20,337 research outputs found
Ab initio studies of structural instabilities in magnesium silicate perovskite
Density-functional simulations are used to calculate structural properties
and high-symmetry phonons of the hypothetical cubic phase, the stable
orthorhombic phase and an intermediate tetragonal phase of magnesium silicate
perovskite. We show that the structure of the stable phase is well described by
freezing in a small number of unstable phonons into the cubic phase. We use the
frequencies of these unstable modes to estimate transition temperatures for
cubic--tetragonal and tetragonal--orthorhombic phase transitions. These are
investigated further to find that the coupling with the strain suggests that
phonons give a better representation than rigid unit modes. The phonons of an
intermediate tetragonal phase were found to be stable except for two rotational
modes. The eigenvectors of the most unstable mode of each of the cubic and
tetragonal phases account for all the positional parameters of the orthorhombic
phase. The phase boundary for the orthorhombic--tetragonal transition
intersects possible mantle geotherms, suggesting that the tetragonal phase may
be present in the lower mantle.Comment: 16 pages, REVTEX, 7 postscript figures (Fig 1 very large, contact
Authors if required); submitted to Physics and Chemistry of Mineral
The Thirty Meter Telescope International Observatory facilitating transformative astrophysical science
The next major advancement in astronomy and cosmology will be driven by deep
observations using very sensitive telescopes with high spatial and spectral
resolution capabilities. An international consortium of astronomers, including
Indian astronomers are building the Thirty Meter Telescope to achieve
breakthroughs in different areas of astronomy starting from studies of the
solar system to that of the early universe. This article provides a brief
overview of the telescope, science objectives and details of the first light
instruments.Comment: 10 page
Simulations of aging and plastic deformation in polymer glasses
We study the effect of physical aging on the mechanical properties of a model
polymer glass using molecular dynamics simulations. The creep compliance is
determined simultaneously with the structural relaxation under a constant
uniaxial load below yield at constant temperature. The model successfully
captures universal features found experimentally in polymer glasses, including
signatures of mechanical rejuvenation. We analyze microscopic relaxation
timescales and show that they exhibit the same aging characteristics as the
macroscopic creep compliance. In addition, our model indicates that the entire
distribution of relaxation times scales identically with age. Despite large
changes in mobility, we observe comparatively little structural change except
for a weak logarithmic increase in the degree of short-range order that may be
correlated to an observed decrease in aging with increasing load.Comment: 9 pages, 12 figure
The Einstein Ring 0047-2808 Revisited: A Bayesian Inversion
In a previous paper, we outlined a new Bayesian method for inferring the
properties of extended gravitational lenses, given data in the form of resolved
images. This method holds the most promise for optimally extracting information
from the observed image, whilst providing reliable uncertainties in all
parameters. Here, we apply the method to the well studied optical Einstein ring
0047-2808. Our results are in broad agreement with previous studies, showing
that the density profile of the lensing galaxy is aligned within a few degrees
of the light profile, and suggesting that the source galaxy (at redshift 3.6)
is a binary system, although its size is only of order 1-2 kpc. We also find
that the mass of the elliptical lensing galaxy enclosed by the image is
(2.910.01) M_{\sun}. Our method is able to achieve
improved resolution for the source reconstructions, although we also find that
some of the uncertainties are greater than has been found in previous analyses,
due to the inclusion of extra pixels and a more general lens model.Comment: Accepted for publication in Ap
Precision measurement of the 5 2S1/2 - 4 2D5/2 quadrupole transition isotope shift between 88Sr+ and 86Sr+
We have measured the isotope shift of the narrow quadrupole-allowed 5 2S1/2 -
4 2D5/2 transition in 86Sr+ relative to the most abundant isotope 88Sr+. This
was accomplished using high-resolution laser spectroscopy of individual trapped
ions, and the measured shift is Delta-nu_meas^(88,86) = 570.281(4) MHz. We have
also tested a recently developed and successful method for ab-initio
calculation of isotope shifts in alkali-like atomic systems against this
measurement, and our initial result of Delta-nu_calc^(88,86) = 457(28) MHz is
also presented. To our knowledge, this is the first high precision measurement
and calculation of that isotope shift. While the measurement and the
calculation are in broad agreement, there is a clear discrepancy between them,
and we believe that the specific mass shift was underestimated in our
calculation. Our measurement provides a stringent test for further refinements
of theoretical isotope shift calculation methods for atomic systems with a
single valence electron
Past and Present Distribution of the Red-Cockaded Woodpecker Picoides borealis and its Habitat in the Ouachita Mountains, Arkansas
Within the past 15 years, at least 41 and probably more active cavity tree clusters (or colonies) of Red cockaded Woodpeckers (Picoides borealis) have existed in remnant, mature shortleaf pine (Pinus echinata) woodlands in the Ouachita Mountains of Arkansas. These clusters were located on both private timberlands and in the Ouachita National Forest. Fewer than half of this number were still active in early 1991, and none remained on private timberlands. The species is presently restricted to the xeric, western margins of the Ouachitas in Scott and Polk counties within the confines of the Ouachita National Forest where it receives protection of the Endangered Species Act. The decline of P. borealis in the Ouachitas resulted from intense logging of old growth pine forests during the timber boom period, ca. 1910-1950, and from the suppression of natural fires, which subsequently allowed hardwoods to invade former pine woodlands
Test results of the Chrysler upgraded automotive gas turbine engine: Initial design
The upgraded engine as built to the original design was deficient in power and had excessive specific fuel consumption. A high instrumented version of the engine was tested to identify the sources of the engine problems. Analysis of the data shows the major problems to be low compressor and power turbine efficiency and excessive interstage duct losses. In addition, high HC and CO emission were measured at idle, and high NOx emissions at high energy speeds
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