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  Partner: UNT Libraries
 Department: Department of Physics
 Decade: 1990-1999
Precision Atomic Spectroscopy with an Integrated Electro- Optic Modulator and DBR Diode Laser at 1083nm

Precision Atomic Spectroscopy with an Integrated Electro- Optic Modulator and DBR Diode Laser at 1083nm

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Date: December 1999
Creator: Castillega, Jaime
Description: We have explored the use of recently developed high speed integrated electro optic modulators and DBR diode lasers as a tool for precision laser studies of atoms. In particular, we have developed a technique using a high speed modulator as a key element and applied it to the study of the fine structure of the 23P state of atomic helium. This state has been of long standing interest in atomic physics and its study has been the aim of several recent experiments using various precision techniques. We present our method and results, which will describe a new method for determining the fine structure constant, and lead to a precision test of atomic theory.
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Characterization and Field Emission Properties of Mo2C and Diamond Thin Films Deposited on Mo Foils and Tips by Electrophoresis

Characterization and Field Emission Properties of Mo2C and Diamond Thin Films Deposited on Mo Foils and Tips by Electrophoresis

Date: August 1999
Creator: Rouse, Ambrosio A., 1960-
Description: In this dissertation M02C and diamond films deposited by electrophoresis on flat Mo foils and tips have been studied to determine their suitability as field emission tips.
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Photoelectric Emission Measurements for CVD Grown Polycrystalline Diamond Films

Photoelectric Emission Measurements for CVD Grown Polycrystalline Diamond Films

Date: August 1999
Creator: Hassan, Tarek
Description: We examined CVD grown polycrystalline diamond films having different methane concentrations to detect defects and study the possible correlation between the methane concentration used during the growth process and the defect density. SEM and Raman results show that the amorphous and sp2 carbon content of the films increases with methane concentration. Furthermore, photoelectric emission from diamond is confirmed to be a two-photon process, hence the electrons are emitted from normally unoccupied states. We found that the photoelectric yield, for our samples, decreases with the increase in methane concentration. This trend can be accounted for in two different ways: either the types of defects observed in this experiment decrease in density as the methane concentration increases; or, the defect density stays the same or increases, but the increase in methane concentration leads to an increase in the electron affinity, which reduces the overall photoelectric yield.
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Picosecond Dynamics of Free-Carrier Populations, Space-Charge Fields, and Photorefractive Nonlinearities in Zincblende Semiconductors

Picosecond Dynamics of Free-Carrier Populations, Space-Charge Fields, and Photorefractive Nonlinearities in Zincblende Semiconductors

Date: August 1999
Creator: Stark, Thomas S.
Description: Generally, nonlinear optics studies investigate optically-induced changes in refraction or absorption, and their application to spectroscopy or device fabrication. The photorefractive effect is a nonlinear optical effect that occurs in solids, where transport of an optically-induced free-carrier population results in an internal space-charge field, which produces an index change via the linear electrooptic effect. The photorefractive effect has been widely studied for a variety of materials and device applications, mainly because it allows large index changes to be generated with laser beams having only a few milliwatts of average power.Compound semiconductors are important photorefractive materials because they offer a near-infrared optical response, and because their carrier transport properties allow the index change to be generated quickly and efficiently. While many researchers have attempted to measure the fundamental temporal dynamics of the photorefractive effect in semiconductors using continuous-wave, nanosecond- and picosecond-pulsed laser beams, these investigations have been unsuccessful. However, studies with this goal are of clear relevance because they provide information about the fundamental physical processes that produce this effect, as well as the material's speed and efficiency limitations for device applications.In this dissertation, for the first time, we time-resolve the temporal dynamics of the photorefractive nonlinearities in two zincblende semiconductors, ...
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Work Function Study of Iridium Oxide and Molybdenum Using UPS and Simultaneous Fowler-Nordheim I-V Plots with Field Emission Energy Distributions

Work Function Study of Iridium Oxide and Molybdenum Using UPS and Simultaneous Fowler-Nordheim I-V Plots with Field Emission Energy Distributions

Date: August 1999
Creator: Bernhard, John Michael
Description: The characterization of work functions and field emission stability for molybdenum and iridium oxide coatings was examined. Single emission tips and flat samples of molybdenum and iridium oxide were prepared for characterization. The flat samples were characterized using X-ray Photoelectron Spectroscopy and X-ray diffraction to determine elemental composition, chemical shift, and crystal structure. Flat coatings of iridium oxide were also scanned by Atomic Force Microscopy to examine topography. Work functions were characterized by Ultraviolet Photoelectron Spectroscopy from the flat samples and by Field Emission Electron Distributions from the field emission tips. Field emission characterization was conducted in a custom build analytical chamber capable of measuring Field Emission Electron Distribution and Fowler-Nordheim I-V plots simultaneously to independently evaluate geometric and work function changes. Scanning Electron Microscope pictures were taken of the emission tips before and after field emission characterization to confirm geometric changes. Measurement of emission stability and work functions were the emphasis of this research. In addition, use of iridium oxide coatings to enhance emission stability was evaluated. Molybdenum and iridium oxide, IrO2, were characterized and found to have a work function of 4.6 eV and 4.2 eV by both characterization techniques, with the molybdenum value in agreement with previous ...
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Evolution of Vacancy Supersaturations in MeV Si Implanted Silicon

Evolution of Vacancy Supersaturations in MeV Si Implanted Silicon

Date: May 1999
Creator: Venezia, Vincent C.
Description: High-energy Si implantation into silicon creates a net defect distribution that is characterized by an excess of interstitials near the projected range and a simultaneous excess of vacancies closer to the surface. This defect distribution is due to the spatial separation between the distributions of interstitials and vacancies created by the forward momentum transferred from the implanted ion to the lattice atom. This dissertation investigates the evolution of the near-surface vacancy excess in MeV Si-implanted silicon both during implantation and post-implant annealing. Although previous investigations have identified a vacancy excess in MeV-implanted silicon, the investigations presented in this dissertation are unique in that they are designed to correlate the free-vacancy supersaturation with the vacancies in clusters. Free-vacancy (and interstitial) supersaturations were measured with Sb (B) dopant diffusion markers. Vacancies in clusters were profiled by Au labeling; a new technique based on the observation that Au atoms trap in the presence of open-volume defects. The experiments described in this dissertation are also unique in that they were designed to isolate the deep interstitial excess from interacting with the much shallower vacancy excess during post-implant thermal processing.
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Charge State Distributions in Molecular Dissociation

Charge State Distributions in Molecular Dissociation

Date: December 1998
Creator: Renfrow, Steven N. (Steven Neal)
Description: The present work provides charge state fractions that may be used to generate TEAMS relative sensitivity factors for impurities in semiconductor materials.
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Fluorine Adsorption and Diffusion in Polycrystalline Silica

Fluorine Adsorption and Diffusion in Polycrystalline Silica

Date: December 1998
Creator: Jin, Jian-Yue
Description: The measurement of fluorine penetration into archeological flint artifacts using Nuclear Reaction Analysis (NRA) has been reported to be a potential dating method. However, the mechanism of how fluorine is incorporated into the flint surface, and finally transported into the bulk is not well understood. This research focuses on the study of the fluorine uptake phenomenon of flint mineral in aqueous fluoride solutions. Both theoretical and experimental approaches have been carried out. In a theoretical approach, a pipe-diffusion model was used to simulate the complicated fluorine transportation problem in flint, in which several diffusion mechanisms may be involved.
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Two-Fold Role of Randomness: A Source of Both Long-Range Correlations and Ordinary Statistical Mechanics

Two-Fold Role of Randomness: A Source of Both Long-Range Correlations and Ordinary Statistical Mechanics

Date: December 1998
Creator: Rocco, A. (Andrea)
Description: The role of randomness as a generator of long range correlations and ordinary statistical mechanics is investigated in this Dissertation. The difficulties about the derivation of thermodynamics from mechanics are pointed out and the connection between the ordinary fluctuation-dissipation process and possible anomalous properties of statistical systems is highlighted.
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On Chaos and Anomalous Diffusion in Classical and Quantum Mechanical Systems

On Chaos and Anomalous Diffusion in Classical and Quantum Mechanical Systems

Date: August 1998
Creator: Stefancich, Marco
Description: The phenomenon of dynamically induced anomalous diffusion is both the classical and quantum kicked rotor is investigated in this dissertation. We discuss the capability of the quantum mechanical version of the system to reproduce for extended periods the corresponding classical chaotic behavior.
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Structural and Photoelectron Emission Properties of Chemical Vapor Deposition Grown Diamond Films

Structural and Photoelectron Emission Properties of Chemical Vapor Deposition Grown Diamond Films

Date: August 1998
Creator: Akwani, Ikerionwu Asiegbu
Description: The effects of methane (CH4), diborone (B2H6) and nitrogen (N2) concentrations on the structure and photoelectron emission properties of chemical vapor deposition (CVD) polycrystalline diamond films were studied. The diamond films were grown on single-crystal Si substrates using the hot-tungsten filament CVD technique. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) were used to characterize the different forms of carbon in the films, and the fraction of sp3 carbon to sp3 plus sp2 carbon at the surface of the films, respectively. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the films. The photoelectron emission properties were determined by measuring the energy distributions of photoemitted electrons using ultraviolet photoelectron spectroscopy (UPS), and by measuring the photoelectric current as a function of incident photon energy.
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Synthesis and Study of Engineered Heterogenous Polymer Gels

Synthesis and Study of Engineered Heterogenous Polymer Gels

Date: August 1998
Creator: Chen, Yuanye
Description: This dissertation studies physical properties and technological applications of engineered heterogenous polymer gels. Such gels are synthesized based on modulation of gel chemical nature in space. The shape memory gels have been developed in this study by using the modulated gel technology. At room temperature, they form a straight line. As the temperature is increased, they spontaneously bend or curl into a predetermined shape such as a letter of the alphabet, a numerical number, a spiral, a square, or a fish. The shape changes are reversible. The heterogenous structures have been also obtained on the gel surface. The central idea is to cover a dehydrated gel surface with a patterned mask, then to sputter-deposit a gold film onto it. After removing the mask, a gold pattern is left on the gel surface. Periodical surface array can serve as gratings to diffract light. The grating constant can be continuously changed by the external environmental stimuli such as temperature and electric field. Several applications of gels with periodic surface arrays as sensors for measuring gel swelling ratio, internal strain under an uniaxial stress, and shear modulus have been demonstrated. The porous NIPA gels have been synthesized by suspension technique. Microstructures of newly ...
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On Delocalization Effects in Multidimensional Lattices

On Delocalization Effects in Multidimensional Lattices

Date: May 1998
Creator: Bystrik, Anna
Description: A cubic lattice with random parameters is reduced to a linear chain by the means of the projection technique. The continued fraction expansion (c.f.e.) approach is herein applied to the density of states. Coefficients of the c.f.e. are obtained numerically by the recursion procedure. Properties of the non-stationary second moments (correlations and dispersions) of their distribution are studied in a connection with the other evidences of transport in a one-dimensional Mori chain. The second moments and the spectral density are computed for the various degrees of disorder in the prototype lattice. The possible directions of the further development are outlined. The physical problem that is addressed in the dissertation is the possibility of the existence of a non-Anderson disorder of a specific type. More precisely, this type of a disorder in the one-dimensional case would result in a positive localization threshold. A specific type of such non-Anderson disorder was obtained by adopting a transformation procedure which assigns to the matrix expressing the physics of the multidimensional crystal a tridiagonal Hamiltonian. This Hamiltonian is then assigned to an equivalent one-dimensional tight-binding model. One of the benefits of this approach is that we are guaranteed to obtain a linear crystal with a ...
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Quantum-Confined CdS Nanoparticles on DNA Templates

Quantum-Confined CdS Nanoparticles on DNA Templates

Date: May 1998
Creator: Rho, Young Gyu
Description: As electronic devices became smaller, interest in quantum-confined semiconductor nanostructures increased. Self-assembled mesoscale semiconductor structures of II-VI nanocrystals are an especially exciting subject because of their controllable band gap and unique photophysical properties. Several preparative methods to synthesize and control the sizes of the individual nanocrystallites and the electronic and optical properties have been intensively studied. Fabrication of patterned nanostructures composed of quantum-confined nanoparticles is the next step toward practical applications. We have developed an innovative method to fabricate diverse nanostructures which relies on the size and a shape of a chosen deoxyribonucleic acid (DNA) template.
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Scanning Tunneling Microscopy of Epitaxial Diamond (110) and (111) Films and Field Emission Properties of Diamond Coated Molybdenum Microtips

Scanning Tunneling Microscopy of Epitaxial Diamond (110) and (111) Films and Field Emission Properties of Diamond Coated Molybdenum Microtips

Date: May 1998
Creator: Lim, Seong-Chu
Description: The growth mechanism of chemical vapor deposition (CVD) grown homo-epitaxial diamond (110) and (111) films was studied using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). In addition, the field emission properties of diamond coated molybdenum microtips were studied as a function of exposure to different gases.
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The Fractal Stochastic Point Process Model of Molecular Evolution and the Multiplicative Evolution Statistical Hypothesis

The Fractal Stochastic Point Process Model of Molecular Evolution and the Multiplicative Evolution Statistical Hypothesis

Date: May 1997
Creator: Bickel, David R. (David Robert)
Description: A fractal stochastic point process (FSPP) is used to model molecular evolution in agreement with the relationship between the variance and mean numbers of synonymous and nonsynonymous substitutions in mammals. Like other episodic models such as the doubly stochastic Poisson process, this model accounts for the large variances observed in amino acid substitution rates, but unlike other models, it also accounts for the results of Ohta's (1995) analysis of synonymous and nonsynonymous substitutions in mammalian genes. That analysis yields a power-law increase in the index of dispersion and an inverse power-law decrease in the coefficient of variation with the mean number of substitutions, as predicted by the FSPP model but not by the doubly stochastic Poisson model. This result is compatible with the selection theory of evolution and the nearly-neutral theory of evolution.
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Microstructural Studies of Dental Amalgams Using Analytical Transmission Electron Microscopy

Microstructural Studies of Dental Amalgams Using Analytical Transmission Electron Microscopy

Date: May 1997
Creator: Hooghan, Tejpal Kaur
Description: Dental amalgams have been used for centuries as major restorative materials for decaying teeth. Amalgams are prepared by mixing alloy particles which contain Ag, Sn, and Cu as the major constituent elements with liquid Hg. The study of microstructure is essential in understanding the setting reactions and improving the properties of amalgams. Until the work reported in this dissertation, optical microscopy (OM), scanning electron microscopy (SEM), and x-ray diffractometry (XRD) were used commonly to analyze amalgam microstructures. No previous systematic transmission electron microscopy (TEM) study has been performed due to sample preparation difficulties and composite structure of dental amalgams. The goal of this research was to carry out detailed microstructural and compositional studies of dental amalgams. This was accomplished using the enhanced spatial resolution of the TEM and its associated microanalytical techniques, namely, scanning transmission electron microscopy (STEM), x-ray energy dispersive spectroscopy (XEDS) and micro-microdiffraction (μμD). A new method was developed for thinning amalgam samples to electron transparency using the "wedge technique." Velvalloy, a low-Cu amalgam, and Tytin, a high-Cu amalgam, were the two amalgams characterized. Velvalloy is composed of a Ag₂Hg₃ (γ₁)/HgSn₇₋₉ (γ₂) matrix surrounding unreacted Ag₃Sn (γ) particles. In addition, hitherto uncharacterized reaction layers between Ag₃Sn(γ)/Ag₂Hg₃ (γ₂) and ...
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Model for Long-range Correlations in DNA Sequences

Model for Long-range Correlations in DNA Sequences

Date: December 1996
Creator: Allegrini, Paolo
Description: We address the problem of the DNA sequences developing a "dynamical" method based on the assumption that the statistical properties of DNA paths are determined by the joint action of two processes, one deterministic, with long-range correlations, and the other random and delta correlated. The generator of the deterministic evolution is a nonlinear map, belonging to a class of maps recently tailored to mimic the processes of weak chaos responsible for the birth of anomalous diffusion. It is assumed that the deterministic process corresponds to unknown biological rules which determine the DNA path, whereas the noise mimics the influence of an infinite-dimensional environment on the biological process under study. We prove that the resulting diffusion process, if the effect of the random process is neglected, is an a-stable Levy process with 1 < a < 2. We also show that, if the diffusion process is determined by the joint action of the deterministic and the random process, the correlation effects of the "deterministic dynamics" are cancelled on the short-range scale, but show up in the long-range one. We denote our prescription to generate statistical sequences as the Copying Mistake Map (CMM). We carry out our analysis of several DNA sequences, ...
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A Study of Nonlinear Dynamics in an Internal Water Wave Field in a Deep Ocean

A Study of Nonlinear Dynamics in an Internal Water Wave Field in a Deep Ocean

Date: December 1996
Creator: Kim, Won-Gyu, 1962-
Description: The Hamiltonian of a stably stratified incompressible fluid in an internal water wave in a deep ocean is constructed. Studying the ocean internal wave field with its full dynamics is formidable (or unsolvable) so we consider a test-wave Hamiltonian to study the dynamical and statistical properties of the internal water wave field in a deep ocean. Chaos is present in the internal test-wave dynamics using actual coupling coefficients. Moreover, there exists a certain separatrix net that fills the phase space and is covered by a thin stochastic layer for a two-triad pure resonant interaction. The stochastic web implies the existence of diffusion of the Arnold type for the minimum dimension of a non-integrable autonomous system. For non-resonant case, stochastic layer is formed where the separatrix from KAM theory is disrupted. However, the stochasticity does not increase monotonically with increasing energy. Also, the problem of relaxation process is studied via microscopic Hamiltonian model of the test-wave interacting nonlinearly with ambient waves. Using the Mori projection technique, the projected trajectory of the test-wave is transformed to a form which corresponds to a generalized Langevin equation. The mean action of the test-wave grows ballistically for a short time regime, and quenches back to ...
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A Study of Some Biological Effects of Non-Ionizing Electromagnetic Radiation

A Study of Some Biological Effects of Non-Ionizing Electromagnetic Radiation

Date: December 1996
Creator: Park, Young C. (Young Chul), 1960-
Description: The experimental studies of this work were done using a microwave cavity spectrometer, Escherichia coli (E-coli) bacteria, and other peripheral equipment. The experiment consists of two steps. First, a general survey of frequencies from 8 GHz to 12 GHz was made. Second, a detailed experiment for specific frequencies selected from the first survey were further studied. Interesting frequency dependent results, such as unusually higher growing or killing rates of E-coli at some frequencies, were found. It is also concluded that some results are genetic, that is, the 2nd, and 3rd subcultures showed the same growing status as the 1st cultures.
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The Physics of Gaseous Exposures on Active Field Emission Microcathode Arrays

The Physics of Gaseous Exposures on Active Field Emission Microcathode Arrays

Date: September 1996
Creator: Chalamala, Babu Reddy
Description: The interaction of active molybdenum field emission microcathode arrays with oxygen, water, carbon dioxide, methane, hydrogen and helium gases was studied. Experiments were setup to measure the emission characteristics as a function of gas exposures. The resulting changes in the surface work function of the tips were determined from the Fowler-Nordheim plots. The kinetics of the FEA-gas interaction were studied by observing the ion species originating from the array during and after gas exposures with a high resolution quadrupole mass spectrometer. With the work function data and the mass spectrometry information, the mechanisms responsible for emission degradation and subsequent device recovery after exposures have been determined. The data obtained was used in estimating the device lifetimes under various vacuum environments. Also it was found that the gas exposure effects are similar in dc and pulsed modes of operation of the arrays, thus permitting the use of dc mode testing as an effective acceleration method in establishing the device lifetimes under various vacuum conditions. The vacuum conditions required for the long term emission current stability and reliability of vacuum microelectronic devices employing FEAs are established. Exposure of Mo field emitter arrays to oxygen bearing species like oxygen, water and carbon dioxide ...
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Charge State Dependence of L-Shell X-Ray Production Cross Sections of ₂₈Ni, ₂₉Cu, ₃₀Zn, ₃₁Ga, and ₃₂Ge by Energetic Oxygen Ions

Charge State Dependence of L-Shell X-Ray Production Cross Sections of ₂₈Ni, ₂₉Cu, ₃₀Zn, ₃₁Ga, and ₃₂Ge by Energetic Oxygen Ions

Date: August 1996
Creator: Azordegan, Amir R. (Amir Reza)
Description: Charge state dependence of L-shell x-ray production cross sections have been measured for 4-14 MeV ¹⁶O^q (q=3⁺-8⁺) ions incident on ultra-clean, ultra-thin copper, and for 12 MeV ¹⁶O^q (q=3⁺-8⁺) on nickel, zinc, gallium and germanium solid foils. L-shell x-ray production cross section were measured using target foils of thickness ≤0.6 μg/cm² evaporated onto 5 μg/cm² carbon backings. Oxygen ions at MeV energies and charge state q were produced using a 3MV 9SDH-2 National Electrostatics Corporation tandem Pelletron accelerator. Different charge states, with and without K-vacancies, were produced using a post acceleration nitrogen striping gas cell or ¹²C stripping foils. L-shell x-rays from ultra-thin ₂₈Ni, ₂₉Cu,₃₀Zn,₃₁Ga, and ₃₂Ge targets were measured using a Si(Li) x-ray detector with a FWHM resolution of 135 eV at 5.9 keV. The scattered projectiles were detected simultaneously by means of silicon surface barrier detectors at angle of 45° and 169° with respect to the beam direction. The electron capture (EC) as well as direct ionization (DI) contributions were determined from the projectile charge state dependence of the target x-ray production cross sections under single collision conditions. The present work was undertaken to expand the measurements of L-shell x-ray production cross sections upon selected elements with low ...
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Temperature Dependency of Some of the Thermodynamical Properties of Aqueous Binary-Mixture Systems

Temperature Dependency of Some of the Thermodynamical Properties of Aqueous Binary-Mixture Systems

Date: August 1996
Creator: Zheng, Yueming
Description: The temperature dependence of the refractive index and the density of aqueous binary mixtures of water and ethyl alcohol (C₂H₅OH) were measured by using a modified Michelson interferometer and a narrow glass capillary tube over the temperature range of 278≤T≤353 K for solutions of 100, 75,65, 50, 25, 10 and 0 volume percent ethyl alcohol. The temperature was cycled over both increasing and decreasing directions to explore hysteresis in the cycling. The data are discussed and compared with the Lorentz-Lorenz (LL) formula. A more accurate formula which fits the experimental data better than the LL relation was derived. An attempt was made to determine the nature of the solvent-solute interaction through any changes that were found in the refractive index for He-Ne laser light and IR diode signals and to analyze the refractive index and density results to test the accuracy of the available mixing rules in predicting the refractive index values and the density of binary systems. Conductivity measurements (d. c.) over the temperature range 278≤T≤353 K of aqueous solutions of NaCl at various concentrations were made and used to establish transport properties of ions in solution. The dynamical properties of the electrolytes were used to establish the nature ...
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Spatiotemporal Properties of Coupled Nonlinear Oscillators

Spatiotemporal Properties of Coupled Nonlinear Oscillators

Date: July 1996
Creator: Chen, Ding
Description: Spatiotemporal properties of classical coupled nonlinear oscillators are investigated in this thesis. Chapter 1 gives an introduction to nonlinear lattices and to the concept of breathers, that are spatially localized and temporally periodic excitation in nonlinear lattices. The concept of anti-continuous limit that provides the basic methodology in probing spatiotemporal breather properties is discussed. In Chapter 2, the general approach for finding exact breather solutions from the anti-continuous limit is examined, and the rotating wave approximation(RWA) is applied to probe the spatial structure of static breathers. Numerical evidence reveals that the RWA relates the spatial structure of stable multi-breathers to a single breather of the same frequency. Chapter 3 presents linear stability analysis of static breathers and gives a systematic way to construct mobile breathers. Formation and collision properties of this moving breathers are also studied. Chapter 4 discusses dynamics of kinks and anti-kinks in hydrogen-bonded chains in the context of two-component soliton model. From molecular dynamics simulations with finite temperature, it is observed that, in a real system (eg. ice), a pair of kink and anti-kink can evolve into a moving-breather-like excitation. Chapter 5 is devoted to the understand of the effects of disorder in the Holstein model. The ...
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