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Full-Text Articles in Physics

Accuracy And Limitations Of Localized Green’S Function Methods For Materials Science Applications, Duane D. Johnson, Andrei V. Smirnov Dec 2001

Accuracy And Limitations Of Localized Green’S Function Methods For Materials Science Applications, Duane D. Johnson, Andrei V. Smirnov

Duane D. Johnson

We compare screened real-space and reciprocal-space implementations of Korringa-Kohn-Rostoker electronic-structure method for their applicability to largescale problems requiring various levels of accuracy. We show that real-space calculations in metals can become impractical to describe energies. We suggest a combined r- and k-space scheme as the most efficient and flexible strategy for accurate energy calculations. Our hybrid code is suitable for (parallel) large-scale calculations involving complex, multicomponent systems. We also discuss how details of numerical procedures can affect accuracy of such calculations.


Absence Of Fine Structure In The Photoemission Spectrum Of The Icosahedral Al-Pd-Mn Quasicrystal, Z. M. Stadnik, D. Purdie, Y. Baer, Thomas A. Lograsso Nov 2001

Absence Of Fine Structure In The Photoemission Spectrum Of The Icosahedral Al-Pd-Mn Quasicrystal, Z. M. Stadnik, D. Purdie, Y. Baer, Thomas A. Lograsso

Ames Laboratory Publications

The results of the low-temperature ultrahigh-energy-resolution photoemission studies of a single-grain icosahedral alloy Al70Pd21.5Mn8.5 have been presented. The existence of the theoretically predicted pseudogap in the density of states at the Fermi energy has been confirmed. No evidence of the theoretically predicted spikiness of the density of states could be observed. It has been suggested that the failure to detect the predicted spikiness of the density of states in icosahedral quasicrystals is consistent with the results of Zijlstra and Janssen [Europhys. Lett. 52, 578 (2000)] who showed that the spikiness occurs only in ...


Classical Density Functional Theory Of Freezing In Simple Fluids: Numerically Induced False Solutions, M. Valera, F. J. Pinski, Duane D. Johnson Nov 2001

Classical Density Functional Theory Of Freezing In Simple Fluids: Numerically Induced False Solutions, M. Valera, F. J. Pinski, Duane D. Johnson

Duane D. Johnson

Density functional theory (DFT) has provided many insights into the freezing of simple fluids. Several analytical and numerical solution have shown that the DFT provides an accurate description of freezing of hard spheres and their mixtures. Compared to other techniques, numerical, grid-based algorithms for solving the DFT equations have more variational freedom and are capable of describing subtle behavior, as that seen in mixtures with multipeaked density profiles. However the grid-based approach is sensitive to the coarseness of the mesh employed. Here we summarize how the granularity of the mesh affects the freezing point within the DFT. For coarse meshes ...


Phonon Softening In Ni-Mn-Ga Alloys, Lluís Mañosa, Antoni Planes, Jerel L. Zarestky, Thomas A. Lograsso, Deborah L. Schlagel, C. Stassis Jun 2001

Phonon Softening In Ni-Mn-Ga Alloys, Lluís Mañosa, Antoni Planes, Jerel L. Zarestky, Thomas A. Lograsso, Deborah L. Schlagel, C. Stassis

Ames Laboratory Publications

The TA2 phonon dispersion curves of Ni-Mn-Ga alloys with different compositions which transform to different martensitic structures have been measured over a broad temperature range covering both paramagnetic and ferromagnetic phases. The branches show an anomaly (dip) at a wave number that depends on the particular martensitic structure, and there is softening of these anomalous phonons with decreasing temperature. This softening is enhanced below the Curie point, as a consequence of spin-phonon coupling. This effect is stronger for systems with higher electronic concentration.


Structural Characterization Of Thin Film Photonic Crystals, G. Subramania, Rana Biswas, Kristen P. Constant, M. M. Sigalas, Kai-Ming Ho May 2001

Structural Characterization Of Thin Film Photonic Crystals, G. Subramania, Rana Biswas, Kristen P. Constant, M. M. Sigalas, Kai-Ming Ho

Materials Science and Engineering Publications

We quantitatively analyze the structure of thin film inverse-opal photonic crystals composed of ordered arrays of air pores in a background of titania. Ordering of the sphere template and introduction of the titania background were performed simultaneously in the thin film photonic crystals. Nondestructive optical measurements of backfilling with high refractive index liquids, angle-resolved reflectivity, and optical spectroscopy were combined with band-structure calculations. The analysis reveals a thin film photonic crystal structure with a very high filling fraction (92–94 %) of air and a substantial compression along the c axis (∼22–25%).


Physical Properties Of Heusler-Like Fe2val, Ye Feng, J. Y. Rhee, T. A. Wiener, David W. Lynch, B. E. Hubbard, A. J. Sievers, Deborah L. Schlagel, Thomas A. Lograsso, L. L. Miller Apr 2001

Physical Properties Of Heusler-Like Fe2val, Ye Feng, J. Y. Rhee, T. A. Wiener, David W. Lynch, B. E. Hubbard, A. J. Sievers, Deborah L. Schlagel, Thomas A. Lograsso, L. L. Miller

Ames Laboratory Publications

A comprehensive characterization of the compound Fe2VAl was carried out. Samples grown by arc melting or the Bridgman method have Al and Fe deficiencies of up to 5 at. %. Czochralski-grown samples were Fe rich and Al deficient. X-ray diffraction implies appreciable antisite disorder in all of our samples. Fourier-transform infrared (FTIR) spectroscopy measurements showed that the carrier density and scattering time had little sample-to-sample variation or temperature dependence for near-stoichiometric samples. FTIR and dc resistivity suggest that the transport properties of Fe2VAl are influenced by both localized and delocalized carriers, with the former primarily responsible for ...


Composite Magnetostrictive Materials For Advanced Automotive Magnetomechanical Sensors, R. William Mccallum, Kevin W. Dennis, David C. Jiles, John E. Snyder, Y. H. Chen Apr 2001

Composite Magnetostrictive Materials For Advanced Automotive Magnetomechanical Sensors, R. William Mccallum, Kevin W. Dennis, David C. Jiles, John E. Snyder, Y. H. Chen

Ames Laboratory Publications

In this paper we present the development of a composite magnetostrictive material for automotive applications. The material is based on cobaltferrite,CoO⋅Fe2O3, and contains a small fraction of metallic matrix phase that serves both as a liquid-phasesintering aid during processing and enhances the mechanical properties over those of a simple sinteredferrite ceramic. In addition the metal matrix makes it possible to braze the material, making the assembly of a sensor relatively simple. The material exhibits good sensitivity and should have high corrosion resistance, while at the same time it is low in cost.


Magnetic Force Microscopy Study Of Magnetization Reversal In Sputtered Fesial(N) Films, Chester C.H. Lo, J. E. Snyder, J. S. Leib, R. Chen, B. Kriegermeier-Sutton, Matthew J. Kramer, David C. Jiles, M. T. Kief Mar 2001

Magnetic Force Microscopy Study Of Magnetization Reversal In Sputtered Fesial(N) Films, Chester C.H. Lo, J. E. Snyder, J. S. Leib, R. Chen, B. Kriegermeier-Sutton, Matthew J. Kramer, David C. Jiles, M. T. Kief

Materials Science and Engineering Publications

The magnetization reversal in a series of rf-sputtered FeSiAl(N) films has been studied using magnetic force microscopy. A system has been developed which has the capability to image domain structure while an in-plane magnetic field is applied in situ. All films exhibited a stripe domain structure in zero applied field which was indicative of a perpendicular component of domain magnetization which alternates in sign. All films showed a similar sequence of magnetization processes: on reducing the applied field from saturation a fine stripe domain structure nucleated and then coarsened as the field was decreased to zero. Local switching of ...


Unbiased Density Functional Solutions Of Freezing In Binary Mixtures Of Hard Or Soft Spheres, M. Valera, R. F. Bielby, F. J. Pinksi, Duane D. Johnson Jan 2001

Unbiased Density Functional Solutions Of Freezing In Binary Mixtures Of Hard Or Soft Spheres, M. Valera, R. F. Bielby, F. J. Pinksi, Duane D. Johnson

Duane D. Johnson

various size ratios, σ2/σ1, using density functional theory. The Grand Potential is minimized using an unbiased, discrete, real-space mesh that does not constrain the shape of the density, and, in many cases, leads to solutions qualitatively different from those using Gaussians and plane-waves. Besides the usual face-centered-cubic solid-solution phase for σ2/σ1≈1.0, we find a sublattice-melt phase for σ2/σ1=0.85–0.5 (where the small-sphere density is nonlocalized and multi-peaked) and the NaCl phase for σ2/σ1=0.45–0.35 (when the small-sphere density again sharpens). For a range of size ratios of soft ...


Theoretical Studies Of The Nucleation And Growth Of Thin Metal Films: A Focus On Ag Deposited On Ag(100) , Kyle John Caspersen Jan 2001

Theoretical Studies Of The Nucleation And Growth Of Thin Metal Films: A Focus On Ag Deposited On Ag(100) , Kyle John Caspersen

Retrospective Theses and Dissertations

Theoretical studies of the nucleation and growth of metal films are performed, where the focus is the Molecular Beam Epitaxial (MBE) growth of Ag on the Ag(100) surface. Ag films grown under MBE, for the temperatures and atomic fluxes considered here (0→300K), are very far from equilibrium structures, due to the breaking of detailed balance during deposition. Included are studies of: metal film growth at very low temperatures; the temperature dependence of "mound" formation; the temperature dependence of kinetic roughening; the effect of the step-edge barrier on very thin films, and the post-deposition time dependence of nucleation. For ...


Photoemission Study Of Mnbi And Gdni2ge2 , Derek Paul Brammeier Jan 2001

Photoemission Study Of Mnbi And Gdni2ge2 , Derek Paul Brammeier

Retrospective Theses and Dissertations

Photoelectron spectroscopy was performed on single crystals of MnBi and GdNi2Ge2 utilizing synchrotron radiation as the light source. MnBi is known for its large magneto-optical Kerr rotation and GdNi 2Ge2 is studied for its intriguing magnetic properties at low temperature, including a spin density wave (SDW) that is reported to occur at 27.1 K. Angle resolved photoemission was used to investigate the electronic band structures of both materials. Resonant photoemission was used to characterize valence band features. The shallow core levels were also investigated using angle integrated photoemission. Results from MnBi measurements are compared with recent theoretical bandstructure and ...


Electromechanical Characterization Of Ultrasonic Nde Systems , Changjiu Dang Jan 2001

Electromechanical Characterization Of Ultrasonic Nde Systems , Changjiu Dang

Retrospective Theses and Dissertations

An ultrasonic nondestructive evaluation (NDE) measurement system is a complex collection of many elements such as the pulser/receiver, the cabling, the transducers, and the material configuration being tested. To completely model an ultrasonic measurement system, a system model, called the electroacoustic measurement (EAM) model , was developed. This model allows one to analyze the measurement system at many different levels, ranging from individual details to the entire system itself. The EAM model has been implemented in software using the MATLAB development environment such that one has control over the specification of the detailed system components. On the other hand, the ...


Theoretical Studies Of Isolated Silicon And Germanium Clusters, And Silicon Clusters On Si(111)-7x7 Surface , Bei Liu Jan 2001

Theoretical Studies Of Isolated Silicon And Germanium Clusters, And Silicon Clusters On Si(111)-7x7 Surface , Bei Liu

Retrospective Theses and Dissertations

We have performed a systematic search for the ground state geometry of Si+n and Si-n in the size range 3 ≤ n ≤ 20 within the framework of density functional theory (DFT) with the local density approximation (LDA) and generalized gradient approximation (GGA). Various properties such as ionization potentials for neutral clusters, vertical detachment energies and photoelectron spectra for anions, fragmentation pathways and dissociation energies for cations, and mobilities for both anions and cations, are calculated and compared with experiments. The structures for medium-sized clusters (10 ≤ n ≤ 20) generally follow the prolate "stacked Si9 tricapped trigonal prism (TTP)" pattern. Both bulk ...