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Physics Faculty Publications

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Articles 2041 - 2070 of 2177

Full-Text Articles in Physical Sciences and Mathematics

In Situ Hall-Effect System For Real-Time Electron-Irradiation Studies, B. S. Ziebro, David C. Look, Joseph W. Hemsky, J. Rice Jan 1990

In Situ Hall-Effect System For Real-Time Electron-Irradiation Studies, B. S. Ziebro, David C. Look, Joseph W. Hemsky, J. Rice

Physics Faculty Publications

A unique system capable of taking in situ Hall‐effect measurements during electron irradiation has been developed. The key element is a small, powerful rare‐earth magnet. Measurements can be taken while the electron beam is on, resulting in a considerable time savings and eliminating problems associated with mounting and demounting the sample. High resolution electron concentration and mobility versus fluence data are quickly and easily obtained, making possible detailed defect production rate studies as functions of energy and flux.

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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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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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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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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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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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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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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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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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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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Cooperative Effects Among Partial Waves In Mie Scattering, James A. Lock Dec 1988

Cooperative Effects Among Partial Waves In Mie Scattering, James A. Lock

Physics Faculty Publications

Observations of an illuminated water droplet at a close distance are described mathematically by the Fourier transform of the Mie-scattering amplitude convolved with the aperture function of the observer’s eye. Most of the sharp enhancements found in the Fourier transform correspond to geometrical rays associated with the various terms in the Debye-series expansion of the Mie amplitude. However, there are some enhancements that cannot be ascribed to any individual Debye-series term. Instead, they arise from a constructive interference cooperation of the phase of a scattering resonance in a single partial wave with the region of the stationary phase corresponding to …

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Transport Critical Current And Magnetization Measurements Of Melt-Processed Yba2cu3o7-X, A. H. Hermann, Z. Z. Sheng, W. Kiehl, D. Marsh, A. Elali, Paul D. Hambourger, C. Almasan, J. Estrada, T. Datta Nov 1988

Transport Critical Current And Magnetization Measurements Of Melt-Processed Yba2cu3o7-X, A. H. Hermann, Z. Z. Sheng, W. Kiehl, D. Marsh, A. Elali, Paul D. Hambourger, C. Almasan, J. Estrada, T. Datta

Physics Faculty Publications

We report magnetic field dependence of the transport critical current and dc magnetic susceptibility measurements on YBa2Cu3O7−x superconductors formed by melt-solid reactions at 950 °C between Ba-Cu-O (or Tb-Ba-Cu-O) and solid nonstoichiometric Y-Ba-Cu-oxide. Four-probe dc critical current measurements at 77, 64, and 4.2 K show strong depression of the critical current density with increasing magnetic field in agreement with a model of weakly linked superconducting regions. Diamagnetic shielding and Meissner flux expulsion measurements in the temperature range 10–300 K show about one third volume fraction of perfect superconductivity. Both shielding and flux expulsion were observed to be approximately temperature independent …

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Possible Origins Of High-Tc, Superconductivity, John R. Hardy, John W. Flocken May 1988

Possible Origins Of High-Tc, Superconductivity, John R. Hardy, John W. Flocken

Physics Faculty Publications

A new mechanism is proposed to explain high-T, superconductivity in copper-oxide-based, open perovskitelike systems. It is shown that, should the oxygen ions be moving in a double-well potential, an order-of-magnitude enhancement of the electron-lattice coupling follows automatically from a consistent treatment of this motion. Both theoretical and experimental evidence for the presence of such double wells is cited.

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Cayley-Tree Ising Model With Antiferromagnetic Nearest-Neighbor And Ferromagnetic Equivalent-Neighbor Interactions, Miron Kaufman, M. Kahana May 1988

Cayley-Tree Ising Model With Antiferromagnetic Nearest-Neighbor And Ferromagnetic Equivalent-Neighbor Interactions, Miron Kaufman, M. Kahana

Physics Faculty Publications

The phase diagram of the Ising model with antiferromagnetic nearest-neighbor interactions and ferromagnetic equivalent-neighbor interactions on the Cayley tree is determined exactly. A nonuniversal critical line separates the disordered and the ordered phases. A line of first-order transitions separating ferromagnetic order from antiferromagnetic order ends in the midst of the ordered phase at a classical ordered critical point. For a small range of values of the ratio of the two couplings, two transitions occur as the temperature is varied. In this case the uniform magnetization is not a monotonic function of the temperature.

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Contact Resistance Measurements In Gaas Mesfet's And Modfet's By The Magneto‐Tlm Technique, David C. Look Jan 1988

Contact Resistance Measurements In Gaas Mesfet's And Modfet's By The Magneto‐Tlm Technique, David C. Look

Physics Faculty Publications

The standard transmission‐line model (TLM) for specific contact resistivity measurements of planar contacts is improved in two ways: (i) the addition of a magnetic field, which gives the mobility and carrier concentration of the bulk material, and the mobility of the material under the contact; and (ii) an extension to two layers, which makes the model applicable to MODFET structures. The results are applied to MESFET material, and MODFET material. One conclusion concerning the latter material is that the electrons directly beneath the annealed Au/Ge/Ni contacts have lower mobility than those in the bulk, but still …

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A New Search For Interstellar C3, Charles L. Joseph, Theodore P. Snow, C Gregory Seab Jan 1988

A New Search For Interstellar C3, Charles L. Joseph, Theodore P. Snow, C Gregory Seab

Physics Faculty Publications

A new, very sensitive search for interstellar triatomic carbon has resulted in upper limits for a few diffuse clouds of order 1010 cm – 2, or about 10 – 11 with respect to hydrogen. These limits are consistent with recent cold diffuse cloud chemistry models, but may be in conflict with shocked cloud models such as those invoked to explain CH + abundances. Our results may also be argue against linear carbon-chain molecules as carriers of the diffuse interstellar bands.

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The Energy-Dependence Of Polarization Observables In The ²H(D,Γ)⁴He Reaction, H. R. Weller, R. M. Whitton, J. Langenbrunner, Evans Hayward, W. R. Dodge, S. Kuhn, D. R. Tilley Jan 1988

The Energy-Dependence Of Polarization Observables In The ²H(D,Γ)⁴He Reaction, H. R. Weller, R. M. Whitton, J. Langenbrunner, Evans Hayward, W. R. Dodge, S. Kuhn, D. R. Tilley

Physics Faculty Publications

Measurements of the tensor and vector analyzing powers, Ayy(130°) and Ay(130°), have been obtained for the 2H(d,γ )4He reaction for energies ranging from Ed(lab) = 0.3 MeV to Ed(lab) = 50 MeV. The Ayy(130°) data are sensitive to the D-state present in the ground state of 4He and are observed to have their maximum value near Ed = 30 MeV. The vector analyzing power data show a maximum near Ed = 3 MeV. The data are compared to the results of a microscopic …

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Theory Of The Observations Made Of High-Order Rainbows From A Single Water Droplet, James A. Lock Dec 1987

Theory Of The Observations Made Of High-Order Rainbows From A Single Water Droplet, James A. Lock

Physics Faculty Publications

Over a dozen rainbows have been observed in a single water droplet. They appear as glare spots on the water droplet which take on coloration at the appropriate rainbow angles. The appearance of rainbows as colored glare spots in this situation is understood in terms of the caustics created in the vicinity of the droplet by the refracting light rays. The angular positions of the glare spots are understood in terms of the Fourier transform of the geometric scattering amplitude. The rainbow glare spots are also found to appear numerically in the Fourier transform of the Mie scattered fields. An …

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Generating Problem Sets With Word Processing Software, Jearl D. Walker Dec 1987

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

Physics Faculty Publications

No abstract provided.

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Moment Analysis Of The Cluster-Size-Distribution Approach To Scaling During Coagulation, Thomas W. Taylor, C. M. Sorensen Dec 1987

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 …

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