Submitted by Irene Nascimento (irene.kessia@ufpe.br) on 2016-07-12T19:45:21Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_MessiasVilbert.pdf: 2165242 bytes, checksum: 21182d23be8e680205d5ea8209ddfe04 (MD5) / Made available in DSpace on 2016-07-12T19:45:21Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_MessiasVilbert.pdf: 2165242 bytes, checksum: 21182d23be8e680205d5ea8209ddfe04 (MD5) Previous issue date: 2015-10-23 / capes / Sistemas de tamanho nito con nados entre geometrias de placas planas e paralelas, cujas superf cies de contorno est~ao sujeitas a condi c~oes de contorno de Dirichlet e Neumann, e separados por uma dist^ancia L foram analisados no espa co dos momentos. N os introduzimos uma representa c~ao modi cada para as autofun c~oes discretas e utilizamos campos escalares renormalizados em termos de partes de v ertice 1PI (do ingl^es \one-particle irreducible") sem massa e tamb em massivos. N os discutimos as multiplicidades nas regras de Feynman que surgem na constru c~ao dos diversos diagramas, o que e devido a escolha da representa c~ao das fun c~oes base, e apresentamos as condi c~oes de normaliza c~ao modi cadas. Para quase-momentos externos n~ao nulos, provamos que as condi c~oes de contorno de Neumann e de Dirichlet podem ser uni cadas em um unico formalismo. Discutimos os regimes de crossover dimensionais para estes e mostramos a correspond^encia com as condi c~oes de contorno peri odicas e antiperi odicas. Em particular, provamos que os efeitos de tamanho nito para Dirichlet e Neumann n~ao requerem necessariamente termos de superf cie, mas s~ao implementados n~ao-trivialmente nas regras de Feynman envolvendo apenas termos de bulk na Lagrangiana. Como uma aplica c~ao, calculamos, via esquema diagram atico, os expoentes cr ticos e , pelo menos, at e a ordem de dois loops. Mostramos que os ndices cr ticos s~ao os mesmos do sistema bulk (in nito), independentemente das condi c~oes de contorno. Em seguida, estendemos o nosso m etodo de an alise do tamanho nito para sistemas competitivos m-axiais no ponto cr tico de Lifshitz. Em uma abordagem inicial, consideramos nita uma das dire c~oes ao longo do subespa co sem competi c~ao e observarmos um comportamento semelhante com rela c~ao ao crossover dimensional de sistemas n~ao-competitivos. Para L su cientemente grande, calculamos os expoentes cr ticos 1, 1, 2 e 2 at e ordens de pelo menos dois loops com aux lio de uma aproxima c~ao especial para a regulariza c~ao das integrais de Feynman. Tais expoentes s~ao id^enticos aos do sistema in nito. Por m, analisamos sistemas competitivos arbitr arios do tipo Lifshitz, os quais apresentam diversos eixos de competi c~ao e podem ser tratados pelo modelo CECI, que e o caso mais geral dentre os modelos que exibem o ponto de Lifshitz como caracter stica. Para formular o problema das transi c~oes de fase nesses exemplos de sistemas complexos, introduzimos uma t ecnica de teoria de campo escalar de massa nula e aplicamos o m etodo de subtra c~ao m nima, como meio de renormaliza c~ao, para calcular, perturbativamente, os expoentes cr ticos do modelo CECI, no caso isotr opico (d = mn). Para o caso isotr opico desse modelo, conseguimos calcular os expoentes cr ticos exatamente at e O( 2 n) (at e O( 3 n) para a dimens~ao an^omala n). / Finite size systems con ned between parallel plate geometries whose boundary surfaces are subject to Dirichlet and Neumann boundary conditions and separated by a distance L are analyzed in momentum space. We introduce a modi ed representation for the discrete eigenfunctions in a renormalized one-particle irreducible (1PI) vertex part scalar eld-theoretic framework using either massless or massive elds. We discuss the appearence of multiplicities in the Feynman rules to construct diagrams due to this choice of representation of the basis functions and present the modi ed normalization conditions. For nonvanishing external quasimomenta, we prove that Dirichlet and Neumann boundary conditions can be uni ed within a single formalism. We discuss the dimensional crossover regimes for these and show a correspondence with those from periodic and antiperiodic boundary conditions. In particular, we prove that nite size e ects for Dirichlet and Neumann boundary conditions do not require surface terms necessarily but are implemented non-trivially from the Feynman rules involving only bulk terms in the Lagrangian. As an application, we compute the critical exponents and at least up to two-loop level through diagrammatic means. We show that the critical indices are the same as those from the bulk (in nite) system irrespective of the boundary conditions. Next, we extend our nite-size method of analysis to m-axial competing systems at the Lifshitz critical point. In an initial approach, we consider as nite one of the directions along the non competitive subspace and we observe a similar behavior in comparison with the dimensional crossover for non competitive sistems. For L great enough, we calculate the critical exponents 1, 1, 2 and 2 up to at least 2 loops order with the aid of a special approximation for regularizing the Feynman integrals. These exponents are identical to those obtained from in nite systems. At last, we analyze competitive systems of arbitrary Lifshitz type, which have di erent axes of competition and can be treated by the CECI model, which is the most general case among the models that exhibit a Lifshitz point critical behavior. In order to formulate the problem of phase transitions in these examples of complex systems, we introduce a technique for scalar eld theory of zero mass and apply the method of minimal subtraction as a means of renormalization to calculate perturbatively the critical exponents of the CECI model for in the isotropic case (d = mn). For the isotropic case of this model, we calculate the critical exponents exactly up to O( 2 n) (up to O( 3 n) for the anomalous dimension n).

Submitted by Vasti Diniz (vastijpa@hotmail.com) on 2017-09-12T12:27:38Z No. of bitstreams: 1 arquivototal.pdf: 987629 bytes, checksum: 073f4a58e014f0b46588edf144d4a42b (MD5) / Made available in DSpace on 2017-09-12T12:27:38Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 987629 bytes, checksum: 073f4a58e014f0b46588edf144d4a42b (MD5) Previous issue date: 2016-02-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this dissertation we work with the Horava-Lifshitz-like Gross-Neveu model in (2+1) dimensions in the Large N expansion. Firstly we make an article revision [6] where it is shown that the Gross-Neveu Model in the 1/N expansion presents a dynamic mass generation by means of the introduction of an auxiliary field, which results in the dynamical parity broken. We calculate the gap equation where we will see the generated mass dependence with the coupling constant. After that, we will put a gauge field to the model and study the polarization tensor which will generate an induced Chern-Simons term in the Effective Lagrangian. As a novelty, we work with the Gross-Neveu Model in the context of Horava-Lifshitz, where anisotropic scaling is done, thus breaking the Lorentz invariance. We introduce an auxiliary field and we study the cases which the value of the critical dynamic exponent Z is even and when it is odd. In the case where z is even, there is no dynamic mass generation so the parity symmetry is conserved and we will not have the term induced of Chern-Simons either. In the case where z is odd, we will have the dynamic mass generation and the dynamic parity symmetry will occur. Finally we couple a gauge field in the model and find the Chern-Simons term, which clearly shows the anisotropy of space and time for values of z> 1 / Nesta dissertacao trabalhamos corn o modelo de Gross-Neveu ern (2+1) dimensoes na expansao 1/N no contexto de Horava-Lifshitz. Primeiro, faremos uma revisao do artigo [6], onde se mostra que o Modelo de Gross-Neveu na expansao 1/N apresenta uma geracao dinamica de massa mediante a introducao de urn campo auxiliar, o que traz como consequencia a quebra dinamica da simetria de paridade. Calculamos a equacao de gap, onde veremos a dependencia da massa gerada corn a constante de acoplamento. ApOs isso, acoplaremos urn campo de gauge ao modelo, estudamos o tensor de polarizacao, o qual vai gerar urn termo induzido de tipo Chern-Simons na lagrangiana efetiva. Como novidade, trabalhamos corn o Modelo de Gross-Neveu no contexto de Horava-Lifshitz, onde se faz urn escalonamento anisotrOpico, quebrando, assim, a invariancia de Lorentz. Introduzimos urn campo auxiliar e estudamos os casos ern que o valor do exponente dinamico critico z é par quando é Impar. No caso ern que z é par, nao ha geracao dinamica de massa pelo que a simetria de paridade é conservada e tambern nao teremos o termo induzido de Chern-Simons. No caso ern que z é impar, vamos ter a geracao dinamica de massa vai ocorrer a quebra dinamica de simetria de paridade. Finalmente, acoplamos urn campo de gauge no modelo e encontramos o termo tipo Chern-Simons, o qual mostra claramente a anisotropia do espaco tempo para valores de z > 1.

Made available in DSpace on 2015-05-14T12:14:07Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 735779 bytes, checksum: 2ffa25e5b45beedc0c8f4f31d5f79b06 (MD5) Previous issue date: 2013-06-14 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this thesis, we review some basic concepts related to Lorentz symmetry violation and Horava-Lifshitz-like theories. We present our contribution to the calculation of the effective potential in two frameworks that exhibit Lorentz invariance violation. First, we calculate the effective potential for some HL-like theories. We obtained results for the pure scalar model, for the scalar QED with critical exponent z = 2 and z generic, and for the Yukawa model also with z = 2 and with arbitrary values of z. For the latter model, we show that the effective potential vanishes for even z and has nontrivial values for odd z. The second framework is the Kostelecký-Berger model that implements the Lorentz symmetry breaking in supersymmetry. We develop the theory of superfields for three and four-dimensional space-time based on this model and show how this development leads to a theory of superfields with ether-like Lorentz violation. We also obtained the one loop contributions to the effective action and effective potential in this theory. / Nesta tese, revisamos alguns conceitos básicos relacionados à violação da simetria de Lorentz e a teorias do tipo Horava-Lifshitz. Apresentamos nossa contribuição ao cálculo do potencial efetivo em duas formulações que exibem violação da invariância de Lorentz. Primeiro, calculamos o potencial efetivo para algumas teorias do tipo HL. Obtivemos resultados para o modelo escalar puro, para a QED escalar com expoente crítico z = 2 e z genérico, e para o modelo de Yukawa também com z = 2 e com valores arbitrários de z. Para este último modelo, demonstramos que o potencial efetivo se anula para z par e possui valores não triviais para z ímpar. A segunda formulação é o modelo de Kostelecký-Berger que implementa a quebra da simetria de Lorentz em supersimetria. Desenvolvemos a teoria de supercampos para três e quatro dimensões do espaço-tempo com base neste modelo e mostramos como este desenvolvimento leva a uma teoria de supercampos com violação de Lorentz do tipo-éter. Obtemos também as contribuições de um loop para a ação efetiva e para potencial efetivo nesta teoria.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-12-22T12:37:32Z No. of bitstreams: 1 josielcarlosdesouzagomes.pdf: 11267373 bytes, checksum: 393a01f57f4f5afaaf46890b84f4a7ac (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-12-22T12:42:22Z (GMT) No. of bitstreams: 1 josielcarlosdesouzagomes.pdf: 11267373 bytes, checksum: 393a01f57f4f5afaaf46890b84f4a7ac (MD5) / Made available in DSpace on 2016-12-22T12:42:22Z (GMT). No. of bitstreams: 1 josielcarlosdesouzagomes.pdf: 11267373 bytes, checksum: 393a01f57f4f5afaaf46890b84f4a7ac (MD5) Previous issue date: 2015-02-26 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A nanotecnologia é uma área de estudo promissora e que nos mostra resultados bastante surpreendentes. Amostras magnéticas (Cobalto e liga de Permalloy (Ni81Fe19), por exemplo) em escala nanométrica, têm como aplicabilidade importante a gravação magnética devido à crescente demanda por meios de gravação cada vez mais rápidos e de alta capacidade de armazenamento. Para determinados tamanhos de nanofios, observa-se a presença de domínios magnéticos e paredes de domínios do tipo vórtice ou transversal que podem ser transportadas para diferentes regiões sem deformação. Pode-se usar tais paredes como bit de informação mas, para isso, precisa-se conhecer com detalhes o comportamento dessas paredes em diversas situações. Neste presente trabalho utilizamos simulações numéricas para estudar o comportamento da magnetização em nanofios retangulares (nanofitas) de Permalloy-79, que apresentam parede de domínio transversal entre domínios “head-to-head”. Utilizamos nestas simulações um modelo no qual os momentos magnéticos interagem através da interação de troca e a interação dipolar. Embora a maioria dos trabalhos encontrados utilizem campo magnético para mover a parede, optamos por aplicar corrente de spin-polarizado na direção do nanofio devido ao fato de ser mais prático de ser produzido. A dinâmica do sistema é regida pelas equações de Landau-Lifshitz-Gilbert e a atuação da corrente é introduzida nessas equações. Fizemos uma abordagem teórica na qual pode-se mostrar como esta equação de Landau-Lifshitz-Gilbert para aplicação de corrente foi obtida. A integração da equação de Landau-Lifshitz-Gilbert é feita utilizando o método de Runge-Kutta e de Predição-Correção. Baseado nessas teorias, escrevemos um programa na linguagem Fortran-90 para realizar as simulações. Em nossos resultados observamos o comportamento da velocidade da parede de domínio em função do tempo e da densidade de corrente. Comparamos estes resultados com a bibliografia. / Nanotechnology is a promising field of study and show us pretty amazing results. Magnetic samples (Cobalt and alloy Permalloy (81NiFe19), for example) at the nanometer scale, have as important applicability the magnetic recording due to the growing demand for recording media ever faster and high storage capacity. For certain sizes of nanowires, it is observed the presence of magnetic domains and vortex domain walls or transverse domain wall which can be transported to different regions without deformation. It can use such walls as bit of information, but for that it is necessary to know in detail the behavior of these walls in various situations. In this work we used numerical simulations to study the behavior of the magnetization in rectangular nanowires (nanostrip) of Permalloy-79, which have transverse domain wall between domains "head-to-head."We used in these simulations a model in which the magnetic moments interact through the exchange interaction and the dipolar interaction. Although most studies found use magnetic field to move the wall, we decided to apply spin-polarized current toward the nanowire due the fact that it is more practical to be produced. The dynamics of the system is governed by the equations of Landau-Lifshitz-Gilbert and the current performance is introduced in these equations. We made a theoretical approach in which you can show how this equation of Landau-Lifshitz-Gilbert for applying current was obtained. The integration of the equation of Landau-Lifshitz-Gilbert is done using the Runge-Kutta and Prediction-Correction methods. Based on these theories, we wrote a program in Fortran-90 language to perform the simulations. In our results we observed the behavior of the domain wall velocity as a function of time and current density. We compare these results with the literature.

L’objectif de cette thèse est d’explorer analytiquement et numériquement la dynamique d’aimantation de nanoparticules magnétiques. Nous montrons qu’il est possible de contrôler efficacement le retournement d’aimantation d’une nanoparticule à l’aide d’une excitation autorésonante. Cette étude révèle que l’amortissement de Gilbert et la température altèrent l’efficacité de ce procédé, tandis que les interactions dipolaires peuvent le faciliter. Les propriétés stationnaires d’une monocouche de nanoparticules sont également étudiées en reproduisant numériquement des courbes ZFC. Nous observons ainsi qu’un désordre structurel ne modifie pas la température de blocage. Enfin, nous étudions le comportement d’un ensemble de nanoparticules en interaction dans un fluide à l’aide de simulations de dynamique moléculaire. Nous retrouvons les configurations à l’équilibre en forme de chaînes et d’anneaux, puis nous examinons les aspects dynamiques en mettant en évidence l’existence d’ondes de spins. / The goal of this thesis is to explore analytically and numerically the magnetization dynamics in magnetic nanoparticles. Firstly, we study the Néel dynamics of fixed. We show that one can efficiently control the magnetization reversal of a nanoparticle by using a chirped excitation (autoresonance). This study reveals that the Gilbert damping and the temperature alter the efficiency of the reversal, while dipolar interactions can improve it. The stationary properties of a monolayer of nanoparticles are then examined by computing ZFC curves with a Monte Carlo method. We observe that structural disorder has no effect on the blocking temperature. Finally, we investigate the behavior of an ensemble of interacting nanoparticles moving in a fluid with a molecular dynamics approach. Our numerical simulations reproduce the usual chain and ring-like equilibrium configurations. We then study the dynamics of these structures and show the existence of super-spin waves.

No description available.

Understanding magnetization reversal is very important in designing high density and high data transfer rate recording media. This research has been motivated by interest in developing new nonvolatile data storage solutions as magnetic random access memories - MRAMs. This dissertation is intended to provide a theoretical analysis of static and dynamic magnetization switching of magnetic systems within the framework of critical curve (CC). Based on the time scale involved, a quasi-static or dynamic CC approach is used. The static magnetization switching can be elegantly described using the concept of critical curves. The critical curves of simple uncoupled films used in MRAM are discussed. We propose a new sensitive method for CC determination of 2D magnetic systems. This method is validated experimentally by measuring experimental critical curves of a series of Co/SiO2 multilayers systems. The dynamics switching is studied using the Landau-Lifshitz-Gilbert (LLG) equation of motion. The switching diagram so-called dynamic critical curve of Stonerlike particles subject to short magnetic field pulses is presented, giving useful information for optimizing field pulse parameters in order to make ultrafast and stable switching possible. For the first time, the dynamic critical curves (dCCs) for synthetic antiferromagnet (SAF) structures are introduced in this work. Comparing with CC, which are currently used for studying the switching in toggle MRAM, dCCs show the consistent switching and bring more useful information on the speed of magnetization reversal. Based on dCCs, better understanding of the switching diagram of toggle MRAM following toggle writing scheme can be achieved. The dynamic switching triggered by spin torque transfer in spin-torque MRAM cell has been also derived in this dissertation. We have studied the magnetization's dynamics properties as a function of applied current pulse amplitude, shape, and also as a function of the Gilbert damping constant. The great important result has been obtained is that, the boundary between switching/non-switching regions is not smooth but having a seashell spiral fringes. The influence of thermal fluctuation on the switching behavior is also discussed in this work.

The process of forcibly dewetting a solid substrate from a bulk liquid so as to leave a thin residual layer on the surface is referred to as forced dewetting. This novel experimental approach helps to investigate interfacial species by minimizing the interference of the bulk liquid when coupled with spectroscopy. In this work, the scope of liquids investigating using this approach has been expanded from simple fluids to one type of complex fluid, a nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB).In order to better understand the interfacial behavior of the simple fluids, water, chloroform, and n-pentane vapors were adsorbed onto omega-terminated SAM-modifed Ag (11-mercaptoundecanoic acid, 11-mercaptoundenanol, and undecanethiol) surfaces under vapor-saturated conditions. The kinetics of solvent adsorption on each of these surfaces were investigated and the thicknesses of the adsorbed layer were compared to predictions from Lifshitz theory of long-range van der Waals interactions. Although the predicted thicknesses do not match the experimental values for adsorbed films, the predicted thicknesses do match those observed experimentally using forced dewetting. The correlation between these predicted and observed thicknesses implies that residual film formation under the conditions of forced dewetting used in this laboratory is dictated by interfacial forces alone.The surface adsorption behavior of 5CB was investigated using surface-enhanced Raman spectroscopy with the aid of localized surface plasmon resonances-surface plasmon polaritron coupling. The results clearly indicate that 5CB is adsorbed to smooth Ag surface in a facial orientation with pi-d orbital interaction suggested.Finally, forced dewetting studies of bare, -NH2-temintaed SAM, and -CH3-temintaed SAM modified-SiO2 substrates from 5CB were undertaken. Residual layer thicknesses were monitored as a function of substrate velocity. The transition from the regime in which interfacial forces dictate residual layer thickness to the regime in which fluid dynamic forces dictate thickness was observed for the first time and was evaluated in terms of the average 5CB director orientation. Unlike simple fluids, 5CB has strong interfacial interactions from surface anchoring depending on the chemical nature of the substrate, which makes the residual layer thicknesses at least 100 times larger than observed in simple fluids.

Scientific development requires profound understandings of micromechanical and nanomechanical systems (MEMS/NEMS) due to their applications not only in the technological world, but also for scientific understanding. At the micro- or nano-scale, when two objects are brought close together, the existence of stiction or adhesion is inevitable and plays an important role in the behavior operation of these systems. Such effects are due to surface dispersion forces, such as the van der Waals or Casimir interactions. The scientific understanding of these forces is particularly important for low-dimensional materials. In addition, the discovery of materials, such as graphitic systems has provided opportunities for new classes of devices and challenging fundermental problems. Therefore, invesigations of the van der Waals or Caismir forces in graphene-based systems, in particular, and the solution generating non-touching systems are needed. In this study, the Casimir force involving 2D graphene is investigated under various conditions. The Casimir interaction is usually studied in the framework of the Lifshitz theory. According to this theory, it is essential to know the frequency-dependent reflection coefficients of materials. Here, it is found that the graphene reflection coefficients strongly depend on the optical conductivity of graphene, which is described by the Kubo formalism. When objects are placed in vacuum, the Casimir force is attractive and leads to adhesion on the surface. We find that the Casimir repulsion can be obtained by replacing vacuum with a suitable liquid. Our studies show that bromobenzene is the liquid providing this effect. We also find that this long-range force is temperature dependent and graphene/bromobenzene/metal substrate configuration can be used to demonstrate merely thermal Casimir interaction at room temperature and micrometer distances. These findings would provide good guidance and predictions for practical studies.

We solved the Landau-Lifshitz equations numerically to examine the time development of a system of magnetic particles. Constant or periodical external magnetic field has been applied. First, the system has been studied without dissipation. Local energy excitations (breathers) and chaotic transients have been found. The behaviour of the system and the final configurations can strongly depend on the initial conditions, and the strength of the external field at an earlier time. We observed some sudden switching between two remarkably different states. Series of bifurcations have been found. When a weak Gilbert-damping has been taken into account, interesting behaviour has been found even in the case of one particle as well: bifurcation series and period multiplication leading to chaos. For a system of antiferromagnetically coupled particles, highly nontrivial hysteresis loops have been produced. The dynamics of the magnetization reversal has been investigated and the characteristic time-scale of the reversal has been estimated. For more particles, the energy spectrum and the magnetization of the system exhibits fractal characteristics for increasing system size. Finally, energy have been pumped into the system in addition to the dissipation. For constant field, complicated phase diagrams have been produced. For microwave field, it has been found that the chaotic behaviour crucially depends on the parity of the number of the particles.