Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A6: Frontiers of Computational Materials |
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Sponsoring Units: DCOMP Chair: Stefano Curtarolo, Duke University Room: Baltimore Convention Center 310 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A6.00001: Multiscale Modeling of Solidification Microstructure: Atomic-Scale Simulations of Crystal-Melt Interfaces and Beyond Invited Speaker: |
Monday, March 13, 2006 8:36AM - 9:12AM |
A6.00002: Reliable First-Principles Alloy Thermodynamics via Optimal, Truncated Cluster Expansions Invited Speaker: Cluster expansions (CE) are increasingly used to combine first- principles electronic-structure structural formation energies and Monte Carlo methods to predict phase stability in alloys, and search for new materials. As a basis-set expansion in terms of lattice geometrical clusters and effective cluster interactions, the CE is exact if infinite, but is tractable only if truncated. We present an optimal truncation for CE basis sets that provides reliable thermodynamics and is easy to implement in mulitcomponent alloys, whereas former truncations were not well defined and sometimes led to unreliable results. We discuss predictive error estimation, error estimation of temperature prediction, Rayleigh- Ritz variation errors associated with basis set truncation for both concentration-dependent and independent version (similar to local compact support in finite-element methods), as well as a means for rapid assessment of transition temperatures without performing Monte Carlo. We exemplify all of the issues in various binary alloys. A Thermo Toolkit (TTK) that automates construction of the optimal, truncated CE, generation of linear-independent unit-cell structures, electronic-structure job submission for the required unit-cell, collection into database, and ultimately Monte Carlo construction of phase diagram is exemplified also. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:48AM |
A6.00003: Realistic nanostructures from first-principles: fluxional handles to control the conductance of carbon nanotubes Invited Speaker: We have combined large-scale, $\Gamma-$point electronic-structure calculations and the maximally-localized Wannier functions approach to calculate efficiently and with full first-principles accuracy the electronic structure and the quantum conductance of complex systems containing thousands of atoms. This approach is applied to study covalent functionalizations in metallic single-wall carbon nanotubes. We find that for most covalent ligands (from hydrogens to aryl moieties) the electronic structure around the Fermi energy is much less dependent on the chemical nature of the ligands than on the $sp^3$ functionalization pattern disrupting the conjugation network. These covalent functionalizations are more stable when paired with saturating hydrogens. Even when paired, they still act as strong scattering centers that degrade the conductance of the tubes already at low degrees of coverage. Instead, we find that cycloadditions of carbenes or nitrenes can preserve metallicity to an unusually high degree, whenever bond cleavage between two sidewall carbon atoms is induced. This process restores the original $sp^2$ hybridization for the sidewall carbons and preserves, even in the presence of significant distortions, the original ``transparency'' of the $\pi$ manifold. The chirality and curvature of the nanotube and the chemistry of the addends determine this bond cleavage and in turn the transport properties. Remarkably, a well-defined range of diameters can be found for which certain addends - such as dicyanocarbene - exhibit a bistable switchable state, where the opening or closing of the bond, and thus the opening and closing of the conduction channels, could be directed with chemical, electrochemical or optical means. This discovery opens the way to novel and promising routes to control and modulate nanotube conductance, with applications ranging from sensors to memories to opto- and nano-electronics. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:24AM |
A6.00004: Evolutionary approach for determining first-principles model Hamiltonians Invited Speaker: The ability to perform accurate solid-state calculations based completely on first principles (for relatively small unit cells) has made it possible to develop model Hamiltonians that can be rapidly ``searched'' for optimal target properties---i.e., true materials-by-design. Recent applications include ferroelectric properties and band-gap engineering. The most difficult step in ``training'' such model Hamiltonians is making choices for the number and types of parameters in the model that insure the \emph{predictive} power of the model. Based on an evolutionary approach, we have developed an algorithm\footnote {Gus~L.~W.~Hart, V.~Blum, M.~J.~Walorski, and A.~Zunger, Nature Materials \textbf{4} 391 (2005); V.~Blum, Gus~L.~W.~Hart, M.~J.~Walorski, and A.~Zunger, Phys. Rev. B \textbf{72}, 165111 (2005).} for selecting the types and number of terms in a Cluster Expansion model for a binary alloy. This approach removes much of the tedium of constructing the model and robustly finds the best possible set of parameters. The approach is general and can be applied to a wide variety of other models as well. I illustrate the success of the new approach first on systems where the best parameter set is known analytically, and second, as applied to several recent ``real-world'' examples, including (1) the role of long-period-superlattices in the Cu-Pd system, (2) predicting configuration-dependent bulk-moduli in transition- metal carbides and nitrides, (3) predicting optimal superlattice stacking/orientations to engineer desired band-gaps in MgO-ZnO wide gap alloys. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 11:00AM |
A6.00005: Prediction of new crystal structure phases in metal borides Invited Speaker: Identification of novel crystal structures is an important step for predicting new stable compounds in alloys, since most theoretical search algorithms are restricted to a given prototype library or a lattice type. Performing {\it ab initio} data mining [1] of intermetallic compounds we have discovered that even in such a well-studied class of systems as metal borides there are previously unknown phases comparable in energy to the existing ones [2]. We demonstrate that even though the new structures are relatively simple, their identification is not straightforward. We systematically investigate the stability and electronic properties of the new metal boride phases. Our calculations show that some phases exhibit electronic features similar to those in the famous MgB$_2$ and could be good superconductors. The new phases are likely to have random stacking faults, so they might not be detected with standard x-ray methods. Our results could thus be used as an important guide in the search for new superconducting metal borides. [1] S. Curtarolo {\it et al.}, Phys. Rev. Lett. {\bf 91}, 135503 (2003). [2] A.N. Kolmogorov {\it et al.}, submitted (2005). [Preview Abstract] |
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