143 research outputs found
Vacancies in SiGe: Jahn–Teller distortion and spin effects
The electronic structure of a vacancy in silicon-germanium is studied using ab initio total-energy minimization methods. The calculations are based on density-functional theory in the local-spin-density approximation. We report ionic relaxations, defect formation energies and ionization levels of Si and Gevacancies in a zinc blende model structure (SiGe). The Gevacancy in SiGe is characterized by symmetry-lowering Jahn–Teller (JT) distortions and a negative-effective-U effect, in those respects resembling the vacancy in elemental silicon. For Si vacancy, the exchange-coupling energy is found to overcome the JT energy, and symmetric high-spin ground states are predicted.Peer reviewe
Seudomicetoma dermatofÃtico en un gato persa : aspectos clÃnicos, patológicos y evolutivos
En este trabajo presentamos los aspectos clÃnicos, anatomopatológicos y evolutivos de un caso de dermatitis piogranulomatosa múltiple causada por hongos del género Microsporum, también conocida como seudomicetoma dermatofÃtico, en un gato persa, no castrado, de un año de edad al comienzo de la enfermedad. Las lesiones se caracterizaron macroscópicamente por la formación de numerosos nódulos de diferentes tamaños, localizados en la cabeza y antebrazo derecho, extendiéndose posteriormente a zona dorso lateral del tronco. La evolución de estos nódulos, que se generalizaron en algo menos de un mes, era hacia la coalescencia y fistulización, con descarga de un material grumoso blanco-grisáceo. El estudio histopatológico reveló una dermatitis piogranulomatosa, supeficial y profunda, caracterizada por presentar en el centro de los granulomas agregados de hongos septados inmersos en una matriz acidófila. La extirpación quirúrgica de un gran número de nódulos proporcionó mejorÃas transitorias, aunque reaparecieron nuevas lesiones. En este trabajo se describe la evolución tras dos años de seguimiento clÃnico. Tras el tratamiento con griseofulvina no se detectan nuevas lesiones después de 6 meses de su aplicación y seguimiento.This paper reports the clinical and histopathological features of a case of pyogranulomatous dermatitis caused by Microsporum fungi, a condition also known as dermatophyte pseudomycetoma, in a eneyear-old Persian cat. Grossly apparent fistulized nodules of varying sizes displaying a granular discharge were localized on the head, back, elbow and tail. Microscopic analysis revealed a deep piogranulomatous dermatitis characterized by centrally-located clusters of fungi within an acidophilic matrix. In just under a month, nodules had beco me generalized. Partial surgical excision and non-specific treatment provided temporary improvement, but new lesions subsequentlyappeared
Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast. II. Numerical study
We implement a phase-field simulation of the dynamics of two fluids with
arbitrary viscosity contrast in a rectangular Hele-Shaw cell. We demonstrate
the use of this technique in different situations including the linear regime,
the stationary Saffman-Taylor fingers and the multifinger competition dynamics,
for different viscosity contrasts. The method is quantitatively tested against
analytical predictions and other numerical results. A detailed analysis of
convergence to the sharp interface limit is performed for the linear dispersion
results. We show that the method may be a useful alternative to more
traditional methods.Comment: 13 pages in revtex, 5 PostScript figures. changes: 1 reference added,
figs. 4 and 5 rearrange
Visual 3-D SLAM from UAVs
The aim of the paper is to present, test and discuss the implementation of Visual SLAM techniques to images taken from Unmanned Aerial Vehicles (UAVs) outdoors, in partially structured environments. Every issue of the whole process is discussed in order to obtain more accurate localization and mapping from UAVs flights. Firstly, the issues related to the visual features of objects in the scene, their distance to the UAV, and the related image acquisition system and their calibration are evaluated for improving the whole process. Other important, considered issues are related to the image processing techniques, such as interest point detection, the matching procedure and the scaling factor. The whole system has been tested using the COLIBRI mini UAV in partially structured environments. The results that have been obtained for localization, tested against the GPS information of the flights, show that Visual SLAM delivers reliable localization and mapping that makes it suitable for some outdoors applications when flying UAVs
ab initio modeling of open systems: charge transfer, electron conduction, and molecular switching of a C_{60} device
We present an {\it ab initio} analysis of electron conduction through a
molecular device. Charge transfer from the device electrodes to the
molecular region is found to play a crucial role in aligning the lowest
unoccupied molecular orbital (LUMO) of the to the Fermi level of the
electrodes. This alignment induces a substantial device conductance of . A gate potential can inhibit charge transfer and
introduce a conductance gap near , changing the current-voltage
characteristics from metallic to semi-conducting, thereby producing a field
effect molecular current switch
Cluster-based density-functional approach to quantum transport through molecular and atomic contacts
We present a cluster-based density-functional approach to model charge
transport through molecular and atomic contacts. The electronic structure of
the contacts is determined in the framework of density functional theory, and
the parameters needed to describe transport are extracted from finite clusters.
A similar procedure, restricted to nearest-neighbor interactions in the
electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)].
Here, we show how to systematically improve the description of the electrodes
by extracting bulk parameters from sufficiently large metal clusters. In this
way we avoid problems arising from the use of nonorthogonal basis functions.
For demonstration we apply our method to electron transport through Au contacts
with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure
Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast. I. Theoretical approach
We present a phase-field model for the dynamics of the interface between two
inmiscible fluids with arbitrary viscosity contrast in a rectangular Hele-Shaw
cell. With asymptotic matching techniques we check the model to yield the right
Hele-Shaw equations in the sharp-interface limit and compute the corrections to
these equations to first order in the interface thickness. We also compute the
effect of such corrections on the linear dispersion relation of the planar
interface. We discuss in detail the conditions on the interface thickness to
control the accuracy and convergence of the phase-field model to the limiting
Hele-Shaw dynamics. In particular, the convergence appears to be slower for
high viscosity contrasts.Comment: 17 pages in revtex. changes: 1 reference adde
Resonant Andreev reflections in superconductor-carbon-nanotube devices
Resonant Andreev reflection through superconductor-carbon-nanotube devices
was investigated theoretically with a focus on the superconducting proximity
effect. Consistent with a recent experiment, we find that for high transparency
devices on-resonance, the Andreev current is characterized by a large value and
a resistance dip; low-transparency off-resonance devices give the opposite
result. We also give evidence that the observed low-temperature transport
anomaly may be a natural result of Andreev reflection process
From Hierarchies to Levels: New Solutions for Games with Hierarchical Structure
Recently, applications of cooperative game theory to economic allocation problems have gained popularity. In many of these problems, players are organized according to either a hierarchical structure or a levels structure that restrict players ’ possibilities to cooperate. In this paper, we propose three new solutions for games with hierarchical structure and characterize them by properties that relate a player’s payoff to the payoffs of other players located in specific positions in the structure relative to that player. To define each of these solutions, we consider a certain mapping that transforms any hierarchical structure into a levels structure, and then we apply the standard generalization of the Shapley Value to the class of games with levels structure. The transformations that map the set of hierarchical structures to the set of levels structures are also studied from an axiomatic viewpoint by means of properties that relate a player’s position in both types of structure
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