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Full-Text Articles in Physics

Lattice-Dynamical Study Of The Premartensitic State Of The Cu-Al-Be Alloys, Lluís Mañosa, Jerel L. Zarestky, Thomas A. Lograsso, Dwight W. Delaney, C. Stassis Dec 1993

Lattice-Dynamical Study Of The Premartensitic State Of The Cu-Al-Be Alloys, Lluís Mañosa, Jerel L. Zarestky, Thomas A. Lograsso, Dwight W. Delaney, C. Stassis

Ames Laboratory Publications

Neutron-scattering techniques have been used to study the premartensitic state of a family of Cu-Al-Be alloys, which transform from the bcc phase to an 18R martensitic structure. We find that the phonon modes of the TA2[110] branch have very low energies with anomalous temperature dependence. A slight anomaly at q=2/3 was observed; this anomaly, however, does not change significantly with temperature. No elastic peaks, related to the martensite structure, were found in the premartensitic state of these alloys. The results are compared with measurements, performed under the same instrumental conditions, on two Cu-Al-Ni and a Cu-Zn-Al ...


Inclusion Of Charge Correlations In Calculations Of The Energetics And Electronic Structure For Random Substitutional Alloys, Duane D. Johnson, F. J. Pinski Oct 1993

Inclusion Of Charge Correlations In Calculations Of The Energetics And Electronic Structure For Random Substitutional Alloys, Duane D. Johnson, F. J. Pinski

Duane D. Johnson

Currently, the coherent-potential approximation (CPA) implemented via the multiple-scattering theory of Korringa, Kohn, and Rostoker (KKR) gives the best first-principles description of the electronic structure for random substitutional alloys. However, the total energy has an important component of electrostatic energy missing, namely, that arising from the correlation of charges with varying atomic environments. We develop a ‘‘charge-correlated’’ CPA method (cc-CPA) which includes (some) local environmental charge correlations within the KKR-CPA method. We investigate the cc-CPA energetics for several alloys and show that the formation energies are in better agreement with experimental results. These calculations show that the excess charge on ...


Photoemission Study Of Ce-Catalyzed Oxidation Of W(110) And W(111), C. Gu, C. G. Olson, David W. Lynch Oct 1993

Photoemission Study Of Ce-Catalyzed Oxidation Of W(110) And W(111), C. Gu, C. G. Olson, David W. Lynch

Physics and Astronomy Publications

Different catalytic oxidation effects of Ce on W(110) and W(111) were observed with photo- emission spectroscopy. On Ce-covered W(110), the top W layer is quickly oxidized to a surface monoxide at room temperature. Oxidation is almost halted after the completion of one monolayer of WO. On Ce/W(111), instead of WO, WO3 grows on the surface under the same conditions. The monolayer suboxide formation found on Ce/W(110) and Ce/Ta(110) (as reported earlier) is a direct consequence of the most densely packed structure of the bcc (110) surfaces. The more effective oxidation promotion ...


Correlation Functions For Ionic Motion From Nmr Relaxation And Electrical Conductivity In The Glassy Fast-Ion Conductor (Li2s)0.56(Sis2)0.44, Ivar Svare, Ferdinando Borsa, D. R. Torgeson, Steve W. Martin Oct 1993

Correlation Functions For Ionic Motion From Nmr Relaxation And Electrical Conductivity In The Glassy Fast-Ion Conductor (Li2s)0.56(Sis2)0.44, Ivar Svare, Ferdinando Borsa, D. R. Torgeson, Steve W. Martin

Materials Science and Engineering Publications

The Li7 NMR spin-lattice relaxation and the electrical conductivity in the typical glassy fast-ion conductor (Li2S)0.56(SiS2)0.44 are discussed from models of Li+ionic motion with distributions of activation energies, as well as from stretched-exponential time-correlation functions. The measured correlation times from the two effects differ by two orders of magnitude, and the derived distributions are shifted greatly relative to each other. We relate the great differences to percolation around the high barriers in the distribution. We present a phenomenological theory that yields good quantitative fits to the observed NMR relaxation with a Gaussian distribution, and ...


Resistivity Of The High-Temperature Metallic Phase Of Vo2, Werner W. Schulz, Philip B. Allen, Renate M. Wentzcovitch, Paul C. Canfield Aug 1993

Resistivity Of The High-Temperature Metallic Phase Of Vo2, Werner W. Schulz, Philip B. Allen, Renate M. Wentzcovitch, Paul C. Canfield

Paul C. Canfield

Measurements are reported on the electrical resistivity ρ(T) along the c axis of a single crystal of VO2, from the metal-insulator transition at T=333 K up to 840 K. The temperature dependence is very linear, and a fit to Bloch-Grüneisen theory gives a residual resistivity ρ0=65 μΩ cm and a ratio ρ(840 K)/ρ0=8. With the help of a local-density-approximation band-structure calculation, we further pursue the conventional (Bloch-Boltzmann) interpretation by extracting the electron-phonon coupling constant λ=1.1 and the mean free path l(800 K)=3.3 Å. The short mean free path implies ...


Dynamical X-Ray Diffraction From An Icosahedral Quasicrystal, S. W. Kycia, Alan I. Goldman, Thomas A. Lograsso, Dwight W. Delaney, D. Black, M. Sutton, E. Dufresne, R. Brüning, B. Rodricks Aug 1993

Dynamical X-Ray Diffraction From An Icosahedral Quasicrystal, S. W. Kycia, Alan I. Goldman, Thomas A. Lograsso, Dwight W. Delaney, D. Black, M. Sutton, E. Dufresne, R. Brüning, B. Rodricks

Ames Laboratory Publications

We present direct evidence of dynamical diffraction of x rays from a quasicrystal. High-resolution x-ray-diffraction measurements of the Al-Pd-Mn face-centered icosahedral quasicrystal were performed, revealing a mosaic full width at half maximum of less than 0.001°. In a second experiment, the anomalous transmission of x rays (the Borrmann effect) was observed. These measurements show that nearly perfect quasicrystals may be grown to centimeter-size dimensions allowing x-ray techniques based upon dynamical diffraction to be brought to bear on the analysis of icosahedral structures.


Tables Of Coincidence Orientations For Ordered Tetragonal L Lo Alloys For A Range Of Axial Ratios, Abha Singh, Alexander H. King Jan 1993

Tables Of Coincidence Orientations For Ordered Tetragonal L Lo Alloys For A Range Of Axial Ratios, Abha Singh, Alexander H. King

Alexander H. King

In this paper we develop and apply techniques for computation of CSL, DSCL and step-vector data for grain boundaries in tetragonal materials for a range of axial ratios. This has application to L10 alloys including TiA1, which is a candidate for lightweight high-temperature structural applications. Our results are compared with others and found to be more accurate and complete. We provide data for a wider range of axial ratios than those considered by previous workers. We have also derived equivalent quaternions for tetragonal crystals in tetragonal-crystal coordinates and listed conditions for selecting a unique reduced rotation in tetragonal-crystal coordinates so ...


Nqr Study Of Local Structure And Cooling Rate‐Dependent Superconductivity In La2cuo4+Δ, A. P. Reyes, E. T. Ahrens, P. C. Hammel, J. D. Thompson, Paul C. Canfield, Z. Fisk, R. H. Heffner, J. E. Schirber Jan 1993

Nqr Study Of Local Structure And Cooling Rate‐Dependent Superconductivity In La2cuo4+Δ, A. P. Reyes, E. T. Ahrens, P. C. Hammel, J. D. Thompson, Paul C. Canfield, Z. Fisk, R. H. Heffner, J. E. Schirber

Paul C. Canfield

Preliminary results of structural studies in oxygen‐annealed La2CuO4+δ(δ∼0.03) using 139La nuclear quadrupole resonance (NQR) spectroscopy are reported. Superconducting critical temperatures were found to depend on the rate of cooling through a narrow temperature range near 195 K. Analysis of the 139La NQR spectra reveal that lanthanum atoms in the oxygen‐rich metallic region occupy two sites having different local environments. One site has a structural configuration closely related to the stoichiometric oxygen‐poor compound. The second appears only below 200 K and exhibits a large shift and broad distribution of NQR frequency ν Q . Shifts ...


Island-Size Scaling In Surface Deposition Processes, M. C. Bartelt, Michael C. Tringides, James W. Evans Jan 1993

Island-Size Scaling In Surface Deposition Processes, M. C. Bartelt, Michael C. Tringides, James W. Evans

Physics and Astronomy Publications

Diffusion-mediated nucleation and growth of islands during deposition occurs essentially irreversibly in a variety of systems. We provide a scaling theory for the full island-size distribution, both with the ratio of surface diffusion to deposition rates and with time. Scaling functions and exponents are determined by simulation and explained analytically by an unconventional rate-equation analysis. Experimental tests for theoretical predictions are discussed, including the scaling of superlattice beam profiles for diffraction studies of heteroepitaxial systems.


A Geometrical Rationalization Of The Special Properties Of The 14° [001] Grain Boundary In Yba2cu3o7−Δ, Alexander H. King, Abha Singh Jan 1993

A Geometrical Rationalization Of The Special Properties Of The 14° [001] Grain Boundary In Yba2cu3o7−Δ, Alexander H. King, Abha Singh

Alexander H. King

14° [001] tilt grain boundaries in YBa2Cu3O7−δ have been found to exhibit anomalously good superconducting critical currents, associated with magnetic flux pinning, although no special interfacial structure has heretofore been associated with this misorientation. We present an analysis of the geometry of the grain boundary in question and show that it can correspond to a special interface, within the definition of constrained lattice coincidence, which may lead to new understanding of the superconducting behavior of grain boundaries. Our analysis shows that the tetragonal‐to‐orthorhombic phase transformation may have important effects upon grain boundary structure and properties, indicating new ...


Synthesis, Characterization, Phase Diagrams And Superconducting And Normal State Magnetic Properties Of La[Subscript 2-X]Sr[Subscript X]Cuo4 (0 [Less Than Or Equal To] X [Less Than Or Equal To] 0.08) And Electrochemically Oxidized La[Subscript 2-X]Sr[Subscript X]Cuo[Subscript 4 + Delta] (0 [Less Than Or Equal To] X [Less Than Or Equal To] 0.33, 0 [Less Than Or Equal To Delta Less Than Or Equal To] 0.12) , Fangcheng Chou Jan 1993

Synthesis, Characterization, Phase Diagrams And Superconducting And Normal State Magnetic Properties Of La[Subscript 2-X]Sr[Subscript X]Cuo4 (0 [Less Than Or Equal To] X [Less Than Or Equal To] 0.08) And Electrochemically Oxidized La[Subscript 2-X]Sr[Subscript X]Cuo[Subscript 4 + Delta] (0 [Less Than Or Equal To] X [Less Than Or Equal To] 0.33, 0 [Less Than Or Equal To Delta Less Than Or Equal To] 0.12) , Fangcheng Chou

Retrospective Theses and Dissertations

La[subscript] 2-xSr[subscript] xCuO[subscript]4 (0 ≤ x ≤ 0.15) can all be intercalated with oxygen by an novel electrochemical oxidation method. Bulk superconductivity is found with an onset T[subscript] c ≈ 40 K for the whole range 0.01 ≤ x ≤ 0.15; for x = 0.25 and 0.33, the electrochemical oxidation did not improve the superconducting properties. The magnetic susceptibility [chi](T = 50-320 K) data for La[subscript]2CuO[subscript]4.11 and La[subscript]1.92Sr[subscript]0.08CuO[subscript]4.07 are nearly identical with those of conventionally prepared La[subscript]1.85Sr[subscript]0 ...


Theoretical Study Of The Noble Metals On Semiconductor Surfaces And Ti-Base Shape Memory Alloys , Yungui Ding Jan 1993

Theoretical Study Of The Noble Metals On Semiconductor Surfaces And Ti-Base Shape Memory Alloys , Yungui Ding

Retrospective Theses and Dissertations

The electronic and structural properties of the (√3 x √3) R30° Ag/Si(111) and (√3 x √3) R30° Au/Si(111) surfaces are investigated using first principles total energy calculations. We have tested almost all experimentally proposed structural models for both surfaces and found the energetically most favorable model for each of them. The lowest energy model structure of the (√3 x √3) R30° Ag/Si(111) surface consists of a top layer of Ag atoms arranged as "honeycomb-chained-trimers" lying above a distorted "missing top layer" Si(111) substrate. The coverage of Ag is 1 monolayer (ML). We find ...


Impedance Spectroscopy And Nmr Studies Of Fast Ion Conducting Chalcogenide Glasses , Hitendra Kumar Patel Jan 1993

Impedance Spectroscopy And Nmr Studies Of Fast Ion Conducting Chalcogenide Glasses , Hitendra Kumar Patel

Retrospective Theses and Dissertations

Two fully automated, temperature controlled, impedance spectroscopy workstations have been developed for this project. Impedance spectroscopy studies of glasses have been performed over the wide frequency (1 Hz-1 GHz) and wide temperature (100-500 K) range;This study yields the first-ever full compositional (three decades) range examination of the Na[subscript]2S + B[subscript]2S[subscript]3 and K[subscript]2S + B[subscript]2S[subscript]3 systems. The activation energy is modeled using a modified Anderson-Stuart model that predicts well the results for the low alkali glasses. a.c. conductivity is examined through the electrical modulus formalism and the Kohlrausch-Williams-Watts decay ...


Investigation Of The Phase Equilibria And Phase Transformations Associated With The Bi2sr2cacu2oy Superconductor , Terry George Holesinger Jan 1993

Investigation Of The Phase Equilibria And Phase Transformations Associated With The Bi2sr2cacu2oy Superconductor , Terry George Holesinger

Retrospective Theses and Dissertations

Compositional measurements of the Bi[subscript]2Sr[subscript]2CaCu[subscript]2O[subscript]y (2212) solid solution region were performed in the TEM in order to separate intergrowth and solid solubility effects. Most of the solid solubility is accounted for by changes in the Sr and Ca concentrations. Changes in the Bi concentration account for the rest. Based on these results, two substitution schemes were determined to occur. The first is just the interchange between Sr and Ca. The second involves the substitution of Bi for Sr or Ca. It appears to be unfavorable for Sr or Ca to substitute for ...


Quantum Dynamical Phenomena Of Independent Electrons In Semiconductor Superlattices Subject To A Uniform Electric Field , Ann Marie Bouchard Jan 1993

Quantum Dynamical Phenomena Of Independent Electrons In Semiconductor Superlattices Subject To A Uniform Electric Field , Ann Marie Bouchard

Retrospective Theses and Dissertations

We solve the time-dependent Schrodinger equation for independent electrons in both periodic and aperiodic semiconductor superlattices with a superimposed static uniform electric field, F, using both analytical and numerical methods. In the case of periodic superlattices, in suitable circumstances the electron exhibits Bloch oscillations in the form of long-lived and virtually time-periodic sinusoidal center-of-mass oscillations. Depending on the miniband structure of the superlattice, the value of F, and the detailed form of the initial wave function, other dynamical phenomena can occur which can coexist with or even totally mask the Bloch oscillations. These include a nearly time-periodic coherent breathing mode ...


Theoretical Studies Of Zirconium And Carbon Clusters With Molecular Dynamics Simulations , Bulin Zhang Jan 1993

Theoretical Studies Of Zirconium And Carbon Clusters With Molecular Dynamics Simulations , Bulin Zhang

Retrospective Theses and Dissertations

With the aim of understanding the anomalous phonon behavior near the martensitic phase transition in Zr, we have simulated the dynamics of atomic motion in the high temperature bcc phase of Zr using an embedded-atom potential. The calculated dynamical structure factors reproduce the strong asymmetry in the scattering cross-sections in different Brillouin zones observed in inelastic neutron scattering experiments. From the real-space atomic picture, we observed the phase fluctuations between bcc and low temperature phase hcp. It is found that the anomalous phonon behavior arises from the incompleteness of the phase fluctuations;Combining an efficient simulated annealing scheme for generating ...


Circular Magnetic X-Ray Dichroism In Rare Earth Compounds , Jonathan Clark Lang Jan 1993

Circular Magnetic X-Ray Dichroism In Rare Earth Compounds , Jonathan Clark Lang

Retrospective Theses and Dissertations

Circular magnetic x-ray dichroism (CMXD) has been used to probe the magnetic properties of rare earth compounds. This study has been undertaken in two distinct parts. First, the angular dependence of the CMXD signal has been examined in order to determine the multiple nature of the transitions involved. Second, the nature of the dichroic spectra of amorphous and crystalline GdFe[subscript]2 has been probed. CMXD offers a unique probe of the magnetic properties of amorphous materials providing information that is not available from any other technique;Previous CMXD measurements at the L edges of rare earth materials have demonstrated ...


Rheed Studies Of The Nucleation, Growth, And Mobility Of Ag Atoms On The Si(111)7x7 Surface , Kelly Ryan Roos Jan 1993

Rheed Studies Of The Nucleation, Growth, And Mobility Of Ag Atoms On The Si(111)7x7 Surface , Kelly Ryan Roos

Retrospective Theses and Dissertations

The low temperature and flux dependent growth of ultrathin Ag films on the Si(111)7 x 7 surface is studied with Reflection High-Energy Electron Diffraction (RHEED). The grazing incidence geometry of RHEED allows for an incident molecular beam normal to the surface, and makes it an ideal surface probe for studying ultrathin film growth in real time. Short-lived oscillations in the diffracted intensity are observed during Ag deposition at 150 K, indicating quasi-layer-by-layer growth mediated by adatom mobility. When the 150 K growth is performed over a wide range of deposition rates F, the peak intensity is observed to ...


Growth And Characterization Of Polysilicon Films Deposited By Reactive Plasma Beam Epitaxy , Behnam Moradi Jan 1993

Growth And Characterization Of Polysilicon Films Deposited By Reactive Plasma Beam Epitaxy , Behnam Moradi

Retrospective Theses and Dissertations

Polycrystalline silicon was deposited at low temperatures (400-550°C) by using a new technique, reactive plasma beam epitaxy. The technique consists of using an intense flux of hydrogen radicals, generated by an electron cyclotron resonance (ECR) source to promote nucleation and crystallinity at low temperatures. The film structure could be smoothly changed from polysilicon to amorphous by changing the flux of incoming H radicals on the surface of the sample. The flux of hydrogen radicals can be controlled by changing the deposition pressure, with lower pressures leading to a higher H radical flux and greater degree of crystallinity. The effect ...


Density Functional Studies: First Principles And Semi-Empirical Calculations Of Clusters And Surfaces , Susan Buthaina Sinnott Jan 1993

Density Functional Studies: First Principles And Semi-Empirical Calculations Of Clusters And Surfaces , Susan Buthaina Sinnott

Retrospective Theses and Dissertations

In the research presented here, various theoretical electronic structure techniques are utilized to analyze widely different systems from silicon clusters to transition metal solids and surfaces. For the silicon clusters, first principles density functional methods are used to investigate Si[subscript] N for N = 2-8. The goal is to understand the different types of bonding that can occur in such small clusters where the coordination of the atoms differs substantially from that of the stable bulk tetrahedral bonding. Such uncoordinated structures can provide a good test of more approximate theories that can be used eventually to model silicon surfaces, of ...