4,979 research outputs found
Density-functional investigation of the rhombohedral to simple cubic phase transition of arsenic
We report on our investigation of the crystal structure of arsenic under
compression, focusing primarily on the pressure-induced A7 to simple cubic (sc)
phase transition. The two-atom rhombohedral unit cell is subjected to pressures
ranging from 0 GPa to 200 GPa; for each given pressure, cell lengths and
angles, as well as atomic positions, are allowed to vary until the fully
relaxed structure is obtained. We find that the nearest and next-nearest
neighbor distances give the clearest indication of the occurrence of a
structural phase transition. Calculations are performed using the local density
approximation (LDA) and the PBE and PW91 generalized gradient approximations
(GGA-PBE and GGA-PW91) for the exchange-correlation functional. The A7 to sc
transition is found to occur at 21+/-1 GPa in the LDA, at 28+/-1 GPa in the
GGA-PBE and at 29+/-1 GPa in the GGA-PW91; no volume discontinuity is observed
across the transition in any of the three cases. We use k-point grids as dense
as 66X66X66 to enable us to present reliably converged results for the A7 to sc
transition of arsenic.Comment: To be published in Physical Review B; material supplementary to this
article is available at arXiv:0810.169
Performance of AAOmega: the AAT multi-purpose fibre-fed spectrograph
AAOmega is the new spectrograph for the 2dF fibre-positioning system on the
Anglo-Australian Telescope. It is a bench-mounted, double-beamed design, using
volume phase holographic (VPH) gratings and articulating cameras. It is fed by
392 fibres from either of the two 2dF field plates, or by the 512 fibre SPIRAL
integral field unit (IFU) at Cassegrain focus. Wavelength coverage is 370 to
950nm and spectral resolution 1,000-8,000 in multi-Object mode, or 1,500-10,000
in IFU mode. Multi-object mode was commissioned in January 2006 and the IFU
system will be commissioned in June 2006.
The spectrograph is located off the telescope in a thermally isolated room
and the 2dF fibres have been replaced by new 38m broadband fibres. Despite the
increased fibre length, we have achieved a large increase in throughput by use
of VPH gratings, more efficient coatings and new detectors - amounting to a
factor of at least 2 in the red. The number of spectral resolution elements and
the maximum resolution are both more than doubled, and the stability is an
order of magnitude better.
The spectrograph comprises: an f/3.15 Schmidt collimator, incorporating a
dichroic beam-splitter; interchangeable VPH gratings; and articulating red and
blue f/1.3 Schmidt cameras. Pupil size is 190mm, determined by the competing
demands of cost, obstruction losses, and maximum resolution. A full suite of
VPH gratings has been provided to cover resolutions 1,000 to 7,500, and up to
10,000 at particular wavelengths.Comment: 13 pages, 4 figures; presented at SPIE, Astronomical Telescopes and
Instrumentation, 24 - 31 May 2006, Orlando, Florida US
Comparing Laser Diffraction and Optical Microscopy for Characterizing Superabsorbent Polymer Particle Morphology, Size, and Swelling Capacity
In this study, we determined the accuracy and practicality of using optical microscopy (OM) and laser diffraction (LD) to characterize hydrogel particle morphology, size, and swelling capacity (Q). Inverse-suspension-polymerized polyacrylamide particles were used as a model system. OM and LD showed that the average particle diameter varied with the mixing speed during synthesis for the dry (10â120 lm) and hydrated (34â240 lm) particles. The LD volume and number mean diameters showed that a few large particles were responsible for the majority of the water absorption. Excess water present in the gravimetric swelling measurements led to larger Qs (8.2 6 0.37 g/g), whereas the volumetric measurements with OM and LD resulted in reduced capacities (6.5 6 3.8 and 5.7 6 3.9 g/g, respectively). Results from the individual particle swelling measurements with OM (5.2 6 0.66 g/g) statistically confirmed that the volumetric methods resulted in a reduced and more accurate measurement of the Q than the gravimetric method
On the investigations of galaxy redshift periodicity
In this article we present a historical review of study of the redshift
periodicity of galaxies, starting from the first works performed in the
seventies of the twentieth century until the present day. We discuss the
observational data and methods used, showing in which cases the discretization
of redshifts was observed. We conclude that galaxy redshift periodisation is an
effect which can really exist. We also discussed the redshift discretization in
two different structures: the Local Group of galaxies and the Hercules
Supercluster. Contrary to the previous studies we consider all galaxies which
can be regarded as a structure member disregarding the accuracy of velocity
measurements. We applied the power spectrum analysis using the Hann function
for weighting, together with the jackknife error estimator. In both the
structures we found weak effects of redshift periodisation.Comment: 10 pages, 4 figures, to be published in Part. and Nucl. Lett. 200
Discovery of Extreme Examples of Superclustering in Aquarius
We report the discovery of two highly extended filaments and one extremely
high density knot within the region of Aquarius. The supercluster candidates
were chosen via percolation analysis of the Abell and ACO catalogs and include
only the richest clusters (R >= 1). The region examined is a 10x45 degree strip
and is now 87% complete in cluster redshift measurements to mag_10 = 18.3. In
all, we report 737 galaxy redshifts in 46 cluster fields. One of the
superclusters, dubbed Aquarius, is comprised of 14 Abell/ACO clusters and
extends 110h^-1Mpc in length only 7 degrees off the line-of-sight. On the
near-end of the Aquarius filament, another supercluster, dubbed Aquarius-Cetus,
extends for 75h^-1Mpc perpendicular to the line-of-sight. After fitting
ellipsoids to both Aquarius and Aquarius-Cetus, we find axis ratios (long-to-
midlength axis) of 4.3 for Aquarius and 3.0 for Aquarius-Cetus. We fit
ellipsoids to all N>=5 clumps of clusters in the Abell/ACO measured-z cluster
sample. The frequency of filaments with axis ratios >=3.0 (~20%) is nearly
identical with that found among `superclusters' in Monte Carlo simulations of
random and random- clumped clusters, however, so the rich Abell/ACO clusters
have no particular tendency toward filamentation. The Aquarius filament also
contains a `knot' of 6 clusters at Z ~0.11, with five of the clusters near
enough togeteher to represent an apparent overdensity of 150. There are
three other R >= 1 cluster density enhancements similar to this knot at lower
redshifts: Corona Borealis, the Shapely Concentration, and another grouping of
seven clusters in Microscopium. All four of these dense superclusters appear
near the point of breaking away from the Hubble Flow, and some may now be in
collapse, but there is little evidence of any being virialized.Comment: 45 pages (+ e-tables), 7 figures, AASTeX Accepted for Publication in
Ap
On the Origins of Starburst and Post-Starburst Galaxies in Nearby Clusters
HST WFPC2 images in B (F450W) and I (F814W) have been obtained for three
starburst (SB) and two post-starburst (PSB) galaxies in the Coma cluster, and
for three such galaxies in the cluster DC2048-52. V (F555W) and I images for an
additional PSB galaxy in Coma have been extracted from the archive. Seven of
these galaxies were previously classified as E/S0 on the basis of ground-based
images, one as Sa, and the other as an irregular.
The HST images reveal these SB/PSB galaxies to be heterogeneous in
morphology. Nevertheless a common theme is that many of them, especially the SB
galaxies, tend to have centralized spiral structure that appears simply as a
bright ``bulge''on ground-based images. In addition, while some PSB galaxies
exhibit distinct spiral structure, on the whole they have smoother morphologies
than the SB galaxies. The morphologies and luminosity profiles are generally
consistent with substantial starbursts in the form of centralized spiral
structure (the SB galaxies) which fade into smoother morphologies (the PSB
galaxies), with lingering spectroscopic evidence for past central starbursts.
An important point is that the PSB galaxies retain disks, i.e, they have not
evolved into spheroidal systems.Comment: 32 pages, 10 figures including 3 jpg images. To appear in the January
1999 Astronomical Journa
Time Optimal Control in Spin Systems
In this paper, we study the design of pulse sequences for NMR spectroscopy as
a problem of time optimal control of the unitary propagator. Radio frequency
pulses are used in coherent spectroscopy to implement a unitary transfer of
state. Pulse sequences that accomplish a desired transfer should be as short as
possible in order to minimize the effects of relaxation and to optimize the
sensitivity of the experiments. Here, we give an analytical characterization of
such time optimal pulse sequences applicable to coherence transfer experiments
in multiple-spin systems. We have adopted a general mathematical formulation,
and present many of our results in this setting, mindful of the fact that new
structures in optimal pulse design are constantly arising. Moreover, the
general proofs are no more difficult than the specific problems of current
interest. From a general control theory perspective, the problems we want to
study have the following character. Suppose we are given a controllable right
invariant system on a compact Lie group, what is the minimum time required to
steer the system from some initial point to a specified final point? In NMR
spectroscopy and quantum computing, this translates to, what is the minimum
time required to produce a unitary propagator? We also give an analytical
characterization of maximum achievable transfer in a given time for the two
spin system.Comment: 20 Pages, 3 figure
Observations of red-giant variable stars by Aboriginal Australians
Aboriginal Australians carefully observe the properties and positions of
stars, including both overt and subtle changes in their brightness, for
subsistence and social application. These observations are encoded in oral
tradition. I examine two Aboriginal oral traditions from South Australia that
describe the periodic changing brightness in three pulsating, red-giant
variable stars: Betelgeuse (Alpha Orionis), Aldebaran (Alpha Tauri), and
Antares (Alpha Scorpii). The Australian Aboriginal accounts stand as the only
known descriptions of pulsating variable stars in any Indigenous oral tradition
in the world. Researchers examining these oral traditions over the last
century, including anthropologists and astronomers, missed the description of
these stars as being variable in nature as the ethnographic record contained
several misidentifications of stars and celestial objects. Arguably,
ethnographers working on Indigenous Knowledge Systems should have academic
training in both the natural and social sciences.Comment: The Australian Journal of Anthropology (2018
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