Die vorliegende Dissertationarbeit ist dem Problem der analytischen Beschreibung des Confinement-Mechanismus in der QCD und in anderen Eichtheorien gewidment. Als Leitprinzip der Arbeit wurde das sogenannte Wilsonsche-Confinement-Kriterium gewählt, gemäss welchem diese Erscheinung durch eine effektive Stringtheorie beschrieben werden kann. Die entstehenden Strings des Eichfeldes verbinden farbige-Objekte (Quarks, Gluonen) miteinander und hindern ihr Auseinandergehen auf makroskopische Abstände. Es werden verschiedene Verfahren der Ableitung dieser Stringstheorien aus unterschiedlichen Eichtheorien, einschliesslich der QCD, vorgestellt. Kapitel 2 enthält die Untersuchung der nichtlokalen effektiven Stringwirkung, die im Rahmen des sogenannten stochastischen Vakuum-Modells der QCD entsteht, wobei die Wechselwirkung zwischen den Elementen der String-Weltfläche durch den phänomenologischen Background-Gluon-Propagator vermittelt wird. Durch Entwicklung dieser Wirkung nach Ableitungen wurden die ersten Terme niedrigster Ordnung bestimmt. Die ersten beiden Terme dieser Entwicklung sind die Nambu-Goto- und Rigidity-Terme mit Kopplungskonstanten, die sich durch das Gluon-Kondensat und die Korrelationlänge des QCD-Vakuums ausdrücken lassen. Die Vorzeichen dieser Konstanten zeigen, dass die durch dieses Verfahren erhaltenen Strings stabil sind. Danach wurde eine mögliche Lösung des ``Crumpling'' Problems auf der Basis eines zusätzlichen topologischen Stringtermes im Instantongas-Modell des QCD-Vakuums vorgestellt. Mittels Störungstheorie im nicht-störungstheoretischen QCD-Background berechneten wir zusätzliche-Korrekturen zur ursprünglichen nicht-störungstheoretischen Stringwirkung. Diese Korrekturen führen zu neuen Formen der nichtlokalen effektiven Stringwirkung, die den störungstheoretischen Gluon-Propagator im Backgroundfeld zwischen den Elementen der Weltfläche enthalten. Durch Ableitungsentwicklung dieser Wirkung bekommen wir eine Korrektur zur Kopplungskonstante des Rigidity-Terms; die Stringsspannung des Nambu-Goto-Terms jedoch bleibt unverändert. Am Ende dieses Kapitels wurde der Hamilton-Operator des QCD-Strings mit spinlosen Quarks hergeleitet, der der effektiven Stringwirkung mit Rigidity-Term entspricht. Dieser Hamilton-Operator liefert einen Korrekturterm zur Wechselwirkung im relativistischen Quarkmodell-Operator. Im Kapitel 3 untersuchten wir das Problem der Stringdarstellung von Abelsch-projezierten Eichtheorien. Als erstes wurde die Herleitung der Stringdarstellung der erzeugenden Funktion für das einfachste Modell dieser Art, d.h. die Abelsch-projezierte SU(2)-QCD gegeben, die einem dualen Abelschen Higgs-Modell mit äusseren elektrisch geladendenen Teilchen äquivalent ist. Der Vorteil dieses Stringszuganges im Vergleich zum Zugang des stochastischen Vakuum-Modells der QCD besteht in der Berücksichtigung der Integration über String-Weltflächen, die auf Grund der Integration über den Singulärteil der Higgsfeld-Phase entsteht. Zusätzlich zur Stringdarstellung der erzeugenden Funktion wurde im London-Limes die Stringdarstellung für die erzeugenden Funktionale der Feldstärke- und Monopolstromkorrelatoren hergeleitet. Dies gab uns die Möglichkeit, die entsprechenden bilokalen Kumulanten zu finden und zu zeigen, dass die bilokalen Kumulanten der Feldstärke für grosse Abstände das gleiche Verhalten wie die entsprechenden eichinvarianten Kumulanten der QCD zeigen. Das Letztere wurde durch das stochastische Vakuum-Modell vorhergesagt und durch Gitterexperimente berechnet. Dieses Ergebnis unterstützt einerseits die Methode der Abelschen Projektion und gibt anderseits dem stochastischen Vakuum-Modell der QCD einen neuen feldtheoretischen Status. Danach erweiterten wir unsere Analyse über den Rahmen des London-Limes hinaus untersuchten den Zusammenhang von quartischen Kumulanten und bilokalen Kumulanten. Anschliessend wurde die Stringdarstellung der SU(3)-Gluodynamik hergeleitet. Dabei wurde die Stringdarstellung für ein entsprechendes duales Modell formuliert, das drei Arten des magnetischen Higgs-Feldes enthält. Infolgedessen liefert das Modell drei Strings, von denen nur zwei wirklich unabhängig sind. Alle diese Strings wechselwirken untereinander durch Austausch zweier massiver dualer Eichbosonen. Ausserdem erhielten wir die bilokalen Kumulanten des effektiven dualen Modells der SU(3)-Gluodynamik. Die entsprechenden bilokalen Kumulanten zeigen für grosse Abstände ein Verhalten wie es durch das stochastische Vakuum-Modell vorhergesagt wurde. Zum Schluss dieses Kapitels geben wir eine nützliche Darstellung für erzeugende Funktionen von Abelsch-projezierten Theorien in Form von Integralen über Monopolströme an. Im Kapitel 4 wurde ein weiteres Modell untersucht, das eine analytische Beschreibung des Confinement-Mechanismus zulässt, nämlich die 3D kompakte QED. Für den Wilson-Loop der entsprechenden Theorie mit Monopoldichten wurde die Äquivalenz zur sogenannten Confining-Stringtheorie demonstriert. Ausserdem wurde das Verhalten der bilokalen Kumulante der Feldstärke im Limes schwacher Felder untersucht. Dieses Verhalten befindet sich ebenfalls in Übereinstimmung mit den Voraussagen des stochastischen Vakuum-Modells. Erwartungsgemäss sind die Stringdarstellungen der erzeugenden Funktionen der 3D kompakten QED im Limes schwacher Felder und der dualen Abelschen Higgs-Modelle sehr ähnlich. Wir zeigten ausserdem, dass diese Entsprechung nicht zufällig ist. Die 3D kompakte QED ergibt sich nämlich im Limes verschwindender Eichbosonmasse aus dem 3D Abelschen Higgs-Modell mit äusseren Monopolen. Zum Schluss wurde ein allgemeines Verfahren der Beschreibung der Anregungen der Stringweltfläche in Abelsch-projezierten Theorien (kompakte QED und QCD) ausgearbeitet. Es ist auf der Methode der nicht-linearen Sigma-Modelle gegründet und gibt eine Möglichkeit, die in diesen Fluktuationen quadratische Effektive Wirkung zu erhalten. In der Dissertation wurden analytische nicht-störungstheoretische Verfahren ausgearbeitet, die neue Informationen über den Confinement-Mechanismus in der QCD und anderen Eichtheorien liefern und zum besseren Verständnis der Vakuumstruktur dieser Theorien beitragen können. Sie sind insbesondere relevant für die Herleitung effektiver Stringtheorien aus Eichtheorien. / The main problem addressed in the present Dissertation was an attempt of an analytical description of confinement in QCD and other gauge theories. As a guiding principle for our investigations served the so-called Wilson's picture of confinement, according to which this phenomenon can be described in terms of some effective theory of strings, joining coloured objects to each other and preventing them from moving apart to macroscopic distances. In this Dissertation, we have proceeded with a derivation of such string theories corresponding to various gauge ones, including QCD, i.e. with the solution of the problem of string representation of gauge theories. We have started our analysis with the nonlocal string effective action, arising within the so-called Stochastic Vacuum Model of QCD, where the interaction between the string world-sheet elements is mediated by the phenomenological background gluon propagator. By performing the derivative expansion of this action, we have derived the first few terms of a string Lagrangian. The first two nontrivial of them turned out to be the Nambu-Goto and rigidity terms with the coupling constants expressed completely via the gluonic condensate and correlation length of the QCD vacuum. The signs of these constants ensure the stability of strings in the so-obtained effective string theory. After that, we have investigated the problem of crumpling for the string world-sheets by derivation of the topological string term in the instanton gas model of the gluodynamics vacuum. Next, by making use of perturbation theory in the nonperturbative QCD vacuum, we have calculated perturbative corrections to the obtained string effective action. Those lead to a new form of the nonlocal string effective action with the propagator between the elements of the world-sheet being the one of a perturbative gluon in the confining background. By the derivative expansion of this action, we got a correction to the rigidity term coupling constant, whereas the string tension of the Nambu-Goto term occurs to get no corrections due to perturbative gluonic exchanges. Finally, we have derived the Hamiltonian of QCD string with spinless quarks at the ends, associated with the obtained string effective action including the rigidity term. In the particular case of vanishing orbital momentum of the system, this Hamiltonian reduces to that of the so-called relativistic quark model, albeit with some modifications due to the rigidity term, which might have some influence on the dynamics of the QCD string with quarks. All these topics have been elaborated on in Section 2, and form the essence of the string representation of QCD within the Stochastic Vacuum Model. In Section 3, we have addressed the problem of string representation of Abelian-projected theories. In this way, we have started with the string representation for the partition function of the simplest model of this kind, namely the Abelian-projected SU(2)-QCD, which is argued to be the dual Abelian Higgs Model with external electrically charged particles. The advantage of this approach to the string representation of QCD w.r.t. the one based on the Stochastic Vacuum Model is a possibility to get an integration over the string world-sheets, resulting from the integration over the singular part of the phase of the Higgs field. After the string representation of the partition function in the London limit, we have proceeded with the string representation for the generating functionals of the field strength and monopole current correlators. Those enabled us to find the corresponding bilocal cumulants and demonstrate that the large-distance asymptotic behaviour of the bilocal field strength cumulant matches the one of the corresponding gauge-invariant cumulant in QCD, predicted by the Stochastic Vacuum Model and measured in the lattice experiments. This result supports the method of Abelian projection on the one hand and gives a new field-theoretical status to the Stochastic Vacuum Model on the other hand. After that, we have extended our analysis beyond the London limit, and studied the relation of the quartic cumulant, which appears there, with the bilocal one in the London limit. Next, by making use of the Abelian projection method, we have addressed the problem of string representation of the SU(3)-gluodynamics. Namely, we have casted the related dual model, containing three types of magnetic Higgs fields, into the string form. Consequently, the latter one turned out to contain three types of strings, among which only two ones were actually independent. As a result, we have found, that both the ensemble of strings as a whole and individual strings display confining properties in a sense that all types of strings (self)interact via the exchanges of the massive dual gauge bosons. We have also derived bilocal cumulants in the effective dual model of confinement, corresponding to the SU(3)-gluodynamics, and they turned out to be also in line with the ones predicted by the Stochastic Vacuum Model. In conclusion of this topic, we have obtained another useful representation for the partition functions of the Abelian-projected theories in the form of an integral over the monopole currents. In Section 4, we have studied another model, allowing for an analytical description of confinement, which is 3D compact QED. In this way, by virtue of the integral over the monopole densities, we have derived string representation for the Wilson loop in this theory and demonstrated the correspondence of this representation to another recently found one, the so-called confining string theory. After that, we have calculated the bilocal cumulant of the field strength tensors in the weak-field limit of the model under study. It also turned out to be in line with the general concepts of the Stochastic Vacuum Model and therefore matches the corresponding results known from the lattice measurements in QCD and found analytically for the effective Abelian-projected theories in the previous Section. Besides that, string representations of the partition functions of the weak-field limit of 3D compact QED and of the dual Abelian Higgs Model turned out to be also quite similar. We have illustrated later on that this correspondence is not accidental. Namely, we have shown that 3D compact QED is nothing else, but the limiting case of 3D Abelian Higgs Model with external monopoles, corresponding to the vanishing gauge boson mass. Finally, we have elaborated on a unified method of description of the string world-sheet excitations in the Abelian-projected theories, compact QED, and QCD, based on the techniques of nonlinear sigma-models, and obtained the effective action, quadratic in the world-sheet fluctuations. In conclusion, the proposed nonperturbative techniques provide us with some new information on the mechanisms of confinement in QCD and other gauge theories and shed some light on the vacuum structure of these theories. They also show the relevance of string theory to the description of this phenomenon and yield several prescriptions for the construction of the adequate string theories from the corresponding gauge ones.

Les glaces de spin, comme Dy2Ti2O7 et Ho2Ti2O7, sont des matériaux présentant un magnétisme particulièrement exotique. Ils constituent les premiers composés cristallins ferromagnétiques frustrés à avoir été découverts. Cette frustration permet la fractionnalisation des degrés de liberté de spin et l’émergence de monopôles magné-tiques, dont la physique est formalisée par le modèle des haltères.Dans cette thèse, nous étudions dans un premier temps le diagramme de phase de ce modèle grâce à un parallèle avec le modèle de Blume-Capel S = 2. On identifie dans ce diagramme la phase fragmentée observée expérimentalement dans Ho2Ir2O7,et on localise le point critique de la transition entre la phase glace de spin et la phase fragmentée.Dans un second temps, on montre numériquement que la dynamique du système autour de ce point critique appartient à la classe d’universalité du modèle d’Ising 3D. On utilise pour cela deux outils : les lois d’échelle de Kibble-Zurek et le rapport de fluctuation-dissipation. L’obtention de ce dernier a nécessité l’introduction d’une méthode novatrice pour le calcul des fonctions de réponse. Nous soulignons également que ces outils sont spécifiquement intéressants dans le cas des glaces de spin où les temps microscopiques sont de l’ordre de 1 μs, rendant le ralentissement critique observable expérimentalement.Dans un troisième temps, nous employons à nouveau la violation du théorème de fluctuation-dissipation pour caractériser un régime fortement hors équilibre de la phase glace de spin, où les degrés de liberté sont cinétiquement bloqués du fait de l’attraction coulombienne entre les monopôles. / Spin ices, such as Dy2Ti2O7 and Ho2Ti2O7, are materials exhibiting exotic magnetic properties. They were the first frustrated ferromagnetic crystalline compounds to be discovered. The frustration leads to the fractionnalisation of the spin degrees of freedom and the emergence of magnetic monopoles, whose physics is formalised in the dumbbell model. In this thesis, we study the full phase diagram of this model in analogy with theS=2 Blume-Capel model. We identify in this diagram the fragmented phase observed experimentally in Ho2Ir2O7, and we localise the critical point of the transition between the spin ice phase and the fragmented phase.In a second part, we show numerically that the dynamics of this system at thecritical point belongs to the 3D Ising university class. We use for this two tools :the Kibble-Zurek scaling law and the fluctuation-dissipation ratio. For the latter, ithas been necessary to introduce a novel method to measure response functions. Wealso emphasize that these tools are specifically interesting for spin ice materials, as the unusually long microscopic time scale (1 μs) should make it possible to experimentallyobserve out-of-equilibrium phenomena related to critical slowing down.In a third part, we use the violation of the fluctuation-dissipation theorem to characterise a strongly out-of-equilibrium regime of spin ice - a thermal quench from high to low temperature, where degrees of freedom are kinetically blocked because ofthe Coulombic attraction between the monopoles.

Made available in DSpace on 2015-03-26T13:35:24Z (GMT). No. of bitstreams: 1 texto completo.pdf: 663701 bytes, checksum: 07cd6e5521c259dcb98087edefcf3bf3 (MD5) Previous issue date: 2007-07-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / We study magnetic monopoles in a Lorentz- and CPT-odd electrodynamical framework in (3+1) dimensions. This is the standard Maxwell model extended by means of a Chern-Simons-like term, bμF μνAv, which respects gauge but violates both Lorentz and CPT symmetries (as a consequence, duality is also lost). Our main interest concerns the analysis of the model in the presence of Dirac monopoles, so that the Bianchi identity no longer holds, which naively yields the non-conservation of electric charge. Since gauge symmetry is respected, the issue of charge conservation is more involved. Actually, the inconsistency may be circumvented, if we assume that the appearance of a monopole induces an extra electric current. The reduction of the model to (2+1) dimensions in the presence of both the magnetic sources and Lorentz-violating terms is presented. There, a quantization condition involving the scalar remnant of bμ, say, the mass parameter, is obtained. We also point out that the breaking of duality may be associated with an asymmetry between electric and magnetic sources in this background, so that the electromagnetic force experienced by a magnetic pole is supplemented by an extra term proportional to bμ, whenever compared to the one acting on an electric charge. / Nesta dissertação estudamos monopolos magnéticos numa estrutura de uma Eletrodinâmica que violam as simetrias de Lorentz e CPT, em (3+1)D. Este é o modelo de Maxwell estendido por um termo do tipo Chern-Simons, bμ.FμνAv, que respeita a transformação de calibre mas viola as simetrias de Lorentz e CPT (como conseqüência, a dualidade é perdida). O interesse principal concentra na análise do modelo na presença de monopolos magnéticos tipo-Dirac, de modo que a identidade de Bianchi seja quebrada, que nos resulta na aparente não-conservação da carga elétrica. A inconsistência pode ser contornada considerando que na presença de monopolos "carregam" com eles uma corrente extra. Uma redução dimensional do modelo para (2+1)D é considerado na presença de ambas as fontes, magnéticas e elétricas, e com o termo de violação da simetria de Lorentz e CPT, onde uma condição da quantização que envolve o remanescente escalar de bμ é obtido. Também, é indicado que a quebra da dualidade pode ser associado com uma assimetria entre fontes elétricas e magnéticas neste cenário, de modo que a força eletromagnética experimentada por um pólo magnético seja suplementada pelo termo extra, proporcional a bμ, quando comparada com uma carga elétrica.

Neste trabalho foram investigadas as propriedades magnéticas (curva de histerese e ruído Barkhausen) de alguns aços (ABNT 1045, 1050 e 1548) cujas superfícies foram endurecidas por indução. O objetivo era de desenvolver um ensaio não-destrutivo para determinar a espessura da camada endurecida, assim evitando a necessidade de realizar medidas (destrutivas) de microdureza. Medidas da susceptibilidade magnética mostraram um pico (ferrita) em torno de 800 A/m e outro (martensita) em torno de 3500 A/m. Assim procuramos correlacionar a amplitude do pico de ferrita com a espessura da camada dura na superfície. Para esta finalidade foram desenvolvidos sensores para medir ruído Barkhausen e um filtro para o processamento do sinal. Foi possível correlacionar medidas de microdureza com algumas características observadas nas medições magnéticas, como, por exemplo, a amplitude do sinal de ruído Barkhausen e o campo magnético do pico do sinal Barkhausen. Deste modo, encontrou-se uma relação entre a amplitude do ruído Barkhausen e a espessura da camada superficial. Seria desejável complementar essas medidas com outras e sugestões para uma continuação do trabalho são detalhadas. / Submitted by Marcelo Teixeira (mvteixeira@ucs.br) on 2014-06-03T20:29:34Z No. of bitstreams: 1 Dissertacao Gustavo Rodrigues da Silva.pdf: 8142476 bytes, checksum: 29c4748e15d0fbcc852d788b9f1fa011 (MD5) / Made available in DSpace on 2014-06-03T20:29:34Z (GMT). No. of bitstreams: 1 Dissertacao Gustavo Rodrigues da Silva.pdf: 8142476 bytes, checksum: 29c4748e15d0fbcc852d788b9f1fa011 (MD5) / This work investigates the magnetic properties (hysteresis loop and Barkhausen noise) in some steels (ABNT 1045, 1050 and 1548) whose surfaces had been hardened by induction. The goal was to develop a non-destructive test procedure to determine the thickness of the hardened layer, thus avoiding the need to perform (destructive) measurements of the microhardness. Measurements of the magnetic susceptibility showed a peak (ferrite) around 800 A / m and other (martensite) at around 3500 A / m. Therefore, a correlation was sought between the peak amplitude of ferrite with the thickness of hard layer on the surface. For this purpose sensors were developed to measure Barkhausen noise as well as a filter for signal processing. It was possible to correlate measurements of microhardness with some features observed in magnetic measurements, for example, the amplitude of the Barkhausen noise and magnetic field of peak Barkhausen signal. Thus, a relationship was found between the amplitude of the Barkhausen noise and the thickness of the surface layer. It would be desirable to supplement these with other measurements and suggestions for further work are detailed.

Neste trabalho foram investigadas as propriedades magnéticas (curva de histerese e ruído Barkhausen) de alguns aços (ABNT 1045, 1050 e 1548) cujas superfícies foram endurecidas por indução. O objetivo era de desenvolver um ensaio não-destrutivo para determinar a espessura da camada endurecida, assim evitando a necessidade de realizar medidas (destrutivas) de microdureza. Medidas da susceptibilidade magnética mostraram um pico (ferrita) em torno de 800 A/m e outro (martensita) em torno de 3500 A/m. Assim procuramos correlacionar a amplitude do pico de ferrita com a espessura da camada dura na superfície. Para esta finalidade foram desenvolvidos sensores para medir ruído Barkhausen e um filtro para o processamento do sinal. Foi possível correlacionar medidas de microdureza com algumas características observadas nas medições magnéticas, como, por exemplo, a amplitude do sinal de ruído Barkhausen e o campo magnético do pico do sinal Barkhausen. Deste modo, encontrou-se uma relação entre a amplitude do ruído Barkhausen e a espessura da camada superficial. Seria desejável complementar essas medidas com outras e sugestões para uma continuação do trabalho são detalhadas. / This work investigates the magnetic properties (hysteresis loop and Barkhausen noise) in some steels (ABNT 1045, 1050 and 1548) whose surfaces had been hardened by induction. The goal was to develop a non-destructive test procedure to determine the thickness of the hardened layer, thus avoiding the need to perform (destructive) measurements of the microhardness. Measurements of the magnetic susceptibility showed a peak (ferrite) around 800 A / m and other (martensite) at around 3500 A / m. Therefore, a correlation was sought between the peak amplitude of ferrite with the thickness of hard layer on the surface. For this purpose sensors were developed to measure Barkhausen noise as well as a filter for signal processing. It was possible to correlate measurements of microhardness with some features observed in magnetic measurements, for example, the amplitude of the Barkhausen noise and magnetic field of peak Barkhausen signal. Thus, a relationship was found between the amplitude of the Barkhausen noise and the thickness of the surface layer. It would be desirable to supplement these with other measurements and suggestions for further work are detailed.

Ένα από τα μεγαλύτερα θεωρητικά κατασκευάσματα της φυσικής είναι αυτό της σκοτεινής ύλης. Στην παρούσα μεταπτυχιακή διπλωματική εργασία αναλύονται οι λόγοι για τους οποίους πιστεύουμε στην ύπαρξη της σκοτεινής ύλης, τα σωματίδια και γενικά τα αντικείμενα που μπορεί να την απαρτίζουν, η σκοτεινή ενέργεια και οι μορφές της και τέλος εναλλακτικές προτάσεις. / One of the most important theoretical creations of physics is dark matter. In this master thesis we describe the reasons we believe in the existence of dark matter, the particles and the objects that consists dark matter, dark energy and finally alternative theories.

Made available in DSpace on 2016-12-23T14:34:49Z (GMT). No. of bitstreams: 1 Diego Henrique Carvalho dos Santos.pdf: 664413 bytes, checksum: 9b89955577d748bc73682dc9f48c6e83 (MD5) Previous issue date: 2012-12-07 / O objetivo deste trabalho é apresentar como se dá a correspondência entre teoria de calibre e conexões em espaços …brados. Mais precisamente estabelecemos um dicionário entre a teoria de calibre da mecânica quântica de uma partícula carregada sujeita a um campo eletromagnético e o estudo das conexões em …brados por círculos e por retas complexas. Em seguida, analisamos dois objetos de estudo em física utilizando o conhecimento adquirido no estudo da geometria de espaços …brados. As classes de Chern e a holonomia de uma conexão nos fornecerão uma visualização geométrica de, respectivamente, monopolos magnéticos e o efeito Aharonov-Bohm / The aim of this work is to present how works the correspondence between the gauge theory and connections in ber bundles. More precisely establishing a dictionary between gauge theory of the quantum mechanics of a charged particle under the in‡uence of an electromagnetic eld and the studies of connections in circle bundles and line bundles. Then, we analyzed two objects of studies in physics using the knowledge acquired in the study of the geometry of ber bundles. The Chern classes and the holonomy of a connection will provide a geometrical visualization of, respectively, magnetic monopoles and the Aharonov-Bohm e¤ect

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-28T14:29:11Z No. of bitstreams: 1 damiaodesousavieirajunior.pdf: 10265456 bytes, checksum: 8b1ceaeb4c4be0e91a46c2d30add7349 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-07T21:17:55Z (GMT) No. of bitstreams: 1 damiaodesousavieirajunior.pdf: 10265456 bytes, checksum: 8b1ceaeb4c4be0e91a46c2d30add7349 (MD5) / Made available in DSpace on 2017-08-07T21:17:55Z (GMT). No. of bitstreams: 1 damiaodesousavieirajunior.pdf: 10265456 bytes, checksum: 8b1ceaeb4c4be0e91a46c2d30add7349 (MD5) Previous issue date: 2016-02-03 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O contínuo desenvolvimento das técnicas de fabricação de estruturas em escala nanométrica, com considerável precisão e reprodutibilidade, tem permitido e estimulado a investigação científica em torno das propriedades básicas e novas aplicações tecnológicas desses sistemas. Especialmente a partir dos anos 90, é crescente o interesse da comunidade científica no comportamento de sistemas magnéticos nano-estruturados. Nestes, a quebra da simetria espacial devido às pequenas dimensões faz com que exibam comportamentos completamente distintos dos observados em amostras macroscópicas. A anisotropia de forma resultante das interações clássicas entre os dipolos magnéticos permite a formação de estruturas magnéticas exóticas em nanomagnetos como vórtices, skyrmions, paredes de domínio individuais e, até mesmo, excitações topológicas similares a monopolos magnéticos. A compreensão e controle do comportamento magnético estático e dinâmico dessas estruturas é fundamental para o desenvolvimento de novos dispositivos tecnológicos baseados em spintrônica. Neste trabalho foram estudadas nanopartículas planares, alongadas na forma elíptica, de material ferromagnético macio, especificamente o Permalloy-79. Tais nanopartículas tem atraído atenção devido ao seu potencial de aplicação prática no desenvolvimento de novos sensores, dispositivos de lógica, mídias de armazenamento de dados de alta densidade e dispositivos MRAM (Magnetic Random Access Memory). Pelo viés do interesse científico básico, tais nano-ilhas ferromagnéticas são a unidade fundamental em arranjos magnéticos bidimensionais geometricamente frustrados, como sistemas de gelo de spin artificiais. Nestes sistemas o arranjo geométrico das ilhas quebra a degenerescência do estado fundamental da rede, caracterizando um estado de frustração geométrica que permite excitações de comportamento análogo ao de monopólos magnéticos. Sob tais aspectos, é essencial caracterizar as configurações magnéticas no estado fundamental e os processos de reversão da magnetização em nanopartículas individuais. A forma elíptica planar gera uma forte anisotropia magnética, definindo duas configurações fundamentais para a magnetização do estado fundamental das nanopartículas: o estado de vórtice ou o estado alinhado ao longo do maior eixo — estado tipo C. A partir de uma razão de aspecto limite, a magnetização do estado fundamental é confinada no plano e ao longo do eixo maior de cada nano ilha, definindo um nanomagneto monodomínio com dois estados degenerados de magnetização, útil às aplicações previamente descritas. Partindo desse intuito estudamos inicialmente, através de simulação por dinâmica de spin, a competição entre os estados de vórtice e os estados alinhados tipo C como uma função da forma de cada nano-ilha elíptica, construindo um diagrama de fases de estados vórtice - tipo C. Cada nanopartícula magnética é modelada por momentos magnéticos que interagem via interação de troca entre primeiros vizinhos e por interação dipolar clássica de longo alcance. Nossos resultados mostram que é possível fabricar nano-ilhas alongadas com estado fundamental alinhado tipo C em razões de aspecto menores que dois. Este é um resultado interessante do ponto de vista tecnológico, pois permite usar ilhas menores que as atuais em pesquisas com gelos de spin e MRAM. Geralmente, os arranjos experimentais são feitos com nanopartículas de razão de aspecto próximas a três para garantir o estado fundamental alinhado da magnetização. Acrescentando ao modelo um termo de interação Zeeman com um campo magnético externo, estudamos o comportamento da reversão da magnetização nas nanopartículas. Consideramos espessuras diferentes e duas razões de aspecto distintas: uma do tamanho experimental usual e outra menor proposta a partir de nossos resultados. Aplicando campo magnético senoidal em diferentes frequências e em direções distintas no plano das nanoilhas, observou-se a dependência dos processos de reversão em função da espessura das partículas e com a direção e frequência do campo aplicado. Os resultados permitem traçar linhas gerais acerca do comportamento da reversão da magnetização nas nanopartículas individuais sob campo magnético externo. Evidentemente para o desenvolvimento das possíveis aplicações tecnológicas, inclusive o controle de excitações como monopólos magnéticos em gelos de spin, é crucial entender os processos ultra rápidos de reversão da magnetização, o que envolve a aplicação de campo externo de alta frequência em direções cuidadosamente definidas. Com esse objetivo, também estudamos a reversão da magnetização nas nano-ilhas por pulsos curtos de campo magnético (da ordem de nanosegundos) aplicados em diferentes direções. Observamos uma forte dependência da coerência da reversão da magnetização com a direção do campo aplicado e uma significante diferença na dependência angular da coercividade em relação ao observado em trabalhos prévios para campos aplicados na condição quase-estática. Finalmente, baseado em nossos resultados, propomos um método para o controle da reversão coerente da magnetização de nanopartículas individuais em matrizes quadradas de gelos de spin artificiais. Acreditamos que nossos resultados poderão ser úteis no desenvolvimento ulterior de arranjos magnéticos artificiais geometricamente frustrados e no controle das excitações topológicas destes sistemas. / The continuous development of structures fabrication techniques at the nanometer scale with considerable precision and reproducibility has allowed and encouraged scientific research around the basic properties and new technological applications of these systems. Especially from the 90's, there is growing interest of the scientific community in the behavior of nanostructured magnetic systems. In these, the breaking of spatial symmetry due to small dimensionality causes quite different behaviors from those observed in the bulk. The resulting shape anisotropy of the classical interaction between magnetic dipoles allows the formation of exotic magnetic structures in nanomagnets as vortices, skyrmions, single domain walls and even topological excitations similar to magnetic monopoles. The understanding and control of static and dynamic magnetic behavior of these structures is essential for the development of new technological devices based on spintronics. In this work we studied planar elongated nanoparticles in the elliptical shape of soft ferromagnetic material, specifically the Permalloy-79. Such nanoparticles have attracted attention because of their potential to practical application in the development of new sensors, logic devices, high density data storage media and MRAM (Magnetic Random Access Memory) devices. By the bias of basic scientific interest, such ferromagnetic nano-islands are the fundamental unit in two-dimensional magnetic arrangements geometrically frustrated as artificial spin ice systems. In these systems, the geometric arrangement of islands break the degeneracy of the network ground state featuring a state of geometrical frustration that allows excitations with analogous behavior of magnetic monopoles. Under these aspects, it is essential to characterize the magnetic configurations in the ground state and the magnetization reversal processes in individual nanoparticles. The elliptical planar shape generates a strong magnetic anisotropy which defines two basic configurations for the magnetization of the ground state of the nanoparticles: the vortex state or the aligned state along the major axis - type C state. As from an aspect ratio limit value, the magnetization of the ground state is confined in the plane and along the major axis of each nano-island defining mono-domain nanomagnet with two degenerate states of magnetization, useful for the applications previously described. Starting from this purpose we study initially, through simulation by spin dynamics, the competition between the vortex states and aligned type C states as a function of the shape of each elliptical nano-island to build a states diagram. Each magnetic nanoparticle is modeled by magnetic moments that interact by exchange interaction between nearest neighbors and by the classical long-range dipolar interaction. Our theoretical results indicate the possibility to manufacture elongated nano-islands with ground state like aligned C state for aspect ratios less than two. This is an interesting result from the technological point of view because it will be possible to use smaller islands in researches on spin ice and MRAM. Generally, the experimental arrangements are made with nanoparticles of aspect ratio close to three to ensure aligned magnetization in the ground state. Adding to the model a Zeeman interaction term between the magnetic moments and an external magnetic field we study the behavior of the magnetization reversal in nanoparticles. We consider different thickness and two different aspect ratios: one in the usual experimental size and a smaller proposed from our results. Applying sinusoidal magnetic field at different frequencies along the anisotropy axis in directions of ten and forty-five degrees from this, we observed the dependence of the reversal processes on the thickness of the particles and with the direction and frequency of the applied field. The results allow to establish general guidelines about the magnetization reversal behavior of the individual nanoparticles under external magnetic field. Evidently, for the development of possible technological applications, including the control of excitation like magnetic monopoles in spin ice, it is crucial to understand the ultrafast magnetization reversal processes which involves the application of high frequency magnetic fields in carefully defined directions. With this aim, we also studied the magnetization reversal of the nano-islands by short pulses of magnetic field (of the nanosecond order) applied in different directions. We observed a strong dependence on the coherence of the magnetization reversal with the direction of the applied field and a significant difference in the angular dependence of the coercivity compared to those seen in previous studies with applied magnetic fields in quasistatic conditions. Finally, based on our results we propose a method for the control of the coherent magnetization reversal of individual nanoparticles in square artificial spin ice arrays. We believe that our results may be useful in further developments of geometrically frustrated magnetic artificial arrangements and in the control of the topological excitations of these systems.

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Au long de ce manuscrit de thèse je présenterai des effets non linéaires émergents dans les condensats d'exciton-polaritons spineurs. Après un chapitre d'introduction amenant les notions de bases nécessaires, je me concentrerai dans une première partie sur les défauts topologiques quantifiés par des nombres demi-entiers et discuterai leur stabilité, accélération et nucléation en présence de champs magnétiques effectifs. Nous verrons que ces objets se comportent comme des charges magnétiques manipulables démontrant une analogie fascinante avec les monopoles de Dirac. De manière remarquable nous verrons également que ces objets peuvent être utilisés comme des signaux stables pour sonder la physique d'analogues acoustiques de trous noirs. Dans une seconde partie j'étudierai des structures de basse dimensions. Plus particulièrement, je décrirai la formation de solitons de bande interdite et les oscillations de Bloch des exciton-polaritons dans des microfils comportant des structures périodiques et d'autre part les oscillations Josephson à température ambiante dans des paires de micropilliers couplés. / Along this thesis manuscript I shall present some emergent nonlinear phenomena in spinor exciton polariton condensates. After an introductory chapter bringing the necessary background, I will in a first part focus on half-integer topological defects discussing their stability, acceleration and nucleation in the presence of effective magnetic fields. We will see that these objects behave as magnetic charges being fascinating dirac's monopole analogues. Remarkably we will see as well how they can be used as stable signals allowing to seed the physics acoustic black holes analogues. In a second part I will concentrate on low dimensional structures. Especially, I'll describe the formation of gap solitons and the Bloch oscillations of exciton-polaritons in periodically patterned microwires, and besides, the room temperature Josephson oscillations within pairs of coupled micropillars.