Physical Sciences and Mathematics Commons^™

Semiclassical Scattering In A Circular Semiconductor Microstructure, C. D. Schwieters, J. A. Alford, John B. Delos

Arts & Sciences Articles

The conductance of a microscopic junction shows fluctuations caused by quantum interference of waves that follow different paths between the leads. We give a semiclassical formula for these fluctuations. The theory utilizes trajectories which travel between the centers of the lead apertures; it also incorporates diffraction at these apertures. We extend the theory to include ‘‘ghost paths,’’ which scatter diffractively off the lead mouths. Semiclassical S-matrix elements are computed for a circular junction over a range of Fermi wave numbers, and the large-scale structure of these matrix elements shows good agreement with quantum results. Finally, we propose a hypothesis …

Raman Spectroscopy Of Carbon Materials, John R. Dennison, M. Holtz, G. Swain

All Physics Faculty Publications

Use of carbon materials is no longer limited to diamond jewelry or graphite pencils and lubricants. The last decade has witnessed an explosion of technological applications driven by the development of fabrication methods and the discovery of several new classes of pure carbon. Structural diversity exhibited by the carbon atoms, from local chemical order to long-range crystalline order, is key to understanding their physical and chemical properties and in future materials development. This article summarizes the use of Raman spectroscopy as a principal tool to investigate the vibrational dynamics of carbon materials and to provide indirect structural characterization of their …

Slow Down, D. Chris Benner, Rodelle A. Benner

Arts & Sciences Articles

No abstract provided.

An Exactly Soluble Fresnel Diffraction Model Of Two-Slit Interference, James A. Lock

Physics Faculty Publications

Using the formalism of Fresnel diffraction, we examine the diffraction and interference of two initially widely separated parallel Gaussian light beams. This two-beam configuration is a version of Young's two-slit interference problem that is free of the mathematical complexity that occurs in most Fresnel diffraction calculations. As a result, it clearly and simply illustrates the transition from ray optics to wave interference. (C) 1996 American Association of Physics Teachers.

Design Of A Two-Chamber-No₂-Fluorescence-Cell, Volker Schmid

Dissertations and Theses

Nitrogen oxides control the photochemical production of ozone O₃ in the lower atmosphere and influence the concentration of the hydroxyl radical HO in the troposphere. As an important member of the nitrogen oxide family, NO₂ plays a significant role in serving as the only known source of ozone (through photolysis) in the lower atmosphere and as sink for HO via formation of nitric acid. Therefore, accurate measurements of nitrogen dioxide are essential for the understanding of the processes that determine the distribution of tropospheric ozone and for assessing the impact of human activity on the troposphere.

Although reliable and …

The Current Energy Situation: Summary Points, David W. Hafemeister, A. Brecher, A. Fainberg, R. Howes, R. Socolow, D. Bodansky, P. Craig, B. Hammer, S. Smith, E. Stechel

Physics

Energy problems are now being largely ignored, despite their continuing importance. The near-disappearance of energy from the U.S. public agenda is apparent in the scant attention given to energy topics by the media and by public figures. Some suggestions for governmental action were stimulated by the rise in gasoline prices in the Spring of 1996, but the matter was treated as a short-term anomaly -- not as a harbinger of more severe difficulties to come. This lack of long term concern is perhaps natural, because fuel supplies are generally ample and prices are still relatively low, with the real cost …

Noncentral Potentials And Spherical Harmonics Using Supersymmetry And Shape Invariance, Ranabir Dutt, Asim Gangopadhyaya, Uday P. Sukhatne

Physics: Faculty Publications and Other Works

It is shown that the operator methods of supersymmetric quantum mechanics and the concept of shape invariance can profitably be used to derive properties of spherical harmonics in a simple way. The same operator techniques can also be applied to several problems with noncentral vector and scalar potentials. As examples, we analyze the bound state spectra of an electron in a Coulomb plus an Aharonov – Bohm field and/or in the magnetic field of a Dirac monopole.

Comment On ``Confirmation Of The Sigma Meson'', Joseph Schechter, Masayasu Harada, Francesco Sannino

Physics - All Scholarship

We comment on the recent paper by N.A. Tornqvist and M. Roos published in Phys. Rev. Lett. 76, 1575 (1996).

Dijet Angular Distributions In Direct And Resolved Photoproduction At Hera, M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. R. Okrasinski, J. Repond, R. Stanek, R. L. Talaga, H. Zhang, Margarita C. K. Mattingly, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, R. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, I. Gialas, R. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam

Faculty Publications

Jet photoproduction, where the two highest transverse energy (ETjet) jets have ETjet above 6 GeV and a jet-jet invariant mass above 23 GeV, has been studied with the ZEUS detector at the HERA ep collider. Resolved and direct photoproduction samples have been separated. The cross section as a function of the angle between the jet-jet axis and the beam direction in the dijet rest frame has been measured for the two samples. The measured angular distributions differ markedly from each other. They agree with the predictions of QCD calculations, where the different angular distributions reflect the different spins of the …

Observation Of Events With An Energetic Forward Neutron In Deep Inelastic Scattering At Hera, M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. R. Okrasinski, J. Repond, R. Stanek, R. L. Talaga, H. Zhang, Margarita C. K. Mattingly, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, R. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, I. Gialas, R. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam

Faculty Publications

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken x and Q2 in the range 3 · 10-4 < xBJ < 6 · 10-3 and 10 < Q2 < 100 GeV2.

Nlo Waveguide "And" Switch And Method Therefor, John J. Kester, Iyad A. Dajani, Peter M. Ranon, Thomas G. Alley

AFIT Patents

Method and apparatus are provided for NLO switching by first providing a phase-matched SHG grating which outputs a reinforced SHG beam only when two input beams of frequency (ω) are present in two modes of such wg. The so encoded NLO switch is operated by directing at least two input pulsed laser beams of frequency (ω) into the two modes of the wg to generate a reinforced pulsed output SHG beam and output same from the wg in an NLO switching process. The two input beams desirably have a separate pulse train and the spatial and temporal overlap of the …

Physical Conditions In Low-Ionization Regions Of The Orion Nebula, J. A. Baldwin, A. Crotts, R. J. Dufour, Gary J. Ferland, S. Heathcote, J. J. Hester, K. T. Korista, P. G. Martin, C. R. O'Dell, R. H. Rubin, A. G. G. M. Tielens, D. A. Verner, D. K. Walter, Z. Wen

Physics and Astronomy Faculty Publications

We reexamine the spectroscopic underpinnings of recent suggestions that [O I] and [Fe II] lines from the Orion H II region are produced in gas where the iron-carrying grains have been destroyed and the electron density is surprisingly high. Our new observations show that previous detections of [O I] 5577 were dominated by telluric emission. Our limits are consistent with a moderate density (≈ 10⁴ cm^-3) photoionized gas. We show that a previously proposed model of the Orion H II region reproduces the observed [O I] and [Fe II] spectrum. These lines are fully consistent with formation …

Observation Of Light Diffusion And Correlation Transport In Nematic Liquid Crystals, M. H. Kao, Kristen Anne Jester , '94, A. G. Yodh, Peter J. Collings

Physics & Astronomy Faculty Works

Light diffusion and temporal correlation transport are studied in an orientationally ordered multiply scattering medium. In particular, we experimentally demonstrate the anisotropic diffusion of light through a turbid nematic liquid crystal, and we measure the temporal correlations of these diffused speckle fields for the first time. The measurements are shown to provide useful information about this material, specifically the average rotational viscosity of the director. Computer simulations corroborate both the experimental observations and a more rigorous analytic theoretical formulation of this problem.

Fundamental Aspects Of Atomic Photoionization With High-Brightness Light Sources, Anthony F. Starace, Steven T. Manson

Anthony F. Starace Publications

The construction of numerous new synchrotron light sources worldwide is providing the means to study atomic and molecular photoionization processes at a level of detail that is unparalleled in even the recent past. Correspondingly, in theory, the availability of supercomputers and, perhaps more important, relatively inexpensive computer workstations, has permitted theoretical calculations to tackle more complex processes than ever before. In short, these technical developments are creating an unprecedented amount of data to be analyzed and understood.

In this volume, many of the experimental and theoretical advances of recent years will be described. We take the point of view in …

Ground-State Roughness Of The Disordered Substrate And Flux Line In D=2, Alan Middleton, Chen Zeng, Y. Shapir

Physics - All Scholarship

We apply optimization algorithms to the problem of finding ground states for crystalline surfaces and flux lines arrays in presence of disorder. The algorithms provide ground states in polynomial time, which provides for a more precise study of the interface widths than from Monte Carlo simulations at finite temperature. Using $d=2$ systems up to size $420^2$, with a minimum of $2 \times 10^3$ realizations at each size, we find very strong evidence for a $\ln^2(L)$ super-rough state at low temperatures.

Rate Coefficients For Charge Transfer Between Hydrogen And The First 30 Elements, J. B. Kingdon, Gary J. Ferland

Physics and Astronomy Faculty Publications

We present analytic fits to charge exchange rate coefficients over the full range of temperatures which occurs in photoionized or shock-heated plasmas. We consider reactions between neutral hydrogen and all elements with parent ion charge q = 1-4 up to Z = 30. Many rates were obtained from various sources in the literature. For reactions for which no data were available, we calculated rates using the Landau-Zener formalism. For these new reactions, we tabulate both total and state-specific rate coefficients. Ml are fitted with a consistent, accurate formula. These fits may be incorporated easily into spectral synthesis codes, and we …

Review Of Dark Sun: The Making Of The Hydrogen Bomb By Richard Rhodes, David W. Hafemeister

Physics

No abstract provided.

Magnetization Reversal Behavior In Cobalt Rare-Earth Thin Films, S.S. Malhotra, Z.S. Shan, D.C. Stafford, Sy_Hwang Liou, David J. Sellmyer

David Sellmyer Publications

Cobalt RareEarth (CoR, R = Pr,Sm) thin films on a Cr underlayer have shown promise as future ultra high density magnetic recording media. The activation volume or the magnetic switching volume (v*) is an important consideration for thermal stability and media noise in high density recording media and V* can also provide information about the magnetization reversal behavior in these films. The magnetic switching volume was estimated for the CoPr films with thicknesses from 28 to 639 nm with a Cr underlayer of 80 nm annealed at 500 ⁰C. The measured switching volume ranges from 1.3 to 1.7x10^{-18 …}

Doubly Differential Cross Sections Of Low-Energy Electrons Emitted In The Ionization Of Molecular Hydrogen By Bare Carbon Ions, Lokesh C. Tribedi, P. Richard, D. Ling, Y. D. Wang, C. D. Lin, R. Moshammer, G. W. Kerby Iii, M. W. Gealy, M. Eugene Rudd

M. Eugene Rudd Publications

We have measured the double differential cross sections (DDCS) (d²σ/dε_edΩ^e) of low-energy electron emission in the ionization of H₂ bombarded by bare carbon ions of energy 30 MeV. The energy and angular distributions of the electron DDCS have been obtained for 12 different emission angles and for electron energies varying between 0.1 and 300 eV. We have also deduced the single differential and total ionization cross section from the measured DDCS. The data have been compared with the predictions of first Born approximations and the CDW-EIS (continuum distorted wave–eikonal …

Simple Measurement Of 300 K Electron Capture Cross Section For El2 In Gaas, David C. Look, Z-Q. Fang

Physics Faculty Publications

A simple experiment involving only the measurement of dark current I_dark and 1.1 μm photocurrent I_PC in semi‐insulating (SI) GaAs allows an accurate determination of the electron capture cross section σ_n for the important defect EL2 in GaAs. For 45 SI GaAs samples, from 12 different boules, grown by three different techniques, we find that I_PC/I_dark=1.96±0.05 at 300 K. This relationship gives σ_n=1.4±0.4×10⁻¹⁶ cm², which is compared to previously estimated values.

Deep-Center Hopping Conduction In Gan, David C. Look, D. C. Reynolds, W. Kim, Ö. Aktas, A. Botchkarev, A. Salvador, H. Morkoç

Physics Faculty Publications

Molecular‐beam‐epitaxial GaN layers change from strongly conductive (ρ≂10⁻² Ω cm at 300 K) to semi‐insulating (ρ≂10⁶ Ω cm) as the N flux is increased. Layers grown at low fluxes show strong n‐type conduction, with transport in the conduction band at high temperatures and in a shallow donor band at low temperatures. For layers grown at high N fluxes, the Hall coefficients become too small to measure, suggesting hopping conduction among deep centers. The temperature‐dependent resistivity data are most consistent with multiphonon, rather than single‐phonon, hopping.

Determination Of Crystal-Field Parameters Of Sm2Fe17N3 And Sm2Fe17C3, S. Wirth, M. Wolf, K.H. Muller, Ralph Skomski, S. Brennan, J.M.D. Coey

Ralph Skomski Publications

The crystal-field parameters A₂₀ and A₄₀ were determined from the magnetocrystalline anisotropy constants K₁ and K₂ in a linear approximation for Sm₂Fe₁₇Z_x (Z = N, C) compounds. For the materials investigated, the values of Iη0 are calculated in two ways: i) from the Kί at zero temperature and ii) from the temperature dependence of the Kί. Both methods give equivalent results. The value A₂₀≈-400 Ka₀^-2 has been obtained which corresponds to an inverse THOMAS-FERMI screening length q≈2.3 Å^-1 calculated within the framework …

Is There Spin-Glass Exchange In Ultrathin Fe(110) Films ?, Ralph Skomski, D. Sander, Axel Enders, J. Kirschner

Ralph Skomski Publications

The magnetism of sesquilayer bcc iron films on W(110) is investigated. Coercivity data and theoretical calculations that the freezing of the magnetization at low temperatures reflects exchange-dominated domain-wall pinning rather than superparamagnetic spin-glass behavior.

Film Stress And Domain Wall Pinning In Sesquilayer Iron Films On W(110), D. Sander, Ralph Skomski, C. Schmidthals, Axel Enders, J. Kirschner

Ralph Skomski Publications

We present an in situ investigation of the correlation between elastic and magnetic properties of monolayer iron films on W(110). Sesquilayers, consisting of two-monolayer patches on a nearly ideal monolayer film, exhibit anomalous elastic properties and a strikingly high in-plane coercivity of order 0.3 T. The sesquilayer coercivity maximum is explained by a novel domain wall pinning mechanism, based on an enhanced exchange interaction in the two monolayer thick patches. This rather unique behavior is restricted to (110) surfaces but does not occur on (100) and (111) thin films.

High Coercivity Smfesic Films Fabricated By Multilayer Sputtering, S.Y. Zhang, Z.S. Shan, Yi Liu, David J. Sellmyer

David Sellmyer Publications

SmFeSiC thin films have been obtained by sputtering SmFe/C(Si) multilayers with a Ta underlayer on Si substrates and subsequently annealing at 700° C. The coercivity of the SmF films strongly depends on the sputtering and annealing conditions. The influence of the thickness of the Ta underlayer, the thickness ratio of SmFe to C(Si), the argon pressure and heat treatment In- plane Coercivities up to 7.2 kOe and squareness of 0.94 were obtained.

Lorentz Microscopy Observation Of Magnetic Grains In Co-Sm // Cr Films, Yi Liu, Z.S. Shan, David J. Sellmyer

David Sellmyer Publications

This paper presents our recent Lorentz microscopy study of magnetic domain structure and its relation to the nanostructure of Co-Sm films. The magnetic domains were imaged by Foucault Lorentz mode and the corresponding nanostructure was revealed by bright field and high resolution images. The nanostructure is composed of clusters of 20 nm grown on Cr grains. Each cluster is formed by the amorphous matrix and about 5 nm crystallites distributed in the matrix. It is found that each cluster forms a magnetic grain. Such magnetic grains show uniform contrast near the magnetic domain wall, and correspond to the switching volume …

An Effective-Viscosity Description Of Domain-Wall Motion In Magnetic Thin Films With Perpendicular Anisotropy, Ralph Skomski, J. Giergiel, J. Kirschner

Ralph Skomski Publications

The expansion of magnetic domains in thin films with perpendicular anisotropy is investigated. To determine the domain-wall velocity as a function of the applied magnetic field a self-=consistent magnetic-viscosity approach is used. The main predictions of the theory are a linear behavior\r in the limit of moderately strong fields, a quasi-exponential behavior for fields close to the propagation field, and a negative velocity for very small and negative fields. The predictions of the theory are compatible with domain-wall investigations on Au/Co/Au sandwiches.

High Coercive Field And Film Stress For Epitaxial Monolayers Of Fe On W(110), D. Sander, Axel Enders, Ralph Skomski, J. Kirschner

Ralph Skomski Publications

Elastic and magnetic properties of ultra-thin iron films on tungsten(110) are investigated. In situ film stress measurements during growth show stress values of 25 GPa per deposited monolayer. Our experiments indicate that the tremendous film stress triggers the formation of a misfit dislocation network at a coverage of 1.5 monolayers. The effect of the film stress and its spatial variation on the magnetic behavior are discussed. We find an high coercivity of order 0.3 T for 1.5 monolayer thick films. A model is presented that explains the high coercivity in terms of strong domain wall pinning.

The Itinerant Limit Of Metallic Anisotropy, Ralph Skomski

Ralph Skomski Publications

The anisotropy contribution of itinerant electrons confined to a quantum-well potential is calculated and compared with the quasi-ionic contribution due to virtually bound electrons. The easy magnetization direction of the non-L-S quantum-well electrons lies in the plane, and the magnitude of the anisotropy, maximally of order 10 J/m³, is too weak to explain anisotropies encountered in practice. This means that itinerant 3d anisotropy in bulk materials and thin films is associated with Hund's rules-type ionic Contributions.

Theory Of The Seebeck Coefficient In Lacro₃ And Related Perovskite Systems, D. B. Marsh, Paul Ernest Parris

Physics Faculty Research & Creative Works

We consider the Seebeck coefficient in LaCrO₃ and related transition-metal-oxide perovskites using a model for electronic conduction based on the electronic structure of the 3d orbitals of the B-site transition-metal cations. Relations for the Seebeck coefficient are presented for those perovskite systems in which electronic conduction is through the t_2g states of the B-site transition-metal cations. High- and low-temperature limits for the Seebeck coefficient are identified for the cases of both strong and weak magnetic coupling between electron spins. In these high- and low-temperature limits, the Seebeck coefficient is determined as a function of carrier concentration. Results are …