117 research outputs found
Transport Properties of an Interacting Quantum Dot with a Non-Uniform Magnetization
We study the influence of the non-homogeneity of a magnetization field on the
behaviour of interacting electrons in a quantum dot. In particular we
investigate the magnetotransport properties when the dot is weakly coupled to
two ferromagnetic leads. We take into account the interactions in the quantum
dot non-perturbatively. For a magnetization which varies slowly on the scale of
the Fermi wave length, the non-homogeneity effect is described by a gauge
potential that can be treated perturbatively.Comment: 6 pages, to be published in EP
Spin and Charge Correlations in Quantum Dots: An Exact Solution
The inclusion of charging and spin-exchange interactions within the Universal
Hamiltonian description of quantum dots is challenging as it leads to a
non-Abelian action. Here we present an {\it exact} analytical solution of the
probem, in particular, in the vicinity of the Stoner instabilty point. We
calculate several observables, including the tunneling density of states (TDOS)
and the spin susceptibility. Near the instability point the TDOS exhibits a
non-monotonous behavior as function of the tunneling energy, even at
temperatures higher than the exchange energy. Our approach is generalizable to
a broad set of observables, including the a.c. susceptibility and the
absorption spectrum for anisotropic spin interaction. Our results could be
tested in nearly ferromagnetic materials.Comment: JETPL class, 6 pages, 2 figure
Amorphous alumina in the extended atmosphere of Alpha Orionis
In this paper we study the extended atmosphere of the late-type supergiant
Alpha Orionis. Infrared spectroscopy of red supergiants reveals strong
molecular bands, some of which do not originate in the photosphere but in a
cooler layer of molecular material above it. Lately, these layers have been
spatially resolved by near and mid-IR interferometry. In this paper, we try to
reconcile the IR interferometric and ISO-SWS spectroscopic results on Alpha
Orionis with a thorough modelling of the photosphere, molecular layer(s) and
dust shell. From the ISO and near-IR interferometric observations, we find that
Alpha Orionis has only a very low density water layer close above the
photosphere. However, mid-IR interferometric observations and a narrow-slit
N-band spectrum suggest much larger extra-photospheric opacity close to the
photosphere at those wavelengths, even when taking into account the detached
dust shell. We argue that this cannot be due to the water layer, and that
another source of mid-IR opacity must be present. We show that this opacity
source is probably neither molecular nor chromospheric. Rather, we present
amorphous alumina (Al2O3) as the best candidate and discuss this hypothesis in
the framework of dust-condensation scenarios.Comment: 15 pages, 18 figures, accepted for publication in A&
Promoting Transparency in Social Science Research
There is growing appreciation for the advantages of experimentation in the social sciences. Policy-relevant claims that in the past were backed by theoretical arguments and inconclusive correlations are now being investigated using more credible methods. Changes have been particularly pronounced in development economics, where hundreds of randomized trials have been carried out over the last decade. When experimentation is difficult or impossible, researchers are using quasi-experimental designs. Governments and advocacy groups display a growing appetite for evidence-based policy-making. In 2005, Mexico established an independent government agency to rigorously evaluate social programs, and in 2012, the U.S. Office of Management and Budget advised federal agencies to present evidence from randomized program evaluations in budget requests (1, 2)
Interferometric properties of pulsating C-rich AGB stars I. Intensity profiles and uniform disc diameters of dynamic model atmospheres
We present the first theoretical study on center-to-limb variation (CLV)
properties and relative radius interpretation for narrow and broad-band
filters, on the basis of a set of dynamic model atmospheres of C-rich AGB
stars. We computed visibility profiles and the equivalent uniform disc radii
(UD-radii) in order to investigate the dependence of these quantities upon the
wavelength and pulsation phase. After an accurate morphological analysis of the
visibility and intensity profiles determined in narrow and broad-band filter,
we fitted our visibility profiles with a UD function simulating the
observational approach. UD-radii have been computed using three different
fitting-methods to investigate the influence of the sampling of the visibility
profile: single point, two points and least square method. The intensity and
visibility profiles of models characterized by mass loss show a behaviour very
different from a UD. We found that UD-radii are wavelength dependent and this
dependence is stronger if mass loss is present. Strong opacity contributions
from C2H2 affect all radius measurements at 3 micron and in the N-band,
resulting in higher values for the UD-radii. The predicted behaviour of
UD-radii versus phase is complicated in the case of models with mass loss,
while the radial changes are almost sinusoidal for the models without mass
loss. Compared to the M-type stars, for the C-stars no windows for measuring
the pure continuum are available.Comment: 13 pages, 9 figures, accepted for publication in A&
Atmospheric dynamics in carbon-rich Miras. I. Model atmospheres and synthetic line profiles
Atmospheres of evolved AGB stars are heavily affected by pulsation, dust
formation and mass loss, and they can become very extended. Time series of
observed high-resolution spectra proved to be a useful tool to study
atmospheric dynamics throughout the outer layers of these pulsating red giants.
Originating at various depths, different molecular spectral lines observed in
the near-infrared can be used to probe gas velocities there for different
phases during the lightcycle. Dynamic model atmospheres are needed to represent
the complicated structures of Mira variables properly. An important aspect
which should be reproduced by the models is the variation of line profiles due
to the influence of gas velocities. Based on a dynamic model, synthetic spectra
(containing CO and CN lines) were calculated, using an LTE radiative transfer
code that includes velocity effects. It is shown that profiles of lines that
sample different depths qualitatively reproduce the behaviour expected from
observations.Comment: accepted by A&A, 12 pages, 9 figure
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