bluuebunny/crossref_metadata_2025 huggingface dataset:
Organization of mesoscale convective systems: 2. Linear theory
Authors: Cheng Anning
Published: June 2005

The anelastic system of equations (3‐D momentum, continuity and thermodynamic energy) is used to investigate the organization of mesoscale convective systems (MCSs) using the reference layer concept presented in part 1. Latent heating is assumed to be proportional to the vertical velocity. The WKBJ method is used to solve this system of equations by perturbing the solution linearly from that at the reference level, which is the top of the reference layer. The reference layer is defined as a layer with maximum wind shear and unstable moist stratification over a minimum thickness of 200 mbar. The characteristics of the reference layer, such as the magnitude of the shear and moist stratification, determine the type of MCS' organization and associated properties in the analysis. This result agrees well with numerical simulations of linear MCSs presented in part 1 of this series of study. Three types of MCSs are identified from the linear theory: one is nonlinear and two are linear. The group speed of all three types turns out to be proportional to the mean wind at the reference level. Nonlinear MCSs occur when the vertical wind shear is weak and the stratification is unstable. Type 1 linear solutions are neutral, shear‐parallel lines; their reference layers are usually in the middle troposphere. Type 2 linear solutions are amplifying, shear‐perpendicular lines; their reference layers are usually in the lower troposphere. The theory predicts the widths and growth rates of the type 2 linear MCSs. The width of an amplifying MCS is determined as a simple function of the Richardson number, while the growth rate is a function of vertical wind shear and Richardson number. Tests of the linear theory using data from several field experiments show that it gives fairly realistic results for a variety of the observed MCSs.
Evaluation of the Hall effect in the substorm current system: An equivalent circuit approach
Authors: Nakai H. , Kamide Y.
Published: June 2005

A simple equivalent circuit model is employed to examine the ionosphere‐magnetotail coupling during substorms. The Pedersen and Hall conductivities are assumed to be finite in the two resistive regions, which represent the mid‐tail plasma sheet and the poleward half of the nightside auroral oval. These two regions are connected through both the wedge‐current and meridional‐current systems. It is demonstrated that this model successfully reproduces substorm changes in the cross‐tail current and the westward electrojet. In consistent with earlier observations, the solutions of the model suggest that about 90% of the westward electrojet is the Hall current. It is shown that at the expansion onset of substorms the overshoot electric field is induced more effectively for a weaker increase in the ionospheric conductivity.
Properties of local plasma injections in Saturn's magnetosphere
Authors: Burch J. L. , Goldstein J. , Hill T. W. , Young D. T. , Crary F. J. , Coates A. J. , André N. , Kurth W. S. , Sittler E. C.
Published: May 2005

Electron and ion drift dispersion events are often observed by the Cassini Plasma Spectrometer (CAPS) in the inner magnetosphere of Saturn (5 to 10 RS). These events appear to result from azimuthally‐limited injections of plasma and persist for at least several hours. During this time, the events can be analyzed to obtain information on the time and azimuthal location of the injections. The CAPS data show evidence of both remote and local injections. In this paper a conceptual model of Saturnian centrifugal interchange is developed based on the characteristics of the local injections.
Nanobump arrays fabricated by laser irradiation of polystyrene particle layers on silicon
Authors: Huang S. M. , Sun Z. , Luk’yanchuk B. S. , Hong M. H. , Shi L. P.
Published: June 2005

Two-dimensional (2D) nanobump arrays were fabricated by laser irradiation of a regular lattice of absorptive polystyrene (PS) microspheres on an undoped (100) Si wafer. The experiments were performed with single-pulse 248nm KrF laser radiation. The structure of the arrays fabricated by this method was characterized by field emission scanning electron microscope and atomic force microscope. The near-field effects under the absorptive particle are studied. The ablation and thermal processes induced by the optical near-field around the particles are investigated. The formation mechanism of nanobumps is discussed.
Single polarized Tm3+ laser in Zn-diffused LiNbO3 channel waveguides
Authors: Cantelar E. , Sanz-García J. A. , Lifante G. , Cussó F. , Pernas P. L.
Published: June 2005

In this work, laser operation at 1.76μm in Tm3+:LiNbO3 Zn-diffused channel waveguides is reported. The laser emission is single polarized with the electric field parallel to the optic axis(π-polarization) and operates in continuous-wave regime, at room temperature. The threshold of laser oscillation is in the range of 40mW, the slope efficiency is around 1%, and both magnitudes are dependent on the channel width, in accordance with the mode overlap between the pump and signal modes within the waveguides.
Intracavity grating-confined all-epitaxial vertical-cavity surface-emitting laser based on selective interface Fermi-level pinning
Authors: Gazula D. , Ahn J. , Lu D. , Huang H. , Deppe D. G.
Published: June 2005

An all-epitaxial GaAs-based vertical-cavity surface-emitting laser is demonstrated with an intracavity mode and current-confining grating. The grating uses selective Fermi-level pinning at a heterointerface to confine the current to the same regions as the optical mode. Despite the grating’s “coarseness,” an increase of efficiency is obtained in side-by-side comparison with devices that lack the grating.
Cholesteric liquid crystal laser with wide tuning capability
Authors: Lin Tsung-Hsien , Chen Yi-Jan , Wu Chun-Hui , Fuh Andy Y.-G. , Liu J.-H. , Yang P.-C.
Published: June 2005

This letter examines a planar cholesteric cell (CLC) doped with two collocated laser dyes as a one-dimensional photonic crystal. Adding phototunable chiral material (AzoB) allows the CLC photonic crystal to be lased at the band edges of the photonic band gap with a tuning range of over 100nm. Tuning is performed by irradiating the chiral AzoB material with UV light, causing the material to undergo trans-cis isomerization in the CLC film. The tuning range is the visible region from 563to667nm. Moreover, the tuning is reversible.
Depletion characteristics of two-dimensional lateral p-n-junctions
Authors: Reuter D. , Werner C. , Wieck A. D. , Petrosyan S.
Published: June 2005

We have fabricated two-dimensional in-plane p-n junctions from a p-doped pseudomorphic GaAs∕In0.1Ga0.9As∕Al0.35Ga0.65As heterostructure employing Si compensation doping by focused ion beam implantation. The current–voltage characteristics at room temperature showed good rectifying behavior, and the current in the reverse direction was below 1nA for voltages up to 10V. The depletion width was measured by optical beam-induced current, and a linear dependence on the reverse bias was found which is in contrast to the square root dependence observed in three-dimensional junctions. The results agree well with theoretical predictions.
Carbon nanotube growth for field-emission cathodes from graphite paste using Ar-ion bombardment
Authors: Hunt Charles E. , Glembocki Orest J. , Wang Yu , Prokes Sharka M.
Published: June 2005

Multiwall carbon nanotubes (MW-CNT) have been synthesized from solid-phase graphite. The graphite is deposited as a thick-film paste and irradiated with a 1.2keV flood Ar-ion beam, transforming the graphite surface to a composite of MW-CNT embedded in the graphite matrix. Micro-Raman measurements have verified that the nanotubes are metallic in nature. The technique was used to make printed field-emission cathodes. Emission from these cathodes demonstrates Fowler–Nordheim tunneling characteristics. The irradiated film emits at an extraction field of 5.0V∕μm, which is less than one-sixth of the minimum extraction field of the nonirradiated graphite film, and exhibit lower noise and greater emission uniformity.
Near-field and far-field dynamics of (Al,In)GaN laser diodes
Authors: Schwarz Ulrich T. , Pindl Markus , Wegscheider Werner , Eichler Christoph , Scholz Ferdinand , Furitsch Michael , Leber Andreas , Miller Stephan , Lell Alfred , Härle Volker
Published: June 2005

Near- and far-field dynamics of edge-emitting (Al,In)GaN laser diodes are measured simultaneously with a 100 nm spatial and a 5 ns temporal resolution using a scanning near-field microscope. We reconstruct the phase distribution at the laser diode facet. Beam steering and near-field mode dynamics are interpreted in terms of thermal and carrier induced change of refractive index in the waveguide.
Atomistic simulation of nanoporous layered double hydroxide materials and their properties. I. Structural modeling
Authors: Kim Nayong , Kim Yongman , Tsotsis Theodore T. , Sahimi Muhammad
Published: June 2005

An atomistic model of layered double hydroxides, an important class of nanoporous materials, is presented. These materials have wide applications, ranging from adsorbents for gases and liquid ions to nanoporous membranes and catalysts. They consist of two types of metallic cations that are accommodated by a close-packed configuration of OH− and other anions in a positively charged brucitelike layer. Water and various anions are distributed in the interlayer space for charge compensation. A modified form of the consistent-valence force field, together with energy minimization and molecular dynamics simulations, is utilized for developing an atomistic model of the materials. To test the accuracy of the model, we compare the vibrational frequencies, x-ray diffraction patterns, and the basal spacing of the material, computed using the atomistic model, with our experimental data over a wide range of temperature. Good agreement is found between the computed and measured quantities.
Molecular dynamics study of thermal phenomena in an ultrathin liquid film sheared between solid surfaces: The influence of the crystal plane on energy and momentum transfer at solid-liquid interfaces
Authors: Ohara Taku , Torii Daichi
Published: June 2005

A molecular dynamics study has been performed on a liquid film sheared between moving solid walls. Thermal phenomena that occur in the Couette-like flow were examined, including energy conversion from macroscopic flow energy to thermal energy, i.e., viscous heating in the macroscopic sense, and heat conduction from the liquid film to the solid wall via liquid-solid interfaces. Four types of crystal planes of fcc lattice were assumed for the surface of the solid wall. The jumps in velocity and temperature at the interface resulting from deteriorated transfer characteristics of thermal energy and momentum at the interface were observed. It was found that the transfer characteristics of thermal energy and momentum at the interfaces are greatly influenced by the types of crystal plane of the solid wall surface which contacts the liquid film. The mechanism by which such a molecular scale structure influences the energy transfer at the interface was examined by analyzing the molecular motion and its contribution to energy transfer at the solid-liquid interface.
Equilibrium limits of coherency in strained nanowire heterostructures
Authors: Ertekin Elif , Greaney P. A. , Chrzan D. C. , Sands Timothy D.
Published: June 2005

Due to their unique boundary conditions, nanowire heterostructures may exhibit defect-free interfaces even for systems with large lattice mismatch. Heteroepitaxial material integration is limited by lattice mismatches in planar systems, but we use a variational approach to show that nanowire heterostructures are more effective at relieving mismatch strain coherently. This is an equilibrium model based on the Matthews critical thickness in which the lattice mismatch strain is shared by the nanowire overlayer and underlayer, and could as well be partially accomodated by the introduction of a pair of misfit dislocations. The model is highly portable to other nanowire material systems and can be used to estimate critical feature sizes. We find that the critical radius of this system is roughly an order of magnitude larger than the critical thickness of the corresponding thin film/substrate material system. Finite element analysis is used to assess some aspects of the model; in particular, to show that the variational approach describes well the decay of the strain energy density away from the interface.
Reduced subpicosecond electron relaxation in GaNxAs1−x
Authors: Sinning S. , Dekorsy T. , Helm M. , Mussler G. , Däweritz L. , Ploog K. H.
Published: June 2005

We report on time resolved femtosecond carrier dynamics in molecular beam epitaxy grown GaNxAs1−x with a nitrogen fraction of 1.3%. The intraband carrier relaxation time in GaNxAs1−x is found to be significantly larger than in GaAs. We compare the experimental results with carrier-polar optical phonon scattering rates calculated within the band anticrossing model. From the results we conclude that the slowing down of the carrier relaxation is a result of the strongly modified band structure in GaNxAs1−x.
Characterization of nanotextured AlN thin films by x-ray absorption near-edge structures
Authors: Suga T. , Kameyama S. , Yoshioka S. , Yamamoto T. , Tanaka I. , Mizoguchi T.
Published: June 2005

AlN thin films have been grown on c-cut sapphire substrates by pulsed-laser deposition. The film epitaxially grown at 1073 K under vacuum of 5×10−4Pa was used to examine the crystallographic orientation dependence of Al K-edge x-ray absorption near-edge structures (XANES), which satisfactorily agrees with theoretical spectra obtained by first-principles calculations. The film grown at 1073 K with N2 backfill of 7×10−2Pa shows nanotextured structure with its c plane parallel to the substrate. Although the nanotexture is not evident by x-ray diffraction, XANES can unambiguously indicate the texturing. Cross-sectional high-resolution electron microscopy provides the evidence of the nanostructure.
Sedimentation of spheres at small Reynolds number
Authors: Felderhof B. U.
Published: May 2005

The effect of fluid inertia on the settling of spheres in a viscous incompressible fluid is studied in the limit of small Reynolds number. The kinetic energy of flow depends on the positions of the spheres, and gives rise to forces on the spheres. In the dilute limit it suffices to study the corresponding pair interaction. The interaction is calculated from the Stokes flow for two spheres settling between plane walls in the point particle limit. The dissipative interaction between a pair of spheres is calculated from the Proudman–Pearson [I. Proudman and J. R. A. Pearson, J. Fluid Mech. 2, 237 (1957)] solution of the Navier–Stokes equations for flow about a sphere in unbounded geometry. The combination of kinetic and dissipative interaction gives rise to a repulsive force of range of the order of the sphere diameter divided by the Reynolds number.
Bonding properties and their relation to residual stress and refractive index of amorphous Ta(N,O) films investigated by x-ray absorption spectroscopy
Authors: Jan J. C. , Babu P. D. , Tsai H. M. , Pao C. W. , Chiou J. W. , Ray S. C. , Kumar K. P. Krishna , Pong W. F. , Tsai M.-H. , Jong C. A. , Chin T. S.
Published: June 2005

This work presents N, O K-edge x-ray absorption near-edge structure (XANES) and Ta L3-edge extended x-ray absorption fine structure (EXAFS) studies of amorphous Ta(N,O) films prepared with various flow rate ratios of N2∕O2. The N and O K-edge XANES and Ta L3-edge EXAFS spectra demonstrate the presence of N2 molecules. These spectra also show that Ta(N,O) films have similar local atomic structure as that of Ta2O5. No evidence of the formation of Ta–N bond was obtained. The intensities of the π* feature in the N K-edge spectra and the features of O2p–Ta5d hybridized states were found to correlate with the residual stress and the refractive index, respectively.
Magnetic properties of MnN: Influence of strain and crystal structure
Authors: Marques M. , Teles L. K. , Scolfaro L. M. R. , Furthmüller J. , Bechstedt F. , Ferreira L. G.
Published: June 2005

For manganese mononitride (MnN), the total energy versus lattice constant is obtained using the spin density functional theory. Instead of the tetragonally distorted NaCl structure, we study the zinc blende and wurtzite structures in which AlN, GaN, and InN crystallize. The ground state with nonmagnetic, antiferromagnetic (AFM), or ferromagnetic (FM) arrangement of spins depends on the polymorph of MnN and on the lattice constant. At equilibrium lattice constants, in zinc blende it is AFM in [100] direction, and in wurtzite it is FM. The zinc blende polytype of MnN under hydrostatic pressure at the InN lattice constant presents FM ground state. For the wurtzite polytype at the GaN and AlN lattice constants, the AFM is the ground state, but goes back to a FM ground state for the InN lattice constants. For both structures, the system presents a half-metallic state at InN lattice constants (with a total magnetic moment of 4μB per Mn atom) instead of the metallic state obtained for smaller lattice constants. Results indicate that the FM or the AFM state of Ga1−xMnxN and In1−xMnxN may be related to, relaxed, or strained, MnN incorporations or Mn-rich composition fluctuations.
Femtosecond laser microprinting of biomaterials
Authors: Zergioti I. , Karaiskou A. , Papazoglou D. G. , Fotakis C. , Kapsetaki M. , Kafetzopoulos D.
Published: June 2005

This Letter demonstrates a laser rapid prototyping method that can be used for fabricating high-density resolution patterns of biomaterials. Ultraviolet femtosecond laser pulses have been used for directly printing a wide range of biomaterials in complicated patterns and structures. The ultrashort laser pulses reduce the thermal effects, thus allowing the effective deposition of sensitive biomaterials at high spatial resolution for microfabricating patterns. We present the microprinting of different biomaterial patterns, such as DNA (deoxyribonucleic acid) and proteins, with spatial resolution down to 50μm and we demonstrate that they maintain their properties and biological functions and, thus, can be practically used as biosensors.
Decomposing total IR spectra of aqueous systems into solute and solvent contributions: A computational approach using maximally localized Wannier orbitals
Authors: Iftimie Radu , Tuckerman Mark E.
Published: June 2005

The theoretical principles underpinning the calculation of infrared spectra for condensed-phase systems in the context of ab initio molecular dynamics have been recently developed in literature. At present, most ab initio molecular dynamics calculations are restricted to relatively small systems and short simulation times. In this paper we devise a method that allows well-converged results for infrared spectra from ab initio molecular dynamics simulations using small systems and short trajectories characteristic of simulations typically performed in practice. We demonstrate the utility of our approach by computing the imaginary part of the dielectric constant ε″(ω) for H2O and D2O in solid and liquid phases and show that it compares well with experimental data. We further demonstrate that maximally localized Wannier orbitals can be used to separate the individual contributions of different molecular species to the linear spectrum of complex systems. The new spectral decomposition method is shown to be useful in present-day ab initio molecular dynamics calculations to compute the magnitude of the “continuous absorption” generated by excess protons in aqueous solutions with good accuracy even when other species present in the solutions absorb strongly in the same frequency window.
A molecular model for cohesive slip at polymer melt/solid interfaces
Authors: Tchesnokov M. A. , Molenaar J. , Slot J. J. M. , Stepanyan R.
Published: June 2005

A molecular model is proposed which predicts wall slip by disentanglement of polymer chains adsorbed on a wall from those in the polymer bulk. The dynamics of the near-wall boundary layer is found to be governed by a nonlinear equation of motion, which accounts for such mechanisms on surface chains as convection, retraction, constraint release, and thermal fluctuations. This equation is valid over a wide range of grafting regimes, including those in which interactions between neighboring adsorbed molecules become essential. It is not closed since the dynamics of adsorbed chains is shown to be coupled to that of polymer chains in the bulk via constraint release. The constitutive equations for the layer and bulk, together with continuity of stress and velocity, are found to form a closed system of equations which governs the dynamics of the whole “bulk+boundary layer” ensemble. Its solution provides a stick-slip law in terms of the molecular parameters and extruder geometry. The model is quantitative and contains only those parameters that can be measured directly, or extracted from independent rheological measurements. The model predictions show a good agreement with available experimental data.
One-step waveguide and optical circuit writing in photopolymerizable materials processed by two-photon absorption
Authors: Klein Stéphane , Barsella Alberto , Leblond Hervé , Bulou Hervé , Fort Alain , Andraud Chantal , Lemercier G. , Mulatier J. C. , Dorkenoo Kokou
Published: June 2005

Two-photon absorption process is known to be a convenient tool to create three-dimensional microstructures in photopolymerizable materials. In this context, we have fabricated stable optical waveguides. The features of these waveguides (in particular, transmission losses) have been compared to the results of numerical simulations. We have also demonstrated the possibility of connecting two optical fibers via a curved guide and to realize Y splitters. The technique allows one to fabricate operational integrated optical circuits in photopolymerizable resins.
Homogeneous nucleation rate measurements of 1-butanol in helium: A comparative study of a thermal diffusion cloud chamber and a laminar flow diffusion chamber
Authors: Brus David , Hyvärinen Antti-Pekka , Ždímal Vladimír , Lihavainen Heikki
Published: June 2005

Isothermal homogeneous nucleation rates of 1-butanol were measured both in a thermal diffusion cloud chamber and in a laminar flow diffusion chamber built recently at the Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Prague, Czech Republic. The chosen system 1-butanol-helium can be studied reasonably well in both devices, in the overlapping range of temperatures. The results were compared with those found in the literature and those measured by Lihavainen in a laminar flow diffusion chamber of a similar design. The same isotherms measured with the thermal diffusion cloud chamber occur at highest saturation ratios of the three devices. Isotherms measured with the two laminar flow diffusion chambers are reasonably close together; the measurements by Lihavainen occur at lowest saturation ratios. The temperature dependences observed were similar in all three devices. The molecular content of critical clusters was calculated using the nucleation theorem and compared with the Kelvin equation. Both laminar flow diffusion chambers provided very similar sizes slightly above the Kelvin equation, whereas the thermal diffusion cloud chamber suggests critical cluster sizes significantly smaller. The results found elsewhere in the literature were in reasonable agreement with our results.
Divergent subcritical convection in magnetized plasma from asymmetric sourcing
Authors: Adler D. T. , Hassam A. B.
Published: June 2005

Asymmetric particle and heat sourcing in a plasma confined in a closed magnetic field line configuration results in convection cells, as is well known. This phenomenon occurs even if the sourcing on average produces density and temperature profiles that are subcritical, i.e., magnetohydrodynamically stable to interchange modes. Such subcritical convection is expected to be small compared to the convection from supercritical driving for which the system is interchange unstable. The ratio of subcritical to supercritical convection is expected to scale as the inverse Reynolds numbers (for large Reynolds numbers). It is shown that this ratio is, in fact, considerably larger. As marginal stability is approached, the subcritical convection grows from very small to almost the unstable convection size, i.e., of order unity. This effect may be similar to why a driven, damped harmonic oscillator increases in amplitude as resonance is approached. A numerical simulation is done to demonstrate this effect. It is also shown that transport from the large convection can be substantial.
Desorption dynamics of oxide nanostructures fabricated by local anodic oxidation nanolithography
Authors: Mori G. , Lazzarino M. , Ercolani D. , Sorba L. , Heun S. , Locatelli A.
Published: June 2005

We studied the properties of GaAs oxides which were grown by local anodic oxidation (LAO) nanolithography using an atomic force microscope. We find that the LAO structures desorb under irradiation with soft x-rays (130eV). We analyzed the desorption process in detail by time-resolved photoelectron spectroscopy. We observe that even in the first stages of light exposure the LAO oxide is mainly composed of Ga2O, with a small fraction of Ga2O3 and As oxides. The As oxides are located only in the surface layers of the LAO oxide where they account for 10% of the oxide. Within 160minutes of exposure they completely desorb. Moreover, we find evidence for the presence of unoxidized GaAs embedded in the LAO oxide.
Direct measurement of the energy thresholds to conformational isomerization in Tryptamine: Experiment and theory
Authors: Clarkson Jasper R. , Dian Brian C. , Moriggi Loïck , DeFusco Albert , McCarthy Valerie , Jordan Kenneth D. , Zwier Timothy S.
Published: June 2005

The methods of stimulated emission pumping-hole filling spectroscopy (SEP-HFS) and stimulated emission pumping population transfer spectroscopy (SEP-PTS) were applied to the conformation-specific study of conformational isomerization in tryptamine [TRA, 3-(2-aminoethyl)indole]. These experimental methods employ stimulated emission pumping to selectively excite a fraction of the population of a single conformation of TRA to well-defined ground-state vibrational levels. This produces single conformations with well-defined internal energy, tunable over a range of energies from near the zero-point level to well above the lowest barriers to conformational isomerization. When the SEP step overcomes a barrier to isomerization, a fraction of the excited population isomerizes to form that product. By carrying out SEP excitation early in a supersonic expansion, these product molecules are subsequently cooled to their zero-point vibrational levels, where they can be detected downstream with a third tunable laser that probes the ground-state population of a particular product conformer via a unique ultraviolet transition using laser-induced fluorescence. The population transfer spectra (recorded by tuning the SEP dump laser while holding the pump and probe lasers fixed) exhibit sharp onsets that directly determine the energy thresholds for conformational isomerization in a given reactant-product conformer pair. In the absence of tunneling effects, the first observed transition in a given X−Y PTS constitutes an upper bound to the energy barrier to conformational isomerization, while the last transition not observed constitutes a lower bound. The bounds for isomerizing conformer A of tryptamine to B(688–748cm−1), C(1)(860–1000cm−1), C(2)(1219–1316cm−1), D(1219–1282cm−1), E(1219–1316cm−1), and F(688–748cm−1) are determined. In addition, thresholds for isomerizing from B to A(<1562cm−1), B to F(562–688cm−1), and out of C(2) to B(<747cm−1) are also determined. The A→B and B→A transitions are used to place bounds on the relative energies of minima B relative to A, with B lying at least 126cm−1 above A. The corresponding barriers have been computed using both density functional and second-order many-body perturbation theory methods in order to establish the level of theory needed to reproduce experimental results. While most of the computed barriers match experiment well, the barriers for the A–F and B–F transitions are too high by almost a factor of 2. Possible reasons for this discrepancy are discussed.
Theoretical analysis of growth of ZnO nanorods on the amorphous surfaces
Authors: Kwon S. Joon , Park Jae-Gwan
Published: June 2005

Semiconductor nanorod arrays on a substrate have a preferential alignment orientation that minimizes the excessive free energy of the system. In the case of wet chemically synthesized zinc oxide (ZnO) nanorod on the amorphous surfaces, the thermodynamic driving force determines the orientation to be normal to the surface. Among the various kinds of amorphous surfaces, the spherical seed layer composed of ZnO precursors gives isotropic radially aligned arrays. For other surfaces, such as wrinkled and planar ZnO precursor thin film, nanorod arrays are aligned to be perpendicular to the tangential line of the surface. The maximum value of the aspect ratio of the nanorod is determined by the thermodynamic relationship. The number density of nanorods per unit precursor particles decreases with increasing contact angle of the seed particles.
Diffusion exchange NMR spectroscopic study of dextran exchange through polyelectrolyte multilayer capsules
Authors: Qiao Y. , Galvosas P. , Adalsteinsson T. , Schönhoff M. , Callaghan P. T.
Published: June 2005

Diffusion exchange of dextran with molecular weights 4.4 and 77 kDa through polyelectrolyte multilayer (PEM) hollow capsules consisting of four bilayers of polystyrene sulfonate/polydiallyldimethylammonium chloride has been investigated using two-dimensional nuclear-magnetic-resonance methods: diffusion–diffusion exchange spectroscopy (DEXSY) and diffusion–relaxation correlation spectroscopy (DRCOSY). Results obtained in DRCOSY experiments show that the diffusion process of dextran 77 kDa exhibits an observation time dependence suggesting a diffusion behavior restricted by confinement. We find evidence for both single capsule and capsule aggregate states, with a partitioning of the 77-kDa dextran between the free and capsule states much larger than that suggested by volume fraction alone. Results from DEXSY experiments show that dextran 77 kDa is in diffusive exchange through the capsules with an exchange time of around 1 s. In contrast, the capsules have no detectable influence on the diffusion process of the dextran 4.4 kDa. This quantitative information may be used in designing PEM capsules as drug carriers.
Characterization of pulsed plasma in unbalanced magnetron argon discharge
Authors: Lee Sang-Won , Seo Sang-Hun , In Jung-Hwan , Chung Chin-Wook , Chang Hong-Young
Published: June 2005

Time-resolved probe measurements have been conducted in a unipolar pulsed dc magnetron system to investigate the temporal evolution of plasma parameters, such as the electron density and the electron temperature, and to determine how the driving frequency and the duty cycle of the dc pulse affect these parameters. A new probe measurement system was employed to perform the time-resolved measurements of I-V characteristic curves and their second derivatives with a maximum time resolution of 100 ns. The measurements were performed in a constant voltage mode, constant current mode, and constant power mode, with various pulse frequencies ranging from 5 kHz to 50 kHz and duty cycles ranging from 10% to 90%, to investigate the detailed temporal evolutions of the electron energy distribution function and the plasma parameters. The results show that as the pulse frequency increases, the electron density and the electron temperature exhibit insignificant changes. However, the reduction of the duty cycles results in a significant increase of the electron temperature, irrespective of the operating mode. A comparison of the measured electron energy distribution functions shows that the increase in the electron temperature is caused by a decrease in the population of trapped low-energy electrons and∕or by an increase in the population of drifting high-energy electrons. This result can be explained by considering the electron heating due to the deep-penetrating cathode sheath.
Helimagnetic order in the re-entrant ferromagnet Ce2Fe15.3Mn1.7
Authors: Prokhnenko O. , Arnold Z. , Kamarád J. , Ritter C. , Isnard O. , Kuchin A.
Published: June 2005

Substitution of Mn for Fe in Ce2Fe17 strongly suppresses the ferromagnetic ground state, changing it into an antiferromagnetic one. However, a state of spontaneous magnetization is re-entered in the Ce2Fe17−xMnx compounds when the Mn content reaches x=1.3. Magnetic structures and magnetic interactions in the re-entrant ferromagnetic Ce2Fe15.3Mn1.7 compound have been studied using both microscopic (neutron powder diffraction in magnetic fields of up to 0.7T) and macroscopic (ac susceptibility and magnetization measurements under high pressures of up to 8kbar in magnetic fields of up to 5T) techniques. A spiral magnetic structure distorted by the presence of an in-plane ferromagnetic component is found below TN=182K. Enhancement of the ferromagnetic interactions leads to a steep transformation of the magnetic spiral to a magnetic structure with nonzero spontaneous magnetization below ΘT=59K. This structure is described by a distorted elliptical spiral model with an incommensurate propagation vector along the c axis. Magnetic field significantly expands the range of stability of this re-entrant ferromagnetic phase. In contrast to the magnetic field, application of pressure does not change ΘT within the achieved pressure range. It leads to a moderate reduction of the Néel temperature, dTN∕dP=−2.3K∕kbar. The preferential placement of Mn on some crystallographic sites in Ce2Fe17−xMnx and the related changes of volume and internal magnetic field will be discussed as possible reasons of the peculiar magnetic behavior of this compound.
The molecular structure and orientation of antiferroelectric liquid crystal using the density-functional theory and two-dimensional correlation-polarized infrared spectroscopy
Authors: Cheng Jianbo , Cheng Yuchuan , Ruan Weidong , Xu Weiqing , Zhao Bing , Zhang Gang
Published: June 2005

The structure and vibrational frequencies of the chiral antiferroelectric liquid-crystal molecule, 4-(1-methyheptyloxycarbonyl) phenyl-4-(4′-octyloxy) benzoate (MHOCPOOB), have been calculated using the density-functional theory (DFT) with the Becke-3 Lee–Yang–Parr/6-31G(d,p) level. The observed vibrational spectra have been resolved and assigned in detail by comparison to the computed values. The results indicate that the computed and observed spectra are in good agreement with each other. The stable molecular structure obtained with the DFT theory shows that the two hydrocarbon chains are all-trans zigzag conformer and nearly perpendicular to each other. The orientation of the mesogen part and the hydrocarbon chains for MHOCPOOB in the Sm–CA* phase are investigated by employing the polarization-angle-dependent infrared spectra in the electric-field induced and the two-dimensional correlation spectroscopy. After combining the experimental and theoretical results, it can be concluded that the azimuth of the achiral and chiral chains is opposite to each other, the orientation of the achiral chain is almost the same direction as the mesogen core, and the orientation of the chiral chain is nearly perpendicular to the mesogen part. The achiral and chiral CH2 chains are both a probable all-trans zigzag conformer.
A model for nonexponential relaxation and aging in dissipative systems
Authors: Pérez-Madrid A.
Published: June 2005

The nonexponential relaxation and aging inherent to complex dynamics manifested in a wide variety of dissipative systems are analyzed through a model of diffusion in phase space in the presence of a nonconservative force. The action of this force establishes a heat flow which maintains the system away from equilibrium. The inability of the system to find its equilibrium state becomes apparent through the presence of an effective temperature field. This is the temperature of the stationary nonequilibrium state reached by the system satisfying a generalized version of the fluctuation-dissipation theorem. The presence of a nonequilibrium temperature leads to a hierarchy of relaxation times responsible for the aging phenomena and to a relation similar to the Vogel–Fulcher–Tammann law [H. Vogel, Phys. Z. 22, 645 (1921); G. S. Fulcher, J. Am. Ceram. Soc. 8, 339 (1925); 8, 789 (1925); G. Tammann and W. Hesse, Z. Anorg. Allg. Chem. 156, 245 (1926)].
Microwave spectra of the Xe–N2 van der Waals complex: A comparison of experiment and theory
Authors: Wen Qing , Jäger Wolfgang
Published: June 2005

Rotational transitions for the Xe–N2 complex were measured in the frequency region from 4to18GHz using a pulsed-nozzle Fourier-transform microwave spectrometer. Twelve (four) a-type transitions were recorded for the Xe132–N214 and Xe129–N214 (Xe131–N215) isotopomers. In addition, the nuclear quadrupole hyperfine structures due to the presence of the N14 (nuclear-spin quantum number I=1) and Xe131 (I=3∕2) nuclei were detected and analyzed. Two ab initio potential-energy surfaces were calculated at the coupled-cluster level of theory with single, double, and pertubatively included triple excitations. Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set was used for the nitrogen atoms. For the first surface, a well-tempered basis set with additional polarization functions was used for the Xe atom; for the second surface, a newly developed augmented correlation-consistent polarized valence quintuple-zeta basis set employing small-core relativistic pseudopotentials was used for the Xe atom. The basis sets were supplemented with bond functions for the van der Waals bond. The counterpoise correction was applied to reduce the basis-set superposition error. The resulting two surfaces both have a single minimum at a T-shaped geometry, with well depths of 122.4 and 119.3cm−1, respectively. Bound-state energies supported by the potential-energy surface were determined. The quality of the ab initio potential-energy surfaces was evaluated by comparison of the experimental transition frequencies and rotational and centrifugal distortion constants with those derived from the bound-state energies. A scaled potential-energy surface was obtained which has excellent agreement with the experimental data.
High depth nondestructive stress measurements on thick steel alloys
Authors: Selim F. A. , Wells D. P. , Harmon J. F. , Williams J.
Published: June 2005

Stress measurements were performed using accelerator-based γ-ray induced positron annihilation spectroscopy technique, which allows probing of defects at high depths in thick materials up to several centimeters. Induced stresses due to tensile, fatigue, cold work, and bending tests were investigated in steel alloys of about 1-cm thickness. The measurements showed the dependence of the line-shape parameter of the annihilation peak S on the induced deformation in the four tests. They also revealed an interesting behavior for the change of S parameter with tensile deformation, related to the engineering stress-strain curve of the material. Transmission electron microscopy measurements of dislocation density in cold work deformation suggested that the saturation of positron annihilation parameters often observed in cold work data is not due to compete positron trapping at defects. It was also shown that the S parameter has a weak sensitivity and quickly saturates in fatigue test when compared with the other mechanical tests, which was interpreted as being due to the brittle nature of fatigue failure and the very little plastic deformation associated with it.
Kinetics of protein binding in solid-phase immunoassays: Theory
Authors: Klenin Konstantin V. , Kusnezow Wlad , Langowski Jörg
Published: June 2005

In a solid-phase immunoassay, binding between an antigen and its specific antibody takes place at the boundary of a liquid and a solid phase. One of the reactants (receptor) is immobilized on a surface. The other reactant (ligand) is initially free in solution. We present a theory describing the kinetics of immunochemical reaction in such a system. A single essential restriction of the theory is the assumption that the reaction conditions are uniform along the binding surface. In general, the reaction rate as a function of time can be obtained numerically as a solution of a nonlinear integral equation. For some special cases, analytical solutions are available. Various immunoassay geometries are considered, in particular, the case when the reaction is carried out on a microspot.
Ion energy spectrum of expanding laser-plasma with limited mass
Authors: Murakami M. , Kang Y.-G. , Nishihara K. , Fujioka S. , Nishimura H.
Published: June 2005

A simple analytical model is presented for hydrodynamic expansion of laser-produced plasma with a limited mass, which expands quasi-isothermally during laser irradiation and quasiadiabatically after turning off the laser. During the isothermal expansion, the masses undergo entire disintegration under a relatively long laser pulse, while the ions are being kept accelerated. This physical picture significantly contrasts with that described by the orthodox self-similar solution for a semi-infinite planar rarefaction wave. The two successive expansions, i.e., isothermal expansion followed by adiabatic expansion, are described, respectively, by different self-similar solutions, which are found to be connected smoothly with each other in time and space. The ion energy spectrum obtained by the model reproduces well experimental results obtained under different geometrical conditions. The maximum ion kinetic energy is also estimated in terms of the laser and target parameters.
Effect of charge fluctuations and collisions on the energy loss of N×M projectiles for a generalized Lorentzian dusty plasma
Authors: Sarwar M. Adnan , Mirza Arshad M.
Published: June 2005

The shielded potential and the energy loss by N×M projectiles passing through a collisional dust-contaminated plasma with dust-charge fluctuations and grain-size distribution are presented. Analytical general expressions are obtained for the shielded potential and for the energy loss by considering two-body correlation effects. An interference contribution of these projectiles to the potential and energy loss is observed which depends upon their orientation and separation distance. The dust-charge fluctuation produces a potential well instead of Coulomb-type potential for a slowly moving test charge with slow charge relaxation rate and energy is gained by the charged projectiles. However, fast charge relaxation enhances the energy loss and some peaks are observed showing the excitation of some electrostatic modes. On the other hand, the dust neutral collisions also enhance the energy loss for projectile velocities greater than the dust acoustic speed for a Maxwellian plasma (for a large value of the spectral index κ).
A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows
Authors: Chaouat Bruno , Schiestel Roland
Published: June 2005

A new subgrid-scale turbulence model involving all the transport equations of the subgrid-scale stresses and including a dissipation rate equation is proposed for large-eddy simulation (LES) of unsteady flows which present nonequilibrium turbulence spectra. Such a situation in flow physics occurs when unsteadiness is created by forced boundary conditions, but also in more complex situations, when natural unsteadiness develops due to the existence of organized eddies. This latter phenomenon explains the instability found in a porous-walled chamber with mass injection. Due to the high value of Reynolds number, the presence of wall boundaries, and the use of relatively coarse grids, the spectral cutoff may be located before the inertial zone of the energy spectrum. The use of transport equations for all the subgrid-scale stress components allows us to take into account more precisely the turbulent processes of production, transfer, pressure redistribution effects, and dissipation, and the concept of turbulent viscosity is no longer necessary. Moreover, some backscatter effects can possibly arise. As a result of modeling in the spectral space, a formally continuous derivation of the model is obtained when the cutoff location is varied, which guaranties compatibility with the two extreme limits that are the full statistical Reynolds stress transport model of Launder and Shima and direct numerical simulation. In the present approach, due to the presence of the subgrid-scale pressure-strain correlation term in the stress equations, the new subgrid model is able to account for history and nonlocal effects of the turbulence interactions, and also to describe more accurately the anisotropy of the turbulence field. The present model is first calibrated on the well-known fully turbulent channel flow. For this test case, the LES simulation reveals that the computed velocities and Reynolds stresses agree very well with the DNS data. The application to the channel flow with wall mass injection which undergoes a transition process from laminar to turbulent regime and the development of natural unsteadiness is then considered for illustrating the potentials of the method. LES results are compared with experimental data including the velocity components, the turbulent stresses, and the transition location. A satisfactory agreement is obtained for both the mean quantities and the turbulent field. In addition, structural information of the flow is provided.
On the thickness uniformity of micropatterns of hyaluronic acid in a soft lithographic molding method
Authors: Jeong Hoon Eui , Suh Kahp Y.
Published: May 2005

A soft lithographic molding is a simple and yet robust method for fabricating well-defined microstructures of a hydrophilic biopolymer such as polyethylene glycol and polysaccharide over a large area. The method consists of three steps: placing a polydimethylsiloxane mold with a bas-relief pattern onto a drop-dispensed polymer solution typically dissolved in water, letting the mold and the solution undisturbed in contact until solvent evaporates completely, and leaving behind a polymer replica after mold removal. In such a molding process, water can only evaporate from the edges of the mold due to impermeable nature of polydimethylsiloxane to water, resulting in a nonuniform distribution of film thickness or pattern height. Here we examine systematically how the evaporation rate affects the thickness distribution of the resulting microstructures by evaporating the solution of hyaluronic acid in various conditions. To compare with a theory, we also present a simple theoretical model based on one-dimensional conservation equation for a liquid film, which is in good agreement with the experimental data.
Imaging and quantifying mixing in a model droplet micromixer
Authors: Stone Z. B. , Stone H. A.
Published: June 2005

Rapid mixing is essential in a variety of microfluidic applications but is often difficult to achieve at low Reynolds numbers. Inspired by a recently developed microdevice that mixes reagents in droplets, which simply flow along a periodic serpentine channel [H. Song, J. D. Tice, and R. F. Ismagilov, “A microfluidic system for controlling reaction networks in time,” Angew. Chem. Int. Ed. 42, 767 (2003)], we investigate a model “droplet mixer.” The model consists of a spherical droplet immersed in a periodic sequence of distinct external flows, which are superpositions of uniform and shear flows. We label the fluid inside the droplet with two colors and visualize mixing with a method we call “backtrace imaging,” which allows us to render cross sections of the droplet at arbitrary times during the mixing cycle. To analyze our results, we present a novel scalar measure of mixing that permits us to locate sets of parameters that optimize mixing over a small number of flow cycles.