Theses and Dissertations

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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.
Open Access
Mesospheric momentum flux studies over Fort Collins CO (41N, 105W)
(Colorado State University. Libraries, 2009) Acott, Phillip Edward, author; She, Chiao Yao, advisor; Krueger, David A., advisor
System upgrades to the Colorado State University Sodium Lidar have enabled over 300 hours of night-time gravity wave momentum flux measurements with concurrent 24-hour measurements of the mean and tidal wind and temperature fields of the mesosphere and lower thermosphere (MLT) region of the atmosphere above Fort Collins, CO. Results include the vertical profile of nighttime zonal momentum flux divided by density (MF/ρ), as well as nighttime wind and temperature variances; the results also provide some insight into the accompanying gravity wave-tidal interactions.
Embargo
Molecular dynamics simulation studies and experimental measurements of radiofrequency heating for strongly coupled and extremely magnetized ultracold neutral plasmas
(Colorado State University. Libraries, 2023) Jiang, Puchang, author; Roberts, Jacob L., advisor; Yost, Dylan, committee member; Lee, Siu Au, committee member; Yalin, Azer, committee member
Ultracold neutral plasmas(UNPs) are good experimental platforms for fundamental plasma physics studies because of their experimentally adjustable parameters, accessible timescales, ability to enter the strong coupling parameter regime, and easy access to large degrees of electron magnetization. The work in this thesis contains both simulation and experimental studies of UNPs. One simulation project describes a new UNP heating mechanism discovered using Molecular Dynamics simulations: DC electric field heating. This DC electric field heating mechanism occurs when a DC electric field is present when the plasma is formed. sets a lower limit of how cold UNP electron temperatures can be reached experimentally. A second simulation project investigates a many-body physics effect on collisional damping in UNPs and a breakdown in standard plasma theory treatments when the plasma is approaching the strongly coupled regime. This breakdown arises due to the increasing significance of three- or many-body electron-ion interactions influencing the plasma transport properties and particle collisions. My simulations find evidence for this being the case. Experimental studies of UNP electron-ion collision physics during the application of high-frequency RF electric fields to the UNP were conducted, and measurements of the RF-induced electron heating rate from the weak magnetized regime to extremely magnetized regime were performed. The results obtained are in qualitative agreement with the theory prediction but there's quantitative disagreement. Possibilities for resolving this disagreement are presented.
Open Access
A measurement of muon neutrino charged-current interactions with a charged pion in the final state using the NOνA near detector
(Colorado State University. Libraries, 2023) Rojas, Paul Nelson, author; Buchanan, Norm, advisor; Lee, Siu Au, committee member; Harton, John, committee member; Kokoszka, Piotr, committee member
The NOνA experiment is a long-baseline neutrino experiment hosted by Fermilab. The intense NuMI neutrino beam, combined with NOνA Near Detector, provides the opportunity to study neutrino interactions at an unprecedented level. The goal of this analysis is to measure the rate of muon-neutrino charged-current interactions in the NOνA near detector resulting in the production of one muon and at least one charged pion. This thesis will present the result of the double differential cross section measurement of this process in muon kinematics of energy and angle. Excesses in the extracted signal (greater than 25%), relative to the simulation, were found at large scattering angles. These excesses were greater than the estimated uncertainties (∼15%).
Open Access
Spin wave characterization in a 1D YIG magnonic crystal
(Colorado State University. Libraries, 2023) Compton, Lia, author; Buchanan, Kristen, advisor; Harton, John, committee member; Prieto, Amy, committee member
In this thesis, I will analyze and discuss features of spin wave propagation characteristics measured in a one-dimensional (1D) yttrium iron garnet (YIG) magnonic crystal using time-resolved Brillouin light scattering (TR-BLS) measurements. Magnonic crystals are a promising candidate to aid in developing spin-based devices that exploit the spin of the electron since magnonic crystals can be used to control the information transmitted by spin waves. In magnonic crystals, periodic modulation of the material properties is used to create a band structure and hence allow or suppress the propagation of spin waves with specific frequencies. To better understand spin wave propagation in a 1D YIG magnonic crystal, (TR-BLS) measurements were used to map out the temporal and spatial evolution of spin wave pulses at different frequencies. By analyzing the TR-BLS data with a cross-correlation method, the group velocities were determined at different frequencies and a better understanding of the changes in the pulse shape is gained. The TR-BLS data show that multiple width-quantized spin wave modes are present and highlights the importance of considering the two-dimensional nature of spin wave propagation, even in a one-dimensional system.
Embargo
A measurement of the double-differential electron antineutrino charged-current inclusive cross section in the NOvA near detector
(Colorado State University. Libraries, 2023) Doyle, Derek, author; Buchanan, Norm, advisor; Gelfand, Martin, committee member; Harton, John, committee member; Norman, Andrew, committee member; Pouchet, Louis-Noël, committee member
The neutrino is a fundamental particle of the universe that was first hypothesized in 1930 by Wolfgang Pauli to explain the observed energy distribution of outgoing electrons produced from beta-decay. Since then, it has been discovered that there are at least three types, or flavors, of neutrinos and that they oscillate between these flavors as they travel through space and time. This discovery proved that neutrinos have a non-zero mass and positioned neutrino oscillations to provide a window into understanding the matter/antimatter asymmetry in the universe. Principle to all neutrino measurements is an accurate and robust interaction model over a large range of energies, and measurements to support the model. Of particular importance to the NuMI Off-axis νe Appearance (NOvA) neutrino oscillation experiment is the energy range from 1 to 10 GeV, where Quasi-Elastic (QE), Meson-Exchange Current (MEC), and Deep Inelastic Scattering (DIS) interactions all contribute significantly. Using neutrinos from the Neutrinos at the Main Injector (NuMI) beam and the NOvA near detector, the first double-differential electron antineutrino charged-current inclusive cross section is measured and compared to various interaction models implemented within the genie Generator framework, version 3. Good agreement is observed between measurement and a genie model tuned to NOvA data.
Embargo
Optical lattice deceleration of a cryogenic metastable atomic hydrogen beam
(Colorado State University. Libraries, 2023) Cooper, Samuel F., author; Yost, Dylan C., advisor; Roberts, Jacob L., committee member; Gelfand, Martin, committee member; Van Orden, Alan, committee member
Hydrogen is the most abundant and simple naturally occurring element in existence, making it an ideal platform for study of fundamental atomic physics. Theoretical physics has the capacity of making extraordinarily precise predictions of atomic hydrogen's energy levels, owing to hydrogen's innate simplicity. To provide valuable new information to the theoretical models, such as definitions of fundamental constants, requires pushing experimental measurement of these energy levels to extreme precision, and obtaining experimental values that agree or disagree with theory provide a rigorous test of fundamental physics. Unfortunately, hydrogen has yet to benefit from the advent of laser cooling and trapping techniques pioneered in other species due to the prohibitive ultraviolet wavelengths required. As a consequence, modern best measurements to-date are limited by uncertainties due to thermal energies of atomic hydrogen samples. The next generation of ultra-high precision experiments will require new ways to obtain slow and or cold atomic hydrogen. This work contains progress made towards this goal, where advent of a novel high power UV radiation source on this experiment opened the viability for exploring new horizons. Specifically detailed in this dissertation are the efforts toward generating a cryogenic helium temperature beam of metastable (2S) atomic hydrogen with velocity characterization and a first ever demonstration of a novel, all-optical deceleration method which utilizes an electro-optically controlled far detuned optical lattice. In the proof-of-principle experiment a velocity selected portion of the atomic hydrogen beam was decelerated from 300 ms–1 to 280 ms–1 in a single 30 ns optical pulse.
Open Access
Investigation of laser cooling and trapping of atomic silicon: towards the development of a deterministic single ion source
(Colorado State University. Libraries, 2023) Ronald, Samuel R., author; Lee, Siu Au, advisor; Fairbank, William M., Jr., advisor; Rocca, Jorge J., committee member; Marconi, Mario C., committee member
The laser cooling and magneto-optical trapping of silicon atoms were investigated experimentally. These are the first steps towards the development of a deterministic single ion source suitable for single ion implantation of a Kane quantum computer. We identified the 3s23p2 3P2 → 3s3p3 3Do3 transition at 221.74nm as a cycling transition suitable for laser cooling. We also identified the 3s23p2 1D2 → 3s3p3 3Do3 at 256.26nm as a repump transition coupling a lower metastable state with the upper cooling state. Two deep ultraviolet (DUV) laser systems were implemented to provide the cooling and repump laser light. Both systems utilized two stage second harmonic generation to quadruple the frequency of a fundamental laser to produce the DUV light. The cooling laser system utilized frequency quadrupling of a tunable cw Ti:Sapphire ring laser to produce up to 90mW at 221.74nm. The repump laser system utilized frequency quadrupling of an external cavity diode laser to produce up to 35mW at 256.26nm. A silicon atomic beam source operating at 1400°C was developed that produced a beam of free silicon atoms for laser studies. The atomic beam characteristics were analyzed, and the velocity distribution was manipulated via laser cooling. Careful spectroscopic studies were performed on the cooling and repump transitions. Frequency references for the DUV lasers were investigated in Te2 and I2 with Doppler free saturated absorption spectroscopy, using the first doubling stage output of the cooling and repump laser, respectively. Specific hyperfine components of the molecular transitions in Te2 and I2, suitable for frequency references, were identified and measured. Locking of the cooling laser on the Te2 reference was demonstrated. A magneto-optic trap (MOT) was implemented in the silicon atomic beam. A CCD optical system to image the fluorescence from atoms in the MOT was developed and achieved single atom ii detection capability. MOT trapping of silicon atoms was attempted. The low flux of atoms in the MOT velocity capture range precluded any observation of trapped atoms. A Zeeman slower, based on a novel design utilizing a variable pitch helical solenoid, was designed, simulated, and constructed to improve the flux of slow atoms. No magneto-optic trap was observed due to insufficient laser power for simultaneous Zeeman slowing and magneto-optic trapping. Investigations were performed for one dimensional laser cooling, via a Zeeman slower, along the atomic beam motion direction. Atomic beam velocity distribution profiles were observed to be modified when the Zeeman slower was on. The parameter space of Zeeman slower currents, laser power and detuning, was explored. A simulation of the atom motion over the 1m long flight path under the influence of the Zeeman slower was carried out and found to agree with the observed results.
Open Access
Luminescence measurements inform a strategy for unlocking the full potential of CdTe-based photovoltaics
(Colorado State University. Libraries, 2023) Jundt, Pascal M., author; Sites, James R., advisor; Sampath, Walajabad S., committee member; Yost, Dylan C., committee member; Gelfand, Martin P., committee member; Kuciauskas, Darius, committee member
Cadmium telluride (CdTe) photovoltaics are characterized by simplicity and speed of fabrication with low usage of materials, all of which translate into low cost. These significant advantages have earned CdTe the second-highest adoption rate of all photovoltaic technologies. However, conversion efficiencies, while functional, are significantly lower than the theoretical limit for this material. This discrepancy is almost entirely a discrepancy in voltage, and the so-called "voltage deficit" of CdTe has stubbornly persisted for decades. While many strategies are being pursued to attempt to reduce the voltage deficit, this issue is fundamentally one of excessive nonradiative recombination due to defects within the absorber material, as will be demonstrated in this dissertation. Recombination is evaluated primarily by luminescence measurements, and as such this class of measurements is particularly relevant to the challenges faced by CdTe research today. The rate of recombination is parameterized by the carrier lifetime, and time-resolved photoluminescence (TRPL) is the most common method of determining this parameter in CdTe. Historically, accurate determination of bulk lifetime was as simple as extracting the time constant of the slowest component of a TRPL decay. However, significant gains in material passivation and doping over the last few years have both decreased the relative influence of trap-assisted recombination and increased the influence of p-n junction fields on TRPL measurements. Consequently, when measurements are performed on complete cells, extracting the tail time constant from a TRPL decay no longer necessarily gives an accurate representation of the bulk material lifetime, and the result is distorted by field effect contributions. This fact is not necessarily well-known by the CdTe community, and extraction of the tail time constant is still the most common way to report lifetimes, even in measurements on complete state-of-the-art cells. This dissertation demonstrates the skewing effects of junction fields, and identifies under which conditions they manifest and how. To probe field effects, external electrical bias was incorporated during TRPL measurements, which allows fairly precise manipulation of fields. Biased TRPL measurements were performed on a variety of samples, and a model was developed to substantiate and better explain the results. It was found that the same characteristics which enable good performance (high lifetime, doping, and mobility) are the same which add complexity to TRPL interpretation. It was also found that field effects can be effectively suppressed by significant forward bias, leading to far more accurate determination of bulk lifetime. TRPL and external radiative efficiency (ERE) luminescence measurement results have indicated very low rates of nonradiative recombination and associated very high lifetime for some CdTe-based materials deposited at Colorado State University, particularly the cadmium selenium telluride (CdSeTe) alloy. While these attributes should allow voltages approaching 1 V and efficiencies on the order of 25%, when incorporated into "traditional" cell architectures these materials typically achieve middling performance at best, and often no performance at all. To unlock the great potential indicated by luminescence measurements, a different cell architecture is proposed which aims to accommodate these materials and take advantage of their characteristics. In an n-i-p configuration, an intrinsic absorber material is sandwiched between two carrier-selective contacts, at least one of which must be transparent. This design eliminates the requirement that the absorber be doped, which penalizes lifetime. Based on the findings of modeling reported here, undoped CdSeTe appears to be an ideal intrinsic layer material. The currently-utilized SnO2:F/MgZnO front contact appears to be excellent as the n-type electron-selective layer. The one missing component is the p-type hole-selective layer; modeling in this dissertation describes in detail what attributes are required of this material. Most important is band alignment with CdSeTe, which should produce a valence band offset as close to zero as possible, and a conduction band offset which forms a sufficiently high electron barrier. Sufficient p-type doping is also quite important. Based on these criteria, ZnTe was identified as a suitable candidate material, and several cells were fabricated with this architecture. While preliminary cells achieved relatively poor performance compared with traditional designs, J-V curves were surprisingly well-behaved, and the almost immediate achievement of functioning cells using an entirely new approach is promising. Luminescence characterization of these structures identified several areas for improvement, namely the use of a p-type dopant other than copper and the replacement of ZnTe with another material with similar band structure but more compatible lattice constant.
Open Access
Development of a liquid argon purity monitoring system
(Colorado State University. Libraries, 2023) Fogarty, Samuel J., author; Harton, John, advisor; Mooney, Michael, committee member; Menoni, Carmen, committee member
Liquid argon time-projection chambers (LArTPCs) are used to detect charged particles and measure their properties. Charged particles that pass through the liquid argon (LAr) in a LArTPC ionize and excite argon atoms, producing ionization electrons and prompt scintillation light. The ionization electrons drift through the LAr volume in a uniform electric field and produce a signal at the anode. The scintillation light is used to determine the drift coordinate of an event, which allows for 3D reconstruction of tracks and interactions. Electro-negative impurities lead to the reduction of the ionization electrons and scintillation light. They worsen a detector's ability to perform event reconstruction by reducing the signal-to-noise ratios. A purity monitor is a device that is often used alongside LArTPCs to monitor the LAr purity. It extracts electrons from a photo-cathode via the photoelectric effect and drifts them through LAr to an anode using an electric field. When traversing the purity monitor, some of the electrons will be lost due to impurities along the way. As a result, the drift-electron lifetime, which is related to the LAr impurity concentration, can be determined by measuring the difference in charge between the cathode and anode. This method allows for continuous purity monitoring of the LAr used in a LArTPC. This thesis describes the development and testing of a purity monitoring system that is used in conjunction with a LArTPC at Colorado State University.
Open Access
Precision measurement and symmetry properties of metastable hydrogen
(Colorado State University. Libraries, 2022) Rasor, Cory M., author; Yost, Dylan, advisor; Roberts, Jacob, committee member; Mooney, Michael, committee member; Bartels, Randy, committee member
Hydrogen has been an indispensable system to study during the development of quantum mechanics due to the simplicity of its atomic structure. Hydrogen maintains its utility today as an important tool for determining fundamental values such as the Rydberg and fine structure constants, as well as the proton charge radius. The work described in this thesis aims to use hydrogen for determining the proton Zemach radius, to search for anomalous spin-dependent forces, and to provide means for measuring the degree of parity violation within this simple system. An overview of a 2S1/2 hyperfine interval measurement is described, followed by a description of the apparatus used and finally a discussion of the systematic effects to be characterized. A proposed parity violation experiment is also described.
Open Access
Search for an anomalous excess of charged-current electron neutrino interactions with the MicroBooNE detector
(Colorado State University. Libraries, 2022) Caro Terrazas, Ivan, author; Mooney, Michael R., advisor; Wilson, Robert J., committee member; Buchanan, Norm, committee member; Kirby, Michael, committee member
MicroBooNE is a liquid argon time projection chamber detector designed to address the excess of low-energy electromagnetic events observed by the MiniBooNE detector. Electron neutrinos can create a wide variety of topologies when interacting with liquid argon; this analysis measures events without pions, both with (1eNp0π) and without (1e0p0π) visible protons. This thesis presents a first measurement of pionless charged-current electron neutrino interactions from the Booster Neutrino Beam at Fermilab in the MicroBooNE detector. A model based on the MiniBooNE result is used to quantify the strength of the electron neutrino excess. The analysis suggests that if an excess is present, it is not consistent with a simple scaling of the electronneutrino contribution to the flux. Combined, the 1eNp0π and 1e0p0π channels do not give a conclusive indication of the tested model, but separately they both disfavor the low-energy excess model at > 90% CL. The observation in the most sensitive 1eNp0π channel is below the prediction and is consistent with no excess. In the less sensitive 1e0p0π channel the observation at low energy is above the prediction, while overall there is agreement over the full energy spectrum.
Open Access
Brillouin light scattering spectroscopy of phonons, magnons, and magnetoelastic waves
(Colorado State University. Libraries, 2022) Nygren, Katherine Elise, author; Buchanan, Kristen S., advisor; Field, Stuart, committee member; Brewer, Samuel, committee member; Shores, Matthew, committee member
This thesis discusses three projects that involve the propagation of waves through the utilization of an optical measurement technique known as Brillouin light scattering (BLS) spectroscopy. BLS spectroscopy measurements were completed using a six pass tandem Fabry-Pérot interferometer to detect light that has inelastically scattered from vibrational, spin, or magnetoelastic waves in a sample. This measurement method is noncontact, so wires do not need to be connected to the sample, nondamaging (unless the laser power is too high, and only for sensitive samples), and can detect nonlinear signals. The first project uses an antenna called an interdigital transducer to produce a surface acoustic wave. This wave travels across a piezoelectric substrate and couples to a spin wave in a nickel film. The coupled wave known as a magnetoelastic wave is then studied using BLS as a function of the external applied magnetic field. These results are used to help us understand how the magnetic resonance contributes to the coupled wave. Further BLS measurements as a function of distance across the nickel film are used to calculate a decay length of the magnetoelastic wave two orders of magnitude larger than the decay length for a pure spin wave in nickel. Second, we explore a device using a thin film of an organic ferrimagnet called vanadium tetracyanoethylene (VTCNE) that is magnetic at room temperature and has low damping, which rivals damping in high quality YIG films commonly used in microwave applications. Because VTCNE is oxygen sensitive it is encapsulated between two pieces of glass using an epoxy. The encapsulation does not change the damping, however due to magnetostriction, the strain of the epoxy may change the magnetic properties of the film. To understand how the epoxy strain can effect this device and others with similar encapsulation, we study thermal phonons in the encapsulation materials using Brillouin light scattering. The thermal phonon measurements along with phonon simulations allow us to calculate both the wave speeds and the elastic properties of the materials. These calculated properties can then be used to model future VTCNE devices. The final major project uses BLS spectroscopy to study spin waves in a Y-shaped structure of an iron nickel alloy. Using an in-plane externally applied magnetic field and an antenna across the top of the Y, we excite magnons in each arm of the Y, which then propagate into the base of the Y. BLS measurements are taken in each arm and the base of the Y, as a function of the driving frequency, and a 2D spatial map of the spin waves in the Y-structure was obtained to gain additional information on the modes that propagate past the junction of the Y. The BLS data in conjunction with simulations, demonstrate an indirect way to efficiently excite Damon-Eshbach spin waves as well as convert low wavevector spin waves in the arms of the Y into higher wavevector spin waves as they propagate into the base of the Y. The wavevector conversion and more efficient method of generating Damon-Eshbach spin waves are tools that can be exploited in magnonic device designs. Three additional spin wave projects are also discussed briefly. The projects include a yttrium iron garnet (YIG) confined structure, a VO2 film with a metal-insulator-transition near room temperature, and a heavy metal-ferrimagnet-heavy metal sample that should have a strong interfacial Dzyaloshinskii-Moriya interaction.
Open Access
Advancing the capability of high energy Yb:YAG lasers: multilayer coatings, pulse shaping and post compression
(Colorado State University. Libraries, 2022) Wang, Hanchen, author; Rocca, Jorge, advisor; Roberts, Jacob, committee member; Lee, Siu Au, committee member; Marconi, Mario, committee member
Recently, cryogenically cooled Yb-doped amplifiers have been successfully scaled both in output energy and in repetition rate. The performance of such laser systems and their applications can be expanded by advancements in the development of optical coatings, that allow for scaling to higher pulse energies; as well as improvements in pulse shaping that include shorter pulse durations and the generation of programmable sequences of ultrashort pulses. This dissertation focuses on realizing the improvements mentioned above for cryogenic Yb:YAG amplifiers. First it reports the evaluation of ion beam sputtering (IBS) dielectric coatings for Yb:YAG at the environmental conditions in which cryogenic amplifiers are operated. The IBS coatings showed consistent performance in ambient, vacuum and cryogenic conditions, with damage threshold measured 20.4±0.6 J/cm2 for anti-reflection (AR) coating, and 27.4±1.3 J/cm2 for high reflector (HR) coating with 280 ps pulse duration at 77 K under the ISO:21254 standard. Second, a method for synthesizing trains of high energy compressed pulses was demonstrated and used to pump an 18.9 nm Ni-like Mo plasma-based soft x-ray laser more efficiently. The synthesized pulse increased the conversion efficiency of this spatially coherent soft x-ray source by 40%. Finally, femtosecond pulses were generated by post compression using a gas filled hollow core fiber (HCF), in which spectral broadening was achieved by self-phase modulation with an additional contribution from stimulated Raman scattering. Utilizing nitrogen gas as the non-liner medium, 300 mJ, 8 ps pulses were broadened to 3.7 nm and re-compressed to 460 fs by a grating compressor. The propagation and spectral broadening of high energy picosecond pulses in gas-filled HCFs were modeled and the results of simulations were compared with experiments.
Open Access
Exotic phenomena in rare-earth based geometrically frustrated magnets
(Colorado State University. Libraries, 2022) Yahne, Danielle Rose, author; Ross, Kate A., advisor; Bradley, Mark, committee member; Buchanan, Kristen, committee member; Zadrozny, Joe, committee member
Rare-earth (RE) based frustrated magnets are ideal systems to explore quantum effects in materials, which are paramount for the development of quantum computers, MRAM, and other next-generation technology. RE based materials are of specific interest due to the strong spin-orbit coupling and crystal electric field effects, which split the degenerate 4f angular momentum states, often leading to an effective spin-1/2 doublet with anisotropic effective exchange models. For this reason, RE materials are paramount to investigating the effects of anisotropic exchange on exotic ground states or quantum phases. Exchange frustration refers to when a system cannot simultaneously satisfy competing interactions, which can lead to a macroscopic degeneracy in the ground state of the system. Materials with geometric frustration, where competing interactions occur due to the crystal geometry alone, have been shown to host a wealth of exotic phenomena, including spin ice phases, quasi-particle excitations, order-by-disorder, and the highly entangled quantum spin liquid (QSL) state, to name a few. In this thesis, we will discuss three RE systems that exhibit geometric frustration in addition to exchange frustration: two RE pyrochlore oxides (RE2TM2O7) and a 2D isosceles triangular lattice material K3Er(VO4)2. Spin-1/2 antiferromagnetic (AFM) 2D triangular lattice magnets are an archetype of geometric frustration. While these materials are theorized to host a variety of different ground states and exotic phases depending on the anisotropies of the system, only a handful of RE material examples have been explored. We report the first deep dive into one such system, K3Er(VO4)2. We have determined the ordered magnetic structure of K3Er(VO4)2, finding an unusual structure with alternating layers comprised of AFM aligned and zero moment. We theorize this unique structure is due to the strong XY single-ion anisotropy, suggested from magnetometry measurements, which acts to suppress (to the point of vanishing completely) the out-of-plane pseudo-spin-1/2 magnetic moments. Next, we explored the effects of phase competition in a well-studied effective spin-1/2 RE pyrochlore oxide, Er2Sn2O7. Previous polycrystalline work has found Er2Sn2O7 to possess a suppressed critical temperature and an AFM Palmer-Chalker ground state. The determined exchange and single-ion anisotropy of Er2Sn2O7 find the ground state lies in close proximity to a competing AFM phase. Through extensive single crystal heat capacity measurements, we discovered a reentrant field vs. temperature phase diagram, where a system that has developed order returns to the original, less ordered (paramagnetic) state as some external parameter (field) is tuned continuously. We investigated the underlying mechanisms behind the reentrance by utilizing Monte Carlo simulations, mean field theory, and classical linear spin-wave calculations. This theory suggests that reentrance is linked to soft modes arising from phase competition, either from enhanced competition of the proximal AFM phase or from competing T=0 field-evolved ground states, depending on the specific applied field direction. In both cases, the soft modes enhance thermal fluctuations which cause the specific ordered phase to be entropically stabilized, thus forming a reentrant phase diagram. Finally, we report recent elastic neutron diffraction results on a RE pyrochlore oxide and candidate octupolar spin-ice, Ce2Sn2O7. The pseudo-spin-1/2 moments in Ce2Sn2O7 are known to possess dipolar-octupolar character and a large parameter space within the phase diagram is theorized to host novel QSL states. Previous powder neutron diffraction found diffuse scattering at high scattering vectors associated with magnetic octupoles. However, our undertaking of a similar measurement on nominally the same sample, found strikingly different results. Our neutron diffraction resulted in a broad, diffuse signal at low scattering vectors, reminiscent of a dipolar spin-ice. Neutron diffraction and atomic PDF measurements have not turned up obvious sample deformities or evidence of oxidation that could explain the differences in the diffuse signals. Further atomic studies and significant theory work is necessary to fully understand the results of this measurements, but the similarities to sister compound Ce2Zr2O7 suggest that Ce2Sn2O7 could lie on a phase boundary that is sensitive to minor distortions.