bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of high speed particle streams in dc glow discharge dusty plasmas
Authors: Thomas Edward
Published: July 2002

This paper reports on measurements of high speed flows of micron-sized particles in dc glow discharge dusty plasmas. In these experimental investigations, the dust particles are observed to flow towards the anode region in streams or channels with peak speeds of νst⩾30 mm/s. These speeds are 5 to 15 times faster than the average speed of the particles (νcl⩽2 mm/s) in the central dust cloud. Through the use of the particle image velocimetry measurement technique, the spatial distributions of particle velocities in these streams are directly measured. From these measurements, estimates of the spatial structure of the plasma potential in the vicinity of the dust cloud boundary are made.
Determination of energy barriers in organic light-emitting diodes by internal photoemission
Authors: Sigaud Philippe , Chazalviel Jean-Noël , Ozanam François , Stéphan Olivier
Published: July 2002

Internal photoemission has been used to determine the energy barriers for electrode/organic-layer interfaces in organic light-emitting diodes. The diodes were polymer or small-molecule single-layer structures made from poly-(9-vinylcarbazole), poly-(9,9-dihexylfluorene), tris-(8-hydroxyquinolinate) aluminum, and N,N′- Bis(3-methylphenyl)-N,N′-diphenylbenzidine with indium tin oxide or metallic electrodes (Al, Au, or Cu). The internal photoemission yield follows the Fowler theory and the energy barriers exhibit a small lowering for increasing internal electric field due to image-force potential. There is an offset of 0.4–0.5 eV between the actual energy barriers and the naive expectations from the difference between the energy levels taken from the literature. Interface dipole effects and uncertainties concerning the electrochemical determination of the highest occupied molecular orbital levels are the most plausible origins of this difference.
Model and experiments of a drop impinging on an immersed wall
Authors: Klaseboer Evert , Chevaillier Jean-Philippe , Maté Annie , Masbernat Olivier , Gourdon Christophe
Published: July 2002

We present a model describing the rebound of a drop impinging on a rigid plane wall immersed in water. This model is based on the resolution of the drop equation of motion in an unbounded fluid in which an additional pressure force is introduced accounting for the wall effect on the drop motion. This force is computed from a film drainage simulation model during the approach of the deformable particle to the wall. Results of the model have been compared with experimental trajectories of drops impinging vertically at terminal rise velocity against a horizontal wall immersed in water at rest. These trajectories have been obtained with the help of an image processing technique. A wide range of experimental conditions has been studied (drop diameter, interfacial tension, drop viscosity, and density). In most of the cases, the model predicts the experimental trajectories within a very good accuracy (height of bouncing, deformation, number of rebounds) even in the case of a significant deformation. The numerical results show that the rebound of a deformable inclusion against a wall in water is essentially governed by the balance between the added mass force and the film pressure force exerted on the drop during the impact. The model has also been successfully tested in the case of an impinging bubble at high particle Reynolds number, based on experimental data taken from Tsao and Koch [Phys. Fluids 9, 44 (1997)].
A particle model of rolling grain ripples under waves
Authors: Andersen Ken Haste
Published: July 2002

A simple model for the formation of rolling grain ripples on a flat sand bed by the oscillatory flow generated by a surface wave is presented. An equation of motion is derived for the individual ripples, seen as “particles,” on the otherwise flat bed. The model accounts for the initial appearance of the ripples, the subsequent coarsening of the ripples, and the final equilibrium state. The model is related to the physical parameters of the problem, and an analytical approximation for the equilibrium spacing of the ripples is developed. It is found that the spacing between the ripples scales with the square-root of the nondimensional shear stress (the Shields parameter) on a flat bed. The results of the model are compared with measurements, and reasonable agreement between the model and the measurements is demonstrated.
The multiscale formulation of large eddy simulation: Decay of homogeneous isotropic turbulence
Authors: Hughes Thomas J. R. , Mazzei Luca , Oberai Assad A. , Wray Alan A.
Published: July 2002

The variational multiscale method is applied to the large eddy simulation (LES) of homogeneous, isotropic flows and compared with the classical Smagorinsky model, the dynamic Smagorinsky model, and direct numerical simulation (DNS) data. Overall, the multiscale method is in better agreement with the DNS data than both the Smagorinsky model and the dynamic Smagorinsky model. The results are somewhat remarkable when one realizes that the multiscale method is almost identical to the Smagorinsky model (the least accurate model!) except for removal of the eddy viscosity from a very small percentage of the lowest modes.
Behavior of oxygen doped SiC thin films: An x-ray photoelectron spectroscopy study
Authors: Avila A. , Montero I. , Galán L. , Ripalda J. M. , Levy R.
Published: July 2002

Thin silicon carbide films have been deposited by chemical vapor deposition on p-type (100) silicon substrates. The composition and bonds formed in these films have been analyzed by x-ray photoelectron spectroscopy (XPS) and infrared spectroscopy. The native surface oxide on the silicon carbide surface induced by air exposure has also been studied. Several phases are detected in the near-surface region: elemental Si, Si oxides (mainly SiO2), Si carbide (SiC) and Si oxicarbides (SiOxCy). Quantitative XPS analysis results indicate that, for atomic oxygen fractions <0.15, the Si–C phases are dominant in the films. Above this value no silicon oxicarbide is observed, but a multiphase material formed by elemental Si, Si oxides and Si carbides is observed. In spite of the film being a complex phase mixture, a simple relationship is found between the overall carbon and oxygen compositions. The carbon atomic fraction in the film decreases quasilinearly as the oxygen content increases, with a slope of about −1. An overall composition of SiOxC3−x in the 0.5<x<2 range is found for the phase mixture. A comparison with silicon carbide obtained by CHn+ ion implantation into monocrystalline silicon is made.
Precipitation of nanometer scale Zn crystalline particles in ZnO–B2O3–SiO2 glass during electron irradiation
Authors: Jiang Nan , Qiu Jianrong , Silcox John
Published: July 2002

In this letter, we report precipitation of nanometer scale Zn crystalline particles in Zn borosilicate glass induced by 100 keV electron irradiation. Nanometer Zn crystalline particles precipitate rapidly in the glass under high intensity electron irradiation. A double ionization mechanism is proposed in interpretation of the formation of Zn particles. Thermal effects are less important than ionization interactions.
Fatigue-free behavior of highly oriented Bi3.25La0.75Ti3O12 thin films grown on Pt/Ti/SiO2/Si(100) by metalorganic solution decomposition
Authors: Chon Uong , Yi Gyu-Chul , Jang Hyun M.
Published: July 2002

Fatigue-free and highly c-axis oriented Bi3.25La0.75Ti3O12 (BLT) thin films were grown on Pt/Ti/SiO2/Si(100) substrates using metalorganic solution decomposition. Films annealed above 500 °C were characterized by strong c-axis preferential growth with an in-plane alignment of grains. The BLT film capacitors with a Pt top electrode showed excellent ferroelectric properties. The remanent polarization (2Pr) and the coercive field (Ec) were in the range of 26–28 μC/cm2and 50–75 kV/cm, respectively. More importantly, the BLT capacitors did not show any significant fatigue up to 3.5×1010 read/write switching cycles at a frequency of 1 MHz.
Energetic proton generation in ultra-intense laser–solid interactions
Authors: Wilks S. C. , Langdon A. B. , Cowan T. E. , Roth M. , Singh M. , Hatchett S. , Key M. H. , Pennington D. , MacKinnon A. , Snavely R. A.
Published: July 2002

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.
Stability of nonconservative hyperbolic systems and relativistic dissipative fluids
Authors: Ortiz Omar E.
Published: July 2002

A stability theorem for general quasi-linear symmetric hyperbolic systems (not necessarily conservation laws) is proved in this work. The key assumption is the “stability eigenvalue condition,” which requires all the eigenvalues of the constant coefficient system symbol to have negative real part for nonzero Fourier frequency, decaying no faster than |ω|2 when |ω|→0. The decay of the solution to zero, as time grows to infinity, is proved when the space dimension is bigger than or equal to 3. As an application of the general theorem, stability is proved for the equations describing relativistic dissipative fluids.
An experimental investigation of the surface temperature field during evaporative convection
Authors: Saylor J. R. , Smith G. B. , Flack K. A.
Published: July 2002

Measurements of the surface temperature field are presented for a water surface undergoing evaporation. These temperature fields were measured using an infrared camera for a range of heat fluxes q″=30–500 W/m2. Experiments were conducted for water surfaces with and without a surfactant monolayer. A statistical analysis of the data is presented which shows the effect of heat flux and surfactants on the root mean square and skewness of the field. The data reveals a linear increase in the rms with increasing heat flux, which is similar for clean and surfactant conditions. In contrast, the skewness is markedly different for the clean and surfactant-covered cases. For clean surface conditions, the skewness attains large, negative values, becoming increasingly negative as q″ increases. When the surface is covered with a surfactant monolayer, however, the skewness exhibits small, negative values which approach zero as the heat flux increases. This behavior is reflected in the pdf which is clearly asymmetric in the clean case and virtually symmetric in the surfactant case. A physical mechanism is presented to explain these results. Temporal power spectra are presented which reveal the role of heat flux and surfactants on the temporal evolution of the surface temperature field.
Instability of dust ion-acoustic waves in a dusty plasma containing elongated and rotating charged dust grains
Authors: Shukla P. K. , Tskhakaya D. D.
Published: July 2002

The dispersion properties of the dust ion-acoustic waves (DIAWs) in an unmagnetized dusty plasma is examined when the plasma constituents are electrons, ions, and charged dust grains which are elongated and rotating. Since the dipole moment of elongated and rotating dust grains is nonzero, significant modifications of the DIAW spectrum emerge. It is found that the DIAWs are subjected to an instability when the DIAW frequency approximately equals the angular rotation frequency of the elongated dust grains. The relevance of our investigation to enhanced fluctuations in space and laboratory dusty plasmas is pointed out.
Synthesis of superhard cubic BC2N
Authors: Solozhenko Vladimir L. , Andrault Denis , Fiquet Guillaume , Mezouar Mohamed , Rubie David C.
Published: July 2002

Cubic BC2N was synthesized from graphite-like BC2N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC2N at ambient conditions is 3.642(2) Å, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC2N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC2N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond.
Photoabsorption spectra in the continuum of molecules and atomic clusters
Authors: Nakatsukasa Takashi , Yabana Kazuhiro
Published: July 2002

We present linear response theories in the continuum capable of describing photoionization spectra and dynamic polarizabilities of finite systems with no spatial symmetry. Our formulations are based on the time-dependent local density approximation with uniform grid representation in the three-dimensional Cartesian coordinate. Effects of the continuum are taken into account either with a Green’s function method or with a complex absorbing potential in a real-time method. The two methods are applied to a negatively charged cluster in the spherical jellium model and to some small molecules (silane, acetylene and ethylene).
Epitaxial thin films of hexagonal BaRuO3 on (001) SrTiO3
Authors: Lee M. K. , Eom C. B. , Lettieri J. , Scrymgeour I. W. , Schlom D. G. , Tian W. , Pan X. Q. , Ryan P. A. , Tsui F.
Published: July 2002

We report the growth, epitaxial arrangement, and electrical and magnetic properties of epitaxial thin films of hexagonal BaRuO3 on (001) cubic perovskite substrates. Four-circle x-ray diffraction reveals that the BaRuO3 films are predominantly grown with two distinct orientations normal to the (001) SrTiO3 substrate: (022̄3) of the four-layered hexagonal structure (4H) in the sputter-grown films and (202̄5) of the nine-layered hexagonal structure (9R) in the pulsed laser deposited films. (022̄3)-oriented 4H films consist of four orthogonal domains with the in-plane relationship of BaRuO3[21̄1̄0]//SrTiO3〈110〉. The temperature dependent resistivity of the (022̄3) 4H film shows metallic behavior. In contrast, a resistivity minimum is observed at low temperatures in the (202̄5) 9R film. Both films exhibit Pauli paramagnetism.
The q-Laplace operator and q-harmonic polynomials on the quantum vector space
Authors: Iorgov N. Z. , Klimyk A. U.
Published: July 2002

The aim of this paper is to study q-harmonic polynomials on the quantum vector space generated by q-commuting elements x1,x2,…,xn. They are defined as solutions of the equation Δqp=0, where p is a polynomial in x1,x2,…,xn and the q-Laplace operator Δq is determined in terms of q-derivatives. The projector Hm:Am→Hm is constructed, where Am and Hm are the spaces of homogeneous (of degree m) polynomials and q-harmonic polynomials, respectively. By using these projectors, a q-analog of classical associated spherical harmonics is constructed. They constitute an orthonormal basis of Hm. A q-analog of separation of variables is given. Representations of the nonstandard q-deformed algebra Uq′(son) [which plays the role of the rotation group SO(n) in the case of classical harmonic polynomials] on the spaces Hm are explicitly constructed.
How does a shock in supersonic flow grow out of smooth data?
Authors: Chen Shuxing
Published: July 2002

This paper is devoted to the precise description of the process of formation and construction of shock waves. If a steady supersonic flow moves above a concavely curved wall, then the flow is compressed by the bending wall, and a suspended shock will be formed in the flow field. This fact with the existence of solution having an assigned singularity structure is rigorously proved. It is indicated that the suspended shock will start from the cusp of the envelope of λ+ characteristics. Moreover, some estimates describing the behavior of the solution near the cusp are also obtained.
Microwave spectra of the Ar–ND3 van der Waals complex and its partially protonated isotopomers
Authors: van Wijngaarden Jennifer , Jäger Wolfgang
Published: July 2002

Rotational spectra of the Ar–ND3 van der Waals complex were measured in the frequency range between 4 and 19 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. The isotopomers studied include those with ND3, NHD2, and NH2D. A tunneling splitting due to the inversion of the ammonia subunit within the ground state of the complex was observed for all three isotopomers. This splitting cannot be measured in Ar–NH3 for spin statistical reasons. Nuclear quadrupole hyperfine structure of rotational transitions arising from the N14 nucleus was resolved and the corresponding nuclear quadrupole coupling constants were determined. A smaller nuclear quadrupole splitting arising from the deuterium nuclei was observed but not resolved. The ground state spectroscopic constants are compared with experimental and theoretical data previously reported for Ar–NH3. Additional transitions were observed for Ar–ND3 and assigned to the two inversion tunnelling states of an excited internal rotor state. A fit of the spectroscopic constants for these two states suggests the presence of a Coriolis perturbation along the lines of that observed in the microwave spectrum of Ar–NH3.
Diffusion-influenced reversible geminate recombination in one dimension. II. Effect of a constant field
Authors: Kim Hyojoon , Shin Kook Joe , Agmon Noam
Published: July 2002

The diffusion-influenced reversible geminate-pair recombination problem is solved exactly in one dimension, in the presence of a constant external field. As the field strength changes sign, the long time asymptotics of the components of the Green function solution show a primary kinetic transition, in which the equilibrium values are changed. At two other critical values of the external field the approach to equilibrium changes, from a t−3/2 power-law to exponential. At the three critical fields, asymptotic t−1/2 decay prevails.
Integrable Yang–Mills–Higgs equations in three-dimensional de Sitter space–time
Authors: Kotecha V. , Ward R. S.
Published: July 2002

This paper describes an integrable Yang–Mills–Higgs system on (2+1)-dimensional de Sitter space–time. It is the curved-space–time analog of the Bogomolnyi equations for monopoles on R3. A number of solutions, of various types, are constructed.
Cationic ordering control of magnetization in Sr2FeMoO6 double perovskite
Authors: Balcells Ll. , Navarro J. , Bibes M. , Roig A. , Martı́nez B. , Fontcuberta J.
Published: July 2002

The role of the synthesis conditions on the cationic Fe/Mo ordering in Sr2FeMoO6 double perovskite is addressed. It is shown that this ordering can be controlled and varied systematically. The Fe/Mo ordering has a profound impact on the saturation magnetization of the material. Using the appropriate synthesis protocol a value of 3.7 μB has been obtained. Mössbauer analysis reveals the existence of two distinguishable Fe sites and a charge density at the Fem+ ions significantly larger than 3d5 suggesting a Fe contribution to the spin-down conduction band. The implications of these findings for the synthesis of Sr2FeMoO6 having optimal magnetoresistance response are discussed.
Time resolved coherent anti-Stokes Raman scattering of I2 isolated in matrix argon: Vibrational dynamics on the ground electronic state
Authors: Karavitis M. , Zadoyan R. , Ara Apkarian V.
Published: July 2002

Time-resolved, electronically resonant, coherent anti-Stokes Raman scattering is used to prepare and interrogate vibronic coherences of molecular iodine in matrix Ar. Coherences that involve evolution on the excited B(3Π0u) state, first- and third-order coherences, decay in less than one vibrational period (τ<300 fs). In contrast, as many as 200 vibrational periods of motion can be observed for Raman-prepared wave packets consisting of zero-phonon vibrational superpositions on the ground electronic state (second-order coherence). Packets consisting of v=4, 5 and v=3, 4, 5 on the X(1Σg) state decay with a half-life of 10±1 ps at 31 K, allowing a more accurate measure of vibrational level spacings and decoherence time than has been possible in frequency domain. The harmonic frequency of the molecule is reduced by 1.5 cm−1 (0.7%) in the matrix. The lack of recurrence in the excited electronic state ensures that the resonant anti-Stokes scattering arises only from the negative momentum component of the Raman packet. This momentum filter, which should be ubiquitous in condensed media, leads to a signal with deeper modulation than in the gas phase.
Scattering on graphs and one-dimensional approximations to N-dimensional Schrödinger operators
Authors: Melnikov Yu. , Pavlov B.
Published: July 2002

In the present article we develop the spectral analysis of Schrödinger operators on lattice-type graphs. For the basic example of a cubic periodic graph the problem is reduced to the spectral analysis of certain regular differential operators on a fundamental star-like subgraph with a selfadjoint condition at the central node and quasiperiodic conditions at the boundary vertices. Using an explicit expression for the resolvent of lattice-type operator we develop in the second section appropriate Lippmann–Schwinger techniques for the perturbed periodic operator and construct the corresponding scattering matrix. It serves as a base for the approximation of the multi-dimensional Schrödinger operator by a one-dimensional operator on the graph: in the third section of the paper for given N-dimensional Schrödinger operators with rapidly decreasing potential we construct a lattice-type operator on a cubic graph embedded into RN and show that the original N-dimensional scattering problem can be approximated in a proper sense by the corresponding scattering problem for the perturbed lattice operator.
Description of the parallel thermal conductance technique for the measurement of the thermal conductivity of small diameter samples
Authors: Zawilski Bartosz M. , Littleton Roy T. , Tritt Terry M.
Published: July 2002

In order to measure the thermal conductivity of small needlelike samples (2.0×0.05×0.1 mm3) such as pentatellurides and single carbon fibers, we have developed a new technique called the parallel thermal conductance (PTC) technique. In the more typical steady state method that is used for measuring thermal conductivity, thermocouples are attached to the sample in order to measure the temperature gradient and a heater in order to supply this gradient. However, attaching thermocouples and heaters directly to small samples may be relatively difficult, and cause large heat losses and errors. Thus, the measurement of the thermal conductivity of small samples and thin films has been a formidable challenge, with only few successes, due, among other factors, to the heat loss. It is also difficult for the small samples to support the heaters and thermocouples without causing damage to the sample. In this paper we describe the recently developed PTC method providing measurements on standards as well as single carbon fibers, in addition to preliminary pentatelluride crystals measurements.
Higher order mode excitations in gyro-amplifiers
Authors: Nguyen K. T. , Calame J. P. , Danly B. G. , Levush B. , Garven M. , Antonsen T.
Published: July 2002

In gyro-devices, a nonlinear output taper is often employed as the transition from the near cutoff radius of the interaction circuit to a much larger output waveguide. The tapers are usually designed to avoid passive mode conversion, and thus do not consider the effect of a bunched beam. However, recent simulations with the self-consistent MAGY code [Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)] indicate that higher order mode interactions with the bunched electron beam can substantially compromise the mode purity of the rf output. The interaction in the taper region is traveling wave in nature, and is strongly dependent on the residual beam bunching characteristics resulting from the upstream operating mode interaction. An experiment has been performed to quantify the rf output mode content from a Ka-band gyroklystron. The agreement between salient theoretical and measured rf output characteristics confirms the existence of higher order mode excitation in output tapers as predicted by theory.
A Reference Equation of State for the Thermodynamic Properties of Nitrogen for Temperatures from 63.151 to 1000 K and Pressures to 2200 MPa
Authors: Span Roland , Lemmon Eric W. , Jacobsen Richard T , Wagner Wolfgang , Yokozeki Akimichi
Published: July 2002

A new formulation for the thermodynamic properties of nitrogen has been developed. Many new data sets have become available, including high accuracy data from single and dual-sinker apparatuses which improve the accuracy of the representation of the pρT surface of gaseous, liquid, and supercritical nitrogen, including the saturation states. New measurements of the speed of sound from spherical resonators yield accurate information on caloric properties in gaseous and supercritical nitrogen. Isochoric heat capacity and enthalpy data have also been published. Sophisticated procedures for the optimization of the mathematical structure of equations of state and special functional forms for an improved representation of data in the critical region were used. Constraints regarding the structure of the equation ensure reasonable results up to extreme conditions of temperature and pressure. For calibration applications, the new reference equation is supplemented by a simple but also accurate formulation, valid only for supercritical nitrogen between 250 and 350 K at pressures up to 30 MPa. The uncertainty in density of the new reference equation of state ranges from 0.02% at pressures less than 30 MPa up to 0.6% at very high pressures, except in the range from 270 to 350 K at pressures less than 12 MPa where the uncertainty in density is 0.01%. The equation is valid from the triple point temperature to temperatures of 1000 K and up to pressures of 2200 MPa. From 1000 to 1800 K, the equation was validated with data of limited accuracy. The extrapolation behavior beyond 1800 K is reasonable up to the limits of chemical stability of nitrogen, as indicated by comparison to experimental shock tube data.
Collisionless shock and supernova remnant simulations on VULCAN
Authors: Woolsey N. C. , Ali Y. Abou , Evans R. G. , Grundy R. A. D. , Pestehe S. J. , Carolan P. G. , Conway N. J. , Dendy R. O. , Helander P. , McClements K. G. , Kirk J. G. , Norreys P. A. , Notley M. M. , Rose S. J.
Published: July 2002

The VULCAN [C. N. Danson et al., Opt. Commun. 103, 392 (1993)] laser at the UK Central Laser Facility is being used for laboratory-based simulations of collisionless shocks. By ensuring that key dimensionless parameters in the experiments have values similar to those of supernova remnants (SNRs), the hydrodynamics and magnetic field of the experiment are scaled to those of a SNR. This makes it possible to investigate experimentally the physics of collisionless magnetized shocks in such objects. The experiments are providing data against which to test current theory. Collisionless shock formation and the interaction of two counterpropagating colliding plasmas permeated by a strong magnetic field are discussed.
Electric field in inductively coupled gas discharges
Authors: Piejak R. , Godyak V. , Alexandrovich B.
Published: July 2002

The electric field in a low-pressure electrodeless discharge has been determined from discharge voltage measurements at constant discharge current as a function of gas pressure. Measurements have been made in neon, argon, and xenon at discharge currents of 1, 3, and 10 A with a driving frequency of 0.45 MHz. The behavior of the electric field as a function of gas pressure has been found to be qualitatively similar for all three gases. At the lowest gas pressures at which a discharge can be sustained, the electric field is relatively high. With increasing gas pressure the field reaches a local minimum, followed by a local maximum and another local minimum. Above 1 Torr, the electric field increases monotonically with gas pressure.
The effect of potential truncation on the gas–liquid surface tension of planar interfaces and droplets
Authors: Napari Ismo , Laaksonen Ari
Published: July 2002

Density functional theory is applied to study the properties of attracting hard-sphere fluids with different cutoffs of the Lennard-Jones potential. The cut-off dependence of the gas–liquid surface tension is not correctly described by a formula which depends only on temperature and critical temperature of the fluid. A better correlation with an explicit dependence on the cut-off radius is developed. The work of nucleus formation in gas–liquid nucleation is shown highly susceptible to potential truncation. The distance between the equimolar surface and the Gibbs surface of tension (δ) decreases for droplets if the potential is truncated. However, for very large droplets (more than 106 molecules) the truncation increases δ. The same holds for δ∞ (Tolman length) at planar interfaces.
Vector correlations in dissociative photoionization of O2 in the 20–28 eV range. I. Electron-ion kinetic energy correlations
Authors: Lafosse A. , Brenot J. C. , Golovin A. V. , Guyon P. M. , Hoejrup K. , Houver J. C. , Lebech M. , Dowek D.
Published: July 2002

Imaging and time resolved coincidence techniques are combined to determine ion-electron (VO+,Ve,P) velocity vector correlations in dissociative photoionization (DPI) of the O2 molecule induced by linearly polarized synchrotron radiation (P). The ion-electron kinetic energy correlations identify each DPI process by its reaction pathway, intermediate molecular state and dissociation limit. The 4π collection of ions and electrons provides their branching ratios. Up to 12 DPI processes are identified in the 20–28 eV range. Photoionization into the O2+(B 2∑g−) in the Franck–Condon (FC) region populating the [O+(4S)+O(3P)] limit is the dominant process. In the 22.3–24 eV region excitation of the O2*(3 2∏u,nsσg) Rydberg series, followed by dissociation and atomic autoionization to the [O+(2D)+O(3P)] limit reaches about 10% of the DPI flux. A new DPI process is identified in the same energy range, which populates the [O+(4S)+O(1D)] limit. At higher energies the relative weight of the four distinct processes which correspond to ionization into the O2+(3 2∏u) and O2+(c 4∑u−) states in the FC region and population of distinct excited limits is evaluated. The spatial analysis of the (VO+,Ve,P) vector correlation for selected processes will be discussed in a companion paper.
Picosecond time-resolved magnetization reversal dynamics in Ni80Fe20 microstructure
Authors: Choi B. C. , Ballentine G. E. , Belov M. , Hiebert W. K. , Freeman M. R.
Published: July 2002

Ultrafast magnetization reversal dynamics in a 15-nm-thick Ni80Fe20 microstructure (10 μm×2 μm) is studied using both time-resolved scanning Kerr microscopy and numerical simulation. The time dependence of the magnetization component along the magnetic easy axis reveals a dramatic reduction in switching time, when the magnetization vector is pulsed by a longitudinal switching field while a steady transverse biasing field is applied. According to the time domain images, the abrupt change of the switching time is due to the change in the magnetization reversal mode; i.e., the nucleation dominant reversal process is replaced by domain wall motion if a transverse biasing field is applied.
Analysis of the elastic deformation of semicrystalline poly(trimethylene terephthalate) by the atomistic-continuum model
Authors: Yang Jae Shick , Jo Won Ho
Published: July 2002

The atomistic-continuum model was validated for a heterogeneous material, and elastic constant of semicrystalline poly(trimethylene terephthalate) (PTT) was calculated by the model. Perfect agreement between simulated strain energy and its theoretical value was observed for a two-phase series model for composites, as the energy minimization proceeded, indicating that the model properly describes heterogeneous materials. The system for semicrystalline PTT consisted of a matrix (amorphous phase), described as a continuum, and a spherical inclusion (crystalline phase), represented in atomistic detail. The degree of crystallinity of semicrystalline PTT could be controlled by varying the volume fraction of an inclusion. The Young’s moduli in all directions of semicrystalline PTT increase with increasing the degree of crystallinity, as expected.
Different transfer paths for thermally activated electrons and holes in self-organized Ge/Si(001) islands in a multilayer structure
Authors: Huang C. J. , Tang Y. , Li D. Z. , Cheng B. W. , Luo L. P. , Yu J. Z. , Wang Q. M.
Published: July 2002

We investigated the temperature dependence (10–250 K) of the photoluminescence (PL) emission spectrum of self-organized Ge/Si(001) islands in a multilayer structure. With elevated temperature, we find that the thermally activated holes and electrons are gathered by the Ge islands in different ways. The holes drift from the wetting layer into the islands, while the electrons, confined in Si due to type-II band alignment, leak into the Ge islands by the electrostatic interaction with the holes accumulated there. It results in an increase of the integrated intensity of island-related PL at a certain temperature range and a reduction of the phonon energy in the phonon-assisted PL of the islands by involving a type-I transition into a type-II transition.
Spin-polarized electron transport in ferromagnet/semiconductor hybrid structures (invited)
Authors: Bland J. A. C. , Hirohata A. , Guertler C. M. , Xu Y. B. , Tselepi M.
Published: July 2002

Two major problems in spin electronics remain to be solved: room temperature spin injection at a source and spin detection at a drain electrode. The lateral size of magnetic contacts and the presence of a potential barrier at the interface are believed to have a key influence on the efficiency of both of these processes. We therefore aimed to clarify these issues by studying spin-polarized transport across epitaxially grown single crystal Fe (001)/GaAs nanoclusters and at the Schottky barrier formed at Ni80Fe20/GaAs interfaces. We observed a negative contribution to the magnetoresistance of an ultrathin (2.5 ML) discontinuous epitaxial Fe film as occurs in tunnel magnetoresistance. This result suggests that spin transport via GaAs is possible on the nanoscale. In the continuous NiFe/GaAs structures, circularly polarized light was used to create a population of spin-polarized electrons in the GaAs substrate and spin-polarized electron transport across the interface at room temperature was detected as an electrical response associated with the field-dependent photocurrent. Surprisingly, highly efficient spin transmission is observed at room temperature, indicating that there is no significant loss of spin polarization for electrons crossing the interface. This result unambiguously demonstrates that spin detection is possible at room temperature in a continuous ferromagnet/semiconductor contact in the presence of the Schottky barrier.
Transitions in morphology observed in nitrogen/methane–hydrogen depositions of polycrystalline diamond films
Authors: Ayres V. M. , Farhan M. , Spach D. , Bobbitt J. , Abdul Majeed J. , Wright B. F. , Wright B. L. , Asmussen J. , Kanatzidis M. G. , Bieler T. R.
Published: July 2002

In this work, we report on a series of transitions in morphology and texture as 5–1000 parts per million of nitrogen were added to 2% and 1% methane–hydrogen depositions of polycrystalline diamond films. Five results are reported. (1) The threshold for transition into the {100}-faceted morphology occurred at lower parts per million nitrogen for the 1% versus the 2% methane–hydrogen series, opposite from the transition thresholds previously reported. (2) At 1000 parts per million nitrogen the film quality of both series had not yet seriously degraded. (3) A well defined sequence of intermediate texture transitions as a function of increasing parts per million nitrogen was observed for both series. (4) A pretransition morphology of large crystallites interspersed among microcrystalline material directly preceding the transitions to the {100}-faceted morphology was observed for both series. (5) A layered growth and/or etched morphology at high nitrogen concentrations was observed for both series. We discuss these observations in terms of the possible influence of our deposition conditions on the accessibility of diamond growth parameter space and chemistry, including possible dynamical effects of the temperature gradients.
Application of the Wolf method for the evaluation of Coulombic interactions to complex condensed matter systems: Aluminosilicates and water
Authors: Demontis Pierfranco , Spanu Silvano , Suffritti Giuseppe B.
Published: July 2002

The application of the method recently proposed by Wolf et al. [J. Chem. Phys. 110, 8254 (1999)] for the evaluation of Coulombic energy in condensed state systems by spherically truncated, pairwise r−1 summation is verified for liquid water and anhydrous and hydrated aluminosilicates. Criteria for the estimation of the optimum values for the truncation radius and the damping parameter are discussed. By several examples it is verified that the new method is computationally more efficient than the traditional Ewald summations. For the considered systems the performances of the new method are good, provided that the truncation radius and the damping parameter are carefully chosen.
Another look at the electron attachment to nitrous oxide
Authors: Kryachko Eugene S. , Vinckier Chris , Nguyen Minh Tho
Published: July 2002

Ab initio molecular orbital calculations up to the coupled-cluster level with the aug-cc-pVQZ basis set allowed us to have a new look at the electron affinity of nitrous oxide (N2O) resulting in a detection of a new N2O− entity, and thereby a novel mechanism for the dissociative electron attachment process, N2O+e−→N2+O−. Addition of an electron to the linear N2O ground state (X 1Σ+) leads first to an open-chain bound anion which lies 25 kJ/mol above the neutral. Upon a cyclization of the open anion with an additional energy barrier of 25 kJ/mol, a cyclic anionic species is formed which is more stable than the open isomer and lies now, at most, 3 kJ/mol above the neutral ground state (the transition structure for cyclization being 50 kJ/mol above neutral N2O). The cyclic anionic species constitutes a weak complex between N2 and O− characterized by a binding energy of only 16 kJ/mol. The electronic structure of the anion complex is analyzed, a number of earlier experimental results are clarified and a resolution for the long-standing disagreement between experiment and theory around the electron affinity of N2O is proposed.
Evolution of structure, morphology, and reactivity of hydrogenated amorphous silicon film surfaces grown by molecular-dynamics simulation
Authors: Ramalingam Shyam , Sriraman Saravanapriyan , Aydil Eray S. , Maroudas Dimitrios
Published: July 2002

The relationship between the structure, H coverage, morphology, and reactivity of plasma deposited hydrogenated amorphous silicon (a-Si:H) film surfaces was investigated using molecular-dynamics simulations. Surfaces of a-Si:H films grown with SiH3 as the sole deposition precursor are found to be remarkably smooth due to a valley-filling mechanism where mobile precursors, such as SiH3 and Si2H6, diffuse and react with dangling bonds in the valleys on the surface. Surface valleys are reactive due to the increased concentration of dangling bonds and decreased H coverage in these regions. The previously speculated physisorbed configuration, where SiH3 is weakly bound to the surface through a H atom, is highly unlikely to be the mobile precursor state.
Mechanism of nanodiamond film formation by stress relaxation on a preferentially oriented vertical basal plane graphitic precursor
Authors: Hoffman A. , Heiman Alexey , Christiansen S. H.
Published: July 2002

Nanocrystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on the substrate temperature, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While in a narrow temperature window around 880 °C a nanodiamond film composed of an agglomerate of diamond particles 3–5 nm in size embedded in an amorphous matrix is obtained, at higher and lower deposition temperatures the films maintain their graphitic character throughout. The nanodiamond film forms on top of a thin graphitic precursor layer of 150–200 nm thickness (critical thickness of the precursor). It was also found that the formation of the nanodiamond phase is initially accompanied by an increase in surface roughness which decreases with film growth. The graphitic precursor film displays a preferred spatial alignment of its basal planes perpendicular to the silicon substrate surface. The reason for this alignment is suggested to be associated to a stress relaxation mechanism in the graphitic films during growth. Beyond a “critical thickness” where compressive stress has built up in the layer to an extent that it must be relaxed, stress relaxation is governed by the formation of a nanodiamond film. By cross sectional and high resolution transmission electron microscopy analysis the microstructure of the films as a function of distance from the silicon substrate interface was investigated. The alignment of the graphitic precursor within the surface near region of the films as a function of deposition time was investigated by angle-resolved near edge x-ray adsorption fine structure. Atomic force microscopy was applied to study the morphological evolution of the films.
Interaction of cavitation bubbles with a free surface
Authors: Robinson P. B. , Blake J. R. , Kodama T. , Shima A. , Tomita Y.
Published: July 2002

The motion of single- and two-cavitation bubbles generated by laser beams directly beneath a free surface is studied experimentally, using high-speed photography, and theoretically using the highly accurate boundary integral method. Favorable comparisons of bubble shape history and centroid motion are observed while the numerical calculations provide information on the pressure field surrounding the bubbles. A range of responses, including the null impulse state, is obtained for the two bubbles depending on the bubble size ratio and the interbubble and bubble-free surface distances, although in all cases reported in this article, the bubble nearest the free surface yields a high-speed liquid jet directed away from the free surface. It is also found that when the free-surface–bubble interaction is strong, a fast free-surface spike is formed for both the single- and two-bubble cases.
Compact generator based on a single-gap polymer-foil switch for 1 ns risetime 10 kV rectangular pulse
Authors: Zeng Zhengzhong , Ma Lianying
Published: July 2002

This article presents a compact rectangular pulse generator based on a single-gap polymer-foil switch triggered by a sharp metal pin fixed on an electromagnet-driven mechanism. This switch makes use of the intrinsic low inductance of the polymer-foil switch to gain a pulse risetime as low as 1 ns. The switch is matched with a Blumlein transmission line composed of standard coaxial cables to readily provide high-voltage output as high as 10 kV with matched load of changeable impedance (typically 50 and 75 Ω). The pulse width can be adjusted by changing the length of the cables. Experiments of a prototype pulser based on this foil switch demonstrate an output pulse of risetime less than 1 ns, amplitude up to 10 kV, and width of about 40 ns.
The effect of molecular orientation in collisions of OH with CO and N2
Authors: van Beek M. C. , ter Meulen J. J.
Published: July 2002

The effect of OH orientation on rotationally inelastic collisions of OH(X2Π) with CO and N2 has been studied in a crossed molecular beam setup at translational energies of 750 and 690 cm−1, respectively. The OH molecules were prepared in the v=0,Ω=32,J=32,f state by hexapole state selection and oriented with their O end or H end toward the collision partner by a static electric field in the collision zone. A degree of orientation of 〈cosθ〉=0.46 has been obtained. In general the cross sections are larger for collisions at the O end in excitation to low rotational states, whereas the cross sections are larger for H end excitation to higher rotational states. OH+CO and OH+N2 behave quite similarly when compared to OH+Ar. Systematic differences between OH+CO and OH+N2 may be attributed to the influence of complex formation on the inelastic collision process. Reanalysis of state-to-state scattering experiments on unoriented OH+CO and OH+N2 indicate that the interaction potential is more head–tail symmetric with respect to OH for OH+N2 compared to OH+CO.
Crosstalk and lateral conduction effects in continuous-sensor amorphous silicon imagers
Authors: Mulato M. , Ready S. , Van Schuylenbergh K. , Lu J. P. , Street R. A.
Published: July 2002

Studies are reported of the image-blur effects caused by lateral crosstalk between neighboring pixels of large-area amorphous silicon (a-Si:H) image sensors. Data are obtained from high fill factor sensor arrays using 512×512 pixels of 75 μm size and a pixel gap of 10 μm. Measurements of the line-spread function determine the charge transfer from the illuminated pixel to neighboring ones along both array orientations, and for different samples and operating conditions. The lateral conduction is attributed to three effects: conduction along the interface between the a-Si:H film and the underlying passivation; field-dependent electron injection at the edge of the sensor; and field enhancement of the interface conduction due to the bias applied to the address lines. We show that the crosstalk can be controlled by the choice of operating conditions and optimization of the materials.
Preparation conditions and optical properties of rare earth ion (Er3+ and Eu3+)-doped alumina films by the aqueous sol–gel method
Authors: Ishizaka T. , Kurokawa Y.
Published: July 2002

Transparent alumina and rare earth ion (Er3+ and Eu3+) highly doped alumina films (15 mol %) were prepared by the aqueous sol–gel method, in order to examine the effects of various preparation conditions on the structure and optical properties of doped films. The results showed that when the gel was heat treated, it lost free and bound water and was converted into the oxide by way of a series of intermediate phases although the oxide as a whole remained structureless. The thermogravimetric analysis curve showed a marked weight loss in the temperature range of 100–500 °C and the slow loss continuing at higher temperature of 800 °C. High doping may reflect noncrystalline structure of alumina. The fluorescence of Eu3+ sensitive of microstructure increased especially corresponding to this weight loss with treatment. The Ω2 indicative of the asymmetric environment of Er3+ showed a similar increase. Concentration quenching for doped films treated at 800 °C was not significant in this concentration range. Lifetimes increased with increasing treatment temperature, similar to above increase.