Theses and Dissertations - Department of Physics & Astronomy

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    Transport in a Strongly Coupled Charged Relativistic Quantum Fluid in the Presence of a Chiral Anomaly & a Strong Magnetic Field Via Hydrodynamics & Holography
    (University of Alabama Libraries, 2020) Koirala, Roshan Nmn; Kaminski, Matthias; University of Alabama Tuscaloosa
    Motivated by applications to the quark gluon plasma state generated in heavy ion collisions, I study a 4-dimensional supersymmetric conformal field theory in the thermal state, carrying axial charge, and subject to a strong external magnetic field, $B$. This theory has a chiral anomaly, which conspires with the magnetic field and axial charges to give rise to a host of novel transport phenomena. According to the gauge/gravity correspondence, also referred to as holography the described field theory state is dual to a charged magnetic black brane which is a solution of Einstein-Maxwell-Chern-Simons theory. I compute the quasinormal modes which are holographically dual to the poles of the two-point functions of the energy-momentum tensor and axial current operators, encoding information about the dissipation and transport of charges in the plasma. I derive general results in the hydrodynamic field theory and confirm them by explicit solutions in the dual gravity theory. Chiral transport is analyzed beyond the hydrodynamic approximation for the five hydrodynamic modes, one of which turns from a purely dissipative diffusion mode into a propagating chiral magnetic wave mode. I derive Kubo formulas for the thermodynamic and hydrodynamic transport coefficients in this plasma state. I analyze the discrete symmetries and Onsager relations of the corresponding correlators. I provide the transport coefficients in the holographic theory including both DC values and their frequency, $\omega$, dependence. The chiral magnetic conductivity and magneto-vortical susceptibilities are independent of $B$ for all $\omega$ while the chiral vortical conductivity and chiral heat conductivity have dependence when $B \neq \omega$. The parallel and perpendicular shear viscosity are found to be cubic in $\omega$, while conductivity is linear in $\omega$ in the conformal limit $\omega \gg T$. AC values of Hall conductivity are found to dominate by $n_0/B$ term when $\omega \gg T$, while that of Hall viscosity quickly decays to zero near $\omega \sim T$.
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    Investigation of Magnetic Anisotropies and Magnetization Dynamics in Soft Magnetic Materials
    (University of Alabama Libraries, 2020) Srivastava, Abhishek; Mewes, Tim; University of Alabama Tuscaloosa
    Magnetic anisotropy and damping are two main properties that determine the characteristics of many magnetic devices such as inductors, transformers, hard drives, GMR sensors, MRAM etc. The magnetic materials used in these devices can be in different forms. For example spintronic devices are made of thin films (single crystalline or polycrystalline), and thus in these systems the effect of the substrate such as lattice mismatch, strain etc plays an important role in determining and manipulating magnetic properties. In case of transformers and inductors magnetic materials are made of bulk ferrite or as thin (~ 20 micrometer) ribbons of nanocomposite alloys. This dissertation gives a basic introduction of the magnetization dynamics and the physics and instrumentation of FMR. The induced anisotropy and magnetization dynamics of Co74.6Fe2.7Mn2.7Nb4Si2B14 (at %) melt-spun, soft magnetic alloy ribbons after various secondary processing treatments was studied by broadband ferromagnetic resonance (FMR) technique. A new method of determining the relative permeability of these ribbons is discussed and compared to the established vibrating sample magnetometry (VSM) and the toroid method. This new method of determining the permeability does not require information about the volume or mass of the sample nor does it require any special sample preparation procedure. Another study presented in this thesis investigates the temperature dependence of the magnetic anisotropy of a single crystal magnetite (Fe3O4) thin film on MgGa2O4 substrate. The aim of this study is to characterize the magnetization dynamics and magnetic anisotropy of this magnetite thin film through the Verwey transition. The FMR study of this film suggest a continuous structural transition from cubic to monoclinic phase as the temperature is decreased. Finally the magnetic properties of polycrystalline semiconducting spinel CdCr2S4 films grown by low-pressure metal organic chemical vapor deposition are studied. This includes the investigation of the paramagnetic to ferromagnetic phase transition using broadband FMR. The effective magnetization vs temperature data shows a relatively sharp transition compared to magnetization vs temperature data obtained from VSM. The study shows that these differences can be traced to the different roles the applied magnetic field has when analyzing the data from these two techniques.
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    Magnetic Anisotropies and Damping in Multilayered Thin Films
    (University of Alabama Libraries, 2021) Rai, Anish; Mewes, Tim; University of Alabama Tuscaloosa
    Magnetic materials are ubiqutous in technology that we use on a daily basis. They are used as storage medium in hard drives, to read out the information with the hard drive read head, and to design sensors used in cars and cell phones to only name a few examples. The dynamic response of magnetic materials to an excitation is a crucial aspect of many of these applications and devices.In the following section a short introduction to the magnetization dynamics will be given with a special emphasis on ferromagnetic resonance phenomena. The concept of magnetic anisotropies and the importance of magnetic relaxation is also discussed in this section. This is followed by a detailed description of the experimental technique of broadband ferromagnetic resonance spectroscopy that is used throughout this dissertation. The last section of this chapter discusses and summarizes the magnetic anisotropy and relaxation studies carried out as part of this dissertation.
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    Accelerated Design of Novel Heusler Compounds for Spintronics Applications
    (University of Alabama Libraries, 2021) KC, Shambhu; LeClair, Patrick; University of Alabama Tuscaloosa
    Material discovery could be defined as the identification of a previously unexplored phases/composition which may exhibit properties that are unique or similar to that of previously explored composition. Historically, this has relied to some extent on serendipity. With the search space getting wider and at the same time an increased global competitiveness, it has become apparent that the material discovery process can be accelerated, which also helps in reducing cost. Spintronics, which utilizes both the spin and charge of an electron, is a technology that has the promise to take over existing charge-based technology. Half-metallic ferromagnets, due to their ability to generate 100% spin polarization, are considered ideal materials to be used in spintronic devices. While many candidate half-metals have been predicted based on theoretical calculations, finding a half-metallic character in experiments is still an open challenge. This provides impetus to search for new candidate materials with robust half-metallic character. In this dissertation, a new substitution scheme has been realized that allows for the design of many new functional materials in a relatively short time. It is also shown that, in many cases, alloy properties can be tuned by counting the total number of valence electrons, which is less dependent on the substitution scheme. Another approach, which paves the way to enhance the magnetic properties of the materials is also discussed. Hence with the identification of new approaches to material design, this dissertation adds value in the quest for the accelerated design of functional materials.
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    Magnetization Dynamics in Ultrathin Films and Multilayers
    (University of Alabama Libraries, 2021) Sapkota, Arjun; Mewes, Claudia; Mewes, Tim; University of Alabama Tuscaloosa
    Robust understanding of magnetization dynamics is very crucial to design and fabricate novelmaterials for next generation spintronic applications. A combination of experimental techniques and theoretical modeling facilitates fundamental research to establish new concepts. This dissertation discusses the magnetization dynamics of ultrathin films and multilayers using broadband ferromagnetic resonance (FMR) techniques and micromagnetic simulations. Chapter one includes the theoretical background on magnetization dynamics, the explanation of FMR experiments with schematic diagrams and data analysis, and insight in micromagnetic simulations using the in-house developed MATLAB-based code M^3. All simulations are performed with M^3. Chapter one also includes a summary of various publications which I co-authored during my time as a graduate student in the Mewes’ Magnetics Laboratory at UA. Chapter two discusses the limitations of the macrospin model of materials with inhomogeneous perpendicular anisotropy. This is a micromagnetic study motivated from experimental FMR measurements of [Co/Ni]_N multilayers. Chapter three is devoted to experimental FMR measurements carried out on FeGa thin films. These results show significantly lower Gilbert damping and linewidth in comparison to previous publications in similar structures, indicating the high quality of these thin films. This is also confirmed by X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements. These films are particularly interesting because of their well known superior magnetostrictive behavior. Motivated from previous FMR experiments in IrMn/CoFe and MnN/CoFeB exchange bias systems, we investigated the anisotropic damping in multilayer spintronic devices using micromagnetic simulations in chapter four. With the introduction of a damping tensor in the Landau-Lifshitz- Gilbert (LLG) equation, this study explains the observed unidirectional relaxation in IrMn/CoFe bilayers and co-existence of unidirectional and uniaxial relaxation in MnN/CoFeB bilayers .
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    Thermal radiation from the sun at 8.6 millimeters wavelengths
    (University of Alabama Libraries) Weaver, Richard Reid; University of Alabama Tuscaloosa
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    Mechanical and Thermal Properties of Linear Carbon Chains Encapsulated by Multi- and Double -Walled Carbon Nanotubes
    (University of Alabama Libraries, 2021) Sharma, Keshav; Araujo, Paulo T.; University of Alabama Tuscaloosa
    As a field of study, optics have been a critical to the development of material’s science. The interaction between light and matter is often non-destructive and non-invasive; making it powerful in the determination of materials’ compositions, and their properties at the electronic and vibrational levels. In the present work, we have broadened the use of light spectroscopy as a technique to determine, accurately, mechanical and thermal properties of molecular systems. More specifically, pressure (P) and temperature (T)-dependent Raman spectroscopy allowed us to access elusive mechanical (Young’s modulus (E), Grüneisen parameter (γ), and mechanical strain (ε)) and thermal properties (coefficient of thermal expansion (α), specific heat capacity (c_v), and thermal strain (ε_T)) of linear carbon chains (LCCs), which are one-atom thick linear carbon molecules. The results show that all these quantities follow universal relations that are solely dependent on P, T, and on the number of carbon atoms (N). In Appendix 01 we also describe how spectral derivative analysis combined with absorption and photo-luminescence spectroscopies allowed for unravelling elusive electronic and vibronic transition in free base 5,10,15,20-meso-tetra(pyridyl)-21H,23H-porphyrin (H2TPyP).
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    Holographic Techniques Applied to Rotating Fluids and Non-Relativistic Fluids
    (University of Alabama Libraries, 2021) Amano, Markus Antonio; Kaminski, Matthias; University of Alabama Tuscaloosa
    With the advent of the influential AdS/CFT correspondence, as a concrete realization of the holographic principle, theoreticians can construct models of strongly coupled quantum systems. In practice the holographic community often constructs toy models qualitatively similar to the Quark Gluon Plasmas (QGP). This dissertation presents three applications of AdS/CFT to model strongly coupled fluids where two symmetries are broken via theory or change of state. As the first application, a model of a 2D non-relativistic strongly coupled fluid is presented. The non-relativistic gravitational dual is is Horava Gravity (HG). A numerical study is conducted to calculate the transport coefficients and Quasinormal Modes (QNM). Second, a model of a strongly coupled rotating plasma is constructed and analyzed. The study is conducted to calculate the transport coefficients. Third, the aforementioned analysis is extended to determine the convergence radius of the hydrodynamic expansion in the rotational case. The convergence radius is determined by the calculation of critical points. This dissertation also covers an international collaboration that studied a 4+1D resonating gravitational soliton. This study focuses on the thermodynamic stability of the spacetime and its dual "glueball phase". The dissertation will close with an exposition of current projects and future prospects.
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    Assessing and Enhancing Seawater Corrosion Performance of Aluminum Alloy 7075 and 2024 Additive Repairs Produced by Cold Gas Dynamic Spray Deposition
    (University of Alabama Libraries, 2021) Agar, Ozymandias; Brewer, Luke N.; University of Alabama Tuscaloosa
    The purpose of this research is to investigate the corrosion behavior of cold sprayed (CS) aluminum alloys 2024 and 7075 in an immersed seawater environment, and improve understanding of CS processing-microstructure-corrosion property relationships. As a low temperature additive spray technique, CS has an attractive ability to deposit heat-sensitive alloys including AA2024 and AA7075 which is useful for additive repairs. Corrosion is a common damage mechanism which accounts for 1/5 of equipment downtime in the US military, but there is insufficient understanding of the corrosion performance of CS repair materials.This research finds that CS aluminum alloys in an immersed seawater environment can have very similar activity, reactivity, and potentiodynamic response compared to their wrought counterparts, and that CS deposits can even have superior pitting performance. CS corrosion properties, especially activity, change depending on powder heat treatment due to the resulting differences in the size and distribution of alloying element intermetallics. Solutionized CS-7075, for instance, is as active as the wrought (within margin of instrument error), but overaged CS-7075 has a less active Ecorr than wrought AA7075-T651 and in a repair scenario may cause a galvanic couple. Modifying spray processing parameters can cause significant changes in CS-2024 pitting performance without having a meaningful impact on potentiodynamic behavior. CS-7075 corrosion behavior relative to its wrought counterpart AA7075-T651 is stable for a range of electrolyte pH ( 6.2 - 8.6) and shows predictable changes in activity and reactivity that follow those observed in wrought AA7075-T651 for a range of salinity (0.5 – 1.5 times that found in ASTM D1141 artificial seawater). This results in this dissertation – the first set of comprehensive corrosion studies of CS-2024 and CS-7075 in an immersed seawater environment – highlight the promising corrosion properties of solid state repairs made from these alloys.
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    Laser assisted cold spray of ferritic alloys: oxide dispersion strengthened fe91ni8zr1 and aisi 4340 steel
    (University of Alabama Libraries, 2020-08) Barton, Dallin James; Thompson, Gregory B.; Brewer, Luke N.; University of Alabama Tuscaloosa
    Oxide dispersion strengthened (ODS) Fe91Ni8Zr1 (at. %) and AISI 4340 steel were successfully deposited via laser assisted cold spray. The laser assisted cold spray technique includes accelerating powder particles such that they strike a substrate at supersonic speeds and metallurgically bond. A high-powered laser irradiated the surface of the deposition area making the substrate surface thermally softer promoting deposition. In situ laser heating of the substrate increased the deposition efficiency of the high strength 4340 steel from 48 % to 72 %. The increased surface temperature from 400 C to 950 C also increased the median ferrite grain size. As the ferrite grain size increased, the hardness decreased; however, at higher surface temperatures, the steel transitioned to martensite and compensated the lost hardness due to grain size with the hardness returning to the same values as the cold spray deposits. The lowest viable surface temperature achieved for multi-layered LACS deposition of ODS materials is 650 C. Increased surface temperatures led to an increase in deposition efficiency up to 32 % at 950 C and resulted in a lower hardness material. Grain sizes and particle sizes increased from the elevated temperatures as well. However, the grains did not grow the same throughout the thickness of the material. Grains near the surface of the deposit are several times larger than grains near the deposit-substrate interface. In addition to LACS processing and microstructure, this work reports compressive surface residual stresses of LACS deposits, dynamic strain aging of ODS materials, and a commentary for improving cluster analysis of nano-scale oxides measured through atom probe tomography.
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    Investigation and rational design of the catalyst-support interface in redox catalysis by ceria
    (University of Alabama Libraries, 2020) Liu, Zhongqi; Wang, Ruigang; University of Alabama Tuscaloosa
    Investigating and controlling the catalyst-support interfacial interaction/structure and their effects on catalytic performance are crucial for optimizing the activity, selectivity, and durability of catalytic materials, as the heterogeneous catalytic reactions typically take place on the catalyst surface and/or at the interface between the catalyst and support. Ceria (CeO2), due to its remarkable redox activity, has been widely adopted as an active support material or promoter in a multitude of redox catalytic reactions and is the focus of this research. With the goal of bridging the predictable catalyst design-fundamental understanding of performance-practical application, we expect to develop uniform and well-defined CeO2 nanostructures as model supports to investigate the underlying mechanism of the catalyst-support interactions, and furthermore establish the correlation between interfacial structure and catalytically active sites. In Chapter 2, reducible CeO2 nanorods and nanocubes, as well as irreducible SiO2 nanospheres supported cobalt oxides (CoOx) catalysts were synthesized and comparatively studied to understand the effects of support morphology, surface defect, support reducibility, in addition to the CoOx-support interactions on their redox and catalytic properties. Chapter 3 focuses on exploring the role of “bimetallic catalysts-support interaction” over highly active CeO2 nanorods supported pure cobalt oxides and cobalt-based bimetallic oxides nanoparticles (Fe-Co, Ni-Co and Cu-Co). The interactions between cobalt with the second transition metals (Fe, Ni and Cu) are discussed as well. Nanoparticle agglomeration issue always exists when using wet-chemical methods to synthesize CeO2 nanomaterials, which is harmful for catalytic applications due to decreased surface area. Therefore, Chapter 4 presents a scalable and facile electrospinning process for designing novel fibrous structured CeO2 and one-pot synthesis of high-surface-area, thermally stable and low-temperature active Ru-CeO2 nanofiber catalysts. Besides, attracted by the great interest of three-dimensional (3D) nanoarray structures fabrication towards novel and high-performance catalyst design, as well as nanodevice applications, electrochemical deposition technique was adopted for fabricating CeO2 nanoarrays in Chapter 5. Processing factors on growing controllable CeO2 nanoarrays, including the current density, reaction temperature, stirring rate, anode and substrate types were comprehensively investigated. A scale-up synthetic strategy for CeO2 nanoarrays fabrication is developed. Besides, possible mechanisms for morphological evolution and growth of CeO2 nanoarrays are discussed.
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    Search for neutrinoless double-beta decay and 42Ar in enriched xenon using the EXO-200 dataset
    (University of Alabama Libraries, 2020) Hughes, Mitchell; Piepke, Andreas; University of Alabama Tuscaloosa
    Observation of neutrinoless double-beta decay (0νββ) would constitute discovery of a new class of particle (the Majorana neutrino), violate lepton number conservation, and provide a constraint on the neutrino mass scale. The Enriched Xenon Observatory experiment (EXO-200) operated an extremely radio-pure tracking calorimeter filled with liquid xenon enriched in the candidate isotope 136Xe and installed underground in New Mexico, USA. The author presents a search for 0νββ with the complete EXO-200 dataset, the second-largest exposure of any 0νββ experiment (234.1 kg∙yr). The analysis applied coupled fits to event energy and position with a classical topological discriminator. The author produced an expanded background model to search for radioactive 42Ar via its β-decay product, 42K. No signal excess was observed near the 0νββ Q-value, leading to a limit on the 0νββ half-life in 136Xe of T_(1/2)^0ν >3.0×10^25 yr and Majorana neutrino mass 〈m_ββ 〉<(100-309) meV, both reported at the 90% confidence level (CL). The corresponding sensitivity of 4.2×10^25 yr (90% CL) represents an improvement of 5.7% over the standard EXO-200 background model, which does not include 42Ar. Notably, the author's expanded model has been used to publish the first constraint on the content of 42Ar in enriched xenon, which provides important input to the design of future low-background experiments. A limit is set on the specific activity of 42Ar in enriched xenon at <0.8 μBq/kg (90% CL). Additionally, the author’s improvements to offline event reconstruction techniques, operational responsibility for the EXO-200 scintillation panel detectors, and principal contributions to a published paper on cosmogenic backgrounds are also discussed.
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    Raman spectroscopy of double- and triple-walled carbon nanotubes: fundamental, combination, and overtone modes
    (University of Alabama Libraries, 2020) Hue, Jia Wern; Araujo, Paulo T.; University of Alabama Tuscaloosa
    A single-walled carbon nanotube (SWNT) is a graphene sheet rolled up into a tube. Double-walled (DWNT) and triple-walled carbon nanotubes (TWNT) are two and three coaxial SWNTs respectively. Isolated species of DWNTs and TWNTs were only recently probed and it can be considered as a new branch in carbon nanotube science. Phonons, and the combination of phonons and overtones are fundamental to understanding optical processes, transport and thermoelectricity in carbon nanotubes. To study these phonons, resonance Raman spectroscopy is employed on DWNT and TWNT bundles, as well as isolated TWNTs. The phonon modes of interest to this dissertation are the radial breathing mode (RBM), the G-band, and the M-band. From the RBM, it is learnt that for inner tubes with a tube diameter less than 1.2 nm, the curvature effects are dominant. Evidence for intertube interactions was found, although for outer tubes the environmental effects dominate. There was also evidence for commensurate and incommensurate tubes. Studying the G-band allowed for possible chiral indices to be identified for the middle and outer tubes of isolated TWNTs. In addition, the frequency shifts of TWNT G-band frequencies relative to SWNT G-band frequencies were studied and compared to TWNT G-band frequency shifts. As for the M-bands, phonon mode assignments for peaks between 1680 cm-1 to 1850 cm-1 were attempted.
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    Production, modification, and characterization of natural bamboo fiber
    (University of Alabama Libraries, 2019) Rocky, AMK Bahrum Prang; Thompson, Amanda J.; University of Alabama Tuscaloosa
    In an effort to extract natural bamboo fiber (NBF) from bamboo for textiles and other uses, four bamboo species Bissetii, Giant Gray, Moso, and Red Margin were chosen for investigation. Conventional fibers such as cotton, polyester, regular rayon, and 12 commercial bamboo viscose were included for comparative study. By using different chemicals and routes, 144 types of NBFs were produced. Assessments on fiber yield percentages (40-77%), average lengths (1.50-37.10 cm), fineness (9.68—93.3 Tex), and overall qualities, determined at least 47 sets were prospective for commercial use. Hand-spinning was executed on three sets of NBFs after blending with cotton fibers. Investigation on moisture regain (M_R) and moisture content (M_C), revealed that bamboo plants and NBFs had M_R=8.0% and M_R=7.5% which was lower than rayon and bamboo viscose fiber (~11% and ~10%) but higher than raw cotton fibers (~5.7% and 5.4%). Among tensile properties, breaking tenacity of longer NBFs (33-37 cm) was 64-140 N/Tex and elongation at break was 2.0-2.5%; these values were 1.50-2.50 N/Tex and 8.0-11.0% respectively for blended yarns. Elemental, chemical, and crystallographic investigations were accomplished by energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infra-Red), Raman spectroscopy (RS) and X-ray diffraction (XRD) techniques. Bamboo plants were estimated to be 70-78% carbon (C) and 20-30% oxygen (O) atoms, and O/C ratio of 0.26-0.33. The NBFs had a higher O/C ratio of 0.58-0.70. Comparisons of the spectra revealed the differences between bamboo-NBF and other fibers. Some distinct lignocellulosic peaks were in NBFs that could be responsible for unique properties. The crystallinity index (CI) of bamboo plants was 63-67% but CI of NBFs was higher 69-73% with crystallite sizes of 35-39 Å (3.5-3.9 nm). Four reflection planes and other properties are also documented. A suitable antibacterial test method was modified for quantitative estimation of bacterial reduction. Results suggest that though 11 out of 12 bamboo viscose products failed to exhibit inhibition against the bacteria, most of the bamboo and NBF specimens successfully showed a bacterial reduction of 8-95% against Klebsiella pneumoniae and 3-50% against Staphylococcus aureus.
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    The spin Seebeck effect in magnetic insulating oxides
    (University of Alabama Libraries, 2019) Li, Zhong; Gupta, Arunava; Mankey, Gary J.; University of Alabama Tuscaloosa
    The spin Seebeck effect (SSE), the generation of a spin current from a thermal gradient, is a novel effect which involves the interaction between charge, spin and heat. Insulating magnetic materials, like yttrium iron garnet (YIG, Y₃Fe₅O₁₂) and nickel ferrite (NFO, NiFe₂O₄), are ideal for the study of this new effect due to avoiding other magnetic effects. Thin films of Y₃Fe₅O₁₂, Ce₀.₇₅Y₂.₂₅Fe₅O₁₂ and NiFe₂O₄ have been grown and optimized on different substrates (MgAl₂O₄, MgGa₂O₄, CoGa₂O₄) using the pulsed laser deposition (PLD) technique, and their crystal structures were investigated using X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). For the magnetocrystalline anisotropy in the thin films, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) measurements are done. We further did spin Seebeck effect measurements on optimized samples. First, for thin films of Ce₀.₇₅Y₂.₂₅Fe₅O₁₂, homogeneous substitution of Ce in YIG results in the enhancement of the signal in magneto-optic Kerr effect (MOKE) without forming CeO₂ when at lower O₂ atmosphere. The spin Seebeck effect measurements on Ce:YIG films show similar trends and comparable results with pure YIG films suggesting potential applications for thermoelectric generation. Second, an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch between NFO thin films and substrates, which also correlates well with the decrease in the Gilbert damping parameter from FMR measurements. Furthermore, we have developed a vector measurement of the spin Seebeck effect in epitaxial NiFe₂O₄ thin films, which were grown by pulsed laser deposition on (011)- or (001)-oriented MgGa₂O₄ and CoGa₂O₄ substrates with varying lattice mismatches. This new method for SSE measurement shows the existence of a magnetic strain anisotropy in NiFe₂O₄ thin films significantly impacts the shape and magnitude of the SSE voltage hysteresis loops, which demonstrates that voltage signals from bidirectional SSE measurements can be utilized as a new vectorial magnetometry technique to reveal the complete magnetization reversal process.
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    Solution-processed doping in CdTe thin film solar cells
    (University of Alabama Libraries, 2019) Montgomery, Angeqlique; Yan, Feng; University of Alabama Tuscaloosa
    Cadmium Telluride (CdTe) is one of the leading photovoltaic (PV) technologies in the world with a world record ~ 22.1%. With a bandgap of 1.45eV and a high absorption coefficient, the theoretical power conversion efficiency limit of 32% is limited by recombination and high resistivities in CdTe devices. Doping CdTe is a necessary way to improve device performance. In this work, it is demonstrated that a cost-effective solution-processed Group I and Group V doping in CdTe thin film solar cells allows for efficiency increases and stability improvement in CdTe devices. By varying the doping concentration, activation annealing temperatures, and deposition parameters, the root cause for the increased power conversion efficiency was investigated. Group I and Group V dopants showed smoother films indicating less resistance through the back contact leading to an increase in power conversion efficiencies.
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    Nanostructural characterization of geopolymers
    (University of Alabama Libraries, 2019) Tuinukuafe, Atolo Aaron; Amirkhanian, Armen N.; University of Alabama Tuscaloosa
    Three investigations were conducted in this article-style dissertation, all sharing the common goal of achieving improved characterization of geopolymers in a manner that provides nanostructural information. The first study revisited the use of statistical nanoindentation for characterizing the micromechanical properties of heterogeneous materials and explores the limitations of the method for future applications with geopolymers. In the second investigation, a novel method was developed for quantitatively analyzing the microsturcture of geopolymers by using electrical resistivity measurements. The method was found to provide insight into the nanoscale porosity. The third investigation utilized atom probe tomography (APT) to observe the effect of elevated temperature of the nanostructure of a fly ash geopolymer. Complimentary analysis techniques from the first two investigations were used to support the findings from APT. The outcomes of this dissertation are additional insight into the nanostructure of geopolymers, as well as several new characterization tools that may see broader use for other cementitious materials in future research.
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    A study of white dwarfs: cataclysmic variables and double-detonation supernovae
    (University of Alabama Libraries, 2019) Caldwell, Spencer; Townsley, Dean M.; University of Alabama Tuscaloosa
    Novae, be it classical, dwarf, or supernovae, are some of the most powerful and luminous events observed in the Universe. Although they share the same root, they are produced by different physical processes. We research systems capable of experiencing novae with the intention of furthering our understanding of these astrophysical phenomena. A cataclysmic variable is a binary star system that contains a white dwarf with the potential of undergoing classical or dwarf novae. A recent observation of a white dwarf within one of these systems was found to have an unusually high surface temperature for its orbital period. The discovery contradicts current evolutionary models, motivating research to determine a theoretical justification for this outlier. Using MESA (Modules for Experiments in Stellar Astrophysics), we simulated novae for a progenitor designed to represent a white dwarf in an interacting binary. We developed post-novae cooling timescales to constrain the temperature value. We found the rate at which classical novae cool post-outburst (< 1 K yr−1) is in general agreement with the four−year follow-up observation (∼ 2 K). The evolution of white dwarfs during double- detonation type Ia supernovae was also studied. The progenitors capable of producing these events are not fully established, requiring a consistent model to be developed for parametric analysis. Three improvements were made to the simulation model used in (Townsley et al., 2019): the inclusion of a de-refinement condition, a new particle distribution, and a burning limiter. The focus here was to enhance the computational efficiency, offer better representation of particles in the supernova ejecta, and control the nuclear energy release. These developments were employed to test double-detonation scenarios capable of producing spectra analogous to type Ia supernovae, which will offer insight into their prevalence and strengthen their use in measuring cosmological distance.
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    A tale of two standard model extensions
    (University of Alabama Libraries, 2019) Villalba, Desmond; Okada, Nobuchika; University of Alabama Tuscaloosa
    The Standard Model (SM) has provided physicists with a nearly complete effective description of the fundamental building blocks for the Universe. While several questions regarding the makeup of our Universe have been resolved, many still remain. One such question deals with the vast energy difference between the electroweak scale and Planck scale (${\cal O}(10^{17}\, {\rm GeV})$). The origin of this large divide has physical implications affecting the running of the Higgs mass, as it receives quantum corrections that are quadratic. Affiliated with this large division of energy scales is an issue that came about upon detection of the Higgs boson at the Large Hadron Collider (LHC), which enabled us to infer the value of the Higgs self-coupling. As a result, the renormalization group equation for the Higgs self-coupling predicts a negative self-coupling at around energies of $10^9 - 10^{11} $ GeV. If true, this would indicate that our vacuum state is unstable. Taking our motivation from stringy effects by modifying the local kinetic term of an Abelian Higgs field by the Gaussian kinetic term, we show that the Higgs field does not possess any instability, and the beta-function of the self-interaction for the Higgs becomes exponentially suppressed at high energies, showing that such class of theory never suffers from a vacuum instability. Another interesting question to consider, is what might be the origin of the large mass difference between the fundamental fermions? As will be shown, the fermion mass hierarchy can be explained through the use of our formalism developed in our setup of the "Domain-Wall Standard Model in a non-compact 5-dimensional space-time", where all the SM fields are localized in certain domains of the 5th dimension. Reproducing the hierarchy is contingent upon the localization positions of the fermions along the extra flat dimension. As a result of these different localization points, the effective 4-dimensional Kaluza-Klein mode gauge couplings become non-universal. This allows for the possibility of interesting experimental considerations which will be discussed. Flavor Changing Neutral Current constraints provide stringent bounds on our model, through these constraints we can glean information about the extra dimension. We have found two possibilities that satisfy these constraints: (1) the KK mode of the SM gauge bosons are extremely heavy and unlikely to be produced at the LHC, however future FCNC measurements can reveal the existence of these heavy modes. (2) the width of the localized SM fermions is very narrow, meaning the 4D KK mode gauge couplings are almost universal. In this case the FCNC constraints can be easily avoided, even for a KK gauge boson mass of order TeV. Such a light KK gauge boson can be discovered at the LHC in the near future.
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    New soft magnetic materials for high frequency applications
    (University of Alabama Libraries, 2019) Wu, Shuang; Mewes, Tim; University of Alabama Tuscaloosa
    Soft magnetic materials are widely used in devices such as inductors, transformers, antennas, magnetic hard drives, etc. Some of those devices will benefit greatly from operating at high frequencies. Thus fundamental study on finding the materials that have better soft magnetic properties is essential for improving the performance of those devices. Fe alloys have been proved to be promising candidates for high frequency applications. In this dissertation, an extensive study of magnetic properties of FeAl, (FeCo)-Al and (FeCo)-Si alloy thin films and their dependence on the film thickness and growth temperature has been presented. These films have body-centered cubic structure and columnar growth morphology. It is shown that the thickness of the film, which has an influence on the stress inside the film, may affect the coercivity through the magnetic-elastic coupling. The same mechanism is observed in the growth temperature dependence study, where reduced stress caused by increased growth temperature leads to a decrease in coercivity. The effective damping parameter shows a huge increase at small thickness due to the spin pumping effect. In-plane rotation ferromagnetic resonance measurements unveil the existence of four-fold anisotropy in (FeCo)-Si films. In addition, a four-fold symmetry is observed in the FMR linewidth vs. in-plane angle plot, which indicates anisotropic damping caused by the two-magnon scattering contribution. The film thickness dependence of FMR linewidth caused by the two-magnon scattering suggests that the origin of the two-magnon scattering is not pure interfacial.