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 Characterization of Quantum Vortex Dynamics in Superfluid Helium(2015) Meichle, David P.; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Liquid helium obtains superfluid properties when cooled below the Lambda transition temperature of 2.17 K. A superfluid, which is a partial Bose Einstein condensate, has many exotic properties including free flow without friction, and ballistic instead of diffusive heat transport. A superfluid is also uniquely characterized by the presence of quantized vortices, dynamical line-like topological phase defects around which all circulation in the flow is constrained. Two vortices can undergo a violent process called reconnection when they approach, cross, and retract having exchanged tails. With a numerical examination of a local, linearized solution near reconnection we discovered a dynamically unstable stationary solution to the Gross-Pitaevskii equation, which was relaxed to a fully non-linear solution using imaginary time propagation. This investigation explored vortex reconnection in the context of the changing topology of the order parameter, a complex field governing the superfluid dynamics at zero temperature. The dynamics of the vortices can be studied experimentally by dispersing tracer particles into a superfluid flow and recording their motions with movie cameras. The pioneering work of Bewley et al. provided the first visualization technique using frozen gases to create tracer particles. Using this technique, we experimentally observed for the first time the excitation of helical traveling waves on a vortex core called Kelvin waves. Kelvin waves are thought to be a central mechanism for dissipation in this inviscid fluid, as they provide an efficient cascade mechanism for transferring energy from large to microscopic length scales. We examined the Kelvin waves in detail, and compared their dynamics in fully self-similar non-dimensional coordinates to theoretical predictions. Additionally, two experimental advances are presented. A newly invented technique for reliably dispersing robust, nanometer-scale fluorescent tracer particles directly into the superfluid is described. A detailed numerical investigation of the particle-vortex interactions provides novel calculations of the force trapping particles on vortices, and a scaling was found suggesting that smaller particles may remain bound to the vortices at much higher speeds than larger particles. Lastly, a new stereographic imaging system has been developed, allowing for the world-first three-dimensional reconstruction of individual particles and vortex filament trajectories. Preliminary data, including the first three-dimensional observation of a vortex reconnection are presented.Item Properties of Metallic Helimagnets(2012) Ho, Kwan-yuet; Kirkpatrick, Theodore R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates various aspects of helimagnets. Helimagnets are magnets with spins aligned in helical order at low temperatures. It exists in materials of crystal structure lacking the spatial inversion symmetry. The helical order is due to the Dzyaloshinskii-Moriya (DM) mechanism. Examples of helimagnets include MnSi, FeGe and Fe1-xCoxSi. A field theory appropriate for such magnets is used to derive the phase diagram in a mean-field approximation. The helical phase, the conical phase, the columnar phase and the non-Fermi-liquid (NFL) region in the paramagnetic phase are discussed. It is shown that the orientation of the helical vector along an external magnetic field within the conical phase occurs via two distinct phase transitions. The columnar phase, believed to be a Skyrmion lattice, is found to exist as Abrikosov Skyrmions near the helimagnetic phase boundary, and the core-to-core distance is estimated. The Goldstone modes that result from the long-range order in the various phases are determined, and their consequences for electronic properties, in particular, the specific heat, single-particle relaxation rate and the electrical conductivity, are derived. In addition, Skyrmion gases and lattices in helimagnets are studied, and the size of a Skyrmion in various phases is estimated. For isolated Skyrmions, the long distance tail is related to the magnetization correlation functions and exhibits power-law decay if the phase spontaneously breaks a continuous symmetry, but decays exponentially otherwise. The size of a Skyrmion is found to depend on a number of length scales. These length scales are related to the strength of DM interaction, the temperature, and the external magnetic field.Item A Scaled Equation of State for the Liquid-Liquid Critical Point in Supercooled Water(2007-09-14) Fuentevilla, Daphne Anne; Anisimov, Mikhail A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The second-critical-point scenario is one of the most popular explanations for the anomalous behavior of supercooled liquid water. According to this scenario, liquid water at ambient conditions is a "supercritical" ?uid that separates into two types of liquid water in the supercooled region. However, experimental confirmation is challenging. In this work we developed a scaled parametric equation of state, based on the principle of critical-point universality, to examine the second-critical-point scenario from a new direction. The equation of state, built on the growing evidence for liquid-liquid water separation, is universal in terms of theoretical scaling fields and belongs to the Ising-model universality class. The theoretical scaling fields are postulated to be analytical combinations of the physical fields, pressure and temperature. The equation of state enables us to accurately locate the "Widom line" (locus of stability minima) and determine that the critical pressure is considerably lower than predicted by computer simulations.