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1993

Physics Faculty Publications

Articles 1 - 27 of 27

Full-Text Articles in Physical Sciences and Mathematics

Evidence For Day-To-Night Ion Transport At Low Solar Activity In The Venus Pre-Dawn Ionosphere, J. F. Brannon, Jane L. Fox, H. S. Porter Dec 1993

Evidence For Day-To-Night Ion Transport At Low Solar Activity In The Venus Pre-Dawn Ionosphere, J. F. Brannon, Jane L. Fox, H. S. Porter

Physics Faculty Publications

Periapsis of the Pioneer Venus spacecraft dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre-dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re-entry period. In particular, we examine here the atomic ion density profiles. …

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Recombination Of 2-Dimensional Electrons With Free Light And Heavy Holes In Alxga1-Xas-Gaas Single Heterojunctions In A Magnetic-Field, D. C. Reynolds, David C. Look, B. Jogai, C. E. Stutz Dec 1993

Recombination Of 2-Dimensional Electrons With Free Light And Heavy Holes In Alxga1-Xas-Gaas Single Heterojunctions In A Magnetic-Field, D. C. Reynolds, David C. Look, B. Jogai, C. E. Stutz

Physics Faculty Publications

Landau-level oscillations are observed in the photoluminescence from an AlxGa1-xAs-GaAs single heterojunction in an applied magnetic field. Extrapolating the oscillations back to zero field gives the energy of the transition from the two-dimensional (2D) electrons to the free heavy holes (hh1 and hh2) and free light holes (lh) weakly confined in the GaAs active layer. The measured energy separation between hh1 and lh is 1.8 meV, which agrees very well with the calculated value of 2.2 meV. The measured energy separation between hh1 and hh2 is 3.8 meV, in good agreement with the calculated value of …

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Mirrorless All‐Optical Bistability In Bacteriorhodopsin, D.V.G.L.N. Rao, F. J. Aranda, B. J. Wiley, J. A. Akkar, D. L. Kaplan, J. F. Roach Sep 1993

Mirrorless All‐Optical Bistability In Bacteriorhodopsin, D.V.G.L.N. Rao, F. J. Aranda, B. J. Wiley, J. A. Akkar, D. L. Kaplan, J. F. Roach

Physics Faculty Publications

We report direct observations of all‐optical mirrorless bistability associated with saturable absorption in three kinds of bacteriorhodopsin (BR) samples: wild‐type BR in water solution and dispersed in thin films of a polymer matrix as well as water solution of the genetically engineered mutant BRD96N. The experiments are carried out with picosecond pulses at 532 nm. The values measured for the saturation intensity are explained in terms of the relaxation of the excited M state population to the B state of the BR photocycle for the three kinds of samples studied.

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On The Escape Of Oxygen And Hydrogen From Mars, Jane L. Fox Sep 1993

On The Escape Of Oxygen And Hydrogen From Mars, Jane L. Fox

Physics Faculty Publications

Escape rates of oxygen atoms from dissociative recombination of O2+ above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured …

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Numerical Study Of A High-Order Quasiconserved Quantity In The Henon-Heiles Problem, Paul Finkler, C. Edward Jones, Glenn A. Sowell Sep 1993

Numerical Study Of A High-Order Quasiconserved Quantity In The Henon-Heiles Problem, Paul Finkler, C. Edward Jones, Glenn A. Sowell

Physics Faculty Publications

Recent efforts to derive and study a quasiconserved quantity K in the Henon-Heiles problem in terms of a single set of variables are discussed. Numerical results are given, showing how the value of such a quantity varies with time and order in a power-series expansion for K in terms of monomials of the coordinates and velocities. The lowest order in the power series for K corresponds to n =4 and the highest order to n =27, so that 24 orders are included in the series. The results are compared with an earlier study by the authors [Phys. Rev. A 42, …

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Semi-Insulating Nature Of Gas Source Molecular Beam Epitaxial Ingap Grown At Very Low Temperatures, David C. Look, Y. He, J. Ramdani, N. Elmasry, S. M. Bedair Aug 1993

Semi-Insulating Nature Of Gas Source Molecular Beam Epitaxial Ingap Grown At Very Low Temperatures, David C. Look, Y. He, J. Ramdani, N. Elmasry, S. M. Bedair

Physics Faculty Publications

InxGa1−xP lattice matched to GaAs (x≂0.51) has proven to be useful in many device applications. Here we show that undoped, semi‐insulating InGaP is possible by growing with gas source molecular beam epitaxy at very low temperatures, 150–250 °C. The material grown at about 200 °C is n‐type with a 296‐K resistivity of 9×105 Ω cm, a mobility of 120 cm2/V s, and a donor activity energy of 0.48 eV. When annealed at 600 °C for 1 h, the resistivity increases to greater than 109 Ω cm and the …

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Electron-Beam Modification Of Gaas Surface-Potential - Measurement Of Richardson Constant, S. M. Lindsay, Joseph W. Hemsky, David C. Look Aug 1993

Electron-Beam Modification Of Gaas Surface-Potential - Measurement Of Richardson Constant, S. M. Lindsay, Joseph W. Hemsky, David C. Look

Physics Faculty Publications

The surface potential of GaAs is strongly modified in the presence of a high‐energy electron beam due to the creation of electron‐hole pairs in the depletion region and the subsequent drift of the holes to the surface where they neutralize surface states. This effect is modeled in terms of a parameter K=A∗T2/Ib(dE/dz)η, where Ib is the beam current density, A∗ is the effective Richardson constant, dE/dz is the beam energy loss per unit length, and η−1 is the average energy required to …

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Electron-Drift Velocities In Argon-Boron Trichloride Gas-Mixtures, Donald L. Mosteller Jr., Merrill L. Andrews, Jerry D. Clark, A. Garscadden Aug 1993

Electron-Drift Velocities In Argon-Boron Trichloride Gas-Mixtures, Donald L. Mosteller Jr., Merrill L. Andrews, Jerry D. Clark, A. Garscadden

Physics Faculty Publications

Electron drift velocities were measured for argon gas mixtures containing 25 ppm to 0.5% boron trichloride additive using a pulsed-Townsend drift tube. These results show a marked sensitivity to mixture ratio and the gas mixtures also show negative differential conductivity and strong attachment at low electric field/gas number density.

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Diffraction Of A Gaussian Beam By A Spherical Obstacle, James A. Lock, Edward A. Hovenac Aug 1993

Diffraction Of A Gaussian Beam By A Spherical Obstacle, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

The Kirchhoff integral for diffraction in the near-forward direction is derived from the exact solution of the electromagnetic boundary value problem of a focused Gaussian laser beam incident on a spherical particle. The diffracted intensity in the vicinity of the particle is computed and the way in which the features of the diffraction pattern depend on the width of the Gaussian beam is commented on.

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Interpretation Of Electron Correlation In Local-Density Approximation For Exchange, Viraht Sahni, Marlina Slamet Jul 1993

Interpretation Of Electron Correlation In Local-Density Approximation For Exchange, Viraht Sahni, Marlina Slamet

Physics Faculty Publications

We provide a new interpretation of how electron correlations are represented within the local-density approximation for exchange by deducing via Harbola-Sahni theory the corresponding pair-correlation density. The expression differs from that in the literature and contains, in addition to the uniform electron-gas result, a term proportional to the gradient of the density. Thus, the nonuniformity of the electron density is explicitly incorporated in the approximation, and constitutes thereby the reason underlying its success.

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Upper Limits To The Nightside Ionosphere Of Mars, Jane L. Fox Jul 1993

Upper Limits To The Nightside Ionosphere Of Mars, Jane L. Fox

Physics Faculty Publications

The nightside ionosphere of Mars could be produced by electron precipitation or by plasma transport from the dayside, by analogy to the Venus, but few measurements are available. We report here model calculations of upper limits to the nightside ion densities on Mars that would be produced by both mechanisms. For the auroral model, we have adopted the downward traveling portions of the electron spectra measured by the HARP instrument on the Soviet Phobos spacecraft in the Martian plasma sheet and in the magnetotail lobes. For the plasma transport case, we have imposed on a model of the nightside thermosphere, …

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High-Voltage Mosfet Bipolar Square-Wave Generator, T. D. Usher, Grant A. Mcauley Jul 1993

High-Voltage Mosfet Bipolar Square-Wave Generator, T. D. Usher, Grant A. Mcauley

Physics Faculty Publications

he construction of a high‐voltage (up to 1000 V) bipolar metal‐oxide‐semiconductor field‐effect transistor square‐wave generator is described. This generator is capable of producing both positive and negative going square waves with variable amplitude, repetition rate, and width. The circuit was designed for ferroelectrics research, however other applications are possible. The rise time of the prototype was 200 ns which was quite satisfactory for the present ferroelectric research project however the rise time can be decreased to 50 ns if necessary. The reader with a modest knowledge of electronics should be able to construct the circuit. Possible pitfalls and critical points …

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Prominent Thermally Stimulated Current Trap In Low-Temperature-Grown Molecular-Beam Epitaxial Gaas, Z-Q. Fang, David C. Look Jul 1993

Prominent Thermally Stimulated Current Trap In Low-Temperature-Grown Molecular-Beam Epitaxial Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

By far, the largest thermally stimulated current trap in molecular beam epitaxial GaAs grown at 200–250 °C is T5, with an activation energy of 0.27 eV and most likely related to VGa. After an anneal at 300–350 °C, another trap T6☒ appears, with an activation energy of 0.14 eV and closely identified with VAs or the complex, VAs‐AsGa. Proposed defect reactions in this As‐rich material include VGa+AsAs→VAs‐AsGa, and VGa+AsGa→VGa‐AsGa.

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Annealing Dynamics Of Molecular-Beam Epitaxial Gaas Grown At 200°C, David C. Look, D. C. Walters, G. D. Robinson, J. R. Sizelove, M. G. Mier, C. E. Stutz Jul 1993

Annealing Dynamics Of Molecular-Beam Epitaxial Gaas Grown At 200°C, David C. Look, D. C. Walters, G. D. Robinson, J. R. Sizelove, M. G. Mier, C. E. Stutz

Physics Faculty Publications

By separating a 2‐μm‐thick molecular‐beam‐epitaxial GaAs layer grown at 200 °C from its 650‐μm‐thick substrate, we have been able to obtain accurate Hall‐effect and conductivity data as functions of annealing temperature from 300 to 600 °C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. …

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Analytical Two-Layer Hall Analysis - Application To Modulation-Doped Field-Effect Transistors, David C. Look, C. E. Stutz, Christopher A. Bozada Jul 1993

Analytical Two-Layer Hall Analysis - Application To Modulation-Doped Field-Effect Transistors, David C. Look, C. E. Stutz, Christopher A. Bozada

Physics Faculty Publications

The classical magnetic‐field‐dependent Hall coefficient and conductivity equations are inverted to give the mobilities μ1 and μ2 and carrier concentrations n1 (or p1) and n2 (or p2) in two degenerate bands. The two‐band solution holds for arbitrary magnetic‐field strength as long as quantum effects can be ignored (i.e., kT≳ℏeB/m∗), and it is argued that the analysis can also be applied to two separate layers up to reasonable field strengths. The results are used to determine the two‐dimensional electron gas mobility and carrier concentration in a modulation‐doped field‐effect …

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Donor And Acceptor Concentrations In Molecular-Beam Epitaxial Gaas Grown At 300-Degrees-C And 400-Degrees-C, David C. Look, G. D. Robinson, J. R. Sizelove, C. E. Stutz Jun 1993

Donor And Acceptor Concentrations In Molecular-Beam Epitaxial Gaas Grown At 300-Degrees-C And 400-Degrees-C, David C. Look, G. D. Robinson, J. R. Sizelove, C. E. Stutz

Physics Faculty Publications

The first Hall‐effect measurements on molecular beam epitaxial GaAs layers grown at the low temperatures of 300 and 400 °C are reported. Two independent methods were used to determine donor ND and acceptor NA concentrations and activation energy ED0, with the following combined results: ND≂3±1×1018, NA≂1.5±1×1017 cm−3, and ED0=0.645±0.009 eV for the 300 °C layer; ND≂2±1×1017, NA≂7±3×1016 cm−3, and ED0=0.648±0.003 eV for the 400 °C layer. Thus, the deep donor is …

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Thermally Activated Escape Over Fluctuating Barriers, Ulrich Zürcher, Charles R. Doering Jun 1993

Thermally Activated Escape Over Fluctuating Barriers, Ulrich Zürcher, Charles R. Doering

Physics Faculty Publications

We investigate the thermally activated escape of a Brownian particle over a potential barrier whose height fluctuates with a rate α between the values E+ and E−. We are mainly interested in the low-temperature behavior where E+/T≫E−/T. We calculate the mean exit time as a function of the rate of the barrier fluctuations for the piecewise linear and the piecewise constant barrier, τ=τ(α). For the piecewise constant potential we find three different regimes: τ∼τ+ for α<τ−1+=exp(-E+/T), τ∼2τ− for α>τ−1−=exp(-E−/T), and τ∼α−1 for τ−1+<α<τ−1−. The mean exit time for the piecewise linear potential has a different behavior for fast barrier fluctuations, α>τ−1−; τ(α) is a monotonously increasing function that approaches the asymptotic value τ∼ √τ+τ− for α→∞. We show that …

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Photoquenching And Thermal Recovery Of A Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look May 1993

Photoquenching And Thermal Recovery Of A Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

A prominent thermally stimulated current peak T5 appearing in semi‐insulating GaAs is shown to photoquench under infrared illumination, and then thermally recover at a rate r=2.0×108 exp(−0.26 eV/kT) s−1, exactly the same as that observed for EL2, within experimental error. Two possible explanations exist: (1) T5 and EL2 are microscopically very similar, probably each with an AsGa core; or (2) T5 is an electron trap that only appears to quench and recover with EL2 because EL2 controls the electron lifetime. Several other traps show similar quenching and recovery behavior.

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The Effect Of Ionizing And Displacive Radiation On The Thermal Conductivity Of Alumina, D. P. White Apr 1993

The Effect Of Ionizing And Displacive Radiation On The Thermal Conductivity Of Alumina, D. P. White

Physics Faculty Publications

The effects of ionizing and displacive radiation on the thermal conductivity of alumina at high temperatures have been studied. The phonon scattering relaxation times for several scattering mechanisms have been used to determine the effect on the thermal conductivity. The scattering mechanisms considered are scattering by electrons excited into the conduction band, vacancies, aluminum precipitates, and voids. It is found that under irradiation conditions where the electrical conductivity and dielectric loss tangent are greatly increased there is not a significant decrease in the thermal conductivity due to phonon-electron scattering. The conditions under which the scattering due to vacancies, aluminum precipitates, …

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Contribution Of High-Order Rainbows To The Scattering Of A Gaussian Laser Beam By A Spherical Particle, James A. Lock Apr 1993

Contribution Of High-Order Rainbows To The Scattering Of A Gaussian Laser Beam By A Spherical Particle, James A. Lock

Physics Faculty Publications

I review the theory of the scattering of a Gaussian laser beam by a dielectric spherical particle and give the details for constructing a computer program to implement the theory. Computational results indicate that if the width of the laser beam is much less than the diameter of the particle and if the axis of the beam is incident near the edge of the particle, the fifth-, sixth-, and ninth-order rainbows should be evident in the far-field scattered intensity. I performed an experiment that yielded tentative evidence for the presence of the sixth-order rainbow.

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Measurement Of The Casimir-Polder Force, C. I. Sukenik, M. G. Boshier, S. Cho, V. Sandoghdar, E. A. Hinds Feb 1993

Measurement Of The Casimir-Polder Force, C. I. Sukenik, M. G. Boshier, S. Cho, V. Sandoghdar, E. A. Hinds

Physics Faculty Publications

The authors have studied the deflection of ground-state sodium atoms passing through a micron-sized parallel-plate cavity by measuring the intensity of a sodium atomic beam transmitted through the cavity as a function of cavity plate separation. This experiment provides clear evidence for the existence of the Casimir-Polder force, which is due to modification of the ground-state Lamb shift in the confined space of a cavity. The results confirm the magnitude of the force and the distance dependence predicted by quantum electrodynamics.

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On The System Of Diffuse Interstellar Bands At 5844 And 5850 Å, J Krelowski, Theodore P. Snow, J Papaj, C G. Seab, B Wszolek Jan 1993

On The System Of Diffuse Interstellar Bands At 5844 And 5850 Å, J Krelowski, Theodore P. Snow, J Papaj, C G. Seab, B Wszolek

Physics Faculty Publications

Two neighboring diffuse interstellar bands (DIBs) at 5844 and 5850 Å are shown to be of different origin. The presence or absence of the 5844 DIB is related to the ratio of the two prominent DIBs at 5780 and 5797 Å as well as to the nature of the ultraviolet extinction curve. The 5844 Å DIB is very sensitive to the extinction, being completely absent in cases where the far-UV extinction is low. This suggests that the carrier is destroyed by photons having energies above 11 or 12 eV. The 5850 Å DIB correlates very well with the well-studied DIB …

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New Asga Related Center In Gaas, David C. Look, Z-Q. Fang, J. R. Sizelove, C. E. Stutz Jan 1993

New Asga Related Center In Gaas, David C. Look, Z-Q. Fang, J. R. Sizelove, C. E. Stutz

Physics Faculty Publications

A new center related to AsGa has been found at relatively high concentrations (1017 cm-;3) in semi‐insulating (2×10;7 Ω cm) molecular beam epitaxial GaAs grown at 400 °C. Although the ir photoquenching and thermal recovery characteristics are nearly identical to those of ;EL2, the thermal activation energy is only 0.65±0.01 eV, much lower than the ;EL2 value of 0.75±0.01 eV. Other properties which are different include the electron‐capture barrier energy, hyperfine constant, and magnetic circular dichroism spectrum.

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The Production And Escape Of Nitrogen Atoms On Mars, Jane L. Fox Jan 1993

The Production And Escape Of Nitrogen Atoms On Mars, Jane L. Fox

Physics Faculty Publications

We have computed the production rates and densities of odd nitrogen species in the Martian atmosphere using updated rate coefficients and a revised ionosphere-thermosphere model. We find that the computed densities of NO are somewhat smaller than those measured by Viking 1, but reasonable agreement can be obtained by assuming that the rate coefficient for loss of odd nitrogen in the reaction of N with NO is smaller at temperatures that prevail in the lower Martian thermosphere (about 130–160 K) than the standard value, which applies to temperatures of 200–400 K. We have also modeled the escape fluxes of N …

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Recovery Of Quenched Hopping Conduction In Gaas-Layers Grown By Molecular-Beam Epitaxy At 200-Degrees-C, David C. Look, Z-Q. Fang, J. R. Sizelove Jan 1993

Recovery Of Quenched Hopping Conduction In Gaas-Layers Grown By Molecular-Beam Epitaxy At 200-Degrees-C, David C. Look, Z-Q. Fang, J. R. Sizelove

Physics Faculty Publications

The dark current at 82 K, in GaAs layers grown by molecular-beam epitaxy at 200 °C and annealed at 550 °C, is reduced by a factor 350 after 5 min of IR (hν<~1.12 eV) light illumination. As temperature is swept upward at 0.2 K/s, the current recovers rapidly near 130 K. A numerical analysis of the current recovery, based on hopping conduction, gives an excellent fit to the data for a thermal recovery rate r=3×108 exp(-0.26/kT), very close to the rate observed for EL2 (AsGa). This proves that the conduction below 300 K in this material is due to hopping between AsGa-related centers in their ground states. Variable-range hopping [exp-(T0/T)1/4] gives a slightly better fit to the data than nearest-neighbor hopping [exp(-ɛ3/kT)] in the range T=82-160 K, but the fitted recovery …

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Determination Of The Neutron Spin Structure Function, P. L. Anthony, R. G. Arnold, H. R. Band, H. Borel, P.E. Bosted, V. Breton, G. D. Cates, T. E. Chupp, F. S. Dietrich, J. Dunne, R. Erbacher, J. Fellbaum, H. Fonvielle, R. Gearhart, R. Holmes, E. W. Hughes, J. R. Johnson, D. Kawall, C. Keppel, S. E. Kuhn, R. M. Lombard-Nelsen, J. Marroncle, T. Maruyama, W. Meyer, Z. E. Meziani, H. Middleton, J. Morgenstern, N. R. Newbury, G. G. Petratos, R. Pitthan, R. Prepost, Y. Roblin, S. E. Rock, S. H. Rokni, G. Shapiro, T. Smith, P. A. Souder, M. Spengos, F. Staley, L. M. Stuart, Z. M. Szalata, Y. Terrien, A. K. Thompson, J. L. White, M. Woods, J. Xu, C. C. Young, G. Zapalac, E142 Collaboration Jan 1993

Determination Of The Neutron Spin Structure Function, P. L. Anthony, R. G. Arnold, H. R. Band, H. Borel, P.E. Bosted, V. Breton, G. D. Cates, T. E. Chupp, F. S. Dietrich, J. Dunne, R. Erbacher, J. Fellbaum, H. Fonvielle, R. Gearhart, R. Holmes, E. W. Hughes, J. R. Johnson, D. Kawall, C. Keppel, S. E. Kuhn, R. M. Lombard-Nelsen, J. Marroncle, T. Maruyama, W. Meyer, Z. E. Meziani, H. Middleton, J. Morgenstern, N. R. Newbury, G. G. Petratos, R. Pitthan, R. Prepost, Y. Roblin, S. E. Rock, S. H. Rokni, G. Shapiro, T. Smith, P. A. Souder, M. Spengos, F. Staley, L. M. Stuart, Z. M. Szalata, Y. Terrien, A. K. Thompson, J. L. White, M. Woods, J. Xu, C. C. Young, G. Zapalac, E142 Collaboration

Physics Faculty Publications

The spin structure function of the neutron g1n has been determined over the range 0.03 < x < 0.6 at an average Q2 of 2 (GeV/c)2 by measuring the asymmetry in deep inelastic scattering of polarized electrons from a polarized 3He target at energies between 19 and 26 GeV. The integral of the neutron spin structure function is found to be f-10 gn1(x)dx = -0.022 ± 0.011. Earlier reported proton results together with the Bjorken sum rule predict f-10 gn1(x)dx = -0.059 ± 0.019.

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²H (D,Γ) ⁴He Polarization Observables At 20, 30, And 50 Mev, R. M. Whitton, H. R. Weller, E. Hayward, W. R. Dodge, S. E. Kuhn Jan 1993

²H (D,Γ) ⁴He Polarization Observables At 20, 30, And 50 Mev, R. M. Whitton, H. R. Weller, E. Hayward, W. R. Dodge, S. E. Kuhn

Physics Faculty Publications

Tensor and vector analyzing powers for the ²H (d,γ) ⁴He reaction have been obtained as a function of angle at Ed(lab) = 20, 30, and 50 MeV. Differential cross sections were extracted at 30 and 50 MeV. Data for Ayy(θ) and Ay(θ) were obtained at all three energies, while T20(θ) was also measured at Ed(lab) = 50 MeV. A direct capture calculation was performed and compared to the data. This calculation assumes point deuterons and that the reaction proceeds primarily via E2 radiation in this energy region. A value of …

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