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Physics Faculty Publications

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Articles 2101 - 2130 of 2177

Full-Text Articles in Physical Sciences and Mathematics

Narrow 0+ State In 20Ne And 06+ And 07+ Rotational Bands, H. T. Richards, G. Caskey, J. H. Billen, S. R. Riedhauser, Daniel J. Steck Jun 1984

Narrow 0+ State In 20Ne And 06+ And 07+ Rotational Bands, H. T. Richards, G. Caskey, J. H. Billen, S. R. Riedhauser, Daniel J. Steck

Physics Faculty Publications

A reanalysis of old data removes the (0+,2+) ambiguity for a very narrow state at Ex(20Ne)=11.55 MeV and gives a unique 0+ assignment. Such a 0+ state corresponds well to a predicted state at 11.494 MeV of unusually small reduced widths for decay to both the ground and first excited state of 16O. This new 0+ state is a better 06+ band head for the 8p-4h states at 15.159 MeV (6+) and 18.538 MeV (8+) than the currently accepted 0+ state …

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A Simple Model For Impurity Photoabsorption In Silicon, Gust Bambakidis Jun 1984

A Simple Model For Impurity Photoabsorption In Silicon, Gust Bambakidis

Physics Faculty Publications

A simple model for absorption of infrared radiation by impurity atoms in silicon crystals has been developed and applied to electronic excitations ofthe Group V donors Bi, Sb, As, and P, and the Group III acceptors B, AI, Ga, and In. The model is based on the quantum-defect method for approximating bound donor or acceptor wave functions outside the core region of the impurity. For each donor species, the relative oscillator strengths have been calculated for the transitions from the ground state to the first four excited levels. For each acceptor species, the relative oscillator strengths were calculated for transitions …

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Feasibility Studies Of Conversion Electron Mössbauer Spectroscopy (Cems) Of Iron-Doped Inp, Gilbert R. Hoy, Desmond C. Cook, Edward Agyekum May 1984

Feasibility Studies Of Conversion Electron Mössbauer Spectroscopy (Cems) Of Iron-Doped Inp, Gilbert R. Hoy, Desmond C. Cook, Edward Agyekum

Physics Faculty Publications

An electron detector for conversion electron Moessbauer spectroscopy CEMS has been designed and built. Tests have been performed to study the effects on the detector resolution with respect to high voltage, gas flow rate, and counting rate. These parameters have been optimized to detect the 7.3keV conversion electrons emitted following resonant absorption of the 14.4keV source gamma rays associated with the Moessbauer effect using Fe57. CEMS spectra have been obtained using standard Fe57 enriched and natural iron samples. CEMS spectra have also been obtained for InPFe samples supplied by the Naval Research Laboratory. So far we have only been successful …

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Relativistic Invariance And Zitterbewegung, James A. Lock Mar 1984

Relativistic Invariance And Zitterbewegung, James A. Lock

Physics Faculty Publications

We examine the question of what it is about the structure of relativistic quantum mechanics that causes the nonintuitive phenomenon of the Zitterbewegung of particle position to exist. Further, we examine various assumptions concerning the measurement process which are implicit in the observation of Zitterbewegung.

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Delineation Of A Coal Burn Edge With Seismic Refraction, Karen D. Sontag, Paul J. Wolfe Jan 1984

Delineation Of A Coal Burn Edge With Seismic Refraction, Karen D. Sontag, Paul J. Wolfe

Physics Faculty Publications

No abstract provided.

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Center-Of-Mass Properties Of Composite Systems Subject To Non-Linear External Interactions, James A. Lock Aug 1983

Center-Of-Mass Properties Of Composite Systems Subject To Non-Linear External Interactions, James A. Lock

Physics Faculty Publications

If two bound nonrelativistic quantum particles are each subject to a nonlinear external potential, their Schrödinger equation does not separate in relative and center-of-mass coordinates. For this case, the details of internal state of the composite affect its center-of-mass properties. The extent of this internal state influence is examined for an exactly soluble one-dimensional model system.

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Hole Transport In Pure And Doped Gaas, H. J. Lee, David C. Look Aug 1983

Hole Transport In Pure And Doped Gaas, H. J. Lee, David C. Look

Physics Faculty Publications

We have used a two-band model (heavy and light holes) to calculate the transport properties ofp-type GaAs. The scattering mechanisms included are acoustic-mode deformation potential, acoustic-mode piezoelectric potential, polar- and nonpolar-mode deformation potential, ionized impurity, and space charge. Interband scattering is included explicitly for the optical phonons and phenomenologically for the acoustic phonons. The intraband polar optical-mode scattering, for which a relaxation time cannot be defined, was calculated by using the numerical method of Fletcher and Butcher. The acoustic deformation-potential parameter and the coupling coefficient for interband scattering were calculated by fitting the theory to Hall-mobility data for both pure …

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Low Compensation Vapor-Phase Epitaxial Gallium-Arsenide, P. C. Colter, David C. Look, D. C. Reynolds Aug 1983

Low Compensation Vapor-Phase Epitaxial Gallium-Arsenide, P. C. Colter, David C. Look, D. C. Reynolds

Physics Faculty Publications

Vapor phase epitaxial gallium arsenide (GaAs) layers, with lower compensation ratios than any reported heretofore, have been reproducibly grown by the Ga/H2/AsCl3 method. One of these samples has been studied extensively by electrical measurements and shows an acceptor concentration of (2.0 ± 0.7)X1013 cm-3 , and a compensation rate of NA/ND = 0.06 ± 0.02. These numbers are supported by magnetophotothermal spectroscopy and photoluminescence measurements. The preparation involves growth on [211A] substrates, and a pregrowth bakeout of the Ga source, which results in a significantly lower Zn acceptor concentration in the layer. …

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Electrical-Properties Of Low-Compensation Gaas, David C. Look, P. C. Colter Jul 1983

Electrical-Properties Of Low-Compensation Gaas, David C. Look, P. C. Colter

Physics Faculty Publications

Recently we have reproducibly grown vapor-phase epitaxial GaAs, with less than 10% compensation, in an AsC13-Ga-H2 reactor. The low-temperature electrical properties of such samples are quite interesting, with neutral-impurity scattering and screening being much more important than usual. The Hall mobility is typically above 105 cm2/V sec at 5 K and has two maxima as a function of temperature, the usual one near 50 K and another near 9 K, The latter phenomenon has not been observed before, to our knowledge, The mobility and carrier concentration temperature dependences for a low-compensation sample and a …

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A Study Of The 0.1-Ev Conversion Acceptor In Gaas, David C. Look, Gernot S. Pomrenke Jun 1983

A Study Of The 0.1-Ev Conversion Acceptor In Gaas, David C. Look, Gernot S. Pomrenke

Physics Faculty Publications

Two semi-insulating liquid-encapsulated Czochralski GaAs cyrstals, one Cr-doped and the other undoped, were annealed at 750°C for 15 min in flowing H2. Each sample converted to conducting p type in the near-surface region, due to the formation of acceptors at E∪ + 0.1 eV. We have studied this phenomenon by electrical, optical, and analytical profiling techniques, and have determined conclusively that the acceptors in our samples are not related to Mn accumulation, a commonly accepted explanation. It is argued that the O.I-e V center may arise from several possible sources, each exhibiting a VGa -like state at …

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An Elementary Development Of Mass-Energy Equivalence, Daniel J. Steck, Frank Rioux May 1983

An Elementary Development Of Mass-Energy Equivalence, Daniel J. Steck, Frank Rioux

Physics Faculty Publications

No abstract provided.

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Defect Nature Of The 0.4-Ev Center In O-Doped Gaas, David C. Look, S. Chaudhuri, J. R. Sizelove May 1983

Defect Nature Of The 0.4-Ev Center In O-Doped Gaas, David C. Look, S. Chaudhuri, J. R. Sizelove

Physics Faculty Publications

We have studied the Ec - 0.4 e V center in O-doped GaAs by a combination of temperature-dependent Hall-effect measurements, spark-source mass spectroscopy, and secondary-ion mass spectroscopy. The conclusion is that neither 0 nor any other impurity can account for the O.4-eV center; therefore, it is a pure defect.

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Seam Wave Characteristics In An Eastern U.S. Coal, Paul J. Wolfe, Timothy G. Holdeman, Benjamin H. Richard Jan 1983

Seam Wave Characteristics In An Eastern U.S. Coal, Paul J. Wolfe, Timothy G. Holdeman, Benjamin H. Richard

Physics Faculty Publications

No abstract provided.

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Nitrogen Escape From Mars, Jane L. Fox, Alexander Dalgarno Jan 1983

Nitrogen Escape From Mars, Jane L. Fox, Alexander Dalgarno

Physics Faculty Publications

The escape rate of nitrogen from Mars is calculated to be 2.3×105 s−1 for low solar flux conditions and 8.9×105 s−1 for high solar flux conditions. The major source of energetic atoms is dissociative recombination of ground state and vibrationally excited N2+ ions. The measured 15N/14N isotope ratio can be reproduced by postulating an early dense atmosphere during which little differentiation occurred.

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Positive Identification Of The Cr(4+)-->Cr(3+) Thermal Transition In Gaas, David C. Look, S. Chaudhuri, L. Eaves Dec 1982

Positive Identification Of The Cr(4+)-->Cr(3+) Thermal Transition In Gaas, David C. Look, S. Chaudhuri, L. Eaves

Physics Faculty Publications

Temperature-dependent Hall-effect measurements on two Cr-doped GaAs samples
show a dominant center at E1 =0.324—1.4 x 10-4T eV, with respect to the valence-band edge. By comparison with secondary-ion mass spectroscopy measurements of the Cr concentration, and recent EPR measurements of the Cr2+, Cr3+, and Cr4+ concentration in several samples, it is shown unambiguously that this energy describes the Cr4+→Cr3+ transition. This is the first conclusive evidence for a charge-state transition involving Cr4+ in GaAs.

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Study Of Lattice Instabilities In Snx Euy-X Mo6s8 (Y=1.0 And 1.2), Paul D. Hambourger, J. C. Ho, C. Y. Huang, H. L. Luo Aug 1982

Study Of Lattice Instabilities In Snx Euy-X Mo6s8 (Y=1.0 And 1.2), Paul D. Hambourger, J. C. Ho, C. Y. Huang, H. L. Luo

Physics Faculty Publications

An anomaly in the heat capacity of Eu1.2Mo6S8 at ∼120 K (T0) has been observed and identified as a structural transformation. The heat capacity, electrical resistance, and Hall coefficient for the Eu-rich pseudoternaries have been measured under pressure to obtain the pressure dependence of T0. The depression of T0 by pressure is used to explain the pressure dependence of the superconducting transition temperature and the anomalous temperature dependence of the critical field.

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Magneto-Hall And Magnetoresistance Coefficients In Semiconductors With Mixed Conductivity, David C. Look Feb 1982

Magneto-Hall And Magnetoresistance Coefficients In Semiconductors With Mixed Conductivity, David C. Look

Physics Faculty Publications

Magneto-Hall and magnetoresistance formulas, correct to order B2, are derived for the case in which both single-carrier and mixed-carrier effects are important. Also, a new magneto-Hall coefficient Is Presented: β=‹π4›‹π›/‹π2›3 - 1. Values of β for various scattering mechanisms are calculated and compared with experiment

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A Stratospheric Chemical Instability, Jane L. Fox, Steven C. Wofsy, Michael B. Mcelroy, Michael J. Prather Jan 1982

A Stratospheric Chemical Instability, Jane L. Fox, Steven C. Wofsy, Michael B. Mcelroy, Michael J. Prather

Physics Faculty Publications

The equations which determine partitioning of Clx in steady state have multiple (three) solutions under conditions which might arise in the high-latitude winter stratosphere. Two of these solutions are stable, one is unstable, to infinitesimal perturbations. The relative stability of solutions is examined by subjecting the system to finite perturbations. The more stable solution is found to eliminate the less stable when semi-infinite volumes of the two solutions are placed in contact. The high-ClO, low-NO2 solution is more stable under most conditions. Transitions from less to more stable states are slow in winter but may occur more rapidly …

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Atomic Carbon In The Atmosphere Of Venus, Jane L. Fox Jan 1982

Atomic Carbon In The Atmosphere Of Venus, Jane L. Fox

Physics Faculty Publications

The densities of atomic carbon in the Venusian thermosphere are computed for a model which includes both chemistry and transport. The maximum density of C is 2.8×107 cm−3 near 150 km for an assumed O2 mixing ratio of 1×10−4. Photoionization of atomic carbon is found to be the major source of C+ above 200 km, and resonance scattering of sunlight by atomic carbon may be the major source of the C I emissions at 1561 Å, 1657 Å, and 1931 Å. The computed C+ densities are found to be in substantial agreement with …

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Statistics Of Multicharge Centers In Semiconductors - Applications, David C. Look Nov 1981

Statistics Of Multicharge Centers In Semiconductors - Applications, David C. Look

Physics Faculty Publications

A general formula is derived for the electron occupation numbers appropriate for multicharge centers in semiconductors, including excited states. The results are used to rederive and generalize several formulas of interest in the literature, in order to show exactly how the degeneracies of individual states enter in. Particular attention is paid to certain subjects which are sometimes confusing, such as how the statistics of band states differ from those those of localized states. Another subject of much recent interest, negative-U centers, is dealt with in some detail. We show how the dependence of the average occupation number on Fermi …

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Glory In Optical Backscattering From Air Bubbles, Dean S. Langley, Philip L. Marston Sep 1981

Glory In Optical Backscattering From Air Bubbles, Dean S. Langley, Philip L. Marston

Physics Faculty Publications

Observations of light backscattered from air bubbles in a viscous liquid demonstrate an enhancement due to axial focusing. A physical-optics approximation for the cross-polarized scattering correctly describes the spacing of regular features observed. The non-cross polarized scattering is not adequately described by a single class of rays.

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Optical Pulse Compression In A Cholesteric Liquid Crystal, D. N. Ghosh Roy, D.V.G.L.N. Rao, H. Bronk Sep 1981

Optical Pulse Compression In A Cholesteric Liquid Crystal, D. N. Ghosh Roy, D.V.G.L.N. Rao, H. Bronk

Physics Faculty Publications

A 20‐ns laser pulse is compressed to nearly 2.5 ns in a 10‐cm‐long sample of liquid‐crystal cholesteryl oleate in the isotropic phase. Pulse compression in a length as short as only 5 cm has been observed. A semiquantitative explanation is given in terms of stimulated Brillouin scattering.

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Negative Acceleration Components For A Relativistic Particle, James A. Lock Jul 1981

Negative Acceleration Components For A Relativistic Particle, James A. Lock

Physics Faculty Publications

Newtonian acceleration is considered in the light of special relativity theory with the result that the component of acceleration in the direction of an arbitrary applied force is always positive.(AIP)

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Observation Of The Transition From Semiconductor To High-Tc Superconductor In (Snxeu1-X)Ymo6s8 Under High Pressure, D. W. Harrison, K. C. Lim, J. D. Thompson, C. Y. Huang, Paul D. Hambourger, H. L. Luo Jan 1981

Observation Of The Transition From Semiconductor To High-Tc Superconductor In (Snxeu1-X)Ymo6s8 Under High Pressure, D. W. Harrison, K. C. Lim, J. D. Thompson, C. Y. Huang, Paul D. Hambourger, H. L. Luo

Physics Faculty Publications

Pressure-induced high-temperature superconductivity is observed in semiconducting (SnxEu1-x)yMo6S8, where 0<~x<~0.1 and y=1.0 and 1.2, having a carrier concentration ≃1019/cm3 at 4.2 K as determined from Hall-effect measurements. Above a threshold pressure ≃7 kbar, superconductivity appears with dTc/dP≃2 K/kbar. The maximum superconducting temperature (Tc∼10 K), reached at ∼ 12 kbar, represents the highest pressure-induced Tc in any semiconductor. For P>~13 kbar, the temperature-dependent resistance appears metallic.

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Hall Effect At High Pressure In (Snxeu1-X)1.2mo6s8, Paul D. Hambourger, C. Y. Huang, H. L. Luo Jan 1981

Hall Effect At High Pressure In (Snxeu1-X)1.2mo6s8, Paul D. Hambourger, C. Y. Huang, H. L. Luo

Physics Faculty Publications

No abstract provided.

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O++ In The Venusian Ionosphere, Jane L. Fox, G. A. Victor Jan 1981

O++ In The Venusian Ionosphere, Jane L. Fox, G. A. Victor

Physics Faculty Publications

It is shown that the revised photochemical theory for O++ in the earth's ionopshere, where double photoionization of O is the dominant source, is consistent with recent O++ measurements on Venus. A rate coefficient of 1.5 × 10−10 cm³ s−1 for O++ + O is obtained, and the possible importance of O++ chemistry on minor atomic ions and airglow emissions is explored.

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An Experiment In Discovery, Daniel J. Steck Dec 1980

An Experiment In Discovery, Daniel J. Steck

Physics Faculty Publications

No abstract provided.

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Automation Of A Popular Monochromator, David C. Look, J. W. Farmer, R. N. Ely Jul 1980

Automation Of A Popular Monochromator, David C. Look, J. W. Farmer, R. N. Ely

Physics Faculty Publications

A wavelength‐scan and intensity‐control system for a widely‐used, high‐intensity monochromator is described. The wavelength scan is bidirectional and variable speed, and may be controlled manually or by TTL logic from a computer. The intensity control is effected by use of a programmable dc power supply and D–A converter. Various filters are described which allow an intensity of up to 2×1014 photons/cm2 s to be achieved in a 1 cm×3 cm area over a wavelength range 0.76–2.50 μm, at 0.07 μm bandwidth, with a single grating. (A lesser intensity is available to 3.4 μm.) This wide range is made …

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Electron Irradiation Defects In N-Type Gaas, J. W. Farmer, David C. Look Apr 1980

Electron Irradiation Defects In N-Type Gaas, J. W. Farmer, David C. Look

Physics Faculty Publications

We have studied the production and annealing of defects produced by 1-MeV electron irradiation of n-type GaAs. Two of these defects lie at 0.15 and 0.31 eV from the conduction band, respectively; in addition, there is at least one acceptor much closer to the valence band. The carrier-removal rate depends upon sample purity but is independent of the irradiation flux. The removal rate is also highly dependent upon the position of the the Fermi level, an effect which is considered in some detail. At about 200°C, the defects recover in two stages, with the respective recovery rates given by λ …

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Hall-Coefficient And Resistivity At High Pressure In 1t-Tas2, Paul D. Hambourger, F. J. Disalvo Jan 1980

Hall-Coefficient And Resistivity At High Pressure In 1t-Tas2, Paul D. Hambourger, F. J. Disalvo

Physics Faculty Publications

No abstract provided.

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