The Apparent Fractal Conjecture

This short communication advances the hypothesis that the observed fractal structure of large-scale distribution of galaxies is due to a geometrical effect, which arises when observational quantities relevant for the characterization of a cosmological fractal structure are calculated along the past light cone. If this hypothesis proves, even partially, correct, most, if not all, objections raised against fractals in cosmology may be solved. For instance, under this view the standard cosmology has zero average density, as predicted by an infinite fractal structure, with, at the same time, the cosmological principle remaining valid. The theoretical results which suggest this conjecture are reviewed, as well as possible ways of checking its validity.Comment: 6 pages, LaTeX. Text unchanged. Two references corrected. Contributed paper presented at the "South Africa Relativistic Cosmology Conference in Honour of George F. R. Ellis 60th Birthday"; University of Cape Town, February 1-5, 199

Potencialidade da cultura do algodão para a produção de biodiesel no Meio-Norte do Brasil.

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Transport properties of a two impurity system: a theoretical approach

A system of two interacting cobalt atoms, at varying distances, was studied in a recent scanning tunneling microscope experiment by Bork et. al.[Nature Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all experimentally analyzed interatomic distances, the physics observed in these experiments. Our proposal is based on the two-impurity Anderson model, with the inclusion of a two-path geometry for charge transport. This many-body system is treated in the finite-U slave boson mean-field approximation and the logarithmic-discretization embedded-cluster approximation. We physically characterize the different charge transport regimes of this system at various interatomic distances and show that, as in the experiments, the features observed in the transport properties depend on the presence of two impurities but also on the existence of two conducting channels for electron transport. We interpret the splitting observed in the conductance as the result of the hybridization of the two Kondo resonances associated with each impurity.Comment: 5 pages, 5 figure

Symmetry, incommensurate magnetism and ferroelectricity: the case of the rare-earth manganites RMnO3

The complete irreducible co-representations of the paramagnetic space group provide a simple and direct path to explore the symmetry restrictions of magnetically driven ferroelectricity. The method consists of a straightforward generalization of the method commonly used in the case of displacive modulated systems and allows us to determine, in a simple manner, the full magnetic symmetry of a given phase originated from a given magnetic order parameter. The potential ferroic and magneto-electric properties of that phase can then be established and the exact Landau free energy expansions can be derived from general symmetry considerations. In this work, this method is applied to the case of the orthorhombic rare-earth manganites RMnO3. This example will allow us to stress some specific points, such as the differences between commensurate or incommensurate magnetic phases regarding the ferroic and magnetoelectric properties, the possible stabilization of ferroelectricity by a single irreducible order parameter or the possible onset of a polarization oriented parallel to the magnetic modulation. The specific example of TbMnO3 will be considered in more detail, in order to characterize the role played by the magneto-electric effect in the mechanism for the polarization rotation induced by an external magnetic field.Comment: Conference: Aperiodic`0

The Two-Component Virial Theorem and the Physical Properties of Stellar Systems

Motivated by present indirect evidences that galaxies are surrounded by dark matter halos, we investigate whether their physical properties can be described by a formulation of the virial theorem which explicitly takes into account the gravitational potential term representing the interaction of the dark halo with the barionic or luminous component. Our analysis shows that the application of such a ``two-component virial theorem'' not only accounts for the scaling relations displayed, in particular, by elliptical galaxies, but also for the observed properties of all virialized stellar systems, ranging from globular clusters to galaxy clusters.Comment: 13 pages, 2 figures, LaTeX, corrected few typos. This version matches the published versio

Resultados de pesquisa obtidos na cultura do algodão nos cerrados da região Meio-Norte do Brasil no período de 1999 a 2008.

O objetivo deste trabalho é informar aos produtores de algodão da região Meio-Norte do Brasil sobre os resultados de pesquisa obtidos no período de 1999 a 2008. Nesse período foram conduzidos 91 experimentos de avaliação de genótipos visando a introdução da cultura algodoeira como uma alternativa para a rotação com as culturas de soja e milho. No Piauí, os trabalhos de pesquisa foram realizados nos municípios de Teresina, Bom Jesus, Uruçuí, Palmeira do Piauí e Baixa Grande do Ribeiro e, no Maranhão, nos municípios de Tasso Fragoso, Sambaíba, São Raimundo das Mangabeiras, Chapadinha, Brejo, Anapurus e Mata Roma. A amplitude para produtividade média de algodão em caroço variou de 2.628 kg ha-1 a 3.598 kg/ha-1 , no Piauí e, de 2.522 kg/ha-1 a 3.360 kg/ha-1, no Maranhão. No Piauí, os municípios de Baixa Grande do Ribeiro e Uruçuí apresentaram as maiores produtividades com uma amplitude de variação de 3.254 kg/ha-1 a 4.356 kg/ha-1 e 2.730 kg/ha-1 a 3.419 kg/ha-1, respectivamente. No Maranhão as melhores produtividades ocorreram nos municípios de Tasso Fragoso com uma amplitude de variação de 2.751 kg/ha-1 a 4.236 kg/ha-1 e, em São Raimundo das Mangabeiras com uma amplitude de variação de 3.313 kg/ha-1 a 3.943 kg/ha-1

Comportamento de genótipos de girassol no município de Mata Roma, MA, no período de 2008 a 2011.

O objetivo deste trabalho foi avaliar o comportamento de genótipos de girassol no município de Mata Roma, com vistas à produção de grãos e óleo.bitstream/item/79281/1/CT231.pd