12,491 research outputs found
An analysis of astronaut performance capability in the lunar environment. Volume 1 - Performance problems and requirements for additional research
Analyzing data on expected astronaut performance in lunar environmen
An Analysis of Astronaut Performance Capability in the Lunar Environment. Volume 2 - Performance Capability Support Data
Astronaut performance capability in lunar environmen
Growth, micro-structuring, spectroscopy, and optical gain in as-deposited waveguides
Deposition and micro-structuring of layers with low background losses (0.11 dB/cm) and lifetimes up to 7 ms have been optimized for active devices. Net gain of 0.7 dB/cm at 1533 nm has been measured.\ud
\u
Observations on the Formation of Massive Stars by Accretion
Observations of the H66a recombination line from the ionized gas in the
cluster of newly formed massive stars, G10.6-0.4, show that most of the
continuum emission derives from the dense gas in an ionized accretion flow that
forms an ionized disk or torus around a group of stars in the center of the
cluster. The inward motion observed in the accretion flow suggests that despite
the equivalent luminosity and ionizing radiation of several O stars, neither
radiation pressure nor thermal pressure has reversed the accretion flow. The
observations indicate why the radiation pressure of the stars and the thermal
pressure of the HII region are not effective in reversing the accretion flow.
The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient
to form massive stars within the time scale imposed by their short main
sequence lifetimes. A simple model of disk accretion relates quenched HII
regions, trapped hypercompact HII regions, and photo-evaporating disks in an
evolutionary sequence
Nuclear Magnetic Relaxation in the Ferrimagnetic Chain Compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O: Three-Magnon Scattering?
Recent proton spin-lattice relaxation-time (T_1_) measurements on the
ferrimagnetic chain compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O are
explained by an elaborately modified spin-wave theory. We give a strong
evidence of the major contribution to 1/T_1_ being made by the three-magnon
scattering rather than the Raman one.Comment: J. Phys.: Condens. Matter 16, No. 49, 9023 (2004
Anomalous Radio-Wave Scattering from Interstellar Plasma Structures
This paper considers scattering screens that have arbitrary spatial
variations of scattering strength transverse to the line of sight, including
screens that are spatially well confined, such as disks and filaments. We
calculate the scattered image of a point source and the observed pulse shape of
a scattered impulse. The consequences of screen confinement include: (1) Source
image shapes that are determined by the physical extent of the screen rather
than by the shapes of much-smaller diffracting microirregularities. These
include image elongations and orientations that are frequency dependent. (2)
Variation with frequency of angular broadening that is much weaker than the
trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including
frequency-independent cases; and (3) Similar departure of the pulse broadening
time from the usually expected \nu^{-4} scaling law. We briefly discuss
applications that include scattering of pulses from the Crab pulsar by
filaments in the Crab Nebula; image asymmetries from Galactic scattering of the
sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active
galactic nuclei by intervening galaxies. We also address the consequences for
inferences about the shape of the wavenumber spectrum of electron density
irregularities, which depend on scaling laws for the image size and the pulse
broadening. Future low-frequency (< 100 MHz) array observations will also be
strongly affected by the Galactic structure of scattering material. Our
formalism is derived in the context of radio scattering by plasma density
fluctuations. It is also applicable to optical, UV and X-ray scattering by
grains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by
ApJ, revised version has minor changes to respond to referee comments and
suggestion
Giant enhancement of anisotropy by electron-phonon interaction
Anisotropic electron-phonon interaction is shown to lead to the anisotropic
polaron effect. The resulting anisotropy of the polaron band is an exponential
function of the electron-phonon coupling and might be as big as . This
also makes anisotropy very sensitive to small changes of coupling and implies
wide variations of anisotropy among compounds of similar structure. The isotope
effect on mass anisotropy is predicted. Polaron masses are obtained by an exact
Quantum Monte Carlo method. Implications for high-temperature superconductors
are briefly discussed.Comment: 5 pages, 4 figure
Aerosol Data Sources and Their Roles within PARAGON
We briefly but systematically review major sources of aerosol data, emphasizing suites of measurements that seem most likely to contribute to assessments of global aerosol climate forcing. The strengths and limitations of existing satellite, surface, and aircraft remote sensing systems are described, along with those of direct sampling networks and ship-based stations. It is evident that an enormous number of aerosol-related observations have been made, on a wide range of spatial and temporal sampling scales, and that many of the key gaps in this collection of data could be filled by technologies that either exist or are expected to be available in the near future. Emphasis must be given to combining remote sensing and in situ active and passive observations and integrating them with aerosol chemical transport models, in order to create a more complete environmental picture, having sufficient detail to address current climate forcing questions. The Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) initiative would provide an organizational framework to meet this goal
Electrically tunable GHz oscillations in doped GaAs-AlAs superlattices
Tunable oscillatory modes of electric-field domains in doped semiconductor
superlattices are reported. The experimental investigations demonstrate the
realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the
temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes
is determined using an analytical and a numerical modeling of the dynamics of
domain formation. Three different oscillatory modes are found. Their presence
depends on the actual shape of the drift velocity curve, the doping density,
the boundary condition, and the length of the superlattice. For most bias
regions, the self-sustained oscillations are due to the formation, motion, and
recycling of the domain boundary inside the superlattice. For some biases, the
strengths of the low and high field domain change periodically in time with the
domain boundary being pinned within a few quantum wells. The dependency of the
frequency on the coupling leads to the prediction of a new type of tunable GHz
oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure
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