756 research outputs found
Microscopic Model for Sequential Tunneling in Semiconductor Multiple Quantum Wells
We propose a selfconsistent microscopic model of vertical sequential
tunneling through a multi-quantum well.The model includes a detailed
description of the contacts,uses the Transfer Hamiltonian for expressions of
the current and it treats the Coulomb interaction within a mean field
approximation. We analyze the current density through a double well and a
superlattice and study the formation of electric field domains and
multistability coming from the Coulomb interaction. Phase diagrams of parameter
regions (bias, doping in the heterostructure and in the contacts,etc) where the
different solutions exist are given.Comment: 4 pages, 8 Postscript Figure
Neutrino telescopes under the ocean: The case for ANTARES
Neutrino telescopes offer an alternative way to explore the Universe. Several
projects are in operation or under construction. A detector under the ocean is
very promising because of the very accurate angular resolution that it
provides. The ANTARES project is intended to demonstrate the feasibilty of such
a detector.Comment: Talk given at the Neutrino98 conference, Takayama, Japan, June 4-9,
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Current self-oscillations, spikes and crossover between charge monopole and dipole waves in semiconductor superlattices
Self-sustained current oscillations in weakly-coupled superlattices are
studied by means of a self-consistent microscopic model of sequential tunneling
including boundary conditions naturally. Well-to-well hopping and recycling of
charge monopole domain walls produce current spikes (high frequency modulation)
superimposed on the oscillation. For highly doped injecting contacts, the
self-oscillations are due to dynamics of monopoles. As the contact doping
decreases, a lower-frequency oscillatory mode due to recycling and motion of
charge dipoles is predicted. For low contact doping, this mode dominates and
monopole oscillations disappear. At intermediate doping, both oscillation modes
coexist as stable solutions and hysteresis between them is possible.Comment: 4 pages, 4 figure
Possibilité d'observation, par le télescope Antares, de neutrinos de haute énergie associés aux sursauts gamma et validation des techniques de détection à l'aide d'un prototype
PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF
Bifurcation behavior of a superlattice model
We present a complete description of the stationary and dynamical behavior of
semiconductor superlattices in the framework of a discrete drift model by means of numerical continuation,
singular perturbation analysis, and bifurcation techniques. The control parameters are the
applied DC voltage (φ) and the doping (ν) in nondimensional units. We show that the organizing
centers for the long time dynamics are Takens–Bogdanov bifurcation points in a broad range of parameters
and we cast our results in a φ-ν phase diagram. For small values of the doping, the system
has only one uniform solution where all the variables are almost equal. For high doping we find
multistability corresponding to domain solutions and the stationary solutions may exhibit chaotic
spatial behavior. In the intermediate regime of ν the solution can be time-periodic depending on the
bias. The oscillatory regions are related to the appearance and disappearance of Hopf bifurcation
tongues which can be sub- or supercritical. These results are in good agreement with most of the
experimental observations and also predict new interesting dynamical behavior.Junta de Andalucía PB97-008
Sociedad civil y política exterior: ¿hacia un nuevo apradigma? : Algunas consideraciones en torno a la cuestión "Papeleras"
En el transcurso de los últimas años—gracias a fenómenos tales como la globalización, la erosión del vínculo representativo y la democracia centrada en los medios— la sociedad civil se ha visto revitalizada como un actor clave en la construcción del espacio público y en la orientación de las políticas públicas en general.
El presente trabajo pretende analizar hasta que punto esta tendencia puede ser observada en el ámbito de las relaciones internacionales, más específicamente dentro de la formulación de la política exterior; y, a su vez, considerar si esto puede significar la consolidación de un nuevo paradigma dentro de la disciplina.
Para tal fin se tomará como estudio de caso, desde la perspectiva de la participación de los actores no estatales —léase: grupos de interés, ONGs, asambleas vecinales, medios de comunicación—, el conflicto argentino-uruguayo por la instalación en este último país de dos plantas de procesamiento de celulosa; para, finalmente, ver su incidencia dentro de la política exterior argentina. Principalmente, se analizará el rol y la relevancia de la Asamblea Ciudadana Ambientalista de Gualeguaychú en la formación de dicha política.Mesa Políticas exteriores latinoamericanas: temas, actores y procesosInstituto de Relaciones Internacionale
The influence of anthropogenic activities on the concentration of pesticides, physicochemical and microbiological properties in the Chumbao river, Andahuaylas, Perú
The province of Andahuaylas is located in the region of Apurimac, a Peruvian city located in the Andes. Andahuaylas has
three important districts: San Jerónimo, Andahuaylas and Talavera. These areas are highly populated, and their buildings are close to
the Chumbao River. This research aimed to determine the influence of anthropogenic activities on the concentration of pesticides,
physicochemical and microbiological characteristics in the Chumbao River. Five agricultural activities, 19 organochlorine pesticides,
and 25 organophosphorus pesticides were studied. Eighteen physicochemical properties and two microbiological properties were also
studied at seven sampling points along the Chumbao River during the rainy and dry seasons. Several veterinary and agricultural
products were identified in the agricultural activities studied. However, no significant concentration values of organochlorine and
organophosphate pesticides were found; no weed control products were found. In the case of livestock farming, albendazole and
ivermectin are frequently used as anthelmintics. Organophosphate and pyrethroid insecticides are used to control flies and other
ectoparasites. Several pesticides have been identified in the surveys in the case of quinoa, potato, and corn crops. Some of the
physicochemical and microbiological properties are above environmental quality standards according to current Peruvian regulations;
these properties increase at points where wastewater is discharged directly into the river. The study evidenced a progressive
deterioration of water quality in the Chumbao River caused by anthropic activities in the basin. These may cause infectious and parasitic
diseases in the urban population of the Chumbao river valley
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