Physical Sciences and Mathematics Commons^™

A Simple Technique To Measure Stress In Ultrathin Films During Growth, D. Sander, Axel Enders, J. Kirschner

Axel Enders Publications

We demonstrate an easy implementation of the cantilever bending beam approach to measure stress during film growth in ultrahigh vacuum. Using a simple and compact optical deflection technique, film stress with sub-monolayer sensitivity can be detected. A stress measurement during FeSi, formation on Si(l11) is presented.

Optical Measurements On The Bp Iii To Isotropic Phase Transition In Highly Chiral Liquid Crystals, Jonathan Brent Becker , '92, Peter J. Collings

Physics & Astronomy Faculty Works

Optical activity and light scattering measurements in the chiral-racemic system of 4-(2-methylbutyl) phenyl-4-hexyloxybenzoate (CE4) reveal a much larger decrease in the discontinuity at the third blue phase (BP III) to isotropic phase transition than reported previously using adiabatic scanning calorimetry on another compound. Whereas these new data show that the transition is still slightly discontinuous for the pure optical isomer of CE4, similar light scattering measurements on the pure optical isomer of 4″-(2- methylbutylphenyl)-4′-(2-methylbutyl)-4-biphenyl-carboxylate (CE2), which is more chiral than CE4, show no discontinuity at all.

Electronic Structures And Curie Temperatures Of Iron-Based Rare-Earth Permanent-Magnet Compounds , J.P. Woods , B.M. Patterson, A.S. Fernando, Sitaram Jaswal, D. Welipitiya, David J. Sellmyer

David Sellmyer Publications

The modification of the electronic structures of Sm₂Fe_17-xAl_xN_y, NdFe₁₁TiN_y, and YFe_12-xMo_x upon alloying and nitriding are examined with self-consistent spin-polarized calculations and soft-x-ray photoemission measurements between 18 and 135 eV. The changes in the Curie temperature T_c with substitutional modifications and nitrogen addition are modeled with self-consistent spin-polarized electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth which relates the electronic structure to T_c. The calculations show that the spin-summed density of states at the Fermi energy is related to …

Failure To Observe Electron Circular Dichroism In Camphor, K. W. Trantham, M. E. Johnston, Timothy J. Gay

Timothy J. Gay Publications

We have searched for evidence of electron circular dichroism, the preferential
transmission of longirudinally polarized electrons lhrough a chiral medium, using stereoisomers of camphor. Such an effect was seen by Campbell and Farago in 1987 for 5 eV elecvons. Within our detection limit (~0.02%) we have not observed asymmehic transmission at either an incident electron energy of 1 eV or 5 eV. ln preparation for this inquiry, we also measured the total scattering cross section for camphor in the energy range 0.5-5 eV.

Femtosecond Laser-Produced Plasma X-Rays From Periodically Modulated Surface Targets, J. C. Gautheir, S. Bastiani, P. Audebert, J. P. Geindre, K. Neuman, Thomas D. Donnelly, M. Hoffer, R. W. Falcone, R. Shepherd, D. Price, B. White

All HMC Faculty Publications and Research

We have studied theoretically and experimentally the x-ray production above 1 keV from femtosecond laser plasmas generated on periodically modulated surface targets. Laser energy coupling to plasma surface waves has been modeled using a numerical differential method. Almost total absorption of incident laser radiation is predicted for optimized interaction conditions. Silicon gratings have been irradiated by a 120fs Ti:sapphire laser at irradiances in excess of 10¹⁶ W/cm². X-ray intensities above 1.5 keV (K-shell lines) have been measured as a function of the incidence angle. Results show a distinct x-ray emission maximum for the first order diffraction …

Temporary Negative-Ion States Of Na, K, Rb, And Cs, A.R. Johnston, Paul Burrow

Paul Burrow Publications

Electron-transmission spectroscopy is used to observe structure in the electron-scattering cross section associated with the formation of temporary negative ions and channel-opening effects. A strong ‘‘family’’ resemblance is apparent among the resonances of the alkali metals. From published calculations and angular scattering studies, the lower-lying features are known with some confidence. Based on these, plausible assignments for some of the higher-lying resonances are put forth. We propose that the absence of resonances near the Cs 5 ²D and possibly the Na 4 ²P and 5 ²P thresholds is associated with the negative polarizabilities of these excited states.

Nonlinear Temporal Diffraction And Frequency Shifts Resulting From Pulse Shaping In Chirped-Pulse Amplification Systems, X Liu, R Wagner, Anatoly Maksimchuk, E Goodman, J Workman, Donald P. Umstadter, A Migus

Donald Umstadter Publications

We present experimental results of the amplif ication of strongly amplitude-modulated chirped pulses resulting from the coherent addition of two delayed short pulses. The nonlinearities in the amplifier chain induce a temporal diffraction resulting in prepulses and postpulses, in addition to the two main pulses when compressed. Simultaneously, temporal-resolved and spectral-resolved output pulses show that the prepulses and postpulses are blue shifted and red shifted, respectively, explaining the causality of the system.

Effects Of Track Structure And Cell Inactivation On The Calculation Of Heavy Ion Mutation Rates In Mammalian Cells, Francis A. Cucinotta, J. W. Wilson, M. R. Shavers, Robert Katz

Robert Katz Publications

It has long been suggested that inactivation severely effects the probability of mutation by heavy ions in mammalian cells. Heavy ions have observed cross sections of inactivation that approach and sometimes exceed the geometric size of the cell nucleus in mammalian cells. In the track structure model of Katz the inactivation cross section is found by summing an inactivation probability over all impact parameters from the ion to the sensitive sites within the cell nucleus. The inactivation probability is evaluated using the dose-response of the system to γ-rays and the radial dose of the ions and may be equal to …

Dynamic Phase Steepening In Alfven Waves, Stephen R. Granade

Honors Theses

Our solar system contains more activity and complexity than can be seen through a telescope. One such "invisible" phenomenon is the solar wind, created by a steady stream of particles blasted away from the sun in all directions. The sun's donut-shaped magnetic field lines channel this stream. Particles moving along the field lines perform an intricate helical dance, with ions winding one way and electrons the other.

The solar wind shapes and is shaped by the magnetic fields of the planets and the sun. If left undisturbed by outside influences, the earth's magnetic field, like the sun's, would resemble a …

A Theoretical Study Concerning The Solar Cycle Dependence Of The Nightside Ionosphere Of Venus, Zoltan Dobe, Andrew F. Nagy, Jane L. Fox

Physics Faculty Publications

We modeled the chemical and physical processes taking place in the nightside ionosphere of Venus by solving the one dimensional coupled continuity and momentum equations for 12 ion species [CO₂⁺, O₂⁺, O⁺, H⁺, NO⁺, CO⁺, N₂⁺, N⁺, He⁺, C⁺, O⁺ (²D) and O⁺ (²P)]. We investigated the relative importance of the two major processes responsible for maintaining the nightside plasma densities: atomic ion transport from the dayside and impact ionization due to energetic electron precipitation. …

Monte-Carlo Simulation Of Bulk Hole Transport In Alxga1-Xas, In1-Xalxas, And Gaasxsb1-X, M. J. Martinez, David C. Look, J. R. Sizelove, F. L. Schuermeyer

Physics Faculty Publications

We report a Monte Carlo study of hole transport in Al_xGa_1−xAs, In_1−xAl_xAs, and GaAs_xSb_1−x. The effects of alloy scattering are significant in all three cases, but mobilities are still high enough to be advantageous in particular device applications. We separately calculate the Hall r factors by a Boltzmann transport method and show that these factors are vitally important when attempting to compare Monte Carlo drift mobilities with experimental Hall data.

Demonstration Of Semiconductor Characterization By Phonon Side-Band In Photoluminescence, D. C. Reynolds, David C. Look, D. N. Talwar, G. L. Mccoy, K. R. Evans

Physics Faculty Publications

In this paper two GaAs samples were investigated; one was a very pure sample grown by chemical-vapor deposition, the other was grown by molecular-beam epitaxy. The dominant optical transition in the high-purity sample was the donor-bound-exciton transition. Phonon sidebands associated with both the free exciton and the donor-bound exciton were observed. The active phonons were the longitudinal-optical (LO) and the transverse-optical (TO) modes associated with both the free exciton and the donor-bound exciton at the Γ point in k space; the TO mode from the donor-bound exciton at the X point, the LO from the free exciton at the L …

Molecular Fluorescence Spectroscopy In The Vicinity Of A Microstructure, P.T. Leung, Thomas F. George

Physics Faculty Publications and Presentations

A brief review is presented on fluorescence spectroscopy of molecules in the vicinity of a microstructure, which could be a surface structure, clustered ions/atoms, a colloidal particle in a solution, or a tip in certain molecular probes. This structure can be viewed as microscopic, mesoscopic or macroscopic, depending on the specific environment in which the molecule is located. An overview is given for selected experimental and theoretical aspects of this phenomenon, emphasizing some of the previous modeling work of the authors. Discussion of possible future attempts to explore the phenomenon in more detail is presented at the end.

Critical Energy Of Torus Knots, Fred Hickling, Wesley Davis, Heather Woolverton

Journal of the Arkansas Academy of Science

The energy of a smoothly parameterized knot y(t) is defined as rr\ i i lp7 \\dn dsdt Jo Jo \||7M-7(0f (D(t(s),T(t))) 2 j\\ds \\dt where D(y (s), y(t)) is the arc length between the two points y (s) and y(t) on the curve. Simple calculus based arguments are used to locate critical values of the energy functional for torus knots. Explicitly the curves given parametrically by °(«*)W = (V2°iSri).JSSBe V2 C s7nS are CriticalP ° intS ° fthe energy functional whenever a and b are relatively prime.

Nonequilibrium Dynamic Conductivity Of Superconductors: An Exploitable Basis For High Energy Resolution X-Ray Detectors, Armen Gulian, D. Van Vechten

Mathematics, Physics, and Computer Science Faculty Articles and Research

A new design for high‐energy radiation/particle detectors is presented. The nonequilibrium response of a superconductor to the absorption of the incident quanta is sensed by electromagnetic measurements of the altered dynamic conductivity. Microwave absorption may be used to amplify the signal. Such a detector will provide better energy resolution than semiconducting charge‐collection devices once the statistical resolution limit is reached.

Energy-Loss Particle Identification In 2-D Silicon Drift Detectors, G. Douglas Maudlin, A. A. Rollefson, Wilfred J. Braithwaite

Journal of the Arkansas Academy of Science

A relatively new type of transducer known as the Silicon Drift Detector (SDD) has been fabricated onto thin silicon wafers. SDD operates like a miniature, high-resolution, 2-D Time-projection chamber. One of these devices can detect two dimensions of an ionizing particle's position, and its integrated electrical charge output level isproportional to the particle's energy loss through the silicon. An array ofSDD's, arranged in three coaxial cylinders, is being considered as part of an instrument surrounding the beam pipe of highly-relativistic colliding beam facility, where it would be used to simultaneously track individual paths of thousands of charged particles emerging from …

The Integral Quantum Hall Effect, John Jens Ryan

Honors Theses, 1963-2015

Temperature dependencies of the integer quantum Hall effect in an AlGaAs sample are reported. We obtained measurements at temperatures between 2.2 and 4.2 K, and magnetic fields up to 8 T. Shubnikov de Hass oscillations were also observed and analyzed. The relation between dp,/dB, the slopes between Hall plateaus, and temperature was also investigated and found to not fit the T-r~e lationship which others have experimentally found. Dependencies of the Hall plateaus on the magnetic field, Hall current, and the temperature are also investigated.

Structure And Function Of Magnetosomes In Magnetotactic Bateria, Richard B. Frankel, Dennis A. Bazylinski

Physics

Magnetotactic bacteria contain magnetosomes, which are mineral particles enclosed my membranes. The particles are ferrimagnetic magnetite, ferrimagnetic greigite, or greigite and non-magnetic pyrite. The particles constitute an elegant biomagnetic compass that orients the cell along the geomagnetic field lines as it swims. This paper discusses the structures of these particles and their possible formation mechanisms.

Vibrational Dynamics And Structure Of Amorphous Carbon Modeled Using The Embedded Ring Approach, T. E. Doyle, John R. Dennison

All Physics Faculty Publications

The Raman spectrum of graphitic amorphous carbon (g-C) is modeled with simple classical methods, using rings as medium-range structural units. g-C provides a simple [nearly two dimensional (2D) with a single type of atom and one dominant bond type] prototypic example of a large class of continuous random network (CRN) solids where ring or cluster vibrations cannot be decoupled from the network in which they are embedded. We determined the in-plane vibrational modes of n=4, 5, 6, 7, and 8-membered symmetric, planar carbon rings using bond-stretching and bond-angle bending force constants; an additional force constant couples each ring node to …

A Correlative Comparison Of The Ring Current And Auroral Electrojets Using Geomagnetic Indices, W. B. Cade Iii, Jan Josef Sojka, Lie Zhu

All Physics Faculty Publications

From a study of the 21 largest geomagnetic storms during solar cycle 21, a strong correlation is established between the ring current index Dst and the time-weighted accumulation of the 1-hour auroral electrojet indices, AE and AL. The time-weighted accumulation corresponds to convolution of the auroral electrojet indices with an exponential weighting function with an e-folding time of 9.4 hours. The weighted indices AE_w and AL_w have correlation coefficients against Dst ranging between 0.8 and 0.95 for 20 of the 21 storms. Correlation over the entire solar cycle 21 database is also strong but not as …

What Is The Source Of Observed Annual Variations In Plasmaspheric Density?, S. M. Guiter, C. E. Rasmussen, T. I. Gombosi, Jan Josef Sojka, Robert W. Schunk

All Physics Faculty Publications

Plasmaspheric densities have been observed previously to be higher in December than in June, with the ratio varying between 1.5 and 3.0 and with larger variations at lower L shells. In order to search for the cause of the observed annual variations, we have modeled plasmaspheric density, using a time-dependent hydrodynamic model. On an L = 2 field line with geomagnetic longitude equal to 300°, the modeled plasmaspheric densities were a factor of 1.5 times higher in December than in June. The modeled December to June density ratio was found to increase slightly with L shell, in contrast to observations; …

Parameterized Ionospheric Model: A Global Ionospheric Parameterization Based On First Principles Models, R. E. Daniell Jr., L. D. Brown, D. N. Anderson, M. W. Fox, P. H. Doherty, D. T. Decker, Jan Josef Sojka, Robert W. Schunk

All Physics Faculty Publications

We describe a parameterized ionospheric model (PIM), a global model of theoretical ionospheric climatology based on diurnally reproducible runs of four physics based numerical models of the ionosphere. The four numerical models, taken together, cover the E and F layers for all latitudes, longitudes, and local times. PIM consists of a semianalytic representation of diurnally reproducible runs of these models for low, moderate, and high levels of both solar and geomagnetic activity and for June and December solstice and March equinox conditions. PIM produces output in several user selectable formats including global or regional latitude/longitude grids (in either geographic or …

Natural Symmetries Of The Yang-Mills Equations, Charles G. Torre

All Physics Faculty Publications

A natural generalized symmetry of the Yang–Mills equations is defined as an infinitesimal transformation of the Yang–Mills field, built in a local, gauge invariant, and Poincaré invariant fashion from the Yang–Mills field strength and its derivatives to any order, which maps solutions of the field equations to other solutions. On the jet bundle of Yang–Mills connections a spinorial coordinate system is introduced that is adapted to the solution subspace defined by the Yang–Mills equations. In terms of this coordinate system the complete classification of natural symmetries is carried out in a straightforward manner. It is found that all natural symmetries …

Some Remarks On Gravitational Analogs Of Magnetic Charge, Charles G. Torre

All Physics Faculty Publications

Existing mathematical results are applied to the problem of classifying closed p-forms which are locally constructed from Lorentzian metrics on an n-dimensional orientable manifold M(0

Optimizing Tracking Software For A Time Projection Chamber, Wilson H. Howe, Christine A. Byrd, Amber D. Climer, Wilfred J. Braithwaite, Jeffrey T. Mitchell

Journal of the Arkansas Academy of Science

International research collaborations will be using accelerators in the U.S. and Europe to produce and detect t5 phase transition in high-density nuclear matter called the Quark-Gluon Plasma, formed in collisions between pairs of A=200 nuclei, for projectiles with kinetic energies large compared to their rest mass energies. Each collaboration will use time projection chambers (TPC) to track thousands ofsecondary charged particles formed in the aftermath ofeach central primary collision. Creating and optimizing TPC tracking software is difficultinsuch a high multiplicity environment, particularly for particles with a low momentum (below 300 MeV/C). A thigh momenta, energy loss is low enough for …

Proforce Waves: The Effect Of Current Behind The Shock Front On Wave Structure, Mostafa Hemmati, Steven Young

Journal of the Arkansas Academy of Science

Recently, the initial boundary conditions for proforce waves with a substantial current behind the shock front have been derived. Computer solutions of the Electron Fluid Dynamical equations meet the expected boundary conditions at the end of the sheath region. This paper will compare the wave structure for proforce waves with and without current behind the shock front.

Drift Chamber Utilizing Microstrip Readout For Testing A New Micro Tpc Concept, H. Weiman, W. G. Gong, S. Margetis, M. T. Burks, Wilfred J. Braithwaite, A. A. Rollefson

Journal of the Arkansas Academy of Science

A drift chamber type radiation detector is being used to examine design criteria for a new type of detector called a micro Time Projection Chamber (micro TPC) which is being proposed for use in high energy nuclear physics experiments. The main advantage of the micro TPC detector is its very low radiation thickness compared to its silicon counterpart. The micro TPC is a charged-particle detector which willbe optimized for good two track resolution which is needed inahigh track density environment. Such performance requires low electron diffusion and high resolution readout. The diffusion willbe reduced bylimiting the drift distance to 15 …

Supralinear Photoconductivity Of Copper Doped Semi-Insulating Gallium Arsenide, K. H. Schoenbach, R. P. Joshi, F. Peterkin, R. L. Druce

Bioelectrics Publications

We report on the intensity dependent supralinear photoconductivity in GaAs:Si:Cu material. The results of our measurements show that the effective carrier lifetime can change over two orders of magnitude with variations in the intensity of the optical excitation. A threshold intensity level has been observed and can be related to the occupancy of the deep copper level. Numerical simulations have also been carried out to analyze the trapping dynamics. The intensity dependent lifetimes obtained from the simulations match the experimental data very well. Finally, based on the nonlinear intensity dependence of the effective lifetimes, a possible low‐energy phototransistor application for …

Quantum Mechanics Of Multi-Prong Potentials, Asim Gangopadhyaya, A Pagnamenta, Uday P. Sukhatme

Physics: Faculty Publications and Other Works

We describe the bound state and scattering properties of a quantum mechanical particle in a scalar N-prong potential. Such a study is of special interest since these situations are intermediate between one and two dimensions. The energy levels for the special case of N identical prongs exhibit an alternating pattern of non-degeneracy and (N−1) fold degeneracy. It is shown that the techniques of supersymmetric quantum mechanics can be used to generate new solutions. Solutions for prongs of arbitrary lengths are developed. Discussions of tunneling in N-well potentials and of scattering for piecewise constant potentials are given. Since our treatment is …

Introduction To Monte Carlo Methods, Sue Ellen Mccloskey, Wilfred J. Braithwaite

Journal of the Arkansas Academy of Science

Monte Carlo computer programming is becoming increasingly popular to those who use it, due to the ease with which complex problems may be formulated and solved. However, the growth of MC programming for small projects is inhibited by a frequent misconception of difficulty, inferred from the high level of complexity of problems solved in High Energy and Nuclear Physics using MC methods. In addition, few students of science and engineering are receiving exposure to the basic issues involved in the Monte Carlo process despite the ease with which MC can be used to solve classical physics problems, especially those problems …