101,129 research outputs found
Knots and Classical 3-Geometries
It has been conjectured by Rovelli that there is a correspondence between the
space of link classes of a Riemannian 3-manifold and the space of 3-geometries
(on the same manifold). An exact statement of his conjecture will be
established and then verified for the case when the 3-manifold is compact,
orientable and closed.Comment: 14p. type-set in AmS-TeX version 2.
Spin-ladders with spin gaps: A description of a class of cuprates
We investigate the magnetic properties of the Cu-O planes in stoichiometric
SrCuO (n=3,5,7,...) which consist of CuO double chains
periodically intergrown within the CuO planes. The double chains break up
the two-dimensional antiferromagnetic planes into Heisenberg spin ladders with
rungs and legs and described by
the usual antiferromagnetic coupling J inside each ladder and a weak and
frustrated interladder coupling J. The resulting lattice is a new
two-dimensional trellis lattice. We first examine the spin excitation spectra
of isolated quasi one dimensional Heisenberg ladders which exhibit a gapless
spectra when is even and is odd ( corresponding to n=5,9,...) and a
gapped spectra when is odd and is even (corresponding to
n=3,7,...). We use the bond operator representation of quantum
spins in a mean field treatment with self-energy corrections and obtain a spin
gap of for the simplest single rung ladder (n=3), in
agreement with numerical estimates.Comment: 21 pages, 5 figures upon request, REVTEX, ETH-TH/93-3
On higher dimensional black holes with abelian isometry group
We consider (n+1)--dimensional, stationary, asymptotically flat, or
Kaluza-Klein asymptotically flat black holes, with an abelian --dimensional
subgroup of the isometry group satisfying an orthogonal integrability
condition. Under suitable regularity conditions we prove that the area of the
group orbits is positive on the domain of outer communications, vanishing only
on its boundary and on the "symmetry axis". We further show that the orbits of
the connected component of the isometry group are timelike throughout the
domain of outer communications. Those results provide a starting point for the
classification of such black holes. Finally, we show non-existence of zeros of
static Killing vectors on degenerate Killing horizons, as needed for the
generalisation of the static no-hair theorem to higher dimensions
Half-Quantum Vortices in Thin Film of Superfluid He
Stability of a half-quantum vortex (HQV) in superfluid He has been
discussed recently by Kawakami, Tsutsumi and Machida in Phys. Rev. B {\bf 79},
092506 (2009). We further extend this work here and consider the A phase of
superfluid He confined in thin slab geometry and analyze the HQV realized
in this setting. Solutions of HQV and singly quantized singular vortex are
evaluated numerically by solving the Ginzburg-Landau (GL) equation and
respective first critical angular velocities are obtained by employing these
solutions. We show that the HQV in the A phase is stable near the boundary
between the A and A phases. It is found that temperature and magnetic
field must be fixed first in the stable region and subsequently the angular
velocity of the system should be increased from zero to a sufficiently large
value to create a HQV with sufficiently large probability. A HQV does not form
if the system starts with a fixed angular velocity and subsequently the
temperature is lowered down to the A phase. It is estimated that the
external magnetic field with strength on the order of 1 T is required to have a
sufficiently large domain in the temperature-magnetic field phase diagram to
have a stable HQV.Comment: 5 pages, 5 figure
Seed production, infestation, and viability in Acacia tortilis (synonym: Vachellia tortilis) and Acacia robusta (synonym: Vachellia robusta) across the Serengeti rainfall gradient
Tree recruitment in savannas proceeds in multiple stages characterized by successive filters occurring at the seed and seedling stages. The âdemographic bottleneckâ hypothesis suggests that such filters ultimately restrict tree density and prevent trees from dominating grasses in savannas, but many of the demographic transitions underlying this assumption have not been quantified. We investigated how short- (1â2 years) and long-term (40 + years) rainfall patterns influenced seed production, infestation, and viability for two dominant species, Acacia robusta and Acacia tortilis across the Serengeti ecosystem mean annual precipitation gradient over a two-year period. We found that neither production, nor infestation, nor viability was influenced by rainfall. Pod production differed between species and increased with tree height in A. robusta. Mean infestation proportion in 2013 was higher (mean ± SE; 0.28 ± 0.08) in A. tortilis than in A. robusta (0.11 ± 0.05) but the trend reversed in 2014, when A. tortilis (0.33 ± 0.10) had lower infestation than A. robusta (0.61 ± 0.09). Under laboratory conditions, A. tortilis and A. robusta seeds had maximum germination (= viability) proportions of 70 and 20%, respectively. Mean seed viability was more than five-fold higher (0.46 ± 0.19) in A. tortilis than in A. robusta (0.08 ± 0.10). Our study has produced important estimates for seed stage demographic dynamics that can be used for modeling tree dynamics in Serengeti system, and savannas in general
The Arecibo HII Region Discovery Survey
We report the detection of radio recombination line emission (RRL) using the
Arecibo Observatory at X-band (9GHz, 3cm) from 37 previously unknown HII
regions in the Galactic zone 66 deg. > l > 31 deg. and |b| < 1 deg. This
Arecibo HII Region Discovery Survey (Arecibo HRDS) is a continuation of the
Green Bank Telescope (GBT) HRDS. The targets for the Arecibo HRDS have
spatially coincident 24 micron and 20 cm emission of a similar angular
morphology and extent. To take advantage of Arecibo's sensitivity and small
beam size, sources in this sample are fainter, smaller in angle, or in more
crowded fields compared to those of the GBT HRDS. These Arecibo nebulae are
some of the faintest HII regions ever detected in RRL emission. Our detection
rate is 58%, which is low compared to the 95% detection rate for GBT HRDS
targets. We derive kinematic distances to 23 of the Arecibo HRDS detections.
Four nebulae have negative LSR velocities and are thus unambiguously in the
outer Galaxy. The remaining sources are at the tangent point distance or
farther. We identify a large, diffuse HII region complex that has an associated
HI and 13CO shell. The ~90 pc diameter of the G52L nebula in this complex may
be the largest Galactic HII region known, and yet it has escaped previous
detection.Comment: Accepted to ApJ Data can be found here: http://go.nrao.edu/hrd
Stage-specific vertical distribution of Alaska plaice (Pleuronectes quadrituberculatus) eggs in the eastern Bering Sea
The stage-specific distribution of Alaska plaice (Pleuronectes quadrituberculatus) eggs in the southeastern
Bering Sea was examined with collections made in mid-May in
2002, 2003, 2005, and 2006. Eggs in the early stages of development were found primarily offshore of the 40-m
isobath. Eggs in the middle and late stages of development were found inshore and offshore of the 40-m isobath. There was some evidence that early-stage eggs occur deeper in the
water column than late-stage eggs, although year-to-year variability in that trend was observed. Most eggs were in the later stages of development; therefore the majority of spawning is estimated to have occurred a few weeks before collectionâprobably Aprilâand may be highly synchronized
among local spawning areas. Results indicate that sampling with continuous underway fish egg collectors(CUFES) should be supplemented with sampling of the entire water column to ensure adequate samples of all egg stages of Alaska plaice. Data presented offer new information on the stage-dependent horizontal and vertical distribution of Alaska plaice eggs in the Bering Sea and provide further evidence that the early life history stages of this species are vulnerable
to near-surface variations in hydrographical conditions and climate forcing
Thermospheric winds and temperatures above Mawson, Antarctica, observed with an all-sky imaging, Fabry-Perot spectrometer
A new all-sky imaging Fabry-Perot spectrometer has been installed at Mawson station (67&deg;36' S, 62&deg;52' E), Antarctica. This instrument is capable of recording independent spectra from many tens of locations across the sky simultaneously. Useful operation began in March 2007, with spectra recorded on a total of 186 nights. Initial analysis has focused on the large-scale daily and average behavior of winds and temperatures derived from observations of the 630.0 nm airglow line of atomic oxygen, originating from a broad layer centered around 240 km altitude, in the ionospheric F-region. <br><br> The 1993 Horizontal Wind Model (HWM93), NRLMSISE-00 atmospheric model, and the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP) model were used for comparison. During the geomagnetically quiet period studied, observed winds and temperatures were generally well modelled, although temperatures were consistently higher than NRLMSISE-00 predicted, by up to 100 K. CTIP temperatures better matched our data, particularly later in the night, but predicted zonal winds which were offset from those observed by 70â180 ms<sup>&minus;1</sup> westward. During periods of increased activity both winds and temperatures showed much greater variability over time-scales of less than an hour. For the active night presented here, a period of 45 min saw wind speeds decrease by around 180 ms<sup>&minus;1</sup>, and temperatures increase by approximately 100 K. Active-period winds were poorly modelled by HWM93 and CTIP, although observed median temperatures were in better agreement with NRLMSISE-00 during such periods. <br><br> Average behavior was found to be generally consistent with previous studies of thermospheric winds above Mawson. The collected data set was representative of quiet geomagnetic and solar conditions. Geographic eastward winds in the afternoon/evening generally continued until around local midnight, when winds turned equatorward. Geographic meridional and zonal winds in the afternoon were approximately 50 ms<sup>&minus;1</sup> weaker than expected from HWM93, as was the transition to equatorward flow around midnight. There was also a negligible geographic zonal component to the post-midnight wind where HWM93 predicted strong westward flow. Average temperatures between 19:00 and 04:00 local solar time were around 60 K higher than predicted by NRLMSISE-00
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