Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H1: DCMP Prize Session
Sponsoring Units: DCMPChair: Julia Phillips, Sandia National Laboratories
Room: Colorado Convention Center Four Seasons 2-3
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H1.00001: Beyond the Quantum Hall Effect: New Phases of 2D Electrons at High Magnetic Field Invited Speaker: In this talk I will discuss recent experiments on high mobility single and double layer 2D electron systems in which collective phases lying outside the usual quantum Hall effect paradigm have been detected and studied. For example, in single layer 2D systems near half-filling of highly excited Landau levels new states characterized by a massive anisotropy in the electrical resistivity of the sample are observed at very low temperature. The anisotropy has been widely interpreted as the signature of a new class of correlated electron phases which incorporate a stripe-like charge density modulation. Orientational ordering of small striped domains at low temperatures accounts for the resistive anisotropy and is reminiscent of the isotropic-to-nematic phase transition in classical liquid crystals. \\ \\ Double layer 2D electron systems possess collective phases not present in single layer systems. In particular, when the total number of electrons in the bilayer equals the degeneracy of a single Landau level, an unusual phase appears at small layer separation. This phase possesses a novel broken symmetry, spontaneous interlayer phase coherence, which has a number of dramatic experimental signatures. The interlayer tunneling conductance develops a strong and very sharp resonance around zero bias resembling the dc Josephson effect. At the same time, both the longitudinal and Hall resistances of the sample vanish at low temperatures when currents are driven in opposite directions through the two layers. These, and other observations are broadly consistent with theories in which the broken symmetry phase can equivalently be described as a pseudospin ferromagnet or an (imperfect) excitonic superfluid. \\ \\ This work reflects a collaboration with M.P. Lilly, K.B. Cooper, I.B. Spielman, M. Kellogg, L.A. Tracy, L.N. Pfeiffer, and K.W. West. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H1.00002: Off-Diagonal Long-Range Order and Collective Excitations in the Fractional Quantum Hall Effect Invited Speaker: The experimental discovery of the fractional quantum Hall effect was a stunning surprise which came to be understood in terms of a novel state of matter in which strongly correlated electrons acquire a new and unprecedented type of collective quantum order. The mystery of superconductivity was first understood macroscopically in terms of Ginsburg-Landau effective theory before the microscopic BCS theory was developed. Here the historical order was reversed. Laughlin discovered his essentially exact microscopic wave function and only subsequently did we begin to understand its implications in terms of a new type of off-diagonal long-range order and an effective Chern-Simons field theory for composite particles carrying magnetic flux. In this gauge theory, the fact that Laughlin's quasi-particle excitations carry sharply quantized fractional charge could be understood as analogous to sharp flux quantization in a superconductor. The fact these vortex excitations have finite energy could be understood as the result of magnetic screening by the gauge field. In addition to discussing this macroscopic picture of the FQHE, I will also discuss the microscopic wave function that Allan MacDonald and I developed in collaboration with Phil Platzman which accurately describes the gapped magneto-phonon and magneto-roton collective excitations of the system. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H1.00003: Oliver E. Buckley Prize Talk Invited Speaker: |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H1.00004: Atom Chains at Surfaces: A Playground for Low-Dimensional Physics Invited Speaker: One-dimensional physics is particularly elegant because of its mathematical transparency. However, it is not easy to realize a one-dimensional system experimentally. Using self-assembly techniques, it has become possible to produce atomic chain structures at silicon surfaces and to control their dimensionality, their band filling, and their magnetic moment [1]. The atoms are locked to the surface, but metallic electrons are de-coupled from the substrate due to the band gap of silicon. In a sense, these are the ultimate nanowires, each consisting of a single chain of orbitals. Angle-resolved photoemission reveals surprising features, such as a fractional band filling [2], a spin-splitting at a non-magnetic surface [3], and the one-dimensional analog of stripes (alternating metallic and semiconducting sections). \newline \newline [1] Crain and Himpsel, Appl. Phys. A \textbf{82}, 431 (2006). \newline [2] Crain et al., Phys. Rev. Lett. \textbf{90}, 176805 (2003). \newline [3] Barke et al. Phys. Rev. Lett., in press (2006). [Preview Abstract] |
Session H2: Polymer Physics Prize
Sponsoring Units: DPOLYChair: Steve Granick, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center Four Seasons 4
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H2.00001: Challenges for Polymer Theory and Simulation Invited Speaker: I will discuss some contemporary topics in polymer physics that represent challenges for theorists and computer simulators in the coming decade. These are: \begin{itemize} \item The challenge of multiple scales -- bridging the atomistic to the continuum \item Structure-property relations for nanocomposites \item Supramolecular polymer assembly \item Science and engineering of conjugated polymer interfaces -- electronic structure meets polymer physics \item Rheology and structure of inhomogeneous polymers \item Ultimate mechanical properties of everything polymeric \end{itemize} There are common aspects to these challenging topics; for example, multiple scales and the curse of dimensionality are pervasive. My presentation will touch on the theoretical tools that are needed to conduct research in these areas, and will highlight a few contributions from my own group. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H2.00002: Self-Assembly of Monolayer and Multilayer Films of Spherical-Domain Diblock Copolymers Invited Speaker: Self-assembly of block copolymers in thin films can yield templates for nanolithographic patterning of substrates on very small length scales as well as ordered multilayer structures for membrane and electronic applications. On a fundamental level these thin block copolymer melts raise many interesting questions about self-assembly in 2D and how the transition from 2D to 3D occurs as the film is increased in thickness. To answer these, scattering techniques such as grazing incidence small angle X-ray scattering (GISAXS) can be a very useful complement to the normal imaging techniques of AFM, SEM and TEM, but in combination with self-consistent field theoretic (SCFT) simulations, these become even more powerful. I highlight one such set of questions, how the packing of spherical block copolymer domains confined to a thin film changes as the thickness of the film is increased layer by layer of spheres, as a concrete example of the usefulness to this combined approach and how the SCFT methods pioneered by Glenn Fredrickson and his colleagues have profoundly influenced experimental polymer physics. Using GISAXS we find hexagonal symmetry in films 1-4 layers thick. Stacking in films 1-4 layers thick is close-packed \textit{AB}, \textit{ABA and ABAB}. At 5 layers, the hexagonal symmetry breaks to form an orthorhombic phase, characterized by second/first nearest-neighbors $a_{1}/a_{2}$ and lattice angle \textit{$\varphi $}. As the number of layers is increased, $a_{1}$ and \textit{$\varphi $} increase continuously to approach that for BCC (110) planes. From measurements collected above and below the critical angle of the polymer, the structure is uniform throughout the depth of the film. SCFT calculations provide a semi-quantitative description of the transition and the insight that it is a consequence of competition between the optimal HEX packing at the film surfaces with the preferred BCC (110) inner layer packing in the bulk. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H2.00003: On the consequences of interacting with Glenn Fredrickson Invited Speaker: Since joining Bell Labs in 1984, and subsequently UC Santa Barbara in 1990, Glenn Fredrickson has contributed many timely and inspiring theories to the polymer physics community. These developments have had a significant impact on the speaker, often resulting in years of experimental research. This lecture will trace forward representative examples of Fredrickson's theoretical insights that have spawned innovative and scholarly research. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H2.00004: Supramolecular concepts in self-assembly of complex polymer systems Invited Speaker: We discuss the complexation, the self-assembly behaviour and nanostructures obtained in comb-like liquid crystalline polymers formed by ionic complexation of cationic dendronized polymers and anionic lipids. The resulting self-assembled materials exhibit thermotropic liquid crystalline behaviour and a rich state diagram. The topology of the LC phases resulting from the self-assembly process, their lattice parameter and the distribution of polymer and lipid domains are discussed via birefringency analysis, small angle x-ray scattering, differential scanning calorimetry and transmission electron microscope. Depending on the generation of the dendronized polymer and the length of the alkyl chains, amorphous, lamellar, columnar hexagonal and a rarely observed columnar tetragonal phase can be obtained, where the long-range ordering of the structures is a function of the generation of the dendronized polymer considered and the lattice space is of the order of 3-6 nm. The selective staining of polymer/lipid domains allows establishing unambiguously the composition of each domain in the observed nanostructures and a structural model is proposed which accounts for the systematic variations of structure in terms of alkyl chain length as well as polymer generation. Furthermore, we discuss our recent efforts towards enhancing long-range order via external applied fields. Owing to the reversible nature of the ionic complexation this process proves high relevance for nanoporous channels, biomimetic, transport and nanotemplating applications. References: Canilho, N.; Kasemi, E.; Mezzenga, R.; Schluter, A.D. \textit{J. Am. Chem. Soc}. \textbf{128}, 13998 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 11:00AM |
H2.00005: PE Crystallization and Rotator Phases. Invited Speaker: Recent work on crystallization of polyethylene (PE) implicates a metastable rotator phase as the nucleating phase. This claim invites the questions: what is the structure of this phase, what is the free energy difference $\Delta F$ driving nucleation, and what is the surface free energy $\Sigma$ of the phase in contact with the melt? Related proposals for critical nuclei in polymer crystallization (``fringed micelles'') were dispensed with long ago, with estimates of $\Delta F$ and $\Sigma$ that were unduly pessimistic. With more recent theoretical tools, we can revisit PE nucleation, comparing crystalline and rotator phase nuclei. To do so requires a model that can describe the bulk and surface free energy of both phases. To compare bulk free energies, we use a 6-state generalized Potts model, in which the disordered phase represents the rotator phase. Using a multiscale approach, coupling constants are obtained from solid-state simulations of domain walls between six degenerate crystalline orderings. The surface free energies, dominated by entropic penalties of melt segments near the nucleus surface (the ``fringe''), are calculated using methods developed in the context of polymer brushes. Combining these ingredients, we can make a more enlightened comparison of the nucleation barrier for crystalline and rotator phase nuclei. [Preview Abstract] |
Session H3: Anomalous Hall Effect: Theory and Experiments
Sponsoring Units: DCMPChair: Qian Niu, University of Texas at Dallas
Room: Colorado Convention Center Korbel 2A-3A
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H3.00001: Intrinsic vs. extrinsic mechanisms of anomalous Hall effect Invited Speaker: Anomalous Hall effect (AHE) in ferromagnets has been a fundamental and intriguing issue in condensed-matter physics. Various mechanisms have been proposed, including the Karplus-Luttinger's band intrinsic mechanism, and extrinsic skew-scattering and side-jump mechanisms. However, the controversy on the mechanism has not been resolved yet. In this talk, a unified theory of the anomalous Hall effect (AHE) is presented for multi-band ferromagnetic metallic systems with dilute impurities [1], using the gauge-covariant formalism for the Keldysh Green's function [2]. In the clean limit, the AHE is mostly due to the extrinsic skew- scattering, and is sensitive to details of impurity potential. When the Fermi level is located around anti-crossing of band dispersions split by spin-orbit interaction, the intrinsic AHE to be calculated ab initio is resonantly enhanced by its non- perturbative nature. Then, an extrinsic-to-intrinsic crossover occurs when the relaxation rate is comparable to the spin-orbit interaction energy. Futher increasing the relaxation rate, a new scaling relation $\sigma_{xy}\propto\sigma_{xx}^{1.6}$ appears in the hopping-conduction regime. Various experimental data on transition-metals and oxcides are understood in terms of this theory [3]. \newline \newline [1] S. Onoda, N. Sugimoto, and N. Nagaosa, Phys. Rev. Lett. {\bf 97}, 126602 (2006). \newline [2] S. Onoda, N. Sugimoto, and N. Nagaosa, Prog. Theor. Phys. {\bf 116}, 61 (2006). \newline [3] T. Miyasato {\it et al.}, cond-mat/0610324. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H3.00002: The Anomalous Hall effect in MnSi and Fe$_x$TaS$_2$ Invited Speaker: In a high-purity ferromagnet with long carrier lifetime $\tau$, e.g. MnSi, the ordinary Hall conductivity $\sigma_H^N$ can dominate the intrinsic Anomalous Hall effect (AHE) conductivity $\sigma_H^A$. We show that the large magnetoresistance provides a way to separate accurately the two Hall currents. Below $T_C$, we find that the AHE conductivity is strictly proportional to the magnetization $M$, viz. $\sigma_H^A = S_HM$ with a parameter $S_H$ that is independent of both temperature $T$ and field $H$. This implies that $\sigma_H^A$ is strictly independent of $\tau$. In the layered, hard ferromagnet Fe$_x$TaS$_2$, the large coercivity leads to abrupt reversals of $M$ when it switches. We show that this provides an accurate way to separate $\sigma_H^A$ from $\sigma_H^N$. Again, $\sigma_H^A$ is independent of $T$ from 5 to 50 K. We compare the observed constancy at low $T$ with theories for the AHE. We also describe a Hall anomaly recently observed in MnSi under pressure. This anomaly appears to arise from strong sensitivity of the Hall current to the spin texture, possibly reflecting its finite chirality. The dependence of the anomaly to $T$ and $H$ will be reported. \newline \newline **This work is done in collaboration with Y. Onose, J. G. Checkelsky, E. Morosan, R. J. Cava, Y. Tokura and N. P. Ong. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H3.00003: Investigation of the Anomalous Hall Effect in Three Unusual Ferromagnets Invited Speaker: The Hall resistivity ($\rho _{xy})$, resistivity ($\rho _{xx})$, and magnetization of three metallic ferromagnets are investigated as a function of magnetic field and temperature [1]. The three ferromagnets, EuFe$_{4}$Sb$_{12}$ (T$_{c} \approx $ 84 K), Yb$_{14}$MnSb$_{11}$ (T$_{c}$ $\approx $ 53 K), and Eu$_{8}$Ga$_{16}$Ge$_{30}$ (T$_{c} \approx $ 36 K) are Zintl compounds with carrier concentrations between 1 x 10$^{21}$cm$^{-3}$ and 3.5 x 10$^{21}$ cm$^{-3}$. The relative decrease in $\rho_{xx}$ below T$_{c}$ [$\rho_{xx}$(T$_{c})$/$\rho_{xx}$(2 K)] is 28, 6.5, and 1.3 for EuFe$_{4}$Sb$_{12}$, Yb$_{14}$MnSb$_{11}$, and Eu$_{8}$Ga$_{16}$Ge$_{30}$ respectively. The low carrier concentrations coupled with low magnetic anisotropies allow a relatively clean separation between the anomalous ($\rho^{'}_{xy})$, and normal contributions to the measured Hall resistivity. For each compound the anomalous contribution in the zero field limit is fit to $a\rho _{xx}+\sigma _{xy} \rho_{xx}^{2}$ for temperatures T $<$T$_{c}$. The anomalous Hall conductivity, $\sigma _{xy}$, is -220 $\pm $ 5 ($\Omega ^{-1}$ cm$^{-1})$, -14.7 $\pm $ 1 ($\Omega ^{-1}$ cm$^{-1})$, and 28 $\pm $ 3 ($\Omega ^{-1}$ cm$^{-1})$ for EuFe$_{4}$Sb$_{12}$, Yb$_{14}$MnSb$_{11}$, and Eu$_{8}$Ga$_{16}$Ge$_{30}$ respectively and is independent of temperature for T $<$ T$_{c}$ if the change in spontaneous magnetization (order parameter) with temperature is taken into account. These data appear to be consistent with recent theories of the anomalous Hall effect that suggest that even for stochiometric ferromagnetic crystals, such as those studied in this work, the intrinsic Hall conductivity is finite at T = 0, and is a ground state property that can be calculated from the electronic structure. New measurements on single crystals of the tetragonal compound Yb$_{14}$MnSb$_{11}$, however, indicate that the intrinsic Hall conductivity can change sign, depending on the direction of the current and magnetic field with respect to the crystallographic axes. These new results will also be discussed within the context of recent theories. Research was done in collaboration with Rongying Jin, David Mandrus and Peter Khalifah. \newline \newline [1] B. C. Sales et al. Phys. Rev. B 73 (2006) 224435. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H3.00004: Ordinary and anomalous Hall effects of ferromagnetic Mn5Ge3 Invited Speaker: It is well know that in ferromagnetic materials, the Hall effect includes two contributions: the ordinary Hall effect (OHE), which is proportional to the applied magnetic field, and the anomalous Hall effect (AHE), which originates from the magnetization of the material. Although both phenomena have been throughly studied, there are still questions about the origins of both OHE and AHE in ferromagnetic materials with complicated band structures. Using ferromagnetic Mn5Ge3 thin films as an example, we investigate the Hall effect experimentally and theoretically. We have separated the intrinsic and extrinsic contributions to the experimental AHE and calculated the intrinsic anomalous Hall conductivity from the Berry curvature of the Bloch states using first-principles methods. The intrinsic anomalous Hall conductivity depends linearly on the magnetization, which can be understood from the long-wavelength fluctuations of the spin orientation at finite temperatures. The \textit{quantitative }agreement between theory and experiment is remarkably good, not only near 0 K but also at finite temperatures, up to about 240 K (0.8\textit{Tc}) [1]. The measured ordinary Hall coefficient is found to change its sign as a function of temperature. From a detailed analysis, which includes magneto-resistance measurements, magnetic characterization, and first-principles calculations, we establish that the sign change of the OHE is mainly caused by the mixing of the AHE with the magneto-resistance and differential susceptibility. This work was done in collaboration with Y. Yao, Di Xiao, Q. Niu, and H.H. Weitering. \newline \newline [1] Changgan Zeng, Yugui Yao, Qian Niu, and Hanno, H. Weitering, Phys. Rev. Lett. 96, 037204 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 11:00AM |
H3.00005: Berry phase, Orbital Magnetization, and Anomalous Hall/Nernst Effect Invited Speaker: It is now well recognized that the Berry phase of the electronic wave function plays an important role in the dynamics of Bloch electrons. For instance, the electron will acquire an anomalous velocity term transverse to the applied electric field, giving rise to an intrinsic contribution to the anomalous Hall effect. We have recently discovered that the Berry phase also modifies the phase-space density of states in the presence of a magnetic field. This surprising result has a number of implications, such as a field-dependent Fermi sea volume, Berry phase correction to the orbital magnetization, and linear (in field) magnetoresistance. Based on a general, finite-temperature, formula for orbital magnetization, we are able to develop a satisfactory theory for anomalous transport in ferromagnets driven by statistical forces (the gradient of temperature or chemical potential). Here a charge Hall current arises from the Berry-phase correction to the orbital magnetization rather than from the anomalous velocity, which does not exist in the absence of a mechanical force. We provide an explicit expression for the off-diagonal thermoelectric conductivity, establish the Mott relation between the anomalous Nernst and Hall effects, and reaffirm the Onsager relation between reciprocal thermoelectric conductivities. A first-principles evaluation of our expression is carried out for the material CuCr$_2$Se$_{4-x}$Br$_x$ , obtaining quantitative agreement with a recent experiment. This work is done in collaboration with Q. Niu, J.-R. Shi, Y.-G Yao, Z. Fang. \\ \\ 1. D. Xiao, J. Shi, and Q. Niu, Phys. Rev. Lett. {\bf 95}, 137204 (2005). \\ 2. D. Xiao, Y. Yao, Z. Fang, and Q. Niu, Phys. Rev. Lett. {\bf 97}, 026603 (2006). \\ 3. Y. Yao, \textit{et. al.}, cond-mat/0609714. [Preview Abstract] |
Session H4: Recent Advances in quantum Monte Carlo Simulations
Sponsoring Units: DCOMP DCMP DMPChair: Richard Martin, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center Korbel 2B-3B
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H4.00001: Recent advances in auxiliary-field methods --- simulations in lattice models and real materials Invited Speaker: We have developed an auxiliary-field (AF) quantum Monte Carlo (QMC) method for many-body simulations. The method takes the form of a linear superposition of independent-particle calculations in fluctuating external fields. ``Entanglement'' of the different field configurations leads to random walks in Slater determinant space. We formulate an approximate constraint on the random walk paths to control the sign/phase problem, which has shown to be very accurate even with simple mean-field solutions as the constraining trial wave function. The same method can be applied to both simplified lattice models and real materials. For realistic electronic Hamiltonians, each random walk stream resembles a density-functional theory (DFT) calculation in random local fields. Thus, the AF QMC method can directly import existing technology from standard electronic structure methods into a many-body QMC framework. We have demonstrated this method with calculations in close to 100 systems, including Si solid, first- and second-row molecular systems, molecules of heavier post-d elements, transition-metal systems, and ultra-cold atomic gases. In these we have operated largely in an automated mode, inputting the DFT or Hartree-Fock solutions as trial wave functions. The AF QMC results showed consistently good agreement with near-exact quantum chemistry results and/or experiment. I will also discuss additional algorithmic advances which can further improve the method in strongly correlated systems. \\ \\ Supported by ARO, NSF, ONR, and DOE-cmsn. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H4.00002: Lattice regularized diffusion Monte Carlo method Invited Speaker: We introduce a lattice regularization scheme for quantum Monte Carlo calculations of realistic electronic systems$[1]$. Our method is based on the discretization of a projection operator (Green's function), constructed upon an effective regularized Hamiltonian$[2]$. In particular, its Laplacian is discretized with two incommensurate mesh sizes, $a$ and $a^\prime$, where $a^\prime/a$ is a fixed irrational number, and the regularized Hamiltonian goes to the continuous limit for $a\to 0$. The use of the double mesh improves significantly the convergence to the $a\to 0$ limit, and allows one to take into account efficiently the different length scales in the system. Another advantage of this framework is the possibility to include non-local potentials in a consistent variational scheme, substantially improving both the accuracy and the computational stability upon previous non-variational diffusion Monte Carlo approaches. However, we have recently shown$[3]$ that also the standard diffusion Monte Carlo algorithm can be made stable and variational even in the presence of non-local pseudopotentials, by including a non-local discrete process in the diffusion operator. This work can open the route for even more reliable and accurate electronic ground state calculations using diffusion Monte Carlo methods. \\ \\ $[1]$ M. Casula, C. Filippi, and S. Sorella, Phys. Rev. Lett. {\bf 95}, 100201 (2005). \\ $[2]$ S. Sorella, cond-mat/0201388. \\ $[3]$ M. Casula, Phys. Rev. B {\bf 74}, 161102(R) (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H4.00003: Pfaffian wave functions and topology of fermion nodes Invited Speaker: Pfaffian is defined as a signed sum of all pair partitions of even number of elements and it can be viewed as a nontrivial generalization of determinant. Pfaffian enables to define the simplest possible antisymmetric wave function based on pair spinorbital(s) and therefore represents a pairing generalization of the Slater determinant of one-particle orbitals. Pfaffians actually accomodate several types of pairing wave functions, for example, one special case is the Bardeen-Cooper- Schrieffer wave function. Using this platform we propose pfaffian wave functions with simultaneous pairings both in singlet and triplet channels and we benchmark their performance in fixed-node quantum Monte Carlo. We implement Gaussian elimination-like algorithm which enables to calculate pfaffians with efficiency similar to calculation of determinants. For a testing set of first row atoms and molecules we show that single pfaffians provide correlation energies systematically at the level of about 95\%. Linear combinations of small number of pfaffians recover another fraction of the missing correlation energy comparable to significantly larger determinantal expansions. In addition, we show that pfaffians possess an important property of fermionic wave functions, namely, the minimal number of two nodal domains defined by fermion nodes. This is related to the proof that under rather general conditions closed-shell ground state wave functions of fermionic systems in d$>$1 have two nodal domains for arbitrary system size. The explicit proofs cover a number of paradigmatic models such as fermions on a sphere surface, in a periodic box, atomic states, etc, and we discuss the implications of this on efficient construction of wave functions and on several types of many-body effects. Supported by NSF and done in collaboration with M. Bajdich, L.K. Wagner, G. Drobny, and K.E Schmidt.\newline Refs: L. Mitas, PRL 96, 240402 (2006); L. Mitas, cond-mat/0605550; M. Bajdich et al, PRL 96, 130201 (2006); cond-mat/0610850. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H4.00004: QMC simulations using backflow correlated wave functions Invited Speaker: An inhomogeneous backflow transformation for many-particle wave functions is presented and applied to electrons in atoms, molecules, and solids. Backflow transformations are compact parametrizations, by which we mean that the number of parameters required to retrieve a given fraction of the correlation energy increases only slowly with system size. We report variational and diffusion quantum Monte Carlo (VMC and DMC) energies for a number of systems and study the computational cost of using backflow wave functions. Backflow transformations alter the nodal surface of the wave function and can therefore be used to reduce the fixed-node error in DMC calculations. Applications to the homogeneous electron gas, the all-electron lithium atom and dimer, and carbon atom and dimer, and pseudopotential calculations for the carbon atom and dimer and carbon diamond are presented. When the initial nodal surface is reasonably accurate, backflow appears to do an excellent job in improving the VMC energy and correcting the remaining errors in the nodal surface. When the initial nodal surface is poor, however, backflow is apparently incapable of making the gross changes to the nodal surface required to correct the flaws, although it still normally lowers both the VMC and DMC energies significantly. Overall, we find that inhomogeneous backflow transformations can provide a substantial increase in the amount of correlation energy retrieved within VMC and DMC calculations. This approach is of considerable generality as it is successful in metals and in insulators, and in large and small systems. Backflow transformations can readily be used with pairing wave functions, and this approach could yield significant improvements when a wave function consisting of a single determinant of one-particle orbitals is a poor starting point. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 11:00AM |
H4.00005: Resonating Valence Bond wavefunctions for electronic simulations Invited Speaker: We discuss several progress for the simulation of strongly correlated electrons, based on an efficient implementation of the Resonating Valence Bond (RVB) theory with Quantum Monte Carlo (QMC). Due to very important advances[1] in the energy optimization of strongly correlated variational wave functions, it is now possible to optimize several variational parameters with remarkable efficiency even within a stochastic approach such as QMC. In this way it is possible to describe very accurately the electronic correlation by a first principle many-body wave function, that can be extended to fairly large electronic systems. Indeed a remarkable improvement of the Hartree-Fock theory is provided by the so called RVB wave function introduced by P.W. Anderson in the context of High-Tc superconductivity[2]. For instance, by means of this paradigm, it has been possible to perform a realistic and accurate simulation of the benzene dimer, where we have found that the RVB correlation of the benzene ring plays a crucial role in the dimer bonding[3,4]. Finally we consider the still controversial low-temperature and high-pressure phase diagram of Hydrogen by using the same RVB wavefunction. We use a novel second order Langevin dynamics by introducing a consistent friction tensor, allowing to remain in thermal equilibrium even with very noisy forces, namely determined by QMC with very short runs. This allows us to simulate finite temperature systems ($\simeq 100$ H) with very high efficiency, while the variational parameters are consistently optimized during the ionic dynamics. \begin{description} \item{[1]} See C. J. Umrigar, J. Toulouse, C. Filippi, S. Sorella and R. G. Hennig, cond-mat/0611094 and references therein. \item{[2]} P. W. Anderson Science 235, 1196 (1987). \item{[3]} M. Casula, C. Attaccalite and S. Sorella J. Chem. Phys. {\bf 121} 7110 (2004). \item{[4]} S. Sorella, M. Casula and D. Rocca in preparation. \item{[5]} C. Attaccalite and S. Sorella in preparation. \end{description} [Preview Abstract] |
Session H5: Exact-Exchange Based DFT Functionals Meet Quasiparticle Energy Calculations: Exciting Prospects for Condensed Systems
Sponsoring Units: DCOMPChair: Patrick Rinke, Fritz Haber Institute
Room: Colorado Convention Center Korbel 1A-1B
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H5.00001: Ab-initio study of the excited state properties of liquid water Invited Speaker: In the last decades, we have witnessed an increasing interest on water and many studies on the geometry and ground state properties have appeared in the literature. Nevertheless, its excited state properties, which are essential to study the chemical and physical behavior of many biological and industrial processes, have not been investigated yet. We present here ab-initio calculations, in the framework of many body Green's function formalism, of liquid water. We use snapshots taken from classical molecular dynamics as input geometries for the study of the electronic and optical spectra. The excitation spectra are first obtained within the Density Functional Theory (DFT) and then corrected within the ``GW'' approximation. The optical absorption spectra are calculated by solving the Bethe-Salpeter equation; they result modified, with respect to the DFT spectra, suggesting the presence of important excitonic effects at low energies. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H5.00002: GW/Bethe-Salpeter Calculations for Solids for Core and Valence Spectra Invited Speaker: The introduction of self-energy corrections to density- functional calculations has greatly improved the one-electron properties of materials. GW calculations are a prime example of this. Excitation spectra, meanwhile, often probe the excited states of materials that involve the excitation of at least one electron across the Fermi level, producing an electron-hole pair. This can be the case for optical absorption and inelastic scattering of x-rays and electrons. Because of interactions in the excited state between the electron and hole, it is preferable to solve the coupled two-particle equation of motion for the electron-hole pair. Bethe-Salpeter-equation (BSE) calculations are a prime example of this. In addition to solving the equation of motion for an interacting electron and hole moving in a static solid, one can try to include lifetime- damping and other effects in the absorption spectrum. In this talk, I would like to present a framework that allows treatment of excitation spectra in a wide variety of materials, ranging from traditional semiconductors and insulators to fairly complicated minerals. This will also facilitate consideration of uniaxial birefringence, spatial-dispersion-induced birefringence, multiplet effects, and electric-quadrupole transitions in x-ray spectra. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H5.00003: New hybrid functionals for ab-initio calculations of properties of solids Invited Speaker: This presentation will address our current efforts to develop more accurate exchange-correlation functionals for Density Functional Theory. The functional to be discussed is a screened Coulomb potential exchange hybrid called \textbf{HSE} [1], which is particularly suited for calculations in solids because it is much faster than regular hybrids and can also be used in metals and systems with negligble band gaps. \textbf{HSE} yields an important improvement in band gap estimates [2] compared to \textbf{LDA}, \textbf{GGAs}, and meta-\textbf{GGAs.} We will also present applications to transition metal oxides, silicon phase transitions and defects [3], and other problems where electron localization seems to play a crucial role [4]. \newline \newline [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, \textit{J. Chem. Phys.} \textbf{118}, 8207 (2003); J. Heyd and G. E. Scuseria, \textit{J. Chem. Phys.} \textbf{120}, 7274 (2004); J. Heyd and G. E. Scuseria, \textit{J. Chem. Phys.} \textbf{121}, 1187 (2005). \newline [2] J. Heyd, J. E. Peralta, G. E. Scuseria, and R. L. Martin, \textit{J. Chem. Phys.} \textbf{128}, 174101 (2005); J. E. Peralta, J. Heyd, G. E. Scuseria, and R. L. Martin, Phys. Rev. \textbf{B 74}, 073101 (2006). \newline [3] E. R. Batista, J. Heyd, R. G. Hennig, B. P. Uberuaga, R. L. Martin, G. E. Scuseria, C. J. Umrigar, and J. W. Wilkins, Phys. Rev. B \textbf{74}, 121102(R) (2006). \newline [4] I. D. Prodan, G. E. Scuseria, and R. L. Martin, Phys. Rev. B \textbf{73}, 045104 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H5.00004: Hybrid functional and selfconsistent GW$\Gamma$ calculations for solids Invited Speaker: $GW$ calculations in various flavors are presented for small gap and large gap systems, comprising typical semiconductors (Si, SiC, GaAs, GaN, ZnO, ZnS, CdS and AlP), small gap semiconductors (PbS, PbSe, PbTe), insulators (C, BN, MgO, LiF) and noble gas solids (Ar, Ne). The general finding is that single shot $G_0W_0$ calculations based on wavefunctions obtained by conventional density functional theory calculations yield too small band gaps, whereas $G_0W_0$ calculations following hybrid Hartree-Fock density functional calculations tend to overestimate the band gaps by roughly the same amount. This is at first sight astonishing, since the hybrid functionals yield very good band gaps themselves. The contradiction is resolved showing that the inclusion of the attractive electron-hole interactions (excitonic effects) are required to obtain good static and dynamic dielectric functions using hybrid functionals. The corrections are usually incorporated in GW using ``vertex corrections'', and, in fact, inclusion of vertex corrections rectifies the predicted band gaps. In order to remove the dependency on the initial wavefunctions we furthermore present selfconsistent GW calculations, again including an approximate treatment of vertex corrections. The results are in excellent agreement with experiment, with a few percent deviations for all considered materials. We conclude that predictive band gap engineering is now possible with the theoretical description approaching experimental accuracy. Finally the relationship between the $GW$ method and hybrid functionals is use to elaborate on the shortcomings of hybrid functionals for large gap systems and metals. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 11:00AM |
H5.00005: Optimized effective potential methods for molecules and solids Invited Speaker: The relation of the optimized effective potential (OEP) method to density-functional theory and in particular to Kohn-Sham (KS) methods is discussed. It is shown that OEP approaches only represent proper KS methods if the basis set for the one-particle functions and the basis set for the effective potential are well balanced. It is shown that exact exchange KS methods based on the OEP approach not only yield band structures that are improved compared to band structures resulting from conventional KS approaches but that exact exchange magnetization-current density-functional theory implemented via an OEP approach represents a framework for a unified treatment of magnetic effects, spin-orbit interactions, and magnetization currents. [Preview Abstract] |
Session H6: Physicists as Entrepreneurs
Sponsoring Units: FIAPChair: Philip Wyatt, Wyatt Technology Corporation
Room: Colorado Convention Center 207
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H6.00001: Wine and Coca Cola: Serendipity and Entrepreneurial success Invited Speaker: Starting a new company from scratch depends critically on identifying a product for which there will be enough demand to generate a profit after a reasonable time period. Many start-ups nowadays obtain early Government grants or contracts ($e. g$. the so-called SBIR programs), but until such contracts begin (if ever!) to provide positive cash flow, a source of other funds become essential. Despite contracts, venture capital, and some friends willing to ``gamble,'' our first entrepreneurial venture ended not with a bang, but a whimper! We had chosen the wrong market. On the day before the company closed its doors, an unanticipated event occurred. We had opened a few bottles of wine for our final farewell but, as we began drinking, we decided to study their laser-scattering properties with our unappreciated and failed instrumentation. The resulting press coverage of a paper reporting that ``tasting'' was phenomenal, though it was published too late to save the company. Starting a second entrepreneurial venture was far more difficult as the first ``angels'' were nowhere to be found. A forgotten proposal by that failed first venture was suddenly funded providing, thereby, the means to start the second. As the second venture began, the success of the wine paper suggested that trying the same method with cola drinks might prove interesting. A new paper reporting on those results was immediately picked up by \textit{Applied Optics} for an issue cover. At first, a particular cola manufacturer was not amused. However, it soon recognized the significance of laser scattering and began to contribute to the Company's support. Complemented by their largesse and the newly funded contract, the Company's instrumentation commercialization programs became sharply focused: refine the development and sale of a new type of absolute light scattering photometer incorporating a laser. We never looked back. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H6.00002: A Mythical History of the Scanning Probe Microscope - How it Could Have Been Invited Speaker: The path from the ground breaking Topografiner by Young et. al. in 1972 to the current Atomic Force Microscopes was tortuous, to say the least. Now as an entrepreneur, they say that you should study the problem, work out a plan, and then execute the plan. Since this rarely works for me in real life, let's follow the mythical history of Phil the physics student whose simple approach to scanning probe microscopes during his summer job may explain life better than real life did. Comparisons between Phil's experience and real life will be made along the way to show how random real life was compared to Phil's straightforward approach. We will follow Phil as he goes from the Scanning Touching Microscope (STM) to the All Fancy Microscope (AFM) and ends up with a current scanning probe microscope. The ``lesson'' in this story is that when you are doing something new, you learn so much while you are doing it that what you thought at the beginning (the plan) is rarely the best way to go. It is more important, I believe, for entrepreneurs to explore possibilities and keep their eyes open along the way rather than pretend the path they are on is the right one. Phil is mythical because he always knew where he was headed and it was always the right direction. So how does Phil's story end? I'm working on it and will tell you at the March Meeting. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H6.00003: A Physicist in Business: Opportunities, Pitfalls, and Lifestyle. Invited Speaker: A traditional education in physics does not normally include business classes or dealing with opportunities to start a company, yet scientists often now start and run small companies. Physicists are mainly interested in technology. However, other factors quickly dominate chances for business success. These include finance, accounting, cash flow analysis, recruiting, interviewing, personnel issues, marketing, investments, retirement plans, patents and other not always so fun activities. Technical decisions are often strongly influenced by company finances and market-analysis. This talk discusses how to recognize opportunity, how to minimize chances for failure, and lifestyle changes one needs to be aware of before entrepreneurship involvement. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H6.00004: Optical View of an Entrepreneur Invited Speaker: The WYKO Corporation was founded on December 27, 1982 to design, manufacture, sell, and service metrology instruments for many applications, with the largest market being in the magnetic data storage industry. WYKO grew out of the research my students and I did at the Optical Sciences Center at the University of Arizona, but it's origins actually started earlier at the Itek Corporation where I went to work after getting my PhD in optics at the University of Rochester. Founding, growing, and cashing out was an unbelievable experience that was more fun than I ever dreamed anything could be. In this talk I will discuss the formation, growth, and eventual selling of the WYKO Corporation to Veeco in 1997. Both the fun parts and the not so fun parts will be discussed. The biggest surprises experienced and what I think are the most important factors in growing a successful high-tech company will be described. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 11:00AM |
H6.00005: Enterpreneurism in Science & Business: A Personal Case Study Invited Speaker: I'll talk about my personal journey from teenage particle physicist to 47-year-old running a company I founded 20 years ago (that happens right now to be poised on the brink of some very exciting developments). I'll talk about personal choices people make, and some of my observations from watching many peoples' careers. I'll talk about my work in building new directions in science, and how it relates to Mathematica and our company. I'll also talk about why we hire a lot of physicists, and what they do. [Preview Abstract] |
Session H7: Nuclear Weapon Missions in the 21st Century
Sponsoring Units: FPSChair: David Hafemeister, Center for International Security and Cooperation, Stanford University
Room: Colorado Convention Center Korbel 4A-4B
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H7.00001: The Future of the U.S. Nuclear Weapons Program Invited Speaker: This paper will examine our plans for the future of the U.S. nuclear weapons program including efforts to ``transform'' the stockpile and supporting infrastructure. We proceed from the premise that the United States will need a safe, secure, and reliable nuclear deterrent for the foreseeable future. Moreover, the Stockpile Stewardship Program is working. Today's stockpile---comprised of legacy warheads left over from the Cold War---is safe and reliable. That said, we see increased risk, absent nuclear testing, in assuring the \textit{long-term safety and reliability} of our current stockpile. Nor is today's nuclear weapons complex sufficiently ``responsive'' to fixing technical problems in the stockpile, or to potential adverse geopolitical change. Our\underline { task} is to work to ensure that the U.S. nuclear weapons enterprise, including the stockpile and supporting infrastructure, meets long-term national security needs. Our \underline {approach} is to develop and field replacement warheads for the legacy stockpile---so-called Reliable Replacement Warheads (RRW)---as a means to transform both the nuclear stockpile and supporting infrastructure. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H7.00002: Strategic Command's Role with Nuclear Weapons Invited Speaker: |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H7.00003: AAAS Assessment of the Role of the Reliable Replacement Warhead in the US Nuclear Weapons Complex Invited Speaker: The American Association for the Advancement of Science sponsored a study of the role of the Reliable Replacement Warhead (RRW) in the US Nuclear Weapons Complex during the latter part of 2006. As the Chair of that study I will report our principal findings and recommendations. Our conclusions are based on the experience and knowledge of the committee members, the information available in numerous reports and related analyses, and on presentations and discussions with DOE/NNSA officials, staff members from the Lawrence Livermore, Los Alamos, and Sandia National Laboratories, and others with special expertise and perspectives. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H7.00004: What Are Nuclear Weapons For? Invited Speaker: Through the decades of the Cold War the prospect of a nuclear holocaust was all too real. With the demise of the Soviet Union and the end of the Cold War, that threat to civilization as we know it had receded. But today we face a grave new danger, the acquisition of nuclear weapons by hostile or unstable governments and terrorists. What can and should we be doing to meet this challenge and prevent the world's most dangerous weapons from falling into very dangerous hands? Are there any reasons for us to still retain thousands of nuclear warheads in our arsenals? What are they for? Can we rekindle the bold vision of a world free of nuclear weapons that President Reagan and General Secretary Gorbachev brought to their remarkable summit meeting at Reykjavik twenty years ago, and define practical steps toward achieving such a goal? [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 11:00AM |
H7.00005: Nuclear Weapon Missions After the Cold War Invited Speaker: |
Session H8: Focus Session: Novel Superconductors III: Mostly MgB2
Sponsoring Units: DMPChair: Anthony Carrington, Bristol University
Room: Colorado Convention Center Korbel 1C
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H8.00001: Large anisotropic normal-state magnetoresistance in clean MgB$_2$ thin films Invited Speaker: MgB$_2$ is arguably the first material which shows clear multiband superconductivity and two energy gaps. The interplay between the interband and intraband scattering as well as electron-phonon coupling has manifested in many physical properties. In this talk, I will review the magnetoresistance measurements of the normal state of MgB$_2$. We have shown large normal-state magnetoresistance with temperature-dependent anisotropy in very clean epitaxial MgB$_2$ thin films (residual resistivity much smaller than 1 $\mu \Omega$cm) grown by hybrid physical-chemical vapor deposition.[1] The magnetoresistance shows a complex dependence on the orientation of the applied magnetic field, with a maximum magnetoresistance (MR~136\%) observed at the field $H$ parallel to $ab$ plane at low temperature. However, the angular dependence changes dramatically as the temperature is increased, and at high temperatures ($T >$ 100 K), the magnetoresistance maximum changes to the $H$ perpendicular to $ab$ direction. We attribute the large magnetoresistance and the evolution of its angular dependence with temperature to the multiple bands with different Fermi surface topology in MgB$_2$ and the relative scattering rates of the $\sigma$ and $\pi$ bands, which vary with temperature due to stronger electron-phonon coupling for the sigma bands. The change of anisotropy with disorder has also been reported in neutron irradiated MgB$_2$ thin films. \newline Work done in collaboration with B. T. Liu, Y. F. Hu, J. Chen, H. Gao, L. Shan, H. H. Wen, A.V. Pogrebnyakov, J. M. Redwing, and X. X. Xi. \newline \newline [1] Qi Li et al., Phys. Rev. Lett., 96, 167003 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H8.00002: Tunneling study of two-band superconductivity in MgB$_{2}$ Ke Chen, Chenggang Zhuang, Yi Cui, Qi Li, Xiaoxing Xi, Zi-Kui Liu Thin film Pb/barrier/MgB$_{2}$ sandwich-type tunnel junctions have been fabricated. The MgB$_{2}$ films were epitaxially grown on (211) MgO substrates with c-axis tilted by 19.5 degrees with respect to the substrate normal by hybrid physical-chemical vapor deposition. Tunneling from both $\sigma $ and $\pi $ bands of MgB$_{2}$ to Pb is observed from the current-voltage characteristics (CVCs). The temperature dependence and the magnetic field dependence of the two gaps were obtained by fitting the CVCs in the framework of two band superconductivity. It shows that at zero temperature and zero field, $\Delta _{\sigma } \quad \approx $ 2.3 mV and $\Delta _{\pi } \quad \approx $ 7.4 mV. As the applied magnetic field normal to the substrate increases, the contribution from the $\pi $ gap is suppressed much more quickly than the $\sigma $ gap, which is in agreement with lower critical field corresponding to the $\pi $ gap. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H8.00003: Mixed State Dissipation in Zero Temperature Limit Enhanced by Two Gap Effect in MgB$_{2}$ Thin Films Ying Jia, Yan Huang, Huan Yang, Lei Shan, Cong Ren, Hai-Hu Wen, Chenggang Zhuang, Yi Cui, Qi Li, Xiaoxing Xi Through the measurements of resistive transition, point contact tunneling spectrum and Hall effect on crystalline MgB$_2$ thin films, the dissipation in the mixed state has been exclusively investigated. It is found that the resistive transition broadens monotonously with the magnetic field leading to a non-vanishing mixed state dissipation in zero temperature limit. Hall effect and point contact tunneling measurements indicate that this dissipation is contributed by the vortex motion and associated with the losing of long range phase coherence induced by the proliferation of the quasiparticles from the $\pi$-band. These results suggest the existence of the vortex quantum liquid enhanced by the two gap effect. Some preliminary results on mesoscopic-bridges will also be reported showing the interesting interplay of the two band superconductivity. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H8.00004: Observation of the Leggett's collective mode in MgB$_2$ two-band superconductor G. Blumberg, A. Mialitsin, B. S. Dennis, N. D. Zhigadlo, J. Karpinski We report observation of novel collective mode in the multi-band MgB$_2$ superconductor by resonant electronic Raman spectroscopy. The mode appears below T$_c$ in the $A_1g$ scattering channel at 9.2 meV, which is in-between the two gap values, 4.5 meV for the fundamental gap in the $\pi$-band and 13.5 meV for the gap in the $\sigma$-band. We attribute this excitation to collective mode first discussed by Leggett: If a system contains two coupled superfluids a simultaneous cross tunneling of a pair of electrons become possible. The mode is caused by dynamical oscillations of the pairs between the two superfluids leading to small fluctuations of the relative phase of two superconducting condensates. For MgB$_2$ the the oscillations between the condensates involve scattering of a pair of $\sigma$-band electrons with momentum $(k, -k)$ into a pair of $\pi$-band electrons with momentum $(k^\prime, -k^\prime)$. The symmetry and energy of observed mode is consistent with theoretical predictions. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H8.00005: Interplay of ballistic and diffusive superconductivity in the vortex core in the model two-band system MgB$_2$ K. Tanaka, M. Eschrig, D. F. Agterberg A revived interest in multi-band superconductivity has emerged due to the unexpected and interesting simultaneous presence of diffusive and ballistic bands in the superconductor MgB$_2$. Motivated by recent experimental data on the vortex state in MgB$_2$ obtained by scanning tunneling spectroscopy, we theoretically study the intriguing effects of superconductivity in a diffusive band (`$\pi$ band') induced by superconductivity in a ballistic band (`$\sigma $ band'). We apply a unique model that has been developed recently [1] for describing such a system, based on coupled Eilenberger and Usadel equations. Results are presented for the spatial variation of the order parameter, the current density, and the vortex core spectrum in the two bands. A particularly interesting result emerging from our studies is the possibility of additional bound states near the gap edge in the `strong' $\sigma$ band, which arise from hybridization with the `weak' $\pi$ band. The development of such gap-edge bound states is examined for various sets of physical parameters that are relevant for MgB$_2$. We will also discuss the induced Kramer-Pesch effect in the $\pi$ band and magnetic-field dependence of vortex core size. [1] K. Tanaka, D. F. Agterberg, J. Kopu, M. Eschrig, Phys. Rev. B {\bf 73}, 220501(R) (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H8.00006: Breakdown of phase-locked states and limit of the upper critical field controlled by interband scattering in two-gap superconductors. Alexander Gurevich The effect of weak interband scattering on the maximum upper critical field $H_{c2}$, which can be achieved in a two-band superconductor by increasing intraband impurity scattering is considered. Using the two-gap Usadel equations, we show how weak interband scattering provides the crossover from the orbitally limited to the paramagnetically limited $H_{c2}$, both in bulk samples and thin films. The results are applied to describe high $H_{c2}$ values in carbon-doped MgB$_2$ films. It is shown that interband scattering produces mixed gradient terms $\propto Re(\Pi^{*}\Psi_\sigma^{*}\Pi\Psi_\pi)$ in the free energy, where $\Pi = \nabla +2\pi A/\phi_0$, and $\Psi_\sigma=\Delta_\sigma\exp(i\theta_\sigma)$ and $\Psi_\pi = \Delta_\pi\exp(i\theta_\pi)$ are intraband order parameters. The mixed gradient coupling can provide parametric excitation of the plasmon Legget mode or large-amplitude interband phase textures by ac currents. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H8.00007: Effect of oxygen alloying on scattering processes in MgB$_{2}$ Raghuram Gandikota, Rakesh Singh, Yi Shen, Nathan Newman, John Rowell The effect of oxygen alloying on T$_{c}$, resistivity, H$_{c2}$, and J$_{c}$ of MBE-grown MgB$_{2}$ films was studied. Oxygen was introduced either during growth or by ex-situ heating in oxygen atmosphere. While the concentration of oxygen increased from 1 to 9{\%}, \textit{dH}$_{c2}^{\vert \vert }$\textit{(0)/dT} of the films, at T$_{c}$, increases from 0.67T/K to 1T/K. J$_{c}$s greater than 400 kA/cm$^{2}$( at 8T, 4.2K) have been observed in these films and increased oxygen alloying changes J$_{c}$ very little. $H_{c2}^{\vert \vert }(0)$ and $H_{c2}^{-}$(0) values, obtained in these films with T$_{c}$s $\sim $ 31K, are as high as 43T and 32T respectively. These values are significantly higher than the maximum $H_{c2}^{\vert \vert }(0)$ value obtained for ion irradiated films, neutron irradiated MgB$_{2}$ bulk, and C-alloyed MgB$_{2}$ bulk. ~While the J$_{c}$ values of the oxygen alloyed films are higher than the C-alloyed Penn State films, the H$_{c2}$(0) values are, however, still significantly smaller than the record values found in the C-alloyed films. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H8.00008: MgB$_{2}$ Tunnel Junctions with Native or Thermal Oxide Barriers Raghuram Gandikota, Rakesh Singh, Yi Shen, Nathan Newman, John Rowell MgB$_{2 }$tunnel junctions (MgB$_{2}$/barrier/MgB$_{2})$ were fabricated using oxides of Mg and stoichiometric MgB$_{2}$, as the tunnel barrier. The sum of the superconducting gaps ($\pi $-gap) observed in conductance-voltage (G-V) measurements was as high as 4.3mV at 4.2 K and a finite value was found for temperatures above 30K. The G-V data exhibit smeared BCS densities of states, indicative of a degraded layer at the electrode/barrier interface. The presence of such an interface might also explain the lack of supercurrents above 20K in junctions exhibiting gap structures above 30K and even in shorted junctions. A subgap current was also observed and was not found to strongly depend on the oxide stoichiometry.~ [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H8.00009: Superconductivity in M$_{n+1}$AX$_{n}$ compounds Michael Osofsky, R. J. Soulen, Jr., S. B. Qadri, M. W. Barsoum We present evidence for the presence of bulk superconductivity in several members of the M$_{n+1}$AX$_{n}$ (M=early transition metal, A=group A element, and X=carbon and/or nitrogen; n=1-3) family of compounds. Samples were synthesized by using standard ceramic techniques. We will present resistivity, susceptibility and specific heat data for these materials. We will also show detailed x-ray diffraction and microscopy data to demonstrate that impurity phases cannot account for the results. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H8.00010: Effects of Grain Size and Doping Level on the Critical Current Density of the Ti-sheathed MgB$_{2}$ Superconducting Wires with SiC Doping Gan Liang, Hui Fang, Cad Hoyt, Z. P. Luo, F. Yen, M. Hanna, A. Alessandrini, K. Salama The effects of the grain size and doping level on the critical current density ($J_{c})$ of the SiC-doped Ti-sheathed MgB$_{2}$ superconducting wires were studied. Two groups of samples were prepared: for the first group, the average size of the SiC grains was 20 nm and the doping levels were 5{\%}, 10{\%}, and 15{\%}; for the second group, the doping level of the SiC dopant was 10{\%} and the average sizes of the SiC particles were 20 nm, 43 nm, and 123 nm. All of the samples were sintered at 800 C for 30 minutes. Contrary to the $J_{c}$ results reported on the SiC-doped Fe-sheathed MgB$_{2}$ wires by some other groups, we found that the $J_{c}$ for the SiC-doped Ti-sheathed MgB$_{2}$ wires decreases with both the increase of SiC concentration and the decrease of the grain size. Only for the wires with average grain size of 123 nm, $J_{c}$ is greater than that of the un-doped MgB$_{2}$ wires. A simple model is proposed to explain the formation of the impurities in the cores of these doped MgB$_{2}$ wires. This unusual dependence of $J_{c}$ on the size and doping-level of the SiC dopant is discussed in association with the magnetization, resisitivity, XRD, TEM, and SEM results. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H8.00011: Superconducting and Normal State Properties of OsB$_2$* Yogesh Singh, A. Niazi, X. Zong, B.J. Suh$^{\dagger}$, M.W. Vannette, R. Prozorov, D.C. Johnston OsB$_{2}$ is a layered superhard metallic material that was found to superconduct below $T_{\rm c} = 2.1$~K.$^1$ We report the first detailed measurements of the static and dynamic magnetic susceptibilities $\chi$, electrical resistivity, heat capacity $C_{\rm p}$, penetration depth, and $^{11}$B NMR on OsB$_{2}$ to characterize its superconducting and normal state properties. The results confirm that OsB$_{2}$ is a bulk superconductor below $T_{\rm c} = 2.1$~K\@. Its properties can be described by a close to weak-coupling \emph{s}-wave BCS model with an electron-phonon coupling constant $\lambda = 0.4$--0.5, $\Delta(0)/(k_{\rm B}T_{\rm c}) \approx 1.9$, a small Ginzburg-Landau parameter $\kappa$ of order 5 or less, and a small zero-temperature critical magnetic field of roughly $500~$Oe. The $^{11}$B NMR measurements in the normal state show a nuclear spin-lattice relaxation time $T_1 = 2.1$~s at room temperature and a Korringa law with $T_1T = 610$~s$\cdot$K at lower $T$, and a correspondingly small $T$-independent Knight shift. These results indicate a small \emph{s} character of the conduction electron wave function at the B site at the Fermi level. Our results will be compared to corresponding data for MgB$_2$.\\ 1. J. K. Vandenberg et al., Mater. Res. Bull. {\bf 10}, 889 (1975).\\ $^*$Supported by the USDOE under Contract No.\ W-7405-Eng-82.\\ $\dagger$ Permanent address: Dept. Phys., The Catholic Univ. Korea. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H8.00012: Hot isostatic pressure synthesis of MgB2 from sonochemically modified precursors Brett A. McCarty, Amanda Toomey, Ruslan Prozorov Close to theoretical density bulk MgB2 was obtained by hot isostatic pressing (HIP) from precursors subject to high-intensity ultrasonic treatment. Comparative results obtained on unmodified and sonicated boron of various purities (from 80{\%} to 99{\%}) and in combination with various phases of magnesium are reported. Samples made from sonicated precursors showed improvements in superconducting properties without affecting Tc. Analysis of magnetization, SEM images, and XRD will be presented. [Preview Abstract] |
Session H9: Superconductivity: Josephson Junctions, Proximity Effect & Squids I
Sponsoring Units: DMPChair: Laura Greene, University of Illinois
Room: Colorado Convention Center Korbel 1D
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H9.00001: Search for second-order Josephson tunneling in Superconductor-Ferromagnet-Superconductor junctions M.J.A. Stoutimore, D.J. Van Harlingen, S.M. Frolov, V.V. Bolginov, V.A. Oboznov, V.V. Ryazanov We have fabricated Nb-CuNi-Nb SFS (Superconductor-Ferromagnet-Superconductor) $\pi $-Josephson junctions in a geometry that allows us to perform transport measurements to determine the uniformity of the sample and subsequent current-phase measurements to look for period doubling on the same junction. Junctions of this type exhibit transitions between the 0-junction and the $\pi $-junction state as a function of the ferromagnetic barrier thickness and temperature. Near the 0-$\pi $ transition points, it is predicted that first-order tunneling will be suppressed and the second-order term, proportional to sin(2$\phi )$, may dominate. The interpretation of past experiments to measure this term have been ambiguous due to concerns that non-uniformity in the ferromagnetic layer could mimic second-order Josephson behavior by producing half-integer Shapiro steps. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H9.00002: Superconducting Proximity Effects in Epitaxial Cr/Nb Bi-Layers: A Novel Approach Using a Three-Terminal Device Architecture Paul B. Welander, Michael R. Vissers, James N. Eckstein We have studied the proximity effects between a superconductor (Nb) and a SDW antiferromagnet (Cr) using epitaxial thin films and a novel three-terminal device structure. Single-crystal Cr/Nb bi-layers are grown on sapphire by means of molecular beam epitaxy. Interface quality is crucial -- oxidation of the Cr surface before Nb deposition can render the proximity effects unseen. Our three-terminal device architecture allows independent measurement of the bi-layer sheet-resistance and the Nb-Cr contact resistance. We find that the contact resistance goes to zero a few tenths of a degree below the Nb critical temperature. Over this temperature range the junction conductance increases at a rate well beyond that predicted by Andreev reflection alone. The bi-layer sheet resistance also shows a slight increase with decreasing temperature. As the devices are cooled further we find both linear and non-linear current-voltage regimes. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H9.00003: Proximity Effect in Nb/Mg/CoFe Trilayers. Seong Kook Choi, Jun Hyung Kwon, Kookrin Char We have fabricated the Nb/Mg bilayer and Nb/Mg/CoFe trilayer samples by varying Mg layer thickness and measured their superconducting transition temperature $T_c $ electrically using the 4-prove method. Mg normal layer was used to investigate the effect of its small atomic number, since we have observed previously the largely different behavior when Au, Cu and Al were used. When the Al layer was used, a very unusual behavior was found. Because of chemical interaction between Nb and Mg, the $T_c $ transition curves did not show sharp $T_c $ transition. In order to prevent this interaction, we inserted 2 nm thick Al layer between Nb and Mg. In the case of Nb/Al(2nm)/Mg, we observed $T_c $ behavior consistent with a conventional SN theory. In the case of Nb/Al(2nm)/Mg/CoFe with fixed thicknesses of Nb and CoFe layer, the $T_c $ values exhibited two distinct behavior as the thickness of Mg increased. The $T_c $ value of S/N/F trialyer increased rapidly until the Mg thickness reached a few nm. As Mg thickness increased further to 200 nm, the $T_c $ value of S/N/F decreased again, following closely those of the S/N data. Overall, the Mg data basically followed those of Al data, suggesting that the low atomic number of the normal layer is important in observing the unusual proximity effect in SNF trilayers. Our analysis of the interface effect using an Usadel picture will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H9.00004: Proximity effect in superconducting/magnetic nanostructures. Jose L. Vicent, Elena Navarro, Nuria O. Nunez, Alfonso Cebollada, Alejandro Perez-Junquera, Jose I. Martin Fe nanoparticles have been grown by sputtering technique on sapphire substrates. The fabrication parameters (substrate temperature, rate, {\ldots}) allow us to change the nanoparticle sizes at will. Different elements have been deposit on top of these Fe nanoparticles. The magnetic behaviour of the Fe nanoislands strongly depends of the capping material, for instance capping with thin Al film does not change the magnetic behaviour of the nanoislands, but capping with thin Pt film induces an enhancement of the magnetic nanoparticles coupling, and finally exchange bias develops in the nanostructured system Cr layer on top of Fe nanoislands. Nb films have been deposited on top of these nanostructures. The superconducting/magnetic proximity effect has been studied for different capping materials (Al, Pt, and Cr) and different Fe nanoparticle sizes. We will show that the superconducting proximity effect strongly depends on the capping materials, with interplay between exchange bias and superconductivity leading to enhancement of the superconducting critical temperature. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H9.00005: Effects of magnetic fluctuation on 0-$\pi$ transition in a superconductor-ferromagnet-superconductor junction Michiyasu Mori, Shin'ichi Hikino, Saburo Takahashi, Sadamichi Maekawa There has been growing interest in a superconductor- ferromagnetic metal-superconductor (SFS) junction, in which the Josephson critical current, $I_{\rm c}$, shows a cusp as a function of thickness of ferromagnetic-layer, $d$, and/or temperature, $T$. Such a non-monotonous behavior, which is in marked contrast to $I_{\rm c}$ in a conventional Josephson junction, originates from the fact that the current-phase relation is shifted by $\pi$. This is called $\pi$-state. We study the influence of magnetic fluctuation on $I_{\rm c}$ in the SFS junction by a tunneling Hamiltonian approach. An analytical formula of $I_{\rm c}$ is given in the fourth order perturbation theory as regards the tunneling matrix element. Electrons propagate diffusively in the FM due to non-magnetic- and magnetic scatterings. The $I_{\rm c}$ exhibits the damped oscillatory dependence on $d$, and shows the transition between ${\it 0}$- and $\pi$-{\it states}. When the superconducting transition temperature is comparable to the ferromagnetic Curie temperature, the period of oscillation is elongated by increasing $T$ due to the magnetic fluctuation, which plays an important role in the $0$-$\pi$ transition, in particular, with $T$. Our results present an appropriate combination of a superconductor and a ferromagnetic metal to control the ${\it 0} $- and the $\pi$-{\it states}. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H9.00006: Theory of long range superconducting proximity effect in half-metallic ferromagnets: the role of disorder Matthias Eschrig, Tomas Lofwander A Josephson supercurrent between two singlet superconducting electrodes separated by half-metallic CrO$_2$ has been recently reported. Only conduction electrons with equal spins can be paired in a half metal. Both the mechanism involved in the conversion process from singlet to equal spin Cooper pairs at the interface, as well as whether the symmetry of the pairing correlations in ferromagnets is odd-frequency s-wave or even frequency p-wave for different amounts of disorder, are subjects currently under debate. We report an extensive theoretical investigation of the Josephson effect through half-metals with arbitrary impurity concentration. We discuss a model for the conversion mechanism between the singlet and triplet supercurrents at the interfaces and we analyze the symmetry of the pairing components that contribute to the supercurrent in the entire range from ballistic to diffusive transport. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H9.00007: Electrical Characterization of Superconducting Microbridge Josephson Junctions with Ferromagnetic Strip Luis Gomez, Shinichi Kitamura, Takahiro Kubo, Haruhisa Kitano, Atsutaka Maeda We have fabricated high temperature superconductor microbridges where thin ferromagnetic strips, with nanometer size widths, have been deposited across the center of the microbridge. We characterized these devices as a function of temperature, magnetic field, and microwave power and frequency in order to evaluate their potential as Josephson junctions. The devices fabricated until now present promising Josephson-like characteristics and could be regarded as $S-S'-S$ weak links, although their $I-V's$ are flux flow type for all temperatures below $T_{c}$. We are currently modifying the bridge as well as the ferromagnetic strip dimensions (width and thickness) in order to improve the shape of the $I-V's$. In this talk, we will present our results to date. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H9.00008: Long range odd frequency triplet components in F/S/F trilayers Han-Yong Choi, Nayoung Lee We calculate the singlet and triplet pairing amplitudes of the ferromagnet/superconductor/ferromagnet (F/S/F) trilayers by solving the Usadel equation linearized near the transition temperature $T_c$ in the diffusive limit. Here, S is a conventional singlet s-wave supercondcutor like Nb and F is a ferromagnet like CoFe. A particular focus is on the odd frequency triplet pairing components which have a long length scale in F region unlike the singlet pairing component. The triplet components in F/S/F are induced by the proximity effects when the two F's have non-colinear orientations. We will identify the fingerprints of the odd frequency triplet pairing components which will help to directly observe them in the F/S/F trilayers. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H9.00009: A Strategy for Fitting a Three Parameter Model of Charge Transport in Ferromagnetic/Superconductor Point Contacts Charles W. Smith, Paul J. Dolan, Jr. We study charge transport in ferromagnet/superconductor point contacts that exhibit effects of spin polarization (P), interface elastic scattering (Z), and bulk inelastic scattering ($\Gamma$). We demonstrate a strategy to extract values for P, Z and $\Gamma$ from conductance data, as a function of temperature. The resulting parameter set can then be used to fit a model that describes charge transport in this type of point contact. Experimental examples will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H9.00010: Tunneling through the metal-insulator transition with Nb/Nb$_{x}$Si$_{1-x}$/Nb Josephson junctions Burm Baek, Alan Kleinsasser, George Kerber, Paul Dresselhaus, Samuel Benz We have successfully fabricated Josephson junctions with Nb$_{x}$Si$_{1-x}$ barriers to study the metal-insulator transition. By tuning the relative concentration of the barrier we can explore the electrical properties as different junctions are controllably tuned to concentrations in either the metallic or insulating states.. By cosputtering Nb and Si, reliable control of barrier properties has been achieved. In the metallic barrier regime that has higher Nb content, the uniformity, reproducibility, and tunability have already proven useful for voltage standard applications. In the insulating regime that has low Nb concentration, we observe hysteretic junction behavior. Details of the crossover from metallic to insulating behavior of the Josephson barrier will be discussed along with the ramifications for superconductive devices using this technology. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H9.00011: Superconducting Proximity Effect in Thin Semiconducting Films Michael Vissers, Soren Flexner, Paul Welander, Kevin Inderhees, James Eckstein The superconducting proximity effect changes both the transport properties of the N-layer as well as the conductance between the N and S layers. We use a novel 3 terminal device structure to probe this which provides two resistance measurements allowing us to measure both the N-layer sheet resistance, Rs, as well as the junction conductance, Gc. When the N-layer is a degenerate semiconductor, the changes in these quantities are large. Gc increases much more than the factor of 2 that Andreev reflection theory predicts, and both Rs as well as Gc exhibit reentrance as a function of temperature. We interpret these changes as the N-S boundary moving into the semiconductor increasing Gc while simultaneously removing volume in the N-layer that had been used in normal transport. Magnetic fields applied both parallel and perpendicular to the junction cause the maximum conductance to increase while the sheet resistance rises. The magnitude of the necessary field implies a local proximity effect. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H9.00012: Effect of heavy electron mass $m^{\star}$ on Andreev reflection (AR) in heavy-fermion/superconductor (HF/S) point-contacts H. Stalzer, W.K. Park, L.H. Greene, J.L. Sarrao, J.D. Thompson, J. Frederick, P. Canfield, L.D. Pham, Z. Fisk We investigate the effect of $m^{\star}$ on Andreev reflection in HF/S point-contacts (PC) by measuring the differential electrical conductance at temperatures between 1.5 and 10\,K. An electrochemically etched Nb tip ($T_{\rm c}$\,=\,9.2\,K) is brought into contact with HF single crystals (CeCoIn$_5$, CeRhIn$_5$, YbAl$_3$) of varying $m^{\star}$. Our conductance signals show a clear superconducting gap structure expected for PC in the Sharvin regime which can be fitted by the Blonder- Tinkham-Klapwijk model. Preliminary results on CeCoIn$_5$ at temperatures above its $T_{\rm c}$\,=\,2.3\,K indicate an enhanced AR signal of similar magnitude as in Au/Nb PC which is in contrast to CeCoIn$_5$/Au PC as reported earlier\,[1]. We discuss this in the context of a two fluid model which considers heaviness and bandstructure of the HF quasiparticles and which may also explain the asymmetric conductance background observed in many heavy-fermions below a characteristic temperature $T^{\star}$. [1] W.~K. Park {\it et al.}, PRB {\bf 72}, 052509 (2005). $-$ This work was supported by the Deutsche Forschungsgemeinschaft, and U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the CMM at UIUC and the NSF-DMR-0503360 at UC. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H9.00013: Density of States measurements of AlMn alloys with tunable superconducting gaps Galen O'Neil, Dan Schmidt, Nathan Miller, Joel Ullom, Anthony Williams, Gerald Arnold, Steven Ruggiero Superconductors with tunable transition temperatures and energy gaps are useful for a variety of device applications. For instance, transition-edge $\mu$-calorimeter sensors and electron-tunneling $\mu$-refrigerators have been made based on Al with various levels of Mn doping. The transition temperature of AlMn can be continuously tuned from about $\sim 1.35~K$ to below 10~mK by Mn concentrations up to several thousand ppm. Here, we present detailed measurements of the superconducting density of states of AlMn made with both normal metal-insulator-superconductor and superconductor-insulator-superconductor tunnel junctions. We compare the data to theoretical expectations for magnetic and non-magnetic impurities in superconductors. In particular we show that the density of states of AlMn is not gapless as we would expect from magnetic impurities. Rather, present evidence indicates that the density of states is essentially BCS-like with an increased Dynes parameter. The increased Dynes parameter corresponds to a broadened peak at the gap and an increase in subgap states. We discuss the implications of this behavior for tunnel junction devices with AlMn electrodes. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H9.00014: Making an Analogy between Forming a Josephson Junction and the Use of Wave Functionals to Form Soliton- Anti Soliton Pairs in Both Biological and Condensed Matter Physics Andrew Beckwith Our paper generalizes techniques initially explicitly developed for CDW applications only with respect to what is needed for multi dimensional instantons forming in complex condensed matter and/or bio physics applications. This involves necessary conditions for formulation of a soliton- anti soliton pair, assuming a minimum distance between charge centers, and discusses the prior density wave physics example as to why a Pierels gap term is added to the tilted washboard potential for insuring the formation of scalar potential fields. We state that much the same methodology is needed for higher dimensional condensed matter systems and bio physics, with strict conditions stated as to necessary potential terms needed to form a Josephson junction interpretation as to how to form wave functionals with necessary Gaussian character which can model instanton physics via a process analogous to Pierels gap and Brillouin zone boundary physics. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H9.00015: Dynamics of a current-biased Bi$_2$Sr$_2$CaCu$_2$O$_{\8+d}$ surface intrinsic Shaoxiong Li, Wei Qiu, Siyuan Han, Y. F. Wei, X. B. Zhu, C. Z. Gu, S. P. Zhao, H. B. Wang The dynamics of switching from superconducting to voltage state of a current-biased Bi$_2$Sr$_2$CaCu$_2$O$_{\8+d}$ surface intrinsic Josephson junction is studied by measuring the temperature-dependent switching current distributions at temperatures ranging from 15 mK to 4.8 K. Crossover from phase diffusion to Kramers switching and thermal activation to quantum tunneling have been observed. Quantitative analysis indicates that the dynamics of a single high T$_c}$ Josephson junction can be described very well by theories whose predictions have been confirmed by experiments performed on low T$_c}$ Josephson junctions. Our result aslo indicate that the d-wave order parameter symmetry has no observable effect on the dynamics of Bi$_2$Sr$_2$CaCu$_2$O$_{\8+d}$ surface intrinsic Josephson junctions where current is along c-axis of the crystal. [Preview Abstract] |
Session H10: Spin Glasses and Other Systems with Disorder
Sponsoring Units: DMPChair: Daniel Silevitch, University of Chicago
Room: Colorado Convention Center Korbel 1E
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H10.00001: Ferromagnetism and Glassiness in a Disordered Ising Magnet C. Ancona-Torres, T.F. Rosenbaum, G. Aeppli LiHo$_{0.20}$Y$_{0.80}$F$_{4}$ is a diluted, dipolar-coupled Ising magnet on the boundary between ferromagnet and spin glass. Application of a magnetic field transverse to the Ising axis introduces quantum tunneling modes that speed relaxation and drive an order-disorder transition. We determine the Transverse Field - Temperature phase diagram through measurements of the linear and nonlinear magnetic susceptibility, as well as the spectral response of the system. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H10.00002: The Disordered Ising Ferromagnet in Transverse Field D.M. Silevitch, D. Bitko, J. Brooke, G. Aeppli, T.F. Rosenbaum The phase diagram and critical behavior of the model quantum magnet LiHo${}_x$Y${}_{1-x}$F${}_4$ were studied for x=0.44, 0.65, and 1.0. Measurements of the phase boundary and critical exponents show mean-field behavior in the classical variable, temperature, but significant deviations from mean-field character emerge in the presence of the quantum variable, transverse magnetic field. Rounding and hysteresis of the magnetic susceptibility in the highly disordered and low temperature limit point to an amplified role for non-equilibrium physics in this regime. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H10.00003: Specific Heat of the Dilute Ising Magnet LiHo$_x$Y$_{1-x}$F$_4$ Jeffrey Quilliam, Chas Mugford, Lauren Lettress, Jan Kycia We will present specific heat results on the dilute dipolar-coupled Ising magnet LiHo$_x$Y$_{1-x}$F$_4$. This material was previously observed to change from a spin glass to an unusual ``anti-glass'' state at a Ho concentration of $x\simeq 0.045$. This state showed dynamics that are very different from those of a spin glass\footnote{S. Ghosh \emph{et al.}, Science {\bf 296}, 2195 (2002)} and also exhibited sharp features in its specific heat at around 100 and 300 mK\footnote{S. Ghosh \emph{et al.}, Nature {\bf 425}, 48 (2003).}. In contrast, our measurements of the heat capacity do not reproduce these sharp features and instead find broad curves for three concentrations (1.8\%, 4.5\% and 8.0\%). Integrating $C/T$ reveals a residual entropy $S_0$ which is 0 for 8.0\% Ho but increases with lower concentration (to $0.31R$ at 1.8\% Ho). This provides some evidence for a change to a different magnetic ground state below 8.0\% Ho and is qualitatively consistent with Monte Carlo simulations\footnote{J. Snider and C. C. Yu, Phys. Rev. B {\bf 72}, 214203 (2005).}. AC susceptibility measurements probing the dynamics of this system are currently being performed and results will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H10.00004: Study of magnetism at the atomic level in highly doped LiHo$_x$Y$_{1-x}$F$_4$ J.A. Rodriguez, A.A. Aczel, S.R. Dunsiger, G.J. MacDougall, G.M. Luke, P.L. Russo, A.T. Savici, Y.J. Uemura, C.R. Wiebe $\mathrm{LiHo_xY_{1-x}F_4}$ is believed to be a physical realization of the ferromagnetic transverse field Ising model. Nevertheless, studies show deviations from the theoretical expectations. One of these differences is the appearance of an anomalous paramagnetic phase after the spin glass state is destroyed by dilution. $\chi_{AC}$ measurements motivated some authors to refer to this phase as an ``anti-glass". Most of the experimental results on the anti-glass phase have been performed at a single Ho concentration ($x=4.5$\%). In order to better understand the magnetic properties of this system, we performed zero field and longitudinal field $\mu$SR measurements in three highly diluted samples (x = 0.02, 0.045 and 0.08), each of which was expected to lie within the anti-glass phase. Our measurements probe the dynamic behavior of the system in a higher frequency window than the existing $\chi_{AC}$ studies, and have the advantage of being performed with a microscopic probe. We found that there is no qualitative difference on the magnetic behavior upon dilution. In this talk we will discuss the fluctuation of magnetic moments down to base temperature ($\sim$20mK) and in the presence of a transverse magnetic field. We will also discuss our results on the basis of an independent-ion picture. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H10.00005: ABSTRACT HAS BEEN MOVED TO H11.00013 |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H10.00006: Substitution effects in the spin-gap compound BiCu$_{2}$PO$_{6}$ A.V. Mahajan, B. Koteswara Rao, J. Bobroff BiCu$_{2}$PO$_{6}$ has a structure where the Cu spins appear to form a two-leg ladder. From our susceptibility measurements, we indeed find an exponential decrease of the spin-susceptibility ($\chi _{spin}$(T)) below a broad maximum with a spin-gap ($\Delta )$ of about 40 K. Analysis of $\chi _{spin}$(T) indicates that the leg and rung exchange couplings are nearly equal and further that the inter-ladder coupling is not negligible. No long-range order was observed down to 1.8 K. Heat capacity measurements yield $\Delta \quad \approx $ 40 K. Isovalent substitutions (Zn and Ni) at the Cu site destroy the spin-gap and induce a transition to a spin-glass/disordered magnetic state below about 5 K, as seen from our susceptibility, heat capacity, and $\mu $SR data. Heterovalent substitutions (Pb, Sr) at the Bi site, which should release one hole per substituent, do not significantly change the magnetic behavior other than a low-T Curie term in the susceptibility. On the other hand, Na doping (which should give rise to two holes per Na) gives rise to an additional peak in $\chi _{spin}$(T) at about 20 K, below the broad maximum at 60 K. The nature of this transition is currently being investigated. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H10.00007: Magnetism of weakly hole-doped, anisotropic CuO$_2$ spin chains in La$_5$Ca$_9$Cu$_{24}$O$_{41}$ R. Leidl, R. Klingeler, B. B\"uchner, M. Holtschneider, W. Selke Experiments suggest that for fields $B > 4\textrm{T}$ (applied along the easy axis) the spin chain system in La$_x$Ca$_{14-x}$Cu$_{24}$O$_{41}$, $x\ge5$, is characterized by short-range spin order and quasistatic (quenched) charge disorder. Using realistic estimates for the interaction parameters, we present extensive Monte-Carlo simulations of a (classical) two-dimensional Heisenberg model with randomly distributed, static holes. We find, in particular, that the spin-flop transition of the pure model (without holes) is destroyed and transformed into a smooth anomaly. A qualitative agreement with the experimentally observed magnetic susceptibility curves in La$_5$Ca$_9$Cu$_{24}$O$_{41}$ is obtained. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H10.00008: Magnetic Phase Separation in La$_{1-x}$Sr$_{x}$CoO$_{3}$ Single Crystals using $^{139}$La NMR. Robert X. Smith, Michael J.R. Hoch, Philip L. Kuhns, William G. Moulton, Gregory S. Boebinger, Arneil P. Reyes, Chris Leighton, John Mitchell Nano-scale phase separation appears to occur in a number of doped transition metal oxides and has been shown to be important in the cobaltites. LSCO is a mixed valence (Co$^{3+}$, Co$^{4+})$ near-cubic perovskite. We report $^{139}$La NMR experiments on nanoscale phase separation in single crystals complementing earlier work on sintered powders$^{1}$. Concentrations in the range x=0.05 to 0.30, spanning the metal-insulator transition at x=0.18, were used. Spectra for x=0.30 reveal a single peak showing long-range FM order with an internal transferred hyperfine field of 2.5T, at the La site. NMR spectra for samples with x$<$0.30 show asymmetric peaks that can be well fitted with two Gaussian's, evidence of FM and spin/cluster-glass magnetic phases. A droplet model has been proposed in the literature, where Co neighbors interact via double exchange in hole-rich regions to form the FM phase. We find magnetic phase separation persists over a narrower x range in the single crystals than in previously studied sintered samples$^{1}$. 1) M.J.R Hoch et al. PRB 70, 174443 (2004) [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H10.00009: Spin-glass ordering in the layered III-VI Diluted Magnetic Semiconductor Ga$_{1-x}$Mn$_{x}$S Tom Pekarek, E.M. Watson, J. Garner, P.M. Shand, I. Miotkowski, A.K. Ramdas A spin-glass transition has been observed in a class of materials based on a layered III-VI semiconducing host. We have performed dc magnetization and ac susceptibility measurements on the diluted magnetic semiconductor Ga1-xMnxS ($x$ = 0.09). A scaling analysis of the nonlinear magnetization just above the transition gives \textit{Tc} = 11.2 +/- 0.2 K, and the critical exponent values gamma~= 4.0 +/- 1.0 and beta = 0.8 +/- 0.2. The non-linear magnetization scaling for Ga1-xMnxS follow the same universal scaling function characterized with the same values of $\gamma $ and $\beta $ as Zn$_{0.49}$Mn$_{0.51}$Te over many orders of magnitude along each axis. The values for the critical exponents $\gamma $ and $\beta $ obtained in this work are in excellent agreement with values reported for other spin-glass materials. These results represent convincing evidence that the III{\-}VI diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$S undergoes a true spin-glass transition and is in a subset of the class of insulating spin-glass materials with short-range interactions. The observed spin-glass transition in Ga1-xMnxS is unprecedented in the published literature on III-VI DMS. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H10.00010: Spin-Glass Like Phase in the Weak-Coupling Limit of the Double-Exchange Model Randy Fishman Recent work has demonstrated that in the weak-coupling or RKKY limit, the double-exchange (or Kondo lattice) model supports a spin-glass like (SGL) phase with short-range but not long-range magnetic order. The magnetic susceptibility and Edwards-Anderson order parameter q of this SGL phase have been evaluated using dynamical mean-field theory (DMFT), which becomes exact in infinite dimensions. We find that q=M(T/T$_{SGL})^{2}$, where M is the classical Brillouin function and T$_{SGL}$ is the SGL transition temperature. The correlation length of the SGL phase is determined by a correlation parameter Q that simultaneously maximizes T$_{SGL}$ and minimizes the free energy. The magnetic susceptibility has a cusp at T$_{SGL}$ and reaches a nonzero value as the temperature goes to zero. Analytic results for the SGL phase of a model with classical spins but without quenched disorder and geometric frustration should provide new avenues of investigation into SGL behavior. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H10.00011: Chaos in spin glasses Helmut G. Katzgraber, Florent Krzakala We study the effects of small temperature as well as disorder perturbations on the equilibrium state of three-dimensional Ising spin glasses via an alternate scaling ansatz. By using Monte Carlo simulations, we show that temperature and disorder perturbations yield chaotic changes in the equilibrium state and that temperature chaos is considerably harder to observe than disorder chaos. Results in two space dimensions are also discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H10.00012: Spin glass phenomena caused by dipole interactions and the Onsager reaction field Derek Walton The effects of dipole interactions have been the subject of study for decades, however with few exceptions the Onsager reaction field has been neglected. It will be shown that this field leads to waiting time efeects and rejuvenation. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H10.00013: Universality in spin glasses: A Monte Carlo study Helmut G. Katzgraber, Mathias Koerner, A. Peter Young We study universality in three-dimensional Ising spin glasses by large-scale Monte Carlo simulations of the Edwards-Anderson Ising spin glass for several choices of bond distributions, with particular emphasis on Gaussian and bimodal interactions. A finite-size scaling analysis suggests that three-dimensional Ising spin glasses obey universality. Results in two space dimensions are briefly discussed. [Preview Abstract] |
Session H11: Focus Session: Multiferroic Heterostructures
Sponsoring Units: DMPChair: Nicola Spaldin, University of California, Santa Barbara
Room: Colorado Convention Center Korbel 1F
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H11.00001: Multiferroic BiFeO$_{3}$/BiCrO$_{3}$ superlattices Mark Huijben, Colleen Kantner, Qian Zhan, Joseph Orenstein, Ramamoorthy Ramesh There is currently an increasing interest into multiferroic materials. Although a large number of potential applications can be envisaged, there are currently no known single-phase materials that show large, robust magnetization and polarization at room temperature. Theoretical calculations of artificially constructed (111) layered double perovskite Bi$_{2}$FeCrO$_{6}$ predict them to be ferrimagnetic (with a magnetic moment of 2 $\mu _{B}$ per formula unit) and ferroelectric (with a polarization of $\sim $80 $\mu $C/cm$^{2})$. A high degree of control over the layer composition is required to accomplish this. In this work we fabricated such epitaxial BiFeO$_{3}$/BiCrO$_{3}$ superlattices by laser-MBE during which the growth was controlled on the atomic scale by reflection high energy electron diffraction. We will report results of structural, chemical, electrical and magnetic measurements of such superlattices. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H11.00002: Electric Field Controlled Magnetism in BiFeO$_{3}$/Ferromagnet Films M. Barry, K. Lee, Y.H. Chu, P.L. Yang, L.W. Martin, C.A. Jenkins, R. Ramesh, A. Scholl, A. Doran BiFeO$_{3}$ is the only single phase room temperature multiferroic that is currently known. Not only does it have applications as a lead-free replacement for ferroelectric memory cells and piezoelectric sensors, but its interactions with other materials are now attracting a great deal of attention. Its multiferroic nature has potential in the field of exchange bias, where it could allow electric-field control of the ferromagnetic (FM) magnetization. In order to understand this coupling, an understanding of the magnetization in BiFeO$_{3}$ is necessary. X-ray linear and circular dichroism images were obtained using a high spatial resolution photoelectron emission microscope (PEEM), allowing elemental specificity and surface sensitivity. A piezoelectric force microscope (PFM) was used to map the ferroelectric state in micron-sized regions of the films, which were then probed using crystallographic measurements and temperature dependent PEEM measurements. Temperature dependent structural measurements allow decoupling of the two order parameters, ferroelectric and magnetic, contributing to the photoemission signal. Careful analysis of linear and circular dichroism images allows determination of magnetic directions in BiFeO$_{3}$ and FM layers. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 9:00AM |
H11.00003: Ferroelectricity in (BaTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_m$ Superlattices Containing as Few as one BaTiO$_{3}$ Layer (n=1) Invited Speaker: The question of how thin a ferroelectric can be and still be ferroelectric has been the source of an intensive research effort over the past decade. Several studies, both theoretical and experimental, have concluded that with appropriate boundary conditions ferroelectricity can exist in superlattices containing BaTiO$_{3}$ or PbTiO$_{3}$ layers as thin as one unit cell. In this talk I will show the results of experiment and theory for BaTiO$_{3}$/SrTiO$_{3}$ superlattices grown by reactive molecular-beam epitaxy (MBE) on three different substrates: TiO$_{2}$-terminated (001) SrTiO$_{3}$, (110) DyScO$_{3}$, and (110) GdScO$_{3}$. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO$_{2}$ were deposited sequentially to create commensurate BaTiO$_{3}$/SrTiO$_{3}$ superlattices with a variety of periodicities. The superlattices consist of an $n$ unit-cell-thick slab of BaTiO$_{3}$ followed by an $m$ unit-cell-thick slab of SrTiO$_{3}$, which are designated [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$, where $q$ is the number of times the bilayer is repeated. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks and the narrowest rocking curves ever reported for oxide superlattices. High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. UV Raman results show that the BaTiO$_{3}$ in these [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$ superlattices is tetragonal and the SrTiO$_{3}$ is polar due to strain. Temperature-dependent UV Raman and XRD reveal the paraelectric-to-ferroelectric transition temperature ($T_{C})$. Our results* demonstrate (1) that [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$ superlattices containing as few as one strained BaTiO$_{3}$ layer ($n$=1) are ferroelectric and (2) the sensitivity of $T_{C}$ to the boundary conditions. Comparisons to \textit{ab initio} and phase-field modeling of the properties of these [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$ ferroelectric superlattices will be made and the importance of strain demonstrated. In addition to probing finite size effects and the importance of mechanical boundary conditions, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control. * D.A. Tenne, A. Bruchhausen, N.D. Lanzillotti-Kimura, A. Fainstein, R.S. Katiyar, A. Cantarero, A. Soukiassian, V. Vaithyanathan, J.H. Haeni, W. Tian, D.G. Schlom, K.J. Choi, D.M. Kim, C.B. Eom, H.P. Sun, X.Q. Pan, Y.L. Li, L.Q. Chen, Q.X. Jia, S.M. Nakhmanson, K.M. Rabe, and X.X. Xi, ``Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy,'' \textit{Science} \textbf{313} (2006) 1614-1616. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H11.00004: Magnetoelectric Phase Control in Epitaxial Oxides from First Principles Craig Fennie, Karin Rabe We propose a design strategy - based on the coupling of spins, optical phonons, and strain - for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO$_3$. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H11.00005: Interfacial multiferroism and giant magnetoelectricity in nano-capacitors James Rondinelli, Massimiliano Stengel, Nicola Spaldin We present results of density functional calculations of the dielectric and magnetic responses of SrRuO$_3$/SrTiO$_3$/SrRuO$_3$ oxide heterostructures. Our calculations indicate the spatial coexistence of magnetic and polar behavior at the metal-insulator interface, suggesting a route to a new type of {\it interfacial multiferroic}. We also find a giant magnetoelectric response, and comment on the control of this magnetoelectric effect as a means to realizing new devices and sensors. Such devices should be feasible as the ability to fabricate and functionalize new complex materials continues to grow. By controlling the interacting charge, spin and lattice degrees of freedom at interfaces, it is then possible to tune the interactions between layers to create new electric or magnetic phases that are accessible with applied fields. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H11.00006: Non-Linear Optical probing of MultiFerroicity and Phase Transitions in BiFeO$_{3}$ Thin Films Amit Kumar, Lane Martin, R. Ramesh, Venkatraman Gopalan Bismuth Iron Oxide BiFeO$_{3}$ is being studied extensively by researchers to utilize its multiferroic properties for designing multi-state memory devices. In this work , we present the first results of the non linear optical probing of this material to study the simultaneous ferroelectric and antiferromagnetic ordering below the Neel's temperature. Optical second harmonic generation (SHG) has been employed to determine crystal and magnetic symmetries of thin BiFeO$_{3}$ films grown in different orientations at temperatures ranging from room temperatures to above the Neel's Temperature. We show that SHG can separate the antiferromagnetic and ferroelectric order parameters cleanly and probe each of these phenomena. The coupling between the ferroelectric and antiferromagnetic ordering under electric and magnetic fields will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H11.00007: Optical spectroscopic study on new magnetoelectric hexagonal \textit{RE}MnO$_{3}$ (\textit{RE}=Gd, Tb, Dy, and Ho) thin films Woo Seok Choi, Sung Seok A. Seo, Jung Hyuk Lee, Daesu Lee, Tae Won Noh, Yunsang Lee Recently, magnetoelectric effects in various oxides have been attracting lots of attentions and are being extensively investigated due to their intriguing couplings between the magnetic and electric order parameters. Here we report optical spectroscopic investigations on new hexagonal \textit{RE}MnO$_{3}$ (\textit{RE} = Gd, Tb, Dy, and Ho) thin films, which are fabricated by epi-stabilization technique [1]. From the in-plane optical conductivity spectra of the hexagonal \textit{RE}MnO$_{3}$, we observe a dramatic increase of the optical transition related to Mn 3d a$_{1g}$ energy level, as the ionic radius of the $R$ ion increases. The optical transition at 1.64 eV for DyMnO$_{3}$ shifts to 1.67 and 1.81 for TbMnO$_{3}$ and GdMnO$_{3}$ respectively. For natural hexagonal \textit{RE}MnO$_{3}$ (\textit{RE} = Y, Er, Lu, and Sc) with smaller ionic sizes, the same optical transitions occur at $\sim $1.6 eV. The large peak shift in new hexagonal \textit{RE}MnO$_{3}$ is understood by local flattening of Mn-O bipyramid, which will enhance the crystal field energy of a$_{1g}$, as the \textit{RE} ionic size increases. \newline [1] J. H. Lee \textit{et al}., Adv. Mat., to be published (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H11.00008: Electronic reconstruction at SrMnO$_{3}$-LaMnO$_{3}$ superlattice interfaces Serban Smadici, Peter Abbamonte, Anand Bhattacharya, Xiaofang Zhai, James Eckstein, Andrivo Rusydi Progress in molecular beam epitaxy made possible the growth of manganese oxide superlattices with the dopant ions arranged in separate regular layers. Little is known about how this ``structural'' doping is reflected into the MnO$_{2}$ planes; for instance what is the plane effective hole concentration. We studied superlattices made of SrMnO$_{3}$ and LaMnO$_{3}$ layers with a doping of x=0.33 using resonant soft x-ray scattering. For scattering momenta at which the non-resonant contribution is suppressed by symmetry, i. e. L=3 in units of the superlattice period, resonant soft x-ray scattering probes the distribution of doped holes, and, in particular, the nature of the interface between the doped and undoped layers. Our measurements at the O K edge show a temperature-dependent hole distribution. This electronic reconstruction which occurs with cooling below T$_{c}$ =220~K is related to the transition of the superlattice to a ferromagnetic state. Resonant scattering spectra at the Mn L edges from spin and orbital distributions have complex shapes providing additional insights into these new materials. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H11.00009: Transport and magnetic properties of digital superlattices of (LaMnO$_{3})_{2n}$/(SrMnO$_{3})_{n}$. A. Bhattacharya, J. Kavich, S.G.E. te Velthuis, X. Zhai, M. Warusawithana, J. Freeland, Sam Bader, J.N. Eckstein Superlattices of (LaMnO$_{3})_{2n}$/(SrMnO$_{3})_{n }$, $n$=1-5, were synthesized using O$_{3}$-assisted MBE. Both constituents are antiferromagnetic insulators at low temperatures. The overall stoichiometry is the same as for La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ random alloys, but the $A$-site disorder is eliminated. For $n$=1, 2 a metallic ferromagnetic state is obtained at low T. For $n$=1, all measured properties are nearly identical to those of the random alloy. The emergence of a metallic state is interpreted in terms of a Mott transition driven by the proximity between LaMnO$_{3}$/SrMnO$_{3}$ interfaces. For $n\ge $3, a transition to an insulating state occurs, with a suppression of $T_{C }$ and $M_{s}$ and an increase in $H_{c}$. Using neutron and resonant x-ray scattering, we observe a modulation of the ferromagnetism commensurate with the superlattice period for $n$=5. We propose that magnetic frustration at the AF/F interfaces drives the insulating state. Ack: DOE BES contract {\#}DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H11.00010: Interface Magnetization in Digitally Layered (LaMnO$_{3})_{2n }$/(SrMnO$_{3})_{n}$ Superlattices J.J. Kavich, A. Bhattacharya, J.W. Freeland, J.N. Eckstein Interfaces in solids have been an enduring theme in materials physics due to the exciting new physics that can emerge in interface regions arising from a variety of competing interactions. Because of this, multilayer materials can provide a novel means to separate disorder physics from other competing interactions. The samples used in this experiment were grown using ozone-assisted atomic layer by layer molecular beam epitaxy (ALL-MBE) on (100) oriented SrTiO$_{3}$. Using X-ray Resonant Magnetic Scattering, we present a comparison of the magnetic properties at interfaces of a series of (LaMnO$_{3})_{2n }$/(SrMnO$_{3})_{n}$ superlattices as a function of n ranging from 1 to 5 unit cells. Combined with transport measurements, the magnetic properties appear to be correlated with a change in the electronic properties of the superlattice. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H11.00011: Optically-induced coherent spin precession in manganite superlattices Haibin Zhao, Kevin Smith, Gunter Luepke The artificial perovskite superlattices composed of LaMnO$_{3}$ (LMO) and SrMnO$_{3}$ (SMO) have been investigated to elucidate the interface ferromagnetic order created by adjoining the two antiferromagnetic Mott insulators. Optically-induced coherent spin precessions are studied by time-resolved magneto-optical Kerr effect in a superlattice composed of 4 u. c. (unit cells) LMO and 2 u. c. SMO for a total of 13 double-layers grown on a single-crystalline (001) substrate of SrTiO$_{3}$ (STO). We found that the demagnetization field and the anisotropy field are very similar in the LMO/SMO superlattice (H$_{d}$ = 0.75 T, H$_{a}$ = -0.14 T) and in a La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ thin film (H$_{d}$ = 0.72 T, H$_{a}$ = -0.2 T) both grown on STO substrates indicating that the strain and magnetic character are very similar in the two manganite structures. However, the low field precession behavior is distinctly different which shows that pinning by antiferromagnetic spins in the LMO layers and/or surface anisotropy of the superlattice may contribute significantly to the effective field at low applied fields, thus modifying the mode profile and precession frequency. We will discuss this exchange interaction in LMO/SMO superlattices with different periods in zero-field cooling and field cooling. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H11.00012: Magnetism and electronic structure at the interface of a metal CaRuO$_{3 }$ and Mott insulator CaMnO$_{3}$. Alexander Boris, John Freeland, Jerald Kavich, Ho Nyung Lee, Petar Yordanov, Giniyat Khaliullin, Bernhard Keimer, Jak Chakhalian Recent advances in fabrication of ultra-thin complex oxide heterostructures have opened new opportunities to investigate possible novel quantum states at the correlated interfaces. With this aim we fabricated ultra-thin superlattices of CaMnO$_{3}$(CMO)/CaRuO$_{3}$(CRO) with the thickness of CRO layers from 1 to 12 unit cells by laser MBE. Electronic properties of CRO/CMO were investigated by soft x-ray spectroscopies at the L-edges of Mn and Ru. SQUID and optical reflectivity revealed a ferromagnetic thickness-independent transition at T$_{c }\approx $100K and CRO thickness-dependent negative magnetoresistance. This behavior is in marked contrast to the individual layers. At the interface we found a clear sign of net magnetic moment on Mn, which saturates only at magnetic field of 5T. Unlike CMO, similar measurements at the Ru L$_{3}$-edge showed no detectable magnetism in the field up to 5T. Comparison with Ru references confirmed Ru(IV) oxidation state. These findings are in the sharp contrast with previously suggested models involving Ru(IV-V) valency exchange and thus reveal intricate nature of the interface between a metal and Mott insulator. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H11.00013: Probing multiferroicity and spin-spin interactions via angular dependent dielectric measurements on $Y$-doped \textit{HoMnO}$_{3}$ in high magnetic fields Relja Vasic, Haidong Zhou, Chris Wiebe, James Brooks Dielectric measurements are used to characterize magnetic phase transitions in the doped ferrielectric oxides \textit{Ho}$_{1-x}Y_{x}$\textit{MnO}$_{3}$ ($x$ = 0, 0.4, 0.5, 0.6, 0.7, 0.8, 1). The \textit{T-B-$\theta $} phase diagram of the ferrielectric material \textit{Ho}$_{1-x}Y_{x}$\textit{MnO}$_{3}$ has been determined from the dielectric constant angular dependence between the crystallographic $c$-axis and applied magnetic field. The re-entrant temperature-magnetic field phase transitions which involve in- plane \textit{Mn} spin rotations in the antiferromagnetic state below the N\'{e}el temperature are driven by the interaction with the \textit{Ho} subsystem. We describe this behavior in terms of the interaction of the \textit{Ho} sublattice spin system with the underlying, robust \textit{YMnO}$_{3}$ antiferromagnetic triangular lattice, where the \textit{Ho}-spin interactions are highly sensitive to $Y$ concentration and field direction. The magnetic field anisotropy study is an important step towards understanding of magnetic and electric phase competition in the diluted $4f$ system by non-magnetic Yttrium($Y)$. [Preview Abstract] |
Session H12: Focus Session: Spin Transport
Sponsoring Units: GMAG DMP FIAPChair: Carsten Timm, Kansas University
Room: Colorado Convention Center Korbel 3C
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H12.00001: Dephasing in (Ga,Mn)As Nanowires {\&} Rings Invited Speaker: Quantum correction to the conductivity of ferromagnetic semiconductors are thus far largely unexplored. But to understand quantum mechanical transport the knowledge of basic material properties like phase coherence length and corresponding dephasing mechanism are indispensable ingredients. The lack of observable quantum phenomena prevented experimental access to these quantities so far. Here we report on the observation of universal conductance fluctuations in ferromagnetic (Ga,Mn)As. The analysis of the length and temperature dependence of the fluctuations in one-dimensional wires reveals a 1/T dependence of the dephasing time. The measurement of the Aharonov-Bohm effect in nanorings as well as a weak localization correction to the conductivity, observed in arrays of wires, are in good agreement with the results obtained from the conductance fluctuations. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H12.00002: Low frequency 1/f and random telegraph noise in (Ga,Mn)As Meng Zhu, Xia Li, Gang Xiang, Nitin Samarth Resistance noise measurements can provide insights into the interplay between charge transport and magnetism in complex physical systems [B. Raquet {\it et al.}, Phys. Rev. Lett. {\bf 84}, 4485 (2000)]. We report the temperature- and magnetic field-dependence of the low frequency electrical noise in (Ga,Mn)As epilayers with different Mn concentrations (and different conductivity). Surprisingly, we do not observe any anomalies in the noise spectra across the Curie temperature. However, we find an enhancement in the integrated noise (over the frequency span 125mHz-11Hz) at temperatures below $\sim 10$ K where the resistivity shows a minimum. For more metallic samples, the normalized power spectrum density is $1/f$-like over the entire temperature range studied, while more insulating samples show Lorentzian spectra accompanied by random telegraph noise (RTN) at low temperatures. The magnetic field dependence of the integrated noise shows distinct correlations with magnetization switching, suggesting changes in scattering during domain wall nucleation/propagation. From the magnetic field driven suppression of the RTN, we infer the existence of nanoscale magnetic clusters that fluctuate between two states separated by a field-tunable barrier. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H12.00003: Investigation of Planar Hall Effect in (Ga,Mn)As/GaAs/(Ga,Mn)As Structures Yingyuan Zhou, Z. Ge, Y.J. Cho, S. Shen, X. Liu, J.K. Furdyna, M. Dobrowolska We present a study of the planar Hall effect in the multilayer structures (Ga,Mn)As/GaAs/(Ga,Mn)As. The planar Hall effect (PHE) in a single (Ga,Mn)As layer yields two electric states (high and low), ideal as a basis for device design. The present paper is motivated by the speculation that a coupled (Ga,Mn) As/GaAs/(Ga,Mn)As system provides the possibility of combining PHE with tunneling magnetoresistance, thus leading to complex multiplets of electric states. Our PHE studies were carried out on coupled structures in which the two (Ga,Mn)As layers were made different by either modulation doping or by low temperature annealing. A series of specimens were prepared with different thicknesses of the GaAs spacer (3nm or 6nm). Experimental results show that for samples with 3-nm spacers the magnetic coupling between the two (Ga,Mn)As layers is so strong that their magnetizations reverse together. The PHE then behaves similar to that of a single (Ga,Mn)As layer, except that in the multilayers the PHE voltage switchings are less abrupt. In samples with 6-nm-thick spacer, however, we see the emergence of switchings with multiple values of the PHE voltage. Such multiple electric states can be qualitatively explained by modeling the coupled structures as a network of resistors. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H12.00004: Quantum corrections to the longitudinal and anomalous Hall conductivity in (Ga,Mn)As. Partha Mitra, Nitesh Kumar, Nitin Samarth Although the canonical ferromagnetic semiconductor (Ga,Mn)As has now been studied extensively for over a decade, the fundamental understanding of the temperature-dependent conductivity and the origins of the anomalous Hall effect still remain open questions. Here, we report measurements of the longitudinal and transverse conductivity in (Ga,Mn)As samples in the regime of ``dirty" diffusive transport ($1 \leq k_F l_e \leq 3$ as calculated from the Drude formula) over a wide temperature range ($50$ mK $\leq T \leq 5$ K). Although we observe a power law temperature dependence of the conductivity, the scaling is inconsistent with standard expectations based upon known quantum corrections to the conductivity. We also examine the scaling of the anomalous Hall conductivity with longitudinal conductivity and compare our observations with theories of the anomalous Hall effect. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H12.00005: Magnetotransport and magneto-optical properties of GaMnAs thin films with high Mn concentrations Kenichi Ohno, Shinobu Ohya, Masaaki Tanaka III-V-based ferromagnetic-semiconductor (FMS) GaMnAs is a good model system for future semiconductor-spintronics devices. For practical applications, it is important to increase the Curie temperature ($T_{C})$ of GaMnAs (the current record is 173 K) to room temperature. The mean field theory predicts that $T_{C}$ of GaMnAs increases in proportion to its Mn concentration $x$. However, it is difficult to grow GaMnAs with $x \quad >$ 10{\%}, because MnAs clusters and Mn interstitial defects are easily formed in such a high $x$ region. Here, we have successfully grown GaMnAs films with $x$ of 12 - 21{\%} by decreasing the growth temperature to 150-200$^{o}$C and by reducing the film thickness to 10 nm. The magnetic circular dichroism and the anomalous Hall effect measurements indicated that these GaMnAs films have the intrinsic FMS features. A high $T_{C}$ value of 170 K was obtained when $x$ = 12{\%}. This work was partly supported by PRESTO/SORST of JST, Grant-in-aid for Scientific Research, IT Program of RR2002 of MEXT. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H12.00006: Electronic transport in diluted magnetic semiconductors: application of the memory function formalism for spin and charge disordered media. F.V. Kyrychenko, C.A. Ullrich To get an expression for electrical conductivity in diluted magnetic semiconductors (DMSs) we employ the memory function formalism and derive a general expression for the current relaxation kernel in spin and charge disordered systems. To illustrate the model we performed simplified calculations of spin and charge scattering rates in the weak-disorder limit for some special cases of interest: (i) In a system with positional correlation of the scattering centers we found a significant enhancement of the charge scattering. The enhancement is sensitive to cluster parameters and may be influenced through post-growth annealing. (ii) In the magnetically ordered system we show that the suppression of localized spins fluctuations results in the reduction of the spin scattering that substantially contributes to the experimentally observed resistivity drop below $T_c$. (iii) Memory function formalism gives the possibility to include electronic many-body effects in a consistent and systematic manner through time-dependent density functional theory. We use this approach to study the combined effect of disorder and electron-electron interaction on the transport properties of DMSs. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H12.00007: Spin transport through individual single-walled carbon nanotubes Suyong Jung, Zhen Yao We have investigated spin transport through individual single-walled carbon nanotubes contacted with ferromagnetic permalloy electrodes. At low temperatures, hysteretic magnetoresistance (MR) is observed in both the Fabry-Perot interference regime and the Kondo regime. Both the sign and magnitude of the MR oscillate as a function of gate and bias voltages. The behavior in the interference regime can be explained well using non-interacting ballistic model incorporating the effect of spin-dependent interfacial phase shift. In the strongly interacting Kondo regime, however, the behavior of the MR is qualitatively different. We will present possible theoretical models and numerical fittings to elucidate the MR features in our data. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H12.00008: Spin Transport through Multilayer Graphene Masaya Nishioka, Allen Goldman We have demonstrated spin valve behavior in structures in which crystals containing multiple graphene layers were positioned between two ferromagnetic contacts. Graphene is a promising candidate for the spacers of spin valves, because of its small spin-orbit interaction and high mobility. We used a 3nm thick crystal which contained several layers of graphene. Cobalt electrodes with a 100nm gap were fabricated on the crystal using electron beam lithography. The device showed $\sim $0.2{\%} magnetoresistance at 10K using an in-plane magnetic field. The effect was found at temperatures as high as 150K. The observed behavior could be explained by the switching of the magnetizations of the Co electrodes, which was inferred from measurements of their anisotropic magnetoresistance. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H12.00009: Charge and spin transport in graphene nanostructures Sungjae Cho, Yung-Fu Chen, Michael S. Fuhrer We have studied spin injection from ferromagnetic (permalloy) electrodes into graphene devices using a non-local four-probe geometry. We observe sign reversal of the non-local resistance upon switching of the magnetization direction of the electrodes, indicating injection and detection of a spin current. We report the temperature and carrier density dependence of the spin valve signal. We observe an unusual reversal of the sign of the spin valve signal at some carrier densities. We have also examined the magnetotranport in the low field and quantum Hall regimes in devices with mobilities differing by an order of magnitude. The results will be discussed in terms of the physics of conduction at the Dirac point. Support provided by the Office of Naval Research and the UMD-MRSEC Shared Equipment Facilities. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H12.00010: Magnetoresistance of Gd doped Carbon films Erik Helgren, Li Zeng, C. Ronning, H. Zutz, J. Ager, F. Hellman The rare earth dopant Gd was introduced into amorphous carbon ($a-$C) by two quite different techniques; mass selected ion beam deposition (MSIBD) of tetrahedral amorphous carbon (\textit{ta-}C) followed by Gd implantation and magnetron co-sputtering of Gd and C targets. Raman, RBS and TEM characterization indicate the films are metastable. Films prepared by sputtering ($a-$Gd$_{x}$C$_{1-x}$ x=4.2 $\sim $15.6 at.{\%}) have a spin-glass freezing with a temperature which (2-6K) scales with Gd concentration. Films prepared by MSIBD followed by ion implantation (\textit{taC}:Gd$_{x}$, x=4, 7, 13 at.{\%}) show no freezing and a paramagnetic Curie-Weiss law down to 1.9K. Transport measurements show typical doped amorphous semiconductor behavior with very large negative magnetoresistance (MR). The MR of the two types of films are similar, which indicates a universal magnetic moment-carrier interaction in these Gd doped amorphous semiconductor systems. A comparison of these films' MR with other Gd doped semiconductors such as $a-$Gd$_{x}$Si and $a-$Gd$_{x}$Ge will be discussed. The MR properties of this type of thin film material indicate the importance of the local materials structure and the consequence of the electron screening effects. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H12.00011: Anti-Weak Localization Measurements in the Ballistic Regime Dilhani Jayathilaka, Aruna Dedigama, Sheena Murphy, Madhavie Edirisooriya, Niti Goel, Tetsuya Mishima, Michael Santos, Kieran Mullen Anti-weak localization dominates at low fields in systems in which spin-orbit coupling is strong. The experimental results are well described by theory [1] in low mobility systems in which the magnetic length ($l_{B})$ is greater than the mean free path; however high mobility systems with strong spin-orbit interactions, such the InSb based two dimensional systems (2DESs) examined here, are not in this diffusive regime. A recently developed theory [2] addresses both the diffusive and ballistic regimes taking into account both the backscattered and non-backscattered contributions to the conductivity. We will discuss the agreement of the new theory to measurements of InSb 2DESs prepared with both strong Dresselhaus and Rashba effects. [1] S.V. Iordanskii, Yu B. Lyanda-Geller, and G.E. Pikus, \textit{JETP Lett.} \textbf{60}, 206 (1994). [2] L.E. Golub, \textit{Phys. Rev. B}. \textbf{71}, 235310 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H12.00012: Electrical measurement of pure spin currents in a two dimensional electron gas Ananth Venkatesan, Sergey Frolov, Joshua Folk, Werner Wegscheider We present an electrical measurement of pure spin currents in a $AlGaAs/GaAs$ two dimensional electron gas. Spin polarized electrons are injected into the centre of a $90\mu$m long channel through a quantum point contact (QPC) in a large in-plane magnetic field. The charge current flows to one end of the channel. A pure spin current flows to the opposite end, driven by a chemical potential difference between the two spin populations. This difference is recorded using spin polarized QPCs along the channel in regions free of charge current. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H12.00013: $^{77}$Se NMR investigation of the paramagnetic metal phase of $\lambda $-(BETS)$_{2}$FeCl$_{4}$ Guoqing Wu, W.G. Clark, S.E. Brown, J.S. Brooks, A. Kobayashi, H. Kobayashi We report $^{77}$Se NMR measurements of the spectrum and the spin-lattice relaxation rate (1/$T_{1})$ in a 7 $\mu$g single crystal of $\lambda $(BETS)$_{2}$FeCl$_{4}$ over the temperature ($T)$ range 2.5-10 K in an applied field of 10.9 T parallel to the $a$-axis (paramagnetic metal phase). A behavior close to 1/$T_{1}T$ = constant is observed. It indicates that for these conditions, 1/$T_{1}$ is dominated by the hyperfine interaction between the $^{77}$Se spins and the conduction electrons, in contrast to 1/$T_{1}$ for the protons, which is driven by the magnetic fluctuations of the Fe$^{3+}$ spins [W.G. Clark et al., Appl. Mag. Res. \textbf{27}, 279 (2004)]. From these proton measurements, we estimate that the contribution of the Fe$^{3+}$ fluctuations to 1/$T_{1}$ of $^{77}$Se is negligible. Work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0520552 (SEB). [Preview Abstract] |
Session H13: Focus Session: NaxCoO2 and AxNiO2
Sponsoring Units: DMP GMAGChair: Zenji Hiroi, University of Tokyo
Room: Colorado Convention Center Korbel 4C
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H13.00001: Novel sodium ordering on a NaxCoO2 surface Woei Wu Pai, S.S. Huang, C.H. Lin, H. S. Hsue, F.C. Chou The conducting layered sodium cobaltate, NaxCoO2, has generated great research interests recently. This material exhibits surprising properties as the Na concentration x is varied. Despite intense studies, Na ordering and its subtle interplay with charge ordering in the CoO2 layer remains unclear. Here we report the first direct observation of Na ordering on a NaxCoO2 surface (x=0.84) with scanning tunneling microscopy. Three distinct Na phases, all of hexagonal symmetry, were identified. These new findings did not fit any theoretical prediction at present. Plausible structure models were proposed. In additional, an one-dimensional stripe modulation on the surface was discovered, which was found to be a bulk phenomenon as well. Our results should prompt more detailed theoretical investigations into the mechanism of Na ordering. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H13.00002: Electronic Structures of Na in $Na_xCoO_2$ Paoan Lin, D. J. Huang, Horng-Tay Jeng, Chen-Shiung Hsue Sodium cobalt oxides ($Na_xCoO_2$) have attracted renewed because of their exceptionally large thermoelectric power recent discovery of superconductivity in their hydrated counterparts. In order to investigate the dependence on th doping-concentration for the electronic structures, we hav carried out a series of LDA+U Ab initio calculation on sod cobalt oxides ($Na_xCoO_2$) of various dopings. The calcul results were compared with experimental results of polarization-dependent soft x-ray absorption spectroscopy. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H13.00003: Pressure effect on magnetic and structural phase transitions in {Na$_x$CoO$_2$} ($x$=0.75, 0.80) O.B. Korneta, S.O. Leontsev, Y.V. Sushko, R. Jin, B.C. Sales, D. Mandrus The sodium-rich metallic compounds of Na$_x$CoO$_2$ family with $x \sim 3/4$ are known to exhibit an order-disorder structural transition at $\sim 340$K and a magnetic transition at $\sim 22$K. We have performed the magnetization and resistivity measurements under hydrostatic pressure to study both phase transitions in compounds with $x=0.75$, $x=0.80$. The data established positive pressure dependence of both the structural and magnetic transitions. Positive pressure effect on the Neel temperature suggests that superexchange interactions of localized moments may play an important role in magnetic properties of these materials. Such a conjecture is further supported by the observation of the metal-insulator transition (and its pressure evolution) in interplane resistivity of the $x=0.80$ compound. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H13.00004: Weak coupling SDW ground state with strong Fermi surface gapping in Na$_{x}$CoO$_{2}$ , x $\approx $ 0.8 M. Bruehwiler, B. Batlogg, S.M. Kazakov, J. Karpinski, D. Sheptyakov In Na$_{x}$CoO$_{2}$ the electrons move on a triangular lattice and in the Na-rich composition range (x $\ge $ 0.75) form a SDW ground state below T$_{c}\approx $ 22.5 K with a small ordered moment. We have studied this Fermi surface instability with heat capacity, magnetic and transport measurements on a series of samples with various nominal Na content. The SDW phase is characterized by a jump $\Delta $C at T$_{c }$ and an associated reduction of the electronic density of states. This removal of DOS has been deduced from the high-temperature value of the Sommerfeld $\gamma $ and the extrapolation from below 1K to T $\to $ 0. Interestingly, the ratio $\Delta $C/($\delta \gamma $.T$_{c })\approx $1.5 is close to the BCS weak coupling value. Even more surprising is the observation that up to $\approx $ 80{\%} of the DOS is removed in this Fermi surface instability. In addition to the gapped electronic excitation spectrum a broad hump in the specific heat is measured above $\approx $5K, consistent with excitations of the gapped spin wave spectrum in the SDW ordered state. Crystal structure analysis reveals for the SDW an orthorhombic symmetry and thus a slight distortion of the triangular lattice. Similarities and differences to the CDW-like state, which forms at x=0.5 also in a distorted triangular lattice, will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H13.00005: Structure prediction and phase diagram calculation of NaxCoO2 by Ab Initio methods Ying Shirley Meng, Yoyo Hinuma, Osman Burak, Gerbrand Ceder The unusual electronic properties of NaxCoO2 make it a material of considerable interest. At high sodium concentration it displays a remarkable combination of high electronic conductivity and high Seebeck coefficient. Understanding these phenomena requires a detailed understanding of the local structure, since different Na-vacancy orderings are strongly coupled to the electronic structure and Co3+/Co4+ arrangement. In this study, we investigate the charge ordering and Na-vacancy ordering using first principles electronic structure methods within the GGA and GGA+U approximations. Na ordering is determined not only by a competition between Na site energies difference and Na-Na repulsion, but also by the Co-Na interlayer interaction induced by charge localization. We believe that in particular at high Na content a quantitative understanding of the coupling is essential in understanding the remarkable electronic properties. Phase diagram calculations of NaxCoO2 to understand the phase stability in the system will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H13.00006: The low-energy ARPES and heat capacity of Na$_{0.3}$CoO$_2$: A DMFT study Chris Marianetti, Olivier Parcollet, Kristjan Haule, Gabriel Kotliar We use DMFT to calculate the ARPES spectrum and heat capacity for Na$_{0.3}$CoO$_2$. Both the traditional Hirsch-Fye (HF) Quantum Monte-Carlo technique and the newly developed continuous time (CT) quantum Monte-Carlo technique are used to solve the DMFT impurity problem. We show that the e$_g$' hole pockets on the Fermi surface are destroyed as the on-site coulomb repulsion is increased. Additionally, we show that quantitative agreement with both ARPES and heat capacity can be achieved. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H13.00007: Fabrication and Control of Sodium Concentration in Cobaltate Na$_{x}$CoO$_{2}$ thin films ($x $= 0.68, 0.75) by Thermal Diffusion W.J. Chang, J.-Y. Lin, C.H. Hsu, Y.K. Kuo, K.H. Wu, T.M. Uen, H.L. Liu, Y.S. Gou, J.Y. Juang We have fabricated Na$_{x}$CoO$_{2}$ ($x\sim $0.68 {\&} 0.75) thin films on sapphire (0001) substrates via lateral diffusion of sodium into Co$_{3}$O$_{4}$ (111) epitaxial films. The environment of thermal diffusion is key to the control of the sodium concent in thin films. From the results of x-ray diffraction and in-plane resistivity \textit{$\rho $}$_{ab}$, the epitaxial growth and the sodium contents of these Na$_{x}$CoO$_{2}$ thin films were identified. The thermoelectric measurements show a large thermoelectric power in our films, similar to that of single crystals. The quasiparticle scattering rate is found to approach zero, consistent with the small residual resistivity, indicating high quality of the Na$_{x}$CoO$_{2}$ thin films. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H13.00008: Charge and spin order on the triangular lattice --- Na$_x$CoO$_2$ at $x=0.5$ Sen Zhou, Ziqiang Wang The nature of charge and spin order of strongly correlated triangular lattice fermions is investigated in connection to the unconventional insulating state of Na$_x$CoO$_2$ at $x=0.5$. We study an extended Hubbard ($t$-$U$-$V$) model of the electron doped Co $a_{1g}$ band using a spatially unrestricted Gutzwiller approximation. We find a new class of charge and spin ordered states at $x=1/3$ and $x=0.5$ where the system alleviates antiferromagnetic (AF) frustration via charge inhomogeneity. We show that the $\sqrt{3}a\times2a$ off-plane Na dopant order at $x=0.5$ plays an important but subtle role. It induces weak $\sqrt{3}a\times1a$ charge order in the Co layer without gapping the Fermi surface and allows successive $\sqrt{3}a\times1a$ AF and $2a\times2a$ charge/spin ordering transitions at low temperatures. The nesting with the $2a\times2a$ hexagonal zone boundary gaps out almost the entire Fermi surface at $x=0.5$. We study the phase structure and compare to the findings of recent experiments. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H13.00009: High Resolution Scanning Tunneling Microscopy of Na$_{x}$CoO$_{2}$ M.C. Boyer, W.D. Wise, Kamalesh Chatterjee, M.A. Zimmermann, E.W. Hudson Since the 2003 discovery of superconductivity in water doped sodium cobaltate (Na$_{x}$CoO$_{2})$, many experimental techniques have been brought to bear on not only the superconducting parent state (x $\sim $ 0.3) but on other dopings as well. Unfortunately, scanning tunneling microscopy, which has shown so much success in the study of the related cuprates, has not been as successful in the study of Na$_{x}$CoO$_{2}$. We will present results from topographic and spectroscopic measurements of Na$_{x}$CoO$_{2}$ made using our variable temperature scanning tunneling microscope, with a focus on changes observed between 130 K and 4 K. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H13.00010: Charge and Spin Order in Na$_0.5$CoO$_2$ Ting-pong Choy, Philip Phillips Several experimental puzzles surround the insulating state of Na$_0.5$CoO$_2$: 1) antiferromagnetic order is observed but with a reduced moment $\mu_B=0.25$, 2) the insulating state occurs at a temperature below which N\'eel order obtains, and 3) static charge ordering is not seen in all NMR experiments. To address these questions, we focus on controlled calculations of the spin-wave spectrum and the magnitude of the local moment in two of the models proposed for the insulating state: 1) a charge-ordered state with 4-fold symmetry and 2) charge-ordering state with only 2-fold symmetry. We present a detailed iso-spin/spin coupling model which demonstrates how the charge and spin order are coupled. The phase diagram suggests that the ground state of Na$_0.5$CoO$_2$ should be both charge and spin ordered. Serveral candidates with different ordering are studied under a generalized spin-wave theory. By comparing the Neutron results with the low energy excitaion and the calculated structure factor, we conclude that the ground state of Na$_0.5$CoO$_2$ is charge with 4-fold symmetry and long-range spin order. In this state, we find that a spin moment of $\mu_B=0.25$ is well described by the experimentally relevant parameters for the exchange couplings. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H13.00011: Unusual valency and magnetic order in silver nickelates Sergey Streltsov, M.D. Johannes, I.I. Mazin, D.I. Khomskii Ag$_2$NiO$_2$ forms as a triangular based layered nickelate, with a structure identical to the well-studied alkali nickelates LiNiO$_2$ or NaNiO$_2$, but with a double layer of Ag between the oxide planes. The metallic intercalant ions give rise to highly unusual valence state for silver: Ag$^{1/2+}$. We show that the reason for the underoxidation is that the two silver ions form extremely strong bonding-antibonding bands, pushing the lowest Ag-$s$ derived band beneath the (filled) O $p$ complex. This additionally preserves metallicity down to the lowest measured temperatures and gives rise to complex, competing magnetic interactions. The resulting spin fluctuations may explain the large discrepancy (too large for phonon renormalization) between calculated and measured linear specific heat coefficients. Our calculations do not support a controversial cooperative Jahn-Teller distortion, but a comparison with calculations and experiments for single-layer AgNiO2 suggests that magnetically driven charge disproportionation may instead explain the observed structural transition. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H13.00012: Structural disorder and magnetic properties of NaNi$_{0.5}$Mn$_{0.5}$O$_{2}$ and LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$. Natasha Chernova, Miaomiao Ma, Jie Xiao, M. Stanley Whittingham, Julien Breger, Jordi Cabana, Clare Grey Magnetic properties of layered O(3) compounds LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$ and NaNi$_{0.5}$Mn$_{0.5}$O$_{2}$ are studied using AC susceptibility and DC magnetization techniques in order to elucidate magnetic interactions within transition metal (TM) layers and between them in compounds with various TM distributions. In ideal layered NaNi$_{0.5}$Mn$_{0.5}$O$_{2}$, antiferromagnetic (AF) ordering transition at 60 K and a spin-flop transition at 5 K in the magnetic field of 2.2 T are found. Upon lost of Na, AF order changes with ferrimagnetic, which may be caused by Ni$^{2+}$ migration to the Na layer. LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$ with flower or zigzag TM order show ferrimagnetic ordering at around 100 K, and significant magnetization hysteresis below this temperature, indicating presence of Ni$^{2+}$ in the Li layer. Magnetic interactions in all compounds are analyzed and models of spin order at low temperatures are proposed. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H13.00013: Curie-Weiss metallic state in sodium cobaltates Ilya Vekhter, Christopher Hooley One of the most intriguing properties of sodium cobaltates, Na$_x$CoO$_2$ is the so-called Curie-Weiss metallic phase appearing at relatively high doping, $x\sim 0.7$. It exhibits Curie-Weiss magnetic susceptibility in a metal not far from the onset of antiferromagnetic order. Surprisingly for a layered quasi-two-dimensional structure, the neutron scattering experiments in the ordered state yield comparable in-plane and interplane magnetic exchange constants. We consider a model layered system on the verge of transition to a type-A antiferromagnet. We investigate whether in such a system fluctuations of the in-plane magnetization may give the apparent Curie-Weiss behavior in analogy with the spin-fluctuation theory for itinerant ferromagnets. We consider the effect of the crossover from incoherent to coherent interplane transport on the magnetic susceptibility and discuss the effect of sodium doping. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H13.00014: Ferromagnetism, paramagnetism and a Curie-Weiss metal in NaxCoO2 Jaime Merino, Ben Powell, Ross McKenzie Motivated by the unconventional properties and rich phase diagram of Na$_x$CoO$_2$ we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative inter-site hopping amplitudes ($t<0$) and an on-site Coulomb repulsion, $U$, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for $t>0$ a large enhancement of the effective mass, itinerant ferromagnetism and a metallic phase with a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. The transport and magnetic properties measured in Na$_x$CoO$_2$ are consistent with DMFT predictions of a metal close to the Mott insulator and we discuss the role of Na ordering in driving the system towards the Mott transition. We propose that the Curie-Weiss metal phase observed in Na$_x$CoO$_2$ is a consequence of the crossover from ``bad metal'' with incoherent quasiparticles at temperatures T$>$T$^*$ and Fermi liquid behavior with enhanced parameters below T$^*$, where T$^*$ is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of Na$_x$CoO$_2$. [Preview Abstract] |
Session H14: Focus Session: Exchange Bias
Sponsoring Units: GMAG DMPChair: Kai Liu, University of California, Davis
Room: Colorado Convention Center Korbel 4D
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H14.00001: Identification and separation of two distinct contributions to the training effect in polycrystalline exchange biased Co/FeMn bilayers M.K. Chan, J.S. Parker, P.A. Crowell, C. Leighton We show that polycrystalline Co/FeMn bilayers display two distinct forms of training and qualitatively explain their FeMn thickness ($t_{FeMn})$ dependence. The two types of training can be identified and separated via their distinctive field cycle and $t_{FeMn}$ dependences, and the degree of asymmetry between the ascending and descending branches of the hysteresis loops. Samples were prepared via UHV dc magnetron sputter deposition onto Si/SiO$_{2}$ substrates at room temperature. The Co thickness was 6 nm while $t_{FeMn}$ was varied between 0 and 20 nm. Upon field cooling, hysteresis loops display two distinct forms of training. The first is single cycle training accompanied by strong reversal asymmetry. The amount of training and degree of asymmetry are correlated and strongly dependent on $t_{FeMn}$. This effect is due to the biaxial anisotropy of the antiferromagnet$^{1}$. Subsequent loops are symmetric and exhibit multi-loop training that follows a $n^{-1/2}$ dependence, where $n$ is the loop number$^{2}$. This effect is attributed to thermally activated depinning of weakly coupled uncompensated interfacial antiferromagnet spins. 1. A. Hoffmann, Phys. Rev. Lett. \textbf{93} 97203, 2004. 2. D. Paccard, C. Schlenker, O. Massenet, R. Montmory, A. Yelon, Phys. Stat. Sol.16, 301, 1966. This work was supported by the NSF MRSEC program and NSF DMR 04-06029. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H14.00002: Evolution of exchange coupling in epitaxial Fe/SmCo during partial SmCo demagnetization* J. E. Davies, Kai Liu, J. S. Jiang, S. D. Bader, E. E. Fullerton In epitaxial Fe/SmCo, irreversible magnetization reversal occurs once the SmCo hard layer starts switching [1,2]. We investigated the evolution of the interlayer exchange coupling as the hard SmCo layer demagnetizes using the first and second order reversal curve methods (FORC and SORC, respectively). The FORC distribution [2,3] shows two distinct features during the hard layer reversal: a single positive peak at high fields and a negative/positive pair of features at low applied fields. The single peak corresponds to the reversal of the hard SmCo layer and yields a SmCo switching field distribution. The negative/positive pair is due to reversal of the soft Fe layer and its position traces the evolving interlayer exchange coupling strength. SORC measurements show that the Fe layer remains mostly reversible during SmCo demagnetization and allow direct determination of the exchange field. The measured exchange field evolution agrees with an analytical model [4]. [1].Fullerton, et al., PRB 58, 12193 (1998). [2].Davies, et al, APL 86, 262503 (2005). [3].Davies, et al, PRB 70, 224434 (2004); PRB 72, 134419 (2005). [4].Vlasko-Vlasov et al. PRL 86, 4386 (2001). * Supported by ACS-PRF, Alfred P. Sloan Foundation and DOE (BES-MS contract {\#}DE-AC02-06CH11357) [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H14.00003: Magnetic configuration in antiferromagnetically coupled [Co/Pd]$_{15}$/TbFeCo with out-of-plane anisotropy S.M. Watson, J.A. Borchers, T. Hauet, S. Mangin, E.E. Fullerton We have used Polarized Neutron Reflectometry (PNR) to investigate the magnetic properties of an exchange-coupled bilayer system with out-of-plane magnetization. These systems show potential for increasing storage densities in magnetic recording media. Magnetization measurements suggest the formation of an in-plane domain wall. The magnetic configuration inside such systems results from the competition between the magnetic field, short-range exchange coupling, and long-range dipolar interactions. This study involved [Co(0.5 nm)/Pd(X)]$_{15}$/TbFeCo(25 nm) (X=3.5, 5, 7 nm) structures. Both the [Co/Pd] and TbFeCo exhibit strong out-of-plane anisotropy and are exchange coupled antiferromagnetically due to the TbFeCo alloy concentration. The magnitude of exchange coupling between the Co layers may be modified by changing the Pd thickness. PNR measurements, which are sensitive to the in-plane component of the magnetization only, confirm the formation of an in-plane domain wall that varies with the exchange stiffness inside the Co/Pd and with the field. The extent of the in-plane domain wall decreases with increasing applied field for the films with Pd thicknesses of 7 and 5 nm whereas the behaviour of the thinnest film (Pd = 3.5nm) suggests the Co/Pd bilayers show no evidence of a domain wall. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H14.00004: Positive Exchange Bias in GdFe/NiCoO Thin Films Justin Olamit, Kai Liu Thin films of GdFe/NiCoO are one of the few systems that exhibit positive exchange bias [1-4]. In this study, we show that the positive bias in Gd$_{x}$Fe$_{1-x}$/NiCoO is sensitive to the GdFe composition and the field cooling sequence. In particular, the hysteresis loops are often bifurcated due to the existence of multiple phases: a low anisotropy phase with a single reversal in small fields and a higher anisotropy phase with a single or double loop, depending on the GdFe stoichiometry. In Fe-rich samples, increasing the cooling field causes the low anisotropy phase to shift from negative to positive bias and the double-loop high anisotropy phase to shift toward negative bias. In Gd-rich samples, the low anisotropy phase is always positively biased and the single-loop high anisotropy phase is always negatively biased for all cooling field strengths. These behaviors are a result of the parallel and antiparallel couplings between different magnetic phases of GdFe with the NiCoO layer. [1] J. Nogues\textit{, et al.}, Phys. Rev. Lett. \textbf{76}, 4624 (1996). [2] S. Mangin\textit{, et al.}, Phys. Rev. B \textbf{68}, 140404 (2003). [3] X. Ke\textit{, et al.}, Appl. Phys. Lett. \textbf{84}, 5458 (2004). [4] D. Z. Yang\textit{, et al.}, Phys. Rev. B \textbf{71}, 144417 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H14.00005: Exchange bias training effect in Co/CoO heterostructures with variation of the ferromagnetic film thickness Srinivas Polisetty, Tathagata Mukherjee, Sarbeswar Sahoo, Christian Binek The exchange bias (EB) training effect is studied in a Co/CoO heterostructure using low temperature longitudinal Kerr rotation. After field cooling the sample to below the N\'{e}el temperature of CoO the EB training effect manifests itself by a decrease of the EB field upon cycling the Co film through consecutive hysteresis loops. We explore the temperature dependence of the training effect and its dependence on the Co-thickness, t$_{Co}$. The latter is studied by locally probing the EB in a wedge Co/CoO system. The gradient of the Co film thickness allows to measure local t$_{Co}$-dependences in a range of $\Delta $t$_{Co}$ = 23.6nm varying over the substrate length of 8.5mm. The Co wedge is prepared by MBE taking advantage of the steep decrease of the Co flux when leaving the center of the Co beam. A wedge angle of $1.6\times 10^{-4}\;^{\circ}$is revealed by local small angle X-ray reflectivity. We compare the measured t$_{Co}$-dependence with our phenomenological theory predicting a t$_{Co}^{2}$-increase of the leading fitting parameter. This behavior is clearly distinguishable from the 1/t$_{Co}$ -decrease of the equilibrium EB field. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H14.00006: Tilted cores of magnetic vortices due to exchange bias K. Y. Guslienko, A. Hoffmann Recently, the influence of exchange bias on magnetic vortices has been investigated experimentally.\footnote{J. Sort {\em et al.}, Phys. Rev. Lett. \textbf{95}, 067201 (2005); Phys. Rev. Lett. \textbf{97}, 067201 (2006).} By generalizing the rigid vortex model\footnote{K. Y. Guslienko et al., Phys. Rev. B \textbf{65}, 024414 (2002).} we develop an analytic model of the magnetization reversal in an exchange-biased ferromagnetic dot. We account explicitly for a non-uniformity of the magnetization reversal mode along the direction perpendicular to the layers. This non-uniformity allows the vortex core position to vary throughout the thickness of the ferromagnetic layer. We show that the geometrical confinement in combination with the interface exchange field leads to new asymmetries of the hysteresis loops.\footnote{K. Y. Guslienko and A. Hoffmann, Phys. Rev. Lett. \textbf{97}, 107203 (2006)} Namely, the critical fields for vortex nucleation and annihilation respond differently to the interfacial exchange bias, resulting in an asymmetry of the irreversible parts of the hysteresis loops in addition to the overall shift due to the exchange bias. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H14.00007: Controlling the sign of the exchange bias in Fe$_{x}$Ni$_{1-x}$F$_2$/Co and Fe$_{x}$Zn$_{1-x}$F$_2$/Co bilayers Miyeon Cheon, Zhongyuan Liu, Hongtao Shi, David Lederman A correlation between the sign of the exchange bias and the sign of the uncompensated magnetization was observed in the Fe$_ {x}$Ni$_{1-x}$F$_2$/Co bilayer system. Due to this correlation and the fact that the uncompensated magnetization was reversed at high fields at low temperatures, the sign of the exchange bias was controlled by controlling the sign of the uncompensated magnetization in this system. The dilute antiferromagnet Fe$_{0.36}$Zn$_{0.64}$F$_2$/Co system, which was previously shown to also have a large uncompensated magnetization, also showed the same effect but at slightly higher temperatures. Using a micromagnetic simulation program (OOMMF) and comparing to the experimental data, the micromagnetic constants were obtained. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H14.00008: Dynamic enhancement of the exchange bias training effect Sarbeswar Sahoo, Andreas Berger, Srinivas Polisetty, Christian Binek Exchange bias in coupled magnetic films and its accompanying training effect are fundamental interface phenomena which impact spintronic applications. Training is referred to as a gradual change of the bias field, which evolves upon cycling the soft layer through consecutive hysteresis loops. We report on its dynamic enhancement in exchange coupled bilayers of soft and hard ferromagnetic materials. Dynamic effects are induced with increasing sweep rate of the applied magnetic field from quasi-static to the fully dynamic range. A dynamically generalized theory based on triggered and partially truncated relaxation is in excellent agreement with the data. Remarkable universality of our theoretical approach is evidenced when applying the approach to the dynamic training effect of a conventional exchange bias system involving an antiferromagnetic pinning layer. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H14.00009: Exchange bias inducing temperature Alexey Dobrynin, Ruslan Prozorov Characteristic temperatures governing behavior of ferromagnetic - antiferromangetic (F-AF) heterostructures are discussed. The inducing temperature, $T_{ind}$, at which the easy direction of magnetization is established, is in general case different from the maximum temperature at which exchange bias may exist $T_B$ (blocking temperature) and the N\'eel temperature of the antiferromagnet $T_N$. The case of $T_{ind} < T_N$ suggests presence of a frustrated interfacial AF spin structure in the system, otherwise $T_{ind} = T_N$. If $T_B = T_{ind} < T_N$, the interfacial F-AF interactions are stronger than that between the interfacial AF spins and the rest of the AF part, assuming rotation of those spins during the magnetization reversal. The exchange bias value in this case is determined by the latter AF exchange coupling. In the case of $T_B < T_{ind} < T_N$, the interfacial AF spins stay stable, and the exchange bias field is determined by the interfacial F-AF coupling. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H14.00010: FORC Study of Magnetization Reversal Asymmetry in Fe/FeF$_{2}$ Exchange Biased Thin Films Justin Olamit, Kai Liu, Zhi-Pan Li, Ivan K. Schuller Asymmetric magnetization reversal in exchange biased ferromagnet/antiferromagnet (FM/AF) thin films have attracted much interest. Recent work has shown that the asymmetry in Ni/FeF$_{2}$ films is due to local incomplete domain walls in the FM parallel to the interface [1, 2]. We have investigated reversal asymmetry in Fe/epitaxial-FeF$_{2}$ using a First Order Reversal Curve (FORC) technique [3]. The major hysteresis loop is asymmetrical. Along the decreasing-field sweep of the hysteresis loop, FORC measurements show that the nucleation of domain structures occurs gradually while the domain annihilations are abrupt. However, along the increasing-field reversal, the domain nucleations are abrupt and the annihilations occur gradually. Rotating the AF easy axis away from the applied field shows that the nucleation and annihilation field distributions also have different angular dependencies along the field sweeps. These different distributions lead to the asymmetry seen in the shape of the major loop. [1] Li, \textit{et al}., PRL \textbf{96}, 217205 (2006). [2] Morales, \textit{et al}., APL \textbf{89}, 072504 (2006). [3] Davies, \textit{et al}., PRB \textbf{70}, 224434 (2004)~; APL \textbf{86,} 262503 (2005); PRB \textbf{72}, 134419 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H14.00011: Magnetic coupling and training effects in Co/NiO/[Co/Pt] structures with orthogonal easy axes S. Adenwalla, A. Baruth In an attempt to broaden our understanding of the unexpected oscillatory coupling seen in perpendicularly magnetized [Co/Pt]/NiO[Co/Pt]$^{1}$ samples we have investigated a series of Co/NiO/[Co/Pt] samples in which the magnetization lies in-plane and perpendicular to plane for the Co and [Co/Pt] layers respectively. Although no preferred coupling is expected, we find a coupling that depends on a variety of parameters including strength of an in-plane magnetic field pulse (the ``set field''), the number of cycles (the training effect) and the NiO thickness. The strength of coupling, as measured by a shift in the Co in-plane hysteresis loop, is directly proportional to the in-plane set field, an effect of the nonzero remanence of the [Co/Pt] layer. On training, the coupling strength drops abruptly by a factor of nearly 3 on the first cycle and drops more slowly thereafter. We attribute this to the presence of domains in the [Co/Pt] layer. [1] A. Baruth et al. Phys. Rev. B \textbf{74}, 054419 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H14.00012: Domain overlap in antiferromagnetically coupled [Co/Pt]/NiO/[Co/Pt] multilayers A. Baruth, L. Yuan, J.D. Burton, K. Janicka, E.Y. Tsymbal, S.H. Liou, S. Adenwalla Antiferromagnetically coupled magnetic thin films with perpendicular anisotropy exhibit domain overlap regions originating from magnetostatic stray fields localized in the vicinity of the domain walls. Using high resolution magnetic force microscopy we investigate these overlap regions in [Co/Pt]/NiO/[Co/Pt] multilayers as a function of the antiferromagnetic interlayer exchange coupling between the two Co/Pt stacks. Our results agree both qualitatively and quantitatively with a simple model that looks at the external fields near the domain wall regions and their magnetostatic interaction. This model gives an overlap $\delta $=8M$_{top}$M$_{bottom}$t$^{2}$/J$_{IEC}$, where M$_{top}$ and M$_{bottom}$ are the respective saturation magnetizations, t is the thickness, and J$_{IEC}$ is the coupling strength. There is excellent quantitative agreement between the data and the model, which is substantiated by independent magnetization and thickness measurements which agree within 5{\%} to those obtained by the 2 parameter fit. Details can be found in [1]. This research was supported by NSF (grants Nos. MRSEC DMR-0213808 and DMR-0203359). [1] A. Baruth et al. Appl. Phys. Lett. \textbf{89}, 202505 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H14.00013: Probing the exchange-biased system of Co/ $\gamma $-Fe$_{50}$Mn$_{50}$ using a large scale micromagnetic model. Jyotirmoy Saha, Randall Victora We have built a micromagnetic model to study the exchange interaction between Co (a ferromagnet) and $\gamma $-Fe$_{50}$Mn$_{50}$ (an antiferromagnet) that has (111) texturing. The antiferromagnet thickness dependence of exchange bias and enhanced coercivity at two different temperatures were obtained and comparison to experiments showed reasonable agreement. Another aspect of this exchange interaction that we looked into was the grain size dependence of exchange bias. Here we obtain a linear relationship between exchange bias and inverse grain size for nearly thermally stable antiferromagnetic grains. This is the trend that is both predicted by theory and produced in experiments. The plot of exchange bias vs. in-plane field cooling angles exhibits a cosine like symmetry that is set by the thermal stability of the antiferromagnetic grains. Most of the values relating to material characteristics used in our model are taken from literature. One input parameter in our model that needs refinement either from experiments or from band structure calculations is the maximum anisotropy value of the antiferromagnet and its distribution amongst the antiferromagnetic grains. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H14.00014: Exchange bias measurement methodologies and the role of hysteresis loop asymmetry Ondrej Hovorka, Andreas Berger, Gary Friedman The phenomenon of exchange bias refers to the observation of a hysteresis loop field shift in ferromagnetic-antiferromagnetic (AFM) compound structures [1] and in all-ferromagnetic bilayer systems [2]. The exchange bias effect is typically quantified by determining the sum of the coercive fields from a hysteresis loop. Such a two-point (TP) measurement is, however, unambiguous only for time reversal symmetric hysteresis loops [3]. To account for the loop asymmetry, frequently observed in experiments, we recently proposed an alternative characterization scheme, called the center of mass method (CM) [3]. In the present study, we correlate the differences between TP and CM methods and the hysteresis loop asymmetry, using measurement data obtained from the all-ferromagnetic bilayer system, which are supported by model calculation results. We find the loop asymmetry to be a reliable indicator for the ambiguity of the conventional TP method. We will also discuss the applicability of the CM method to conventional AFM structures. [1] A. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200, 552 (1999). [2] A. Berger et. al., Appl. Phys. Lett. 85, 1571 (2004). [3] O. Hovorka et. al., Appl. Phys. Lett. 89, 142513 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H14.00015: Ferrimagnetism in Gd-Ni bilayers Marta Marszalek, J. Jaworski, W.E. Evenson, A. Barth, F. Treubel, M. Albrecht, G. Schatz Physical properties of thin films play an increasingly important role in technical applications. With controlled growth and the production of layered systems, interesting and novel mechanical, optical, electrical and chemical characteristics can be obtained. Here we present studies of structural and magnetic propterties of Gd-Ni bilayers. Temperature-dependent SQUID magnetization measurements show antiferromagnetic coupling between Gd and Ni films, with compensation temperature determined for various bilayer structures. They were complemented by field-dependent magnetization measurements by SQUID and XMCD, revealing the typical switching behavior of an artificial ferrimagnet with two exchange-coupled layers. Structural investigations have been performed using STM, XRD and XRR to determine crystallinity and morphology of the system. [Preview Abstract] |
Session H15: Molecular Based Magnets
Sponsoring Units: GMAGChair: Arthur Hebard, University of Florida
Room: Colorado Convention Center Korbel 4E
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H15.00001: Molecular Magnets based on Fe and TCNE (Tetracyanoethylene, C$_{2}$(CN)$_{4}$): structures from powder diffraction Jae-Hyuk Her, Peter Stephens, Konstantin Pokhodnya, Michael Bonner, Joel Miller There are many interesting organic-based magnets for which understanding is limited because they are not available as single crystals. However, in many cases it has proved possible to determine complete crystal structures from high resolution powder diffraction data. We discuss two specific systems with Fe(II): ~Fe[TCNE]$_{2}$ (which is unexpectedly Fe[TCNE][C$_{4}$(CN)$_{8}$]$_{1/2})$, and Fe[TCNE][MeCN]$_{2}$FeCl$_{4}$. ~Both structures contain a previously unobserved configuration of $\mu_{4}$-[TCNE]$^{-}$ anion bonded to transition metal ions. High resolution powder diffraction patterns were collected at the X16C beamline, National Synchrotron Light Source, Brookhaven National Laboratory.~ Simulated annealing and direct methods programs (FOX, Topas-Academic, EXPO) were used to solve and refine the structures. ~One cannot hope to understand properties of systems such as molecular magnets without knowing their structures, and the present work demonstrates the utility of powder diffraction to obtain that information. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H15.00002: Neutron diffraction and specific-heat studies of magnetic ordering in [Fe$^{II}(\Delta )$Fe$^{II}(\Lambda )$(ox)$_{2}$(Phen)$_{2}$]$_{n}$ molecular magnet C.J. Ho, Chia Pin Sun, C.C. Yang, C.L. Huang, C.C. Chou, Lu-Lin Li, K.J. Lin, W.H. Li, J.W. Lynn, H.D. Yang The magnetic characteristics of molecular magnet [Fe$^{II}(\Delta )$Fe$^{II}(\Lambda )$(ox)$_{2}$(Phen)$_{2}$]$_{n}$ (\textbf{1}), its chemical formula is C$_{28}$H$_{16}$Fe$_{2}$N$_{4}$O$_{8}$ for unity, has been studied by neutron powder diffraction and field dependence of specific heat and magnetization. The intrinsic antiferromagnetic ordering between magnetic Fe ions and magnetic hysteresis below $T_{m}\approx $8.6 K are observed by analyzing Bragg pattern of neutron scattering and isothermal magnetization, respectively. The long-range magnetic ordering (LRMO) is also confirmed from the observation of a small $\lambda $-type anomaly at $T_{m}$ in specific heat measurement. However, the magnetic entropy due to this anomaly is estimated as 0.03R, which is much smaller then expected Rln5 (S=2 for Fe$^{II})$ indicating the spin fluctuations as short-range ordering at $T>T_{m}$. In addition, another magnetic anomaly located at 1K at zero field is increased in temperature and became broadening when applying magnetic field. It might be explained by quantum spin and Zeeman splitting phenomena. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H15.00003: High Frequency Electron Paramagnetic Resonance Studies of High Spin Co(II) Complexes Jon Lawrence, Chris Beedle, En-Che Yang, James Ma, Stephen Hill, David Hendrickson Variable-High-Frequency Electron Paramagnetic Resonance (HFEPR) data have been collected for single crystals of [Zn(hmp)(dmb)Cl]$_{4}$ (\textbf{1}) doped with a small quantity of high-spin Co(II), and an isostructural tetranuclear cobalt complex [Co$^{II}$(hmp)(dmb)Cl]$_{4}$ (\textbf{2}). Crystals of complex \textbf{2 }exhibit low temperature hysteresis, suggesting that it may be a single molecule magnet (SMM).$^{1}$ However, HFEPR data for complex \textbf{2} cannot be fit to a standard Giant Spin model, as is routinely the case for other SMMs. HFEPR data obtained for complex \textbf{1} indicate that the ground state of the Co$^{II}$ ions is an effective spin \textit{S$\prime $}~=~$^{1}$/$_{2}$ Kramers' doublet with a highly anisotropic $g$-tensor. The anisotropy is of the easy-axis type, with the individual easy axis directions tilted away from the crystallographic $c$ direction by 58$^{o}$. We will attempt to rationalize the EPR spectrum obtained for complex \textbf{2} (as well as its possible SMM behavior) in terms of a simple model involving anisotropic exchange coupling between four effective spin \textit{S$\prime $}~=~$^{1}$/$_{2}$ Co$^{II }$ions, with the local anisotropy entering only through the anisotropic g-tensor at each Co$^{II}$ site. $^{1}$ E.-C. Yang, J. Appl. Phys. \textbf{91}, 7382 (2002). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H15.00004: Tunnelling transport through a Mn$_{12}$ molecular magnet in an external magnetic field. L. Michalak, C.M. Canali, V.G. Benza, M.R. Pederson Recent single-electron-transistor experiments with Mn$_{12}$ as a central island exhibit a puzzling behaviour in the tunnelling conductance as a function of the external magnetic field, such as the lack of magnetic hysteresis. We describe the system in terms of a phenomenological giant spin model with two charge states: the $N$-electron state (neutral molecule) and the $(N+1)$-electron state (one extra electron added). The parameters of the model, such as the total spin and the magnetic anisotropy barrier, are calculated by state-of-the-art DFT. The addition of the tunnelling electron's spin to the giant spin is represented in terms of Schwinger bosons. We compute transport by means of a quantum rate equation in the sequential tunnelling regime, which is appropriate for experimental conditions. We find that the model cannot display hysteresis in the differential conductance as a function of the magnetic field when the coupling with the leads is the only source of relaxation. Coherence effects and cotunnelling are further analyzed by means of a master equation for the full density matrix. DFT calculations can also shed light on the influence of the tunnelling electron orbital degree of freedom on the tunnelling amplitudes. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H15.00005: Exchange constants and second-order magnetic anisotropy in cyanide-bridged Fe2M2 single-molecule magnets Kyungwha Park, Stephen Holmes Electronic structure and intramolecular exchange constants are calculated for three cyanide-bridged single-molecule magnets, {[Tp$^{\star}$Fe$^{\mathrm{III}}$(CN)$_3$M$^{\mathrm{II}}$(DMF)$_4$]$_2$(OTf)$_2$}$\cdot$2DMF (M$^{\mathrm{II}}$=Mn, Co, Ni) (abbreviated as Fe$_2$Mn$_2$, Fe$_2$Co$_2$, and Fe$_2$Ni$_2$) that have been recently synthesized, within a generalized-gradient approximation in spin-polarized density-functional theory (DFT). Due to strong ligand fields present in the molecules, the Fe ions exhibit a low ground-state spin of $S=1/2$ in which the orbital angular momentum may not be quenched even without spin-orbit coupling. Based on the calculated electronic structures, the magnetic anisotropy for Fe$_2$Mn$_2$, Fe$_2$Co$_2$, and Fe$_2$Ni$_2$ is computed including single-electron spin-orbit coupling within a DFT formalism. The theoretical values of the induced orbital angular momentum and of the magnetic anisotropy parameters are compared to those for a single-molecule magnet Mn$_{12}$. The total magnetic anisotropy present in the three single-molecule magnets is due to competition between the magnetic anisotropy of the Fe and of the M ions. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H15.00006: Observation of self-assembled Mn$_{12}$-ac Molecules on Highly Ordered Pyrolytic Graphite Dongmin Seo, Winfried Teizer Thin films of the single molecule magnet Mn$_{12}$-ac have been deposited on Highly Ordered Pyrolytic Graphite (HOPG) by a solution evaporation method [1-3]. Mn$_{12}$-ac molecules in a well-ordered self-assembled triangular lattice were subsequently observed in these films by Scanning Tunneling Microscopy (STM) at room temperature under ambient conditions. STM images show typical center to center intermolecular separations of $\sim $ 5 nm. X-ray photoelectron spectroscopy shows that the self-assembled compound on the HOPG surface is consistent with Mn$_{12}$-ac and a control experiment demonstrated that it cannot be another species that may be present in the solvent. \newline [1] K. Kim et al., Appl. Phys. Lett. 85, 3872 (2004). \newline [2] D. M. Seo et al., J. Mag. Magn. Mater. 301, 31 (2006). \newline [3] D. Seo et al., J. Mag. Magn. Mater. in press (doi:10.1016/j.jmmm.2006.09.034). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H15.00007: Dispersive Frequency Shifts in the EPR spectra of the Single-Molecule Magnet Fe$_8$ Jonathan Friedman, Mustafa Bal, Christopher Beedle, David Hendrickson High-frequency electron paramagnetic resonance (EPR) has been used to study single-molecule magnets (SMMs) for more than a decade. We observe dispersive effects in a cylindrical cavity when a single crystal of the Fe$_8$ SMM is tuned to resonance with millimeter-wave radiation. The reflected power from the cavity is measured as a function of the radiation frequency at magnetic fields from 0 to 1.5 Tesla and temperatures between 2.0 and 20.0 K. Although the sample/cavity filling factor is small, $\sim$0.1\%, we observe a substantial sample-induced frequency shift of the cavity resonance when the field brings a dipole- allowed transition near resonance with the applied radiation. At 2.0 K, the resonant frequency of the cavity ($\sim$117.5 GHz) exhibits a shift on the order of 10 MHz (comparable to the width of the cavity resonance). At the same time, we observe a reduction in both the cavity Q and the amount of power absorbed by the cavity. The data allows us to gain both dispersive and absorptive information about the material. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H15.00008: Spatial Determination of Magnetic Avalanche Ignition points Reem Jaafar, S. Mchugh, Yoko Suzuki, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou Using time-resolved measurements of local magnetization [1], we report studies of the propagation of magnetic avalanches (fast magnetization reversals) in Mn12-ac crystals triggered stochastically in response to a time-varying (swept) magnetic field. The spherical nature of the fronts produced by avalanches originating within the bulk is reflected in the time-of-arrival at an array of micro-Hall sensors placed on the surface of the sample. By treating the propagating front as a spherical bubble of radius $r\propto t$, we locate the approximate ignition points in a two-dimensional cross-section of the crystal. The trigger points are stochastically distributed (some in the bulk and some at the edges), with higher density regions that vary from sample to sample. This suggests that avalanches originate preferentially in weak regions of a crystal where the defect density is high. \newline \newline [1] Yoko Suzuki et al. Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H15.00009: Local Temperature Measurement of Avalanche Front in Mn12-Acetate Sean McHugh, M.P. Sarachik, Y. Suzuki, R. Jaafar, Y. Myasoedov, E. Zeldov, A. Finkler, R. Bagai, G. Christou Local magnetization measurements using micro-Hall bars have recently revealed that a magnetic avalanche propagates as a narrow front moving with subsonic speed. In analogy with chemical deflagration (or combustion), we have proposed ``magnetic deflagration,'' a thermodynamic process by which the reversing spins produce heat stimulating the reversal of neighboring spins in the crystal [1]. Based on this model, a simple calculation yields a ``flame front'' temperature of the order of 10 K or higher. Although it has been established that the average temperature exhibits a (small) increase, confirmation of this model requires local, fast measurements of the temperature of the front. We describe experimental progress using $\mu$m sized arrays of germanium thermometers. \newline \newline [1] Y. Suzuki, et. al., Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H15.00010: Conditions for Triggering Avalanches in Mn$_{12}$-acetate. Yoko Suzuki, S. Mchugh, R. Jaafar, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, N.E. Chakov, G. Christou Recent measurements in Mn$_{12}$-acetate have shown that magnetic avalanches (corresponding to fast magnetization reversal) propagate as a narrow front with a velocity that is roughly two orders of magnitude smaller than the speed of sound. This phenomenon is closely analogous to the propagation of a flame front through a flammable chemical substance (deflagration) [1]. The conditions for nucleation of avalanches triggered in response to a time-varying (swept) magnetic field were studied for different fields and temperatures. In these crystals, avalanches happened only at low temperatures and were found to occur stochastically at fields ranging from 1.0 T to 4.5 T. There is no apparent structure in the distribution of avalanches for fields below $\approx$ 3.5 T; at higher fields we find evidence that the probability is lower at ``nonresonant'' magnetic fields where tunneling across the anisotropy barrier is suppressed. This provides evidence that lowering the barrier by quantum mechanical tunneling facilitates the ignition of avalanches. Based on these and other measurements, we suggest that avalanches are triggered below 3.5 T by defects with lower energy barriers. [1] Y. Suzuki, et al., Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H15.00011: Frequency Domain Magnetic Resonance Spectroscopy in Molecular Magnetism Joris van Slageren We have shown over the past years that frequency domain magnetic resonance spectroscopy (FDMRS) is excellently suited to the determination of zero-field splittings (ZFS) in molecular magnets. Among its merits are: the lack of necessity of an external magnetic field, and easy access to very large zero-field splittings. Several examples will be shown. The magnetic resonance lineshapes give information on distributions in the sample as well as excited spin state dynamics. The theoretical analysis of the origin of the cluster ZFS has shown that antisymmetric exchange interactions can play a large role. Because frequency and magnetic field are independent experimental parameters in the FDMRS technique, more sophisticated experiments can be performed. For example, we have spectroscopically studied the relaxation of the magnetization including quantum tunneling. We have also studied the dipolar interaction between single-molecule magnets using magnetic resonance measurements in solutions of various concentrations. Finally, we have shown that single molecule magnets can function as tunable radiation polarization rotators in the THz range. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H15.00012: Hydrogen Bonding and Multiphonon Structure in One- and Two-Dimensional Polymeric Magnets J.L. Musfeldt, S. Brown, J. Cao, M.M. Conner, A.C. McConnell, H.I. Southerland, J.L. Manson, J.A. Schlueter, M.D. Phillips, M.M. Turnbull, C.P. Landee We report a systematic investigation of the temperature dependent infrared vibrational spectra of a family of chemically related coordination polymeric magnets based upon two different bridging anions, fluoride (F$^-$) and bifluoride (HF$_2^-$), in copper-pyrazine complexes including Cu(HF$_2$)(pyz)$_2$BF$_4$, Cu(HF$_2$)(pyz)$_2$ClO$_4$, and CuF$_2$(H$_2$O)(pyz)). We compare our results with several one- and two-dimensional prototype materials including Cu(NO$_3$)$_2$(pyz) and Cu(ClO$_4$)(pyz) $_2$. Unusual low temperature hydrogen bonding, local structural transitions associated with stronger low-temperature hydrogen bonding, and striking multiphonon effects that derive from coupling of an infrared-active fundamental with strong Raman-active modes of the pyrazine building-block molecule are observed. Based on the spectroscopic evidence, these interactions are common to this family of coordination polymers and work to stabilize the low temperature magnetic state. Similar interactions are likely to be present in other molecule-based magnets. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H15.00013: Preliminary Studies of the Metal Organic Molecule $C_{24} H_{36} N_2 O_4 Cu$ David Wiseby, Danqin Feng, Peter Dowben, Carter Silvernail, John Belot, Anthony Caruso We have investigated the metal organic molecule $C_{24} H_{36} N_2 O_4 Cu$, (CuII) and have characterized some of its electronic and magnetic properties. The molecule is of interest because it has a small magnetic moment of 1.03$\mu _b $ per molecule, expected of a Cu spin 1/2 system. There is some preliminary evidence that vapor deposited thin films of the Cu(II) molecule on Cu(111) and Co(111) are crystalline, with some evidence of band structure ultra violet photoemission spectroscopy (UPS). There is generally good agreement between the photoemission and model calculations performed using restricted Hartree-Fock under the semiempirical PM3 methodology. This new molecule has a HOMO-LUMO gap, but is well screened in the photoemission final state. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H15.00014: Low Temperature Magnetic Behavior of Manganese Carboxylates Marshall Bremer, Shengming Liu, Bridger Anderson, Joseph Sandstrom, Doug Schulz, Anthony Caruso Antiferromagnetic ordering has proved to be very useful in producing high temperature remnant moments in organic-based compounds. A manganese carboxylate-based complex demonstrating strong antiferromagnetic coordination has been discovered and shown to exhibit exotic magnetic behavior at low temperatures. The complex is comprised of two-dimensional sheets containing 12 member, edge sharing hexagons featuring carboxylate bridged manganese. These honeycomb sheets provide the structure which supports several magnetic phase transitions. ac and dc magnetometry data support spin glass, metamagnetic and ferrimagnetic behavior at low temperatures. The interaction parameter J is estimated by comparing mean field theory models to the high temperature susceptibility data. The magnetic states and supporting evidence will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H15.00015: Phonon Bottleneck in the Single-Molecule Magnet Fe$_8$ Induced by Pulsed Millimeter-Wave Radiation M. Bal, Jonathan Friedman, Wei Chen, Mark Tuominen, Evan Rumberger, David Hendrickson We report measurements of the magnetization dynamics of the Fe$_8 $ single-molecule magnet on timescales as short as $\sim$10 ns using millimeter-wave radiation to drive transitions between the ground ($m$ = 10) and first excited ($m$ = 9) states. We find that during the radiation pulse the magnetization decreases linearly, while afterwards it decays exponentially back to its initial value with a long time constant of $\sim$10 $\mu$s. We interpret these results as evidence of a phonon bottleneck in which a non-equilibrium number of phonons resonant with the 10- to-9 transition builds up in the crystal, leading to an population increase in the m = 9 state. The time for these phonons to decay (either by escaping the crystal or through scattering) is interpreted to be the measured $\sim$10 $\mu$s. We observe that the phonon bottleneck is established in less than $\sim$15 ns, which suggests that the spin-phonon relaxation time $T_1$ is (rather unexpectedly) shorter than this value. [Preview Abstract] |
Session H16: Focus Session: 2D Quantum Magnetism
Sponsoring Units: GMAGChair: Igor Zaliznyak, Brookhaven National Laboratory
Room: Colorado Convention Center Korbel 4F
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H16.00001: Competing Impurities in an Antiferromagnetic Background. Lyudmyla Adamska, Marcello B. Silva Neto, Cristiane Morais Smith We study the order-from-disorder transition and reentrant magnetism in a doped crystal La$_{2-x}$Sr$_{x}$Cu$_{1-z}$Zn$_{z}$O$_{4}$ within the framework of a long-wavelength nonlinear sigma model that properly incorporates the Dzyaloshinskii- Moriya and XY anisotropies. The effect of doping La$_{2}$CuO$_{4}$ with nonmagnetic impurities, such as Zn, is considered according to classical percolation theory, while the effect of the Sr-doped charge carriers is described as a dipolar frustration of the antiferromagnetic order. We derive the expressions for several magnetic, thermodynamic, and spectral properties of the system, such as the N\'{e}el temperature, the spin-stiffness, and the anisotropy gaps, as well as their evolution with both Zn and Sr doping for cases of ballistic and diffusive vacancies. We solve the problem analytically in the limits of low and high temperatures. The connection to several experiments will be discussed [Phys. Rev. B59, R725 (1999), Science 295, 1691 (2002), Phys. Rev. Lett. 93, 027001 (2004)]. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H16.00002: Quantum effects of impurity-induced frustrations in diluted antiferromagnets Shiu Liu, Sasha Chernyshev We show that in an antiferromagnet doped with non-magnetic impurities, e.g. Zn-doped La$_2$CuO$_4$, an impurity can induce substantial frustrating interactions among the next- and next-next-nearest neighbor Cu spins around the impurity site, $J'_{Zn}$ and $J''_{Zn}$, respectively. Such interactions arise from the virtual transitions through the Zn and O orbitals. We study the 2D, square lattice, $S=1/2$ Heisenberg antiferromagnet in which the frustration is induced by the dilution with such impurities. We use the $T$-matrix approach to calculate the quantum effect of such dilution on the antiferromagnetic order parameter, staggered magnetization $M(x)$, as a function of the doping concentration $x$. In the experimentally relevant range of $J'_{Zn}$ and $J''_{Zn}$ we find substantial deviation of our results for $M(x)$ from the non-frustrated site-dilution theories. We argue that the frustration effect explains discrepancies between the experimental data and the non-frustrated site-dilution theory of the copper-oxide plane with Zn impurities. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H16.00003: Frustration in the Hubbard model: a quantum cluster study Andriy Nevidomskyy, Christian Scheiber, David Senechal The role of frustration in the Hubbard model is studied on the square lattice with nearest and next-nearest neighbour hoppings $t$ and $t'$ using the Variational cluster perturbation theory (VCPT, see [1]). We find two phases with long-range magnetic order: the usual antiferromagnet (AF1) phase, stable at small $t'/t,$ and the so-called superantiferromagnetic phase (AF2) for large frustration. These are separated by a phase with no magnetic order. We also find d-wave superconductivity (d$_{x2-y2})$ for small values of $U\le $4$t$ and sufficiently weak frustration. The Mott-Hubbard transition is discussed in this context. We also compare the classical phase diagram obtained from the large-U expansion with that of the frustrated J$_{1}$-J$_{2}$ Heisenberg model. \newline \newline [1] M. Potthoff, M. Aichhorn, C. Dahnken, Phys. Rev. Lett. \textbf{91}, 206402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H16.00004: High Frequency ESR study of the two-dimensional Heisenberg antiferromagnet copper pyrazine perchlorate. Johan van Tol, Saritha Nellutla, Chris Landee Copper pyrazine perchlorate, Cu(Pyrazine)$_{2}$(ClO$_{4})_{2}$, is thought to be an almost prefect 2D Heisenberg antiferromagnet. We have performed single crystal electron magnetic resonance experiments at 120, 240, and 336 GHz on this system. Both the line-width and resonance shift are field-dependant and indicate that the 3D antiferromagnetic ordering temperature increases from 4.2 K at zero field to about 7 K at 12 Tesla. This can be interpreted by a the field-induced XY-behavior, as has been predicted[1]. A. Cuccoli et al., \textit{Phys. Rev. B} \textbf{68}, 060402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H16.00005: Field dependent ordering temperature in copper pyrazine perchlorate, Cu(pz)$_{2}$(ClO$_{4}$)$_{2}$ Christopher Landee, Fan Xiao, Mark Turnbull, N. Tsyrulin, Michel Kenzelmann, Hans Van Tol Copper pyrazine perchlorate is a molecular-based 2D S=1/2 Heisenberg antiferromagnet (QHAF) with a moderate exchange constant (J/k = 17.5 K) and a saturation field of 60 T. The zero-field ordering temperature, as recently determined by muon spin relaxation experiments [1], is 4.3 K corresponding to excellent isolation (J$'$/J $\approx $ 8x10$^{-4})$ between magnetic layers [2]. Recent studies of Cu(pz)$_{2}$(ClO$_{4})_{2}$ in applied fields (specific heat and ESR) show the ordering transition to increase with field by as much as 30{\%} in a field of nine tesla. This effect will be discussed in terms of a field-induced anisotropy crossover model [3]. 1. T. Lancaster, S. J. Blundell et al, submitted for publication. 2. P. Sengupta, A. W. Sandvik, and R. R. P. Singh, Phys. Rev. B \textbf{68}, 094423 (2003). 3. A. Cuccoli et al, Phys. Rev. B \textbf{68}, 060402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H16.00006: Field-induced anisotropy crossover in copper pyrazine perchlorate Cu(pz)$_{2}$(ClO$_{4}$)$_{2}$ Fan Xiao, Christopher Landee, Mark Turnbull The temperature-dependent magnetization of a single crystal of the 2D QHAF copper pyrazine perchlorate was studied at different fields along all three crystalline orientations. The crystal has identical response when the applied field lies within the layer, only showing a low temperature minimum for fields larger than 3 kOe. The temperature of the minimum increases smoothly with the field strength. The effect is explained as a field-induced 2D Heisenberg to 2D XY anisotropy crossover [1]. \newline \newline [1] A. Cuccoli et al, Phys. Rev. B \textbf{68}, 060402 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H16.00007: Spontaneous spin-lattice coupling in the geometrically frustrated triangular lattice antiferromagnet CuFeO2 Invited Speaker: We use high-resolution synchrotron x-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet CuFeO$_{2}$. On cooling from room temperature, CuFeO$_{2}$ undergoes two antiferromagnetic phase transitions with incommensurate (IC) and commensurate magnetic order at $T_{N1}$=14 K and $T_{N2}$=11 K, respectively. The occurrence of these two magnetic transitions is accompanied by second- and first-order structural phase transitions from hexagonal to monoclinic symmetry. Application of a 6.9 T magnetic field lowers both transition temperatures by 1 K, and induces additional IC structural and magnetic modulations in the temperature region where magnetoelectric behavior has been observed.$^{\S }$ These results show that a strong magneto-elastic coupling is intimately related to the multiferroic effect. It is believed that the IC magnetic structure is noncollinear because this would break the inversion symmetry, as required for the appearance of multiferroic behavior. Because the multiferroic phase appears only in a high magnetic field it is not possible to perform a full crystallographic study to demonstrate this noncollinearity, and an indirect method is being pursued that involves the measurement and characterization of the spin dynamics of this system using neutron scattering.~ This novel approach probes the link between multiferroelectricity and the appearance of noncollinear IC magnetic structures. $^{\S }$Ye et al, Phys. Rev. B 73, 220404 (R) (2006) [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H16.00008: Quantum and Classical Spins on the Distorted Kagome (volborthite) Lattice Fa Wang, Ashvin Vishwanath, Yong Baek Kim The spin-1/2 quantum antiferromagnet on the distorted Kagome lattice (with bonds along one direction stronger than along the other two directions) is realized in the mineral volborthite [F.Bert, et.al. Phys.Rev.Lett. 95, 087203 (2005)]. Here we study properties of antiferromagnetic spin systems on this lattice, in particular how the distortion affects the extreme frustration of the Kagome structure. We first consider ground states of classical O(3) spins on this lattice and show that there is a very large (although probably sub-extensive) number of them. Order-by-disorder effects resulting from thermal or quantum spin waves will be discussed. Finally, we consider approaching the problem directly from the quantum limit via Schwinger boson and fermion mean field theories. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H16.00009: Spin chirality and high-field phase diagram of the kagom\'e lattice antiferromagnet $A$Fe$_{3}$(OH)$_{6}$(SO$_{4}$)$_{2}$ K. Matan, J. S. Helton, D. Grohol, B. M. Bartlett, V. Sikolenko, D. G. Nocera, Y. S. Lee We have used neutron scattering to study spin chirality and high-field spin re-oriention in single crystal and powder samples of the antiferromagnetic kagom\'e lattice compounds, $A$Fe$_{3}$(OH)$_{6}$(SO$_{4}$)$_{2}$ ($A$ = K and Ag). The $\frac{5}{2}$ Fe$^{3+}$ spins on the kagom\'e lattice order three dimensionally for temperatures below the N\'eel temperature, T$_{N}$ = 65 K. Above the N\'eel temperature, we observed the spin chiral order. Neutron measurements reveal critical spin fluctuations above T$_N$, which indicate that the spin-rotational symmetry and the vector chiral symmetry are not broken simultaneously at T$_N$. Below the N\'eel temperature, neutron scattering measurements in high fields show a spin re-orientation transition, which appear to be first-order in nature. Details of the high field phase diagram will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H16.00010: Ground state and excitation properties of the quantum kagom\'{e} system ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ investigated by local probes. Oren Ofer, Amit Keren, Emily Nytko, Matthew Shores, Bart Bartlett, Daniel Nocera, Alex Amato, Chris Baines We present a comprehensive study on the ground state and excitation spectrum of the $S=1/2$, analytically pure and perfect kagom\'{e} system ZnCu$_{3}$% (OH)$_{6}$Cl$_{2}$ using the following measurements: magnetization, muon spin rotation frequency shift $K$, transverse relaxation time $T_{2}^{\ast }$% , and zero field relaxation, and Cl nuclear spin-lattice relaxation $T_{1}$. Using our data we address four questions which are at the heart of the investigation of the quantum kagom\'{e} system: Do $S=1/2$ spins on kagom% \'{e} lattice freeze? Is the ground state magnetic? What is the density of excited states, and is there a gap in the spin energy spectra? Finally, does the lattice distort in order to accommodate spin-Peierls state? We found no sign of singlet formation, no long range order nor spin freezing, and no sign of spin-Peierls transition even at temperatures as low as $60$~mK. The density of states has an $E^{1/4}$ energy dependence with a negligible magnetic gap to excitation. Thus ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is an exotic magnet with no broken continuous symmetry but gapless excitations. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H16.00011: Spin dynamics of the S=1/2 kagom\'e lattice antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ J.S. Helton, K. Matan, M.P. Shores, E.A. Nytko, B.M. Bartlett, Y. Yoshida, Y. Takano, A. Suslov, Y. Qiu, J.-H. Chung, D.G. Nocera, Y.S. Lee An important challenge in condensed matter physics is the search for quantum spin liquid states in two dimensional frustrated systems. We have performed thermodynamic and neutron scattering measurements on the $S=1/2$ kagom\'e lattice antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$. The susceptibility indicates a Curie-Weiss temperature of $\theta_{CW} \simeq -300$~K; however, no magnetic order is observed down to 50~mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1~meV. The specific heat at low-$T$ follows a power law with exponent $\alpha \leq 1$. These results suggest that an unusual spin-liquid state with essentially gapless excitations is realized in this kagom\'e lattice system. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H16.00012: Theory of spin liquid on the kagome lattice and application to ZnCu$_3$(OH)$_6$Cl$_2$: Projected wavefunction study Ying Ran, Michael Hermele, Patrick A. Lee, Xiao-Gang Wen Recent experiments on ZnCu$_3$(OH)$_6$Cl$_2$ have drawn new attention to the ground state of the spin-1/2 kagome lattice antiferromagnet. We have examined this issue using Gutzwiller projected fermion wavefunctions, and propose that the ground state is described by the projection of fermions hopping on the kagome lattice in a background $\pi$-flux on the hexagons and zero flux on the triangles. This state has gapless Dirac points and is an algebraic spin liquid. Properties of the wavefunction and low-energy excitations will be discussed, with a focus on application to the experiments. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H16.00013: Theory of spin liquid on the kagome lattice and application to ZnCu$_3$(OH)$_6$Cl$_2$ : Quantum field theory Michael Hermele, Ying Ran, Patrick A. Lee, Xiao-Gang Wen We study the properties of an algebraic spin liquid on the kagome lattice, which is suggested by projected wavefunction calculations to be the ground state. Various quantities of relevance to ZnCu$_3$(OH)$_6$Cl$_2$ will be discussed, including specific heat, magnetic susceptibility and spin correlations. The role of magnetic impurities in this spin liquid state and the implications for experiments will also be discussed. [Preview Abstract] |
Session H17: Focus Session: Theory and Simulation - Polyelectrolytes & Brushes
Sponsoring Units: DPOLY DCOMPChair: Kevin Cavicchi, University of Akron
Room: Colorado Convention Center 102
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H17.00001: BREAK
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Tuesday, March 6, 2007 8:36AM - 8:48AM |
H17.00002: Wigner Crystallization of Chiral Polyelectrolyte Bundles Gregory Grason, Robijn Bruinsma We construct an effective model to describe the thermodynamics of the Wigner- crystalline state of polyvalent counterions absorbed within hexagonal bundles of chiral, rod-like macroions (such as DNA or filamentous actin). We argue that the ground state counterion configuration as well as the associated long-wavelength fluctuations about this state are characterized by a frustrated, antiferromagnetic XY {\it spin} Hamiltonian defined on the kagom\'e lattice. The unusual statistical mechanics associated with this model can be treated within a generalized, dual description of interacting vortices (or screw defects), revealing that generically Wigner-crystalline ground states are constructed of arrays of screw-like configurations of counterions which wind helically around the constitute macroions. Further, the dual description reveals that the ground state admits both {\it integer} and {\it fractional} screw-like defects as well as Ising- like fluctuations between domains of unlike chirality. We find that the molecular chirality of the macroions plays an important role in suppressing these domain fluctuations, and hence chirality itself becomes a critical parameter in melting behavior of the Wigner-crystalline state. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H17.00003: Variable length condensing agents in polyelectrolyte condensation Richard Guaqueta, Erik Luijten We use grand-canonical Monte Carlo simulations to study the role of the condensing agent in polyelectrolyte condensation. The rigid polyelectrolytes are modeled on the M13 and fd viruses, and (following recent experiments) the condensing agents are short divalent chains of variable length $\delta$ modeled on diamine molecules. We observe two different regimes of condensation as the length of the condensing agent varies, with the behavior at large $\delta$ characterized by significant alignment of the diamines with the polyelectrolytes. We also study the effect of the polyelectrolyte surface charge density $\sigma$, and find that the stability of the condensed phase varies nonmonotonically with $\sigma$, in accordance with the observation of two \emph{different} trends in experiments. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H17.00004: Field Theory of Polyelectrolyte Complexation Yuri Popov, Glenn Fredrickson We study polyanion-polycation solutions using a field-theoretic approach formulated in terms of auxiliary fields (conjugate to mass and charge densities). Within this framework, we derive exact Hamiltonians for a wide variety of systems: with implicit or explicit solvents, for symmetric or asymmetric polyions, with or without salt. By systematic expansion, we analytically obtain one-loop fluctuation corrections to the mean-field results for these systems in arbitrary dimensions. As an example, we study the symmetric salt-free polyanion-polycation mixture in implicit solvent. We demonstrate that this basic system and its phase diagram are described by three universal reduced variables. We obtain simple analytical expressions for thermodynamic quantities and structure factors, including two correlation lengths - Edwards's length and a polymer electrostatic length. We also conduct scaling analysis in dilute and semi-dilute regimes and show that the concentration of pair formation is exponentially small in polymer length. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H17.00005: Complexation in poly-electrolyte solutions: field theoretic simulations of fluctuation induced phase transition Jonghoon Lee, Yuri Popov, Glenn Fredrickson We study complexation phenomenon in symmetric poly-electrolyte solutions using field theoretic framework. The mean field approximation is incapable to capture the phase transition in the system. We performed large-scale field theoretic simulations using complex Langevin dynamics algorithm to include the field fluctuation effect. This allows us to study thermodynamics and structural properties of the complexes in detail and, ultimately, construct a phase diagram of the complexation transition, which is compared with the beyond-mean-field (one-loop) analytic result. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H17.00006: Rouse Dynamics of Polyelectrolyte Solutions: Molecular Dynamics Study Andrey Dobrynin, Qi Liao, Michael Rubinstein We performed molecular dynamics simulations of dilute and semidilute polyelectrolyte solutions to study Rouse dynamics of polyelectrolytes. Polyelectrolyte solutions are modeled by an ensemble of bead-spring chains of charged Lennard-Jones particles with explicit counterions. We show that the simulations of the Rouse dynamics give qualitatively similar results to the experimentally observed dynamics of polyelectrolyte solutions. Our simulations showed that the chain relaxation time depends nonmonotonically on polymer concentration. The chain relaxation time decreases with increasing polymer concentration in dilute solution. This decrease in the chain relaxation time is due to counterion condensation. In the semidilute solution regime the chain relaxation time decreases with polymer concentration as inverse square root of polymer concentration. In this concentration range the chain relaxation time follows the usual Rouse scaling dependence on the chain degree of polymerization. At very high polymer concentrations the chain relaxation time begins to increase with increasing the polymer concentration. The crossover polymer concentration to the new scaling regime is independent on the chain degree of polymerization. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H17.00007: Effect of Interfacial Curvature on the Miscibility of Mixed Charged and Neutral Polymer Brushes You-Yeon Won, Kevin Witte We present a theoretical study of the phase behavior of a mixture of neutral and polyelectrolyte polymers attached to a surface or interface in a brush configuration. The Edward's formalism for the Green's function is extended to incorporate electrostatic effects and allow for mixtures of mutually incompatible brushes. The resultant self-consistent field (SCF) equations are numerically evaluated for spherical and cylindrical geometries within the mean field approximation. Phase behavior of the surface constrained polymer mixture (assuming mobile grafting points) is explored by calculating the system free energy and applying the standard free energy of mixing analysis. The effect on the brush mixture miscibility of varying the surface/interface curvature at constant grafting density is extensively investigated. It is further demonstrated that the correlation between brush miscibility and curvature vary with the charge of the polyelectrolyte brushes. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H17.00008: Anisotropic Fluctuation Effects in Polyelectrolyte Adsorption Ying Jiang, Qiang Wang We have examined the fluctuation effects on the adsorption of flexible polyelectrolytes on flat substrates using the theory of anisotropic composition fluctuation. We expand the free energy functional in terms of perturbations around the self-consistent mean-field solution of the inhomogeneous system. Setting the functional derivatives with respect to the perturbations of the field variables to zero produces a set of self-consistent equations; the zeroth-order expansion corresponds to the mean-field result and the 2nd-order expansions represent the Gaussian fluctuations in the system. The composition fluctuations result in stronger charge inversion than obtained in our previous self-consistent field calculations for the same system. Our study shows that the fluctuation and correlation effects in the system give the predominant contributions to charge inversion, in agreement with other theoretical and experimental studies. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H17.00009: Local algorithms for Coulomb's law in molecular dynamics Joerg Rottler The computation of Coulombic interactions still forms one of the major bottlenecks in molecular dynamics simulations of soft and biologial materials. Most current fast Coulomb techniques rely on Fourier methods, but their parallel efficiency on standard compute clusters is not always ideal. In addition, they have difficulty dealing with inhomogeneous dielectric environments that one might like to treat in implicit solvent models. We discuss an attractive alternative, real-space approach that does not rely on Poisson's equation, but mediates the Coulomb interaction through a thermalized auxiliary field that is dynamically constrained to obey Gauss' law. Instead of globally optimizing the field configuration as in conventional approaches, the algorithm performs a partial integration over the transverse degrees of freedom of the electric field, which requires only local operations. The locality leads directly to linear (i.e. O(N)) scaling with the number of particles, implies excellent parallelizability and generalizes easily to inhomogenenous dielectrics without substantial overhead. We show that recent implementations of this method can yield an accuracy sufficient for atomistic simulations, calibrate its parallel efficiency and compare to standard Fourier methods. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H17.00010: ABSTRACT HAS BEEN MOVED TO N21.00014 |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H17.00011: Transitions of tethered polymer chains Jutta Luettmer-Strathmann, Federica Rampf, Wolfgang Paul, Kurt Binder Polymer chains near surfaces with attractive monomer surface interactions undergo a transition from three-dimensional to two- dimensional conformations as the temperature is lowered. In good solvent conditions, this is a well-known adsorption transition, which corresponds to a bicritical point in the infinite chain limit; it is continuous and the adsorption temperature is independent of the (net) interaction strength between monomers. In poor solvent, on the other hand, the transition is not well understood. In this work, we present simulation results for a flexible lattice model of a single chain tethered to a surface. A two-dimensional Wang- Landau algorithm was employed to obtain a density of states in the space of surface and monomer-monomer contacts for several chain lengths. The density of states was evaluated with interaction parameters spanning the range from good to poor solvent conditions and from repulsive to strongly attractive surfaces. Our results for good-solvent conditions show the expected adsorption transition. In poor solvent, we find a splitting of the adsorption transition into two main branches, which may be interpreted in terms of a drying and wetting transition, where the wetting transition proceeds through a series of layering transitions. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H17.00012: Finite-Stretching Corrections to the Strong-Stretching Theory of Polymer Brushes in Solvent Jaeup Kim, Mark Matsen Grafted polymers in solvent are naturally stretched and form a brush. Earlier theoretical approach known as strong-stretching theory (SST) has been very successful in predicting fundamental properties such as parabolic density profile and broad chain end distribution. A more rigorous self-consistent-field theory (SCFT) has shown good agreement with SST but it also revealed new features. For instance, there exists a proximal layer next to the substrate ($z=0$) where the polymer concentration $\phi(z)$ vanishes. Furthermore, a brush has an exponentially decaying tail region beyond the brush height $h$ predicted by SST. Due to the complexity of numerical approach few previous studies focused on these features. We have made a systematic analysis of the proximal layer shape and its effect on the free energy. The size of the proximal region $\mu$ scales as $1/h$ and the profile has a scaling symmetry. Polymer concentration $\phi(z)$ grows linearly near the grafting surface with a slope $6/Na^2$ when the integral of $\phi(z)$ is normalized to unity. Here $a$ is the statistical segment length and $N$ is its total number of segments per chain. A universal function $\overline{\phi}(x)$ is numerically found so that $\phi(z) \approx \mu \overline{\phi}(z/\mu)$ independent of $h$. We also investigated the shape of the tail region to which entropically excited chains contribute. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H17.00013: ABSTRACT WITHDRAWN |
Session H18: De Novo Designed Peptides as Building Nanostructural Blocks
Sponsoring Units: DPOLY DBPChair: Darrin Pochan, University of Delaware
Room: Colorado Convention Center 103
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H18.00001: BREAK
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Tuesday, March 6, 2007 8:36AM - 8:48AM |
H18.00002: Responsive Polypeptide-based Block Copolymer Assemblies Daniel A. Savin, Gopal Venkatachalam, Sandeep S. Naik, Kay E. Gebhardt Amphiphilic block copolymers of poly(butadiene) and poly(L- lysine) (PB-P(Lys)) as well as poly(propylene oxide) and P(Lys) (PPO-P(Lys)) were synthesized and their solution properties studied using dynamic light scattering and transmission electron microscopy. We exploit secondary structure changes that occur in the P(Lys) chain to observe changes in solution morphology as a function of solution conditions. At high pH, the P(Lys) chain assumes either an $\alpha$-helical or a $\beta$-sheet conformation depending on temperature, while at lower pH the side chains become protonated, resulting in an expanded coil configuration. In these studies, we explore the pH and temperature responsiveness for a series of block copolymers with varying morphology. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H18.00003: Early Stages of De Novo Designed Beta-Hairpin Peptide Self-Assembly Tuna Yucel, Joel P. Schneider, Darrin J. Pochan In aqueous solution, MAX 1 peptide is unfolded and does not self-assemble. The peptide intramolecularly folds into a beta-hairpin when the electrostatic interactions between charged residues are screened through increasing the ionic strength at neutral pH. Beta-hairpin molecules supramolecularly assemble via hydrophobic collapse and hydrogen bonding into a 3-D hydrogel network. By combining the results of CD, cryo-TEM, DLS, and oscillatory rheology, we understand that the self-assembly proceeds by nucleation of monodisperse (3 nm wide) beta-sheet fibrils, which elongate, branch and cross-link to form clusters of fibrils. Assembly kinetics at this early stage indicates power law growth with assembly time. Eventually, clusters of fibrils interpenetrate to form a percolated network, as evidenced by the increasing network rigidity. The early stage assembly process will be discussed and compared to published gelation models. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H18.00004: Effect of Strand Symmetry on the Nanostructure and Material Properties in Beta-Hairpin Peptide Hydrogels Rohan Hule, Darrin Pochan, Radhika Nagarkar, Joel Schneider Hydrogels have been established as promising biomaterials for applications such as scaffolds for tissue engineering, controlled drug delivery and cell encapsulation. De novo designed beta hairpin peptides, capable of undergoing self assembly and hydrogel formation, were investigated that contain asymmetric beta strand arms surrounding a turn sequence. The stimuli responsive self assembly of the hydrogels occurs via an intramolecular folding and strand interdigitation mechanism. CD and FTIR indicate a beta sheet secondary structure. WAXS shows a fibril structure reminiscent of the cross beta spine. SANS has been employed to globally quantify the local structure as being rod-like. Modification of the strand registry results in fibrils with non-twisting, laminated vs. twisted nanostructure. Fibril dimensions as measured by TEM and AFM corroborate the interdigitated assembly. Bulk material properties of these hydrogels studied using oscillatory rheology vary significantly for the different morphologies. Differences in the peptide registry that drive hydrogel nanostructure and the consequent material properties can be potentially utilized for usage in specific biomaterial applications. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H18.00005: Self-assembling, bioactive protein hydrogels via engineered coiled-coil aggregation. James Harden, Stephen Fischer, Lixin Mi We describe associating triblock proteins with that self-assemble into reversible, nanostructured hydrogels with a regular network structure and specific biofunctional attributes. These fibrilar, telechelic designs consist of a hydrophilic random coil (denoted R) flanked by associating coiled-coil end domains (denoted A, B, C). The central R domain also encodes specific cell binding and signaling functions of extracellular matrix (ECM) constituents. We will discuss a series of proteins with complimentary associating end blocks that preferentially form heterotrimer aggregates of A, B, and C domains. Mixtures of symmetric triblocks ARA, BRB, and CRC in aqueous solution self assemble into reversible viscoelastic network structures, which we characterize using microscopy, light scattering techniques and computer simulations. Supporting circular dichroism and analytical ultracentrifugation studies of the secondary structure and association behavior of the A, B, C domains will also be presented. Through the use of microscopic and cell proliferation assays, we also show that these hydrogels are capable of inducing biomimetic responses of ECM constituents in cell culture experiments. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H18.00006: Planar peptide processing Kirk Baldwin, Robert Willett Spatial manipulation on small length scales of biological materials, in particular peptide based substances, is important both for implementing assays and for exploiting the properties of the materials set. In this talk we describe methods for patterning peptides in planar manipulations much as is exercised with materials in semiconductor processing: Controlled deposition into small length-scale patterns is accomplished through selective adhesion to patterned substrates or deposition through patterned masks, and removal of peptide films can be achieved through wet or dry etching techniques. These methods are shown to be applicable to at least the micron scale, and this technique summary presents an elemental tool-box for planar processing of this set of biological films. Collectively these techniques provide a ``toolbox'' of methods to accomplish rudimentary planar processing with peptides. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H18.00007: Self-Assembling Octa-peptides Aline Miller, Antonios Konstantopolous, Laurent Caron, Alberto Saiani In this work we have focused on examining systematically the effect of hydrophobicity, charge distribution and size of amino acid on the self assembly behavior of a series of octa-peptides that have been synthesized in our laboratory: AEAEAKAK, AEAKAEAK, FEFKFEFK. FEFKFEFK, FDFDFRFR, FDFRFDFR, FDFDFKFK, FDFKFDFK, FKFDFDFK and FDFKFKFD. The structure of our systems have been elucidated using a combination of Fourier transform infra-red spectroscopy, atomic force microscopy and small angle neutron scattering. This work has shown that the peptides form beta-sheet rich fibrils that have circa 4-6 nm in diameter, and these can associate further along their length scales depending on the amino acid sequence. In some cases these fibrils, or thicker fibers, then become physically entangled to give rise to a 3-dimensional fibrillar hydrogel that does not flow upon inversion of the sample vial. The mechanical properties of all resulting hydrogels have been explored using oscillatory rheometry and results related back to hydrogel structure across the length scales. Here we will present phases diagrams, propose a generalized gelation mechanism and link molecular structure to macroscopic properties. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H18.00008: Sequence Dependent Peptide Self-Assembly and Beta-Sheet Fibrils as Templates for Inorganic Material Matthew Lamm, Darrin Pochan, Joel Schneider Synthetic peptides have been designed to self-assemble into beta-sheet fibrils of varying morphology depending on the peptide sequence. Incorporation of a diproline sequence between two beta-sheet forming strands is used to affect peptide conformation and thus the self-assembly mechanism and resulting fibrillar morphology (e.g. twisted vs. untwisted). Peptide length, proline stereochemistry, diproline sequence position, and assembly kinetics are shown to significantly affect fibril morphology. Furthermore, fibrils of varying morphology are employed as templates for inorganic material such as amorphous silica. In addition, metal nanoparticles were synthesized and functionalized to interact with the fibrils resulting in laterally spaced, linear particle arrays. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H18.00009: Incorporation of Designed Extended Chromophores into Amphiphilic 4-helix Bundle Peptides for Biomolecular Materials Ting Xu, Jiayu Wang, Joe Strzalka, Thomas Russell, Michael Therien, J. Kent Blasie De novo designed peptides, together with synthetic non-biological cofactors, could lead to peptide-based~systems with novel properties not exhibited by biological systems. Extended chromophores can be designed and tailored, with appropriate donors, acceptors and constituents,~to exhibit selected nonlinear optical responses and light-induced electron transport and/or proton~translocation over large distances. Designed extended chromophores can be incorporated into the amphiphilic 4-helix bundle peptides via bis-histidyl ligation. Amphiphilic 4-helix bundle peptide monolayer, both the apo- and holo-form, can be oriented vectorially at the air/water interface. Nanoporous thin films made from diblock copolymers are ideal templates to assemble the artificial proteins with laterally hexagonal order. We will also discuss the efforts on re-designing the artificial proteins and incorporate them into block copolymer based nanoporous templates. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H18.00010: Turning protein into room temperature molecular magnet Chia-Ching Chang, Shang-Fan Lee, Kien-Wen Sun, Lou-Sing Kan Metallothionein-2 (MT-2) is a cysteine-rich protein that binds seven divalent transition metal ions avidly via its metal-thiol linkages. A magnetic MT-2 containing two Mn and five Cd (Mn,Cd-MT-2) has been synthesized by protein refolding process. No trace of Fe was detected by ICP mass spectroscopy. The uniform size distribution, tested by dynamic light scattering, indicated that each Mn,Cd-MT-2 molecule is a single molecular magnet. Its coercive field of ferromagnetic signals changed slightly from 50 to 300K, but dropped rapidly when the temperature rose from 330 to 395 K. The blocking temperature T$_{B}$ is around 410K, in powder form. These results indicated that the un-paired electron of both Mn$^{2+}$ might be aligned by electron hoping of the bridging sulfurs in the $\beta $-metal binding cluster of MT-2 and when the protein deformed at 410K the ferromagnetic signals disappear correspondingly. This engineered molecule exhibits both molecular magnetization and bio-compatibility. These features make Mn,Cd-MT-2, a good candidate for biological applications and sensing sources of new nano-devices. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H18.00011: Interaction of the synthetic polypeptide poly(FFDD) with single-walled carbon nanotubes Yachin Cohen, Merav Granite, Amram Mor, Wim Pyckhout-Hintzen Dispersion of bulk-synthesized single-walled carbon nanotubes (SWCNT) and their subsequent assembly into beneficial structures, especially in aqueous medium, requires the interaction of amphiphilic moieties. Among these, proteins as well as \textit{de-novo} polypeptides have been found to provide useful functional SWCNT dispersions. The synthetic polypeptides reported so far have rather elaborate sequences, which are deemed necessary for the specific conformations that successfully interact with the SWCNT surface. We have sought to study simple oligo-peptides and their basic interactions with SWCNTs in water. An oligo-peptide: poly(FFDD) [F=phenyl alanine, D = aspartic acid] with 30 amino-acid units, exhibiting and alternating hydrophobic/hydrophilic motif, was synthesized and used successfully to disperse SWCNTS. Small-angle neutron scattering (SANS) measurements with contrast variations were performed in different D$_{2}$O/H$_{2}$O mixtures. The SANS patterns show that poly(FFDD) alone in water assembles into a complex structure. However, an open conformation which is loosely attached to the SWCNT surface is indicated by SANS. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H18.00012: Investigating the specificity of adsorption of onto gold by gold-binding peptides using molecular dynamics simulations Ana Vila Verde, Janna Maranas It is possible to engineer artificial peptide sequences showing high specificity of adsorption for surfaces like gold, platinum or other solid materials. However, the reasons behind that high specificity are not clear. We investigate the adsorption of a genetically engineered peptide with high gold specificity using all-atom molecular dynamics simulations. Accurate Lennard-Jones parameters describing the interactions of gold with both water and amino acids are not currently available, so thus we discuss assignment of appropriate values. Two sets of simulations are presented: one using peptides made of a gold-binding motif (MHGKTQATSGTIQS) and another using peptides made of a non gold-binding motif (AIRRDVNCIGASMH). Adsorption onto the (111) and the (100) crystalline faces of gold is investigated. We discuss our results in light of the features of the peptide (sequence, charge, structure, nature of the amino acids) that may be responsible for the specificity of the gold-binding motif for gold. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H18.00013: Direct Assembly of Periplasmic Binding Proteins on Gold Surfaces Cristian Staii, David Wood, Giacinto Scoles We present a new and very promising approach to protein based biosensor design, which uses a technique called nanografting to immobilize proteins at addressable locations on Au surfaces. In nanografting, an Atomic Force Microscope (AFM) tip is used to disrupt a preexisting monolayer of alkanethiol molecules on a gold surface, thereby facilitating exchange with alternative thiol-linked proteins from the surrounding solution. This technique opens the possibility of preparing highly ordered, nanometer size protein arrays that can be patterned at different addressable locations on the surface. We also use the AFM to monitor the ligand-induced conformational changes of periplasmic binding proteins nanografted on Au substrates. [Preview Abstract] |
Session H19: Focus Session: New Frontiers in Imaging IV
Sponsoring Units: DCPChair: Jeffrey Reimer, University of California, Berkeley
Room: Colorado Convention Center 104
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H19.00001: Molecular Dynamics Underlie the Nature of MRI Signals: The NMR Shutter-Speed Invited Speaker: Motions of the spin-bearing molecules can have profound effects on the very nature (the exponentiality) of the macroscopic NMR signal. Quantitative mechanistic protocols often involve varying the equilibrium molecular kinetics (usually by temperature~change) relative to the ``NMR time-scale'' (SS$^{-1})$, usually ill-defined as the absolute difference of resonance frequencies [$\vert \Delta \omega \vert $] in sites between which spins are exchanged. This holds true for the equilibrium water molecule exchange between tissue compartments and distinct populations. However, \textit{in vivo} studies must [by regulation] be isothermal, and the tissue $^{1}$H$_{2}$O MRI signals remain essentially isochronous [$\Delta \omega $ = 0]. In NMR, an equilibrium process is manifest in the context of its ``exchange condition.'' It only ``appears'' to be fast or slow by comparison of its actual rate~constant with its \textit{system} ``shutter{\-}speed'' (SS). [A~nonzero $\Delta \omega $ is the first, but not only, SS: its~dimension is reciprocal time.] The process kinetics can be measured only if its NMR condition is varied at least partway between the fast- and slow exchange limits. In an isothermal study with no catalyst, this can be accomplished only by varying the pertinent SS. An MRI contrast reagent (CR) increases the laboratory frame $^{1}$H$_{2}$O relaxation rate constant, R$_{i}$ [$\equiv $~(T$_{i})^{-1}$; i = 1,2]. For an isochronous exchange process, the SS is the intrinsic $\vert \Delta $R$_{i}\vert $ for the sites. In~quantitative dynamic-contrast-enhanced (DCE) studies, analytical pharmacokinetic modeling is accomplished on region-of-interest (ROI) or pixel by pixel $^{1}$H$_{2}$O signal time-courses following bolus CR injections. Accounting for the equilibrium transendothelial and transcytolemmal water interchange processes (a three-site exchange situation) is crucial for modeling accuracy: the relevant SS values vary during the CR bolus passage. This is so for DCE studies of cancer, multiple sclerosis, and myocardial blood flow variation. It is necessary for the successful discrimination of malignant and benign breast and prostate lesions. One can expect a SS for almost any NMR experiment. This includes diffusion weighted and rotating-frame longitudinal relaxation of\textit{ in vivo} $^{1}$H$_{2}$O signals. In~these latter cases, the pertinent SS can be manipulated solely by adjustment of pulse sequence parameters, leading to completely non-invasive protocols. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H19.00002: Human Regional Pulmonary Gas Exchange with Xenon Polarization Transfer (XTC) Iga Muradian, James Butler, Mirko Hrovat, George Topulos, Elizabeth Hersman, Iulian Ruset, Silviu Covrig, Eric Frederick, Stephen Ketel, F.W. Hersman, Samuel Patz Xenon Transfer Contrast (XTC) is an existing imaging method (Ruppert et al, Magn Reson Med, 51:676-687, 2004) that measures the fraction F of $^{129}$Xe magnetization that diffuses from alveolar gas spaces to septal parenchymal tissue in lungs in a specified exchange time. As previously implemented, XTC is a 2-breath method and has been demonstrated in anesthetized animals. To use XTC in humans and to avoid issues associated with obtaining identical gas volumes on subsequent breath-hold experiments as well as precise image registration in post-processing, a single breath XTC method was developed that acquires three consecutive gradient echo images in an 8s acquisition. We report here initial measurements of the mean and variance of F for 5 normal healthy subjects as well as 7 asymptomatic smokers. The experiments were performed at two lung volumes ($\sim $45 and 65{\%} of TLC). We found that both the mean and variance of F increased with smoking history. In comparison, standard pulmonary function tests such as DLCO FEV1 showed no correlation with smoking history. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H19.00003: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H19.00004: Human Pulmonary Diffusion Weighted Imaging at 0.2T with Hyperpolarized 129Xe Adrian Sindile, Iga Muradian, Mirko Hrovat, Christina Johnson, William Hersman, Sam Patz Unlike hyperpolarized 3He inhalations, which achieve a high degree of gas mixture homogeneity due to the higher diffusion constant, hyperpolarized 129Xe requires additional precautions to assure gas mixture homogeneity. A homogeneous concentration of Xe inside the human lungs is necessary to allow the use of ADC values as a reproducible measure of lung physiology and structure. To determine whether observed ADC differences are due to regional variations in Xe dilution, which would affect diffusitivity, we measured ADC as a function of a number of exhaling/rebreathing cycles (breaths). The results of our investigations into these differences will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H19.00005: Large Production of Hyperpolarized 129-Xe for MRI Applications Iulian Ruset, F.W. Hersman, Jan Distelbrink, Stephen Ketel, Silviu Covrig, Iga Muradian, Adrian Sindile Although 129-Xe was the first hyperpolarized gas to be used in MRI studies, the research community has focused on 3-He, mainly because of the larger quantities of hyperpolarized gas available. Xenon has advantages over helium, such as natural abundance, lower diffusion, and high solubility in blood. It presents a large frequency chemical shift when dissolved in blood, tissue, brain, or trapped in molecular cages. A new design of a high-flow low-pressure spin-exchange optical pumping Rb-Xe polarizer was recently demonstrated by our group. The concept of counterflowing the gas mixture against laser light and dividing the polarizing cell in three operational zones has resulted in an increase with over an order of magnitude in the output magnetization compared with previously reported polarizers. We were able to produce hyperpolarized xenon at 64{\%} polarization for 0.3 liters/hour flow rate and 22{\%} polarization at 6 liters/hour. We also demonstrated a new design of freezing and thawing hyperpolarized xenon with minimum losses. We will present the concept of the high-flow low-pressure counterflowing xenon polarizer, its performance, as well as new optical pumping laser technologies. We will discuss optimization plans for xenon polarizing systems based on experimental observed limitations and theoretical modeling. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H19.00006: Rapid Production of Hyperpolarized $^3$He Gas for MRI Benjamin C. Anger, Richard E. Jacob, Kevin R. Minard, Brian T. Saam Hyperpolarized (HP) $^3$He gas created via spin-exchange optical pumping (SEOP) is widely used as a signal source in MRI applications. One drawback to conventional SEOP is the time required for polarization. The process normally requires 10 - 20 hours to achieve 40-50\% polarization in enough gas ($\sim$1 L) for a single imaging experiment. Two recent advances in the physics of SEOP have led to dramatic enhancements in polarization efficiency: the use of spectrally narrowed diode-laser arrays and hybrid SEOP, which employs both potassium and rubidium as alkali-metal intermediaries. We have combined these techniques in constructing two polarizers, a prototype system at Utah and a more fully engineered system at PNNL. We report $>$60\% $^3$He polarization in 0.5 bar{\Large{$\cdot$}}L of gas in valved and refillable glass cells, achieved in under 4 h. With the apparatus described we are able to produce several liters of polarized $^3$He per day. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 10:12AM |
H19.00007: Double Quantum Filtered NMR Spectroscopy and Imaging Invited Speaker: As a result of the anisotropic motion of water molecules interacting with ordered biological tissues the proton-proton dipolar interaction and the deuteron quadrupolar interaction do not average to zero leaving some residual splittings. The technique of double quantum filtered (DQF) NMR capitalizes on this phenomenon, opening new possibilities to probe biological processes and to obtain a new kind of contrast in MRI. In the talk new applications of the DQF pulse sequences to the study of nerves, enabling the measurement of intercompartmental water exchange in sciatic and optic nerves, the study of the fiber architecture in cartilage under normal, compressed and diseased conditions and the imaging of tendons, enabling the monitoring their healing process following injury. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H19.00008: Hyperpolarized Krypton-83 as a MRI Contrast Agent Zackary Cleveland, Galina Pavlovskaya, Karl Stupic, John Repine, Thomas Meersmann Hyperpolarized krypton-83 (I = 9/2) yields NMR signal enhancements [1] of 1200 to 4500 times that of thermal equilibrium value depending on the composition of the optical pumping gas mixture. The quadrupolar relaxation of krypton-83 provides surface sensitive contrast in MRI [2] and yields information about surface hydrophobicity [3], surface-to-volume ratio, surface temperature, and surface hydration. These characteristics make hp krypton-83 MRI a promising technique for materials science applications and medical diagnosis. Experimental hp krypton-83 results in model systems with biomedically relevant coatings (e.g. lung surfactant and cigarette tar) are presented. Additionally, preliminary results from hp krypton-83 in excised rodent lungs are discussed. (1) ZI Cleveland, et al., Chem. Phys., 2006. 124(4) 044311. (2) GE Pavlovskaya, et al., Proc. Natl. Acad. Sci. U.S.A.,2005. 102: 18275-18279. (3) KF Stupic, et al., Solid State Nucl. Magn. Reson., 2006. 29: 79-84. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H19.00009: Introducing Hyperpolarized Xenon-131 Directly Detected by NMR Spectroscopy Karl Stupic, Zackary Cleveland, Galina Pavlovskaya, Thomas Meersmann Previously, high-field NMR and MRI applications of hyperpolarized (hp) noble gasses focused on the isotopes helium-3 (spin I = 1/2), xenon-129 (spin I = 1/2) [1], and more recently krypton-83 (spin I = 9/2) [2]. In this contribution, hp xenon-131 (spin I = 3/2) was generated by spin-exchange optical pumping and separated from the rubidium vapor for high field NMR detection at 14.1 T field strength. Xenon-131 is of particular interest because of its quadrupolar nature that can be utilized for the study of surfaces [3] and for the investigation of high magnetic field effects on the electronic structure of the noble gas atom [4]. In addition, this isotope is a useful probe for quadrupolar processes during gas transfer and during NMR/MRI detection. Experiments with xenon-131, including multiple quantum filtered NMR spectroscopy [3], provides insights into similar processes present in krypton-83 and its more complicated spin system [5]. [1] D. Raftery \textit{Ann. Rep. NMR Spec.}, \textbf{57}, 208 (2006). [2] G. Pavlovskaya, \textit{et al}., \textit{Pro. Natl. Acad. Sci. U.S.A.} \textbf{102}, 18275 (2005). [3] T. Meersmann \textit{et al.}, \textit{Phys. Rev. Lett. }\textbf{80}, 1398 (1998). [4] T. Meersmann and M. Haake, \textit{Phys. Rev. Lett.} \textbf{81}, 1211 (1998). [5] Z. Cleveland, \textit{et al.}, \textit{J. Chem. Phys.}\textbf{124}, 044312 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H19.00010: Spin relaxation in hyperpolarized krypton-83 and xenon-129 Thomas Meersmann, Zackary Cleveland, Karl Stupic, Galina Pavlovskaya The potential medical application of hyperpolarized (hp) krypton-83 (spin S = 9/2) [1] make a better insight into the NMR relaxation behavior of this isotope desirable, in particular since the relaxation limits the observed signal intensity but also provides a source for MRI contrast. The quadrupolar relaxation of krypton-83 is shown to be highly dependent on temperature, optical pumping gas mixture, the nature of surrounding surfaces and the applied magnetic field strength [2, 3]. The relaxation is mainly caused by quadrupolar interactions during brief surface adsorption periods of the krypton atoms onto the surrounding container walls. In contrast to xenon-129, interactions with paramagnetic impurities in the surface or with gas phase oxygen are not significant. 1) Pavlovskaya, et al. Proc. Natl. Acad. Sci. U.S.A.,2005. 102: 18275-18279; 2) Cleveland, Z.I., et al. J. Chem. Phys., 2006. 124(4) 044311; 3) Stupic, K.F et al. Solid State Nucl. Magn. Reson., 2006. 29: 79-84. [Preview Abstract] |
Session H20: Electronic and Optical Properties of Insulators
Sponsoring Units: DMPChair: Keivan Esfarjani, University of California, Santa Cruz
Room: Colorado Convention Center 105
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H20.00001: Quasiparticle Energies of Liquid Water Deyu Lu, Giulia Galli Understanding the electronic structure of liquid water is of great interest for a broad range of physical, chemical and biological processes occurring in solution. In particular, the ability to obtain an accurate description of water electronic states from first principle plays an important role in interpreting experimental observations. We present results of water electronic properties obtained using Many-Body Green's function approaches within the GW approximation. In particular we compare results obtained using the full dielectric matrix and various model dielectric functions. We analyzed the band structure and the absorption spectrum, with focus on the blue shift with respect to density functional theory results, which has been reported in a recent theoretical study [Garbuio {\it et al.}, PRL: 97, 137402, 2006]. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H20.00002: Temperature- and pressure-induced changes in optical properties of silica Andrea Trave, Babak Sadigh, Eric Schwegler, Jeff Bude Silica is among the most commonly studied materials in its crystalline and amorphous forms, yet it still presents several obscure aspects in its behavior under heating and densification. First-principles simulations of silica glass and quartz are conducted to explain the experimentally observed rapid increase in optical absorption and consequent defect formation when silica glass is heated. Prior to full gap closure, we observe the appearance of optically active localized states, which will be studied in terms of their origin and characterization. Additional analysis of the effect of localized structural defects and of the optical properties of the material under compression will also be undertaken and discussed. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H20.00003: Sum-frequency spectroscopic study of optical phonons in alpha-Quartz Wei-Tao Liu, Y. R. Shen Sum-frequency vibrational spectroscopy (SFVS) was used to probe optical phonons of alpha-quartz. As a second-order nonlinear optical process, SF generation is allowed only with phonon modes that are both infrared and Raman active. This, together with dependences on input/output polarization combination and crystal orientation, enabled us to examine more closely the symmetry properties of such phonons. In the experiment, a tunable infrared ($\sim $7-14$\mu $m) and a visible (532nm) input beam were overlapped in a crystalline alpha-Quartz, and the SF output in the reflected direction was detected. Resonant enhancement of the output when the infrared frequency scanned over phonon modes yielded the phonon spectra. Analysis of the spectra with the help of existing infrared and Raman spectra of alpha-quartz provides concrete information about the observed phonon modes. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H20.00004: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H20.00005: Optical properties of the 1D antiferromagnet KCuF$_3$ Joachim Deisenhofer, Paolo Ghigna, Franz Mayr, Alois Loidl, Dirk van der Marel We present measurements of the optical properties of KCuF3, a paradigm for orbital ordering [1] and one of the best realizations of a quasi-one dimensional spin chain [2]. We can identify the d-d excitations on the Cu sites and the charge- transfer gap of the system. The observed crystal-field level splitting and the gap value will be compared to recent results obtained by LDA+U calculations [3]. Moreover, we find anomalies in the optical properties already above the magnetic phase transition. These features appear concomitantly with a change in the orbital order parameter as reported by resonant x-ray scattering [4] and indicate a symmetry change already above the Neel temperature [5]. [1] K. I. Kugel and D. I. Khomskii, Sov. Phys. Usp. 25, 231 (1982). [2] B. Lake et al., Nature Materials 4, 329 (2005). [3] S.V. Streltsov et al., Phys.~Rev.~B {\bf 71}, 245114 (2005); I. Leonov, unpublished. [4] L. Paolasini et al., PRL 88, 106403 (2002). [5] N. Binggeli and M. Altarelli, PRB 70, 085117 (2004). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H20.00006: LDA+U Models of Polarons in LaBr3 and CsI John Jaffe, Sebastien Kerisit, Kevin Rosso We describe calculations of the formation and hopping energies of hole polarons (holes self-consistently localized in lattice distortions) in the wide-bandgap ionic materials LaBr3 and CsI. Both one-center (breathing mode) and two center (anion dimer, also known as Vk center) polarons were treated. The LDA+U method based on the VASP code was employed, since standard DFT methods often fail to represent localized electronic states in solids. We used a 72-atom supercell of the UCl3 structure for LaBr3, and a 54-atom CsCl-structure unit cell for CsI. We attempt to correlate differences in electronic transport between these two compounds with different energy nonproportionality behavior that they exhibit as Ce-activated scintillators in gamma-ray spectroscopy. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H20.00007: Electrostatic resonances and optical responses of cylindrical clusters Chun Wing Choy We develop a Green function formalism (GFF) for computing the electrostatic resonance in clusters of cylindrical particles. In the GFF, we take advantage of a surface integral equation to avoid matching the complicated boundary conditions on the surfaces of the particles. Numerical solutions of the eigenvalue equation yield a pole spectrum in the spectral representation. The pole spectrum can in turn be used to compute the optical response of these particles. For two cylindrical particles, the results are in excellent agreement with the exact results from the multiple image method and normal mode expansion method. The results of this work can be extended to investigate the enhanced nonlinear optical responses of metal-dielectric composites, as well as optical switching in plasmonic waveguides. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H20.00008: Crystal and nonlinear optical properties of triphenylguanidine: theory and experiments Cl\'{a}udia Cardoso, Pedro Silva, Fernando Nogueira, Jos\'{e} Ant\'{o}nio Paix\~{a}o Guanidine compounds have attracted much interest due to the donor and acceptor abilities of the nitrogen and their potencial nonlinear optical properties. Octupolar molecules are particularly interesting from the point of view of nonlinear optics. Their null dipole moment does not stand as a drawback for its crystallization and still allows the crystal to present large third order susceptibilities if some symmetry requirements are fulfilled. In the present work we focused on the triphenylguanidine (TPG) octupolar molecule and its crystalline forms. We present computational and experimental results both for the isolated molecule and TPG crystals. The structural properties as well as optical spectra and response properties will be presented from the point of view of the requirements to obtain octupolar molecules-based materials with enhanced nonlinear optical properties. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H20.00009: Enhanced Luminescence in the Layered Single Crystal Ce[Ag(CN)$_2$]$_3$ Christie Larochelle Single crystals of the form Ln[M(CN)$_2$]$_3$ (Ln=trivalent rare earth; M=Ag, Au, or both) have a layered structure consisting of alternating layers of M(CN)$_2^-$ ions and Ln$^{3+}$ ions. Recent work on this type of crystal has focused on energy transfer from the metal dicyanide donor to the rare earth acceptors, specifically Tb$^{3+}$, Eu$^{3+}$, and Sm$^{3+}$. Crystals of Ln[Ag(CN)$_2$]$_3$ are particularly interesting because they exhibit site-selective excitation. However, the luminescence intensity in these crystals is vanishingly weak at ambient temperatures. We present preliminary luminescence results from a new sample, Ce[Ag(CN)$_2$]$_3$. This crystal displays strong luminesence at all temperatures between 78 K and 295 K, in contrast with all other rare earth dicyanoargentates we have studied. We present steady-state excitation and emission results along with time-resolved measurements of both the cerium doped crystal and the single crystal La[Ag(CN)$_2$]$_3$ for comparison. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H20.00010: Polarizability of Optically Trapped Nanorods Douglas Bonessi, Keith Bonin, Thad Walker We optically trapped C60 polymer nanorods with diameters of 300-500 nm and lengths of 1-3 microns in water in a single beam trap. While in the trap, the nanorods were optically torqued by rotating the plane of polarization of the trapping light. The polarizability of the rod can be found by measuring the rod rotation rate as a function of the polarization rotation rate, and then finding a theoretical fit to this curve that uses a computation of the applied torque as a function of polarizability. We used a discrete dipole approximation (DDA) routine to calculate torques on these trapped C60 rods. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H20.00011: Dynamical structure factor of CaF$_{2}$: Striking coherent dynamical screening of ``atomic'' Ca-derived excitations O.D. Restrepo, M.C. Troparevsky, A.G. Eguiluz, B.C. Larson, J.Z. Tischler, P. Zschack, Y.Q. Cai, H. Ishii, P. Chow, E.L. Shirley, C.C. Kao We report \textit{ab initio} calculations of the dynamical structure factor of CaF$_{2, }$performed within time-dependent density functional theory, together with non-resonant inelastic x-ray scattering measurements. The ``effective'' dielectric function has also been determined. The excitations derived from the ``atomic'' Ca 3p--$>$ 3d process display a striking wave vector dependence. Such dipole-allowed excitation would be expected to lie at about 27 eV. \textit{However, for small q's the leading Ca 3p--$>$ 3d feature lies at about 35 eV.} We demonstrate that this feature corresponds to a collective mode, whose physics embodies a remarkable manifestation of crystal local-field effects induced by charge localization and their interplay with the dynamical screening at the ``natural'' 3p--$>$ 3d energy. For intermediate $q's$, the 27 eV excitation emerges and coexists with the collective mode ---thus highlighting the physics of the ``atomic'' 3p--$>$3d excitation in the condensed matter environment, which is controlled by dynamical coherent screening. For large $q's$ the Ca-derived spectrum consists of the ``single-particle'' Ca 3p--$>$3d excitation, together with the dipole-forbidden Ca 3s--$>$3d excitation. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H20.00012: Vibrational effects on SrTiO$_3$ by K edge X-ray absorption using first-principle methods Silvia Tinte, Eric L. Shirley Using the Bethe-Salpeter-equation methodology has recently become possible to calculate core and valence excited electronic states and spectra, which are usually computed in the ground-state atomic positions. However, vibrational effects can shift and broaden the spectrum through coupling atomic displacements to electron states and electronic excitations. In this work, we analize the phonon effects on the Ti 3d states in cubic SrTiO$_3$ by Ti K edge X-ray absorption fine structure using first-principle methods. LDA total energies and coupled electron-hole Bethe-Salpeter equation calculations are performed for different ionic configurations following relevant normal modes of SrTiO$_3$. As result, we obtain gradients of the excited-state energy and electron-phonon coupling coefficients. Our final goal is to include the Franck-Condon effect on the broadening of the calculated spectra. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H20.00013: Cu $K$-edge Resonant Inelastic X-Ray Scattering in Edge-Sharing Cuprates Thomas Devereaux, Francois Vernay, Brian Moritz, George Sawatzky We present calculations for resonant inelastic x-ray scattering (RIXS) in edge-shared copper oxide systems, such as CuGeO$_{3}$ and Li$_{2}$CuO$_{2}$, appropriate for hard x-ray scattering where the photoexcited electron lies above oxygen 2p and copper 3d orbital energies. We perform exact diagonalizations of the multi-band Hubbard model with and without the presence of a photoexcited 1s core hole, and determine the energies, orbital character and resonance profiles of excitations which can be probed via RIXS. We find excellent agreement with recent results on Li$_{2}$CuO$_{2}$ and CuGeO$_{3}$ in the 2-7 eV photon energy loss range. [Preview Abstract] |
Session H21: General Theory: Density Functional Theory
Sponsoring Units: DCOMPChair: Giulia Galli, University of California, Davis
Room: Colorado Convention Center 106
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H21.00001: Effects of semicore d-electrons in screened-exchange density functional methods Byounghak Lee, Lin-Wang Wang We report a theoretical study on the role of shallow $d$ states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that the inaccurate pseudo-wavefunctions can lead to 1) an overestimation of the screened-exchange interaction between the localized $d$ states and the delocalized higher energy $s$ and $p$ states and 2) an underestimation of the screened-exchange interaction between the $d$ states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of $d$ states by including the $s$ and $p$ states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and the $d$ state binding energy in good agreement with experiments. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality band gaps but also much better $d$ state binding energy. As an example, we present sX-LDA results of $s-d$ coupling in zinc-blende semiconductors and compare them with LDA+U results. We also present an efficient method to correct the pseudo-wavefunction exchange-integral error by using projection of wavefunctions onto atomic orbitals. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H21.00002: Asymptotic Near Nucleus Structure of the Electron-Interaction Potential in Local Effective Potential Theories Viraht Sahni, Zhixin Qian In previous work, it has been shown that for spherically symmetric or sphericalized systems, the asymptotic near nucleus structure of the electron-interaction potential is $v_{ee}(r) = v_{ee}(0) + \beta r + \gamma r^{2}$. In this paper we prove via time-independent Quantal Density Functional Theory[1](Q-DFT): (i) correlations due to the Pauli exclusion principle and Coulomb repulsion do not contribute to the linear structure;(ii) these Pauli and Coulomb correlations contribute quadratically; (iii) the linear structure is \emph{solely} due to Correlation-Kinetic effects, the coefficient $\beta$ being determined analytically. By application of adiabatic coupling constant perturbation theory via QDFT we further prove: (iv) the Kohn-Sham (KS-DFT) `exchange' potential $v_{x}(r)$ approaches the nucleus linearly, this structure being due \emph{solely} to lowest- order Correlation-Kinetic effects: (v) the KS-DFT `correlation' potential $v_{c}(r)$ also approaches the nucleus linearly, being \emph{solely} due to higher-order Correlation-Kinetic contributions. The above conclusions are equally valid for system of arbitrary symmetry, provided spherical averages of the properties are employed. \\ 1 \emph{Quantal Density Functional Theory}, V. Sahni (Springer-Verlag 2004) [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H21.00003: Wave Function Functional via the Normalization Constraint Xiao-Yin Pan, Viraht Sahni, Lou Massa In this paper we extend our prior work [1] on the construction of approximate wave function functionals $\Psi = \Psi[\chi]$ for the ground state of the $He$ atom by attempting to expand the space of variations of the functional. As in our prior work, we assume $\Psi[\chi] = \Phi\{\phi_{i} \}[1 - f(\chi)]$, where $\Phi\{\phi_{i}\}$ is a normalized Slater determinant of the orbitals $\phi_{i}({\bf x})$, ${\bf x} = {\bf r} \sigma$, and $f(\chi)$ a correlation factor, and employ normalization as the constraint to determine the function $\chi$. However, we expand the space of variations from $\chi = \chi(s)$ to $\chi = \chi(st)$, where $s = r_{1} + r_{2}, t = r_{1} - r_{2}$. Although the constrained search over the entire requisite (\emph{st}) space is not achieved, two solutions of the integral equation for the function $\chi$ have been obtained. These two wave functions have the properties that they are normalized independent of the prefactor, with the density being that of the prefactor. The significance of these attributes of the wave function functionals will be discussed. \\ 1 . X.-Y. Pan et al, Phys. Rev. Lett. \textbf{93}, 130401 (2004) [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H21.00004: A new approach to in full potential multiple scattering theory electronic structure calculations G.M. Stocks, Yang Wang, Aurelian Rusanu, Don M. Nicholson, Markus Eisenbach, Yevgeniy Puzyrev Despite the wide use of first principles electronic multiple scattering theory methods there realization as full potential methods has proved problematical with the consequence that atomic relaxation in not typically performed due to the lack of accurate forces. Here we describe some new techniques that facilitate an easy implementation of these full potential methods. In the determination of the scattering path matrix(t-matrix) we eschew the expansion of the shape function and use surface integrals to determine scattering t-matrix. We also use a new method is the treatment of Poisson problem where the charge density is divided in a spherical non-overlapping charge, which is treated using standard methods, and a pseudo charge which is treated by FFT-methods. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H21.00005: Optical properties of real surfaces: local field effects at oxidized Si(100)(2x2) computed with an efficient numerical scheme Lucia Caramella, Giovanni Onida, Fabio Finocchi, Lucia Reining, Francesco Sottile We show the application of an efficient numerical scheme to obtain the independent-particle dynamic polarizability matrix $\chi^{(0)}({\bf r}, {\bf r}', \omega)$, a key quantity in modern \emph{ab initio} excited state calculations. The method has been applied to the study of the optical response of a realistic oxidized silicon surface, including the effects of the local fields. The latter are shown to substantially increase the surface optical anisotropy in the energy range below the bulk bandgap. Our implementation in a large-scale \emph{ab initio} computational code allows us to make a quantitative study of the CPU time scaling with respect to the system size, and demonstrates the real potential of the method for the study of excited states in large systems. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H21.00006: Quantum mechanical models of energy dependence on fractional charge Steven Valone, Susan Atlas When subsystems interact sufficiently, the subsystems can exchange electrons and the effective number of electrons on each subsystem can take on fractional values. The energy of the system can be expressed as a function of that fractional charge. Pioneering work of Perdew, Parr, Levy, and Balduz [1] showed that when the subsystems interact weakly, the energy depends linearly with the fractional charge. We explain recently derived energy dependencies, based on a 2-state model, for the case when the subsystems interact strongly [2]. Those results are extended to a more general 3-state case. Insights into the properties of the chemical potentials of the subsystems are discussed. \newline \newline [1] J. P. Perdew, R. G. Parr, M. Levy, and J. L. Balduz, Jr., Phys. Rev. Lett. {\bf 49}, 1691 (1982). \newline [2] S. M. Valone and S. R. Atlas, Phys. Rev. Lett. accepted. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H21.00007: Wave Function Functionals via the Constraint of the Expectations of Hermitian Single-Particle Operators Marlina Slamet, Xiao-Yin Pan, Viraht Sahni Recently, the idea [1] of expanding the space of variations in calculations of the energy by considering the approximate wave function $\Psi$ to be a functional of functions $\chi$, $\Psi = \Psi[\chi]$, rather than a function, has been proposed. A constrained search over all functions $\chi$ such that $\Psi[\chi]$ satisfies a physical constraint or leads to a known value of an observable, is performed. A rigorous upper bound to the energy is then obtained via the variational principle. In this paper we extend this work on the ground state of the He atom by imposing the constraint that $\Psi[\chi]$ reproduce the exact expectations of the Hermitian single-particle operators $W = \sum_{i} r_{i}^{n}, n = -2, -1, 1, 2$, and $W =\sum_{i}\delta ({\bf r}_{i})$. By employing the form $\Psi[\chi] = \Phi [1 - f (\chi)]$, where $\Phi$ is a prefactor and $f(\chi)$ a correlation factor, two solutions to the resulting integral equation for the functions $\chi$ are determined for each operator $W$. In each case, the two wave function functionals lead to upper bounds to the energy that differ minimally from those of [1] in which only the constraint of normalization is imposed, while simultaneously reproducing the exact expectation of the operator $W$ .\\ 1. X.-Y. Pan, et al, Phys. Rev. Lett. \textbf{93}, 130401 (2004) [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H21.00008: Molecular grand-canonical ensemble density functional theory O. Anatole von Lilienfeld, Mark E. Tuckerman The fundamental challenge of compound design, \textit{i.e.}~the reverse engineering of stable chemical compounds with predefined specific properties, originates in the high-dimensional and combinatorial nature of the \textit{chemical} space [1] which is spanned by the grand-canonical variables (superimposed particle densities of electrons and nuclei). A rigorous description of chemical space, using a grand-canonical multi-component density functional theory framework, will be presented [2]. Specifically, a total energy density functional for molecular systems in contact with an electron and a proton bath is introduced using Lagrange multipliers which correspond to the energetic response to changes of the elementary particle densities. Results will be shown for a molecular Fukui function, for finite temperature estimates of the redox potential of ammonia, and for alchemical variation of the intermolecular energy of formic acid interacting non-covalently with 10 proton systems [2,3]. Implications for rational compound design [4] and multi-scale modeling shall be discussed.\newline [1] P Kirkpatrick, C Ellis \textit{Nature} {\bf 432} 823 (2004) \newline [2] OAvL, M E Tuckerman \textit{J Chem Phys} {\bf 125} 154104 (2006) \newline [3] OAvL, M E Tuckerman \textit{submitted} \newline [4] OAvL, R Lins, U Rothlisberger \textit{Phys Rev Lett} {\bf 95} 153002 (2005) [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H21.00009: The Accuracy of calculated electronic ground state energies Bernard Delley, Russell D. Johnson Ground-state enthalpies, calculated by a wide variety of electronic structure methods, are compared with experimentally well established values across a sizable database of 577 molecules and 15 atoms. With the diversity of species and bonding types available in this compilation it is possible to detect deficiencies that may escape with smaller test sets. The present analysis relying on ``Database optimized Atomic Enthalpies of Formation'' (DAtEF) yields error statistics which relate to reaction enthalpies among the species much more directly than extrapolations based on atomization enthalpies. The evaluation is applied to methods ranging from high level first principles wavefunction calculations to density functionals and to semiempirical approaches. It is found that computationally efficient and broadly applicable density functional methods with relatively small but adequate numerical basis sets can provide ground state enthalpies within $\approx$ 20 kJ/mol RMS ( $\approx$ 4.8 kcal/mol). This must be considered an excellent result, as presently only the heaviest available methods may provide about a factor of 2 more accuracy, if the conclusions inferred from a subset of the database hold up. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H21.00010: Time-dependent optimized effective potential for quantum wells Harshani Wijewardane, Carsten A. Ullrich Most present applications in time-dependent density-functional theory employ adiabatic approximations for the exchange- correlation (XC) potential, ignoring all functional dependence on densities at previous times. In this talk, we describe the electron dynamics in quantum wells beyond the adiabatic approximation, using the time-dependent optimized effective potential (TDOEP) method. In TDOEP, the XC potential is a functional of the time-dependent orbitals, and follows from an integral equation over space and time. We solve the full TDOEP integral equation for quantum well intersubband dynamics in exact exchange as well as self-interaction corrected ALDA. Various properties of the resulting time-dependent XC potential, such as its asymptotics, memory dependence, and discontinuity upon population of a new subband level are discussed. This work is supported by NSF DMR-0553485 and Research Corporation. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H21.00011: Time-dependent density functional approach for ultrafast optical phenomena Volodymyr Turkowski, Carsten A. Ullrich We present a formulation of time-dependent density functional theory (TDDFT) in a density-matrix framework, which can be applied to study ultrafast optical phenomena in semiconductor bulk systems and heterostructures. In particular, we derive and analyze the TDDFT version of the semiconductor Bloch equations and study the resulting absorption spectra of simple model insulators for different types of exchange-correlation potentials within and beyond the adiabatic LDA. We discuss the demands that the time-dependent exchange-correlation potential needs to satisfy in order to obtain physically correct absorption spectra, including excitonic features. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H21.00012: Dynamical Effects in the Interaction of Energetic Ions and Matter Ryan M. Hatcher, Matthew J. Beck, Sokrates T. Pantelides A theoretical description of the microscopic processes that underlie the interaction of energetic ions traversing a solid faces unique challenges as it is intrinsically a dynamic phenomenon. Here we use time-dependent density-functional theory to explore the exchange of energy between channeled ions, which interact weakly with the solid's nuclei, and electrons in a silicon crystal. We find that the \textbf{dynamic} response of the electron gas is characterized by a drag effect where there is an average accumulation of dynamical electron charge density behind the ion. The drag effect is superposed on additional dynamical patterns. We report the ``stopping powers'' for a number of ion species that are in excellent agreement with experimentally observed oscillations in the stopping powers as a function of the atomic number of the ions. We analyze the result by comparing with results obtained for an ion traversing a thin layer of homogeneous electron gas of various densities. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H21.00013: Accelerating self consistent field convergence by rubber sheeting of initial electronic wave functions. G. Eric Matthews, N.A.W. Holzwarth, George Martin, Briana Keeling, Douglas Agopsowicz We develop an algorithm for generating better initial electronic wave function estimates for density functional theory calculations following atomic movement. First principles molecular dynamics and atomic relaxation calculations involve successive movements of atoms followed by self consistent field (SCF) solutions for electronic wave functions. The SCF solutions converge most rapidly when starting from reasonably good estimates. Often estimates are generated directly from the wave functions of the previous atomic positions without adjustments for effects of position changes. Such estimates result in fast convergence to the correct wave function for small atomic movements, but for larger movements, convergence may be much slower. We present a method for improving the estimates of the new wave functions by using information from the movement of the atoms. Our algorithm is based on the ``rubber-sheeting'' method used in overlaying satellite imagery on geographic maps. A warping function is calculated that stretches and shrinks different regions of the wave function so that regions near nuclei are dragged along with the atoms. These estimates yield faster convergence for cases studied thus far. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H21.00014: Current-density functional theory of the friction of ions in an interacting electron gas. V. U. Nazarov, J. M. Pitarke, Y. Takada, G. Vignale, Y.-C. Chang Recently [1], the dynamical contribution to the friction coefficient of an electron gas for ions has been obtained quite generally in terms of the exchange and correlation (xc) kernel of the time-dependent density-functional theory (TDDFT). To implement this approach practically, an efficient approximation, like the local-density approximation (LDA), is needed for the dynamical xc kernel. It is, however, known that the {\em scalar} xc kernel of the TDDFT is a nonlocal quantity for which the LDA is not only inaccurate, but also contradictory [2]. Here we recast the theory into the terms of the {\em tensorial} xc kernel of the current-density functional theory [3] in which form the LDA can be applied. Our numerical results are in a considerably better agreement with the experimental stopping power of Al than it has been the case within the LDA to the TDDFT. [1] V.U.Nazarov {\it et al.}, Phys. Rev. B71, 121106 (2005). [2] G.Vignale, Phys. Lett. A209, 206 (1995). [3] G.Vignale and W.Kohn, Phys. Rev. Lett. 77, 2037 (1996). [Preview Abstract] |
Session H22: GSNP Student Award Session and Exactly Solvable Models
Sponsoring Units: GSNPChair: Sidney Redner, Boston University
Room: Colorado Convention Center 108
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H22.00001: Probing large length scale behavior of spin glasses with patchwork dynamics Creighton Thomas See MAR07-2006-004072 in Session X29. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H22.00002: The Product Space and its Consequences for Economic Growth Cesar Hidalgo See MAR07-2006-000468 in Session A22. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H22.00003: Strain Hardening and Plastic Deformation in Polymer Glasses Robert S. Hoy See MAR07-2006-005404 in Session X25. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H22.00004: A Statistical Ensemble for Soft Granular Matter Silke Henkes See MAR07-2006-002889 in Session J22. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H22.00005: Liquid metal flow in a spherical shell: recent results Santiago Andres Triana See MAR07-2006-005434 in Session K1. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H22.00006: Universality of Synchrony Kevin Wood See MAR07-2006-006788 in Session S22. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H22.00007: Diameter of Random Clusters in Potts Models D. W. Blair, Jon Machta We report measurements of cluster diameter -- the maximum over all pairs of connected vertices of the minimum path length between the vertices -- in numerical simulations of random clusters in q-state Potts models in two and three dimensions. Although the diameter is not a thermodynamic quantity, it is expected to display critical behavior for Potts models models as the size of the largest cluster diverges at the critical point. We have developed an efficient algorithm for measuring the diameter, and have obtained results using the Swendsen-Wang algorithm both for equilibrating the model and for identifying clusters. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H22.00008: DNA unzipping and the unbinding of directed polymers in a random media Yariv Kafri, Anatoli Polkovnikov We consider the unbinding of a directed polymer in a random media from a wall in $d=1+1$ dimensions and a simple one-dimensional model for DNA unzipping. Using the replica trick we show that the restricted partition functions of these problems are {\em identical} up to an overall normalization factor. Our finding gives an example of a generalization of the stochastic matrix form decomposition to disordered systems; a method which effectively allows to reduce the dimensionality of the problem. The equivalence between the two problems, for example, allows us to derive the probability distribution for finding the directed polymer a distance $z$ from the wall. We discuss implications of these results for the related Kardar-Parisi-Zhang equation and the asymmetric exclusion process. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H22.00009: Molecular-field theory method for evaluating critical points of Ising model Kirill K. Zhuravlev The molecular-field theory is one of the most common approximations used to calculate properties of materials with Ising model. A generalization, improving the previous results of molecular-field theory, is proposed. This method distinguishes between two lattices with different geometries but equal number of nearest neighbors, such as square and diamond, and triangular and simple cubic lattices, a result, which is missing from most other mean-field approaches. Spontaneous magnetization as a function of temperature shows remarkable deviations from mean-field type of behavior and is in good agreement with exact results. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H22.00010: Extended universality and information theory Cintia Lapilli, Peter Pfeifer, Carlos Wexler Recently, we have discovered the {\em extended universality}, where entire families of systems behave identically both near and away from a critical point [1] if the temperature and a parameter describing the interaction between neighboring units of the system exceed a certain value. In the regime where the extended universality is present $T > T_{\rm eu}$, the thermodynamics of the system is {\em degenerate} in the sense that all thermodynamic observables of each system are independent of the interaction parameter, and a system with discrete degrees of freedom (i.e. the $p$-state clock model) behaves (from the point of view of all thermodynamic observables) exactly as if these degrees of freedom were continuous (i.e. the planar rotor or XY model). To the best of our knowledge there is only one comparable case where a similar sharp switchover between a discrete and a continuum description of a system is observed: this is in the areas of telecommunications, digital signal processing, and information theory: the Nyquist-Shannon sampling theorem [2]. In this talk we will give an interpretation of the extended universality in terms of the Nyquist–Shannon sampling theorem. \noindent [1] Universality away from critical points in two-dimensional phase transitions, C.M. Lapilli, P. Pfeifer, and C. Wexler, Phys. Rev. Lett. {\bf 96}, 140603 (2006). \noindent [2] H. Nyquist, Trans. AIEE {\bf 47}, 617 (1928); [3] C.E. Shannon, Proc. Institute of Radio Engineers {\bf 37}, 10 (1949). [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H22.00011: Interacting anyons in one dimension: The Fibonacci chain Andreas Ludwig, Adrian Feiguin, Simon Trebst, Matthias Troyer, Alexei Kitaev, Zhenghan Wang, Michael Freedman We discuss generalizations of quantum spin chains using anyonic degrees of freedom. The simplest model for interacting anyons in one dimension is closely related to the Fibonacci topological quantum field theory. The Hamiltonian favors neighboring anyons to fuse into the trivial channel, similar to the quantum Heisenberg spin chain favoring neighboring spins to form spin singlets. Numerical simulations show that the model is critical with a dynamical critical exponent z=1. It is described by a conformal field theory with central charge c=7/10. An exact solution of this model is given by mapping to a Temperley-Lieb algebra. We discuss the excitation spectra for finite systems, and generalizations to dimerized chains and ladders. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H22.00012: Zeros of the dispersion relation of the elementary excitation and the correlation length of strongly correlated quantum systems Yuichi Nakamura We argue that the imaginary part of a zero of the dispersion relation of the elementary excitation of quantum systems is equal to the inverse correlation length. We confirm the relation for the Hubbard model[1] in the half-filled case; it has been confirmed only for the S=1/2 antiferromagnetic XXZ chain[2]. In order to search zeros of the dispersion relation in the complex momentum space efficiently, we introduce a non-Hermitian generalization of quantum systems by adding an imaginary vector potential ig to the momentum operator[3]. We also show for the half-filled Hubbard model the reason why the non-Hermitian critical point[4] is equal to the inverse correlation length[5] by noting the dispersion relation of the charge excitation. \newline [1] Y. Nakamura and N. Hatano, in preparation. \newline [2] K. Okunishi, Y. Akutsu, N. Akutsu and T. Yamamoto, Phys. Rev. B 64 (2001) 104432. \newline [3] Y. Nakamura and N. Hatano, Physica B 378-380 (2006) 292; J. Phys. Soc. Jpn. 75 (2006) 114001. \newline [4] T. Fukui and N. Kawakami, Phys. Rev. B 58 (1998) 16051. \newline [5] C. A. Stafford and A. J. Millis, Phys. Rev. B 48 (1993) 1409. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H22.00013: Gauge covariant Keldysh formulation of Wigner representation through deformational quantization Naoyuki Sugimoto, Shigeki Onoda, Naoto Nagaosa Non-linear responses such as nonlinear optical effects are of great current interests from the fundamental physics and application viewpoints. Therefore a microscopic quantum theory for these non-linear processes in non-equilibrium state is called for. The extension of the Kubo formula or the Keldysh formula to the nonlinear response preserving gauge-covariance is not straightforward. We developed a gauge-covariant Keldysh formulation with a general electromagnetic field[1]. Such a formulation is realized by replacing the Moyal product in the Wigner space by the star product which is given by deformational quantization. We derived the explicit form of this star product. Our formula has the following merits. (1) The star product facilitates an order-by-order calculation of an observable in terms of the electromagnetic field. (2) The gauge-invariance of the formula is clearly seen, and we do not have to worry about the Ward identity, because the formula is given by gauge-covariant Wigner space. We will mention about an application of this method to Zener tunneling problem in the presence of dissipation. [1] N. Sugimoto, S. Onoda and N. Nagaosa, cond-mat/0611142. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H22.00014: Combining the density matrix renormalization group and truncated spectrum approach for two-dimensional strongly correlated systems Yury Adamov, Robert Konik We propose a combined numerical and analytical approach to two dimensional strongly correlated systems which are representable as arrays of one-dimensional exactly solvable systems. In our approach the exact solution provides us a compact representation of one-dimensional subsystems that makes it numerically feasible to treat the interactions between subsystems using a DMRG algorithm. This compact representation comes about through a simple truncation of the spectrum. To illustrate our approach, we consider an array of interacting quantum Ising chains. The results are then compared with an analytical RPA treatment of the same system. [Preview Abstract] |
Session H23: Low-D Metals
Sponsoring Units: DCMPChair: Alison Baski, Virginia Commonwealth University
Room: Colorado Convention Center 110
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H23.00001: The growth of Manganese wires on Si(100): observation of the sub-monolayer coverage regime with STM Hui Liu, Petra Reinke The study of thin film magnetic materials and the doping of semiconductors with magnetically active dopant atoms has received increased attention due their potential applications in magnetic memory devices and spintronics. We observe the deposition of Mn on the Si(100) 2x1 reconstructed surface in the sub-monolayer regime with STM. Short Mn wires with a length of 5 to about 20 atoms are formed an oriented perpendicular to the Si-dimer rows. At higher coverage some Mn wires are anchored with one end of the wire at the edge and extend onto the lower lying surface. The region in between the wires is particularly interesting: if the Mn wire distance is reduced the dimers change their orientation and are tilted, or begin to form zig-zag lines. The wire length and dimer deformation is likely governed by local strain. We will discuss the wire statistics (lengths, orientation, and position), control of their growth and present first data on the electronic structure of the wires. The growth of Si and Ge overlayers and incorporation of Mn wires in Ge-quantum dots is currently explored. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H23.00002: Growth and Characterization of a Combinatorial Array of Magnesium-Aluminum Alloys Daad Haddad, Charles Olk We have used combinatorial gradient controlled sputter deposition to produce a library of thin films with a wide range of compositions within the Mg-Al alloy system. We have successfully isolated the {\ss} (Mg$_{17}$Al$_{12})$ phase. The importance of understanding the physical properties of the {\ss} phase becomes apparent when one realizes the contrary effects associated with its presence in these alloys. The presence of the {\ss} phase is desirable for increasing corrosion resistance while undesirable as it generally produces reduced mechanical strength of the alloy. We present details of the growth procedure, as well as structural and compositional characterization. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H23.00003: Atomic structures of 13-atom clusters by density functional theory Hsin-Yi Chen, Ching-Ming Wei The 13-atom cluster structures of the alkaline metals, alkaline earth metals, boron group, 3d, 4d, and 5d transition metals in the periodic table, and Pb are investigated by density functional theory with three kinds of exchange correlation approximation: i) LDA (Local Density Approximation), ii) GGA (Generalized Gradient Approximation) [1], and iii) PBE (Perdew-Burke-Ernzerhof) [2]. The results mainly focus on five 3-D structures: icosahedral, cuboctahedral, hexagonal-closed packed, body-center cubic, decahedral, and the other two layer structures: buckled biplanar (bbp) and garrison-cap biplanar (gbp) structures. Limited by accuracy of exchange correlation approximation, two interesting results are found. The ground states of Ca$_{13}$, Sr$_{13}$, Ba$_{13}$, Sc$_{13}$, Y$_{13}$, La$_{13}$, Ti$_{13}$, Zr$_{13}$, and Hf$_{13}$ are icosahedral structures. The clusters of Ir$_{13}$, Pt$_{13}$, Cu$_{13}$, Ag$_{13}$, and Au$_{13}$ are more favorable for layer structures (i.e. bbp and gbp) than the other five 3-D structures. \newline [1] J. P. Perdew et al., Phys. Rev. B 46, 6671 (1992). \newline [2] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H23.00004: Length characterization of DNA-wrapped carbon nanotubes using Raman Spectroscopy Shin G. Chou, Hyungbin Son, Aurea Zare, Ado Jorio, Riichiro Saito, Mildred Dresselhaus, Gene Dresselhaus The systematic resonance Raman study has been carried out on DNA-wrapped SWNTs of different lengths using several different values of laser excitation energy. The correlation observed between the intensity ratio of the D -band and G-band features (I$_{D}$/I$_{G})$ and the average nanotube energy lengths indicates that nanotube length can be used as the dominant structural parameter in Raman characterization, and that the I$_{D}$/I$_{G}$ ratio can be used as a qualitative gauge for estimating the average nanotube length. By systematically varying the laser excitation energy, we have also found that the I$_{D}$/I$_{G}$ ratio strongly depends on whether the tubes are metallic or semiconducting, as well as on the laser excitation energy. Further directions for this research will be presented. The authors gratefully acknowledge support for this research from the National Science Foundation grant DMR-04-05538. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H23.00005: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H23.00006: Gold Cluster Formation on C$_{60}$ Surfaces: Au-Cluster Beads and Self-Organized Structures Petra Reinke, Hui Liu Petra Reinke, Hui Liu, Department of Materials Science and Engineering, University of Virginia The investigation of C$_{60}$-Au interaction is central to the advancement of solar cell and nanotechnology applications of C$_{60}$. C$_{60}$ grows in a quasi-layer-by-layer mode on a pristine graphite surface and form a special surface structure (coexistence of round and fractal islands). The deposition of Au leads to the formation of a complex array of different surface structures, while the basic island structure of the C$_{60}$ is preserved. The Au-clusters nucleate preferentially at the graphite-first fullerene layer islands edge forming beadlike structures. A roughness analysis of the fullerene surface indicates the presence of Au atoms embedded in the fullerene surface, situated in the troughs in between the large molecules. The analysis of the spatial and size distributions of Au clusters provides the basis for the development of a qualitative model which describes the relevant surface processes in the Au-fullerene system. The simultaneous deposition of Au and C$_{60 }$leads to the formation of organized structures, in which Au clusters are embedded in a ring of fullerene molecules with a constant width. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H23.00007: Self-Assembly of Thiol Adsorbates on the Au(111)surface Frank Hagelberg, Quinton Williams, Jian-Ge Zhou A long-standing controversy related to the dimer pattern formed by methanethiol (CH3SH) and methylthiolate (CH3S) on the Au(111) surface has been resolved using density functional theory within periodic boundary conditions. It is found that the S atoms of methanethiol adsorbates on the Au(111) surface form Van der Waals dimers. For methylthiolate, it is shown that no dimerization occurs at high coverage. At intermediate coverage, however, a Van der Waals dimer pattern emerges. The presence of defects in the Au(111) surface does not change this conclusion. Molecular dynamics simulation at high coverage demonstrates that the observed dialkyl disulfide species emerge during the desorption process, and thus are not attached to the surface. A meta-stable monomer pattern has been shown to be only marginally higher in adsorption energy than the dimer configuration which explains the observed fragility of the dimers. For the understanding of these results, it is of crucial importance that methanethiol molecules, contrary to a widely held assumption, remain stable when deposited on clean Au(111) surfaces /1, 2/. In the presence of defects, however, methanethiol adsorbates dissociate and form methylthiolate. /1/ I. Rzeznicka, J. Lee, P. Maksymovych, J. Yates, Jr., J. Phys. Chem. B109, 15992 (2005). /2/ J. Zhou, F. Hagelberg, Phys. Rev. Lett. 97, 45505 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H23.00008: Chemical pressure and hidden one-dimensional behavior Andrea Sacchetti, Leonardo Degiorgi, Thierry Giamarchi, Nancy Ru, Ian Fisher We report on the first optical measurements of the rare-earth tri-telluride charge-density-wave systems. Our data, collected over an extremely broad spectral range, allow us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the charge-density-wave gap excitation, respectively. The data analysis displays a diminishing impact of the charge-density- wave condensate on the electronic properties with decreasing lattice constant across the rare-earth series. We propose a possible mechanism describing this behavior and we suggest the presence of a one-dimensional character in these two-dimensional compounds. We also envisage that interactions and umklapp processes might play a relevant role in the formation of the charge-density-wave state in these compounds. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H23.00009: Fermi-liquid effects in the magnetization oscillations in quasi-two-dimensional conductors Alexander Zimbovsky, Natalya Zimbovskaya In this work we present the results of theoretical analysis of the Haas-van Alphen oscillations in quasi-two-dimensional metals. We have been studying the effect of the Fermi-liquid correlation of charge carriers on the above oscillations. It was shown that at reasonably low temperatures and weak electron scattering the Fermi-liquid interactions may cause noticeable changes in both amplitude and shape of the oscillations even at realistically small values of the Fermi-liquid parameters. Also, we show that the Fermi-liquid interactions in the system of the charge carriers may cause magnetic instability of a quasi-two-dimensional metal near the peaks of quantum oscillations in the electron density of states at the Fermi surface, indicating the possibility for the diamagnetic phase transition within the relevant ranges of the applied magnetic fields. The obtained results are applicable to strongly anisotropic organic metals, and to other quasi-two-dimensional compounds. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H23.00010: Interface mobility from interface random walk Zachary Trautt, Moneesh Upmanyu, Alain Karma Computational studies aimed at extracting interface mobilities require driving forces orders of magnitude higher than those occurring experimentally. We present a computational methodology that extracts the absolute interface mobility in the zero driving force limit by monitoring the one-dimensional random walk of the mean interface position along the interface normal. The method exploits a fluctuation-dissipation relation similar to the Stokes-Einstein relation, which relates the diffusion coefficient of this Brownian-like random walk to the interface mobility. Atomic-scale simulations of grain boundaries in model crystalline systems validate the theoretical predictions, and also highlight the profound effect of impurities. The generality of this technique combined with its inherent spatial-temporal efficiency should allow computational studies to effectively complement experiments in understanding interface kinetics in diverse material systems. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H23.00011: Ab-initio investigations on electronic and lattice dynamical properties on intercalation of Cu (copper) into hexagonal boron nitride(hBN). Bahadir Altintas, Cihan Parlak, Resul Eryigit, Cetin Bozkurt Layered structure of hexagonal boron nitride(hBN) and its intercalation with transition metals have been the subject of many recent studies. In this work, we investigate the electronic structure and the lattice dynamical properties of copper intercalated hBN by using Density Functional Theory(DFT) with a plane-wave basis set for the electronic wave functions and periodic boundary conditions.The interaction between valance electrons, the nuclei and the core electrons is described by norm-conserving pseudopotentials. We report the result of calculations on lattice geometry, electronic and lattice dynamical properties of the compound. Possible effects of Cu-incorporation on the structure of hBN were determined from a consideration of minimizing the quantum mechanical total energy and forces. Intercalated Cu atom is found to prefer the position between B and N atoms of two layers. The signature of intercalated Cu states were determined from the calculated electronic local density of states. The phonon frequencies were computed at the center of the Brillouin zone and four Cu-related bands were found at 187, 560, 960, 1206 cm$^{-1}$ which can be measured by IR spectroscopy. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H23.00012: Current-Driven Phase Oscillation and Domain-Wall Propagation in W$_{x}$V$_{1-x}$O$_{2}$ Nanobeams Abram Falk, Qian Gu, Junqiao Wu, Lian Ouyang, Hongkun Park We report the observation of a current-driven metal (M)-insulator (I) phase oscillation in two-terminal devices incorporating individual W$_{x}$V$_{1-x}$O$_{2}$ nanobeams connected in parallel with a shunt capacitor. The phase oscillation frequency reaches above 5 MHz for $\sim $1-$\mu $m-long devices. The M-I phase oscillation coincides with the charging/discharging of the capacitor and occurs through the axial drift of a single M-I domain wall driven by Joule heating and the Peltier effect. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H23.00013: Coincidence Measurements of the Auger Cascade Process in MnO, Ag and Pd. R. Sundaramoorthy, A.H. Weiss, S.L. Hulbert, R.A. Bartynski The Auger spectra associated with Auger cascade processes provides a probe of many-electron phenomena, the effects of screening and correlation in the intermediate and final many hole states. Here we present the first direct measurements of the energy spectra of electrons emitted in the later steps of Auger cascade processes in MnO, Pd and Ag performed using Auger-Auger coincidence spectroscopy (AACS). The Auger spectra resulting from the decay of core holes generated by a previous Auger cascade step (as measured by AACS) are shown to be broadened and shifted as compared to the Auger spectra resulting from the direct photo excitation of the corresponding core holes as measured by Auger photoelectron coincidence (APECS). The large differences between the Auger spectra resulting from the different origins of the core hole excitation are discussed in terms of the correlation effects in many hole excited states. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H23.00014: Scandium Oxide Thin Films and Their Optical Properties in the EUV Guillermo Acosta, David Allred, Steve Turley, Richard Vanfleet In recent years, it was conjectured that scandium thin films could be used to produce highly reflective coatings in the Extreme Ultraviolet (EUV). However, scandium's affinity to form new compounds prevents such coatings from achieving calculated reflectivities. In this project, thin films of scandium oxide are studied to supplement the understanding and use of scandium, and possibly as a substitute for scandium in multilayer coatings. This study reports on the physical and optical characterization of scandium oxide thin films. Thin films of scandium oxide, 15-50 nanometers thick, were deposited on silicon photodiodes by reactively sputtering scandium in an oxygen environment. These samples were measured using EUV synchrotron radiation at the Lawrence Berkeley National Laboratory Advanced Light Source, Beamline 6.3.2. Reflection and transmission measurements, at variable angles, were taken simultaneously from 2.7-50 nanometers. Analysis of the data has provided experimentally determined optical constants. Additional characterization of the samples includes ellipsometry, scanning transmission electron microscopy, energy dispersive x-ray analysis, and high resolution transmission electron microscopy. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H23.00015: Quantum statistics for a finite number of polarons in a neutralizing background Fons Brosens, S.N. Klimin, J.T. Devreese The ground state energy of an $N$-polaron system, confined to a spherical quantum dot with a neutralizing background charge, is investigated within an all-coupling many-body path-integral variational principle, taking into account both the Fermi statistics of the polarons and the electron-electron interaction. The treatment of the ground-state energy is performed for both closed-shell and open-shell systems. The average fermion density in the neutral spherical dot is characterized by the Wigner-Seitz parameter $r_s$. For a sufficiently large but finite number of polarons, the dependency of the ground state energy on $r_s$ is similar to that for a polaron gas in bulk. Herefrom, we can conclude that the ground state properties of a polaron gas in bulk can be qualitatively described using a model of a finite number of polarons in a confinement potential provided by a neutralizing background charge. [Preview Abstract] |
Session H24: Molecular Electronics and Quantum Dots
Sponsoring Units: DPOLY DMPRoom: Colorado Convention Center 201
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H24.00001: BREAK |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H24.00002: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H24.00003: Thermopower and Electrical Conductance Measurements of Single Molecule Junctions Pramod Sangi Reddy, Sung-Yeon Jang, Rachel Segalman, Arun Majumdar The thermopower and electrical conductance of metal-molecule-metal junctions is studied by trapping single molecules between two gold electrodes with either a temperature differential (thermopower) or voltage differential (electrical conductance) applied across the electrodes. The voltage differential generated due to a temperature differential across a single molecule of Benzenedithiol, Dibenzenedithiol and Tribenzenedithiol trapped between Au electrodes is measured. The sign of the measured thermopower is used to show unambiguously that electrical conduction in these single molecule junctions is p-type (hole). The electrical current in a metal-molecule-metal junction due to a voltage differential of $\sim $100 mV is measured. The effect of molecular structure on electrical conductance is studied by 1) systematically varying the length of aliphatic molecules and aromatic molecules 2) changing the end groups binding to the electrodes 3) by adding substituents to the molecules. It is seen that the electrical resistance of aliphatic and aromatic molecules increases exponentially with length, while there was little effect of end groups and substituents for the molecules that we studied. Further, aromatic molecules are found to be much less resistive than aliphatic molecules of similar length. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H24.00004: Quantum Dots Tailored with Conjugated Polymer Jun Xu, Zhiqun Lin Placing conjugated polymers (CPs) in direct contact with a quantum dot (QD) (i.e., preparing QD-CP nanocomposites) carries advantage over cases where QD aggregation dominates. Such QD-CP nanocomposites possess a well-defined interface that significantly promotes the charge or energy transfer between these two components. However, very few studies have centered on such direct integration and QD-CP nanocomposites confined in nanoscopic geometries have never been explored. Here we demonstrate an approach to graft vinyl functionalized poly(3-hexylthiophene) (P3HT) onto aryl-bromide functionalized CdSe QD surfaces. The photophysical properties of nanocomposites in nanoscopic confined geometries are studied. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H24.00005: Plasmonically Enhanced Second-Harmonic Generation from Metallic/Organic Hybrid Self-Assembled Films Kai Chen, Cemil Durak, Randy Heflin, Hans Robinson We have fabricated a new class of second order nonlinear optical materials by combining ionic self-assembled multilayer (ISAM) films with silver nanoparticle arrays in a non-centrosymmetric geometry. These hybrid films exhibit second-harmonic generation (SHG) efficiencies as much as 1600 times larger than unmodified, conventional ISAM films, which makes a three bilayer hybrid film perform at the same level as a micron thick, 700-1000 bilayer film. This was accomplished by using nanosphere lithography to deposit silver nanoparticles on the ISAM film, tuning the geometry of the particles to make their plasmonic resonances overlap the frequency of optical excitation. Even though the enhancement is already large, we suggest that further refinements of the techniques are expected to lead to additional enhancements of similar or larger magnitude. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H24.00006: Reversible Photomechanical Switching of Individual Engineered Molecules at a Surface Matthew Comstock, Niv Levy, Armen Kirakosian, Jongweon Cho, Frank Lauterwasser, Jessica Harvey, David Strubbe, Jean Fr\'echet, Dirk Trauner, Steven Louie, Michael Crommie We have spatially resolved reversible light-induced mechanical switching in a single organic molecule bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of an individual azobenzene molecule on a gold surface before and after reversibly cycling its mechanical structure between \textit{trans} and \textit{cis} states via photo-actuation (i.e., photoisomerization). Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of \textit{tert}-butyl (TB) ligands (``legs'') to the azobenzene phenyl rings. We find that azobenzene molecules lacking TB legs or having only two legs do not switch on a gold substrate under UV irradiation, while molecules synthesized with four TB legs can be photoswitched on gold. STM images of the functionalized molecules show that increasing the number of TB legs ``lifts'' the azobenzene molecules from the substrate, thereby increasing their photomechanical activity. The reversibility of the photoreaction, along with comparison of experimental data to \textit{ab initio} simulation of isomerized azobenzene, confirms the photo-induced \textit{trans-cis} conversion of single molecules. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H24.00007: Magnetic Field Effect on Hybrid Exciton in a Quantum Dot Coated by an Organic Shell Justin Angus, Que Huong Nguyen We investigate the effect of magnetic field perturbations on the hybrid exciton in a semiconductor quantum dot coated by an organic material. The spatial confinement effect of electron and holes of the heterostructures have been considered together with the quantum confined Zeeman effect and the magnetic confinement. Upon the application of magnetic field the coupling term between the two kinds of excitons increases. An important result is the possibility of tuning the Wannier-Frenkel exciton resonance by applied magnetic fields. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H24.00008: Theoretical study of photoisomerization of azobenzene derivatives on Au(111) David A. Strubbe, Matthew J. Comstock, Niv Levy, Armen Kirakosian, Jongweon Cho, Michael F. Crommie, Steven G. Louie Azobenzene and its various substituted derivatives are organic molecules that can be made to photoisomerize reversibly in solution between the \textit{cis} and \textit{trans} isomers. Scanning tunneling microscopy (STM) experiments have recently shown that photoisomerization is also possible in vacuum on a Au(111) surface. We use \textit{ab initio} pseudopotential density-functional theory to confirm and analyze the experimental results by simulating STM images of the isomers, and we also study how the molecules adsorb on the surface and why some azobenzene derivatives can photoisomerize on the surface while others cannot. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H24.00009: Reliable and Versatile Molecular Electrodes Pawan Tyagi, Dongfeng Li, Stephen Holmes, Bruce Hinds Further advancements of molecular electronics will require a reliable and easily scalable electrode fabrication scheme with dimensional control to molecular lengths. We have produced versatile molecular junction (MJ) with high yield ( 90{\%}) long device life ($>$1year) using simple photolithography and thin film methods. The critical electrode dimension is readily set to the length of a molecule by the thickness of an insulator film at a pattern edge. A variety of MJs were prepared by attaching paramagnetic molecular clusters to span the exposed edge of metal-insulator-metal tunnel junctions. Magnetic (Co, NiFe and Ni) and nonmagnetic (Cu, Pd, Ta and Au) metal electrodes and Al$_{2}$O$_{3}$ insulator were utilized. After molecule attachment $\sim $5000{\%} increase in current over bare tunnel junction current was observed. Control experiments including the use of neat solvents, using junction widths longer than molecules, use of insulating molecules, and the reversible binding of molecule to top electrode confirm the successful fabrication of molecular electrodes. MJs were photoactive producing $\sim $60mV photo voltage with white light irradiation. Large magneto-resistance effects were seen with magnetic electrodes. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H24.00010: Manipulation of Kondo Effect via Two-Dimensional Molecular Self-Assembly Violeta Iancu, Aparna Deshpande, Saw-Wai Hla We report manipulation of a Kondo resonance originated from the spin-electron interactions between a two dimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K using a low temperature scanning tunneling microscope. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a small hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with a decreasing the number of nearest neighbor molecules from six to zero. This Kondo temperature variation is originated from the scattering of surface electrons by the molecules located at the edges of the molecular layer, which reduces spinelectron coupling strength for the molecules inside the layer. Investigations on different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice. This work is financially supported by US-DOE grant DE-FG02-02ER46012. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H24.00011: Spatial correlation of photoisomerization of functionalized azobenzene molecules on a surface Niv Levy, Matthew J. Comstock, Jongweon Cho, Armen Kirakosian, Luis Berbil-Bautista, Frank Lauterwasser, Jean M. J. Fr\'echet, David Strubbe, Steven G. Louie, M. F. Crommie Photoactive azobenzene molecules have great potential for nanoscale opto-mechanical applications. We report a scanning tunneling microscopy (STM) study of the time-dependence of photo-switching tetra-\emph{tert}-butyl-azobenzene (TTB-AB) molecules on Au(111). ``Switched'' molecule concentrations were measured as a function of exposure time to various incident light wavelengths until stationary concentrations were reached. We examined the spatial correlations of the photo-switching rates. Scanning tunneling spectroscopy was used to reveal the possible dependence of switching dynamics on the electronic structure of the islands. Implications for organic photoactive devices will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H24.00012: Electron transport through the building blocks of proteins David Cardamone, George Kirczenow We investigate two-terminal charge transport through single oligopeptide molecules, thiol-bonded to gold leads. Applying \emph{ab initio} and semi-empirical techniques, we calculate equilibrium and non-equilibrium results in the Landauer formalism. The conductance and current thus obtained are consistent with the recent experimental results of X. Y. Xiao, B. Q. Xu, and N. J. Tao (\emph{J. Am.\ Chem.\ Soc.} {\bf 126}, 5370; \emph{Angew.\ Chem.\ Int. \ Ed.} {\bf 43}, 6148). This theory furthermore provides a straightforward explanation of the striking current rectification seen in those experiments. [Preview Abstract] |
Session H25: Focus Session: Medical Radiation Biology
Sponsoring Units: DBPChair: Paul Gueye, Hampton University
Room: Colorado Convention Center 203
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H25.00001: Proteomic determination of the biological sequelae of electron irradiation. Invited Speaker: Radiobiological-based treatment planning, where radiation dose is varied according to the regional biological variations in tumor tissue e.g. hypoxia, is becoming increasingly available and represents a radically different approach to improving the radiocurability of tumors. However, many of the current algorithms are based upon radiobiological phenomenon that have been studied for decades, e.g., the oxygen effect, and few utilize recent information on biological parameters that influence radiation response, e.g. EGFR status. With regard to electron treatment planning, there is a paucity of studies that have looked at the biological consequence of exposure to electrons of differing energies. The assumption is that there is a uniform cell killing per unit dose within the treatment volume. We have recently applied proteomic analysis to determine the impact that exposure to low and high-energy electrons have on the proteome of tumor cells; preliminary data suggests that a completely different spectrum of proteins are expressed 24 hours after exposure to 50 cGy of high versus low LET electrons. Changes in the cellular proteome provide an indication of the different cellular responses elicited in response to damage induced by high and low energy electrons. Should these protein changes reflect a different high versus low energy electron mediated cell inactivation, then algorithms may have to be developed that take into account the energy distribution within the dose field. A new technique called MALDI-imaging is capable of resolving proteomic differences at various sites in a tissue slice, thus allowing for a spatial determination of proteins within an irradiated tumor volume. In the future it may thus be feasible to determine the exact dose distribution with an irradiated field and determine the efficacy with which radiation kills tumor or normal cells. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H25.00002: Energy Spectra Reconstruction from Beta Emitters: A Study of the 90SR/90Y Case Ariano Munden, Paul Gueye, Cynthia Keppel, Chris Soares Reconstruction of individual electron energies from a 25$\mu $Ci $^{90}$Sr/$^{90}$Y radioactive source was performed using a dipole magnet and a scintillating fiber based detector. The dipole was constructed from two 5.08x5.08x2.54cm permanent magnets separated by a distance of 2 cm and having a maximum field of about 5kG. The electron beam leaving the source has a 2.28MeV maximum energy and was collimated within a 1cm at the entrance face of the magnet. Mapping of the magnetic field was done using a hall probe with an accuracy of about 2G. An electron detector consisted of blue shifted scintillating fibers with thicknesses of 1mm was used to detect the particles exiting the magnet. The data was compared with the ICRU energy distribution data for 90Sr, 90Y, and the composite 90Sr/90Y sources. The comparison was performed using a chi squared test. The setup provides an energy resolution less than 10{\%}. Such system could be used to reconstruct the energy distribution of any beta emitter for various types of calibrations as used in experimental physics (nuclear/high energy, medical, material sciences etc.). . [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H25.00003: Validity Of the Low Energy Electromagnetic Physics As Implemented Using the Geant4 Toolkit Using A Sr$^{90}$/Y$^{90 }$Beta Emitter Source Rachel Black, Paul Gueye Calibration procedures in experimental physics (nuclear physics, material sciences, medical physics etc.) usually require the use of a low activity radioactive source. A model of the setup is most often performed to understand and optimize system performances. We have investigated the validity of the low energy electromagnetic physics models up to a 2.3 MeV as implemented in the Geant4 simulation toolkit. For this, a set of experiments was done using a beta emitter source consisting of a Sr$^{90}$/Y$^{90}$ in secular equilibrium. The electrons enter a permanent dipole magnet made of two 5.08x5.08x2.54 cm$^{3}$ blocks of Neodymium Iron Boron encased within an iron support frame and separated by a distance of 2cm. The measured Gaussian-like magnetic field separates the energies of the beta particles exiting the magnet. These electrons were then collected on an array made from sixteen 1mm thick scintillating fibers. The experimental data were compared against the ICRU database. The Geant4 simulation was developed to understand the energy loss and spectra obtained during the actual experiment. Forward (backward) simulation were done to generate (reconstruct) the (secondary) primary energy distribution of the source. Preliminary results of this study will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H25.00004: Comparison of Fluid Attenuated Inversion Recovery Sequence with Spin Echo T$_{2}$ Weighted MRI for Characterization of Brain Pathology Indra Sahu, Sheshkant Aryal, Shanta Shrestha, Ram Ghimire, Keith Earle Twenty cases of different brain pathology have been studied via MRI using an open resistive magnet with magnetic field strength of 0.2 Tesla. The relative signal intensity with respect to the repetition time (TR) at fixed echo time (TE) 0.11 sec. has been studied. It was found that the signal intensity saturates for most lesions beyond a certain TR$\sim $6 sec in the T$_{2}$ - weighted image. The signal intensity differs with respect to the inversion time (TI) for fat and cerebrospinal fluid (CSF). It was found that the intensity is nulled for CSF at TI $\sim $1.5 sec. and for Fat at TI $\sim $ 0.10 sec in the FLAIR imaging sequence. Thus the intensity of the lesions is qualitatively different for the two sequences. From the radiological diagnostic point of view, it was concluded that the FLAIR sequence is more useful for the detection of lesions compared to T$_{2}$ sequences. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H25.00005: Experimental test of specific predictions of a model for the oscillatory response of p53 to DNA damage. Gustavo Stolovitzky, John Wagner, J. Jeremy Rice, Lan Ma, Wenwei Hu, Zhaohui Feng, Arnold Levine We have proposed a model for radiation-induced oscillations of the p53-mdm2 system that makes specific predictions about the range of both p53 and mdm2 transcription rates that support oscillation. Our model predicts that in cells with a polymorphism in the mdm2 gene (SNP309) that enhances mdm2 transcription levels, oscillations disappear. The kinetics of the p53 and Mdm2 levels measured in cells with different genotype at the SNP309 locus show that oscillations of p53 and Mdm2 are observed in the cells wild type for mdm2 SNP309 but not in cells homozygous for mdm2 SNP309. By using H1299 cell line expressing wild-type p53 under a tetracycline-regulated promoter we found that only when p53 levels are in a certain range, oscillation can be observed after stress. This study provides evidence that proper range of the p53 and Mdm2 levels are required for the coordinated p53-Mdm2 oscillation upon stress. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H25.00006: Impedance Analysis of Ovarian Cancer Cells upon Challenge with C-terminal Clostridium Perfringens Enterotoxin Geoffrey Gordon, Chun-Min Lo Both in vitro and animal studies in breast, prostate, and ovarian cancers have shown that clostridium perfringens enterotoxin (CPE), which binds to CLDN4, may have an important therapeutic benefit, as it is rapidly cytotoxic in tissues overexpressing CLDN4. This study sought to evaluate the ability of C-terminal clostridium perfringens enterotoxin (C-CPE), a CLDN4-targetting molecule, to disrupt tight junction barrier function. Electric cell-substrate impedance sensing (ECIS) was used to measure both junctional resistance and average cell-substrate separation of ovarian cancer cell lines after exposure to C-CPE. A total of 14 ovarian cancer cell lines were used, and included cell lines derived from serous, mucinous, and clear cells. Our results showed that junctional resistance increases as CLDN4 expression increases. In addition, C-CPE is non-cytotoxic in ovarian cancer cells expressing CLDN4. However, exposure to C-CPE results in a significant (p$<$0.05) dose- and CLDN4-dependent decrease in junctional resistance and an increase in cell-substrate separation. Treatment of ovarian cancer cell lines with C-CPE disrupts tight junction barrier function. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 10:12AM |
H25.00007: Understanding Radiotherapy-Induced Second Cancers Invited Speaker: There is increasing concern regarding radiation-related second-cancer risks in long-term radiotherapy survivors, and a corresponding need to be able to predict cancer risks at high radiation doses. While cancer risks at moderately low radiation doses are reasonably understood from A-bomb survivor studies, there is much more uncertainty at the high doses used in radiotherapy. It has generally been assumed that cancer induction decreases rapidly at high doses due to cell killing. However, most recent studies of radiation-induced second cancers in the lung and breast, covering a very wide range of doses, contradict this assumption. A likely resolution of this disagreement comes from considering cellular repopulation during and after radiation exposure. Repopulation / proliferation with a significant number radiation-induced pre-malignant cells, tends to counteract the effect of cell killing, and keeps the induced cancer risks higher at high doses. We describe and apply a biologically based, minimally parameterized model of dose-dependent cancer risks, incorporating carcinogenic effects, cell killing and, additionally, proliferation / repopulation effects. The situation is somewhat different for radiation-induced leukemia, as repopulation via the blood stream tends to be with cells that originated father away from the treatment volume than is the case for solid second cancers, thus containing a smaller proportion of radiation-damaged cells. The model predictions agree well with recent data on second cancer risks, both for radiation-induced solid cancers and for radiation-induced leukemias. Incorporating repopulation effects provides both a mechanistic understanding of cancer risks at high doses, as well as providing a practical methodology for predicting, and therefore potentially minimizing, cancer risks in organs exposed to high radiation doses during radiotherapy. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H25.00008: Photoelectric Effect, Bremsstrahlung, and Compton Effect Formulas Should Contain Rotational and Vibrational Energies Stewart Brekke The kinetic energy element in the Photoelectric Effect, Bremsstrahlung and Compton Effect formulas should also include besides the linear kinetic energy element rotational (spin) and vibrational kinetic energy elements. In the photoelectric effect the formula should be $[ h\nu = 1/2mv^2 + 1/2I\omega_r^2 + (n + 1/2)\hbar\omega_v + \phi]$ where $\omega_r$ is the rotational angular velocity and $\omega_v$ is the vibrational angular frequency. Similarly, in Bremsstrahlung the kinetic energy lost to photon creation at total braking shoudl be $[1/2mv^2 + 1/2I\omega_r^2 + (n +1/2)\hbar\omega_v =eV =h\nu_{max}]$. The resulting kinetic energy of a recoil particle in the Compton Effect should be $[1/2mv^2 + 1/2I\omega_r^2 + (n +1/2)\hbar\omega_v = (h\nu)\Delta\lambda/(\lambda +\Delta\lambda)]$. Also, in pair production and annhiliation the kinetic energies of the annhiliated pair and created pair should include the spin and vibrational energies. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H25.00009: Exploring the Role of Calcium in Cardiac Cell Dynamics Carolyn Berger, Salim Idriss, Ned Rouze, David Hall, Daniel Gauthier Bifurcations in the electrical response of cardiac tissue can destabilize spatio-temporal waves of electrochemical activity in the heart, leading to tachycardia or even fibrillation. Therefore, it is important to understand the mechanisms that cause instabilities in cardiac tissue.Traditionally, researchers have focused on understanding how the transmembrane voltage is altered in response to an increase in pacing rate, i.e. a shorter time interval between propagating electrochemical waves. However, the dynamics of the transmembrane voltage are coupled to the activity of several ions that traverse the membrane. Therefore, to fully understand the mechanisms that drive these bifurcations, we must include an investigation of the ionic behavior. We will present our recent investigation of the role of intracellular calcium in an experimental testbed of frog ventricle. Calcium and voltage are measured simultaneously, allowing for the previous research regarding voltage to guide our understanding of the calcium dynamics. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H25.00010: Biological Response of Cancer and Normal Cells on Irradiation from Electrons with Energies up to 200 keV. Yuriy Prilepskiy This paper presents continuation data of the series of experiments with the electron gun of the CEBAF machine at Jefferson Lab (Newport News, VA), which is capable of delivering electrons with energies up to 200 keV. This 1.5 GHz beam permits to generate cellular damage within minutes. We have performed irradiation of cancer and normal cells with different electron energies and currents to investigate cell biological responses. The biological response is measured through proteomics analysis before and after irradiation. The living cells are encased in special air containers allowing proper positioning in vacuum where the electrons are present. The containers receive the irradiation from the mono energetic electrons with energy up to 120 keV, resulting in an irradiation from both electrons and a small number of photons from the original beam passing through the thin container window. This window allows approximately half of the beam to come through. The study will permit to address the physical processes involved in the RBE and LET at a level that supersedes current data listed in the literature. We will discuss the experimental setup and the second stage of data collected with the new more developed system. This research is part of a global program to provide detailed information for the understanding of radiation based cancer treatments. [Preview Abstract] |
Session H26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections I
Sponsoring Units: DCPChair: Todd Martinez, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center 205
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H26.00001: Electronically Excited States and Conical Intersections in Cytosine and its Analogs Invited Speaker: Conical intersections between two and three electronic states of the same symmetry have been found to play a key role in nonadiabatic processes. In recent years many studies have shown that conical intersections are important in the photophysics of nucleobases and facilitate radiationless decay to the ground state. Interestingly, there are molecules very similar in structure to the nucleobases which show very different photophysical behavior, i.e., longer excited state lifetimes and high quantum yields of fluorescence. An important question that arises is what causes the different behavior between nucleobases and their fluorescent analogs. In this work we present studies of cytosine and several of its analogs in an effort to correlate the molecular structure to the photophysical behavior. Large scale ab initio multireference configuration interaction methods (MRCI) are being used. Our results show the presence of many seams of two- or three-state conical intersections in both types of systems and energetic differences seem to be the cause of the different photophysical behavior. A mixed quantum mechanical/ classical mechanical (QM/MM) approach where the solute is described with the MRCI method will also be presented as a means to study the effect of the solvent on excited states. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H26.00002: Coherent Control of the Photo-isomerization of Retinal in Bacteriorhodopsin Invited Speaker: Proteins are highly evolved structures in which their central role is to direct chemical or solar energy into functions. A central tenet in biology is that proteins have evolved to stabilize the transition states processes directing energy transduction into functions. In the transition state region, the motions are fairly localized such the wave properties of matter can lead to destructive and constructive interferences that have a pronounced effect on transmission probabilities along reaction coordinates. Further, the time scale for motion through a transition state, often involving a conical intersection, is comparable to the currently believed decoherence times for electronic and vibrational degrees of freedom governing this motion. The question arises whether the phases of the underlying matter waves could play a role in directing biological processes. In order to address this question, we exploited coherent control protocols using shaped laser fields to determine whether or not the absolute quantum yield of the photo-isomerization of retinal in bacteriorhodopsin (bR) could be achieved using weak field conditions to probe the natural function. Through feedback-controlled amplitude and phase variation of the spectral components comprising the excitation pulse, we could selectively enhance or suppress the isomerization quantum yield by 20{\%} in either direction. Our experimental observation illustrates that the wave properties of matter, as manifest on vibrational quantum coherences, can play a role in biological processes to the point that they can even be manipulated. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H26.00003: Anion Photoelectron Spectroscopy of Deprotonated Cytosine and Thymine at 5.826 eV Terry Yen, Bradley F. Parsons, Sean M. Sheehan, Daniel M. Neumark UV radiation from sunlight is one of the most ubiquitous and lethal forms of environmental carcinogens. Luckily, perhaps as a result of selective pressure by molecular evolution, DNA is remarkably stable to photochemical decay. The solar UV photons are absorbed primarily by the building blocks of DNA, the nucleobases, which undergo ultrafast nonradiative relaxation processes back to the ground state by internal conversion. It has been suggested that the nucleobases' excited singlet states of pi-sigma-star character, which are dissociative along the NH stretch coordinate, play a key role in the relaxation pathways. Studying the dynamics of the excited states of deprotonated nucleobases is therefore of great importance in understanding the photostability of DNA. Excited state dynamics of the deprotonated nucleobases cytosine and thymine has been investigated using anion photoelectron spectroscopy at 5.826 eV. In the experiments, a laser is used to detach an electron from a mass selected deprotonated nucleobase anion and the electron kinetic energy (eKE) is measured using velocity-map imaging (VMI). Through VMI, information is obtained on the nature of the excited states involved in the non-radiative processes of DNA. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H26.00004: Comparing the electronic relaxation of pyrimidine bases and nucleosides in aqueous solution Stephen Bradforth, Askat Jailaubekov, Delmar Larsen, Christi Chester The ultrafast deactivation of DNA bases excited in the ultraviolet is known to occur by rapid nuclear motion through conical intersections between different electronic potential energy surfaces. How the intersections between these surfaces and the dynamics over these surfaces are modified by surrounding the base with water is a significant open question in DNA photophysics. Using a broadband transient absorption apparatus with 30 fs time resolution, we observe dispersed spectra from 300 -- 700 nm revealing excited-state dynamics originating for U and T both as nucleobases and nucleosides. New sub-100fs dynamics is observed, including stimulated emission. The deactivation pathways and spectral signatures of the various intermediates are compared to data from gas-phase time-resolved photoelectron spectroscopy and non-adiabatic quantum-classical simulations. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H26.00005: ABSTRACT HAS BEEN MOVED TO L26.00002 |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H26.00006: Photoisomerization selectivity in conjugated $\pi$-bond systems through local microenvironment Aaron Virshup, Todd Martinez Photoisomerization represents one of the simplest means to convert light energy into mechanical motion on the molecular scale. Theoretical models of photobiology often require description of not only a small photochemically active chromophore, but also the effects of the much larger solvent or protein environment containing the chromophore. We have recently developed a program for carrying out excited state QM/MM studies of photodynamics using \textit{ab initio} quantum chemistry techniques for the QM region, and modeling the time evolution of the system with the Full Multiple Spawning method for molecular dynamics. With this method, we show how local charge environments can be used to manipulate and enhance the photoisomer selectivity of small conjugated molecules. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H26.00007: Excited State Dynamics of DNA and RNA bases Hanneli Hudock, Benjamin Levine, Todd Martinez Recent ultrafast spectroscopic experiments have reported excited state lifetimes for DNA and RNA bases and assigned these lifetimes to various electronic states. We have used theoretical and simulation methods to describe the excited state dynamics of these bases in an effort to provide a mechanistic explanation for the observed lifetimes. Our simulations are based on ab initio molecular dynamics, where the electronic and nuclear Schrodinger equations are solved simultaneously. The results are further verified by comparison to high-level ab initio electronic structure methods, including dynamic electron correlation effects through multireference perturbation theory, at important points along the dynamical pathways. Our results provide an explanation of the photochemical mechanism leading to nonradiative decay of the electronic excited states and some suggestions as to the origin of the different lifetimes. Comparisons between pyrimidines illustrate how chemical differences impact excited state dynamics and may play a role in explaining the propensity for dimer formation in thymine. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H26.00008: Probing solvation effects at conical intersections by ultrafast photoelectron imaging Benoit Soep, Lionel Poisson, Kevin Raffael, Jean Michel Mestdagh The electronic excitation of polyatomic molecules is generally followed by relaxation of the electronic energy to the ground state or to metastable, low lying states such as triplet states in hydrocarbons. It can be extremely rapid whenever conical intersections between the surfaces are at play, owing to their structural changes. Since, in general, relaxation is observed in condensed phases, it is essential to conduct the relevant experiments in the presence of a perturbing medium, here the surface of an argon cluster. We address the coupling of two excited configurations in a molecule possessing charge transfer intermediates thus prone to medium effects. We shall compare here the observation of the free and deposited molecule at the surface of argon clusters. The effect of the cluster and the possibility to record significant photoelectron spectra is thus described that represents an innovation for large systems. We made use of the anisotropy of the photoelectron angular distribution of the electrons to unravel the dynamics of the several excited configurations that are traversed during the electronic relaxation. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H26.00009: Azobenzene optical switch controlled by external force Martin Konopka, Nikos Doltsinis, Dominik Marx, Ivan Stich External pulling force can be employed to manipulate optical switching properties of azobenzene molecule which is a promising system for molecular electronics devices. We perform density functional treatment of azobenzene terminated by S-H groups with pulling force applied via restraints on the sulfur atoms. We treat the system both at zero and room temperature and among other quantities focus on separation between ground (S0) and first excited (S1) singlet states. The separation is crucial for cis $\leftrightarrow$ trans reisomerization rate. For trans isomer we observe decrease of the S1-S0 separation with increased stretching force and the molecule length. For cis isomer we find opposite: the separation increases thus lowering photoisomerization rate. Another interesting point is mechanically induced cis $\rightarrow$ trans inversion in the ground state which occurs for extensions above 5~\AA. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H26.00010: Control of molecular fragmentation using shaped femtosecond pulses Marcos Dantus, Vadim Lozovoy The ability to tailor the excitation laser pulse by pulse shaping has inspired a number of scientist to search for special pulses that would be capable of inducing selective bond fragmentation or specific molecular rearrangements. This presentation will summarize results from a comprehensive look at the interaction of shaped laser pulses with para-nitrotoluene molecules. We have performed exhaustive experimental evaluation over very different pulse shaping strategies such as chirp, sinusoidal modulation, sinusoidal phase, binary phase, binary amplitude, binary phase and amplitude. For all these methods we recorded hundreds of mass spectra together with the integrated second harmonic generation (SHG) as an independent parameter related to pulse complexity or pulse duration. As expected, the integrated SHG of a given laser pulse correlates linearly with the total amount of ions detected. We were surprised, however, that the fragmentation patterns observed varied simply and predictably with the integrated SHG regardless of pulse shaping strategy. This implies that the integrated SHG fraction compared to a transform limited pulse is an excellent predictor of the fragmentation pattern for a given molecule. The implications of our findings for this and other molecules will be discussed from the fundamental point of view of bond selective chemistry. The development of applications for molecular recognition will also be discussed. [Preview Abstract] |
Session H27: Focus Session: Computational Nanoscience III - DNA Translocation / Nanopores
Sponsoring Units: DMP DCOMPChair: Abdelkader Kara, University of Central Florida
Room: Colorado Convention Center 301
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H27.00001: Ionic conductance in nanopores Johan Lagerqvist, Michael Zwolak, Massimiliano Di Ventra We study ionic transport through nanopores from the perspective of the microscopic electrostatics. We predict the existence of step-like structures in the ionic conductance as a function of both nanopore diameter and ionic concentration. This is due to the formation and breakage of hydration layers around the ions. In bulk water, there are a few of these layers around each ion. As an ion goes through the pore, some of the hydration layers break and thus create an energetic barrier for ion transport. The number of broken layers depends on the pore diameter and the ionic concentration. In this talk, we discuss various properties of the hydration layers and the parameter range necessary to experimentally observe quantized conductance of ions through nanopores. Work supported in part by NSF and NIH. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H27.00002: Electrophoretic speed of a polyelectrolyte in a nanopore Sandip Ghosal A hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through a nanopore is presented. It is assumed that the speed is determined by a balance of electrical and viscous forces arising from within the pore in the presence of co and counter ions. Further, classical continuum electrostatics and hydrodynamics as well as the mean field description of Poisson-Boltzmann is assumed to be applicable after accounting for Manning condensation on the polyelectrolyte. An explicit formula for the translocation speed as a function of the pore geometry and other physical parameters is obtained and is shown to be consistent with recent experimental measurements on DNA translocation through nanopores in silicon membranes. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H27.00003: ABSTRACT HAS BEEN MOVED TO A41.00013 |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H27.00004: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H27.00005: Transcription of ribosomal RNA: the role of antitermination of RNA polymerase Stefan Klumpp, Terry Hwa The genes encoding ribosomal RNA are transcribed at high rates of 1-2 transcripts per second. These high transcription rates are crucial to maintain the large concentration of ribosomes necessary in fast growing bacteria. To understand how transcription is regulated under these conditions, we developed a model for the traffic of transcribing RNA polymerases (RNAP). Our simulations show that the transcription rate is limited by the elongation stage of transcription rather than by transcript initiation. The maximal transcription rate is severly impaired by RNAP pausing with pause durations in the second range which is ubiquitous under single-molecule conditions. We propose that ribosomal antitermination reduces pauses and thereby increases the transcription rate. This idea is in quantitative agreement with the observed increase of the elongation rate due to antitermination and predicts a two-fold increase of the transcription rate. Antitermination must be highly efficient, since incomplete antitermination with only a few percent of non-antiterminated, i.e. slow, RNAPs completely abolishes its effect. This result suggests that rho-dependent termination may selectively terminate slow RNAPs. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H27.00006: Mechanically induced cis to trans reisomerization of azobenzene Robert Turansky, Martin Konopka, Ivan Stich, Dominik Marx Using density functional techniques we study mechanochemistry of the azobenzene molecule. Azobenzene is an optically switchable molecule. Laser light is normally used to achieve molecular switching between the cis and trans isomers. We use mechanochemistry to achieve the switching. Thiolate-gold bond can used to exert mechanical energy on the molecule bonded between two gold electrodes in static AFM apparatus. Our model consists of two realistic gold electrodes bridged by dithioazobenzene. We find that pulling the transisomer leads just to formation of gold nanowires and mechanical breakage of the electrodes. However, mechanochemistry with modest applied forces leads to cis $\leftrightarrow$ trans reisomerization via rotation mechanism. Contrary, use of simple constraints instead of realistic gold electrodes, leads to cis $\leftrightarrow$ trans reisomerization, albeit with significantly larger applied forces and via inversion mechanism. Important experimental and theoretical ramifications of these simulations will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H27.00007: DNA translocation through protein and synthetic nano pores Invited Speaker: DNA translocation through narrow protein channels is recognized as an important process in biology. Recently it has attracted lot of attention in the biophysical community following several experiments on DNA translocation through protein nano-pores, and more recently, through synthetic silicon nano-pores. A fundamental understanding is needed for various biological processes, {\em e.g.}, entry and exit of a DNA in and out of a cell, efficient separation methods for macromolecules, and, possibly fast DNA sequencing. In this talk I will be presenting results for the DNA translocation using a coarse-grained model for an idealized DNA as well as the pore. I will consider several scenarios for the DNA translocation. First, I will show scaling of translocation time of a homopolymer as it escapes from the {\em trans} side to the {\em cis} side of an idealized thin membrane\footnote{A. Milchev, K. Binder, and Aniket Bhattacharya, J. Chem. Phys. {\bf 121}, 6042 (2004).} Then I will consider DNA dynamics subject to a driving force inside the pore. Next, I will consider heteropolymer threading through a nano-pore. Specifically we will consider both highly ordered and completely random sequences of the chain and relate specific sequences to the distribution of the translocation time and the residence time inside the pore. These studies also will include effects due to different environment on either side of the pore, specific DNA-pore interactions located at selective sites, {\em etc.}. I will discuss relevance of these simulation results to recent experiments and theoretical models. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H27.00008: DNA nucleoside interaction and identification with carbon nanotubes Sheng Meng, Paul Maragakis, Costas Papaloukas, Efthimios Kaxiras DNA and carbon nanotubes (CNTs) are prototypical one-dimensional structures. Segments of single-strand DNA are extremely flexible, strongly hydrophilic biopolymers while CNTs are extremely stiff, strongly hydrophobic nanorods. The interaction between DNA and CNTs is being intensely investigated for possible use in, e.g., DNA transporters or biosensors. Recent success in detecting DNA conformational changes and hybridization by near-infrared fluorescence of CNTs or CNT-- field-effect transistors has opened the possibility of DNA sequencing through electronic means. Here we investigate the interaction of individual DNA nucleosides with a CNT in vacuum and in the presence of external gate voltage. We propose a scheme to discriminate between nucleosides on CNTs based on measurement of electronic features through a local probe such as scanning tunnelling spectroscopy. We demonstrate through quantum mechanical calculations that these measurements can achieve 100\% efficiency in identifying DNA bases. Our results support the practicality of ultrafast DNA sequencing using electrical measurements. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H27.00009: Probing the Structure of DNA-Carbon Nanotube Hybrids with Molecular Dynamics Simulations Robert R. Johnson, Alan T. Johnson, Michael L. Klein DNA-carbon nanotube hybrids (DNA-NT) consist of a single-walled carbon nanotube (SWNT) wrapped with a self-assembled monolayer of single-stranded DNA (ssDNA). Recent experiments involving DNA-NT have shown that this material holds a wide range of technologically useful properties. However, a detailed understanding of its microscopic structure and interactions is lacking. To assist the interpretation of contemporary experiments, we have performed atomistic molecular dynamics (MD) simulations using empirical force fields. MD reveals the nature of the interactions and structural arrangements involved in DNA-NT. We find that the hybrid material spontaneously self-assembles via the attractive $\pi -\pi $ stacking interaction between ssDNA nucleobases and SWNT sidewall. Under ambient conditions, ssDNA adopts various wrapping conformations about SWNT including right- and left- handed helices as well as disordered, kinked structures. These conformations are energetically distinct with the compact right-handed helix the most favorable. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H27.00010: Finite size effect on hydrogen bond cooperativity in (Ala)$_n$ polypeptides: A DFT study using numeric atom-centered orbitals Volker Blum, Joel Ireta, Matthias Scheffler An accurate representation of the energetic contribution $E_{\rm hb}$ of hydrogen bonds to structure formation is paramount to understand the secondary structure stability of proteins, both qualitatively and quantitatively. However, $E_{\rm hb}$ depends strongly on its environment, and even on the surrounding peptide conformation itself. For instance, a short $\alpha$-helical polypeptide (Ala)$_4$ can not be stabilized by its single hydrogen bond, whereas an infinite $\alpha$-helical chain (Ala)$_\infty$ is clearly energetically stable over a fully extended conformation. We here use all-electron density functional calculations in the PBE generalized gradient approximation by a recently developed, computationally efficient numeric atom-centered orbital based code$^1$ to investigate this H-bond \emph{cooperativity} that is \emph{intrinsic} to Alanine-based polypeptides (Ala)$_n$ ($n$=1-20,$\infty$). We compare finite and infinite prototypical helical conformations ($\alpha$, $\pi$, 3$_{10}$) on equal footing, with both neutral and ionic termination for finite (Ala)$_n$ peptides. Moderately sized NAO basis sets allow to capture $E_{\rm hb}$ with meV accuracy, revealing a clear jump in $E_{\rm hb}$ (cooperativity) when two H-bonds first appear in line, followed by slower and more continuous increase of $E_{\rm hb}$ towards $n\rightarrow\infty$. $^1$ V. Blum, R. Gehrke, P. Havu, V. Havu, M. Scheffler, \emph{The FHI Ab Initio Molecular Simulations (aims) Project}, Fritz-Haber-Institut, Berlin (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H27.00011: Electronic Structure of DNA: A Maximally-Localized Wannier Function Approach Arash Mostofi, Nicola Marzari We combine large-scale, \textit{ab initio} electronic structure calculations and the maximally-localized Wannier function approach in order to study the electronic properties of DNA strands. By performing full first-principles calculations on stacked DNA base-pairs, we determine the optimally localized, real-space basis set that is able to describe the infinite one-dimensional system efficiently and accurately. This work opens the way to obtaining a detailed understanding of charge transport and conductance in DNA, bringing closer the prospect of engineering its electronic structure for use in nano-electronic circuits and biotechnology applications. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H27.00012: Theoretical study the physical contribution to the signal to noise ratio (SNR) and sensitivity of Extraordinary Magnetoresistance (EMR) quantum well structures Y. Shao, S. A. Solin, A. Girgis , L. R. Ram-Mohan, Keon-Ho Yoo The application of EMR sensor performance requires ultra-thin films with very high mobility \textit{$\mu $}, and high electron concentration \textit{ n}, because the sensitivity of the EMR device is proportional to the \textit{$\mu $}$ ^{2}$ and the SNR depends on the \textit{ n}$_{3D}$$^{1/2}$\textit{$\mu $}$ ^{2}$ (1/\textit{f} noise) or \textit{ n}$_{3D}$$^{1/2}$\textit{$\mu $}$ ^{5/2}$ (thermal noise). We have modeled the electron concentration and mobility in a two dimensional electron gas (2DEG) layer located in a delta-doped InSb/AlInSb heterostructure. The non-parabolic band structure due to the nature of the small energy band gap of InSb is explicitly accounted for. The subband energy levels, electron wave functions and band-edge profiles were obtained using the $\textbf {\textit{k}$\cdot$ \textit{p}}$ method. The electron transport properties were calculated by including contributions of scattering from ionized impurities, the background neutral impurities, the deformation potential acoustic phonons, and the polar optical phonons. We have calculated the dependencies of \textit{$\mu $}$ ^{2}$, \textit{ n}$_{3D}$$^{1/2}$\textit{$\mu $}$ ^{2}$ and \textit{ n}$_{3D}$$^{1/2}$\textit{$\mu $}$ ^{5/2}$ on temperature, spacer layer thickness, doping density, and the quantum well thickness. This work will impact EMR sensor design. [Preview Abstract] |
Session H28: Focus Session: Graphene I
Sponsoring Units: DMPChair: Jisoon Ihm, Korean Institute of Advanced Studies
Room: Colorado Convention Center 302
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H28.00001: Theory of quantum transport in graphene and nanotubes Invited Speaker: In graphene, electronic states are described by Weyl's equation for a massless neutrino [1,2]. The system has a topological singularity at the origin of the wave vector (${\bf k}\!=\!0$), giving rise to nontrivial Berry's phase when ${\bf k}$ is rotated around the origin [3]. The singularity causes various zero-mode anomalies such as discrete jumps in the diagonal [4], off-diagonal Hall [5], and dynamical conductivity [6] at the Fermi energy corresponding to ${\bf k}\!=\!0$. In the presence of a magnetic field, a Landau level with zero energy exists independent of the strength of the field [7], giving rise to a singular diamagnetism of graphene and the large magnetic anisotropy of the carbon nanotube [8] used extensively for the observation of the Aharonov-Bohm effect [9,10]. In the absence of a magnetic field, the system belongs to a symplectic universality class even in the presence of scatterers unless their potential range is smaller than the lattice constant. Being combined with the presence of an odd number of current carrying channels, this leads to the absence of backward scattering [11] and the presence of a perfectly conducting channel [12], making a metallic carbon nanotube a perfect conductor with ideal conductance exhibiting intriguing frequency dependence [13,14]. In the presence of scatterers with range smaller than the lattice constant, the system crossovers from the symplectic to an orthogonal class [15,16], and to a unitary class if higher order \boldmath{$k \cdot p$} terms causing trigonal warping are considered [17] or in magnetic fields [18]. These symmetry crossovers manifest themselves as strong difference in localization effects due to disorder in both graphene [18,19] and a carbon nanotube [20]. \par References: [1] J. C. Slonczewski and P. R. Weiss, Phys. Rev. {\bf 109}, 272 (1958). [2] T. Ando, J. Phys. Soc. Jpn. {\bf 74}, 777 (2005). [3] T. Ando, T. Nakanishi, and R. Saito, J. Phys. Soc. Jpn. {\bf 67}, 2857 (1998). [4] N. H. Shon and T. Ando, J. Phys. Soc. Jpn. {\bf 67}, 2421 (1998). [5] Y. Zheng and T. Ando, Phys. Rev. B {\bf 65}, 245420 (2002). [6] T. Ando, Y. Zheng, and H. Suzuura, J. Phys. Soc. Jpn. {\bf 71}, 1318 (2002). [7] J. W. McClure, Phys. Rev. {\bf 104}, 666 (1956). [8] H. Ajiki and T. Ando, J. Phys. Soc. Jpn. {\bf 62}, 2470 (1993); {\bf 63}, 4267 (1994) (Erratum). [9] H. Ajiki and T. Ando, J. Phys. Soc. Jpn. {\bf 62}, 1255 (1993). [10] S. Zaric, G. N. Ostojic, J. Kono, J. Shaver, V. C. Moore, M. S. Strano, R. H. Hauge, R. E. Smalley, and X. Wei, Science {\bf 304}, 1129 (2004). [11] T. Ando and T. Nakanishi, J. Phys. Soc. Jpn. {\bf 67}, 1704 (1998). [12] T. Ando and H. Suzuura, J. Phys. Soc. Jpn. {\bf 71}, 2753 (2002). [13] T. Ando, J. Phys. Soc. Jpn. {\bf 71}, 2505 (2002). [14] Y. Asada and T. Ando, J. Phys. Soc. Jpn. {\bf 75}, 094711 (2006). [15] H. Suzuura and T. Ando, Phys. Rev. Lett. {\bf 89}, 266603 (2002). [16] T. Ando and K. Akimoto, J. Phys. Soc. Jpn. {\bf 73}, 1895 (2004). [17] K. Akimoto and T. Ando, J. Phys. Soc. Jpn. {\bf 73}, 2194 (2004). [18] T. Ando, J. Phys. Soc. Jpn. {\bf 73}, 1273 (2004). [19] E. McCann, K. Kechedzhi, V. I. Falko, H. Suzuura, T. Ando, and B. L. Altshuler, Phys. Rev. Lett. {\bf 97}, 146805 (2006). [20] T. Ando, J. Phys. Soc. Jpn. {\bf 75}, 054701 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H28.00002: Charge-Tunable Electron-Phonon Coupling in Single Layer Graphene Jun Yan, Yuanbo Zhang, Philip Kim, Aron Pinczuk We report the observation of electron-phonon coupling in single layer graphene via gate-modulated Raman spectroscopy. The doubly-degenerate long-wavelength optical phonon of graphene (the G-band) is found to be very sensitive to charging of the single atomic layer by the electric-field-effect. The functional dependences of frequency and line-width on gate voltage are explained in terms of charge-tunable interactions of G-band phonons with particle-hole transitions across a vanishing band gap. The phonon dynamics uncovers, from a unique perspective, the intriguing physics of Dirac fermions residing in this two dimensional hexagonal lattice of carbon atoms. The striking symmetry manifested in the spectra offers an optical venue for the determination of the charge-neutral Dirac-point. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H28.00003: Electron-Phonon Interactions in Graphene and Graphene Layers Jia-An Yan, W.Y. Ruan, M.Y. Chou We have performed first-principles calculations of the phonon linewidth due to the electron-phonon coupling in one and two layers of graphene using the density-functional perturbation theory. For single-layer graphene, we find that the calculated linewidth is dominated by electron interaction with the two highest optical phonon modes near the $\Gamma$ point and by the highest optical phonon mode near the Brillouin zone boundary corners K and K$'$. A value of the mass enhancement parameter, $\lambda$= 0.3, is obtained for the one layer when we extrapolate the smearing temperature to zero. As for the case of bilayer graphene, although the phonon dispersion relations are almost identical to those of single layer, significant enhancement of electron interaction with some phonon modes is observed due to interlayer coupling, leading to distinct phonon linewidths. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H28.00004: Electron-phonon interaction and valley splittings in graphene W. Y. Ruan, Jia-An Yan, Li Yang, M. Y. Chou Based upon first-principles calculations, a two-valley effective mass theory has been developed for graphene in a strong magnetic field and the electron-phonon couplings calculated using density-functional perturbation theory. We showed that the electron interaction with phonons about the Brilouin zone corners can lead to valley-splittings which increases linearly with the magnetic field, in agreement with a recent experimental observation. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H28.00005: Electron Self-Energy Corrections to Quasiparticle Excitations in Graphene and Large Diameter Single-Walled Carbon Nanotubes Jack Deslippe, David Prendergast, Steven Louie Recent experimental measurements of the band structure and band velocity at the Dirac point in graphene highlight many novel effects due to the existence of Dirac fermions in this system. The low energy electronic states are measured to have Fermi velocity of approximately $1.1\times 10^6$ m/s, with energy dispersion obeying the 2D massless Dirac equation. Motivated by this work, we explore in detail the importance of an accurate description of the electron self-energy in determining the quasiparticle band structures of graphene, graphite, and armchair single-walled carbon nanotubes near the Fermi energy, using the GW approximation to the electron self energy. \\[1.0ex] This work was supported by National Science Foundation Grant No. DMR04-39768 and by the US DOE under Contract No. DE-AC02-05CH11231. Computational resources were provided by SDSC and NERSC. Jack Deslippe acknowledges funding from the DOE Computational Science Graduate Fellowship (CSGF). [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H28.00006: Direct observation of Landau levels of massless and massive Dirac fermions. Guohong Li, Eva Y. Andrei The low energy quasiparticles in graphene resemble massless relativistic particles (Dirac fermions): they have a linear energy-momentum spectrum and possess internal degrees of freedom arising from the crystal symmetry of the honeycomb lattice, leading to particle anti-particle pairs. When two layers of graphene are coupled together, the quasiparticles acquire a band-mass and are transformed into chiral massive fermions. Both types of quasiparticles develop unusual Landau levels in a magnetic field which profoundly alter the magneto-transport properties. We will report the direct observation of the Landau levels associated with these quasiparticles using a low temperature STM in fields up to 12 Tesla. The experiments reveal two independent sequences of Landau levels that provide evidence for the coexistence of massless and massive Dirac fermions. The energy levels of the former exhibit a square-root dependence on both field and Landau-level index $n$, while the latter are linear in field with a Landau-level index dependence of $[n(n+1)]^{1/2}$. Both sequences exhibit a zero energy Landau level which is a unique and direct consequence of the quantum-relativistic nature of these quasiparticles. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H28.00007: Electron-electron and spin-orbit interactions in graphene Mehdi Zarea, Nancy Sandler Electron-electron and spin-orbit interactions in graphene nanoribbons. Narrow graphene ribbons with armchair edges exhibit insulating or metallic behavior depending on the ratio of the ribbon width to the lattice constant. Metallic behavior arises from the presence of non-degenerate localized boundary states with a linear dispersion relation.[1,2] Furthermore, in the presence of spin-orbit interactions, the edge states become spin-filtered states.[3,4] In this work, we studied a Dirac model for armchair ribbons with an effective low-energy Hamiltonian for the edge states that contains intrinsic spin-orbit and Coulomb interactions. By using the bosonization technique we obtain the phase diagram and correlation functions of the model and present a detailed comparison with results derived for armchair nanotubes. We also address the stability of the edge states in the presence of electron-electron interactions. Extensions for zigzag ribbons are discussed. [1] M.Fujita et al J.Phys.Soc.Jpn 65, 1920 (1996) [2] L. Brey and H. A. Fertig, Phys. Rev. B 73, 235411 (2006) [3] C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005) [4] K. Sengupta, R. Roy, and M. Maiti, Phys. Rev. B 74, 094505 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H28.00008: Spin density wave formation in graphene facilitated by the in-plane magnetic field Sebastian Reyes, Alexei Tsvelik We suggest that by applying a magnetic field lying in the plane of graphene layer one may facilitate an excitonic condensation of electron-hole pairs with opposite spins and chiralities. The provided calculations yield a conservative estimate for the transition temperature $T_c \sim 0.1~ B$. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H28.00009: Is Graphene a Fermi Liquid? Wang-Kong Tse, Sankar Das Sarma, Euyheon Hwang In this talk, we answer the question posed in the title above by considering theoretically the electron-electron interaction induced many-body effects in undoped (`intrinsic') and doped (`extrinsic') 2D graphene layers. We find that (1) intrinsic graphene is a \textit{marginal} Fermi liquid with the imaginary part of the self-energy, $\mathrm{Im}\Sigma(\omega)$, going as linear in energy $\omega$ for small $\omega$, implying that the quasiparticle spectral weight vanishes at the Dirac point as $(\mathrm{ln}\omega)^{-1}$; and, (2) extrinsic graphene is a well-defined Fermi liquid with $\mathrm{Im}\Sigma(\omega)\sim \omega^2\mathrm{ln}\omega$ near the Fermi surface similar to 2D carrier systems with parabolic energy dispersion. We provide analytical and numerical results for quasiparticle renormalization in graphene, concluding that all experimental graphene systems are ordinary 2D Fermi liquids since any doping automatically induces generic Fermi liquid behavior. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H28.00010: Inelastic Coulomb scattering of 2D graphene Euyheon Hwang, B. Y. K Hu, Sankar DasSarma The inelastic quasiparticle lifetime of 2D graphene is calculated using the full dynamically screened Coulomb interaction. We calculate the imaginary part of the quasiparticle self-energy for doped (or gated) graphene, using the $G_0W$ and random phases approximations. At low energy regimes, the intraband single particle excitation (SPE) and plasmon contribute to the self energy, but the interband SPE does not contribute to the self energy due to the phase space restrictions. At higher energies ($\omega \agt E_F$) interband SPE contribution increases sharply, overwhelming the intraband SPE and plasmon contribution. The calculated inelastic quasiparticle lifetime is significantly different from semiconductors with parabolic bands because of linear energy dispersion and chiral properties of graphene. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H28.00011: Evidence for weak antilocalization in epitaxial graphene Xiaosong Wu, Xuebin Li, Zhimin Song, Claire Berger, Walt A. de Heer Transport in ultrathin graphite on silicon carbide is graphene-like and appears to be dominated by the electron-doped epitaxial graphene layer at the interface. Weak antilocalization in 2D samples manifests itself as a broad cusp-like depression in the longitudinal resistance for magnetic fields 10 mT $<$ B $<$ 5 T. An extremely sharp weak-localization resistance peak at B = 0 is also observed. These features quantitatively agree with recent graphene weak-localization theory. Scattering contributions from charges in the substrate and from trigonal warping due to the graphite layer are tentatively identified. The Shubnikov-de Haas oscillations show an anomalous Berry's phase. Their small amplitudes may be related to graphene scattering processes. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H28.00012: Spin-polarized states in zigzag-edge graphene nanostrips John W. Mintmire, Junwen Li, Daniel Gunlycke, Carter T. White Zigzag-edge graphene nanostrips (GNSs) are known to exhibit localized edge states in the vicinity of the Fermi level. It has previously been reported that these edge states are ferrimagnetic. We present a study based on first-principle DFT and Hubbard model calculations that confirm the ferrimagnetic nature of the edge states. By comparing the results, we have estimated the Hubbard U to be approximately 2.7 eV. Energy dispersions, spin polarizations, and total energies are calculated for various widths of the nanostrips. In both our approaches, we find that the ferrimagnetic states have lower energy than the spin-restricted solution. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H28.00013: Study of spin-polarized transport in layers of graphene W.-H. Wang, K. Pi, H. Choi, P. Wei, J. Shi, R. Kawakami Electron transport in graphene layers has drawn great attention recently due to the observation of 2D behavior and relativistic dispersion in these systems. Our attention is focused on spin-polarized transport in ferromagnet(FM)/graphene/FM devices in which the FM electrodes act as spin injectors and spin detectors. Specifically, the spin-polarized transport across graphene should be manifested as a dependence of resistance on the relative alignment of the FM electrode magnetizations (i.e. spin valve effect). Few-layer graphene (FLG) are extracted from kish graphite by sonication. FLGs are then dispersed and dried onto SiO2/Si substrate with pre-patterned electrodes. Atomic force microscopy, scanning electron microscopy and optical microscopy are used to characterize topographic properties and surface quality of FLG. FM electrodes are fabricated onto selected FLG using a combination of electron beam lithography and molecular beam epitaxy deposition in ultrahigh vacuum to ensure high quality magnetic materials and interfaces. We have performed initial electrical measurements and results from our studies will be discussed. [Preview Abstract] |
Session H29: Glassy Dynamics and Jamming I
Sponsoring Units: DFDChair: Karin Dahmen, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center 303
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H29.00001: Dynamic Facilitation in Colloidal Glasses Scott V. Franklin, Eric R. Weeks Dense colloidal suspensions share many characteristics with molecular glasses and, because easily visualized, are a model system for investigating the transition to the glassy state. An important feature of glasses is the presence of spatially heterogeneous dynamics; at any given time only a small subset of particles (clusters) are significantly mobile. To explain the origin and spatiotemporal correlation of clusters, Garrahan and Chandler proposed {\it dynamic facilitation}, in which motion at one location facilitates subsequent (in time) motion at adjacent regions. We use confocal microscopy to investigate dynamic facilitation in binary mixtures of micron-sized PMMA spheres in two and three dimensions. Dynamic facilitation is identified with spatial correlations between the most mobile particles at two subsequent time intervals, a measure used previously by Vogel and Glotzer to analyze simulations of glass-forming liquids. This provides a critical test of how mobility propagates through the sample in space-time and the spatial and temporal correlation of mobile clusters. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H29.00002: Excess Vibrational Modes in Model Glasses Ning Xu, Matthieu Wyart, Andrea Liu, Sidney Nagel We performed both theoretical analysis and computer simulations to study the excess low-frequency normal modes (boson peak) for two widely-used model glasses at zero temperature. The onset frequencies for the anomalous modes from the simulations agree very well with predictions from variational calculations based on minimizing the vibrational energy cost of the lowest-frequency anomalous mode. This energy cost originates from the excess interactions per particle over $z_c$, where $z_c=2d$ is the minimum number required for mechanical stability in $d$ dimensions. The total $z$ interactions per particle are divided into two classes: $z_1$ ``stiff'' interactions determine the structure of the anomalous mode by adding extra nodes; the remaining $z-z_1$ interactions act as a perturbation and increase the vibrational energy of the mode by increasing the restoring force for displacements. Even though both glasses studied have a high number of interactions per particle, $(z_1-z_c)/z_c$ is always smaller than $0.6$, which indicates that the physics of jamming is relevant to the study of the excess normal modes in glasses. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H29.00003: Combining Coarse-Graining and Density of States Monte Carlo: Application to Ortho-terphenyl Jayeeta Ghosh, Roland Faller The non equilibrium transition from liquid to glass is a challenging problem in condensed matter physics. Various techniques have been applied to elucidate the nature of transition without reaching consensus. The relevant time scales near the glass transition are so long that Molecular Dynamics fails. There is much debate whether standard Monte Carlo succeeds can sample phase space near or below the glass transition temperature. We therefore combine advanced techniques to study the system near the glass transition temperature. Based on atomistic models of the small organic glass former Ortho-terphenyl (OTP) we develop a mesoscale model in which each phenyl ring is replaced by a single interaction center. We obtain a structurally coarse-grained model based on Boltzmann inversion of atomistic radial distribution function at various temperatures. As atomistic radial distribution functions are only weakly temperature dependant, the optimization can be performed at any temperature and can be used for a range of temperatures. It turned out that in the glassy range we need to optimize the potential below the glass transition temperature. Once we have a valid mesoscale model we apply the Wang-Landau Density of States Monte Carlo technique to find the density of states for the system. This novel Monte Carlo technique has already been applied to model glass forming materials but not yet to a chemically explicit model. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H29.00004: Geodesic path picture for slow dynamics in supercooled liquids Chengju Wang, Richard M. Stratt How does dynamics dramatically slow down with decreasing temperature in supercooled liquids? We suggest that the answer can be deduced from the geometry of the potential energy landscape. Instead of looking at real dynamical processes associated with barriers hoping, the landscape is characterized by the geodesic (shortest) paths in the \textit{energy landscape ensemble}, which was defined to include all the configurations with a potential energy less than a given value. Within our geodesic path theory, the diffusion constants depend on the typical ratio of the Euclidean distance to the geodesic path length. Computer simulations show that using only this geometric property of the landscape, one can reproduce the dramatic slow down in diffusion constants for the Kob-Andersen model, a typical glassy system. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H29.00005: Random Field Ising Model In and Out of Equilibrium Yang Liu, Karin Dahmen We present numerical studies of zero-temperature Gaussian random-field Ising model (zt-GRFIM) in both equilibrium and non-equilibrium. We compare the no-passing rule, mean-field exponents and universal quantities in 3D (avalanche critical exponents, fractal dimensions, scaling functions and anisotropy measures) for the equilibrium and non-equilibrium disorder-induced phase transitions. We show compelling evidence that the two transitions belong to the same universality class. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H29.00006: Probing large length scale behavior of spin glasses with patchwork dynamics Creighton Thomas, Olivia White, Alan Middleton Glassy systems equilibrate on timescales that are difficult to reach with direct simulation of dynamics. The characteristic length scale over which fluctuations occur grows sub-logarithmically in time, so examining aging and rejuvenation effects is problematic. In order to probe large length scale dynamics, we use ``patchwork dynamics,'' in which a patch of size $a$ is selected out of the spin glass and optimized subject to fixed boundaries provided by the neighboring spins to the patch. Using this method, we investigate the large length scale dynamics of the 2D Ising spin glass with Gaussian $J_{ij}$, as well as the random bond Ising ferromagnet, where equilibration is found on scales longer than $a$, and the critical point between these two states. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H29.00007: Aging and non-Gaussian dynamics in a colloidal glass Gianguido C. Cianci, Eric R. Weeks As a hallmark of the glassy state of matter, aging has attracted substantial attention, yet it remains a poorly understood phenomenon. It manifests itself by a dependence of the dynamical properties of the sample on the time elapsed since vitrification. The glassy state is also marked by dynamics that are heterogeneous in both time and space, and that exhibit non-Gaussian statistics over moderate to long timescales. We use a density and refractive index matched suspension of micron sized PMMA colloids as a model glassy material. At these length scales, laser scanning confocal microscopy allows us to follow the motion of a few thousand particles in real time and real space. We study the interplay between the timescales at which the dynamics are non-Gaussian and the age of the sample. We also analyze the spatial extent over which the dynamics are heterogeneous and examine the age dependence of this length scale. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H29.00008: Replica theory for fluctuations of the activation barriers in glassy systems Joerg Schmalian, Maxim Dzero, Peter Wolynes Using an effective potential approach to self generated glasses we determine the nucleation of entropic droplets in systems with random first order transition and entropy crisis. We demonstrate that fluctuations of the configurational entropy and of the liquid glass surface tension are crucial for an understanding of the barrier fluctuations in glassy systems and thus are responsible for the broad spectrum of excitations and heterogeneous dynamics in glasses. In particular we derive a relation between the length scale for dynamic heterogeneity and the related barrier fluctuations. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H29.00009: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H29.00010: Transport in Disordered Reaction-Diffusion Systems Andrew Missel, Karin Dahmen The effects of quenched spatial disorder in the reaction rates on the behavior of reaction-diffusion (RD) models have been difficult to discern, but understanding these effects is essential for predicting the behavior of any real system reasonably well-described by such a model. We present here a step towards an understanding of these effects on transport in RD systems, taking as our model a $1$D system in which particles compete ($2A\to A$) and diffuse with spatially homogeneous rates, reproduce ($A\to2A$) on certain sites (``oases''), and die ($A\to0$) on all others. We show that predictions from a simplified linear model for the first passage properties between two oases match the results of Monte Carlo simulations; these results, along with some ideas from percolation theory, can be used to make some predictions about the nature of transport across a disordered (many oases) system in higher dimensions. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H29.00011: Is microscopic description of inherent structures possible? Valentin A. Levashov, Takeshi Egami, Rachel S. Aga, James R. Morris Description of relaxation in a supercooled liquid of N particles using 3N dimensional potential energy landscape (PEL) implicitly favors the idea that the structure is too complex to be described by any microscopic local structural parameters. We addressed this issue by using atomic level stresses (ALS) introduced a while ago as local structural parameters. With MD simulations and the steepest decent method on a model of liquid iron we studied how the distributions of ALS in inherent structures (IS) depend on the original temperature. We found that the ALS of the IS clearly show the crossover and glass transition temperatures. Thus we conclude that relaxation in inherent structures could be described not only by macroscopic, but also by microscopic topological quantities (MTQ). We also found a way to relate the character of fluctuations in MTQ at real temperatures to the energies of the corresponding IS. We found that the mode-coupling temperature is located significantly below the crossover temperature, closer to the glass transition temperature. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H29.00012: Self-Organized Criticality Below The Glass Transition: A Computer Simulation Katharina Vollmayr-Lee, Elizabeth A. Baker We obtain evidence that the dynamics of a glassy system below the glass transition is characterized by self-organized criticality. To investigate the dynamics of a binary Lennard-Jones system we use molecular dynamics simulations. To study cooperative motion we define single particle jump events via single particle trajectories and identify clusters of jump events which are correlated in space and time. We find string-like clusters whose size is power-law distributed not only close to $T_{{\mbox{\scriptsize {c}}}}$ but for {\em all} temperatures below $T_{{\mbox{\scriptsize {c}}}}$, indicating self-organized criticality which is suggestive of a freezing in of critical behavior. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H29.00013: Dynamics and effective temperature for a steady-state sheared glass Thomas Haxton, Andrea Liu In a model sheared glass, the slow dynamics near the onset of jamming are shown to be controlled by a well-defined effective temperature $T_{\rm eff}$. We conduct two-dimensional nonequilibrium molecular dynamics simulations of steadily-sheared, densely-packed, bidisperse disks with soft repulsive pairwise interactions in contact with a heat reservoir. We calculate the viscosity and $T_{\rm eff}$ as functions of shear rate $\dot \gamma$ and bath temperature $T_{\rm bath}$. At $\dot \gamma=0$, the system undergoes a glass transition at $T_{\rm bath}=T_g$. We study the steady state at $\dot \gamma \ne 0$ and $T_{\rm bath} < T_g$. At low $\dot \gamma$, $T_{\rm eff}$ decreases extremely slowly with $\dot \gamma$ and is nearly independent of $T_{\rm bath}$, while the viscosity continues to increase rapidly. The dramatic change in dynamics with a gradual change in effective temperature is reminiscent of the behavior of the quiescent system as temperature is lowered towards $T_g$. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H29.00014: Observations of shear-induced clusters seen near the colloidal glass transition Dandan Chen, Denis Semwogerere, Eric R. Weeks Many studies of the glass transition focus on structural relaxation arising from thermally induced dynamics. Several of these studies observed isolated clusters of fast-moving particles. We present experimental work that finds similar heterogeneities from mechanically-induced motion applied to a dense colloidal suspension. We study micron-sized PMMA spheres with a volume fraction of approximately 50\%. The sample is subjected to shear while simultaneously the dynamics are recorded using fast confocal microscopy. From the resulting 3D movie the trajectories of the individual particles are tracked and the macroscopically imposed shear is subtracted to study the mechanically-induced non-affine particle motion. We find fast-moving cooperative clusters with pronounced motion in the shear direction. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H29.00015: System-size dependence of dynamical heterogeneity in a glass-forming liquid Chandan Dasgupta, Smarajit Karmakar Dynamical heterogeneity in supercooled liquids is often characterized by a space- and time-dependent higher-order correlation function of local density fluctuations and the corresponding susceptibility (the so-called four-point susceptibility). If the growth of this susceptibility as the temperature is decreased towards the ideal glass transition temperature of mode-coupling theory is a consequence of a growing dynamical correlation length, the dependence of this quantity on the system size should exhibit finite-size scaling behavior. We have used constant-temperature molecular dynamics simulations to study the temperature and sample-size dependence of this quantity for a well-known glass-forming liquid (the Kob-Anderson mixture). Our results show the expected finite-size scaling behavior of the four-point susceptibility in the temperature range over which it exhibits a power-law growth. However, the sample-size dependence of the time scale at which the susceptibility peaks does not exhibit conventional finite-size scaling, possibly indicating the presence of effects not captured in mode-coupling theory. [Preview Abstract] |
Session H30: Liquid Crystals: Experiments and Theory
Sponsoring Units: DFDChair: Robin Selinger, Kent State University
Room: Colorado Convention Center 304
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H30.00001: Nonphotochemical Laser Induced Nucleation From a Supercooled Thermotropic Liquid Crystal Xiaoying Sun, Bruce Garetz, Michele Moreira, Peter Palffy-Muhoray A nonphotochemical laser induced phase transition was studied in a supercooled 4'-n-pentyl-4-cyanobiphenyl(5CB, also referred to as PCB and K15 ) liquid crystal system, using linearly polarized ps pulses from a Nd:YAG laser at a wavelength of 532nm. The result shows that light could induce nucleation from the metastable supercooled isotropic phase to the nematic phase in the case of a slow cooling rate and high laser intensity. The director of the induced nematic phase tends to align along the direction of polarization of the light. At the intensities used, there is no observable reorientation of the director once it is in the nematic phase. These experimental results are consistent with a mechanism based on optical Kerr alignment. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H30.00002: Stressed liquid crystals for fast phase retardation switching Anatoliy Glushchenko, Kevin Wood, Alex Rockwood, John West, Guoqiang Zhang, Ke Zhang Liquid crystalline materials are an attractive medium for many devices because they produce large electrically controllable shifts of the phase retardation. Relatively thick (10-100 micron) liquid crystal layers are needed for modern devices. However, the speed of these devices is inversely proportional to the square of the LC layer thickness. We must therefore decouple the speed of switching and thickness of the layer. Here we report on the development of stressed liquid crystals (SLCs) to achieve this goal. These new materials consist of a stressed polymer network that imposes unidirectional alignment of a liquid crystal matrix. We induced the stress by shearing the material. As expected from earlier research in PDLCs, shearing greatly reduces the switching time. Unlike the PDLCs the scattering of the films decreases drastically upon shearing. Indeed the SLC materials are optically clear throughout the visible and infrared. The SLC materials are also characterized by essentially no hysteresis and a completely linear voltage response. This greatly simplifies design of the electronics and driving schemes driving. The SLC's performance is determined by the component materials used, their relative concentration and the preparation conditions. We have produced SLC cells that produce almost 20 microns of phase retardation, that switch on and off in several milliseconds. In general the SLC cells require a driving voltage of about 1V/ $\mu $m. The SLCs produce the largest phase retardation at the fastest speeds. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H30.00003: Orientational multiplicity and transitions in liquid crystalline droplets Rajesh Goyal, Morton Denn Orientation distributions in droplets of liquid crystals with homeotropic anchoring are computed with a simulated annealing algorithm that minimizes the free energy of the Oseen-Frank continuum theory. The droplets exhibit multiple orientational steady states that are separated by finite energy barriers over the entire range of the dimensionless ratio of surface to elastic forces, with maximum transition energy densities of the order of 2,000 Pa for a typical liquid crystalline droplet with a spherical radius of 1 micron. The transition energy densities decrease with elongation to spheroidal droplets with aspect ratios of four or more, indicating that droplet elongation is favored to drive surface-induced transitions. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H30.00004: Optical Activity Produced by Layer Chirality in Bent-Core Liquid Crystals Loren Hough, Chenhui Zhu, Michi Nakata, Nattaporn Chattham, Gert Dantlgraber, Carsten Tschierske, Noel Clark Recent observations of large optical activity in chiral smectic liquid crystalline phases formed from achiral bent-core molecules have been attributed to both a helical superstructure and to layer optical chirality (LOC). The LOC model predicts that optical activity is produced by the local chiral layer structure formed by the simultaneous tilt and polar ordering of bent-core molecules (Hough and Clark PRL, 95, 107802 (2005)). The LOC model predicts that optical activity should be present in the well ordered B2 phase. However, in most materials, the optical activity is masked by birefringence. We studied the SmC$_{A}$P$_{A }$subphase of GDa104 (Dantlgraber, et al. Chem. Mater. 14, 1149 (2002).), which has a tilt angle of $\sim $45 degrees (orthoconic), and thus very low birefringence. In this system, we directly demonstrate that layer chirality produces optical activity consistent with the LOC model. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H30.00005: Observation of a possible tetrahedratic phase in bent-core David Wiant, Krishna Neupane, Sunil Sharma, Antal Jaklu, James Gleeson, Samael Sprunt Combined measurements of optical birefringence induced by high magnetic fields and dynamic light scattering have been performed on bent-core liquid crystals at temperatures above their clearing points. The results of these measurements provide compelling evidence of a phase transition between two optically isotropic phases that is consistent with the development of a novel ``tetrahedratic" form of orientational order. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H30.00006: Giant flexoelectricity of bent-core nematic liquid crystals John Harden, Badel Mbanga, Nandor Eber, Katalin Fodor-Csorba, Samuel Sprunt, James Gleeson, Antal Jakli Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLC's gigantic (more than three orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic (BCN) materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H30.00007: Bent-core fiber structure: Experimental and theoretical studies of fiber stability C. Bailey, E. Gartland, A. Jakli Recent studies have shown that bent core liquid crystals in the B7 and B2 phases can form stable freestanding fibers with a so called ``jelly-roll'' layer configuration, which means that the smectic layers would be arranged in concentric cylindrical shells. This configuration shows layer curvature is necessary for fiber stability. Classically this effect would destabilize the fiber configuration because of the energy cost of layer distortions and surface tension. We propose a model that can predict fiber stability in the experimentally observed range of a few micrometers, by assuming that layer curvature can be stabilized by including a term dealing with the linear divergence of the polarization direction if the polarization is allowed to have a component normal to the smectic layers. We show that this term can stabilize the fiber configuration if its strength is larger than the surface tension. We also propose an entropic model to explain the strength of this term by considering steric effects. Finally we will take results from this model and apply them to better understand experimental findings of bent-core fibers. Financial support by NSF FRG under contract DMS-0456221. Prof. Daniel Phillips, Particia Bauman and Jie Shen at Purdue Univ., Prof. Maria Carme Calderer at Univ. of Minnesota, and Prof. Jonathan Selinger at Kent State Univ. Liou Qiu and Dr. O.D. Lavrentovich, Characterization Facilities, Liquid Crystal Institute, Kent State Univ. Julie Kim and Dr. Quan Li, Chemical Synthesis Facilities, Liquid Crystal Institute, Kent State Univ. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H30.00008: Field-induced phase transitions in SmCP phases of bent-core liquid crystals S. Dhakal, Jonathan V. Selinger Liquid crystals composed of bent-core molecules have been studied extensively because they exhibit a wide variety of phases with potential applications. The smectic phases of these molecules have polar order in the layer plane due to the close packing. If the molecules are tilted with respect to the layer normal, the combination of tilt and polar order gives the layer chirality. The resulting liquid crystal can be either chiral or racemic (antichiral), even if the molecules are achiral. The reversibility of these two states with the application of electric field implies the possibility of making light shutters from antiferroelectric liquid crystals of bent-core molecules [1]. Extending an earlier model of chiral order [2], we develop a phenomenological theory involving three order parameters: chirality ($\chi )$, polarity (P) and tilt ($\theta )$ within each layer. By minimizing the free energy, we predict multiple phase transitions (SmC$_{A}$P$_{A}\to $SmC$_{S}$P$_{F}$, SmC$_{A}$P$_{A}\to $ SmC$_{A}$P$_{F})$ as a function of electric field, consistent with experiments. [1] A. Jakli \textit{et al}., \textit{Liq. Cryst.} \textbf{29}, 377 (2002). [2] J.V.Selinger, \textit{Phys. Rev. Lett.} \textbf{90,} 165501(2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H30.00009: Theory of ``scar'' defects in liquid-crystalline films Jonathan V. Selinger, Zhao Lu Recently, several researchers have studied crystalline order on the surface of a sphere, both theoretically [1] and experimentally [2]. In this system, one might expect to see twelve point disclinations, as required by topology. Instead, they find that the point disclinations extend into ``scar'' defects, which are finite grain boundaries. Our goal is to determine how general is the phenomenon of scar formation. Does it require crystalline order, and does it require curvature? For that reason, we investigate an xy model in a flat disk geometry, where boundary conditions require a total topological charge of +1, i.e. the vector order parameter must rotate through a total angle of 2$\pi $. In the classical xy model, the ground state would have a single vortex of charge +1. However, for certain slight variations on the xy model, the ground state has a scar defect, which looks like two vortices of charge +1/2 connected by an orientational domain wall. The formation of scars depends on details of the interaction energy in a lattice or continuum system. We discuss possible opportunities to observe these scar defects in experiments. [1] M. J. Bowick et al., \textit{Phys. Rev. B} \textbf{62}, 8738 (2000). [2] A. R. Bausch et al., \textit{Science} \textbf{299}, 1716 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H30.00010: Semi-soft Nematic Elastomers and Nematics in Crossed Electric and Magnetic Fields Fangfu Ye, Ranjan Mukhopadhyay, Olaf Stenull, Tom Lubensky Ideal nematic elastomers exhibit soft elasticity in which an elastic modulus of a uniaxial solid vanishes and in which it takes no stress to produce strains, up to a critical strain, in the direction perpendicular to the uniaxial direction. Elastomers crosslinked while stretched exhibit semi-soft elasticity with a nearly linear stress-strain curve at small strain followed by a nearly flat stress-strain relation up to a critical strain. Semi-soft elasticity above the so-called mechanical critical point, which terminates the line of coexistence of nematic phases, has not been established. We calculate global phase diagrams and stress-strain curves for a series of models, including a generalization of de Gennes-Maier-Saupe (dGMS) model and the neolclassical model for nematic elastomers, and demonstrate that semi-soft behavior can persist well above the mechanical critical point. The dGMS model also describes a normal nematic in perpendicular electric and magnetic fields and exhibits a Blume-Emery-Griffiths tricitical point topology. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H30.00011: Simulation studies of liquid crystal elastomers: response to light Robin Selinger, Jonathan V. Selinger, Badel L. Mbanga Azo-dye doped nematic elastomers bend and flex when subject to photoexcitation. We model this mechanical response at the continuum level using nonlinear finite element simulation. Our finite element algorithm uses explicit dynamics based on a Hamiltonian which couples mechanical strain and nematic order. Our explicit dynamics algorithm is structured essentially like molecular dynamics, and we discuss how this continuum level code can be easily integrated into a multiscale model. We use the model further to explore potential applications of nematic elastomers in microfluidics, fiber orientation control, and biologically-inspired robots. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H30.00012: Simulation studies of liquid crystal elastomers: soft elasticity Badel Mbanga, Jonathan V. Selinger, Robin Selinger Liquid crystal elastomers combine the elastic properties of rubbers with the order inherent in nematic liquid crystals. Stretching a monodomain strip of nematic elastomer in a direction transverse to the nematic director results in an energy-free rotation of the director, giving rise to a soft elastic response. In building a simulation model of this mechanism, we consider the limit in which the orientational order equilibrates rapidly compared to the strain, so that the local order tensor remains in continuously evolving quasi- static equilibrium as the strain relaxes. The order tensor in each volume element is determined by minimizing a free energy functional in the form of a Landau expansion that includes a term coupling the local orientational order with the local strain. The strain evolves via nonlinear finite element explicit dynamics. We intend through this model to further our understanding of the basic physics governing the dynamic mechanical response of nematic elastomers and also provide a useful computational tool for design and testing of potential engineering device applications. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H30.00013: A dynamical approach for liquid crystal simulations Jones Wan A novel, efficient simulation scheme is proposed to determine the liquid crystal configurations under complex physical environments. In this simulation scheme, the liquid crystal elastic energy, the electrostatic interaction, and the surface anchoring effect are calculated by a dynamical approach, which is analogous to molecular dynamics simulation. As a result, various techniques established for molecular dynamics are readily adopted to the proposed scheme. We demonstrate the new method by calculating the director field of a liquid crystal under the effects of an external electric field and patchy pattens. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H30.00014: Smectic Defects with Riemann Reason Elisabetta Matsumoto, Randall Kamien, Christian Santangelo Minimal surfaces are natural starting points when considering smectics because they minimize the bending energy of the layers. Alternatively, ``sums'' of screw dislocations have been used to model twist-grain boundaries, and often lead to layers with the same topology as classic minimal surfaces. We use Riemann's minimal surface as a model for a smectic whose layers are joined by pores. We evaluate the energetics of this surface using an explicit phase field representation in terms of elliptic functions. We also build a surface with the same topological structure as Riemann's minimal surface using a configuration of oppositely-charged screw dislocations. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H30.00015: Angular momentum transport in soft anisotropic matter Mark Warner, Peter Palffy-Muhoray, Michael Shelley, Xiaoyu Zheng If an anisotropic crystal is placed in a magnetic field, it will tend to rotate into alignment as a rigid body. An anisotropic sample of soft matter, such as a liquid crystal or a liquid crystal elastomer, will behave differently; initially, only the anisotropic constituents rotate about their centers of mass, then interactions between them give rise to material currents and eventually to rigid body rotation. Since the external field exerts only a body torque -- but no force - on the sample, it is interesting to ask how the forces which drive translational motion arise. To gain insight into this problem, we consider the response of a cylindrical liquid crystal sample in a magnetic field in a geometry where both the director and the applied field are perpendicular to the cylinder axis. We solve the equations of motion for low molecular weight liquid crystals and for liquid crystal elastomers and trace the flow of angular momentum in order to understand the details of the dynamics. [Preview Abstract] |
Session H31: Transport in Carbon Nanotubes: Theory
Sponsoring Units: DCMPChair: Saiful Khondaker, University of Central Florida
Room: Colorado Convention Center 401
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H31.00001: First-principles studies of electrical transport in metal-contacted semiconducting carbon nanotubes. Juan Palacios, P Tarakeshwar, Dae Kim We present first-principles calculations of the transport properties of semiconducting carbon nanotubes (CNT's), coupled to metallic electrodes. Our results indicate that, for realistic end-contact geometries, including atomic relaxation, the Fermi level position within the gap differs between palladium-contacted CNT's and gold-contacted CNT's. More interestingly, the contact resistance for the valence band in the case of Pd is much smaller than in the case of Au, while no significant difference is observed for the conduction band. This could explain experimental results showing that hole conduction is favored in the case of Pd contacts. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H31.00002: First Principle Study of Electronic Transport in Carbon Nanotubes and Copper Nanowires for Interconnect Applications Yu Zhou, Yiming Zhang, Subbalakshmi Sreekala, Pulickel Ajayan, Saroj Nayak We will present our recent first principles calculation modeling work on carbon nanotubes (CNT) and copper wires for Interconnect applications. In particular we have calculated the ballistic transport properties of nanotubes based on their density of states and band structures, and compared with that of copper wires of similar dimension. By using Ohm's law and Landauer Formalism, we computed the resistance of them in mesoscopic sizes. The effect of correlation in the transport properties are discussed in detail. We will present our work on the nanowires and nanotubes packing and their impact on the resistance, while taking into account the surface scattering based on Fuchs-Sondheimer model. The performance of CNT for both local and global Interconnects will be discussed in detail. Our results show that nanotube bundles can outperform copper wires for long intermediate and global interconnects. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H31.00003: Interwall interactions and electrical conductance in telescoping carbon nanotubes Yong-Ju Kang, Yong-Hoon Kim, Kee Joo Chang Telescopically aligned carbon nanotubes, where the inner core shells are pulled out from the house shells with larger diameters in multi-walled nanotubes, are good systems to interwall interactions and their effect on electron conduction. In several tight-binding calculations, there exists some controversy in the quantum conductance of telescoping nanotubes. In this work, using the non-equilibrium matrix Green function approach within the first principles local-density-functional approximation, we study the quantum transport behavior of the (5,5)/(10,10) telescoping nanotube. Varying the hybridized double wall region, we investigate the effect of interwall interactions on the electron transport and compare the results with those obtained from tight-binding calculations. Although individual tubes have two conducting channels at the Fermi level, only one channel gives rise to electrical conduction with antiresonance dips in transmission, while the other channel is suppressed. Thus, the maximum conductance is close to $G_{0}$, in contrast to single $\pi $-orbital tight-binding calculations, which showed the maximum conductance close to 2$ G_{0}$. Our first-principles calculations indicate that the tight-binding model significantly overestimates the interwall coupling between the inner and outer shells. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H31.00004: Quantum transport in carbon nanotube field effect transistors Kalman Varga, S.T. Pantelides We have investigated the transport properties of carbon nanotube field effect transistors using the recently developed source-and-sink method [1]. We report first-principles results on the current-voltage characteristics of semiconducting carbon nanotubes in transverse electric field, highlighting differences with Si-based devices, e.g., band mixing caused by the gate electric field. We also find that the source-drain current exhibits an intrinsic saturation as function of the gate voltage. The calculated results are in good overall agreement with pertinent experiments. \newline \newline [1] K. Varga and S. T. Pantelides, Phys. Rev. Lett. submitted for publication. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H31.00005: Signature of the electron-electron interaction in the magnetic field dependence of the nonlinear I-V characteristics in non-centrosymmetric conductors Eric Deyo, Boris Spivak, A. Yu. Zyuzin In non-centrosymmetric media, there exists a contribution to the nonlinear I-V characteristics which is linear in magnetic field and quadratic in voltage. This effect is entirely due to electron-electron interaction, and its magnitude is proportional to the electron-electron interaction constant. We present calculations of the magnitude of this effect in mesoscopic samples and in chiral carbon nanotubes as a function of temperature. In the case of a magnetic field oriented parallel to plane of a mesoscopic sample, the effect is proportional to both the electron-electron interaction constant and the spin-orbit scattering amplitude. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H31.00006: \textit{Ab initio} investigations of formation of the poly-bromine anions encapsulated inside the carbon nanotube. Dongchul Sung, Noejung Park, Suklyun Hong We have performed \textbf{\textit{ab initio}} density-functional calculations to investigate the electronic and geometric structure of the bromine adsorbates inside the carbon nanotube. It is found that the charged odd-membered molecular species (Br$_{3}$ or Br$_{5}$ ) are energetically favored inside the carbon nanotube rather than common Br$_{2}$ molecule. Vapor phase of bromine molecules (Br$_{2})$ could exothermically adsorb into the nanotubes, and in turn, transform into the Br$_{3}$ or Br$_{5}$ structures without a significant energy barrier. Such a formation of the poly-bromine anions accompanies a strong charge transfer from the nanotube to the adsorbates, rendering the encapsulating nanotube strongly hole-doped. We suggest that an exposure of the tip-opened carbon nanotube samples to a modest Br$_{2 }$partial pressure could result in strong hole-doped, and thus nearly metallic nanotube samples. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H31.00007: Electron transport through molecular-carbon nanotube interfaces Nicolas Bruque, Rajeev Pandey, Khairul Alam, Roger Lake Investigations have focused on electron transport through metal-molecule systems. Less effort has been directed towards semiconductor-molecule systems, and the least attention has been given to electron transport through carbon nanotube-molecule systems. A specific implementation of the latter system consists of two CNTs joined by a molecule, or a CNT-molecule-CNT system. Such a system can provide the electronic functionality of a resonant tunnel diode. The molecular contacts, i.e. the CNTs, are a $\pi $-bond surface and, as such, they are both chemically and geometrically different from metal contacts or sp$^{3}$ semiconductor contacts. A model system is studied to focus solely on the interface geometry of two simple $\pi $-bond systems, CNTs and polyacetylene (CH)$_{n}$. The system is CNT-(CH)$_{n}$-CNT. At the interface, in the relaxed structure, the (CH)$_{n}$ is oriented coplanarly with the tangential plane of the CNT. The transmission, calculated with our DFT (FIREBALL)-NEGF code is, on average, 3 or more orders of magnitude larger than the transmission of an unrelaxed structure in which the (CH)$_{n}$ is perpendicular to the CNT at the point of contact. This is also true when the (CH)$_{n}$ of the relaxed structure undergoes a 180$^{o}$ twist. Interface geometry plays a crucial role in the electron transport. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H31.00008: ABSTRACT HAS BEEN MOVED TO V31.00015 |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H31.00009: Effects of Electron-phonon scattering on Conductance of Carbon nanotubes using Time-dependent wave-packet approach Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose The application of single-walled carbon nanotubes as the ideal ballistic conductors is expected. However, the electronic current saturates at the high-bias regime due to electron-phonon scattering. In order to improve the conductivity, understanding of the scattering mechanism is highly required. We investigated the electron-phonon coupling effect on the conductance in single-walled carbon nanotubes using the time-dependent wave-packet approach under a tight-binding approximation [1]. The vibrational atomic displacements in real space are introduced through the time-dependent change of the transfer energies. We solve the time-dependent Schr\"odinger equation and obtain the time-dependent diffusion coefficients of the electronic wave packets. From these data, we can extract the coherence length and then the conductance. We found that the optical phonon decreases the conductance of metallic carbon nanotubes, because the propagating speed of electron is reduced by the electron-phonon scattering. Furthermore, we clarify the difference of the scattering effects on the conductivity of the metallic nanotube and the semiconducting one. [1] S. Roche \textit{et al.}, PRL 95 (2005) 076803 [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H31.00010: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H31.00011: Model Calculation for Molecular Junctions in between Carbon Nanotube Leads. Yiing-Rei Chen, Lei Zhang, Mark Hybertsen We present analytical and numerical calculations for several prototypes of molecules in a CNT-molecule-CNT junction. The properties of the transmission function at the Fermi level reveal the influence from the CNT symmetry and the nature of the molecules. In particular, we discuss and compare both one-point contact and two-point contact cases, so as to illustrate how the Fermi level transmission of a multi-molecule junction can be either larger or smaller than that in the single-molecule case, depending on the choice of contact sites. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H31.00012: Quantum electron transport in toroidal carbon nanotubes with metallic leads. Mark Jack, Mario Encinosa Carbon nanotubes and carbon nanotori possess all the interesting new electronic features seen in graphene e.g. massless Dirac fermion characteristics, small spin-orbit coupling effects, and quantized conductance, along with interesting curvature and boundary condition effects closing the tube to form a torus. The authors calculate electronic transport properties such as density-of-states and transmissivity for toroidal carbon nanotubes with attached metallic or carbon nanotube leads as functions of the lead positions. A tight-binding Hamiltonian for the nanotorus is applied to a 24-carbon-atom armchair unit cell. The closure of the straight tube to a toroidal geometry introduces an additional off-diagonal coupling term, not encountered for the straight case. The device Green's function is then evaluated in tight-binding approximation using a recursion method to systematically determine its diagonal and off-diagonal matrix elements. {\it References:} 1. M. Encinosa and M. Jack, Phys. Scr. 73 (2006) 439-442. 2. M. Encinosa and M. Jack, Excitation of surface dipole and solenoidal modes on toroidal structures. Photonics and Nanostructures (Elsevier), May 2006. (Submitted) 3. M. Encinosa and M. Jack, Dipole and solenoidal magnetic moments of electronic surface currents on toroidal nanostructures. J. of Computer-Aided Materials Design (Springer), May 2006. (In Press) [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H31.00013: Mechanical and electronic properties at the interface between the Si(100) surface and semiconducting carbon nanotubes Salvador Barraza-Lopez I discuss the \emph{ab initio} mechanical and electronic properties of semiconducting carbon nanotubes adsorbed on the Si(100) surface. After revising results from nanotubes on the fully unpassivated surface[1], the interaction between a semiconducting nanotube and a fully H-passivated Si(100) surface with dopants is examined[2]. As silicon wafers are ordinarily doped, the model closely resembles experimental onditions[2,3,4], allowing for qualitative comparison. The single H-monolayer prevents electronic states in nanotubes from energetically shifting along with those of the doped supporting substrate, permitting the engineering of the relative positions of the slab and nanotube band edges. Finally, and following experimental work, we study adsorption characteristics of nanotubes on partially passivated surfaces. Surface states in the unpassivated regions modify the electronic structure of the interface and provide for the anchoring of nanotubes, deforming them in some cases. Results with and without dopants will be given[2].\\ 1 S. Barraza-Lopez et al. J. Appl. Phys. (in press). 2 Submitted. 3 Appl. Phys. Lett 83, 5029 (2003). 4 P. M. Albrecht and J. W. Lyding, Small (in press). [Preview Abstract] |
Session H32: Quantum Fermi Gases
Sponsoring Units: DAMOPChair: Cindy Regal, University of Colorado/JILA
Room: Colorado Convention Center 402
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H32.00001: Superfluid-Insulator Transition of Fermions in Optical Lattices across a Feshbach Resonance Hui Zhai, Tin-Lun Ho We study superfluid-insulator (SI) transition of fermions in an optical lattice as a function of scattering length and fermion density. For systems with two fermions (hence one boson) per site, SI transition is the usual Mott transition between bosonic molecules on the BEC side of resonance. On the BCS side, the insulating phase is the band insulator. SI transition is caused by the energy gain in promoting two fermions in valance band to various valance bands to form Cooper pairs. This phenomena become even more intriguing at higher fermion densities. In this talk, we shall present the phase diagram for SI transition across a Feshbach resonance for different densities. Our results directly related to the recent MIT experiment on SI transition of lattice fermions with two fermion per site. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H32.00002: p-wave Feshbach Molecules. John Gaebler, J. T. Stewart, J. L. Bohn, D. S. Jin We present evidence for the production and detection of molecules using a p-wave Feshbach resonance between 40K atoms. We have measured the binding energies and lifetimes for these molecules. We find that the binding energies scale linearly with magnetic field near the resonance. At magnetic fields above the resonance we detect quasi-bound molecules with lifetimes set by the tunneling rate through the centrifugal barrier. We discuss the possibility of using a p-wave Feshbach resonance to study BEC-BCS crossover physics with finite angular momentum pairing. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H32.00003: Quantum Phase Transitions of Ultra Cold Gases in the Fermi-Bose Hubbard Hamiltonian D. G. Schirmer, L.D. Carr, I. Danshita, J.E. Williams, Charles Clark The experimental realization of ultracold fermions has stimulated work on theoretical models of zero-temperature quantum phase transitions and the BCS-BEC crossover. Ultracold gases confined in optical lattices can demonstrate a wide range of different phases by varying controllable system parameters, such as optical lattice intensity, particle number, spin composition and atomic interactions. We perform numerical studies of a Fermi-Bose-Hubbard Hamiltonian with the Vidal algorithm (Time Evolving Block Decimation). Our Hamiltonian treats a one dimensional system of fermions coupled to a bosonic molecular state, as occurs in Feshbach resonances, and encompasses a very large parameter space. We present the quantum phase diagram, focusing on small systems and the most experimentally relevant parameters. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H32.00004: Destruction of $p$-wave weakly bound molecules in a gas of spin-polarized fermions Jose P. D'Incao, Chris H. Greene We have studied collisional aspects which might affect the lifetime of $p$-wave molecules created in ultracold spin-polarized fermi gases near a Feshbach resonance [1]. Atom-molecule inelastic collisions might be the main process in which the collision products can be released from typical traps. Our study allows us describe the dependence of the collision rates on the $p$-wave scattering length, which is crucial for understanding the stability of such molecules in the strongly interacting regime. [1] H. Suno, B. D. Esry, and C. H. Greene, Phys. Rev. Lett. 90, 053202 (2003). This work was supported in part by the National Science Foundation. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H32.00005: The potential energy of a $^{40}$K Fermi gas in the BCS-BEC crossover John Stewart, John Gaebler, Cindy Regal, Deborah Jin We present a measurement of the potential energy of an ultracold trapped gas of $^{40}$K atoms in the BCS-BEC crossover and investigate the temperature dependence of this energy at a wide Feshbach resonance, where the gas is in the unitarity limit. In particular, we study the ratio of the potential energy in the region of the unitarity limit to that of a non-interacting gas, and in the $T=0$ limit we extract the universal many-body parameter $\beta$. We find $\beta = -0.54^{+0.05}_{-0.12}$; this value is consistent with previous measurements using $^{6}$Li atoms and also with recent theory and Monte Carlo calculations. This result demonstrates the universality of ultracold Fermi gases in the strongly interacting regime. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H32.00006: Phase separation in a mixture of two species of fermionic atoms in one-dimensional optical lattice Shi-Jian Gu, Rui Fan, Hai-Qing Lin In this work, we study the ground-state phase diagram of a mixture of two species of fermionic atoms in one-dimensional optical lattice, as described by an asymmetric Hubbard model. We investigate the quantum phase transition from density wave to phase separation by studying both the corresponding charge order parameter and quantum entanglement, and present phase diagram as function of band-filling. A rigorous prove, that even for the case of a single hole doping, the density wave is unstable to the phase separation in the infinite U limit, will be presented. We also discuss experimental feasibility of observing such a phase separation. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H32.00007: Natural orbits of atomic Cooper pairs in a nonuniform Fermi gas Y.H. Pong, C.K. Law We present the natural orbits of atomic Cooper pairs in an inhomogeneous Fermi gas. These orbits provide the pairing mode functions of constructing BCS states in finite systems. We further exploit such orbits to study Cooper pair wave functions in various trapping situations. In particular, we quantify and characterize the quantum entanglement between atoms in a Cooper pair associated with the spatial degrees of freedom. \newline (Reference : Y.H. Pong, C.K. Law, Phy. Rev. A \textbf{74}, 013618 (2006)) [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H32.00008: Spontaneous Magnetization of Harmonically Trapped Ultracold Fermions in an Optical Lattice R.C. Brown, L.D. Carr, I. Danshita, J.E. Williams, Charles W. Clark We use a single-band Fermi Hubbard Hamiltonian to study the ground states of a system of ultracold fermions in a one dimensional optical lattice with an external harmonic trap. We perform simulations using exact diagonalization for small systems with one to five wells and we employ Vidal's algorithm (Time Evolving Block Decimation) for larger systems with up to a hundred wells. As the trapping frequency increases we observe spontaneous transverse magnetization at the edges of the trap. We present a theoretical interpretation of this intriguing result, and discuss how it can be observed in experiments. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H32.00009: Spin Drag and Spin-Charge Separation in Cold Fermi Gases Marco Polini, Giovanni Vignale Low-energy spin and charge excitations of one-dimensional interacting fermions are completely decoupled and propagate with different velocities. These modes however do not live forever and can decay due to several possible mechanisms, even in the complete absence of impurities. In the spin channel the main mechanism of decay at finite temperature is related to a distinctive mechanism of friction that dominates spin transport: the spin drag. In this work we show how two component cold Fermi gases confined inside a tight atomic waveguide offer the unique opportunity to measure directly the spin-drag relaxation time that controls the broadening of a spin packet. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H32.00010: Dynamics of particle density and noise correlators of a cold Fermi system expanding from a harmonic trap Pavel Nagornykh, Victor Galitski We have studied dynamics of an atomic Fermi system with a finite number of particles $N$ after it is released from a harmonic trapping potential. We consider two different initial states: the Fermi sea state and the projected BCS (PBCS) state described by the projection of the grand-canonical BCS wave function onto the subspace with a fixed number of particles. In the former case, we derive exact and simple analytic expressions for the particle density $n({\bf r},t)$ and density-density correlation functions $\left\langle n({\bf r},t) n({\bf r}',t)\right\rangle$ taking into account the level quantization and possible anisotropy of the trap. In the latter case of the PBCS state, we obtain analytic expressions for the density and its correlators in the leading order with respect to the ratio of the trap frequency and the superconducting gap (the ratio assumed small). We discuss several interesting dynamic features, which may be used to distinguish between the Fermi sea and BCS states. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H32.00011: Pairing with multiple flavors of fermions in ultracold atoms Robert Cherng, Gil Refael, Eugene Demler We use Ginburg-Landau formalism to discuss s-wave pairing in ultracold Fermi gases with N flavors and with SU(N) symmetric interactions. We show that when the number of flavors is greater than two, the uniform superfluid state is unstable since the magnetization or flavor imbalance couples directly to the superfluid order parameter. We study the case of three flavors in detail to analyze the competition between phase separation and non-uniform FFLO-like superfluid states. Implications of our results for experiments will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H32.00012: Two-species fermion mixtures with mass and population imbalance Menderes Iskin, Carlos Sa de Melo We analyze the phase diagram of uniform superfluidity for two-species fermion mixtures from the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) limit as a function of the scattering parameter and population imbalance. We find at zero temperature that the phase diagram of population imbalance versus scattering parameter is asymmetric for unequal masses, having a larger stability region for uniform superfluidity when the lighter fermions are in excess. In addition, we find topological quantum phase transitions associated with the disappearance or appearance of momentum space regions of zero quasiparticle energies. Lastly, near the critical temperature, we derive the Ginzburg- Landau equation, and show that it describes a dilute mixture of composite bosons and unpaired fermions in the BEC limit. [Preview Abstract] |
Session H33: Focus Session: Superconducting Qubits I
Sponsoring Units: GQIChair: Frank Wilhelm, University of Waterloo
Room: Colorado Convention Center 403
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H33.00001: A quantum trajectory approach to circuit QED Invited Speaker: Circuit QED is a promising area of research as it opens the possibility to realize the extreme strong coupling limit of cavity QED using superconducting electrical circuits. In this regime, quantum effects become dominant and a full quantum treatment of the system is essential. In this talk, I will show how continuous-in-time measurement theory can be used to obtain a quantum trajectory description of the qubit evolution conditioned on the measurement output. This conditional quantum trajectory describes the quantum non-demolition measurements used by the Schoelkopf group at Yale. Using this approach, we investigate measurement-induced dephasing, the effect of relaxation on measurement fidelity and the extreme strong coupling limit where effects such as number splitting occur. This description also opens the door to the investigation of non-linearities present in the Yale expriment. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H33.00002: Theoretical and Experimental Studies of Circuit QED Systems (Part I) Matteo Mariantoni, Frank Deppe, Rudolf Gross The formalism of circuit QED explains the coupling between a superconducting qubit (charge or flux) and a microwave resonator. Instead of focusing on the well-known resonant and dispersive regimes, we investigate a deeply dispersive regime, where qubit and resonator are strongly detuned and the transition frequency of the resonator is almost negligible compared to the qubit one. This regime has been exploited experimentally in several different implementations, e.g., the reading-out of a superconducting qubit by means of a low frequency resonator. In this framework, we have developed a simple formalism which encompasses the many explanations given in the literature on the experiments mentioned above. Furthermore, our results shed new light on decoherence studies of dephasing mechanisms due to low frequency noise as well as photon noise. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H33.00003: Theoretical and Experimental Studies of Circuit QED Systems (Part II) Frank Deppe, Matteo Mariantoni, Shiro Saito, Takayoshi Meno, Kouichi Semba, Hideaki Takayanagi, Rudolf Gross In recent years, the interaction between superconducting qubits and on-chip microwave resonators has been investigated in theory and in experiment. We performed microwave spectroscopy on a system composed of a superconducting flux qubit and the single mode of an LC circuit resonator. The latter is formed by the shunting capacitance and its associated line inductance of the dc SQUID used to read-out the qubit state. Our implementation provides a counterpart to experiments in which the state of the microwave field is detected. The data shows clear evidence of the coupled system (coupling constant: few tens of MHz). Simulations of a dissipationless driven Jaynes-Cummings-like model allow us to estimate the effective number of photons present in the resonator. One possible interesting application would be the generation of microwave single photons. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H33.00004: Phase Purcell Effect and the Crossover to Strong Coupling in Dispersive Circuit QED Ioana Serban, Enrique Solano, Frank Wilhelm We study the decoherence of a superconducting qubit due to the dispersive coupling to a damped harmonic oscillator. We go beyond the weak qubit-oscillator coupling, which we associate with a {\it phase Purcell effect}, and enter into an unexplored decoherence regime, solving a theoretical inconsistency in existing models: the divergence of the qubit dephasing rate in the absence of environment. Our results can be applied, with small adaptations, to a large variety of other physical systems, e.g. trapped ions and cavity QED, boosting theoretical and experimental decoherence studies. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H33.00005: Single microwave photon source using circuit QED Alexandre Blais, Jay Gambetta, Clifford Cheung, Andreas Wallraff, David Schuster, Steven Girvin, Robert Schoelkopf In circuit QED, a superconducting charge qubit is fabricated inside a high-quality factor transmission line resonator [1]. This opens the possibility to probe the regime of strong coupling of cavity QED using microwave photons and artificial atoms. This physics was experimentally investigated in the resonant [2] and in the dispersive [3] regime. In this talk, we will show how this setup could be used to generate single microwave photons on demand. Moreover, by using a different circuit QED layout with two resonators, photon generation could be tagged. The possibility to use a similar setup to detect single photons will also be discussed. [1] A. Blais, R.-S. Huang, A. Wallraff, S. M. Girvin and R. J. Schoelkopf, Phys. Rev. A \textbf{69}, 062320 (2004). [2] A. Wallraff, D. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf, Nature \textbf{431}, 162 (2004). [3] D. I. Schuster, A. A. Houck, J. A. Schreier, A. Wallraff, J. M. Gambetta, A. Blais, L. Frunzio, B. Johnson, M. H. Devoret, S. M. Girvin and R. J. Schoelkopf, Cond-mat/0608693. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H33.00006: Dispersive single photon non-linear optics with circuit QED David I. Schuster, Steven M. Girvin, Robert J. Schoelkopf Circuit quantum electrodynamics couples a superconducting qubit to a high quality factor microwave cavity[1]. The strong coupling limit is reached when the cavity is resonant with the qubit, and the interaction between them dominates over decoherence[2]. If in addition, when the qubit and cavity are far off resonance, the dispersive frequency shifts are still larger than the decay rates, then the strong dispersive limit will be reached. In this regime, the qubit absorption spectrum resolves into individual photon number peaks[3]. The dual of this photon numbersplitting is that the cavity inherits some of the non-linearity of the qubit. This inherited non-linearity can be used to create photonic qubits and create quantum states of light in the cavity. [1] Blais, et. al. PRA, 2004, 69, 062320 [2] Wallraff, et. al. Nature, 2004, 431, 162 [3] Schuster, et. al. Nature, 2006, in press [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H33.00007: Coupling two superconducting qubits via a cavity Johannes Majer, Steven Girvin, Robert Schoelkopf In a recent experiment [1] we have demonstrated that a superconducting qubit can be coupled strongly to a transmission line cavity. The qubit is able to swap its state with the cavity. Presently we place two qubits in the cavity which both can exchange their state with the cavity. This exchange establishes a coupling between qubits placed at the opposite end of the cavity. We discuss possible realizations of two-qubit gates and provide preliminary experimental results. [1] A. Wallraff et al., Nature (London) 431, 162 (2004) [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H33.00008: Cavity QED with a Josephson Phase Qubit E. M. Weig, M. Ansmann, R. Bialczak, N. Katz, E. Lucero, R. McDermott, M. Neeley, A. D. O'Connell, M. Steffen, J. M. Martinis, A. N. Cleland, M. R. Geller A superconducting qubit coupled to a microwave resonator is a solid state implementation of cavity quantum electrodynamics. This system allows a study of the coherent interaction of a macroscopic two-level system with a single photon in the strong coupling limit. We have investigated a Josephson phase qubit capacitively coupled to a superconducting coplanar waveguide resonator (CPW). The phase qubit is tunable over a wide frequency range and can thus be brought in and out of resonance with the CPW. Vacuum Rabi oscillations and cavity quantization can be probed spectroscopically as well as in the time domain. An arbitrary quantum state can be initialized in the phase qubit and transferred to the CPW. Using the qubit as sensitive probe of the resonator the relaxation time T$_1$ as well as the dephasing time T$_2$ of the resonator can be measured directly. With lifetimes of the order of several microseconds, high Q resonators are envisioned to act as storage elements for the quantum state of a qubit or as inter-qubit communication bus. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H33.00009: Probing Multiphoton Dressed States of a Superconducting Qubit C.M. Wilson, T. Duty, F. Persson, M. Sandberg, G. Johansson, L. Tornberg, P. Delsing There has been great interest in the new field of circuit QED, where the interaction of photons and matter are studied in the context of superconducting qubits. In this work, we create dressed states of a superconducting qubit, the single Cooper-pair box (SCB), with an intense microwave ($\sim $7 GHz) drive. The dressed states represent the hybridization of the qubit and photon degrees of freedom, and appear as avoided level crossings (ALC) in the combined qubit-photon energy diagram. The ALC occur when the energy of n photons is resonant with the charging energy of the SCB. By embedding the circuit in an rf resonator ($\sim $650 MHz), we can directly probe the dressed states. When the dressed states are off resonance, we see a purely reactive response, analogous to the quantum capacitance. On resonance, we see that the dressed qubit absorbs energy from the resonator. For some conditions, we also see evidence of population inversion in the dressed states, indicated by amplification of the reflected rf field and a negative quantum capacitance. All these effects can be explained by including relaxation in the dressed state picture. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H33.00010: 2D cavity grid quantum computing Jan von Delft, Ferdinand Helmer, Matteo Mariantoni, Florian Marquardt, Enrique Solano We propose a novel scheme for scalable solid state quantum computing, where superconducting microwave transmission line resonators (cavities) are arranged in a two-dimensional grid on the surface of a chip, coupling to superconducting qubits (charge or flux) at the intersections. We analyze how tasks of quantum information processing can be implemented in such a topology, including efficient two-qubit gates between any two qubits, initialization and read-out. The effects of decoherence, fabrication imperfections and inhomogeneities will be addressed. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H33.00011: Quantum teleportation using circuit cavity-QED systems Jae Park, Mika Sillanpaa, Ray Simmonds We investigate the experimental prospects for a probabilistic quantum teleportation scheme. In particular, we consider a system composed of coupled superconducting Josephson-junction phase qubits and superconducting transmission line resonators. The relatively short lived phase qubits are used only to prepare states, mediate entanglement, and readout. The longer lived resonators play the traditional role of qubits, providing the entanglement channel and logical basis. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H33.00012: Cavity QED and magnetic field modulated coupling between two-level resonators and a Josephson junction Lin Tian, Raymond Simmonds A superconducting Josephson junction can be explored as a novel probe of the amorphous two-level systems (TLSs) inside the junction. In recent experiments, TLSs have been demonstrated in the energy splittings of the superconducting phase qubits. However, the mechanism of the coupling between the TLSs and the junction remains unresolved. Possible mechanisms include coupling of the TLSs with the critical current and coupling of the TLSs with the dielectric field in the junction. In this talk, we present a scheme that can distinguish the two mechanisms. The key idea is to apply a magnetic field inside the junction, which is treated as a high-Q cavity, and study the cavity transmission in the presence of the TLS. When the TLS couples with the critical current, the magnitude of the coupling will be strongly modulated by the magnetic field; when the TLS couples with the dielectric field, the magnitude of the coupling will not be affected. The change of the coupling can be observed through the cavity transmission. We calculate the cavity transmission under the magnetic field and show that the dependence of the coupling on the field can be extracted from the amplitude and the spectrum of the transmission. We also show that spatial location of the TLS can be resolved by this scheme. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H33.00013: Coherent coupling between a Josephson phase qubit and an LC resonant cavity Mika Sillanpaa, Jae Park, Raymonds Simmonds We have taken the first step towards the implementation of cavity QED quantum information processing with Josephson phase qubits. We have observed for the first time a coherent interaction between a phase qubit and an LC cavity formed by a 7 mm long coplanar waveguide resonant at 9 GHz. At the co- resonant point of the qubit and cavity, we observe splitting of the qubit's spectral line. In a time-domain measurement, we observe coherent vacuum Rabi oscillations between the qubit and oscillator. [Preview Abstract] |
Session H34: DNA and Protein Analysis with Nanofluidics
Sponsoring Units: DBPChair: Bob Austin, Princeton University
Room: Colorado Convention Center 404
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H34.00001: Single Molecule Manipulation and Analysis in Nanofluidic Systems Invited Speaker: We have used simple small-scale structures to isolate and manipulate individual biomolecules in order to observe there activity and identity. Nanofluidic devices with dimensions, smaller than a relevant molecular length scale, have been used to sort or control the confirmation of long biopolymers such as DNA. Structurally-derived entropic and frictional forces balanced against the forces resulting from applied fields can elongate and controllably move a selected molecule. This can be used for measuring the length of the DNA or presenting it in an oriented manner for analysis. We have also employed metallic apertures a few tens of nanometers in diameter to confine a region of optical excitation to a volume on the order of 10$^{-20}$ liters, which allows the observation of single molecule motion and binding activity at meaningful rates and concentrations. This approach enables measuring the motility of proteins and binding of individual molecules in lipid layers and cell membranes. Small fluid channels have also been used to isolate individual optically detected molecules for evaluation in flowing systems. The measurement of mobility and detection of discrete molecular binding events can be done at the individual molecule level in such fluid systems. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H34.00002: Single-molecule manipulation of genomic DNA in extensional flow for haplotyping applications Rebecca Dylla-Spears, Lydia Sohn, Susan Muller We have developed a method amenable to haplotyping and manipulation of single molecules of double-stranded genomic DNA. Fluorescent polystyrene beads that are surface-functionalized with site-specific probes are incubated with fluorescently labeled double-stranded lambda-DNA. The solution is introduced into a microfluidic cross slot where the DNA molecules are trapped and elongated at the stagnation point of the planar extensional flow. The degree of elongation can be controlled using the flow strength in the device, as demonstrated by Perkins, Smith, and Chu (Science 1997). Beads bound along the stretched DNA may be directly observed and their locations along the backbone determined using fluorescence microscopy. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H34.00003: A Single--Step Photolithographic Interface for Cell-Free Gene Circuits and Active Biochips Amnon Buxboim, Maya Bar-Dagan, Veronica Frydman, David Zbaida, Margherita Morpurgo, Roy Bar-Ziv We developed a biochip platform technology suitable for controlled cell-free gene expression at the micron scale. A new hybrid molecule, ``daisy,'' was designed and synthesized to form in a single step a bio-compatible lithographic interface on silicon dioxide. A protocol was formulated for immobilization of linear DNA molecules thousands of base pairs long on daisy-coated surfaces to submicron spatial resolution and up to high densities. On-chip protein synthesis can be obtained with dynamic range of up to four orders of magnitude and minimal nonspecific activity. En route to on-chip artificial gene circuits, a simple two-stage gene cascade was built in which the protein synthesized at the first location diffuses to regulate the synthesis of another protein at a second location. The current approach opens possibilities for laboratories not proficient in surface chemistry to design active biochips based on cell-free gene expression with applications in artificial systems and synthetic biology. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H34.00004: Mode Transition of RNA Trap by Electric and Hydraulic Force Field in Microfluidic Taper Shape Channel Yuzuru Takamura, Kunimitsu Ueno, Wako Nagasaka, Yuichi Tomizawa, Eiichi Tamiya We have discovered a phenomenon of accumulation of DNA near the constricted position of a microfluidic chip with taper shaped channel when both hydro pressure and electric field are applied in opposite directions. However, RNA has not been able to trap so far, unlike huge and uniformly double stranded DNA molecules, RNAs are smaller in size and single stranded with complicated conformation like blocks in lysed cell solution. In this paper, we will report not only large but also small RNA (100$\sim $10b) are successfully trapped in relatively large microfluidic taper shape channel (width $>$10um). RNA are trapped in circular motion near the constricted position of taper shape channel, and the position and shape of the trapped RNA are controlled and make mode transition by changing the hydraulic and the electric force. Using this technique, smaller size molecule can be trapped in larger micro fluidic structure compared to the trap using dielectrophoresis. This technique is expected to establish easy and practical device as a direct total RNA extraction tool from living cells or tissues. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H34.00005: The Physics of Nanoconfined DNA: Varying Temperature and Ionic Conditions Walter Reisner, Anders Kristensen, Jonas Tegenfeldt, Henrik Flyvbjerg, Niels B. Larsen Top-down approaches to nanotechnology have the potential to revolutionize biology by making possible the construction of chip-based devices with nanoscale features that can not only detect, separate and analyze single DNA molecules by size but also--it is hoped in the future--actually sequence at the single molecule level. Using electron beam lithography we have fabricated nanochannel devices in fused silica with dimensions on order of 100x100nm and lengths of 100s of micrometers. Both dsDNA and ssDNA molecules, imaged via fluorescence microscopy, are observed to stretch out in these effectively one dimensional systems. We present measurements of the DNA extension as a function of ionic strength. We also demonstrate how the DNA melting transition can be probed in real time by heating the nanochannel extended DNA. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H34.00006: Modeling DNA Separation in Entropic Trap Device Alex Vaughn, Yongmei Wang DNA electrophoresis in the entropic trap device fabricated by Craighead and coworkers has some interesting properties that allow long chains to be separated; moreover, their results showed that long chains had higher mobility than short chains, a counter-intuitive result. The mechanism by which the device works is not well understood. This study seeks therefore to understand the device's mechanism more thoroughly with a desire to provide the knowledge necessary to optimize the separation of long chains of DNA. The study uses dynamic Monte Carlo simulations on a simple-cubic lattice to model the separation of DNA. The simulation algorithm was first tested by confirming the chain length independence of the electrophoretic mobility of DNA in bulk solution, a well-known experimental fact. When DNA chains are constrained in a slit channel, the electrophoretic mobility is still independent of chain length. If DNA-wall interactions are added to the model, then the mobility decreases with the chain length for short chains and reaches a plateau for long chains. In a channel with entropic traps, the mobility is found to increase with the chain length, consistent with experimental results by Craighead and coworkers. We also found that a better separation was achieved when the trap was made deeper. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H34.00007: Imaging Biological Systems using Dielectric Near-Field Microscopy Keith Brown, David Issadore, Tom Hunt, Robert Westervelt We have developed a dielectric spectrometer for use on biological systems. The spectrum of dielectric response to RF electric fields is analogous to color as an optical response. Measurement of the dielectric spectrum from $\sim $ 10 kHz to $\sim $ 3 GHz will reveal information about the structure and conditions of protein solutions, protein crystals and biological tissues. We designed and built a system to test biological samples in a microfluidic chamber mounted on a circuit board. The apparatus measures the RF dielectric spectrum directly, or by analyzing the pulse response in the time domain. We have constructed several versions of the hardware for sensitive capacitive measurements, including two types of capacitive bridges, and a transmission line, incorporating precision electronics and local generation of pulses.~ A goal is to scale the system down and implement many dielectric spectrometers as an array of pixels on a CMOS chip for dielectric near-field microscopy of biological samples. This work made possible by NSEC NSF grant PHY-0117795 and the NCI MIT-Harvard CCNE. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H34.00008: Fundamental limits of detection with nanowire FET chem/bio sensors in subthreshold and linear regimes Xuan Gao, Gengfeng Zheng, Charles Lieber Nanowire field effect transistors (NW-FETs) have been demonstrated to be powerful sensors for the detection of biological and chemical species, and thus understanding and pushing their intrinsic sensitivity limits could have a significant impact on a broad-range of applications of these devices. We report studies of the response of silicon-NW-FET sensors as the devices are tuned from linear to subthreshold regimes by electrochemical gating. Conductance versus solution pH data show that operation in the subthreshold regime can increase both the percentage change in conductance and the signal to noise ratio of the device by over ten times compared to the linear regime. We also demonstrate that operating in the subthreshold regime yields improvement in the detection limit for the cancer marker protein PSA with detection down to $\sim $1.5 fM for a device with $\sim $0.75 pM detection limit in the linear regime. Analysis of these results shows that the sensitivity improvement is due to the more effective surface charge gating resulting from the reduced screening by carriers. In addition, the effect of NW diameters and the intrinsic charge detection limit for using NW-FET devices will be described. Our work shows that optimization of NW-FET structure and operating conditions can provide a significant enhancement as well as a fundamental understanding of the sensitivity limits for nano-FET sensors. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H34.00009: Modeling PCR in Natural Convection Systems Kevin Dorfman, Ehud Yariv, Guy Ben Dov Polymerase chain reaction (PCR) is a biochemical protocol for making many copies of a DNA template by thermal cycling between a hot temperature (where the strands are separated) and a cool temperature (where primers are annealed). In natural convection PCR, the requisite thermal cycling is provided by a buoyancy-driven circulating flow of the carrying buffer between a lower hot plate (at the denaturing temperature) and an upper cold plate (at the annealing temperature). We present a multi-component convection-diffusion-reaction model for natural convection-driven PCR when both primers and PCR enzyme are in excess. The evolution of the DNA population achieves a stationary state, wherein the problem is recast as an eigenvalue problem for computing the exponential amplification rate. With a realistic choice of parameters, the model predicts a doubling time on the order of two minutes, in agreement with experiments and much slower than the fluid cycling time. In contrast to what might be expected, the doubling time increases monotonically with the diffusion coefficient. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H34.00010: Tethered DNA molecules stretched by an electric field: A Molecular Dynamics Study. Gary Slater, Martin Bertrand It has been predicted by Long, Ajdari and Viovy (Phys. Rev. Lett., 1996, 76:3858) that the mechanical force necessary to stall a DNA molecule during electrophoresis is substantially smaller than the sum of the electrical forces applied on all of its monomers. In fact, it should be proportional to its hydrodynamic friction coefficient, which may vary with the molecular conformation. We have tested this prediction using coarse-grained Molecular Dynamics simulations in which we explicitly included the polymer, the solvent, the counterions and the salt. Our results show that the above prediction is indeed valid. In fact, our data demonstrate that there is a universal linear relationship between the stall force and the product of the electrical field and the radius of gyration of the polyelectrolyte. This remarkable relationship holds even when the electric forces stretch the DNA molecule near full extension. We thus conclude that an electrophoretic field is equivalent to a fluid flow, as suggested by Long, Ajdari and Viovy. This has profound implications for the development of a theoretical framework that can explain the electrophoresis of hybrid DNA-protein molecules. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H34.00011: Electrical Noise Characterization of Noble Gas Ion Beam Fabricated Nanopore Detectors Ryan Rollings, Bradley Ledden, Eric Krueger, Greg Salamo, Jiali Li, John Chervinsky, Jene Golovchenko Nanopores fabricated with low energy noble gas ion beams in a silicon nitride membrane can be employed as the fundamental element of single biomolecule detection and characterization devices. The effect of morphology, annealing, and physical surface treatments are systematically studied to determine their effect on the electrical noise characteristics of the nanopore when used as part of a nanofluidic detector. Atomic Force Microscopy (AFM) is used to measure the morphology of the region near the pore, while X-ray Photoelectron Spectroscopy (XPS) and Rutherford Backscattering (RBS) are used to measure the change in the surface composition with annealing as well as initial depth profiles of imbedded ions. We qualitatively discuss the underlying physical processes that contribute to the electrical noise characteristics of the pore in comparison with our measurements and present optimized conditions for fabricating the quietest pores. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H34.00012: Self-trapping and stretching of DNA using single nano-height micropillar Po-Keng Lin, Chi-Cheng Fu, Y.-L. Chen, W. S. Fann We propose a novel method to trapping ds-DNA molecules in 30-100nm slit-like nanochannel with single micropillar. In this environment the DNA molecules unusually extend around obstacles such as pillars or walls. The DNA molecules appear to have quasi-one dimensional dynamics even though the confinement is quasi-two dimensional. The trapping process can occur only when the channel height below the Kuhn length of ds-DNA. We experimentally observe the Brownian motions of the DNA using wide-field fluorescence microscopy. The static and dynamic scaling with DNA length (9.4$\sim $166 kbps) and channel height (30$\sim $240nm) have been analyzed and compared with the experimental results of DNA confined in the square nanochannels in the literatures (W. Reisner et al., Phys. Rev. Lett. 94 196101 (2005)). This micro/nano fluidic device can be applied to study the multi-step biochemical reactions in confinements such as DNA folding induced by protein and restriction mapping of DNA in the future. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H34.00013: Spatial Detection of Submicron Particles with Integrated Circuit Charge Sensors David Issadore, Tom Hunt, Robert Westervelt Using a standard MOSIS 0.35 micron Integrated Circuit process, we have built a position sensor for use in all-electrical feedback traps for submicron particles. The device has four transistors in a square, with floating gates that capacitively detect a charged particle in a microfluidic chamber above. The four transistors form the front ends of two independent differential amplifiers that report the x and y position of the particle. Future work towards integration of dielectrophoretic feedback forces for an all-electrical ``Anti-Brownian motion'' trap will be discussed. [Preview Abstract] |
Session H35: Emerging Spectroscopic Techniques
Sponsoring Units: DBP DCPChair: Marilyn Gunner, The City College of New York
Room: Colorado Convention Center 405
Tuesday, March 6, 2007 8:00AM - 9:48AM |
H35.00001: BREAK
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Tuesday, March 6, 2007 9:48AM - 10:00AM |
H35.00002: Real-time detection of multiple biomolecular reactions on a functionalized glass surface using a scanning oblique-incidence optical reflectivity difference (an ellipsometric technique). Yung-Shin Sun, James P. Landry, Xiangdong Zhu One of the enabling platforms in proteomic research is parallel (high-throughput) detection of multiple biomolecular interactions on a microarray. To keep conformational and in turn functional integrity of protein molecules, label-free detection is desirable. We have developed an oblique-incidence optical reflectivity difference (OI-RD) technique for label-free measurements of protein reactions with molecular targets in microarray format immobilized on functionalized glass surface. As an ellipsometric technique, OI-RD measures changes in thickness and/or optical dielectric response instead of fluorescence. By incorporating total internal refection geometry and a multi-element photodiode array detector, we demonstrate how such the OI-RD technique can be efficiently used to measure multiple protein reactions in real time with surface-immobilized molecules or molecular groups on a glass substrate. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H35.00003: In Situ X-ray Reflectivity Studies of Protein Adsorption onto Functionalized Surfaces Andrew Richter The adsorption of protein films onto solid surfaces, both artificial and naturally occurring, have been widely studied using a variety of techniques due to their importance in medicine, biomedical applications, and the general understanding of protein structure and function. What have yet to be performed are in situ, time-resolved, high-resolution structural studies of these systems. We have begun a project that uses the technique of in situ x-ray reflectivity to obtain highly resolved structural information with time resolution on the order of minutes. This talk will present our first findings of serum albumin and immunoglobulin G films on hydrophobic self-assembled monolayers. The protein films are readily observable, showing extensive denaturing after adsorption with a slow decay of density into the aqueous solution. Additionally, a thin low-density region that occurs between the hydrophobic film and the solution persists after protein deposition. Comparisons to films that are removed from solution, the influence of solution concentration, the effects of x-ray damage, and the time scales for protein film formation and evolution will also be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H35.00004: Mid-IR spectra of the bio-related molecules in the gas phase Yongjun Hu, Elliot R. Benstein Mid-IR spectra of gas phase bio-related molecules R-OH, R-COOH and simple non-aromatic amino acids, such as glycine and valine, detected by vacuum ultraviolet (VUV), 10.5 eV single photon ionization of supersonically expanded and cooled samples, are presented and discussed. Molecules and their fragment species, generated by a proton transfer reaction following ionization, are identified by time of flight mass spectroscopy. The fundamentals and overtones of the CH and OH stretches and some combination bands are identified in the spectra. Rotational resolution for the OH mode and its first overtone yield an estimate of $\sim $50 K for the methanol monomer in the supersonic beam. Two neutral C$_{2}$H$_{5}$OH conformers can be identified by high sensitivity IR plus VUV nonresonant ionization and fragmentation detected (NRIFD-IR) vibrational spectroscopy. Free OH and NH stretches are missing in the spectrum of glycine and valine, indicating that the strong intra-molecular hydrogen bonds are formed in these gas phase species. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H35.00005: Single Quantum Dots Imaged with Resonance Rayleigh Scattering Do Not Blink David W. Ward, Wei Min, Ethan S. Karp, Xiaoliang Sunney Xie Semiconductor quantum dots have become a robust fluorescent marker for the life sciences. Two key issues limit the broad use of quantum dots as fluorescent markers: heterogeneous emission and non-radiant dark populations. All bright quantum dots blink stochastically, have considerable heterogeneity in their emission, and have fluctuations in their fluorescence lifetimes, limiting their utility as single particle trackers by introducing potentially large interruptions in particle trajectories. Further, a significant fraction does not fluoresce at all, undermining biophysical studies such as immuno-fluorescence. We present an alternative or complement to fluorescent imaging of quantum dots. We have developed a new technique, resonant Rayleigh scattering (RRS) microscopy, for imaging single quantum dots which does not exhibit blinking. Detection of individual quantum dots, both surface immobilized and freely diffusing in aqueous solution, is demonstrated. Non-fluorescent populations of quantum dots are visible through RRS microscopy. Though other non-fluorescence detection techniques exist they are significantly more complicated than our technique, which requires minimal alteration of a conventional confocal fluorescence microscope. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H35.00006: Development of 0.24 THz pulsed electron paramagnetic resonance to ``film'' proteins in action with the UCSB free electron laser Susumu Takahashi, Dan G. Allen, Kiyotaka Akabori, Melissa Anholm, Hieu Nguyen, Sangwoo Kim, Mark S. Sherwin, Johan van Tol, Louis-Claude Brunel Pulsed electron paramagnetic resonance (EPR) is extremely useful to study the fast dynamics of molecules. Currently, most high-power pulsed EPR experiments are performed near 10 GHz, with a time resolution of 100 ns. The spin dephasing times of spin labels on proteins in aqueous solution are tens of ns. Thus, conventional pulsed EPR measurements of proteins are performed on frozen samples. There exist instruments which operate at 95 GHz with time resolution shorter than 100 ns. We present the development of a 0.24 THz pulsed EPR system which is expected to have sub-ns time resolution, enabling the EPR study of proteins in solution. The system uses the UCSB free electron laser (FEL) to produce kW-level pulses at 240 GHz. A ``pulse-slicer'' shortens the FEL's microsecond pulses to the ns range. Sequences of two or three pulses separated by up to 25 ns will be made using a home-made delay line. A superheterodyne detection system is being fabricated to be sensitive enough to detect 1nW signals and also protected from kW FEL inputs. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H35.00007: Three-Dimensional Imaging of Single Large Macromolecules Using Equally Sloped Tomography E. Lee, B. Fahimian, J. Ma, C. Iancu, C. Suloway, E. Wright, G. Jensen, J. Miao We report the development of equally sloped tomography for the reconstruction of the 3D structure of single large macromolecules. In a combination of pseudo-polar fast Fourier transform and the oversampling method with an iterative algorithm, equally sloped tomography makes superior 3D reconstruction to conventional tomography which has an intrinsic drawback due to the use of equally angled 2D projections. By employing equally sloped tomography and cryo electron microscopy, we have obtained the 3D structure of single hemocyanin protein molecules and HIV viruses at $\sim$ 5 nanometer resolution. Preliminary analysis based on cross- correlation has indicated that the 3D images using equally sloped tomography are superior to those of the conventional method. We believe this general approach will find broad applications in high-resolution 3D imaging of large macromolecules. [Preview Abstract] |
Session H38: Focus Session: X-ray and Neutron Instruments and Sciences I
Sponsoring Units: GIMSChair: Carolyn MacDonald, State University of New York at Albany
Room: Colorado Convention Center 501
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H38.00001: Probing Magnetic Materials Using Synchrotron Radiation and Phase Retarding Optics Invited Speaker: Synchrotron radiation has become an essential tool in the study of magnetic materials. The utility of x-ray measurements arises from the fact that the resonant and polarization properties observed near core-level resonances probe the valence-electron spin and orbital properties in an element specific manner. Critical to enabling such studies, however, has been the ability to easily manipulate the polarization of the x-ray beam. Circularly polarized x-rays pay a particularly important role, due to their coupling to the net ferromagnetic moment in a material. This talk will focus on how phase retarding optical elements can be used to tailor the focus on how phase retarding optical elements can be used to tailor the x-ray beam polarization in order to enable various types of magnetic measurements. Examples of magnetic spectroscopy, scattering, and imaging measurements employing such optics will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H38.00002: Probing the breakup of high-speed liquid jet by ultrafast x-ray microimaging. Jin Wang High-pressure high-speed sprays have vast industrial and consumer applications that penetrate to very aspect of the society. Despite their longstanding multitude of uses, the fundamental physics that governs the spray flow formation in high-speed jets is not well understood. Experimentally, the difficulty is due, in large part, to a lack of information about the composition of spray plumes close to the nozzle, such as liquid breakup mechanism and spray mass distribution. Traditional visualization tools like visible-light-based imaging have not been effective. To date, theoretical and computational studies of the sprays have proven to be extremely difficult, if not impossible, to carry out. We report here the development of x-ray-based microimaging technique to visualize the breakup of optically opaque high-speed jets in the near-nozzle region. The quantitative near-nozzle spray characteristics can serve to validate primary liquid breakup models and be used as indispensable initial and boundary conditions for spray atomization processes in further downstream areas. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H38.00003: X-ray imaging of nickel-based microstructured superalloys using synchrotron radiation Naji Husseini, Divine Kumah, Codrin Cionca, Roy Clarke, Jianzhang Yi, Christopher Torbet, J. Wayne Jones Nickel-based superalloys are used in harsh environments such as airplane turbines and nuclear power plants for their high temperature stability and resilience to oxidation and corrosion. These superalloys grow via directional solidification along the $<$001$>$ orientation and assume a dendritic morphology along $<$100$>$, concentrating Ni into the dendrites and TaC elsewhere. 200 $\mu $m thick samples of Rene N5 were imaged in a transmission setup at Sector 7 of the Advanced Photon Source with high-intensity synchrotron radiation. The recorded intensity maps contain information about the elemental concentration with sub-micron resolution, enhanced by phase contrast near sharp compositional variations. These maps show vacancies and cracks in addition to linearly decreasing concentrations of Re and W out from the center. Interferences seen while rotating the sample reveal misorientations of the cores and strain between dendrites, while a full rotation permits 3D tomography. One-second exposure times allow observation of \textit{in situ} crack propagation induced by an ultrasonic fatiguer. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H38.00004: Full-Field Microscopy with Synchrotron Radiation Christoph Rau, Vasilica Crecea, Wenjun Liu, Ian Robinson A full-field X-ray microscope has been built at the UNICAT-beamline 34 ID-C at the Advanced Photon Source (APS), working with a Kirkpatrick-Baez mirror (KB) as condenser and a micro-Fresnel-zone plate (FZP) as objective lens. 50~nm-features have been resolved in a Nickel structure operating the microscope at a photon energy of 9keV. The KB system used as a condenser focuses approximately 63{\%} of the incoming intensity onto the sample spot, matching the aperture of the objective lens. For the latter we have a choice of gold micro-FZP having outer zone widths from 40 to 70~nm. Under these conditions the X-ray microscope provides 50-85~nm resolution and short exposure times due to the high efficiency of the KB-system. A field of view of 20x40 micron$^{2}$ can be imaged within a minute by scanning the condenser optic. First tomography experiments have been performed. We will also discuss other techniques such as cone and in-line phase contrast imaging. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H38.00005: Quantitative strain analysis of single crystals using x-ray topography Y. Zhong, Y.S. Chu, A. T. Macrander, S.F. Krasnicki The x-ray topography technique images diffraction intensity variations of a crystal. The use of a CCD camera enables the measurement of different spatial resolutions. Currently an x-ray topograph with spatial resolution of 1 micron has been achieved, but the quantitative data analysis has not been explored widely. Quantitative strain analysis on these images extends new capabilities in crystal study. We have developed methods to quantify strain information through topography data. We will present these methods and discuss related practical issues, such as advantages, sensitivities, and limitations. We first introduce the azimuthal rotation method, suitable for strain components along the surface normal direction. The analysis requires accurate image registration; therefore we use the cross correlation method. Next we introduce a method to obtain quantitative strain tensor using bright field lattice refinement. The application of these methods on materials study is shown. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H38.00006: Absorber Materials for Transition-Edge Sensor X-ray Microcalorimeters Ari-David Brown, Simon Bandler, Regis Brekosky, James Chervenak, Fred Finkbeiner, Naoko Iyomoto, Richard Kelley, Caroline Kilbourne, Frederick Porter, Enectali Figueroa-Feliciano, Tarek Saab, John Sadleir Arrays of superconducting transition-edge sensors (TES) can provide high spatial and energy resolution necessary for x-ray astronomy. High quantum efficiency and uniformity of response can be achieved with a suitable absorber material, in which absorber x-ray stopping power, heat capacity, and thermal conductivity are relevant parameters. Here we compare these parameters for bismuth and gold. We find that the thermal conductivity of these materials is highly dependent upon the thin film deposition technique. Furthermore, we briefly discuss the performance of our x-ray detectors when they possess cantilevered evaporated Bi/Au, electroplated Bi/Au, and electroplated Au absorbers. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H38.00007: Electronic Structure Studies of Ce-doped Gamma Detector Materials Andrew Canning, Rostyslav Boutchko, Stephen Derenzo, Lin-Wang Wang, Marv Weber Cerium doped materials such as the Lanthanum Halides represent some of the brightest known scintillators for the detection of gamma rays. The scintillation process in Cerium doped materials corresponds to the transition from a 5d to 4f state on the Cerium atom where the 5d and 4f states must lie in the gap of the host materials. We have performed electronic structure calculations for many different Cerium doped materials using density functional based methods to determine the positions of the 5d and 4f states relative to the valence and conduction bands of the host materials. We find good agreement with experimental results for the systems studied in particular for the Lanthanum Halides. Our theoretical calculations will be used as a first step screening for candidate new detector materials. This work is funded by the Dept. of Homeland Security, Domestic Nuclear Detection Office. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H38.00008: Gamma-Ray Compton Light Source Development at LLNL Frederic Hartemann, Scott Anderson, David Gibson, Chris Hagmann, Micah Johnson, Igor Jovanovic, Mike Messerly, Jason Pruet, Miro Shverdin, Aaron Tremaine, Dennis McNabb, Craig Siders, Chris Barty A new class of tunable, monochromatic gamma-ray sources capable of operating at high peak and average brightness is currently being developed at LLNL for nuclear photo-science and applications. These novel systems are based on Compton scattering of laser photons by a high brightness relativistic electron beam produced by an rf photoinjector. Key technologies, basic scaling laws, and recent experimental results will be presented, along with an overview of future research and development directions. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H38.00009: Design and characterization of a compact multi-detector gamma array for studies of induced gamma emission: spontaneous decay of 178m2Hf as a test case P. Ugorowski, R. Propri, S.A. Karamian, D. Gohlke, J. Lazich, N. Caldwell, R.S. Chakrawarthy, M. Helba, H. Roberts, J.J. Carroll Recent scientific attention has focused on the m2 isomeric state of Hafnium, 178m2Hf. The spontaneous decay of 178m2Hf takes the form of a cascade of gamma photons, totaling 2.4 MeV of energy per nucleus, or approximately 1.3 GigaJoules/gram. If all the decays were simultaneous, exawatt (10$^18$) energy outputs could be realized. A class of isomers called ``K-isomers'' has been studied to determine the possibility of xray-induced decay of the excited isomeric state. The purpose of the ``miniball'' detector system was to separate out possible induced cascades from the spontaneous decay cascades using nuclear calorimetry, in order to settle a recent scientific controversy involving claims of induced decay and counter-claims of null results. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H38.00010: Submicron Resolution Neutron Radiography R. Gregory Downing Imaging diverse materials such as biological and electronic samples at nanometer scales is of current importance; however, capable analytical tools are few. An entirely novel position sensitive neutron detector was conceived based upon illumination of a thin converter causing reactions that diametrically emitted two particles. The converter is carefully aligned between facing position-sensitive particle detectors. The neutron-induced reaction particles strike both detectors in near temporal unison. The nanosecond difference in arrival time uniquely reveals the energy of each particle. Knowing the initial energies of the particles from fundamental physics, the geometry of the system, and the residual energy of the particles then the precise spatial coordinates of the neutron reaction are determined. The data are deconvolved to form a temporal and spatial map of the neutron field illuminating the area of the converter. This detector promises spatial resolution that ranges from a few micrometers to tens of nanometers, an improvement 10 to over 100 times existing systems. Applications for the detector include radiography and tomography for a host of organic and inorganic material studies. A trial demonstration at the NCNR will utilize an intense conditioned neutron beam and high speed data processing capabilities. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H38.00011: Scientific Opportunities at OPAL, the New Australian Research Reactor Robert Robinson Australian physics is entering a new ``golden age,'' with the startup of bright new neutron and photon sources in Sydney and Melbourne, in 2006 and 2007 respectively. The OPAL reactor and the Australian Synchrotron can be considered the greatest single investment in scientific infrastructure in Australia's history. They will essentially be ``sister'' facilities, with a common open user ethos, and a vision to play a major role in international science. Fuel was loaded into the reactor in August 2006, and full power (20MW) achieved in November 2006. It is our plan to commence the formal user program in mid 2007, but commissioning experiments will have taken place well before then. The first three instruments in operation will be a high-resolution powder diffractometer (for materials discovery), single-crystal diffractometer (for small-molecule crystallography) and a strain scanner (for mechanical engineering and industrial applications). These will be closely followed by four more instruments with broad application in nanoscience, condensed- matter physics and other scientific disciplines. Instrument performance will be competitive with the best research-reactor facilities anywhere. To date there is committed funding for 9 instruments, with a capacity to install a total of $\sim$18 beamlines. An update will be given on the status of OPAL, its thermal and cold neutron sources, its instruments and hopefully the first data. [Preview Abstract] |
Session H39: Focus Session: Phase Transitions and Domains in Ferroelectric Nanostructures II
Sponsoring Units: FIAPChair: Alex Bratkovsky, Hewlett Packard
Room: Colorado Convention Center 502
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H39.00001: Unusual phenomena in ferroelectric nanostructures Invited Speaker: First-principles based computations are nowadays capable of tackling really complex challenges of fundamental and technological importance. For example, a dipole vortex structure has been discovered, by means of these methods, in isolated nanoparticles of ferroelectrics --which may lead to a new generation of efficient nanoscale memory devices [1]. Here, we use first-principles-based approaches to address the following issues: (i) what are the elastic signatures and field charateristics of dipole vortices in isolated ferroelectric nanodots? (ii) how to control the chirality of such dipole vortices (which is an important challenge to solve for future applications)? and (iii) what are the possible ground states of arrays of ferroelectric dots embedded in a crystal lattice? Regarding item (i), we found that the tetragonal axial ratio in the vortex state is lower than 1 (unlike in ``normal'' ferroelectric or antiferrodistortive phases), and that the electric field produced by the dipole vortex outside the dot oscillates in space when changing the polar angle of the cylindrical coordinate system. Such features can serve as fingerprints of the vortex state to experimentally confirm the existence of such unusual state. Moreover, we demonstrate that, and explain why, using inhomogeneous electric fields is an efficient solution to item (ii) [2]. Finally, it is predicted that array of embedded dots can adopt new phases, depending on the temperature and difference in polarizability between the dots and medium [3]. Atomistic details of such phases, as well as their governing mechanisms, will be provided. \newline \newline Coauthors: I. Ponomareva, I. Kornev, I. Naumov, and L. Bellaiche, University of Arkansas. \newline \newline [1] I. Naumov, L. Bellaiche, \& H. Fu. ``Unusual phase transitions in ferroelectric nanodisks and nanorods, '' Nature 432, 737 (2004). \newline [2] S. Prosandeev, I. Ponomareva, I. Kornev, I. Naumov, \& L. Bellaiche. ``Controlling toroidal moment by means of an inhomogeneous static field: an ab initio study,'' Phys. Rev. Lett. 96, 237601-1-4 (2006). \newline [3] S. Prosandeev \& L. Bellaiche. ``Properties of ferroelectric nanodots embedded in a polarizable medium: atomistic simulations.,'' Phys. Rev. Lett. 97, 167601 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H39.00002: Influence of the growth direction on properties of ferroelectric ultrathin films Inna Ponomareva, Laurent Bellaiche Ferroelectric thin films have been intensively studied recently because of their potential applications in nonvolatile memories, infrared detectors and microelectromechanical systems. Many interesting features of these systems, such as the strong dependency of their dipole patterns on boundary conditions and size thickness, are now well understood. However, an overwhelming majority of past studies focused on films grown along the [001] direction. As a result, one remaining mystery in thin films is the influence of the {\it growth direction} on their properties. Here we report results of first-principles-based calculations of PZT ferroelectric thin films that are grown along different directions and subject to different boundary conditions [1]. A wide variety of dipole patterns is discovered, including ferroelectric phases absent in the bulk and complex periodic stripe nanodomains. Moreover, a large enhancement of dielectricity is found in ultrathin films exhibiting a growth direction that differs from a possible direction of the polarization in the corresponding bulk. A set of two general and simple rules is provided to analyze and understand all these features. [1] I. Ponomareva and L. Bellaiche, Phys. Rev. B {\bf 74}, 064102 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H39.00003: Phase field modeling of domain structures in nano-composite ferroelectric multilayers Julia Slutsker, Andrei Artemev, Alexander Roytburd The formation of domain structures in differently patterned multilayers with a nano-composite structure containing ferroelectric structure components has been studied by using a phase field method based on the microelasticity theory and the Fourier spectral analysis of electrostatic interactions. The effects of the depolarizing electric field, the thickness of the film, and the misfit between a film and a substrate on the domain pattern and switching properties have been analyzed. The effect of a relative strength of elastic interactions in the multilayer on the dielectric response and effective piezo coefficients has been studied. A correlation between the results of the phase field modeling and the existing results of first principle calculations has been demonstrated, thus allowing us to describe ferroelectric films on different scale levels. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H39.00004: Depolarizing Field and ``Real'' Hysteresis Loops in Nanometer Scale Ferroelectric Films A.M. Bratkovsky, A.P. Levanyuk We give detailed analysis of the effect of depolarizing field in nanometer-size ferroelectric capacitors studied by Kim \textit{et al.} [Phys. Rev. Lett. \textbf{95}, 237602 (2005)]. We calculate a critical thickness of the homogeneous state and its stability with respect to domain formation for strained thin films of BaTiO$_3$ on SrRuO$_3$/SrTiO$_3$ substrate within the Landau theory. While the former (2.5nm) is the same as given by ab- initio calculations, the actual critical thickness is set by the domains at 1.6nm [1]. There is a large Merz's activation field for polarization relaxation. Remarkably, the results show a \emph {negative} slope of the ``actual'' hysteresis loops, a hallmark of the domain structures in ideal thin films with imperfect screening[2]. \newline [1] A.M. Bratkovsky and A.P. Levanyuk, Appl. Phys. Lett. (in print), cond-mat/0608283. \newline [2] A.M. Bratkovsky and A.P. Levanyuk, Phys. Rev. B {\bf 63}, 132103 (2001). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H39.00005: Size Effects in Ferroelectric Thin Films: The Role of 180 degree domains Rajeev Ahluwalia, David Srolovitz The depolarization fields set up to due to uncompensated surface charges in a ferroelectric thin film can suppress the ferroelectric phase below a critical size. However, recent experiments show that 180 degree domain structures can help to stabilize ferroelectricity in films which are as small as about 3 unit cells thick. We study the influence of these domain structures on the size dependent properties of ferroelectric thin films using a Ginzburg-Landau theory. The model incorporates the effect of depolarization field by considering non-ferroelectric passive layers at the top and bottom surfaces. It is shown that the wavelength of the 180 degree domains decreases as the film thickness is reduced and eventually the film abruptly becomes paraelectric below a critical size. It is also shown that 180 degree domains appear during the process of polarization switching causing a time dependent relaxation of the remnant polarization , consistent with recent experiments. Further, it is observed that the depolarization induced domain wall motion significantly alters the shape of the polarization vs electric field (P-E) loops at small thicknesses. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 10:00AM |
H39.00006: Domain structure and polarization of ferroelectric graded multilayers and nanograded films. Invited Speaker: The equilibrium domain structure and its evolution under an electric field in ferroelectric graded films and multilayers are considered. The equilibrium graded film is self-poled and contains a single-domain and a polydomain (with 180\r{ } domains) layers. The polarization of a graded multilayer and films proceeds by movement of wedge-like domains as a result of progressive transformation of polydomain layers to a single-domain state. The change of the energy of domain walls accompanying growth and shrinkage of wedge domains can be taken into account by introducing a local effective field on the boundary between polydomain and single-domain parts of a film. It makes the movements of this boundary irreversible and contributes to the hysteresis of polarization. The theory provides the principal explanation of dielectric and pyroelectric behavior of graded ferroelectric films including enhanced dielectric constant and giant pyroeffect. The theory will be applied to the discussion of several related problems: (1) elastic domains as a result of structural transformations in graded materials; (2) domains in graded ferromagnetic films; (3) domain structures under non-uniform fields, particularly, the formation of zigzag interdomain and interfacial walls under a local field. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H39.00007: Dielectric Permittivity and Tunability of Ferroelectric Bilayers and Multilayer Heterostructures S. P. Alpay, I. B. Misirlioglu, G. Akcay, S. Zhong, J. V. Mantese, M. W. Cole Ferroelectric multilayers and superlattices have gained interest for many applications in the telecommunications industry. A thermodynamic analysis is presented to demonstrate that ferroelectric multilayers interact through internal elastic, electrical, and electromechanical fields and the ``strength'' of the coupling can be quantitatively described using Landau theory, theory of elasticity, and principles of electrostatics. The modeling indicates that the electrostatic coupling between the layers leads to the suppression of ferroelectricity at a critical paraelectric layer thickness for ferroelectric-paraelectric bilayers. This bilayer is expected to have a huge dielectric response and tunability at this critical thickness. We carry out a numerical analysis for prototypical BaTiO$_{3}$-SrTiO$_{3}$ bilayers (40 to 800 nm total thickness) as a function of SrTiO$_{3}$ fraction. There exists a critical fraction of SrTiO$_{3}$ at which the polarization is suppressed due to the depolarization field and a large dielectric response is predicted. It is shown that this critical fraction decreases with decreasing total bilayer thickness indicating that the interfacial effects are more pronounced in thinner bilayers. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H39.00008: Phase diagrams of epitaxial BaTiO$_3$/SrTiO$_3$ superlattices from first principles Sergey Lisenkov, Laurent Bellaiche The fabrication of artificial superlattices (SL) consisting of alternating layers of two or more ferroelectric oxides is nowadays possible. Such heterostructures may possess properties that dramatically differ from those of bulk ferroelectrics, since, e.g., physical properties of ferroelectric SL should be very sensitive to the epitaxial strain arising from the substrate on which the SL is grown. Determining the temperature-misfit strain diagram of SL is thus of considerable importance. Here, a first-principles-based effective Hamiltonian approach (that has been recently developed and successfully tested for disordered or ordered Ba$_x$Sr$_{1-x}$TiO$_3$ systems [1]) is used within Monte-Carlo simulations to determine such phase diagram in epitaxially grown (001) BaTiO$_3$/SrTiO$_3$ SL of different period. We found that, unlike in the short SL, that exhibit a phase diagram that resemble that of (001) BaTiO$_3$ thin films under short-circuit-like conditions, original domain patterns with unusual inhomogeneous atomistic features occur in the longer SL. The reason behind such dramatic difference is revealed. [1] L. Walizer {\em et al,}, {\em Phys. Rev. B}, {\bf 73}, 144105, (2006). [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H39.00009: Super-pyroelectric effect in nanocrystalline films of BaTiO3 with macro-domain organization Yahin Yvry, Vera Lyahovitskaya, Ilya Zon, Igor Lubomirsky, Alexander Roytburd Self-supported films of nanocrystalline BaTiO3 exhibit an earlier unreported phenomenon: orders of magnitude increase of pyroelectric effect (``super-pyroelectricity''). The measured value ($\sim $ 1$\mu $C/(cm2$\cdot $K)) is one the highest ever reported for ferroelectric materials. The super-pyroelectricity arises due to self-organization of hundreds of millions nanocrystalline ferroelectric grains in macro-domains: the regions where directions of the crystallographic axes are strongly correlated. Small temperature variations cause reversible changes in the direction of spontaneous polarization of ferroelectric grains in macro-domains producing gigantic pyroelectric current. In contrast to regular pyroelectricity observed in ferroelectrics, super-pyroelectric effect reaches maximum at 80-100 K below the temperature of the para-to-ferroelectric phase transition. This work demonstrates that polycrystalline macro-domains are capable of fast ($<$10 $\mu $sec!) and reversible adaptation to minute changes of external conditions, which promises creation of pyroelectric and piezoelectric devices with previously unattainable performance. [Preview Abstract] |
Session H40: Semiconductors: Growth and Characterization
Sponsoring Units: FIAPChair: Howard Branz, Energy Renewable Research Laboratory
Room: Colorado Convention Center 503
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H40.00001: Breakdown physics of low-temperature silicon epitaxy Charles W. Teplin, Pauls Stradins, Eugene Iwaniczko, Qi Wang, Kim M. Jones, Robert Reedy, Bobby To, Dean H. Levi, Helio Moutinho, Howard M. Branz We describe new insights into the mechanisms that affect low-temperature silicon epitaxy growth (T$<$630\r{ }C) by chemical vapor deposition (CVD). Experiments using hot-wire CVD show that below 500\r{ }C epitaxial growth is limited to relatively small thicknesses ($\sim $0.5$\mu $m), after which an amorphous nucleates and takes over film growth. Above $\sim $600\r{ }C, however, epitaxy is possible to very large thicknesses (at least 11 $\mu $m) and no breakdown is observed. We present an isotropic model for growth that explains the morphologies observed after breakdown to a-Si:H at low temperatures. The cause of breakdown, however, is still not well understood. However, our hot-wire CVD experiments over a large range of temperatures (200-700\r{ }C) provide important insights into the roles of roughness (that is implicated in the failure of low-T molecular beam silicon epitaxy) and hydrogen in epitaxy failure. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H40.00002: Silicon epitaxy onto silicon wafers above 600\r{ }C by 100 nm/min hot-wire chemical vapor deposition Paul Stradins, Charles W. Teplin, Kim Jones, Robert C. Reedy, Qi Wang, Howard M. Branz We study a new silicon epitaxy regime by hot-wire chemical deposition onto silicon surfaces above 600\r{ }C. In this regime, epitaxy proceeds at high growth rates ($>$100 nm/min) compared with lower-T growth, and does not appear to be thickness-limited. With a tantalum hot-wire operating at 1900\r{ }C in SiH$_{4}$, we obtain phase-pure Si at 77 nm/min on (100)-oriented wafers at 650\r{ }C. With a tungsten filament at 2100 \r{ }C, phase-pure epitaxy proceeds faster than 100 nm/min from 620 to 700\r{ }C. Epitaxial growth up to 11{\-}$\mu $m thick is confirmed by transmission electron microscopy, x-ray diffraction and in-situ ellipsometry. This relatively low T epitaxial growth regime could be utilized for photovoltaic devices made by epitaxial thickening of c-Si seeds on low-cost substrate such as borosilicate glass. Temperature above 600$^{o}$C and corresponding thermal dehydrogenation of the growing surface is critical for the high-quality, rapid epitaxial growth: between 450 and 600\r{ }C, there appears to be an intermediate region where epitaxy is poor or nonexistent -- even in comparison with epitaxy at 200 to 400\r{ }C. The role of gas depletion chemistry, as well as structural/electronic quality is discussed. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H40.00003: Study on the growth of ZnO micro and nano-structures at low temperature and atmospheric pressure M. Morales, B. Claflin, G.C. Farlow, D.C. Look Deposition of ZnO from the vapor in flowing carrier gases has been studied for use in the growth of micro- and nano- structures. We have investigated how variations in the carrier gas composition and flow rate and the position of the substrate control the morphology of the nanostructures. Source material was either Zn powder or Zn acetate, either evaporated (powder) or decomposed (acetate) in the temperature range $500^oC$ to $650^oC$ in flowing $Ar/O_2$ at atmospheric pressure. It was also found that Zn powder must be washed in HCl to achieve reliable deposition at the lower temperatures. Scanning electron microscopy (SEM) images of samples grown from a Zn acetate source show micron-sized chimneys forming at 5 cm from the source, to 100 nm dispersed crystals at 7 cm or greater distance from the source. SEM images of samples grown from a Zn powder source show forrested needles $\sim$ 100 nm in diameter by 1 micron long.Photoluminescence measurements from these samples show a dominate line at 3.36 eV with additional features at 3.32 and 3.37 eV. The line widths are $\sim$ 3.5 meV, indicating good quality material. The usual gree-band emission is also observed. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H40.00004: Novel ZnO nanostructures grown by the templation of SiO$_{2}$ photonic crystals Hsia Yu Lin, Yang Fang Chen ZnO nanostructures were grown on assembled silica nanoparticles by vapor-liquid-solid (VLS) method. It is found that high aspect ratio of nanotips can be obtained. Besides, the nanotips can only be grown along some restricted axial orientations. As the tip length increases, the tip shape gradually transfers from hexagonal to circular. The optical measurements show that in addition to the band edge emission, two defect states are found, which locate at different regions along the growth direction. Moreover, we show that the ZnO nanotips exhibit an excellent waveguide for steering light beam generated from the Tb nanoparticle incorporated in the central core of SiO$_{2}$ particle. Therefore, it is pointed out that the core-shell Tb- SiO$_{2}$ with the growth of ZnO nanotips on its surface can act as an optical hub. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H40.00005: P-type Doping and Electroluminescence in ZnO Thin Films David Norton, Hyun-Sik Kim, Jean-Marie Erie, Patrick Sadik, Stephen Pearton, Fan Ren As a direct bandgap material with emission in the ultraviolet, ZnO is being actively pursued in the areas of ultraviolet light emitting diodes and laser diodes. The critical issues in developing such optoelectronic devices include p-type doping, minority carrier injection, and defects. In this talk, the focus will be doping and transport properties of phosphorus-doped ZnO films and heterostructures, including Hall measurement characterization. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H40.00006: The effects of vacuum annealing on the top-most layer of 6H-SiC measured by Positron annihilation induced Auger Spectroscopy S. Mukherjee, M. Nadesalingam, B. Davis, G. Brauer, A. H. Weiss Silicon Carbide (SiC) in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel application in electronics. The wide range of the band gap offered by different polytype with very little lattice mismatch can be utilized to grow smooth heterojunctions. Till now it has not been achieved and hence the surface characterization of such crystals is critical. Positron Annihilation induced Auger Electron Spectroscopy (PAES) is an established tool to characterize the top most atomic surface layer of solids. Here, PAES has been used to study the surface of 6H- SiC after annealing under different thermal and ambient conditions. The PAES measurements indicate that top-most atomic layer becomes C rich after vacuum annealing at 800 C. In additional a large chemical shift in the Si peak of approximately -12 eV was observed with PAES. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H40.00007: Improved Schottky Contacts on n-type SiC using ZrB$_{2}$ Tom Oder, Pamela Martin, Adetayo Adedeji, Tamara Isaacs-Smith, John Williams We present results on ZrB$_{2}$ Schottky contacts deposited on n-type SiC by DC magnetron sputtering at temperatures between 20 $^{o}$C and 800 $^{o}$C. The Schottky barrier heights determined by current-voltage measurements, increased with the deposition temperature from 0.87 eV for contacts deposited at 20 $^{o}$C to 1.07 eV for those deposited at 600 $^{o}$C. The RBS spectra of these contacts revealed a substantial decrease in oxygen peak with increase in the deposition temperature and showed no reaction at the ZrB$_{2}$/SiC interface. The barrier heights of the contacts annealed in nitrogen for 20 mins at 200 $^{o}$C to 500 $^{o}$C using a rapid thermal processor revealed only a slight increase. These results indicate improvement in the electrical properties and thermal stability of ZrB$_{2}$ on n-type SiC when the contacts are deposited at elevated temperatures, making them attractive for high temperature applications. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H40.00008: Step Arrays on Vicinal SiC Formed by Hydrogen-Etching S. Nie, R.M. Feenstra, Y. Ke, R.P. Devaty, W.J. Choyke, G. Gu SiC is a useful substrate for heteroepitaxy, with step arrays on the surface used to minimize defects in the film [1]. We have studied the formation of steps on SiC surfaces using H-etching at 1600\r{ }C. Both Si-face, (0001), and C-face, (000\underline {1}), surfaces are used, with miscut angles of 0, 3.5\r{ }, and 12\r{ } towards $<$1\underline {1}00$>$ or $<$11\underline {2}0$>$ directions. For H-etched surfaces it is known that steps tend to form with full unit-cell height (1.5 nm for 6H-SiC) and with step edges perpendicular to $<$1\underline {1}00$>$ [2]. Accordingly, we find that miscuts towards $<$1\underline {1}00$>$ result in ordered arrays of steps. On the Si-face step bunching is observed, with typical step heights of 4.5 nm for 12\r{ } miscut. In contrast, for the C-face, little step bunching is observed, with the surface forming well ordered arrays of single-unit-cell-high steps. For the case of miscut towards $<$11\underline {2}0$>$ the situation is more complicated, with meandering steps observed. We conclude that the C-face is most ideal as a vicinal template. Supported by NSF. \newline [1] C.D. Lee et al. MIJ-NSR \textbf{7}, 2 (2002). \newline [2] V. Ramachandran et al. JEM \textbf{27}, 308 (1998). [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H40.00009: Controlling Degree of Crystalline Boron Carbide by Plasma Enhanced Chemical Vapor Deposition Joseph Sandstrom There has been a recent resurgence in the interest of semiconducting boron carbide, based on its use as a radiation hard semiconductor. Here, we present growth character and commensurate structural and electronic properties from the low temperature but large area (6" wafer) deposition of boron carbide from the solid source precursor, 1,2 - dicarbadodecaborane. Of special interest is the control over the degree of crystallinity as provided from changing plasma pressure growth. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H40.00010: A comparative study of dissociation pathways of silane {\&} germane on Si (001) using cluster {\&} slab formalism in density functional theory. Abir De Sarkar, Li Qiang, Hway Chuan Kang, David J. Srolovitz Si-Ge thin films are of enormous importance in view of their wide gamut of applications in electronic, opto-electronic, photonic devices, etc. Therefore, a concrete understanding of the elementary processes that lead to film growth is indispensable for an optimum control of film growth. Thin films are typically grown by chemical vapor deposition (CVD) using gas phase precursors like silane {\&} germane. Dissociation of the gas phase precursor (e.g. silane, germane) on the substrate (e.g. Si, Ge) is the first and the most crucial elementary step in film growth. In order to obtain a clear understanding and insight into this mechanism, we have investigated the different possible pathways for silane and germane dissocation on Si(001). (001) surface of Si is typically and preferably used as substrate in film growth. Both germane and silane have been found to exhibit a preference for the intradimer mode of adsorption to the interdimer mode of adsorption on Si(001) surface. Germane shows a lower barrier for dissociative adsorption and the difference of barriers between these 2 modes of adsorption is higher for silane. Unlike the barriers for hydrogen desorption from Si-Ge surface (where the cluster approach predicts a higher barrier), the barriers predicted by the slab formalism for dissociative adsorption has been observed to be higher than the cluster values. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H40.00011: Kinetically constraint 0-D and 1-D heteroepitaxial islands growth Zhipeng Li, Manish Kumar Singh, Eng Soon Tok, Joyce Pei Ying Tan, Min Lin, Yong-Lim Foo Direct observation of the dynamics, formation and selective growth of low dimensional epitaxial Fe$_{13}$Ge$_{8}$ structures (0-D compact islands or 1-D wires of different aspect ratios) was conducted in \textit{real time} using \textit{in-situ }ultra-high vacuum transmission electron microscopy at 350, 430, 480 and 510$^{\circ}$C. Different types of Fe$_{13}$Ge$_{8}$ islands (0-D/1-D) and aspect ratio were formed at all the different deposition temperatures. Both types of island share the same epitaxial relation to the underlying Ge substrate. The compact islands are formed preferentially at lower deposition temperature while wires, which are kinetically constrained to grow, at higher temperature. The length/width growth rate was investigated by measuring the island size/shape evolution at different deposition temperatures during growth. The effective E$_{a}$ for growth along two orthogonal azimuths of an Fe$_{13}$Ge$_{8}$ island are 0.17 and 0.95 eV. The temperature dependence in morphology evolution is due to anisotropy in ledge diffusion on orthogonal azimuth during growth. We illustrate that temperature provides an avenue to selectively control the dimensionality (O-D or 1-D) of Fe$_{13}$Ge$_{8}$growth on Ge(001) surface. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H40.00012: Structure and Optical Properties of Ge-Sb-Te films Heng Li, Tong Ju, Don Williamson, P. Craig Taylor Amorphous and Crystalline films in the system Ge-Sb-Te are of interest because of their use in reversible phase change optical storage media or electrical switches. These applications utilize differences in optical or electrical properties between the crystalline and amorphous phases of the same material. The most commonly employed composition is Ge$_{2}$Sb$_{2}$Te$_{5}$. We present data on amorphous films of Ge$_{2}$Sb$_{2}$Te$_{4}$, Ge$_{2}$Sb$_{2}$Te$_{5 }$ and Ge$_{2}$Sb$_{2}$Te$_{7}$ grown by RF sputtering and examine the effects of growth rate, different growth systems and growth pressure on the structure and optical properties. Small Angle X-ray Scattering results show there are elliptical ``voids'' in the films with the long axis along the growth direction. The dependent of this void structure on growth parameters will be discussed. Lower Ar Pressure during growth produce less oxygen contamination in the films. The effects of oxygen impurities on the optical properties will be discussed. [Preview Abstract] |
Session H41: Semiconductors: Many-body and Correlated Systems
Sponsoring Units: FIAPChair: Allan MacDonald, University of Texas at Austin
Room: Colorado Convention Center 504
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H41.00001: A charge droplet picture of magnetotransport in disordered delta-doped heterostructures Malcolm Kennett, Vikram Tripathi We discuss theoretically how electrons confined to two dimensions in a delta-doped heterostructure can arrange themselves in a droplet-like spatial distribution due to disorder and screening effects when their density is low. We apply this droplet picture to magnetotransport and derive the expected dependence on electron density of several quantities relevant to this transport, in the regimes of weak and moderate magnetic fields. We find good qualitative and quantitative agreement between our calculations and recent experiments on delta-doped heterostructures. In particular we show that in the regime of magnetic fields where the resistivity $\rho$ varies with magnetic field $B$ as $\rho(B)\propto\exp[\alpha B^{2}]$, that $\alpha\propto n_{e}^{-\frac{3}{2}}$, where $n_{e}$ is the electron density in the 2DEG, even though the average tunneling distance between droplets is much larger than the average inter-electron spacing. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H41.00002: ABSTRACT HAS BEEN MOVED TO U32.0012 |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H41.00003: Topological defects, ``magnetic charges'' and coherence in dipolar condensates Egor Babaev The current progress in fabrication technology raised the expectation for the creation of excitonic condensates in semiconductor bilayers. We discuss an effective model for such bilayers and topological defects paying special attention to dynamics of the gauge field. We also discuss different physical systems which allow generalizations of the concept of dipolar superfluidity [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H41.00004: Wigner Supersolid of Excitons in Electron-Hole Bilayers Yogesh Joglekar, Alexander Balatsky, Sankar Das Sarma Bilayer electron-hole systems, where carriers in one layer are electrons and carriers in the other are holes, are expected to undergo Bose-Einstein condensation of excitons when the layer separation $d$ is much smaller than the interparticle distance $r_s a_B$ within each layer. We show, based on general principles, that there are two distinct ground states in this regime. The first, a uniform Bose condensate of excitons, has been studied in the literature. We predict the existence of a second state, a Wigner supersolid of excitons, that occurs in the region $\sqrt{r_s}\leq d/a_B\leq r_s$. In this phase, the excitons are phase coherent but form a Wigner crystal due to dipolar repulsion. We present a qualitative phase diagram of the bilayer system, and discuss properties and possible signatures of the Wigner supersolid phase. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H41.00005: Kinematics of Cold Excitons in the Laser Induced Exciton Trap A.T. Hammack, L.V. Butov, L.E. Smallwood, L. Mouchliadis, A.L. Ivanov, A.C. Gossard We have recently demonstrated laser induced trapping of indirect excitons in coupled quantum wells [1]. An important advantage of the laser induced exciton trapping is the possibility of controlling the trap in-situ by varying the laser intensity in space and time. Moreover, the excitons at the trap center are cold since they are far from the hot area of the laser excitation. Here, we report the studies of exciton kinetics in the laser induced traps. In particular, the kinetics of the trap formation and the exciton collection to the trap center when the excitation is switched on, as well as the kinetics of the exciton cloud when the excitation and trap are switched off have been measured by ultrafast imaging spectroscopy. The dynamics of the degenerate Bose gas of excitons in the trap was measured and analyzed for the first time. [1] A.T. Hammack, M. Griswold, L.V. Butov, L.E. Smallwood, A.L. Ivanov, and A.C. Gossard, Phys. Rev. Lett. 96, 227402 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H41.00006: Coherence Length of Cold Exciton Gases in Coupled Quantum Sen Yang, A.T. Hammack, M.M. Fogler, L.V. Butov, A.C. Gossard We report on emergence of spontaneous coherence of excitons at low temperatures. A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length reaches about 2-3 microns at the lowest temperature(1.5K), this corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H41.00007: Phase diagrams of the three-electron quantum dots Angbo Fang, Xuguang Chi, Ping Sheng We determine the rich phase diagrams for three-electron quantum dots in both the strong-correlation and the high-magnetic-field regimes, by employing an accurate and efficient non-variational approach. Through the complete separation of spatial rotation from kinematic rotation, the hidden symmetry of the zero angular momentum state is revealed, which is related to the space inversion properties of the system. We also show that although the ground state for arbitrary total angular momentum (L) forms an electronic Wigner molecule, the highest low-energy state for large L is liquid- like in character and beyond the classical description. Our results are compared to recent experimental results and other calculations. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H41.00008: Optical Detection of Bilayer Exciton Condensation Jung-Jung Su, Allan H. MacDonald The generation of quantum degenerate exciton clouds in semiconductor bilayers and the detection of their anticipated Bose condensation has generated both theoretical and experimental interest. Recently the angular distribution of luminescence from a coherent trapped bilayer exciton system was recently calculated based on a bosonic description of the excitions. Motivated by the expectation that the condensation temperature in these systems will be maximized when the exciton density exceeds the area per isolated exciton, we have examined how the underlying Fermi statistics, which is relevant in this regime, alters the luminescence properties. We will report on a luminescence spectrum calculation which starts from a Bogoliubov description of the condensed ground state. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H41.00009: Density-functional theory of interacting electrons in inhomogeneous quantum wires Saeed H. Abedinpour, Marco Polini, Gao Xianlong, Mario P. Tosi Motivated by the experimental evidence of electron localization in cleaved edge overgrowth quantum wires and by the recent interest in the development of density-functional schemes for inhomogeneous Luttinger and Luther-Emery liquids, we present a novel density-functional study of a few interacting electrons confined by power-law external potentials into a short portion of a thin quantum wire. The theory employs the quasi-one-dimensional (Q$1D$) homogeneous electron liquid as the reference system and transfers the appropriate Q$1D$ ground-state correlations to the confined inhomogeneous system through a suitable local-density approximation (LDA) to the exchange and correlation energy functional. The LDA describes accurately ``liquid-like'' phases at weak coupling but fails in describing the emergence of ``Wigner molecules'' at strong coupling. A local spin-density approximation allowing for the formation of antiferromagnetic quasi-order with increasing coupling strength is proposed as a first step to overcome this problem. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H41.00010: Optical Manipulation of collective spin correlations in semiconductors with polarization squeezed vacuum Eran Ginossar, Yehoshua Levinson, Shimon Levit We calculate the transfer rate of correlations from polarization entangled photons to the collective spin of a many-electron state in a two-band system. It is shown that when a semiconductor absorbs pairs of photons from a two-mode squeezed vacuum, certain fourth order electron-photon processes correlate the spins of the excited electron pairs of different quasi-momenta. Different distributions of the quantum Stokes vector of the light lead to either enhancement or reduction of the collective spin correlations, depending on the symmetry of the distribution. We find that as the squeezing of the light becomes non- classical, the spin correlations exhibit a crossover from being positive with a $\sim N^2$ ($N$ is average photon number) scaling, to being negative with $\sim N$ scaling, even when $N$ is not small. Negative spin correlations mean a preponderance of spin singlets in the optically generated state. We discuss the possibility to measure the collective spin correlations in a combined measurement of the Faraday rotation fluctuation spectrum and excitation density in a steady-state configuration. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H41.00011: Many-body interaction in semiconductors probed with 2D Fourier spectroscopy Mikhail Erementchouk, Michael Leuenberger, Lu Sham A particular difficulty in studying many-body interactions in a solid is the absence of an experimental technique that can directly probe their key characteristics. We show that 2D Fourier spectroscopy provides an efficient tool for the measurement of critical parameters describing the effect of many-body interactions on the optical response of semiconductors. We develop the basic microscopic theory of 2D Fourier spectroscopy of semiconductors in the framework of the three-band model (heavy holes, light holes, and electrons). The theory includes many-body correlations nonperturbatively. We show, in particular, that 2D Fourier spectrum allows one to make a distinction between the diffraction on the gratings created by the heavy- and the light-hole excitons. We apply the theory to an analysis of the available experimental data. Based on this analysis we are able to deduce the relative contributions to four-wave mixing of the interaction between the excitons with different and the same helicities. Experiments providing more detailed information are suggested. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H41.00012: Many-body Interactions in Semiconductors Probed by Optical Two-dimensional Fourier Transform Spectroscopy T. Zhang, X. Li, S. T. Cundiff, R. P. Mirin, I. Kuznetsova Optically excited semiconductors with dominant near band-gap features provide an ideal laboratory for the study of fundamental many-body problems in condensed matter physics. Optical two-dimensional (2D) Fourier transform spectroscopy, which correlates the phase evolution of the nonlinear polarization during the initial evolution and final emission periods, has been utilized to investigate the interactions of excitons (the quasi-particles of bound electron-hole pairs) in semiconductor quantum wells. The strength and lineshape of the excitons and continuum states in 2D spectra show the influences of various exciton interactions, including excitation-induced dephasing, excitation-induced shift, biexciton formation, and local field corrections. The 2D spectra are sensitive to experimental conditions such as tuning, excitation density and polarization. The signatures of exciton interactions under different excitation conditions, including co-linear, co-circular and cross-linear polarizations, are identified in a microscopic theory with nonlinearities up to the third order in coherent $\chi ^{(3)}$-limit beyond the Hartree-Fock level and achieve qualitative agreement with the experiments. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H41.00013: Crystal and Electronic Structure of Copper Sulfides Pavel Lukashev, Walter R. L. Lambrecht Because of the complexity of the crystal structure of Cu$_{2-x}$S, no electronic band structure studies have been performed in the past. These materials have S atoms on a (hcp) or at high-temperature (fcc) close packed lattice but the Cu atoms occupy various low-symmetry Wyckoff sites of which only the statistical distribution is known from X-ray diffraction experiments. Here, we constructed supercell models for the cubic and hexagonal phases with the Cu positions determined by a weighted random number generator. The electronic structure of both these models and the monoclinic structure are studied using the full-potential linearized muffin-tin orbital method in the local density approximation (LDA). Both LDA and GW quasiparticle calculations give a zero band gap for the latter. The supercell models gave small band gaps of order 0.1--0.2 eV. Adding a Cu-s shift as suggested by the antifluorite structure GW calculation and an analysis in terms of atomic orbitals, increases the gap to about 0.5 eV. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H41.00014: Influence of size effect and electron correlation to the energy band gap of CuFeO$_{2}$ and AgFeO$_{2}$ Khuong Ong, Kewu Bai, Peter Blaha, Ping Wu We have calculated the electronic structure of delafossite type oxides CuFeO$_{2}$ and AgFeO$_{2}$ using the Full Potential Linearlized Augmented Plane Wave (FP-LAPW) method within Perdew-Burke-Ernzerhof Generalized-Gradient Approximation (PBE-GGA). A metallic state instead of true insulating state is obtained for CuFeO$_{2}$ and AgFeO$_{2}$. The insulating state is reproduced when electron correlations have been taken into account. An effective Hubbard parameter for Fe, U$_{eff}$=7.86eV, has been derived based on an ab initio constraint calculation. This value is an over estimation for the optical band gaps of CuFeO$_{2}$ and most probably for AgFeO$_{2}$ as well. One reasonable U$_{eff}$ has been derived by comparing between computational and experimental X-Ray emission spectra. The energy band gap of CuFeO$_{2}$ and AgFeO$_{2}$ within the PBE-GGA+U is found as charge transfer gap. Theoretical optical band gaps $\Delta _{0}$=1.30eV, $\Delta _{1}$=2.06eV, and $\Delta _{2}$=3.20eV for CuFeO$_{2}$ are quite compatible with experimental data. For AgFeO$_{2}$ an optical band gap $\Delta _{0}$=1.90eV has been predicted. The size effect is considered as the origin of the increase in optical and energy band gaps of AgFeO$_{2}$ in comparison with CuFeO$_{2}$. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H41.00015: Electronic structure of chalcopyrite CuInSe$_2$: LDA and GW Cihan Parlak, Tanju Gurel, Resul Eryigit CuInSe$_2$ is an important ternary semiconductor for solar cell applications with the highest demonstrated conversion efficiency. We have investigated its electronic structure by using pseudopotential density functional theory at the local density approximation (LDA) as well as by Hedin's GW approximation schemes. GW calculations are performed in self- consistent form as well as at the static COHSEX approximation level. The COHSEX approach results are found to be in reasonable agreement with the experimental data. The role of Copper semicore states (3$s^2$3$p^6$) in the band structure is found to be negligible for the LDA calculations while its crucial for obtaining a correct ordering of the bands at the GW level. The overall GW band structure is found to be similar to the LDA one with an almost dispersionless scissor shift along the $Z-\Gamma-X$ direction of the Brillouin zone. [Preview Abstract] |
Session H42: Metals on Metals
Sponsoring Units: DCMPChair: Ted Einstein, University of Maryland
Room: Colorado Convention Center 505
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H42.00001: Effects of shadowing and steering in oblique-incidence metal growth J.G. Amar, Y. Shim, V. Borovikov The effects of oblique incidence on the surface morphology in metal (100) epitaxial growth are studied using a simplified model which includes shadowing but not the effects of short-range and long-range attraction of depositing atoms to the surface. Surprisingly, we find that many of the qualitative features observed in oblique incidence Cu(100) growth, including the existence of anisotropy in the submonolayer regime, as well as of a transition from anisotropic mounds to ripples perpendicular to the beam with increasing deposition angle, can be explained primarily by geometrical effects. We also find that the formation of (111) facets is crucial to the development of well-ordered ripples at large angles of incidence. A second transition from ripples oriented perpendicular to the beam to `rods' with (111) facets oriented parallel to the beam is also found at very high deposition angles and film thicknesses. When the effects of short- and long-range interactions are included in our simulations, we find two main effects. In the submonolayer regime, attraction tends to weaken the effects of shadowing and reduce the submonolayer anisotropy. However, in the multilayer regime `flux-focusing' due to long-range attraction tends to enhance the anisotropy and reduce the critical thickness/angle for the ripple transition. Near the transition from ripples to rods, sideways attraction also tends to stabilize the isotropic phase as is observed experimentally. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H42.00002: Multiscale simulations of oblique-incidence Cu/Cu(100) epitaxial growth V. Borovikov, Y. Shim, J.G. Amar We present the results of multiscale simulations of oblique-incidence Cu/Cu(100) epitaxial multilayer growth carried out in order to explain the experimentally observed ripple formation at large deposition angles with respect to the substrate normal. Our method combines a kinetic Monte Carlo (KMC) simulation for the thermal surface diffusion with a small-scale one-atom molecular dynamics (MD) simulation of every deposition event, in order to account correctly for shadowing as well as the short-range and long-range attraction of depositing atoms to the surface. These simulations are particularly challenging both because of the existence of significant finite-size effects at large deposition angles, as well as because of the computational time required to simulate the deposition process, and therefore an efficient algorithm for carrying out parallel simulations of deposition will be described. The relative importance of shadowing and long-range attraction, as well as the dependence of the surface morphology on such parameters as the Ehrlich-Schwoebel barrier, edge- and corner-diffusion, and deposition flux will be discussed. Preliminary results in which both the substrate atoms and the depositing atom undergo molecular dynamics in the final stages of deposition will also be presented for comparison. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H42.00003: Molecular Dynamics Simulations of Upward Diffusion of Adatoms and Clusters on Facetted fcc Metal (110) Surfaces Haili Yang, Qiang Sun, Songyou Wang, Shunfang Li, Zhenyu Zhang, Yu Jia We study upward self-diffusion of adatoms and small clusters across the outer edges of the mounds formed in fcc metal(110) homoepitaxy using molecular dynamics simulations with interatomic potentials described by the embedded-atom method. Our results show that both single adatoms and small clusters on the (111) and (100) facets of Al and Cu(110) can readily diffuse upwards and cross the outer edge of the mounds, but with different atomistic mechanisms. An adatom crosses the outer edge via a simple place exchange or indirect exchange mechanism. In contrast, the upward diffusion and outer-edge crossing of small clusters is realized by their dissociation at the edge of the mound after one or two cluster atoms are incorporated into the edge. Our simulations reveal that there truly exists efficient upward mass transport in homoepitaxy on facetted fcc metal (110) surfaces. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H42.00004: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H42.00005: Ab initio calculation of atomic interactions on Al(110): implications for epitaxial growth Kristen Fichthorn, Yogesh Tiwary Using first-principles calculations based on density-functional theory, we resolved atomic interactions between adsorbed Al atoms on Al(110). Relevant pair and trio interactions were quantified. We find that pair interactions extend to the third in-channel and second cross-channel neighbor on the anisotropic (110) surface. Beyond these distances, pair interactions are negligible. The nearest-neighbor interaction in the in-channel direction is attractive, but nearest-neighbor cross-channel interaction is repulsive. While nearest-neighbor, cross-channel repulsion does not support the experimental observation of 3D hut formation in Al/Al(110) homoepitaxial growth [1], we find that trio interactions can be significant and attractive and they support cross-channel bonding. The pair and trio interactions have direct and indirect components. We have quantified the electronic and elastic components of the indirect, substrate-mediated interactions. We also probe the influence of these interactions on the energy barriers for adatom hopping. [1] F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. Wang, and Z. Zhang, Phys. Rev. Lett. 91, 016102 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H42.00006: Multi-scale simulation of quantum dot formation in Al/Al (110) homoepitaxy Yogesh Tiwary, Kristen Fichthorn In experimental studies of Al(110) homoepitaxy, it is observed that over a certain temperature window (330-500K), 3D huts, up to 50 nm high with well defined and smooth (111) and (100) facets, form and self-organize over the micron scale [1]. The factors leading to this kinetic self-organization are currently unclear. To understand how these structures form and evolve, we simulated multi-layer, homoepitaxial growth on Al(110) using ab initio kinetic Monte Carlo (KMC). At the high temperatures, where nano-huts form, the KMC simulations are slow. To tackle this problem, we use a technique developed by Devita \& Sander [2], in which isolated adatoms make multiple moves in one step. We achieve high efficiency with this algorithm and we explore very high temperatures on large simulation lattices. We uncover a variety of interesting morphologies (Ripples, mounds, smooth surface, huts) that depend on the growth temperature. By varying the barriers for various rate processes, we discern the factors that determine hut sizes, aspect ratios, and self-organization. [1] F. Buatier de Mongeot, W. Zhu, A. Molle, R. Buzio, C. Boragno, U. Valbusa, E. Wang, and Z. Zhang, Phys. Rev. Lett. 91, 016102 (2003). [2] J.P. Devita \& L.M. Sander, Phys. Rev. B 72, 205421 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H42.00007: Positron trapping at quantum-dot like Cu nano-particles embedded in Fe and submonolayer films of Au and Pd deposited on Cu(100) surface. N. G. Fazleev, A. H. Weiss Recently clear evidence has been provided that positron spectroscopy can be used to characterize the properties of quantum-dot-like nano-particles embedded in host material even at dilute levels as a result of the preferential trapping of positrons in the nano-particles. The results of studies of sputtered surfaces of the Fe-Cu alloy with quantum-dot like Cu nano-particles embedded in the top atomic layers of Fe and submonolayer films of Au and Pd deposited on Cu(100) using Positron-Annihilation-Induced Auger-Electron Spectroscopy are analyzed by performing calculations of positron surface states and annihilation characteristics. Estimates of the positron binding energy, work function and annihilation characteristics performed for studied surfaces reveal their sensitivity to nano-particle size and coverage. Trapping of positrons at nano-particles on studied surfaces is determined from calculated positron surface state wave functions and comparison of theoretical core annihilation probabilities with experimental ones estimated from the measured Auger peak intensities. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H42.00008: Adatom Diffusion on Ag(100) and Cu (100) Surface with Steps: insights from \textit{ab initio} electronic structure calculations Handan Yildirim, Abdelkader Kara, Talat Rahman We present results of calculations of the activation barriers for an adatom (Ag or Cu) diffusing on terraces of Cu(100) and Ag(100), with steps on them. Our results based on the density functional theory (DFT) with the generalized gradient approximation, present systematic differences with those obtained using semi-empirical potentials. For the Ag systems, the latter are always higher than the former by about 40 meV, except for the case of hopping over the step where it is higher by 130 meV. The opposite is the case for Cu for which lower values are obtained by semi-empirical potentials than by DFT, except for the case of hopping over the step where this value is overestimated by the semi-empirical potentials. In examining the subtleties in the differences between the Ag and Cu systems, analysis of the d-band of the adatom in the hollow and bridge site show a systematic shift of the band towards higher energies when moving from hollow to saddle points. Moreover, Ag systems show a large narrowing of the d-band as opposed to the Cu cases where it is not noticeable. For adatom diffusion on the terraces (without steps) of Ag(100) and Cu(100), our calculations are in agreement with previous results. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H42.00009: Quantum Size Effect on Adatom Surface Diffusion Jinfeng Jia, Liying Ma, Lin Tang, Xucun Ma, Qikun Xue, Y. Han, Steve Huang, Feng Liu Using scanning tunnelling microscopy, we demonstrate the nucleation density of Fe islands on surface of nanoscale Pb films oscillates with film thickness, providing a direct manifestation of quantum size effect on surface diffusion. The Fe adatom diffusion barriers were derived to be 204 and 187 meV on a 21 and 26 monolayer (ML) Pb film, respectively, by matching the kinetic Monte Carlo simulations to the experimental island densities. The effect is further illustrated by growth on wedged Pb films, where the Fe island density is consistently higher on the odd-layer films than on the even-layer films in the thickness range of 11 to 15 ML. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H42.00010: Multisite Interactions in Lattice-Gas Models of Adsorbates: Reconciling Adatom Relaxations at Steps T.L. Einstein, Rajesh Sathiyanarayanan, T.J. Stasevich In a lattice-gas (LG) framework for (111) cubic surfaces, pair interactions cannot distinguish A and B steps, but an orientation-dependent three adatom (trio) interaction can, as we verify with VASP for Cu(111).\footnote{TJS et al., PRB a){\bf 70}('04)245404, b){\bf 71}('05)245414, c){\bf 73}('06)115426.}$^b$ However, on Pt(111), small clusters considerably underestimate the difference.\footnote{Feibelman, Surf.\ Sci.\ {\bf 463}('00)L661; Michely et al., ibid.\ {\bf 256}('91)217.} For a sequence of overlayer configurations, we explore the role of lateral relaxations and how they complicate LG analysis. On Cu(100) our prior VASP calculations of a particular trio interaction energy (E$_{\rm d}$) gave a large positive value.$^{3c}$ This nearly cancels the attractive second-neighbor interaction energy (E$_2$), leading to a discrepancy between theory and experiment of step stiffness anisotropy.$^{3a}$ Relaxations at step edges greatly reduce this repulsion. Since position-dependent interactions are improper in LG models, we show how to deal with this phenomenon using a quarto interactions. We comment on extensions to (110) faces and analytic expressions for step stiffness. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H42.00011: Heterogeneity in ultrathin films simulated by Monte Carlo method Jiebing Sun, James B. Hannon, Gary L. Kellogg, Karsten Pohl The 3D composition profile of ultra-thin Pd films on Cu(001) has been experimentally determined using low energy electron microscopy (LEEM).$^{[1]}$ Quantitative measurements of the alloy concentration profile near steps show that the Pd distribution in the 3$^{rd}$ layer is heterogeneous due to step overgrowth during Pd deposition. Interestingly, the Pd distribution in the 2$^{nd}$ layer is also heterogeneous, and appears to be correlated with the distribution in the 1$^{st}$ layer. We describe Monte Carlo simulations that show that correlation is due to Cu-Pd attraction, and that the 2$^{nd}$ layer Pd is, in fact, laterally equilibrated. By comparing measured and simulated concentration profiles, we can estimate this attraction within a simple bond counting model. [1] J. B. Hannon, J. Sun, K. Pohl, G. L. Kellogg, \textit{Phys. Rev. Lett.} \textbf{96}, 246103 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H42.00012: Diffuse x-ray scattering study of vacancy nanoclusters incorporated into thin Ag(001) homoepitaxial films Shawn Hayden, Chinkyo Kim, Craig Jeffrey, Rui Feng, Edward Conrad, Philip Ryan, Michael Gramlich, Paul Miceli In situ diffuse x-ray scattering was used to investigate vacancies that are incorporated into Ag films on Ag(001). Knowledge of the kinetic pathways through which these vacancies are incorporated is fundamental to our understanding of how films and nanostructures grow at the atomic scale. Unexpectedly large vacancy clusters having a volume of 750 ang$^3$ were observed in 100 ML films grown at 150K. Vacancy clusters are also found to incorporate at 300K where the films grow layer-by-layer. Studies of thinner films (5, 10, 20 ML grown at 150K) indicate that the vacancy clusters begin their formation immediately upon deposition and that the vacancies, once initiated, are stable to both their enlargement and subsequent burial. This has important implications for the mechanisms that control the growth and organization of nanostructures. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H42.00013: Modeling of the Kinetics of Metal Film Growth on 5-Fold Surfaces of Icosohedral Quasicrystals J.W. Evans, B. Unal, V. Fournee, C. Ghosh, D.-J. Liu, C.J. Jenks, P.A. Thiel During submonolayer deposition of metals on 5-f icosohedral Al- Pd-Mn and Al-Cu-Fe surfaces, experimental evidence for several system points to heterogeneous nucleation of islands at specific ``dark star'' trap sites. We model this phenomenon using a mean-field rate equation formulation for Ag on Al-Pd-Mn, where data is available for both the flux and temperature dependence of the island density. We also utilize a more sophisticated kinetic Monte Carlo simulation approach to analyze an atomistic lattice-gas model (for an appropriate ``disordered-bond-network'' of nearest-neighbor adsorption sites) describing nucleation of starfish islands observed by STM for Al on Al-Cu-Fe. Finally, we briefly describe multilayer growth morphologies (which can display kinetic roughening or quantum size effects), but which also generally reflect the submonolayer island distribution. B. Unal et al. PRB 75 (2007); C. Ghosh et al. Phil. Mag. 86 (2006) 831; Surf. Sci. 600 (2006) 1110; V. Fournee et al. PRL 95 (2005) 155504. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H42.00014: Surface Self-Assembly Driven by Dislocation Annihilation and Glide Bogdan Diaconescu, Karsten Pohl The process of growing nanometer sized ordered arrays of clusters on the misfit dislocation networks of strained metallic thin films [1, 2] requires a detailed understanding of the nucleation and film-adsorbate interaction at the atomic level. In the case of sulfur adsorption on submonolayer silver films on 0001 surface of ruthenium, the Ag's short herring bone rectangular misfit dislocation unit cell of 19(18)x16(15) Ag atoms reconstructs into a well-ordered triangular array of S filled Ag vacancy islands 18 Ag atoms apart. Atomically and time resolved variable temperature STM measurements correlated with 2D Frenkel-Kontorova models based on first principles interaction parameters show how a sequential process of Ag vacancy island formation, followed by annihilation of adjacent and opposite pairs of threading dislocation and glide of Shockley partial dislocations generates the uniform triangular array of Ag vacancy islands. We conclude that the strain in the Ag film is the driving force responsible for the surface self-assembly process. 1. Pohl \textit{et al.,} Nature 397, 238 (1999) 2. K. Th\"{u}rmer \textit{et al.}, Science 311, 1272 (2006) [Preview Abstract] |
Session H43: Focus Session: Physics of Thermoelectric Materials and Phenomena II
Sponsoring Units: FIAP DMPChair: Qiang Li, Brookhaven National Laboratory
Room: Colorado Convention Center 506
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H43.00001: Solid State Thermionic Energy Conversion Invited Speaker: An overview of the activities at the Thermionic Energy Conversion Center will be given. This is a consortium of twelve research groups that are working to optimize thermoelectric properties of embedded metallic nanoparticles and multilayers. Hot electron filtering using heterostructure barriers is used to break the trade off between high Seebeck coefficient and high electrical conductivity. Embedded ErAs nanoparticles and metal/semiconductor multilayers are used to reduce the lattice thermal conductivity without significant effect on electrical conductivity. The implication of the superlattice transport on the electronic thermal conductivity and Lorenz number will also be discussed. Cross-plane and in-plane thermoelectric properties are characterized in a wide temperature range. The effective ZT of the thin film is measured using the transient Harman technique. Integrated circuit fabrication techniques are used to transfer the n- and p-type thin films on AlN substrates and make power generation modules with hundreds of thin film elements. Potential for energy conversion efficiency exceeding 20{\%} and high power density $>$1W/cm2 will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H43.00002: High temperature cross-plane Seebeck coefficient measurement of ErAs:InGaAs/InGaAlAs superlattice Zhixi Bian, Mona Zebarjadi, Ali Shakouri, Gehong Zeng, John Bowers The 3$\omega $ technique is used to measure the Seebeck coefficient across 2.4 micron superlattices made of 80$\times $((InGaAs)$_{0.6}$(lnAlAs)$_{0.4}$ -10nm / InGaAs-20 nm) films lattice matched to InP substrate. ErAs nanoparticles are randomly distributed inside the 20 nm InGaAs layer. We characterized 4 samples with different doping concentrations (from 2$\times $10$^{18}$ cm$^{-3}$ to 10$^{19}$cm$^{-3})$ in a temperature range of 300K to 600 K. A significant increase in the cross plane Seebeck coefficient compared to the in plane one is observed. Comparison with DC measurement shows that the 3$\omega $ method is more accurate especially at high temperatures. Theoretical analysis based on the solution of the coupled Schr\"{o}dinger and Poisson equations, together with modified Boltzmann transport equation is used to explain the experimental results. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H43.00003: Electronic Properties of Layered Cobaltates. Qing Jie, Qiang Li Electronic properties of layered cobaltates are strikingly different from those in conventional metals, that result in water-induced superconductivity and large thermoelectric power factor. Here we report a transport study of thermoelectric power and electrical conductivity as a function of temperature and magnetic field in several cobaltate single crystals and thin films: Ca$_{3}$Co$_{4}$O$_{9}$ and highly doped Na$_{x}$CoO$_{2}$. There are three temperature regions where the electronic transport properties of these cobaltates exhibit distinctive behaviors. At low temperatures, the transport property is strongly influenced by the magnetism. At the elevated temperatures, it shows the feature of a correlated metal. At high temperatures, it has weak temperature dependence. These results will be compared with the infrared studies. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H43.00004: Crystal Structure of Misfit Thermoelectric Compound [Ca$_{2}$CoO$_{3}$]$_{0.62}$CoO$_{2}$ by Electron Diffraction and High Resolution Electron Microscopy P. Oleynikov, V. Volkov, Q. Jie, Q. Li, Y. Zhu Layered cobaltates are of great interest from a physics point of view, as shown by their thermoelectric and magnetoresistance properties. In order to understand the origin of physical properties of layered cobaltates and, in particular, of misfit cobalt oxide [Ca$_{2}$CoO$_{3}$]$_{0.62}$CoO$_{2}$ with high thermoelectric power, an accurate determination of the crystal structure is required. Ambiguities of the structure analysis of this compound performed by X-ray methods stimulated us to re-examine its structure by the electron diffraction patterns (DP) and high resolution electron microscopy (HREM) methods. Single crystal [Ca$_{2}$CoO$_{3}$]$_{0.62}$CoO$_{2}$ grown under SrCl$_{2}$-flux has a misfit lattice structure with rock-salt type [Ca$_{2}$CoO$_{3}$] slabs and interpenetrating CdI$_{2}$-type [CoO$_{2}$] layers stacked along $c$-axis and incommensuration along $b$-axis. The nearest commensurate structure with ratio of sublattice parameters $b_{1}$/$b_{2}$ = 5:8 closely matches the 0.62 composition index. Analysis of HREM images and the presence of lines with diffuse scattering and weak spots on the ($h$0$l)$ DP lead to the tripled $a$ unit cell parameter, which was significantly different from the average structure determined by X-ray and neutrons. Work supported by the U.S. DOE, BES (DE-AC02-98CH10886). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H43.00005: Infrared Studies of Charge Dynamics in Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films and Single Crystals Jiufeng Tu, Zhijun Xu, Weidong Si, Qiang Li In recent years, the 2D-layered cobaltates have emerged as promising p-type thermoelectric materials. These systems show high thermoelectric figure of merit and are ideal candidates as the materials of choice at elevated temperatures. We have carried out infrared reflectivity studies of Ca$_{3}$Co$_{4}$O$_{9}$ thin films and single crystals as a function of frequency and temperature with the emphasis on the coupling between the lattice, the charge and the spin degrees of freedom. Several important features have been observed: (1) the overall reflectivity is low as a result of a small carrier density in this system (the plasma frequency around 100 meV); (2) several phonon features are observed and some exhibit evidence of strong electron-phonon coupling; (3) a gap-like structure is seen in the low frequency region below 100 K that could be responsible to the insulating behavior observed in transport measurements at low temperatures. Further infrared studies will be performed in magnetic field (both parallel and perpendicular to the CoO$_{2}$ layers). A good understanding of our infrared results should shed light on the origin of high thermo-power in these 2D-layered cobaltates. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H43.00006: Thermoelectric properties of epitaxial Li$_{x}$CoO$_{2}$ thin films . Zhigang Ma, A. Venimadhav, Qi Li, X. X. Xi, H. P. Sun , Xiaoqing Pan We have studied the thermoelectric properties of layered cobaltate Li$_{x}$CoO$_{2}$ since the similar compound Na$_{x}$CoO$_{2}$ has shown exceptionally high thermoelectric power. Both \textit{in situ} epitaxial grown and topotaxial Li$_{x}$CoO$_{2}$ films have been achieved. Epitaxial films were grown by pulsed-laser deposition technique and topotaxial films were prepared by converting an epitaxial Co$_{3}$O$_{4}$ film to Li$_{x}$CoO$_{2 }$by annealing in Li vapor. X-ray diffraction analysis showed the films are $c$-axis oriented. For topotaxial Li$_{x}$CoO$_{2 }$the largest thermoelectric power of the samples is found to be around 380 $\mu $V/K at room temperature, while\textit{ in situ} films show thermopower of 100 $\mu $V/K. Both show semiconducting behaviors. The difference on the thermopower of the two types of samples will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H43.00007: Thermal Conductivity Minima in Superlattices and Localization-like Phenomena Rama Venkatasubramanian It is becoming clear in many 2-dimensional superlattice (SL) material systems that there exists a minimum lattice thermal conductivity for an optimal SL period. These have been first observed and reported in the Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$, PbTe/PbTeSe and Si/Ge SL systems by us in RTI. These minima become evident when the electronic thermal conductivity, using Lorentz parameter, is subtracted from the total thermal conductivity to monitor the lattice thermal conductivity as a function of SL period. The basis for the numerical value of Lorentz parameter, observed from many facets of material and device characteristics, will be presented. Such a lattice thermal conductivity minimum has also been recently observed in other SL material systems. Recently, a similar behavior has also been observed in the thermal conductivity of superlattices embedded with an ordered array of nanoparticles. We will explore the commonality of these results in terms of a localization-like behavior for phonons. The arguments for the complex relationship between the SL period and the low-frequency cut-off wavelength, traceable to a cut-off frequency originating from diffusive transport of a temperature wave, will be presented. The physics behind what triggers the localization-like phenomena of phonons in such nanostructures will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H43.00008: Si/Ge Superlattice Structures for Thermoelectric Power Generation James Caylor, Rama Venkatasubramanian Research conducted at RTI into the use of thin films, in particular superlattice materials, for thermoelectric power generation has focused on three materials families: Bi$_{2}$Te$_{3}$, PbTe, and Si/Ge. The Bi$_{2}$Te$_{3}$-based superlattice materials have already produced record ZT (thermoelectric figure-of-merit) and devices using these low temperature materials ($\sim $200\r{ }C) have surpassed bulk performance during power generation. RTI has also developed the growth of Si/Ge superlattice films as well as their fabrication into power generation devices applications at higher temperatures ($\sim $500\r{ }C). Results presented will include confirmation of superlattice structure via X-ray diffraction, showing well formed satellite peaks indicative of a high-quality superlattice. In addition, data will be presented that shows the lowering of thermal conductivity by the superlattice structure with nearly a 5x reduction in lattice thermal conductivity compared to the alloy film. Initial thin-film power device results showing $>$2{\%} efficiency and 2x improvement of ZT over SiGe alloys, at $\Delta $T of 378K and T$_{mean}$ of 484K, will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H43.00009: Ultra-low thermal conductivity in disordered, layered tungsten diselenide Catalin Chiritescu, David Cahill, Nguyen Ngoc, David Johnson, Arun Bodapati, Pawel Keblinski, Paul Zschack Ultra low thermal conductivity of tungsten diselenide (WSe$_{2})$ thin films is achieved by controlling order and disorder of two dimensional WSe$_{2}$ sheets. We prepared highly textured nanocrystalline WSe$_{2}$ films by modulated elemental reactant (MER) method. Synchrotron X-ray diffraction shows the WSe$_{2}$ sheets are well aligned with the Si (100) substrate and the films have completely random crystalline orientation in the a-b plane. The cross-plane thermal conductivity of thin films of WSe$_{2}$ is as small as 0.05 W/m-K at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe$_{2}$ and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. Molecular dynamics simulation on model structures suggests that the ultra-low thermal conductivity in layered, disordered crystals is a general phenomenon and not restricted to WSe$_{2}$. Ion irradiation of the samples disrupted the layered structure and the crystallinity of the WSe$_{2}$ sheets and lead to an increase with a factor of 3 in thermal conductivity. We attribute the ultra-low thermal conductivity to the localization of lattice vibrations induced by the random stacking of two-dimensional crystalline WSe$_{2}$ sheets. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H43.00010: Micro- and nanomachined tools for measuring in-plane thermal conductivity of thermoelectric thin films R. Rahman, R. Sultan, F. Baset, B. L. Zink Many of the potential next-generation thermoelectric materials being studied are either thin films or nanostructures that are expected to have anisotropic properties. For example, the thermal conductivity of a layered thin film in the plane of the film, $k_{\parallel}$, is likely to be different from that perpendicular to the layers, $k_{\perp}$. Techniques such as the $3\omega$ method and picosecond thermoreflectance allow accurate measurements of $k_{\perp}$ at temperatures relevant to thermoelectrics, but measuring $k_{\parallel}$ is often difficult. In this talk we discuss our efforts to design and demonstrate accurate measurements of $k_{\parallel}$ of thin films from $77-475$ K using micro- and nanomachined thermal isolation platforms. Using thin-film structures to support the thin-film sample reduces background contributions, and careful control of the geometry keeps radiation errors small. We will also discuss plans for a next-generation device that will simultaneously measure thermal conductivity, thermopower, and electrical conductivity of a thin-film or nanostructure, allowing determination of the thermoelectric figure-of-merit, $ZT$. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H43.00011: Figure of merit for thermoelectric power generation estimated from enhanced mobility in [100] oriented \textit{$\beta $}-FeSi$_{2}$ thin film Hirofumi Kakemoto, Hajime Shibata, Satoshi Wada, Takaaki Tsurumi \textit{$\beta $}-FeSi$_{2}$ has been attracted to be applied to the thermoelectric device, for instance, the Seebeck coefficient shows the maximum value about 500$^{o}$C that it is good for thermoelectric power generation. However low figure of merit (Z) has been reported about 5x10$^{-4}$ K$^{-1}$. The Z is represented as $m$*$^{2/3}$(\textit{$\mu $}/\textit{$\kappa $}$_{ph})$, where $m$*, \textit{$\mu $} and \textit{$\kappa $}$_{ph}$ are effective mass, mobility and thermal conductivity, respectively. Although \textit{$\kappa $}$_{ph}$ is good for thermoelectric power, low \textit{$\mu $} has been reported as polaronic-conduction in \textit{$\beta $}-FeSi$_{2}$ crystal. In 3D electron density distribution of \textit{$\beta $}-FeSi$_{2}$ crystal, Si layer in the crystal shows covalent bonding network with Si atoms, and it suggests the new possibility for enhancement of \textit{$\mu $}. In this report, the objective is to exhibit the possibility for enhancement of Z in order to control the crystallographic orientation of \textit{$\beta $}-FeSi$_{2}$ crystal by means of film formation. \textit{$\beta $}-FeSi$_{2}$ thin film was prepared on Si(100) substrate using molecular beam epitaxy method. The crystallographic orientation of sample showed about 80{\%} of [100] direction from x-ray diffraction pattern. The transport properties were investigated using Hall measurement with van der Pauw electrode configuration. The resistivity and \textit{$\mu $} were also measured, and they were compared with \textit{$\beta $}-FeSi$_{2}$ polycrystal. In addition, enhancement of Z was estimated using above formula. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H43.00012: High-temperature ZT of InGaAlAs Thin Films with Embedded ErAs Nanoparticles Rajeev Singh, Zhixi Bian, Younes Ezzahri, Ali Shakouri, Gehong Zeng, John Bowers, Joshua Zide, Art Gossard We have measured the thermoelectric (TE) figure-of-merit (ZT) of InGaAlAs thin films with embedded ErAs nanoparticles over a wide temperature range (300K - 650K). This material system is currently being explored for use in power generation applications such as waste heat recovery. A novel high-speed measurement system was developed to measure the ZT of thin films of thicknesses on the order of 1um with a transient thermal signal resolution of 200ns at temperatures up to 900K. In order to resolve the intrinsic ZT of thin-film materials, TE devices were fabricated to minimize electrical and thermal parasitics and differential measurement was employed on TE devices of varying film thicknesses. The improvement in ZT of the material with ErAs nanoparticles embedded in the semiconductor matrix is verified throughout the temperature range. The increase in TE ZT is found to be mainly due to the reduction in material thermal conductivity due to phonon scattering by the ErAs nanoparticles. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H43.00013: Thermal Conductance Measurements of Aromatic and Aliphatic Self-Assembled Monolayers Robert Wang, Sung-Yeon Jang, R. A. Segalman, Arun Majumdar Thermal conductance measurements of solid-solid junctions separated by an interfacial organic self-assembled monolayer (SAM) suggest that molecular heterostructures are a promising new class of thermoelectric materials. Au-SAM-GaAs junctions were fabricated by nanotransfer printing and their thermal conductance was measured by the 3$\omega $ technique. SAMS investigated in this study consisted of quaterphenyldithiol and alkanedithiols of varying length. This study explores two key aspects of thermal transport in molecules: 1) the effect of aromatic versus aliphatic molecular structure 2) the effect of molecular length in the aliphatic molecule. Measurements of thermal conductance using the 3$\omega $ method were very robust to processing history and no thermal dependence on alkane chain length was observed. [Preview Abstract] |
Session H44: Focus Session: Optical Properties of Nanocavities and Structured Materials
Sponsoring Units: DMPChair: Glenn Solomon, National Institute of Standards and Technology
Room: Colorado Convention Center 507
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H44.00001: Directed Self-Assembly of Single Quantum Dot Optical Devices Invited Speaker: Techniques that allow one to control the nucleation site of individual, self-assembled quantum dots are necessary for the fabrication of optical and electronic devices that must align subsequent control structures to the dots. In this talk I will describe a ``nanotemplate deposition'' technique that allows one to direct the surface migration of deposited Indium species during crystal growth so that InAs quantum dot nucleation sites can be controlled \textit{a-priori} with nanometre precision. Using this technique one can construct high finesse, two-dimensional photonic crystal microcavity membranes in which the optical defect mode is aligned both spectrally and spatially to a single InAs/InP, site-selected quantum dot emitting at 1550nm. I will describe the fabrication and optical characterisation of such cavities, with Purcell factors in excess of 4,000 and mode volumes of approximately $0.5\left( {\lambda /n} \right)^3$, and I will illustrate how coupling to these cavities using a tapered nanowire micro-loop fiber can be used to optimize the extraction efficiency for single photon source applications. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H44.00002: Photoluminescence up-conversion of single quantum dots in a microcavity. E. B. Flagg, A. Muller, X. Y. Wang, D. G. Deppe, W. Ma, J. Zhang, G. J. Salamo, M. Xiao, C. K. Shih We have studied photoluminescence upconversion in single self-assembled InGaAs quantum dots that are embedded in a planar optical microcavity. Upconversion is generally thermal and can be characterized unambiguously due to the absence of inhomogeneous broadening. A side-excitation photoluminescence technique allows us to detect arbitrarily close to the laser line thereby distinguishing otherwise unresolvable energy splittings. This allows the investigation of upconversion as a function of both temperature and energy separation. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H44.00003: Low-threshold few-emitter quantum dot lasing. Glenn Solomon, Wei Fang, Stephan Goetzinger, Zhigang Xie Ultra-low threshold lasing via a single emitter is of strong fundamental interest in solid-state and atomic physics. While lasing from a single emitter has not yet been observed in solid-state systems, a quantum dot (QD) gain medium of only a few QD states can be coupled to an optical cavity mode and lase. We describe such an ultra-low threshold lasing system here utilizing a microdisk cavity and a dilute QD gain medium. The microdisk is GaAs and supports high quality-factor whispering gallery modes. The QD gain medium is composed of InAs-based QDs formed epitaxially through lattice mismatch strain. Our systems show lasing even in the smallest, sub-2 micrometer disk diameters. Because of the high cavity quality factor, we observe nondegenerate modes due to broken symmetry. A typical QD spectrum of discrete emission lines observed at lower pump power is often highly modified near transparency leading to pump power dependent absorption. Changes in the cavity linewidth, second-order correlation measurements, and output emission versus input pumping are used to verify lasing. The system has sub-microwatt CW lasing thresholds and exhibits lasing from a small number of emitters. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H44.00004: Resonance photoluminescence from a single semiconductor quantum dot in a microcavity. A. Muller, E.B. Flagg, X.Y. Wang, D.G. Deppe, W. Ma, J. Zhang, G.J. Salamo, M. Xiao, C.K. Shih The analogue of resonance fluorescence in atomic physics is demonstrated for the first time in a zero-dimensional solid-state system consisting of self-assembled InGaAs quantum dots. The dots were embedded in a planar microcavity so that the quantum dot emission, coupled to the resonant cavity modes, was effectively decoupled from the excitation field. The latter was introduced via waveguide modes with a fiber in a side-excitation configuration. The result is a background-free detection of a single quantum dot's photoluminescence which shows antibunched photon emission and can be driven into Rabi oscillations using pulsed excitation. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H44.00005: Optical microtube ring resonators formed by rolled-up strained semiconductor bilayers Tobias Kipp, Christian Strelow, Holger Welsch, Christian Heyn, Detlef Heitmann Starting from an epitaxially grown InGaAs/GaAs bilayer and using optical lithography and wet-etching processes, we utilize the self-rolling mechanism of strained bilayers to fabricate self-supporting microtubes with diameters of about 5 $\mu m$ and wall thicknesses of only 100 -– 200 nm. We demonstrate these structure to act as optical ring resonators by measuring the photoluminescence of an optically active material, either quantum wells or self-assembled quantum dots, which is embedded into the tubes' walls. We find spectra of sharp modes arising from constructive interference of light running circularly around the micotube's axis inside its wall. The mode structure is in very good agreement with the result of a theoretical modeling of the microtube as a thin dielectric waveguide forming a closed ring. These novel microtube ring resonators, in which the optically active material is intrinsically located close to the optical field maximum, are good candidates for both, new optoelectronic devices and cavity quantum electrodynamic experiments. We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft via the SFB 508 ``Quantum Materials'' and the Graduiertenkolleg 1286 ``Functional Metal-Semiconductor Hybrid Systems.'' [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H44.00006: Voltage-controlled Deformation of Photonic Crystal Membranes Hubert J. Krenner, H. Kim, S. M. Thon, D. Bouwmeester, N. G. Stoltz, P. M. Petroff We present a novel photonic device consisting of a free-standing Photonic Crystal (PC) membrane which can be mechanically deformed by an external voltage. This is realized by introducing doped layers in the membrane and the underlying substrate. We embed self-assembled InAs quantum dots (QDs) in the membrane as active emitters. In a first step metal contacts are fabricated for both doped layers of the device. Two-dimensional PC microcavities are defined by electron beam lithography and ICP etching. The PC membrane is finalized by selectively removing an AlGaAs layer underneath the patterned region. By applying a bias voltage between the two contacts we are able to change the electrostatic force between the substrate and the membrane analogous to a plate capacitor. Due to the small thickness of the membrane the electrostatic force leads to a deformation with vertical displacements up to 250nm at room temperature. We demonstrate that at low temperatures this displacement can be reversibly changed over a wide range by an external voltage leading to a visible deformation of the membrane. We present first results of micro-PL experiments to probe the influence of the deformation on the optical modes of PC microcavities. - Supported by the Alexander-von-Humboldt Foundation [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H44.00007: Controlling the Optical Properties of Self-Assembled Quantum Dots Using External Strain M. Zielinski, W. Jaskolski, G. W. Bryant, J. Diaz, J. Aizpurua Passive control of the optics of self-assembled quantum dots is achieved by controlling dot size, shape and composition via growth. Local strain from lattice mismatch between the dot and barrier influences the electronic properties. Dynamical control could be achieved via imposed external strain to change level degeneracies, polarize transitions, or modify coupling between dots. Moreover dots could be coupled to bending modes to optically cool nanomechanical oscillators to the quantum limit. To understand the impact of externally imposed strain on the electronic states of self-assembled dots, we use a tight- binding theory of dots that incorporates local strain from lattice mismatch and externally imposed strain from applied stressors or the bend in a nanomechanical oscillator. Energy level shifts depend on the position of the dot in a nanomechanical oscillator and how the oscillator is bent. Energy levels can red-shift and blue-shift depending on how the external strain is imposed. Shifts in the electronic levels due to different bending modes are determined. This allows us to assess how much active control is possible. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H44.00008: Strong Second Order Piezoelectric Effect in InGaAs/GaAs Nanostructures Gabriel Bester, Alex Zunger, Xifan Wu, David Vanderbilt We show that the piezoelectric effect that describes the emergence of an electric field in response to a crystal deformation has strong contributions in III-V semiconductors such as GaAs and InAs from second-order effects that have been neglected so far. We calculate the first and second-order piezoelectric tensors using density functional theory. Applying these calculated tensors to quantum wells [1] gives piezoelectric fields that agree well with experiments, whereas neglect of non-linearities leads to qualitative disagreements. We find that the linear and the quadratic piezoelectric coefficients have the opposite effect on the field. Which term dominates is stronlgy dependend on concentration $x$ for quantum wells and for large $x$ the quadratic terms strongly dominates. Applying our theory to quantum dots [2] shows that both term nearly cancel each other so neglecting piezoelectricity is a better approximation than using only the linear term. Thus, the piezoelectric field turns out to be a rare example of a physical quantity for which the first-order and second-order contributions are of comparable magnitude.\\[.1cm] [1] G. Bester, A. Zunger, X. Wu and D. Vanderbilt, Phys. Rev. B. {\bf 74}, 081305({\bf R}) (2006). [2] G. Bester, X. Wu, D. Vanderbilt and A. Zunger, Phys. Rev. Lett. {\bf 96}, 187602 (2006).\\[.1cm] [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H44.00009: Single molecule fluorescence decay rate statistics in clusters of nanoparticles Luis Froufe, Juan Jose Saenz, Remi Carminati In this work, we study the fluorescence rate statistics of a single emitter in a finite size (nanoscopic) random medium (cluster), made of small spherical particles. For a given configuration of the system, we calculate numerically the Green tensor of the system. We deduce the spontaneous decay rate $\Gamma$, as well the radiative and the nonradiative contributions. Repeating the calculation for the configuration distribution allows to compute the full statistics. These numerical experiments are used as a basis for a physical discussion. We focus on the regime in which the statistics is determined by near field interactions, with negligible multiple scattering. The decay rate statistics is influenced by the local environment of the emitter. In particular, we show that for moderate absorption, the nonradiative contribution is proportional to the imaginary part of the dielectric function of the particles, while the radiative contribution is almost constant. An important result is that the standard deviation exhibits diferent regimes dominated by either near-field scattering or absorption. This quantity could be used for nanoscale imaging in complex media. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H44.00010: Diffusion limit in complex media Lev Deych, Mikhail Eremetnchouk, Hui Cao, Heeso Noh, Alexander Lisyansky Structures with pre-engineered spatial modulations of the dielectric function attract a lot of attention because they provide possibility to control effectively propagation of light. Recently it has been realized that unusual optical characteristics of such structures have a deep impact on their general physical properties. Even such well-studied phenomenon as light transport in disordered media is strongly affected by the regular modulation of the dielectric function. As a result, the transport in complex media has specific features, which can not be understood in the framework of the standard theory. We present the general theory of the diffusive (completely incoherent) limit in disordered structures with regular modulation of the dielectric function. We establish a relation between this limit and equilibrium understood from the statistical physics point of view. We show, in particular, that in the case of weak disorder the diffusion limit is virtually independent on the disorder and is nontrivially determined by properties of the ideal structure. We demonstrate how the diffusion of light appears as the perturbation of equilibrium. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H44.00011: Aperiodic nanostructured optical devices fabricated with a femtosecond laser Timothy Gerke, Jeremy Brown, Wenjian Cai, Ariel Libertun, Rafael Piestun Periodic three-dimensional (3D) structures have increasingly caught the attention of the scientific community. Aperiodic 3D structures, however, have remained relatively unexplored. We present structural and optical characterization of 3D aperiodic nanostructures created by scanning focused femtosecond (fs) laser pulses to produce permanent refractive index changes inside glass. We created polarization-sensitive devices using the effect of fs-laser-induced birefringence in fused silica. In this regime, the laser-created plasma gives origin to subwavelength structures that generate anisotropy by the effect of form birefringence. We demonstrated polarization-selective computer-generated holograms using this effect in three dimensions. These holograms form different reconstructions for different illuminating polarization states [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H44.00012: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H44.00013: Azobenzene-based photomechanical monolayers as light-addressable nano-engineered structures. J.M. Dahdah, T.E. Furtak, D.M. Walba, G. Fang, Y. Yi, J.E. Maclennan, N.A. Clark Azobenzene-based photomechanical monolayers have received a great deal of attention for their potential as platforms for light-addressable nano-engineered structures in bioscience, photonics, and display technologies. We have developed an aminoazobenzene material (d-MR), derived from methyl red, which forms high-quality, covalently anchored monolayers on glass. These monolayers demonstrate unusually high sensitivity to polarized light, which controls the molecular orientation distribution through optical anisotropy of the trans-cis isomerization. In an effort to understand and optimize this phenomenon we are studying the influence of the two-dimensional molecular field on the dynamics of the light-driven reorganization. We have correlated the behavior of d-MR monolayers, as determined by spectral studies of dichroism and differential reflection ellipsometry, to dilute solutions of d-MR in a variety of solvents, as characterized by absorption cross sections, quantum yields, and characteristic time constants. The resulting information has helped to clarify the details of how these molecules respond to light leading to design strategies for even higher performing monolayers. [Preview Abstract] |
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