Theses and Dissertations

Permanent URI for this collectionhttps://hdl.handle.net/10217/100500

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Open Access
A complex of interactions in rare-earth based quantum honeycomb magnets
(Colorado State University. Libraries, 2025) Treglia, Andrew, author; Gelfand, Martin, advisor; Neilson, James, advisor; Chen, Hua, committee member; Prieto, Amy, committee member
Quantum spin liquids (QSLs) represent an exotic state of matter characterized by long-range quantum entanglement and fractionalized excitations. Although predicted theoretically by models such as the Kitaev honeycomb model, experimentally verifying these states in real materials remains a significant challenge in condensed matter physics. This dissertation investigates the magnetic interactions in rare-earth honeycomb materials as potential candidates for realizing QSLs, with a focus on the exchange mechanisms governing their magnetic properties. Through a combination of magnetization, heat capacity, and inelastic neutron scattering experiments, this work systematically explores the magnetic phase behavior of Yb2Si2O7 and ErCl3, two rare-earth compounds with effective spin-1/2 degrees of freedom on geometrically frustrated lattices. The first part of this dissertation examines Yb2Si2O7, which exhibits field-induced quantum phase transitions. Magnetization measurements reveal strong anisotropy consistent with spin-orbit coupled physics, while neutron diffraction and inelastic spectroscopy provide insight into the formation of a Bose-Einstein condensate (BEC) of triplons. However, the absence of staggered magnetization in field-dependent neutron scattering data challenges prior theoretical models and suggests a more complex underlying exchange mechanism. The second part of this work focuses on ErCl3, a material whose honeycomb lattice structure and strong single-ion anisotropy make it a promising candidate for realizing bond-dependent exchange interactions. Single-crystal neutron scattering experiments, performed in a controlled air-free environment, reveal deviations from conventional isotropic exchange, indicating the presence of anisotropic interactions. A systematic analysis of spin wave excitations enables the extraction of exchange parameters, offering experimental constraints for theoretical models describing Kitaev-like interactions in this system. By integrating experimental results with theoretical models, this dissertation advances the understanding of rare-earth-based quantum magnets. The findings underscore the importance of strong spin-orbit coupling and crystal field effects in stabilizing anisotropic exchange, while highlighting key challenges in the experimental verification of QSL behavior. These results provide critical insights for the ongoing search for materials that host quantum spin liquid states and topological excitations, paving the way for future investigations into the realization of quantum matter in real materials.
Open Access
Emergent topological phenomena in low-dimensional systems induced by gauge potentials
(Colorado State University. Libraries, 2025) Winblad, Aidan, author; Chen, Hua, advisor; Eykholt, Richard, committee member; Gelfand, Martin, committee member; Pinaud, Olivier, committee member
In this dissertation we discuss how gauge potentials can be used as a key ingredient for inducing topological phase transitions in condensed matter systems, such as conductors, insulators, and superconductors. The first chapter covers some important background physics: Maxwell's equations, gauge invariance, minimal coupling, and Peierls phase, etc. It then presents a review of how one can realize Majorana fermions (MFs) in superconductors and their importance to topological quantum computing. In the end of chapter 1, we give an overview of the basics of Landau levels (LLs) and their relation to the Chern number. Chapter 2 presents a theoretical proposal for inducing topological phase transitions that allow for MFs to be hosted and rotated along the corners of a hollow equilateral triangle, which can serve as a basic building block for topological quantum logic gates. This provides a potential new avenue for achieving a topological quantum computation where a network of interconnected triangular islands allows for braiding of MFs. In chapter 3 we show using Floquet theory and high-frequency expansion, that oblique incident, circularly polarized light can give rise to spectral features analogous to Landau levels in the quantum Hall effect (QHE), where the effective magnetic field is related to the electric field of the laser light. Outside of having the electric field as a useful parameter for achieving a QHE device, this finding enables us to explore non-equilibrium systems exhibiting topological phenomena in the absence of spatial periodicity. Chapter 4 concludes and discusses further implications of the work in this dissertation.
Open Access
Back-contact layers on CdTe solar cells and loss analysis of top-performing devices
(Colorado State University. Libraries, 2025) Kasik, Camden, author; Sites, James, advisor; Buchanan, Kristen, committee member; Harton, John, committee member; Sampath, W. S., committee member; Topič, Marko, committee member
Photovoltaic technologies provide a nearly infinite energy source and have become cost competitive in recent years, driving them to be the most installed energy source. Cadmium telluride (CdTe) thin-film photovoltaics has the advantage of a well matched energy band-gap to the solar spectrum, as well as rapid production capabilities, making it the second most adopted solar-energy technology. Relatively low realized power conversion efficiencies, with the record being 23.1% (compared to the limit of 33%), has held CdTe back from expanding its market share. For comparison, silicon photovoltaics has reached an efficiency of 26.6% and has ~95% of the solar energy market share. The limited efficiency for CdTe is attributed in large part to the voltage deficit, which is a problem CdTe researcher have struggled with for years. Analysis to better understand the loss contributions in CdTe, and other technologies, as well as the addition of back-buffer layers will be presented in this work. Understanding the losses limiting solar-cell performance is essential to determining pathways to increased efficiencies. Comparisons to the theoretical limits of different parameters, which vary depending on the band-gap of the absorbing material, show which parameters cause different devices to have the most loss in efficiency, and thus the most room for improvement. These comparisons are done on record-efficiency devices, both single and multi-junction, from different technologies to determine which technologies have the most room for efficiency growth, and what parameter is the limiting factor. Detailed diode analysis was done to quantify the parasitic losses that negatively impact device performance. Determining these parameters produced valuable information from current-voltage data that is already available, allowing for more comparisons to be made when investigating the losses for different devices. Fill-factor losses were further analyzed using the diode parameters. The losses in fill-factor were quantified by splitting the total loss into its different diode parameter contributions. This detailed diode and fill-factor analysis gave a better understanding of what parameters caused the larger losses in photovoltaic devices. The results of this analysis on the best single-junction cells from different technologies, different CdTe laboratories, and for CdTe record devices over time are discussed. In addition to detailed loss analysis, back-buffer layers for CdTe devices were also investigated. A deposited layer of tellurium oxide was added to the native oxide present on the CdTe in the device structure as a dopant free passivating layer. Including a second CdCl2 passivation treatment was found to be crucial for good performance from the tellurium-oxide devices. Copper-doping treatments negatively impacted the device performance when additional tellurium oxide was included, with temperature-dependent current-voltage measurements suggesting the formation of an energy barrier which limited performance. Photoluminescence measurements on tellurium-oxide devices showed the impact of high-temperature treatments on absorbers with dopants already present, and the formation of defect states after an energetic sputtering deposition. Cadmium zinc telluride was also investigated as an electron-reflection layer to reduce recombination at the back surface. Optimization of the CdZnTe layer, including doping and substrate temperature, was investigated to create the best possible devices. Photoluminescence measurements, and the fact that devices including the CdZnTe layer outperformed control devices, indicated reduced recombination at the back surface. Film characterization measurements showed small crystal formation in the CdZnTe layer which highlights the importance of doping in this layer to improve conductivity for hole extraction. Experiments on First Solar absorbers again showed the impact that high-temperature processes can have on previously doped absorbers. The limitations of these back-contact layers are discussed, along with suggestions for future research to overcome them.
Open Access
Measurements of the motions of atoms: flow velocities and diffusion constants
(Colorado State University. Libraries, 1981) Prodan, John V., author; Fairbank, William M., Jr., advisor; She, Chiao-Yao, advisor
Using Fluorescence Correlation Spectroscopy (FCS), the motion of atoms have been measured in real time. Single sodium atoms have been detected with this method and their flow velocity crudely measured. By averaging over many atoms, very clear signals were obtained and velocities up to 85 m/sec were measured. These velocities were determined by measuring the transit time of atoms across two resonantly tuned laser beams. Diffusion constants, D, of sodium in helium, neon and argon buffer gases were also measured using FCS. The products, Dp, where p is the pressure of the buffer gas, for these three cases were determined to be 427 ± 7 Torr cm2/sec, 277 ± 10 Torr cm2/sec and 200 ± 9 Torr cm2/sec, respectively. These numbers were obtained from measuring the average duration of the fluorescence burst from individual sodium atoms diffusing through the laser beam. Also measured was the diffusion constant for the excited sodium atom (2P3/2) in a helium buffer gas at 200 Torr pressure, with a result of 598 ± 84 Torr cm2/sec.
Open Access
Detection and transit time measurements of individual sodium atoms diffusing in a helium flow by the laser resonance fluorescence correlation technique
(Colorado State University. Libraries, 1979) Pan, Ci-Ling, author; She, C. Y., advisor
This thesis describes the laser resonance fluorescence correlation technique for single-atom velocity measurement. Using this technique, we have detected individual sodium atoms diffusing through a laser beam in a slow helium flow. From the width of the fluorescence bursts detected, the transit time for the diffusing atom is determined. This is the first measurement of the motion of a single atom in a buffer gas. A probability analysis was developed which allowed us to estimate the average burst size of emitted fluorescence photons by an atom traversing the laser beam. All these results were in general agreement with the theoretical predictions. With improvements, we will be able to measure the velocity of a single-atom either in a flow or in a vacuum. By averaging over many sodium atoms, the diffusion coefficients of sodium atoms in helium and argon buffer gases were investigated using this technique. The measured diffusion coefficients were found to be in reasonable agreement with theoretical predictions and previous experimental results. To our knowledge, this is the first application of resonance fluorescence correlation technique to the measurement of diffusion coefficient of fast moving atoms in gases.
Open Access
Two-dimensional oxygen clusters and films adsorbed on graphite
(Colorado State University. Libraries, 1981-01) Pan, Ru-Pin Chao, author; Etters, Richard D., advisor; Sites, James R., committee member; Johnson, Gearold R., committee member; Gillis, Nelson S., committee member
The orientations, structures and phase transitions of two-dimensional oxygen molecular clusters adsorbed on the graphite plane were studied using the Monte-Carlo method. Orientational phase transitions have been found for all cluster sizes studied. Melting temperatures show agreement with measured values for extremely low coverage films. Two different approximations were used in the cluster studies. In one approximation, the interaction between a cluster and the surface was ignored and the center of mass of each oxygen molecule was fixed on a plane. In the other case an atom-atom Lennard-Jones potential was used to determine the surface interaction. An analytic expression was used for the summation over all the carbon atoms in the graphite. The minimal energies and structures of infinite adsorbed films were studied using an energy minimization method. Two stable phases have been found. The molecules in the low density phase are all oriented parallel to the substrate, while the molecules in the high density phase are perpendicular to the substrate. These two phases are identified as the experimentally measured δ and β phases, respectively. The lattice constants are found to be in agreement with the measured values within 3%.
Open Access
Investigating spin wave dynamics in YIG microstrips using micromagnetic simulations
(Colorado State University. Libraries, 2024) Pikul, Md Abu Jafar, author; Buchanan, Kristen, advisor; Berger, Josh, committee member; Menoni, Carmen, committee member
The goal of this thesis is spin wave propagation in magnetic microstrips with non-standard magnetic field directions using micromagnetic simulations. Spin waves, quasi-particles known as magnons, are waves that are generated when the electron spins oscillate collectively in a magnetic lattice. These waves can be used to transport energy. Spin waves have potential applications for microwave signal processing, logic operations, filtering, and biomedical applications. Spin waves are often launched using microstrip antennas in rectangular magnetic strips that serve as waveguides, and understanding how spin waves behave in these waveguides is critical to developing magnonics devices. The existing analytical theories that describe spin-wave behavior in microstrips are, however, limited to high-symmetry configurations (i.e., for the standard case, typically the static magnetic field direction is either parallel or perpendicular to spin waves' propagation direction) and fail to address spin-wave behavior in practical, lower-symmetry scenarios. In this thesis, field directions of 70° and 80° are examined and compared to the surface spin wave configurations. The angle is measured with respect to the long axis (i.e., x-axis) of the microstrip antenna, and the magnetic fields are applied in the plane of the microstrip. The lower-symmetry scenarios show complex, titled diamond patterns because anisotropic dispersion relations in magnetic thin films govern spin-wave propagation. In this study, micromagnetic simulations were used to model spin wave dynamics in a series of rectangular Yttrium Iron Garnet (YIG) microstrips in low-symmetry configurations. YIG is an ideal material for spin wave investigations because it has exceptionally low damping. The spin wave behavior was analyzed in a series of simulations in which the static magnetic field direction, driving frequency, excitation type (stripline antenna or point source), and microstrip dimensions (i.e., ranges of widths 1.28 μm to 2252.8 μm) were systematically altered. For stripline antennas, diamond-shaped spin wave propagation patterns are observed when the field is applied in-plane at 90° with respect to the long axis of the magnetic thin film, and this diamond pattern can be understood by considering the interference of width-quantized modes. As the static magnetic field direction is reduced, the diamond pattern tilts, and the outline angles are similar to what is seen for point source excitations. The micromagnetic investigations explain the discrepancy and fill the gap between the experimental observations and analytical calculations. These findings enhance our understanding of spin wave dynamics and will be useful for the development of advanced magnonics devices and information processing technologies.
Open Access
Muon neutrino reconstruction with machine-learning techniques at the ICARUS detector
(Colorado State University. Libraries, 2024) Mueller, Justin J., author; Mooney, Michael, advisor; Harton, John, committee member; Brandl, Alexander, committee member; Brewer, Samuel, committee member; Terao, Kazuhiro, committee member
The ICARUS T600 LArTPC detector successfully ran for three years at the underground LNGS laboratories, providing a first sensitive search for LSND-like anomalous electron neutrino appearance in the CNGS beam. After a significant overhauling at CERN, the T600 detector has been placed in its experimental hall at Fermilab, fully commissioned, and the first events observed with full detector readout. Regular data-taking began in May 2021 with neutrinos from the Booster Neutrino Beam (BNB) and neutrinos six degrees off-axis from the Neutrinos at the Main Injector (NuMI). Modern developments in machine learning have allowed for the development of an end-to-end machine-learning-based event reconstruction for ICARUS data. This reconstruction folds in 3D voxel-level feature extraction using sparse convolutional neural networks and particle clustering using graph neural networks to produce outputs suitable for physics analyses. The analysis presented in this thesis demonstrates a high-purity and high-efficiency selection of muon neutrino interactions in the BNB suitable for the physics goals of the ICARUS experiment and the Short-Baseline Neutrino Program.
Open Access
ICARUS cosmic ray tagger efficiency
(Colorado State University. Libraries, 2024) Boone, Tyler N., author; Wilson, Robert, advisor; Fairbank, William, committee member; Mooney, Michael, committee member; Brandl, Alexander, committee member
The ICARUS Cosmic Ray Tagger (CRT) was constructed with the goal to tag cosmogenic muons passing through the ICARUS Time Projection Chamber (TPC). Construction and commissioning of the detector began in Fall 2019 with the Side CRT North wall and continued for several years through the installation of the Top CRT. In this thesis I will summarize my contributions to the CRT system and describe a measurement of the installed CRT detection efficiency using the TPC.
Open Access
Long range fiber noise cancellation
(Colorado State University. Libraries, 2024) Helburn, Noah, author; Sanner, Christian, advisor; Brewer, Sam, committee member; Wilson, Jesse, committee member
Optical atomic clocks are beyond timekeeping applications an increasingly important tool for testing fundamental physics and pushing the quantum science frontier. Being able to compare remote optical clocks by sharing coherent laser light between them opens exciting scientific perspectives. Optical fibers are almost ideal guides for sending light over long distances, but they induce phase noise on the light travelling through them. We demonstrate an actively phase-stabilized optical fiber link over a distance of 80km. A fractional frequency instability of 7 × 10−15 at 1 s and 6.3 × 10−18 after 1800 s was achieved.
Open Access
Measurement of cadmium telluride bilayer solar cells
(Colorado State University. Libraries, 2024) Chime, Chinecherem Agnes, author; Sites, James, advisor; Buchanan, Kristen, committee member; Sampath, Walajabad, committee member
Photovoltaic (PV) technology is a green technology that uses devices and semiconducting materials to generate power by converting the absorbed energy from solar to electrical energy. Understanding the performance and behavior of a fabricated device is essential for enhancing their efficiency for future commercialization. Cadmium-telluride (CdTe) technology is a PV technology that uses CdTe as the semiconductor layer for absorbing and converting sunlight into electricity. Incorporating a bilayer of cadmium selenium telluride (CdSexTe1-x) alloy and CdTe into solar cell devices have shown particularly good performance, enhanced passivation, and higher efficiency. In this research, cadmium telluride solar cells were fabricated with a focus on improving the performance of the absorber layers. Radio frequency (RF) magnetron sputtering and close-space sublimation were adopted in preparing the front and back contact layers respectively. The fabricated device comprises of Tec-10 glass/100-nm magnesium-doped zinc oxide (MZO)/0.5-μm CST40/2.5-µm CdTe/ cadmium-chloride passivation/ Cu-doping/ 40-nm Te/ carbon and nickel paint back contact. As part of the performance improvement measures, the bilayer surface was passivated with cadmium chloride (CdCl2) and doped afterwards with copper. The fabricated CdSexTe1-x/CdTe device was subjected to room temperature and low temperature current density-voltage (J-V), capacitance, phase angle, quantum efficiency (QE), reflectance, electroluminescence (EL), and photoluminescence (PL) measurements. The J-V characteristics gave 15% device efficiency and showed diode curves which rolled over at lower temperatures, but were more ideal at higher temperatures. Capacitance measurements gave a hole density of 4x1014 cm-3 and a phase angle of 88o. The cells recorded high quantum efficiency of about 85% which is indicative of reduced recombination rate. Few defects were observed from the EL images while the PL emission peaks were obtained at 875 nm corresponding to an approximate energy band gap value of 1.42 eV. The measurement results show good performance for use in commercial solar cells for energy sustainability. Future implications encompass module fabrication, flexible devices, and affordability for enhancing green energy production and minimizing environmental pollution. Prospects envisage fabricating CdTe devices with higher efficiencies which would continue to compete successfully with other solar cell technologies.
Open Access
Precision measurements on a single trapped beryllium ion
(Colorado State University. Libraries, 2024) Fairbank, David M., author; Brewer, Samuel M., advisor; Yost, Dylan, committee member; Sanner, Christian, committee member; Van Orden, Alan, committee member
Precision laser spectroscopy of transitions in simple atoms can be used as a stringent test of many-body quantum electrodynamics (QED) calculations, or to extract subtle information about internal nuclear structure. 9Be+ is a three electron ion which has been the focus of study in ion trap and high energy beam experiments dating back several decades. We present the first measurements of the D-lines in 9Be+ using a single trapped ion, which reduced the experimental uncertainty of both the D1 and D2 transitions by an order of magnitude. A framework for characterization of systematic shifts due to effects like photon recoil and quantum interference in ion trap-based measurements of strong transitions is presented. From the D2 lineshape data, a 2P excited state lifetime was extracted with reduced uncertainty and better agreement with theory, compared to previous work. The first experimental measurement of the unresolved 2P3/2 hyperfine splittings is reported, which helped to uncover a sign error in the theoretical prediction of the 2P3/2 electric quadrupole hyperfine constant. This measurement required development of techniques to selectively isolate and measure the unresolved components, utilizing the exceptional state preparation and control available for trapped ions. The 1.25 GHz 2S1/2 ground state hyperfine splitting was measured with a relative uncertainty of 1.6×10−11 using microwave Ramsey spectroscopy and is in good agreement with previous measurements made in Penning traps at NIST. The technique can be extended to the rare isotope 7Be+, for which the current hyperfine constant uncertainty is four orders of magnitude larger. This planned measurement could enable extraction of an improved value of the 7Be nuclear Zemach radius. D-line measurements on the rare isotopes 7,10Be+ are also planned using the techniques developed for 9Be+. A comparison of the fine structure splitting across the isotope chain can be used to extract the relative nuclear charge radii or test the many-body QED contributions to theory in Li-like ions. A new ion trap was built and direct ablation loading of the ion trap from small 9BeCl2 salt deposits was demonstrated in preparation for loading the rare isotopes from evaporated aqueous solution.
Open Access
Brillouin light scattering: a powerful tool for magnonics research
(Colorado State University. Libraries, 2024) Swyt, Mitchell S., author; Buchanan, Kristen S., advisor; Patton, Carl, committee member; Menoni, Carmen, committee member; Field, Stuart, committee member
The slow down in generation-over-generation improvement in CMOS based logic and storage devices has spurred recent exploration into magnonic devices, those based on propagating perturbations of magnetic order called magnons, or spin waves. These devices are of particular interest due to their chargeless, low-power operation, scalability to the nanoscale, and compatibility with existing CMOS technologies. By exploiting spin waves, information may be transferred and operated upon without electrical currents. Magnetic textures like Neel domain walls, chiral transitions between magnetic domains, or skyrmions, magnetic vortices, represent additional avenues in magnonics for data storage and logic devices. Magnonic crystals, artificial crystals made by modulating magnetic properties in a periodic fashion, are one example of magnonic devices that have seen recent interest. With applicability in logic and signal processing, study of how spin waves propagate through these crystals is a necessity in the pursuit of new crystal designs. Brillouin light scattering (BLS) spectroscopy, an inelastic light scattering technique, is a powerful tool in this pursuit, as it allows for the spatial and temporal mapping of spin wave propagation. In this thesis, we will discuss three studies of spin waves by BLS: a 1D magnonic crystal, a 2D magnonic crystal, and a study of the interfacial Dzyaloshinskii-Moriya interaction. First, time-resolved BLS was used to study the band gap formation in a 1D magnonic crystal. By mapping the propagation of spin wave pulses through the crystal, complex two dimensional interference patterns were observed. These patterns are ignored by the simple models used to understand the behavior of this crystal design, and we provide a model to calculate these patterns from the spin wave dispersion relation. The temporal development of interference that forms the basis for band gap formation in this system is also observed. Second, time-resolved BLS was used to study spin wave caustic beams in a 2D magnonic crystal. This crystal design represents a new regime in magnonic crystals, in which the patterning dimensions are much smaller than the spin wave wavelength and generate caustic beams. The formation of a narrow (3 MHz) wide rejection band is observed and the possible mechanisms, including edge effects and interference between caustic beams, are explored. Third, the temperature dependence of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) is measured in a Pt/Co film for temperatures ranging from 15 K to room temperature. Previous studies have been reported for temperatures above room temperature and this study serves to test theory over a greater range of temperatures. The iDMI parameter was quantitatively measured by measuring the frequency difference for counter-propagating surface spin waves by BLS. These three studies demonstrate that BLS is a versatile and powerful tool in the field of magnonics.
Open Access
A study of the feasibility of detecting primordial microscopic black hole remnants with the NOvA far detector
(Colorado State University. Libraries, 2024) Wrobel, Megan, author; Buchanan, Norm, advisor; Berger, Josh, committee member; Adams, Henry, committee member
Several papers have argued that microscopic black holes may be stable against complete evaporation and may be a viable dark matter candidate [1–3]. This paper assesses the practicality of detecting these objects using long-baseline neutrino facilities, such as the NuMI Off-Axis νe Appearance (NOvA) experiment and the Deep Underground Neutrino Experiment (DUNE). The origin, stability, properties, and energy loss mechanism of such objects are examined. The signals produced from the detectors should allow for discrimination between these microscopic black holes and other particles traversing the detector. Potential challenges that could arise and next steps are also identified and considered.
Open Access
Oblique pumping, resonance saturation, and spin wave instability processes in thin Permalloy films
(Colorado State University. Libraries, 2008) Olson, Heidi M., author; Patton, Carl E., advisor
The study of nonlinear dynamics in metal films is of increasing importance as advancements are made in magnetic recording. In this dissertation, these interactions are examined by the study of first order spin wave instability (SWI) processes that occur for external static magnetic fields well below ferromagnetic resonance (FMR), and second order SWI processes that occur for static fields over the full FMR field range. This work is concerned specifically with the study of the high power resonance saturation and oblique pumping responses in thin Permalloy films, the microwave threshold amplitudes at which the instabilities occur, and the theoretical analysis of the relevant SWI processes. To greatly increase measurement accuracy and reduce measurement time, the high power FMR system has been modified and new calibration techniques implemented. The modifications to the system allow for fully automated and calibrated microwave threshold amplitude vs. static field measurements, termed butterfly curves. Resonance saturation butterfly curves have been measured for an in-plane field configuration for 35 - 123 nm thin Permalloy films. The butterfly curves show a jump on the low field side associated with a low field shift of the FMR profile and a foldover like asymmetry development. Apart from the jump, the second order Suhl SWI theory, suitably modified for thin films, provides good fits to the butterfly curve data through the use of constant spin wave relaxation rates that are on the same order as expected for intrinsic magnon-electron scattering processes. The FMR in-plane precession cone angles at threshold are small. Oblique pumping butterfly curves have been measured at different in-plane field configurations for 104 and 123 nm thin Permalloy films. The butterfly curves show thickness dependent high field cutoffs that agree with the field points at which the bottom of the spin wave band moves above one half the pump frequency. A combination of parallel and perpendicular first order SWI theory, suitably modified for thin films, shows good fits to the data except at low fields where the thin film approximation is not applicable. The damping trial functions used for the fits correspond to magnon-electron and three-magnon scattering processes.
Open Access
The cosmic ray energy spectrum from 1-10 EXA electron volts measured by the Pierre Auger Observatory
(Colorado State University. Libraries, 2009) Knapik, Robert, author; Harton, John L., advisor
The observed decrease in flux of cosmic rays as the energy increases can be described by power law with an almost constant spectral index for 12 decades of energy. Observing spectral index changes are used to constrain models for the sources of cosmic rays. The Pierre Auger Observatory was built to study the highest energy cosmic rays and combines two complementary techniques, a fluorescence detector and a surface detector. The surface detector is 100% efficient for energies above 3 EeV allowing for a flux measurement with low systematic uncertainties. This thesis describes the techniques developed to measure the flux of cosmic rays below 3 EeV while maintaining low uncertainties. The resulting energy spectrum confirms the previously measured change in spectral index observed by other experiments. Systematic differences in the measured energy spectra between experiments exist. Possible reasons for these differences and the astrophysical implications are discussed.
Open Access
Excited electronic state decomposition mechanisms and dynamics of nitramine energetic materials and model systems
(Colorado State University. Libraries, 2007) Greenfield, Margo, author; Guo, Yuanqing, advisor; Bernstein, Elliot R., advisor
Energetic materials play an important role in aeronautics, the weapon industry, and the propellant industry due to their broad applications as explosives and fuels. RDX (1,3,5-trinitrohexahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and CL- 20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane)-j are compounds which contain high energy density (J/cm3) or (J/g). Although RDX and HMX have been studied extensively over the past several decades, a complete understanding of their decomposition mechanisms and dynamics is unknown. This work describes the novel approach taken to assist in the overall understanding of the decomposition of these energetic materials, namely their gas phase single molecule excited state decomposition. Excited electronic states can be generated by shock and compression and therefore play an important role in the initiation/decomposition of RDX, HMX, and CL-20. Energy (ns lasers) and time resolved (fs lasers) UV-photodissociation experiments have been performed to elucidate the mechanisms and dynamics of gas phase energetic material decomposition from excited electronic states. Time of flight mass spectroscopy (TOFMS), laser induced fluorescence (LIF), and pump-probe experiments performed on three energetic materials, as well as five model systems, illustrate the unique behavior of energetic materials. TOFMS UV photodissociation (ns) experiments of gas phase RDX, HMX, and CL-20 generate the NO molecule as the initial decomposition product. Four different vibronic transitions of the initial decomposition product, the NO molecule, are observed: A2Σ(υ'=0)<—X2Π(υ"=0,l,2,3). Simulations of the rovibronic intensities for the A<— Xtransitions demonstrate that NO dissociated from RDX, HMX, and CL-20 is rotationally cold (~ 20 K) and vibrationally hot (~ 1800 K). Conversely, experiments on the five model systems (nitromethane, dimethylnitramine, nitropyrrolidine, nitropiperidine and dinitropiperazine) produce rotationally hot and vibrationally cold NO spectra. LIF experiments are performed to rule out the possible decomposition product OH, generated along with NO, perhaps from the suggested HONO elimination mechanism. The OH radical is not observed in the fluorescence experiments, indicatingthe HONO decomposition intermediate is not an important pathway for the excited electronic state decomposition of cyclic nitramines. The NO molecule is also employed to measure the dynamics of the excited statedecomposition. A 226 nm, 180 fs light pulse is utilized to photodissociate the gas phase systems. Stable ion states of DMNA and nitropyrrolidine are observed while the energetic materials and remaining model systems present the NO molecule as the only observed product. Pump-probe transients of the resonant A<—X (0-0) transition of the NO molecule show a constant signal indicating these materials decompose faster than the time duration of the 226 nm laser light. Comparison of NO from the three energetic materials to NO from NO2 gas generated by a 180 fs light pulse at 226 nm indicates that NO2 is not an intermediate product of the excited electronic state photodissociation of RDX, HMX, or CL-20. Two possible excited state decomposition mechanisms are suggested for the three energetic materials. The first mechanism involves a dissociative excited electronic statein which the nitramine moieties (CNNO2) in the electronically excited energetic material isomerize (CNONO) and further dissociate. In the second possible decomposition mechanism the electronically excited molecules undergo internal conversion to very highly excited (~5 eV of vibrational energy) vibrational states of their ground electronic state. Once in the ground state, isomerization of the nitramine moieties occurs and thematerial further decomposes. Calculational results together with the experimental results indicate the energetic materials decompose according to the second mechanism, relaxation to the ground state, while the model systems follow the excited electronic state decomposition pathway. An additional path in which the -NO2 moiety loses an O atom, becomes linear with the CN attachment, and then NO is released, is also consistent with experimental observations but is, as yet, not supported by calculations. The keys to generating better cyclic nitramine energetic materials would then beto enhance the propensity to form Si - So conical intersections, improve Si - So Franck-Condon factors for internal conversion near the Si zero point level, and to enhance the So density of vibronic states at high So vibrational energy. Additionally, one would like to generate NO with less internal vibrational excitation, so altering the NONO vibrational excitation in the dissociation process could be important. These ideas would suggest that more flexible cyclic nitramines, with increased internal degrees of freedom, might be useful to explore for new energetic systems. Perhaps larger ring structures along the lines of CL-20 might be useful compounds to explore.
Open Access
Development of a very compact high repetition rate soft x-ray laser
(Colorado State University. Libraries, 2010) Furch, Federico Juan Antonio, author; Rocca, Jorge J., advisor; Marconi, Mario, advisor
Over the last 25 years, the field of soft x-ray lasers has evolved from facility size devices delivering a few shots per day, to table-top lasers operating at several shots per second. In these lasers the gain medium is a highly ionized, hot and dense plasma created by a sequence of short, high energy pulses from an optical laser. Current table-top soft x-ray lasers have enabled numerous applications such as nano-scale imaging, nano-fabrication and dense plasma diagnosis among others. However these lasers are still limited in repetition rate, and therefore average power, owing to thermal effects originated in the flash lamp pumped amplifiers of the optical driver laser. Direct diode-pumping of the driver laser opens the possibility of developing more compact, higher repetition rate optical laser systems to pump soft x-ray lasers. Directly pumping small quantum defect materials such as Yb:YAG with a narrow bandwidth source of the optimum wavelength allows to significantly increase the efficiency and then reduce the thermal load in the gain materials. In addition, cryogenic cooling of the laser materials significantly improves their thermal performance. This approach will allow for soft x-ray laser operation at much higher repetition rates. In this work I present the results of the demonstration of an all diode-pumped soft x-ray laser that constitutes the first of a new generation of more compact, higher repetition rate soft x-ray lasers in the spectral region between 10 and 20 nm. To pump these lasers we developed an all diode-pumped chirped pulse amplification laser system based on cryogenically cooled Yb:YAG. This optical laser generates pulses of 1 J of energy in 8.5 ps pulses at 10 Hz, the highest energy per pulse for sub-10 ps pulses from a diode-pumped system at the present time. This soft x-ray laser has the potential to operate at unsurpassed repetition rates in a reduced footprint.
Open Access
Soft x-ray laser interferometry of dense plasmas
(Colorado State University. Libraries, 2007) Filevich, Jorge, author; Rocca, Jorge J. G., advisor
This Dissertation presents the results of the study of plasmas using soft x-ray laser interferometry. The use of soft x-ray wavelengths (14.7 nm and 46.9 nm) permits probing plasmas that are denser and that have steeper density gradients than those that can be probed using optical interferometry. The use of diffraction gratings as beam splitters permitted the construction of a novel interferometer design that is robust, stable and with high throughput. The measurements conducted include the first demonstration of soft x-ray laser interferometry with picosecond resolution. The first set of results presented herein are the observation of an unexpected on-axis density depression in narrow-focus laser-created plasmas. It is caused by plasma-radiation-induced ablation of target material outside of the region irradiated by the plasma-heating laser. This colder material expands at a slower velocity than the hotter central region, resulting in the observed on-axis density depression. The effect is shown to be a general phenomenon, present in many narrow focus plasmas under different irradiation conditions. The second set of results unveiled the significant contribution of bound electrons to the index of refraction of multiply ionized plasmas. Experiments that mapped the density of aluminum plasmas using a λ=14.7 nm laser beam showed interference fringes that bent in the direction opposite to that expected, contradicting the widely accepted assumption that the index of refraction for multiply ionized plasmas at soft x-ray wavelengths only depends on the free electrons. The contribution of bound electrons to the index of refraction is shown to be significant, and to affect a broad range of wavelengths due to numerous bound-bound and bound-free transitions present in the plasma. Moreover, the contribution of bound electrons to the index of refraction was shown to be important in several materials at different probe soft x-ray wavelengths, in particular for tin, silver and carbon plasmas probed at λ=46.9 nm. This fundamental result affects not only the interpretation of soft x-ray interferograms for plasma density measurements, but also the propagation of soft x-ray light in plasmas in general.
Open Access
Effects of contact-based non-uniformities in cadmium sulfide/cadmium telluride thin-film solar cells
(Colorado State University. Libraries, 2008) Davies, Alan R., author; Sites, James R., advisor
To strongly contribute to the near-term electricity supply, CdTe-based photovoltaic devices must continue to improve in performance under the constraint of simple and cost efficient fabrication methods. This dissertation focuses on characterization and modeling of devices with non-uniform performance induced by the cell contacts. Devices were obtained from a commercially viable pilot-scale fabrication system at Colorado State University. Current versus voltage (J-V), quantum efficiency (QE) and laser-beam-induced current (LBIC) were the main characterization techniques applied in this work. The p-type CdTe semiconductor has a large work-function and thus tends to form a Schottky barrier when the back-electrode is formed. A common strategy of mitigating the performance-limiting contact barrier is to prepare the CdTe surface with a chemical etch, and include Cu to reduce the effective barrier. Non-uniformity of the etch or Cu inclusion, or insufficient application of Cu can result in a non-uniform contact, with regions of high- and low-energy Schottky barriers participating in the cell performance. Barrier non-uniformities in devices with little or no Cu were identified with the LBIC measurement and a model for their influence was developed and tested using PSpice circuit modeling software. Because of their superstrate configuration, CdTe cells feature front contacts made from transparent-conducting oxides (TCOs). Fluorine-doped tin oxide (F:SnO2) is a common choice because of its availability and acceptable optical and electrical properties. When the n-CdS layer of the CdS/CdTe structure is thinned to encourage greater current generation, non-uniformities of the solar cell junction arise, as CdTe comes into sporadic contact with the TCO layer. Device simulations suggest that the SnO2/CdTe junction is weaker than CdS/CdTe because of a large conduction-band offset induced by the differing electron affinities in the heterojunction. LBIC was used to verify increasing non-uniformity in devices with thin CdS and whole-cell performance followed the trends predicted by simulations. An empirical relationship between CdS thickness and relative influence the weaker junction was developed. The practical limit of CdS thickness was determined to be about 120 nm for CSU devices.