15,698 research outputs found
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
Clustering, Angular Size and Dark Energy
The influence of dark matter inhomogeneities on the angular size-redshift
test is investigated for a large class of flat cosmological models driven by
dark energy plus a cold dark matter component (XCDM model). The results are
presented in two steps. First, the mass inhomogeneities are modeled by a
generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is
characterized by a smoothness parameter and a power index ,
and, second, we provide a statistical analysis to angular size data for a large
sample of milliarcsecond compact radio sources. As a general result, we have
found that the parameter is totally unconstrained by this sample of
angular diameter data.Comment: 9 pages, 7 figures, accepted in Physical Review
Estrutura de uma área de caatinga invadida por algarobeira na Fazenda Gavião, Petrolina-PE.
Objetivando a análise dessas formações vegetais no município de Petrolina-PE, comparou-se duas vegetações de caatinga, sendo uma invadida por algarobeira (Prosopis juliflora (S w) DC)
Estrutura genética populacional de bacurizeiros na região Meio-Norte do Brasil.
A região Meio-Norte caracteriza-se pela sua biodiversidade, com destaque para espécies frutíferas nativas, como, por exemplo, o bacurizeiro, cujos frutos são apreciados na alimentação pelas populações locais. A fragmentação e/ou desmatamento que ocorre nessa área causa danos para populações de plantas, afetando a estrutura genética destas. Assim esse estudo verificou a estruturação genética em cinco populações de bacurizeiro dos Estados do Maranhão e Piauí por meio de marcadores entre simples sequências repetidas (ISSR) e da análise de variância molecular. Da diversidade genética total, 28,88% foi dividido entre as populações e 77,16% dentro das populações, portanto há estruturação significativa entre as populações
Relação genética baseada em análises ISSR em bacurizeiros oriundos de Barras - PI.
A utilização do bacurizeiro, uma espécie frutífera, com finalidade madeireira começou a comprometer a variabilidade genética da espécie. Assim, marcadores entre simples sequências repetidas (ISSR) foram usados para caracterizar 15 genótipos de bacurizeiro que compõem à Coleção de Germoplasma da Embrapa Meio-Norte. O coeficiente de distância Euclidiana foi usado para a construção de uma matriz de distância genética e dendrograma. Foram amplificados 226 locos de DNA, utilizando-se 17 primers. Foi obtida uma média de 8,38 de distância entre os genótipos, o que indica uma alta diversidade genética. A maior dissimilaridade foi obtida entre os genótipos BGB 7 e BGB 19 e BGB 7 e BGB 70 (12,77) e podem constituir bons genitores em programas de melhoramento
Flat Cosmology with Coupled Matter and Dark Energies
Three models of a flat universe of coupled matter and dark energies with
different low-redshift parameterizations of the dark energy equation of state
are considered. The dark energy is assumed to vary with time like the trace of
the energy-momentum tensor of cosmic matter. In the radiation-dominated era the
models reduce to standard cosmology. In the matter-dominated era they are, for
modern values of the cosmological parameters, consistent with data from SNe Ia
searches and with the data of Gurvits et al.(1999)for angular sizes of ultra
compact radio sources. We find that the angular size-redshift tests for our
models offer a higher statistical confidence than that based on SNe Ia data. A
comparison of our results with a recent revised analysis of angular
size-redshift legacy data is made,and the implications of our models with
optimized relativistic beaming in the radio sources is discussed. In particular
we find that relativistic beaming implies a Lorentz factor less than 6,in
agreement with its values for powerful Active Galactic Nuclei.Comment: Version to appear in The Astronomical Journal, with a modified name-
Flat Cosmology with Coupled Matter and Dark Energies. Expanded and Modified
conten
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
Influencia da embalagem e condicoes de armazenamento na longevidade de sementes florestais.
Permite conhecer as condicoes de embalagens de armazenamento que proporcionem maior longevidade das sementes de angico (Anadenanthera macrocarpa (Benth) Brenan, Pau d'arco (Tabebuia impetiginosa Mart) e imbirucu (Pseudobombax simpliciflolium A. Robyns)
Thermodynamics of Decaying Vacuum Cosmologies
The thermodynamic behavior of vacuum decaying cosmologies is investigated
within a manifestly covariant formulation. Such a process corresponds to a
continuous irreversible energy flow from the vacuum component to the created
matter constituents. It is shown that if the specific entropy per particle
remains constant during the process, the equilibrium relations are preserved.
In particular, if the vacuum decays into photons, the energy density and
average number density of photons scale with the temperature as and . The temperature law is determined and a generalized
Planckian type form of the spectrum, which is preserved in the course of the
evolution, is also proposed. Some consequences of these results for decaying
vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon
creation are discussed.Comment: 21 pages, uses LATE
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