44,855 research outputs found
Ferromagnetism and temperature-dependent Electronic Structure of hcp Gadolinium
We use a combination of a many-body model analysis with an ab initio band
structure calculation to derive the temperature dependent electronic
quasiparticle structure of the rare-earth metal Gadolinium. As a local-moment
system Gd is properly represented by the ferromagnetic (multiband)
Kondo-lattice model (s-f (d-f) model). The single-particle part of the
model-Hamiltonian is taken from an augmented spherical wave (ASW) band
calculation. The proposed method avoids the double counting of relevant
interactions by exploiting an exact limiting case of the model and takes into
account the correct symmetry of atomic orbitals. The a priori only weakly
correlated 5d conduction bands get via interband exchange coupling to the
localized 4f levels a distinct temperature dependence which explains by a
Rudermann-Kittel-Kasuya-Yosida (RKKY) -type mechanism the ferromagnetism of Gd.
We get a self-consistently derived Curie temperature of 294.1 K and a
T=0-moment of 7.71 , surprisingly close to the experimental
values. The striking induced temperature-dependence of the 5d conduction bands
explains respective photoemission data. The only parameter of the theory
(interband exchange coupling J) is uniquely fixed by the band calculation.Comment: 12 pages, 9 figure
Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach
Light is affected by local inhomogeneities in its propagation, which may
alter distances and so cosmological parameter estimation. In the era of
precision cosmology, the presence of inhomogeneities may induce systematic
errors if not properly accounted. In this vein, a new interpretation of the
conventional Dyer-Roeder (DR) approach by allowing light received from distant
sources to travel in regions denser than average is proposed. It is argued that
the existence of a distribution of small and moderate cosmic voids (or "black
regions") implies that its matter content was redistributed to the homogeneous
and clustered matter components with the former becoming denser than the cosmic
average in the absence of voids. Phenomenologically, this means that the DR
smoothness parameter (denoted here by ) can be greater than unity,
and, therefore, all previous analyses constraining it should be rediscussed
with a free upper limit. Accordingly, by performing a statistical analysis
involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a
flat CDM model we obtain for the extended parameter,
(). The effects of are also
analyzed for generic CDM models and flat XCDM cosmologies. For both
models, we find that a value of greater than unity is able to
harmonize SNe Ia and cosmic microwave background observations thereby
alleviating the well-known tension between low and high redshift data. Finally,
a simple toy model based on the existence of cosmic voids is proposed in order
to justify why can be greater than unity as required by supernovae
data.Comment: 5 pages, 2 figures. Title modified, results unchanged. It matches
version published as a Brief Report in Phys. Rev.
Sparse spatial selection for novelty-based search result diversification
Abstract. Novelty-based diversification approaches aim to produce a diverse ranking by directly comparing the retrieved documents. However, since such approaches are typically greedy, they require O(n 2) documentdocument comparisons in order to diversify a ranking of n documents. In this work, we propose to model novelty-based diversification as a similarity search in a sparse metric space. In particular, we exploit the triangle inequality property of metric spaces in order to drastically reduce the number of required document-document comparisons. Thorough experiments using three TREC test collections show that our approach is at least as effective as existing novelty-based diversification approaches, while improving their efficiency by an order of magnitude.
Revisiting the correlation between stellar activity and planetary surface gravity
Aims: We re-evaluate the correlation between planetary surface gravity and
stellar host activity as measured by the index log(). This
correlation, previously identified by Hartman (2010), is now analyzed in light
of an extended measurements dataset, roughly 3 times larger than the original
one.
Methods: We calculated the Spearman's rank correlation coefficient between
the two quantities and its associated p-value. The correlation coefficient was
calculated for both the full dataset and the star-planet pairs that follow the
conditions proposed by Hartman (2010). In order to do so, we considered
effective temperatures both as collected from the literature and from the
SWEET-Cat catalog, which provides a more homogeneous and accurate effective
temperature determination.
Results: The analysis delivers significant correlation coefficients, but with
a lower value than those obtained by Hartman (2010). Yet, the two datasets are
compatible, and we show that a correlation coefficient as large as previously
published can arise naturally from a small-number statistics analysis of the
current dataset. The correlation is recovered for star-planet pairs selected
using the different conditions proposed by Hartman (2010). Remarkably, the
usage of SWEET-Cat temperatures leads to larger correlation coefficient values.
We highlight and discuss the role of the correlation betwen different
parameters such as effective temperature and activity index. Several additional
effects on top of those discussed previously were considered, but none fully
explains the detected correlation. In light of the complex issue discussed
here, we encourage the different follow-up teams to publish their activity
index values in the form of log() index so that a comparison across
stars and instruments can be pursued.Comment: 11 pages, 3 figures, accepted for publication in A&
Is CDM an effective CCDM cosmology?
We show that a cosmology driven by gravitationally induced particle
production of all non-relativistic species existing in the present Universe
mimics exactly the observed flat accelerating CDM cosmology with just
one dynamical free parameter. This kind of scenario includes the creation cold
dark matter (CCDM) model [Lima, Jesus & Oliveira, JCAP 011(2010)027] as a
particular case and also provides a natural reduction of the dark sector since
the vacuum component is not needed to accelerate the Universe. The new cosmic
scenario is equivalent to CDM both at the background and perturbative
levels and the associated creation process is also in agreement with the
universality of the gravitational interaction and equivalence principle.
Implicitly, it also suggests that the present day astronomical observations
cannot be considered the ultimate proof of cosmic vacuum effects in the evolved
Universe because CDM may be only an effective cosmology.Comment: 6 pages, 2 figures, changes in the abstract, introduction, new
references and typo correction
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