Bulletin of the American Physical Society
2007 APS Four Corners Section/SPS Zone 16 Joint Fall Meeting
Volume 52, Number 14
Friday–Saturday, October 19–20, 2007; Flagstaff, Arizona
Session J1: Condensed Matter: Structure and Theory |
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Chair: Clark Snow, Sandia National Laboratories Room: Chemistry (Bldg. 20) Room 224 |
Saturday, October 20, 2007 11:00AM - 11:12AM |
J1.00001: Simulated models of perturbed angular correlation (PAC) spectroscopy in a 4-state+S system Jeffery A. Hodges, Michael A. Stufflebeam, William E. Evenson, P. Matheson, M.O. Zacate Cerium oxide has a cubic crystal structure. A vacancy in CeO$_2$ can be trapped by a probe atom and hop among equivalent 1st or 2nd neighbor sites of the probe, producing a fluctuating electric field gradient (EFG) at the probe nucleus. We have simulated the perturbed angular correlation (PAC) spectrum due to such a changing EFG (4-state model), as well as the case with an additional static background EFG (4-state+S). We have studied the effect of changing the defect hopping rates on the resulting spectrum and the inferred hyperfine parameters. We have analyzed these data to determine experimental conditions under which nonequilibrium initial probe distributions can be detected by PAC. [Preview Abstract] |
Saturday, October 20, 2007 11:12AM - 11:24AM |
J1.00002: Simulated models of inhomogeneous broadening in perturbed angular correlation spectroscopy (PAC) Michael A. Stufflebeam, Jeffery A. Hodges, William E. Evenson, P. Matheson, M.O. Zacate All real crystals have defects: missing atoms (vacancies), impurities, atoms out of place, etc. In perturbed angular correlation (PAC), these defects produce damping of the correlation signal in time and broadening of the frequency spectrum. This broadening is termed ``inhomogeneous broadening'' since it is due to the inhomogeneities (i.e. defects) in the crystal. We have simulated PAC spectra for various concentrations (0.1\% to 5\%) of distant randomly distributed defects plus a near-neighbor vacancy in simple cubic and face-centered cubic crystal structures. For every particular set of defects, the randomly distributed defects and the near-neighbor vacancy together produce a net electric field gradient (EFG), from which we obtain the PAC spectrum. We fit average PAC spectra to study the effects of defect concentration and crystal structure on inhomogeneous broadening as an aid to analyzing experimental data. [Preview Abstract] |
Saturday, October 20, 2007 11:24AM - 11:36AM |
J1.00003: Structural phase transformations with plasticity using the Landau-Ginzburg theory Roman Gr\"oger, Turab Lookman Technologically useful materials, such as shape memory alloys (NiTi, FePd, U$_6$Nb) undergo displacive transformations from a high-temperature austenitic phase to a low-temperature, lower symmetry martensitic phase. The Landau-Ginzburg approach has been successfully applied to describe these transformations when the associated strains are purely elastic. In this work we consider both elastic and plastic strains and generalize the Landau-Ginzburg theory to also describe the elastic-plastic regime. In our model the elastic strains satisfy the Saint Venant compatibility constraints and, provided they are known, the applied stress can be calculated from the generalized Hooke's law. When this stress reaches the yield stress, the plastic strains evolve according to relations derived from the model of von Mises plasticity that we complement by a simple phenomenological model of strain hardening. The unloading proceeds purely elastically and the last stress attained is identified as the yield stress for subsequent reloading. We illustrate our approach with a two-dimensional version of the cubic to tetragonal structural transformation in FePd. [Preview Abstract] |
Saturday, October 20, 2007 11:36AM - 11:48AM |
J1.00004: How Does Nature Choose? A Combinatorial Approach to Preferred Chemical Ordering in Perovskite Structures. Matthew Lords, Erin Gilmartin, Gus Hart Material properties are intimately tied to crystal structure. Many materials, alloys in particular, share a common, underlying ``motif'', such as fcc/bcc/hcp ``parent lattice'', but have different chemical orderings. Among the nearly infinite possibilities for chemical orderings, why does nature choose the few it does? We answer this question generally and give an example of the perovskite structure, important in ferroelectrics, catalysts, and superconductors. [Preview Abstract] |
Saturday, October 20, 2007 11:48AM - 12:00PM |
J1.00005: AC Conductivity in Hopping Systems David H. Dunlap The ubiquitous power-law frequency dependence observed for the ac conductivity in highly disordered semiconductors has a number of competing explanations. We review these mechanisms and discuss their relation to a simple Langevin theory for conductivity in confined regions. The theory is used to interpret recent measurements of the ac conductivity in nanowire arrays. [Preview Abstract] |
Saturday, October 20, 2007 12:00PM - 12:12PM |
J1.00006: Fluxon relaxation in superconductors Kurt Stangel, Ralph Chamberlin, Nathan Newman, Jianyi Jiang, Brian Moeckly Using a high-speed SQUID magnetometer, we measure the relaxation of magnetic flux quanta (fluxons) in superconductors as a function of time after removing an applied field. When relatively small fields are removed, the relaxation is accurately described by a logarithmic time dependence, consistent with the Anderson-Kim theory for fluxon motion. However, when larger fields are removed, we see faster fluxon motion at short times and slower relaxation at long times, similar to a collective pinning model. This non-logarithmic relaxation often appears with a saturation in the relaxation slope, which may come from a crossover to strongly interacting fluxons when their average separation becomes less than the London penetration depth. [Preview Abstract] |
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