24,747 research outputs found
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
Area Quantization in Quasi-Extreme Black Holes
We consider quasi-extreme Kerr and quasi-extreme Schwarzschild-de Sitter
black holes. From the known analytical expressions obtained for their
quasi-normal modes frequencies, we suggest an area quantization prescription
for those objects.Comment: Final version to appear in Mod. Phys. Lett.
Awaking the vacuum with spheroidal shells
It has been shown that well-behaved spacetimes may induce the vacuum
fluctuations of some nonminimally coupled free scalar fields to go through a
phase of exponential growth. Here, we discuss this mechanism in the context of
spheroidal thin shells emphasizing the consequences of deviations from
spherical symmetry.Comment: 10 pages, 7 figures. Minor changes, version published on Phys. Rev.
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
From quantum to classical instability in relativistic stars
It has been shown that gravitational fields produced by realistic
classical-matter distributions can force quantum vacuum fluctuations of some
nonminimally coupled free scalar fields to undergo a phase of exponential
growth. The consequences of this unstable phase to the background spacetime
have not been addressed so far due to known difficulties concerning
backreaction in semiclassical gravity. It seems reasonable to believe, however,
that the quantum fluctuations will "classicalize" when they become large
enough, after which backreaction can be treated in the general-relativistic
context. Here we investigate the emergence of a classical regime out of the
quantum field evolution during the unstable phase. By studying the appearance
of classical correlations and loss of quantum coherence, we show that by the
time backreaction becomes important the system already behaves classically.
Consequently, the gravity-induced instability leads naturally to initial
conditions for the eventual classical description of the backreaction. Our
results give support to previous analyses which treat classically the
instability of scalar fields in the spacetime of relativistic stars, regardless
whether the instability is triggered by classical or quantum perturbations.Comment: 16 pages. Minor changes to match the published versio
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
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