12,453 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
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
Asymptotic Entanglement Dynamics and Geometry of Quantum States
A given dynamics for a composite quantum system can exhibit several distinct
properties for the asymptotic entanglement behavior, like entanglement sudden
death, asymptotic death of entanglement, sudden birth of entanglement, etc. A
classification of the possible situations was given in [M. O. Terra Cunha,
{\emph{New J. Phys}} {\bf{9}}, 237 (2007)] but for some classes there were no
known examples. In this work we give a better classification for the possibile
relaxing dynamics at the light of the geometry of their set of asymptotic
states and give explicit examples for all the classes. Although the
classification is completely general, in the search of examples it is
sufficient to use two qubits with dynamics given by differential equations in
Lindblad form (some of them non-autonomous). We also investigate, in each case,
the probabilities to find each possible behavior for random initial states.Comment: 9 pages, 2 figures; revised version accepted for publication in J.
Phys. A: Math. Theo
Melting temperature of screened Wigner crystal on helium films by molecular dynamics
Using molecular dynamics (MD) simulation, we have calculated the melting
temperature of two-dimensional electron systems on \AA-\AA helium
films supported by substrates of dielectric constants
at areal densities varying from cm to cm. Our results are in good agreement with the available
theoretical and experimental results.Comment: 4 pages and 4 figure
A thorough analysis of the short- and mid-term activity-related variations in the solar acoustic frequencies
The frequencies of the solar acoustic oscillations vary over the activity
cycle. The variations in other activity proxies are found to be well correlated
with the variations in the acoustic frequencies. However, each proxy has a
slightly different time behaviour. Our goal is to characterize the differences
between the time behaviour of the frequency shifts and of two other activity
proxies, namely, the area covered by sunspots and the 10.7cm flux. We define a
new observable that is particularly sensitive to the short-term frequency
variations. We then compare the observable when computed from model frequency
shifts and from observed frequency shifts obtained with the Global Oscillation
Network Group (GONG) for cycle 23. Our analysis shows that on the shortest
time-scales the variations in the frequency shifts seen in the GONG
observations are strongly correlated with the variations in the area covered by
sunspots. However, a significant loss of correlation is still found. We verify
that the times when the frequency shifts and the sunspot area do not vary in a
similar way tend to coincide with the times of the maxima of the quasi-biennial
variations seen in the solar seismic data. A similar analysis of the relation
between the 10.7cm flux and the frequency shifts reveals that the short-time
variations in the frequency shifts follow even more closely those of the 10.7cm
flux than those of the sunspot area. However, a loss of correlation between
frequency shifts and 10.7cm flux variations is still found around the same
times.Comment: 7 pages, 6 figures, accepted for publication in MNRA
On the relation between activity-related frequency shifts and the sunspot distribution over the solar cycle 23
The activity-related variations in the solar acoustic frequencies have been
known for 30 years. However, the importance of the different contributions is
still not well established. With this in mind, we developed an empirical model
to estimate the spot-induced frequency shifts, which takes into account the
sunspot properties, such as area and latitude. The comparison between the model
frequency shifts obtained from the daily sunspot records and those observed
suggests that the contribution from a stochastic component to the total
frequency shifts is about 30%. The remaining 70% is related to a global,
long-term variation. We also propose a new observable to investigate the short-
and mid-term variations of the frequency shifts, which is insensitive to the
long-term variations contained in the data. On the shortest time scales the
variations in the frequency shifts are strongly correlated with the variations
in the total area covered by sunspots. However, a significant loss of
correlation is still found, which cannot be fully explained by ignoring the
invisible side of the Sun when accounting for the total sunspot area. We also
verify that the times when the frequency shifts and the sunspot areas do not
vary in a similar way tend to coincide with the times of the maximum amplitude
of the quasi-biennial variations found in the seismic data.Comment: 4 pages, 2 figures, proceedings of the Joint TASC2 - KASC9 Workshop -
SPACEINN - HELAS8 Conference "Seismology of the Sun and the Distant Stars
2016: Using Today's Successes to Prepare the Future". To be published by the
EPJ Web of Conference
Carregando dados na arquitetura data warehousing - armazém de dados de frutas.
Data warehouse / Data Mart. Arquiteturas. O processo ETT (Extração, Transformação e carga de dados).bitstream/CNPTIA/9176/1/COMNICADOTEC3_0.pdfAcesso em: 29 maio 2008
Manejo florestal na Chapada do Araripe: uma técnica de combate à desertificação.
O presente trabalho apresenta o procedimento técnico denominado Manejo Florestal Sustentável como uma ferramenta eficiente no combate à desertificação. O manejo garante a cobertura florestal da área, retém a maior parte da diversidade vegetal original e pode ter impactos pequenos sobre a fauna. A adoção do manejo permite produção sustentável de madeira na área e requer um menor tempo para recuperação em relação à exploração não manejada. A região do Araripe tem consumido cada vez mais a matéria prima de base florestal como principal insumo na sua matriz energética. As empresas ali instaladas usam a vegetação nativa para o beneficiamento e transformação da gipsita. Assim, existe a necessidade de se investir na oferta de madeira por meio das práticas de manejo, reflorestamentos e recuperação de áreas degradadas. Esta região apresenta uma área de 332.361 ha disponíveis para a prática do Manejo Florestal que se manejada, poderá atender a demanda atual (da ordem de 2milhões mst/ano) e também demandas projetadas para os próximos anos (milhões mst/ano). Trata-se de um procedimento técnico, econômico, ambiental e socialmente viável. É inteiramente compatível com outros usos tradicionais da terra como a pecuária, apicultura e a obtenção de produtos florestais não madeireiros
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