13,414 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
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
Kinematic Constraints to the Transition Redshift from SNe Ia Union Data
The kinematic approach to cosmological tests provides a direct evidence to
the present accelerating stage of the universe which does not depend on the
validity of general relativity, as well as on the matter-energy content of the
Universe. In this context, we consider here a linear two-parameter expansion
for the decelerating parameter, , where and are
arbitrary constants to be constrained by the Union supernovae data. By assuming
a flat Universe we find that the best fit to the pair of free parameters is
() = ( whereas the transition redshift is () (). This
kinematic result is in agreement with some independent analyzes and
accommodates more easily many dynamical flat models (like CDM).Comment: 10 pages, 4 figures, 1 tabl
New approaches for estimating risk from exposure to diethylstilbestrol.
A subgroup from a National Institute of Environmental Health Sciences, workshop concerned with characterizing the effects of endocrine disruptors on human health at environmental exposure levels considered the question, If diethylstilbestrol (DES) were introduced into the market for human use today and likely to result in low-dose exposure of the human fetus, what would be required to assess risk? On the basis of an analysis of the quality of data on human DES exposure, the critical times and doses for inducing genital tract malformations and cancer must be determined. This would be facilitated through analysis of the ontogeny of estrogen receptor expression in the developing human genital tract. Models of low-dose estrogenic effects will have to be developed for human and rodent genital tract development. Mouse models offer many advantages over other potential animal models because of the wealth of the earlier literature, the availability of sensitive end points, the availability of mutant lines, and the possibility of generating genetically engineered model systems. Through multidisciplinary approaches, it should be possible to elucidate the cellular and molecular mechanisms of endocrine disruption elicited by estrogens during development and facilitate an assessment of risk to humans
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
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