23,045 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
Mechanical properties of several nickel alloys in hydrogen at elevated temperatures
Tests were performed to determine low cycle fatigue and crack growth rate properties of one iron-base and two forms of one cast nickel-base alloy. The alloys were tested in various forms and/or heat-treat conditions that are proposed for use in a high-pressure hydrogen or a hydrogen-water vapor environment. Some general conclusions can be made comparing the results of tests in a hydrogen environment with those in a hydrogen-water vapor environment. The hydrogen-water vapor environment caused a 50 percent average reduction in fatigue life, indicating extreme degradation when compared with tests conducted in air, for Incoloy 903 at 1033 K (1400 F). Crack growth rates increased significantly for all materials with increasing test temperature. A very significant increase (three orders of magnitude) in crack growth rate occurred for Incoloy 903 tested in the hydrogen-water vapor environment when compared with testing done in hydrogen along at 922 K (1200 F)
Nonmetallic Low-Temperature Normal State of K0.70Fe1.46Se1.85Te0.15
The normal-state in-plane resistivity below the zero-field superconducting
transition temperature and the upper critical field Hc2 were measured by
suppressing superconductivity in pulsed magnetic fields for
K0.70Fe1.46Se1.85Te0.15. The normal-state resistivity is found to
increase logarithmically with decrasing temperature as
. Similar to granular metals, our results suggest
that a superconductor - insulator transition below zero-field T may be
induced in high magnetic fields. This is related to the intrinsic real-space
phase-separated states common to all inhomogeneous superconductors.Comment: 6 pages, 4 figure
The effects of ganglionic and adrenergic blockade on the circulation of the young chimpanzee Final technical report, 1 Oct. 1963 - 30 Apr. 1965
Ganglionic and adrenergic blockade effects on circulatory system studied on young chimpanze
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
Precision Determination of the Mass Function of Dark Matter Halos
The predicted mass function of dark matter halos is essential in connecting
observed galaxy cluster counts and models of galaxy clustering to the
properties of the primordial density field. We determine the mass function in
the concordance CDM cosmology, as well as its uncertainty, using
sixteen -particle nested-volume dark-matter simulations, spanning a
mass range of over five orders of magnitude. Using the nested volumes and
single-halo tests, we find and correct for a systematic error in the
friends-of-friends halo-finding algorithm. We find a fitting form and full
error covariance for the mass function that successfully describes the
simulations' mass function and is well-behaved outside the simulations'
resolutions. Estimated forecasts of uncertainty in cosmological parameters from
future cluster count surveys have negligible contribution from remaining
statistical uncertainties in the central cosmology multiplicity function. There
exists a potentially non-negligible cosmological dependence (non-universality)
of the halo multiplicity function.Comment: 4 pages, 3 figures, submitted to ApJ
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