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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.
161

Intégration et mesures de magnéto-transport de nano-objets magnétiques obtenus par voie chimique / Integration and magneto-transport measurements of magnetic nano-objects obtained by chemical way

Dugay, Julien 13 December 2012 (has links)
L'étude du transport électronique dans des nano-objets métalliques et magnétiques issus de la chimie est un challenge en spintronique. En particulier, le manque de résultats expérimentaux révèle la difficulté à positionner ces nano-objets entre des électrodes de mesures tout en préservant leurs propriétés (magnétisme, intégrité des barrières tunnel organiques...). Ce travail de thèse vise à contourner ces difficultés et à étudier le magnétotransport dans ces systèmes. Pour cela, nous avons conçu et développé à l'intérieur d'une boîte à gants couplée à un bâti de pulvérisation cathodique des systèmes expérimentaux d'assemblages de nano-objets. Nous avons étudié les mécanismes mis en jeu lors de l'assemblage par la technique de dip coating, et réussi à déposer des monocouches de nanoparticules (NPs) de natures différentes (FeCo, Fe, Co) sur des surfaces d'Au, de SiO2 et de résine fine (40 nm). Ces résultats, couplés à une technique de nanoindentation, ont permis de mesurer quelques - voire une- NP(s). Une autre technique, la diélectrophorèse, s'est révélée simple et efficace pour piéger et orienter des nano-objets de taille, de nature, et de forme différentes entre des électrodes. Grâce à cette technique et au dépôt d'une couche protectrice d'alumine, nous avons étudié les propriétés de magnétotransport de plusieurs types de nano-objets sensibles à l'oxydation ou à la vapeur d'eau: NPs de Fe, de Co, FeCo et [Fe(H-trz)2(trz)](BF4)] (composés à transition de spin). Trois jeux de barrières tunnel organiques greffés sur des NPs de fer ont présenté de la magnétorésistance tunnel jusqu'à température ambiante. De plus, des nano-objets de [Fe(H-trz)2(trz)](BF4)] de facteurs de forme variable, ont montré une variation de la conductance liée à la transition de spin. Enfin, nous avons étudié l'influence de la longueur des ligands sur les propriétés de conductions de NPs de Cobalt, qui a validé nos méthodes d'échange de ligands et ont pu être analysées quantitativement. Nos travaux rendent désormais envisageable l'utilisation de NPs issues de la chimie dans différents domaines de la spintronique / The study of charge transport in metallic and magnetic nano-objects chemically synthesized is a challenge in spintronic. Particularly, the lack of experimental results reveals the difficulty in locating such nano-objects in between electrodes while preserving their good properties. This thesis aims to overcome these difficulties in order to study the magnetotransport in such systems. Therefore, we have designed and developed technical processes which induce the self-assembly of the nano-objects inside a glove box-sputtering system. After studying the mechanisms involved in the self-assembly obtained by dip coating, we succeeded to deposit monolayers of nanoparticles (NPs) of different materials (FeCo, Fe, Co) on gold surfaces, SiO2 and thin resin film (40 nm). These results, coupled with a nanoindentation technique allows us to measure a few or a unique NP(s). Another technique, called dielectrophoresis, has been proved to be a simple and versatile way to trap (and align) nano-objects with different (aspect ratio), size, nature, and shape in between the electrodes. Thanks to this technique and the deposit of a protective capping layer of alumina, we studied the magnetotransport properties of a large number of nano-objects sensitive to oxidation or humidity: Fe, Co, FeCo and [Fe(H-trz)2(trz)](BF4)] (spin crossover compounds). Three sets of organic tunnel barriers surrounding different Fe NPs presented tunnel magnetoresistance up to room temperature. Moreover, [Fe(H-trz)2(trz)](BF4)] nano-objects with different aspect ratio, highlighted a change in conductance connected to the spin transition. Finally, we validated our ligands exchange methods by studying the influence of the ligands length on the conduction properties of Co NPs, which have been analyzed quantitatively. Our works demonstrate the possibility to use the chemical NPs in different fields of spintronics
162

The inelastic Cooper pair tunneling amplifier (ICTA) / Un amplificateur basé sur le tunneling inélastique de paires de Cooper

Jebari, Salha 26 June 2017 (has links)
Les amplificateurs paramétriques Josephson (JPA) se sont révélés être un outil indispensablepour l’étude expérimentale de dispositifs quantiques dans le régime micro-onde ; car ilsrajoutent uniquement le minimum de bruit imposé par la mécanique quantique[1]. Cependant,ces amplificateurs sont beaucoup plus difficiles à utiliser et optimiser que leurs homologuesclassiques. Récemment, plusieurs expériences réalisées avec des circuits supraconducteurs,composés d’une jonction Josephson polarisée en tension en série avec un résonateur microonde,ont montré qu’une paire de Cooper peut traverser la barrière de la jonction par effettunnel en émettant un ou plusieurs photons avec une énergie totale de 2e fois la tensionappliquée. Dans cette thèse, nous montrerons qu’un tel circuit permet de mettre en place unamplificateur micro-onde préservant la phase que nous appelons « Amplificateur basé sur letunneling inélastique de paires de Cooper » (ICTA). Il est alimenté par une tension continueet peut fonctionner avec un bruit très proche de la limite quantique.Nous commencerons en présentant le principe du fonctionnement de l’ICTA. Par analogieavec la théorie quantique des JPAs[2], nous avons étudié les performances de cet amplificateurcomme le gain, la bande passante et le bruit. Ensuite, nous présenterons la premièrepreuve expérimentale d’une amplification proche de la limite quantique sans utilisation d’unepompe micro-onde externe, mais simplement d’une tension continue dans une configurationextrêmement simple. Ces mesures ont été faites sur des échantillons avec des jonctionsen aluminium, dénommés ICTA de première génération. Selon nos résultats théoriques etexpérimentaux, nous avons conçu des circuits hyperfréquences où l’impédance présentéeà la jonction dépend de fréquences spécifiques afin d’optimiser les performances de notreamplificateur. Ces échantillons, dénommés ICTA de seconde génération, ont été fabriquésavec du nitrure de niobium. Une amélioration significative du gain et du bruit a été prouvée.Un tel amplificateur, alimenté par une simple tension continue, pourrait rendre la mesurede signaux micro-ondes au niveau du photon unique beaucoup plus faciles et permettred’intégrer plusieurs amplificateurs sur une seule puce. Il pourrait donc être un élémentimportant pour la lecture de qubit dans les processeurs quantiques à grande échelle. / Josephson parametric amplifiers (JPA), have proven to be an indispensable tool for awide range of experiments on quantum devices in the microwave frequency regime, becausethey provide the lowest possible noise. However, JPAs remain much more difficult to use andoptimize than conventional microwave amplifiers. Recent experiments with superconductingcircuits consisting of a DC voltage-biased Josephson junction in series with a resonator,showed that a tunneling Cooper pair can emit one or several photons with a total energyof 2e times the applied voltage. In this thesis we show that such q circuit can be used toimplement a new type of phase preserving microwave amplifier, which we call InelasticCooper pair Tunneling Amplifier (ICTA). It is powered by a simple DC bias and offers nearquantum-limited noise performance.We start this work by presenting a brief and simple picture of the basic ICTA operatingprinciples. In analogy with the quantum theory of JPAs, we calculate the performances ofthis amplifier such as the gain, bandwidth and noise. Then, we present the first experimentalproof that amplification close to the quantum limit is possible without microwave drive inan extremely simple setup. These measurements are made on a first generation of samplesbased on aluminium junctions. According to our theoretical and experimental results, wehave designed microwave circuits presenting specific frequency-dependent impedances tothe junction in order to optimize the performances of our amplifier. This second generationof ICTA samples is fabricated from niobium nitride and provide significantly lower noiseand higher gain.We expect that once fully optimized, such an amplifier, powered by simple DC voltagescould then make measuring microwave signals at the single photon level much easier andallow to deploy many amplifiers on a chip. It could therefore be an important ingredient forqubit readout in large-scale quantum processors.
163

Retroanálise de uma escavação de vala escorada a céu aberto de uma linha do metrô de São Paulo / Back analyses of on open trench excavation for the São Paulo subway

Frigerio, Giulio Peterlevitz 23 March 2004 (has links)
Esta dissertação apresenta em primeira etapa o trabalho desenvolvido para averiguar a adequação dos modelos reológicos de Mohr-Coulomb e de Endurecimento de solo, para representar as deformações e distorções que ocorrem no sistema soloestrutura, quando do processo de escavação de valas escoradas. Além disto, em uma segunda etapa fazem-se estimativas de previsão de danos causados em edificações, em decorrência das escavações de uma vala escorada da linha 1 do Metropolitano de São Paulo (Metrô S.P.). A primeira e a segunda etapa foram feitas através de 810 simulações numéricas, em elementos finitos utilizando-se o software PLAXIS, associadas a retroanálise por processo direto do módulo de deformabilidade a 50% da tensão de ruptura dos solos utilizando-se para isto o processo direto. Apresenta-se também uma síntese da formação e dos tipos de solos que constituem a bacia sedimentar de São Paulo, onde se localiza a seção experimental nº1 objeto de estudo desta dissertação. Faz-se uma breve revisão bibliográfica a respeito das técnicas de retroanálise. Apresentam-se critérios para escolha de intervalos de parâmetros geotécnicos que representem o sistema solo-estrutura no processo de escavação. Foram feitas análises paramétricas para determinar os parâmetros geotécnicos que mais influenciam o sistema solo-estrutura. Comparam-se os modelos constitutivos de Mohr-Coulomb e de endurecimento na previsão das deformações. Por fim, faz-se a previsão do nível de danos causados pelas escavações da vala, a um edifício hipotético / This dissertation presents, in a first part, the work done to verify how appropriate are the Mohr-Coulomb and hardening soil constitutive models to represent the strains and the distortions associated with escavations of braced wall process. In the second part, estimates are made of the damages in constructions due to the braced excavations of line one of Sao Paulo Subway (Metrô S.P.). In the two phases, 810 numeric simulations were made, in finite elements using the software PLAXIS, associated the back analysis for direct process. It is presented a synthesis of the formation and the types of soils that constitute the basin of the sediments of the city of São Paulo, where is located the section experimental nº1, object of study of this dissertation. It is presented an short bibliographical revision regarding the back analysis techniques. Criteria for choice of intervals of parameters geotechnical that represent the system soil-structure in the excavation process are presented. Parametric analyses are made to determine which the parameters have larger influence in the behavior of the system soil-structure. The behavior of the soil-structure system is simulated using the Mohr-Coulomb and hardening soil constitutive models. The Mohr-Coulomb and hardening soil constitutive models are compared in the forecast of the deformations. Finally, it is made the forecast of the level of damages to a hypothetical building caused by the braced excavations
164

Spin-orbit interaction in quantum dots and quantum wires of correlated electrons - A way to spintronics? / Spin-Bahn-Wechselwirkung in Quantenpunkten und Quantendrähten korrelierter Elektronen - Ein Weg Richtung Spintronik?

Birkholz, Jens Eiko 06 October 2008 (has links)
No description available.
165

Atomic and molecular clusters in intense laser pulses

Mikaberidze, Alexey 07 October 2011 (has links) (PDF)
We have investigated processes of ionization, energy absorption and subsequent explosion of atomic and molecular clusters under intense laser illumination using numerical as well as analytical methods. In particular, we focused on the response of composite clusters, those consisting of different atomic elements, to intense light pulses. Another major theme is the effect of the molecular structure of clusters on their Coulomb explosion. The action of intense laser pulses on clusters leads to fundamental, irreversible changes: they turn almost instantaneously into nanoplasmas and subsequently disintegrate into separate ions and electrons. Due to this radical transformation, remarkable new features arise. Transient cluster nanoplasmas are capable of absorbing enormous amounts of laser energy. In some cases more than 90 % of incident laser energy is absorbed by a gas of clusters with a density much smaller than that of a solid. After the efficient absorption, the energy is transformed into production of energetic ions, electrons, photons, and even neutrons. Composite clusters show especially interesting behavior when they interact with intense laser pulses. Nanoplasmas formed in composite clusters may absorb even more laser energy, than those formed in homogeneous clusters, as we demonstrate in this work. One of the most important results of this thesis is the identification of a novel type of plasma resonance. This resonance is enabled by an unusual ellipsoidal shape of the nanoplasma created during the ionization process in a helium droplet doped with just a few xenon atoms. In contrast to the conventional plasma resonance, which requires significant ion motion, here, the resonant energy absorption occurs at a remarkably fast rate, within a few laser cycles. Therefore, this resonance is not only the most efficient (like the conventional resonance), but also, perhaps, the fastest way to transfer laser energy to clusters. Recently, dedicated experimental studies of this effect were performed at the Max Planck Institute in Heidelberg. Their preliminary results confirm our prediction of a strong, avalanche-like ionization of the helium droplet with a small xenon cluster inside. A conventional plasma resonance, which relies on the cluster explosion, also exhibits interesting new properties when it occurs in a composite xenon-helium cluster with a core-shell geometry. We have revealed an intriguing double plasma resonance in this system. This was the first theoretical study of the influence of the helium embedding on the laser- driven nanoplasma dynamics. Our results demonstrate the important role of the interaction between xenon and helium parts of the cluster. Understanding this interaction is necessary in order to correctly interpret the experimental results. We have elucidated several important properties of Coulomb explosion in atomic and molecular clusters. Specifically, it was found that the kinetic energy distribution of ions after the Coulomb explosion of an atomic cluster is quite similar to the initial potential energy distribution of ions and is only weakly influenced by ion overtake effects, as was believed before. For the case of molecular hydrogen clusters, we have shown that the alignment of molecules inside the cluster affects its Coulomb explosion. Investigation of the dynamical processes in composite and molecular clusters induced by intense laser pulses is a step towards understanding them in more complex nano-objects, such as biomolecules or viruses. This is of great interest in the context of x-ray diffractive imaging of biomolecules with atomic resolution, which is one of the main goals of new x-ray free electron laser facilities.
166

[en] ELECTRONIC TRANSPORT AND THERMOELECTRIC PROPERTIES OF STRONGLY CORRELATED NANOSCOPIC SYSTEMS / [pt] TRANSPORTE ELETRÔNICO E PROPRIEDADES TERMOELÉTRICAS DE SISTEMAS NANOSCÓPICOS FORTEMENTE CORRELACIONADOS

GUILLERMO ANTONIO MAXIMILIANO GOMEZ SILVA 10 January 2019 (has links)
[pt] Nesta tese foram estudados três sistemas nanoscópicos compostos de pontos quânticos (PQs). No primeiro deles foi analisada a denominada nuvem Kondo, ou a extensão da blindagem que os spins da banda de condução fazem do spin de uma impureza magnética embebida em uma matriz metálica e representada, no nosso caso, por um PQ. As propriedades da nuvem Kondo foram obtidas através da manifestação da ressonância Kondo na densidade de estados local nos sítios da matriz metálica e também através das correlações de spin entre o spin do elétron no PQ e os spins da banda de condução. Foi possível encontrar uma concordância entre as extensões da nuvem Kondo obtidas com ambos métodos. O segundo sistema estudado consiste em uma estrutura de três PQs alinhados e com o PQ central acoplado a dois contatos metálicos. Foi analisada a operação deste sistema como uma porta lógica quântica cujo funcionamento depende do estado de carga do PQ central. Foi feito um estudo dependente do tempo das propriedades do sistema e, em particular, da correlação dos spins dos PQs laterais. Mostramos que o efeito Kondo, refletido na condutância do sistema, pode ser uma ferramenta fundamental para conhecer o estado da porta quântica. Os primeiros dois sistemas foram tratados usando o método dos Bósons Escravos na aproximação de campo médio. Finalmente, foi estudado o transporte termoelétrico em um sistema de dois PQs quando um deles está acoplado a contatos metálicos unidimensionais. O sistema foi analisado no regime de resposta linear e não linear a um potencial externo no regime de bloqueio de Coulomb. Mostramos que a presença de ressonâncias Fano e de uma singularidade de Van-Hove na densidade de estados dos contatos unidimensionais perto do nível de Fermi são ingredientes fundamentais para o aumento da eficiência termoelétrica do dispositivo. O problema de muitos corpos foi resolvido na aproximação de Hubbard III que permite um estudo correto das propriedades de transporte deste sistema para T maior que TK, onde TK é a temperatura Kondo. / [en] In this thesis, were studied three nanoscopic quantum dot (QD) systems. First, the so-called Kondo cloud was analyzed, the extension of the conduction band spin screening of a magnetic impurity embedded in a metallic matrix and represented, in our case, by a QD. The Kondo cloud properties were obtained studying the way in which the local density of states of the metallic matrix sites reflects the Kondo resonance and also through the spin-spin correlations between the QD and the conduction band spins. It was possible to find a good agreement between the Kondo cloud extensions obtained using both methods. The second system consists of three aligned QDs with the central QD connected to two metallic leads. The operation of this system as a quantum gate was studied, which depends on the central QD charge. A time dependent study of the system properties and, in particular, of the lateral QDs spin correlation was developed. We showed that the Kondo effect, reflected in the conductance, could be a fundamental tool to measure the information contained in the quantum gate state. The first two systems were treated using the Slave Bosons Mean Field Approximation method. Finally, we studied the thermoelectric transport of a two QD system when one of them is connected to two onedimensional leads. The system was analyzed in the linear and nonlinear response to an external applied potential, always in the Coulomb blockade regime. It was found that the presence of Fano resonances and a Van-Hove singularity in the one-dimensional lead density of states near the Fermi level are fundamental ingredients to enhance thermoelectric efficiency. The many-body problem was treated in the Hubbard III approximation, which is a correct approach to study the transport properties for T greater than TK, where TK is the Kondo temperature.
167

Charge transport in the assemblies of magnetic, non-magnetic and spin-cross over nano-structures / Transport de charge dans les assemblages de nanostructures magnétiques, non magnétiques et à spin cross-over complexes

Usmani, Suhail 05 April 2018 (has links)
La compréhension des propriétés de transport de charge des nanostructures métalliques et magnétiques est très importante pour le développement et la miniaturisation des dispositifs fonctionnels modernes. En particulier, les nanostructures synthétisées chimiquement sont intéressant car elles permettent de mieux contrôler leur forme et leur taille, ce qui peut être utilisé pour ajuster leurs propriétés de transport de charge. L'objectif de cette thèse est d'étudier les aspects différents des propriétés de transport de charge qui résultent de la petite taille et de la nature magnétique de différents types de nanostructures comprenant des nanoparticules de Pt (1,3-3 nm), des particules magnétiques FeCo (⁓10 nm), et complexe de coordination à base de triazole Fe (II). Pour préciser davantage, des phénomènes tels que le blocage de Coulomb, la magnétorésistance tunnel et la transition de spin seront mis en évidence. En fonction de la propriété souhaitée, ces nanostructures peuvent être exploitées pour leurs applications dans divers capteurs, actionneurs et dispositifs spintroniques, etc. / Understanding charge transport properties of metallic and magnetic nano-structures is highly important for the development and miniaturization of modern functional devices. In particular, chemically synthesized nano-structures are in focus as they provide better control over their shape and size, which can be used to tune their charge transport properties. The aim of this thesis is to study the various aspects of charge transport properties which emerge due to the small size and magnetic nature of different types of nanostructures which include Pt nanoparticles (1.3-3 nm), FeCo magnetic particles (⁓10 nm), and Fe (II) triazole based coordination complex. To further specify, phenomenon such as Coulomb blockade, tunnel magnetoresistance and spin-transition will be in focus. Depending on the desirable property, these nanostructures can be exploited for their applications in a variety of sensors, actuators and spintronic devices etc.
168

Retroanálise de uma escavação de vala escorada a céu aberto de uma linha do metrô de São Paulo / Back analyses of on open trench excavation for the São Paulo subway

Giulio Peterlevitz Frigerio 23 March 2004 (has links)
Esta dissertação apresenta em primeira etapa o trabalho desenvolvido para averiguar a adequação dos modelos reológicos de Mohr-Coulomb e de Endurecimento de solo, para representar as deformações e distorções que ocorrem no sistema soloestrutura, quando do processo de escavação de valas escoradas. Além disto, em uma segunda etapa fazem-se estimativas de previsão de danos causados em edificações, em decorrência das escavações de uma vala escorada da linha 1 do Metropolitano de São Paulo (Metrô S.P.). A primeira e a segunda etapa foram feitas através de 810 simulações numéricas, em elementos finitos utilizando-se o software PLAXIS, associadas a retroanálise por processo direto do módulo de deformabilidade a 50% da tensão de ruptura dos solos utilizando-se para isto o processo direto. Apresenta-se também uma síntese da formação e dos tipos de solos que constituem a bacia sedimentar de São Paulo, onde se localiza a seção experimental nº1 objeto de estudo desta dissertação. Faz-se uma breve revisão bibliográfica a respeito das técnicas de retroanálise. Apresentam-se critérios para escolha de intervalos de parâmetros geotécnicos que representem o sistema solo-estrutura no processo de escavação. Foram feitas análises paramétricas para determinar os parâmetros geotécnicos que mais influenciam o sistema solo-estrutura. Comparam-se os modelos constitutivos de Mohr-Coulomb e de endurecimento na previsão das deformações. Por fim, faz-se a previsão do nível de danos causados pelas escavações da vala, a um edifício hipotético / This dissertation presents, in a first part, the work done to verify how appropriate are the Mohr-Coulomb and hardening soil constitutive models to represent the strains and the distortions associated with escavations of braced wall process. In the second part, estimates are made of the damages in constructions due to the braced excavations of line one of Sao Paulo Subway (Metrô S.P.). In the two phases, 810 numeric simulations were made, in finite elements using the software PLAXIS, associated the back analysis for direct process. It is presented a synthesis of the formation and the types of soils that constitute the basin of the sediments of the city of São Paulo, where is located the section experimental nº1, object of study of this dissertation. It is presented an short bibliographical revision regarding the back analysis techniques. Criteria for choice of intervals of parameters geotechnical that represent the system soil-structure in the excavation process are presented. Parametric analyses are made to determine which the parameters have larger influence in the behavior of the system soil-structure. The behavior of the soil-structure system is simulated using the Mohr-Coulomb and hardening soil constitutive models. The Mohr-Coulomb and hardening soil constitutive models are compared in the forecast of the deformations. Finally, it is made the forecast of the level of damages to a hypothetical building caused by the braced excavations
169

Etudes des sources locales de contraintes et des variations spatio-temporelles de l'activité sismique à l'intérieur de la plaque européenne / Study of local stress sources and spatio-temporal variations of seismic activity within european plate

Kusters, Dimitri 12 December 2014 (has links)
Les causes de l’activité sismique à l’intérieur des plaques tectoniques sont encore mal comprises, que ce soit l’origine des contraintes responsables des séismes ou leur relation avec la variation dans le temps et dans l’espace de l’activité sismique.<p>Les contraintes à l’intérieur des plaques résultent de l’action de forces de longueurs d’onde différentes, qui se superposent et s’additionnent. En utilisant une nouvelle méthode (Camelbeeck et al. 2013), déterminant les contraintes générées localement (échelle de 10 à 100 km), nous estimons l’importance relative de cette composante locale du champ de contrainte. En comparant ces contraintes locales avec les contraintes déduites des mécanismes au foyer des tremblements de terre en Europe occidentale, nous suggérons que celles-ci semblent jouer un rôle non-négligeable dans l’occurrence de l’activité sismique. C’est le cas dans des régions où les contraintes locales étaient déjà reconnues, mais également dans des régions précédemment identifiées comme dominées par les contraintes à plus grandes longueurs d’onde.<p>Le champ de contrainte généré localement est constant à l’échelle de temps des catalogues sismiques, ce qui ne permet pas d’expliquer l’occurrence dans le temps des séismes. Il est cependant modifié par les variations des contraintes locales générées par l’activité séismique elle-même, ce qui explique les séquences de répliques des séismes de Roermond (13/04/1992, Mw=5.4) et d’Alsdorf (22/07/2002, Mw= 4.7) dans le graben de la Roer. Nous y suggérons également l’importance de ces variations à une échelle de temps plus longue (millier d’années) à partir des données de paléoséismologie.<p>Pour mieux comprendre les relations spatio-temporelles des séismes, nous avons également analysé dans quelle mesure l’occurrence de l’activité sismique dans le graben de la Roer est un processus de Poisson, ou si l’activité future est localisée à proximité des séismes du passé, ou située dans des régions dénuées de séismes à ce jour. L’emploi des méthodes linéaires classiques et de méthodes non-linéaires dans cette région mais aussi dans les îles britanniques et dans le sud de la Norvège montrent que les séismes du passé ne peuvent expliquer les taux d’activité sismique actuellement mesurés. Une partie de l’activité sismique correspond ainsi à une activité de background, indépendante de l’occurrence des séismes du passé. La méthode des multifractales permet aussi de caractériser régionalement l’importance, la variété et continuité des processus responsables de l’activité sismique sans pour autant en identifier les causes. <p>Notre travail nous a permis d’identifier l’importance relative de certaines causes de l’activité sismique, par exemple l’importance des variations locales des contraintes générées par l’activité séismique elle-même, mais n’a pas permis par exemple d’identifier l’origine de l’activité de background, clairement mise en évidence par l’analyse multifractale./<p><p> / Doctorat en sciences, Spécialisation géologie / info:eu-repo/semantics/nonPublished
170

Coulomb breakup of halo nuclei by a time-dependent method

Capel, Pierre 29 January 2004 (has links)
Halo nuclei are among the strangest nuclear structures.<p>They are viewed as a core containing most of the nucleons<p>surrounded by one or two loosely bound nucleons. <p>These have a high probability of presence at a large distance<p>from the core.<p>Therefore, they constitute a sort of halo surrounding the other nucleons.<p>The core, remaining almost unperturbed by the presence<p>of the halo is seen as a usual nucleus.<p><p><P><p><p>The Coulomb breakup reaction is one of the most useful<p>tools to study these nuclei. It corresponds to the<p>dissociation of the halo from the core during a collision<p>with a heavy (high <I>Z</I>) target.<p>In order to correctly extract information about the structure of<p>these nuclei from experimental cross sections, an accurate<p>theoretical description of this mechanism is necessary.<p><p><P><p><p>In this work, we present a theoretical method<p>for studying the Coulomb breakup of one-nucleon halo nuclei.<p>This method is based on a semiclassical approximation<p>in which the projectile is assumed to follow a classical trajectory.<p>In this approximation, the projectile is seen as evolving<p>in a time-varying potential simulating its interaction with the target.<p>This leads to the resolution of a time-dependent Schrödinger<p>equation for the projectile wave function.<p><p><P><p><p>In our method, the halo nucleus is described<p>with a two-body structure: a pointlike nucleon linked to a<p>pointlike core.<p>In the present state of our model, the interaction between<p>the two clusters is modelled by a local potential.<p><p><P><p><p>The main idea of our method is to expand the projectile wave function<p>on a three-dimensional spherical mesh.<p>With this mesh, the representation of the time-dependent potential<p>is fully diagonal.<p>Furthermore, it leads to a simple<p>representation of the Hamiltonian modelling the halo nucleus.<p>This expansion is used to derive an accurate evolution algorithm.<p><p><P><p><p>With this method, we study the Coulomb breakup<p>of three nuclei: <sup>11</sup>Be, <sup>15</sup>C and <sup>8</sup>B.<p><sup>11</sup>Be is the best known one-neutron halo nucleus.<p>Its Coulomb breakup has been extensively studied both experimentally<p>and theoretically.<p>Nevertheless, some uncertainty remains about its structure.<p>The good agreement between our calculations and recent<p>experimental data suggests that it can be seen as a<p><I>s1/2</I> neutron loosely bound to a <sup>10</sup>Be core in its<p>0<sup>+</sup> ground state.<p>However, the extraction of the corresponding spectroscopic factor<p>have to wait for the publication of these data.<p><p><P><p><p><sup>15</sup>C is a candidate one-neutron halo nucleus<p>whose Coulomb breakup has just been studied experimentally.<p>The results of our model are in good agreement with<p>the preliminary experimental data. It seems therefore that<p><sup>15</sup>C can be seen as a <sup>14</sup>C core in its 0<sup>+</sup><p>ground state surrounded by a <I>s1/2</I> neutron.<p>Our analysis suggests that the spectroscopic factor<p>corresponding to this configuration should be slightly lower<p>than unity.<p><p><P><p><p>We have also used our method to study the Coulomb breakup<p>of the candidate one-proton halo nucleus <sup>8</sup>B.<p>Unfortunately, no quantitative agreement could be obtained<p>between our results and the experimental data.<p>This is mainly due to an inaccuracy in the treatment<p>of the results of our calculations.<p>Accordingly, no conclusion can be drawn about the pertinence<p>of the two-body model of <sup>8</sup>B before an accurate reanalysis of these<p>results.<p><p><P><p><p>In the future, we plan to improve our method in two ways.<p>The first concerns the modelling of the halo nuclei.<p>It would be indeed of particular interest to test<p>other models of halo nuclei than the simple two-body structure<p>used up to now.<p>The second is the extension of this semiclassical model to<p>two-neutron halo nuclei.<p>However, this cannot be achieved<p>without improving significantly the time-evolution algorithm so as to<p>reach affordable computational times. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

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