3,020 research outputs found
Application of photoluminescence and electroluminescence techniques to the characterization of intermediate band solar cells
The intermediatebandsolarcell (IBSC) is a photovoltaic device with a theoretical conversion efficiency limit of 63.2%. In recent years many attempts have been made to fabricate an intermediateband material which behaves as the theory states. One characteristic feature of an IBSC is its luminescence spectrum. In this work the temperature dependence of the photoluminescence (PL) and electroluminescence (EL) spectra of InAs/GaAs QD-IBSCs together with their reference cell have been studied. It is shown that EL measurements provide more reliable information about the behaviour of the IB material inside the IBSC structure than PL measurements. At low temperatures, the EL spectra are consistent with the quasi-Fermi level splits described by the IBSC model, whereas at room temperature they are not. This result is in agreement with previously reported analysis of the quantum efficiency of the solarcell
On inhibiting Auger intraband relaxation in InAs/GaAs quantum dot intermediate band solar cells
Wavelet transforms for non-uniform speech recognition
An algorithm for nonuniform speech segmentation and its application in speech recognition systems is presented. A method based on the Modulated Gaussian Wavelet Transform based Speech Analyser (MGWTSA) and the subsequent parametrization block is used to transform a uniform signal into a set of nonuniformly separated frames, with the accurate information being fed into a speech recognition system. The algorithm needs a frame characterizing the signal where necessary, trying to reduce the number of frames per signal as much as possible, without an appreciable reduction in the recognition rate of the system.Peer ReviewedPostprint (published version
S5 0836+710: An FRII jet disrupted by the growth of a helical instability?
The remarkable stability of extragalactic jets is surprising, given the
reasonable possibility of the growth of instabilities. In addition, much work
in the literature has invoked this possibility in order to explain observed jet
structures and obtain information from these structures. For example, it was
recently shown that the observed helical structures in the jet in S5 0836+710
could be associated with helical pressure waves generated by Kelvin-Helmholtz
instability. Our aim is to resolve the arc-second structure of the jet in the
quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found
by present observing arrays, as this lack implies a loss of jet collimation
before interaction with the intergalactic medium. In this work, we use an
observation performed in 2008 using EVN and MERLIN. The combined data reduction
has provided a complete image of the object at arc-second scales. The lack of a
hot-spot in the arc-second radio structure is taken as evidence that the jet
losses its collimation between the VLBI region and the region of interaction
with the ambient medium. This result, together with the previous identification
of the helical structures in the jet with helical pressure waves that grow in
amplitude with distance, allow us to conclude that the jet is probably
disrupted by the growth of Kelvin-Helmholtz instability. This observational
evidence confirms that the physical parameters of jets can be extracted using
the assumption that instability is present in jets and can be the reason for
many observed structures. Interestingly, the observed jet is classified as a
FRII object in terms of its luminosity, but its large-scale morphology does not
correspond to this classification. The implications of this fact are discussed.Comment: Accepted for publication in Astronomy & Astrophysic
Bioactividad de extractos de Melia azedarach L. sobre el taladro del maíz Sesamia nonagrioides Lef.
A partir de un extracto metanólico de semillas de Melia azedarach se han aislado los compuestos responsables de su actividad biológica como agentes inhibidores de la alimentación y reguladores del desarrollo sobre Sesamia nonagrioides. Las pruebas biológicas de actividad se han realizado incorporando cada fracción a la dieta, hojas de maíz o dieta sintética, de larvas de 2°estadio del insecto. La técnica de cromatografía líquida de alta resolución a escala semipreparativa, utilizando un cartucho de fase reversa, ha permitido separar dos componentes con cantidad y pureza suficientes para poder ser analizados con posterioridad mediante técnicas
espectroscópicas.A systematic investigation of methanolic extracts of the seeds of Melia azedarach, has
been carried out to isolate the main compounds, biologically active as feeding inhibitors and as insect growth regulators, for the Mediterranean Corn Borer, Sesamia nonagrioides. Biological activity of each fraction was evaluated by presenting it to the 2 on. instar larvae of the insect into its feeding diet, maize leaves or synthetic diet. A high performance liquid chromatographic reversed phase procedure has been developed whereby the two main active products of the extract could be isolated in amount enough to be later analyzed using spectroscopic techniques
Optical spectroscopy of microquasar candidates at low galactic latitudes
We report optical spectroscopic observations of a sample of 6 low-galactic
latitude microquasar candidates selected by cross-identification of X-ray and
radio point source catalogs for |b|<5 degrees. Two objects resulted to be of
clear extragalactic origin, as an obvious cosmologic redshift has been measured
from their emission lines. For the rest, none exhibits a clear stellar-like
spectrum as would be expected for genuine Galactic microquasars. Their
featureless spectra are consistent with being extragalactic in origin although
two of them could be also highly reddened stars. The apparent non-confirmation
of our candidates suggests that the population of persistent microquasar
systems in the Galaxy is more rare than previously believed. If none of them is
galactic, the upper limit to the space density of new Cygnus X-3-like
microquasars within 15 kpc would be 1.1\times10^{-12} per cubic pc. A similar
upper limit for new LS 5039-like systems within 4 kpc is estimated to be
5.6\times10^{-11} per cubic pc.Comment: 7 pages, 7 figures. Published in A&A, see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...413..309
Relativistic simulations of rotational core collapse. I. Methods, initial models, and code tests
We describe an axisymmetric general relativistic code for rotational core
collapse. The code evolves the coupled system of metric and fluid equations
using the ADM 3+1 formalism and a conformally flat metric approximation of the
Einstein equations. The relativistic hydrodynamics equations are formulated as
a first-order flux-conservative hyperbolic system and are integrated using
high-resolution shock-capturing schemes based on Riemann solvers. We assess the
quality of the conformally flat metric approximation for relativistic core
collapse and present a comprehensive set of tests which the code successfully
passed. The tests include relativistic shock tubes, the preservation of the
rotation profile and of the equilibrium of rapidly and differentially rotating
neutron stars (approximated as rotating polytropes), spherical relativistic
core collapse, and the conservation of rest-mass and angular momentum in
dynamic spacetimes. The application of the code to relativistic rotational core
collapse, with emphasis on the gravitational waveform signature, is presented
in an accompanying paper.Comment: 18 pages, 12 figure
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