Quantum phase transitions (QPT) on the border of magnetism have provided a fertile hunting ground for the discovery of new states of matter, for example; the marginal Fermi Liquid and non Fermi Liquid states as well high T$_C$ cuprate and magnetically mediated superconductivity. In this thesis I present work on three materials in which it may be possible to tune the system through a magnetic QPT with the application of hydrostatic pressure. Although the details of the underlying physics are different in each of the materials, they are linked by the possibility of finding new states on the border of magnetism. Applying hydrostatic pressure, we have suppressed the ferromagnetic (FM) transition in metallic Fe$_2$P to very low temperature and to a potential QPT. Counter-intuitive broadening of the magnetic hysteresis leading up to the FM-AFM QPT may well be a crucial clue as to the nature of the model needed to understand this phase transition. A sharp increase in the quasi-particle scattering cross-section as well as the residual resistivity accompany a departure from the quadratic temperature dependence of the resistivity. This possible deviation from Fermi liquid behaviour is stable over a significant range of temperature. The unexplained upturn in the resistivity of CeGe that accompanies the AFM transition was studied under pressure. Pressure increased the residual resistivity as well as decreasing the relative size of the upturn, but had a moderate effect on the Neel temperature. The insensitivity of the N$\acute e$el temperature to pressure has been compared to its relative sensitivity to applied feld. The existence of the upturn and its evolution with pressure and applied feld can reasonably be argued to be due to the details of the electron band structure in the system. By applying pressure we have drastically reduced the resistivity of the insulating antiferromagnet NiPS$_3$. Concurrent work on FePS$_3$ has shown metallisation under pressure. It seems reasonable to speculate that NiPS$_3$ may also metallise at higher pressure. The energy gap is narrowed in both materials as pressure is increased. Magnetisation measurements have revealed a low temperature upturn indicating some possible ferromagnetic component or proximity to another magnetic state. A peak in the magnetisation is also seen at 45K in zero-feld cooled measurements. Both of these features point to a system with a complex magnetic ground state.

Orientadores: Cris Adriano, Eduardo Granado Monteiro da Silva / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-02T06:12:12Z (GMT). No. of bitstreams: 1 Tasca_KelinRegina_D.pdf: 6364872 bytes, checksum: 7c505073990103d19ade00e5fcace964 (MD5) Previous issue date: 2017 / Resumo: Neste trabalho a dinâmica da transição de fase em sistemas de nanopartículas de óxido de titânio e de supercondutores de altas temperaturas críticas à base de Fe e a dinâmica da magnetização do NiO foram investigadas através de experimentos de difração de raios-X resolvida no tempo... O resumo poderá ser visualizado no texto completo da tese digital / Abstract: In this work the dynamics of the phase transition in systems of titanium oxides nanoparticles and hight Tc iron based superconductors and the magnetization dynamics of NiO were investigated by experiments of time resolved x-ray diffraction... The abstract is available with the full electronic document / Doutorado / Física / Doutora em Ciências / 1094215/2012 / CAPES

Among the great breakthroughs in nanoscience and nanotechnology is the emergence of synthetic polymers that demonstrate biological activity and thus can be exploited for biomedical applications, extending from agents in therapeutics to drug delivery and tissue engineering. A key factor in the fabrication of such polymeric materials is the ability to tune and control their properties. To this end, an insight into the mode of interactions with biological systems is imperative. Computer simulations have proved to be a valuable tool that can compliment experiments and provide -otherwise inaccessible- information. In the context of this thesis, different aspects of the polymeric biological activity were investigated by studying two polymeric materials suitable for different types of applications, aiming to clarify yet undisclosed mechanisms that govern the polymers' behaviour either in solution or in conjunction with model lipid membranes. The first part of the thesis is dedicated to a nonionic amphiphilic copolymer known as Pluronic L64 that is considered as a candidate for the design of novel hybrid polymer-lipid vesicles that will act as carriers for drugs or genes. The hybrid bilayers are subjected to mechanical stress and their properties are compared to those of pure lipid bilayers. The simulations showed that the hybrid membranes can sustain increased surface tension prior to rupture, are stiffer, thicker and the polymers can induce higher lipid tail packing and also reduce the lipid mobility, rendering the membranes more ordered and less fluid. At high values of lateral pressure, which leads to pore formation, the copolymer chains decelerate the pore growth. The examination of the defect formation mechanism reveals that the hydrophilic PEO segment plays the most vital role. The same systems were also observed in varying temperatures and the impact of the inserted polymers on the phase behaviour was investigated. The data suggested that the polymers change the nature of the phase transition from a discontinuous to a continuous one. The hybrid membranes transform between the ordered and the disordered phase in a continuous manner and not at a critical melting temperature. Interestingly, the effect of polymers is different at the low and high temperature regions, as proved by the analysis of the mechanical, structural and dynamic membrane properties. The second part is focused on the study of polyhexamethylene biguanide (PHMB), a biguanide-based polyelectrolyte, that possesses remarkable biocidal properties. Even though PHMB's activity is known, the specific mode of action against bacterial membranes is still puzzling. Our work revealed that the polyelectrolyte assumes a counterintuitive behaviour in aqueous solution tending to self-organise into ordered compact structures, despite the repulsive electrostatic interactions of its positively charged segments. The formed nano-objects are thermodynamically stable, as was confirmed by free energy calculations and could be linked to PHMB's antibacterial mechanism. These findings pave the way for further computational and experimental exploration of these fascinating and promising materials that could lead to the design of novel smart biologically active nanoparticles.

This PhD project is focused on three lead-free ferroelectric solid solutions, which are specifically Na0.5Bi0.5TiO3-KNbO3(NBT-KN), Na0.5Bi0.5TiO3-NaNbO3(NBT-NN) and Na0.5Bi0.5TiO3-BaTiO3(NBT-BT), to evaluate the effects of composition, electric field and temperature on structural and electrical properties. Novel observations of both reversible and irreversible electric field-induced phase switching were made in both NBT-KN and NBT-NN ceramics. The NBT-KN solid solution is the primary focus of this thesis. All compositions were observed to be cubic in the as-sintered, unpoled state. However, a well-defined ferroelectric hysteresis P-E loop was obtained for compositions with low KN contents, indicating that an irreversible phase transition from a weak-polar relaxor ferroelectric (RF) to a long-range ordered metastable ferroelectric (FE) state had occurred during the measurement procedure. Both the unpoled and poled ceramic powders were examined using high resolution synchrotron XRD. For the poled state, a rhombohedral R3c structure was identified for compositions with low KN content, confirming the occurrence of the irreversible electric field-induced structural transformation from cubic to rhombohedral. In contrast, a cubic structure was retained for high KN contents, giving rise to reversible phase switching evidenced by constricted P-E hysteresis loops. Similar behaviour was observed for NBT-NN system. An 'in-situ' electric field poling experiment was conducted using high energy synchrotron XRD. In certain NBT-KN compositions the structural transformation, from cubic to mixed phase cubic+rhombohedral and finally single phase rhombohedral, occurred progressively with increasing cycles of a bipolar electric field. Similar behaviour was observed for NBT-NN compositions having low NN contents. Furthermore, the distributions of domain orientation and lattice strain over a range of orientations relative to the poling direction were determined for NBT-KN, NBT-NN and NBT-BT ceramics exhibiting the rhombohedral phase. By combining the structural information with the results of dielectric and ferroelectric measurements, a phase diagram was constructed to illustrate the influence of temperature and composition on the stability of the metastable ferroelectric and relaxor ferroelectric states for the NBT-KN system. Furthermore, the phase transition temperatures obtained from dielectric measurements were correlated with the ferroelectric and thermal depolarisation characteristics for each of the NBT-KN, NBT-NN and NBT-BT systems.

Ce travail de thèse vise à modéliser et simuler la séparation de phase d’un système polymère solvant afin de comprendre, prédire et maitriser la dynamique de formation de membranes poreuses. L’équation phénoménologique proposée de Cahn-Hilliard, couplée à un champ de vitesse (Modèle H dans la classification de Hohenberg et Halperin) sera utilisée pour décrire l’inversion de phase induite par un changement de température (procédé TIPS), le système PMMA/cyclohexanol étant pris comme système de référence.Dans un premier temps, nous nous sommes attachés à étudier de façon systématique l’influence du terme de mobilité sur la dynamique de séparation de phase, sans couplage avec l’hydrodynamique en 2D. Un analyse des lois de croissance des structures formées a été réalisée via une analyse des images par transformée de Fourier et une analyse spécifique des patterns a été effectuée via l’analyse originale des descripteurs de Minkowski. Dans une seconde partie de la thèse, nous avons étudié le couplage entre la séparation de phase et l’hydrodynamique, en 2D et 3D. Enfin, nous avons intégré dans une dernière partie le couplage entre la séparation de phase, l’hydrodynamique et les phénomènes de transferts de matière aux interfaces induits notamment par l’évaporation du solvant au cours de la formation d’une membrane. / The thesis aimed at modeling and simulating the phase separation dynamics of a polymeric system (polymer/solvent or polymer/solvent/non-solvent) for better understanding and controlling the formation mechanisms of porous polymeric membranes. The equation of Cahn and Hilliard (1956)[1] was used and coupled to a hydrodynamic model (H model in the classification of Hohenberg et Halperin (1977)[2]) to simulate the phase inversion in closed system.In a first stage, hydrodynamics was not coupled to Cahn-Hilliard equation and the work focused on the mobility term in the Cahn-Hilliard equation and its influence on the phase separation dynamics (pattern evolution, growth law of the characteristic structures). The patterns were analyzed by Fourier transform and an original analysis was also performed using the Minkowski descriptors.In a second part of the thesis, we coupled the Cahn-Hilliard equations with Navier-Stokes equations and the influence of this coupling on the membrane formation dynamics was investigated.Then, in order to simulate the membrane formation in a context closer to reality, transfer phenomena were simulated at the upper interface of the domain to describe solvent evaporation during demixing process.Finally, the simulations were extended to ternary systems (polymer/solvent/non-solvent) systems since most of industrial polymer membranes involve are prepared from ternary system, with NIPS process (Non-solvent Induced Phase Separation) or dry casting process (differential evaporation between solvent and non-solvent).

Foi desenvolvido o aparato experimental necessário à realização de experimentos de RMN em baixas temperaturas, com o objetivo de investigar fenômenos críticos nos sistemas antiferromagnéticos diluídos FexZn1-xF2. Estudou-se a dependência do segundo momento da linha de ressonância do F0 com a temperatura, desde T &#8776 TN até T &#8776 250K, através de medidas experimentais e simulações numéricas baseadas num modelo de campo médio. Também foi estudada a dependência angular da meia largura e da forma da linha. Os resultados das simulações numéricas concordam qualitativamente, mas não quantitativamente, com os resultados experimentais. Encontramos também a indicação de uma distribuição em T2 ao longo da linha de ressonância do F0, quando H0 é orientado perpendicularmente ao eixo C. O expoente crítico da magnetização do REIM foi medido diretamente a partir do segundo momento da linha do F0, após um esfriamento da amostra sem campo aplicado (Zero Field Cooling). O resultado obtido &#946=0.36 &#177 0.01 concorda com o valor esperado &#946= 0.35 / The experimental apparatus required for Nuclear Magnetic Resonance low temperatures experiments was developed for the investigation of critical phenomena in the diluted antiferromagnetic FexZn1-xF2. The dependence of the second moment of the F0 resonance line with the temperature was studied from T &#8776 TN to T &#8776 250K, both by experimental measurements and numerical mean field simulations. Angular dependence of the line width and line shape were also investigated. The numerically simulated results qualitatively agreed, but quantitatively didn´t agree with the experimental results. We found an indication of a distribution in T2 through the F0 resonance line, when H0 is oriented perpendicularly to the C axis. The magnetization critical exponent &#946 of the REIM was measured directly from the second moment of the F0 resonance line, after a zero field cooling procedure. We obtained &#946= 0.36 &#177 0.01 which is to be compared with the theoretical value &#946= 0.35

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Na natureza há vários fenômenos envolvendo transições de fase com quebra ou restauração de simetrias. Tipicamente, mudanças de fase, são associadas com uma quebra ou restauração de simetria, que acontecem quando um determinado parâmetro de controle é variado, como por exemplo temperatura, densidade, campos externos, ou de forma dinâmica. Essas mudanças que os sistemas sofrem podem levar a formação de defeitos topológicos, tais como paredes de domínios, vórtices ou monopolos magnéticos. Nesse trabalho estudamos particularmente mudanças de fase associadas com quebras ou restaurações dinâmicas de simetria que estão associadas com formação ou destruição de defeitos do tipo paredes de domínio em modelos de campos escalares com simetria discreta. Nesses processos dinâmicos com formação ou destruição de domínios, estudamos a possibilidade de formação de estruturas do tipo oscillons, que são soluções não homogêneas e instáveis de campo, mas que podem concentrar nelas uma quantidade apreciável de energia e terem uma vida (duração) suficientemente grande para serem de importância física. Estudamos a possibilidade de formação dessas soluções em modelos de dois campos escalares interagentes em que o sistema é preparado em diferentes situações, com a dinâmica resultante nesses sistemas estudada numa rede discreta. / In nature there are several phenomena involving phase transitions with symmetries breaking or restoration. Typically,the phase state changes associated with a break or a restoration of the sistem's symmetry, that occur when a particular control parameter is varied, such as as temperature, density, external fields or dynamically. These systems undergo changes which can lead to formation of defects, such as domain walls, vortices or magnetic monopoles. In this work we study particularly phase changes, breaks or restorations, associated with dynamic symmetry that are associated with the formation or destruction of defects, as domain walls in models of scalar fields with discrete symmetry. In these dynamical processes with formation or destruction of domains we studied the possibility of forming oscilons type structures, which are not homogeneous and unstable field solutions, but can concentrate there in an appreciable amount of energy and have a lifetime (duration) large enough to have physical importance. We study the possibility of formation of oscillon kind solutions in models with scalar fields interacting in which the system is prepared for different situations, with the resulting dynamics of these systems studied in a discrete lattice.

Apresentamos nesse trabalho uma proposta de dois modelos aleatórios para a evolução de espécies. O primeiro para a evolução via seleção natural por meio de competição entre espécies e o segundo por mutação. No primeiro caso consideramos uma versão do modelo proposto por Guiol, Machado e Schinazi no caso subcrítico. Para tal modelo derivamos a distribuição da aptidão de sobrevivência ao meio do indivíduo mais forte ao longo de uma excursão ao zero. Para o segundo modelo proposto, consideramos a mutação como uma estratégia para sobrevivência ao meio e provamos condições para sobrevivência e morte quando o modelo evolui sobre a árvore binária orientada com raiz, provando a existência de transição de fase no modelo. / Here we present a proposal for two random models for the evolution of species. To the first one we consider the evolution via natural selection through competition between species and to the second one by mutation. In the first case we consider a version of the model proposed by Guiol, Machado and Schinazi at the subcritical case. For this model we derive the distribution of fitness of the strongest individual along a excursion to zero. For the second one we consider mutation as a strategy for survival and we prove conditions for death and survival when the model evolves on the binary tree with root oriented and we prove the existence of phase transitions in the model.

Associamos independentemente a cada vértice v de un grafo infinito G um raio de infecção aleatório R_v e definimos um modelo de percolação sujeito às seguintes regras: (1) no tempo zero só a raiz é declarada infectada, (2) um vértice é declarado infectado em um instante t, t>0, se está a uma distância no maximo R_v de algum vértice v previamente infectado, e (3) vértices infectados permanecem infectados para sempre. Dizemos que há sobrevivência em uma realização particular do modelo se o número final de vértices infectados é infinito. Neste trabalho damos condições suficientes sobre o grafo G para a transição de fase deste modelo, estabelecendo limitantes não triviais para o parâmetro crítico quando os raios R_v têm distribuição geometrica de parâmetro 1-p. Além disto, restringindo nosso estudo para o caso das árvores esfericamente simétricas, obtemos um melhor limitante superior para este parâmetro. Finalmente, concluímos que o parâmetro crítico para o modelo nas árvores homogêneas de grau d+1 se comporta assintoticamente como 1/(2d). / We assign independently to each vertex v of an infinite graph G, a random radius of infection R_v and define a percolation model subject to the following rules: (1) at time zero, only the root is declared infected, (2) a vertex is declared infected at time t, t>0, if it is at distance at most R_v of some vertex v previously infected, and (3) infected vertices stay infected forever. We say that there is survival in a particular realization of the model if the final number of infected vertices is infinite. In this work, we give sufficient conditions on the graph G for the phase transition of this model, by stating non-trivial bounds for the critical parameter when the radii have geometrical distribution with parameter 1-p. In addition, restricting our study to the case of the spherically symmetric trees, we obtain an improved upper bound for this critical parameter. Finally, we conclude that the critical parameter for the model on homogeneous trees of degree (d+1) behaves asymptotically as 1/(2d).

Estudamos um sistema de partículas a tempo discreto cuja dinâmica é a seguinte. Considere que no instante inicial sobre cada inteiro não negativo há uma partícula, inicialmente inativa. A partícula da origem é ativada e instantaneamente ativa um conjunto aleatório contíguo de partículas que estão a sua direita. Como regra, no instante seguinte ao que foi ativada, cada partícula ativa realiza esta mesma dinâmica de modo independente de todo o resto. Dizemos que o processo sobrevive se em qualquer momento sempre há ao menos uma partícula ativa. Chamamos este processo de Firework, associando a dinâmica de ativação de uma partícula inativa a uma infecção ou explosão. Nosso interesse é estabelecer se o processo tem probabilidade positiva de sobrevivência e apresentar limites para esta probabilidade. Isto deve ser feito em função da distribuição da variável aleatória que define o raio de ação de uma partícula. Associando o processo de ativação a uma infecção, podemos pensar este modelo como um modelo epidêmico. Consideramos também algumas variações dessa dinâmica. Dentre elas, variantes com partículas distribuídas sobre a semirreta dos reais positivos (nesta vertente, existem condições para as distâncias entre partículas consecutivas) e também com as partículas distribuídas sobre vértices de árvores. Estudamos também para esses casos a transição de fase e probabilidade de sobrevivência. Nesta variante os resultados obtidos são funções da sequência de distribuições dos alcances das explosões e da estrutura dos lugares onde se localizam as partículas. Consideramos também variações do modelo onde cada partícula ao ser ativada, permanece ativa durante um tempo aleatório e nesse período emite explosões que ocorrem em instantes aleatórios. / We studied a discrete time particle system whose dynamic is as follows. Consider that at time zero, on each non-negative integer, there is a particle, initially inactive. A particle which is placed at origin is activated and instantly activates a contiguous random set of particles that is on its right. As a rule, the next moment to what it has been activated, each active particle carries the same behavior independently of the rest. We say that the process survives if the amount of particles activated along the process is infinite. We call this the Firework process, associating the activation dynamic of a particle to an infection or explosion process. Our interest is to establish whether the process has positive probability of survival and to present limits to this probability. This is done according to the distribution random variable that defines the radius of infection of each active particle, Associating the activation process to an infection, we think this model as a model epidemic. We also consider some variations of this dynamic. Among them, variants with particles distributed over the half line (there are conditions for the distances between consecutive particles) and also with particles distributed over the vertices of a tree. We studied phase transitions and the correspondent survival probability. In this variant the results depend on the sequence of probability distributions for the range of the explosions and on the particles displacement. We also consider a variation where each particle after activated, remains active during a random time period emitting explosions that occur in random moments.