bluuebunny/crossref_metadata_2025 huggingface dataset:
Equation of state of the rigid disk fluid from its triangle distribution
Authors: Stillinger Frank H. , Stillinger Dorothea K. , Torquato Salvatore , Truskett Thomas M. , Debenedetti Pablo G.
Published: July 2002

The distribution function f(3) for triplets of mutual nearest neighbors offers a description of local order for many-particle systems confined to a plane. This paper proposes a self-consistent theory for f(3) in the case of the classical rigid disk model, using three basic identities for closure. Numerical analysis of the resulting coupled nonlinear integral equations yields predictions for the pressure, the boundary tension, and the Kirkwood superposition defect for three disks in mutual contact. The approximation employed implicitly constrains the disk system to remain in the fluid phase at all densities up to close packing (ρa2=2/31/2). The pressure and boundary tension agree reasonably well with the corresponding predictions of the two-dimensional scaled particle theory, but the former agrees even better with a rational approximant due to Sanchez that reproduces eight virial coefficients.
Angular distributions of scattered electrons from gaseous benzene molecules
Authors: Gianturco F. A. , Lucchese Robert R.
Published: July 2002

Recent measurements of the angular distributions for scattered electrons in gaseous benzene have produced elastic (rotationally summed) differential cross sections over a broad range of angular values and of collision energies. The present calculations examine the corresponding computed quantities over the same energy and angular values and compare them with the measurements. The necessary exchange interaction is included both exactly and via approximate local models in order to test the reliability of the local model potentials for such delicate quantum dynamics features as angular distributions. Very good overall agreement is found between measurements and the calculations presented here. The local model potentials are found to provide good agreement with the exact exchange calculations at energies above 6.0 eV.
Monte Carlo study of the CO-poisoning dynamics in a model for the catalytic oxidation of CO
Authors: Albano Ezequiel V. , Marro Joaquín
Published: July 2002

The poisoning dynamics of the Ziff–Gulari–Barshad [Phys. Rev. Lett. 56, 2553 (1986)] model, for a monomer–dimer reaction, is studied by means of Monte Carlo simulations. Studies are performed within the monomer absorbing state and close to the coexistence point. Analysis of the average poisoning time (τp) allows us to propose a phenomenological scaling approach in which τp diverges logarithmically with the lattice side and algebraically with the distance to the coexistence point. The structure of monomer clusters during poisoning is analyzed and compared with observations at coexistence.
Forced vibrations of the fundamental family of modes in AT-cut quartz strip resonators
Authors: Tiersten H. F. , Sham T.-L.
Published: July 2002

Mindlin’s equations for the vibrations of elastic crystal plates are employed in the description of AT-cut quartz strip resonators. The electrically driven three-dimensional piezoelectric pure thickness solution is incorporated in the treatment. The driving voltage appearing in this thickness solution is included in the variational principle from which the plate equations are obtained. In this way the resulting Mindlin equations contain the driving voltage and hold for plates with small piezoelectric coupling. The equations are applied in the analysis of strip resonators. The eigensolutions are obtained by solving a sequence of one-dimensional problems that are defined by utilizing the results from the previous problem variationally. The driven solution is obtained by means of an expansion in the eigensolutions and a lumped parameter representation of the admittance, which is valid in the vicinity of a resonance, is obtained. Calculated results are presented for a range of geometries and the influence of the couplings is exhibited.
The classical Kepler problem and geodesic motion on spaces of constant curvature
Authors: Keane Aidan J. , Barrett Richard K. , Simmons John F. L.
Published: July 2002

In this paper we clarify and generalize previous work by Moser and Belbruno concerning the link between the motions in the classical Kepler problem and geodesic motion on spaces of constant curvature. Both problems can be formulated as Hamiltonian systems and the phase flow in each system is characterized by the value of the corresponding Hamiltonian and one other parameter (the mass parameter in the Kepler problem and the curvature parameter in the geodesic motion problem). Using a canonical transformation the Hamiltonian vector field for the geodesic motion problem is transformed into one which is proportional to that for the Kepler problem. Within this framework the energy of the Kepler problem is equal to (minus) the curvature parameter of the constant curvature space and the mass parameter is given by the value of the Hamiltonian for the geodesic motion problem. We work with the corresponding family of evolution spaces and present a unified treatment which is valid for all values of energy continuously. As a result, there is a correspondence between the constants of motion for both systems and the Runge–Lenz vector in the Kepler problem arises in a natural way from the isometries of a space of constant curvature. In addition, the canonical nature of the transformation guarantees that the Poisson bracket Lie algebra of constants of motion for the classical Kepler problem is identical to that associated with geodesic motion on spaces of constant curvature.
A mobile charge densities in harmonic oscillators (MCDHO) molecular model for numerical simulations: The water–water interaction
Authors: Saint-Martin Humberto , Hernández-Cobos Jorge , Bernal-Uruchurtu Margarita I. , Ortega-Blake Iván , Berendsen Herman J. C.
Published: July 2002

In this work we present a new proposal to model intermolecular interactions and use it for water molecules. The parameters of the model were fitted to reproduce the single molecule’s electrostatic properties, a sample of 352 points in a refined ab initio single molecule deformation potential energy surface (PES), and the theoretical limit of the dimerization energy, −20.8 kJ/mol. The model was able to reproduce a sample of 180 additional points in the single molecule deformation PES, and 736 points in a pair-interaction surface computed at the MP2/aug-cc-pVQZ′ level with the counterpoise correction. Though the model reproduced the diagonal of the polarizability tensor, it could account for only 60% of the three-body nonadditive contributions to the interaction energies in 174 trimers computed at the MP2/6-311++(2d,2p) level with full counterpoise correction, but reproduced the four-body nonadditivities in 34 tetramers computed at the same level as the trimers. The model’s predictions of the structures, energies, and dipoles of small clusters resulted in good agreement with experimental data and high quality ab initio calculations. The model also reproduced the second virial coefficient of steam at various temperatures, and the structure and thermodynamical properties of liquid water. We found that the short-range water–water interactions had a critical influence on the proper performance of the model. We also found that a model based on the proper intermolecular interactions requires the inclusion of intramolecular flexibility to be adequate.
Effect of the impact ionization of deep traps on the field distribution in planar thin-film GaAs structures
Authors: Prokhorov Eugenio F. , González-Hernández Jesus , Gorev Nikolai B. , Kodzhespirova Inna F. , Kovalenko Yury A.
Published: July 2002

The distribution of the electric field in planar GaAs structures made up of a thin film and semi-insulating compensated substrate is considered allowing for the impact ionization of deep traps in the substrate near the film–substrate interface. It is shown that there exists a critical film thickness below which the impact ionization of deep traps can make the film exhibit a long-length region of a uniform electric field exceeding the threshold of N-type negative differential mobility without recourse to special doping profiles.
X-ray diffraction investigation of n-type porous silicon
Authors: Chamard V. , Dolino G.
Published: July 2002

High resolution x-ray diffraction has been used to study the effect of doping level and illumination on the formation of n-type porous silicon. For highly doped n+-type porous layers prepared in darkness, an unusual increase of the lattice mismatch with the formation time is observed. When the samples are prepared under illumination, a degradation of the crystal quality, increasing with a decrease of the material doping level, is observed. The possible origins of these effects are discussed.
Interfacial stability and structure in InAs/GaAs(111)A heteroepitaxy: Effects of buffer layer thickness and film compositional grading
Authors: Zepeda-Ruiz Luis A. , Pelzel Rodney I. , Weinberg W. Henry , Maroudas Dimitrios
Published: July 2002

Interfacial stability and the morphology of the epitaxial film surface have been studied in InAs/GaAs(111)A heteroepitaxy based on atomistic simulations and scanning tunneling microscopy. Effects of buffer layer thickness were examined by analyzing two heteroepitaxial systems consisting of thin and thick GaAs buffer layers. In both cases, one monolayer of In0.50Ga0.50As is grown initially on the buffer layer prior to InAs growth. Our results indicate that film compositional grading and the resulting segregation of In atoms at defects in the semicoherent interface can be used effectively in conjunction with the mechanical compliance of thin buffer layers to delay the completion of the coherent-to-semicoherent interfacial transition.
Formation of β-C3N4 crystals at low temperature
Authors: Wei J.
Published: July 2002

Carbon nitride films with β-C3N4 crystals of 200 nm grain size were grown on Si (100) substrates using magnetron sputtering. Reactive deposition was achieved using a graphite target in an argon/nitrogen plasma at room temperature. These films were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. Micro level β-C3N4 crystal grains were observed with TEM. According to calculations from electron diffraction pattern, these crystalline structures were in a good agreement with hypothetical b-C3N4 structure. AFM measurement also indicated the grain size was around 200 nm and the carbon nitride films had low surface roughness. From XPS data, maximum N/C ratio of 0.5 was achieved in the films. XPS spectra of the films typically showed three peaks in the C 1s core level spectrum (centered at 284.6, 285.9, and 287.2 eV) and two peaks in the N 1s core level spectrum (centered at 398.7 and 400.2 eV). This indicates that there are two types of C–N bonds; N is bonded to sp2- or sp3-coordinated C atoms in the as-deposited films. FTIR spectra showed three absorption bands in the range of 1000–3000 cm−1. The absorption band around 2367 cm−1 can be attributed to C≡N nitrile bond. The absorption bands around 1559 and 1201 cm−1 demonstrate the existence of C=N (sp2) and C–N (sp3) bonds. Analysis of Raman spectrum further demonstrated the coexistence of sp, sp2, and sp3 bonds. All of XPS, FTIR, and Raman measurements showed the presence of fourfold coordinated β-C3N4 crystals in the films, which is in good agreement with TEM and electron diffraction results.
Ultraviolet photochemistry of hydrogen-bonded HBr⋅acetone complexes in argon matrices
Authors: McCurdy Patrick R. , Vorpagel Erich R. , Hess Wayne P.
Published: July 2002

We have studied the ultraviolet photoreaction of HBr⋅acetone complexes isolated in argon matrices using Fourier-transform infrared spectroscopy. Photoexcitation of the acetone π*←n0 transition, using either 266 or 309 nm, results in the reaction of hydrogen-bonded HBr⋅acetone complexes to produce HBr⋅propen-2-ol complexes through a novel photoenolization mechanism. Only the hydrogen-bonded HBr⋅acetone complexes undergo reaction following mid-UV irradiation. No significant loss of isolated reactant molecules occurs using either 266 or 309 nm excitation. Detailed ab initio calculations of HBr⋅propen-2-ol complex are presented to support these findings. Photoexcitation of similar matrices at 193 nm, which excites the acetone 3s←n0 transition and HBr directly, shows extensive reaction of HBr, acetone, and their complexes by a photofragmentation mechanism. We discuss the mechanism of HBr assisted acetone photoenolization in detail and report experimental and calculated band frequencies and structures of HBr⋅acetone complexes.
Theory of sub-Doppler Autler–Townes splitting in molecules: Alignment and orientation of the angular momentum in nonpolar molecules
Authors: Spano Frank C.
Published: July 2002

Within a density matrix framework, an analytical expression for the optical–optical double resonance (OODR) line shape for a molecule in the presence of a strong coupling laser is derived. The coupling laser is tuned to a transition between the upper level in the OODR sequence and another excited level which is not part of the OODR sequence. The pump and probe lasers are treated perturbatively, while the coupling laser is treated exactly. The line shape reveals sub-Doppler Autler–Townes (AT) splitting of the states |J,m〉 within the highest excited level. The splitting depends on |m|(m) for linearly (circularly) polarized coupling fields. m-state selectivity and therefore alignment and orientation of the angular momentum can be achieved by tuning the probe laser in the presence of a coupling field which is sufficiently strong to resolve the m-dependent AT splittings. m selectivity is analyzed for e and f parity 1Πg levels in diatomic molecules. A method to orient nonpolar diatomics using circularly polarized lasers is proposed.
A Monte Carlo study of symmetry breaking of I3− in aqueous solution using a multistate diabatic Hamiltonian
Authors: Margulis C. J. , Coker D. F. , Lynden-Bell R. M.
Published: July 2002

A model for the description of the electronic ground state of the triiodide ion in solution is developed. It is based on the “diatomics in molecules” technique and is parametrized from experimental data. The solvent molecules are treated by classical intermolecular potentials. The solvent–ion interaction, which depends on the instantaneous positions of the solvent molecules, enters into the Hamiltonian matrix elements as a spatially varying external electrostatic potential. We use the model to investigate the distribution of the bond lengths of a linear triiodide ion in water at 300 K using Monte Carlo calculations. We find that under these conditions the molecule is significantly distorted with considerable redistribution of charge and bond lengths of 2.95 Å and 3.38 Å. The free energy barrier to switching bond lengths at room temperature is quite high (of the order of 10 kT) so that the distortion is predicted to have a long lifetime. The distribution of instantaneous vibrational frequencies is investigated and shows that the solvent has a greater effect on the frequency of the antisymmetric stretch than on that of the symmetric stretch vibration.
Lifetime of ferroelectric Pb(Zr, Ti)O3 ceramic cathodes with high current density
Authors: Einat M. , Shur D. , Jerby E. , Rosenman G.
Published: July 2002

Electron emission from ferroelectric cathodes is investigated, it is commonly suggested as an electron source for different applications due to its special characteristics such as high current density, easy treatment, and operation. In this experimental research, a lifetime of lead zirconate-titanate ceramic cathode with composition related to a ferroelectric phase was studied. The strong plasma emission from the cathode was excited in a nonreversal (nonswitching) mode by application of unipolar high stress. Severe damage to the cathodes was observed, especially in a high repetition rate. An upper limit of the lifetime of the ferroelectric cathode with plasma-induced emission was estimated at about ∼106 pulses of ∼200 ns each at ∼100 Hz repetition rate. Possible applications of the limited lifetime ferroelectric cathode are discussed.
Spectral and intensity dependence of spatially resolved two-photon conductivity defects on a GaAsP photodiode
Authors: Osborn David L. , Leone Stephen R.
Published: July 2002

Femtosecond laser excitation through a near-field scanning optical microscope is used to study spatially resolved defects in the two-photon conductivity of a GaAs0.6P0.4 diffusion type photodiode. Two types of defects are observed when the photodiode is excited with femtosecond pulses below the bulk band gap energy. Photocurrent enhancement defects show a higher photocurrent than the surrounding areas and are driven by both one-and two-photon processes. These defects are often correlated with pits in the photodiode surface. Photocurrent depression defects have the normal two-photon power dependence and are not associated with surface pits. Based on the low measured coverage of both defect sites, the performance of GaAsP in auto- and cross correlators will be unaffected in most situations. A deviation in alloy stoichiometry, in which localized areas are rich in arsenic while poor in phosphorus, provides the best explanation for the origin of the enhancement defects.
Using mean field theory to determine the structure of uniform fluids
Authors: Vollmayr-Lee Katharina , Katsov Kirill , Weeks John D.
Published: July 2002

The structure of a uniform simple liquid is related to that of a reference fluid with purely repulsive intermolecular forces in a self-consistently determined external reference field (ERF) φR. The ERF can be separated into a harshly repulsive part φR0 generated by the repulsive core of a reference particle fixed at the origin and a more slowly varying part φR1 arising from a mean field treatment of the attractive forces. We use a generalized linear response method to calculate the reference fluid structure, first determining the response to the smoother part φR1 of the ERF alone, followed by the response to the harshly repulsive part. Both steps can be carried out very accurately, as confirmed by computer simulations, and good agreement with the structure of the full Lennard-Jones fluid is found.
The supersymmetric Camassa–Holm equation and geodesic flow on the superconformal group
Authors: Devchand Chandrashekar , Schiff Jeremy
Published: July 2002

We study a family of fermionic extensions of the Camassa–Holm equation. Within this family we identify three interesting classes: (a) equations, which are inherently Hamiltonian, describing geodesic flow with respect to an H1 metric on the group of superconformal transformations in two dimensions, (b) equations which are Hamiltonian with respect to a different Hamiltonian structure and (c) supersymmetric equations. Classes (a) and (b) have no intersection, but the intersection of classes (a) and (c) gives a system with interesting integrability properties. We demonstrate the Painlevé property for some simple but nontrivial reductions of this system, and also discuss peakon-type solutions.
Measurement of the d36 coefficient of mercury cadmium telluride by reflection second harmonic generation
Authors: Wark A. W. , Pugh D. , Berlouis L. E. A. , Cruickshank F. R. , Brevet P. F.
Published: July 2002

The second order nonlinear coefficient (d36) of the narrow band gap semiconductor, mercury cadmium telluride (MCT), is measured. Because MCT is strongly absorbing at a 1.06 μm wavelength, the measurement was performed by comparing the second harmonic intensity reflected from the material surface to the second harmonic intensity measured for a quartz sample in transmission. The analysis depends on the derivation of comparable expressions for the reflected and transmitted intensities. Using this approach a value of d36=350±40 pm/V is obtained, a value much larger than those reported for similar zinc-blende type materials. The large magnitude of the MCT d36 is attributed to an electronic resonance enhancement.
Local crystal analysis using near-field optical second harmonic microscopy: Application to thin ferroelectric films
Authors: Smolyaninov I. I. , Liang H. Y. , Lee C. H. , Davis C. C.
Published: July 2002

Application of near-field optical second harmonic microscopy to nanometer scale crystal and strain analysis of thin polycrystalline ferroelectric films is described. The near-field signal is shown to be sensitive to local crystal orientation by experimental and theoretical studies of polarization properties of second harmonic generation from a poled single crystal of BaTiO3. Spatial resolution of the order of 80 nm has been demonstrated by recovery of the local poling direction of individual ferroelectric domains in polycrystalline thin Pb(ZrxTi1−x)O3 films. Near-field strain induced second harmonic generation from thin BaxSr1−xTiO3 films has also been studied, and an agreement between theory and experiment has been established. Thus, the technique described provides a tool for nanometer scale crystal and strain analysis of polycrystalline samples.
Native donors and compensation in Fe-doped liquid encapsulated Czochralski InP
Authors: Zhao Y. W. , Luo Y. L. , Fung S. , Beling C. D. , Sun N. F. , Chen X. D. , Cao L. X. , Sun T. N. , Bi Keyun , Wu Xiang
Published: July 2002

Undoped and Fe-doped liquid encapsulated Czochralski (LEC) InP has been studied by Hall effect, current–voltage (I–V), and infrared absorption (IR) spectroscopy. The results indicate that a native hydrogen vacancy complex donor defect exists in as-grown LEC InP. By studying the IR results, it is found that the concentration of this donor defect in Fe-doped InP is much higher than that in undoped InP. This result is consistent with the observation that a much higher concentration of Fe2+ than the apparent net donor concentration is needed to achieve the semi-insulating (SI) property in InP. By studying the I–V and IR results of Fe-doped InP wafers sliced from different positions on an ingot, the high concentration of Fe2+ is found to correlate with the existence of this hydrogen complex. The concentration of this donor defect is high in wafers from the top of an ingot. Correspondingly, a higher concentration of Fe2+ can be detected in these wafers. These results reveal the influence of the complex defect on the compensation and uniformity of Fe-doped SI InP materials.
A novel simplified laser diode driver
Authors: Chang Chun-Ming , Liao Tai-Shan
Published: July 2002

A novel, low-cost, and simple circuit for driving a laser diode with low supply voltage was demonstrated. A laser diode was driven with a supplied voltage as low as 1.5 V using only two transistors and minimum passive components.
Bundle structure and sliding of single-walled carbon nanotubes observed by frictional-force microscopy
Authors: Miura K. , Ishikawa M. , Kitanishi R. , Yoshimura M. , Ueda K. , Tatsumi Y. , Minami N.
Published: July 2002

The bundle structure and the sliding of single-walled carbon nanotubes (SWNTs) have been observed by frictional-force microscopy. The diameter of the nanotube and the nearest distance between any two nanotubes were estimated to be 1.4 and 0.3 nm, respectively. The frictional force required to move SWNTs on a KCl(001) surface and its energy dissipation were estimated to be 11 nN and 0.75×10−16 J, respectively.
Analytical theory of frequency-multiplying gyro-traveling-wave-tubes
Authors: Nusinovich G. S. , Chen W. , Granatstein V. L.
Published: July 2002

The theory is developed which describes analytically the gain and bandwidth in frequency-multiplying gyro-traveling-wave-tubes. In this theory the input waveguide is considered in the small-signal approximation. Then, in the drift region separating the input and output waveguides, the electron ballistic bunching evolves which causes the appearance in the electron current density of the harmonics of the signal frequency. The excitation of the output waveguide by one of these harmonics is considered in a specified current approximation. This makes the analytical study of a large-signal operation possible. The theory is illustrated by using it to analyze the performance of an existing experimental tube.
Kick-starting the cell cycle: From growth-factor stimulation to initiation of DNA replication
Authors: Aguda Baltazar D.
Published: July 2002

The essential genes, proteins and associated regulatory networks involved in the entry into the mammalian cell cycle are identified, from activation of growth-factor receptors to intracellular signal transduction pathways that impinge on the cell cycle machinery and ultimately on the initiation of DNA replication. Signaling pathways mediated by the oncoproteins Ras and Myc induce the activation of cyclin-dependent kinases CDK4 and CDK2, and the assembly and firing of pre-replication complexes require a collaboration among E2F, CDK2, and Cdc7 kinase. A proposed core mechanism of the restriction point, the major checkpoint prior to commitment to DNA synthesis, involves cyclin E/CDK2, the phosphatase Cdc25A, and the CDK inhibitor p27Kip1.
Sum rules for generalized electron-pair moments
Authors: Koga Toshikatsu
Published: July 2002

For many-electron atoms, the generalized electron-pair density function g(q;a,b) represents the probability density function for the magnitude |ari+brj| of two-electron vector ari+brj to be q, where a and b are real-valued parameters. It is pointed out that the second moments 〈q2〉(a,b), associated with g(q;a,b), satisfy a rigorous sum rule which connects one- and two-electron properties of atoms and molecules for any exact and approximate wave functions. The same is also true in momentum space.
Coupled-cluster characterization of the ground and excited states of the CH2N and CH2P radicals
Authors: Brinkmann Nicole R. , Wesolowski Steven S. , Schaefer Henry F.
Published: July 2002

High-level coupled-cluster theory with large basis sets was used to determine the optimized geometries and harmonic vibrational frequencies for the ground and low-lying excited electronic states of the CH2N and CH2P radicals. Additionally, isotropic hyperfine splitting constants were determined for the C2v CH2X, trans-HCXH and cis-HCXH (where X=N and P) isomers as a gauge of the delocalization of the unpaired electron. The geometrical parameters of X̃ 2B2 CH2X, the trans-HCXH and cis-HCXH conformers, and the first three excited states are reported at the coupled-cluster level with single, double, and perturbatively applied triple excitations [CCSD(T)] using Dunning’s correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ). The C2v structures on the ground state surface are predicted to lie 9.3 and 13.5 kcal/mol lower than the trans- and cis-isomers, respectively, for CH2N and 28.1 and 30.0 kcal/mol, respectively, for CH2P. There are many other important properties of CH2N and CH2P which are not known from experiment. The geometrical parameters of the CH2N ground state [r(C–N)=1.2462 Å, r(C–H)=1.0921 Å, and θ(HCH)=119.4°] and the CH2P ground state [r(C–P)=1.6583 Å, r(C–H)=1.0842 Å, and θ(HCH)=118.9°] agree well with the C–N and C–P bond distances of the r0 structures derived from microwave data, although notable differences were observed in the C–H bond distance and HCH bond angle. This research resolves an earlier discrepancy between theory and experiment for the ground state C–P distance in CH2P. The Fermi contact terms for 1H, 13C, 14N, and 31P were determined at CCSD(T) level of theory with the cc-pVTZ and cc-pVQZ basis sets and are in reasonable agreement with the experimental values with a maximum deviation of 26 MHz for CH2N and 11 MHz for CH2P. The excited states of CH2N are predicted to lie 33 000 cm−1 (Ã 2B1), 36 000 cm−1 (B̃ 2A′), and 38 000 cm−1 (C̃ 2A1) above the ground state, and the excited states of CH2P to lie approximately 21 000 cm−1 (Ã 2A′), 26 000 cm−1 (B̃ 2B1), and 33 000 cm−1 (C̃ 2A1) above the ground state.
Persistent diffusion in nanopores
Authors: ten Bosch A.
Published: July 2002

Brownian dynamics are used to relate the time dependent concentration of particles constrained in a pore to the observed flux and to study the flow induced by wall effects. The joint probability for a particle is calculated, not only to be at a certain position but with a given velocity as well. The persistence of the velocity direction and the resulting coupling between position and velocity alter the particle dynamics. In constrained geometries, transport though the pore is enhanced. With time, an average particle velocity flow field is set up along with the initial slow evolution by diffusion. For reflecting boundary conditions on the wall, the increase of the density along the walls causes particles to flow toward the center of the pore.
Photoluminescence properties of δ-doped barrier layers in modulation-doped InAlAs/InGaAs field-effect transistor structures
Authors: Watanabe Kazuo , Yokoyama Haruki
Published: July 2002

A photoluminescence peak from the Si δ-doped InAlAs barrier layers (δ peak) is found around the InAlAs exciton peak from the buffer layer in 15 modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structure wafers. The δ peak position ordinarily shifts to shorter wavelength with increasing δ doping concentration. The intensity reduction of the δ peak due to raising the temperature from 6 K is considerably slower than that of the InAlAs exciton peak. The excitation spectrum of the δ peak is clearly different from that of the InAlAs exciton peak and seems to reflect the optical absorption dominated by the potential slope in the upper side of the barrier layer and the quasi-Fermi level. The δ peak is not detected from HFET structures without the contact layers. The δ peak is attributed to the recombination of electrons from the δ-doped layer and photogenerated holes weakly confined in the upper side of the barrier layer.
Mode dynamics in a free electron maser with broadband frequency-dispersive feedback
Authors: Savilov A. V. , Bratman V. L. , Denisov G. G. , Bongers W. A. , van der Geer C. A. J. , Manintveld P. , Militsyn B. L. , Verhoeven A. G. A. , Urbanus W. H.
Published: July 2002

For a free-electron oscillator, problems of mode dynamics are studied for a case when the feedback has a wide, but finite band (the feedback coefficient depends on the frequency). It is shown that even a broadband frequency-dispersive feedback can significantly affect both the electron-current threshold of stability of the single-mode operation and the process of frequency tuning.
Calibration facility for solar wind plasma instrumentation
Authors: Marti Adrian , Schletti Reto , Wurz Peter , Bochsler Peter
Published: July 2002

Space-borne analysis of the composition of the solar wind offers the unique possibility of direct measurement of material originating from the sun. For development, testing, and calibration of solar wind particle instrumentation, particle beams of highly charged ions for all elements from hydrogen up to iron are needed. Although sources for these ions have been available for some time, the special demands of space instrumentation made it necessary to build a dedicated facility. We built an electron-cyclotron-resonance ion source operating at 2.45 GHz. The ion source is installed on a high voltage platform allowing for postacceleration potentials of up to 100 kV. Ions are produced from elements in gaseous and solid phase; the latter from vapor emitted from a high temperature furnace closely attached to the ion source.
Robust Langmuir probe circuitry for fusion research
Authors: Boedo J. , Gunner G. , Gray D. , Conn R.
Published: July 2002

Langmuir probes attached to the plasma facing components of fusion experiments are biased with constant or swept voltages to obtain measurements of plasma parameters such as electron temperature and density. The circuitry used must be rugged and protect the power supplies and electronics against generally harsh conditions and sudden discharge terminations, or disruptions. Modularity, ease of repair and expandability are important because short-lived radiation from neutron activation is often present after the discharges, preventing access to the circuitry. We report the implementation of modular probe circuitry featuring robust protection, remote testing and reset and easy maintenance and expandability, achieved by using DIN-rail modules.
A noncontact laser technique for circular contouring accuracy measurement
Authors: Wang Charles , Griffin Bob
Published: July 2002

The worldwide competition in manufacturing frequently requires the high-speed machine tools to deliver contouring accuracy in the order of a few micrometers, while moving at relatively high feed rates. Traditional test equipment is rather limited in its capability to measure contours of small radius at high speed. Described here is a new noncontact laser measurement technique for the test of circular contouring accuracy. This technique is based on a single-aperture laser Doppler displacement meter with a flat mirror as the target. It is of a noncontact type with the ability to vary the circular path radius continuously at data rates of up to 1000 Hz. Using this instrument, the actual radius, feed rate, velocity, and acceleration profiles can also be determined. The basic theory of operation, the hardware setup, the data collection, the data processing, and the error budget are discussed.
Impulsive stimulated scattering of surface acoustic waves on metal and semiconductor crystal surfaces
Authors: Zhao Liang , Baer Bruce J. , Yamaguchi Masashi , Than Ha Trong , Yarmoff Jory , Chronister Eric L.
Published: July 2002

Impulsive stimulated scattering (ISS) is used to obtain the orientational dependence of the velocity of surface acoustic waves (SAW) on single crystal metal and semiconductor surfaces. Mechanically polished surfaces of aluminum(111), nickel(100), and germanium(100) samples were examined, as well as a comparison of mechanically polished versus high vacuum sputtering/annealing of a Ni(100) surface. The ISS technique offers an accurate and robust method of obtaining surface acoustic velocities of metal and semiconductor crystalline surfaces without physical contact. The orientationally dependent ISS results on Ni(100) are compared with recent classical Brillouin scattering measurements, and with velocities calculated using bulk elastic constant data. Finally, ISS measurements on tilted (i.e., lower symmetry surfaces) reveal coupling of the ISS excited SAW with bulk transverse modes.
Determination of grain-boundary diffusion of Ag in nanocrystalline Cu by the Hwang–Balluffi method
Authors: Erdélyi Z. , Girardeaux Ch. , Langer G. A. , Beke D. L. , Rolland A. , Bernardini J.
Published: July 2002

From the Ag and Cu Auger signals measured on the surface of nanocrystalline Cu/Ag thin bilayers, the temperature dependence of the parameter ω′ for Ag grain-boundary diffusion in Cu (ω′=δkbDb/δsks, where δ and δs are the width of the grain boundary and the segregated layer and kb and ks are the grain-boundary and surface segregation factors, respectively) have been determined by the Hwang–Balluffi method in the C-kinetics regime over the temperature range of 393–428 K. These values have been compared with triple products, P=δkbDb, determined in the temperature range 584–804 K by Bernardini et al. [Phil. Mag. A 73, 237 (1996)] using radio tracer technique in the B-kinetics regime and the temperature dependence of the surface segregation factor has been extracted. The surface segregation energy (34±16 kJ/mol) agrees well with other data published in the literature.
Crossover behavior of star polymers in good solvents
Authors: Lue L. , Kiselev S. B.
Published: July 2002

We perform Monte Carlo calculations for the mean-square center-to-end distance, mean-square radius of gyration, and second virial coefficient of f=3 to 41 arm star polymers composed of rigidly bonded hard spheres of varying diameters. As with linear chains, there are two different crossover regimes: (i) crossover from the Gaussian chain to the Kuhnian chain limit, where the penetration function Ψ(f) increases monotonically with increasing polymer molecular weight, and (ii) crossover from the rigid-rod to the Kuhnian chain limit, where the penetration function decreases with increasing molecular weight. We propose a phenomenological approach for the extension of our previous crossover theory for linear polymers to star polymers. We show that the theoretical crossover function obtained earlier by Douglas and Freed [Macromolecules 16, 1854 (1984)] fails to reproduce the simulation data for the penetration function with f⩾6, while the phenomenological crossover model is in good agreement with the simulation data up to f⩽41. We also obtain a generalized crossover equation for the penetration function for linear and star polymers in good solvents. The crossover equation is able to accurately describe the variation of the infinite molecular weight limit of the penetration function Ψ*(f) with the number of arms f on the star polymer, and it predicts that Ψ*(f) approaches 2.39 in the limit f→∞.
How gap genes make their domains: An analytical study based on data driven approximations
Authors: Gursky Vitaly V. , Reinitz John , Samsonov Alexander M.
Published: July 2002

We consider a mathematical formulation of the problem of protein production during segment determination in the Drosophila blastoderm, together with some preliminary results of its analytical study. We reformulate the spatial difference equations as a set of nonlinear partial differential equations and obtain their dimensionless form in the continuum limit. Using previous results obtained by the gene circuit method, we find an asymptotic statement of the problem with a small parameter. Some results of the comparison method applied to the model are obtained, and exact stationary upper solutions are derived. They exhibit distinctive features of localized bell-shaped structures.
Kinetics of intrachain reactions of supercoiled DNA: Theory and numerical modeling
Authors: Klenin Konstantin V. , Langowski Jörg
Published: July 2002

We considered an irreversible biochemical intrachain reaction of supercoiled DNA as a random event that occurs, with some probability, at the instant of collision between two reactive groups attached to distant sites of the DNA molecule. For sufficiently small intrinsic rate constant kI, the dominant process contributing to the productive collisions is the quasione-dimensional reptation of the strands forming the superhelix. The mean reaction time is then given by τF+1/kIcL, where τF is the mean time of the first collision caused by reptation, and cL is the local concentration of one reactive group around the other. The internal reptation of DNA strands was simulated by the repton model, in which a superhelix branch is approximated by a string of beads placed in a row of cells. This simple model allows semiquantitative estimation of τF and cL (in some arbitrary units) for a large range of the DNA lengths L. The repton chain was calibrated with the help of the data available for small supercoiled plasmids from Monte Carlo and Brownian dynamics simulations. The repton model and the Brownian dynamics give the same form of the distribution of the first collision time. Our estimations show that, for opposite sites of the chain, the mean first collision time τF varies from 5 ms (L=2.5 kb) to 1 s (L=20 kb). The corresponding cL values (for the reaction radius 10 nm) are 3×10−6 and 2×10−7 M.
Indices of refraction and their dispersion characteristics of ZnMgCdSe thin films grown by molecular beam epitaxy
Authors: Peiris F. C. , Furdyna J. K. , Guo S. P. , Tamargo M. C.
Published: July 2002

We have measured the dependence of the indices of refraction n on alloy composition of Zn1−x−yMgxCdySe films grown by molecular beam epitaxy on InP substrates for a series of alloy compositions x and y. The compositions of the Zn1−x−yMgxCdySe thin films were determined by photoluminescence and x-ray diffraction experiments. A prism coupler technique, capable of measuring n with an accuracy of at least 0.1% at discrete wavelengths, was then used to measure n of each of the thin films. In all of the samples, at least three guided modes were observed in the spectrum obtained by the prism coupler method. The accurate values of n obtained by this method show an inverse relationship with respect to their band gaps. In addition, by comparing the n values obtained for the quaternary Zn1−x−yMgxCdySe alloys with the previously obtained values for the ternary Zn1−xCdxSe and Zn1−xMgxSe alloys, it is concluded that n of the quaternary system is almost completely dictated by the content of Mg.
Simulation of optical spectra of Eu3+ ion in fluorozirconate glasses by molecular dynamics simulation and point charge crystal field
Authors: Soga Kohei , Inoue Hiroyuki , Makishima Akio
Published: July 2002

Eu 3+ -doped fluorozirconate glasses with two compositions Zr:Ba=1:1(ZB11) and Zr:Ba =3:1(ZB31) were prepared. The emission, excitation, and fluorescence line narrowing spectra of those glasses were measured. Certain differences between the two glasses were observed in the observed spectra. Structural models of the glasses were simulated by molecular dynamics (MD) simulation. The optical spectra of Eu3+ in those glasses were simulated from the simulated glass structures using a point charge crystal field. The differences in spectral characteristics between the two glasses in the observed spectra were reproduced in the simulated spectra. The MD simulation showed that the barium ions were more apt to coordinate to EuFa clusters than the zirconium ions were.
Effect of emitter metallization on thermal properties of small AlGaAs heterojunction bipolar transistors
Authors: Kleckner T. C. , Reid A. R. , St. Denis A. R. , Jackson M. K.
Published: July 2002

We have extended an experimental technique to measure thermal resistance by pulsed current–voltage measurements to small bipolar transistors with thermal time constants of several hundred nanoseconds. Results for a 3×3 μm2 AlGaAs/GaAs heterojunction bipolar transistor are compared with detailed finite element method simulations, and show good agreement when heat flow into the emitter metallization is accounted for. Simulation results indicate that emitter metallization plays a significant role in reducing temperature nonuniformity across the emitter.
Ab initio investigation of the potential energy profiles for the gas phase CH4+O2+(2Πg) reaction system
Authors: Irle Stephan , Morokuma Keiji
Published: July 2002

High level ab initio and density functional calculations of the ground state potential energy profiles were carried out to study the mechanism of the ion–molecule reaction CH4+O2+(X 2Πg) for four reaction channels: insertion of O2+ into the C–H bond of methane (INS), hydride abstraction from methane (HA), charge transfer (CT), and O–O cleavage path (OO) after INS process to give CH2OH++OH. Common to these channels are initial encounter complexes, and our calculations match very closely experimental estimates for binding energies. The INS channel proceeds through CH4OO+ and gives a deep minimum corresponding to the exothermic and metastable intermediate CH3OOH+. This species can easily eliminate H to give CH2OOH+, a product observed experimentally. For the slightly endothermic HA channel to give CH3++OOH, two pathways were found: a direct pathway (likely to dominate at higher collision energy) from the encounter complex via a HA transition state at 5.8 kcal/mol above the reactants, and an indirect pathway with a slightly smaller energy requirement consisting of elimination of OOH from the INS intermediate CH3OOH+. A transition state with a high energy requirement of 15 kcal/mol was found for O–O cleavage from CH3OOH+, consistent with the experimental finding that O–O cleavage occurs at high energies. It was also found that the seam of crossing between two potential surfaces is facilitated and therefore the CT channel is promoted by the O–O stretching and the methane deformation vibrations, again consistent with the experimental results.
Finite element method for two-dimensional vibrational wave functions: Theory and application to van der Waals molecules
Authors: Dudley Timothy J. , Pandey Rajeev R. , Staffin Philip E. , Hoffmann Mark R. , Schatz George C.
Published: July 2002

A variational formulation finite element method is developed for calculation of vibrational wave functions in a domain spanned by close-coupled, or Jacobi, coordinates R and γ. C1 tensor-product basis functions, which allow straightforward separation of kinetic and overlap integrals into products of one-dimensional integrals, are used. Furthermore, representation of the potential energy surface in terms of the same tensor-product basis functions used to represent the wave functions allows the potential energy integrals to also be written as a sum of products of one-dimensional integrals. Factorization of the integrals, together with expression of one-dimensional integrals in analytic or rapidly convergent power series form, reduces the computational effort of calculation of all matrix elements to a small, and arguably insignificant, level. It is shown that the theoretical error in eigenvalue, i.e., O(h6) for bicubic Hermite functions, is achieved for a number of rare gas van der Waals triatomics for which surfaces have been previously published. We also present illustrative calculations on NeHCl and A′2 and A″2 NeHCl+, which have not been previously studied, for surfaces calculated at the CCSD(T)/cc-pVTZ level.
Laminated core modeling under rotational excitations including eddy currents and hysteresis
Authors: Bottauscio Oriano , Chiampi Mario
Published: July 2002

This article presents a numerical model for the electromagnetic analysis of hysteretic-laminated cores under rotational excitations. The computational approach is based on the finite element solution of a two-dimensional field magnetic problem in a homogeneous structure, where the skin effect due to macroscopic eddy currents in the lamination depth is included through a generalized dynamic constitutive relationship between B and H. The proposed model, after validation, is applied to the analysis of a laminated disk, evaluating the effects of the supply frequency and distorsion on power losses and B–H loops.
Benzene adsorption on Ir{100} studied by low-energy electron diffraction I–V analysis: Evidence for formation of tilted benzyne
Authors: Johnson K. , Sauerhammer B. , Titmuss S. , King D. A.
Published: July 2002

In order to increase understanding of the structure and bonding of aromatic molecules and their fragments on transition metal surfaces, a low-energy electron diffraction (LEED) study of benzene adsorption on Ir{100} has been carried out. Following benzene adsorption at 465 K, a c(2×4) LEED pattern is observed. Its formation is accompanied by the loss of two hydrogen atoms as H2, indicating that benzyne (C6H4) is formed. This is the first time an ordered overlayer of benzyne has been observed on a transition metal surface. It makes the structure accessible to LEED I–V analysis, providing the most reliable structural information for benzyne adsorbed on a transition metal surface to date. The benzyne species was found to be di-σ bonded to the bridge site with a 47° tilt angle to the surface normal.
Structural and magnetotunneling investigations on La0.7Ca0.3MnO3/Al2O3 multilayers
Authors: Luo Yuansu , Samwer Konrad
Published: July 2002

Measurements of tunneling magnetoresistance (TMR) in La0.7Ca0.3MnO3/Al2O3 magnetic multilayers are reported with the emphasis on effects of the sample microstructure, arising from different substrates or varied deposition temperatures. Observed superlattice structures reveal a detailed structural information especially from the thin Al2O3 layers. Magnetotransport behaviors were measured below the Curie temperature of La0.7Ca0.3MnO3, showing a clear structural dependence. A large low-field TMR of about 65% was observed at 40 K from a trilayer junction. The improved temperature dependence of TMR is attributed to improved microstructure both in electrodes and thin barrier.