UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item Enhanced transport of spin-orbit-coupled Bose gases indisordered potentials(2020) Yue, Yuchen; Spielman, Ian; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Anderson localization is a single particle localization phenomena in disordered media that is accompanied by an absence of diffusion.Spin-orbit coupling (SOC) describes an interaction between a particle's spin and its momentum that directly affects its energy dispersion, for example creating dispersion relations with gaps and multiple local minima. We show theoretically that combining one-dimensional spin-orbit coupling with a transverse Zeeman field suppresses the effects of disorder, thereby increasing the localization length and conductivity. This increase results from a suppression of backscattering between states in the gap of the SOC dispersion relation. Here, we focus specifically on the interplay of disorder from an optical speckle potential and SOC generated by two-photon Raman processes in quasi-1D Bose-Einstein condensates. We first describe back-scattering using a Fermi's golden rule approach, and then numerically confirm this picture by solving the time-dependent 1D Gross Pitaevskii equation for a weakly interacting Bose-Einstein condensate with SOC and disorder. We find that on the 10's of millisecond time scale of typical cold atom experiments moving in harmonic traps, initial states with momentum in the zero-momentum SOC gap evolve with negligible back-scattering, while without SOC these same states rapidly localize.Item Theoretical studies of the interplay between superconductivity and disorder(2012) Gangopadhyay, Anirban; Galitski, Victor; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, I explore a variety of disordered condensed matter systems and investigate questions pertinent to transport in such systems. In the first part of the thesis, I seek explanations for the strange feature of a giant magnetoresistance peak seen in the vicinity of superconductor-insulator transitions. To this end, I propose a semiclassical two-component Coulomb glass model for 2D insulators close to such transitions. I show that a local pairing attraction in Coulomb glasses can lead to crucial modification of the low-energy density of states which may affect transport. In another explanation for the peak, I consider an Anderson insulator of localized pairs and develop a theory of their transport. I study the change in localization length of the pairs (treated as bosons) on applying a magnetic field and the consequent change in transport properties. I show that from a statistical consideration alone, one can predict a nonmonotonicity in magnetoresistance. In the process, I also revisit the classic problem of directed polymers in a random media (DPRM) and propose a toy model for magnetoresistance in bosonic insulators based on DPRM scalings. In the second part of the thesis, I derive a class of exact solutions for two-level systems driven by a time-periodic external field which pertains to loss mechanisms in superconducting charge qubits.Item Energy Localization and Transport in Binary Isotopically Disordered Fermi-Pasta-Ulam Chains(2005-05-26) Snyder, Kenneth Alan; Kirkpatrick, Theodore R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energy transport in binary isotopically disordered (BID) nonlinear Fermi-Pasta-Ulam (FPU) chains is a competition between localization and mode transitions. Starting from an arbitrary localized pulse, energy will dissipate ballistically until either Anderson localization (a disorder effect) or phonon scattering (a nonlinearity effect) slow the rate of dissipation. To reduce computational effort, we propose starting from a localized energy eigenstate so that in the absence of anharmonicity the energy is stationary and there is no transport. The second moment of the site energies is used to characterize an effective thermal conductivity as a function of impurity concentration and nonlinearity strength. Calculating the properties of harmonic BID chains at arbitrary impurity concentration is complicated by the pure-disordered-pure transition that occurs as the impurity concentration varies from zero to one. The localization length of dilute impurity harmonic BID chains is calculated exactly using scaling laws and the scattering cross section of a single impurity, which is calculated for discrete systems, differs from the continuum result. For arbitrary impurity concentration, the localization length is estimated by assuming independent contributions from the two limiting cases of pure material. Information entropy was used to show that the number of modes excited by phonon scattering decreased with increasing impurity concentration, a fact that consistent with density of states calculations. At all impurity concentrations, the second moment of the site energies increases linearly in time, a fact that is corroborated by the number of masses participating in energy transport, as calculated from the localization parameter. The dilute concentration dependence of the effective thermal conductivity was consistent with kinetic theory. At the highest concentrations the thermal conductivity was proportional to the original localization length because mode suppression and dense impurities meant that the same length scale remained dominant over a long period of time.