Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G31: Focus Session: Simulation of Complex Materials I |
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Sponsoring Units: DCOMP DMP Chair: Jeff Grossman, University of California, Berkeley Room: Baltimore Convention Center 328 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G31.00001: Orbital-order-induced metal-insulator transition in manganites Sanjeev Kumar, Arno Kampf, Pinaki Majumdar We study a two-band double-exchange model with adiabatic Jahn-Teller phonons, weak antiferromagnetic superexchange and quenched disorder. This model for perovskite manganites is studied using a recently developed real space Monte-Carlo method (cond-mat/0406082). We present results for the optical conductivity and the density of states and provide a microscopic picture of the spin and orbital structures. The electron lattice coupling leads to orbital ordering at zero hole doping ($x=0$), while the magnetism is controlled by the interplay of superexchange and double exchange interactions. We track the evolution upon hole doping of this orbital ordered insulator and locate the critical doping concentrations required for (i) the disappearance of orbital order and (ii) the emergence of the metallic behavior. We compare our results in detail with the experimental data on $ La_{1-x}Ca_xMnO_3 $ in the low $x$ regime. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G31.00002: Sequencing-Independent Delocalization in a DNA-Like Double Chain with Base Pairing Rodrigo Caetano, Peter Schulz The question of whether DNA is intrinsically conducting or not is still a challenger. Experiments on DNA conductivity are very controversial: Metallic, semiconducting, insulating and even superconductor behaviors have been reported. The theoretical studies of the electronics properties of DNA range from strictly one-dimensional tight-binding chains, up to ab initio calculations. Ab initio calculations give useful insight on environment influence but have to be limited to a reduced number of model DNA molecules. On the other hand strictly one-dimensional calculations deal with effectives sites instead of a base pair structure, underestimating the paring effects. Here, we focus in that intrinsic factor to show that double chains with base pairing with DNA-like correlation leads to delocalized states. We use a double-chain nearest-neighbor hopping tight-binding model to describe the system. The double chains are constituted by four different sites, representing the nucleotides (A, C, G and T). The sites in the first chain are randomly assigned, while the sites in the second chain obey the DNA pairing. The main consequence of the correlation is a localization-delocalization transition. The present result suggest the DNA is intrinsically a promising electronic material and the hidrance to DNA nanoelectronics is solely of technological nature. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G31.00003: Orientational Defects in Ice Ih: An Interpretation of Electrical Conductivity Measurements Maurice de Koning, Alex Antonelli, Antonio J.R. da Silva, Adalberto Fazzio While the isolated water molecule is one of the simplest in Nature, the condensed phases of H$_{2}$O reveal many complex features that still elude complete understanding. An example concerns the role of crystal defects in the peculiar electrical properties of proton-disordered hexagonal ice Ih. When an electric field is applied to an ice specimen, it becomes polarized by the thermally activated reorientation of the molecular dipoles through the motion of Bjerrum defects, which represent local disruptions of the hydrogen-bond network. While the conceptual picture of these Bjerrum defects is now well established, a quantitative understanding of their properties is still lacking, rendering a direct interpretation of experimental electrical conductivity data difficult. In this work we present a first-principles study of the structure and energetics of Bjerrum defects in ice Ih and compare the results to experimental electrical conductivity data. While the DFT result for the activation energy is in good agreement with experiment, we find that its two components have quite different values. Aside from providing new insight into the fundamental parameters of the microscopic electrical theory of ice, our results suggest the activity of traps in doped ice in the temperature regime typically assumed to be controlled by the free migration of L-type Bjerrum defects. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G31.00004: Scaling behavior of Film growth mechanism Mina Yoon, Lee Ho Nyung, Zhigang Suo, Wei Hong, Hans M. Christen, Doug Lowndes, Zhenyu Zhang Experimental evidence has accumulated that a strained film can grow stably on a vicinal surface. Linear perturbation analysis of the step-flow regime results in a dispersion relation which determines the persistence of the step-flow growth. The dispersion relation can also be used to probe the system parameters. Investigating the growth dynamics in the step-bunching regime, we found that there is a critical film thickness above which step-bunching occurs. The critical thickness shows a scaling behavior depending on the terrace width and the deposition flux. Experiments show a qualitative agreement with the theory. Our results may open a way to grow films in a desired way. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G31.00005: One-dimensional Hartree-Fock calculations for electrons in a constriction Jiang Qian, Bertrand Halperin, Walter Hofstetter, Gergely Zarand There are a number of puzzling experimental results associated with electron transport through a narrow constriction of finite length, such as the so-called 0.7 structure in the quantum point contact and the experiments on tunneling between two parallel quantum wires. To gain deeper insights into these experiments, it may be important to understand how a constriction region containing electrons at a very low-density is connected to higher density leads at each end. We have formulated Hartree-Fock methods applicable to one-dimensional models of such systems, in equilibrium or with a finite source-drain voltage. We will present results of the electron density profile and spin structures around the constriction for several models of interaction and constriction potentials. This work is supported in part by NSF grand DMR-02-33773 and PHY-01-17795. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G31.00006: Mixed Wannier-Bloch functions for electrons and phonons in extended disordered systems Alfredo Pasquarello, Feliciano Giustino A major difficulty in the study of disordered systems arises from the inadequacy of the Bloch description of quasiparticle and collective excitations. The main alternative to the crystal momentum representation is provided by the Wannier picture. However, despite the success of the optimally localized Wannier functions in crystalline environments, their applicability to disordered systems is hindered by the lack of an explicit energy label, the latter being essential for interpreting spectroscopic data. We here introduce mixed Wannier-Bloch functions for studying electronic and vibrational spectra of extended disordered solids. These functions are designed to carry both spatial localization (Wannier character) and limited spectral broadening (Bloch character). Hence, they are best suited for associating specific spectral features to the underlying atomic-scale mechanisms. The new approach is demonstrated through applications to vitreous silica. The use of mixed Wannier-Bloch functions leads to a straightforward interpretation of the electronic spectrum of $v$-SiO$_2$ in terms of $s$ and $p$ atomic orbitals, and of the corresponding vibrational spectrum in terms of the molecular modes of the Si-O-Si bridge. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:48AM |
G31.00007: Quantum Monte Carlo Calculations of Transition Metal Oxides Invited Speaker: Quantum Monte Carlo is a powerful computational tool to study correlated systems, allowing us to explicitly treat many-body interactions with favorable scaling in the number of particles. It has been regarded as a benchmark tool for first and second row condensed matter systems, although its accuracy has not been thoroughly investigated in strongly correlated transition metal oxides. QMC has also historically suffered from the mixed estimator error in operators that do not commute with the Hamiltonian and from stochastic uncertainty, which make small energy differences unattainable. Using the Reptation Monte Carlo algorithm of Moroni and Baroni(along with contributions from others), we have developed a QMC framework that makes these previously unavailable quantities computationally feasible for systems of hundreds of electrons in a controlled and consistent way, and apply this framework to transition metal oxides. We compare these results with traditional mean-field results like the LDA and with experiment where available, focusing in particular on the polarization and lattice constants in a few interesting ferroelectric materials. This work was performed in collaboration with Lubos Mitas and Jeffrey Grossman. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G31.00008: First Principles Study of Formation Energies and Diffusion Mechanisms of Native Point Defects in Rutile TiO$_{2-x}$ Hakim Iddir, Serdar Ogut, Nigel Browning We present results from first principles calculations for the formation energies and diffusion mechanisms of the oxygen vacancy (V$_{\rm O}$), titanium interstitial (Ti$_I$) and titanium antisite (Ti$_{\rm O}$) point defects in reduced rutile TiO$_{2-x}$. The calculations are performed in large supercells containing 240 to 270 atoms using the {\em ab initio} pseudopotential total energy method. As expected, the formation energy of Ti$_{\rm O}$ is found to be significantly higher than V$_{\rm O}$ and Ti$_I$, which have competitive formation energies as a function of the sample stoichiometry. The dominant charge states of V$_{\rm O}$ and Ti$_I$ are found to be $(2+)$ and $(4+)$, respectively, for a wide range of the electron chemical potential. The calculated diffusion barriers for Ti$_I^{4+}$ are lower than those of V$_{\rm O}^{2+}$, and do not present a significant anisotropy between the (100) open channels and the (110) direction. In fact, the barrier for the diffusion of Ti$_I^{4+}$ along the (100) open channels has an unexpectedly 0.1 eV higher barrier than along (110). The reasons for this observation and the implications of the present results are discussed in relation to bulk-defect assisted surface phenomena in rutile TiO$_2$. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G31.00009: Multifunctionality and Fluid Permeabilities of Triply Periodic Minimal Surfaces Youngjean Jung, Salvatore Torquato It has recently been shown that triply periodic two-phase bicontinuous composites with interfaces that are the Schwartz P and D minimal surfaces are not only geometrically extremal but extremal for simultaneous transport of heat and electricity. The multifunctionality of such two-phase systems has been further established by demonstrating that they are also extremal when a competition is set up between the effective bulk modulus and electrical (or thermal) conductivity of the bicontinuous composite. We computed the fluid permeabilities of these and other triply periodic bicontinuous structures arranged on the sites of a simple cubic lattice at a porosity $\phi=1/2$ [Y. Jung and S. Torquato, Phys. Rev. E {\bf 72}, 056319 (2005)]. We found that the Schwartz P porous medium has the largest fluid permeability among all of the six triply periodic porous media in consideration. The fluid permeabilities are shown to be inversely proportional to the corresponding specific surfaces for these structures. This leads to the conjecture that the maximal fluid permeability for a triply periodic porous medium with a simply connected pore space at a porosity $\phi=1/2$ is achieved by the structure that globally minimizes the specific surface. We will also present specific single-length-scale, two-dimensional, isotropic multifunctional optimal microstructures at the phase volume fraction $\phi = 1/2$. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G31.00010: Ab Initio Structural Energetics of Bare $\beta-$Si$_3$N$_4$ Surfaces and the Interface with Sm$_2$O$_3$ Juan Carlos Idrobo, Hakim Iddir, Serdar Ogut, Alexander Ziegler, Nigel Browning, R.O. Ritchie Motivated by recent electron microscopy studies on the Si$_3 $N$_4$/rare-earth oxide interfaces, the atomic and electronic structures of bare $\beta-$Si$_3$N$_4$ surfaces are investigated from first principles. The equilibrium shape of a Si$_3$N$_4$ crystal is found to have a hexagonal cross section and a faceted domelike base in agreement with experimental observations. The large atomic relaxations on the prismatic planes are driven by the tendency of Si to saturate its dangling bonds, giving rise to resonant-bond configurations or planar sp$^2$-type bonding. We predict three bare surfaces with lower energies than the open-ring ($10\overline{1}0$) surface observed at the interface, which indicate that non- stoichiometry and the presence of the rare-earth oxide play crucial roles in determining the termination of Si$_3$N$_4$ matrix grains. We also present preliminary first principles results for the Si$_3$N$_4$/Sm$_2$O$_3$ interface and compare our findings to experimental observations. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G31.00011: Manipulation/Extraction of an Adatom from a Mound Handan Yildirim, Abdelkader Kara, Talat S. Rahman We present results of manipulation/extraction of Ag/Cu atoms from small Ag/Cu mounds on Ag(111)/Cu(111), rescpectively. Molecular dynamics (MD) and molecular static (MS) simulations were carried out using interaction potentials from the embedded atom method. Evaluation of the energy landscape in the absence of tip shows that for a Ag atom, lateral (sliding downwards) and vertical (climbing upwards) diffusion,the activation barrier is about 0.3 eV. We show that the presence of a Ag tip lowers the energy barrier for both lateral and vertical diffusion. We find that the optimum position of the tip to achieve extraction is when the tip is at the edge of the mound in front of the adatom, and at a height of 2.43 {\AA} from the Ag adatom, for which the barriers for diffusion drop to 0.032 eV for lateral and 0.182 eV for vertical diffusion. In this case, the adatom is manipulated in the pulling mode. In the case of Cu, we found that the adatom manipulation/extraction is performed in the sliding/dragging mode. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G31.00012: Viscoelastic Stokes Flow M. Gregory Forest, Ke Xu, Isaac Klapper We extend the formalism of Stokes flow from viscous to viscoelastic fluids, and then illustrate this correspondence with a variety of applications, including flow past a moving point source and flow around a driven oscillating sphere. [Preview Abstract] |
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