Orientador: Alex Antonelli / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-21T01:39:17Z (GMT). No. of bitstreams: 1 Garcez_KarlMarxSilva_D.pdf: 2581955 bytes, checksum: 2dfa2df8f3ea59af33216b429c53d794 (MD5) Previous issue date: 2012 / Resumo: Neste trabalho, investigamos os efeitos da pressão sobre as transições entre as fases líquidas e amorfas em silício sobre regime super-resfriado. A investigação é feita através de simulações computacionais, onde técnicas eficientes para cálculo da energia livre, são implementadas sobre o método Monte Carlo. Nossos cálculos, utilizam um potencial interatômico dependente do ambiente local para o Si e confirmam a existência, na região super-resfriada, de uma transição líquido-líquido, entre um líquido de alta densidade e um líquido de baixa densidade. Este líquido de baixa densidade, se mantido sob resfriamento, sofre uma transição vítrea e torna-se um amorfo de baixa densidade. Em pressão nula, a transição líquido-líquido ocorre a cerca de 325 K abaixo da temperatura de fusão encontrada em nossas simulações. Nossos resultados mostram que temperatura de transição líquido- líquido diminui com o aumento da pressão. O aumento da pressão leva a curva de coexistência líquido-líquido à região onde o líquido de baixa densidade torna-se um vidro. De maneira que para pressões acima de 5 GPa, os resultados mostram que a transição líquido-líquido é suprimida pela dinâmica do sistema vítreo. Nós também descobrimos que acima de 5 GPa, a temperatura de transição vítrea é menor do que em pressões mais baixas, sugerindo que sob estas condições a transição vítrea ocorre entre um líquido de alta densidade e um amorfo de alta densidade / Abstract: In this study, we investigated the effects of pressure on the transitions between liquids and amorphous phases of silicon on supercooled regime. Research is done through computer simulations, where efficient techniques for calculating the free energy, are implemented on the Monte Carlo method. Our calculations uses an environment-dependent interatomic potential for Si and confirms, in the supercooled region, the existence of a liquid-liquid transition between a high density liquid and a low density liquid. This low density liquid, if kept under cooling, undergoes a glass transition and becomes a low density amorphous. At zero pressure, the liquid-liquid transition occurs at about 325 K below the melting temperature found in our simulations. Our results show that the liquid- liquid transition temperature decreases with increasing pressure. The increased pressure leads the liquid-liquid coexistence curve to a region where the low-density liquid becomes a glass. So that for pressures above 5 GPa, the results show that the liquid-liquid transition is suppressed by the glassy dynamics of system. We also found that above 5 GPa, the glass transition temperature is lower than at lower pressures, suggesting that under these conditions the glass transition occurs between a high density liquid and a high density amorphous / Doutorado / Física / Doutor em Ciências

Orientador: Maurice de Koning / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-13T12:16:33Z (GMT). No. of bitstreams: 1 CarvajalJara_DiegoAlejandro_M.pdf: 4649880 bytes, checksum: d014cd5c835938e4643e478fa70353be (MD5) Previous issue date: 2009 / Resumo: Recentemente tem sido proposta a existência de uma transição líquido-líquido em substâncias puras com o propósito de explicar alguns comportamentos anômalos como os incrementos de funções resposta (compressibilidade isotérmica, coeficiente de expansão térmica, calor específico) ao diminuir a temperatura de um líquido. A existência deste tipo de transição foi demostrada experimentalmente para o fósforo por dispersão de raios X, e através de simulações atomísticas ou de primeiros princípios para a água, o silício, o carbono, etc. A compreensão detalhada deste tipo de transição está intimamente relacionada à questão fundamental de quais fatores físicos controlam as propriedades de um líquido, e portanto o estudo desta é de grande importância para o desenvolvimento de novas tecnologias, a síntese de novos materiais e o controle de suas propriedades. Neste trabalho realizamos simulações atomísticas sobre um sistema de 1152 partículas de Gálio submetidas a um potencial semi-empírico MEAM, com condições periódicas de contorno. Com estas simulações procuramos uma transição líquido-líquido no Gálio através de um processo de eliminação de três teorias. Inicialmente mostramos que o limite de metaestabilidade do líquido super-aquecido tem um comportamento monótono decrescente no plano de fase P-T. Posteriormente nosso sistema apresenta histerese, uma descontinuidade no volume, estruturas locais diferentes, duas fases que fluem, e um calor latente característico de uma transição de fase de primeira ordem. Por todas estas razões concluímos que o sistema simulado apresenta um transição líquido-líquido de primeira ordem. Adicionalmente foram realizadas compressões e expansões isotérmicas para temperaturas diferentes, observando que estes processos também apresentam histerese e que ela diminui com o aumento da temperatura, indicando assim a possível existência de um segundo ponto crítico e a finalização da transição líquido-líquido. Finalizamos o trabalho com a obtenção e a caracterização de uma possível nova fase cristalina do Gálio cuja estrutura ainda não tem sido obtida experimentalmente. Esta fase foi obtida por casualidade durante o estudo da existência de uma transição líquido-líquido no Gálio ao tencionar o Gálio a -1.6GPa. Sua estrutura é do tipo ortorrômbica com uma simetria Cmcm (grupo espacial 63) e sua principal diferença do Gálio-I é a orientação dos dímeros de Gálio, que nesta nova fase estão dispostos paralelamente. Simulações por DFT mostraram que esta nova fase é metaestável a pressão nula e chega a ser estável a pressões negativas abaixo de ~ 1.5GPa. / Abstract: Recently, the existence of a liquid-liquid transition in pure substances has been proposed as an explanation of anomalous behaviors such as the increase of response functions (isothermal compressibility, coefficient of thermal expansion, specific heat) with decreasing temperature displayed by some liquids. The existence of this type of transition has been demonstrated experimentally for phosphorous by X-ray diÿraction, and through atomistic simulations for water, silicon and carbon. The detailed understanding of this type of transition is closely related to the fundamental question of which physical factors control the properties of a liquid. Therefore, the study of this phenomenon is of great importance for the development of new technologies, the synthesis of new materials and the control of their properties. In this work, we carry out a series of atomistic simulations of a system containing 1152 Gallium atoms described by a semi-empirical Modified Embedded-Atom Model subject to periodic boundary conditions. By means of these simulations we search for a liquid-liquid transition in Gallium by means of a process of elimination of three theories. Initially we show that the limit of metastability of the superheated liquid has a decreasing monotonous behavior in the pressure-temperature phase diagram. Subsequently, our system presents hysteresis, a discontinuity in volume, two phases that have different local atomic structures and display diffusion, and a latent heat, all characteristic of a first-order phase transition. For all these reasons we conclude that the simulated system presents a liquid-liquid phase transition of first-order in the supercooled regime. In addition, we carried out several simulations of isothermal compressions and expansions for different temperatures. These results also show hysteresis although it is found to decrease with increasing temperature, thus indicating the possible existence of a second critical point at which the liquid-liquid transition ends. We finish our studies with the discovery and characterization of a crystalline phase of Gallium whose structure has not been observed experimentally. This phase was obtained by chance during the study of the existence of a liquid-liquid transition in Gallium under tension of -1.6GPa. Its structure is of the orthorhombic type with Cmcm symmetry (space group 63). Its main difference with respect to the Gallium-I phase is that in the new phase the Gallium dimers are disposed in a parallel fashion. Subsequent DFT simulations show that this new phase is metastable at zero pressure and predict it to become stable with respect to Gallium-I arrives at negative pressures below ~1.5GPa. / Mestrado / Física da Matéria Condensada / Mestre em Física

Dans un contexte d’augmentation constante des volumes de boues d’épuration à traiter, l’optimisation du traitement des boues est un enjeu primordial. Les étapes de traitement, et de transport mettent en jeu des écoulements qu’il est nécessaire de comprendre et de prédire afin, par exemple, de pour pouvoir estimer les pertes de charges en conduite ou bien pour dimensionner les installations de pompage. D’un point de vue physique, les boues peuvent être considérées comme une suspension de particules dans un gel suspendant. Ainsi, le comportement rhéologique des boues d’épuration présente des similitudes importantes avec les suspensions colloïdales et les gels polymériques. Ces trois types de matériaux, i.e. les boues d’épuration, les gels colloïdaux et les suspensions polymériques, présentent un comportement rhéologique complexe dépendant du temps et de la sollicitation imposée. Ils présentent un comportement dual, solide aux contraintes faibles, et liquide pour des contraintes élevées. La transition solide-liquide est généralement modélisée par la définition d’un seuil de contrainte ou de déformation, supposé séparer les régimes solide et liquide. Cependant, cette notion de seuil suppose une transition abrupte, et s’oppose aux observations expérimentales qui mettent en évidence une transition continue et progressive. L’étude de la littérature a permis de mettre en évidence une nécessité d’améliorer la compréhension et la modélisation du phénomène de transition solide-liquide. De plus, il est nécessaire d’unifier la description des régimes solide et liquide sous un même modèle, afin de mettre en lien une continuité mathématique avec le caractère continu et progressif du phénomène physique modélisé. Une analyse des résultats disponibles dans la littérature nous a permis de construire un modèle mathématique unique pour décrire le comportement solide et le comportement liquide des matériaux étudiés. Les hypothèses posées à partir de la littérature pour construire ce modèle ont ensuite été validées expérimentalement. Le modèle proposé est basé sur la décomposition de la complaisance du matériau en la somme d’une contribution solide et d’une contribution liquide, dépendant du temps, de la sollicitation appliquée et de l’histoire du matériau. Ce modèle permet une description commune des comportements solides et liquides du matériau, en tenant compte de l’existence d’une élasticité résiduelle y compris pour des contraintes élevées, et d’une dissipation visqueuse faible pour les contraintes faibles, conformément aux résultats expérimentaux. Ces travaux de thèse ont permis de mettre en évidence le fait que le mécanisme de transition solide-liquide était piloté non pas par la contrainte ou par la déformation, mais par la complaisance du matériau. De plus, ils ont permis d’ouvrir la voie à une nouvelle manière d’appréhender la thixotropie et la transition solide-liquide des matériaux pâteux. En effet, le comportement d’un matériau pâteux est piloté par deux paramètres : un module élastique plateau correspondant à un état totalement structuré, et une viscosité infinie correspondant à un état totalement déstructuré. Ces paramètres intrinsèques au matériau sont alors pondérés par des évolutions de la microstructure, menant à une compétition entre les effets élastiques et les effets visqueux. Ainsi, la différence entre un comportement de type loi de puissance et un comportement de type loi de puissance à seuil peut être expliquée simplement par l’apparition d’effets élastiques non négligeables. / In a context of constant increasing volumes of wastewater treatment sludge, optimizing the treatment of sludge appears to be crucial. Each step of treatment and transportation involves flows. It appears necessary to understand and predict these flows in order, for example, to estimate pressure drops in pipes or to size properly pumping facilities. In a physical point of view, sludge can be considered as a suspension of particles in a gel. Thus, its rheological behaviour presents significant similarities to that of colloidal suspensions of polymeric gels. These three types of materials, i.e. wastewater treatment sludge, colloidal suspensions and polymeric gels, present a complex rheological behaviour which depend on both time and the applied solicitation. They exhibit a dual behaviour, solid at low shear stresses, and liquid when the applied shear stress is high. The solid-liquid behaviour is generally modelled by defining a critical shear stress or a critical strain, supposed to be the limit between the solid and liquid regimes. Nevertheless, this concept implies an abrupt transition, unlike experimental observations showing a continuous and progressive transition. The study of the literature permitted to highlight the need to improve the understanding and modelling of the solid-liquid transition. Moreover, it appears necessary to unify the description of the solid and liquid regime in a unique model, in order to link a mathematical continuity with thecontinuous and progressive nature of the physical phenomenon to model. The study of the results available in the literature permited us to build a unique mathematical model to describe both the solid behaviour and the liquid behaviour of the studied materials. The assumptions made from the literature results have thus been experimentally validated. The proposed model is based on the decomposition of the compliance of the material in the sum of a solid contribution and a liquid contribution, depending on time, the applied solicitation and the story of the material. This model permits a unique description of solide and liquid regimes of the material, taking into account the existence of a residual elasticity at high shear stresses, and a viscous dissipation for low shear stresses, in accordance with experimental results. This work permitted to highlight the fact that the solid-liquid transition mecanism is controlled by the compliance of the material, and not the shear stress or the strain. Moreover, it opened the way to a new way of understanding the thixotropy and the solid-liquid transition of pasty materials. Thus, the behaviour of a pasty material is controlled by two parameters : a plateau elastic modulus corresponding to a totally structured state, and an infinite viscosity corresponding to a totally destructured state. These parameters intrinsic to the material are pondered by the evolutions of the microstructure, leading to a competition between elastic and viscous effects. Thus, the difference between the power law behaviour and the Herschel-Bulkley behaviour can be simply explained by the apparition of elastic effects that can’t be neglected.

Diffusion plays an important role in a number of processes like heterogeneous catalysis, corrosion, separation and purification of chemicals of industrial importance, steel hardening, fuel cells, and solid electrolytes for batteries. It also plays a vital role in several biological processes like transport across biomembranes, nerve impulse, flow of blood and permeation of ingested drug. The elementary process of diffusion in solids is quite different from those in liquids. Similarly, the mode of diffusion in porous solid where different regimes such Knudsen regime exists bears little similarity to those in a dense close-packed crystalline solid. Chapter 1 provides a brief introduction to basics of diffusion in different phases of condensed matter. Among the various phases discussed are liquids, close-packed crystalline solids (e.g., body-centered cubic solids), amorphous solids (e.g. glasses) and microporous crystalline solids (e.g., zeolites). Diffusion in these widely differing phases often bears no resemblance to each other; the rate of diffusion in these phases varies over many orders of magnitude and the elementary step and mechanism in the diffusion process are very different. Brief introduction to theories for diffusion in these phases is provided. Various experimental techniques to measure diffusivities are discussed. Different microscopic models to explain the Quasi Elastic Neutron Scattering (QENS) spectra of these phases yield an insight into the elementary step of the diffusion process. Notwithstanding the fact that completely different models are invoked to explain diffusion in different phases, there are certain underlying generic behaviour across these widely differing phases as the recent work on size dependence of diffusion in these phases demonstrate. Diffusion of a molecule or species (in the context of diffusion within condensed phases) without loss of generality may be said to occur in a medium. A universal behaviour observed is that self diffusivity exhibits a maximum as a function of the size of the diffusant when the diffusant is confined to a medium, as a result of what is known as the Levitation Effect. Such a maximum in self diffusivity has been seen in widely differing medium: microporous solids, dense liquids, ions in polar solvents, etc. The aim of the thesis is to investigate and further explore such universal behaviour and demonstrate for the first time the existence of common trends across different condensed phases in spite of difference in the detail at the microscopic level. In Chapter 2, we report a molecular dynamics study of diffusion of diatomic species AB within zeolite Y. The bond length of A-B as well as the interaction of A and B with the host zeolite atoms are varied. The results demonstrate that for the symmetric case (when A=B or AA), there exists a preferred bond length (determined by the bottleneck or window diameter) when the diffusivity is maximum. This is in agreement with previous results on monatomic species which also exhibit a similar diffusivity maximum. More importantly, no such maximum is seen when the interaction asymmetric is introduced in AB. Slight asymmetry in the interaction gives rise to a weak maximum while large asymmetry in interaction obliterates the diffusivity maximum. These results suggest that the importance of interaction between the diffusant and the medium in Levitation Effect or size-dependent diffusivity maximum. Further, it also demonstrates for the first time the close association between an inversion centre (in a statistical sense and not in the crystallographic sense) and the Levitation Effect. In Chapter 3, a study of size dependence of solutes in a Lennard-Jones liquid is reported. Einstein and others derived the reciprocal dependence of the self-diffusivity D on the solute radius ru for large solutes based on kinetic theory. We examine here (a) the range of ru over which Stokes-Einstein (SE) dependence is valid and (b) the precise dependence for small solutes outside of the SE regime. We show through molecular dynamics simulations that there are two distinct regimes for smaller solutes: (i) the interaction or Levitation Effect (LE) regime for solutes of intermediate sizes and (ii) the D 1/ru2 for still smaller solutes. We show that as the solute-solvent size ratio decreases, the breakdown in the Stokes-Einstein relationship leading to the LE regime has its origin in dispersion interaction between the solute and the solvent. These results explain reports of enhanced solute diffusion in solvents existing in the literature seen for small solutes for which no explanation exists. Several properties have been computed to understand the nature of solute motion in different regimes. We investigate in Chapter 4, the dependence of self diffusivity on the size of the diffusant in a disordered medium with the objective of understanding the experimentally observed correlation between self diffusivity and activation energy seen in a wide variety of glasses. Typically, it is found in many ionic glasses that a higher conductivity is associated with lower activation energy and vice versa. Our understanding of transport in glasses as provided by existing theories does not offer an explanation of this correlation. We have carried out molecular dynamics simulation as a function of the size of the impurity atom or diffusant (both neutral and charged) in a model host amorphous matrix. We find that there is a maximum in self diffusivity as a function of the size of the impurity atom suggesting that there is an appropriate size for which the diffusivity is maximum. The activation energy is found to be the lowest for this size of the impurity. A similar maximum has previously been found in other condensed phases such as confined fluids and dense liquids and has its origin in the Levitation Effect. The implications of this result for understanding ionic conductivity in glasses are discussed. Our result suggests that there is a relation between microscopic structure of the amorphous solid, diffusivity or conductivity and activation energy. The nature of this relationship is discussed in terms of the Levitation parameter showing that diffusivity is maximum when the size of the neck or doorway radius is comparable with the size of the diffusant. Our computational results here are in excellent agreement with independent experimental results which show that structural features of the glass are important in determining the ionic conductivity. In Chapter 5, we report results of molecular dynamics investigations into neutral impurity diffusing within an amorphous solid as a function of the size of the diffusant and density of the host amorphous matrix. We find that self diffusivity exhibits an anomalous maximum as a function of the size of the impurity species. An analysis of properties of the impurity atom with maximum diffusivity shows that it is associated with lower mean square force, reduced backscattering of velocity autocorrelation function, near-exponential decay of the intermediate scattering function (as compared to stretched-exponential decay for other sizes of the impurity species) and lower activation energy. These results demonstrate the existence of well known size-dependent diffusivity maximum in disordered solids. Further, we show that the diffusivity maximum is observed at lower impurity diameters with increase in density. This is explained in terms of the levitation parameter and the void structure of the amorphous solid. We demonstrate that these results imply contrasting dependence of self diffusivity (D) on the density of the amorphous matrix, . D increases with  for small sizes of the impurity but shows an increase followed by a decrease for intermediate sizes of the impurity atom. For large sizes of the impurity atom, D decreases with increase in . These contrasting dependence arises naturally from the existence of Levitation Effect. In Chapter 6, we discuss size dependence of impurity diffusion in an ordered system. We report molecular dynamics simulation studies to understand the role of impurity size and impurity-host interaction on impurity diffusivity in a body centered cubic solid. The simulation studies have been performed for a set of impurity-host interaction parameter ih (i=impurity, h=host atom) for a range of impurity sizes in rigid and flexible bcc solids. A double maximum is seen corresponding to two different sizes of the impurity species. Anomalous maximum is seen for a larger size of the impurity species in the case of the rigid host as compared to flexible host. The second anomalous diffusivity disappears with decrease in ih in flexible bcc solid. For one of the ih where double diffusivity maxima are observed, various properties are analysed to understand the anomalous diffusion behaviour. The impurity with anomalous diffusion has lower activation energy as compared to other impurities. Among the two anomalous impurities, the impurity with higher diffusivity has lower activation energy. The anomalous regime impurities as associated with velocity autocorrelation function with little or no backscattering, minimum average mean square force due to host atoms, lower activation energy. The self intermediate scattering function shows faster decay and a single relaxation time for anomalous regime impurity and two relaxation times for other impurity sizes. The wavenumber dependence of diffusivity of impurities shows oscillatory behaviour except for the anomalous regime impurities which show monotonic dependence on wavenumber. Chapter 7 discusses the influence of temperature induced solid-liquid phase transition on the size-dependent diffusivity. We report results for two distinct cases: (a) when the phase change is associated with corresponding changes in density and (b) when the phase change occurs at constant density. The latter is carried out so as to obtain the influence of disorder on the size-dependent diffusion or Levitation Effect. Studies with variable density are useful to understand the effect of disorder as well as change in density on size-dependent diffusivity. Two diffusivity maxima in the solid face-centred cubic phase is seen when the impurity-medium interaction is sufficiently large. One of these diffusivity maximum disappears with decrease in h. The impurities near the diffusivity maximum show velocity autocorrelation function with little backscattering, minimum in the average mean square force, lower activation energy, faster decay of self intermediate scattering function with a single relaxation time and a monotonic decay in wavevector dependence of diffusivity. Chapter 8 reports molecular dynamics simulations of a model guest tetrahedral molecule AX4 with differing bond lengths lAX have been carried out in a sphere with different surface roughness. The rotational-diffusion coefficient Dr shows a maximum for a particular value of lAX. This corresponds to the distance at which the interaction of the guest with the atoms of the host is most favourable. Although, the intensity of the maximum decreases with increase in the roughness of the confining surface, it is seen that the maximum exists even for a reasonably high degree of roughness. The observed maximum arises from the minimum in the torque on the tetrahedral molecule from its interaction with the confining medium due to mutual cancellation of forces. Activation energy for rotation is seen to be also a minimum for the bond length for which Dr is a maximum. These results suggest that there is a maximum in the rotational-diffusion coefficient when the rotating molecule is confined to a sphere of comparable size similar to the maximum in translational diffusion coefficient seen in porous solids and known as the Levitation Effect. On increase in the roughness of the sphere surface, the value of lAX at which the maximum in Dr is seen decreases. This is similar to the shift seen in the size of the diffusant corresponding to maximum diffusivity in the case of translational diffusivity. In Chapter 9 possible extensions to the work reported in the previous chapters and the directions to take are discussed. Symmetry plays an important role in size dependent diffusivity maximum in microporous crystalline solids; it would be interesting to investigate if similar role of symmetry exists in case of liquids and other disordered solids. Previous work from this laboratory on ions in water has shown the importance of electrostatic interactions. In the light of this, it would be interesting to see the influence of long-range interactions in breakdown of Stokes-Einstein relationship in liquids. Effect of density of the medium on impurity diffusion can be studied over a wide range of densities in case of supercritical fluids such as ions in water (where electrostatic interactions are present) and these can provide greater insight into dependence of diffusion on density. The origin of two diffusivity maxima in case of body-centered and face-centred cubic solids needs a detailed investigation to understand its origin. Quantification of disorder and its effect on size dependence of diffusion would be of interest. A detailed comparison with experimental data of matrix isolated molecules to understand and verify the dependence of rotational diffusivity on the size of the molecule as well as the cavity radius would be instructive.

<p>In high-temperature superconductors a large region of the magnetic phase diagram is occupied by a vortex phase that displays a number of exciting phenomena. At low temperatures, vortices form a truly superconducting solid phase which at high temperatures turns into a dissipative vortex liquid. The character of the transition between these two phases depends on the amount and type of disorder present in the system. For weak point disorder the vortex solid-to-liquid transition is a first-order melting. In the presence of strong point disorder the solid is thought to be a vortex-glass and the transition into the liquid is instead of second order. When the disorder is correlated, like twin boundaries or artificially introduced columnar defects, the transition is also second order, but has essentially different properties. In this work, the transition between the solid and liquid phases of the vortex state has been studied by resistive transport measurements in mainly YBa2Cu3O7-[delta](YBCO) single crystals with different types of disorder.</p><p>The vortex-glass transition has been investigated in an extended model for the vortex-liquid resistivity close to the transition that takes into account both the temperature and magnetic field dependence of the transition line. The resistivity of samples with different properties was measured with various contact configurations at several magnetic fields and analyzed within this model. For each sample, attempts were made to scale the transition curves to one curve according to a suitable scaling variable predicted by the model. Good scaling was found in a number of different situations. The influence of increasing anisotropy and angular dependence of the magnetic field in the model were also considered.</p><p>The vortex solid-to-liquid transition was also studied in heavy-ion irradiated YBCO single crystals. The ions create columnar defects in the sample that act as correlated disorder. A magnetic field was applied at a tilt angle with respect to the direction of the columns. At the transition the resistance disappears as a power law with different exponents in the three orthogonal directions considered. This provides evidence for a new type of critical behavior with fully anisotropic critical scaling properties not previously found in any physical system.</p><p>The effect on the vortex solid-to-liquid transition of high magnetic fields applied parallel to the superconducting layers of underdoped YBCO single crystals was also studied. Some novel features were observed: a sharp kink appearing close to Tc at high magnetic fields and a triple dip in the angular dependence of the resistivity close to B||ab in some regions of the phase diagram.</p> / <p>I högtemperatursupraledare består en stor del av det magnetiska fasdiagrammet av en vortexfas som uppvisar ett flertal spännande fenomen. Vid låga temperaturer bildar vortexarna en fast vortexfas utan elektriskt motstånd. Vid högre temperatur övergår denna fas till en dissipativ vortexvätska. Egenskaperna hos denna fasövergång beror på oordningen i form av defekter. Vid svag punktoordning är fasomvandlingen mellan det fasta och flytande vortextillståndet en första ordningens smältövergång. Vid stark punktoordning anses den fasta fasen vara ett vortexglas och övergången till vortexvätskan är istället av andra ordningen. När oordningen är korrelerad, som för tvillinggränser eller artificiellt skapade kolumndefekter, är övergången också av andra ordningen men med väsentligt annorlunda egenskaper. I detta arbete har övergången mellan det fasta och det flytande vortextillståndet studerats med resistiva transportmätningar i framförallt enkristaller av YBa2Cu3O7-[delta] (YBCO) med olika typer av oordning.</p><p>Vortexglasövergången har undersökts i en utvidgad modell för resistansen i vortexvätskan nära fasövergången där hänsyn tas till såväl temperatur- som fältberoendet. Resistansen hos prover med olika egenskaper mättes i varierande magnetfält och i flera kontaktkonfigurationer och analyserades inom denna modell. Övergångskurvorna skalades till en kurva med en skalningsvariabel som givits av modellen. God skalning uppnåddes i flera olika fall. Effekten av ökande anisotropi och vinkelberoendet i modellen undersöktes också.</p><p>Vortexövergången mellan det fasta och det flytande vortextillståndet undersöktes även i enkristaller av YBCO bestrålade med tunga joner. Jonerna skapade kolumndefekter som fungerar som korrelerad oordning. Vinkeln mellan pålagt magnetfält och dessa kolumndefekter varierades. Vid fasövergången avtar resistansen som en potenslag med olika exponenter i de tre undersökta ortogonala riktningarna. Detta ger experimentell belägg för en ny typ av kritiskt beteende med fullständigt anisotropa kritiska skalningsegenskaper.</p><p>Egenskaparna hos på vortexövergången mellan fast och flytande fas vid höga magnetfält parallella med de supraledande lagren hos underdopade YBCO enkristaller undersöktes också. Några nya effekter observerades: en skarp knyck uppstod nära Tc vid höga magnetfält och en tredubbel dipp i den vinkelberoende resistiviteten nära B||ab i några regioner av fasdiagrammet.</p>

In high-temperature superconductors a large region of the magnetic phase diagram is occupied by a vortex phase that displays a number of exciting phenomena. At low temperatures, vortices form a truly superconducting solid phase which at high temperatures turns into a dissipative vortex liquid. The character of the transition between these two phases depends on the amount and type of disorder present in the system. For weak point disorder the vortex solid-to-liquid transition is a first-order melting. In the presence of strong point disorder the solid is thought to be a vortex-glass and the transition into the liquid is instead of second order. When the disorder is correlated, like twin boundaries or artificially introduced columnar defects, the transition is also second order, but has essentially different properties. In this work, the transition between the solid and liquid phases of the vortex state has been studied by resistive transport measurements in mainly YBa2Cu3O7-[delta](YBCO) single crystals with different types of disorder. The vortex-glass transition has been investigated in an extended model for the vortex-liquid resistivity close to the transition that takes into account both the temperature and magnetic field dependence of the transition line. The resistivity of samples with different properties was measured with various contact configurations at several magnetic fields and analyzed within this model. For each sample, attempts were made to scale the transition curves to one curve according to a suitable scaling variable predicted by the model. Good scaling was found in a number of different situations. The influence of increasing anisotropy and angular dependence of the magnetic field in the model were also considered. The vortex solid-to-liquid transition was also studied in heavy-ion irradiated YBCO single crystals. The ions create columnar defects in the sample that act as correlated disorder. A magnetic field was applied at a tilt angle with respect to the direction of the columns. At the transition the resistance disappears as a power law with different exponents in the three orthogonal directions considered. This provides evidence for a new type of critical behavior with fully anisotropic critical scaling properties not previously found in any physical system. The effect on the vortex solid-to-liquid transition of high magnetic fields applied parallel to the superconducting layers of underdoped YBCO single crystals was also studied. Some novel features were observed: a sharp kink appearing close to Tc at high magnetic fields and a triple dip in the angular dependence of the resistivity close to B||ab in some regions of the phase diagram. / I högtemperatursupraledare består en stor del av det magnetiska fasdiagrammet av en vortexfas som uppvisar ett flertal spännande fenomen. Vid låga temperaturer bildar vortexarna en fast vortexfas utan elektriskt motstånd. Vid högre temperatur övergår denna fas till en dissipativ vortexvätska. Egenskaperna hos denna fasövergång beror på oordningen i form av defekter. Vid svag punktoordning är fasomvandlingen mellan det fasta och flytande vortextillståndet en första ordningens smältövergång. Vid stark punktoordning anses den fasta fasen vara ett vortexglas och övergången till vortexvätskan är istället av andra ordningen. När oordningen är korrelerad, som för tvillinggränser eller artificiellt skapade kolumndefekter, är övergången också av andra ordningen men med väsentligt annorlunda egenskaper. I detta arbete har övergången mellan det fasta och det flytande vortextillståndet studerats med resistiva transportmätningar i framförallt enkristaller av YBa2Cu3O7-[delta] (YBCO) med olika typer av oordning. Vortexglasövergången har undersökts i en utvidgad modell för resistansen i vortexvätskan nära fasövergången där hänsyn tas till såväl temperatur- som fältberoendet. Resistansen hos prover med olika egenskaper mättes i varierande magnetfält och i flera kontaktkonfigurationer och analyserades inom denna modell. Övergångskurvorna skalades till en kurva med en skalningsvariabel som givits av modellen. God skalning uppnåddes i flera olika fall. Effekten av ökande anisotropi och vinkelberoendet i modellen undersöktes också. Vortexövergången mellan det fasta och det flytande vortextillståndet undersöktes även i enkristaller av YBCO bestrålade med tunga joner. Jonerna skapade kolumndefekter som fungerar som korrelerad oordning. Vinkeln mellan pålagt magnetfält och dessa kolumndefekter varierades. Vid fasövergången avtar resistansen som en potenslag med olika exponenter i de tre undersökta ortogonala riktningarna. Detta ger experimentell belägg för en ny typ av kritiskt beteende med fullständigt anisotropa kritiska skalningsegenskaper. Egenskaparna hos på vortexövergången mellan fast och flytande fas vid höga magnetfält parallella med de supraledande lagren hos underdopade YBCO enkristaller undersöktes också. Några nya effekter observerades: en skarp knyck uppstod nära Tc vid höga magnetfält och en tredubbel dipp i den vinkelberoende resistiviteten nära B||ab i några regioner av fasdiagrammet. / QC 20110125

Orientador: Alex Antonelli / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T11:09:53Z (GMT). No. of bitstreams: 1 Michelon_MateusFontana_D.pdf: 2538667 bytes, checksum: 7cf68d701030ed6e318c7312b4f25a03 (MD5) Previous issue date: 2009 / Resumo: Um dos objetivos da simulação atomística na ciência dos materiais é calcular as propriedades de um material virtual e propor rotas para sua fabricação em laboratório. Uma das principais propriedades que o material deve apresentar antes de ser sintetizado é a estabilidade termodinâmica. Como a estabilidade é determinada pela energia livre, o cálculo preciso desta quantidade é de fundamental importância na construção de um laboratório virtual. Neste contexto, desenvolvemos uma metodologia alternativa para a determinação da energia livre de ligas substitucionais, que leva em conta os graus de liberdade vibracionais e configuracionais com precisão controlada. A metodologia utiliza o método de Monte Carlo para simular a dinâmica de trocas e vibrações atômicas e determina a energia livre através dos métodos de ligação adiabática e escalamento reversível. Além disso, a metodologia é capaz de avaliar a influência de mecanismos associados à entropia vibracional, através da relaxação sucessiva de vínculos na dinâmica. Especificamente, permite quantificar os mecanismos de a) proporção de ligações entre átomos distintos, b) discrepância entre volumes atômicos e c) relaxação volumétrica, e identificar a origem da diferença de entropia vibracional na transição ordem-desordem. Testamos e aplicamos a metodologia para estudar um modelo semiempírico da liga Ni3Al. Observamos um aumento da entropia vibracional na transição ordem-desordem comparável com o aumento da entropia configuracional e explicado pelo aumento de volume na transição. Outra característica de um laboratório virtual é possuir modelos que descrevam satisfatoriamente os sistemas de interesse. Investigamos um potencial do tipo tight-binding e descobrimos que não é transferível para descrever fenômenos de ordem-desordem em diversas ligas. Além de investigar o fenômeno ordem-desordem em ligas, estudamos transições de fase líquido-líquido em substâncias puras. Apresentamos uma evidência teórica de transição líquido-líquido de primeira ordem em um modelo semiempírico do gálio, fornecendo suporte a uma recente evidência experimental de transição líquido-líquido no regime super-resfriado deste elemento. Além disso, as simulações atomísticas sugerem um mecanismo microscópico para esta transição. Outras características de um laboratório virtual são a possibilidade de estudar sistemas em condições experimentais inacessíveis e a capacidade de propor novos experimentos. Neste contexto, apresentamos uma evidência teórica de transição líquido-líquido em um modelo ab initio para o dióxido de carbono. A transição ocorre entre um líquido molecular e um líquido polimérico em uma região do diagrama de fases atualmente inacessível experimentalmente. Em um futuro próximo, esperamos que seja possível testar esta previsão teórica e sintetizar fases poliméricas por meios físicos. / Abstract: One of the goals of atomistic simulation in materials science is to calculate properties of a virtual material and suggest routes for its fabrication in laboratory. One of the main properties that the material must have before being synthesized is the thermodynamical stability. As the thermodynamical stability is determined by the free energy, its accurate calculation is of fundamental importance for the construction of a virtual laboratory. In this context, we developed an alternative methodology to determine the free energy of substitutional alloys, which takes into account both the vibrational and configurational degrees of freedom with controlled accuracy. The methodology uses the Monte Carlo method to simulate both the vibrational and exchange dynamics and uses the adiabatic switching and reversible scaling methods to calculate the free energy efficiently. In addition, the methodology is able to evaluate the effect of three mechanisms in the vibrational entropy, through successive relaxations of constraints associated with the dynamics. Specifically, it allows to quantify the mechanisms of a) bond proportion, b) atomic size mismatch and c) bulk volume, and thus identify the origin of the vibrational entropy difference at the order-disorder transition. We tested and applied the methodology to study a semiempirical model of the Ni3Al alloy. We observed an increasing of the vibrational entropy at the order-disorder transition comparable to the configurational entropy increasing and explained by an increasing of the bulk volume. Another expected feature of a virtual laboratory is to offer models that describe satisfactorily the systems of interest. We investigated a tight-binding potential and found out that it is not transferable to describe the order-disorder phenomena in several alloys. In addition to the study of the order-disorder phenomena in alloys, we investigated phase transitions between two liquids of a pure substance. We present a theoretical evidence of a first-order liquid-liquid phase transition in a semiempirical model of gallium, which lend support to the recent experimental evidence of a first-order liquid-liquid transition in the supercooled regime of this element. Moreover, the atomistic simulations suggest a microscopic mechanism for this phase transition. Another expected features of a virtual laboratory are the possibility to investigate systems in unreachable experimental conditions and the capacity to suggest new experiments. In this context, we present an ab initio theoretical evidence of a liquid-liquid phase transition in carbon dioxide. We predict a transition between a molecular liquid and a polymeric liquid at a temperature and pressure which are presently unreachable experimentally. We hope that in the near future it will be possible to test this theoretical prediction and synthesize polymeric phases through physical means. / Doutorado / Física da Matéria Condensada / Doutor em Ciências

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The thesis involves computer simulation and theoretical studies of dynamics of water under confinement and structural transformation in different complex systems. Based on the systems and phenomena of interest, the work has been classified in to three major parts: I. Dynamics of water under confinement II. Dynamics of water in presence of amphiphilic solutes III. Structural transformation in complex systems The three parts have further been divided into nine chapters. Brief chapter wise outline of the thesis is discussed below. Part I deals with the dynamics of water in confined systems. In Chapter I.1, we provide a brief introduction of water dynamics inc on fined systems. We also give a brief outline of relevant experimental and theoretical techniques used to study the water dynamics under confinement. Chapter I.2 describes a model based analytical study of dynamical correlation in confined systems. Here, we introduce a novel one dimensional Ising model to investigate the propagation and annihilation of dynamical correlations in confined systems and to understand the intriguing shortening of the orientational relaxation time that has been reported for small sized reverse micelles (RMs).In our model, the two spins located at the two end cells are oriented in the opposite directions to mimic the surface effects present in the real systems. These produce opposing polarizations which propagate from the surface to the center, thus producing bulk like condition at the center. This model can be solved analytically for short chains. For long chains, we solve the model numerically with Glauber spin flip dynamics (and also with Metropolis single-spin flip Monte Carlo algorithm).We show that the model satisfactorily reproduces many of the features observed in experiments. Due to the destructive interference among correlations that propagate from the surface to the core, one of the rotational relaxation time components decays faster than the bulk. In general, the relaxation of spins is non-exponential due to the interplay between various interactions. In the limit of strong coupling between the spins or in the limit of low temperature, the nature of the relaxation of spins undergoes a change with the emergence of homogeneous dynamics, where the decay is predominantly exponential. In Chapter I.3, layer-wise distance dependent orientation relaxation of water confined in reverse micelle s(RM)is studied using theoretical and computational tools. We use both a newly constructed spins on a ring (SOR) Ising-type model with modified Shore-Zwanzig rotational dynamics and atomistic simulations with explicit water. Our study explores the size effect of RMs and the role of intermolecular correlations, compromised by the presence of a highly polar surface, on the distance (from the surface) dependence of water relaxation. The SOR model can capture some aspects of distance dependent orientation relaxation, such as acceleration of orientation relaxation at intermediate layers. In atomistic simulations, layer-wise decomposition of hydrogen bond (H-bond) formation pattern clearly reveal that the H-bond arrangement of water at a certain distance away from the surface can remain frustrated due to interaction with the polar surface head groups. We show that this layer-wise analysis also reveals the presence of a non-monotonic, slow relaxation component which can be attributed to the frustration effect and is accentuated in small to intermediate size RMs. For larger RMs, the long-time component decreases monotonically from the interface to the interior of the RMs with slowest relaxation observed at the interface. In ChapterI.4, we present theoretical two dimensional infrared spectroscopic (2D-IR) studies of water confined within RMs of various sizes. Here we focus again mainly on the altered dynamics of confined water by performing a layer-wise decomposition of water. We aim to quantify the relative contributions to the calculated 2D-IR spectra by water molecules located in different layers. The spectra of 0-1 transition clearly show substantial elongation along the diagonal, due to in homogeneous broadening and incomplete spectral diffusion, in the surface water layer of different size of RMs studied in this work. Our study reveals that the motion of the surface water molecules is sub-diffusive, establishing the constrained nature of their dynamics. This is further supported by the two peak nature of the angular analogue of the van Hove correlation function. With increasing system size the motion of water molecules becomes more diffusive in nature and the structural diffusion is observed to be almost completed in the central layer of larger RMs. Comparisons between experiment and simulation help establishing the correspondence between the spectral decomposition available in experimental 2D-IR with the spatial decomposition of simulated 2D-IR. Simulations also allow a quantitative exploration of the relative role of water, sodium ions and sulfonate head groups in irrational dephasing. Interestingly, the negative cross correlation between forces on oxygen and hydrogen of O-H bond in bulk water significantly decreases in the surface layer of different RMs. This negative cross correlation gradually increases in the central layer with increasing size of the RMs and this is found to be partly responsible for the faster relaxation rate of water in the central layer. Part II consists of two chapters and focuses on the dynamics of water in presence of amphiphilic solutes. In Chapter II.1, we present a brief introduction of water – DMSO binary mixture and various anomalous properties of the same. In Chapter II.2, we present theoretical IR study of water dynamics in water–DMSO binary mixtures of different compositions. We show that with increasing DMSO concentration, the IR absorption peak maxima show the presence of structural transformation in similar concentration range, observed in earlier studies. Analysis of H-bonded network near hydrophilic and hydrophobic part of DMSO also suggests that average number of hydrogen bonds near the hydrophobic parts possess maxima at the same concentration range. We also show that with increasing DMSO concentration water dynamics becomes very slow. This has been supported by the diagonal elongation of the 2D-IR spectra and also the slow decay of frequency fluctuation correlation n function (FFCF) and the orientation time correlation function (OTCF). The decoupling of the OTCF establishes that water-DMSOH-bond is much stronger than that of water-water. The last part (Part III) consists of three chapters that deal with structural transformation in various complex systems. In Chapter III.1, we introduce polydisperse systems and present existing theoretical, computer simulation and experimental studies. It also contains the importance and diversity of polydisperse system in nature. In Chapter III.2, we present computer simulation study of melting of polydisperse Lennard-Jones (LJ) system with Gaussian polydispersity in size. The phase diagram reproduces the existence of an early temperature in variant terminal polydispersity (δt0.11), with no signature of re-entrant melting. The absence of re-entrant melting can be attributed to the influence of attractive part of the potential on melting. We find that at terminal polydispersity the fractional density change approaches zero that seems to arise from vanishingly small compressibility of the disordered phase. At constant temperature and volume fraction system undergoes a sharp transition from crystalline solid to disordered state with increasing polydispersity. This has been quantified by second and third order rotational invariant bond orientational orders as well as by the average inherent structure energy. The translational order parameter also indicates similar structural change The free energy calculation further supports the nature of the transition. The third order bond orientational order shows that with increasing polydispersity, local cluster favors more icosahedral-like arrangements and thus the system loses its crystalline symmetry. In Chapter III.3, we present study of phase transition and effect of confinement on it in SOR model. This system is similar to our SOR model discussed in Chapter I.3. The spins execute continuous rotation under a modified XY Hamiltonian. In order to understand the nature of phase transition in such confined spin systems we have performed extensive Monte Carlo simulations. The system size dependence of Binders cumulant, specific heat, order parameter and finite size scaling of order parameter universally suggest the existence of a phase transition. The absence of hysteresis and Scaling of Binders energy cumulant minimum confirm the continuous nature of the transition. The finite size scaling analyses give rise to the mean field nature of the transition. Plausible applications of the proposed model in modeling dipolar liquids in confined systems are also discussed. In Appendix A, we discuss a preliminary study of front propagation in a non-equilibrium system. The model system analogous to the super cooled liquid shows non-Avrami domain growth during rejuvenation. The origin of the non-Avrami nature of the domain growth and the presence of cross over are also discussed. In Appendix B, we discuss umbrella a sampling technique and WHAM analysis which is used in ChapterIII.2 to get the free energy of polydisperse LJ system.