665 research outputs found
Galactic Escape Speeds in Mirror and Cold Dark Matter Models
The mirror dark matter (MDM) model of Berezhiani et al. has been shown to
reproduce observed galactic rotational curves for a variety of spiral galaxies,
and has been presented as an alternative to cold dark matter (CDM) models. We
investigate possible additional tests involving the properties of stellar
orbits, which may be used to discriminate between the two models. We
demonstrate that in MDM and CDM models fitted equally well to a galactic
rotational curve, one generally expects predictable differences in escape
speeds from the disc. The recent radial velocity (RAVE) survey of the Milky Way
has pinned down the escape speed from the solar neighbourhood to
km s, placing an additional constraint on dark
matter models. We have constructed an MDM model for the Milky Way based on its
rotational curve, and find an escape speed that is just consistent with the
observed value given the current errors, which lends credence to the viability
of the MDM model. The Gaia-ESO spectroscopic survey is expected to lead to an
even more precise estimate of the escape speed that will further constrain dark
matter models. However, the largest differences in stellar escape speeds
between both models are predicted for dark matter dominated dwarf galaxies such
as DDO 154, and kinematical studies of such galaxies could prove key in
establishing, or abolishing, the validity of the MDM model.Comment: Accepted for publication in the European Physical Journal
Three-dimensional MgB-type superconductivity in hole-doped diamond
We substantiate by calculations that the recently discovered
superconductivity below 4 K in 3% boron-doped diamond is caused by
electron-phonon coupling of the same type as in MgB, albeit in 3
dimensions. Holes at the top of the zone-centered, degenerate -bonding
valence band couple strongly to the optical bond-stretching modes. The increase
from 2 to 3 dimensions reduces the mode-softening crucial for reaching
40 K in MgB Even if diamond had the same \emph{bare} coupling constant
as MgB which could be achieved with 10% doping, would only be 25
K. Superconductivity above 1 K in Si (Ge) requires hole-doping beyond 5% (10%).Comment: revised version, accepted by PR
Electron-phonon interaction in Graphite Intercalation Compounds
Motivated by the recent discovery of superconductivity in Ca- and
Yb-intercalated graphite (CaC and YbC) and from the ongoing debate
on the nature and role of the interlayer state in this class of compounds, in
this work we critically study the electron-phonon properties of a simple model
based on primitive graphite. We show that this model captures an essential
feature of the electron-phonon properties of the Graphite Intercalation
Compounds (GICs), namely, the existence of a strong dormant electron-phonon
interaction between interlayer and electrons, for which we
provide a simple geometrical explanation in terms of NMTO Wannier-like
functions. Our findings correct the oversimplified view that
nearly-free-electron states cannot interact with the surrounding lattice, and
explain the empirical correlation between the filling of the interlayer band
and the occurrence of superconductivity in Graphite-Intercalation Compounds.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
Gutzwiller theory of band magnetism in LaOFeAs
We use the Gutzwiller variational theory to calculate the ground-state phase
diagram and quasi-particle bands of LaOFeAs. The Fe3d--As4p Wannier-orbital
basis obtained from density-functional theory defines the band part of our
eight-band Hubbard model. The full atomic interaction between the electrons in
the iron orbitals is parameterized by the Hubbard interaction U and an average
Hund's-rule interaction J. We reproduce the experimentally observed small
ordered magnetic moment over a large region of (U,J) parameter space. The
magnetically ordered phase is a stripe spin-density wave of quasi-particles.Comment: 4 pages, 5 figure
Inclusion for Cultural Education in Museums, Audio and Touch Interaction
Inclusive access to culture for all people in institutions, such as museums, is an important issue specified in French laws and is also recognized internationally. This article investigates inclusion of blind and partially blind visitors in museums. The pilot study conducted involves blind, partially blind, and sighted people and observes their perception of audio descriptions and different tactile representations within a museum. 12 participants were asked to experience three different conditions for 3 scenes of the Bayeux Tapestry using inclusive and co-created audio descriptions, simplified swell paper representations, and high relief representations. Overall, a high level of interest was found across all conditions, with multimodality through audio and tactile stimulus found to have enriched participants’ experience. However, more guided tactile exploration would be better. From participants’ feedback, some observations have emerged which could be explored for the development of new technologies to better respond to museum visitors’ expectations
História e Etnologia. Lévi-Strauss e os embates em região de fronteira
O objetivo desse texto é recuperar, a partir de dois textos específicos de Lévi-Strauss, o debate que esse autor trava com a História. Tratam-se mais exatamente de dois ensaios que, apesar de apresentarem título idêntico - "História e etnologia" -, foram redigidos em momentos diferentes e, mais interessante, não se remetem um ao outro. O primeiro deles, e o mais conhecido, é parte da famosa coletânea de artigos escritos por Lévi-Strauss durante os anos 40 e 50, intitulada Antropologia Estrutural. O artigo em questão fora publicado originalmente com o mesmo título na Revue de Métaphysique et de Morale, número 54, no ano de 1949. Já o segundo artigo, é o resultado de uma palestra apresentada por Lévi-Strauss, em 2 de julho de 1983, na Sorbonne, por ocasião do quinto ciclo de conferências em homenagem a Marc Bloch e editado na revista dos Annales, no mesmo ano. A intenção é, portanto, tomar as análises de Lévi-Strauss, entendido nessas searas como o mais radical dos antropólogos em seu método sincrônico e sem sujeito, e perceber como na delimitação disciplinar, sobretudo no campo da Antropologia, pareceu necessária a contraposição com a História.The aim of this text is to recover, from two specific texts by Lévi-Strauss, the debate the author leads with History. They are precisely two essays that, although having identical titles - History and Ethnology - were written in different moments, and more interestingly, do not refer to one another. The first and most known is part of the famous collection of essays written by Lévi-Strauss during the forties and fifties, entitled Structural Anthropology. The article in question was originally published with the same title in the Revue de Métaphysique et de Morale, number 54, in the year of 1949. The second article, in its turn, is the result of a lecture held by Lévi-Strauss on 2 July 1983 at Sorbonne, on the occasion of the fifth cycle of conferences in honour of Marc Bloch and edited in the Annales magazine, in the same year. The intention is, therefore, to take Lévi-Strauss analysis, understood in these areas as the most radical of the anthropologists in his synchronal method and without subject, and to perceive how necessary, in the disciplinary delimitation, the contraposition of History was, mainly in the Anthropology field
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