Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P31: Focus Session: Computational Nanoscience V: Mechanical Properties and General Methods |
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Sponsoring Units: DMP DCOMP Chair: Murilo L. Tiago, Oak Ridge National Laboratory Room: Morial Convention Center 223 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P31.00001: Materials under Extreme Conditions: Ultrahigh Strength under Shock Loading Invited Speaker: Eduardo M. Bringa |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P31.00002: Effects of nanoscale structure on the mechanical properties of nanoporous silica. Daniel Lacks Porosity in materials can create materials with superior function, by reducing the weight and dielectric constant of the material -- e.g., in nature, porous bone has evolved to reduce the weight of the skeleton and thus minimize the energy required for animals to move. However, the introduction of porosity is not without a downside, as porosity generally compromises the mechanical behavior of a material, which could preclude its utility. We use molecular dynamics simulations, in conjunction with an experimental investigation, to show that nanoscale structuring can be used to make porous silica with mechanical properties that remain favorable as the porosotity increases. In particular, the elastic modulus scaling with density is much weaker in these materials than in conventional porous materials. Our simulations show that this scaling occurs because the nanoscale structure induces a change in the atomic level structure, where the new structure has a higher local stiffness. (Fan et al, Nature Materials 6, 418 (2007)). [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P31.00003: Phase-field simulations of martensite-martensite nanocomposites Mathieu Bouville, Rajeev Ahluwalia We study composites made of two martensite-forming materials, with different transition temperatures, $T_l < T_h$. The system remains austenitic at high temperature, and if the temperature is very low then the difference between the two materials is negligible. We therefore focus on intermediate temperatures, i.e.\ $T_l \le T < T_h$. Then only one material can transform to martensite~--- the other transformation may occur only if it is triggered by the martensite already formed (volume changes will then play a key role). We study the effect of reducing the size of the system, in particular how martensite can form in nanocomposites at temperatures at which no martensitic transformation can exist at the macroscopic scale. This work has relevance to multiferroics, where the phase transformation in one material (e.g.\ ferroelectric) triggers a transformation in another material, for instance magnetostrictive. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P31.00004: The dynamical activation-relaxation technique (DART): an on-the-fly kinetic Monte-Carlo algorithm Fadwa El-Mellouhi, Michel Cote, Laurent J. Lewis, Normand Mousseau We present DART, the dynamical activation-relaxation technique, that combines the activation-relaxation technique (ART nouveau) with a non-lattice KMC method that allows the on-the-fly identification of barriers and the full treatment of lattice deformations. Most KMC schemes rely on the use of a fixed list of events and barriers, which are drawn with the proper weight during the simulation. While this works well for a number of problems (such as metal-on-metal growth), it cannot be used for processes where the events may change with time. DART overcomes this limitation. ART nouveau has been used extensively for the study of activated mechanisms in different materials within both an empirical and an ab-initio description of the systems. In the DART implementation, KMC moves are based on a catalog of events constructed on-the-fly using ART. After each KMC move, this catalog is updated so as to take into account new environments that may appear. A topological description of the structure of the system at each moment allows the method to identify rapidly these new environments and to move forward efficiently. In this talk, we will describe the method and present the case of interstitial diffusion in Si. Our results are compared with previous molecular-dynamics and on-lattice KMC simulations. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P31.00005: Charge patching method for the calculation of electronic structure of organic semiconductors Nenad Vukmirovic, Lin-Wang Wang The electronic structure of organic semiconducting conjugated polymers and molecular crystals is essential in determining their optical and transport properties. Such organic semiconductor systems have potential applications for solar cells, field-effect transistors and luminescent devices. However, for such systems containing a large number of atoms, the direct calculations based on density functional theory (DFT) are often not feasible. Here, we present the development of the charge patching method for the calculations of organic systems, a method which was previously successful in treating inorganic semiconductor materials [1]. The results of the calculations for alkane and alkene chains using this approach yield the difference in Kohn-Sham wavefunction eigen energies of the order of only 30 meV compared with direct DFT calculations. Further investigations involving aromatic compounds, as well as elements such as sulfur, nitrogen, and oxygen, will be presented. [1] L.W. Wang, Phys. Rev. Lett. 88, 256402 (2002). [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P31.00006: Real-space grid representation of momentum and kinetic energy operators for electronic structure calculations. Domenico Ninno, Giovanni Cantele, Fabio Trani The development of computational methods based on real-space grids is contributing to the advances and understanding of nanoscale materials. Real-space grids methods, particularly within the pseudopotential density functional theory, have the advantage of producing highly structured matrices paving the way towards grid-based O(N) methods for both DFT total energy and molecular dynamics calculations. However, a limitation seems to be the lack of a basis set putting these approaches on a different ground with respect to standard methods. We prove that this limitation is only apparent showing that the position operator eigenkets are the natural basis set for the finite difference representation of momentum and kinetic energy operators. Some conceptual points and unpublished results related to the connection between the discrete and the continuum representations will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P31.00007: Single molecular NDR: A first principles study Ranjit Pati The demonstration of single molecule switch with a negative differential resistance (NDR) feature has drawn considerable attention in recent years. The NDR feature is described by a steady increase followed by a decrease in current with the increase in applied bias. Here we report a single molecular NDR in a strongly coupled metal-molecule junction, with peak to valley ratio (PVR) of 2.7 at both positive and negative bias. The bias dependent screening effect is explicitly included in our calculation through a rigorous self-consistent many body approach. The non equilibrium Green function approach is used to calculate the quantum transport. The origin of high PVR will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P31.00008: Electron correlations in molecular wires: e-e interactionion both in leads and bridge Yuri Dahnovsky, Vince Ortiz Molecular systems (molecules) with strong electron-electron interaction both in leads and a bridge are described in terms of the time-dependent electron Green functions. We prove that the Meir-Wingreen expression holds if one assumes that the bridge and lead electron subsystems are strongly separated. We develop a diagrammatic technique in a well determined, noncrossing cluster approximation. Within this approach, Dyson equations for various nonequilibrium Green functions are derived, and the validity conditions are found. In addition, we rigorously prove that despite strong electron-electron or electron-viration interaction in the systems with the finite number of quantum states, the Landauer-Buttiker expression for electric current is true. The ab initio electron propagator method is applied to the calculations of I-V characteristics in molecular electronic devices with the bridge composed from 1,4--benzene--dithiolate molecule. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P31.00009: Characterizing Capture-Zone Distributions: Generalized Wigner vs.\ Alternative Forms, and Experimental Fits T.L. Einstein, Alberto Pimpinelli In problems of growth of islands or of dots, it is often advantageous to consider the distribution of the areas of proximity (Voronoi) cells of nucleation sites, i.e.\ the capture zones (CZ). Extending results for terrace-width distributions on vicinal surfaces, \footnote{AP et al., PRL 95, 246101 (2005); TLE, Appl. Phys. A 87, 375 (2007)} we have shown that the (non-equilibrium, steady-state) CZ distribution is well described by the generalized Wigner expression $P_\varrho(s)=as^\varrho\exp(-bs^2)$ [with $a$ and $b$ being constants assuring normalization and unit mean, and $s$ here the CZ area divided by its mean], which accounts for a strikingly broad range of fluctuation phenomena. For CZ distributions we find that the single adjustable parameter $\varrho = (2/d)(i + 1)$, where $i$ is the size of the critical nucleus and $d$ (= 1 or 2) the spatial dimensionality.\footnote{A. Pimpinelli and T.L. Einstein, PRL 99, 226102 (2007).} We emphasize comparisons with other fitting expressions (esp.\ gamma functions), new applications to experimental data, and the generality of $P_\varrho(s)$. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P31.00010: Molecular simulations of capillary adhesion Shengfeng Cheng, Mark Robbins Adhesion due to capillary condensation is ubiquitous in nature, and dominates the adhesion force between particles in many experiments. Traditional models are based on continuum theory and may not describe nanoscale capillaries in microelectromechanical systems (MEMs) or at an atomic force microscope (AFM) tip. We employ molecular dynamics simulations to investigate the capillary adhesion between a nominally spherical tip and a flat substrate with a liquid bridge of fixed volume. The atomic scale roughness on the tip, contact angle and volume are varied. The adhesive force-distance curve and the separate contributions from Laplace pressure and surface tension are compared to continuum theory using independently measured parameters. Continuum theory provides a good description down to separations of a few molecular diameters. Atomic scale roughness affects the contact angle that enters the continuum theory and alters the adhesive force. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P31.00011: Molecular propellers and tunneling-driven motors Lela Vukovic, Boyang Wang, Petr Kral We design molecular propellers with carbon nanotube rotors and aromatic blades that allow selective pumping of hydrophobic and hydrophilic liquids [1]. Our molecular dynamics studies show that the pumping efficiency strongly depends on the chemistry of the liquid-blade interface. We also discuss several prototypes of highly efficient molecular motors driven by electron tunneling that could drive such rotary molecular machines [2]. These systems might pump liquids and provide motility at the nanoscale. [1] B. Wang and P. Kr\'al, Phys. Rev. Lett. 98, 266102 (2007). [2] L. Vukovic, B. Wang and P. Kr\'al, submitted. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P31.00012: An algebraic approach to computer program design and memory management James Raynolds, Lenore Mullin Beginning with an algebra of multi-dimensional arrays and following a set of reduction rules embodying a calculus of array indices, we translate (in a mechanizable way) from the high-level mathematics of any array-based problem and a machine specification to a mathematically-optimized implementation. Raynolds and Mullin introduced the name \it{Conformal Computing}\,${}^\star$\rm \ to describe this process that will be discussed in the context of data transforms such as the Fast Fourier, Wavelet Transforms and QR decomposition. We discuss the discovery that the access patterns of the Wavelet Transform form a sufficiently regular subset of those for our cache-optimized FFT so that we can be assured of achieving similar efficiency improvements to the Wavelet Transform as those that were found for the FFT. We present recent results in which careful attention to reproducible computational experiments in a dedicated/non-shared environment is demonstrated to be essential in order to optimally \it{measure} \rm the response of the \it{system} \rm (in this case the computer itself is the object of study) so as to be able to optimally tune the algorithm to the numerous cost functions associated with all of the elements of the memory/disk/network hierarchy. ${}^\star$ The name Conformal Computing is protected: \copyright ~2003, The Research Foundation, State University of New York. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P31.00013: Giant Hall Effect in Laterally Inhomogeneous 2D Electron Gas Hang Xie, Ping Sheng Giant Hall effect has been observed in non-magnetic granular metals at concentration close to the quantum percolation threshold [1], attributable to quantum interference effect. In this work we numerically simulate the Hall effect for 2D electron gas in a laterally inhomogeneous structure. At scales smaller than the electron dephasing length, we obtain the Hall coefficient of 2DEG by solving Schrodinger's equation, with 4 leads connected to the sample. It is shown that for special (laterally) nano-scaled structures, the Hall coefficient can be enhanced by at least 3 orders of magnitude. We have also simulated the effect of assembling such structures into a macroscopic sample, by solving the Laplace equation. \newline [1] X. X. Zhang, C. Wan, H. Liu, Z. Q. Li, P. Sheng and J. J. Lin, Phys. Rev. Lett. 86, 5562-65, (2001). [Preview Abstract] |
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