Physical Sciences and Mathematics Commons^™

Comment On "Observation Of Superluminal Behaviors In Wave Propagation", Harry Ringermacher, Lawrence R. Mead

Faculty Publications

No abstract provided.

43rd Rocky Mountain Conference On Analytical Chemistry

Rocky Mountain Conference on Magnetic Resonance

Final program, abstracts, and information about the 43rd 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 Denver, Colorado, July 29 - August 2, 2001.

The Three-Dimensional Random Field Ising Magnet: Interfaces, Scaling, And The Nature Of States, Alan Middleton, Daniel S. Fisher

Physics - All Scholarship

The nature of the zero temperature ordering transition in the 3D Gaussian random field Ising magnet is studied numerically, aided by scaling analyses. In the ferromagnetic phase the scaling of the roughness of the domain walls, $w\sim L^\zeta$, is consistent with the theoretical prediction $\zeta = 2/3$. As the randomness is increased through the transition, the probability distribution of the interfacial tension of domain walls scales as for a single second order transition. At the critical point, the fractal dimensions of domain walls and the fractal dimension of the outer surface of spin clusters are investigated: there are at least …

Microcavity Surface Emitting Laser, Stewart Feld, John P. Loehr, James A. Lott

AFIT Patents

A three-dimensional waveguiding structure for a microcavity surface-emitting laser is described in which native aluminum oxide layers provide control of intracavity waveguiding and the laser optical mode structure of the emitted beam. Microcavity lasers described herein account for the blueshift of the emission wavelength as the laser lateral dimensions are reduced to or below the emission wavelength.

An All Optical Laser Wakefield Electron Injector, Donald P. Umstadter

Donald Umstadter Publications

The personnel who were supported by the grant included the P.I. (Prof. Umstadter), several research scientists (A. Maksimchuk and V. Yanovsky), a postdoc (P. Zhang) and several graduate and undergraduate students. Although there were several setbacks in developing the novel laser technology required to produce a monoenergetic beam of electrons from an all-optical accelerator, several important steps were taken towards reaching that ultimate goal. The most important outcome of this project was that we demonstrated the principle of optical control of laser accelerators, namely, that one laser pulse could modify the properties (e.g., emittance and electron number) of an electron …

Microwave Ionization Of An Atomic Electron Wave Packet, Michael Noel, Lung Ko, T. F. Gallagher

Physics Faculty Research and Scholarship

A short microwave pulse is used to ionize a lithium Rydberg wave packet launched from the core at a well-defined phase of the field. We observe a strong dependence on the relative phase between the motion of the wave packet and the oscillations of the field. This phase dependent ionization is also studied as a function of the relative frequency. Our experimental observations are in good qualitative agreement with a one-dimensional classical model of wave packet ionization.

Measurement And Interpretation Of Deuterium-Line Emission In The Orion Nebula, C. R. O'Dell, Gary J. Ferland, W. J. Henney

Physics and Astronomy Faculty Publications

We present new observations of the deuterium and hydrogen Balmer lines in the Orion Nebula. There is a real variation in the deuterium-to-hydrogen line ratios across the nebula, being greatest in the emission from the largest proplyd (Orion 244-440). We also present the results of a detailed model for the emission of these lines, the hydrogen lines being the result of photoionization and recombination while the deuterium lines are produced by fluorescent excitation of the upper energy states by the far-UV radiation from θ¹ Ori C. Comparison of the observations and predictions of the line intensities shows good agreement, …

Crystal-Size Distributions And Possible Biogenic Origin Of Fe Sulfides, Mihály Pósfai, Krisztina Cziner, Emö Márton, Péter Márton, Peter R. Buseck, Richard B. Frankel, Dennis A. Bazylinski

Physics

Sedimentary greigite (Fe₃S₄) can form either by ''biologically controlled'' or by ''biologically induced mineralization'' (BCM and BIM, respectively). In order to identify the origin of magnetic Fe sulfides, we studied and compared the sizes and morphologies of greigite crystals produced by a magnetotactic microorganism (previously described and referred to as the ''many-celled magnetotactic prokaryote'', MMP) and Fe sulfides from two specimens of Miocene sedimentary rocks (from Łaka, in the foredeep of the Western Carpathians and from Michalovce, in the Transcarpathian Depression). Greigite grains from the MMP and the Łaka rock show nearly Gaussian crystal-size distributions (CSDs), …

Off-Axis Electron Holography Of Magnetotactic Bacteria: Magnetic Microstructure Of Strains Mv-1 And Ms-1, R. E. Dunin-Borkowski, Martha R. Mccartney, Mihály Pósfai, Richard B. Frankel, Dennis A. Bazylinski, Peter R. Buseck

Physics

Off-axis electron holography in the transmission electron microscope is used to characterize the magnetic microstructure of magnetotactic bacteria. The practical details of the technique are illustrated through the examination of single cells of strains MV-1 and MS-1, which contain crystals of magnetite (Fe₃O₄) that are ~50nm in size and are arranged in chains. Electron holography allows the magnetic domain structures within the nanocrystals to be visualized directly at close to the nanometer scale. The crystals are shown to be single magnetic domains. The magnetization directions of small crystals that would be superparamagnetic if they were isolated …

Evidence For Temperature Dependent Moments Ordering In Ferromagnetic Nimnsb(100), C.N. Borca, Takashi Komesu, Hae-Kyung Jeong, Peter A. Dowben, Delia Ristoiu, Ch. Hordequin, J. P. Nozie`Res, J. Pierre, Shane Stadler, Y. U. Idzerda

Peter Dowben Publications

From magnetic circular dichroism measurements of the Heusler alloy NiMnSb, in combination with other techniques, we show that a dramatic increase in the Mn and Ni moments occurs below a ‘‘crossover’’ transition temperature of about 80–100 K. This phase transition is well below the Curie temperature of NiMnSb (~730 K). While the spin polarization near the Fermi level, from spin-polarized inverse photoemission, remains very high for k_II50, it is unlikely that NiMnSb is a half-metallic ferromagnet near room temperature.

Microscopic Kinetics And Time-Dependent Structure Factors, T. Aspelmeier, Beate Schmittmann, R. K. P. Zia

Beate Schmittmann

The time evolution of structure factors (SF) in the disordering process of an initially phase-separated lattice depends crucially on the microscopic disordering mechanism, such as Kawasaki dynamics (KD) or vacancy-mediated disordering (VMD). Monte Carlo simulations show unexpected “dips” in the SFs. A phenomenological model is introduced to explain the dips in the odd SFs, and an analytical solution of KD is derived, in excellent agreement with simulations. The presence (absence) of dips in the even SFs for VMD (KD) marks a significant but not yet understood difference of the two dynamics.

High-PT Jets In Pp̅ Collisions At √S =630 And 1800 Gev, B. Abbott, Gregory R. Snow, D0 Collaboration

Gregory Snow Publications

Results are presented from analyses of jet data produced in pp̅ collisions at √s =630 and 1800 GeV collected with the DØ detector during the 1994–1995 Fermilab Tevatron Collider run. We discuss the details of detector calibration, and jet selection criteria in measurements of various jet production cross sections at √s = 630 and 1800 GeV. The inclusive jet cross sections, the dijet mass spectrum, the dijet angular distributions, and the ratio of inclusive jet cross sections at√s =630 and 1800 GeV are compared to next-to-leading order QCD predictions. The order α_s³ calculations are in …

Magnetic States Of Granular Layered Cofe-Al2O3, J.B. Sousa, G.N. Kakazei, Y.G. Pogorelov, J.A.M. Santos, O. Petracic, Wolfgang Kleemann, Christian Binek, Susana Cardoso De Freitas, P.P. Freitas, M.M. Pereira De Azevedo, N.A. Lesnik, Stanislav Rokhlin, P.E. Wigen

Christian Binek Publications

The granular layered magnetic system Co₈₀Fe₂₀(t)/Al₂ O₃ (3 nm), where the Co₈₀Fe₂₀ layers of nominal thickness t form separate, almost spherical magnetic granules of typical diameter 2-3 nm between the Al₂O₃ spacers, was studied. We discuss measurements of the dc and ac magnetic susceptibility χ for 1 nm

Dynamics Of Symmetry Breaking And Tachyonic Preheating, Gary Felder, Juan García-Bellido, Patrick B. Greene, Lev Kofman, Andrei Linde, Igor Tkachev

Physics: Faculty Publications

We reconsider the old problem of the dynamics of spontaneous symmetry breaking (SSB) using 3D lattice simulations. We develop a theory of tachyonic preheating, which occurs due to the spinodal instability of the scalar field. Tachyonic preheating is so efficient that SSB typically completes within a single oscillation as the field rolls towards the minimum of its effective potential. We show that, contrary to previous expectations, preheating in hybrid inflation is typically tachyonic. Our results may also be relevant for the theory of the formation of topological defects and of disoriented chiral condensates in heavy ion collisions.

Relativistic Effects On Interchannel Coupling In Atomic Photoionization: The Photoelectron Angular Distribution Of Xe, Oliver Hemmers, S. T. Manson, M. M. Sant'anna, P. Focke, H. Wang, I. A. Sellin, Dennis W. Lindle

Environmental Studies Faculty Publications

Measurements of the photoelectron angular-distribution asymmetry parameter β for Xe 5s photoionization have been performed in the 80–200 eV photon-energy region. The results show a substantial deviation from the nonrelativistic value of β=2 and provide a clear signature of significant relativistic effects in interchannel coupling.

Characterization Of Near-Surface Traps In Semiconductors: Gan, David C. Look, Z-Q. Fang

Physics Faculty Publications

We present a simple a criterion, based on deep-level transient spectroscopy peak heights S(V_r) at two or more values of reverse bias V_r, to unequivocally determine whether or not a particular semiconductor trap is of bulk or near-surface nature. Moreover, we present an expression for S(V_r) with fitting parameters ϕ_B, the Schottky barrier height; δ, the trap penetration depth; and N_T, the trap density. Application of the method to a thick, high-quality, epitaxial GaN layer, reveals two common traps which penetrate only 2700±300 Å into the …

Local Polariton Modes And Resonant Tunneling Of Electromagnetic Waves Through Periodic Bragg Multiple Quantum Well Structures, Lev I. Deych, Alexey Yamilov, Alexander A. Lisyansky

Physics Faculty Research & Creative Works

We study analytically defect polariton states in Bragg multiple quantum well structures and defect-induced changes in transmission and reflection spectra. Defect layers can differ from the host layers in three ways: exciton-light coupling strength, exciton resonance frequency, and interwell spacing. We show that a single defect leads to two local polariton modes in the photonic band gap. These modes cause peculiarities in reflection and transmission spectra. Each type of defect can be reproduced experimentally, and we show that each of these plays a distinct role in the optical properties of the system. For some defects, we predict a narrow transmission …

A Theoretical Study Of The Effects Of Insulators On Electron Transport Through Electrostatic Tube Lenses, T. G. Anderson, B. G. Birdsey, S. M. Woeher, Mark A. Rosenberry, Timothy J. Gay

Timothy J. Gay Publications

Using the computer program SIMION 6, we have studied the effects of spurious insulator charging on the trajectories of electrons through electrostatic tube lenses. We considered lens elements with flat ends, spaced a distance g apart, whose inner diameter D=10g. For the standard cases of drift tubes, two-element lenses, and einzel lenses, we found that charging effects are eliminated if t/g>3.5, where t is the distance between the lens inner diameter and the charged spacing insulator.

High-Temperature Magnetic Properties Of Mechanically Alloyed Smco 5 And Yco5 Magnets, I.A. Al-Omari, Ralph Skomski, R.A. Thomas, D. Leslie-Pelecky, David J. Sellmyer

David Sellmyer Publications

The high-temperature coercivity of mechanically alloyed and subsequently annealed RCo₅ (R=Sm and Y) is studied. The annealed materials have the hexagonal CaCu₅ structure with 2 : 17 (or 1 : 7) regions as a minor phase. High-temperature magnetic measurements showthat the coercivities of materials decrease with increasing temperature fromroom-temperature to 873 K, but that the temperature coefficient of the coercivity of YCo₅ is much smaller than that of SmCo₅. This behavior is explained in terms of the intrinsic temperature variation of the magnetocrystalline anisotropy.

Coercivity Of Titanium-Substituted High-Temperature Permanent Magnets, Jian Zhou, Ralph Skomski, David J. Sellmyer

David Sellmyer Publications

The temperature dependence of the coercivity of Sm–Co based magnets is investigated by magnetization measurements and model calculations. The Zr-free titanium-substituted Sm–Co material exhibits a positive temperature coefficient dH_{c/dT of the coercivity (TCC) above room temperature, a reasonable hysteresis-loop shape, and an appreciable coercivity of 12.3 kOe at 500 C for the nominal composition Sm(Co6.2Cu0.8Ti0.3). The samples were produced by heat-treating the disordered 1 : 5 alloy commonly referred to as the TbCu7 (or 1 : 7) phase. X-ray diffraction analysis shows that, upon annealing at 1165 C, …}

Grain-Boundary Micromagnetism, Ralph Skomski, H. Zeng, David J. Sellmyer

David Sellmyer Publications

Continuum and layer-resolved calculations are used to investigate the spin structure in the vicinity of grain boundaries. Reduced exchange in the grain-boundary region gives rise to a quasidiscontinuity of the magnetization and yields a perturbation which decays exponentially inside the grains. An effective intergranular exchange is obtained as a micromagnetically well-defined function of the grain-boundary exchange, and it is discussed how grain boundaries affect the hysteresis loops of nanostructures.

The Blazhko Effect Of The Rr Lyrae Star Fm Persei, Kevin M. Lee, Edward G. Schmidt

Edward Schmidt Publications

We have obtained 615 new V and R observations of the RR Lyrae star FM Per. A period search identified a primary period of 0.489256 days; however, the light curve of FM Per cannot be well described by a single period. We conclude that FM Per demonstrates the Blazhko effect with a Blazhko period of approximately 122 days.

Ferromagnetic Co-C Nanodot Arrays Produced By Direct Interferometric Laser Annealing, M. Zheng, M. Yu, Yi Liu, Ralph Skomski, David J. Sellmyer, V.N. Petryakov, Yu. K. Verevkin, Nikolay Polushkin, N.N. Salashchenko

Sy-Hwang Liou Publications

Magnetic properties of Co-C nanodot arrays produced by direct interferometric laser annealing are investigated by magnetic force microscopy (MFM) and magnetization measurements. The dots are formed by locally annealing sputtered amorphous Co-C films in regions where the laser intensity is highest. As-sputtered Co-C films do not exhibit ferromagnetic order at room temperature, but MFM shows that the dots become magnetic upon annealing, possibly due to the agglomeration or phase separation of Co-rich clusters. The dots are embedded in either a paramagnetic or weakly magnetic matrix. The magnetic properties of the generated pattern can be changed by varying the laser power. …

Magnetic Properties And Nanostructures Of Composite, M.L. Yan, Yi Liu, M.J. Yu, Sy_Hwang Liou, David J. Sellmyer

Sy-Hwang Liou Publications

Nanocomposite Co_χPt100_100-χ :C thin films with χ = 57 and χ = 50 were prepared by magnetron sputtering. Structures and activation volumes were studied systematically in order to better understand the magnetic behavior.We found that activation volumes of these two samples were almost the same. However, physical grain sizes of these two samples were different. Magnetic grains in the sample with equiatomic Co and Pt composition were almost independently switched and magnetic grains in the sample with higher Co content were exchange coupled and switched together. The reduction of the physical grain size in the sample …

Measurement Of The Two-Jet Differential Cross Section In Pp̅ Collisions At √S=1800 Gev, T. Affolder, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

A measurement is presented of the two-jet differential cross section d³σ/dE_Td η₁d η₂ at center of mass energy √s =1800 GeV in pp̅ collisions. The results are based on an integrated luminosity of 86 pb^-1 collected during 1994 to 1995 by the CDF Collaboration at the Fermilab Tevatron collider. The differential cross section is measured as a function of the transverse energy E_T of a jet in the pseudorapidity region 0.1 < | η₁| < 0.7 for four different pseudorapidity bins of a second jet restricted to 0.1 < |η₂| < 3.0. The results are compared with next-to-leading order QCD calculations determined using the CTEQ4 and MRST sets of parton distribution functions. None of the sets examined in this analysis provides a good description of the data.

Multiplewavelength Optical Observations Of A Long-Lived Meteor Trail, B. R. Clemesha, A. F. Medeiros, D. Gobbi, H. Takahashi, P. P. Batista, Michael J. Taylor

All Physics Faculty Publications

A long‐lived meteor trail has been observed at wavelengths of 572.5 nm, 557.7 nm, 630.0 nm, 865.5 nm and in the near infrared band from 715 to 930 nm. The trail was detected at all these wavelengths, with the possible exception of 865.5 nm, where its identification was marginal. It was seen longest (17 minutes) through the wide band NIR 715‐930 nm filter. The fact that the trail was only marginally visible in the 865.5 nm (0–1) band of molecular oxygen, and was strongest in the wide‐band NIR image, raises serious doubts about an earlier suggestion that the infrared light …

Separation Of Recollision Mechanisms In Nonsequential Strong Field Double Ionization Of Ar: The Role Of Excitation Tunneling, Bernold Feuerstein, Daniel Fischer, Alexander Dorn, Claus Dieter Schroter, J. Deipenwisch, Jose R. Crespo Lopez-Urrutia, C. Hohr, Paul B. Neumayer, Horst Rottke, Christoph Trump, M. Wittmann, Georg Korn, Wolfgang Sandner, Robert Moshammer, Joachim Hermann Ullrich

Physics Faculty Research & Creative Works

Vector momentum distributions of two electrons created in double ionization of Ar by 25 fs, 0.25PW/cm² laser pulses at 795 nm have been measured using a “reaction microscope.” At this intensity, where nonsequential ionization dominates, distinct correlation patterns are observed in the two-electron momentum distributions. A kinematical analysis of these spectra within the classical “recollision model” revealed an (e,2e)-like process and excitation with subsequent tunneling of the second electron as two different ionization mechanisms. This allows a qualitative separation of the two mechanisms demonstrating that excitation-tunneling is the dominant contribution to the total double ionization yield.

Resummation Of The Divergent Perturbation Series For A Hydrogen Atom In An Electric Field, Ulrich D. Jentschura

Physics Faculty Research & Creative Works

We consider the resummation of the perturbation series describing the energy displacement of a hydrogenic bound state in an electric field (known as the Stark effect or the LoSurdo-Stark effect), which constitutes a divergent formal power series in the electric-field strength. The perturbation series exhibits a rich singularity structure in the Borel plane. Resummation methods are presented that appear to lead to consistent results even in problematic cases where isolated singularities or branch cuts are present on the positive and negative real axis in the Borel plane. Two resummation prescriptions are compared: (i) a variant of the Borel-Pad'e resummation method, …

General Features Of The Intrinsic Ferroelectric Coercive Field, V. M. Fridkin, Stephen Ducharme

Stephen Ducharme Publications

The value of the intrinsic ferroelectric coercive field is obtained independently, from general energy considerations and from the predictions of several models of the ferroelectric state. All predictions yield a value of the order of the depolarization field, which is equal to the spontaneous polarization divided by the dielectric permittivity, and are consistent with the recent measurements of the intrinsic ferroelectric coercive field in ultrathin Langmuir–Blodgett films of copolymers of polyvinylidene fluoride with trifluoroethylene. Prior studies succeeded only in measuring the much smaller extrinsic coercive fields, which are limited by nucleation processes and domain motion.

Feedback Correction Of Angular Error In Grating Readout, Monish Ranjan Chatterjee, Sundaram Ramachandran

Electrical and Computer Engineering Faculty Publications

Angular and wavelength READ beam errors in holographic interconnection systems are often a recurrent problem. Several strategies have been proposed to minimize or eliminate such READOUT misalignments.

Some years ago, Chatterjee and co-workers proposed a method involving READ beam wavelength tuning to correct output angular errors. In this paper, we investigate the possibility of using an acousto-optic (A-O) Bragg cell with optoelectronic feedback to dynamically correct the scattered beam for deviations in the incidence direction of the READ beam of a hologram. The concept here is based on an acoustic frequency feedback strategy used recently by Balakshy and Kazaryan for …