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

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.

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

Alterações em características químicas de amostras de compostos de bagaço de coco-verde resultantes do método de estocagem.

Para fins de pesquisa científica, por vezes o armazenamento de amostras de composto orgânico se faz necessária. Para tanto, o conhecimento da eficiência do método de estocagem utilizado, quanto à preservação das características iniciais da amostra, é imprescindível. O presente trabalho teve por objetivo avaliar os efeitos de duas formas de estocagem, sobre suas características químicas originais de amostras de compostos de resíduos agroindustriais do coco-verde estocadas há seis anos. Os compostos utilizados foram obtidos a partir da mistura de bagaço de coco-verde com torta de filtro, esterco bovino e fosfato natural de Gafsa, umedecidas com soluções com concentrações crescentes de vinhaça (0, 50 e 75%). As amostras foram coletadas após 131 dias de compostagem e analisadas após seis anos de armazenamento em potes herméticos (secas a 65ºC) ou sob congelamento em freezer (a -20ºC). Foram determinadas o pH em CaCl2, carbono orgânico (CO), nitrogênio total, relação C:N e teor de cinzas das amostras. Os métodos de estocagem distintamente influenciaram as variáveis consideradas, embora não tenham mostrado efeito sobre o pH. Significativos efeitos foram observados sobre os teores de carbono orgânico erelação C:N e, em menor grau, sobre os de nitrogênio total e cinzas. Em amostras conservadas por seis anos, as duas formas de estocagem podem produzir resultados os quais sugerem a maior ou menor maturidade dos compostos (respectivamente, para amostras secas e congeladas) que aquela observada por ocasião da coleta das amostras. Os resultados deste estudo revelam a necessidade da avaliação temporal da eficiência dos métodos de forma a evitar os problemas detectados decorrentes da estocagem prolongada de amostras de composto orgânico