Submitted by Gustavo Caixeta (gucaixeta@gmail.com) on 2017-02-21T11:44:40Z No. of bitstreams: 1 texto completo.pdf: 2365238 bytes, checksum: a140cdba64721e32f2327df62fbcccb3 (MD5) / Made available in DSpace on 2017-02-21T11:44:40Z (GMT). No. of bitstreams: 1 texto completo.pdf: 2365238 bytes, checksum: a140cdba64721e32f2327df62fbcccb3 (MD5) Previous issue date: 2013-02-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES, Brasil / Neste trabalho estudamos alguns processos de desmagnetização de um gelo de spin artificial em geometria triangular através de dois processos utilizando campo magnético externo: O primeiro processo similar ao processo de histerese, campo magnético em uma única direção, e o segundo através de um campo magnético rotacional e decrescente. Para isto, utilizamos simulações computacionais Monte Carlo, considerando que os spins da rede eram tipo Ising e que estes seriam invertidos somente quando a condição −si.Bi ≤ bi fosse satisfeita. Como tentativa de aproximar nossas simulações dos experimentos feitos em laboratório, adaptamos os valores de nossas simulações para serem proporcionais aos valores obtidos em laboratório e consideramos uma desordem nos spins da rede, fazendo com que o valor do campo magnético mínimo para a inversão bi siga uma distribuição Gaussiana (bi → bc ± b'i ). Esta última consideração seria reflexo da dificuldade de construir os gelos de spins em laboratório, gerando várias ilhas diferentes em uma mesma rede. Além disto, consideramos uma outra desordem no processo similar ao de histerese: o desalinhamento do campo magnético externo com a rede devido suas pequenas dimensões. Os resultados de nosso trabalho foram obtidos através de rede quadradas com 30 espaçamentos de rede e as médias foram tomadas de 50 × 10 amostras (50 redes diferentes e 10 amostras de cada rede). Nestes resultados, percebemos que no processo similar ao de histerese, as redes foram desmagnetizadas e conseguiram alcançar o estado fundamental, mesmo com grandes desordens. Enquanto no processo de campo magnético rotacional percebemos que as redes eram desmagnetizadas mas nenhuma alcançava o estado fundamental. No processo rotacional, o estado fundamental era somente alcançado no interior das redes sem desordem. / In this work we studied some demagnetization protocols for an arti cial spin ice in a triangular lattice through two processes using external magnetic elds: The fi rst process is similar to the process of hysteresis, using a magnetic field in one direction, and the second through a rotational and decrasing magnetic field.For this, we used Monte Carlo computer simulations, considering that the lattice spins are fl ipped only when the condition −si·Bi≤bi is satis ed. In an attempt to relate our simulations to laboratory experiments, we adapted our simulations values to be proportional to the values obtained in laboratory and we have considered a disorder in the lattice's spins, in such a way that the minimum reversing magnetic field bi, follows a Gaussian distribution (bi→bc±b'i). This last consideration would be a re flection of the di fficulty of constructing the spin ices in the laboratory, generating several di erent islands in a lattice. Moreover, we considered another kind of disorder in the hysteresis-like process: the misalignment of the external magnetic field with the lattice due to its small dimensions. The results of our work were obtained through square lattices with 30 lattice spacings and averages were taken from 50 × 10 samples (50 dif erents lattices and 10 sample to each lattice). In these results, we realized that the process similar to hysteresis, the lattice were demagnetized and managed to reach the ground state, even with major disorders. While in the process of rotational magnetic field we realized that the lattice were demagnetized but none reached the ground state. In this rotational process, only the ground state was achieved in a small region of the lattice without the disorder.

Made available in DSpace on 2018-07-27T18:26:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-09-04. Added 1 bitstream(s) on 2018-07-27T18:30:19Z : No. of bitstreams: 1 000866848.pdf: 631406 bytes, checksum: 951f24cae0a92bf26987f68a9faaef16 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Nesta dissertação apresentamos uma descrição da dualidade elétrica-magnética e seus aspectos clássicos e quânticos. Nosso análise se inicia com os monopolos magéticos sugeridos por Dirac em 1931[1] e vai até o trabalho do Seiberg e Witten em 1994 [27]. Na descrição clássica, precisamos introduzir os monopolos magnéticos a fim de obter a dualidade elétrica-magnética manifesta. Mais tarde, a origem dos monopolos se mais torna mais clara quando começamos com uma teoria de Yang-Mills. Os aspectos clássicos da teoria foram explicados pela conjetura de Montonen e Olive 1977 [7]. Explorando os aspectos quânticos da teoria, notamos a importância de introduzir supersimetria, principalmente supersimetria estendida, onde tiramos vantagem da propiedade de holomorficidade, a qual nos leva aos teoremas não renormalizáveis, onde o cálculo é mais simples. Focamos na teoria de gauge supersimétrica N = 2 SU(2). A teoria é completamente resolvível para baixas energias. A maior parte do conteúdo deste trabalho é baseada nas várias revisões da dualidade de Seiberg-Witten [30],[31],[32] / In this dissertation we present a description of the electric-magnetic duality and their classical and quantum aspects. Our analysis starts from the suggested magnetic monopoles by Dirac in 1931 [1] and goes until the work of Seiberg and Witten in 1994 [27]. In the classical description, we need to introduce the magnetic monopoles in order to make manifest the electricmagnetic duality. Later, the origin of monopoles becomes clear when we start from a Yang-Mills theory. The classical aspects of the E-M duality are covered in the Montonen-Olive conjecture 1977[7]. Working on the quantum aspects of the theory, we note the importance of introducing supersymmetry. Specially for extended supersymmetry, where we take advantage of the holomorphicity property, which leads us to the non-renormalizable theorems, where the computation is easier. We focus on theN = 2SU(2) supersymmetric gauge theories. It turns out that the theory is fully solvable at the low energies regime[27]. Most of this work is based on reviews about the Seiberg and Witten duality [30],[31],[32]

Orientador: Nathan Jacob Berkovits / Banca: Horatiu Stefan Nastase / Banca: Diego Trancanelli / Resumo: Nesta dissertação apresentamos uma descrição da dualidade elétrica-magnética e seus aspectos clássicos e quânticos. Nosso análise se inicia com os monopolos magéticos sugeridos por Dirac em 1931[1] e vai até o trabalho do Seiberg e Witten em 1994 [27]. Na descrição clássica, precisamos introduzir os monopolos magnéticos a fim de obter a dualidade elétrica-magnética manifesta. Mais tarde, a origem dos monopolos se mais torna mais clara quando começamos com uma teoria de Yang-Mills. Os aspectos clássicos da teoria foram explicados pela conjetura de Montonen e Olive 1977 [7]. Explorando os aspectos quânticos da teoria, notamos a importância de introduzir supersimetria, principalmente supersimetria estendida, onde tiramos vantagem da propiedade de holomorficidade, a qual nos leva aos teoremas não renormalizáveis, onde o cálculo é mais simples. Focamos na teoria de gauge supersimétrica N = 2 SU(2). A teoria é completamente resolvível para baixas energias. A maior parte do conteúdo deste trabalho é baseada nas várias revisões da dualidade de Seiberg-Witten [30],[31],[32] / Abstract: In this dissertation we present a description of the electric-magnetic duality and their classical and quantum aspects. Our analysis starts from the suggested magnetic monopoles by Dirac in 1931 [1] and goes until the work of Seiberg and Witten in 1994 [27]. In the classical description, we need to introduce the magnetic monopoles in order to make manifest the electricmagnetic duality. Later, the origin of monopoles becomes clear when we start from a Yang-Mills theory. The classical aspects of the E-M duality are covered in the Montonen-Olive conjecture 1977[7]. Working on the quantum aspects of the theory, we note the importance of introducing supersymmetry. Specially for extended supersymmetry, where we take advantage of the holomorphicity property, which leads us to the non-renormalizable theorems, where the computation is easier. We focus on theN = 2SU(2) supersymmetric gauge theories. It turns out that the theory is fully solvable at the low energies regime[27]. Most of this work is based on reviews about the Seiberg and Witten duality [30],[31],[32] / Mestre

abstract: In this dissertation, I present the results from my recent investigations into the interactions involving topological defects, such as magnetic monopoles and strings, that may have been produced in the early universe. I performed numerical studies on the interactions of twisted monopole-antimonopole pairs in the 't Hooft-Polyakov model for a range of values of the scalar to vector mass ratio. Sphaleron solution predicted by Taubes was recovered, and I mapped out its energy and size as functions of parameters. I also looked into the production, and decay modes of $U(1)$ gauge and global strings. I demonstrated that strings can be produced upon evolution of gauge wavepackets defined within a certain region of parameter space. The numerical exploration of the decay modes of cosmic string loops led to the conclusions that string loops emit particle radiation primarily due to kink collisions, and that their decay time due to these losses is proportional to $L^p$, where $L$ is the loop length and $p \approx 2$. In contrast, the decay time due to gravitational radiation scales in proportion to $L$, and I concluded that particle emission is the primary energy loss mechanism for loops smaller than a critical length scale, while gravitational losses dominate for larger loops. In addition, I analyzed the decay of cosmic global string loops due to radiation of Goldstone bosons and massive scalar ($\chi$) particles. The length of loops I studied ranges from 200-1000 times the width of the string core. I found that the lifetime of a loop is approximately $1.4L$. The energy spectrum of Goldstone boson radiation has a $k^{-1}$ fall off, where $k$ is the wavenumber, and a sharp peak at $k\approx m_\chi/2$, where $m_\chi$ is the mass of $\chi$. The latter is a new feature and implies a peak at high energies (MeV-GeV) in the cosmological distribution of QCD axions. / Dissertation/Thesis / Doctoral Dissertation Physics 2020

Dans un contexte où l'utilisation d'un antibiotique provoque une hausse de la résistance bactérienne, ce mémoire évalue théoriquement et grâce à la simulation numérique, de quelle façon un monopoleur maximize son profit, à la suite de l'obtention d'un brevet. Du point de vue théorique, le monopoleur souhaite d'une part maximiser son profit à chaque instant et de l'autre maximiser son profit tout au long de la durée de vie de son brevet. À l'aide de la simulation numérique, la valorisation faite par le monopoleur de son antibiotique s'avère être le point central de l'analyse, de sorte que si la valorisation faite par le monopoleur est élevée, le monopoleur conserve l'efficacité de son antibiotique en diminuant la quantité produite. Dans le cas contraire, le monopoleur produira une plus grande quantité et conservera une moins grande efficacité de son antibiotique.

A way to obtain materials that show novel phenomena is to explore the interplay between geometry and interactions. When it is not geometrically possible to satisfy all the interactions by a given configuration, then to find the ground state becomes very complicated. This interplay between geometry and interactions defines geometrical frustration. One of the most popular examples of geometrical frustration in magnetism is spin ice. In this system, nearest neighbour ferromagnetic interactions between Ising spins in a pyrochlore structure emulate water ice by showing the same degree of frustration. This is manifested by the same ground state residual entropy. Although the clearest example of spin ice among magnets is shown by Dy₂Ti₂O₇, the behaviour of this material is richer than that of pure spin ice. The large magnetic moments of the rare earth Dy form a spin ice that also interacts via dipolar interactions. These long range interactions give rise to monopolar excitations which dramatically affect the dynamics of the system with respect to the pure spin ice case. In this thesis magnetisation experiments and numerical methods are used to explore the properties of the magnetic insulator Dy₂Ti₂O₇. We study its excitations at low temperature and describe the out-of-equilibrium characteristics of the magnetisation processes, below a temperature where the system freezes out. For temperatures above the freezing temperature, we describe and measure a 3D Kasteleyn transition and the concomitant Dirac strings associated to it, for the field in the [100] crystallographic direction. For temperatures below the freezing temperature, we find new out-of-equilibrium phenomena. Magnetic jumps are measured and their sweep rate dependence analysed. A deflagration theory is proposed and supported by simultaneous magnetisation and sample temperature measurements obtained by a new design of a Faraday magnetometer.

This thesis is concerned with solitons (solutions of certain nonlinear partial differential equations) in certain cases when the underlying space is either a lattice or curved. Chapter 2 of the thesis is concerned with the outcome of collisions between a kink (a 1-dimensional soliton) and an antikink for certain topological discrete (TD) systems. The systems considered are the TD sine-Gordon and the TD ø(^4) For the TD sine-Gordon system it is found that the kink can support an internal shape mode which plays an important role during the collisions. In particular, this mode can be excited during collisions and this leads to spectacular resonance effects. The outcome of any particular collision has sensitive dependence on the initial conditions and could be either a trapped kink-antikink state, a "reflection" or a "transmission”. Such resonance effects are already known to exist for the conventional discrete ø(^4) system, and the TD ø(^4) system is no different, though the results for the two are not entirely similar. Chapter 3 considers the question of the existence of explicit travelling kink solutions for lattice systems. In particular, an expression for such a solution for the integrable lattice sine-Gordon system is derived. In Chapter 4, by reducing the Yang-Mills equations on the (2 + 2)-dimensional ultrahyperbolic space-time, an integrable Yang-Mills-Higgs system on (2 + 1) dimensional de Sitter space-time is derived. It represents the curved space-time version of the Bogomolny equations for monopoles on R(^3) . Using twister methods, various explicit solutions with gauge groups U(l) and SU(2) are constructed. A multi-solution SU(2) solution is also presented.

Cette thèse explore l’histoire des entreprises coopératives dans l’État du Wisconsin entre les années 1870 et les années 1930 et les manières dont différents groupes d’acteurs les ont mobilisées afin de défendre leurs intérêts. En croisant les questionnements de l’histoire des entreprises et ceux de l’histoire politique, elle étudie le recours aux coopératives à travers trois grands moments de réforme dans l’histoire des États-Unis – les luttes antimonopolistes du Gilded Age, le progressisme et le New Deal. Ce travail montre que les coopératives font partie de l’horizon réformateur américain de manière continue dans ces décennies. Bien que les coopérateurs ne parviennent pas à construire un « mouvement », ou un « système » économique qui permettrait de servir d’alternative au capitalisme industriel dominé par les grandes entreprises, ils dessinent avec succès les contours d’une forme d’entreprise différente, plus démocratique et dont le modèle est relativement bien identifié à la fin des années 1930. Afin de comprendre les mécanismes d’institutionnalisation de cette forme d’entreprise, on observe les différentes manières dont elle est mobilisée et appropriée au cours du temps – en premier lieu par les fermiers, mais également par les consommateurs, et dans une moindre mesure par les ouvriers. Le rôle de l’État, dont les agents sollicitent les coopératives et en font des outils de politique publique, fait également l’objet d’une attention particulière. L’analyse de ces mobilisations est constamment replacée dans l’environnement au sein duquel les coopérateurs évoluent : en mesurant l’écart qui distingue les coopératives des formes d’organisation économique contre lesquelles elles se forment, celles qui leur servent de référence, ou celles avec lesquelles elles sont en concurrence. Penser les coopératives dans leur environnement signifie également interroger l’importance des territoires dans lesquelles elles s’inscrivent, et en particulier la spécificité des zones rurales, lieu privilégié du développement des coopératives américaines à cette période. / This dissertation explores the history of cooperative businesses in the state of Wisconsin between the 1870s and the 1930s, and the ways various groups resorted to those organizations in order to protect their interests. My study involves both business history and political history, and looks at the uses of cooperatives through three important reform movements in US history: the antimonopolist struggles of the Gilded Age, the Progressive era and the New Deal. I show that cooperatives were continuously part of the American reform horizon in those decades. Although cooperators did not succeed in creating a “movement” or an economic “system” that would serve as an alternative for an industrial capitalism dominated by big corporations, they were able to build a specific and more democratic form of business enterprise, whose model was easy to identify at the end of the 1930s. In order to understand the mechanisms that led to the institutionalization of this form of business, I observe the various ways farmers, consumers, and, to a lesser extent, workers used and understood cooperatives over time. The role of the state, whose agents called upon cooperatives and turned them into public policy tools, is also a subject of inquiry. The analysis of the uses of the cooperative form of business is constantly replaced in the environment in which their members operated, especially by measuring the distance that separated them from the economic organizations against which they were formed, those that were their models, or those against which they competed. Understanding cooperatives in their environment also means questioning the importance of the territories where they were created, and more specifically the characteristics of rural areas, where cooperatives were especially successful.

As improving the performance of turbofan designs becomes increasingly difficult, manufacturers are looking to new technologies for the next generation of jet engines. An 'open rotor' replaces the fan of the turbofan with a set of external rotors. This has the potential to offer a significant improvement in propulsive efficiency, but the design for low noise is a key challenge. Hence, high fidelity noise prediction methods are needed to accurately predict and compare the noise of different designs. This thesis focuses on one set of methods based on the Ffowcs Williams-Hawkings (\fwh) equation. This equation is considered to be the most realistic description of aeroacoustic noise generation, as it is a direct rearrangement of the Navier-Stokes equations. The \fwh\ equation is difficult to solve for realistic test cases such as an open rotor, and is susceptible to several types of error. This thesis categorises these errors as ``input'', ``neglection'' and ``discretisation'' errors. Discretisation errors arise from the need to integrate a discretised source field for the total noise, neglection errors result from needing to ignore part of the source field for practical reasons, and input errors relate to any errors caused by inaccurate input to the solver. The fundamental motivation of this thesis is to advance the understanding of neglection and discretisation errors and how they can be mitigated, in order to develop best practice solvers and methodologies for application to open rotors. Dimensional analysis is combined with analytical flow solutions to develop a process for isolating and quantifying discretisation errors. This process is used to study a wide range of solver methodologies and select a best practice solver methodology for open rotor noise prediction. This first-of-a-kind study produces a solver methodology that reduces discretisation errors by an order of magnitude compared to an industry standard solver. Previous research into neglection errors has shown that avoiding density perturbations in acoustic source terms can be beneficial. This thesis uses a generic aeroacoustic analogy to provide a new, physically intuitive method of incorporating a surface discontinuity that enables density perturbations to be avoided in a far more elegant manner than previous research. The above method improvements are investigated using a modern open rotor rig test case. The results demonstrate that discretisation and neglection errors can be severe in realistic cases and the potential of the method improvements to significantly mitigate them.

Made available in DSpace on 2015-03-26T13:35:18Z (GMT). No. of bitstreams: 1 texto completo.pdf: 2056393 bytes, checksum: 6c718958e16dde37636b88ed5dbf583f (MD5) Previous issue date: 2012-02-14 / Fundação de Amparo a Pesquisa do Estado de Minas Gerais / We study the magnetic excitations in two-dimensional spin ices on square lattices, by means of computer simulations. These materials are artificially produced by lithographic techniques, using ferromegnetic nano-islands arranged in a lattice especially organized in such a way, that, the lattice geometry together with the dipole-dipole interactions lead to an intrinsic frustration of the system. Our analysis, based only on the dipolar interaction between the islands, correctly reproduces the ground state experimentally observed recently. In addition, we verify the emergence of magnetic excitations that behave like magnetic monopoles, carrying isolated magnetic charge. These quasi-particles interact by means of the Coulomb potential plus a linear confining potential, which is attributed to the emergence of string-like excitations, linking the pair of opposite charges. However, we argue that the string has a configurational entropy, so that the string tension may decrease as the temperature is increased and then, free magnetic monopoles should be found in two-dimensional spin ices at a critical temperature. We also study the case where the artificial spin ice is built with missing nano-island (vacancy). For the case with only one vacancy, we found that the defect modify the potential obtained for the normal lattice. The vacancy behaves like a pair of opposite charges, carrying half charge of the usual magnetic monopole excitation. Our calculations show that when the smallest distance between the charges (or the string) is greater than 2a (a is the lattice spacing), the interaction of the string with the defect is negligible. However, for distances smaller than 2a, the string interacts with the defect more appreciable, in a very complicated way, which depends on the shape and size of the string. Our results indicate that, on each vertex where the string passes, there is an additional quadrupole moment and so the field created by the string decays very quickly. / Nós estudamos as excitações magnéticas que surgem nos gelos de spin bidimensionais, por meio de simulações computacionais em redes quadradas. Esses materiais são produzidos artificialmente, através de técnicas litográficas utilizando nanopartículas ferromagnéticas arranjadas em uma rede especialmente organizada, de modo que a geometria da rede juntamente com a interação dipolo-dipolo leva a uma frustração intríseca do sistema. A análise, baseada somente na interação dipolar entre as nanoilhas, reproduz corretamente o estado fundamental recentemente observado experimentalmente. Foi verificado a emergência de excitações magnéticas que se comportam como monopolo magnético, carregando carga magnética isolada. Essas quasi-partículas interagem via potencial coulombiano adicionado a um potencial confinante linear, atribuído ao aparecimento de excitações do tipo corda, que conectam o par monopolo-antimonopolo. Contudo, verificamos que a corda possui uma entropia configuracional, de modo que a tensão da corda pode diminuir à medida em que a temperatura é aumentada, fazendo com que os monopolos magnéticos possam ser encontrados livres nos gelos de spin bidimensionais em uma temperatura crítica. Também estudamos o caso em que o gelo de spin é construído com uma única ilha faltante em uma dada posição da rede. Foi visto que o defeito pertuba o potencial obtido em uma rede normal. A vacância se comporta como um par de cargas opostas carregando metade da carga do monopolo magnético usual. Nossos cálculos mostram que quando a menor distância entre uma carga e a vacância é maior que 2a (a é o espaçamento de rede), a interação entre a corda e o defeito é negligenciável. Contudo, para distâncias menores que 2a, a corda interage mais apreciavelmente com o defeito, de uma forma muito complicada que depende de seu tamanho e forma. Os resultados indicam que em cada vértice por onde a corda passa, existe um momento de quadrupolo magnético adicional, de forma que o campo criado pela corda decai muito rapidamente.