O estudo do sistema antiferromagnético diluído K2Fe1-xInxCl5.H2O com a finalidade de se estudar efeitos de campos aleatórios, mostrou o aparecimento de uma magnetização remanente (Mr) quando o composto é resfriado sujeito a um campo magnético externo. Esta magnetização remanente surge ao longo do eixo de fácil magnetização, abaixo de TN, para campos tão baixos quanto alguns mOe. A presença deste efeito foi observada também em outros compostos como por exemplo no Rb2Fe1-xInxCl5.H2O. A origem de Mr está provavelmente associada à presença de domínios antiferromagnéticos que se formam mesmo em campos muito baixos, e cujas paredes são presas por íons não magnéticos e defeitos. No caso do composto com rubídio, as paredes se encontram aparentemente menos presas, podendo levar a outros efeitos como a observação de fenômenos de relaxação magnética quando o campo em que a amostra é resfriada é retirado abaixo de TN. É sugerido neste trabalho que a magnetização remanente está provavelmente armazenada nas paredes de domínio formadas, já que o comportamento de Mr não segue aquele observado para a sub-rede antiferromagnética. Estudamos também o efeito da diluição na transição entre as fases antiferromagnética e spin-flop. Observamos a presença de uma histerese para o caso dos compostos K2Fe1-xInxCl5.H2O, além de um alargamento da transição. A presença destes efeitos está provavelmente associada à presença de uma fase mista onde as fases antiferromagnética e spin-flop coexistem em uma estrutura de domínios. No caso dos compostos Rb2Fe1-xInxCl5.H2O, de uma modo geral é observado apenas um alargamento da transição, sem o aparecimento de histerese. Neste caso, é possível a existência de uma fase intermediária induzida por campos aleatórios, similar àquela proposta por Aharony. As medidas de calor específico realizadas com e sem campo magnético externo aplicado no sistema (Nd1-xSmx)2CuO4 mostrou em todos os casos a presença de um máximo. Através do estudo do comportamento deste máximo, observamos a presença de duas regiões de diluição de comportamento distinto. Para x > 0.7, o comportamento é aquele observado para um sistema antiferromagnético simples diluído. A aplicação de campo neste caso, leva a uma pequena diminuição na temperatura em que o máximo ocorre. Para x < 0.7 temos que o efeito dominante é o mesmo que aquele observado para o caso do Nd2CuO4. O comportamento do Nd2CuO4 não é o de um antiferromagneto simples, já que as interações entre as sub-redes de Cu e de Nd levam a um “splitting” do estado de energia fundamental da sub-rede de Nd, e consequentemente a um comportamento do tipo Schottky no calor específico. / Magnetization measurements performed on diluted antiferromagnetic compounds K2Fe1-xInxCl5.H2O in order to study the Random Fields Ising Model revealed the existence of a remnant magnetization (Mr) when the sample is cooled below TN in an applied axial magnetic field. This remnant magnetization along the easy axis direction is observed in magnetic fields as low as a few mOe. Other diluted compounds, like Rb2Fe1-xInxCl5.H2O, also show the same behavior. The origin of this remnant magnetization is probably related to the presence of low field antiferromagnetic domains, which walls are pinned by non-magnetic ions and defects. For the Rb-based compound, the walls are probably not strongly pinned. In this case, when the field is turned off bellow TN relaxation effects are sometimes observed. It is suggested in this work that the remnant magnetization is stored in the domain walls, since the Mr behavior is quite different from the one observed for the antiferromagnetic sublattice. We have also studied the dilution effect in the spin-flop transition. It is possible to observe the presence of an anomalous widening and a hysteresis in the transition region for the K2Fe1-xInxCl5.H2O compound. These effects are probably related to the presence of a mixed intermediate phase, where the spin-flop and the antiferromagnetic phase coexist in a domain structure. For the Rb2Fe1-xInxCl5.H2O compound, it is usually observed just a widening of the transition, without hysteresis. In this case it is possible the existence of a random fields induced intermediate phase, similar to the one proposed by Aharony. Specific heat measurements performed with and without applied fields in samples of (Nd1-xSmx)2CuO4 showed the presence of a maximum at low temperatures in all samples studied. The Sm concentration (x) dependence of the temperature of this maximum showed two regions with different behavior. For x > 0.7, the specific heat curve is similar to the one observed in ordinary diluted antiferromagnetic compounds. The temperature of the maximum decreases with increasing applied magnetic field. On the other hand, for x < 0.7, the effect is similar to the one observed for the pristine Nd2CuO4. The Nd2CuO4 behavior is not the one observed in ordinary antiferromagnets, since the Nd-Cu interaction causes a splitting of the ground state level of the Nd sublattice and, consequently, a Schottky like behavior in the specific heat measurements.

Este trabalho apresenta o estudo das propriedades físicas de amostras de KxMoO2-? e MoOy. São reportados resultados de difratometria de raios X e de nêutrons, cálculo de densidade de estados pela teoria do funcional da densidade, medidas de espectroscopia de fotoemissão, resistividade elétrica, magnetização, calor específico e expansão térmica. Resultados de difratometria de raios X e de nêutrons, com ajustes baseados em simulações realizadas com o programa GSAS, permitiram selecionar as amostras monofásicas e de boa qualidade. As medidas de espectroscopia de fotoemissão mostram que a existência de alta fração Mo3+/Mo4+ em diversas amostras de KxMoO2-? e MoOy é a responsável pela anomalia da resistividade elétrica descrita pela lei de potência prevista no modelo do líquido de Luttinger (LL). A observação da anomalia nas curvas de resistividade elétrica em amostras de MoO2 deficientes em oxigênio suporta que tal comportamento está relacionado à presença de íons Mo3+ e não com a presença de K. Resultados de magnetização e resistividade elétrica confirmaram a existência de supercondutividade no sistema K-Mo-O. Foi observada que a temperatura crítica (TC) é função da composição inicial de K, sendo que foi encontrada a mais alta TC reportada para este sistema (TC ~9,5 K). Medidas de magnetização mostraram ainda a coexistência de supercondutividade e magnetismo no composto KxMoO2-?. Foi ainda observada uma transição do tipo metal de Bose para isolante em uma amostra de KxMoO2-? que reforça a evidência da formação de pares de Cooper nesse sistema. Por fim, medidas de calor específico, expansão térmica e resistividade elétrica evidenciaram a existência de uma transição de fase de primeira ordem em altas temperaturas (entre 250 e 270 K) tanto em amostras policristalinas de KxMoO2-? e MoOy quanto em monocristais de MoO2. A transição observada no MoO2 nunca foi reportada para este material. Esta transição é discutida com base na movimentação dos átomos de Mo existentes nas ligações unidimensionais de Mo-Mo e nas distorções dos octaedros de MoO6 presentes na sua estrutura cristalina. / This work presents the study of physical properties in polycrystalline samples of KxMoO2-?, and MoOy. We present results of X-ray and Neutrons diffraction patterns, density of states calculations by density functional theory, photoemission spectroscopy, electrical resistivity, magnetization, specific heat, and thermal expansion measurements. X-ray and neutrons diffraction results as well as calculations of the structure and diffractograms carried out by simulations using GSAS shown which the samples are high quality. Photoemission spectroscopy measurements shown the existence of Mo3+/Mo4+ in high fraction for KxMoO2-? and MoOy which are responsible to electrical anomalous behavior described by powder law predicted in the Luttinger Liquid theory. The observation of anomalous is related to Mo3+ ions in the samples. Magnetization and electrical resistivity confirm the existence of superconductivity in K-Mo-O system. It was observed that the critical temperature (TC) is a function of the starting composition of K and the higher TC found out and reported for this system (TC ~9.5 K). Magnetization measurements also have shown the coexistence of superconductivity and magnetism in KxMoO2-?. It was observed a Bose metal-insulator transition in K0.05MoO2-? which further evidence of the formation of Cooper pairs in the system. Finally, specific heat measurements, thermal expansion and electrical resistivity evidenced the existence of first order phase transition in high temperatures (between 250 and 270 K) in KxMoO2-? and MoOy polycrystalline as well as MoO2 single crystal. This transition is discussed looking for Mo atoms moving in the one-dimensional links Mo-Mo and of the distortions of MoO6 octahedra in the crystalline structure.

Estudamos nesta tese três sistemas desordenados distintos das áreas de vidros de spins e fluidos complexos, por meio de modelos estatísticos no contexto da aproximação de campo médio. Analisamos os efeitos da inclusão de graus de liberdade elásticos sobre o diagrama de fases do modelo de Sherrington-Kirkpatrick, que é a versão de campo médio de um modelo popular de vidros de spins, paradigmas de sistemas com desordem temperada. Analisamos em seguida alguns problemas típicos da física dos fluidos complexos. Investigamos o diagrama de fases de um modelo de Maier-Saupe (MS), que é uma espécie de arquétipo das transições nemáticas, numa versão de rede muito simples, denominada modelo de Maier-Saupe-Zwanzig (MSZ), com a introdução de uma variável binária de desordem para representar uma mistura de discos e cilindros. Mostramos que o aparecimento de uma fase nemática biaxial, termodinamicamente estável, que tem sido intensamente procurada na literatura, depende da forma de tratamento das variáveis de desordem. Finalmente, utilizamos o modelo MSZ, na presença de termos elásticos não lineares e de elementos de desordem, a fim de reproduzir diversas características do comportamento termodinâmico dos elastômeros nemáticos, novos materiais poliméricos, com propriedades dos cristais líquidos nemáticos e das borrachas, tema de grande interesse na física da matéria mole. / We study three distinct disordered systems in the areas of spin glasses and complex fluids, by means of mean-field statistical models. We first analyze the effects of compressibility on the phase diagram of the Sherrington-Kirkpatrick model, a mean-field version of a popular model of spin glasses, which are paradigmatic examples of systems with quenched disorder. We then analyze some typical problems in the area of physics of complex fluids. We investigate the phase diagram of a Maier-Saupe model (MS), which is a sort of archetype of nematic transitions, in a simple lattice version called Maier-Saupe-Zwanzig model (MSZ), with the introduction of a binary variable of disorder to mimic a mixture of rod-like and plate-like mesogens. We show that the emergence of a stable nematic biaxial phase, which has been intensely pursued in the literature, depends on the form of treatment of the disorder variables. Finally, we use the MSZ model, in the presence of non-linear elastic terms and elements of disorder, to reproduce several aspects of the thermodynamic behavior of nematic elastomers, new polymeric materials with the properties of liquid crystals and rubber, and of great importance in the area of soft-matter physics.

In this dissertation, we will study solvent-vapor induced spherulitic growth in multicomponent thin films modeled as prestressed elastic solids. The interface between the crystalline phase and the amorphous phase will be treated as an evolving thermodynamic system and no diffusion of any component will be considered. The dissertation is divided into three parts. In Part I we will determine necessary conditions of thermodynamic equilibrium between the two solid phases, the inter- face, and the vapor. In Part II we will derive the thermodynamic driving force for spherulitic growth in multicomponent elastic thin films. In Part III we will investigate the effect of prestress on the directional dependence of the growth. There a formula that delineates how the prestress affects the shape of the spherulite will be proposed.

In this thesis, we study several different renormalization group (RG) methods, including the conventional Wilson renormalization group, Monte Carlo renormalization group (MCRG), exact renormalization group (ERG, or sometimes called functional RG), and tensor renormalization group (TRG). We use the two dimensional nearest neighbor Ising model to introduce many conventional yet important concepts. We then generalize the model to Dyson's hierarchical model (HM), which has rich phase properties depending on the strength of the interaction. The partition function zeros (Fisher zeros) of the HM model in the complex temperature plane is calculated and their connection with the complex RG flows is discussed. The two lattice matching method is used to construct both the complex RG flows and calculate the discrete β functions. The motivation of calculating the discrete β functions for various HM models is to test the matching method and to show how physically relevant fixed points emerge from the complex domain. We notice that the critical exponents calculated from the HM depend on the blocking parameter b. This motivated us to analyze the connection between the discrete and continuous RG transformation. We demonstrate numerical calculations of the ERG equations. We discuss the relation between Litim and Wilson-Polchinski equation and the effect of the cut-off functions in the ERG calculation. We then apply methods developed in the spin models to more complicated and more physically relevant lattice gauge theories and lattice quantum chromodynamics (QCD) like theories. Finite size scaling (FSS) technique is used to analyze the Binder cumulant of the SU(2) lattice gauge model. We calculate the critical exponent nu and omega of the model and show that it is in the same universality class as the three dimensional Ising model. Motivated by the walking technicolor theory, we study the strongly coupled gauge theories with conformal or near conformal properties. We compare the distribution of Fisher zeros for lattice gauge models with four and twelve light fermion flavors. We also briefly discuss the scaling of the zeros and its connection with the infrared fixed point (IRFP) and the mass anomalous dimension. Conventional numerical simulations suffer from the critical slowing down at the critical region, which prevents one from simulating large system. In order to reach the continuum limit in the lattice gauge theories, one needs either large volume or clever extrapolations. TRG is a new computational method that may calculate exponentially large system and works well even at the critical region. We formulate the TRG blocking procedure for the two dimensional O(2) (or XY ) and O(3) spin models and discuss possible applications and generalizations of the method to other spin and lattice gauge models. We start the thesis with the introduction and historical background of the RG in general.

A blue sky, a white cloud or a red sunset are explained by classical multiple scattering theory of light. However, these phenomena neglect interference occurrence. Once it is taken into account, interference in a disordered medium may actually put a halt to the propagation of light, an effect known as Anderson Localization. Until now, experimental reports of Anderson Localization of light in 3D systems have not been conclusive. Our goal is to understand what are the underlying obstacles, and look for new insights from a theoretical point of view. In this dissertation, the properties of a cloud of two-level atoms scattering light are investigated. The dipole-dipole interaction generates collective modes, some of them, being localized. We found that finite-size effects dominate the lifetime of the localized modes, specifically by the ratio of localization length to their distance to the system boundaries. Localized modes saturates at maximum of 20% even above phase transition. Studying the steady-state regime, the coupling between localized modes and light is weak. Both results agrees with the difficulty of experimental evidence of light localization and promote the link of experiments and theory. / Um céu azul, uma nuvem branca ou um por do sol vermelho são explicados pela teoria clássica de espalhamento múltiplo da luz. No entanto, esses fenômenos negligenciam a ocorrência de interferências. Uma vez levada em conta, a interferência em um meio desordenado pode interromper a propagação da luz, um efeito conhecido como Localização de Anderson. Até agora, relatos experimentais de Anderson Localização de luz em sistemas 3D não foram conclusivos. Nosso objetivo é entender quais são os obstáculos fundamentais, e buscar novos insights do ponto de vista teórico. Nesta dissertação, as propriedades de uma nuvem de átomos de dois níveis espalhando luz de é investigado. A interação dipolo-dipolo gera modos coletivos, alguns deles, sendo localizados. Descobrimos que os efeitos de tamanho finito dominam o tempo de vida dos modos localizados, especificamente pela razão entre o comprimento da localização e sua distância até os limites do sistema. Os modos localizados saturam no máximo 20%, mesmo acima da transição de fase. Estudando o regime de estado estacionário, o acoplamento entre modos localizados e luz é fraco. Ambos os resultados concordam com a dificuldade da evidência experimental da localização da luz e promovem a ligação entre experimentos e teoria.

A liga de FeRh apresenta uma transição de fase magneto-estrutural de primeira ordem próxima da temperatura ambiente, fazendo deste material um forte candidato para aplicações de gravação magnética termicamente assistida ou em refrigeradores magnéticos. Em baixas temperaturas o FeRh apresenta uma fase antiferromagnética enquanto em altas temperaturas apresenta uma fase ferromagnética. Cientes disto, neste trabalho foram analisados as transições de fase de filmes de FeRh, porém quando na presença de nanofios de Ni. Para isso foram fabricados filmes de FeRh, próximos da composição equiatômica, sobre uma matriz nanoporosa de Al 2 O 3 com nanofios de Ni. A matriz nanoposora foi produzida por um processo de anodização em dois passos, sendo preenchido os poros com Ni via eletrodeposição. O filme de FeRh foi crescido por deposição via magnetron sputtering, em duas temperaturas distintas, 525 e 600 o C. Um tratamento térmico in situ a 600 o C, por uma hora, foi feito no filme depositado na mesma temperatura. Curvas de magnetização em função de campo magnético aplicado, em temperaturas fixas, mostraram comportamentos característicos de filmes ou nanofios quando medidos nas direções preferenciais de cada componente, respectivamente, sugerindo um forte acoplamento via magnetoestrição entre os elementos. Foi analisada a dependência da transição de fase com relação a condições de magnetização dos nanofios de Ni, sendo observado clara dependência do comportamento da transição de fase com relação à estas condições. A transição de fase do FeRh é dada por nucleações e, posteriormente, por um deslocamento das paredes de domínio adjacentes a nucleação. Este comportamento é bem descrito pela somatória de duas distribuições gaussianas, sendo ajustadas na primeira derivada da magnetização em função da temperatura. Estes ajustes mostraram um claro comportamento da distribuição de nucleação com relação a magnetização remanente dos nanofios. Tal comportamento não tinha sido relatado até este momento. O filme depositado em 525 o C se mostrou mais suscetível a magnetização remanente dos nanofios do que o filme depositado em 600 o C. Posteriormente, medidas de transição de fase sob campo magnético intenso mostraram uma forte relação entre as temperaturas críticas de transição e a intensidade de campo aplicado, sendo deslocada a curva de transição para temperaturas mais baixas devido o incremento do campo. O filme depositado em 525 o C apresentou uma transição mais larga e uma menor magnetização em altas temperaturas do que o filme depositado em 600 o C. / The FeRh alloy presents a first order magnetic-structural phase transition close the room temperature, making this material a strong candidate to heat-assisted magnetic recording or magnetic refrigerators. At low temperaturas the FeRh the FeRh shows a antiferromagnetic phase while at high temperatures shows a ferromagnetic phase. Aware of this, in this work were analyzed the FeRh films phase transition, but under the Ni nanowires presence. For this were maked FeRh films, close the equiatomic composition, over a nanoporous matrix of Al 2 O 3 with Ni nanowires. The nanoporou matrix were produced by a two-step anodization process and the nanoporou fille with Ni via eletrodeposition. The FeRh film were grown by deposition via magnetron sputtering at two different temperatures, 525 e 600 o C. Annealing in situ at 600 o C for one hour was done in the film deposited at same temperature. Magnetization curves as function of applied magnetic field, at fixed temperatures, showed characteristic behaviors of the films or nanowires where measured in the preferential direction of each componente, respectively, suggest a strong magnetostriction coupling between elements. The phase transition dependence was analyzed with respect to magnetization conditions of Ni nanowires, been observed a clear dependence of the phase transition behavior with these conditions. The FeRh phase transition is given by nucleations and followed by a displacement of domain walls adjacents to the nucleation. This behavior is well defined for the sum of two gaussians distributions, been fitted to the first derivative of magnetization as function of temperature. These fittings showed a clear behavior of the nucleation distribution with repect to a nanowires remanent magnetization. Such behavior has never been reported before. The film deposited at 525 o C showed more susceptible to nanowires remanentc magnetization than film deposited at 600 o C. Subsequently meadures of phase transition under high magnetic fields showed a strong relation between the critical temperatures of depostion and the field intensity, being displaced to lower temperatures the transition curve with the field increment. The film deposited at 525 o C presented a broader transition with lower magnetization at high temperatures than film deposited at 600 o C.

Doctor of Philosophy(PhD) / This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.

<p>This thesis presents a theoretical study of bulk materials using <i>ab initio</i> methods based on the density functional theory (DFT).</p><p>Crystallographic structural phase transformations and phase stability for 5f-dioxides, ABO₃ perovskites, and ABO₄ compounds have been extensively studied. Different approaches such as static total energy calculations, elastic stability and dynamical stability (phonon calculations) criteria have been used to determine the phase stability. As a special case, the lattice dynamics of solid Xe has been studied as a function of pressure.</p><p>Dielectric functions and optical constants have been calculated for solar energy cell system CuIn_1-xGa_xSe₂ with concentrations x=0, 0.25, 0.5 and 1.0 as well as for C₆₀, PbWO₄and δ-AlOOH. The absorption coefficient provides information about the optimum solar energy conversion efficiency. We have derived absorption coefficients for a number of compounds. Comparisons between the calculated and experimental dielectric functions and absorption coefficients have been made.</p><p>The main part of this thesis focuses on the nanolayered ternary compounds M _N+1AX_N(MAX), where N = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N. These ternary carbides and nitrides combine unusual properties of both metals and ceramics. They exhibit high hardness, but fully reversible plasticity, and negligible thermoelectric power. These excellent properties make the MAX phases another new class of materials with versatile technological applications. Our work presents a systematic study of the electronic, bonding, elastic and optical properties of the MAX phases. A new MAX phase-Ti₄SiC_3,is calculated to be stable, and at the same time also been synthesized by experimentalists. Surface energy calculations have also been performed for the (0001) surface of the Ti-Si-C system. The general relations between the electronic structure and materials properties of the MAX phases have been elaborated in the thesis.</p>

Ab Initio calculations suggest that partially lithiated layered LixMnO₂ transforms to spinel in a two-stage process. In the first stage, a significant fraction of the Mn and Li ions rapidly occupy tetrahedral sites, forming a metastable intermediate. The second stage involves a more difficult coordinated rearrangement of Mn and Li ions to form spinel. This behavior is contrasted to LixCoO₂. The susceptibility of Mn for migration into the Li layer is found to be controlled by oxidation state which suggests various means of inhibiting the transformation. These strategies could prove useful in the creation of superior Mn based cathode materials. / Singapore-MIT Alliance (SMA)