31,500 research outputs found
Charge Distribution Near Oxygen Vacancies in Reduced Ceria
Understanding the electronic charge distribution around oxygen vacancies in
transition metal and rare earth oxides is a scientific challenge of
considerable technological importance. We show how significant information
about the charge distribution around vacancies in cerium oxide can be gained
from a study of high resolution crystal structures of higher order oxides which
exhibit ordering of oxygen vacancies. Specifically, we consider the
implications of a bond valence sum analysis of CeO and
CeO. To illuminate our analysis we show alternative
representations of the crystal structures in terms of orderly arrays of
co-ordination defects and in terms of flourite-type modules. We found that in
CeO, the excess charge resulting from removal of an oxygen atom
delocalizes among all three triclinic Ce sites closest to the O vacancy. In
CeO, the charge localizes on the next nearest neighbour Ce atoms.
Our main result is that the charge prefers to distribute itself so that it is
farthest away from the O vacancies. This contradicts \emph{the standard picture
of charge localisation} which assumes that each of the two excess electrons
localises on one of the cerium ions nearest to the vacancy. This standard
picture is assumed in most calculations based on density functional theory
(DFT). Based on the known crystal structure of PrO, we also
predict that the charge in CeO will be found in the second
coordination shell of the O vacancy. Although this review focuses on bulk
cerium oxides our approach to characterising electronic properties of oxygen
vacancies and the physical insights gained should also be relevant to surface
defects and to other rare earth and transition metal oxides.Comment: 20 pages, 23 figures. The replacement file has a new format for the
figures are the document layout but no change in content. v3 has the
following main changes: 1. The abstract and introduction were extensively
revised. 2. Sec. IV was removed. 3. The Conclusion was rewritte
Exploring the challenges of implementing e-health: a protocol for an update of a systematic review of reviews.
There is great potential for e-health to deliver cost-effective, quality healthcare and spending on e-health systems by governments and healthcare systems is increasing worldwide. However, the literature often describes problematic and unsuccessful attempts to implement these new technologies into routine clinical practice. To understand and address the challenges of implementing e-health, a systematic review was conducted in 2009, which identified several conceptual barriers and facilitators to implementation. As technology is rapidly changing and new e-health solutions are constantly evolving to meet the needs of current practice, an update of this review is deemed necessary to understand current challenges to the implementation of e-health. This research aims to identify, summarise and synthesise currently available evidence, by undertaking a systematic review of reviews to explore the barriers and facilitators to implementing e-health across a range of healthcare settings
Reliability physics
Speakers whose topics relate to the reliability physics of solar arrays are listed and their topics briefly reviewed. Nine reports are reviewed ranging in subjects from studies of photothermal degradation in encapsulants and polymerizable ultraviolet stabilizers to interface bonding stability to electrochemical degradation of photovoltaic modules
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What Does It Take for a Family to Afford to Pay for Health Care?
Addresses how much Californians can afford to pay for health care based on their current insurance premiums and out-of-pocket expenses as well as other basic necessities, such as housing, child care, transportation, food, and taxes
Higher-Order Angular Galaxy Correlations in the SDSS: Redshift and Color Dependence of non-Linear Bias
We present estimates of the N-point galaxy, area-averaged, angular
correlation functions () for = 2,...,7 for
galaxies from the fifth data release of the Sloan Digital Sky Survey. Our
parent sample is selected from galaxies with , and is the
largest ever used to study higher-order correlations. We subdivide this parent
sample into two volume limited samples using photometric redshifts, and these
two samples are further subdivided by magnitude, redshift, and color (producing
early- and late-type galaxy samples) to determine the dependence of
() on luminosity, redshift, and galaxy-type. We
measure () using oversampling techniques and use them
to calculate the projected, . Using models derived from theoretical
power-spectra and perturbation theory, we measure the bias parameters and
, finding that the large differences in both bias parameters ( and
) between early- and late-type galaxies are robust against changes in
redshift, luminosity, and , and that both terms are consistently
smaller for late-type galaxies. By directly comparing their higher-order
correlation measurements, we find large differences in the clustering of
late-type galaxies at redshifts lower than 0.3 and those at redshifts higher
than 0.3, both at large scales ( is larger by at ) and
small scales (large amplitudes are measured at small scales only for ,
suggesting much more merger driven star formation at ). Finally, our
measurements of suggest both that and is negative.Comment: 46 pages, 19 figures, Accepted to Ap
Prewetting transitions of Ar and Ne on alkali metal surfaces
We have studied by means of Density-Functional calculations the wetting
properties of Ar and Ne adsorbed on a plane whose adsorption properties
simulate the Li and Na surfaces. We use reliable ab-initio potentials to model
the gas-substrate interactions. Evidence for prewetting transitions is found
for all the systems investigated and their wetting phase diagrams are
calculated.Comment: 6 pages, 8 figures, submitted for publication in Phys. Rev.
Definitive Evidence for Order-by-Quantum-Disorder in Er2Ti2O7
Here we establish the systematic existence of a U(1) degeneracy of all
symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice,
at the mean-field level. By extracting the Hamiltonian of Er2Ti2O7 from
inelastic neutron scattering measurements, we then show that the
U(1)-degenerate states of Er2Ti2O7 are its classical ground states, and
unambiguously show that quantum fluctuations break the degeneracy in a way
which is confirmed by experiment. This is the first definitive observation of
order-by-disorder in any material. We provide further verifiable consequences
of this phenomenon, and several additional comparisons between theory and
experiment.Comment: 4.5 pages, 3 figures, 7.5 pages of Supplemental Material, 8
supplemental figure
UV Imaging Polarimetry of the Seyfert 2 Galaxy Mrk 3
We present UV imaging polarimetry data of the Seyfert 2 galaxy Mrk 3 taken by
the Hubble Space Telescope. The polarized flux is found to be extended to ~1
kpc from the nucleus, and the position angles of polarization are
centrosymmetric, confirming that the polarization is caused by scattering. We
determine the location of the hidden nucleus as the center of this
centrosymmetric pattern. From the polarization images taken in two broad bands,
we have obtained the color distribution of the polarized flux. Some regions
have blue polarized flux, consistent with optically-thin dust scattering, but
some bright knots have a color similar to that of Seyfert 1 nucleus. Also, the
recent Chandra X-ray observation suggests that the ratio of scattered UV flux
to scattered X-ray flux is rather similar to the intrinsic UV/X-ray ratio in a
Seyfert 1 nucleus, if the observed extended X-ray continuum is scattered light.
While the scattered X-ray would be essentially from electron scattering, the UV
slope and UV/X-ray ratio both being similar to Seyfert 1's would lead to two
possibilities as to the nature of the UV scatterers. One is that the UV may
also be scattered by electrons, in which case the scattering gas is somehow
dust-free. The other is that the UV is scattered by dust grains, but the
wavelength-independent UV scattering with low efficiency indicated by the UV
slope and UV/X-ray ratio would suggest that the grains reside in UV-opaque
clouds, or the dust might be mainly composed of large grains and lacks
small-grain population.Comment: 15 pages, 8 figures (plus 2 color versions of grayscale figures), To
appear in ApJ; minor corrections for the proofs of the manuscrip
Accessibility of the resources of near Earth space using multi-impulse transfers
Most future concepts for exploration and exploitation of space require a large initial mass in low Earth orbit. Delivering this mass requires overcoming Earth's natural gravity well, which imposes a distinct obstacle to space-faring. An alternative for future space progress is to search for resources in-situ among the near Earth asteroid population. This paper examines the scenario of future utilization of asteroid resources. The near Earth asteroid resources that could be transferred to a bound Earth orbit are determined by integrating the probability of finding asteroids inside the Keplerian orbital element space of the set of transfers with an specific energy smaller than a given threshold. Transfers are defined by a series of impulsive maneuvers and computed using the patched-conic approximation. The results show that even moderately low energy transfers enable access to a large mass of resources
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