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 INTERFACIAL INTERACTIONS OF NANOTUBES: AN IN-SITU STUDY OF STRUCTURE AND REACTIONS WITH THEIR ENVIRONMENTS(2021) Chao, Hsin-Yun; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanotubes have the potential to be a revolutionary material for many different applications. Though often touted as versatile and tunable materials, the difficulty of their reliable large-scale production for any specified property is a hurdle in their wide-scale implementation. Interactions at nanotube interfaces dictate overall performance of their growth, radiation resistance, and nanofluidics properties. In this dissertation, I present in-situ experiments using an environmental transmission electron microscope (ETEM). Numerous aspects of interfacial mechanisms of nanotubes are examined at the atomic scale and models considered for the observed behavior. First, I study the interface between nanotubes and catalyst particles during single-walled carbon nanotube (SWCNT) growth. The structure and phase transformation of cobalt catalysts are elucidated for inactive, active, and deactivated nanoparticles by ETEM imaging. Through in-depth studies of multiple distinct cobalt nanoparticles, I establish the dominant nanoparticle phase for SWCNT growth. I also identify the preferred lattice planes and a threshold for work of adhesion for the anchoring and liftoff of SWCNTs. Second, the nanotubes are tested for their radiation resistance properties. I study the resistance of nanotube degradation in an ionizing environment with oxygen pressure, where the damage initiates at the interface with the gas phase. Observations show boron nitride nanotubes (BNNTs) have a higher resistance to damage than carbon nanotubes (CNTs). By computing knock-on threshold energies for the atoms impacted by incoming electrons, a model can be formulated for the oxygen-assisted radiation damage pathway. I provide further validation to the model with heating experiments that demonstrate a surprising increase in damage resistance. Lastly, interfaces between nanotubes and water are studied. The goal is to capture the ordering dynamics of water at the BNNT interface using in-situ characterization at cryogenic temperatures. Water is hyper-quenched to liquid nitrogen temperatures for the formation of low density amorphous (LDA) ice. High resolution images are then acquired, preserving the original water structure. Crystallization of LDA ice is induced by both environmental heating and electron beam irradiation. I present a comparison of the structural evolutions of LDA ice with and without the presence of BNNT, which indicates the presence of nascent ordering at the interface.Item Many-Body Dephasing in a Cryogenic Trapped Ion Quantum Simulator(2019) Kaplan, Harvey B.; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)While realizing a fully functional quantum computer presents a long term technical goal, in the present, there are small to mid-sized quantum simulators (up to $\sim 100$ qubits), that are capable of approaching specialized problems. The quantum simulator discussed here uses trapped ions to act as qubits and is housed in a cryogenically cooled vacuum chamber in order to reduce the background pressure, thereby increasing ion chain length and life-time. The details of performance and characterization of this cryogenic apparatus are discussed, and this system is used to study many-body dephasing in a finite-sized quantum spin system. How a closed quantum many-body system relaxes and dephases as a function of time is important to understand when dealing with many-body spin systems. In this work, the first experimental observation of persistent temporal fluctuations after a quantum quench is presented with a tunable long-range interacting transverse-field Ising Hamiltonian. The fluctuations in the average magnetization of a finite-size system of spin-$1/2$ particles are measured presenting a direct measurement of relaxation dynamics in a non-integrable system. This experiment is in the regime where the properties of the system are closely related to the integrable Hamiltonian with global coupling. The system size is varied in order to investigate the dependence on finite-size scaling, and the system size scaling exponent extracted from the measured fluctuations is consistent with theoretical prediction.Item SPACE-DEPLOYED, THIN-WALLED ENCLOSURE FOR A CRYOGENICALLY-COOLED HIGH TEMPERATURE SUPERCONDUCTING COIL(2015) Porter, Allison Kyeong; Sedwick, Raymond J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interaction of magnetic fields generated by large superconducting coils has multiple applications in space, including actuation of spacecraft or spacecraft components, wireless power transfer, and shielding of spacecraft from radiation and high energy particles. These applications require coils with major diameters as large as 20 meters and a thermal management system to maintain the superconducting material of the coil below its critical temperature. Since a rigid thermal management system, such as a heat pipe, is unsuitable for compact stowage inside a 5 meter payload fairing, a thin-walled thermal enclosure is proposed. A 1.85 meter diameter test article consisting of a bladder layer for containing chilled nitrogen vapor, a restraint layer, and multilayer insulation was tested in a custom toroidal vacuum chamber. The material properties found during laboratory testing are used to predict the performance of the test article in low Earth orbit. Deployment motion of the same test article was measured using a motion capture system and the results are used to predict the deployment in space. A 20 meter major diameter and coil current of 6.7 MA is selected as a point design case. This design point represents a single coil in a high energy particle shielding system. Sizing of the thermal and structural components of the enclosure is completed. The thermal and deployment performance is predicted.Item Development of a dual-tip millikelvin Josephson scanning tunneling microscope(2014) Roychowdhury, Anita; Lobb, Christopher J; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, I first describe the design and construction of a dual-tip millikelvin STM system. The STM is mounted on a dilution refrigerator and the setup includes vibration isolation, rf-filtered wiring, an ultra high vacuum (UHV) sample preparation chamber and sample transfer mechanism.
Next I describe a novel superconducting tip fabrication technique. My technique involves dry-etching sections of 250 μm diameter Nb wire with an SF6 plasma in a reactive ion etcher. I present data taken with these tips on various samples at temperatures ranging from 30 mK to 9 K. My results demonstrate that the tips are superconducting, achieve good spectroscopic energy resolution, are mechanically robust over long time periods, and are atomically sharp.
I also show data characterizing the performance of our system. This data is in the form of atomic resolution images, spectroscopy, noise spectra and simultaneous scans taken with both tips of the STM. I used these to examine the tip-sample stability, cross talk between the two tips, and to extract the effective noise temperature (∼185 mK) of the sample by fitting the spectroscopy data to a voltage noise model.
Finally, I present spectroscopy data taken with a Nb tip on a Nb(100) sample at 30 mK. The enhanced spectroscopic resolution at this temperature allowed me to resolve peaks in the fluctuation-dominated supercurrent at sub-gap voltages. My analysis indicates that these peaks are due to the incoherent tunneling of Cooper pairs at resonant frequencies of the STM's electromagnetic environment. By measuring the response of the STM junction to microwaves, I identified the charge carriers in this regime as Cooper pairs with charge 2e. The amplitude of the response current scales as the square of the Bessel functions, indicating that the pair tunneling originates from photon assisted tunneling in the incoherent regime, rather than the more conventionally observed Shapiro steps in the coherent regime.Item Cryogenic test of gravitational inverse square law below 100-micrometer length scales(2010) Yethadka Venkateswara, Krishna Raj; Paik, Ho Jung; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The inverse-square law is a hallmark of theories of gravity, impressively demonstrated from astronomical scales to sub-millimeter scales, yet we do not have a complete quantized theory of gravity applicable at the shortest distance scale. Problems within modern physics such as the hierarchy problem, the cosmological constant problem, and the strong CP problem in the Standard Model motivate a search for new physics. Theories such as large extra dimensions, ‘fat gravitons,’ and the axion, proposed to solve these problems, can result in a deviation from the gravitational inverse-square law below 100 μm and are thus testable in the laboratory. We have conducted a sub-millimeter test of the inverse-square law at 4.2 K. To minimize Newtonian errors, the experiment employed a near-null source, a disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, were positioned on the two sides of the source mass at a nominal distance of 280 μm. As the source was driven sinusoidally, the response of the test masses was sensed through a superconducting differential accelerometer. Any deviations from the inverse-square law would appear as a violation signal at the second harmonic of the source frequency, due to symmetry. We improved the design of the experiment significantly over an earlier version, by separating the source mass suspension from the detector housing and making the detector a true differential accelerometer. We identified the residual gas pressure as an error source, and developed ways to overcome the problem. During the experiment we further identified the two dominant sources of error - magnetic cross-talk and electrostatic coupling. Using cross-talk cancellation and residual balance, these were reduced to the level of the limiting random noise. No deviations from the inverse-square law were found within the experimental error (2σ) down to a length scale λ = 100 μm at the level of coupling constant |α|≤2. Extra dimensions were searched down to a length scale of 78 μm (|α|≤4). We have also proposed modifications to the current experimental design in the form of new tantalum source mass and installing additional accelerometers, to achieve an amplifier noise limited sensitivity.