8,212 research outputs found
Impact of electronic correlations on the equation of state and transport in -Fe
We have obtained the equilibrium volumes, bulk moduli, equations of state of
the ferromagnetic cubic and paramagnetic hexagonal phases
of iron in close agreement with experiment using an ab initio dynamical
mean-field theory approach. The local dynamical correlations are shown to be
crucial for a successful description of the ground-state properties of
paramagnetic -Fe. Moreover, they enhance the effective mass of the
quasiparticles and reduce their lifetimes across the
transition leading to a step-wise increase of the resistivity, as observed in
experiment. The calculated magnitude of the jump is significantly
underestimated, which points to non-local correlations. The implications of our
results for the superconductivity and non-Fermi-liquid behavior of
-Fe are discussed.Comment: 6 pages, 3 figure
Photometric and spectroscopic variations of the Be star HD 112999
Be objects are stars of B spectral type showing lines of the Balmer series in
emission. The presence of these lines is attributed to the existence of an
extended envelope, disk type, around them. Some stars are observed in both the
Be and normal B-type spectroscopic states and they are known as transient Be
stars. In this paper we show the analysis carried out on a new possible
transient Be star, labelled HD 112999, using spectroscopic optical observations
and photometric data.Comment: 10 pages, 5 figures, accepted for publication in IBV
Structure of metastable states in spin glasses by means of a three replica potential
We introduce a three replica potential useful to examine the structure of
metastables states above the static transition temperature, in the spherical
p-spin model. Studying the minima of the potential we are able to find which is
the distance between the nearest equilibrium and local equilibrium states,
obtaining in this way information on the dynamics of the system. Furthermore,
the analysis of the potential at the dynamical transition temperature suggests
that equilibrium states are not randomly distributed in the phase space.Comment: plain tex, 26 pages, 6 postscript figure
Overall evaluation of Skylab imagery for mapping of Latin America
The author has identified the following significant results. Skylab imagery is both desired and needed by the Latin American catographic agencies. The imagery is cost beneficial for the production of new mapping and maintenance of existing maps at national topographic series scales. If this information was available on a near time routine coverage basis, it would provide an excellent additional data base to the Latin American cartographic community, specifically Argentina, Bolivia, Chile, Colombia, Dominican Republic, Guatemala, Paraguay, and Venezuela
A spectroscopic survey of the youngest field stars in the solar neighbourhood. I. The optically bright sample
We present the first results of a ground-based programme conducted on 1-4m
class telescopes. Our sample consists of 1097 active and presumably young
stars, all of them being optical counterparts of RASS X-ray sources in the
northern hemisphere. We concentrate on the 704 optically brightest
(V_Ticho<=9.5 mag) candidates. We acquired high-res spectroscopy in the
Halpha/Li spectral regions for 426 of such stars without relevant literature
data. We describe the sample and the observations and we start to discuss its
physical properties. We used a cross-correlation technique and other tools to
derive accurate radial/rotational velocities and to perform a spectral
classification for both single and SB2 stars. The spectral subtraction
technique was used to derive chromospheric activity levels and Li abundances.
We estimated the fraction of young single stars and multiple systems in stellar
soft X-ray surveys and the contamination by more evolved systems, like RS
CVn's. We classified stars on the basis of Li abundance and give a glimpse of
their sky distribution. The sample appears to be a mixture of young
Pleiades-/Hyades- like stars plus an older Li-poor population (~1-2 Gyr). 7
stars with Li abundance compatible with the age of IC 2602 (~30 Myr) or younger
were detected as well, although 2 appear to be Li-rich giants. The discovery of
a large number of Li-rich giants is another outcome of this survey. The
contamination of soft X-ray surveys by old systems in which the activity level
is enhanced by tidal synchronisation is not negligible, especially for K-type
stars. 5 stars with Li content close to the primordial abundance are probably
associated with known moving groups in the solar neighbourhood. Some of them
are PTTS candidates according to their positions in the HR diagram.Comment: 16 pages, 12 figures, 6 tables; 2 figures and 2 tables in electronic
form only. Paper accepted by Astronomy and Astrophysic
Benchmarking of debris flow experimental tests using combined finite-discrete element method, FEMDEM
An investigation of the hidden structure of states in a mean field spin glass model
We study the geometrical structure of the states in the low temperature phase
of a mean field model for generalized spin glasses, the p-spin spherical model.
This structure cannot be revealed by the standard methods, mainly due to the
presence of an exponentially high number of states, each one having a vanishing
weight in the thermodynamic limit. Performing a purely entropic computation,
based on the TAP equations for this model, we define a constrained complexity
which gives the overlap distribution of the states. We find that this
distribution is continuous, non-random and highly dependent on the energy range
of the considered states. Furthermore, we show which is the geometrical shape
of the threshold landscape, giving some insight into the role played by
threshold states in the dynamical behaviour of the system.Comment: 18 pages, 8 PostScript figures, plain Te
Orthogonal Polynomial Representation of Imaginary-Time Green's Functions
We study the expansion of single-particle and two-particle imaginary-time
Matsubara Green's functions of quantum impurity models in the basis of Legendre
orthogonal polynomials. We discuss various applications within the dynamical
mean-field theory (DMFT) framework. The method provides a more compact
representation of the Green's functions than standard Matsubara frequencies and
therefore significantly reduces the memory-storage size of these quantities.
Moreover, it can be used as an efficient noise filter for various physical
quantities within the continuous-time quantum Monte Carlo impurity solvers
recently developed for DMFT and its extensions. In particular, we show how to
use it for the computation of energies in the context of realistic DMFT
calculations in combination with the local density approximation to the density
functional theory (LDA+DMFT) and for the calculation of lattice
susceptibilities from the local irreducible vertex function.Comment: 14 pages, 11 figure
Revealing species assembly rules in nematode communities
Species assemblages are not randomly assembled from a local species pool; they often show segregated or aggregated distribution patterns. These patterns may be attributed to both biotic and abiotic factors. On a large scale abiotic factors may be important, while on a smaller scale other factors such as species interactions may become essential. Here we will focus on small-scale patterns in nematode communities. Species patterns are generally revealed by null models based on presence/absence data. Since there is an increasing chance of falsely rejecting the null hypothesis of a random assembled community with increasing matrix size, we used an algorithm generating independent null matrices and applied a large number of swap attempts to build a null matrix. Moreover, we applied an additional test to reveal the susceptibility of the analyses of checker and the C-, T- and Vscore to a Type I error for randomised data. To minimise the influence of the abiotic environment, we restricted the swapping algorithm of the null model to the replicate samples of one sampling event. Since stronger species interactions are expected for species of the same functional type, the nematode data was split according to the four feeding types defined by Wieser (1953). Our data indicate that species tend to aggregate and co-occur more often in some replicate samples than would be expected from a random species distribution of the local species pool. This is in accordance with the patchy distribution patterns known for nematode species. These aggregated patterns are also found for the different feeding types. The factors causing these aggregated patterns cannot be established since they are not included in the data, but the data do indicate that competitive exclusion is unlikely at the scale of a sample core
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