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1
COLOMBO, EDUARDO HENRIQUE FILIZZOLA. "COLLECTIVE BEHAVIOR OF LIVING BEINGS UNDER SPATIOTEMPORAL ENVIRONMENT FLUCTUATIONS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2018. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=36051@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCOORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
FUNDAÇÃO DE APOIO À PESQUISA DO ESTADO DO RIO DE JANEIRO
PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTS. DE ENSINO
BOLSA NOTA 10
PROGRAMA DE DOUTORADO SANDUÍCHE NO EXTERIOR
Organismos vivos têm seus próprios meios de locomoção e são capazes de se reproduzir. Além disto, o habitat no qual os organismos estão inseridos é tipicamente heterogêneo, de modo que as condições ambientais variam no tempo e no espaço. Nesta tese, são propostos e investigados modelos teóricos para compreender o comportamento coletivo de organismos vivos, visando responder questões relevantes sobre a organização e preservação da população utilizando técnicas analíticas e numéricas. Inicialmente, considerando um habitat homogêneo, em que as propriedades estatísticas das condições ambientais são independentes do tempo e do espaço, estudamos como padrões espaço-temporais podem emergir na distribuição da população devido a interações não-locais e investigamos o papel das flutuações ambientais neste processo. Em seguida, assumindo um meio ambiente heterogêneo, analisamos o caso de um único domínio de habitat. Considerando uma classe de equações não lineares, introduzindo flutuações temporais e interações entre os organismos, fornecemos uma perspectiva geral da estabilidade de populações neste caso, desafiando os conceitos ecológicos anteriores. Em um segundo passo, assumindo uma paisagem complexa fragmentada, consideramos que os indivíduos têm acesso a informações sobre a estrutura espacial do meio. Mostramos que os indivíduos sobrevivem quando as regiões espaciais viáveis estão suficientemente aglomeradas e observamos que o tamanho da população é maximizado quando os indivíduos utilizam parcialmente a informação do meio ambiente. Finalmente, como resultados exatos analíticos não são factíveis em muitas situações importantes, propomos uma abordagem efetiva para interpretar os dados experimentais. Assim, somos capazes de conectar a heterogeneidade do ambiente e a persistência da população, caracterizada pela distribuição de probabilidade para os tempos de vida.
Living entities have their own means of locomotion and are capable of reproduction. Furthermore, the habitat in which organisms are embedded is typically heterogeneous, such that environment conditions vary in time and space. In this thesis, theoretical models to understand the collective dynamics of living beings have been proposed and investigated aiming to address relevant questions such as population organization and persistence in the environment, using analytical and numerical techniques. Initially, considering an homogeneous habitat, in which the statistical properties of the environmental conditions are time and space independent, we study how spatiotemporal order can emerge in the population distribution due to nonlocal interactions and investigate the role of environment fluctuations in the self-organization process. Further, we continue our investigation assuming an heterogeneous environment, starting with the simplest case of a single habitat domain, and we obtain the critical conditions for population survival for different population dynamics. Considering a class of nonlinear equations, introducing temporal oscillations and interactions among the organisms, we are able to provide a general picture of population stability in a single habitat domain, challenging previous ecological concepts. At last, assuming a fragmented complex landscape, resembling realistic properties observed in nature, we additionally assume that individuals have access to information about the spatial structure. We show that individuals survive when patches of viable regions are clustered enough and, counter-intuitively, observe that population size is maximized when individuals have partial information about the habitat. Finally, since, analytical exact results are not feasible in many important situations, we propose an effective approach to interpret experimental data. This way we are able to connect environment heterogeneity and population persistence.
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Linnér, Erik. "Interplay of collective fluctuations in strongly correlated fermionic systems." Electronic Thesis or Diss., Institut polytechnique de Paris, 2023. http://www.theses.fr/2023IPPAX090.
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Les systèmes fortement corrélés présentent souvent des diagrammes de phase riches avec différentes phases ordonnées impliquant des degrés de liberté de spin, de charge, d'appariement ou d'orbitale. La description théorique de la compétition entre les différentes instabilités dans les systèmes fortement corrélés, qui donne lieu à cette phénoménologie, reste l'un des Saint-Graal de la théorie moderne de la matière condensée. Elle pose un énorme défi de complexité à la fois conceptuelle et numérique, et l'interaction des fluctuations électroniques concurrentes constitue donc un obstacle à la compréhension des diagrammes de phase complexes d'une large gamme de matériaux quantiques corrélés. Cela motive la recherche de méthodes simplifiées pour étudier l'interaction des fluctuations collectives.Nous présentons une extension multicanal de l'approche du champ fluctuant récemment développée pour les fluctuations collectives concurrentes dans les systèmes électroniques corrélés. La méthode est basée sur une optimisation variationnelle d'une action d'essai qui contient explicitement les paramètres d'ordre des principaux canaux de fluctuation. Elle donne un accès direct à l'énergie libre du système, facilitant la distinction entre les phases stables et métastables du système. Nous appliquons notre approche au modèle de Hubbard étendu, un modèle de fermions sur réseau paradigmatique, qui occupe une place de choix dans la théorie de la matière condensée en raison de la pertinence potentielle de ses versions répulsives et attractives pour les matériaux électroniques et les systèmes artificiels. En utilisant notre technique pour étudier le régime de couplage faible à intermédiaire de l'interaction répulsive, nous constatons qu'elle capture la compétition entre les fluctuations d'onde de densité de charge et des fluctuations antiferromagnétiques en accord qualitatif avec des méthodes numériquement plus coûteuses. En outre, cette méthode permet d'accéder aux propriétés des états excités et aux effets de corrélation à plusieurs corps, directement sur l'axe des fréquences réelles sans utiliser de techniques de continuation analytique numériques. L'approche du champ fluctuant multicanal offre donc une voie prometteuse pour un traitement numériquement peu coûteux de l'interaction entre les fluctuations collectives dans les systèmes de petite et grande taille.En utilisant l'approche introduite du champ fluctuant multicanal, nous explorons le diagramme de phase du modèle de Hubbard étendu dans les régimes répulsif et attractif, en abordant l'interaction des fluctuations dans les canaux antiferromagnétiques, de l'onde de densité de charge, de l'onde s supraconductrice et de la séparation de phases. Bien que ce modèle ait été étudié de manière intensive depuis des décennies, notre nouvelle approche nous permet d'identifier une nouvelle phase caractérisée par la coexistence de fluctuations collectives de l'onde s supraconductrice et de la séparation de phases. Ces résultats sont en accord avec les observations précédentes de phases supraconductrices et de séparation de phases dans les systèmes électroniques, notamment dans les supraconducteurs à haute température critique. En outre, la méthode des champs fluctuants multicanaux permet de mettre en évidence la quintessence du modèle de Hubbard étendu grâce à la grande variété de types de compétitions qui émergent des différentes instabilités. La nature générale de la théorie proposée, qui permet d'incorporer une grande variété de modes collectifs, en fait un outil prometteur pour l'étude de l'interaction des fluctuations collectives dans les systèmes fermioniques fortement corrélésStrongly correlated systems often display rich phase diagrams exhibiting different ordered phases involving spin, charge, pairing, or orbital degrees of freedom. The theoretical description of the competition between different instabilities in strongly correlated systems giving rise to this phenomenology, remains one of the holy grails of modern condensed matter theory. It poses a tremendous challenge of both conceptual and computational complexity, and thus the interplay of competing electronic fluctuations constitutes a roadblock to the understanding of the complex phase diagrams of a wide range of correlated quantum materials. This motivates the search for constructing simplified methods to study interplaying collective fluctuations.We introduce a multichannel extension of the recently developed fluctuating field approach to competing collective fluctuations in correlated electron systems. The method is based on a variational optimization of a trial action that explicitly contains the order parameters of the leading fluctuation channels. It gives direct access to the free energy of the system, facilitating the distinction between stable and metastable phases of the system.We apply our approach to the extended Hubbard model, a paradigmatic fermionic lattice model, occupying a prime place in condensed matter theory due to the potential relevance of its repulsive and attractive versions for both electronic materials and artificial systems.Utilising the technique to study the weak to intermediate coupling regime of the repulsive interaction, we find it to capture the interplay of competing charge density wave and antiferromagnetic fluctuations with qualitative agreement with more computationally expensive methods. In addition, the method allows access to excited-state properties, through the one-particle excitation spectrum, and many-body correlation effects, through the self-energy, directly on the real-frequency axis without using numerical analytic continuation techniques. The multichannel fluctuating field approach thus offers a promising route for a numerically low-cost treatment of the interplay between collective fluctuations in small to large systems.Using the introduced multichannel fluctuating field approach, we explore the phase diagram of the extended Hubbard model in both repulsive and attractive regimes, addressing the interplay of fluctuations in the antiferromagnetic, charge density wave, s-wave superconducting, and phase separation channels. Despite the fact that this model has been intensively studied for decades, our novel approach allows us to identify a novel phase that is characterised by the coexistence of collective s-wave superconducting and phase separation fluctuations. These findings resonate with previous observations of interplaying phase separation and superconducting phases in electronic systems, most importantly in high-temperature superconductors. In addition, the multichannel fluctuating field method allows to display the quintessential nature of the extended Hubbard model through the large variety of types of competitions which emerges from the interplaying instabilities. The general nature of the proposed theory, allowing to incorporate a variety of collective modes, makes it a promising tool for studying the interplay of collective fluctuations in strongly correlated fermionic systems
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Luo, Yu, and 罗郁. "Suppression of collective fluctuations and generation of entanglement in a spin ensemble." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2012. http://hub.hku.hk/bib/B48329794.
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Spin degrees of freedom have been extensively explored in the context of quantum information processing. Many proposals of quantum computation architectures use spins as carriers of quantum of information. A central problem is to efficiently generate quantum entanglement between spin qubits which proves to be a crucial resource for quantum information tasks. On the other hand, uncontrollable spin degrees of freedom in the environment of spin qubits are the major causes of errors at low temperature, for example, the lattice nuclear spins hyperfine coupled to single electron spin qubit localized in semiconductor nano-structures. An outstanding problem for scalable quantum computation is to suppress the collective fluctuations from such spin baths so that the coherence time of the spin qubit can be improved. With these two motivations, the problems of suppressing collective spin fluctuations and generating entanglement in various spin ensembles are addressed in this thesis.
In the first half of the thesis, two approaches are introduced for suppressing the collective fluctuations in the nuclear spin bath so that the quantum coherence time of electron spin qubit in semiconductor quantum dots can be improved. The first approach works for a coupled double dot system. A theory for the interaction with the nuclear spins is developed when the two-electron singlet state is in resonance with one of the triplet state in moderate external magnetic field. At this resonance condition, the nuclear-electron flip-flop process caused by the hyperfine interaction can lead to a feedback mechanism, which can be used to suppress the nuclear hyperfine field. The second approach works for a single dot system. It is shown that strong pumping of the nuclear spins in dynamic nuclear polarization processes can saturate the nuclear spin bath towards the collective “dark states”. In such dark states, the transverse nuclear field fluctuation can be substantially suppressed compared to the value at thermal equilibrium. Two physical schemes are proposed to realize the nuclear dark states for suppression of the nuclear field fluctuations.
In the second half of the thesis, schemes are presented for generating large scale quantum entanglement in two types of spin qubit systems. For atomic spin qubits in optical lattices, schemes are proposed on how to prepare pure spin coherent state (SCS) with low collective spin by incoherent pumping with collective spin raising and lowering operations. Such SCS realize networks of mutually entangled spins which can be idea resources for the quantum telecloning algorithm. For donor nuclear spin qubits in silicon architecture, proposals are shown on how to deterministic prepare Dicke states which constitute an important class of multipartite entangled states. Our scheme is capable of preparing both symmetric and asymmetric Dicke states which form a complete basis set of the spin Hilbert space. The required controls are in situ to the prototype Kane’s quantum computer. The preparation is robust because each desired Dicke state is the steady state under designed pumping process. The schemes presented here also make possible the construction of decoherence free subspaces where quantum information is protected from collective noises.published_or_final_version
Physics
Doctoral
Doctor of Philosophy
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Tsikata, Sedina. "Small-scale electron density fluctuations in the hall thruster, investigated by collective light scattering." Palaiseau, Ecole polytechnique, 2009. http://tel.archives-ouvertes.fr/docs/00/48/40/27/PDF/thesis.pdf.
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Les oscillations dans le plasma d'un propulseur à effet Hall sont susceptibles de provoquer le transport anormal à travers des lignes du champ magnétique. La théorie cinétique linéaire montre qu'en particulier certaines oscillations, de fréquence de l'ordre du mégahertz et de longueur d'onde millimétrique, peuvent jouer un rôle important dans le transport anormal. Les échelles caractéristiques de ces fluctuations ne sont pas détectables par des outils standards comme les sondes. Ce travail décrit pour la première fois l'utilisation d'un diagnostic de diffusion collective (PRAXIS) conçu pour l'étude du plasma du propulseur, qui a mené à l'identification des modes instables dans le plasma. Deux modes hautes fréquences ont été identifiés, se propageant dans les directions azimutale et axiale, avec des longueurs d'onde millimétriques et des fréquences de l'ordre du mégahertz. Les directions de propagation et les ouvertures angulaires de ces modes ont été déterminées. Le mode azimutal, identifié dans la théorie comme agent principal du transport, possède des composantes anti-parallèles au champ magnétique et parallèles au champ électrique et se propage dans une ouverture angulaire très restreinte. Le mode axial montre des caractéristiques liées à la vitesse et la divergence du faisceau d'ions. Le niveau de fluctuation de la densité est associé à une grande amplitude du champ électrique fluctuant. Les résultats des expériences sont en accord avec les prévisions théoriques et apportent de nouvelles informations, permettant ainsi d'améliorer et de développer des modèles pour les deux modesAnomalous electron transport across magnetic field lines in the Hall thruster plasma is believed to be due in part to plasma oscillations. Oscillations of frequencies on the order of a few megahertz and of wavelengths on the order of a millimeter have been shown to be likely to lead to transport. Measurements of fluctuations at these length scales is, however, beyond the reach of conventional thruster diagnostics such as probes. This work describes the first application of a specially-designed collective light scattering diagnostic (PRAXIS) to the measurement of electron density fluctuations and the subsequent identification of unstable modes in the thruster plasma. Two main high frequency modes are identified, propagating azimuthally and axially, of millimetric length scales and megahertz frequencies. The propagation directions and angular openings of the modes are determined. The azimuthallypropagating mode, believed to be responsible for transport, is shown to have wave vector components antiparallel to the magnetic field and parallel to the electric field, and to propagate within an extremely limited region. The axially-propagating mode is shown to have features closely related to the ion beam velocity and divergence. The electron density fluctuation level is calculated and is associated with a high electric field amplitude. The experiments, confirming a number of predictions arising from linear kinetic theory, also provide much additional information permitting the improvement and development of models for both modes
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Mirebrahimi, Seyedmeghdad. "Interacting stochastic systems with individual and collective reinforcement." Thesis, Poitiers, 2019. http://www.theses.fr/2019POIT2274/document.
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L'urne de Polya est l'exemple typique de processus stochastique avec renforcement. La limite presque sûre (p.s.) en temps existe, est aléatoire et non dégénérée. L'urne de Friedman est une généralisation naturelle dont la limite (proportion asymptotique en temps) n'est plus aléatoire. De nombreux modèles aléatoires sont fondés sur des processus de renforcement comme pour la conception d'essais cliniques au design adaptatif, en économie, ou pour des algorithmes stochastiques à des fins d'optimisation ou d'estimation non paramétrique. Dans ce mémoire, inspirés par de nombreux articles récents, nous introduisons une nouvelle famille de systèmes (finis) de processus de renforcement où l'interaction se traduit par un phénomène de renforcement collectif additif, de type champ moyen. Les deux taux de renforcement (l'un spécifique à chaque composante, l'autre collectif et commun à toutes les composantes) sont possiblement différents. Nous prouvons deux types de résultats mathématiques. Différents régimes de paramètres doivent être considérés : type de la règle (brièvement, Polya/Friedman), taux du renforcement. Nous prouvons l'existence d'une limite p.s. coommune à toutes les composantes du système (synchronisation). La nature de la limite (aléatoire/déterministe) est étudiée en fonction du régime de paramètres. Nous étudions également les fluctuations en prouvant des théorèmes centraux de la limite. Les changements d'échelle varient en fonction du régime considéré. Différentes vitesses de convergence sont ainsi établiesThe Polya urn is the paradigmatic example of a reinforced stochastic process. It leads to a random (non degenerated) almost sure (a.s.) time-limit.The Friedman urn is a natural generalization whose a.s. time-limit is not random anymore. Many stochastic models for applications are based on reinforced processes, like urns with their use in adaptive design for clinical trials or economy, stochastic algorithms with their use in non parametric estimation or optimisation. In this work, in the stream of previous recent works, we introduce a new family of (finite) systems of reinforced stochastic processes, interacting through an additional collective reinforcement of mean field type. The two reinforcement rules strengths (one componentwise, one collective) are tuned through (possibly) different rates. In the case the reinforcement rates are like 1/n, these reinforcements are of Polya or Friedman type as in urn contexts and may thus lead to limits which may be random or not. We state two kind of mathematical results. Different parameter regimes needs to be considered: type of reinforcement rule (Polya/Friedman), strength of the reinforcement. We study the time-asymptotics and prove that a.s. convergence always holds. Moreover all the components share the same time-limit (synchronization). The nature of the limit (random/deterministic) according to the parameters' regime is considered. We then study fluctuations by proving central limit theorems. Scaling coefficients vary according to the regime considered. This gives insights into the different rates of convergence
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Moran, José. "Statistical physics and anomalous macroeconomic fluctuations." Thesis, Paris, EHESS, 2020. http://www.theses.fr/2020EHES0106.
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Comment des perturbations à l'échelle de l'individu peuvent-elle devenir des grandes fluctuations à l'échelle de l'économie? Cette question, malgré son étude depuis des décennies, est encore ouverte. Dans cet ouvrage, j'étudie cette énigme à l'aide de méthodes issues de la physique statistique. En partant d'une analyse approfondie des distributions en loi de puissance, je montre qu'une compréhension claire de leur origine et de leurs propriétés permet de mieux appréhender leurs conséquences socioéconomiques. Je propose ensuite un modèle d'économie en réseau, où les entreprises dépendent les unes des autres pour produire, de sorte que la nature même de leurs interactions peut les rendre susceptibles d'amplifier des fluctuations. Ensuite, je soumets au lecteur une étude empirique des propriétés statistiques des taux de croissance et fournis un cadre permettant d'étudier leur dynamique. Dans la partie finale je m'intéresse à des modèles qui exhibent des phénomènes collectifs non-triviaux parce qu'ils considèrent des effets d'imitation ou de mémoire dans les décisions prises par les individus, montrant bien la nécessité de tenir compte de la possible complexité des différentes parties constitutives des modèles économiquesHow do microscopic perturbations at the level of an individual grow to become macroscopic fluctuations of the whole economy? Despite decades of effort, this puzzle remains open. In this work, I tackle this problem using methods and techniques from statistical physics. Beginning with a thorough analysis of power law distributions, I argue that understanding their origin and properties helps in elucidating their socio-economic consequences. I then explore a model of an economy where firms interact through a {\em production network} in a way that causes them to be intrinsically prone to amplify fluctuations. Later on, I conduct an empirical survey of the statistical properties of firm growth rates and provide a framework to study their dynamics. I finally move onto models where non-trivial collective phenomena arise from imitation and memory effects at the level of the individual, highlighting the need of accounting for complexity in economic modelling
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Tovazzi, Daniele. "Self-sustained periodic behaviors in interacting systems: macroscopic limits and fluctuations." Doctoral thesis, Università degli studi di Padova, 2018. http://hdl.handle.net/11577/3425244.
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In this thesis we study the appearance of self-sustained periodic behavior in the macroscopic dynamics of some interacting systems and related critical phenomena. The thesis is organized as follows. In Chapter 1 we focus on the emergence of periodicity in cooperative mean field models whose interaction potential undergoes a dissipative evolution. We define a generalized Curie-Weiss model with dissipation and we analyse its macroscopic dynamics: we show that not only a periodic behavior is present at sufficiently low temperature, but also that, in certain regimes, any (finite) number of stable limit cycles can coexist. Chapter 2 is concerned with a two-population Curie-Weiss model: we define two types of microscopic dynamics, one with delay and the other without. We identify configurations of the interaction network which can enhance macroscopic oscillations in the case without delay; we also show that delay allows the appearance of a collective periodic behavior in configurations in which periodicity was otherwise absent. In Chapter 3 we consider again the mechanism of dissipation, this time dropping the mean field hypothesis. We study a short-range interacting system obtained introducing dissipation in a 1-dimensional Ising model. We prove that, in a suitable zero-temperature infinite-volume limit, the total magnetization of the system displays regular oscillations between polarized phases. Finally, Chapter 4 is dedicated to the analysis of critical fluctuations for systems exhibiting a Hopf bifurcation in the dynamics of the macroscopic law. The behavior of critical fluctuations around the macroscopic limit reflects the type of bifurcation and the observables display fluctuations evolving at different time scales. We identify the slow and the fast variable and we obtain the convergence of the slow variable to its limiting dynamics via an averaging principle.In questa tesi studiamo comportamenti periodici auto-sostenuti che appaiono nella dinamica macroscopica di certi sistemi interagenti e alcuni fenomeni critici collegati a questo comportamento. La tesi è organizzata come segue. Nel primo capitolo ci concentriamo sulla comparsa di periodicità in modelli cooperativi a campo medio il cui potenziale di interazione è soggetto a una dissipazione. Definiamo un modello di Curie-Weiss generalizzato con dissipazione ed analizziamo la sua dinamica limite: mostriamo che non solo il comportamento periodico è presente a temperature sufficientemente basse, ma anche che, in certi regimi, diversi cicli limite stabili possono coesistere, purché in numero finito. Nel secondo capitolo ci occupiamo di un modello di Curie-Weiss bipopolato: definiamo due tipi di dinamiche microscopiche, una con ritardo e l'altra senza. Identifichiamo le configurazioni della rete di interazione che possono dare luogo ad oscillazioni macroscopiche nel caso senza ritardo; mostriamo inoltre che il ritardo permette la comparsa di periodicità in configurazioni nelle quali sarebbe altrimenti assente. Nel terzo capitolo consideriamo nuovamente il meccanismo della dissipazione, questa volta lasciando cadere l'ipotesi di interazione a campo medio. Studiamo un sistema di particelle con interazione a corto raggio ottenuto introducendo la dissipazione in un modello di Ising 1-dimensionale. Mostriamo che, in un opportuno limite di temperatura zero e volume infinito, la magnetizzazione totale del sistema presenta oscillazioni regolari tra fasi polarizzate. Infine, il quarto capitolo è dedicato all'analisi delle fluttuazioni critiche di sistemi che esibiscono una biforcazione di Hopf nella dinamica della legge macroscopica. Il comportamento delle fluttuazioni critiche attorno al limite macroscopico riflette il tipo di biforcazione e gli osservabili mostrano fluttuazioni che evolvono su scale temporali differenti. Identifichiamo la variable lenta e quella veloce ed otteniamo la convergenza della variabile lenta alla sua dinamica limite tramite un averaging principle.
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Tsikata, Sedina. "Fluctuations de densité électronique à petite échelle dans un propulseur à effet Hall, investigué par la méthode de diffusion collective." Phd thesis, Ecole Polytechnique X, 2009. http://tel.archives-ouvertes.fr/tel-00484027.
Full textAbstract:
Les oscillations dans le plasma d'un propulseur à effet Hall sont susceptibles de provoquer le transport anormal à travers des lignes du champ magnétique. La théorie cinétique linéaire montre qu'en particulier certaines oscillations, de fréquence de l'ordre du mégahertz et de longueur d'onde millimétrique, peuvent jouer un rôle important dans le transport anormal. Les échelles caractéristiques de ces fluctuations ne sont pas détectables par des outils standards comme les sondes. Ce travail décrit pour la première fois l'utilisation d'un diagnostic de diffusion collective (PRAXIS) conçu pour l'étude du plasma du propulseur, qui a mené à l'identification des modes instables dans le plasma. Deux modes hautes fréquences ont été identifiés, se propageant dans les directions azimutale et axiale, avec des longueurs d'onde millimétriques et des fréquences de l'ordre du mégahertz. Les directions de propagation et les ouvertures angulaires de ces modes ont été déterminées. Le mode azimutal, identifié dans la théorie comme agent principal du transport, possède des composantes anti-parallèles au champ magnétique et parallèles au champ électrique et se propage dans une ouverture angulaire très restreinte. Le mode axial montre des caractéristiques liées à la vitesse et la divergence du faisceau d'ions. Le niveau de fluctuation de la densité est associé à une grande amplitude du champ électrique fluctuant. Les résultats des expériences sont en accord avec les prévisions théoriques et apportent de nouvelles informations, permettant ainsi d'améliorer et de développer des modèles pour les deux modes.
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Cavalier, Jordan. "Modèles cinétiques et caractérisation expérimentale des fluctuations électrostatiques dans un propulseur à effet Hall." Thesis, Université de Lorraine, 2013. http://www.theses.fr/2013LORR0130/document.
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L'étude des phénomènes turbulents se développant en sortie du propulseur de Hall est nécessaire pour pouvoir modéliser le transport anormal (par opposition au transport diffusif) des électrons à travers les lignes de champ magnétique. Les relations de dispersion de deux instabilités pouvant être responsables de ce transport ont été mesurées à des échelles millimétriques à l'aide du diagnostic de diffusion collective de la lumière. Ce travail de thèse s'attache à en donner une description aussi bien théorique qu'expérimentale, pierre à l'édifice de la compréhension du transport dans le propulseur. Une instabilité se propageant majoritairement dans la direction azimutale du propulseur y est caractérisée comme étant l'instabilité de dérive électronique ExB et un modèle analytique décrivant la fréquence expérimentale y est dérivé et validé. De plus, le manuscrit présente une méthode de déconvolution du signal de la diffusion collective de la fonction d'appareil pour ce mode. Une fois déconvoluées, les relations de dispersion expérimentales peuvent être ajustées par la fréquence du modèle analytique, ce qui permet de mesurer expérimentalement et de manière originale la température et la densité électronique dans le jet d'ions énergétiques du plasma du propulseur. Enfin, la seconde instabilité, se développant autour de la direction axiale du propulseur, est caractérisée comme l'instabilité double faisceau entre les ions simplement et doublement chargés du plasmaThe study of turbulent phenomena that grow at the exit plane of the Hall thruster is required to modelize the anomalous transport (in contrast to the diffusion transport) of electrons across the magnetic field lines. The dispersion relations of two instabilities that can be responsible for this transport have been mesured at millimetric scales by mean of the collective light scattering diagnostic. The aim of the thesis is to describe them theoretically as well as experimentally, improving the understanding of the Hall thruster transport. In the thesis, an instability that propagates principally azimuthally is caracterized as the ExB electron drift instability and an analytical model that describes the experimental frequency is derived and validated. In addition, the manuscript presents an original method to unfold the signal of the collective scattering diagnostic from the instrumental function of this mode. Once corrected, the experimental dispersion relations can be adjusted by the frequency given by the analytical model, allowing to measure experimentally and in an original way the electron temperature and density in the energetic ion jet of the Hall thruster plasma. The second instability that is mainly propagating in the axial direction is caracterized as the two-stream instability between the simply and doubly charged ions of the plasma
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Delmotte, Blaise. "Modeling and Simulation of Individual and Collective Swimming Mechanisms in Active Suspensions." Phd thesis, Toulouse, INPT, 2015. http://oatao.univ-toulouse.fr/14354/1/delmotte_partie_1_sur_2_2.pdf.
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We have all witnessed the flocking of starlings in the sky and the schools of fish that form in the ocean. This kind of organization of living creatures is not limited to those that we see, but also occurs for those that we don’t : swimming microorganisms. Suspen- sions of micro-swimmers exhibit a rich dynamics. Their behaviors can play an important role in the survival of the group, its development, the balance between species, their trophic strategies and even animal fertility. They can form coherent structures due to collective motion, mix the surrounding fluid or modify its rheological properties. Such diversity results from the complex interplay between swimming strategies, physiological processes, chemical reactions and hydrodynamic interactions. Fluid Mechanics is there- fore essential to understand and master the mechanisms involved in these phenomena. While experimental studies bring out new findings and, sometimes, provide physical ex- planations, modeling remains essential. Yet, including an accurate description of the micro-swimmers in a suspension containing thousands (nay millions) individuals, requires considering a wide range of coupled scales (from one micron 10^−6m to several millimeters 10^−3m). What happens on large scales depends on sophisticated mechanisms occurring two or three orders of magnitude below. Therefore, the multiscale modeling of such phenomena is still a major challenge for the state-of-the-art numerical methods. This thesis aims at providing a contribution in that direction. In a first part, we will show that reproducing swimming mechanisms at the scale of the micro-swimmer can be achieved with various models spanning different levels of complexity. We will then present our developments to incorporate these models in an efficient framework for large scale simulations. We will show how to simultaneously account for the Brownian motion of the smallest particles (10^−6m). Our code reproduces known results from the literature with the same accuracy, but at lower cost and at larger scales, thus bridging a gap between particle-based models, experiments and continuum formulations from kinetic theory. Using the capabilities afforded by our method, we eventually address two open problems in the experimental literature : the origins of orientational correla- tions between interacting self-propelled micro-droplets and the mechanisms at play in the nonlinear enhancement of Brownian particle diffusion in active suspensions.
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Falcon, Claudio. "Quelques effets des fluctuations sur les systèmes collectifs." Paris 6, 2008. http://www.theses.fr/2008PA066435.
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Chanderdeo, Kerisha. "Librarians’ book selection practices and budget fluctuations in the City of Cape Town, South Africa." University of Western Cape, 2019. http://hdl.handle.net/11394/7543.
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Magister Artium - MAOne of the main purposes of a library is to disseminate information to the community it serves. Collection development, which is the process of building and maintaining a library’s collection is, thus an essential part of library services. The selection of materials that align with community needs and interests is an especially important aspect of collection development. With reference to the Helen Haines theory of book selection which is community-centred and highlights the role of lifelong learning in book selection, this research investigated the book selection practices of librarians from the City of Cape Town Library and Information Service
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Bouriquet, Bertrand. "Relaxation en forme et multifragmentation nucléaire." Phd thesis, Université de Caen, 2001. http://tel.archives-ouvertes.fr/tel-00003803.
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Ce travail concerne la caractérisation des réactions de multifragmentation induites dans les collisions centrales de Xe+Sn à 32 MeV /A mesurées avec le détecteur INDRA. Cette caractérisation informe sur le degré d'équilibration. Un aspect crucial pour ces études est la sélection des événements. Dans cette étude deux approches ont été utilisées pour la sélection des collisions d'intérêt. La première approche s'appuie sur des sélecteurs de centralité (angle de flot, angle d'émission du plus gros fragment, multiplicité totale de particules chargées). La seconde approche qui constitue une innovation dans le domaine de la physique nucléaire est l'utilisation des cartes auto-organisées de Kohonen. Cette méthode permet un classement en topologie des événements sans utiliser explicitement des variables reliées à la forme. Les deux méthodes de sélection indiquent la coexistence d'au moins deux niveaux de dissipation dans les collisions les plus centrales. La méthode des cartes auto-organisées permet de déconvoluer ces processus. L'échantillon associé au processus de multifragmentation et à la plus grande conversion de l'énergie initiale représente 5% (250 mb) de la section efficace totale de réaction. Quelle que soit la méthode de sélection la forme moyenne de la source est allongée dans la direction du faisceau. Ce résultat signe une relaxation incomplète du degré de liberté de forme. Les données expérimentales sont comparées avec un modèle de multifragmentation statistique (SMM) incluant la déformation de la source et une énergie collective d'expansion. La caractérisation des réactions de multifragmentation en terme de taille, d'énergie d'excitation d'énergie collective d'expansion et de déformation est indépendante du mode de sélection utilisé. De cette comparaison on déduit que 70% de l'énergie disponible et 83% des nucléons du système initial ont été équilibré durant la réaction.
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Ruiz, Mora Africa. "Bounded Surfatron Acceleration in the Presence of Random Fluctuations." Master's thesis, Temple University Libraries, 2015. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/345317.
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Mechanical EngineeringM.S.M.E.
The mechanisms of acceleration and transport of collisionless plasma in the presence of electromagnetic turbulence (EMT) still remains not fully understood. The particle-EMT interaction can be modelled as the interaction of the particle with a particular wave in the presence of random noise. It has been shown that in such a model the acceleration of the charged particles can be almost free. This effect is known as resonance, which can be explained by the so-called “surfatron” mechanism. We have conducted several numerical simulations for the models with and without the presence of EMT. The turbulence has been modeled as small random fluctuations on the background magnetic field. Particles dynamics consist of two regimes of motion: (i) almost free (Larmor) rotation and (ii) captured (resonance) propagation, which are given by two different sets of invariants. We have determined the necessary conditions for capture and release from resonance for the model without fluctuations, as well as the intrinsic structure of the initial conditions domain for particles in order to be captured. We observed a difference in the orders of magnitude of the dispersion of adiabatic invariant due to the effects of the added fluctuations at the resonance. These results are important to describe the mixing of the different energy levels in the presence of EMT. To understand the impact of the EMT on the system dynamics, we have performed statistical analysis of the effects that different characteristics of the random fluctuations have on the system. The particles' energy gain can be viewed as a random walk over the energy levels, which can be described in terms of the diffusion partial differential equation for the probability distribution function. This problem can be reverse-engineered to understand the nature and structure of the EMT, knowing beforehand the energy distribution of a set of particles.
Temple University--Theses
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Reel, Andrew. "APPROXIMATING THE SPECTRAL WIDTH OF IRRADIANCE FLUCTUATIONS WITH QUASI-FREQUENCY." Master's thesis, University of Central Florida, 2008. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2607.
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Under weak turbulence theory, we will use the random thin phase screen model and the Kolmogorov power-law spectrum to derive approximate models for the scintillation index, covariance function of irradiance fluctuations, and temporal spectrum of irradiance fluctuations for collimated beams. In addition, we will provide an expression for the quasi-frequency of a collimated beam and investigate the relationship between the quasi-frequency and the maximum width of the normalized temporal spectrum of irradiance for a collimated beam.M.S.
Department of Mathematics
Sciences
Mathematical Science MS
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McCracken, James. "DECOHERENCE IN QUANTUM DOT CHARGE QUBITS: THE ROLE OFELECTROMAGNETIC FLUCTUATIONS." Master's thesis, University of Central Florida, 2006. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2796.
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Lateral semiconductor quantum dot structures have been proposed as an effective quantum bit (qubit) for quantum computation. A single excess electron with the freedom to move between two capacitively coupled quantum dots creates a `pseudo'-spin system with the same qubit behavior as the more natural two level system of a single electron spin. The excess electron in the double dot system is restricted to one of the two dots, thereby creating two separate and distinct states (usually referred to as |L> and |R>). The benefit of these charge qubits lie in the relative ease with which they can be manipulated and created. Experiments have been performed in this area and have shown controllable coherent oscillations and thus efficient single-qubit operations. However, the decoherence rates observed in the experiments is still quite high, making double dot charge qubits not very appealing for large-scale implementations. The following work describes the effect of the electromagnetic (EM) environment of the double quantum dot system on the decoherence of the charge state. Sources of decoherence in similar systems have been extensively investigated before and this paper follows a close theoretical framework to previous work done in the area. The effect of the EM environment can been seen in the calculations discussed below, although it is clear that the decoherence seen in experiments cannot be fully explained by the voltage fluctuations as they are investigated here. The limitations of the calculations are discussed and improvements are suggested.M.S.
Department of Physics
Sciences
Physics
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Xia, Xiang Ph D. Massachusetts Institute of Technology. "Collective excitations in low dimensional systems and stochastic control of population growth in a fluctuating environment." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/38620.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.Vita.
Includes bibliographical references.
In this thesis, I study several problems in the following areas: collective excitations in condensed matter physics, noise in gene network and stochastic control in biophysics. In the first area, I construct an effective field theory to describe Bose-Einstein Condensate (BEC) realized in an external potential. This theory explicitly explores the idea of spontaneous symmetry breaking and its application in the description of phase transitions of confined systems. Based on the effective lagrangian, I calculate the excitation spectrum and Matsubara Green's functions using the method of functional integrals. The theory also shows that in one dimension the collective excitation of a bosonic system can be unified with that of a fermionic system, which is described by Luttinger liquid theory. The unified theory of collective excitations of low dimensional quantum systems motivates my study of collective excitations of interacting classical particles confined in one dimension. It is shown in my paper that the structure of Hamiltonian or Lagrangian for one dimensional constrained systems is uniquely determined by conservation laws. Therefore the excitations of bosonic, fermionic and classical particles are strikingly similar in one dimension.
(cont.) In the second area, i. e., noise in gene networks and phenotypic switching in a fluctuating environment, I study the noise propagation in a gene network cascade using the method of master equations which examines the validity of the more popular methods such as the Langevin equation. To further explore the applications of stochastic processes for complex systems, I study phenotypic switches in a fluctuating environment. By combining the techniques of stochastic differential equation and stochastic dynamical programming, I propose a simple framework which can be used to study phenotypic growth dynamics. Another work is to explore the influence of environment on the dynamical properties of small systems is directed to the unusual blinking statistics of semiconductor quantum dots. I show in a model system that a broad spectrum of decay rates is possible when disorder is present in the environment.
by Xiang Xia.
Ph.D.
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Vanhala, Juuso. "Essays on labor market frictions, technological change and macroeconomic fluctuations /." Helsinki, 2007. http://ethesis.helsinki.fi/julkaisut/val/kansa/vk/vanhala/essayson.pdf.
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Bonamy, Daniel. "Phénomènes collectifs dans les matériaux granulaires : écoulements de surface et réarrangements internes dans des empilements modèles." Paris 11, 2001. https://tel.archives-ouvertes.fr/tel-00003916.
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Les matériaux granulaires possèdent des propriétés rhéologiques et mécaniques peu communes. Ils peuvent en effet s'écouler comme des liquides mais, sous certaines conditions, se bloquer et résister à des contraintes extérieures sans se déformer. Dans ce mémoire, nous étudions cette dualité solide/liquide à travers deux expériences permettant de faire le lien entre le comportement individuel des grains et celui de l'empilement. Tout d'abord on considère les écoulements à la surface d'un empilement incliné. Nous dérivons tout d'abord une description continue de leur dynamique à partir des équations de conservation intégrées sur l'épaisseur (approche de Saint-Venant) en étudiant les écoulements stationnaires dans un tambour tournant. Nous nous intéressons ensuite à la rhéologie du matériau et analysons les corrélations spatiales du champ de vitesse et les fluctuations de compacité. Nous montrons l'existence d'une forte concentration d'agrégats de grains trop denses pour pouvoir se déformer. Leur taille est distribuée en loi de puissance, sans échelle caractéristique. Nous discutons les implications de ces observations sur les approches théoriques proposées pour décrire les écoulements denses. La deuxième partie porte sur les réarrangements internes engendrés par une petite sollicitation extérieure dans un empilement apparemment statique. Une petite dilatation thermique engendre des fluctuations de conductance géantes dans un empilement de grains conducteurs. L'analyse statistique de ces fluctuations nous permet de relier cette sensibilité électrique à des fluctuations tribologiques locales et non à des réorganisations collectives du réseau des contacts comme cela a pu être suggéré par le passé. Nous visualisons dans un deuxième temps les micro-déplacements des grains engendrés par l'ajout d'une petite surcharge en surface. Cela nous permet de discuter la validité des différents modèles décrivant la répartition des contraintes dans un empilement statiqueGranular media exhibit non-intuitive rheological and mechanical properties. They can flow as liquids but, under specific conditions, they can jam and resist to external stress without being sheared. In this thesis, the solid/liquid duality is investigated through two experiments whose main features are to allow to study the relationships between the individual behavior of beads and the one of the packing. In the first part, we consider the surface flows in an inclined packing. A continuous description based on depth averaged conservation equations (Saint-Venant approach) is first derived from the study of steady surface flows in a rotating drum. We then focus on the internal rheology of these flows and investigate the spatial correlation of the instantaneous velocity field and the fluctuations of the volume fraction. We demonstrate the existence of rigid clusters of grains embedded in the flows. Their size is power-law distributed from the grain size scale up to the thickness of the flowing layer. The implications of the absence of a characteristic length scale on available theoretical models of dense granular flows are discussed. The second part of the thesis deals with internal rearrangements in a packing submitted to a small perturbation. A small thermal dilation leads to giant electrical fluctuations in a pile of metallic beads. The statistical analysis of these fluctuations allows us to relate this electrical sensitivity to local tribological fluctuations rather than collective reorganizations of the contacts network as proposed in the past. We then visualize the grains micro-displacements entailed by adding a small overload to the surface. This experiment allows us to discuss the validity of the different approaches proposed to describe stress distribution in static packing
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Griffin, Alison. "FUNDAMENTAL STUDY OF FC-72 POOL BOILING SURFACE TEMPERATURE FLUCTUATIONS AND BUBBLE BEHAVIOR." Doctoral diss., University of Central Florida, 2008. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3153.
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A heater designed to monitor surface temperature fluctuations during pool boiling experiments while the bubbles were simultaneously being observed has been fabricated and tested. The heat source was a transparent indium tin oxide (ITO) layer commercially deposited on a fused quartz substrate. Four copper-nickel thin film thermocouples (TFTCs) on the heater surface measured the surface temperature, while a thin layer of sapphire or fused silica provided electrical insulation between the TFTCs and the ITO. The TFTCs were micro-fabricated using the liftoff process to deposit the nickel and copper metal films. The TFTC elements were 50 microns wide and overlapped to form a 25 micron by 25 micron junction. TFTC voltages were recorded by a DAQ at a sampling rate of 50 kHz. A high-speed CCD camera recorded bubble images from below the heater at 2000 frames/second. A trigger sent to the camera by the DAQ synchronized the bubble images and the surface temperature data. As the bubbles and their contact rings grew over the TFTC junction, correlations between bubble behavior and surface temperature changes were demonstrated. On the heaters with fused silica insulation layers, 1-2 C temperature drops on the order of 1 ms occurred as the contact ring moved over the TFTC junction during bubble growth and as the contact ring moved back over the TFTC junction during bubble departure. These temperature drops during bubble growth and departure were due to microlayer evaporation and liquid rewetting the heated surface, respectively. Microlayer evaporation was not distinguished as the primary method of heat removal from the surface. Heaters with sapphire insulation layers did not display the measurable temperature drops observed with the fused silica heaters. The large thermal diffusivity of the sapphire compared to the fused silica was determined as the reason for the absence of these temperature drops. These findings were confirmed by a comparison of temperature drops in a 2-D simulation of a bubble growing over the TFTC junction on both the sapphire and fused silica heater surfaces. When the fused silica heater produced a temperature drop of 1.4 C, the sapphire heater produced a drop of only 0.04 C under the same conditions. These results verified that the lack of temperature drops present in the sapphire data was due to the thermal properties of the sapphire layer. By observing the bubble departure frequency and site density on the heater, as well as the bubble departure diameter, the contribution of nucleate boiling to the overall heat removal from the surface could be calculated. These results showed that bubble vapor generation contributed to approximately 10% at 1 W/cm^2, 23% at 1.75 W/cm^2, and 35% at 2.9 W/cm^2 of the heat removed from a fused silica heater. Bubble growth and contact ring growth were observed and measured from images obtained with the high-speed camera. Bubble data recorded on a fused silica heater at 3 W/cm^2, 4 W/cm^2, and 5 W/cm^2 showed that bubble departure diameter and lifetime were negligibly affected by the increase in heat flux. Bubble and contact ring growth rates demonstrated significant differences when compared on the fused silica and sapphire heaters at 3 W/cm^2. The bubble departure diameters were smaller, the bubble lifetimes were longer, and the bubble departure frequency was larger on the sapphire heater, while microlayer evaporation was faster on the fused silica heater. Additional considerations revealed that these differences may be due to surface conditions as well as differing thermal properties. Nucleate boiling curves were recorded on the fused silica and sapphire heaters by adjusting the heat flux input and monitoring the local surface temperature with the TFTCs. The resulting curves showed a temperature drop at the onset of nucleate boiling due to the increase in heat transfer coefficient associated with bubble nucleation. One of the TFTC locations on the sapphire heater frequently experienced a second temperature drop at a higher heat flux. When the heat flux was started from 1 W/cm^2 instead of zero or returned to zero only momentarily, the temperature overshoot did not occur. In these cases sufficient vapor remained in the cavities to initiate boiling at a lower superheat.Ph.D.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering PhD
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Pond, Jarrad W. T. "Perturbation analysis of fluctuations in the universe on large scales, including decaying solutions and rotational velocities." Honors in the Major Thesis, University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETH/id/1309.
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This item is only available in print in the UCF Libraries. If this is your Honors Thesis, you can help us make it available online for use by researchers around the world by following the instructions on the distribution consent form at http://library.ucf.edu/Systems/DigitalInitiatives/DigitalCollections/InternetDistributionConsentAgreementForm.pdf You may also contact the project coordinator, Kerri Bottorff, at kerri.bottorff@ucf.edu for more information.Bachelors
Sciences
Physics
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Hershberger, Craig. "COMPARISON OF THEORETICAL MODELS OF POWER SPECTRAL DENSITY TO THE EXPERIMENTAL VALUE FOR SPECTRUM OF IRRADIANCE FLUCTUATIONS." Master's thesis, University of Central Florida, 2008. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2737.
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A propagation experiment was designed, assembled, and conducted on an extended range to verify theoretical temporal models for weak to strong fluctuation theory. Laser light intensity was propagated over terrain at the Kennedy Space Center (Florida), and detected using optical receivers at a distance of 13.3 Km from the optical source. The intensity data from the experiment was used to generate an experimental Power Spectral Density (PSD) function. The theoretical Mutual Coherence Function (MCF) and Wave Structure Function (WSF) as set forth by Andrews/Phillips , were evaluated to determine the effective relationship between the statistical moments of the random optical field and the laser light intensity. Two scales of interest were identified (refractive large-scale and diffractive small-scale) and plotted revealing the characteristic shape of each component. In addition, statistical principles applied to the correlation/covariance function relationship and a graphical convolution process were used to generate a theoretical PSD function. Further, utilizing Taylor's "frozen turbulence" hypothesis an analysis of the theoretical temporal covariance function was performed. Functional forms for refractive and diffractive log-irradiance components were developed and used to generate a second theoretical PSD function. Finally, the experimental and theoretical Power Spectral Density functions are plotted on the same graph and a comparison is performed.M.S.E.E.
School of Electrical Engineering and Computer Science
Engineering and Computer Science
Electrical Engineering MSEE
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Thiagarajan, Raghavan. "Collective effects in living matter : from cytokinetic rings to epithelial monolayers." Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAF039/document.
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L’émergence de comportements collectifs cellulaires n’est pas bien comprise. Nous l’abordons dans deux systèmes biologiques. A l'échelle du micromètre lors de la constriction de l’anneau cytokinétique, nous montrons que des complexes d’acto-myosine s’auto-organisent sous forme d’agrégats dans la levure à fission et dans la cellule de mammifères. Ces auto-organisations découlent de règles d'interactions communes mais pour des fonctions distinctes, le transport et la génération de stress respectivement. A l'échelle de 100 micromètres, nous observons des pulsations corrélées de cellules épithéliales. Nous montrons les rôles du frottement avec la surface, et le couplage entre l’aire cellulaire, sa hauteur et sa contractilité. Nous présentons aussi deux études, des polyamines synthétiques pour étudier la polymérisation d'actine in vivo, puis l’inversion de sens dans la migration - la ratchetaxie. Cette thèse illustre l'importance des phénomènes physiques dans la dynamique cellulaireThe emergence of collective behavior from the interaction of individual units is not clear. In this thesis, we address this question in two different systems at different scales. At the micrometer scale during cytokinetic ring constriction, we show that acto-myosin self-organizes into rotating and static clusters in fission yeast and mammalian cells. These self-organizations arise from common interaction rules, but to serve distinct functions, transport and stress generation respectively. At 100 micrometers scale, we report correlated pulsations of cells in an epithelial monolayer. We show the key roles of substrate friction, and the tight coupling between cell area, cell height and contractility. We also present two other studies: synthetic polyamines for studying actin polymerization in vivo, and direction reversal in single cell migration during ratchetaxis. Altogether, this PhD illustrates the importance of physical phenomena in cellular dynamics
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Stromqvist, Vetelino Frida. "FADE STATISTICS FOR A LASERCOM SYSTEM AND THE JOINT PDF OF A GAMMA-GAMMA DISTRIBUTED IRRADIANCE AND ITS TIME DERIVATIVE." Doctoral diss., University of Central Florida, 2006. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3091.
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The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics for lasercom systems are determined from the probability density function (PDF) of the irradiance fluctuations. The expected number of fades per second and their mean fade time require the joint PDF of the fluctuating irradiance and its time derivative. Theoretical integral expressions, as well as closed form, analytical approximations, were developed for the joint PDF of a gamma-gamma distributed irradiance and its time derivative, and the corresponding expression for the expected number of fades per second. The new approximation for the conditional PDF of the time derivative of a gamma-gamma irradiance is a zero mean Gaussian distribution, with a complicated irradiance depending variance. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma-gamma distributions. A Gaussian beam wave was propagated through the atmosphere along a horizontal path, near ground, in the moderate-to-strong optical turbulence. To characterize the propagation path, a new method that infers atmospheric propagation parameters was developed. Scintillation theory combined with a numerical scheme was used to infer the structure constant, Cn2, the inner scale and the outer scale from the optical measurements. The inferred parameters were used in calculations for the theoretical PDFs. It was found that fade predictions made by the gamma-gamma and lognormal distributions provide an upper and lower bound, respectively, for the probability of fade and the number of fades per second for irradiance data collected in the moderate-to-strong fluctuation regime. Aperture averaging effects on the PDF of the irradiance fluctuations were investigated by comparing the irradiance distributions for the three receiver apertures at two different values of the structure parameter and, hence, different values of the coherence radius. For the moderate-to-strong fluctuation regime, the gamma-gamma distribution provides a good fit to the irradiance fluctuations collected by finite-sized apertures that are significantly smaller than the coherence radius. For apertures larger than or equal to the coherence radius, the irradiance fluctuations appear to be lognormally distributed.Ph.D.
Department of Mathematics
Sciences
Mathematics
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Krishnan, Vaidyanathan. "The nature of Turbulence in a Narrow Apex Angle Isosceles Triangular Duct." Doctoral diss., University of Central Florida, 2007. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3811.
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An experimental investigation was performed to ascertain the nature of turbulence in a narrow apex angle isosceles triangular duct. The study involved the design and construction of a low noise, low turbulence wind tunnel that had an isosceles triangular test section with an apex angle of 11.5[degrees]. Experiments involved the measurement of velocity fluctuations using hot wire anemometry and wall pressure fluctuations using a condenser microphone. Measurement of the velocity fluctuations reconfirms the coexistence of laminar and turbulent regions at a given cross section for a range of Reynolds numbers. The laminar region is concentrated closer to the apex while the turbulent region is found closer to the base. The point of transition is a function of the Reynolds number and moves closer to the apex as the flow rate is increased. Moreover, it was found in this investigation that traditional scaling of the turbulent statistical quantities do not hold good in this geometry. Although velocity fluctuations showed distinctive flow regimes, no such distinction could be seen in the dynamic wall pressure data. The nature of the dynamic wall pressure was uniform throughout the entire cross section suggesting that wall pressure fluctuations, unlike the velocity fluctuations, are able to travel from the base to the apex, without being damped. This implies that the relationship between the velocity and the pressure fluctuations applicable in the other systems does not hold well in a narrow apex angle isosceles triangular duct. Further, the typical scaling relationships applied to wall pressure spectra of other geometries doesn't apply in this scenario and the ratio of the RMS pressure fluctuation to the mean shear is much higher compared to a flat plate or pipe flow situation.Ph.D.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Mechanical Engineering PhD
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Wayne, David. "THE PDF OF IRRADIANCE FOR A FREE-SPACE OPTICAL COMMUNICATIONS CHANNEL: A PHYSICS BASED MODEL." Doctoral diss., University of Central Florida, 2010. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3812.
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An accurate PDF of irradiance for a FSO channel is important when designing a laser radar, active laser imaging, or a communications system to operate over the channel. Parameters such as detector threshold level, probability of detection, mean fade time, number of fades, BER, and SNR are derived from the PDF and determine the design constraints of the receiver, transmitter, and corresponding electronics. Current PDF models of irradiance, such as the Gamma-Gamma, do not fully capture the effect of aperture averaging; a reduction in scintillation as the diameter of the collecting optic is increased. The Gamma-Gamma PDF of irradiance is an attractive solution because the parameters of the distribution are derived strictly from atmospheric turbulence parameters; propagation path length, Cn2, l0, and L0. This dissertation describes a heuristic physics-based modeling technique to develop a new PDF of irradiance based upon the optical field. The goal of the new PDF is three-fold: capture the physics of the turbulent atmosphere, better describe aperture averaging effects, and relate parameters of the new model to measurable atmospheric parameters. The modeling decomposes the propagating electromagnetic field into a sum of independent random-amplitude spatial plane waves using an approximation to the Karhunen-Loeve expansion. The scattering effects of the turbulence along the propagation path define the random-amplitude of each component of the expansion. The resulting PDF of irradiance is a double finite sum containing a Bessel function. The newly developed PDF is a generalization of the Gamma-Gamma PDF, and reduces to such in the limit. An experiment was setup and performed to measure the PDF of irradiance for several receiver aperture sizes under moderate to strong turbulence conditions. The propagation path was instrumented with scintillometers and anemometers to characterize the turbulence conditions. The newly developed PDF model and the GG model were compared to histograms of the experimental data. The new PDF model was typically able to match the data as well or better than the GG model under conditions of moderate aperture averaging. The GG model fit the data better than the new PDF under conditions of significant aperture averaging. Due to a limiting scintillation index value of 3, the new PDF was not compared to the GG for point apertures under strong turbulence; a regime where the GG is known to fit data well.Ph.D.
School of Electrical Engineering and Computer Science
Engineering and Computer Science
Electrical Engineering PhD
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Gombeaud, Clément. "Thermalisation dans les collisions d’ions lourds ultrarelativistes." Paris 6, 2010. http://www.theses.fr/2010PA066179.
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Lors des collisions d'ions lourds au Relativistic Heavy Ion Collider (RHIC) de Brookhaven, les partons composant tous les nucléons mis en jeu interagissent. L'étude de ces interactions est un domaine important de la physique des particules. Jusqu'à présent, on utilisait le plus souvent le modèle hydrodynamique des fluides parfaits (non visqueux) pour rendre compte de l'évolution du système créé dans les collisions. Mais ce modèle est incapable de reproduire l'ensemble des observations expérimentales. Afin de mieux comprendre l'évolution du système créé dans une collision d'ions lourds, j'ai réalisé une résolution numérique de l'équation de Boltzmann à (2+1)dimensions afin de simuler la dynamique d'un gaz de particules relativistes. Nous avons ainsi pu montrer que l'introduction de déviations à l'état d'équilibre thermique local permet de comprendre certaines des mesures expérimentales que les calculs d'hydrodynamique idéale ne sont pas à même d'expliquer. L'étude de la dépendance en centralité du flot elliptique nous a permis d'évaluer le degré de thermalisation de (le nombre de Knudsen correspondant à) la matière créée dans les collisions centrales, Au-Au à sqrt s=200~GeV, au RHIC. Nous avons alors pu montrer que la prise en compte des effets de thermalisation partielle permettait d'expliquer le ``HBT Puzzle''. Néanmoins, la thermalisation partielle n'est pas, à elle seule, capable de faire s'accorder prédictions théoriques et données expérimentales. D'autres effets physiques semblent avoir été sous-estimés comme les fluctuations dans les conditions initiales dont on a montré qu'elles étaient un ingrédient clef dans l'étude du rapport {4}/(v_{2})^2.
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Deseigne, Julien. "Système expérimental modèle d'un fluide actif polaire." Phd thesis, Université Paris-Diderot - Paris VII, 2010. http://tel.archives-ouvertes.fr/tel-00567513.
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Les mouvements collectifs observés dans la nature, tels les nuées d'étourneaux ou les bancs de poissons, peuvent être décrits dans le cadre d'un nouveau type de matière condensée fondamentalement hors d'équilibre : la matière active polaire. Elle est constituée de particules, dites polaires, capables d'utiliser de l'énergie qu'on leur injecte pour se mouvoir dans une direction propre. Nous avons réalisé un système bidimensionnel de disques polaires soumis à une vibration homogène qui n'interagissent que par contact. Ces disques se comportent comme des marcheurs aléatoires, dont les trajectoires présentent une longueur de persistance grande devant leur taille et contrôlée par les fluctuations angulaires de la polarité des disques. Les modes d'alignement résultant du couplage entre la persistance du mouvement et l'interaction de coeur dur entre les particules sont complexes. En particulier, nous observons que seules 10% des collisions se traduisent par un alignement effectif de type ferromagnétique. Pourtant, nous observons l'émergence de mouvements collectifs spontanés au sein du système caractérisés par des fluctuations géantes de densité. Ces résultats montrent la robustesse de l'ordre polaire observé dans les modèles théoriques et numériques de la matière active polaire 2D sur substrat.
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Clément, David. "PROPRIETES STATIQUES ET DYNAMIQUES D'UN CONDENSAT DE BOSE-EINSTEIN DANS UN POTENTIEL ALEATOIRE." Phd thesis, Université Pierre et Marie Curie - Paris VI, 2007. http://tel.archives-ouvertes.fr/tel-00262463.
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Ce mémoire présente différents aspects des condensats de Bose-Einstein atomiques placés dans un potentiel aléatoire 1D. Nous décrivons en détail les propriétés et la caractérisation du potentiel optique aléatoire utilisé, issu d'un champ de tavelures. Nous abordons alors trois aspects des condensats désordonnés. En premier lieu, quelques propriétés des condensats désordonnés piégés sont caractérisées. En particulier, nous étudions le développement de modulations de densité sur le condensat au cours d'un temps de vol et nous montrons que leur origine n'est pas liée à des fluctuations de phase initiales. Dans un deuxième temps, nous décrivons des propriétés de transport d'un condensat dans le désordre. Nous avons observé la suppression de l'expansion de l'onde de matière dans un potentiel aléatoire 1D et développé un scénario de piégeage, où les interactions jouent un rôle central, permettant de comprendre ces observations. Nous présentons ensuite un travail théorique sur l'expansion du condensat dans un régime dit de faible désordre et nous montrons que le phénomène de localisation d'Anderson peut alors être à l'origine du piégeage de l'onde de matière. Enfin, dans une dernière partie nous abordons l'étude des modes collectifs d'un condensat désordonné. Des mesures sur l'absence de décalage en fréquence des modes dipolaires et quadrupolaires et l'amortissement du mode dipolaire en présence de désordre sont présentées. Enfin, nous mesurons la vitesse de propagation d'un pic de densité et nous en concluons que la vitesse du son n'est pas modifiée par la présence d'un potentiel aléatoire de faible amplitude.
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Geier, Kevin Thomas. "Probing Dynamics and Correlations in Cold-Atom Quantum Simulators." Doctoral thesis, Università degli studi di Trento, 2022. http://hdl.handle.net/11572/351120.
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Cold-atom quantum simulators offer unique possibilities to prepare, manipulate, and probe quantum many-body systems. However, despite the high level of control in modern experiments, not all observables of interest are easily accessible. This thesis aims at establishing protocols to measure currently elusive static and dynamic properties of quantum systems. The experimental feasibility of these schemes is illustrated by means of numerical simulations for relevant applications in many-body physics and quantum simulation. In particular, we introduce a general method for measuring dynamical correlations based on non-Hermitian linear response. This enables unbiased tests of the famous fluctuation-dissipation relation as a probe of thermalization in isolated quantum systems. Furthermore, we develop ancilla-based techniques for the measurement of currents and current correlations, permitting the characterization of strongly correlated quantum matter. Another application is geared towards revealing signatures of supersolidity in spin-orbit-coupled Bose gases by exciting the relevant Goldstone modes. Finally, we explore a scenario for quantum-simulating post-inflationary reheating dynamics by parametrically driving a Bose gas into the regime of universal far-from-equilibrium dynamics. The presented protocols also apply to other analog quantum simulation platforms and thus open up promising applications in the field of quantum science and technology.I simulatori quantistici ad atomi freddi offrono possibilità uniche per preparare, manipolare e sondare sistemi quantistici a molti corpi. Tuttavia, nonostante l'alto livello di controllo raggiunto negli esperimenti moderni, non tutte le osservabili di interesse sono facilmente accessibili. Lo scopo di questa tesi è quello di stabilire protocolli per misurare delle proprietà statiche e dinamiche dei sistemi quantistici attualmente inaccessibili. La fattibilità sperimentale di questi schemi è illustrata mediante simulazioni numeriche per applicazioni rilevanti nella fisica a molti corpi e nella simulazione quantistica. In particolare, introduciamo un metodo generale per misurare le correlazioni dinamiche basato su una risposta lineare non hermitiana. Ciò consente test imparziali della famosa relazione fluttuazione-dissipazione come sonda di termalizzazione in sistemi quantistici isolati. Inoltre, sviluppiamo tecniche basate su ancilla per la misura di correnti e correlazioni di corrente, consentendo la caratterizzazione della materia quantistica fortemente correlata. Un'altra applicazione è orientata a rivelare l'impronta della supersolidità nei gas Bose con accoppiamento spin-orbita eccitando il corrispondente modo di Goldstone. Infine, esploriamo uno scenario per la simulazione quantistica della dinamica di riscaldamento post-inflazione modulando parametricamente un gas Bose e portandolo nel regime della dinamica universale lontana dall'equilibrio. I protocolli presentati si applicano anche ad altre piattaforme di simulazione quantistica analogica e aprono quindi applicazioni promettenti nel campo della scienza e della tecnologia quantistica.
Quantensimulatoren auf Basis ultrakalter Atome eröffnen einzigartige Möglichkeiten zur Präparation, Manipulation und Untersuchung von Quanten-Vielteilchen-Systemen. Trotz des hohen Maßes an Kontrolle in modernen Experimenten sind jedoch nicht alle interessanten Observablen auf einfache Weise zugänglich. Ziel dieser Arbeit ist es, Protokolle zur Messung aktuell nur schwer erfassbarer statischer und dynamischer Eigenschaften von Quantensystemen zu etablieren. Die experimentelle Realisierbarkeit dieser Verfahren wird durch numerische Simulationen anhand relevanter Anwendungen in der Vielteilchenphysik und Quantensimulation veranschaulicht. Insbesondere wird eine allgemeine Methode zur Messung dynamischer Korrelationen basierend auf der linearen Antwort auf nicht-hermitesche Störungen vorgestellt. Diese ermöglicht unabhängige Tests des berühmten Fluktuations-Dissipations-Theorems als Indikator der Thermalisierung isolierter Quantensysteme. Darüber hinaus werden Verfahren zur Messung von Strömen und Strom-Korrelationen mittels Kopplung an einen Hilfszustand entwickelt, welche die Charakterisierung stark korrelierter Quantenmaterie erlauben. Eine weitere Anwendung zielt auf die Enthüllung spezifischer Merkmale von Supersolidität in Spin-Bahn-gekoppelten Bose-Einstein-Kondensaten ab, indem die relevanten Goldstone-Moden angeregt werden. Schließlich wird ein Szenario zur Quantensimulation post-inflationärer Thermalisierungsdynamik durch die parametrische Anregung eines Bose-Gases in das Regime universeller Dynamik fern des Gleichgewichts erschlossen. Die dargestellten Protokolle lassen sich auch auf andere Plattformen für analoge Quantensimulation übertragen und eröffnen damit vielversprechende Anwendungen auf dem Gebiet der Quantentechnologie.
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Rossi, Enrico MacDonald Allan H. "Fluctuations and dissipation of collective dynamics in spin and pseudospin ferromagnets." 2005. http://repositories.lib.utexas.edu/bitstream/handle/2152/2148/rossie90948.pdf.
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Rossi, Enrico. "Fluctuations and dissipation of collective dynamics in spin and pseudospin ferromagnets." Thesis, 2005. http://hdl.handle.net/2152/2148.
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Watnick, Paula Ivonne. "Cooperative properties of lipid bilayers : collective director fluctuations and the effects of hydrophobic mismatch in protein/lipid membrane systems." Thesis, 1989. https://thesis.library.caltech.edu/8065/1/Watnick%201989.pdf.
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Cooperative director fluctuations in lipid bilayers have been postulated for many years. ^2H-NMR T_1^(-1), T_(1P)^(-1) , and T_2^(-1); measurements have been used identify these motions and to determine the origin of increased slow bilayer motion upon addition of unlike lipids or proteins to a pure lipid bilayer.
The contribution of cooperative director fluctuations to NMR relaxation in lipid bilayers has been expressed mathematically using the approach of Doane et al.^1 and Pace and Chan.^2 The T_2^(-1)’s of pure dimyristoyllecithin (DML) bilayers deuterated at the 2, 9 and 10, and all positions on both lipid hydrocarbon chains have been measured. Several characteristics of these measurements indicate the presence of cooperative director fluctuations. First of all, T_2^(-1) exhibits a linear dependence on S2/CD. Secondly, T_2^(-1) varies across the ^2H-NMR powder pattern as sin^2 (2, β), where , β is the angle between the average bilayer director and the external magnetic field. Furthermore, these fluctuations are restricted near the lecithin head group suggesting that the head group does not participate in these motions but, rather, anchors the hydrocarbon chains in the bilayer.
T_2^(-1)has been measured for selectively deuterated liquid crystalline DML hilayers to which a host of other lipids and proteins have been added. The T_2^(-1) of the DML bilayer is found to increase drastically when chlorophyll a (chl a) and Gramicidin A' (GA') are added to the bilayer. Both these molecules interfere with the lecithin head group spacing in the bilayer. Molecules such as myristic acid, distearoyllecithin (DSL), phytol, and cholesterol, whose hydrocarbon regions are quite different from DML but which have small,neutral polar head groups, leave cooperative fluctuations in the DML bilayer unchanged.
The effect of chl a on cooperative fluctuations in the DML bilayer has been examined in detail using ^2H-NMR T_1^(-1), T_(1P)^(-1) , and T_2^(-1); measurements. Cooperative fluctuations have been modelled using the continuum theory of the nematic state of liquid crystals. Chl a is found to decrease both the correlation length and the elastic constants in the DML bilayer.
A mismatch between the hydrophobic length of a lipid bilayer and that of an added protein has also been found to change the cooperative properties of the lecithin bilayer. Hydrophobic mismatch has been studied in a series GA' / lecithin bilayers. The dependence of 2H-NMR order parameters and relaxation rates on GA' concentration has been measured in selectively deuterated DML, dipalmitoyllecithin (DPL), and DSL systems. Order parameters, cooperative lengths, and elastic constants of the DML bilayer are most disrupted by GA', while the DSL bilayer is the least perturbed by GA'. Thus, it is concluded that the hydrophobic length of GA' best matches that of the DSL bilayer. Preliminary Raman spectroscopy and Differential Scanning Calorimetry experiments of GA' /lecithin systems support this conclusion. Accommodation of hydrophobic mismatch is used to rationalize the absence of H_(II) phase formation in GA' /DML systems and the observation of H_(II) phase in GA' /DPL and GA' /DSL systems.
1. J. W. Doane and D. L. Johnson, Chem. Phy3. Lett., 6, 291-295 (1970). 2. R. J. Pace and S. I. Chan, J. Chem. Phy3., 16, 4217-4227 (1982).
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Brüning, Beate-Annette. "Collective Short Wavelength Dynamics in Phospholipid Model Membranes - with Inelastic Neutron Scattering." Doctoral thesis, 2008. http://hdl.handle.net/11858/00-1735-0000-0006-B492-D.
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KU, KENG-CHIA, and 古耿嘉. "Period Influence Maximization under Fluctuation Aware Collective Behavior." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/na4wjt.
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Fuentes, Miguel Andrés. "Microeconomic effects of exchange rate fluctuations /." 2004. http://www.gbv.de/dms/zbw/547388349.pdf.
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"Parallel field-induced universal conductance fluctuations in open quantum dots." Université catholique de Louvain, 2005. http://edoc.bib.ucl.ac.be:81/ETD-db/collection/available/BelnUcetd-03072005-122529/.
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