Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W9: Theoretical Methods and Applications |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Alexander Tchernatinsky, University of Minnesota Room: Morial Convention Center RO7 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W9.00001: Polarization model revisited Michael Galperin, Abraham Nitzan, Mark A. Ratner We revisit a polaron model proposed by us as a possible mechanism for nonlinear conductance, and discuss difference in polaron formation within isolated system vs. molecular junction situation. Within one-level model we present approximate expression for electronic Green function corresponding to inelastic transport case, which in appropriate limits reduces to expressions presented previously for isolated molecule and for molecular junction coupled to slow vibration (static limit). Relevance of the isolated molecule-type consideration to describe properties of molecular junctions is discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W9.00002: A theoretical investigation of the porphyrin-gold junction: applications to molecular wires. Matt McKenzie, Zorabel LeJeune, Jayne Garno, Bin Chen An important step in the miniaturization of electronic devices involves molecular wires and junctions at the nanoscale level. Porphyrins are a promising material for such objects because of their unique electronic, chemical, and optical properties. The model porphyrin used in this study is a free based tetra-substituted with two phenyl rings and two pyridyl rings as peripheral groups which could provide a mechanism for enhanced electron transfer. The goal of this study is to elucidate the electron transfer paths between the model porphyrins and Au(111). The orbital structures and properties are determined using Car-Parrinello molecular dynamics. The geometry of the porphyrin on the gold surface is explored; from a complete reorientation of the molecule with respect to the surface to different orientations of the pyridyl groups. The calculated electronic conductivity, using the Kubo-Greenwood formula, will be compared to experimental findings using conductive probe Atomic Force Microscopy. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W9.00003: Ion-Ion Interactions in Simple Metallic Systems: Beyond Linear Response James Porter, Neil Ashcroft, Geoffrey Chester We extend the formalism of electronic response theory to second order in perturbing pseudopotentials and examine the physical consequences on effective ionic pair potentials for certain simple metals, under standard conditions. The pseudopotentials, assumed to be transferable, are of the Ashcroft empty-core form. Our results show that inclusion of second-order response terms in the pair potentials leads to the deepest potential minima having locations that are within 8\% of the experimental nearest-neighbor distances for crystalline sodium, magnesium, aluminum, and metallic silicon, all in their standard one-atmosphere structures. Second-order response effects are found to become increasingly important as the valence increases. We briefly discuss two natural extensions of this research, namely to three-body potentials and to changes expected in pair potentials at higher densities. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W9.00004: Algorithm for extraction of quantum oscillation orbits from band structure data Patrick Rourke, Stephen Julian In determining the Fermi surface of a material, quantum oscillation measurements are often compared to band structure calculations. Each oscillation frequency corresponds to an electron (or hole) orbit on the Fermi surface, perpendicular to the applied magnetic field; only orbits enclosing areas that are locally extremal are detected. To facilitate comparisons between theory and experiment, we have developed an algorithm, ``SKEAF,'' which finds extremal orbits in band structure calculations and determines quantum oscillation frequencies, effective masses and band specific heat contributions. Our code uses a k-space supercell approach, and can successfully locate geometrically-complicated orbits. Example results will be presented for the heavy fermion material UPt$_{3}$. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W9.00005: Development of the relativistic tight-binding model for Platinum Alexander Tchernatinsky, J. Woods Halley As a first step in a program to understand the mechanism of oxygen reduction on a platinum surface in an aqueous environment, we developed a relativistic self consistent tight- binding model for platinum. We applied a scheme that we successfully used previously for the description of titanium (S.~Erdin,~et~al.,~PRB, \textbf{72}, 035405 (2005)) in which the electronic structure problem is described by an energy functional containing onsite terms depending self consistently on the local charge and interatomic terms. Due to the high atomic number of platinum, relativistic effects are known to be significant in the electronic structure. We include relativistic effects in the onsite functions of the tight binding model by making them self consistently dependent on the local Mulliken charge (as before) and also on the expectation values of the total atomic angular momentum number J of the atom and the occupation numbers of the 5d$_{5/2}$ and 5d$_{3/2}$ atomic orbitals in the tight binding basis. We find that this set of variables can uniquely describe the low energy states of the isolated platinum atom including relativistic effects. Their values are calculated self consistently in the tight binding model for the metal. The model was used to calculate the electronic structure of relaxed, low index platinum surfaces. Results will be compared with DFT results and with experiment. This work was supported in part by Minnesota Supercomputing Institute and U.S.DOE. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W9.00006: Equivalence of dipole correction and Coulomb cutoff techniques in supercell calculations Liping Yu, V. Ranjan, W. Lu, J. Bernholc, M. Buongiorno Nardelli In ab initio calculations for surfaces or non-periodic systems one often relies on the supercell approximation, where periodic replicas are separated by enough empty space to avoid spurious interactions between successive images. However, a vacuum separation is not sufficient to screen dipolar interactions that appear in asymmetrically charged or polar systems. Two solutions have been proposed in the literature: (i) the dipole correction, and (ii) Coulomb cutoff formalism that eliminates interactions between periodic replicas. We compare these methods under the same conditions in the framework of plane wave DFT calculations. It is found that the two methods produce equivalent results for the total energy, force, charge density and self-consistent potential. In band structure calculations, the results coincide for occupied states but differ for delocalized unoccupied ones in small supercells. This discrepancy can be used as a criterion to identify supercell sizes that are sufficiently large to obtain converged results. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W9.00007: Issues related to Convergence Properties of First Principles Full Potential Multiple Scattering Electronic Structure Calculations Aurelian Rusanu, G. Malcolm Stocks, Markus Eisenbach, Don M. Nicholson, Yang Wang Despite some clear advantages for specific problems, the implementation of full potential electronic structure methods based on the use of multiple scattering theory (MST) (KKR derived approaches) has received much less investment than alternate electronic structure methods. Here we describe some new techniques that facilitate an easy and accurate implementation of first principles full potential MST methods. The method consists of solving: (1) the full-potential single site scattering problem, where we avoid the usage of the shape function by surface integrals methods to determine the scattering matrices, and (2) the Poisson problem, where the site centered full-potential is constructed from a sphere bounded non-overlapping charge density and a smooth space-filling charge density. Specifically, we discuss issues related to accuracy and convergence properties of these techniques within the context of the order-N Locally Self-consistent Multiple Scattering (LSMS) method. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W9.00008: First-principles investigation of electronic band gap in multiwalled carbon nanotube: Role of mechanical deformations Pavan K. Valavala, Gregory M. Odegard, Ranjit Pati The carbon nanotube (CNT) structures have been the subject of intense research in recent years. Some studies have shown that the electronic band gap in single walled CNT can be modulated through mechanical deformations such as flattening. It has been shown that single walled CNTs undergo a semiconducting-metallic and semiconducting-metallic-semiconducting transition when subjected to deformations. However, the modulation of electronic band gap of multiwalled CNT under mechanical deformations has not been studied. We have used first- principles gradient density functional approach to explore the role of flattening on the electronic properties of MWCNT structures. The influence and the effect of flattening on the electronic properties of the constituent single walled CNTs are also explored. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W9.00009: Atomic-scale Analysis of the Interactions Between Atomic Hydrogen and Multi-walled Carbon Nanotubes Andre R. Muniz, Tejinder Singh, Dimitrios Maroudas We present a detailed atomic-scale analysis of the interactions of atomic hydrogen with the internal layers of multi-walled carbon nanotubes (MWCNTs). The analysis is based on a synergistic combination of classical molecular dynamics (MD) with first-principles density functional theory (DFT). The Adaptive Interatomic Reactive Empirical Bond Order (AIREBO) potential is employed in the MD simulations of H-MWCNT interactions and the resulting structural relaxations. Parameters that have been varied in our analysis include nanotube diameters, number of nanotube walls, inter-wall spacing, and temperature. The MD simulations reveal that, under certain conditions, H chemisorption onto internal MWCNT walls and H diffusion in the space between walls can induce the formation of inter-shell sp$^{3}$ C-C bonds. The MD mechanisms are in good agreement with our DFT calculations of optimal pathways of C-C bond formation and provide interpretations for the formation of nanocrystalline carbon, which has been observed experimentally upon H$_{2}$ plasma exposure of MWCNTs. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W9.00010: Electronic Property Control of Single-Walled Carbon Nanotubes by Functionalization Chiayun Wu, Young-Kyun Kwon Single-walled carbon nanotubes exhibit remarkable electronic properties. It is well known that there are two types of single-walled carbon nanotubes: metallic and semiconducting. However, separating semiconducting carbon nanotubes from metallic ones is a ``holy grail'' problem in nanoelectronics fields. Using ab initio density functional theory, we will present the effects of various functional groups, such as 4-bromobenzene diazonium tetrafluoroborate, on SWNTs. Modifications in electronic and transport properties due to such functionalization will be discussed. Possible mechanism converting metallic tubes to semiconducting ones will be addressed. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W9.00011: Design of Janus Nanoparticles with Atomic Precision Qiang Sun, Qian Wang, Puru Jena, Yoshi Kawazoe Janus nanoparticles, characterized by their anisotropic structure and interactions have added a new dimension to nanoscience because of their potential applications in biomedicine, sensors, catalysis and assembled materials. The technological applications of these nanoparticles, however, have been limited as the current chemical, physical, and biosynthetic methods lack sufficient size and shape selectivity. We report a technique where gold clusters doped with tungsten can serve as a seed that facilitates the natural growth of anisotropic nanostructures whose size and shape can be controlled with atomic precision. Using ab initio simulated annealing and molecular dynamics calculations on Au$_{n}$W (n$>$12) clusters, we discovered that the W@Au$_{12}$ cage cluster forms a very stable core with the remaining Au atoms forming patchy structures on its surface. The anisotropic geometry gives rise to anisotropies in vibrational spectra, charge distributions, electronic structures, and reactivity, thus making it useful to have dual functionalities. In particular, the core-patch structure is shown to possess a hydrophilic head and a hydrophobic tail. The W@Au$_{12}$ clusters can also be used as building blocks of a nano-ring with novel properties. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W9.00012: Uniaxial compression of group-IV nanoparticles from ab-initio molecular dynamics simulations Prasanjit Samal, Matteo Cococcioni Uniaxial compressions of isolated systems from ab-initio molecular dynamics are made possible through the extension of the electronic enthalphy method [Phys. Rev. Lett. 94, 145501 (2005)] previously introduced for finite systems under hydrostatic pressure. Through this novel approach experimental settings with nanoparticles indented between parallel plates can be reproduced and simulated more realistically, thus allowing for more reliable comparisons between experimental data and simulation results. Molecular dynamics simulations for some group-IV nanoparticles under uniaxial compression have been performed using this extended scheme. Comparison with the deformation of the same systems under hydrostatic loads will elucidate the differences and similarities in the nucleation events of structural transformations and in their kinetic pathways. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W9.00013: Explaining the electroluminescence of single molecules John Buker, George Kirczenow Experimental studies of electroluminescence from molecules on complex substrates have yielded intriguing relationships between the current-voltage characteristics and optical emission from such systems.\footnote{X. H. Qiu, G. V. Nazin, W. Ho, Science {\bf 299}, 542 (2003), S. W. Wu, N. Ogawa, W. Ho, Science {\bf 312}, 1362 (2006).} In this talk we propose a theoretical model that is able to account for many observed properties of these systems. We obtain distinct photon emission spectra and corresponding I-V curves for different couplings of the electrodes to the molecule, that are consistent with experimental data. We find emission to be highly dependent on the details of the tip/molecule and molecule/substrate coupling, and make further photon emission predictions for systems not yet achieved experimentally. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W9.00014: Toward quantitative acoustic molecular gas sensing Andi Petculescu Laser spectroscopy techniques provide unrivaled tools for detailed molecular sensing studies. Critical applications such as gas monitoring in life-support systems often require sensors that are not only fast and sufficiently accurate but also rugged and not needing extensive maintenance and calibration. Acoustic sensors are widely used to this end, based on measuring sound speed changes. These, however, can only constrain the overall molecular mass. For reasons unknown, researchers have habitually disregarded acoustic attenuation. Beside the classical sound loss mechanisms due to viscosity, heat conduction, and diffusion, the non-classical contribution to attenuation arises from the inability of internal molecular degrees of freedom to follow the acoustic temperature fluctuations. This connects acoustic attenuation to the molecular relaxation times. In the laboratory, acoustic studies of molecular relaxation in fluids require that the ambient pressure be varied over a wide range at a given frequency in order to cover the relaxation processes. In a fast-sensing device, this would be highly impractical. A novel algorithm is presented here relying on measuring sound speed and attenuation at one pressure and two frequencies to reconstruct the full frequency dependence of the effective specific heat for the main relaxation processes. This would enable a smart sensor to infer the concentration and nature of contaminant molecules in a base gas. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W9.00015: A delta-function model for three-dimensional axisymmetric crystals Ping Du, Harris Wong A surface energy polar plot contains two possible singularities: the cusps that give facets on an equilibrium crystal, and the circular arcs connecting the cusps that can lead to missing orientations. The common approach of specifying the surface energy usually cannot handle both singularities simultaneously. We model the surface stiffness to avoid missing orientations. Furthermore, a facet is represented by the Dirac delta function with the weight of the delta function equal to the width of the facet. Thus, both singularities are treated precisely. This approach has been shown to work for two-dimensional symmetric [1] and axially symmetric [2] crystals. Here, we apply the delta function to model three-dimensional axisymmetric crystals and obtain analytic solutions to the nonlinear differential equation governing the crystal shape. We find that at every point on the crystal surface the chemical potential is equipartitioned between the axial and the azimuthal component. \newline [1] Xin, T. and H. Wong, \underline {Surface Science} \textbf{487}, L529 (2001). \newline [2] Du, P. and H. Wong, \underline {Scripta Materialia} \textbf{55}, 1171 (2006). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700