26,063 research outputs found
Recommended from our members
A Double Error Dynamic Asymptote Model of Associative Learning
In this paper a formal model of associative learning is presented which incorporates representational and computational mechanisms that, as a coherent corpus, empower it to make accurate predictions of a wide variety of phenomena that so far have eluded a unified account in learning theory. In particular, the Double Error Dynamic Asymptote (DDA) model introduces: 1) a fully-connected network architecture in which stimuli are represented as temporally clustered elements that associate to each other, so that elements of one cluster engender activity on other clusters, which naturally implements neutral stimuli associations and mediated learning; 2) a predictor error term within the traditional error correction rule (the double error), which reduces the rate of learning for expected predictors; 3) a revaluation associability rate that operates on the assumption that the outcome predictiveness is tracked over time so that prolonged uncertainty is learned, reducing the levels of attention to initially surprising outcomes; and critically 4) a biologically plausible variable asymptote, which encapsulates the principle of Hebbian learning, leading to stronger associations for similar levels of cluster activity. The outputs of a set of simulations of the DDA model are presented along with empirical results from the literature. Finally, the predictive scope of the model is discussed
SPICE modelling of photoluminescence and electroluminescence based current-voltage curves of solar cells for concentration applications
Quantitative photoluminescence (PL) or electroluminescence (EL) experiments can be used to probe fast and in a non-destructive way the current-voltage (IV) characteristics of individual subcells in a multi-junction device, information that is, otherwise, not available. PL-based IV has the advantage that it is contactless and can be performed even in partly finished devices, allowing for an early diagnosis of the expected performance of the solar cells in the production environment. In this work we simulate the PL- and EL-based IV curves of single junction solar cells to assess their validity compared with the true IV curve and identify injection regimes where artefacts might appear due to the limited in-plane carrier transport in the solar cell layers. We model the whole photovoltaic device as a network of sub-circuits, each of them describing the solar cell behaviour using the two diode model. The sub-circuits are connected to the neighbouring ones with a resistor, representing the in-plane transport in the cell. The resulting circuit, involving several thousand subcircuits, is solved using SPICE
Rotationally resolved spectroscopy of dwarf planet (136472) Makemake
Context. Icy dwarf planets are key for studying the chemical and physical
states of ices in the outer solar system. The study of secular and rotational
variations gives us hints of the processes that contribute to the evolution of
their surface. Aims. The aim of this work is to search for rotational
variability on the surface composition of the dwarf planet (136472) Makemake
Methods. We observed Makemake in April 2008 with the medium-resolution
spectrograph ISIS, at the William Herschel Telescope (La Palma, Spain) and
obtained a set of spectra in the 0.28 - 0.52 {\mu}m and 0.70 - 0.95 {\mu}m
ranges, covering 82% of its rotational period. For the rotational analysis, we
organized the spectra in four different sets corresponding to different
rotational phases, and after discarding one with low signal to noise, we
analyzed three of them that cover 71% of the surface. For these spectra we
computed the spectral slope and compared the observed spectral bands of methane
ice with reflectances of pure methane ice to search for shifts of the center of
the bands, related to the presence of CH 4 /N 2 solid solution. Results. All
the spectra have a red color with spectral slopes between 20%/1000 {\AA} and
32%/1000 {\AA} in accordance with previously reported values. Some variation in
the spectral slope is detected, pointing to the possibility of a variation in
the surface content or the particle size of the solid organic compound. The
absorption bands of methane ice present a shift toward shorter wavelengths,
indicating that methane (at least partially) is in solid solution with
nitrogen. There is no variation within the errors of the shifts with the
wavelength or with the depth of the bands, so there is no evidence of variation
in the CH 4 /N 2 mixing ratio with rotation. By comparing with all the
available data in the literature, no secular compositional variations between
2005 and 2008 is found
On the galactic chemical evolution of sulfur
Sulfur abundances have been determined for ten stars to resolve a debate in
the literature on the Galactic chemical evolution of sulfur in the halo phase
of the Milky Way. Our analysis is based on observations of the S I lines at
9212.9, 9228.1, and 9237.5 A for stars for which the S abundance was obtained
previously from much weaker S I lines at 8694.0 and 8694.6 A. In contrast to
the previous results showing [S/Fe] to rise steadily with decreasing [Fe/H],
our results show that [S/Fe] is approximately constant for metal-poor stars
([Fe/H] < -1) at [S/Fe] = +0.3. Thus, sulfur behaves in a similar way to the
other alpha elements, with an approximately constant [S/Fe] for metallicities
lower than [Fe/H] = -1. We suggest that the reason for the earlier claims of a
rise of [S/Fe] is partly due to the use of the weak S I 8694.0 and 8694.6 A
lines and partly uncertainties in the determination of the metallicity when
using Fe I lines. The S I 9212.9, 9228.1, and 9237.5 A lines are preferred for
an abundance analysis of sulfur for metal-poor stars.Comment: Accepted by A&A, 12 pages. Full article with figures in A&
Infrared 3D Observations of Nearby Active Galaxies
We present multi-wavelength imaging observations of three nearby and famous
active galaxies obtained with NICMOS, ISOCAM and the MPE near-IR integral field
spectrometer. The data reveal a variety of features and properties that are
missed in optical studies and in traditional IR monodimensional spectroscopy.Comment: 6 pages, to appear in "Imaging the Universe in Three Dimensions:
Astrophysics with Advanced Multi-Wavelength Imaging Devices", eds. W. van
Breugel and J. Bland-Hawthorn, needs pasp3D.st
- …