200 research outputs found
Superconductivity in the SU(N) Anderson Lattice at U=\infty
We present a mean-field study of superconductivity in a generalized N-channel
cubic Anderson lattice at U=\infty taking into account the effect of a
nearest-neighbor attraction J. The condition U=\infty is implemented within the
slave-boson formalism considering the slave bosons to be condensed. We consider
the -level occupancy ranging from the mixed valence regime to the Kondo
limit and study the dependence of the critical temperature on the various model
parameters for each of three possible Cooper pairing symmetries (extended s,
d-wave and p-wave pairing) and find interesting crossovers. It is found that
the d- and p- wave order parameters have, in general, very similar critical
temperatures. The extended s-wave pairing seems to be relatively more stable
for electronic densities per channel close to one and for large values of the
superconducting interaction J.Comment: Seven Figures; one appendix. Accepted for publication in Phys. Rev.
Palaeontological framework from Pirabas Formation (North Brazil) used as potential model for equatorial carbonate platform
The Pirabas Formation (early to middle Miocene) from the equatorial margin of North Brazil is characterized by a shallow-marine carbonate platform with high fossil diversity and abundant micro- and macrofossil remains. The Pirabas Formation represents a unique carbonate system along the Atlantic margin of South America that developed before the onset of the Amazon delta. We studied the palaeontology and lithofacies of outcrops of the uppermost Pirabas Formation and found that was deposited in a coastal marine environment with marginal lagoons under the influence of a tidal regime and tropical storms. The remains of calcareous algae, molluscs, crustaceans, echinoiderms, bryozoans, solitary corals, fish and marine mammals, together with foraminifera, ostracods and other marine microfossils, shaped a biogenic framework, that together with the post-depositional processes of dissolution of skeletal grains, is responsible for the mean packstone-floatstone porosity of 14.9%. The palaeontological framework and the petrophysical characterization of the carbonate rocks from the uppermost Pirabas Formation outcrop represent a baseline to interpret the entire Pirabas Formation in the subsurface stratigraphic sections (cores) of this important Neogene unit. Considering that carbonate rocks account for ~50% of oil and gas reservoirs around the world, this research provides a model for Neogene tropical carbonate deposits useful for carbonate petroliferous reservoirs in the Brazilian equatorial basins.publishe
The Metric of the Cosmos from Luminosity and Age Data
This paper presents the algorithm for determining the Lemaitre-Tolman (LT)
model that best fits given datasets for maximum stellar ages, and SNIa
luminosities, both as functions of redshift. It then applies it to current
cosmological data. Special attention must be given to the handling of the
origin, and the region of the maximum diameter distances. As with a previous
combination of datasets (galaxy number counts and luminosity distances versus
redshift), there are relationships that must hold at the region of the maximum
diameter distance, which are unlikely to be obeyed exactly by real data. We
show how to make corrections that enable a self-consistent solution to be
found. We address the questions of the best way to approximate discrete data
with smooth functions, and how to estimate the uncertainties of the output -
the 3 free functions that determine a specific LT metric. While current data
does not permit any confidence in our results, we show that the method works
well, and reasonable LT models do fit with or without a cosmological constant.Comment: 25 pages, 8 figures; matches published versio
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