Digital Commons Network^™

Strain Variation With Sample Thickness In Gan Grown By Hydride Vapor Phase Epitaxy, D. C. Reynolds, David C. Look, B. Jogai, J. E. Hoelscher, R. E. Sherriff, Richard J. Molnar

Physics Faculty Publications

High quality GaN crystals can be grown on sapphire by hydride vapor phase epitaxy. The thermal expansion mismatch between sapphire and GaN produces strain in the GaN crystal as it is cooled from the growth temperature to room temperature. The strain is evidenced by shifts in the photoluminescence and reflectance line positions. By analyzing the surface strain as the crystal thickness is increased, the thickness required to obtain zero surface strain can be estimated. This structure might provide a lattice matched and thermally matched substrate for further epitaxial growth of GaN.

Observation Of Stratospheric Ozone Depletion Associated With Delta Ii Rocket Emissions, Martin N. Ross, Darin W. Toohey, W. T. Rawlins, E. C. Richard, K. K. Kelly, A. F. Tuck, M. H. Proffitt, Donald E. Hagen, Alfred R. Hopkins, Philip D. Whitefield, J. R. Benbrook, W. R. Sheldon

Physics Faculty Research & Creative Works

Ozone, chlorine monoxide, methane, and submicron particulate concentrations were measured in the stratospheric plume wake of a Delta II rocket powered by a combination of solid (NH₄ClO₄/Al) and liquid (LOX/kerosene) propulsion systems. We apply a simple kinetics model describing the main features of gas-phase chlorine reactions in solid propellant exhaust plumes to derive the abundance of total reactive chlorine in the plume and estimate the associated cumulative ozone loss. Measured ozone loss during two plume encounters (12 and 39 minutes after launch) exceeded the estimate by about a factor of about two. Insofar as only the …

Molecular-Dynamics Study Of Phase Transitions In Alkali Thiocyanates, M. M. Ossowski, John R. Hardy, R. W. Smith

John R. Hardy Papers

An account is presented of our studies of the order-disorder phase transitions in KSCN, RbSCN, and CsSCN. These are based on parameter-free interionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions. We performed static structural relaxations and supercell molecular dynamics and predicted with reasonable accuracy the temperatures for the onset of the transitions. In particular, we address the question of how the SCN^- ions disorder to yield subsequent structural transformations. We found high-temperature phases of average Fm3m symmetry for both KSCN and RbSCN. We argue that in reality the full appearance of …

Effect Of Disorder On Perpendicular Magnetotransport In Co/Cu Multilayers, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

We investigate the spin-dependent conductance of the Co/Cu multilayers in the current-perpendicular-tothe- plane (CPP) geometry. Using a realistic tight-binding model for the electronic band structure of the multilayer, and introducing disorder in the on-site atomic energies we calculate the conductance and giant magnetoresistance (GMR) within the quantum-mechanical linear response theory by performing averaging over random disorder configurations. By varying the thickness and the number of individual layers and the degree of disorder in the multilayers, we analyze factors influencing the CPP GMR. In particular, we show the importance of the thickness-dependent interface resistance, which depends on the mean free path …

Theory Of Magnetostatic Coupling In Thin-Film Rectangular Magnetic Elements, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

A theory of magnetostatic coupling in thin-film uniformly magnetized arrays of rectangular magnetic elements is presented. Analytic expressions for the magnetostatic energy and the dipolar fields are derived. The influence of the aspect ratio and the spacing between the elements on the magnetostatic coupling is investigated. It is found that increasing the aspect ratio reduces the critical distance between the elements, above which the magnetostatic inter-element coupling can be neglected.

Limits On Anomalous Ww Γ And Wwz Couplings From Ww/Wz→Eνjj Production, B. Abbott, Gregory Snow, D0 Collaboration

Gregory Snow Publications

Limits on anomalous WW γ and WWZ couplings are presented from a study of WW/WZ→eνjj events produced in pp̅ collisions at √s = 1.8 TeV. Results from the analysis of data collected using the DØ detector during the 1993–1995 Tevatron collider run at Fermilab are combined with those of an earlier study from the 1992–1993 run. A fit to the transverse momentum spectrum of the W boson yields direct limits on anomalous WW γ and WWZ couplings. With the assumption that the WW γ and WWZ couplings are equal, we obtain -0.34 < λ < 0.36 (with Δk=0) and -0.43< Δk<0.59 (with λ =0) at the 95% confidence level for a form-factor scale Λ = 2.0 TeV.

42nd Rocky Mountain Conference On Analytical Chemistry

Rocky Mountain Conference on Magnetic Resonance

Abstracts from the 42nd annual meeting of the Rocky Mountain Conference on Analytical Chemistry, co-sponsored by the Colorado Section of the American Chemical Society and the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Broomfield, Colorado, July 30 - August 3, 2000.

Driven Vortices In Confined Geometry: The Corbino Disk, M. Cristina Marchetti

Physics - All Scholarship

The fabrication of artificial pinning structures allows a new generation of experiments which can probe the properties of vortex arrays by forcing them to flow in confined geometries. We discuss the theoretical analysis of such experiments in both flux liquids and flux solids, focusing on the Corbino disk geometry. In the liquid, these experiments can probe the critical behavior near a continuous liquid-glass transition. In the solid, they probe directly the onset of plasticity.

Final Quantum Nf State Analysis For Multiply Charged Ion-Circular Rydberg Target Collisions, K. R. Cornelius, Ronald E. Olson

Physics Faculty Research & Creative Works

The classical trajectory Monte Carlo method has been used to simulate fully stripped ion-circular Rydberg atom collisions. We report that the mechanisms that govern the capture process for ions colliding with aligned Rydberg targets, namely direct and indirect velocity matching, alternate as to which is dominant. Previous experimental and theoretical results for singly charged ion-Rydberg atom collisions demonstrate that direct capture processes dominate at intermediate collision speeds. We have found that for highly charged ion-Rydberg atom collisions electron capture via indirect velocity matching is dominant at intermediate collision speeds and contributes up to 90% of the total capture cross-section.

Static-Electric-Field-Induced Polarization Effects In Harmonic Generation, Bogdan Borca, A.V. Flegel, M.V. Frolov, N. L. Manakov, Dejan B. Milosevic, Anthony F. Starace

Anthony F. Starace Publications

Two static-electric-field-induced effects on harmonic generation are demonstrated analytically and numerically: elliptic dichroism (in which the harmonic yield is different for right and left elliptically polarized laser fields) and elliptical polarization of harmonics produced by linearly polarized driving laser fields. Both effects stem from interference of real and imaginary parts of the nonlinear atomic susceptibilities. Possibilities for experimentally measuring these effects are discussed.

Energetics And Geometry Of Excitations In Random Systems, Alan Middleton

Physics - All Scholarship

Methods for studying droplets in models with quenched disorder are critically examined. Low energy excitations in two dimensional models are investigated by finding minimal energy interior excitations and by computing the effect of bulk perturbations. The numerical data support the assumptions of compact droplets and a single exponent for droplet energy scaling. Analytic calculations show how strong corrections to power laws can result when samples and droplets are averaged over. Such corrections can explain apparent discrepancies in several previous numerical results for spin glasses

Surface Electronic Phase Transition In Colossal Magnetoresistive Manganese Perovskites: La0.65Sr0.35Mno3, Hani Dulli, E. Ward Plummer, Peter A. Dowben, Jaewu Choi, Sy_Hwang Liou

Peter Dowben Publications

We have observed a distinct surface phase transition for an important class of giant magnetoresistance materials [La_1-x Sr_x MnO₃ (x = 0.35)]. The surface phase transition occurs at 240 K compared to 370 K for the bulk and is fundamentally different. In the bulk, a ferromagnetic metal to paramagnetic bad-metal transition occurs, while the lower-temperature surface transition is from an insulator to a semimetal. The surface of this manganese perovskite is electronically and compositionally quite different from the bulk with important implications for the behavior of artificially grown layered transition-metal oxides and for the use of surface …

Thermolectric Materials Based On Intercalated Layered Metallic Systems, Peter C. Eklund, Leonid Grigorian, Keith A. Williams, Gamini U. Sumanasekera

Physics and Astronomy Faculty Patents

A novel thermoelectric material in the form of a unit cell including a first reagent intercalated as a semiconducting layer into a metallic layered host and a method for producing the thermoelectric material are disclosed. The unit cell is characterized by a Seebeck coefficient S of 80-140 μV/K° and an electrical conductivity σ of 10³ -10⁴ (Ωcm)^-1, as well as a figure of merit Z of about 2×10^-3 K^-1 at 100° K.

Freezing Field Dependance Of The Exchange Bias In Uniaxial Fef2-Copt Heterosystems With Perpendicular Anisotropy, Christian Binek

Christian Binek Publications

The exchange bias effect is measured for the first time in FeF2–CoPt heterosystems with perpendicular anisotropy. The exchange previous field exhibits a strong dependence on the axial previous freezing field. This behavior is explained in terms of the microscopic spin structure at the interface, which is established on cooling to below TN. We calculate the dependence of the spin structure on the previous freezing field within the framework of an Ising model. It takes into account the Zeeman energy as well as an antiferromagnetic exchange coupling between the adjacent layers at the interface.

Comment On “Direct Observation Of Stretched-Exponential Relaxation In Low-Temperature Lennard-Jones Systems Using The Cage Correlation Function”, Ulrich Zürcher, T. Keyes

Physics Faculty Publications

A Comment on the Letter by Eran Rabani, J. Daniel Gezelter, and D. J. Berne, Phys. Rev. Lett. 82, 3649 (1999). The authors of the Letter offer a Reply.

Plasma Oscillations And Expansion Of An Ultracold Neutral Plasma, Scott D. Bergeson, S. Kulin, T. C. Killian, S. L. Rolston

Faculty Publications

We report the observation of plasma oscillations in an ultracold neutral plasma. With this collective mode we probe the electron density distribution and study the expansion of the plasma as a function of time. For classical plasma conditions, i.e., weak Coulomb coupling, the expansion is dominated by the pressure of the electron gas and is described by a hydrodynamic model. Discrepancies between the model and observations at low temperature and high density may be due to strong coupling of the electrons.

Kinetic Energy Release In Electron-Induced Decay Reactions Of Molecular Ions: C3h8+ And C3h7+, S. Matt, Olof E. Echt, A. Stamatovic, T. D. Mark

Physics & Astronomy

We have measured the kinetic energy release (KER) distributions for electron-induced dissociation of mass-selected molecular parent and fragment ions of propane. They are compared with distributions determined for spontaneous (metastable) dissociation. The average KER for induced dissociation of C3H8+ into C3H7+ is 13.2 +/- 1.2 meV, about 42% larger than for the spontaneous reaction. This large difference is attributed to the dramatically reduced time at which the induced reaction can be sampled. In contrast, the KER for dissociation of C3H7+ into C3H5+, which is characterized by a large reverse activation energy, is hardly affected by the experimental time scale. (C) …

Static Quark Potential In Three Flavor Qcd, James E. Hetrick, C. Bernard

All Faculty Articles - School of Engineering and Computer Science

We study the effects of dynamical quarks on the static quark potential at distances shorter than those where string breaking is expected. Quenched calculations and calculations with three flavors of dynamical quarks are done on sets of lattices with the lattice spacings matched within about one percent. The effect of the sea quarks on the shape of the potential is clearly visible. We investigate the consequences of these effects in a very crude model, namely solving Schrödinger’s equation in the resulting potential.

Effective Surface Debye Temperature For Nimnsb.100. Epitaxial Films, C.N. Borca, Takashi Komesu, Hae-Kyung Jeong, Peter A. Dowben, D. Ristoiu, Ch. Hordequin, J. Pierre, J. P. Nozie`Res

Peter Dowben Publications

The surface Debye temperature of the NiMnSb (100) epitaxial films has been obtained using low energy electron diffraction, inverse photoemission, and core-level photoemission. The normal dynamic motion of the (100) surface results in a value for the effective surface Debye temperature of 145613 K. This is far smaller than the bulk Debye temperature of 31265 K obtained from wave vector dependent inelastic neutron scattering. The large difference between these measures of surface and bulk dynamic motion indicates a soft and compositionally different (100) surface. © 2000 American Institute of Physics. [S0003-6951(00)05127-5]

Quantum Computation Through Entangling Single Photons In Multipath Interferometers, John C. Howell, John A. Yeazell

Mathematics, Physics, and Computer Science Faculty Articles and Research

Single-photon interferometry has been used to simulate quantum computations. Its use has been limited to studying few-bit applications due to rapid growth in physical size with numbers of bits. We propose a hybrid approach that employs n photons, each having L degrees of freedom yielding Lⁿ basis states. The photons are entangled via a quantum nondemolition measurement. This approach introduces the essential element of quantum computing, that is, entanglement into the interferometry. Using these techniques, we demonstrate a controlled-NOT gate and a Grover's search circuit. These ideas are also applicable to the study of nonlocal correlations in many dimensions.

Fluid Flow In Micro-Channels: A Stochastic Approach, Hilda Marino Black

Doctoral Dissertations

In this study free molecular flow in a micro-channel was modeled using a stochastic approach, namely the Kolmogorov forward equation in three dimensions. Model equations were discretized using Central Difference and Backward Difference methods and solved using the Jacobi method. Parameters were used that reflect the characteristic geometry of experimental work performed at the Louisiana Tech University Institute for Micromanufacturing.

The solution to the model equations provided the probability density function of the distance traveled by a particle in the micro-channel. From this distribution we obtained the distribution of the residence time of a particle in the micro-channel. Knowledge of …

Search For The Charged Higgs Boson In The Decays Of Top Quark Pairs In The EΤ And Μτ Channels At √S =1.8 Tev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

Top quark production offers the unique opportunity to search for a charged Higgs boson (H^±), as the contribution from t→H₊b→ τ ₊ νb can be large in extensions of the standard model. We use results from a search for top quark pair production by the Collider Detector at Fermilab (CDF) in the e τ +E_T+jets and μ τ+E_T+jets signatures to set an upper limit on the branching ratio of Ɓ (t→H₊b) in 106 pb^-1 of data. The upper …

Resolution Of Overlapping Spectra By Wavelength Modulation Spectroscopy, Audra Michiele Bullock

Electrical & Computer Engineering Theses & Dissertations

Wavelength modulation absorption spectroscopy is a highly sensitive, non-intrusive technique for probing gaseous species, which employs the well-known principles of modulation spectroscopy in a novel way. With this technique, parameters such as velocity, density, and temperature can be measured with a high degree of precision. The research presented here shows that wavelength modulation is a convenient means of increasing the sensitivity of an absorption spectroscopy measurement because it allows for harmonic detection. The focus of the dissertation is resolution of overlapping spectra by harmonic detection and the advantages gained by performing detection at the higher harmonics, e.g., sixth and eighth. …

Phenomenological Description Of Double-Pion Photoproduction, Andry M. Rakotovao

Physics Theses & Dissertations

Double pion photoproduction off proton targets is studied, in the framework of an Effective Lagrangian approach at tree level, based on the coupling of photons and pions to nucleons and resonances. We consider N, Δ(1232), N(1440), N(1520) and N(1535) as intermediate baryonic states and ρ-meson as the intermediate 2π resonance. The general form of the amplitude is presented and total cross sections of the γp → π⁺π− p, γp → π⁰π⁰p and γp → π ⁺π⁰n processes are evaluated up to Eγ …

Changes In Electron-Phonon Coupling Across A Bulk Phase Transition In Copolymer Films Of Vinylidene Fluoride (70%) With Trifluoroethylene (30%), C. N. Borca, Shireen Adenwalla, Jaewu Choi, Lee Robertson, H. You, V. M. Fridkin, S. P. Palto, N. Petukhova, Stephen Ducharme, Peter A. Dowben

Peter Dowben Publications

We present evidence for a change in electron – phonon coupling across a bulk ph=e transition. Below the lattice stiffening transition at around 160 K, there is a change in the diffracted peak width observed by neutron and X-ray scattering techniques. Also, the electronic band structure of the copolymer is shifting in binding energy below 160 K, decreasing the density of states near the Fermi level. The value of the effective Debye temperature above the transition temperature is approximately 50 K, while below 160 K, the value of the Debye parameter is 245 K. We postulate that the coupling between …

Thermal Lattice Boltzmann Simulation For Multispecies Fluid Equilibration, Linda L. Vahala, Darren Wah, George Vahala, Jonathan Carter, Pavol Pavlo

Electrical & Computer Engineering Faculty Publications

The equilibration rate for multispecies fluids is examined using thermal lattice Boltzmann simulations. Two-dimensional free-decay simulations are performed for effects of velocity shear layer turbulence on sharp temperature profiles. In particular, parameters are so chosen that the lighter species is turbulent while the heavier species is laminar-and so its vorticity layers would simply decay and diffuse in time. With species coupling, however, there is velocity equilibration followed by the final relaxation to one large co- and one large counter-rotating vortex. The temperature equilibration proceeds on a slower time scale and is in good agreement with the theoretical order of magnitude …

Direct Measurement Of Oscillations Between Degenerate Two-Electron Bound-State Configurations In A Rapidly Autoionizing System, Heider N. Ereifej, J. Greg Story

Physics Faculty Research & Creative Works

In this paper we report a direct observation of the oscillation between bound-state configurations in a rapidly autoionizing system. Calcium atoms were excited to a pure 4p_3/2nd two-electron configuration using a 500-fsec laser pulse. The initial 4p_3/2nd doubly excited state is energy degenerate with the 4p_1/2n'd states and several continuum channels. Because of the short-pulse excitation, the initial state of the atom is not an energy eigenstate, but a nonstationary wave packet. As a result, oscillations between the two bound configurations were produced. These oscillations were measured by scanning the timing of a second …

Neutral Beam Stopping And Emission In Fusion Plasmas. I: Deuterium Beams, H. Anderson, M. G. Von Hellermann, R. Hoekstra, L. D. Horton, A. C. Howman, R. W.T. Konig, R. Martin, Ronald E. Olson, H. P. Summers

Physics Faculty Research & Creative Works

The charge transfer reaction of neutral deuterium beams with impurities enables one of the principle quantitative diagnostic measurements of the hot core fusion plasma; that is, charge exchange spectroscopy. The complementary measurement of beam emission spectroscopy has been fruitful in motional Stark wavelength shift and fluctuation studies, but less so in using absolute measured intensities. In the last two years we have achieved substantial improvement in the quantitative analysis and agreement between the observed and modelled beam emission at the JET Joint Undertaking. This has depended on improved spectral fitting of the overlayed Dα motional Stark multiplet, self-consistent beam emission …

Electroweak Structure Of Three-And Four-Body Nuclei, Laura Elisa Marcucci

Physics Theses & Dissertations

This work reports results for (i) the elastic electromagnetic form factors of the trinucleons; (ii) the nuclear response functions of interest in [e → e'] experiments, at excitation energies below the deuteron breakup threshold; (iii) the astrophysical S-factor for proton weak capture on ³He (the hep reaction). The initial and final state wave functions are calculated using the correlated hyperspherical harmonics method, from a realistic Hamiltonian consisting of the Argonne ^v18 two-nucleon and Urbana IX three-nucleon interactions. The nuclear electroweak charge and current operators include one- and many-body components. The predicted magnetic form factor of …

Changes In Electron-Phonon Coupling Across A Bulk Phase Transition In Copolymer Films Of Vinylidene Fluoride (70%) With Trifluoroethylene (30%), C. N. Borca, Shireen Adenwalla, Jaewu Choi, Lee Robertson, H. You, V. M. Fridkin, S. P. Palto, N. Petukhova, Stephen Ducharme, Peter A. Dowben

Peter Dowben Publications

We present evidence for a change in electron - phonon coupling across a bulk phase transition. Below the lattice stiffening transition at around 160 K, there is a change in the diffracted peak width observed by neutron and X-ray scattering techniques. Also, the electronic band structure of the copolymer is shifting in binding energy below 160 K, decreasing the density of states near the Fermi level. The value of the effective Debye temperature above the transition temperature is approximately 50 K, while below 160 K, the value of the Debye parameter is 245 K. We postulate that the coupling between …