Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P1: Optoelectronic Manipulation and Control of Charges and Spins in Quantum Dots
Sponsoring Units: DCMPChair: Alexander Efros, Naval Research Laboratory
Room: Spirit of Pittsburgh Ballrom A
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P1.00001: Optical control of spin coherence in singly charged quantum dots Invited Speaker: The most promising candidate for implementation of quantum information technologies in semiconductors is the spin of an electron confined in a quantum dot because of its good coherence properties. Our approach is based on using an electron spin ensemble for defining a robust macroscopic quantum bit. Typically such an ensemble suffers from inhomogeneities. Using tailored pulsed laser excitation protocols this ensemble can be homogenized, such that the involved electrons appear to be identical when precessing about an external magnetic field. In this contribution problems and perspectives related to this approach will be discussed. In particular collective initialization and manipulation of the electron spin ensemble will be addressed. The use of all-optical techniques ensures that the manipulation can be performed on picosecond time scales. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P1.00002: The quantum dot molecule from an optical point of view Invited Speaker: For over ten years the techniques of single quantum dot optical spectroscopy has enabled rapid progress in the fundamental understanding of quantum dots and in the application of quantum information concepts [1]. We now apply these ever improving optical techniques to two self-assembled InAs/GaAs quantum dots that are coherently coupled through tunneling – that is, a quantum dot molecule [2]. The optical spectrum of a quantum dot molecule is much richer than that of a single quantum dot. As one might expect, there is both new physics and enhanced opportunity for quantum information applications. We find that the optical spectrum of single QD molecules charged with 0, 1, or 2 electrons or holes show intriguing and unique patterns of anti-crossings and spin exchange splittings that are readily understood in terms of a few simple concepts. Closer inspection is revealing new information and opportunity, however. For example, on the fundamental side, we have recently discovered evidence that the ground state of the molecule can be an anti- bonding state when it is the hole that tunnels between the dots – a new effect not found in atoms. On the quantum information side, we have engineered a quantum dot molecule in which we can simultaneously control and nondestructively measure the spin of a single electron. This solves a serious limitation in the optical control of single quantum dots. These studies are laying the groundwork necessary to enable optically controlled entanglement of two spins. Here I give an overview of our current understanding of this system from an optical point of view. \\[4pt] [1] ``Optical Studies of Single Quantum Dots'' D. Gammon and D.G. Steel, Physics Today 55, 36 (2002). \\[0pt] [2] ``Optically Mapping the Electronic Structure of Coupled Quantum Dots,'' M. Scheibner et al. Nature Physics 4, 291 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P1.00003: Spectroscopy of Collective Modes in Few-electron Quantum Dots Invited Speaker: Quantum correlations among electrons confined in semiconductor quantum dots (QDs) are expected to lead to exotic states of matter, such as an electron molecule. In the limit of vanishing electron density, the distances between the confined electrons are rigidly fixed like those of nuclei in conventional molecules. The electronic excitations of such a molecule are quantized normal modes of roto-vibration, whose quanta have either a rigid-rotor or relative-motion character. Recent progress on the emergence of molecular roto-vibrational modes at experimentally attainable densities will be discussed. Signatures of the roto-vibrational spectrum are observed even if the localization in space of the electron wave functions is not yet fully achieved. I will present a joint experimental and theoretical investigation of the neutral electronic excitations of nanofabricated AlGaAs/GaAs QDs that contain four electrons. We use inelastic light scattering to probe electronic charge and spin excitations in an array of identical nanofabricated QDs. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibrational modes of a conventional molecule- do not depend on the rotational state represented by the angular momentum, which can be controlled by the application of a magnetic field. A theoretical model, based on full configuration-interaction method, offers an excellent quantitative agreement with the experimental findings. I will also demonstrate optical control of the number of electrons and lateral confining potential in our GaAs/AlGaAs QDs. This is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of the energy-level structure and number of electrons is manifested in the evolution of low-lying spin and charge excitations probed by inelastic light scattering. Our findings open a new venue towards the all-optical manipulation of single electrons in QDs. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P1.00004: Quantum coherence of electron spins in semiconductor quantum dots Invited Speaker: |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P1.00005: Ultrafast Coherent Control of a Single Electron Spin in a Quantum Dot Invited Speaker: Practical quantum information processing schemes require fast single-qubit operations. For spin-based qubits, this involves performing arbitrary coherent rotations of the spin state on timescales much faster than the spin coherence time. While we recently demonstrated the ability to initialize and monitor the evolution of single spins in quantum dots (QDs)\footnote{M. H. Mikkelsen, J. Berezovsky, N. G. Stoltz, L. A. Coldren, D. D. Awschalom, {\em Nature Physics} \textbf{3}, 770 (2007); J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, {\em Science} \textbf{314}, 1916 (2006).}, here we present an all-optical scheme for ultrafast manipulation of these states through arbitrary angles. The GaAs QDs are embedded in a diode structure to allow controllable charging of the QDs and positioned within a vertical optical cavity to enhance the small single spin signal. By applying off-resonant optical pulses, we coherently rotate a single electron spin in a QD up to $\pi$ radians on picosecond timescales \footnote{J. Berezovsky, M. H. Mikkelsen, N. G. Stoltz, L. A. Coldren, D. D. Awschalom, {\em Science} \textbf{320}, 349 (2008).}.We directly observe this spin manipulation using time-resolved Kerr rotation spectroscopy at $T=10\mathrm{K}$. Measurements of the spin rotation as a function of laser detuning and intensity confirm that the optical Stark effect is the operative mechanism and the results are well-described by a model including the electron-nuclear spin interaction. Using short tipping pulses, this technique enables one to perform a large number of operations within the coherence time. This ability to perform arbitrary single-qubit operations enables sequential all-optical initialization, ultrafast control and detection of a single electron spin for quantum information purposes. [Preview Abstract] |
Session P2: Critical Spin Liquids in Strongly Correlated Systems
Sponsoring Units: DCMPChair: T. Senthil, Massachusetts Institute of Technology
Room: Spirit of Pittsburgh Ballrom BC
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P2.00001: Quantum spin liquid in the spin-1/2 triangular antiferromagnet EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$ Invited Speaker: EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$ (Et=C$_{2}$H$_{5}$-, Me=CH$_ {3}$-, dmit=C$_{3}$S$_{5})$ is one of molecular conductors derived from an anion radical of the Pd(dmit)$_{2}$ molecule and closed-shell monocations (Et$_{x}$Me$_{4-x}$Z)$^{+ }$(Z=N, P, As, Sb; $x$=0, 1, 2) [1]. A common feature of these Pd(dmit)$_{2}$ salts is a conducting anion layer where the Pd(dmit)$_{2}$ anions form a dimer unit [Pd(dmit)$_{2}$]$_{2}^{-} $. Electronic structure around the conduction band can be described by a simple tight-binding calculation based on the dimer unit. The conduction band is half-filled and two-dimensional. At ambient pressure, all the Pd (dmit)$_{2}$ salts behave as Mott insulators where one spin is localized on each dimer. Interdimer transfer integrals indicate that the dimers form a quasi (isosceles) triangular lattice. Interdimer transfer integrals can be tuned by the choice of the cation, which deeply affects the electronic state. The EtMe$_{3}$Sb salt has a nearly regular-triangular lattice. The EtMe$_{3}$Sb cations are located between conduction layers and exhibit orientational disorder. The temperature dependence of the magnetic susceptibility is described in terms of the Pad\'e approximant expression based on the high temperature series expansion of $\chi$ of the antiferromagnetic spin-1/2 Heisenberg model on the triangular lattice with an exchange interaction $J$=220-250 K. The $^{13}$C-NMR measurements show no indication of either spin ordering/freezing or an appreciable spin gap down to 1.37 K, which is lower than 1{\%} of $J$ [2]. The specific heat measurements indicate gapless spin excitation. These results strongly suggest that the ground state of the EtMe$_{3}$Sb salt is a gapless spin liquid state. On the other hand, the Et$_{2}$Me$_{2}$Sb salt, which has also a nearly regular-triangular lattice, shows a first-order transition toward a charge separation state (2Dimer$^{-} \quad \to $ Dimer$^{0}$ + Dimer$^{2-})$ at 70 K [3]. \\[0pt] References\\[0pt] 1) R. Kato, \textit{Chem. Rev}., \textbf{104}, 5319 (2004).\\[0pt] 2) T. Itou et al., \textit{Phys. Rev. B}, \textbf{77}, 104413 (2008).\\[0pt] 3) M. Tamura and R. Kato, \textit{Chem. Phys. Lett}., \textbf {387}, 448 (2004). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P2.00002: Spin Liquid State of in the $S=1/2$ Hyper-kagome Antiferromagnet Na$_{4}$Ir$_{3}$O$_{8}$ Invited Speaker: A new Ir oxide, Na$_{4}$Ir$_{3}$O$_{8}$, with a cation-ordered (Ir and Na) spinel structure, was discovered [1]. This compound is an $S$=1/2 Mott insulator with d$^{5}$ (low spin state) Ir4+. As a result of the ordering of Ir and Na within spinel B-sites, magnetic Ir$^{4+}$ ions form a three-dimensional network of corner shared triangles, called hyper-kagome lattice, which provides us with a novel playground for the physics of frustration in $S$=1/2 hyper-kagome A new Ir oxide, Na$_{4}$Ir$_{3}$O$_{8}$, with a cation-ordered (Ir and Na) spinel structure, was discovered [1]. This compound is an $S = 1/2$ Mott insulator with d$^{5}$ (low spin state) Ir$^{4+}$. As a result of the ordering of Ir and Na within spinel B-sites, magnetic Ir$^{4+}$ ions form a three-dimensional network of corner shared triangles, called hyper-kagome lattice, which provides us with a novel playground for the physics of frustration in $S$=1/2 hyper-kagome antiferromagnet. It may be interesting to infer that hyper-kagome lattice has a chirality. The result of magnetization measurements indicates the presence of strong antiferromagnetic coupling (Curie-Weiss temperature $\theta_{\rm CW} \sim -650$ K) between $S = 1/2$ spins. Nevertheless, we find no evidence for long range magnetic ordering in this $S = 1/2$ hyper-kagome antiferro-magnet at least down to 2 K, apparently due to the presence of geometrical frustration. The absence of long range ordering was firmly established by the persistence of $^{23}$Na NMR lines down to 2 K without intensity change [2]. These results strongly suggest that the ground state of this system is a three dimensional $S = 1/2$ spin liquid. Unusual spin excitations of this $S = 1/2$ hyper kagome system will be discussed, based on the specific heat and the NMR data at low temperatures. \\[4pt] [1] Y. Okamoto, M. Nohara, H. Aruga-Katori, and H. Takagi, Phys. Rev. Lett., 99, 137207 (2007). \\[0pt] [2] S. Fujiyama, K. Kanoda, Y. Okamoto, and H. Takagi, in preparation. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P2.00003: Critical ``metal''-like phases of frustrated spins and bosons in two dimensions Invited Speaker: I will review recent theoretical progress in understanding quantum phases of 2D correlated boson or spin systems which exhibit ``Bose-metal'' type phases with gapless excitations residing on surfaces in momentum space. A spin liquid phase with a Fermi surface of spinons is one example of interest, being potentially relevant to the organic spin liquid materials $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$. I will discuss frustrated spin and boson models with ring exchanges that may stabilize such phases, in particular in the vicinity of the Mott transition as is the case in the organic materials. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P2.00004: Gapless spin liquids on the three dimensional hyper-kagome lattice of Na$_4$Ir$_3$O$_8$ Invited Speaker: Recent experiments indicate that Na$_4$Ir$_3$O$_8$, a material in which s=1/2 iridium local moments form a three dimensional network of corner-sharing triangles, may have a quantum spin liquid ground state with gapless spinon excitations. Using a combination of various theoretical approaches, we propose a quantum spin liquid with spinon Fermi surfaces as a favorable candidate for the ground state of the Heisenberg model on the hyper-kagome lattice of Na$_4$Ir$_3$O$_8$. We also present a theory of the bandwidth-controlled metal-insulator transition that may occur as a pressure-tuned transition in this material. We discuss our predictions in relation to the current and future experiments. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P2.00005: Numerical Evidence of Gapless Spin Liquids on Ladders Invited Speaker: I will present numerical evidence of strong-coupling phases for quasi-one-dimensional systems as ladder descendants of candidate models for 2D Bose metal and spin liquid states which possess surfaces of gapless excitations. I will discuss the phase diagrams for two concrete models (square lattice boson and triangular lattice spin models with ring-exchange) based on controlled numerical approaches (DMRG and ED), where such strong-coupling phases are realized in a wide regime of parameters. A close comparison between numerical results and slave-particle descriptions will allow us to characterize these phases in detail and identify signatures reflecting the parent 2D states. [Preview Abstract] |
Session P3: Fe-based Superconductors: Correlation Effects
Sponsoring Units: DCMPChair: Steve Kivelson, Stanford University
Room: 301/302
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P3.00001: ARPES studies of the electronic structure of Fe-based superconductors Invited Speaker: The recent discovery of superconductivity in Fe-based layered compounds has created renewed interest in high temperature superconductivity. With a superconducting transition temperature as high as 55 K, this discovery provides a new direction to understand the essential ingredients for achieving a high superconducting transition temperature. In this talk, I will present our recent angle-resolved photoemission spectroscopy (ARPES) studies on LaOFeP and (Ba,K)Fe$_{2}$As$_{2}$ systems, with special emphasis on the basic electronic structure of the parent compounds. For LaOFeP, quantitative agreement can be found between our ARPES data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound [1]. On the other hand, the picture for (Ba,K)Fe$_{2}$As$_{2}$ system is more complicated. I will discuss two important issues in these FeAs compounds: 1) the unexpected Fermi surface topology in both undoped and doped compounds; 2) the peculiar signature of the SDW transition in ARPES spectra for the parent compound. \\[4pt] [1] D. H. Lu, M. Yi, S.-K. Mo, A. S. Erickson, J. Analytis, J.-H. Chu, D. J. Singh, Z. Hussain, T. H. Geballe, I. R. Fisher {\&} Z.-X. Shen, Nature 455, 81 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P3.00002: Charge dynamics in the normal state of the iron oxypnictide superconductor LaFePO Invited Speaker: Many-body interactions in materials govern diverse and complex phenomena like high-$T_c$ superconductivity, density wave instabilities and metal-insulator transitions. Infrared spectroscopy is an ideal tool for identifying the fingerprints of the various interactions in a material. We present infrared and optical properties in the normal state of $ab$-plane oriented single crystals of the iron oxypnictide superconductor LaFePO. Prominent Drude peaks and low scattering rates indicate the presence of coherent quasiparticles. We find that this metal has a relatively low carrier density compared to MgB$_2$, for example. An important result is that the Drude spectral weight i.e. the kinetic energy of itinerant quasiparticles is reduced by correlation effects to 50\% of the band theory value. Even though LaFePO is among the most conducting of the iron-pnictides, we find that electronic correlations cannot be ignored in any realistic physical description of this material. We classify LaFePO as a moderately correlated metal. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P3.00003: Electronic Correlations and Magnetic Frustration in the Iron Pnictides Invited Speaker: An important question about the high Tc iron pnictides is the extent to which they are strongly correlated. Based on the fact that they are ``bad metals,'' we propose that these materials are in proximity to a Mott insulator [1,2]. In other words, the degree of the electronic correlations here is closer to that of intermediately-coupled metallic systems like V2O3, which lies near the Mott transition, rather than that of simple antiferromagnetic metals such as Cr. Consequently, we model the incoherent electronic excitations in terms of localized moments, whose superexchange interactions contain the competing nearest-neighbor (J1) and next-nearest-neighbor (J2) components [1]. The magnetic frustration of the J1-J2 model in the relevant parameter range leads to a (pi,0) antiferromagnet and a reduced ordered moment. The model also features an Ising transition that naturally yields a structural phase transition. All these are consistent with the neutron scattering results. With the coupling of the local moments to the coherent electronic excitations, the strength of the antiferromagnetic order is tuned, leading to a magnetic quantum critical point [2]. The ordered moment should therefore vary across the undoped iron arsenides. In addition, the magnetic quantum criticality can be probed by P doping for As in the parent iron pnictides, where the disruption to the quantum criticality caused by superconductivity is likely to be less compared to the electron or hole doped cases. Finally, the implications of the electronic correlations and magnetic frustration on the multi-band superconductivity will be briefly discussed. \\[4pt] [1] Q. Si and E. Abrahams, PRL 101, 076401 (2008); \\[0pt] [2] J. Dai, Q. Si, J-X Zhu and E. Abrahams, arXiv:0808.0305. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P3.00004: Pairing symmetry and Antiferromagnetic Exchange Coupling in Fe-Based Superconductors Invited Speaker: I discuss the existence of strikingly identical paradigms applicable to both cuprates and iron-based superconductors in understanding magnetism, superconductivity and the interplay between the two. The magnetic states and transitions in iron- based superconductors are well described by a $J_1-J_2-J_z$ magnetic exchange model where $J_1$, $J_2$ and $J_z$ are nearest neighbour, next nearest neighbour and inter-layer couplings respectively. Differing from the t-J model for cuprates where d-wave pairing symmetry is favored, the magnetic exchange in the iron based supercondunctors predicts an unconventional s-wave $cosk_x cosk_y$ pairing. I will show that the predicted pairing symmetry is supported by many experimental results, and also discuss new predictions associated with the pairing symmetry. \\[4pt] References: \\[0pt] [1]: Chen Fang, Hong Yao, Wei-Feng Tai, Jiangping Hu and S. Kivelson ``Theory of Electron Nematic Order in LaOFeAs''; Phys. Rev. B 77 224509 (2008).\\[0pt] [2]: Kangjun Seo, A. B. Bernevig and JiangPing Hu, ``Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides''; PRL 101, 206404 (2008) ArXiv: 0805.2958.\\[0pt] [3]: Meera M. Parish, Jiangping Hu and B. Andrei Bernevig ``Experimental Consequences of the S-wave cos(kx)cos (ky) Superconductivity in the Iron-Pnictides'', Phys. Rev. B 78, 144514 (2008) ArXiv:0807.4572.\\[0pt] [4]: Chen Fang, B. Andrei Bernevig, Jiangping Hu,``Theory of Magnetic Order in $ Fe_{1+y}Te_{1-x}Se_x$'', arXiv:0811.1294 (2008). [Preview Abstract] |
Session P4: Physics of Polymer Membranes for Water Purification
Sponsoring Units: DPOLYChair: Alamgir Karim, National Institute of Standards and Technology
Room: 306/307
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P4.00001: Transport of water and solutes in reverse osmosis and nanofiltration membranes Invited Speaker: The polyamide active layers of reverse osmosis and nanofiltration membranes used for water purification are real-world examples of nanoscale functional materials: the active layer is only $\sim $100 nm thick. Because the active layer is formed by a process of interfacial polymerization, the structure and composition of the membrane is highly inhomogeneous and even such basic physical and chemical properties as the atomic density, swelling in water, the distribution of charged species between water and membrane, and the mobility of water and ions, are poorly understood. We are using Rutherford backscattering spectrometry (RBS) to determine the composition, roughness, and thickness of the membrane; reveal the surprisingly high solubility of salt ions in the polymer active layer; analyze the acid-base chemistry of charged functional groups; and determine the degree of polymer cross-linking. Measurements of mass-uptake and adsorption-induced mechanical stress of membranes in humid air enable us to determine the water solubility, specific volume of water, and the mechanical strength of the membrane. Comparisons between these equilibrium data and the permeability of the membrane to water and salts show that the mobility of water molecules in the membrane approaches the mobility of bulk water, and that the rejection of salt ions is accomplished by low mobility, not low solubility. My collaborators in this work are Xijing Zhang, Orlando Coronell, and Prof. Benito Mari\~{n}as. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P4.00002: High Flux Nanofibrous Membranes for Water Purification Invited Speaker: Recently, nanofibrous materials have been made more readily available in large part due to advances in electro-spinning and related technologies, including the use of a combination of electrostatic and gas-blowing forces. The non-woven structure has unique features, including interconnected pores, very large surface-to-volume ratio, and ease of surface modifications which enable such scaffolds to have many biomedical and industrial applications. The chemical composition of electrospun membranes can be adjusted by using different polymers, polymer blends or nanocomposites, made of organic or inorganic materials. In this talk, we demonstrate a breakthrough technology on thin-film nanocomposite membranes for high-flux water purification based on nanofiber scaffolds. The breakthrough incorporates two new and unique concepts of the membrane design: (1) the replacement of the conventional flux-limited porous substrate with a highly porous nanofibrous scaffold, and (2) the creation of a very thin, strong and functional nanocomposite barrier layer, imbedded with interconnected and directed water channels. Preliminary experiments on the hierarchical design and assembly of this unique nanofibrous membrane have already revealed very promising potentials. By using a hydrophilic nanocomposite barrier layer, an asymmetric electrospun nanofibrous mid-layer scaffold and a non-woven microfibrous support, the flux rate of this not yet optimized membrane system is 3-10 times better than that of the best among all known conventional ultrafiltration/nanofiltration media without losing the high rejection and low fouling criteria. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P4.00003: Polymers for Waste Water Treatment Invited Speaker: |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P4.00004: Carbon Nanotube Membranes for Water Purification Invited Speaker: Carbon nanotubes are an excellent platform for the fundamental studies of transport through channels commensurate with molecular size. Water transport through carbon nanotubes is also believed to be similar to transport in biological channels such as aquaporins. I will discuss the transport of gas, water and ions through microfabricated membranes with sub-2 nanometer aligned carbon nanotubes as ideal atomically-smooth pores. The measured gas flow through carbon nanotubes exceeded predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeded values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations and measured for aquaporins. More recent reverse osmosis experiments reveal ion rejection by our membranes. Based on our experimental findings, the current understanding of the fundamentals of water and gas transport and of ion rejection will be discussed. The potential application space that exploits these unique nanofluidic phenomena will be explored. The extremely high permeabilities of these membranes, combined with their small pore size will enable energy efficient filtration and eventually decrease the cost of water purification.\\[4pt] In collaboration with Francesco Fornasiero, Biosciences and Biotechnology Division, PLS, LLNL, Livermore, CA 94550; Sangil Kim, NSF Center for Biophotonics Science \& Technology, University of California at Davis, Sacramento CA 95817; Jung Bin In, Mechanical Engineering Department, UC Berkeley, Berkeley CA 94720; Hyung Gyu Park, Jason K Holt, and Michael Stadermann, Biosciences and Biotechnology Division, PLS, LLNL; Costas P. Grigoropoulos, Mechanical Engineering Department, UC Berkeley; Aleksandr Noy, Biosciences and Biotechnology Division, PLS, LLNL and School of Natural Sciences, University of California at Merced. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P4.00005: Hierarchical Fiber Structures Made by Electrospinning Polymers Invited Speaker: A filter for water purification that is very thin, with small interstices and high surface area per unit mass, can be made with nanofibers. The mechanical strength of a very thin sheet of nanofibers is not great enough to withstand the pressure drop of the fluid flowing through. If the sheet of nanofibers is made thicker, the strength will increase, but the flow will be reduced to an impractical level. An optimized filter can be made with nanometer scale structures supported on micron scale structures, which are in turn supported on millimeter scale structures. This leads to a durable hierarchical structure to optimize the filtration efficiency with a minimum amount of material. Buckling coils,\footnote{Tao Han, Darrell H Reneker, Alexander L. Yarin, Polymer, Volume 48, issue 20 (September 21, 2007), p. 6064-6076.} electrical bending coils\footnote{Darrell H. Reneker and Alexander L. Yarin, Polymer, Volume 49, Issue 10 (2008) Pages 2387-2425, DOI:10.1016/j.polymer.2008.02.002. Feature Article.} and pendulum coils\footnote{T. Han, D.H. Reneker, A.L. Yarin, Polymer, Volume 49, (2008) Pages 2160-2169, doi:10.1016/jpolymer.2008.01.0487878.} spanning dimensions from a few microns to a few centimeters can be collected from a single jet by controlling the position and motion of a collector. Attractive routes to the design and construction of hierarchical structures for filtration are based on nanofibers supported on small coils that are in turn supported on larger coils, which are supported on even larger overlapping coils. ``Such top-down'' hierarchical structures are easy to make by electrospinning. In one example, a thin hierarchical structure was made, with a high surface area and small interstices, having an open area of over 50\%, with the thinnest fibers supported at least every 15 microns. [Preview Abstract] |
Session P5: Fluid Dynamics and Computational Science
Sponsoring Units: DFD DCOMPChair: Pui-Kuen Yeung, Georgia Institute of Technology
Room: 401/402
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P5.00001: Inertial Particles in Turbulent Flows and the Clustering Instability of Interstellar Dust Invited Speaker: The dynamics of dust grains in turbulent flows plays an important role in many astrophysical processes. I will review the problem of the formation of planetesimals (precursors of full-fledged planets) in turbulent circumstellar disks. I will then discuss some fundamental aspects of the physics of heavy particles in turbulent flows, and specifically the phenomenon of small scale clustering, an effect verified by laboratory experiments and in situ terrestrial cloud sampling. I will present results of large numerical simulations of particle-laden compressible turbulence, including statistics of clustering and of particle velocity differences. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P5.00002: Multiscale modeling of the human arterial tree on the TeraGrid. Invited Speaker: A multiscale model of the human arterial tree will be presented consisting of the macrovascular network (MaN, arteries above 1-2 mm), the mesovascular network (MeN, arterioles above 10 micro-m) and the microvascular network (MiN, capillaries). Coupling conditions between the MaN-MeN-MiN will be discussed and three different methods in modeling each network will be presented. Specific examples will be shown for the intracranial arterial tree for healthy subjects but also for patients with hydrocephalus. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P5.00003: Terascale Direct Numerical Simulations of Turbulent Combustion Invited Speaker: The rapid growth in computational power in the past decade has presented both opportunities and challenges for high-fidelity simulations of turbulent reacting flows. The advent of terascale computing power has made it possible to glean fundamental physical insight into fine-grained ``turbulence-chemistry'' interactions in simple laboratory-scale turbulent flames from direct numerical simulation at moderate Reynolds numbers with detailed chemistry. Recent DNS results are presented to elucidate the role of autoignition and large-eddy mixing on the stabilization of a lifted ethylene-air jet flame in a heated coflow. The role of scalar dissipation rate on modulating ignition delays or lift-off heights is discussed. The simulations were performed at a jet Reynolds number of 10,000 and required 1.3 billion grid points to resolve the turbulence and flame structure. In a second related topic, the morphology of the scalar dissipation rate field in a turbulent jet flame is examined using topological methods, in particular the Morse-Smale Complex, which provides a natural segmentation of dissipation rate elements or ``features.'' These features are tracked in time, and conditional feature statistics are presented. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P5.00004: Prediction and predictability of hurricanes with high-performance computers and cloud-resolving ensembles Invited Speaker: This talk will be primarily devoted to the use of high-performance computing facilities to perform ensemble-based state estimation of hurricanes with cloud-resolving numerical weather prediction models. I will be sharing our recent experience in using approximately 30,000 cluster cores simultaneously at the Texas Advanced Computing Center which successfully assimilates high-resolution airborne Doppler radar observations in realtime and subsequently delivers 2 deterministic and 60-member ensemble forecasts running at 4.5/1.5-km effective horizontal grid spacings in a timely fashion. Since the predictability of hurricanes may be fundamentally limited by chaotic moist convection and subsequent upscale error growth, I would advocate that besides the need of~continuously~improving the hurricane forecast models and~ingesting high-resolution observations into the models to better initialize the storm, the hurricane state estimation is fundamentally probabilistic that demands cloud-solving ensemble-based data assimilation and forecasting. Improvements of forecast models may come from ever increasing computer power to better resolve the storms numerically and from improved~fundamental understanding of the dynamics and impact of subgrid-scale turbulence in hurricanes. Improvements of better state estimation may also come from development of new theories that are applicable for~high-dimensional, non-linear, non-Gaussian dynamic systems such as in hurricanes. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P5.00005: On the two-way interactions between dispersed particles and turbulent flows Invited Speaker: Particle-laden turbulent flows are ubiquitous in nature (e.g. dust storms on Earth and Mars) and in industrial applications (e.g. liquid fuel and pulverized coal sprays in combustion chambers). Experimental and numerical studies of these flows are quite challenging due to the wide spectra of length- and time-scales of the dispersed particles in addition to the spectra of scales intrinsic to the carrier fluid turbulence. The two-way nonlinear interactions between the dispersed particles and the turbulence result in complex multi-scale physical phenomena. The lecture focuses on the physical mechanisms of the two-way interactions between dispersed spherical particles and simple turbulent flows using Direct Numerical Simulation (DNS). Particles whose diameter is smaller than the Kolmogorov length scale of turbulence are simulated as point particles. Results of particle-laden isotropic and homogeneous shear turbulent flows are presented. Particles with diameter larger than the Kolmogorov length scale are fully resolved using the Immersed Boundary method. Results of fully resolved particle-laden isotropic turbulence are presented. [Preview Abstract] |
Session P6: Ultracold Gases with Dipolar Interactions
Sponsoring Units: DAMOPChair: Han Pu, Rice University
Room: 406
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P6.00001: Ultracold polar molecules Invited Speaker: Polar molecules - molecules exhibiting a permanent electric dipole moment - have bright perspectives as systems with long-range and anisotropic interaction. These interactions have been the basis for numerous exciting theoretical proposals ranging from ultra-cold chemistry, precision measurements, quantum phase transitions to novel systems for quantum information processing and quantum control with external magnetic and electric fields. We will present our recent work on the creation of a near quantum degenerate gas of rovibrational ground state polar $^{40}$K$^{87}$Rb molecules. Using a single step of two photon coherent transfer, we transfer weakly bound KRb molecules to the rovibrational ground state of the singlet electronic ground molecular potential. The polar molecules have a permanent electric dipole moment, which we measure with Stark spectroscopy to be 0.566(17) Debye. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P6.00002: Effects of magnetic dipolar interactions on collective modes and instabilities of alkali BECs Invited Speaker: In this talk I will review the phenomenon of roton softening in systems with dipolar interactions. Special emphasis will be on magnetic dipolar interactions in Bose condensates of alkali atoms, when fast Larmor precession and spin dynamics strongly modify the character of unstable modes. I will also discuss the enhancement of roton softening in multilayer stacks of two dimensional condensates. Implications of these theoretical results for recent experiments with Rb-87 and K-39 atoms will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P6.00003: Evidence for supersolid behaviour in a spin-1 rubidium gas Invited Speaker: I will present experimental evidence of the coexistence of two types of symmetry-breaking long-range order in a quantum degenerate gas of spin-1 $^{87}$Rb atoms: crystalline magnetic ordering, which spontaneously breaks translational symmetry, and long-range phase coherence of a macroscopic wavefunction, which spontaneously breaks the $U(1)$ gauge symmetry associated with the particle number. Such a gas was prepared by gradually cooling non-degenerate, unpolarized, optically trapped gases into the regime of quantum degeneracy. Using a high-resolution magnetization-sensitive imaging method, we observe a phase transition below which the quantum gas forms a crystalline array of magnetic domains. Based on our previous experiments on the evolution of helical spin textures in such a gas, we ascribe this crystalline order to the competition between a short-range isotropic ferromagnetic interaction and a long-range anisotropic dipolar interaction. We confirm the phase coherence of this gaseous crystal by atom interferometry. Specifically, we use a form of Bragg spectroscopy to measure the first-order correlation function of the superfluid order parameter at variable distance. The coexistence of translational symmetry breaking, characteristic of a solid, and long-range phase coherence, characteristic of a superfluid, are hallmarks of the sought-after supersolid phase. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P6.00004: Dynamics and coherence in a collapsing dipolar BEC Invited Speaker: Chromium atoms in a Bose-Einstein condensate (BEC) interact - in addition to s-wave scattering - via magnetic dipole-dipole interaction. Although the magnetic forces between the atoms which carry a large magnetic moment of 6 Bohr magnetons are still rather weak, they can become the dominant interaction when a Feshbach resonance is used to reduce the contact interaction to zero [1]. In this regime, the stability of a chromium Bose-Einstein condensate depends on the geometry of the trap. This is an intrinsic and unique effect of an anisotropic interaction. We have measured the stability diagram of such a dipolar BEC by exploring the border between stable and unstable regions [2]. When we cross this border with an initially stable condensate by a sudden change of the scattering length into the unstable regime, we observe the collapse and subsequent explosion due to dipole-dipole interaction [3]. The anisotropy of the underlying interaction reveals in the formation of a non-trivial structure during the collapse. I will discuss the dynamics of the collapse depending on the geometry of the trapping potential which we have studied experimentally and compare the results to three dimensional numerical simulations of the Gross-Pitaevskii equation. By interfering several condensates collapsing at the same time, we also study the coherence properties of the collapsing clouds. \\[4pt] [1] T. Lahaye et al. {\it Nature} {\bf 448}, 672 (2007)\\[0pt] [2] T. Koch et al. {\it Nature Physics} {\bf 4}, 218 (2008)\\[0pt] [3] T. Lahaye et al. {\it Phys. Rev. Lett.} {\bf 101}, 080401 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P6.00005: Condensed Matter Physics and Quantum Simulations with Cold Polar Molecules and Rydberg Atoms Invited Speaker: Polar molecules prepared in their electronic and vibrational ground state are characterized by large dipole moments associated with rotational excitations. This gives rise to large dipole-dipole interactions between molecules, which can be manipulated with external DC and AC microwave fields. The possibility to tune these strong, long-range and anisotropic interactions, combined with the trapping in reduced dimensions, raises interesting prospects for cold ensembles of polar molecules as \textit{strongly correlated} condensed matter system, i.e. provides \textit{analog quantum simulation} of strongly interacting systems and quantum phases. Specific examples to be discussed include the engineering of various spin and Hubbard models for polar molecules in optical lattices, e.g. the Kitaev model and three body interactions, and the tailoring effective molecular interaction potentials based on ``blue-shields'' with microwave fields. Furthermore, dipolar gases in 1D and 2D trapping geometries can form self-assembled lattice structures in single layer and bilayer systems. In addition, a mixture of cold atoms and polar molecules forming a self-assembled lattice gives rise to a new realization of Hubbard models with widely tunable lattice parameters and small lattice spacing, which represents a systems with both strong correlation and phonon dynamics. Extremely strong dipolar or Van der Waals interactions can also be obtained in cold atomic gases excited to Rydberg states. We will briefly discuss new ideas of developing a \textit{digital quantum simulator} for spin models. It is based on performing a stroboscopic sequence of many particle quantum gates based on manipulating dipolar interactions between groups of Rydberg atoms in large spacing optical lattices. [Preview Abstract] |
Session P7: Forging Effective Partnerships with Your Local Science Center: Outcomes from the Workshop on University/Science Center Collaborations
Sponsoring Units: FEd FPSChair: Philip Hammer, Franklin Institute
Room: 407
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P7.00001: Lessons Learned from the APS/TFI Workshop on University/Science Center Collaborations: Outreach Strategies for Faculty Working with their Local Science Museum Invited Speaker: On May 31 -- June 1, 2008, The Franklin Institute (TFI) hosted the American Physical Society/Franklin Institute Workshop on University/Science Center Collaborations. This Workshop brought together forty leaders from science centers, universities, and federal funding agencies to explore what works and what doesn't work in university-science center collaborations. The goal was to explore the outreach motivations of academic institutions, their scientists and students, the characteristics and needs of small vs. large science centers, and the goals for and outcomes expected from reaching out to the general public from the perspectives of universities and science centers. The result was a convergence of viewpoints on how a good collaboration is established, built upon, sustained, and evaluated. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P7.00002: University Perspectives on Science Center/University Interactions Invited Speaker: A program bringing graduate students into science museums is described.~ Practical, nuts and bolts, methods for making the program work are outlined. Questions are asked about the somewhat uncomfortable relation between graduate education, research, and informal~education.~~ [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P7.00003: University/Science Center Collaborations (A Science Center Perspective): Developing an Infrastructure of Partnerships with Science Centers to Support the Engagement of Scientists and Engineers in Education and Outreach for Broad Impact Invited Speaker: Science centers, professional associations, corporations and university research centers share the same mission of education and outreach, yet come from ``different worlds.'' This gap may be bridged by working together to leverage unique strengths in partnership. Front-end evaluation results for the development of new resources to support these (mostly volunteer-based) partnerships elucidate the factors which lead to a successful relationship. Maintaining a science museum-scientific community partnership requires that all partners devote adequate resources (time, money, etc.). In general, scientists/engineers and science museum professionals often approach relationships with different assumptions and expectations. The culture of science centers is distinctly different from the culture of science. Scientists/engineers prefer to select how they will ultimately share their expertise from an array of choices. Successful partnerships stem from clearly defined roles and responsibilities. Scientists/engineers are somewhat resistant to the idea of traditional, formal training. Instead of developing new expertise, many prefer to offer their existing strengths and expertise. Maintaining a healthy relationship requires the routine recognition of the contributions of scientists/engineers. As professional societies, university research centers and corporations increasingly engage in education and outreach, a need for a supportive infrastructure becomes evident. Work of TryScience.org/VolTS (Volunteers TryScience), the MRS NISE Net (Nanoscale Informal Science Education Network) subcommittee, NRCEN (NSF Research Center Education Network), the IBM On Demand Community, and IEEE Educational Activities exemplify some of the pieces of this evolving infrastructure. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P7.00004: Perspective of NSF-MPS Program Directors on Educational Outreach Invited Speaker: The National Science Foundation Broader Impacts review criterion (often known as Criterion 2) has been subject to much discussion since first implemented by NSF. The broader impact of different proposals can vary widely, based on different factors such as the particular research activities proposed, the interests of the PI(s), the type of institution involved in the proposal, and the different opportunities available on the local area, to name just a few. In this talk the Broader Impacts review criterion will be discussed from the viewpoint of the NSF Program Officers and will include different examples of potential Broader Impact activities. In Collaboration with Uma Venkateswaran, and Kathleen V. McCloud. [Preview Abstract] |
Session P8: Centenary of Lev Landau
Sponsoring Units: FHPChair: Gloria Lubkin, American Institute of Physics
Room: 414/415
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P8.00001: Lev Landau: A View from the West Invited Speaker: The tragic accident which ended Landau's scientific career at an early age meant that Lev Landau was known personally to only a small number of western scientists. His remarkable influence on twentieth century physics thus came from his published work and indirectly from the members of the famed Landau school, who are so well represented at this Symposium. Regarding the published work, I would distinguish three distinct ways in which Landau's influence has been felt. The most obvious is the set of seminal papers on a broad set of topics ranging from Landau diamagnetism, to the phonon-roton theory and two-fluid hydrodynamics of $^{4}$He, Fermi-liquid theory and zero sound, the theory of second-order phase transitions, the Landau-Hopf theory of fluid turbulence and many more. The second class of contributions consists of the famed Landau-Lifshitz Course of Theoretical Physics, which first appeared in the West in the late fifties and early sixties. In many ways the third aspect of Landau's influence, although more difficult to define, is probably even more significant. This is Landau's pervasive presence in a large number of the major theoretical advances in condensed matter and statistical physics throughout the second half of the twentieth century. So many major developments can be viewed as elaborations, advances and - yes - corrections to the foundational theories and points of view laid down by Landau. One example is the theory of superfluidity in Bose liquids, for which one may ask why Landau resisted London's explanation in terms of Bose condensation, which has turned out to be important after all. A second example is the Fermi liquid theory and important later developments stemming from superfluid transitions or effects of strong correlations. A third example is the theory of second-order phase transitions which lays the foundations for the study of critical phenomena using the renormalization group. In each case one marvels at the important foundational role played by Landau's work and one may ask to what extent he himself anticipated the later developments. It is hoped that the subsequent speakers might address some of these questions. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P8.00002: Landau and theory of quantum liquids Invited Speaker: General conceptions and history of the Landau Theory of superfluidity and the Theory of Fermi liquid will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P8.00003: Landau and Theory of Phase Transitions Invited Speaker: Landau's theory of phase transitions is probably his most general and most influential work. I describe history of its creation, its basic ideas and their developments and extensions and its deep influence on modern science. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P8.00004: Landau and Feyman diagrams Invited Speaker: Landau considered the Feynman diagrams and the Dyson concept of their visual summation as a breakthrough in the physics of particles. This visual perception in his opinion activated the person intuition. In this intuitive way Landau introduced the concept of partial summation which led to major results in the particle physics and the condensed matter theory. Some of his and of the members of his school results will be presented in a pure visual way. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P8.00005: Landau's Contributions to Applied Physics Invited Speaker: |
Session P9: Jamming: Theory and Experiment I
Sponsoring Units: GSNPChair: Daniel Blair, Georgetown University
Room: 303
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P9.00001: Cooperative Particle Dynamics in the Manhattan Model Prasanta Pal, Corey O'Hern, Jerzy Blawzdziewicz We study Brownian dynamics of hard rods in a Manhattan-like traffic grid, in which a series of narrow one-dimensional horizontal and vertical channels intersect at right angles, and particles are forbidden from turning at the intersections. We measure the intermediate scattering function (ISF) and mean-square displacement (msd) as a function of packing fraction $\phi$ and determine $\phi_g$ at which dynamical arrest occurs as a function of the system size, number of intersections, and topology of the grid. As a particular example, we explicitly characterize the cooperative particle dynamics required for structural relaxation for symmetric systems in which all lobes between junctions contain the same number of particles. In these systems, we predict the scaling behavior of the structural relaxation time and self-diffusion coefficient versus $\phi_g - \phi$. We will also quantify the extent to which these systems age and determine whether there is a characteristic number of junctions above which glassy dynamics occurs. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P9.00002: Universal Scaling Relation Near Point J Thomas Haxton, Andrea Liu Recently, several studies (P. Olsson and S. Teitel. \textit{Phys. Rev. Lett.} \textbf{99}, 178001 (2007); T. Hatano. arXiv:0803.2296; L. Berthier and T. A. Witten. arXiv:0810.4405) have indicated the existence of a dynamical phase transition at or near Point J, the point at zero temperature, zero shear stress, and a critical density where repulsive amorphous sphere packings lose rigidity. However, a universal scaling relation connecting the rheology of the jammed solid to that of the viscous liquid has been lacking. We control the temperature, strain rate, and pressure in molecular dynamics simulations to show that the steady-state rheology is described by a universal scaling relation near Point J. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P9.00003: Scaling of Rheology Near the Colloidal Jamming Transition Zexin Zhang, Anindita Basu, Thomas Haxton, Andrea Liu, Arjun Yodh Recent simulations have proposed that the zero-temperature, zero-shear-stress jamming transition can be understood in the framework of critical phenomena, and thus can be described by various asymptotic scaling laws. We carry out rheology experiments in the vicinity of the jamming transition to study the scaling of flow properties of a bidisperse colloidal soft sphere system. We find, both below and above the jamming transition, a scaling collapse of the rheological data when the shear stress and shear rate are rescaled by proximity to the jamming transition. We extract critical scaling exponents and compared with simulations. C. S. O'Hern et al. Phys. Rev. E 68, 011306 (2003). P. Olsson, S. Teitel, Phys. Rev. Lett., 99, 178001 (2007). T. Hatano, arXiv:0803.2296v4 (2008), arXiv:0804.0477v2 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P9.00004: Quenched Disorder as a Fourth Axis to the Jamming Phase Diagram Cynthia Olson Reichhardt, Evan Groopman, Zohar Nussinov, Charles Reichhardt We propose that the general jamming phase diagram proposed by Liu and Nagel [Nature 396, 21, 1998] as a function of shear, density and temperature could also have a fourth axis which is the density of quenched disorder. This could represent jamming in porous media. Using numerical simulations we show that the density at which jamming occurs in a two-dimensional system of disordered disks decreases as the amount of quenched disorder in the sample increases. We argue that when the jamming correlation length is on the same length scale as the average distance between disorder sites, the system will jam. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P9.00005: Critical behavior from geometric confinement in shear thickening suspensions Eric Brown, Heinrich Jaeger We performed rheometry measurements on shear thickening suspensions. The viscosity is measured as a shear stress over shear rate in the shear thickening region to have divergent scalings of both the magnitude and slope at a critical packing fraction $\phi_c$. The yield stress also has a divergent scaling at $\phi_c$. This is qualitatively different form jamming models in which the yield stress grows gradually from an onset packing fraction. The value of $\phi_c$ depends only on particle shape and equals $0.56$ for hard spheres, corresponding to random loose packing and the onset of dilation. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P9.00006: Stress Fluctuations and Nonlinear Dynamical Modes Near Jamming David A. Egolf, Edward J. Banigan The jamming transition is often considered a dynamical transition, but how to properly and quantitatively characterize the changes in dynamical behavior is an open question. We perform numerical simulations of a two-dimensional sheared granular layer over a range of packing fractions spanning the transition. Within these simulations, we calculate a partial spectrum of Lyapunov exponents and vectors, which (at least in one sense) is an optimal decomposition of the dynamics of the system. We find that the Lyapunov exponents and vectors corresponding to the most important dynamical modes of the system tend to localize in space and time near important physical events, such as cooperative rearrangements or redistributions of stresses. In addition, we find that the magnitudes of Lyapunov exponents are directly linked to the size of relative stress fluctuations of the system. At high densities, the system changes from chaotic to non-chaotic, and we measure dynamical time and length scales that diverge as the system jams. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P9.00007: From Spheres to Ellipsoids: The Story of the Density of States Zorana Zeravcic, Ning Xu, Sidney R. Nagel, Andrea J. Liu Packings of frictionless ellipsoids have not only captured the imagination of the public, but also bring up a number of fundamental issues regarding the properties of jammed media. For instance, the average contact number $Z$ of such packings at jamming varies continuously between the spherical isostatic value $Z_{\rm iso}=6$ and the value $Z_{\rm iso}=10$ for ellipsoids of revolution if the ellipticity $\epsilon-1$ is turned on. Here we study the vibrational spectra of soft ellipsoids both as a function of density and $\epsilon$. Our spectra show a two-band structure. For small aspect ratios there is first a rotational band, then a gap and then a second band of translational character. As we increase the aspect ratio, the gap closes and the remaining band has a mixed character. We discuss various surprising features of the spectrum in detail and show how the changes in the gap are related with the change of $Z$ with $\epsilon$. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P9.00008: Growing length scale in gravity-driven dense granular flows Shubha Tewari, Melanie Finn, Allison Ferguson, Bulbul Chakravarty We report on simulations of a two-dimensional, dense, bidisperse system of inelastic hard disks falling down a vertical tube under the influence of gravity. We examine the approach to jamming as the average flow of particles down the tube is slowed by making the outlet narrower. Defining coarse- grained velocity and stress fields, we find a length scale and a time scale can be extracted from two-point spatial and temporal correlations of these fields. Both length and time scales are found to grow as jamming is approached \footnote{ S. Tewari, B. Tithi, A. Ferguson and B. Chakraborty, arXiv:0806.2413}. In an ongoing effort to understand the origin of the growing length and time scales, we have been investigating velocity profiles and distributions, and we will report on these results. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P9.00009: Experimental Study of the 2D Jamming Transition Xiang Cheng We can study a jammed system of particles by following a loosely-packed configuration as the individual particles increase their size until all the particles are constrained by their neighbors. Because tapioca pearls swell to over twice their initial diameter when submerged in water, they offer an ideal medium with which to study properties of the jamming transition in the presence of frictional interactions. Using an array of $\sim $ 10,000 tapioca pearls, we study several static and dynamic signatures of the two-dimensional jamming transition. The amplitude of the first peak of the pair-correlation function changes non-monotonically as the packing fraction of the system increases. This is consistent with recent experiments in a colloidal system of NIPA particles at finite temperatures [1]. This signature is a vestige of the divergence of this peak in the frictionless-sphere limit [2]. A length scale, defined by the spatial velocity correlation function, and the number hexagons in the Voronoi tessellation have pronounced maxima at the transition. [1] Z. Zhang, D. T. N. Chen, A. G. Yodh, K. B. Aptowicz and P. Habdas, Bull. Am. Phys. Soc. Volume 53, Number 2 (2008). [2] C. S. O'Hern, L. E. Silbert, A. J. Liu and S. R. Nagel, Phys. Rev. E 68, 011306 1-19 (2003). [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P9.00010: Influence of Confinement on the Structure of Random Close Packing Kenneth Desmond, Eric R. Weeks We study the structure of many simulated random closing packings confined between two walls. Each packing consists of a binary mixture in equal number with a sizes ratio of 1.4. Our aim is to quantify how a confining boundary and the thickness between the boundaries alters the structure of randomly close packed disks in 2D and spheres in 3D. We find that confinement lowers the packing fraction, and induces heterogeneity in particle density where particles show strong layering near the wall. Both the particle density and the structure of the local packing show oscillations that decay outward from the wall. The decay in the oscillations is rapid, with a characteristic length scale less than the largest particle diameter. We have also developed a simple model for describing the decrease in packing fraction with confinement. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P9.00011: Influence of Confinement on Dynamical Heterogeneities in Dense Colloidal Samples Kazem Edmond, Eric R. Weeks We study a colloidal suspension confined between two parallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of the colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. Like particles in an unconfined near-glassy system, groups of particles in our confined system move together cooperatively. Normally these groups would be spatially isotropic. However, the confining boundaries induce a layering of the particles. We show that the layering modifies the shapes of the mobile groups within the sample so that they are planar. We investigate how the planar restriction of the shapes of the mobile groups may be the cause of the sample's glassy behavior. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P9.00012: Influence of Boundary Mobility on the Dynamics of Confined Colloidal Suspensions Gary L. Hunter, Kazem V. Edmond, Eric R. Weeks We use fast confocal microscopy to study the influence of interfacial mobility and confinement on the dynamics of dense colloidal suspensions. Experiments on confined molecular super-cooled liquids have shown that hard/immobile boundaries result in an increase in relaxation times relative to bulk measurements, whereas soft/mobile boundaries lead to a decrease in relaxation times. We confine suspensions of PMMA microspheres within emulsion droplets of different sizes, thereby probing the consequences of confinement. By changing the viscosity of the external, continuous phase, we also control the interfacial mobility of our samples. In this way, we separate the two effects and draw comparisons between mobility within colloidal suspensions and molecular liquids. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P9.00013: Jamming of Foams Gijs Katgert, Martin van Hecke We experimentally investigate jamming of soft frictionless spheres at zero stress and zero temperature, using static two-dimensional packings of foam bubbles. As a function of the distance to the jamming point, we obtain the distribution of interparticle forces, the scaling of the contact number and the distribution of free Voronoi area per bubble. Our results compare favorably to earlier predictions for soft discs and grains from theory and numerics. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P9.00014: Controlled jamming of particle-laden interfaces using a spinning drop tensiometer Hsin-Ling Cheng, Sachin Velankar Partially-wettable particles often adsorb nearly irreversibly at liquid/liquid interfaces. Under conditions when the interface is crowded with a particle monolayer, ``2D jamming'' can occur, i.e. the interface loses mobility and displays solid- like characteristics. We studied the jamming of iron oxyhydroxide (FeOOH) particles adsorbed at the interface between ethylene glycol and mineral oil using a spinning drop tensiometer (SDT). With decreasing rotational rate, the cylindrical drop retracted due to interfacial tension, thus reducing the interfacial area and increasing interfacial particle concentration. Accordingly, when the specific interfacial area became comparable to that for a close packing of particles, interfacial jamming occurred and drop retraction was arrested. Fast interfacial contraction or low capillary pressures led to less compact jammed monolayers, i.e. with a larger specific interfacial area. There was also significant hysteresis between compressing vs. expanding the jammed monolayer, suggesting that a certain minimum force is required for unjamming. Limited experiments with the same particles at a mineral oil/ silicone oil interface showed altogether different behavior: a particle-free portion of the interface coexisted with a particle-covered portion, suggesting that the monolayer behavior at this non-polar/non-polar interface is dominated by interparticle attraction. [Preview Abstract] |
Session P10: Focus Session: Multiferroics II
Sponsoring Units: FIAPChair: Serge Nakhmanson, Argonne National Laboratory
Room: 304
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P10.00001: Exchange interactions between soft ferromagnetic thin films and multiferroic BiFeO$_{3}$. Invited Speaker: Controlling the magnetization of a thin ferromagnetic (FM) film using an electric field is a Holy Grail of nowadays spintronics as it would revolutionize the addressing of magnetic memory elements. One strategy is to combine the magnetoelectric coupling of multiferroic materials like BiFeO$_{3}$ (BFO) [1] with the exchange coupling (EC) observed in FM / antiferromagnetic (AFM) systems such as in BFO/CoFeB bilayers [2]. BFO is a material of choice as it is one of the very few room-temperature AFM multiferroics. The two types of studied structures consist in FM layers of CoFeB deposited on BFO/STO films as well as thick permalloy layers sputtered onto BFO single crystals. They have been investigated by MagnetOptic Kerr Effect (MOKE) measurements. A macroscopic shift H$_{E}$ of the FM loops is a signature of exchange-bias (EB) in bilayers where the FM spins are coupled to the uncompensated AFM ones. We will show that the complex angular dependences of H$_{E}$ and H$_{C}$ result from the competition between the anisotropies of the FM and AFM layers and the strength of the EC. We will also compare the magnetic properties of the FM layers in relation with the ferroelectric structure of the underlying BFO. In their virgin state, the crystals are in a single ferroelectric and AFM domain with a cycloidal magnetic structure whereas thin films, in which the cyloid is suppressed, are in a highly multidomain state. This comparative study allows us to determine the nature and location of the spins involved in the mechanism of EC. Finally, we present the electric field effect on H$_{E}$ and H$_{C}$ of these systems. Our previous work on BFO crystals demonstrated that during electrical poling, any change of the polarisation direction induces a spin flop of the AFM moments. We will show here that a 90\r{ } rotation of the anisotropy axes can be obtained in domains where the polarisation is electrically flipped. \\[4pt] [1] PRL, \textbf{100}, 227602 (2008). \\[0pt] [2] PRL, \textbf{100}, 017204 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P10.00002: First principles study of magnetostructural instabilities in magnetic perovskite oxides Jun Hee Lee, Lucia Palova, Karin M. Rabe First principles phonon dispersion relations are reported for a range of magnetic perovskite oxides in cubic high-symmetry reference structures. Materials considered include EuTiO$_3$ and BiFeO$_3$. For each system, the dominant lattice instabilities are identified. These are frozen-in, singly and in combination, and the structures are optimized in the resulting space groups. From this, we identify distinct low-energy alternatives to the ground-state structure. We focus particularly on the dependence of the lattice instabilities and structural energetics of low-lying phases on the magnetic order, and extract key magnetostructural coupling parameters. The analysis is applied to predict possible structural and magnetic phase transitions as a function of epitaxial strain and/or of composition in low-concentration solid solutions ($A_{1-x}A'_xB$O$_3$, $AB_{1-x}B'_x$O$_3$, $A_{1-x}A'_xB_{1-y}B'_y$O$_3$ for small $x$,$y$). [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P10.00003: Absence of Critical Thickness in an Ultrathin Improper Ferroelectric Film Na Sai, Craig J. Fennie, Alexander A. Demkov We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO$_3$ ultra-thin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface, is found to be metastable in a single domain state despite the presence of a depolarizing field. We establish a connection between the result and the role of improper ferroelectric transition. Our results imply that improper ferroelectric ultrathin films can have rather unique properties that are distinctive from those of very thin films of ordinary ferroelectrics. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P10.00004: Influence of step heights and terrace lengths of bottom electrodes on structural and ferroelectric properties of BiFeO$_{3}$ thin films Jae-Wan Park, Seung-Hyub Baek, Chang-Beom Eom The ferroelastic domain structures of epitaxial ferroelectric thin films are critical to control the ferroelectric properties. We have already demonstrated the selection of ferroelastic domain structure variants in epitaxial BiFeO$_{3}$ films and consequently significant improvement in ferroelectric switching behavior and leakage current by employing miscut in cubic (001) SrTiO$_{3 }$substrates. We have also observed significant step bunching in the SrRuO$_{3}$ bottom electrode and BiFeO$_{3}$ thin films on high miscut substrates resulting in significant surface roughness. In particular, the formation of step bunching causes different step heights and terrace lengths which can affect the growth behaviors of subsequent epitaxial films. We have studied the influence of step bunching of SrRuO$_{3}$ bottom electrodes on structural and ferroelectric properties of BiFeO$_{3}$ thin films grown on SrRuO$_{3}$ bottom electrodes by sputtering. We have varied the step heights and terrace lengths of SrRuO$_{3}$ bottom electrodes on 0.2$^{o}$ miscut (001) SrTiO$_{3}$ substrates by controlling of the growth conditions such as laser energy and pulse frequency in pulsed laser deposition. We will discuss the relationship between underlying step structures and the ferroelectric properties of epitaxial BiFeO$_{3}$ films. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P10.00005: Functional Properties at Domain Walls in BiFeO$_{3}$: Electrical, Magnetic, and Structural investigations Qing He, C.-H. Yang, P. Yu, M. Gajek, J. Seidel, R. Ramesh, F. Wang, Y.-H. Chu, L. W. Martin, N. Spaldin, A. Rother BiFeO$_{3}$ (BFO) is a widely studied robust ferroelectric, antiferromagnetic multiferroic. Conducting-atomic force microscopy studies reveal the presence of enhanced conductivity at certain types of domain walls in BFO. We have completed detailed TEM studies of the physical structure at these domain walls as well as in-depth DFT calculations of the evolution of electronic structure at these domain walls. These studies reveal two major contributions to the observed conduction: the formation of an electrostatic potential at the domain walls as well as a structurally-driven change in the electronic structure (i.e., a lower band gap locally) at the domain walls. We will discuss the use of optical characterization techniques as a way of probing this change in electronic structure at domain walls as well as detailed IV characterization both in atmospheric and UHV environments. Finally, the evolution of magnetism at these domain walls has been studied through the use of photoemission measurements. Initial findings point to a significant change in the magnetic order at these domain walls in BFO. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P10.00006: Electronic conduction in doped multiferroic BiFeO$_{3}$ Chan-Ho Yang, Jan Seidel, Sang-Yong Kim, M. Gajek, P. Yu, M.B. Holcomb, L.W. Martin, R. Ramesh, Y.H. Chu Competition between multiple ground states, that are energetically similar, plays a key role in many interesting material properties and physical phenomena as for example in high-$T_{c}$ superconductors (electron kinetic energy vs. electron-electron repulsion), colossal magnetoresistance (metallic state vs. charge ordered insulating state), and magnetically frustrated systems (spin-spin interactions). We are exploring the idea of similar competing phenomena in doped multiferroics by control of band-filling. In this paper we present systematic investigations of divalent Ca doping of ferroelectric BiFeO$_{3}$ in terms of structural and electronic conduction properties as well as diffusion properties of oxygen vacancies. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P10.00007: Ferroelectric switching behavior of (001) mono-domain BiFeO$_{3}$ thin films Seung-Hyub Baek, Ho-Won Jang, Chad Folkman, Chang-Beom Eom, Yulan Li, Benjamin Winchester, Long-qing Chen, Christopher Nelson, Xiao-qing Pan, Ramamoorthy Ramesh BiFeO$_{3}$ has drawn a great deal of attention as a single phase multiferroic material for the magnetoelectric device and non-volatile memory applications. BiFeO$_{3}$ has a magnetoelectric coupling effect between [111] polarization and (111) anti-ferromagnetic order, which allows manipulation of magnetic property by electric field. However, the anti-ferromagnetic plane can be switched only by non-180$^{o}$ polarization switching due to this coupling geometry. Thus, for magnetoelectric device applications, it is crucial to selectively control 71$^{o}$ (or 109$^{o})$ switching. Here, we report the selective control of ferroelectric switching by the size of switched area; local field by AFM tip and uniform field by large area top electrodes. The origin of this behavior will be discussed using phase-field simulations. This result implies that the geometry of magnetoelectric devices should be determined by considering the size of switching area. Moreover, this result can be expanded to other rhombohedral systems such as PMN-PT and PZT. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P10.00008: Systematic investigation of morphotropic phase boundaries in rare-earth doped BiFeO3 Daisuke Kan, Shigehiro Fujino, Vartharajan Anbusathaiah, Valanoor Nagarajan, Makoto Murakami, Sung–Hwan Lim, Arun Luykx, Dwight Hunter, Manfred Wuttig, Ichiro Takeuchi We have investigated structural and ferroelectric properties of BiFeO$_{3}$ doped with rare-earth (RE) elements La, Sm, Gd, Dy, Lu using thin film composition spreads. Previously, we had reported on discovery of a morphotropic phase boundary in (Bi,Sm)FeO$_{3}$.[1] Thin film composition spreads of (Bi,RE)FeO$_{3}$ were fabricated by combinatorial pulsed laser deposition. From scanning xray diffraction, rhombohedral to pseudo-orthorhombic structural transitions are observed. The composition at which the structural transition takes place changes with radii of the RE element: the smaller the ion, the smaller the substitution concentration. This shows that the chemical pressure effect is the cause of the transition. The ferroelectric - antiferroelectric transition is observed for RE doped BiFeO$_{3}$ with smaller radii than the Bi$^{3+}$ ion at the compositions which coincide with the structural transition for each RE dopant. The detailed correlation between the structural properties and ferroelectric and piezoelectric properties will be discussed. This work is supported by DMR, NSF DMR, ARO and the W. M. Keck Foundation.\\[3pt][1] S. Fujino et al., APL 92, 202904 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P10.00009: Ideal Nanocheckerboard $BiFeO_3-BiMnO_3$ from First Principles Lucia Palova, Karin Rabe, Premala Chandra Motivated by recent nanocheckerboard patternings of oxide materials, we use first principles calculations to characterize a prototypical atomic-scale checkerboard of $BiFeO_3-BiMnO_3$ and to compare its properties to those of its bulk constituents. We find this checkerboard has a multiferroic ground state with nonzero ferroelectric polarization and a nonzero magnetic moment, thereby combining desirable features of bulk $BiFeO_3$ and $BiMnO_3$. Unlike either of its parent compounds, structural distortion of the checkerboard stabilizes different magnetic states; this magnetostructural effect can be used to switch between states with zero and nonzero magnetization. The role of oxygen-octahedron rotations and strain in the magnetic ordering of the nanocheckerboard will be examined in detail. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P10.00010: Mechanism of Spontaneous Electric Polarization Flop in TbMnO$_{3}$ Hajime Sagayama, Nobuyuki Abe, Taka-hisa Arima, Kazuaki Iwasa Orthorhombic perovskite TbMnO$_{3}$ is one of the typical multiferroic systems. Spontaneous electric polarization (P) along the $c$-axis which originates from the spiral configuration of Mn$^{3+}$ spins rotating in the \textit{bc}-plane appears below $T_{C}$ ($\sim $27K). P//$c$ is turned to the direction along the a-axis by applying a magnetic field along $a$- or $b$-axis. Magnetic structure analysis and a spin-polarized neutron diffraction study of $^{160}$Gd$_{0.7}$Tb$_{0.3}$MnO$_{3}$ strongly suggest that P//$a$ in TbMnO$_{3}$ in high magnetic fields also originates from spin spiral rotating in the \textit{ab}-plane as in the case of P//$c$. It has been pointed out that anisotropic Tb $f$-electron magnetic moments play an important role for the complicated electric polarization flop. In this study, we have confirmed the change of spin basal plane of TbMnO$_{3}$ from the \textit{bc-} to \textit{ab-} plane by applying a magnetic field along the $b$-axis using spin-polarized neutron diffraction technique. We observed that a magnetic fields induce a C-type antiferromagnetic structure caused by the local anisotropy of Tb magnetic moments. We have succeeded in explaining the electric polarization flop of TbMnO$_{3}$ in terms of a coupling between Mn$^{3+}$ spins and anisotropic Tb magnetic moments. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P10.00011: Non-$d^0$ Mn-driven ferroelectricity in antiferromagnetic BaMnO$_3$ James Rondinelli, Nicola Spaldin Using first-principles density functional theory calculations we predict a ferroelectric ground state structure -- driven by off-centering of the magnetic Mn ion -- for perovskite-structure BaMnO$_3$. Our finding is surprising, since the competition between energy-lowering covalent bond formation, and energy-raising Coulombic repulsions (the 2nd order Jahn-Teller effect) usually only favors off-centering for non-magnetic $d^0$ ions. It is consistent, however, with the recent observation [S. Bhattacharjee, E. Bousquet and P. Ghosez, Arxiv e-prints {\bf 811}, 0811.2344 (2008)], that large lattice constants can stabilize polar off-centering of magnetic ions, by lowering the short-range Coulomb repulsions that favor the centrosymmetric phase. We calculate the Born effective charges for the compound, and find anomalously large values for Mn and O, consistent with the large calculated ferroelectric polarization of 30 $\mu$C/cm$^2$. We also describe the changes that occur in the electronic structure when the system transitions from a centrosymmetric to polar state. Finally, we suggest possible routes by which the cubic perovskite phase can be stabilized over the usual hexagonal phase. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P10.00012: Antiferromagnetic Resonance in Multiferroic YMnO$_3$ and LuMnO$_3$ Sergei Zvyagin Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic orders. Combining conventional far-infrared Fourier-transform and THz-range free electron laser electron spin resonance (ESR) spectroscopy techniques, magnetic excitations in hexagonal multiferroic YMnO$_3$ and LuMnO$_3$ in the antiferromagnetically (AFM) ordered phase have been studied. The gap in the excitation spectrum ($\sim 42$ and $\sim 48$ cm$^{-1}$ for YMnO$_3$ and LuMnO$_3$, respectively) was observed directly. Similar slope of the frequency-field dependences of AFM resonance modes, $\sim$ 0.5 cm$^{-1}$/T, was revealed for both compounds. A fine structure of AFM resonance absorption has been revealed by means of high-resolution ESR techniques, which can be explained taking into account a finite interaction between the neighboring Mn$^{3+}$ layers. The work was done in collaboration with M. Ozerov, D. Kamensky, E. \v{C}i\v{z}m\'{a}r, J. Wosnitza, A.K. Kolezhuk, D. Smirnov, H.D. Zhou, and C.R. Wiebe. [Preview Abstract] |
Session P11: Focus Session: Transport Properties of Nanostructures IV: Correlation Effects
Sponsoring Units: DCMPChair: Harold Baranger, Duke University
Room: 305
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P11.00001: Incorporating Exchange-Correlation Effects in Quantum Transport through Nano-scale Junctions Invited Speaker: State-of-the-art computational methods for modelling electron transport in nano-scale junctions are based on effective single- particle approximations such as the Kohn-Sham theory of density functional theory. This methodology has been successfully applied to junctions with strong coupling to the metallic electrodes, but has proved insufficient for less homogeneous junctions where the distinction between the nano-device and the electrodes is more pronounced. In order to obtain a more accurate and rigorous description of exchange-correlation effects in weakly correlated molecular junctions, we have implemented the many-body GW approximation within a transport framework suitable to treat non-periodic systems consisting of an interacting region coupled to infinite non-interacting leads with different chemical potentials. Fundamental trends in the properties of metal-molecule-metal junctions are identified on the basis of simple model calculations. These include renormalization of molecular QP levels due to dynamical polarization effects both in- and out of equilibrium as well as the reduction of QP life-times due to enhanced QP scattering under finite bias conditions. As will be shown, these genuine many-body effects can have a large influence on the junction IV characteristics even for weakly correlated systems. The importance of using a fully self-consistent GW self-energy for quantum transport calculations will be demonstrated. Finally (preliminary) results for more realistic molecular junctions will be discussed. References: K. S. Thygesen and A. Rubio, PRB 77, 115333 (2008); K. S. Thygesen, PRL 100, 166804 (2008); K. S. Thygesen and A. Rubio, arXiv:0810.5214. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P11.00002: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P11.00003: Two-channel Kondo effect and phonon-assisted transport in single-molecular junctions Luis Dias da Silva, Elbio Dagotto The interplay between vibrational modes and Kondo physics is a fundamental aspect of transport properties of correlated molecular conductors. In this theoretical work, we study such interplay in a system consisting of a single molecule in a metallic break junction tuned (by gate voltages) to be in an ``odd-N'' coulomb blockade valley (Kondo-prone). The connection to left and right metallic leads creates the usual coupling to a conduction channel with left-right symmetry (the ``even"-parity channel). A center-of-mass vibrational mode introduces an additional, phonon-assisted tunneling through the asymmetric (``odd''-parity channel). Our numerical renormalization-group calculations reveal that the phonon-mediated coupling to the odd channel leads to the appearance of a two-channel Kondo (2chK) effect, characterized by a non-Fermi-liquid (NFL) fixed point. The ground-state has NFL properties for a critical value of the phonon-mediated coupling strength and critical lines are present for wide range of parameters, including the regime away from particle-hole symmetry. Signatures of this 2chK non-Fermi-liquid behavior are prominent in the thermodynamic properties as well as in the linear conductance. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P11.00004: Universal Scaling of Zero-Bias Conductance Peaks in Single-Molecule Transistors Incorporating Tetra[2,3-thienylene] Zachary Keane, Gavin Scott, Douglas Natelson There is significant interest in exploring universal scaling laws as they apply to the Kondo state in diverse physical systems. One such system is GaAs quantum dots, in which Grobnis et al. have demonstrated that the conductance follows a universal scaling function in temperature and source-drain bias. More recently, Scott et al. have demonstrated that the same scaling law applies to single molecule transistors incorporating both C60 and bis(2,5-di-[2]pyridyl-3,4-dithiocyanto-pyrrolate)Cu(II), despite the fact that the relevant energy scales in these systems can differ by 3 orders of magnitude. We will report measurements and universal scaling analysis of the Kondo conductance as a function of temperature and source-drain bias in a fourth system, single molecule transistors incorporating tetra[2,3-thienylene]. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P11.00005: Tunneling spectra of individual magnetic endofullerene molecules E. S. Tam, J. E. Grose, J. J. Parks, B. Ulgut, H. D. Abru\~{n}a, D. C. Ralph, C. Timm, M. Scheloske, W. Harneit We report measurements of electron tunneling spectra for individual N@C$_{60}$ molecules, a spin-3/2 endohedral fullerene. The molecules were measured at low temperature in electromigrated break-junctions in the single-electron transistor configuration. We observe that the N@C$_{60}$ devices exhibit a spin-state transition as a function of applied magnetic field which was not observed in C$_{60}$ control devices. The nature of this transition enables us to identify the charge and spin states of the molecule. The spectra of N@C$_{60}$ devices also exhibit low-energy excited states and signatures of non-equilibrium spin excitations predicted for this molecule. The experimental spectra can be reproduced theoretically by accounting for the exchange interaction between the nitrogen spin and electron(s) on the C$_{60}$ cage. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P11.00006: Spin-dependent effects in transport through individual molecules and nanoparticles J. J. Parks, E. S. Tam, S. Flores-Torres, H. D. Abruna, D. C. Ralph We report measurements of electron transport through individual molecules and nanoparticles incorporated into electromigrated break junction devices. In low-temperature studies of a thiol-terminated organometallic complex using a mechanically controllable break junction, we have studied the effects of molecular distortions. We find that as a function of stretching the molecule, a zero-bias Kondo peak can split into two finite-bias peaks, reminiscent of singlet-triplet transitions in other types of quantum dots. We discuss possible mechanisms in terms of coupling between broken spatial symmetries and the spin state of the molecule. We also measure devices in which molecules and nanoparticles are contacted by ferromagnetic electrodes, so as to study the interplay of spin polarization with single-electron charging effects. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P11.00007: Universal Scaling of Nonequilibrium Transport in the Kondo Regime of Single Molecule Devices Gavin Scott, Zachary Keane, Jacob Ciszek, James Tour, Douglas Natelson Scaling laws and universality are often associated with systems exhibiting emergent phenomena possessing a characteristic energy scale. We report nonequilibrium transport measurements on two different types of single-molecule transistor (SMT) devices in the Kondo regime. The conductance at low bias and temperature adheres to a scaling function characterized by two parameters. This result, analogous to that reported recently in semiconductor dots with Kondo temperatures two orders of magnitude lower, demonstrates the universality of this scaling form. We compare the extracted values of the scaling coefficients to previous experimental and theoretical results. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P11.00008: Spatial range of the Kondo effect C.A. B\"usser, G. B. Martins, L. Costa Ribeiro, E. V. Anda, E. Dagotto The objective of this work is to discuss the spatial range of the effect caused by the Coulomb interaction localized at an impurity center. The numerical method we use, the embedded cluster approximation (ECA) and the finite U slave bosons mean field (FU-SBMF), are developed to treat localized impurity systems. It is important to note that, contrary to other techniques, ECA and FUSB can work in real space. Instead of using the spin-spin correlation to determine the length of the Kondo cloud, we will use the local density of states (LDOS) on the lead, far from the impurity. The presence of the impurity produce a disturbance in the LDOS of sites away from it. In this work, we propose to use this distortion to evaluate the spatial range of the Kondo effect. We observe that the effect of the distortion decays exponentially as a function of the distance from the impurity. With that in mind, a characteristic length $\hat{R}_{\rm K}$ can be easily defined. When the coupling between the impurity and the metal is increased, we verify that $\hat{R}_{\rm K} \sim 1/ T_{\rm K}$. We will also discuss how the magnetic field and temperature affect the length $R_{\rm K}$. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P11.00009: 1/N expansion of the nonequilibrium single-impurity Anderson Model Zurab Ratiani, Aditi Mitra Results are presented for the nonequilibrium single-impurity Anderson model using a large-N approach, where N is the degeneracy of the impurity level. Using the Keldysh formalism, we extend the slave-boson functional integral method of Read and Newns to the out of equilibrium current carrying case. The correlation function for the slave boson is shown to exhibit a long time power law behavior along with an exponential decay whose origin is current induced decoherence, a result consistent with nonequilibrium X-ray edge physics. Expressions for the impurity susceptibility and the conductance through the device are presented to O(1/N) and for an applied voltage less than the Kondo temperature. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P11.00010: Interplay of Rashba and lateral spin-orbit interactions on the spin polarized conductance of quantum point contacts.* A. Ngo, P. Debray, S. E. Ulloa In this work, we study the conductance properties of semiconductor quantum point contacts (QPCs) created by laterally confining a two-dimensional electron gas via side-gating. The electric field due to the gradients of the lateral confining potential results in lateral spin-orbit coupling. Our experimental observations in QPCs fabricated in InGaAs/InAs exhibit a plateau in conductance at half-quantization, G $\cong $ 0.5(2e$^{2}$/h)$,$ in the \textit{absence} of applied magnetic field. To understand our experimental results, we carry out calculations of ballistic transport through QPCs in the presence of Rashba \textit{and} lateral spin-orbit coupling. Using a scattering matrix approach, we calculate the spin-dependent conductance for different confinement and applied electric fields. High spin polarization can be obtained in the absence any external magnetic field by controlling the tunable perpendicular applied electric field and the shape of lateral confining potential, but only at high spin-orbit interaction strength. We also study the possibility that the strong asymmetric confining potential creates an effective spin-dependent term due e.g. to electron-electron interactions. This term breaks the time reversal symmetry and is able to produce the 0.5 conductance plateau, similar to that seen in our experiments. Our results might provide a new approach to explore spin polarized electron sources. * Supported by NSF-DMR. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P11.00011: Spin control in Rashba-Aharonov-Bohm quantum dot ring in the Kondo regime. E. Vernek, N. Sandler, S. E. Ulloa Application of small magnetic fields in QDs embedded in Aharonov-Bohm (AB) ring geometries, as well as gate voltages that modify the Rashba spin-orbit interaction (RSOI), are possible experimental probes to control spin transport. One important feature of charge transmission through QDs is the Kondo effect, resulting from the strong Coulomb interactions in the dot and carrier hopping between dot and current leads. Although much work has focused on the Kondo regime in QDs, not much is known on how RSOI modifies charge transport through the dot or its role in spin-transport. Full understanding of RSOI on the Kondo regime is fundamental, as it studies the competition of different coherent phenomena and has potential applications in devices such as spin-filters. A study of this geometry included the role of RSOI perturbatively [1]. However, the full features of Kondo physics are subtle and not captured in perturbation theory. In this work, we present a numerical renormalization group study that addresses charge and spin transport properties in the zero-bias regime, and allows comparisons with perturbation results. We find that the presence of both AB fields and RSOI results in an intrinsic polarizing field that breaks the spin degeneracy. This allows a delicate control of spin polarization of the conductance in the system, while strong RSOI suppresses the Kondo effect. [1] R. J. Heary et al., PRB 77, 115132 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P11.00012: Decoherence due to contacts in ballistic nanostructures I. Knezevic In quasiballistic nanoscale electronic structures, the process of relaxation towards a steady state cannot be attributed to carrier scattering. Rather, the active region of a nanostructure is an open quantum-mechanical system, whose nonunitary evolution (decoherence) toward a nonequilibrium steady state is determined by carrier injection from the rapidly dephasing contacts. I will present a technique for the treatment of contact-induced decoherence in ballistic nanostructures, which is established within the framework of the open system theory. Efficient electron-electron scattering in the contacts enables one to consider them nearly ``memoryless'' and derive a Markovian kinetic equation for the active region's statistical operator through coarse graining over the contacts' short memory- retention time. By incorporating a first-principles model interaction between the active region and the contacts into the Markovian dynamics derived, nonequilibrium steady-state distribution functions of the forward- and backward-propagating states in the active region are derived analytically. The approach is illustrated on several two- terminal nanostructures. I will also discuss the relationship between the present approach and the Landauer-B\"{u}ttiker formalism, as well as the inclusion of scattering. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P11.00013: High frequency measurements of shot noise suppression in atomic-scale metal contacts Patrick J. Wheeler, Kenneth Evans, Jeffrey Russom, Nicholas King, Douglas Natelson Shot noise provides a means of assessing the number and transmission coefficients of transmitting channels in atomic- and molecular-scale junctions. Previous experiments at low temperatures in metal and semiconductor point contacts have demonstrated the expected suppression of shot noise when junction conductance is near an integer multiple of the conductance quantum, $G_{0}\equiv 2e^2/h$. Using high frequency techniques, we demonstrate the high speed acquisition of such data at room temperature in mechanical break junctions. In clean Au contacts conductance histograms with clear peaks at $G_{0}$, $2G_{0}$, and $3G_{0}$ are acquired within hours, and histograms of simultaneous measurements of the shot noise show clear suppression at those conductance values. We describe the dependence of the noise on bias voltage and analyze the noise vs. conductance histograms in terms of a model that averages over transmission coefficients. [Preview Abstract] |
Session P12: Focus Session: Characterization and Modeling of Complex Surfaces and Interfaces
Sponsoring Units: DMP DCMPChair: Yue Qi, General Motors
Room: 308
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P12.00001: Atomistic Structure and Energy of Solid-Liquid Interfaces Invited Speaker: As microstructural length-scales are reduced, the role of interfaces in determining the properties of materials becomes more dominant. The importance of the correlation between interface structure and chemistry with interface (and bulk) properties is evident in a range of material systems, and is a topic of intense experimental and theoretical work for solid-solid interfaces. While detailed thermodynamic analysis of solid-liquid interfaces is routinely conducted, knowledge of the local structure at solid-liquid interfaces is still incomplete. To be more specific, the correlation between the structure of the solid, and the structure in the liquid near the interface, has not been fully addressed. In this presentation, in-situ ($\sim $750\r{ }C) high resolution transmission electron microscopy (HRTEM) of liquid Al in contact with sapphire ($\alpha $-Al$_{2}$O$_{3})$ will be presented. Contrast perturbations in the liquid Al adjacent to the crystalline substrate were determined to be due to ordering of the liquid, via detailed multi-slice dynamic electron scattering simulations. Details on the type of ordering were interpreted by molecular dynamics simulations of liquid Al in contact with crystalline substrates, and compared to sessile drop studies of liquid Al on sapphire. These results are compared with recent HRTEM investigations of equilibrium amorphous films at metal-Al$_{2}$O$_{3}$ interfaces, where partial ordering of the film plays an important entropic role in reaching stable nanometer-thick films. This will then be extended to equilibrium segregation, and the concept of extremely small volumes of liquids confined by crystals. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P12.00002: Calculation of excess interfacial entropy, stress and energy for solid-liquid interfaces Brian B. Laird, Ruslan L. Davidchack, Mark Asta, Yang Yang The solid-liquid interfacial free energy, $\gamma_{\rm sl}$, governs a number of important phenomena, e.g., crystal nucleation and growth, and wetting. For an equilibrium crystal-melt interface, $\gamma_{\rm sl}$ can be calculated via simulation using thermodynamic integration or capillary fluctuations [Phys. Chem. B {\bf 109}, 17802 (2005)]. The calculation of $\gamma_{\rm sl}$ away from coexistence requires the temperature and strain dependence of $\gamma_{\rm sl}$, which can be determined from the excess interfacial entropy, $\eta_{\rm sl}$, and stress tensor, $\mbox{\boldmath$\tau$}_{\rm sl}$. We determine $\eta_{\rm sl}$ and $\mbox{\boldmath$\tau$}_{\rm sl}$ for a system of Lennard-Jones particles and for particles with an inverse-power interaction [$\phi(r) = \epsilon (\sigma/r)^{n}$] for $n = $ 6, 8 (fcc and bcc) and 12, 20 (fcc). We determine $\eta_{\rm sl}$ and $\mbox{\boldmath$\tau$}_{\rm sl}$ for the (100), (110) and (111) orientations. We calculate $\eta_{\rm sl}$ using two methods, both using the Gibbs dividing surface defined so that the excess interfacial particle number is zero. In the first, we calculate $\eta_{\rm sl}$ from the temperature dependence of $\gamma_{\rm sl}$, $\mbox{\boldmath$\tau$}_{\rm sl}$ and the number density, $\rho$, along the coexistence curve. In the second, we calculate the excess interfacial energy, $e_{\rm sl}$, and use the equation $\gamma_{\rm sl} = e_{\rm sl} - T \eta_{\rm sl}$. The results agree within estimated errors. One surprising observation is that $\eta_{\rm sl}$, $e_{\rm sl}$ and $\mbox{\boldmath$\tau$}_{\rm sl}$ are significantly more anisotropic than $\gamma_{\rm sl}$. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P12.00003: Determination of Spin Order in Magnetic Organic Semiconductor V[TCNE]$\sim $2 Hailemariam Ambaye, Valeria Lauter, Stephen Nagler, Christina Hoffmann, Hal Lee, Andrew Payzant, Arthur Epstein, Chen Chi-Yi, Richard Goyette These Organic-based magnets are new area of materials research. The discovery of V[TCNE]$\sim $2 with its high Tc $\sim $ 400 K and semiconducting behavior similar to silicon, as well as its photonic response unique for magnetic materials, opens up many issues of fundamental physics and chemistry as well as the potential opportunities for use of these and related materials in technologies ranging from spintronics to sensing. To understand the magnetic state and the evolution of the magnetic at and near interfaces with other magnetic and non magnetic materials we have performed a polarized neutron measurement at the SNS magnetism reflectometer instrument. The measurements show the presence of magnetic response at 5K temperature. The room temperature measurements show no magnetic responses. The systems considered are V[TCNE]$\sim $2(1500A) and 6000A on Si substrate. Work supported by DOE. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P12.00004: Stabilization of a ferromagnetic insulating phase with colossal magnetoresistance at the interface of manganite bilayers Jonathan Laverdi\`ere, Serge Jandl, Patrick Fournier The charge ordered phase observed in colossal magnetoresistive manganites motivated many theoretical and experimental efforts. Charge order is an insulating electronic phase that becomes metallic in sufficiently high magnetic field. This high ``melting'' field hinders any applications for magnetic storage devices. Here, we present the study of the proximity effect in Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (NSMO) / Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (NCMO) bilayers. NSMO is dominated by ferromagnetic double exchange, producing a ferromagnetic metallic phase, while NCMO is strongly influenced by the Jahn-Teller lattice distortion, localizing charges on the Mn$^{3+}$ sites. Our study addresses the following question: Which one will dominate at the NSMO/NCMO interface? We will present Raman scattering and magnetotransport measurements on NCMO/NSMO bilayers grown on SrTiO$_{3}$ substrate. A ferromagnetic insulating phase has been observed for very thin NSMO films. This phase becomes metallic and gives rise to colossal magnetoresistance at a low field compared to the usual melting field. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P12.00005: Effects of surfactant to the physical properties of single-walled carbon nanotube buckypaper. Jin Gyu Park, Charlie Lin, Jesse Smithyman, Adam Cooke, Shu Li, Richard Liang, Chuck Zhang, Ben Wang, Ade Kismarahardja, James Brooks Single-walled carbon nanotubes (SWCNTs) were dispersed in aqueous medium using surfactant and filtered to get an entangled network, called buckypaper (BP). Thermogravimetric analysis shows that the remaining surfactant has significant weight percentage and has effects on the physical properties. Raman spectrum of BP, especially the radial breathing mode is related to the entanglement degree and residual surfactant. The G-band peak shift shows different temperature dependence with the reduction of residual surfactant in the BP and oxidation of nanotube. The electrical conductivity was improved after removing surfactant and temperature dependence of electrical resistivity followed variable range hopping type conduction. Mechanical properties are also affected from their integration degree, alignment, and residual surfactant. Therefore, tensile modulus and strength were improved after washing surfactant. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P12.00006: Low-Temperature Absorption Studies in Bismuth Nanowires Jason Reppert, Malcolm Skove, Marcie Black, Mildred Dresselhaus, Apparao Rao Bulk bismuth has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Previously, we have shown evidence for strong quantum confinement in Bi nanorods with diameters $\sim $10 nm which undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. These quantum confinement effects can be potentially useful in optical and electro-optical devices. Here, we report the low temperature (77 K) optical absorption properties of $\sim $10 nm diameter Bi nanorods using Fourier Transform Infrared spectroscopy. The Bi nanorods exhibit a strong absorption peak ($\sim $1000 -- 1400 cm$^{-1}$, depending on the diameter) in the mid-IR that is not present in bulk bismuth. The full width at half maximum intensity of the IR absorption peaks decrease from 26 cm$^{-1}$ at 300 K to 15 cm$^{-1}$ at 77 K. No significant blue-shift in energy was observed, and these changes will be discussed in terms of the temperature dependence of the L-point and T-point electron energies. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P12.00007: Nanopatterned Biomimetic Surfaces to Probe the Role of cytoskeletal Proteins in cell Adhesion Justin Abramson, Matteo Palma, Mark Schvartzman, Shalom Wind, Michael Sheetz, James Hone Nanometer level spatial organization has been shown to play a crucial role in cell mechanics, in particular in cell adhesion to the extracellular matrix. Combining nanolithography and biomolecular self-assembly strategies, we report on the fabrication of nanopatterned biomimetic surfaces to probe the importance of both the spatial ordering of transmembrane proteins as well as the role played by peptide sequences as cell binding domains in the formation of cell focal adhesions. We have fabricated arrays of Au/Pd nano-dots using electron-beam and nanoimprint lithography. Different chemical strategies have been pursued to biofunctionalize such nanostructures, both through the formation of mixed Self Assembled Monolayers as well as via chemical reactions at surfaces. Fluorescence microscopy allowed us to monitor single-molecule chemisorption of cell-adhesion proteins in vitro, as well as to follow cell spreading on the nanopatterned bio-arrays, in order to investigate cytoskeletal protein binding interactions in vivo. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P12.00008: Molecular Dynamics Simulations of Interfaces in Complex Materials Invited Speaker: Molecular details of structure and chemistry play and important role in the properties and engineering performance of composites and complex materials. Experimental investigation of the interfaces most often presents itself as a major challenge. This, in turn, becomes an opportunity in disguise for the molecular level simulation approaches. However, in order to convincingly address to the problem of elucidating the structure and chemistry at the interfaces, one must employ reliable and accurate and transferrable interaction potentials for dissimilar materials - this is the case for composites and complex materials. In this, talk we will present examples of molecular dynamics studies on the structure and properties of silicon nano-crystals in a silica matrix, piezoelectric CNT-polyimide nano-composites, and on the role of super lattice structures for enhancing mechano-electric coupling in ferro-electric ceramic alloys. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P12.00009: Evaluation of dispersion interactions in general geometries Anthony Maggs, Pasquali Samuela Dispersion interactions are very often approximated by pairwise van der Waals interactions between molecules. In dense media, however, there are important corrections due to the many-body nature of fluctuations. These many-body forms can be calculated in closed form in the simplest of geometries using the methods of Casimir and Lifshitz. Here we study dispersion interactions between bodies in general geometries. We map the calculation of the partition function onto a determinant which we discretize and evaluate with the help of Cholesky factorization. We study the efficiency of the factorization in two and in three dimensions and conclude that accuracies of the order of one per cent are readily achieved in the total interaction energy. We compare the approximations of pairwise additivity and proximity force with our numerical methods. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P12.00010: Phase-field Simulation of Phase Coarsening at Ultra-high Volume Fractions Ke-Gang Wang, Xueru Ding The study of phase coarsening kinetics during microstructure evolution is critical to a variety of industrial applications involving two-phase systems in which the dispersed phase controls the properties of the material. Liquid-phase sintering, casting and spray deposition are just a few examples of processes in which the coarsening process has important technological implications. In this talk, the dynamics of phase coarsening at ultra-high volume fractions (V$_{V}>$0.9) will be presented based on 2-D phase-field simulations. Kinetics of phase coarsening and spatial correlations in microstructures will be revealed. Pair distribution functions in microstructures will be shown. The scaled particle-size distribution as functions of the dispersoid volume fraction will be demonstrated. Finally, computational results are compared with experimental observations. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P12.00011: The impact of self-healing on the life-time of materials John Gaddy, Wouter Montfrooij, Alexander Schmets Structural materials that are attributed with the (new) property of ``self-healing'' will obviously lead to safer, longer lasting and more reliable structures. The property of ``self-healing'' can be defined as the ability of a material to mitigate autonomously early stages of damage such as micro cracks, and many examples of materials with this properties have been reported in recent years [1]. In this contribution we investigate the effect of healing on the expected service life time of a model material. We apply a statistical mechanics' inspired computational approach to model the process of damage accumulation and on-site healing of a material under well defined loading conditions. We define a material as being at the end of its service life when a percolative path of damaged cells has passed a prescribed length. The variation of service life for various scenarios, such as healing times and distribution of healing centers is investigated. Finally we show how this type of models may be useful for the design of optimized self healing materials. \\[3pt] [1] S. van der Zwaag (editor), (2007). \textit{Self Healing Materials: An Alternative Approach to 20 Centuries of Materials Science}. Springer Netherlands. [Preview Abstract] |
Session P13: Focus Session: Extreme Conditions and High Pressure I: Chemistry
Sponsoring Units: DCOMP GSCCMChair: Michelle Weinberger, Geophysical Laboratory, Carnegie Institution of Washington
Room: 309
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P13.00001: Shock-induced Reactions in Pentaerythritol Tetranitrate Studied by Molecular Dynamics Simulation Joanne Budzien, Aidan P. Thompson, Sergey V. Zybin Molecular dynamics simulations were performed using the reactive force field, ReaxFF, as implemented in the General Reactive Atomistic Simulation Program code for systems consisting of a single crystal of PETN with not fewer than 237000 atoms. The crystals were shocked along the [100] direction using two different piston velocities. The resulting chemical reactions were tracked in an attempt to elucidate short-time initiation mechanisms. Here, we present the primary, secondary, and intermediate products as a function of time and position behind the shock front. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P13.00002: Molecular dynamics simulations of uniaxial shock compression of RDX crystals Dmitry Bedrov, Justin Hooper, Grant Smith Using the Hugoniostat methodology atomistic molecular dynamics simulations of uniaxial shock compressions along [001], [100], and [010] directions of RDX crystal have been conducted over a wide range of shock pressures. The Hugoniostat simulations allow modeling of shocked material without the necessity to have extremely large simulation cell required to explicitly resolve the shock wave propagation. Hugoniostat simulations on systems containing only few thousand molecules allowed us to determine Hugoniot elastic limit and to investigate shock-induced shear banding and phase transition in RDX crystal. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P13.00003: A molecular dynamics study of the role of pressure on the response of reactive materials to thermal initiation N. Scott Weingarten, William D. Mattson, Betsy M. Rice, Anthony D. Yau, Timothy P. Weihs Reactive materials have the potential for implementation into a wide variety of commercial and military applications. However, the fundamental physical and chemical processes that control the energy release are not well understood. To elucidate the mechanisms of energy release, we simulated the exothermic alloying reactions of a Ni/Al bilayer with initial temperatures of 1100 K and 1500 K using both microcanonical (NVE) and isoenthalpic (NPH) molecular dynamics simulations with an embedded atom method (EAM) potential. The mechanism of the mixing is the same for all simulations: as mixing and reaction occurs at the interface, the heat generated first melts the Al layer, and subsequent mixing leads to further heat generation after which the Ni layer melts, leading to additional mixing until the alloying reactions are completed. The results indicate that pressure has a significant influence on the rates of atomic mixing and alloying reactions. In addition, two-phase coexistence simulations were used to determine the melting temperatures of pure Al and pure Ni at various pressures using this potential, and these values are discussed within the context of the Ni/Al bilayer results. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P13.00004: Mbar Chemistry: Novel States of Matter in Extreme Conditions Invited Speaker: Compression energy at 100 GPa often exceed several eV/atom, rivaling the energy of strong chemical bonds. Therefore, the application of such a high pressure significantly alters the chemical, electronic/optical, thermomechanical properties of solids and, in turn, provides a way to test condensed matter theory and to exploit novel materials with advanced properties. Furthermore, reeent advances in diamond-anvil cell high-pressure technologies coupled with advanced third-generation synchrotron x-ray offer unprecedented opportunities to discover exotic states of matter at high pressure-temperature conditions of the Earth and planetary interiors. In this paper, I will discuss several recent results of high-pressure chemistries that occur in simple low Z molecular solids to novel novel nonmolecular extended solids. Broadly speaking, these molecular-to-nonmolecular transitions occur as a result of the pressure-induced electron delocalization arising from a rapid increase in electron kinetic energy at high density. Yet, the details are substantially more complicated because of the phase metastability, large lattice strain, and governing kinetics. As a result, there are many outstanding questions regarding the exact nature of chemical bonding, phase stability, and transition mechanisms. Also, presented are several future directions of high pressure materials research in an complementary phase and time scales of dynamic and static high pressures. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P13.00005: Novel Catalytic Behavior of Dense Hot Water in PETN Decomposition Reactions Christine Wu, Laurence Fried, Lin Yang, Nir Goldman, Sorin Bastea Under extreme conditions, water is known to exhibit fascinating physical behaviors. Its remarkable structural and phase complexity strongly suggests that its chemical properties may be unusual as well, which have remained largely unrevealed. Using \textit{ab inito} molecular dynamics simulations, we have recently discovered that water plays an unexpected role in catalyzing complex reactions of a high explosive pentaerythritol tetranitrate (PETN). This finding is in contrary to the current view of water as a stable final product of high explosive reactions, and has possible implications in geochemistry, such as reactions in planetary interiors. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P13.00006: The formation of carbon nitride clusters in shocked insensitive explosives Laurence Fried, Riad Manaa, Evan Reed, Nir Goldman Many high explosives are organic molecular crystals that contain both oxidizing and reducing functional groups. These solids rapidly release their energy in supersonic detonation waves. It has been observed that explosives rich in carbon tend to have much longer reaction zones than those that do not. These explosives form graphitic or diamond-like carbon particles during detonation. The slow diffusion-limited process of forming the bulk solid from carbon clusters is believed to play a central role in determining the reaction zone length of a given explosive. In this work, we identify an altogether new mechanism for the slow reactivity of carbon rich explosives. Quantum–based multi-scale simulations of shocked 1,3,5-triamino- 2,4,6-trinitrobenzene (TATB) provide the first evidence for the formation of nitrogen-rich heterocyclic clusters that impede the formation of fluid nitrogen and solid carbon. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P13.00007: Ab initio molecular dynamics of hypervelocity chemistry Igor Schweigert, Brett Dunlap Resolving chemical dynamics of decomposition of energetic molecules is crucial for understanding detonation initiation in energetic materials and predicting their sensitivity to shock and impact stimuli. We employ Born-Oppenheimer molecular dynamics driven by density-functional methods to identify possible decomposition pathways in nitric esters (including pentaerythritol tetranitrate) and to understand the effect of collision orientation and velocity. Studies of the potential energy surface in the bond-breaking region, unimolecular decomposition, and binary hypervelocity collisions of model nitric esters (methyl and ethyl nitrates) will be reported. Methodological challenges in describing extensive changes in the electronic structure that accompany decomposition will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P13.00008: Boron carbides from first principles Emmanuel Betranhandy, Jelena Sjakste, Nathalie Vast In this work, we focus on the understanding gained from the investigation of the physical properties of boron-carbides with theoretical methods based on density functional theory (DFT). Comparison of computed and experimental vibrational or NMR spectra has shown that the atomic structure of B$_{4}$C consists in C-B-C chains linking mostly B$_{11}$C icosahedra, and a few percent of B$_{10}$C$_{2}$ icosahedra. In particular, C-C-C chains are excluded and can not be responsible for B$_{4}$C amorphization under shockwaves. In this work we find that at lower carbon concentration all models are metastable with respect to B$_{4}$C plus $\alpha $-boron. This could explain actual difficulties in the synthesis of clean samples. Furthermore we discuss effects of temperature and/or pressure on stabilities and properties. Finally, the idea of combining high hardness and superconductivity in the same material by doping boron-rich solids has emerged. We show results on the strength of the electron-phonon coupling constant obtained with DFT-based methods in B$_{13}$C$_{2}$. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P13.00009: Dynamical stability of the cubic metallic phase of AlH3 at ambient pressure Duck Young Kim, Ralph H. Scheicher, Rajeev Ahuja We have characterized the high-pressure cubic phase of AlH$_{3}$ using density functional theory to determine mechanical as well as electronic properties and lattice dynamics from the response function method [1]. Metallization in AlH$_{3}$ under pressure has been studied, which is of great interest not only from a fundamental physics point of view for the study of phenomena related to metallic hydrogen, but also, because metallic AlH$_{3}$ possesses weaker Al-H bonds than other insulating phases [2]. Our phonon calculations show the softening of a particular mode with decreasing pressure, indicating the onset of a dynamical instability that continues to persist at ambient conditions. We find from analyzing the atomic and electronic interactions using theoretical calculations that finite-temperature effects lead to the desired stabilization of metallic AlH$_{3}$ at ambient conditions.\\[0pt] [1] PRB \textbf{78}, 100102(R) (2008). \\[0pt] [2] APL \textbf{92}, 201903 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P13.00010: Tight binding multi-scale simulations of detonating energetic materials Evan Reed, M. Riad Manaa, Laurence Fried We present density-functional tight-binding (DFTB) molecular dynamics simulations of shock and detonation waves propagating through a series of explosives ranging from insensitive TATB to sensitive hydrogen azide and identify key differences in behavior. The simulations are performed using the Multi-Scale Shock Method (MSST) which we have extended to maintain thermodynamic equilibrium between electrons and ions to correctly treat electronic heat capacity. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P13.00011: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P13.00012: \textit{Ab Initio} Simulation of the Equation of State and Kinetics of Shocked Water Nir Goldman, Evan J. Reed, I-F. William Kuo, Laurence E. Fried, Christopher J. Mundy, Alessandro Curioni We report herein first principles simulations of water under shock loading and the chemical reactivity under these hot, compressed conditions. Using a novel simulation technique for shock compression, we observe that water achieves chemical equilibrium in less than 2 ps for all shock conditions studied. The decomposition occurs through the reversible reaction H$_{2}$O $\Delta $ H$^{+}$ + OH$^{-}$. We make comparison to the experimental results for the Hugoniot pressure and density final states. We develop and employ a new quantum correction method to the calculated temperatures which provides validation of both previous experiments and our simulations. Near the approximate intersection of the Hugoniot and the Neptune isentrope, we observe high concentrations of negatively charged species that contribute electronic states near the band gap. *This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P13.00013: Dynamical (in)stabilities of high-pressure H2O ices VII, VIII and X Razvan Caracas We study high-pressure solid H2O ice: the lattice dynamical properties of ice X and the transition path between molecular ices VII/VIII and the ionic ice X with first-principles calculations using density functional theory in the ABINIT implementation. Our work [PRL 101, 085502] defines the dynamical stability of ice X between about 120 GPa up to about 400 GPa. Based on phonon band dispersion we show that the phase transition sequence at low temperature and high pressures in ice is ice VIII - disordered ice X - ice X - ice Pbcm. The disordered ice X is due to a phonon collapse in the whole Brillouin zone at pressures below 120 GPa, phonon that corresponds to hydrogen atoms bouncing back and forth between every two oxygen neighbors in a double well potential. Post-ice X is orthorhombic Pbcm and appears due to a phonon instability in M at pressures higher than 400 GPa that distorts the bcc cubic sublattice of oxygen atoms into a hcp-like structure. Our calculations validate earlier theoretical predictions for a phase transition to a post-ice X structure in H2O [Benoit et al. PRL 76, 2934]. We also identify and discuss the (meta)stability of several intermediate phases between ice VIII and ice X. [Preview Abstract] |
Session P14: Emulsions and Foams
Sponsoring Units: DFDChair: Douglas Durian, University of Pennsylvania
Room: 315
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P14.00001: Measurement of the Force Network in a Dense Emulsion under Shear S. K. Dutta, E. Knowlton, D. L. Blair We have investigated the properties of a dense oil-in-water emulsion under shear stress. Measurements of the jammed emulsion were taken with a customized confocal rheometer, which is capable of acquiring three-dimensional images while simultaneously applying a precise shear. Images acquired deep inside the emulsion are detailed enough to determine the position and shape of individual emulsion droplets. The forces on each droplet were calculated from the deformation due to neighbors, making it possible to link the bulk rheological properties of the emulsion to local structural relaxation and the force distribution measured at the single droplet level. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P14.00002: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P14.00003: Flow-Induced Droplet Deformation and Unjamming in Concentrated Emulsions under Large-Amplitude Oscillatory Shear Jung-Ren Huang, Thomas G. Mason We employ the technique of shear oscillation light scattering to study concentrated oil-in-water emulsions subjected to oscillatory shear that causes droplet deformation and restructuring. Three dimensionless scattering intensity anisotropy factors, defined using the primary and secondary Bragg peak intensities, reflect the degree of droplet deformation caused by the applied shear. These factors distinguish the soft-jamming regime, where shear causes positional disorder, from the sliding hexagonally closed-packed layer regime, where shear induces positional order. Furthermore, near and above the jamming limit of spherical particles, the shear-induced droplet structure depends sensitively on the droplet volume fraction and the shear history. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P14.00004: Structure and Rheology of Stimuli-Responsive Pickering Emulsions Dan Ho, Prasad Sarangapani, Yingxi Elaine Zhu Self-assembly of micro-and nano-spheres and their stability at liquid-liquid interfaces are important due to their broad range of applications from emulsion polymerization to heavy oil transportation. In this work, we employ temperature-responsive poly(N-isopropyl acrylamide) (PNIPAM) microspheres to form Pickering emulsions and directly visualize the dynamics and rheology at the droplet interfaces in response to varied temperature using confocal laser scanning microscopy. Destabilization of the interface is observed as increasing temperature across the lower critical solution temperature (LCST) around 42-44 degree C for this system, where the coarsening at the oil-water interface occurs due to the shrinkage of PNIPAM particle size and results in the onset of coalescence of droplets. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P14.00005: Shear Induced Dynamics of Polydisperse Jammed Emulsion Systems Joaquim Clara Rahola, Eric R. Weeks We study polydisperse decane-in-water emulsions at droplet volume fractions ranging from $\phi$ = 0.65 to $\phi$ = 0.9. At such concentrations emulsions are jammed and thus droplet rearrangements are limited. To induce droplet displacements, an oscillatory strain is applied. We use confocal microscopy to track the trajectories of the droplets in real time and space. Almost all the droplets move periodically, but due to the polydispersity many of them move non-affinely as they are pushed around by other droplets. In these glassy suspensions, the motions of nearby droplets are correlated within a characteristic distance. This length is independent of particle volume fraction while it exhibits an increasing trend with increasing strain amplitude. Moreover, despite the disordered structure of our system, droplets' motions are correlated over ranges longer than the average particle diameter. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P14.00006: Colloidal Hydrodynamics with Arbitrary Boundary Conditions Jonathan K. Whitmer, Erik Luijten Hydrodynamic interactions are essential to the understanding of colloidal dynamics. Due to their complexity and computational cost, they are often ignored in simulations. Over the past decade, coarse-grained methods such as Stochastic Rotation Dynamics\footnote{A. Malevanets and R. Kapral, J. Chem.\ Phys. \textbf{112}, 7260 (2000)} (an example of the larger family of Multi-Particle Collision (MPC) methods\footnote{H. Noguchi and G. Gompper, Phys.\ Rev. E \textbf{78} 016706 (2008)}) have been developed to include these interactions efficiently in simulation. To use these methods for the study of self-assembly dynamics of particles with anisotropic surface chemistry, we extend previously implemented methods for stick boundary conditions\footnote{I. Gotze, H. Noguchi, and G. Gompper Phys.\ Rev. E \textbf{76} 046705 (2007)} to arbitrarily slipping surfaces on the curved surfaces of spherical colloids. We present a mapping from an easily tunable simulation parameter onto the slip length as defined by Navier, and discuss the dynamics of anisotropic particles simulated using this method. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P14.00007: Reversible Rayleight-to-MIe Scattering Transition in a Core-Shell Colloidal System Guangnan Meng, Adeline Perro, Vinothan Manoharan We present a study of light scattering from colloidal particles with small polystyrene cores and large shells of poly(\emph{N'}-isopropylacrylamide-\emph{co}-acrylic acid). When swollen in deionize water at room temperature, the shell is nearly index-matched to pure water, and the scattering is dominated by Rayleigh scattering from the polystyrene cores. As we change the solvent condition by increasing temperature or salt concentration, the shell starts to shrink and scatter light. Both the scattering cross section and the forward scattering of the particles increase, characteristic of Mie scatterers. We use optical microscopy, static light scattering and turbidimetry to study this optical transition. Such core-shell particles might be used as aqueous index-matched tracer colloids, as model scatterers for self-assembly studies, or as optical filters with tunable opacity. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P14.00008: Pressure driven foam flow rheology C.D. Jones, K. Nordstrom, D.J. Durian We probe the complex rheology of 3d foams by flowing them through a narrow column. The foam flows upward through one of two vertical rectangular columns with a 4:1 cross-sectional aspect ratio, by bubbling gas through a soapy solution at the base of our apparatus. One column is clear acrylic sheet on all sides, which is slippery to the foam, and results in plug flow. The other column has the narrow surfaces covered with sandpaper, giving them a sticky surface, which creates shear due to the zero velocity boundary condition. As expected, the flow profile between the slippery broad faces is flat, however the profile between the narrow, sticky faces exhibits a curved velocity profile that is strongly dependent on flow rate. We are able to analyze a 2d velocity profile from a 3d bulk system, whereas other recent foam rheology work has been constrained to the 2d system. We employ particle image velocimetry to measure the strain rate, and compute the stress from the pressure drop along the channel, to investigate the local stress-strain relationships in a flowing foam. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P14.00009: Effective temperature of a sheared foam Daniel Valdez-Balderas, Peter Olsson, Stephen Teitel We perform computer simulations of a model for an overdamped, sheared foam in two dimensions at zero temperature. We measure an effective temperature with the use of an embedded oscillator, in manner analogous to experiments done by Abate and Durian on a different system [arXiv:0806.0765v2]. Our oscillator is one of the bubbles in the foam, which, in addition to its interaction with other bubbles, is also subject to a harmonic potential. We define an effective temperature based on the fluctuations in the position of the oscillator. We compare our results to the effective temperatures computed with the use of measurements of the fluctuations of the shear stress and fluctuations of the energy, respectively. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P14.00010: Realization spaces of bubble clusters and coarsening trajectories Bryan Chen, Randall Kamien In the search for a more unified description of the geometry of equilibrium foams, we study the space of all realizations of equilibrium bubble clusters of fixed topology. The geometry of foam is highly constrained due to the area minimization property - in two dimensions, this means that all interfaces must be portions of circles and interfaces intersect in threes at angles of $120^{\circ}$. This results in a finite dimensional space of bubble clusters, and the dynamics of coarsening via gas diffusion induces a vector flow on it. The boundaries and singularities of the realization space may be identified with topological transitions and instabilities in coarsening. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P14.00011: Experimental studies of low-density fluid phases in tunable dipolar colloids Anand Yethiraj, Ning Li, Hugh Newman, Manuel Valera, Ivan Saika-Voivod Experiments of low-density colloidal fluid phases in the presence of an external electric field are presented. We obtain angular order parameters as a function of the applied electric field. When plotted against a dimensionless dipolar strength parameter, the order parameters for different particle sizes fall on a single curve, suggesting that colloids in a fluid phase in the presence of electric fields do indeed interact by an effective point dipolar interaction. We then explore the statistics of particle packings at low-density and extract the experimental compressibilities and equation of state for these dipolar colloids. [Preview Abstract] |
Session P15: Biologically Inspired Physics: Swimming, Propulsion, Bio-fluids
Sponsoring Units: DFDChair: Silas Alben, Georgia Institute of Technology
Room: 316
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P15.00001: Swimming in a vortex street Silas Alben Recent studies showed that a trout swimming in a cylinder wake can save energy by ``slaloming'' through a vortex street. We present a simple model using a flexible body with vortex sheets, and find swimming shapes which maximize output power and efficiency. We find analytic solutions and compare the optimal swimming phase between the body and vortices with previous experiments and numerics. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P15.00002: Symmetry and Hydrodynamic Interactions of Linked-Sphere Swimmers Gareth Alexander, Julia Yeomans The motile behavior of micron-sized organisms offers an insight into a physical environment very different to our own. Micron length scales correspond to low Reynolds number conditions where viscous forces dominate over the effects of inertia [1]. A topic of growing interest is the role played by hydrodynamic interactions, both with confining walls and between organisms as a means to generate collective motility. We shall describe the form and properties of swimmer-swimmer interactions for simple models consisting of a small number of linked-spheres [2,3]. These interactions do not follow the naively expected dipolar form and moreover exhibit a strong sensitivity to the relative phase of the swimmers. Several of these features have a natural interpretation in terms of the kinematic reversibility of Stokes flows and we shall describe in particular an exact result for the scattering of two swimmers related by time reversal. [1] G. I. Taylor, Proc. R. Soc. A 209, 447 (1951); 211, 225 (1952). [2] A. Najafi and R. Golestanian, Phys. Rev. E 69, 062901 (2004). [3] C. M. Pooley, G. P. Alexander, and J. M. Yeomans, Phys. Rev. Lett. 99, 228103 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P15.00003: Collective locomotion of non-swimmers Eric Lauga, Denis Bartolo To achieve propulsion at low Reynolds number, a swimmer (e.g. a biological cell such as a bacterium, or a spermatozoon) must deform its shape in time in a way that is not invariant under time-reversal symmetry (non-reciprocal); this is Purcell's scallop theorem. We show here explicitly that there is no many-scallop theorem. Two active bodies undergoing reciprocal deformations - and therefore incapable of swimming when considered separately - can exploit hydrodynamic interaction to swim. If the bodies are polar, we also show that they experience effective long-range interactions. We derive our results analytically for a minimal dimers model, and generalize them to more complex geometries on the basis of symmetry and scaling arguments. Furthermore, we explain how such cooperative locomotion can be realized experimentally by shaking a collection of soft particles with a homogeneous external field, thereby making non-swimmers swim. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P15.00004: Self-Assembled Magnetic Surface Swimmers: Theoretical Model Igor Aranson , Maxim Belkin, Alexey Snezhko The mechanisms of self-propulsion of living microorganisms are a fascinating phenomenon attracting enormous attention in the physics community. A new type of self-assembled micro-swimmers, {\it magnetic snakes}, is an excellent tool to model locomotion in a simple table-top experiment. The snakes self-assemble from a dispersion of magnetic microparticles suspended on the liquid-air interface and subjected to an alternating magnetic field. Formation and dynamics of these swimmers are captured in the framework of theoretical model coupling paradigm equation for the amplitude of surface waves, conservation law for the density of particles, and the Navier-Stokes equation for hydrodynamic flows. The results of continuum modeling are supported by hybrid molecular dynamics simulations of magnetic particles floating on the surface of fluid. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P15.00005: Accumulation of microswimmers near surface due to steric confinement and rotational Brownian motion Guanglai Li, Jay Tang Microscopic swimmers display some intriguing features dictated by Brownian motion, low Reynolds number fluid mechanics, and boundary confinement. We re-examine the reported accumulation of swimming bacteria or bull spermatozoa near the boundaries of a fluid chamber, and propose a kinematic model to explain how collision with surface, confinement and rotational Brownian motion give rise to the accumulation of micro-swimmers near a surface. In this model, an elongated microswimmer invariably travels parallel to the surface after hitting it from any incident angle. It then takes off and swims away from the surface after some time due to rotational Brownian motion. Based on this analysis, we obtain through computer simulation steady state density distributions that reproduce the ones measured for the small bacteria E coli and Caulobacter crescentus, as well as for the much larger bull spermatozoa swimming near surfaces. These results suggest strongly that Brownian dynamics and surface confinement are the dominant factors for the accumulation of microswimmers near a surface. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P15.00006: Modeling the Behavior of Self-Propelled Microcapsules Amitabh Bhattacharya, O. Berk Usta, Anna C. Balazs Biological cells can perform complex tasks by signaling and moving autonomously in their environment. We study a system of self-propelled microcapsules, first proposed by Usta et al (2008), that mimics this process. It consists of a signaling and target microcapsule placed close to an adhesive substrate and immersed in fluid. The signaling microcapsule encases nanoparticles, which, when released, modifies the adhesive strength of the substrate. The adhesion gradients in the substrate, along with hydrodynamic interactions among the capsules, gives rise to a sustained motion of the microcapsules. In this work, we perform simulations (based on lattice Boltzmann method for the fluid and random walk simulation for nanoparticles) of several signal-target configurations, consisting of two or more rigid capsules. In particular, we examine a configuration consisting of a single signaling capsule pushing multiple target capsules in a single file. For a constant release rate of nanoparticles, the velocity of the train of capsules asymptotes to a constant value at large times. Using a low-order analytical model for this system, we show that there is a simple relationship between this asymptotic velocity and the parameters in the system (e.g. number of capsules, release rate of nanoparticles, viscosity of fluid, adhesive strength of substrate etc.). [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P15.00007: Mixing fluid by self-propelled objects Maxim Belkin, Alexey Snezhko, Igor Aranson, Wai-Kwong Kwok Magnetic microparticles suspended at the water-air interface and subjected to an ac external driving self-assemble into dynamic structures (magnetic snakes). The snakes are accompanied by four large hydrodynamic vortices. At high enough frequencies and amplitudes of driving the snakes transform into self-propelled swimmers. Moving erratically, these swimmers mix the surface of fluid at a very high rate. We performed detailed experimental studies of these self-organized mixing. We studied space and time correlation and diffusion process in such systems. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P15.00008: Enskog-theory for stochastic models with self-propelled and passive particles Alemayehu Gebremariam, Thomas Ihle Macroscopic evolution equations for interacting many-body systems do not just ``emerge''; they follow from microscopic laws. However, it is often difficult to quantitatively establish this link, especially for systems which cannot be described by a Hamiltonian and which do not have pairwise additive interactions. Therefore, the general form of the macroscopic equations is usually obtained by symmetry arguments. Here, using a particle-based model with discrete time evolution steps for fluid flow I show how the macroscopic transport equations can be rigorously derived from microscopic collision rules. The approach starts with the full N-particle Liouville equation and leads to a multi-particle Enskog-equation which is treated by a Chapman-Enskog expansion. No linearization or single-relaxation time approximation of the collision operator are needed. The obtained thermo-hydrodynamic equations show excellent agreement with previous numerical results. The same approach is used to study a simple model of self-propelled, swarming birds. This model was proposed by T. Vicsek et al. [Phys. Rev. Lett. {\bf 75} (1995) 1226]; it has ``multi-particle collisions'' where birds within some interaction range align their flying directions. I analytically analyze the collision-operator for small and large bird density, and derive the hydrodynamic equations for the density and velocity fields. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P15.00009: The ``caterpillar'' simulation model for a biological filament Aimee Bailey, Christopher Lowe, Adrian Sutton We present a simulation model for an elastic filament in a viscous fluid, relevant for systems ranging from suspensions of paper pulp to micro-organism motility. It incorporates the Stokeslet treatment of the hydrodynamic force. We show that a non-arbitrary choice of the hydrodynamic radius is necessary to recover known dynamic behavior of a fiber with a finite cross-section. Our simulations explore configurations inaccessible by theory. We illustrate the utility of the model by considering the simple scenario of a charged filament in an electric field. Results suggest a circularly polarized electric field is a viable means for aligning microtubules in solution. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P15.00010: Flow and nutrient transport through porous scaffolds used for the culture of bone cells in perfusion bioreactors Dimitrios Papavassiliou, Roman Voronov, Vassilios Sikavitsas, Samuel VanGordon The goal is to understand via computation the behavior of the flow inside porous scaffolds that are used in bone tissue bioreactors. Fluid shear is an important stimulatory factor in preosteoblastic cells seeded in scaffolds and cultured under continuous flow perfusion. A Lattice Boltzmann method has been employed to simulate the flow field within porous scaffolds obtained with high resolution micro-CT. Lagrangian methods have also been used to determine the nutrient dispersion inside the scaffolds. The shear stresses calculated inside the scaffold architecture indicate that the shear stresses experienced by cells inside the scaffold can vary by orders of magnitude. This is important when designing scaffolds for bone tissue growth, since osteoblastic cells require to be stimulated by shear for growth. Moreover, cell detachment can occur when the fluid shear is too high, thus, placing a limit on the stresses that a particular scaffold design should allows. The talk will address the methodology, the validation and the correlation of scaffold structure characteristics with the shear stresses and with the rate of mass transfer. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P15.00011: Instabilities and waves in thin films of living fluids Sumithra Sankararaman, Sriram Ramaswamy We formulate the thin-film hydrodynamics of a suspension of polar self-driven particles and show that it is prone to several instabilities through the interplay of activity, polarity and the existence of a free surface. Our approach extends, to self-propelling systems, the work of Ben Amar and Cummings [Phys Fluids {\bf 13} (2001) 1160] on thin-film nematics. Based on our estimates the instabilities should be seen in bacterial suspensions and the lamellipodium, and are potentially relevant to the morphology of biofilms. We suggest several experimental tests of our theory. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P15.00012: Tuning inter-virus interactions in natural aquatic environments Nathan W. Schmidt, Andrew K. Udit, Leonardo Gutierrez, Thanh H. (Helen) Nguyen, M.G. Finn, Gerard C.L. Wong Polymeric natural organic matter (NOM) originating from plants and animals is ubiquitous in natural aquatic environments. Many water-borne pathogens, including viruses, readily associate with NOM, which has a statistical distribution of charged and hydrophobic groups. Virus-NOM association influences the transport of viruses in groundwater environments, but little is known about this interaction, or how NOM can induce new inter-virus interactions. To better understand the interaction between NOM and aqueous contaminants, we use the MS2 and Qbeta viruses (diameters $\sim $ 27nm) as surrogate water-borne pathogens. Small Angle X-ray Scattering is used to characterize the inter-particle interaction between viruses over a range of NOM concentrations and different salt types and concentrations. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P15.00013: Computational studies on characteristic fluid behavior in the stented cerebral aneurysm Miki Hirabayashi, Makoto Ohta, Daniel A. R\"ufenacht, Bastien Chopard We present a computational analysis of the fluid behavior in the stented aneurysm. It is important to reveal the complex mechanism of the velocity reduction of the flow in the stented aneurysm in order to design the effective stent, which is a tubular mesh of wires placed for the treatment of the cerebral aneurysm. To understand the effect of a stent we already proposed a qualitative analysis of the flow pattern in the stented aneurysm. Here we present a quantitative analysis of the transition of the pressure and the shear stress caused by the changes of the flow pattern to verify the velocity reduction mechanism of the stent. We expect that our study will lead to a new suggestion for the effective treatment of the cerebral aneurysm by the stent. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P15.00014: Run length is the dimension that characterizes path integrals useful for designing passive bacterial pumps David Liao, Guillaume Lambert, Peter Galajda, Robert Austin Asymmetric funnels have been used as passive pumps to concentrate \textit{E. coli} in nanofabricated devices (Austin 2007). Funnel geometry changes pump efficiency, which could be important when driving cell sorters (Whitesides 2008). The large set of funnel geometries that could be considered when designing pumps motivated us to derive a path-integral-like formula to predict the flux produced by arbitrary funnel geometries. We applied this equation to a two-dimensional wedge-shaped funnel. Model and experiment agree that the steady-state ratio between concentrations on two sides of a funnel open to $60^{\circ}$ is 3 when the aperture is one fifth the bacterial run length and 1 when the aperture is 16 times the run length, an example of how the run length here has a role loosely analogous to the wavelength in quantum mechanical path integrals. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P15.00015: Selective transport through nano-channels: do we understand it? Anton Zilman, T. Jovanovic-Talisman, B. Chait, M. Rout, S. Di Talia, M. Magnasco Functioning of living cells requires selective molecular transport, which is provided by transport channels that are able to selectively transport certain molecular species while filtering others, even similar ones. Such channels can selectively transport their specific molecules in the presence of vast amounts of non-specific competition. In many cases, efficient and selective transport occurs without direct input of metabolic energy and without transitions from an `open' to a `closed' state during the transport event. Examples include selective permeability of porins and transport through the nuclear pore complex. Mechanisms of selectivity of such channels have inspired design of artificial selective nano-channels, which mimic the function of selective biological channels. Mechanisms of selectivity of such nano-channels are still unknown. I present a theoretical model to explain the selectivity of transport through nano-channels, which contains only the essentials of stochastic kinetics inside the channel. The theory provides a mechanism for selectivity based on the differences in the kinetics of transport through the channel between different molecules. The theory explains how the specific molecules are able to filter out the non-specific competitors - and proposes a mechanism for sharp molecular discrimination. The theoretical predictions account for previous experimental results and have been verified in ongoing experiments [Preview Abstract] |
Session P16: Solid Helium: Theory
Sponsoring Units: DAMOPChair: Henry Glyde, University of Delaware
Room: 317
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P16.00001: Simulating the Melting Transition of Helium in Two Dimensions Keola Wierschem, Efstratios Manousakis We study the melting behavior of helium in two dimensions with the path integral Monte Carlo method. Two dimensional melting theory predicts two melting transitions: solid to hexatic and hexatic to isotropic liquid, described by a loss of translational and orientational order, respectively. We calculate the translational and orientational order parameters, and use finite size scaling to determine the two melting transitions in the thermodynamic limit. We also study the superfluid/normal phase boundary of 2D helium relative to the above mentioned two stage melting boundaries. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P16.00002: Superfluid networks with mesoscopic structure as models of supersolid 4He Burcu Yucesoy, Jon Machta, Nikolai Prokof'ev, Boris Svistunov One proposal for understanding supersolidity is that grain boundaries and/or defect lines in solid 4He may support superfluidity. To understand the consequences of this proposal, we carry out simulations of the XY model with mesoscale structure corresponding to grain boundaries and/or defect lines. In the absence of disorder, we find a sharp phase transition unlike the gradual transition seen in experiments on supersolids. However, with disorder we find results that are qualitatively similar to the experiments. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P16.00003: Absence of Dislocation Quantum Roughening in Solid $^4$He Darya Aleinikava, Eugene Dzedzits, Anatoly Kuklov, David Schmeltzer Dislocations in quantum crystals are shown to be smooth at zero temperature because of the effective Coulomb-type interaction between kinks induced by exchange of bulk phonons. We provide heuristic Kosterlitz-Thouless and Renormalization Group arguments against quantum roughening and confirm them by Monte Carlo simulations of the effective model of edge dislocation moving in its gliding plane –- a quantum string (or classical membrane in $d=2$) subjected to periodic Peierls potential and Coulomb-type interaction. Simulations of such Sine-Gordon type action have been conducted in the Villain approximation in terms of the J-current formulation. Renormalized stiffness as a function of the long-range interaction strength $C$ and dislocation length $L$ is shown to be described by a master curve $F(C \ln L)$, where $F(x) \to 0$, as $x \to \infty$. We also discuss a mechanism of suppression of superfluidity along the dislocation core by thermal kinks and show that it leads to locking in of the mechanical and superfluid responses at finite temperature, which is consistent with the recent experiment of Day and Beamish (Nature {\bf 450}, 853 (2007)). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P16.00004: Classical roughening of dislocations and the effect of shear modulus softening in solid $^4$He. Eugene Dzedzits, Darya Aleinikava, Anatoly Kuklov, David Schmeltzer We propose that shear modulus $\mu(T)$ softening with increasing temperature $T$ observed by Day and Beamish [1] is due to a crossover experienced by dislocations from quantum smooth to classically rough state in the Peierls potential. Quantum dislocation is described by the Sine-Gordon model in dimensions $d=1+1$ with long-range interactions between kinks (induced by exchanging bulk phonons). Monte Carlo simulations of this model show that finite $T$ response on external stress can fit well the data $\mu(T)$ [1] for the parameters typical for $^4$He. We compare this model with the one proposed in Ref. [1]: the $^3$He impurities boiling off from the dislocations. Good fit of $\mu(T)$ cannot be achieved within this model for realistic values of the dislocation densities and relative fractions of $^3$He atoms. [1] J. Day and J. Beamish, Nature {\bf 450}, 853(2007) [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P16.00005: Quantum Glass in Solid He? Alexander Balatsky, Matthias Graf Recent discovery of a possible supersolid state by Kim and Chan has stimulated an active debate about true nature of a low temperature state of solid $^{ 4}$He. We will discuss possible glassy component that could be present in solid $^{ 4}$He. We will focus on i) the role of tunneling systems (TS) as a component that freezes out at lowest temperatures and ii) interactions between TS. We will address possible quantum effects and the role of TS statistics in solid $^{4}$He vs solid $^{ 3}$He-$^{ 4}$He mixtures. Implications for the torsional oscillator and for thermodynamics will be discussed as well. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P16.00006: The glassy response of torsion oscillators of solid $^{4}$He Matthias J. Graf, Zohar Nussinov, Alexander V. Balatsky We have calculated the glassy response of a torsional oscillator filled with solid $^{4}$He assuming a phenomenological glass model. Making only a few assumptions about the distribution of glassy relaxation times in a small subsystem of otherwise rigid solid $^{4}$He, we can account for the bulk of the magnitude of the observed period shift and dissipation peak as reported in several torsion oscillator experiments. The glass model places stringent constraints on dynamic and thermodynamic responses of solid $^{4}$He and the magnitude of a possible supersolid phase. We also discuss the implications for a superglass state proposed recently by the Cornell group. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P16.00007: A `Superglass' State in Solid $^{4}$He Benjamin Hunt, Ethan Pratt, Vikram Gadagkar, Minoru Yamashita, Alexander V. Balatsky, J. C. Davis We study the relaxation dynamics of both the resonance frequency $f(T)$ and the
dissipation rate $D(T)=Q^{-1}(T)$ of a torsional oscillator (TO) containing
solid $^{4}$He. Abruptly at the temperature $T*$ characteristic of the proposed
supersolid phase, the relaxation times within $f(T)$ and $D(T)$ begin to increase
precipitously together. Moreover, for all $T |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P16.00008: Viscoelastic Behavior of Solid $^4$He Chi-Deuk Yoo, Alan T. Dorsey We model the torsional oscillator experiments by using the Kelvin-Voigt model of viscoelasticity for solid $^4$He~[1]. With this model we find that a relaxation time which grows rapidly as the temperature is lowered can produce both a peak in the inverse of $Q$-factor and a decrease in the resonant period of the torsional oscillator. We also identify two different regimes of the relaxation in temperature: the activation energy is found to be about 260 mK at high temperatures and 18.6 mK at low temperatures. By using the derived relaxation time we fit to the torsional oscillator result obtained by Clark {\it et al}.~[2]. We find that the viscoelastic solid model provides a good agreement with the observed dissipation; however, it only accounts for a part of the measured resonant period shift, suggesting a possibility of the onset of superfluidity in solid $^4$He. \newline \newline \noindent[1] C.-D. Yoo and A. T. Dorsey, arXiv:0810.2525. \newline \noindent[2] A. C. Clark, J. T. West, and M. H. W. Chan, Phys. Rev. Lett. {\bf 99}, 135302 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P16.00009: Binding energy of $^3$He to dislocations in solid $^4$He Debajit Goswami, Kinjal Dasbiswas, Chi-Deuk Yoo, Alan T. Dorsey Recent heat capacity experiments on solid $^4$He [1] show a peak in the specific heat which is interpreted as the signature of the supersolid transition. We pursue an alternative explanation for the heat capacity feature in which $^3$He impurities desorb from dislocations in solid $^4$He; the peak temperature scales with the binding energy of $^3$He to dislocations in $^4$He. Within a continuum elastic model for solid $^4$He, we make quantum mechanical estimates for the binding energy, using a combination of variational and numerical methods. We find for a short distance cut-off of one lattice constant of $^4$He, the binding energy is about 70 mK for edge and 60 mK for a screw dislocation.\newline\newline \noindent [1] X. Lin, A. C. Clark, and M. H. W. Chan, Nature {\bf 449}, 1025 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P16.00010: Specific heat due to the binding of $^3$He impurities to dislocations in solid $^4$He Kinjal Dasbiswas, Debajit Goswami, Chi-Deuk Yoo, Alan T. Dorsey A statistical lattice model is used to study the binding of $^3$He impurities to dislocations in solid $^4$He. By considering a chemical equilibrium between the $^3$He atoms in the bulk and those adsorbed onto the dislocations, we are able to calculate the equilibrium thermodynamic properties of the system. The specific heat, as expected, exhibits a Schottky bump whose attributes depend on parameters like the binding energy and the concentrations of $^3$He atoms as well as defect sites. The calculated specific heat for typical values of these parameters shows a close match with experiment \footnote{X. Lin, A. C. Clark, and M. H. W. Chan, Nature {\bf 449}, 1025 (2007).}, the peak magnitude being of the order of 10 $\mu$J mol$^{-1}$ K$^{-1}$ and peak being located at around 50 mK. We show that the essential features of our model are independent of the exact lattice structure and derive an expression to estimate the shift in peak position from the binding energy value, which is an effect of the chemical potential. [Preview Abstract] |
Session P17: Semiconducting Qubits I
Sponsoring Units: GQIChair: Christie Simmons, University of Wisconsin
Room: 318
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P17.00001: Practical elimination of leakage in superconducting qubits by pulse shaping Felix Motzoi, Jay Gambetta, Patrick Rebentrost, Frank Wilhelm In trying to develop high-fidelity control of superconducting and optical lattice devices, many techniques have been borrowed from the NMR literature, such as shaped, computer-generated, and composite pulses. However, unwanted coupling to higher energy levels cause simple spin state control to fail. We have shown that we can effectively remove any coupling to the third level in specifically phase and transmon qubits by adding a second control proportional to the derivative of the first along a rotation axis perpendicular to that of the first control. The 2-control strategy gives implementations with basic pulse-shaping as small as 4 pixels. Using realistic values of decoherence(5 $\mu$s), we find errors as small as $10^{-4}$ for such a 4ns pulse, about an order of magnitude better than using one quadrature. An easy to implement analytic formula can also be applied that handily improves on any existing single-control analog or pixelated pulse. These results demonstrate that experimental calibration and decoherence effects are the limiting factors in achieving high-fidelity quantum gates, and the focus of attention should be on these rather than on increasing the anharmonicity. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P17.00002: Quantum Sensing in the Presence of Realistic Attenuation Yaakov Weinstein, Gerald Gilbert Quantum entangled states can be used to beat the standard quantum limit on the variance of a measurement and to beat the Rayleigh limit on resolution. These phenomena are known as supersensitivity and superresolution, respectively. We demonstrate that photonic implementation of either supersensitivity or superresolution will not be successful in the presence of realistic attenuating atmospheres. This is true even when superresolution is attempted with unentangled photons. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P17.00003: Free-Time and Fixed End-Point Optimal Control Theory in Quantum Mechanics: Application to Entanglement Generation Kenji Mishima, Koichi Yamashita We have constructed \textit{free-time} and fixed end-point optimal control theory for quantum systems and applied it to entanglement generation between rotational modes of two polar molecules coupled by dipole-dipole interaction. The motivation of the present work is to solve optimal control problems more flexibly by extending the popular \textit{fixed-time} and fixed end-point optimal control theory for quantum systems to \textit{free-time} and fixed end-point optimal control theory. Our theory can not only achieve high transition probabilities but also determine exact temporal duration of the laser pulses. As a demonstration, our theory is applied to entanglement generation in rotational modes of NaCl-NaBr polar molecular systems that are sensitive to the strength of entangling interactions. Using the tailored laser pulses, we discuss the fidelity of entanglement distillation and quantum teleportation. Our method will significantly be useful for the quantum control of non-local interaction such as entangling interaction, and other time-sensitive general quantum dynamics, chemical reactions. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P17.00004: Quantum multiobservable control Raj Chakrabarti, Rebing Wu, Herschel Rabitz We present deterministic algorithms for the simultaneous control of an arbitrary number of quantum observables. Unlike optimal control approaches based on cost function optimization, quantum multiobservable tracking control (MOTC) is capable of tracking predetermined homotopic trajectories to target expectation values in the space of multiobservables. The convergence of these algorithms is facilitated by the favorable critical topology of quantum control landscapes. Fundamental properties of quantum multiobservable control landscapes that underlie the efficiency of MOTC, including the multiobservable controllability Gramian, are introduced. The effects of multiple control objectives on the structure and complexity of optimal fields are examined. With minor modifications, the techniques described herein can be applied to general quantum multiobjective control problems. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P17.00005: Quantum speed limit and optimal control Tommaso Caneva, Michael Murphy, Tommaso Calarco, Rosario Fazio, Simone Montangero, Vittorio Giovannetti, Giuseppe E. Santoro The Heisenberg uncertainty principle, $\Delta E\Delta t\geq \hbar$, implies that a system cannot pass through distinguishable, i.e. orthogonal, states within arbitrarily short time. In the case of a time-independent Hamiltonian, the presence of this ultimate bound has been well established and summarized in the concept of a maximum allowed velocity, called \emph{quantum speed limit} (QSL). On other hand for a time-dependent Hamiltonian the problem started to be addressed only very recently and is still open. Optimal control theory offers a valuable tool to explore this issue: we test its performance in two paradigmatic cases, Landau-Zener model and transfer of information along a chain of coupled spins, and show that the results are compatible with the ultimate limits enabled by quantum mechanics. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P17.00006: Performance Gains for Superconducting Qubits by Means of Optimal Control Theory Robert Roloff, Walter Poetz Superconducting circuits are promising candidates for the successful implementation of qubit--arrays and qubit--gates within solid--state systems. However, despite recent progress within coherent control of charge, phase and flux qubits, considerable improvement in gate fidelities is needed to build large--scale quantum information processing devices. We present an optimal control scheme based on process tomography, capable of taking into account relaxation, dephasing and unwanted state--leakage within the qubit (array). We apply this theory to explore the performance limits of Josephson charge qubits within current experimental means. Environmental effects, as well as state--leakage, are modeled microscopically, using a full quantum mechanical description and taking into account 1/f and Ohmic fluctuations based on experimental noise spectra. Within time--optimal control theory, we show that under typical conditions gate fidelities of $F=1-10^{-3}$ should be possible for a Josephson charge qubit. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P17.00007: Towards non-adiabatic control of a superconducting qubit Jonas Bylander$^1$, Mark S. Rudner$^1$, Andrey V. Shytov$^2$, Sergio O. Valenzuela$^1$, David M. Berns$^1$, Karl K. Berggren$^1$, Leonid S. Levitov$^1$, William D. Oliver$^1$ Transitions in a qubit driven through an energy-level avoided crossing can be controlled by carefully engineering the driving protocol. With the driving rate chosen to optimize the coupling strength, an arbitrary rotation of a qubit's quantum state on the Bloch sphere can be performed. This regime, if realized experimentally, may lead to fast quantum-logic gates with times of operation much shorter than those achieved by using Rabi transition-based protocols. We have performed an experiment with a superconducting persistent-current qubit in the non- adiabatic regime, driven by a large-amplitude radio-frequency field. By applying a waveform consisting of two harmonic components generated by a digital source, we demonstrate a mapping between the amplitude and phase of the harmonics produced at the source and those received by the device. This mapping allows us to image the actual waveform at the device and accurately produce the desired time dependence. Our method constitutes a step towards non-adiabatic control with arbitrary waveforms. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P17.00008: Universal quantum control of two electron spin qubits via dynamic nuclear polarization Hendrik Bluhm, Sandra Foletti, Diana Mahalu, Vladimir Umansky, Amir Yacoby Encoding a single logical qubit in the collective spin states of two electrons in a double quantum dot can provide sub-nanosecond electrically controlled gates that are fast enough to refocus dephasing due to slow fluctuations of the hyperfine field from the nuclei of the host material [1]. In this work, we experimentally demonstrate full quantum control of a GaAs two electron logical spin qubit. One fast electrical control axis resulting from coherently exchanging the two electrons has already been demonstrated [2]. We achieve coherent evolution around a second axis caused by a difference in the nuclear hyperfine fields felt by the two electrons. This field difference is obtained by dynamically polarizing the Ga and As nuclei by transferring spin from the electrons to the nuclei. It can reach up to several hundred mT and can be maintained in a steady state. We demonstrate rotations around this axis with a programmable frequency that can exceed 1 GHz. Using quantum state tomography enabled by both control axes, we characterize the evolution of the qubit state around a fixed but tunable combined axis. Our results establish full electrical quantum control at the single qubit level with gate times of a few nanoseconds. [1] Taylor et al., Nature Physics \bf{1}, 177 (2005). [2] Petta et al. Science \bf{309}, 2180 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P17.00009: Control of exchange coupling in Si double quantum dots Dimitrie Culcer, L. Cywinski, Qiuzi Li, Sankar Das Sarma We determine the exchange coupling in a Si double quantum dot in the Heitler-London approximation. Qubit manipulation in bulk Si is hindered by the sixfold valley degeneracy of conduction band electrons which causes the exchange interaction between qubits to oscillate as a function of their separation. We demonstrate that in quantum dots these oscillations are suppressed by quantum confinement. We determine the dependence of the exchange coupling on the barrier potential between the dots and examine the role of charge fluctuations. Our results suggest that together with long Si spin lifetimes Si quantum dots could lead to improved control of spin qubits. Within the Heitler-London approximation the work presented is completely general and the results are valid for any ground state. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P17.00010: Optimal experiment design for parameter estimation as applied to dipole- and exchange-coupled qubits Kevin Young, Mohan Sarovar, Birgitta Whaley, Robert Kosut We consider the problem of quantum parameter estimation with the constraint that all measurements and initial states are separable. Two qubits are presumed coupled through the dipole and exchange interactions. The resulting Hamiltonian generates a unitary evolution which, when combined with arbitrary single-qubit operations,contributes to a universal set of quantum gates. However, while the functional form of the Hamiltonian is known, a particular experimental realization depends on several free parameters - in this case, the position vector relating the two qubits and the magnitude of the exchange interaction. We use the Cramer-Rao bound on the variance of a point estimator to construct the optimal series of experiments to estimate these free parameters. Our method of transforming the constrained optimal estimation problem into a convex optimization is powerful and widely applicable to other systems. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P17.00011: Measurement of the nonadiabatically-induced coherent time evolution of a single-electron wavefunction in a surface acoustic wave dynamic quantum dot Adam Thorn, Masaya Kataoka, Michael Astley, Daniel Oi, Crispin Barnes, Chris Ford, Dave Anderson, Geb Jones, Ian Farrer, Dave Ritchie, Michael Pepper Observation of coherent single-electron dynamics is severely limited by experimental bandwidth. We present a method to overcome this using moving quantum dots defined by surface acoustic waves. Each dot holds a single electron, and travels through a static potential landscape. When the dot moves abruptly between regions of different confinement, the electron is excited into a superposition of states, and oscillates unitarily from side to side. These oscillations are measured almost non-invasively, by allowing a small amount of tunnelling out of the dot each time the wavefunction approaches a tunnel barrier. We have modelled this in detail by solving the single-particle time-dependent Schr\"odinger equation for a realistic potential, and find good agreement between the measurements and the simulations. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P17.00012: Weak measurement of a solid-state qubit revealed in low-frequency noise Alexander Korotkov Weak quantum measurement becomes a subject of experimental study with solid-state qubits. Partial collapse, quantum uncollapsing, and persistent Rabi oscillations have been already demonstrated with superconducting qubits by the UCSB and Saclay groups. Now we propose an experiment, in which the features of a weak quantum measurement are revealed in the low- frequency noise of the detector signal. (Here we mean a frequency much lower than the Rabi frequency, though sufficiently high to avoid 1/f noise.) The idea is to use two detectors measuring the same qubit, so that one detector collapses the qubit, while the other detector senses the result of the collapse. Then the cross-correlation of low-frequency noises in outputs of the two detectors carries information about the collapse process. The experiment can be realized with superconducting or semiconductor qubits. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P17.00013: Error Accounting in Electron Counting Experiments Michael Wulf, Alexander B. Zorin Electron counting experiments attempt to provide a current of a known number of electrons per unit time. We propose architectures utilizing a few readily available electron-pumps or turnstiles with error rates of 1 part in $10^4$ with common sensitive electrometers to achieve the desireable accuracy of 1 part in $10^8$. This is achieved not by counting electrons but by counting the errors of individual devices; these are less frequent and therefore readily recognized and then accounted for. We thereby ease the route towards quantum based standards of current and capacitance. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P17.00014: High-fidelity universal quantum gates through quantum interference Frank Gaitan, Ran Li Numerical simulation results are presented which suggest that a class of non-adiabatic rapid passage sweeps first realized experimentally in 1991, and which give rise to controllable quantum interference effects observed in 2003 using NMR, should be capable of implementing a universal set of quantum gates $\mathcal{G}$ that operate with high-fidelity. $\mathcal{G}$ consists of the Hadamard and NOT gates, together with variants of the phase, $\pi /8$, and controlled-phase gates. Sweep parameter values are provided which simulations indicate will produce the different gates in $\mathcal{G}$, and for each gate, yield an error probability $P_{e} < 10^{-4}$. These simulations suggest that the universal gate set produced by these rapid passage sweeps show promise as possible elements of a fault-tolerant scheme for quantum computing. We discuss current challenges facing experimental implementation of this approach to universal quantum computing. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P17.00015: Dynamics of Quantum Control for Bosons in Optical Lattices Analabha Roy, Linda Reichl We investigate the possibility of quantum control in an ultracold atom Bose gas in an optical lattice by looking at numerical simulations of the dynamics of controlled excitations in these systems. These excitations are mediated by pulsed signals that cause Stimulated Raman Adiabatic passage (STIRAP) from the ground state to excited states. The transition to chaos affects the quantum dynamics of such systems as has been demonstrated for single-particle and mesoscopic-systems in optical potentials. We determine the influence of Bose statistics on this dynamics, as well as the effects of controlling quantum phase transitions in this manner for interacting cold atom systems. [Preview Abstract] |
Session P18: Bulk Block Copolymers II
Sponsoring Units: DPOLYChair: Bradley Olsen, California Institute of Technology
Room: 319
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P18.00001: Directed Self-Assembly of Cadmium Selenide Nanocrystals in Conjugated Rod-Coil Block Copolymers B. L. McCulloch, J. Urban, R. A. Segalman Semiconducting polymer/nanocrystal composites are attractive for many applications; however their performance relies crucially on nanoscale morphology. We demonstrate that a conjugated rod-coil diblock copolymer can be used both to absorb light and template the location of CdSe nanocrystals. A combination of the liquid crystallinity of the conjugated rod block and the interactions of the nanocrystal ligand coat with the block copolymer control self-assembly. For example, incorporation of the nanocrystal in the rod nanodomain disrupts liquid crystallinity. In the case of a poly(alkoxy-phenylene vinylene-b-2-vinyl pyridine) (PPV-b-P2VP) block copolymer and CdSe nanocrystals, self-assembly leads to a bulk lamellar structure on the 10nm length scale. Small angle X-ray scattering confirms the addition of nanocrystals swells the domain size. We demonstrate via transmission electron microscopy the nanocrystals reside preferentially in the P2VP domain, presumably due to the strong nanocrystal surface interactions with polar P2VP and exclusion effects of the crystalline PPV phase. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P18.00002: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P18.00003: Nano-porous Poly(3-hexylthiophene) films: A novel route to prepare bulk heterojunction photovoltaic devices Tirtha Chatterjee, Kulandaivelu Sivanandan, Craig J. Hawker, Edward J. Kramer Conjugated polymers are excellent candidates for use in low-cost electronics and photovoltaics applications. Bulk heterojunction (BHJ) morphologies are promising device architecture as the close proximity of the electron donor and acceptor micro-domains (with domain size comparable with the exciton diffusion length) facilitates the charge transport process. In order to achieve a well ordered BHJ architecture, poly(3-hexylthiophene) (P3HT) based rod-coil copolymers are synthesized where coil blocks are grafted to the P3HT chain through a cleavable linker. The linker and the attached sacrificial coil block can easily be cleaved and removed by chemical treatment leaving a rough nano-porous P3HT film. Scanning force microscopy and grazing incidence small angle X-ray scattering convincingly show the nano-pore formation. Further, depth profiling using dynamic secondary ion mass spectroscopy indicates that nano-pores probably penetrate the entire depth of the film (device thickness). Subsequently refilling of the nano-pores by electron transporting component (fullerene derivatives) provides the required device morphology. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P18.00004: The Influence of Electric Fields on the Order-Disorder Transition Temperature of Block Copolymer Systems Heiko Schoberth, Kristin Schmidt, Kerstin Schindler, Alexander B\"oker We investigate the influence of electric fields on the phase behavior of diblock copolymers in concentrated solutions using synchrotron small-angle X-ray scattering (synchrotron SAXS). When heating the solutions through the order-disorder transition temperature {$T_{\mathrm{ODT}}$}, we find a significant decrease in {$T_{\mathrm{ODT}}$} with increasing electric-field strength. In addition we found a temperature regime in which it is possible to switch between the mixed and phase separated state at constant temperature upon application of a moderate electric field. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P18.00005: Phase Behavior of Polystyrene-block-Poly(n-alkyl-ran-n'alkyl methacrylate) Copolymers Hong Chul Moon, Junhan cho, Jin Kon Kim The phase behavior of polystyrene-block-poly(n-butyl-ran-n- hexyl) methacrylate copolymers and polystyrene-block-poly(n- octyl-ran-methyl) methacrylate copolymers were investigated by using small angle X-ray scattering, birefringence and rheometry. When the total molecular weight and the composition of the random copolymers were judiciously controlled, the closed-loop phase behavior with both a lower disorder-to-order transition and an upper order-to-disorder transition was observed. These block copolymers exhibited excellent baroplasticity. The observed phase behavior was explained by a compressible mean field approach. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P18.00006: Gas Pressure Effect on Phase Behavior of Deuterated Polystyrene-block-poly(n-pentyl methacrylate) Hye Jeong Kim, Jin Kon Kim, Du Yeol Ryu The pressure effect of various gases on the phase transitions of deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymer was investigated by small angle neutron scattering (SANS) and birefringence. With increasing helium gas pressure, the size of closed-loop consisting of both the lower disordered- to-ordered transition and the upper ordered-to-disordered transition was decreased, which is similar to the hydrostatic pressure effect. On the other hand, when nitrogen gas was used, the size of the closed-loop became larger with increasing pressure. These interesting results are explained by the binding energy calculation. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P18.00007: Pressure Jump Studies of Block Copolymer Phase Transition in Selective Solvent Yongsheng Liu, Rama Bansil, Milos Steinhart Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the order-disorder transition (ODT) in a 30{\%} (w/v) solution of a diblock copolymer of poly(styrene -- isoprene) (SI 18-12) in diethylphthalate (DEP), a selective solvent for the PS block using pressure jump methods. The results show that the ODT temperature increases at about 20C/kbar with pressure. Time resolved pressure jump SAXS experiments were done to study the kinetics of disorder to BCC phase transition and the reverse transition. Pressure jump from 100 bar to 800 bar at 108 C from disordered state displayed a BCC structure at 30 seconds. Results of experiments with solvent viscosity increased by adding low molecular weight polystyrene will also be presented. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P18.00008: Self-assembled Oniontype Multiferroic Nanostructures Shenqiang Ren, Robert M. Briber, Manfred Wuttig Spontaneously self-assembled oniontype multiferroic nanostructures based on block copolymers as templating materials are reported. Diblock copolymer containing two different magnetoelectric precursors separately segregated to the two microdomains have been shown to form well-ordered templated lamellar structures. Onion-type multilamellar ordered multiferroic (PZT/CoFe$_{2}$O$_{4})$ nanostructures have been induced by room temperature solvent annealing in a magnetic field oriented perpendicular to the plane of the film. The evolution of the onion-like microstructure has been characterized by AFM, MFM, and TEM. The structure retains lamellar periodicity observed at zero field. The onion structure is superparamagnetic above and antiferromagnetic below the blocking temperature. This templating process opens a route for nanometer-scale patterning of magnetic toroids by means of self-assembly on length scales that are difficult to obtain by standard lithography techniques. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P18.00009: Periodic Polymers for PhoXonics Edwin Thomas Exploiting the size and shape dependence of material properties and accessing multi-functionality holds great promise for the development of materials that will contribute to novel future technologies. Polymers can act as hosts for metallic and dielectric nanoparticles as well as organic molecules, resulting in nanocomposites with combinations of properties not available by other means. \textit{Periodic} structural assemblies are of particular interest, due to their interesting interactions with waves: especially light and mechanical waves. Progress in this exciting area requires excellent control of structure formation. A top-down, bottom-up approach, involving interference lithography and self assembly is demonstrating good success in fabricating the requisite structures and desired properties for photonics and phononics. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P18.00010: Hydration and phase separation of polyethylene glycol in copolymers of tyrosine derived carbonates. N. Sanjeeva Murthy, Wenjie Wang, Joachim Kohn Effect of PEG fraction and its block size on the temperature-induced phase transitions and the hydration-induced phase separation were investigated in a copolymer of desaminotyrosyl tyrosine ethyl ester (DTE) and PEG using simultaneous SAXS/WAXS/DSC. The PEG segments crystallized when the block size was at least 2000 Daltons and present at $\sim $ 40 wt{\%}, and raised the T$_{g}$ of the polymer by $\sim $ 15 $^{\circ}$C. The PEG blocks in dry polymers with up to 50 wt{\%} PEG, even when crystalline, were found to be uniformly distributed with no evidence of phase separation at 10 nm length scales. The non-iodinated PEG-rich sample with 30 mole{\%} PEG$_{2k}$ showed the lower critical solution temperature (LCST) behavior with PEG blocks forming a separate phase above -21 $^{\circ}$C. In the iodinated version of this polymer, the PEG$_{2k}$ blocks were phase separated in the solid phase. In all samples, whether PEG was crystalline or not, hydration induced PEG to separate into 15 nm hydrated domains. Phase behavior was dependent on whether poly(DTE) or the PEG was the major (matrix) phase. Changes in the mobility of the chains brought about by water-mediated hydrogen-bonding, and modulated by heat, appear to be the common underlying explanation for the range of observed phase behavior. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P18.00011: Robustness of Pluronic Block Copolymer Nanostructure to Structural Changes in Dispersed Nanoparticles Theresa A. LaFollette, Lynn M. Walker Thermoreversible block copolymers [(PEO)n-(PPO)m-(PEO)n; trade name Pluronic] self assemble into ordered micelle gels. Nanoparticles (3-10nm) are templated in the interstitial spaces of Pluronic micelle gels to form nanocomposite systems. Globular hydrophilic proteins have served as model monodisperse nanoparticles in this work. We have shown that these proteins are templated in the interstitial sites of the cubic packed micelle gels at room temperature. By raising the temperature, the proteins are denatured to study the robustness of the micelle gel to structural changes due to the unfolded protein. Nanoscale structure is determined from small angle neutron scattering (SANS). It was expected that any change in the nanoparticle size would cause a change in the packing of the Pluronic micelle gel. However in SANS experiments, the FCC and BCC Pluronic templates show no nanoscale structural differences between a room temperature sample and a sample that has been heated to denature the protein and then cooled back to room temperature. There is a change in the template at longer length scales as evidenced by a low q upturn in the scattered intensity. The robustness of the micelle gel at different length scales will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P18.00012: Phase behavior of block copolymer nanocomposites George Papakonstantopoulos Incorporating nanoparticles in block copolymers can allow the creation of a material with tailored properties. In addition, the control of the nanoparticle location in a nanometer scale, can lead to novel applications for these materials. Although, the phase behavior of block copolymers in the bulk is well established, the effects of nanoparticles on their phase behavior, especially under confinement, are not well understood. We carried out a systematic study to investigate the self-assembly of block copolymer-nanoparticle composites using a coarse grain model. These systems are studied in the bulk and under confinement. The dependence of the location and distribution of the nanoparticles within the block copolymer as a function of particle-polymer interaction, size and shape were examined. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P18.00013: Effect of Chain Architecture on Nanoparticle Miscibility in Block Copolymer Nanocomposites Jessica Listak, Hyung Ju Ryu, Ilhem F. Hakem, Rangou Sofia, Politakos Nikolaos, Misichronis Konstantinos, Apostolos Avgeropoulos, Michael R. Bockstaller This contribution will present a combined experimental and theoretical analysis of the effect of block copolymer chain architecture on the miscibility and morphology of enthalpically neutralized particle additives. The chain architecture is found to be a critical parameter in facilitating particle dispersion imposing both direct as well as indirect constraints on the particle distribution. Continuous block configurations (such as the bridged midblocks in triblock copolymers) are found to inhibit particle compatibilization. Interestingly, the particle miscibility is found to be strongly affected by the configuration of the block adjacent to particle-filled domains (indirect constraint). In particular, incompatibility is observed for high branching densities in the adjacent domains (such as miktoarm chain architectures). A mean-field model will be presented to rationalize this observation as a consequence of segmental crowding that counteracts changes in the layer dimensions induced by particle sequestration. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P18.00014: Rheological and Mechanical Properties of Crosslinked Block Copolymer Nanofiber and Polystyrene Blends. Sungwon Ma, Yonathan Thio The mechanical and rheological properties of blends of crosslinked and uncrosslinked poly(styrene)-\textbf{\textit{b}}-poly(isoprene) copolymer with commercially available polystyrene were studied. Cylindrical morphology of PS-\textbf{\textit{b}}-PI copolymer was employed for generating nanofiber morphology. Cold vulcanization process using sulfur monochloride (S$_{2}$Cl$_{2})$ was used to preserve the morphology. Blends of uncrosslinked PS-\textbf{\textit{b}}-PI copolymer with neat polystyrene were also prepared. Both blend samples were prepared by solvent casting method with the filler contents varying between 0.5 and 10 wt{\%}. The mechanical and rheological properties were characterized and the microstructures of the fiber and the systems were imaged. The dynamic moduli (G' and G'') of the crosslinked system increased with increasing the fiber content compared to the uncrosslinked system. The results were compared to the rheological model by fitting to Cross-Williamson. This blend study indicated critical volume concentration of nanofiber between 5 and 10 wt{\%} of nanofiber content. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P18.00015: Well Ordered Polymer Melts with Sub 5-Nanometer Domains upon Blending Surfactants with Selectively Associating Additives Vikram Daga, Vijay Tirumala, Curran Chandler, Alvin Romang, Eric Anderson, Eric Lin, James Watkins Applications employing block copolymers such as templating mesoporous inorganic structures and patterning would benefit from reduction in domain size formed in well-ordered block copolymer templates. The extent to which the domain size can be reduced is limited by the minimum required segregation strength, $\chi N$, where $N$ determines the size of block copolymer chains and the domain size. We have shown that disordered block copolymer surfactants with molar mass less than 15 kg/mol, can be made to undergo disorder-to-order transition by blending selectively associating homopolymers as well as small molecule additives with multi-point, non-ionic interactions. Blending with selectively associating additives result in an increase in segregation strength $\chi N$ through an increase in apparent $\chi$. The resulting domain sizes were found to be as low as 5 nm which is significantly lower than that seen for a typical block copolymer template. [Preview Abstract] |
Session P19: The Physics of Polymer Nanocomposites: Properties
Sponsoring Units: DPOLYChair: Jan Obrzut, National Institute of Standards and Technology
Room: 320
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P19.00001: Tuning optical properties of gold nanorods in polymer films through thermal reshaping Russell Composto, Eric Mills, Yu Liu The thermal reshaping of gold nanorods (NRs) in a poly(methyl methacrylate) (PMMA) nanocomposite film is investigated by UV-vis and TEM. To ensure dispersion, the NRs are modified with PEG brushes, and then dispersed in PMMA. Thermal annealing of the PMMA:NR film results in a blue shift of the longitudinal plasmon resonance, caused by a decrease in the length of the NR. The rate of the blue shift increases as temperature increases from 100 $^{\circ}$C to 200 $^{\circ}$C, and the longitudinal absorption peak approaches a constant value that scales linearly with temperature. We demonstrate a potential application by fabricating a device with a gradient in optical properties. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P19.00002: Quenching Photoluminescence in Single-Walled Carbon Nanotube/Copolymer Composite Materials Andrew Schoch, L. Catherine Brinson, Kenneth R. Shull Single-walled carbon nanotubes (SWNTs) stabilized by A-B diblock and A-B-A triblock copolymers are excellent model systems for studying the relationship between nanotube dispersion and mechanical response. The SWNTs cannot be dispersed in the alcoholic solvent used here without the addition of copolymer. However, the B blocks are in good solvent conditions for all temperatures and the A blocks solvent quality decreases with decreasing temperature. This solvent quality difference drives the formation of micelles with A block cores at low temperatures. As verified by AFM, the micelles form heterogeneous micelles in solutions by incorporating the SWNTs. The dispersion has also been verified with near-IR photoluminescence spectroscopy (NIR-PLS) and the mechanical properties of these materials have been examined with rheological methods. The elastic contribution to the shear modulus increases while at high temperatures which we attribute to an increase in the number of NT-NT contacts. We have attempted to verify this observation by simulating the high temperature environment in the NIR-PLS measurements and looking for quenching. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P19.00003: Temperature measurements of inverse micelles coated in gold nanoparticles using fluorescence Chad Daley, James A. Forrest, Ryan Speller, Toews William, Patrick McVeigh, Todd Emrick When nanoparticles are subject to laser radiation they have the ability to efficiently absorb energy from the beam and transform this energy into heat. Photothermal therapy uses this phenomenon to irreparably damage tissue surrounding nanoparticle conjugates. Despite the promise of this technique, there is no concensus on the damage mechanism or even the local heating. Here we present an experiment designed to measure local temperatures achieved in such processes. Ligand covered Gold nanoparticles are used to stabalize inverse micelles containing fluorescence dye in the water component. The fluorescence intensity being temperature dependent provides us with a means of measuring the temperature of the micelles as a function of time immediately following a laser pulse. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P19.00004: Magnetic and Optical anisotropy thiol-capped Au NPs embedded into a polymer Jose de la Venta, Miguel Angel Garcia, Virginia Bouzas, Andrea Pucci, Giacomo Ruggeri The anisotropy at the nanoscale is achieved when the shape of the objects is not spherical such as in the case of nanorods, nanotubes or nanowires. However, when they are embedded in a macroscopic matrix, the random distribution destroys the anisotropy. In this work we study the possibility of induce optical and magnetic anisotropy in a system consisting of spherical thiol capped Au NPs embedded in a polymeric matrix. The ferromagnetic-like behavior arises from the bond between the Au-S atoms and the optical response is also highly dependent on these bonds. So, modifications in the environment and in these bonds could alter the behavior of the whole system. When the NPs are embedded in a polymeric matrix, which is stretched even 40 times in one direction, SQUID and UV/Vis measurements show that arise a macroscopic anisotropy in spite of the spherical shape of the NPs. EXAFS measurements confirm that there are modifications in the Au-S bonds along the stretched direction that are responsible of the induced macroscopic anisotropy. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P19.00005: Stability of the double gyroid phase to nanoparticle polydispersity in polymer tethered nanosphere systems Carolyn Phillips, Christopher Iacovella, Sharon Glotzer Recent simulations have shown that aggregating nanospheres functionalized with polymer ``tethers'' can self-assemble to form the double gyroid phase also seen in block copolymer and surfactant systems. Within the gyroid domain, the nanoparticles pack in icosahedral motifs, stabilizing the gyroid phase in a small region of the phase diagram[1]. We study the impact of nanoparticle polydispersity on the properties of the double gyroid phase [2]. Here we show that a low amount of polydispersity lowers the energy of the phase. A large amount of polydispersity raises the potential energy of the system, disrupts the icosahedral packing, and eventually, destabilizes the gyroid. A study of binary gyroids indicates that the inclusion of a small population of either smaller or larger nanospheres encourages low-energy icosahedral clusters. Using a new measure for determining the volume of a component in a microphase-separated system based on the Voronoi-tessellation, we show that polydispersity compacts the gyroid domain and lowers the average coordination of the nanospheres. [1] Iacovella, et al., PRE, 2007 [2] Phillips, et al., ``Stability of the double gyroid phase to nanoparticle polydispersity in polymer tethered nanosphere systems, preprint. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P19.00006: Experimental and computational investigation of percolation in complex polymer nanocomposites Derrick Stevens, Lori Downen, Russell Gorga, Laura Clarke The continuing development of polymer nanocomposites has led to increasingly complex morphology, such as the mats of composite nanofibers formed from electrospinning. The formation of particle networks within the composite volume that leads to enhanced properties, such as electrical conductivity, may be influenced by this complex sample geometry. In this work, experimental and computational efforts are utilized to understand and predict the percolation threshold (critical volume fraction) for two cases: single ultra-high aspect ratio fibers (where fiber diameter can be similar to the particle dimensions) and these same fibers arranged in a random mat with up to 80{\%} porosity. 2D and 3D Monte Carlo simulations, modeled on the actual parameters of our experimental system [1], are utilized and the results are compared with our experimental findings. In particular, confinement to fibers increases the percolation threshold; however the multi-fiber pathways available in mats partially reduce this constraint [2]. [1] S.S. Ojha, D.R. Stevens, K. Stano, T. Hoffman, L.I. Clarke, R.E. Gorga, \textit{Macromolecules} \textbf{41}, 2509 (2008). [2] D.R. Stevens, L.N. Downen, L.I. Clarke, \textit{Phys. Rev. B} \textbf{in press }(2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P19.00007: Influence on Thermal Diffusivity through a Transformation of Nanotube-like Clay Platelets in Polymer Blends Seongchan Pack, Takashi Kashiwagi, Tadanori Koga, Jonathan Sokolov, Miriam Rafailovich We have previously demonstrated that large aspect ratio nanoparticles such as clays or nanotubes can form in-situ grafts which become universal compatiblizing agents for polymer blends. Here we show how the same mechanism could be applied to producing flame retardant materials in the polymer matrix. In particular, the large aspect nanoclays prevent thermally induced phase segregation and disperse the flame retardants, which greatly decrease flammability and increase efficiency of the flame retardants during combustion due to a formation of ribbons-like structures. These structures could produce a lager thermal differential gradient between the two polymer phases, which could change a heat specific of the system during combustion. Therefore, a small addition of the nanoclays affects the huge reduction on heat release rate and the mass loss rates. Furthermore, using a small angle X-ray scattering (SAXS), a transmission electron microscopy (TEM), and a scanning electron microscopy (SEM) shows that the clay platelets could be transformed into tubular-like rods during combustion, which would increase of the thermal diffusivity in the polymer blend. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P19.00008: Electrical Conductivity in Polymer Composites Containing Metal Nanowires: Simulation and Experiment Sadie White, Tarun Vemulkar, John Fischer, Karen Winey The study of rod percolation behavior has resurfaced in recent years, because it explains electrical conductivity in polymer nanocomposites containing carbon nanotubes and metal nanowires. Common processing techniques result in fillers with L/D $<$ 50, so traditional models, which are only strictly correct in the limit of L/D $\sim \quad \infty $, are ineffective at predicting percolation in these systems. We present a simulation that constructs percolated networks of finite-aspect ratio rods and calculates their electrical conductivity. We will compare our simulation results with polymer composites containing silver nanowires with aspect ratios of $\sim $10 and $\sim $30. Finally, we will present the temperature-dependent electrical conductivity of these composites and interpret the results using the thermal expansion coefficients of polystyrene and silver. These materials act as ``thermal switches,'' wherein electrical conductivity of certain composites can be manipulated by several orders of magnitude over the temperature range from 80K-425 K. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P19.00009: Polymer Nanocomposites Made with Unmodified Graphite or Carbon Nanotubes: Role of Dispersion in Optimizing Mechanical and Thermal Properties and Electrical Conductivity Junichi Masuda, Katsuyuki Wakabayashi, Philip Brunner, Cynthia Pierre, John Torkelson Polymer nanocomposites made with carbon-based nanofiller have the potential to achieve unprecedented, multifunctional property enhancements in comparison with other nanocomposite systems. Here, we describe research in which we prepare nanocomposites with polymers that are not amenable to solution-based processing, such as polypropylene and poly(ethylene terephthalate). Solid-state shear pulverization is used singly or in conjunction with melt processing to obtain well-dispersed polymer/graphite and polymer/carbon nanotube nanocomposites. We report record improvements in properties of unoriented films of polypropylene nanocomposites, including Young's modulus, crystallization rate, and thermal degradation temperature. We also characterize electrical conductivity of such nanocomposites and note that the dispersion characteristics necessary to achieve maximum mechanical and thermal properties differ from those needed to maximize electrical conductivity. The potential of and challenges with using unmodified graphite as a filler in polymer nanocomposites will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P19.00010: Conductive Paper by LBL Assembly of PSS and ITO onto Wood Fibers and its Electrical Properties through Impedance Spectroscopy and I-AFM Chunqing Peng, Yonathan Thio, Rosario Gerhardt Conductive paper has been fabricated by layer-by-layer (LBL) assembly of polyelectrolytes and indium tin oxide (ITO) nanoparticles onto wood fibers, followed by traditional paper making method. The wood fibers were first coated with polyethyleneimine (PEI) and then LBL assembled with poly(sodium 4-styrenesulfonate) (PSS) and ITO for several bilayers. The AC electrical properties, measured for frequencies ranging from 0.01 Hz to 1 MHz, will be reported for the in-plane (IP) and through-the-thickness (TT) directions. With 10 bilayers of PSS/ITO assembly on wood fibers, the conductivity of as-prepared paper was improved by more than six orders of magnitude and reach to 5.2$\times $10$^{-6}$ S cm$^{-1}$ in IP direction and 1.9$\times $10$^{-8}$ S cm$^{-1}$ in TT direction. The percolation phenomenon of ITO nanoparticles through the handsheet in both directions was observed through current atomic force microscopy (I-AFM). By applying a bias voltage, either on one end of the paper stripes or on one side of the paper handsheet, the current can be detected on the other end of the paper stripes or on the other side of the paper handsheet. PEI can be used to modify the ITO suspension and significantly improve the LBL procedure. The mechanism of PEI modifying ITO colloidal suspension will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P19.00011: Placement Control of Nanomaterial Arrays on Surface-Reconstructed Block Copolymer Thin Films Jeong Gon Son, Wan Ki Bae, Huiman Kang, Paul F. Nealey, Kookheon Char Nanomaterials such as nanoparticles, quantum dots and nanorods/wires have recently attracted considerable attention not only because of their unique electronic, optical, and magnetic properties depending on their size and chemical structure but also due to their possible applications to optoelectronic devices, next-generation memory devices, and biological sensors. In order to take full advantage of these useful properties for highly integrated fabrication, precise control of such nanomaterials on patterned substrates is inevitably required. In this presentation, we demonstrate a straightforward and reproducible method for the placement of nanomaterials such as nanoparticles and nanorods on patterned PS-$b$-PMMA block copolymer (BCP) thin films. This concept is based on the properties of surface-reconstructed BCP thin films, which could induce topographical nanopatterns induced by selective solvent vapor treatment without any etching process. The deposition conditions for high density nanomaterial patterns in the grooves of BCP nanopatterns were optimized. By treating the surface under electron beam irradiation, the pattern inversion of nanomaterial-containing BCP nanopatterns was also observed, which can be further processed to realize the dual nanomaterial patterning. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P19.00012: Confinement and Ordering of Au Nanorods in Polymer Films Michael J. A. Hore, Eric Mills, Yu Liu, Russell J. Composto Ordered arrays of gold nanorods (Au NRs) possess interesting optical properties that might be utilized in future devices. Au NRs functionalized with a poly(ethylene glycol)-thiol brush are incorporated into homopolymer or block copolymer (BCP) films. NR distribution and orientational correlations are studied as a function of nanorod concentration and spacial confinement via Rutherford backscattering spectrometry (RBS) and transmission electron microscopy, respectively. In particular, differences in the degree of nanorod ordering are presented for PMMA homopolymer films ($d \quad \sim $ 45 nm) versus PS-$b$-PMMA BCP films (L/2 $\sim $ 40 nm), where higher ordering is seen in the case of BCP films. At moderate volume fractions of NRs, $\phi $ = 1{\%} to 10{\%}, the degree of ordering is moderate, and increases with increasing $\phi $ . However, coexistence between regions of higher ordering and isotropic orientations is observed. In addition to the planar confinement considered above, orientation of Au NRs confined to cylindrical P2VP domains is studied in PS-$b$-P2VP BCP films. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P19.00013: Carbon nanotubes nucleate the growth of graphitic layers during carbonization.of electrospun poly(acrylonitrile) nanofibers. Sabina Prilutsky, Yachin Cohen, Eyal Zussman Hybrid nanofibers with varying concentration of multiwalled carbon nanotubes (MWCNTs) in polyacrylonitrile (PAN) were fabricated using the electrospinning technique and subsequently carbonized. The morphology of the fabricated carbon nanofibers (CNFs) at different stages of the carbonization process was characterized by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. In-situ morphological changes during heating were followed by HRTEM using a heated stage. The polycrystalline nature of the CNFs was shown, with increasing content of ordered crystalline regions having enhanced orientation with increasing content of MWCNTs. The results indicate that MWCNTs embedded within the PAN nanofibers nucleate the growth of graphitic layers during PAN carbonization. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P19.00014: Crystallization and melting behavior of isotactic polypropylene and carbon nanotube nanocomposites Georgi Georgiev, Yaniel Cabrera, Lauren Wielgus, Zarnab Iftikhar, Michael Mattera, Peter Gati, Austin Potter, Peggy Cebe Polymer nanocomposites (PNCs) are the most recent development in the field of polymer science and technology. Geared toward creating novel polymer based materials, PNCs are the largest commercial application for nanotubes. Spherulitic polymer crystal growth was changed by inducing new fibrillar crystals on the surface of carbon nanotubes. Upon isothermal melt crystallization at 135$^{o}$C, CNTs lead to monoclinic crystal growth perpendicularly to the long axis of the nanotubes, explained by the multiple nucleation centers formed at the interface of the carbon nanotube and the polymer chains. Using Microscopic Transmission Ellipsometry (MTE), the sign of the alpha crystallographic phase was determined as positive. Using Differential Scanning Calorimetry (DSC), a decrease in the Avrami exponent was measured with increase of concentration of nanotubes. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P19.00015: Crystallization kinetics in poly(ethylene oxide) / layered silicates nanocomposites Eleni Pavlopoulou, Sapfo Fotiadou$^{1}$, Eleni Papananou, Kiriaki Chrissopoulou, Spiros H. Anastasiadis, Giuseppe Portale, Wim Bras We investigate the effect of inorganic clay on the crystalline characteristics and the crystallization kinetics of PEO in its intercalated nanocomposites with natural montmorillonite (Na+- MMT). The structure of the hybrids was investigated over multiple length scales by X-ray diffraction, small-angle X-ray scattering (SAXS) and polarizing optical microscopy (POM) as well as by DSC. The PEO within the galleries is completely amorphous whereas only the excess polymer outside the completely full galleries can crystallize at high PEO concentrations. The time resolved measurements reveal the effect of clay on crystallization. Even very small amount of the inorganic can cause a significant decrease of the spherulite size. The crystallization mechanism varies from sporadic nucleation for pure PEO to two-dimensional growth with predetermined nuclei at 10wt\% clay with a higher activation barrier for low clay concentration. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU. [Preview Abstract] |
Session P20: Melts and Solutions
Sponsoring Units: DPOLYChair: Chang Ryu, Rensselaer Polytechnic Institute
Room: 321
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P20.00001: Dynamics and rheology of high molar mass polyethylene oxide solutions Abhishek Shetty, Michael Solomon We report dynamic light scattering (DLS), bulk rheology and turbulent drag reduction (TDR) measurements that investigate the structure and dynamics of high molar mass PEO solutions. Steady shear rheology of high molar mass PEO solutions, when modeled by the FENE-P constitutive equation, was consistent with viscoelastic relaxation times much larger than predicted by single polymer, dilute solution theory. DLS of dilute PEO solutions showed a single relaxation mode in the decay time distribution, which scales as q$^{-3}$ rather than the q$^{-2}$ scaling expected of diffusive dynamics. We interpret this result as consistent with the internal dynamics of large multichain domains, clusters or aggregates in the high molar mass PEO solutions. By means of DLS, we also show that the aggregation state of dilute solutions of high molar mass PEO can be manipulated by addition of the chaotropic salt guanidine sulfate or the divalent salt magnesium sulfate. Addition of these salts shifts the power law scaling of the relaxation time from q$^{-3}$ to q$^{-2}$. This shift of relaxation time scaling from one indicative of aggregate dynamics (q$^{-3})$ to one characteristic of polymer center-of-mass diffusion (q$^{-2})$ shows that these salts are effective de-aggregation agents for PEO. We discuss the results in light of the potential connection between aggregation behavior and polymer TDR of high molar mass PEO. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P20.00002: Diffusive Properties of Dilute HPC Solutions: Comparative Study with DLS and FPR Ryan McDonough, Kiril Streletzky, Paul Russo The dynamics of HPC (Hydroxy-propyl-cellulose) solutions were studied by two fundamentally different methods: FPR (Fluorescence Photo-bleaching and Recovery) and DLS (Dynamic Light Scattering). FPR captures diffusive processes by establishing a photo-bleached boundary and ``seeing'' only tagged particles diffusing back into bleached area, which yields a contrast function. DLS auto-correlates scattered light intensity from particles in order to determine a statistical decay function. Inverse Laplace transform (CONTIN) and stretch exponential line shape analysis (LSA) serve to quantitatively decompose decay data into different diffusion processes or modes. The first finding is that the CONTIN and LSA results on the same sample are fairly consistent. The second finding is that the modal distributions for FPR and DLS spectra on the same sample show consistent dissimilarities. This indicates a comparative limitation or sensitivity in range of detectable diffusive processes between FPR and DLS in a complex system. The third finding is that the fluorescent tag and tagging process seem to alter the diffusion processes seen by DLS in a way that is consistent; there is a slower mode apparent in non-tagged sample which does not appear in the tagged sample. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P20.00003: Helix formation via kinetic assembly of charged block copolymer cylinders in solution Sheng Zhong, Ke Zhang, Karen Wooley, Darrin Pochan A multi-micrometer-long, cylinder with helical superstructure is created from coassembly of poly(acrylic acid)-\textit{block}-poly(methyl acrylate)-\textit{block}-polystyrene (PAA-$b$-PMA-$b$-PS) triblock copolymers with excessive triethylenetetramine or diethylenetriamine in the mixture of 67{\%} volume ratio of water in tetrahydrofuran (THF). The stable pitch distance of the formed helices is due to the balance of long range electrostatic association and uniaxial tension along the cylinder, which can be efficiently tuned by varying the type and amount of the multivalent amine molecules. Double and triple helices are also formed with characteristic interhelical cynlinder distances similar to what is observed as the pitch in single helices. A kinetic study shows that the formation of a helix undergoes a complex, but reproducible, nanostructure evolution, including a starting stacked structure, a transition state of very short helices with the length of tens of nanometers and a final multi-micrometer-long mature helix by connecting those short helices. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P20.00004: Enhancing Polymer-Fullerene Miscibility Through Enthalpic Interactions Katie Campbell, David Bucknall, Yonathan Thio, Haskell Beckham, Uwe Bunz, Adam Hannon, Andrew Zappas, Bilge Hatiboglu, Michael Kempf Using both theoretical and experimental methods, the use of functional groups in controlling the miscibility between various polymers and fullerenes has been investigated. Molecular dynamics simulations with unmodified C$_{60}$ and C$_{60}$ dimers indicated that the number and connectivity of phenyl rings as functional groups, polymer backbone spacing, and aromaticity are all factors in fullerene miscibility. To distinguish between entropic and enthalpic factors, UV-visible spectroscopy was used to determine fullerene solubility with a variety of solvents and to also determine association constants with solvents and polymers. A distinct time dependency for complex formation with many of the fullerene- organic materials investigated was observed as evidenced by a change in solution color with time. Stern-Volmer approximations and fluorescence quenching were used to examine the association of C$_{60}$ with a series of poly(\textit{para}-phenylene ethynylene)s, cyclic polystyrene (PS), and linear PS. The fluorescence quenching of these materials by C$_{60}$ indicates an association between C$_{60}$ and the polymer. WAXS studies have shown the formation of C$_{60}$ aggregates in PS at concentrations of C$_{60}$ as low as 1 wt{\%}. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P20.00005: Modeling Thermodynamic Behavior of Nonionic Surfactants in Water Valeriy Ginzburg, Pierre Varineau Aqueous solutions of nonionic surfactants exhibit a non-trivial phase behavior known as lower critical solution temperature (UCST), where solutions are homogeneous at lower temperatures but become cloudy (two-phase) at higher temperatures. Conventional Flory-Huggins\footnote{M. L. Huggins, \textit{J. Chem. Phys. }\textbf{9}, 440 (1941); P. J. Flory, \textit{J. Chem. Phys. }\textbf{9}, 660 (1941).} theory of polymer solutions fails to describe such phase behavior. We utilize the approach suggested by Dormidontova\footnote{E. E. Dormidontova, \textit{Macromolecules }\textbf{35}, 987 (2002).} and modify Flory-Huggins theory by explicitly accounting for water-water and water-alkylene oxide hydrogen bonding. While the Dormidontova model was restricted to aqueous solutions of polyethylene oxide (PEO), we extend it to include other monomers and their copolymers. With the new approach, we can semi-quantitatively predict cloud points of various nonionic surfactants (Tergitol{\texttrademark} L and Ecosurf{\texttrademark} series) as functions of their molecular structures. We also discuss extensions of this model to calculate micellar phase behavior and oil/water/surfactant interfacial tensions. {\texttrademark} Trademark of The Dow Chemical Company [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P20.00006: Role of surfactants on the assembly of amphiphilic copolymers through instabilities of organic/water interfaces Jintao Zhu, Ryan C. Hayward We have studied the influence of aqueous surfactants on the assembly of amphiphilic copolymers through hydrodynamic instabilities of organic/water interfaces. Micropipette aspiration measurements on evaporating chloroform droplets containing polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers revealed that the onset of interfacial instability and subsequent growth in surface area corresponded to a near vanishing of the interfacial tension. By adding another surfactant, such as sodium dodecyl sulfate (SDS), to the aqueous phase, the chloroform/water interfacial tension was reduced and the onset of instability shifted to lower concentration of PS-PEO. Varying amounts of SDS also led to qualitatively different mechanisms of growth in interfacial area and correspondingly different morphologies of the resulting copolymer assemblies. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P20.00007: Influence of Intermolecular Interactions on Fragility of Polymers Kumar Kunal, Alexei Sokolov Glass transition in polymers is a result of slowing down of segmental relaxation. Steepness of the temperature-dependence of segmental relaxation times close to the glass transition temperature, Tg is expressed in terms of fragility parameter. A strongly non-Arrhenius temperature dependence of segmental relaxation times with steep variations close to Tg is called a `fragile' behavior, and a nearly Arrhenius behavior is called `strong'. The existing theoretical models and experimental investigations on polymers with weak van der Waal's interactions suggest that fragile behavior of polymers may be linked to their poor packing ability. However, the effect of strong intermolecular interactions on fragility such as polar interactions and hydrogen bonds remains unexplored. It has been predicted that polymers composed of polar monomers are likely to be highly fragile. We have studied polymers with strongly polar interactions and hydrogen bonds and found that although polar polymers do seem to have a higher Tg than their non-polar counterparts, no such conclusion can be drawn about their fragility. The different effects of polar interactions on different classes of polymers may be attributed to the difference in their Tgs. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P20.00008: Random Walk of Chain Molecules Along Pore Axis Guiduk Yu, Sergei Obukhov, Jiun-Tai Chen, June Huh, Yoontae Hwang, Soonchun Mok, Priyanka Dobriyal, Pappannan Thiyagarajan, Thomas P. Russell*, Kyusoon Shin* We investigated the overall conformation of polymer chain in cylindrical nanopores using small-angle neutron scattering. The mixture of hydrogenous PS and deuterated PS is confined in nanopores. Surprisingly, the overall conformation of polymer chains along the pore axis is observed to be the same as that in bulk. Even though the chain dimension is larger than the radius of the pores, the chains along the pore axis are not stretched, but sustain to be in unperturbed state. The SANS results implicate that the interpenetration of polymer chains decreases as polymer enters nanopores. We expect the reduction of intermolecular entanglement possibly alters other physical properties of polymer under nanoconfinement. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P20.00009: Dynamics of Cyclic Molecules Threaded into a Linear Polymer Chain Koichi Mayumi, Hitoshi Endo, Michihiro Nagao, Noboru Osaka, Hideaki Yokoyama, Mitsuhiro Shibayama, Kohzo Ito Dynamics of polyrotaxane (PR), in which cyclic molecules, cyclodextrins (CDs), are threaded on an axis linear polymer chain, poly(ethylene glycol) (PEG), are first studied by contrast variation neutron spin echo (CV-NSE). By comparing PRs of hydrogenated and deuterated PEG with different scattering contrasts, we successfully extract two diffusive modes of CDs, corresponding to self diffusion and relative motion to the axis PEG in PR. The self-diffusion constant of CD in PR is determined to be about one-third of the free one in the absence of the axis polymer, which would reflect the space dimension of diffusion with the topological restriction on the axis chain path. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P20.00010: Diffusion of Water through Methyl- and Hydroxyl-Terminated Poly(Dimethylsiloxane) Ahmed E. Ismail, Gary S. Grest, David R. Heine, Mark J. Stevens, Mesfin Tsige Both experimental and numerical reports of the diffusion constant of water through poly(dimethylsiloxane) (PDMS) show variances of nearly an order of magnitude. We use molecular dynamics simulations to calculate the diffusion constant for both methyl- and hydroxyl-terminated PDMS chains. We examine the effects of both concentration and chain length. For a single water molecule, we find that diffusion depends strongly on the initial location of the molecule, as the ``caging'' phenomenon reported by Müller-Plathe can occur. At intermediate concentrations, we observe the formation of dimer and trimer water clusters, leading to lower diffusion rates; at concentrations above the reported aggregation limit of 0.1 wt \%, we observe the onset of phase segregation. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P20.00011: Effect of Stereochemistry and Polydispersity on Diffusion in Polypropylene Ernst von Meerwall, Numan Waheed, Wayne Mattice We have performed dynamic Monte-Carlo (MC) simulations and pulsed-gradient diffusion (D) experiments to study the effect of stereochemical composition in linear polypropylene (PP) melts. The coarse-grained simulations were based on the rotational isomeric state model and Lennard-Jones potentials. For the proton NMR diffusion measurements we obtained three PP specimens of differing molecular weight M and dispersity, with the probability of a meso diad Pm = 0.02 (syndiotactic), 0.23 (atactic), and 0.89 (nearly isotactic). The experiment supplied the fixed conversion between MC steps and real time; no dependence on Pm is expected. Both simulation and M-scaled experiment found D at high Pm several times faster than at low Pm. The constant-M simulation also showed a maximum near Pm = 0.8 due to quenched randomness. To find the source of the remaining disagreement with experiment, new simulations tracked the samples' Pm, mean M, and polydispersity, producing modest improvement. We suspect that the GPC determination of M and its distribution, based on linear polyethylene calibration, is somewhat dependent on PP stereochemistry (via D), generating misleading results. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P20.00012: On chain statistics and entanglement of flexible linear polymer melts Shi-Qing Wang In this work the chain statistics of most linear flexible polymers have been found to be rather universal, allowing chain entanglement to be depicted with few parameters. We first show, to our surprise, based on the literature data of most familiar linear polymers that (a) at the same number of backbone bonds most linear polymers have comparable coil size and are similarly flexible in spite of widely varying chain thickness and (b) the Kuhn length involves a similar number of backbone bonds. The packing model is found to describe the onset molecular weight Me obtained from the elastic plateau modulus whereas all other models in the literature fail to provide good correlation. It is chain thickness not stiffness that correlates with Me for over one hundred flexible linear polymers. On the other hand, other models such as percolation model appear to provide some crude correlation for Mc, to which the packing model does not apply well, where Mc denotes the point of departure in the molecular weight scaling from Rouse like to reptation like. Thus, our analysis clarified the apparent contradiction among the various models. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P20.00013: Translational Diffusion in a Confined Polymer Melt Janet Wong, Liang Hong, Sung Chul Bae, Steve Granick At the University of Illinois, a new experimental platform has been developed that combines the surface forces apparatus with spatially-resolved fluorescence recovery after photobleaching, giving direct measurements of translational diffusion when polymer melts are confined between mica sheets to controlled thicknesses comparable to the size of the molecules themselves. Applying this platform to polydimethylsiloxane (PDMS), we find not only the anticipated dependence on film thickness but also a dependence on the local pressure: when mica sheets are pressed together so that they flatten, the diffusion of chains confined between them depends on the local pressure, being slowest near the center of the contact. The confined chains split into two populations: those that are immobile on the scale of hours, and those whose mobility is close to that of the unperturbed polymer melt. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P20.00014: Entangled Polymer Melt Dynamics Studied By Low-Field NMR Fabian Vaca Chavez, Patrick Huebsch, Ronald Zirbs, Wolfgang Binder, Kay Saalwaechter Proton Multiple-Quantum (MQ) NMR is a powerful technique to investigate polymer dynamics due to its sensitivity to molecular motions on very different timescales. Entangled melts exhibit dynamic processes that cover a wide range of timescales, starting from fast ps-scale segmental reorientation up to diffusive and cooperative motions on the ms-s-scale. In this work, we apply MQ NMR to linear poly(cis-1,4-isoprene) and poly(isobutylene) of different molecular weight above the glass transition over suitable ranges of temperature, in order to establish the dynamic regimes predicted by the tube model, and, for the first time, to extract actual time scale information. This directly complements many neutron scattering studies, which are restricted to the sub-$\mu$s-timescale. Measurements on PIB-grafted silica particles with different molecular weights and different chain densities on the surface of the particle are also shown. The data is analyzed by establishing scaling laws which can be directly associated with different dynamic regimes predicted by the tube/reptation model. Full analytical analyses based on a correlation function which explicitly includes segmental, Rouse, and reptation dynamics are discussed. [Preview Abstract] |
Session P21: Computational Study of Semiconductor Band Structures
Sponsoring Units: FIAP DCMPChair: Chris Van de Walle, University of California, Santa Barbara
Room: 323
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P21.00001: First-Principles determination of deformation potentials in nitrides Qimin Yan, Patrick Rinke, Matthias Scheffler, Chris G. Van de Walle Group-III nitrides and their alloys are now commonly used in optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs). In these devices strain plays a crucial role since it affects the band structure near the valence- and conduction-band edge and thus the optical properties and the device characteristics. The deformation potentials that describe the change in band structure under strain have not yet been reliably determined, either experimentally and theoretically. Here we present a systematic study of the strain effects in AlN, GaN and InN in the zinc-blende and wurtzite phase. Besides density functional theory (DFT) in the most commonly applied local- and gradient corrected density approximation (LDA/GGA) we also apply the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional [1] and $G_{0}W_{0}$ quasiparticle corrections to address the band gap problem. We present a complete set of deformation potentials that allows us to predict the band positions under realistic strain conditions. For the wurtzite phase we observe non-linearities in the strain dependence that may, in parts, explain the appreciable scatter in previous theoretical work on deformation potentials of group- III-nitrides. [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003) Work supported by the UCSB Solid State Lighting and Energy Center. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P21.00002: Band-gap bowing, band offsets, and electron affinity for InGaN alloys: A DFT study. Poul Moses, Chris Van de Walle InGaN alloys are successfully being used in optical, electronic, and photovoltaic devices; a novel application is for photochemical water splitting. In order to further improve InGaN-based devices a detailed understanding of the materials properties as a function of alloy composition is needed. To obtain such insight we have investigated the band bowing and absolute band positions of InGaN alloys using density functional theory. The HSE exchange correlation functional has been used in order to accurately calculate the electronic band structure [1]. Detailed surface calculations have been performed that, combined with bulk calculations for alloys, yield information about the positions of valence and conduction bands on an absolute energy scale. We will discuss bowing effects, band offsets, and electron affinities in light of the application of InGaN alloys for photochemical hydrogen production. [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003) [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P21.00003: A Hybrid Look at Band Offsets in AlN/GaN Heterostructures Jeremy Nicklas, John Wilkins Hybrid functionals have been gaining traction for their better estimation of band gaps in semiconductors. Recently, a screened hybrid functional, HSE, has been introduced that improves upon the hybrid functionals by essentially screening out the Fock exchange after a given radius. This study compares how well the HSE functional does with the technologically important band offsets in the AlN and GaN wide bandgap heterostructures compared to experiment and other previous theoretical calcuations. Both the strained polar hexagonal and the nonpolar cubic phases of these III-V semiconductors are taken into consideration. Due to the large induced electric field in the polar hexagonal strucuture, a multipole decomposition will be discussed as well. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P21.00004: Reassessing the Description of the Electronic Structure of a Semiconductor Alloy Yong Zhang, A. Mascarenhas, L.-W. Wang Although an electronic state in an alloy like Ga$_{x}$In$_{1-x}$P is not a Bloch state, it is generally considered to be reasonably close to a Bloch state in the sense of a virtual crystal approximation (VCA), and it is often referred to as $\Gamma $-like, L-like, or X-like. We have find that within certain context one could call a band edge state as $\Gamma $-like, if the dominant component of its wavefunction is indeed the $\Gamma $ state of the VCA, but globally the alloy states are in general very different from those of the VCA in two important aspects: (1) a $\Gamma $-like state, for instance, could in fact have a very small $\Gamma $ component of the VCA state, and (2) if the corresponding VCA states are degenerate, for instance, a X-like band edge alloy state, there will be strong coupling among the degenerate valleys. These new insights have major impacts on our understanding of the optical and electronic properties[1], and the ordering effects [2] in a semiconductor alloy.\\[0pt] [1] Y. Zhang, A. Mascarenhas, and L.-W. Wang, Phys. Rev. Lett. 101, 036403 (2008). \\[0pt] [2]Y. Zhang, A. Mascarenhas, and L.-W. Wang, Phys. Rev. B (in press). [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P21.00005: \textit{Ab initio} study of the optical properties of Si-XII Brad Malone, Jay Sau, Marvin Cohen We present a first-principles calculation of the optical excitation spectrum of Si-XII, a high-pressure, metastable phase of silicon in the R8 structure. Recent calculations of the quasiparticle spectrum have shown Si-XII to be semiconducting with a small, indirect band gap. In this paper we solve the Bethe-Salpeter equation (BSE) to obtain the optical spectrum of this material. We then compare our calculated optical spectrum with experimental data for other forms of silicon commonly used in photovoltaic devices. These include cubic, polycrystalline, and amorphous forms of silicon. We find that the calculated values of the optical functions relevant to photovoltaic absorption in Si-XII show greater overlap with the incident solar spectrum than those found in these other silicon phases. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P21.00006: First-Principles Hartree-Fock Study of Locations and Hyperfine Interactions of Transition Metal Impurities in Silicon R.H. Pink, S.R. Badu, Archana Dubey, R.H. Scheicher, Lee Chow, M.B. Huang, T.P. Das The study of the magnetic properties of transition metal ions in silicon is currently of great interest because of their potential applications in spintronics. An understanding of the ferromagnetism associated with the interactions between these impurities requires a knowledge of their locations in the lattice. Three possible locations of Mn$^{2+}$, V$^{2+}$, and Cr$^{+}$ ions have been investigated, namely, the interstitial hexagonal (H$_{i})$ and tetrahedral (T$_{i})$ and substitutional (S) sites. Both binding energies and hyperfine interactions are being studied using the Hartree-Fock Cluster procedure with many-body effects included by the many-body perturbation theory (MBPT) procedure. For Mn$^{2+}$ ion, the Hi site is found to be unstable while the Ti and S sites have positive binding energies. Our calculated 55Mn hyperfine constant favors the Ti site\footnote{G.W. Ludwig and H.H. Woodbury, Phys. Rev. Lett. 5, 98 (1960).} which is also supported by channeling measurements.\footnote{J. LaRose and M.B. Huang (to be published).} [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P21.00007: Theoretical analysis on the effect of tip-induced band bending on scanning tunneling spectroscopy measurements on H-terminated Si(100) surface Hideomi Totsuka, Satoshi Watanabe Scanning tunneling spectroscopy (STS) is widely used experimental technique. However, theoretical study on STS is not sufficient yet, in the sense that the effects of important factors such as the tip-induced band bending (TIBB) in measurements on semiconductor surfaces have not been examined yet. In this study, we have analyzed the STS spectra on a H-terminated Si(100) surface theoretically using a method [1] which can calculate the electron states under applied bias voltages self-consistently. We found that the band gap in the STS spectra is larger than that in the density of states in our calculation. Furthermore, we found that this cannot be understood from TIBB, while the bias voltage dependence of TIBB in our calculation corresponds well with experimental result [2]. [1] Y. Gohda et al., Phys. Rev. Lett. 85, 1750 (2000). [2] M. McEllisterm, et al., Phys. Rev. Lett. 70, 2471 (1993) [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P21.00008: Band gaps and band offsets in the SiO$_2$/Si interface calculated by including the self-energy of electrons and holes Luiz Ferreira, Leonardo Fonseca, Mauro Ribeiro, Jr. Density Functional theory, as formulated by Kohn and Sham (Phys. Rev. {\bf 140}, A1133 (1965)), is insufficient when it comes to the calculation of one-particle excitations (electrons and holes). In this case, one has to include the self-energy of the particle (see for instance R. G\'{o}mez- Abal. et al Phys. Rev. Lett. {\bf 101}, 106404 (2008)). This self-energy is mostly the classical electrostatic self-energy of the particle charge density but has an important contribution from exchange and correlation. In a recent paper (L. G. Ferreira et al, Phys. Rev. B {\bf 78}, 125116 (2008)), it is shown that the self-energy can be calculated with the help of a ``self-energy potential'', wholly derived from pure atomic calculations. The band gaps calculated with those self-energies are precise, in no way worse than the GW band gaps, and yet the calculation is very simple and fast. Next challenge we faced was the calculation of the band-offsets of the all important Si/SiO$_2$ system. Notice that the ``self-energy potential'' is centered between two covalent bonded atoms in the Si side and centered at the O in the SiO$_2$ side. Then the question is whether these self-energy potential perturbations create a wrong charge density at the interface. The answer is that both gaps and band offsets were calculated with outstanding quality. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P21.00009: The influence of pressure on defects in amorphous silicon Jeffrey Grossman, Lucas Wagner Amorphous silicon(a-Si) thin-film solar cells are promising materials for solar cells, but they suffer from the Staebler-Wronski effect (SWE), in which the efficiency degrades over the course of a few hours of light exposure. While there has been progress in mitigating this effect through sample preparation, there is still no clear microscopic explanation for the degradation. We have used first principles density functional theory and highly accurate quantum Monte Carlo calculations to investigate the effect of pressure on different types of defects present in a-Si. Our calculations show that the effect of pressure on a-Si is strongly dependent on the particular type of defect, and they further may provide new ways to experimentally determine the dominant defect type. We also report on the effect of pressure on the simplest reaction in a-Si: a bond switch between two neighboring Si atoms, which could be an important element in the understanding of the SWE [1]. [1] L.K. Wagner and J.C. Grossman. PRL (in press) [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P21.00010: Ab initio calculations of the dielectric functions of semiconductors and alloys including excitonic effect via LASTO method Hyejung Kim, Yia-Chung Chang We calculate dielectric functions of semiconductors and alloys including the electron-hole interactions within the ab initio framework. The Bethe-Salpeter equation is constructed using a full-potential linear augmented-Slater-type orbital (LASTO) method. The electron-hole interaction is computed with a sufficiently dense k-point mesh, which shows good convergence. Point group symmetry has been utilized to speed up the computation significantly. Dielectric functions of alloys are calculated by the configurational average of special quasirandom structures. The inclusion of the electron-hole interaction both shifts the peak positions and changes peak heights of the imaginary part of the dielectric functions, resulting in better agreement with ellipsometry data than the spectra obtained without including the electron-hole interactions. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P21.00011: The Electronic and Optical Properties of Manganese-doped Wurtzite ZnO Yiming Mi, Xinxin Zhao, Shuichi Iwata The electronic and optical properties of Manganese-doped Wurtzite ZnO were studied by the first principles pseudopotential plane wave method within density functional theory formalism. The electronic structures, density of states, and optical absorption spectra were investigated for different doping concentration. The acquired results reveal that the energy gap of the Mn-doped ZnO increases with the increase of Mn-doping concentration, and the UV absorption of the system gets stronger with the Mn-doping concentration augmented, which are consistent with others' calculational and experimental results fairly well. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P21.00012: Electronic Structures of Zn(Te,O) Byounghak Lee, Lin-Wang Wang It was recently observed that highly mismatched semiconductor alloys display multiple band gaps of solar spectra, opening a possibility of higher efficiency photovoltaic materials in bulk systems [1]. The highest solar radiation conversion efficiency in these materials is expected to reach 56{\%}, almost twice as high as the best existing single crystal material. The existing empirical model explains observed band structures [2], but it lacks details of atomic level information and cannot explain the constituent atomic composition dependence. We present a theoretical study of Zn(Te,O) using newly developed Linear Scaling 3-Dimensional Fragment method [3] to answer technologically important questions, such as existence of nonradiative recombination centers, charge transfer, and wavefunction localization. [1] K. M. Yu, W. Walukiewicz, J. Wu, W. Shan, J. W. Beeman, M. A. Scarpulla, O. D. Dubon, and P. Becla, Phys. Rev. Lett. 91, 246403 (2003). [2] W. Shan et al. Phys. Rev. Lett. 82, 1221 (1999). [3] L.-W. Wang, Z. Zhao, and J. Mexa, Phys. Rev. B 77, 165113 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P21.00013: Dielectric Function and Critical Point of GeSbTe Pseudo-binary Compound Thin Films Hosun Lee, Jun-Woo Park, Youn-Seon Kang, Tae-Yon Lee, Dong-Seok Suh, Ki-Joon Kim, Cheol Kyu Kim, Yoon Ho Kang, Juarez L. F. Da Silva We measure the dielectric functions of GeSbTe pseudo-binary thin films by using spectroscopic ellipsometry. We anneal the thin films at various temperatures. According to x-ray diffraction, the as-grown thin films are amorphous and the annealed films have metastable and stable crystalline phases. By using standard critical point model, we obtain the accurate values of the energy gap of the amorphous phase as well as the critical point energies of the crystalline thin films. The critical point energies are compared to the band gap energies determined by the method of linear extrapolation of the optical absorption. As the Sb to Ge atomic ratio increases, the optical (band) gap energy of amorphous (crystalline) phase decreases. Standard critical point fitting show several higher band gaps. The electronic band structures and the dielectric functions of the thin films are calculated by using density functional theory and are compared to the measured ones. The band structure calculations show in stable phase that GeTe, Ge$_{2}$Sb$_{2}$Te$_{5}$, and Ge$_{1}$Sb$_{2}$Te$_{4}$ have indirect gap whereas Ge$_{1}$Sb$_{4}$Te$_{7}$ and Sb$_{2}$Te$_{3}$ have direct gap. The measured indirect band gap energies match well with the electronic band structure calculations. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P21.00014: Origin of the unusually large band gap bowing and the breakdown of the band-edge distribution rule in the SnxGe1-x alloys Wan-Jian Yin, Xin-Gao Gong, Su-Huai Wei Most semiconductor alloy AxB1-x has a non-linear dependence of its band gap Eg(x) as a function of the alloy composition x, and the variation is usually described by a parabolic function Eg(x) = xEg$^{A }$+ (1-x)Eg$^{B}$ - bgx(1-x), where Eg$^{A }$and Eg$^{B}$ are the band gaps of A and B at their respective equilibrium lattice constants and bg is the so-called bowing parameter. The conventional band-edge distribution of bg is usually described by the equation bVBM(CBM) = $\Delta $EVBM(CBM)/$\Delta $Egbg , where $\Delta $EVBM(CBM) and $\Delta $EVBM(CBM) are VBM and CBM natural band offsets. Using first-principles calculations, we investigate the unusual nonlinear behaviors of the band gaps in SnxGe1-x alloys. We show that the large bowing of the direct band gap is induced by the disordering effect. Moreover, we calculated individual contribution of the band edge states and find that the bowing of the conduction band edge is much larger than the bowing of the valence band edge, although the natural valence band offset between Ge and Sn is larger than the natural conduction band offset. The breakdown of the band-edge distribution rule is explained by the large lattice mismatch between Ge and Sn and the large deformation potential of the band edge states. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P21.00015: Accurate electronic structure calculations of lead chalcogenides by QSGW method Sergey Faleev, Oleg Mryasov Enhancement of the energy conversion efficiency of thermoelectric materials has been a long goal of materials physics. Recent experiments show that distortion of the electronic structure of PbTe by adding small amount of impurities results in enhancement of the Seebeck coefficient and doubling of the ZT factor [Heremans et al., Science 321, 554 (2008)]. This enhancement is thought to be due to the impurity-induced increase of the energy dependence of density of states near the Fermi level. The reliable theoretical prediction of the energy states of impurities in host matrix (which often very difficult to measure experimentally) are required in order to explain the experiments and predict and optimize properties of new materials. Accurate calculations for bulk system is a first necessary step required for further study of systems with impurities. Here we report results for bulk lead chalcogenides obtained with recently developed QSGW method [Faleev et al., PRL 93, 126406 (2004)]. We found that electronic structure of lead chalcogenides and, in particular, the band gaps and effective masses are predicted with much higher accuracy within the QSGW approach than within most commonly used DFT theory. This result opens way for predictive search of novel thermoelectric materials. [Preview Abstract] |
Session P22: Focus Session: Spin Hall and other Spin-Orbit Effects in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Jairo Sinova, Texas A and M University
Room: 324
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P22.00001: Beller Lectureship Talk: Spin Hall Effect Invited Speaker: The Spin Hall Effect (SHE) and related transport phenomena originating from the coupling of the charge and spin currents due to spin-orbit interaction were predicted [1] in 1971. Following the suggestion in [2], the first experiments in this domain were done at Ioffe Institute in Saint Petersburg [3], providing the first observation of what is now called the Inverse Spin Hall Effect. As to the SHE itself, it had to wait for 33 years before it was experimentally discovered by two groups [4] in Santa Barbara (US) and in Cambridge(UK). The phenomenon consists in spin accumulation at the lateral boundaries of a current-carrying non-magnetic conductor, the spin directions being opposite at the opposing boundaries. The boundary spin polarization is proportional to the current and changes sign when the direction of the current is reversed. It exists in relatively wide \textit{spin layers} determined by the spin diffusion length, typically on the order of 1 $\mu $m. I will discuss the phenomenology of spin-charge coupling, the underlying microscopic mechanisms, and the existing experimental results obtained in semiconductors and in metals at cryogenic, as well as at room temperatures. I will also address a related, but as yet unknown phenomenon, the \textit{swapping} of spin currents, which is due to the correlation between spin rotation during a scattering event and the direction of scattering. \\[4pt] [1] M.I. Dyakonov and V.I. Perel, JETP Lett. \textbf{13}, 467 (1971); , Phys. Lett. \textbf{A35}, 459 (1971)\\[0pt] [2] N.S. Averkiev and M.I. Dyakonov, JETP Lett. \textbf{35}, 196 (1983)\\[0pt] [3] A.A. Bakun et al., JETP Lett. \textbf{40}, 1293 (1984)\\[0pt] [4] Y.K. Kato et al., Science \textbf{306}, 1910 (2004); J. Wunderlich et al., Phys. Rev. Lett. \textbf{94}, 047204 (2005) [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P22.00002: Spin Hall frequency doubling and spin memristive effects Yuriy V. Pershin, Massimiliano Di Ventra It is shown that when a time dependent voltage is applied to a system with inhomogeneous electron density in the direction perpendicular to main current flow, the spin Hall effect results in a transverse voltage containing a double frequency component. We demonstrate that there is a phase shift between applied and transverse voltage oscillations, related to memristive behavior of semiconductor spintronics systems. It is interesting that spin memristive effects in this system are manifested directly in the voltage response. A different method to achieve the second harmonic generation, based on the inverse spin Hall effect, is also discussed. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P22.00003: Enhanced Spin Hall Effect by Single Antidot Potential Mikio Eto, Tomohiro Yokoyama We theoretically investigate an extrinsic spin Hall effect in semiconductor heterostructures due to the scattering by an artificial potential created by a single antidot, STM tip, etc. The strength of the potential is electrically tunable. First, we formulate the spin Hall effect in terms of phase shifts in the partial wave expansion for two-dimensional electron gas. For scattered electrons in $\theta$ direction, we obtain a spin polarization $P(\theta)$ perpendicular to the two-dimensional plane [$P(-\theta)=-P (\theta)$]. The spin polarization $P(\theta)$ is significantly enhanced by an attractive potential when the resonant condition of a partial wave is satisfied by tuning the potential strength. Second, we study the spin Hall effect in a three-terminal device with an antidot at the junction. The conductance and spin polarization are evaluated numerically.\footnote{M.\ Yamamoto and B.\ Kramer, J.\ Appl.\ Phys.\ {\bf 103}, 123703 (2008), for repulsive potential.} We obtain a spin polarization of more than 50\% due to the resonant scattering when the attractive potential is properly tuned. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P22.00004: Competiting interplay between Rashba and cubic-k Dresselhaus spin-orbit interaction in spin Hall effect C.S. Chu, R.S. Chang, A.G. Mal'shukov We study the interplay between the Rashba and cubic-$k$ Dresselhaus spin-orbit interactions (SOI) in a diffusive two-dimensional electron gas (2DEG). Within the spin Hall configuration, we perform a systematic calculation of the spin accumulation $S_{z}$ and the spin polarizations $S_{i}^{B}$ at the lateral edges and in the bulk of the 2DEG, respectively. Both the relative coupling strength of the Rashba and the Dresselhaus SOI, and the electron densities are varied. The spin accumulation exhibits strong competiting features, including in the Dresselhaus-dominant regime the sign change in $S_{z}$ when electron density is large enough, and in the Rashba-dominant regime the complete suppression of $S_{z}$. Most surprisingly is our finding that the Rashba-dominant regime occurs as early as $\alpha\approx 2\tilde{\beta}$, where $\alpha$, $\tilde{\beta}$ are the Rashba and the effective linear-$k$ Dresselhaus SOI coupling constant, respectively. Similar Rashba-dominant regime is found in the spin polarizations, when $\alpha\geq \tilde{\beta} $. Our results point out that decreasing $|\alpha|$ leads to the restoration of the spin accumulation. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P22.00005: Monte Carlo Simulation of Spin-Injection Hall Effect Liviu P. Z\^arbo, Jairo Sinova, J\"org Wunderlich, Toma\v s Jungwirth, Shou-Cheng Zhang The spin-injection Hall effect,which is the newest addition to the spintronic Hall effect family, consists in the transversal deflection of a charge spin-polarized current injected in a spin-orbit coupled semiconductor channel which results in transverse Hall voltage whose magnitude varies along the channel direction. Just as in the case of spin Hall effect, the phenomenon is due to both intrinsic and extrinsic (impurity driven) spin-orbit scattering. We develop a semiclassical spin-dependent Monte Carlo simulation technique which enables us to quantitatively explain the mechanisms of spin-injection Hall effect in experimentally relevant systems. This is achieved by incorporating both intrinsic and extrinsic contributions to anomalous Hall effect (AHE) which are rigorously derived within the recently developed gauge invariant semiclassical theory of AHE. The advantage of this approach over a fully quantum mechanical treatment is that it enables us to investigate the spin-injection Hall effect in micrometer-size devices while still retaining the essential physics. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P22.00006: Calculations of Spin-orbit Splittings in Two-Dimensional Heterostructures Marta Prada, Mark Friesen, Robert Joynt We present calculations of the of the wave-vector dependent spin-orbit level splittings in GaAs/InGaAs and Si/SiGe quantum wells. We use both an effective mass approach and a numerical tight-binding approach (NEMO-3D) that includes the effects of the interfaces on atomic scales. We are able to separate the Rashba and Dresselhaus contributions. The calculations are done as a function of applied electric field and well width. We find good agreement of theory and experiment for the measurements of L. Meier {\it{et al.}}, (Nature Physics \textbf{3}, 650 (2007)) on GaAs/InGaAs. In Si/SiGe wells, we find significant valley- spin-orbit mixing and also that the Dresselhaus term is substantial, and can even be larger than the Rashba term for realistic parameters. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P22.00007: Coherent ultrafast spin flip in a 2D electron gas Carey Phelps, Timothy Sweeney, Hailin Wang We report the experimental demonstration of ultrafast electron spin flip in a modulation doped CdTe quantum well. Complete spin flip is realized with an off-resonant laser pulse of 2 ps in duration. The effective pi-pulse flips the electron spins with respect to an axis that is orthogonal to both the external magnetic field (Voigt geometry) and the sample growth axis. The realization of the ultrafast pi-pulse opens up a new avenue for protecting electron spins from decoherence with dynamical decoupling. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P22.00008: Analysis of Electron Spin Relaxation Momentum Time in Narrow Gap Semiconductor Quantum Well and Dots: Including Rashba and Dresselhaus Effects Yung-Sheng Huang, Jung-Sheng Huang A model of GaAs quantum dots embedded in a quantum wire is studied. We want to investigate how the electron spin relaxation momentum time (SRT) is varying with some physical parameters. We find that SRT decreases while the four parameters, external magnetic field, surrounding temperatures, both quantum wire width and thickness increase. The reason is caused by more and more phonons resulted in a higher scattering probability between electrons and phonons. Thus the SRT is reduced. Besides, Lommer and Silva showed that in narrow gap semiconductor bulk materials, the Rashba effect is larger than Dresselhaus effect. Our results show that Dresshaus effect is larger than Rashba effect for the quantum well under electric field, especially when the quantum well width is small. The authors are interested in studying whether the same characteristics exist in quantum dots. We are working on this line. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P22.00009: Spin accumulation in a Rashba-type two-dimensional electron gas due to a nonuniform driving electric field Lu-Yao Wang, Chon-Saar Chu, Mal'shukov Anatoly It is well understood that a Rashba-type two-dimensional electron gas (2DEG) does not support spin accumulation, or spin Hall effect, in the diffusive regime when the driving electric field is uniform. In this work we address the issue about a possible restoration of the spin Hall effect when the driving field is nonuniform. Toward this end, we consider the spin accumulation in the vicinity of a circular hole, with radius $R\sim l_{so}$, where the driving field becomes nonuniform. Here $l_{so}$ is the spin relaxation length, and $l_{so} \quad >> l_{e}$, the electron mean free path. Our result shows that the nonuniform driving field gives rise to nonuniform in-plane spin densities S$_{x}$ and S$_{y}$, which in turn contribute to a finite spin current via the combined processes of spin diffusion and spin-precession. The spin accumulation thus obtained is proportional to the Rashba coupling constant $\alpha $, and its spatial pattern is one of spin-dipole form, aligned perpendicular to the driving field. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P22.00010: Scattering approach in calculating Rashba Spin-orbit coupling in asymmetric Chih-Piao Chuu, Qian Niu The Rashba Spin-orbit coupling plays a crucial role in spin manipulation in semiconductor heterostructures. We study the underlying physics through scattering approach with the Kane model. Several physical parameters are considered, including potential barrier asymmetries, quantum well inclination, as well as band structure parameters. This may provide a better understanding in designing spintronic devices. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P22.00011: 2D Holstein polarons in the presence of spin-orbit interactions Lucian Covaci, Mona Berciu The electron-phonon interaction in the presence of spin-orbit interactions (of either Rashba or Dresselhaus type) must be taken in account for GaAs quantum dots or for spintronic devices. The possibility of tuning the electron-phonon interaction by coupling to a substrate (e.g. in organic transistors) requires an accurate treatment of this problem in all coupling regimes. We apply a recently developed approximation (the Momentum Average Approximation) to this specific theoretical question. We have shown that this method is exact in various asymptotic regimes while being accurate for all coupling strengths. We calculate the self-energy at the MA(2) level of the approximation. From ground state properties (energy and effective mass) we conclude that in the presence of spin-orbit interactions, the polaron is harder to trap -- the crossover from large to small polarons is shifted to higher couplings. From the spectral function, we show that there are two distinct regimes, depending on relation between the phonon frequency and the strength of the spin-orbit interaction. When the latter is larger we find that the polaron character is dominated by only one band (the `-' band). We also show that the off-diagonal part of the self-energy plays an essential role in obtaining the polaron + one phonon continuum correctly. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P22.00012: Full spin control in 2DEGs with no magnetic fields B.J. Moehlmann, M. E. Flatt\'e A properly chosen closed spin transport path in the plane of a III-V semiconductor quantum well suffices for arbitrary spin manipulation of conduction electrons about any desired axis. This feature of spin transport relies on the non-commutativity of the precession matrices associated with non-colinear path segments. The electron spin rotation depends solely on the path geometry, not the speed of the spin along the path. Simple closed paths have been found which will perform arbitrary spin rotations along arbitrary axes with no net spatial displacement of the spins. The paths differ depending on the form of the internal effective magnetic fields induced by crystal asymmetry, growth asymmetry, and strain and electric fields. This work was supported by an ONR MURI. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P22.00013: Many-body effects on $\rho_{xx}$ Ringlike Structures via SDFT Gerson J. Ferreira, Henrique J. P. Freire, J. Carlos Egues In the quantum Hall regime, the longitudinal resistivity $\rho_{xx}$ plotted in a density--magnetic-field diagram displays ringlike structures due to the crossings of spin split Landau levels of distinct subbands. We theoretically investigated the dependence of the magnetoresistance on the magnetic field tilt angle and on the temperature using Spin Density Functional Theory (SDFT). Assuming a temperature dependence of the Landau levels broadenings, we show that the ringlike structures are broken at sufficiently low temperatures due to a ferromagnetic quantum phase transition. Additionally, for tilt magnetic field, the momentum in the growth direction ($z$) also couples to the magnetic field $zP_x$ coupling, thus giving rise to anticrossings between consecutive Landau levels and subbands, collapsing the ring with increasing tilt angle. We find that the interplay of these anticrossings and many-body interactions (via SDFT) leads to a reduced $zP_x$ coupling, increasing the collapsing angle at which the ring fully disappears. Our results explain some of the physical mechanisms behind ring formation and collapse which have been experimentally observed. [Preview Abstract] |
Session P23: Lattice and Mechanical Properties
Sponsoring Units: DCMPChair: Daniel Finkenstadt, University of Illinois at Urbana-Champaign
Room: 325
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P23.00001: Temperature dependence of the phonon density of states in FeSi and CoSi. Olivier Delaire, Matthew Lucas, Matthew Stone, Douglas Abernathy The phonon density of states (DOS) of the B20 compounds Fe(1-x)Co(x)-Si (x=0.0, 0.03, 0.5, 1.0) was measured as function of temperature from 10K to 773K using inelastic neutron scattering. The phonon DOS of FeSi exhibits an excess softening compared to the predictions of the quasiharmonic model, in agreement with previous measurements of elastic constants as function of temperature [1]. The phonon DOS of CoSi softens less, on the other hand, and appears in better agreement with the pure volume effect of the quasiharmonic model. These trends are compared to previous measurements of the temperature dependence of the phonon DOS in the A15 compounds V3Si and V3Ge [2]. Using first-principles electronic structure calculations, the observed anomalies are related to the details of the band structure in these compounds. It is shown that sharp features in proximity to the Fermi level lead to anomalous phonons through a sensitivity to thermal disorder, or adiabatic electron-phonon coupling. [1] D. Mandrus et al., Phys. Rev. B 51, 4763 (1994) [2] O. Delaire et al. , Phys. Rev. Lett. 101, 105504 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P23.00002: Effects of temperature and chemical order on phonons in Fe-V alloys Matthew Lucas, Jorge Munoz, Olivier Delaire, Brent Fultz, Douglas Abernathy, Matthew Stone, Mark Loguillo Inelastic neutron-scattering spectra were measured on body-centered-cubic Fe-V alloys as a function of temperature and composition. These data were reduced from time-of-flight histograms to spectra that resemble the phonon density of states (DOS), but were distorted by differences in efficiencies of the atom species for phonon scattering. Nuclear resonant inelastic x-ray scattering spectra were measured for the 57-Fe isotope in a similar set of alloys at room temperature to compliment the neutron spectra. With temperature the 50-50 alloy undergoes an ordering transition from A2 to B2, as evidenced by increasing intensity in the superlattice peaks from the elastic regime of the neutron spectra. This ordering is accompanied by a change in the phonon DOS. The Connolly-Williams cluster inversion method is performed on the DOS of the disordered Fe-V alloys in order to correlate changes in the chemical order with changes in the DOS for the 50-50 alloy. The temperature dependence of the DOS of the disordered alloys is used to determine the anharmonic phonon entropy, and the chemical dependence to determine the phonon entropy of mixing. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P23.00003: Ambipolar diffusion and recombination of photoexcited carriers in bismuth films Yu-Miin Sheu, Yi-Jiunn Chien, Ctirad Uher, Stephen Fahy, David Reis Recent experimental and theoretical studies on bismuth show that intense ultrafast photoexcitation leads to a large- amplitude, softened coherent A1g phonon. Thus, the subsequent dynamics of the photoexcited carriers will strongly influence the dynamics of the phonon. However, little is known about the nonequilibrium carrier dynamics due to difficulty in separating carrier relaxation and other processes. Here we report ultrafast counter propagating optical pump-probe experiments, measuring photoexcited carrier transport across optically thick single crystal bismuth films at room temperature. The films are grown on transparent sapphire substrates with thicknesses varying between 185 and 385 nm, sufficient to separate the carrier dynamics from the effects of lattice heating and strain, when pumped and probed on opposite faces. The measured recombination time is about 14-30 ps and ambipolar diffusivity between 22 and 28 cm$^2$/s for the different films. The carrier recombination time is much longer than the A1g phonon period, supporting a two chemical potential model for the photoexcited electronic system and phonon dynamics, in which carrier diffusion (rather than electron-hole plasma cooling or recombination substantially reduces the carrier density over the lifetime of the phonon, leading to a chirped mode. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P23.00004: Magnetotransporet in ultra quantum Bismuth and related alloys Doron Bergman, Karyn Le Hur Recent studies of elemental Bismuth and related alloys in strong magnetic fields, have uncovered peculiar electric and thermal transport behavior (Behnia et al. Science 317, 1729 (2007), Banerjee PRB 78, 161103 (2008) ). In particular, the Hall resistivity resembles that of the fractional quantum Hall effect, in exhibiting quasi-plateaus, corresponding to fractional filling factors. At the same time anomolous features appear in the Nernst and Seebeck coefficients. Recent efforts to address possible interaction effects (Burnell et al., Alicea et al. preprints 2008), while suggesting interesting electronic states of Bismuth in this ultra quantum regime, have not explained the unusual transport phenomena. We investigate the transport phenomena in more detail, by using a microscropic model of the Bismuth band structure. We first explore Landau level physics in this model, and then go on to develop a theory of the anomoulous transport phenomena, using Boltzmann kinetic theory. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P23.00005: Angular dependent Nernst effect in Bi$_{x}$Sb$_{1-x }$ across the quantum limit Kamran Behnia, Zengwei Zhu, Benoit Fauqu\'e, Aritra Banerjee, Bertrand Lenoir The Fermi surface of bismuth occupies a tiny (10$^{-5})$ fraction of the Brillouin zone. Therefore, a field of 9 T oriented along the trigonal axis pushes the electrons to their lowest Landau level. Alloying bismuth with antimony reduces the carrier density and lowers this threshold field known as the quantum limit. Approaching this limit, the Nernst-Ettingshausen effect was found to display giant quantum oscillations whose magnitude is yet to be understood. The Nernst response presents sharp peaks each time a Landau level of hole-like quasi-particles meets the chemical potential. In addition to these peaks, a number of anomalies of unidentified origin were detected. Here, we present the first study of Nernst effect as a function of a rotating magnetic field in Bi$_{x}$Sb$_{1-x}$ up to 12 T. The results highlight the role played by the Dirac quasi-particles of the electron pockets in the generation of the unidentified anomalies. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P23.00006: Thermal transport properties of two-dimensional Dirac fermion Yousef Romiah, Xin-Zhong Yan, Chin-Sen Ting The self consistent Born approximation is utilized to obtain an expression for the electric and thermal currents needed for the calculation of thermal transport properties, including the Seebeck coefficient and the thermal conductivity. The validity of the Wiedemann-Franz law is checked. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P23.00007: Grain coarsening in crystals from evolution of dislocation densities : Results from a continuum theory of dislocation dynamics Woosong Choi, Yong Chen, Stefanos Papanikolaou, James Sethna, Surachate Limkumnerd Continuum theories of grain growth and coalescence dynamics currently use phase-field and other models without direct connection to the underlying dislocations which form the polycrystal grain boundaries. We extend a recently proposed wall-forming continuum dislocation dynamics theory\footnote{S. Limkumnerd and J. P. Sethna, Phys. Rev. Letters \textbf{96}, 095503 (2006)} to incorporate dislocation line tension energy, and explore the resulting coarsening dynamics in two dimensions. We report initial results both on scaling behavior, coarsening and coalescence mechanisms emerging from our theory, and compare to experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P23.00008: Smooth versus jerky motion of packets of dislocations across fields of obstacles. Catalin Picu, Renge Li We report on the transition from smooth (``unzipping'') to jerky motion of multiple interacting dislocations (elastic manifolds) moving across a field of randomly located obstacles under constant applied stress. The transition is controlled by the stress, the obstacle strength and distribution. The system exhibits spatial and temporal correlations (intermittency) similar to those observed experimentally at much larger scale in dislocation avalanches. Power law distributions of jump amplitudes and separation times emerge. Comparison of the simulation results with experimental data indicates that the jerky motion is more relevant for plastic deformation of real crystals than unzipping. The strain rate sensitivity parameter, $m$, decreases sharply when the system enters the jerky mode and becomes independent of the obstacle strength, presence of obstacles of various strengths and the way those are mixed, and of temperature. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P23.00009: Molecular-dynamics analysis of the mechanical behavior of face-centered cubic metallic ultrathin films Kedarnath Kolluri, M. Rauf Gungor, Dimitrios Maroudas We report results of large-scale molecular-dynamics simulations for the dynamic deformation under biaxial tensile strain of nanometer-scale-thick films of various face-centered cubic metals. Our results indicate that films of metals with moderate to high propensity for formation of stacking faults (e.g., Ni and Cu) exhibit an extended easy glide regime followed by a sharp increase in the material stress, whereas those with low propensity for stacking-fault formation (such as Al) exhibit a monotonic increase in the stress during dynamic loading. We find that the plastic strain rate in Cu and Ni thin films is far greater than that in Al thin films, leading to stress dissipation and an extended easy glide regime. Analysis of defect interaction mechanisms during dynamic deformation reveals dislocation annihilation, which is due to stacking-fault-mediated cross-slip mechanisms in Ni and Cu films and due to collinear interactions between dislocations in Al films. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P23.00010: Composition dependence of the elastic constants of $\beta $ and ($\alpha $~+~$\beta {\rm p}$-phase PdH$_{x}$ Douglas Safarik, Ricardo Schwarz, Stephen Paglieri, Dale Tuggle, Robert Quintana Previously [1], we measured the room-temperature elastic constants of
PdH$_{x}$ for 0~$<$~$x$~$<$~0.75. These measurements were done on single
crystals of $\alpha $-phase Pd(H) solid solution ($x$~$<$~0.01), of $\beta
$-phase Pd-H hydride ($x$~$>$~0.62), and of coherent two-phase mixtures of
$\alpha +\beta $ phases (0.01$ |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P23.00011: Adhesion Enhancement by Interfacial Microcrack Toughening Yue Qi, Haibo Guo, Xingcheng Xiao, Zhihui Xu, Xiaodong Li In this study, we reported a novel approach to enhance the adhesion of diamond coatings on titanium substrate by interfacial toughening. An array of oriented and confined micro-cracks around the interface was found to have the ability of opening and self-healing to release strain energy, by which to enhance macro-adhesion. Density functional theory calculations explained that cracks are energetically preferred to initiate and propagate along the (100) plane in titanium carbide interlayer. Thus by controlling the orientation of the interlayer for the failure associated with the delaminatation can be avoided for the coating/interlayer/substrate system. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P23.00012: Oxide-Dispersion Strengthened Nanoparticulate Composites with Application to Magnetic Materials Robert Cammarata, Stephen Farias, Chai-Ling Chien Metal matrix nanocomposites have been fabricated by a novel electrochemical deposition method in order to produce enhanced yield strength and creep resistant materials. Metals have been synthesized from an electrolytic solution containing a suspension of oxide nanoparticles. Using a rotating disk electrode, metal samples with a uniform dispersion of oxide nanoparticles are obtained. By controlling the concentration of particles in the solution, the electrode rotation rate, and deposition current density, the volume fraction of oxide in the nanocomposite can be sensitively controlled. Low load indentation testing reveals a substantial increase in room temperature yield strength compared to single phase metals that is close to that predicted from classical hardening models. Particular attention has given to magnetic materials such as Ni and FeCo with the aim of producing materials with improved mechanical behavior without significant degradation of the magnetic properties. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P23.00013: High pressure thermoelasticity of vanadium Daniel Orlikowski To support a larger effort for a multi-phase constitutive strength model for vanadium, we discuss the calculations performed to determine the anharmonic thermoelasticity for bcc and rhombohedral phases of vanadium. In this investigation, we have performed extensive calculations of the elastic moduli over broad ranges of temperature ($<$10,000 K) and pressure ($<$3 Mbar), accounting for both the electron-thermal and ion-thermal contributions. Using density functional theory (DFT) with the projector augmented-wave (PAW) methodology to calculate the electron-thermal component, we have combined this with the ion-thermal component, which is calculated from Monte Carlo (MC) canonical distribution averages of the strain derivatives on a multi-ion potential itself. The ion-potential is described through a many-body, quantum-based interatomic potential---the model generalized pseudopotential theory (MGPT). We suggest regions of stability for the rhombohedral structure in the phase diagram. The resulting elastic moduli are compared to available experimental results and to sound speeds measured along the Hugoniot. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P23.00014: Shock-wave dispersion and attenuation in discrete media, effect on source localization. Hasson Tavossi Shock wave speed and attenuation in a non-linear discrete media are investigated. The goal of the study is to identify parameters that control shock wave or impulsive wave speed and energy dissipation in the discrete non homogeneous binary media. Material properties of solid constituents and the elastic behavior of contact points are shown to depend on the wave frequency. The behavior at both high frequency and low frequency limits are analyzed. The effects of depth on wave velocity profile, wave spectral content and attenuation are also considered. Among applications are; accurate near ground shock wave source localization by microseism waves. [Preview Abstract] |
Session P24: Focus Session: Electron Transport in Nanotubes
Sponsoring Units: DMPChair: Marco Buongiorno Nardelli, North Carolina State University
Room: 326
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P24.00001: Coupling of spin and orbital motion of electrons in carbon nanotubes Invited Speaker: Electrons in atoms possess both spin and orbital degrees of freedom. In non-relativistic quantum mechanics, these are independent, resulting in large degeneracies in atomic spectra. However, relativistic effects couple the spin and orbital motion, leading to the well-known fine structure in atomic spectra. The electronic states in defect-free carbon nanotubes are widely believed to be four-fold degenerate, owing to independent spin and orbital symmetries, and also to possess electron--hole symmetry. Here we report measurements demonstrating that the spin and orbital motion of electrons are coupled, thereby breaking all of these symmetries. This spin--orbit coupling is directly observed as a splitting of the four-fold degeneracy of a single electron in ultra-clean quantum dots. The coupling favors parallel alignment of the orbital and spin magnetic moments for electrons and antiparallel alignment for holes. Our measurements are consistent with recent theories that predict the existence of spin--orbit coupling in curved graphene and describe it as a spin-dependent topological phase in nanotubes. Work done in collaboration with F. Kuemmeth, S. Ilani, and D. C. Ralph [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P24.00002: Sensitivity of carbon nanotube and graphene transistors to local ionic structure Iddo Heller, Sohail Chatoor, Jaan Mannik, Marcel A. G. Zevenbergen, Cees Dekker, Serge G. Lemay Transistors based on single-walled carbon nanotubes (SWNTs) and graphene can be operated in aqueous solution where the electrolyte acts as a highly efficient gate. We show that the composition and spatial distribution of ions in the electrolyte intricately affect the conductance of SWNT and graphene transistors. Changes in the ionic strength, pH, and surprisingly, the type of ions affect the electronic transport through the electrostatic gating effect. In addition, changing pH leads to Schottky-barrier modification, while changing ionic strength affects the gate capacitance. Interestingly, the observed electrostatic gating effect for graphene is larger than for SWNTs. Most of our data is explained by a model that considers ionizable groups on both the underlying substrate and on the carbon surfaces. Our findings have significant implications for optimizing sensing experiments with nanocarbon transistors. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P24.00003: Boosting electronic transport in carbon nanotubes by isotopic disorder Michele Lazzeri, Niels Vandecasteele, Francesco Mauri The current/voltage curve of metallic carbon nanotubes (CNTs) displays at high bias a sudden increase of the resistivity due to the scattering of electrons with phonons [1] having an anomalously-high population (hot phonons) [2,3]. Indeed, the rate at which hot-phonons are excited by the electrons is faster than the rate at which they are deexcited. Here, we show that it is possible to improve the electrical performances of metallic CNTs by 13C isotope enrichment. In fact, isotopic disorder creates additional channels for the hot-phonon deexcitation, reduces their population and, thus, the nanotube high-bias differential-resistance. This is an extraordinary case where disorder improves the electronic transport, with important technological consequences in view of the use of metallic CNTs as interconnects in future electronic devices. [1] Z. Yao et al., Phys. Rev. Lett. 84, 2941 (2000). [2] M. Lazzeri et al., Phys. Rev. Lett. 95, 236802 (2005); M. Lazzeri and F.Mauri, Phys. Rev. B 73, 165419 (2006). [3] M. Oron-Carl and R. Krupke, Phys. Rev. Lett. 100, 127401 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P24.00004: Electrical Transport in Long Bundles of Carbon Nanotube-Metal Hybrids Saikat Talapatra, Rakesh Shah, Clayton Schenk, Xianfeng Zhang, Swastik Kar Although a number of works have proposed that bundles of carbon nanotubes can withstand high current densities at low resistances for high-performance applications, such structures have been demonstrated to fall short of proposed expectations. This is chiefly due to limited access to all nanotubes in a bundle in conventional two-terminal device configurations, with low number of effective conducting channels. By depositing a small quantity of high-conductance metal alloys that wet the nanotube surface, our CNT-Au/Pd alloy hybrid conductors show improved performance in terms of failure current density and resistivity. Low temperature transport measurements show that the nanotube bundles with metal coating and especially after the high-bias treatment show more and more metallic nature, with decreased negative temperature coefficient of resistance. The results obtained will be discussed in the framework of transport theories of quasi-one dimensional systems. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P24.00005: Phonon populations in a biased carbon nanotube transistor Mathias Steiner, Marcus Freitag, Vasili Perebeinos, James Tsang, Joshua Small, Megumi Kinoshita, Dongning Yuan, Jie Liu, Phaedon Avouris We present a comprehensive picture of the phonon populations in an electrically-driven carbon nanotube transistor, including the Raman-active G and radial breathing modes (RBM), and also the Raman-inactive zone boundary mode (K), and intermediate-frequency mode (IFP), populated by anharmonic decay. The effective temperature of the RBM is considerably lower than the intermediate- and high-frequency mode temperatures, which we explain by a phonon-decay bottleneck. We include substrate polar phonon scattering to fully account for the device electronic characteristics. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P24.00006: Electronic and thermal transport in carbon nanostructures: the role of low-frequency modes Nicola Bonini, Nicola Marzari Low-frequency phonon modes play an important role in the electronic and thermal transport properties of carbon nanotubes and ultrathin graphitic films. Not only they determine the very high thermal conductivity of these materials, but they also affect the electrical transport: at low bias they weakly scatter electrons, while at high bias they concur to determine the population of those optical phonon modes that most strongly limit the electrical conductivity. Quite interestingly, these low frequency phonons are also expected to couple to the vibrational modes of a surrounding medium more efficiently than high frequency phonons, providing an effective channel for the exchange of vibrational energy between the nanostructure and the environment. Here we use density functional theory and density functional perturbation theory to characterize the inelastic relaxation mechanisms---phonon-phonon and electron-phonon interactions---that determine the lifetime of these phonon modes. We will discuss the relevance of these results to estimate the transport properties of carbon nanomaterials. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P24.00007: Temperature-dependant current saturation in double-wall carbon nanotubes Delphine Bouilly, Matthieu Paillet, Richard Martel Current saturation is known to occur at high voltage bias in carbon nanotubes, for single-wall as well as multi-wall configurations. This saturation is generally attributed to the backscattering of carriers by optical phonons. Here we report transport measurements performed on single double-walled carbon nanotubes as a function of temperature between 77K and 400K. The good quality of the contacts between the nanotubes and the electrodes allows to observe a temperature dependence in the I-V curves. At high temperature, the saturation current shows a value around 25$\mu $A, as expected from the energy of optical phonons, but then increases non-linearly with decreasing temperature. The low-bias conductance is also measured to increase with decreasing temperature. Phenomenological models are investigated in order to explain the observed trends. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P24.00008: Conductance Switching in Gated Graphene Nanoribbons Jesse M. Kinder, Jonathon J. Dorando, Garnet K.L. Chan We have investigated transport through locally gated metallic graphene nanoribbons using a numerical tight-binding method. We consider a device in which the orientation of the gate with respect to the axis of the ribbon is variable. We find that the conductance, as calculated within the nonequilibrium Green function formalism, depends strongly on the gate voltage and the orientation of the gate. In particular, we identify specific angles at which a small change in gate voltage results in a large change in the probability of transmission. This response occurs in ribbons with zigzag or armchair edges and could provide a mechanism for a nanometer-scale electronic switch. Using the effective Dirac Hamiltonian for electrons in graphene, we provide a qualitative explanation for the ON-OFF response at particular angles and characterize its dependence on the geometry of the ribbon. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P24.00009: Variable Range Hopping Conductivity in Carbon Nanotube Threads. Chaminda Jayashinghe, David Mast, Mark Schulz, Vesselin Shanov We have measured the low temperature, dc electrical transport in threads spun from long multi-wall carbon nanotubes (MWCNT). The electrical transport in these threads shows variable range hopping (VRH) behavior at low temperatures, as well as non-linear IV characteristics at high applied electric fields. The MWCNT used to make the threads have an outer diameter from about 6 nm to 30 nm; these MWCNT's have been grown in lengths up to 18mm. The diameter of the CNT threads in this study have diameters of 15 and 25 microns; the threads were spun using 2mm long MWCNT's. The room temperature (RT) resistivity of the threads is on the order of 5 mOhm cm and can be changed by post-spinning treatment strategies such as high temperature annealing. From 300K down to 4.2K, the resistivities show an exponential dependence with temperature consistent with VRH conduction. As the RT resistivity decreases, the low temperature transport shifts from being dominated, at low temperatures, by Coulomb-Gap VRH described by Efros and Shklovskii [1] to 3D-VRH as first formulated by Mott [2]. Analysis will be given of how the VRH behavior changes with intrinsic and post-treatment thread resistivity. [1] A.L. Efros and B.I. Shklovskii, J. Phys. C: Solid State Physics. \textbf{8}, L49 (1975). [2] N.F. Mott, J. Non-Cryst. Solids\textbf{ 1}, 1 (1968). [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P24.00010: Experimental and computational study of 1/f noise scaling in single-walled carbon nanotube percolation films Ashkan Behnam, Gijs Bosman, Ant Ural We study the scaling of 1/f noise in single-walled carbon nanotube percolation films as a function of device parameters and film resistivity both experimentally and computationally. The results suggest that the noise generated by tube-tube junctions dominates the total 1/f noise in nanotube films and that the noise amplitude depends strongly on device dimensions, nanotube degree of alignment, and the film resistivity, following a power-law relationship with resistivity near the percolation threshold after properly removing the effect of device dimensions. We also find that the critical exponents associated with the noise-resistivity and noise-device dimension relationships are not universal invariants, but rather depend on the specific parameter that causes the change in the resistivity and 1/f noise, and the values of the other device parameters. Since 1/f noise is a more sensitive measure of percolation than resistivity, these results not only provide important fundamental physical insights into the complex interdependencies associated with percolation transport in nanotube films, but also help understand and improve the performance of these nanomaterials in potential device applications, such as nanoscale sensors, where noise is an important figure of merit. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P24.00011: Anisotropic Terahertz Response of Highly Aligned Single-Walled Carbon Nanotubes L. Ren, X. Wang, L. Booshehri, D. Hilton, J. Kono, C. Pint, R. Hauge, D. Rana, K. Takeya, I. Kawayama, M. Tonouchi Dynamic conductivities of degenerate 1-D electrons are expected to provide a wealth of information on quantum confinement, electron interactions, and disorder. Here, we use terahertz time-domain spectroscopy (THz-TDS) to determine the complex dielectric function of a thin film of highly aligned single-walled carbon nanotubes (SWNTs) on sapphire. The THz electromagnetic wave used was linearly polarized, and the measured dielectric function was very anisotropic. As the angle between the nanotube axis and the THz electric filed changed, anisotropy was clearly observed for both the real and imaginary parts of the dielectric function. The absorption of the THz wave decreased monotonically with increasing angle from 0 to 90 degrees, with maximum and minimum absorption at 0 and 90 degrees, respectively. Through a proper model, the complex dynamic conductivity was extracted and showed a non-Drude-like frequency dependence, with the real part increasing monotonically with increasing frequency between 0.2 and 1.8 THz. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P24.00012: Carbon nanotube diode performance and photovoltaic response Daner Abdula, Moonsub Shim Due to their unique electronic properties, carbon nanotubes have been at the forefront in the development of next generation electronic devices. The p-n diode is arguably the most pivotal electronic and photovoltaic device. Up to now, nanotube diodes have had major drawbacks including complex quad-terminal device geometries to achieve electrostatic doping, large series resistances from the inclusion of an intrinsic region at the junction, unstable n-type doping, and Zener breakdown. We have developed a method to create two terminal abrupt junction diodes from single semiconducting carbon nanotubes with simple photo-patterned polymer layers defining air-stable p- and n-regions. These intratube diodes show nearly ideal behavior with relatively low series resistance and no sign of Zener breakdown at room temperature. Spatial doping profiles measured by micro-Raman spectroscopy and selective electrochemical gating of the n-region indicate that diode performance depends strongly on relative doping levels. A short circuit current of 1.4 nA with an open circuit voltage of 205 mV are measured when illuminated to saturation. [Preview Abstract] |
Session P25: Focus Session: Graphene VII: Electronic Properties
Sponsoring Units: DMPChair: Andre Geim, Manchester University
Room: 327
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P25.00001: Cyclotron Resonance at the Dirac Point Paul Cadden-Zimansky, Erik Henriksen, Zhigang Jiang, Li-Chun Tung, Mollie Schwartz, Yong-Jie Wang, Philip Kim, Horst Stormer We present high field infrared spectroscopy data on the n = -1 $\rightarrow$ 0 and n = 0 $\rightarrow$ 1 Landau Level (LL) transitions in graphene. At high magnetic fields, up to 31 T, measurements of single layer graphene show large shifts in the transition energies, which are suggestive of a gap in the n = 0 LL. The magnitude of these shifts are strongly enhanced over the bare Zeeman splitting naively expected for a spin-split state. We compare the field dependence of the shifts with proposed degeneracy-breaking mechanisms at the Dirac point. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P25.00002: Hydrodynamic approach to transport in graphene Rafi Bistritzer, Allan MacDonald Exploiting the strong electron-electron interactions in graphene we construct a hydrodynamic theory in which the carrier dynamics is described by three parameters: the electronic temperature, the chemical potential and the drift velocity. We use this theory to describe both linear and non linear transport in graphene. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 9:00AM |
P25.00003: Graphene Update Invited Speaker: Graphene is now a bright and still rapidly rising star on the horizon of materials science and condensed matter physics, revealing a cornucopia of new physics and potential applications [1]. I will overview our recent experimental work on graphene concentrating on its exotic electronic properties and speculate about potential applications. References [1] For review, see A. K. Geim, K. S. Novoselov. Nature Mater. 6, 183 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P25.00004: Energy Relaxation of Hot Dirac Fermions in Graphene Wang-Kong Tse, Sankar Das Sarma We develop a theory for the energy relaxation of hot Dirac fermions in graphene. We obtain a generic expression for the energy relaxation rate of hot Dirac fermions in graphene due to electron-phonon interaction and calculate the power loss due to both optical and acoustic phonon emission as a function of electron temperature $T_{\mathrm{e}}$ and density $n$. We find an intrinsic power loss weakly dependent on carrier density and non-vanishing at $n = 0$, originating from interband electron-optical phonon scattering from the intrinsic electrons in the graphene valence band. We also obtain the total power loss per carrier to be $\sim 10^{-12}\,-\,10^{-7}\,\mathrm{W}$ within the range of electron temperatures $\sim 20\,-\,1000\,\mathrm{K}$, finding that the temperature for the optical phonon emission to overtake acoustic phonon emission as the dominant energy loss mechanism ranges $\sim 200-300\,\mathrm{K}$ for $n = 10^{11}-10^{13}\,\mathrm{cm}^{-2}$. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P25.00005: Electron and hole puddles in monolayer graphene on SiO$_{2}$ B.J. LeRoy, A. Deshpande, W. Bao, F. Miao, C.N. Lau We have performed spatially resolved scanning tunneling spectroscopy measurements on single layer graphene. The graphene was prepared on SiO$_{2}$ by the mechanical exfoliation technique and an electrode was attached by electron beam lithography. Atomically resolved topography images over 40 nm areas show that the graphene conforms to the SiO$_{2}$ surface as well as having intrinsic ripples. In addition to the topography measurements, we have mapped the local density of states as a function of position and energy. We observe a spatially varying Dirac point which leads to electron and hole puddles at low energy. These puddles have a characteristic size scale of about 5 nm. The puddles arise due to curvature in the graphene film which induces shifts in the chemical potential as well as long range scattering from charged impurities. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 10:00AM |
P25.00006: Ripples on graphene and their effect on lattice and electronic properties Invited Speaker: A discovery of graphene, a new allotrope of carbon [1], representing the simplest, one-atom thick, membrane, opens exciting perspectives in statistical physics of two-dimensional systems in general. As expected from theory of flexible membranes [2], free suspended graphene is corrugated (rippled) due to thermal bending fluctuations, which was confirmed by experiment [3] and atomistic simulations [4]. This makes graphene strongly anharmonic crystal leading to anomalous temperature dependences of its thermal expansion, elastic moduli and other thermodynamic and mechanical properties. The ripples are also a source of pseudomagnetic gauge field [5] acting on Dirac fermions which leads to important consequences for the electronic structure such as a formation of midgap states [6,7]. Quenched ripples can be also important sources of electron scattering limiting charge-carrier mobility in graphene [8]. Possible mechanisms of this quenching are discussed. \\[3pt] [1] K.S. Novoselov et al, Science \textbf{306}, 666 (2004). \\[0pt] [2] \textit{Statistical Mechanics of Membranes and Surfaces}, ed. by D. R. Nelson, T. Piran, and S. Weinberg (World Sci, Singapore 2004). \\[0pt] [3] J.C. Meyer et al, Nature \textbf{446}, 60 (2007). \\[0pt] [4] A. Fasolino, J.H. Los, and M.I. Katsnelson, Nature Mater. \textbf{6}, 858 (2007). \\[0pt] [5] S.V. Morozov et al, Phys. Rev. Lett. \textbf{97}, 016801 (2006). \\[0pt] [6] F. Guinea, M.I. Katsnelson, and M.A.H. Vozmediano, Phys. Rev. B \textbf{77}, 075422 (2008). \\[0pt] [7] T.O. Wehling et al, Europhys. Lett. \textbf{84}, 17003 (2008). \\[0pt] [8] M.I. Katsnelson and A.K. Geim, Phil. Trans. Royal Soc. A \textbf{366}, 195 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P25.00007: Rippling of Graphene Maria Moura, Rebecca Thompson-Flagg, Michael Marder Experiments found that free standing single-layer graphene sheets display ripples (see ref. Meyer et al. Nature 446, 60 2007). Here we show that these ripples can be a consequence of adsorbed molecules sitting on random sites. The adsorbates cause the bonds between the carbon atom to lengthen slightly. Static buckles caused by roughly 20 \% coverage of adsorbates are consistent with experimental observations. We explain why this mechanism is more likely to explain ripples than are thermal fluctuations or the Mermin-Wagner theorem (previously invoked). [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P25.00008: Spectromicroscopy study of surface morphology and quasiparticle dynamics in suspended graphene Kevin Knox, Mehmet Yilmaz, Shancai Wang, Alberto Morgante, Dean Cvetko, Andrea Locatelli, Onur Mentes, Miguel Nino, Philip Kim, Richard Osgood We report angle-resolved photoemission and electron diffraction measurements of single crystal exfoliated graphene obtained at the Nanospectroscopy beamline at the Elettra synchrotron light source. Although typical exfoliated graphene sample sizes prohibit the use of conventional UHV techniques, we have used micro-spot low-energy electron diffraction ($\mu $LEED) and micro-spot angle-resolved photoemission ($\mu $ARPES) to probe this unique 2D system. $\mu $LEED measurements provide information about the surface morphology of monolayer and multilayer graphene sheets, which are not atomically flat, but microscopically corrugated. Our photoemission measurements reveal details of the quasiparticle spectrum in the vicinity of the Fermi level. We will discuss modifications to the bare band dispersion due to electron-electron interactions and departure from the standard Fermi liquid model for quasiparticle lifetime. Results from suspended graphene will be compared to results from samples supported on SiO$_{2}$. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P25.00009: Kohn-Luttinger superconductivity in graphene Jose Gonzalez We address the development of superconductivity in graphene when the Fermi level becomes close to one of the Van Hove singularities of the electron system. The different segments of the Fermi line show then an approximate nesting, which enhances unconventional superconducting and magnetic correlations for a dominant repulsive interaction. The origin of the pairing instability lies in the strong anisotropy of the e-e scattering along the Fermi line, leading to a channel with attractive coupling when making the projection of the BCS vertex on the symmetry modes with nontrivial angular dependence. We show that the superconducting instability is particularly strong at the Van Hove singularity in the valence band of graphene, where the critical scale may be pushed up to temperatures larger than 1 K, depending on the ability to tune the Fermi level to the proximity of the singularity. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P25.00010: Polarization and Spectral Properties of Massive Graphene Valeri Kotov, Vitor Pereira, Bruno Uchoa, Antonio Castro Neto We discuss the situation when a finite gap exists in the Dirac fermion spectrum, due to either external factors (substrate-induced), or the mesoscopic (finite size) nature of the sample. The gap could also be generated dynamically via chiral symmetry breaking. We will overview: (1.) The behavior of the polarization charge, induced by an external Coulomb source. The charge density exhibits an unconventional distribution is space, which we present in detail. The density variation could be observable in real experiments when an external ion is placed on the graphene sheet. (2.) The modification of the fermion self-energy; in particular, the large-N limit of the theory. We find that in this limit, which could be relevant for Graphene, the electronic dispersion is strongly renormalized at small energies. We discuss possible consequences of this behavior. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P25.00011: High-Mobility Dual-gated Graphene Field-Effect Transistors with Al$_{2}$O$_{3 }$Dielectric Seyoung Kim, Junghyo Nah, Insun Jo, Davood Shahrjerdi, Luigi Colombo, Zhen Yao, Emanuel Tutuc, Sanjay Banerjee The carrier mobility in graphene field-effect transistors (GFETs) is primarily dominated by the extrinsic impurity scattering, such as charged impurities in the dielectric. Therefore, the impact of a top-gate dielectric stack on the transport characteristics of graphene represents a key issue for high-performance GFETs. Here, we present the fabrication and characterization of dual-gated graphene FETs and dual-gated graphene devices with Hall bar geometry using Al$_{2}$O$_{3}$ as top-gate dielectric. We use a thin Al film as a nucleation layer to enable the atomic layer deposition of Al$_{2}$O$_{3}$. Our FETs show mobility values of over 6,000 cm$^{2}$/Vs at room temperature, a finding which indicates that the top-gate stack does not significantly increase the carrier scattering, and consequently degrade the device characteristics. We propose a device model to fit the experimental data with a single mobility value. [Preview Abstract] |
Session P26: Focus Session: Graphene VIII: Electronic Properties
Sponsoring Units: DMPChair: Michael Fuhrer, University of Maryland
Room: 328
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P26.00001: Experimental studies of the transport in graphene in a parallel magnetic field at low temperatures Liyuan Zhang, Jorge Camacho, Helin Cao, Isaac Childres, Yong Chen, Alexei Tsvelik, Dmitri Kharzeev, Maxim Khodas, Myron Strongin, Tonica Valla, Igor Zaliznyak Graphene has remarkable electric properties, and it is also a very promising material for spintronic applications. Most previous experiments, however, were focused on studying graphene devices in perpendicular magnetic field, which quantizes the real-space motion of Dirac electrons in graphene and leads to an unusual quantum Hall effect. Here we will present the results of experimental studies of electric transport in single- and few-layer graphene devices in parallel magnetic field and at low temperatures. The Dirac-point resistance of our graphene devices~was studied as a function of magnetic field and temperature. The effect of tuning the chemical potential under different magnetic fields was also investigated and will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P26.00002: The Image Potential for Graphene with an electrostatic Grating Godfrey Gumbs, Danhong Huang, Pedro Echenique We calculate the surface response function and the image potential of a layered structure of two-dimensional (2D) electron systems (ES). A point charge is placed at a distance away from the surface which is in the $xy$-plane. These 2D layers are coupled through the Coulomb interaction and {\em there is no interlayer electron hopping\/}. The separation between adjacent layers can be adjusted to investigate the role which layer separation and the number of layers play on both the surface response function and the image potential. Specifically, we consider the system composed of graphene layers or the layered 2D electron gas (EG) formed at the interface of a semiconductor heterostructure such as GaAs/AlGaAs. We show that the image potential for graphene is qualitatively the same as for the 2DEG. We examine the way in which the image potential is modified by applying a one-dimensional periodic electrostatic potential (through a gated grating for modulation). These results indicate that the charge screening for graphene is not much different from the 2DEG. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P26.00003: Adsorption of Ammonia on Graphene Prasoon Joshi, Hugo Romero, Awnish Gupta, Humberto Gutierrez, Milton Cole, Srinivas Tadigadapa, Peter Eklund We report on experimental studies of NH$_{3}$ adsorption/desorption kinetics on graphene surfaces. The study employs bottom-gated graphene field effect transistors (FETs) supported on Si/SiO$_{2}$ substrates. Detection of NH$_{3}$ occurs through the shift of the source-drain resistance maximum (``Dirac peak'') with gate voltage. The observed shift of the Dirac peak toward negative gate voltages in response to NH$_{3}$ exposure is attributed to the charge transfer from adsorbed NH$_{3}$, with the amount of charge estimated to be $\sim $ 0.06 electrons per molecule. The desorption kinetics of our FET devices is well described by the sum of two exponential terms corresponding to a fast and a much slower process, whose time constants differ by a factor of $\sim $ 9. The two-time constant desorption kinetics is consistent with Fickian-type diffusion of NH$_{3}$ from the interstitial pockets formed at the interface between the graphene and the supporting SiO$_{2}$ gate dielectric. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P26.00004: Resist-free method for contacting graphene and few-layer graphene Caterina Soldano, Ather Mahmood, Jeremie Grisolia, Veronica Savu, Juergen Brugger, Erik Dujardin In recent years, discovery of graphene has offered the scientific community an important tool to investigate a variety of fundamental phenomena. Exceptional electronic transport render graphene a promising candidate for future high-performance electronic devices. Conventional techniques to fabricate graphene devices include lithographic approaches, which tend to alter the structure and surface of graphene, and therefore its properties. A graphene contacting method free of any surface damaging and/or modification is indeed in need. Here, we present a simple resist-free non-invasive approach for contacting graphene and/or few-layers graphene. SiN membrane were custum-made and used to mask samples, previously deposited on substrate. Then, evaporation of metal allows to fabricate electrical leads for testing. Further, we present preliminary measurements of the electronic properties (room- and low-temperatures) of one of our typical sample contacted by such technique. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P26.00005: Transport studies of highly oriented pyrolytic graphite Aruna N. Ramanayaka, Bhaskar Kaviraj, Ramesh G. Mani Highly Oriented Pyrolytic Graphite (HOPG) consists of stacked sheets of single layers of carbon with weak interlayer interactions, which gives rise to anisotropic transport with striking differences between in-plane and perpendicular transport. Transport studies of single layers of carbon, known as Graphene, have shown striking new features in two dimensional transport, arising from the linear dispersion relation and analogies to quantum electrodynamics. A question of interest for our experiments is to examine the crossover from Graphite to Graphene and trace the three-dimensional to two-dimensional evolution in the transport properties of the layered carbon system. Hence, we report here on our efforts to fabricate specimens starting from commercially available HOPG, and present measurements in magnetic fields upto 12 Tesla, down to 1.5 K. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P26.00006: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P26.00007: Semiclassical model for the magnetoresistance and Hall coefficient of inhomogeneous graphene Rakesh Tiwari, David Stroud We show that when bulk graphene breaks into n-type and p-type puddles, the in-plane resistivity becomes strongly field dependent in the presence of a perpendicular magnetic field, even if homogeneous graphene has a field-independent resistivity. We calculate the longitudinal resistivity $\rho_{xx}$ and Hall resistivity $\rho_{xy}$ as a function of field for such a system, using the effective-medium approximation. The conductivity tensors of the individual puddles are calculated using a standard Boltzmann approach suitable for the band structure of graphene near the Dirac points. The resulting resistivity saturates, provided that the area fractions $f_n$ and $1 - f_n$ of n and p type puddles are slightly unequal, and agrees with experiments if the relaxation time is weakly field-dependent. The Hall resistivity $\rho_{xy}$ found to change sign at $f_n = 1/2$. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P26.00008: Anomalous Quantum Oscillations in Graphene Hybrid Structures Conor Puls, Neal Staley, Ying Liu It is well recognized that the edge states of graphene are interesting and important for both fundamental inquiry and potential practical applications of graphene. However, states associated with a step found in a continuous sheet of graphene with two different thicknesses have not been explored to date. We report a study of graphene hybrid structures featuring such a step. In a bulk hybrid featuring two large-area one- and two-layer graphene, two sets of Landau levels and features related to the interface were found. In edge hybrids featuring a large two-layer graphene with narrow one-layer graphene edges, we observed an anomalous suppression in quantum oscillation amplitude due to the locking of one- and two-layer graphene Fermi energies by charge transfer across the interface. These findings demonstrate the existence of unique interface states and related phenomena deserving of further studies. We will also report relevant studies on epitaxially-grown graphene films. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P26.00009: Graphene mediated exchange bias Yuriy Semenov, John Zavada, Ki Wook Kim We have theoretically investigated the role of graphene in mediating the indirect exchange interaction when it is placed between two ferromagnetic dielectric materials. The calculation based on a tight-binding model illustrates that the magnetic interactions at the interfaces affect not only the graphene band structure but also the thermodynamic potential of the system. This induces an indirect exchange interaction between the magnetic layers that can be considered in term of an effective exchange bias field. The analysis clearly indicates a strong dependence of the effective exchange bias on the properties of the mediating layer. Through the dependence on the graphene electro-chemical potential, the effective exchange bias can be modulated electrically over a wide range even at room temperature. This dependence also results in qualitatively different characteristics for the cases involving monolayer and bilayer graphene. The numerical estimation indicates the potential significance of the proposed phenomenon in practical applications. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P26.00010: Conductivity engineering of graphene by defect formation Biplab Sanyal Graphene exhibits exotic electronic properties revealed in transport measurements. The possibility to influence the electronic structure and hence control the conductivity by physisorption or doping with adatoms is crucial in view of electronics applications. Here we show that in contrast to expectation, the conductivity of graphene increases with increasing concentration of vacancy defects, with an amount of over one order of magnitude. We obtain a pronounced enhancement of the conductivity after insertion of defects by ab-initio electronic structure calculations. The theoretical results are supported by the experimental studies on carbon nano-sheets. Our finding is attributed to defect induced mid-gap states, which create a region exhibiting metallic behavior around the vacancy defects. The modification of the conductivity of graphene by implementation of stable defects is crucial for the creation of electronic junctions in graphene-based electronics. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P26.00011: A computational study of electrical transport in graphene-based films and composites Jeremy Hicks, Ashkan Behnam, Ant Ural We study the electrical behavior of films and composites composed of a mesh of graphene sheets using Monte Carlo simulations. We take into account the sheet-sheet junctions with different areas as well individual graphene sheets in calculating the film/composite transport properties. We find that the resistivity of composites varies by many more orders of magnitude than films approaching the percolation threshold due to tunneling-percolation through the sheet network, but otherwise the two exhibit many of the same scaling trends. Furthermore, we find that resistivity in both cases is a strong function of graphene sheet aspect ratio, density, volume fraction, and device dimensions. We extract important parameters such as the critical exponents near the percolation threshold and compare them with the available experimental data. These results, explained through physical and geometrical arguments, give valuable insights into the factors influencing the electrical transport in graphene sheet films and nanocomposites. Such graphene-based nanomaterials might find applications in many fields such as photovoltaics, sensors, and multifunctional nanocomposites. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P26.00012: Investigating the quantum behavior of a graphene-based Josephson Junction Joseph Lambert, Zechariah Thrailkill, Roberto Ramos Recent experiments have demonstrated the Josephson effect in superconducting mesoscopic graphene devices consisting of two superconducting leads separated by a few hundred nanometers, contacted by single and multiple layers of graphene [1]. We report on the progress of low temperature experiments that study the temperature dependence of switching currents in this device. The motivation is to explore the presence of macroscopic quantum metastable states similar to those found in current-biased Josephson junctions. These states are interesting and have been used as basis states for superconducting qubits. [1] H.B. Heersche, P.D. Jarillo-Herrero, J.B. Oostinga, L.M.K. Vandersypen, and A.F.Morpurgo, Induced superconductivity in grapheme, Solid state communications, 143(1-2), 72-76 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P26.00013: Study on the Electronic Band Structures of Doped Graphene C.G. Hwang, Kevin T. Chan, D. Siegel, A.V. Fedorov, Marvin L. Cohen, J.B. Neaton, A. Lanzara Graphene, a carbon sheet self-assembled on a SiC substrate, has been found to undergo changes in an electronic property as a function of doping concentration. Depending on the species of dopants, charge carrier density is gradually modified with increasing dopant coverage. By using angle resolved photoemission spectroscopy, we study how the graphene pi bands are modified by changing doping concentration and we discuss the effect of doping on many-body interaction such as electron-phonon coupling. These results provide new information on the role of electron-phonon coupling for superconductivity in the graphite intercalated compounds. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P26.00014: Anomalous thermoelectric transport of Dirac particles in graphene Peng Wei, Wenzhong Bao, Yong Pu, Chun Ning (Jeanie) Lau, Jing Shi We report a thermoelectric transport property study of single layer graphene devices in both classical and quantum Hall regimes. In zero magnetic field, by sweeping the gate voltage V$_{g}$ to vary the carrier density $n_{2D} $, we demonstrate a diverging behavior in the Seebeck coefficient $S_{xx}$, i.e.$S_{xx} \sim 1/\sqrt {n_{2D} } $, which is a direct consequence of the linear energy dispersion of the massless particles. At low temperatures and high carrier densities, the Seebeck coefficient depends linearly on temperature, indicating the validity of the Mott relation. In the applied magnetic fields, we observe an anomalously large Nernst signal($\sim $ 6 $\mu $V/K*T) at the Dirac point. This is another unusual property expected for massless particles. At low temperatures where the quantum Hall effect is observed, both the Seebeck and Nernst signals show oscillations corresponding to the Landau levels manifested in the quantum Hall plateaus. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P26.00015: Structure, Stability, Edge States and Aromaticity of Graphene Ribbons Tobias Wassmann, Ari Paavo Seitsonen, A. Marco Saitta, Michele Lazzeri, France Mauri We determine the stability, the geometric, the electronic and magnetic structure of hydrogen-terminated graphene-nanoribbons edges as a function of the hydrogen content of the environment by means of density functional theory [1]. Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum pressures. Under more standard conditions, the most stable structures are the mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with one di- and two mono-hydrogenated sites. At high hydrogen-concentration ``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in analogy to the spontaneous breaking of graphene into small-width nanoribbons observed experimentally in solution [2]. The stability and the existence of exotic edge electronic-states and/or magnetism is rationalized in terms of simple concepts from organic chemistry (Clar's rule). [1] T. Wassmann, et al. Phys. Rev. Lett. 101, 096402 (2008). [2] X. Li et al., Science 319, 1229 (2008); X. Wang et al., Phys. Rev. Lett. 100, 206803 (2008). [Preview Abstract] |
Session P27: Focus Session: Advances in Scanned Probe Microscopy III: High Frequency and Optical Techniques
Sponsoring Units: GIMSChair: Chris Hammel, Ohio State University
Room: 329
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P27.00001: Nanoscale Spectroscopy with Optical Antennas Invited Speaker: Antennas are devices that efficiently convert localized energy to free propagating radiation, and vice versa. They are a key enabling technology in the microwave and radiowave regime but their optical analogue is greatly unexplored. In order to understand antenna-coupled light emission and absorption we use a single molecule as an elementary light emitting device. With an optical antenna in the form of a simple gold particle we are able to increase the emission efficiency by more than a factor of 10. However, for very short distances between particle and molecule the fluorescence yield drops drastically because of nonradiative energy transfer. A simple gold particle is not an efficient optical antenna and it can be expected that favorably designed nanoplasmonic structures will yield much higher enhancement. Optical antennas can be employed as light sources for high-resolution optical microscopy and spectroscopy. We demonstrate vibrational (Raman scattering) and nonlinear imaging with spatial resolutions down to 10nm. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P27.00002: \textit{Nano-Prism} Probe for Nano-Optical Applications Taekyeong Kim, Byung Yang Lee, Seunghun Hong, Deok-Soo Kim, Zee Hwan Kim Recently, a \textit{nano-prism} structure has drawn attention as an optical nano-antenna due to its exotic optical properties, while it has been extremely difficult to prepare a probe terminated with a nano-prism for nano-optical applications. Herein, we report a method to mass-produce pristine nano-prism probes. Our fabrication process resulted in \textit{nano-prism with sharp edge at the end of the probe,} which significantly enhanced the electric field around it and made nano-prism probes ideal for nano-optical applications. We performed the apertureless near-field scanning optical microscopy on gold nanoparticles using a nano-prism probe, revealing the field localization at the vertices of the nano-prism. We also demonstrated the fabrication of multiple nano-prism probes in a parallel fashion. This method could be a major breakthrough and provide tremendous flexibility for SPM optical applications such as nano-TERS (tip enhanced Raman scattering) or FRET (fluorescent resonance energy transfer) because it allows one to mass-produce nano-probes terminated virtually general nanostructures. (Advanced Materials, in press) [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P27.00003: Optical Response of Absorbates in the STM Environment; The Influence of the STM Tip and Plasmonic Effects Ping Chu, D.L. Mills STM is widely used to explore the excited states and related optical properties of adsorbates on metal surfaces. The adsorbate may be placed on a thin oxide layer that is grown on the metallic substrate. One may ignore the direct hybridization between the adsorbate electrons and those in the substrate. We have developed the theory of the optical response of adsorbates in such an environment. Electrons in the adsorbate may interact with the electronic degrees of freedom in the tip/substrate complex through the fluctuating electric fields generated by the zero point motions of electrons in the substrate and the tip. The coupled plasmons of the tip/substrate complex contribute to these fluctuating fields. We have developed a formalism which allows us to describe energy level shifts of the adsorbate orbitals and the non radiative decay rate of excited states from coupling to the electronic degrees of freedom in the tip and substrate. We have also developed a theory of plasmon enhanced radiation emission under the tip, where the coupled plasmons of the tip/substrate complex are responsible for the enhancement of the emission. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P27.00004: Ultrafast Stroboscopic Optical Interferometry of Nanoelectromechanical Devices in Damping Pressurized Gas Environment O. Svitelskiy, V. Sauer, N. Liu, K.M. Cheng, M.R. Freeman, W.K. Hiebert A broad range of prospective applications of nanoelectromechanical devices necessitates understanding their performance under varying external conditions. We report a comprehensive gas damping study of a series of Si nanobridges and nanocantilevers with thickness of 0.147 $\mu $m, widths ranging from 0.1 to 1 $\mu $m, and lengths from 0.5 to 12 $\mu $m. Free ring-down oscillations of the resonators, capacitively excited by 1 ns 50 V electric pulses, were measured via instantaneous optical interference pictures snapped by a femtosecond laser. The devices response to a range of damping environments was studied, including response to different gases (He, N$_{2}$, CO$_{2}$) in widely ranging pressures from deep vacuum up to 200 bar, all done in a specially designed scanning optical microscopy chamber [1]. The resonator parameters demonstrate three distinct regions of pressure behavior: high vacuum, free molecular flow, and viscous. For each region a qualitative model is presented.\newline [1] O.Svitelskiy et al, Rev.Sci.Instr,\textbf{79} 093701, 2008 [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P27.00005: An Ultra High Vacuum Radio Frequency Scanning Tunneling Microscope Utku Kemiktarak, Keith Schwab, Kamil Ekinci Radio frequency scanning tunneling microscope (RF-STM) utilizes a \textit{LC} resonant circuit to achieve impedance matching between the STM tunnel junction and 50-$\Omega $ high frequency electronics. This technique allows measurement bandwidths up to 10 MHz. We have built an ultra high vacuum (UHV) RF-STM system with in-situ tip and surface treatment as well as sample, tip and matching circuit exchange. In this talk, we will describe the basic operation of this system and discuss the application of UHV RF-STM to high frequency displacement detection. We will argue that UHV RF-STM is a suitable tool to measure back-action forces of tunneling electrons and other tip-sample interactions. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P27.00006: Development of a scanning probe microscope for localized ferromagnetic resonance measurements Christian Long, Naoyuki Taketoshi, Haitao Yang, Ichiro Takeuchi We present an update on our research into the development of a scanning probe microscope capable of performing localized ferromagnetic resonance measurements. The system is based on near-field microwave microscopy using a resonant microwave cavity. Using near-field microwave microscopy allows us to produce a GHz frequency magnetic field which is confined to the region around the probe tip. By recording the change in the transmission coefficient of the resonator (S12) as a function of applied DC magnetic field, we measure the absorption of RF energy by the sample. The resulting ferromagnetic resonance spectrum allows us to map the magnetic properties of the material under the probe tip. The possibility to perform localized ferromagnetic resonance measurements using a scanning probe geometry promises to yield new insight into the properties of magnetic thin films. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P27.00007: Scanning magnetic resonance microscopy: Spatially resolved imaging of ferromagnetic resonance on yttrium iron garnet disk. Toshu An, Toyoaki Eguchi, Yukio Hasegawa We developed a radio frequency (RF) probe which can be implemented into scanning probe microscope aiming for its spatially resolved imaging. The probe is composed of a sharp tip attached at the end of a semi-rigid coaxial cable which transmits RF over 10 GHz. To measure ferromagnetic resonance (FMR) of a sample, the probe is set close to the sample, and the S$_{11}$ parameter was measured by using a network analyzer. As a test magnetic sample, a 10 mm-diameter and 1 mm-thickness polycrystalline YIG (yttrium iron garnet) disk was used. By locating the RF probe at the center of the YIG disk, FMR signal was detected as an absorption dip at 2.8 GHz in the S$_{11}$ measurements under in-plane static magnetic field of 458 Oe. The detected FMR signal has a sharper dip compared with that obtained in the coplanar wave guide method, and by moving the RF probe to the edge of the YIG disk, two different frequencies of FMR signal appears depending on the moving direction parallel or perpendicular to the applied magnetic field. The detected spatially dependent FMR signals are well explained by the magnetostatic waves. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P27.00008: Design and Fabrication of a Nanoscale Force Sensor for High-Speed Atomic Force Microscopy J. M. Campbell, B. Lucht, R. G. Knobel The atomic force microscope (AFM) has become an important tool in many fields ranging from materials science to biology. Conventional microfabricated AFM cantilevers have resonance frequencies of 10--300~kHz; some specialized cantilevers are available with frequencies up to 2~MHz. However, this represents the practical limit of the resonance frequency of microcantilevers. Three modeling methods were used to design a 200~MHz silicon nitride cantilever suitable for integration into an atomic resolution, frequency-modulation AFM. A process was developed to fabricate the cantilever coupled to an atomic point contact (APC) displacement detector, a device first demonstrated by Flowers-Jacobs et al.~(2007). The cantilever mask and APC electrodes were defined through electron-beam lithography and triple-angle evaporation. The cantilever pattern was transferred to the nitride layer through focused ion beam milling and a subsequent wet etch into the underlying Si substrate suspended the structure. Then, using an active feedback system similar to that developed by Strachan et al.~(2005), electromigration was used to form the APC at 77~K and $10^{-6}$~Torr. Progress toward measuring cantilever motion with the APC displacement detector through microwave reflectometry will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P27.00009: Interferometric and Synthetic Aperture Real-Time Terahertz Imaging Ke Su, Zhiwei Liu, Dale E. Gary, John F. Federici, Robert B. Barat, Zoi-Heleni Michalopoulou Over the past several years, several methods of real-time THz imaging have been developed. In this presentation, we describe a synthetic aperture imaging method of THz imaging. A 4-element THz detector array is used to reconstruct 2-D images of a point source through the interferometric synthetic aperture imaging method. A capture rate up to 63frames/s can be achieved. The recorded video showing the movement of the terahertz source in real time can be viewed at http://www.njit.edu/$\sim $ks265/imagingvideo.html after baseline and phase correction. Furthermore, a high power THz source will be integrated in this CW THz system for longer stand-off imaging distances. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P27.00010: Fabrication of MEMS Bimaterial Sensors for Uncooled THz Imaging. Dragoslav Grbovic, Gamani Karunasiri Recently, there has been a significant interest in Terahertz (THz) technology, primarily its applications in concealed object detection and medical imaging. THz region of the spectrum has been underutilized due to lack of compact and efficient sources and detectors. THz imaging has recently been achieved using uncooled, microbolometer infrared (IR) camera and quantum cascade laser (QCL) operating as a THz illuminator. However, bolometer IR cameras are not optimized for the THz band and fabrication of their focal plane arrays (FPAs) is complex due to requred monolithic integration of detectors and readout electronics. Recent developments in bi-material based IR FPAs with optical readout, substantially simplify the fabrication process by decoupling readout from sensing. This presentation describes the design and fabrication of THz-optimized bi-material FPAs, as well as integration of the real-time imaging system. The detection scheme involves detector deformation to minute temperature changes due to absorption of THz radiation. Individual detector deformations are simultaneously probed by shining visible light on entire FPA and reflecting it into a CCD camera. Optical readout eliminates the self-heating effects, enabling longer integration times and, better signal-to-noise ratio. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P27.00011: Detection of terahertz radiation from 410 GHz CMOS circuit and other high-frequency oscillators using a Fourier Transform Interferometer Eunyoung Seok, Daniel J. Arenas, Dongha Shim, Kenneth K. O, David B. Tanner Recently, a record-setting operating frequency of 410 GHz was reported for a CMOS circuit, fabricated using 45 nm technology. To measure the emission from this and related devices, we employed a Bruker 113v fourier transform interferometer. The radiation from an on-chip patch antenna attached to the 410 GHz push-push oscillator circuit was measured by placing the chip in the lamp housing of the interferometer. Emission was detected in the first and second harmonics of the oscillator fundamental. Power was estimated by comparison to that from quasi-blackbody sources (globar and mercury lamp). Possible applications will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P27.00012: Measuring Spin-Lattice and Spin-Spin Relaxation Times Using a Continuous Wave Electron Paramagnetic Resonance Spectrometer Michael R. Page, M.R. Herman, K.C. Fong, D.V. Pelekhov, P.C. Hammel The spin-spin and spin-lattice relaxation times, known as $T_2$ and $T_1$, respectively, provide information about the spins in a material. This information can be used as an imaging technique in Magnetic Resonance methods. $T_1$ and $T_2$ can be measured by a Continuous Wave Electron Paramagnetic Resonance Spectrometer, provided the relationship between the input power and oscillating magnetic field is known. The advantage to this is that Continuous Wave Electron Paramagnetic Resonance Spectrometers are much cheaper than Pulse Electron Paramagnetic Resonance Spectrometers. The relationship between the input power and the oscillating magnetic field is determined by using a sample with known $T_1$ and $T_2$, measuring the absorption at different power levels, and fitting the distribution of absorptions to a curve. We show the results of this measurement with a Bruker EMX 2.7 Continuous Wave Electron Paramagnetic Resonance Spectrometer. This work was supported by The U.S. Army Research Office MURI under contract W911NF-05-1-0414 and by The U.S. Army Research Office DURIP under contract W911NF-07-1-0305. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P27.00013: Terahertz Near-Field Nanoscopy of Mobile Carriers in Single Semiconductor Nanodevices Andreas J. Huber, Fritz Keilmann, J. Wittborn, Javier Aizpurua, Rainer Hillenbrand We introduce ultraresolving Terahertz (THz) near-field microscopy based on THz scattering at atomic force microscope tips. Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm, which is in good agreement with full electro-dynamic calculations. Imaging semiconductor transistors, we provide first evidence of 40 nm ($\lambda$/3000) spatial resolution at 2.54 THz (wavelength $\approx$ 118 $\mu$m) and demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. We find that the mobile carrier contrast can be directly related to near-field excitation of THz-plasmons in the doped semiconductor regions. This opens the door to quantitative studies of local carrier concentration and mobility at the nanometer scale. The THz near-field response is extraordinary sensitive, providing contrast from less than 100 mobile electrons in the probed volume. Future improvements could allow for THz characterization of even single electrons or biomolecules. [Preview Abstract] |
Session P28: Focus Session: Thermoelectricity in Si-containing Materials
Sponsoring Units: FIAPChair: Udo Pernisz, Dow Corning Corporation
Room: 330
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P28.00001: Atomistic Simulations of Heat Transport in Silicon Nanowires Invited Speaker: Silicon is one of the best known materials of our age, cheap and readily available, being the basic constituent of semiconductor electronics. It would therefore be highly desirable to broaden its utilization for, e.g. renewable energy applications. Recently, it has been proposed that Silicon may be engineered to be an efficient thermoelectric material for use in solid state devices. Although a rather inefficient thermoelectric in its bulk form, at the nanoscale Si may become a poor heat conductor, while retaining good electronic conduction properties, and thus exhibit high efficiency in converting heat into electric current. However the fundamental reasons for the reported low heat conduction in Si nanowires (NW) are not yet understood, and different interpretations has so far appeared in the literature. Here we present atomistic simulations of heat conduction in Si NW of 1 to 3 nm diameter. Our results show that, depending on their surface structure, these wires may exhibit values of the thermal conductivity varying by two orders of magnitude, and as high as those of bulk Si. This clearly indicates that the increased surface to bulk ratio at the nanoscale may be only partially responsible for the decreased thermal conductivity observed experimentally. We also find that diffusive, yet extended, vibrational modes present in the case of wires with amorphous surfaces, are responsible for a dramatic decrease of a factor of 100 in the conductivity of purely crystalline NWs. Our findings suggest ways of engineering wires with even lower thermal conductivity, by increasing surface disorder, in particular by alloying Si with, e.g. Ge at the crystalline-amorphous interface. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P28.00002: Ultralow thermal conductivity in Electrolessly Etched (EE) Silicon Nanowires Kedar Hippalgaonkar, Renkun Chen, Bair Budaev, Jinyao Tang, Sean Andrews, Padraig Murphy, Subroto Mukerjee, Joel Moore, Peidong Yang, Arun Majumdar EE process produces single-crystalline Silicon nanowires with rough walls. We use suspended structures to directly compute the heat transfer through single nanowires. Nanowires with diameters less than the mean free path of phonons impede transport by boundary scattering. The roughness acts as a secondary scattering mechanism to further reduce phonon transport. By controlling the amount of roughness it is possible to push limits to the extent that nanowire conductance close to quanta of thermal conductance,${\pi k_B^2 T} \mathord{\left/ {\vphantom {{\pi k_B^2 T} {6\hbar }}} \right. \kern-\nulldelimiterspace} {6\hbar }$ is observed. Traditionally, the lower limit of conductivity is amorphous Silicon at 1 W/mK at room temperature. The measured conductivity of our nanostructures challenges even this amorphous limit pointing towards previously unstudied mechanisms of thermal resistance. We measure thermal conductivity of $\sim $150nm diameter EE wires to be $\sim $1 W/mK. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P28.00003: Thermoelectric Power of Silicon Nanowires Hyuk Ju Ryu, Deborah Paskiewicz, Shelley Scott, Max Lagally, Mark Eriksson Thermoelectric nanomaterials have been attracting considerable interest for the cooling of hotspots and the conversion of waste thermal energy into useful electrical energy. There is special interest in silicon thermoelectrics because of potential monolithic integration on microchips as well as opportunities in nanofabrication and bandstructure engineering. We present measurements of the thermoelectric power of silicon nanowires with different doping concentrations and the gate field effect. Because silicon heterostructures can have modulation in charge density and mobility along the charge path, we have fabricated and measured silicon/silicide, silicon/silicon-germanium, and hybrid orientation silicon heterostructures in the form of nanowires. Measurements of the thermoelectric power of these structures and the effects of the internal interfaces will be presented and compared with theoretical calculations. This work is supported by DOE, AFOSR, NZ-FRST, NDSEG, NSF, and SOITECH. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P28.00004: Thermoelectric Properties of Higher Manganese Silicide Nanowires Arden Moore, Jeremy Higgins, Feng Zhou, Song Jin, Li Shi Higher manganese silicides (HMS) have a relatively high thermoelectric figure of merit (\textit{ZT}) of about 0.7. HMS nanowires have been synthesized using a chemical vapor deposition method. In this work, the thermoelectric properties of individual HMS nanowires are measured and analyzed to determine the role of size effects on electron and phonon transport as well as potential \textit{ZT }enhancement. Measurements of Seebeck coefficient, electrical conductivity, and thermal conductivity were performed using both suspended and substrate-based microdevices. Results show that the Seebeck coefficient of two as-synthesized 60 nm diameter nanowires between 300-400K is about 25-50{\%} lower than that of single crystal bulk parallel to the $c$-axis, while the electrical conductivity values are about 25{\%} lower than bulk single crystal in the same direction. The thermal conductivity of one 60 nm diameter nanowire at room temperature was found to be four times smaller than the bulk value along the $c$-axis. The large reduction in thermal conductivity and small to moderate impact on electrical transport may lead to HMS nanowires with enhanced \textit{ZT}. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P28.00005: Thermal conductivity of silicon-germanium alloys from first-principles Jivtesh Garg, Nicola Bonini, Nicola Marzari Thermoelectric materials will become commercially viable for converting heat into electricity and for refrigeration once their figure of merit (ZT) is improved. One key approach to increase performance is to reduce thermal conductivity - e.g. in alloys it is lower than the binary endpoints due to increased scattering induced by strain and disorder. Understanding the thermal conductivity of complex materials is also important in other applications - from reducing hot-spot temperatures in electronic chips to better thermal-insulation materials. Here, we have calculated the thermal conductivity of silicon-germanium alloys using ab-initio density functional perturbation theory. The electronic strucure of the alloy is studied with the virtual crystal approximation and the single mode relaxation time approximation; perturbation theory up to the third order provides phonon lifetimes, and disorder effects are taken into account by ensemble averages over configurations with random mass disorder. The contribution of acoustic and optical phonons to the thermal conductivity is also presented, together with the phonon mean free paths. These calculations could be used to estimate the size of the nanostructures that could reduce the thermal conductivity below bulk values through increased scattering of phonons. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P28.00006: ABSTRACT HAS BEEN MOVED TO S1.00260 |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P28.00007: Thermoelectric Properties of Silicon Nanowires: a Computational Study E. B. Ramayya, I. Knezevic We present a detailed simulation of electronic and thermal transport in thin, highly doped silicon nanowires, surrounded by a native oxide. Electronic states are found from a self- consistent Poisson-Schr\"{o}dinger solver within the effective mass framework. Confined acoustic phonon dispersions are calculated from the elastic continuum equation with the free- standing boundary conditions, appropriate for Si surrounded by the acoustically softer SiO$_2$. Transport of charge and heat is described by solving the Boltzmann transport equations for both electrons and acoustic phonons using the ensemble Monte Carlo technique. We see little increase in the phonon-drag portion of the Seebeck coefficient over the bulk value, and obtain the total Seebeck coefficient in agreement with experiment. Boundary roughness scattering indeed proves to have a significant effect on both electronic and thermal transport, and we discuss a novel method to account for the phonon boundary scattering, which supplants the use of the phenomenological specularity parameter. We demonstrate that indeed the room-temperature figure of merit in thin wires reaches values close to 1, and discuss options for its further enhancement. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P28.00008: Lattice thermal conductivity of nanostructured semiconductors from atomistic simulations Yuping He, Davide Donadio, Joo-Hyoung Lee, Jeffrey C. Grossman, Giulia Galli We present an atomistic analysis of the thermal conductivity (k) of nanoporous silicon (np-Si) [1, 2], and we compare our results with those obtained for bulk crystalline (c-Si) and amorphous Si. We computed k using equilibrium molecular dynamics and Green Kubo relations; we then analyzed our results by solving the Boltzmann Transport Equation in the single mode relaxation time approximation, and by using an approach devised [3] to describe thermal transport in disordered semiconductors. We observe that in np-Si the phonon mean free path is reduced by up to a factor of 10 with respect to c-Si, yielding a reduction of the k of about 2 orders of magnitude. The predominant phonon scattering processes contributing to k can be modeled by the same non-perturbative [3] approach that describes thermal transport in a-Si. \\[0pt] [1] J-H. Lee, et al. Appl. Phys. Lett, 91, 223110 (2007)\\[0pt] [2] J-H. Lee, et al., Nano. Lett., 8(11), 3750 (2008)\\[0pt] [3] P, B. Allen and J. L. Feldman, Phys. Rev. B 48, 12581 (1993) [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P28.00009: Direct Measurements of Figure-of-Merit in Amorphous Silicon-based Thermoelectric thin films Rubina Sultan, Azure Avery, Barry Zink Thermoelectric materials may play an important role in the solution of the urgent global need for energy. The dimensionless figure of merit ($ZT=\sigma {\alpha}^2 T/\textit{k}$) depends on three fundamental transport properties (thermal conductivity, electrical conductivity and thermoelectric power) of the material and optimizing the efficiency relies on effective measurement techniques of these quantities. These material properties may change from bulk to thin film form. The primary challenge is to search for materials with optimized electrical transport while minimizing the thermal conductivity. Amorphous materials and their alloys are relatively new functionally important materials that demonstrate superior properties in wide range of applications such as in thermoelectricity because of their low thermal conductivity due the higher degree of disorder. Previously, we reported in plane thermal conductivity of amorphous Silicon Nitride (\textit{a}-Si-N) membranes. In this talk we present our measurement technique and recent results of thermoelectric properties of thin film amorphous Si and its alloys by direct measurement of in plane thermal conductivity, thermopower and electrical conductivity on one platform and discuss the thermoelectric figure of merit. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P28.00010: Enhancement in power factor in p-type bulk SiGe alloys Giri Joshi, Austin Minnich, Xiaowei Wang, Gaohua Zhu, Yucheng Lan, Dezhi Wang, Bed Poudel, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Silicon Germanium (SiGe) alloys have been used for high temperature power generation in thermoelectric generators that provided the onboard electrical power to several US space vehicles. Since their performance is related to dimensionless figure-of-merit (ZT), material scientists have focused their attention on possible improvements in ZT of SiGe alloys through an increase in power factor and decrease in thermal conductivity. We have improved peak ZT to 0.95 in p-type SiGe bulk alloys by reducing thermal conductivity with nanostructuring approach. Now, we have been perusing modulation doping technique to improve ZT by increasing power factor in SiGe bulk alloys. We have observed significant improvement in power factor but ZT is not improved due to increase in thermal conductivity. The enhancement of power factor is mainly due to increase in mobility of carriers without much affecting the Seebeck coefficient. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P28.00011: Thermal Conductivity of Silicon/Germanium Nanostructures H.-Y. Chang, L. Tsybeskov, A. Sirenko, D.J. Lockwood, J.-M. Baribeau, X. Wu, M.W.C. Dharma-Wardana, T.I. Kamins, A.M. Bratkovsky The efficiency of thermoelectric devices can be enhanced by increasing electrical conductivity and lowering thermal conductivity. Semiconductor nanostructures, whose electrical and thermal conductivities could be optimized by changing their electronic and structural properties, are ideal candidates for such device applications. However, complete understanding of their device properties and limitations requires a technique allowing temperature measurements with a nanoscale spatial resolution. In this work, we studied the thermal conductivities of two groups of Si/Ge nanostructures: Si/SiGe multilayer samples prepared by molecular beam epitaxy, and Si/Ge nanowire heterojunctions prepared by chemical vapor deposition based vapor-solid-liquid process. Sample temperatures during irradiation by a laser beam were measured using Stokes and Anti-Stokes modes of Raman scattering of different vibration modes, and thermal conductivity was calculated by using the temperature gradient between different parts of SiGe nanostructures. We find clear correlations between samples' structural properties and their thermal conductivity. This work suggests a novel approach toward high-efficiency Si/SiGe nanostructure-based thermoelectric generators. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P28.00012: High Temperature Elastic Moduli Measurements and Phase Transition Studies of Novel Thermoelectric Materials Guangyan Li, Resheed Adebisi, Josh Gladden Thermoelectric (TE) materials can be used to convert heat including waste heat to electrical power. They are one component to energy savings and independence. Silicon germanium (SiGe) and Zintl phase compounds are excellent candidates for high temperature applications. The mechanical properties of these materials need to be known before their actual applications in high temperature (1000C) environments. The temperature dependent elastic moduli of five different SiGe alloys were successfully measured using a high temperature resonant ultrasound spectroscopy (RUS) technique. A linear trend is generally observed up to 600C, a downward curvature especially in two n-type samples is noticeable at higher temperatures. Hysteresis is only observed in one of the n-type SiGe samples. Phase transitions, indicated by shifts in the natural frequencies of a Zintl sample, were observed near 792, 892, 931C. The nature of these transitions will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P28.00013: Preparation and thermoelectric properties of Magnesium compounds Xiunu Lin, George Nolas, Dongli Wang We report on the synthesis and low temperature transport properties measurements of environmental-friendly thermoelectric materials Mg$_{2}$B (B=Si, Ge, Sn) and their alloys. These semiconductors are prepared through solid-state reaction of constituent elements. The effect of electrons doping and structure vacancies on thermoelectric properties are studied by substituting trivalent Sb for tetravalent Ge or Si on Mg$_{2}$Si and Mg$_{2}$Ge compounds. For Mg$_{2}$Ge system, both the Seebeck coefficient and electrical resistivity first decrease and then increase with increasing Sb content, whereas the thermal conductivity decrease monotonically. The Mg$_{2}$Si system displays similar tendency in seebeck coefficient, resistivity, and thermal conductivity but shows smaller magnitudes. Our Hall measurement at room temperature indicates that the modulation in these thermoelectric properties can be accounted for by the variance of electron concentration. The Mg$_{2}$Si$_{1-x}$Sn$_{x}$ solid solutions were prepared and investigated to study the dependence of thermoelectric properties on carrier types and carrier concentrations. [Preview Abstract] |
Session P29: Focus Session: Manganites
Sponsoring Units: DMP GMAGChair: Gang Cao, University of Kentucky
Room: 333
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P29.00001: Recent studies of models for manganites in the bulk and in superlattices Invited Speaker: In this talk, we will review our recent double-exchange model studies for manganites and some related experimental discoveries. First, we will briefly address the existence of a clear CMR effect in numerical simulations, and the short-range spin and charge correlations and local density of states in this CMR regime [1,2]. Second, we will analyze manganite superlattices (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$. The reconstruction of charge density, spin order, and orbital order at the interfaces and the relation with a novel experimentally observed metal-insulator transition (MIT) at $n=3$ will be discussed [3]. Finally, the multiferroic spiral spin order in the undoped manganite $R$MnO$_3$ ($R$=Tb, Dy) will also be briefly studied under the double-exchange framework [4]. \\[4pt] [1] R. Yu, S. Dong, C. \c{S}en, G. Alvarez, and E. Dagotto. Phys. Rev. B 77, 214434 (2008).\\[0pt] [2] C. \c{S}en, G. Alvarez, and E. Dagotto, Phys. Rev. Lett. 98, 127202 (2007).\\[0pt] [3] S. Dong, R. Yu, S. Yunoki, G. Alvarez, J.-M. Liu, and E. Dagotto, Phys. Rev. B 78, in press(R) (2008).\\[0pt] [4] S. Dong, R. Yu, S. Yunoki, J.-M. Liu, and E. Dagotto, Phys. Rev. B 78, 155121 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P29.00002: Small dissimilarity in lattice distortion triggers anomalously large anisotropic magnetoresistance in manganite perovskite. R. Li, H. Wang, X.Z. Wang, Y. Matsui, X. Wang, Z. Cheng, B. Shen, E.W. Plummer, Jiandi Zhang Anisotropic magnetoresistance (AMR) effects are of fundamental importance not only for providing information on spin-orbital coupling and magneto-crystalline anisotropy, but also for enabling technological applications. Here, we report an anomalous AMR effect in a prototype manganite single crystal---La$_{0.69}$Ca$_{0.31}$MnO$_{3}$. We demonstrate that the broken symmetry, through cubic to orthorhombic structural distortion in the crystal, leads to profound anisotropic magneto-transport behavior. The measured AMR behavior shows a direct correlation with the anisotropic field-tuned metal-insulator transition (MIT) in the system and can be understood via a phenomenological uniaxial anisotropy model. It is revealed that a small crystalline anisotropy can trigger a large AMR near the MIT phase boundary of the system. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P29.00003: Doping dependent evolution of the polaron metal N. Mannella, K. Tanaka, S.-K. Mo, W. Yang, H. Zheng, J. Mitchell, J. Zaanen, T.P. Deveraux, N. Nagaosa, Z. Hussain, Z.-X. Shen Experimental and theoretical evidence has already suggested that the ferromagnetic metallic (FM) phase in colossal magnetoresistive manganites is not a conventional metal but rather a polaronic conductor. In the bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (LSMO), Angle Resolved Photoemission (ARPES) experiment revealed that the FM phase is a polaronic metal with a strong anisotropic character of the electronic excitations [1,2]. A small but well-defined quasiparticle (QP) with heavy mass along the [110] or ``nodal'' direction is found to account for the metallic properties and their temperature dependent evolution [2]. In this talk, we will discuss recent ARPES results on the x = 0.60 composition and contrast them to the x = 0.40 results. Recent work has shown that the region in proximity of x = 0.60 constitute the most metallic bilayer manganite with DC conductivity about one order of magnitude higher than that corresponding to the region 0.30 $<$ x $<$ 0.40. Much as in the x = 0.40 composition, for x = 0.60 along the nodal direction we observe a peak-dip-hump structure with QP of heavy effective mass. Quantitative differences in the electron-phonon coupling constant $\lambda $, the QP spectral weight and the hump energy are fully consistent with the doping evolution of the transport properties. [1] Nature \underline {438}, 474 (2005), [2] Phys. Rev. B \underline {76}, 233102 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P29.00004: Metal-insulator transition in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.59) revealed by ARPES Zhe Sun, J. F. Douglas, Q. Wang, A. Fedorov, Y. -D. Chuang, H. Zheng, J. F. Mitchell, D. S. Dessau Using angle-resolved photoemission spectroscopy (ARPES), we studied the metal-insulator transition of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.59). Below T$_{C}$, there is significant metallic weight at the Fermi level, while a gap opens above T$_{C}$, in excellent agreement with resistivity measurements. We also found that in this compound the metal-insulator transition is associated with a remarkable coherent-incoherent weight transfer from the dispersive band to a non-dispersive feature over a large energy scale. The band dispersion also shows an unusual change with increasing temperature, suggesting complicated interactions in this material. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P29.00005: Role of Oxygen Electrons in the Metal-Insulator Transition in the Mangnetoresistive Oxide La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ W. Al-Sawai, B. Barbiellini, A. Koizumi, P.E. Mijnarends, T. Nagao, K. Hirota, M. Itou, Y. Sakurai, A. Bansil We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e. the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metallic-like to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated from the area enclosed by the Compton profile anisotropy differences. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition. Work supported in part by the USDOE. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P29.00006: Charge/Orbital Ordered Phases of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7-\delta }$ Kenneth Gray, Hong Zheng, Qing'An Li, John Mitchell Our studies have significantly modified the conventionally-held view of the phase diagram of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7-\delta }$ for two compositions exhibiting charge (and orbital) order (CO), i.e., at hole doping levels, h=x-$\delta $, of $\sim $0.5 and $\sim $0.6. These CO states are stable over very narrow doping ranges ($\Delta $h$\sim \pm $0.005) at the lowest temperatures, but those ranges increase at higher temperatures (to $\Delta $h$\sim \pm $0.02) in a manner consistent with simple entropy considerations. Such narrow ranges dictate the crucial need for crystal homogeneity. Attesting to such homogeneity is a conductivity ratio of $>$10$^{10}$ upon crossing the first-order phase boundary from CO at h=0.60 to AAFM at h$\sim $0.59 or h$\sim $0.61 plus two findings that were missed in the existing literature: that these CO phases are the ground state at the lowest temperatures and, for h$\sim $0.5, that coexistence of the CO and AAFM phase is absent at any temperature. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P29.00007: Resonant Inelastic X-ray Scattering in CE-ordered bilayer manganite Frank Weber, Stephan Rosenkranz, John-Paul Castellan, John Mitchell, Hong Zheng, Diego Casa, Thomas Gog Resonant Inelastic X-ray Scattering (RIXS) has recently emerged as valuable tool in the study of orbital excitations in transition metal oxides.Stepah We have performed RIXS measurements at the Mn K-edge in the half doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$. Our sample was a non-reentrant single crystal with long range CE order down to lowest temperatures. We made wave vector dependent energy loss scans with $\Delta $E$\le $15eV in the (110) direction at three different temperatures, i.e. T=75K (AFM CE ordered), 175K (PM CE ordered) and 250K (PM and no orbital order). In particular, we compare the temperature dependence of the 2eV peak with previous results on manganite perovskites [1]. Work supported by US DOE BES-DMS DE-AC02-06CH11357.\\[4pt] [1] S. Grenier et al. Phys. Rev. Lett. 94, 047203 (2005) [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P29.00008: Time-resolved Optical Study of Charge-ordered Manganites. Takahisa Tokumoto, Judy Cherian, Ryan deRosa, Paula Sahanggamu, Sanhita Ghosh, Stephen McGill, Tesfaye Gebre, Haidong Zhou, Christopher Wiebe We study the effects of applied electric fields and large magnetic fields on the optical properties of Pr$_{(1-x)}$Ca$_{x} $MnO$_{3}$ (x$\sim $0.5) (PCMO) and La$_{(1-x)}$Ca$_{x}$MnO$_{3} $ (x$\sim $0.18) (LCMO) using time-resolved techniques. Our measurements are performed down to 4 K and in dc magnetic fields up to 31 T. The conductivity of the low-temperature strong charge/orbital ordering in PCMO is altered by the application of an electric field and a magnetic field. We demonstrate that time- resolved optical reflection and Kerr effect measurements are capable of capturing these mixed electronic and magnetic effects to gain further insight into the change of the ordering. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P29.00009: Lattice-form dependent charge- and orbital- ordered states in perovskite-related mangananites Daisuke Okuyama, Yusuke Tokunaga, Reiji Kumai, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura Charge and orbital order in half-doped manganites has been extensively studied since the magnetic-field induced melting of charge and orbital order (CO-OO) results in colossal magnetoresistance phenomena. However, there remain two points to be clarified, concerning the CO-OO states. First issue is the degree of charge disproportionation (CD); Full CD between Mn$^{3+}$ and Mn$^{4+}$ ions has been widely believed while charge density wave ordering with less distinct CD has also been recently proposed. Another issue is the orbital shape (OS) at Mn$^{3+}$ ion in the CO-OO phase. The reason why the OSs of (La,Ca)MnO$_3$ ((3y$^2$-r$^2$)/(3$x^2$-r$^2$)) and (La,Sr)$_2$MnO$_4$ ((y$^2$-z$^2$)/(z$^2$-x$^2$)) are different is not clarified. In our study, we tried to clarify the CDs and OSs in (Eu,Ca)$_2$MnO$_4$ and (Pr,Sr,Ca)$_3$Mn$_2$O$_7$ by means of x-ray crystal structural analyses and well established methods of bond valence sum and Kanamori diagram, and to compare with those of (Pr,Ca)MnO$_3$. We found that the CD of all the samples is much smaller than unity. In addition, the CDs and OSs are systematically dependent on the dimension of MnO$_6$ network. From simple consideration, we concluded that apical oxygens play an important role. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P29.00010: Charge and orbital order effects in La$_{x}$Sr$_{1-x}$MnO$_{2.6}$ B. Dabrowski, L. Suescun, S. Kolesnik, S. Remsen, J. Mais Low temperature annealing in hydrogen have been used to obtain oxygen vacancy ordered manganites Sr$_{4+n}$Mn$^{3+}_{4}$Mn$^{4+}_{n}$O$_{10+3n}$ (n=0, 1, 3) displaying charge and orbital ordering. For the La-substituted n=1 phase four Mn$^{3+}$ cations exhibit elongated pyramidal coordination while the fifth one in octahedral coordination shows decreasing formal valence Mn$^{(4- 5x)+}$. This selective doping produces structural strain resulting in unusual apically compressed coordination leading to complex magnetic interactions and frustration. Similar structures have been previously observed for the (La,Ba)CuO$_{3-d}$ cuprates revealing common vacancy ordering relationships in perovskites for which highly distorted Mn$^{3+}$ (Cu$^{2+})$ and symmetric Mn$^{4+}$ (Cu$^{3+})$ ions are present simultaneously. Work at NIU was supported by the NSF-DMR-0706610 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P29.00011: Phase separations in La$_{1-x}$Ca$_{x}$MnO$_{3}$ at critical doping levels J. Tao, Q. Jie, Q. Li, Y. Zhu, D. Niebieskikwiat, M.B. Salamon, S.J. Pennycook La$_{1-x}$Ca$_{x}$MnO$_{3}$ specimens have been widely studied for their rich and complex physics. There is a boundary in the phase diagram at $x$ = 0.50. At low temperatures, the system is ferromagnetic for $x$ less than 0.5 while charge ordering phase is favored for $x$ equal to or greater than 0.5. The mechanism for this drastic change over the continuous doping still remains unclear. Here we report our electron diffraction study on bulk La$_{1-x}$Ca$_{x}$MnO$_{3}$ at the critical doping $x$ = 0.48 and 0.50. Lorentz microscopy is also employed in the study to obtain the magnetic domain information during the phase transitions in these two specimens. The observed structure is integrated with the measured properties and it shows novel phenomena of the materials at nanoscale. This work is funded by U.S. DOE/BES under Contract No. DE-AC02-98CH10886. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P29.00012: High Pressure Neutron Diffraction Study on a Self-doped CMR Manganite Peng Gao, Trevor A. Tyson, Zhiqiang Chen, Chris Stock, Matthew G. Tucker High-pressure neutron diffraction measurements were conducted on the self-doped CMR material La$_{0.85}$MnO$_{3-\sigma }$ up to $\sim $ 7 GPa above and below the magnetic ordering temperatures. The diffraction data show no abrupt structure change (space group) in the whole pressure and temperature range studied. The detailed atomic structural changes are examined. In addition the magnetic structure as a function of pressure and temperature was explored at high and low pressures. Peaks that could be attributed to magnetic scattering appear at $\sim $230K under $\sim $ 0.7 GPa and persist at high pressure ($\sim $7GPa). The pressure dependent distortion of the MnO6 polyhedra is discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P29.00013: Universal magnetic behavior the electron-doped SrMnO$_{3}$ cubic perovskite S. Kolesnik, B. Dabrowski, O. Chmaissem SrMnO$_{3}$ is the end member of a widely explored family of colossal magnetoresistive manganites R$_{x}$Sr$_{1-x}$MnO$_{3}$ (R=rare earth elements). Low-level R$^{3+}$ substitutions change the antiferromagnetic order from G-type in cubic SrMnO$_{3}$ to C-type in tetragonal R$_{x}$Sr$_{1-x}$MnO$_{3}$ through first-order resistive and structural transitions. From the magnetization, transport, and neutron diffraction experiments we observe that a similar change can be induced by B-site substitutions in SrMn$_{1-x}$M$_{x}$O$_{3}$ (M=Ru$^{5+}$,Mo$^{6+})$ both generating Mn$^{3+}$ in the Mn$^{4+}$ matrix. For both A-site and B-site substitutions, the N\'{e}el temperature is dependent on the Mn$^{3+}$ concentration in a universal way. These observations reveal that the magnetic and electronic properties of low-level substituted SrMnO$_{3}$ are controlled by the band filling throughout the increasing role of local distortions of Mn$^{3+}$O$_{6}$ octahedra changing from randomly diluted to cooperative character of the entire lattice. Work at NIU was supported by the NSF (DMR-0706610) and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Session P30: Focus Session: Oxide Surfaces and Interfaces
Sponsoring Units: DMP GMAGChair: Jian Shen, Oak Ridge National Laboratory
Room: 334
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P30.00001: Structural Stabilization of Antiferromagnetism at the Surface of a Layered Manganite E.W. Plummer, V.B. Nascimento, R.G. Moore, H. Liu, M.H. Pan, D. Mazur, J.W. Freeland, K.E. Gray, R.A. Rosenberg, H. Zheng, J.F. Mitchell, R. Saniz, A.J. Freeman, J. Rundgren Here, we present evidence to support the idea that the distinct surface electronic and magnetic state seen in the double-layered manganites (La$_{2}-_{2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, with dopings of 0.3 $< \quad x \quad <$ 0.4) is driven solely by a subtle change in the lattice at the surface, which is consistent with the strong coupling between the lattice, charge, and spin degrees of freedom in these doped transition-metal oxides. A combined experimental and theoretical approach was essential in understanding the origin of the nonmagnetic surface phase. The new measurements and calculations give an insightful explanation of the previous x-ray and tunneling data, which indicated a nonmagnetic and insulating surface bilayer. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P30.00002: Applying the extended Drude model for the mid-infrared complex conductivity tensor of SrRuO$_3$ M.-H. Kim, G. Acbas, M.-H. Yang, C. T. Ellis, M. Eginligil, J. Cerne, P. Khalifah, I. Ohkubo, H. Christen, D. Mandrus, Z. Fang The complex longitudinal ($\sigma_{xx}$) and Hall ($\sigma_{xy} $) conductivities in a SrRuO$_3$ film are determined using Faraday and Kerr measurements in the mid-infrared (100 $\sim$ 1000 meV) energy and 10 K $\sim$ 300 K temperature ranges. The extended Drude model (EDM) for ($\sigma_{xx}$) is used to subtract the ordinary part of $\sigma_{xy}$. Using the EDM parameters, the ordinary contribution to $\sigma_{xy}$ is found to be significantly smaller than the anomalous contribution at most temperatures and wavelengths, decreasing strongly as the wavelength decreases. We suggest that the paramagnetic part of the $\sigma_{xy}$ results from the unsaturated magnetization, not from the ordinary part of $\sigma_{xy}$. This work was supported by the Research Crop. Cottrell Scholar Award (UB), NSF-CAREER-DMR0449899 (UB), and an instrumentation award from the CAS (UB). Oak Ridge Natl. Lab. Is managed by UT- Battelle, LLC, for the U.S. DOE (contract DE-ACO5-00OR22725). [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P30.00003: Antiferromagnetism and insulating nature of ultrathin films of SrRuO3 Priya Mahadevan, F. Aryasetiawan, A. Janotti, T. Sasaki Metallic oxides form an integral part of oxide-based technologies, constituting the connecting electrode material. In this context SrRuO3 is a material that has been widely studied as it in addition to being metallic is also ferromagnetic. Recent experiments have found that ultrathin films of SrRuO$_3$ are insulating and hence 5-6 monolayers are required before metallic character is observed. In this work we theoretically examine the origin of the insulating state with first principle GGA+U calculations. The value of U is calculated from first principles. Ru has a formal configuration of d$^4$ in SrRuO$_3$. In bulk SrRuO$_3$ this translates into a low spin state with an electronic configuration of $t_{2g\uparrow}t_{2g\downarrow}$. Hence at the surface/ultrathin film limit one expects the observed insulating nature to come from a transition into the nonmagnetic state with the lowest crystal-field levels contributed by $d_{yz}$ and $d_{xz}$ orbitals being completely filled. However one finds that the system undergoes an unusual structural distortion which is accompanied by a spin state transition. This spin state transition is accompanied by a transition into an antiferromagnetic state which drives the system insulating. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P30.00004: Structure and Transport in ultrathin SrRuO$_{3}$ Arthur P. Baddorf, Junsoo Shin, Von Braun Nascimento, Albina Y. Borisevich, Sergei V. Kalinin, Vincent Meunier, Peter Maksymovych SrRuO$_{3}$ (SRO) is a common electrode material in oxide thin film growth, and is also representative ``bad'' metal. Although bulk SRO is orthorhombic, a typical study grows a pseudocubic film on SrTiO$_{3}$(100) substrate. We have measured electrical conductivity and structure of in-situ grown SRO films using scanning probe microscopy, low energy electron diffraction (LEED) and scanning transition electron microscopy. The films remain conducting down to the thickness of one layer. LEED displayed sharp (1x1) patterns at 80 and 300K up to 5 layers, but revealed strong half-order spots with weak intensity at quarter-order by 20 layers. STM images show the surfaces of thick films to have many vacancies, which first principles theory suggests are missing SrO molecules. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P30.00005: Atomic control and characterization of surface defect states of TiO$_{2}$ terminated SrTiO$_{3}$. A. Kareev, M. Kareev, S. Prosandeev, J. Liu, C. Gan, J.W. Freeland, Min Xiao, J. Zhang, L. Brillson, J. Chakhalian By using a new wet-etch procedure$^{1}$ we have obtained high-quality atomically flat TiO$_{2}$ terminated surfaces of STO (100) single crystals with the surface morphology equivalent or better to that of the conventional routes. By applying a combined power of CL and PL, RHEED, AFM, and resonant XAS, we are able to identify and monitor the complex evolution of oxygen defect states and Ti ion valency at the surface and near-surface regions. Our data revealed a high level of local defects resulting in the presence of the Ti$^{3+\delta }$ states at the surface in the conventionally treated STO surface. We have developed an efficient method to control the defect states capable of a marked reduction of the defect concentration. We have demonstrated that the PL, CL and XAS are able to distinguish the surface-related Ti states from oxygen vacancies trapping charge transfer vibronic excitons. $^{1}$M. Kareev et al., Appl. Phys. Lett. 93, 061909 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:36AM |
P30.00006: Polar and non-polar oxide interfaces: charge and spin behaving badly Invited Speaker: Interfaces between two dissimilar materials are well known to lead to new behavior and often useful properties. Whereas covalent semiconductors have been studied and used for decades, and the interfaces of magnetic metals also have assumed great importance, the use of oxides in such juxtaposed systems is much more recent. Oxides add the huge impact of ionicity, and the correlated electron behavior that occurs if open shells are present. Even without correlation effects, finite overlayers (slabs) involving a polar discontinuity can sustain a surprisingly large separation of charge, as will be illustrated with calculational results on LaAlO$_3$ slabs on SrTiO$_3$ substrates: a strong {\it polar distortion}, uniform over several unit cells, creates the necessary screening. The most recent results on the mechanism and character of the insulator-to-metal transition with thickness will be discussed. Polar discontinuities are not necessary to create exotic behavior, as we illustrate with rutile-structure VO$_2$/TiO$_2$ multilayers, where a topologically protected zero-gap two-dimensional half-metal arises in a thickness regime between thin insulating and thick conducting VO$_2$ slabs. Work done in collaboration with R. Pentcheva, V. Pardo, and K. Otte. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P30.00007: Prediction of a switchable two-dimensional electron gas at KNbO3/ATiO3 (A = Sr, Ba, Pb) interfaces Yong Wang, Manish Niranjan, Sitaram Jaswal, Evgeny Tsymbal The demonstration of a quasi-two dimensional electron gas (2DEG) at the (LaO)$^{+}$/(TiO$_{2})^{0}$ interface in LaAlO$_{3}$/SrTiO$_{3}$ heterostructure has fuelled intense research activity in recent years. The 2DEG has a high carrier mobility and electron density that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control the properties of the 2DEG by external stimulus, e.g., by an electric field. In this study we use density functional calculations to explore a ferroelectric KNbO$_{3}$/SrTiO$_{3 }$heterostructure for this purpose. The polar discontinuity at the (NbO$_{2})^{+}$/(SrO)$^{0 }$interface in KNbO$_{3}$/SrTiO$_{3}$ heterostructure is similar to that at the (LaO)$^{+}$/(TiO$_{2})^{+}$ interface in LaAlO$_{3}$/SrTiO$_{3}$ heterostructure. Our results suggest that a 2DEG is created at the (NbO$_{2})^{+}$/(SrO)$^{0 }$interface due to the electronic reconstruction with properties strongly determined by the orientation of the electric polarization. We further explore the formation of 2DEG at the interfaces of all ferroelectric KNbO$_{3}$/BaTiO$_{3}$ and KNbO$_{3}$/PbTiO$_{3}$ heterostructures and its dependence on polarization orientation. Finally, we discuss how the properties of 2DEG in aforementioned heterostructures are influenced by the compensation of polarization charges by free carriers, rendering it switchable. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P30.00008: A Novel Quantum Well of Embedded SrTiO$_3$ in LaAlO$_3$ Hanghui Chen, Alexie Kolpak, Sohrab Ismail-Beigi Inspired by the novel behavior of $\textrm{LaAlO}_3/\textrm{SrTiO}_3$(001) heterointerfaces, we propose new sets of interfaces in which one TiO$_2$ or SrO layer is embedded in LaAlO$_3$. These interfaces can form quantum wells which trap electrons or holes in a single atomic layer of TiO$_2$ or SrO, respectively, when the ``polar catastrophe'' occurs. This narrow confinement, in contrast to the situation at the $\textrm{LaAlO}_3/\textrm{SrTiO}_3$(001) interfaces, sheds light on the still uncertain origin of the charge carriers and provides ideas for engineering their properties. In addition, we study the field effect on these systems to predict the critical external electric field required to induce an insulating-to-metallic transition. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P30.00009: Resonant Tunneling Through a Two-Dimensional Electron Gas in All-Oxide Heterostructure Tunnel Junctions J.D. Burton, E.Y. Tsymbal, J.P. Velev Oxide heterostructures exhibit a variety of very interesting physical properties and have tremendous potential for new types of multifunctional device applications. For example, very recently it was discovered that a two-dimensional electron gas (2DEG) is formed at the (001) interface between two perovskite oxides that are otherwise insulating in the bulk. We examine, within the framework of first-principles density functional theory, the effect of a complex SrTiO3-LaO-SrTiO3 barrier forming a 2DEG on conductance and TMR in all-oxide magnetic tunnel junctions. The replacement of one SrO atomic layer by LaO in the otherwise pure SrTiO3 barrier can be understood as precision substitutional doping of trivalent La for divalent Sr, leading to the formation of a 2DEG. Such precision atomic layering is within the reach of current experimental fabrication techniques. Our calculations reveal that compared to the pure SrTiO3 barrier, the tunneling conductance can be substantially enhanced due to resonant tunneling through the 2DEG. However, this effect is sensitive to lattice polarization effects in the SrTiO3 barrier as well as the choice of electrode material. We will discuss these effects with the goal of stimulating experimental studies. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P30.00010: Modulation of the low-temperature magnetoresistance of Ar-irradiated SrTiO$_3$ via field-effect gate doping J.H. Ngai, Y. Segal, J. Hoffman, F.J. Walker, C.H. Ahn Recent experiments have shown that irradiating single crystal SrTiO$_3$ with Ar ions can create an amorphous surface layer with a quasi-2-dimensional electron gas (Q2DEG) below. We present low-temperature magnetotransport measurements of this Q2DEG system, as a function of gate doping. The magnetoresistance can be tuned as n-type carriers are doped into the interface between the amorphous and crystalline SrTiO$_3$ layers. Anisotropy in the magnetoresistance is also measured with respect to the direction of the applied magnetic field. These results will be compared with the magnetotransport properties of LaAlO$_3$/ SrTiO$_3$ heterostructures, where the possibility of novel magnetic behavior will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P30.00011: Nanoscale Electrical Properties of Oxide Heterostructures Revealed Via Introspection Cheng Cen, Stefan Thiel, Jochen Mannhart, Jeremy Levy Previous work shows that conductive regions can be formed via lateral nanoscale confinement of a quasi-two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ interface$^{2}$. Here we demonstrate how structures constructed in this method serve not only as novel nanoelectronic devices but also as tools for studying fundamental physics in the underlying material system. Nanowires, tunnel junctions, field effect transistors (FETs), together with associated phenomena that we observed such as negative differential resistance, provide insight into the mechanism responsible for the existence and spatial confinement of the interfacial metal-insulator transition. We discuss several examples of nanodevices and the constraints they place on models and mechanisms that govern their properties. $^{2}$Cen et al, Nature Materials 7, 298 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P30.00012: A metal-insulator transition tunable by lattice deformation in LaTiO$_{3}$ thin films Franklin Wong, Seung-Hyub Baek, Ho Won Jang, Rajesh Chopdekar, Virat Mehta, Chang-Beom Eom, Yuri Suzuki Strong electron-electron and electron-lattice correlations play critical roles in electronic transitions of complex oxides. Since LaTiO$_{3}$ is a narrow bandgap Mott insulator on the verge of a metal-insulator transition, substrate-induced lattice distortions offer a route to tuning its electronic properties. We have observed metallic to insulating behavior in LaTiO$_{3}$ films depending on choice of (001) substrates: SrTiO$_{3}$, LSAT, and LaAlO$_{3}$. Tetragonal distortions induced by epitaxial in-plane compression from the SrTiO$_{3}$ substrates result in metallicity in LaTiO$_{3}$ films, while films on LSAT substrates exhibit a range of electronic properties depending on the degree of lattice relaxation. Whereas thinner LaTiO$_{3}$ films on LSAT exhibit ``semimetallic'' behavior, in thicker films, the out-of-plane lattice parameters surprisingly converge to values greater than the bulk lattice constant, and the films become more insulating. We will discuss the profound consequences thin-film lattice deformation has on electrical transport. We speculate that stabilization of lattice distortions via epitaxy may open a new avenue for materials engineering of oxides through careful control of structural perturbations. [Preview Abstract] |
Session P31: Focus Session: Spin Ice
Sponsoring Units: GMAGChair: Oleg Tchernyshyov, Johns Hopkins University
Room: 335
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P31.00001: Magnetothermodynamics of spin ice and related compounds Invited Speaker: Geometrically magnetic frustration, which results from the competition of spin-spin interactions of magnetic ions on a regular magnetic lattice, leads to a variety of exotic low temperature states including ``spin ice.'' ``Spin ice'' refers to a magnetic state wherein the two-in/two-out spin configurations of rare earth pyrochlore compounds mimic the proton positions in the water ice, characterized by the ``zero point entropy'' of (R/2) ln(3/2). In this study, we examine how structural disorder affects spin dynamics and the magnetic ``zero point entropy.'' By diluting the ``spin ice'' materials with nonmagnetic ions on the rare earth sites, we have found that the entropy of the diluted species depends non-monotonically on the dilution concentration, and we explain this behavior using a generalized Pauling approximation. Nonmagnetic doping on B sites leads to only a small decrease of the ``zero point entropy,'' indicating the robust nature of ``spin-ice.'' We have also studied Dy2Ge2O7, which has the same chemical formula as ``spin ice'' materials and Ising-like spins but a tetragonal structure. Dy2Ge2O7 undergoes a long range antiferromagnetic ordering transition, but the spin dynamics at temperatures above the order transition is similar to that observed in the canonical ``spin ice'' systems, suggesting that such dynamics are generic to a broader class of Ising-like rare earth systems. \\[4pt] References: \\[0pt] [1] X. Ke \textit{et al}., Phys. Rev. Lett. \textbf{99}, 137203 (2007).\\[0pt] [2] X. Ke \textit{et al}., Phys. Rev. B. \textbf{76}, 214413 (2007).\\[0pt] [3] X. Ke \textit{et al}., Phys. Rev. B. 7\textbf{8}, 104411 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P31.00002: Magnetic charge order in kagome spin ice with dipolar interactions Gia-Wei Chern, Paula Mellado, Oleg Tchernyshyov Dipolar interactions in spin ice are described most effectively in terms of magnetic charges residing on the dual lattice [1]. While spin ice on the pyrochlore lattice contains no magnetic monopoles at low temperatures, spin ice on kagome [2] contains a unit magnetic charge ($\pm 1$) on every triangle. With the aid of Monte-Carlo simulations, we show that long-range Coulomb interaction between the monopoles lifts the degeneracy of the spin-ice states and induces a phase transition into a state with ordered magnetic charges but no spin order. The residual entropy is reduced from the spin-ice value but remains extensive. The phase transition is continuous with critical exponents close to the two-dimensional Ising universality class. [1] C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature {\bf 451,} 42 (2008). [2] A. S. Wills, R. Ballou, and C. Lacroix, Phys. Rev. B {\bf 66}, 144407 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P31.00003: Comparing artificial frustrated magnets: geometric effects in nanomagnet arrays Jie Li, Xianglin Ke, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied arrays of single-domain ferromagnetic islands arranged on lattices such that the magnetostatic interactions between the islands are frustrated by the geometry of the arrays. We compare results for three different lattice geometries: the previously studied square ``artificial spin ice'' lattice[1,2], a hexagonal lattice, and a ladder lattice which is topologically-equivalent to the former one. After the ac demagnetization the magnetic moment configurations are imaged via Magnetic Force Microscopy (MFM). We find that the ladder lattice shows local correlations which are similar to those of the square lattice, suggesting it as a basis for comparison of the energetics of the other two lattices. The normalized magnetostatic energy of all three geometries decreases with decreasing demagnetization step size, but the lattices approach their ground states at different rates. 1. R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi, and P. Schiffer, Nature 439, 303 (2006). 2. X. Ke, J. Li, C. Nisoli, P. E. Lammert, W. McConville, R. F. Wang, V. H. Crespi, and P. Schiffer, Phys. Rev. Lett. 101, 037205 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P31.00004: Cooperative Paramagnetic Behavior in the Stuffed Pyrochlore Tb$_{2+ \textit{x}}$Ti$_{2-2 \textit{x}}$Nb$_{ \textit{x}}$O$_{7}$ B.G. Ueland, J.S. Gardner, M.L. Dahlberg, P. Schiffer, A.J. Williams, J.G. Kim, R.J. Cava The pyrochlore Tb$_{2}$Ti$_{2}$O$_{7}$ is a cooperative paramagnet that has generated much interest in the frustrated magnetism community due to the presence of persistent short range spin-spin correlations and its apparent lack of long range magnetic order down to temperatures below $\textit{T}$ = 0.05 K, despite an effective spin-spin interaction strength given by \textit{$\theta_{W}$} $\sim$ 10 K. Motivated by recent work on the stuffed spin ices, we have begun investigations into stuffed variants of Tb$_{2}$Ti$_{2}$O$_{7}$, in which we replace some of the nonmagnetic Ti$^{4+}$ with magnetic Tb$^{3+}$, thus gradually changing the geometry of the magnetic sublattice from a lattice of corner sharing tetrahedra to a disordered lattice of side sharing tetrahedra. Here, we present results from magnetization and neutron scattering studies on powder samples of Tb$_{2+ \textit{x}}$Ti$_{2-2 \textit{x}}$Nb$_{ \textit{x}}$O$_{7}$, where $\textit{x}$ = 0.2, 0.4, 0.6, or 1, where diffraction data indicate that the $\textit{x}$ = 1 material has a disordered fluorite lattice. Preliminary results indicate that short range magnetic correlations similar to those present in Tb$_{2}$Ti$_{2}$O$_{7}$ exist in all of the materials studied, and that long range antiferromagnetic order may exist in the $\textit{x}$ = 1 material. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P31.00005: Quantized Berry phase and entanglement entropy for a spin ladder system Isao Maruyama, Shou Tanaya, Mitsuhiro Arikawa, Yasuhiro Hatsugai We have demonstrated that quantized Berry phases and entanglement can be used as a new tool for exploring gapped systems which do not exhibit symmetry breaking. Especially, using quantized Berry phases in several gapped systems we identified location of singlet pairs successfully, which illustrate the valence bond solid (VBS)[1], the itinerant singlet [2], the Kondo singlet. Recently, we have studied four-spin ring exchange interaction in a spin-1/2 two-leg ladder, which introduces frustration and generates various phases, such as the dominant vector-chirality (DVC) phase. In the DVC phase, the Berry phase detects the plaquette singlet[3] while entanglement entropy shows that degree of freedom of the edge state is non-zero. It means that singlets are localized at every links and separated as a free spin by adopting the boundary as in the VBS phase. In fact, under the open boundary condition, we found S=1 three-fold degenerated excited states with the small gap depends on the system-size as in the Kennedy triplet. [1] T.Hirano, H.Katsura, Y.Hatsugai, PRB 77 (2008) 094431 [2] I.Maruyama, Y.Hatsugai, JPSJ 76 (2007) 113601 [3] I.Maruyama, T.Hirano, Y.Hatsugai, AX.0806.4416 [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P31.00006: Tuning magnetic frustration of nanomagnets in triangular-lattice geometry X. Ke, J. Li, S. Zhang, C. Nisoli, V. Crespi, P. Schiffer We study the configuration of magnetic moments on triangular lattices of single-domain ferromagnetic islands, examining the consequences of magnetostatic interactions in this frustrated geometry. By varying the island-island distance along one direction, we are able to tune the ratio of different interactions between neighboring islands, resulting in a corresponding variation in the local correlations between the island moments. Unlike other artificial frustrated magnets, this lattice geometry displays regions of ordered moment orientation, possibly resulting from a higher degree of anisotropy leading to a reduced level of frustration. Reference: X. Ke \textit{et al}., Appl. Phys. Lett., in press (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P31.00007: Magnetization Reversal in Artificial Kagome Ice Stephen Daunheimer, Yi Qi, Todd Brintlinger, Paula Mellado, Oleg Tchernyshyov, John Cumings Lorentz-force transmission electron microscopy (LTEM) and ferromagnetic resonance (FMR) have been used to examine magnetization reversal of artificial kagome ice in detail. Previously, no ice rule violations have been observed in this novel magnetic metamaterial [1], in contrast to artificial square ice, where ice rule violations appear to be a robust feature [2]. Theoretical considerations support this observation but predict the generation of transient defects violating the ice rules during magnetization reversal. Such defects are expected to self-annihilate upon completion of the reversal process or removal of the applied magnetic field. We will present results on the experimental exploration for ice-rule violations in this system both during field rotation and uniaxial field reversal using both FMR and LTEM. [1] Y. Qi et al., Phys. Rev. B 77, 094418 (2008) [2] X. Ke et al., Phys. Rev. Lett. 101, 037205 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P31.00008: Equilibration and response properties in spin ice systems Claudio Castelnovo, Roderich Moessner, Shivaji Sondhi It was recently argued that magnetic monopoles emerge in a class of exotic magnets known as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into deconfined monopoles. Here we investigate analytically and numerically the effects that these peculiar excitations have on the equilibration and response properties of a system. In particular, we study temperature quenches in exhaustive detail. The implications of these results on the possibility of finding new experimental signatures of magnetic monpole excitations in rare earth titanates (Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$) are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P31.00009: Monopole and Dirac string constrained Dynamics in Spin Ice Ludovic Jaubert, Peter Holdsworth Since its discovery approximately 10 years ago [1], spin ice has proved to be an abounding source of exotic collective phenomena, in particular long-range dipolar correlations and unconventional phase transitions. But its most impressive property is undoubtedly the occurrence of 3d fractionalisation through excitations out of its topological ground state, behaving like effective \textit{magnetic monopoles} [2]. I shall present an experimental signature of their influence in magnetic relaxation measurements for a spin ice material Dy$_{2}$Ti$_{2}$O$_{7}$ [3] and show that the observed low temperature dynamical slow down can be explained quantitatively by the Coulomb interactions between monopoles and the overlapping of Dirac strings filling the quasi-particle vacuum [4]. \\[3pt] [1] M. Harris \& al. Phys. Rev. Lett. \textbf{79}, 2554 (1997). \\[0pt] [2] C. Castelnovo \& al. Nature \textbf{451}, 42-45 (2008). \\[0pt] [3] J. Snyder \& al. Phys. Rev. B \textbf{69}, 064414 (2004). \\[0pt] [4] L.D.C. Jaubert \& P.C.W. Holdsworth (submitted) [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P31.00010: Slow spin relaxation in dipolar spin ice. Martin Orendac, Lucia Sedlakova, Alzbeta Orendacova, Peter Vrabel, Alexander Feher, Daniel M. Pajerowski, Justin D. Cohen, Mark W. Meisel, Masae Shirai, Steven T. Bramwell Spin relaxation in dipolar spin ice Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$ was investigated using the magnetocaloric effect and susceptibility. The magnetocaloric behavior of Dy$_{2}$Ti$_{2}$O$_{7 }$at temperatures where the orientation of spins is governed by ``ice rules`` (T $<$ T$_{ice})$ revealed thermally activated relaxation; however, the resulting temperature dependence of the relaxation time is more complicated than anticipated by a mere extrapolation of the corresponding high temperature data [1]. A susceptibility study of Ho$_{2}$Ti$_{2}$O$_{7 }$ was performed at T $>$ T$_{ice }$and in high magnetic fields, and the results suggest a slow relaxation of spins analogous to the behavior reported in a highly polarized cooperative paramagnet [2]. [1] J. Snyder et al., Phys. Rev. Lett. 91 (2003) 107201. [2] B. G. Ueland et al., Phys. Rev. Lett. 96 (2006) 027216. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P31.00011: Quantum Spin Ice for Pr Pyrochlore Magnets Shigeki Onoda, Yoichi Tanaka We theoretically propose a new state comprising a quantum-mechanical analogue of the spin ice for pyrochlore magnets. In classical spin-ice systems like Dy$_2$Ti$_2$O$_7$, the ice rule is mainly driven by the magnetic dipolar interaction, which is proportional to the square of the total angular momentum $J$. Therefore, for Pr$^{3+}$ ions having two $f$ electrons forming the $J=4$ localized moment, the dipolar interaction becomes an order of magnitude smaller than that for Dy ions. Then, the magnetic superexchange interaction should play a important role. In fact, the form of the exchange interaction is nontrivial because of the highly relativistic nature of $f$ electrons with strong LS coupling and crystal-field effect. Here, we present a microscopic derivation of the effective relativistic spin-orbital Hamiltonian for the pyrochlore magnets Pr$_2TM_2 $O$_7$ with a transition-metal element $TM$. Then, it is shown that the nearest-neighbor exchange interaction is significantly modified from antiferromagnetic to ferromagnetic by quantum- mechanical processes through excited states split by the crystal field. This bears a quantum-mechanical formation of the ice rule for Pr magnetic moments. Solving the Hamiltonian for a Pr$_4$O tetrahedral cluster, we obtain a further small energy splitting of the low-energy states, leaving doubly degenerate ground states per tetrahedron. Implications for the lattice model and possible relevance to experiments are also discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P31.00012: SrEr$_2$O$_4$ in an applied magnetic field --a quantum phase transition? O.A. Petrenko, G. Balakrishnan, T.J. Hayes, P. Manuel, D.T. Adroja, L.C. Chapon SrEr$_2$O$_4$ belongs to a family of materials with the formula Sr$Ln_2$O$_4$, where $Ln=$ Gd, Dy, Ho, Er, Tm and Yb. In these compounds the magnetic $Ln$ ions are linked through a network of triangles and hexagons~[1]. Despite the strong exchange interaction ($\Theta_{CW}\!\! \approx \! -12$~K), long range ordering develops in SrEr$_2$O$_4$ only at 0.75~K~[2]. The structure consists of FM chains running along the {\it c}~axis, two adjacent chains being stacked antiferromagnetically. The moments point along the {\it c} direction, but only one of the two Er sites has a sizeable moment of 4.5~$\rm \mu_B$. An unusual behaviour in SrEr$_2$O$_4$ is observed in an applied field, where for $H\!\! \parallel \! c$ axis, a field of 0.5~T completely destroys long range magnetic order and introduces instead some sort of state with short range magnetic correlations. This conclusion is reached on the basis of neutron diffraction experiment at ISIS, where a replacement of the sharp Bragg peaks by broad diffuse scattering features is observed. A further increase in magnetic field causes a restoration of the long range order and a disappearance of the diffuse scattering. These observations resemble the behaviour seen around a quantum critical phase transition, although additional investigations are required to prove the presence of a QCP in SrEr$_2$O$_4$. [1]. H.~Karunadasa {\it et al.}, Phys. Rev. B {\bf 71}, 144414 (2005). [2]. O.A.~Petrenko {\it et al.}, Phys. Rev. B {\bf 78}, 184410 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P31.00013: Dynamic spin ice: Pr$_{2}$Sn$_{2}$O$_{7}$ Christopher Wiebe, Haidong Zhou, John Janik, Luis Balicas, Youn-joon Jo, Yiming Qiu, John Copley, Jason Gardner In this presentation, we report a new spin ice - Pr$_{2}$Sn$_{2}$O$_{7}$ - which appears to have enhanced residual entropy due to the dynamic nature of the spins. Neutron scattering experiments show that at 200 mK, there is a significant amount of magnetic diffuse scattering which can be fit to the dipolar spin ice model. However, these short-ranged ordered spins have a quasielastic response that is atypical of the canonical spin ices, and suggests that the ground state is dynamic (ie. composed of locally ordered 2-in, 2-out spin configurations that can tunnel between energetically equivalent orientations). We report this as an example of a \textit{dynamic} spin ice down to 200 mK. [Preview Abstract] |
Session P32: Focus Session: Spin Dependent Physics in Organic Materials
Sponsoring Units: GMAG DMPChair: Frank Tsui, University of North Carolina
Room: 336
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P32.00001: Organic Magnetoresistance Invited Speaker: In recent years a broad range of magnetoresistance phenomena have been reported for organic-based semiconductors, conductors and magnets. Organic systems illustrating magnetoresistance, include molecular- and polymer-based nonmagnetic semiconductors[1], organic-based spin polarized magnetic semiconductors,[2] nonmagnetic conducting polymers, and ferromagnet/organic semiconductor/ferromagnet heterojunctions. Examples of each of these organic-based systems will be presented together with a discussion of the roles of magnetotransport mechanisms including interconversion of singlets and triplets, compression of the electronic wavefunction in presence of a magnetic field, quantum interference phenomena, effects of a ``Coulomb gap'' in $\pi $* subbands of organic magnetic semiconductors with resulting near complete spin polarization in conduction and valence bands of magnetic organic semiconductors.[2,3] Opportunities for magnetotransport in Ferromagnet/Organic Semiconductor/Ferromagnet heterojunctions will be discussed.[4] \\[4pt] [1] V.N. Prigodin \textit{et al.}, Synth. Met. \textbf{156}, 757 (2006); J.D. Bergeson \textit{et al.}, Phys. Rev. Lett. \textbf{100}, 067201 (2008) \\[0pt] [2] V.N. Prigodin \textit{et al}., Adv. Mater. \textbf{14}, 1230 (2002. \\[0pt] [3] J.B. Kortright \textit{et al}., Phys. Rev. Lett., \textbf{100}, 257204 (2008). \\[0pt] [4] J.D. Bergeson, \textit{et al.}, Appl. Phys. Lett. \textbf{93}, 172505 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P32.00002: Magnetic and magnetotransport properties of organic trilayers of alkanedithiol self-assembled monolayers sandwiched between ferromagnetic thin films William Rice, Jeremy Niskala, Jeff Haller, Paul Hoertz, Wei You, Frank Tsui Magnetic and magnetotransport properties of organic spin valve structures have been studied. The organic trilayer structure consists of a self-assembled monolayer (SAM) of alkanedithiol sandwiched between two ferromagnetic metal contacts, a Ni film as the bottom contact and a Co film as the top contact. The SAM was formed using novel methods on the Ni surface on the bottom of the vertical structure. Two alternative designs have been developed, one uses an additional conducting polymer layer for electrical isolation during thermal evaporation of the top Co contact and another uses nanotransfer printing to directly apply the top Co contact. Each trilayer was examined in vacuum using 4-terminal transport measurements. Both designs have indicated tunneling as the transport mechanism between contacts. Magnetooptic Kerr Effect (MOKE) measurements show independent switching of the ferromagnetic layers at approximately 50 and 100 Gauss. Magnetotransport measurements were carried out as a function of bias voltage, temperature and field, in order to explore spin-dependent transport through the organic interlayer. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P32.00003: Effects of interface microstructure on magnetotransport in organic spin valve structures Yaohua Liu, Taegweon Lee, H. E. Katz, D. H. Reich, S. M. Watson, J. A. Borchers Organic semiconductors hold promise for spintronics because of their potentially long spin diffusion length. We have studied Fe/Alq$_3$ (tris(8-hydroxyquinoline) aluminum)/Co multilayer films with Alq$_3$ thickness in the range 50 to 150 nm.[1] Similar to previously reported results, we found considerable variability in the magnetotransport properties for cross junctions made in nominally identical conditions. To explore the sources of these effects, we studied the microstructure of such multilayer films by X-ray reflectometry and polarized neutron reflectometry (PNR). We found that the films show well-defined layers with limited chemical intermixing (3-5 nm) at the Alq$_3$/ferromagnet (FM) interfaces. However, larger magnetoresistance (MR) is associated with sharper Alq$_3$/FM interfaces, and with a magnetically dead Fe-rich region at the Alq$_3$/Fe interface, which may potentially circumvent the resistivity mismatch problem. The PNR data also show that the Co layer on top of the Alq$_3$ can adopt a multi-domain magnetic structure at low field and a perfect anti-parallel state is not obtained. [1] Y. Liu et al., arXiv:0810.0289v1. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P32.00004: Enhanced Magnetoresistance in Alq3-based spin valve using buffer-layer assisted growth Dali Sun, Chengjun Sun, Lifeng Yin, Hangwen Guo, Zheng Gai, Xiaoguang Zhang, Zhao-hua Cheng, Jian Shen In the field of organic spintronics, interfacial diffusion between magnetic electrodes and organic spacer layers is a serious problem for both understanding the underlying mechanism and achieving high magnetoresistance. Using buffer layer assist growth, we have successfully fabricated vertical organic spin valves with much sharper interface. Spin valves prepared by this method exhibit considerably larger magnetoresistance. The spacer layer thickness-dependent magnetoresistance suggests that field-dependent interfacial barrier plays the crucial role for the observed magnetoresistance. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P32.00005: Spin and Charge Injection and Transport in Ferromagnet/Organic Semiconductor/Ferromagnet Heterejunction Jung-Woo Yoo, H. W. Jang, C. B. Eom, Chi-Yueh Kao, A. J. Epstein We studied the spin injection and transport in organic semiconductor by employing LSMO and Fe as an anode and cathode in hybrid spin valve structure. Using thin layer ($t$ $<$ 10 nm) of rubrene as a spacer, our device displays MR as high as 50 $\% $ at low temperature and at low bias voltage. The charge injection into organic spacer in our devices is injection limited. At high applied voltage ($V$ $>$ $V_{th}$) field-driven drift current prevails in current density through the organic semiconductor. At low bias $V$, inelastic hopping followed by thermionic emission is dominant at high $T$, which decreases significantly as $T$ lowered. And eventually the current density through the device becomes purely tunneling at low $T$ and $V_b$. The spin and charge injection, effects of inclusion of tunneling barrier, and the effects of crystallinity of organic layer will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P32.00006: Spin filtering of Photo-excited charge from Organic Nanostructures Aditya Mohite, Bruce Alphenaar, Tiffany Santos, Jagadeesh Moodera In organic materials, coupling between the incident photon and the electron spin is very weak. Here we demonstrate that spin filtering materials can be used to induce intersystem crossing, and allow the spin polarized triplet excitonic states to be probed. A thin layer of EuS was deposited at the interface formed between a single-wall nanotube and an aluminum contact. EuS is a ``spin filtering material,'' a ferromagnetic insulator with large spin-orbit coupling, allowing preferential tunneling by electrons of a preferred spin direction. A small magnetic field is applied to align the electron spin in the EuS with the carbon nanotube. The enhanced spin-orbit interaction allows for observation of a low-energy peak in the capacitive photocurrent scan. The energy spacing between the two peaks matches closely with the theoretical predictions for the S$_{1}$--T$_{1}$ spacing in nanotubes. Further measurements of the triplet peak reveal that its magnitude depends on the orientation of the B-field with maximum peak height occurring when the spins in EuS line up with the nanotube axis. These measurements suggest that introduction of a spin filtering layer could be used to study the triplet formation in organic solar cell materials. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P32.00007: Spin and charge transport study in single crystal organic semiconductors Karthik V. Raman, Carlijn L. Mulder, Marc A. Baldo, Jagadeesh S. Moodera Spin transport studies in amorphous rubrene films have shown exciting and promising results [1]. A large spin diffusion length in these amorphous films has increased the motivation to perform spin transport study in high purity single crystal rubrene. This will provide the fundamental understanding on the spin transport behavior in OS; not influenced by defects or traps. We will present work on small channel single crystal rubrene FET device with magnetic electrodes. For example, our preliminary studies have show mobility for FET with Co electrode to be 0.014cm$^{2}$/V-s. A study on the spin and charge transport properties in single crystals of OS with magnetic electrodes is being done and the results will be reported. The influence of gate voltage and applied magnetic field on the transport properties will be discussed. [1] J.H. Shim et al., PRL \textbf{100}, 226603 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P32.00008: Inter-Molecular Spin-Orbital Coupling Effects on Magnetoresistance and Spin-Dependent Excited Processes in Organic Semiconductors Liang Yan, Bin Hu A low magnetic field can change electrical current and electroluminescence in organic semiconductors, leading to magnetoresistance and magnetic field effects due to magnetic field-dependent singlet/triplet ratio involved in charge transport and excited states. In general, an external magnetic field can change singlet and triplet ratios through two major pathways: spin-dependent electron-hole pairing and field-dependent intersystem crossing. We found that tuning inter-molecular spin-orbital coupling leads to a significant change in magnetoresistance, electro-fluorescence, and electro-phosphorescence. These experimental findings indicate that (i) inter-molecular and intra-molecular electron-hole pairs account for magnetoresistance and magnetic field effects, respectively, (ii) spin mixing occurs in inter-molecular excited states, and (iii) spin-mixing is a function of both spin-orbital coupling and singlet-triplet energy difference. This presentation will discuss the effects of magnetic field on both spin-dependent electron-hole pairing and spin mixing in magnetoresistance and magnetic field effects in organic semiconductors. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P32.00009: Magnetoresistance in the High Magnetic Field Regime for Organic Semiconductors J.L. Martin, V.N. Prigodin, J.D. Bergeson, C.Y. Kao, A.J. Epstein While there has been much study of the low field (of order 100 Oe) magnetoresistance in organic semiconductor (OSC) materials, very little has been done in high fields of order 10kOe. Magnetoresistance studies in the high field were conducted on the OSC materials tris-(8-hydroxyquinoline) aluminum (Alq$_{3})$ and alpha sexithiophene ($\alpha $-6T). The high field shows a different response from that of the low field and displays several features suggesting that multiple mechanisms are at work. In addition, the two materials demonstrate behaviors that differ from one another, suggesting different classes of OSC. The experimental results are discussed in the context of the MIST model [1], which attributes magnetoresistance to the recombination of electron-hole pairs with interconversion of singlets and triplets. \\[0pt] [1] J.D. Bergeson, \textit{et al.}, PRL \textbf{100}, 067201 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P32.00010: Iron Nanoparticle Driven Spin-valve Behavior in Aligned Carbon Nanotube Arrays* Mark B. Murphey, Jeremy D. Bergeson, Stephen J. Etzkorn, Liangti Qu, Junbing Yang, Liming Dai, Arthur J. Epstein Spin-valve structures have been constructed from aligned arrays of carbon nanotubes, yielding a magnetoresistance reaching 25 {\%}$^{1}$. In addition to including vertically aligned carbon nanotube arrays, iron catalyst nanoparticles that form the array function as the second ferromagnetic electrode. Reversal of the magnetization of the electrode in an applied magnetic field results in a clear peak in the resistance of the device. A spin scattering length in excess of 9 $\mu $m shows excellent spin transport through the nanotube array. The effect of oxide barriers and device patternability are explored. 1. Bergeson, et al., Appl. Phys. Lett. \textbf{93}, 172505 (2008) *This work is supported in part by DOE Grant Nos. DE-FG02-86ER45271 and DE-FG02-01ER45931, and AFOSR Grant No. FA9550-06-1-0175 and FA9550-06-1-0384, NSF Grant No. CMS-0609077, and IMR Grant Nos. FG0004 and FG 0036. The support of the Materials and Manufacturing Directorate of the Air Force Research Laboratory is gratefully acknowledged. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P32.00011: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P32.00012: Electrically controlled $g$-factor and magnetism in conjugated metallorganic molecules Zhi-Gang Yu Conjugated metallorganic molecules have localized spins at the central transition-metal ions and mobile $\pi$-electrons in the surrounding ligands. Here we construct model Hamiltonians based on first-principles calculations to describe spins at the ions and $\pi$-electrons in the ligands. It is shown that the $g$-factor and magnetic susceptibility in such a molecule can be tuned to a great extent by an electrical voltage across one of the ligands. The underlying physics is that the voltage modifies the charge distribution of the ligand, which in turn changes the interplay of the the ion's spin-orbit coupling and the energy splitting among its $d$ orbitals. The capability of controlling the $g$-factor and magnetism at the molecular level has great implications in quantum information storage and processing. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P32.00013: Magnetic-Field-Driven Ising Quantum Criticality of Two-Dimensional Square-Lattice Antiferromagnet Cr(dien)(O$_{2})_{2}$.H$_{2}$O N. Kaur, G. Preamplume, N. Dalal, A. Kumar, Y. H. Kim, Y. Takano, S. Nellutla, Y. J. Jo, L. Balicas We report on a systematic study of magnetically driven quantum phase transition in a new compound based on Cr(IV). The compound, Cr(dien)(O$_{2})_{2}$.H$_{2}$O, is a low dimensional antiferromagnet with a Neel temperature T$_{N}$ of 2.55 K in zero field. We have used torque magnetometry, heat capacity and magnetocaloric-effect measurements down to 200 mK, to obtain a complete magnetic phase diagram. A detailed analysis of the dependence of T$_{N}$ on magnetic field using the power law T$_{N}$ $\sim $ (H$_{c}$-H)$^{\alpha }$ yielded the critical exponent $\alpha $ = 2.01$\pm $0.02, with H$_{c}$ = 12.392$\pm $0.003 T, indicating that this system behaves like a 3-d Ising magnet at low temperatures. [Preview Abstract] |
Session P33: Superconductivity: Electronic Structure II
Sponsoring Units: DCMPChair: Goran Karapetrov, Argonne National Laboratory
Room: 403
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P33.00001: Universality of momentum-dependent charge-transfer excitations in the undoped cuprates measured by resonant inelastic X-ray scattering Guillaume Chabot-Couture, Jason Hancock, Li Lu, Patrick Mang, Owen Vajk, Diego Casa, Thomas Gog, Martin Greven From its conception, the study of charge-transfer excitations in the cuprates has driven the majority of resonant inelastic X-ray scattering (RIXS) research. Still, to this day, there exists more variation among published RIXS spectra and their interpretation than common ground. By carefully studying the momentum-dependent spectral weight of three of the most important undoped cuprates -- La$_2$CuO$_4$, Nd$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$ -- we present evidence of universality of the observed charge-transfer excitations. To clarify and highlight this observation, we construct dispersion diagrams and study the incident energy dependence. Comparison to Hubbard model predictions suggests that the essence of this universality is captured already by one-band physics, while material dependence and cross-sectional effects only obfuscate this behavior. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P33.00002: Momentum-Resolved Cu K-edge RIXS Spectra in the Insulating Parent Compounds of High Tc Superconductors C-C. Chen, B. Moritz, F. Vernay, S. Johnston, J. Hancock, G. Chabot-Couture, M. Greven, I. Elfimov, G. A. Sawatzky, T.P. Devereaux Resonant inelastic X-ray scattering (RIXS) has the ability to highlight various many-body excitations that can be characterized by photon momentum transfer and energy loss. Exact Diagonalization calculations on small clusters were carried out to investigate the nature of the excitations seen in RIXS spectra in the parent compounds of high Tc superconductors. The model many-body calculation includes electronic orbitals necessary to highlight Zhang-Rice singlets, charge transfer and d-d excitations, as well as states with significant apical character. The influence of different orbitals on the RIXS spectra is studied, and the character of the excitations in different regions of the Brillouin zone is determined as well. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P33.00003: Momentum dependence of the electron-phonon coupling, phonon-induced pairing interaction, and self-energy effects in YBa$_2$Cu$_3$O$_7$ within the local density approximation Rolf Heid, Klaus-Peter Bohnen, Dirk Manske, Roland Zeyher Using the local density approximation (LDA) and a realistic phonon spectrum we calculate the momentum and frequency dependence of the electron-phonon coupling in YBa$_2$Cu$_3$O$_7$ and determine its consequences for the phonon-induced pairing interaction and for the electronic self-energy in the normal state. The phonon-induced interaction has a pronounced peak for large momentum transfers and the interband contributions between bonding and antibonding band are of the same magnitude as the intraband ones. The dimensionless coupling constant in the d-wave channel $\lambda^d$, relevant for superconductivity, is only 0.022, i.e., even about ten times smaller than the small value of the s-wave channel. For electronic states at the Fermi energy, the maximum in the real part of the phonon-induced self-energy at low frequencies is about a factor 5 too small compared to the experiment, resulting in a very small and smooth change in the slope of the electronic dispersion [1]. These findings suggest that phonons are not the important low-energy excitations, and cannot produce well-pronounced kinks in YBa$_2$Cu$_3$O$_7$, at least, within LDA. [1] Heid, Bohnen, Zeyher,Manske, PRL {\bf 100}, 137001 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P33.00004: Electronic Properties of Rocksalt Copper Monoxide Paul Michael Grant Rocksalt copper monoxide, although not yet synthetically realized in bulk form, can be studied computationally as a proxy for the family of layered HTSC copper oxides. We report results for a series of tetragonal CuO rocksalt structures with c/a lattice parameter ratios ranging from 1.0 to 1.5, employing the plane-wave pseudopotential method with exchange/correlation LDA+U. As expected, we obtain a metallic state for U = 0 at all values of c/a given that the nominal valence electron configuration for Cu in copper monoxides is 3d$^{9}$ yielding a partially occupied conduction band. However, completely unexpected was our finding similar metallic properties in rocksalt CuO for all physically plausible values of U (up to 10 eV) and c/a between 1.0 to approximately 1.2. Only for c/a $>$ 1.2 do our calculations reveal the opening of a Mott-Hubbard charge-transfer gap. We interpret our results$^{1}$ as supporting the original motivations of Bednorz and Mueller that high temperature superconductivity in the layered copper oxide perovskites may begin with their tendency to exhibit Jahn-Teller strong electron-phonon coupling$^{2}$. \newline $^{1}$P. M. Grant, J. Phys: CS \textbf{129} (2008) 01242. \newline $^{2}$J. G. Bednorz and K. A. Mueller, Rev. Mod. Phys. \textbf{60} (1988) 585. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P33.00005: Electronic Structure of Superconducting FeSe Studied by Photoemission Spectroscopy Rikiya Yoshida, Takanori Wakita, Hiroyuki Okazaki, Yoshikazu Mizuguchi, Shunsuke Tsuda, Yoshihiko Takano, Hiroyuki Takeya, Kazuto Hirata, Takayuki Muro, Mario Okawa, Kyoko Ishizaka, Shik Shin, Hisatomo Harima, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya We have performed soft x-ray and ultrahigh-resolution laser photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed an ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P33.00006: Generic phase diagram of ``electron-doped'' T' cuprates M. Naito, O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe The electronic phase diagram of cuprate superconductors is a key ingredient to understand the still unresolved mechanism of high-temperature superconductivity. A particular interesting question is the differences and similarities between the hole- and electron-doped sides. The phase diagram of hole-doped high-$T_{c}$ cuprates has been well established, and shows a well-known ``dome'' shape with maximal superconductivity at a doping level of about 0.15. In contrast, the phase diagram of the electron-doped side is controversial. This is because the superconductivity in the T$'$ cuprates deteriorates seriously by the presence of impurity oxygen (O$_{ap})$ atoms, which have to be cleaned up in order to unveil the generic phase diagram of the $T'$-cuprates. We investigated the generic phase diagram of the electron doped superconductor, Nd$_{2-x}$Ce$_{x}$CuO$_{4}$, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O$_{ap}$ atoms, we found that the $T_{c}$ increases monotonically from 24 K to 29 K with decreasing $x$ from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of O$_{ap}$ ``impurities'' on superconductivity and also magnetism in T' cuprates. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P33.00007: Band-theory description of hole localization and singlet polarons in doped cuprates Alessio Filippetti, Danilo Puggioni, Vincenzo Fiorentini We use an advanced ab-initio band theory (the pseudo-self interaction corrected local density approach, pSIC) to describe spin-compensated polarons (e.g. Zhang-Rice singlets (ZRS)) typical of low-dimensional doped cuprates. Despite their many-body nature, ZRS can be transparently interpreted via (and, in fact, constructed from) single-particle states, provided that band theory describes accurately enough their localization in the limit of vanishing band dispersion. We provide examples of polarons in real materials, specifically chain-like Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10 }$and the high-T$_{c}$ superconductor (HTSC) Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{6+y}$. The former is the ideal prototype of dopable one-dimensional cuprate with zig-zag Cu-O interactions. Studying the electronic and magnetic properties over the full range of possible doping, we identify several different polaron-dominated ground states and the attendant phase transitions. Furthermore, ZRS are key to the behavior of doped CuO$_{2}$ units in HTSC. We show how their occurrence can dramatically affect the electronic properties (e.g. the Fermi surface) in underdoped Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{6+y}$. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P33.00008: Interplay of Ca and O doping in Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{6+y}$ studied by first-principles calculations Vincenzo Fiorentini, Alessio Filippetti, Danilo Puggioni Experiments reveal an impressive asymmetry in most aspects (involving e.g. magnetic, superconducting, or structural properties) of high-Tc superconductors upon cation or oxygen doping, respectively. A thorough understanding of this asymmetry cannot eschew a rigorous description of the fundamental mechanism ruling electronic and structural properties for each (x,y) doping combination. Here we present results obtained by the pseudo-self-interaction free density functional (pSIC) method, which is capable to describe metal-insulating transitions in several cuprate materials. We describe in detail the chemistry of the distinct insulating-metal transitions occurring in the CuO chains and in the CuO$_{2}$ planes in underdoped YBa$_{2}$Cu$_{3}$O$_{6+y }$for y=[0,0.5]. We then show that interactions with chains crucially affect the ability of Ca doping to inject holes in CuO$_{2}$ planes. The dramatic effects of this double-doping interplay on the magnetic and superconducting properties of underdoped Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{6+y}$ cannot be understood by the disentangled action of the individual doping sources. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P33.00009: Possible enhancements of AFM spin-fluctuations in high-T$_C$ cuprates Thomas Jarlborg Ab-initio band calculations for high-T$_C$ cuprates, together with modelling based of a free electron like band, show a strong interaction between anti-ferromagnetic (AFM) spin waves and periodic lattice distortions as for phonons, even though this type of spin-phonon coupling (SPC) is underestimated in calculations using the local density approximation. The SPC has a direct influence on the properties of the HTC cuprates and it can explain many observations. The strongest effects are seen for modulated waves in the CuO bond direction, and a band gap is formed near the X,Y points, but unusal band dispersion (like ``waterfalls'') might also be induced below the Fermi energy (E$_F$) in the diagonal direction. The band results are used to propose different ways of increasing AFM spin-fluctuations locally, and to have a higher density-of-states (DOS) at E$_F$. Static potential modulations, via periodic distribution of dopants or lattice distortions, can be tuned to increase the DOS. This opens for possibilities to enhance coupling for spin fluctuations ($\lambda_{sf}$) and superconductivity. The exchange enhancement is in general increased near a surface, which suggests a tendency towards static spin configurations. The sensivity of the band results to corrections of the local density potential are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P33.00010: Interband Transitions in La$_{2-x}$Sr$_{x}$CuO$_{4}$ observed by Resonant Inelastic X-Ray Scattering D.S. Ellis, Jungho Kim, H. Zhang, S. Wakimoto, J.P. Hill, Y. Ando, S. Komiya, D. Casa, T. Gog, Y.-J. Kim Resonant inelastic x-ray scattering measures the energy and momentum dependence of electronic excitations, whose probabilities are resonantly enhanced, in this study, by utilizing hard x-rays at the Cu K-edge absorption energy. Three main features in the resonant inelastic x-ray scattering spectrum of La$_{2-x}$Sr$_{x}$CuO$_{4}$ were observed to develop as the doping x increased from the underdoped to the overdoped region of the high-temperature superconductor phase diagram. Measured at the zone-boundary momentum transfer ($\pi $ 0), the spectra consist of three main peaks: one peak below an isosbestic point at 2.2 eV which strengthens at high doping, and two broad peaks above - one at 3.3 eV increasing in energy and decreasing in intensity, and the other stationary at higher energy. Taking a cue from existing band structure calculations, these peaks are interpreted as the transitions between stationary bands of non-bonding Oxygen, a Zhang-Rice singlet type band at the Fermi level, and the upper Hubbard band. These transitions are also discussed in the context of existing angle-resolved photoemission data. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P33.00011: Off gap interface reflectivity of electron waves in Fabry Perot resonators Aritz Leonardo, P.M. Echenique, E. Chulkov, F. Schiller, J.E. Ortega The interface reflectivity in the MgW(110) metallic quantum well is from line shape analysis of high resolution photo emission. A quick reflectivity drop is found away from projected band of the appropriate symmetry near Ef, such that the interface overcomes the bulk like quasi particle lifetime as the line broadening mechanism. A nearly free electron model for the W(110) substrate band structure demonstrates coherent wave function scattering is the relevant mechanism determines the interface reflectivity in the resonator. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P33.00012: Quasiparticle dispersion anomaly induced by the spin excitation in electron-doped cuprates Tao Zhou, Chin-Sen Ting It is proposed that the $50-70$ meV dispersion anomaly (kink) in electron-doped cuprates revealed by recent angle-resolved photoemission spectroscopy experiments is caused by coupling with the spin fluctuation. We elaborate that the kink exists both along nodal and antinodal directions, and both in the superconducting and normal state. The renormalized effect for the density of states is also studied and the hump feature outside the superconducting coherent peak is established, consistent with recent scanning tunnelling microscopy experiments. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P33.00013: Weak flux pinning in the phase separated La$_{2-x}$Sr$_{x}$CuO$_{4+y }$ system Hashini Mohottala, Barrett O. Wells, Joseph I. Budnick, William A. Hines We have studied the magnetic characteristics of a series of super-oxygenated La$_{2-x}$Sr$_{x}$CuO$_{4+y}$ samples. According to our previous studies, these samples spontaneously phase separate to give an oxygen rich superconducting phase with a T$_{C}$ near 40 K and an oxygen poor magnetic phase that also orders near 40 K.\footnote{Mohottala et al Nature Mat. 5, 377 pg. 377 (2006)} All our samples showed a large reversibility in magnetization at different temperatures. Although the internal magnetic regions were expected to behave as pinning sites, our present study shows that they do not favor flux pinning. In terms of the matching principle between the defect and the coherence length, the regions that are larger than the coherence length cannot act as flux pinning centers. Thus our results imply that the magnetic regions are too large to act as pinning centers. Overall less flux pinning in the oxygen rich system also suggests that the separate superconducting regions in the system are more homogeneous. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P33.00014: Preparation of undoped superconducting T'-\textit{RE}$_{2}$CuO$_{4}$ by MBE with ex-situ post-reduction Hideki Yamamoto, Osamu Matsumoto, Michio Naito It has recently been revealed that the optimally-reduced end-member compounds T'-\textit{RE}$_{2}$CuO$_{4}$ (\textit{RE} = Pr, Nd, Sm, Eu, Gd) show superconductivity with $T_{c}$ over 30K [1] although they are commonly believed as Mott insulators. The superconducting specimens were produced by metal organic decomposition (MOD) with elaborated reduction procedures, where the advantage of thin-films, large surface-to-volume ratio, is fully utilized to achieve the optimal oxygen configuration. Their single-crystalline thin films prepared by UHV-based process may have a further advantage of providing a good opportunity for elucidating the highly important controversy in these materials using powerful but surface-sensitive probes such as ARPES and STM. With this motivation in mind, we grew 100-nm-thick T'-\textit{RE}$_{2}$CuO$_{4}$ (\textit{RE} = Pr, Nd, Sm) films on SrTiO$_{3}$ substrates by MBE. The as-grown films were semiconducting. However, with a post-reduction treatment in a tubular furnace, which is essentially identical to that for the MOD films, the MBE films became metallic and showed superconductivity at $\sim $ 30K, indicating that the superconducting films are potentially obtainable through in-vacuo process by tuning up the reduction conditions. [1] O. Matsumoto et al., Physica C 468 (2008) 1148; M. Naito et al., J. Phys: Conf. Ser. 108 (2008) 012037. [Preview Abstract] |
Session P34: Superconductivity: STM
Sponsoring Units: DCMPChair: Maria Iavarone Iavarone, Argonne National Laboratory
Room: 404
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P34.00001: Heating Effects in Interlayer Tunneling Spectroscopy of Bi$_{2.1}$Sr$_{1.4}$Ca$_{1.5}$Cu$_{2}$O$_{8+\delta }$ as Inferred from Single Junction Methods C. Kurter, J.F. Zasadzinski, L. Ozyuzer, D.G. Hinks, K.E. Gray In order to study Joule-heating effects on small intrinsic Josephson junction (IJJ) stacks or mesas, we compare their current-voltage curves, I(V), with those of single junctions, both using Bi$_{2.1}$Sr$_{1.4}$Ca$_{1.5}$Cu$_{2}$O$_{8+\delta }$ (Ca-Bi2212) intercalated by HgBr$_{2}$. Even for small volume stacks with reduced dissipation by intercalation, there can be self-heating despite the absence of the commonly seen backbending of I(V). This conclusion is based on distinctive features of I(V) of intermediate size mesas which were absent in single junctions. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P34.00002: Development of a novel variable temperature scanning tunneling microscope and discovery of spectral weight shift between two bands across Tc in underdoped Bi2212. Jhinhwan Lee, K. Fujita, C.K. Kim, A. Schmidt, H. Eisaki, S. Uchida, J.C. Davis We investigated the quasiparticle interference as a function of temperature for underdoped Bi2212 with Tc=42K, using the newly developed variable temperature STM. Due to increased S/N and resolution, we could observe for the first time the dispersing octet peaks well above Tc. With novel high momentum resolution analysis we also found that each octet peak actually consists of two bands with distinct dispersions and observed clear spectral weight shift from one band, corresponding to the Bogoliubov quasiparticle whose dispersion depends sensitively on the temperature, to the other, with dispersion roughly following the normal state band structure with no significant temperature dependence, as we cross Tc from the superconducting state to the pseudogap state. This new discovery may shed new insight to our understanding of the pseudogap state of the underdoped cuprate superconductor. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P34.00003: Imaging the Vortex Liquid State in Bi$_{2}$Sr$_{2}$CuO$_{6+\delta}$ T.L. Williams, M. Zech, Yi Yin, T. Kondo, T. Takeuchi, H. Ikuta, J.E. Hoffman We use a low temperature scanning tunneling microscope (STM) to study the vortex state of the high-T$_c$ superconductor Bi$_{2}$Sr$_{2}$CuO$_{6+\delta}$ in magnetic fields up to 9 T. At a temperature of 6 Kelvin, we find no localized vortices down to H = 0.25 T. However, the gap depth from the spatially averaged dI/dV spectrum decreases with increasing magnetic field, which indicates a vortex liquid state. By tracking atomically resolved locations at different magnetic fields, we apply a normalization technique to remove inhomogeneities in the underlying density of states, revealing a more homogeneous superconducting state. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P34.00004: Coexistence of competing orders with two energy gaps in real and momentum space in the High T$_{c}$ Superconductor Bi$_{2}$Sr$_{2-x}$La$_{x}$CuO$_{6+\delta }$ Jihua Ma, Z.-H. Pan, F.C. Niestemski, M. Neupane, Y.-M. Xu, Zqiang Wang, Vidya Madhavan, P. Richard, K. Nakayama, T. Sato, T. Takahashi, H.-Q. Luo, L. Fang, H.-H. Wen, H. Ding We have performed scanning tunneling microscopy and angle-resolved photoemission spectroscopy on optimally doped and overdoped Bi$_{2}$Sr$_{2-x}$La$_{x}$CuO$_{6+\delta }$. We observe two distinct energy gaps that coexist both in real space and in the antinodal region of momentum space below T$_{c. }$We find that the small gap is associated with superconductivity. The large gap persists above T$_{C}$ and seems to be linked to observed charge order. We also find a strong correlation between these two gaps suggesting they are affected by similar physical processes. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P34.00005: Coexistence of superconducting and pseudogap quasiparticles in underdoped Bi2212: Studies of STM/STS and ultra-fast optical spectroscopy Y. H. Liu, T. Kurosawa, Y. Toda, K. Shimatake, N. Momono, M. Oda, M. Ido At present, the relationship between superconducting (SC) gap and psuedogap (PG) of cuprate superconductors is still under intense debate. Here, we present our recent results of the electronic structure and quasiparticle dynamics measured by STM/STS and ultra-fast optical spectroscopy on underdoped Bi2212 crystals, which provide direct evidence that SC and PG quasiparticles coexist below Tc. We will also discuss the origins of the periodic charge order and the nano-scale electronic inhomogeneity. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P34.00006: Origin of electron-hole asymmetry in the scanning tunneling spectrum of $Bi_2Sr_2CaCu_2O_{8+\delta}$ A. Bansil, Jouko Nieminen, Hsin Lin, R. S. Markiewicz We have developed a material specific theoretical framework for modelling scanning tunneling spectroscopy (STS) of high temperature superconducting materials in the normal as well as the superconducting state. Results for $Bi_2Sr_2CaCu_2O_{8+\delta}$ (Bi2212) show clearly that the tunneling process strongly modifies the STS spectrum from the local density of states (LDOS) of the $d_{x^2-y^2}$ orbital of Cu. The dominant tunneling channel to the surface Bi involves the $d_{x^2-y^2}$ orbitals of the four neighbouring Cu atoms. In accord with experimental observations, the computed spectrum displays a remarkable asymmetry between the processes of electron injection and extraction, which arises from contributions of Cu $d_{z^2}$ and other orbitals to the tunneling current. Work supported in part by the USDOE. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P34.00007: The Impact of an Oxygen Dopant in an ideal Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Crystal Steve Johnston, Francois Vernay, T. P. Devereaux Scanning tunneling microscopy studies have shown that local nanoscale pairing inhomogeneities are correlated with interstitial oxygen dopants in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Combining electrostatic and cluster calculations, we examine the impact of a dopant on the local Madelung and charge transfer energies, magnetic exchange J , Zhang-Rice mobility, and interactions with the lattice. It is found that electrostatic modifications locally increases the charge transfer energy and slightly suppresses J. It is further shown that coupling to c-axis phonons is strongly modified near the dopant. The combined e?ects yield broadened spectral features, reduced charge gap energies, and a sizable local increase of J implying a strong local interplay between antiferromagnetism, polarons, and superconducting pairing. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P34.00008: Tunneling Spectral Dip Feature in High Tc Cuprates: Experiment and Analysis John Zasadzinski, Liam Coffey, Cihan Kurter, Ken Gray A fully self-consistent Eliashberg analysis is presented to analyze the spectral dip feature observed in tunnel junctions on Bi2212. Methods include SIS break junctions, intrinsic Josephson junctions in mesas and SIN junctions from STM. This analysis is presented for a variety of doping levels and the resulting electron-boson spectral function and self-energy is compared with other spectroscopic probes. Evidence of spectral dip features in other high Tc cuprates is presented including Tl2212 to demonstrate the universality of the spectral dip and its relation to the mechanism of pairing. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P34.00009: Phenomenological model of the bipartite electronic structure of Bi2Sr2CaCu2O8+d: Predicting bulk thermodynamic quantities from tunneling spectroscopy J.W. Alldredge, K. Fujita, Jinho Lee, M. Wang, H. Eisaki, S. Uchida, P.J. Hirschfeld, J.C. Davis, K. McElroy Using high quality local STM maps with corresponding quasiparticle interference data, we develop a complete phenomenological description of the density of states in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$. This not only describes the local density of states but also consistently describes the spectral density of states derived from the QPI. The model consists of a d-wave gap structure at high energy. At low energies is has an additional higher harmonic term in the d-wave gap. Using this we capture not only the high energy gap signature but also the low energy features in the LDOS which accompany the termination of the QPI signal and this allows us to quantitatively measure the features across a wide series of dopings showing consistence between real and k-space. The use of this simple model allows us to successfully predict superfluid density, confirming that our model can successfully determine bulk physics from a local measurement. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P34.00010: Scanning tunneling spectroscopic evidence for a magnetic field-revealed microscopic order in the high-T$_{C}$ superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ A.D. Beyer, M.S. Grinolds, M.L. Teague, N.-C. Yeh, S. Tajima We present spatially resolved scanning tunneling spectroscopic measurements of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ as a function of magnetic field and at T$<<$T$_{C}$. The observed \textit{intra}-vortex quasiparticle (QP) spectra appear pseudogap (PG)-like, with an energy gap of V$_{PG}\approx $32meV. The value of V$_{PG}$ is significantly larger than the observed \textit{inter}-vortex superconducting (SC) gap, $\Delta _{SC}$=20meV, and equal to the incommensurate spin fluctuation gap observed by neutron scattering. We also observe a secondary and less pronounced intra-vortex gap at $\Delta $'$\sim $7-10meV. Fourier transformation of QP spectra reveals two sets of non-dispersive, field-enhanced conductance modulations with periods of 3.4\underline {+}0.5 and 7.3\underline {+}0.5 lattice constants. Energy histograms of QP spectra show a significant shift from SC to primarily PG-like spectra and a growing enhancement of spectral weight at $\Delta $' as magnetic field increases, implying a significant interplay between SC and a field-enhanced microscopic order. Ref.: Beyer, \textit{et.al. }[arxiv:0808.3016]. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P34.00011: STM studies of Co$_{x}$NbSe$_{2}$ and Mn$_{x}$NbSe$_{2}$ Maria Iavarone, Goran Karapetrov, Roberto Di Capua, Alex Koshelev, Daniel Rosenmann, Terukazu Nishizaki, Norio Kobayashi The effect of the intercalation of Co and Mn into the 2H phase transition-metal dichalcogenide NbSe2 has been investigated with a low temperature scanning tunneling microscope (STM). The effect of individual atomic impurities on the superconduting state has been studied. Tunneling spectroscopy at 0.4 K reveals clear spectroscopic signature of the magnetic impurities at atomic scale. We find that Co is in the weak scattering limit and the tunneling spectra are homogeneous on the sample surface with sharp coherent superconducting peaks. Mn instead acts as a strong scatterer destroying superconductivity at atomic scale, even when the number of impurities is limited to just a few in a correlation volume. The effect of intercalation on the charge density waves will be discussed as well. This work was supported by UChicago Argonne, LLC, Operator of Argonne National Laboratory ("Argonne"). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P34.00012: Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic (STS) studies of electron-doped cuprate superconductor La$_{0.1}$Sr $_{0.9}$CuO$_{2}$ (La-112) M.L. Teague, A.D. Beyer, N.-C. Yeh, S.-I. Lee We present STS studies on the electron-doped cuprate superconductor La-112 as a function of magnetic field (H). The spatially resolved spectra manifest vortices, and the average vortex lattice constant scales consistently with Abrikosov's theory. A hidden pseudogap (V$_{CO})$ smaller than the superconducting gap ($\Delta _{SC})$ is revealed inside the vortex core, and the core radius is comparable to the superconducting coherence length \textit{$\xi $}$_{ab}$ = 4$.$86 nm. Analysis of the energy histograms reveals that $\Delta _{eff}$, where $\Delta _{eff}$=[($\Delta _{SC})^{2 }$+ (V$_{CO})^{2}$]$^{1/2}$, shifts downward with increasing H from $\Delta _{eff}$= 12.2 $\pm $ 0.8 meV at H = 0 to a base value of V$_{CO}$=8.5 $\pm $0.6 meV at H $>$ 0. This finding differs from the behavior of conventional superconductors where the vortex-state spectral weight would shift continuously to lower energies with increasing H and show peaks at zero energy due to suppression of $\Delta _{SC}$ inside vortices. Finally, Fourier transformation of the vortex-state tunneling spectra will be reported and compared with results from other cuprates. Ref.: Teague et al., arxiv:0809.0541. Work supported by NSF Grant DMR-0405088. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P34.00013: Noise Predictions for STM in Systems with Local Electronic Nematic Order Erica Carlson, Yen Lee Loh, Karin Dahmen We propose that thermal noise in local stripe orientation should be readily detectable via STM on systems in which local stripe orientations are strongly affected by quenched disorder. Stripes, a unidirectional, nanoscale modulation of electronic charge, are strongly affected by quenched disorder in two-dimensional and quasi-two-dimensional systems. While stripe orientations tend to lock to major lattice directions, dopant disorder locally breaks rotational symmetry. In a host crystal with otherwise C4 rotational symmetry, stripe orientations in the presence of quenched disorder map to the random field Ising model. While the low temperature state of such a system is generally a stripe glass in two dimensional or strongly layered systems, as the temperature is raised, stripe orientational fluctuations become more prevalent. We propose that these thermally excited fluctuations should be readily detectable in scanning tunneling spetroscopy as telegraph noise in the high voltage part of the local I(V) curves. We predict the spatial, temporal, and thermal evolution of such noise, including the circumstances under which such noise is most likely to be observed. In addition, we propose an in-situ test for assessing whether such noise is due to correlated fluctuations rather than independent switchers. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P34.00014: STM Study of Bosonic Modes in the Cuprate Superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4}$ Vidya Madhavan, Francis Niestemski, Shiliang Li, Pengcheng Dai We use a low temperature (4 K) ultra-high vacuum scanning tunneling microscope (STM) to investigate the electron-doped high temperature superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4}$ (PLCCO). We examine the superconducting gap and the satellite features identified as bosonic modes. We investigate these modes with increasing oxygen reduction which represents the third dimension in the electron-doped superconducting phase diagram. We relate our findings to neutron scattering results performed on the same sample. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P34.00015: Collective action of nanopatterned pins: barrier towards creating interstitial vortices Gorky Shaw, Shyam Mohan, Jaivardhan Sinha, Satyajit Banerjee We show that by nano-patterning a superconductor (NbSe$_{2 }$single$_{ }$crystal) with an array of blind holes produces significant magnetic field sweep rate dependent metastable magnetization response[1]. Our results are explained on the basis of a unique collective action of the blind holes pins which creates a barrier against vortex redistribution inside the sample. We propose that this barrier leads to a phase separation creating distinct population of vortices viz., those pinned on blind holes and those confined in the interstitials between the holes [1]. We find that due to the barrier, there is a significant enhancement in the stability of vortices against thermal fluctuations. [1] Gorky Shaw, Shyam Mohan, Jaivardhan Sinha and S. S. Banerjee* (submitted; xxx.lanl.gov/abs/0811.1256) \textit{*satyajit@iitk.ac.in} [Preview Abstract] |
Session P35: Focus Session: Iron Pnictides and Other Novel Superconductors IX: Mostly Transport
Sponsoring Units: DCMPChair: Sergey Budko, Ames Laboratory
Room: 405
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P35.00001: Low-temperature thermal transport properties of BaNi$_2$As$_2$ Nobuyuki Kurita, Filip Ronning, Yoshifumi Tokiwa, Eric D. Bauer, Alaska Subedi, David J. Singh, Joe D. Thompson, Roman Movshovich Low-temperature specific heat $C$($T$) and thermal conductivity $\kappa$($T$) measurements under magnetic field have been performed on a recently discovered Ni-based superconductor BaNi$_2$As$_2$ ($T_{\mathrm{c}}$\,=\,0.7\,K). In zero field, $\kappa$($T$) shows $T$-linear behavior in the normal state and exhibits a BCS-like exponential decrease below $T_{\mathrm{c}}$. The field dependence of the residual thermal conductivity extrapolated to zero temperature is indicative of a fully gapped superconductor. This conclusion is supported by the analysis of $C$($T$) data, which are well fit based on the BCS theory. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P35.00002: Anisotropy of electronic transport in the iron-pnictide superconductor Ba(Fe,Co)$_2$As$_2$ Makariy A. Tanatar, N. Ni, C. Martin, R.T. Gordon, H. Kim, V.G. Kogan, G.D. Samolyuk, S.L. Bud'ko, P.C. Canfield, R. Prozorov The anisotropy of electrical resistivity in the iron pnictide superconductor Ba(Fe,Co)$_2$As$_2$ has been studied using the Montgomery technique and direct transport measurements in single crystals cut along principal directions of the conductivity tensor (tetragonal c-axis and a-direction perpendicular to it). A good quantitative agreement is found between two sets of data, with a $\rho_c/ \rho_a$ anisotropy of 5 $\pm$ 1 just above the superconducting transition temperature. This is in very good agreement with expectations based on the anisotropy of the critical fields, suggestive of orbital limiting of superconductivity at T$_c$. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P35.00003: C-axis transport of pnictide single crystals. Yuri Koval, Paul Mueller, Guenter Behr, Bernd Buechner Mesa-type structures of $\sim $1 $\mu $m$^{2}$ area were fabricated on the (ab) plane of small LaO$_{0.9}$F$_{0.1}$FeAs single crystals. Resistance vs. temperature measurements showed a metallic behavior with a residual resistance ratio higher than 10. Both magnetic susceptibility and transport measurements showed the same value for the critical temperature, i.e. $\sim $20K. Current-voltage characteristics are typical for overdamped Josephson junctions with a critical current density of $\sim $10$^{5}$ A/cm$^{2}$. Moreover, the critical current vs. temperature dependence follows the Ambegaokar-Baratoff relation for the maximum dc Josephson current. One possible explanation could be that we have observed an intrinsic Josephson effect in $<$c$>$-direction. This is supported by recent measurements of radiation emission between 11 and 12 GHz. Finally, we discuss current injection effects on Josephson critical current and T$_{c}$. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P35.00004: Thermoelectric Power of RO$_{1-x-y}$F$_{x}$FeAs Feng Chen, Kalyan Sasmal, Melissa Gooch, Fengyan Wei, Bernd Lorenz, Yuyi Xue, C.W. Chu, Bing Lv, Zhongjia Tang, Arnold Guloy The thermoelectric power $S$($T$) has been measured on the RO$_ {1-x-y}$F$_{x}$FeAs samples with $0 \leq x \leq 0.3$ and $0 \leq y \leq 0.5$ and with the rare earth R = La, Ce, Sm and Pr. Together with Hall and lattice parameter studies, systematical $x$-dependency is observed, although rather weak for samples with $x > 0$ and $y = 0$. The $S$($T$) of the undoped samples with $x = y = 0$, however, appears to have rather different shape and amplitude. By comparing with the resistivity drop around 150 K, the change seems to be associated with the spin/lattice instability previously proposed. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P35.00005: A novel non-Fermi-liquid state in the iron-pnictide FeCrAs Wenlong Wu, Alix McCollam, Ian Swainson, Patrick Rourke, Denis Rancourt, Stephen Julian We report transport and thermodynamic properties of stoichiometric single crystals of the hexagonal iron-pnictide FeCrAs. The in-plane resistivity shows an unusual ``non-metallic" dependence on temperature $T$, rising continuously with decreasing $T$ from $\sim$ 800~K tobelow 100 mK. The $c$-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at $T_N \sim 125$~K. Below 10~K the resistivity follows a non-Fermi-liquid power law, $\rho (T) =\rho_0 -AT^x$ with $x<1$. The specific heat, on the other hand, shows typical Fermi liquid behaviour with a linear temperature dependence and a large Sommerfeld coefficient, $ \gamma \sim 30 {\rm\ mJ/mol\,K^2}$. The magnetic susceptibility does not follow Curie-Weiss law and it is rather weakly temperature dependent at low temperature. The high temperature properties of FeCrAs are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the $T \rightarrow 0{\rm ~K}$ properties suggest a new class of non-Fermi liquid. This low temperature state has some features expected of a fractionalized electron system, in which conduction electrons break up into a charge carrying part that scatters anomalously and a spin part that has the thermodynamic properties of a Fermi liquid. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P35.00006: Large quasiparticle thermal Hall conductivity in the superconductor $\rm Ba_{1-x}K_xFe_2As_2$ Joseph Checkelsky, Lu Li, G.F. Chen, J.L. Luo, N.L. Wang, N.P. Ong We have measured the thermal conductivity $\kappa_{xx}$ and thermal Hall conductivity $\kappa_{xy}$ in single-crystal $\rm Ba_{1-x}K_xFe_2As_2$. Below the superconducting transition temperature $T_{c}$ ($\sim$ 30 K), we observe a large peak in the weak-field $\kappa_{xy}$ . A corresponding peak in the zero-field thermal conductivity $\kappa_{xx}$ is also observed. Together, these imply the existence of a large population of hole-like quasiparticles below $T_{c}$ . In magnetic fields $H$ approaching 35 T, the peaks in $\kappa_{xx}$ are strongly suppressed. A fit of the $\kappa_{xx}$ vs. $H$ curves shows that the data are consistent with the scattering of long-lived quasiparticles by vortices. Using these fits, we have extracted estimates of the quasiparticle mean-free-path, and separated the zero-field electronic and phononic terms $\kappa_{e}$ and $\kappa_{ph}$, respectively. We discuss the origin of the large quasiparticle population in terms a strongly anisotropic gap parameter or a gap with nodes. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P35.00007: Fe pnictides in high magnetic fields. Invited Speaker: High magnetic fields provide invaluable tool in probing such complex and phase-rich materials as novel pnictide superconductors, where upper critical field estimates surpass 100 Tesla for some of the compositions. We have investigated a number of tertiary and quaternary iron pnictides in pulsed magnetic fields exceeding 60 T and will present the latest data on their superconducting and normal state properties at high fields. We probe the importance of reduced dimensionality for high temperature superconductivity across different families of pnictides, where we find contrasting behavior between 122 and 1111 compounds. We find clear signature of the Fermi surface reconstruction in magnetotransport properties of the field-induced normal state, which coincides with reported structural and magnetic phase transitions. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P35.00008: Spin density wave instability and pseudogap formation in EuFe2As2 Jong Hoon Shin, Soon Jae Moon, Ju-Young Kim, Seung Hyun Kim, Woo Seok Choi, Byung Chul Jeon, Yun Sang Lee, Beong Ki Cho, Kee Hoon Kim, Tae Won Noh Recently, iron arsenide superconductors have aroused great amount of interest. In these compounds, by doping electron or hole, the superconductivity arises with the suppression of spin density wave (SDW) order. The close relation between the superconducting state and SDW instability suggests that the magnetic fluctuation might play an important role. Therefore, it is imperative to study the magnetic ground state of the parent compounds to understand the mechanism of the superconductivity. We investigated optical conductivity spectra of EuFe2As2 single crystals, which showed SDW order below about 190 K. Across the transition temperature, the optical spectral weight transferred from low energy (below 900cm-1) to higher energy (above 900 cm-1), forming a pseudogap. In the SDW phase, the sharp Drude-like response still remained. Our results indicate that the SDW formation induce the partial gap opening in the Fermi surface of EuFe2As2. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P35.00009: Electronic anisotropy from magneto-transport near T$_{c}$ in SmFeAs(O$_{0.7}$F$_{0.25})$ and (Ba,Rb)Fe$_{2}$As$_{2}$ single crystals Philip Moll, Karsten Kunze, Zbigniew Bukowski, Nikolai Zhigadlo, Janusz Karpinski, Bertram Batlogg We derived thermally activated flux flow (TAFF) activation energies E$_{a}$(H) and the upper critical fields \textbf{Hc}$_{2}$(T) parallel to the c-axis and in the Lorentz-force free configuration (\textbf{H} $\vert \vert $ \textbf{ab} $\vert \vert $ \textbf{j}) of SmFeAs(O$_{0.7}$F$_{0.25})$ and (Ba,Rb)Fe$_{2}$As$_{2}$ single crystals from resistance measurements and compare them to the ones reported for other REFeAs(OF). A perfectly rectangular rod (67x11x4 $\mu $m), aligned with the crystal axes, was cut from a larger SmFeAs(O$_{0.7}$F$_{0.25})$ single crystal ($\sim $ 200 $\mu $m) by a Focused Ion Beam (FIB) which allowed us to precisely control its geometry factor L/A = 0.89 1/$\mu $m. The FIB was also used to deposit 4 Pt contacts. We found a slope of \textbf{H}$_{c2, 50\% }$ (T), parallel to the c-axis, of 1.9 T/K for SmFeAs(O$_{0.7}$F$_{0.25})$ and 3.7 T/K for (Ba,Rb)Fe$_{2}$As$_{2}$ near T$_{c}$. The electronic anisotropy, derived from magneto-transport, is significantly larger in the REFeAs(OF) crystals than in (Ba,Rb)Fe$_{2}$As$_{2}$. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P35.00010: Quantum critical regime in the phase diagram of K$_x$Sr$_{1-x}$Fe$_2$As$_2$ Bernd Lorenz, Melissa Gooch, Bing Lv, Arnold M. Guloy, Ching-Wu Chu The electrical and thermoelectric properties of K$_x$Sr$_{1-x}$Fe$_2$As$_2$ are investigated. While the temperature dependence of the resistivity of SrFe$_2$As$_2$ (x=0) and KFe$_2$As$_2$ (x=1) is strongly nonlinear over a large temperature range it becomes surprisingly linear for x close to x$_c$ = 0.4 above the superconducting transition. This apparent deviation from the Fermi liquid behavior is similar to the high-T$_c$ cuprate superconductors and may indicate the existence of a quantum critical regime above the superconducting dome. We show that the temperature dependence of the thermoelectric power S follows a logarithmic scaling, S/T = const.*log(T) at the critical value x$_c$. The experimental results are consistent with a Ginzburg-Landau model for FeAs compounds predicting quantum critical scaling with a dynamical exponent z=2 and an effective dimension d+z=4. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P35.00011: Low temperature thermal conductivity of single crystal Ba$_{1-x}$K$_x$Fe$_2$As$_2$ J.-P. Reid $^1$ , M.A. Tanatar $^2$ , X. Luo $^1$ , N. Doiron-Leyraud $^1$ , N. Ni $^3$ , S.L. Bud'ko $^{2,3}$ , P.C. Canfield $^{2,3}$ , H. Lup $^4$ , Z. Wang $^4$ , H.H. Wen $^4$ , R. Prozorov $^{2,3}$ , Louis Taillefer $^1$ Novel iron-arsenic based superconductors with T$_c$ $>$ 50 K form a good reference for studying physical processes, underlyning high-T$_c$ superconductivity. Indeed, the importance of unique features of the superconductivity in cuprates, like proximity to Mott insulating state in the phase diagram, magnetism, d-wave superconducting pairing can be tested experimentally. Here we report a study aiming at understanding the symmetry of the superconducting gap in iron-arsenic compounds. Temperature and magnetic field dependence of low temperature thermal conductivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals was studied down to 60 mK (T$_c$/500) and in magnetic fields up to 15 T (H/H$_{c2}\cong$~ 0.25 ). We find no residual linear term in zero magnetic field and can therefore exclude a superconducting gap with a line of nodes. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P35.00012: Effects of Co substitution on thermodynamic and transport properties and anisotropic H$_{c2}$ in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. Ni Ni, M. E. Tillman, J-Q. Yan, A. Kracher, S. L. Bud'ko, P. C. Canfield, S. T. Hannahs Single crystal samples of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$, x$<$ 0.12, have been characterized by microscopic, thermodynamic and transport measurements. With increasing Co concentration, the features of the structural and magnetic transitions are suppressed at a rate of roughly 15K per percent of Co. Superconductivity is stabilized at low temperatures for 0.038 $\le $x and up through our highest doping level of x = 0.114. The superconducting region has a dome like appearance with maximum T$_{c}$ values ($\sim $ 23 K) found near x $\sim $ 0.07. The T -- x phase diagram shows that either the existence of superconductivity in both the tetragonal and the orthorhombic (AFM) phase or there is a structural phase separation. Anisotropic H$_{c2}$ data clearly show that the superconductivity which occurs in samples that show features associated with the transition to the low temperature orthorhombic state is 50{\%} smaller than that found in samples that remain in the tetragonal phase. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P35.00013: Log-T upturn in resistivity of lightly-doped oxypnictide superconductor in an intense magnetic field Z. Stegen, S.C. Riggs, J.B. Kemper, Y. Jo, L. Balicas, G.S. Boebinger, F.F. Balakirev, A. Migliori, H. Chen, R.H. Liu, X.H. Chen We report the resistivity of a series of fluorine-doped SmFeAsO$_{1-x}$F$_x$ polycrystalline superconductors in magnetic fields up to 60 T. For underdoped samples ($x < 0.15 $), the low-temperature resistive state is characterized by pronounced magneto-resistance including an upturn in the resistivity at low temperatures. The ``insulating behavior'' is characterized by a log-T divergence observed over a decade in temperature. In contrast, samples with doping $x > 0.15$ display metallic behavior with little magnetoresistance, where intense magnetic fields serve to broaden the superconducting transition rather than significantly suppress T$_c$. The doping regime for the log-T behavior coincides with the spin density wave (SDW) in the phase diagram for SmFeAsO$_{1-x}$F$_x$. [Preview Abstract] |
Session P36: Tutorial for Authors and Referees of Physical Review; followed by Physics Journal Question and Answer
Room: 408
Wednesday, March 18, 2009 8:00AM - 9:30AM |
P36.00001: Tutorial for Authors and Referees Editors from Physical Review Letters and Physical Review will provide information and tips for our less experienced referees and authors. This session is aimed at anyone looking to submit to or review for any of the APS journals, as well as anyone who would like to learn more about the authoring and refereeing processes. Topics for discussion will include advice on how to write good manuscripts, similarities and differences in writing referee reports for PRL and PR, and other ways in which authors, referees, and editors can work together productively. Following a short presentation from the editors, there will be a moderated discussion. Refreshments will be served. [Preview Abstract] |
Wednesday, March 18, 2009 9:30AM - 11:00AM |
P36.00002: Physics Q \& A APS's latest publication, Physics ( http://physics.aps.org/ ), highlights exceptional papers from the Physical Review journals. Each week, editors from the journals choose papers that merit this treatment, aided by referee comments and internal discussion. Physics features expert commentaries written by active researchers who are asked to explain the results to physicists in other subfields. These commissioned articles are edited for clarity and readability across fields and are accompanied by explanatory illustrations. Please join us for a session that will include a brief presentation by the editors of Physics followed by a Q and A. [Preview Abstract] |
Session P37: Focus Session: Structure and Dynamics of Interfacial Water I
Sponsoring Units: DCPChair: Hrvoje Petek, University of Pittsburgh
Room: 409
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P37.00001: Electron Solvation Dynamics at D$_{2}$O Ice and Na/D$_{2}$O/Metal Interfaces Invited Speaker: Electron transfer (ET) across interfaces is of vital importance in different areas of physics, chemistry and biology. Using time-resolved two-photon-photoemission spectroscopy we have studied the ultrafast dynamics of interfacial ET and solvation processes in amorphous and crystalline D$_{2}$O layers on single crystal metal substrates and the influence of coadsorbed Na ions. In these experiments, photoinjection of electrons from the metal into the adsorbate conduction band is followed by ultrafast localization and solvation of the excess electrons. The subsequent energetic stabilization of these solvated electrons due to nuclear rearrangements of the polar molecular environment is accompanied by an increasing degree of localization. The observed ET rates strongly depend on the local structure of the ice. In crystalline D$_{2}$O layers we monitor the stabilization of trapped electrons at the ice vacuum interface continuously from femtoseconds up to minutes. This behavior observed for crystalline ice is fundamentally different from amorphous D$_{2}$O layers where the excess electrons have a much lower survival probability, which lifetimes of the order of 100 fs, which extend to several 10 ps if Na ions are coadsorbed at the ice surface. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P37.00002: Electron dynamics and intermolecular energy transfer in aqueous solutions studied by X-ray electron spectroscopy Invited Speaker: X-ray photoelectron spectroscopy measurements from a vacuum liquid microjet are performed to investigate the electronic structure of aqueous solutions. Here, focus is on the excited-state dynamics of chloride and hydroxide anions in water, following core-level excitation. A series of Cl$^{-}$(aq) charge-transfer-to-solvent (CTTS) states, and their ultrafast relaxation, on the time scale of the core hole, is identified from the occurrence of spectator Auger decay. Resonant oxygen 1s excitation of aqueous hydroxide, in contrast, leads to non-local decay, involving energy transfer into a neighboring water molecule. This channel is argued to arise from the weak hydrogen donor bond of OH$^{-}$(aq), and thus identifies a special transient hydration configuration, which can explain hydroxide's unusual and fast transport in water. Analogous measurements from pure water point to a similar relaxation channel, which is concluded from a strong isotope effect. The characteristic resonance spectral features are considerably stronger for H$_{2}$O(aq) than for D$_{2}$O(aq). As for OH$^{-}$(aq) the results can be understood in terms of energy transfer from the excited water molecule to a neighbor water molecule. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P37.00003: Electrons tunneling through fluctuating water and proteins Invited Speaker: We have analyzed the characteristics of electron tunneling through thermally-fluctuating water and protein media [1]. A metric is defined that indicates when the tunneling propagation is well described by the average donor-acceptor tunneling interaction, as opposed to being dominated by medium fluctuations. Indeed, there is a transition distance that establishes a change in mechanism, and this distance is different for water-mediated compared to protein-mediated tunneling. Even in the fluctuation-dominated regime, we find that the three-dimensional protein fold controls the tunneling interactions. We also find that pairs of proteins in near contact may establish particularly strong water-mediated tunneling routes [2]. \\[4pt] [1] I.A. Balabin, D.N. Beratan, and S.S. Skourtis, Phys. Rev. Lett., 101, 158102 (2008). \\[0pt] [2] J. Lin, I.A. Balabin, and D.N. Beratan, Science, 310, 1311-1313 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P37.00004: Ultraviolet Induced Disordering of the Rutile TiO2 (110) Surface Daniel Hennessy, Michael Pierce, Kee-Chul Chang, Satoru Takakusagi, Kohei Uosaki, Hoydoo You We present x-ray crystal truncation rod (CTR) data collected on the water/rutile TiO2 (110) surface before and after ultraviolet illumination. It is known that UV illumination transforms the surface superhydrophilic, with contact angle 0 degrees. [1] The wet, slightly hydrophilic surface of the clean, prepared samples exhibits a laterally ordered water adlayer that disorders under UV illumination. Contact angle measurements (CAM) show the surface exposed to ambient air is slightly hydrophobic, with contact angle 61(5) degrees. The well-protected dry surface is slightly hydrophilic, with contact angle 32(5) degrees. We propose a model based on domain growth of hydrophilic phases with laterally disordered water adatoms, consistent with some previous studies. [2] [1] R. Wang et al, Nature, 388, 431 (1997). [2] K. Hashimoto, H. Irie, and A. Fujishima, Jap J Appl Phys, 44, 8269 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P37.00005: X-ray studies of the Density Depletion at Hydrophobic Water-Solid Interfaces Markus Mezger, Harald Reichert, Heiko Schroder, John Okasinski, Roland Roth, Helmut Dosch, Sebastian Schoder, Veijo Honkimaki, John Ralston Deeply buried hydrophobic solid-water interfaces were probed with high-energy x-ray reflectivity. The experimental data provide clear evidence for a thin density depletion with an integrated deficit corresponding to approximately 40\% of a water monolayer extending over a maximum of two molecular layers. In addition, measurements on the influence of gases (Ar, Xe, Kr, N$_2$, O$_2$, CO, CO$_2$) dissolved in the water have been performed. No effect on the hydrophobic water gap was found. The presence of nanobubbles at the interface could also be excluded. By comparing the experimental results with an generic DFT model we can give a quantitative estimation for different contributions to the observed gap size. \newline [1] M. Mezger {\em et al.}, {\em Proc. Natl. Acad. Sci. USA} {$\mathbf{103}$}, 18401 (2006). \newline [2] M. Mezger {\em et al.}, {\em J. Chem. Phys.} {$\mathbf{128}$}, 244705 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P37.00006: First-principles study of water on Cu (110) surface Jun Ren, Sheng Meng The persistent demand for cheaper and high efficient catalysts in industrial chemical synthesis, such as ammonia, and in novel energy applications, hydrogen generation and purification in fuel cells motivated us to study the fundamental interaction involved in water-Cu system, with an intension to examine Cu as a possible competitive candidate for cheaper catalysts. Water structure and dissociation kinetics on a model open metal surface: Cu (110), have been investigated in detail based on first-principles electronic structure calculations. We revealed that in both monomer and overlayer forms, water adsorbs molecularly, with a high tendency for diffusion and/or desorption rather than dissociation on clean surfaces at low temperature. With the increase of the water coverage on the Cu (110) surface, the H-bond pattern lowers the dissociation barrier efficiently. More importantly, if the water molecule is dissociated, the hydrogen atoms can diffuse freely along the [110] direction, which is very useful in the hydrogen collection. In addition, we extended to study water on other noble metal (110) surfaces. The result confirms that Cu (110) is the borderline between intact and dissociative adsorption, differing in energy by only 0.08 eV. This may lead to promising applications in hydrogen generation and fuel cells. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P37.00007: A self-consistent polarizable electron water potential: applications to clusters and bulk Leif Jacobson We have recently re-parameterized the electron--water potential due to Turi and Borgis [J.~Chem.\ Phys.\textbf{117}, 6186 (2002)] to be used with the polarizable water potential AMOEBA for use in hydrated electron simulations. In our model the single electronic wave function polarizes the water molecules and vice versa in a fully self-consistent manner. Comparison to binding energies and relative energetics of $(\mbox{H}_2\mbox{O})_n^-$ isomers (with $n<33$) to $ab\ intio$ results show a significant increase in accuracy over the previous parameterization which used a fixed charge water model. The relative importance of polarization in various binding motifs as well as cluster and bulk molecular dynamics simulations will be presented. The simulated optical absorption spectra will also be discussed. [Preview Abstract] |
Session P38: Focus Session: Nanomaterials for Energy Applications I
Sponsoring Units: DCPChair: Nathaniel Rosi, University of Pittsburgh
Room: 410
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P38.00001: Nanoscale Architectures for Energy Applications Invited Speaker: In my group, we have developed a number of different potential architecture systems for gaining insights into energy storage and photovoltaics. In one manifestation of our efforts, generating a heterojunction comprising nanotubes and nanocrystals, externally bound and connected, has been significant. The unique, innovative, and important aspect of this particular nanoscale architecture is that it takes advantage of the tunability, in terms of size, shape, and chemistry, of nanotubes and nanocrystals, to create a sharp junction interface, whose properties are inherently manipulable, tailorable, and hence, predictable. For example, the electrical resistance of nanotube-nanoparticle networks is dependent on the nanoscale junctions that exist between these constituent nanomaterials as well as on microscale and macroscale connectivity. Thus, rational design of these nanomaterials is critical to a fundamental understanding of charge transport in single molecules and the determination of their conductance. Results on these systems can therefore be used to increase understanding of intrinsic factors affecting carrier mobility, such as electronic structure, carrier trapping, and delocalization. In a second manifestation, three-dimensional, dendritic micron- scale spheres of alkali metal hydrogen titanate 1D nanostructures (i.e.: nanowires and nanotubes) have been generated using a modified hydrothermal technique in the presence of hydrogen peroxide and an alkali metal hydroxide solution. Sea-urchin-like assemblies of these 1D nanostructures have been transformed into their hydrogen titanate analogues by neutralization as well as into their corresponding semiconducting, anatase titania nanostructured counterparts through a moderate high-temperature annealing dehydration process without destroying the 3D hierarchical structural motif. The as-prepared hollow spheres of titanate and titania 1D nanostructures have overall diameters, ranging from 0.8 $\mu$m to 1.2 $\mu$m, while the interior of these aggregates are vacuous with a diameter range of 100 to 200 nm. We have demonstrated that these assemblies are useful for example as active photocatalysts for the degradation of synthetic Procion Red dye under UV light illumination. In a third set of experiments, a size- and shape-dependent morphological transformation was demonstrated during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts. Our results indicate that as-synthesized titania nanostructures possessed higher photocatalytic activity than the commercial titania precursors from whence they were derived. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P38.00002: High Capacity Hydrogen Sorption in Nanoscale Transition Metal Based Organometallic Complexes Invited Speaker: Using a highly sensitive nanobalance we have discovered high capacity hydrogen absorption in a range of transition-metal(TM) based organometallic complexes prepared using physical vapor deposition techniques. Hydrogen absorption upto 14 wt{\%} has been measured in titanium ethylene complexes and confirmed by mass doubling when deuterium is employed instead of hydrogen. I will present results of comprehensive measurements on other TM-complexes of alkenes, alkanes and ring compounds. I will also discuss these measurements in the context of theoretical calculations based on first principles quantum mechanics that have appeared in the recent literature. Finally, I will discuss the prospects for practical applications of these materials and the problems that might be encountered. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P38.00003: Electron Dynamics in Nanocrystalline TiO2 and ZnO Measured by Terahertz Spectroscopy Charles Schmuttenmaer, Jason Baxter Understanding the microscopic details of carrier transport in nanocrystalline colloidal thin films is required for complete understanding of a variety of photochemical and photoelectrochemical cells utilizing interpenetrating networks. Measuring the photoconductivity in these materials, however, is a challenging problem because of the inherent difficulty of attaching wires to nanometer-sized objects. Furthermore, picosecond carrier dynamics play an important role in efficient charge separation and transport, but the low temporal resolution of traditional methods used to determine their photoconductivity precludes their use in studying sub-ps to ps dynamics. This talk will present recent advances utilizing THz spectroscopy to investigate and elucidate the microscopic behavior of carrier dynamics within the context of materials for energy applications such as dye-sensitized solar cells and solar-driven cells for catalytic chemistry. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P38.00004: Theoretical investigation of the high energy excitations in silicon nanocrystals Adam Gali, Marton Voros, Dario Rocca, Gerely Zimanyi, Giulia Galli Recently, efficient multi-exciton generation (MEG) has been reported for several nanoclusters including silicon nanocrystals (SiNC), too. However, the existence of MEG has been disputed in the literature. The reported bi-exciton states in SiNC involve high energy empty states of SiNC. These states are expected to be very delocalized, and thus easily modified by the environment surrounding the SiNCs. In addition, the SiNCs are fabricated in a solution that usually contains CnHm molecules, e.g. hexane, that may bind to the surface of SiNCs, and modify their absorption spectrum. We have studied the absorption spectra of hydrogenated SiNCs by first principles calculations. The geometry was optimized within density functional theory (DFT), while absorption spectra were determined by time-dependent DFT. The effect of the environment on the SiNC was modeled by i) varying the distance between the nanoparticles ii) allowing for surface reconstruction and iii) monitoring the effect of absorption of CnHm groups on absorption spectra. We found that the high energy spectrum of SiNCs strongly depends on the environment. Our findings indicate that taking into account effects of surface states and SiNCs concentration in solution is crucial, in order to understanding multi exciton generation. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P38.00005: Fast exciton relaxation and multiple exciton generation (MEG) in semiconductor nanocrystals: the role of defects Christophe Delerue, Guy Allan Recent works have concluded that a single high-energy photon could generate multiple excitons in semiconductor nanocrystals but these results are debated and are not well understood theoretically. More generally, the physics of the relaxation of excitations in semiconductor nanocrystals receives growing interest. We show that surface defects must play an important role in these processes. We calculate the rate for the relaxation of hot carriers by impact ionization and we show that the presence of surface defects leads to an increase of the relaxation rate at lower excitation energy. We present simulations of the carrier multiplication in Si nanocrystals and we discuss the results of recent experiments in light of these results. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P38.00006: First-principles study of LaSn$_{3 }$ as an anode for lithium-ion batteries Dongwon Shin, Christopher Wolverton, John Vaughey, Michael Thackeray Using both density functional theory (DFT) calculations and experiment, we investigate the tin-rich intermetallic compound LaSn$_{3 }$as a possible anode for lithium-ion batteries. We use DFT calculations to compare the relative energies of hypothetical insertion- and displacement-type reactions in an effort to elucidate the energetically-preferred reaction mechanism of Li with LaSn$_{3}$. From our DFT calculations, we find: (i) lithium insertion reactions with LaSn$_{3}$ are predicted to be energetically unfavorable and highly unlikely to occur; (ii) in contrast, the energetically preferred reaction is a displacement reaction in which La is partially displaced from LaSn$_{3}$ to yield La$_{3}$Sn$_{5}$ and Li reacts with the residual Sn to form Li$_{17}$Sn$_{4}$, corresponding to an electrochemical capacity of 307 mAh/g (iii) this partial displacement reaction is preferred relative to the complete displacement and lithiation of Sn; and (iv) the lithiated-tin compound, Li$_{17}$Sn$_{4}$, is energetically more favored than the commonly reported Li$_{22}$Sn$_{5}$ composition. Electrochemical and structural data largely confirm the DFT predictions; they demonstrate that lithium reacts with LaSn$_{3}$ via a displacement reaction to provide a reversible specific capacity of 200-250 mAh/g. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P38.00007: Ab Initio Prediction of the Size-Dependence of Nano-scale Platinum Dissolution in Water Kristin Persson, Byungchan Han, Gerbrand Ceder In low-temperature fuel cells, the mechanism behind the observed performance loss of the platinum catalyst is not well understood. Using ab initio methods, we calculate 0.5 - 2 nm diameter Pt nanoparticles with varying degrees of O and OH surface absorbates, optimized by site and particle surface structure. In fuel cells, the oxidation of the particle surface origins from the breakup of water molecules. To mimic these conditions we employ a grand canonical ensemble treatment of water as a source of O and H. Additionally, pH effects and dissolved species (from experiments) are incorporated, the latter by changing the experimental element reference state to that of calculated solids. This formalism allows us to determine the stability regions of nanoparticle Pt in equilibrium with water, as a function of particle size, potential and pH. As a result we find enhanced dissolution for the smaller Pt nano-particles, compared to the larger. Furthermore, surface passivation effects from O and OH adsorption do not significantly increase the stability of the nano-particle phases in the potential-pH region relevant for fuel cell operating conditions. Thus, we can identify size-dependent dissolution as a mechanism which will promote the growth of larger particles at the expense of smaller ones and ultimately cause a degradation in the nanoparticle Pt catalyst performance. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P38.00008: First-principles theory of capacitive and electrochemical energy storage Joongoo Kang, Yong-Hyun Kim Recently there has been much interest in development of new electrochemical capacitors to meet high-power and high-energy applications. Pseudo-capacitors using fast surface redox reactions can store electrical energy of 10 to 100 times larger than supercapacitors and still exhibit fast and reversible charge-discharge responses in contrast to batteries. Yet, energy storage mechanisms in super- and pseudo-capacitors have not been fully understood at the level of electrons. Here we have performed first-principles calculations for electrical double layers of a TiO$_{2}$ (101) electrode and solvated lithium ions on the surface, with the ethylene carbonates (EC) as solvent molecules. As Li ions are desolvated from Li-EC$_{4}$ to Li-EC$_{3}$ and bare Li ions, the capacitance gets larger due to the reduced distance between the Li ions and the electrode. When Li ions are intercalated into the subsurface of the TiO$_{2}$ electrode as supposed in pseudocapacitors, the electrostatic energy due to charge separation is reduced for a given stored charge, but the electrochemical reaction starts to occur causing a large increase in the capacitance. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P38.00009: Steps in hydrogen production from methanol on sub-nanometer palladium clusters Faisal Mehmood$^{1}$, Jeffrey P. Greeley$^{2}$, Peter Zapol$^{1}$, Larry A. Curtiss$^{1,2}$ Extensive experimental and theoretical work has been done to understand the decomposition of methanol on various metal and metal oxide nanoparticles for hydrogen production. The activity of sub-nanometer sized particles $<$ 1nm however is not very well known, primarily because of technical challenges involved in preparation and stabilization of the clusters. To explore the properties of the Pd clusters computationally, we have carried out density functional calculations for the methanol decomposition reaction on Pd$_{4}$ and Pd$_{8}$ clusters. The thermodynamics and kinetics of three decomposition routes involving C--O, C--H and O--H scission were investigated; activation energy barriers were determined with the nudged elastic band method. A detailed analysis of the PES for methanol decomposition shows C--O activation to be the least favorable step. In addition, all possible reaction paths for the Pd$_{4}$ cluster are much lower in comparison to single crystal surface and large nanoparticles. To understand how particle size affects the elementary reaction steps, we also present a comparison of methanol decomposition on Pd$_{4}$ with Pd$_{8}$ clusters. Finally, we will discuss the implication of a linear correlation between the transition state and final state energies that is followed for all elementary reaction steps on Pd$_{4}$ and Pd$_{8}$ clusters. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P38.00010: Two-Dimensional Boron-Nitride Layers as Flexoelectric Nanogenerators I. Naumov, A. Bratkovsky, V. Ranjan The direct conversion of ambient motion into electrical energy, especially at nanoscale, is fundamental and technological challenge. Boron-Nitride non-centrosymmetric monolayers are piezoelectrics that can sustain much larger structural and produce very large (a few Volts) voltage drop across flexed nanostrips. We show, with the use of ab-initio calculations, the existence of giant nonlinear flexoelectric effect in BN 2D strips. The induced polarization is quadratic in amplitude of atomic displacements $A$, yet the dipole moment per unit cell is about four times larger compared to PbZrTiO3 [1]. The resulting voltage drop across the BN nanostrip is set by bandgap in material $E_g/q\sim 5$ Volts and nearly independent of the strip width. The large voltage produced by this inert bio-compatible material may find a variety of applications and, in particular, as nanogenerators and sensors powered by an ambient motion or agitation. Prior alternatives, like ZnO, GaN and CdS, are leaky, generate much smaller voltage, and impractical [2]. [1] I.Naumov, A.Bratkovsky, V.Ranjan, arXiv:0810.1775 (2008). [2] Y. Qin, X. Wang, Z.L. Wang, Nature {\bf 451}, 809 (2008); M.A. Schubert et al, Appl. Phys. Lett. {\bf 316}, 122904 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P38.00011: Electrochemical Double Layer Capacitors Using Aligned Multiwall Carbon Nanotubes Grown Directly on Conductive Substrates Rakesh Shah, Xianfeng Zhang, Saikat Talapatra We report on the properties of Electrochemical Double Layer Capacitor (EDLCs) electrodes fabricated using aligned multiwall carbon nanotubes (MWNT) grown on Inconel sheets. Air assisted chemical vapor deposition technique was employed to synthesize the aligned MWNT on these substrates. The capacitive behavior of the EDLC's fabricated using different lengths of aligned MWNTs was examined using cyclic voltammetry, constant current charge/discharge, and impedance spectroscopy. These measurements show that the charge storage phenomenon was non-Faradic with equivalent series resistance in the range of 0.13-0.4$\Omega $. The maximum values of specific capacitance of the carbon nanotube materials used in these devices were in the range of 14.6-21.57 F/g. The maximum value of power density and energy density of the whole supercapacitor devices were 1.48 Wh/Kg and 2.7 KW/Kg, respectively. These results show that the multiwall carbon nanotubes grown directly on conductive substrates are promising candidates as electrodes for electrochemical energy store device applications. [Preview Abstract] |
Session P39: Self-Organization in Biological Cells and Tissues I
Sponsoring Units: DBPChair: Andras Czirok, University of Kansas Medical Center
Room: 411
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P39.00001: Statistical analysis and modeling of collective cell motion and pattern formation Andras Czirok, Andras Szabo Cell motility and its guidance through cell-cell contacts is instrumental in vasculogenesis and in several other morphogenic processes as well. During vasculogenesis multicellular sprouts invade rapidly into avascular areas, eventually creating an interconnected network pattern. Epithelial cell sheets migrate during organogenesis or wound healing. These phenomena were studied with time-lapse microscopy both in vivo and in vitro. Statistical analysis of cell trajectories reveals that motile confluent cultures may behave either as vortical fluids or as deforming elastic sheets. The observed flow fields and pattern formation can be explained by our generalized cellular Potts model -- representing cell polarization and self-propulsion, links between the cytoskeleton of adjacent cells as well as an asymmetric preferential attraction to the surface of adjacent cells. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P39.00002: Self-organization in Systems ofTreadmilling Filaments. Konstantin Doubrovinski, Karsten Kruse The cytoskeleton is an active intracellular network of polar filaments responsible for maintenance of cell shape, cell division, and cell locomotion. A broad variety of cellular processes depend critically on the ability of cyoskeletal filaments to treadmill, i.e. to move by growing at one end while simultaneously shrinking at the other end. In particular, treadmilling is indispensable for cell crawling as well as for generation of various cellular appendages including stereocilia, microvilli, and filipodia. Quantitative modeling of systems involving treadmilling filaments is challenging since it requires describing long-range interactions of particles with many degrees of freedom. We introduce a novel framework for describing systems of treadilling filaments. Within our framework, we identify a class of systems that admit exact solution of the underlying dynamic equations. We compare the corresponding solutions to those obtained by coarse-graining, an approximation which is valid on large length-scales. We apply our new framework to treat two biological systems: cytoskeletal dynamics in fish melanophores and locomotion of human neutrophil cells. In both cases our theory faithfully accounts for the qualitative and semi-quantitative properties of the intracellular structures observed in the corresponding experiments. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P39.00003: Self-assembly of the yeast actomyosin contractile ring as an aggregation process: kinetics of formation and instability regimes Nikola Ojkic, Dimitrios Vavylonis Fission yeast cells assemble an equatorial contractile ring for cytokinesis, the last step of mitosis. The ring assembles from $\sim $ 65 membrane-bound ``nodes''' containing myosin motors and other proteins. Actin filaments that grow out from the nodes establish transient connections among the nodes and aid in pulling them together in a process that appears as pair-wise attraction (Vavylonis et al. Science 97:319, 2008). We used scaling arguments, coarse grained stability analysis of homogeneous states, and Monte Carlo simulations of simple models, to explore the conditions that yield fast and efficient ring formation, as opposed to formation of isolated clumps. We described our results as a function of: number of nodes, rate of establishing connections, range of node interaction, distance traveled per node interaction and broad band width, $w$. Uniform cortical 2d distributions of nodes are stable over short times due to randomness of connections among nodes, but become unstable over long times due to fluctuations in the initial node distribution. Successful condensation of nodes into a ring requires sufficiently small $w$ such that lateral contraction occurs faster then clump formation. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P39.00004: Discrete and continuous models of protein sorting in the Golgi Haijun Gong, Russell Schwartz The Golgi apparatus plays an important role in processing and sorting proteins and lipids. Golgi compartments constantly exchange material with each other and with other cellular components, allowing them to maintain and reform distinct identities despite dramatic changes in structure and size during cell division, development and osmotic stress. We have developed two minimal models of membrane and protein exchange in the Golgi --- a discrete, stochastic model [1] and a continuous ordinary differential equation (ODE) model --- both based on two fundamental mechanisms: vesicle-coat-mediated selective concentration of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins during vesicle formation and SNARE-mediated selective fusion of vesicles. Both show similar ability to establish and maintain distinct identities over broad parameter ranges, but they diverge in extreme conditions where Golgi collapse and reassembly may be observed. By exploring where the models differ, we hope to better identify those features essential to minimal models of various Golgi behaviors. [1] H. Gong, D. Sengupta, A. D. Linstedt, R. Schwartz. Biophys J. 95: 1674-1688, 2008. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P39.00005: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P39.00006: Studying the Transition to Multicellular Life by Altering a Chemical Signaling Channel Carl Franck, Kayvon Daie The starvation response of the eukaryotic microbial system Dictyostelium discoideum has continued to provide opportunities to explore the transition from solitary to collective life. Specifically, one observes a change of behavior from random to synchronized cellular motion reflecting successful long-ranged chemical signaling that leads to aggregation. In the typical experimental universe life goes on upon a flat substrate underneath an ocean of liquid media through which these chemical signals pass. In our observations of starvation development we have uniquely exploited the possibilities afforded by varying the depth of this signaling channel over an interesting range: from essentially infinitely thick (mm's of depth) to an extremely thin wetting layer (below 1 micron). We also examine the development system over a wide range of surface density: from almost a full monolayer to a few percent areal coverage. Our key observation is a striking reduction of the time from the beginning of starvation to the onset of synchronized movement when we reduce the aqueous overlayer thickness to the thinnest values. We provide an interpretation for our observations by combining an exact solution to the diffusive transport problem with a rough dynamical theory for multiagent synchronization. This work was supported by the NIH (P01 GM078586). [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P39.00007: Simulation of cellular shapes on micro-patterned substrate using the Cellular Potts Model Benoit Vianay, Herve Guillou Cell adhesion and motility are processes involved in fundamental biological phenomena using biological structures as anchorage points and cytoskeleton filaments which are very dynamical and at non-equilibrium. We study cell adhesion on micro-patterned substrate where an introduction of a finite distance between anchorage points of the cell modifies drastically the organization of the cytoskeleton and the anchorage point's distribution. Some of statistically most used shapes represent stationary states of the system which should minimize the energy dissipation. We verified this hypothesis reproducing morphologies by simulation of Monte Carlo using the Cellular Potts Model (Graner and Glazier, PRL69 p2013 (1992)). Shapes obtained by simulation depend of four phenomenological parameters as interaction between cell and ECM and are in excellent qualitative agreement with experimental shapes. The aim of this presented work is to link model parameters to physico-chemical properties of cells and to establish phenomenological relations between interesting parameters controlling the cytoskeleton organization. Collaborations : J. Kafer {\&} F. Graner : Laboratoire de spectrometrie Physique -- Grenoble; E. Plannus {\&} M. Block~: Institut Albert Bonniot -- Grenoble. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P39.00008: Turing instabilities on curved surfaces with applications to postsynaptic domain formation Christoph Haselwandter, Martin M. M\"uller, Jemal Guven, Mehran Kardar, Roya Zandi Postsynaptic receptor molecules are one of the key regulators of signal transmission across synapses. Receptors mostly populate postsynaptic domains, which also comprise stabilizing scaffold molecules. The formation of receptor-scaffold domains can be understood as a Turing instability arising from the interactions between receptors and scaffolds. The curvature of the membrane modifies the developing patterns which will be explored using analytical and numerical methods. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P39.00009: Perturbing Streaming in \textit{Dictyostelium discoidium} Aggregation Erin Rericha, Gene Garcia, Carole Parent, Wolfgang Losert The ability of cells to move towards environmental cues is a critical process allowing the destruction of intruders by the immune system, the formation of the vascular system and the whole scale remodeling of tissues during embryo development. We examine the initial transition from single cell to group migration in the social amoeba \textit{Dictyostelium discoidium}. Upon starvation, \textit{D. discoidium} cells enter into a developmental program that triggers solitary cells to aggregate into a multicellular structure. The aggregation is mediated by the small molecule, cyclic-AMP, that cells sense, synthesize, secrete and migrate towards often in a head-to-tail fashion called a stream. Using experiment and numerical simulation, we study the sensitivity of streams to perturbations in the cyclic-AMP concentration field. We find the stability of the streams requires cells to shape the cyclic-AMP field through localized secretion and degradation. In addition, we find the streaming phenotype is sensitive to changes in the substrate properties, with slicker surfaces leading to longer more branched streams that yield large initial aggregates. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P39.00010: Matrix Production in Response to Nutrient Depletion in Bacillus Subtilis Biofilms Thomas Angelini, Michael Brenner, David Weitz Encasing the cells that comprise a bacterial biofilm, the extracellular polysaccharide (EPS) matrix may serve several purposes in biofilm development and survival. One class of examples involves adhesion; the EPS can contribute to cell-cell adhesion and cell substrate adhesion. In contrast to biofilm expansion by proliferation, which produces more nutrient consumers, EPS production could be an alternative, more efficient method of biofilm expansion. The recent work of Vlamakis, et al (2008) demonstrated a transition in the rate of EPS production during biofilm growth. At early stages of development, when the biofilm is thin, a low level of matrix is expressed. At later stages, when the biofilm has thickened, EPS production is dramatically increased. This transition could be a response to nutrient depletion, as there must be a critical biofilm thickness, above which nutrients cannot diffuse into the center of the biofilm before being consumed by cells at the edge. Here we quantify biofilm size and shape during the early stages of Bacillus Subtilis biofilm growth, simultaneously monitoring matrix expression levels. We show that the critical biofilm size scales with nutrient concentration as expected by a simple nutrient depletion model. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P39.00011: Degenerate polygonal tilings in simple animal tissues Primoz Ziherl, Ana Hocevar We study 2D polygonal tilings as models of the en-face structure of single-layer biological tissues. Using numerical simulations, we explore the phase diagram of equilibrium tilings of equal-area, equal-perimeter convex polygons whose energy is independent of their shape. We identify 3 distinct phases, which are all observed in simple epithelial tissues: The disordered phase of polygons with 4-9 sides, the hexatic phase, and the hexagonal phase with perfect 6-fold coordination. We quantify their structure using Edwards' statistical mechanics of cellular systems. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P39.00012: Non-equilibrium self-assembly of a filament coupled to ATP hydrolysis Padinhateeri Ranjith We study the stochastic dynamics of growth and shrinkage of single actin filaments or microtubules taking into account insertion, removal, and ATP/GTP hydrolysis of subunits. The resulting phase diagram contains three different phases: two phases of unbounded growth : a rapidly growing phase and an intermediate phase, and one bounded growth phase. We analyze all these phases, with an emphasis on the bounded growth phase. We also discuss how hydrolysis affects force-velocity curves. The bounded growth phase shows features of dynamic instability, which we characterize in terms of the time needed for the ATP/GTP cap to disappear as well as the time needed for the filament to reach a length of zero ({\it i.e.} to collapse) for the first time. We obtain exact expressions for all these quantities, which we test using Monte Carlo simulations. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P39.00013: Chemotaxis of catalytically-driven nanorods Young-Moo Byun, Paul Lammert, Vincent Crespi, Yiying Hong, Ayusman Sen Chemotaxis, a kind of taxis, is the directed motion of nanoscale organisms such as bacteria along the gradient of chemical concentration. Chemists have created non-biological nanorods, made of gold at one end and platinum at the other, which move autonomously through a solution of hydrogen peroxide due to a catalytic reaction,1 and showed that those metallic nanorods mimic chemotaxis by moving towards regions in a solution with a high concentration of hydrogen peroxide.2 In this talk, we present a theoretical model for chemotaxis and a way of how to analyze the motion of nanorods, and then compare our theory to the experimental data. 1. Paxton et al, J. Am. Chem. Soc., 126, 13424-13431 (2004) 2. Hong et al, Phys. Rev. Lett. 99, 178103 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P39.00014: Self-organization of the MinE ring in subcellular Min oscillations Julien Derr, Jason T. Hopper, Anirban Sain, Andrew D. Rutenberg In the bacterium {\it Escherichia coli}, the mid-cell positioning of division is achieved by the sub-cellular oscillation of Min proteins. MinD interacts with the membrane and polymerizes into filaments. MinE binds to membrane bound MinD leading to the depolymerization of the MinD filaments. It has been observed experimentally that MinE forms a ring, known as the E-ring, near the end of the MinD polymers. We model and solve the self-organization of the E-ring. Rebinding of MinE to depolymerizing MinD filament tips controls MinE ring formation. We find two types of E-ring profiles near the filament tip: a strong plateau-like E-ring as seen {\it in vivo}, controlled by 1D diffusion along the bacterial length, or a weak cusp-like E-ring controlled by 3D diffusion near the filament tip. We discuss the initial instability that leads to MinD filament depolymerization and the formation of the E-ring. We also discuss the duration of transients leading towards strong or weak E-rings. We compare with experiment both in {\it vivo} and {\it in vitro}. [Preview Abstract] |
Session P40: Theoretical and Computational Biophysics
Sponsoring Units: DBPChair: Sonya Bahar, University of Missouri--St. Louis
Room: 412
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P40.00001: Noise-Optimized Speciation in a Simple Evolutionary Model Nathan Dees, Sonya Bahar A simple computational model for Darwinian evolution is constructed based on three minimal requirements: inheritance, variability, and overpopulation. The fitness of organisms is based on their position in a two-dimensional fitness landscape which is changed periodically either by random fluctuations, or via a feedback mechanism based on the number of organisms in close proximity. The clustering of organisms in a morphospace overlaid on this landscape is considered an analog of speciation and is investigated as a function of the degree of variability, or ``noise'', allowed in the morphology of new (children) organisms with respect to their parents. We find that a maximum number of species are formed at an intermediate value of this noise parameter, suggesting a stochastic resonance-like effect. We also address the spread of inherited traits through the overall population, finding an ``all or none'' effect in which the properties of a traced organism either die out completely or percolate through the entire population, leading to what might be considered as ``homologous'' traits even in species widely separated in morphospace. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P40.00002: Cyclic Process as a Tool for Considering Evolution Victor Bondarenko Evolution is the process. The primary question is which tools we use to consider the process. In this work, basing on the original results of investigation of the intrinsic bistability in quantum systems, the concept of the cyclic process is developed for qualitative and quantitative consideration of processes as following: Everything that happens is the process of changing; the process is the cyclic process of ``... $\to $ seed $\to $ plant $\to $ seed $\to $...'' type; the cyclic process is formed by two complement phase transitions of ``seed $\to $ plant'' and ``plant $\to $ seed'' type; the cyclic process is the manifestation of self-consistent interaction of interdependent two-state system, environment, and radiation, so that the whole Universe is involved in each process; the cyclic process can be described qualitatively and quantitatively by a real cubic equation with four generalized dimensionless real parameters, provided that one of the parameters undertakes cyclic change of its value and all four parameters belong to limited interdependent intervals to maintain cyclic process. Using the cyclic process approach as a powerful tool a variety of issues is considered. Preference of evolution, extinction, adaptation, and relation between microscopic structure and macroscopic behavior of the system are addressed. Seeing the evolution is the most transcending seeing of existence. The cyclic process approach is suggested to be a corner stone for scientific approach to seeing evolution. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P40.00003: Stepping in the bacterial flagellar motor Thierry Mora, Howard Yu, Ned S. Wingreen Many bacteria swim by virtue of tiny rotary motors that drive rotation of helical flagella. These motors are powered by a proton flux that is converted into torque by a mechanism which remains largely unknown. Recently, it has been reported that at low speed, the bacterial flagellar motor proceeds by steps. To account for these steps, we propose a physical model in which the stator drives a ``bumpy'' rotor through a viscous medium. Our model is consistent with most of the available data, and allows us to make testable predictions, in particular on the speed and diffusion properties of the rotor. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P40.00004: Actin-Polymerization-Driven Motility with Site Specific Tethering Edward Banigan, Andrea Liu A recent numerical simulation by Lee and Liu (2008) has demonstrated a new possible mechanism for actin-polymerization-driven motility. The simulation is a physically consistent version of the Brownian dynamics formulation of the dendritic nucleation model. The model shows that motility can indeed be achieved with the constituent proteins of the dendritic nucleation model, but that motility arises from a mechanism completely different from those proposed before. In the simulations, the build-up of F-actin behind the moving surface drives the surface forwards if the surface has a net repulsion with actin. In this work, we extend the model to include a site specific tethering interaction between the moving surface and actin, to imitate, for example, the effects the ActA or N-Wasp protein. We study the effects of varying binding strength and binding site coverage. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P40.00005: FKBP binding free energies obtained via non-equilibrium simulation F. Marty Ytreberg We discuss the advantages and disadvantages of estimating binding free energies (i.e., absolute binding affinities) via non-equilibrium unbinding simulations. The study utilizes the FKBP protein bound to two different ligands as a model system. The non-equilibrium methodology utilized is straight-forward, requiring little or no modification to modern molecular simulation packages, and is trivially parallelizable. The approach makes use of a physical pathway, eliminating the need for complicated alchemical decoupling schemes. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P40.00006: Computational Research Needs for Renewable and Alternative Energy: Studies of Natural and Artificial Photosynthesis Victor Batista The atomspheric oxygen that sustains life on earth has been generated by plants during the light period of photosynthesis. At the molecular level, the reaction involves catalytic water splitting into dioxygen, protons and electrons in the subunit D1 of photosystem II (a transmembrane complex of about 20 proteins found in the thylakoid membranes of green plant chloroplasts). Both the reaction mechanism and the structure of the catalytic center responsible for this important reaction remain poorly understood. This talk will present recent advances in experimental and computational studies towards the development of rigorous models of the oxomangenese catalytic complex and the catalytic cycle responsible for oxygen evolution, as well as recent progress on studies of biomimetic systems for artificial photosynthesis. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P40.00007: Systematic Coarse-Graining of Peptides to Understand their Effective Molecular Interactions Luca Larini, Gregory A. Voth The process of building reliable coarse-grained models is a major challenge for both theory and simulation. Force matching is a systematic method to produce quantitatively accurate coarse-grained potentials from atomistic simulation data. This method provides a sound theoretical background that can also be used to gain deeper insight into the system under examination. In this way, force matching can be employed as a tool for analysis. Application to a simple biological molecule will be described in order to gain a better understanding of the effective forces acting on the system. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P40.00008: Rigorous treatment of electrostatics for spatially varying dielectrics: how far can one go using energy minimization? Yi-Kuo Yu, Oleg Obolensky, Rajarshi Ray, T. Doerr A novel energy minimization formulation of electrostatics that allows computation of the electrostatic energy and forces to any desired accuracy in a system with arbitrary dielectric properties is presented. An integral equation for the scalar charge density is derived from an energy functional of the polarization vector field. This energy functional represents the true energy of the system even in non-equilibrium states. Arbitrary accuracy is achieved by solving the integral equation for the charge density via a series expansion in terms of the equation's kernel, which depends only on the geometry of the dielectrics. The streamlined formalism operates with volume charge distributions only, not resorting to introducing surface charges by hand. Therefore, it can be applied to arbitrary spatial variation of the dielectric susceptibility. The simplicity of application of the formalism to real problems is shown with three examples. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P40.00009: Ab-Initio Based Computation of Rate Constants of Spin Forbidden Transitions in (Bio)inorganic Complexes and Metalloproteins Abdullah Ozkanlar, Jorge H. Rodriguez Some (bio)chemical reactions are non-adiabatic processes whereby the total spin angular momentum, before and after the reaction, is not conserved. These are named spin- forbidden reactions. The application of spin density functional theory (SDFT) to the prediction of rate constants is a challenging task of fundamental and practical importance. We apply non-adiabatic transition state theory in conjunction with SDFT to predict the rate constant of the spin- forbidden dihydrogen binding to iron tetracarbonyl. To model the surface hopping probability between singlet and triplet states, the Landau-Zener formalism is used. The lowest energy point for singlet-triplet crossing, known as minimum energy crossing point (MECP), was located and used to compute, in a semi-quantum approach, reaction rate constants at 300 K. The predicted rates are in good agreement with experiment. In addition, we present results which are relevant to the ligand binding reactions of metalloproteins. This work is supported in part by NSF via CAREER award CHE-0349189 (JHR). [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P40.00010: Electrostatic properties of two finite width charged dielectric slabs in water Y.S. Jho, M. Kanduc, A. Naji, M.W. Kim, R. Podgornik, F.L.H. Brown, P.A. Pincus We study the electrostatic interaction between two like-charged membranes of finite thickness embedded (composed of five parallel dielectric interfaces) in a medium of higher dielectric constant. A generalized SC theory is applied along with extensive Monte-Carlo simulations which applied numerical algorithm based on the image charge method to calculate accurate electrostatic potential or forces. We found the dielectric discontinuity is important in a SC regime. They drive strong counterion crowding in the central region of the inter-surface space upon increasing the solvent/membrane dielectric mismatch and change the membrane interactions from attractive to repulsive at small separations. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P40.00011: Electronic Structure Analysis for Proteins on the FMO Method Tomoki Kobori, Shinji Tsuneyuki, Keitaro Sodeyama, Kazuto Akagi, Kiyoyuki Terakura, Hidetoshi Fukuyama The enormity and complexity of proteins have rendered their electronic structure calculation very costly. Although recently established Fragment Molecular Orbital (FMO) method enables us to calculate total energy of a huge protein precisely based on quantum mechanics, the method does not refer to one-electron orbitals and one-electron energy spectrum. In this paper we propose a method of analyzing electronic structure of a protein based on first principles calculation with reasonable accuracy and CPU cost. We construct one- electron Hamiltonian of proteins by assembling the output of the FMO method: fragment orbitals are determined by fragment monomer calculation, while interaction and overlap between fragment orbitals in different fragments are obtained from dimer calculation. After one-electron Hamiltonian matrix of the whole system is fabricated with the fragment orbital basis, one- electron energy spectrum is obtained by its diagonalization. If the matrix dimension is too large, unimportant orbitals are eliminated from the matrix so that the diagonalization of the Hamiltonian becomes feasible. The method is applicable to both the Hartree-Fock method and the density functional theory. In this paper, validity of the method is verified by some test calculations of small peptides. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P40.00012: Helical secondary structure of polyalanine peptides in vacuo: Ac-Ala$_n$-LysH$^+$ ($n$=5,10,15), experiment and theory Mariana Rossi, Volker Blum, Peter Kupser, Gert von Helden, Frauke Bierau, Gerard Meijer, Matthias Scheffler The presence of a solvent is often viewed as indispensable to explain the structure of peptides and proteins. However, well defined \emph{secondary} structure motifs (helices, sheets, ...) also exist \emph{in vacuo}, offering a unique ``clean room'' condition to quantify the stabilizing interactions. We here unravel the structure of LysineH$^+$ capped polyalanine peptides Ac-Ala$_n$-LysH$^+$ ($n$-5,10,15), by combining experimental multi-photon IR spectra obtained using the FELIX free-electron laser at room-temperature with van der Waals-corrected all-electron density-functional theory (DFT) in the generalized gradient approximation in the FHI-aims code [1]. Earlier ion mobility studies of these molecules indicate helical structure [2], which we here demonstrate quantitatively. For $n$=5, we find a close energetic competition of different helix motifs ($\alpha$, 3$_{10}$), with similar and good agreement between measured and calculated vibrational spectra. We show how the LysH$^+$ termination acts to induce helices also for longer peptides, and how vibrational modes develop with helix length ($n$=10,15), yielding, e.g., a softening of collective modes towards the infinite helix limit. [1] V. Blum \emph{et al}, Comp. Phys. Comm. (2008), accepted. [2] M. Kohtani \emph{et al.}, JACS 120, 12975 (1998). [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P40.00013: Hofmeister effect and the phase diagram of lysozyme Steven Lettieri, Xiaofei Li, James Gunton The phase diagrams for lysozyme are calculated for two different precipitant salts, NaCl and NaSCN, using a potential of mean force that takes into account contributions from ion-dispersion forces (J.Phys.Chem.B, 110, 24757). Our results are consistent with a recent perturbation theory calculation (J.Phys.Chem.B, 110, 24757) in that the phase diagram for lysozyme with NaCl is quite different than for lysozyme with NaSCN for the same molar concentration (0.2M). However, in contrast to the perturbation theory calculation, we find that the lysozyme phase diagram with NaCl has a metastable fluid-fluid coexistence curve and that the metastability gap in the case of NaSCN is much larger than predicted by perturbation theory. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P40.00014: Infrared Spectroscopy with ab initio molecular dynamics simulations : gas phase floppy peptides of increasing size and complexity, in relation with IR-MPD experiments Marie-Pierre Gaigeot We present finite temperature DFT-based Car-Parrinello molecular dynamics (MD) simulations for the calculation of infrared spectra of complex molecular systems, either in the gas phase or in the condensed phase. We will review the fundamentals of the method, as well as the applicability and originality of finite temperature MD simulations for the purpose of modeling infrared spectra. Illustrations are taken from the infrared spectroscopy of alanine peptides of increasing size and complexity (from dipeptides to an octo-peptide) in the gas phase, in relation with IR-MPD (Infrared Multi Photon Dissociation) experiments : 300-400 K gas-phase action spectroscopy as devised on the CLIO platform at the University of Orsay-France or on the platform developed in the group of L. Snoek at Oxford-UK. A special emphasis on vibrational anharmonicities and how they can be extracted from molecular dynamics simulations will be put forward. Furthermore, band assignments in terms of atomic movements from MD is challenging and we have introduced a general method for obtaining effective normal modes of molecular systems from MD simulations. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P40.00015: Linear irreversible thermodynamics, efficiency and coefficient of performance of a thermal Brownian motor in tight coupling Mulugeta Bekele, Anteneh Getachew, Arun Jayannavar We analytically study a thermal Brownian motor and calculate the Onsager's coefficients near a \textit{finite} stall force in the spirit of recent development in non-equilibrium steady state thermodynamics. We show that the reciprocity relation holds and the determinant of the Onsager's matrix vanishes \textit{when heat leakage is neglected}. This condition implies that the device is built with tight coupling and hence Carnot's efficiency can be achieved for the quasi-static process. We also show that the efficiency at maximum power to be exactly half of Carnot's efficiency. Under similar condition we explore the coefficient of performance when our model works as a refrigerator. [Preview Abstract] |
Session P41: Models of Strongly Correlated Electrons
Sponsoring Units: DCMP DMPChair: Paul Fendley, University of Virginia
Room: 413
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P41.00001: Time-Reversal Symmetry Breaking and Spontaneous Anomalous Hall Effect in Fermi Fluids Kai Sun, Eduardo Fradkin We study the spontaneous non-magnetic time-reversal symmetry breaking in a 2D Fermi liquid without breaking either the translational symmetry or the $U(1)$ charge symmetry. Using a Berry phase approach, we found that for a large class of models, including all one- and two-band models, the time-reversal symmetry breaking states can be classified into two classes, dubbed type I and II, depending on the accompanying spatial symmetry breaking patterns. The properties of each class are studied. In particularly, we show that the states breaking both time-reversal and chiral symmetries (type II) are described by spontaneously generated Berry phases and exhibit anomalous Hall effect in the absence of magnetic fields and magnetic impurities. We also show examples of the time-reversal symmetry breaking phases in several different microscopically motivated models and calculate their associated Hall conductance within a mean-field approximation. In particularly, we found a simple lattice structure in which the time-reversal symmetry breaking phases is stabilized by infinitesimal interactions. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P41.00002: String-nets, quantum loop gases and the sign problem for non-abelian anyons Andrea Velenich, Claudio Chamon, Xiao-Gang Wen Hamiltonians giving rise to topological ground states can be constructed explicitly as sums of local operators acting on Hilbert spaces where distinct classical string-net configurations are orthogonal. We show explicitly the connection beteewn string-nets and quantum loop gas models with their non-orthogonal inner product. Also we emphasize the role of the ``sign problem'' for a Hamiltonian in enforcing the topological character of its ground state. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P41.00003: Hydrodynamic description of spin Calogero-Sutherland model Alexander Abanov, Manas Kulkarni, Fabio Franchini We study a non-linear collective field theory for an integrable spin-Calogero-Sutherland model. The hydrodynamic description of this $SU(2)$ model in terms of charge density, charge velocity and spin currents is used to study non-perturbative solutions (solitons) and examine their correspondence with known quantum numbers of elementary excitations [1]. A conventional linear bosonization or harmonic approximation is not sufficient to describe, for example, the physics of spin-charge (non)separation. Therefore, we need this new collective bosonic field description that captures the effects of the band curvature. In the strong coupling limit [2] this model reduces to integrable $SU(2)$ Haldane-Shastry model. We study a non-linear coupling of left and right spin currents which form a Kac-Moody algebra. Our quantum hydrodynamic description for the spin case is an extension for the one found in the spinless version in [3].\\[3pt] [1] Y. Kato,T. Yamamoto, and M. Arikawa, J. Phys. Soc. Jpn. 66, 1954-1961 (1997).\\[0pt] [2] A. Polychronakos, Phys Rev Lett. 70,2329-2331(1993).\\[0pt] [3] A.G.Abanov and P.B. Wiegmann, Phys Rev Lett 95, 076402(2005) [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P41.00004: Supersymmetry in strongly correlated fermion models Dimitris Galanakis, Stefanos Papanicolaou, Chris Henley We investigate the Fendley and Schoutens~\footnote{ P. Fendley and K. Schoutens, Phys. Rev. Lett. 90, 120402 (2003).} model of hard core fermions on lattice which have hopping elements $t$, and potential terms $V$ which include a second-neighbor repulsion with some multi-particle terms. At the special point $t=V$, they showed that the Hamiltonian is $H = \{Q^\dagger(r), Q\}$ with $Q = \sum_r q(r)= \sum_r c(r)P(r)$, where $c(r)$ is an annihilation operator and $P(r)$ enforces the hardcore. That means the system acquires an exact non-relativistic supersymmetry, and for a range of fillings has a large number of zero-energy ground states~$^1$. To obtain insights on the nature of the zero-energy states and excitations, we perform exact diagonalization studies on finite clusters for various interaction strengths, fillings and lattice geometries. We note that for fillings beyond $n\approx 0.3$, we find coexisting domains of the inert crystal at $n=1/2$, in contrast to a related non-supersymmetric model~\footnote{ N.G. Zhang and C.L. Henley, Phys. Rev. B 68, 014506 (2003).} Moreover, using both numerical and analytical tools, we investigate perturbative limits where $q(r)$ is changed so as to preserve supersymmetry but a particular class of ground-states becomes trivial. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P41.00005: Hexatic and Microemulsion Phases in the 2d Quantum Plasma Bryan Clark, Michele Casula, David Ceperley It has been long known that the two-dimensional one component plasma supports both a Wigner-crystal and liquid phase. Classically [1,2], it is known that a hexatic phase exists but it is not known how this hexatic phase extends into the quantum regime. Moreover, at low temperature, phenomenological arguments [3] from Jamei, et. al. suggest the existence of microemulsion phases including stripes and bubbles. We use diffusion and path integral Monte Carlo to map out this phase diagram. We are able to extend the hexatic phase into the quantum regime as well as quantify the nature of the defects and exponents in the long range quantum system. We also specify the the nature, extent and existence (or lack thereof) of the expected low-T microemulsion phases. \\[0pt] [1] Muto, S. \& Aoki, H. Crystallization of a classical two-dimensional electron system: Positional and orientational orders. Phys. Rev. B 59, 14911(1999).\\[0pt] [2] He, W.J. et al. Phase transition in a classical two-dimensional electron system. Phys. Rev. B 68, 195104(2003).\\[0pt] [3] Jamei, R., Kivelson, S. \& Spivak, B. Universal Aspects of Coulomb-Frustrated Phase Separation. Phys. Rev. Lett. 94, 056805-4(2005). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P41.00006: Quantum phase transition in a staggered flux phase Christoph Puetter, Hae-Young Kee We study the quantum critical point inside the staggered flux phase. We present the dynamics of the fermions at the critical point and discuss their relevance for the phenomena observed in high-Tc cuprates. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P41.00007: Quantum correlated percolation Liang Cao, M. Jeng, J. M. Schwarz Abstract: Quantum percolation is the study of hopping transport of a quantum particle on randomly diluted percolation clusters. Inspired by correlated percolation models of geometrical jamming, we extend quantum percolation to investigate hopping transport on percolation clusters with geometric constraints on the occupation of bonds/sites. An example of a geometric constraint is each occupied site must have at least $k$ occupied neighboring sites to remain occupied ($k$-core percolation). Another example is particular sets of neighboring sites containing at least one occupied site for an occupied site to remain occupied (spiral model). Both models exhibit long-range geometrical correlations differing from ordinary percolation and give rise to a discontinuous phase transition (in high dimensions for $k$-core percolation). To investigate how these atypical long-range geometrical correlations affect the hopping transport of a quantum particle, we numerically study the level statistics of quantum $k$-core percolation on the Bethe lattice and the two-dimensional quantum spiral model. While the quantum $k$-core model exhibits an insulator-to-metal transition as the occupation probability is increased, preliminary results indicate that there is no insulator-to-metal transition in the two-dimensional quantum spiral model. Studies of a three- dimensional quantum spiral model will also be addressed as will possible physical applications of quantum jamming. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P41.00008: Frustration of dissipation in a spin-boson model Kevin Ingersent, Alper Duru The spin-boson model (SBM), in which a quantum two-level system couples via one component of its effective spin to a dissipative bosonic bath, has many realizations. There has been much recent interest in the SBM with a sub-Ohmic bath characterized by a power-law spectral exponent $0 < s < 1$, where at zero temperature a quantum critical point separates delocalized and localized phases. Numerical renormalization group calculations have called into question [1] the validity of the long-assumed mapping between the SBM and the classical Ising chain with interactions decaying with distance $|i-j|$ as $1/|i-j|^{1+s}$. Attention has also fallen on a variant of the SBM in which two components of the impurity spin couple to different bosonic baths. For Ohmic case ($s = 1$), competition between the baths has been shown to frustrate the dissipation and reduce the coupling of the impurity to the environment [2]. The present study addresses the SBM with two sub-Ohmic baths, where dissipative effects are intrinsically stronger than for $s=1$. Numerical renormalization group methods are used to identify a continuous quantum phase transition in this model and to evaluate critical exponents characterizing the quantum-critical behavior in the vicinity of the transition. [1] M. Vojta et al., Phys. Rev. Lett. 94, 070604 (2005). [2] E. Novais et al., Phys. Rev. B 72, 014417 (2005). Supported by NSF Grant DMR-0710540. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P41.00009: Fidelity susceptibility and quantum phase transitions Shi-Jian Gu In this talk, I will introduce the quantum fidelity approach to quantum phase transitions based on its leading term, i.e. the fidelity susceptibility. The fidelity susceptibility denotes the adiabatic leading response of the ground state to the driving parameter. Differ from traditionally approach based on the ground-state energy, the fidelity susceptibility shows distinct scaling and singular behaviours around the critical point. I will present also the ground-state fidelity approach to both Landau's phase transition and topological phase transition, as illustrated by the Lipkin-Meshkov-Glick model and the Kitaev honeycomb model, respectively. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P41.00010: Scaling of logarithmic quantum fidelity in the Lipkin-Meshkov-Glick model Ching Yee Leung, Ho-Man Kwok, Shi-Jian Gu, Hai-Qing Lin The quantum fidelity is used to describe quantum phase transitions in many works. As the classical expression of logarithmic fidelity is shown to be an extensive value, it was suggested that the logarithmic fidelity can be averaged over the system size and named as fidelity per site. However, illustrated by the anisotropic Lipkin-Meshkov-Glick model, which exhibits different scaling behaviour in different phases, we show that the logarithmic fidelity in the ground state of the model scales like $N$ in the symmetry-broken phase and $N^0$ in the polarizing phase. It is suggested to be a pure quantum effect and generalization of fidelity per site is proposed. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P41.00011: Multicanonical Monte Carlo simulations of anisotropic SU(3) and SU(4) Heisenberg models Kenji Harada, Naoki Kawashima, Matthias Troyer We present the results of multicanonical Monte Carlo simulations on two-dimensional anisotropic SU(3) and SU(4) Heisenberg models. In our previous study [K.~Harada, et al., J.~Phys.~Soc.~Jpn. \textbf{76}, 013703 (2007)], we found evidence for a direct quantum phase transition from the valence-bond-solid(VBS) phase to the SU(3) symmetry breaking phase on the SU(3) model and we proposed the possibility of deconfined critical phenomena (DCP) [T.~Senthil, et al., Science \textbf{303}, 1490 (2004); T.~Grover and T.~Senthil, Phys. Rev. Lett. \textbf{98}, 247202 (2007)]. Here we will present new results with an improved algorithm, using a multicanonical Monte Carlo algorithm. Using a flow method-like technique [A.B.~Kuklov, et al., Annals of Physics \textbf{321}, 1602 (2006)], we discuss the possibility of DCP in both models. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P41.00012: Topological stability of q-deformed quantum spin chains Charlotte Gils, Eddy Ardonne, Simon Trebst, Andreas Ludwig, Matthias Troyer, Zhenghan Wang Quantum mechanical systems, whose degrees of freedom are so-called $su(2)_k$ anyons, form a bridge between ordinary spin systems and systems of interacting non-Abelian anyons. Such a connection can be made for arbitrary spin-S systems, and we explicitly discuss spin-$1/2$ and spin-$1$ systems. Anyonic spin-$1/2$ chains exhibit a topological protection mechanism that stabilizes their gapless ground states and which vanishes only in the limit ($k \to \infty$) where the system turns into the ordinary spin-$1/2$ Heisenberg chain. For anyonic spin-$1$ chains we show that their phase diagrams closely mirror the one of the biquadratic spin-$1$ chain. This includes generalizations of the Haldane phase, of the AKLT point, and the appearance of several stable critical phases described by (super)conformal field theories. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P41.00013: Momentum distribution of the one-dimensional hard-core boson Hubbard model Min-Chul Cha, Jong-Geun Shin, Ji-Woo Lee We investigate the momentum distributions, $n_k$, of the one- dimensional hard-core boson Hubbard model as a function of the nearest-neighbor interaction strength by exact diagonalizations for lattices up to 30 sites. It is well known that the ground state of this model shows a quantum phase transition between the Ising-ordered insulating phase and the XY-ordered superfluid phase at $V=2t$. Predetermination of the critical point helps us to investigate various critical behaviors. At the critical point, the momentum distribution shows a linear dependence ($n_k \sim |k-\pi|$). $n_k (k=\pi)$ shows different critical behaviors upon appoaching the critical point in the Ising or XY regions. Some other properties of the momentum distributions and the crtical behaviors are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P41.00014: Ring-exchange interaction in doubly degenerate orbital system with strong electron correlation Joji Nasu Orbital degree of freedom is one of the attractive themes in transition-metal oxides. Since the inter-site orbital interaction depends explicitly on the bond direction, one orbital configuration which minimizes the bond energy in one direction does not minimize in other directions. This is a kind of frustration. We study the e$_{g}$ orbital model (EOM) where the e$_{g}$ orbital is represented by the pseudo-spin (PS) with nearest neighbor (NN) interaction in a cubic lattice. Due to this frustration, this model shows a macroscopic number of degenerate states in the classical ground states. It is known that these states are lifted by thermal and quantum fluctuations. We examine the long-range interaction effect in the EOM. This interaction is derived by the higher-order perturbational processes of the electron transfer under strong on-site Coulomb repulsion in the two orbital Hubbard model. In particular, roles of the orbital ring-exchange interaction are focused on. This includes the magnetic octupole operator which does not appear in the previous EOM with NN interaction. We analyzed this model by the mean field approximation and the classical Monte-Carlo method. We found that PS canted state is stabilized rather than PS collinear state which is realized in the previous EOM due to thermal and quantum fluctuations. It is also shown that the magnetic octupole polarization appears in a wide parameter region. [Preview Abstract] |
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