bluuebunny/crossref_metadata_2025 huggingface dataset:
Amorphous fluoropolymers as insulators for reversible low-voltage electrowetting
Authors: Seyrat Emilie , Hayes Robert A.
Published: July 2002

Amorphous fluoropolymers have been used as a hydrophobic “top coat” of insulating materials for electrowetting, to confer the reversibility required for practical applications. The electrical properties of such fluoropolymers (in particular breakdown voltages of less than 20 V/μm) are often implicated in poor electrowetting performance. However, we have recently found that the intrinsic electrical properties, including the breakdown voltage, of appropriately prepared fluoropolymer coatings are sufficient to allow the amorphous fluoropolymer to function both as the insulator and hydrophobic surface for electrowetting. A typical electrowetting material system is then reduced to the basic material components of electrode, insulator, and conducting liquid. This simplicity facilitates both the fabrication of electrowetting devices, the soluble fluoropolymer insulator being directly wet coated onto the electrode materials, as well as the study of charging mechanisms. Reversible electrowetting on insulators thinner than 1 μm and at less than 50 V is also routinely achievable.
Experimental study of MgB2 decomposition
Authors: Fan Z. Y. , Hinks D. G. , Newman N. , Rowell J. M.
Published: July 2002

The thermal stability of MgB2 has been studied experimentally to determine the role of thermodynamic and kinetic barriers in the decomposition process. The MgB2 decomposition rate approaches one monolayer per second at 650 °C and has an activation energy of 2.0 eV. The evaporation coefficient is inferred to be ∼10−4, indicating that this process is kinetically limited. These values were inferred from in situ measurements using a quartz crystal microbalance and a residual gas analyzer, in conjunction with ex situ measurements of redeposited material by Rutherford backscattering spectroscopy and secondary ion mass spectroscopy. The presence of a large kinetic barrier to decomposition indicates that the synthesis of MgB2 thin films conditions may be possible with vacuum processing, albeit within a narrow window in the reactive growth conditions.
Development of a fast fiber-optic two-color pyrometer for the temperature measurement of surfaces with varying emissivities
Authors: Müller B. , Renz U.
Published: July 2002

A two-color pyrometer has been developed to measure the temperature of surfaces with unknown emissivities during high speed turning processes. Quartz fibers enable measurements at locations with limited optical access. The sensitivity of the pyrometer has to be high enough to measure temperatures down to 300 °C of an aluminum alloy with an emissivity as low as 0.2. The accuracy of the two-color pyrometer has been compared with the accuracy of monochromatic pyrometers for different metallic surfaces. The different arguments for the choice of the two pyrometer wavelengths 1.7 and 2.0 μm are explained. The influences of the surface emissivities, the digitization, and the noise on the absolute and relative measurement error have been determined. Fast amplifiers and data acquisition allow a maximum time resolution of a few microseconds and a local resolution of ∼0.5 mm2. Some test measurements of an aluminum alloy surface are presented.
Diatomic molecules between very heavy elements of group 13 and group 17: A study of relativistic effects on bonding
Authors: Fægri Knut , Saue Trond
Published: July 2002

The effects of relativity on the bonding between heavy elements of groups 13 and 17 have been investigated. Using extensive energy minimized basis sets, calculations have been carried out within several models which include relativistic effects at various levels—4-component Dirac–Hartree–Fock, spin-free relativistic, Lèvy–Leblond, mass–velocity-Darwin perturbation theory, as well as nonrelativistic Hartree–Fock calculations. The relativistic effects on spectroscopic constants are dramatic, and dominated by the spin–orbit splitting and the relativistic contraction of the valence p-orbitals. Particularly noteworthy is the spin–orbit induced change of sign of the dipole moment for [113][117]. This highlights the importance of including the spin–orbit effect in the variational optimization of the wave function for heavy elements.
Photoelectron spectroscopy of gold–silver binary cluster anions (AunAgm−; 2⩽n+m⩽4)
Authors: Negishi Yuichi , Nakamura Yoshiaki , Nakajima Atsushi , Kaya Koji
Published: July 2002

The electronic properties of gold–silver binary cluster anions (AunAgm−; 2⩽n+m⩽4) were studied by using photoelectron spectroscopy (PES) with a magnetic-bottle-type electron spectrometer. The AunAgm− cluster anions were generated by a laser vaporization of a gold–silver alloy rod. For triatomic AunAg3−n clusters, monotonous increases of electron affinity (EA) were observed by the replacement of Ag atom by Au atom. In contrast, the change in EA of tetratomic AunAg4−n clusters was irregular; for n=0–2 their EAs are almost the same around 1.5 eV, whereas for n=3,4 they increase to around 2.7 eV. This irregularity in EA of AunAg4−n can be attributed to the contribution of an ionic bonding. For Au1Ag3− and Au2Ag2−, moreover, the PES spectra show two components whose intensity ratio depends on cluster source conditions, showing that two isomers should coexist at these two compositions.
High pressure crystal phases of benzene probed by infrared spectroscopy
Authors: Ciabini Lucia , Santoro Mario , Bini Roberto , Schettino Vincenzo
Published: July 2002

In this work we present an infrared study of the high pressure phases of benzene crystal up to 25 GPa. The far infrared portion of the spectrum has been studied for the first time above 4 GPa. From the pressure behavior of the frequencies and the integrated peak absorptions we are able to confirm, beside the transition between phases II–III at 4.8 GPa, the transition between phases III–III′ at 11.2 GPa. The spectral pattern changes very smoothly among the different phases and the number of bands is always consistent with a crystal structure having two molecules per cell and the inversion center as a symmetry element. At 17.4 GPa a structural change is suggested from the slope decrease of the frequency evolution with pressure of almost all the crystal modes. Particularly important are the changes of the doubly degenerate ν18 bending mode, which could be strongly involved in the irreversible transformation occurring at higher pressure.
Effect of tacticity on the segmental dynamics of polypropylene melts investigated by C13 nuclear magnetic resonance
Authors: Lippow S. M. , Qiu XiaoHua , Ediger M. D.
Published: July 2002

C 13 nuclear magnetic resonance (NMR) T1 and nuclear Overhauser effect measurements are reported for syndiotactic and isotactic polypropylene from their melting points to 525 K. These results indicate that the segmental dynamics of syndiotactic polypropylene are 1.7 times slower than for isotactic polypropylene at 500 K. Recent molecular dynamics computer simulations [Antoniadis, Samara, and Theodorou, Macromolecules 32, 8635 (1999)] predict this trend qualitatively but predict too large a dependence of dynamics upon tacticity. The contribution of normal mode relaxation to the decay of the C–H vector autocorrelation function is significantly larger for syndiotactic polypropylene than for either isotactic or atactic polypropylene.
Oscillatory magnetic bead rheometer for complex fluid microrheometry
Authors: Keller M. , Schilling J. , Sackmann E.
Published: July 2002

The dynamic range of the microrheometry based on the analysis of the enforced motion of colloidal force probes by video microscopy has been extended to 40 Hz. For that purpose a novel rapid image processing system has been developed enabling the real-time data acquisition and analysis thus improving the time resolution of particle tracking to 6 ms. A second advancement consists of the development of a correction procedure accounting for two effects: First, for the smearing out of the diffraction image of the beads due to the finite data acquisition time and, second, for systematic phase shifts of magnetic bead deflection with respect to the force due to the finite response time of the superparamagnetic beads in the direction of the gradient of the magnetic field. The new method has been applied to re-study the dynamic scaling law of the frequency dependence of the viscoelastic impedance of entangled actin solutions in the frequency regime determined by the conformational dynamics and entropic tension of single filaments. The frequency dependence of the storage and loss modulus obeys the scaling laws G′(ω)∝G″(ω)∝ωα with α=0.83(8) which is only slightly higher than the theoretical prediction (α=0.75).
Finite size effects in the anisotropic (λ/4!)(φ14+φ24)d model
Authors: Fosco C. D. , Svaiter N. F.
Published: July 2002

We consider the (λ/4!)(φ14+φ24) model on a d-dimensional Euclidean space, where all but one of the coordinates are unbounded. Translation invariance along the bounded coordinate, z, which lies in the interval [0,L], is broken because of the boundary conditions (BCs) chosen for the hyperplanes z=0 and z=L: DD and NN, where D denotes Dirichlet and N Neumann, respectively. The renormalization procedure up to one-loop order in the two-point function is applied, obtaining two main results. The first is the fact that the renormalization program requires the introduction of counterterms which are surface interactions. The second one is that the tadpole graphs for DD and NN have the same z dependent part in modulus but with opposite signs. We investigate the relevance of this fact to the elimination of surface divergences.
Dynamics of He4 droplets
Authors: Krotscheck E. , Zillich R.
Published: July 2002

Optimized variational calculations have been carried out for clusters of He4 between N=20 and N=1000 atoms. For small cluster sizes with less or equal to 112 particles, where comparisons with existing diffusion Monte Carlo results are possible, we find good agreement for the ground state energies and densities. Using a somewhat simpler approximation, we also calculate the bound state energies of He3 atoms attached to these clusters. We then calculate excitations and the dynamic structure function. The complex and nonlocal self-energy introduced for that purpose gives access to the calculation of both elastic and inelastic scattering processes for He4 and He3 atoms impinging on the clusters.
Effects of O2, Ar, and H2 gases on the field-emission properties of single-walled and multiwalled carbon nanotubes
Authors: Wadhawan A. , Stallcup R. E. , Stephens K. F. , Perez J. M. , Akwani I. A.
Published: July 2002

We compare the effects of O2, Ar, and H2 gases on the field-emission (FE) properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after operation in UHV. The higher operating voltages in an O2 environment cause a permanent decrease of the FE current and an increase in the turn-on field of MWNTs. The ratios of the slopes before and after O2 exposure are approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively.
Measurements of optical refrigeration in ytterbium-doped crystals
Authors: Epstein R. I. , Brown J. J. , Edwards B. C. , Gibbs A.
Published: July 2002

We measured anti-Stokes fluorescence cooling (optical refrigeration) in ytterbium-doped yttrium aluminum garnet (YAG). Pumping the 2.3% Yb3+:YAG crystal with 1.8 W of 1030 nm laser light produced a temperature drop of 8.9 °C from room temperature. The high thermal conductivity and ruggedness of this crystal make it an attractive material for use in optical refrigerators. Our spectral studies show that pure crystals of this material could be efficient for optical refrigeration at temperatures above ∼100 K. Photothermal deflection measurements show that our current crystals can cool at ∼250 K. Additionally, we measured optical refrigeration in a 5% Yb3+:Y2SiO5 crystal when pumped at 1050 nm.
Quantification of the orientational disorder in ortho-dichlorotetramethylbenzene: A single crystal deuterium nuclear magnetic resonance and x-ray study of the site populations
Authors: Bräuniger Thomas , Poupko Raphy , Luz Zeev , Zimmermann Herbert , Haeberlen Ulrich
Published: July 2002

The title compound, 1,2-dichloro-3,4,5,6-tetramethylbenzene (DCTMB) exhibits three solid phases, III, II, and I, of which two, Phase III (<170 K) and Phase II (170–381 K), have been investigated in the present study by means of x-ray diffraction and deuterium NMR. The latter measurements were performed on powder and single crystal samples of perdeuterated DCTMB (DCTMB-d12). Phase III is “right–left” disordered, with the molecular para axes (the axes having two methyls in para positions) well ordered in the crystal. The right–left disorder is manifested by the fact that the chlorine and ortho methyls have each an occupancy number of 12, thus ensuring an average inversion symmetry at the molecular site. The NMR results also indicate that at least one of the ortho methyls, and probably the second one too, are slightly distorted, randomly up and down, relative to the benzene plane. Except for fast methyl group rotation the molecules in this phase are static on the NMR time scale. Phase II is much more mobile and disordered, with the molecular para axes distributed over all six local crystallographic orientations. A detailed analysis of the single crystal NMR results shows that the fractional populations in the various orientations are biased with some orientations more populated than others (while keeping average inversion symmetry). It is shown that this bias is due to differences in both the enthalpy and entropy associated with the various orientations. NMR line shape and T1 relaxation data in Phase II (and I) indicate rapid jumps between the different orientations in the high temperature range of the measurements. The estimated activation parameters for the reorientational jumps are Ea=33 kJ/mol and k(260 K)=4.5×107 s−1.
dc and rf performance of proton-irradiated AlGaN/GaN high electron mobility transistors
Authors: Luo B. , Johnson J. W. , Ren F. , Allums K. K. , Abernathy C. R. , Pearton S. J. , Dwivedi R. , Fogarty T. N. , Wilkins R. , Dabiran A. M. , Wowchack A. M. , Polley C. J. , Chow P. P. , Baca A. G.
Published: July 2002

AlGaN/GaN high electron mobility transistors (HEMTs) with a range of gate lengths (0.8–1.2 μm) and widths (100–200 μm) were exposed to 40 MeV protons at fluences of 5×109 or 5×1010 cm−2. The drain–source currents in the devices decreased by 15%–20% at the higher fluence, while the extrinsic transconductance decreased by ∼30% under the same conditions. Based on the increases in the reverse breakdown voltage and the channel resistance, the main degradation mechanism is believed to be creation of deep trap states in the band gap which remove electrons from the channel. The maximum frequency of oscillation, fMAX, also decreased as a result of the proton-induced damage, with a change of −20% at the shorter gate widths and −50% at the largest widths. The reverse recovery switching time was essentially unaffected by the irradiation, remaining at ∼1.6×10−8 s. Postradiation annealing at 800 °C was successful in restoring the dc and rf performance parameters to ⩾90% of their original values. The AlGaN/GaN HEMTs are much more robust than their AlGaAs/GaAs counterparts to displacement damage and appear well-suited to radiation environment applications.
Blistering of silicon crystals by low keV hydrogen and helium ions
Authors: Qian C. , Terreault B.
Published: July 2002

The blistering of Si wafers with (100), (110), and (111) orientations, induced by implantation of low energy (5–8 keV) H and/or He ions, was studied. Our earlier work had used either pure H implants at high dose (5×1016 cm−2) or low dose coimplantation (1×1016 H cm−2+1×1016 He cm−2). Here we study pure He implantation, and coimplantation with variable He/H dose ratio, in order to understand the roles of H and He, in the hope of improving the blistering efficiency (as used, e.g., in the Smart-cut® process). After room temperature implantation, the samples were vacuum annealed in three steps at 500, 550, and 600 °C. At each step, the development of blistering and exfoliation was quantified by scanning electron microscopy (“exfoliation” is here used to mean blister explosion). We found that the thermal budget required for blistering is higher than for comparable doses of high energy (>30 keV) ions. On the Si(110) surface, H blistering is very weak, He blistering is the strongest, but exfoliation absent. On Si(111), H blistering is the strongest, He the weakest, but exfoliation a maximum. Si(100) shows intermediate behavior. Weak H blistering correlates with premature H2 desorption. H+He coimplantation efficiently increases the degree of blistering and, to some extent, exfoliation.
Fusion pathways of vesicles: A Brownian dynamics simulation
Authors: Noguchi Hiroshi , Takasu Masako
Published: July 2002

We studied the fusion dynamics of vesicles using a Brownian dynamics simulation. Amphiphilic molecules spontaneously form vesicles with a bilayer structure. Two vesicles come into contact and form a stalk intermediate, in which a necklike structure only connects the outer monolayers, as predicted by the stalk hypothesis. We have found a new pathway of pore opening from stalks at high temperature: the elliptic stalk bends and contact between the ends of the arc-shaped stalk leads to pore opening. On the other hand, we have clarified that the pore-opening process at low temperature agrees with the modified stalk model: a pore is induced by contact between the inner monolayers inside the stalk.
Pd/AlN/SiC thin-film devices for selective hydrogen sensing
Authors: Serina F. , Ng K. Y. S. , Huang C. , Auner G. W. , Rimai L. , Naik R.
Published: July 2002

A hydrogen sensing device has been fabricated using a 1500- Å-thick AlN layer interposed between a SiC substrate and a Pd gate. This device behaves as a rectifying diode. Hydrogen in the gas flow causes a shift in the forward conduction and the sensitivity of the device is found to increase with the total flow rate. At a device temperature of 300 °C and a flow rate of 1000 sccm, the typical forward current of ∼1 mA increases by 10% for 10 ppm hydrogen, independent of the presence of oxygen or CO in the gas flow. The shift is also independent of the presence of propane. However, for very large concentrations of propane a slight response can be discerned, due to the presence of some hydrogen resulting from the incipient decomposition of the hydrocarbon at elevated temperature.
Temperature dependence of electroluminescence intensity of green and blue InGaN single-quantum-well light-emitting diodes
Authors: Hori A. , Yasunaga D. , Satake A. , Fujiwara K.
Published: July 2002

Temperature dependence of electroluminescence (EL) spectral intensity of the super-bright green and blue InGaN single-quantum-well (SQW) light-emitting diodes has been studied over a wide temperature range (T=15–300 K) under a weak injection current of 0.1 mA. It is found that when T is slightly decreased to 140 K, the EL intensity efficiently increases, as usually seen due to the improved quantum efficiency. However, with further decrease of T down to 15 K, it drastically decreases due to reduced carrier capture by SQW and trapping by nonradiative recombination centers. This unusual temperature-dependent evolution of the EL intensity shows a striking difference between green and blue SQW diodes owing to the different potential depths of the InGaN well. The importance of efficient carrier capture processes by localized tail states within the SQW is thus pointed out for enhancement of radiative recombination of injected carriers in the presence of the high-density dislocations.
Composition dependence of the energy gap of Zn1−x−yMgxBeySe quaternary alloys nearly lattice matched to GaAs
Authors: Godo K. , Makino H. , Cho M. W. , Chang J. H. , Yamazaki Y. , Yao T. , Shen M. Y. , Goto T.
Published: July 2002

The composition dependence of the energy gap (Eg) of Zn1−x−yMgxBeySe quaternary alloys grown by molecular-beam epitaxy was investigated. The energy gap of Zn1−x−yMgxBeySe can be controlled in a range of 2.7 eV<Eg<3 eV under nearly lattice-matching conditions to GaAs(001). The phenomenological formula for the energy gap of ZnMgBeSe quarterly is obtained using the bowing parameters theoretically or experimentally estimated for the ternary compounds. These results show that nonlinear behavior was observed on the composition dependence of the energy gap as the Be and Mg compositions increased.
Role of femtosecond pulses in distinguishing third- and fifth-order nonlinearity for semiconductor-doped glasses
Authors: Bindra K. S. , Kar A. K.
Published: July 2002

We report Z-scan experiments at a frequency below the band gap of semiconductor-doped glasses using 100 fs laser pulses. A large contribution from the positive third-order nonlinearity is observed in complete contrast with the picosecond z-scan experiments. The third- and fifth-order nonlinearity for semiconductor-doped glasses can be estimated from z-scans for ps and fs pulses.
Repetitive plasma discharges in saline solutions
Authors: Stalder K. R. , Woloszko J. , Brown I. G. , Smith C. D.
Published: July 2002

We describe some observations of plasma formation in aqueous electrolyte solutions (containing either sodium chloride or barium chloride), using a small planar electrode configuration driven by a repetitive bipolar voltage source. Plasma pulses of a few microseconds duration were established by a 100 kHz square wave of a few hundred volts peak to peak, drawing a current during the plasma period of several hundred milliamperes. Spectral measurements show the presence of strong ion lines, confirming that plasma is formed in the liquid. The process produces chemical radicals such as OH and, when placed in proximity to biological tissue, causes biological material volumetric removal, opening possibilities for new kinds of plasma-driven electrosurgery such as tissue debulking or cutting.
Residue packing in proteins: Uniform distribution on a coarse-grained scale
Authors: Bagci Zerrin , Jernigan Robert L. , Bahar Ivet
Published: July 2002

The high packing density of residues in proteins ought to be manifested in some order; to date this packing order has not been thoroughly characterized. The packing regularity in proteins is important because the internal organization of proteins can have a dominant effect on functional dynamics, and it can aid in the design, simulation and evaluation of structures. Packing metrics could also inform us about normal sequence variability, an issue that, with the accumulating genome data, becomes increasingly important. Other studies, indicating a possible correlation between packing density, sequence conservation, and folding nucleation [O. B. Ptitsyn, J. Mol. Biol. 278, 655 (1998)], have emphasized the importance of packing. Here, residue clusters from protein databank structures, each comprised of a central residue and all neighbors located within the first coordination shell, have been rigidly re-oriented and superimposed in a self-consistent optimization. About two-thirds of residues are found to follow approximately the relative orientation preferences of face-centered-cubic packing, when examined on a coarse-grained scale (one site per residue), while the remaining one-third occupy random positions. The observed regularity, which becomes more pronounced after optimal superimposition of core residues, appears to be the result of uniform sampling of the coordination space around each residue on a coarse-grained scale with hydrophobic clustering and volume exclusion, to achieve packing densities close to that of the universal closest packing of identical spheres.
High resolution low-temperature superconductivity superconducting quantum interference device microscope for imaging magnetic fields of samples at room temperatures
Authors: Baudenbacher F. , Peters N. T. , Wikswo J. P.
Published: July 2002

We have developed a microscope to image weak magnetic fields using submillimeter pickup coils made from conventional low-temperature superconducting niobium wire coupled to the input circuit of a superconducting quantum interference device (SQUID). The pickup coil and the SQUID sensor are mounted in the vacuum space of the cryostat and are thermally anchored to the liquid helium reservoir. A 25 μm thick sapphire window separates the room temperature (RT) sample and the vacuum space. The spacing between the pickup coil and RT sample was typically less than 130 μm. The spatial resolution is limited by the diameter of the pickup coil. The pickup coils are easily interchangeable, allowing us to adapt the SQUID microscope to a variety of different measurements. We have achieved a spatial resolution of 250 μm with a magnetic field sensitivity of 850 fT/Hz1/2 or a spatial resolution of 500 μm with a magnetic field sensitivity of 330 fT/Hz1/2. We have used this instrument to measure various biomagnetic and paleomagnetic fields.
Subsurface reactions of silicon nitride in a highly selective etching process of silicon oxide over silicon nitride
Authors: Ito Masafumi , Kamiya Kiyoshi , Hori Masaru , Goto Toshio
Published: July 2002

In situ subsurface reaction measurements of silicon nitride in a selective etching process of silicon oxide over silicon nitride using an x-ray photoelectron spectroscopy (XPS) and a Fourier transform infrared reflection absorption spectroscopy (FTIR RAS) have been carried out. Under low selectivity etching conditions using an electron cyclotron resonance plasma employing a pure octafluorocyclobutane (C4F8) gas, a clear difference has been observed between time-evolution spectra of FTIR RAS and those of XPS on the etched silicon nitride films. From these results it has been found that the etching reaction layer is thicker than that under highly selective etching conditions and that SiF3 bonds are located in the deeper region rather than in the shallow region of the reaction layer. On the other hand, under highly selective etching conditions employing C4F8 gas diluted by Ar gas, it has been observed that the shallow region of the reaction layer in the etched silicon nitride films becomes C–C cross-linking-rich and CN sp2 bond-rich. From these results, it is concluded that the C–C cross linking prevents CN sp2 bonds from reacting with F atoms, resulting in the suppression of the etch by-products such as FCN.
Integration schemes for dissipative particle dynamics simulations: From softly interacting systems towards hybrid models
Authors: Vattulainen I. , Karttunen M. , Besold G. , Polson J. M.
Published: July 2002

We examine the performance of various commonly used integration schemes in dissipative particle dynamics simulations. We consider this issue using three different model systems, which characterize a variety of different conditions often studied in simulations. Specifically, we clarify the performance of integration schemes in hybrid models, which combine microscopic and mesoscale descriptions of different particles using both soft and hard interactions. We find that in all three model systems many commonly used integrators may give rise to surprisingly pronounced artifacts in physical observables such as the radial distribution function, the compressibility, and the tracer diffusion coefficient. The artifacts are found to be strongest in systems, where interparticle interactions are soft and predominated by random and dissipative forces, while in systems governed by conservative interactions the artifacts are weaker. Our results suggest that the quality of any integration scheme employed is crucial in all cases where the role of random and dissipative forces is important, including hybrid models where the solvent is described in terms of soft potentials. Regarding the integration schemes, the best overall performance is found for integrators in which the velocity dependence of dissipative forces is taken into account, and particularly good performance is found for an approach in which velocities and dissipative forces are determined self-consistently. Remaining temperature deviations from the desired limit can be corrected by carrying out the self-consistent integration in conjunction with an auxiliary thermostat, in a manner that is similar in spirit to the well-known Nosé–Hoover thermostat. Further, we show that conservative interactions can play a significant role in describing the transport properties of simple fluids, in contrast to approximations often made in deriving analytical theories. In general, our results illustrate the main problems associated with simulation methods in which dissipative forces are velocity dependent, and point to the need to develop new techniques to resolve these issues.
Band gap engineering, band edge emission, and p-type conductivity in wide-gap LaCuOS1−xSex oxychalcogenides
Authors: Ueda Kazushige , Hosono Hideo
Published: July 2002

The preparation of LaCuOS1−xSex solid solutions (x=0.0, 0.25, 0.5, 0.75, and 1.0) was attempted to control their energy gap and band edge emission energy. X-ray diffraction analysis revealed that the lattice constant of LaCuOS1−xSex increased linearly with increasing x, indicating the formation of a complete solid solution in the LaCuOS–LaCuOSe system. The energy gap estimated from the diffuse reflectance spectra varied continuously from ∼3.1 eV for x=0 to ∼2.8 eV for x=1. The sharp emission near the absorption edge was observed in all samples at room temperature under ultraviolet light irradiation. p-type electrical conduction in these materials was confirmed by Seebeck measurements, and the conductivity was enhanced by substitution of Sr for La. These results demonstrated that the formation of the solid solutions enabled band gap engineering in LaCuOS1−xSex oxychalcogenides keeping their band edge emission feature and p-type conductivity.
A comparison between potentiostatic circuits with grounded work or auxiliary electrode
Authors: Busoni L. , Carlà M. , Lanzi L.
Published: July 2002

Potentiostatic circuit configurations with work electrodes and auxiliary electrodes at ground potential have been reviewed and compared. Though the former is by far the best known and most used, the latter was more convenient and accurate in interfacial capacity measurements.
Large sonic band gaps in 12-fold quasicrystals
Authors: Lai Yun , Zhang Xiangdong , Zhang Zhao-Qing
Published: July 2002

The sonic band-gap structures of 12-fold symmetry quasicrystals consisting of rigid cylinders in air are investigated by using the multiple scattering method. Large full gaps are found in this system owing to its high symmetry. At filling fractions between 0.2 and 0.4, this 12-fold square–triangle tiling is much better for the realization of sonic band gaps than the square or triangular lattice. This makes the 12-fold quasicrystal a promising structure for acoustic-wave band-gap materials.
Continuous-wave all-optoelectronic terahertz imaging
Authors: Siebert Karsten J. , Quast Holger , Leonhardt Rainer , Löffler Torsten , Thomson Mark , Bauer Tobias , Roskos Hartmut G. , Czasch Stephanie
Published: July 2002

We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.
Phase field microelasticity theory and simulation of multiple voids and cracks in single crystals and polycrystals under applied stress
Authors: Wang Yu U. , Jin Yongmei M. , Khachaturyan Armen G.
Published: July 2002

The phase field microelasticity theory of a three-dimensional elastically anisotropic single crystal with multiple voids and cracks is developed. It is extended to the case of elastically isotropic polycrystal. The theory is based on the exact equation for the strain energy of the “equivalent” continuous elastically homogeneous body presented as a functional of the phase field. This field is the equivalent stress-free strain. It is proved that the equivalent stress-free strain minimizing the strain energy of the elastically homogeneous body fully determines the elastic strain and displacement of the body with voids/cracks. The geometry and evolution of multiple voids and cracks are described by the phase field, which is a solution of the stochastic time-dependent Ginzburg–Landau equation. Other stress-generating defects, such as dislocations and precipitates, are trivially integrated into this theory. The proposed model does not impose a priori constraints on the configuration of multiple voids and cracks or on possible configurations and evolutions of cracks along their propagation paths. Examples of computations of the elastic equilibrium of systems with voids and/or cracks, the evolution of cracks in single crystals and polycrystals, and material toughening due to second-phase obstacles are considered.
A critical note on density functional theory studies on rare-gas dimers
Authors: van Mourik Tanja , Gdanitz Robert J.
Published: July 2002

In recent literature, some authors claim to have successfully applied density functional theory (DFT) methods to the attractive interaction between rare-gas atoms. In this note, we make a critical survey of these works and come to the conclusion that, in contrast to the claims made, state-of-the-art DFT methods are incapable of accounting for dispersion effects in a quantitative way.
The use of a hybrid resonator consisting of one-dimensional and two-dimensional Bragg reflectors for generation of spatially coherent radiation in a coaxial free-electron laser
Authors: Ginzburg N. S. , Peskov N. Yu. , Sergeev A. S. , Phelps A. D. R. , Cross A. W. , Konoplev I. V.
Published: July 2002

The use of a novel hybrid scheme of a two-mirror Bragg resonator consisting of one-dimensional (1D) and two-dimensional (2D) reflectors is suggested for generation of powerful spatially coherent radiation in a coaxial free-electron laser driven by a large-size (102–103 wavelengths) high-current annular relativistic electron beam. The 2D Bragg reflector is positioned at the cathode side of the oscillator to provide synchronization of radiation from the oversized electron beam via transverse energy fluxes, which arise due to scattering on the 2D Bragg structure. The conventional 1D Bragg reflector is positioned at the collector side to complete the feedback.
Varying the fraction of orbital exchange in density functional theory: Influence on nuclear magnetic resonance shielding constants
Authors: Wilson Philip J. , Tozer David J.
Published: July 2002

A series of hybrid exchange-correlation functionals containing varying fractions of orbital exchange ξ=0.0, 0.1, 0.2, … 1.0, are determined using conventional molecular thermochemical fits. The functionals are used to determine Kohn–Sham nuclear magnetic resonance shielding tensors for a series of small molecules involving first- and second-row atoms; results are compared with experimental values. On average, isotropic and anisotropic shieldings determined using the conventional coupled approach become progressively less accurate as ξ increases from 0.0 to 1.0. By contrast, isotropic and anisotropic shieldings determined from the hybrid Kohn–Sham densities using the uncoupled multiplicative Kohn–Sham (MKS) approach [Chem. Phys. Lett. 337, 341 (2001)] improve significantly as ξ increases from 0.0 to 0.2–0.3; optimal results are more than three times as accurate as the corresponding coupled results. As ξ is further increased, the MKS results degrade. The quality of the Kohn–Sham highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) eigenvalue difference in the MKS calculations is investigated by comparing it with values determined from coupled cluster Brueckner doubles densities. In line with the shielding observations, optimal HOMO–LUMO differences are obtained near ξ=0.3.
The semiclassical helium atom
Authors: Wintgen D. , Richter K. , Tanner G.
Published: July 2002

Recent progress in the semiclassical description of two-electron atoms is reported herein. It is shown that the classical dynamics for the helium atom is of mixed phase space structure, i.e., regular and chaotic motion coexists. Semiclassically, both types of motion require separate treatment. Stability islands are quantized via a torus–quantization-type procedure, whereas a periodic-orbit cycle expansion approach accounts for the states associated with hyperbolic electron pair motion. The results are compared with highly accurate ab initio quantum calculations, most of which are reported here for the first time. The results are discussed with an emphasis on previous interpretations of doubly excited electron states
Time scale to ergodicity in the Fermi–Pasta–Ulam system
Authors: De Luca J. , Lichtenberg A. J. , Lieberman M. A.
Published: July 2002

We study the approach to near-equipartition in the N-dimensional Fermi–Pasta–Ulam Hamiltonian with quartic (hard spring) nonlinearity. We investigate numerically the time evolution of orbits with initial energy in some few low-frequency linear modes. Our results indicate a transition where, above a critical energy which is independent of N, one can reach equipartition if one waits for a time proportional to N2. Below this critical energy the time to equipartition is exponentially long. We develop a theory to determine the time evolution and the excitation of the nonlinear modes based on a resonant normal form treatment of the resonances among the oscillators. Our theory predicts the critical energy for equipartition, the time scale to equipartition, and the form of the nonlinear modes below equipartition, in qualitative agreement with the numerical results.
The role of cardiac tissue structure in defibrillation
Authors: Trayanova Natalia , Skouibine Kirill , Aguel Felipe
Published: July 2002

The purpose of this paper is to investigate the relationship between cardiac tissue structure, applied electric field, and the transmembrane potential induced in the process of defibrillation. It outlines a general understanding of the structural mechanisms that contribute to the outcome of a defibrillation shock. Electric shocks defibrillate by changing the transmembrane potential throughout the myocardium. In this process first and foremost the shock current must access the bulk of myocardial mass. The exogenous current traverses the myocardium along convoluted intracellular and extracellular pathways channeled by the tissue structure. Since individual fibers follow curved pathways in the heart, and the fiber direction rotates across the ventricular wall, the applied current perpetually engages in redistribution between the intra- and extracellular domains. This redistribution results in changes in transmembrane potential (membrane polarization): regions of membrane hyper- and depolarization of extent larger than a single cell are induced in the myocardium by the defibrillation shock. Tissue inhomogeneities also contribute to local membrane polarization in the myocardium which is superimposed over the large-scale polarization associated with the fibrous organization of the myocardium. The paper presents simulation results that illustrate various mechanisms by which cardiac tissue structure assists the changes in transmembrane potential throughout the myocardium.
Visualizing the evolution of a star on a graphic workstation
Authors: Noels Arlette , Papy Roland , Remy François
Published: July 2002

Theoretical models of a one solar mass star from the gravitational contraction onto the main sequence of the Hertzsprung–Russell diagram are visualized on a Tektronix 4337 workstation.
A hierarchical approach to stochastic simulation of correlated systems
Authors: Visscher P. B. , Dye Judy , Fang Jen-Ho
Published: July 2002

A description is given of a new algorithm for simulating systems with known density correlations, a problem that has been intensively studied in the context of petroleum reservoirs. Given an assumed spatial correlation function (covariance) and known densities at an arbitrary set of constrained points, it generates members of an ensemble defined by Gaussian statistics, the covariance function, and the known constraints. In the case of a power-law correlation function, this is a fractal distribution. Our algorithm is significantly more efficient than existing techniques for doing this—in the fractal case, the computer time requirement is proportional to N, rather than N3, where N is the number of grid points. In applications, where N can be 104 or larger, this is a significant difference.
Comparison of three methods for determining fit parameter uncertainties for the Marquardt Compromise
Authors: Cunningham Robert W.
Published: July 2002

Monte Carlo simulation was used to study the fit parameter uncertainties obtained from the Marquardt Compromise for a noisy data set, data that deviates from the parent function. A known Gaussian was the parent function in this work. Each data point was also weighted by either statistical or uniform weights. The three uncertainties were the Monte Carlo estimate, equal to the standard deviation of a fit parameter, the error matrix estimate, and the propagation of errors estimate. All simulations produced equality of the error matrix and the propagation of errors estimates for the same fit parameter. Each Monte Carlo estimate was linearly dependent on a parameter that determined the amount of noise and was independent of the weights. However, the error matrix uncertainties depended linearly on the weights and were independent of the noise for statistical weights, but depended linearly on the noise and were independent of the weights for fixed weights. Because the error matrix estimates depend on chi square, it was also studied and varied quadratically with the noise and as the inverse square of the weights. Each of these results was also derived from the theory of the Marquardt Compromise.
Exponents in lifetime and power spectral density forms in self-organized critical systems—A Mathematica application
Authors: Meisel L. V. , Cote P. J.
Published: July 2002

Bak, Tang, and Wiesenfeld (BTW) [Phys. Rev. Lett. 59, 381 (1987); Phys. Rev. A 38, 36 (1988)] established that power law frequency dependencies in the power spectral density (PSD) and size effect modified power law distributions of lifetimes are the fingerprints of self-organized critical systems. Jensen, Christensen, and Fogedby (JCF) [Phys. Rev. B 40, 7425 (1989)] clarified the ideas introduced by BTW and established the connection between the distribution of lifetimes and the PSD for the case of exponentially cutoff (‘‘size effect’’ modified) distributions of lifetimes. Here the JCF connection between the PSD and the distribution of lifetimes is established for sharp cutoff distributions, which supports the idea that the JCF connection holds for quite general size effect modified lifetime distributions. The PSD may be expressed in terms of generalized hypergeometric functions in this case. A detailed discussion of the JCF connections is presented for a subset of values of the lifetime distribution exponent, for which the generalized hypergeometric functions reduce to Fresnel integrals and sine and cosine integrals, which were the subject of a recent ‘‘Numerical Recipes’’ column [Press and Teukolsky, Comput. Phys. 6, 670–672 (1992)]. All calculations were performed in Mathematica.
Nonlinear effects in a stretched string—a numerical computation
Authors: Biswas Tarun
Published: July 2002

In most undergraduate physics courses, differential equations that arise from the description of physical systems are, very often, summarily linearized. This is done to find analytical solutions using standard methods. With the present day availability of microcomputers, such approximations are not always necessary. In this article a simple-minded finite difference numerical method is shown to be able to handle the nonlinear partial differential equation of a standard physical system—a stretched string. The method is general enough to be applicable for a large class of systems. The numerical computation shows that large wave pulses will distort and sinusoidal standing waves will develop higher harmonics. A computer animated presentation is used to make the results look more realistic.
A computer program for the characterization of solar cells
Authors: Casados-Cruz Gaspar , Morales-Acevedo Arturo
Published: July 2002

A quick and efficient computer program was developed in order to determine important parameters of solar cells, such as the series and shunt resistances, the ideality diode factor, the dark current, and the light current from I versus V measurements. A generalized, single-exponential model was assumed and, instead of solving nonlinear equations, we used accurate analytical approximations for the calculation of the above parameters. The program was designed to be used easily on any IBM-PC compatible computer by providing a user-friendly interface. The required input parameters, such as the open circuit voltage (Voc), short circuit current (Isc), maximum power voltage (Vm), or maximum power current (Im), were determined directly from the I–V pairs by numerical polynomial least-squares fitting, so that the user does not have to determine these parameters graphically. Therefore, our program is also compatible with a data acquisition system attached to the same computer. The program was tested with simulated data and also with real measurements for typical production silicon solar cells; in all cases, good agreement between the experimental curve and the model’s theoretical curve was achieved as determined by a high correlation factor and also by the graphical comparison displayed by the program.
Distributed visual programming environment: Possibilities and limitations
Authors: Rijnders Frank M. , Spoelder H. J. W. , Groen F. C. A.
Published: July 2002

Distributed processing is an essential ingredient for the data processing in the next decade. Surprisingly therefore most state-of-the-art environments lack generic support for distributed processing. It can be realized in a highly portable way using standard tools like RPC and XDR. However the feasibility of such an approach critically depends on the effort the user has to undertake to use these tools and the overhead that is introduced by them. We have developed a visual programming environment which uses an RPC/XDR based client/multiple server approach as execution model and allows, through its sophisticated user-interface, easy utilization of distributed processing. For a typical heterogenous network, the overhead of communication is measured. The measurements can be described by a linear model: setup times are around 10 ms; per-8-byte transfer times, around 10 μs. From these data it can be concluded that distribution of tasks taking less than 50 ms will lead to severe performance degradation. So a coarse grain distribution has to be used. Clearly the influence of the overhead decreases with increasing computation complexity of the tasks distributed. For instrumentation the overhead of this approach is comparable to IEEE-488 transfer times. Therefore this scheme can readily be used in existing acquisition setups. The combination of distributed processing and visual presentation in this environment can therefore conveniently be exploited in a number of applications within physics.
Tracking artificial satellites with the Macintosh
Authors: Houlahan Padraig
Published: July 2002

Satellite Tracker, the software described here, simulates the satellite orbital dynamics problem in order to help the student understand many of the underlying aeronautical concepts by permitting him or her to manipulate orbital data, or to make predictions of satellite passes verifiable through observation. A brief description of its capabilities and configurations along with some applications is presented.