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Iowa State University

1999

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Articles 1 - 30 of 33

Full-Text Articles in Physics

Quasicrystalline Nature Of Quasicrystal Surfaces: A Photoemission Study, D. Naumović, P. Aebi, :L. Schlapbach, C. Beeli, Thomas A. Lograsso, Dwight W. Delaney Dec 1999

Quasicrystalline Nature Of Quasicrystal Surfaces: A Photoemission Study, D. Naumović, P. Aebi, :L. Schlapbach, C. Beeli, Thomas A. Lograsso, Dwight W. Delaney

Ames Laboratory Publications

Differently prepared surfaces of quasicrystalline i-Al-Pd-Mn are analyzed using angle-resolved photoemission in the x-ray andultraviolet range of photon energies. Depending on the preparation, we find both surfaces with crystalline structure and metallic character, and surfaces with quasicrystalline structural fingerprints and a suppressed density of states at the Fermi level, compatible with a pseudogap.


Evidence For Replicate 5p Core Levels In Photoelectron Spectra Of Eu Metal Due To Nonconstant Kinetic-Energy Resonant Auger Decay, S. Haffner, C. G. Olson, David W. Lynch Dec 1999

Evidence For Replicate 5p Core Levels In Photoelectron Spectra Of Eu Metal Due To Nonconstant Kinetic-Energy Resonant Auger Decay, S. Haffner, C. G. Olson, David W. Lynch

Physics and Astronomy Publications

Satellites on the low-binding-energy side of core-level photoelectron emission due to extra 4f screening are a well-known feature in the x-ray photoelectron spectra of valence fluctuation materials and rare-earth metals. A notable exception is Eu metal, where up to now no low-binding-energy satellite has been observed. In this paper we show that in Eu metal the 4d−4f resonance can decay via a resonant Auger decay, which is not a constant kinetic-energy feature due to a rapid change of the strength of 4f screening with excitation energy, establishing a low-binding-energy replica of the 5p core-level photoelectron emission.


Co-Oxidation Model With Superlattice Ordering Of Adsorbed Oxygen. I. Steady-State Bifurcations, E. W. James, C. Song, James W. Evans Oct 1999

Co-Oxidation Model With Superlattice Ordering Of Adsorbed Oxygen. I. Steady-State Bifurcations, E. W. James, C. Song, James W. Evans

Mathematics Publications

We analyze a model for CO oxidation on surfaces which incorporates both rapid diffusion of adsorbed CO, and superlattice ordering of adsorbed immobile oxygen on a square lattice of adsorption sites. The superlattice ordering derives from an “eight-site adsorption rule,” wherein diatomic oxygen adsorbs dissociatively on diagonally adjacent empty sites, provided that none of the six additional neighboring sites are occupied by oxygen. A “hybrid” formalism is applied to implement the model. Highly mobile adsorbed CO is assumed randomly distributed on sites not occupied by oxygen (which is justified if one neglects CO–CO and CO–O adspecies interactions), and ...


On The Temperature Dependence Of Multiple- And Single-Scattering Contributions In Magnetic Exafs, H. Wende, F. Wilhelm, P. Poulopoulos, K. Baberschke, J. W. Freeland, Y. U. Idzerda, A. Rogalev, Deborah L. Schlagel, Thomas A. Lograsso, D. Arvanitis Sep 1999

On The Temperature Dependence Of Multiple- And Single-Scattering Contributions In Magnetic Exafs, H. Wende, F. Wilhelm, P. Poulopoulos, K. Baberschke, J. W. Freeland, Y. U. Idzerda, A. Rogalev, Deborah L. Schlagel, Thomas A. Lograsso, D. Arvanitis

Ames Laboratory Conference Papers, Posters, and Presentations

We demonstrate that the temperature dependence of structural as well as magnetic fluctuations can be probed by the use of the Magnetic Extended X-ray Absorption Fine Structure (MEXAFS) spectroscopy. We compare those to the dynamic disorder as probed by the EXAFS. Here we present temperature-dependent MEXAFS investigations carried out at the L-edges of a thin Fe film and a Gd single crystal. By comparing the experimental results to ab initio calculations the single-scattering contributions are separated from multiple-scattering contributions. It is found that the multiple-scattering contributions are enhanced for the MEXAFS compared to the normal EXAFS.


Optical Photonic Crystals Fabricated From Colloidal Systems, G. Subramania, Kristen P. Constant, Rana Biswas, M. M. Sigalas, Kai-Ming Ho Jun 1999

Optical Photonic Crystals Fabricated From Colloidal Systems, G. Subramania, Kristen P. Constant, Rana Biswas, M. M. Sigalas, Kai-Ming Ho

Materials Science and Engineering Publications

Photonic crystals of close-packed arrays of air spheres in a dielectric background of titania have been fabricated with a ceramic technique. Unlike previous methods, ordering of the spheres and the formation of the titania network are performed simultaneously. The photonic crystals exhibit a reflectance peak and a uniform color at the position of the first stop band. The wavelength of the reflectance peak scales very well with the sphere size.


Novel One-Phase Synthesis Of Thiol-Functionalized Gold, Palladium, And Iridium Nanoparticles Using Superhydride, Chanel K. Yee, Rainer Jordan, Abraham Ulman, Henry White, Alexander H. King, Miriam Rafailovich, Jonathan Sokolov May 1999

Novel One-Phase Synthesis Of Thiol-Functionalized Gold, Palladium, And Iridium Nanoparticles Using Superhydride, Chanel K. Yee, Rainer Jordan, Abraham Ulman, Henry White, Alexander H. King, Miriam Rafailovich, Jonathan Sokolov

Alexander H. King

A new, facile, general one-phase synthesis for thiol-functionalized gold, palladium, and iridium nanoparticles, using tetrahydrofuran (THF) as the solvent and lithium triethylborohydride (Superhydride) as the reducing agent, is presented. For octadecanethiol-functionalized gold (Au/ODT) nanoparticles, HRTEM of drop-cast particle-films revealed the formation of spherical particles of d = 4 ± 0.3 nm average size. Electron diffraction shows fcc packing arrangement, similar to that of bulk gold. The crystalline gold cores are surrounded with closely packed n-alkyl chains mainly in an all-trans conformation, adopting orthorhombic packing as confirmed by FTIR spectroscopy. Particles are arranged in a discrete solidlike assembly with a correlation ...


Incommensurate And Commensurate Antiferromagnetic Spin Fluctuations In Cr And Cr Alloys From Ab Initio Dynamical Spin Susceptibility Calculations, Duane D. Johnson, J. Pulter, B. Ginatempo, E. Bruno, J. B. Staunton Apr 1999

Incommensurate And Commensurate Antiferromagnetic Spin Fluctuations In Cr And Cr Alloys From Ab Initio Dynamical Spin Susceptibility Calculations, Duane D. Johnson, J. Pulter, B. Ginatempo, E. Bruno, J. B. Staunton

Duane D. Johnson

A scheme for making ab initio calculations of the dynamic paramagnetic spin susceptibilities of solids at finite temperatures is described. It is based on time-dependent density functional theory and employs an electronic multiple scattering formalism. Incommensurate and commensurate antiferromagnetic spin fluctuations in paramagnetic Cr and compositionally disordered Cr95V5 and Cr95Re5 alloys are studied together with the connection with the nesting of their Fermi surfaces. We find that the spin fluctuations can be described rather simply in terms of an overdamped oscillator model. Good agreement with inelastic neutron scattering data is obtained.


Microstructure And Magnetic Properties Of As-Quenched And Heat-Treated (Nd,Dy)Feb Powders Produced By High Pressure Gas Atomization, John E. Snyder, Chester C.H. Lo, X. Fang, B. Kriegermeier, David C. Jiles Apr 1999

Microstructure And Magnetic Properties Of As-Quenched And Heat-Treated (Nd,Dy)Feb Powders Produced By High Pressure Gas Atomization, John E. Snyder, Chester C.H. Lo, X. Fang, B. Kriegermeier, David C. Jiles

Ames Laboratory Publications

The magnetic properties and microstructures of a series of as-quenched and heat-treated inert gas atomized (IGA) rare-earth rich (Nd,Dy)–Fe–B particles have been investigated. Heat treatment was found to substantially improve magnetic properties, with effects most pronounced in samples with higher Dy content and higher total rare earth (RE) content. The as-quenched particles consisted of an underquenched dendritic-like structure with the majority phase RE2Fe14B, and a fine network of RE-rich material between the grains. The heat-treated particles showed a change in microstructure which correlated with magnetic property changes. Particles which showed little change in magnetic properties showed no ...


Application Of The Preisach And Jiles–Atherton Models To The Simulation Of Hysteresis In Soft Magnetic Alloys, M. Pasquale, G. Bertotti, David C. Jiles, Y. Bi Apr 1999

Application Of The Preisach And Jiles–Atherton Models To The Simulation Of Hysteresis In Soft Magnetic Alloys, M. Pasquale, G. Bertotti, David C. Jiles, Y. Bi

Ames Laboratory Publications

his article describes the advances in unification of model descriptions of hysteresis in magnetic materials and demonstrates the equivalence of two widely accepted models, the Preisach (PM) and Jiles–Atherton (JA) models. Recently it was shown that starting from general energy relations, the JA equation for a loop branch can be derived from PM. The unified approach is here applied to the interpretation of magnetization measured in nonoriented Si–Fe steels with variable grain size ⟨s⟩, and also in as-cast and annealed Fe amorphous alloys. In the case of NO Fe–Si, the modeling parameter k defined by the volume ...


Monitoring Fatigue Damage In Materials Using Magnetic Measurement Techniques, Chester C.H. Lo, F. Tang, Y. Shi, David C. Jiles, S. B. Biner Apr 1999

Monitoring Fatigue Damage In Materials Using Magnetic Measurement Techniques, Chester C.H. Lo, F. Tang, Y. Shi, David C. Jiles, S. B. Biner

Center for Nondestructive Evaluation Publications

Measurements of hysteresis and Barkhausen effect (BE) have been made on 0.1 wt % C Fe–C alloys subjected to strain-controlled fatigue at various strain amplitudes. A relationship between the fatigue lifetime and strain amplitude was observed. The hysteresis properties of the samples cycled at different strain amplitudes were found to vary systematically with expended fatigue life. These properties showed significant changes in the initial and final stages of fatigue, while between these stages they remained stabilized. In the stable stage the remanence was found to decrease, whereas the coercivity increased with increasing strain amplitude. Variations in BE signal during ...


Core Loss Reduction In Electrical Steels Through Materials Processing, B. Verbrugge, David C. Jiles Apr 1999

Core Loss Reduction In Electrical Steels Through Materials Processing, B. Verbrugge, David C. Jiles

Electrical and Computer Engineering Publications

Research has been conducted into the effects of laser scribing on the core losses of soft magnetic materials. Various types of lamination steels for use in electric motors and transformers were studied including iron based alloys containing carbon,silicon,nickel, and cobalt. The results show that the change in core losses for carbon,nickel, and cobalt alloys was small for the scribing conditions examined, but core loss reductions of up to 18% were achieved with iron–silicon alloys.


Dynamical Diffraction And X-Ray Standing Waves From Atomic Planes Normal To A Twofold Symmetry Axis Of The Quasicrystal Alpdmn, Terrence Jach, Y. Zhang, R. Colella, M. De Boissieu, Alan I. Goldman, Thomas A. Lograsso, D. W. Delaney, S. Kycia Apr 1999

Dynamical Diffraction And X-Ray Standing Waves From Atomic Planes Normal To A Twofold Symmetry Axis Of The Quasicrystal Alpdmn, Terrence Jach, Y. Zhang, R. Colella, M. De Boissieu, Alan I. Goldman, Thomas A. Lograsso, D. W. Delaney, S. Kycia

Ames Laboratory Publications

We have observed dynamical diffraction in the [[(02402̅ 4) and [[(04604̅ 6) reflections of the icosahedral quasicrystal AlPdMn in the back-reflection geometry ( θB = 90°). The x-ray fluorescence from the Al and Pd atoms exhibits strong standing wave behavior, similar to that observed in crystalline materials. The data indicate a long-range order of each species of atoms, with the coherent positions attributable to distributions of the Al and Pd, which we compare to a centrosymmetric model. We observe deviations from the model which imply small departures from inversion symmetry along the twofold symmetry axis and from the expected coherent fractions ...


Fluctuation-Induced Transitions In A Bistable Surface Reaction: Catalytic Co Oxidation On A Pt Field Emitter Tip, Yu Suchorski, J. Beben, E. W. James, James W. Evans, R. Imbihl Mar 1999

Fluctuation-Induced Transitions In A Bistable Surface Reaction: Catalytic Co Oxidation On A Pt Field Emitter Tip, Yu Suchorski, J. Beben, E. W. James, James W. Evans, R. Imbihl

Mathematics Publications

Fluctuations which arise in catalytic CO oxidation on a Pt field emitter tip have been studied with field electron microscopy as the imaging method. Fluctuation-driven transitions between the active and the inactive branch of the reaction are found to occur sufficiently close to the bifurcation point, terminating the bistable range. The experimental results are modeled with Monte Carlo simulations of a lattice-gas reaction model incorporating rapid CO diffusion.


Effect Of The Elastic Modulus Of The Matrix On Magnetostrictive Strain In Composites, Y. Chen, John E. Snyder, Carl R. Schwichtenberg, Kevin W. Dennis, D. K. Falzgraf, R. William Mccallum, David C. Jiles Feb 1999

Effect Of The Elastic Modulus Of The Matrix On Magnetostrictive Strain In Composites, Y. Chen, John E. Snyder, Carl R. Schwichtenberg, Kevin W. Dennis, D. K. Falzgraf, R. William Mccallum, David C. Jiles

Ames Laboratory Publications

The effect of the matrix material on the magnetostriction of composites containing highly magnetostrictive particles has been studied. Experimental results showed that the elastic modulus of the matrix is an important factor determining the bulk magnetostriction of the composite. For a series of composites with the same volume fraction of magnetostrictive particles but different matrix materials, the bulk magnetostriction was found to increase systematically with decreasing elastic modulus of the matrix. A modeltheory for the magnetostriction of such composites has been developed, based on two limiting assumptions: uniform strain or uniform stress inside the composite. The theory was then used ...


Magneto-Optic Kerr Effect Of Strongly Correlated Electron Compounds , Rüdiger Johannes Lange Jan 1999

Magneto-Optic Kerr Effect Of Strongly Correlated Electron Compounds , Rüdiger Johannes Lange

Retrospective Theses and Dissertations

The optical conductivity and the magneto-optic polar Kerr effect of RAl 2 (R = La, Ce, Pr), RFe2 (R = Y, Ce), RNi 2B2C (R = Tm, Yb), CeB6, and YbBiPt were measured between 1.4 and 5.4 eV using a rotating analyzer ellipsometer and a normal incidence Kerr spectrometer. Optical absorption of RAl2, RFe 2, and RNi2B2C shows metallic behavior. For the low carrier concentration metals CeB6 and YbBiPt the spectrum is dominated by interband transitions. Spectra of RAl2 and RFe 2 show absorption at 2 eV which is also found in heavier RFe2 compounds. This structure is attributed to R-derived ...


Resistivity, Thermopower And The Correlation To Infrared Active Vibrations Of Mn1.56co0.96ni0.48o4 Spinel Films Sputtered In An Oxygen Partial Pressure Series, Rand Dannenberg, S. Baliga, R. J. Gambino, Alexander H. King, A. P. Doctor Jan 1999

Resistivity, Thermopower And The Correlation To Infrared Active Vibrations Of Mn1.56co0.96ni0.48o4 Spinel Films Sputtered In An Oxygen Partial Pressure Series, Rand Dannenberg, S. Baliga, R. J. Gambino, Alexander H. King, A. P. Doctor

Alexander H. King

Mn1.56Co0.96Ni0.48O4 spinel was sputter deposited using a series of oxygen partial pressures. Electrical resistivity versus temperature and thermopower versus temperature measurements at each oxygen partial pressure were made. The variations of the thermopower and resistivity with oxygen partial pressure are consistent with a change in the ratio of Mn3+ to Mn4+ cations, which occurs due to changes of oxygen content of the material. The weak temperature dependence of the thermopower indicates small polaron hopping is the charge transport mechanism. Combining the models of Mott and Schnakenberg to analyze the transport data, we find that the Debye temperature ...


Optical Properties And Electronic Structures Of B2 And B19′ Phases Of Equiatomic Ni-Ti Alloys, Joo Yull Rhee, Bruce N. Harmon, David W. Lynch Jan 1999

Optical Properties And Electronic Structures Of B2 And B19′ Phases Of Equiatomic Ni-Ti Alloys, Joo Yull Rhee, Bruce N. Harmon, David W. Lynch

Physics and Astronomy Publications

The dielectric functions of equiatomic Ni-Ti alloys were measured by spectroscopic ellipsometry in the energy range of 1.5–5.4 eV at ∼423 and at ∼25 K. The peak at ∼2.26 eV in the B19′ (monoclinic structure) optical conductivity spectrum has a slightly larger magnitude than in the B2 (cubic CsCl structure) phase, while the shoulder at ∼3.5 eVbecomes weaker and almost indiscernible upon martensitic transformation. A new structure develops at ∼2.85 eV in the B19′ phase; however, it is also very weak. The band structures and the optical conductivity were calculated in both phases using ...


On Black Hole Horizon Fluctuations, Kirill Tuchin Jan 1999

On Black Hole Horizon Fluctuations, Kirill Tuchin

Kirill Tuchin

A study of the high angular momentum particles ‘atmosphere’ near the Schwarzschild black hole horizon suggested that strong gravitational interactions occur at invariant distance of the order of 3√M [2]. We present a generalization of this result to the Kerr-Newman black hole case. It is shown that the larger charge and angular momentum black hole bears, the larger invariant distance at which strong gravitational interactions occur becomes. This invariant distance is of order 3√r+2(r+ − r−). This implies that the Planckian structure of the Hawking radiation of extreme black holes is completely broken.


Infrared Optical Properties Of Mn1.56co0.96ni0.48o4 Spinel Films Sputter Deposited In An Oxygen Partial Pressure Series, Rand Dannenberg, S. Baliga, R. J. Gambino, Alexander H. King, A. P. Doctor Jan 1999

Infrared Optical Properties Of Mn1.56co0.96ni0.48o4 Spinel Films Sputter Deposited In An Oxygen Partial Pressure Series, Rand Dannenberg, S. Baliga, R. J. Gambino, Alexander H. King, A. P. Doctor

Alexander H. King

Mn1.56Co0.96Ni0.48O4 spinel films were sputter deposited onto silicon substrates using a series of oxygen partial pressures. Fourier transform infrared transmission and reflectance, and Raman scattering measurements were made. The 1–25 μm wavelength range was examined using these optical techniques. The complex index of refraction was calculated for this entire wavelength range. Infrared active vibrations were analyzed using multiple oscillator analysis, Kramers–Kronig analysis, and derivative reflectance spectroscopy. The Raman and infrared active lattice vibrations were observed to shift with increasing oxygen partial pressure during film deposition, and were consistent with the earlier published shift in Debye ...


Hard Single Diffraction In Pp̄ Collisions At 1800 And 630 Gev Center Of Mass Energies , Kristal Monika Mauritz Jan 1999

Hard Single Diffraction In Pp̄ Collisions At 1800 And 630 Gev Center Of Mass Energies , Kristal Monika Mauritz

Retrospective Theses and Dissertations

A search for evidence of Hard Single Diffraction (HSD) in forward and central jet systems is described for center-of-mass energies of 1800 and 630 GeV. Events are examined for the presence of a forward rapidity gap with concurrent jet production. Relative event rates are estimated for both processes as compared to an inclusive dijet data sample, and the distributions of Pomeron x in the proton, called xi, are also measured for both forward and central HSD events, The data measurements are compared to the Ingelman-Schlein model.


Wave Propagation In Ordered, Disordered, And Nonlinear Photonic Band Gap Materials , Elefterios Efstathiou Lidorikis Jan 1999

Wave Propagation In Ordered, Disordered, And Nonlinear Photonic Band Gap Materials , Elefterios Efstathiou Lidorikis

Retrospective Theses and Dissertations

Photonic band gap materials are artificial dielectric structures that give the promise of molding and controlling the flow of optical light the same way semiconductors mold and control the electric current flow. Their basic property is the photonic gap, a frequency range in which wave propagation is not allowed in any direction, in a close analogy with the electronic energy band gap in semiconductors;In this dissertation we study two areas of photonic band gap materials. The first area is focused on the properties of one-dimensional PBG materials doped with Kerr-type nonlinear material. Such systems have been shown to exhibit ...


Development Of Highly Magnetostrictive Composites For Applications In Magnetomechanical Torque Sensors , Yonghua Chen Jan 1999

Development Of Highly Magnetostrictive Composites For Applications In Magnetomechanical Torque Sensors , Yonghua Chen

Retrospective Theses and Dissertations

The objective of this work was to investigate and develop a new type of magnetomechanical material with high magnetomechanical response and low hysteresis. This material will be used in electronic torque sensors. A major appreciation could be for advanced steering systems in automobiles which will replace the fuel inefficient hydraulic steering systems currently in use;The effect of the matrix material on the magnetostriction of composites containing highly magnetostrictive particles was studied. Both experimental and modeled results showed that the elastic modulus of the matrix is an important factor determining the magnetostriction of the composite. For a series of composites ...


Resonance Production In Quark And Gluon Jets , Derek William Lane Jan 1999

Resonance Production In Quark And Gluon Jets , Derek William Lane

Retrospective Theses and Dissertations

The formation of hadrons from free quarks and gluons is poorly understood and can not be fully explained by current theories. This hadronization process is described using phenomenological models, each reflecting possible scenarios for the QCD dynamics. Electron-positron annihilation at the Z0 resonance provides an ideal environment in which to study hadron production because of the combination of high event rates and clean final states due to the absence of beam and target remnants. Using the particle identification capabilities of the DELPHI detector at the Large Electron-Positron collider (LEP), it is possible to differentiate the production of different hadron species ...


Anisotropic Magnetization And Transport Properties Of Ragsb₂ (R=Y, La-Nd, Sm, Gd-Tm) , Kenneth Duane Myers Jan 1999

Anisotropic Magnetization And Transport Properties Of Ragsb₂ (R=Y, La-Nd, Sm, Gd-Tm) , Kenneth Duane Myers

Retrospective Theses and Dissertations

Anisotropic measurements of the magnetic and transport properties have been performed on high quality single crystals of the RAgSb2 series of compounds with R = Y, La-Nd, Sm, Gd-Tm. For all of these compounds except for ErAgSb2 and TmAgSb2, strong magnetic anisotropy created by CEF splitting of the Hund's rule ground state confines the moments to the basal plane. Additional anisotropy within the basal plane is also observed in DyAgSb2, leading to a series of metamagnetic transitions. Detailed analysis of the saturated moment and critical fields allows a determination of the net distribution of magnetic moments within each phase. The ...


Ultrasonic Beam Models In Anisotropic Media , Matthias Rudolph Jan 1999

Ultrasonic Beam Models In Anisotropic Media , Matthias Rudolph

Retrospective Theses and Dissertations

This dissertation models the sound fields produced by ultrasonic transducers for immersion inspections where the sound fields pass through general curved interfaces into a general anisotropic material;The foundations for these models are discussed, including slowness surfaces for anisotropic materials and the reflection/transmission of plane waves at a planar fluid/anisotropic solid interface;Three beam models are developed. Two of these models---the boundary diffraction wave model and the multi-Gaussian model---rely on the paraxial approximation, resulting in models that are computationally very efficient. As a consequence, those models can be used to conduct parametric studies in a highly effective manner ...


Transport Processes In Directional Solidification And Their Effects On Microstructure Development , Prantik Mazumder Jan 1999

Transport Processes In Directional Solidification And Their Effects On Microstructure Development , Prantik Mazumder

Retrospective Theses and Dissertations

The processing of materials with unique electronic, mechanical, optical and thermal properties plays a crucial role in modern technology. The quality of these materials depend strongly on the microstructures and the solute/dopant fields in the solid product, that are strongly influenced by the intricate coupling of heat and mass transfer and melt flow in the growth systems. An integrated research program is developed that include precisely characterized experiments and detailed physical and numerical modeling of the complex transport and dynamical processes. Direct numerical simulation of the solidification process is carried out that takes into account the unsteady thermo-solutal convection ...


The Interplay Of Long-Range Magnetic Order And Single-Ion Anisotropy In Rare Earth Nickel Germanides , Zahirul Islam Jan 1999

The Interplay Of Long-Range Magnetic Order And Single-Ion Anisotropy In Rare Earth Nickel Germanides , Zahirul Islam

Retrospective Theses and Dissertations

This dissertation is concerned with the interplay of long-range order and anisotropy in the tetragonal RNi2Ge2 (R = rare earth) family of compounds. Microscopic magnetic structures were studied using both neutron and x-ray resonant exchange scattering (XRES) techniques;The magnetic structures of Tb, Dy, Eu and Gd members have been determined using high-quality single-crystal samples. TbNi2Ge2 and DyNi2Ge2 order in an amplitude modulated structure below TN, with propagation vectors (0 0 ∼ 0.758) and (0 0 ∼ 0.75), respectively. Below Tt < TN, the structures are equal moment commensurate , described by three modulations, (0 0 ¾) and its third harmonic (0 0 ¼), (½ ½ 0) and (½ ½ ½), respectively. Whereas the Tb compound is uniaxial with moments along the c&d4; axis in both phases, its Dy neighbor has an ordered in-plane component. XRES is the probe of choice for the study of neutron-opaque Eu and Gd members. While EuNi2Ge2 has a simple antiferromagnetic state, (0 0 1), with moments in the basal plane, GdNi2Ge 2 orders with an incommensurate modulation, (0 0 ∼ 0.793) at TN. There is an ordered component along c&d4; in the Gd compound below TN but above Tt. Below Tt, Gd moment is locked to the basal plane;This work has correlated a strong Fermi surface nesting to the magnetic ordering in the RNi2Ge2 compounds. Generalized susceptibility, c0 (q), calculations found nesting to be responsible for both incommensurate ordering wave vector in GdNi2Ge 2, and the commensurate structure in EuNi2Ge 2. A continuous transition from incommensurate to commensurate magnetic structures via band filling is predicted. The surprisingly higher TN in EuNi2Ge2 than that in GdNi 2Ge2 is also explained;Next, all the metamagnetic phases in TbNi2Ge2 with an applied field along the c&d4; axis have been characterized with neutron diffraction measurements. A "mixed phase" model for the first metamagnetic structure consisting of fully-saturated as well as reduced-moment Tb ions is presented. The moment reduction may be due to moment instability which is possible if the exchange is comparable to the low-lying CEF level splitting and the ground state is a singlet. In such a case, certain Tb sites may experience a local field below the critical value needed to reach saturation.


Backscattering And Attenuation During The Propagation Of Ultrasonic Waves In Duplex Titanium Alloys , Paul Donald Panetta Jan 1999

Backscattering And Attenuation During The Propagation Of Ultrasonic Waves In Duplex Titanium Alloys , Paul Donald Panetta

Retrospective Theses and Dissertations

This dissertation reports results from studies of the interactions between the microstructure/macrostructure of titanium alloys and propagating ultrasonic waves. Attention is focused on the interactions which cause backscattering and attenuation. Experimental studies will be reported which show that the phase aberration contribution to attenuation does not remove energy from the propagating beam, and a physical picture is developed. A simple ray model which encompasses the ideas presented in the physical picture is used to predict attenuation for propagation in single phase materials with elongated grains. In addition, a general theory for attenuation that includes some degree of multiple scattering ...


Scanning Tunneling Microscopy Studies Of Submonolayer And Multilayer Ag Films Deposited On Ag(100) , Conrad Robert Stoldt Jan 1999

Scanning Tunneling Microscopy Studies Of Submonolayer And Multilayer Ag Films Deposited On Ag(100) , Conrad Robert Stoldt

Retrospective Theses and Dissertations

A comprehensive analysis of submonolayer and multilayer Ag films deposited on Ag(100) is performed. Extensive ultra-high vacuum Scanning Tunneling Microscopy (STM) experiments are presented revealing exotic equilibration pathways in the Ag adlayer, as well as novel growth modes in multilayer Ag films;Real-time STM imaging of adlayer dynamics at 295 K allows for the elucidation and quantification of various kinetic processes controlling the coarsening of two-dimensional (2D) submonolayer island distributions. Coarsening kinetics are analyzed using a mean-field Smoluchowski rate equation, which gives both a strongly size dependent island diffusion coefficient and an estimate of the size scaling exponent. Details ...


Investigation Of Single Abrikosov Vortex Pinning In Superconducting Josephson Junctions Using Artificially Induced Pinning Sites , Matthew Joseph Breitwisch Jan 1999

Investigation Of Single Abrikosov Vortex Pinning In Superconducting Josephson Junctions Using Artificially Induced Pinning Sites , Matthew Joseph Breitwisch

Retrospective Theses and Dissertations

Cross-strip superconductor-normal-insulator-superconductor Josephson junctions have been fabricated in order to study the pinning of a single Abrikosov vortex trapped within thin films of Nb. The vortex would be induced to move either by thermal depinning or by pushing on the vortex with a transport current in one of the films. Thermal depinning studies show that the temperature of the initial motion of the vortex is independent of applied fields up to +/-20 mG but the temperature where the vortex exits the junction seems to be about 50 mK lower in +/-20 mG than in zero field. Lorentz force depinning studies ...