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Physics Faculty Publications

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Articles 1861 - 1890 of 1942

Full-Text Articles in Physical Sciences and Mathematics

Magnetoresistance Method To Determine Gaas And Alxga1-Xas Mobilities In Alxga1-Xas/Gaas Modulation-Doped Field-Effect Transistor Structures, David C. Look, George B. Norris, W. Kopp, T. Henderson, H. Morkoç Aug 1985

Magnetoresistance Method To Determine Gaas And Alxga1-Xas Mobilities In Alxga1-Xas/Gaas Modulation-Doped Field-Effect Transistor Structures, David C. Look, George B. Norris, W. Kopp, T. Henderson, H. Morkoç

Physics Faculty Publications

Charge carrier mobilities are conveniently measured in simple, homostructure field-effect transistors (FET's) by means of the geometric magnetoresistance (GMR) technique. Heterostructure FET's, however, are more complicated because of multiple conducting regions, as well as multiple conducting bands within a given region. We apply a multilayer GMR mobility model to a frequently used heterostructure FET design, namely, the Al0.3Ga0.7As conduction band. In the particular MODFET structure studied here, the lowest GaAs subband mobility ranges from 5.7X103 cm2/Vs at threshold to 6.9X103 cm2/Vs at saturation while Al0.3Ga0.7As mobility …

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Incident Beam Polarization For Laser Doppler Velocimetry Employing A Sapphire Cylindrical Window, James A. Lock, Harold J. Schock Jul 1985

Incident Beam Polarization For Laser Doppler Velocimetry Employing A Sapphire Cylindrical Window, James A. Lock, Harold J. Schock

Physics Faculty Publications

For laser Doppler velocimetry studies employing sapphire windows as optical access ports, the birefringency of sapphire produces an extra beam intersection volume which serves to effectively smear the acquired velocity flow field data. It is shown that for a cylindrical window geometry, the extra beam intersection volume may be eliminated with minimal decrease in the fringe visibility of the remaining intersection volume by suitably orienting the polarizations of the initial laser beams. For horizontally incident beams, these polarizations were measured at three intersection locations within the cylinder. It was found that the measured polarization angles agreed with the theoretical predictions.

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First-Principles Study Of Structural Instabilities In Halide-Based Perovskites: Competition Between Ferroelectricity And Ferroelasticity, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer Jun 1985

First-Principles Study Of Structural Instabilities In Halide-Based Perovskites: Competition Between Ferroelectricity And Ferroelasticity, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer

Physics Faculty Publications

We have made a systematic theoretical survey of the competition between ferroelastic and ferroelectric instabilities in the family of halide-based perovskites of formula ABX3, where A is an alkali-metal ion, B is a Be, Mg, or Ca ion, and X is a halide ion. Initially we surveyed the whole series of such compounds, making a theoretical lattice-dynamical study using first-principles interionic potentials composed of a long-range pure Coulomb interaction between the spherically symmetric free ions, and a short-range component calculated by the Gordon-Kim approach from the overlapping free-ion charge densities. We then proceeded to examine in more detail three compounds, …

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Beyond The Rigid-Ion Approximation With Spherically Symmetric Ions, L. L. Boyer, M. J. Mehl, J. L. Feldman, John R. Hardy, C. Y. Fong Apr 1985

Beyond The Rigid-Ion Approximation With Spherically Symmetric Ions, L. L. Boyer, M. J. Mehl, J. L. Feldman, John R. Hardy, C. Y. Fong

Physics Faculty Publications

Ab initio calculations show that a spherically symmetric charge relaxation of ions in a crystal, in response to the long-range electrostatic potential, is important for understanding the splitting between longitudinal- and transverse-optic-mode frequencies, and the violation of the Cauchy relations among elastic constants.

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Summary Abstract: Capacitance-Voltage Characteristics In Modulation Doped Heterojunction Fets, George B. Norris, David C. Look, W. Koop, J. Klem, H. Morkoç Mar 1985

Summary Abstract: Capacitance-Voltage Characteristics In Modulation Doped Heterojunction Fets, George B. Norris, David C. Look, W. Koop, J. Klem, H. Morkoç

Physics Faculty Publications

No abstract provided.

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Schottky-Barrier Profiling Techniques In Semiconductors - Gate Current And Parasitic Resistance Effects, David C. Look Jan 1985

Schottky-Barrier Profiling Techniques In Semiconductors - Gate Current And Parasitic Resistance Effects, David C. Look

Physics Faculty Publications

The theory for obtaining mobility and carrier concentration profiles by the Hall-effect, magnetoresistance, and capacitance-conductance methods is developed in the relaxation-time approximation. This theory is then applied to semiconductors in which a Schottky barrier is used to control a depletion region. Particular emphasis is given to field-effect transistor structures which are ideally suited for geometric magnetoresistance measurements. A unique feature of the present model is the correction for finite gate (Schottky-barrier) current, which can be very important under forward-gate-bias conditions. The ability to use forward-bias makes the near-surface region more accessible. Also, parasitic resistance effects are treated. We apply these …

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The Vibrational Distribution Of N2+ In The Terrestrial Ionosphere, Jane L. Fox, Alexander Dalgarno Jan 1985

The Vibrational Distribution Of N2+ In The Terrestrial Ionosphere, Jane L. Fox, Alexander Dalgarno

Physics Faculty Publications

The densities and vibrational distributions of N2+ in the X²Σg+, A²Πu, and B²Σu+ states in the daytime terrestrial ionosphere are computed for both low and high solar activity. Altitude profiles of the relative populations of the vibrational levels of N2+X²Σg+ are presented. The fraction of vibrationally excited N2+ varies from 5% at 100 km to 50% at 450 km. Several models are examined in which loss of N2+(v) is enhanced for v > 0 and in …

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The Rotated Diffraction Grating: A Laboratory Experiment, James A. Lock Jan 1985

The Rotated Diffraction Grating: A Laboratory Experiment, James A. Lock

Physics Faculty Publications

No abstract provided.

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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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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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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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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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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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