Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Microwave Photos in High Impedance Transmission line: Dispersion, Disorder and Localization(2017) Mehta, Nitish Jitendrakumar; Murphy, Thomas E; Manucharyan, Vladimir E; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we will describe the theoretical and experimental studies of a TEM on-chip superconducting transmission line with a wave impedance as high as 20 $\mathrm{k}\Omega$, phase and group velocity of waves simultaneously reduced by a factor of 100 in a broad range of frequencies from 0 to about 10 $\mathrm{GHz}$. A conventional microwave coaxial transmission line gets its inductance and capacitance from magnetic and electric fields stored in the space between its inner and outer conductors. This in turn limits its impedance to around 50 $\Omega$ and group velocity of waves very close to the speed of light in vacuum. In this work we are able to increase the impedance by over two orders of magnitude and reduce the group and phase velocity of waves by over two orders of magnitude as well, by constructing a coplanar transmission line out of a pair of long Al/AlOx/Al Josephson tunnel junction chains. A Josephson junction gets its inductance not from the magnetic energy but rather from the much larger kinetic energy of tunneling Cooper pairs, which is unrelated to the electromagnetic properties of vacuum. In this work we present a design of such a transmission line and low-temperature measurement of its dispersion relation. We then study and characterize the disorder present in the circuit parameters of our system and using this, we conclude that for frequencies up to 12 GHz, there is no evidence of Anderson localization of waves, even for chains exceeding 30,000 junctions. Low dissipation and absence of localization make this transmission line ideal for use in various experiments where high impedance can enable strong coupling between light and matter.Item Majorana Zero Modes in Solid State Systems(2015) Hui, Hoi Yin; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Majorana zero modes are zero-energy excitations that are their own anti-particles, and obey non-Abelian statistics which could be harnessed for topological quantum computation. There are many theoretical proposals to realize them in solid state systems, but experimental realizations are confronted by a number of non-idealities. In this thesis, we theoretically investigate such complications, thereby suggesting improvement and directions that could be pursued. We first develop a theoretical framework to analyze the effect of ensemble-averaged disorder on the Majorana zero modes, generalizing the Eilenberger theory to handle 1D systems while retaining short-distance fluctuations. We then consider disordered topological insulator-based heterostructures, showing that extra subgap states are potentially induced, obscuring the density-of-states signature of the Majorana zero mode. We also analyze in depth the experimentally observed soft gap feature, suggesting that a cleaner interface in the semiconductor-based proposal can harden the gap. In view of some of the limitations of the proposals based on semiconductors or topological insulators, we look into a new class of systems in which a ferromagnetic atomic chain is put on the surface of a bulk spin-orbit-coupled superconductor. This system is analyzed in two limits, corresponding to weak or strong inter-atomic hopping on the chain. In each of these cases, the topological criteria are obtained. We also find that in the limit of strong chain-superconductor coupling, the length scales of the effective Hamiltonian of the chain are significantly suppressed, potentially explaining some of the recent observations in experiments.Item A Multilevel Exploration of Neighborhood Disorder, Family Management and Antisocial Behavior(2012) Lobo Antunes, Maria; Simpson, Sally S; Criminology and Criminal Justice; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tremendous research has been dedicated to unpacking the relationship between neighborhood characteristics and youth development. Despite these efforts conclusions have been generally mixed and it is the lack of consensus regarding the importance of community that has in part fueled this dissertation. Much of the research dedicated to examining community and neighborhood effects on parenting and child behavior have been focused on community structural characteristics. Even though there is much evidence to suggest that disorder can affect both individuals and communities alike there is a paucity of literature on how neighborhood disorder may simultaneously influence family practices and child problem behavior. Using data from the Project on Human Development in Chicago Neighborhoods (PHDCN) and borrowing from the conceptual framework developed by Furstenberg, Cook, Eccles, Elder and Sameroff (1999), the current investigation explores the relationship between disorder, family management and youth antisocial behavior by posing four research questions. First, what are the effects of neighborhood disorder on family management practices? Second, how do family management strategies influence youth involvement in antisocial behavior? Third, how does neighborhood disorder affect youth antisocial behavior? Fourth, what is the multilevel relationship between disorder, family management and antisocial behavior? A series of models analyzed using Hierarchical Linear Model indicate that although disorder significantly influences several parenting strategies, in the end, family management practices tend to have a greater impact on youth involvement in antisocial behavior. Parents living in disordered neighborhood are more likely to limit their child's access to the surrounding neighborhood which in turn is shown to reduce antisocial behavior. These findings suggest that protective family management practices can be effective in curbing youth deviant behavior. Moreover, the analyses also revealed a significant relationship between proximal mechanisms of antisocial behavior (exposure to violence and peer deviance), family management and antisocial behavior. In essence, parental efforts in reducing exposure to violence and peer deviance have a protective effect in reducing antisocial behavior, especially in highly disordered neighborhoods. In keeping with these findings, several avenues for future research are discussed, as are theoretical and policy implications.Item MAKING SENSE OF BROKEN WINDOWS: THE RELATIONSHIP BETWEEN PERCEPTIONS OF DISORDER, FEAR OF CRIME, COLLECTIVE EFFICACY AND PERCEPTIONS OF CRIME(2009) Hinkle, Joshua Conard; Weisburd, David; Criminology and Criminal Justice; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The broken windows thesis has had a profound impact on policing strategies around the world. The thesis suggests that police can most effectively fight crime by focusing their efforts on targeting disorder--minor crimes and nuisance behaviors such as loitering, public drinking and vandalism, as well as dilapidated physical conditions in a community. The strategy was most prominently used in New York City in the 1990s, and has been often credited for the crime drop observed in the city over that decade. Despite the widespread influence of the broken windows thesis, there has been relatively little rigorous empirical research which has sought to test the validity of its theoretical propositions. This dissertation aimed to address this shortcoming by using structural equation modeling to test the relationships between perceived disorder, fear of crime, collective efficacy and perceptions of crime suggested by the broken windows thesis using survey data collected during a randomized, experimental evaluation of broken windows policing in three cities in California. The results are supportive of the broken windows thesis, but also raise some challenges. Perceptions of disorder were found to increase fear of crime, reduce collective efficacy and lead to crime as suggested. However, fear of crime was not significantly related to collective efficacy as suggested, and the direct effect of perceived social disorder on perceptions of crime was the strongest effect in every model. Nevertheless, the findings do suggest that a reduction of disorder in a community may have positive effects in the form of reducing fear and promoting collective efficacy, and suggest the limitations of studies which only test for direct effects of disorder on crime and/or do not examine the variables at the perceptual level. Future research needs to further examine the broken windows thesis, ideally involving a prospective longitudinal study of crime at place.