Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K31: Focus Session: Simulation of Complex Materials III |
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Sponsoring Units: DCOMP DMP Chair: Alberto Franceschetti, National Renewable Energy Laboratory Room: Baltimore Convention Center 328 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K31.00001: Integrated Density of States Lloyd's Formula for Disordered Alloys with Short-Range Order via a KKR-based Dynamical Cluster Approximation D. A. Biava, Subhradip Ghosh, W. A. Shelton, D. D. Johnson Within Korringa, Kohn and Rostoker (KKR) multiple scattering theory, we have formulated {\footnote{D.A. Biava et al, Phys. Rev. B 72 113105 (2005)}} a static version of the Dynamic Cluster Approximation (DCA) {\footnote{M. Jarrell and H R Krishnamurthy, Phys. Rev. B 63 125102 (2001)}}, which is a cluster (or non-local) generalization of the coherent potential approximation (NLCPA) {\footnote{D.A. Rowlands et al, Phys. Rev. B 67 115109 (2003)}} that includes environmental effects, including short-range order. Within our KKR-DCA/NL-CPA we present an analytic expression for configurationally-averaged integrated density of states $<$$N(E)$$>$, or generalized Lloyd's formula. We show also that this Lloyd's formula is stationary with respect to changes in the DCA/NLCPA effective medium, providing a rigorous electronic density functional theory and total-energy formalism for (partially) disordered alloys with(out) short-range order that is exact for infinite sized clusters and reduces to the single-site CPA for a cluster size of one. We show applications to various alloys with(out) short-range order. [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K31.00002: New modeling approach for the deformation of fractal hierarchical structures. Catalin Picu, Monica Soare A new formulation is proposed to address the deformation of multiscale hierarchical structures with a self-similar geometry across scales for which the classical formalism of continuum mechanics either does not apply or it is too expensive from computational point of view. Examples of such structures are porous materials (rocks, aerogels, glasses) in which pores with a wide range of dimensions are found, many biological materials and some of the newly developed nanostructured hierarchical composites. To this end, the equations of solid mechanics are reformulated to include information about the geometry. The procedure to solve boundary value problems with the new formulation will be presented. Examples will be discussed in which the variation of the solution with the number of scales in the hierarchy is studied. [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K31.00003: Medium-Range Structural Properties of Vitreous Germania Obtained through First-Principles Analysis of Vibrational Spectra Luigi Giacomazzi, Paolo Umari, Alfredo Pasquarello We analyse the principal vibrational spectra of vitreous GeO$_2$ and derive therefrom structural properties referring to length scales beyond the basic tetrahedral unit. We generate a model structure which yields a neutron structure factor in accord with experiment. The inelastic-neutron, the infrared, and the Raman spectra, calculated within a density-functional approach, also agree with respective experimental spectra. The accord for the Raman spectrum supports a Ge-O-Ge angle distribution centered at 135$^\circ$. The Raman feature $X_2$ is found to result from vibrations in three-membered rings, and therefore constitutes a distinctive characteristic of the medium-range structure. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:42PM |
K31.00004: Predicting complex ground state structures from first principles: genetic algorithm for finding accurate coarse-grained Hamiltonians Invited Speaker: First-principles quantum-mechanical (QM) calculations allow to evaluate many interesting properties of a \emph{given} nano-scale configuration of atoms with high accuracy. However, \emph{predicting} stable structures or finite-$T$ thermodynamic configurational averages with QM accuracy remains a challenge: even for a binary solid with $N$ atoms per unit cell, $2^N$ distinct configurations must be evaluated. Such large numbers of calculations can be made affordable by mapping the QM Hamiltonian onto a computationally simpler ``coarse-grained'' Hamiltonian. The ability to predict ground state structures then depends on the shape of the coarse-grained Hamiltonian, but this shape is not a priori clear. For instance, a few simple assumed generic interactions will allow only a few simple ground states, but any more complex structures will be missed. For the generalized Ising model for binary alloys (cluster expansion), I show how a genetic algorithm$^{1,2}$ can identify the leading interactions which characterize a given system. I illustrate the method for the bcc binary alloys of Nb, Ta, Mo, W. A rich spectrum of ground state structures is found, including both well-known and unsuspected complex structures, far beyond what is envisioned from ``usual-suspect'' structure listings or from simple generic interactions. At the same time, order-disorder temperatures are significantly lower than those from simple intuition-based interactions, in agreement with experimental observations for these systems. This work was done at the National Renewable Energy Laboratory, supported by DOE-SC-BES, in collaboration with A. Zunger and G. Hart. \\ $^1$G. Hart, V. Blum, M. Walorski and A. Zunger, Nature Materials \textbf{4}, 391 (2005); $^2$V. Blum, G. Hart, M. Walorski and A. Zunger, Phys. Rev. B \textbf{72}, 165113 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K31.00005: Accurate prediction of x-ray absorption spectra using density functional theory Giulia Galli, David Prendergast The increasing availability of x-ray absorption spectroscopy measurements for materials in the condensed phase is providing new opportunities to explore the local structure of disordered materials. However, the spectra produced by such experiments rely heavily on theoretical interpretation to infer the underlying atomic structure. We make use of density functional theory calculations to accurately approximate the initial and final state electronic structure associated with the absorption of an x-ray photon in the condensed phase. We outline some efficient computational approaches applied to ordered and disordered systems, concentrating on the K-edge absorption of oxygen. We report simulated x-ray absorption spectra for ice I, liquid water and magnesium oxide. Our results indicate excellent agreement with experiment for ice I and magnesium oxide, and our results for water, modelled using the classical TIP4P potential, indicate a reasonable qualitative agreement with experiment. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K31.00006: A First-principles Investigation of Superprotonic Activity in CsHSO$_4$ Brandon Wood, Nicola Marzari With the recent push to realize the hydrogen economy, there has been a surge of interest in viable solid-state materials for use as fuel cell electrolytes. Among the candidates are various derivatives of the anhydrous superprotonic conductor CsHSO$_4$, which exhibits reasonably high ionic conductivity at relevant operating temperatures while remaining electronically insulating. However, despite being widely characterized experimentally, a truly atomistic picture of the diffusion pathways and mechanisms in the material is missing. To this end, we have characterized this material with extensive first-principles static and dynamical calculations on 112-atom supercells of CsHSO$_4$. We isolate the dominant atomistic mechanisms involved in the superprotonic behavior and discuss the effect of correlated diffusive events in enhancing proton transport. We also offer a detailed description of the dynamics of the hydrogen bond network topology as the diffusing protons propagate through the lattice structure. The role of our findings in understanding superionic behavior are discussed. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K31.00007: First-principles study of the Hume-Rothery electron concentration rule in Al-Cu-(Fe,Ru)-Si 1/1-cubic approximants Ryoji Asahi, O.Y. Kontsevoi, U. Mizutani, T. Takeuchi, A.J. Freeman To elucidate the Hume-Rothery electron concentration rule, we determined the self-consistent electronic structures of the Al$_{108}$Ru$_{24}$Cu$_{6}$Si$_{6}$ and Al$_{108}$Fe$_{24}$Cu$_ {6}$Si$_{6}$ 1/1-1/1-1/1 approximants containing 144 atoms in each \textit {Pm}-3 cubic unit cell using the full-potential linearized augmented plane wave (FLAPW) method [1], now running on massively parallel computer platforms. A significant pseudogap was found around the Fermi level for both alloys in the calculated densities of states, which should contribute to stabilization of the system. The FLAPW wave functions provide a direct observation of the Brillouin zone resonance in the Fermi surface [2]: a Fourier analysis of the wave functions confirms the Hume-Rothery matching rule 2$k_{F}=K$ where the reciprocal lattice vectors $K$ consist of {\{}543{\}}, {\{}550{\}}, and {\{}710{\}} planes highly degenerate at the $N$ point. Consequently, an effective electron concentration per atom ($e/a)$ was evaluated to be 0.8 for both Ru and Fe in these structures making a sharp contrast with the previously assumed empirical value of -2.7 proposed by Raynor [3]. [1] Wimmer et al., Phys. Rev. B \textbf{24}, 864 (1981). [2] Asahi et al., Phys. Rev. B \textbf{72}, 125102 (2005). [3] Raynor, Prog. Metal Phys. \textbf{1}, 1 (1949). [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K31.00008: High-precision mixed-space cluster expansion for Cu-rich Cu-Pd alloys: Explaining the "$L1_2" phase S. B\"arthlein, S. M\"uller, G.L.W. Hart, A. Zunger A remarkable feature of Cu-Pd alloys is the existence of long-periodic superlattices (LPS) on the Cu-rich side of the phase diagram. Whereas earlier studies did not include necessary information about the diversity of important, but until then inaccessible formation enthalpies, we are able by combining DFT calculations with a mixed-space cluster expansion, genetic algorithms [1] and Monte Carlo to predict the phase stability from millions of possible candidates. Effective interactions were constructed, enabling us to predict the ground state line in order to determine the stable configurations a $T=0$K. As a matter of special interest, we investigate the so-called "$L1_2$" phase, which emerges as a domain-mixture between the LPS3 and a newly discovered low-temperature phase at $x_{Pd}=0.125$. Examination of the system's short-range order reveals a continuous transition from the domain-mixture to the disordered solid solution. Hence a natural explanation for the existence of this off-stoichiometry phase can be given. [1] G.L.W. Hart et al., Nat. Mater. {\bf 4}, 391 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K31.00009: Nanoscale Patterning of L1$_{2}$ Chemical Order in Ni$_{3}$Al Alloy Processed by Energetic Ions Jia Ye, Youhong Li, Robert Averback, Pascal Bellon, Jianmin Zuo We recently predicted that alloys forming chemical ordered phases at equilibrium can be forced into stable nanoscale patterns of chemical order by irradiation with energetic particles. We have tested this prediction on the Ni$_{3}$Al compound by combining MD and KMC simulations. For 1MeV Kr and 70KeV He ions, MD simulation is used to simulate the disordered zones, which are then incorporated into KMC simulations to reach long irradiation times. We introduce a new method based on scaling behavior of structure factor to identify reliably the transition boundary between patterning and disordered states. These simulations indicate that 1MeV Kr ion irradiation can lead to patterning, whereas 70KeV He ion cannot. These results are compared to experimental results, obtained by performing ion irradiations on Ni$_{3}$Al thin films grown by sputtering. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K31.00010: Maximally fast coarsening algorithms Mowei Cheng, Andrew Rutenberg We present maximally-fast numerical algorithms for conserved coarsening systems that are stable and accurate with a growing natural time-step $\Delta t=At_s^{2/3}$. We compare the scaling structure obtained from our maximally-fast conserved systems directly against the standard fixed-timestep Euler algorithm, and find that the error scales as $\sqrt{A}$ --- so arbitrary accuracy can be achieved. For non-conserved systems, only effectively finite timesteps are accessible for similar unconditionally stable algorithms. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K31.00011: Mesoscopic modeling of the response of human dental enamel to mid-infrared radiation Ana Vila Verde, Marta Ramos, A.M. Stoneham Ablation of human dental enamel, a composite biomaterial with water pores, is of significant importance in minimally invasive laser dentistry but progress in the area is hampered by the lack of optimal laser parameters. We use mesoscopic finite element models of this material to study its response to mid-infrared radiation. Our results indicate that the cost-effective, off-the-shelf CO$_{2}$ laser at $\lambda $ = 10.6 $\mu $m may in fact ablate enamel precisely, reproducibly and with limited unwanted side effects such as cracking or heating, provided that a pulse duration of $\approx $ 10 $\mu $s is used. Furthermore, our results also indicate that the Er:YAG laser ($\lambda $ = 2.94 $\mu $m), currently popular for laser dentistry, may in fact cause unwanted deep cracking in the enamel when regions with unusually high water content are irradiated, and also provide an explanation for the large range of ablation threshold values observed for this material. The model may be easily adapted to study the response of any composite material to infrared radiation and thus may be useful for the scientific community. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K31.00012: The isotope effect in the ferroelectric phase transition of KDP using {\it ab-initio} path intergal simulations Varadharajan Srinivasan, Roberto Car, Daniel Sebastiani We perform {\it ab-initio} path integral simulations on protonated and deuterated KDP at different temperatures and lattice constants in order to probe the origin of the isotope effect of the ferroelectric phase transition in this material. By taking into account the quantum nature of the proton/deuteron our simulations are capable of distinguishing the direct effects of a pure mass change versus the indirect structural effect in the hydrogen bonding geometry upon deuteration. In reality, the direct and indirect effects amplify each other in a self-consistent manner, leading to the huge isotope effect on the transition temperature. With our calculation we can selectively investigate the manisfestation of the two phenomena. We characterize the ferro and paraelectric phases with the help of a recent modification of the path integral implementation in the CPMD package which enables us to compute momentum distributions of the proton/deuteron both above and below the transition temperature in order to characterize the extent of proton/deuteron delocalization in both phases. [Preview Abstract] |
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