Digital Commons Network^™

New Singularity In Anisotropic, Time-Dependent Solutions To Maximally Gauss-Bonnet Extended Gravity, T. Kitaura, James Thomas Wheeler

All Physics Faculty Publications

Among the solutions for anisotropic, time-dependent, maximally Gauss-Bonnet extended gravity, we find a class of curvature singularities for which the metric components remain finite. These new singularities therefore differ in type from the standard Kasner-like divergences expected for this class of theories. We study perturbative solutions near the singularity and show that there exist solutions with timelike paths that reach the singularity in finite proper time. Solving the equation of geodesic deviation in the same approximation, we show that the comoving coordinate system does not break down at the singularity. A brief classification of the corresponding singularity types in Robertson-Walker …

Symmetry Selection Rules For Vibrational Excitation By Resonant Electron Impact And A Unified Treatment Of Vibronic Coupling Between Resonances And To The Continuum: A Complete Symmetry Analysis Of Vibrational Excitation In Benzene, Gordon A. Gallup

Gordon Gallup Publications

A symmetry analysis of the contributions to the transition operator from vibronic coupling provides symmetry selection rules for vibrational excitation by resonant electron impact in polyatomic molecules. In the context of electron scattering vibronic coupling operates in two modes. These are named internal and external vibronic coupling. Internal vibronic coupling operates within and among the quasibound states giving rise to the resonances. External vibronic coupling operates between the quasibound states and the continuum. We discuss the differences, particularly with respect to the angular distribution of electron scattering. A complete symmetry analysis of vibrational excitation is given using benzene as an …

High-Voltage Mosfet Bipolar Square-Wave Generator, T. D. Usher, Grant A. Mcauley

Physics Faculty Publications

he construction of a high‐voltage (up to 1000 V) bipolar metal‐oxide‐semiconductor field‐effect transistor square‐wave generator is described. This generator is capable of producing both positive and negative going square waves with variable amplitude, repetition rate, and width. The circuit was designed for ferroelectrics research, however other applications are possible. The rise time of the prototype was 200 ns which was quite satisfactory for the present ferroelectric research project however the rise time can be decreased to 50 ns if necessary. The reader with a modest knowledge of electronics should be able to construct the circuit. Possible pitfalls and critical points …

Prominent Thermally Stimulated Current Trap In Low-Temperature-Grown Molecular-Beam Epitaxial Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

By far, the largest thermally stimulated current trap in molecular beam epitaxial GaAs grown at 200–250 °C is T₅, with an activation energy of 0.27 eV and most likely related to V_Ga. After an anneal at 300–350 °C, another trap T₆^☒ appears, with an activation energy of 0.14 eV and closely identified with V_As or the complex, V_As‐As_Ga. Proposed defect reactions in this As‐rich material include V_Ga+As_As→V_As‐As_Ga, and V_Ga+As_Ga→V_Ga‐As_Ga.

Annealing Dynamics Of Molecular-Beam Epitaxial Gaas Grown At 200°C, David C. Look, D. C. Walters, G. D. Robinson, J. R. Sizelove, M. G. Mier, C. E. Stutz

Physics Faculty Publications

By separating a 2‐μm‐thick molecular‐beam‐epitaxial GaAs layer grown at 200 °C from its 650‐μm‐thick substrate, we have been able to obtain accurate Hall‐effect and conductivity data as functions of annealing temperature from 300 to 600 °C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged As_Ga concentrations. …

Analytical Two-Layer Hall Analysis - Application To Modulation-Doped Field-Effect Transistors, David C. Look, C. E. Stutz, Christopher A. Bozada

Physics Faculty Publications

The classical magnetic‐field‐dependent Hall coefficient and conductivity equations are inverted to give the mobilities μ₁ and μ₂ and carrier concentrations n₁ (or p₁) and n₂ (or p₂) in two degenerate bands. The two‐band solution holds for arbitrary magnetic‐field strength as long as quantum effects can be ignored (i.e., kT≳ℏeB/m∗), and it is argued that the analysis can also be applied to two separate layers up to reasonable field strengths. The results are used to determine the two‐dimensional electron gas mobility and carrier concentration in a modulation‐doped field‐effect …

Long-Range Random Walks On Energetically Disordered Lattices, B. D. Bookout, Paul Ernest Parris

Physics Faculty Research & Creative Works

Although the master equation describing long-range random walks on an energetically disordered lattice is governed by a nonsymmetric transition matrix W, it may be mapped through a similarity transform onto an imaginary-time Schrödinger equation governed by a Hermitian (Hamiltonian) operator H₀ having a nondegenerate ground state. Under this mapping the diffusion constant D can be expressed in terms of the exact ground state energy of operators that are infinitesimally perturbed from H₀.

Light Injection From An Active Cladding To The Core Of A Fiber Optic Fluorosensor, Alvin Leon Bryant

Electrical & Computer Engineering Theses & Dissertations

A fluorosensor works on the basis of light injection from the clad into the core of an optical fiber. The sensitivity of such a sensor depends on the efficiency of light injection. Conditions necessary for a high injection efficiency are experimentally determined. The refractive index difference between the core and the clad,ⁿcore ^-nclad, must be maximized to achieve a high injection efficiency. Since an air clad has the least refractive index, a bare core fiber will have a maximum efficiency. Standard optical fibers used for communication employ a minimum value for ⁿcore ^-nclad, in order …

Absorption Imaging Studies On Intrinsic Gaas, John Samuel Kenney

Electrical & Computer Engineering Theses & Dissertations

A diagnostic technique has been developed which allows recording of the temporal and spatial development of electric field structures in GaAs photoconductive switches. The optical method is based on the Franz-Keldysh effect. Measurements were performed with a Nd:YAG activation laser and a GaAs laser diode as the probe laser. At low applied voltages a 100 atm wide region of high electric field is seen at the cathode only. With increasing voltage strong domain like structures emerge at the anode also. A calibration measurement shows that the electric fields in these domains are well above 50 kV/cm. At a threshold voltage, …

Second-Order Flexoelectric Effect In Chiral Nematic Liquid Crystals, D. W. Cronin, Rolfe G. Petschek, Eugene M. Terentjev

Faculty Scholarship

A molecular-statistical model for the second-order flexoelectric effect in chiral nematics is developed, accounting for additional chiral terms in the pair intermolecular potential U(1,2). It is shown by symmetry analysis, that there are four independent chiral terms in the flexoelectric polarization, proportional to the square of director gradients, ℘∼λ∇ n∇n. A molecular-statistical theory is developed for the second-order chiral flexoelectric constants λ. Their values, obtained in the mean-field approximation, depend on molecular dimensions, interaction parameters, and the degree of molecular chirality. Possible experimental situations are discussed where this effect can be observed.

Measurement Of The Microwave Dielectric Constant For Low‐Loss Samples With Finite Thickness Using Open‐Ended Coaxial‐Line Probes, G. Q. Jiang, W. H. Wong, E. Y. Raskovich, W. G. Clark, W. A. Hines, Jeff Sanny

Physics Faculty Works

This work addresses the effect of finite sample thickness on microwave dielectric constant measurements for thin, planar, low-loss samples using the open-ended coaxial-line probe method. Detailed measurements of the dielectric constant were carried out on a wide range of thicknesses of air samples which were backed by infinitely thick teflon and alumina dielectric media. The measurements were made at room temperature for various (50 Ω) coaxial-line dimensions, microwave frequencies 4-8 GHz, and power levels near a fraction of a mW. The results provide strong support for previously published theoretical calculations based on a boundary value problem which uses a spectral …

Photopolarimeter Based On Planar Grating Diffraction, R. M.A. Azzam, K. A. Giardina

Electrical Engineering Faculty Publications

A division-of-amplitude photopolarimeter (DOAP) is described that employs a diffraction grating in the conventional spectrometer orientation with the grating grooves normal to the plane of incidence. Four coplanar diffracted orders are used for polarimetric analysis to determine all four Stokes parameters of incident light simultaneously and virtually instantaneously (with the speed being determined solely by the photodetectors and their associated electronics); a fifth order is used for alignment by autocollimation or by use of a position-sensing quadrant detector. To sensitize the instrument for the +45° and -45° azimuths of incident linearly polarized light and for the handedness of incident circular …

Enhanced Retroreflection Of Light From Rough Metal Surfaces, Sivakumar Natarajan

Masters Theses

The optical constants n and k measured by Johnson and Christy (1972) for the noble metals (gold, silver and copper) are used to analyze the relationship between the surface characteristics and the peak height and half-width of the backscattering peak which is observed in the angular distribution of the intensity of diffusely reflected light at a vacuum-metal interface in a direction antiparallel to the original incident beam. Surfaces with surface roughness of heights less than 200 A to 300 A will be considered and a recently developed perturbation treatment will be used in these studies.

The backscattering of light from …

Avalanches And The Renormalization Group For Pinned Charge-Density Waves, Alan Middleton, Onuttom Narayan

Physics - All Scholarship

The critical behavior of charge-density waves (CDWs) in the pinned phase is studied for applied fields increasing toward the threshold field, using recently developed renormalization group techniques and simulations of automaton models. Despite the existence of many metastable states in the pinned state of the CDW, the renormalization group treatment can be used successfully to find the divergences in the polarization and the correlation length, and, to first order in an $\epsilon = 4-d$ expansion, the diverging time scale. The automaton models studied are a charge-density wave model and a ``sandpile'' model with periodic boundary conditions; these models are found …

Photoemission Studies Of Co- And Fe-Based Compounds With The Thmn12 Structure, A.S. Fernando, J.P. Woods, Sitaram Jaswal, B.M. Patterson, D. Welipitiya, A.S. Nazareth, David J. Sellmyer

David Sellmyer Publications

The electronic structures of NdFe₁₁Ti, NdCo₁₀V₂, and YCo₁₀Cr₂ have been studied with photoemission and spin-polarized calculations. The changes in these electronic structures upon nitrogenation have also been investigated. In the Fe compound, the Fe 3d states dominate the calculated density of states near the Fermi-edge, and the N(2p) peak is evident at around 6.3 eV. There is no shift in Fe 3d peaks visible in these compounds upon nitrogenation. Other than small energy shifts in the peak positions, there is an overall agreement between experimental data and the calculated …

Magnetic Properties Of Epitaxial Single Crystal Ultrathin Fe3Si Films On Gaas (001), Sy_Hwang Liou, S.S. Malhotra, J.X. Shen, M. Hong, J. Kwo, H.S. Chen, J.P. Mannaerts

Sy-Hwang Liou Publications

Magnetic properties of Fe₃Si films with thickness from 2 to 210 monolayers (ML) epitaxially grown on GaAs (001) were studied using a superconducting quantum interference device and alternating gradient force magnetometers. Growth of these single-crystal intermetallic compound films were carried out in a multichamber molecular beam epitaxy (MBE) system. The samples were covered in situ with Au 50 Å thick to prevent oxidation when the samples were removed from the MBE chamber. All the films are ferromagnetic even for samples as thin as 2 ML. The easy magnetization direction of the films is parallel to the film surface. …

Nanostructural Effects And Interface Magnetism In Co/Pd Multilayers, Ping He, Z.-S. Shan, John A. Woollam, David J. Sellmyer

David Sellmyer Publications

A series of Co/Pd multilayers were made by dc magnetron sputter deposition on Al foil substrates. For these multilayered samples, Co layer thicknesses were less than 4 Å and Pd layers were varied from 4 to 22 Å. Sputtering rates were controlled by either sputtering power (10–50 W) or Ar sputtering pressure (3–15 mTorr). In both cases, lower deposition rates yielded higher perpendicular coercivity up to 2.6 kOe. Structures of the samples were studied using conventional θ -2θ x-ray diffractometry (XRD). It has been found that magnetic properties such as coercivity and saturation magnetization are sensitive to interfacial structures. A …

Far-Ultraviolet Stellar Photometry: A Field In Monoceros, Edward G. Schmidt, George R. Carruthers

Edward Schmidt Publications

Far-ultraviolet photometry of stars in a field in Monoceros in the wavelength rang from 1230 Å to 1600 Å has been carried out using data from an electrographic Schmidt camera carried on a sounding rocket. Ultraviolet magnitudes were extracted for 602 objects in the field. Fifty-eight percent were tentatively identified with visible stars using the SIMBAD data base while another 25% are blends of objects too close together to separate with our resolution. Eleven of the uv objects coincide with parts of the star clusters NGC 2169, NGC 2244, and KGC 2264 in which individual stars cannot be resolved. As …

Photoquenching And Thermal Recovery Of A Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

A prominent thermally stimulated current peak T₅ appearing in semi‐insulating GaAs is shown to photoquench under infrared illumination, and then thermally recover at a rate r=2.0×10⁸ exp(−0.26 eV/kT) s⁻¹, exactly the same as that observed for EL2, within experimental error. Two possible explanations exist: (1) T₅ and EL2 are microscopically very similar, probably each with an As_Ga core; or (2) T₅ is an electron trap that only appears to quench and recover with EL2 because EL2 controls the electron lifetime. Several other traps show similar quenching and recovery behavior.

120kev Ar8+-Li Collisions Studied By Near Uv And Visible Photon Spectroscopy, E. Jacquet, P. Boduch, M. Chantepie, M. Druetta, D. Hennecart, X. Husson, D. Lecler, Ronald E. Olson, J. Pascale, N. Stolterfoht, M. Wilson

Physics Faculty Research & Creative Works

A spectroscopic analysis of light emitted in the 200-600 nm wavelength range by Ar⁷⁺, Ar⁶⁺ and Ar⁵⁺ ions after charge exchange in 120keV Ar⁸⁺-Li collisions is performed. Transitions with Δn = 1 and Δn = 2 for n = 8, 9, 10 and 11 states of Ar8 following single electron capture are identified and the production cross sections for n = 8 and n = 9 are deduced from emission cross sections and compared with those calculated by the three-body classical trajectory Monte-Carlo method. Lines due to double capture process were observed and identified …

Two Dimensional Power Spectral Estimation Using Constrained Iterative Spectral Deconvolution, Jerome Leslie Coggins

LSU New Orleans Theses and Dissertations

Two-dimensional power spectral estimation is an important tool for seismic data analysis and other applications. Some datasets, however, have a limited number of points in one or both dimensions. In seismic applications, there are typically fewer points in the spatial domain as compared to the temporal domain. Conventional spectral estimation techniques suffer from poor resolution on short datasets due to inherent smoothing or bias.

Ramaswamy and loup developed a one-dimensional method for the estimation of power spectra for short datasets. Constrained Iterative Spectral Deconvolution (CISD) greatly improves the power spectral resolution using a straight-forward algorithm. In a comparison to other …

Investigation Of Spin-Orbit Effects In The Excitation Of Noble Gases By Spin-Polarized Electrons, J. E. Furst, W. M. K. P. Wijayaratna, D. H. Madison, Timothy J. Gay

Timothy J. Gay Publications

We have measured the integrated Stokes parameters of the light emitted following the impact excitation by polarized electrons of the np⁵ (n+1)p [5/2] ₃ states in Ne (n=2), Ar (n=3), Kr (n=4), Xe (n=5), and the np⁵ (n+1) p[5/2] ₂ state in Kr. The near-threshold linear-polarization fractions η1 were consistent with zero for all of the J=3 states measured, providing no evidence of Mott scattering. For J=2 excitation, η₁ shows the clear influence of the intermediately coupled nature of the state. …

Unusual Thermal Stability Of A Site-Ordered Mc60 Rocksalt Structure (M=K, Rb, Or Cs), Q. Zhu, O. Zhou, J. E. Fischer, Andrew R. Mcghie, W. R. Romanow, Robert M. Strongin, M. A. Cichy, Amos B. Smith Iii

Chemistry Faculty Publications and Presentations

X-ray diffraction and differential scanning calorimetry of M_xC₆₀, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to K_xC₆₀ with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas …

Metastability In Two Dimensions And The Effective Potential, Mark Alford, Marcelo Gleiser

Dartmouth Scholarship

We study analytically and numerically the decay of a metastable phase in (2+1)-dimensional classical scalar field theory coupled to a heat bath, which is equivalent to two-dimensional Euclidean quantum field theory at zero temperature. By a numerical simulation we obtain the nucleation barrier as a function of the parameters of the potential, and compare it to the theoretical prediction from the bounce (critical bubble) calculation. We find the nucleation barrier to be accurately predicted by theory using the bounce configuration obtained from the tree-level (“classical”) effective action. Within the range of parameters probed, we found that using the bounce derived …

Contribution Of High-Order Rainbows To The Scattering Of A Gaussian Laser Beam By A Spherical Particle, James A. Lock

Physics Faculty Publications

I review the theory of the scattering of a Gaussian laser beam by a dielectric spherical particle and give the details for constructing a computer program to implement the theory. Computational results indicate that if the width of the laser beam is much less than the diameter of the particle and if the axis of the beam is incident near the edge of the particle, the fifth-, sixth-, and ninth-order rainbows should be evident in the far-field scattered intensity. I performed an experiment that yielded tentative evidence for the presence of the sixth-order rainbow.

Filtered Noise Can Mimic Low-Dimensional Chaotic Attractors, P. E. Rapp, Alfonso M. Albano, T. I. Schmah, L. A. Farwell

Physics Faculty Research and Scholarship

This contribution presents four results. First, calculations indicate that when examined by the Grassberger-Procaccia algorithm alone, filtered noise can mimic low-dimensional chaotic attractors. Given the ubiquity Of signal filtering in experimental investigations, this is potentially important. Second, a criterion is derived which provides an estimate of the minimum data accuracy needed to resolve the dimension of an attractor. Third, it is shown that a criterion derived by Eckmann and Ruelle [Physica D 56, 185 (1992)] to estimate the minimum number of data points required in a Grassberger-Procaccia calculation can be used to provide a further check on these dimension estimates. …

Monte Carlo Simulations Of Water-Ice Layers On A Model Silver Iodide Substrate: A Comparison With Bulk Ice Systems, James H. Taylor, Barbara N. Hale

Physics Faculty Research & Creative Works

Two water layers adsorbed on a model silver iodide basal face are simulated at nine temperatures from 150 to 425 K using Monte Carlo methods. The periodic unit cell of 96 internally rigid water molecules (interacting via the revised central-force potentials) couples to the rigid-substrate atoms via effective pair potentials with Lennard-Jones short-range and Coulomb long-range terms. The distribution of molecules perpendicular to the substrate exhibits layering, and individual layer structure factors, dipole moments, and "pseudodiffusion" coefficients are calculated. A complex temperature dependence with the two layers taking on different solidlike, quasiliquid, or liquid properties at the same T is …

New Magnets From Interstitial Intermetallics, J.M.D. Coey, Ralph Skomski

Ralph Skomski Publications

The outlook for improving the energy product of permanent magnets beyond the theoretical limit of 516 kJ/m³ for Nd₂Fe₁₄B is discussed. Most promising are nanostructured two-phase magnets consisting of an aligned hard-magnetic skeleton phase possessing exceptionally high uniaxial anisotropy and a soft-magnetic phase which assures a high magnetization. Brown’s paradox is discussed for inhomogeneous magnets, and micromagnetic calculations are used to predict energy products as high as 1090 kJ/m3 in suitable nanostructured Sm₂Fe₁₇N₃/Fe₆₅Co₃₅ composites, such as multilayers or disordered two-phase magnets with common c axis.

Non-Resonant Transfer And Excitation For S^13+ Projectiles Colliding With Neutral Gas Targets, Liang Duo

Masters Theses

Non-resonant Transfer and Excitation (NTE) by 20-70 Mev S,3+ projectiles in collisions with neutral gas targets of He, Ne and Ar is investigated. For NTE, positively charged projectiles undergo excitation and electron capture in a single collision in which the capture and excitation processes are independent events. The mechanisms involved in the NTE process are electron-nucleus interactions, and the combination of projectile excitation and capture is a two-step process. In this work, NTE cross sections, along with total K x-ray production cross sections and total single-electron capture and loss cross sections, were measured as a function of projectile energy. The …

Impurity Modes In Photonic Band Structures, Yi-Fen Chang

Masters Theses

In this thesis I study a theoretical model of one-dimensional dielectric array (a layered system of dielectric slabs) which forms a periodic dielectric system in which band gaps are opened in the frequency spectrum. Specifically, we study such a one-dimensional array with linear impurities introduced in the periodic dielectric system. In these impurity structures narrow resonant modes are found in the band gaps as impurity modes and these impurity modes change with the dielectric constant. This impurity problem is similar to the donor and acceptor state problem in electronic semiconductors. The calculations presented in this thesis are based on numerically …