Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B35: Energy Landscapes in Clusters, Materials, and Biology II |
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Sponsoring Units: DCP Chair: D. Leitner, University of Nevada - Reno Room: LACC 511B |
Monday, March 21, 2005 11:15AM - 11:51AM |
B35.00001: Magnetic Dynamics of Co Nanoparticle Dispersions Invited Speaker: Meigan Aronson We have used inelastic neutron scattering studies to study the dynamics and spatial correlations of monodisperse suspensions of Co nanoparticles. At high temperatures, the scattering is broad and quasielastic, reflecting the reversal of nanoparticle moments between states separated by an anisotropy barrier. Surprisingly, the scattering is strong even at wave vectors q as large as 2-3 A$^{-1}$, suggesting that the reversal of the overall nanoparticle moment involves localized and rapid fluctuations of smaller groups of Co moments. Spectral weight is transferred to increasingly small energies as the blocking temperature T$_{B}$ is approached, with the slowest fluctuations associated with the smallest wave vectors. Below T$_{B}$ the quasielastic scattering in the magnetically frozen state becomes inelastic, growing in intensity and hardening with decreasing temperature. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:27PM |
B35.00002: What can global optimisation and coarse-graining reveal about energy landscapes? Invited Speaker: The description of biological systems often requires a large number of degrees of freedom. Exploring their energy landscape is an important but computationally demanding task. We apply a new methodology that identifies global or local minima of interest without sampling or exhaustive searching. Rather, it relies on a relaxation of the interactions of interest to a special sum of squares convex function that can be solved in polynomial time by exploiting computationally efficient semidefinite programming methods. Another advantage of the method is that the level of any approximation is controllable and hence known. We will apply the method to model problems and discuss how it can be used to guide coarse-graining to few but relevant degrees of freedom in protein-protein interactions. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B35.00003: Solid to liquid transition and the potential energy surface of sodium clusters Ignacio L. Garzon, Juan A. Reyes-Nava, Luis A. Perez Thermal stability properties, the melting-like transition, and heat capacities of Na$_{n}$, n=13-309, clusters have been investigated using a many-body potential energy surface and molecular dynamics simulations. The characteristics of the solid-liquid transition in the sodium clusters are analyzed by calculating physical quantities like caloric curves, heat capacities, and root-mean-square bond length fluctuations using simulation times of several nanoseconds. Distinct melting mechanism are obtained for the sodium clusters in the size range investigated. The calculated melting temperatures show an irregular variation with the cluster size, in qualitative agreement with recent experimental results. By performing a survey of the cluster potential energy landscape, it is found that the width of the distribution function of the kinetic energy and the spread of the distribution of the potential energy minima (isomers), are useful features to determine the different behavior of the heat capacity of the function of the cluster size. Density functional calculations were performed to test the accuracy of the many-body potential energy surface to model the bonding existing in sodium clusters. Acknowledgments: This work was supported by CONACyT No. 43414-F, UNAM-DGAPA No. IN4402, and DGSCA UNAM Supercomputing Center. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B35.00004: Driving force for the WO3 (001) surface relaxation Maciej Gutowski, Ivan Yakovkin The optimized structure of the (001) WO$_{3}$ surface with various types of termination has been studied at the density functional theory level with a Perdew-Wang exchange-correlation functional. In contrast to transformations of bulk WO3, surface relaxation results in a significant decrease of the total energy. Indeed, the surface energy decreases from 10.2x10$^{-2}$ eV/A$^{2}$ for the ideal (ReO$_{3}$-like) to 2.2x10$^{-2}$ eV/A$^{2}$ for the relaxed c(2x2) O-terminated surface. The surface relaxation is accompanied by a dramatic redistribution of the density of states near the Fermi level, associated with transformations of surface states. This redistribution lowers the electronic energy of the slab and therefore is suggested to be the driving force for surface relaxation of the (001)WO$_{3}$. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B35.00005: Understanding the Low-Energy Dynamics of Inorganic Fullerene-Like WS$_2$ Nanoparticles R.D. Luttrell, S. Brown, J. Cao, J.L. Musfeldt, R. Rosentsveig, R. Tenne Inorganic fullerene-like nanoparticles are attracting attention due to their outstanding solid-state lubricating behavior. We present the vibrational response of inorganic fullerene-like WS$_2$ nanoparticles and discuss the effects of local strain and effective charge on the dynamics of this material. We compare these results to those of the chemically identical (but morphologically different) layered solid. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:39PM |
B35.00006: Invited Speaker: |
Monday, March 21, 2005 1:39PM - 1:51PM |
B35.00007: An Alternative Reaction Scheme for Calcium Phosphate Nucleation on Silica Glass Ceramics: Molecular Dynamics Simulations Bernard Delley, Rene Windiks The silica mineral $\alpha$-wollastonite exposed to physiological aqueous solutions induces the nucleation of calcium phosphate (CaP) on its surface. The nucleation is the initial step of the formation of biological-like CaP layers which finally provide an ideal environment for a strong bonding to biological tissues and mature bone. However,the mechanism of nucleation and the molecular structure of the first inorganic precipitates is still under investigation. We have performed density functional molecular dynamics simulations on model systems and propose a nucleophilic substitution reaction type which includes the formation of covalent Si--O--P linkages. Initially, oxygen atoms of phosphate ions attach to silicon atoms of the solvated silica species at the surface of the mineral. Subsequently, a Si--O bond is formed and the silicon atom has a penta-oxo coordination. Finally, another Si--O bond of the penta- coordinated silicon is broken.The calculated change of the Helmholtz free energy is negative. That is, the reaction ends with CaP strongly bonded to the mineral. The reaction barriers along this pathway remain fairly low, as the pentacoordinated Si is part of a metastable intermediate state. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B35.00008: Heterogeneity in classical and non-classical nucleation Natali Gulbahce, W. Klein, Harvey Gould A crossover from heterogeneous to homogeneous nucleation has been observed experimentally (e.g., in ADP) when the quench depth is increased. To understand this crossover behavior in detail, we have simulated heterogeneous and homogeneous nucleation in nearest-neighbor and long-range Ising models for various quench depths. We find that for a fixed system size the system crosses over from heterogeneous to homogeneous nucleation for increasing quench depth only if the interaction is sufficiently long-range. We determined the survival curve, the fraction of systems that remain in the metastable state after a given time. We find that the survival curves as a function of quench depth have different slopes for heterogeneous and homogeneous nucleation when the interaction is short-range, but have identical slopes within the accuracy of our data for long-range interactions. We conclude that if the extent of the heterogeneity is smaller than the range of interaction, the survival curves will be the same. [Preview Abstract] |
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