1,605 research outputs found
Efficient Computation of Dendritic Microstructures using Adaptive Mesh Refinement
We study dendritic microstructure evolution using an adaptive grid, finite
element method applied to a phase-field model. The computational complexity of
our algorithm, per unit time, scales linearly with system size, rather than the
quadratic variation given by standard uniform mesh schemes. Time-dependent
calculations in two dimensions are in good agreement with the predictions of
solvability theory, and can be extended to three dimensions and small
undercoolingsComment: typo in a parameter of Fig. 1; 4 pages, 4 postscript figures, in
LateX, (revtex
Covalent bond shortening and distortion induced by pressurization of thorium, uranium, and neptunium tetrakis aryloxides
Covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of the actinides, but the ionic versus covalent nature of metal-ligand bonding in actinide complexes remains controversial. The tetrakis 2,6-di-tert-butylphenoxide complexes of Th, U and Np form an isostructural series of crystal structures containing approximately tetrahedral MO(4) cores. We show that up to 3 GPa the Th and U crystal structures show negative linear compressibility as the OMO angles distort. At 3 GPa the angles snap back to their original values, reverting to a tetrahedral geometry with an abrupt shortening of the M-O distances by up to 0.1 Å. The Np complex shows similar but smaller effects, transforming above 2.4 GPa. Electronic structure calculations associate the M-O bond shortening with a change in covalency resulting from increased contributions to the M-O bonding by the metal 6d and 5f orbitals, the combination promoting MO(4) flexibility at little cost in energy
Neutrinos in Non-linear Structure Formation - The Effect on Halo Properties
We use N-body simulations to find the effect of neutrino masses on halo
properties, and investigate how the density profiles of both the neutrino and
the dark matter components change as a function of the neutrino mass. We
compare our neutrino density profiles with results from the N-one-body method
and find good agreement. We also show and explain why the Tremaine-Gunn bound
for the neutrinos is not saturated. Finally we study how the halo mass function
changes as a function of the neutrino mass and compare our results with the
Sheth-Tormen semi-analytic formulae. Our results are important for surveys
which aim at probing cosmological parameters using clusters, as well as future
experiments aiming at measuring the cosmic neutrino background directly.Comment: 20 pages, 8 figure
Crossover Scaling in Dendritic Evolution at Low Undercooling
We examine scaling in two-dimensional simulations of dendritic growth at low
undercooling, as well as in three-dimensional pivalic acid dendrites grown on
NASA's USMP-4 Isothermal Dendritic Growth Experiment. We report new results on
self-similar evolution in both the experiments and simulations. We find that
the time dependent scaling of our low undercooling simulations displays a
cross-over scaling from a regime different than that characterizing Laplacian
growth to steady-state growth
Measurement of the Induced Proton Polarization P_n in the 12C(e,e'\vec{p}) Reaction
The first measurements of the induced proton polarization, P_n, for the 12C
(e,e'\vec{p}) reaction are reported. The experiment was performed at quasifree
kinematics for energy and momentum transfer (\omega,q) \approx (294 MeV, 756
MeV/c) and sampled a recoil momentum range of 0-250 MeV/c. The induced
polarization arises from final-state interactions and for these kinematics is
dominated by the real part of the spin-orbit optical potential. The
distorted-wave impulse approximation provides good agreement with data for the
1p_{3/2} shell. The data for the continuum suggest that both the 1s_{1/2} shell
and underlying l > 1 configurations contribute.Comment: 5 pages LaTeX, 2 postscript figures, accepted by Physical Reveiw
Letter
Phenotypic variation of larks along an aridity gradient:Are desert birds more flexible?
We investigated interindividual variation and intra-individual phenotypic flexibility in basal metabolic rate (BMR), total evaporative water loss (TEWL), body temperature (T-b), the minimum dry heat transfer coefficient (h), and organ and muscle size of five species of larks geographically distributed along an aridity gradient. We exposed all species to constant environments of 15degreesC or 35degreesC, and examined to what extent interspecific differences in physiology can be attributed to acclimation. We tested the hypothesis that birds from deserts display larger intra-individual phenotypic flexibility and smaller intern individual variation than species from mesic areas.Larks from arid areas had lower BMR, TEWL, and h, but did not have internal organ, sizes different from birds from mesic habitats. BMR of 15degreesC-acclimated birds was 18.0%, 29.1%, 12.2%, 25.3%, and 4.7% higher than of 35degreesC-acclimated Hoopoe Larks, Dunn's Larks, Spike-heeled Larks, Skylarks, and Woodlarks, respectively. TEWL of 15degreesC-acclimated Hoopoe Larks exceeded values for 35degreesC-acclimated individuals by 23% but did not differ between 15degreesC- and 35degreesC-acclimated individuals in the other species. The dry heat transfer coefficient was increased in 15degreesC-acclimated individuals of Skylarks and Dunn's Larks, but not in the. other species. Body temperature was on average 0.4degreesC +/- 0.15degreesC (mean +/- 1 SEM) lower in 15degreesC-acclimated individuals of all species. Increased food intake in 15degreesC-acclimated birds stimulated enlargement of intestine (26.9-38.6%), kidneys (9.8-24.4%), liver (16.5-27.2%), and. stomach (22.0-31.6%). The pectoral muscle increased in 15degreesC-acclimated Spike-heeled Larks and Skylarks, remained unchanged in Hoopoe Larks, and decreased in 15degreesC-acclimated Woodlarks and Dunn's Larks. We conclude that the degree of intra-individual flexibility varied between physiological traits and among species, but that acclimation does not account for interspecific differences in BMR, TEWL, and h in larks. We found no general support for the hypothesis that species from desert environments display larger intra-individual phenotypic flexibility than those from mesic areas.The coefficient of variation of larks acclimated to their natural environment was smaller in species from and areas than in species from mesic areas for mass-corrected BMR and surface-specific h, but not for mass-corrected TEWL. The high repeatabilities of BMR, TEWL, and h in several species indicated a within-individual consistency on which natural selection could operate.</p
Designing a new science-policy communication mechanism for the UN Convention to Combat Desertification
The United Nations Convention to Combat Desertification (UNCCD) has lacked an efficient mechanism to access scientific knowledge since entering into force in 1996. In 2011 it decided to convene an Ad Hoc Working Group on Scientific Advice (AGSA) and gave it a unique challenge: to design a new mechanism for science-policy communication based on the best available scientific evidence. This paper outlines the innovative 'modular mechanism' which the AGSA proposed to the UNCCD in September 2013, and how it was designed. Framed by the boundary organization model, and an understanding of the emergence of a new multi-scalar and polycentric style of governing, the modular mechanism consists of three modules: a Science-Policy Interface (SPI); an international self-governing and self-organizing Independent Non-Governmental Group of Scientists; and Regional Science and Technology Hubs in each UNCCD region. Now that the UNCCD has established the SPI, it is up to the worldwide scientific community to take the lead in establishing the other two modules. Science-policy communication in other UN environmental conventions could benefit from three generic principles corresponding to the innovations in the three modules-joint management of science-policy interfaces by policy makers and scientists; the production of synthetic assessments of scientific knowledge by autonomous and accountable groups of scientists; and multi-scalar and multi-directional synthesis and reporting of knowledge
A large scale hearing loss screen reveals an extensive unexplored genetic landscape for auditory dysfunction
The developmental and physiological complexity of the auditory system is likely reflected in the underlying set of genes involved in auditory function. In humans, over 150 non-syndromic loci have been identified, and there are more than 400 human genetic syndromes with a hearing loss component. Over 100 non-syndromic hearing loss genes have been identified in mouse and human, but we remain ignorant of the full extent of the genetic landscape involved in auditory dysfunction. As part of the International Mouse Phenotyping Consortium, we undertook a hearing loss screen in a cohort of 3006 mouse knockout strains. In total, we identify 67 candidate hearing loss genes. We detect known hearing loss genes, but the vast majority, 52, of the candidate genes were novel. Our analysis reveals a large and unexplored genetic landscape involved with auditory function
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