bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhancement of the quantum-confined stark effect utilizing asymmetric quantum well structures
Authors: Gug R. K. , Hagston W. E.
Published: July 2002

The potential profile of a rectangular quantum well is optimized so as to maximize the quantum confined stark effect. Digitization of this profile gives a structure which can be grown experimentally. As an example, a well of width 100 Å in Al0.4Ga0.6As/GaAs is optimized giving a structure which is both asymmetric and double quantum well like. The associated redshift is increased at large fields by a factor of more than 4 compared with the original structure, and the associated oscillator strengths are a much more sensitive function of the applied field. This example illustrates the utility of such optimized structures for device applications.
Organic electroluminescent devices by high-temperature processing and crystalline hole transporting layer
Authors: Gao Z. Q. , Lai W. Y. , Wong T. C. , Lee C. S. , Bello I. , Lee S. T.
Published: July 2002

Crystallization and interdiffusion of organic layers have been widely considered to be important causes for performance degradation of organic electroluminescent devices. By using high substrate temperature during vapor deposition, we have fabricated organic electroluminescent devices with crystalline organic layers. Contrary to the common belief, substantial increases in luminescent efficiency and brightness have been observed in the device thus fabricated. Such devices also showed much improved storage stability against high humidity compared to the conventional ones with amorphous organic layers.
Observation of tip-induced gap states in lightly doped Si(100) using scanning tunneling spectroscopy
Authors: Lin Hai-An , Jaccodine Ralph J. , Freund Michael S.
Published: July 2002

Scanning tunneling spectroscopy without shifts related to band bending was utilized to study tip-induced gap states in lightly doped Si(100) (ρ=12–25 Ω cm). The separation dependence of scanning tunneling spectroscopy revealed a reversible interaction between the tip and sample. A “U” shape curve of normalized differential conductivity versus sample bias in the band gap was also observed as the tip approached the sample, suggesting the evolution of a continuum of tip-induced gap states. These results can be explained in terms of an emission dominant-tunnel model where the tunneling current is controlled by electron emission from traps in the band gap. The experiments described herein demonstrate that scanning tunneling microscopy can be used as a powerful tool for probing the origin and evolution of the surface states.
Thermal poling of silica in air and under vacuum: The influence of charge transport on second harmonic generation
Authors: Pruneri V. , Samoggia F. , Bonfrate G. , Kazansky P. G. , Yang G. M.
Published: July 2002

A comparison between thermal poling of silica in air and in vacuum is reported. It is shown that the second-order susceptibility and thickness of the nonlinear layer as well as their time evolution are highly dependent on the surrounding poling atmosphere. In the vacuum case a charge distribution (under the anode) more complex and broader than that for the air case has also been revealed by laser induced pressure pulse measurements. A multiple charge carrier model can explain the formation and evolution of the depletion region under the anode. The findings are relevant to achieve improved nonlinearities in fiber and waveguide devices.
Optically switchable gratings based on polymer-dispersed liquid crystal films doped with a guest–host dye
Authors: Fuh Andy Y.-G. , Tsai M.-S. , Huang L.-J. , Liu T.-C.
Published: July 2002

In this work, we add a small amount of a guest–host dye G-206 in the polymer-dispersed liquid crystal mixtures to fabricate holographic gratings. Following formation, the grating is irradiated by a single Ar+ laser beam while probed by a He–Ne laser. Experimental results indicate that the first-order diffracted intensity of the probe beam increases significantly. Such an optically switchable diffraction effect is attributed to thermal expansion. The G-206 dye absorbs the Ar+ laser, subsequently resulting in thermal expansion. The liquid crystal (LC) droplets are then squeezed so that the LC molecules within the droplets are reoriented collectively. Thus, the refractive index difference between the LC-rich and polymer-rich stripes Δn increases.
Distinctive observation of valence-band electron direct-tunneling current in a nanometer-thick silicon oxide film on monocrystalline silicon
Authors: Omura Yasuhisa , Nakatsuji Hiroshi
Published: July 2002

This letter describes the experimental observation of valence-band electron-tunneling current in a nanometer-thick SiO2 film formed on a monocrystalline silicon substrate. The tunneling effective mass of the valence-band electrons is about 1.5 times that for conduction-band electrons.
Quantitative determination of hydrogen adsorption site on the Pt(111)-(1×1) surface by low energy ion channeling
Authors: Lui K. M. , Kim Y. , Lau W. M. , Rabalais J. W.
Published: July 2002

Ion channeling using low keV energies is introduced as a sensitive probe for determining the adsorption site of hydrogen on surfaces. The technique is shown to be capable of quantitatively probing the positions of light elements on heavy substrates with analysis by simple geometrical constructs. Quantitative analysis of the Pt(111)-(1×1)–H surface using 5 keV Ne+ ions shows that the hydrogen atoms preferentially populate the fcc site with a height of 0.9±0.1 Å above the first-layer Pt atoms and a corresponding Pt–H bond length of 1.9±0.1 Å.
Size stabilization of arsenic precipitates in nonstoichiometric GaAs-based compounds
Authors: Hung C.-Y. , Harris J. S. , Marshall A. F. , Kiehl R. A.
Published: July 2002

A narrowing of the size distribution of arsenic precipitates in nonstoichiometric GaAs-based compounds is observed during annealing for precipitates in the small size regime of a few nanometers. This improvement in size uniformity is in marked contrast to what is observed for larger precipitates, which coarsen with a widening distribution similar to that of classical Ostwald ripening. Inverse coarsening caused by an elastic interaction between small precipitates due to coherency strain is a possible mechanism for this interesting and potentially useful behavior.
Solid-state gadolinium–magnesium hydride optical switch
Authors: Armitage R. , Rubin M. , Richardson T. , O’Brien N. , Chen Yong
Published: July 2002

The optical switching properties of gadolinium–magnesium hydride have been demonstrated in a solid-state electrochromic device. With positive polarization of the hydride electrode, the visible reflectance approaches 35% with virtually zero transmission, while with negative polarization, the visible transmission exceeds 25% at 650 nm. The switching is reversible, with intermediate optical properties between the transparent and reflecting states.
Effect of growth interruptions on the interfaces of InGaAs/AlAsSb superlattice
Authors: Georgiev Nikolai , Mozume Teruo
Published: July 2002

The effect of growth interruption times combined with selective group-V species exposure of InGaAs/AlAsSb short-period superlattice structure was investigated with photoluminescence, x-ray diffraction, and reflection high-energy electron diffraction. Reflection electron diffraction shows surface reconstruction transitions dependent on the time and species type exposure procedure. A shift in the photoluminescence peak position is observed from samples grown under different species type exposure compared to the samples grown without interruption, Sb termination being at a lower energy and As termination at higher energy, respectively. This is interpreted in terms of increased mixing of Sb in the interface InGaAs layers.
Theory of the charge-transport properties of naphthyl diamine used in organic light-emitting devices
Authors: Zhang R. Q. , Lee C. S. , Lee S. T.
Published: July 2002

The electronic structures of a prototypical electroluminescent molecule, N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), with various extra charges (+6 to −6 atomic units), have been theoretically studied by means of the PM3 and ab initio molecular orbital theories as well as density functional theory in combination with a decomposition of the density of states. It was found that, under positive charging, the essential distribution feature of the molecular orbitals at constituent atoms in the NPB molecule can still favor carrier transport, but cannot do so under negative charging. By explaining the efficient hole-transporting property of NPB, the present study elucidates the potential of the theoretical approach for the selection of optimum carrier-transporting organic materials.
The role of oxygen on the stability of gettering of metals to cavities in silicon
Authors: Williams J. S. , Conway M. J. , Wong-Leung J. , Deenapanray P. N. K. , Petravic M. , Brown R. A. , Eaglesham D. J. , Jacobson D. C.
Published: July 2002

The effect of oxygen implanted into epitaxial Si layers on the ability to getter Au to nanocavities, previously formed by H implantation and annealing, has been studied by Rutherford backscattering, transmission electron microscopy, and secondary ion mass spectrometry. We demonstrate that oxygen is gettered to cavities during extended annealing at 950 °C. Furthermore, the arrival of oxygen at cavities is not only shown to inhibit subsequent attempts to getter Au to cavities, but also to eject chemisorbed Au from the cavity walls. Similar behavior is observed in Czochralski Si, where the source of oxygen is within the Si itself.
Quantum interference control of currents in CdSe with a single optical beam
Authors: Laman N. , Shkrebtii A. I. , Sipe J. E. , van Driel H. M.
Published: July 2002

We show that ballistic current generation can occur in a semiconductor via quantum interference between absorption pathways for orthogonal polarization components of a single-frequency beam. This effect occurs for a subset of noncentrosymmetric materials, is macroscopically associated with a second-order nonlinear optical susceptibility, and produces current injection linearly proportional to the beam intensity. We demonstrate this in wurtzite CdSe (Eg=1.75 eV) at 295 K using cw and femtosecond optical sources of wavelength 600–750 nm (2.07–1.66 eV). The intensity and spectral dependence are in reasonable agreement with a first-principles calculation. Continuous current density of 30 μA cm−2 is produced for 60 mW cm−2 intensity at 633 nm.
Imaging near-contact transport in the planar-collector geometry for a Schottky contact on high-purity GaAs
Authors: Record K. A. , Palmieri D. R. , Haegel N. M. , Wynne D.
Published: July 2002

Variable temperature electron beam induced current (EBIC) and cathodoluminescence (CL) were combined to image electric fields and charge transport for a Schottky contact on high purity epitaxial GaAs in the planar-collector geometry. Simultaneous EBIC and CL imaging proves that the near-contact EBIC signal is dominated by depletion effects, even in material where the bulk diffusion length greatly exceeds the intercontact distance. In forward bias, an EBIC dipole is observed, providing direct spatial indication of the transition between drift and diffusion transport of locally generated charge.
Shear piezoelectric coefficients of gallium nitride and aluminum nitride
Authors: Muensit S. , Goldys E. M. , Guy I. L.
Published: July 2002

We have developed a direct method of measuring the shear strain using a laser-based interferometer. The method was applied to the measurement of the d15 coefficient for wurtzite GaN and AlN. A value for d15 of 3.6±0.2 pm V−1 for AlN has been obtained, in good agreement with values quoted in the literature. The value of d15 for GaN has also been measured to be 3.1±0.2 pm V−1.
Energy-dispersive, x-ray reflectivity density measurements of porous SiO2 xerogels
Authors: Windover D. , Lu T.-M. , Lee S. L. , Kumar A. , Bakhru H. , Jin C. , Lee W.
Published: July 2002

X-ray reflectivity has been used to measure nondestructively the density of thin, porous, silica xerogels used for interlayer dielectric applications. The critical angle, defined through total external reflection, was measured for multiple x-ray energies to correct for sample misalignment error in the determination of the density for the films. This density was used to extrapolate the percentage porosity, assuming a bulk SiO2 density standard. The results were compared to those obtained by Rutherford backscattering and ellipsometry techniques.
Scanning gate spectroscopy on nanoclusters
Authors: Gurevich L. , Canali L. , Kouwenhoven L. P.
Published: July 2002

A gated probe for scanning tunneling microscopy (STM) has been developed. The probe extends normal STM operations by means of an additional electrode fabricated next to the tunneling tip. The extra electrode does not make contact with the sample and can be used as a gate. We report on the recipe used for fabricating the tunneling tip and the gate electrode on a silicon nitride cantilever. We demonstrate the functioning of the scanning gate probes by performing single-electron tunneling spectroscopy on 20-nm-gold clusters for different gate voltages.
Strain-induced second-harmonic generation in pseudocubic Ba0.48Sr0.52TiO3 thin films
Authors: Oh U. C. , Ma Jing , Wong G. K. L. , Ketterson J. B. , Je Jung Ho
Published: July 2002

We report the observation of a strong second-order nonlinear optical response from epitaxial Ba0.48Sr0.52TiO3 thin films grown on MgO(001) by magnetron sputtering. The result is unexpected since the crystal structure of Ba0.48Sr0.52TiO3 is centrosymmetric under ambient conditions. A second-order nonlinear susceptibility d33 as large as 11 pm/V at a fundamental wavelength of 1.064 μm is observed in a 300 Å thick film. By studying a series of films with different strains, induced by a film–substrate lattice mismatch, we conclude that the resulting tetragonal distortion, presumably accompanied by a loss of inversion symmetry, is responsible for the second-harmonic generation.
Low-dose nitrogen implants in 6H–silicon carbide
Authors: Saks N. S. , Agarwal A. K. , Mani S. S. , Hegde V. S.
Published: July 2002

Low-dose n-type nitrogen implants in 6H–SiC have been studied using the Hall effect. Previous studies of doping by implantation in SiC have concentrated on heavily doped layers such as required for transistor sources and drains. Here, we focus on more lightly doped layers, e.g., such as required for the active regions of high-voltage power devices. The low-dose N implants are found to activate more readily than high doses. Almost ideal N-implanted layers with a donor density of ∼1×1017/cm3 and a low residual acceptor density from implant damage of only ∼1.5×1015/cm3 have been obtained after a 1400 °C anneal.
Thermal equilibrium and stability of copper complexes in silicon crystal
Authors: Nakamura Minoru
Published: July 2002

Formation of the photoluminescence (PL) Cu center (1.014 eV) for silicon crystals diffused with Cu at 700 °C was observed. For the samples with a Cu concentration lower than 1×1014 atom/cm3, almost the same amount of the Cu center was formed for the same concentration of Cu for all the cooling conditions employed without forming Cu precipitation, and the Cu center was very stable for a long storage time at room temperature. More frequent Cu precipitation occurred for the samples with higher Cu concentrations. Quenching of the equilibrium states of the Cu complexes at high temperatures did not occur by rapid cooling. Thermal equilibrium and stability of Cu complexes for the samples containing no Cu precipitates differed from those of the samples containing precipitates.
Origin of preferential orthorhombic twinning in SrRuO3 epitaxial thin films
Authors: Maria Jon-Paul , McKinstry H. L. , Trolier-McKinstry S.
Published: July 2002

In order to elucidate the driving forces which promote oriented in-plane crystallographic texture in SrRuO3 thin films deposited on stepped SrTiO3 substrates, a high-temperature x-ray analysis of both SrRuO3 thin films and powders was conducted. Structural phase transitions were found at temperatures near 350 °C and slightly above 600 °C. The transitions are tentatively indexed as orthorhombic to tetragonal and tetragonal to cubic, respectively. These results suggest that SrRuO3 thin films grow with cubic symmetry. As such, film–substrate interfacial characteristics, rather than a preferred growth direction, are believed to determine the orientation of orthorhombic twins.
Array detector for the atomic force microscope
Authors: Schäffer Tilman E. , Richter Michael , Viani Mario B.
Published: July 2002

We present a method for measuring the deflection of the optical beam in an atomic force microscope (AFM) that yields an increased signal-to-noise ratio, compared to the conventional two-segment detection. This increase is achieved by distributing the optical power from the beam across an array of photodetector segments and splitting it into multiple channels. Each channel has an adjustable gain factor that is set dynamically to weigh the contribution from each channel. We find a mathematical condition for the gain factors that allows detection of cantilever deflections with maximum signal-to-noise ratio and demonstrate this for the case of a 12-μm-long cantilever in an AFM for small cantilevers.
Selective excitation and photoinduced bleaching of defects in InAlGaAs/GaAs high-power diode lasers
Authors: Tomm J. W. , Bärwolff A. , Elsaesser T. , Luft J.
Published: July 2002

Mounting-induced defects in semiconductor quantum-well (QW) lasers are investigated by photocurrent spectroscopy. The defects are located in the laser waveguides and give rise to an absorption band below the QW band gap with a maximum absorption cross section of σ=2×10−15 cm2. We observe a nonlinear fully reversible photobleaching of the defects and a resulting increase of QW photocurrent upon continuous wave irradiation of the devices, demonstrating a direct interaction between quantum-confined carriers and a defect level.
The complex Bateman equation in a space of arbitrary dimension
Authors: Fairlie D. B. , Leznov A. N.
Published: July 2002

The general solution to the complex Bateman equation in a space of arbitrary dimensions is constructed.
The structure of cosmic strings
Authors: Low Robert J.
Published: July 2002

It is normal to treat cosmic strings as line defects in space–time. The interpretation of mass as the deficit angle associated with such a defect has the consequences of mass being nonadditive, and Lorentz boosts acting as magnifying glasses of arbitrarily high resolution. Herein, an alternative is proposed which has neither of the above properties.
Cyclical behavior in early universe cosmologies
Authors: Billyard Andrew P. , Coley Alan A. , Lidsey James E.
Published: July 2002

This paper studies early universe cosmologies derived from a scalar–tensor action containing cosmological constant terms and massless fields. The governing equations can be written as a dynamical system which contains no past or future asymptotic equilibrium states (i.e., no sources nor sinks). This leads to dynamics with very interesting mathematical behavior such as the existence of heteroclinic cycles. The corresponding cosmologies have novel characteristics, including cyclical and bouncing behavior possibly indicating chaos. The connection between these early universe cosmologies and those derived from the low-energy string effective action is discussed.
Combination band spectroscopy of H3+
Authors: McCall Benjamin J. , Oka Takeshi
Published: July 2002

Thirty rovibrational transitions of H3+ have been observed near 1.25 μm using a tunable diode laser and a positive column discharge. In addition to the H3+ transitions, over 200 transitions between Rydberg states of H2 were observed—these could be discriminated against by using a discharge dominated by He, which apparently collisionally quenches the Rydberg states of H2 without affecting H3+. Twenty-eight of the H3+ transitions have been assigned to the ν1+2ν22←0 band, and provide experimentally determined energy levels for most of the levels up to J=4 in the ν1+2ν22 state. The remaining two H3+ transitions have been assigned to the 2ν1+ν2←0 band. These bands represent a crucial test of ab initio calculations, as they reach higher vibrational levels of H3+ than any yet observed. We have compared our experimental results with recent variational calculations by several groups.
An equipment for three-dimensional spatially resolved gas analysis
Authors: Johansson M. , Lundström I. , Ekedahl L.-G.
Published: July 2002

The apparatus described is able to measure three-dimensional temperature and partial pressure distributions in gas mixtures with a spatial resolution better than 0.5 mm. The measurements are performed in a well defined laminar gas flow in a duct of rectangular cross section. The gas mixture is sampled locally with a quartz tube orifice leak movable in three dimensions. Mass spectrometry is used for partial pressure measurements. The total pressure in the duct can be regulated in the range 50–760 Torr and the gas temperature can be varied between room temperature and 500 °C. The apparatus is primarily designed for studies of the distribution of reactants and products over catalytic surfaces. Because of the well defined gas flow in the duct the partial pressure distributions of reactants and products over the catalytic surface can be calculated with good accuracy. By comparing calculated pressure distributions to measured ones the local reaction rate on the catalytic surface as well as the pressures of reactants and products at the surface can be obtained. As an example it is shown how the hydrogen distribution over a platinum surface exposed to a mixture of hydrogen, oxygen, and argon can be used to calculate the local water formation rate on the platinum surface.
Experimental studies of the vapor phase nucleation of refractory compounds. V. The condensation of lithium
Authors: Ferguson Frank T. , Nuth Joseph A.
Published: July 2002

Lithium nucleation was studied over the range of 830–1100 K in a gas evaporation apparatus yielding supersaturation ratios of approximately 300 to 7 over this temperature range, respectively, at an estimated flux of 109 cm−3 s−1. During runs we observed the same fluorescence phenomena due to atomic lithium vapor and the lithium dimer as noted by other researchers. The measured supersaturations are much higher than the values predicted by Classical Nucleation Theory. Modifications to Classical Nucleation Theory to account for the presence of dimers in the lithium vapor do not seem to account for this discrepancy. The data and Scaled Nucleation Theory agree fairly well at the lower temperature range, but at the higher end of the temperature range the data have a steeper drop in the supersaturation values than predicted by Scaled Nucleation Theory.
Coupled harmonic oscillator systems: An elementary algebraic decoupling approach
Authors: Feldmann Tova , Cohen M. , Burrows B. L.
Published: July 2002

We present simple explicit coordinate transformations which serve to decouple the Schrödinger equation for a pair of (not necessarily identical) harmonic oscillators in the presence of bilinear perturbing potentials. We derive general conditions for the decoupling, and give some examples of physical interest. These include the much studied example with just a static perturbation, the parallel problem with a dynamic coupling term, and the classic example of an isotropic two-dimensional oscillator in a transverse magnetic field, first solved by Fock (1928) by separation of variables.
24 segment high field permanent sextupole magnets
Authors: Vassiliev A. , Nelyubin V. , Koptev V. , Kravtsov P. , Lorentz B. , Marik H. J. , Mikirtytchiants M. , Nekipelov M. , Rathmann F. , Paetz gen. Schieck H. , Seyfarth H. , Steffens E.
Published: July 2002

We report on the design, construction, and magnetic field measurements of a system of high field sextupole magnets made from NdFeB compounds. The magnets are utilized as a focusing system for neutral hydrogen (or deuterium) atoms in a polarized atomic beam source based on Stern–Gerlach spin separation. Each magnet consists of 24 segments of permanently magnetized material differing in remanence and coercivity to reduce demagnetization. According to quadratic extrapolation to the pole tip the magnetic flux density reaches values of up to B0=1.69 T. Three-dimensional field calculations using the MAFIA code were carried out to optimize the magnet performance and to avoid demagnetization by selecting appropriate materials for the individual segments. Measurements of the radial, azimuthal, and longitudinal magnetic flux density distributions were carried out by means of a small Hall probe (100×200×15 μm3). The measurements with the small probe permitted to extract experimentally higher order multipole components very close (∼100 μm) to the inner surface. Experimental values obtained are compared to predictions based on MAFIA calculations and on the Halbach formalism.
Structure, properties, and photodissociation of O4−
Authors: Aquino Adelia J. A. , Taylor Peter R. , Walch Stephen P.
Published: July 2002

We present the results of an ab initio quantum-chemical investigation of the structure of the anion O4− and potential energy surfaces for several electronic states. In addition to ground-state vibrational frequencies, which are in good agreement with other calculations and with matrix isolation experiments, we have investigated both photodissociation of O4− into various states of O2 and O2− and dissociative photodetachment leading to a free electron and various states of two O2 molecules. Our surfaces lead to predictions for both photodissociation and photodetachment processes that are in excellent agreement with experiment.
Brueckner based generalized coupled cluster theory: Implicit inclusion of higher excitation effects
Authors: Nooijen Marcel , Lotrich Victor
Published: July 2002

A generalization of the single reference Coupled Cluster parameterization for the ground state wave function is proposed that includes substitution operators that annihilate the reference determinant, but which act nontrivially on the correlated part of the wave function. It is shown that an inclusion of such two-body operators can mimic the effect of conventional connected triple and higher excitation operators. Results obtained with Brueckner based Generalized Coupled Cluster Doubles theory (BGCCD-version x) are found to be comparable in accuracy to CCSD(T) and CCSDT for a number of difficult test cases. In the current version of the BGCCD approach we obtain correlated ionization potentials and electron affinities as a by-product of a ground state calculation. This multistate nature of the BGCCD-X approach can give rise to problems with intruder states similar as in Fock Space Coupled Cluster theory.
Useful work and the thermal efficiency in the ideal Lenolr cycle with regenerative preheating
Authors: Georgiou Demos P.
Published: July 2002

In the existing thermal engine concepts negative work transfer (usually needed to drive a compression process) is supplied by the work produced by the engine itself. The remaining difference (i.e., the net work transfer) becomes the useful work, since it is available for external consumption. The thermal efficiency is the parameter that compares this against the heat input into the system. It forms the main optimization parameter in any engine design. The objective of the present study is to show that for the case of the Lenoir cycle with regenerative preheating the entire positive work is available for external consumption, since the negative (i.e., the compression) work is supplied by the atmospheric air. Not only this, but, during the compression process and due to the pressure difference across the two sides of the moving piston, an additional (useful) work transfer may be generated. Thus, the proposed power plant may be considered as a combination of a thermal engine and a wind turbine. In the ideal cycle limit (at least), the total amount of useful work exceeds the heat entering the system. This leads to the definition of a new parameter for the efficiency (called the technical efficiency), which compares the combined positive work transfer (i.e., the useful one) against the heat entering the system and which may exceed the 100% level.
Glass transition behavior, crystallization kinetics, and microstructure change of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass under high pressure
Authors: Wang Wei Hua , Zhuang Yan Xin , Pan Ming Xiang , Yao Yu Su
Published: July 2002

Glass transition behavior, crystallization kinetics, and the microstructural change of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass (BMG) are studied in ambient, annealing, and high pressure conditions. Significant structural relaxation induced by preannealing and high pressure annealing has been observed in the BMG obtained with a low cooling rate. The experimental results indicate that the BMG contains a large amount of vacancy-like free volume. The relaxation results in the microstructural transformation from short-range order to medium-range order and significant effects on the subsequent glass transition and crystallization. The role of relaxation in the glass transition and the crystallization is phenomenologically explained.
A scanning dual-capillary-tube viscometer
Authors: Kim Sangho , Cho Young I. , Kensey Kenneth R. , Pellizzari Roberto O. , Stark Peter R. H.
Published: July 2002

The present study introduces the concept of a new scanning dual-capillary-tube viscometer (SDCV) for viscosity measurements of both Newtonian and non-Newtonian fluids, including whole blood, over a wide range of shear rates. The flow rate and pressure drop measurements that are usually required for the operation of a capillary-tube viscometer are replaced with two measurements of liquid-height variation with time. Using a charge-coupled device sensor array, one could measure the variation of fluid levels at each riser tube, h1(t) and h2(t), from which the test fluid viscosity was calculated. The feasibility and accuracy of the SDCV technique has been demonstrated for a standard-viscosity oil and for bovine blood by comparing results against established viscosity measurement techniques. A power-law viscosity model was used to correlate the data for the latter. The dual-capillary-tube viscometer extends the shear rate range as low as 0.1 s−1 for both the viscosity of mineral oil (9.9 cP at 25 °C) and bovine blood with 3% ethylenediaminetetraacetic acid at room temperature.
A molecular orbital study on H and H2 elimination pathways from methane, ethane, and propane
Authors: Irle Stephan , Morokuma Keiji
Published: July 2002

Decomposition pathways for propane, as well as methane and ethane for comparison, in its ground electronic state were studied using density functional and high accuracy ab initio calculations. The reaction pathways were characterized by locating the transition states and following the intrinsic reaction coordinate. Atomic hydrogen as well as molecular hydrogen elimination pathways were investigated, including three deuterated propane species for comparison with experiment. The methyl and ethyl groups in propane are found to stabilize transition states and radical/biradical intermediates along the reaction pathways. For propane, 2,2-elimination of an hydrogen molecule is found to be the dominant molecular elimination pathway, in agreement with recent photochemical experiments. We find 1,1-elimination to be the next important molecular elimination pathway, followed by 1,2-elimination, which disagrees with the experimental result favoring 1,2- over 1,1-elimination.
Microscopic model of carbon monoxide binding to myoglobin
Authors: McMahon Benjamin H. , Stojković Branko P. , Hay P. Jeffrey , Martin Richard L. , Garcı́a Angel E.
Published: July 2002

We present a microscopic model of carbon monoxide (CO) binding to myoglobin which reproduces the experimentally observed Arrhenius pre-exponential factor of 109 s−1 and activation enthalpy distribution centered at 12 kJ/mol. The model is based on extensive ab initio calculations of CO interacting with a model heme-imidazole group which we performed using a fully quantum mechanical Hartree–Fock/density functional theory (HF/DFT) hybrid method. We fit the HF/DFT calculated energies, obtained for over 1000 heme-CO structures with varied CO and iron positions and orientations for both high (S=2) and low (S=0) spin states, to a model potential function which includes a bonding interaction in both of the spin states, electrostatic, and anisotropic Lennard-Jones-type interactions. By combining the x-ray determined protein structure with this potential and protein-CO interactions and internal heme interaction potentials obtained from established molecular dynamics literature, we calculate the energy required for the CO to reach the spin crossing from the heme pocket. We find that the transition between the two spin states occurs when CO and iron have activation enthalpies of 8 kJ/mol and 3 kJ/mol, respectively, which are necessary to move CO towards the iron and the iron atom relative to the heme plane Npyr. At the same time we find that 1 kJ/mol is needed to move Nε of His-64 and Cγ of Val-68 relative to the heme group. The requirement that these motions be synchronized reduces the Arrhenius pre-exponential by a factor of 150 from the 1012 s−1 obtained from CO motion across the heme pocket, leaving a factor of ∼ 6 to account for CO orientation and nonadiabaticity of the electronic spin change. The observed width of the enthalpy distribution is reproduced by assuming a Gaussian distribution of the heme positions with a standard deviation of 0.2 Å. We characterize the conformational relaxation by calculating an enthalpy barrier using x-ray structures of myoglobin in both the MbCO photoproduct and deoxy conformations, and we find a small difference, ∼ 5 kJ/mol, between the two conformations.
Solid effect in the electron spin dressed state: A new approach for dynamic nuclear polarization
Authors: Weis V. , Bennati M. , Rosay M. , Griffin R. G.
Published: July 2002

We describe a new type of solid effect for dynamic nuclear polarization (DNP) that is based on simultaneous, near resonant microwave (mw) and radio frequency (rf) irradiation of a coupled electron nuclear spin system. The interaction of the electron spin with the mw field is treated as an electron spin dressed state. In contrast to the customary laboratory frame solid effect, it is possible to obtain nuclear polarization with the dressed state solid effect (DSSE) even in the absence of nonsecular hyperfine coupling. Efficient, selective excitation of dressed state transitions generates nuclear polarization in the nuclear laboratory frame on a time scale of tens of μs, depending on the strength of the electron–nuclear coupling, the mw and rf offset and field strength. The experiment employs both pulsed mw and rf irradiation at a repetition rate comparable to T1e−1, where T1e is the electronic spin lattice relaxation time. The DSSE is demonstrated on a perdeuterated BDPA radical in a protonated matrix of polystyrene.
A statistical approach for the study of singlet–triplet interactions in small polyatomic molecules
Authors: Altunata Selen , Field Robert W.
Published: July 2002

Surface electron ejection by laser excited metastable (SEELEM) and laser induced fluorescence (LIF) spectroscopies are complementary techniques that can be employed to provide both qualitative and quantitative insights into the mechanisms of singlet–triplet coupling in small polyatomic molecules. Two qualitatively distinct singlet–triplet coupling mechanisms are examined here in order to reveal the statistical signatures by which they can best be distinguished. These interaction mechanisms are: direct coupling between a “bright” S1 state and an entire background manifold of “dark” triplet states; and “doorway-mediated” indirect coupling in which the bright state couples to the manifold of dark states through the intermediary of one doorway state of unique vibronic character. Our goal in this paper is to present and explain the basis for the effectiveness of statistical methods with which complex LIF/SEELEM spectra may be processed so as to distinguish qualitatively between these two mechanistic possibilities. The trends obtained from these statistical methods are also shown to encode information about some characteristic quantitative features of the triplet perturbers, such as their coupling strength to the bright state S1. The development of the statistical methods described in this paper was motivated by our experiments on acetylene. Acetylene has certain useful dynamical features which make it a good model system for this study. The statistical measures developed distinguish conclusively between the direct and doorway-mediated coupling schemes, because each scheme is shown here to give rise to characteristic statistical signatures in the SEELEM and LIF spectra. Qualitative results from a preliminary real data set analyzed using the statistical approach proposed here are also presented in order to demonstrate the effectiveness of these statistical measures.
Reorientational disorder of anions in the hydrogen sulfides of sodium and potassium (NaDS and KDS) investigated by neutron diffraction between T=4 K and T=470 K
Authors: Haarmann F. , Jacobs H. , Kockelmann W.
Published: July 2002

High resolution neutron powder diffraction experiments on NaDS and KDS were carried out as a function of temperature between T=4 K and T=470 K at ROTAX (ISIS, UK). The probability density function (pdf) of the atoms was studied in detail, in particular that of deuterium. Several forms of temperature dependent reorientational disorder of the anions are the reason for structural polymorphism of these compounds. A monoclinic low (LTM), a rhombohedral middle (MTM), and a cubic high temperature (HTM) modification are the polymorphs. For the structures of the different modifications the parameters were refined by the use of split-atom models, Fourier synthesis, and/or cubic harmonics. As a main result of this study the anisotropy of the pdf of deuterium in NaDS and KDS was evaluated. It is due to large amplitudes of librations of the anions.
Exciton luminescence linewidth due to correlated compositional fluctuations in group-III nitride quantum wells
Authors: Litvinov V. I.
Published: July 2002

The model for finite-range correlated compositional disorder is proposed, and the calculation of the energy domain of the exciton localization in alloy quantum well is carried out. The approach suggests an interpolation scheme between an uncorrelated and a finite-range correlated disorder and describes experimental data on excitonic pholuminescence in group III-nitride alloy quantum wells.
The vibrational energy pattern in acetylene (VI): Inter- and intrapolyad structures
Authors: Zhilinskiı́ B. I. , El Idrissi M. I. , Herman M.
Published: July 2002

Intra- and interpolyad structures are investigated in the vibrational energy pattern of acetylene, using the spectroscopic Hamiltonian presented in previous papers in this series [see El Idrissi et al., J. Chem. Phys. 110, 2074 (1999)]. The existence of two constants of the motion is shown to generate very regular patterns in the manifold of vibrational energy levels. Distinct regular and oscillatory contributions are evidenced in the number of vibrational levels in the main polyads, which are fully reproduced using the generating function presented in Sadovskiı́ and Zhilinskiı́ [J. Chem. Phys. 103, 10520 (1995)]. Further developments of this approach are outlined.
Molecular dynamic simulation of the interaction, at high energy, between the N2 molecule and polyethylene
Authors: Duffour Emmanuel
Published: July 2002

In this paper, molecular dynamic simulation is used to study the behavior of polyethylene, in orthorhombic phase, under atomic collision. Simulation of the interaction between a N2 molecule, at high initial velocity, and polyethylene is made to understand the destruction process of the macromolecule. The calculations show two phases during the ablation. First, the collision produces CH2 units emission out of the crystal and second, there is a long relaxation time where the remaining energy is dissipated into the polymer.
Piezoelectric coefficient of aluminum nitride and gallium nitride
Authors: Lueng C. M. , Chan H. L. W. , Surya C. , Choy C. L.
Published: July 2002

The piezoelectric coefficient d33 of aluminum nitride (AlN) and gallium nitride (GaN) thin films grown on silicon substrates by molecular beam epitaxy have been measured using a laser interferometer. X-ray diffraction reveals that the AlN and GaN films consist mainly of crystals with a hexagonal wurtzite structure. In order to grow epitaxial GaN films, an AlN film was first deposited on silicon as the buffer layer, so the d33 measurement for GaN was actually performed on GaN/AlN/Si multilayer systems. The relative permittivity and electrical resistivity of each constituent layer of the film and the potential drop across each layer were determined as a function of frequency. The potential drops were then used to calculate the piezoelectric coefficient d33 of GaN. After correcting for substrate clamping, d33 of AlN and GaN were found to be (5.1±0.1) and (3.1±0.1) pm V−1, respectively.
The method of adjoint operators and a symmetry operator for the Maxwell equations in type-D vacuum backgrounds
Authors: Silva-Ortigoza Gilberto
Published: July 2002

Since in all the type-D solutions of the Einstein vacuum field equations with a cosmological term each maximal spin-weighted component of the electromagnetic spinor field satisfies a decoupled equation and there exists a two-index Killing spinor field; we show, via the adjoint operators method, that a symmetry operator for the Maxwell equations can be constructed.
Bridging the gap between homopolymer and protein models: A discontinuous molecular dynamics study
Authors: Voegler Smith Anne , Hall Carol K.
Published: July 2002

A series of seven off-lattice protein models is analyzed that spans a range of chain geometry from a simple, low-resolution homopolymer model to an intermediate-resolution model that accounts for the presence of side chains, the varied character of the individual amino acids, the rigid nature of protein backbone angles, and the length scales that characterize real protein bead sizes and bond lengths. Discontinuous molecular dynamics is used to study the transition temperatures and physical structures resulting from simulations with each protein model. Our results show that each protein model undergoes multiple thermodynamic transitions that roughly correlate with protein transitions during folding to the native state. Other realistic protein behavior, such as burial of hydrophobic side chains and hindered motion due to backbone rigidity, is observed with the more-detailed models. The results suggest that, despite their simplicity when compared with all-atom protein models, the models presented here display a significant amount of protein character and, when coupled with the efficient discontinuous molecular dynamics algorithm, may enable simulation of multiprotein systems over long times.
On the use of relaxation methods for localized dynamic models
Authors: Ducros F. , Sagaut P. , Quéméré P.
Published: July 2002

An analogy with the use of relaxation methods is identified in both the Lagrangian dynamic model [J. Fluid Mech. 319, 353 (1996)] and the localized dynamic model [Phys. Fluids 7, 839 (1995)]. A general way to introduce a relaxation method for dynamic models is proposed, together with the ability of deriving constants without using Germano’s identity [Phys. Fluids A 3, 1760 (1991)]. The resulting family of models contains a relaxation parameter ra and leads to a strictly local and stable modeling, as shown by a posteriori tests in channel flow for Reτ=180. The influence of ra is briefly discussed.
On the atmospheric interfacial waves
Authors: Choi J. W.
Published: July 2002

We consider waves at the interface of a two-layer fluid. The upper layer is assumed to be isothermal and compressible with a density profile decreasing exponentially with height up to infinity. The lower layer is assumed to be incompressible with constant density bounded below by a rigid bottom with an obstruction of compact support. By a unified asymptotic method, a stationary forced modified KdV equation is derived when the KdV theory fails. New steady solutions are discovered and numerical results are also presented.
Stability of shock wave reflections in nonequilibrium steady flows and hysteresis
Authors: Grasso F. , Paoli R.
Published: July 2002

In the present work we have addressed the issue of the stability of shock wave reflection in the presence of vibrational and chemical relaxation phenomena and its relation with the occurrence of the hysteresis. In order to better understand the physics of the shock wave reflections we have first formulated an evolution equation for the entropy of a mixture of gases in thermal and chemical nonequilibrium by invoking the shifting equilibrium assumption and the concepts of irreversible thermodynamics, and assuming (i) that all diatomic molecules behave as harmonic oscillators; and (ii) finite rate chemistry. A perturbation analysis of the total entropy evolution equation has then been carried out to analyze the stability of shock wave configurations (either regular or Mach) both for ideal and real gases. The analysis shows that a Mach reflection is more stable than a regular one; furthermore, its stability is enhanced by nonequilibrium effects. In order to clarify the occurrence of the hysteresis phenomenon in light of the conclusions reached through the stability analysis, we have also carried out multidimensional simulations (both at flight and wind tunnel conditions) by developing a pseudotransient procedure to span a (hysteresis) loop dual solution domain → Mach reflection domain → dual solution domain. The simulations show that the total entropy of the system exhibits an abrupt change along the path dual solution domain → Mach reflection domain, while it is continuous along the reverse path. An argument is then developed to prove that hysteresis is the natural consequence of the different stability properties of regular and Mach reflections and the Prigogine minimum total entropy production principle.
The study of fabrication of the x-ray mirror by numerically controlled plasma chemical vaporization machining: Development of the machine for the x-ray mirror fabrication
Authors: Mori Yuzo , Yamamura Kazuya , Sano Yasuhisa
Published: July 2002

A numerically controlled plasma chemical vaporization machining (CVM) machine has been developed in order to fabricate ultrahigh accuracy x-ray mirror efficiently without making the deformed layer. In this machine, a gas bearing was applied to both of the rotary electrode for plasma generation and an XY table in order to maintain the cleanliness of the machining atmosphere. A silicon flat mirror for the x-ray was made by using developed machine. A value of 22.5 nm (p–v) was achieved as flatness. The CVM technique is more fully described in the accompanying article in this issue.