17,168 research outputs found
Substrate induced proximity effect in superconducting niobium nanofilms
Structural and superconducting properties of high quality Niobium nanofilms
with different thicknesses are investigated on silicon oxide and sapphire
substrates. The role played by the different substrates and the superconducting
properties of the Nb films are discussed based on the defectivity of the films
and on the presence of an interfacial oxide layer between the Nb film and the
substrate. The X-ray absorption spectroscopy is employed to uncover the
structure of the interfacial layer. We show that this interfacial layer leads
to a strong proximity effect, specially in films deposited on a SiO
substrate, altering the superconducting properties of the Nb films. Our results
establish that the critical temperature is determined by an interplay between
quantum-size effects, due to the reduction of the Nb film thicknesses, and
proximity effects
Understanding chemical evolution in resolved galaxies -- I The local star fraction-metallicity relation
This work studies the relation between gas-phase oxygen abundance and
stellar-to-gas fraction in nearby galaxies. We first derive the theoretical
prediction, and argue that this relation is fundamental, in the sense that it
must be verified regardless of the details of the gas accretion and star
formation histories. Moreover, it should hold on "local" scales, i.e. in
regions of the order of 1 kpc. These predictions are then compared with a set
of spectroscopic observations, including both integrated and resolved data.
Although the results depend somewhat on the adopted metallicity calibration,
observed galaxies are consistent with the predicted relation, imposing tight
constraints on the mass-loading factor of (enriched) galactic winds. The
proposed parametrization of the star fraction-metallicity relation is able to
describe the observed dependence of the oxygen abundance on gas mass at fixed
stellar mass. However, the "local" mass-metallicity relation also depends on
the relation between stellar and gas surface densities.Comment: 10 pages, 4 figures. Matches accepted version (significant typo
corrected
Exact Nonequilibrium Work Generating Function for a Small Classical System
We obtain the exact nonequilibrium work generating function (NEWGF), for a
small system consisting of a massive Brownian particle connected to internal
and external springs. The external work is provided to the system for a finite
time interval. The Jarzynski equality (JE), obtained in this case directly from
the NEWGF, is shown to be valid for the present model, in an exact way
regardless of the rate of external work
On the Electronic Transport Mechanism in Conducting Polymer Nanofibers
Here, we present theoretical analysis of electron transport in polyaniline
based (PANi) nanofibers assuming the metalic state of the material. To build up
this theory we treat conducting polymers as a special kind of granular metals,
and we apply the quantum theory of conduction in mesoscopic systems to describe
the transport between metallic-like granules. Our results show that the concept
of resonance electron tunneling as the predominating mechanism providing charge
transport between the grains is supported with recent experiments on the
electrical characterization of single PANi nanofibers. By contacting the
proposed theory with the experimental data we estimate some important
parameters characterizing the electron transport in these materials. Also, we
discuss the origin of rectifying features observed in current-voltage
characteristics of fibers with varying cross-sectional areas.Comment: 5 pages, 1 figure, accepted for publication in Phys. Rev. B, Vol.72,
xxxx (2005
Energy-momentum Density of Gravitational Waves
In this paper, we elaborate the problem of energy-momentum in general
relativity by energy-momentum prescriptions theory. Our aim is to calculate
energy and momentum densities for the general form of gravitational waves. In
this connection, we have extended the previous works by using the prescriptions
of Bergmann and Tolman. It is shown that they are finite and reasonable. In
addition, using Tolman prescription, exactly, leads to same results that have
been obtained by Einstein and Papapetrou prescriptions.Comment: LaTeX, 9 pages, 1 table: added reference
Exercise-Induced Asthma in Children a Contribution for the Standardization of Challenge Tests
A asma induzida pelo exercÃcio (AIE), apesar de entidade muito prevalente na criança asmática,apresenta aspectos etiopatogénicos não totalmente esclarecidos. De igual modo, os aspectos metodológicos das provas de provocação utilizadas para o diagnóstico
da AIE não estão normalizados.
Objectivos: Foram objectivos deste trabalho estudar sensibilidade e especificidade de diferentes parâmetros funcionais (DEMI vs VEMS) e equipamentos(bicicleta ergométrica vs tapete rolante) utilizados nas provas de esforço, monitorizar a ocorrência de
respostas tardias ao esforço e correlacionar os resultados com os de uma prova de provocação brônquica inespecÃfica normalizada (metacolina).
Métodos: Foram incluÃdas 22 crianças com asma e 10 com rinite (grupo controle), com 7-15 anos. Todas as crianças foram submetidas a três provas de provocação efectuadas no intervalo de 1 mês, provas de esforço em tapete rolante (T) e com bicicleta
ergométrica (B) e prova de metacolina, efectuadas sob condições ambientais controladas e com interrupção da medicação relevante.
Resultados: Sensibilidade: T - redução VEMS
>=10%=82%; B - redução VEMS>=10%=57%;
T - redução DEMI>=10%=86%; B - redução DEMI
>=10%=81%. Especificidade: T & B - redução VEMS >=10%=100%; T - redução DEMI>=10%=40%;
B - redução DEMI>=10%=33%. Não se identificaram respostas broncoconstritoras tardias ao exercÃcio. As crianças asmáticas com prova de esforço positiva apresentaram uma reactividade à metacolina
significativamente superior à dos asmáticos com prova negativa.Conclusões: A prova de esforço em tapete rolante representa o melhor teste para diagnóstico da AIE na
criança. O parâmetro funcional normalizado a ser utilizado deverá ser o VEMS, aceitando-se uma redução de 10% como critério de positividade. Os doentes com resposta positiva ao esforço foram igualmente mais
reactivos à prova de metacolina, isto é apresentam hiperreactividade brônquica inespecÃfica mais grave, o que substancia o interesse da terapêutica preventiva
anti-inflamatória com corticóides tópicos e/ou antileucotrienos no controle da AIE
Determination of phytoextraction potential of plant speciesfor toxic elements in soils of abandoned sulphide-mining areas
This study has determined contamination levels in soils and plants from the SaËœo Domingos mining area, Portugal, by k0-INAA. Total concentrations of As, Sb, Cr, Hg, Cu, Zn and Fe in soils were very high, exceeding the maximum limits in Portuguese legislation. Concentrations of toxic elements like As, Sb and Zn were highest in roots of Erica andevalensis, Juncus acutus, Agrostis castellana and Nicotiana glauca. Additionally, As, Br, Cr, Fe, Sb and Zn in all organs of most plants were above toxicity levels.
Those species that accumulated relatively high concentrations of toxic elements in roots (and tops) may be cultivated for phytostabilisation of similar areas
Quantum Cosmology in Scalar-Tensor Theories With Non Minimal Coupling
Quantization in the minisuperspace of non minimal scalar-tensor theories
leads to a partial differential equation which is non separable. Through a
conformal transformation we can recast the Wheeler-DeWitt equation in an
integrable form, which corresponds to the minimal coupling case, whose general
solution is known. Performing the inverse conformal transformation in the
solution so found, we can construct the corresponding one in the original
frame. This procedure can also be employed with the bohmian trajectories. In
this way, we can study the classical limit of some solutions of this quantum
model. While the classical limit of these solutions occurs for small scale
factors in the Einstein's frame, it happens for small values of the scalar
field non minimally coupled to gravity in the Jordan's frame, which includes
large scale factors.Comment: latex, 18 page
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