1,444 research outputs found
Vacuum polarization by topological defects in de Sitter spacetime
In this paper we investigate the vacuum polarization effects associated with
a massive quantum scalar field in de Sitter spacetime in the presence of
gravitational topological defects. Specifically we calculate the vacuum
expectation value of the field square, . Because this investigation
has been developed in a pure de Sitter space, here we are mainly interested on
the effects induced by the presence of the defects.Comment: Talk presented at the 1st. Mediterranean Conference on Classical and
Quantum Gravity (MCCQG
Lorentz-breaking effects in scalar-tensor theories of gravity
In this work, we study the effects of breaking Lorentz symmetry in
scalar-tensor theories of gravity taking torsion into account. We show that a
space-time with torsion interacting with a Maxwell field by means of a
Chern-Simons-like term is able to explain the optical activity in syncrotron
radiation emitted by cosmological distant radio sources. Without specifying the
source of the dilaton-gravity, we study the dilaton-solution. We analyse the
physical implications of this result in the Jordan-Fierz frame. We also analyse
the effects of the Lorentz breaking in the cosmic string formation process. We
obtain the solution corresponding to a cosmic string in the presence of torsion
by keeping track of the effects of the Chern-Simons coupling and calculate the
charge induced on this cosmic string in this framework. We also show that the
resulting charged cosmic string gives us important effects concerning the
background radiation.The optical activity in this case is also worked out and
discussed.Comment: 10 pages, no figures, ReVTex forma
Self-similar magnetoresistance of Fibonacci ultrathin magnetic films
We study numerically the magnetic properties (magnetization and
magnetoresistance) of ultra-thin magnetic films (Fe/Cr) grown following the
Fibonacci sequence. We use a phenomenological model which includes Zeeman,
cubic anisotropy, bilinear and biquadratic exchange energies. Our physical
parameters are based on experimental data recently reported, which contain
biquadratic exchange coupling with magnitude comparable to the bilinear
exchange coupling. When biquadratic exchange coupling is sufficiently large a
striking self-similar pattern emerges.Comment: 5 pages, 5 EPS figures, REVTeX, accepted for publication in Phys.
Rev.
Wakes in Dilatonic Current-Carrying Cosmic Strings
In this work, we present the gravitational field generated by a cosmic string
carrying a timelike current in the scalar-tensor gravities. The mechanism of
formation and evolution of wakes is fully investigated in this framework. We
show explicitly that the inclusion of electromagnetic properties for the string
induces logarithmic divergences in the accretion problem.Comment: Revised version to be published in the Phys. Rev.
Avaliação de leguminosas arbóreas para recuperação de solos e repovoamento em áreas degradadas, Irauçuba-CE.
Resumo: O modelo atual de exploração agropecuária na caatinga vem causando conseqüências desastrosas sobre os recursos naturais, com perdas consideráveis da biodiversidade de sua cobertura arbórea. Faz-se necessário o desenvolvimento de alternativas para recuperação florestal de imensas áreas degradadas nos sertões nordestinos. Assim, foi conduzido, no período de 2003 a 2005, em Irauçuba, CE, um experimento visando selecionar espécies leguminosas arbóreas simbiontes para a recuperação de áreas degradadas. Foram estabelecidas, por meio de mudas: Albizia lebeck, Prosopis juliflora, Piptadenia macrocarpa, Caesalpinia bracteosa, Gliricidia sepium, Caesalpinia ferrea, Mimosa hostilis, Leucaena leucocephala, Bauhinia cheilantha, Erythrina velutina, Mimosa caesalpinifolia e Parkinsonia aculeata. Os resultados sugerem que para Irauçuba e outras regiões semelhantes no Nordeste Brasileiro, a P. juliflora, a M. hostilis, a C. ferrea e a G. sepium são recomendadas para programas de recomposição florestal. [Leguminous trees evaluation for recuperation of soils and forestry recomposition of degraded areas, Irauçuba-CE]. Abstract: The present model of agriculture and livestock exploration in the caatinga region is resulting in érrea1dia consequences on the natural resources, with high losses of the biodiversity of its tree cover. The development of alternatives for the forestry recuperation of immense 1érre in the Northeastern hinterland is strongly needed. Thus, na experiment was carried out in Irauçuba, Ceará in order to select symbiont leguminous trees for reclamation of degraded 1érre. Albizia lebeck, Prosopis juliflora, Piptadenia macrocarpa, Caesalpinia bracteosa, érrea1dia sepium, Caesalpinia 1érrea, Mimosa hostilis, Leucaena leucocephala, Bauhinia cheilantha, Erythrina velutina, Mimosa caesalpinifolia e Parkinsonia aculeata were established by seedlings. The results indicated that P. juliflora, M. hostilis, C. ferrea and G. sepium may be recommended for forestry recuperation for Irauçuba and other similar regions in Northeastern Brazil.Edição dos Resumos do Congresso Brasileiro de Agroecologia, 5., Guarapari, ES de 01 a 04 de outubro de 2007
Gravitation: Global Formulation and Quantum Effects
A nonintegrable phase-factor global approach to gravitation is developed by
using the similarity of teleparallel gravity with electromagnetism. The phase
shifts of both the COW and the gravitational Aharonov-Bohm effects are
obtained. It is then shown, by considering a simple slit experiment, that in
the classical limit the global approach yields the same result as the
gravitational Lorentz force equation of teleparallel gravity. It represents,
therefore, the quantum mechanical version of the classical description provided
by the gravitational Lorentz force equation. As teleparallel gravity can be
formulated independently of the equivalence principle, it will consequently
require no generalization of this principle at the quantum level.Comment: Latex (IOP style), 14 pages, 3 figures. To appear in Classical and
Quantum Gravit
Avaliação de leguminosas arbóreas, para recuperação de solos e repovoamento em áreas degradadas, Quixeramobim-CE.
Resumo: O modelo atual de exploração agropecuária na caatinga vem ausando conseqüências desastrosas sobre os recursos naturais, com perdas consideráveis dabiodiversidade de sua cobertura arbórea. Faz-se necessário o desenvolvimento de alternativas para recuperação florestal de imensas áreas degradadas nos sertões nordestinos. Assim, foi conduzido, no período de 2003 a 2005, em Quixeramobim, CE, um experimento visando selecionar espécies leguminosas arbóreas simbiontes para a recuperação de áreas degradadas. Foram estabelecidas, por meio de mudas: Albizia lebeck, Gliricidia sepium, Caesalpinia ferrea, Mimosa hostilis, Leucaena leucocephala, Mimosa caesalpinifolia e Parkinsonia aculeata. Os resultados obtidos sugerem que para Quixeramobim, com exceção da Albizia lebeck, as demais leguminosas podem ser utilizadas em programas de recuperação florestal de áreas degradadas. [Leguminous trees avaliation for recuperation of soils and forestry recomposition of degraded areas, Quixeramobim-CE]. Abstract: The present model of agriculture and livestock exploration in the caatinga region is resulting in desastrous consequences on the natural resources, with high losses of the biodiversity of its tree cover. The development of alternatives for the forestry recuperation of immense areas in the Northeastern hinterland is strongly needed. Thus, an experiment was carried out in Quixeramobim, Ceará in order to select symbiont leguminous trees for reclamation of degraded areas. Albizia lebeck, Gliricidia saepium, Caesalpinia ferrea, Mimosa hostilis, Leucaena leucocephala, Mimosa caesalpinifolia e Parkinsonia aculeata were established by seedlings. The results indicated that except A. lebeck, all the species may be recommended for forestry recuperation for Quixeramobim and other similar regions in Northeastern Brazil.Edição dos Resumos do Congresso Brasileiro de Agroecologia, 5., Guarapari, ES de 01 a 04 de outubro de 2007
Loop variables in the geometry of a rotating black string
In this paper we analyze in the Wilson loop context the parallel transport of
vectors and spinors around a closed loop in the background space-time of a
rotating black string in order to classify its global properties. We also
examine particular closed orbits in this space-time and verify the Mandelstam
relations.Comment: 14 pages, iopart styl
Gravitational field around a time-like current-carrying screwed cosmic string in scalar-tensor theories
In this paper we obtain the space-time generated by a time-like
current-carrying superconducting screwed cosmic string(TCSCS). This
gravitational field is obtained in a modified scalar-tensor theory in the sense
that torsion is taken into account. We show that this solution is comptible
with a torsion field generated by the scalar field . The analysis of
gravitational effects of a TCSCS shows up that the torsion effects that appear
in the physical frame of Jordan-Fierz can be described in a geometric form
given by contorsion term plus a symmetric part which contains the scalar
gradient. As an important application of this solution, we consider the linear
perturbation method developed by Zel'dovich, investigate the accretion of cold
dark matter due to the formation of wakes when a TCSCS moves with speed and
discuss the role played by torsion. Our results are compared with those
obtained for cosmic strings in the framework of scalar-tensor theories without
taking torsion into account.Comment: 21 pages, no figures, Revised Version, presented at the "XXIV-
Encontro Nacional de Fisica de Particulas e Campos ", Caxambu, MG, Brazil, to
appear in Phys. Rev.
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