Bulletin of the American Physical Society
Annual Meeting of the Four Corners Section of the APS
Volume 55, Number 9
Friday–Saturday, October 15–16, 2010; Ogden, Utah
Session D2: Computational Condensed Matter |
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Chair: J.R. Dennison, Utah State University Room: 404A |
Friday, October 15, 2010 3:30PM - 3:42PM |
D2.00001: Approximation of Range in Materials as a Function of Incident Electron Energy Gregory Wilson, J.R. Dennison The range, or maximum distance an electron of a given incident energy can penetrate through a material before all kinetic energy is lost and the electron comes to rest, is a common way to parameterize electron interactions with materials. We have developed a simple composite analytic expression to approximate the range, R using standard materials properties (e.g., density, atomic number, atomic weight, stochiometry). This is accomplished by fitting tabulated values from the NIST ESTAR database with well established relativistic semi-empirical models for the high energy range (greater than 30 keV) and relating this to the NIST IMFP (inelastic mean free path) database for lower energy ranges (less than 10 meV) by using the continuous slow down approximation (CSDA). The resulting function is valid for most conducting, semiconducting and insulating materials using a single fitting parameter and represents a large increase in estimated range extending over more than seven orders of magnitude in energy (approximately 3 eV to 10 MeV) and range (1 nm to 1 cm), with uncertainty of less than approximately 20 percent. [Preview Abstract] |
Friday, October 15, 2010 3:42PM - 3:54PM |
D2.00002: Computational Study of Carbon Nanotubes Embedded in Silicon Jaren Norris Inspired by recent experimental success in coating carbon nanotubes with silicon, I am computationally modeling carbon nanotubes embedded in silicon using density functional theory. I am studying mechanical, electrical and optical properties as a function of nanotube size and orientation in the silicon lattice. [Preview Abstract] |
Friday, October 15, 2010 3:54PM - 4:06PM |
D2.00003: Cluster Expansion for Pt/Pd-Al binary alloys Derek Carr Pure platinum and pure palladium are too soft for typical jewelry applications. Adding small amounts of other metals can significantly increase their performance. However, international hallmarking standards require the alloys to be 95{\%} pure by weight. How does one achieve significant improvements in performance adding only small amounts (5 wt-{\%}) of other metals? Significant improvements are possible with small additions when the added element forms an ordered array in the Pt/Pd matrix. Our task is to identify, among an infinite set of possibilities, arrangements that are stable and which will form easily. One solution is to use a cluster expansion. A cluster expansion is a fast method which can calculate the energy of all candidate crystal superstructures. Using the cluster expansion, we identify the ``ground states,'' the atomic arrangements that are the most stable. After the ground states are identified, Monte Carlo simulations are used to predict the order-disorder transition temperatures. The transition temperatures indicate the feasibility of making the ordered alloys in the laboratory. [Preview Abstract] |
Friday, October 15, 2010 4:06PM - 4:18PM |
D2.00004: Novel Occurences of L1$_1$ and L1$_3$ found using the synnergy between High Throughput and Cluster Expansion Lance Nelson, Gus Hart, Stefano Curtarolo Despite their geometric simplicity, L1$_1$(CuPt) and L1$_3$ (CdPt$_3$) fail to appear as groundstates in experimental systems. ( L1$_1$ appears in CuPt only) Are these crystal structures actually energetically unfavorable, or have they simply been overlooked in experimental studies? Here we investigate, using computational methods, the energetic stability of these phases in all binary inter-metallic systems. We combine the results of two techniques,namely High Throughput (HT) and Cluster Expansion (CE), to maximize efficiency and ensure thoroughness. HT results show L1$_1$(L1$_3$) to be stable in the following systems: AgPd, AgPt, CuPt, PdPt(CdPt,CuPt,PdPt,LiPd,LiPt). Cluster expansions constructed for these systems verify the HT findings in all cases, with the exception of the HT groundstate PdPt-L1$_1$.(D$_4$ is found to be energetically more favorable) Monte Carlo simulations, which are used to identify order-disorder transition temperatures, were performed for all occurences of these two phases. While the transition temperatures for some systems are found to be extremely low, others appear to be physically realizable. [Preview Abstract] |
Friday, October 15, 2010 4:18PM - 4:30PM |
D2.00005: EFG Component Distribution Functions in Inhomogeneous Broadening in PAC Spectroscopy Mike Adams, P. Matheson, W.E. Evenson, M.O. Zacate Perturbed Angular Correlation (PAC) spectroscopy is used to study the distribution and mobility of defects within crystals. The angular correlation of multiple gamma rays emitted from probe nuclei, affected by the net electric field gradient (EFG) in a probe's vicinity, are used to produce the PAC spectrum, G$_{2}$(t). The distribution of EFGs from many random defects in a crystal, results in inhomogeneous broadening (IHB) of G$_{2}$(t). Our EFG component probability distribution functions are found by summing 20,000 net EFGs, each found from taking a random distribution of vacancies of a particular concentration, combined with a single trapped vacancy in a near neighbor position to a probe nucleus. The derived EFG component distributions allow us to reconstruct the G$_{2}$(t) as a function of defection concentration. The EFG component distribution functions are characterized by weighted sums of either Gamma, Lorenztian or Gaussian distributions. A systematic change in the type and number of distribution functions required to model IHB is apparent as defect concentration increases. In particular, the EFG distributions become increasingly skewed with increasing defect concentration. Results for the EFG components in simple cubic (SC), face-centered cubic (FCC) and body-centered cubic (BCC) lattices are presented. [Preview Abstract] |
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