Biological membranes mainly constituent lipid molecules along with some proteins and steroles. The properties of the pure lipid bilayers as well as in the presence of other constituents (in case of two or three component systems) are very important to be studied carefully to model these systems and compare them with the realistic systems. Molecular dynamic simulations provide a good opportunity to model such systems and to study them at microscopic level where experiments fail to do. In this thesis we study the structural and dynamic properties of the pure phospholipid bilayers and the phase behavior of phospholipid bilayers when other constituents are present in them. Material and structural properties like area per lipid and area compressibility of the phospholipids show a big scatter in experiments. These properties are studied for different system sizes and it was found that the increasing undulations in large systems effect these properties. A correction was applied to area per lipid and area compressibility using the Helfrich theory in Fourier space. Other structural properties like order of the lipid chains, electron density and radial distribution functions are calculated which give the structure of the lipid bilayer along the normal and in the lateral direction. These properties are compared to the X-ray and neutron scattering experiments after Fourier transform. Thermodynamic properties like heat capacity and heat of melting are also calculated from derivatives of energies available in molecular dynamics. Heat capacity on the other hand include quantum effect and are corrected for that by applying quantum correction using normal mode analysis for a simple as well as ambiguous system like water. Here it is done for SPC/E water model. The purpose of this study is to further apply the quantum corrections on macromolecules like lipids by using this technique. Furthermore the phase behavior of two component systems (phospholipids/cholesterol) is also studied. Phase transition in these systems is observed at different cholesterol concentrations as a function of temperature by looking at different quantities (as an order parameter) like the order of chains, area per molecule and partial specific area. Radial distribution functions are used to look at the in plane structure for different phases having a different lateral or positional order. Adding more cholesterol orders the lipid chains changing a liquid disordered system into a liquid ordered one and turning a solid ordered system into a liquid ordered one. Further more the free energy of domain formation is calculated to investigate the two phasecoexistence in binary systems. Free energy contains two terms. One is bulk freeenergy which was calculated by the chemical potential of cholesterol moleculein a homogeneous system which is favorable for segregation. Second is thefree energy of having an interface which is calculated from the line tension of the interface of two systems with different cholesterol concentration which in unfavorable for domain formation. The size of the domains calculated from these two contributions to the free energy gives the domains of a few nm in size. Though we could not find any such domains by directly looking at our simulations. / <p>QC 20120913</p>

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-11-05T18:32:08Z No. of bitstreams: 1 OZÓRIO BEZERRA HOLANDA NETO – DISSERTAÇÃO (PPGFísica) 2015.pdf: 1866080 bytes, checksum: a15052e3dc0bbf5bba2c805913e20d6a (MD5) / Made available in DSpace on 2018-11-05T18:32:08Z (GMT). No. of bitstreams: 1 OZÓRIO BEZERRA HOLANDA NETO – DISSERTAÇÃO (PPGFísica) 2015.pdf: 1866080 bytes, checksum: a15052e3dc0bbf5bba2c805913e20d6a (MD5) Previous issue date: 2015-07 / Capes / Nesta dissertação estudamos os aspectos clássicos e quânticos da extensão derivativa do modelo de Chern-Simons Abeliano na eletrodinâmica em (2+1) dimensões. No contexto clássico, descrevemos suas principais propriedades, tais como a invariância de calibre e a estrutura do propagador associado quando este modelo é adicionado à teoria de Maxwell. A principal característica desse modelo é a de que ele nos fornece um par de excitações (uma não massiva e outra massiva) para o modo de propagação das ondas eletromagnéticas. No contexto quântico, estudamos a possibilidade de induzir esse termo na ação efetiva da eletrodinâmica quântica via correções radiativas de determinante fermiônico em um laço. Neste caso, analisamos sua ocorrência em temperatura zero e nita. O resultado oriundo da temperatura nita tem como propriedade gerar novas excitações para os modos de propagação das ondas eletromagnéticas de pendentes da temperatura. / In this work we studied the classical and quantum aspects of derivative extension of the Chern-Simons Abelian model in electro dynamics in (2+1) dimensions. In classical context, we describe their main properties such as gauge in variance and the structure of the associated propagator when this template is added to Maxwell's theory. The main feature of this model is that it provides us a pair of excitation (one not massive and another massive) for the propagation mode of the electromagnetic waves. In the quantum context, we studied the possibility of inducing this term in thee active action of quantum electrodynamics via radiative corrections of fermionic determinant in loop. In this case, we analyze its occurrence at zero and nite temperature. The result a rising from the nite temperature has the property to generate new excitement for the modes of propagation of electromagnetic waves dependent of temperature.

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-10-09T18:55:52Z No. of bitstreams: 1 ARTHUR GONÇALVES CAVALCANTI – DISSERTAÇÃO (PPGFísica) 2016.pdf: 845402 bytes, checksum: dbdfb2a26834c477a45e9e735fa670d3 (MD5) / Made available in DSpace on 2018-10-09T18:55:52Z (GMT). No. of bitstreams: 1 ARTHUR GONÇALVES CAVALCANTI – DISSERTAÇÃO (PPGFísica) 2016.pdf: 845402 bytes, checksum: dbdfb2a26834c477a45e9e735fa670d3 (MD5) Previous issue date: 2016-02 / Capes / A teoria da relatividade geral prevê soluções tipo buraco negro, as quais são caracterizadas pela existência de um horizonte de eventos. Como exemplo, podemos citar a métrica obtida por Bãnados-Teitelboim-Zanelli (BTZ), que é uma solução da gravitação em (2+1)- dimensões, em que se considera uma constante cosmológica negativa. Nos últimos anos, buracos negros não-comutativos têm sido investigados na literatura por muitos autores. Em particular, a métrica BTZ não-comutativa foi obtida considerando-se a equivalência, que existe em três dimensões, entre gravitação e a teoria de Chern-Simons, que e uma teoria quântica de campos topológica em três dimensões, e usando-se o mapeamento de Seiberg-Witter com a solução em (2+1)-dimensões. A presença de divergências na teoria quântica de campos leva a considerar a possibilidade de modificar o princípio da incerteza de Heisemberg, introduzindo uma escala de comprimento fundamental, e esta modificação geram correções nas propriedades termodinâmica de buracos negros. Um dos efeitos associados as soluções tipo buraco negro, independente da dimensão do espaço-tempo, e a emissão térmica (Radiação Hawking), a qual e vista como um processo de tunelamento devido as flutuações do vácuo que acontece na região próxima ao horizonte de eventos. Neste trabalho, com o objetivo de investigar as correções devido a não comutatividade e ao princípio da incerteza generalizado, consideramos a métrica BTZ não-comutativa. Para tanto, usamos o formalismo de tunelamento via método de Hamilton-Jacobi. / The general relativity theory predicts black hole type solutions, which are characterized by the existence of an event horizon. As an example, the metric obtained by Ba~nados- Teitelboim-Zanelli (BTZ), which is a soluton of the gravitation in (2 + 1)-dimensions in what is considered a negative cosmological constant. In recent years, noncommutative black holes have been investigated by many authors in the literature. In particular, the BTZ metric non-commutative was obtained considering the equivalent, which exists in three dimensions, between gravitation and Chern-Simons theory, which is a quantum theory topological elds in three dimensions, and using it mapping Seiberg-Witter with the solution of (2 + 1)-dimensions. The presence of divergences in quantum eld theory leads to consider the possibility of modifying the principle of Heisenberg uncertainty by introducing a fundamental length scale, and this modi cation generate corrections to the thermodynamic properties of black holes. One of the e ects associated with the black hole type solutions, regardless of the space-time dimension is the thermal emission (Hawking radiation), which is seen as a process of tunneling due to vacuum uctuations that happens in the region near the event horizon . In this work, in order to investigate the corrections due to noncommutativity and the principle of widespread uncertainty, we consider the metric BTZ noncommutative. For this, we use tunneling formalism via Hamilton-Jacobi method.

The presented thesis spans over a number of related topics with a com- mon theme - the so-called universality. Classical fields exhibiting this property serve as exact solutions to virtually any higher-order theory irrespective of the particular form of the field equations, being thus of particular interest in ef- fective field theories. The aim of this work is to study various aspects of such solutions in the context of gravity, electrodynamics, as well as more general nonabelian gauge theories. The results are concentrated in four chapters, the first of which is devoted to what we call the almost universal spacetimes. Due to their nice curvature properties, these spacetimes provide an efficient method for simplifying and solving the field equations of higher-order gravity theories. We illustrate this feature of almost universal metrics by finding new vacuum solutions to quadratic gravity and six-dimensional conformal gravity. In the second chapter, we shift our attention towards electrodynamics. Following up on recent results on universal electromagnetic fields, we deal with Einstein- Maxwell fields which simultaneously solve also any higher-order modification of the Einstein-Maxwell theory. In particular, we identify solutions with this remarkable property as plane-fronted gravitational and...

In this thesis the properties of proteins and membranes are studied by molecular dynamics simulations. The subject is decomposed into parts addressing free energy calculations in proteins, mechanical inclusion models for lipid bilayers, phase transitions and structural correlations in lipid bilayers and atomistic lipid bilayer models. The work is based on results from large scale computer simulations, quantum mechanical and continuum models. Efficient statistical sampling and the coarseness of the models needed to describe the ordered and disordered states are of central concern. Classical free energy calculations of zinc binding, in metalloproteins, require a quantum mechanical correction in order to obtain realistic binding energies. Classical electrostatic polarisation will influence the binding energy in a large region surrounding the ion and produce reasonable equilibrium structures in the bound state, when compared to experimental evidence. The free energy for inserting a protein into a membrane is calculated with continuum theory. The free energy is assumed quadratic in the mismatch and depend on two elastic constants of the membrane. Under these circumstances, the free energy can then be written as a line tension multiplied by the circumference of the membrane inclusion. The inclusion model and coarse grained particle simulations of the membranes show that the thickness profile around the protein will be an exponentially damped oscillation. Coarse-grained particle simulations of model membranes containing mixtures of phospholipid and cholesterol molecules at different conditions were performed. The gel-to-liquid crystalline phase transition is successively weakened with increasing amounts of cholesterol without disappearing even at a concentration of cholesterol as high as 60%. A united atom parameterization of diacyl lipids was constructed. The aim was to construct a new force field that retains and improves the good agreement for the fluid phase and at the same time produces a gel phase at low temperatures, with properties coherent with experimental findings. The global bilayer tilt obtains an azimuthal value of 31◦ and is aligned between lattice vectors in the bilayer plane. It is also shown that the model yield a correct heat of melting as well as heat capacities in the fluid and gel phase of DPPC. / <p>QC 20140919</p>

The work presented in this thesis consists of two distinct parts. The first years of my work focused on the development and improvement of a new equipment built to study magnetic and electrical properties, particularly applying microwaves in reflection and transmission conditions. The sample space in conventional cryostats with superconducting magnets is usually smaller than 10-mm-diameter. Our equipment consists of a hollow cylindrical cryostat having a 33-mm-diameter hole all along its vertical axis. These characteristics enable the measurement of large samples and the use of big resonant cavities to get to a wider microwave (MW) range, particularly in transmission measurements. The cryostat has a superconducting magnet made of a solenoid that applies a magnetic field from -5 T to 5 T, and a temperature controller that works in the range 1.8 - 300 K. The system is cooled down with nitrogen and helium and the temperature can be controlled with the precision required by each experiment using a heater and a needle valve. Different probes for a wide range of experiments in our cryostat have been developed in order to be as versatile as possible. Following this idea each one has been divided in two halves that can be combined as it is preferred in each experiment. Each probe is made of a 8-to-10-mm-diameter stainless steel tube that is used to protect and give some stiffness to the measuring device. A coaxial cable and different waveguides are added to these stainless steel jackets, so we end up having nine halves, four upper parts that can be combined with five lower parts. There are three waveguides working in the frequency ranges 33-50 GHz (WR22), 50-75 GHz (WR15) and 75-110 GHz (WR10), and a coaxial cable that maintains the fundamental mode at a frequency of 60 GHz. In the extra lower part a 16-pin Fischer connector is added at the bottom in order to be able to perform more resistance experiments. Once the probes have been built, they have been tested to make sure the system is able to reach high vacuum and to be cooled down. Problems found along the way have been solved and at the end all the probes work properly. Different sample holders have been designed and built according to the needs in each experiment. The system has been tested by reproducing experimental results with Mn12-acetate, as quantum tunneling and magnetic avalanches, and by obtaining new results on microwave transmission in thin TiN films. The second part of the thesis focuses on the measurements of thin TiN films in a dilution refrigerator working with a mixture of 3He and 4He that enables experiments at a few tens of millikelvins. The cryostat also contains a superconductor magnet which can apply a magnetic field up to 5 T. Low-temperature transport properties of nanoperforated superconducting TiN films have been experimentally studied. Resistance measurements have been performed in the critical region of the superconductor-insulator transition (SIT), applying the magnetic field perpendicular to the plane of the structure or the dc current through the sample. SIT is a transition from a superconductor to an insulator state by localizing the Cooper pairs. The evolution of the SIT with temperature, magnetic field and dc current has been investigated in detail. Characteristic parameters have been determined for as-cast thin films using the theory of quantum corrections to conductivity. Disorder-driven and field-induced SITs have been measured. Commensurability effects have been observed down to the lowest experimental temperature, and are emphasized in the more disordered samples. The SIT has been observed for a dc current applied across the sample as changes in the curvature at zero bias current. Experiments prove that electronic transport in the nanoperforated samples is mediated by Andreev conversion. Finally, the existence of the superinsulator state has been experimentally proved. / El treball que es presenta en aquesta tesi consta de dues parts ben diferenciades. La primera pretén el desenvolupament d’un equip experimental concebut per a l’estudi de propietats magnètiques i elèctriques en materials diversos i, en especial, el treball amb radiació de microones en condicions de reflexió i transmissió. La segona s’ha centrat en les mesures de transport d’una capa fina superconductora de nitrur de titani (TiN) de 5 nm de gruix en un criòstat de dilució.