7,883 research outputs found
Kinetic Monte Carlo simulation of the nitridation of the GaAs (100) surfaces
We present, in this work, our preliminary results of a systematic theoretical
study of the adsorption of N over As-terminated GaAs (100) (21)
surfaces. We analyzed the changes in the bond-lenghts, bond-angles and the
energetics involved before and after deposition. Our results show that the
N-atoms will prefer the unoccupied sites of the surface, close to the As dimer.
The presence of the N pushes the As dimer out of the surface, leading to the
anion exchange between the N and As atoms. Based on our results, we discussed
about the kinetics of the N islands formation during epitaxial growth of the
III-Nitrides.Comment: 4 pages, 7 figures, accepted for publication in Braz. J. Phys.,
special number, Proceedings of BWSP-12, 12th Brazilian Workshop on
Semiconductor Physic
Age-related increase of kynurenic acid in human cerebrospinal fluid-IgG and beta(2)-microglobulin changes
Kynurenic acid (KYNA) is an endogenous metabolite in the kynurenine pathway of tryptophan degradation and is an antagonist at the glycine site of the N-methyl-D-aspartate as well as at the alpha 7 nicotinic cholinergic receptors. In the brain tissue KYNA is synthesised from L-kynurenine by kynurenine aminotransferases (KAT) I and II. A host of immune mediators influence tryptophan degradation. In the present study, the levels of KYNA in cerebrospinal fluid (CSF) and serum in a group of human subjects aged between 25 and 74 years were determined by using a high performance liquid chromatography method. In CSF and serum KAT I and II activities were investigated by radioenzymatic assay, and the levels of β2-microglobulin, a marker for cellular immune activation, were determined by ELISA. The correlations between neurochemical and biological parameters were evaluated. Two subject groups with significantly different ages, i.e. 50 years, p < 0.001, showed statistically significantly different CSF KYNA levels, i.e. 2.84 ± 0.16 fmol/μl vs. 4.09 ± 0.14 fmol/μl, p < 0.001, respectively; but this difference was not seen in serum samples. Interestingly, KYNA is synthesised in CSF principally by KAT I and not KAT II, however no relationship was found between enzyme activity and ageing. A positive relationship between CSF KYNA levels and age of subjects indicates a 95% probability of elevated CSF KYNA with ageing (R = 0.6639, p = 0.0001). KYNA levels significantly correlated with IgG and β2-microglobulin levels (R = 0.5244, p = 0.0049; R = 0.4253, p = 0.043, respectively). No correlation was found between other biological parameters in CSF or serum. In summary, a positive relationship between the CSF KYNA level and ageing was found, and the data would suggest age-dependent increase of kynurenine metabolism in the CNS. An enhancement of CSF IgG and β2-microglobulin levels would suggest an activation of the immune system during ageing. Increased KYNA metabolism may be involved in the hypofunction of the glutamatergic and/or nicotinic cholinergic neurotransmission in the ageing CNS
A Nuclear Physics Program at the ATLAS Experiment at the CERN Large Hadron Collider
The ATLAS collaboration has significant interest in the physics of
ultra-relativistic heavy ion collisions. We submitted a Letter of Intent to the
United States Department of Energy in March 2002. The following document is a
slightly modified version of that LOI. More details are available at:
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/SM/ionsComment: Letter of Intent submitted to the United States Department of Energy
Nuclear Physics Division in March 2002 (revised version
The Asymptotics of Wilkinson's Iteration: Loss of Cubic Convergence
One of the most widely used methods for eigenvalue computation is the
iteration with Wilkinson's shift: here the shift is the eigenvalue of the
bottom principal minor closest to the corner entry. It has been a
long-standing conjecture that the rate of convergence of the algorithm is
cubic. In contrast, we show that there exist matrices for which the rate of
convergence is strictly quadratic. More precisely, let be the matrix having only two nonzero entries and let
be the set of real, symmetric tridiagonal matrices with the same spectrum
as . There exists a neighborhood of which is
invariant under Wilkinson's shift strategy with the following properties. For
, the sequence of iterates exhibits either strictly
quadratic or strictly cubic convergence to zero of the entry . In
fact, quadratic convergence occurs exactly when . Let be
the union of such quadratically convergent sequences : the set has
Hausdorff dimension 1 and is a union of disjoint arcs meeting at
, where ranges over a Cantor set.Comment: 20 pages, 8 figures. Some passages rewritten for clarit
Logarithmic periodicities in the bifurcations of type-I intermittent chaos
The critical relations for statistical properties on saddle-node bifurcations
are shown to display undulating fine structure, in addition to their known
smooth dependence on the control parameter. A piecewise linear map with the
type-I intermittency is studied and a log-periodic dependence is numerically
obtained for the average time between laminar events, the Lyapunov exponent and
attractor moments. The origin of the oscillations is built in the natural
probabilistic measure of the map and can be traced back to the existence of
logarithmically distributed discrete values of the control parameter giving
Markov partition. Reinjection and noise effect dependences are discussed and
indications are given on how the oscillations are potentially applicable to
complement predictions made with the usual critical exponents, taken from data
in critical phenomena.Comment: 4 pages, 6 figures, accepted for publication in PRL (2004
Brucelose bovina no Pantanal Sul-Matogrossense: dados preliminares.
Com o objetivo de levantar a freqüência preliminar de bovinos de corte não vacinados do Pantanal, foram visitados, no período de 1994 a 1996, 16 rebanhos e 309 animais, sorteados dentre as fazendas cadastradas na Embrapa Pantanal. A freqüência foi estimada com base no número de animais que apresentavam resultados positivos em ambos os testes (AAT e soroaglutinação lenta/2 mercaptoetanol (2ME)) Os resultados do testes diagnósticos confirmatórios foram positivos, negativos ou inconclusivos e classificou-se como positiva a propriedade que apresentou pelo menos um animal positivo no teste confirmatório (2ME); negativa, quando todas os animais apresentaram-se negativos nos testes sorológicos e inconclusiva, quando existiram resultados sorológicos negativos e inconclusivo em pelo menos um dos animais.bitstream/CPAC/28669/1/COT58.pd
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