Physical Sciences and Mathematics Commons^™

Scanning Tunneling Microscopy Study Of Intermediates In The Dissociative Adsorption Of Closo-1,2-Dicarbadodecaborane On Si(111), J.M. Carpinelli, E. Ward Plummer, Dongjin Byun, Peter A. Dowben

Peter Dowben Publications

Closo-1,2-dicarbadodecaborane (C₂B₁₀H₁₂) is a source compound found to be suitable for the deposition of a high resistivity form of boron-carbide (B₅C), and the fabrication of boron-rich semiconductor devices. A scanning tunneling microscope (STM) was used to image these molecular icosahedra on Si(111)-(7x7). Molecular decomposition (tip induced and otherwise! produced a boron-carbide/silicon interface with pronounced heterojunction electronic characteristics. In STM, this interface is characterized by a disordering of the Si(111)-(7x7) reconstruction. We suggest, based on Auger electron spectroscopy data and low-energy electron diffraction observations, that boron atoms from the dissociated source molecules …

Resonant Photoemission Studies Of The Thickness Dependence Of The Unoccupied Gd 5D Bands, Peter A. Dowben, Dongqi Li, Jiandi Zhang, M. Onellion

Peter Dowben Publications

Ultrathin Gd films have been studied with constant initial-state spectroscopy (CIS) of photoemission utilizing linearly polarized light of synchrotron radiation. The photoemission cross section of the 5d surface state near E_F shows strong photon energy dependence, i.e., sharp peaks in CIS spectra near the Gd 5p_1/2 absorption edge. These peaks originate from resonant photoemission processes involving the occupied 5p and unoccupied 5d levels of the surface atoms. The symmetry of the unoccupied states were determined experimentally by changing the polarization of light, where p-polarized light excites only to the unoccupied d_{z^{2 …}}

Scanning Tunneling Microscopy Study Of Intermediates In The Dissociative Adsorption Of Closo-1,2-Dicarbadodecaborane On Si(111), J.M. Carpinelli, E.W. Plummer, Dongjin Byun, Peter A. Dowben

Peter Dowben Publications

Closo-1,2-dicarbadodecaborane (C₂B₁₀H₁₂) is a source compound found to be suitable for the deposition of a high resistivity form of boron-carbide (B₅C), and the fabrication of boron-rich semiconductor devices. A scanning tunneling microscope (STM) was used to image these molecular icosahedra on Si(111)-(7 x 7). Molecular decomposition (tip induced and otherwise) produced a boron-carbide/silicon interface with pronounced heterojunction electronic characteristics. In STM, this interface is characterized by a disordering of the Si(111)-(7 x 7) reconstruction. We suggest, based on Auger electron spectroscopy data and low-energy electron diffraction observations, that boron atoms from the …

The Paramagnetic Correlation Length Of Mn Thin Films, D.N. Mcilroy, Peter A. Dowben

Peter Dowben Publications

We have utilized Ginzberg-Landau mean field theory to analyze the branching ratio and soft X-ray magnetic circular dichroism measurements of the L₃ and L₂ adsorption edges of thin Mn overlayers on ferromagnetic fcc Co(001) at room temperature. We have determined a short range magnetic correlation of 4.25Å and that the long range magnetic order has a decay length of 1.29Å. These results indicate that the first monolayer of Mn ferromagnetically couple with the Co substrate. The paramagnetic Mn layer exhibits short range magnetic order, but lacks long range magnetic order. Due to the exponential decrease of both the …

Visible Luminescence From Single Crystal‐Silicon Quantum Wells, Peter N. Saeta, A. C. Gallagher

All HMC Faculty Publications and Research

Single crystal‐silicon quantum wells with SiO₂ barriers have been grown from SIMOX silicon‐on‐insulator substrates. Photoluminescence in the red and near‐infrared is observed for average well width <8 >nm, with peak signal for 2‐nm average width. The luminescence spectrum is independent of well width for SiO₂ barriers, but shifts 0.3 eV to higher energy upon removal of the upper oxide layer with HF. Both results suggest the importance of radiation from surface states.

Electrical Field Enhanced Thermal Quenching Of A Prominent Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

Detailed experimental results are presented for a ‘‘thermal quenching’’ of thermal stimulated current signals in the most prominent trap in undoped semi‐insulating (SI) GaAs, T₅ with an activation energy of 0.27–0.31 eV. A possible model for the thermal quenching of T₅ is discussed, emphasizing the thermally stimulated nature of the quenching process, the effect of electric field and the formation of high‐field domains. The thermal quenching of T₅ can frequency be observed in SI GaAs grown by the vertical gradient freeze (VGF) technique, or by the liquid encapsulated Czochralski (LEC) technique under certain conditions.

Observation Of Postcollision Effects In The Scattered Projectile Spectra For Ionizing Proton-Helium Collisions, Tibor Vajnai, A. D. Gaus, Joachim A. Brand, W. T. Htwe, Don H. Madison, Ronald E. Olson, Jerry Peacher, Michael Schulz

Physics Faculty Research & Creative Works

We have measured and calculated doubly differential single ionization cross sections as a function of the scattering angle and the projectile energy loss for 50 to 150 keV proton-helium collisions. These cross sections show unexpected structures as a function of both the energy loss and the scattering angle, which are interpreted as due to the postcollision interaction. Although the effects of postcollision interactions have previously been observed in electron spectra, this is the first observation of such effects for the scattered protons.

Interpretation Of Extinction In Gaussian-Beam Scattering, James A. Lock

Physics Faculty Publications

The extinction efficiency for the interaction of a plane wave with a large nonabsorbing spherical particle is approximately 2.0. When a Gaussian beam of half-width w(0) is incident upon a spherical particle of radius a with w(o)/a < 1, the extinction efficiency attains unexpectedly high or low values, contrary to intuitive expectations. The reason for this is associated with the so-called compensating term in the scattered field, which cancels the field of the Gaussian beam behind the particle, thereby producing the particle's shadow. I introduce a decomposition of the total exterior field into incoming and outgoing portions that are free of compensating terms. It is then shown that a suitably defined interaction efficiency has the intuitively expected asymptotic values of 2.0 for w(o)/a >> 1 and 1.0 for w(o)/a << 1.

The Evolution Of Density-Driven Circulation Over Sloping Bottom Topography, G. H. Wheless, J. M. Klinck

CCPO Publications

The short timescale temporal evolution of buoyancy-driven coastal flow over sloping bottom topography is examined using a two-dimensional, vertically averaged numerical model. Winter shelf circulation driven by a coastal ''point source'' buoyancy flux is modeled by initiating a coastal outflow with density anomaly epsilon into well-mixed shelf water. The nonlinear interaction between the time-varying velocity and density field is represented by an advection-diffusion equation. Three cases are discussed: that of a buoyant (epsilon < 0) outflow, a neutral (epsilon = 0) outflow, and a dense (epsilon > 0) outflow. Results are similar to observations from well-mixed shelf areas and show that density-topography interactions are capable of substantially influencing coastal circulation. A negative (buoyant) coastal …

On The Mechanism Of Giant Electromotility In Polyelectrolyte Gels, Kimberly Ann Branshaw '95

Honors Projects

Electromotility, i.e. bending in response to an electric field, of polyelectrolyte gels in ionic solutions has recently been investigated at a few leading academic and industrial labs as a potential chemomechanical engine. We have discovered that the underlying physics in these systems is more complex than previously believed. We have found that the bending, which seems to obey a {t power law, is inconsistent with the simple idea of a bending speed, but is consistent with a diffusion mechanism. Evidence of diffusion was independently provided by experiments on gels grown or bent in the presence of dyes. We have explored …

Novel Swelling Structures And Electromotility Response In Polyelectrolyte Gels, Dana Deardorff

Honors Projects

Electromotility, the bending in response to an electric field, of polyelectrolyte gels in ionic solutions has been identified as a candidate for a potential chemomechanical engines such as muscles. We discovered that the underlying physics of these systems is more complex than previously believed. We found that the bending as a function of time obeys a square root power law. This points strongly towards a diffusion mechanism for the bending. Kinetic evidence for diffusion was independently corroborated by experiments on gels grown or bent in the presence of dyes. We explored the effects of varying poly-ion concentration in the backbone …

Fourier Series And Optical Transform Techniques In Contemporary Optics: An Introduction, Raymond Wilson, Sean Mccreary

Raymond Wilson

Partial-Wave Representations Of Laser Beams For Use In Light-Scattering Calculations, Gérard Gouesbet, James A. Lock, Gérard Gréhan

Physics Faculty Publications

In the framework of generalized Lorenz-Mie theory, laser beams are described by sets of beam-shape coefficients. The modified localized approximation to evaluate these coefficients for a focused Gaussian beam is presented. A new description of Gaussian beams, called standard beams, is introduced. A comparison is made between the values of the beam-shape coefficients in the framework of the localized approximation and the beam-shape coefficients of standard beams. This comparison leads to new insights concerning the electromagnetic description of laser beams. The relevance of our discussion is enhanced by a demonstration that the localized approximation provides a very satisfactory description of …

Van Hove Singularity Induced L11 Ordering In Cupt, J. F. Clark, F. J. Pinski, Duane D. Johnson, P. A. Sterne, J. B. Staunton, B. Ginatempo

Duane D. Johnson

We describe an ordering mechanism that arises due to coupling between electronic states at van Hove singularities. This novel mechanism, which naturally leads to ordered structures with small unit cells, couples states near the Fermi energy which are localized at high-symmetry points in k space, in contrast to the conventional mechanism which relies on large parallel sheets of Fermi surface. Using first-principles calculations of the electronic structure of ordered and disordered alloys, we show that this mechanism drives the unusual short- and long-range order found in fcc CuPt.

Microstructure Of The Cr Underlayer And Its Effect On Sm-Co//Cr Thin Films, Yi Liu, B. W. Robertson, Z.S. Shan, Sy_Hwang Liou, David J. Sellmyer

Sy-Hwang Liou Publications

Sm-Co film is a potential candidate for the future high density recording media of 10 Gb/in² which requires bit sizes of the order of 300 nm and grain sizes of about 10 nm. This article investigates the microstructure of the Cr underlayer in Sm-Co thin films and its effect on Sm-Co thin films prepared by the dc magnetron sputtering technique. The grain size of the Cr underlayer is found to be about 25 nm. Grains with small angle misorientation usually form local agglomerates. Studies by transmission electron microscopy (TEM) bright field images at different defocus settings and by high …

Conduction-Electron Screening In The Bulk And At Low-Index Surfaces Of Ta Metal, D. Mark Riffe, W. Hale, B. Kim, J. L. Erskine

All Physics Faculty Publications

High-resolution core-level photoemission spectra from Ta(100) have been measured. The well-resolved peak from the first atomic layer allows a separate assessment of bulk and surface-layer screening responses: singularity indices α_B=0.10±0.01 and α₁₀₀=0.205±0.025, respectively, are obtained. The measured surface-atom lifetime broadening of 70±20 meV (compared to 37±5 meV in the bulk) is consistent with published Auger–photoemission coincidence measurements. The result for the bulk singularity index has been applied in the analysis of previous data in order to extract the screening response in the first atomic layer of Ta(111) (α₁₁₁=0.16±0.01) and Ta(110) (α₁₁₀=0.150±0.015). The …

Scaling And The Fractal Geometry Of Two-Dimensional Quantum Gravity, Simon Catterall, G. Thorleifsson, Mark Bowick, V. John

Physics - All Scholarship

We examine the scaling of geodesic correlation functions in two-dimensional gravity and in spin systems coupled to gravity. The numerical data support the scaling hypothesis and indicate that the quantum geometry develops a non-perturbative length scale. The existence of this length scale allows us to extract a Hausdorff dimension. In the case of pure gravity we find d_H approx. 3.8, in support of recent theoretical calculations that d_H = 4. We also discuss the back-reaction of matter on the geometry.

Target Dependence Of Binary Encounter Electron Peak Anomalies In Collisions Of Partially Stripped Heavy Ions With Molecular Hydrogen And Noble Gases, W. Wolff, J. Wang, H. E. Wolft, J. L. Shinpaugh, Ronald E. Olson, D. H. Jakubassa-Amundsen, S. Lencinas, U. Bechthold, R. Herrmann, H. Schmidt-Bocking

Physics Faculty Research & Creative Works

A systematic search was performed for the manifestation of quantum interference effects in the shape and angular distribution of the binary-encounter electron peak in collisions of partially stripped, or structured, heavy ions with noble gases and molecular hydrogen. The ionic species investigated were Cu5+,19+, I7+,23+, Au11+,29+ and U13+, all at the same nominal velocity equivalent to 0.6 MeV amu-1. Experimental double-differential cross sections for secondary electron emission in the binary encounter energy region are compared with a simple model based on the elastic scattering of quasi-free target electrons in the projectile field as well as with results of impulse approximation …

Collisional Effects In He I: An Observational Analysis, J. B. Kingdon, Gary J. Ferland

Physics and Astronomy Faculty Publications

Accurate and reliable helium abundances can test modern theories of galactic and primordial nucleosynthesis. Unfortunately, there is some question whether current theory can account for collisional contributions to He I. We present new observations of two planetary nebulae (PNs) in the range λ850-λ9650, which we use to assess the importance of collisonal effects in the He I spectrum. The first object, NGC 7027, is expected to show relatively strong collisional enhancement, while the second, NGC 7026, should display only small effects. We derive new collision-to-recombination correction factors, based on new collision strengths from the 29-state quantal calculation …

Monitoring Nuclear Test Ban Treaties, David W. Hafemeister

Physics

With the end of the Cold War, the emphasis on monitoring nuclear weapons tests has shifted from the 1976 Threshold Test Ban Treaty (TTBT), which confined the US and USSR to a limit of 150 kilotons (kt), to the Comprehensive Test Ban (CTB) Treaty, which would ban all nuclear tests by the signatories. 1995 will be a watershed year to control the proliferation of nuclear weapons, as the Nuclear Non-Proliferation Treaty (NPT) must be considered for renewal. The fate of the NPT is dependent on a successful CTB negotiation, because the two treaties are politically linked. This paper will cover …

Effects Of Imperfect Unipolarity On The Ionization Of Rydberg Atoms By Subpicosecond Half-Cycle Pulses, N. E. Tielking, Thomas J. Bensky, Robert R. Jones

Physics

Rydberg atoms are ionized by nearly unipolar, subpicosecond electromagnetic pulses. Deviations from a perfectly unidirectional pulse are found to alter substantially the ionization probability as a function of peak field. Quantitative agreement between classical theory and experiment is achieved if the pulse imperfections are significantly attenuated.

Resonantly Laser-Driven Plasma Waves For Electron Acceleration, Donald P. Umstadter, J. Kim, Eric Esarey, E. Dodd, T. Neubert

Donald Umstadter Publications

A method for generating large-amplitude nonlinear plasma waves, which utilizes an optimized train of independently adjustable, intense laser pulses, is analyzed in one dimension both theoretically and numerically (using both Maxwell-fluid and particle-in-cell codes). Optimal pulse widths and interpulse spacings are computed for pulses with either square or finite-rise-time sine shapes. A resonant region of the plasma wave phase space is found where the plasma wave is driven most efficiently by the laser pulses. The width of this region, and thus the optimal finite-rise-time laser pulse width, was found to decrease with increasing background plasma density and plasma wave amplitude, …

On Science Schooling, Seminar Style, Peter J. Collings

Physics & Astronomy Faculty Works

No abstract provided.

An Experimental Apparatus For Studying Differential Scattering Of Multiply-Charged Ions From Neutral Targets Following Capture, John Edward Edens

Masters Theses

An experimental apparatus has been designed, enabling measurement of angular differential cross sections for single-electron capture (SEC). The design consists of a microchannel plate detector with a one-dimensional position sensitive anode. A bow-time shaped aperture was employed to convert a radially scattered distribution into an approximate linear one. An electrostatic retarding grid was used to separate SEC contributions from the reaction products. Measurements were made of the angular differential cross sections for SEC of Ar^q+ (q=4-6,8) ions scattering from He and Ar at impact energies of 600 to 3000 eV and angles between 0 and 23 mrad. The experimental …

Dynamics Of Optical-Field-Ionized Plasmas For X-Ray Lasers, Thomas D. Donnelly, R. W. Lee, R. W. Falcone

All HMC Faculty Publications and Research

The success of recombination-pumped x-ray laser schemes is determined by the kinetics of ions in plasmas with relatively dense, cold-electron distributions. We examine how laser gain in such systems is affected by a multipeaked electron distribution generated by sequential ionization of atoms using high-intensity, ultrashort-pulse lasers. We also investigate the role of heating processes that modify electron energy distributions during the recombination and the accompanying collisional cascade. We find that conditions for the success of these schemes are critically modified by the inclusion of these effects.

Computer-Simulation Study Of The Permeability Of A Porous Sediment Model, Ras B. Pandey, Jeffrey L. Becklehimer

Faculty Publications

A computer simulation model is used to study the permeability of fluid flow through porous media generated by the random distribution of the sediments in a two-dimensional lattice. Fluid particles are confined to pore space with the sediments forming the rigid barriers at the pore boundaries. An interaction between the fluid particles and the pore substrate is introduced to incorporate the additional drag in the pore space. The pressure gradient causes a bias to drive the fluid and the Metropolis algorithm is used to hop the fluid particles. The permeability of this system is studied as a function of the …

Review: 'Integrated Optics: Design And Modeling,' By Reinhard Marz, Bradley D. Duncan

Electro-Optics and Photonics Faculty Publications

My overall impression is that Integrated Optics: Design and Modeling will make a fine addition to almost anyone's collection of books on integrated optics. It will, however, serve its readers better as a reference book than as a text from which to first learn the basic concepts of integrated optics. I say this because the book is written at a fairly sophisticated technical level, though the author often moves rather quickly to the "bottom line" without providing material much beyond what is first necessary to introduce a problem or concept, and then a statement of the results and/or implications. It …

Time Dependent Response Of Equatorial Ionospheric Electric Fieldsto Magnetospheric Disturbances, Bela G. Fejer, L. Scherliess

Bela G. Fejer

We use extensive radar measurements of F region vertical plasma drifts and auroral electrojet indices to determine the storm time dependence of equatorial zonal electric fields. These disturbance drifts result from the prompt penetration of high latitude electric fields and from the dynamo action of storm time winds which produce largest perturbations a few hours after the onset of magnetic activity. The signatures of the equatorial disturbance electric fields change significantly depending on the relative contributions of these two components. The prompt electric field responses, with lifetimes of about one hour, are in excellent agreement with results from global convection …

Coulomb Holes And Correlation Potentials In The Helium Atom, Marlina Slamet, Viraht Sahni

Physics Faculty Publications

In this paper we investigate Coulomb correlation effects in the He atom by studying the structure of the static and dynamic Coulomb hole charge distributions as determined by the analytical 39-parameter correlated wave function of Kinoshita. The static Coulomb hole, which is defined in terms of the radial electron-electron distribution function, shows that as a result of Coulomb repulsion there is a reduction in probability of electron approach within a distance of one atomic unit and an increase in this probability for greater separations. The dynamic Coulomb hole defined directly in terms of the pair-correlation density describes the probability of …

Thermal Effects On Y2O3:Eu3+ For Fiber Optic Temperature Sensing, Arnel C. Lavarias

Electrical & Computer Engineering Theses & Dissertations

Fiber optic methods have gained acceptance in the past several years as non­ intrusive optical techniques of measuring environmental variables, particularly under hostile conditions. Fiber optic temperature sensors are required for many applications which have a large operating range. One way to implement this sensor is to introduce rare earth-doped materials into a fiber showing temperature-dependent optical emission characteristics. The thermal effects on optical emission characteristics of Y₂O₃:Eu³⁺ were studied. The thermal broadening and temperature dependence of the lifetime (τ) of the 611.4 nm emission due to the ⁵D₀ ➔ 7F …