Bulletin of the American Physical Society
74th Annual Meeting of the Southeastern Section
Volume 52, Number 13
Thursday–Saturday, November 8–10, 2007; Nashville, Tennessee
Session GA: Computational Science |
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Chair: Marco Buongiorno-Nardelli, North Carolina State University Room: Scarritt-Bennett Center Laskey Great Hall |
Friday, November 9, 2007 8:30AM - 9:00AM |
GA.00001: First Principles Search for New Superconducting Layered Borides Invited Speaker: The identification of novel crystal structures is a fundamental step for predicting new stable compounds in alloys. While performing ab initio data mining of intermetallic compounds [1], we discover a new family of layered metal borides [2], of which MgB$_{2}$ is one particular element (the new phases are called Metal Sandwich (MS)). Thermodynamic stability and electronic properties of these MS phases are investigated in details, leading to the prediction of a hypothetical novel superconductor MS-LiB [2,3]. Calculations show that the MS phases in the Li-B system exhibit electronic features similar to those of MgB$_{2}$ [2,3] and CaC$_{6}$ [4]. Although the predicted critical temperature of LiB is lower than that of MgB$_{2}$ (references [4] and [5] for MS2-LiB and MS1-LiB, respectively), the peculiarities of MS-LiB in terms of electronic structure, layer arrangements and doping capabilities allow a lot of freedom in the search for higher T$_{c}$ systems [5,6]. We acknowledge the Teragrid-Partnership for computational resources. Research supported by ONR and NSF. [1] Phys. Rev. Lett. \textbf{9}1, 135503 (2003). [2] Phys. Rev. B \textbf{73}, 180501(R) (2006). [3] Phys. Rev. B \textbf{74}, 224507 (2006). [4] Phys. Rev. B \textbf{75}, 064510 (2007). [5] Phys. Rev. B \textbf{75}, 144506 (2007). [6] A. N. Kolmogorov, M. Calandra, and S. Curtarolo, \textit{Engineering superconductors with ab initio methods: ternary metal borides}, (2007). [Preview Abstract] |
Friday, November 9, 2007 9:00AM - 9:30AM |
GA.00002: Hole Transfer in Hydrated DNA Driven by Spatial Correlated Solvent Fluctuations Invited Speaker: We use a combination of computational approaches to address the transport properties of an electron hole in hydrated DNA. Molecular dynamics simulations are employed to study at the base level the nature and the strength of the solvent-induced electrostatic fluctuations. A hybrid first-principles/molecular-mechanics scheme designed for periodic calculations of biological systems is then used to determine the hole energy at DNA bases embedded in hydrated DNA chains. Thus, the combined effect of solvent fluctuations and sequence on energetics and transfer characteristics is addressed through the use of 1-D lattice models. In this study, we find that stacking and hydration introduce at each base normal energy deviations correlated in both space and time. Solvent-induced energy disorder yields localized hole states and supplies the driving force for transfer processes over distances exceeding the correlation length. Sequence, on the other hand, is found to determine the transfer rate across short DNA traits. Our investigation shows that correlated solvent fluctuations and sequence compete evenly to define hole energetics and transfer character in hydrated DNA. [Preview Abstract] |
Friday, November 9, 2007 9:30AM - 10:00AM |
GA.00003: A computational study of the role of defects and single molecules on transport properties in low dimensional systems Invited Speaker: Nanoscale electronic devices have been extensively investigated as a new frontier beyond conventional microelectronics. At the same time as the typical size of practical devices is shrinking, there is a tremendous expansion of the available computational resources, in terms of scalability and speed. The result is that we are now rapidly approaching the point where the typical length scales of systems available experimentally are becoming similar to the ones that can be treated accurately on state-of-the-art computers. In this talk, I will present two recent examples where large-scale calculations have been used to understand and predict novel phenomena at the molecular and nanoscale. In the first illustration, I will show how a combination of scanning tunneling microscopy measurements and large-scale density functional theory calculations can be used to elucidate the fundamental role and formation process of defects on TiO$_{2}$ (110) surface. In the second part of the talk, I will show how it is possible to couple large-scale quantum electronic structure calculations with non-equilibrium Green function formulation for determining the quantum conductance of a number of molecular systems The switching behavior in systems based on individual molecules embedded in a conducting nanotube is analyzed in detail and a novel paradigm for nanoscale non-volatile memory element is presented. [Preview Abstract] |
Friday, November 9, 2007 10:00AM - 10:30AM |
GA.00004: Simulations of Li ion diffusion in the electrolyte material -- Li$_3$PO$_4$ Invited Speaker: Solid-state lithium ion electrolytes are becoming increasingly important in batteries and in related technologies. We have used first-principles modeling techniques based on density functional theory and the nudged elastic band method to examine possible Li ion diffusion mechanisms in terms of their migration energies $E_m$. Simulations were performed in idealized crystals of the electrolyte material Li$_3$PO$_4$, considering both vacancy and interstitial processes. We find that an ``interstitialcy'' mechanism, involving the concerted motion of an interstitial Li ion and a neighboring lattice Li ion, is likely to provide the most efficient ion transport in Li$_3$PO$_4$. Ion transport in pure crystals involves the formation of vacancy-interstitial pairs requiring an additional energy $E_f$, resulting in a thermal activation energy of $E_A=E_m+E_f/2$. Calculated values of $E_A$ are in excellent agreement with single crystal experiments on $\gamma$-Li$_3$PO$_4$. Our simulations examine similarities and differences between diffusion processes in the $\gamma$ and $\beta$ crystal structures. In addition, we analyze zone center phonon modes in order to further validate our calculations with available experimental measurements and to determine the range of vibrational frequencies associated with Li ion motion. [Preview Abstract] |
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