bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of SR irradiation on tin-doped indium oxide thin film prepared by rf magnetron sputtering
Authors: Sasaki M. , Hanamoto K. , Kimura Y. , Kaito C. , Miki H. , Nakayama Y.
Published: July 2002

The effects of synchrotron radiation (SR) irradiation on the electrical properties of tin-doped In2O3 (ITO) thin films prepared by rf magnetron sputtering have been studied. A white SR beam focused by a cylindrical mirror was used to irradiate ITO thin films at doses of 0.35, 1.98, 3.50, and 6.64 mA h. We have achieved resistivity of 2×10−4 Ω cm on ITO thin film at an irradiation dose of 3.5 mA h without extra heating. The mobility of the ITO films did not decrease after SR irradiation. The electrical properties in connection with the structural change due to SR are discussed.
Alloy composition and electronic structure of Cd1−xZnxTe by surface photovoltage spectroscopy
Authors: Yang Jihua , Zidon Y. , Shapira Yoram
Published: July 2002

The alloy composition of a Cd1−xZnxTe(111) sample and its spatial homogeneity have been determined by surface photovoltage spectroscopy (SPS) and compared to conventional energy dispersive x-ray spectroscopy measurements. Experimental improvements of the former technique yield a contactless, surface sensitive, and highly accurate spectral resolution of the band gap (error<4 meV) and consequently of the Zn concentration (error<0.6% in comparison with the latter technique). In addition, SPS is capable of determining the face and type of the Cd1−xZnxTe as well as identifying gap states at its surface. The electronic structure has been investigated in comparison with n-CdTe(111), before and after various surface chemical treatments. An acceptor surface state has been observed at 1.21 eV below the conduction band edge and attributed to TeO2. A donor surface state (with a lower concentration relative to the corresponding state in CdTe) associated with Cd atom displacement has been found at 1.42 eV above the valence band maximum. A chemically induced surface state at 0.72 eV below the conduction band edge may be due to Zn vacancies, as supported by x-ray photoelectron spectroscopy measurements.
Role of Ge on film quality of SiC grown on Si
Authors: Sarney W. L. , Wood M. C. , Salamanca-Riba L. , Zhou P. , Spencer M.
Published: July 2002

We find that the incorporation of Ge into SiC during the metalorganic chemical vapor deposition growth process improves the crystalline quality of SiC films grown on Si (111) substrates at 1000 °C. Secondary ion mass spectroscopy results indicate that Ge does not act as a surfactant, but rather it incorporates throughout the entire film. Transmission electron microscopy results show that high quality, single crystalline SiC films of up to 80 nm thickness are repeatably obtained for GeH4 flow rates ranging from 20 to 30 sccm. Higher GeH4 flow rates induce twinning and result in a reduced growth rate and increased surface roughness. Lower GeH4 flow rates result in polycrystalline and/or amorphous-like SiC films similar to what is normally obtained at such a low growth temperature. We discuss the role of Ge during growth and how its presence in the reactor during Si substrate carbonization results in higher quality SiC films than normally achieved for growth temperatures of 1000 °C.
Photodetachment and theoretical study of free and water-solvated nitrate anions, NO3−(H2O)n (n=0–6)
Authors: Wang Xue-Bin , Yang Xin , Wang Lai-Sheng , Nicholas John B.
Published: July 2002

We investigated free and water-solvated gas phase nitrate anions, NO3−(H2O)n (n=0–6), by photodetachment photoelectron spectroscopy and theoretical calculations. We obtained the electronic structure and electron binding energies of the free and solvated NO3− at three detachment photon energies, 4.661, 6.424, and 7.866 eV. The ground and two low-lying electronic excited states of the NO3 radical (X 2A2′,A 2E″,B 2E′) were observed at the 6.424 and 7.866 eV photon energies. The photoelectron spectra of the solvated nitrate complexes are similar to that of the bare NO3−, except that they become broadened and diffuse due to the solvation. The spectrum of NO3−(H3O)3 showed a resolved vibrational progression of the N–O symmetric stretching (1000 cm−1), suggesting the cluster possesses a high symmetry. NO3− and NO3 were calculated at various levels of theory. Based on the good agreement between density functional theory calculations and experiment for NO3− and NO3, we carried out systematic calculations for NO3−(H2O)n (n=1–6) using primarily density function theory methods. The calculations indicate that NO3−(H2O)n (n=1–3) are all planar, with the first three H2O forming the first solvation shell around NO3−, giving rise to a highly symmetric C3h NO3−(H2O)3. The next three waters form a second solvation shell without direct contact with NO3−. The C3h NO3−(H2O)3 solvation structure was observed to be rather robust and largely preserved in the larger clusters.
Lorentz transmission electron microscopy and magnetic force microscopy characterization of NiFe/Al-oxide/Co films
Authors: Yu Andrew C. C. , Lo Chester C. H. , Petford-Long Amanda K. , Jiles David C. , Miyazaki Terunobu
Published: July 2002

Magnetization reversal process of NiFe/Al-oxide/Co junction films was observed directly using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). In situ magnetizing experiments performed in both LTEM and MFM were facilitated by a pair of electromagnets, which were mounted on the sample stages. A two-stage magnetization reversal process for the junction film was clearly observed in LTEM with NiFe magnetization reversed first via domain wall motion followed by Co magnetization reversal via moment rotation and domain wall motion. Reversal mechanism and domain characteristics of the NiFe and Co layers showed very distinctive features. The magnetization curve of the junction film measured using alternating gradient force magnetometry showed a nonzero slope at the antiparallel magnetization configuration region, which implies that magnetization directions of the NiFe and Co layers were not exactly antiparallel due to Co moment rotation existed in that region. After the magnetization reversal of the Co was complete, MFM images revealed some magnetic contrast, which suggests that an out-of-plane magnetization component remained in the Co layer. Such magnetic contrast disappeared at higher magnetic fields when the Co moments further rotated and aligned parallel to the applied field direction.
Determination of coverage in passivated porous silicon by Brillouin spectroscopy
Authors: Fan H. J. , Kuok M. H. , Ng S. C. , Boukherroub R. , Lockwood D. J.
Published: July 2002

Brillouin scattering has been performed to probe surface acoustic waves in porous silicon films that have been chemically modified with various surface passivators. The surface Rayleigh mode and two film acoustic modes are observed. Unlike the Rayleigh mode, the two film modes are found to exhibit velocity dispersion. The effect of passivation is manifested as a shift of Brillouin peaks to lower frequencies relative to those of the freshly prepared porous silicon samples. The coverage of the surface area of the pores of the entire porous layers by the respective chemical passivators has been estimated from the phase velocity of one of the film modes.
Variational method for solving the contracted Schrödinger equation through a projection of the N-particle power method onto the two-particle space
Authors: Mazziotti David A.
Published: July 2002

The power method for solving N-particle eigenvalue equations is contracted onto the two-particle space to produce a reduced “variational” method for solving the contracted Schrödinger equation (CSE), also known as the density equation. In contrast to the methods which solve a system of approximate nonlinear equations to determine the two-particle reduced density matrix (2-RDM) nonvariationally, the contracted power method updates the 2-RDM iteratively through a “gradient” of the N-particle energy. After each power iteration we modify the 2-RDM to satisfy certain N-representability conditions through an extension of purification to correlated RDMs. The contracted power method is illustrated with a variety of molecules. Significant features of the present calculations include (i) accurate results for both first- and second-order functionals for building the 3- and the 4-RDM’s from the 2-RDM’s; (ii) the first molecular implementation of the Mazziotti correction within the CSE [Mazziotti, Phys. Rev. A 60, 3618 (1999)]; (iii) a spin–orbital formulation; (iv) the treatment of both core and valence orbitals as active; and; (v) a reduction of the CSE computational scaling through fast summation and the natural-orbital transformation.
Photoluminescence of ultrasmall Ge quantum dots grown by molecular-beam epitaxy at low temperatures
Authors: Dashiell M. W. , Denker U. , Müller C. , Costantini G. , Manzano C. , Kern K. , Schmidt O. G.
Published: July 2002

Low-temperature epitaxial growth of Si–Ge heterostructures opens possibilities for synthesizing very small and abrupt low-dimensional structures due to the low adatom surface mobilities. We present photoluminescence from Ge quantum structures grown by molecular-beam epitaxy at low temperatures which reveals a transition from two-dimensional to three-dimensional growth. Phononless radiative recombination is observed from 〈105〉 faceted Ge quantum dots with height of approximately 0.9 nm and lateral width of 9 nm. Postgrowth annealing reveals a systematic blueshift of the Ge quantum dot’s luminescence and a reduction in nonradiative recombination channels. With increasing annealing temperatures Si–Ge intermixing smears out the three-dimensional carrier localization around the dot.
Closure of orbits and dynamical symmetry of screened Coulomb potential and isotropic harmonic oscillator
Authors: Zeng Bei , Zeng Jin-Yan
Published: July 2002

It is shown that for any central potential V(r) there exist a series of conserved aphelion and perihelion vectors R̃=p×L−g(r)r, g(r)=rV′(r). However, if and only if V(r) is a pure or screened Coulomb potential, R̃ and L constitute an SO4 algebra in the subspace spanned by the degenerate states with a given energy eigenvalue E<0 at the aphelia and perihelia (ṙ=0). For a pure Coulomb potential, R̃ is reduced to the Pauli–Runge–Lenz (PRL) vector R and for a screened Coulomb potential R̃ is reduced to the extended PRL vector R′. While dR/dt=0 always holds, dR′/dt=0 holds only at the aphelia and perihelia. Moreover, the space spanning the SO4 algebra for a screened Coulomb potential is smaller than that for a pure Coulomb potential. The relation of closed orbits for a screened Coulomb potential with that for a pure Coulomb potential is clarified. The ratio of the radial frequency ωr and angular frequency ωφ, ωr/ωφ=κ=1 for a pure Coulomb potential irrespective of the angular momentum L and energy E(<0). For a screened Coulomb potential κ is determined by the angular momentum L, and when κ is any rational number (κ<1), the orbit is closed. The situation for a pure or screened isotropic harmonic oscillator is similar.
Enhancement of remanent polarization in epitaxial BaTiO3/SrTiO3 superlattices with “asymmetric” structure
Authors: Shimuta Toru , Nakagawara Osamu , Makino Takahiro , Arai Seiichi , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

Enhancement of remanent polarization has been demonstrated in epitaxial “asymmetric” BaTiO3/SrTiO3 strained superlattices, in which the thickness ratio of BaTiO3 to SrTiO3 layers is changed at molecular layer order accuracy. The superlattices have been prepared on Nb-doped SrTiO3 (100) single-crystal substrates by a pulsed-laser deposition technique. The superlattice with a stacking periodicity of 15 unit cells BaTiO3/3 unit cells SrTiO3 shows the largest remanent polarization 2Pr of 46 μC/cm2, which is about three times that of the BaTiO3 single-phase film formed under the same condition. The increase in the remanent polarization is attributed both to the BaTiO3-rich structure and to the increase in lattice parameter c due to the mismatch of in-plane lattice parameters between BaTiO3 and SrTiO3.
A model of solar flares based on arcade field reconnection and merging of magnetic islands
Authors: Choe G. S. , Cheng C. Z.
Published: July 2002

Solar flares are intense, abrupt releases of energy in the solar corona. In the impulsive phase of a flare, the intensity of hard x-ray emission reaches a sharp peak indicating the highest reconnection rate. It is often observed that an x-ray emitting plasma ejecta (plasmoid) is launched before the impulsive phase and accelerated throughout the phase. Thus, the plasmoid ejection may not be an effect of fast magnetic reconnection as conventionally assumed, but a cause of fast reconnection. Based on resistive magnetohydrodynamic simulations, a solar flare model is presented, which can explain these observational characteristics of flares. In the model, merging of a newly generated magnetic island and a preexisting island results in stretching and thinning of a current sheet, in which fast magnetic reconnection is induced. Recurrence of homologous flares naturally arises in this model. Mechanisms of magnetic island formation are also discussed.
Recent design upgrades of the APS beamline standard components
Authors: Shu Deming , Ramanathan Mohan , Haeffner Dean R.
Published: July 2002

Over the last five years, based on the operating experiences, many upgrades have been made to the beamline standard components to continue improving beamline performance and reliability at the Advanced Photon Source (APS). In this article, the particular design upgrades, as well as the new component design specifications of APS beamline standard components, are summarized.
Large magnetoresistance using hybrid spin filter devices
Authors: LeClair P. , Ha J. K. , Swagten H. J. M. , Kohlhepp J. T. , van de Vin C. H. , de Jonge W. J. M.
Published: July 2002

A magnetic “spin filter” tunnel barrier, sandwiched between a nonmagnetic metal and a magnetic metal, is used to create a magnetoresistive tunnel device, somewhat analogous to an optical polarizer-analyzer configuration. The resistance of these trilayer structures depends on the relative magnetization orientation of the spin filter and the ferromagnetic electrode. The spin filtering in this configuration yields a previously unobserved magnetoresistance effect, exceeding 100%.
Insertion device operating experience at the Advanced Photon Source
Authors: Grimmer John , Ramanathan Mohan , Smith Martin , Merritt Michael
Published: July 2002

The Advanced Photon Source has 29 insertion devices (IDs) installed in the 7 GeV electron storage ring; 28 of these devices, most of which are 3.3 cm period undulators, use two horizontal permanent magnet structures positioned over a straight vacuum chamber. A support and drive mechanism allows the vertical gap between the magnet structures to be varied, thus changing the x-ray energy produced by the ID [J. Viccaro, Proc. SPIE 1345, 28 (1990); E. Gluskin, J. Synchrotron Radiat. 5, 189 (1998)]. Most of these IDs use a drive scheme with two stepper motors, one driving each end through a mechanism synchronizing the upper and lower magnet structures. Our experience in almost 5 yr of operating this system will be discussed. All of the IDs are in continuous operation for approximately 10 weeks at a time. Reliability of operation is of paramount importance, as access to the storage ring for servicing of a single ID inhibits operation for all users. Our experience in achieving highly reliable ID operation is reviewed. Accuracy of operation and repeatability over time are also vital. To this end, these devices use absolute optical linear encoders with submicron resolution for primary position feedback. Absolute rotary encoders are used as a backup to the linear encoders. The benefits and limitations of each type of encoder, and our experience dealing with radiation and electrical noise are reviewed. The insertion devices operate down to gaps as small as 8.5 mm, with clearance over the vacuum chamber as small as 200 μm. The vacuum chamber has a minimum wall thickness of only 1 mm. A number of levels of safeguards are used to prevent contact between the magnet structure and the vacuum chamber. These safeguards and their evolution after gaining operational experience are presented.
Center for the Advancement of Natural Discoveries using light emission: A new project for 3 GeV intermediate energy light source in the Republic of Armenia
Authors: Tsakanov V. , Avagyan V. , Ayvazyan V. , Amatuni G. , Grigoryan B. , Ivanian M. , Laziev E. , Martirosyan Y. , Mikaelyan R. , Tatikian S. , Vardanyan A.
Published: July 2002

We are planning to construct a 3 GeV synchrotron light source in the Republic of Armenia called Center for the Advancement of Natural Discoveries using light emission—CANDLE. The purpose of the machine is to produce high flux and high brightness photon beams from bends and insertion devices in the soft and hard x-ray spectrum range from 1 to 50 keV. The main elements of the light source are an S-band linac, a full energy booster, and a 16-cell double bend achromatic storage ring. We will discuss the main parameters of the machine, the insertion devices, and the preliminary beamlines instrumentation.
Reducing the sensitivity of Fourier transform infrared spectroscopy to line-frequency source variations
Authors: Bosch R. A. , Julian R. L.
Published: July 2002

Audio-frequency variations of the electron beam size and position produce noise when performing Fourier transform infrared (FTIR) spectroscopy with synchrotron radiation from an electron storage ring. The undesirable sensitivity of FTIR spectroscopy to periodic line-frequency source variations may be decreased when a line trigger initiates the motion of the interferometer mirror. Several methods of reducing line-frequency noise are described.
A theoretical study on the ionization of NO2 with analysis of vibrational structure of the photoelectron spectrum
Authors: Takeshita Kouichi , Shida Norihiro
Published: July 2002

Ab initio calculations have been performed to study the molecular structures and the vibrational levels of the low-lying ionic states (1A1, B23, A23, A21, B21, B11, 2 1A1, B13, and A13) of nitrogen dioxide. The global regions of the potential energy surfaces have been obtained by multireference single and double excitation configuration interaction calculations. Vibrational calculations using vibrational Hamiltonians have been used for vibrational analysis. The equilibrium molecular structures and a vibrational analysis of these states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states are also presented and are compared with the photoelectron spectra. A new assignment of the photoelectron spectra has been proposed.
Thermally activated self-alignment of exchange coupling in NiO/NiFe bilayers
Authors: Paetzold A. , Röll K.
Published: July 2002

Long-term and temperature stability of the exchange coupling are important conditions for the practical application of spin-valve devices. Here we present magnetic measurements on the properties of sputtered NiO/NiFe bilayers. After deposition a temperature dependent magnetic self-alignment process was observed where the exchange bias field Heb increased according to a ln(t) law. The temperature dependence of this aftereffect indicates that a thermally activated process takes place which can be explained by a two energy-level relaxation model. In addition the long-term stability of the exchange coupling during a forced antiparallel alignment of the NiFe layer was investigated. A significant improvement of the coupling stability can be reached by cooling the samples through the Neél temperature in a magnetic field. Finally, the temperature dependence of Heb was determined for differently treated samples.
A simple powerful computing system for tomography imaging
Authors: Lee Te-Hui , Yin Gung-Chian , Wei Der-Hsin , Tsang King-Long
Published: July 2002

A simple, but powerful, parallel image computing system based on a filtered back-projection algorithm [Dowd et al., Proc. SPIE 3772, 224 (1999)] has been implemented for computerized tomography (CT) in a distributed computing framework by connecting several personal computers (PC) via the Internet. The computing architecture consists of four major components: experimental control, publisher, subscribers, and 3D shaper. The system performance is optimized by an unequal data distribution scheme that takes the computation capability of each subscriber into account. Our result shows that it takes 4.4 s to calculate one two-dimensional charge coupled device projection image of 512×512 pixels when six Pentium III 800 MHz computers each equipped with 512 MB RAM are included in the system. The computation efficiency can be further improved if more computers are utilized. Under this framework, a faster-than-data-acquisition image process for computerized tomography is achievable. The performance of the system is tested using a He–Ne laser as the light source. Despite the artifacts and noises caused by strong absorption, diffraction, and deflection, the image of a 3-pin connector test sample can still be reconstructed clearly and correctly.
Picosecond time-resolved x-ray diffraction probe of coherent lattice dynamics (abstract) (invited)
Authors: Reis D. A. , DeCamp M. F. , Bucksbaum P. H. , Clarke R. , Dufresne E. , Merlin R.
Published: July 2002

The short pulses of hard x rays from synchrotron and laser based sources are sensitive probes of lattice dynamics on an ultrafast time scale. Using pump–probe time-resolved x-ray diffraction, we are able to follow the propagation of a picosecond coherent acoustic pulse in an ultrafast laser-strained single crystal. Comparison of the data with dynamical diffraction simulations allows for the quantitative determination of both the surface and bulk components of the associated strain. This technique is scalable to femtosecond and shorter time scales as x-ray pulses become shorter in duration, such as in fourth generation light sources. In addition, the diffraction of x rays off of coherent optical phonons may lead to the production of a femtosecond x-ray switch.
Studies of nanoscale dynamics using coherent x rays from an x-ray free electron laser (abstract) (invited)
Authors: Stephenson G. Brian
Published: July 2002

This talk will describe proposed studies of atomic- and nanoscale dynamics in condensed matter which take advantage of the high coherent x-ray flux to be produced by future x-ray free-electron laser (FEL) sources, such as the Linac Coherent Light Source at Stanford. In particular, I will focus on the current status and future prospects for photon correlation spectroscopy using coherent x-rays (XPCS), and the use of the ultrashort pulse structure of the x-ray FEL to observe dynamics into the femtosecond range.
Applications of hard x-ray microprobes with suboptical resolution to biomedical and environmental sciences (abstract)
Authors: Maser J. , Lai B. , Cai Z. , Ilinski P. , Legnini D.
Published: July 2002

Hard x-ray microprobes with suboptical resolution have emerged as powerful probes of the elemental constituents of biomedical and environmental specimens. In particular, trace element distributions in the native state of many of these systems can be quantitatively mapped. This has a wide range of applications, particularly in following the metal distribution under changing conditions. If systems such as cells are exposed to known environmental changes, or if the interaction between pathogens and host cells lead to a change of trace metal distribution, important information about the underlying mechanism can be gained. We will present several biomedical and environmental applications of the hard x-ray microprobes at sector 2 of the Advanced Photon Source.
Double crystal bent Laue “beam cleaner” (abstract)
Authors: Karanfil C. , Chapman L. D. , Segre C. U. , Bunker G. B.
Published: July 2002

We describe the performance of double crystal “beam cleaner” that allows one to extract a specific harmonic from all the reflections transmitted through a monochromator. This effectively can be used to extend the useful energy range of double crystal Bragg monochromators and to provide a spectrally pure beam for x-ray absorption fine structure and other experiments. Advantages of this device include high efficiency, simple alignment, and low cost. [See C. Karanfil et al. AIP Conf. Proc. 521, 276 (2000)].
Polarization analyzer for resonant x-ray diffraction (abstract)
Authors: Caliebe W. A. , Lenhard A. , Siddons D. P. , Pindak R.
Published: July 2002

A polarization analyzer has been designed for soft x-ray resonant diffraction experiments at the S K edge. The Bragg angle of graphite (0001) at the S K edge is 48°, which makes it perfectly suited as polarization analyzer. Two graphite (0001) crystals (12×12×2 mm3) are mounted on small tangent-arm driven spring loaded stages with their diffraction planes in the horizontal and vertical plane for σ and π analysis of polarized x rays. The diffracted x rays are detected by small peltier cooled Si detectors with dimensions 1×12 mm2 with the longer dimension in the direction of diffraction, so that the detector can remain fixed. These two polarization analyzers and an additional detector for the directly diffracted beam are mounted on a stage, so that the polarization of the diffracted x rays can be determined just by moving the appropriate analyzer into the beam. The tangent arm and the detector allow us to scan ±50 eV around the S K edge. This setup avoids mounting of the polarization analyzer on a rotation stage, whose axis has to be aligned with the diffracted beam. The tangent-arm is driven by motorized micrometers, which are equipped with encoders to obtain an angular resolution of 0.001°.
Reactive oxygen beam generation system using pulsed laser evaporation of highly concentrated solid ozone
Authors: Nishiguchi Tetsuya , Morikawa Yoshiki , Kekura Mitsuru , Miyamoto Masaharu , Nonaka Hidehiko , Ichimura Shingo
Published: July 2002

A reactive oxygen beam generation system is described for the formation of high-quality and high-precision films. This system utilizes pulsed laser evaporation of highly concentrated solidified ozone (O3). The equipment for safely generating and handling a large amount of high-purity liquid and solid O3 was also developed for this purpose. The beam is characterized by its high concentration of oxygen atoms in an excited state [O(1D)], constant flux per laser shot (4×1017 molecules cm−2 shot−1), appropriate level of kinetic energy (KE) for enhancing the surface reaction (mean KE of 0.4 eV, maximum KE of 2 eV) and small angular spread (6°). These characteristics enabled us to precisely control the SiO2 film thickness by the number of laser shots, and achieve an enhanced Si oxidation rate and new local oxidation process.
Quiet direct simulation of plasmas
Authors: Albright B. J. , Daughton W. , Lemons Don S. , Winske Dan , Jones Michael E.
Published: July 2002

A new approach to particle simulation, called “quiet direct simulation Monte Carlo” (QDSMC), is described that can be applied to many problems of interest, including hydrodynamics, magnetohydrodynamics (MHD), and the modeling of collision plasmas. The essence of QDSMC is the use of carefully chosen weights for the particles (e.g., Gauss–Hermite, for Maxwellian distributions), which are destroyed each time step after the particle information is deposited onto the grid and reconstructed at the beginning of the next time step. The method overcomes the limited dynamical range and statistical noise typically found in particle simulations. In this article QDSMC is applied to hydrodynamics and MHD test problems, and its suitability for modeling semi-collisional plasma dynamics is considered.
Emission properties of a dual ion/electron source based on Au–In alloy
Authors: Sheu B. L. , Wang Y. L.
Published: July 2002

A very stable dual ion/electron emitter based on a tungsten tip coated with Au–In alloy has been fabricated. When melted, the alloy is used as a liquid metal ion source; after being solidified under a prescribed ion emission condition, it is used as a cold field electron emitter. Compared to the first dual ion/electron source made of In-coated tungsten tip, the Au–In source exhibits much enhanced electron emission stability and maintains a point-like electron emitter to a much higher emission current. With its stable ion/electron emission properties as well as good electron emission brightness (∼2×108 A/sr⋅cm2), the Au–In dual emitter is a potential source for a single-column-focused ion/electron beam system.
Magnetic force microscopy using focused ion beam sharpened tip with deposited antiferro–ferromagnetic multiple layers
Authors: Liu Zhiyong , Dan You , Jinjun Qiu , Wu Yihong
Published: July 2002

The resolution of magnetic force microscopy can be improved by sharpening the tip using a focused ion beam (FIB). However, when the tip is too sharp it is difficult to deposit continuous films on the tip, and the magnetization of a certain portion of the tip may become unstable due to the small thickness or particle size of the magnetic coating. This in turn may degrade the signal to noise ratio of the detection signal. In this work, a novel tip coated with antiferromagnet–ferromagnet exchanged coupled multiple layers was developed. It was found that the newly developed tip exhibits a much higher resolution than the conventional trimmed and nontrimmed tips.
Lagrangian stochastic modeling of anomalous diffusion in two-dimensional turbulence
Authors: Reynolds A. M.
Published: July 2002

It is shown that at intermediate times, the Langevin equation corresponding to the nonlinear Fokker–Planck equation with exponents μ=1 and ν>1 produces trajectories with multifractal scaling and anomalous power-law dispersion, in common with observations of drifters in the ocean and numerical simulations of tracer particles in two-dimensional turbulence. The extent of this regime and the occurrence of anomalously large normal diffusion at much later times are shown to be in close agreement with dispersion data from numerical simulations of two-dimensional turbulence. In analogy with the dynamics of point vortices in two-dimensional turbulence, the modeled dynamics are nonergodic and effectively comprise of a background Ornstein–Uhlenbeck process punctuated by occasional fast long flights. It is shown that these dynamics optimize the nonextensive (Tsallis) entropy. It is tentatively suggested that the anomalous dispersion in two-dimensional turbulence is a consequence of smaller than average Lagrangian accelerations in regions of the flow with faster than average velocities.
Nonaxisymmetric flow between an air table and a floating disk
Authors: Cox Stephen M.
Published: July 2002

Steady flow in the thin air cushion that supports a disk floating above an air table is examined. The flow is assumed to be of similarity form, and a new, nonaxisymmetric flow is found, in addition to the well-known axisymmetric flow. In the limit of large air-injection rate, the new flow becomes approximately axisymmetric in the bulk, nonaxisymmetric effects being confined to the boundary layer near the disk. For a given air-injection rate and air-cushion thickness, the nonaxisymmetric flow can support a greater weight than can the axisymmetric flow.
Microstructures and mechanical properties of tungsten wire/particle reinforced Zr57Nb5Al10Cu15.4Ni12.6 metallic glass matrix composites
Authors: Choi-Yim Haein , Schroers Jan , Johnson William L.
Published: July 2002

Tungsten wire or particle reinforced metallic glass matrix composites are produced by infiltrating liquid Zr57Nb5Al10Cu15.4Ni12.6 (Vit106) into tungsten reinforcements at 1150 and at 1425 K. X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy are carried out to characterize the composite. The matrix of the composite processed at 1150 K is mostly amorphous, with some embedded crystals. During processing, tungsten dissolves in the glass-forming melt and upon quenching precipitates over a relatively narrow zone near the interface between the tungsten and matrix. In the composites processed at 1425 K, tungsten dissolves in the melt and diffuses through the liquid medium, and then reprecipitates upon quenching. The faster kinetics at this high temperature results in a uniform distribution of the crystals throughout the matrix. Mechanical properties of the differently processed composites containing wires and particles are compared and discussed. The composites exhibit a plastic strain failure of up to 16% without sacrificing the high-failure strength, which is comparable to monolithic Vit106.
Strong coupling of the single excitations in the Q-like bands of phenylene-linked free-base and zinc bacteriochlorin dimers: A time-dependent density functional theory study
Authors: Yamaguchi Yoichi , Yokoyama Shiyoshi , Mashiko Shinro
Published: July 2002

Time-dependent density-functional theory (TDDFT) calculations were carried out to predict the Q-like bands of the two structural isomers of the phenylene-linked free-base (FBBC) and zinc (ZnBC) bacteriochlorin dimers. The calculated singlet excitation energies and oscillator strengths for the low-lying excited states of the reference monomers, FBBC and ZnBC, are quite consistent with recent ab initio calculations. The 1,3 and 1,4-phenylene-linked dimers have monomer-like Q bands and new Q′ bands comprised of the cross-linked (i.e., charge transfer) excitations from the FBBC (ZnBC) ring to the ZnBC (FBBC) ring, whose excitation energies are slightly and considerably red-shifted by about 0.03 eV and 0.5–0.7 eV compared to the monomers, respectively. The monomer-like Qx bands are of the mixed highest occupied molecular orbital (HOMO)→lowest unoccupied molecular orbital (LUMO) excitations in the Qx bands from the different bacteriochlorin rings, whose origin is the coupling of the Qx transition dipole moments through the π–π interaction between the unreduced pyrroles situated across the phenylene group. This mixing character is still maintained in the monomers separated by the same distance in the dimer, which is quite different from previous TDDFT calculations for the phenylene-linked free-base and zinc porphyrin dimers. The present dimers may show a more efficient Qx excitation energy transfer between the bacteriochlorins via each’s through-space overlapped LUMOs than the conventional porphyrin dimers.
Effect of hot electrons on stimulated Compton scattering of a laser in a self-sustained plasma channel
Authors: Upadhyay A. , Tripathi V. K. , Pant H. C.
Published: July 2002

The presence of drifting energetic electrons in laser produced plasmas influences the stimulated Compton scattering process in two ways. First, it weakens the guiding process as these electrons quickly pass through the length of the channel before they could be driven out of the channel by the ponderomotive force. Thus a self-guided laser could have a larger spot size at a given power. Second, energetic electrons cause stronger Landau damping of the Langmuir wave produced in the Compton process, leading to strengthening of the growth rate. The nonlocal effects reduce the growth rate.
The role of neutrals in high-mode (H-mode) pedestal formation
Authors: Groebner R. J. , Mahdavi M. A. , Leonard A. W. , Osborne T. H. , Porter G. D. , Colchin R. J. , Owen L. W.
Published: July 2002

An analytic model, derived from coupled continuity equations for the electron and neutral deuterium densities, is consistent with many features of edge electron density profiles in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research, 1986, Vol. I (International Atomic Energy Agency, Vienna, 1987), p. 159]. For an assumed constant particle diffusion coefficient, the model shows that particle transport and neutral fueling produce electron and neutral density profiles that have the same characteristic scale lengths at the plasma edge. For systematic variations of density in high-mode (H-mode) discharges, the model predicts that the width of the electron density transport barrier decreases and the maximum gradient increases, as observed in the experiments. The widths computed from the model agree quantitatively with the experimental widths for conditions in which the model is valid. These results support models of transport barrier formation in which the H-mode particle barrier is driven by the edge particle flux and the width of the barrier is approximately the neutral penetration length.
Isotropic collision induced light scattering spectra from gaseous SF6
Authors: Godet J.-L. , Rachet F. , Le Duff Y. , Nowicka K. , Bancewicz T.
Published: July 2002

The experimental binary isotropic collision-induced light scattering spectrum of the gaseous sulfur hexafluoride is measured in absolute units in the 30–210 cm−1 frequency range. The contribution of dipole–multipole mechanisms is computed in a semi-classical way. From comparison with experiment, the independent component E of the dipole–octopole polarizability tensor is estimated. This evaluation is compared to a recent theoretical ab initio calculation.
Laser envelope solitons in cold overdense plasmas
Authors: Poornakala S. , Das A. , Sen A. , Kaw P. K.
Published: July 2002

Some questions pertaining to the existence and nature of one-dimensional envelope pulse solitons propagating into an overdense plasma are examined by a numerical investigation of the relativistic cold plasma equations. Finite amplitude single hump solitons with significant density cavitation are obtained for both immobile and mobile ions. For the immobile ion case the eigenvalue spectrum has a continuum nature and there is a smooth transition from standing single pulse solitons to moving solitons. A composite spectrum of moving multipeak solitons is also obtained and approximate analytical estimates of their amplitudes are provided.
Linear coupling of electromagnetic and Jeans modes in self-gravitating plasma streams
Authors: Yaroshenko Victoria V. , Voitenko Yuriy , Goossens Marcel
Published: July 2002

A new mechanism of linear coupling between electromagnetic (nonpotential) and gravitational disturbances is found for oblique propagation relatively to particle streams. The general dispersion law is derived and applied to the case of two countersteaming dust beams of equal strength and quiasiperpendicular propagation. It reveals a strong linear coupling between the low-frequency aperiodically unstable electromagnetic (AEM) and the Jeans (JM) modes. The coupling is of a mode conversion type, resulting in a frequency gap in the dispersion, and thus significantly modifies the instability criteria. It is shown that, in contrast to the electrostatic case, AEM and JM coupling in streaming self-gravitating plasmas can actually appear even if the plasma frequencies of the dust species greatly exceed the corresponding Jeans frequencies.
Memory effects related to deep levels in metal–oxide–semiconductor structure with nanocrystalline Si
Authors: Kwon Young Hae , Park C. J. , Lee W. C. , Fu D. J. , Shon Y. , Kang T. W. , Hong C. Y. , Cho H. Y. , Wang Kang L.
Published: July 2002

Nanocrystalline (nc) -Si was grown on SiO2 by rapid thermal chemical vapor deposition. The tunneling oxide layer of a thickness of 4 nm was formed on p-type Si(100) by rapid thermal oxidation at 1050 °C for 30 s. Metal–oxide–semiconductor (MOS) structures were fabricated and capacitance–voltage characterization was carried out to study the memory effects of the nc-Si embedded in the MOS structure. We found the memory effect to be dominantly related to hydrogen-related traps, in addition to being influenced by the three-dimensional quantum confinement and Coulomb charge effects. Deep level transient spectroscopy reveal that the activation energies of the hydrogen-related traps are Ev+0.29 eV (H1) and Ev+0.42 eV (H2), and the capture cross sections are 4.70×10−16 cm2 and 1.44×10−15 cm2, respectively. The presence of Si–H and Si–H2 bonds was confirmed by Fourier transform infrared spectroscopy.
Controlling chaos in the current-driven ion acoustic instability
Authors: Fukuyama T. , Taniguchi K. , Kawai Y.
Published: July 2002

Control of intermittent chaos caused by the current-driven ion acoustic instability is attempted and the controlling mechanism is investigated. When a small negative dc voltage is applied to the chaotic system as a perturbation, the system changes from a chaotic state to a periodic state while maintaining the instability, indicating that the chaotic state caused by the ion acoustic instability is well controlled by applying a small negative dc voltage. A hysteresis structure is observed on the V–I curve of the mesh grid to which the negative dc voltage to control is applied. Furthermore, when a negative dc voltage is applied to the state which shows a laminar structure existing under same experimental conditions, the system becomes chaotic via a bifurcation. Driven-chaos is excited when a negative dc voltage is applied to the laminar state. Applying a small negative dc voltage leads to controlling intermittent chaos while exciting driven-chaos.
Hydrogen storage in different carbon nanostructures
Authors: Ritschel M. , Uhlemann M. , Gutfleisch O. , Leonhardt A. , Graff A. , Täschner Ch. , Fink J.
Published: July 2002

Carbon nanostructures of different kinds have been synthesized by chemical vapor deposition. By modifying the deposition temperature, the catalyst material, and the hydrocarbon, nanofibers with herringbone structure, multi-walled nanotubes with tubular structure, and single-walled nanotubes were deposited. The nanostructures were purified with different treatment methods. The carbon nanostructures were characterized using scanning and transmission electron microscopy. The hydrogen storage capability was investigated for all obtained nanostructures. The measurements show that the storage capacity of hydrogen is very limited in all the carbon nanostructures.
Interaction of runaway electrons with lower hybrid waves via anomalous Doppler broadening
Authors: Martín-Solís J. R. , Sánchez R. , Esposito B.
Published: July 2002

Due to the relativistic decrease of the electron cyclotron frequency, a cyclotron resonance may appear between runaway electrons and lower hybrid waves. A single particle description of the runaway dynamics [J. R. Martín-Solís et al., Phys. Plasmas 5, 2370 (1998)] is extended to analyze the effect of the interaction of runaway electrons with lower hybrid waves via anomalous Doppler broadening. The conditions under which the resonant interaction can play a role in limiting the runaway energy are established and it is shown that, under typical lower hybrid current drive operation parameters, an efficient wave-particle coupling may occur. Observations of a fast pitch angle scattering event during the current decay phase of Ohmic discharges in the Toroidal Experiment for Technically Oriented Research (TEXTOR) [R. J. E. Jaspers, Ph.D. thesis, Technical University Eindhoven (1995)] are interpreted in terms of such interaction.
Global optimization analysis of water clusters (H2O)n (11⩽n⩽13) through a genetic evolutionary approach
Authors: Guimarães Freddy F. , Belchior Jadson C. , Johnston Roy L. , Roberts Christopher
Published: July 2002

The structures and stabilities of water clusters (H2O)n with 11⩽n⩽13 are determined by a genetic algorithm approach with two new evolutionary operators—namely annihilator and history operators. These studies show that the modified genetic algorithm provides an efficient procedure for calculating global minima with an especial attention to molecular water clusters. The actual results are in quantitative agreement with previous calculations using the basin hopping Monte Carlo method.
Investigations of the electron field emission properties and microstructure correlation in sulfur-incorporated nanocrystalline carbon thin films
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: July 2002

Results are reported on the electron field emission properties of sulfur (S)-incorporated nanocrystalline carbon (n-C:S) thin films grown on molybdenum (Mo) substrates by hot-filament chemical vapor deposition (HFCVD) technique. In addition to the conventionally used methane (CH4) as carbon precursor with high hydrogen (H2) dilution, hydrogen sulfide–hydrogen (H2S)/H2 premix gas was used for sulfur incorporation. The field emission properties for the S-incorporated films were investigated systematically as a function of substrate temperature (TS) and sulfur concentration. Lowest turn-on field achieved was observed at around 4.0 V/μm for the n-C:S sample grown at TS of 900 °C with 500 ppm of H2S. These results are compared with those films grown without sulfur (n-C) at a particular TS. The turn-on field was found to be almost half for the S-assisted film thus demonstrating the effect of sulfur addition to the chemical vapor deposition process. An inverse relation between turn-on field (EC), growth temperature and sulfur concentration was found. The S incorporation also causes significant microstructural changes, as characterized with non-destructive complementary ex situ techniques: scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy (RS). S-assisted films show relatively smoother and finer-grained surfaces than those grown without it. These findings are discussed in terms of the dual role of sulfur in enhancing the field emission properties by controlling the sp2 C cluster size and introducing substantial structural defects through its incorporation. The in-plane correlation length (La) of sp2 C cluster was determined from the intensity ratio of the D- and G-bands [I(D)/I(G)] in the visible RS as a function of deposition temperature and sulfur concentration using a phenomenological model. The turn-on field was found to decrease with increasing sp2 C cluster size in general ranging from 0.8 to 1.4 nm. The films having sp2 C clusters of around 1.4 nm had the lowest turn-on field and steep rising emission currents, providing an estimate of optimum size for La for the material grown hereby. These findings are assessed in terms of a reduced field emission barrier brought about by the sulfur addition and the need for relatively longer conductive paths capable of withstanding the relatively large emission currents. It is because the sp2 C cluster size predominate the chemical environment, chemical order, sp3 content or local conductivity. Besides, although most of the S is expected to be electrically inactive, under the high doping conditions (larger S/C) hereby employed, there may be some amount of S in donor states, an indication of the availability of conduction electrons. These results also suggest that the behaviors of sulfur-incorporated nanocrystalline carbon thin films are closer to that grown with phosphorus (P) and Nitrogen (N) elements.
Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in situ SixNy masking
Authors: Dadgar A. , Poschenrieder M. , Bläsing J. , Fehse K. , Diez A. , Krost A.
Published: July 2002

Thick, entirely crack-free GaN-based light-emitting diode structures on 2 in. Si(111) substrates were grown by metalorganic chemical-vapor deposition. The ∼2.8-μm-thick diode structure was grown using a low-temperature AlN:Si seed layer and two low-temperature AlN:Si interlayers for stress reduction. In current–voltage measurements, low turn-on voltages and a series resistance of 55 Ω were observed for a vertically contacted diode. By in situ insertion of a SixNy mask, the luminescence intensity is significantly enhanced. A light output power of 152 μW at a current of 20 mA and a wavelength of 455 nm is achieved.