16,902 research outputs found
Extended plasticity in commercial-purity zinc
90% rolling-reduction of annealed commercial-purity zinc sheet
(grain size 100 - 150 μ) results in the fragmentation of the large grains
into, finally, stable micro-grains, 1 - 211 in diameter. The stability of
the micro-grains is due to the presence of soluble and insoluble impurities
which prevent recrystallization.
This micro-grain material is strain-rate sensitive, and elongations of
200% have been obtained at room temperature.
Although this as-rolled, 90% reduction zinc sheet is not super-plastic
according to the current definition, its behaviour has led to the coining of
the phase 'extended plasticity'.
Evidence of grain-boundary sliding is found on examination of the surface
by scanning electron microscopy, while the examination of thin foils and
activation energy measurements support the dynamic softening (recovery) theory;
thus, both these mechanisms must be operating, to a greater or less extent,
to confer on this material the observe mechanical behaviour.
It is finally concluded that it is dangerous to draw conclusions regarding
the mechanism of plastic deformation from surface observations alone
A Comparison of Semi-Analytic and Smoothed Particle Hydrodynamics Galaxy Formation
We compare the statistical properties of galaxies found in two different
models of hierarchical galaxy formation: the semi-analytic model of Cole et al.
and the smoothed particle hydrodynamics (SPH) simulations of Pearce et al.
Using a `stripped-down' version of the semi-analytic model which mimics the
resolution of the SPH simulations and excludes physical processes not included
in them, we find that the two models produce an ensemble of galaxies with
remarkably similar properties, although there are some differences in the gas
cooling rates and in the number of galaxies that populate halos of different
mass. The full semi-analytic model, which has effectively no resolution limit
and includes a treatment of star formation and supernovae feedback, produces
somewhat different (but readily understandable) results. Agreement is
particularly good for the present-day global fractions of hot gas, cold dense
(i.e. galactic) gas and uncollapsed gas, for which the SPH and stripped-down
semi-analytic calculations differ by at most 25%. In the most massive halos,
the stripped-down semi-analytic model predicts, on the whole, up to 50% less
gas in galaxies than is seen in the SPH simulations. The two techniques
apportion this cold gas somewhat differently amongst galaxies in a given halo.
This difference can be tracked down to the greater cooling rate in massive
halos in the SPH simulation compared to the semi-analytic model. (abridged)Comment: 19 pages, 13 figures, to appear in MNRAS. Significantly extended to
explore galaxy progenitor distributions and behaviour of models at high
redshift
Optimal traps in graphene
We transform the two-dimensional Dirac-Weyl equation, which governs the
charge carriers in graphene, into a non-linear first-order differential
equation for scattering phase shift, using the so-called variable phase method.
This allows us to utilize the Levinson Theorem to find zero-energy bound states
created electrostatically in realistic structures. These confined states are
formed at critical potential strengths, which leads to us posit the use of
`optimal traps' to combat the chiral tunneling found in graphene, which could
be explored experimentally with an artificial network of point charges held
above the graphene layer. We also discuss scattering on these states and find
the zero angular momentum states create a dominant peak in scattering
cross-section as energy tends towards the Dirac point energy, suggesting a
dominant contribution to resistivity.Comment: 11 pages, 5 figure
Chandra and XMM-Newton Observations of the Double Cluster Abell 1758
Abell 1758 was classified as a single rich cluster of galaxies by Abell, but
a ROSAT observation showed that this system consists of two distinct clusters
(A1758N and A1758S) separated by approximately 8\arcmin (a projected
separation of 2 Mpc in the rest frame of the clusters). Only a few galaxy
redshifts have been published for these two clusters, but the redshift of the
Fe lines in the Chandra and XMM-Newton spectra shows that the recessional
velocities of A1758N and A1758S are within 2,100 km s. Thus, these two
clusters most likely form a gravitationally bound system, but our imaging and
spectroscopic analyses of the X-ray data do not reveal any sign of interaction
between the two clusters. The Chandra and XMM-Newton observations show that
A1758N and A1758S are both undergoing major mergers.
A1758N is in the late stages of a large impact parameter merger between two 7
keV clusters. The two remnant cores have a projected separation of 800 kpc.
Based on the measured pressure jumps preceding the two cores, they are receding
from one another at less than 1,600 km s. The two cores are surrounded
by hotter gas (--12 keV) that was probably shock heated during
the early stages of the merger. The gas entropy in the two remnant cores is
comparable with the central entropy observed in dynamically relaxed clusters,
indicating that the merger-induced shocks stalled as they tried to penetrate
the high pressure cores of the two merging systems.Each core also has a wake of
low entropy gas indicating that this gas was ram pressure stripped without
being strongly shocked (abridged). (A copy of the paper with higher resolution
images is available at http://asc.harvard.edu/~lpd/a1758.ps).Comment: paper plus 13 figure
Do early-life exposures explain why more advantaged children get eczema? Findings from the U.K. Millennium Cohort Study
Background:
Atopic dermatitis (eczema) in childhood is socially patterned, with higher incidence in more advantaged populations. However, it is unclear what factors explain the social differences.
Objectives:
To identify early-life risk factors for eczema, and to explore how early-life risk factors explain any differences in eczema.
Methods:
We estimated odds ratios (ORs) for ever having had eczema by age 5 years in 14 499 children from the U.K. Millennium Cohort Study (MCS), with a focus on maternal, antenatal and early-life risk factors and socioeconomic circumstances (SECs). Risk factors were explored to assess whether they attenuated associations between SECs and eczema.
Results:
Overall 35·1% of children had ever had eczema by age 5 years. Children of mothers with degree-level qualifications vs. no educational qualifications were more likely to have eczema (OR 1·52, 95% confidence interval 1·31–1·76), and there was a gradient across the socioeconomic spectrum. Maternal atopy, breastfeeding (1–6 weeks and ≥ 6 months), introduction of solids under 4 months or cow's milk under 9 months, antibiotic exposure in the first year of life and grime exposure were associated with an increased odds of having eczema. Female sex, Pakistani and Bangladeshi ethnicity, smoking during pregnancy, exposure to environmental tobacco smoke and having more siblings were associated with reduced odds for eczema. Controlling for maternal, antenatal and early-life characteristics (particularly maternal smoking during pregnancy, breastfeeding and number of siblings) reduced the OR for eczema to 1·26 (95% confidence interval 1·03–1·50) in the group with the highest educational qualifications compared with the least.
Conclusions:
In a representative U.K. child cohort, eczema was more common in more advantaged children. This was explained partially by early-life factors including not smoking during pregnancy, breastfeeding and having fewer siblings
Fundamental Limits of Classical and Quantum Imaging
Quantum imaging promises increased imaging performance over classical
protocols. However, there are a number of aspects of quantum imaging that are
not well understood. In particular, it has so far been unknown how to compare
classical and quantum imaging procedures. Here, we consider classical and
quantum imaging in a single theoretical framework and present general
fundamental limits on the resolution and the deposition rate for classical and
quantum imaging. The resolution can be estimated from the image itself. We
present a utility function that allows us to compare imaging protocols in a
wide range of applications.Comment: 4 pages, 3 figures; accepted for Physical Review Letters, with
updated title and fixed typo
On the structure of the scalar mesons and
We investigate the structure of the scalar mesons and
within realistic meson-exchange models of the and
interactions. Starting from a modified version of the J\"ulich model for
scattering we perform an analysis of the pole structure of the
resulting scattering amplitude and find, in contrast to existing models, a
somewhat large mass for the ( MeV,
MeV). It is shown that our model provides a description of
data comparable in quality with those of
alternative models. Furthermore, the formalism developed for the
system is consistently extended to the interaction leading to a
description of the as a dynamically generated threshold effect
(which is therefore neither a conventional state nor a
bound state). Exploring the corresponding pole position the
is found to be rather broad ( MeV,
MeV). The experimentally observed smaller width results from the influence of
the nearby threshold on this pole.Comment: 25 pages, 15 Postscript figure
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