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Neutron Scattering Studies Of Reni2b2c (Re=Lu,Y,Ho,Er): Lattice Dynamics , Max Bullock Jan 1997

Neutron Scattering Studies Of Reni2b2c (Re=Lu,Y,Ho,Er): Lattice Dynamics , Max Bullock

Retrospective Theses and Dissertations

Inelastic neutron scattering techniques have been used to study the low energy phonon excitations in superconducting RENi2B2C (RE=Lu, Y) to further characterize the anomalous features observed by Kawano et al. (Phys. Rev. Lett., 77, 4628 (1996)) for RE=Y and Stassis et al. (Phys. Rev. B, 55, R8678 (1997)) for RE=Lu, when these systems enter the superconducting ground state. We find that above T c the frequencies of the [delta]4[[xi] 00] lowest-lying acoustic and optic phonon modes decrease with decreasing temperature, for [xi] close to the nesting vector [xi]m. In addition there is a shift ...


Modelling Polarized Light For Computer Graphics , Sairam Sankaranarayanan Jan 1997

Modelling Polarized Light For Computer Graphics , Sairam Sankaranarayanan

Retrospective Theses and Dissertations

The quality of visual realism in computer generated images is largely determined by the accuracy of the reflection model. Advances in global illumination techniques have removed to a large extent, some of the limitations on the physical correctness achievable by reflection models. While models currently used by researchers are physically based, most approaches have ignored the polarization of light. The few previous efforts addressing the polarization of light were hampered by inherently unphysical light transport algorithms. This paper, besides taking polarization of light into account in the reflection computation, also provides a basis for modelling polarization as an inherent attribute ...


The Gamma-Ray Energy Spectrum Of The Active Galaxy Markarian 421 , Jeffrey Alan Zweerink Jan 1997

The Gamma-Ray Energy Spectrum Of The Active Galaxy Markarian 421 , Jeffrey Alan Zweerink

Retrospective Theses and Dissertations

This is a study of the very high energy gamma-ray emission from Markarian 421. The data were taken by the Whipple Observatory 10 meter imaging atmospheric Cherenkov telescope (IACT) during the 1995/96 observing season. A method of extracting energy spectra using data obtained from IACTs has been previously developed at Iowa State University. This method is reviewed in the context of finding the spectrum of the Crab Nebula, the "standard candle" of very high energy gamma-ray astronomy. One important quantity that sets the energy scale of the Whipple 10 meter telescope is the electronic gain. This quantity has been ...


Chemical Diffusivity And Wave Propagation In Surface Reactions , Michael Joseph Tammaro Jan 1997

Chemical Diffusivity And Wave Propagation In Surface Reactions , Michael Joseph Tammaro

Retrospective Theses and Dissertations

Surface reactions on single crystal substrates have received a great deal of recent interest. These reactions are far-from-equilibrium, and can be modeled using a variety of contemporary techniques in statistical physics. Various models for CO-oxidation and a model for NO reduction by CO are studied using mean-field rate equations, reaction-diffusion equations, and Monte-Carlo simulations. Techniques are developed and applied to study the phase diagram and critical phenomena in the CO-oxidation models. Detailed theories of transport in disordered systems are developed and applied to describe chemical wave propagation in both the CO-oxidation models and the NO-reduction model.


Rheed Studies Of Vicinal Si(111) Surfaces And Ag Films Grown On Si(111) , Michael Kevin Stanley Jan 1997

Rheed Studies Of Vicinal Si(111) Surfaces And Ag Films Grown On Si(111) , Michael Kevin Stanley

Retrospective Theses and Dissertations

Reflection high energy electron diffraction (RHEED) was used to study the growth of silver films and the evolution of step structures on the silicon (111) surface. Silver films were deposited by molecular beam epitaxy onto the Si(111) 7 x 7 surface. Films deposited below room temperature showed RHEED intensity oscillation whose quality improved with decreasing temperature. RHEED oscillations were also improved by the application of an initial burst in the deposition flux. Such improvement and the temperature dependence of the oscillations is attributed to an increase in the island nucleation density;Vicinal silicon samples miscut from the (111) plane ...


Production Of [Sigma]0 In [Square Root Of S] = 91.2 Gev Qq̄ Events At Lep, Christopher Kenneth Legan Jan 1997

Production Of [Sigma]0 In [Square Root Of S] = 91.2 Gev Qq̄ Events At Lep, Christopher Kenneth Legan

Retrospective Theses and Dissertations

This thesis presents a measurement of one of the three isospin states of the JP = 1[over] 2+ octet [sigma] baryons, the [sigma]0. In addition, the analysis yields the first differential cross-section measurement of the [sigma]0 hyperon in e+e-→ q[bar]q events. The unique particle identification capabilities of the DELPHI detector at LEP are used to obtain an increased efficiency by extending the standard [lambda]-finding algorithm.;The average number of [sigma]0's produced per Z0 decay is calculated to beN([sigma]0)/Zsphad0=0.101 ± 0.008( stat) ± 0.014(syst) ± 0.007(extrap ...


Nonlinear Response Of Superconductors To Alternating Fields And Currents , Jason R. Mcdonald Jan 1997

Nonlinear Response Of Superconductors To Alternating Fields And Currents , Jason R. Mcdonald

Retrospective Theses and Dissertations

Nonlinearities caused by vortex pinning and weak links in superconductors are considered. The effects of vortex pinning will be treated in both the parallel and perpendicular geometries using the critical state model. The results for the parallel geometry will be used to calculate the nonlinear surface impedance of a hard type-II superconductor. The results for the perpendicular geometry will be used to treat harmonic generation (HG) and intermodulation distortion (IM) in superconducting transmission lines and resonators. Nonlinearites due to weak links will be modelled using both the resistively shunted junction model (RSJ) for small junctions, and the damped sine-Gordon equation ...


Simulational Studies Of Epitaxial Semiconductor Superlattices: Quantum Dynamical Phenomena In Ac And Dc Electric Fields , Joseph Patrick Reynolds Jan 1997

Simulational Studies Of Epitaxial Semiconductor Superlattices: Quantum Dynamical Phenomena In Ac And Dc Electric Fields , Joseph Patrick Reynolds

Retrospective Theses and Dissertations

Using high-accuracy numerical methods we investigate the dynamics of independent electrons in both ideal and realistic superlattices subject to arbitrary ac and/or dc electric fields. For a variety of superlattice potentials, optically excited initial wave packets, and combinations of ac and dc electric fields, we numerically solve the time-dependent Schrodinger equation. In the case of ideal periodic superlattice potentials, we investigate a long list of dynamical phenomena involving multiple miniband transitions and time-dependent electric fields. These include acceleration effects associated with interminiband transitions in strong fields, Zener resonances between minibands, dynamic localization with ac fields, increased single-miniband transport with ...


Two-Dimensional Pancake Vortices In A Finite Stack Of Magnetically Coupled Thin Superconducting Films , Thomas Lim Pe Jan 1997

Two-Dimensional Pancake Vortices In A Finite Stack Of Magnetically Coupled Thin Superconducting Films , Thomas Lim Pe

Retrospective Theses and Dissertations

This dissertation investigates the structure and behavior of two-dimensional (2D) pancake vortices in a stack of N Josephson-decoupled superconducting thin films under a magnetic field applied prependicular to the layers. This system of magnetically coupled superconducting films is proposed as a model for the multilayered high-Tc superconductors. Starting with the properties of one pancake vortex located in any of the N superconducting layers, an analytical expression is derived for the magnetic coupling force between pairs of 2D pancake vortex lattices residing in different layers, each having the same lattice structure, but displaced (not rotated) relative to each other. Using this ...


Dynamical Processes In The Middle Atmosphere As Observed From Upper Atmosphere Research Satellite Limb-Sounding Data , Douglas Ray Allen Jan 1997

Dynamical Processes In The Middle Atmosphere As Observed From Upper Atmosphere Research Satellite Limb-Sounding Data , Douglas Ray Allen

Retrospective Theses and Dissertations

This study uses data from the Upper Atmosphere Research Satellite (UARS) to investigate two topics in middle atmosphere dynamics: the "4-day wave" and transport processes deduced from atmospheric carbon monoxide. The 4-day wave is an eastward moving quasi-nondispersive feature with 4 day period occurring near the winter polar stratopause. Evidence of the 4-day feature is presented in UARS Microwave Limb Sounder (MLS) temperature, geopotential height, quasigeostrophic potential vorticity (PV), and ozone from the late southern winters of 1992 and 1993. Space-time spectral analyses reveal a double-peaked vertical temperature structure with an out-of-phase relationship between the two peaks. The height variation ...


X-Ray Resonant Exchange Scattering Of Rare-Earth Nickel Borocarbides , Carsten Detlefs Jan 1997

X-Ray Resonant Exchange Scattering Of Rare-Earth Nickel Borocarbides , Carsten Detlefs

Retrospective Theses and Dissertations

Investigations of the physical properties of the recently discovered rare-earth nickel borocarbides, RNi2B2C, continue to provide further insight into the interplay between superconductivity and magnetism. In several of these materials, long range magnetic order coexists with superconductivity over some temperature range, and the large variety of magnetic structures observed in neutron and x-ray experiments indicate long range order of localized magnetic moments coupled via a RKKY-type interaction. The strong anisotropic behavior present in some members of the family indicates that crystalline electric field energies also play an important role in the formation of the magnetically ordered state;We performed x-ray ...


A Search For High Mass Photon Pairs In Pp̄ [Right Facing Arrow][Small Greek Letter Gamma][Small Greek Letter Gamma]Jj Events At [Square Root Of S] = 1.8 Tev , Bryan Adrian Lauer Jan 1997

A Search For High Mass Photon Pairs In Pp̄ [Right Facing Arrow][Small Greek Letter Gamma][Small Greek Letter Gamma]Jj Events At [Square Root Of S] = 1.8 Tev , Bryan Adrian Lauer

Retrospective Theses and Dissertations

A search for new physics in the channel p[bar] p→[gamma][gamma] jj has been carried out. In some extended Higgs models, a light neutral scalar Higgs boson is produced with suppressed couplings to fermions and standard model (SM) strength couplings to vector bosons (bosonic Higgs), thus enhancing the H→[gamma][gamma] channel. We required one photon in the event with EspT[gamma]>30 GeV, |[eta][gamma] |15 GeV, |[eta][gamma] |30 GeV, |[eta]jet |15 GeV, |[eta]jet |<2.25. The final M[gamma][gamma] distribution is consistent with background and no resonance is observed. A 90(95)% confidence level (C.L.) upper limit cross section vs M[gamma][gamma] is calculated, which ranges from 0.60(0.73) pb for M[gamma][gamma] = 65GeV/c2 to 0.24(0.32) pb for M[gamma][gamma]=150GeV/c2. With standard model coupling strengths between the bosonic Higgs and vector bosons, and coupling strengths of zero between the bosonic Higgs and fermions a 90(95)% C.L. bosonic Higgs lower mass limit is set at 90(85) GeV/c2.


Tropospheric Constituent Variability Associated With Baroclinic Waves , Elizabeth Mary Stone Jan 1997

Tropospheric Constituent Variability Associated With Baroclinic Waves , Elizabeth Mary Stone

Retrospective Theses and Dissertations

Baroclinic waves are fundamental to tropospheric dynamics, and therefore it is of interest to understand their impact on atmospheric constituents. Here we examine the dynamics of the troposphere during baroclinic wave life cycles and study in detail their influence on constituent transport. We incorporate two methods of studying the atmosphere. The first is the study of observations made by remote sensing of the atmosphere by satellites. The second is the examination of the atmosphere through the use of an atmospheric general circulation model;In an observational case study, we used upper tropospheric water vapor measurements from the Upper Atmosphere Research ...


Optically Detected Magnetic Resonance Studies On [Pi]-Conjugated Polymers And Novel Carbon Allotropes , Jonathan Farel Partee Jan 1997

Optically Detected Magnetic Resonance Studies On [Pi]-Conjugated Polymers And Novel Carbon Allotropes , Jonathan Farel Partee

Retrospective Theses and Dissertations

The photophysics of poly(p-phenylene)-type ladder polymers (m-LPPP) and 2,5-dibutoxy poly(p-phenylene ethynylene) (DBO-PPE) films and solutions were studied by X-band photoluminescence detected magnetic resonance (PLDMR). Frequency resolved PLDMR measurements on LPPP, DBO-PPE, poly(3-hexylthiophene) (P3HT), poly(p-phenylene-vinylene) (PPV), and C70 are also reported and discussed;All the polymer samples exhibit three distinct features when excited at wavelengths [lambda]> 458nm: (i) A narrow PL-enhancing spin-1/2 polaron resonance, (ii) broad full- and (iii) half-field spin-1 triplet exciton powder patterns due to the [delta]m s = 1 and [delta]m s = 2 transitions among the triplet sublevels, respectively;The ...