8,226 research outputs found
Surface Structure in an Accretion Disk Annulus with Comparable Radiation and Gas Pressure
We have employed a 3-d energy-conserving radiation MHD code to simulate the
vertical structure and thermodynamics of a shearing box whose parameters were
chosen so that the radiation and gas pressures would be comparable. The upper
layers of this disk segment are magnetically-dominated, creating conditions
appropriate for both photon bubble and Parker instabilities. We find little
evidence for photon bubbles, even though the simulation has enough spatial
resolution to see them and their predicted growth rates are high. On the other
hand, there is strong evidence for Parker instabilities, and they appear to
dominate the evolution of the magnetically supported surface layers. The disk
photosphere is complex, with large density inhomogeneities at both the
scattering and effective (thermalization) photospheres of the evolving
horizontally-averaged structure. Both the dominant magnetic support and the
inhomogeneities are likely to have strong effects on the spectrum and
polarization of thermal photons emerging from the disk atmosphere. The
inhomogeneities are also large enough to affect models of reflection spectra
from the atmospheres of accretion disks.Comment: ApJ, in pres
Direct administration of 2-hydroxypropyl-beta-cyclodextrin into guinea pig cochleae: Effects on physiological and histological measurements
<p>Cochlear response measurements from two different animals made before (red) and after (blue) treatment with HPβCD (Panel A) and TTX (Panel B) to 80 dB SPL 4 kHz tone bursts. Cochlear response waveform maintained CAP-like morphology after HPβCD treatment, consistent with reduced mechanical drive to neural excitation (Panel B, blue). In contrast, response waveform is EPSP-like following TTX treatment. Unlike TTX, results from HPβCD do not support the hypothesis that the auditory nerve is a site of action for 13 mM HPβCD.</p
SuprimeCam Observation of Sporadic Meteors during Perseids 2004
We report the serendipitous findings of 13 faint meteors and 44 artificial
space objects by Subaru SuprimeCam imaging observations during 11-16 August
2004. The meteors, at about 100km altitude, and artificial satellites/debris in
orbit, at 500km altitude or higher, were clearly discriminated by their
apparent defocused image sizes. CCD photometry of the 13 meteors, including 1
Perseid, 1 Aquarid, and 11 sporadic meteors, was performed. We defined a peak
video-rate magnitude by comparing the integrated photon counts from the
brightest portion of the track traversed within 33ms to those from a 0-mag star
during the same time duration. This definition gives magnitudes in the range
4.0< V_{vr} <6.4 and 4.1< I_{vr}<5.9 for these 13 meteors. The corresponding
magnitude for virtual naked-eye observers could be somewhat fainter especially
for the V-band observation, in which the [OI] 5577 line lasting about 1 sec as
an afterglow could contribute to the integrated flux of the present 5-10 min
CCD exposures. Although the spatial resolution is insufficient to resolve the
source size of anything smaller than about 1 m, we developed a new estimate of
the collisionally excited column diameter of these meteors. A diameter as small
as a few mm was derived from their collisionally excited photon rates, meteor
speed, and the volume density of the oxygen atoms at the 100km altitude. The
actual column diameter of the radiating zone, however, could be as large as few
100m because the excited atoms travel that distance before they emit forbidden
lines in 0.7 sec of its average lifetime. Among the 44 artificial space
objects, we confirmed that 17 were cataloged satellites/space debris.Comment: 14 pages, 13 figures, 5 tables, submitted to PAS
Si/Ge hole-tunneling double-barrier resonant tunneling diodes formed on sputtered flat Ge layers
We have demonstrated Si/Ge hole-tunneling double-barrier resonant tunneling diodes (RTDs) formed on flat Ge layers with a relaxation rate of 89% by our proposed method; in this method, the flat Ge layers can be directly formed on highly B-doped Si(001) substrates using our proposed sputter epitaxy method. The RTDs exhibit clear negative differential resistance effects in the static current–voltage (I–V) curves at room temperature. The quantized energy level estimation suggests that resonance peaks that appeared in the I–V curves are attributed to hole tunneling through the first heavy- and light-hole energy levels
First-Principles Study on Peierls Instability in Infinite Single-Row Al Wires
We present the relation between the atomic and spin-electronic structures of
infinite single-row atomic wires made of Al atoms during their elongation using
first-principles molecular-dynamics simulations. Our study reveals that the
Peierls transition indeed occurs in the wire with magnetic ordering: it
ruptures to form a trimerized structure with antiferromagnetic ordering and
changes from a conductor to an insulator just before forming a linear wire of
equally-spaced atoms. The formation of the trimerized wire is discussed in
terms of the behavior of the -symmetry bands of the Al wire.Comment: 10 pages, 4 figure
Sparse Exploratory Factor Analysis
Sparse principal component analysis is a very active research area in the last decade. It produces component loadings with many zero entries which facilitates their interpretation and helps avoid redundant variables. The classic factor analysis is another popular dimension reduction technique which shares similar interpretation problems and could greatly benefit from sparse solutions. Unfortunately, there are very few works considering sparse versions of the classic factor analysis. Our goal is to contribute further in this direction. We revisit the most popular procedures for exploratory factor analysis, maximum likelihood and least squares. Sparse factor loadings are obtained for them by, first, adopting a special reparameterization and, second, by introducing additional [Formula: see text]-norm penalties into the standard factor analysis problems. As a result, we propose sparse versions of the major factor analysis procedures. We illustrate the developed algorithms on well-known psychometric problems. Our sparse solutions are critically compared to ones obtained by other existing methods
Quasiharmonic elastic constants corrected for deviatoric thermal stresses
The quasiharmonic approximation (QHA), in its simplest form also called the
statically constrained (SC) QHA, has been shown to be a straightforward method
to compute thermoelastic properties of crystals. Recently we showed that for
non-cubic solids SC-QHA calculations develop deviatoric thermal stresses at
high temperatures. Relaxation of these stresses leads to a series of
corrections to the free energy that may be taken to any desired order, up to
self-consistency. Here we show how to correct the elastic constants obtained
using the SC-QHA. We exemplify the procedure by correcting to first order the
elastic constants of MgSiO-perovskite and MgSiO-post-perovskite, the
major phases of the Earth's lower mantle. We show that this first order
correction is quite satisfactory for obtaining the aggregated elastic averages
of these minerals and their velocities in the lower mantle. This type of
correction is also shown to be applicable to experimental measurements of
elastic constants in situations where deviatoric stresses can develop, such as
in diamond anvil cells.Comment: 4 figures, 1 table, submitted to Phys. Rev. B, July 200
First-principles study of electron transport through cages
Electron transport properties of C molecules suspended between gold
electrodes are investigated using first-principles calculations. Our study
reveals that the conductances are quite sensitive to the number of C
molecules between electrodes: the conductances of C monomers are near 1
G, while those of dimers are markedly smaller, since incident electrons
easily pass the C molecules and are predominantly scattered at the
C-C junctions. Moreover, we find both channel currents locally
circulating the outermost carbon atoms.Comment: 8 pages and 3 figure
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