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1993

Iowa State University

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Articles 91 - 107 of 107

Full-Text Articles in Physics

Rheed Studies Of The Nucleation, Growth, And Mobility Of Ag Atoms On The Si(111)7x7 Surface , Kelly Ryan Roos Jan 1993

Rheed Studies Of The Nucleation, Growth, And Mobility Of Ag Atoms On The Si(111)7x7 Surface , Kelly Ryan Roos

Retrospective Theses and Dissertations

The low temperature and flux dependent growth of ultrathin Ag films on the Si(111)7 x 7 surface is studied with Reflection High-Energy Electron Diffraction (RHEED). The grazing incidence geometry of RHEED allows for an incident molecular beam normal to the surface, and makes it an ideal surface probe for studying ultrathin film growth in real time. Short-lived oscillations in the diffracted intensity are observed during Ag deposition at 150 K, indicating quasi-layer-by-layer growth mediated by adatom mobility. When the 150 K growth is performed over a wide range of deposition rates F, the peak intensity is observed to ...


Growth And Characterization Of Polysilicon Films Deposited By Reactive Plasma Beam Epitaxy , Behnam Moradi Jan 1993

Growth And Characterization Of Polysilicon Films Deposited By Reactive Plasma Beam Epitaxy , Behnam Moradi

Retrospective Theses and Dissertations

Polycrystalline silicon was deposited at low temperatures (400-550°C) by using a new technique, reactive plasma beam epitaxy. The technique consists of using an intense flux of hydrogen radicals, generated by an electron cyclotron resonance (ECR) source to promote nucleation and crystallinity at low temperatures. The film structure could be smoothly changed from polysilicon to amorphous by changing the flux of incoming H radicals on the surface of the sample. The flux of hydrogen radicals can be controlled by changing the deposition pressure, with lower pressures leading to a higher H radical flux and greater degree of crystallinity. The effect ...


Density Functional Studies: First Principles And Semi-Empirical Calculations Of Clusters And Surfaces , Susan Buthaina Sinnott Jan 1993

Density Functional Studies: First Principles And Semi-Empirical Calculations Of Clusters And Surfaces , Susan Buthaina Sinnott

Retrospective Theses and Dissertations

In the research presented here, various theoretical electronic structure techniques are utilized to analyze widely different systems from silicon clusters to transition metal solids and surfaces. For the silicon clusters, first principles density functional methods are used to investigate Si[subscript] N for N = 2-8. The goal is to understand the different types of bonding that can occur in such small clusters where the coordination of the atoms differs substantially from that of the stable bulk tetrahedral bonding. Such uncoordinated structures can provide a good test of more approximate theories that can be used eventually to model silicon surfaces, of ...


High-Spin [Gamma]-Ray Spectroscopy In 198bi, Superdeformation In 191au, And Multi-Photon Resonances In Low Energy E+E- Scattering , Duc Ta Vo Jan 1993

High-Spin [Gamma]-Ray Spectroscopy In 198bi, Superdeformation In 191au, And Multi-Photon Resonances In Low Energy E+E- Scattering , Duc Ta Vo

Retrospective Theses and Dissertations

The properties of low-and high-spin levels in [superscript]198Bi have been studied with the [superscript]181Ta([superscript]22Ne,5n)[superscript]198Bi reaction at 116 MeV and 120 MeV bombarding energies. Levels along and near the yrast lines, including one isomer, were established up to spin J ≈ 30\hbar. The main configurations of these states are suggested to be [pi] h[subscript]9/2 coupled to the known excited states in [superscript]197Pb. A sequence of states built on the excitation of the odd proton (coupled to one, two, or three quasi neutron-holes) was observed. In addition, at least two collective ...


Relativistic Two-Body Wave Equations , Alan James Sommerer Jan 1993

Relativistic Two-Body Wave Equations , Alan James Sommerer

Retrospective Theses and Dissertations

The history and conceptual development of relativistic two-body formalisms leading up to the Bethe-Salpeter equation are reviewed. The various three dimensional reductions of the Bethe-Salpeter equation, in particular the quasipotential equations, and methods for solving them are discussed. Previous studies which attempted to assess the relative merits of the various equations are examined, and a new criterion for judging the equations is proposed. The bound state energy predictions of the traditional quasipotential equations are compared with the predictions of fourth order perturbation theory at small coupling. The equations are ranked according to how well they reproduce perturbation theory at low ...


An Axial Approach To Detection In Capillary Electrophoresis , John Aaron Taylor Jan 1993

An Axial Approach To Detection In Capillary Electrophoresis , John Aaron Taylor

Retrospective Theses and Dissertations

The purpose of this research has been to develop and demonstrate visualization schemes which further the capabilities of capillary electrophoresis instrumentation. Our approach involves on-axis illumination of the compounds inside the capillary detection region and is applied to absorbance and fluorescence detection;Absorbance measurements were accomplished by focussing an incident beam of laser light into one end of the separation capillary. By utilizing signals collected over the entire length of the analyte band, this technique enhances the analytical path length of conventional absorbance detection sixty fold. The demonstrated instrument offers a fifteen-fold improvement in concentration limits of detection;Three fluorescence ...


Effects Of Randomly Rough Surfaces On Ultrasonic Inspection , Mehmet Bilgen Jan 1993

Effects Of Randomly Rough Surfaces On Ultrasonic Inspection , Mehmet Bilgen

Retrospective Theses and Dissertations

Surface finish of an industrial part affects its ultrasonic inspection and consequently the surfaces are often machined smooth before the evaluation. Ultrasonic inspection through smooth surfaces has been well studied and understood. A theoretical basis has been established for the characterization of interior cracks, voids and inclusions, and vast amount of literature exists. However, much less is known about quantitative ultrasonic inspection of such flaws in parts with rough surfaces, e.g. machine marks or "as-cast" surfaces. A question arises "When can an industrial part with randomly rough surfaces be inspected robustly using ultrasound?" This dissertation is aimed at answering ...


Topics In Electroweak Baryogenesis: The Sphaleron And T-Violation , Seungkoog Lee Jan 1993

Topics In Electroweak Baryogenesis: The Sphaleron And T-Violation , Seungkoog Lee

Retrospective Theses and Dissertations

Assuming that beyond the standard model physics can be parametrized in terms of high dimensional operators, we examine their effects on the energy of the sphaleron and the classical solution of the gauge and Higgs fields. In the absence of fermions, all of the six dimension 6 operators which are SU(2) symmetric have a small effect when calculated perturbatively. However, calculated non-perturbatively, one of the operators alters the boundary conditions of the equations of motion of the Higgs and gauge fields involved, and another operator gives rise to an abrupt change in the sphaleron energy at a small but ...


Random And Cooperative Sequential Adsorption, James W. Evans Jan 1993

Random And Cooperative Sequential Adsorption, James W. Evans

Physics and Astronomy Publications

Irreversible random sequential adsorption (RSA) on lattices, and continuum "car parking" analogues, have long received attention as models for reactions on polymer chains, chemisorption on single-crystal surfaces, adsorption in colloidal systems, and solid state transformations. Cooperative generalizations of these models (CSA) are sometimes more appropriate, and can exhibit richer kinetics and spatial structure, e.g., autocatalysis and clustering. The distribution of filled or transformed sites in RSA and CSA is not described by an equilibrium Gibbs measure. This is the case even for the saturation "jammed" state of models where the lattice or space cannot fill completely. However exact analysis ...


Transitions In The Kinetics And Steady States Of Irreversible A+Bc Surface-Reaction Models, B. Weng, W. H. Weinberg, James W. Evans Jan 1993

Transitions In The Kinetics And Steady States Of Irreversible A+Bc Surface-Reaction Models, B. Weng, W. H. Weinberg, James W. Evans

Physics and Astronomy Publications

The three-component irreversible surface-reaction model A+BCAC+1/2B2 with infinite reaction rates between nearest-neighbor adspecies is studied by Monte Carlo simulations. For a square lattice the system evolves to a degenerate poisoned state exponentially in time, except for a narrow range of pressures in which a reactive quasi-steady-state exists. The latter poisons very slowly in time. For a hexagonal lattice a true reactive steady state occurs for a range of pressures bordered by continuous and discontinuous transitions to poisoned states. The latter behavior is observed by adding the reaction channel A+BAB on a square lattice. Some ...


The Car‐Parking Limit Of Random Sequential Adsorption: Expansions In One Dimension, M. C. Bartelt, James W. Evans, M. L. Glasser Jan 1993

The Car‐Parking Limit Of Random Sequential Adsorption: Expansions In One Dimension, M. C. Bartelt, James W. Evans, M. L. Glasser

Physics and Astronomy Publications

We consider the irreversible random sequential adsorption of particles taking ksites at a time, on a one‐dimensional lattice. We present an exact expansion for the coverage, θ(t,k)=A0(t)+A1(t)k−1+A2(t)k−2+..., for times, 0≤tO(k), and at saturation t=∞. The former is new and the latter extends Mackenzie’s results [J. Chem. Phys. 37, 723 (1962)]. For these expansions, we note that the coefficients Ai≥1(∞) are not obtained as large‐t limits of the Ai≥1(t). Finally, we comment on the ...


Island-Size Scaling In Surface Deposition Processes, M. C. Bartelt, Michael C. Tringides, James W. Evans Jan 1993

Island-Size Scaling In Surface Deposition Processes, M. C. Bartelt, Michael C. Tringides, James W. Evans

Physics and Astronomy Publications

Diffusion-mediated nucleation and growth of islands during deposition occurs essentially irreversibly in a variety of systems. We provide a scaling theory for the full island-size distribution, both with the ratio of surface diffusion to deposition rates and with time. Scaling functions and exponents are determined by simulation and explained analytically by an unconventional rate-equation analysis. Experimental tests for theoretical predictions are discussed, including the scaling of superlattice beam profiles for diffraction studies of heteroepitaxial systems.


Zgb Surface Reaction Model With High Diffusion Rates, James W. Evans Jan 1993

Zgb Surface Reaction Model With High Diffusion Rates, James W. Evans

Physics and Astronomy Publications

The diffusionless ZGB (monomer–dimer) surface reaction model exhibits a discontinuous transition to a monomer‐poisoned state when the fraction of monomer adsorption attempts exceeds 0.525. It has been claimed that this transition shifts to 2/3 with introduction of rapid diffusion of the monomerspecies, or of both species. We show this is not the case, 2/3 representing the spinodal rather than the transition point. For equal diffusion rates of both species, we find that the transition only shifts to 0.5951±0.0002.


Kinetics Of The Monomer-Monomer Surface Reaction Model, James W. Evans, T. R. Ray Jan 1993

Kinetics Of The Monomer-Monomer Surface Reaction Model, James W. Evans, T. R. Ray

Physics and Astronomy Publications

The two-dimensional monomer-monomer (AB) surface reaction model without diffusion is considered for infinitesimal, finite, and infinite reaction rates k. For equal reactant adsorption rates, in all cases, simulations reveal the same form of slow poisoning, associated with clustering of reactants. This behavior is also the same as that found in simulations of the two-dimensional voter model studied in interacting-particle systems theory. The voter model can also be obtained from the dimer-dimer or monomer-dimer surface reaction models with infinitesimal reaction rate. We provide a detailed elucidation of the slow poisoning kinetics via an analytic treatment for the k=0+ AB reaction ...


Support Minimized Inversion Of Acoustic And Elastic Wave Scattering , Ali Safaeinili Jan 1993

Support Minimized Inversion Of Acoustic And Elastic Wave Scattering , Ali Safaeinili

Retrospective Theses and Dissertations

Inversion of limited data is common in many areas of NDE such as X-ray Computed Tomography (CT), Ultrasonic and eddy current flaw characterization and imaging;In many applications, it is common to have a bias toward a solution with minimum (L[superscript]2)[superscript]2 norm without any physical justification. When it is a priori known that objects are compact as, say, with cracks and voids, by choosing "Minimum Support" functional instead of the minimum (L[superscript]2)[superscript]2 norm, an image can be obtained that is equally in agreement with the available data, while it is more consistent ...


Acoustically Driven Integrated Microstrip Antennas And Electromagnetic Radiation From Piezoelectric Devices , Charles Forrest Campbell Jan 1993

Acoustically Driven Integrated Microstrip Antennas And Electromagnetic Radiation From Piezoelectric Devices , Charles Forrest Campbell

Retrospective Theses and Dissertations

For a receiver system to be considered integrated, the antenna and electronics need to be fabricated on a single semiconductor wafer. To avoid EMI problems, the circuitry must somehow be shielded from the environment where the antenna resides. The required shielding may be accomplished by separating the antenna on one side of the wafer from the electronics on the other side with a conducting ground plane. Energy may be acoustically coupled from the antenna to the circuitry through the ground plane via thin film piezoelectric transducers on either side of the wafer. The radiating side of the wafer would then ...


Frequency Conversion Analysis Of Noise In Heterojunction Bipolar Transistor Oscillators Including Periodically Modulated Noise Sources , Bradley Allen Kramer Jan 1993

Frequency Conversion Analysis Of Noise In Heterojunction Bipolar Transistor Oscillators Including Periodically Modulated Noise Sources , Bradley Allen Kramer

Retrospective Theses and Dissertations

A procedure which may be used to analyze the noise characteristics of HBT oscillators is presented. A large signal model of a HBT is developed based largely on the Gummel-Poon transistor model. A new base-emitter diffusion capacitance model is also generated which shows improved accuracy over conventional bipolar transistor models in characterizing HBTs. The large signal characteristics of the oscillator are then established using a time domain simulator. Conversion matrix theory is applied to the large signal model to generate an equivalent linear oscillator model which accounts for all of the frequency conversion effects which occur due to circuit nonlinearities ...