bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical dynamics of cyclopropynylidyne (c-C3H; X2B2) formation from the reaction of C(1D) with acetylene, C2H2(X 1Σg+)
Authors: Kaiser R. I. , Mebel A. M. , Lee Y. T.
Published: July 2002

The reaction between electronically excited carbon atoms, C(1D), and acetylene was studied at two average collision energies of 45 kJ mol−1 and 109 kJ mol−1 employing the crossed molecular beam technique. The time-of-flight spectra recorded at mass to charge m/e=37(C3H+) and m/e=36(C3+) show identical patterns indicating the existence of a carbon versus atomic hydrogen exchange pathway to form C3H isomer(s); no H2 elimination to the thermodynamically favorable tricarbon channel was observed. Forward-convolution fitting of our data shows that the reaction proceeds via direct stripping dynamics on the A′1 surface via an addition of the carbon atom to the π-orbital of acetylene to form a highly rovibrationally, short lived cyclopropenylidene intermediate which decomposes by atomic hydrogen emission to c-C3H(X 2B2). The dynamics of this reaction have important impact on modeling of chemical processes in atmospheres of comets approaching the perihelon as photolytically generated C(1D) atoms are present.
Ultraviolet Raman study of A1(LO) and E2 phonons in InxGa1−xN alloys
Authors: Alexson Dimitri , Bergman Leah , Nemanich Robert J. , Dutta Mitra , Stroscio Michael A. , Parker C. A. , Bedair S. M. , El-Masry N. A. , Adar Fran
Published: July 2002

We report on ultraviolet Raman spectroscopy of InxGa1−xN thin films grown on sapphire by metal-organic chemical vapor deposition. The A1(LO) and E2 phonon mode behavior was investigated over a large compositional range (0<x<0.50). Compelling evidence is presented for one-mode behavior for the A1(LO) phonon, and data suggestive of two-mode behavior are presented for the E2 phonon.
Local degradation of selectively oxidized AlGaAs/AlAs distributed Bragg reflectors in lateral-injection vertical-cavity surface-emitting lasers
Authors: De Vittorio Massimo , Vaccaro Pablo O. , De Giorgi Milena , De Rinaldis Sergio , Cingolani Roberto
Published: July 2002

We show the local degradation of a selectively oxidized top distributed Bragg reflector (DBR) in a lateral-junction vertical-cavity surface-emitting laser (LJ-VCSEL) working at room temperature in continuous-wave operation. The measurements were carried out by a scanning microluminescence system used in reflection mode. The injection of a few milliamps in continuous-wave operation at room temperature in the LJ-VCSEL induces damage both in the DBRs and in the active area. The submicron resolution maps of the reflected laser intensity, recorded from the top surface of the LJ-VCSEL, show a strong local change in the top DBR reflectivity before and after current injection. The μ-photoluminescence map, recorded after the device failure, shows that the radiative recombination is strongly decreased in the damaged area of the device.
Deconvolution of the intrinsic spontaneous spectrum of vertical-cavity surface-emitting devices
Authors: Royo P. , Stanley R. P. , Ilegems M. , Streubel K. , Gulden K. H.
Published: July 2002

A simple method for determining the intrinsic spontaneous spectrum of vertical-cavity surface-emitting devices is presented. The procedure is based on angle-resolved measurements of the top-emission spectra and comparison with numerical simulations, which corresponds to a deconvolution. The use of this technique is demonstrated on AlGaInP-based microcavity light-emitting diodes designed for top emission at 650 nm. Measurement on reference light-emitting diodes having the same active region confirmed the accuracy of this method which is nondestructive, easy to implement, and can be applied to vertical-cavity surface-emitting lasers operating below threshold.
Reactions at suprathreshold energy: Evidence of a kinematic limit to the internal energy of the products
Authors: Picconatto Carl A. , Srivastava Abneesh , Valentini James J.
Published: July 2002

In numerous state-to-state dynamics studies of atom transfer reactions at energies substantially in excess of the barrier height, population is not observed in many of the energetically accessible product quantum states. Frequently, the experimental state distributions end for internal energies well below the total energy limit. The lack of observable population in these high energy states has led us to conclude that not all of the total available energy is accessible to the rovibrational states of the products. To explain this behavior we describe a model in which the internal energy is constrained by kinematic factors. We use this model to show that the energy constraint is quantitatively determined by the skew angle for the reaction. We apply this model to the results of state-to-state experiments on a large number of reactions and get quantitative agreement between our predictions and actual observations.
Chemoconvection: A chemically driven hydrodynamic instability
Authors: Bees M. A. , Pons A. J. , Sørensen P. G. , Sagués F.
Published: July 2002

We describe theory and experiments concerning a chemical reaction, the alkaline oxidation of glucose with methylene blue as a catalyst, that is hypothesized to drive fluid motion via an overturning instability, as an example of a “chemoconvective” process. A theoretical model is developed to explain this phenomenon and linear analyses from steady and pseudosteady states are used to predict the basic length and time scales of the patterns which initially appear. These theoretical predictions, using kinetic parameters from recent independent experiments, are contrasted with results from pattern initiation experiments. Preliminary comparisons indicate good qualitative and quantitative agreement.
Enhanced photoemission of Ag–O–Cs composite thin films with an internal electric field
Authors: Zhang Q. F. , Liu W. M. , Xue Z. Q. , Wu J. L.
Published: July 2002

Ag–O–Cs thin films with internal field-assisted structure were fabricated, and enhanced photoemission was observed when the internal electric field was applied to the thin films. The increase of photoelectronic quantum yield, corresponding to the applied 30 V bias, was about 15.7%, while the thin films were irradiated by the light with wavelength of 510 nm. From an analysis of the electric potential distribution in the Ag–O–Cs thin films with the applied internal electric field, it is found that the interfacial barrier between the Ag nanoparticles and the Cs2O matrix is decreased and the vacuum level at the surface is degraded. The calculated barrier curves for various applied biases are illustrated to show the thinning effect of internal electric field on the interfacial barrier width. The theoretical lowering of interfacial barrier height is obtained as 0.08 and 0.22 eV when the thin films are stimulated by applied bias of 1 and 30 V, respectively. Further, a group of formulas as well, based upon the electric potential distribution in the Ag–O–Cs thin films, is deduced to describe the relationship between the applied bias and the degradation of the surface vacuum level. The enhanced photoemission of Ag–O–Cs thin films is attributed to the field-induced variations in the energy band structure which are considered to result in the increased probabilities for the photoexcited electrons to travel through the interfacial barrier and escape into the vacuum.
Plasma oxidation of thin aluminum layers for magnetic spin-tunnel junctions
Authors: Kuiper A. E. T. , Gillies M. F. , Kottler V. , ’t Hooft G. W. , van Berkum J. G. M. , van der Marel C. , Tamminga Y. , Snijders J. H. M.
Published: July 2002

This article presents results of a study initiated to characterize the plasma-oxidation process of very thin Al films, a technology commonly used to produce good barrier layers for magnetic spin-tunnel junctions. The behavior of oxygen in the oxidizing Al layer is determined using both quantitative (Rutherford backscattering spectrometry, transmission electron microscopy) and qualitative (x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry) analytical techniques. We have applied in situ XPS and experimented with O218 to unravel details of the oxidation mechanism. In addition, the influence of the oxygen pressure on the oxidation rate was established, both with and without a plasma being present. From optical emission spectra it is concluded that this pressure has a minor effect on the relative abundance of excited species in the oxygen plasma. When combined, these data constitute the basis of a model that distinguishes several steps in the plasma oxidation of Al. At the start, oxygen penetrates rapidly throughout the total Al layer, followed by a period of increasing oxygen concentration but constant oxide thickness. Finally, the Co underlayer becomes involved in the oxidation process, which marks the deterioration of the spin-tunnel junction. Evidence is obtained that for the thicker initial Al layers the Co electrode layer starts to oxidize before completion of the Al oxidation. This explains why for 0.8-nm-thick Al films the highest tunnel-magnetoresistance effect is obtained for stoichiometric Al2O3, whereas for 1.5 nm Al this occurs while the oxide is still substoichiometric.
A complete look at a multiple pathway reaction: The reaction of O(1D) with ethane
Authors: Shu Jinian , Lin Jim J. , Lee Yuan T. , Yang Xueming
Published: July 2002

The O(1D) atom reaction with ethane has been investigated using crossed molecular beam techniques. Four different reaction channels have been observed. Distinctive product angular distributions have also been observed for these reaction channels, indicating these reaction channels proceed via different dynamical pathways. These detailed experimental investigations provide a full dynamical picture of this reaction and also an important link between chemical reaction dynamics and kinetics studies. The experimental results presented here also provide an excellent test ground for developing quantitative theoretical understandings of multiple channel reaction dynamics for this interesting reaction.
The reaction N2++N2→N3++N from thermal to 25 eV
Authors: Tosi Paolo , Lu Wenyun , Bassi Davide , Tarroni Riccardo
Published: July 2002

The endothermic reaction N2++N2→N3++N has been investigated both experimentally and by ab initio calculations. Integral cross sections are presented as a function of collision energy. For excited N2+ ions the reaction onset shifts towards lower energies, indicating that the internal energy of the reactant ions promotes the reaction. Analysis of the energy dependence of the cross section does not allow an unequivocal determination of the reaction endothermicity. However, calculations of the ground doublet surface indicate that the minimum energy path from reactants to products proceeds through strongly bent geometries, with an endothermicity equivalent to the thermodynamic value of about 5 eV. For linear geometries an additional barrier of about 1 eV is found in the product channel. The structure of N4+ in its first excited quartet state has been also calculated by ab initio methods.
Magnetohydrodynamic simulation of gas embedded plasma discharge
Authors: Esaulov Andrey , Sasorov Pavel , Soto Leopoldo , Zambra Marcelo
Published: July 2002

A pathbreaking theoretical investigation of the dynamics of gas embedded plasma discharge has been performed within the scope of a one-dimensional, two-temperature magnetohydrodynamic model that includes plasma dynamics, as well as the accretion of the plasma part of the discharge from the adjoining gas. Good agreement between the simulation results and experimental data has been found. New information about the discharge structure and the distributions of plasma parameters has been obtained. It was revealed that the structure of initially preionized region greatly affects pinch dynamics even at the late stages of the discharge evolution. Present investigation opens new horizons in solving various sophisticated problems of modern plasma physics that require consideration of plasma dynamics at the conditions, critical for current breakdown.
Comparative optical studies of p-type and unintentionally doped GaN: The influence of annealing
Authors: Guha S. , Keller R. C. , Yang V. , Shahedipour F. , Wessels B. W.
Published: July 2002

We present Raman studies of p-type and unintentionally doped GaN epitaxial layers grown by metalorganic vapor phase epitaxy onto c-plane sapphire substrates. The E2 (high) Raman mode from a series of thermally annealed p-type samples shows that a compressive lattice distortion is induced with increasing annealing temperature. This is further corroborated by our photoluminescence measurements which show that the blue luminescence at 2.8 eV undergoes a redshift upon increasing the annealing temperatures beyond 650 °C. In comparing the Raman and photoluminescence spectra from the various samples we discuss the importance of two possible mechanisms: local distortion and longitudinal optical phonon–plasmon coupling.
Texture inheritance in Al(Cu) interconnect materials
Authors: Murray C. E. , Rodbell K. P.
Published: July 2002

Deposition surface morphology plays a major role in governing the texture within sputter deposited Al (0.5% wt., Cu) films. The presence of a thin Ti barrier layer provides an epitaxial template for Al(Cu) atoms to grow. However, the topography of the surface on which the Ti is deposited also influences Al(Cu) film texture. Through a combination of x-ray pole figure analysis and scanning electron microscopy, we observed that the behavior of (111) texture of Al(Cu) films deposited on Ti is dictated by the underlying surface roughness. A quantitative link between the offset in maximum Al (111) texture from the substrate normal and the average surface orientation of the deposition surface has been established. Information from atomic force microscope scans of the underlying surface has been used to construct a surface offset distribution (SOD), which contains the distribution of local orientations within a surface. Above a threshold roughness of the deposition surface, the SOD maxima correspond to the observed offset Al (111) texture in Al(Cu)/Ti films deposited on a variety of interlevel dielectric layers.
Thermal stability of magnetic tunnel junctions studied by x-ray photoelectron spectroscopy
Authors: Keavney David J. , Park Sungkyun , Falco Charles M. , Slaughter J. M.
Published: July 2002

We have studied the evolution of chemical state of the metallic layers in NiFe/Al oxide/NiFe tunnel junction structures in as-deposited films and after postdeposition annealing. Both top and bottom NiFe layers in as-deposited films show significant Fe oxidation, but no Ni oxidation. This Fe is reduced in annealed samples, implying that oxygen migrates from the FeNi layers, possibly into the Al oxide layer. We also find that both top and bottom electrodes are significantly oxidized even in optimally annealed films.
Postnucleation surface transport-kinetical phenomena and morphological instability in film deposition from vapor
Authors: Louchev Oleg A. , Sato Yoichiro , Kanda Hisao
Published: July 2002

The problem of transition of the postnucleation surface profile to continuous growth in the chemical vapor deposition process is considered. The developed numerical model includes a comprehensive set of transport-kinetical phenomena ongoing on the growth interface taking into account the effect of surface self-shadowing from the impinging species feeding the growth. This effect takes place when the characteristic length scale of surface profiles provided by the nucleation stage is lower than the free mean path of the gas species. The effect is shown to be able to destabilize the surface morphology in cases when the surface concentration is held far from the adsorption–desorption equilibrium and the surface profile length provided by the nucleation stage is considerably larger than the surface diffusion length of the growth species. Conditions under which the postnucleation surface smoothing may take place are specified. Experimentally feasible steps for postnucleation surface smoothing are discussed.
Emission spectrum of hard x-rays, generated from iron, copper, and molybdenum targets by subpicosecond KrF laser pulses
Authors: Takeyasu N. , Hirakawa Y. , Imasaka T.
Published: July 2002

K -shell emission spectra of iron, copper, and molybdenum in the range of 1.5–20 keV using a 500 fs KrF excimer laser have been observed and are reported herein.
Modeling of the multiple-toroidicity stellarator/heliotron magnetic fields
Authors: Smirnova M. S.
Published: July 2002

A typical feature of all current stellarator-type configurations with discrete toroidal or modular field coils is a multiple-toroidicity character of the magnetic field. It manifests itself in the presence of distant harmonics with high-order N⩾2 toroidal mode numbers in a Fourier-decomposition of the magnetic field. The main purpose of the present paper is to derive convenient magnetic-field models, which reflecting adequately all specific features of the magnetic field in multiple-toroidicity stellarators, could allow one to reduce the adiabatic invariants of particle motion to elliptic integrals. Such models are derived in the paper for a number of current stellarator-type devices on the basis of the analysis of the structure of ripple wells along field lines. The use of these magnetic-field models can simplify investigations of the neoclassical transport in stellarators.
CoNi/Pt interface roughness probed by nonlinear magneto-optics, x-ray scattering and atomic force microscopy
Authors: Bal K. , Kirilyuk A. , Rasing Th. , Luo Y. , Samwer K. , Haast M. A. M. , Lodder J. C.
Published: July 2002

The crystallographic contribution of the nonlinear magneto-optical response from CoNi/Pt interfaces appears to scale linearly with increasing interface roughness as determined by small angle x-ray scattering and atomic force microscopy. From the magnetic contribution it follows that the increased interface roughness causes the interface moment to turn out of plane while the bulk of the film has an in-plane magnetization.
Rotating magnetic field current drive in a hollow plasma column with a steady toroidal field
Authors: Farengo Ricardo , Clemente Roberto A.
Published: July 2002

The effect of a steady azimuthal magnetic field on rotating magnetic field current drive is studied. The configuration considered consists of an infinitely long plasma column with a finite radius conductor, which carries a steady longitudinal current, running along its axis. The ions are assumed to be fixed and the electrons are described using an Ohm’s law that contains the Hall term. A fully two-dimensional computer code is developed to solve the resulting time-dependent equations. For some values of the steady azimuthal field, two steady-state solutions with different efficiencies are found.
A diffusion quantum Monte Carlo method based on floating spherical Gaussian orbitals and Gaussian geminals: Dipole moment of lithium hydride molecule
Authors: Lu Shih-I
Published: July 2002

A diffusion quantum Monte Carlo method, based on a trial wave function, built from floating spherical Gaussian orbitals and geminals, is reported. The ability to precisely sample the Gaussian function by using the Ornstein–Uhlenbeck random walks process, to treat electron correlation effect explicitly, and to apply the Hellmann–Feynman theorem are the important consequences of this approach. Floating functions, which remedy deficiency of electron density around nuclei, satisfy the Hellmann–Feynman theorem. Gaussian geminals, which correlate the electron pair directly, are used to treat correlation energy and to improve the quality of the trial wave function. Applying this type of trial wave function, within fixed-node approximation, in diffusion quantum Monte Carlo (QMC) calculation of ground state energies of lithium hydride molecule, provides very good estimates of the electronic energy and dipole moment. We also compare the results with several previous QMC results and experiments.
Construction of reproducing kernel Hilbert space potential energy surfaces for the 1 A″ and 1 A′ states of the reaction N(2D)+H2
Authors: Hollebeek Timothy , Ho Tak-San , Rabitz Herschel , Harding Lawrence B.
Published: July 2002

We present in detail the construction procedure of two high quality global potential energy surfaces that have recently been used in quasiclassical and quantum dynamics studies for the reaction N(2D)+H2→NH+H. The procedure is based on the reproducing kernel Hilbert space method to interpolate high-level multireference configuration interaction ab initio calculations using augmented polarized triple zeta basis sets. A particular molecular coordinate system has been adopted to treat the nonadditive three body interaction as well as to guarantee the triangle inequalities of three molecular bond lengths and permutation symmetry of the two hydrogen atoms. A new radial reproducing kernel that remains finite at short distances while decaying to zero at large distances has been introduced, especially, for treating the molecule in the linear H–N–H configurations. Moreover, a robust procedure has been devised to handle the cusp in the two-body H2 interaction associated with the 1 A″ potential energy surface. The resulting surfaces are smooth, accurate, efficient to evaluate, exactly reproducing the input data upon which they are based and represent a significant improvement over previous surfaces for this reaction.
Topologies to geometries in protein folding: Hierarchical and nonhierarchical scenarios
Authors: Fernández Ariel , Colubri Andrés , Berry R. Stephen
Published: July 2002

This work presents a method to portray protein folding dynamics at a coarse resolution, based on a pattern-recognition-and-feedback description of the evolution of torsional motions of the backbone chain in the hydrophobic collapse of the protein. The approach permits theory and computation to treat the search of conformation space from picoseconds to the millisecond time scale or longer, the time scales of adiabatic evolution of soft-mode dynamics. The procedure tracks the backbone torsional coordinates modulo the basins of attraction to which they belong in the Ramachandran maps. The state and history of the backbone are represented in a map of local torsional states and hydrophobicity/hydrophilicity matching of the residues comprising the chain, the local topology matrix (LTM). From this map, we infer allowable structural features by recognizing patterns in the LTM as topologically compatible with particular structural forms within a level of frustration tolerance. Each such 3D realization of an LTM leads to a contact map, from which one can infer one or more structures. Introduction of energetic and entropic terms allow elimination of all but the most favored of these structures at each new juncture. The method’s predictive power is first established by comparing “final,” stable LTMs for natural sequences of intermediate length (N⩽120) with PDB data. The method is extended further to β-lactoglobulin (β-LG, N=162), the quintessential nonhierarchical folder.
Demagnetizing factors of long cylinders with infinite susceptibility
Authors: Chen D.-X.
Published: July 2002

Simple analytic formulas are proposed, that provide a close approximation to the fluxmetric and magnetometric demagnetizing factors of long cylinders with infinite susceptibility as functions of aspect ratio.
An interatomic potential for mercury dimer
Authors: Munro Lindsey J. , Johnson J. Karl , Jordan Kenneth D.
Published: July 2002

The potential energy curve of the ground electronic state of the Hg dimer has been calculated using the CCSD(T) procedure and relativistic effective core potentials. The calculated binding energy (0.047 eV) and equilibrium separation (3.72 Å) are in excellent agreement with experiment. A variety of properties, including the second virial coefficient, rotational and vibrational spectroscopic constants, and vibrational energy levels, have been calculated using this interatomic potential and agreement with experiment is good overall.
Shear Alfvén mode resonances in nonaxisymmetric toroidal low-pressure plasmas. II. Singular modes in the shear Alfvén continuum
Authors: Salat A. , Tataronis J. A.
Published: July 2002

The spatial dependence of modes in the shear Alfvén continuum of nonaxisymmetric toroidal magnetohydrodynamic plasma equilibria of zero pressure is investigated theoretically. As in configurations with plane or axial symmetry it is found that these modes may exhibit a singularity extended across whole “singular” magnetic surfaces. However, with nonaxisymmetry in the zero pressure approximation a logarithmic singularity appears in general rather than the oscillatory singularity of an axisymmetric plasma. If the nonaxisymmetry is too strong, the surface-covering property of the singular modes may be lost.
Bead-break instability
Authors: Gaskell P. H. , Kapur N. , Savage M. D.
Published: July 2002

Flow is considered in a “fluid bead” located in the nip between two contra-rotating rolls, and bounded by two curved menisci. Such a flow arises in meniscus roll coating where fluid is transferred from the lower applicator roll to a substrate, in contact and moving with the upper metering roll, by means of a transfer jet (snake). Equilibrium of the bead is maintained through a balance of hydrodynamic and capillary stresses, the stability of which is considered experimentally by increasing the speed of the metering roll while that of the applicator roll remains constant. At a critical speed ratio, the upstream meniscus becomes unstable; the bead contracts as the meniscus accelerates forward and merges with its downstream counterpart—giving rise to “bead-break.” A mathematical model, based on lubrication theory, exhibits multiple solutions and a limit point for the existence of steady solutions. A linear stability analysis identifies the stable solution and shows that the flow becomes unstable at the limit point—which is taken to be the onset of bead-break. The criterion for bead-break is (d/dX)[P+σ/Ru]<0 at the upstream meniscus, where P, σ, and Ru represent fluid pressure, surface tension, and radius of curvature of the upstream meniscus, respectively. The effect of the pressure gradient is shown to stabilize the fluid bead whereas that of the surface tension term is to destabilize once the upstream meniscus—and hence the whole bead—is located downstream of the nip. For a given geometry and flow rate, the critical speed ratio decreases with increasing characteristic capillary number. Theoretical predictions compare well with experimental data.
Injection- and space charge limited-currents in doped conducting organic materials
Authors: Jain S. C. , Geens W. , Mehra A. , Kumar V. , Aernouts T. , Poortmans J. , Mertens R. , Willander M.
Published: July 2002

Most conducting organic materials have a background p-type doping varying in the range 1015–1017 cm−3. We report results of a theoretical and experimental study of carrier transport in p-doped organic Schottky diodes. The theory given in this article shows that in a doped organic material with ohmic contacts the current is ohmic at low voltages. If the ohmic contact at the cathode is replaced by an Al Schottky contact the current varies exponentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltages. The voltage at which the change takes place depends on the doping concentrations. In the SCLC regime the current varies according to the well-known V2 law if there are no traps and the mobility is independent of the electric field. If either trapping or effect of field on mobility is important, the current varies as Vm, where m>2. We have investigated experimentally the I–V characteristics of Schottky diodes fabricated using the PPV-based oligomer 2,5-di-n-octyloxy-1,4-bis (4′, 4″-bis-styryl) styrylbenzene (Ooct-OPV5) blended with polystyrene (PS) and the PPV-based polymer poly(2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10). As predicted by the theory, Al/Ooct-OPV5:PS/ITO (indium tin oxide) and Al/OC1C10/ITO Schottky diodes do show that the current varies exponentially with V at low voltages and as SCLC according to the Vm law (with m=3) at high voltages. The V3 variation of the current in the SCLC regime can be due to trapping or field dependent mobility. It is not possible to distinguish unambiguously between the two mechanisms using the experimental results. The voltage at which transition from the Shockley current to SCLC takes place can be used to determine the background doping concentration. The p-type background doping concentration in the Ooct-OPV5 is found to be ∼1017 cm−3. From the temperature variation of the hole current at low voltages, a value 0.53±0.1 eV is determined for the Schottky barrier height at the Al/Ooct-OPV5:PS contact. When image barrier lowering for 1017 cm−3 doping is taken into account, this value of the barrier height is in good agreement with the difference in the Al work function and highest occupied molecular orbital of the organic material. Finally we suggest that if the background doping concentration can be eliminated, the SCLC and light emission in the light-emitting diodes should occur at lower voltages.
Effective medium theory for large particulate size composites
Authors: Stoyanov Aleksandr J. , Fischer Eugene C. , Überall Herbert
Published: July 2002

The present study extends our previous work that concerned the effective permittivities and absorptivities of particulate composites consisting of graphite-type microsphere inclusions in a dielectric matrix, and their suitability as radar-absorbing materials. Whereas in that work spheres of one size (radius a=50 μm) had been considered only, we here study the dependence of the results on the sphere radius of up to one or several millimeters, and find sizable effects on permittivities for a⩾1 mm, and below that for the absorptivities, indicating the increasing importance of multiple scattering effects. While the multiple-scattering formulas utilized here are based on a low-frequency approximation, we devise a modification of these formulas in order to extend their validity to higher frequencies (or larger particle sizes).
Spin wave instability in single crystal Zn–Y hexagonal ferrite at 8.93 GHz
Authors: Cox Richard G. , Patton Carl E. , Wittenauer Michael A. , Kabos Pavel , Chen Liang
Published: July 2002

Resonance saturation (RS), subsidiary absorption (SA), and parallel pump (PP) spin wave instability threshold measurements have been made on single crystal easy plane disks of Mn substituted Zn–Y type hexagonal ferrite materials at 8.93 GHz and room temperature. For each configuration, “butterfly curves” of the spin wave instability threshold microwave field amplitude hcrit as a function of the static field applied in the disk plane were obtained. The previous theory for these instability processes was also extended to include planar magnetocrystalline anisotropy and a wave vector k dependent spin wave linewidth, ΔHk. The RS butterfly curve had a characteristic “V” shape with a rounded minimum at the ferromagnetic resonance (FMR) field. The nominal ΔHk needed to fit the data at the ferromagnetic resonance field was 7 Oe, but the butterfly curve shape indicated a k-dependent ΔHk. The butterfly curves for the PP configuration were flat at low field and then diverged rapidly at the cutoff field for first order instability processes, Hcut. The SA butterfly curves were also flat over the field interval for first order processes, but then decreased as the field was increased above Hcut. This decrease is attributed to the onset of second order processes due to the proximity of the FMR and additional magnetostatic mode peaks as one moves to and then above Hcut. The flat portions of the PP and SA butterfly curves could be fitted with a single k-independent ΔHk value of 18 Oe.
Mode dependent intracluster vibrational energy redistribution rate in size-selected benzonitrile–(CHCl3)n=1–3 clusters
Authors: Yamamoto Ryousuke , Ebata Takayuki , Mikami Naohiko
Published: July 2002

The rate constants of intracluster vibrational energy redistribution (IVR) of benzonitrile–(CHCl3)1 for the 121 (1000 cm−1), 11 (760 cm−1), and 6a1 (460 cm−1) levels have been measured by time-resolved stimulated Raman-UV double resonance spectroscopy. It was found that the observed rate constants are independent of the energies but strongly dependent on the vibrational modes. In order to find a relationship between the structure and the IVR rate, structures of benzonitrile–(CHCl3)n=1–3 have been determined based on the results of the Raman spectra and the high resolution S1–S0 electronic spectra for the size-selected clusters, and ab initio MO calculations. The Raman spectra were observed for both CHCl3 and benzonitrile sites. It was found that the CH stretching vibration of the CHCl3 moiety showed a higher frequency shift in the clusters, whose magnitude depends on the binding site to benzonitrile. For the benzonitrile moiety, the Raman spectra of CH stretch (ν2), CN stretch (νCN), ring breathing (ν12 and ν1), and CCC in-plane bending (ν6a) vibrations were investigated. From those results, it was concluded that the clusters have the form such that the CH hydrogen of the first CHCl3 is hydrogen-bonded to the N end of the CN group, while second and third CHCl3 are hydrogen-bonded to the phenyl ring. The observed mode dependence of the IVR rate constants will be discussed based on the cluster structure and the vibrational motion.
MP2 correlation effects upon the electronic and vibrational properties of polyyne
Authors: Poulsen Tina D. , Mikkelsen Kurt V. , Fripiat Joseph G. , Jacquemin Denis , Champagne Benoı̂t
Published: July 2002

The linear infinite periodic chain of carbon atoms (polyyne) is studied at the MP2 level employing the crystal orbital approach. The equilibrium structure, the bond length alternation, the energy band gap, the Young Modulus, the force constants, the vibrational frequencies, and the phonon dispersion curves are determined and compared to Hartree–Fock results.
Copper gettering by aluminum precipitates in aluminum-implanted silicon
Authors: Petersen G. A. , Myers S. M.
Published: July 2002

Copper in Si is shown to be strongly gettered by Al-rich precipitates formed by implanting Al to supersaturation and then annealing. At temperatures ranging from 600 to 800 °C a layer containing Al precipitates is found to getter Cu from Cu silicide located on the opposite side of a 0.25 mm Si wafer, indicating a substantially lower chemical potential for the Cu in the molten-Al phase. Cu gettering proceeds rapidly until an atomic ratio of approximately 2 Cu atoms to 1 Al atom is reached in the precipitated Al region, after which the gettering process slows down. Redistribution of Cu from one Al-rich layer to another at low Cu concentrations demonstrates that a segregation-type gettering mechanism is operating. Cu gettering occurs primarily in the region containing the precipitated Al rather than in the region where the Al is entirely substitutional.
Effects of phase transformation and interdiffusion on the exchange bias of NiFe/NiMn
Authors: Lai Chih-Huang , Lien W. C. , Chen F. R. , Kai J. J. , Mao S.
Published: July 2002

The correlation between the exchange field of NiFe/NiMn and the phase transformation of NiMn was investigated. Transmission electron microscopy (TEM) dark-field images, contributed by the order phase of NiMn, were used to identify the location and volume fraction of the order phase. TEM selected area diffraction patterns showed the (110) superlattice diffraction rings of NiMn, verifying the existence of the order phase in the annealed samples. The order volume fraction can be calculated by the dark field image contributed by the (110) diffraction. The exchange field increased almost linearly with increasing order volume fraction. Energy dispersive x-ray spectroscopy attached to TEM indicated that Mn diffused into NiFe for annealing at 280 °C, leading to a larger coercivity and small coercivity squareness. Part of the NiMn still maintains the paramagnetic phase even after annealing at 280 °C.
Simultaneous electroluminescence and photoluminescence aging studies of tris(8-hydroxyquinoline) aluminum-based organic light-emitting devices
Authors: Popovic Zoran D. , Aziz Hany , Hu Nan-Xing , Ioannidis Andronique , dos Anjos Paulo N. M.
Published: July 2002

Using experimental devices which do not show electroluminescence, we have previously found that photoluminescence of tris(8-hydroxyquinoline) aluminum (AlQ3), a widely used organic electroluminescent material, decreases under prolonged transport of holes. This leads to a conclusion that AlQ3 cations are unstable, and to our proposal that the same mechanism is responsible for degradation of organic light-emitting devices (OLEDs) based on AlQ3. In this communication, we broaden our studies to include simultaneous electroluminescence and photoluminescence measurements on OLEDs containing thin AlQ3 as the emitter layer. Results show that the decrease in the electroluminescence efficiency is indeed associated with degradation of the AlQ3 in the vicinity of the hole transport layer interface.
Wire vibration, bowing, and breakage in wire electrical discharge machining
Authors: Arunachalam C. , Aulia M. , Bozkurt B. , Eubank P. T.
Published: July 2002

This article provides an analysis of the wire electrical discharge machining (EDM) process. The causes of wire vibration, bowing, and breakage are identified. The cross sectional configuration of an eroded wire is derived from basic physics under the assumption that the erosion rate is constant. This configuration is verified experimentally with further explanation as to why numerous experimental studies have yielded different configurations. A computational model has been developed that can evaluate the systematic effects that lead to wire breakage by determining the stress induced by wire erosion and the stress induced by the sparks during the operation of a wire-cutting EDM machine. This model is also capable of determining the extent of wire bowing and vibrations in these machines but only in the frontal direction. The model is supported by data from experiments performed on an AGIECUT 612 wire machine cutting a 10 mm high copper bar with a 0.15 mm brass wire to acquire wire breakage data. The nearly parabolic shape of the bowed wire agrees with the results of other authors making more restrictive assumptions.
Spectroellipsometric study of sol–gel derived potassium sodium strontium barium niobate films
Authors: Mak C. L. , Lai B. , Wong K. H. , Choy C. L. , Mo D. , Zhang Y. L.
Published: July 2002

Spectroscopic ellipsometry (SE) was used to characterize the sol–gel derived (K0.5Na0.5)0.4(Sr0.6Ba0.4)0.8Nb2O6 (KNSBN) thin films as a function of sol concentration. In the analysis of the measured SE spectra, a modified double-layer Forouhi–Bloomer model was adopted to represent the optical properties of the KNSBN films. In this model, the films were assumed to consist of two layers—a bottom bulk KNSBN layer and a surface layer that composed of bulk KNSBN as well as void. Good agreement was obtained between the measured spectra and the model calculations in the chosen spectral region. Effective medium approximation theory was used to evaluate the effective refractive index for the surface layer. The results of SE have been correlated with atomic force microscopy measurements of surface roughness. Our analyses have shown that the surface layer had a lower refractive index than the bottom one. In addition, the refractive index and the surface roughness of the KNSBN films increase with the sol concentration.
Epitaxial growth of ZnO films on Si substrates using an epitaxial GaN buffer
Authors: Nahhas Ahmed , Kim Hong Koo , Blachere Jean
Published: July 2002

We report on epitaxial growth of ZnO films on Si(111) substrates using an epitaxial GaN buffer layer. A rf magnetron sputtering process has been developed and utilized in growing epitaxial GaN buffers on Si, and then ZnO films on the GaN-buffered Si substrates. X-ray diffraction analysis shows that both the ZnO and GaN films are of a monocrystalline wurtzite structure with an epitaxial relationship of ZnO[0001]//GaN[0001]//Si[111] along the growth direction and ZnO[112_01]//GaN[112_0]//Si[11_0] along the in-plane direction. The successful growth of epitaxial ZnO/GaN films on Si demonstrates the feasibility and promise of integrating various functional devices on the same substrate.
Influence of beam head effects on high-power klystron amplifier
Authors: Uhm Han S. , Kim Hyoung S. , Park Gun S.
Published: July 2002

Transient behavior of the cavity excitation caused by beam head effects in high-power klystrons is investigated. Recognizing that the input cavity in a klystron can be excited by the input microwave signal and the head of the electron beam, it is shown that the frequency of the induced cavity voltage Vs driven by the signal is the same as the signal frequency ωs. On the other hand, the frequency of the induced cavity voltage Vh driven by the beam head is the cavity resonance frequency ω0. An expression of the induced voltage Vh is obtained by making use of the Laplace transformation. The induced voltage Vh caused by the beam head is expressed in terms of the current-change profile in time. The magnitude of the induced voltage caused by the beam head is inversely proportional to the beam rise time, thereby decreasing drastically for a large value of the beam rise time. It is also shown that the beam head effects are completely negligible whenever the beam rise time τr satisfies ω0τr=2nπ where n is an integer. The amplitude of the whole cavity voltage oscillates with the beat wave frequency Δω=ω0−ωs. This amplitude oscillation may play an important role in pulsed high-power klystrons.
Investigating individual chromopores within single porous silicon nanoparticles
Authors: Mason M. D. , Sirbuly D. J. , Carson P. J. , Buratto S. K.
Published: July 2002

We use single nanoparticle luminescence microscopy to determine a distribution of individual chromophores present in porous Si nanoparticles. From these distributions, we determine the average number of emitting chromophores in each nanoparticle and the fluorescence emission count rate of a single chromophore within the porous silicon nanoparticle. We also show that the same size nanoparticles prepared under two different electrochemical conditions have different fluorescence peak maxima, and exhibit different chromophore number distributions, consistent with the quantum confinement model for the luminescence in porous silicon.
Tunnel magnetoresistance on ferromagnetic single-electron transistors with multiple tunnel junction
Authors: Shirakashi Jun-ichi , Takemura Yasushi
Published: July 2002

Single-electron transistors with ferromagnetic multiple tunnel junctions are studied theoretically. Tunnel magnetoresistance under the Coulomb blockade regime is modulated by the gate voltage and is increased with increasing the number of tunnel junctions. Higher-order tunneling processes, especially for inelastic macroscopic quantum tunneling of charge, play an important role in the enhancement of the tunnel magnetoresistance. This implies that the tunnel magnetoresistance of the ferromagnetic single-electron transistor could be controlled by the gate and drain voltages and the number of the tunnel junctions.
Effect of particle size on the photoluminescence from hydrogen passivated Si nanocrystals in SiO2
Authors: Cheylan S. , Elliman R. G.
Published: July 2002

The effect of hydrogen passivation on the photoluminescence from Si nanocrystals prepared in SiO2 by ion implantation and annealing is examined as a function of nanocrystal size (implant fluence). Passivation is shown to produce a significant increase in emission intensities as well as a redshift of spectra, both of which increase with increasing fluence. These results are shown to be consistent with a model in which larger nanocrystals are assumed to contain more nonradiative defects (i.e., the defect concentration is assumed to be proportional to the nanocrystal surface area or volume). Since this results in a smaller fraction of larger nanocrystals contributing to the initial luminescence, emission spectra are initially blueshifted relative to that that might be expected from the physical nanocrystal size distribution. The contribution from larger crystallites is then disproportionately increased by passivation resulting in the observed redshift.
Evaluation of a governing equation for the macroscopic dynamics of electrorheological fluids
Authors: Pompeo Neto Luiz B. , Arruda Antonio Celso F. , Vieira Sheila L.
Published: July 2002

An analytical equation of motion is proposed to model the dynamics of change of electrorheological fluids under the influence of simultaneous shear (γ̇) and electrostatic (E) fields of force. This is accomplished with the use of constitutive relations written down by Mason and collaborators [A. Okagawa, R. G. Cox, and S. G. Mason, J. Colloid Interface Sci. 47, 536 (1974)] for the electrostatic and hydrodynamic stresses developed in the fluid. The properties of the suspension are considered in a way that both the dielectric and conductive characters of particles and suspending fluid are comprised. From the model results an equation relating η to E and γ̇, a characteristic apparent viscosity, that is not restricted to a simple quadratic dependence with E, a usual degree of functional dependence from which more recent experimental results, thus far inexplicably, seem to deviate. Experimental results will be compared to the theoretical predictions.
Analysis of amplification of thermal vibrations of a microcantilever
Authors: Muralidharan G. , Mehta A. , Cherian S. , Thundat T.
Published: July 2002

We examine the conditions under which the small amplitude of thermal vibrations of cantilevers typically used for atomic force microscopy and sensor applications can be enhanced through a feedback mechanism. Using a simple mathematical model with two independent measurable physical parameters, a time delay τ and a gain factor G, we show that for certain values of these two parameters, such amplification is feasible. Experimental measurements of the two parameters when amplification succeeded show that these fall in the range predicted by the calculations.
Electron paramagnetic resonance studies of Mn2+ ions in β-Ga2O3 single crystal
Authors: Kim I. G. , Yeom T. H. , Lee S. H. , Yu Y. M. , Shin H. W. , Choh S. H.
Published: July 2002

A Mn2+ ion-doped β-Ga2O3 single crystal was grown by using a floating zone method. By employing an X-band electron paramagnetic resonance (EPR) spectrometer, Mn2+ EPR spectra were recorded at room temperature. The rotation patterns in the crystallographic planes together with spin-Hamiltonian parameters of Mn2+ show unequivocally that the actual local site symmetry of the Mn2+ ion is monoclinic. A Mn2+ ion lies on a site with a two-fold rotation symmetry, parallel to the monoclinic axis of the crystal. The spectroscopic splitting tensor g, zero-field splitting parameters Bkqs, and the hyperfine tensor A are determined with the effective spin Hamiltonian. It turns out that the Mn2+ ion lies at only one site, the substitutional site for the Ga3+ ion in the oxygen octahedron rather than in tetrahedron. Energy levels of the ground state for a Mn2+ ion embedded in the β-Ga2O3 crystal are calculated.