3,027 research outputs found
Magnification relations in gravitational lensing via multidimensional residue integrals
We investigate the so-called magnification relations of gravitational lensing
models. We show that multidimensional residue integrals provide a simple
explanation for the existence of these relations, and an effective method of
computation. We illustrate the method with several examples, thereby deriving
new magnification relations for galaxy lens models and microlensing (point mass
lensing).Comment: 16 pages, uses revtex4, submitted to Journal of Mathematical Physic
Study of the local field distribution on a single-molecule magnet-by a single paramagnetic crystal; a DPPH crystal on the surface of an Mn12-acetate crystal
The local magnetic field distribution on the subsurface of a single-molecule
magnet crystal, SMM, above blocking temperature (T >> Tb) detected for a very
short time interval (~ 10-10 s), has been investigated. Electron Paramagnetic
Resonance (EPR) spectroscopy using a local paramagnetic probe was employed as a
simple alternative detection method. An SMM crystal of
[Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate) and a crystal of 2,2-
diphenyl-1-picrylhydrazyl (DPPH) as the paramagnetic probe were chosen for this
study. The EPR spectra of DPPH deposited on Mn12-acetate show additional
broadening and shifting in the magnetic field in comparison to the spectra of
the DPPH in the absence of the SMM crystal. The additional broadening of the
DPPH linewidth was considered in terms of the two dominant electron spin
interactions (dipolar and exchange) and the local magnetic field distribution
on the crystal surface. The temperature dependence of the linewidth of the
Gaussian distribution of local fields at the SMM surface was extrapolated for
the low temperature interval (70-5 K)
Astrometric Microlensing as a Method of Discovering and Characterizing Extra-Solar Planets
We introduce a new method of searching for and characterizing extra-solar
planets. We show that by monitoring the center-of-light motion of microlensing
alerts using the next generation of high precision astrometric instruments the
probability of detecting a planet orbiting the lens is high. We show that
adding astrometric information to the photometric microlensing lightcurve
greatly helps in determining the planetary mass and semi-major axis. We
introduce astrometric maps as a new way for calculating astrometric motion and
planet detection probabilities. Finite source effects are important for low
mass planets, but even Earth mass planets can give detectable signals.Comment: 9 pages includes 8 postscript figures, AAS Latex, submitted to Ap
Coherent manipulation of electron spins up to ambient temperatures in Cr(S=1/2) doped KNbO
We report coherent spin manipulation on Cr (\emph{S} = 1/2, \emph{I} =
0) doped KNbO, which constitutes a dilute two-level model relevant for
use as a spin qubit. Rabi oscillations are observed for the first time in a
spin system based on transition metal oxides up to room temperature. At liquid
helium temperature the phase coherence relaxation time \emph{} reaches
s and, with a Rabi frequency of 20 MHz, yields a single qubit
figure of merit \emph{} of about 500. This shows that a diluted ensemble
of Cr (\emph{S} = 1/2) doped KNbO is a potential candidate for
solid-state quantum information processing.Comment: 4 page
Links across disabilities:unveiling associations between functional domains
Background: Persons with disabilities experience higher risks of mortality as well as poorer health as compared to the general population. The aim of this study is to estimate the correlations between functional difficulties across several domains in six countries. Methods: National census data with questions on disability from six countries (Mauritius, Morocco, Senegal, Myanmar, Vietnam, and Uruguay) was used in this study. We performed logistic regressions to assess the extent to which having a functional difficulty in one domain is correlated with having a functional difficulty in each of the other domains and report weighted odds ratios (ORs) overall and within age-groups (‘18–44’ years and ‘45+’ years). Models adjust for age, sex, and location (rural or urban). Sensitivity analyses around different choices of predictors and response variables were conducted. Findings: For all countries, reporting a functional difficulty in one domain was consistently and significantly positively correlated with reporting a functional difficulty in other domains (overall) and for each of the two age-groups considered - ‘18–44’ years and ‘45+’ years. All ORs were greater than one. Cognition, mobility, and hearing were the domains that were the most correlated ones with other domains. The highest pairwise correlations were for i/ hearing and cognition; ii/ mobility and cognition. Results were robust to changing the severity thresholds for functional difficulties. Across countries, Uruguay, the only high-income country among the six countries under study, had the lowest correlations between functional domains. Conclusions: There are consistent positive associations in the experience of functional difficulties in various domains in the six countries under study. Such correlations may reflect barriers to social services including healthcare services and resources (e.g. assistive devices) that may lead to an avoidable deterioration of functioning across domains. Further research is needed on the trajectories of functional difficulties and on structural barriers that people with functional difficulties may experience in their communities and in healthcare settings in particular. This is important as some functional difficulties may be preventable.</p
Characterizing the Cluster Lens Population
We present a detailed investigation into which properties of CDM halos make
them effective strong gravitational lenses. Strong lensing cross sections of
878 clusters from an N-body simulation are measured by ray tracing through
13,594 unique projections. We measure concentrations, axis ratios,
orientations, and the amount of substructure of each cluster, and compare the
lensing weighted distribution of each quantity to that of the cluster
population as a whole. The concentrations of lensing clusters are on average
34% larger than the typical cluster in the Universe. Despite this bias, the
anomalously high concentrations (c >14) recently measured by several groups,
appear to be inconsistent with the concentration distribution in our
simulations, which predict < 2% of lensing clusters should have concentrations
this high. No correlation is found between lensing cross section and the amount
of substructure. We introduce several types of simplified dark matter halos,
and use them to isolate which properties of CDM clusters make them effective
lenses. Projections of halo substructure onto small radii and the large scale
mass distribution of clusters do not significantly influence cross sections.
The abundance of giant arcs is primarily determined by the mass distribution
within an average overdensity of ~ 10,000. A multiple lens plane ray tracing
algorithm is used to show that projections of large scale structure increase
the giant arc abundance by a modest amount <7%. We revisit the question of
whether there is an excess of giant arcs behind high redshift clusters in the
RCS survey and find that the number of high redshift (z > 0.6) lenses is in
good agreement with LCDM, although our simulations predict more low redshift (z
< 0.6) lenses than were observed. (abridged)Comment: 19 pages, 15 figures. Submitted to Ap
Electrical expression of spin accumulation in ferromagnet/semiconductor structures
We treat the spin injection and extraction via a ferromagnetic
metal/semiconductor Schottky barrier as a quantum scattering problem. This
enables the theory to explain a number of phenomena involving spin-dependent
current through the Schottky barrier, especially the counter-intuitive spin
polarization direction in the semiconductor due to current extraction seen in
recent experiments. A possible explanation of this phenomenon involves taking
into account the spin-dependent inelastic scattering via the bound states in
the interface region. The quantum-mechanical treatment of spin transport
through the interface is coupled with the semiclassical description of
transport in the adjoining media, in which we take into account the in-plane
spin diffusion along the interface in the planar geometry used in experiments.
The theory forms the basis of the calculation of spin-dependent current flow in
multi-terminal systems, consisting of a semiconductor channel with many
ferromagnetic contacts attached, in which the spin accumulation created by spin
injection/extraction can be efficiently sensed by electrical means. A
three-terminal system can be used as a magnetic memory cell with the bit of
information encoded in the magnetization of one of the contacts. Using five
terminals we construct a reprogrammable logic gate, in which the logic inputs
and the functionality are encoded in magnetizations of the four terminals,
while the current out of the fifth one gives a result of the operation.Comment: A review to appear in Mod. Phys. Lett.
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