Bulletin of the American Physical Society
2006 Texas Section of the APS Joint Fall Meeting
Thursday–Saturday, October 5–7, 2006; Arlington, Texas
Session CMP1: Condensed Matter I |
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Chair: Alex Weiss, University of Texas at Arlington Room: UT Arlington, University Center Concho |
Friday, October 6, 2006 10:30AM - 10:42AM |
CMP1.00001: A Hybrid Density Functional Study of Armchair SiC Nanotubes Kazi Alam, Asok Ray As a continuation of our previous work on SiC nanoclusters, we report here first principles calculations on the electronic and geometric structures of armchair silicon carbide nanotubes from (3,3) to (9,9). The finite cluster approach with dangling bonds terminated with hydrogen has been used. The theoretical formalism used is hybrid density functional theory incorporating Hartree-Fock exchange with density functional theory exchange-correlation. In particular, we have used the B3LYP hybrid functional and the Los Alamos pseudopotential LANL2DZ as implemented in the Gaussian 03 suite of programs. For silicon, the 1s, 2s, and 2p electrons have been treated as core states and the remaining electrons as valence states. For carbon and hydrogen, all electron basis sets have been used. A detailed comparison of the structures and stabilities of the nanotubes has been performed and the dependence of the electronic band gaps on the respective tube diameters has also been investigated. Results will be compared with other published data in the literature. [Preview Abstract] |
Friday, October 6, 2006 10:42AM - 10:54AM |
CMP1.00002: Structural and electronic properties of the germanium-based clathrates Ge$_{46}$ and Ba8Ge$_{46}$ Emmanuel Nenghabi, Charles Myles Semiconductor clathrates have been of considerable interest in recent years. This is mainly due to structural characteristics that give rise to unique electrical and thermal transport properties. There are two structural clathrate phases, Type I and Type II. Here, we focus on Ge-based Type I clathrates. The Type I structure is simple cubic, with 46 atoms per unit cell. It has 20-atom cages and 24-atom cages combined in a 2 to 6 ratio. The cages can host ``guests''; usually alkali or alkaline earth atoms. Using the local density approximation, we have studied the structural and electronic properties of the pristine Type I clathrate Ge$_{46}$ and the Ge clathrate with all cages filled with Ba guests; Ba$_{8}$Ge$_{46}$. The equilibrium lattice structures were obtained by ab initio pseudo potential calculations combined with dynamic minimizations. As found by others [1], the pure Ge$_{46}$ clathrate structure has a slightly higher total energy than that of diamond phase Ge. Further, band structure calculations show that the this material is a semiconductor with a fundamental band gap that is considerably larger than that of the diamond phase. We find that Ba$_{8}$Ge$_{46}$ is metallic with the conduction bands slightly modified by the Ba guest atoms in the clathrate cages. \newline \newline [1] J. Dong and O. F. Sankey, J. Phys: Condens. Matter 11(1999)-6129-6145 [Preview Abstract] |
Friday, October 6, 2006 10:54AM - 11:06AM |
CMP1.00003: Heat capacity anharmonicity in elemental vanadium and V93Co7 Jorge Munoz, Olivier Delaire, Brent Fultz The specific heats of elemental vanadium and V93Co7 with body-centered cubic crystal structure were measured by differential scanning calorimetry from room-temperature to 1400°C. The results were compared to the harmonic phonon contribution, the electronic contribution, and the expansion against the bulk modulus contribution to the total heat capacity. The harmonic phonon contribution was obtained from the phonon densities-of-states, measured using inelastic neutron scattering. The electronic contribution to the heat capacity was obtained from ab-initio electronic band structure calculations. For the thermal expansion contribution, the room-temperature bulk and shear moduli were calculated after sound velocity measurements done by ultrasonic transduction; the coefficients of thermal expansion were measured from room-temperature up to 700°C using thermomechanical analysis. The study shows that the source of the heat capacity anharmonicity in the measured samples is phonon-phonon interaction. [Preview Abstract] |
Friday, October 6, 2006 11:06AM - 11:18AM |
CMP1.00004: ABSTRACT WITHDRAWN |
Friday, October 6, 2006 11:18AM - 11:30AM |
CMP1.00005: Theoretical Investigation of Nonradiative Transitions in Polysiloles Meghann Palmer, Murilo Tiago, Alex Demkov Silacyclopentadienes, or siloles, exhibit aggregation-enhanced photoluminescence. Quenching of luminescent nano-aggregates by electron deficient nitroaromatic explosives, such as trinitrotoluene, may be monitored to detect explosives [1]. Detection relies on both electronic and structural interactions between the sensing material and the analyte. These interactions can be studied using state-of-the-art theoretical methods thus aiding the screening of potentially useful polymers. In particular, we consider 2,3,4,5-tetraphenylsiloles which have been shown experimentally to be only slightly luminescent as individual monomers in solution and highly luminescent when aggregated into oligomers [2]. Using density functional theory along with quasi-particle methods we investigate the electronic structure of the molecule and its luminescent properties. We consider the possibility that the relatively unhindered motion of the phenyl rings may provide a nonradiative transition channel. \newline \newline [1] H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, \textit{J. Am. Chem. Soc.} \textbf{125}, 3821 (2003). \newline [2] J. Luo, \textit{et. al.}, \textit{Chem. Communications}, 1740 (2001). [Preview Abstract] |
Friday, October 6, 2006 11:30AM - 11:42AM |
CMP1.00006: Extended Electron States in DNA Molecules Kemal Bagci An important class of disordered 1D conductor is the DNA macromolecules. A DNA strand is a random sequence of four nucleotides (guanine G, adenine A, cytosine C, and thymine T). For a conduction electron the sequence of nucleotides is mapped into the sequence of sites with on-site energies, \textit{$\varepsilon $}$_{A}$ = 8.24 eV, \textit{ $\varepsilon $}$_{T }$= 9.14 eV, \textit{$\varepsilon $}$_{C}$ = 8.87 eV, and \textit{$\varepsilon $}$_{G}$ = 7.75 eV. Due to the overlapping of the electron orbitals, there is hopping transport between the neighboring sites. The hopping constant is about $t\le 1$eV. Since the potential profile fluctuates randomly, the electron diffusion along the chain is limited by localization length $l(E)$. However, the long-range correlations may give rise to a continuum of extended states. These states may provide long-distance transport. Here we report the results of our analytical study of the localization length of DNA. We have generalized the analytical result in Ref.[1] to two channel systems. The method is based on the generalized result, where $l(E)$ is related to the binary correlation function of the nucleotide sequence. We calculated localization for DNAs, which exhibit long-range and oscillatory correlations, those that may possess the mobility edge.. Our study shows that the structure of the correlations in some of the human DNA's favors formation of the sharp mobility edges in the energy spectrum. [1] F.M. Izrailev, A.A. Krokhin, Phys. Rev. Lett. \textbf{ 82,} 4062 (1999). [Preview Abstract] |
Friday, October 6, 2006 11:42AM - 11:54AM |
CMP1.00007: Debye Model of Steps on Vicinal Crystal Surfaces Howard L. Richards, Clint A. Greene The steps on a vicinal crystal surface can be mapped onto the world lines of spinless fermions, with the average direction of the steps (the $y$-direction) being mapped to time. If the interaction energy per unit length between neighboring steps is given by $V(L) \! = \! A/L^2$ (as is common), this resulting quantum system is integrable for only three values of $\tilde {A} \! \equiv \! \tilde{\beta}A/(k_{\rm B}T)^2$. For other values of $\tilde{A}$, the Pairwise Einstein Model gives an excellent approximation for the Terrace Width Distribution (TWD, the histogram of $x_{i+1}(y)-x_{i}(y)$) but is severely limited in describing $g_x(\Delta y) \! \equiv \! \langle [x_i (y+\Delta y) - x_i(y)]^2\rangle$, particularly for $\Delta y \! > \! \xi$, the correlation length. Here we show how the one- dimensional Debye model correctly gives $g_x(\Delta y)$ even for large $\Delta y$. The Pairwise Einstein Model also suggests a relationship between the compressibility of the steps and the tails of the TWD, a relationship we clarify using the Debye model. [Preview Abstract] |
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