Bulletin of the American Physical Society
2007 Ohio Section of the APS Spring Meeting
Volume 52, Number 5
Friday–Saturday, May 4–5, 2007; Ypsilanti, Michigan
Session C2: Condensed Matter Physics |
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Chair: Michael Crescimanno, Youngstown State University Room: EMU Student Center 310A |
Saturday, May 5, 2007 8:12AM - 8:24AM |
C2.00001: Stereochemical Effects on Diffusion in Polypropylenes: Simulation and Experiment Ernst von Meerwall, Numan Waheed, Wayne Mattice In support of our dynamic Monte-Carlo (MC) simulations we performed pulsed-gradient NMR diffusion (D) measurements at 180$^{o}$C on moderately disperse polypropylene (PP) melts to study the effects of stereochemical composition. The coarse-grained simulations were based on the rotational isomeric state model and Lennard-Jones potentials. For the proton NMR diffusion measurements we obtained three PP specimens, with probabilities of a meso dyad P$_{m}$ = 0.02 (syndiotactic), 0.23 (atactic), and 0.89 (nearly isotactic). After accounting for differences in polydispersity, results were normalized to a common molecular weight M, using a scaling law taken from our earlier work with n-alkanes. Conversion between MC steps and real time was derived from experiment; no dependence on P$_{m}$ was expected. Both experiment and simulation found D at high P$_{m}$ several times faster than at low P$_{m}$, but the simulation also showed a maximum in D near P$_{m}$ = 0.8. The latter effect is attributed to quenched randomness; no specimen was available for experimental verification. For reasons not well understood, the experimental D-distribution on the P$_{m}$ = 0.89 sample exceeded that expected from the known M-distribution. [Preview Abstract] |
Saturday, May 5, 2007 8:24AM - 8:36AM |
C2.00002: Resonance sharpening of Aharanov-Bohm (AB) oscillations in an AB ring with double quantum dots Eric Hedin, Yong Joe, Arkady Satanin In a balanced AB ring with an identical QD in each arm, a combination of Breit-Wigner and Fano resonances presents a sensitively balanced region of parameter space in which arbitrarily sharp transmission resonance spikes manifest. The transmission becomes a resonance zero when the electron Fermi energy matches the quasi-bound state energy of the matched QD's. However, for values of magnetic flux through the AB ring which are integral multiples of the flux quantum, the analytical expression of the transmission amplitude (obtained from an exactly solvable tight-binding formalism) becomes indeterminate (zero over zero). The resulting transmission resonances near this critical point can show delta-function like full-transmission spikes as a function of magnetic flux. This effect may have practical application as a sensitive monitor of magnetic flux. [Preview Abstract] |
Saturday, May 5, 2007 8:36AM - 8:48AM |
C2.00003: Nucleation switching in phase change memory. Y.A. Kryukov, V.G. Karpov, S.D. Savransky, I.V. Karpov We propose a simple physical model of threshold switching in phase change memory cells (PCM), based on the electric field induced nucleation of conductive cylindrical crystallites in amorphous media. We consider a flat plate capacitor filled with the high resistive amorphous material $Ge_2 Sb_2 Te_5$ (GST). When the voltage $V$is applied the corresponding field lowers the free energy and nucleation barrier for the amorphous/crystalline transition. This increases the probability of creating crystalline nuclei, which are much less resistive than the amorphous ones. Hence the low resistive crystalline path between two electrodes is created, causing the memory switching. The model is solved analytically and leads to a number of predictions. They include correlations between the threshold voltage $V_{th}$ and material parameters, such as the nucleation barrier and radius, amorphous layer thickness, as well as $V_{th}$ versus temperature and switching delay time. In particular our calculations show linear dependence between threshold voltage and thickness of a memory cell and that the threshold voltage inversely proportional to the temperature to the power $1.5$. We have carried out verifying experiments: good agreement is achieved. [Preview Abstract] |
Saturday, May 5, 2007 8:48AM - 9:00AM |
C2.00004: Symmetry and Circularization in the Damped Kepler Problem Michael Crescimanno, Brian Hamilton Generically, a Hamiltonian system to which damping (non-Hamiltonian) forces are added loses its symmetry. It is a non-trivial fact that the eccentricity vector of lightly damped Kepler orbits is a constant for linear damping only. We describe the group theoretic background necessary to understand this fact and to relate it to that analogue of the Landau criterion for superfluidity associated with the general problem of orbit circularization. [Preview Abstract] |
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