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1989

Physics

Physics Faculty Publications

Articles 1 - 19 of 19

Full-Text Articles in Physical Sciences and Mathematics

Dielectric Response Spectrum Of A Damped One- Dimensional Double-Well Oscillator, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer Dec 1989

Dielectric Response Spectrum Of A Damped One- Dimensional Double-Well Oscillator, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer

Physics Faculty Publications

The characteristic features of the dielectric response spectrum associated with the condensation of a "soft" phonon mode during a ferroelectric phase transition can be reproduced by an oscillator moving in a damped linear double-well potential. The behavior of the response function below the transition temperature T, can be simulated by introducing a linear mean-field coupling which destroys the symmetry of the well. Potentials derived from physically realistic parameters are shown to result in dielectric responses which are in order-of-magnitude agreement with experimental values.

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Observability Of Atmospheric Glories And Supernumerary Rainbows, James A. Lock Dec 1989

Observability Of Atmospheric Glories And Supernumerary Rainbows, James A. Lock

Physics Faculty Publications

The finite spatial coherence width of sunlight at the Earth imposes restrictions on the production of scattering phenomena based on the interference of light waves. With the spatial coherence properties of sunlight taken into account, the visibility of the supernumerary rainbow sequence adjacent to the primary rainbow and the radii of the water droplets that produce the optimum glory intensity were calculated. A substantial reduction was found in the contrast of all the supernumeraries beyond the first few, and the peak observability of the glory occurred for water droplets with radii between 10 and 20 µm.

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Deviations From Bulk Transport Measurements In Semi-Insulating Gaas, N. C. Halder, David C. Look Nov 1989

Deviations From Bulk Transport Measurements In Semi-Insulating Gaas, N. C. Halder, David C. Look

Physics Faculty Publications

Because of the high bulk resistivity of semi‐insulating GaAs, surface or near‐surface effects can change the apparent magnitudes of resistivity, mobility, and carrier concentration. We consider the following causes: (1) above‐surface conduction, such as that due to impurities in a porous oxide; (2) subsurface conduction, due to sawing and polishing damage; (3) tunneling conduction in surface states; and (4) changes in near‐surface conduction due to the modification of surface potential by surface states or absorbates. The most important of these effects appear to be subsurface damage and surface potential changes.

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Hall-Effect Depletion Corrections In Ion-Implanted Samples: Si29 In Gaas, David C. Look Sep 1989

Hall-Effect Depletion Corrections In Ion-Implanted Samples: Si29 In Gaas, David C. Look

Physics Faculty Publications

The sheet free‐carrier concentration in a thin, conducting layer on an insulating substrate is lower than the net, sheet‐dopant concentration because of free‐carrier depletion in the surface and interface regions. Here we develop an algorithm to give the true, net sheet‐donor concentration from the measured sheet‐Hall concentration under the assumption of a Gaussian donor profile, which is usually sufficiently accurate for ion‐implanted samples. Correction curves are generated for Si29 ions implanted into GaAs at energies of 60, 100, 130, 150, and 200 keV, and at doses of 1×1011–2×1014 ions/cm2. Also, the …

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Silicon Crystallite Formation In Ion-Implanted Quartz, U. B. Ramabadran, H. E. Jackson, Gary C. Farlow Sep 1989

Silicon Crystallite Formation In Ion-Implanted Quartz, U. B. Ramabadran, H. E. Jackson, Gary C. Farlow

Physics Faculty Publications

Rapid thermally annealed silicon‐implanted x‐cut α‐quartz samples have been examined by Rutherford backscattering and Raman microprobe spectroscopy. The data indicate that the silicon has diffused at 1200 °C to form a buried layer of crystallites of size 1–10 μm. The crystallites are preferentially oriented and under substantial stress.

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Uniformity Of 3-In, Semi-Insulating, Vertical-Gradient-Freeze Gaas Wafers, David C. Look, D. C. Walters, M. G. Mier, J. S. Sewell, J. S. Sizelove, A. Akselrad, J. E. Clemans Jul 1989

Uniformity Of 3-In, Semi-Insulating, Vertical-Gradient-Freeze Gaas Wafers, David C. Look, D. C. Walters, M. G. Mier, J. S. Sewell, J. S. Sizelove, A. Akselrad, J. E. Clemans

Physics Faculty Publications

We have evaluated the uniformity in [EL2], dislocation (or etch‐pit) density (EPD), resistivity, mobility, and carrier concentration for 3‐in., semi‐insulating GaAs wafers grown by the vertical‐gradient‐freeze (VGF) technique. Although slight W or U patterns were observed in [EL2] and EPD along the 〈110〉 directions, for the first time, nevertheless the overall uniformity was excellent, and comparable to that in the best In‐doped and whole‐boule‐annealed ingots grown by the liquid‐encapsulated Czochralski (LEC) technique. Based on results from implant‐activation studies on LEC wafers, it is estimated that the measured nonuniformities in EPD and [EL2] for the VGF wafers would contribute only about …

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An Improved Correction Algorithm For Number Density Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac Jun 1989

An Improved Correction Algorithm For Number Density Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

A correction factor to the number density measured by the Forward Scattering Spectrometer Probe (FSSP) which compensates for dead time and coincidence errors was determined by calculating the probabilities of and the average number of particles in the six possible types of dead time and coincidence events. These probabilities and averages were calculated by means of a probabilistic model based on Poisson statistics. A Monte Carlo computer simulation of the FSSP operation was also carried out and the number density correction factor was compared with the Monte Carlo data. For an actual number density of 2000/cm3, it was …

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A Correction Algorithm For Particle-Size Distribution Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac Jun 1989

A Correction Algorithm For Particle-Size Distribution Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

Multiparticle coincidence events in the scattering volume of the forward-scattering spectrometer probe (FSSP) cause the instrument to bias the measurement of the particle size distribution of atmospheric aerosols toward large diameters. We employ a probabilistic model based on Poisson statistics to determine the average diameter and rms width of the actual size distribution as functions of the average diameter and rms width of the measured distribution. We compare our predictions to a Monte Carlo simulation of the FSSP operation

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An Alternative Approach To The Teaching Of Rotational Dynamics, James A. Lock May 1989

An Alternative Approach To The Teaching Of Rotational Dynamics, James A. Lock

Physics Faculty Publications

An alternative approach to the stability of torque-free rotations and to top nutation that emphasizes conservation laws and physical reasoning is examined. It is felt that such an approach, which is largely free of sophisticated mathematics, would be useful in teaching these topics in rotational motion at the introductory physics level.

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Ferroelectricity In Perovskitelike Nacaf3 Predicted Ab Initio, P. J. Edwardson, L. L. Boyer, R. L. Newman, D. H. Fox, John R. Hardy, John R. Flocken, R. A. Guenther, Wai-Ning Mei May 1989

Ferroelectricity In Perovskitelike Nacaf3 Predicted Ab Initio, P. J. Edwardson, L. L. Boyer, R. L. Newman, D. H. Fox, John R. Hardy, John R. Flocken, R. A. Guenther, Wai-Ning Mei

Physics Faculty Publications

The ability of zero-stress simulations, using Gordon-Kim pair potentials, to describe the structures and transformations of known fluoride-based perovskites is demonstrated for the case of KCaF3. When K+ is replaced by Na+ a new ferroelectric crystal isomorphous with LiNbO3 is predicted. The equivalent relationships of the ferroelectric lithium niobate structure with the perovskite and antiperovskite structures are examined. A polarization of 21 jµC/cm2 at room temperature and a transition temperature of 550 K are predicted for NaCaF3. Surface effects are examined in simulations of a 1080-ion cluster.

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Determination Of Shallow Minority-Acceptor Concentration In Multiply Doped Silicon, Gust Bambakidis, G. J. Brown Apr 1989

Determination Of Shallow Minority-Acceptor Concentration In Multiply Doped Silicon, Gust Bambakidis, G. J. Brown

Physics Faculty Publications

A method is presented, based on photothermal ionization spectroscopy (PTIS), for determining the shallow minority‐acceptor concentration in multiply doped silicon, over the concentration range 1013/cm3–1015/cm3. The method is an extension of a model developed previously for the PTIS response in a multiply doped semiconductor, which accounts for the experimentally observed change in signature, from negative to positive, of the lower‐energy lines of the deeper acceptor as the temperature is increased. It uses a calculated curve of the dependence on the shallow‐acceptor concentration of the temperature at which the change in …

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Photodissociation Of Co In The Thermosphere Of Venus, Jane L. Fox Apr 1989

Photodissociation Of Co In The Thermosphere Of Venus, Jane L. Fox

Physics Faculty Publications

Recent investigations of CO photoabsorption demonstrate that photodissociation longward of the ionization threshold at 88.5 nm occurs primarily through line absorptions rather than continuous processes. We have constructed high resolution photoabsorption cross sections for CO at rotational temperatures near 250 K from the improved data on dissociating transitions. We examine the effects of the new cross sections on the rate of solar photodissociation of CO in the thermosphere of Venus and compare the results to values obtained with the lower resolution cross sections available previously. We find that the photodissociation profile peaks slightly higher in the atmosphere and the peak …

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Equilibrium Polymerization On The Equivalent-Neighbor Lattice, Miron Kaufman Apr 1989

Equilibrium Polymerization On The Equivalent-Neighbor Lattice, Miron Kaufman

Physics Faculty Publications

The equilibrium polymerization problem is solved exactly on the equivalent-neighbor lattice. The Flory-Huggins entropy of mixing is exact for this lattice. We verify the discrete version of the n-vector model when n→0 is equivalent to the equal reactivity polymerization process in the whole parameter space, including the polymerized phase. The polymerization processes for polymers satisfying the Schulz distribution exhibit nonuniversal critical behavior. A close analogy is found between the polymerization problem of index r and the Bose-Einstein ideal gas in d=-2r dimensions, with the critical polymerization corresponding to the Bose-Einstein condensation.

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An Analysis Of Two Unusual Reflection Caustics, James A. Lock, Judith R. Woodruff Mar 1989

An Analysis Of Two Unusual Reflection Caustics, James A. Lock, Judith R. Woodruff

Physics Faculty Publications

A reflection caustic consisting of an x or a cross surrounded by a circle was observed using two different reflector geometries. Double-glazed insulating windows that have become bowed, and circularly symmetric cookie trays and cake covers possessing two different curvatures at different distances from the symmetry axis, produce similar caustics when plane waves are incident far off-axis. For the case of the insulating windows, it was found that the concavely bowed outer pane produced the x portion of the caustic and the convexly bowed inner pane produced the circle portion. For the case of the circularly symmetric reflectors, the x …

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A New Technique For Whole-Wafer Etch-Pit Density Mapping In Gaas, David C. Look, D. C. Walters, J. S. Sewell, S. C. Dudley, M. G. Mier, J. S. Sizelove Feb 1989

A New Technique For Whole-Wafer Etch-Pit Density Mapping In Gaas, David C. Look, D. C. Walters, J. S. Sewell, S. C. Dudley, M. G. Mier, J. S. Sizelove

Physics Faculty Publications

The pits formed on an etched GaAs surface, due to the anisotropic etching around dislocations, are efficient light scatterers, and thus reduce transmission. We have derived a quantitative relationship between the fractional transmission and the etch‐pit density (EPD) and have shown that the same absorption apparatus which is commonly used to obtain a whole‐wafer [EL2] map can also be used to generate an EPD map. The technique is verified by comparing the fractional transmission with the actual EPD count at 166 points on a three‐inch, low‐pressure, liquid‐encapsulated Czochralski wafer. Also, [EL2] and EPD maps, with more than 3500 points each, …

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Non-Debye Enhancements In The Mie Scattering Of Light From A Single Water Droplet, James A. Lock, Judith R. Woodruff Feb 1989

Non-Debye Enhancements In The Mie Scattering Of Light From A Single Water Droplet, James A. Lock, Judith R. Woodruff

Physics Faculty Publications

The glare spots usually seen on a single water droplet which has been illuminated by a plane wave are produced by geometrical rays which correspond to the different terms of the Debye series expansion of the Mie scattered field. Recently other glare spot enhancements have been predicted which correspond to scattering resonances coupling to the orbiting rays associated with high-order geometrical rainbows. We observed the non-Debye enhancement of the eleventh-order rainbow glare spot at an observation angle of 90° on a 3.5-mm water droplet illuminated by polarized He-Ne laser light.

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On The Size Distribution Of Newly Formed Grains In Red Supergiant Atmospheres, C Gregory Seab, Theodore P. Snow Jan 1989

On The Size Distribution Of Newly Formed Grains In Red Supergiant Atmospheres, C Gregory Seab, Theodore P. Snow

Physics Faculty Publications

Theoretical ultraviolet extinction curves have been calculated for comparison with observed curves for circumstellar dust in M supergiants. The theoretical curves assumed a silicate grain composition, because silicate grains are expected in the oxygen-rich environments that are observed. Calculations were performed with and without the inclusion of scattering into the beam, with largely similar results. A comparison of the computed curves with the observed ultraviolet extinction curve for circumstellar dust in Scorpii indicates that the size distribution of the circumstellar grains must cut off near 800 Å that is, there are few or no grains smaller than this. Our conclusion …

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Predicting Peculiar Interstellar Extinction From Gaseous Abundances, Charles L. Joseph, Theodore P. Snow, C Gregory Seab Jan 1989

Predicting Peculiar Interstellar Extinction From Gaseous Abundances, Charles L. Joseph, Theodore P. Snow, C Gregory Seab

Physics Faculty Publications

Molecular and atomic abundances are examined for 19 lines of sight through dense clouds, each with a peculiar selective extinction curve. The interstellar clouds in the present study appear to fall into two distinct categories: CN-rich, with relatively small amounts of neutral iron, or CN-poor, with large amounts of neutral iron. Lines of sight, having a CN/(Fe i) abundance ratio about two (~ 0.3 dex) or greater, are found to have a shallow (2.57 ± 0.55 mag) 2175 Å feature relative to the underlying extinction, while the strength of the bump is 3.60 ± 0.36 for the other dense clouds …

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A Search For Interstellar And Circumstellar C60, T P. Sno, C G. Seab Jan 1989

A Search For Interstellar And Circumstellar C60, T P. Sno, C G. Seab

Physics Faculty Publications

It has recently been suggested that the diffuse interstellar bands may be formed by ionized polyhedral carbon molecules such as C60(+). While specific laboratory measurements of absorption bands of this molecular ion have not been made, a feature due to the neutral molecule C60 has been discovered at 3860A. Examination of spectra of several reddened stars, as well as one star known to have circumstellar carbonaceous dust, shows no sign of the feature, leading to upper limits of the order of 10 to the 14th/sq cm for the column density of C60. These limits are not yet sensitive enough to …

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