650 research outputs found
The effects of chronic inorganic and organic phosphate exposure on bactericidal activity of the coelomic fluid of the sea urchin Lytechinus variegatus (Echinodermata: Echinoidea)
Slabs of stabilized jellium: Quantum-size and self-compression effects
We examine thin films of two simple metals (aluminum and lithium) in the
stabilized jellium model, a modification of the regular jellium model in which
a constant potential is added inside the metal to stabilize the system for a
given background density. We investigate quantum-size effects on the surface
energy and the work function. For a given film thickness we also evaluate the
density yielding energy stability, which is found to be slightly higher than
the equilibrium density of the bulk system and to approach this value in the
limit of thick slabs. A comparison of our self-consistent calculations with the
predictions of the liquid-drop model shows the validity of this model.Comment: 7 pages, 6 figures, to appear in Phys. Rev.
Episodic rainfall influences the distribution and abundance of the regular sea urchin Lytechinus variegatus in Saint Andrew Bay, northern Gulf of Mexico
The distribution and abundance of Lytechinus variegatus (Lamarck) were determined at three shallow-water stations in Saint Andrew Bay, FL, in the northern Gulf of Mexico. Populations were monitored at 4-mo intervals from Aug. 1997 to Aug. 1999 along 2- X 10-m transects, with four transects at each station. In Aug. 1997 individuals ranged in size from 15- to 63-mm diameter (mean = 38 mm; density= 1.4 individuals (ind.) m-2) at Station 1 (Stations 2 and 3 were not sampled on that date). By Oct. 1997 individuals at Station 1 ranged in size from 26 to 62 mm (mean = 48 mm; density = 0.6 ind. m-2), suggesting the growth of individuals within the population. At Station 2, individuals ranged between 50 and 70 mm (mean = 59 mm; densities = 1.0 ind. m-2) and at Station 3 between 30 and 79 mm (mean= 51 mm; densities = 1.4 ind, m-2) in Oct. 1997, indicating a population with no recent recruits. A mass mortality event was observed in April 1998 and was attributed to reduced salinities resulting from above-average rainfall in the previous month. Sea urchins were absent at Stations 1 and 2, whereas at Station 3 postdisturbance recruits were smaller (mean = 28 mm) and densities lower. Sea urchin tests were also observed at beach stations proximate to Stations 1 and 2. By July 1998 the populations had partially recovered at Stations 2 and 3 (densities = 0.6 and 1.1 ind. m-2, respectively), but test diameters were small at both stations. Test diameters had increased in Dec. 1998 at Station 3 when compared with those of July, but individuals at Station 2 showed overall smaller test diameters (mean = 28 mm), and densities had decreased again, presumably after another mass mortality caused by another major rainfall in Sep. In April 1999 test diameters had decreased further at Stations 2 and 3, indicating emigration or death of larger animals. We suggest that the shallow-water Saint Andrew Bay population of L. variegatus is reduced for several years at a time by episodic heavy rainfalls. This observation emphasizes the importance of density-independent processes controlling the distribution and abundance of marine organisms
Verification of the mixed layer depth in the OceanMAPS operational forecast model for Austral autumn
The ocean mixed layer depth is an important parameter
describing the exchange of fluxes between the atmosphere and ocean. In ocean
modelling a key factor in the accurate representation of the mixed layer is
the parameterization of vertical mixing. An ideal opportunity to investigate
the impact of different mixing schemes was provided when the Australian
Bureau of Meteorology upgraded its operational ocean forecasting model,
OceanMAPS to version 3.0. In terms of the mixed layer, the main difference
between the old and new model versions was a change of vertical mixing
scheme from that of Chen et al. (1994) to the General Ocean Turbulence Model.The model estimates of the mixed layer depth were compared with those
derived from Argo observations. Both versions of the model exhibited a deep
bias in tropical latitudes and a shallow bias in the Southern Ocean,
consistent with previous studies. The bias, however, was greatly reduced in
version 3.0, and variance between model runs decreased. Additionally, model
skill against climatology also improved significantly. Further analysis
discounted changes to model resolution outside of the Australian region
having a significant impact on these results, leaving the change in vertical\ud
mixing scheme as the main factor in the assessed improvements to mixed layer
depth representation.</p
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Simple DFT-LSDA modeling of the molecular-like aspects of ultra-thin film properties
Ordered ultra-thin films (UTF`s) are atomic n-layers (n = 1,2,3,...) with translational symmetry in-plane and molecular-like inter-planar spacings. Though commonly used (especially at relatively large n-values) as models of crystalline surfaces, they are intrinsically interesting and of growing technological significance as the basic building blocks of multi-layer electronic devices. Predicting the structure and properties of even a simple diatomic 1-layer means addressing aspects of molecular binding (and boundary conditions) in the context of an extended, periodically bounded system. At the level of refinement provided by the local spin density approximation to Density Functional Theory, the baseline standard of today`s predictive, chemically specific solid-state calculations, a number of technical and fundamental issues arise. The authors focus on treatment of the isolated atoms, on basis sets, and on numerical precision, as illustrated by the Fe atom and BN 1- and 2-layer calculations. Computational requirements are illustrated by a brief summary of recently completed calculations on crystalline sapphire, {alpha}-Al{sub 2}O{sub 3}, which used the same code
A Density Functional Study of O2 Adsorption on (100) Surface of gamma-Uranium
We have studied the chemisorption processes of O2 on the (100) surface of
uranium using generalized gradient approximation to density functional theory.
Dissociative adsorptions of O2 are significantly favored compared to molecular
adsorptions. We found interstitial adsorption of molecular oxygen to be less
probable, as no bound states were found in this case. Upon oxygen adsorption, O
2p orbitals is found to hybridize with U 5f bands, and part of the U 5f
electrons become more localized. Also there is a significant charge transfer
from the first layer of the uranium surface to the oxygen atoms, which made the
bonding partly ionic. For the most favored site the dissociative chemisorption
energy is approximately 9.5 eV, which indicates a strong reaction of uranium
surface with oxygen. Spin polarization does not have a considerable effect on
the chemisorption process. For most of the sites and approaches, chemisorption
configurations are almost same at both spin-polarized and non-spin-polarized
cases. For the most favored chemisorption sites of oxygen on uranium,
paramagnetic adsorption is slightly stronger, by 0.304 eV, than the magnetic
adsorption
Evolving properties of two dimensional materials, from graphene to graphite
We have studied theoretically, using density functional theory, several
materials properties when going from one C layer in graphene to two and three g
raphene layers and on to graphite. The properties we have focused on are the
elastic constants, electronic structure (energy bands and density of state s),
and the dielectric properties. For any of the properties we have investigated
the modification due to an increase in the number of graphene layers is within
a few percent. Our results are in agreement with the analysis presented
recently by Kopelevich and Esquinazi (unpublished)
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