, contour plots in two and three dimensions, and animation. In this article, these tools are applied to the manipulation and visualization of atomic orbitals, with one example of molecular orbitals. © 1995 American Institute of Physics.
Efficient visualization of a plasma spray simulation
Authors: Hansen G. A. , Chang C. H.
Published: July 2002

A transient particle-laden flow was analyzed using a prototype three-dimensional scientific visualization package developed to provide a high-quality image at acceptable animation rates without the use of specialized graphics hardware. The tool was designed for interactive visualizations of velocity vectors, entrained particles, and volumetric scalar data such as temperature, pressure, etc., on a domain that may be time-dependent. This software is implemented using X11/Motif for portability. The prototype was used to visualize a transient simulation of a plasma spraying process using the LAVA code in which entrained particles are modeled by a stochastic particle spray model, fully coupled to the plasma flow. The visualization tool displays important statistical information regarding particle spray characteristics including particle temperature and degree of melting. A parallel implementation using PVM software on FDDI-interconnected workstations shows some promise in extending reasonable interactive response to larger datasets.
Numerical simulations of an atomic beam focused by an optical potential
Authors: Kidan T. , Adler Joan , Ron A.
Published: July 2002

We present a model to describe the behavior of an atomic beam in a focusing experiment and discuss its implementation. Such modeling is relevant to the development of atom lithography. For simplicity, the light-atom interaction may be treated as a conserving potential (neglecting spontaneous emission) so that the Schrödinger equation is sufficient for a description of the problem. We start from a two-dimensional time-dependent Schrödinger equation that is reduced to a one-dimensional equation by a paraxial approximation, and then solved iteratively via Padé approximation. The solution is implemented in an interactive routine with Matlab(5.x) to compute and visualize the atomic wave packet development as it propagates through the laser light optical potential. We present examples of our implementation, starting with cases that have known analytic solutions such as propagation of the atomic wave packet through free space and through a parabolic potential. We also present a case that does not have a simple analytic solution, namely the propagation of the atomic wave packet through a focusing optical potential, which has a Gaussian profile in the propagation direction and a parabolic profile in the perpendicular direction. © 1998 American Institute of Physics.
Direct formation of ferrite films in wet process
Authors: Abe M. , Tanno Y. , Tamaura Y.
Published: July 2002

We have prepared a polycrystalline Co-ferrite film by the electroless ferrite-plating technique of our invention [J. Appl. Phys. 55, 2614 (1984)] in an aqueous solution at 70 °C on substrates of organic compounds (polyethylene terephthalate, polymethyl methacrylate, polycarbonate, Teflon), a stainless steel, and a polyester fiber. We used no intermediate layer, which we needed previously to enhance the adhesion of the ferrite film to the surface. By exposing the organic substrates to an rf-excited air plasma, the wettability of the surface improved, which enhanced the adhesion of the film to the substrate. Here we have measured the adhesion by a cross-cut test. The plasma forms hydrophilic groups such as -OH and -COOH, which improve the wettability. The OH group adsorbs the metal ions in the reaction solution, initiating the ferrite film formation. The glass shows a strong adhesion to the film even when it is not exposed to the plasma, because the glass has the OH group on the surface inherently. The stainless steel does not enhance the adhesion by the plasma treatment because the plasma forms no OH group on the metal surface. The adhesion power higher than ∼10 kg/cm2 has been obtained between the Co-ferrite film and the glass (not plasma treated), the polyethylene terephthalate (plasma treated) and the Teflon (plasma treated).
Optical properties of (AlAs)n(GaAs)n superlattices grown by metalorganic chemical vapor deposition
Authors: Ishibashi A. , Mori Y. , Itabashi M. , Watanabe N.
Published: July 2002

Optical properties were investigated on the superlattices with a unit lattice period of (AlAs)n(GaAs)n (n=1–24) which were grown by atmospheric-pressure metalorganic chemical vapor deposition. Raman spectroscopy indicated that superlattice structure is realized for each n without collapsing into alloys. Photoluminescence measurement indicated that the ultrathin-layer superlattice (with n larger than 2) has a direct energy gap, which is in good agreement with a tight-binding calculation.
Direct measurement of space-charge injection from a needle electrode into dielectrics
Authors: Hibma T. , Zeller H. R.
Published: July 2002

We show that space-charge injection from a needle into a dielectric can be determined with a resolution of 10−15 °C, by using guarded needle electrodes and a charge compensating bridge circuit. The technique is applied to epoxy resin samples. A well-defined homogeneous space-charge cloud formation is observed in quantitative agreement with a field-limiting space-charge concept.
Doping reaction of PH3 and B2H6 with Si(100)
Authors: Yu Ming L. , Vitkavage D. J. , Meyerson B. S.
Published: July 2002

The reaction of phosphine PH3 and diborane B2H6 on Si(100) surfaces was studied by surface analytical techniques in relation to the in situ doping process in the chemical vapor deposition of silicon. Phosphine chemisorbs readily either nondissociatively at room temperature or dissociatively with the formation of silicon–hydrogen bonds at higher temperatures. Hydrogen can be desorbed at temperatures above 400 °C to generate a phosphorus layer. Phosphorus is not effective in shifting the Fermi level until the coverage reaches 2×1014/cm2. A maximum shift of 0.45 eV toward the conduction band was observed. In contrast, diborane has a very small sticking coefficient and the way to deposit boron is to decompose diborane directly on the silicon surface at temperatures above 600 °C. Boron at coverages less than 2×1014/cm2 is very effective in shifting the Fermi level toward the valence band and a maximum change of 0.4 eV was observed.
Use of high-T
c superconductors for the determination of absolute thermoelectric power
Authors: Uher Ctirad
Published: July 2002

The use of the recently discovered high-Tc superconductors for purposes of precision determination of the absolute thermoelectric power is illustrated on commercially available wires of copper and Nb-Ti. High-Tc superconductors offer a wide range of temperatures where precision metrology can be carried out, and their unusually steep upper critical field allows for an accurate determination of the effect of magnetic field on the thermopower of test leads.
Magnetization curves of Pd/Co multilayers with perpendicular anisotropy
Authors: Draaisma H. J. G. , de Jonge W. J. M.
Published: July 2002

In a ferromagnetic thin film with strong perpendicular anisotropy, saturation may be reached at fields lower than the magnetization. This field is calculated for a multilayer with alternating ferromagnetic and nonmagnetic layers assuming that the stripe domains are oriented only up or down along the anisotropy axis. The results are compared with experimental data on Pd/Co multilayers with ultrathin Co layers. The agreement is very good if we take σw=1×10−3 J/m2 as the energy of walls between the domains. It is argued also that with other materials, high perpendicular relative remanence may be achieved when multilayers with suitable parameters are used.
26.1% solar cell efficiency for Ge mechanically stacked under GaAs
Authors: Partain L. D. , Kuryla M. S. , Weiss R. E. , Ransom R. A. , McLeod P. S. , Fraas L. M. , Cape J. A.
Published: July 2002

We have processed a diffused Ge wafer into a Ge concentrator solar cell and mechanically stacked it under a GaAs cell fabricated by Varian. We measured this stack’s efficiency to be 26.1% for terrestrial air mass 1.5 direct (AM1.5D) conditions at a 285× concentration ratio. We showed that this efficiency is limited by optical absorption in the Varian GaAs cell caused by high 2–4 (1018) cm−3 substrate doping. We used a 2×1017 cm−3 doped GaAs filter to estimate the stack efficiency as 27.4%, which would be achieved with the same Varian GaAs cell formed on a lower doped substrate. We project efficiencies assuming the best properties reported for a GaAs device. This gives a 29.6% efficiency for an improved, planar Ge cell and 31.6% efficiency for a proposed point contact geometry for the Ge cell. The corresponding space (AM0) efficiencies at a 159× concentration ratio range from the 23.4% value we measured on the stack up to 28.4% projected for the point contact Ge place under the best GaAs cell. We showed that Ge cells give higher efficiencies than Si when stacked under GaAs.
Structural properties of bismuth-bearing semiconductor alloys
Authors: Berding M. A. , Sher A. , Chen A.-B. , Miller W. E.
Published: July 2002

Materials currently used for detection in the infrared spectral region have notoriously poor structural properties. In search of a better narrow-gap material, we have addressed the structural properties of bismuth-bearing III-V semiconductor alloys theoretically. Because the Bi compounds are not known to form zinc-blende structures, only the anion-substituted alloys InPBi, InAsBi, and InSbBi are considered candidates as narrow-gap semiconductors. We calculate the bond energies and lengths for the zinc-blende Bi compounds and their diluted and concentrated alloys. Strain coefficients for the compounds are calculated, and predictions for the mixing enthalpies, miscibility gaps, and critical temperatures are made. Miscibility calculations indicate that InSbBi will be the most miscible, and because of the large lattice mismatch of the constituents, InPBi will be the most difficult to mix. Tendencies toward cluster formation and deviations from randomness in the alloys are considered. Calculations of the hardness of the Bi compounds indicate that, once formed, the InPBi alloy will be harder than the other Bi alloys and substantially harder than the currently favored narrow-gap semiconductor HgCdTe. Thus, although InSbBi may be an easier material to prepare, InPBi promises to be a harder material. Vacancy formation energies are calculated and compared with those of the constituent compounds of narrow-gap II-VI alloys.
Physical kinetics
Authors: Walker Franklin E.
Published: July 2002

The elucidation of the chemistry and physics of shock-induced chemical reactions and the initiation and detonation of chemical explosives has been slow and difficult. The processes involved often are not in thermal equilibrium, and they do not obey Arrhenius kinetics in the usual sense. Some calculations show a slow step in the transfer of shock energy to intramolecular vibrons from the lattice phonons that may limit reaction rates to around 107–109 s−1. Data and arguments are presented from Hugoniot and detonation studies that support a new concept of high-temperature kinetics. It is proposed that the averaged vibrational velocities of the atoms in condensed systems under the influence of high-velocity shock waves constrain the chemical reaction rates in the systems. This new concept is compared with alternative explanations and contrasted with Arrhenius processes.
Externally modulated intense relativistic electron beams
Authors: Friedman M. , Krall J. , Lau Y. Y. , Serlin V.
Published: July 2002

The physics of modulation of an intense relativistic electron beam by an external microwave source is studied in this paper via experiment, theory, and simulation. It is found that the self-fields of the electron beam, in general, intensify the current modulation produced by the external source. The linear and nonlinear theory, together with the simulation, show that the classical klystron description in the drift tube region is substantially modified by the beam’s high density. In the modulating gap, electron bunches may be generated instantaneously without the necessity of propagating the beam through a long drift tube. These properties, which have no counterparts in low-density beams, lead to the generation of large amplitude, coherent, and monochromatic current modulation on an intense beam. The excellent amplitude stability and the phase-locking characteristics (<2°) of the modulated current, demonstrated in experiments, open new areas of research in high-power microwave generation and compact particle accelerators.
Dependence on the Schottky metal and crystal orientation of the Schottky diode characteristics of β-SiC single crystals grown by chemical vapor deposition
Authors: Fujii Yoshihisa , Shigeta Mitsuhiro , Furukawa Katsuki , Suzuki Akira , Nakajima Shigeo
Published: July 2002

Schottky barrier contacts have been made on β-SiC single crystals grown by chemical vapor deposition on Si substrates, and their capacitance-voltage and current-voltage characteristics are measured. Dependence of the Schottky characteristics on Si substrate orientation [(n11), n=6, 5, 4, 3, 1, and (100)] is examined. The current-voltage characteristics of the Schottky diodes of the β-SiC films on Si(611) and Si(411) are excellent compared with the conventional Schottky diodes on Si(100). The barrier height is determined by the pulsed capacitance-voltage method. Capacitance-voltage characteristics are measured by this method avoiding the effects of deep levels which are not ionized at room temperature. The dependence on the Schottky metal (Pt and Au) is examined. Both Pt-Schottky diodes and Au-Schottky diodes show good rectification characteristics. The barrier height of the Pt-Schottky diodes is 1.3–1.8 eV and that of the Au-Schottky diodes 1.0–1.6 eV, depending on the substrate orientation. For the same substrate orientation the barrier height for Pt is found to be 0.1–0.3 eV larger than that for Au.
Molybdenum deposition from the decomposition of molybdenum hexacarbonyl
Authors: Cho C. C. , Bernasek S. L.
Published: July 2002

Molybdenum metal deposition from the decomposition of Mo(CO)6 adsorbed on Si(100), Mo, and Cu surfaces was studied by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, thermal desorption spectroscopy, and low-energy electron diffraction. Pyrolytic, photolytic, and electron-induced Mo(CO)6 decomposition were observed and indicated different dissociation mechanisms. Thermally decomposed Mo(CO)6 was found to leave metallic Mo in the presence of C and O. Electron-induced decomposition resulted in the formation of molybdenum carbide on the surfaces. Ultraviolet (UV) irradiation of adsorbed Mo(CO)6 induced new peaks in XPS and TDS spectra, suggesting the formation of an unsaturated molybdenum carbonyl adsorbate. Mo(CO)6 was found to form a multilayer on these surfaces at low temperatures, and desorb with zero-order kinetics. Although both adsorbate desorption and decomposition took place when the samples were heated, desorption was the dominant reaction path. UV irradiation of gaseous and coadsorbed Mo(CO)6 and O2 was also investigated. UV irradiation of the gas-phase mixture leads to MoO2 and MoO3 deposition; however, UV irradiation of coadsorbed Mo(CO)6 and O2 resulted in unsaturated molybdenum carbonyl. The effects of annealing and Ar+ bombardment on the Mo-deposited Si(100) surface are also reported.
The deposition and study of hard carbon films
Authors: Zou J. W. , Reichelt K. , Schmidt K. , Dischler B.
Published: July 2002

Carbon films have been deposited by rf plasma decomposition of methane at 50–1400 V negative bias and 1.3×10−3–1.3×10−1 mbar pressure. Hardness, internal stress, density, hydrogen content, and infrared absorption depending on the preparation parameters have been measured. From the IR measurements the ratio of sp3 to sp2 bonds was calculated and the total amount of hydrogen in the films was determined by elastic recoil detection (0.5>H/C>0.15). We found in the range of bias voltages 0<‖−VB ‖<100 V polymerlike films, in the range 100 V<‖−VB ‖<600 V diamondlike hard carbon films with high internal stress, and in the range 600<‖−VB ‖<1400 V graphitelike soft films with low stress. The density of the diamondlike films was about 2 g/cm3 and of the graphitelike films about 1.4 g/cm3. The microhardness seems to be correlated to the internal mechanical stress.
Piezoelectric properties and elastic constants of 4H and 6H SiC at temperatures 4–320 K
Authors: Karmann S. , Helbig R. , Stein R. A.
Published: July 2002

4H and 6H silicon carbide (SiC) crystals are piezoelectric and can be excited to resonant vibrations with quality factors up to Q≊100 000 and an electromechanical coupling factor of k31≊0.03 for 6H material. The resonance frequencies are used for the determination of elastic constants from 4 to 320 K. One finds a Poisson’s ratio σ=0.212 and low-temperature values s11=2.035×10−12 m2/N and c33=55.12×1010 N/m2 for 6H, s11=2.114×10−12 m2/N, and c33=60.52×1010 N/m2 for 4H crystals. All elastic constants show a nonmonotonic dependence from the temperature at about 50 K.
The G value in plasma and radiation chemistry
Authors: Baird James K. , Miller George P. , Li Ning
Published: July 2002

The G value refers to the number of molecules of reactant consumed or product formed per 100 eV of energy absorbed. Although ubiquitous in radiation chemistry, the G-value concept appears only rarely in plasma chemistry. We derive a formula for the G value for a general plasma chemical reaction as a function of the electrical power absorbed, the fraction of molecules transformed, and the flow rate of the gas entering the reactor. Applying our formula to the ammonia plasma radiofrequency discharge data of d’Agostino et al. [Plasma Chem. Plasma Process. 1, 19 (1981)], we find that G(-NH3) depends in general upon the conditions but lies in the range of 6.0–20 molecules/100 eV. By comparison, the values of G(-NH3) reported by Peterson for the gas-phase radiolysis of ammonia lie in the range 2.7–10 molecules/100 eV. We suggest that this similarity with respect to order of magnitude may have its origin in a common reaction mechanism initiated by inelastic electron-molecule collisions. We use this hypothesis and the Boltzmann transport theory to derive a formula which expresses G(-NH3) in terms of electron-molecule scattering cross sections, the distribution of electron velocities, and the rate constants for secondary reactions. In principle, given a knowledge of the reaction mechanism and the electron-energy-loss channels, our method may be used to express the G value of any gas.
Domain observation on nanocrystalline material
Authors: Schäfer R. , Hubert A. , Herzer G.
Published: July 2002

Bulk polycrystalline material with a grain size in the nanometer range can be prepared by controlled crystallization of certain metallic glass alloys. The material shows excellent soft magnetic properties which are attributed to an effective averaging of the local crystal anisotropy over the scale of a Bloch wall width. The general appearance of observed domain patterns resembles those of metallic glasses. Wide in-plane domains as well as the characteristic stress patterns are observed. On annealing beyond the optimum temperature the coercivity rises which manifests itself by an irregular domain appearance. An interesting behavior is found if the samples are observed at temperatures beyond ∼320 °C. At this temperature the amorphous ferromagnetic grain boundary phase which couples the iron-rich crystallites becomes paramagnetic. The domains reversibly show a similar character as the overannealed samples. This observation indicates that overannealing leads in this material not only to grain growth beyond the optimum, but also to an interruption of the ferromagnetic coupling between the grains.
Multicritical behavior in the antiferromagnetic Blume–Capel model
Authors: Wang Yung-Li , Kimel J. D.
Published: July 2002

The existence of the fourth-order critical point has been found in the spin-1 Ising antiferromagnet with a single-ion potential and an external magnetic field involved by using the mean-field approximation calculation. The Monte Carlo simulation gives more accurate results and shows that no such phase transition occurs in the square lattice system. To show definitely that the fourth-order phase transition appears in three-dimensional systems, the simple cubic system is investigated by using Monte Carlo simulations. It was found that the decomposition of the tricritical point into a critical end point and a double critical end point appears in the phase diagram within a small range of the single-ion potential parameter. The location of the fourth-order critical point can also be estimated.
Hysteresis, microstructure, and magneto-optical recording in Co/Pt and Co/Pd multilayers
Authors: Zeper W. B. , van Kesteren H. W. , Jacobs B. A. J. , Spruit J. H. M. , Carcia P. F.
Published: July 2002

The factors that influence the coercive field Hc and the shape of the magnetic hysteresis loop for Co/Pt multilayers (MLs), applied as a magneto-optical (MO) recording medium, have been investigated. The hysteresis loop of MLs, made by evaporation of Kr sputtering, became more rectangular when the total film thickness was reduced, i.e., the saturation field Hs decreased and the nucleation field Hn increased. Hs was calculated using a stripe domain model and Hn was modeled assuming that nucleation took place at a field that destabilized small vestigial magnetic domains. Both models assumed that domain walls can move in the ML film and the agreement with experiment was good. However, these models could not explain the hysteresis loops for the MLs deposited on a thick Pt base layer or made at high sputter gas pressure. These MLs showed a more particulate microstructure and the hysteresis loop was sheared, consistent with magnetic reversal by uniform rotation rather than domain-wall motion. From thermomagnetic recording experiments it was concluded that these latter MLs were less suitable as a MO recording medium because of higher noise in spite of the larger Hc. Co/Pt disks made by either evaporation or Kr sputtering showed lower noise and a higher carrier-to-noise ratio than those deposited on a Pt base layer or made by Ar sputtering.
Comparison of the pressure-induced frequency shift of Sm:YAG to the ruby and nitrogen vibron pressure scales from 6 to 820 K and 0 to 25 GPa and suggestions for use as a high-temperature pressure calibrant
Authors: Hess Nancy J. , Schiferl David
Published: July 2002

The pressure-induced frequency shift of the Sm:YAG Y1 peak at elevated temperature is calibrated against the temperature-corrected Raman shift of the nitrogen vibron and, at temperatures less than 673 K, the R1 shift of ruby. The results presented here indicate that pressure can be determined from the Y1 and Y2 peak frequencies, without temperature correction, from 6 to 820 K and from 1 bar to 25 GPa by using the equations: P(GPa) =−0.12204 (ωY1obs−16187.2) and P(GPa)=−0.15188 (ωY2obs−16232.2). However, pressure determinations based on Y2 are less accurate, especially at high temperature. At elevated temperature, the Sm:YAG Y1 and Y2 peak frequencies are most accurately determined by curve fitting a spectral window at least 400 cm−1 wide. The spectral range was chosen in order to include the decay of the intensity of the Lorentzian Y1 peak to a background value and incorporate a third peak at 16360 cm−1.
Nonlinear spectroscopy near half-gap in bulk and quantum well GaAs/AlGaAs waveguides
Authors: Islam M. N. , Soccolich C. E. , Slusher R. E. , Levi A. F. J. , Hobson W. S. , Young M. G.
Published: July 2002

We study the nonlinear properties of bulk AlGaAs and GaAs/AlGaAs multiple quantum wells (MQW) below the half-band-gap energy using subpicosecond pulses between 1.65 and 1.7 μm. In the bulk material we find a value for the nonlinear index n2 = +3.6× 10−14 cm2/W and a two-photon absorption coefficient β = 0.26 × 10−4 cm/MW. In the MQW we measure an n2 up to 2.4 times larger, and we attribute this enhancement to a stronger 1S-exciton intermediate state. The β value is up to 25 times larger in the MQW. This larger value may result from midgap states that resonantly enhance the virtual intermediate state in two-photon absorption and act as a real transition in a two-step absorption process. The resulting figure of merit (2n2/βλ) for the bulk (MQW) material is 17 (1.6), which means that these semiconductors below half band gap are appropriate for all-optical switching and quantum optics applications. We confirm that n2 is instantaneous on the 300 fs time scale of our pulses from self-phase-modulation spectra as well as time-resolved pump-probe measurements. However, we find an intriguing exchange of energy between the two orthogonal axes as evidenced by the signal along the probe axis following the negative derivative of the pump intensity. This result may be explained by self-phase modulation of the pump combined with a low-frequency Raman process that couples the modes along orthogonal axes.
Stability analysis, finite current effects, and experimental results in the Autoresonance Microwave Accelerator
Authors: Shpitalnik Reuven
Published: July 2002

The details of the autoresonance acceleration scheme suggested recently are being investigated below. The eikonal perturbation analysis is presented as well as some numerical results on the finite current effects. An electron cyclotron maser as well as a peniotron-like interactions are discussed for the acceleration with a circularly polarized TE21 mode in a round waveguide. Experimental data, resulting from an improvement of the axial magnetic field, showing acceleration to 200 keV out of the expected 250 keV, are reported.
Kinetics of laser-induced phase transitions in Ni-Al alloys
Authors: Bostanjoglo O. , Penschke V.
Published: July 2002

Time-resolved transmission electron microscopy (TEM) was applied to study phase transitions in free-standing Ni0.75Al0.25 films, induced by pulses from an Nd:YAG laser (20 ns, 532 nm). At fluences above 0.23 J/cm2 melting occurred within the laser pulse, the liquid was set into violent motion during 300 ns after melting, and the film solidified 1–2 μs later. The liquid motion was triggered by expansion of the melting film. Solidification proceeded by heterogeneous nucleation of crystals at the periphery of the laser molten spot and by centripetal growth of large plates with velocities 0.9–2.4 m/s, from which a supercooling of 2–20 K at the liquid/crystal interface is inferred. The final texture consisted of large γ and γ′ crystals plus fine-grained Al precipitates. The dynamics were either continuously traced or intermediate stages were visualized by double-frame TEM with exposure times of some 10 ns.
Optimal epilayer thickness for InxGa1−xAs and InyAl1−yAs composition measurement by high-resolution x-ray diffraction
Authors: Bennett Brian R. , del Alamo Jesús A.
Published: July 2002

The composition of InxGa1−xAs and InyAl1−yAs epitaxial layers on InP substrates can be measured by high-resolution x-ray diffraction (HRXRD) in many cases. If layers are too thick, however, substantial lattice relaxation will occur, requiring multiple asymmetric scans to determine composition. If layers are too thin, they will not produce a distinct Bragg peak. Based upon measurements of both coherent and relaxed layers as well as simulations, we have determined the range of epilayer thickness over which a single HRXRD scan yields the composition of InxGa1−xAs and InyAl1−yAs layers to within 1%. Calibration layers grown within this range allow fast and accurate characterization.
Inverse modeling of impact ionization rate through comparison of Monte Carlo simulation of Si metal-oxide-semiconductor device characteristics and experimental results
Authors: Imanaga Syunji , Hane Kunio , Hayafuji Yoshinori
Published: July 2002

Direct comparison of the characteristics of a Si metal-oxide-semiconductor device provided by Monte Carlo simulation and experimental results is rare. This paper appraises the degree of agreement between simulated and experimental results of drain current versus drain voltage (Id-Vd). It also derives the impact ionization rate formula inversely by comparing the simulated and experimental dependence of the substrate current (Isub) on the gate voltage (Vg). We found that (1) for Id-Vd characteristics, the agreement in the linear region was off, but overall agreement was fairly good, and (2) the simulated Isub-Vg characteristics were in fairly good agreement with the experimental characteristics when the modified Keldish formula Pii=P0[(E−1.12)/1.12]n with an n of 7 and P0 of 2.8×1011 s−1 was used as the formula for the impact ionization rate.
Low temperature growth of ZnSxSe1−x alloys fabricated by hydrogen radical enhanced chemical vapor deposition in an atomic layer epitaxy mode
Authors: Fujiwara Hiroyuki , Gotoh Jun , Shirai Hajime , Shimizu Isamu
Published: July 2002

We have grown ZnSxSe1−x alloys on a GaAs(100) substrate in a wide compositional range (0≤ x≤0.3) by the insertion of the ZnS monolayer into the ZnSe multilayers. We have succeeded in growing these alloys at temperatures as low as 200 °C using the hydrogen radical enhanced chemical vapor deposition (HRCVD) in the atomic layer epitaxy (ALE) mode. In the ALE-HRCVD, it is the self-limiting action, realized on both surfaces covered with Se and Zn, which leads to the high-quality crystals. In the photoluminescence spectra of these ZnSxSe1−x alloys we have observed only the sharp emission lines at the band edges (2.80–2.90 eV) which we attribute predominantly to the excitons. The structural fluctuations have been remarkably reduced by the layer-by-layer method. In addition, we have found very low defect density in these crystals despite the large lattice deformation due to the lattice mismatch. An ordered alloy [(ZnS)2(ZnSe)24]69 has been made by the optimization of the growth conditions.