Browsing by Author "Ross, Kate, committee member"
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Item Open Access A framework for simultaneous photon detector readout system simulations for the deep underground neutrino experiment(Colorado State University. Libraries, 2020) Christensen, Anne R., author; Buchanan, Norm, advisor; Ross, Kate, committee member; Bangerth, Wolfgang, committee memberThis thesis will discuss the changes to the coding framework for the Deep Underground Neutrino Experiment (DUNE). DUNE is simulated in a coding framework, called Liquid Argon Software (LArSoft). The framework simulates the particle event, the photons produced due to interactions and the electronics. The electronic simulation framework for DUNE has been changed to improve functionality and ease of use. The electronics simulation has been modularized so electronic readout models can be directly compared. The changes to the framework will be described and validated in this thesis.Item Open Access An analysis of noise in the NOvA near detector(Colorado State University. Libraries, 2016) Judah, Matthew, author; Buchanan, Norm, advisor; Ross, Kate, committee member; Pallickara, Sangmi, committee memberThe NOvA (NuMI Off-axis νe Appearance) long-baseline neutrino experiment utilizes neutrino oscillations to gain insight into the unanswered questions in neutrino physics and further our knowledge of particle physics. The answers can only be arrived at through precise and accurate measurements of neutrino properties. To obtain these high precision results using the NOvA experiment background signals and noise must be understood and characterized. The work described is a study of noise characteristics from the electronics and photosensors used in the near detector of the NOvA experiment. A number of methods for the identification and elimination of noise in the electronics are discussed.Item Open Access Compositional tuning, crystal growth, and magnetic properties of iron phosphate oxide(Colorado State University. Libraries, 2017) Tarne, Michael, author; Neilson, James, advisor; Shores, Matthew, committee member; Ross, Kate, committee memberIron phosphate oxide, Fe3PO4O3, is a crystalline solid featuring magnetic Fe3+ ions on a complex lattice composed of closely-spaced triangles. Previous work from our research group on this compound has proposed a helical magnetic structure below T = 163 K attributed to J1-J2 competing interactions between nearest-neighbor and next-nearest-neighbor iron atoms. This was based on neutron powder diffraction featuring unique broad, flat-topped magnetic reflections due to needle-like magnetic domains. In order to confirm the magnetic structure and origins of frustration, this thesis will expand upon the research focused on this compound. The first chapter focuses on single crystal growth of Fe3PO4O3. While neutron powder diffraction provides insight to the magnetic structure, powder and domain averaging obfuscate a conclusive structure for Fe3PO4O3 and single crystal neutron scattering is necessary. Due to the incongruency of melting, single crystal growth has proven challenging. A number of techniques including flux growth, slow cooling, and optical floating zone growth were attempted and success has been achieved via heterogenous chemical vapor transport from FePO4 using ZrCl4 as a transport agent. These crystals are of sufficient size for single crystal measurements on modern neutron diffractometers. Dilution of the magnetic sublattice in frustrated magnets can also provide insight into the nature of competing spin interactions. Dilution of the Fe3+ lattice in Fe3PO4O3 is accomplished by substituting non-magnetic Ga3+ to form the solid solution series Fe3-xGaxPO4O3 with x = 0, 0.012, 0.06, 0.25, 0.5, 1.0, 1.5. The magnetic susceptibility and neutron powder diffraction data of these compounds are presented. A dramatic decrease of the both the helical pitch length and the domain size is observed with increasing x; for x > 0.5, the compounds lack long range magnetic order. The phases that do exhibit magnetic order show a decrease in helical pitch with increasing x as determined from the magnitude of the magnetic propagation vector. This trend can be qualitatively reproduced by increasing the ratio of J2/J1 in the Heisenberg model. Intriguingly, the domain size extracted from peak broadening of the magnetic reflections is nearly equal to the pitch length for each value of x, which suggests that the two qualities are linked in this unusual antiferromagnet. The last chapter focuses on the oxyfluoride Fe3PO_7-xFx. Through fluorination using low-temperature chimie douce reactions with polytetrafluoroethylene, the magnetic properties show changes in the magnetic susceptibility, isothermal magnetization, and neutron powder diffraction. The magnetic susceptibility shows a peak near T = 13 K and a zero field cooled/field cooled splitting at T = 78 K. The broad, flat-topped magnetic reflections in the powder neutron diffraction exhibit a decrease in width and increase in intensity. The changes in the neutron powder diffraction suggest an increase in correlation length in the ab plane of the fluorinated compound. Iron phosphate oxide is a unique lattice showing a rich magnetic phase diagram in both the gallium-substituted and fluorinated species. While mean-field interactions are sufficient to describe interactions in the solid solution series Fe{3-xGaxPO4O3, the additional magnetic transitions in Fe3PO7-xFx suggest a more complicated set of interactions.Item Open Access Growth and characterization of ultra-low damping Co₂₅Fe₇₅ thin films(Colorado State University. Libraries, 2020) Swyt, Mitchell, author; Buchanan, Kristen, advisor; Ross, Kate, committee member; Menoni, Carmen, committee memberThis thesis focuses on the growth and characterization of ultra-low damping Co25Fe75 thin films. Ultra-low damping in a metal is of interest for the design of new spintronic devices because this offers the opportunity to move both electrons and spin waves over appreciable distances. In this work, the effects of seed and capping layers on the damping parameter and magnetization are investigated. A series of thin films were deposited using DC magnetron sputtering. A combination of X-ray reflectometry (XRR), vibrating sample magnetometry (VSM), and ferromagnetic resonance spectroscopy (FMR) were used to determine the film quality, saturation magnetization, and damping parameters of each film. The results show that the Ta seed layers promoted smooth film growth for Co25Fe75, but direct interfaces with Ta or Pt resulting in enhanced damping. Cu spacer layers can be used to disrupt the enhancement but promote rough film growth for the studied sample growth conditions. Damping values in agreement with published results were achieved for two films from the set, with α=0.0064 ± 0.0004 for Ta/Co25Fe75 and α=0.0063 ± 0.0011 for Ta/Cu/Co25Fe75/Cu/Ta.Item Open Access Metal oxides as buffer layers in polycrystalline CdTe thin-film solar cells(Colorado State University. Libraries, 2021) Pandey, Ramesh, author; Sites, James, advisor; Sampath, W.S., committee member; Ross, Kate, committee member; Harton, John, committee memberThe optical band-gap of 1.5 eV and absorption coefficient the order of 105 cm-1 makes CdTea very attractive absorber for thin-film solar cells. This dissertation explores methods to improve both the front, or emitter, part of the cell and the back contact to the CdTe-based thin-film solar cells. The choice of an n-type emitter partner for CdTe based solar cells is crucial to the overall power conversion efficiency. In comparison to the traditional CdS emitter, metal oxides such as ZnO, MgO, and the ternary alloy MgxZn1-xO have large optical band-gaps making them transparent to most of the solar spectrum and an ideal emitter layer adjacent to light-facing side of the absorber in a superstrate configuration. The optical and electrical properties of MgxZn1-xO emitters can be modulated by varying the elemental ratio of x = Mg:(Mg + Zn) in the ternary alloy. Tracing the variation of the conversion efficiency as a function of Mg fraction in MgxZn1-xO emitter, an optimal Mg fraction of x = 0.15 was found to produce highest efficiency for the CdTe-based thin-film solar cells. Photoelectron spectroscopy demonstrated the conduction band offset at the emitter/absorber interface transitions from a cliff like -0.1 eV for x = 0.00 to a spike like 0.2 eV at the optimal x = 0.15. Photoluminescence and low-temperature current-voltage measurements showed that the interface between MgZnO and the CdSeTe is well passivated for x = 0.15. Further increase in the Mg fraction however increases the band offset between the emitter/absorber leading to distortions of J-V curves under various illumination conditions. Light soaking experiments and numerical simulations show that an insufficient density of carriers in the MgZnO due to the compensating defects causes these distortions: a failure of superposition of light and dark curves referred to as cross over, and distortion from normal current voltage behavior under spectra filtered illumination. An extrinsic doping of the emitter is critical to rectify these distortions and Ga-doped MgZnO was employed to experimentally demonstrate a cure to these J-V distortions characteristic of an undoped MgZnO emitter. It paves pathway to increase the n-type carrier density in the MgZnO emitter. The group-V doping of CdTe has shown potential to improve open circuit voltage, with level of doping in absorber the order of 1016 cm-3 and lifetimes of hundreds of ns. Numerical device simulations demonstrate that doping the emitter layer is essential and a particular challenge if the doping in the absorber is high. The results find the carrier concentration in emitter should be higher than the doping in the absorber to attain high open-circuit voltage in the highly doped CdTe-absorbers possible with arsenic doping. Various back contact metals like Ag, Co, Pt and metalloids like Te, and Se with different work functions were used to make an ohmic contact with the CdTe back surface. The use of a buffer layer behind the bare CdTe surface is found to be critical to the device performance. A thin 30-nm layer of Te have become the preferred choice of back buffer layer. Metal oxides like TeOx has been introduced as back buffer between the CdTe absorber and Te back contact to study their effect in device performance. The study finds that a double CdCl2 passivation procedure before and after the deposition of oxides is critical to the performance of these solar cells. Devices with the TeOx and the Te layer as back buffer demonstrated a power conversion efficiency in excess of 17 % without the incorporation of dopant in the absorber. Such a result is significant, as extrinsic dopants in CdTe-based absorbers often introduce defects in the absorber leading to increased recombination and degradation of cell performance particularly if the absorber is doped with Cu. Spectral and time resolved photoluminescence measurements carried out with illumination from front glass side show such cells have improved minority carrier lifetimes. The rear TRPL illumination to probe a CdTe/TeOx surface measured lifetimes of few ns indicative of the TeOx as a back buffer layer to mitigate the effects of large defects on a free CdTe surface. These results demonstrate metal oxides as a promising candidates for back buffer layers, and passivating back contact for hole selectivity in the CdTe-based solar cells.Item Open Access On automorphism groups of p-groups(Colorado State University. Libraries, 2017) Maglione, Joshua, author; Wilson, James B., advisor; Hulpke, Alexander, committee member; Penttila, Tim, committee member; Ross, Kate, committee memberWe provide the necessary framework to use filters in computational settings, in particular for finitely generated nilpotent groups. The main motivation for this is to construct automorphisms of the group from derivations and Lie automorphisms of an associated Lie algebra. The key application of our work is a naively parallelizable algorithm to compute Aut(G), for finite p-groups G of exponent p. This algorithm comes as a consequence of several structure theorems on filters; one, which allows for parallelism, is a theorem about general decompositions of groups (e.g. central decompositions) and their automorphisms.