Bulletin of the American Physical Society
2006 Texas Section of the APS Joint Fall Meeting
Thursday–Saturday, October 5–7, 2006; Arlington, Texas
Session CMP2: Condensed Matter II |
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Chair: Asok Ray, University of Texas at Arlington Room: UT Arlington, University Center Concho |
Friday, October 6, 2006 1:30PM - 1:42PM |
CMP2.00001: Positron Annihilation Induced Auger and Gamma Spectroscopy of Catalytically Important Surfaces A.H. Weiss, M.P. Nadesalingam, R. Sundaramoorthy, S. Mukherjee, N.G. Fazleev The annihilation of positrons with core electrons results in unique signatures in the spectra of Auger-electron and annihilation-gamma rays that can be used to make clear chemical identification of atoms at the surface. Because positrons implanted at low energies are trapped with high efficiency in the image-correlation well where they are localized just outside the surface it is possible to use annihilation induced Auger and Gamma signals to probe the surfaces of solids with single atomic layer depth resolution. In this talk we will report recent applications of Positron Annihilation Induced Auger Electron Spectroscopy (PAES) and Auger-Gamma Coincidence Spectroscopy (AGCS) to the study of surface structure and surface chemistry. Our research has demonstrated that PAES spectra can provide new information regarding the composition of the top-most atomic layer. Applications of PAES to the study of catalytically important surfaces of oxides and wide band-gap semiconductors including TiO$_{2}$, SiO$_{2,}$Cu$_{2}$O, and SiC will be presented. We conclude with a discussion of the use of Auger-Gamma and Gamma-Gamma coincidence spectroscopy for the study of surfaces at pressures closer to those found in practical chemical reactors. Research supported by the Welch Foundation Grant Number Y-1100. [Preview Abstract] |
Friday, October 6, 2006 1:42PM - 1:54PM |
CMP2.00002: Magnetization Measurements of $Mn_{12}$-acetate Thin Films T. Wellington, A. Yamaguchi, K. Suzuki, H. Ishimoto, J. Means, W. Teizer We are reporting magnetization measurements obtained for thin films of the molecular magnet $Mn_{12} $-acetate,$[Mn_{12} O_{12} (CH_3 COO)_{16} (H_2 O)_4 ]\cdot 2CH_3 COOH\cdot 4H_2 O$. This molecular magnet behaves like a single S = 10 system with very high anisotropy. It exhibits macroscopic quantum tunnelling of the magnetization. At temperatures below 3K, there is an observable magnetic bistability with hysteresis, as well as splitting of zero field and field cooled magnetization. The thin film samples were prepared using the Laser Ablation and the Dip and Dry Technique. Measurements were taken using a magnetometer at magnetic fields up to 6T and temperatures of up to 300K. [Preview Abstract] |
Friday, October 6, 2006 1:54PM - 2:06PM |
CMP2.00003: Study of the Effects of the SiC Doping on the Critical Current Density of the Ti-sheathed MgB$_{2}$ Superconducting Wires Gan Liang, Hui Fang, Shelley Keith, Cad Hoyt, Mina Hanna, F. Yen, B. Lv, M. Alessandrini, K. Salama The effects of the SiC doping on the magnetic critical current density ($J_{c})$ of the Ti-sheathed MgB$_{2}$ superconducting wires were studied for the first time. Two groups of Ti-sheathed MgB$_{2}$ wire samples were prepared and studied: for the first group, the size of the SiC particles was 20 nm and the concentrations were 5{\%}, 10{\%}, and 15{\%}; for the second group, the concentration of the SiC dopant was 10{\%} and the sizes of the SiC particles were 20 nm, 45 nm, and 100-200 nm. Contrary to the $J_{c}$ results reported on the SiC-doped Fe-sheathed MgB$_{2}$ wires, we found that the $J_{c}$ for the SiC-doped Ti-sheathed MgB$_{2}$ wires decreases with both the concentration and particle size of the SiC dopant. We found that only for the wires with 100-200 nm SiC size, the $J_{c}$ is greater than that of the undoped MgB$_{2}$ wires. This unusual dependence of $J_{c}$ on the size and concentration of the SiC dopant is discussed in association with the results from the magnetization, electrical resisitivity, x-ray diffraction and scanning electron microscopy measurements. [Preview Abstract] |
Friday, October 6, 2006 2:06PM - 2:18PM |
CMP2.00004: Comparisons of Different Particle-Chain Methods for Path Integral Monte Carlo Methods Terrence Reese, Bruce Miller In previous work we have used Path Integral Monte Carlo methods to simulate a Positronium atom in a Lennard-Jones fluid. Trial positions are created for sub-chains of particles on the polymer chain to allow for proper exploration of the configuration space. Different methods can be used to determine how the different chains are selected. In this report we compare the results from simulations of Positronium in Xenon at 300 and 340K using our leap frog method and another method where the selection of the sub-chains for trial movements is done randomly. The results indicate that a random selection of sub-chains leads to more accurate simulation results at higher densities. [Preview Abstract] |
Friday, October 6, 2006 2:18PM - 2:30PM |
CMP2.00005: Time Reversible Linear Scaling ab initio Molecular Dynamics C.J. Tymczak, Anders Niklasson, Matt Challacombe We present a time-reversible Born-Oppenheimer molecular dynamics scheme, based on self-consistent Hartree-Fock or density functional theory, where both the nuclear and the electronic degrees of freedom are propagated in time. We show how a time-reversible adiabatic propagation of the electronic degrees of freedom is possible despite the non-linearity and incompleteness of the self-consistent field procedure. With a time-reversible lossless propagation the simulated dynamics is stabilized with respect to a systematic long-term energy drift and the number of self-consistency cycles can be kept low (often only 2-4 cycles per nuclear time step) thanks to a good initial guess given by the adiabatic propagation of the electronic degrees of freedom. The proposed molecular dynamics scheme therefore combines a low computational cost with a physically correct time-reversible representation of the dynamics, which preserves a detailed balance between propagation forwards and backwards in time. [Preview Abstract] |
Friday, October 6, 2006 2:30PM - 2:42PM |
CMP2.00006: Influence of Hamiltonian on the properties of NaCl Ryan Jacob, A.R. Chourasia Sodium chloride has been investigated using the CRYSTAL98 code to determine its properties. The geometry and the basis set have been optimized prior to analysis. The analysis was carried out using the different Hamiltonians: Restricted Hartree-Fock, Unrestricted Hartree-Fock, Restricted open shell Hartree-Fock, and the DFT. In the DFT the B3PW and B3LYP potentials have been used. The physical properties have been calculated and compared with the experimental values. The present investigation is aimed at studying the usefulness and limitations of the code. [Preview Abstract] |
Friday, October 6, 2006 2:42PM - 2:54PM |
CMP2.00007: Relaxation of Actinide Surfaces: An All Electron Study Raymond Atta-Fynn, Pratik Dholabhai, Asok Ray Fully relativistic full potential density functional calculations with a linearized augmented plane wave plus local orbitals basis (LAPW + lo) have been performed to investigate the relaxations of heavy actinide surfaces, namely the (111) surface of fcc $\delta $-Pu and the (0001) surface of dhcp Am using WIEN2k. This code uses the LAPW + lo method with the unit cell divided into non-overlapping atom-centered spheres and an interstitial region. The APW+lo basis is used to describe all s, p, d, and f states and LAPW basis to describe all higher angular momentum states. Each surface was modeled by a three-layer periodic slab separated by 60 Bohr vacuum with four atoms per surface unit cell. In general, we have found a contraction of the interlayer separations for both Pu and Am. We will report, in detail, the electronic and geometric structures of the relaxed surfaces and comparisons with the respective non-relaxed surfaces. [Preview Abstract] |
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