Bulletin of the American Physical Society
2008 Joint Meeting of the APS Ohio-Region Section, the AAPT Southern Ohio Section, and the ACS Dayton-Section
Volume 53, Number 8
Friday–Saturday, October 10–11, 2008; Dayton, Ohio
Session C6: Condensed Matter |
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Chair: Matthew Bohn, Air Force Institute of Technology Room: Oelman Hall 302 |
Saturday, October 11, 2008 8:00AM - 8:12AM |
C6.00001: Temporal and spectral photoluminescence from HVPE grown GaAs Matthew Bohn, William Guiney Defects discovered in low temperature photoluminescence (PL) spectrum recorded from Hydride-Vapor Phase Epitaxial growth of GaAs were measured as a function of secondary HCl flow. An Argon ion laser illuminated the GaAs samples, which were held at 10 K using a liquid He cold finger in a cryogenic dewar. Transitions involving the point defects due to e-Si$_{As}$, V$_{As}$-Si$_{As}$, and e-V$_{Ga}$ are identified in the PL spectrum. The Si defects are plotted as a function of HCl secondary flow rate and carrier concentration. It is postulated that the Si defects are introduced into the growth due to the quartz tube used in the secondary flow. In addition, time resolved PL measurements were made using a femtosecond Ti:sapphire laser and a synchronized Hamamatsu streak camera capable of picosecond temporal resolution. The time resolved PL measurements of the samples substantiated the excellent quality of the crystalline growth. [Preview Abstract] |
Saturday, October 11, 2008 8:12AM - 8:24AM |
C6.00002: Diffusion in Polypropylenes: Effects of Stereochemistry and Polydispersity Ernst von Meerwall, Numan Waheed, Wayne Mattice We have performed pulsed-gradient diffusion (D) experiments at 180 deg. C and dynamic Monte-Carlo (MC) simulations to study the effect of stereochemical composition of linear propylene (PP) melts. The coarse-grained simulations were based on the rotational isomeric state model and Lennard-Jones potentials. For the proton NMR difusion measurements we obtained three PP specimens of differing molecular weight M and dispersity, with the probability of a meso diad Pm = 0.02 (syndiotactic), 0.23 (atactic), and 0.89 (nearly isotactic). Conversion between MC steps and real time was derived from experiment; no dependence on Pm is expected. Both simulation and M-scaled experiment found D at high Pm several times faster than at low Pm, but the constant-M simulations showed a maximum in D near Pm = 0.8 due to quenched randomness. To find the source of the remaining disagreement with experiment, new simulations tracked the samples' mean M and polydispersity, but the disagrement actually increased. We suspect that the GPC determination of M and its distribution, based on linear polyethylene scaling, is strongly dependent on PP stereochemistry, producing the misleading result. [Preview Abstract] |
Saturday, October 11, 2008 8:24AM - 8:36AM |
C6.00003: Calculating Young's modulus for a carbon nanotube Feras Alzubi, Ronald Cosby Young's modulus for an armchair single-wall carbon nanotube was calculated using an atomistic approach and density functional theory (DFT). Atomic forces and total energies for strained carbon nanotube segments were computed using Atomistix's Virtual NanoLab (VNL) and ToolKit (ATK) software. For a maximum strain of one percent, elastic moduli were calculated using both force-strain and energy-strain data. The average values found for Young's modulus were in the range 1.2 to 3.9 TPa depending on the cross-sectional area taken for the carbon nanotube, consideration of Poisson's ratio, and the calculation method used. Three possible choices of cross-sectional area for the carbon nanotube are discussed and parameter and convergence tests for the DFT computations are described. [Preview Abstract] |
Saturday, October 11, 2008 8:36AM - 8:48AM |
C6.00004: Multiconfiguration Hartree-Fock autoionization calculations near the 3p excitation edge of the transition metals Kofi Nuroh We report electron-impact excitation relative scattering cross sections for the transition metals $^{21}$SC through $^{27}$Ni near the 3p excitation edge.\footnote{K. Nuroh, Phys. Rev. B\textbf{ 77}, 125137 (2008)} The first set of calculations is based on the theoretical model of random-phase approximation for core-electron scattering in solids in which only electrostatic interactions are taken into account.\footnote{K. Nuroh, Phys. Rev. B \textbf{70}, 205115 (2004)} The second set of calculations is based on an analysis that hinges on the Bethe-Born approximation\footnote{K. Nuroh, J. Phys. C \textbf{20}, 5305 (1987)} in which both electrostatic and magnetic interactions are incorporated. Both sets of calculations show trends in Z that are manifested in available measurements. [Preview Abstract] |
Saturday, October 11, 2008 8:48AM - 9:00AM |
C6.00005: Terrace Width Distributions in the Limit $\tilde{\beta}_B / \tilde{\beta}_A \to \infty$: Numerical Transfer Matrix Results Howard Richards With a few physical and mathematical simplifications, the Terrace Width Distributions (TWDs) for a stepped crystal surface with typical step interactions have been shown to be Generalized Wigner Distributions (GWDs).\footnote{H.~L.\ Richards and T.~L.Einstein, \textit{Phys.\ Rev.\ E} {\bf 76}, 016124 (2005).} This is true even when steps have different stiffnesses ($\tilde{\beta}_A$ and $\tilde{\beta}_B$) that alternate, as has been confirmed by Monte Carlo simulations.\footnote{J.~A.\ Yancey, H.~L.\ Richards, and T.~L.Einstein, \textit{Surf.\ Sci.\/} {\bf 598}, 78--87 (2005).} Monte Carlo simulations have three serious drawbacks for studying very unequal stiffnesses: (1) the simulated steps have only finite length, which may be close to or smaller than the correlation length; (2) the time required to equilibrate may be prohibitively long; and (3) statistical uncertainties are unavoidable. Additionally, the simulations in Ref.~2 were problematic, since $\tilde{\beta}_A$ approached zero as $\tilde{\beta}_B$ became large. This work avoids all those problems by finding TWDs from numerical transfer matrices with $\tilde{\beta}_A$ held constant as $\tilde{\beta}_B \to \infty$. This is a necessary step before the GWD can be analyzed as an ensemble average of Gruber-Mullins TWDs. [Preview Abstract] |
Saturday, October 11, 2008 9:00AM - 9:12AM |
C6.00006: Simulation of Ferromagnetic Properties for the Two-Dimensional Ising Model Christopher Lemon, Ronald Johns A Matlab program was written that uses Monte Carlo methods and the heat bath algorithm to simulate the two-dimensional Ising model. Several program variations were written to see if the simulation would accurately predict the existence of the Curie temperature and ferromagnetic domains. For a variety of square lattices, the net magnetization was calculated as the applied magnetic field was varied. It was found that the simulated Curie temperature is in good agreement with that predicted from the theory of the Ising model. Additionally, simulations show that lattice sites of the same spin tend to cluster together; the size of these domains is dependent on the external magnetic field. Based on these results, this program successfully simulates two hallmarks of the Ising model of ferromagnetism: Curie temperature and ferromagnetic domains. [Preview Abstract] |
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