890 research outputs found
Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111)
Using a van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92,
246401 (2004)], we perform ab initio calculations for the adsorption energy of
benzene (Bz) on Cu(111) as a function of lateral position and height. We find
that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive
interactions) changes the relative stability of eight binding-position options
and increases the binding energy by over an order of magnitude, achieving good
agreement with experiment. The admolecules can move almost freely along a
honeycomb web of "corridors" passing between fcc and hcp hollow sites via
bridge sites. Our diffusion barriers (for dilute and two condensed adsorbate
phases) are consistent with experimental observations. Further vdW-DF
calculations suggest that the more compact (hexagonal) Bz-overlayer phase, with
lattice constant a = 6.74 \AA, is due to direct Bz-Bz vdW attraction, which
extends to ~8 \AA. We attribute the second, sparser hexagonal Bz phase, with a
= 10.24 \AA, to indirect electronic interactions mediated by the metallic
surface state on Cu(111). To support this claim, we use a formal
Harris-functional approach to evaluate nonperturbationally the asymptotic form
of this indirect interaction. Thus, we can account well for benzene
self-organization on Cu(111).Comment: 13 pages, 7 figures, 3 tables, submitted for publication Accepted for
publication in Phys. Rev. B. This version contains improved notation (with
corresponding relabeling of figures), very small corrections to some
tabulated values, and corrections concerning lattice lengths and subsequent
discussion of commensurability of unit-cell dimension
High Current Diffusion Type Diodes at Cryogenic Temperatures for the LHC Superconducting Magnet Protection
High-current by-pass diodes are required for the protection of the superconducting magnets for the Large Hadron Collider LHC at CERN . These diodes are at liquid helium tem-perature and will be exposed to irradiation. With the re-location of the by-pass diodes for the main dipoles underneath the iron yoke and of those for the quadrupoles at the bottom of the cryostat the new estimations for the irradiation dose amounts to about 30 Gy and a neutron fluence of about 1.5 x 1011 n/cm2 for the dipole di-odes and about 100 Gy and5 x 1011 n/cm2 for the quadrupole diodes during 10 years. These relatively low doses may allow the use of diffusion type diodes in-stead of epitaxial diodes. The electrical characteristics of several diodes were measured at temperatures in the range between 1.8K and 300K. Diffu-sion type diodes from three manufacturers were submitted to high current endurance tests in liquid helium. Electrical characteristics and temperatures were measured versus time and showed acceptable results. First irradiation tests show that modified diffusion diodes can be used at least for the dipole by-pass
Response of the Shockley surface state to an external electrical field: A density-functional theory study of Cu(111)
The response of the Cu(111) Shockley surface state to an external electrical
field is characterized by combining a density-functional theory calculation for
a slab geometry with an analysis of the Kohn-Sham wavefunctions. Our analysis
is facilitated by a decoupling of the Kohn-Sham states via a rotation in
Hilbert space. We find that the surface state displays isotropic dispersion,
quadratic until the Fermi wave vector but with a significant quartic
contribution beyond. We calculate the shift in energetic position and effective
mass of the surface state for an electrical field perpendicular to the Cu(111)
surface; the response is linear over a broad range of field strengths. We find
that charge transfer occurs beyond the outermost copper atoms and that
accumulation of electrons is responsible for a quarter of the screening of the
electrical field. This allows us to provide well-converged determinations of
the field-induced changes in the surface state for a moderate number of layers
in the slab geometry.Comment: 11 pages, 6 figures, 4 tables; accepted for publication by Phys. Rev.
B; changes from v1 in response to referee comments, esp. to Sections I and
V.B (inc. Table 4), with many added references, but no change in results or
conclusion
Atresia folicular en peces teleósteos: Una revisión
Many fish species with economic importance have been heavily exploited, causing the collapse of their fisheries. For this reason, there have been attempts to cultivate fish species for commercial purposes, resulting in varying success. It has been observed that when some species are kept in captivity or under intensive exploitation in its natural environment, they present reproductive dysfunctions that leads to the appearance of follicular atresia. This process is scarcely studied in fish and the characterization by several authors shows differences in nomenclature and criteria that make even more difficult its understanding. The objective of this work is to present a literature review of follicular atresia in fish that will contribute to its understanding and study. The follicular atresia is a degenerative process observed in some oocytes that can occur at any time of its development. The presence of follicular atresia has been associated with normal degenerative conditions due to seasonal changes in the gonadal activity, health disorders or inadequate management conditions in fish culture. It has been suggested that this process is a mechanism that allows the recycling of components and energy. However, in certain species such as puye (Galaxias maculatus) and cutthroat trout (Salmo clarki), among others, it has been noted the presence of follicular atresia with pathological characteristics, in which the yolk reabsorption does not occur causing the hardening of the more developed follicles that leads finally to the female death.Muchas especies de peces de importancia económica han sido intensamente explotadas, provocando el colapso de sus pesquerías. Por este motivo, en algunas de estas especies se han hecho intentos por lograr cultivar individuos con fines comerciales, obteniendo relativo éxito. Se ha observado que cuando algunas especies son mantenidas en cautiverio o bajo intensa presión de explotación en su medio natural presentan disfunciones reproductivas que conducen a la aparición de atresia folicular. Este es un proceso poco estudiado en peces y la caracterización realizada por varios autores, arroja diferencias de criterios y nomenclatura que dificultan aún más su comprensión. El objetivo de la presente investigación es realizar una revisión bibliográfica de la atresia folicular en peces y contribuir a su comprensión y estudio. La atresia folicular es un proceso degenerativo observado en algunos oocitos en cualquier momento de su desarrollo. La aparición de atresia folicular se ha relacionado con condiciones degenerativas normales debido a cambios estacionales en la actividad gonadal, afecciones sanitarias, o bien, a condiciones de manejo inadecuadas en peces mantenidos en cultivo. Este proceso se ha postulado como un mecanismo que permite reciclar componentes y energía. Sin embargo, en especies como el puye (Galaxias maculatus), la trucha garganta cortada (Salmo clarki), entre otras, se ha observado la presencia de una atresia folicular con características patológicas, en la cual la reabsorción del vitelo no ocurre, causando el endurecimiento de los folículos más desarrollados, lo que puede conducir finalmente a la muerte de la hembra
Conservation of phase space properties using exponential integrators on the cubic Schrödinger equation
The cubic nonlinear Schrödinger (NLS) equation with periodic boundary conditions is solvable using Inverse Spectral Theory. The nonlinear spectrum of the associated Lax pair reveals topological properties of the NLS phase space that are difficult to assess by other means. In this paper we use the invariance of the nonlinear spectrum to examine the long time behavior of exponential and multisymplectic integrators as compared with the most commonly used split step approach. The initial condition used is a perturbation of the unstable plane wave solution, which is difficult to numerically resolve. Our findings indicate that the exponential integrators from the viewpoint of efficiency and speed have an edge over split step, while a lower order multisymplectic is not as accurate and too slow to compete. © 2006 Elsevier Inc. All rights reserved
How Environmental Justice Patterns are Shaped by Place: Terrain and Tree Canopy in Cincinnati, Ohio, USA
Understanding the spatial distribution of environmental amenities requires consideration of social and biogeophysical factors, and how they interact to produce patterns of environmental justice or injustice. In this study, we explicitly account for terrain, a key local environmental factor, while assessing whether tree canopy is distributed equally in Cincinnati, Ohio, USA. We conducted separate analyses for all land and for residential land only. For all land, terrain alone accounted for 59% of the variation in tree canopy cover. In our spatial autoregressive model, socioeconomic variables describing race, wealth, and education did not explain significant variation in canopy cover. In other words, terrain is the primary factor related to tree canopy in Cincinnati. In our analysis of residential land only, terrain was again the dominant predictor of tree canopy cover, and percent black population and median home value were also positive, significant explanatory variables. Tree canopy was abundant in two hilly areas with dissimilar socioeconomic characteristics, with proportionally larger black populations in the western hills and higher home values in the eastern hills. In summary, the overwhelming importance of terrain may obscure subtler patterns between tree canopy and socioeconomic variables. Although general social processes may drive environmental injustice across disparate cities, our study highlights the need to account for local biogeophysical context
Theory of Polar Corrections to Donor Binding
We calculate the optical phonon correction to the binding energy of electrons
to donors in cubic materials. Previous theories calculated the Rydberg energy
reduced by the effective mass and the static dielectric function. They omitted
an important energy term from the long-range polarization of the ionized donor,
which vanishes for the neutral donor. They also omitted the donor-phonon
interaction. Including these terms yields a new formula for the donor binding
energy
Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces.Comment: 8 pages, 6 figures, submitted to J. Phys.: Condens. Matte
Benchmarking van der Waals Density Functionals with Experimental Data: Potential Energy Curves for H2 Molecules on Cu(111), (100), and (110) Surfaces
Detailed physisorption data from experiment for the H_2 molecule on low-index
Cu surfaces challenge theory. Recently, density-functional theory (DFT) has
been developed to account for nonlocal correlation effects, including van der
Waals (dispersion) forces. We show that the functional vdW-DF2 gives a
potential-energy curve, potential-well energy levels, and difference in lateral
corrugation promisingly close to the results obtained by resonant elastic
backscattering-diffraction experiments. The backscattering barrier is found
selective for choice of exchange-functional approximation. Further, the DFT-D3
and TS-vdW corrections to traditional DFT formulations are also benchmarked,
and deviations are analyzed.Comment: 15 pages, 9 figure
A de novo Ser111Thr variant in aquaporin-4 in a patient with intellectual disability, transient signs of brain ischemia, transient cardiac hypertrophy, and progressive gait disturbance
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