Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 Electronic and Magnetic Properties of MnP-Type Binary Compounds(2019) Campbell, Daniel James; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interactions between electrons, and the resulting impact on physical properties, are at the heart of present-day materials science. This thesis looks at this idea through the lens of several compounds from a single family: the MnP-type transition metal pnictides. FeAs and FeP show long range magnetic order with some similarities to the high temperature, unconventional iron-based superconductors. CoAs lies on the border of magnetism, with strong fluctuations but no stable ordered state. CoP, in contrast, shows no strong magnetic fluctuations but serves as a useful baseline in determining the origin (from composition, structure, or magnetic order) of behavior in the other materials. For this work, single crystals were grown with two different techniques: solvent flux and chemical vapor transport. In the case of FeAs the flux method resulted in the highest quality crystals yet produced. Extensive work was then performed on these samples at the University of Maryland and the National High Magnetic Field Laboratory. Quantum oscillations observed in high magnetic fields, in combination with density functional theory calculations, give insight into the Fermi surfaces of these materials. Large magnetoresistance in the phosphides, but not the arsenides, demonstrates differences in the choice of pnictogen atom that cannot be simply a product of electron count. Angle-dependent linear magnetoresistance in FeP is a sign of a possible Dirac dispersion and topological physics, as has been hinted at in other MnP-type materials. Ultimately, it is possible to examine results for all four compounds and draw conclusions on the role of each of the two elements in the formula, which can be extended to other members of this family.Item Multiscale Modeling and Simulation of Stepped Crystal Surfaces(2016) Schneider, Joshua Peter; Margetis, Dionisios; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A primary goal of this dissertation is to understand the links between mathematical models that describe crystal surfaces at three fundamental length scales: The scale of individual atoms, the scale of collections of atoms forming crystal defects, and macroscopic scale. Characterizing connections between different classes of models is a critical task for gaining insight into the physics they describe, a long-standing objective in applied analysis, and also highly relevant in engineering applications. The key concept I use in each problem addressed in this thesis is coarse graining, which is a strategy for connecting fine representations or models with coarser representations. Often this idea is invoked to reduce a large discrete system to an appropriate continuum description, e.g. individual particles are represented by a continuous density. While there is no general theory of coarse graining, one closely related mathematical approach is asymptotic analysis, i.e. the description of limiting behavior as some parameter becomes very large or very small. In the case of crystalline solids, it is natural to consider cases where the number of particles is large or where the lattice spacing is small. Limits such as these often make explicit the nature of links between models capturing different scales, and, once established, provide a means of improving our understanding, or the models themselves. Finding appropriate variables whose limits illustrate the important connections between models is no easy task, however. This is one area where computer simulation is extremely helpful, as it allows us to see the results of complex dynamics and gather clues regarding the roles of different physical quantities. On the other hand, connections between models enable the development of novel multiscale computational schemes, so understanding can assist computation and vice versa. Some of these ideas are demonstrated in this thesis. The important outcomes of this thesis include: (1) a systematic derivation of the step-flow model of Burton, Cabrera, and Frank, with corrections, from an atomistic solid-on-solid-type models in 1+1 dimensions; (2) the inclusion of an atomistically motivated transport mechanism in an island dynamics model allowing for a more detailed account of mound evolution; and (3) the development of a hybrid discrete-continuum scheme for simulating the relaxation of a faceted crystal mound. Central to all of these modeling and simulation efforts is the presence of steps composed of individual layers of atoms on vicinal crystal surfaces. Consequently, a recurring theme in this research is the observation that mesoscale defects play a crucial role in crystal morphological evolution.