Physical Sciences and Mathematics Commons^™

Shifted X-Ray Photoelectron Peak In Molecular-Beam Epitaxial Gaas Grown At 200 Degrees C, David C. Look, J. T. Grant, J. R. Sizelove

Physics Faculty Publications

X‐ray photoelectron spectroscopic results show that molecular beam epitaxial GaAs grown at 200 °C has a reduced effective surface potential energy, about 0.5 eV, compared with the usual 0.7 eV. A Poisson analysis of the data, using parameters from Hall effect and absorption measurements, requires that the Fermi‐level‐controlling defect in this material must have a significantly lower activation energy than that of EL2, an unexpected result.

Photoquenching Of Hopping Conduction In Low-Temperature-Grown Molecular-Beam-Epitaxial Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

We have observed IR photoquenching of the hopping conduction in GaAs samples grown by molecular beam epitaxy at the low temperature of 250 °C and annealed at temperatures from 300 to 600 °C. A key element in the success of this study is removal of the layers from their substrates. The hopping conduction recovers at about 140 K, with a thermal activation energy of about 0.3 eV.

Gas Phase Interstitial Modification Of Rare-Earth Intermetallics, J.M.D. Coey, Ralph Skomski, S. Wirth

Ralph Skomski Publications

The gar phase interstitial modification of rare-earth intermetallics is studied. Net reaction energies for nitrogen in Sm₂Fe₁₇ and Nd(Fe₁₁T1) are U₀ = - 57 kJ/mole and U₀ = - 51 kJ/mole, respectively. The equilibrium nitrogen concentration is calculated as function of temperature and gas pressure using a simple lattice gas model.

For nitrogen in Sm₂Fe₁₇, refined diffusion parameters D₀ (N) = 1.02 mm²/r and Ea(N) = 133 kJ/mole, determined by thermopiezic analysis of the initial stage of nitrogen absorption, are used to calculate nitrogen profiles and …

Absolute Triple Differential Cross Section For Ionization Of Helium Near Threshold, T. Rösel, Jochen Röder, Leslie S. Frost, Kurt Jung, Hendrik Ehrhardt, Stephenie J. Jones, Don H. Madison

Physics Faculty Research & Creative Works

Absolute measurements with an accuracy of 22% and theoretical results in a distorted-wave Born approximation (DWBA) are reported for the triple-differential cross section for 26.6-eV electron-impact ionization of helium. An apparatus is used that allows all scattering angles to be independently varied for both coplanar and noncoplanar geometries. The measurements are compared with a DWBA calculation that includes exchange distortion in the calculation of the distorted waves, as well as with earlier calculations by Crothers [J. Phys. B 19, 463 (1986)] and Pan and Starace [Phys. Rev. Lett. 67, 185 (1991)]. Emphasis is placed on understanding the mechanisms for near-threshold …

Arnold M. Karo, John R. Creighton, John R. Hiskes, Fredrick Mcmurphy, John R. Hardy

John R. Hardy Papers

Arnold M. Karo died on 16 June 1991 at age 63, after a yearlong battle with leukemia. He was a theoretical chemist and solid-state physicist with the chemistry and materials science department at Lawrence Livermore National Laboratory.

The Cosmological Kibble Mechanism In The Laboratory: String Formation In Liquid Crystals, Mark Bowick, L. Chandar, Eric A. Schiff, Ajit M. Srivastava

Physics - All Scholarship

We have observed the production of strings (disclination lines and loops) via the Kibble mechanism of domain (bubble) formation in the isotropic to nematic phase transition of a sample of uniaxial nematic liquid crystal. The probablity of string formation per bubble is measured to be $0.33 \pm 0.01$. This is in good agreement with the theoretical value $1/ \pi$ expected in two dimensions for the order parameter space $S^2/{\bf Z}_2$ of a simple uniaxial nematic liquid crystal.

Binary Mixtures Near Surfaces, Patrick Dunn

Dissertations and Theses

This paper presents an approach to modeling a binary mixture near a surf ace. The approach used is based on statistical mechanics. The Cluster Variation (CV) method is used to solve an Ising-like model with a cluster size of two (neighboring lattice points). The free energy of the system is expressed in terms of the probabilities of occurrences of particular clusters. The CV method is first developed for a homogeneous system which leads to a set of three non-linear partial differential equations. The surface boundary is then introduced and the CV method is developed for this system which leads to …

Geometrical Phases And Symmetries In Dissipative Systems, Adam S. Landsberg

WM Keck Science Faculty Papers

A geometrical phase is constructed for dissipative dynamical systems possessing continuous symmetries. It emerges as the natural analog of the holonomy associated with the adiabatic variation of parameters in quantum-mechanical and classical Hamiltonian systems. In continuous media, the physical manifestation of this phase is a spatial shift of a wave pattern, typically a translation or rotation. An illustration associated with pattern formation in fluids is provided.

34th Rocky Mountain Conference On Applied Spectroscopy

Rocky Mountain Conference on Magnetic Resonance

Program and registration information for the 34th annual meeting of the Rocky Mountain Conference on Applied Spectroscopy, co-sponsored by the Colorado Section of the American Chemical Society and the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Denver, Colorado, August 2-6, 1992.

Review: 'Theory Of Dielectric Optical Waveguides,' 2nd Edition, By Dietrich Marcuse, Bradley D. Duncan

Electro-Optics and Photonics Faculty Publications

I suppose I ought to say up front that while preparing this review I often found myself feeling very much like a student evaluating his teacher. After all, it was, in part, the first edition of Dietrich Marcuse's Theory of Dielectric Optical Waveguides (among a handful of other similar texts) from which I first studied the principles of optical waveguide theory under the demanding, yet patient and graceful guidance of Dr. Ahmad Safaai-Jazi. Thus with the utmost respect for a teacher whom I have never met, I shall try to faithfully share my thoughts and feelings regarding the second edition …

Closed-Orbit Theory Of Oscillations In Atomic Photoabsorption Cross Sections In A Strongelectric Field. I. Comparison Between Theory And Experiments On Hydrogen And Sodium Above Threshold, J. Gao, John B. Delos, M. Baruch

Arts & Sciences Articles

Using a simple analytic formula from closed-orbit theory, we calculate photoabsorption cross sections of hydrogen and sodium in a strong electric field. The theoretical spectra show good agreement with experimental results. A scaled variable measurement is also suggested.

Construction Of A Lithium Vapor Oven For Electron Capture And Multiple Ionization Experiments, Oliver Woitke

Masters Theses

A Li vapor oven, to be used as a source of Li target atoms in electron capture and multiple ionization experiments, was designed and installed in the beamline of the Van de Graaff accelerator at Western Michigan University. The oven was tested using two different projectile beams, protons and He+. The test results indicate that Li evaporated from the oven in the form of a cloud rather than as a jet spray. For He+ colliding with Li it was found that the projectile single-electron loss yield is about three orders of magnitude larger than the single-electron capture yield, as expected …

Charge Conservation In The Incommensurate-Commensurate Transition Of Charge Density Waves, Jörg Kastrup

Masters Theses

W ith the complex continuation method, introduced by Gupta and Sutherland (1976), the grand partition function for the sine-Gordon model of charge density waves may be calculated without violation of charge conservation. Contrary to previous works by Okwamoto, Takayama and Shiba (1979) and Turkevich and Doniach (1982) the chemical potential becomes temperature dependent. By direct comparison of numerical results with those of the above mentioned authors it is shown that the incommensurate-commensurate transition does not occur in the previously predicted temperature range.

A Study Of The Transfer Matrix Method For The Classical Statistical Mechanics Of One Dimensional Systems, Dietmar R.A. Johlen

Masters Theses

Two formalisms using the transfer matrix technique, the first one by Gupta and Sutherland (1976) and the second one by Guyer and Miller (1979), are investigated and a proof is given for their equivalence.

Furthermore, it is pointed out that previous studies neglected the difficulties that arise from nonhermitian pseudo Hamiltonian. This work proves that the same results are obtained by taking into account the nonhermiticity of the pseudo Hamiltonian.

Thus, the transfer integral technique is extended to nonhermitian pseudo Hamiltonians.

Localization Of Light In One Dimensional Randomly Disordered Dielectric Medium, K. Balaji

Masters Theses

The concept of localization of electrons in semiconductors has been known ever since the 1960's when P. W. Anderson and N. F. Mott made detail investigations on the transport properties of alloys. At the end of the last decade, however, physicist started looking for localization in other systems. Specifically, they looked at the localization of light and acoustic waves in disordered systems. Localization is a property which arises for all wavelike excitations in disordered media.

In this study we analyze the localization of light in one dimensional randomly disordered dielectric medium using computer simulation (numerical integration method) technique. The results …

Traps In Semi-Insulating Inp Studied By Thermally Stimulated Current Spectroscopy, Z. Q. Fang, David C. Look, J. H. Zhao

Physics Faculty Publications

Traps in Fe‐doped semi‐insulating InP samples have been studied by thermally stimulated current spectroscopy with IR (hν≤1.12 eV) excitation at 81 K. The possible involvement of native defects in determining the compensation mechanisms is suggested based on the observation of other than the usual 0.64 eV Fe‐related activation energy for the dark current in one of the four samples supplied from different sources. A metastable behavior of traps in another sample was found and explained by a charge‐controlled defect reaction model. Three out of the six traps observed are suggested to be electron traps and one among the …

Development Of Membrane Film Fiber Optic Based Sensors For The Remote Monitoring Of The Quality Of Surface And Groundwater, Leonidas G. Bachas

KWRRI Research Reports

The objective of this study was the development of chromo- and fluoroionophores and their subsequent use in sensors capable of surface and groundwater monitoring. Several reagents (modified crown ethers and porphyrins) were synthesized based on principles of chemical recognition and used for metal and pH sensing. The modified crown ether reagents include a chromogenic or fluorogenic group. The selectivity of these reagents is controlled by the size of the crown ether ring and the type of the chromogenic or fluorogenic side-arm. In addition, a fluorogenic crown ether was synthesized that incorporates a fluorogenic side-arm and a perfluorinated carbon chain. The …

Ultrahigh Vacuum Chamber For Synchrotron X-Ray Diffraction From Films Adsorbed On Single-Crystal Surfaces, John R. Dennison, S. K. Wang, P. Dai, T. Angot, H. Taub, S. N. Ehrlich

All Physics Faculty Publications

An ultrahigh vacuum chamber has been developed for structural analysis of adsorbed films and single‐crystal surfaces using synchrotron x‐ray diffraction. It is particularly well suited for investigations of physisorbed and other weakly bound films. The chamber is small enough to transport and mount directly on a standard four‐axis diffractometer and can also be used independently of the x‐ray diffractometer. A low‐current, pulse‐counting, low‐energy electron diffraction/Auger spectroscopy system with a position‐sensitive detector enables in situ characterization of the film and substrate while the sample is located at the x‐ray scattering position. A closed‐cycle He refrigerator and electron bombardment heater provide controlled …

Adiabatic Cooling Of Atoms By An Intense Standing Wave, Jian Chen, J. Greg Story, Jeffrey J. Tollett, Randall G. Hulet

Physics Faculty Research & Creative Works

Lithium atoms channeled in the nodes of an intense standing-wave radiation field are cooled to near the recoil limit by adibatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root-mean-squared momentum of 2ħk in one dimension, where ħk is the momentum of a radiation-field photon. The data are compared with the results of a Monte Carlo simulation using a two-level atom model. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.

Closed-Orbit Theory Of Oscillations In Atomic Photoabsorption Cross Sections In A Strong Electric Field. Ii. Derivation Of Formulas, J. Gao, John B. Delos

Arts & Sciences Articles

A formula for photoabsorption cross sections of hydrogen and alkali-metal atoms in a static electric field is derived, based on the closed-orbit theory previously used to study hydrogen in a magnetic field. Electric fields are simpler than magnetic fields, because the classical motion is regular and closed orbits can be enumerated. In alkali metals the core modifies the relevant dipole matrix elements, and it produces additional phase shifts.

Critical Behavior Of Charge Density Waves Below Threshold: Numerical And Scaling Analysis, Alan Middleton, Daniel S. Fisher

Physics - All Scholarship

The critical behavior of pinned charge density waves (CDW's) is studied as the threshold for sliding is approached. Using the Fukuyama-Lee-Rice Hamiltonian with relaxational dynamics, the polarization and linear response are calculated numerically. ... On the irreversible approach to threshold, the response due to avalanches triggered by local instabilities dominates the polarizability, which diverges in one and two dimensions. Characteristic diverging length scales are studied using finite-size scaling of the sample-to-sample variations of the threshold field in finite systems and finite-size effects in the linear polarizability and the irreversible polarization. A dominant diverging correlation length is found which controls the …

Planar H2O Masers In Star-Forming Regions, Moshe Elitzur, David J. Hollenbach, Christopher F. Mckee

Physics and Astronomy Faculty Publications

The planar geometry of shocked material is the key property in enabling the high brightness temperatures of H20 masars in star-forming regions. We solve for the brightness temperature, the beaming angle, and the maser spot size for thin, saturated planar masers under the assumption that the velocity change across the maser due to ordered motions is small compared with the thermal or microturbulent line width. For a given set of physical parameters, the brightness temperature is essentially fully determined by the length of the velocity-coherent region in the shocked plane along the line of sight. The geometry in the transverse …

Electron Mean-Free Paths In The Alkali Metals, G. K. Wertheim, D. Mark Riffe, N. V. Smith, P. H. Citrin

All Physics Faculty Publications

Photoemission data in which the signal from the first atomic layer is well resolved from that of the bulk are used to determine accurately the kinetic-energy dependence of the inelastic-electron mean free path in the alkali metals. At the higher kinetic energies, the data are in very good agreement with the theory of Penn. Below about 10 eV, the mean free path in the heavier alkali metals drops markedly below the theoretical values. This is attributed to electron decay processes involving the unoccupied d bands.

Flux Trapping In Superconducting Pellets, Andreas Straub

Dissertations and Theses

This research concerns the effects on samples of nominal composition Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O_y which were exposed to hot, dense argon in a ballistic compressor.

The investigations were concentrated on two specimens which were exposed to hot, dense argon at about 1800 K (peak pressure 330 atm) and 1500 K (peak pressure 230 atm), respectively. Sample Bi #1 showed a completely melted surface structure after triple exposure in the ballistic compressor at 1800 K while the surface of sample Bi #7 was just partly melted after double exposure at 1500 K.

Changes …

Modification Of The Magnetic Flux-Line Interaction At A Superconductor's Surface, M. Cristina Marchetti

Physics - All Scholarship

The pair interaction between magnetic flux lines in a semi-infinite slab of an anisotropic type-II superconductor in an external field is derived in the London limit. The case where the applied field is normal to the superconductor/vacuum interface is considered. The presence of stray fields near the surface leads to an additional contribution to the repulsive interaction between flux lines that vanishes exponentially with the distance from the interface. The pair interaction is used to obtain the continuum elastic energy of a distorted semi-infinite flux-line array. The presence of the superconductor/vacuum interface yields surface contributions to the compressional and tilt …

Work-Function Anisotropies As An Origin Of Long-Range Surface Forces, Nancy Burnham, R Colton, H Pollock

Nancy A. Burnham

Unusual effects noticed in previous force microscopy data are explained by a model based on work-function anisotropies and their associated patch charges. Measurable forces between macroscopic bodies can be due to the interaction of patch charges, with important consequences in the fields of surface forces, contact mechanics, adhesion, Schottky barriers, and the surface properties of insulators.

Defect Models In Electron-Irradiated N-Type Gaas, B. Ziebro, Joseph W. Hemsky, David C. Look

Physics Faculty Publications

1 MeV electron irradiation has been performed in degenerate, n‐type (n≂2×10¹⁷ cm^-3), molecular beam epitaxial GaAs layers, and Hall effect measurements have been carried out during the irradiation in order to get accurate defect production data. The results have been fitted with statistical models, and are most consistent with the usual E1 (E_C-0.045 eV) and E2 (E_C-0.15 eV) levels being the (-/0) and (0/+) transitions of the As vacancy, respectively. Also, an acceptor well below EC-0.15 eV is produced at a much higher rate than that of E1 and E2.

Analysis Of Multifrequency Dispersive Optical Bistability And Switching In Nonlinear Ring Cavities With Large Medium-Response Times, Pawel Pliszka, Partha P. Banerjee

Electrical and Computer Engineering Faculty Publications

Using a simple model of a ring cavity comprising a cubically nonlinear medium, we analyze dispersive optical bistability in the presence of more than one spectral component. We show the phenomenon of so-called competition for resonance. In addition to presenting cavity characteristics for the cases of two and three different frequencies, we also discuss the general method for finding steady-state solutions and checking their stability. A simple and efficient algorithm, based on a relaxation method, is devised to find steady-state solutions satisfying appropriate boundary conditions. The relaxation dynamics is physically related to a finite response time of the medium.

Structures Of Selected Boranes And Carboranes, Sunwoo Lee, Peter A. Dowben, A.T. Wen, A.P. Hitchcock, John A. Glass Jr., James T. Spencer

Peter Dowben Publications

Nido-pentaborane(9) (B₅H₉), 1, nido-2,3-diethyl-2,3-dicarbahexaborane(8)[( C₂H₅)₂C₂B₄H₆], 2, nido-decaborane (14) (B₁₀H₁₄), 3, closo- 1,2-dicarbadodecaborane(12) (H₂C₂B₁₀H₁₀), 4, can be used as possible source compounds for boron and boron carbide thin film deposition. Inner shell electron energy-loss spectroscopy (ISEELS) studies of the boron Is and carbon Is core excitations of gas phase species have been undertaken so as to characterize these molecular precursors at solid surfaces. The near edge structure of ISEELS provides a good "fingerprint" …

Electronic Structure Of Borane Cage Molecules Adsorbed On Si(111), Sunwoo Lee, Dongqi Li, Sophie M. Cendrowski-Guillaume, Peter A. Dowben, F. Keith Perkins, S.P. Frigo, R.A. Rosenberg

Peter Dowben Publications

We compare the valence band photoemission resulting from molecular adsorption of nido-decaborane(14) (B₁₀H₁₄) and nido-2,3-diethyl-2,3-dicarbahexaborane on Si(111) at 100 K. The relative shift of the apparent valence band edge with increasing molecular coverage is consistent with the relative energy difference in the highest occupied molecular orbital for these two molecules as inferred from modified neglect of differential overlap calculations. The results suggest that the screening is very similar, despite the very different molecular structures of these two nido cage molecules.