2,500 research outputs found
Learning Design: reflections on a snapshot of the current landscape
The mounting wealth of open and readily available information and the swift evolution of social, mobile and creative technologies warrant a re-conceptualisation of the role of educators: from providers of knowledge to designers of learning. This need is being addressed by a growing trend of research in Learning Design. Responding to this trend, the Art and Science of Learning Design workshop brought together leading voices in the field and provided a forum for discussing its key issues. It focused on three thematic axes: practices and methods, tools and resources, and theoretical frameworks. This paper reviews some definitions of Learning Design and then summarises the main contributions to the workshop. Drawing upon these, we identify three key challenges for Learning Design that suggest directions for future research
Fictive Impurity Approach to Dynamical Mean Field Theory: a Strong-Coupling Investigation
Quantum Monte Carlo and semiclassical methods are used to solve two and four
site cluster dynamical mean field approximations to the square lattice Hubbard
model at half filling and strong coupling. The energy, spin correlation
function, phase boundary and electron spectral function are computed and
compared to available exact results. The comparision permits a quantitative
assessment of the ability of the different methods to capture the effects of
intersite spin correlations. Two real space methods and one momentum space
representation are investigated. One of the two real space methods is found to
be significantly worse: in it, convergence to the correct results is found to
be slow and, for the spectral function, nonuniform in frequency, with
unphysical midgap states appearing. Analytical arguments are presented showing
that the discrepancy arises because the method does not respect the pole
structure of the self energy of the insulator. Of the other two methods, the
momentum space representation is found to provide the better approximation to
the intersite terms in the energy but neither approximation is particularly
acccurate and the convergence of the momentum space method is not uniform. A
few remarks on numerical methods are made.Comment: Errors in previous versions corrected; CDMFT results adde
On four numerical schemes for a unipolar degenerate drift-diffusion model
International audienceWe consider a unipolar degenerate drift-diffusion system where the relation between the concentration of the charged species c and the chemical potential h is . For four different finite volume schemes based on four different formulations of the fluxes of the problem, we discuss stability and existence results. For two of them, we report a convergence proof. Numerical experiments illustrate the behaviour of the different schemes
Detailed analysis of Balmer lines in cool dwarf stars
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and
subgiant stars is presented using MARCS model atmospheres based on the most
recent calculations of the line opacities. A detailed quantitative comparison
of the solar flux spectra with model spectra shows that Balmer line profile
shapes, and therefore the temperature structure in the line formation region,
are best represented under the mixing length theory by any combination of a low
mixing-length parameter alpha and a low convective structure parameter y. A
slightly lower effective temperature is obtained for the sun than the accepted
value, which we attribute to errors in models and line opacities. The programme
stars span temperatures from 4800 to 7100 K and include a small number of
population II stars. Effective temperatures have been derived using a
quantitative fitting method with a detailed error analysis. Our temperatures
find good agreement with those from the Infrared Flux Method (IRFM) near solar
metallicity but show differences at low metallicity where the two available
IRFM determinations themselves are in disagreement. Comparison with recent
temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who
employed a different description of the wing absorption due to self-broadening,
does not show the large differences predicted by Barklem et al. (2000). In
fact, perhaps fortuitously, reasonable agreement is found near solar
metallicity, while we find significantly cooler temperatures for low
metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&
Very Long Baseline Array observations of the Intraday Variable source J1128+592
Short time-scale flux density variations of flat spectrum radio sources are
often explained by the scattering of radio waves in the turbulent, ionized
Interstellar Matter of the Milky Way. One of the most convincing observational
arguments in favor of this is the annual modulation of the variability
time-scale caused by the Earth orbital motion around the Sun. J1128+592 is an
IDV source with a possible annual modulation in its variability time-scale. We
observed the source in 6 epochs with the VLBA at 5, 8 and 15 GHz in total
intensity and polarization. The VLBA observations revealed an east-west
oriented core-jet structure. Its position angle agrees with the angle of
anisotropy derived from the annual modulation model. No significant long-term
structural changes were observed with VLBI on mas-scales, however, compared to
archival data, the VLBI core size is expanded. This expansion offers a possible
explanation to the observed decrease of the strength of IDV. VLBI polarimetry
revealed significant changes in the electric vector position angle and Rotation
Measure of the core and jet. Part of the observed RM variability could be
attributed to a scattering screen (37 pc distance), which covers the source
(core and jet) and which may be responsible for the IDV. Superposition of
polarized sub-components below the angular resolution limit may affect the
observed RM as well.Comment: accepted for A&A (11 pages, 11 figures
From Solar Proton Burning to Pionic Deuterium through the Nambu-Jona-Lasinio model of light nuclei
Within the Nambu-Jona-Lasinio model of light nuclei (the NNJL model),
describing strong low-energy nuclear interactions, we compute the width of the
energy level of the ground state of pionic deuterium. The theoretical value
fits well the experimental data. Using the cross sections for the reactions
nu_e + d -> p + p + e^- and nu_e + d -> p + n + nu_e, computed in the NNJL
model, and the experimental values of the events of these reactions, detected
by the SNO Collaboration, we compute the boron neutrino fluxes. The theoretical
values agree well with the experimental data and the theoretical predictions
within the Standard Solar Model by Bahcall. We argue the applicability of the
constraints on the astrophysical factor for the solar proton burning, imposed
by helioseismology, to the width of the energy level of the ground state of
pionic deuterium. We show that the experimental data on the width satisfy these
constraints. This testifies an indirect measurement of the recommended value of
the astrophysical factor for the solar proton burning in terrestrial
laboratories in terms of the width of the energy level of the ground state of
pionic deuterium.Comment: 10 pages, no figures, Late
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