bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for preferential concentration
Authors: Sigurgeirsson H. , Stuart A. M.
Published: November 2002

The preferential concentration of inertial particles in a turbulent velocity field occurs when the particle and fluid time constants are commensurate. We propose a straightforward mathematical model for this phenomenon and use the model to study various scaling limits of interest and to study numerically the effect of interparticle collisions. The model comprises Stokes’ law for the particle motions, and a Gaussian random field for the velocity. The primary advantages of the model are its amenability to mathematical analysis in various interesting scaling limits and the speed at which numerical simulations can be performed. The scaling limits corroborate experimental evidence about the lack of preferential concentration for a large and small Stokes number and make new predictions about the possibility of preferential concentration at large times and lead to stochastic differential equations governing this phenomenon. The effect of collisions is found to be negligible for the most part, although in some cases they have an interesting antidiffusive effect.
Changes in flexibility upon binding: Application of the self-consistent pair contact probability method to protein-protein interactions
Authors: Canino Lawrence S. , Shen Tongye , McCammon J. Andrew
Published: November 2002

We extend the self-consistent pair contact probability method to the evaluation of the partition function for a protein complex at thermodynamic equilibrium. Specifically, we adapt the method for multichain models and introduce a parametrization for amino acid-specific pairwise interactions. This method is similar to the Gaussian network model but allows for the adjusting of the strengths of native state contacts. The method is first validated on a high resolution x-ray crystal structure of bovine Pancreatic Phospholipase A2 by comparing calculated B-factors with reported values. We then examine binding-induced changes in flexibility in protein–protein complexes, comparing computed results with those obtained from x-ray crystal structures and molecular dynamics simulations. In particular, we focus on the mouse acetylcholinesterase:fasciculin II and the human α-thrombin:thrombomodulin complexes.
Hypersonic characterization of sound propagation velocity in AlxGa1−xN thin films
Authors: Riobóo R. J. Jiménez , Rodrı́guez-Cañas E. , Vila M. , Prieto C. , Calle F. , Palacios T. , Sánchez M. A. , Omnès F. , Ambacher O. , Assouar B. , Elmazria O.
Published: November 2002

Sound propagation velocity of surface acoustic waves (SAWs) and bulk acoustic waves (BAWs) has been investigated by means of high resolution Brillouin spectroscopy. The results show a linear dependence of the BAW propagation velocity with the Al concentration. There is no relevant influence of the substrate chosen in the propagation velocity of BAWs in AlxGa1−xN thin films. SAW propagation velocity of epitaxially grown AlN is clearly lower than the observed one in AlN prepared by reactive dc magnetron sputtering. Numerical simulation results of SAW propagation velocity are compared with the experimental results.
Two-dimensional ion flux distributions in inductively coupled plasmas: Effect of adding electronegative gases to Ar
Authors: Kim Tae Won , Aydil Eray S.
Published: November 2002

A two-dimensional array of planar Langmuir probes built on a 200 mm diam silicon wafer was used to measure the radial and azimuthal variation of ion flux impinging on the wafer surface in Ar/SF6 and Ar/Cl2 discharges maintained in an inductively coupled plasma etching reactor. The spatial variation of ion flux in a pure Ar discharge is approximately radially symmetric and peaks at the center of the wafer for pressures between 10 and 60 mTorr. The spatially averaged ion flux in a pure Ar discharge increases with increasing pressure and the corresponding uniformity degrades with increasing pressure within the pressure range studied. Addition of small amounts of electronegative gases to an Ar discharge flattens the radial and azimuthal ion flux distribution and accentuates azimuthal variations due to subtle asymmetries in the reactor geometry such as pumping ports. At fixed power, pressure, and flow rate, the spatially averaged ion current density decreases with increasing mole fraction of the electronegative gases in the feed gas.
Temperature dependence of the band gap of GaAsSb epilayers
Authors: Lukic-Zrnic R. , Gorman B. P. , Cottier R. J. , Golding T. D. , Littler C. L. , Norman A. G.
Published: November 2002

We have optically characterized a series of GaAs1−xSbx epilayers (0.19<x<0.71) grown by molecular bean epitaxy on semi-insulating GaAs substrates, with surface orientations of (001), (001) 8° toward (111)B, (001) 8° toward (111)A, and (115)B. For each of these samples, we have investigated the absorption as a function of temperature (4 K<T<300 K) using Fourier transform infrared spectroscopy techniques. The band gap at each temperature was determined from the photon energy dependence of the absorption coefficient and compared with theoretical predictions. From our results we have obtained the Varshni coefficients, α=(4.2±0.1)×10−4 eV/K and β=(189±9) K, which describe well not only the temperature dependence of the band gap for the entire alloy range of our samples, but also for the past experimental work of others. These values differ significantly from what we believe are the only other reported values by K. G. Merkel et al. [K. G. Merkel et al., Appl. Phys. Lett. 65, 2442 (1994)].
Electron temperature and mechanisms of hot carrier generation in quantum cascade lasers
Authors: Harrison P. , Indjin D. , Kelsall R. W.
Published: November 2002

A technique for calculating the temperature of the nonequilibrium electron distribution functions in general quantum well intersubband devices is presented. Two recent GaAs/Ga1−xAlxAs quantum cascade laser designs are considered as illustrative examples of the kinetic energy balance method. It is shown that at low current densities the electron temperature recovers the expected physical limit of the lattice temperature, and that it is also a function of current density and the quantised energy level structure of the device. The results of the calculations show that the electron temperature Te can be approximated as a linear function of the lattice temperature Tl and current density J, of the form Te=Tl+αe−lJ, where αe−l is a coupling constant (∼6–7 K/kA cm−2 for the devices studied here) which is fixed for a particular device.
Determination of the valence band dispersions for Bi2Se3 using angle resolved photoemission
Authors: Greanya V. A. , Tonjes W. C. , Liu Rong , Olson C. G. , Chung D.-Y. , Kanatzidis M. G.
Published: November 2002

The valence band dispersions for Bi2Se3 have been determined using angle resolved photoelectron spectroscopy. The experimentally determined band dispersions are compared with theoretical band structure calculations. In general, good agreement between experiment and theory was found. However, energy bands in the Γ-Z direction are significantly flatter than those predicted by theory.
Microwave and magnetic properties of double-sided hexaferrite films on (111) magnesium oxide substrates
Authors: Wang S. G. , Yoon S. D. , Vittoria C.
Published: November 2002

We have deposited hexaferrite films of BaFe12O19 on both sides of a magnesium oxide, (111) MgO, substrate by the liquid phase epitaxy (LPE) deposition technique. The procedure entailed the deposition of seed layers of BaFe12O19 onto the substrate by laser ablation deposition and then dipping of the substrate into a molten flux. Whereas the seed layers were the order of 0.5 μm, the total thickness after LPE deposition was ∼45 μm. The ferrimagnetic resonance linewidth at 60 GHz, 27 Oe, is as low as it has ever been measured before in BaFe12O19 to the best of our knowledge. The static field measurements of the saturation magnetization and anisotropy field were in agreement with previous measured bulk values.
Relativistic correlating basis functions for the Ga–Kr, In–Xe, and Tl–Rn atoms
Authors: Osanai You , Noro Takeshi , Miyoshi Eisaku
Published: November 2002

Generally contracted Gaussian-type function sets were developed for relativistic correlating functions of the Ga–Kr, In–Xe, and Tl–Rn atoms using an atomic natural orbital (ANO) approach. The ANOs were constructed based on configuration interaction calculations of the ground states of respective atoms including the major relativistic effects through model core potentials (MCPs). Both atomic and molecular applications are presented. The use of the present basis sets in conjunction with the MCPs gave the excitation energies, ionization potentials, and electron affinities in good agreement with the observed values for several atoms. The spectroscopic constants of the TlH molecule yielded by relativistic MCP calculations including spin–orbit effects were in excellent agreement with the experimental results.
Intermolecular-interaction effects on quantum-phase dynamics of dimer systems interacting with a two-mode squeezed vacuum field
Authors: Nakano Masayoshi , Yamaguchi Kizashi
Published: November 2002

We investigate the intermolecular-interaction (dipole–dipole interaction) effects on the quantum dynamics of dimer density matrices and photon-phase distributions using several dimer models with different intermolecular distances in the presence of a two-mode squeezed vacuum field. In this photon field, each mode is initially correlated and the reduced one-mode photon distribution is equivalent with that of a thermal field. For comparison, we perform parallel studies, in which the initial fields are two types of noncorrelated two-mode fields, i.e., a two-mode coherent field and a two-mode thermal field. It is found that although the two-mode squeezed vacuum field causes the random oscillations of dimer populations in the noninteracting dimer, the periodic oscillations like the collapse–revival behavior emerge as the intermolecular distance decreases (the intermolecular interaction increases). Similar and dissimilar features among quantum dynamics caused by these three types of fields are investigated by analyzing the dynamical behavior of two-mode Pegg–Barnett photon-phase distributions and off-diagonal dimer density matrices, which indicate the coherency between dimer states. In addition to the quantum statistical properties of initial photon field, the change in the degree of contribution between one- and two-photon processes caused by the intermolecular interaction is found to be important to determine these features.
Reaction probabilities and reaction cross sections for three-dimensional He+H2+(v) collisions: A time-dependent quantum mechanical study
Authors: Maiti Biswajit , Kalyanaraman C. , Panda Aditya Narayan , Sathyamurthy N.
Published: November 2002

Vibrational (v) state-selected reaction probabilities (PvjJ) have been calculated for the reaction He+H2+(v=0,1,2,3)→HeH++H, for H2+ in its ground rotational state (j=0) for a range of total angular momentum (J) values, for total energy (E) in the range 0.94–1.4 eV, using time-dependent quantum mechanical (TDQM) approach and centrifugal sudden approximation. The number of oscillations in Pv0J(E) decreases with increase in J. In addition, there is a noticeable increase in the threshold energy (Eth) with increase in J. The magnitude of Pv0J decreases with increase in J resulting in converged cross section values by the time J becomes 35–45, depending upon v and E under investigation. The resulting (converged) reaction cross section values are in excellent agreement with the experimental results at E=1.14 eV, both in terms of the magnitude and in terms of the observed vibrational enhancement. Interestingly, there is a noticeable discrepancy between our TDQM results and earlier time-independent quantum mechanical studies at higher energies, for v=1 and 2. For v=3, the difference between the two theoretical studies is the largest near the threshold. Our results for v=2 and 3 suggest that the oscillations in the vibrational state-selected reaction cross section values are amenable to experimental observation.
Finite size effects in pressure measurements for Monte Carlo simulations of lattice polymer models
Authors: Stukan M. R. , Ivanov V. A. , Müller M. , Paul W. , Binder K.
Published: November 2002

We report on a careful analysis of finite size effects on pressure measurements in Monte Carlo computer simulations of isotropic athermal solutions of flexible polymer chains by means of the repulsive wall method. We find finite size corrections to the pressure due to surface effects. These corrections are inversely proportional to the thickness of the simulation box, both if we keep the average density (in the canonical ensemble) or the chemical potential (in the grand canonical ensemble) constant in course of the preparation of the starting conformation. We propose a modification of the repulsive wall method which allows avoidance of these finite size effects and to estimate the pressure for an infinite system when running simulations in a finite box.
Model for viscoelasticity in a binary mixture
Authors: Harbola Upendra , Das Shankar P.
Published: November 2002

The viscoelastic response of a binary mixture in the dense supercooled state is studied close to the dynamic transition point of the self-consistent mode-coupling theory. Relaxation of the transverse current correlation functions for the isotropic liquid shows a crossover from the propagating shear waves to diffusive shear mode at a critical wave number q0. q0 decreases with increasing density indicating growing correlations, similar to what is observed in computer simulation studies of the same systems. Anomalous stretching of the frequency-dependent shear modulus G(ω) with density is also observed.
Complexity of classical dynamics of molecular systems. I. Methodology
Authors: Nerukh Dmitry , Karvounis George , Glen Robert C.
Published: November 2002

Methods for the calculation of complexity have been investigated as a possible alternative for the analysis of the dynamics of molecular systems. “Computational mechanics” is the approach chosen to describe emergent behavior in molecular systems that evolve in time. A novel algorithm has been developed for symbolization of a continuous physical trajectory of a dynamic system. A method for calculating statistical complexity has been implemented and tested on representative systems. It is shown that the computational mechanics approach is suitable for analyzing the dynamic complexity of molecular systems and offers new insight into the process.
Complexity of classical dynamics of molecular systems. II. Finite statistical complexity of a water–Na+ system
Authors: Nerukh Dmitry , Karvounis George , Glen Robert C.
Published: November 2002

The computational mechanics approach has been applied to the orientational behavior of water molecules in a molecular dynamics simulated water–Na+ system. The distinctively different statistical complexity of water molecules in the bulk and in the first solvation shell of the ion is demonstrated. It is shown that the molecules undergo more complex orientational motion when surrounded by other water molecules compared to those constrained by the electric field of the ion. However the spatial coordinates of the oxygen atom shows the opposite complexity behavior in that complexity is higher for the solvation shell molecules. New information about the dynamics of water molecules in the solvation shell is provided that is additional to that given by traditional methods of analysis.
ND
4
+
and NH3D+ dynamics in ammonium persulphate. II. Transition from low-to-high-temperature regime
Authors: Schmidt T. , Schmitt H. , Haeberlen U. , Olejniczak Z. , Lalowicz Z. T.
Published: November 2002

The transition of the dynamics of ND4+ ions in ammonium persulphate, dominated at low temperatures by coherent uniaxial rotational tunnelling about one specific N–D bond (the preferred bond) and, at high temperatures, by frequent stochastic jumps about all N–D bonds is elucidated with deuteron spin-lattice relaxation measurements, selective saturation experiments and deuteron NMR line shape analyses. Between 20 and 35 K, the coherent uniaxial tunnelling is superseded by thermally activated stochastic jumps about the same bond with kinetic parameters kdyn0=10(11.5±0.5) s−1 and Edyna=(3.6±0.3) kJ/mol. At higher temperatures, thermally activated stochastic jumps about the other N–D bonds set in. Their kinetic parameters are kst0=10(12.2±0.5) s−1 and Esta=(7.8±0.5) kJ/mol. From the primary and secondary tunnelling observed at low temperatures we infer the heights of the potentials which hinder rotations of the ND4+ ions about the preferred and any other N–D bond. These heights, minus the rotator’s ground state energy, are about 25% larger than, respectively, Edyna and Esta. The kinetic parameters of the two stochastic processes are such that the essentially uniaxial coherent and then incoherent dynamics at low temperatures is superseded at the decomposition temperature of the compound by stochastic reorientational jumps which reflect the basic tetrahedral symmetry of the ammonium ion.
X-ray photoemission determination of the Schottky barrier height of metal contacts to n–GaN and p–GaN
Authors: Rickert K. A. , Ellis A. B. , Kim Jong Kyu , Lee Jong-Lam , Himpsel F. J. , Dwikusuma F. , Kuech T. F.
Published: November 2002

Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the surface Fermi level position within the band gap for thin metal overlayers of Au, Al, Ni, Ti, Pt, and Pd on n–GaN and p–GaN. Nonequilibrium effects were taken into account by measuring the Fermi edge of the metal overlayer. There are two different behaviors observed for the six metals studied. For Au, Ti, and Pt, the surface Fermi level lies about 0.5-eV higher in the gap for n-type than for p-type GaN. For Ni, Al, and Pd, the surface Fermi level position is independent of doping, but varies from one metal to the other. Results for Ni, Pd, and Al fit a modified Schottky–Mott theory, while Au, Ti, and Pt demonstrate a more complex behavior. Atomic force microscopy was used along with photoemission to investigate the growth mode of each metal on the GaN surface.
Dielectric relaxation behavior and its relation to microstructure in relaxor ferroelectric polymers: High-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) copolymers
Authors: Cheng Z.-Y. , Zhang Q. M. , Bateman Fred B.
Published: November 2002

The dielectric behavior of the high-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) 68/32 mol % copolymer was characterized in the dose range from 0 to 175 Mrad. It was found that as the dose increases, the copolymer evolves from a normal ferroelectric to a relaxor ferroelectric and then to a simple relaxor. Correspondingly, the crystalline morphology changes from a two-phase coexistence between a polar and nonpolar phases at low doses (<50 Mrad) to a macroscopically uniform nonpolar phase for doses beyond 50 Mrad, as revealed by x-ray data. Interestingly, it was observed that in the whole dose range, the Vogel–Fulcher (V–F) relationship could describe the measured relationship between the frequency and the peak temperature of dielectric constant well. For the copolymers irradiated with dose near 75 Mrad, which show a typical relaxor ferroelectric response, the V–F behavior is a result of freezing of the relaxation time (into polar glass state). For the copolymers irradiated with doses lower that 50 Mrad where a polar phase and nonpolar phase coexist, the dielectric data at near the ferroelectric-paraelectric temperature show a dielectric peak with V–F type relaxation followed by a transition from a relaxor ferroelectric and normal ferroelectric as the temperature is lowered, suggesting a possible freezing process even in this dose range.
Characterization of the response of chromium-doped alumina screens in the vacuum ultraviolet using synchrotron radiation
Authors: McCarthy K. J. , Baciero A. , Zurro B. , Arp U. , Tarrio C. , Lucatorto T. B. , Moroño A. , Martín P. , Hodgson E. R.
Published: November 2002

We have measured the response of chromium-doped alumina screens to vacuum ultraviolet radiation and derived quantum efficiency curves for the energy range from 30 to 300 eV. A model is presented to explain the structure in this curve. In addition, the radiation hardness of such screens, which have found application as narrow-band radiation detectors for a hot fusion plasma diagnostic, is reported here for MeV electrons. Finally, a simple model is constructed to obtain the carrier diffusion length and the bulk efficiency of this material.
Dynamics of laser-ablation Ti plasmas studied by laser-induced fluorescence imaging spectroscopy
Authors: Sasaki K. , Matsui S. , Ito H. , Kadota K.
Published: November 2002

The distributions of Ti+ and Ti densities in laser-ablation Ti plasmas were visualized by laser-induced fluorescence imaging spectroscopy. The absolute Ti density was determined by ultraviolet absorption spectroscopy. The absolute Ti+ density was evaluated by comparing the laser-induced fluorescence intensity from Ti+ with that from Ti. In addition, we evaluated the total numbers of Ti+ and Ti in plasmas by integrating the density distributions spatially. As a result, it was found that the total number of Ti+ was higher than that of Ti in the initial stage of laser-ablation plasmas. In ambient He gas, we observed the temporal increase in the total number of Ti, indicating the gas-phase production of Ti due to neutralization of Ti+. The increase in the total number of Ti+ was also observed, suggesting the production of Ti+ due to neutralization of Ti2+. The present experimental results show that Ti+ and Ti2+ are major species produced by laser ablation of a metal Ti target.
Formation of hydrogenated carbon nitride films by reactive sputtering
Authors: Wei J.
Published: November 2002

Hydrogenated carbon nitride films have been grown on Si (100) substrates using magnetron sputtering. Reactive deposition is achieved using a graphite target in an argon/nitrogen/hydrogen plasma at room temperature. These films are characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. At lower-hydrogen fraction, the films include 10–20 nm β–C3N4 crystal grains which are identified by TEM analysis. With a further increase in hydrogen fraction, the films change to be totally amorphous. AFM measurement indicates the hydrogenated carbon nitride films have low-surface roughness. From XPS data, the hydrogen promotes the C–H and N–H bonds. XPS spectra of the films typically shows three major peaks in the C 1s core-level spectrum (centered at 284.64, 285.96, and 287.45 eV) and three major peaks in the N 1s core-level spectrum (centered at 398.3, 399.1, and 400.2 eV). FTIR spectra showed five absorption bands in the range of 1000–3500 cm−1. The absorption band between 3100 and 3500 cm−1 can be assigned to the N–H bond. The absorption band between 2800 and 3000 cm−1 can be assigned to the C–H bond. The absorption band around 2200 cm−1 can be attributed to the C≡N nitrile bond. The absorption bands around 1600 and 1080 cm−1 demonstrate the existence of C=NH (sp2) and C–NH (sp3) bonds. The analysis of Raman spectrum further demonstrates the coexistence of these chemical bonds.
Microwave measurements of the photonic band gap in a two-dimensional photonic crystal slab
Authors: Hickmann J. M. , Solli D. , McCormick C. F. , Plambeck R. , Chiao R. Y.
Published: November 2002

We have measured the photonic band gap in the transmission of microwaves through a two-dimensional photonic crystal slab. The structure was constructed by cementing acrylic rods in a hexagonal array to form rectangular stacks. We find a band gap centered at approximately 11 GHz, whose depth, width, and center frequency vary with the number of layers in the slab, angle of incidence, and microwave polarization.
Ground state and excitation dynamics in Ag doped helium clusters
Authors: Mella Massimo , Colombo Maria Carola , Morosi Gabriele
Published: November 2002

We present a quantum Monte Carlo study of the structure and energetics of silver doped helium clusters AgHen for n up to 100. Our simulations show the first solvation shell of the Ag atom to include roughly 20 He atoms, and to possess a structured angular distribution. Moreover, the P1/22←2S1/2 and P3/22←2S1/2 electronic transitions of the embedded silver impurity have been studied as a function of the number of helium atoms. The computed spectra show a redshift for n⩽15 and an increasing blueshift for larger clusters, a feature attributed to the effect of the second solvation shell of He atoms. For the largest cluster, the computed excitation spectrum is found in excellent agreement with the ones recorded in superfluid He clusters and bulk. No signature of the direct formation of the proposed AgHe2 exciplex is present in the computed spectrum of AgHe100. To explain the absence of the fluorescent D2 line in the experiments, a relaxation mechanism between the P3/22 and the P1/2He2 states is proposed on the basis of the partial overlap of the excitation bands in the simulated spectra.
The effects of high-frequency ultrasound on turbulent liquid mixing with a rapid chemical reaction
Authors: Ito Yasumasa , Nagata Kouji , Komori Satoru
Published: November 2002

The effects of high-frequency ultrasound and mean fluid shear on turbulent mixing with a rapid chemical reaction were experimentally investigated in three types of liquid mixing-layer flow downstream of a turbulence-generating grid; pure grid-generated turbulence, grid-generated turbulence with high-frequency ultrasonic irradiation, and grid-generated turbulence with mean fluid shear. Instantaneous velocity and concentration were simultaneously measured using the combination of a laser-Doppler velocimeter and a laser-induced fluorescence method. The results show that turbulent mixing and chemical reaction are promoted by ultrasonic irradiation and mean fluid shear. The amount of chemical product in grid-generated turbulence with high-frequency ultrasonic irradiation is much larger than that in grid-generated turbulence with mean fluid shear, despite turbulent mass transport being enhanced at an equivalent level in both flows. This is attributed to the difference in turbulent mass transport at small scales. Ultrasonic irradiation more enhances the mass transport at smaller scales than by mean fluid shear, whereas mean shear can promote the mass transport only at larger scales. As a result, high-frequency ultrasound can be regarded as a better tool for promoting turbulent mixing and chemical reaction than mean fluid shear.
Monte Carlo calculation of second and third virial coefficients of linear and star polymers on lattice
Authors: Shida Kazuhito , Ohno Kaoru , Kawazoe Yoshiyuki , Nakamura Yo
Published: November 2002

An efficient algorithm for counting contributing terms in the calculation of second and third virial coefficients of the lattice polymer model was proposed. The algorithm was applied to linear and three-arm star polymers. The algorithm’s efficiency was demonstrated, and the obtained results were compared to both experimental and computational results already reported. To the authors’ best knowledge, the estimation of the third virial coefficient of the three-arm star polymer is the first reported.
Finite amplitude Faraday waves induced by a random forcing
Authors: Repetto R. , Galletta V.
Published: November 2002

In the present contribution we study the waves arising on the free surface of a liquid in a rectangular container undergoing vertical oscillations. Our aim in the work is to investigate the role of a random forcing characterized by a narrow-band spectrum on the wave amplitude close to subharmonic resonant conditions. The analysis is carried out theoretically by means of a weakly nonlinear analysis, assuming the ratio a0 between the acceleration of the tank and the gravitational acceleration to be small. We consider irrotational flow and take into account viscous effects by adding a linear dissipative term to the amplitude equation following Miles [“Nonlinear Faraday resonance,” J. Fluid Mech. 146, 285 (1984)]. Comparing the results with those obtained in the case of a monochromatic forcing, it appears that the range of unstable frequencies significantly widens. This finding agrees with the theoretical results found in the linear context by Zhang, Casademunt, and Viñals [“Study of the parametric oscillator driven by a narrow band noise to model the response of a fluid surface to time-dependent accelerations,” Phys. Fluids A 5, 3147 (1993)]. The maximum equilibrium amplitude of the free surface waves for the random forcing case turns out to be smaller than that of the monochromatic forcing and it decreases as the spectrum width is increased.
Shock-induced ignition for a two-step chain-branching kinetics model
Authors: Sharpe Gary J.
Published: November 2002

The ignition of reactive materials by a shock wave, when the chemistry is governed by chain-branching kinetics, is investigated using a combination of high activation temperature asymptotics and numerical simulations. A two-step chemical model is used. The first step is a thermally neutral induction time, representing chain initiation and chain branching, which has a temperature-sensitive Arrhenius form of the reaction rate. At the end of the induction time is a transition point where the fuel is instantaneously converted into chain radicals. The second step is a temperature-insensitive exothermic reaction, representing chain recombination. It is found that the initiation process is qualitatively different from that for a temperature-sensitive one-step reaction considered previously. Three different cases, when the rate of heat release is slow, comparable and fast compared to the initial induction time, are considered. In each case ignition first occurs at the piston and the transition point (which marks the end of the induction zone and the start of the main heat release zone) initially propagates away from the piston at subsonic speeds, so that pressure and temperature disturbances from the exothermic region overtake the transition path and accelerate it. For rapid rates of heat release, a secondary shock is very promptly formed near the piston, which is subsequently amplified into a strong detonation propagating through the induction zone behind the leading shock. However, unlike for one-step kinetics the formation of the secondary shock does not involve quasisteady weak detonations. For moderate rates of heat release a secondary shock still eventually forms at the front of the disturbed region of the induction zone behind the leading shock, but a detonation is not formed until after the collision of the shocks. When the rate of heat release is slow, the transition point is continuously accelerated, but its speed remains subsonic until disturbances due to the heat release overtake the shock. No secondary shock forms for this case, completely unlike the case for one-step kinetics.
Converging swirling liquid jets from pressure swirl atomizers: Effect of inner air pressure
Authors: Sivakumar D. , Raghunandan B. N.
Published: November 2002

Converging swirling liquid jets from pressure swirl atomizers injected into atmospheric air are studied experimentally using still and cine photographic techniques in the context of liquid–liquid coaxial swirl atomizers used in liquid rocket engines. The jet exhibits several interesting flow features in contrast to the nonswirling liquid jets (annular liquid jets) studied in the literature. The swirl motion creates multiple converging sections in the jet, which gradually collapse one after the other due to the liquid sheet breakup with increasing Weber number (We). This is clearly related to the air inside the converging jet which exhibits a peculiar variation of the pressure difference across the liquid sheet, ΔP, with We. The variation shows a decreasing trend of ΔP with We in an overall sense, but exhibits local maxima and minima at specific flow conditions. The number of maxima or minima observed in the curve depends on the number of converging sections seen in the jet at the lowest We. An interesting feature of this variation is that it delineates the regions of prominent jet flow features like the oscillating jet region, nonoscillating jet region, number of converging sections, and so on. Numerical predictions of the jet characteristics are obtained by modifying an existing nonswirling liquid jet model by including the swirling motion. The comparison between the experimental and numerical measurements shows that the pressure difference across the liquid sheet is important for the jet behavior and cannot be neglected in any theoretical analysis.
The Kolmogorov constant for the Lagrangian velocity spectrum and structure function
Authors: Lien Ren-Chieh , D’Asaro Eric A.
Published: November 2002

The inertial subrange Kolmogorov constant for the Lagrangian velocity structure function C0 is related to the inertial subrange constant for the Lagrangian acceleration spectrum β by C0=πβ. However, Rλ must be greater than about 105 for the inertial subrange of the structure function to be sufficiently wide to accurately determine C0, while values of Rλ greater than 102 are sufficient to determine β. Taking these Rλ limitations into account, the only two known high-quality independent measurements of C0 are 5.5 and 6.4.
Interfacial structure in α−Fe/Sm2(Fe,Si)17Cx nanocomposites prepared by crystallization of amorphous alloy
Authors: Zhang X. Y. , Guan Y. , Zhang J. W.
Published: November 2002

The interfacial structure of α−Fe/Sm2(Fe,Si)17Cx nanocomposites prepared by the annealing of amorphous Sm8Fe85Si2C5 has been investigated by positron lifetime spectroscopy. Results show that there are two kinds of interface structures in the α−Fe/Sm2(Fe,Si)17Cx nanocomposites prepared at 750 °C. One characterized by a positron lifetime of 153 ps is determined to be the interfacial amorphous layer. The other has a loosely packed atomic structure in which structural free volumes have a larger size than that of one or two lattice vacancies of Fe. The volume fraction of the former decreases with increasing annealing temperature, and only the interface structure with vacancy-sized free volumes is detected after annealing at temperatures T⩾950 °C. The loosely packed interface structure is believed to weaken the magnetic exchange coupling between the α–Fe and Sm2(Fe,Si)17Cx grains in the nanocomposites.
Mechanical property of electrorheological fluid under step compression
Authors: Tian Yu , Meng Yonggang , Mao Hairong , Wen Shizhu
Published: November 2002

Stepwise compression of electrorheological (ER) fluids based on zeolite and silicone oil under constant voltages was experimentally investigated. The difference between peak compressive stress and stable compressive stress obtained during the stepwise compression changed with the change of applied voltages and compressive strains. The decay ratio of compressive stress, which may depict the solidification level of ER fluids under external electric fields, decreased with the increase of electric field and compressive strains. Also both the peak compressive stress and the stable compressive stress were shown to be determined by the electric field under low applied voltages. With the increase of the applied voltage, ER fluids were shown to be described by the mechanics of compressing a continuous fluid. At a much stronger ER effect, a deviation from the prediction of the continuous media theory occurred, and structure strengthening of ER fluids by compression should be considered.
Electron and hole transport through quantum dots
Authors: Li Shu-Shen , Abliz Ahmad , Yang Fu-Hua , Niu Zhi-Chuan , Feng Song-Lin , Xia Jian-Bai , Hirose Kenji
Published: November 2002

The transmission through quantum dots (QDs) is calculated using the recursion method. In our calculation, the effect of finite offset is taken into account. The results show that the shapes of the QDs determine the number of resonant tunneling peaks and the distances between the peaks decrease as the radii of the QDs increase. The intensities of the conductance are strongly dependent on the barrier widths. The conductance peaks are split when transmitting through two QDs. The theoretical results qualitatively agree with the available experimental data. Our calculated results should be useful for the application of QDs to photoelectric devices.
Frequency response of quartz crystal shear-resonator during an adhesive, elastic contact in a surface forces apparatus
Authors: Berg Steffen , Johannsmann Diethelm , Ruths Marina
Published: November 2002

Contact mechanics experiments on a single asperity contact between two dry mica surfaces have been performed with a surface forces apparatus where one mica surface was excited to oscillatory shear movements by a quartz-crystal resonator. We directly obtain the resonance parameters of the quartz and the radius of the adhesive contact (measured by optical interferometry) as a function of the external load. The frequency shift was found to increase linearly with increasing contact radius as predicted by the elastic point contact model by Laschitsch and Johannsmann [J. Appl. Phys. 85, 3759 (1999)]. The bandwidth increased more strongly than linearly with the contact radius, but not quadratically as predicted by the model. We attribute the differences to dissipative processes in the glue layers supporting the contacting surfaces.
Optical phonons in self-assembled Ge quantum dot superlattices: Strain relaxation effects
Authors: Liu J. L. , Wan J. , Jiang Z. M. , Khitun A. , Wang K. L. , Yu D. P.
Published: November 2002

We present Raman scattering by optical phonons in self-assembled Ge quantum dot superlattices grown by solid-source molecular beam epitaxy. The Ge quantum dots are vertically correlated and have different average sizes and dot morphologies. The GeGe optical phonon frequency was mainly caused by strain relaxation effects. Experimentally observed GeGe optical phonon modes were compared with calculated values using the deformation potential theory, indicating that the strain relaxation of Ge quantum dot superlattices arises not only from atomic intermixing but also from the morphology transition in dot formation.
Magnetic properties of SiO2-coated Fe nanoparticles
Authors: Wu Mingzhong , Zhang Y. D. , Hui S. , Xiao T. D. , Ge Shihui , Hines W. A. , Budnick J. I. , Yacaman M. J.
Published: November 2002

SiO 2 -coated Fe nanoparticles were synthesized using a wet chemical method, and their structural and magnetic properties were studied. The SiO2 material was in an amorphous state. The Fe nanoparticles were in a bcc state and contained an inner ferrihydrite core whose size decreased with increasing calcination temperature. The nanoparticles were basically in the ferromagnetic state. Their saturation magnetization increased with increasing calcination temperature, whereas their coercivity decreased with increasing calcination temperature. Different from bulk Fe, the nanoparticles exhibited strong temperature-dependent magnetic behaviors. The Bloch exponent fell from 1.5 to smaller values and decreased with increasing ferrihydrite content, while the Bloch constants were much bigger than that for bulk and increased significantly with ferrihydrite content. The value of coercivity decreased notably with increasing temperature. The exchange anisotropy arising from the exchange coupling across the Fe/ferrihydrite interfaces was examined and was used to interpret the observed temperature behaviors.
Effect of group II impurity and group III native defect on disordering Cu–Pt type ordered structures in In0.5(AlxGa1−x)0.5P layers
Authors: Chang K. L. , Hsieh K. C. , Grillot P. N. , Pai R. D. , Huang J.-W. , Höfler G. E.
Published: November 2002

Cu–Pt ordering is studied with cross-sectional transmission electron microscopy in p-type, n-type, and nominally undoped InAlP and In(AlGa)P layers. These different doping conditions allow us to investigate the effect of acceptor doping with magnesium and donor doping with tellurium on removing Cu–Pt ordering in In(AlGa)P. Even more significant, however, is the ability to investigate the effect of native group III self-diffusion in p-type, n-type, and nominally undoped InAlP and In(AlGa)P layers. Our data indicate that the rapidly diffusing Mg acceptor species is less effective on removing Cu–Pt ordering than the relatively slow diffusing Te donor species. Also, our results indicate that the native group III defects and group III self-diffusion play a more important role in removing Cu–Pt ordering than either group II acceptor diffusion or group VI donor diffusion.
Second harmonic generation from copper-tetratert-butyl-phthalocyanine Langmuir–Blodgett film/metal interface: Electric quadrupole or electric field induced second harmonic generation effect?
Authors: Li Cheng-Quan , Manaka Takaaki , Cheng Xiao-Man , Iwamoto Mitsumasa
Published: November 2002

It has been found that a high electric field is created at the interface between copper-tetratert-butyl-phthalocyanine (CuttbPc) Langmuir–Blodgett (LB) film/metal due to the exchanged charges. The symmetry of the CuttbPc molecule is broken by this space charge induced electric field (SCIEF), and optical second harmonic (SH) signal is generated from the CuttbPc LB film/metal interface. A theoretical expression of the reflected SH signal intensity is derived using a four-layer model with the SCIEF being treated as a mean field. The origins of the SHG are considered as the quadrupole effect and the electric field induced SHG (EFISHG) due to the SCIEF. On the basis of experimental results and the theoretical analysis, the nonlinear susceptibilities of these two effects are calculated. It is found that at the wavelength of incident fundamental light around 1100 nm the electric quadrupole effect makes a main contribution to the SHG from the interface, while at the wavelength around 1200 nm it is attributed to the EFISHG. It is expected that the SCIEF could be determined by SHG.
Formation and thermal stability of ultra-shallow p+ junctions in Si and Si1−x Gex formed by molecular beam epitaxy
Authors: Thompson P. E. , Bennett J.
Published: November 2002

Ultra-shallow junction layers are required for deep-submicron complementary metal-oxide-semiconductor transistors and quantum devices. The use of low temperature Si molecular beam epitaxy (MBE) has been investigated for the formation of 10 nm ultra-shallow doped layers. Uniformly B-doped Si layers, B delta-doped Si, and uniformly B-doped Si1−x Gex (x=0, 0.2, or 0.4) layers have been studied. Regardless of the growth technique (uniform versus delta-doped) or Ge concentration, the sheet resistance of the optimally-doped, as-grown samples were equivalent to idealized, box profiles having a carrier concentration equal to the B solid solubility limit in Si, 6×1020/cm3, which is a substantial improvement over the best reported, ion implanted shallow layers. The B in the as-grown, optimally-doped MBE layers is fully electrically active and does not require a post-growth anneal. The thermal budget of the layers was established so that the layers can be employed in a device fabrication process. The MBE-grown shallow-doped layers were not affected by 10 min thermal processing up to 700 °C. While substantial B redistribution occurred as a result of the 800 °C 10 min anneal, the sheet resistances of the MBE-grown layers are still less than ion implanted samples having the same junction depth. In addition, it was observed that the Ge alloy layers impeded the diffusion of B at 800 °C.
Signal-to-noise deterioration in perpendicular storage media by thermal and magnetic field aging as determined by magnetic force microscopy
Authors: Svedberg Erik B. , Khizroev Sakhrat , Chang Chunghee H. , Litvinov Dmitri
Published: November 2002

The long-term stability of a CoCrPt perpendicular storage media has been investigated by thermal aging at temperatures ranging from 95 to 280 °C during otherwise normal ambient conditions. The signal-to-noise ratio from magnetic force microscopy images has been extracted by the autocorrelation method and compared for the different aging conditions. During aging at elevated temperatures, the media stack undergoes some structural and crystallographic changes. It is found that the severity of the aging process is higher for low-density bit patterns due to the higher value of demagnetizing fields for large bits. A comparison to the thermal aging is done with a set of disks in which a deterioration of the magnetic bit pattern was induced by an external magnetic field. Two experiments, including fields with a single polarity as well as with a cycled polarity, mimic different magnetic situations for the disk. During the external field experiments, no structural or crystallographic changes occur in the media. The magnetic background noise of the disks is also analyzed. For aging in a magnetic field, it is found that below a certain threshold value, mostly low-density bit patterns are affected by the aging process. However, above the threshold value, both low and high-density bit patterns are affected.
Glass-forming ability and crystallization behavior of Co62−xFexNb6Zr2B30 (x=0,16) amorphous alloys with large supercooled liquid region
Authors: Borrego J. M. , Conde C. F. , Conde A. , Roth S. , Grahl H. , Ostwald A. , Eckert J.
Published: November 2002

The kinetics of the glass transition and the crystallization process of Co62−xFexNb6Zr2B30 (x=0,16) amorphous alloys with large supercooled liquid region was investigated by differential scanning calorimetry. The dependence of the glass transition on the heating rate was analyzed in terms of the Vogel–Fulcher–Tamman equation. The glass-forming ability of these alloys is discussed with respect to the width of the supercooled liquid region, the reduced glass transition temperature, and the fragility parameter. The studied alloys are found to be rather strong glass formers. The crystalline phases formed after the devitrification are identified by means of x-ray diffraction and thermomagnetic measurements. The complicated crystallization process requiring long-range atomic rearrangements seems to contribute to the high stability of the supercooled liquid against crystallization.
Magnetization reversal of the ferromagnetic layer in IrMn/CoFe bilayers
Authors: Wang Y. G. , Petford-Long A. K.
Published: November 2002

The magnetization reversal of the ferromagnetic layer in IrMn/CoFe exchange-coupled bilayer films with different antiferromagnetic (AFM) layer thicknesses (dAFM) has been investigated using Lorentz microscopy and bulk magnetometry. These films exhibit very complex magnetization processes and the reversal mechanism is dependent on dAFM. Holding the film at negative saturation of the ferromagnetic layer for up to 87 h results in no change in the reversal mechanism or coercivity, but in a decrease in the exchange field. We believe that two energy barrier distributions with different time constants coexist. Thermally activated reversal of the antiferromagnetic layer with a large time constant results in an increasing shift of the entire hysteresis loop towards zero field with increased period of time spent at negative saturation, because of a reduction in the overall unidirectional anisotropy in the films. Thermal activation with a small time constant contributes to loop broadening, an asymmetry in reversal, and training effects. As dAFM decreases, the energy barriers for thermally activated reversal of the antiferromagnetic layer decrease so the changes in the antiferromagnetic layer become more significant, resulting in a greater effect on the reversal of the ferromagnetic layer.