26,617 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
On-demand microwave generator of shaped single photons
We demonstrate the full functionality of a circuit that generates single
microwave photons on demand, with a wave packet that can be modulated with a
near-arbitrary shape. We achieve such a high tunability by coupling a
superconducting qubit near the end of a semi-infinite transmission line. A dc
superconducting quantum interference device shunts the line to ground and is
employed to modify the spatial dependence of the electromagnetic mode structure
in the transmission line. This control allows us to couple and decouple the
qubit from the line, shaping its emission rate on fast time scales. Our
decoupling scheme is applicable to all types of superconducting qubits and
other solid-state systems and can be generalized to multiple qubits as well as
to resonators.Comment: 10 pages, 7 figures. Published versio
Electron and Ion Acceleration in Relativistic Shocks with Applications to GRB Afterglows
We have modeled the simultaneous first-order Fermi shock acceleration of
protons, electrons, and helium nuclei by relativistic shocks. By parameterizing
the particle diffusion, our steady-state Monte Carlo simulation allows us to
follow particles from particle injection at nonthermal thermal energies to
above PeV energies, including the nonlinear smoothing of the shock structure
due to cosmic-ray (CR) backpressure. We observe the mass-to-charge (A/Z)
enhancement effect believed to occur in efficient Fermi acceleration in
non-relativistic shocks and we parameterize the transfer of ion energy to
electrons seen in particle-in-cell (PIC) simulations. For a given set of
environmental and model parameters, the Monte Carlo simulation determines the
absolute normalization of the particle distributions and the resulting
synchrotron, inverse-Compton, and pion-decay emission in a largely
self-consistent manner. The simulation is flexible and can be readily used with
a wide range of parameters typical of gamma-ray burst (GRB) afterglows. We
describe some preliminary results for photon emission from shocks of different
Lorentz factors and outline how the Monte Carlo simulation can be generalized
and coupled to hydrodynamic simulations of GRB blast waves. We assume Bohm
diffusion for simplicity but emphasize that the nonlinear effects we describe
stem mainly from an extended shock precursor where higher energy particles
diffuse further upstream. Quantitative differences will occur with different
diffusion models, particularly for the maximum CR energy and photon emission,
but these nonlinear effects should be qualitatively similar as long as the
scattering mean free path is an increasing function of momentum.Comment: Accepted for publication in MNRA
Wind tunnel balance
A flow-through balance is provided which includes a non-metric portion and a metric portion which form a fluid-conducting passage in fluid communication with an internal bore in the sting. The non-metric and metric portions of the balance are integrally connected together by a plurality of flexure beams such that the non-metric portion, the metric portion and the flexure beams form a one-piece construction which eliminates mechanical hysteresis between the non-metric and the metric portion. The system includes structures for preventing the effects of temperature, pressure and pressurized fluid from producing asymmetric loads on the flexure beams. A temperature sensor and a pressure sensor are located within the fluid-conducting passage of the balance. The system includes a longitudinal bellows member connected at two ends to one of the non-metric portion and the metric portion and at an intermediate portion thereof to the other of (1) and (2). A plurality of strain gages are mounted on the flexure beams to measure strain forces on the flexure beams. The flexure beams are disposed so as to enable symmetric forces on the flexure beams to cancel out so that only asymmetric forces are measured as deviations by the strain gages
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)
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