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Physics Faculty Publications

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Articles 1831 - 1860 of 1942

Full-Text Articles in Physical Sciences and Mathematics

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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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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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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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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Numerical Methods In Optics: A Course About Learning Physics Through Computing, James A. Lock Dec 1987

Numerical Methods In Optics: A Course About Learning Physics Through Computing, James A. Lock

Physics Faculty Publications

Many advanced undergraduates find it difficult to connect abstract mathematical formalisms with the concrete physical phenomena that they describe. A course in optics from a numerical methods point of view is described. Its purpose is to exploit the students’ familiarity with computng in order to more effectively learn the physics involved in a number of realistic phenomena. The combination of demonstrations, computer calculations, and computer graphics display of the results can prove to be a useful tool in developing physical intuition in students.

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Equivalence Of Donor And Acceptor Fits To Temperature-Dependent Hall Data: General Case, David C. Look Nov 1987

Equivalence Of Donor And Acceptor Fits To Temperature-Dependent Hall Data: General Case, David C. Look

Physics Faculty Publications

Recently, it was shown that the usual statistical-mechanical formulation used to fit carrier concentration versus temperature data cannot distinguish between the donor or acceptor nature of one single-charge-state center. Here we generalize that result to include any number of donor and acceptor centers, of arbitrary charge multiplicity, and also show how that by fitting one particular case (e.g., every center assumed to be a donor), all of the other possible cases can be immediately solved by inspection.

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Defect Production In Electron-Irradiated, N-Type Gaas, David C. Look, J. R. Sizelove Nov 1987

Defect Production In Electron-Irradiated, N-Type Gaas, David C. Look, J. R. Sizelove

Physics Faculty Publications

Temperature-dependent Hall-effect measurements have been performed on pure, n-type, vapor-phase epitaxial GaAs, irradiated by 1-MeV electrons at room temperature. The energies and production rates of two dominant defect centers, C2 and C3, are as follows: E2 = EC - 0.148, E3 = EC2 = 2.0 and τ3 = 0.5 +/1 0.2 cm-1, in good agreement with deep level transient spectroscopy (DLTS) data. However, the most important result of this study is a very high production rate, τAS ≅ +/- 1 cm-1, for "shallow" acceptors (C …

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Photoresistivity And Photo-Hall-Effect Topography On Semi-Insulating Gaas Wafers, David C. Look, E. Pimentel Nov 1987

Photoresistivity And Photo-Hall-Effect Topography On Semi-Insulating Gaas Wafers, David C. Look, E. Pimentel

Physics Faculty Publications

By placing a semi-insulating GaAs wafer on a fiat, rare-earth magnet, and irradiating the surface with two perpendicular slits of light to form a Greek cross configuration, it is possible to perform photoresistivity and photo-Hall-effect topography on the wafer. The technique is nondestructive in that the contacts are tiny, removable In dots which are placed only on the periphery. By varying the wavelength of the light, selective centers, such as EL2, can be mapped. We compare a 1.1-μ, photoexcited electron concentration map with a quantitative EL2 map on a 3-in. undoped, liquid-encapsulated Czochralski wafer.

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Photoluminescence In Electrically Reversible (Semiconducting To Semiinsulating) Bulk Gaas, Phil W. Yu, David C. Look, W. Ford Oct 1987

Photoluminescence In Electrically Reversible (Semiconducting To Semiinsulating) Bulk Gaas, Phil W. Yu, David C. Look, W. Ford

Physics Faculty Publications

A photoluminescence study has been made of electrically reversible, bulk, liquid-encapsulated Czochralski GaAs at temperatures 2-300 K. The reversibility from the semiconducting to the semi-insulating state is made by slow or fast cooling, respectively, following a 5-h, 950°C heat treatment in an evacuated quartz ampoule. A donor level at Ec - 0.13 eV and two acceptor levels at Ev + 0.069 eV and Ev + 0.174 eV are produced after the heat treatment. Only the acceptor levels were detected by photoluminescence. A tentative model assigning the acceptor to the intrinsic defect pair VGa-GaAs is discussed.

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High Acceptor Production Rate In Electron-Irradiated N-Type Gaas - Impact On Defect Models, David C. Look Sep 1987

High Acceptor Production Rate In Electron-Irradiated N-Type Gaas - Impact On Defect Models, David C. Look

Physics Faculty Publications

Defect production rates have been studied in electron-irradiated GaAs by temperature-dependent Hall-effect (TDR) measurements. The TDH results agree well with deep level transient spectroscopy (DLTS) results for the wen-known electron traps E1, E2, and E3, but conclusively demonstrate a much higher production rate (4 ± 1 cm-1) of acceptors below E3 than the total of all other DLTS traps. These findings strongly affect current defect models, and, e.g., are consistent with the existence of Ga sublattice damage, not seen before.

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Square-Lattice Ising Model In A Weak Uniform Magnetic Field: Renormalization Group Analysis, Miron Kaufman Sep 1987

Square-Lattice Ising Model In A Weak Uniform Magnetic Field: Renormalization Group Analysis, Miron Kaufman

Physics Faculty Publications

For the two-dimensional ferromagnetic Ising critical point, I show that the known values of the critical exponents imply the absence of logarithms of the reduced temperature in the leading contributions to any field derivative of the free energy at zero magnetic field. For the square-lattice Ising antiferromagnet in a weak magnetic field, I compute the critical line Tc(H)=Tc0(1-0.038 023 259H2) and the leading contribution to the susceptibility χ=0.014 718 006 6H2ln(1/ǁtǁ), where t is the reduced temperature.

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Comparison Of Low-Energy Seismic Sources In Till, Paul J. Wolfe, B. H. Richard, Dale V. Dailey, James R. Plomer Mar 1987

Comparison Of Low-Energy Seismic Sources In Till, Paul J. Wolfe, B. H. Richard, Dale V. Dailey, James R. Plomer

Physics Faculty Publications

No abstract provided.

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Multicritical Susceptibility Sum Rules, Miron Kaufman, Michael Ma Mar 1987

Multicritical Susceptibility Sum Rules, Miron Kaufman, Michael Ma

Physics Faculty Publications

Asymptotically close to the Nth-order multicritical point of an N-phase system, there are N-1 sum rules involving the mean-field susceptibilities measured in each of the coexisting phases. These sum rules provide the experimentalist a convenient and stringent test of the theory. In particular, they facilitate the detection of nonclassical effects, especially valuable for N>3 where the fluctuation effect is dominated by the classical contribution for three-dimensional systems.

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Fresnel Diffraction Effects In Misfocused Vision, James A. Lock Mar 1987

Fresnel Diffraction Effects In Misfocused Vision, James A. Lock

Physics Faculty Publications

If a narrow line stimulus on an opposite contrast background is observed with appropriately misfocused vision, the stimulus appears as a number of alternating bright and dark fringes. A numerical calculation of this effect was performed. As a result it was found that the banding phenomenon is a Fresnel diffraction effect depending on the details of both the narrow line stimulus and the eye pupil.

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Equivalence Of Donor And Acceptor Fits Of Temperature-Dependent Carrier-Concentration Data, David C. Look, J. R. Sizelove Feb 1987

Equivalence Of Donor And Acceptor Fits Of Temperature-Dependent Carrier-Concentration Data, David C. Look, J. R. Sizelove

Physics Faculty Publications

It is shown that the usual charge-balance analysis of temperature-dependent carrier-concentration data cannot distinguish between the donor and acceptor behavior of a center which is emitting carriers to a particular band, even though the statistics are different in the two cases. Other data, such as mobility or analytical results are needed to make the distinction.

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Electrical Characterization Of Ion Implantation Into Gaas, David C. Look Jan 1987

Electrical Characterization Of Ion Implantation Into Gaas, David C. Look

Physics Faculty Publications

Recent advances in the characterization of ion‐implanted samples have included whole wafer mapping (topography) and depth profiling techniques. We review several methods for mapping electrical parameters, including the dark‐spot resistance (DSR), and the microwave photoconductance techniques. In addition, we suggest a new photo‐Hall technique which would allow mobility and carrier‐concentration mapping as well as that of resistivity . Finally, we review methods for obtaining ρ, μ, and depth profiles, with particular emphasis on the application of the magnetoresistance techniques in actual field‐effect transistor structures.

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Semiconducting/Semi-Insulating Reversibility In Bulk Gaas, David C. Look, Phil W. Yu, W. M. Theis, W. Ford, G. Mathur, J. R. Sizelove, D. H. Lee, S. S. Li Oct 1986

Semiconducting/Semi-Insulating Reversibility In Bulk Gaas, David C. Look, Phil W. Yu, W. M. Theis, W. Ford, G. Mathur, J. R. Sizelove, D. H. Lee, S. S. Li

Physics Faculty Publications

Bulk, liquid-encapsulated Czochralski GaAs may be reversibly changed from semiconducting (ρ~1 Ω cm) to semi-insulating (ρ~107 Ω cm) by slow or fast cooling, respectively, following a 5 h, 950°C soak in an evacuated quartz ampoule. This effect has been studied by temperature-dependent Han-effect, photoluminescence, infrared absorption, mass spectroscopy, and deep level transient spectroscopy measurements. Except for boron, the samples are very pure, with carbon and silicon concentrations less than 3 X 1014 cm-3. Donor and acceptor concentrations, on the other hand, are in the mid 1015 cm-3 range, which means …

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Semiconducting/Semi-Insulating Reversibility In Bulk Gaas, David C. Look, Phil W. Yu, W. M. Theis, W. Ford, G. Mathur, J. R. Sizelove, D. H. Lee, S. S. Li Oct 1986

Semiconducting/Semi-Insulating Reversibility In Bulk Gaas, David C. Look, Phil W. Yu, W. M. Theis, W. Ford, G. Mathur, J. R. Sizelove, D. H. Lee, S. S. Li

Physics Faculty Publications

Bulk, liquid‐encapsulated Czochralski GaAs may be reversibly changed from semiconducting (ρ∼1 Ω cm) to semi‐insulating (ρ∼107 Ω cm) by slow or fast cooling, respectively, following a 5 h, 950 °C soak in an evacuated quartz ampoule. This effect has been studied by temperature‐dependent Hall‐effect, photoluminescence, infrared absorption, mass spectroscopy, and deep level transient spectroscopy measurements. Except for boron, the samples are very pure, with carbon and silicon concentrations less than 3×1014 cm−3. Donor and acceptor concentrations, on the other hand, are in the mid 1015 cm−3 range, which means that …

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Multicritical Points In An Ising Random-Field Model, Miron Kaufman, Philip E. Klunzinger, A. Khurana Oct 1986

Multicritical Points In An Ising Random-Field Model, Miron Kaufman, Philip E. Klunzinger, A. Khurana

Physics Faculty Publications

The phase diagram of the mean-field Ising model in a random field obeying a symmetric three-peak distribution is determined. This distribution is relevant to diluted antiferromagnets in a uniform magnetic field. The phase diagram includes a fourth-order point, tricritical points, ordered critical points, critical end points, and a double critical end point. An ordered phase persists for arbitrarily large random fields at low temperatures.

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Origin Of Nonuniversality In Micellar Solutions: Comment, R. G. Caflisch, Miron Kaufman, J. R. Banavar Jun 1986

Origin Of Nonuniversality In Micellar Solutions: Comment, R. G. Caflisch, Miron Kaufman, J. R. Banavar

Physics Faculty Publications

No abstract provided.

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A Priori Predictions Of Phase Transitions In Kcaf3 And Rbcaf3: Existence Of A New Ground State, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer Apr 1986

A Priori Predictions Of Phase Transitions In Kcaf3 And Rbcaf3: Existence Of A New Ground State, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer

Physics Faculty Publications

We have made an a prior; theoretical study of the potential-energy surfaces for KCaF3 and RbCaF3, and have examined the relative stability of the various lower-symmetry structures generated from the cubic perovskite phase by "rotations" of the CaF6 octahedra. A completely new ground state was discovered which, when included in the sequence of energy levels, allows us to give a full account both qualitatively and, in the case of RbCaF3, quantitatively of the phase transition sequences in both systems.

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Summary Abstract - Theoretical And Experimental Capacitance-Voltage Behavior Of Modulation-Doped Al(0.3)Ga(0.7)As/Gaas Heterojunctions, George B. Norris, David C. Look, W. Koop, J. Klem, H. Morkoç Mar 1986

Summary Abstract - Theoretical And Experimental Capacitance-Voltage Behavior Of Modulation-Doped Al(0.3)Ga(0.7)As/Gaas Heterojunctions, George B. Norris, David C. Look, W. Koop, J. Klem, H. Morkoç

Physics Faculty Publications

No abstract provided.

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Ar+ In The Terrestrial Ionosphere, Jane L. Fox Jan 1986

Ar+ In The Terrestrial Ionosphere, Jane L. Fox

Physics Faculty Publications

The Ar+ densities in the terrestrial ionosphere are computed for both low and high solar activity models. The reaction N2+(υ > 0) + Ar → N2+ Ar+ is found to be a significant source of Ar+, nearly equal to photoionization and electron impact ionization in the high solar activity model. Peak densities of Ar+ of 11 cm−3 near 190 km and 22 cm−3 near 220 km are predicted for the low and high solar activity models. It is suggested that a simultaneous measurement of Ar+ and Ar densities …

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The Temporary Capture Of Light By A Dielectric Film, James A. Lock Oct 1985

The Temporary Capture Of Light By A Dielectric Film, James A. Lock

Physics Faculty Publications

When a wave packet of light passes through a dielectric film it is found that at a transmission maximum, the group velocity of the light wave packet within the film attains its minimum value. Similarly, at a transmission minimum, it is found that the group velocity of the light wave packet within the film attains its maximum value. The measurability of this effect is also discussed.

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Theoretical And Experimental Capacitance-Voltage Behavior Of Al0.3ga0.7as Gaas Modulation-Doped Heterojunctions - Relation Of Conduction-Band Discontinuity To Donor Energy, George B. Norris, David C. Look, W. Kopp, J. Klem, H. Morkoç Aug 1985

Theoretical And Experimental Capacitance-Voltage Behavior Of Al0.3ga0.7as Gaas Modulation-Doped Heterojunctions - Relation Of Conduction-Band Discontinuity To Donor Energy, George B. Norris, David C. Look, W. Kopp, J. Klem, H. Morkoç

Physics Faculty Publications

For the first time, we show that the capacitance-voltage behavior of modulation-doped heterojunctions may be accurately described by a first-principles theory that includes selfconsistent quantum two-dimensional (2-D) electron subbands in the GaAs, numerical solution of Poisson's equation for band bending and space charge in the (Al,Ga) As, and series resistance in the 2-D channel and heterointerface. The excellent agreement found between the theory and measurements on selected high-quality Al0.3Ga0.7 As/GaAs heterojunctions allows accurate determination of the maximum 2-D carrier concentration. From this, we find a strong relationship between the conduction-band discontinuity and donor binding energy, giving offsets. …

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