Physical Sciences and Mathematics Commons^™

Ion-Ion Recombination In Mixtures Of Methane And Sulphur Hexafluoride, M. C. Cornell, Ian M. Littlewood

Physics Faculty Research & Creative Works

The authors have measured the recombination rate for positive and negative ions in Sulphur hexafluoride and in mixtures of methane and Sulphur hexafluoride in an electron beam sustained discharge. Measurements were made as a function of gas composition, gas pressure in the range 100 to 600 Torr, and reduced electric field strength in the range 12 to 150 Td. When plotted as a function of an effective ion temperature, defined by the drift velocities of the ions in the applied electric field, the rates in each of the gas mixtures follow the same Teff-1.6 power law. From comparisons with theories …

Electrooptic And Piezoelectric Measurements In Photorefractive Barium Titanate And Strontium Barium Niobate, Stephen Ducharme, Jack Feinberg, R.R. Neurgaonkar

Stephen Ducharme Publications

We have measured the low-frequency ("unclamped") electrooptic and piezoelectric coefficients in undoped BaTiO₃and Sr_xBa_1-xNb₂O₆ (x = 0.61) crystals using interferometric techniques. The contribution of the piezoelectric effect to the Pockels measurements is discussed. For an applied ac electric field in the range 0.1-200 V/cm, the electrooptic and piezoelectric effects are linear in the magnitude of the applied field and independent of its frequency in the range 10 Hz-100 kHz. The unclamped electrooptic coefficients of poled BaTiO₃ single crystals are r₁₃= 19.5 pm ±1 pm/V and r₃₃ = …

Theoretical Thermodynamic Properties Of Low Temperature Fluids, Zul Azhar Zahid Jamal

Masters Theses

The thermodynamic functions for low temperature fluids are computed using parametric integral equations and perturbation theory. Parametric integral equation N is applied to a low temperature Lennard-Jones gas. It is found that there is no significant improvement over the better known parametric integral equation C. The two parameter integral equation T is applied to a low temperature square-well potential and is found to be unsatisfactory at reduced temperatures of T*= 1.4 and 1.6, but quite accurate at T* = 2.2. The equation T results for T* = 2.2 are used as the reference system in perturbation theory computations. Tables of …

Effects Of A Magnetic Field On The Thermodynamics Of Dilute Classical Spin Chains, Guoqing Hu

Masters Theses

The thermodynamics of a diluted chain of ferromagnetic classical spin in the presence of an external magnetic field is determined in the limit of the continuum model. The magnetization, susceptibility, specific heat are determined for both the isotropic Heisenberg and the classical XY models as functions of temperature, magnetic field and magnetic concentration. Scaling law for the behavior of the thermodynamic quantities in field, temperature, and magnetic concentration are obtained.

The calculations will be based on the continuum model of classical spin which was solved for the infinite chain system by McGurn and Scalapino.

Thermodynamics Of The Diluted Spin Heisenberg Chain With Single Ion Anisotropy, Salmah Ahmed

Masters Theses

The thermodynamics of the one-dimensional diluted Heisenberg magnet of classical spins in the presence of single ion anisotropy is calculated. The magnetization, susceptibility, energy and specific heat are determined as functions of the magnetic concentration and single ion anisotropy for both ferromagnetic and antiferromagnetic couplings. Spin-spin correlation functions and the elastic scattering are also calculated for various values of anisotropy and spin concentration for the ferromagnetic system.

Generating Problem Sets With Word Processing Software, Jearl D. Walker

Physics Faculty Publications

No abstract provided.

The O-O Collision Cross-Section: Can It Be Inferred From Aeronomical Measurements?, R G. Burnside, C A. Tipley, Vincent B. Wickwar

All Physics Faculty Publications

No abstract provided.

Electric Field And Plasmadensity Measurements In The Strongly-Driven Daytime Equatorial Electrojet: 1. The Unstablelayer And Gradient Drift Waves, R. F. Pfaff, M. C. Kelley, E. Kudeki, Bela G. Fejer, K. D. Baker

Bela G. Fejer

Electric field and plasma density instrumentation on board a sounding rocket launched from Punta Lobos, Peru, detected intense electrostatic waves indicative of plasma instabilities in the daytime equatorial electrojet. Simultaneous measurements taken by the Jicamarca radar showed strong 3-m type 1 electrojet echoes as well as evidence of kilometer scale horizontally propagating waves. The in situ electric field wave spectra displayed three markedly different height regions within the unstable layer: (1) a two-stream region on the topside between 103 and 111 km where the electron current was considered to be strongest, (2) a gradient drift region between 90 and 106.5 …

The Condor Equatorial Electrojetcampaign: Radar Results, E. Kudeki, Bela G. Fejer, D. T. Farley, C. Hanuise

Bela G. Fejer

A review of the experimental and theoretical background to the Condor equatorial electrojet campaign is followed by the presentation and discussion of VHF radar interferometer and HF radar backscatter data taken concurrently with two rocket in situ experiments reported in companion papers (Pfaff et al., this issue (a, b). Both experiments were conducted in strongly driven periods with the on-line radar interferometer displaying signatures of what has been interpreted in earlier radar work (Kudeki et al., 1982) as kilometer scale gradient drift waves. Low-frequency density fluctuations detected by in situ rocket sensors confirm the earlier interpretation. VHF radar/rocket data comparisons …

Electric Field And Plasmadensity Measurements In The Strongly-Driven Daytime Equatorial Electrojet: 2. Two-Streamwaves, R. F. Pfaff, M. C. Kelley, E. Kudeki, Bela G. Fejer, K. D. Baker

Bela G. Fejer

Both primary and secondary two-stream (Farley-Buneman) waves have been detected by in situ electric field and plasma density probes in the strongly driven daytime equatorial electrojet over Peru. Simultaneous Jicamarca radar observations showed strong vertical and oblique 3-m type 1 echoes, also indicative of the two-stream mechanism. The rocket data show the two-stream region on the topside of the unstable layer to be situated between 103 and 111 km where the electron current was the strongest. This region was characterized by broadband plasma oscillations extending past 1 kHz in the rocket frame. Furthermore, above 106.5 km, where the electron density …

Dynamics Of Multiply Charged Ion-Atom Collisions: U³²⁺+Ne, Ronald E. Olson, J. Ullrich, H. Schmidt-Bocking

Physics Faculty Research & Creative Works

Measurements and calculations are presented for the mean recoil-ion energies of Nei+ produced in 1.4 MeV u^-1 (0.33 GeV) collisions of U³²⁺ with Ne. Recoil-ion charge states i=1-8 have been observed; the mean recoil energies are low and do not exceed 1 eV until i>6. Calculations employing a newly developed n-body classical trajectory Monte Carlo method are found to yield results in qualitative agreement with the recoil-ion experiment. Calculations also are presented for the ionisation and charge exchange cross sections, the projectile energy loss and the ejected-electron energy and angular spectra. The importance of fast ejected electrons …

Moment Analysis Of The Cluster-Size-Distribution Approach To Scaling During Coagulation, Thomas W. Taylor, C. M. Sorensen

Physics Faculty Publications

We study the temporal approach of a cluster size distribution to its asymptotic scaling form. By enforcing consistency between the distribution’s zeroth moment derived from both the Smoluchowski equation and the scaling distribution ansatz, we find values for the scaling exponents w and z in terms of the scaling exponent τ and the kernel homogeneity λ which are not equivalent to their asymptotic, scaling forms. The predicted values do agree well, however, with intermediate time values found in simulations by Kang, Redner, Meakin, and Leyvraz [Phys Rev. A 33, 1171 (1986)]. By enforcing consistency between all moment orders, the asymptotic …

Numerical Methods In Optics: A Course About Learning Physics Through Computing, James A. Lock

Physics Faculty Publications

Many advanced undergraduates find it difficult to connect abstract mathematical formalisms with the concrete physical phenomena that they describe. A course in optics from a numerical methods point of view is described. Its purpose is to exploit the students’ familiarity with computng in order to more effectively learn the physics involved in a number of realistic phenomena. The combination of demonstrations, computer calculations, and computer graphics display of the results can prove to be a useful tool in developing physical intuition in students.

Hypernuclear Photoproduction Via (Γ,K*), Kimberly Ann Hodgkinson

Masters Theses

The nuclear (γ,K^*) reaction is studied using plane wave impulse approximation (PWIA) and a distorted wave impulse approximation (DWIA), which includes full Coulomb and optical distortions. The model and calculations for each model are presented. For ¹²C(γ K^*)¹²B, the two approximation methods are compared at 2 energies: 1.47 Gev/c and 1.84GeV/c. ²⁰⁸Pb(γ, K^*)²⁰⁸ i s a lso examined. Distortion effects are found to greatly alter the magnitude of the cross sections, but the shape of the angular distributions remains. the same. Conclusions suggest further study.

A Study Of Ion Implanted Silicon Using Rutherford Backscattering Spectroscopy With Ion Channeling, Thomas J. Pollock

Masters Theses

Rutherford Backscattering Spectroscopy with channeling is the only nondestructive method for analyzing crystals which have been ion implanted. An RBS with channeling procedure has been established at the Accelerator Lab of Western Michigan University. It utilizes a tandem Van de Graaff accelerator for an incident helium beam, a goniometer for crystal alignment, and a computer for experimental control and data aquisition. State of the art measurements can be made with accuracies comparable to other labs. Specifically, ion implanted silicon was studied wherein crystal damage due to implanting was found to vary directly with implanting energy and fluence and was also …

Observation Of Substitutional Site Preference In A Quasicrystal And Implication On Local Structure, M. Eibschütz, M. E. Lines, H. S. Chen, J. V. Waszczak, G. C. Papaefthymiou, Richard B. Frankel

Physics

A combination of magnetic susceptibility and Mössbauer measurements on quasicrystalline i-Al₇₄Mn_20-xFe_xSi₆ (0.02≤x≤7.5) establishes that Mn atoms in i-Al₇₄Mn₂₀Si₆ occupy two distinct classes of sites, and that Fe substitutes for only one of them. The two classes are distinguished by the possession or otherwise of a localized magnetic moment. The data are consistent with a structure of interconnecting Mackay icosahedra (MI) in which localized moments are possessed only by Mn atoms adjacent to "broken" MI connections. The implied connectivity of the resulting MI network is close to …

Anaerobes Pumping Iron, Richard B. Frankel

Physics

No abstract provided.

Theoretical Study Of The Environmental Effects On The Hyperfine Fields Of Ni And Fe In Ni0.75fe0.25, H. Ebert, H. Winter, B. Gyorffy, Duane D. Johnson, F. J. Pinski

Duane D. Johnson

The dependence of the hyperfine fields Bhf of Ni and Fe in Ni0.75Fe0.25 on the surrounding atomic configuration has been studied by performing charge selfconsistent 6 shell-cluster Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) bandstructure calculations. By replacing the CPA- scatterers in the various shells around the central atom by Ni - or Fe-atoms, respectively, it could be shown that the hyperfine fields vary linearly with the number of Fe-atoms within a given shell and that the changes of Bhf due to simultaneous changes of the atomic configurations of different shells are additive. The changes of the hyperfine fields upon ordering of …

Equivalence Of Donor And Acceptor Fits To Temperature-Dependent Hall Data: General Case, David C. Look

Physics Faculty Publications

Recently, it was shown that the usual statistical-mechanical formulation used to fit carrier concentration versus temperature data cannot distinguish between the donor or acceptor nature of one single-charge-state center. Here we generalize that result to include any number of donor and acceptor centers, of arbitrary charge multiplicity, and also show how that by fitting one particular case (e.g., every center assumed to be a donor), all of the other possible cases can be immediately solved by inspection.

Defect Production In Electron-Irradiated, N-Type Gaas, David C. Look, J. R. Sizelove

Physics Faculty Publications

Temperature-dependent Hall-effect measurements have been performed on pure, n-type, vapor-phase epitaxial GaAs, irradiated by 1-MeV electrons at room temperature. The energies and production rates of two dominant defect centers, C₂ and C₃, are as follows: E₂ = E_C - 0.148, E₃ = E_C2 = 2.0 and τ₃ = 0.5 +/1 0.2 cm^-1, in good agreement with deep level transient spectroscopy (DLTS) data. However, the most important result of this study is a very high production rate, τ_AS ≅ +/- 1 cm^-1, for "shallow" acceptors (C …

Photoresistivity And Photo-Hall-Effect Topography On Semi-Insulating Gaas Wafers, David C. Look, E. Pimentel

Physics Faculty Publications

By placing a semi-insulating GaAs wafer on a fiat, rare-earth magnet, and irradiating the surface with two perpendicular slits of light to form a Greek cross configuration, it is possible to perform photoresistivity and photo-Hall-effect topography on the wafer. The technique is nondestructive in that the contacts are tiny, removable In dots which are placed only on the periphery. By varying the wavelength of the light, selective centers, such as EL2, can be mapped. We compare a 1.1-μ, photoexcited electron concentration map with a quantitative EL2 map on a 3-in. undoped, liquid-encapsulated Czochralski wafer.

Numerical Methods For Free-Free Radiative Transition Matrix Elements, Bo Gao, Anthony F. Starace

Anthony F. Starace Publications

Increasing interest in multiphoton absorption processes above the ionization threshold has led theorists to reexamine numerical techniques for calculating radiative transition matrix elements between states of a continuum electron moving at large radial distances in the field of an atom or an ion. Here it is shown that accurate free-free radial matrix elements may be obtained using the usual dipole length formula by means of a rotation at finite distance in the complex coordinate plane together with solution of the free-electron wave function's phase and amplitude at finite distance in the complex coordinate plane. The procedure is designed for use …

Normal-Metal Aharonov-Bohm Effect In The Presence Of A Transverse Electric Field, S. Washburn, H. Schmid, D. Kern, Richard A. Webb

Faculty Publications

The effects of transverse electric fields on the conductance fluctuations in an Sb loop have been studied. We show that the electric field can be used to tune the position (or phase) of Aharonov-Bohm oscillations as well as to alter the aperiodic conductance fluctuation patterns. We disucss two mechanisms which might cause the observed dependence of the fluctuation pattern on transverse electric field. The first is the electrostatic Aharonov-Bohm effect, and the second is the spatial shifting of the electron trajectories by the electric field.

Photoluminescence In Electrically Reversible (Semiconducting To Semiinsulating) Bulk Gaas, Phil W. Yu, David C. Look, W. Ford

Physics Faculty Publications

A photoluminescence study has been made of electrically reversible, bulk, liquid-encapsulated Czochralski GaAs at temperatures 2-300 K. The reversibility from the semiconducting to the semi-insulating state is made by slow or fast cooling, respectively, following a 5-h, 950°C heat treatment in an evacuated quartz ampoule. A donor level at E_c - 0.13 eV and two acceptor levels at Ev + 0.069 eV and Ev + 0.174 eV are produced after the heat treatment. Only the acceptor levels were detected by photoluminescence. A tentative model assigning the acceptor to the intrinsic defect pair V_Ga-Ga_As is discussed.

Angular-Differential Studies Of Excitation In Quasi-One-Electron Collisions At "High" Energy, E. Redd, Timothy Gay, D. M. Blankenship, John T. Park, Jerry Peacher, Denver G. Seely

Physics Faculty Research & Creative Works

Qualitative differences have been observed between two types of "quasi-one-electron" collision systems. We have studied valence-electron excitation at "high" energy (relative collision velocities up to 0.5 a.u.) in the Mg⁺+He and Na⁺+H collision systems, and find that while Mg⁺+He collisions are dominated by "direct" excitation, the Na⁺+H collisions exhibit significant "molecular" excitation, even at the highest velocities. This behavior can be understood in terms of the molecular structure of the respective collision complexes, and the energy separation between the ground and first excited states of the valence electron.

Angular-Differential Studies Of Excitation In Quasi-Oneelectron Collisions At "High" Energy, E. Redd, Timothy J. Gay, D. M. Blakenship, J. T. Park, J. L. Peacher, D. G. Seely

Timothy J. Gay Publications

Qualitative differences have been observed between two types of "quasi-one-electron" collision systems. We have studied valence-electron excitation at "high" energy (relative collision velocities up to 0.5 a.u.) in the Mg⁺+He and Na⁺+H collision systems, and find that while Mg⁺+He collisions are dominated by "direct" excitation, the Na⁺+H collisions exhibit significant "molecular" excitation, even at the highest velocities. This behavior can be understood in terms of the molecular structure of the respective collision complexes, and the energy separation between the ground and first excited states of the valence electron.

Raman Detection Of The Superconducting Gap In Ba-Y-Cu-O Superconductors, K.B. Lyons, Sy_Hwang Liou, M. Hong, H.S. Chen, J. Kwo, T.J. Negran

Sy-Hwang Liou Publications

We have utilized an iodine absorption cell to enable detailed Raman studies at low frequency (down to 15 cm^-1) in a thin film of the high-temperature superconductor Ba₂YCu₃O₇. Subtraction of spectra just above and well into the superconducting phase reveals a weak broad feature (maximum near 400 cm^-1, half width at half maximum approximately 250 cm^-1), which we tentatively interpret as due to scattering near the superconducting gap. The large width of the feature may be partially related to smearing of the transition, or to gap anisotropy. Its …

Site-Diagonal T-Matrix Expansion For Anisotropic Transport And Percolation On Bond-Disordered Lattices, Paul Ernest Parris

Physics Faculty Research & Creative Works

A study is made of the dynamical behavior of an electron or exciton undergoing anisotropic hopping on a d-dimensional bond-disordered lattice. Starting with a master equation for the site probabilities, an exact equation of motion is obtained for the probability currents that flow along the bonds connecting nearest-neighbor sites. Unlike the original master equation, the equation of motion which couples the microscopic currents contains the randomly distributed hopping rates in a form which is strictly site diagonal. The simplification that results leads to a new and exact expansion for the diffusion tensor in powers of an appropriately defined single-bond t …

A Study Of Flow Separation In Transonic Flow Using Inviscid And Viscous Cfd Schemes, James Andrew Rhodes

Mechanical & Aerospace Engineering Theses & Dissertations

A comparison of flow separation in transonic flows is made using various computational schemes which solve the Euler and the Navier-Stokes equations of fluid mechanics. The flows examined are computed using several simple two-dimensional configurations including a backward facing step and a bump in a channel. Comparison of the results obtained using shock fitting and flux vector splitting methods are presented and the results obtained using the Euler codes are compared to results on the same configurations using a code which solves the Navier-Stokes equations.

High Acceptor Production Rate In Electron-Irradiated N-Type Gaas - Impact On Defect Models, David C. Look

Physics Faculty Publications

Defect production rates have been studied in electron-irradiated GaAs by temperature-dependent Hall-effect (TDR) measurements. The TDH results agree well with deep level transient spectroscopy (DLTS) results for the wen-known electron traps E1, E2, and E3, but conclusively demonstrate a much higher production rate (4 ± 1 cm^-1) of acceptors below E3 than the total of all other DLTS traps. These findings strongly affect current defect models, and, e.g., are consistent with the existence of Ga sublattice damage, not seen before.