bluuebunny/crossref_metadata_2025 huggingface dataset:
Single motor unit variability with threshold stimulation in patients with amyotrophic lateral sclerosis and normal subjects
Authors: Jillapalli Devanand , Shefner Jeremy M.
Published: September 2004

Repetitive nerve stimulation often shows responses with an abnormal decrement in patients with amyotrophic lateral sclerosis (ALS), suggesting instability of the neuromuscular junction; however, the pathophysiology and response characteristics of this instability are not clear. We evaluated response variability of 47 single motor units from 16 patients with ALS and 51 units from 10 normal subjects, acquired by delivering threshold stimuli sporadically at 0.5 HZ or less. In addition, in 46 other different single motor units obtained from 21 patients with ALS, variability was studied at both 1‐ and 3‐HZ stimulation rates. Motor units from patients with ALS were significantly more variable than those from normal subjects, even when their larger amplitude was accounted for. This increased variability was not rate dependent. Response variability is a critical measure in the statistical method of motor unit number estimation and is attributed to variability in the number of units activated; the fact that variability of single motor units varies with disease state may be a potentially confounding factor in the application of the technique. Muscle Nerve, 2004
SPICE for SPACE trials, risk analysis, and process improvement
Authors: Cass Ann , Völcker Christian , Ouared Rafik , Dorling Alec , Winzer Lothar , Carranza Juan María
Published: September 2004

As part of a programme for software process improvement sponsored by the European Space Agency (ESA), an ISO 15504 (1998) conformant method for software process assessment has been developed. [Work supported under ESTEC Contract Number 10 662/93/NL/NB CCN5.] Called SPICE for SPACE, or S4S (Völcker and Cass 2002, Cass et al. 2000), the method includes an assessment model based on the ISO/IEC TR 15504 exemplar model with space software practices incorporated.

To promote SPICE for SPACE, ESA has sponsored a series of trial assessments using the S4S method. Since February 2000, eight space software suppliers have been assessed in Austria, Belgium, Denmark, Germany, Great Britain, Italy, and Sweden.

The first part of this paper describes the trial assessment programme to date, summarising the assessments that have taken place, the assessment results, the improvement efforts initiated in the host organisations, and the feedback gathered to improve the S4S method.

The second part of this paper presents a new approach for software process improvement based on a risk analysis of assessment results. Developed within the same initiative, this method sets the basis for integrating process‐oriented risk analysis into the S4S method, where, along with the process and capability dimensions, risk can be considered as a new third dimension. The outcomes of the risk analysis lead to the selection and prioritisation of pre‐defined generic improvement actions. Alternatively, the risk analysis presented here can be applied in the context of supplier evaluation or risk estimation for investors in software organisations. Copyright © 2004 John Wiley & Sons, Ltd.
Role of locus coeruleus α1‐adrenoceptors in motor activity in rats
Authors: Stone Eric A. , Lin Yan , Ahsan Rashedul , Quartermain David
Published: September 2004

The question of whether or not the locus coeruleus (LC) participates in the control of motor activity has been controversial due to difficulties in demonstrating permanent motor deficits after neurotoxic lesions of this nucleus or of the dorsal noradrenergic bundle (DNB). In the present experiments it was shown in rats that acute local blockade (with terazosin) or stimulation (with phenylephrine) of LC α1‐adrenoceptors respectively blocked or stimulated exploratory behavior in a novel cage and the home cage. Moreover, previous lesion of the DNB by i.p. DSP4 abolished the behavioral changes to local LC α1‐receptor manipulation but did not affect motor activity in the novel or home cage by itself. These findings are consistent with the hypothesis that the intact LC does contribute to motor activity control, exerted in part by its α1‐receptors; however, the permanent loss of this nucleus is compensated for by remaining CNS motor structures. Synapse 54:164–172, 2004. © 2004 Wiley‐Liss, Inc.
Electron spin resonance spectroscopy reveals alpha‐phenyl‐N‐tert‐butylnitrone spin‐traps free radicals in rat striatum and prevents haloperidol‐induced vacuous chewing movements in the rat model of human tardive dyskinesia
Authors: Rogoza Raina M. , Fairfax David F. , Henry Paul , N‐Marandi Sevil , Khan Rao F. , Gupta Suresh K. , Mishra Ram K.
Published: September 2004

The typical antipsychotic drug haloperidol causes vacuous chewing movements (VCM) in rats, which are representative of early‐Parkinsonian symptoms or later‐onset extrapyramidal side effects of tardive dyskinesia (TD) in humans. Haloperidol (HP) has been hypothesized to potentiate increases in oxidative stress or free radical‐mediated levels of toxic metabolites in rat striatum while simultaneous upregulating dopamine (DA)‐D2 receptors leading to presumed DA supersensitivity. Alpha(α)‐Phenyl‐N‐tert‐butylnitrone (PBN) is an antioxidant used to combat oxidative stress and measure increases in PBN spin‐adduct activity. Thus, the aim of this study was to investigate whether VCMs are related to upregulation of DA‐D2 receptors or to increased levels of free radicals produced during oxidative stress, and whether PBN had any protective effects. Rats received daily chronic (28 day) i.p. injections of saline, haloperidol (2 mg/kg), PBN (150 mg/kg), or haloperidol + PBN. The VCM model was used to measure extrapyramidal side effects of drug treatments. Electron spin resonance (ESR) spectroscopy was performed to compare concentrations of free radical species in rats receiving injections of HP + PBN. To examine the upregulation of DA‐D2 receptors, binding assays were carried out to assess the increase in DA‐D2 receptor numbers with respect to VCMs following treatment of rats injected with HP, PBN, and HP + PBN. Results of these experiments show that HP‐induced VCMs in rats results from increases in oxidative cellular events and may not be related to increases in striatal DA‐D2 receptors. Synapse 54:156–163, 2004. © 2004 Wiley‐Liss, Inc.
Cell shape and growth of budding yeast cells in restrictive microenvironments
Authors: Suzuki Masaya , Asada Yosuke , Watanabe Daisuke , Ohya Yoshikazu
Published: September 2004

Effects of limited growth space on the cell morphology and cell growth are investigated by creating rigid outside environments. The cube‐shaped holes big enough for a single cell of the budding yeast Saccharomyces cerevisiae were prepared with a focused ion‐beam (FIB), commonly used for processing semiconductors. We demonstrated that the outline of the cells changes their ellipsoidal morphology into a cubic form when the daughter cells are grown in the holes, indicating that yeast cells change their shape in response to external limited space. The yeast cells grown in the microenvironments exhibit neither bud formation nor nuclear division. Although restricted growth caused by the physical barriers leads to the block of cell cycle progression in the wild‐type cells, swe1Δ cells defective in the morphogenesis checkpoint become binucleate after being grown in the microenvironments. These results suggest that yeast cells under spatial restriction arrest cell cycle progression in a Swelp‐dependent manner. Copyright © 2004 John Wiley & Sons, Ltd.
Role of OGG1 and NTG2 in the repair of oxidative DNA damage and mutagenesis induced by hydrogen peroxide in Saccharomyces cerevisiae: relationships with transition metals iron and copper
Authors: Melo R. G. M. , Leitão A. C. , Pádula M.
Published: September 2004

The base excision repair pathway of Saccharomyces cerevisiae possesses three DNA N‐glycosylases, viz. Ogg1p, Ngt1p and Ntg2p, involved in the repair of oxidative DNA damage. It was previously reported that inactivation of any of these activities, in most cases, did not generate a sensitive mutant phenotype to a variety of oxidative agents. Only the ntg1 mutant appeared to be more sensitive to hydrogen peroxide (H2O2) than a wild‐type (WT) strain. In the present study we evaluated the role of S. cerevisiae OGG1 and NTG2 genes in the repair of oxidative lesions induced by high H2O2 concentrations (5–100 mM for 20 min), followed by catalase treatment (500 IU/ml). In these conditions, the ogg1 mutant was more sensitive than the WT strain to H2O2 (concentration 40–60 mM). Unexpectedly, the inactivation of NTG2 in an ogg1 background was able to suppress both sensitivity and mutagenesis induced by H2O2. Indeed, even the ntg2 single mutant was more resistant than the WT (60–100 mM H2O2). The use of metal ion chelators dipyridyl and neocuproine allowed us to evaluate the participation of iron and copper ions in the production of lethal and mutagenic lesions during H2O2 treatment in different DNA repair‐deficient S. cerevisiae strains. The roles of OGG1 and NTG2 genes in the repair of lethal and mutagenic oxidative lesions induced by H2O2 and their relationships with iron and copper ions are discussed. Copyright © 2004 John Wiley & Sons, Ltd.
Chr4, a Schizosaccharomyces pombe homologue of the Saccharomyces cerevisiae Chs4p/Skt5p protein, is related to septum formation and is required for the proper localization of Chs2
Authors: Matsuo Yasuhiro , Matsuura Yoshinori , Tanaka Katsunori , Matsuda Hideyuki , Kawamukai Makoto
Published: September 2004

In Saccharomyces cerevisiae, Chs4p directly interacts with chitin synthase III (Chs3p) to act as a post‐translational regulator of the Chs3p complex. We identified four Chs4p homologous proteins in Schizosaccharomyces pombe which we named Chr1, Chr2, Chr3 and Chr4 (putative chitin synthase regulatory factor). We assessed the functions of these proteins and found that while overproduction of Chr1, Chr2 or Chr3 did not affect the cellular morphology of wild‐type Sz. pombe cells, overproduction of Chr4 caused the cells to form multi‐septa and delayed their growth. All multiple disruptants of chr1, chr2, chr3 and chr4 grew normally under a variety of growth conditions. However, while chitin synthase II (Chs2) normally localizes exclusively at the septum, in many chr4‐disrupted cells it was found in the cytoplasm and the septa. Chs2 did localize at the abnormal septa caused by the overproduction of chr4+. Chr4‐13Myc expression was unaffected by the different media or growth conditions in both wild‐type and the chs2 disruptant. Chs2 expression was also unaltered by the absence of Chr4. Moreover, Chr4‐13Myc localized mostly at the tips and the septum during vegetative growth in chs2, chr1, chr2 and chr3 disruptants as well as in wild‐type. Thus, chr4+ is involved in septum formation and is required for the proper localization of Chs2 at the septum in Sz. pombe. Copyright © 2004 John Wiley & Sons, Ltd.
Expression of the expanded YFL007w ORF and assignment of the gene name BLM10
Authors: Doherty Kevin , Pramanik Ajay , Pride Leah , Lukose James , Wood Moore Carol
Published: September 2004

This report assigns a gene name to an extended ORF in Saccharomyces cerevisiae, and provides the first evidence from Northern analyses that the ORF is expressed and the transcript is the predicted size. Copyright © 2004 John Wiley & Sons, Ltd.
Tandem affinity purification of the Candida albicans septin protein complex
Authors: Kaneko Aki , Umeyama Takashi , Hanaoka Nozomu , Monk Brian C. , Uehara Yoshimasa , Niimi Masakazu
Published: September 2004

A novel vector was constructed to enable the integrative marking of individual genes and the affinity purification of interacting molecules within protein complexes from Candida albicans using a tandem 6×histidine and FLAG epitope tag. The system was verified by purifying the C. albicans septin complex (a self‐associating complex of cytoskeletal proteins) from both yeast and hyphal cells. One‐step affinity purification was insufficient for purification of the protein complex, whereas tandem affinity purification (TAP) gave an extensively purified protein complex with a very low background. Electrophoretic and mass spectrometry analysis showed that the affinity‐purified C. albicans septin complex, which comprises predominantly CaCdc3p, CaCdc10p, CaCdc11p, CaCdc12p and CaSep7p, was not affected by cell morphology. The purified septin complex appeared to have a stoichiometry of 2 CaCdc3p, 1–2 CaCdc10p, 1 CaCdc11p, 2 CaCdc12p and ≤1 CaSep7p. The successful application of TAP to the purification and analysis of the C. albicans septin complex indicates that this technology will have much wider application to proteomic studies of this pathogenic fungus. Copyright © 2004 John Wiley & Sons, Ltd.
The clinical implications of aldosterone escape in congestive heart failure
Authors: Struthers Allan D.
Published: September 2004

Angiotensin converting enzyme (ACE) inhibitor therapy does not reliably suppress aldosterone production, and ‘aldosterone escape’ occurs in up to 40% of patients with congestive heart failure (CHF). Aldosterone levels correlate with the risk of cardiovascular events. Aldosterone adversely affects the risk of cardiovascular events via mineralocorticoid receptors in the heart, blood vessels and other sites. Notably, aldosterone contributes to endothelial dysfunction and attenuates endothelium‐dependent vasodilatation, at least partly by reducing nitric oxide bioavailability. Aldosterone also promotes myocardial fibrosis and cardiac remodelling by enhancing collagen synthesis, resulting in increased myocardial stiffness and increased left ventricular mass. These mechanisms mediated by aldosterone contribute to increased risk of ventricular arrhythmias and sudden cardiac death. Two major prospective trials, including one in which patients routinely received ACE inhibitor and beta blocker therapy, have shown that the use of an aldosterone blocker significantly reduces all‐cause mortality, sudden cardiovascular death and hospitalisation in patients with acute or chronic left ventricular dysfunction or CHF. Inhibition of aldosterone's effect on mineralocorticoid receptors should now be considered standard therapy in these patient populations.
On the limiting aerodynamic roughness of the ocean in very strong winds
Authors: Donelan M. A. , Haus B. K. , Reul N. , Plant W. J. , Stiassnie M. , Graber H. C. , Brown O. B. , Saltzman E. S.
Published: September 2004

The aerodynamic friction between air and sea is an important part of the momentum balance in the development of tropical cyclones. Measurements of the drag coefficient, relating the tangential stress (frictional drag) between wind and water to the wind speed and air density, have yielded reliable information in wind speeds less than 20 m/s (about 39 knots). In these moderate conditions it is generally accepted that the drag coefficient (or equivalently, the “aerodynamic roughness”) increases with the wind speed. Can one merely extrapolate this wind speed tendency to describe the aerodynamic roughness of the ocean in the extreme wind speeds that occur in hurricanes (wind speeds greater than 30 m/s)? This paper attempts to answer this question, guided by laboratory extreme wind experiments, and concludes that the aerodynamic roughness approaches a limiting value in high winds. A fluid mechanical explanation of this phenomenon is given.
Testing a numerical glacial hydrological model using spring speed‐up events and outburst floods
Authors: Kessler Mark A. , Anderson Robert S.
Published: September 2004

We describe a numerical model of a glacial‐hydrologic system that reproduces the salient features of two hydrological and ice motion events commonly observed on temperate alpine glaciers. These include the pattern of ice sliding and water discharge characteristic of spring speed‐up events, and the triggering and structure of outburst floods from ice‐marginal lakes. These behaviors result from strong internal feedbacks between water storage in englacial macro‐pores or an ice‐marginal lake, ice motion by sliding and deformation, and a subglacial hydrologic system consisting of a slow‐flow linked cavity network and a fast‐flow centerline conduit.
Island mass effect in the Marquesas Islands: Time variation
Authors: Martinez Elodie , Maamaatuaiahutapu Keitapu
Published: September 2004

Chlorophyll a in the Marquesas islands has been analyzed for the period 1997–2002 as well as simultaneous sea surface height anomaly, sea surface temperature, wind and rainfall. For the first time, the seasonality of phytoplankton blooms is shown around the Marquesas Islands. Clearly, the data show three types of blooms in this area: seasonal blooms, episodic ones, and La Niña (related bloom). Initiation of blooms is due to the island mass effect and has been explained by the dynamic interaction of the circulation and the topography. The island mass effect blooms are strongly correlated with the total (Ekman plus geostrophic) surface current and episodically correlated with the geostrophic current. Strong correlation is also observed with sea surface temperature. Analysis of rainfall data suggests that land drainage can enhance a bloom if the current dispersing the nutrients is sufficiently strong.
High‐energy protons, electrons, and positrons trapped in Earth's radiation belts
Authors: Fiandrini E. , Esposito G. , Bertucci B. , Alpat B. , Ambrosi G. , Battiston R. , Burger W. J. , Caraffini D. , Di Masso L. , Dinu N. , Ionica M. , Ionica R. , Pauluzzi M. , Menichelli M. , Zuccon P.
Published: September 2004

Accurate measurements of under cutoff proton, electron, and positron fluxes in the energy range 0.07 ÷ 9.1 GeV have been performed with the Alpha Magnetic Spectrometer at altitudes of 370 ÷ 390 km in the geographic latitude interval ±51.7°. The flux maps as a function of the canonical adiabatic variables L, αo, and energy E are presented and discussed.
Drainage basin morphometrics for depressional landscapes
Authors: Lindsay John B. , Creed Irena F. , Beall Frederick D.
Published: September 2004

Measures of the size, position, and connectivity of depressional wetlands are related to runoff variations among 12 forested catchments on the Canadian Shield under varying moisture conditions. A fine‐resolution digital elevation model was used to delineate wetlands within the catchments. Analyses showed that wetland metrics as predictors of runoff variation were superior to catchment area and mean slope, two common basin metrics. The most useful metrics were the area of wetlands in bottomland positions, total wetland area, and volume. During wet periods, catchments containing extensive wetlands were marked by a significant decrease in maximum peak discharge and increase in duration of flow. During mesic and dry periods, catchments containing extensive wetlands were marked by an increase in rise and recession times of peak discharge events and the duration of flows. These characteristics resulted from the interaction between wetlands, their hydrologic connectivity to surface flow paths, and runoff.
On the influence of short and medium range order on the material band gap in hydrogenated amorphous silicon
Authors: Mahan A. H. , Biswas R. , Gedvilas L. M. , Williamson D. L. , Pan B. C.
Published: September 2004

We examine different types of order measured in hydrogenated amorphous silicon (a-Si:H) and their effect on the optical absorption (band gap). We first review previous experimental work determining order on a short-range scale as probed by Raman spectroscopy, and provide, using molecular dynamics simulations, a theoretical explanation for why the band gap increases when this type of ordering is improved. We then present results on a-Si:H films deposited by hot wire chemical vapor deposition (CVD) and plasma enhanced CVD where the short-range order, from Raman spectroscopy, does not change, but order on a larger or medium-range scale does. This order is determined by measuring the width of the first x-ray diffraction peak, and was varied by depositing films at different substrate temperatures and∕or different hydrogen dilutions. We find that the film band gap also increases when this type of ordering improves, and we provide a possible mechanism to explain these trends. We also suggest that much of the previous literature showing an increase in band gap with increasing film hydrogen content should be treated with caution, as these works may not have accurately deconvoluted the effects of optical adsorption due to film hydrogenation from those due to both types of lattice ordering. Finally, we argue that this same trend may apply, to a limited extent, to microcrystalline silicon.
Elastic constants of lotus-type porous magnesium: Comparison with effective-mean-field theory
Authors: Tane Masakazu , Ichitsubo Tetsu , Hirao Masahiko , Ikeda Teruyuki , Nakajima Hideo
Published: September 2004

Lotus-type porous (LTP) metals possess a markedly anisotropic porous structure, in which long straight pores are homogeneously aligned unidirectionally. In this paper, we first describe a procedure for the determination of reliable elastic constants of LTP metals with high porosity, and next apply the technique to LTP magnesium, and finally compare the measurement results with those from the effective-mean-field (EMF) theory that has been recently proposed by Tane and Ichitsubo [Appl. Phys. Lett. 85, 197 (2004)]. Young’s modulus in the direction parallel to the longitudinal axis of pores decreases virtually linearly with porosity p, i.e., E‖≈45.3(1−p)1.33, whereas the normal Young’s modulus E⊥, falls rapidly, i.e., E⊥≈45.3(1−p)2.64. The Mori-Tanaka mean-field theory yields values that are close to the measured elastic constants in a wide porosity range, but cannot explain this power-law behavior. In contrast, the EMF theory gives more accurate values in the high-porosity range, and can reproduce the power-law behavior.
Evidence of energy transfer in an aluminosilicate glass codoped with Si nanoaggregates and Er3+ ions
Authors: Enrichi F. , Mattei G. , Sada C. , Trave E. , Pacifici D. , Franzò G. , Priolo F. , Iacona F. , Prassas M. , Falconieri M. , Borsella E.
Published: September 2004

The enhancement of the Er3+ ions’ photoluminescence (PL) emission at 1.54μm in a Si and Er coimplanted aluminosilicate glass is investigated in detail. A postimplantation thermal treatment has been performed to recover the damage induced by the implantation process and to promote Si aggregation. It will be shown that 1h treatment in N2 atmosphere is not sufficient to induce Si precipitation for temperatures up to 500°C. Nevertheless, the most intense Er3+ PL emission at 1.54μm is achieved after a thermal treatment at 400°C. Such emission has been investigated by pumping in and out of resonance, showing a very efficient energy transfer process in the whole excitation wavelength range (360–515nm). These results suggest that good energy transfer mediators could be small Si aggregates and not only crystalline clusters. For the best performing sample, the effective Er excitation cross section has been measured to be higher than 10−17cm2 at 379 and 390nm and about 2×10−16cm2 at 476nm, that is, several orders of magnitude higher than the Er direct absorption cross section (of the order of 10−21cm2 in this glass). Moreover the coefficient of cooperative upconversion has been evaluated to be 2.7×10−18cm3s−1. The structural and optical properties of this material are discussed and compared to those found for Si and Er codoped silica.
Exclusion zone of convex brushes in the strong-stretching limit
Authors: Belyi Vladimir A.
Published: September 2004

We investigate asymptotic properties of long polymers grafted to convex cylindrical and spherical surfaces, and, in particular, distribution of chain free ends. The parabolic potential profile, predicted for flat and concave brushes, fails in convex brushes, and chain free ends span only a finite fraction of the brush thickness. In this paper, we extend the self-consistent model developed by Ball, Marko, Milner, and Witten [Macromolecules 24, 693 (1991)] to determine the size of the exclusion zone, i.e., size of the region of the brush free from chain ends. We show that in the limit of strong stretching, the brush can be described by an alternative system of integral equations. This system can be solved exactly in the limit of weakly curved brushes, and numerically for the intermediate to strong curvatures. We find that going from melt state to θ solvent and then to marginal solvent decreases relative size of the exclusion zone. These relative differences grow exponentially as the curvature decreases to zero.
Surface-emitting stimulated emission in high-quality ZnO thin films
Authors: Zhang X. Q. , Suemune Ikuo , Kumano H. , Wang J. , Huang S. H.
Published: September 2004

High-quality ZnO thin films were grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates. Three excitonic transitions associated with the valence bands A, B, and C were clearly revealed in the reflectance spectrum measured at 33K. This result indicates that the ZnO thin films have the wurtzite crystalline structure. The emission spectra were measured with backscattering geometry at room temperature. When the excitation exceeded a certain value, linewidth narrowing, nonlinear rise of emission intensity, and the shortening of the carrier lifetime were clearly observed and these demonstrate the onset of stimulated emission. Together with the ZnO thickness dependence, we conclude that the observation of a stimulated emission in a direction perpendicular to the film surface is predominantly due to scattering of the in-plane stimulated emission by slightly remaining surface undulations in the ZnO films.
Photoluminescence of silver in glassy matrices
Authors: García M. A. , García-Heras M. , Cano E. , Bastidas J. M. , Villegas M. A. , Montero E. , Llopis J. , Sada C. , De Marchi G. , Battaglin G. , Mazzoldi P.
Published: September 2004

This work studies the behavior of Ag+ ions incorporated in different silica-based glassy matrices. To this end, Ag-doped silica coatings, prepared via sol-gel and deposited on pure silica and soda-lime glasses, are investigated by means of structural and optical spectroscopy techniques. Silver tends to segregate towards the interface during the annealing process, but in the case of soda-lime glassy substrates the exchange process favors its diffusion into the substrate. The environment of Ag+ ions during the annealing process determines their final oxidation state. In the pure silica matrix, Ag+ ions are found to be unstable and tend to reduce to Ag0, with the subsequent formation of metallic nanoparticles. However, the presence of network formers and modifiers gives rise to the appearance of nonbridging oxygen, which allows the stabilization of Ag+ ions in the matrix.
An analytical density of states and joint density of states analysis of amorphous semiconductors
Authors: O’Leary Stephen Karrer
Published: September 2004

We study the relationship between the density of states and joint density of states functions in amorphous semiconductors. Introducing an elementary empirical model for the density of states functions that captures the basic expected features, we determine analytical and asymptotic joint density of states results, relating the parameters characterizing the underlying density of states functions with the resultant joint density of states. Numerical joint density of states results, corresponding to the specific case of hydrogenated amorphous silicon, are also presented. It is suggested that this density of states and joint density of states analysis will prove of use to the experimentalist.
Laser induced crystallization of hydrogenated amorphous silicon-carbon alloys
Authors: Summonte C. , Rizzoli R. , Servidori M. , Milita S. , Nicoletti S. , Bianconi M. , Desalvo A. , Iencinella D.
Published: September 2004

Laser induced crystallization of hydrogenated amorphous silicon carbon alloy (a-Si1−xCx:H) films has been investigated by means of synchrotron x-ray diffraction. The a-Si1−xCx:H films were deposited on (100) silicon wafers by very high frequency plasma enhanced chemical vapor deposition at 100MHz in hydrogen diluted silane-methane gas mixtures. The substrate was kept at 250°C or 350°C and the stoichiometry was changed from x=0.20 to 0.63. The structural characterization of the as-grown films has been carried out by Rutherford backscattering (hydrogen concentration) and infrared spectroscopy (film ordering). The films were irradiated by a KrF excimer laser (248nm) with varying energy density and number of pulses. After irradiation, the formation of SiC crystallites has been revealed by synchrotron x-ray diffraction. Besides SiC nanocrystals, the formation of crystalline Si and graphite is observed for under- (x<0.50) and over-stoichiometric (x>0.50) samples, respectively. The essential role played by hydrogen concentration and hydrogen bonding configuration in determining the melting threshold and the consequent SiC grain formation is highlighted.
Photothermal Radiometry with Solid Cylindrical Samples
Authors: Wang Chinhua , Mandelis Andreas , Liu Yue
Published: September 2004

We extend the applications of the photothermal diagnostics by means of laser-infrared photothermal radiometry (PTR) to samples with curved surfaces. Specifically, we present both the theoretical and experimental PTR studies on solid cylindrical samples (surfaces). Based on the Green function method, the thermal-wave field distribution of a cylindrical surface that is under the excitation of a periodically modulated uniform beam is obtained. The laser beam, which is of a cylindrical symmetry, impinges uniformly, partially or fully, on the cylindrical surface, which is practically infinitely long along the axial direction. The characteristics of the thermal-wave field with respect to the thermal diffusivity of the material, the diameter of the sample, the size of the incident beam, and the measurement of the angle are discussed. Experimental results are in good agreement with the theory.
A centroid molecular dynamics study of liquid para-hydrogen and ortho-deuterium
Authors: Hone Tyler D. , Voth Gregory A.
Published: September 2004

Centroid molecular dynamics (CMD) is applied to the study of collective and single-particle dynamics in liquid para-hydrogen at two state points and liquid ortho-deuterium at one state point. The CMD results are compared with the results of classical molecular dynamics, quantum mode coupling theory, a maximum entropy analytic continuation approach, pair-product forward- backward semiclassical dynamics, and available experimental results. The self-diffusion constants are in excellent agreement with the experimental measurements for all systems studied. Furthermore, it is shown that the method is able to adequately describe both the single-particle and collective dynamics of quantum liquids.
Quantum dynamics of H2 formation on a graphite surface through the Langmuir Hinshelwood mechanism
Authors: Morisset S. , Aguillon F. , Sizun M. , Sidis V.
Published: September 2004

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar, and a model potential is obtained in a semiempirical way to reproduce the experimental properties of H physisorption on graphite. The reaction probability has been computed in the case when the angular momentum of the relative H-H motion lies parallel to the surface plane. Three-dimensional wave packet calculations have been performed for collision energies ranging from 2 to 50 meV. It is shown that the reaction occurs with a significant probability and produces the H2 molecule with a considerable amount of vibrationnal energy. A simple mechanical model is presented, where desorption of the nascent H2 molecule results from two successive binary elastic collisions.
Island morphology statistics and growth mechanism for oxidation of the Al(111) surface with thermal O2 and NO
Authors: Sexton J. Z. , Kummel A. C.
Published: September 2004

Scanning tunneling microscopy (STM) was employed to study the mechanism for the oxidation of Al(111) with thermal O2 and NO in the 20%–40% monolayer coverage regime. Experiments show that the islands formed upon exposure to thermal O2 and NO have dramatically different shapes, which are ultimately dictated by the dynamics of the gas surface interaction. The circumference-to-area ratio and other island morphology statistics are used to quantify the average difference in the two island types. Ultrahigh-vacuum STM was employed to make the following observations: (1) Oxygen islands on the Al(111) surface, formed upon exposure to thermal oxygen, are elongated and noncompact. (2) Mixed O/N islands on the Al(111) surface, formed upon exposure to thermal nitric oxide (NO), are round and compact. (3) STM movies acquired during thermal O2 exposure indicate that a complex mechanism involving chemisorption initiated rearrangement of preexisting oxygen islands leads to the asymmetric and elongated island shapes. The overall mechanism for the oxidation of the Al(111) surface can be summarized in three regimes. Low coverage is dominated by widely isolated small oxygen features (<3 O atoms) where normal dissociative chemisorption and oxygen abstraction mechanisms are present. At 20%–40% monolayer coverage, additional oxygen chemisorption induces rearrangement of preexisting islands to form free-energy minimum island shapes. At greater than ∼40% monolayer coverage, the apparent surface oxygen coverage asymptotes corresponding to the conversion of the 2D islands to 3D Al2O3 surface crystallites. The rearrangement of oxygen islands on the surface to form the observed islands indicates that there is a short-range oxygen-oxygen attractive potential and a long-range oxygen-oxygen repulsive potential.
Magnetotransport in C-doped AlGaAs heterostructures
Authors: Grbić B. , Ellenberger C. , Ihn T. , Ensslin K. , Reuter D. , Wieck A. D.
Published: September 2004

High-quality C-doped p-type AlGaAs heterostructures with mobilities exceeding 150000cm2∕Vs are investigated by low-temperature magnetotransport experiments. We find features of the fractional quantum Hall effect as well as a highly resolved Shubnikov–de Haas oscillations at low magnetic fields. This allows us to determine the densities, effective masses, and mobilities of the holes populating the spin-split subbands arising from the lack of inversion symmetry in these structures.
Dynamics of the F atom reaction with propene
Authors: Ran Qin , Yang Chung-Hsin , Lee Yuan T. , Shen Guanlin , Yang Xueming
Published: September 2004

The F+C2H3CH3 reaction has been investigated using the crossed molecular beam technique. Three reaction channels have been observed in this reaction: H+C3H5F, CH3+C2H3F, and HF+C3H5. Time-of-flight spectra as well as product laboratory angular distributions have been measured for the HF, C2H3F, and C3H5F products from these three channels. Relative branching ratios of the three observed reaction channels have also been estimated. Experimental results indicate that these different channels exhibit significantly different reaction dynamics.
Transition path sampling study of classical rate-promoting vibrations
Authors: Antoniou Dimitri , Abolfath Mohammad Ramin , Schwartz Steven D.
Published: September 2004

It is now widely accepted that there is a class of enzymatic proton transfer reactions, which proceed through quantum tunneling. In a series of papers we have argued that some experimental features of these reactions can be explained by assuming the presence of a “rate-promoting” vibration which brings donor and acceptor closer together, thus leading to rate enhancement. There has never been a study of this effect for classical systems. We used transition path sampling to study the equivalent classical problem and found a complicated dynamical behavior that cannot be captured by transition state theory. Slow promoting vibrations lead to reactive trajectories that overshoot the saddle point, but on the other hand the short period of fast oscillations allows the reactants to stay only briefly in a low-barrier regime. There is a competition between these effects, which results to an intermediate value for the frequency of the rate-promoting vibration that is optimal for enhancing the rate.
Fast hydrogen elimination from the [Ru(PH3)3(CO)(H)2] and [Ru(PH3)4(H)2] complexes in the first singlet excited states: A diabatic quantum dynamics study
Authors: Vendrell Oriol , Moreno Miquel , Lluch José M.
Published: September 2004

The photodissociation dynamics of [Ru(PH3)3(CO)(H)2] and cis-[Ru(PH3)4(H)2] is theoretically analyzed in the lowest two excited singlet states. Energies obtained through electronic density functional theory calculations that use the time-dependent formalism are fitted to analytical reduced two-dimensional potential energy surfaces (2D-PES). The metal-H2 (R) and H-H (r) distances are the variables of these 2D-PES, the rest of the parameters being kept frozen at the values of the minimum energy structure in the ground electronic state. The time evolution in these 2D-PES is exactly followed by means of a fast Fourier transform algorithm applied to solve the time-dependent Schrödinger equation. A simple diabatization scheme is devised to take into account the probability of transitions between both excited states. The quantum dynamics results point out that photoelimination is almost inexistent if the H2 fragment is to be expelled without further rearrangement of the rest of the complex. Conversely, when the geometries of the complex are optimized by keeping r and R frozen at the hydrogen elimination barrier coordinates, the new 2D-PES so obtained are highly dissociative, the H2 fragment being expelled in less than 100 fs. Finally the picture of the whole reaction that emerges from our theoretical results is described and the main differences between both complexes are examined.
Prediction of the thermophysical properties of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton by Monte Carlo simulation using ab initio potentials
Authors: Nasrabad A. E. , Laghaei R. , Deiters U. K.
Published: September 2004

Gibbs ensemble Monte Carlo simulations were used to test the ability of intermolecular pair potentials derived ab initio from quantum mechanical principles, enhanced by Axilrod-Teller triple-dipole interactions, to predict the vapor-liquid phase equilibria of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton. The interaction potentials for Ne-Ne, Ar-Ar, Kr-Kr, and Ne-Ar were taken from literature; for Ar-Kr a different potential has been developed. In all cases the quantum mechanical calculations had been carried out with the coupled-cluster approach [CCSD(T) level of theory] and with correlation consistent basis sets; furthermore an extrapolation scheme had been applied to obtain the basis set limit of the interaction energies. The ab initio pair potentials as well as the thermodynamic data based on them are found to be in excellent agreement with experimental data; the only exception is neon. It is shown, however, that in this case the deviations can be quantitatively explained by quantum effects. The interaction potentials that have been developed permit quantitative predictions of high-pressure phase equilibria of noble-gas mixtures.
Interdiffusion of Indium in piezoelectric InGaAs∕GaAs quantum wells grown by molecular beam epitaxy on (11n) substrates
Authors: Yee-Rendón C. M. , Pérez-Centeno A. , Meléndez-Lira M. , de la Cruz G. González , López-López M. , Furuya Kazuo , Vaccaro Pablo O.
Published: September 2004

Pseudomorphic In0.2Ga0.8As∕GaAs quantum wells (QWs) were grown by molecular beam epitaxy on GaAs substrates oriented along (11n) directions, with n=1,2,3,4. The optical and structural properties of the heterostructures were studied by photoluminescence (PL), photoreflectance (PR) spectroscopy, and atomic force microscopy measurements. The energy transitions in the QWs have two contributions, a blueshift due to the compressive strain, and a redshift due to the quantum confined Stark effect produced by the piezoelectric field present in the QWs. A variational approach was employed to calculate the QWs ground energy transitions employing an ideal potential well with sharp interfaces. The theoretical energy transitions were fitted to the PL peaks energy to obtain the electric fields in the InGaAs QWs. The obtained electric fields show discrepancies with theoretical piezoelectric fields calculated from the strain present in the QWs. In order to overcome these discrepancies, we propose to include interdiffusion effects of In at the well interfaces. The matrix transfer method was implemented to numerically solve the Schrödinger equation taking into account In interdiffusion effects by including an asymmetric potential well with a profile depending on the details of the In incorporation. With interdiffusion effects included in the energy level calculations, and assuming the expected piezoelectric fields, the theoretical results reproduce very well the experimental values of PL and PR.
Grand canonical Monte Carlo simulations of water in protein environments
Authors: Woo Hyung-June , Dinner Aaron R. , Roux Benoît
Published: September 2004

The grand canonical simulation algorithm is considered as a general methodology to sample the configuration of water molecules confined within protein environments. First, the probability distribution of the number of water molecules and their configuration in a region of interest for biochemical simulations, such as the active site of a protein, is derived by considering a finite subvolume in open equilibrium with a large system serving as a bulk reservoir. It is shown that the influence of the bulk reservoir can be represented as a many-body potential of mean force acting on the atoms located inside the subvolume. The grand canonical Monte Carlo (GCMC) algorithm, augmented by a number of technical advances to increase the acceptance of insertion attempts, is implemented, and tested for simple systems. In particular, the method is illustrated in the case of a pure water box with periodic boundary conditions. In addition, finite spherical systems of pure water and containing a dialanine peptide, are simulated with GCMC while the influence of the surrounding infinite bulk is incorporated using the generalized solvent boundary potential [W. Im, S. Bernèche, and B. Roux, J. Chem. Phys. 114, 2924 (2001)]. As a last illustration of water confined in the interior of a protein, the hydration of the central cavity of the KcsA potassium channel is simulated.
In-plane refractive-index anisotropy in porous silicon layers induced by polarized illumination during electrochemical etching
Authors: Koyama Hideki
Published: September 2004

Porous silicon (PSi) layers have been anodically etched under polarized illumination, and the degree of linear polarization of their photoluminescence (PL) was measured. The etching conditions were chosen such that the resulting PSi layers were thin enough for interference fringes to appear in their PL spectra. Experimental results show a sinusoidal variation in the degree of linear polarization as a function of the emission photon energy. The amplitude and phase of the sinusoidal variation depend largely on the polarization direction of the excitation light. These observations give strong evidence that there is a significant in-plane anisotropy of the refractive indices for these PSi samples. Values ranging from 0.1% to 0.67% have been obtained for the magnitude of the birefringence. The maximum refractive index is obtained when the excitation-light polarization direction is perpendicular to that of the polarized illumination used during etching. This can be explained by assuming that the polarized photoelectrochemical etching causes the Si structures to be thinner in the polarization direction.
Correlation of strain, wing tilt, dislocation density, and photoluminescence in epitaxial lateral overgrown GaN on SiC substrates
Authors: Gmeinwieser N. , Engl K. , Gottfriedsen P. , Schwarz U. T. , Zweck J. , Wegscheider W. , Miller S. , Lugauer H.-J. , Leber A. , Weimar A. , Lell A. , Härle V.
Published: September 2004

Epitaxial lateral overgrown (ELOG) gallium nitride (GaN) on SiC is being studied as a possible substrate for blue laser diodes. A defect density below 2.2×107cm−2 in the wings, compared to 2×109cm−2 in the windows, was achieved. Interaction of the overgrown GaN with the SiO2 mask causes a few degree wing tilt and a transition region of high defect density between windows and wings. Diminished PL, strong tensile stress, and a defect correlated line at around 3.4eV emerge in this up to two-micron-wide transition region. By changing the mask material from SiO2 to SiN we were able to reduce the wing tilt drastically to below 0.7°. This eliminates the defective transition region and extends the low strain and the low defect density area of the ELOG wings. The methods used to study strain, wing tilt, and threading dislocations in the ELOG samples are microphotoluminescence (μPL), transmission electron microscopy, x–ray diffraction, and scanning electron microscope. We also demonstrate the use of the first momentum of the μPL spectra as an effective means to measure strain distribution.
Fluorine-enhanced boron diffusion induced by fluorine postimplantation in silicon
Authors: Noda Taiji
Published: September 2004

In this article, the postimplanted fluorine effect on boron transient-enhanced diffusion (TED) and dose loss during a 750°C annealing is shown. F19 implants at 2keV, after B11 implant at an energy of 1keV,3×1014∕cm2, have been investigated in the dose range between 1×1013∕cm2 and 6×1014∕cm2 without a preamorphizing implant. Below a F-implant dose of 1×1014∕cm2, the reduction or non-enhancement of boron-TED effect is observed. In the case of a F-implant dose of 6×1014∕cm2, the enhanced boron TED (∼2.6×) in crystalline Si and the increased dose loss(∼2×) than that of a normal boron TED is shown, and this anomalous diffusivity enhancement persists for 120min at 750°C. The B+F6×1014∕cm2 consecutive implant damage is smaller than that of the BF25keV implant. In the case of the B+F6×1014∕cm2, a high content of fluorine is retained around the end-of-range (EOR) damage region within 120min. These results indicate that the fluorine retained around the EOR region may affect the enhanced boron TED in crystalline Si at 750°C. Boron-diffusion model, which describes both the native interstitial fluorine and the boron-fluorine chemical effect, explains that the suppression of boron TED with a low F-implant dose is due to the reduction of the interstitial supersaturation.
Real-time spectroscopic ellipsometry study of ultrathin diffusion barriers for integrated circuits
Authors: Aouadi S. M. , Shreeman P. K. , Williams M.
Published: September 2004

The objective of this work is to monitor the growth process and the thermal stability of ultrathin tantalum nitride barrier nanostructures against copper diffusion in integrated circuits using real-time spectroscopic ellipsometry (RTSE). Single layers of copper and bilayer films of copper and tantalum nitride were produced on Si(111) substrates using unbalanced magnetron sputtering. The RTSE data was simulated using the Bruggeman effective medium approximation and a combined Drude-Lorentz model to obtain information about the growth process, film architecture, interface quality, and the conduction electron transport properties for these structures. The results deduced from the RTSE were verified by characterizing the structural and the chemical properties of the fabricated films using x-ray diffraction, Auger electron spectroscopy, and Rutherford backscattering. The effectiveness of the tantalum nitride barrier to stop the diffusion of copper into silicon was evaluated, monitoring their optical properties when annealed at 720°C. The dielectric function of the films changed from a metallic to an insulating character when the diffusion proceeded. Also, the RTSE provided valuable information about the microstructure and the kinetics of the phase transformations that occur during heat treatment.