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
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Item SUPERCONDUCTORS THAT BREAK TIME-REVERSAL SYMMETRY(2019) Boyer, Lance L.; Yakovenko, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since 2006 it has been discovered experimentally that the superconducting state spontaneously breaks time-reversal symmetry (TRS) in several materials, such as Sr2RuO4, UPt3, URu2Si2, PrOs4Sb12, and Bi/Ni bilayers. This dissertation studies three physical phenomena related to time-reversal symmetry breaking (TRSB) in these superconductors. The experimental evidence for TRSB comes from the magneto-optical polar Kerr effect, which is determined by the high frequency ac Hall conductivity. However, these superconductors are also expected to exhibit a spontaneous dc Hall effect in the absence of an applied magnetic field. In the first part of this dissertation we propose a method for measuring the low frequency Hall conductivity in superconductors with TRSB. The method is based on a Corbino disk geometry where an oscillating co-axial magnetic field induces circular electric field, which, in turn, induces radial charge oscillations due to the Hall conductivity. In the second part, we propose an explanation for the polar Kerr effect observed in the Hidden-Order phase of the heavy-fermion superconductor URu2Si2. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state, which produces the Kerr signal, even if the Hidden-Order state respects TRS. We predict that applying a reversed magnetic field should reset the system to the non-magnetic ground state, resulting in zero Kerr signal. In the third part of the dissertation, we investigate the conditions for the existence of a Majorana bound state on a vortex in a 2D d+id superconductor with strong spin-orbit coupling. This TRSB pairing was proposed earlier for the Ni/Bi bilayer. We find that the Majorana bound state can exist for a d+id pairing under conditions similar to those for s-wave pairing.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 Search for the Neutrinoless Double Beta Decay of Xenon-136 with Improved Sensitivity from Denoising(2014) Davis, Clayton G.; Hall, Carter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The EXO-200 detector is designed to search for the neutrinoless double beta decay of 136Xe. ββ0ν decay, if it occurs in nature, would demonstrate the fundamental nature of neutrino mass; set the mass scale of the neutrino sector; and demonstrate lepton number non-conservation. Since the ββ0ν decay produces a monoenergetic peak, the energy resolution of the detector is of fundamental importance for the sensitivity of the experiment. The present work describes a new analysis technique which improves the energy resolution of EXO-200 through a combination of waveform denoising and weighting of waveform components based on their expected signal-to-noise ratio. With this method, the energy resolution of the detector is improved by 21% and the expected background in the 2σ region of interest is reduced by 32%. Applying this technique to 99.8 kg*years of exposure collected by EXO-200 between October 5, 2011 and September 1, 2013, we find no statistically significant evidence for the presence of ββ0ν in the data. We set a half-life limit T1/2 > 1.1 × 1025 years at 90% confidence. We also describe further improvements which could impact the energy resolution of EXO-200, and consider implications for the planned nEXO experiment.