Thesis (doctoral)--Technische Universiteit Delft, 1997. / Includes bibliographical references.

Thesis (Ph. D.)--Florida State University, 2006. / Advisor: James S. Brooks, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Sept. 20, 2006). Document formatted into pages; contains xiii, 106 pages. Includes bibliographical references.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 83-97). Also available in electronic version.

Orientador: Rafael Zadorosny / Resumo: O estudo de materiais supercondutores (SCs) em escalas nanométricas tem se destacado na área da Física de Matéria Condensada nas últimas décadas. O foco tem-se dado aos estudos das propriedades elétricas e magnéticas desses materiais, além da intensificação do desenvolvimento de técnicas para a produção e manipulação em nanoescala. Nos ditos supercondutores mesosocópicos, efeitos de confinamento influenciam na dinâmica de vórtices, ocasionando, inclusive, estados de vórtices múltiplos e de vórtices gigantes. Neste trabalho, usamos a teoria de Ginzburg-Landau para estudar supercondutores mesoscópicos com uma rede de defeitos. Foram analisados o comportamento magnético das amostras para dois tipos diferentes de defeitos, sendo eles, buracos que transpassam o material (antidots, ADs) e buracos com uma fina camada supercondutora (Blind Holes, BHs). As amostras foram expostas a campos externos variáveis e o loop de histerese das mesmas fora levantado. Os sistemas foram simulados com temperatura T=0,9Tc, e possuindo geometria quadrada de tamanho lateral L=54 ξ(0), defeitos também quadrados de lado l=2ξ(0) e espaçados por 2ξ(0). Nestas condições, efeitos de confinamento são muito fortes, inclusive devido ao tamanho dos vórtices, cujo ξ(T=0,9) é aproximadamente 3ξ(0). Com isso, há pouca diferença no comportamento global das diferentes amostras, contudo, a dinâmica de vórtices é alterada devida à natureza distinta entre ADs e BHs. Outro fato interessante é que, sendo os vórtices muito... (Resumo completo, clicar acesso eletrônico abaixo) / Mestre

Evacuation processes can be evaluated using different simulation models. However, recently, microscopic simulation models have become a more popular tool for this purpose. The objectives of this study are to model multiple evacuation scenarios and to compare the INTEGRATION microscopic traffic simulation model against the MATSim mesoscopic model. Given that the demand was the same for both models, the comparison was achieved based on three indicators: estimated evacuation time, average trip duration, and average trip distance. The results show that the estimated evacuation times in both models are close to each other since the Origin-Destination input file has a long tail distribution and so the majority of the evacuation time is associated when travelers evacuate and not the actual evacuation times. However, the evaluation also shows a considerable difference between the two models in the average trip duration. The average trip duration using INTEGRATION increases with increasing traffic demand levels and decreasing roadway capacity. On the other hand, the average trip duration using MATSim decreases with increasing traffic demand and decreasing the roadway capacity. Finally, the average trip distance values were significantly different in both models. The conclusion showed that the INTEGRATION model is more realistic than the MATSim model for evacuation purposes. The study concludes that despite the large execution times of a microscopic traffic simulation, the use of microsimulation is a worthwhile investment. / Master of Science

[Truncated abstract] In this dissertation we present a theoretical study of electron transport through domain walls, with a particular focus on conductance properties, in order to understand various transport measurements that have been carried out recently on ferromagnetic nanowires. The starting point for our work is a ballistic treatment of transport through the domain wall. In this case conduction electrons are generally only weakly reflected by the domain wall, and the principal effect is a mixing of transmitted electron spins between up and down states. For small spin-splitting of conductance electrons the latter can be characterized by an appropriate adiabaticity parameter. We then incorporate the effect of spin-dependent scattering in the regions adjacent to the domain wall through a circuit model based on a generalization of the two-resistor theory of Valet and Fert. Within this model we find that the domain wall gives rise to an enhancement of resistance similar to the giant magnetoresistance effect found in ferromagnetic multilayer systems. The effect is largest in the limit of an abrupt wall, for which there is complete mistracking of spin, and decreases with increasing wall width due to the reduction of spin mistracking. For reasonable physical parameter values we find order-of-magnitude agreement with recent experiments. Going beyond the assumption of ballistic transport, we then consider the more realistic case of a domain wall subject to impurity scattering. A scattering matrix formalism is used to calculate conductance through a disordered region with either uniform magnetization or a domain wall. By combining either amplitudes or probabilities we are able to study both coherent and incoherent transport properties. The coherent case corresponds to elastic scattering by static defects, which is dominant at low temperatures, while the incoherent case provides a phenomenological description of the inelastic scattering present in real physical systems at room temperature. It is found that scattering from impurities increases the amount of spin mistracking of electrons travelling through a domain wall. This leads, in the incoherent case, to a reduction of conductance through the domain wall as compared to a uniformly magnetized region. In the coherent case, on the other hand, a reduction of weak localization and spin-reversing reflection amplitudes combine to give a positive contribution to domain wall conductance, which can lead to an overall enhancement of conductance due to the domain wall in the diffusive regime. A reduction of universal conductance fluctuations is found in a coherent disordered domain wall, which can be attributed to a decorrelation between spin-mixing and spin-conserving scattering amplitudes.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The Multi-resolution Assignment and Loading of Transport Activities (MALTA) system is a simulation-based Dynamic Traffic Assignment model that exploits the advantages of multi-processor computing via the use of the Message Passing Interface (MPI) protocol. Spatially partitioned transportation networks are utilized to estimate travel time via alternate routes on mega-scale network models, while the concurrently run shortest path and assignment procedures evaluate traffic conditions and re-assign traffic in order to achieve traffic assignment goals such as User Optimal and/or System Optimal conditions.Performance gain is obtained via the spatial partitioning architecture that allows the simulation domains to distribute the work load based on a specially designed Recursive On-line Load Balance model (ROLB). The ROLB development describes how the transportation network is transformed into an ordered node network which serves as the basis for a minimum cost heuristic, solved using the shortest path, which solves a multi-objective NP Hard binary optimization problem. The approach to this problem contains a least-squares formulation that attempts to balance the computational load of each of the mSim domains as well as to minimize the inter-domain communication requirements. The model is developed from its formal formulation to the heuristic utilized to quickly solve the problem. As a component of the balancing model, a load forecasting technique is used, Fast Sim, to determine what the link loading of the future network in order to estimate average future link speeds enabling a good solution for the ROLB method.The runtime performance of the MALTA model is described in detail. It is shown how a 94% reduction in runtime was achieved with the Maricopa Association of Governments (MAG) network with the use of 33 CPUs. The runtime was reduced from over 60 minutes of runtime on one machine to less than 5 minutes on the 33 CPUs. The results also showed how the individual runtimes on each of the simulation domains could vary drastically with naïve partitioning methods as opposed to the balanced run-time using the ROLB method; confirming the need to have a load balancing technique for MALTA.

This thesis is devoted to quantum confinement effects in low-dimensional Dirac materials. We propose a variety of schemes in which massless Dirac fermions, which are notoriously diffcult to manipulate, can be trapped in a bound state. Primarily we appeal for the use of external electromagnetic fields. As a consequence of this endeavor, we find several interesting condensed matter analogues to effects from relativistic quantum mechanics, as well as entirely new effects and a possible novel state of matter. For example, in our study of the effective Coulomb interaction in one dimension, we demonstrate how atomic collapse may arise in carbon nanotubes or graphene nanoribbons, and describe the critical importance of the size of the band gap. Meanwhile, inspired by groundbreaking experiments investigating the effects of strain, we propose how to confine the elusive charge carriers in so-called velocity barriers, which arise due to a spatially inhomogeneous Fermi velocity triggered by a strained lattice. We also present a new and beautiful quasi-exactly solvable model of quantum mechanics, showing the possibilities for confinement in magnetic quantum dots are not as stringent as previously thought. We also reveal that Klein tunnelling is not as pernicious as widely believed, as we show bound states can arise from purely electrostatic means at the Dirac point energy. Finally, we show from an analytical solution to the quasi-relativistic two-body problem, how an exotic same-particle paring can occur and speculate on its implications if found in the laboratory.