Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W26: First-Principles Simulation of Complex Materials |
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Sponsoring Units: DCOMP DMP Chair: Steven Erwin, NRL Room: LACC 501B |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W26.00001: Diffusion, Coalescence, and Reconstruction of Vacancy Defects in Graphene Layers Gun-Do Lee, Euijoon Yoon, Nong-Moon Hwang Recently, vacancy defects in graphene layers have attracted a lot of interest due to the formation of vacancy defects by irradiation, which is the important process in the synthesis of nanotube junctions of various types. In this talk, the diffusion of vacancy defects in single graphene layer is discussed by tight-binding molecular dynamics simulations using the environmet-dependent tight-binding carbon potential. We also check the energetics by using ab-intio method. In this study, we find that two single vacancies coalesce into a divacancy and those are reconstructed into a new structure, which is composed of carbon pentagons and heptagons. This reconstructed structure is energetically more favorable than other structures of carbon divacancy. The calculated STM image is compared with experiment. The diffusion of vacancy defects in singe-wall carbon nanotubes will also be discussed. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W26.00002: Alloying Contributions to Twinnability in Nickel: Testing the Rule of Thumb Donald Siegel It is well-known that the addition of small quantities of solute atoms can dramatically alter the mechanical properties of an unalloyed metal. An example is the change in a metal's tendency to twin after solute incorporation. The rule of thumb describing a solute's effect on twinning is based on changes to the intrinsic stacking fault energy (SFE) of the unalloyed phase: If a solute lowers the SFE, then the twinnability of the alloy should be greater than that of the pure phase. In FCC metals, it is furthermore thought that alloying will lower the SFE if the first intermetallic phase in the binary phase diagram is HCP. By combining \textit{ab initio} simulations with an elasticity theory expression for twinnibility [1], I examine whether the conventional wisdom regarding alloying and twinnability are correct. Using Nickel as a test case, it is first shown that the theoretical twinnability of pure Ni falls within the expected experimental range [1]. I then evaluate the generalized stacking fault surface for Ni with Nb, Mn, and W additions, and compare their twinnability with the rule-of-thumb predictions.\newline [1] Bernstein and Tadmor, Phys Rev B 69, 094116 (2004) [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W26.00003: First principles studies of self-polarization in electroactive polymers Serge Nakhmanson, Marco Buongiorno Nardelli, Jerry Bernholc The efficiency and physical transparency of the Wannier function formalism makes it especially suitable for studies of polarization and piezoelectricity in large systems ( $>$ 100 atoms). We have used this approach to examine the polar properties of polyvinylidene fluoride (PVDF) and its copolymers with tri- and tetrafluoroethylene in different VDF-to-copolymer ratios. In the former case the monomer dipole moment is increased by 50\% (from 2 to 3 Debye) as the isolated PVDF chains are brought together to form a crystal. In PVDF crystals containing copolymers we observe polarization enhancement, compared to pure PVDF, at copolymer concentrations of 10-16\%. In general, our calculations show that polarization in electroactive polymers is described by cooperative, quantum-mechanical interactions between polymer chains, which cannot be viewed as a superposition of rigid dipoles. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W26.00004: Theoretical proposal and investigation of ferroelectric superlattices incorporating head-to-head and tail-to-tail 180$^\circ$ domain walls Xifan Wu, David Vanderbilt In ferroelectric superlattice structures, it is normally assumed that interfaces and ferroelectric domain walls obey electrical boundary conditions involving the matching, or near matching, of the normal component of the electrical polarization across the interface. If this requirement is not met, the interface would normally be metallic, or charged, or both, leading to structures that tend to be unstable. However, the recent development of techniques for atomic-layer epitaxial growth of ferroelectric perovskites opens a novel possibility of synthesizing superlattice structures in which doping layers are intentionally inserted to compensate the polarization charges at the interfaces. For example, using first-principles density-functional methods, we investigate PbTiO$_3$ superlattices in which the polarization alternates from up to down {\it along the growth direction}, and in which the large polarization charges at the 180$^\circ$ head-to-head and tail-to-tail domain walls are compensated by heterovalent substitution (e.g., Sc or Nb substituting all Ti atoms in one layer). We show that it is theoretically possible to construct insulating superlattice structures of this kind, and investigate their novel properties. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W26.00005: A First Principles Study of Graphitic-Like Structures of SiC Ming Yu, C.S. Jayanthi, S.Y. Wu Bulk SiC has many superior inherent properties. These outstanding properties are expected to be enhanced in SiC-based one-dimensional structures. Recently, SiC multiwall nanotubes were synthesized by two experimental groups. However, there is not yet any detailed study on their structures or properties. As a first step towards such studies, we have carried out structural optimization and the calculation of the energetics of SiC graphitic-like structures using the DFT-based VASP. We intentionally started with an initial configuration of buckled SiC graphitic-like sheet to model the possible effect of the dangling bonds associated with Si atoms. To our surprise, the configuration eventually relaxed to a regular flat graphitic- like form. Furthermore, we found that the energy of the relaxed graphitic-like sheet of SiC is only $\sim$0.48 eV/atom higher than that of bulk ?-SiC, indicating that the presence of carbon atoms in the network indeed has a stabilizing effect on the graphitic-like sheet of SiC. A detailed discussion on the structural and electronic properties of SiC graphitic-like structures will also be presented. This study is expected to pave the way for the understanding of structure and properties of SiC-based nanotubes. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W26.00006: First Principles Study of Nanotubes of Boron. Kah Chun Lau, Ranjit Pati, Ravindra Pandey Current trends in miniaturization of electronic devices have motivated a growing interest in various nanoscale structures. Besides carbon nanotubes, boron nanotubes are believed to be another stable ``homonuclear'' nanotubes, which has recently been synthesized. Understanding of the structural stability, electronic properties and chemical bonding of boron nanotubes can be set as another baseline for the evolutionary changes from carbon nanotubes to hybrid C$_x$B$_y$N$_z$ nanotubes to boron-nitride nanotubes. We will present the results on structural stability, electronic properties and chemical bonding of boron nanotubes using first principles periodic approach. Specifically, we will discuss the configurational stability and dependence of the band structure on the diameter of the boron nanotubes. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W26.00007: First-principles simulations at constant electric polarization Oswaldo Dieguez, David Vanderbilt We develop a formalism to perform first-principles calculations for insulators at fixed electric polarization. As shown by Sai, Rabe, and Vanderbilt (SRV),\footnote{N.~Sai, K.M.~Rabe, and D.~Vanderbilt, Phys.~Rev.~B {\bf 66}, 104108 (2002).} such an approach allows one to map out the energy landscape as a function of polarization, providing a powerful tool for the theoretical investigation of polar materials. While the SRV method is only approximate because the effect of electric field is described using low-order Taylor expansions, our method is exact because we use the finite-fields approach of Souza, \'I\~niguez, and Vanderbilt.\footnote{I.~Souza, J.~\'I\~niguez, and D.~Vanderbilt, Phys.~Rev.~Lett.~{\bf 89}, 117602 (2002).} We apply our method both to systems where the ionic contribution to the polarization dominates, and to systems where this is not the case. We show that the SRV method gives rather accurate results in the former case as expected, while the present exact method provides substantial improvements in the latter case. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W26.00008: Adsorption and Diffusion of Pt and Au on the Stoichiometric and Reduced TiO$_2$ Rutile (110) Surfaces Hakim Iddir, Serdar Ogut, Nigel Browning, Mark Disko A comparative first principles pseudopotential study of the adsorption and migration profiles of single Pt and Au atoms on the stoichiometric and reduced TiO$_2$ rutile (110)surfaces is presented. Pt and Au behave similarly with respect to (i) most favorable adsorption sites, which are found to be the hollow and substitutional sites on the stoichiometric and reduced surfaces, respectively, and (ii) the large increase in their binding energy (by $\sim$ 2 eV) when the surface is reduced. Pt, on the other hand, binds more strongly (by $\sim$ 2 eV) to both the stoichiometric and reduced surfaces. The migration profiles of the two metals also display interesting similarities and differences. In particular, although the energy barrier for both Pt and Au diffusion on the stoichiometric surface is rather low (0.1 - 0.2 eV), Pt displays a one-dimensional migration pattern, while Au migration is two-dimensional. In contrast, the energy barrier on the reduced surface is significantly higher ($\sim$ 0.5 eV) for Pt than Au. These results for Pt,Au/TiO$_2$ are discussed in connection with the activity of the two metals and the origin of the strong-metal-support-interaction phenomenon. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W26.00009: Characterization of Silicon Nitride/Lanthanide-Oxide Interfaces at the Atomic Scale by Scanning Transmission Electron Microscopy and Density Functional Theory J. C. Idrobo, A. Ziegler, M. Cinibulk, C. Kisielowski, R. Ritchie, N. Browning, S. Ogut $\beta-$Si$_3$N$_4$ ceramics are good candidates for heat- intensive structure applications. However, the wide use of the material is limited by its brittleness. This can be compensated by the addition of secondary rare-earth oxide phases into the Si$_3$N$_4$ matrix. We investigate the bonding of different rare-earth atoms to the interface between the Si$_3 $N$_4$ matrix grains and the intergranular phase doped with La, Sm, Er, Yb, and Lu. Using atomic resolution Z-contrast imaging and EELS in the STEM and density functional theory, we find, for the first time, how each rare-earth element attaches to the interface differently depending on atomic size, electronic configuration, and the presence of oxygen along the interface. Atomic resolution EELS taken at the interface suggest that the electronic structure of the rare-earth dopants plays a secondary effect on the positioning of the atoms at the interface. The Si $L_{23}$ edge signal from the termination position has Si$-$O-like features indicating that the open hexagonal Si$_3$N$_4$ rings are oxygen terminated. This result is also confirmed by DFT calculations. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W26.00010: Supercell issues in density functional calculations P.A. Schultz, R.M. van Ginhoven, T.R. Mattsson, A.E. Mattsson Simulations within density functional theory (DFT) are a common component of research into the physics of materials. With the broad success of DFT, it is easily forgotten that computational DFT methods invariably do not directly represent simulated properties, but require careful construction of models that are computable approximations to a physical property. Perhaps foremost among these computational considerations is the routine use of the supercell approximation to construct finite models to represent infinite systems. Pitfalls in using supercells (k-space sampling, boundary conditions, cell sizes) are often underappreciated. We present examples (e.g. vacancy defects) that exhibit a surprising or significant dependence on supercells, and describe workable solutions. We describe procedures needed to construct meaningful models for simulations of real material systems, focusing on k-space and cell size issues. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04- 94AL85000. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W26.00011: Optimal Cluster Expansions: Predicting Alloy Thermodynamics with Desired Accuracy Nikolai Zarkevich, Duane D. Johnson We show how to achieve a desired accuracy in first-principles prediction of alloy thermodynamics using an optimal cluster expansion [1], and discuss reliability of error estimates. We illustrate the scaling of computational complexity versus accuracy, and demonstrate that with controlled accuracy a reliably predicted phase-transition temperature converges to the experimental value. We also show there is a simple and rapid means to estimate transition temperatures using the optimal cluster expansion. [1] N.A. Zarkevich and D.D. Johnson, Phys. Rev. Lett. {\bf 92}, 255702 (2004). [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W26.00012: Theoretical Study of Encapsulated Alkali Metal Atoms in Nanoporous Channels of ITQ-4 Zeolite: One-Dimensional Metals and Inorganic Electrides Hong Li, S. D. Mahanti Electronic structure calculations within density functional theory have been carried out in a class of {\it M}-ITQ-4 zeolite ({\it M} = Na, K, Rb, Cs) to understand the competing effects of guest-guest ({\it M}-{\it M}) and guest-host ({\it M}-ITQ-4) interactions. These compounds are known as inorganic electrides because the state of the valence electron of the alkali atom is manipulated by trapping the alkali atoms inside inorganic zeolite channels $^{1,2}$. We find that the arrangements of alkali atoms in the ITQ-4 zeolite channel change dramatically in going from Cs to Na. In Na-ITQ-4, the Na atoms form a nearly perfect 1D metal undergoing Peierls distortion and concomitant dimerization. However, in Cs-ITQ-4, the Cs atoms form a zig-zag chain and couple rather strongly to the host. The calculated geometry for Cs-ITQ-4 zeolite is in very good agreement with the pair distribution function (PDF) measurement $^3$. Optical absorptions have also been calculated which are in qualitative agreement with experiment. In addition to the guest-host high energy excitations ranging from 0.54 eV to 2.10 eV, we also find an infrared peak at 3300 nm, which should be carefully tested by experiments.\\ 1. A. S. Ichimura {\it et. al.}, J. Am. Chem. Soc. {\bf 124}, 1170 (2002).\\ 2. D. P. Wernette, {\it et. al.}, Chem. Mater. {\bf 15}, 1441 (2003).\\ 3. V. Petkov {\it et. al.}, Phys. Rev. Lett. {\bf 89}, 075502 (2002). [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W26.00013: Geometric simulation of structures containing rigid units Stephen Wells Much insight into the behaviour of the framework silicates can be obtained from the Rigid Unit model. I review results from geometric analyses [1] of framework structures, quantifying the significance of rigid unit motion in thermal disorder and in defect accomodation, and from a method of simulation [2,3] based on a whole-body `geometric potential' rather than on interatomic potentials. I show the application of the geometric potential to the symmetry-constrained generation of hypothetical zeolite frameworks [4], and to the rapid generation of protein conformations using insights from rigid cluster decomposition [5]. 1. Wells, Dove and Tucker, Journal of Applied Crystallography, 37:536--544 (2004). 2. G.D. Gatta and S.A. Wells, Phys. Chem. Min. 31:1--10 (2004). 3. A. Sartbaeva, S. A. Wells, S. A. T. Redfern, J. Phys.: Condens. Matter 16, 8173 (2004) 4. M. M. J. Treacy, I. Rivin, E. Balkovsky, K. H. Randall and M. D. Foster, Micropor. Mesopor. Mater. 74, 121-132 (2004). 5. M.F. Thorpe, Ming Lei, A.J. Rader, Donald J. Jacobs, and Leslie A. Kuhn, Journal of Molecular Graphics and Modelling 19, 1:60 - 69, (2001). [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W26.00014: A theoretical measure for deformation twinning in FCC metals Noam Bernstein, Ellad Tadmor The factors that control plasticity in ductile crystals are not well understood. For example the competition between slip (dislocation motion) and deformation twinning is usually quantified by its empirically observed correlation with the stacking fault energy. However, experimental measurements show significant scatter from this correlation, and materials such as Al fail to follow the nominal trend. We describe a theoretical measurement for the tendency of a material to deformation twin in terms of parameters that can be computed atomistically. We use a quantum-mechanical tight-binding method to compute this twinnability parameter for 8 fcc metals. The ordering of these materials by twinnability agrees with available experimental evidence. Our physically motivated, atomistically based criterion allows us to explain the low incidence of deformation twinning in Al, and predicts that Pd should twin as easily as Cu. [Preview Abstract] |
Thursday, March 24, 2005 5:18PM - 5:30PM |
W26.00015: A First-principles Investigation of Superionic Behavior: $\alpha$-AgI as a Case Study Brandon Wood, Nicola Marzari First-principles molecular dynamics simulations are well suited to the study of conductivity in superionic solids, thanks to the relatively high frequency of observable diffusion events. AgI offers a paradigmatic example of superionic behavior and as such presents itself as an excellent candidate for our investigation. We combine results from Car-Parrinello simulations of $\alpha$-AgI with static and linear-response calculations in order to characterize various structural, dielectric, and vibrational properties of the system. The dynamics simulations offer a unique and unbiased characterization of the atomistic mechanisms involved in fast-ion diffusion, including a statistical analysis of most-frequented conduction pathways and sublattice ordering; and direct examination of the maximally localized electronic orbitals offers insight into chemical interactions present between ionic sublattices. We are further able to extract information on the nature of the superionic phase transition, including an investigation of possible thermodynamic and structural motivations for superionic behavior. [Preview Abstract] |
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W26.00016: Excited spin multiplets of magnetic molecules from a DFT-based many-spin Hamiltonian Kyungwha Park, Mark Pederson, C. Stephen Hellberg It has been recently demonstrated within spin-polarized density-functional theory that the electronic structure and magnetic properties of large magnetic molecules consisting of up to 200 atoms can be calculated and that the calculated ground-state properties agree well with experiment. However, the excited spin multiplets of magnetic molecules, due to strongly coupled metal ions, are not well understood especially for the case with large magnetic anisotropy. In this talk, we address the necessity of using a many-spin Hamiltonian to explore the excited spin multiplets and present our method to model the Hamiltonian for an isolated Mn$_{12}$-acetate molecule using spin-polarized density-functional theory. We identify the total magnetic moments of low-lying excited spin multiplets by diagonalizing the model Hamiltonian (dimension of $10^8 \times 10^8$). We also show our calculated energy gaps between the excited and the ground spin multiplets and predict magnetic anisotropy barriers of the spin multiplets. Finally, we compare our results with experimental data. [Preview Abstract] |
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