836 research outputs found
Metallic properties of magnesium point contacts
We present an experimental and theoretical study of the conductance and
stability of Mg atomic-sized contacts. Using Mechanically Controllable Break
Junctions (MCBJ), we have observed that the room temperature conductance
histograms exhibit a series of peaks, which suggests the existence of a shell
effect. Its periodicity, however, cannot be simply explained in terms of either
an atomic or electronic shell effect. We have also found that at room
temperature, contacts of the diameter of a single atom are absent. A possible
interpretation could be the occurrence of a metal-to-insulator transition as
the contact radius is reduced, in analogy with what it is known in the context
of Mg clusters. However, our first principle calculations show that while an
infinite linear chain can be insulating, Mg wires with larger atomic
coordinations, as in realistic atomic contacts, are alwaysmetallic. Finally, at
liquid helium temperature our measurements show that the conductance histogram
is dominated by a pronounced peak at the quantum of conductance. This is in
good agreement with our calculations based on a tight-binding model that
indicate that the conductance of a Mg one-atom contact is dominated by a single
fully open conduction channel.Comment: 14 pages, 5 figure
Electronic and atomic shell structure in aluminum nanowires
We report experiments on aluminum nanowires in ultra-high vacuum at room
temperature that reveal a periodic spectrum of exceptionally stable structures.
Two "magic" series of stable structures are observed: At low conductance, the
formation of stable nanowires is governed by electronic shell effects whereas
for larger contacts atomic packing dominates. The crossover between the two
regimes is found to be smooth. A detailed comparison of the experimental
results to a theoretical stability analysis indicates that while the main
features of the observed electron-shell structure are similar to those of
alkali and noble metals, a sequence of extremely stable wires plays a unique
role in Aluminum. This series appears isolated in conductance histograms and
can be attributed to "superdeformed" non-axisymmetric nanowires.Comment: 15 pages, 9 figure
The mass-metallicity gradient relation of early-type galaxies
We present a newly observed relation between galaxy mass and radial
metallicity gradients of early-type galaxies. Our sample of 51 early-type
galaxies encompasses a comprehensive mass range from dwarf to brightest cluster
galaxies. The metallicity gradients are measured out to one effective radius by
comparing nearly all of the Lick absorption-line indices to recent models of
single stellar populations. The relation shows very different behaviour at low
and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10
M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a
tight relation with mass, such that metallicity gradients become shallower with
decreasing mass and positive at the very low-mass end. Above the mass
transition point several massive galaxies have steeper gradients, but a clear
downturn is visible marked by a broad scatter. The results are interpreted in
comparison with competing model predictions. We find that an early star-forming
collapse could have acted as the main mechanism for the formation of low-mass
galaxies, with star formation efficiency increasing with galactic mass. The
high-mass downturn could be a consequence of merging and the observed larger
scatter a natural result of different merger properties. These results suggest
that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed
initially by mergers of gas-rich disc galaxies and then subsequently evolved
via dry merger events. The varying efficiency of the dissipative merger-induced
starburst and feedback processes have shaped the radial metallicity gradients
in these high-mass systems.Comment: 5 pageg, 3 figures, accepted by ApJ Lette
Plant growth bacteria as promoters of Betula pubescens establishment in anthropogenic sediments
Optimización de uniones y estructuras de autobuses
El objetivo principal del proyecto de investigación presentado es el de obtener una mejorar en el comportamiento y la predictibilidad de las tensiones de los modelos FEM de vehÃculos de grandes dimensiones para el transporte de pasajeros (autobuses y autocares). Dadas las dimensiones de estos vehÃculos y el tipo de estructura de los mismos (tubular de sección rectangular hueca) los modelos más adecuados desde el punto de vista técnico, económico y de coste de modelización computacional son los generados con elementos de tipo viga. Dichos elementos debido a la simplicidad de su formulación presentan una serie de limitaciones en la caracterización del comportamiento de las uniones (infinitamente rÃgidas), debido a esto los modelos de autobuses realizados con este tipo de elementos presentan mayores rigideces estructurales. Teniendo en cuenta lo anteriormente mencionado las estimaciones de vida a fatiga o la estimación de las tensiones se ven afectadas. Para resolveré este problema se han realizando una serie de investigaciones mediante modelización y experimentación para diferentes modelos de uniones que se encuentran de manera frecuenta en las estructuras de autobuses urbanos. Por otra parte fue ideada una metodologÃa y se realizo un programa mediante el cual se pueden realizar de manera rápida simulaciones para una serie entera de configuraciones de los parámetros del modelo con elementos finitos: tipos de elementos utilizados, tipo de mallado, dimensión del mallado caracterÃsticas de las constantes reales. Conjuntamente fueron realizadas una serie de análisis modales para diferentes uniones en las cuales se intento aislar y evidenciar las diferencias de comportamiento vibracional influenciado exclusivamente por la configuración de la unión. Los resultados obtenidos serán utilizados para poder realizar un modelo con elementos finitos que reproduzca de manera más real el comportamiento de las uniones rÃgidas que se utilizan en los autobuses y autocares (uniones soldadas
Influence of bacterial dynamics upon the final characteristics
The microbiological profile in raw milk cheeses is typically characterized by a multitude of microbial
groups, with interactions among them throughout ripening that are not fully understood to date. Incidence
of undesired microorganisms in raw cheesemaking milk, as is the case of either spoilage or even
pathogenic ones, is a common trait in Portuguese traditional cheeses. Hence, they will likely contribute
to the physicochemical changes occurring therein and, consequently, to the characteristics of the final
product. In order to gain insight into their role, model cheese systems, manufactured as far as possible
according to artisanal practices (except that the initial microbial load and biodiversity were controlled),
were experimentally tested. Single contaminants, or a consortium thereof, were inoculated at two levels
in sterilized raw ewe's milk, and duly combined with inocula containing one or two lactic acid bacteria
normally found in those traditional cheeses. The physicochemical composition, organic acid profile, and
evolution of both protein breakdown and rheology were monitored throughout a 60 d-ripening period.
Modifications brought about within the cheese matrix as a result of microbial metabolism, especially
those arising from the interaction between lactic acid bacteria and unwanted microorganisms, included
the enhanced release of peptides and free amino acids, which in turn led to higher viscoelastic moduli.
The final model cheeses could be well discriminated, based on the impact of the various inocula
considered upon the levels of organic acids. Conversely, proteolysis and viscoelastic properties appeared
to be essentially independent of the initial microflora.info:eu-repo/semantics/publishedVersio
R125H, W240S, C386R, and V507I SLC4A11 mutations associated with corneal endothelial dystrophy affect the transporter function but not trafficking in PS120 cells
SLC4A11 mutations are associated with Fuchs’ endothelial corneal dystrophy (FECD), congenital hereditary endothelial dystrophy (CHED) and Harboyan syndrome (endothelial dystrophy with auditory deficiency). Mice with genetically ablated Slc4a11 recapitulate CHED, exhibiting significant corneal edema and altered endothelial morphology. We recently demonstrated that SLC4A11 functions as an NH3 sensitive, electrogenic H+ transporter. Here, we investigated the properties of five clinically relevant SLC4A11 mutants: R125H, W240S, C386R, V507I and N693A, relative to wild type, expressed in a PS120 fibroblast cell line. The effect of these mutations on the NH4Cl-dependent transporter activity was investigated by intracellular pH and electrophysiology measurements. Relative to plasma membrane expression of NaK ATPase, there were no significant differences in plasma membrane SLC4A11 expression among each mutant and wild type. All mutants revealed a marked decrease in acidification in response to NH4Cl when compared to wild type, indicating a decreased H+ permeability in mutants. All mutants exhibited significantly reduced H+ currents at negative holding potentials as compared to wild type. Uniquely, the C386R and W240S mutants exhibited a different inward current profile upon NH4Cl challenges, suggesting an altered transport mode. Thus, our data suggest that these SLC4A11 mutants, rather than having impaired protein trafficking, show altered H+ flux properties
Quantum size effects in Pb islands on Cu(111): Electronic-structure calculations
The appearance of "magic" heights of Pb islands grown on Cu(111) is studied
by self-consistent electronic structure calculations. The Cu(111) substrate is
modeled with a one-dimensional pseudopotential reproducing the essential
features, i.e. the band gap and the work function, of the Cu band structure in
the [111] direction. Pb islands are presented as stabilized jellium overlayers.
The experimental eigenenergies of the quantum well states confined in the Pb
overlayer are well reproduced. The total energy oscillates as a continuous
function of the overlayer thickness reflecting the electronic shell structure.
The energies for completed Pb monolayers show a modulated oscillatory pattern
reminiscent of the super-shell structure of clusters and nanowires. The energy
minima correlate remarkably well with the measured most probable heights of Pb
islands. The proper modeling of the substrate is crucial to set the
quantitative agreement.Comment: 4 pages, 4 figures. Submitte
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