Thesis (Ph.D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Gokhale, Arun; Committee Member: Gall, Kenneth; Committee Member: Garmestani, Hamid; Committee Member: Kurtis, Kimberly; Committee Member: Thadhani, Naresh

Thesis (Ph. D.)--Drexel University, 2005. / Includes abstract and vita. Includes bibliographical references (leaves [165]-180).

Orientador: Rogério Rosenfeld / Banca: José Geraldo Pereira / Banca: Hélio Vasconcelos Fagundes / Banca: Elcio Abdalla / Banca: Alberto Vazquez Saa / Resumo: Fazemos uma revisão do modelo cosmológico padrão, apresentando suas bases observacionais e mostrando os aspectos conceituais mais relevantes. Depois realizamos uma revisão da teoria de in ação, indicando as motivações conceituais que levaram à formulação da teoria, o mecanismo que faz possível a in ação cósmica e como esse processo resolve os problemas clássicos da cosmologia padrão. Após mostrar que a in ação é um mecanismo bem-sucedido para explicar a origem das perturbações de densidade primordiais, concentramo-nos em descrever a evolução das perturbações de densidade cosmológica, tanto na sua fase linear como não-linear. Além disso, mostramos como o campo de perturbações de densidade linear permite predizer estatisticamente a abundância e a distribuição das estruturas cósmicas. Posteriormente, consideramos a expansão acelerada do universo e discutimos os candidatos que têm sido propostos para tentar explicar a origem dessa aceleração, especialmente o candidato da energia escura, no qual nos detemos para revisar os modelos básicos propostos com respeito à sua natureza. Adicionalmente, mostramos como sua presença afeta a evolução das perturbações de densidade. Finalmente, baseandonos no modelo de Lemaître-Tolman-Bondi, fazemos uma generalização do modelo do colapso esférico para estudar a evolução não-linear de perturbações de densidade inhomogêneas, tanto em um universo Einstein-de Sitter como em um universo CDM / Abstract: We present a review of the standard cosmological model, showing both its observational basis as well as the most revelant conceptual aspects. Subsequently, we give an overview of the in ation theory , pointing out the conceptual motivations that led to its formulation, the mechanism that allow the cosmic in ation and how that process resolves the classical problems of the standard cosmology. After showing that the in ation theory provides a successful mechanism to explain the origin of the primordial density perturbations, we focus on describing the evolution of the cosmological density perturbations, both in linear and nonlinear phase. On the other hand, we show how the linear density perturbation eld allows to predict statistically the abundance and distribution of the cosmic structures. Next, we consider the accelerated expansion of the universe and mention the candidates that have seen proposed to try to explain the origin of the acceleration; especially the dark energy candidate, in which we pause to examine the basic models proposed about its nature. Further, we expose how its presence a ects the evolution of the density perturbations. Finally, based on the Lemaître-Tolman-Bondi, we make a generalization of the spherical collapse model to study the evolution of inhomogeneous nonlinear density perturbations, both in an Einstein-de Sitter as CDM universe / Doutor

Magnetic Resonance Imaging (MRI) has become one of the most important medical imaging modalities over the past few decades because of its flexibility and low risk, along with other useful attributes. For traditional MRI, the static magnetic field, B_0, must be highly homogeneous. Obtaining this homogeneity can be difficult. Traditional MRI also requires linear gradient fields that are directed along the static field direction. Under these conditions a Fourier transform relationship exists between sampled data and the image to be reconstructed. In the case of an inhomogeneous static field, gradient fields that are not linear, or gradients that are not pointed along B0, there will be no Fourier transform relationship, but a linear relationship does exists and imaging is still possible. This thesis explores the possibilities of inhomo- geneous field imaging and presents the development of hardware for inhomogeneous MRI research. Two techniques for inhomogeneous imaging are derived and presented. Matrix operators are found for these two imaging methods which are analyzed using a singular value decomposition. This analysis shows that reconstructing an image from an inhomogeneous system is possible if a field map is available.

<p> Thick-film resistors are being used in the hybrid microelectronics industry. One characteristic that has been considered for the thick film resistor i s the reproduceability in resistor value associated with the production techniques employed in the reproduceability in resistor value associated with the production techniques employed in the resistor manufactured. </p> <p> One parameter that has been associated with reproduceability is the yield figure; another parameter is the width of the resistance distribution for a given resistor production. These parameters can be used to identify the degree of reproduceability for the resistor production. A high yield figure or a small spread figure indicates a high degree of reproduceability. </p> <p> The effect of aspect ratio and sheet resistivity on the reproduceability has been evaluated for paste system A. Aspect ratios of 10, 5, 2 and 0.7 for a range of sheet resistivities (100, 1K, 10K, 100K, and 1M ohms per square) were investigated. Results show that for a given aspect ratio the amount of variation in spread for the range of sheet resistivities is negligible within the limits of error in the experiment. For a given sheet resistivity, the variation in spread for the range of aspect ratios is considerable. The spread of an aspect ratio of 10 is 25 ± 4%. For an aspect ratio of 0.7 the spread is 59± 7%. </p> <p> The effect of resistor location on spread for a 2" x 2" substrate has been studied. Results indicate that a higher degree of reproduceability can be associated with resistors located near the central (1" x 1") region of the substrate than the resistors located near the perimeter. </p> <p> The effect of increasing the resistor width on the reproduceability has been evaluated for resistors having aspect ratios near unity. Results show that as the resistor width is increased the spread in resistance decreases. For a width of 30 mil the spread in resistance is 37%. When the width is increased to 100 mil, the spread decreases to 21%. </p> / Thesis / Master of Engineering (MEngr)

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published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

Two-dimensional electron gases (2DEG) located at GaAs/Al0.33Ga0.67As heterojunctions are modulated by a periodic magnetic field generated by a magnetic grating fabricated at the surface of the heterostructure and are irradiated by microwaves. The devices were investigated for the detection of excitations of both paramagnetic and ferromagnetic spins in the magnetoresistance. Electron spin antiresonance was electrically detected, where spin flips are propelled by two transitions: one activated by snake orbit oscillations in the slanted magnetic field, the other by the microwaves. The double resonance forms a dark state which blocks spin flips, decreases Overhauser shift and freezes snake orbit drift, therefore changing the conduction in the 2DEG. The antiresonance is quantitatively described in the coherent population trapping framework. Collective and localised spin wave modes in dysprosium and cobalt gratings were detected as well in the 2DEG. Such effects were investigated as a function of microwave power, temperature, tilt angle of the applied magnetic field, and by varying the structural and material parameters to change the strength of dipolar interactions. The data reveal two types of spin waves. Dipolar magnetisation waves propagate across the grating through the magnetostatic interaction between stripes. An analytical expression of their dispersion curve was derived and a good fit of the ferromagnetic resonance broadening was obtained. The second type are dipolar edge spin waves which manifest through a series of sharp resonances at lower magnetic field. These waves are confined near the pole surfaces in spin wave ”wells”. The eigenfrequencies of the quantised modes were calculated and a qualitative explanation of the low field resonances was obtained. The experiments show that photovoltage measurements in hybrid semiconductor-ferromagnetic structures provide a sensitive and non-invasive tool for probing the spin waves of small magnets

Multiple-input multiple-output (MIMO) systems are a mature technology that has been incorporated into current wireless broadband standards to improve the channel capacity and link reliability. Nevertheless, due to the continuous increasing demand for wireless data traffic new strategies are to be adopted. Very large MIMO antenna arrays represents a paradigm shift in terms of theory and implementation, where the use of tens or hundreds of antennas provides significant improvements in throughput and radiated energy efficiency compared to single antennas setups. Since design constraints limit the number of usable antennas, virtual systems can be seen as a promising technique due to their ability to mimic and exploit the gains of multi-antenna systems by means of wireless cooperation. Considering these arguments, in this work, energy efficient coding and network design for large virtual MIMO systems are presented. Firstly, a cooperative virtual MIMO (V-MIMO) system that uses a large multi-antenna transmitter and implements compress-and-forward (CF) relay cooperation is investigated. Since constructing a reliable codebook is the most computationally complex task performed by the relay nodes in CF cooperation, reduced complexity quantisation techniques are introduced. The analysis is focused on the block error probability (BLER) and the computational complexity for the uniform scalar quantiser (U-SQ) and the Lloyd-Max algorithm (LM-SQ). Numerical results show that the LM-SQ is simpler to design and can achieve a BLER performance comparable to the optimal vector quantiser. Furthermore, due to its low complexity, U-SQ could be consider particularly suitable for very large wireless systems. Even though very large MIMO systems enhance the spectral efficiency of wireless networks, this comes at the expense of linearly increasing the power consumption due to the use of multiple radio frequency chains to support the antennas. Thus, the energy efficiency and throughput of the cooperative V-MIMO system are analysed and the impact of the imperfect channel state information (CSI) on the system's performance is studied. Finally, a power allocation algorithm is implemented to reduce the total power consumption. Simulation results show that wireless cooperation between users is more energy efficient than using a high modulation order transmission and that the larger the number of transmit antennas the lower the impact of the imperfect CSI on the system's performance. Finally, the application of cooperative systems is extended to wireless self-backhauling heterogeneous networks, where the decode-and-forward (DF) protocol is employed to provide a cost-effective and reliable backhaul. The associated trade-offs for a heterogeneous network with inhomogeneous user distributions are investigated through the use of sleeping strategies. Three different policies for switching-off base stations are considered: random, load-based and greedy algorithms. The probability of coverage for the random and load-based sleeping policies is derived. Moreover, an energy efficient base station deployment and operation approach is presented. Numerical results show that the average number of base stations required to support the traffic load at peak-time can be reduced by using the greedy algorithm for base station deployment and that highly clustered networks exhibit a smaller average serving distance and thus, a better probability of coverage.