Bulletin of the American Physical Society
Fall 2009 Meeting of the Four Corners Section of the APS
Volume 54, Number 14
Friday–Saturday, October 23–24, 2009; Golden, Colorado
Session H7: Materials Physics II |
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Chair: Mark Riffle, Utah State University Room: Hill Hall 202 |
Saturday, October 24, 2009 11:20AM - 11:32AM |
H7.00001: Inhomogeneous Broadening in Perturbed Angular Correlation Spectroscopy Austin Bunker, Mike Adams, Jeffery Hodges, Tyler Park, Michael Stufflebeam, William Evenson, Phil Matheson, Matthew Zacate Our research concerns the effect of a static distribution of defects on the net electric field gradient (EFG) within crystal structures. Defects and vacancies perturb the distribution of gamma rays emitted from radioactive probe nuclei within the crystal. These defects and vacancies produce a net EFG at the site of the probe which causes the magnetic quadrupole moment of the nucleus of the probe to precess about the EFG. The net EFG, which is strongly dependent upon the defect concentration, perturbs the angular correlation (PAC) of the gamma rays, and is seen in the damping of the perturbation function, $G_2(t)$, in time and broadening of the spectral peaks in the Fourier transform. We have used computer simulations to study the probability distribution of EFG tensor components in order to uncover the concentration dependence of $G_2(t)$. This in turn can be used to analyze experimental PAC data and quantitatively describe properties of the crystal. [Preview Abstract] |
Saturday, October 24, 2009 11:32AM - 11:44AM |
H7.00002: Finding probability distributions for electric field gradient components with inhomogeneous broadening in perturbed angular correlation spectroscopy Tyler Park, Mike Adams, Austin Bunker, Jeffery Hodges, Michael Stufflebeam, William Evenson, Phil Matheson, Matthew Zacate Materials contain defects, which affect crystal properties such as damping of the correlation signal,$G_2(t)$, in time and broadening of the frequency spectrum in perturbed angular correlation (PAC) experiments. We attribute this inhomogeneous broadening (IHB) to the random static defects that produce a distribution of electric field gradients (EFGs). Our goal is to find a relationship between the amount of broadening and the concentration of defects. After simulating the EFGs from random configurations of defects, we map our results from the $V_{zz}$-$V_{xx}$ plane to a coordinate system optimized for the EFG distribution through a Czjzek transformation, followed by a conformal mapping. From histograms in this space, we can define probability distribution functions with parameters that vary according to defect concentration. This allows us to calculate the broadened $G_2(t)$ spectrum for any concentration, and, in reverse, identify concentrations given a broadened $G_2(t)$ spectrum. [Preview Abstract] |
Saturday, October 24, 2009 11:44AM - 11:56AM |
H7.00003: Defect Concentration Dependence of Inhomogeneous Broadening in PAC Spectroscopy Mike Adams, Austin Bunker, Jeffery Hodges, Tyler Park, Michael Stufflebeam, William Evenson, Phil Matheson, Matthew Zacate Defects in crystals affect the electric field gradient (EFG) tensor components at radioactive probe nuclei. We consider the net EFG from a random distribution of vacancies combined with a single trapped vacancy in a near neighbor position. The net EFG perturbs angular correlation (PAC) and provides information about the concentration of vacancies. For various concentrations (.1 to 15 percent) we have simulated PAC spectra in simple cubic, body centered and face centered cubic crystal structures. Using the probability distributions we found for the EFG tensor components we reconstruct $G_2(t)$ for various defect concentrations. We take these reconstructions and compare them with the simulated $G_2(t)$ functions to check for self-consistency. We can then use the simulated probability distributions to examine the concentration dependence of experimental broadened PAC spectra. This work will be applied initially to broadened PAC data from $\beta$-Mn, Al-doped $\beta$-Mn, and Sr$_2$RuO$_4$. [Preview Abstract] |
Saturday, October 24, 2009 11:56AM - 12:08PM |
H7.00004: Role of Grain Boundaries in the Conductivity of Vanadium Dioxide Thin Films Felipe Rivera, Richard Vanfleet, Robert Davis Vanadium dioxide (VO$_{2}$) single crystals undergo a structural first-order metal to insulator phase transition at approximately 68$^{\circ}$C. This phase transition exhibits a resistivity change of up to 5 orders of magnitude in bulk specimens. We observe a 2-3 order of magnitude change in thin films of VO$_{2}$ presumably due to the large number of grain boundaries in the film. The interface between grains was studied by TEM and appears amorphous. Electron energy loss spectroscopy shows VO$_{2}$ like spectra with no additional surface oxidation. [Preview Abstract] |
Saturday, October 24, 2009 12:08PM - 12:20PM |
H7.00005: Molecular Dynamic Simulation of Laser Melting of Nanocrystalline Gold Zhibin Lin, Elodie Leveugle, Eduardo Bringa, Leonid Zhigilei We present the mechanisms and kinetics of short pulse laser melting of single crystal and nanocrystalline Au films based on the results of atomic-scale simulations. The simulations are performed for a broad range of laser fluences with a computational approach that combines the molecular dynamics method with a continuum-level description of laser excitation and subsequent relaxation of conduction band electrons. At high fluences, grain boundary melting in nanocrystalline films results in a decrease of the size of crystalline grains at the initial stage of the laser heating and is followed by a rapid (within 1-3 ps) collapse of remaining crystalline parts of the film as soon as the lattice temperature exceeds the limit of the crystal stability against the onset of homogeneous melting. At low laser fluences, the initiation of melting at grain boundaries can steer the melting process along the path where the melting continues below the equilibrium melting temperature and the crystalline regions shrink and disappear under conditions of substantial undercooling. This unusual melting behavior is explained based on thermodynamic analysis of the stability of small crystalline clusters surrounded by undercooled liquid. [Preview Abstract] |
Saturday, October 24, 2009 12:20PM - 12:32PM |
H7.00006: Vibrational structure of the alkali metal surfaces R. Wilson, D.M. Riffe Many physical properties of solids are phonon dependent. While numerous theoretical and experimental investigations have successfully characterized bulk vibrational structure, further characterization of surface phonons is needed. The behavior of surface phonons is important to the understanding of nano-structures, the interpretation of experimental measurements used to study solids, and the understanding of a variety of physical phenomena that solids exhibit. We present the calculation of vibrational modes on the (110) and (100) of alkali metal surfaces with the use of an Embedded Atom Method (EAM) inter-atomic potential. Vibrational dependent properties in the bulk that are calculated by the EAM potential are in good agreement with experiment, providing credibility to the EAM potential's accuracy. Surface properties such as entropy, specific heat, vibrational density of states, and Debye temperatures are calculated from the vibrational modes. The surface phonons are found to be highly polarized, resulting in anisotropic thermal behavior at and near the surface. A database of bulk and surface Debye temperatures is being created as a resource. [Preview Abstract] |
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