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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Waves and instabilities in quantum plasmas

Ali, Shahid January 2008 (has links)
The study of waves and instabilities in quantum plasmas is of fundamental importance for understanding collective interactions in superdense astrophysical objects, in high intense laser-plasma/solid-matter interactions, in microelectronic devices and metallic nanostructures. In dense quantum plasmas, there are new pressure laws associated with the Fermi-Dirac distribution functions and new quantum forces associated with the quantum Bohm potential and the Bohr magnetization involving electron ½ spin. These forces significantly alter the collective behavior of dense quantum plasmas. This thesis contains six papers, considering several novel collective modes and instabilities at quantum scales. In Paper I, we have used the quantum hydrodynamical (QHD) model for studying the one-dimensional dust-acoustic (DA) waves incorporating the Fermi pressure law and the quantum Bohm potential. The latter modifies the DA wave dispersion relation in a collisional plasma. In Paper II, we have calculated the electrostatic potential of a test charge in an unmagnetized electron-ion quantum plasma. It is found that the Debye-Hückel and oscillatory wake potentials strongly depend upon the Fermi energy at quantum scales. The results can be of interest for explaining the charged particle attraction and repulsion in degenerate quantum plasmas, such as those in semiconductor and microelectronic devices. Paper III presents the parametric study of nonlinear electrostatic waves in two-dimensional collisionless quantum dusty plasmas. A reductive perturbation method has been employed to the QHD equations together with the Poisson equation, obtaining the cylindrical Kadomtsev-Petviashvili (CKP) equations and their stationary localized solutions. We have numerically examined the quantum mechanical and geometrical effects on the profiles of nonplanar quantum dust-ion-acoustic (DIA) and DA solitary waves. The role of static as well as mobile (negatively or positively charged) dust particles on the low-frequency electrostatic waves has also been highlighted for metallic nanostructures. Paper IV introduces the nonlinear properties of the ion-sound waves in a dense electron-ion Fermi magnetoplasma. The computational analysis of the nonlinear system reveals that the Sagdeev-like potential and the ion-sound density excitations are significantly affected by the wave direction cosine and the Mach number at quantum scales. Paper V considers the nonlinear interactions of electrostatic upper-hybrid (UH), ion-cyclotron (IC), lower-hybrid (LH), and Alfvén waves in a quantum magnetoplasma. The nonlinear dispersion relations have been analyzed analytically to obtain the growth rates for both the decay and modulational instabilities involving the dispersive IC, LH, and Alfvén waves. In Paper VI, we have identified a new drift-like dissipative instability in a collisional quantum plasma. The modified unstable drift-like mode can cause cross-field anomalous ion-diffusion at quantum scales.
2

Effets mécaniques de la lumière sur des particules anisotropes micrométriques et dynamique du mouillage à l’interface eau-air / (Mechanical effects of light on anisotropic micron-sized particles and their wetting dynamics at the water-air interface

Mihiretie, Besira 05 July 2013 (has links)
Nous présentons une série d’expériences sur des particules micrométriques de polystyrène de formes ellipsoïdales. Les rapports d’aspects (k) des particules sont variables, de 0.2 à 8 environ. Ces ellipsoïdes sont manipulés dans l’eau par faisceau laser modérément focalisé. On observe la lévitation et l’équilibre dynamique de chaque particule, dans le volume et au contact d’une interface, solide-liquide ou liquide-liquide. Dans une première partie, nous montrons que des particules de k modéré sont piégées radialement. Par contre, les ellipsoïdes allongés (k>3) ou aplatis (k<0.3) ne peuvent pas être immobilisés. Ces particules « dansent » autour du faisceau, dans un mouvement permanent associant translation et rotation. Les mouvements sont périodiques, ou irréguliers (chaotiques) selon les caractéristiques de la particule et du faisceau. Un modèle en 2d est proposé qui permet de comprendre l’origine des oscillations. La seconde partie est une application de la lévitation optique pour une étude de la transition mouillage total-mouillage partiel des particules à l’interface eau-air. Nous montrons que la dynamique de la transition ne dépend pratiquement pas de la forme de particule, et qu’elle est déterminée par le mécanisme d’accrochage-décrochage de la ligne de contact. / We report experiments on ellipsoidal micrometre-sized polystyrene particles. The particle aspect ratio (k) varies between about 0.2 and 8. These particles are manipulated in water by means of a moderately focused laser beam. We observe the levitation and the dynamical state of each particle in the laser beam, in bulk water or in contact to an interface (water-glass, water-air, water-oil). In the first part, we show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k>3) or flattened (k<0.3) ellipsoids never come to rest, and permanently “dance” around the beam, through coupled translation-rotation motions. The dynamics are periodic or irregular (akin to chaos) depending on the particle type and beam characteristics. We propose a 2d model that indeed predicts the bifurcation between static and oscillating states. In the second part, we apply optical levitation to study the transition from total to partial wetting of the particles at the water-air interface. We show that the dynamics of the transition is about independent of particle shape, and mainly governed by the pinning-depinning mechanism of the contact line.
3

Manipulation of a Large Magneto-Optical Trap:<br />application to Four-Wave Mixing

Gattobigio, Giovanni Luca 29 February 2008 (has links) (PDF)
Dans cette thèse, nous présentons une recherche expérimentale détaillée des lois caract érisant la taille et la densité atomique de grands pièges magnéto-optiques contenant typiquement 10^10 atomes. Il est bien connu que les interactions induites par laser, produites par la diffusion multiple, y constituent la limitation principale à l'obtention de densités atomiques élevées. Pour contourner cette limite, nous avons réalisé un piège magnéto-optique 'noir' combiné un piège dipolaire. Les forces de répulsion assistées par la lumière ont ainsi pu être considérablement réduites et une augmentation de près de deux ordres des grandeur sur le densité (jusqu'à 10^12 atm/cm^3) a été corrélativement observée. Nous avons proposé également un modèle qualitatif pour rendre compte de la densité atomique (et également pour la taille) o`u nous avons introduit les interactions mécaniques induites par les photons du laser repompeur quand son épaisseur optique devient importante. Ce travail fait partie d'un projet de recherche qui vise à terme à observer la localisation d'Anderson de la lumière dans des nuages d'atomes froids, mais pour laquelle des densités de l'ordre (10^13-10^14 atm/cm^3) sont requises. Pour des densités inférieures, des mécanismes de gain peuvent toutefois être mis à profit pour amplifier des modes pre-localisés. Nous avons, ainsi, mis en place une technique pompe-sonde pour la création d'un gain dans l'échantillon atomique. La configuration des pompes a autorisé l'observation du signal de mélange a quatre ondes. Une analyse détaillée de ses propriétés et de ses caractéristiques a été présentée. En particulier, une asymétrie rouge-bleu dans le spectre, pour des intensités élevées des faisceaux pompes, a été mise en évidence. Cette asymétrie a pu tre expliquée en tenant compte du regroupement des atomes dans les noeuds ou dans les ventres de l'onde stationnaire des faisceaux de pompe.
4

Modélisation, simulation et caractérisation de dispositifs TFET pour l'électronique à basse puissance / Modelling, simulation and characterization of tunnel-fet devices for ultra-low power electronics

Revelant, Alberto 15 May 2014 (has links)
Dans les dernières années, beaucoup de travail a été consacré par l’industrie électronique pour réduire la consommation d’énergie des composants micro-électroniques qui représente un fardeau important dans la spécification des nouveaux systèmes.Afin de réduire la consommation d’énergie, nombreuses stratégies peuvent être adoptées au niveau des systèmes micro-électroniques et des simples dispositifs nano-électroniques. Récemmentle Transistor Tunnel `a effet de champ (Tunnel-FET) s’est imposé comme un candidat possible pour remplacer les dispositifs MOSFET conventionnels pour applications de tr`es basse puissance à des tensions d’alimentation VDD < 0.5V. Nous présentons un modèle Multi-Subband Monte Carlo modifié (MSMC) qui a été adapté pour la simulation de TFET Ultra Thin Body Fully Depleted Seminconductor on Insulator (FDSOIUTB) avec homo- et hétéro-jonctions et des matériaux semi-conducteurs arbitraires. Nous prenons en considération la quantification de la charge avec une correction quantique heuristique mais précise, validée via des modèles quantiques complets et des résultats expérimentaux.Le modèle MSMC a été utilisé pour simuler et évaluer la performance de FD-SOI TFET sidéealisées avec homo- et hétéro-jonction en Si, alliages SiGe ou composés InGaAs. Dans la deuxième partie de l’activité de doctorat un travail de caractérisation à basse températurea été réalisé sur les TFETs en Si et SiGe homo- et hétéro-jonction fabriqués par le centre de recherche français du CEA -LETI. L’objectif est d’estimer la présence de l’effet Tunnel comme principal mécanisme d’injection et la contribution d’autres mécanismes d’injection comme le Trap Assisted Tunneling. / In the last years a significant effort has been spent by the microelectronic industry to reducethe chip power consumption of the electronic systems since the latter is becoming a majorlimitation to CMOS technology scaling.Many strategies can be adopted to reduce the power consumption. They range from thesystem to the electron device level. In the last years Tunnel Field Effect Transistors (TFET)have imposed as possible candidate devices for replacing the convential MOSFET in ultra lowpower application at supply voltages VDD < 0.5V. TFET operation is based on a Band-to-BandTunneling (BtBT) mechanism of carrier injection in the channel and they represent a disruptiverevolutionary device concept.This thesis investigates TFET modeling and simulation, a very challenging topic becauseof the difficulties in modeling BtBT accurately. We present a modified Multi Subband MonteCarlo (MSMC) that has been adapted for the simulation of Planar Ultra Thin Body (UTB)Fully Depleted Semiconductor on Insulator (FD-ScOI) homo- and hetero-junction TFET implementedwith arbitrary semiconductor materials. The model accounts for carrier quantizationwith a heuristic but accurate quantum correction validated by means of comparison with fullquantum model and experimental results.The MSMC model has been used to simulate and assess the performance of idealized homoandhetero-junction TFETs implemented in Si, SiGe alloys or InGaAs compounds.In the second part of the thesis we discuss the characterization of TFETs at low temperature.Si and SiGe homo- and hetero-junction TFETs fabricated by CEA-LETI (Grenoble,France) are considered with the objective to identify the possible presence of alternative injectionmechanisms such as Trap Assisted Tunneling. / Negli ultimi anni uno sforzo significativo `e stato speso dall’industria microelettronica per ridurreil consumo di potenza da parte dei sistemi microelettronici. Esso infatti sta diventando unadelle limitazioni pi`u significative per lo scaling geometrico della tecnologia CMOS.Diverse strategie possono essere adottate per ridurre il consumo di potenza considerando ilsistema microelettronico nella sua totalit`a e scendendo fino a giungere all’ottimizzazione delsingolo dispositivo nano-elettronico. Negli ultimi anni il transistore Tunnel FET (TFET) si`e imposto come un possibile candidato per rimpiazzare, in applicazioni a consumo di potenzaestremamente basso con tensioni di alimentazione inferiori a 0.5V, i transistori convenzionaliMOSFET. Il funzionamento del TFET si basa sul meccanismo di iniezione purament quantisticodel Tunneling da banda a banda (BtBT) e che dovrebbe permettere una significativa riduzionedella potenza dissipata. Il BtBT nei dispositivi convenzionali `e un effetto parassita, nel TFETinvece esso `e utilizzato per poter ottenere significativi miglioramenti delle performance sottosogliae pertanto esso rappresenta una nuova concezione di dispositivo molto innovativa erivoluzionaria.Questa tesi analizza la modellizazione e la simulazione del TFET. Questi sono argomenti moltocomplessi vista la difficolt`a che si hanno nel modellare accuratamente il BtBT. In questo lavoroviene presentata una versione modificata del modello di trasporto Multi Subband Monte Carlo(MSMC) adattato per la simulazione di dispositivi TFET planari Ultra Thin Body Fully DepletedSilicon on Insulator (UTB FD-SOI), implementati con un canale composto da un unicosemiconduttore (omogiunzione) o con differenti materiali semiconduttori (eterogiunzione). Ilmodello proposto tiene il conto l’effetto di quantizzazione dovuto al confinamento dei portatoridi carica, con un’euristico ma accurato sistema di correzione. Tale modello `e stato poivalidato tramite una comparazione con altri modelli completamente quantistici e con risultatisperimentali.Superata la fase di validazione il modello MSMC `e utilizzato per simulare e verificare le performancedi dispositivi TFET implementati come omo o eterogiunzione in Silicio, leghe SiGe,o composti semiconduttori InGaAs.Nella seconda parte della tesi viene illustrato un lavoro di caratterizazione di TFET planari abassa temperatura (fino a 77K). Sono stati misurati dispositivi in Si e SiGe a omo o eterogiuzioneprodotti nella camera bianca del centro di ricerca francese CEA-LETI di Grenoble. Tramite talimisure `e stato possibile identificare la probabile presenza di meccanismi di iniezione alternativial BtBT come il Tunneling assistito da trappole (TAT) dimostrando come questo effetto `e,con ogni probabilit`a, la causa delle scarse performance in sottosoglia dei dispositivi TFETsperimentali a temperatura ambiente.
5

Structural distortions in molecular-based quantum cellular automata: a minimal model based study

Santana Bonilla, Alejandro, Gutierrez, Rafael, Medrano Sandonas, Leonardo, Nozaki, Daijiro, Bramanti, Alessandro Paolo, Cuniberti, Gianaurelio 10 January 2020 (has links)
Molecular-based quantum cellular automata (m-QCA), as an extension of quantum-dot QCAs, offer a novel alternative in which binary information can be encoded in the molecular charge configuration of a cell and propagated via nearest-neighbor Coulombic cell–cell interactions. Appropriate functionality of m-QCAs involves a complex relationship between quantum mechanical effects, such as electron transfer processes within the molecular building blocks, and electrostatic interactions between cells. The influence of structural distortions of single m-QCA are addressed in this paper within a minimal model using an diabatic-to-adiabatic transformation. We show that even small changes of the classical square geometry between driver and target cells, such as those induced by distance variations or shape distortions, can make cells respond to interactions in a far less symmetric fashion, modifying and potentially impairing the expected computational behavior of the m-QCA.
6

Analýza mechanického namáhání při zkratu ve vzduchem izolovaném rozváděči vn / Analysis of Mechanical Stress during Short Circuit in an Air-Insulated MV Switchgear

Bártů, Jan January 2017 (has links)
This Master thesis is based on problems of force effects of the magnetic field of short-circuit current. Firstly, I analysed the force action of the three phase busbars during the three-pole short circuit. The analysis was performed in the Ansys Maxwell program and I verified the correctness of the analysis by numerical calculation. The second practical part was executed for ABB company. The main task was to analyse the force effects of the short-circuit current in the HV switchgear with focusing on the arms and contact system of the circuit breaker. The simulations of the following configurations were performed in more details: three configurations of the simplified VD4 circuit breaker, two types of VD4 circuit-breaker contact systems and a linear circuit breaker configuration model. The objective was to calculate the forces acting on the current path of the arms and contact system of the VD4 circuit breaker during the three-phase short-circuit and to simulate the effects of forces on these current parts. For modelling of individual assemblies, calculations and simulations I used: SolidWorks 2014, Ansys Maxwell 16.0 and Ansys Workbench 14.5 programs. With SolidWorks 2014 3D CAD software were modelled simplified assemblies of circuit breakers and contact systems. These systems were imported into the Ansys Maxwell software, where the force effects of the magnetic field of the short-circuit current were calculated. In Ansys Workbench programme was simulated the effect of forces on the current circuit breakers. Moreover, the final deformations of the materials (with respect to the mechanical properties of the structure) were plotted. The analysis of force effects of the magnetic field was performed for the specified static short - circuit current value.

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