bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory for coil impedance of a conducting half space: Analytic results for eddy current analysis
Authors: Coffey Mark W.
Published: July 2002

The general formulation of the coil impedance of a conducting but nonmagnetic half space is examined. An intermediate kernel function is used to recast the theory in a form which gives a fuller physical interpretation and at the same time provides avenues for the solution of the ill-conditioned inverse conductivity problem. In particular, reflection and transmission coefficients depending upon the kernel function are introduced, which yield expressions for the induced and penetration fields, respectively. Correspondences between the forward and inverse conductivity problems, and problems in the London theory of superconductivity for the penetration depth, are established. The use of the kernel function aids in the development of analytic expressions for the single- and multiturn coil impedance and their instance for a half space of constant conductivity. Analytic results are obtained for various integral representations for both general and special impedance cases.
Quantitative evaluation of fabrication processes of proton-exchanged layers in LiTaO3 optoelectronic devices by the line-focus-beam ultrasonic material characterization system
Authors: Kushibiki J. , Miyashita M.
Published: July 2002

Experimental investigations are conducted in order to collect basic data for evaluating proton-exchanged LiTaO3 optical waveguides and their fabrication processes and systems using the line-focus-beam ultrasonic material characterization system, in the frequency range 100–300 MHz. Seven Z-cut LiTaO3 substrates are proton exchanged at several process temperatures (220–280 °C) and times (5–30 min) in a pyrophosphoric acid solution. Leaky surface acoustic wave (LSAW) velocities, measured for all specimens, decrease for all propagation directions. The decrease rate is at maximum in the Y-axis propagation direction, in which the measurement sensitivity to the process conditions is highest. The fH dependences of LSAW velocities, obtained from frequency dependences of LSAW velocities and proton-exchanged layer depths analyzed by secondary-ion mass spectrometry, have almost constant gradients of −0.78 (m/s)/(Hz m). Normalized depth distributions of the elastic properties of proton-exchanged layers are nearly equal; only the depths differ. Also, the relationships among LSAW velocities, layer depths, process times, process temperatures, and diffusion coefficients are experimentally obtained. Homogeneity evaluation of a proton-exchanged, 2-in., Z-cut LiTaO3 wafer processed at 260 °C for 14 min is demonstrated, resulting in a maximum LSAW velocity variation of 1.3 m/s. This corresponds to a depth variation of 7.4 nm and a temperature variation of 0.8 °C for the whole surface.
Long living excited states in boron doped diamond
Authors: Nebel C. E. , Rohrer E. , Stutzmann M.
Published: July 2002

Long living excited states in boron doped diamond are detected by photoconductivity and photoconductivity excitation spectroscopy at temperatures T<190 K in the energy regime 3.2–3.5 eV. The photoconductivity spectra show pronounced maxima in highly doped [(4–8)×1018 cm−3] diamond which decrease with decreasing doping density. A model is presented where holes are optically excited from compensating deep defects, 3.36 eV above the valence band edge, into the valence band. Here they get captured by LO phonon emission into long living excited levels of boron. The long living excited states are 200, 240, and 266 meV above the 1s ground level. Maxima in photocurrent can be detected at high doping concentrations as the wave function overlap between neighboring excited states is significant to allow hopping transitions between neighboring excited states.
Studies of damage in low-power reactive-ion etching of III–V semiconductors
Authors: Rahman M. , Deng L. G. , Wilkinson C. D. W. , van den Berg J. A.
Published: July 2002

Measurements of photoluminescence intensity from GaAs and InGaAs quantum well heterostructures have been used as a noninvasive probe of the damage distribution due to very low-power dry etching. Samples were etched using SiCl4 reactive-ion etching. Comparative studies were made on samples bombarded by the separate constituent ions of a SiCl4 discharge using a mass-resolving ion implanter fitted with a deceleration lens. We also examined the influence of more complex multicomponent discharges. We found that molecular ions contribute less to deep damage than do atomic ions. The reason is that molecular ions fragment upon impact and the secondary atomic ions do not channel very far. Studies of laser illumination on the sample during etching show that a form of radiation-enhanced diffusion can modify the damage distribution. The net picture emerges of a complex process underlying dry-etch damage penetration at very low energies.
Surface analysis of ultraprecise polished chemical vapor deposited diamond films using spectroscopic and microscopic techniques
Authors: Weima J. A. , Job R. , Fahrner W. R. , Kosaca G. C. , Müller N. , Fries T.
Published: July 2002

Chemical vapor deposited (CVD) diamond films of varying crystallographic orientations and sizes were thermochemically polished on steel plates. The sizes of the grains constituting the films used in this work ranged between 10 and 100 μm. Images of the surface morphologies of the films were obtained by the scanning electron microscope (SEM) before and after polishing. A stylus profilometer was used to determine the surface roughness (arithmetic mean deviation) of the as-grown diamond films. The average surface roughness was found to be about 30 μm on the growth side and about 7 μm on the substrate side. Polishing for several hours at temperatures between 750 and 1000 °C thinned the surface roughness down to about 1.3 and 1.5 nm on the growth and substrate sides, respectively, of the film as measured by the atomic force microscope. Raman spectroscopy revealed low energy nondiamond carbon lines in the frequency range between 200 and 700 cm−1 on polished surfaces of optical graded diamond films in addition to the usual graphite bands situated between 1350 and 1580 cm−1. Photoluminescence and cathodoluminescence spectra of both polished and unpolished films revealed nitrogen and silicon as the only detectable defect centers in the films. The Raman spectra of polished optical grade films also contained lines that are attributed to molecular nitrogen complexes (N2) at a frequency of 2438 cm−1, carbon–hydrogen (C–Hn) stretching vibrations at frequencies ranging from 2700 to 3200 cm−1 and molecular hydrogen complexes (H2) at a frequency of 4350 cm−1.
An exact second-order expression for the density functional theory correlation potential for molecules
Authors: Ivanov Stanislav , Bartlett Rodney J.
Published: July 2002

In this communication we present the exact, local, one-electron, second-order correlation potential for molecules, for use in density functional studies. The correlation potential is represented in a basis set, and when combined with the exact exchange potential, it provides an exchange–correlation potential that is derived exclusively from exact, orbital-dependent expressions. In this sense, such potentials provide an ab initio density functional theory (DFT) that permits convergence to the exact answer as higher order terms are introduced, just as is the case for ab initio correlated methods. Furthermore, this potential includes some dispersion effects that are missing from other DFT potentials.
Magnetic force microscopy study of magnetization reversal in sputtered FeSiAl(N) films
Authors: Lo C. C. H. , Snyder J. E. , Leib J. , Chen R. , Kriegermeier-Sutton B. , Kramer M. J. , Jiles D. C. , Kief M. T.
Published: July 2002

The magnetization reversal in a series of rf-sputtered FeSiAl(N) films has been studied using magnetic force microscopy. A system has been developed which has the capability to image domain structure while an in-plane magnetic field is applied in situ. All films exhibited a stripe domain structure in zero applied field which was indicative of a perpendicular component of domain magnetization which alternates in sign. All films showed a similar sequence of magnetization processes: on reducing the applied field from saturation a fine stripe domain structure nucleated and then coarsened as the field was decreased to zero. Local switching of domain contrast was observed along the steepest part of the hysteresis loop as the perpendicular component reversed. As the reverse field was increased toward saturation, the stripe domains disintegrated into smaller regions. This observation is consistent with an interpretation that the domain magnetization rotated locally into the sample plane. The saturation field and the film stress exhibited similar trends with nitrogen partial pressure. The results suggest that the perpendicular anisotropy that caused the formation of the stripe domain structure could be induced by the film stress via magnetoelastic coupling.
A variable-director C13 NMR analysis of lyotropic aramide solutions
Authors: Grinshtein Julia , McElheny Dan , Frydman Veronica , Frydman Lucio
Published: July 2002

The order and dynamics of two aromatic polyamides in their lyotropic phases were investigated with the aid of variable-director nuclear magnetic resonance (NMR). In these experiments polymers were dissolved in concentrated sulfuric acid and allowed to equilibrate inside the main NMR magnetic field B0 to yield macroscopically-aligned liquid crystalline solutions. These ordered fluids were then rotated away from equilibrium for brief periods of time, and their natural abundance C13 NMR spectra collected as a function of different angles between the liquid crystalline director and B0. The resulting spectra showed peaks shifting as well as broadening as a function of the director's orientation, variations that were also found to be concentration- and temperature-dependent. All such changes could be successfully accounted for on the basis of an exchange model involving molecular reorientations of the polymer chains that are occurring in the intermediate NMR time scale. Based on this assumption, the experimental line shapes could be used to extract a detailed description of the macromolecular order and dynamics in these fluids. The former appeared substantially high, and not very different from the one characterizing order in commercial extruded aramide fibers. The latter enabled an estimation of the hydrodynamic radii adopted by the macromolecules in their mesophases, which ended up in close agreement with dimensions recently reported on the basis of small-angle neutron scattering analyses.
The dynamical structure factor in topologically disordered systems
Authors: Martin-Mayor Victor , Mézard Marc , Parisi Giorgio , Verrocchio Paolo
Published: July 2002

A computation of the dynamical structure factor of topologically disordered systems, where the disorder can be described in terms of Euclidean random matrices, is presented. Among others, structural glasses and supercooled liquids belong to that class of systems. The computation describes their relevant spectral features in the region of the high frequency sound. The analytical results are tested with numerical simulations and are found to be in very good agreement with them. Our results may explain the findings of inelastic x-ray scattering experiments in various glassy systems.
Comparison between integral equation method and molecular dynamics simulation for three-body forces: Application to supercritical argon
Authors: Bomont Jean-Marc , Bretonnet Jean-Louis , van der Hoef Martin A.
Published: July 2002

The prediction of the structural and thermodynamic properties of supercritical argon has been carried out by two independent routes: semianalytical calculations and numerical simulations. The first one is based on the hybridized mean spherical approximation (HMSA) conjugated with an effective pair potential that incorporates multipole dispersion interactions. The second one uses a very recent numerical simulation technique, inspired by the Car–Parrinello method [van der Hoef et al., J. Chem. Phys. 111, 1520 (1999)], which contains an effective quantum-mechanical representation of the underlying electronic structure. The latter approach allows us to treat the contribution of the three-body effects as well, and to validate the use of an effective pair potential for them in the framework of the self-consistent integral equation method. For all the supercritical argon states studied, the results obtained with the semianalytical approach are in good agreement with the predictions of the numerical simulation. Here it is shown that HMSA remains competitive with molecular dynamics simulation when the triple-dipole and the dipole–dipole–quadrupole three-body terms are taken into account.
Global breakdown in an alternating current plasma display panel
Authors: Ikeda Y. , Suzuki K. , Fukumoto H. , Shibata M. , Ishigaki M. , Verboncoeur J. P. , Christenson P. J. , Birdsall C. K.
Published: July 2002

In the alternating current plasma display panel, a widespread discharge can take place accidentally in a large number of cells in the upper or lower panel ends. The undesirable discharge is referred to as global breakdown. This is a serious problem, with consequences not only for display quality, but also for product reliability. The mechanism of the global breakdown was examined experimentally using a surface electrometer and theoretically using plasma simulation. From these examinations, the global breakdown mechanism was clarified as follows. Global breakdown was accompanied by charge separation in the horizontal direction of the panel. This charge separation was caused by electron transport downward in the panel during the address discharge. The electron transport formed a negative wall charge on the phosphor surface. When the wall voltage exceeded the insulation voltage of the protective layer, global breakdown occurred. Furthermore, it was clarified that increasing the front dielectric thickness or reducing the back dielectric thickness could suppress the electron transport during the address discharge. These optimizations of cell geometry could eliminate the global breakdown.
Compensation of charge fluctuations in quantum wells with dual tunneling and photon-assisted escape paths
Authors: Huang Danhong , Singh Anjali , Cardimona D. A. , Morath Christian
Published: July 2002

In our previous article [D. H. Huang, A. Singh, and D. A. Cardimona, J. Appl. Phys. 87, 2427 (2000)], we explained the experimentally observed zero-bias residual tunneling current [A. Singh and D. A. Cardimona, Opt. Eng. 38, 1424 (1999)] in quantum-well photodetectors biased by an ac voltage. In this article, we extend our theory to include the photoemission current and reproduce our recent findings on the dynamical drop of photoresponsivity Rph(t) from its static value Rph0 in quantum-well photodetectors as a function of the chopping frequency of the incident optical flux. In this theory, we derive a dynamical equation for a nonadiabatic space-charge field Ena(t) in the presence of an applied electric field Eb(t) and an incident optical flux Φop(t). From it, a compensation of the charge fluctuations in quantum wells is predicted as a result of dual tunneling and photon-assisted escaping paths. We also find a suppression of the nonadiabatic deviation of Rph(t) from Rph0 due to a charge-depletion effect in the quantum wells.
Simulation of CO adsorption and oxidation on Pt(110)
Authors: Zhdanov V. P. , Kasemo B.
Published: July 2002

The clean Pt(110) surface exhibits missing-row (1×2) periodicity. CO adsorption on this surface may lift the (1×2) reconstruction. During CO oxidation, the interplay between the CO-induced surface restructuring and reaction steps may result in oscillations. Our Monte Carlo simulations, executed with a qualitatively correct ratio of the rates of elementary steps involved into the game, show that these phenomena can be described by using a lattice-gas model with Pt–Pt, Pt–CO, and CO–CO lateral interactions.
Self-consistent corrections to the equation of state and chemical potentials of hard chain fluid mixtures
Authors: Omelyan Igor P. , Ben-Amotz Dor
Published: July 2002

A general procedure is proposed for improving the self-consistency of binary mixed hard body fluid equations of state in both the small and large particle size ratio limits. The resulting equations correct both the total pressure and the solute chemical potential in a binary mixture with low solute concentration. The results are applied to the Bonded Hard Sphere (BHS) and Scaled Particle Theory (SPT) equations of state for mixtures of hard chains, to produce new SC-BHS and SC-SPT expressions with improved self-consistency. Comparison with computer simulation measurements of the chemical potentials of hard dumbbells in hard dumbbell fluids with solute-solvent sphere diameter ratios between 0 and 10 demonstrate the improved accuracy of the new equations. The improvement is most marked for SC-SPT, as the original SPT was nonself-consistent at both small and large solute sizes, while BHS was already self-consistent at small, but not at large, solute sizes.
Effect of low-temperature annealing on transport and magnetism of diluted magnetic semiconductor (Ga, Mn)As
Authors: Hayashi Takashi , Hashimoto Yoshiaki , Katsumoto Shingo , Iye Yasuhiro
Published: July 2002

We report improvements in the crystallinity of a III–V-based diluted magnetic semiconductor (Ga, Mn)As by heat treatment (annealing) after growth at comparatively low temperatures. This method can be used to raise the Curie temperature to 100 K without the need for severe optimization of growth conditions, as well as to adjust the material parameters to desired values.
Data reduction in 3ω method for thin-film thermal conductivity determination
Authors: Borca-Tasciuc T. , Kumar A. R. , Chen G.
Published: July 2002

The 3ω method has been proven to be very useful for determining the thermal conductivity of thin films and their substrates. Several simplifications are often used in determining the thermal conductivity of the films based on the experimentally measured 3ω signal. These simplifications, however, have limited range of applicability. In this work, we present a detailed analysis and mathematical modeling of the 3ω method applied for different experimental conditions. Effects considered include the finite substrate thickness, anisotropic nature of the film and substrate thermal conductivity, the film-substrate thermal property contrasts, the effect of heat capacitance of the heater, and the effect of thermal boundary resistance. Several experimental results are analyzed using the models presented. This work shows that the 3ω method can be extended to a wide range of sample conditions, with anisotropic conductivities in both the substrate and the film, and with small film-substrate conductivity contrast.
Integral encounter theories of the multistage reactions. III. Reversible intramolecular energy transfer
Authors: Ivanov K. L. , Lukzen N. N. , Doktorov A. B. , Burshtein A. I.
Published: July 2002

The matrix Integral Encounter Theory (IET) and its modified version (MET) developed earlier are used to study the kinetics of the reversible transitions between two metastable (as singlet and triplet) states of the fluorescent particle. Induced by binary encounters with inert catalysts, these transitions result in either (a) energy quenching or (b) energy conservation, depending on what lifetime of these two states is longer. Integral encounter theory describes adequately the kinetics of energy conservation followed by delayed fluorescence while differential or Markovian versions of the same theory fail. Another advantage of the IET is the natural accounting for arbitrary strong light pumping which makes the Stern–Volmer constant dependent on light intensity, differently for cases (a) and (b).
Energy dependence of electronic energy relaxation in poly(p-phenylenevinylene)
Authors: McCutcheon M. W. , Young Jeff F. , Pattantyus-Abraham A. G. , Wolf M. O.
Published: July 2002

Femtosecond time-resolved photoluminescence spectroscopy is used to study the dynamics of optical emission from poly(p-phenylenevinylene) at 77 K over an energy range of 2.19–2.37 eV. All of the decay curves in this energy range can be described by the summation of two exponential functions. Analysis of the spectral dependence of the time constants extracted from these fits indicates that the transient photoluminescence at any given wavelength is due in general to the superposition of three transitions emanating from a single set of inhomogeneously broadened excitons. The three distinct transitions correspond to processes in which two, one, or zero phonons are excited in the host polymer during the emission event. By separating out these three contributions, it is determined that the effective lifetime of the inhomogeneously broadened excitons increases monotonically from 980 fs at 280 meV above the peak in the density of states (DOS), to 114 ps near the peak in the DOS.
Elastic properties, intrinsic and photoinduced stress in hydrogenated amorphous-silicon thin films with different hydrogen content
Authors: Spanakis E. , Stratakis E. , Tzanetakis P. , Wang Qi
Published: July 2002

All device-quality hydrogenated amorphous-silicon (a-Si:H) thin films on a substrate exhibit intrinsic compressive stress, induced primarily during growth. Furthermore, it has been established by several previous studies that exposure to intense light at room temperature leads to increase of the compressive stress in addition to the creation of dangling-bond defects, well known as the Staebler–Wronski effect [D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977)]. We present here the results of our investigation of the intrinsic and photoinduced stress as well as the Young’s modulus of a series of samples with different hydrogen content prepared by hot-wire (HW) chemical-vapor deposition. These film properties have been measured with high sensitivity through the bending and flexural oscillation of crystalline silicon microcantilevers upon which we deposited the a-Si:H films. The intrinsic stress decreases with H content in the series. This result complements other investigations linking the saturated defect density to H content in HW samples and validates the generally observed correlation between stress and device performance. The photoinduced stress shows almost an identical initial increase with illumination time in all samples but tends to saturate to a lower value for the lower H concentration. We discuss our results on the variation of the Young’s modulus with H content together with those of a previous study by a different measurement technique and sample growth method and compare both to theoretical calculations. A unified picture emerges clearly suggesting that the microstructure of device quality material is not homogeneous.
Laser-cooled fluorescence mass spectrometry using laser-cooled barium ions in a tandem linear ion trap
Authors: Baba Takashi , Waki Izumi
Published: July 2002

We report on a mass spectrometry that uses laser-cooled barium ions to optically detect sample ions in a linear radio-frequency-quadrupole ion trap. To obtain a mass spectrum, we observe the intensity modulation of laser-induced fluorescence emitted by the barium ions, as we electrically excite the secular motions of sample ions that are sympathetically cooled by the barium ions. In a fluorescence mass spectrum of sample xenon ions, we identified the major isotopes of xenon. To estimate the absolute amount of the sample ions that contribute to the mass spectrum, we detect the ions using an electron multiplier that is absolutely calibrated by crystallized laser-cooled barium ions. We enhance transfer efficiency to the electron multiplier using a linear radio-frequency-quadrupole ion guide that is placed in tandem with the ion trap. Typically 30 ions of xenon isotope 131Xe+ were detected with a signal-to-noise ratio of ∼30 in the fluorescence mass spectrum.
Spatially nonuniform local magnetization switching behavior in Co/Pd multilayer films
Authors: Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

We report the experimental finding that magnetization reversal of ferromagnetic thin films exhibits a spatially nonuniform dependence on the strength of a reversing applied field on a submicrometer scale. Field dependence of local magnetization switching time was determined from the local magnetization viscosity curves triggered by various strengths of a reversing applied field, respectively on spatially-resolved local regions by means of a magneto-optical Kerr microscope. It is found that the nonuniform local variation of the field dependence enhances with increasing repeats of layers in Co/Pd multilayer films, and its distribution could be well analyzed by a simple analytic function.
Novel optical sensor for the measurement of surface texture
Authors: Tomassini Paolo , Rovati Luigi , Sansoni Giovanna , Docchio Franco
Published: July 2002

In this article we describe a novel optical sensor designed to measure surface texture, i.e., the roughness and waviness. The system integrates a surface profiler and a light scattering unit in a compact setup suitable for industrial applications. This sensor could represent a unique surface inspection tool to be integrated in grinding stations used for reconditioning rolls for milling. The spatial wavelength measuring range for waviness is 1–100 mm, whereas the amplitudes of the measured defects can be in the range 200 nm–5 μm for waviness and 100 nm–1.5 μm for roughness. The system exhibits a measurement uncertainty of less than 2% of the measuring range. The industrial version of the sensor has been realized and characterized in field.
Complexation of a polyelectrolyte with oppositely charged spherical macroions: Giant inversion of charge
Authors: Nguyen Toan T. , Shklovskii Boris I.
Published: July 2002

Complexation of a long flexible polyelectrolyte (PE) molecule with oppositely charged spherical particles such as colloids, micelles, or globular proteins in a salty water solution is studied. PE binds spheres winding around them, while spheres repel each other and form almost periodic necklace. If the total charge of PE in the solution is larger than total charge of spheres, repulsive correlations of PE turns on a sphere lead to inversion of the net charge of each sphere. In the opposite case when the total charge of spheres is larger, we predict another correlation effect; spheres bind to the PE in such a great number that they invert the charge of the PE. The inverted charge by absolute value can be larger than the bare charge of PE even when screening by monovalent salt is weak. At larger concentrations of monovalent salt, the inverted charge can reach giant proportions. Near the isoelectric point where total charges of spheres and PE are equal, necklaces condense into macroscopic bundles. Our theory is in qualitative agreement with recent experiments on micelles–PE systems.
Spectroscopy and reactivity of size-selected Mg+–methanol clusters
Authors: Lee James I. , Sperry David C. , Farrar James M.
Published: July 2002

This work presents photodissociation spectra of Mg+(CH3OD)n (n=1–5). Mass spectrometry of the parent cluster ions shows that C–H bond cleavage occurs in the ground electronic state. The branching ratios for products of photodissociation show strong selectivity; methyl loss is the exclusive quenching channel in n=1, while a unique pathway that eliminates CH3D occurs in n=2. Methyl loss does not occur for clusters with more than three solvent molecules. The maximum of spectral intensity shifts to the red with increasing cluster size until halting at n=3. These data signal the formation of a solvent shell at a relatively small size. As the cluster size increases, ligand loss and D atom loss become overwhelmingly favored. We address the selectivity in the products in terms of the initial photoexcited state, nonadiabatic coupling to the ground state potential energy surface, and subsequent dissociation and product formation dictated by dynamics on the ground state surface.
Characteristic dynamic behavior of dc arc near graphite bar electrodes with short gap
Authors: Tanaka Shin-ichi , Matsumura Toshiro
Published: July 2002

Periodic sawtooth patterns were observed for arc voltages of dc free burning air arcs between 16 mm diameter graphite bar electrodes with a gap length of 15 mm. Observation with a high-speed video camera showed that the sawtooth wave forms of the arc voltages were brought about by the dynamic motions of the arc spots on the electrodes. The anode spot showed a periodic movement on the electrode edge a few hundreds milliseconds after arc ignition, while no movement of the cathode spot was observed. The deflection of the arc column near the anode spot also showed a characteristic pattern during the periodic movement of the anode spot. In order to investigate the factors influencing the dynamic behavior of the arc column near the anode spot, the Lorentz force applied to the arc column near the anode spot was calculated by taking into account the flux induced by the current distributed inside the anode. It was pointed out that the dynamic behavior of the arc column near the anode spot was interpreted as caused by the Lorentz force.
Degenerate conical intersections: The interaction between the 3 2A′ and 4 2A′ electronic states of C2H as a case study
Authors: Mebel A. M. , Baer M. , Lin S. H.
Published: July 2002

In this Letter are presented and analyzed conical intersections which appear on the two symmetric sides of the C2v line of the C2H molecule. Two conical intersections (CI) of this kind, between the 3 2A′ and 4 2A′ electronic states, were found to be only a short distance apart, e.g., ∼0.3 Å for the CC distance of 1.25 Å. It is shown that these two CIs—to be termed CI twins—have opposite “charges” thus forming altogether a weak interaction. By increasing the CC distance, to 1.35 Å, the two twins coalesce to form a single CI. The interaction of this merged pair varies with the distance as q−1 (as is the case for conical intersections) but, in contrast to ordinary CIs, does not exhibit any topological effects and its intensity is shown to be zero. These features led us to term it as a degenerate CI or concisely DCI.
Critical phase transitions of three-dimensional Ising-like dipolar ferromagnets
Authors: Nicolaides Demetris
Published: July 2002

In ferromagnets with low critical temperatures, dipolar interactions are the reason for the reduction of the critical dimension above which mean-field behavior prevails. Specifically, it is shown that a three-dimensional uniaxial system with dipole–dipole interactions falls into the universality class of a four-dimensional strictly short-range system, whose behavior is mean field with subtle logarithmic corrections which reflect the nonmean-field behavior that sets in fully below four space dimensions.
Magnetic ground states in Pr1−xSrxMnO3 (x=0.48−0.75)
Authors: Jirák Z. , Hejtmánek J. , Pollert E. , Martin C. , Maignan A. , Raveau B. , Savosta M. M. , Tomioka Y. , Tokura Y.
Published: July 2002

The electric transport properties of single-crystal and ceramic samples of Pr1−xSrxMnO3 perovskites were investigated and correlated with structural and magnetic data. Analysis of the experimental data leads to the following conclusions: (i) the phase boundary between the ferromagnetic and A-type antiferromagnetic ground states is observed at x=0.49, (ii) the A-type systems are polaronic conductors in two dimensions, i.e., in the ferromagnetic planes, and approach the metallic conductivity with increasing x, (iii) an appreciable CMR effect in medium fields is observed for x∼0.50 due to the field-induced ferromagnetic transition, (iv) the A type–C type phase boundary occurs for x∼0.60, and, finally, (v) the charge carrier transport in the C-type antiferromagnets is reliably characterized by variable range hopping in one dimension i.e., along the ferromagnetic chains.
Physical model and analysis of quantum dot infrared photodetectors with blocking layer
Authors: Ryzhii V.
Published: July 2002

A device model for quantum dot infrared photodetectors (QDIPs) with the blocking layer (BL) between quantum dots is presented. Explicit analytical formulas for the dark current and the responsivity in the QDIP of this type are obtained as functions of device parameters, including the doping level of the QDIP active region, and the applied bias voltage. It is shown that there are three voltage ranges in which the dark current and the responsivity of QDIPs reveal different behavior. The obtained results qualitatively explain some features of the QDIPs with BL observed experimentally. The characteristics of the QDIPs under consideration are compared with those of other detectors.
Ferromagnetic resonance observation of exchange and relaxation effects in CrO2
Authors: Lubitz P. , Rubinstein M. , Osofsky M. S. , Nadgorny B. E. , Soulen R. J. , Bussmann K. M. , Gupta A.
Published: July 2002

Ferromagnetic resonance (FMR) was observed in epitaxial thin films of CrO2 grown on TiO2. FMR spectra were taken at 9.5 and 35 GHz and at temperatures from 4 K to above the ordering temperature of 393 K. The spectra of these films are generally complicated because of surface roughness and the distribution of stresses and anisotropies through their thickness. The thinnest films, however, display typical spin wave spectra, which could be approximated using uniform magnetic material analyses. The exchange constant at 300 K, D∼70 meV Å2, is consistent with values derived from the temperature dependence of M at low temperatures. The temperature dependence of D was extracted from the spin wave spacing and is similar to other magnetic metals. Our data indicate a room temperature Gilbert damping parameter of at most 0.0023 near 300 K, which is less than those of metallic magnetic materials except possibly Fe. The small damping parameter suggests that intrinsic losses seen in other magnetic metals, which may arise from electronic transitions between bands of different spin character, are small in CrO2.
A statistical model for the length distribution of meshes in a polymer network
Authors: Lang M. , Michalke W. , Kreitmeier S.
Published: July 2002

A statistical model is introduced which allows estimation of the length distribution of meshes in a unimodal polymer network. The length distribution is responsible for the type and number of entanglements in a network and can thus provide information about the deformation behavior of polymers. The model can also predict the influence of certain simulation parameters such as the degree of cross linkage, the chain length, or the density of the melt from which the network is built. Both a reaction- and a diffusion-controlled cross-linking process can be mapped. We found that a shorter chain length implies a smaller number of chains per mesh. An increase of the degree of cross linkage as well as a lowering of the density of the melt also leads to a smaller average length of the meshes.
Glyoxal photodissociation. An ab initio direct classical trajectory study of C2H2O2→H2+2 CO
Authors: Li Xiaosong , Millam John M. , Schlegel H. Bernhard
Published: July 2002

Unimolecular dissociation of glyoxal via a three-body fragmentation channel has been studied by direct classical trajectory calculations using Hartree–Fock (HF) and hybrid density functional methods (BH&HLYP, B3LYP) with split valence and polarized basis sets [HF/3-21G, BH&HLYP/6-311G(d,p) and B3LYP/6-311G(d,p)]. The transition state for C2H2O2→H2+2 CO has a dihedral angle of 90–110° between the carbonyl groups and a calculated barrier of ∼59 kcal/mol above the trans conformer. To simulate the experimental conditions, trajectories were started from a microcanonical ensemble at the transition state with 4, 8, and 16 kcal/mol excess energy distributed among the vibrational modes and the transition vector. In agreement with experiment, the CO rotational distribution is very broad with a high 〈J〉. However, the calculations yielded more CO vibrational excitation for the triple dissociation channel than observed for all channels combined. Hydrogen is produced with low J but significant vibrational excitation, in accord with experiment. Similar to trajectory studies on H2CO→H2+CO, there is a good correlation between the energy released along the part of the reaction path where most of the H2 bond length change occurs and the average vibrational excitation of the H2 products.
Photodissociation of water in the Ã band revisited with new potential energy surfaces
Authors: van Harrevelt Rob , van Hemert Marc C.
Published: July 2002

Theoretical calculations on the photodissociation of water in the first absorption band have been used to test the accuracy of three available potential energy surfaces for the first excited state of water: the well-known coupled electron pair approximation potential of Staemmler and Palma [Chem. Phys. 93, 63 (1985)], and two new multireference double excitation configuration interaction surfaces: the Dobbyn–Knowles surface (unpublished), and the Leiden surface [R. van Harrevelt and M. C. van Hemert, J. Chem. Phys. 112, 5777 (2000)]. Exact quantum mechanical calculations, using the wave packet approach, have been performed for J″>0, where J″ is the initial rotational state of the water molecule. The cross section was found not to depend strongly on the rotational state, so that it is reasonable to compare calculated cross sections for J″=0 with experimental room temperature cross sections. Small and simple corrections were applied to the potential energy surface to improve the agreement between theory and experiment for the cross section of H2O. Spectra for D2O and vibrationally excited water molecules calculated with all three corrected potential energy surfaces were in good agreement with experiments. A comparison between calculated OH(X) or OD(X) vibrational distributions, and recent kinetic energy release measurements of the H or D atoms produced in the 157.6 nm photodissociation of water and its isotopomers [Yang et al., J. Chem. Phys. 113, 10597 (2000)], however, suggests that the Leiden surface is more accurate than the two other surfaces.
Energy level control for self-assembled InAs quantum dots utilizing a thin AlAs layer
Authors: Kim J. S. , Yu P. W. , Leem J. Y. , Lee J. I. , Noh S. K. , Kim Jong Su , Kim S. M. , Son J. S. , Lee U. H. , Yim J. S. , Lee D.
Published: July 2002

Ground-state energy of InAs quantum dots (QDs) in the GaAs matrix can be changed significantly by introducing a thin AlAs layer (1 nm). The photoluminescence (PL) peak position of the QDs grown directly on the thin AlAs layer is blueshifted by 171 meV from that of the QDs grown without the AlAs layer. QDs grown on an additional GaAs thin layer on top of the AlAs layer have PL peaks systematically redshifted to lower energy as the GaAs layer becomes thicker. Time-resolved PL shows that the QDs have similar lifetimes, attesting to the fact that all the QDs grown in this way are of high quality, although the energy level change is large and a thin AlAs layer is introduced.
D-branes, categories and N=1 supersymmetry
Authors: Douglas Michael R.
Published: July 2002

We show that boundary conditions in topological open string theory on Calabi–Yau (CY) manifolds are objects in the derived category of coherent sheaves, as foreseen in the homological mirror symmetry proposal of Kontsevich. Together with conformal field theory considerations, this leads to a precise criterion determining the supersymmetry preserving branes at any point in CY moduli space, completing the proposal of II-stability.
Photodissociation spectroscopy of Mg+–acetaldehyde
Authors: Lu W.-Y. , Kleiber P. D.
Published: July 2002

We have investigated the photodissociation spectroscopy of the Mg+–acetaldehyde bimolecular complex over the spectral range 220–400 nm. We find evidence for four distinct absorption bands in the near ultraviolet that correlate with Mg+-based and acetaldehyde-based transitions. Our results suggest that the Mg+-centered 3pπ(A″)←3sσ(A′) and acetaldehyde-centered π*(A″)←n(A′) transitions are mixed, and result in significant vibrational excitation in the complex leading to broad and unresolved absorption bands. In contrast, the predominantly Mg+-based excitation bands, assigned as 3pπ(A′)←3sσ(A′) and 3pσ(A′)←3sσ(A′), each show prominent vibrational progressions identified with intermolecular wagging modes of the complex. These intermolecular wag progressions are short and anharmonic indicating low barriers to isomerization. In the 3pσ(A′)←3sσ(A′) band several higher frequency vibrational modes are also active and have been assigned to the Mg–OCHCH3 intermolecular stretch, a CCO deformation mode, and the C–O stretch.
Order, disorder, and quasiorder in the hydrogen-bond networks of diamond–NH2 surfaces
Authors: Miller John B.
Published: July 2002

The structures of the ordered diamond(100)-2×1-2NH2, disordered diamond(110)-1×1-2NH2, and quasiordered diamond(111)-1×1-NH2 surfaces were calculated using molecular mechanics and density functional approaches. The surface infrared spectra were simulated using force field molecular dynamics and compared to experimental observations. Hydrogen bonding was found to play a significant role in the structures and spectroscopic properties of these surfaces.
Control of the fragmentation of excited ammonia clusters by femtosecond infrared laser pulses
Authors: Farmanara P. , Stert V. , Ritze H.-H. , Radloff W. , Hertel I. V.
Published: July 2002

Applying photoion and coincident photoelectron detection in femtosecond pump–probe experiments, we have studied the change of the fragmentation behavior of ammonia clusters excited by femtosecond (fs) laser pulses at 200 nm to the electronic Ã state which absorb an additional fs control photon 1–2 ps after the pump photon. Only a few 100 fs after the primary excitation, the (NH3)n clusters are partially transferred to the vibrationally highly excited H-transfer state (NH3)n−2NH4NH2 with a lifetime of a few ps. By irradiating the clusters in this state with control photons of a wavelength in the range of 1200–1400 nm, we were able to excite the clusters resonantly to the next higher electronic state in the H-transfer configuration with a strongly reduced vibrational energy. The excited H-transfer state corresponds to the 3s→3p transition in the NH4 component of the internally hydrogenated clusters. Due to the strong reduction of the vibrational energy after the control photon absorption, the fragmentation probability in the excited H-transfer state and correspondingly in the ionic proton transfer state is drastically reduced. For example, for the ammonia dimer the signal ratio of [(NH3)2+] to [NH4+] has been enlarged by nearly one order of magnitude by the resonant control photon absorption. Whereas the lifetime of the ammonia clusters in the nonexcited H-transfer state is nearly identical for all cluster sizes (2–4 ps) we found distinct lifetimes τ6 for the excited H-transfer state of the dimer and the trimer. For the dimer a lifetime τ6=130±50 fs has been obtained for undeuterated as well as for deuterated ammonia molecules. In contrast, for the trimer the lifetime τ6 is significantly larger and depends on the control wavelength as well as on the isotope composition.
Spectroscopy and dynamics of I2(B)–Ne
Authors: Burroughs Amy , Kerenskaya Galina , Heaven Michael C.
Published: July 2002

The I2–Ne complex has been examined using double resonance and fluorescence depletion techniques. Action spectra for I2(B,v)–Ne, detected by monitoring the I2(B,v−1) predissociation fragments, show that the Δν=−1 predissociation channel is less efficient for levels above v=32 (with no excitation of the intermolecular vibrations), and closed for v>36. From these data we obtained a revised estimate for the dissociation energy for I2(B)–Ne of D0=57.6±1.0 cm−1. Action spectra for I2(B,v=34)–Ne, detected by monitoring I2(B,v=33) fragments, revealed a progression of intermolecular vibrational levels that had not been observed previously. These levels have been assigned to T-shaped, linear, and delocalized states of I2(B,v=34)–Ne. Assignments were based on characteristic vibrational distributions exhibited by the I2(B,v−Δv) predissociation fragments. Fluorescence depletion measurements show that all of the bands in the action spectrum originate from a common ground state level. Furthermore, the one atom cage effect fluorescence from I2(B)–Ne can be depleted by transitions from the zero-point level of I2(X)–Ne. These observations indicate that the ground state wave function is delocalized, sampling both the T-shaped and linear configurations of the complex.
Diffusion influence on Michaelis–Menten kinetics
Authors: Kim Hyojoon , Yang Mino , Choi Myung-Un , Shin Kook Joe
Published: July 2002

Influence of diffusion on the Michaelis–Menten kinetics is investigated with the renormalized kinetic theory recently proposed by Yang et al. [J. Chem. Phys. 108, 117; 108, 8557; 108, 9069 (1998)]. The nonlinearity predicted previously by Zhou [J. Phys. Chem. 101, 6642 (1997)] in the Lineweaver–Burk plot for the high concentration of substrate with his empirical expression and simulation is correctly obtained by the kinetic theory. We discuss possible errors in the estimation of reaction parameters caused by ignoring this nonlinearity in an experimental analysis (performed at even lower concentrations of the substrate). The time evolution of the production rate shows a peak before it reaches the steady-state value. The long time asymptotic relaxation of the deviation of the enzyme concentration from the steady-state value shows t−1/2 power-law behavior instead of the exponential decay predicted by the classical kinetics.
Atomic properties from energy-optimized wave functions
Authors: Gálvez F. J. , Buendı́a E. , Sarsa A.
Published: July 2002

Most of the variational Monte Carlo applications on quantum chemistry problems rely on variance-optimized wave functions. Recently, M. Snajdr and S. M. Rothstein, [J. Chem. Phys. 112, 4935 (2000)] have concluded that energy optimization allows one to obtain wave functions that provide better values for a wide variety of ground state properties. In this work we study the quality of energy-optimized wave functions obtained by using the methodology of Lin, Zhang, and Rappe [J. Chem. Phys. 112, 2650 (2000)], as compared with variance-optimized ones for He to Ne atoms. In order to assess this problem we calculate the energy and some other selected properties. The accuracy and performance of the energy-optimization method is studied. A comparison of properties calculated with energy-optimized wave functions to those existing in the literature and obtained by means of variance-optimized wave functions shows a better performance of the former.
Electron cross section set for CHF3
Authors: Morgan W. Lowell , Winstead Carl , McKoy Vincent
Published: July 2002

We describe the development of a consistent set of low-energy electron collision cross sections for trifluoromethane, CHF3. First-principles calculations are used to obtain key elastic and inelastic cross sections. These are combined with literature values of the ionization cross section and with vibrational excitation cross sections obtained from the Born approximation to form a preliminary set, which is then adjusted to achieve consistency with measured swarm parameters.