bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomagnetic conjugate observations of large‐scale traveling ionospheric disturbances using GPS networks in Japan and Australia
Authors: Tsugawa Takuya , Shiokawa Kazuo , Otsuka Yuichi , Ogawa Tadahiko , Saito Akinori , Nishioka Michi
Published: February 2006

The geomagnetic conjugacy of large‐scale traveling ionospheric disturbances (LSTIDs) was studied using total electron content (TEC) data derived from GPS networks in Japan and Australia. The number of simultaneous occurrences of LSTIDs within ±1 hour in both hemispheres was 5 out of 20 (21) events in Japan (Australia) when Kp ≥ 5− and 0 out of 15 (10) when Kp ≤ 4+. As for the LSTIDs observed simultaneously in both hemispheres, the propagation velocities of equatorward LSTID were comparable between the two hemispheres, with differences of 10–40%. The crossing times at 30° geomagnetic latitude of the simultaneous LSTIDs over Japan and Australia were also different by several tens of minutes for all five events. These observational results indicate that the LSTIDs observed almost simultaneously in both hemispheres are not connected electromagnetically through the geomagnetic field but are generated by atmospheric gravity waves propagating to the equator independently in the two hemispheres.
Lithospheric structure of Tasmania from a novel form of teleseismic tomography
Authors: Rawlinson N. , Reading A. M. , Kennett B. L. N.
Published: February 2006

In 2001 and 2002, a temporary array of 72 seismic recorders was deployed across northern Tasmania (SE Australia), with the aim of imaging the underlying crust and upper mantle using three‐dimensional (3‐D) teleseismic tomography. Using a recently developed adaptive stacking technique, 6520 relative P wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period. A novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method, a grid‐based eikonal solver, is used to map the residual patterns as P wave velocity anomalies. The new scheme proves to be both fast and robust, making it well suited to large data sets and the reconstruction of complex anomalies. The resultant tomographic images of Tasmania exhibit significant lateral perturbations in P wave velocity structure (≤5%) from a 1‐D reference model. A marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern Tasmania is underlain by dense rocks with an oceanic crustal affinity, contrasting with the continentally derived siliciclastic core of western Tasmania. Significantly, the Tamar Fracture System does not overlie the narrow transition from relatively fast to slow velocities, which suggests that it may be a near‐surface feature rather than a manifestation of deeper crustal‐scale suturing as previously thought. Farther west, an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group and Arthur Lineament may be related to remnant subduction of oceanic lithosphere associated with the mid‐Cambrian Delamerian Orogeny.
A 44 kyr paleoroughness record of the Antarctic surface
Authors: Barnes P. R. F. , Wolff E. W. , Mulvaney R.
Published: February 2006

Two 788 m conductivity records from ice cores drilled at Dome C, Antarctica, provide an unprecedented opportunity to examine the past roughness of the Antarctic surface. By measuring the distribution of depth differences between synchronous events in the cores, the surface height distribution can be estimated during time intervals in the past. For the first time we publish a record of this type and consider its paleoclimatic implications. The paleoroughness, originating from sastrugi and dunes on the ice sheet surface, is hypothesized to be related to the wind speed, temperature, and accumulation rate during the period of preservation. The roughness record indicates only a slight decrease in the preserved surface roughness from the last glacial period (0.031 m root mean square (r.m.s.) of surface deviation) to the present (0.029 m r.m.s.). This result is surprising given the large change in temperature and accumulation rate that occurred during the last climatic transition. However, it could be consistent with modeling results suggesting low wind speeds on the East Antarctic Plateau during the last ice age if the possible influences of the accumulation and temperature changes are ignored. Additionally, the reliability of the Dome C ice core record is assessed, and the probability of short‐term events being missing from the profile is determined. These results are of particular interest when constraining the proportion of events that may be synchronously matched with other ice cores. The principles and results present here allow inferences about the natural variation in ice core records generally.
Negative cloud‐to‐ground lightning properties from high‐speed video observations
Authors: Saba M. M. F. , Ballarotti M. G. , Pinto O.
Published: February 2006

From analysis of digital high‐speed video records of 233 negative cloud‐to‐ground (CG) lightning flashes associated with 27 thunderstorms in southeastern Brazil, various lightning properties have been determined. The analysis of the video records showed that although 20% of them were single‐stroke flashes and the average number of strokes per flash was 3.8, a significant variation was observed in these parameters from storm to storm. In a smaller subset containing 138 flashes, 70 (51%) had multiple terminations on the ground. As 138 flashes produced 235 different strike points, the average number of strike points per CG flash was 1.70. Considering that in this study the missing of strokes is practically negligible, we can say from the average multiplicity and from the average number of strike points per flash that each ground contact point is, on average, struck 2.2 times. The 608 time intervals between strokes of 186 negative multiple‐stroke flashes presented a geometric mean value of 61 ms. Although time intervals preceding subsequent strokes that create a new termination tend to be greater than intervals between subsequent strokes that follow the previously formed channel, the difference was not statistically significant. A strong positive correlation between the number of subsequent strokes in a flash and the flash minimum duration may indicate that some processes concerning the time requisite for the channel decay and for the positive leader in the cloud to provide more charges for the next stroke do not permit multiple strokes to occur under a certain minimum time duration.
Changes in the character of stream water dissolved organic carbon during flushing in three small watersheds, Oregon
Authors: Hood Eran , Gooseff Michael N. , Johnson Sherri L.
Published: February 2006

The hydrologic and biogeochemical responses of forested watersheds to inputs of rainfall and snowmelt can be an indicator of internal watershed function. In this study, we assess how the quantity and quality, both chemical and spectroscopic, of stream water DOC changes in response to a 6‐day storm event during the wet season of 2003 in three small (<1 km2) basins in the H. J. Andrews Experimental Forest, Oregon. The watersheds included one old‐growth watershed (WS02) and two previously logged watersheds (WS01 and WS10). Prestorm concentrations of DOC ranged from 1.5 to 2.2 mg C L−1 in the three watersheds and increased approximately threefold on the ascending limb of the storm hydrograph. Concentrations of DOC were both highest in the unharvested, old‐growth watershed. The specific UV absorbance (SUVA, 254 nm) of DOC in the three watersheds increased by 9 to 36% during the storm, suggesting that DOC mobilized from catchment soils during storms is more aromatic than DOC entering the stream during baseflow. The increase in SUVA was most pronounced in the previously harvested catchments. Chromatographic fractionation of DOC showed that the percentage of DOC composed of non‐humic material decreasing by 9 to 22% during the storm. Shifts in the fluorescence properties of DOC suggest that there was not a pronounced change in the relative proportion of stream water DOC derived from allochthonous versus autochthonous precursor material. Taken together, these results suggest that spectroscopic and chemical characterization of DOC can be used as tools to investigate changing sources of DOC and water within forested watersheds.
Space Weather & Telecommunications
Authors: Kunches Joseph M.
Published: February 2006

Kluwer International Series in Engineering and Computer Science, Volume 782; Springer; 388 pp.; hardcover; ISBN 0‐387‐23670‐8; 2005; $159.00
Extension of thickened and hot lithospheres: Inferences from laboratory modeling
Authors: Tirel C. , Brun J.‐P. , Sokoutis D.
Published: February 2006

The extension of a previously thickened lithosphere is studied through a series of analogue experiments. The models deformed in free and boundary‐controlled gravity spreading conditions that simulate the development of wide rift‐type and core complex‐type structures. In models, the development of structures mainly depends on boundary velocity and therefore on bulk strain rate. Wide rifts are of tilted block‐type at high strain rate and of horst‐ and graben‐type at low strain rate. The development of metamorphic core complex‐type structures is enhanced by low strain rates and by the presence of weak heterogeneities within the ductile crust. Core complexes result from a necking instability of the upper crust creating a graben, which further widens, allowing the rise and exhumation of a ductile layer dome. An upward convex detachment, flat on top of the dome and steeper on dome limb, appears not to be the primary cause of the core complex development but its consequence.
Generation of protein kinase Ck1α mutants which discriminate between canonical and non-canonical substrates
Authors: Bustos Victor H. , Marin Oriano , Meggio Flavio , Cesaro Luca , Allende Catherine C. , Allende Jorge E. , Pinna Lorenzo A.
Published: February 2006

Protein kinase CK1 denotes a family of pleiotropic serine/threonine protein kinases implicated in a variety of cellular functions. Typically, CK1 acts as a ‘phosphate-directed’ kinase whose targeting is primed by a single phosphorylated side chain at position n−3 or n−4 relative to serine/threonine, but increasing evidence is accumulating that CK1 can also engage some of its substrates at sites that do not conform to this canonical consensus. In the present paper, we show that CK1α phosphorylates with the same efficiency phosphopeptides primed by a phosphoserine residue at either n−3 [pS(−3)] or n−4 [pS(−4)] positions. The phosphorylation efficiency of the pS(−4) peptide, and to a lesser extent that of the pS(−3) peptide, is impaired by the triple mutation of the lysine residues in the K229KQK232 stretch to alanine residues, promoting 40-fold and 6-fold increases of Km respectively. In both cases, the individual mutation of Lys232 is as detrimental as the triple mutation. A kinetic alanine-scan analysis with a series of substituted peptide substrates in which the priming phosphoserine residue was effectively replaced by a cluster of four aspartate residues was also consistent with a crucial role of Lys232 in the recognition of the acidic determinant at position n−4. In sharp contrast, the phosphorylation of β-catenin and of a peptide including the non-canonical β-catenin site (Ser45) lacking acidic/phosphorylated determinants upstream is not significantly affected by mutations in the KKQK stretch. These data provide a molecular insight into the structural features that underlie the site specificity of CK1α and disclose the possibility of developing strategies for the preferential targeting of subsets of CK1 substrates.
Computing the starting state for Gibbs-Duhem integration
Authors: van ’t Hof A. , de Leeuw S. W. , Peters C. J.
Published: February 2006

Gibbs-Duhem integration implies the numerical integration of a Clapeyron equation. To start the numerical integration, an initial coexistence point and a corresponding initial slope of the Clapeyron equation are needed. In order to apply Gibbs-Duhem integration to all kinds of systems at diverse physical conditions, one has to investigate and assess the available methods that can be used to compute these initial values. This publication focuses on vapor-liquid equilibria in binary mixtures comprising chain molecules. The initial coexistence point is either computed with the NVβ Gibbs ensemble or with the Npβ+test molecule method with overlapping distributions, which is introduced in this publication. Although computationally demanding, the Npβ+test molecule method with overlapping distributions is applicable at conditions where the NVβ Gibbs ensemble fails. We investigated three methods that can be employed to compute the initial slope of the Clapeyron equation. The Widom method and the overlapping-distributions difference method provide correct values for the initial slope. The difference method does only provide the correct answer in special cases. The possibility to judge the reliability of the results makes the overlapping-distributions difference method the safest route to the initial slope. Gibbs-Duhem integration requires the frequent computation of the slope of the Clapeyron equation. This slope depends on ensemble averages of the composition. A new bias method for efficient sampling of the composition in a semigrand-canonical simulation of chain molecules is presented. This bias method considerably enhances the composition sampling in systems comprising chain molecules of different sizes.
An advanced Gibbs-Duhem integration method: Theory and applications
Authors: van ’t Hof A. , Peters C. J. , de Leeuw S. W.
Published: February 2006

The conventional Gibbs-Duhem integration method is very convenient for the prediction of phase equilibria of both pure components and mixtures. However, it turns out to be inefficient. The method requires a number of lengthy simulations to predict the state conditions at which phase coexistence occurs. This number is not known from the outset of the numerical integration process. Furthermore, the molecular configurations generated during the simulations are merely used to predict the coexistence condition and not the liquid- and vapor-phase densities and mole fractions at coexistence. In this publication, an advanced Gibbs-Duhem integration method is presented that overcomes above-mentioned disadvantage and inefficiency. The advanced method is a combination of Gibbs-Duhem integration and multiple-histogram reweighting. Application of multiple-histogram reweighting enables the substitution of the unknown number of simulations by a fixed and predetermined number. The advanced method has a retroactive nature; a current simulation improves the predictions of previously computed coexistence points as well. The advanced Gibbs-Duhem integration method has been applied for the prediction of vapor-liquid equilibria of a number of binary mixtures. The method turned out to be very convenient, much faster than the conventional method, and provided smooth simulation results. As the employed force fields perfectly predict pure-component vapor-liquid equilibria, the binary simulations were very well suitable for testing the performance of different sets of combining rules. Employing Lorentz-Hudson-McCoubrey combining rules for interactions between unlike molecules, as opposed to Lorentz-Berthelot combining rules for all interactions, considerably improved the agreement between experimental and simulated data.
Explicitly correlated second-order perturbation theory using density fitting and local approximations
Authors: Werner Hans-Joachim , Manby Frederick R.
Published: February 2006

Three major obstacles in electronic structure theory are the steep scalings of computer time with respect to system size and basis size and the slow convergence of correlation energies in orbital basis sets. Three solutions to these are, respectively, local methods, density fitting, and explicit correlation; in this work, we combine all three to produce a low-order scaling method that can achieve accurate MP2 energies for large systems. The errors introduced by the local approximations into the R12 treatment are analyzed for 16 chemical reactions involving 21molecules. Weak pair approximations, as well as local resolution of the identity approximations, are tested for molecules with up to 49 atoms, over 100 correlated electrons, and over 1000 basis functions.
Geometric noise reduction for multivariate time series
Authors: Mera M. Eugenia , Morán Manuel
Published: February 2006

We propose an algorithm for the reduction of observational noise in chaotic multivariate time series. The algorithm is based on a maximum likelihood criterion, and its goal is to reduce the mean distance of the points of the cleaned time series to the attractor. We give evidence of the convergence of the empirical measure associated with the cleaned time series to the underlying invariant measure, implying the possibility to predict the long run behavior of the true dynamics.
Soliton ratchets in homogeneous nonlinear Klein-Gordon systems
Authors: Morales-Molina Luis , Quintero Niurka R. , Sánchez Angel , Mertens Franz G.
Published: February 2006

We study in detail the ratchetlike dynamics of topological solitons in homogeneous nonlinear Klein-Gordon systems driven by a biharmonic force. By using a collective coordinate approach with two degrees of freedom, namely the center of the soliton, X(t), and its width, l(t), we show, first, that energy is inhomogeneously pumped into the system, generating as result a directed motion; and, second, that the breaking of the time shift symmetry gives rise to a resonance mechanism that takes place whenever the width l(t) oscillates with at least one frequency of the external ac force. In addition, we show that for the appearance of soliton ratchets, it is also necessary to break the time-reversal symmetry. We analyze in detail the effects of dissipation in the system, calculating the average velocity of the soliton as a function of the ac force and the damping. We find current reversal phenomena depending on the parameter choice and discuss the important role played by the phases of the ac force. Our analytical calculations are confirmed by numerical simulations of the full partial differential equations of the sine-Gordon and ϕ4 systems, which are seen to exhibit the same qualitative behavior. Our results show features similar to those obtained in recent experimental work on dissipation induced symmetry breaking.
The effect of impurities on jamming in random sequential adsorption of elongated objects
Authors: Kondrat Grzegorz
Published: February 2006

We consider the jamming aspect of random sequential adsorption of extended particles onto two-dimensional lattice by computer Monte Carlo simulations. The initial presence of impurities on the substrate disturbs this phenomenon significantly and we study here how the size and density of impurity particles affect the resulting jamming threshold. We present the formula for jamming threshold as a closed function of all important parameters (the size of primary particles, the size of impurity particles, and the final density of impurities). The fractal dimension of the space free of impurities is also discussed.
Theoretical investigation of the dissociation dynamics of vibrationally excited vinyl bromide on an ab initio potential-energy surface obtained using modified novelty sampling and feed-forward neural networks
Authors: Doughan D. I. , Raff L. M. , Rockley M. G. , Hagan M. , Agrawal Paras M. , Komanduri R.
Published: February 2006

The reaction dynamics of vibrationally excited vinyl bromide have been investigated using classical trajectory methods on a neural network potential surface that is fitted to an ab initio database of 12 122 configuration energies obtained from electronic structure calculations conducted at the MP4(SDQ) level of theory using a 6-31G(d,p) basis set for the carbon and hydrogen atoms and Huzinaga’s (4333∕433∕4) basis set augmented with split outer s and p orbitals (43321∕4321∕4) and a polarization f orbital with an exponent of 0.5 for the bromine atom. The sampling of the 12-dimensional configuration hyperspace of vinyl bromide prior to execution of the electronic structure calculations is accomplished by combining novelty-sampling methods, chemical intuition, and trajectory sampling on empirical and neural network surfaces. The final potential is obtained using a two-layer feed-forward neural network comprising 38 and 1 neurons, respectively, with hyperbolic tangent sigmoid and linear transfer functions in the hidden and output layers, respectively. The fitting is accomplished using the Levenberg-Marquardt algorithm with early stopping and Bayesian regularization methods to avoid overfitting. The interpolated potentials have a standard deviation from the ab initio results of 0.0578eV, which is within the range generally regarded as “chemical accuracy” for the purposes of electronic structure calculations. It is shown that the potential surface may be easily and conveniently transferred from one research group to another. The files required for transfer of the vinyl bromide surface can be obtained from the Electronic Physics Auxiliary Publication Service. Total dissociation rate coefficients for vinyl bromide are obtained at five different excitation energies between 4.50 and 6.44eV. Branching ratios into each of the six open reaction channels are computed at 24 vibrational energies in the range between 4.00 and 6.44eV. The distribution of vibrational energies in HBr formed via three-center dissociation from vinyl bromide is determined and compared with previous theoretical and experimental results. It is concluded that the combination of ab initio electronic structure calculations, novelty sampling with chemical intuition and trajectories on empirical analytic surfaces, and feed-forward neural networks provides a viable framework in which to execute purely ab initio molecular-dynamics studies on complex systems with multiple open reaction channels.
Anomalous potentials from inverse analyses of interfacial polydisperse attractive colloidal fluids
Authors: Pangburn Todd O. , Bevan Michael A.
Published: February 2006

This paper investigates effects of using monodisperse inverse analyses to extract particle-particle and particle-surface potentials from simulated interfacial colloidal fluids of polydisperse attractive particles. Effects of polydispersity are investigated as functions of particle concentration and attractive well depth and range for van der Waals and depletion potentials. Forward Monte Carlo simulations are used to generate particle distribution functions for polydisperse interfacial colloidal fluids from which inverted potentials are obtained using an inverse Ornstein-Zernike analysis and an inverse Monte Carlo simulation method. Attractive potentials are successfully recovered for monodisperse colloidal fluids, but polydispersity that is unaccounted for in inverse analyses produces (1) apparent softening of strong forces, (2) anomalous repulsive and attractive interactions, and (3) aphysical particle overlaps. This investigation provides insights into the role of polydispersity in altering the equilibrium structure and corresponding inverted potentials of attractive colloidal fluids near surfaces. These findings should assist the design and interpretation of optical microscopy experiments involving interfacial colloidal fluids similar to the simulated experiments reported here.
Molecular mass and dynamics of poly(methyl acrylate) in the glass-transition region
Authors: Metin Burak , Blum Frank D.
Published: February 2006

The segmental dynamics of bulk poly(methyl acrylate) (PMA) were studied as a function of molecular mass in the glass-transition region using H2 NMR and modulated differential scanning calorimetry (MDSC). Quadrupole-echo H2 NMR spectra were obtained for four samples of methyl-deuterated PMA-d3 with different molecular masses. The resulting spectra were fit using superpositions of simulated spectra generated from the MXQET simulation program, based on a model incorporating nearest-neighbor jumps from positions on the vertices of a truncated icosahedron (soccer-ball shape). The lower molecular-mass samples, influenced by the presence of more chain ends, showed more heterogeneity (broader distribution) and lower glass transitions than the higher molecular-mass samples. The MDSC experiments on both protonated and deuterated samples showed behavior consistent with the NMR results, but temperature shifted due to the different frequency range of the measurements in terms of both the position and breadth of the glass transition as a function of molecular mass.
Electron-impact ionization of helium clusters close to the threshold: Appearance energies
Authors: Denifl S. , Stano M. , Stamatovic A. , Scheier P. , Märk T. D.
Published: February 2006

We have investigated the ionization threshold behavior of small helium cluster ions (cluster size n=2−10) formed via electron-impact ionization of neutral helium droplets and derive appearance energies for mass-selected cluster ions using a nonlinear least-square-fitting procedure. Moreover, we report magic numbers in the mass spectrum observed at the electron energy of 70eV. The apparatus used for the present measurements is a hemispherical electron monochromator combined with a quadrupole mass spectrometer. Our experiment demonstrates that helium clusters are not only exclusively formed via direct ionization above the atomic ionization potential but also indirectly via autoionizing Rydberg states. The present results are compared with previous electron-impact and photoionization results.
Theory of electron solvation in polar liquids: A continuum model
Authors: Zharikov Anatoly A. , Fischer Sighart F.
Published: February 2006

The solvation of electrons in polar liquids is analyzed on the basis of an extended continuum model. In addition to the long-range electron-dipole interaction two short-range interactions are introduced. Among others one accounts for interactions with groups capable of forming hydrogen bonds and the second for quadrupolar characteristics of the liquid molecules. Both are induced by the orientation of the molecular dipole. Applying the scaling method a proper reaction coordinate is introduced and the solvation dynamics are discussed for the electron in the electronic ground state and after excitation to the p-type excited state. The observed spectral evolution of the transient absorption spectra, after two photon excitations for electrons in water and in methanol, is well described by this theory. An analytic estimate for the nonradiative deactivation from the electronically excited solvated electron is found to be consistent with an observed lifetime of 50fs for the electron in water. The theory predicts an about three times slower internal conversion in methanol as solvent in comparison with water.
Photofragment coincidence imaging of small I−(H2O)n clusters excited to the charge-transfer-to-solvent state
Authors: Szpunar David E. , Kautzman Kathryn E. , Faulhaber Ann Elise , Neumark Daniel M.
Published: February 2006

The photodissociation dynamics of small I−(H2O)n(n=2–5) clusters excited to their charge-transfer-to-solvent (CTTS) states have been studied using photofragment coincidence imaging. Upon excitation to the CTTS state, two photodissociation channels were observed. The major channel (∼90%) is a two-body process forming neutral I+(H2O)n photofragments, and the minor channel is a three-body process forming I+(H2O)n−1+H2O fragments. Both processes display translational energy [P(ET)] distributions peaking at ET=0 with little available energy partitioned into translation. Clusters excited to the detachment continuum rather than to the CTTS state display the same two channels with similar P(ET) distributions. The observation of similar P(ET) distributions from the two sets of experiments suggests that in the CTTS experiments, I atom loss occurs after autodetachment of the excited [I(H2O)n−]* cluster or, less probably, that the presence of the excess electron has little effect on the departing I atom.
Vibrationally promoted electron emission from low work-function metal surfaces
Authors: White Jason D. , Chen Jun , Matsiev Daniel , Auerbach Daniel J. , Wodtke Alec M.
Published: February 2006

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9⩽v⩽18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10−2 electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces.
Local stress and heat flux in atomistic systems involving three-body forces
Authors: Chen Youping
Published: February 2006

Local densities of fundamental physical quantities, including stress and heat flux fields, are formulated for atomistic systems involving three-body forces. The obtained formulas are calculable within an atomistic simulation, in consistent with the conservation equations of thermodynamics of continuum, and can be applied to systems with general two- and three-body interaction forces. It is hoped that this work may correct some misuse of inappropriate formulas of stress and heat flux in the literature, may clarify the definition of site energy of many-body potentials, and may serve as an analytical link between an atomistic model and a continuum theory. Physical meanings of the obtained formulas, their relation with virial theorem and heat theorem, and the applicability are discussed.
Negative refraction of spintronics and spin beam splitter
Authors: Zhang Xiangdong
Published: February 2006

The spin-polarized injections through the lateral interface between two regions with different strengths of spin-orbit interactions have been investigated. In the presence of the spin-orbit coupling of both Rashba and Dresselhaus types, the phenomena of negative refraction of spintronics have been found. An electron beam with some angles of incidence after passing through such an interface splits into two beams with different spin polarizations propagating in negative and positive directions, respectively. Thus, a spin beam splitter has been proposed based on such an interface.
Carbon nanostructures on silicon substrates suitable for nanolithography
Authors: Abdi Y. , Mohajerzadeh S. , Hoseinzadegan H. , Koohsorkhi J.
Published: February 2006

We report the application of vertically grown carbon nanotubes (CNTs) for submicron and nanolithography. The growth of CNTs is performed on silicon substrates using a nickel-seeded plasma-enhanced chemical vapor deposition method at a temperature of 650 °C and with a mixture of C2H2 and H2. The grown CNTs are encapsulated by a titanium-dioxide film and then mechanically polished to expose the buried nanotubes, and a plasma ashing step finalizes the process. The emission of electrons from the encapsulated nanotubes is used to write patterns on a resist-coated substrate placed opposite to the main CNT holding one. Scanning electron microscope has been used to investigate the nanotubes and the formation of nano-metric lines. Also a novel approach is presented to create isolated nanotubes from a previously patterned cluster growth.
Quantum-well infrared photodetector with voltage-switchable quadratic and linear response
Authors: Maier Thomas , Schneider Harald , Liu H. C. , Walther Martin , Koidl Peter
Published: February 2006

We present a quantum-well infrared photodetector that allows us to switch between a quadratic and a linear detection mode. We employ an asymmetric detector design where intersubband transitions in a three-level system give rise to a quadratic power dependence down to very low excitation densities. Upon reversing the bias voltage, however, the intermediate subband acts as the ejection channel due to highly efficient tunneling, thus leading to a two-level system with linear photoresponse.
Magnetic and electromagnetic wave absorption properties of α-Fe∕Z-type Ba-ferrite nanocomposites
Authors: Liu Jiu Rong , Itoh Masahiro , Machida Ken-ichi
Published: February 2006

The saturation magnetization values (Ms) of α-Fe∕Ba3Co1.8Fe23.6Cr0.6O41 nanocomposites prepared by mechanically alloying α-Fe with Ba3Co1.8Fe23.6Cr0.6O41 powders increased with increasing the concentration of α-Fe. α-Fe∕Ba3Co1.8Fe23.6Cr0.6O41 nanocomposites showed higher coercivity values than α-Fe and Ba3Co1.8Fe23.6Cr0.6O41 because of the effects of shape anisotropy and exchange bias. The resin compacts with 33.5 vol % α-Fe∕Ba3Co1.8Fe23.6Cr0.6O41 (38, 70, 85 vol % α-Fe) powders provided good electromagnetic wave absorption performances in ranges of 7.5–16.0, 5.4–10.5, and 4.3–8.3 GHz over the absorber thicknesses of 1.3–2.5, 1.6–3.0, and 1.7–3.2 mm, respectively.
Photorefractive effect in iron-doped lithium niobate crystals induced by femtosecond pulses of 1.5μm wavelength
Authors: Beyer O. , Breunig I. , Kalkum F. , Buse K.
Published: February 2006

Illumination of iron-doped lithium crystals (LiNbO3:Fe) with femtosecond pulses of 1.5μm wavelength results in large refractive index changes Δn in the order of 10−3. The sign of the refractive index changes depends on the polarization of the recording light. The results can be very useful for fabrication of tailored holographic components for telecommunication.
Time-resolved measurements of Cl2 density in high-density plasmas and application
Authors: Cunge Gilles , Mori Masahito , Kogelschatz Martin , Sadeghi Nader
Published: February 2006

Absorption at 355 nm, with a pulsed frequency tripled yttrium-aluminum-garnet laser as light source, is used to monitor the time evolution of the Cl2 density in high-density inductively coupled plasmas. The detection limit over a 0.1 s acquisition time is about 0.2 mTorr of Cl2. This technique is well suited for monitoring chlorine density when studying elementary processes in Cl2 containing plasmas. Furthermore, it can be applied to control the process drift in industrial etch reactors resulting from the modification of the chamber walls conditions: by measuring the Cl2 density in a reference Cl2 plasma before etching a wafer, it can be determined if the chamber wall conditions are kept identical from one wafer to another.
Current flow and energy dissipation in low-dimensional semiconductor superlattices
Authors: Fowler D. , Patané A. , Ignatov A. , Eaves L. , Henini M. , Mori N. , Maude D. K. , Airey R.
Published: February 2006

By applying high magnetic and electric fields to a semiconductor superlattice (SL) we create quasi-one-dimensional or quasi-zero-dimensional electronic states. This reduced dimensionality restricts the range of inelastic scattering processes available to the conduction electrons, leading to an increase of the inelastic scattering time and a corresponding decrease of the electrical conductance. Our study reveals the fundamental link between current flow and energy dissipation in low-dimensional conductors, which is relevant to the exploitation of artificial nanowires and quantum dot SLs for novel applications, including recently proposed thermoelectric devices.
Preparation of perovskite Pb(Zn1∕3Nb2∕3)O3-based thin films from polymer-modified solution precursors
Authors: Yao Kui , Yu Shuhui , Tay Francis Eng Hock
Published: February 2006

Epitaxial perovskite Pb(Zn1∕3Nb2∕3)O3 (PZN)-based ferroelectric thin films were prepared through a chemical solution approach, in which the solution precursors were appropriately modified with polyethylene glycol (PEG). The addition of PEG in the solution dramatically promoted the perovskite phase while suppressed the pyrochlore phase, although PEG was pyrolyzed before the crystallization. The interactions between PEG and metal ions were identified through examining the chemical states of the ions using x-ray photoelectron spectroscopy. The ferroelectric and piezoelectric properties of an obtained thin film sample with a composition of 0.462PZN–0.308PMN–0.23PT [PT:PbTiO3;PMN:Pb(Mg1∕3Nb2∕3)O3] were characterized, showing a very low coercive field, high dielectric constant, and strong piezoelectric effect.
Electromigration in self-organized single-crystalline silver nanowires
Authors: Stahlmecke B. , Meyer zu Heringdorf F.-J. , Chelaru L. I. , Horn-von Hoegen M. , Dumpich G. , Roos K. R.
Published: February 2006

We present electromigration experiments on single-crystalline silver nanowires. The wires were grown on 4° vicinal silicon (100) substrates by self-organization and were contacted by electron beam lithography. The electromigration experiments were performed in situ in a scanning electron microscope at room temperature with constant dc conditions. In contrast to other experiments we observe void formation at the anode side of the wires. If the current is reversed, the electromigration behavior is also reversed.
Multidimensional manipulation of carbon nanotube bundles with optical tweezers
Authors: Zhang Jianlong , Kim Hyun Ik , Oh Cha Hwan , Sun Xiudong , Lee Haiwon
Published: February 2006

Optical manipulation of carbon nanotubes (CNTs) in aqueous solution was performed using a linearly polarized infrared tweezers system. Vertical and horizontal manipulation of single-walled and multiwalled carbon nanotubes (SWNTs, MWNTs) was carried out by changing the size of the CNTs and the trapping position. Rotation of MWNT bundles was confirmed using a circular polarized infrared optical tweezers system. Patterning of dots and letters with CNTs was successfully carried out on glass substrates.
Improved quality (112¯0)a-plane GaN with sidewall lateral epitaxial overgrowth
Authors: Imer Bilge M. , Wu Feng , DenBaars Steven P. , Speck James S.
Published: February 2006

We demonstrate a technique to reduce the extended defect densities in a-plane GaN deposited on r-plane sapphire. The SiO2 lateral epitaxial overgrowth mask consisted of ⟨11¯00⟩GaN stripes. Both the mask and GaN were etched through the mask openings and the lateral growth was initiated from the etched c-plane GaN sidewalls, and the material was grown over the mask regions until a smooth coalesced film was achieved. Threading dislocation densities in the range of 106–107cm−2 were realized throughout the film surface. The on-axis and off-axis full width at half maximum value and surface roughness were 0.082°, 0.114°, and 0.622 nm, respectively.
Sub-200-fs pulsed erbium-doped fiber laser using a carbon nanotube-polyvinylalcohol mode locker
Authors: Rozhin Aleksey G. , Sakakibara Youichi , Namiki Shu , Tokumoto Madoka , Kataura Hiromichi , Achiba Yohji
Published: February 2006

We study the difference in mode-locked operations of different saturable absorber films made of single wall carbon nanotube-polyvinylalcohol nanocomposite. The films have different nanotube concentrations and thicknesses. For the study, a mode-locked erbium-doped fiber laser in a ring cavity configuration was constructed by setting the films in a microgap between a pair of fiber end facets. With the optimum film among those we have tested, the shortest pulses with a width of 178fs were achieved at 1.56μm with a repetition rate of 22.8MHz and an average power of 1.55mW.
Capacitance-voltage investigation of high-purity InAs∕GaSb superlattice photodiodes
Authors: Hood Andrew , Hoffman Darin , Wei Yajun , Fuchs Frank , Razeghi Manijeh
Published: February 2006

The residual carrier backgrounds of binary type-II InAs∕GaSb superlattice photodiodes with cutoff wavelengths around 5μm have been studied in the temperature range between 20 and 200K. By applying a capacitance-voltage measurement technique, a residual background concentration below 1015cm−3 has been found. At temperatures below 100K carrier freeze-out is observed with a thermal activation energy of 4.5meV, leading to net carrier concentrations at 77K in the mid 1014cm−3.
Long-range surface plasmon-polariton mode cutoff and radiation
Authors: Breukelaar Ian , Charbonneau Robert , Berini Pierre
Published: February 2006

Using a fully vectorial formulation, the normal mode analysis method is used to model the radiative spreading of surface plasmon-polariton modes input into regions where the bound surface mode is cutoff, or radiative. The analysis was performed at λ0=1.55μm for embedded finite-width metal waveguides where the metal was Au and the surrounding dielectric was SiO2. Mode cutoff is induced by an asymmetry between the index of refraction of the top clad and the bottom clad. Results show that a change in insertion loss of better than 20dB is possible for an appropriate waveguide and dielectric asymmetry. Comparisons between predictions and experimental results show very good agreement.
Energy transfer between Pr3+ and Mn2+ in SrB4O7:Pr,Mn
Authors: Chen Yonghu , Shi Chaoshu , Yan Wuzhao , Qi Zeming , Fu Yibing
Published: February 2006

The luminescence of Pr3+ or Mn2+ singly doped, as well as Pr3+ and Mn2+ codoped, SrB4O7 powder microcrystalline samples were investigated using synchrotron radiation. The photon cascade emission originating from the S01 level was observed in the SrB4O7:Pr3+ (0.1mol%) sample upon 206nm excitation. There are desirable spectral overlaps in the region of 330–430nm between the emission spectra of the SrB4O7:Pr3+ sample and the excitation spectra of the SrB4O7:Mn2+ sample monitoring the Mn2+ luminescence at 640nm. These spectral overlaps are in favor of the energy transfer from Pr3+ to Mn2+, converting the first step photon from Pr3+:S01 in the unpractical ultraviolet or near-ultraviolet regions into the red Mn2+ emission. A comparison of the emission spectra of the SrB4O7:Pr3+, Mn2+ sample with that of the SrB4O7:Pr3+ sample revealed the existence and the efficiency of the proposed energy transfer between Pr3+ and Mn2+, suggesting an promising vacuum ultraviolet phosphor based on Pr3+ and Mn2+ combination with visible quantum efficiency greater than unit.
Differential response of vacuolar proton pumps to osmotica
Authors: Chiu Fan S. , Hsu Shen H. , Chen Jiun H. , Hsiao Yi Y. , Pan Yih J. , Van Ru C. , Huang Yun T. , Tseng Fang G. , Chou Wing M. , Fan Shih K. , Pan Rong L.
Published: February 2006

The vacuole is a fundamental and dominant organelle and occupies a large part of the total cell volume in most mature plant cells. The higher-plant vacuole contains two types of proton-translocating pumps, H+-ATPase (EC 3.6.1.3) and H+-pyrophosphatase (EC 3.6.1.1), residing on the same membrane. These two enzymes generate roughly equal proton gradients across the vacuolar membrane for the secondary transport of ions and metabolites. However, the pumps respond differentially to stress in order to maintain critical functions of the vacuole. In this work, tonoplasts from etiolated mung bean seedlings (Vigna radiata L.) were used to investigate the function of these two enzymes under high osmotic pressure. At high concentrations of sucrose or sorbitol, the light scattering and volume of isolated vesicles were progressively changed. Concomitantly, enzymatic activities, proton translocation, and coupling efficiencies of these two proton-pumping enzymes were inhibited to various extents under high osmotic pressure. No significant change in enzymatic activities of purified vacuolar H+-PPase and H+-ATPase under similar conditions was observed. We thus believe that the membrane structure is an important determinant for proper function of proton pumping systems of plant vacuoles. Furthermore, kinetic analysis shows different variation in apparent Vmax but not in KM values of vacuolar H+-PPase and H+-ATPase at high osmolarity of sucrose and sorbitol, respectively, suggesting probable alterations in substrate hydrolysis reactions but not substrate-binding affinity of the enzymes. A working model is proposed to interpret supplemental roles of vacuolar H+-PPase and H+-ATPase to maintain appropriate functions of plant tonoplasts.
Changes in photosynthetic parameters and antioxidant activities following heat-shock treatment in tomato plants
Authors: Camejo Daymi , Jiménez Ana , Alarcón Juan José , Torres Walfredo , Gómez Juana María , Sevilla Francisca
Published: February 2006

Seedlings of two tomato genotypes, Lycopersicon esculentum Mill. var. Amalia and the wild thermotolerant type Nagcarlang, were grown under a photoperiod of 16 h light at 25°C and 8 h dark at 20°C. At the fourth true leaf stage, a group of plants were exposed to a heat-shock temperature of 45°C for 3 h, and measurements of chlorophyll fluorescence, gas-exchange characteristics, dark respiration and oxidative and antioxidative parameters were made after releasing the stress. The heat shock induced severe alterations in the photosynthesis of Amalia that seem to mitigate the damaging impact of high temperatures by lowering the leaf temperature and maintaining stomatal conductance and more efficient maintenance of antioxidant capacity, including ascorbate and glutathione levels. These effects were not evident in Nagcarlang. In Amalia plants, a larger increase in dark respiration also occurred in response to heat shock and the rates of the oxidative processes were higher than in Nagcarlang. This suggests that heat injury in Amalia may involve chlorophyll photooxidation mediated by activated oxygen species (AOS) and more severe alterations in the photosynthetic apparatus. All these changes could be related to the more dramatic effect of heat shock seen in Amalia than in Nagcarlang plants.
Research note: Low-boron acclimation induces uptake of boric acid against a concentration gradient in root cells of Olea europaea
Authors: Stavrianakou Sotiria , Liakopoulos Georgios , Karvonis Evangelos , Resta Evangelia , Karabourniotis George
Published: February 2006

Low concentrations of boron (B) in the external medium can induce uptake mechanisms whereby plants can develop a concentration gradient for B against the external medium. These mechanisms seem to be widespread among herbaceous species. In this study, olive (Olea europaea L.) plants were acclimated to either high (23 μm; controls) or low (0.5 μm; low-B plants) concentrations of B for 45 d, in a hydroponic culture. Afterwards, a 7-h uptake experiment was conducted by transferring plants of both groups to a series of nutrient solutions with B concentrations ranging from 0.5 to 23 μm. Analysis of B concentration in cell sap of root and xylem exudate was performed by the borate–chromotropic acid HPLC assay. Plants acclimated to high-B concentration showed root cell and xylem exudate B concentrations that were comparable to those of the external medium. In contrast, plants acclimated to low-B concentration were able to develop concentrations of B in root cells up to 2-fold higher than those of the external medium. Moreover, B concentrations in xylem exudate for both plant groups corresponded to those of the root cell sap, indicating diffusion equilibrium. These results support the existence of a mechanism that concentrates B in the root cell sap against the nutrient solution when olive plants are acclimated to low-B conditions.