29,179 research outputs found
Magnetic ordering in GaAlAs:Mn double well structure
The magnetic order in the diluted magnetic semiconductor barrier of double
AlAs/GaAs: Mn quantum well structures is investigated by Monte Carlo
simulations. A confinement adapted RKKY mechanism is implemented for indirect
exchange between Mn ions mediated by holes. It is shown that, depending on the
barrier width and the hole concentration a ferromagnetic or a spin-glass order
can be established.Comment: 3 figure
On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagnetic layer
A self-consistent calculation of the density of states and the spectral
density function is performed in a two-dimensional spin-polarized hole system
based on a multiple-scattering approximation. Using parameters corresponding to
GaMnAs thin layers, a wide range of Mn concentrations and hole densities have
been explored to understand the nature, localized or extended, of the
spin-polarized holes at the Fermi level for several values of the average
magnetization of the Mn ystem. We show that, for a certain interval of Mn and
hole densities, an increase on the magnetic order of the Mn ions come together
with a change of the nature of the states at the Fermi level. This fact
provides a delocalization of spin-polarized extended states anti-aligned to the
average Mn magnetization, and a higher spin-polarization of the hole gas. These
results are consistent with the occurrence of ferromagnetism with relatively
high transition temperatures observed in some thin film samples and
multilayered structures of this material.Comment: 3 page
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
A Bayesian estimate of the CMB-large-scale structure cross-correlation
Evidences for late-time acceleration of the Universe are provided by multiple
probes, such as Type Ia supernovae, the cosmic microwave background (CMB) and
large-scale structure (LSS). In this work, we focus on the integrated
Sachs--Wolfe (ISW) effect, i.e., secondary CMB fluctuations generated by
evolving gravitational potentials due to the transition between, e.g., the
matter and dark energy (DE) dominated phases. Therefore, assuming a flat
universe, DE properties can be inferred from ISW detections. We present a
Bayesian approach to compute the CMB--LSS cross-correlation signal. The method
is based on the estimate of the likelihood for measuring a combined set
consisting of a CMB temperature and a galaxy contrast maps, provided that we
have some information on the statistical properties of the fluctuations
affecting these maps. The likelihood is estimated by a sampling algorithm,
therefore avoiding the computationally demanding techniques of direct
evaluation in either pixel or harmonic space. As local tracers of the matter
distribution at large scales, we used the Two Micron All Sky Survey (2MASS)
galaxy catalog and, for the CMB temperature fluctuations, the ninth-year data
release of the Wilkinson Microwave Anisotropy Probe (WMAP9). The results show a
dominance of cosmic variance over the weak recovered signal, due mainly to the
shallowness of the catalog used, with systematics associated with the sampling
algorithm playing a secondary role as sources of uncertainty. When combined
with other complementary probes, the method presented in this paper is expected
to be a useful tool to late-time acceleration studies in cosmology.Comment: 21 pages, 15 figures, 4 tables. We extended the previous analyses
including WMAP9 Q, V and W channels, besides the ILC map. Updated to match
accepted ApJ versio
Anisotropic superconducting properties of aligned MgB2 crystallites
Samples of aligned MgB2 crystallites have been prepared, allowing for the
first time the direct identification of an upper critical field anisotropy
Hc2^{ab}/Hc2^{c}= xi_{ab}/xi_{c} ~ 1.73; with xi_{o,ab} ~ 70 A, xi_{o,c} ~ 40
A, and a mass anisotropy ratio m_{ab}/m_{c} ~ 0.3. A ferromagnetic background
signal was identified, possibly related to the raw materials purity.Comment: 4 pages, 4 figures; Revised version to appear in Phys. Rev. Let
Comportamento do tamarix (Tamarix sp. - Tamaricaceae) na recuperação de áreas degradadas por mineração.
As atividades de mineração alteram as áreas naturais pelo acúmulo de rejeitos finos e estéreis e pela retirada da camada superior de solo, causando impactos ambientais indesejáveis. Para reduzir estes danos, vem sendo realizado o plantio de espécies arbóreas e arbustivas para a recuperação das áreas degradadas. O presente trabalho teve como objetivo, avaliar o comportamento do tamarix (Tamarixsp.) em ensaio de seleção de espécies florestais na recuperação de área da barragem de rejeito da Mineração Caraíba S/A, situada no município de Jaguarari, Bahia, região semi-árida do Nordeste do Brasil. O tamarix foi introduzido do Peru, pela Embrapa Semi-Árido na década de 80, sendo utilizado na formação de quebra-ventos em regiões desérticas
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