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Articles 33181 - 33210 of 36554

Full-Text Articles in Physical Sciences and Mathematics

Pyro-Electro-Optic Phase Gratings, Stephen Ducharme Nov 1991

Pyro-Electro-Optic Phase Gratings, Stephen Ducharme

Stephen Ducharme Publications

A new physical mechanism is proposed for generating transient phase gratings in transparent pyroelectric materials. The mechanism combines the pyroelectric and electro-optic effects to convert spatial intensity variations into transient phase gratings. The grating diffraction efficiency increases with a figure of merit that is proportional to the pyroelectric, electro-optic, and absorption coefficients and is inversely proportional to the specific heat. The grating response time is proportional to the specific heat and inversely proportional to the thermal conductivity. Diffraction efficiencies that approach unity are predicted in several readily available electro-optic materials with modest pulse energies from Q-switched Nd:YAG lasers. Observation …

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The Behlen Observatory Variable Star Survey: First Results, Edward G. Schmidt Nov 1991

The Behlen Observatory Variable Star Survey: First Results, Edward G. Schmidt

Edward Schmidt Publications

Various light curve parameters are presented for the first 93 objects in a photometric survey of poorly studied variable stars. Stars from the General Catalogue of Variable Stars (GCVS) fainter than tenth magnitude, north of the equator which were classified as pulsating variables (but excluding the long period, semiregular, and irregular variables) were included. It is shown that two parameters, the acuteness and the rise time, are useful for characterizing the light curves. Several criteria are applied to the photometric data to test the classification of the stars. For 26 stars new or improved classifications are suggested. In particular, there …

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Change Of Magnetic Properties Of Th₂Fe₁₇ Due To Interstitial Solution Of C And N, T. H. Jacobs, Gary J. Long, Oran Allan Pringle, Fernande Grandjean, K. H. Buschow Nov 1991

Change Of Magnetic Properties Of Th₂Fe₁₇ Due To Interstitial Solution Of C And N, T. H. Jacobs, Gary J. Long, Oran Allan Pringle, Fernande Grandjean, K. H. Buschow

Chemistry Faculty Research & Creative Works

The changes in magnetic and crystallographic properties in the series Th2Fe17Cx and Th2Fe 17Nx have been studied. The changes in the latter series were also studied by 57Fe Mössbauer spectroscopy. The ultimate enhancements of the saturation moments in these series are 13% and 24%, respectively. This is much larger than observed previously in the corresponding rare-earth compounds, although the Curie temperature enhancements are of comparable magnitude. In the Th-Fe-C system a novel compound of the BaCd 11 type was found, having a Curie temperature around 370 K.

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Dmsp F7 Observations Of A Substorm Field‐Aligned Current, R. E. Lopez, Harlan E. Spence, C. Meng Nov 1991

Dmsp F7 Observations Of A Substorm Field‐Aligned Current, R. E. Lopez, Harlan E. Spence, C. Meng

Physics & Astronomy

In this paper we present observations of a substorm field-aligned current (FAC) system that DMSP F7 traversed just after 0300 UT on April 25, 1985. Ground magnetometer data show that a major substorm was in progress at that time and that DMSP F7 flew through a region of predominantly upward FAC. The DMSP F7 magnetic field data are consistent with this interpretation. The precipitating particle data suggest that there were three distinct large-scale FAC systems. In ascending latitude these were a downward current, an upward current, and a paired upward/downward current system. We identify the first current, which was coincident …

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Anyonic Superconductivity In A Modified Large-U Hubbard Model, Asim Gangopadhyaya, Prasanta K. Panigrahi Nov 1991

Anyonic Superconductivity In A Modified Large-U Hubbard Model, Asim Gangopadhyaya, Prasanta K. Panigrahi

Physics: Faculty Publications and Other Works

A modified large-U Hubbard model at half filling is analyzed by a mean-field approach. Preserving a local U(1) symmetry of the action, the fluctuations about half filling are studied in the spirit of the commensurate-flux-phase condition. The fluctuations then contribute a Chern-Simons term to the tree-level Lagrangian with a coefficient appropriate to that of a half fermion. With the Coulomb repulsion term, we study the low-energy excitations of the model and show the existence of superconductivity in the presence of a four-Fermi interaction term.

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The Effect Of A Tilted Magnetic Field On The Equilibrium Of A Pure Electron Plasma, Grant W. Hart Nov 1991

The Effect Of A Tilted Magnetic Field On The Equilibrium Of A Pure Electron Plasma, Grant W. Hart

Faculty Publications

If the magnetic field in a pure electron plasma containment device is not aligned with the axis of the conducting walls, the electrons in the device will accumulate at the ends of the plasma where the magnetic field lines come closest to the walls and the electrons bound to the field lines can be closest to their image charges. If the plasma is also offset radially from the center (as with an l=1 diocotron mode), then more density will accumulate at one end than the other. As the plasma revolves around the center, the electrons will slosh from one end …

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Inorganic Particles Produced By Microorganisms, Richard B. Frankel Oct 1991

Inorganic Particles Produced By Microorganisms, Richard B. Frankel

Physics

The processes by which microorganisms, particularly bacteria, produce inorganic mineral particles are reviewed.

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Structure, Morphology And Growth Of Biogenic Greigite (Fe3S4), Brigid R. Heywood, Stephen Mann, Richard B. Frankel Oct 1991

Structure, Morphology And Growth Of Biogenic Greigite (Fe3S4), Brigid R. Heywood, Stephen Mann, Richard B. Frankel

Physics

Several species of aquatic bacteria are known to exploit the earth's geomagnetic field as a means of directing their motion towards suitable habitats. A feature common to these bacteria is the presence of discrete intracellular magnetic inclusions, magnetosomes, aligned in chains along the long axis of the organism. The size and orientation of the individual magnetic particles imparts a permanent magnetic dipole moment to the cell which is, in turn, responsible for the magnetotactic response. In all species examined to date the magnetic particles have been found to be well-ordered, single domain, membrane-bounded crystals with reproducible, species-specific morphologies. Until recently, …

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Elastic Magnetic Electron Scattering From ' C At Q2= 1 Gev2/C, R A. Miskimen, H Baghaei, M Frodyma, R S. Hicks, K S. Lee, Gerald Alvin Peterson Oct 1991

Elastic Magnetic Electron Scattering From ' C At Q2= 1 Gev2/C, R A. Miskimen, H Baghaei, M Frodyma, R S. Hicks, K S. Lee, Gerald Alvin Peterson

Gerald Alvin Peterson

Electron scattering from C was measured at a momentum transfer of 5.08 fm . No electron events were observed in the vicinity of the elastic peak, giving an upper limit for the elastic cross section of 1.2 x 10 cm /sr with a confidence level of 90%. At this momentum transfer, the square of the elastic M1 form factor apparently continues to fall exponentially with q. Comparison of the data with shell-model calculations indicates that admixtures of 2hu configurations in the C ground state cannot entirely explain the high-q enhancement of the Ml form factor relative to 1p-shell calculations.

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Scaling Behavior Of Atomic Trajectories In Confined Fluids, Amy Lisa Graves, B. J. Berne Oct 1991

Scaling Behavior Of Atomic Trajectories In Confined Fluids, Amy Lisa Graves, B. J. Berne

Physics & Astronomy Faculty Works

We study the dependence of D, a quantity that has previously been associated with the fractal dimension of an atomic trajectory, on the scale on which it is measured. Single-particle and relative trajectories are generated by molecular-dynamics simulations of Lennard-Jones atoms permanently trapped in a spherical pore. Transient trapping is studied with a generalized Langevin model of dynamics. Confinement of trajectories allows D to exceed 2; in the case of permanent trapping, D diverges.

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Internal Caustic Structure Of Illuminated Liquid Droplets, James A. Lock, Edward A. Hovenac Oct 1991

Internal Caustic Structure Of Illuminated Liquid Droplets, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

The internal electric field of an illuminated liquid droplet is studied in detail with the use of both wave theory and ray theory. The internal field attains its maximum values on the caustics within the droplet. Ray theory is used to determine the equations of these caustics and the density of rays on them. The Debye-series expansion of the interior-field Mie amplitudes is used to calculate the wave-theory version of these caustics. The physical interpretation of the sources of stimulated Raman scattering and fluorescence emission within a liquid droplet is then given.

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Solid State Proton Spin Relaxation And T-Butyl And Methyl Group Reorientation In 1-Bromo-2,4,6-Tri-T-Butylbenzene, Anne M. Fry, Peter A. Beckmann, Albert J. Fry, Peter C. Fox, Ari Isenstadt Oct 1991

Solid State Proton Spin Relaxation And T-Butyl And Methyl Group Reorientation In 1-Bromo-2,4,6-Tri-T-Butylbenzene, Anne M. Fry, Peter A. Beckmann, Albert J. Fry, Peter C. Fox, Ari Isenstadt

Physics Faculty Research and Scholarship

We have used the solid state proton spin relaxation technique to investigate the barriers for methyl and t-butyl group reorientation in polycrystalline 1-bromo-2,4,6-tri-t-butylbenzene. The barriers in the range of 15-19 kJ/mol (3-5 kcal/mol) are compared with those found in related molecules. It is shown that the neighboring ring bromine atom has an effect on the barrier for t-butyl group reorientation similar to that of a neighboring hydrogen atom despite the significantly larger van der Waals' radius of a bromine atom. This most likely occurs because of the relatively long carbon-bromine bond, the distorted ring geometry, and the relatively high polarizability …

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Study Of Cathodoluminescence In Semiconductors, Mandakini V. Kale Oct 1991

Study Of Cathodoluminescence In Semiconductors, Mandakini V. Kale

Electrical & Computer Engineering Theses & Dissertations

Cathodoluminescence in Zn doped GaAs and ZnSe, the materials of interest for electron-beam controlled semiconductor switches, has been investigated. A new diagnostic technique, using telescopic setup of lenses, was developed to study the spatially resolved cathodoluminescence. The setup eliminated the light, other than rays from the sample, parallel to the optical axis, resulting in an enhanced spatial resolution. At high e-beam energies, light was detected, from the Zn layer region in the GaAs. It is proposed to be due to the radiative recombination of electron-hole pairs, created by the e-beam. For the polycrystalline ZnSe no light emission was detected. This …

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First-Principles Study Of Phase Stability In Cu-Zn Substitutional Alloys, P. E. A. Turchi, M. Sluiter, F. J. Pinski, Duane D. Johnson, D. M. Nicholson, G. M. Stocks, J. B. Staunton Sep 1991

First-Principles Study Of Phase Stability In Cu-Zn Substitutional Alloys, P. E. A. Turchi, M. Sluiter, F. J. Pinski, Duane D. Johnson, D. M. Nicholson, G. M. Stocks, J. B. Staunton

Duane D. Johnson

A parameter-free approach to phase stability in Cu-Zn substitutional alloys is shown to describe order-disorder phenomena and structural transformations with remarkable accuracy. The method is based on a multiple-scattering description of the electronic structure properties of the random alloy. Configurational order is treated within the generalized perturbation method and the concentration-functional theory. Thermodynamical properties of α and β brasses are derived from the cluster variation method. This advanced scheme is of general validity and is expected to yield similarly accurate results for other Hume-Rothery alloys.

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Auxiliary Field In Conformal Gauge Theory, James Thomas Wheeler Sep 1991

Auxiliary Field In Conformal Gauge Theory, James Thomas Wheeler

All Physics Faculty Publications

When the full conformal algebra is gauged there arises a gauge field for inverse translations in addition to the usual translational gauge field. By considering every scale-invariant action constructible from the curvatures of the conformal group and the metric, we show that when the gauge field of the usual translations is identified as the vierbein, the gauge field of inverse translations may always be eliminated by its own field equation. After this elimination, every torsion-free, scale-invariant action reduces to a linear combination of the square of the conformal curvature tensor Cαβμν and the square of the Weyl field strength …

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Track Physics Model Of Radiation Effects, Robert Katz Sep 1991

Track Physics Model Of Radiation Effects, Robert Katz

Robert Katz Publications

Radiation effects induced by heavy ions in many materials with diverse end points are well described by the conceptual structure and equations of track physics, first developed for heavy ion tracks in nuclear emulsions. The model describes scintillators, biological cell inactivation and mutation, radiation chemistry, latent tracks in insulators, the response of resists to heavy ions, and other systems. A detector is taken to be composed of small targets whose response to ionizing radiation is principally to secondary electrons. The response is calibrated through determination of the probability of target (in)activation as a function of the absorbed dose of γ …

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Paper Or Plastic?, Matthew J. Moelter Sep 1991

Paper Or Plastic?, Matthew J. Moelter

Physics

Letter to the Editor.

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Equatorial Thermospheric Wind Changes During The Solar Cycle: Measurements At Arequipa,Peru From 1983 To 1990, M. A. Biondi, J. W. Meriwether, Bela G. Fejer, S. A. Gonzalesz, D. C. Hallenbeck Sep 1991

Equatorial Thermospheric Wind Changes During The Solar Cycle: Measurements At Arequipa,Peru From 1983 To 1990, M. A. Biondi, J. W. Meriwether, Bela G. Fejer, S. A. Gonzalesz, D. C. Hallenbeck

Bela G. Fejer

Fabry-Perot interferometer measurements of Doppler shifts in the nightglow 630-nm emission line have been used to determine near-equatorial thermospheric wind velocities at Arequipa, Peru, over ∼2/3 of a solar cycle. Monthly-average nocturnal variations in the meridional and zonal wind components were calculated from the nightly data to remove short term (day-to-day) variability, facilitating display of seasonal changes in the wind patterns, as well as any additional changes introduced by the progression of the solar cycle. The measured seasonal variations in the wind patterns are more pronounced than the solar cycle variations and are more readily understandable in terms of the …

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On Compensation And Conductivity Models For Molecular-Beam-Epitaxial Gaas Grown At Low-Temperature, David C. Look Sep 1991

On Compensation And Conductivity Models For Molecular-Beam-Epitaxial Gaas Grown At Low-Temperature, David C. Look

Physics Faculty Publications

Molecular‐beam‐epitaxial GaAs grown at 200 °C has an extremely high (≳1019 cm−3) concentration of AsGa defects and, after an anneal at 550–600 °C, a high concentration of As precipitates. The relative roles of the AsGa defects and As precipitates in compensation and conductivity is controversial. Here criteria are developed to distinguish between two existing models.

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Can An Economic Approach Solve The High-Level Nuclear Waste Problem, Herbert Inhaber Sep 1991

Can An Economic Approach Solve The High-Level Nuclear Waste Problem, Herbert Inhaber

RISK: Health, Safety & Environment (1990-2002)

Building on the work of Professors O'Hare and Kunreuther, Dr. Inhaber proposes and argues for a non-coercive siting strategy that he calls a "reverse Dutch auction."

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Mie Theory Model Of The Corona, James A. Lock, Leiming Yang Aug 1991

Mie Theory Model Of The Corona, James A. Lock, Leiming Yang

Physics Faculty Publications

We performed a calculation of the corona colors that employed Mie theory to obtain the scattered light intensity. The scattered intensity was integrated over the visible spectrum for a number of different cloud droplet size distributions. The results were converted to chromaticity coordinates, convolved with the angular size of the sun, and plotted on the 1931 CIE chromaticity diagram. The results were compared to observations of multiple-ring coronas. It was found that, when using Mie theory to estimate cloud droplet sizes, water droplets with diameters in the 7-mu-m less-than-or-similar-to D less-than-or-similar-to 15-mu-m range produced the 13 multiple-ring coronas that were …

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Light And Color In The Open Air: Introduction By The Feature Editor, James A. Lock, Craig F. Bohren Aug 1991

Light And Color In The Open Air: Introduction By The Feature Editor, James A. Lock, Craig F. Bohren

Physics Faculty Publications

No abstract provided.

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An Experimental Investigation Of The Finite Time Efficiency Of A Peltier Refrigeration Device, Thomas Schneider Aug 1991

An Experimental Investigation Of The Finite Time Efficiency Of A Peltier Refrigeration Device, Thomas Schneider

Dissertations and Theses

Since the need of energy conservation has become more and more urgent in the past decades, there has been an increased interest in the study and development of more efficient energy conversion systems. One of the fields that have arisen from that endeavor is a branch of physics called Finite Time Thermodynamics (FIT). It may be said that FIT was initiated through the famous paper by Curzon and Ahlborn (1975) that established new bounds on the efficiency of a finite time Carnot heat engine. Before, the traditional treatments gave a fundamental upper limit on the efficiency of any heat engine. …

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Binary Peak Enhancement And Structure In Partially Stripped Ion-Atom Collisions, D. R. Schultz, Ronald E. Olson Aug 1991

Binary Peak Enhancement And Structure In Partially Stripped Ion-Atom Collisions, D. R. Schultz, Ronald E. Olson

Physics Faculty Research & Creative Works

Recent experiments have found both an unexpected enhancement and structure of the binary peak in the spectrum of electrons ejected in partially stripped ion-atom collisions. Utilizing the theoretical approach which has been used to elucidate the origin of this behavior, a survey of their manifestation is presented over a broad range of nuclear and ionic charges (C, F, Fe, I and U ions) and impact energies (0.1 to 100 MeV u-1). For forward binary electron emission it is shown that the greatest enhancement occurs for the lowest projectile ion charge states (highly screened nuclei). The magnitude of the enhancement maximizes …

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Differential And Total Cross Sections For Ionization Of Helium And Hydrogen By Electrons, M. Eugene Rudd Aug 1991

Differential And Total Cross Sections For Ionization Of Helium And Hydrogen By Electrons, M. Eugene Rudd

M. Eugene Rudd Publications

A comprehensive semiempirical model for singly and doubly differential and total ionization cross sections for electron production by electron impact is presented. The model is discussed in relation to several theoretically required constraints. Available experimental data for hydrogen and helium targets are examined, and recommended values of the data are given for all ranges of primary and secondary energies and angles of ejection.

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Average Vertical And Zonal F-Region Plasma Drifts Over Jicamarca, Bela G. Fejer, E. R. De Paula, S. Gonzalez, R. F. Woodman Aug 1991

Average Vertical And Zonal F-Region Plasma Drifts Over Jicamarca, Bela G. Fejer, E. R. De Paula, S. Gonzalez, R. F. Woodman

Bela G. Fejer

The seasonal averages of the equatorial F region vertical and zonal plasma drifts are determined using extensive incoherent scatter radar observations from Jicamarca during 1968–1988. The late afternoon and nighttime vertical and zonal drifts are strongly dependent on the 10.7-cm solar flux. We show that the evening prereversal enhancement of vertical drifts increases linearly with solar flux during equinox but tends to saturate for large fluxes during southern hemisphere winter. We examine in detail, for the first time, the seasonal variation of the zonal plasma drifts and their dependence on solar flux and magnetic activity. The seasonal effects on the …

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X-Ray Emission From Slow Highly Charged Ar Ions Interacting With A Ge Surface, Michael Schulz, C. Lewis Cocke, Siegbert Hagmann, Martin P. Stockli, Horst Schmidt-Bocking Aug 1991

X-Ray Emission From Slow Highly Charged Ar Ions Interacting With A Ge Surface, Michael Schulz, C. Lewis Cocke, Siegbert Hagmann, Martin P. Stockli, Horst Schmidt-Bocking

Physics Faculty Research & Creative Works

We have measured K x-ray spectra and yields from Ar17+ ions slowly approaching a single-crystal Ge surface. The yields were measured as a function of the projectile velocity component perpendicular to the surface. From the data a characteristic time of approximately 1 psec was extracted for the in-flight filling above the surface of the Ar K vacancy.

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Inhibited Spontaneous Emission By Photonic Band Gaps In A Square Lattice Of Periodic Dielectric Medium, Abdullah Al-Ghamdi Aug 1991

Inhibited Spontaneous Emission By Photonic Band Gaps In A Square Lattice Of Periodic Dielectric Medium, Abdullah Al-Ghamdi

Masters Theses

The concepts of band theory for electrons can be employed to describe the behavior of electromagnetic waves propagating in two-dimensional, periodic dielectric structures. These two-dimensional, periodic structures can produce photonic band gaps in which the spontaneous emission by atoms embedded in the structure is prohibited and as a result the performance of many semiconductor devices can be enhanced. The calculations are based on finding the eigenvalues of the algebraic equation for the frequencies of electromagnetic waves moving in a square lattice composed of square dielectric rods with different dielectric constants.

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Eta Photoproduction Via 2H(Γη)2H*, Yan Zhang Aug 1991

Eta Photoproduction Via 2H(Γη)2H*, Yan Zhang

Masters Theses

The reaction H(γη)2H* is studied using the impulse approximation. A recent fit to the H(γη)2H* elementary amplitude is employed in the calculation. The n-p final state is described by Reid's soft core potentials. The calculation shows very different cross sections for the final isospin 0 and isospin 1 break-up channels. The S11 resonance is demonstrated to dominate the eta cross sections in the 740 MeV photon laboratory energy region. Fermi motion in the deuteron does not significantly spread the effect of the S11 . All these results demonstrate that the reaction H(γη) …

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Ultrafast Electronic Disordering During Femtosecond Laser Melting Of Gaas, Peter N. Saeta, J.-K. Wang, Y. Siegal, N. Bloembergen, E. Mazur Aug 1991

Ultrafast Electronic Disordering During Femtosecond Laser Melting Of Gaas, Peter N. Saeta, J.-K. Wang, Y. Siegal, N. Bloembergen, E. Mazur

All HMC Faculty Publications and Research

We have observed an ultrarapid electronic phase transformation to a centrosymmetric electronic state during laser excitation of GaAs with intense femtosecond pulses. Reflection second-harmonic intensity from the upper 90 atomic layers vanishes within 100 fs; reflectivity rises within 0.5 ps to a steady value characteristic of a metallic molten phase, long before phonon emission can heat the lattice to the melting temperature.

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