bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature- and species-dependent quenching of CO B probed by two-photon laser-induced fluorescence using a picosecond laser
Authors: Settersten T. B. , Dreizler A. , Farrow R. L.
Published: September 2002

We report measurements of the temperature- and species-dependent cross sections for the quenching of fluorescence from the B 1Σ+(v=0) state of CO. Cross sections were measured for gas temperatures ranging from 293 K to 1031 K for quenching by H2, N2, O2, CO, H2O, CO2, CH4, He, Ne, Ar, Kr, and Xe. The CO B 1Σ+(v=0) state was populated via two-photon excitation (B 1Σ+←←X 1Σ+), and the B 1Σ+→A 1Π fluorescence was collected. Quenching cross sections were determined from the dependence of the fluorescence-decay rate on quencher-gas pressure. The temperature dependence of the cross sections is well described by a power law for all but the two weakest quenchers, He and Ne.
Photoreflectance spectroscopy of wurtzite CdS
Authors: Imada Akinori , Ozaki Shunji , Adachi Sadao
Published: September 2002

Photoreflectance (PR) spectra have been measured to determine the lowest direct-band edge E0α (α=A, B, and C) of wurtzite CdS single crystal in the temperature range T=13–300 K for both E⊥c and E∥c polarizations. The measured PR spectra can be interpreted by the three-dimensional (3D) excitonic plus one-electron line shapes over the entire temperature range. The temperature dependence of the excitonic and critical-point parameters (energy, amplitude, and broadening parameter) have been determined and analyzed using the Varshni [Physica (Amsterdam) 34, 149 (1967)] equation and an analytical four-parameter expression recently developed for the explanation of the band-gap shrinkage effect in semiconductors. The 3D- exciton binding energies have also been determined to be 27 (A), 31 (B), and 30 meV (C), respectively.
Silicon–on–insulator waveguide photodetector with Ge/Si self-assembled islands
Authors: El kurdi M. , Boucaud P. , Sauvage S. , Fishman G. , Kermarrec O. , Campidelli Y. , Bensahel D. , Saint-Girons G. , Sagnes I. , Patriarche G.
Published: September 2002

We have investigated a silicon-based near-infrared photodetector using a waveguide with strong optical confinement. The high-difference index waveguide is obtained with a silicon–on–insulator substrate. The optically active region consists of self-assembled Ge/Si islands embedded in a p-i-n junction. The Ge/Si islands grown by high-pressure chemical-vapor deposition exhibit a broad photoluminescence and electroluminescence which are resonant around 1.5 μm. The photoluminescence and electroluminescence energies are correlated to the island size and to the island composition using a six-band k⋅p calculation. The spectral responsivity of the detectors is measured in a front facet coupling geometry with a broadband source and with semiconductor laser diodes. For a 0 V applied bias, responsivities of 25 and 0.25 mA/W are measured at room temperature at 1.3 and 1.55 μm, respectively.
Computational study of the dissociation of H–X acids (X=F, Cl, Br, I) in water clusters
Authors: Cabaleiro-Lago Enrique M. , Hermida-Ramón Jose M. , Rodrı́guez-Otero Jesús
Published: September 2002

The ionic dissociation of H–X acids (X=F, Cl, Br, I) in water was examined by conducting a theoretical study on the properties of the clusters formed by the acids with up to five water molecules: X–H(H2O)n (n=1–5). Calculations were done using the DFT/B3LYP and MP2 methods in conjunction with the TZVP basis set and allowed the identification of several minima on the potential surfaces for the clusters. Based on the results, the MP2 method predicts a lower tendency to ionization than does the DFT/B3LYP method; however, both methods provide similar results. The dissociation characteristics of the acids were examined in terms of various parameters including the lengths of the bonds involved in the proton transfer and the frequencies associated with the X–H and O–H stretching modes in the bonds taking part in the proton transfer. The successive incorporation of water molecules to the cluster was found to lengthen X–H distances and simultaneously decrease O⋯H distances. In addition, the X–H stretching frequency underwent a marked redshift; the signal disappeared in the ionized structures, at the expense of a new series of bands around 2800 cm−1 due to stretching vibrations of the O–H bond in the H3O+ ion. Hydrogen fluoride failed to dissociate in the clusters considered; in fact, while some structures were ionized, they were not the most stable configurations for the corresponding clusters. In HCl and HBr, the dissociated structure was the most stable in the clusters of four or more water molecules (n=4–5); however, HBr exhibited a stronger tendency to dissociating above n=3. Finally, HI exhibited dissociation at n>2.
Statistical geometry of packing defects of lattice chain polymer from enumeration and sequential Monte Carlo method
Authors: Liang Jie , Zhang Jinfeng , Chen Rong
Published: September 2002

Voids exist in proteins as packing defects and are often associated with protein functions. We study the statistical geometry of voids in two-dimensional lattice chain polymers. We define voids as topological features and develop a simple algorithm for their detection. For short chains, void geometry is examined by enumerating all conformations. For long chains, the space of void geometry is explored using sequential Monte Carlo importance sampling and resampling techniques. We characterize the relationship of geometric properties of voids with chain length, including probability of void formation, expected number of voids, void size, and wall size of voids. We formalize the concept of packing density for lattice polymers, and further study the relationship between packing density and compactness, two parameters frequently used to describe protein packing. We find that both fully extended and maximally compact polymers have the highest packing density, but polymers with intermediate compactness have low packing density. To study the conformational effects of void formation, we characterize the conformational reduction factor of void formation and found that there are strong end-effect. Voids are more likely to form at the chain end. The critical exponent of end-effect is twice as large as that of self-contacting loop formation when existence of voids is not required. We also briefly discuss the sequential Monte Carlo sampling and resampling techniques used in this study.
Enhanced growth of CoSi2 on epitaxial Si0.7Ge0.3 with a sacrificial amorphous Si interlayer
Authors: Wu W. W. , Chiang T. F. , Cheng S. L. , Lee S. W. , Chen L. J. , Peng Y. H. , Cheng H. H.
Published: September 2002

Enhanced growth of CoSi2 on epitaxial Si0.7Ge0.3 has been achieved with an interposing amorphous-Si (a-Si) layer. The a-Si layer was used as a sacrificial layer to prevent Ge segregation, decrease the growth temperature, as well as maintain the interface flatness and morphological stability in forming CoSi2 on Si0.7Ge0.3 grown by molecular beam eptiaxy. The process promises to be applicable to the fabrication of high-speed Si–Ge devices.
Superfast fronts of impact ionization in initially unbiased layered semiconductor structures
Authors: Rodin P. , Ebert U. , Hundsdorfer W. , Grekhov I. V.
Published: September 2002

A mode of impact ionization breakdown of a p–n junction is suggested: We demonstrate that when a sufficiently sharp voltage ramp is applied in reverse direction to an initially unbiased equilibrium p+–n–n+ structure, after some delay the system will reach a high conductivity state via the propagation of a superfast impact ionization front. The front travels towards the anode with a velocity vf several times larger than the saturated drift velocity of electrons vs leaving a dense electron–hole plasma behind. The excitation of the superfast front corresponds to the transition from the common avalanche breakdown of a semiconductor structure to a collective mode of streamer-like breakdown. We propose that similar fronts can be excited not in layered structures but in plain bulk samples without p–n junctions. Our numerical simulations apply to a Si structure with typical thickness of W∼100 μm switched in series with a load R∼100 Ω, with a voltage ramp of A>1012 V/s applied to the whole system. Our simulations show that first there is a delay of about 1 ns during which the voltage reaches a value of several kilovolts. Then, as the front is triggered, the voltage abruptly breaks down to several hundreds of volts within ∼100 ps. This provides a voltage ramp of up to ∼2×1013 V/s hence up to 10 times sharper than the externally applied ramp. We unravel the source of initial carriers which trigger the front, explain the origin of the time delay in triggering the front, and we identify the mechanism of front propagation.
Chemical depth profile of ultrathin nitrided SiO2 films
Authors: Herrera-Gómez A. , Hegedus A. , Meissner P. L.
Published: September 2002

Ultrathin nitrided SiO2/Si(001) films were studied using angle-resolved x-ray photoemission spectroscopy. The structure of the oxynitride depended on the nitridation process. Under one type of nitridation the film kept the structure of the SiO2, with N assuming O sites. By taking advantage of the nonuniformity on the chemical depth profile, the Si 2p chemical shift was determined for those Si atoms bonded to three O and one N atom, and for those bonded to two O and two N atoms. The stoichiometry depth profile was recognized through a simple method that allowed the input of physical constrains.
Laser spectroscopy of NiBr: Ground and low-lying electronic states
Authors: Leung J. W-H. , Wang Xianghuai , Cheung A. S-C.
Published: September 2002

Four electronic states of NiBr have been studied using the technique of laser vaporization/reaction with supersonic cooling and laser induced fluorescence (LIF) spectroscopy. NiBr molecules were produced by reacting laser ablated nickel atoms and ethyl bromide (C2H5Br). High resolution LIF spectrum between 724 and 810 nm was recorded, which consists of the (2,0), (1,0), and (0,0) bands of the [13.2] Π3/22–X 2Π3/2 system and the [13.2] Π3/22–A 2Δ5/2 system, and also the (v,0) with v=0–4 bands of the [12.6] Σ+2–X 2Π3/2 system. Spectra of four isotopic molecules: Ni58Br79, Ni58Br81, Ni60Br79, and Ni60Br81 were observed and analyzed. Least squares fit of rotationally resolved transition lines yielded accurate molecular constants for the X 2Π3/2, A 2Δ5/2, [12.6] Σ+2, and [13.2] Π3/22 electronic states of NiBr. The bond length, r0, measured for the X 2Π3/2 and A 2Δ5/2 states is 2.196 28 and 2.164 45 Å, respectively. A molecular orbital diagram has been constructed to explain the four observed electronic states. This work represents the first high-resolution spectroscopic study of NiBr.
Glass-forming ability and soft magnetic properties of FeCoSiAlGaPCB amorphous alloys
Authors: Borrego J. M. , Conde A. , Roth S. , Eckert J.
Published: September 2002

The glass-forming ability of (FexCoyBzCu)80Si3Al5Ga2P10 with x=5–70, y=0–63, z=5–12, and u=0–5 amorphous alloys has been analyzed in terms of the width of the supercooled liquid region, the reduced glass transition temperature, and the Vogel–Fulcher–Tammann parameters. Substitution of Fe by Co slightly decreases the glass-forming ability of the studied alloys. The value of the fragility parameter m is discussed in the frame of the general classification scheme of glass-forming liquids. The crystalline phases formed during the first crystallization step are identified. Magnetic moment at low and room temperature, Curie temperature, room temperature magnetostriction, and coercivity decrease with increasing Co content.
Thermal equilibrium governing the formation of negatively charged excitons in resonant tunneling diodes
Authors: Vercik A. , Gobato Y. Galvão , Brasil M. J. S. P.
Published: September 2002

The thermal equilibrium of the formation of negatively charged excitons is studied in this work, by measuring the current and quantum-well photoluminescence in biased resonant tunneling double barrier diodes. We observe that the intensity ratio of negatively charged and neutral excitons depends linearly on the current for fixed temperature and illumination conditions. We propose that the results can be interpreted in terms of a mass-action law governing the concentrations of neutral and charged excitons and free electrons. Measurements at different temperatures and bias yield an electron concentration and a dwell time in the well that are in good agreement with the values previously reported in the literature. We also analyze the dependence of the luminescence on excitation intensity.
Design calculations for flexure hinges
Authors: Wu Yingfei , Zhou Zhaoying
Published: September 2002

This article presents design equations for calculating the compliance or spring rate of rectangular single-axis hinges and right circular hinges. The design equations are the result of an exact deduction and are concise in their expressions, so it is more convenient to use them in calculating and analyzing flexure hinges and flexure hinge mechanisms. Comparing these with the exact design equations of Paros and Weisboro [Mach. Des. 37, 151 (1965)], it is suggested that the equations of this article are effective. Limitations of the simplified equations of Paros and Weisboro and the approximate equation of Smith et al. [J. Phys E 20, 977 (1987)] are discussed. While calculating the compliance of flexure hinges, several important points are mentioned.
Raman and infrared linewidths of CO in Ar
Authors: Thibault F. , Martinez R. Z. , Domenech J. L. , Bermejo D. , Bouanich J.-P.
Published: September 2002

We present measurements of Raman linewidths in the fundamental Q branch of CO for mixtures with Ar at temperatures of 77, 195, and 300 K, recorded using an inverse Raman spectrometer. Starting from a recent ab initio potential energy surface, theoretical values of Ar broadening coefficients for CO infrared and Raman lines (isotropic and anisotropic components) at temperatures in the range 77 to 1100 K are calculated via quantum-mechanical methods. The relative merits of the close coupling theoretical results over the coupled states results are underlined. Finally, a comparison of the calculated pressure broadening coefficients is made with the present experimental data as well as with recently available infrared data. There is general agreement between the calculated and measured values of the broadenings for all the temperatures probed. We conclude that the temperature dependence of the infrared and Raman broadening coefficients have been correctly determined theoretically and may be used to test a common temperature scaling law.
Ion-induced secondary electron emission behavior of sol–gel-derived MgO thin films used for protective layers in alternating current plasma display panels
Authors: Jung Hyun Suk , Lee Jung-Kun , Hong Kug Sun , Youn Hyuk-Joon
Published: September 2002

MgO thin films were prepared using two sols (hydrolyzed sol and stabilized sol) and the ion-induced secondary electron emission behavior of the resultant thin films was investigated. A severe fluctuation in the secondary electron emission current was found in MgO films from hydrolyzed sol. The instability of the ion-induced current was due to the nanosized pores, which were formed during the topotactic reaction of Mg(OH)2 to MgO. Nonhydrolyzed MgO films, however, showed a stable ion-induced current. The ion-induced secondary electron emission coefficients (γi) of the MgO films had a maximum of 0.95±0.02 when the films were heat treated at 550 °C in O2. The change in γi of nonhydrolyzed films was discussed from the viewpoint of crystallinity, residual organics, and surface roughness. The high γi and low processing temperature of nonhydrolyzed MgO films revealed that the sol–gel process is suitable to prepare MgO films for use as a protective layer in ac plasma display panel cells.
The β–δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Thermodynamics
Authors: Henson B. F. , Smilowitz L. , Asay B. W. , Dickson P. M.
Published: September 2002

In this and the accompanying paper [L. Smilowitz et al., J. Chem. Phys. 117, 3789, 2002] we present a theoretical treatment and experimental study, respectively, of the β–δ solid state phase transition in the organic nitramine molecule octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The transition is thermodynamically first order with a measured latent heat, occurs via nucleation and growth, and exhibits a thermally activated rate of transformation. We construct a two state kinetic model of the system consisting of equilibrium terms first order in the β or δ mole fraction simulating nucleation, and second order in β and δ simulating growth. The model has four rate constants, the temperature dependence of which is described by eight parameters. We use the transition state formulation of the rate constants and apply a thermodynamic model of the activated state that associates the difference in activated state free energy in forward and reverse directions with the equilibrium transition free energy, and identifies the activated state of the growth process with a metastable melt. By associating components of the activated state free energy with independently measured thermodynamic energies we reduce the degrees of freedom to three, which we fix initially by comparison with previously published kinetic data. We apply the model to both the β–δ and δ–β transformations over a temperature range from 300 to 700 K in order to assess the theoretical validity of the model. The model reproduces the half time of the transition over this entire range, spanning conversion times from 106 to 10−4 s. In the accompanying paper we present an experimental study of the kinetics and mechanism of the phase transition based on second harmonic generation spectroscopy. We use second harmonic generation to verify the nucleation and growth mechanism of the transition and measure the mole fraction change with time over a wide range of temperatures. We use the set of parameters established by theoretical considerations in this paper as an initial parameter set and determine an optimized set by comparison with these data.
Conical intersections and the nonadiabatic reactions H2O+O(3P)↔OH(A 2Σ+)+OH(X 2Π)
Authors: Matsika Spiridoula , Yarkony David R.
Published: September 2002

In this work the reaction O(3P)+H2O→OH(X 2Π)+OH(A 2Σ+) and the reverse reaction were studied. Π3–3Δ and Π3–3Σ+ conical intersection seams were found that tie two reactant-channel potential energy surfaces to the two excited state potential energy surfaces correlating with OH(X 2Π)+OH(A 2Σ+) products. Near their lowest energy points, which occur for collinear geometries, these accidental symmetry-allowed conical intersection seams have very similar geometries and are separated by <90 cm−1. This close proximity may reflect a three state conical intersection not required by symmetry. The conical intersection seams were shown to be accessible from both the reactant and product channels without a barrier, above any reaction endoergicity, and provide in the case of the reverse reaction for branching to either H2O+O(3P) or OH(X 2Π)+OH(X 2Π).
Domain reorientation anisotropy in ferroelectric polycrystals
Authors: Lente M. H. , Eiras J. A.
Published: September 2002

The domain reorientation process in polycrystalline materials was investigated through hysteresis loop measurements in soft and hard lead zirconate titanate (PZT) bulk ceramics. In order to investigate the degree of alignment of domains with the electric field the ferroelectric characterizations were performed in two perpendicular directions for each sample. It was found that for the soft PZT, the kinetics of the fatigue process due to space charges in a specific direction is not affected by the previous fatigue story induced in the other perpendicular one thus showing a fatigue anisotropy. This fact was attributed to no displacement of part of space charges during the electric field switching. However, for the hard PZT, an influence of the previous electric story could be observed due to the continuous realignment of the complex defects perpendicularly to the electric field.
Competitive transport and percolation in disordered arrays of molecularly-linked Au nanoparticles
Authors: Trudeau P.-E. , Orozco A. , Kwan E. , Dhirani A.-A.
Published: September 2002

We explore the electrical characteristics of disordered films of strongly coupled, molecularly-linked gold nanoparticles (NPs). dc conductivity vs temperature (g vs T) measurements exhibit features that can track a number of competing transport mechanisms. Films with fewer than 6 layers show clear signatures of both activated tunneling and thermionic emission. Our linked NPs admit locally metallic transport, likely through strong quantum interactions, and at room temperature, films with 6 or more layers exhibit a transition to metallic dominated behavior. Observed g vs T dependencies have been modeled treating arrays as disordered resistor networks and using an effective medium approximation (EMA). Our results show that percolation phenomena can play critical roles in transport through NP films, particularly near metal–insulator transitions.
Molecular dynamics description of a layer of water molecules on a hydrophobic surface
Authors: Gordillo M. C. , Martı́ J.
Published: September 2002

Static and dynamic properties of a layer of water molecules on top of a graphite surface are studied by means of molecular dynamics simulations. The water molecules are described by a simple point charge flexible model, and the graphite is taken to be a set of featureless parallel sheets separated 3.4 Å in the z direction. Our results indicate that, even at the lower temperatures considered, the water layer is not flat, with some hydrogen atoms pointing perpendicularly to the surface plane. The O–H stretching frequencies are also different than those of bulk water, appearing a new peak in the simulated spectra at a frequency approximately 200 wave numbers higher than the main peak. This peak is associated with the presence of non-H-bonded molecules.
Heating in current carrying molecular junctions
Authors: Segal Dvira , Nitzan Abraham
Published: September 2002

A framework for estimating heating and expected temperature rise in current carrying molecular junctions is described. Our approach is based on applying the Redfield approximation to a tight binding model for the molecular bridge supplemented by coupling to a phonon bath. This model, used previously to study thermal relaxation effects on electron transfer and conduction in molecular junctions, is extended and used to evaluate the fraction of available energy, i.e., of the potential drop, that is released as heat on the molecular bridge. Classical heat conduction theory is then applied to estimate the expected temperature rise. For a reasonable choice of molecular parameters and for junctions carrying currents in the nA range, we find the temperature rise to be a modest few degrees. It is argued, however, that using classical theory to describe heat transport away from the junction may underestimate the heating effect.
Theory of solutions in the energy representation. II. Functional for the chemical potential
Authors: Matubayasi Nobuyuki , Nakahara Masaru
Published: September 2002

An approximate functional for the chemical potential of a solute in solution is presented in the energy representation. This functional is constructed by adopting the Percus–Yevick-like approximation in the unfavorable region of the solute–solvent interaction and the hypernetted-chain-like approximation in the favorable region. The chemical potential is then expressed in terms of energy distribution functions in the solution and pure solvent systems of interest, and is given exactly to second order with respect to the solvent density and to the solute–solvent interaction. In the practical implementation, computer simulations of the solution and pure solvent systems are performed to provide the energy distribution functions constituting the approximate functional for the chemical potential. It is demonstrated that the chemical potentials of nonpolar, polar, and ionic solutes in water are evaluated accurately and efficiently from the single functional over a wide range of thermodynamic conditions.
Görtler vortices in the Rayleigh layer on an impulsively started cylinder
Authors: MacKerrell Sharon O. , Blennerhassett P. J. , Bassom Andrew P.
Published: September 2002

Centripetal instabilities in two flows involving time-dependent Rayleigh layers on a rotating circular cylinder are examined. In one case we consider the stability of the flow induced in an infinite expanse of quiescent fluid when the cylinder is impulsively given a constant angular velocity; in the other problem the angular velocity increases as the square root of time so that the undisturbed flow has a constant wall shear. For both situations linear neutral stability curves for vortex motions are calculated by quasi-steady (or frozen-time) methods, with these results justified, where possible, by Wentzel–Kramers–Brillouin techniques. The topology of the neutral curve for the ramped angular velocity configuration allows a rigorous description of small wavelength, weakly and fully nonlinear vortex structures to be obtained. Our results are compared with the equivalent cases that arise in the study of unsteady thermal Rayleigh layers induced by the sudden heating of a horizontal flat plate.
The addition of Sb as a surfactant to GaN growth by metal organic vapor phase epitaxy
Authors: Zhang L. , Tang H. F. , Schieke J. , Mavrikakis M. , Kuech T. F.
Published: September 2002

The role and effect of the isoelectronic center Sb on the structure and properties of GaN epilayers have been investigated. The gas phase Sb concentration was varied by changing the triethyl antimony/trimethyl gallium mole ratio over a wide range of concentrations while keeping other growth parameters constant. The Sb addition slightly improved the optical and structural properties of GaN epilayer at a low level of Sb incorporation, especially for the films grown under a high group V/III ratio conditions. The addition of Sb resulted in changes in GaN surface morphology, which was further explored by the lateral epitaxy overgrowth (LEO) technique through the changes in the growth rates and the facet formation. The presence of Sb in the gas phase greatly enhanced the lateral overgrowth rate and altered the formation of the dominant facets. Vertical facets to the LEO growth appeared with the addition of Sb under conditions that normally produced sloped sidewalls. While Sb altered the growth facet present during LEO, only a small amount of Sb was incorporated into the GaN, suggesting that Sb acts as a surfactant during the GaN metal organic vapor phase epitaxy growth. Sb addition produces surface conditions characteristic of a Ga-rich surface stoichiometry indicating both a possible change in the reactivity of NH3 and/or enhanced surface diffusion of Ga adatom species in the presence of Sb.
Subpicosecond laser ablation of dental enamel
Authors: Rode A. V. , Gamaly E. G. , Luther-Davies B. , Taylor B. T. , Dawes J. , Chan A. , Lowe R. M. , Hannaford P.
Published: September 2002

Laser ablation of dental enamel with subpicosecond laser pulses has been studied over the intensity range of (0.1–1.4)×1014 W/cm2 using 95 and 150 fs pulses at a pulse repetition rate of 1 kHz. The experimentally determined ablation threshold of 2.2±0.1 J/cm2 was in good agreement with theoretical predictions based on an electrostatic ablation model. The ablation rate increased linearly with the laser fluence for up to 15 times the ablation threshold. The absence of collateral damage was observed using optical and scanning electron microscopy. Pulpal temperature measurements showed an increase of about 10 °C during the 200 s course of ablation. However, air cooling at a rate of 5 l/min resulted in the intrapulpal temperature being maintained below the pulpal damage threshhold of 5.5 °C. The material removal rates for subpicosecond precision laser ablation of dental enamel are compared with other techniques.
Structural stability and electrical properties of Sr2FeMoO6 under high pressure
Authors: Zhao P. , Yu R. C. , Li F. Y. , Liu Z. X. , Jin M. Z. , Jin C. Q.
Published: September 2002

The structural stability and electrical properties of Sr2FeMoO6 under high pressure at room temperature have been studied using energy dispersive x-ray diffraction with synchrotron radiation and resistance and capacitance measurements. The x-ray diffraction results show that the structure of Sr2FeMoO6 remains stable up to 40 GPa. The equation of state of Sr2FeMoO6 is obtained from the V/V0–P relationship. The bulk modulus B0 and its first-order derivative B0′ of Sr2FeMoO6 were calculated based on the Birch–Murnaghan equation. The electrical resistance undergoes a metallic transition at about 2.1 GPa. The metallic transition may be caused by a change in the electronic structure induced by high pressure.
Carbon nanotube scanning probe for profiling of deep-ultraviolet and 193 nm photoresist patterns
Authors: Nguyen Cattien V. , Stevens Ramsey M. D. , Barber Jabulani , Han Jie , Meyyappan M. , Sanchez Martha I. , Larson Carl , Hinsberg William D.
Published: September 2002

The continual scaling down of complementary metal–oxide semiconductor feature size to 100 nm and below necessitates a characterization technique to resolve high-aspect-ratio features in the nanoscale regime. We report the use of atomic force microscopy coupled with high-aspect-ratio multiwalled carbon nanotube (MWCNT) scanning probe tip for the purpose of imaging surface profile of photoresists. MWCNT tips of 5–10 nm in diameter and about a micron long are used. Their exceptional mechanical strength and ability to buckle reversibly enable resolution of steep, deep nanoscale features. Images of photoresist patterns generated by 257 nm interference lithography as well as 193 nm lithography are presented to demonstrate MWCNT scanning probe tips for applications in metrology.
Ab initio molecular dynamics study of proton mobility in liquid methanol
Authors: Morrone Joseph A. , Tuckerman Mark E.
Published: September 2002

The transport of protons through aqueous, partially aqueous, or nonaqueous hydrogen-bonded media is a fundamental process in many biologically and technologically important systems. Liquid methanol is an example of a hydrogen-bonded system that, like water, supports anomalously fast proton transport. Using the methodology of ab initio molecular dynamics, in which internuclear forces are computed directly from electronic structure calculations as the simulation proceeds, we have investigated the microscopic mechanism of the proton transport process in liquid methanol at 300 K. It is found that the defect structure associated with an excess proton in liquid methanol is a hydrogen-bonded cationic chain whose length generally exceeds the average chain length in pure liquid methanol. Hydrogen bonds in the first and second solvation shells of the excess proton are considerably shorter and stronger than ordinary methanol–methanol hydrogen bonds. Along this chain, proton transfer reactions occur in an essentially random manner described by Poisson statistics. Structural diffusion of the defect structure is possible if the proton migrates toward an end of the defect chain, which causes a weakening of the hydrogen bonds at the opposite end. The latter can, therefore, be easily ruptured by ordinary thermal fluctuations. At the end of the chain where the proton resides, new hydrogen bonds are likely to form due to the strong associative nature of the excess proton. It is through this “snake-like” mechanism that the defect structure is able to diffuse through the hydrogen-bond network of the liquid. The estimated activation enthalpy of this proposed mechanism is found to be in reasonable agreement with the experimentally determined activation enthalpy.
Structure of the (001) surface of γ alumina
Authors: Vijay Amrendra , Mills Greg , Metiu Horia
Published: September 2002

Using density functional theory, we have studied the structure and energetics of the (001) face of γ alumina. Our results address several experimental issues: (1) When the face with tetrahedral aluminum is exposed in the bulk-terminated system, the surface reconstructs extensively, leading to exposure of the higher-density layer. When only a few layers are present, this reconstruction may even lead to the collapse of the system into a different structure. (2) We find that the lowest energy is obtained if the vacant spinel sites lie on octahedral positions. We also find that vacancies are less preferred on the surface than in the bulk. (3) Migration to and from the surface of vacant spinel sites, by hopping of Al atoms between octahedral and tetrahedral cation sites has a rather high barrier. This suggests the vacancy distribution may not reach equilibrium if the material is not annealed carefully during preparation.
On the transverse magnetization of the anisotropic superconductor 2H–NbSe2
Authors: Sirenko V. A. , Makedonska N. I. , Shabakayeva Yu. A. , Schleser R.
Published: September 2002

Torque measurements were performed on a high-quality single crystal of the uniaxial superconductor 2H–NbSe2 in a tilted magnetic field 0–200 kG, in the temperature range 1.5–4.2 K. The transverse component of the absolute magnetization was derived in a magnetic field directed at an angle of 77° to the axis of anisotropy, and its field dependence was analyzed in a reversible domain of the mixed state. The penetration depth and anisotropy characteristics were obtained for the sample under study.
Constant-pH molecular dynamics using stochastic titration
Authors: Baptista António M. , Teixeira Vitor H. , Soares Cláudio M.
Published: September 2002

A new method is proposed for performing constant-pH molecular dynamics (MD) simulations, that is, MD simulations where pH is one of the external thermodynamic parameters, like the temperature or the pressure. The protonation state of each titrable site in the solute is allowed to change during a molecular mechanics (MM) MD simulation, the new states being obtained from a combination of continuum electrostatics (CE) calculations and Monte Carlo (MC) simulation of protonation equilibrium. The coupling between the MM/MD and CE/MC algorithms is done in a way that ensures a proper Markov chain, sampling from the intended semigrand canonical distribution. This stochastic titration method is applied to succinic acid, aimed at illustrating the method and examining the choice of its adjustable parameters. The complete titration of succinic acid, using constant-pH MD simulations at different pH values, gives a clear picture of the coupling between the trans/gauche isomerization and the protonation process, making it possible to reconcile some apparently contradictory results of previous studies. The present constant-pH MD method is shown to require a moderate increase of computational cost when compared to the usual MD method.
Effect of Eu3+ concentration on the grating efficiency and ionic conductivity in sodium–magnesium–aluminosilicate glasses
Authors: Hamad Abdulatif Y. , Wicksted James P. , Ascio Robert , Martin Joel J. , Hunt Charles , Dixon George S.
Published: September 2002

We report a systematic study of a grating formation in which the [Eu2O3] in sodium–magnesium–aluminosilicate glasses is varied from 0.76 to 8.11 mol %. The growth, decay, and erasure of the grating are reported as functions of the [Eu2O3]. The maximum persistent change in the index of refraction was 3×10−5. The persistent change in the index of refraction was initially a quadratic function of the [Eu2O3] and showed a limiting behavior at the highest [Eu2O3]. The transient change in the index of refraction Δntran was a quadratic function of [Eu2O3] throughout the range of concentrations studied here. The grating buildup rate increased linearly with Δntran. The results of this study are consistent with the model published recently by Dixon et al. Ionic conductivities were also measured to help separate the effect of the Eu3+ on the glass network from its active role in transferring the optical energy into ionic motion.
Uniformity tests of individual segments of interband cascade diode laser Nanostacks®
Authors: Malyarchuk V. , Tomm J. W. , Lienau Ch. , Behringer M. , Luft J.
Published: September 2002

We investigate optoelectronic properties of monolithically stacked diode lasers, so-called Nanostack® devices that include two nominally identical waveguide segments separated by a specially designed tunnel junction. Near-field optical microscopy provides straightforward and separate access to the properties of both optically active segments. Device emission, namely electroluminescence and lasing, as well as photoluminescence and photocurrent data, are recorded with high spatial resolution and consistently interpreted. We find reduced laser emission from the laser segment that is situated closer to the substrate. We show that this is not caused by thermal effects but most likely due to a larger trap concentration within or in the vicinity of the quantum wells of this laser segment. Furthermore, we show that in the unbiased devices the potential gradient in this segment is significantly larger than in the one close to the heat sink. In addition it is shown that the coupling between both waveguides is marginal. The results underline the potential and unique advantages of near-field optical microscopy for nondestructive analysis of optoelectronic device.
Experimental and theoretical studies on Rydberg states of CH2CO in the region 120–220 nm
Authors: Chiang Su-Yu , Bahou Mohammed , Wu Yu-Jong , Lee Yuan-Pern
Published: September 2002

Absorption spectra of CH2CO and CD2CO in the region 120–220 nm were recorded with a continuously tunable light source of synchrotron radiation. Our spectra show improved sensitivity and resolution with no interference from continuous absorptions reported previously. Transitions to Rydberg states associated with excitation to 3s–9s, 3px–5px, and 3d orbitals are assigned based on observed vibronic patterns, isotopic shifts, effective quantum numbers, and comparison with vertical excitation energies predicted with theoretical calculations using time-dependent density functional theory (TD-DFT). Several transitions are reassigned and new vibrational progressions are identified for some Rydberg states. The major active modes for the ns Rydberg series are the C=C stretching (ν4) and the C=O stretching (ν2) modes; the CH2 scissor (ν3) mode is also active in several transitions. TD-DFT using a polarized basis set (PBS) proves to be an excellent method for predictions of vertical excitation energies and oscillator strengths, especially for higher Rydberg states.
Pt – WO x electrode structure for thin-film fuel cells
Authors: Park Kyung-Won , Ahn Kwang-Soon , Choi Jong-Ho , Nah Yoon-Chae , Kim Young-Min , Sung Yung-Eun
Published: September 2002

An electrode structure consisting of two phases of Pt and WOx for use in thin-film fuel cells was designed and fabricated using a cosputtering system with a Pt metal and a tungsten oxide target. The coexistence of a polycrystalline Pt nanosized phase and an amorphous tungsten oxide phase in the electrode layer was confirmed by transmission electron microscopic images and x-ray diffraction data. In addition, compared with a Pt thin-film electrode, the two-phase electrode of Pt and WOx showed excellent performance for the devices because of the improved activity of the Pt metallic phase and the spill-over effect of porous tungsten oxides.
Integration of suspended carbon nanotube arrays into electronic devices and electromechanical systems
Authors: Franklin Nathan R. , Wang Qian , Tombler Thomas W. , Javey Ali , Shim Moonsub , Dai Hongjie
Published: September 2002

A synthetic strategy is devised for reliable integration of long suspended single-walled carbon nanotubes into electrically addressable devices. The method involves patterned growth of nanotubes to bridge predefined molybdenum electrodes, and is versatile in yielding various microstructures comprised of suspended nanotubes that are electrically wired up. The approach affords single-walled nanotube devices without any postgrowth processing, and will find applications in scalable nanotube transistors (mobility up to 10 000 cm2/V s) and nanoelectromechanical systems based on nanowires.
Theoretical investigation of intersubband hole transitions in Si/SiGe/Si quantum wells
Authors: Boujdaria K. , Ridene S. , Radhia S. Ben , Zitouni O. , Bouchriha H. , Fishman G.
Published: September 2002

We study the effects of the pc–pc coupling in intersubband hole optical transitions in SiGe/Si quantum wells for x and z polarizations. We have used a k⋅p model taking into account both the p-like first conduction band and the s-like second conduction band. First, we have found a unitary transformation that block diagonalizes the 14×14 Hamiltonian (or 12×12 Hamiltonian) into two 7×7 (or 6×6) blocks that are real symmetric in the finite difference formulation. We find that pc–pc interaction plays a minimal role in intersubband optical transition for x and z polarizations. Moreover, our calculations clearly confirm that the pc–pv coupling favors intersubband transitions for an optical polarization parallel to the layer plane (x polarization). In addition, for z polarization, both s–pv and pc–pv interactions play an equal footing role in intervalence band transitions.
Hollow cathode sustained plasma microjets: Characterization and application to diamond deposition
Authors: Sankaran R. Mohan , Giapis Konstantinos P.
Published: September 2002

Extending the principle of operation of hollow cathode microdischarges to a tube geometry has allowed the formation of stable, high-pressure plasma microjets in a variety of gases including Ar, He, and H2. Direct current discharges are ignited between stainless steel capillary tubes (d=178 μm) which are operated as the cathode and a metal grid or plate that serves as the anode. Argon plasma microjets can be sustained in ambient air with plasma voltages as low as 260 V for cathode-anode gaps of 0.5 mm. At larger operating voltage, this gap can be extended up to several millimeters. Using a heated molybdenum substrate as the anode, plasma microjets in CH4/H2 mixtures have been used to deposit diamond crystals and polycrystalline films. Micro-Raman spectroscopy of these films shows mainly sp3 carbon content with slight shifting of the diamond peak due to internal stresses. Optical emission spectroscopy of the discharges used in the diamond growth experiments confirms the presence of atomic hydrogen and CH radicals.
Formation and crystallization of Ti–Hf–Ni metallic glasses
Authors: Huett V. T. , Kelton K. F.
Published: September 2002

The formation of a Ti25Hf50Ni25 metallic glass and the phase evolution upon crystallization (devitrification) are reported. The glass is strongly metastable, with a 65 °C separation between the glass transition temperature, Tg=335 °C, and the onset temperature for primary crystallization to a nanoscale microstructure consisting of an icosahedrally symmetric ordered phase, that can not be reported as the icosahedral phase (i-phase). There is no distinction between an i-phase and a high-order crystal rational approximant for such small grains, since the unit cell size of a reasonable high-order approximant is of the order of the grain size. Upon annealing at higher temperatures, the metastable icosahedrally symmetric phase transforms to a stable Ti2Ni-type crystal phase.
On the apparent permeability of regular arrays of nonuniform fibers
Authors: Markicevic B. , Papathanasiou T. D.
Published: September 2002

Numerical calculations for fluid flow across arrays of fibers of equal as well as of varying size have been carried out for conditions of creeping flow as well as for conditions of flow with substantial inertia. Based on numerical results for pressure drop and flow rate we compute the apparent hydraulic permeability of many such arrays. These apparent permeabilities are compared to the predictions of a generalized explicit model proposed by Knupp and Lage [J. Fluid Mech. 299, 97 (1995)]. A good agreement is observed, indicating that the above-mentioned explicit model holds for regular fibrous media composed of fibers of varying size.
An extension of Prandtl–Batchelor theory and consequences for chaotic advection
Authors: Mezić Igor
Published: September 2002

We extend the Prandtl–Batchelor theory of steady laminar motion at large Reynolds number to derive conditions that steady three-dimensional Navier–Stokes flows have to satisfy. We combine these results with ergodic theory to show that flows with strong Beltrami property (e.g., ABC flows) cannot be a paradigm for chaotic advection in inertia-dominated boundary-driven three-dimensional flows. Our results indicate that viscous forces are responsible for chaotic advection in steady, three-dimensional boundary-driven Navier–Stokes flows at large Reynolds numbers.
Lower bound for the quantum capacity of a discrete memoryless quantum channel
Authors: Matsumoto Ryutaroh , Uyematsu Tomohiko
Published: September 2002

We generalize the random coding argument of stabilizer codes and derive a lower bound on the quantum capacity of an arbitrary discrete memoryless quantum channel. For the depolarizing channel, our lower bound coincides with that obtained by Bennett et al. We also slightly improve the quantum Gilbert–Varshamov bound for general stabilizer codes, and establish an analog of the quantum Gilbert–Varshamov bound for linear stabilizer codes. Our proof is restricted to the binary quantum channels, but its extension of to l-adic channels is straight forward.
Ferroelastic phase in SrBi2Ta2O9 and study of the ferroelectric phase-transition dynamics
Authors: Kamba S. , Pokorný J. , Porokhonskyy V. , Petzelt J. , Moret M. P. , Garg A. , Barber Z. H. , Zallen R.
Published: September 2002

Polarized microscope observation of ferroelastic domains in a SrBi2Ta2O9 (SBT) single crystal reveals the presence of domains up to Tc1≃770 K, which supports the ferroelasticity and Amam symmetry of the intermediate phase between ferroelectric and paraelectric phases. Far-infrared spectra of SBT ceramics, single crystal and thin films show a well underdamped optical soft mode at 28 cm−1, which partially softens to 21 cm−1 near the ferroelectric transition temperature (Tc2≅600 K). This softening does not explain the entire anomaly of low-frequency permittivity observed near Tc2. On the basis of high-frequency measurements, which do not show a significant dispersion, central-mode-type dispersion in the 10–100 GHz range is proposed as an explanation. So, the phase transition at Tc2 apparently shows a crossover behavior between the displacive and the order–disorder type.
A boundary-layer model of thermocapillary flow in a cold corner
Authors: Canright D. , Huber M.
Published: September 2002

This work develops a boundary-layer model for the thermocapillary feedback mechanism that can occur at the edge of weld pools and other materials processes, in the limit where convection dominates the heat transport but the Prandtl number is small. Previously, Canright [Phys. Fluids 6, 1415 (1994)] showed that in this regime the dynamics of this “cold corner” region is locally determined through the thermal gradient driving the convection that contains the gradient. The present work applies standard boundary-layer approximations to construct a model that captures this dominant feedback mechanism and agrees well with a purely numerical model.
The electrophoretic mobility of an eccentrically positioned spherical particle in a cylindrical pore
Authors: Yariv E. , Brenner H.
Published: September 2002

The electrophoretic mobility of an eccentrically positioned finite-size sphere in a circular cylinder is calculated for the case of a small sphere to cylinder radii ratio using the thin Debye layer approximation.