Theses and Dissertations - Department of Physics & Astronomy

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125 gev higgs boson mass from 5d gauge-higgs unification
(University of Alabama Libraries, 2015) Carson, Jason Carl; Okada, Nobuchika; University of Alabama Tuscaloosa
In the context of a simple gauge-Higgs unification (GHU) scenario based on the gauge group SU(3)×U(1)′ in a 5-dimensional flat space-time, we investigate a possibility to reproduce the observed Higgs boson mass of around 125 GeV. We introduce bulk fermion multiplets with a bulk mass and a (half) periodic boundary condition. In our analysis, we adopt a low energy effective theoretical approach of the GHU scenario, where the running Higgs quartic coupling is required to vanish at the compactification scale. Under this "gauge-Higgs condition," we investigate the renormalization group evolution of the Higgs quartic coupling and find a relation between the bulk mass and the compactification scale so as to reproduce the 125 GeV Higgs boson mass. Through quantum corrections at the one-loop level, the bulk fermions contribute to the Higgs boson production and decay processes and deviate the Higgs boson signal strengths at the Large Hadron Collider (LHC) experiments from the Standard Model (SM) predictions. Employing the current experimental data which show the the Higgs boson signal strengths for a variety of Higgs decay modes are consistent with the SM predictions, we obtain lower mass bounds on the lightest mode of the bulk fermions.
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.
Antineutrino neutral current interactions in MiniBooNE
(University of Alabama Libraries, 2012) Dharmapalan, Ranjan; Stancu, Ion; University of Alabama Tuscaloosa
The antineutrino nucleon neutral current elastic scattering cross section measured at the MiniBooNE experiment is reported. The data set corresponds to 10.1 × 10^20 protons on target which is a world record neutral current elastic antineutrino sample. An antineutrino to neutrino neutral current scattering cross section ratio is measured after accounting for all associated errors. This is the first time such a ratio has been experimentally reported. Previous MiniBooNE neutrino cross section measurements have indicated a higher value for the axial mass, M_A, as compared to the nominal value of M_A=1.0 GeV. A Χ^2 test was performed to find the best value of M_A which matches the antineutrino neutral current elastic data. Finally, an exciting possibility to search for dark matter in the MiniBooNE experiment, using the neutral current interactions is discussed
Applications of methods beyond density functional theory to the study of correlated electron systems
(University of Alabama Libraries, 2013) Sims, Hunter Robert; Butler, W. H.; University of Alabama Tuscaloosa
The difficulty in accurately treating systems in which electron-electron interactions are the dominant physics has plagued condensed matter physics for decades. Currently, there exist many different computational techniques designed to improve upon density functional theory to varying degrees of accuracy. To date, no unified, parameter-free method exists that is guaranteed to yield the correct answer for all materials. Consequently, proper treatment of such systems often requires a combination of several methods, allowing one to check them against one another when their regions of validity overlap and to expand one's reach when a single method cannot reliably describe all of the physics at work. In this dissertation, I present discussion and, when appropriate, brief derivations of several of the most prominent electronic structure methods currently in use---from the local density approximation through LDA+DMFT. I then present several investigations into the electronic and magnetic structure of materials of potential interest for information technology that also illustrate the current state of affairs in computational condensed matter physics. I explore the intersite exchange interactions in CrO_2 within density functional theory (with and without Hubbard “+U” corrections) and evaluate these results through analytic and numerical means. I study the dependence of the mysterious magnetization of Fe_16 N_2 on crystal and electronic structure and employ a wide range of techniques in an attempt to bring greater rigor and deeper understanding to the widely-varying reports on this material. In conjunction with others' careful experimental analysis, I provide a picture of the band structure of the magnetic insulator NiFe_2 O_4 that reveals a novel hierarchy in its band gaps and suggests applications in spintronics and possibly other areas. Finally, I employ dynamical mean-field theory to study the behavior of impurity states in elemental semiconductors, using H impurities in Ge as a base system.
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.
Atom probe tomography study of wide bandgap semiconductor materials
(University of Alabama Libraries, 2014) Dawahre Olivieri, Nabil Farah; Kung, Patrick; University of Alabama Tuscaloosa
This dissertation focuses on developing atom probe tomography (APT) for semiconductors. APT is quickly gaining interest in the field of material characterization because of its unique ability to provide 3D nanoscale studies. APT has been widely used in metals and conductive materials but design changes in the tool in recent years have made atom probe a suitable tool for semiconductor analysis. Because research in atom probe tomography of semiconductors is still in its infancy, it is still unclear whether this characterization method is suitable for semiconductor and how the added knowledge can be different than other accessible tools. This work will utilize APT as a characterization tool for wide bandgap semiconductors, specifically zinc oxide (ZnO) nanowires and GaN-based epitaxial sample. Wide bandgap semiconductor materials have attracted considerable attention in recent years because of the practical applications such as green and blue light emitting and laser diodes, solid-state lighting, photovoltaics, RF and microwave electronics, and gas sensors. Although silicon has remained the industry standard for many of these applications, its limitations have made way into the research of wide bandgap semiconductor materials, such as zinc oxide (ZnO) and gallium nitride (GaN). Because of their large direct bandgap, these materials show excellent promise in the field of optoelectronics, high frequency, high temperature and high power applications. First, we understand the behavior of the material to achieve field evaporation under APT conditions and the mechanisms behind, as well as ways to overcome the different artifacts introduced during sample preparation and data collection. Following this understanding, we can begin to apply APT to device structures to understand the effects of radiation on materials at the atomic scale, as well as the cluster formation of some of the elements along the material. At the conclusion of this dissertation, APT will deliver the results necessary to maximizing device efficiency as well as build the pathway for future APT analysis.
Bacterium E. coli- and phage P22-templated synthesis of semiconductor nanostructures
(University of Alabama Libraries, 2010) Shen, Liming; Gupta, Arunava; University of Alabama Tuscaloosa
The properties of inorganic materials in the nanoscale are found to be size- and shape-dependent due to quantum confinement effects, and thereby nanomaterials possess properties very different from those of single molecules as well as those of bulk materials. Assembling monodispersed nanoparticles into highly ordered hierarchical architectures is expected to generate novel collective properties for potential applications in catalysis, energy, biomedicine, etc. The major challenge in the assembly of nanoparticles lies in the development of controllable synthetic strategies that enable the growth and assembly of nanoparticles with high selectivity and good controllability. Biological matter possesses robust and precisely ordered structures that exist in a large variety of shapes and sizes, providing an ideal platform for synthesizing high-performance nanostructures. The primary goal of this thesis work has been to develop rational synthetic strategies for high-performance nanostructured materials using biological templates, which are difficult to achieve through traditional chemical synthetic methods. These approaches can serve as general bio-inspired approaches for synthesizing nanoparticle assemblies with desired components and architectures. CdS- and TiO_2 -binding peptides have been identified using phage display biopanning technique and the mechanism behind the specific affinity between the selected peptides and inorganic substrates are analyzed. The ZnS- and CdS-binding peptides, identified by the phage display biopanning, are utilized for the selective nucleation and growth of sulfides over self-assembled genetically engineered P22 coat proteins, resulting in ordered nanostructures of sulfide nanocrystal assemblies. The synthetic strategy can be extended to the fabrication of a variety of other nanostructures. A simple sonochemical route for the synthesis and assembly of CdS nanostructures with high yield under ambient conditions has been developed by exploiting the chemical characteristics and structure of permeabilized E. coli bacteria. The crystal phase, morphology, micro/nanostructure, optical absorption, and photocatalytic properties of the CdS nanostructures are tailored over a wide range by merely changing the synthetic conditions. Photoanodes fabricated using the nanoporous hollow CdS microrods exhibit excellent performance for the photocatalytic hydrogen production. This facile approach has been extended to the synthesis and assembly of other semiconducting sulfides, including PbS, ZnS, and HgS.
Bounds on large extra dimensions from the simulation of black hole events at the Large Hadron Collider
(University of Alabama Libraries, 2016) Hou, Shaoqi; Harms, B.; University of Alabama Tuscaloosa
Large extra dimensions were originally proposed to solve the hierarchy problem of the Standard Model (SM) of elementary particle physics. The presence of large extra dimensions dilutes gravity, lowering the Planck scale, while SM particles are required to propagate only in the usual 4 dimensional spacetime, leaving the electroweak scale unchanged. If large extra dimensions exist and they are large enough, the Planck scale may be as low as a few TeV’s, so that the hierarchy problem is solved. A smaller Planck scale will bring about numerous phenomenological consequences; in particular, microscopic black holes may be produced in high-energy particle collisions at this energy scale. The decay of black holes, via the Hawking effect, into elementary particles enables the detection of the black hole events, which can be used to infer the existence of large extra dimensions. In this work, we simulate microscopic black hole formation at the Large Hadron Collider with the black hole event generator CATFISH, and compare the simulation results with the experimental data published by the Compact Muon Solenoid collaboration in 2013 at a center of mass energy $\sqrt{s}= 8$ TeV, corresponding to an integrated luminosity of 12.1 fb$^{-1}$. The goal of this work is to test the large extra dimension model and to determine the value of the Planck scale if large extra dimensions exist. The absence of observed black hole events in the experimental data allows us to set lower bounds on the Planck scale and various parameters related to microscopic black hole formation for a number (3 - 6) of large extra dimensions. Assuming no energy loss during high-energy particle collisions, our analysis sets lower bounds on the fundamental Planck scale ranging from 0.8 TeV to 4.9 TeV for black holes fully decaying into SM particles and 0.5 TeV to 3.0 TeV for black holes settling down to a charge neutral, invisible remnant, depending on the minimum allowed black hole mass at formation. Formation of black holes with mass less than 5.2 TeV to 6.5 TeV (SM decay) and 2.2 TeV to 4.0 TeV (remnant) is excluded at 95\% C.L. Further investigation takes into account the effects of the Generalized Uncertainty Principle (GUP), which is expected to play an important role because the mass of a microscopic black hole is only a few fundamental Planck masses. An analysis similar to the one carried out without including GUP effects reveals smaller lower bounds on the fundamental Planck scale ranging from 0.8 TeV to 1.4 TeV for black holes fully decaying into SM particles, only when $\alpha\ge0.9$, depending on the minimum allowed black hole mass at formation. Therefore, this work constrains not only the sizes of the large extra dimensions and the masses of the microscopic black holes, but also sets the lower limits on the energy scale where the effects of quantum gravity start to become significant.
Collider phenomenology of heavy neutrinos
(University of Alabama Libraries, 2016) Das, Arindam; Okada, Nobuchika; University of Alabama Tuscaloosa
The existence of the neutrino mass has been established by the neutrino oscillation experiments. The so-called seesaw extension of the Standard Model is probably the simplest idea to naturally explain the existence of tiny neutrino mass through the lepton number violating Majorana mass term. There is another alternative way, commonly known as the inverse seesaw mechanism, where the small neutrino mass is obtained by the tiny lepton number violating parameters. In this work we investigate the signatures of such heavy neutrinos, having mass in the Electroweak scale at the high energy colliders. Based on a simple realization of inverse seesaw model we fix the model parameters to reproduce the neutrino oscillation data and to satisfy the other experimental constraints. We assume two flavor structures of the model and the different types of hierarchical light neutrino mass spectra. For completeness we consider the general parameterization for the model parameters by introducing an arbitrary orthogonal matrix and the nonzero Dirac and Majorana phases. Due to the smallness of the lepton number violating parameter this model can manifest the trilepton plus missing energy at the Large Hadron Collider(LHC). Using the recent LHC results for anomalous production of the multilepton events at $8$ TeV with a luminosity of $19.5$ fb$^{-1}$, we derive the direct upper bounds on the light-heavy neutrino mixing parameter as a function of the heavy neutrino mass. Using a variety of initial states such as quark-quark, quark-gluon and gluon-gluon as well as photon mediated processes for the Majorana heavy neutrinos we obtain direct upper bounds on the light-heavy neutrino mixing angles from the current LHC data at $8$ TeV. For the pseudo-Dirac heavy neutrinos produced from the various initial states using the recent anomalous multilepton search by the LHC at $8$ TeV with $19.5$ fb$^{-1}$ luminosity, we obtain upper bounds on the mixing angles.
Coupling semi-analytic models and N-body simulations: a new way of making galaxies and stellar halos
(University of Alabama Libraries, 2017) McCord, Krista; Bailin, Jeremy; University of Alabama Tuscaloosa
Stellar halos give insight into the initial conditions that existed when a host galaxy first formed and provide details on disrupted satellites via their different stellar populations. An algorithm that is computationally inexpensive compared to hydrodynamic simulations is necessary in order to theoretically study the structure and formation of galactic stellar halos in sufficient detail to probe substructure. CoSANG (Coupling Semi-Analytic/N-body Galaxies) is a new computational method that we are developing which couples pure dark matter N-body simulations with a semi-analytic galaxy formation model. At each timestep, results from the N-body simulation feed into the semi-analytic code, whose results feed back into the N-body code making the evolution of the dark matter and baryonic matter dependent on one another. CoSANG will enable a variety of galaxy formation science, including analysis of stellar populations, halo merging, satellite accretion, supermassive black holes, and indirect and direct dark matter detection. In this dissertation, I will describe the new simulation code CoSANG. The results from the extensive testing phase on CoSANG will be presented which indicate CoSANG is properly simulating feedback from galaxies within a dark matter halo. I used this validated code to analyze a CoSANG zoom simulation of a $10^{12} \mathrm{M_{\odot}}$ dark matter halo. Results showed a flatter inner halo near the disk and a more spherical outer halo which is expected when a galaxy exists at the center of a dark matter halo. A comparison is made with a simulation run with the same initial conditions, but with the baryonic component simulated using a hydrodynamic algorithm. The semi-analytic model predicted galaxy types better than the hydrodynamic simulation leading to the conclusion that the CoSANG halo is more accurate. I also present a dark matter direct detection analysis on the CoSANG zoom halo to measure the dark matter velocity distributions and modulation amplitudes. The CoSANG results show that the dark matter velocity distribution does not fit well to a Maxwell Boltzmann distribution and the modulation amplitudes derived indicate an anisotropic dark matter velocity distribution. Future work will include tagging dark matter particles with stellar properties to build and evolve a stellar halo.
Dark matter decays from the galactic center using IceCube-86
(University of Alabama Libraries, 2017) Pepper, James Alan; Williams, Dawn R.; University of Alabama Tuscaloosa
Most searches for Dark Matter primarily focus on the WIMP paradigm, which predicts dark matter masses in the GeV - 10 TeV range. However, these relatively low energy searches continue to produce null results, possibly suggesting that dark matter is something other than WIMPs. Gravitinos, on the other hand, can satisfy the cosmological constraints on dark matter, and decay with a lifetime orders of magnitude longer than the age of the universe, producing extremely high energy neutrinos. The IceCube Neutrino Observatory has already had success detecting EHE extragalactic neutrinos, and is well suited to search for dark matter in this high energy regime. This analysis sets limits on the gravitino lifetime from the high energy neutrino events observed at IceCube using three possible astrophysical explanations of the neutrino flux. The most conservative limit on the gravitino lifetime using the softest two-body decay mode was found to be $\tau_{DM} = 10^{27.6}s$. This is the first analysis developed to place a limit on the gravitino lifetime using IceCube software and simulation files, and the results are comparable to theoretical limits based on the same data set.
Development of a highly sensitive magnetometer with a wide temperature range for the study of heterostructured transition metal-gadolinium alloys
(University of Alabama Libraries, 2016) Chahil, Narpinder Singh; Mankey, Gary J.; University of Alabama Tuscaloosa
The design, construction, and testing of a highly sensitive magnetometer with a wide temperature range for the study of magnetic phase transitions and the effect of thermal processing on heterostructured magnetic films is reported. The magnetometer is based on the magneto-optic Kerr effect which produces a change in polarization of light reflected from a magnetic surface. A laser with a wavelength of 632 nm is used, and the magnetometer measures the magnetic orientation of layers within the skin depth of the incident radiation. For the systems studied in this work, the probed magnetization is assumed to follow the magnetic behavior of the topmost magnetic layer. The apparatus has a temperature range from 65 K to 760 K, enabling the study of a wide range of heterostructured thin films. Alloys of Gd with transition metals have a strong temperature dependence since pure Gd has a ferromagnetic Curie temperature around 300 K. Single-layers of Co, Gd and Co-Gd alloy were used as prototype magnetic system for commissioning the apparatus. The phase transition behaviors as determined by the magnetometer are reported. For testing the thermal processing, bilayers incorporating antiferromagnetic NiMn and NiMn-Gd with the above ferromagnetic layers were studied. These bilayers exhibited magnetic behaviors that were dependent on the thermal processing history, as annealing at 573K and cooling in a static magnetic field produced a change in the exchange bias, coercivity, and shape of the magnetic hysteresis loops.
Diffuse ionized regions in the vicinity of active galaxies
(University of Alabama Libraries, 2011) Darnell, Erin Kay; Keel, W. C.; University of Alabama Tuscaloosa
Using a sample of 39 Active Galactic Nuclei (AGN), we investigated the incidence of giant ionized clouds in the vicinity of active galaxies. We carried out remote observations of the sample using the Southeastern Association for Research in Astronomy (SARA) North telescope at Kitt Peak and the SARA South telescope on Cerro Tololo. Frames were taken in continuum V and using a narrowband filter that transmits the redshifted [O III] 5007 Å line. We formed an emission line image by subtracting the combined and scaled V frames from the combined [O III] frames. To reduce uncorrelated noise, the emission line images are median filtered with a 1".9 x 1".9 box size. To bring out large diffuse regions, we convolve the emission line image with a circular Gaussian function of 3".42 FWHM. Emergent structures are determined to be starlight or ionized gas. 21 members of our sample were recently shown to be tidally disrupted in an atomic Hydrogen (HI) study. No extended [O III]-line emission clouds were seen in the vicinity of any from this group. We found one new instance of extended ionized emission clouds near the Seyfert 1 galaxy RX J1103.2-0654.
Dust mapping of spiral galaxy disks from the Hubble starsmog survey
(University of Alabama Libraries, 2016) Bradford, Sarah; Keel, W. C.; University of Alabama Tuscaloosa
Dust in spiral galaxy disks plays an important role in observations through its interaction with starlight. While arguments have been made in favor of completely opaque spiral disks as a result of the dust (see Valentijn, 1990), most subsequent studies agree that spiral galaxies include both optically thick and optically thin regions. Using the occulting-galaxies technique introduced by White & Keel (1992), we created transmission maps of nine partially overlapped galaxy systems taken from the STARSMOG (STarlight Absorption Reduction through a Survey of Multiple Occulting Galaxies) catalog to probe the optical depth in regions of backlit dust. The measurements of all nine targets show a wide range of opacities, and therefore none of them can be definitively classified as either "optically thick" or "optically thin". In the analysis of the transmission maps of predominantly highly inclined systems, we noticed a distinct edge to the dust structure beyond which no dust was present in the foreground galaxy disk. This radius varied amongst systems but was distinct enough to be measured for three of the systems. The transmission maps also reveal a web-like pattern to the dust structure in some galaxy disks. Four of the targets showed dust structure which not only followed the pitch angle of the spiral arms, but also crossed at other various angles. This web-like pattern was only observed in systems where the foreground galaxy was of Hubble type SBc/Sc/Sbc. While there appears to be no real trend in the statistics of transmission through backlit dust with respect to stellar mass of the galaxies, there does appear to be a correlation with respect to star formation rates. In this nine-system sample, the targets with the least star formation appear more opaque. By continuing this study for the rest of the STARSMOG targets, we will be able to determine how representative this sample is of the entire target set.
Dynamic transport measurements of vo2 thin films through the metal-to-insulator transition
(University of Alabama Libraries, 2018) Jones, Joshua Michael; LeClair, Patrick R.; University of Alabama Tuscaloosa
VO2 is a transition metal oxide material well known for its high magnitude metal-to-insulator transition (MIT) with a corresponding change in crystal structure [1]. At room temperature, VO2 is found in an insulating monoclinic phase (P21/c) that upon heating through the transition temperature (Tc, ~341 K in bulk material) changes to a metallic rutile phase (P42/mnm) [2]. The MIT can be activated thermally by heating or cooling through Tc, but has also been shown to be sensitive to electric field [3], infrared radiation [4], pressure [5], and strain [6]. The value of Tc is also highly tunable through doping [7] and growth of strained epitaxial thin films [8]. The massive 3-4 order of magnitude change in electrical resistivity (ρ) has drawn interest for possible device level applications. The transition is characterized by the coexistence of rutile metallic domains and a monoclinic insulating matrix that results in a smooth progression of the DC transport and dielectric properties as the MIT is induced. In this thesis, we present an overview of three novel transport experiments all of which involve epitaxial TiO2/VO2 films grown in a home-built low-pressure chemical vapor deposition system. The first experiment looks at the time evolution of the film resistance and capacitance as it settles for an extended period very near Tc. We report evidence that this settling process is characterized by at least two underlying relaxation processes. The second experiment involves the deposition and ferromagnetic resonance (FMR) characterization of TiO2/VO2/Ru/Py heterostructures. Our analysis indicates enhanced spin pumping into the VO2 layer when in the metallic state that is associated with an increase in the effective Gilbert damping parameter. Finally, we discuss the results of 1/f noise spectroscopy measurements collected on Hall-bar patterned VO2(100) films. We show that the processes governing noise along both crystallographic axes are identical and, in the metallic rutile state, follows a unique R-3 scaling behavior.
Electrical studies on nickel ferrites and hybrid organic-inorganic semiconductors
(University of Alabama Libraries, 2013) Hannan, Brian; LeClair, Patrick R.; University of Alabama Tuscaloosa
Electrical characterization measurements are performed on ferrite samples and a hybrid organic-inorganic p-n junction. Impedance and resistivity measurements are performed to investigate conducting properties of nickel ferrite and their dependence on growth temperature, composition, and sample dimensions. Impedance measurements are performed on an organic-inorganic semiconductor device and is demonstrated to behave as a p-n junction for potential use in technological applications.
Electron tunneling in the tight-binding approximation
(University of Alabama Libraries, 2016) Mackey, Frederick D.; Butler, W. H.; University of Alabama Tuscaloosa
In this thesis, we treat tunneling similar to a scattering problem in which an incident wave on a barrier is partially transmitted and partially reflected. The transmission probability will be related to the conductance using a model due to Landauer. Previously tunneling has been treated using a simple barrier model, which assumes the electron dispersion is that of free electrons. In this model it is not possible to investigate tunneling in the gap between a valence band and a conduction band. We shall remedy this limitation by using the tight-binding model to generate a barrier with a gap separating a valence band and a conduction band. To do this, we constructed a model consisting of semi-infinite chains of A atoms on either side of a semi-infinite chain of B-C molecules. The B-C chain has a gap extending between the onsite energy for the B atom and the onsite energy for the C atom. Tunneling through the gap has been calculated and plotted. We present exact closed form solutions for the following tunneling systems: (i) A-B interface, (ii) A-(B-C) interface, (iii) A-B-A tunnel barrier, (iv) A-(B-C) interface with the orbitals on B having s-symmetry and those on C having p-symmetry, (v) A-(B-C)-A tunnel barrier.
Exploring the magnetic phases in dysprosium by neutron scattering techniques
(University of Alabama Libraries, 2014) Yu, Jian; Mankey, Gary J.; University of Alabama Tuscaloosa
With one of the highest intrinsic magnetic moments (10.6 μ_B) among the heavy rare-earth elements, dysprosium (Dy) exhibits a rich magnetic phase diagram, including a few modulated magnetic phases. Aided by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the magnetic modulations propagate coherently over a long range, even with intervening non-magnetic layers. Neutron diffraction experiments were performed to determine the microscopic magnetic origin of the field induced phases in bulk Dy as a function of temperature, covering regions of the well-known ferromagnetic, helical antiferromagnetic, fan phases and several possible new phases suggested by previous studies. A short range ordered (SRO) fan phase was identified as the intermediate state between ferromagnetism and long range ordered (LRO) fan. The temperature range of a coexisting helix/fan phase was also determined. The magnetic phase diagram of Dy was thus refined to include the newly determined magnetic structures and the associated phase boundaries. Based on the period of the magnetic modulation and the average magnetization, the evolution of the spin arrangement upon heating was derived quantitatively for the modulated magnetic phases. To gauge the effect of nanostructuring on the magnetic phases with nonmagnetic Y layers, epitaxial Dy/Y superlattices of various layer thicknesses and repeats were fabricated by magnetron sputtering under carefully controlled conditions. X-ray characterizations confirmed that the crystallographic and interfacial qualities of the superlattices are comparable to those grown by MBE in previous studies. The macroscopic magnetization was characterized by magnetometry, whereas the microscopic magnetic structures were extracted from neutron diffraction and polarized neutron reflectometry (PNR) measurements. The ordering of helical modulation is sensitive to the interfacial roughness of the multilayer as well as the cooling histories. Off-specular PNR was applied the first time to characterize the helical domain structures in Dy/Y multilayers. The lateral correlation length in the helical magnetic structure was in the order of 100 nm.
Fabrication and ferromagnetic resonance study of epitaxial spinel ferrite films for microwave device applications
(University of Alabama Libraries, 2014) Pachauri, Neha; LeClair, Patrick R.; Gupta, Arunava; University of Alabama Tuscaloosa
Single crystalline nickel ferrite and lithium ferrite thin films have attracted a lot of research attention recently, because of their unique physical properties for practical applications in next generation technologies, such as monolithic microwave integrated circuits (MMIC) and multiferroic heterostructures. The properties of these materials are closely related to the specific growth method and can be tailored by factors like surface morphology, microstructure and chemical composition. Different thin film growth techniques have been investigated in the past few decades for the fabrication of single crystalline thin films of both these spinel ferrites. However, the difficulty to attain high quality, homogeneous epitaxial films with limited surface and bulk defects and low microwave loss still remains a challenging task. Moreover, there have been very limited reports on the detailed ferromagnetic resonance (FMR) studies of these single crystalline nickel and lithium ferrite thin films, which is an essential aspect to understand the relaxation in magnetization precession (microwave damping) in these materials. In this dissertation work, fabrication and study of structural, magnetic and FMR properties of single crystalline lithium ferrite (LiFe5O8) and nickel ferrite (NiFe2O4) films by direct liquid injection chemical vapor deposition (DLI-CVD) are studied in detail. The growth conditions, which play a crucial role in attaining the desired film morphology and stoichiometry, are optimized to achieve epitaxial, single crystalline lithium ferrite films having low ferromagnetic resonance linewidth coupled with excellent magnetic properties. A detailed ferromagnetic resonance (FMR) study has been done to identify as well as quantify the magnetic relaxation mechanisms in the `as-grown' nickel ferrite films. The broadband frequency, angle and temperature dependent measurements reveal the existence of two-magnon scattering as the active relaxation mechanism for the films.
Finding fossil galaxy system progenitors using strong gravitational lensing
(University of Alabama Libraries, 2018) Johnson, Lucas Edward; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Fossil galaxy systems have been described as some of the oldest systems in the universe, where the central galaxy has cannibalized most nearby member galaxies over cosmic time. While the progenitors to fossil systems have been predicted to exist in numerical simulations, little effort has gone into locating them until now. The discovery of fossil progenitors in the CASSOWARY catalog of strong gravitational lensing demonstrates that not all fossils are old, and their formation histories are more complex than originally thought. These progenitors have optical characteristics consistent with them being the transition phase between non-fossils and fossils, as we are observing the central galaxies in mid-assembly. We also identify a bias where systems acting as strong gravitational lenses are ~5 times more likely to be seen as fossils than non-lensing systems. Chandra X-ray images of eight CASSOWARY fossil progenitors show them being significantly over-luminous and hotter than comparable non-fossils which could be due to the strong lensing bias in our data, or fossils have characteristically deeper potential wells than non-fossils. Two progenitors were luminous enough to see a rise in gas temperature toward their cores which suggests these may be undergoing group mergers akin to the previously studied progenitor CSWA 2 verifying this as a viable fossil formation mechanism. Refinements to our original CASSOWARY data using the Hubble Space Telescope allowed us to disentangle complex merging environments at the centers of these eight progenitors, which further solidified the notion that progenitors are indeed transitioning toward fossil systems.

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