Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V1: Noise in Biological Systems
Sponsoring Units: DBPChair: Yin Yeh, University of California, Davis
Room: Spirit of Pittsburgh Ballroom A
Thursday, March 19, 2009 8:00AM - 8:36AM |
V1.00001: to be determined by you Invited Speaker: |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V1.00002: to be determined by you Invited Speaker: |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V1.00003: to be determined by you Invited Speaker: |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V1.00004: to be determined by you Invited Speaker: |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V1.00005: Activity-dependent stochastic resonance in recurrent neuronal networks Invited Speaker: An important source of noise for neuronal networks is that of the stochastic nature of synaptic transmission. In particular, there can occur spontaneous asynchronous release of neurotransmitter at a rate that is strongly dependent on the presynaptic Ca2+ concentration and hence strongly dependent on the rate of spike induced Ca2+. Here it is shown that this noise can lead to a new form of stochastic resonance for local circuits consisting of roughly 100 neurons - a ``microcolumn''- coupled via noisy plastic synapses. Furthermore, due to the plastic coupling and activity-dependent noise component, the detection of weak stimuli will also depend on the structure of the latter. In addition, the circuit can exhibit short-term memory, by which we mean that spiking will continue to occur for a transient period following removal of the stimulus. These results can be directly tested in experiments on cultured networks. [Preview Abstract] |
Session V2: Constrained Polymers
Sponsoring Units: DCMP DPOLYChair: Xaio Lun Wu, University of Pittsburgh
Room: Spirit of Pittsburgh Ballroom BC
Thursday, March 19, 2009 8:00AM - 8:36AM |
V2.00001: The low-force elasticity of single-stranded DNA Invited Speaker: Single-molecule manipulation experiments, in which a single polymer is stretched with a known force while its extension is measured, are typically described by ideal models (e.g. the worm-like chain or freely-jointed chain) that account only for the polymer's local stiffness, but ignore long-range 'excluded-volume' interactions. Yet, the basic (and successful) Flory scaling theory indicates that long-range interactions must be included to describe the zero-force self-avoiding walk structure of a polymer. Here, we reconcile single-molecule force-extension data with scaling theories of polymer elasticity: measurements of denatured single-stranded DNA show a regime where the extension grows as a non-linear power law with force, in accord with previously-unproven `tensile blob' models. Analysis of the salt dependence of this regime indicates that the polymer's Kuhn length is proportional to the Debye length. This contradicts the classic Odijk-Skolnick-Fixman theory; I will discuss possible explanations for this discrepancy. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V2.00002: Confinement Effects on the Structure of Complex Fluids Invited Speaker: Actin is a key component of the protein complex responsible for producing contractile force in skeletal muscle. Filamentous actin, called F-actin, is a two-stranded helical protofilament with a diameter of $\sim $8nm and a contour length of $\sim $10m. The experimental results show that the persistence length of the F-actin is 4 -20 m. One of interesting problems is to find the structure of a semiflexible filament in a confined space [1], such as a channel width less than the persistence length. The other interesting problem is to find the surface treatment effect on the liquid crystal structure in a confined space. The boundary conditions imposed by the walls of the microchannel generate the spatial patterning of defect domains in a smectic liquid crystal [2] and the formation of a large-area ordered structure [3] by using the structure of smectic liquid in the microchannels. We found that the F-actin undergoes a transition from a 2D randomly oriented regime to a 1D biaxially confined regime with the effective persistence length. We were able to generate defect domains that are nearly uniformly arranged in 2D ordered patterns by controlling the surface hydrophobicity. Furthermore, the formation of a large-area ordered structure of toric focal conic domains was generated. This work was done with C. R. Safinya's group at UCSB and Hee-Tae Jung's group at KAIST. \\[4pt] [1] M.C Choi at. al, Macromolecules 2005,38, 9882-9884\\[0pt] [2] M. C. Choi at. al, PNAS 2004, 101, 17340-17344\\[0pt] [3] D. K. Yoon at. al, Nature Materials, 2007, 6, 866-870 [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V2.00003: Is your brain wired optimally? Invited Speaker: |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V2.00004: Gene brushes on a chip: From crowding and the search problem to synthetic systems Invited Speaker: We assemble DNA polymer brushes coding for entire genes on a surface by means of a new photolithographic approach. The gene density can be controlled from dilute to high density where the local concentration -- Megabase pairs per micron cubed -- is comparable to that in a bacterium. The gene brush, therefore, emulates the crowded medium of the cell, allowing us to study DNA transactions in vitro under native conditions. We find that transcription/translation from these gene brushes is highly sensitive to DNA density, orientation and composition. As a step towards multi-gene synthetic systems, we integrated on a chip two spatially separated gene brushes, and implemented a two-stage transcription/translation cascade. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V2.00005: Confinement effects on folding of proteins and RNA Invited Speaker: |
Session V3: Correlated Phases in Fermi Gases of Ultracold Atoms
Sponsoring Units: DCMP DAMOPRoom: 301/302
Thursday, March 19, 2009 8:00AM - 8:36AM |
V3.00001: A Mott insulator of fermionic atoms in an optical lattice Invited Speaker: In a solid material strong interactions between the electrons can lead to surprising properties. A prime example is the Mott insulator, where the suppression of conductivity is a result of interactions and not the consequence of a filled Bloch band. The proximity to the Mott insulating phase in fermionic systems is the origin for many intriguing phenomena in condensed matter physics, most notably high-temperature superconductivity. Compared to real materials, a fermionic quantum gas trapped in an optical lattice offers a very pure realisation of the Hubbard model, giving a new approach to understand the physics of strongly correlated systems. We report on the formation of a Mott insulator of a repulsively interacting two-component Fermi gas in an optical lattice. It is signalled by three features: a drastic suppression of doubly occupied lattice sites, a strong reduction of the compressibility inferred from the response of double occupancy to atom number increase, and the appearance of a gapped mode in the excitation spectrum. In collaboration with Robert J\"ordens, Niels Strohmaier, and Daniel Greif, ETH Zurich; Kenneth G\"unter, ETH Zurich, ENS Paris; Leticia Tarruell and Tilman Esslinger, ETH Zurich. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V3.00002: The phase-diagram of a superfluid two-component Fermi gas Invited Speaker: What is the benefit of realizing superfluidity in a gas a million times more dilute than air? Such systems consist of well-separated atoms which can be observed and manipulated with the control and precision of atomic physics, and which can be treated with first-principles calculations. By implementing scattering resonances, we have realized the strong-coupling limit of the Bardeen Schrieffer-Cooper (BCS) mechanism and observed a normalized transition temperature of 20\% of the Fermi temperature, higher than in any superconductor. When the population of the two spin states is imbalanced, pairing is frustrated; and superfluidity is quenched at the Chandrasekhar-Clogston limit. When the fermions can form molecules, we observe the emergence of bosonic behavior, and an imbalanced two-component Fermi system can be described as a boson-fermion mixture. Pairing correlations have been studied by rf spectroscopy, determining the fermion pair size and the pairing gap energy in a resonantly interacting superfluid. These studies illustrate a new approach to condensed-matter physics where many-body Hamiltonians are realized in dilute atomic gases. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V3.00003: Trapping and cooling fermionic atoms into the Mott and N\'eel states Invited Speaker: Atomic gases cooled to Nanokelvin temperatures are a new exciting tool to study a broad range of quantum phenomena. In particular, the outstanding degree of control which has been achieved over these quantum systems facilitates access to strongly correlated quantum many body physics. For example, optical lattices have been created to mimic condensed matter systems. We perform a theoretical study of a fermionic gas with two repulsively interacting hyperfine states confined to an optical lattice. We determine a generic state diagram in the presence of a harmonic confining potential. We discuss implications for current experiments. Further we outline different strategies to reach the antiferromagnetic phase. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V3.00004: Polarized Fermi condensates Invited Speaker: We theoretically investigate the polarized two-component Fermi gas, which is the simplest fermion system displaying both superfluidity and ``magnetism''. In particular, we show that the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulated superfluid phase may be realized by embedding the polarized Fermi gas in an array of weakly-coupled parallel 1D ``tubes'' produced by a two-dimensional optical lattice. We argue that the most promising regime for observing the FFLO phase is in the quasi-1D regime, where the atomic motion is largely 1D but there is weak tunneling in the transverse directions that stabilizes long range order. Moreover, within this system, there is an additional phase transition in the FFLO phase, where the quasiparticle spectrum changes from gapless near the 3D limit to gapped in the quasi-1D regime. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V3.00005: A Three-Component Degenerate Fermi Gas. Invited Speaker: We have realized a three-component Fermi gas consisting of the three lowest spin states ($|1\rangle$, $|2\rangle$, $|3\rangle$) of the $^6$Li atom. Interactions in this system are governed by three different scattering lengths ($a_{12}$, $a_{23}$, $a_{13}$) between the three states, which all exhibit broad and overlapping Feshbach resonances. This enables us to tune the interactions of the system to become both strongly repulsive and strongly attractive, making it a generic three-component system. It is therefore ideally suited to study predictions of exotic phases such as color superconductivity that are expected for example inside of neutron stars. It is also possible to tune the scattering lengths to very small values simultaneously, which facilitates the preparation of the mixture: Starting from a degenerate mixture of atoms in states $|1\rangle$ and $|2\rangle$, we simultaneously apply radio frequencies resonant with the $|1\rangle$-$|2\rangle$ and $|2\rangle$-$|3\rangle$ transitions. This causes the three states to be mixed. To obtain an incoherently mixed sample within a few hundred milliseconds we apply a small magnetic field gradient along the weak axis of our trap. In first experiments we studied the collisional stability of our gas with respect to the magnetic field [1]. We observe an intriguing three-body loss resonance that occurs where all two-body scattering lengths are negative and no two-body bound state exists. The mixture is stable where the scattering lengths are relatively small, which is an important prerequisite for the preparation of the gas. The stable gas is also a good starting point for experiments in the strongly interacting regime, where we aim to observe many-body effects. Progress on this effort will be reported. \\[4pt] [1] T.B. Ottenstein et al., Phys. Rev. Lett. 101, 203202. [Preview Abstract] |
Session V4: Microgels: Colloidal Properties of Gels
Sponsoring Units: DPOLY DCMPChair: Weitz David, Harvard University
Room: 306/307
Thursday, March 19, 2009 8:00AM - 8:36AM |
V4.00001: Melting and Frustration in Temperature-Sensitive Colloids Invited Speaker: I will review experiments from my laboratory that employ temperature-sensitive microgel particles to induce novel phase behavior in suspension. This phenomenon offers a fantastic new variable for control of lyotropic suspensions. Recent experiments, for example, have enabled us to learn how three-dimensional crystals first begin to melt [1], to directly observe melting in 2-D wherein intermediate hexatic phases form [2], and to create geometrically frustrated colloidal ``anti-ferromagnets'' [3]. \\[4pt] References: \\[0pt] [1] Alsayed, A.M., Islam, M.F., Zhang, J., Collings, P.J., Yodh, A.G., Science 309, 1207-1210, (2005). \\[0pt] [2] Han Y, Ha NY, Alsayed AM and Yodh AG, Phys. Rev. E, Vol. 77 (2008). \\[0pt] [3] Y. Han, Y. Shokef, A. M. Alsayed, P. Yunker, T. C. Lubensky, and A. G. Yodh, ``Geometric frustration in buckled colloidal monolayers,'' to be published in Nature (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V4.00002: Equilibrium features in the arrested phase separation of PNiPAM microgels Invited Speaker: We investigate the arrested phase separation of poly-N-isopropylacrylamide (PNiPAM) microgels. At large enough concentrations we observe the formation of a macroscopic gel-body that exhibits a peculiar temperature dependence. In a temperature-range, where the volume of the individual particles no longer changes, the final dimension of the macroscopic gel body depends on the depth of the quench into the phase separation regime. Increasing the quench depth results in a decrease of the dimension of the gel-body; this is reminiscent of a thermodynamically driven phenomenon and contrasts with the fact that the formation of the gel-bodies is due to the arrest of a phase separation process. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V4.00003: New materials with microgels Invited Speaker: This talk introduces a flexible and straightforward method for generating responsive microgel materials with new structures by using a microfluidic technique. We demonstrate that this approach enables tight control over the size and monodispersity of droplets as well as the interfacial structures, which is essential for determining release and transport kinetics of encapsulated components. We also show that responsiveness of microgel materials is controllable by tuning their structure, thereby allowing us to overcome the limitation of length scales, since the diffusion of water molecules through the structured gel phase is much faster than through a bulk gel phase of similar dimensions. We have generated a variety of novel gel structures: microgels with complex structures, microgel shells, 3D gel network with a truly fast response, and responsive colloidosomes. The robustness and versatility of this approach are expected to generate more complex systems and create new possibilities to develop novel materials in practical applications, including drug delivery, foods, and cosmetics. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V4.00004: Sublimation dynamics of colloidal microgel crystals Invited Speaker: Polymer microspheres in suspension serve as a powerful model system for probing thermodynamic phase transitions. These particles are large enough to visualize using optical microscopy and the particle trajectories can be obtained with nanometer-scale resolution from the images. Equally important is the ability to tune the interactions among the particles using charge or adsorbed polymers to induce repulsion, or non-adsorbing polymers to induce attraction by the depletion effect. I will focus on short-ranged depletion attraction induced by micelles of molecular surfactants or triblock copolymer (Pluronic). Because the micellar size and concentration depend sensitively on temperature, the magnitude and range of the attraction can be tuned in situ. This approach lets us track individual microspheres as they form crystals following a quench, or as crystals sublimate when superheated. We focus on systems where the particles are attracted to a flat surface by depletion and thus confined to two dimensions. We find that when crystallites are superheated, they first sublimate by thermally-excited bond-breaking at the perimeter. Below a cross-over size, however, the crystallites rapidly become amorphous throughout, then evaporate very fast at an approximately diffusion-limited rate. The cross-over size varies from 20-100, depending on temperature and concentation. A similar two-stage process is followed during crystallization. During nucleation, we measure the free energy as a function of cluster size and thereby obtain interfacial tensions and chemical potentials. The results point to a thermodynamically meta- or unstable fluid phase, which is not found in equilibrium but which plays a key role in phase-separation dynamics according to Ostwald's Rule. Results will be compared to experiments, simulations, and theory of crystallization of globular proteins. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V4.00005: Phase behavior and rheology of ionic microgels Invited Speaker: Our aim is to understand and control the mechanical properties of dense microgel suspensions, where the softness of the constituent particles can have important effects over the macroscopic behavior. In particular, we are using ionic microgel particles based on poly(vinylpyridine), a monomer that ionizes with pH. When de-swollen, the particles are essentially charged hard spheres and crystallize at high enough volume fractions. By contrast, when the microgels are swollen, light and neutron scattering experiments show that the suspension does not crystallize, irrespective of particle density. But even more remarkably, these highly packed systems remain essentially liquid and do not seem to exhibit glassy behavior. This phenomenology is markedly different to that of ordinary colloids and suggests that the properties of the single particle can dramatically affect the phase behavior and mechanical properties of the packed suspension. [Preview Abstract] |
Session V5: Computational Design of Hydrogen Storage Materials
Sponsoring Units: FIAPChair: Chris Van de Walle, University of California, Santa Barbara
Room: 401/402
Thursday, March 19, 2009 8:00AM - 8:36AM |
V5.00001: Computational Discovery of Novel Hydrogen Storage Materials and Reactions Invited Speaker: Practical hydrogen storage for mobile applications requires materials that exhibit high hydrogen densities, low decomposition temperatures, and fast kinetics for absorption and desorption. Unfortunately, no reversible materials are currently known that possess all of these attributes. Here we present an overview of our recent efforts aimed at developing a first-principles computational approach to the discovery of novel hydrogen storage materials. We have developed computational tools which enable accurate prediction of decomposition thermodynamics, crystal structures for unknown hydrides, and thermodynamically preferred decomposition pathways. We present examples that illustrate each of these three capabilities. Specifically, we focus on recent work on crystal structure and dehydriding reactions of borohydride materials, such as Mg(BH$_4$)$_2$, MgB$_{12}$H$_{12}$, and mixtures of complex hydrides such as the ternary LiBH$_4$/LiNH$_2$/MgH$_2$ system.\\ \\ {\textbf References:}\\[0pt] (1) V. Ozolins, E. H. Majzoub, and C. Wolverton, ``First-Principles Prediction of a Ground State Crystal Structure of Magnesium Borohydride'', Phys. Rev. Lett. {\textbf 100}, 135501 (2008).\\ (2) C. Wolverton, D. J. Siegel, A. R. Akbarzadeh, and V. Ozolins, ``Discovery of Novel Hydrogen Storage Materials: An Atomic Scale Computational Approach'', J. Phys. Condens. Matt. {\textbf 20}, 064228 (2008).\\ (3) J. Yang, et al., ``A Self-Catalyzing Hydrogen Storage Material'' Angew. Chem. Int. Ed., {\textbf 47}, 882 (2008).\\ (4) A. R. Akbarzadeh, V. Ozolins, and C. Wolverton, ``First-Principles Determination of Multicomponent Hydride Phase Diagrams: Application to the Li-Mg-N-H System'', Advanced Materials {\textbf 19}, 3233 (2007).\\ (5) D. J. Siegel, C. Wolverton, and V. Ozolins, ``Thermodynamic Guidelines for the Prediction of Hydrogen Storage Reactions and their Application to Destabilized Hydride Mixtures'', Phys. Rev. B {\textbf 76}, 134102 (2007). [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V5.00002: Combining computation and experiment to accelerate the discovery of new hydrogen storage materials Invited Speaker: The potential of emerging technologies such as fuel cells (FCs) and photovoltaics for environmentally-benign power generation has sparked renewed interest in the development of novel materials for high density energy storage. For applications in the transportation sector, the demands placed upon energy storage media are especially stringent, as a potential replacement for fossil-fuel-powered internal combustion engines -- namely, the proton exchange membrane FC -- utilizes hydrogen as a fuel. Although hydrogen has about three times the energy density of gasoline by weight, its volumetric energy density (even at 700 bar) is roughly a factor of six smaller. Consequently, the safe and efficient storage of hydrogen has been identified as one of the key materials-based challenges to realizing a transition to FC vehicles. This talk will present an overview of recent efforts at Ford aimed at developing new materials for reversible, solid state hydrogen storage. A tight coupling between first-principles modeling and experiments has greatly accelerated our efforts, and several examples illustrating the benefits of this approach will be presented. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V5.00003: Novel hydrogen storage approaches using organometallics Invited Speaker: Storing molecular hydrogen in organometallics can ensure fast kinetics, low heat management, high energy efficiency, and superb reversibility. The gravimetric density is, however, low for room temperature storage. The reason for the too low density is because the binding is too weak. First-principles calculations [1,2] suggested that organometallics may significantly increase the binding, which is also correlated with decreasing inter-molecular distances and hence a significantly increased volumetric density [3]. Current experimental difficulties are twofold: a) how to synthesize the organometallics and b) how to avoid the transition metal atoms from clustering [4]? Recent experiment [5] on titanium- doped porous silica and theoretical predictions on calcium doping [3,6] may shed new lights on these difficult problems. \newline [1] Y. Zhao, et al., Phys. Rev. Lett.\textbf{ 94}, 155504 (2005). \newline [2] T. Yildirim and S. Ciraci, Phys. Rev. Lett. \textbf{94}, 175501 (2005). \newline [3] Y.-H. Kim, Y. Y. Sun, and S. B. Zhang, unpublished. \newline [4] Q. Sun, et al., J. Am. Chem. Soc., \textbf{127}, 14582 (2005). \newline [5] A. Hamaed, et al., J. Am. Chem. Soc. \textbf {130}, 6992 (2008). \newline [6] M. Yoon, et al., Phys. Rev. Lett. \textbf{100}, 206806 (2008). [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V5.00004: Computational methods to determine the structure of hydrogen storage materials Invited Speaker: To understand the mechanisms and thermodynamics of material-based hydrogen storage, it is important to know the structure of the material and the positions of the hydrogen atoms within the material. Because hydrogen can be difficult to resolve experimentally computational research has proven to be a valuable tool to address these problems. We discuss different computational methods for identifying the structure of hydrogen materials and the positions of hydrogen atoms, and we illustrate the methods with specific examples. Through the use of ab-initio molecular dynamics, we identify molecular hydrogen binding sites in the metal-organic framework commonly known as MOF-5 [1]. We present a method to identify the positions of atomic hydrogen in imide structures using a novel type of effective Hamiltonian. We apply this new method to lithium imide (Li$_{2}$NH), a potentially important hydrogen storage material, and demonstrate that it predicts a new ground state structure [2]. We also present the results of a recent computational study of the room-temperature structure of lithium imide in which we suggest a new structure that reconciles the differences between previous experimental and theoretical studies. \\[4pt] [1] T. Mueller and G. Ceder, Journal of Physical Chemistry B 109, 17974 (2005). \\[0pt] [2] T. Mueller and G. Ceder, Physical Review B 74 (2006). [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V5.00005: Kinetics of bulk and surface mass transport in complex metal hydrides Invited Speaker: Metal hydrides can be used to store hydrogen in high gravimetric and volumetric densities. However, the kinetics of hydrogen release and uptake are slow in complex metal hydrides. Clarification of the mechanism of hydrogen release and uptake in complex metal hydrides can aid in a rational design of new hydrogen storage materials with fast kinetics or catalysts that will catalyze the rate of hydrogen release from the existing materials. The release of hydrogen in metal hydrides requires the transport of hydrogen and/or heavier species. The kinetics of such mass transport in metal hydrides can be the rate-limiting process for the release of hydrogen. For example, the rate-determining step for the release of hydrogen from NaAlH4 is the creation and diffusion of neutral AlH3 defects in NaAlH4. The release of hydrogen from LiH destabilized LiNH2 also proceeds via the creation of neutral point defects. The mechanism of mass transport in prototypical hydrogen storage materials such as NaAlH4 and LiNH2 and the mechanism of hydrogen diffusion in aluminum will be discussed. [Preview Abstract] |
Session V6: Recent Advances in Biomolecular Simulations
Sponsoring Units: DCOMPChair: Jerry Bernholc, North Carolina State University
Room: 406
Thursday, March 19, 2009 8:00AM - 8:36AM |
V6.00001: Metadynamics simulation of large scale motion in proteins Invited Speaker: Understanding large scale protein motions is essential in the study of many biological processes. Molecular dynamics simulations could provide important clues, but they are hampered by the fact, that they can only explore time scales, smaller than most interesting phenomena. To circumvent this problem, our group has developed a number of enhanced sampling techniques that allow exploring many such long time scale phenomena. In particular, a very recent development permits reconstructing complex pathways without the need of introducing collective variable. A number of large scale applications will be presented. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V6.00002: Quantum Simulations of Solvated Biomolecules Using Hybrid Methods Invited Speaker: One of the most important challenges in quantum simulations on biomolecules is efficient and accurate inclusion of the solvent, because the solvent atoms usually outnumber those in the biomolecule of interest. We have developed a hybrid method that allows for explicit quantum-mechanical treatment of the solvent at low computational cost. In this method, Kohn-Sham (KS) density functional theory (DFT) is combined with an orbital-free (OF) DFT. Kohn-Sham (KS) DFT is used to describe the biomolecule and its first solvation shells, while the orbital-free (OF) DFT is employed for the rest of the solvent. The OF part is fully O(N) and capable of handling 10$^5$ solvent molecules on current parallel supercomputers, while taking only $\sim$ 10 \% of the total time. The compatibility between the KS and OF DFT methods enables seamless integration between the two. In particular, the flow of solvent molecules across the KS/OF interface is allowed and the total energy is conserved. As the first large-scale applications, the hybrid method has been used to investigate the binding of copper ions to proteins involved in prion (PrP) and Parkinson's diseases. Our results for the PrP, which causes mad cow disease when misfolded, resolve a contradiction found in experiments, in which a stronger binding mode is replaced by a weaker one when concentration of copper ions is increased, and show how it can act as a copper buffer. Furthermore, incorporation of copper stabilizes the structure of the full-length PrP, suggesting its protective role in prion diseases. For alpha-synuclein, a Parkinson's disease (PD) protein, we show that Cu binding modifies the protein structurally, making it more susceptible to misfolding -- an initial step in the onset of PD. In collaboration with W. Lu, F. Rose and J. Bernholc. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V6.00003: Urea's action on the hydrophobic interaction in physical and biophysical systems Invited Speaker: For more than a century, urea has been commonly used as an agent for denaturing proteins. However, the mechanism behind its denaturing power is still not well understood. The mechanism of denaturation of proteins by urea is explored using all-atom microseconds molecular dynamics simulations of hen lysozyme generated on BlueGene/L. Accumulation of urea around lysozyme shows that water molecules are expelled from the first hydration shell of the protein. We observe a two stage penetration of the protein, with urea penetrating the hydrophobic core before water, forming a ``dry globule." The direct dispersion interaction between urea and the protein backbone and sidechains is stronger than for water, which gives rise to the intrusion of urea into the protein interior and also to urea's preferential binding to all regions of the protein. This is augmented by preferential hydrogen bond formation between the urea carbonyl and the backbone amides which contributes to the breaking of intra-backbone hydrogen bonds. Our study supports the ``direct interaction mechanism" whereby urea has a stronger dispersion interaction with protein than water. We also show by molecular dynamics simulations that a 7 M aqueous urea solution unfolds a chain of purely hydrophobic groups which otherwise adopts a compact structure in pure water. The unfolding process arises due to a weakening of hydrophobic interactions between the polymer groups. Again the action of urea is found to be direct, through its preferential binding to the polymer or plates. It is, therefore, acting like a surfactant capable of forming hydrogen bonds with the solvent. The preferential binding and the consequent weakened hydrophobic interactions are driven by enthalpy and are related to the difference in the strength of the attractive dispersion interactions of urea and water with the polymer chain or plate. We also show that the indirect mechanism, in which urea acts as a chaotrope, is not a likely cause of urea's action as a denaturant. These findings suggest that, in denaturing proteins, urea (and perhaps other denaturants) forms stronger attractive dispersion interactions with the protein side chains and backbone than does water and, therefore, is able to dissolve the core hydrophobic region. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V6.00004: Systematic Coarse-Graining of Biomolecular Systems Invited Speaker: A multiscale theoretical and computational methodology will be presented for studying biomolecular systems across multiple length and time scales. The approach provides a systematic connection between all-atom molecular dynamics, coarse-grained modeling, and mesoscopic phenomena. At the heart of the methodology is the multiscale coarse-graining method for rigorously deriving coarse-grained models from the underlying molecular-scale forces. Applications of the multiscale approach will be given for membranes and proteins. Recent advances in coarse-graining large protein complexes will also be described along with key applications. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V6.00005: Multi-resolution protein modeling by combining theory and experiment Invited Speaker: The detailed characterization of the overall free energy landscape associated with the folding process of a protein is the ultimate goal in protein folding studies. Modern experimental techniques provide accurate thermodynamic and kinetic measurements on restricted regions of a protein landscape. Although simplified protein models can access larger regions of the landscape, they are oftentimes built on assumptions and approximations that affect the accuracy of the results. We present new methodologies that allows to combine the complementary strengths of theory and experiment for a more complete characterization of a protein folding landscape at multiple resolutions. Recent results and possible applications will be discussed. [Preview Abstract] |
Session V7: Applications of Magnetic Thin Films with Tilted Anisotropy
Sponsoring Units: GMAGChair: Johan Åkerman, Royal Institute of Technology, Sweden
Room: 407
Thursday, March 19, 2009 8:00AM - 8:36AM |
V7.00001: Magnetic Thin Films for Perpendicular Recording Invited Speaker: Magnetic recording technology has shown a rapid growth over the past decade or more with the areal density growth rate more than 40\% due to introduction of giant magneto-resistive (GMR), tunneling magneto-resistive (TMR) head and perpendicular recording technology. Current products are in the range of 300- 400Gb/in$^2$ and demonstrations of 600-800Gb/in$^2$ have occurred for perpendicular recording. Recent progresses are mainly due to improvement of magnetic thin film media signal to noise ratio. In this talk, we give a brief summary of recent progresses on perpendicular recording media technology and look at possible extension for ultra high areal density recording based on recording physics and media designs. First we are going to give a brief review on the advantages of the 1st generation of perpendicular recording media as compared to longitudinal recording media. Then the effect of media switching field and switching field distribution will be discussed. We show several different media designs and show its impact to the recording technology, including media with tilted anisotropy, media with coupled granular continuous (CGC) structure, media with exchange coupled composite structure and media with exchange spring system. In addition, we will discuss the impact of media grain size on achievable areal density, the ultimate limit for magnetic recording based on continuous media. Finally, a possible extension and its impact are discussed. As stated before, here we will not try to dig into very specific detail of each topic but look at the basic concept and physics behind each topic that may potentially lead to new technology break through. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V7.00002: Approaches and Applications of Tilted Magnetic Anisotropy in Hard Disk and STT-RAM Magnetic Recording for Extremely High Areal Density Invited Speaker: Using tilted magnetic anisotropy in magnetic recording media has been proposed and demonstrated to address several key challenges for future hard disk magnetic recording several years ago. Recent simulations show the potential benefit of using tilted magnetic anisotropy in patterned composite magnetic media that can supports the recording areal density up to several Terabit per square inch. In this talk, I will first present the fundamental physics of the advantages of using tilted magnetic anisotropy in magnetic recording media and spin torque transfer random access memory (STT RAM). Then several approaches to experimentally make magnetic films with tilted anisotropy will be presented and discussed in details. These include 1) crystallographically controlling the tilted magnetic easy axis by using proper underlayers and seedlayers; 2) physically engineering the tilted magnetic easy axis by oblique sputtering on smooth or curved substrates; 3) mechanically engineering the tilted magnetic easy axis by self --assembling magnetic nanoparticles with special shape (such as octahedral-shaped L10 phase FePt nanoparticles); 4) micromagnetically engineering the titled magnetic easy axis by exchange-coupling two magnetic layers with different magnetic easy axis and anisotropy constant. Results based on hcp phase Co-alloy and L10 phase FePt and FePd films will be presented in details. Finally, I will propose and discuss how to use tilted magnetic anisotropy for the magnetic free layer in the magnetic tunnel junction (MTJ) to reduce the critical switching current density while keeping the thermal stability, which can extend the recording density of the proposed STT RAM further. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V7.00003: Magnetic nanocap arrays with tilted magnetization Invited Speaker: In modern magnetic recording materials the ``superparamagnetic effect'' has become increasingly important as new magnetic hard disk drive products are designed for higher storage densities. In this regard, patterned media [1], where two-dimensional arrays of nanostructures are used, is one of the concepts that might provide the required areal density in future magnetic recording devices. However, also nanostructure arrays will ultimately need high anisotropy material such as L10-FePt to provid enough thermal stability and thus much higher writing fields than currently obtainable from perpendicular magnetic recording heads. One proposed solution to this problem is the use of tilted magnetic recording media [2]. The basic idea is to tilt the easy axis of the magnetic medium from the perpendicular direction to 45 degree. In this case, the switching field will be reduced by a foctor of two in the Stoner-Wohlfarth limit. Recently, this approach was realized by oblique film deposition onto arrays of self-assembled spherical particles [3-5]. In this presentation, recent results on different film systems including Co/Pt multilayers, FePt and CoPtCr-SiO$_{2}$ alloys which have been deposited onto SiO$_{2}$ particle monolayers will be presented. It turned out that by tuning the growth conditions single domain nanocaps with enhanced magnetic coercivity and tilted anisostropy axis can be achieved even for particle sizes below 50 nm. \\[4pt] [1] B. D. Terris and T. Thomson, J. Phys. D: Appl. Phys. 38 (2005) R199 \\[0pt] [2] J.-P. Wang, Nat. Mater. 4, 191 (2005). \\[0pt] [3] M. Albrecht et al., Nat. Mater. 4, 203 (2005). \\[0pt] [4] T. Ulbrich et al., Phys. Rev. Lett. 96 (2006) 077202. \\[0pt] [5] D. Makarov et al., Appl. Phys. Lett. 93, 153112 (2008). [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V7.00004: Spin-torque oscillators with tilted fixed layer magnetization Invited Speaker: One of the promising applications based on the spin transfer effect [1-3] is the Spin Torque Oscillator (STO) with signal generation at microwave frequencies related to ferromagnetic resonance. The STO may be thought of as a nanoscopic Yttrium Iron Garnet (YIG) oscillator with a similarly broad frequency range, but significant advantages such as easy on-chip integration, and current tunability instead of only field tunability. However, STOs still typically require a large, static, magnetic field for operation; removing the need for this field is currently an intensely researched topic. Three different STO designs have been attempted to address zero field operation: $i)$ the perpendicularly polarized STO [4], \textit{ii}) the wavy torque STO [5], and \textit{iii}) the vortex STO [6]. Recently we proposed the Tilted Polarizer STO (TP-STO) having a fixed layer with an out-of-plane magnetic easy-axis tilted a finite angle away from the film normal [7]. In this talk, I will review our simulation work of the TP-STO and show its potential to generate large output signal in zero field. I will present detailed structural and magnetic characterization of single layer L1$_{0}$ (111) FePt with tilted magnetic anisotropy and show how we have fabricated FePt/Cu/NiFe pseudo spin valves with magnetoresistance values of about 0.5{\%}, and as much as 5{\%} if each interface is dusted with CoFe. Finally, I will present our preliminary work on observing actual microwave signal generation in nano-contact TP-STOs and discuss their potential for applications. \\[4pt] References \\[0pt] [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, 1 (1996). \\[0pt] [2] L. Berger, Phys. Rev. B 54, 9353 (1996). \\[0pt] [3] J. A. Katine,et al., Phys. Rev. Lett. 84, 3149 (2000). \\[0pt] [4] D. Houssameddine, et al, Nat. Mater. 6, 447 (2007). \\[0pt] [5] O. Boulle, et al., Nat. Phys. 3, 492 (2007). \\[0pt] [6] V. S. Pribiag, et al., Nat. Phys. 3, 498 (2007). \\[0pt] [7] Yan Zhou, et al, Appl. Phys. Lett. 92, 262508 (2008); idem, accepted, J. Appl. Phys., (2009). [Preview Abstract] |
Session V8: The Physics of Imaging and Radiotherapy
Sponsoring Units: FPS DBPChair: Barry Berman, George Washington University
Room: 414/415
Thursday, March 19, 2009 8:00AM - 8:36AM |
V8.00001: Dedicated breast CT Invited Speaker: Dedicated breast computed tomography (CT) systems were designed and fabricated in our laboratory, and patient scanning commenced in November 2004. The breast CT scanner was designed utilizing several off-the-shelf components, including the x-ray system, the flat-panel detector, and a position encoder - bearing - motor system. These components were integrated into a custom designed scanner frame and gantry. The breast CT scanners utilize a 17 second acquisition during patient breath-hold, and during this time 500 projection images are acquired over 360 degrees around the breast. The radiation levels are adjusted such that the mean glandular dose is equal to that of two-view mammography for each woman. As of November 2008, over 180 patients have been scanned. Of these, about 40 were imaged with and without contrast agent injection. We have also imaged 4 patients using an integrated PET system. Initial evaluation indicates that high-quality tomographic images of the breast can be achieved at dose levels comparable to two view mammography. The ultimate utility of breast CT may include breast cancer screening, diagnostic imaging, robotically controlled biopsy, and other interventional procedures. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V8.00002: Tomographic Imaging: Visualization of the Unseeable Invited Speaker: Tomographic imaging is a noninvasive approach to acquiring information within the subject under study, and it plays an increasingly important role in the improvement of health care by providing valuable information for diagnosis of diseases, for guidance of disease treatment and therapy, and for assessment/monitoring of treatment response. It has also found a wide variety of applications in other disciplines, ranging from molecular imaging to material sciences to security scan to paleontology. Over the last 30 years, biomedical imaging has involved into an important discipline in its own right. Physics and mathematics form the foundation of advanced tomographic imaging. Computed tomography (CT) and magnetic resonance imaging (MRI) represent two well-known tomographic imaging modalities. In this talk, I will first introduce the basic physics and mathematics principles on which some of the advanced tomographic imaging techniques such as CT are based, with an emphasis on what and how physical signals are detected, how they are used for producing images, and what physical information is that can be extracted from these images. I will then touch upon some of the recent exciting advances in tomographic imaging technology, followed by a brief discussion of some of the important applications of advanced tomographic imaging in medicine and other areas. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:48AM |
V8.00003: Planning and Delivery of Radiation Therapy—Principles and Recent Developments Invited Speaker: |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V8.00004: Image-Guided Radiation Therapy--Application and Advancement Invited Speaker: |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V8.00005: Panel Discussion on Medical Imaging and Radiotherapy Invited Speaker: This panel consists of the invited speakers in this session, and its discussion will be moderated by its organizer. [Preview Abstract] |
Session V9: Focus Session: Structure and Dynamics of Complex Networks
Sponsoring Units: GSNPChair: Beate Schmittmann, Virginia Polytechnic Institute and State University
Room: 303
Thursday, March 19, 2009 8:00AM - 8:12AM |
V9.00001: Hierarchical Link Clustering in Complex Networks Yong-Yeol Ahn, Sune Lehmann, James Bagrow, Albert-L\'aszl\'o Barab\'asi Identifying modular network structure is generally a problem of finding the correct community membership of each node in a network. An alternative approach, clustering links, naturally accounts for real world characteristics such as strong community overlap, multi-partite structure, and hierarchical organization. By introducing a pair-wise link similarity, we use a hierarchical clustering method to identify relevant communities in real-world examples such as biological networks. Our results reveal previously hidden organization of communities. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V9.00002: Enhancing the scale-free network's attack tolerance Zehui Qu, Pu Wang, Zhiguang Qin, Albert-Laszlo Barabasi Despite the large size of most communication systems such as the Internet and World Wide Web (WWW), there is a relatively short path between two nodes, revealing the networks' small world characteristic which speeds the delivery of information and data. While these networks have a surprising error tolerance, their scale-free topology makes them fragile under intentional attack, leaving us a challenge on how to improve the networks' robustness against attack without losing their small world merit. Here we try to enhance scale-free network's tolerance under attack by using a method based on networks' topology re-constructing. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V9.00003: Optimality and Directionality in Network Synchronization Takashi Nishikawa, Adilson Motter In a network of dynamical elements, one of the most fundamental issues is the relationship between the network structure and the collective dynamics of the system. The study of complete synchronization, a simplest form of collective dynamics in a network, in which all oscillators behave in precisely the same way, provides an excellent starting point for understanding how collective behavior arises in a network. The stability of complete synchronization in a weighted directed network of oscillators can be formulated using the well known master stability function and the eigenvalues of the Laplacian matrix encoding the topological structure of the network. In this talk, I will use this formulation to address an interesting optimization problem: which network topology has the highest synchronizability? I will first show that the optimality condition can be expressed solely in terms of the Laplacian eigenvalues. The class of optimal networks contains all directed trees with appropriate connection weights, and most in the class have well-defined directionality. I will also discuss the robustness of optimality against the structural perturbation, as well as the role of directionality in the connectivity patterns in enhancing the synchronizability. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V9.00004: Correlation Networks of Earthquakes Joel Tenenbaum Earthquake events are complex spatiotemporal phenomena, the space and time dependence of which are still not understood. Recently work has been done to explain these events using network modeling, defining links by successive events or probabilities. Our novel approach defines a new kind of network model which defines links through correlation. We find broad correlations across large distances and memory-like signal self-similarity, with statistically significant ``synching'' of different locations to each other. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V9.00005: Conjecture of Alexander and Orbach. Jayanta Rudra, Curtis Doiron The dynamical properties of fractal networks have received wide range of attention. Works on this area by several pioneering authors$^{1-2}$ have led to the introduction of the \textit{spectral dimension} that dictates the \textit{dynamic} properties on a fractal lattice. Most of the studies involving spectral dimension have been performed on a type of fractal lattice known as \textit{percolation} network. Alexander and Orbach$^{2}$ conjectured that the spectral dimension might be exactly 4/3 for percolation networks with Euclidean dimension $d_{e }\ge $ 2. Recent numerical simulations, however, could not decisively prove or disprove this conjecture, although there are other indirect evidences that it is true. We apply a stochastic approach$^{3}$ to determine the spectral dimension of percolation network for d$_{e }\ge $ 2 $a$nd check the validity of the Alexander-Orbach conjecture. Our preliminary results on 2- and 3-dimensional percolation networks indeed show that Alexander-Orbach conjecture is true, resolving a long-standing debate. References: 1. P. G. deGennes, La Recherche 7 (1976) 919. 2. S. Alexander and R. Orbach, J. Phys. Lett. (Paris) 43 (1982) L625. 3. J. Rudra and J. Kozak, Phys. Lett A 151 (1990) 429. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V9.00006: Dense Random Fiber Networks Deform as Stochastic Fractal Objects. Catalin Picu, Hamed Hatami-Marbini The mechanical behavior of random fiber networks is essential in many biological and non-biological systems such as the cytoskeleton, tissue scaffolds and cellulose structures. Here we show that random fiber networks of densities much larger than that of the stiffness percolation threshold are stochastic heterogeneous elastic media with fractal distribution of elastic constants. The elasticity of these networks, both elastic constants and fields, while fluctuating significantly with position, is long-range correlated. The range of scales for stochastic self-similarity is bounded below by the mean fiber segment length and above, by the fiber length. This implies that no scale decoupling exists and no representative volume elements can be identified on scales below the upper cut-off scale, which provides an explanation for the observed delocalized effect of local mechanical perturbations in systems of semi-flexible fibers such as the cytoskeleton. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V9.00007: Voter dynamics on an adaptive network with finite average connectivity Abhishek Mukhopadhyay, Beate Schmittmann We study a simple model for voter dynamics in a two-party system. The opinion formation process is implemented in a random network of agents in which interactions are not restricted by geographical distance. In addition, we incorporate the rapidly changing nature of the interpersonal relations in the model. At each time step, agents can update their relationships, so that there is no history dependence in the model. This update is determined by their own opinion, and by their preference to make connections with individuals sharing the same opinion and with opponents. Using simulations and analytic arguments, we determine the final steady states and the relaxation into these states for different system sizes. In contrast to earlier studies, the average connectivity (``degree'') of each agent is constant here, independent of the system size. This has significant consequences for the long-time behavior of the model. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V9.00008: Dynamics of priority-queue networks Byungjoon Min, Kwang-Il Goh, In-mook Kim Recent application of priority queue models for human dynamics opened a way to study the human behavior under quantitative framework. Given the evident active engagement in social networking of individuals, dynamics of priority queues forming networks needs to be understood. Along this line, here we study the dynamics of priority-queue networks by generalizing the binary interacting priority queue model of Oliveira and Vazquez (OV). We found that the OV model with AND-type protocol for interacting tasks is in general not scalable for the queue networks with more than two queues, because the dynamics for interacting tasks become quickly frozen due to the priority conflicts. We then consider a scalable interaction protocol, an OR-type one, and examine the effects of the number of queues and the network topology on the waiting time dynamics of the priority-queue networks, finding that its distribution exhibits power-law tail in all cases considered, yet with exponents dependent on the network topology. We also show that when the tasks in the queue network are executed synchronously, priority conflicts affect the waiting time dynamics strongly, resulting in a different power law. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V9.00009: About human activity, long-term memory, and Gibrat's law Diego Rybski, Sergey V. Buldyrev, Shlomo Havlin, Fredrik Liljeros, Hernan A. Makse A central research question in the social sciences for several centuries has been whether any law like patterns in the unintended outcomes of human action exist. Here we investigate the existence of scaling laws in the human activity of communication, considering the number of messages sent by individuals as a growth process in time. We analyze millions of messages sent in two social online communities and uncover power-law relations between fluctuations in the growth rate and the activity of the members. We attribute this scaling law to a long-term persistence of human activity beyond daily or weekly cycles holding up to more than a year. Based on such an underlying long-term correlated dynamics, we elaborate a consistent framework for the empirical evidences, establishing a missing link between the scaling behavior in the growth and long-term persistence. Our results indicate that large fluctuations in communication activity can be expected as an unintended consequence of human interaction. This finding is of importance for both designing communication systems and for understanding the dynamics of social systems. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V9.00010: Tour de Sys: The traveler's view of a network Daniel Grady, Christian Thiemann, Dirk Brockmann The plight of the Flatlander is imperfect information about a high-dimensional object. Yet even so, the clever inhabitant of a low-dimensional world can gain a great deal of information about such an object by examining it from many perspectives. We analyze complex transportation networks by using shortest-path trees to measure universal network properties from different locations. Furthermore, by defining a measure of a node's geographical access area we give a more realistic characterization of the centrality or remoteness of a location. The network topology indicates a clear distinction between the center and edge of a network, but we find that examining the weights of links is crucial, as the distinction in the weighted network for some quantities is even more pronounced. Often prior research has not focused on the weightedness of transportation networks, in spite of the fact that this property has an obvious bearing on how the networks are actually used. We show that measuring networks with weighted edges significantly affects their statistics. Our analysis indicates dynamical processes occurring on these networks should behave in a manner very different than what is predicted by considering topology alone. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V9.00011: Universality and the lack of it in multiscale human mobility networks Rafael Brune, Christian Thiemann, Dirk Brockmann Although significant research effort is currently devoted to the understanding of complex human mobility and transportation networks, their statistical features are still poorly understood. Specifically, to what extent geographical scales impose structure on these networks is largely unknown. In particular, in light of the use of human mobility models in the development of quantitative theories for spatial disease dynamics, a comprehensive understanding of their structure is of fundamental importance. The large majority of statistical properties (degree distributions, centrality measures, clustering, etc.) of these networks have been obtained either for large scale networks or on small scale systems, indicating significant yet poorly understood deviations. We will present the first investigation of multiscale and multi-national mobility networks, covering length scales of a few to a few thousand kilometers. We will report that certain properties such as mobility flux distribution are universal and independent of length scale, whereas others vary systematically with scale. In particular, controversial properties such as scale-free degree distributions lose their heavy tails in small to intermediate length-scale windows. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V9.00012: Monte Carlo Studies of the Isoperimetric Dimension of Growing Droplets in Metastable Decay of the Ising Model on Small-World Graphs Howard L. Richards For the Ising model on a regular, nearest-neighbor lattice of less than 6 dimensions, metastable decay occurs via the nucleation of critical droplets; subcritical droplets are biased toward shrinkage, whereas supercritical droplets are biased toward growth. The size of a critical droplet is governed by the competition between the coupling of the magnetic field to the volume of the droplet, which lowers the free energy, and the coupling of the droplet of the stable state to metastable state at the boundary of the droplet, which increases the free energy. This competition between volume effects and surface effects makes the isoperimetric dimension relevant to metastable decay. The simulations discussed here are for a triangular lattice with a small percentage of ``small-world'' connections. The system initially has only one ``down'' spin, from which the droplet grows; switching is irreversible and only occurs for ``up'' spins adjacent to at least one ``down'' spin. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 11:00AM |
V9.00013: Functional vs. Structural Modularity: do they imply each other? Invited Speaker: While many deterministic and stochastic processes have been proposed to produce heterogeneous graphs mimicking real-world networks, only a handful of studies attempt to connect structure and dynamics with the function(s) performed by the network. In this talk I will present an approach built on the premise that structure, dynamics, and their observed heterogeneity, are implementations of various functions and their compositions. After a brief review of real-world networks where this connection can explicitly be made, I will focus on biological networks. Biological networks are known to possess functionally specialized modules, which perform tasks almost independently of each other. While proposals have been made for the evolutionary emergence of modularity, it is far from clear that adaptation on evolutionary timescales is the sole mechanism leading to functional specialization. We show that non-evolutionary learning can also lead to the formation of functionally specialized modules in a system exposed to multiple environmental constraints. A natural example suggesting that this is possible is the cerebral cortex, where there are clearly delineated functional areas in spite of the largely uniform anatomical construction of the cortical tissue. However, as numerous experiments show, when damaged, regions specialized for a certain function can be retrained to perform functions normally attributed to other regions. We use the paradigm of neural networks to represent a multitasking system, and use several non-evolutionary learning algorithms as mechanisms for phenotypic adaptation. We show that for a network learning to perform multiple tasks, the degree of independence between the tasks dictates the degree of functional specialization emerging in the network. To uncover the functional modules, we introduce a method of node knockouts that explicitly rates the contribution of each node to different tasks (differential robustness). Through a concrete example we also demonstrate the potential inability of purely topology-based clustering methods to detect functional modules. The robustness of these results suggests that similar mechanisms might be responsible for the emergence of functional specialization in other multitasking networks, as well, including social networks. [Preview Abstract] |
Session V10: Artificially Structured Materials for Optical Manipulation
Sponsoring Units: DCMPChair: Ian Sellers, Sharp
Room: 304
Thursday, March 19, 2009 8:00AM - 8:12AM |
V10.00001: Photon-phonon excitations in single acousto-optical microcavities Martin Maldovan, Edwin Thomas Photonic and phononic crystals are periodic structures, which, for certain wavelengths, exhibit a bandgap that does not allow the propagation of either light (photons) or sound (phonons). These periodic materials can also localize and guide light or sound by means of microcavities and waveguides, or allow for negative refraction and behave like superlenses. Photonic and phononic crystals have been realized individually, although as yet there has been no combined study that exploits their singular properties. We propose the development of a new class of physical system that combines photonic and phononic properties. In this paper, we show novel excitations between photons and phonons localized in the same area at the same time within these novel materials. The simultaneous localization of light and sound creates a strong influence on photon-phonon interactions and we anticipate the realization of a coherent monochromatic source of very high-frequency phonons. Design guidelines are provided for these novel acousto-optical microcavities in terms of structural morphologies and material platforms. This research helps to develop physical mechanisms for light-induced generation of coherent phonons and acousto-optical effects that can result in the optical cooling of materials. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V10.00002: Near field and diffraction of an electromagnetic wave at a metallic grating with slits Raul Garcia-Llamas, Ramon Mungu\'Ia-Arvayo, Jorge Gaspar-Armenta The interference of surface Plasmons between two slits is studied by using the near-field intensity. The problem of diffraction of an electromagnetic wave at a metallic grating with slits is treated. A rigorous solution of diffraction based on the method of multimodal expansion by Burckhardt$^{1}$ is presented. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V10.00003: Bi-directional Scatter Distribution Function (BSDF) Measurements of Guided Mode Resonance Filter Optical Limiters Robert Lamott, Michael Marciniak, Brian Cunningham Guided Mode Resonance Filters (GMRF) are 2- and 3-D photonic-crystal structures designed to provide a specific photonic band gap. This narrow stop band makes it a suitable candidate for dealing with laser illumination directed at optical sensors, protecting the sensor while allowing the sensor to continue collecting other wavelengths. Since absorbing light is not their primary method of filtering, GMRFs are even suitable for higher power lasers, which may cause thermal damage and failure in an absorptive chromatic filter. However, investigation into where the light is scattered, both through the filter and reflected off of the filter, is necessary to ensure scatter is not damaging other elements of the sensor or other nearby sensors. We analyzed three GMRF samples designed for different wavelengths, using a Complete Angle Scatter Instrument (CASI) to provide in-plane measurements of the strength of the scatter in all transmitted and reflected angles. This data is used to generate Bi-directional Scatter Distribution Functions (BSDFs), which can be either physically or empirically based, to model the reflected and transmitted scatter for all incident angles. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V10.00004: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V10.00005: Theory of Plasmonic Wave Propagation Along a Periodic Chain of Nanoscale Pores in a Metal David Stroud, Kwangmoo Kim We have calculated the dispersion relations and group velocities of plasmonic waves propagating along a periodic chain of nanoscale pores in a Drude metal, using a tight- binding formalism. The propagating modes are Bloch waves constructed from linear combinations of electromagnetic modes of the individual pores embedded in a metallic host. In contrast to the analogous plasmonic waves propagating along periodic chains of metallic nanoparticles in a dielectric, the pore waves do not suffer radiative losses, and the tight- binding approach is not restricted to the quasistatic approximation or to particles small compared to a wavelength. We have also calculated the plasmonic band structure for waves propagating through a three-dimensional inverse opal structure of pores in a metallic host. We discuss the possibility of generating and detecting these waves in porous metals and porous superconductors. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V10.00006: Long wavelength limit of the 2D photonic crystal S.T. Chui, Z.F. Lin We solve {\bf analytically} the multiple scattering equations for the two dimensional photonic crystals in the long wavelength limit. Different approximations of the electric and magnetic susceptibilities are presented from a unified pseudopotential point of view. The nature of the so called plasmon-polariton bands are clarified. Its frequency as a function of the wire radius is discussed. The corresponding tunable ``magnetic surface plasmon'' band is pointed out. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V10.00007: Electro-photonic enhancements in organic solar cells with photonic crystal photoactive layers John Tumbleston, Doo-Hyun Ko, Edward Samulski, Rene Lopez Organic photovoltaics with nanopatterned photonic crystal photoactive layers offer an alternative to conventional planar devices that suffer from a competition between optical absorption and electronic processes. Our recent studies have shown that nanopatterned devices exhibit enhanced absorption and exciton creation profiles as compared to planar cells. Improved absorption results in part from the excitation of resonant optical modes where certain photon energies near the semiconducting band edge are enhanced 20-fold. Prerequisites for their excitation include an index of refraction contrast of 0.3 for the two photonic crystal materials and a periodicity comparable to the band edge wavelength. Resonant mode dispersion determined via photonic band calculations and variable angle absorption measurements indicate that both fast and slow modes exist in nanopatterned devices. Quantum efficiency measurements also confirm improved optical and electrical performance for photonic crystal organic solar cells. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V10.00008: Photonic Quasicrystals for Nonlinear Quantum Optics Susanna Thon, William Irvine, Dirk Bouwmeester Certain semiconductors, such as GaAs and GaP, have very high values for the second order optical nonlinear susceptibility. This makes them promising materials for applications in nonlinear (quantum) optics. However, phase matching conditions must be achieved through microstructuring of the materials or in cavities because they possess no intrinsic birefringence which is the conventional method for achieving phase matching. It has been reported that photonic crystal lattices based on quasicrystal geometries can support multiple photonic bandgaps at widely spaced frequencies. We report on the results of simulations to identify suitable semiconductor quasicrystal structures for nonlinear optics applications such as frequency conversion and the observation of novel cavity QED effects. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V10.00009: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V10.00010: Transformation media with negative refractive indices C.T. Chan, Y. Lai, H.Y. Chen, Jack Ng, Z.Q. Zhang Artificially structured materials with a negative refractive index designed by transformation optics can have interesting properties. Based on merging the concept of complementary media and transformation media, we propose an invisibility cloak operating at a finite frequency that can make an object invisible with a pre-specified shape and size within a certain distance outside the shell. The cloak is comprised of a dielectric core, a negative index metamaterial shell and an ``anti-object'' embedded inside the shell. The cloaked object is not blinded by the cloaking shell since it lies outside the cloak. Full-wave simulations in two dimensions have been performed to verify the cloaking effect. We also show that a positive index core coated with a negative index shell can result in a frequency selective super-absorber which has an absorption cross section that is significantly higher than the geometric cross section. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V10.00011: Electromagnetic transparency by graded metallic coating L. Sun, K.W. Yu Recently there has been an increasing interest in achieving cloaking or invisible devices for electromagnetic fields. The study has been based on Pendry's transformation media concept. In this work, we have studied electromagnetic scattering by coated spheres with a homogeneous core and a radially inhomogeneous dielectric shell described by the lossless graded Drude model $\epsilon(r)=1-\omega_p^2(r)/\omega^2 $. The plasma frequency depends on $r$ as $\omega_p^2=1-cr^k$, where $c$ and $k$ are positive constants. The electromagnetic field distribution has been calculated within the fully electromagnetic Mie scattering theory. When $k=2$, exact analytic solutions can be obtained for the field distribution in terms of Whittaker functions. The total scattering cross section can be obtained from the scattering field amplitudes and is found to be dependent on both the graded profile and the cross-shell ratio. The analytic expressions of the total scattering cross section allow us to assess the conditions for achieving better transparency$^{[1]}$, resulting in tunable electromagnetic cloaking. [1]. A. Al\`u and N. Engheta, Phys. Rev. E 72, 016623 (2005) [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V10.00012: Wave-Front Engineering by Huygens-Fresnel Principle for Nonlinear Optical Interactions in Domain Engineered Structures Zhu Yongyuan, Qin Yiqiang, Zhang Chao Wave-front engineering for nonlinear optical interactions was discussed. Using Huygens-Fresnel principle we developed a general theory and technique for domain engineering with conventional quasi-phase-matching (QPM) structures being the special cases. We put forward the concept of local QPM, which suggests that the QPM is fulfilled only locally not globally. Experiments agreed well with the theoretical prediction. The proposed scheme integrates three optical functions: generating, focusing, and beam splitting of second-harmonic wave, thus making the device more compact. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V10.00013: Electromagnetic Response of Josephson Junction Metamaterials in Positive and Negative Permittivity Regimes Steven Anlage, Laura Adams Negative index of refraction metamaterials have shown strikingly different behavior than their positive index of refraction counterparts, enabling for example cloaking and super lensing. Josephson junction (JJ) metamaterials which are tunable and have low loss are a distinct advantage not only because of their nonlinearity but also due to their ability to be scaled down in size. We will present microwave measurements of JJ arrays that behave differently depending on whether the arrays resonate above or below the cutoff frequency of an electromagnetic waveguide. Below cutoff, resonances seem to indicate a macroscopic phase coherence of the JJ arrays with emission of photons. Above cutoff, we interpret the interaction between the arrays and the electromagnetic waves as indications of vortex-anti-vortex physics. We will describe how the JJ arrays respond to controlled changes of the input power, temperature and dc magnetic fields and how these responses depend on the sign of the permittivity. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V10.00014: Anomalous infrared monochromatic transmission through a film of type-II superconductor in magnetic field and a superconducting multiple conductor system Roman Ya. Kezerashvili, Oleg L. Berman, Vladimir S. Boyko, Yurii E. Lozovik Anomalous far infrared monochromatic transmission through a lattice of Abrikosov vortices in a type-II superconducting film is found. The transmitted frequency corresponds to the photonic mode localized by the defects of the Abrokosov lattice. These defects are formed by extra vortices placed out of the nodes of the ideal Abrokosov lattice. The extra vortices can be pinned by crystal lattice defects of a superconductor. The frequency is studied as a function of magnetic field and temperature in the framework of the Dirac-type two-band model. The control of the transmitted frequency by varying magnetic field and/or temperature is analyzed. Besides, anomalous infrared monochromatic transmission through a superconducting multiple conductor system consisting of parallel superconducting cylinders is found. The transmitted frequency corresponds to the localized photonic mode in the forbidden photonic band, when one superconducting cylinder is removed from the node of the ideal two-dimensional lattice of superconducting cylinders. Our approach is valid for all type-II superconductors but the specific calculations have been performed for the YBCO film in the magnetic field and for the YBCO superconducting cylinders. [Preview Abstract] |
Session V11: Transport Phenomena and Electronic Properties of Nanostructures
Sponsoring Units: DCMPChair: Luis da Silva, Oak Ridge National Laboratory
Room: 305
Thursday, March 19, 2009 8:00AM - 8:12AM |
V11.00001: Momentum statistics of tunneling electrons in nanoelectromechanical systems Steven D. Bennett, Aashish A. Clerk When a mesoscopic conductor is used to measure the position of a nanomechanical oscillator, electrons in the conductor exert a fluctuating back-action force on the oscillator. What is the statistical distribution of the momentum transferred to the oscillator by this force? Motivated by recent experiments that studied a mechanical oscillator coupled to a single tunnel junction \footnote{N. E. Flowers-Jacobs {\it et al.}, Phys. Rev. Lett. {\bf 98}, 096804 (2007).} or a quantum point contact \footnote{M. Poggio {\it et al.}, Nat. Phys. {\bf 4}, 635 (2008).}, we investigate theoretically the statistics of back-action force in these systems as well as correlations between the force and the current. Our approach is based on a scattering matrix that depends parametrically on the oscillator position, allowing us to go beyond weak tunneling and study conductors with arbitrary transmission. We identify two mechanisms of momentum transfer: one involves forces exerted in the scattering region and dominates in the limit of weak tunneling; the other is associated with transferred electron momentum and dominates in the limit of perfect transmission. We also discuss the effects of a spatially asymmetric conductor on the force noise and on the quantum limit of position detection. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V11.00002: Mechano-electronic Superlattices in Silicon Nanoribbons M. Huang, C.S. Ritz, B. Novakovic, F. Flack, D.E. Savage, P.G. Evans, I. Knezevic, D. Yu, Y. Zhang, F. Liu, M.G. Lagally Single-crystal silicon nanomembranes (SiNMs) have the mechanical compliance fundamentally different from bulk materials or supported thin films that can produce unique structural and electronic effects. The growth of nanostressors on SiNMs utilizes this mechanical compliance to create a ``strain lattice'' consisting of tiny regions of local curvature in the SiNMs, the order occurring because the locally bending SiNMs provides a strong feedback for self-organization of the nanostressors. We demonstrate that a high degree of order occurs when Ge or SiGe nanostressors are grown on both sides of free-standing but end-tethered Si nanoribbons. Our calculations prove that the strain lattice in the Si produces a modulation in the electronic band structure, and thus an electronic superlattice. Our calculations also demonstrate that discrete minibands can be observable in such an electronic superlattice at 77K. It is expected that an electric conductivity will be increased in the superlattice. We predict that it is possible to observe discrete minibands at room temperature if other nanostressors are used. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V11.00003: `Spring-Like' and Photo-actuated Molecular-Junctions between Nanoparticles Kabeer Jasuja, Vikas Berry Here we present a study on (1) ``molecular-spring'' nano-device, where controllable and confined forces are applied on collective molecular-junctions between nanoparticles and (2) photo-actuated nano-junction system where azo-molecules incorporated between nanoparticles apply confined forces to displace them. Both systems are built by using covalently/electrostatically crosslinked polyelectrolyte (cPE) molecules sandwiched between gold nanoparticles (GNP), where cPE molecular-junctions are reversibly compressed and stretched by applying electrically and centrifugally induced forces respectively. The GNPs play a dual role (a) of movable connectors to apply forces and (b) of nanoelectrodes to measure molecular deformation via electron tunneling change. The `molecular-spring' junctions were found to have a spring constant between 10$^{-4}$ to 10$^{-3}$ N/m depending on the thickness of the junction. We will also demonstrate the dynamics of these junctions via a motion-in-viscous-media model. The ability to store the compression energy in a molecular-device-architecture and to manipulate these by actuating junctions has the potential to power future molecular devices by stored molecular-energy and controlling properties of nanocomponent based devices. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V11.00004: Noise characterization of metal oxide nanowire FETs with electronic properties controlled by surface geometry Wenyong Wang, Hao Xiong, Curt Richter, Woong-ki Hong, Takhee Lee In this talk we present the results of low-frequency noise and random telegraph signal (RTS) characterization of metal oxide nanowire (NW) field-effect transistors (FETs). ZnO nanowires with different surface geometry properties such as corrugated and smooth surfaces have been synthesized. FETs fabricated from these NWs exhibit different electronic transport characteristics. Noise characterization has been performed on NW FET devices with different surface properties. The obtained noise power spectra at room temperature show 1/f frequency dependences, and the Hooge's constants have been calculated from the gate voltage dependence fo the 1/f noise for the devices with different surface geometries. The characteristics of low frequency noise in the drain current have been further investigated through random telegraph signals measurements at 4.2 K, where the channel current RTSs can be attributed to the correlated carrier number and mobility fluctuation due to the trapping and detrapping of the carriers by discrete border/surface traps. The effects of the NW surface properties on the RTS behaviors will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V11.00005: A Biotransistor: Modulating the Current of a Nanodevice with a Living Cell Jennifer Kane, Jason Ong, Ravi Saraf Isolated single nanoparticles and array of nanoparticles act as switching devices sensitive to charging by a single electron. For a typical 5 nm Au particle, the switching barrier energy due to Coulomb blockade from a single electron charging is approximately 100 meV, making room temperature switching difficult and very noisy. In an array, the switching energy can be a few eV (at cryogenic temperatures), but unlike a single nanoparticle, the energy barrier reduces linearly and vanishes at room temperature. We have developed a ``reactive self-assembly'' method to make a network of one-dimensional necklaces of nanoparticles that behaves as a single-electron device at room temperature. Furthermore, upon cementing the particles with an inorganic semiconductor, the switching behavior at room temperature is significantly improved. To demonstrate an interesting application of room temperature single-electron switching, we couple the network to a living microorganism to modulate the device current by regulating the cell's metabolic activity. In the talk we will describe fabrication of the necklace and biotransistor device. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V11.00006: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V11.00007: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V11.00008: Bistable tunneling current through a quantum dot array junction Yia-Chung Chang, David M.T. Kuo We investigate the tunneling current through a six-fold degenerate $p$ --like states of a one- dimensional (1D) or two-dimensional (2D) quantum dot (QD) array in the x-y plane. Due to the coupling of$p_x $ and $p_y $orbitals at neighboring QDs, a 1D or 2D conduction band ($\varepsilon _p )$ is formed, whereas the $p_z $ orbitals remain localized due to their weak in-plane coupling. The on-site repulsive Coulomb interaction in the $p_z $ levels ($U)$ and that between the $p_z $ level and $p_x $/$p_y $ level ($U_{dc} )$ are taken into account in an extended Anderson model, which is used to investigate the tunneling characteristics of the system. Tunneling current through localized $p_z $ state is calculated in the framework of the Green function technique. Due to the effect of $U_{dc} $, the 1D/2D conduction band states are shifted by a self-energy term $2NU_{dc} $. We find that bistable current can be observed for this system in the Coulomb blockade regime, which makes the system a valid candidate for ultra high-density memory device. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V11.00009: Tunneling Between a Quantum Wire and a Two-Dimensional Electron Gas Dominique Laroche, John Reno, Guillaume Gervais, Mike Lilly We study 1D-2D tunneling between a quantum wire and a 2D electron gas as a function of magnetic field, source drain bias, temperature and 1D subband occupation. The transition from 2D-2D to 1D-2D tunneling is clearly observed through a sharpening of the tunneling resonance, confirming that the measurements are performed in a 1D-2D state. The device used is fabricated in a GaAs/AlGaAs parallel double quantum well heterostructure with an 11 nm wide Al$_{0.9}$GA$_{0.1}$As barrier separating the quantum wells. Quantum wires are created via electron beam lithography defined split gates fabricated on both sides of the sample, albeit only one of the wires is used in the experiment.. The design is such that the 1D density can be independently controlled over a large conduction range and is uniform over the length of the quantum wire. Both wires show non-ballistic quantum steps up until a conductance of 10 $\times $ 2e$^{2}$/h. Magnetotranpsport results are compared to tunneling in the 1D-1D and 2D-2D regimes. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V11.00010: Temperature Dependence of Electron Tunneling between Quantum Dots and Electron Gas Yoko Sakurai, Shintaro Nomura, Yukihiro Takada, Kenji Shiraishi, Masakazu Muraguchi, Tetsuo Endoh, Mitsuhisa Ikeda, Katsunori Makihara, Seiichi Miyazaki We report that gate voltages required for electron injection to quantum dots (QDs) from electron gas and for emission from QDs strongly depend on temperature. For this experiment, Si-QDs floating gate MOS capacitors were designed and fabricated. Displacement current (I) was measured as a function of the gate voltage (V). Peaks in I-V curves appear as a result of charging and discharging of Si-QDs. We have found that the gate voltages for the electron injection to and for the emission from QDs shift toward more positive and negative values with decreasing temperature, respectively. Theoretical study predicts that electron tunneling is strongly enhanced when initial state of 2DES is localized below the QD [1]. Based on this discussion, the experimentally obtained results suggest that localization of electron gas induced by thermal fluctuation is responsible for enhanced electron tunneling. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V11.00011: New insight into Tunneling Process between Quantum Dot and Electron Gas Masakazu Muraguchi, Tetsuo Endoh, Yoko Sakurai, Shintaro Nomura, Yukihiro Takada, Kenji Shiraishi, Mitsuhisa Ikeda, Katsunori Makihara, Seiichi Miyazaki, Yasuteru Shigeta We have theoretically investigated the time-evolution of electron wave function in tunneling from a two-dimensional electron gas (2DEG) to a quantum dot (QD). We have revealed that the electronic state in the electron gas significantly influences the electron tunneling. We clearly showed that the electron tunneling is modified depending on the initial electronic state in the 2DEG. The electron tunneling from 2DEG to QD is strongly enhanced when the initial state of the electron in the 2DEG is localized below the QD. This result indicates that the temporal and spatial fluctuation of electron distribution, which depends on temperature, plays a crucial role in the tunneling process. We will show that the obtained temperature dependence of electron tunneling coincides with our recent experiments of capacitance-voltage characteristic in a QD floating gate MOS capacitor. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V11.00012: Hole Transport and Spin Effects in Cleaved-Edge-Overgrowth Quantum Wires Joseph Sulpizio, Charis Quay, Rafi de Picciotto, K.W. West, L.N. Pfeiffer, David Goldhaber-Gordon Transport measurements on ballistic GaAs electron wires have revealed a rich set of phenomena associated with one-dimensional (1D) quantum systems. Studies of transport in hole systems are a natural extension of these experiments due to the enhanced effective mass, g-factor, and spin-orbit coupling of holes over their electron counterparts. However, only recently has the creation of ballistic hole wire devices been possible due to breakthroughs in molecular beam epitaxy using the cleaved-edge-overgrowth (CEO) technique. We present measurements of hole transport in CEO GaAs quantum wires in magnetic field in a dilution refrigerator. Based on a simple model, we extract the g-factor for different field orientations, and also discuss evidence for observing spin-orbit coupling in a 1D system. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V11.00013: High-quality quantum point contact in two-dimensional GaAs (311)A hole system Javad Shabani, Jason Petta, Mansour Shayegan We studied ballistic transport across a quantum point contact (QPC) defined in a high-quality, GaAs (311)A two-dimensional (2D) hole system using shallow etching and top-gating. The QPC conductance exhibits up to 11 quantized plateaus and the ``0.7'' structure. The ballistic one-dimensional subbands are tuned by changing the lateral confinement and the Fermi energy of the holes in the QPC. We demonstrate that the positions of the plateaus (in gate-voltage), the source-drain data, and the negative magneto- resistance data can be understood in a simple model that takes into account the variation, with gate bias, of the hole density and the width of the QPC conducting channel. Spacings between the quantized energy levels in this geometry are about 2 to 7 times larger than in previous reports of QPCs in other GaAs hole systems. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V11.00014: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V11.00015: Determination Hanbury-Brown Twiss correlations with electrons and photons Eva Zakka-Bajjani, J. Dufouleur, P. Roche, D.C. Glattli, F. Portier What is the statistics of the photons emitted by a quantum
conductor? We
present the first experiment addressing this question, where the
electronic
shot noise power of a 500$\Omega $ tunnel junction is measured in
the 4-8
GHz frequency range in an Hanbury-Brown Twiss geometry (E.
Zakka-Bajjani \textit{et al.}
Phys. Rev. Lett. \textbf{99}, 236803 (2007)). The emitted noise
is analyzed
in two different manners. The fluctuations of the transmitted and
reflected
electronic currents are shown to be anti-correlated. The
auto-correlated
power fluctuations reveal that the junction emits power into the
detection
in the form of photons, and the emitted powers show positive
cross-correlation, proportional to the squared emitted power. The
photons
emitted by a biased low impedance (i.e. R$_{tunnel}< |
Session V12: Semiconductor Growth and Etching
Sponsoring Units: DMP DCMPChair: Melissa Hines, Cornell University
Room: 308
Thursday, March 19, 2009 8:00AM - 8:12AM |
V12.00001: Morphological Study of MBE Grown Iron Nitride Films on Zinc-Blende GaN(001) Jeongihm Pak, Wenzhi Lin, Yinghao Liu, Kangkang Wang, Abhijit Chinchore, Arthur Smith, Kai Sun Iron nitrides are attractive materials for their high magnetic moments, corrosion, and oxidation resistance. We present the successful epitaxial growth of iron nitride (FeN) film on zinc-blende gallium nitride ($c$-GaN) using molecular beam epitaxy with Fe $e$-beam evaporation and rf N-plasma source at substrate temperature of 210 \r{ }C. The film growth is monitored \textit{in-situ} using reflection high energy electron diffraction (RHEED) and the samples are analyzed \textit{ex-situ} using x-ray diffraction (XRD), cross-sectional TEM and atomic force microscopy (AFM). By monitoring the structure, morphology, and lattice constant evolution of the FeN films, the crystal phase and orientation with respect to the $c$-GaN substrate are deduced to be zinc-blende phase with the epitaxial relationship [001]$_{FeN} \quad \vert \vert $ [001]$_{GaN}$ and [100]$_{FeN }\vert \vert $ [100]$_{GaN}$. Surface morphological studies by AFM show cubic structures ranging from 250-400 nm in size and having smooth plateaus with roughness of 6 {\AA}. This work has been supported by DOE (Grant {\#}DE-FG02-06ER46317). [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V12.00002: Iron and Iron Nitride Layers on Wurtzite Gallium Nitride Studied Using MBE/STM Wenzhi Lin, Jeongihm Pak, Yinghao Liu, Kangkang Wang, Abhijit Chinchore, David Ingram, Arthur Smith, Kai Sun It is of interest to study epitaxial growth of iron and iron nitride (FeN) layers on wurtzite gallium nitride (w-GaN) as a possible magnetic/wide-gap semiconductor system for spintronics. X-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED) of Fe deposited on GaN(0001) suggest the existence of the epitaxial relationship [110]$_{Fe}\vert \vert $ [0001]$_{GaN}$. Furthermore, multi-streak RHEED patterns indicate the formation of a multi-domain but smooth film. Also, we have investigated the growth of $\sim $ 1:1 iron nitride on w-GaN(0001) using nitrogen plasma-assisted molecular beam epitaxy (MBE). Both reciprocal and real space techniques were used to study the growth of FeN, including RHEED and scanning tunneling microscopy/spectroscopy (STM/STS). Bulk characterization was also applied, including XRD and transmission electron microscopy (TEM). The results indicate that zinc-blende FeN grows on GaN(0001) with the epitaxial relationship [111]$_{FeN }\vert \vert $ [0001]$_{GaN}$, and initial atomically-smooth FeN layers are formed. This work has been supported by DOE (Grant {\#}DE-FG02-06ER46317). [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V12.00003: Stability of Mg-incorporated InN surfaces: first-principles study T. Akiyama, K. Nakamura, T. Ito, J. -H. Song, A. J. Freeman InN films are attractive materials for electronic and optelectronic applications. The growth of InN eptitaxial films with $n$-type and $p$-type conductivity has traditionally been performed along the polar $<$0001$>$ direction\footnote{R.E. Jones et al., Phys. Rev. Lett, {\bf 96}, 125505 (2006)}, which may result in large polarization fields along the growth direction, reducing the radiative efficiency of quantum-well light emitters. To overcome this drawback, the growth along nonpolar orientation such as (10$\bar{1}$0) and (11$\bar{2}$0) planes and its $p$-type doping have been recently carried out. We have addressed this issue by performing first-principles pseudopotential calculations for Mg-incorporated InN surfaces in various orientations, including (10$\bar{1}$0) and (11$\bar{2}$0) as well as (0001) and (000$\bar{1}$) surfaces\footnote{J.-H. Song et al., Phys. Rev. Lett. {\bf 101}, 106803 (2008)}. The calculated surface energies demonstrate that qualitative trends in the stability of Mg-incorporated surfaces agree with those on GaN surfaces \footnote{J.E. Northrup, Appl. Phys. Lett. {\bf 86}, 122108 (2005)}, although several surface reconstructions different from those on GaN surfaces are obtained. The effects of growth conditions on $p$-type doping are also discussed. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V12.00004: Ferromagnetic $\delta -MnGa$ On Wurtzite GaN(0001): Interface Formation And Film Properties Kangkang Wang, Abhijit Chinchore, Wenzhi Lin, Jeongihm Pak, Arthur Smith, Kai Sun Ferromagnetic (FM) metal/wide band-gap semiconductor contacts are of great interest due to their potential for novel spintronics applications, such as blue and ultra-violet spin light-emitting diodes$^{[1]}$. One promising candidate is $\delta -MnGa$ on wurtzite GaN, whose epitaxial growth has recently been reported$^{[2]}$ , with controllable magnetism via controlling of the Mn:Ga flux ratio. Here we report further studies on MnGa/GaN system grown by N$_{2}$-plasma equipped molecular beam epitaxy (MBE). Reflection high-energy electron diffraction (RHEED) data suggests a quicker and more abrupt interface formation when grown on Ga-polar GaN surface as compared to N-polar. \textit{In-situ} scanning tunneling microscopy (STM) measurements on the first few monolayer's as well as thicker MnGa films will be presented, revealing details of interface formation and other film properties. Stoichiometry dependence of the growth and magnetic properties will also be discussed. This work is supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No. 0730257). Equipment support from ONR is also acknowledged. [1] S.A.Wolf \textit{et al}, Science \textbf{294}, 1488 (2001) [2] E.Lu \textit{et al}, Phys.Rev.Lett. \textbf{97}, 146101 (2006) [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V12.00005: Low-temperature transport properties of disordered tantalum and tantalum nitride films Nicholas Breznay, Mihir Tendulkar, Aharon Kapitulnik Tantalum nitride thin films are used in a wide range of electronic applications, such as in thin film resistors and diffusion barriers in silicon microstructures. Growth and thorough characterization of ultrathin tantalum nitride films may prove useful in potential applications and also facilitate the study of disordered, low-dimensional systems. We will discuss the low-temperature transport properties of reactively sputtered tantalum and tantalum nitride ultrathin films as a function of film structure and composition, and connect our results to recent studies of both these and other similar two-dimensional disordered systems. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V12.00006: First Principles Phases of Sub-Monolayer Sr and La on Si (001) Kevin Garrity, J.W. Reiner, F.J. Walker, C.H. Ahn, S. Ismail-Beigi The epitaxial integration of complex oxides with semiconductors is a key requirement for many emerging technologies. In the short term, the scaling down of the dielectric layer in current CMOS technology will soon require new materials with higher dielectric constants to prevent quantum mechanical leakage currents. More generally, the epitaxial integration of complex oxides with semiconductors would allow new devices to take advantage of the wide range of oxide properties. To date, the first step of the only known method to grow complex oxides on silicon epitaxially has required 1/2 ML of an alkaline earth metal, usually Sr, to be deposited on a clean silicon surface at about 600 C. Using first-principles density functional theory calculations, we examine the growth of sub-monolayer coverages of both Sr and La on Si (100). For Sr on Si, we report on a novel 1/6 ML structure which explains the complex temperature dependence observed experimentally below 1/2 ML Sr. We compare these results to the case of La on Si and elucidate some differences which hinder the growth of epitaxial oxides on La template layers. Our results predict an experimentally verified low temperature path to epitaxy using a Sr template layer. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V12.00007: From STM Images to Chemical Understanding: Kinetic Monte Carlo Simulations of Si(100) Etching Ankush Gupta, Ian T. Clark, Brandon S. Aldinger, Melissa A. Hines Etching reactions literally write a record of their chemical reactivity in the morphology of the etched surface -- a record that can be read using scanning tunneling microscopy (STM) and decoded with the help of simulation. We have developed a fully atomistic kinetic Monte Carlo simulation of Si(100) etching that is appropriate for aqueous etchants that produce fully H-terminated surfaces [e. g., NH$_{4}$F(aq), KOH(aq) and even H$_{2}$O]. The model assumes that the reactivity of individual surface sites is determined by the local geometry. As an example, we simulate the production of near-atomically flat Si(100) surfaces, recently observed experimentally, and show that interadsorbate stress plays a crucial role in determining the steady-state etch morphology. The simulated morphologies are in good agreement with experimental observations. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V12.00008: Atomic-Scale Effects of Applied Strain on Etching of Si(100) Marc F. Faggin, Brandon S. Aldinger, Ankush Gupta, Melissa A. Hines An ideal hydrogen-terminated Si(100) surface would be highly stressed due to unfavorable steric interactions between neighboring surface sites (i.e. neighboring silicon dihydride species). As a result, some aqueous etchants selectively remove every other silicon dihydride to relieve the stress, producing an unusual striped morphology. In these experiments, we apply a uniaxial strain to the surface during etching. Using a combination of scanning tunneling microscopy and surface infrared spectroscopy, we show that applied strains dramatically alter the etch morphology and the surface species, in some cases promoting the formation of hillocks. These effects are explained in terms of the site-specific reactivity of the etching silicon surface. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V12.00009: Aqueous Etching Produces Si(100) Surfaces of Near-atomic Flatness Brandon S. Aldinger, Ankush Gupta, Ian T. Clark, Melissa A. Hines The production of atomically flat Si(100) surfaces is a long-standing technological challenge, as these surfaces are the basis for today's microelectronic devices. We use a combination of STM and vibrational spectroscopy to show that a simple aqueous etch can produce Si(100) surfaces of suprising and unprecedented smoothness. The etched surfaces are characterized by long rows of H-terminated Si atoms. The chemical origins of this perfection are uncovered, in part, by a new polarization-based, spectral deconvolution technique that significantly simplifies the analysis of the well-know H/Si(100) vibrational spectrum. Kinetic Monte Carlo simulations yield further insights into the site-specific chemical reactions that govern the steady-state etch morphology. The effects of interadsorbate stress, etchant pH, and gas evolution will also be discussed as time permits. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V12.00010: Quantitative Correlations of Stress Field and Threading Dislocation Configurations in Si/Si$_{1-x}$Ge$_{X}$/Si(100) Strained Layers Chi-Chin Wu, Robert Hull The equilibrium configurations of the threading dislocation connecting misfit dislocation dipoles in Si/Si$_{1-x}$Ge$_{x}$/Si(100) heterostructures have been analyzed by simulation, and compared to experimental images to investigate the correlations of stress field variations and dislocation configuration in capped heteroepitaxial thin films. Calculations are based on the energy equations for dislocations in elastically isotropic crystals over all possible dislocation angular configurations, and the orientations with the minimum total energy are determined as functions of the magnitude and variations of the film stresses. It is determined that the configuration with the misfit dislocation leading at the cap/film interface (as opposed to the film/substrate interface) is increasingly favored either with increasing magnitude of local film stress/strain or with the variation in calculated geometry of the threading dislocation. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V12.00011: Ordering mechanisms of periodic stripe arrays on boron-doped Si(100) Ivan Ermanoski, Gary Kellogg, Norman Bartelt We have used low energy electron microscopy to determine the factors that control the degree of order in self-assembled periodic stripe arrays on the atomically flat Si(100) with high boron doping. The stripes consist of extremely elongated vacancy islands of single atomic height, formed at $\sim $900C, confined in micrometer-sized pits. ``Perfect'' arrays of parallel stripes (in pits of up to $\sim $10um in size) were formed by allowing various defects to heal over relatively long periods of time. Sublimation was compensated for by an external Si doser, allowing observation of stripe evolution over the course of hours, with no net loss or gain of Si from the area of interest. Stripe formation and ordering mechanisms include spontaneous nucleation and growth of new islands, longitudinal splitting, as well as coarsening due to surface diffusion. Stripe periodicity depends on temperature, allowing for control of this property. Stripes are stable in a range of $\sim $100C, outside of which they assume the familiar shape of elongated islands, shaped by the anisotropy in step energy. Stripe order can be preserved to room temperature by quenching. References: [1] J.-F. Nielsen et al., Appl. Phys. Lett. 79 (2001) 3857 [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V12.00012: On the Growth Mechanisms of Plasma Deposited Amorphous Silicon Thin Films Dimitrios Maroudas, Sumeet Pandey, Tejinder Singh We present a theoretical study of the growth mechanism of plasma deposited amorphous silicon (Si) thin films based on kinetic Monte Carlo simulations according to a transition probability database constructed by first-principles density functional theory calculations. Based on the results of the study, we propose a comprehensive mechanism of amorphous Si thin film growth by plasma deposition under conditions that make the silyl radical the dominant deposition precursor. The growth mechanism consists of various surface kinetic events including radical-surface interactions, adsorbed radical-radical interactions, radical surface diffusion, and surface hydride dissociation reactions. Of particular importance to the Si film growth process and the resulting surface composition is the radical dissociative adsorption mediated by Si over-coordination defects along the reaction pathway. The proposed mechanism explains the experimentally measured surface composition of plasma deposited Si films under the deposition conditions considered. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V12.00013: Fundamental Study of Boron Carbide Sputtering Sudarshan Karki, Dae Yeoun, Saad Janjua, Marcus Driver, Anthony Caruso Boron-rich carbides belong to a special class of solids whose main structural unit is the twelve atom icosahedra. When depositing thin films of boron carbide (nominally B$_{4}$C) by RF or pulsed DC magnetron sputtering, the individual sputtered or ablated cluster size and the temperature of the substrate to which the clusters adsorb to form the film, greatly affects the bulk film physical and electronic structure. This talk will present mass spectrometry data of the target clusters as a function of RF power, DC bias and chamber pressure toward the goal of modeling and understanding how the icosahedral based boron-rich materials sputter and the resultant control over the final film properties. Argon trapped into the film during the deposition as determined by X-Ray photoemission will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V12.00014: Correlation between Bonding Geometry and Band Gap States at Organic -- inorganic interfaces: Catechol on Rutile TiO$_{2}$ (110) Ulrike Diebold, Shao-Chun Li, Jian-guo Wang, Peter Jacobson, Xue-Qing Gong, Annabella Selloni Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical (DFT) and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO$_{2}$ is presented. Using the organic catechol on rutile TiO$_{2}$(110) as a model system it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 $\times $ 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal by about $\pm $27\r{ } in an alternating fashion. In the lowest-energy structure one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, while the other OH group forms a H-bond to the next catechol neighbor. Through proton exchange with the surface this structure transforms into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO$_{2.}$ [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V12.00015: Theoretical adhesion strength of diamond coating with metallic interlayers. Haibo Guo, Yue Qi, Xiaodong Li Metallic interlayers are often needed to enhance the adhesion of diamond coatings to substrates and to promote diamond nucleation and growth. The interfaces between diamond coatings and metallic interlayer materials with different carbide formation enthalpies, Cu, Al, and Ti, are studied using density functional theory. The ideal interface strength or the work of separation is found to decrease with the carbide formation enthalpy. Analysis to the electronic structure shows that covalent metal-carbon bonds form at the interface, and the perturbation from the interface weakens nearby metal phase. Comparing the work of separation at the interface with the fracture energy of the metal, a fracture is likely to initiate in the metal phase near the interface, therefore a tough metal with a large surface energy is needed to achieve a higher overall toughness. In addition, when the surface energy is larger than the interface energy, a wetted diamond/metal interface is formed during diamond nucleation, which also contributes to good adhesion. The interface energy, which is an energy barrier to diamond nucleation, is found to decrease with the carbide formation enthalpy. These results indicate strong carbide formability and a large surface energy of the interlayer enhance the adhesion and the fracture resistance of the interface, and also conduce to the diamond nucleation on the interlayer. [Preview Abstract] |
Session V13: Physics Education and History
Sponsoring Units: FEd FHPChair: Michael Pettersen, Washington and Jefferson College
Room: 309
Thursday, March 19, 2009 8:00AM - 8:12AM |
V13.00001: Physics Education in Sub-Sahara African Schools and Universities Tilahun Deressu This talk will give an overview of physics teaching and research in Sub-Saharan African Schools and Universities. Secondary School Physics Curricula and physics teacher training curricula from 10 African countries are compared from content, methods of delivery and evaluation perspectives. The characteristics of physics study and research in universities, both at the undergraduate and the graduate levels, are described. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V13.00002: Physics That Works: Shift in Physics Education Paradigm Based on Work-Integrated Physics Education. Bahram Roughani A nontraditional work integrated undergraduate physics degree program that balances the option of pursuing a career path after graduation versus graduate studies will be presented. The main components of this undergraduate physics education model, which will be presented are; work-integrated physics education, emphasis on industrial and applied physics, nontraditional undergraduate research and thesis, an altered academic calendar, required technical sequence courses, and flexibility for each student to engage in courses that best complement physics. The impact on the growth of the physics program, the challenges and rewards involved, as well as pros and cons of such program in contrast with the traditional physics degree programs will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V13.00003: Strengthen Instructional Components in Peer-Lead Problem-Solving Workshop with eTEACH Jia-Ling Lin, B. Choe, P. Fowler, R. Gilsdorf, T. Kirchdoerfer, A. Kokemoor, G. Lucas, T.J. Madsen, A. Romens, M. Springer Physics and mechanics are core topics that sometimes frighten beginning engineering students. Statistics have shown that an increasing number of students have given up engineering because of struggle in these courses. One common realization is a consensus among educators that the quality of teaching needs to be improved, and that traditional lectures, even with stellar instructors, fail to motivate students to meet learning challenges. One of the responses from the College of Engineering at UW-Madison has been comprehensive academic support services, specifically the Supplementary Instruction (SI) Program. SI, a student-lead problem-solving workshop has made a significant impact on teaching and learning in engineering for the past eight years. In this report, we describe how we develop eTEACH (an on-line teaching tool) to integrate features of subject- and problem-based learning. We discuss how we incorporate eTEACH to promote broader interactions between instructors and students, as well as among students. Our initial results showed that this change has assisted students with different learning preferences to improve problem-solving and learning strategies. We acknowledge support from Dr. G. Moses, Dr. D. Woolston, and the 2010 project in engineering college. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V13.00004: Student thinking regarding derivative and slope concepts in multivariable calculus Warren Christensen, John Thompson Previous work on students' understanding of graphical interpretation of slope, derivative, and area under curves in various physics contexts has shown substantial difficulties, most notably in kinematics. Concurrently, several reports point toward a lack of algebraic acumen as a likely cause for low achievement in a physics classroom. As part of ongoing research on mathematical challenges that may underlie documented physics difficulties, we developed and administered a brief survey on single- and multivariable calculus concepts to students near the end of a Calculus III course. Some of the questions are based on our earlier work in thermal physics that are essentially stripped of their physics content. Initial findings show that as many as one in five students encounter some type of difficulty when asked to rank the slopes at five different points along a single path. Students asked to rank the derivatives of three different functions at a single value of the variable face additional difficulties. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V13.00005: An Engineer's Physics Lab -- using a Large Force Frame Christy Heid, Donald Rampolla We have constructed very economical, easy to assemble force frames that are used by students in our general physics laboratory at Chatham University. The force frame is used at the beginning of the semester to study vector properties of forces. The force frame can be used as a horizontal or vertical force table. Angles of forces are measured using a large movable (rotation and translation) Cartesian coordinate board attached to the frame with large binder clips. The force frame is a versatile device which is used for a number of other experiments, including beam bending and torsion, mechanical resonance, projectile trajectories, torque, mechanical equilibrium, an isolated non-magnetic support for magnetic field experiments, easily adjustable support for inclined plane experiments, support for traveling wave experiments with heavy rope, and support for large scale fluid flow experiments. One advantage to a wood frame is that things can be easily stapled, nailed, screwed or glued just about anywhere on the frame, and damaged frame members can be replaced easily. As one of the few remaining women's undergraduate institutions, we have found the use of these frames to provide an additional advantage in helping women overcome their fear of simple power tools and assembly of mechanical parts as they become comfortable with these through working with the force frames throughout the semester. We intend to describe and model these applications during the session. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V13.00006: Explain the latent heat and specific heat of water, ammonia, and methanol with degrees of freedom Lianxi Ma There are 15 known crystalline solid phases of water and all of them are called ice. But here the ice in our context is the one when water is cooled down to 0 \r{ }C at 1 atmosphere. It is well known that at 0 \r{ }C and 1 atmosphere, the density of water is 0.9998 g/cm3 and the density of ice is 0.9162 g/cm3. Question: For 0 \r{ }C water and ice, which has a higher internal energy? Because they have same temperature, their molecules should have same kinetic energies. Therefore their potential energies among molecules need to be compared. Because ice's density is lower so it should have larger potential energy, which indicates an incredible conclusion that ice has higher internal energy. How do we explain this paradox? The internal energies of 0 \r{ }C water and ice are considered from the perspective of degree of freedom and latent heat of fusion of water is calculated, which is in good agreement with the published value. With the same consideration, the latent heats of fusion of ammonia and methanol are calculated and the results are in reasonable agreement with the published values. This simple strategy can give specific heats of water, liquid ammonia, and methanol, which are in good agreement with known data. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V13.00007: Introducing Raman Spectroscopy of Crystalline Solids in the Undergraduate Curriculum Bahram Roughani, David Warner, Uma Ramabadran We describe an experiment designed as an upper level physics laboratory that introduces students to Raman Scattering of electronic materials and research methodology. This experiment is an effective approach in demonstrating the relationship between the Raman intensity of the scattered light from crystals and symmetry dependent Raman selection rules. In our measurements we alter the angle between the crystal axis and the polarization of the incident laser beam by sample rotation. The three dimensional plot of the intensity profile versus the theoretical model is used to distinguish between various crystal plans of the same electronic material. This experiment combines knowledge regarding properties of materials with optical characterization. It is suitable as an upper level physics laboratory or for introducing new graduate student to use Raman spectroscopy as a research tool. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V13.00008: Civic Engagement through Differential Equations? Shafiqur Rahman In a technological society such as ours, optimal allocations of limited resources frequently require a clear understanding of the sciences. However, policy makers often lack background in this area, and physics majors almost never get exposed to ideas that lie at the intersection of science and society, certainly not in a quantitative way. As a result, the latter show little interest in such issues. To address this problem, we have developed a short course titled \textit{Civic Engagement for Physicists}. A substantial part of the course is quantitative. For example, when covering issues connected to energy, a topic of major current interest, we use a differential equation from population dynamics to study predictions about when the peak in world oil production might occur, and what the true amount of world oil reserve might be. On the other hand, topics such as \textit{Characteristics of Science} and \textit{National Science Policy} are covered in a qualitative way. In this talk, I'll present details of both the quantitative and the qualitative areas covered by the course, as well as reaction of students. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V13.00009: Challenges and opportunities of undergraduate research Daniela M. Topasna, Gregory A. Topasna Undergraduate research at small schools is becoming the norm rather than the exception that it was years ago. Faculties are now faced with the challenges of incorporating students with varying degrees of academic preparedness and motivation in their research. This coupled with the students' own constraints within the academic schedule can make undergraduate research a challenge for both students and faculty. Like many small undergraduate schools, VMI's faculty and students are faced with these obstacles when engaging in undergraduate research. However, such difficulties can lead to creative solutions that lead to multiple benefits for students and faculty mentors. We present our unique perspective and experiences for this challenging yet rewarding experience as related to thin film research performed at VMI. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V13.00010: Reacting to Galileo: Introducing a New Approach for Gen Ed Science Michael Pettersen Either Galileo was right, or he was wrong; either way, why was there ever any debate about it? And why should we care today about the opposing ideas, which proven wrong so long ago? In the ``Reacting to the Past'' series of curricular materials, students engage with key turning points in human intellectual history by taking sides and recreating the original debate. In this way, students personally identify with points of view that they would otherwise find wrong, boring, and incomprehensible \--- and they learn how we test ideas by challenging them, and defend them by marshalling evidence, which is the core of critical thinking. Students almost universally report that the ``Reacting'' experience is tremendously engaging. I shall describe an application of the ``Reacting'' format to the case of Galileo. The scientific issues involved are comprehensible to non-science majors, the cultural context of Renaissance Italy is rich and wonderful, and Galileo's personal history is tremendously moving. The materials include labs designed to be taught by non-scientists teaching cross-disciplinary liberal arts courses. Other ``Reacting'' science materials have been published or are under development. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V13.00011: Flint and the British Tradition of Relativity Theory James Beichler Most scientists and scholars are familiar with Sir Arthur Eddington's role in verifying General Relativity in 1919. A few less are aware of his work introducing the theory to the English scientific community. Still less know of Eddington's extensions of relativity theory, especially his attempts to develop a unified field theory. But very few scholars, historians or even physicists are aware of the important role played by other English scientists in the acceptance and development of relativity. In fact, H.T. Flint and his colleagues published more than thirty-five articles in peer reviewed journals in Britain over a period of four decades in an attempt to extend relativity to include electromagnetism and the quantum. Yet his work and that of his close associates is almost completely unknown today, in spite of the fact that he published a book describing his complete unified field theory in the 1960s, well before most quantum theorists even began thinking along the lines of unification. In a world filled with speculations about gravitons, superstrings, quantum loops and other unification models, Flint did it first, but his work has all but disappeared from the scientific consciousness. From Eddington to Flint, the English school of relativists has produced ardent supporters of relativity and numerous advances beyond the standard interpretations of general relativity. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V13.00012: Ukrainian Physical-Technical Institute (UFTI) in the 1930's D.H. McNeill, Yu. N. Ranyuk, O.S. Shevchenko UFTI (Ukrainian Physical-Technical Institute; now Kharkiv Institute of Physics and Technology, KhFTI), founded in 1928, was among the first national laboratories. In the 1930's, L. Shubnikov, B. Podolsky, G. Placzek, L. Tisza, F. Houtermans, A. Weissberg, V. Weisskopf, and others worked there on important and interesting research in many areas (low-temperature, electronics, nuclear physics, theory). 2008 was the centenary of Lev Landau, who established his school of theoretical physics and began his \textit{Course of Theoretical Physics} in Kharkiv. It is now $\sim $70 years since the Great purge at UFTI (and, simultaneously, throughout the USSR). UFTI's history, a stark reminder of politics in science, is less known than that of institutions in Moscow and St. Petersburg. \textit{``Delo'' UFTI 1935-1938} [\textit{The UFTI Affair}, Yu. V. Pavlenko, Yu. N. Ranyuk, and Yu. A. Khramov, Kyiv, 1998] is a study, using documents available after 1990, of the lab's early years and its near destruction in the Stalinist purges. Many scientists at UFTI were killed or imprisoned. Documents from this time will be shown. A timeline of the 1930's at UFTI will be presented. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V13.00013: Experience in teaching intensive course of thermal physics for undergraduate physics students Farkhad Aliev This talk of non-technical nature describes experience of the author in teaching the intensive course of thermal physics for the undergraduate physics students at the Universidad Autonoma de Madrid, Spain. After brief introduction to the program, description of the WEB support of the course, I shall describe practical classes ( home-works, visits to the Laboratories, experimental demonstrations, typical problems and typical topics for presentations on the advanced thermodynamics, etc. ). I shall further discuss different possible actions to wake up an interest of the students to the thermal physics and ways to simulate their active participation in the class discussions. I also describe different schemes employed in the last few years to evaluate effectively and clearly the students work and knowledge. Finally, I will analyze the efficiency of our methodic in improving teaching of thermal physics at University level. [Preview Abstract] |
Session V14: Disordered Systems, Glassy Dynamics, and Jamming II
Sponsoring Units: DFD GSNPChair: Peter Schall, University of Amsterdam
Room: 315
Thursday, March 19, 2009 8:00AM - 8:12AM |
V14.00001: Specific Heat Anomalies in Glassy Fluids Due to Cluster Micro-Melting George Hentschel, Valery Ilyin, Itamar Procaccia, Nurith Schupper We will discuss the specific heat anomalies observed in new simulations of equimolar mixtures of particles interacting via soft core repulsive potentials under external pressure that are known to show glassy dynamics at low temperatures. The simulations show both long-lived states of microcrystalline clusters that do not nucleate a crystalline ground state and also the appearance of two specific heat peaks which were not observable in earlier simulations. We argue that the appearance of two peaks is due to the micro-melting of two types of clusters and discuss the form of the resulting specific heat anomalies. Our arguments suggest that the glass transition will typically show non universal features. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V14.00002: The building blocks of Dynamical Heterogeneities in dense granular media Raphael Candelier, Olivier Dauchot, Giulio Biroli Unveiling the connection between the short term relaxation and the long term dynamical heterogeneities as observed near the glass transition in super-cooled liquids and the jamming transition in granular materials remains a major challenge in the physics of glassy systems. On one hand, KCM models relate dynamical heterogeneities to a non trivial structure in the trajectory space, inherited from the local dynamical rules. On the other hand, recent studies of hard spheres close to isostaticity suggest that the collective aspect of the relaxation would stem from the extended character of the softest degrees of freedom, along which the system yields from one meta-bassin to another. There is still no direct experimental evidence in favour of one or the other mechanism in super-cooled liquids nor in dense granular media. Here we will show that for a dense granular layer under cyclic shear dynamical heterogeneities result from a two timescales process. Short time but already collective events consist in clustered cage jumps, which concentrate most of the non affine displacements. Such clusters aggregate both temporally and spatially within an avalanche process, which ultimately builds the large scales dynamical heterogeneities. The typical timescale of the dynamical heterogeneities appears as the crossover between the short time separating successive event within the avalanches and the long time separating the successive jumps of any given particle. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V14.00003: Role of Shape Anisotropy on the Glassy Dynamics of Colloidal Suspensions Mukta Tripathy, Kenneth Schweizer Center-of-mass ideal mode coupling theory (MCT), the nonlinear Langevin activated barrier hopping theory, and the Reference Interaction Site Model have been employed to investigate the structure and slow dynamics of suspensions composed of hard and rigid nonspherical particles. Objects of dimensionality one (rods, rings), two (discs), and three (polyhedra) have been studied. For non-compact particles the volume fraction of ideal kinetic arrest, corresponding to a crossover to activated dynamics, decreases with particle dimensionality and/or aspect ratio. On the other hand, the ideal vitrification volume fraction of compact 3-dimensional objects is a complex and subtle function of particle shape. Calculations of the entropic barrier for activated transport, mean relaxation time, transient localization length, diffusion constant, elastic modulus, and effective fragility have been performed. Deep in the ideal glassy region the barrier height and mean hopping time are controlled by the shape-dependent mean square confining force exerted on a particle by its surroundings. A nearly universal collapse of many dynamical properties is achieved based on a dimensionless difference variable that quantifies the magnitude of the mean square force compared to its critical value at the ideal MCT transition. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V14.00004: Coupled Translational and Rotational Glassy Dynamics in Hard Dicolloid Suspensions Rui Zhang, Kenneth Schweizer Naive mode coupling theory (NMCT) and the nonlinear Langevin equation (NLE) theory of activated glassy dynamics have been generalized to treat the coupled center-of-mass translation and rotational motions of hard linear objects of variable aspect ratio. Two types of ideal nonergodicity transitions are predicted corresponding to localization of only translation (plastic glass for small aspect ratios) or simultaneous arrest of translation and rotation (double glass). The NMCT kinetic arrest transition signals a crossover to activated dynamics controlled by entropic barriers. Specifically, a two-dimensional dynamical free energy surface (function of translational displacement and rotational angle) is constructed which quantifies effective forces in the NLE theory. For all aspect ratios the most efficient activated process corresponding to the trajectory of lowest entropic barrier is associated with a system-specific translation-rotation cooperative motion. Mean alpha relaxation times as a function of dicolloid aspect ratio and volume fraction are computed using multidimensional Kramers theory. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V14.00005: Constitutive relations in dense granular flows John Drozd, Colin Denniston We use simulations to investigate constitutive relations in dry granular flow. Our system is comprised of mono- and poly-disperse sets of spherical grains falling down a vertical chute under the influence of gravity. We observe three phases or states of granular matter: a free-fall dilute granular gas region at the top of the chute, a granular fluid in the middle and then a glassy region at the bottom. We test various proposed constitutive relations to provide a basis for analytically solving for the stresses in granular flows. Finally, we examine the energy conservation and heat flow in our systems and show that the heat conductivity constitutes distinct power-law dependencies on the granular temperature in the glassy and fluid regions of our system. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V14.00006: Polytetrahedral Frustration of Crystallization: A Study of 4d Hard Spheres Patrick Charbonneau, Jacobus A. van Meel, Daan Frenkel Geometrical frustration is thought to ease the supercooling of a liquid. In $3d$ hard spheres the preferred local cluster is icosahedral and the densest packing is tetrahedral, but no periodic lattice is consistent with either symmetry in Euclidian space, so a crystal phase with a different symmetry nucleates upon compression. For $2d$ disks in contrast triangular or hexagonal order is both locally and globally preferred and crystallization of a metastable fluid is quasi-instantaneous. Yet the precise origin of geometrical frustration remains unclear, because in $2d$ and $3d$ polytetrahedral structures are often equated conceptually to the optimal local cluster. Here, we conduct a computational study of the $4d$ analogue, where the optimal local cluster and global order are commensurate, but the polytetrahedral order is not. We observe no sign of facile crystal formation, which support the polytetrahedral frustration scenario. We also find the fluid to be structurally very different from the crystal. The resulting high interfacial free energy sheds new light on $3d$ geometrical frustration and its role in glass formation. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V14.00007: Noise as a Probe of Ising Spin Glass Transitions Zhi Chen, Clare Yu Noise is ubiquitous and and is often viewed as a nuisance. However, we propose that noise can be used as a probe of the fluctuations of microscopic entities, especially in the vicinity of a phase transition. In recent work we have used simulations to show that the noise increases in the vicinity of phase transitions of ordered systems. We have recently turned our attention to noise near the phase transitions of disordered systems. In particular, we are studying the noise near Ising spin glass transitions using Monte Carlo simulations. We monitor the system as a function of temperature. At each temperature, we obtain the time series of quantities characterizing the properties of the system, i.e., the energy and magnetization. We look at different quantities, such as the noise power spectrum and the second spectrum of the noise, to analyze the fluctuations. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V14.00008: Shear banding in colloidal glasses Vijayakumar Chikkadi, Andrew Schofield, Peter Schall We perform slow shear of colloidal glasses using a confocal microscope and shear-cell set up. The particles are tracked in time and space to construct the local strain field, which is observed to be non-uniform with high strain and low strain zones interspersed in space. Our measurements at a volume fraction $\sim $ 59{\%} show the existence of homogeneously sheared regime at a shear rate $\sim $5x10$^{-5}$ s$^{-1}$ and shear localization at higher shear rates ($>$10$^{-4}$ s$^{-1})$. The set-up offers a unique opportunity to elucidate the evolution of shear-bands using the concept of shear transformations. In particular, the aim is to understand the role of correlation between the shear transformations in the growth of shear bands. We present an overview over the homogeneous versus inhomogeneous shear regime in terms of a deformation map for these systems. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V14.00009: Correlations between Dynamical Heterogeneity and Viscoelasticity of Confined Colloidal Suspensions under Oscillatory Shear Prasad Sarangapani, Yingxi Elaine Zhu In this talk, we present a recent rheological study of confined amorphous colloidal thin films under oscillatory shear using a home-designed micron-gap rheometer interfaced with a confocal microscope. We visualize the response of ``hard-sphere'' poly-(methyl methacrylate) (PMMA) particles of 1.2 $\mu $m in diameter to applied shear deformation and simultaneously measure the viscous and elastic moduli of PMMA colloidal thin films of bulk volume fraction, \textit{$\phi $}= 0.43-0.57, confined at gaps ranging from 50 $\mu $m to 1-2 $\mu $m. For confined PMMA colloids under shear at gaps where an applied deformation is sufficiently large to induce non-linear responses, we find commonality in particle dynamics where strongly non-affine motion causes particles to move as cooperatively rearranging groups. However, on average the length scale of these groups is larger than the typical length scales of dynamical heterogeneities for the un-sheared thin films and typically approaches the order of confining dimension. We quantify the nature of shear induced flow cooperativity and its relation to a shear thickening transition observed in the limit of large strain amplitudes. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V14.00010: Aggregation of athermal particles induced by capillarity Michael Berhanu, Arshad Kudrolli Aggregation of cohesive particles floating in a medium is a very broad physical phenomena occurring in colloidal systems, soot particles, and intergalactic dust under gravitation. We investigate the geometrically constrained dynamics of aggregation with new experiments using floating spheres at the air-liquid interface. A short range attractive force can be induced by careful choice of buoyancy and capillarity to create self-assembled particle structures which can be tracked by imaging. First, the particles are placed randomly at the interface, and then aggregation is induced by smoothly decreasing the area of the interface which causes the particles to come within the attractive force range caused by capillarity. We measure the area fraction at which the connectivity and rigidity percolation transitions are observed and further characterize the aggregates with two-point correlation functions. We then compare and contrast our results with gelation and jamming transitions reported with colloids and granular matter. Finally, we study the reverse phenomena, where we probe the response of the aggregate to an increase in interface area. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V14.00011: Correlation range in a supercooled liquid via Green-Kubo expression for viscosity, local atomic stresses, and MD simulations Valentin A. Levashov, Takeshi Egami, James R. Morris We present a new approach to the issue of correlation range in supercooled liquids based on Green-Kubo expression for viscosity. The integrand of this expression is the average stress-stress autocorrelation function. This correlation function could be rewritten in terms of correlations among local atomic stresses at different times and distances. The features of the autocorrelation function decay with time depend on temperature and correlation range. Through this approach we can study the development of spatial correlation with time, thus directly addressing the question of dynamic heterogeneity. We performed MD simulations on a single component system of particles interacting through short range pair potential. Our results indicate that even above the crossover temperature correlations extend well beyond the nearest neighbors. Surprisingly we found that the system size effects exist even on relatively large systems. We also address the role of diffusion in decay of stress-stress correlation function. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V14.00012: Dynamics in Complex Fluids Formed by Conjugated Polymers Naresh Osti, Madhusudan Tyagi, Dilru Ratnaweera, Uwe Bunz, Dvora Perahia Alkyl di-substituted \textit{para-polyphenyleneethylene} (PPE) associates into several complex fluids in dilute solutions of toluene. At high temperature the molecules are isolated and assume extended configuration. As the temperature decreases, the molecules associate and eventually jam in to a fragile gel. These phases are optically active where the dynamic processes affect their optical characteristics. Inelastic Neutron Back Scattering conjunction with Neutron Spin Echo was used to characterize the dynamics on multiple length scales at different temperatures. The current talk will introduce the neutron backscattering results that follow internal dynamics within the PPE molecules as they are confined into aggregates and jams to form a fragile phase. The data will be discussed in terms of Kohlrausch-Williams-Watt model that provides characteristics time constants for the different dynamic processes. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V14.00013: Spontaneous transition in TiNiFe strain glass system Jian Zhang, Xiaobing Ren, Yu Wang, Kazuhiro Otsuka, Jun Sun Glass has been considered as one major challenge for the statistic mechanics, for the presumption of ergodicity is no longer valid. Hence, glass transition was normally viewed as solely kinetic driven process, including ferroic cluster glasses. Whereas, the ferroic cluster glasses appear once the thermodynamics driven ferroic phase transitions are suppressed by the point defects. It seems quite intriguing why the power of thermodynamics is immediately eliminated when the system reaches the threshold composition, and taken over completely by kinetics. We demonstrate the power of the thermodynamics on the glass state by presenting for the first time the existence of spontaneous transition from strain glass to long range phase in the newly found strain glass in TiNiFe, evidenced with various experimental methods. In addition, we provide one phenomenological model to reveal the underlying mechanism due to the competition between the thermodynamic and kinetic factors. Our results on this relatively simply glass system may bring new ideas in other fields of science and technology, i.e. biophyscis and biochemistry. [Preview Abstract] |
Session V15: Liquid Crystals I
Sponsoring Units: DFDChair: Jonathan Selinger, Kent State University
Room: 316
Thursday, March 19, 2009 8:00AM - 8:12AM |
V15.00001: Interaction and Response of a Smectic-A liquid crystal to a 2 nm Nanometer Particle: Phase transition due to the Functionalization Compound Luz J Martinez-Miranda, Lynn K Kurihara We have studied the in-plane (parallel to the magnetic field) alignment of 8CB mixed with FeCo nanoparticles covered with different funtionalization compounds. The functionalization compounds are Polyethelene glycol (PEG (3000)), hydroxyl succinimide (NHS), aminopropyl tri-ethoxy silane (APTS) and mercapto hexa-decanoic acid (MHDA). We have studied them using X-ray scattering. We have found that the inverse integrated intensity of the X-ray scans in the plane of the magnetic field is a good measure of how much energy the system (liquid crystal, nanoparticles, functionalization compound) will need to reorient the liquid crystal in the magnetic field. In addition, we have observed that the orientation the liquid crystal adopts with respect to the nanoparticles can result in a phase transition that takes the liquid crystal to a more disordered and symmetric phase that favors the rotation, as happens in the smectic-nematic transition, observed in the sample with APTS. We relate the disordering to the changes observed in the transition for the liquid crystal and this termination to recent heat capacity measurements by Cordoyiannis et al. [1]. \textbf{References} [1] Cordoyiannis, G., Kurihara, L.K., Martinez-Miranda, L. J, Glorieux, C., Thoen, J., submitted to PRE (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V15.00002: Theory of Ferroelectric Nanoparticles in Nematic Liquid Crystals Lena Lopatina, Jonathan Selinger Many recent experiments have reported that ferroelectric nanoparticles have drastic effects on nematic liquid crystals. Low concentrations of such particles increase the isotropic-nematic transition temperature by over 10 C, and greatly increase the sensitivity of the nematic phase to applied electric fields. To understand these effects, we develop a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals. In this theory, the key issue is the distribution of orientations for the electrostatic dipole moments of the nanoparticles. This distribution is characterized by an orientational order parameter, which interacts with the orientational order of the liquid crystals and stabilizes the nematic phase. We estimate the coupling strength and calculate the resulting enhancement in the transition temperature, in good agreement with experiments. We also predict the response to applied electric fields, showing that the Kerr effect is enhanced above the isotropic-nematic transition. These predictions apply even when the electrostatic interactions are partially screened by moderate concentrations of ions. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V15.00003: Unconventional dimerization in mesogenic semi-phasmidic type carboxylic acid* Shin-Woong Kang, Seung-Yeon Jeong, Dena Mae Agra-Kooijman, Satyendra Kumar, Veena Prasad, Sanjay Varshney The nematic and columar mesophases formed by a semi-phasmidic type carboxylic acid are investigated by DSC, X-ray scattering, FTIR spectroscopy, and polarizing optical microscopy. FTIR spectroscopy and X-ray results confirm that two semi-phasmidic type carboxylic acid molecules form a mesogenic dimer through inter-molecular hydrogen bonding. X-ray diffraction results obtained under \textit{in-situ} magnetic field reveal unique characteristics that set them apart from conventional linear dimers formed via hydrogen bonding. These dimers possess negative dielectric and diamagnetic anisotropies. Values of the length scales corresponding to the diffraction peaks and their orientation relative to the magnetic field strongly suggest the formation of a ``bent-core-like'' mesogenic dimers rather than conventional coplanar linear dimers. *Supported by grant NSF/DMR-086991. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V15.00004: Rheological Properties of T-Shaped Liquid Crystals Nicholas Diorio, Christopher Bailey, Carsten Tschierske, Antal J\'akli The rheological properties of ``T-shaped'' liquid crystal molecules are investigated. These T-shaped molecules show novel liquid crystal phases with a variety of lamellar and columnar structures [1,2,3]. We examined the viscoelastic behavior of these materials over varying temperatures and shear rates. Because of the limited quantities of these materials, a home- made nanoliter rheometer [4] is used that only requires a few nanoliters of material. [1] M. Prehm, X.H. Cheng, S. Diele, M. K. Das, and C. Tschierske; J. AM. CHEM. SOC. 2002, 124, 12072-12073 [2] X.Cheng, M. K. Das, U. Baumeister, S. Diele, and C. Tschierske; J. AM. CHEM. SOC. 2004, 126, 12930-12940 [3] M. Prehm, F. Liu, U. Baumeister, X. Zeng, G. Ungar, and C. Tschierske; Angew. Chem. Int. Ed. 2007, 46, 7972 –7975 [4] C. Bailey, A. J\'akli, ``Broad range nanoliter rheometer'', Provisional patent , KSU 325 (2008) [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V15.00005: The shape and conformation of the mesogenic group in tetrapodic liquid crystals Hyungguen Yoon, Shinwoong Kang, George H. Mehl, Satyendra Kumar The nematic phases formed by tetrapodic mesogens based on Si- or Ge- core have been investigated by various methods [1]; deuterium NMR, polarized IR spectroscopy, and light scattering. In these studies, biaxiality of the nematic phase has been the central issue. The average statistical shape that such a complex molecule adopts and how the four mesogens attached to Si/Ge atom are oriented in the nematic and lower temperature phases are naturally important questions. However, these have not been sufficiently discussed. We performed synchrotron x-ray diffraction experiments on magnetic aligned Ge-tetrapodes, augmented by conoscopy, capacitance, and electro-optical experiments. The results of these studies and the inferences drawn for the mesogenic group formation will be presented. [1] K. Neupane, S.W. Kang, S. Sharma, D. Carney, T. Meyer, G. H. Mehl, D.W. Allender, S. Kumar, and S. Sprunt, \textit{Phys. Rev. Let.} \textbf{97}, 207802 (2006), and references there in. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V15.00006: Quantum dot dispersion in nematic liquid crystal J. Kirchhoff, R.H. Inman, D.S. Chandhok, S. Ghosh, L.S. Hirst Optical and electrical properties of quantum dots can be~significantly altered~by aligning the dots in a linear chain. ~Dispersing quantum dots in liquid crystals can lead to the formation of linear chains due to the partial ordering of the liquid crystal molecules. ~Typically, this results in a red shift in the emission spectrum of the dots as the induced order leads to enhanced~dipolar interactions, resulting in electronically coupled states.~Dispersions of quantum dots are studied as a function of the concentration, size, and shape of the dots in a nematic rod-like liquid crystal material. ~The quantum dots~are seen to aggregate if the concentration of the dots~is too high, leading to little correlation between the quantum dot dispersion and liquid crystal texture. ~In decreasing the quantum dot concentration the aggregates lessen in size and are more uniformly distributed within the liquid crystal. ~Spherical, rod-like, and disc-like quantum dots with emission peaks ranging from 490 nm to 640 nm were studied using polarized optical microscopy and scanning microscopy photoluminescence measurements. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V15.00007: Dielectric and Optical Properties of Nematic ODBP-Ph-C7 Bohdan Senyuk, Hugh Wonderly, Sergii Shiyanovskii, Oleg Lavrentovich, Victor Pergamenshchik Thermotropic biaxial nematic liquid crystals are promising for application in fast switching electro-optical devices.$^{1}$ In the present work, we study the optical and dielectric properties of the nematic phase of thermotropic LC material 4,4'(1,3,4{\-}oxadiazole{\-}2,5{\-}diyl)~di{\-}$p${\-}heptylbenzoate (ODBP-Ph-C7) with boomerang-like molecules, reported to exhibit the biaxial nematic phase$^{2}$. We study ODBP-Ph C7 in well-aligned ``monocrystalline'' states. The experiments show unusual optical and dielectric properties of the material, such as high dielectric permittivity and dielectric relaxation time that increases with temperature. We also analyze defect structures formed by ODBP-Ph- C7 in different geometries of confinement and boundary conditions. $^{1 }$G. R. Luckhurst, Thin Solid Films \textbf{393}, 40 (2001); $^{2}$ B. R. Acharya, A. Primak, T.J. Dingemans, E.T. Samulski and S. Kumar, Pramana J.Phys. \textbf{61}, 231 (2003). [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V15.00008: Highly Oriented Neurofilament Liquid Crystalline Gels for Imaging and Scattering Studies H.C. Holger, R. Beck, C. Ding, J.B. Jones, J. Deek, N.C. MacDonald, Y. Li, C.R. Safinya The neuronal cytoskeleton is composed of a variety of filamentous proteins including, neurofilaments (NFs), microtubules (MTs) and actin. These components form networks that maintain the cell's structure and shape. At high filament concentration, the proteins self-assemble in-vitro and form liquid crystalline phases maintained by their spatial anisotropy and interfilament interactions. Under physical confinement in microchannel with widths less than few persistence lengths, both MT and actin bundles align parallel to the surface$^{1}$. AFM imaging shows that despite being shorter and more flexible filamentous protein, NF networks maintain larger oriented domains over much longer length scales and unexpectedly align perpendicular to the microchannel walls$^{2}$. We discuss the extended orientation order in NFs in terms of the inter-filaments interactions originating from their polyampholyte side-chains. Supported by DOE DE-FG-02-06ER46314, NSF DMR-0803103, and the Human Frontier Science Program organization. [1] N.F.Bouxsein \textit{et al.}, APL \textbf{85} (2004) 5775; L.S.Hirst \textit{et al.} Langmuir \textbf{21} (2005) 3910 [2] H.C.Hesse, R.Beck, J.Deek \textit{et al.} Langmuir \textbf{24} (2008) 8397 [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V15.00009: Theory and simulation of two-dimensional nematic and tetratic phases Jun Geng, Jonathan V. Selinger Recent experiments and simulations have shown that two-dimensional systems can form tetratic phases with four-fold rotational symmetry, even if they are composed of particles with only two-fold symmetry. To understand this effect, we propose a model for the statistical mechanics of particles with almost four-fold symmetry, which is weakly broken down to two-fold. We introduce a coefficient $\kappa$ to characterize the symmetry breaking, and find that the tetratic phase can still exist even up to a substantial value of $\kappa$. Through a Landau expansion of the free energy, we calculate the mean-field phase diagram, which is similar to the result of a previous hard-particle excluded-volume model. To verify our mean-field calculation, we develop a Monte Carlo simulation of spins on a triangular lattice. The results of the simulation agree very well with the Landau theory. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V15.00010: Quadrupolar particles in a nematic liquid crystal: Effects of particle size and shape Francisco Hung We investigate the effects of particle size and shape on the quadrupolar (Saturn ring-like) defect structures formed by a nematic liquid crystal (NLC) around nm- and micron-sized particles with spherical, spherocylindrical and cubic shapes. Our calculations, based on a Landau-de Gennes expansion in terms of the tensor order parameter Q, indicate that for pairs of nanoparticles in close proximity, the most stable defect structure is the ``entangled hyperbolic'' [1]. For pairs of micron-sized particles the NLC forms entangled 'figure of eight' defects [1] around pairs of spheres and spherocylinders. In contrast, we only observed unentangled defect structures around pairs of micron-sized cubic particles. For pairs of spherical and spherocylindrical particles, the transition between ``entangled hyperbolic'' and ``figure of eight'' structures occurs when the particle diameter is between 100 nm and 1 micron. Our calculations suggest that the NLC-mediated interactions between the nanoparticles are fairly strong (up to 5700 kT). These interactions can bind the particles together at specific locations, and thus could be used to assemble the particles into ordered structures with different morphologies. [1] M. Ravnik et al., Phys. Rev. Lett. 99, 247801 (2007) [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V15.00011: A Model Liquid Crystalline System Based on Rodlike Viruses with Tunable Chirality Daniel Beller, Edward Barry, Zvonimir Dogic Filamentous bacteriophages such as the \textit{fd} virus have long been used as ideal model systems to investigate the phase behavior of suspensions of rodlike particles. We study the structure and phase behavior of a mutant, \textit{fd} Y21M, and compare them to the properties of conventional \textit{fd} wild-type (\textit{wt}). These two viruses exhibit dramatically different phase behavior despite differing only by a single amino acid of the major coat protein pVIII. We find that this is attributable to significant differences in the flexibility of the viruses. Using the more rigid \textit{fd} Y21M, we are able for the first time to quantitatively test the Onsager description of the isotropic-nematic phase transition of rigid rods. Even more surprising are the differences in the behavior of the cholesteric phase of \textit{fd }Y21M and \textit{fd wt}. While \textit{fd} \textit{wt} forms a cholesteric pitch with a left-handed helix, \textit{fd} Y21M forms a cholesteric pitch with the opposite handedness. In addition, the magnitude of the cholesteric pitch changes by almost fivefold. Using mixtures of the two viruses, we are able to create liquid crystal systems with tunable control over the macroscopic chiral behavior. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V15.00012: Apparent Broken Reciprocity in Chiral Liquid Crystals Michele Moreira, Nithya Venkataraman, Peter Palffy-Muhoray, Lorenzo Marrucci Reciprocity in optics is predicated on bounded scattering media with symmetric and linear permittivity, conductivity and permeability. Due to their anisotropy and chirality, cholesteric liquid crystals (CLCs) form periodic dielectric structures. If the periodicity is comparable to the wavelength of light, these phases are self-assembled photonic band gap structures. There appear in the permittivity odd powers of the wave vector resulting from nonlocality and broken inversion symmetry. Evidence of non-reciprocity has been found in optically active crystals by Bennett [1] and in stacks of cholesteric and nematic liquid crystal cells by Takezoe [2]. We investigate experimentally and theoretically the possibility of a violation of optical reciprocity of a hetero-photonic-bandgap structure made of two CLC cells of diferent pitch. We have observed a significant apparent violation, but we show that the effect is due to light scattering, and in fact these choleseric structures are reciprocal. \\[3pt] [1] P.J. Bennett et al .Opt. Lett. 21, 1955 (1996) \\[0pt] [2] J. Hwang; et al. Nat. Mat. 4, 383 (2005). [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V15.00013: Raman scattering study of orientation order parameters in thermotropic biaxial nematic LC Min Sang Park, Bum Jin Yoon, Jung Ok Park, Veena Prasad, Mohan Srinivasarao There has been considerable interest in the liquid crystalline phases formed by bent-core molecules, since these molecules were considered to have a biaxial nematic phase. In an effort to understand the orientation behavior and the degree of phase biaxiality in bent-core mesogen, Raman spectroscopy has been used to measure the scattering intensities for orthogonal linear polarization. This straightforward methodology allows the values of both uniaxial, $<$P$_{200}>$ and $<$P$_{400}>$, and biaxial order parameters, $<$P$_{220}>$, $<$P$_{420}>$, and $<$P$_{440}>$ to be quantified. From experimentally derived order parameters, the most probable orientation distribution functions are constructed. The results of these measurements will be presented in the context of experimental evidence of phase biaxiality obtained by other methods including X-ray diffraction and NMR [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V15.00014: Liquid crystalline behaviors of H-bonded dimer formed from the semi-phasmidic type carboxylic acid* Seung-Yeon Jeong, Shin-Woong Kang, Satyendra Kumar, Veena Prasad, Sanjay Varshney Liquid crystalline properties of acid-functionalized semi-phasmidic azo-compound were characterized by DSC, polarizing optical microscopy, and electro-optical measurements. The results suggested that this unconventional mesogenic dimer has a non-typical effective ``bend'' angle where two monomers form the hydrogen bond. To confirm this we performed electro-optical experiments in the nematic phase with strong external electric and magnetic fields. Cells with different surface geometries and treatments we used. The results reveal a behavior that is very distinct from that expected of conventional linear mesogenic dimers formed by a hydrogen bonding. The results confirm negative values of dielectric and diamagnetic anisotropies. Our observations indicate that ``bent-core-like'' dimeric mesogens are formed through an unconventional inter-molecular association. *Work supported by grant NSF/DMR-086991. [Preview Abstract] |
Session V16: Solid Helium: Experiment
Sponsoring Units: DCMPChair: Haruo Kojima, Rutgers University
Room: 317
Thursday, March 19, 2009 8:00AM - 8:12AM |
V16.00001: Bose-Einstein condensation in solid helium Richard Azuah, Souleymane Diallo, Oleg Kirichek, Jon Taylor, Henry Glyde We report new measurements of the Bose-Einstein condensate fraction in solid helium. The goal is to reveal whether there is BEC associated with the superfluid fractions that have been observed in solid helium [1,2]. The condensate fraction, n$_0$, is obtained from neutron scattering measurements of the momentum distribution, n(k), of the atoms in the solid. We use commercial grade helium ($^3$He concentration of 0.3 $\%$) where the T$_c$ for superflow is T$_c$ = 200 mK and have measured the n(k) at 3 temperatures, 500 mK, 150 mK and 65 mK. We use a sample cell that has a large surface to volume ratio (S/V) = 40 cm$^{-1}$ where large superfluid fractions have recently been reported[2]. We use a large sample volume (100 cm$^3$) and high instrument resolution to improve precision beyond that of previous measurements [3]. No clear sign of BEC has been observed but the data is being analyzed so that specific values of n$_0$ can be reported. [1] E. Kim and M.H.W. Chan. Science, 305:1941 (2004); Nature, 427:225, 2004. [2]A. S. C. Rittner, and J. D. Reppy, Phys. Rev. Lett., 98:175302, 2007. [3] Diallo et al. Phys. Rev Lett. 98, 205301 (2007). [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V16.00002: Pressure-driven mass flow in solid 4He Ann Sophie C. Rittner, Wonsuk Choi, John D. Reppy We report on two experiments that explore pressure-driven flow in solid 4He. In the first experiment, two pancake-shaped chambers are connected by a narrow slit. The pressure in one chamber is varied periodically at 2 mHz and we look for a superfluid pressure response in the second chamber. When the cell is filled with liquid, we observe a large pressure signal in the second chamber as expected for superfluid flow. In solid helium, no pressure response is detected outside the noise level. In a second experiment, we generate an oscillating pressure in a torsional oscillator by blocking an annulus. We expect the pressure difference to drive the supersolid component through a radial channel orthogonal to the rotational motion of the oscillator. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V16.00003: Observation of Mass Flux through solid $^4$He Robert Hallock, Michael Ray We have developed a novel apparatus and technique that allows us to maintain an interface between superfluid helium and hcp solid $^4$He at pressures greater than $\approx$ 25 bar, the low temperature solid-liquid coexistence pressure. We use this apparatus to inject helium into one side of the solid, creating a chemical potential difference across the solid, and we then look for a response in the pressure on the other side. We observe a flux of atoms through the solid[1] which tends to decrease with increasing solid pressure. There is also a complicated temperature dependence, which suggests hysteretic behavior. We will describe the experimental apparatus, and some of our results. \\ \\ $[1]$ M.W. Ray and R.B. Hallock Phys. Rev. Lett. \textbf{100} 235301 (2008) [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V16.00004: Growth of hcp Solid $^4$He from the Superfluid Michael Ray, Robert Hallock Using the same experimental apparatus that we developed to search for mass flux in hcp solid $^4$He at pressures greater than 25 bar[1], we study the growth of solid helium from the superfluid at constant temperature. As the pressure of the solid is driven above the melting curve, with helium continuously being added to the sample cell, we observe apparently random events during which the pressure of the solid drops. These pressure drops are accompanied by a sharp transient rise in the temperature of the cell. We will present the data, along with some discussion of what might cause these transients. \\ \\ $[1]$ M.W. Ray and R.B. Hallock Phys. Rev. Lett. \textbf{100} 235301 (2008) [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V16.00005: Velocity dependence of supersolid He-4 in a torsion oscillator. Ethan Pratt, Benjamin Hunt, Vikram Gadagkar, Minoru Yamashita, J.C. Seamus Davis We have developed a free-inertial-decay mapping technique which allows us to acquire the complete velocity-temperature “phase diagram” of supersolid He-4. A striking new feature of the resulting supersolid response map is the appearance of an enhanced dissipation superpeak. We discuss these results in context of various microscopic mechanisms for the velocity-induced suppression of the supersolid response, including a superfluid critical velocity, defect network critical shear, and a glassy dynamical susceptibility. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V16.00006: Torsional oscillator measurements on solid helium in a partially blocked annular channel Duk Young Kim, Sang-Il Kwon, Hyoungsoon Choi, Eunseong Kim We employed a torsional oscillator technique to investigate the non-classical response of solid helium. We grow solid helium samples within an annular channel that is divided by two Vycor blocks. The melting pressure of helium in narrow pores of Vycor glass is elevated up to about 37 bar due to the strong confinement. This effect is used to engineer various combinations of adjacent helium states. When a helium sample is prepared in the pressure range of 25 and 37 bar a solid-liquid-solid sandwich is formed in and outside the Vycor. Non-classical rotational inertia fraction (NCRIF) of 0.02{\%} is observed under this circumstance below about 200mK. For the pressure higher than 37 bar Vycor glasses in the channel can be utilized as partial blocks and the strength of blockage can be modified by gas adsorption on to the pore wall. Supersolid behavior with a number of different partial blockages will be presented. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V16.00007: Simultaneous Oscillation of Annular Solid $^4$He Samples at Two Mode Frequencies in Compound Torsion Pendulum Michael C. Keiderling, Harry Kojima We have extended our studies on the non-classical behavior of solid $^4$He contained in compound torsional oscillator (TO) cell below 1 K. Our unique TO design allows observations on the identical sample at two distinct frequencies(f$_1$=493 and f$_2$=1165 Hz). The sample was grown by blocked capillary method in an annular cell(id = 8.0 mm, od = 10.0 mm, height = 9.0 mm). We focus here on experiments in which the two modes are excited simultaneously. While keeping the drive of f$_2$ mode at a very low level, the drive of f$_1$ mode was varied from high to low levels to produce substantial variations in the non-classical rotation inertia fraction (NCRIf). When the NCRIf seen by f$_1$ mode is reduced by 89, 91 and 94 \% at 9.7, 23.5 and 56.5 mK, respectively, the NCRIf seen by f$_2$ mode (driven at low level) is reduced by 62, 68 and 80 \%. The discrepancies and their temperature dependence in the observed reductions in NCRIf are not yet understood. Similar Measurements with the roles of the drive levels of the modes reversed as well as the changes in the dissipation of the torsional oscillator during the simultaneous drive will be reported. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V16.00008: Effects of $^3$He Impurities on the Non-Classical Rotation Inertia of Solid $^4$He as Measured by Compound Torsion Pendulum David Ruffner, Michael Keiderling, Patryk Gumann, Harry Kojima An intriguing observation related to the discovery of non-classical rotational inertia(NCRI) of solid $^4$He at low temperatures is the extreme sensitivity to $^3$He concentration(x$_3$). Both the magnitude and temperature dependence of the NCRI are affected by relatively low x3 introduced into solid $^4$He samples.[1] We are exploring the $^3$He impurity effect using our compound torsional pendulum which allows probing the NCRI of the identical solid $^4$He sample at two different frequencies ($\sim$493 and $\sim$1165 Hz). The NCRI fractions were derived from the measured shifts in the oscillator frequency of the two modes as functions of temperature. The NCRI fraction derived from the higher frequency mode is greater than that derived from the lower frequency mode at all temperatures. If the NCRI fractions of both modes are normalized at their maximums, the temperatures at which they decrease to 50 \% of the maximum are greater in the higher mode by $\sim$9, 31 and 56 mK when the nominal x$_3$ added is 0.3, 3 and 10 ppm, respectively. Greater values of x$_3$ are currently being studied.[1]Kim, et al., Phys. Rev. Lett. \textbf{100} 065301(2008). [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V16.00009: Sample quality dependence of the specific heat peak in solid $^{4}$He Xi Lin, Z. G. Cheng, M. H. W. Chan We reported a broad peak in specific heat of solid $^{4}$He[1] at a temperature near the onset of Non-Classical Rotational Inertia (NCRI). This peak is likely to be a thermodynamic signature of the supersolid phase. Since it was found that the supersolid fraction is dependent upon the sample quality[2,3], we are motivated to study the sample quality influence on solid $^{4}$He specific heat. In our recent measurements, we have confirmed the presence of a broad peak in the specific heat of solid $^{4}$He below $\sim $200mK in addition to the phonon contribution. We also show how the excess specific heat peak changes with sample quality. 1 X. Lin, A. C. Clark, and M. H. W. Chan, Nature 449, 1025 (2007). 2 A. S. C. Rittner and J. D. Reppy, Phys. Rev. Lett. 98, 175302 (2007). 3 A. C. Clark, J. T. West and M. H. W. Chan, Phys. Rev. Lett. 99, 135302 (2007). [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V16.00010: Torsional oscillator and specific heat studies of 4He in Vycor Z.G. Cheng, X. Lin, J.T. West, M.H.W. Chan Non-classical rotational inertia (NCRI) was first reported in solid helium confined in Vycor using the torsional oscillator technique [1]. Most of the work since then has focused on bulk solid helium. Recent specific heat measurements of bulk solid helium show a peak centering near the NCRI onset temperature [2], a good indication that the two may be related. We report on a series of experiments to study the NCRI fraction and specific heat of solid helium in Vycor. The torsional oscillator experiment is revisited to study the stability of the NCRI fraction. The purpose of the specific heat measurement is to probe the relation between the specific heat peak and NCRI. \\[3pt] [1] E. Kim, M. H. W. Chan, Nature 427, 225 (2004). \\[0pt] [2] X. Lin, A. C. Clark, M. H. W. Chan, Nature 449, 1025 (2007). [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V16.00011: Second sound in supersolid $^{4}$He Norbert Mulders, Sangil Kwon, Eungseong Kim In a system that consists of two interpenetrating continuous phases that are free to move with respect to each other, one would expect to find two longitudinal sound modes. A good example is fourth sound in superfluid helium in a porous matrix, but the phenomena can also be observed with water in rocks. If one interprets the observation of a non-zero NCRIF in solid helium below $\sim $100 mK as due to the independent motion of a supersolid fraction with respect to the crystal, it follows that two longitudinal sound modes should exist, with the slow mode disappearing at the supersolid transition. We will report on our efforts to find this slow mode. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V16.00012: Supersolidity of Solid $^4$He Confined in 25 Angstrom Nanopores Keiya Shirahama, Hitomi Yoshimura, Yoshiyuki Shibayama There has been growing consensus that dislocations play an important role on the supersolid behaviors observed in torsional oscillator experiments for solid $^4$He. When solid He is confined in nanopores, dislocations may not exist, or even if they exist their motion is suppressed, and supersolidity should therefore be greatly modified. Study of solid 4He in confined geometries may therefore give an important clue to elucidate the mechanism of supersolidity. Earlier studies of solid 4He in Vycor by Kim and Chan [1] found no difference in the supersolid behaviors from bulk solid. We have performed torsional oscillator experiments for solid $4$He in a porous Gelsil glass that has nanopores of 25 angstroms in diameter. We have observed an increase in oscillator frequency indicating supersolidity below 200 mK. This result suggests that the existence of dislocations is not a necessary condition for supersolidity. [1] E. Kim, M. H. W. Chan, Nature 427, (2004) 225. [Preview Abstract] |
Session V17: Superconducting Transmons and Circuit QED
Sponsoring Units: GQIChair: Ben Palmer, Laboratory for Physical Sciences, University of Maryland
Room: 318
Thursday, March 19, 2009 8:00AM - 8:12AM |
V17.00001: Experimental study of transmon type qubits coupled to a fast tunable transmission line resonator. Martin Sandberg, Chris Wilson, Fredrik Persson, Io-Chun Hoi, Per Delsing We experimental study a high quality factor (Q-value) transmission line resonator terminated in a superconducting quantum interference device (SQUID). Using an on-chip fast flux bias line we show that we can tune the frequency of such a resonator by hundreds of line widths on a time scale faster the photon lifetime of the resonator. Such a resonator could then be used for dynamic coupling of superconducting quantum bits (qubits). We present preliminary data of two transmon type of qubits coupled to a fast tunable resonator. We show spectroscopy and Rabi oscillations of the qubits far detuned from the resonator. Data showing a relaxation time of 1$\mu $s and a Rabi time of 200 ns have so far been obtained. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V17.00002: Superconducting qubits can be coupled and addressed as trapped ions Y.X. Liu, L.F. Wei, J.R. Johansson, J.S. Tsai, F. Nori Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting qubits (SCQs) which is very similar to the successful one now being used for trapped ions. The SCQs are coupled to the ``vibrational'' mode provided by a superconducting LC circuit or its equivalent (e.g., a superconducting quantum interference device). Both single-qubit rotations and qubit-LC-circuit couplings and/or decouplings can be controlled by the frequencies of the time-dependent magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the LC circuit, can be selectively performed, and the information transfer can be realized in a controllable way. \\[4pt] Y.X. Liu, L.F. Wei, J.R. Johansson, J.S. Tsai, F. Nori, Superconducting qubits can be coupled and addressed as trapped ions, Phys. Rev. B 76, 144518 (2007). URL: http://link.aps.org/abstract/PRB/v76/e144518 [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V17.00003: Second order dispersive regime of circuit QED with a transmon qubit Maxime Boissonneault, J. M. Gambetta, Alexandre Blais In many recent circuit QED experiments [1,2], a transmon-type [3] qubit is fabricated inside a high-Q transmission line resonator. Compared to the Cooper-pair box (CPB), the transmon has both a stronger coupling to the resonator and a significantly longer dephasing time [4]. By going to the dispersive regime, where the qubit-resonator detuning is much larger than the coupling strength, the qubit can be controlled and measured through the resonator. In previous work [5], we have shown that one must include non-linear corrections to the dispersive approximation in strong measurements of a CPB qubit. These corrections cause a saturation of the signal-to-noise ratio and photon-dependant qubit decay and dephasing rates. In this talk, we will show how these non-linear corrections come into play with the transmon, and how they could be used to improve the measurement. [1] Houck et al, Nature, 2007, 449, 328 [2] Majer et al, Nature, 2007, 449, 443 [3] Koch et al, PRA, 2007, 76, 042319, [4] Schreier et al, PRB 77, 180502 (2008), [5] Boissonneault et al, PRA, 2008, 77, 060305(R). [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V17.00004: Two Cavity Circuit QED Blake Johnson, Andrew Houck, Jay Gambetta, Alexandre Blais, Steven Girvin, Michel Devoret, Robert Schoelkopf The circuit QED architecture has proven useful for dispersive manipulation and measurement of superconducting qubits. Previous experiments have shown how to use the AC-Stark shift to spectroscopically extract information about the photon number in the cavity[1]. Here we will show how to extend this toward building a photon statistics analyzer by adding a second cavity to the circuit QED architecture. The second cavity allows for decoupling of the preparation and readout of the cavity field state, opening the way for a measurement of the full photon statistics and reconstruction of the Wigner distribution. \newline \newline [1] Resolving photon number states in a superconducting circuit. D. I. Schuster, et. al. Nature (London) 445 515 (2007). [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V17.00005: Multi-Transmon circuit QED using local and fast flux biasing Leonardo DiCarlo, Jerry Chow, Johannes Majer, Luigi Frunzio, Jay Gambetta, Alexandre Blais, Steven Girvin, Robert Schoelkopf We report local and fast flux tuning of Transmon qubits in circuit QED by means of proximal short-circuited coplanar waveguides. We characterize the effect of these additional microwave channels on qubit lifetime. We demonstrate one-qubit $Z$-gates and time-domain control of two-qubit interaction via virtual photon exchange. Gate performance is characterized by process tomography and compared to gating by AC Stark shift as previously investigated by the Yale cQED team [1]. Research supported by NSF, NSA and ARO. [1] Majer \textit{et al.,} \textit{Nature} \textbf{449}, 443 (2007). [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V17.00006: Inline Cavity Qubit with Bifuraction Readout Markus Brink, Nicholas A. Masluk, Kurtis L. Geerlings, Michael Metcalfe, Vladimir Manucharyan, Luigi Frunzio, Steven M. Girvin, Robert J. Schoelkopf, Michel H. Devoret We present the design and data from a new, strongly coupled superconducting qubit based on Josephson junctions and a strictly 1-dimensional distributed element geometry that operates in the Transmon regime. A cavity bifurcation amplifier is used to read the state of the qubit. The same circuit also supports a linear dispersive readout, which enables direct comparison between the latching and dispersive scheme. Most recent results will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V17.00007: Generation of entangled states in circuit QED using sideband transitions P.J. Leek, S. Filipp, P. Maurer, A. Blais, A. Wallraff The deterministic generation of entanglement between pairs of distant qubits is an important goal in the development of a quantum information processor. A promising and potentially scalable method of achieving this is through the use of sideband transitions between qubits and a globally coupled harmonic oscillator. Here we present an experimental demonstration of such a scheme implemented in an electrical circuit. We generate entanglement between a superconducting transmon qubit and a microwave coplanar waveguide resonator, and transfer this entanglement to a second qubit. We show generation of the four Bell states by this method, extracting the full two-qubit density matrix using a joint dispersive two-qubit readout and quantum state tomography. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V17.00008: Generating entanglement by measurement in circuit QED Jay Gambetta, Chantal Hutchison, Alexandre Blais, Frank Wilhelm In this talk, I will show theoretically how to induce entanglement by measurement in circuit QED. I will use quantum trajectory theory to derive an equation for the conditional state of a two qubit system, conditioned on continuous-in-time measurement of the amplitude and phase of the field leaving the resonator. I will show that with experimental parameters, we can use a decoherence-free subspace to generate an entangled state with a high concurrence and with a success probability of 1/2. Finally I will show that with a simple feedback scheme the same concurrence can be achieved with a success probability of one. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V17.00009: Towards proving non-classicality with a 3-qubit GHZ state in circuit QED Lev S. Bishop, Jay M. Gambetta, Eran Ginossar, Steven M. Girvin, Andrew A. Houck, Jens Koch, Andreas Nunnenkamp, David J. Price, Robert J. Schoelkopf, Lars Tornberg The demonstration of violation of Bell-type inequalities remains challenging for superconducting qubits, due to short coherence times and limited measurement fidelity. Here, we propose to utilize 3-qubit GHZ states in a circuit QED system to accomplish this key step. In contrast to other schemes where the qubits are measured individually, circuit QED offers the advantage that a single dispersive measurement can directly reveal the parity $\langle\sigma^1_z\otimes\sigma^2_z\otimes\sigma^3_z\rangle$. When combined with appropriate 1-qubit rotations, this provides the necessary ingredients to obtain an effective measurement of Mermin's Bell operator with less stringent requirements on the measurement fidelity. Generation of the GHZ state can proceed via either 1- and 2-qubit gates or preparation by measurement. We present results from quantum trajectory calculations and estimate the resulting violation of the Mermin inequality, based on experimentally feasible parameters. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V17.00010: Exploration of the Tavis-Cummings Model with Multiple Qubits in Circuit QED J.M. Fink, A. Blais, A. Wallraff Superconducting qubits in coplanar waveguide resonators provide an unprecedentedly large dipole coupling strength to microwave frequency photons confined in an on-chip waveguide resonator [1]. In contrast to atoms in traditional cavity QED a controlled number of qubits remain at fixed positions with constant coupling to the cavity field at all times. Utilizing these properties we have performed measurements with up to three independently flux-tunable qubits to study cavity mediated multi-qubit interactions. By tuning the qubits in resonance with the cavity field individually, we demonstrate the square root of $N$ scaling of the collective dipole coupling strength with the number of resonant atoms $N$ as described by the Tavis-Cummings model. To our knowledge this is the first observation of this nonlinearity in a system in which the atom number can be changed one by one in a discrete fashion. In addition, the energies of both bright and dark coupled multi-qubit / photon states are well explained by the Tavis-Cummings model over a wide range of detunings. On resonance we obtain an equal superposition of a photon and a Dicke state with an excitation equally shared among the $N$ qubits.\\ $[1]$ J.~M.~Fink et al.~\textit{Nature} 454, 315 (2008). [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V17.00011: Two-photon probe of the Jaynes-Cummings model and controlled symmetry breaking in circuit QED Frank Deppe, Matteo Mariantoni, E. P. Menzel, A. Marx, R. Gross, S. Saito, K. Kakuyanagi, H. Tanaka, K. Semba, T. Meno, H. Takayanagi, E. Solano Superconducting qubits behave as artificial two-level atoms. Coupling them to on-chip microwave resonators has given rise to the field of circuit quantum electrodynamics (QED). In this work, we report on the observation of key signatures of a two-photon driven Jaynes-Cummings model, which unveils the upconversion dynamics of a superconducting flux qubit coupled to an on-chip resonator. Our experiment and theoretical analysis show clear evidence for the coexistence of one- and two-photon driven level anticrossings of the qubit-resonator system. This results from the controlled symmetry breaking of the system Hamiltonian, causing parity to become a not well-defined property. Our study provides deep insight into the interplay of multiphoton processes and symmetries in a qubit-resonator system. We acknowledge support from SFB631, NIM, CREST-JST, JSPS-KAKENHI (18201018), MEXT-KAKENHI (18001002), EuroSQUIP, and the Ikerbasque Foundation. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V17.00012: Dynamics of dispersive single qubit read-out in circuit QED R. Bianchetti, S. Filipp, A. Blais, A. Wallraff In a circuit quantum electrodynamics setup the qubit state is inferred from the response of the coupled qubit-cavity system to a microwave signal applied close to the cavity resonance. We experimentally investigate the frequency dependence of the response for both weak continuous and pulsed measurement signals. We find excellent agreement with theoretical predictions from a generalized Jaynes-Cummings model which includes dissipation and dephasing. The quantitative understanding of the system response is used to optimize the measurement frequency maximizing the signal-to-noise ratio. This allows for an accurate determination of the qubit excited state population from the measured field response. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V17.00013: Two-qubit tomography with joint dispersive read-out in circuit QED S. Filipp, P. Maurer, P. J. Leek, A. Blais, A. Wallraff Quantum state tomography is an important tool in quantum information science for complete characterization of multi-qubit states and their correlations. We demonstrate that for two superconducting qubits coupled to a common resonator mode a reconstruction of the full density matrix can be achieved by measuring the transmission through the resonator -- without the need for single-shot measurement of individual qubits. Since the resonator transmission depends non-linearly on the dispersive cavity pull of different qubit states, information about qubit-qubit correlations is intrinsically present in the averaged measurement signal. It is shown that this simultaneous two-qubit read-out can be used for quantum state tomography of both separable and entangled states. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V17.00014: Measurement of Autler-Townes and Mollow transitions in a strongly driven superconducting qubit M. Baur, S. Filipp, A. Blais, A. Wallraff The spectrum of a multilevel atom can be significantly modified when interacting with electromagnetic fields. In the simplest case where a two-level atom is driven on resonance, two sidebands offset from the main atomic line by the Rabi frequency $\Omega$ appear in the fluorescence spectrum, referred to as the Mollow triplet. Similarly, when probing transitions into a third atomic level, the absorption spectrum shows two spectral lines separated by $\Omega$, called the Autler-Townes doublet. Here we present a measurement of the Autler-Townes doublet and the sidebands of the Mollow triplet in a strongly driven superconducting qubit. The corresponding transitions are detected using dispersive read-out of the qubit coupled off-resonantly to a microwave transmission line resonator. The observed frequencies of the Autler-Townes and Mollow spectral lines are in excellent agreement with a generalized dispersive Jaynes-Cummings model. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V17.00015: Microwave Photon Detector in Circuit QED Juan Jose Garcia-Ripoll, Guillermo Romero, Enrique Solano In this work we propose a design for a microwave photodetector based on elements from circuit QED such as the ones used in qubit designs. Our proposal consists on a microwave guide in which we embed circuital elements that can absorb photons and irreversibly change state. These incoherent absorption processes constitute the measurement itself. We first model this design using a general master equation for the propagating photons and the absorbing elements. We find that the detection efficiency for a single absorber is limited to 50\%, and that this efficiency can be quickly increased by adding more elements with a moderate separation, obtaining 80\% and 90\% for two and three absorbers. Our abstract design has at least one possible implementation in which the absorbers are current biased Josephson junction. We demonstrate that the coupling between the guide and the junctions is strong enough, irrespectively of the microwave guide size, and derivate realistic parameters for high fidelity operation with current experiments. Patent pending No. 200802933, Oficina Espanola de Patentes y Marcas, 17/10/2008. [Preview Abstract] |
Session V18: Focus Session: Physics of Green Polymers and of Biocompatibility
Sponsoring Units: DPOLY DBPChair: John Torkelson, Northwestern University
Room: 319
Thursday, March 19, 2009 8:00AM - 8:36AM |
V18.00001: Spectroscopic Analyses of Microstructures Associated with Plant Based Polymers Invited Speaker: Currently, less than 0.02\% of polymers used are plant based with the rest originating from petroleum feedstock. There are a number of applications that require some of these new polymers, independent of the cost of the petroleum. Because of their size scale, it is most appropriate to use vibrational and NMR spectroscopy to characterize the microstructure of these plant based polymers. We present a number of examples in order to illustrate the use of these alternative polymers. Soybean is one of the most promising alternatives. Both its saturated and unsaturated components can be utilized. In various applications, the saturated component is important because the rapid crystallization directly controls the rheological behaviors. This is especially significant if cocrystallization with other polymers, especially statistically random copolymers, is an important consideration. Crystallization kinetics and subsequent morphological units formed have yet to be characterized. In addition, the unsaturated component can be modified to form various polyols for use in reactive mixtures. The miscibility behavior of such polymers with other oligomers or polymers strongly influences the reaction kinetics and the products formed. The extreme hydrophobic nature of soybean based polymers is reflected in that it has opposite physical properties to that of the hydrophilic polyols used in the current formulations. We also have significant interest in poly(lactic acid). We have characterized the inherent structural rigidity, correlating the changes in chain conformation to the chain conformation. We have identified the intermolecular forces which stabilized the crystalline units. In addition, we have been able to control the crystallization process resulting from addition configurational defects. These applications illustrate the opportunities we have available in a world which may embrace such a set of polymers. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V18.00002: Toughening and reinforcing degradable polymers to extend their properties and applications SuPing Lyu, Jianbin Zhang, Adam Buckalew, Jim Schley, Bryant Pudil, Lian Luo, Chris Hobot, Mike Benz, Randy Sparer, Julie Trudel Polymer materials made from renewable feedstocks mainly are cellulose derivatives and aliphatic polyesters such as polylactide. There are two challenges in the use of these materials to replace petroleum based polymers. One is how to easily process these materials to make them into needed shapes and other is how to broaden the properties of these materials so that they can be used for applications where petroleum based polymers play major roles. Most of the renewable source based materials are brittle. This abstract presents a method of how to toughen and reinforce polylactide to make a family of polymers that cover broad ranges of toughness and strength for various applications such as biomedical device manufacturing. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V18.00003: A QCM-D Study of the Enzymatic Degradation of Cellulose Thin Films Dan Glickman, Oleh Tanchak, Michael Reid, Amanda Quirk, Darrell Cockburn, Colin Macdougall, Anthony Clarke, Jacek Lipkowski, John Dutcher A sophisticated surface-sensitive technique, the quartz crystal microbalance with dissipation monitoring (QCM-D), was used to study the interaction of a mixture of cellulolytic enzymes from the fungus T. reesei with cellulose thin films deposited onto polycrystalline gold surfaces. It was found that the QCM experiment was sensitive to two processes that occur during the enzyme mixture-cellulose thin film experiment: adsorption of the enzyme to the film surface, and the subsequent degradation of the cellulose thin film. A model describing the measured frequency shift in the QCM data will be described, which gives excellent fits to the experimental data. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V18.00004: An Atomic Force Microscopy Study of the Mechanism of Cellulose Biodegradation Amanda Quirk, Maohui Chen, Darrell Cockburn, Sarah Regli, Anthony Clarke, John Dutcher, Jacek Lipkowski, Sharon Roscoe Cellulose, a biopolymer consisting of long chain $\beta $-(1$\to $4) linked glucose sugars, is used as structural material by plants and bacteria. Degradation of cellulose to glucose, a sugar easily fermented to ethanol, occurs by the enzymatic hydrolysis of cellulose by cellulase enzymes. The enzymes have a~complex structure including carbohydrate binding modules and catalytic domains responsible for the binding and degradation of cellulose, respectively. Atomic force microscopy (AFM) was used to study native cellulose films prepared from Acetobacter xylinum using a novel application of the Langmuir-Blodgett technique. These films allowed AFM images of single fibers and their microfibril structure to be obtained. Further \textit{in situ} AFM studies of single fibers were performed in solution using cellulolytic enzymes. The \textit{in situ} degradation of cellulose fibers was monitored over 20-hours using AFM. These studies provided insight into the degradation timeline of a single fiber. Complementary studies of proteins adsorbed on cellulose fibers revealed information about the binding of the enzymes to the substrate. Studying the modular enzyme action separately will provide insight into the mechanism of cellulose binding and contribute to our understanding of the degradation process. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V18.00005: Observation of Biodegradation of Cellulose Fibers Using Surface Plasmon Resonance Imaging Oleh M. Tanchak, Scott Allen, Darrell Cockburn , Anthony J. Clarke, Jacek Lipkowski, John R. Dutcher Cellulose is the most abundant biopolymer on Earth and can provide a renewable supply of ethanol fuel to replace fossil fuels. A fundamental understanding of the mechanisms of the biodegradation of cellulose is essential to the development novel enzyme systems that can efficiently and selectivity degrade a variety of biomass substrates. A novel Surface Plasmon Resonance Imaging (SPRI) instrument was used to study the biodegradation of cellulose fibers anchored to a thiolated gold surface. The kinetics of binding of the inactive enzymes to cellulose fibers and their digestion by catalytically-active homologs will be presented. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V18.00006: Sustainable Engineering and Improved Recycling of PET for High-Value Applications: Transforming Linear PET to Lightly Branched PET with a Novel, Scalable Process Cynthia Pierre, John Torkelson A major challenge for the most effective recycling of poly(ethylene terephthalate) concerns the fact that initial melt processing of PET into a product leads to substantial degradation of molecular weight. Thus, recycled PET has insufficient melt viscosity for reuse in high-value applications such as melt-blowing of PET bottles. Academic and industrial research has tried to remedy this situation by synthesis and use of ``chain extenders'' that can lead to branched PET (with higher melt viscosity than the linear recycled PET) via condensation reactions with functional groups on the PET. Here we show that simple processing of PET via solid-state shear pulverization (SSSP) leads to enhanced PET melt viscosity without need for chemical additives. We hypothesize that this branching results from low levels of chain scission accompanying SSSP, leading to formation of polymeric radicals that participate in chain transfer and combination reactions with other PET chains and thereby to in situ branch formation. The pulverized PET exhibits vastly enhanced crystallization kinetics, eliminating the need to employ cold crystallization to achieve maximum PET crystallinity. Results of SSSP processing of PET will be compared to results obtained with poly(butylene terephthalate). [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V18.00007: Spontaneously Formed Biocompatible Surfaces in Water by Segregation of Amphiphilic Block Copolymers Hideaki Yokoyama, Takashi Ishizone, Naoya Torikai, Jaroslaw Majewski, Ayako Oyane Reduction of hydrophobic interaction in water is important in biological interfaces. We have found that poly(styrene-\textit{b}-oligo ethylene glycol methyl ether methacrylate) (PS-PMENMA) segregates the PMENMA block to the surface in hydrophobic environment such as in air or in a vacuum, and shows remarkable resistance against adsorption or adhesion of proteins, platelets and cells in water. We studied the interfacial structures between PS modified by the spontaneous segregation of PS-PMENMA and water using neutron reflectivity and adhesion force measurement using atomic force microscope with hydrophobic probes. The interfacial structure and hydrophobic interaction depend on the number of ethylene glycol (EO) units in PMENMA. PMENMAs with two or more EO units show distinct swollen layers with two sharp interfaces at polymer/water interfaces, which effectively reduce hydrophobic interaction in water, while PMENMA with one unit of EO diplays broader single interface with unsatisfactory reduction. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:24AM |
V18.00008: Electrospun Nanofibers for Neural and Tissue Engineering Invited Speaker: Electrospinning has been exploited for almost one century to process polymers and other materials into nanofibers with controllable compositions, diameters, porosities, and porous structures for a variety of applications. Owing to its small size, high porosity, and large surface area, a nonwoven mat of electrospun nanofibers can serve as an ideal scaffold to mimic the extra cellular matrix for cell attachment and nutrient transportation. The nanofiber itself can also be functionalized through encapsulation or attachment of bioactive species such as extracellular matrix proteins, enzymes, and growth factors. In addition, the nanofibers can be further assembled into a variety of arrays or architectures by manipulating their alignment, stacking, or folding. All these attributes make electrospinning a powerful tool for generating nanostructured materials for a range of biomedical applications that include controlled release, drug delivery, and tissue engineering. This talk will focus on the use of electrospun nanofibers as scaffolds for neural and bone tissue engineering. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V18.00009: Control of Protein Adsorption on Surfaces with Grafted Polymers Igal Szleifer, Jan Genzer Non-specific protein adsorption is the first process in the foreign body response. The molecular design of surface modifiers that prevent non-specific adsorption requires the understanding of the factors that determine protein adsorption. The hierarchy of time and length scales present in the adsorption requires a multiscale approach to treat the complexity of the process. We will discuss the driving forces that determine protein adsorption and how end-grafted polymers can be used to modify the ability of the proteins to reach the surface. We will show the differences between preventing protein adsorption thermodynamically and kinetically. For practical applications the relevant time scales are hours or days. We will show how a molecular approach can be used to study these time scales. In particular we will show two different levels of approximations based on a molecular understanding of the adsorption process that enables, through the proper integration of degrees of freedom, to determine the kinetics of adsorption over 16 orders of magnitude in time. This approach is applied to explain recent experimental observations carried out on orthogonal modified surfaces that suggest that protein adsorption is a universal function of the product of grafted polymer surface coverage and molecular weight. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V18.00010: Optimization of Polymer Surfaces for Specific Targeting Elena Dormidontova, Matthew Hagy, Shihu Wang Using Monte Carlo simulations we studied reversible binding between a polymer layer functionalized by ligands and a receptor surface. By analyzing distance-dependent profiles for the average number of ligands bound to receptors, the total free energy of polymer layer-cell surface interaction and the interaction force the influence of different design parameters of a polymer layer on the affinity and specificity of binding were investigated. We show that planar polymer layers with a smaller chain length and grafting density, larger degree of functionalization, and larger absolute binding energy exhibit higher affinity to the cell surfaces with a large density of mobile receptors. A high binding specificity can be achieved by the polymer layers with intermediate ligand-receptor binding energies or an intermediate number of ligands, as a larger binding energy or number of ligands lacks specificity while a smaller binding energy or number of ligands provides inadequate affinity. As a result, the optimal design of the polymer layers can be achieved by using several different strategies, which will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V18.00011: Cluster structure in urea aqueous solution and it's effect on DNA denature He Cheng, Charles C. Han, Boualem Hammouda The existence of large cluster structure in urea aqueous solution is proved by Small Angle Neutron Scattering (SANS). Our results indicate that urea is a water- structure-breaker, and large urea cluster will be formed when it's concentration is higher than 20 w{\%}. This cluster is very stable, and almost do not change with temperature. The helix-to-coil denaturation transition of DNA was studied with various urea concentrations, to testify the solvent structure influence on this process. [Preview Abstract] |
Session V19: Focus Session: Hierarchically Ordered Systems
Sponsoring Units: DPOLYChair: Albertu Striolo, University of Oklahoma
Room: 320
Thursday, March 19, 2009 8:00AM - 8:12AM |
V19.00001: Semi-crystalline PMMA Stereocomplex Fibers Matija Crne, Shin-Woong Kang, Jung Ok Park, Satyendra Kumar, Mohan Srinivasarao A mixture of isotactic and syndiotactic PMMA polymers forms a supramolecular helical structure, called ``stereocomplex'' PMMA, which is held together by non-covalent bonding. The helices can pack together, resulting in a semi-crystalline material with a melting point of 172$^{o}$C. Furthermore, the solutions exhibit gelation behavior in appropriate solvents. We have utilized these properties to make oriented stereocomplex PMMA fibers by three different methods -- wet spinning, gel spinning and electrospinning. These fibers are highly oriented and crystalline. They are resistant to high temperatures up to 160$^{\circ}$C. They are also resistant to the PMMA polymerization conditions. We have examined the fibers using X-ray diffraction and Raman spectroscopy. The results agree with the recently proposed triple helical structure where two isotactic PMMA chains wrap around each other in a double helix and then the syndiotactic chain wraps around this double helix. The resulting structure is a triple helix with a 1:2 molar ratio of isotactic : syndiotactic PMMA. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V19.00002: Hierarchically Organized Peptide Core-Silica Shell Hybrids Aysegul Altunbas, Nikhil Sharma, Darrin J. Pochan A biomimetic approach was applied for the fabrication of a 3D hybrid network that displays hierarchical organization of an inorganic layer around an organic self-assembled peptide fibril template. The 20 amino acid peptide used in this study consisted of alternating hydrophilic (lysine) and hydrophobic (valine) residues flanking a four amino acid turn sequence in the center. After intramolecular folding into a beta-hairpin conformation on addition of a desired solution stimulus, this peptide self-assembles into a 3D network of entangled fibrils rich in beta-sheet with a high density of lysine groups exposed on the fibril-surfaces. The lysine-rich surface chemistry was utilized to create a silica shell around the fibrils. The mineralization process of the fibrils was initiated under physiological conditions by adding the silica precursor, tetramethyl orthosilicate, to the pre-assembled hydrogel, which results in a porous silica network that retains the mesoscale structure of the peptide fibril network. Structural characterization via Transmission Electron Microscopy, cryogenic-Scanning Electron Microscopy, Small Angle Neutron and X-ray Scattering and mechanical characterization via oscillatory rheology will be presented. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V19.00003: Hierarchically Structured Regioregular Conjugated Polymer via Evaporative Self-Assembly Myunghwan Byun, Robyn Laskowski, Feng Qiu, Malika Jeffries-EL, Zhiqun Lin Regioregular conjugated polymers, poly (3-hexylthiophene) (P3HT) toluene solution was confined in a sphere-on-flat geometry. The geometrically constrained P3HT solution led to the formation an axially symmetric liquid capillary bridge, from which the consecutive ``stick-slip'' motion of the contact line of the solution due to the solvent evaporation was effectively regulated. As a result, hierarchical ``snake-skin'' like structures of high regularity were yielded, namely, the microscopic structures were composed of P3HT nanofibers. This facile, \textit{one-step} technique based on evaporative self-assembly opens up a new avenue for organizing semicrystalline conjugated polymers into two-dimensional patterns in a cost-effective and nondestructive manner [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V19.00004: Combining Small Molecule with Block Copolymer: a Facile Approach to Direct Hierarchical Assembly of Nanoparticles Invited Speaker: Precise control over the spatial organization of nanoscopic building blocks over multiple length scales is a bottleneck in the ``bottom-up'' approach to generate technologically important materials. We demonstrate a new paradigm to control the hierarchical assembly of nanoparticles through the synergistic co-assembly of block copolymers (BCP), small molecules and readily available nanoparticles. Organizations of nanoparticles into one, two and three-dimensional arrays with controlled inter-particle separation and ordering were achieved without any chemical modification of either the nanoparticles or BCPs. The ordering and distribution of small molecules between different BCP blocks are temperature dependent, leading to responsive materials where the spatial distribution of the nanoparticles can be varied, changing the local environment and the areal density of the nanoparticles. The approach described is versatile; compatible with existing fabrication processes and enables a nondisruptive approach for the generation of functional devices. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V19.00005: Hierarchical volume gratings by combining holographic-patterning and block copolymer self-assembly Michael Birnkrant, Russell Marron, Christopher Li, Lalgudi Natarajan, Vincent Tondiglia, Timothy Bunning A novel hierarchical photonic crystal (HPC) was fabricated by combining top-down and bottom-up nanomanufacturing techniques. The hierarchical structure was fabricated from a volume of material by combining holographic patterning (HP) and block copolymer (BCP) self assembly. The structure of the HPC was investigated as a function of the BCP architecture, BCP concentration and crystallization temperature. Upon heating the photonic crystal a red shift in the reflected wavelength occurs; but, an initial decrease in diffraction efficiency (DE) followed by an increase in DE indicates a non-monotonic change in the structure of the HPC. Upon cooling the reverse occurs reflecting the dynamic change in the hierarchical structure. Transmission electron microscopy, in-situ FTIR and optical spectroscopy were used to correlate the optical property change with BCP/HPC morphology. This approach could open a gateway to fabricating multifunctional hierarchical nanostructures. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V19.00006: Heteroarm Star Block Copolyampholytes as Templates for Hierarchically-Ordered Polyelectrolyte-Surfactant Complexes Matthew Hammond, Chaoxu Li, Constantinos Tsitsilianis, Raffaele Mezzenga We report on the hierarchical ordering observed in dry, solid samples of polyelectrolyte surfactant complexes based upon a novel heteroarm star block terpolymer bearing short polystyrene (PS) arms and an equal number of longer poly(2-vinylpyridine)- \emph{block}-poly(acrylic acid) (P2VP-\emph{b}-PAA) arms. The ampholytic nature of the star block copolymer allowed for complexation to be carried out on either the P2VP blocks (with negatively charged surfactants) or on the PAA blocks (with positively charged surfactants), depending only on the pH at which the complexation reaction was carried out. X-ray scattering and transmission electron microscopy data reveal that the various complexes display self-organization on the length scale of the polyelectrolyte-surfactant complex (ca. 3 - 4 nm) and on that of the overall copolymer (ca. 20 - 40 nm), with the specific repeat distances and self-organized morphologies being dramatically affected by the choice of block to be complexed. This study clearly illustrates how topological design possibilities in hierarchical self-assembly of block copolymer-based supramolecular complexes can be greatly enhanced by increasing the level of complexity of the macromolecular templates used. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V19.00007: Assembly of Organic/Nanoparticle Hybrid Systems Stephen Z.D. Cheng, Yingfeng Tu, Chun Ye, Wenbin Zhang, Xinfei Yu, Ryan M. Van Horn, Chien-Lung Wang The structure, dispersion, and chemical functionality of particles in a material are critical to the material's properties. We are working to build the scientific and technological foundations of using particles such as C60 and POSS to develop new, highly functional, self-assembled materials. These efforts have involved developing new synthetic techniques to efficiently and precisely manipulate particles to control their dispersion and structure within the organic material. First, C60-polymer and POSS-polymer molecules have been synthesized. These materials are capable of crystallizing in solution, enabling the formation of highly conducting or insulating sheets on the basal surfaces of the crystals. Additionally, these molecules may form micelles in solution. Next, POSS-C60-porphyrin molecules were synthesized and were found to self-assemble into discotic columnar structures where the intimately arranged porphyrin core harvests photons and the C60 enhances charge transport, making these materials ideal for organic photovoltaic applications. Finally, C60-POSS conjugate molecules have been synthesized that crystallize into a bilayer structure with alternating conducting and insulating layers. From these investigations, we will develop how best to chemically incorporate particles into materials from a fundamental level for significant technological advancements. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V19.00008: Quantification of the Molecular Topology for Hierarchical Macromolecules Gregory Beaucage Hierarchical structures are often produced from ramified macromolecules such as comb, star, hyperbranched and dendritic polymers. We have recently derived a method for the description of complex molecular and nanostructural topologies based on a statistical analysis [1,2]. The method has been applied to a wide range of hierarchical materials from long chain branched polyolefins, hyperbranched polymers [3], star polymers, H-branched polymers to cyclics, biopolymers [4], and branched nanostructured aggregates. This method, when applied to neutron scattering data, yields the mole fraction of a structure involved in branching, the number of branch sites, the average branch length, and the number if inner chain segments. Further, quantitative measures of the convolution or tortuosity of the structure and the connectivity of the branching network can be made, opening a new window for our understanding of complex molecular topologies. This understanding has recently been applied to biological chain molecules to understand protein and RNA folding [4] for example as well as to aggregated, nanostructured, carbon soot. \\[0pt] [1] Beaucage, G, \textit{Phys. Rev. E} \textbf{2004}, 70, 031401. [2] Kulkarni, AS {\&} Beaucage, G, \textit{J. Polym. Sci. Part B: Polym. Phys.} \textbf{2006}, 44, 1395. [3] Kulkarni, AS {\&} Beaucage, G, \textit{Macromol. Rapid Comm.} \textbf{2007}, 28, 1312.?4) Beaucage, G, \textit{Biophysical J.} \textbf{2008}, 95, 503. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V19.00009: Nanoparticle Assemblies via Self-Assembling Peptide Molecules Nikhil Sharma, Matthew Lamm, Joel Schneider, Kristi Kiick, Darrin Pochan The bottom up approach towards nano-scale patterning presents the possibility of creating hierarchical architectures through simple self-assembly strategies. Herein, we elucidate the self-assembly of different types of peptide molecules into unique nano-scale morphologies and demonstrate their application in the construction of linear arrays of inorganic nanoparticles. A 20 amino acid peptide, consisting of alternating hydrophilic (lysine) and hydrophobic (valine) residues flanking a central diproline turn sequence (VKVKVKVKVPPTKVKVKVKV-NH$_{2})$ was employed as a nano-scale template for the organization of 2nm gold particles. This peptide self assembles into a laminated morphology in solution and has a periodic nanostructure consisting of alternating hydrophobic and hydrophilic layers with a lateral periodicity of 2.5 nm. Negatively charged gold nanoparticles are templated into the positively charged lysine layer through electrostatic interaction and are aligned within the template to form laterally spaced (2D) linear arrays. Also, a long chain alanine-rich polypeptide was used to create 1D nanoparticle assemblies. This peptide assembles into fibrils with monodisperse widths and presents its charged functional groups periodically along the length of the fibril. These functional groups bind nanoparticles that results in their spatially modulated linear arrangement. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V19.00010: Robust Three Dimensional Liquid Films through Nanoparticle Assembly Tzu-chia Tseng, Erin McGarrity, Phillip Duxbury, Amalie Frischknecht, Michael Mackay We create three-dimensional thin liquid films that cover rough surfaces, typically thought to be un-wettable, by employing a nanoparticle self-assembly technique. In this technique, the nanoparticles assemble at the liquid-substrate interface during annealing and they stabilize the liquid-air interface by screening its interactions with the substrate. This results in robust liquid films capable of wetting surface protrusions that are greater than their thicknesses. In this work, blends of polystyrene and CdSe nanoparticles were spincoated onto silicon substrates containing sparsely distributed SiO2 particles (circa 110 nm) and thermally annealed. Film profiles of different thicknesses (40-180 nm) were characterized using atomic force microscopy (AFM). Calculations based on a continuum theory were performed and found to be in agreement with the AFM profile data. Cross-sectional transmission electron microscopy (TEM) was performed to provide validation of the film profile contours and the 3D-assembly of the nanoparticles. This method could be used, for example, to enlarge the interfacial area for exciton dissociation in organic photovoltaic cells. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V19.00011: The path and motion of an electrospinning jet observed with videography and stereography Kaiyi Liu, Camden Ertley, Darrell Reneker An electrospinning jet illuminated with both a steady intense light and a short flash was stereoscopically recorded through a pair of prisms in a video, producing images of both traces of moving glints reflected from the surface of a jet and the instantaneous positions of the path of the jet. The relationship between the visual observation and the jet path described in the Reneker-Yarin model$^{1,2}$ was explained by analyzing the stereographic images. Computer modeling was used to elucidate the relationship between the onset of the bending instability and the bifurcation of a glint trace. The velocities and positions, in 3-dimensional space, of segments of a jet were calculated from the stererographic images. The distributions of velocities and positions of segments along the vertical direction were analyzed. A novel and facile method was used to observe the handedness of the coiled path of an electrospinning jet. 1. D.H. Reneker, A.L. Yarin, Polymer, Vol. 49, (2008) pp 2387-2425. 2. D.H. Reneker, A.L. Yarin, E. Zussman, H. Xu, Advances in Applied Mechanics, Vol. 41 (2006) pp 43-195. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V19.00012: Electrospinning Semicrystalline Block Copolymer Assemblies into Microfibers Kristen Roskov, Ian Manners, Richard Spontak Cylindrical micelles consisting of a diblock copolymer composed of poly(ferrocenyldimethylsilane) (PFDMS)-$b-$poly(2-vinylpyridine) (P2VP) develop in dimethylformamide (DMF), a P2VP-selective solvent, with lengths exceeding one micron. These self-assembled micelles are then incorporated into P2VP homopolymer solution and electrospun. Addition of the cylindrical micelles is found to improve the ability of P2VP to be electrospun and dramatically decrease the bead density that appears in the electrospun microfiber mat. Scanning and transmission electron microscopies are used to investigate both the surface and internal morphology of these fibers, along with the robustness of the micelles. The combination of self-assembled structures within a polymer matrix can lead to fascinating response behavior dependent on temperature; if the sample is heated and the melting point of PFS is surpassed, the micelles will melt and then form classical morphologies. In the case of self-assembled, conductive cylinders of PFS block copolymers, heating the sample destroys conductive pathways. The PFS-$b$-P2VP self-assembled cylinders have also been incorporated into other DMF-selective polymers to verify that the micelles remain intact upon electrospinning. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V19.00013: Electrospinning of native cellulose from nonvolatile solvent system Shanshan Xu, Aihua He, Charles C. Han Electrospinning of cellulose in a highly efficient RTIL of 1-allyl-3-methylimidazolium chloride (AMIMCl) was investigated. It was found that the introduction of co-solvent dimethyl sulfoxide (DMSO) contributed to a continuous jet. The problems lying in nonvolatility and the high ionic strength of the RTIL were successfully resolved using a rotating copper-wire drum as a collector and solidifying the jet under high relative humidity. The water vapor played an important role in leading to ``skin formation'' which helped to stabilize the fibrous morphology, and finally smooth ultra-thin regenerated cellulose fibers were obtained [Preview Abstract] |
Session V20: Thin Films and Adhesion II
Sponsoring Units: DPOLYChair: Connie Roth, Emory University
Room: 321
Thursday, March 19, 2009 8:00AM - 8:12AM |
V20.00001: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V20.00002: Temperature-Modulated Ellipsometry in Vacuum: A New Tool for Probing Glass Transition in Thin Supported Polymer Films Mikhail Efremov, Shauheen Soofi, Anna Kiyanova, Franco Cerrina, Paul Nealey Observation of glass transition in thin polymer films is a good example of experimental challenges that measurements at the nanoscale may present. The standard technique in the field, ellipsometry, has been advanced by state-of-the-art approaches: measurements in vacuum and temperature modulation. Glass transition in 5 -- 200 nm thick spin-cast polystyrene (PS) and poly(methyl methacrylate) (PMMA) films on silicon has been studied at 10$^{-6}$ -- 10$^{-8}$ torr residual gas pressure, using both linear and modulated temperature scans [1, 2]. A well-defined glass transition in 5 nm thick PS and 10 nm thick PMMA films is observed. Factors that can alter glass transition temperature assignment will be discussed. Residual gas can affect data even at the pressures mentioned above [1]. An ionization gauge, the standard vacuum equipment, causes sample deterioration [1]. Temperature modulation effectively separates reversible glass transition from accompanying irreversible processes [2]. [1]. M. Yu. Efremov, S. S. Soofi, A. V. Kiyanova, C. J. Munoz, P. Burgardt, F. Cerrina, and P. F. Nealey, Rev. Sci. Instr., \textbf{79}, 043903 (2008). [2]. M. Yu. Efremov, A. V. Kiyanova, and P. F. Nealey, Macromolecules, \textbf{41}, 5978 (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V20.00003: Physical Aging in Nanoconfined Polymer Films: Importance of 3D vs. 1D Thermal Contraction in the Resulting Physical Aging Response Connie B. Roth, Elizabeth A. Baker, Perla Rittigstein, John M. Torkelson Studies of physical aging in confined geometries have reported conflicting observations of changes in physical aging rates with decreasing polymer film thickness. Accelerated physical aging with decreasing film thickness has been observed with gas permeation and ellipsometry in stiff backbone, so-called ``high free volume,'' polymers traditionally used in gas separation membranes. In contrast, no change or suppressed physical aging has been observed with fluorescence in flexible carbon-carbon backbone polymers. We have developed a new streamlined ellipsometry procedure that allows us to relatively quickly ($\sim$6 hrs) evaluate the physical aging characteristics of both stiff and flexible backbone polymers in a thin film geometry. We present measurements of physical aging rates using our new approach and compare these to existing results in the research literature. In addition, we also address the importance of 3D vs. 1D thermal contraction in the resulting physical aging response, which we believe is one of the key factors accounting for the observed qualitative differences in physical aging rate upon confinement between the existing studies. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V20.00004: Segmental dynamics of supported and freestanding polystyrene thin films probed by dye reorientation. Keewook Paeng, Hau-Nan Lee, Stephen Swallen, Mark Ediger The dynamics of both freestanding and supported polystyrene thin films (down to 15 nm) were studied by measuring the reorientation of dilute dye molecules. Well below T$_{g, }$dye molecules were photobleached using intense linearly polarized light creating an anisotropic distribution. The anisotropy decay was measured using circularly polarized light and probing fluorescence parallel and perpendicular to the bleaching beam during linear temperature ramping. Temperature ramping anisotropy measurements allow us to compare both dynamics and the distribution of relaxation times in thin and thick films. Both freestanding and supported thin films show faster and more broadly distributed dynamics than thick films. For 17.5 nm supported films, temperature ramping experiments show up to 14K shift in dynamics. The corresponding shift for 16.5nm freestanding films was 22K. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V20.00005: Modeling Dielectric Relaxation in Simulations of Polymer Glasses and Thin Films Hendrik Meyer, Simone Peter, J. Baschnagel We perform molecular dynamics simulations to study the dielectric relaxation of a bead-spring model for a polymer melt in the bulk and in supported films [1]. By assigning dipole moments parallel and perpendicular to the backbone of all chains in the completed simulation trajectories we calculate the dielectric spectra of so-called type-A polymers which exhibit relaxation processes due to the local motion of chain segments (``segmental mode'') and due to fluctuations of the end-to-end vector (``normal mode''). We find that the relaxation of both modes is enhanced in the films relative to the bulk. For the segmental mode this difference between film and bulk dynamics increases on cooling toward the glass transition. By a layer-resolved analysis of the segmental relaxation we show that the acceleration of the average film dynamics is a consequence of a smooth gradient in relaxation, originating from both interfaces where the segmental dipoles relax much faster than in the bulk. \\[3pt] [1] S. Peter et al Macromolecules 41 (2008) 7729. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V20.00006: Neighboring Domains Perturb Glass Transition Temperature on Multilayer Films and Nanostructured Polymer Blend Systems Soyoung Kim, Connie Roth, Rodney Priestley, John Torkelson The impact of free surface and polymer-substrate interfaces on the glass transition temperature (Tg) in nanoconfined geometries has been studied for over a decade. Free surfaces reduce the requirement for cooperative dynamics and tend to decrease Tg; attractive interactions with a substrate interface reduce mobility and tend to increase Tg. Employing a multilayer fluorescence technique, we show how the Tg dynamics of PS layers are perturbed by immiscible polymer-polymer interfaces. We determine the length scale over which adjoining layers can perturb the PS layer. Finally, we demonstrate the tunability of the Tg of ultrathin PS layers atop different types of polymers. Our results indicate that the cooperative segmental dynamics of an ultrathin PS layer are strongly coupled to the neighboring domains through the narrow polymer-polymer interface. These results suggest a novel route to create new material properties controlled by the type and thickness of polymers in a multilayer film geometry. Studies with nanostructured blends to monitor Tg perturbation by neighboring domain are also underway. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V20.00007: Near Surface Dynamics of Polymers Probed with Nanoparticle Embedding Dongping Qi, James A. Forrest We use nanoparticle embedding to probe the dynamics of the near surface layer of glassy polymer films. We observe evidence for heterogeneous dynamics in the first 5-10 nm near the free surface of glassy polymers. We observe that the relaxation into the polymer immediately below the free surface is irreversible, even after a period of 1 year. On the other hand, further embedding (5 -10 nm) appears to be reversible. The results are discussed in terms of posible models of near surface mechanical properties. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V20.00008: Glass Transition of Thin Star Polymer Films Emmanouil Glynos, Peter Green The thickness dependence of the glass transition, $T_{g}$, of thin film polystyrene (PS) star molecules, supported by SiO$_{x}$ substrates, has been examined using spectroscopic ellipsometry and compared to the behavior of linear PS chains. Linear PS chains exhibit a film thickness dependence on SiO$_{x}$ substrates, decreasing with decreasing film thickness, for thicknesses $h$ less than approximately 45 nm. This thickness dependence, when normalized by the bulk $T_{g}$, is observed for chains with a wide range of degrees of polymerization $N$, from $N < \quad N_{e}$ (molecular weight between entanglements) to very large values of $N$. The $T_{g}$s of long chain star molecules, of low functionalities, $f$, exhibit the same thickness dependence. However, as the degree of polymerization of an arm length, \textit{Na}, decreases the thickness dependence undergoes a transition, wherein $T_{g}$ increases with decreasing $h$. These effects are discussed in terms of the role of architecture and entropic effects on the structure of the system. Implications on the chain dynamics will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V20.00009: The Glass Transition in Ultra-Thin Polymer Filrms Confined between Structured Surfaces Vikram Kuppa, Gregory Rutledge Molecular Dynamics simulations are used to probe the structure and dynamics of polymers in ultra-thin slit pores. The simulation setup follows the structure of polymer nanocomposites, depicting chains intercalated between layered inorganic silicates. The structure and dynamics of bead-spring oligomers are studied for different film thicknesses, surface-segment interactions and temperature. In particular, we focus on the glass transition of the confined films as a function of layer thickness, interaction strength and density profiles: Tg is demonstrated to increase with confinement and attraction of the polymer with adjacent surfaces. The fragility of the polymer glasses is drastically different from the corresponding bulk system, and is shown to be dependent on the effective co-ordination number. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V20.00010: Apparent changes in the molecular dynamics of thin polymer layers due to the impact of interfacial layers Anatoli Serghei, Martin Tress, Friedrich Kremer Possible mechanisms leading to an apparent faster glassy dynamics in thin polymer layers, as investigated by means of Broadband Dielectric Spectroscopy, are analyzed in detail. It is shown that manifold experimental findings can be traced back to the influence of interfacial sub-layers, where -- due to the proximity to solid interfaces -- the dielectric function of the polymer is altered and modifies, by that, the overall dielectric response of the polymer films. A large amount of experimental data is analyzed to evidence how the contribution of the interfacial dynamics combines with that of the bulk in order to give the total response of a thin polymer film. It is shown that the non-linear character of this combination could lead to apparently discrepant experimental results. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V20.00011: Post-confinement Relaxation Behavior of Nanostructures on Polymer Surface Hua-Gen Peng, Yen Peng Kong, Albert Yee Dimensional relaxation of nanostructures on polymer film was studied to understand dynamics at surfaces and \textit{post-}confinement relaxation. Line gratings from 33 nm and up on PS surface were formed by nanoimprint lithography, and AFM was used to monitor their relaxation with time and temperature. When annealed at temperatures in the vicinity of the bulk Tg, the grating height slumps -- or shrinks -- as surface tension and other driving forces overcome the viscosity. The temperature for rapid slumping decreases at smaller and smaller gratings of all molecular weights, but a simple explanation based on enhanced surface mobility due to increased surface to volume ratio fails to explain the results. Analysis of viscosity shows that the stress from surface tension may cause shear thinning and thus contribute to the reduced nanostructure stability. More importantly, confinement of polymer chains to spatial dimensions comparable to or even smaller than the radius of gyration seems to enhance molecular relaxation, which may be the major factor for the surprisingly low slumping temperature. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V20.00012: Molecular weight and chain architecture dependence of glassy compliance of ultrathin freely standing polymer films Shanhong Xu, Gregory B. McKenna Glassy compliance of ultrathin linear polystyrene films of different molecular weights was investigated with the novel nanobubble inflation techniques developed in our lab. Previous work by O'Connell, Hutcheson and McKenna [1] indicates that the glassy compliance decreases as the film thickness decreases for a polystyrene sample with molecular weight of 1M g/mol. However the glassy stiffening is not nearly as dramatic as that observed in the rubbery plateau regime [2]. Preliminary results in the present work show that the glassy compliance decreases as the molecular weight increases. We are now examining a three-arm star polystyrene with branch molecular weight the same as that of the lower molecular weight linear material and eight-arm stars will also be investigated with the purpose of determining chain architecture effects on the glassy and rubbery behaviors of ultrathin polymer films. \\[4pt] [1] P.A. O'Connell, S.A. Hutcheson and G.B. McKenna, J. Polym. Sci. Part B. Polym. Phys. Ed., \textbf{46}, 1952 (2008). \\[0pt] [2] P.A. O'Connell and G.B. McKenna, Eur. Phys. J. E, \textbf{20}, 143 (2006). [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V20.00013: Surface Chemistry Effects on the Reactivity and Properties of Nanoconfined Bisphenol M Dicyanate Ester in Controlled Pore Glass Sindee Simon, Qingxiu Li The influence of nanoconfinement on the cure kinetics and glass transition temperature of a bisphenol M dicyanate ester/polycyanurate material is investigated as a function of surface chemistry and nanoconfinement size in controlled pore glass (CPG). The glass transition temperature and conversion as a function of cure time is investigated using differential scanning calorimetry. The native CPG surface accelerates the cure of bisphenol M dicyanate to a larger extent compared to the silanized hydrophobic CPG presumably because of the catalytic nature of surface hydroxyl groups of the native pores. Two T$_{g}$s are observed for both monomer and polycyanurates confined in the native CPGs. For the fully cured polycyanurate, the primary T$_{g}$ is depressed by 60 K at 11.5 nm, and the secondary T$_{g}$ is 10 to 33 K above the primary T$_{g}$. The length scale associated with the secondary T$_{g}$ is approximately 0.8 nm. Based on the measurements of both T$_{g}$ and sol content as a function of conversion, the network structure does not change upon nanoconfinement. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V20.00014: Relation Between Glass Transition Temperatures in Polymer Nanocomposites and Polymer Thin Films Jamie Kropka, Victor Pryamitsyn, Venkat Ganesan Motivated by recent experiments, we examine within a percolation model whether there is a quantitative equivalence in the glass transition temperatures of polymer thin films and polymer nanocomposites (PNCs). Our results indicate that while the qualitative behaviors of these systems are similar, a quantitative equivalence cannot be established in general. However, we propose a phenomenological scaling collapse of our results which suggests a simple framework by which the results of the thin films may be used to quantitatively predict the properties of PNCs. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V20.00015: Finite Element Analysis of Nanoparticle embedding into glassy polymers Mark Ilton, James A. Forrest The embedding of rigid nanospheres into the surface of thin film glassy polymers was modelled using a Finite Element Analysis. This method of analysis readily adapts to various material models, film thicknesses, and substrates. Moreover, it allows for modelling of the related hole relaxation process when embedded nanospheres are removed from the film. Calculations using a viscoelastic constitutive model with depth dependent material properties are compared to recent experimental results. The results are discussed in the context of the proposed near surface enhanced dynamics in thin film glassy polymers. [Preview Abstract] |
Session V21: Semiconductor Growth and Material Processing
Sponsoring Units: FIAPChair: Nelson Tansu, Lehigh University
Room: 323
Thursday, March 19, 2009 8:00AM - 8:12AM |
V21.00001: Gallium beam analysis and implications for the growth of ultra-pure GaAs/AlGaAs heterostructures Stefan Schmult, Sam Taylor, Werner Dietsche The low temperature mobility of a two-dimensional electron gas (2DEG) is a perfect probe of the residual level of charged impurities in GaAs/AlGaAs heterostructures grown by molecular beam epitaxy (MBE) [1]. Besides the need of an ultra-high vacuum environment to prevent impurity incorporation during the growth process, pollution may still result from the source materials themselves. We evaluate the purity of Gallium by channeling the evaporated beam through the detection head of a mass spectrometer mounted in cross-beam geometry. It is found, that at the beginning of a growth campaign Gallium-Oxygen and Gallium-Hydrogen compounds represent the sole contaminations resulting from above-average MBE-grade Gallium at our present detection sensitivity [2]. Utilizing the aforementioned Gallium, we obtain in simple single-interface heterostructures 2DEG mobilities exceeding 10$^{7}$cm$^{2}$/Vs. This value is not limited by charged residual impurities, as confirmed in 2DEG-density-dependent transport measurements. [1] F. Stern, Appl. Phys. Lett. \textbf{43}, 974, (1983). [2] S. Schmult, et al., J. Crystal Growth (2008), doi:10.1016/j.jcrysgro.2008.10.014. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V21.00002: Crystal orientation control of CdTe epitaxial layers grown on (100) GaAs with ZnSe buffer layer by molecular beam epitaxy Qiang Zhang, William Charles, Bingsheng Li, Aidong Shen, Carlos Meriles, Maria Tamargo Based on our previous experience with the growth of ZnSe on GaAs, we have improved the substrate, on which we deposit CdTe by first depositing a ZnSe buffer on the bulk GaAs crystals. This allows us to control the II-VI/III-V heterovalent epitaxy prior to the CdTe deposition. Depending on the control of the interface between ZnSe and CdTe, it was possible to stabilize the growth of either (100) or (111) CdTe epitaxial layers on the (100) ZnSe/GaAs substrate. Reflection high-energy electron diffraction was used to observe the nucleation of the epitaxial layers in situ during the growth, while x-ray diffraction and photoluminescence measurements indicate that the CdTe is of high structural quality despite the large lattice constant mismatch of 14.3{\%} between CdTe and ZnSe. To explore the full impact of controllable-orientation growth technique, optical pumping and time-resolved Faraday rotation experiments were performed on CdTe films grown in different crystal orientation. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V21.00003: Development of a Wafer Fusion Process for Producing Patterned GaP Templates Krongtip Termkoa, Vaibhav Mathur, Xifeng Qian, William Goodhue, David Bliss, Rita Peterson Quasi-phase-matching (QPM) is an effective technique for nonlinear optical frequency conversion to generate IR wavelengths not readily available from direct laser sources. The QPM gratings can be produced by electric field poling in ferroelectric oxide materials, or by crystal growth of alternating phase domains in the case of semiconductors. For semiconductor materials GaAs and GaP we are developing new patterning methods to produce optical gratings for QPM. The state of the art for producing orientation-patterned GaAs material is already well developed. Gallium phosphide (GaP), also transparent at IR wavelengths, is attracting interest for nonlinear optical frequency conversion due to its high second-order nonlinear susceptibility, high thermal conductivity, wide band gap and low optical loss. Here we report a method to fabricate a GaP periodic domain inversion template using a process combining wafer fusion, substrate removal, lithographic patterning, and wet/dry etching. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V21.00004: Growth of GaN from Ga:In and Ga:Sb liquid alloys Kathleen Kash, Challa Bekele, John Angus We grew GaN from Ga:Sn and Ga:In melt solutions that varied in composition from pure Ga to a few at{\%} Ga. Growth was done at 900 \r{ }C and 100 mtorr pressure by exposing the melt surface to a nitrogen plasma. A fit to a model of growth rate versus melt composition yielded estimates of the reaction rates for the formation of GaN versus composition of the melt that were, within experimental uncertainty, independent of the choice of diluent. Near-band-edge emission features were prominent in the photoluminescence spectra at both room temperature and 10 K for material grown from the entire range of melt compositions for both diluents. Lattice parameters measured by powder x-ray diffraction spectroscopy revealed an interesting dependence on melt composition; the ``a'' lattice parameter varied by as much as 1{\%} and exhibited a minimum for material grown from melts with compositions between 60 and 70 at{\%} Ga, for both diluents. One motivation for this work is growth of large area, high quality single crystal substrates. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V21.00005: Epitaxial Growth and Characterization of Void-Free 3C-SiC Films on Germanium-Modified Si Substrates using RTCVD Domingo Ferrer, Shagandeep Kaur, Sayan Saha, Seyoung Kim, Emanuel Tutuc, Sanjay Banerjee, Nilratan Mazumder Cubic silicon carbide (3C-SiC) is an attractive wide band gap semiconductor, frequently employed under extreme conditions such as high temperature, high frequency and high power, due to its superior physical and chemical properties. The electronic properties of epitaxial graphene grown on SiC integrated on silicon substrates also offer great potential as a viable candidate for ``\textit{beyond CMOS''} devices. A detailed understanding of both the structure and growth of epitaxial graphene, and the SiC/Si interfaces is very important for designing feasible devices. To this end, the work will analyze the growth and characterization of 3C-SiC on Si(100) and Si(111) substrates. 3C--SiC epitaxial crystal growth was carried out at temperatures as low as 750$^{\circ}$C using Rapid Thermal CVD. A thin germanium buffer layer was deposited on Si substrates prior to epitaxial growth of SiC to suppress the formation of voids. The precursors utilized were (CH3)3SiH and GeH4, for silicon carbide and germanium deposition, respectively. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V21.00006: The role of strontium in oxide epitaxy on silicon (001). James Reiner, Kevin Garrity, Fred Walker, Sohraib Ismail-Beigi, Charles Ahn The integration of crystalline oxides and semiconductors has been made possible by the development of techniques that allow crystalline SrTiO$_{3}$ to be grown on the silicon (001) surface. The most successful approach to realizing these epitaxial oxide-silicon (001) heterostructures requires manipulating substrate temperature and oxygen pressure on a layer-by-layer basis during the deposition of the metal oxide layers. The transition layer between the semiconductor and crystalline oxide is an alkaline earth metal, most often strontium, that is deposited on the silicon surface at around 650\r{ }C. Motivated by the desire to develop a fundamental understanding of this important transition layer, we have studied the surface structures formed by strontium on miscut silicon wafers, which, unlike regular silicon wafers, have a unique surface termination. At high temperatures, this reaction rearranges the top layer of silicon to replace the original top layer with strontium. At low temperatures, this reaction is suppressed, leading to a different, but symmetry related, ordered surface structure. We find that complex oxides can be grown on either surface with comparable crystallinity. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V21.00007: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V21.00008: Continuum model of surface roughness evolution of a-Si:H films grown by low-temperature PECVD Yevgen Kryukov, Nik Podraza, Robert Collins, Jacques Amar Using real-time spectroscopic ellipsometry the evolution of the surface roughness in a-Si:H thin-films grown on smooth c-Si/native oxide substrates by low-temperature plasma-enhanced chemical vapor deposition (PECVD) has been studied as a function of the $H_2$ dilution ratio $R = [H_2]/[SiH_4]$ with $15 \le R \le 60$. The best amorphous Si photovoltaic films correspond to a relatively high dilution ratio ($R \simeq 60$) such that the surface roughness is minimized but close to the amorphous-microcrystalline transition. After a brief period of nucleation of 3D islands, the roughness decreases but then eventually increases rapidly at large bulk layer thicknesses. Using a 3D continuum equation which includes a negative surface tension to take into account the destabilizing effects of short-range attraction and/or shadowing, as well as a smoothing term to take into account surface diffusion we have been able to obtain excellent agreement with experimental results for the evolution of the surface roughness. The dependence of our model parameters on the dilution ratio $R$ is also discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V21.00009: In-situ growth of two-terminal silicon nanowires from locally heated cantilevers Christian Kallesoe, Frances Ross, Chen-Yen Wen, Kristian Molhave, Peter Boggild Resistively heated crystalline silicon cantilevers extending over the edge of a chip offer excellent control of local growth of nanowires, without heating the entire micro-system. Besides being CMOS compatible, the cantilevers also have a rapid temperature cycling, and furthermore the freestanding cantilevers are suitable for in-situ studies of nanowire growth inside a TEM, offering the possibility of applying electrical fields to direct the growth and growing bridging wires between cantilevers thereby making two-terminal in-situ electrical measurements of nanowires possible. We have used such cantilever loops to study the growth of nanowires in-situ in UHVTEM. Epitaxial growth was observed from the crystalline cantilevers and the rapid temperature cycling ensured a very fast reaction time when crystallizing or melting the catalytic particle. The silicon wires were grown towards a cold cantilever loop, thereby forming bridging nanowires and the nanowire contact was seen to depend on the wetting ability of the gold catalytic particle to the cold cantilever. Furthermore various two-terminal measurements were performed on the bridging silicon nanowires in-situ in UHV. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V21.00010: Time-Evolution of the Grain Size Distribution in Random Nucleation and Growth Crystallization Processes Andreas Bill, Anthony V. Teran, Ralf B. Bergmann The micromorphology of solids impacts in an essential way their mechanical, electronic, optical or magnetic properties. Hence, it is an important task to characterize properly the granularity of materials. One central quantity providing such information is the grain size distribution. We propose an analytical derivation of this distribution during the random nucleation and growth crystallization process of a $d-$dimensional solid ($d=1,2,3$). We describe how the grain size distribution evolves from early stages of crystallization to its final form when complete crystallization is achieved. We also discuss the remarkable result that for certain classes of nucleation and growth rates the asymptotic limit of large times is a logarithmic-normal (lognormal) type distribution. Finally, we apply the theory to the time-evolution of the grain size distribution during solid-phase crystallization of Si-films. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V21.00011: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V21.00012: Nanocrystalline Silicon Films by HWCVD Method Purabi Gogoi, Himanshu S. Jha, Pratima Agarwal High quality nano-crystallites embedded amorphous silicon films are deposited using HWCVD technique by varying the substrate temperature T$_{S}$ (100-350 $^{\circ}$C) and chamber pressure P (0.08-.8mbar). The precursor gas used is semiconductor grade hydrogen diluted silane (10 {\%} silane in hydrogen) from Matheson Inc. Filament temperature (T$_{F})$ is kept constant at 1800 $^{\circ}$C. The films are characterized by different tools viz. XRD, SEM, TEM, UV-Vis NIR spectroscopy, FTIR and PL. Both XRD and HRTEM studies indicate the presence of Nanocrystallite (size 5-10nm) in these films. The deposition rate ranges from 5-13 {\AA}/Sec, whereas the hydrogen content varies between 2.5-5 atomic percent. The band gap of films is in the range 2.0-2.37eV. Formation of Nanocrystallites is also supported by the PL and Raman scattering studies. Deposition rate decreases with increase in T$_{S}$, whereas the crystallinity increases with T$_{S}$. The films deposited at high pressure have high deposition rate, low hydrogen content and high band gap. The films are found to be of superior electronic properties suitable for photovoltaic device applications. The tunable band gap, high deposition rate and low hydrogen content is ideal for cost effective device fabrication. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V21.00013: Growth and physical properties of epitaxial GdN and SmN Ben Ruck, Bart Ludbrook, Joe Trodahl, Andrew Preston, Claire Meyer, Ian Farrell, Steve Durbin, Roger Reeves, Matthias Kubel, Laurent Ranno The epitaxial growth and passivation of GdN and SmN by pulsed laser deposition on [100] YSZ substrates has been refined. We report on the growth kinetics, including the relaxation of the films, and the incorporation of oxygen at the substrate-film interface. Magnetic susceptibility measurements confirm a Tc of 70K and small coercive field for GdN. Resistivity measurements show an anomalous peak at Tc, while magnetic transport measurements indicate the films are electron doped to degeneracy, and show a strong anomalous Hall contribution below Tc. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V21.00014: Synthesis and Magnetism of High Curie Temperature Prussian Blue Analogue Molecular Nanomagnet-Chromium Cyanide Molecule Nanowire Arrays Pingheng Zhou, Desheng Xue, Jinli Yao The goal to synthesize molecular nanomagnets that exhibit spontaneous magnetic ordering close to room temperature might enable one to apply them in the fields of magnetic memory devices and microelectronics. Chromium cyanide molecule nanowire arrays with diameters of about 50 nm and lengths up to 4$\mu $m have been synthesized by an electrodepositing technology based on anodizing anodic aluminum oxide films. Characterization measurements show that the oxidation state of the chromium ions in the chromium cyanide nanowires can be expressed as Cr$^{3+}$--CN--Cr$^{3+}$. Magnetic properties measurements indicate that the Curie temperature of chromium cyanide nanowire is 200 K, which is closer room temperature compared with current molecular nanomagnet systems. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V21.00015: ABSTRACT WITHDRAWN |
Session V22: Focus Session: Dilute Magnetic Oxide Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Berend Jonker, Naval Research Laboratory
Room: 324
Thursday, March 19, 2009 8:00AM - 8:12AM |
V22.00001: A few ways in which LDA et al can produce a wrong description of magnetism in oxides Alex Zunger, S. Lany, H. Raebiger Part of the anarchy that exist in the literature regarding conflicting predictions of ferromagnetism in oxides can be attributed to problems in LDA and its application:(1) LDA et al tend to place the oxide conduction band minima CBM) at much too deep energies; transition-metal impurity levels then incorrectly appear within the (delocalized) LDA continuum (instead of in the gap). This leads to overly optimistic result of long -range magnetism. This is not fixable via LDA+U.(2) Oxides are naturally off-stoichiometric, exhibiting significant concentration of indigenous free-carriers that greatly affect the occupancy of impurity levels. Yet, such non-stoichiometry is ignored in many calculations using instead a perfect host crystals. This alters the magnetic properties. (3) LDA- Predicted magnetism in defected oxides without TM impurities often rely on the defect orbitals (e.g, cation vacancy) being equally delocalized over all of its ligand atoms. While inherent to LDA, experiment, as well as more correlated approaches reveal that such orbitals are ``locked'' instead onto a single ligand. This symmetry-breaking causes them to become too localized for (percolative) magnetism. We show here how such problems might be fixed. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V22.00002: First-principles study of the effect of vacancies on magnetic properties Yoshi Kawazoe, Qian Wang, Qiang Sun, Puru Jena Due to the high solubility of Co in ZnO, the magnetic properties of Co-doped ZnO thin films have been extensively studied experimentally. Unfortunately, these results have led to diverse conclusions. To better understand the origin of the controversial experimental findings, we have carried out detailed theoretical studies, focusing on the role of concentration and distribution of Zn and O vacancies on the magnetism of Zn1-xCoxO thin films. We find that when Co atoms are substitutionally doped in ZnO thin films without any defects, the magnetic coupling between Co atoms is intrinsically anti-ferromagnetic. The coupling, however, changes to ferromagnetic when sufficient oxygen vacancies are introduced. On the other hand, Zn vacancies stabilize the anti-ferromagnetic coupling, in sharp contrast with that found in Zn1-xMnxO thin films. Our theoretical studies explain the origin of the different magnetic behavior observed experimentally. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V22.00003: On the possibility of ferromagnetism in MgX (X = O, S, Se) with/without conventional magnetic atoms Van An Dinh, Kazunori Sato, Hiroshi Katayama-Yoshida We present a first principle study on the half-metallicity and ferromagnetism in rock salt MgZ (Z = O, S and Se) with and without conventional magnetic elements. The electronic structure, effective exchange coupling constant and chemical pair interaction are calculated within SIC-LSDA. The Curie temperature is predicted by performing Monte Carlo simulation. A possibility of spinodal decomposition is investigated and simulated. For transition-metal-doped MgO, our results reflect the observation in the experiment [1]. Without oxygen vacancy, Co- and Ni-doped MgO is anti-ferromagnetic. Except Ti and Cr, another transition metals cause the anti-ferromagnetic behavior in MgO. The ferromagnetism can be stabilized by oxygen vacancies. Oxygen vacancies also change the chemical interaction trend of transition metal atoms, and cause an inhomogeneous distribution in MgO. It is also found that N atoms which substitute for anions can introduce the half- metallic ground state and ferromagnetism in MgZ. Monte Carlo simulation shows the above room temperature ferromagnetism in Mg$_{0.8}$N$_{0.2}$Z. Nitrogen atoms in MgO and MgS have a tendency to create clusters, whereas the distribution of N atoms in MgSe is homogeneous at dilute regime. [1]. J. Narayan {\it et al.}, Appl. Phys. Lett. {\bf 93} (2008) 082507. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V22.00004: Ferromagnetism in MgO by Nitrogen Doping Cheng-Han Yang, Mahesh Samant, Stuart Parkin The new group, dilute ferromagnetic oxide and nitride, provides a promising technology potential to combine the magnetic and electronic properties. Studies in creating ferromagnetism in thin films of doped oxide materials without the inclusion of transition metal (TM) or rare-earth (RE) metal have been speculated in recent years. However, there have been many reports of ferromagnetism in large numbers of different material systems with poor control and reproducibility of these data. Our interest is exploring the possibility of ferromagnetism in oxide and nitride films without the introduction of any TM or RE elements. In this study, we have successfully incorporated substantial amounts of nitrogen upto 13 at{\%} into MgO films using molecular beam expitaxy (MBE) technique. N-doped MgO films were carried out by evaporating the Mg in the presence of both atomic oxygen and nitrogen from two different RF sources. Upon the post-annealing, N atoms generate holes on oxygen and so create magnetic moment as high as 0.4 $\mu $B/per N. In undoped MgO films, no magnetic signature was seen either in the as-deposited film or the film after annealing, which is a strong indication that the moment arises from the N dopants. The pre-edge feature of oxygen K-edge measured in Near Edge X-ray Absorption Fine Structure (NEXAFS) shows apparent evidence for the substitution of nitrogen for oxygen after post-annealing. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V22.00005: Electric field control of magnetic semiconductor (Zn,Co)O Hyeon-Jun Lee, Erik Helgren, Frances Hellman The magnetic transport of a conducting Al(2{\%}) doped (Zn,Co)O-field-effect transistor is investigated at low temperature (2-10 K). The Al doped (Zn,Co)O channeling layer ($\sim $ 26 nm-thick) was deposited by magnetron sputtering at 550oC and processed into a 40 $\mu $m thick Hall bar geometry by photolithography and wet etching. An 80nm-thick AlOx layer was deposited at room temperature as the insulating barrier and Cr/Au was used as electrodes. The Hall effect and sheet resistance were measured from 2 - 10 K as a function of temperature, magnetic field and gate electric field. For gate electric field E=0V/cm, the electron concentration is 2.58 x 1014/cm2 at 5 K and there is no anomalous Hall effect. This carrier concentration is experimentally shown (by the Hall effect) to be tuned by $\pm$ 7.0 x 1012/cm2 with E=$\pm$4MV/cm. Application of E=+4 MV/cm induces magnetism in the channel layer as seen by an anomalous Hall effect. These results show that the magnetic properties of (Zn,Co)O with Al-doping can be modulated by gate electric field at low temperature. This research was supported by both DOE and WIN. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V22.00006: Electron mediated ferromagnetism in cobalt doped ZnO Su-Huai Wei, Aron Walsh, Juarez L.F. Da Silva The potential to simultaneously tune both charge and spin in spintronic materials has lead to great interest in searching room temperature dilute ferromagnetic semiconductors. Among them, cobalt doped ZnO has become a focus of attention due to its reported high T$_C$ and reversible cycling of FM ordering. Coupled with existing optical and electrical properties of ZnO, this makes it a potentially technologically essential material. However, the mechanism that leads to ferromagnetism in ZnO:Co is still under debate. Using band structure methods, we show that pure ZnO:Co has a weak preference for antiferromagnetic ordering. Stabilization of ferromagnetism is achieved only after the spin- down Co t$_{2d}$ states are occupied through n-type doping. Our results are compared with available experimental data and results obtained from different levels of theoretical calculations. The limitations of the methods adopted in previous calculations are discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V22.00007: Origin of the co-dopant induced enhancement of ferromagnetism in (Zn,Mn)O Yan Zhu, Juexian Cao, Z.Q. Yang, Ruqian Wu Using the density functional calculations, we elucidate the mechanism of co-dopant induced enhancement of ferromagnetism of (Zn,Mn)O. Li and Cu atoms tends to segregate toward Mn atoms and strongly promote the ferromagnetic coupling via either RKKY or superexchange interaction. The hole states produced by either Li or Cu are rather delocalized and they are efficient in mediating magnetic ordering. These findings shed new light for the design of dilute magnetic semiconductors with co-dopants for spintronic applications. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V22.00008: Synthesis and Magnetic Properties of Cobalt doped ZnO Nanowires Ramakrishna Podila, Jian He, Amar Nath, Apparao Rao Here we report the synthesis of cobalt (Co) doped ZnO nanowires using a chemical vapor deposition technique. About 50 mM aqueous solution of ZnCl$_{2}$ and Co (CH$_{3}$COO)$_{2}$ was injected (rate of 0.1ml/min)into a quartz tube reactor maintained at 550$^{^{\circ}}$C. A constant flow (10:1) of O$_{2}$ and H$_{2}$ was maintained at 500 sccm. The as-prepared nanowires are typically $\sim $1-2 micrometers in length and tens of nanometers in diameter. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were employed to confirm the presence of the Co atoms in the nanowires. Significant ferromagnetism was observed in the 2 atomic {\%} Co-doped ZnO nanowires with a coercive field of 50 Oe. Furthermore, no saturation of magnetic moment was observed up to a field of 5T and 5K. A Curie-Weiss law fit to the temperature dependent magnetic susceptibility data yielded a magnetic moment $\mu $=1.99$\mu _{B}$ for Co$^{2+}$ ion, consistent with the low spin state. Based on Hall measurements and Seebeck coefficient data, the nature of the carriers and origin of magnetism in Co doped ZnO nanowires will be presented. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V22.00009: Search for ferromagnetism in transition metal doped ZnO nano-clusters Indra Dasgupta, Nirmal Ganguli, Biplab Sanyal We present a comprehensive study of the energetics and magnetic interactions in 3d transition metal doped ZnO clusters by ab-initio density functional calculations. We find an important evidence that the charge state of the dopant transition element located at the surface of the cluster is different from that expected in the bulk and has a crucial impact on their magnetic properties. In addition, our calculations clearly reveal defects namely Zn and O vacancies in ZnO in neutral and charged states can induce ferromagnetic interactions between the transition metal atoms whereas anti ferromagnetic coupling dominates in a neutral defect-free cluster. Our results can have significant contributions in the nano-engineering of defects to achieve desired ferromagnetic properties in spintronics applications. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V22.00010: Magnetism of TiO and TiO$_{2}$ Clusters Xiaohui Wei, Ralph Skomski, B. Balamurugan, Z. Sun, D.J. Sellmyer Ferromagnetism in wide-bandgap semiconductors has sparked interest due to its potential applications in spintronic devices$^{1}$. Previous research has focused on doped TiO$_{2}$ thin films, with little work on undoped TiO$_{2}$ and no report on TiO clusters. To investigate the magnetism of small TiO$_{x}$ particles, TiO$_{2}$ and TiO clusters with sizes from 15 to 50 nm were produced by inert gas condensation and examined with TEM, AFM, XRD, and SQUID. Ferromagnetism was found within the investigated temperature range of 10 to 400 K for all clusters including TiO. Interestingly, all clusters exhibit a magnetization enhancement after exposure in air for an extended time, which is probably due to the generation of oxygen vacancies and of Ti$^{3+}$ or Ti$^{2+}$, whose coupled spins may create the observed ferromagnetism$^{2}$. The relationship between cluster structure, size, defects and magnetism will be discussed. - This research is supported by NSF-MRSEC, ONR and NCMN. $^{1}$J. M. D. Coey, Curr. Opin. Solid State Mater. Sci. \textbf{10,} 83 (2006) $^{2}$S. Yoon, J. Phys.: Condens. Matter \textbf{18}, L355 (2006) [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V22.00011: Structural and magnetic characterization of (TM=Co, Fe) doped SnO$_{2 }$ nanostructures A. Parra Palomino, M.S. Rzchowski, O. Perales Perez Recent indications of intrinsic room-temperature (RT) ferromagnetism in transition metal doped-SnO$_{2}$ have increased its attractiveness as promising material for nano-optoelectronic and spintronics-based devices. A control over dopant speciation and the determination of the size-dependence of the properties at the nanoscale, become then indispensable. We present here the conditions for the room-temperature synthesis of doped SnO$_{2}$ in ethanol using SnCl$_{2}$ and LiHO precursors, and the characterization of the resulting bare, Co and Fe- doped SnO$_{2}$ powders. X-ray diffraction patterns of bare and doped SnO$_{2}$ showed the formation of an amorphous structure from the produced powder at RT. However, a pure phase of rutile structure was observed when the samples were annealed in air or Ar at 400$^{\circ}$C. 100nm diameter wires were observed after annealing using scanning electron microscopy (SEM). The results of magnetic characterization of the materials using a vibrating sample magnetometer (VSM) will also be presented and discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V22.00012: Structure and Magnetotransport Properties of High-T$_{C}$ Ferromagnetic Semiconductors, (Ba,Sr)\textit{M}$_{2\pm x}$Ru$_{4\pm x}$O$_{11}$ with \textit{M} = Mn, Fe, Co Larysa Shlyk, Rainer Niewa, Barbara Schuepp-Niewa, Lance De Long We have grown single crystals of R-type ferrites, (Ba,Sr)M$_{2\pm x}$Ru$_{4\pm x}$O$_{11}$ (M = Mn, Fe, Co), with compositions determined from X-ray refinements and microprobe analysis. The hexagonal crystal structure (P6$_{3}$/mmc, Z = 2) consists of (001) layers of edge-sharing (M,Ru)O$_{6}$ octahedra connected within [001] by face-sharing pairs of (M,Ru)$_{2}$O$_{9}$ octahedra and MO$_{5 }$trigonal bipyramids. A significant homogeneity range is generated by variable occupation of octahedral sites by 3d and 4d elements. These compounds are soft ferromagnetic semiconductors with T$_{C}$'s that can substantially exceed room-temperature, depending on composition. The temperature-dependent, in-plane (current parallel to \textbf{ab-}plane) resistivity of SrFe$_{2.51}$Ru$_{3.42}$Al$_{0.07}$O$_{11}$ indicates semiconductivity, and exhibits activated behavior with narrow gap of $\Delta \quad \approx $ 30 meV for T $>$180 K. Hall measurements show the predominant charge carriers are holes; our results suggest these materials are promising for spintronic devices. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V22.00013: Hall effect in magnetic semiconductor InMnSb epitaxial thin films Nikhil Rangaraju, Nidhi Parashar, Bruce Wessels The magnetotransport properties of ferromagnetic $In_{1-x}Mn_{x}Sb$ semiconductor films with x=0.01 to 0.035 were measured from 1.5 K to 298K and magnetic fields up to 18T. The vapor phase epitaxial films are p-type with a hole concentration of $10^{19}~cm^{-3}$ and mobility of $10^{2}~cm^{2}/Vs$. The Hall resistivity is described by the equation $\rho_{xy}=R_{0}B+R_{A}M$ where $R_{0}$ and $R_{A}$ are the normal and anomalous hall coefficients, $B$ is the applied magnetic field and M is the magnetization. The films exhibited an anomalous Hall effect over entire temperature range. It was observed that $R_{A}$ is proportional to the longitudinal resistivity $(\rho_{xx})$ leading to a magnetoresistance dependant Hall voltage. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V22.00014: Magnetotranport properties of magnetic InMnSb semiconductor films. Nidhi Parashar, Nikhil Rangaraju, Bruce Wessels Magnetotransport properties of the magnetic semiconductor In$_{1-x}$M$_{x}$nSb were investigated for temperatures from 1.4 to 300 K and magnetic fields up to 18 T. Films are $p$-type, with carrier concentration $\sim $ 10$^{19}$ cm$^{-3}$, and exhibit anomalous Hall Effect at room temperature. At low temperatures and low fields, negative magnetoresistance of 4 percent was observed, for a film with x = 0.035. For higher fields, a positive magnetoresistance of 9 percent was observed. At 300 K, positive magnetoresistance with hysteretic behavior was observed. The magneto-resistive properties are analyzed with respect to recent models of spin-dependent scattering in magnetic semiconductors. [Preview Abstract] |
Session V23: Focus Session: Extreme Conditions and High Pressure III: Electronic Transitions and Mixtures
Sponsoring Units: DCOMP GSCCMChair: Stephane Mazevet, French Atomic Energy Commission
Room: 325
Thursday, March 19, 2009 8:00AM - 8:12AM |
V23.00001: Phase separation in hydrogen-helium mixtures at high pressure Miguel Morales, Eric Schwegler, David Ceperley, Carlo Pierleoni, Sebastian Hamel, Kyle Caspersen We study the properties of hydrogen-helium mixtures at Mbar pressures and intermediate temperatures (4000 to 10000 K) using first-principles molecular dynamics simulations. Our main goal is to calculate the temperature, as a function of pressure, at which helium becomes insoluble in dense metallic hydrogen. We perform an extensive study of the equation of state of the mixture as a function of density, temperature, and composition and, together with a variety of thermodynamic integration techniques, we calculate the Gibbs free energy of mixing. We will show how to calculate the entropic contribution of the free energy using coupling constant integration methods, which allows us to directly calculate immiscibility temperatures without the need to resort to approximations of the entropy of mixing. These results are relevant to models of the interior structure and evolution of Jovian planets. We find demixing temperatures that are sufficiently high to cross the planetary adiabat of Saturn at pressures around 5 Mbar, implying the existence of partially miscible regions over a significant portion of the interior of the planet. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V23.00002: Molecular dynamics simulations of electron-ion temperature equilibration in an SF6 plasma Lorin X. Benedict, James N. Glosli, David F. Richards, Frederick H. Streitz, Stefan P. Hau-Riege, Frank R. Graziani, Michael S. Murillo, John F. Benage We use classical MD to investigate electron-ion temperature equilibration in a two- temperature SF$_{6}$ plasma. We choose a density of 1.0$\times 10^{19}$ (dissociated) SF$_{6}$ molecules per cm$^{3}$ and initial temperatures of $T_{e}\sim 100$ eV and $T_{S}\sim T_{F}\sim 15$ eV in accordance with experiments currently underway at Los Alamos National Laboratory. Our computed relaxation time is significantly longer than that predicted by the classic theory of Landau and Spitzer. Similar discrepancies are found when comparing to predictions made be more recent theories of electron-ion equilibration. These differences should be large enough to be measured in the upcoming experiments. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V23.00003: Optical signature of the OCP crystallization in a boron plasma Flavien Lambert, St\'ephane Mazevet, Jean Cl\'erouin We have calculated the DC conductivity of a boron plasma along the 1 eV isotherm up to 25 times the normal density. We use both the quantum and orbital free molecular dynamics coupled with, respectively, the Kubo-Greenwood formulation and the semi-classical Ziman theory. We find that the DC conductivity obtained using a full quantum mechanical treatment exhibits a significant jump at the one component plasma phase transition -- specifically the OCP crystallization -- jump that is not reproduced using the semi-classical Ziman description. This difference -- reaching up to a factor of four -- remains well beyond the phase transition and up to the highest density explored. This shows that a full quantum mechanical treatment of the optical and electrical quantities is required in this regime even if semi-classical theories are reliable to obtain both the thermodynamical, and, ionic dynamical and structural properties. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V23.00004: Mixtures in the Warm, Dense Matter Regine Invited Speaker: The bulk of normal matter from planets to the intergalactic medium exists as a composite of various elemental constituents. The interactions among these different species determine the basic properties of such diverse environments. For dilute systems, simple gas laws serve well to describe the mixing. However, once the density and temperature increase, more sophisticated treatments of the electronic component and dynamics become necessary. For the warm, dense matter (WDM) region [10$^{22}$-10$^{25}$ atoms/cm$^3$ and 300K - 10$^6$ K], quantum Monte Carlo and molecular dynamics, utilizing finite-temperature density functional theory (DFT), have served as the basic exploratory tools and benchmarks for other methods. The computational intensity of both methods, especially for mixtures, which require large sample sizes to attain statistical accuracy, has focused considerable attention on mixing prescriptions based on the properties of the pure atomic constituents. Though extensively utilized in many disciplines, these rules have received very little verification [1,2]. We examine the validity of two such rules, density and pressure mixing, for several systems and concentrations by comparing against quantum calculations for the fully-interacting composite. We find considerable differences in some regimes, especially for optical properties. We also probe dynamical properties such as diffusion and viscosity as well as the role of impurities. Finally, as a means of extending DFT results to higher temperature regimes, we also study orbital-free molecular dynamics (OFMD) approaches [3] based on various approximations to the basic density functional. These OFMD schemes permit a smooth transition from the WDM region to simpler one-component plasma and ideal gas models. Research in collaboration with J.D. Kress (LANL), D.A. Horner (LANL), and Flavien Lambert (CEA). \\[4pt] [1] D.A. Horner, J.D. Kress, and L.A. Collins, Phys. Rev. B {\bf{77}}, 064102 (2008).\\[0pt] [2] F. Lambert {\em{et. al.}} Phys. Rev. E {\bf{77}}, 026402 (2008); J. Clerouin {\em{et. al.}} Phys. Rev. B {\bf{76}}, 064204 (2007).\\[0pt] [3] F. Lambert, J. Clerouin, and G. Zerah, Phys. Rev. E {\bf{73}}, 016403 (2006). [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V23.00005: Electrical Conductivity of Synthetic Uranus Sebastien Hamel, Eric Schwegler Mixtures of accreted water, ammonia, and methane at high pressures and temperatures are thought to be major components of the giant planets such as Uranus and Neptune. The pressures and temperatures in their deep interiors can reach several Mbar and several thousands Kelvin, conditions corresponding to the fluid phase. At such extreme interior conditions it is expected that these molecules react chemically to produce a complex mixture. Observables properties such as the magnetic field of these planets are thought to be determined by the physical and chemical properties of matter within this water mixture layer. Using quantum molecular dynamics, we explore the properties of water mixtures at planetary conditions. In particular we discuss the electrical conductivity at high pressure and high temperature of those mixtures in comparison to pure water. 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] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V23.00006: Magnetism and vibrations in the phase $\epsilon$ of Oxygen Tuan Anh Pham, Ralph Gebauer, Sandro Scandolo Sandwiched between a set of magnetic phases at lower pressure, and a non magnetic phase at higher pressure, the magnetic state of phase $\epsilon$ of oxygen has so far been elusive, together with its crystal structure. Neutron diffraction data indicate absence of antiferromagnetism, but do not exclude a ferromagnetic order. The recent refinement of the internal atomic positions from single-crystal diffraction finally provides us with a correct structural model to study the possible occurrence of a magnetic ground state. By employing non-collinear spin-polarized density-functional theory we show that the ground state of $\epsilon$-O$_2$ is non magnetic. We also calculate vibrational spectra and show that $\epsilon$-O$_2$ possesses an additional vibron mode with large Raman cross section, not seen in experiments yet. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V23.00007: Ab-initio calculations of the X-ray absorption spectra of shocked compressed aluminum Stephane Mazevet, Vanina Recoules, Gilles Zerah Molecular dynamics (MD) simulations, using density functional (DF) electronic structure techniques, provide a powerful, predictive tool for examining materials from solids to plasmas over a wide range of densities and temperatures. Using the Kubo-Greenwood formulation, we can access to the frequency-dependent electrical conductivity as well as additional optical properties consistent with the Equation of State. Due to the use of pseudopotentials, the calculations of these properties have, so far, been limited to low photon frequencies, i.e. below 100eV, where only the valence electrons contribute. We recently extended those calculations into the X-ray domain within the PAW formalism. This allows us to describe for the first time, X-ray absorption spectra of shocked compressed aluminum from an ab-initio standpoint. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V23.00008: High-pressure spectroscopic studies on solid germane Xiaojia Chen, Viktor V. Struzhkin, Zhen-Xian Liu, Muhetaer Aihaiti, Yue Meng, Ho-kwang Mao, Russell J. Hemley, Chao Zhang, Rui-Qin Zhang, Yanling Li, Hai-Qing Lin We performed extensive spectroscopic studies on dense germane up to 110 GPa. Pressure -- temperature phase diagram has been established from Raman and infrared spectra. There is no any trace of decomposition of Ge and H$_{2}$ over the pressure range studied. Infrared measurements provide spectroscopic evidence of the metallization of this material at pressure around 16 GPa which is much lower than that observed in sister system - silane. Angle-dispersive powder x-ray diffraction studies reveal that only a structural transition is accompanying when germane enters its metallic state. These experimental observations are examined by \textit{ab initio} calculations. The theoretical results of the electronic, lattice dynamical, and superconducting properties of metallic phase of this material are also presented. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V23.00009: High pressure optical properties of sodium Amy Lazicki, Alexander Goncharov, Viktor Struzhkin, Zhenxian Liu, Eugene Gregoryanz, Christophe Guillaume, Ho-Kwang Mao, Russel Hemley Sodium displays significant complexity at high pressure. The melting temperature drops above a critical pressure, nearly reaching ambient temperature by 120 GPa. In the same pressure regime, phase transitions to low-symmetry and incommensurate structures are observed. Computational studies predict a decrease in the metallic character, and experimental observations have indicated this as well. We present infrared reflectivity for Na through the high pressure incommensurate phase, revealing a significant drop near 120 GPa. First principles calculations of the optical properties are compared to values derived from a Kramer's Kronig analysis of the experimental IR reflectivity, revealing the detailed nature of the pressure-induced deviations from simple metallic behavior. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V23.00010: Influence of Fermi Surface Topology on Superconductivity in High-Pressure Phases of Silane Tian Cui, Xilian Jin Both a semimetallic molecular phase with P2$_{1}$/m symmetry and a metallic atomic phase with P2$_{1}$/c symmetry are found using \textit{ab initio} geometry optimization method from initial configurations of random molecular SiH$_{4}$ and atoms Si and H, respectively. The molecular phase shows a pressure-induced metallization, which can be described quantitatively by Fermi surface (FS) filling constant defined in our work. The lower superconducting critical temperature (\textit{Tc}) about 16.2 K at 175 GPa and its peculiar superconductive behavior that its \textit{Tc} decreases initially and increases later with pressure agree with a recent experimental results. Different electron-phonon coupling mechanisms are uncovered during the increase and decrease of \textit{Tc} with pressure. The atomic phase shows a higher Tc of about 47 K at 190GPa and its \textit{Tc} increases with pressure in its dynamically stable range. The FS filling constant and FS topology transitions under pressure mostly account for the different superconductivity between the molecular and atomic phases. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V23.00011: Structural Properties of Superconducting CaLi$_{2}$ At High Pressures Hahnbidt Rhee, Warren Pickett, Richard Scalettar, William Evans, David Young The hexagonal Laves phase of CaLi$_{2}$, a superconductor at high pressures, has been studied in the diamond anvil cell at varying pressures and temperatures. CaLi$_{2}$ is known to have a maximum superconducting transition temperature of 13 K at 40 GPa. X-ray diffraction measurements were done up to 40 GPa, from room temperature down to 10 K, and phase stability in relation to pressure and temperature has been examined. We present our study to provide more insight into phonon-mediated superconductors and simple-metal systems such as Li.\\[4pt] This work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344 and under Contract DE-FG01-06NA26204. HPCAT use is supported by DOE-BES, DOE-NNSA, NSF, and the W.M. Keck Foundation. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V23.00012: Fe K pre-edge of Fe$_{2}$O$_{3}$ at High Pressure Shibing Wang, Wendy Mao, Yong Cai, Nozomu Hiraoka, Hirofumi Ishii, Yang Ding, Yuming Xiao, Paul Chow, Ho-kwang Mao, Jinfu Shu, Chichang Kao Hematite ($\alpha$-Fe$_{2}$O$_{3}$), as an archetypal 3d transition metal oxide and important earth mineral, undergoes a series of electronic transitions and structural changes at high pressure. At ambient conditions, Fe$_{2}$O$_{3}$ adopts the $\alpha$-Al$_{2}$O$_{3}$ structure and is an antiferromagnetic Mott insulator, with five 3d electrons in the high-spin state. Upon increasing pressure, it transforms from a high-spin state to a low-spin state in the 40-70 GPa range. Here we report experimental results for the Fe K-edge spectra of Fe$_{2}$O$_{3}$ collected in-situ at high pressure using synchrotron x-ray absorption spectroscopy in partial fluorescence yield geometry. The pre-edge features give explicit information about the crystal field splitting energy (CFSE) of octahedrally coordinated Fe$^{3+}$ in Fe$_{2}$O$_{3}$ as a function of pressure, mapping the electronic structure (high-spin to low-spin) transition. The K-$\alpha$ emission spectra at high pressure are also presented. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V23.00013: High Pressure Phase Transitions in FeO from Density Functional Theory, Quantum Monte Carlo and Dynamical Mean Field Theory Luke Shulenburger, Ken Esler, Sergej Savrasov, Jeongnim Kim, R. E. Cohen FeO has a rich behavior under pressure, exhibiting a structural phase transition as well as an insulator-metal transition and a spin collapse. The electronic transitions have been particularly difficult to explain because of the failure of Density Functional Theory (DFT) to capture the electronic state of FeO. We present results from three different methods to better understand the nature of this material. First, from DFT calculations we explore competing explanations for the spin collapse, finding that the increase in bandwidth is at least as important as the crystal field splitting. Additionally, we find that the ligand field effects are responsible for the majority of the change in the local energy levels on the Fe rather than the electrostatic crystal field effect. Secondly, we have performed Dynamical Mean Field Theory (DMFT) calculations. From these we find that the metal insulator transition involves the reorganization of the existing bands and not the appearance of new states at the Fermi level. Finally we test the validity of the approximate results obtained by DFT and DMFT by performing highly accurate Diffusion Monte Carlo calculations. [Preview Abstract] |
Session V24: Focus Session: Nanotube Related Hybrid Structures
Sponsoring Units: DMPChair: Daniel Finkelstadt, Naval Research Laboratory
Room: 326
Thursday, March 19, 2009 8:00AM - 8:36AM |
V24.00001: Broken Optical Symmetry in DNA-SWNT Hybrids: Spectroscopic Signaling of the Helical Wrap Invited Speaker: Functionalizing single-stranded DNA on a single-wall carbon nanotube (SWNT) has allowed isolating individual tubes, making them soluble, and separating SWNTs according to their chirality. Such strong technological impact motivated our study of the optical properties of the DNA-SWNT hybrids, commonly used now for the solution-based fabrication and experiments. The helicity of the DNA wrap may interfere with the intrinsic Hamiltonian of the SWNT and result in bandstructure modulation. Our modeling predicts a symmetry lowering in the hybrid due to the Coulomb potential of the regular helical wrap of the ionized backbone of the ssDNA, followed by the qualitative changes in the cross- or circularly polarized SWNT absorption spectrum (with no or little change in the parallel polarization). In particular, we predict the appearance of a new peak in the cross-polarized absorption of the ssDNA-SWNT at a frequency lower than that of all allowed transitions in the bare tube. Such effect can be used for optical identification of the wrap at sufficient ssDNA coverage. Wrap signaling happens also via another optical effect, a strong circular dichroism even in the complex with an achiral SWNT, and even at the frequencies where ss-DNA has no absorption features at all. Symmetry of the wrap is central to determine such a circular dichroism of the hybrid. Having in mind that the exact geometry of a DNA wrap for an arbitrary tube is not precisely known yet, we put forward a general model capable of tracking optical effects, varying the parameters of the wrap and/or tube diameter. For various ssDNA backbone helical angles and for various tubes we predict different absorption spectra, though a general qualitative feature of the helical symmetry breaking, the appearance of new van Hove singularities and circular dichroism, must be present. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V24.00002: Color Detection Using Chromophore-Nanotube Hybrid Devices Xinjian Zhou, Thomas Zifer, Bryan Wong, Karen Krafcik, Fran\c{c}ois L\'eonard , Andrew Vance In this talk, we will present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. We will also present our all-electron ab initio calculations that are used to study the chromophore-nanotube hybrids and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V24.00003: Spectral Features of Carbon Nanotube Films Changeable With Increasing Thickness John H. Lehman, Katalin Kamar\'as, \'Aron Pekker, Katherine E. Hurst, D.B. Tanner We have investigated an apparent contradiction with respect to optical spectroscopy of carbon nanotube films. The measured absorbance shows ``inverted'' features: local maxima for dilute suspensions correspond to local minima for thick films. The inversion is reconciled by taking into account the saturation of absorption in thick films, when all the light is absorbed in the sample. In this case, the measured direct absorption is (1-R), and independent of thickness. Kramers-Kronig analysis of transmittance data, which provides a means to model absorbance for material ranging from nearly transparent to almost completely opaque, provides values of refractive index for a film of nanotubes. (Borondics et al., PRB 74, 045431 (2006)). From these values, we determined (1- R- T) at many values of the film thickness. This calculation is corroborated with measurements based on the spectral responsivity of a pyroelectric detector and the absolute absorbance of the films. These results are consistent with data on nanotube suspensions by Zhang et. al. (J. Phys. Chem B, 108, 8136 (2004)), representing the limiting case of transparent samples, as well as films by Lehman et. al. (J. Phys. Chem. C, 112, 11776--11778 (2008)) representing the other limiting case of completely opaque samples. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V24.00004: Light emitting peapods: A first-principles study Matus Milko, Tayebeh Movlarooy, Claudia Ambrosch-Draxl Nanohybrids i. e. single wall carbon nanotubes with encapsulated organic molecules have been proposed for opto-electronic devices as they combine the unique mechanical and electronic properties of nanotubes with the desired optical properties of $\pi$-conjugated molecules. We investigate structural, cohesive and optical properties of these hybrid systems using density functional theory. Including the contributions for the non-local dispersive interactions, we examine in a systematic manner the effect of tube diameter and chirality on the stability and bonding characteristics of the peapod as well as the position of the molecule inside the tube. We find that these systems are almost exclusively van-der-Waals bound. Based on a series of oligo-thiophenes encapsulated into zig-zag nanotubes, we explore how the presence of the molecules inside the cavity can alter the electronic and optical properties. In particular, we inspect new features in the dielectric and loss functions due to transitions between the states of the molecule and the nanotube. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V24.00005: Friction and Stability of Carbon Onions Films in Vacuum R.A. Al-Duhaileb, B.W. Jacobs, M.A. Crimp, V.M. Ayres, A. Hirata, M. Horikoshi, M.G.I. Galinato, N. Lehnert Planar graphite is a state-of-the-art low friction solid lubricant. However, in vacuum environments, graphite-based solid lubricants require potentially harmful heavy metal additives to maintain tribological performance. Recent experiments by our group indicate that carbon onions show excellent tribological performance ($\sim $0.03 friction coefficient) without the addition of heavy metal additives. They are therefore good candidates for environmentally benign solid lubricants for use in vacuum. Carbon onion film stability to prevent exposure of un-lubricated surfaces is also an important issue. Tribological performance assessed using ball-on-disk friction measurements equivalent to a 10 m sliding distance indicated that a 15 wt.{\%} solution at 80\r{ }C peroxide oxidation treatment improved the film stability. In the present work, carbon onion friction and film stability are correlated with fundamental structural and chemical properties investigated by high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and multi-wavelength micro-Raman spectroscopy at 77K. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V24.00006: Computational Study on the Structural and Electronic Properties of Various Fullerene Derivatives Sora Park, Jeung Sun Ahn, Young-Kyun Kwon Using $\em{ab~initio}$ density functional theory, we investigate the structural and electronic properties of various fullerene derivates. The equilibrium structures of various additives adsorbed on a fullerene molecule forming fullerene adducts are computed through the geometry relaxation. For a given fullerene adduct, we also calculate the optimum configuration with a different number of additives. In tandem with the structural studies, we calculate the dependence of the HOMO-LUMO gap of each fullerene adduct on the number of additives, and on their relative positions and orientations. Further, using the GW approximation, we also examine the quasiparticle electronic structure of various fullerene derivates. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V24.00007: Diffusion of Various Molecules through Crystalline C$_{60}$ Solid and their Electronic and Vibrational Properties Young-Kyun Kwon First-principles density functional theory is used to study the diffusion of various molecules including diatomic molecules, inert gas molecules , and small metal atoms, and so on, through the C$_{60}$ solid. For each case, the energy surface of a diffusion path is calculated while performing full geometry relaxation of the whole system. Such studies are performed while changing the concentrations of diffusing molecules. The effects of these molecules on the electronic properties of C$_ {60}$ solid are also examined. Especially for diatomic molecules, such as H$_2$, N$_2$, and O$_2$, their frequency shifts are calculated relative to their corresponding counterparts. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V24.00008: Simulation of Fe$_{n}$-doped C$_{60}$ Monolayer on \textit{h}-BN/Ni (111) Lan Li, Hai-Ping Cheng We have performed first-principles calculations based on density functional theory to investigate the structure, electronic structure and magnetic properties of Fe$_{n}$-C$_{60}$ complexes. Interfaces that consist of a C$_{60}$ monolayer, a supporting h-BN/Ni (111) layers, and the transition metal Fe$_{n}$ ($n$ = 1-4 {\&} 15) have been thoroughly characterized. Electron transfer has been observed from the Fe ions to the C$_{60}$ molecules, which leads to the domination of ionic character on the Fe-C$_{60}$ bonding. Furthermore, the Fe$_{n}$-doped C$_{60}$ systems show strong hybridizations between s-, d- orbitals of Fe atoms and $p$-orbital (\textit{$\pi $}-like) of C atoms. The spin of the net transferred electrons from Fe$_{n}$ to C$_{60}$ is spin minority, which leads to a magnetic moment in C$_{60}$ opposite to the total magnetic moment of the system. All of the electronic structure calculations have been performed in generalized gradient approximation (GGA) and local density approximation (LDA). In Fe$_{4}$C$_{60}$ and Fe$_{15}$C$_{60}$ systems, we have also performed GGA+U and LDA+U calculations for comparison. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V24.00009: Superatom states in an endohedral fullerene Tian Huang, Min Feng, Jin Zhao, Hrvoje Petek, Shangfeng Yang, Lothar Dunsch Motivated by the recent discovery of superatom states in C$_{60}$[1], we studied the electronic structures of an endohedral fullerene, Sc$_{3}$N@C$_{80}$, adsorbed on copper surface by LT-STM experiment and DFT calculation. Both experimental and calculated results show that superatom states also exist in Sc$_{3}$N@C$_{80}$. Different from the C$_{60}$, the encapsulated cluster (Sc$_{3}$N-) in Sc$_{3}$N@C$_{80}$ distorts the nearly-spherical potential of the carbon cage, making the atom-like orbitals look asymmetric in the STM images. The adsorbed molecules exhibit various shapes of superatom orbitals due to the different orientation of the Sc$_{3}$N@C$_{80}$ on the surface. When two molecules form a dimer, however, the strong intermolecular hybridization overcomes the perturbation induced by the inside clusters, making all the dimers to have similar H$_{2 }$like molecular orbitals with clear bonding and anti-bonding characteristics. [1] Min Feng, Jin Zhao, Hrvoje Petek Science, 320,359, 2008. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V24.00010: Isotropic Wave Function Delocalization in C$_{60}$ Molecular Assemblies Min Feng, Jin Zhao, Hrvoje Petek Electronic wave function delocalization in a molecular material is highly surprising. Here, we describe a new paradigm of strong intermolecular hybridization of a hollow core-bound molecular state in C$_{60}$ assemblies. In 1D C$_{60}$ wire and 2D C$_{60}$ island, LT-STM revealed extensive, isotropic wave function delocalization at energy above 3.5eV, in contrast with the poor intermolecular wave function overlap of the $\pi $-molecular orbitals. DFT indicates that a new kind of molecular orbital, which is derived from the central potential of the hollow cage shape of C$_{60}$, is responsible for this NFE like wave function delocalization. This central potential derived from the screening interaction and gives rise to s, p, d, etc., symmetry atom-like orbitals, which we dub the superatom molecular orbitals (SAMOs). Studies show how these atomlike orbitals hybridize into H$_{2}$ molecule-like $\sigma $ and $\pi $ symmetry bonding/antibonding orbitals of C$_{60}$ dimmers, and for larger aggregates, with alkali atom-like NFE dispersions. As a common consequence of a hollow topology, we expect that similar SAMO states will exist in other molecules derived by wrapping and rolling molecular sheets into hollow cages and nanotubes. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V24.00011: First-principles calculation of electronic structures of new C$_{60}$ polymers Taichi Kosugi, Shinji Tsuneyuki C$_{60}$ fullerene molecules form fcc crystalline structure at an ambient pressure and temperature. It has been both theoretically and experimentally confirmed that this structure undergoes phase transitions into various structures at high temperatures and high pressures. Yamanaka et al. experimentally found that the individual C60 molecules in fcc structure, which are weakly bonded via van der Waals interactions, are connected to its neighbouring molecules under high temperature and high pressure, leading to the two-dimensional layered insulating rhombohedral polymer 2D-r, which is further polymerized under higher pressure and temperature into the three-dimensional polymer 3D-r. We searched for a new rhombohedral structure of C$_{60}$ polymer using ab initio calculations of the electronic structures and compared it with the experimentally observed data. In addition we quantitatively analyzed how the differenece between the chemical bondings in these structures affect their energetics. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V24.00012: Role of OH Adsorption on the Properties of MRI contrast agent Gd$_{3}$N@C$_{80}$ Vince Ong, Shiv Khanna, Panos Fatouros Endohedral metallofullerenes Gd$_{3}$N@C$_{80}$ decorated with hydroxyl groups are now known to be excellent contrast enhancement agents for Magnetic Resonance Imaging (MRI) leading to strong relaxivity enhancements. One of the outstanding issues is the nature of OH adsorption and its effect on the properties of endohedral Gd$_{3}$N motif. We have carried out theoretical studies on the electronic structure and magnetic properties of the endohedral metallofullerenes functionalized with hydroxyl groups to demonstrate that the nature of OH can have significant effect on the magnetic spin density. The new findings may provide physical insight into the observed strong relaxivity enhancements. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V24.00013: Imaging and Spectroscopy of Diamondoid-Fullerene Hybrid Molecules J.C. Randel, H.C. Manoharan Diamondoids have attracted attention as potential building blocks for nanometer-scale electronic and mechanical devices. The ability to functionalize diamondoids with various atomic and molecular groups enables customizable chemistry, as well as tunable electronic properties. Recently, the library of realizable functional groups has expanded beyond a few atoms, and now includes C$_{60}$ fullerenes. This addition provides a novel opportunity to study a material that combines the sp$^{2}$ and sp$^{3}$ forms of carbon bonding in one hybrid molecule. We investigate these molecules using scanning tunneling microscopy and spectroscopy. We find that thin films of the molecules pack in a well-ordered lattice on Au(111), and report on spectral measurements with single-molecule resolution. We comment on the connection between strong features in these electronic structure measurements and the nature of electron transport through single hybrid molecules. [Preview Abstract] |
Session V25: Focus Session: Graphene XIII: Spectroscopic and Transport Properties
Sponsoring Units: DMPChair: Shaffique Adam, University of Maryland
Room: 327
Thursday, March 19, 2009 8:00AM - 8:12AM |
V25.00001: Charged Impurity Scattering in Graphene induced by adsorption of calcium Masahiro Ishigami, Jyoti Katoch We have measured the impact of charged impurity scattering induced by adsorbed calcium atoms on the transport properties of graphene sheets. We vary the density of adsorbed atoms on the surface of graphene based-devices which are otherwise devoid of any surface adsorbates in ultra high vacuum environment. We will discuss the impact of calcium atoms on the charge carrier mobility, gate-dependent conductivity and minimum conductivity in comparison with earlier measurements performed using potassium. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V25.00002: Current-Phase Measurements in Single Layer Graphene Cesar Chialvo, Ion Moraru, Daniel Bahr, Nadya Mason, Dale Van Harlingen The current-phase relationship (CPR) of a Josephson junction can provide key information about the microscopic processes that make up a supercurrent. However, CPR has not been previously measured in graphene. We have successfully fabricated a variety of Josephson junctions containing single-layer graphene as a weak link, and with different junction width to length ratios. We present results of measurements based on a phase-sensitive SQUID technique, where we determine the supercurrent amplitude and phase, as well as a possibly anomalous shape of the CPR. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 9:00AM |
V25.00003: Landau level spectroscopy of Dirac fermions in multilayer epitaxial graphene and graphite Invited Speaker: The results of magneto-absorption studies of epitaxial multilayer graphene on SiC and of graphite will be presented. The talk will be focused on inter Landau level transitions characteristic of electronic states with Dirac-like dispersion relations which are distinctive of graphene but persist in multilayer epitaxial graphene and are also present at the H-point of bulk graphite. The high energy limits of the Dirac-cone in epitaxial graphene will be discussed from experiments carried out in the near-infrared spectral range. Probing the nearest vicinity of the Dirac point with far-infrared light will be testified. An exceptional quality of essentially neutral multilayers of graphene on SiC (low temperature carrier mobilities $\sim $250 000cm\^{}2/Vs) will be discussed from cyclotron resonance absorption resonance. This resonance will be shown to persist up to room temperature with negligible changes of the width what indicates no relevant thermally activated scattering process in this material. M.L. Sadowski, et al, Phys. Rev. Lett. 97, 266405 (2006), P. Plochocka et al., ibid., 100, 087401 (2008), M. Orlita et al., ibid., 100, 136403 (2008), M. Orlita et al., ibid, accepted, (arXiv:0808.3662). [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V25.00004: Charged Impurity Scattering in Bilayer Graphene Shudong Xiao, Jianhao Chen, Ellen D. Williams, Michael S. Fuhrer Materials Research Science and Engineering Center and Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, MD, 20742, USA We have examined the impact of charged impurity scattering on the charge carrier transport in bilayer graphene in ultra-high vacuum (UHV) at low temperatures. Bilayer graphene sheets are mechanically exfoliated on Si/SiO$_{2}$ substrates, and the number of layers is verified by micro-Raman spectroscopy. Charged impurity density is varied over a wide range (up to 2x10$^{13}$ cm$^{-2})$ by deposition of potassium atoms on clean bilayer graphene in UHV. At a gate-induced charge carrier density of 4.3 x 10$^{12}$ cm$^{-2}$, the mobility is inversely proportional to the charged impurity density $\mu $ = 5x10$^{15}$ V$^{-1}$s$^{-1}$/$n_{imp}$. Surprisingly, the coefficient relating $\mu $ to 1/$n_{imp}$ has a similar magnitude to that for single-layer graphene, indicating a similar strength for charged impurity scattering at this carrier density. The magnitude of charged impurity scattering, as well as the implications for the source of disorder in undoped bilayer graphene, will be discussed in the context of Boltzmann transport theory. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V25.00005: Band structure asymmetry of bilayer graphene revealed by infrared spectroscopy Zhiqiang Li, Erik Henriksen, Zhigang Jiang, Zhao Hao, Matt Zhang, Michael Fogler, Michael Martin, Philip Kim, Horst Stormer, Dimitri Basov We report on infrared spectroscopy of bilayer graphene integrated in gated structures. The dominant feature of the optical conductivity is a resonance peak due to interband transitions between the two conduction bands or two valence bands. Both the frequency and the voltage dependence of the peak show a significant asymmetry upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 10:00AM |
V25.00006: On the dynamical and dc conductivity of graphene Invited Speaker: It was only recently shown that a simply-observable quantity as the optical transparency of suspended graphene is defined solely by the fine structure constant [1]. In this talk, I will give the theoretical explanation to this experiment, i.e., show that even in the visible-optics regime the corrections to the Dirac cone approximation are small (a few percent) and the effect of next-nearest neighbor hopping is negligible [2]. I will also discuss the infrared conductivity of graphene on a substrate where electron-phonon and impurity scattering become important [3]. Finally, I will comment on the still unsettled question of dc conductivity in graphene and discuss - apart from Coulomb scattering - an alternative scattering mechanism based on midgap states [4]. \\[4pt] [1] R. R. Nair et al., Science 320, 1308 (2008).\\[0pt] [2] T. Stauber et al., Phys. Rev. B 78, 085432 (2008).\\[0pt] [3] T. Stauber et al., Phys. Rev. B 78, 085418 (2008).\\[0pt] [4] T. Stauber et al., Phys. Rev. B 76, 205423 (2007). [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V25.00007: Thermoelectric and Magnetothermoelectric Transport Measurements of Graphene Yuri M. Zuev, Willy Chang, Philip Kim We investigated the electronic, thermoelectric, and magnetothermoelectric transport properties of graphene as a function of temperature and carrier density. Microfabricated heater and thermometer electrodes were used to simultaneously measure conductance and thermoelectric power (TEP) of graphene in the temperature range of 4-300K. Graphene exhibits both positive and negative values of TEP, with a peak value on the order of kB/e, when the Fermi energy is below and above the charge neutrality point, respectively. A quantitative comparison of the conductance and TEP can be made using the semiclassical Mott relation. We observed an excellent quantitative agreement between the measured TEP and the Mott relation based on the mesoscopic two terminal conductance in the low temperature regime (T $<$ 30K). At higher temperatures, the Mott relation employing the local conductivity is necessary. Upon applying magnetic field, the magneto-thermopower exhibits characteristic oscillations in accordance with the Shubnikov-de Hass oscillations in conductance. In the quantum Hall regime at high B field, we observed the quantizing transverse and longitudinal thermpower components which are also in good agreement with the generalized Mott relation, except near the charge neutral Dirac point. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V25.00008: Piezoresistivity of graphene-based thin films Rahul Raveendran Nair, K.S. Navoselov, Da Jiang, Soren Neubeck, Leonid Ponomarenko, A.K. Geim Large-scale production of graphene films is of particular interest because of graphene's extraordinary electronic, mechanical and optical properties. We report the properties of graphene films produced by spraying or spinning of a graphene suspension obtained by ultrasound cleavage of graphite in organic solvents, the route that does not involve graphene oxide [1]. Wafer-scale uniform films of overlapping submicron graphene crystallites were made on transparent substrates and exhibited sheet resistivity of a few k$\Omega $ with more than 80{\%} transmission with respect to white light. Electric measurements and Raman studies suggest that the films are p-doped. In particular, we have investigated the piezoresistive effect in such films by depositing them on flexible plastic substrates. Fully reversible changes in the resistance were observed as a function of strain that could exceed 8{\%} before the films started loosing their continuity. The piezoresistive gauge factor was up to $\sim $30 for our films. [1] Peter Blake \textit{et al.}, Nano Lett. 2008, 8,1704-1708. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V25.00009: Graphite in the bi-layer regime: in-plane transport Hridis Pal, Sefaattin Tongay, Dmitri Gutman, Dmitrii Maslov, Arthur Hebard The dependence of in-plane resistivity of HOPG graphite on temperature is studied both experimentally and theoretically over a wide range of temperatures(up to $\sim$900 K). For temperatures larger than the next-to-nearest-plane coupling which gives rise to an overlap of the valence and conduction bands, but still below the nearest-plane coupling, graphite can be viewed as a stack of bilayers. In this regime, the in-plane conductivity $\sigma$ is supposed to scale as $T\tau$, where $\tau$ is the scattering time. For conventional electron-phonon scattering, $\tau\propto 1/T$ and $\sigma$ is supposed to saturate at higher $T$. However, we observe experimentally that $\sigma$ decreases monotonically without any sign of saturation up to the highest temperature measured. We propose two additional scattering mechanisms which lead to a decrease of $\sigma$: intervalley scattering by phonons and multiple intravalley scattering by phonons due to anharmonicity of a layered lattice at high temperatures. A reasonable agreement between theory and experiment is obtained by using this model. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V25.00010: Tuning the Infrared Absorption of a Bilayer Graphene Field-Effect Transistor Chun Hung Lui, Kin Fai Mak, Matthew Sfeir, James Misewich, Tony Heinz Bilayers of graphene have attracted intense interest because of the possibility of tuning of their band gap by the application of a perpendicular electric field [Taisuke Ohta et al. Science 313, 951 (2006)]. Indeed, such gate electric fields induce both 1) tuning of the chemical potential and 2) modification of the bilayer electronic structure by the development of potential difference across the two layers. These effects have significant consequences for the infrared absorption, which probes the interband transitions, of bilayer samples. We have examined these issues by measuring the evolution of the optical conductivity (for photon energies of 0.2 - 0.8 eV) of graphene bilayer field-effect transistors constructed with a transparent top gate. The infrared absorption shows a significant and reproducible variation with gate voltage. The behavior for positive and negative gate voltages reveals an electron/hole asymmetry, reflecting corresponding differences in the band structure. The role of the development of a band gap in these structures and the effect of electrostatic screening will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V25.00011: Local Transport Measurements on Graphene Using Scanning Tunneling Potentiometry Weigang Wang, Ko Munakata, Michael Rozler, Francoise Kidwingira, Malcolm Beasley Scanning tunneling potentiometry (STP) is a local transport measurement that was demonstrated some time ago, but has only recently been developed in a generally useful form. Near equilibrium, STP measures the electrochemical potential along a sample surface with near nanometer spatial resolution. With our newly developed STP system, we report preliminary results on few-layer graphene at room temperature and 4.2K. Room temperature STP data show a constant drift in the electric properties. At low temperature, however, our data show no such drift. Possible evidence for Landauer resistivity dipoles will be presented. Work supported by the AFOSR. [Preview Abstract] |
Session V26: Focus Session: Graphene XIV: Magnetism and Bilayers
Sponsoring Units: DMPChair: Bruno Uchoa, University of Illinois at Urbana-Champaign
Room: 328
Thursday, March 19, 2009 8:00AM - 8:12AM |
V26.00001: Defect-induced magnetism in graphene nanoflakes E. Martinez-Guerra, M.E. Cifuentas-Quintal, R. de Coss The interaction between electron spin and the magnetic moments of vacancies in graphene could open new opportunities for spintronic and quantum computation. In that direction, we have studied the magnetic properties of graphene nanoflakes (C$_{6n2}$H$_{6n})$ with vacancies within the framework of density functional theory, using the pseudopotential LCAO method with a Generalized Gradient Approximation (GGA) for the exchange-correlation energy functional. In particular, we have calculated the magnetic moment of graphene nanoflakes of different diameters with a simple vacancy. We have found that the total spin-polarization of the graphene nanoflakes with a simple vacancy decreases as the diameter increases. In particular, we show that the vacancy induces the appereance of a midgap state at Fermi level. Thus, the spin degeneracy is broken, being only one of the spin channels of the midgap state occupied, the other being empty. This feature could be exploited for future spintronic applications. This research was supported by Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grant No. 83604. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V26.00002: Magnetism in Nanopatterned Graphene and Graphite Film Li Chen, Decai Yu, Elisabeth Lupton, Feng Liu We report first-principles calculations of magnetic properties of nanopatterned graphene-based nanostructures (GBNs) and nanopatterned graphite films (NPGFs). We introduce a simple geometric rule to design several novel magnetic GBNs: 0D FM nanodots with the highest possible magnetic moments, 1D FM nanoribbons, and 2D magnetic superlattices, whose predicted ground-sate magnetic ordering is confirmed by first-principles calculations. Furthermore, we show that nanopatterned graphite films (NPGFs) can exhibit magnetism similar to GBNs. In particular, graphite films with patterned nanoscale triangular holes and channels with zigzag edges all have ferromagnetic ground states. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V26.00003: Tunable band structure in double gated trilayer graphene Monica Craciun, Saverio Russo, Michihisa Yamamoto, Jeroen Oostinga, Alberto Morpurgo, Seigo Tarucha Graphene based materials are promising candidates for nano electronic applications. It is currently unclear which layer thickness is better suited for a given application, as only the properties of monolayers and bilayers have been investigated systematically. For the optimization of future devices, it is important to understand how the electronic properties of graphene based materials evolve from Dirac particles, in monolayer, to massive particles in bulk graphite. We experimentally address this question by investigating trilayer graphene, the thinnest few layer graphene system in which all the parameters determining the band structure of graphite are first found. Contrary to monolayer and bilayer (which are both zero gap semiconductors), we find that trilayer is a semimetal with a finite overlap of conduction and valence bands. We show that the low energy band structure of trilayer graphene can be tuned by a large amount by means of an external perpendicular electric field, achieving 100{\%} change in band overlap a property not known to occur in any other semimetal. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V26.00004: Can Carbon Be Ferromagnetic? H. Ohldag, T. Tyliszczak, R. {H\"{o}hne}, P. Esquinazi, D. Speman, M. Ungureneau, T. Butz While conventional wisdom says that magnetic materials have to contain some metallic atoms, the possibility of intrinsic magnetic order in pure metal free carbon materials is of fundamental interest because of the role of carbon as an elemental building block of organic as well as inorganic matter and last but not least of the tremendous interest in the electronic properties of graphene based structures. The common controversy raised across all disciplines in this matter is whether the magnetism of carbon is intrinsic or induced by other elements. We address this controversy by providing clear experimental evidence that metal free carbon can be ferromagnetic at room temperature using dichroism x-ray absorption spectro-microscopy. For this purpose we acquired soft x-ray microscopy images of magnetic structures on a thin carbon film that have been produced by irradiation with a focused 2.25MeV proton beam. Our element specific magnetic probe shows no indication of magnetically ordered Fe, Co or Ni impurities in these samples. Combination of the microscopy data with element specific spectroscopy and hysteresis measurements shows furthermore that only the carbon $\pi$-electronic states contribute to the long range ferromagnetic order of the sample. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V26.00005: Low-frequency magnetooptical spectra of bilayer Bernal graphene Yen-Hung Ho, Y. H. Chiu, M. F. Lin The low-frequency magnetoabsorption spectra of bilayer Bernal graphene are investigated within the gradient approximation. The interlayer interactions significantly alter the Landau- level energies, state wave function, and thus enrich the ptical excitation spectra. There exist four kinds of absorption peaks, mainly owing to the optical transitions between two groups of Landau levels with valence and conduction states. The number, intensity, and frequency of absorption peaks strongly depend on the field strength. Such features quite differ from those of monolayers. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V26.00006: Tunneling Measurements in Single- and Multi-layer Graphene Yung-Fu Chen, Travis Dirks, Cesar Chialvo, Nadya Mason We have fabricated novel single- and multi-layer graphene devices with both normal metal ohmic contacts and superconducting tunnel probes. The superconducting gap of the tunnel probes in the graphene devices is well formed at 250 mK, showing that the probes are good for tunneling spectroscopy studies of graphene electronic structure. We have observed oscillations as a function of bias and gate voltages, possibly due to electron phase interference among the layers and/or tunnel probe interfaces. Those oscillations die out in the presence of magnetic fields. Unexpectedly, two distinct and symmetric peaks, which have weak dependence on gate voltages, exist in the superconducting gap. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V26.00007: Resistance noise in electrically biased bilayer graphene Atindra Nath Pal, Arindam Ghosh The growing interest in bilayer graphene (BLG) is fueled by the ability to control the energy gap between its valence and conduction bands through external means. Here, we demonstrate experimentally that the low-frequency resistance fluctuations, or noise, in bilayer graphene is strongly connected to its band structure, and displays a minimum when the gap between the conduction and valence band is zero. Using double-gated bilayer graphene devices we have tuned the zero gap and charge neutrality points independently, which offers a unique mechanism to investigate the low-energy band structure, charge localization and screening properties of bilayer graphene. We show: (1) the noise to be minimum when band gap ($\Delta _{g})$ = 0 even if it corresponds to a nonzero carrier density (n), (2) the evidence of localized states near the band tails even at $\Delta _{g}$ = 0, with a mobility edge that strongly depends on the external electric field E, and finally, (3) a method to directly determine the dielectric properties of BLG in both electron and hole-doped regimes. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V26.00008: Ab-initio study of doped bilayer graphene Paola Gava, Michele Lazzeri, A. Marco Saitta, Francesco Mauri The recent discovery that the application of an external electric field induces a band gap opening in bilayer graphene [1], attracted a lot of interest on this system, due to important applications in nanoelectronics. By means of ab-initio calculations, we investigated the electronic properties of doped bilayer graphene, in presence of different bottom and top gate. In particular, the dependence of the band gap on the doping, on the average external electric field and temperature has been analysed. We find that our ab-initio results differ with respect to those obtained with standard Tigth Binding (TB) calculations [2]. In particular, we show important charge effects, which are crucial for the description of the electronic properties of bilayer graphene, and which are not included in TB models. Moreover, we compare our results with experimental measurements of the band gap, cyclotron mass and work function. [1] Ohta et al., Science v.313 , 951 (2006). [2] Castro Neto et al., Pys. Rev. Lett. v.99, 216802 (2007). [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V26.00009: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V26.00010: Vortices, zero modes and fractionalization in bilayer-graphene exciton condensate Babak Seradjeh, Heidrun Weber, Marcel Franz A real-space lattice formulation is given for the recently discussed exciton condensate in a symmetrically biased graphene bilayer. We show that in the continuum limit an oddly-quantized vortex in the condensate binds exactly one zero mode per valley index of the bilayer. In the full lattice model the zero modes are split slightly due to intervalley mixing. We support these results by an exact numerical diagonalization of the lattice Hamiltonian. We also discuss the effect of the zero modes on the charge content of these vortices and deduce some of their interesting properties, including their fractional exchange statistics. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V26.00011: Ultrafast electroic-state dynamics of graphite probed by time-resolved photoemission spectroscopy Y. Ishida, T. Togashi, K. Yamamoto, M. Tanaka, T. Taniuchi, T. Kiss, A. Shimoyamada, K. Ishizaka, M. Nakajima, T. Suemoto, S. Shin Time-resolved photoemission spectroscopy (250-kHz repetition of 1.5-eV pump and 5.9 eV probe pulses with durations of $\sim $170 fs) using a hemisphereical electron-energy analyzer (VG Scienta SES2002) is employed to investigate the ultrafast electronic-state dynamics of highly oriented pyrolytic graphite. We directly observe electrons excited to 0.75 eV above the Fermi level within 0.1 ps after the pump, reflecting the conical dispersion of graphite about the Fermi level [1]. The excited state decays over $\sim $20 ps with at least two time scales. The longer time scale shows little pump-power dependence, indicating that the decay is independent of the excitation population. We also find a peculiar increase of the spectral weight at the Fermi level throughout the transient state, which can be modeled by a dynamical broadening of the electronic states due to hot optical phonons generated by the pump [2]. [1] S.Y. Zhou \textit{et al.}, \textit{Nature Phys.} \textbf{2}, 595 (2006). [2] T. Krampfrath \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{95}, 187403 (2005); D. Sun \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{101}, 157402 (2008). [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V26.00012: Plasmon Dispersion and Damping in Electrically-isolated Two-dimensional Charge Sheets Yu Liu, Roy Willis Using high resolution reflection electron-energy-loss- spectroscopy (HREELS), we compare experimental results for the wavevector-dependent behavior of plasmons in a graphene sheet on SiC(0001) with that due to a filled band of surface states on semiconducting silicon. There are significant differences in behavior between the two systems, and the behavior predicted for a classical two-dimensional sheet of electrons. In particular, the damping increases with wavevector independent of any obvious inelastic scattering channel. The results illustrate the importance of finite-momentum, non-local potential effects for the dynamical behavior of electrically- isolated charge sheets.\footnote{Y. Liu, et al. Phys. Rev. B 78, 201403(R)(2008)} [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V26.00013: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V26.00014: Edge Phonons of Graphene from Tight-Binding. Daniel Finkenstadt, N. Bernstein, D. Gunlycke, M.J. Mehl Edge-states in graphene can affect the band-gap and carrier group velocities in narrow ( $<$ 5 nm) graphene nanoribbons. As a first, tight-binding approximation from simple nearest-neighbor hopping, it is shown that armchair nanoribbons have large band-gaps compared to zigzag nanoribbons, which are metallic, unless certain crucial effects are included in the calculation, e.g. magnetic- , quasiparticle- , charge-self-consistent- and/or relaxation- based degeneracy lifting. All of these effects open a small band gap, and the interplay between relaxation and electronic-structure may be examined by calculating the edge phonons of graphene. To this end, we expand on our previous, all-neighbor tight-binding Hamiltonian [\textit{Phys. Rev. B} \textbf{76}, 121405(R) (2007)] and include charge self-consistency at the edge of a zigzag nanoribbon. By allowing charge transfer and structural-relaxation at zigzag edges, we are able to remove imaginary phonons and verify the opening of a small band-gap in zigzag ribbons, which is characterized by the phonon density-of-states and normal modes of carbon-hydrogen edge bonds. These calculations are relevant to ribbons cut along non-ideal directions, as well, and we will discuss edge-disorder. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V26.00015: Optical Conductivity and quasiparticle properties of Bilayer graphene Yafis Barlas, Kun Yang The low energy properties of Bernal stacked bilayer graphene can be adequately described by chiral quasiparticles exhibiting a Berry phase of $2 \pi$ with a parabolic dispersion. When the Fermi energy lies at the neutrality point the Fermi surface consists of a pair of points where dominant inter-band excitations determine the effect on electronic correlations. The particle-hole continuum due to the inter-band excitations is given by $\Omega > q^2/(4m)$ in frequency-momentum space. The full wavevector and frequency dependent polarization bubble and optical conductivity is calculated within the RPA. We also calculate the quasiparticle properties for short-ranged interactions and comment on the breakdown of Fermi liquid theory. [Preview Abstract] |
Session V27: Focus Session: Emerging Scanning Probe Microscopy Methods for Biological Applications
Sponsoring Units: GIMSChair: Igor Sokolov, Clarkson University, Sergei Kalinin, Oak Ridge National Laboratory
Room: 329
Thursday, March 19, 2009 8:00AM - 8:36AM |
V27.00001: Kinetic parameters of association and dissociation between single molecules measured by single-molecule force spectroscopy Invited Speaker: This presentation is focused on development of experimental scanning probe microscopy (SPM) approaches to quantify kinetic parameters of association and dissociation between receptor--ligand pairs. The potential of mean force (pmf) between interacting molecules is quantified by single molecule force spectroscopy (SMFS) approach. In SMFS molecules are allowed to interact and form molecular bond. Consequent measurements of rupture forces are used to characterize the attractive part of the pmf by extracting the distance from the equilibrium to the transition state, the rate of dissociation at no force and the activation energy. Factors affecting accuracy of the measured kinetic parameters are discussed including effects of the polymeric tether stiffening and possible contribution of non-single molecule events to the statistics of rupture forces. The developed SMFS method accounts for pertinent systematic errors and is tested using specific biotin-streptavidin interactions. The measured kinetic parameters show quantitative agreement with theoretical predictions. In addition, a new single-molecule approach to measure the activation energy of association is proposed. This approach uses the dependence of the probability to form molecular bonds on probe velocity when one of the interacting molecules is tethered by a flexible polymeric linker to the AFM probe. The application of the developed method to study interactions between biomolecules is demonstrated with measurements of the activation energy of biotin-streptavidin association. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V27.00002: Coaxial and Triaxial Atomic Force Microscope Probes for Nanoassembly Keith A. Brown, Jonathan Aguilar, R.M. Westervelt We present a technique for the controlled three dimensional assembly of nanoscale objects using a modified atomic force microscope (AFM) probe. A conducting AFM probe is coated with alternating insulating and metal layers then etched at the tip to expose coaxial electrodes. The fabrication allows freedom to specify the size of the tip and therefore the length scale of objects to manipulate. An RF voltage is applied to the electrodes to trap objects at the tip with dielectrophoresis (DEP). The object may be released when it has been moved to the desired location by turning off the field. We present a two electrode coaxial configuration capable of positive DEP and a three electrode triaxial configuration for negative DEP which holds the trapped object away from the tip to overcome the ''sticky finger'' problem. The integration of three dimensional assembly with the nanometer precision and force-imaging capability of an AFM creates a platform for imaging and constructing structures at the nanoscale. We describe initial experiments and fabrication. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V27.00003: Protein bond rupture measured by AFM and the energy landscape problem Peter Hoffmann, Essa Mayyas, Lindsay Runyan The measurement of protein interaction provides an intriguing opportunity for Atomic Force Microscopy (AFM)-based force measurements. The AFM has the advantage that it is relatively easy to use and widely available. However, the interpretation of the force data is lagging behind the experimental capabilities of the technique. In this talk, I will present some recent results of rupture force measurements between two proteins, and discuss our efforts to interpret the resulting data in terms of the underlying energy landscape. We performed measurements on matrix metalloproteases and their natural inhibitors at pulling speeds ranging over 3 orders of magnitude (30-48000 nm/s). However, we found that commonly used theory to interpret such data is inadequate and does not capture the physics of the problem. Consequently, data analysis based on such theories leads to highly erroneous results. We will discuss our attempts to improve the theory and present parameters extracted from the data that reflect the underlying energy landscape of the studied protein-protein interaction. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V27.00004: AFM method to detect differences in adhesion of silica bids to cancer and normal epithelial cells Igor Sokolov, Swaminathan Iyer, Ravi Gaikwad, Craig Woodworth To date, the methods of detection of cancer cells have been mostly based on traditional techniques used in biology, such as visual identification of malignant changes, cell growth analysis, specific ligand-receptor labeling, or genetic tests. Despite being well developed, these methods are either insufficiently accurate or require a lengthy complicated analysis. A search for alternative methods for the detection of cancer cells may be a fruitful approach. Here we describe an AFM study that may result in a new method for detection of cancer cells in vitro. Here we use atomic force microscopy (AFM) to study adhesion of single silica beads to malignant and normal cells cultured from human cervix. We found that adhesion depends on the time of contact, and can be statistically different for malignant and normal cells. Using these data, one could develop an optical method of cancer detection based on adhesion of various silica beads. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V27.00005: Rapid Cellular Identification by Dynamic Electromechanical Response Vladimir Reukov, Maxim Nikiforov, Alexei Vertegel, Gary Thompson, Stephen Jesse, Sergei Kalinin Quick and reliable identification of individual prokaryotic organisms and cellular types is of utmost importance for various applications. A number of strategies for cellular identification are currently used to meet this challenge. All of the existing techniques require culturing bacteria prior to measurement, which increases the time needed for experimentation and analysis considerably. Here, we report on a method for rapid cellular identification and mapping using the detection of broadband electromechanical response. Electromechanical spectra from \textit{M. Lysodeikticus} and \textit{P. Fluorescens} deposited on PLL-coated mica were collected over wide frequency range. Principal component analysis of the spectra bundled with neural network analysis provides a robust algorithm for identification of the cellular organisms based on their electromechanical properties. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 10:00AM |
V27.00006: Simultaneous Nanomechanical and 3d Optical Microscopy: Cellular Distortions and Structural Dynamics Invited Speaker: Combined SPM and Optical systems are increasingly used to study biological structures, including living cells. Here, an AFM is employed to expose cells to foreign bodies and biochemicals, and to measure the resulting attractive and repulsive forces exerted by the cell. 3d optical fluorescence measurements are simultaneously performed revealing distortions and/or restructuring of the cell, membrane, actin cystoskeleton, etc. The work focuses primarily on MH-S cells (mouse lung macrophages) transfected with GAP-43 GFP to identify cell membranes and/or mCherry Actin to identify cytoskeleton dynamics. During standard AFM, optical-cross sections reveal drastic cell distortions up to 50 percent. The viscoelastic response of the cells to nanoNewton induced forces by Silica and Polystyrene beads is also quantified via Structural Recovery After Probing (STrAP), which monitors the rate of cellular recovery following nanoindentation. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V27.00007: Spectral Oscillations in Backscattering of Light from a Biological Cell Alexander Heifetz, Alexander Patashinski, Vadim Backman Based on general electrodynamics principles, we provide an explanation for spectral oscillations in the intensity of visible light elastic backscattering from a live epithelial biological cell. We suggest that the source of spectral oscillations in backscattering from a cell is the nucleus, which is a spheroidal particle several times larger then the incident wavelength, and has a sharp boundary. Because of the small optical contrast of the nucleus relative to surrounding cytoplasm, contribution of single scattering to the overall signal is comparable to that of multiple scattering. We show that the high frequency spectral oscillations in backscattering are due to single scattering, which can be obtained in the first Born approximation. Multiple scattering effects result in slow envelope spectral oscillations. We expand the Mie backscattering cross-section of a uniform sphere in power series to show that the equivalence between the first order Mie backscattering and first order in Born series. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V27.00008: Single-molecule detection of near-infrared phthalocyanine dyes You Li, Brian Canfield, Lloyd Davis The major advantage associated with near-IR monitoring is the fact that few compounds show intrinsic fluorescence in this region of the spectrum. Phthalocyanine dyes provide excellent photostability and hence are an attractive candidate for fluorescence bioassay applications. However, because of their small Stokes shifts, non-standard methods are needed for separation of fluorescence from scattered laser light. We have developed a custom confocal microscope that uses a low-cost laser diode operating at 665.8 nm for sample excitation and an angle-tuned Raman notch filter to block scattered laser radiation and provide high-throughput of fluorescence. Also, a diffraction grating is used to isolate the laser excitation wavelength from the block broadband luminescence of the laser. We have used the system to observe photon bursts from single molecules of zinc phthalocyanine fluorophores in an ethanol solution. The autocorrelation function of the photon trace provides a measure of the signal-to-noise ratio. We also discuss ongoing experiments to characterize the limits of detection of near-infrared fluorophores in aqueous solution using the microscope. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V27.00009: Chemical Recognition Tunneling via Hydrogen Bond Jin He, Shuai Chang, Lisha Lin, Shuo Huang, Ashley Kibel, Myeong Lee, Peiming Zhang, Otto Sankey, Stuart Lindsay Hydrogen bonds enhance electron tunneling rates over vacuum tunneling as well as making chemically selective attachments to target molecules when patterns of donors and acceptors match. This raises the possibility of a completely new approach to transducing chemical information into electrical signals, based on forming an electrical circuit via a target molecule that bridges a gap between two electrodes by means of hydrogen bonding. Hydrogen-bond sensitive contrast has recently been demonstrated in scanning-tunneling microscope (STM) images of DNA bases. In this presentation, I will first show that the tunnel-current vs. distance decay curves acquired by STM change shape with the number of hydrogen bonds mediating an interaction. [1] Base composition of DNA oligomers can be resolved by this method. Further studies demonstrate that these tunnel-current decay signals can be used to count the number of hydrogen bonds in interactions between DNA bases and related compounds. The signals are partially mechanical in origin, reflecting the tensile strength of a tunnel junction held together with hydrogen bonds. \\[4pt] [1] He, J., Lin, L., Zhang, P. {\&} Lindsay, S. M. \textit{Nano Letters} \textbf{7}, 3854-3858 (2007). [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V27.00010: Nanoscale Properties of Neural Cell Prosthetic and Astrocyte Response D.A. Flowers, V.M. Ayres, R. Delgado-Rivera, I. Ahmed, S.A. Meiners Preliminary data from in-vivo investigations (rat model) suggest that a nanofiber prosthetic device of fibroblast growth factor-2 (FGF-2)-modified nanofibers can correctly guide regenerating axons across an injury gap with aligned functional recovery. Scanning Probe Recognition Microscopy (SPRM) with auto-tracking of individual nanofibers is used for investigation of the key nanoscale properties of the nanofiber prosthetic device for central nervous system tissue engineering and repair. The key properties under SPRM investigation include nanofiber stiffness and surface roughness, nanofiber curvature, nanofiber mesh density and porosity, and growth factor presentation and distribution. Each of these factors has been demonstrated to have global effects on cell morphology, function, proliferation, morphogenesis, migration, and differentiation. The effect of FGF-2 modification on the key nanoscale properties is investigated. Results from the nanofiber prosthetic properties investigations are correlated with astrocyte response to unmodified and FGF-2 modified scaffolds, using 2D planar substrates as a control. [Preview Abstract] |
Session V28: Focus Session: Graphene Device and Applications III
Sponsoring Units: FIAPChair: Phaedon Avouiris, IBM
Room: 330
Thursday, March 19, 2009 8:00AM - 8:36AM |
V28.00001: Graphene field-effect transistors for RF applicatoins Invited Speaker: There has been growing interest in graphene as a replacement for III-V materials in MMIC applications because of its high mobility, its potential for high saturation velocity, and its nearly perfect two-dimensional electrostatics. We present results from the first experimental high-frequency measurements of graphene field-effect transistors (GFETs), demonstrating an f$_{T}$ of 14.7 GHz for a 0.5-$\mu $m-length device with a 30-nm-thick HfO$_{2}$ top-gate. Despite I$_{on}$/I$_{off} \quad \sim $7, high transconductances ($>$833 $\mu $S/$\mu $m) and current saturation are achieved. We present detailed measurement and analysis of velocity saturation in GFETs, demonstrating the potential for velocities approaching 10$^{8 }$cm/sec and the effect of an ambipolar channel on current-voltage characteristics. We find that the saturation velocity is sheet-carrier dependent and limited by interfacial phonon scattering from the SiO$_{2}$ substrate upon which the graphene is fabricated. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V28.00002: Tunable spin-polarized terahertz excitations in graphene nanoribbons Jun-Qiang Lu, Xiaoguang Zhang, Sokrate T. Pantelides Graphene nanoribbons have an energy gap that is tunable from zero to terahertz (THz) regime by an external gate field. The indirect energy gap in a nanoribbon of infinite length, however, is unsuitable for optical excitations. We report a theoretical investigation of such nanoribbons with a finite, nanoscale length. We show that such nanoribbons can be excited optically and exhibit unique electronic excitations in the THz regime. The results unveil THz radiation-induced edge standing spin waves with different wavelengths at the two edges and a resonant frequency that can be controlled by an external gate voltage, opening the possibility of THz ``opto-spintronic'' applications. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V28.00003: THz Emission from Graphite Surfaces Chen Xia, Jie Shan Graphite formed by Van der Waals force between adjacent graphene sheets has been studied for more than six decades due to its relatively simple quasi-two-dimensional structure. Lately, because of its close relationship to carbon nanotubes and new physics originating from graphene's linear excitation spectrum and the 4-fold degeneracy graphitic materials have attracted much of research attention. However, still little is known about the high-freqency transport properties of these materials. In this work, we investigate graphite materials in THz regime by THz emission spectroscopy. Picosecond THz pulses were observed from a highly oriented pyrolytic graphite (HOPG) surface when it was illuminated by intenseultrafast optical pulses at an oblique angle. The emission was mostly p-polarized and increased linearly with pump fluence. Several potential mechanisms for the emission including surface nonlinearility will be discussed in the talk. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V28.00004: Low - Frequency Noise in Graphene Transistors Guanxiong Liu, Qinghui Shao, Desalegne Teweldebrhan, Alexander Balandin, Serguei Roumyantsev, Michael Shur We present the results of the experimental investigation of the low-frequency noise in three-terminal bilayer graphene devices. The quality of graphene layers has been verified with micro-Raman spectroscopy. Back-gated devices were fabricated using electron beam lithography and evaporation. The back-gate was used to adjust electrical conductivity through the graphene layer placed on top of Si/SiO$_{2}$ substrate. The charge neutrality point for examined devices was$\sim $10 V. The noise spectral density was rather low (on the order of $\sim $10E$^{-23}$--10E$^{-22}$ A$^{2}$/Hz at frequency of 1 kHz).The noise reveals generation-recombination (G-R) bulges. Presence of G-R bulges and deviation from the 1/f spectrum suggest that the noise is of carrier-number fluctuation origin due to carrier trapping by defects [1].The low values of the low-frequency noise add validity to the proposed electronic applications of graphene. [1] Q. Shao et al., IEEE EDL (2008). [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V28.00005: Environmental Effects on 1/f Noise in Graphene and CNT Based Devices Brett R. Goldsmith, Ye Lu, Zhengtang Luo, A.T. Charlie Johnson Graphite related materials such as carbon nanotubes, graphene and graphene oxide show promise for future electronic and chemical sensor applications. Nanotubes and graphene, in particular, have been shown to make exquisitely sensitive chemical sensors. Due to their low carrier density, the 1/f noise in these nanomaterials is very high. Understanding the cause of this noise is particularly important for chemical sensing applications, and the noise common to these materials may be one barrier to current practical success for graphitic sensors outside the lab. We have compared the noise power spectral density (PSD) of these three materials in different chemical environments and at different temperatures. This information should play a key role in guiding the development of future sensing devices as well as helping to illuminate the atomic scale interactions which lead to enhanced or suppressed 1/f noise in graphitic materials. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 10:00AM |
V28.00006: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V28.00007: Progress towards graphene as a quantum-limited electro-mechanical resonator R. G. Knobel, A. Chia With its high stiffness, low density and relatively simple fabrication, graphene promises to be an ideal system for exploring the quantum limits of mechanical measurements. In particular, electronic transport through a graphene sheet suspended over an electrode can be strongly modulated by vibrations of the sheet -- whether through the standard field effect which changes the carrier density in the sheet, or through modulation of the Coulomb blockade in quantum dots formed in the sheet. In this work we present the novel fabrication scheme we are using for this work, which involves exfoliation and identification of single-layer graphene sheets on a PMMA layer above a silicon substrate, cross-linking of the PMMA to form supports for the graphene and metal electrodes, and subsequent lithography to form electrodes. Raman scattering measurements before and after patterning confirm the single-layer nature of the graphene, and preliminary low-temperature transport measurements show the feasibility of this system for quantum-limited sensitivity of resonant motion of the sheet. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V28.00008: Developing resonant tunneling devices based on graphene Eric Yu, Sandip Tiwari, Derek Stewart We present an \textit{ab-initio} study of the electronic properties of patterned graphene structures as candidate resonant tunneling devices. We consider graphene nanoribbons that have been modified with one or more narrow constrictions or patterned with periodic nanoscale antidotes[1]. Specifically, we focus on semi-metallic armchair nanoribbons with narrow semiconducting regions and semi-metallic zigzag nanoribbons patterned with antidots. Using a first-principles density functional theory (DFT) approach, we investigate the induced band-gap opening and transmission coefficients. We examine how varying the lengths of the constrictions, changing the separation between dots and their sizes affect transport properties. We will also discuss I-V characteristics of these graphene structures and evaluate the possibility of a negative differential resistance in these devices. [1] T. G. Pedersen \textit{et al.}, \textit{Physical Review Letters}, \textbf{100} 136804 (2008) [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V28.00009: Coherent Graphene Devices: Movable Mirror, Buffer and Memory L. Zhao, S. F. Yelin We theoretically report that, at a sharp electrostatic step potential in graphene, massless Dirac fermions can obtain a Goos-H\"{a}nchen-like shift under total internal reflection. Based on these results, we study the coherent propagation of the quasiparticles along a sharp graphene \emph{p-n-p} waveguide and derive novel dispersion relations for the guided modes. Consequently, coherent graphene devices (e.g. movable mirror, buffer and memory) induced only by the electric field effect can be proposed. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V28.00010: Graphene interferometry Daniel Gunlycke, Carter White Ballistic transport calculations of graphene connected to two contacts are presented. The calculations are based on the nearest-neighbor, tight-binding approximation but are otherwise treated exactly within a Green function formalism. It is shown that under certain circumstances stable collective resonances emerge from a resonant structure that in general could be quite complicated. These collective resonances originate from a large number of non-equivalent conduction channels and are evenly spaced, except for a region close to the Fermi level. The separation between neighboring collective resonances depends to first order only on the contact separation. Their contrast, on the other hand, is affected by the width of the sample, temperature, and unevenness in the contact interfaces. Despite the existence of many potential sources that could degrade the collective resonances, these resonances could still prove to be observable experimentally. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V28.00011: Nanotube Films and Their Application For Mode-Locked Lasers Alex G. Rozhin, A.C. Ferrar Carbon nanotubes (CNTs) exhibit strong saturable absorption, i.e. they become transparent under sufficiently intense light. This has great potential for applications in photonics. By tuning the nanotube diameter it is easy to tune the saturable absorption in a broad optical range of interest for telecommunications, medicine and military applications. The performance of CNTs based saturable absorbers depends on concentration, bundle size, and transparency of the matrix where CNTs are dispersed. CNT saturable absorbers can be produced by cheap wet chemistry and can be easily integrated into polymer photonic systems. Here, we review the fabrication and characterization of saturable absorber based on CNT-polymer optical composites [1,2,3]. We use strong ultrasonication to obtain CNT solutions. Such solutions with different nanotube bundle sizes are then studied by photoluminescence excitation spectroscopy [4]. We find that exciton energy transfer between semiconducting CNTs is an efficient carrier relaxation channel in the bundles [4]. This fingerprints and quantifies the presence of small bundles and allows us to optimize the solutions used for composites preparation. We demonstrate picosecond pulse generation in a nanotube mode-locked waveguide laser [5], as well as 125 fs generation in an erbium doped fiber laser. We also report a novel SWNT- polycarbonate polymer composite, with a absorption maximum at 1550 nm and a bandwidth of about 300 nm [6]. This has strong saturable absorption with saturation intensity of 7 MW/cm$^{2}$. We demonstrate the first SWNT-mode-locked widely tunable fibre ring laser [7]. This is achieved through the control of amplification at the specific transitions of the Er$^{3+}$ gain medium by placing a band-pass filter in a laser cavity [7]. [1] A. G. Rozhin et al. Phys. Stat. Sol. (b) \textbf{243}, 3551 (2006). [2] V. Scardaci et al. Physica E \textbf{37}, 115 (2007) [3] T. Hasan et al. J. Phys. Chem C \textbf{111}, 12549 (2007) [4] P. H. Tan et al. Phys. Rev. Lett. \textbf{99}, 137402 (2007) [5] G. Della Valle et al., Appl. Phys. Lett. \textbf{89}, 231115 (2006) [6] V. Scardaci et. al. Adv. Mat. \textbf{20}, 4040 (2008 [7] F. Wang et. al. Nature Nano. Nov (2008). [Preview Abstract] |
Session V29: Correlated Electrons
Sponsoring Units: GMAGChair: Meigan Aronson, Brookhaven National Laboratory
Room: 333
Thursday, March 19, 2009 8:00AM - 8:12AM |
V29.00001: Emergence of spin structure in quantum wires under strong magnetic fields Gilad Barak, Georg Schusteritsch, Amir Yacoby, Loren Pfeiffer, Ken West We study the effects of a perpendicular magnetic field on the spin and charge structure of a quantum wire. Using momentum resolved tunneling between two parallel wires we measure the dispersion relation for different perpendicular magnetic fields. We find that as the magnetic field increases, charges with opposing spin separate in the cross section of the wire giving rise to strips of polarized and unpolarized electrons. We argue that this structure results from the exchange interaction between electrons in the wire. We discuss the applicability of these results to the structure of Quantum Hall edge states. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V29.00002: Nonequilibrium transport in the Anderson model of a biased quantum dot Sung Chao, Guillaume Palacios, Andres Jerez, Carlos Bolech, Pankaj Mehta, Natan Andrei We derive the transport properties of a quantum dot subject to a source-drain bias by means of the Scattering Bethe Anstaz, a generalization of the traditional Thermodynamic Bethe Ansatz to open systems out of equilibrium, which allows a description of the the system in nonequilibrium steady state over the full range of its parameters. Solving the equations at zero temperature and magnetic field we present here the non-linear conductance against the bias voltage with arbitrary tunneling rate and with the gate voltage varying from the mixed valence to the Kondo regime. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V29.00003: Imaging the Fano lattice in the heavy fermion material URu$_{2}$Si$_{2}$ by scanning tunneling spectroscopy Andrew Schmidt, Mohammad Hamidian, Peter Wahl, Focko Meier, Graeme Luke, J.C. Davis We present scanning tunneling spectroscopy measurements of the heavy fermion material URu$_{2}$Si$_{2}$. Two dimensional differential conductance maps of mechanically cleaved surfaces reveal a narrow Fano lineshape about the Fermi level that is pervasive across the field of view. Such a lineshape is expected whenever a discrete energy level is coupled to a continuum of levels. By fitting the spectra to a Fano function, we produce maps showing the variation of the Fano parameters across the surface. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V29.00004: Theory of the Unusual Quasiparticle Excitations in USb$_2$ Peter Riseborough, Xiaodong Yang A band of long-lived quasiparticles with a renormalized dispersion relation that does not cross the Fermi surface has been observed in USb$_2$ by angle resolved photoemission measurements. The existence of a kink in the quasiparticle dispersion relation of a band that does not cross the Fermi energy is unprecedented. We show that the observed renormalization does not come from the standard theory of electron-phonon renormalizations. We consider the effect of the interband self-energy and vertex corrections as possible causes for the formation of the renormalized quasiparticles. The effect of temperature is also considered. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V29.00005: Kink in the dispersion of f-electrons Tomasz Durakiewicz, Peter S. Riseborough, Clifford G. Olson, John J. Joyce, Peter M. Oppeneer, Saad Elgazzar, Eric D. Bauer, John L. Sarrao, Ela Guziewicz, David P. Moore, Martin T. Butterfield, Kevin S. Graham Many-body interactions may result in the formation of long- lived heavy quasi-particles that exhibit kinks in their energy dispersion. Kinks are often seen in d-electron correlated systems and are attributed to many different origins, such as coupling to phonons, extended spin-fluctuations, or electron- electron correlations. We have found that the renormalization of a 5f electron band in USb2 leads to the formation of a kink characterized by two distinct regions with different quasiparticle masses, peak asymmetries, lifetimes and a record- breaking small width of 3meV. The kink energy scale of 17meV originates from renormalization of a point-like Fermi surface, and is a factor of two smaller than previously measured in correlated materials. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V29.00006: Localized $5f$ antiferromagnetism in cubic UIn$_{3}$: $^{115}$In-NMR/NQR Study H. Sakai, S. Kambe, Y. Tokunaga, H. Chudo, Y. Tokiwa, D. Aoki, Y. Haga, Y. \={O}nuki, H. Yasuoka $^{115}$In nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on an antiferromagnet UIn$_3$ with the cubic AuCu$_3$-type structure. The NQR frequency ($\nu_{\rm Q}$) and Knight shift ($K$) of $^ {115}$In in UIn$_3$ have been estimated in the paramagnetic state from NMR experiments under applied field. The perpendicular component of transferred hyperfine coupling constant ($A_\perp$) has been deduced from scaled behavior of $K$ to the static susceptibility ($\chi$). Under zero field, the observation of the NQR spectrum has led to an estimated $\nu_ {\rm Q}$ of 11.8 MHz at 90 K. The temperature variation of the NQR relaxation rates ($1/T_1$) far above the N{\'e}el temperature $T_{\rm N}$=88 K approaches a constant value, which indicates a localized nature for the $5f$- electrons in this system. On the other hand, in the antiferromagnetically ordered state at 4 K (well below $T_ {\rm N}$), the $^{115}$In-NMR spectrum has been scanned over frequencies ranging from $\sim$20 to $\sim$70 MHz under zero applied field. From the analysis of the NMR spectrum, we propose that the direction of U moments in the AF state is neither $\langle 100 \rangle$ nor $\langle 111 \rangle$, but may be $\langle 110 \rangle$. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V29.00007: Studies of the Ferromagnetic Superconductors URhGe and UCoGe Travis Williams, Adam Aczel, Weiqiang Yu, Yasutomo Uemura, Jeremy Carlo, Tatsuo Goko, Jim Garrett, Graeme Luke Superconductivity (SC) cannot cooperatively exist with ferromagnetism (FM) in conventional superconductors, since ferromagnetism would act to destroy Cooper pairs. Thus, in FM superconductors such as URhGe and UCoGe, a more exotic pairing type must exist. I will outline the growth and characterization of URhGe and UCoGe crystals, and our measurements of the FM and SC properties. Our combined results from DC Resistivity, Bulk Magnetometry and Muon Spin Relaxation show FM properties in the samples, including a clear FM transition at 9.5K in the URhGe crystal. We will discuss our results and their implications for the nature of the SC state in these materials. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V29.00008: Renormalization of electronic structure close to Fermi level of CeIrIn$_{5}$ at 20K Yinwan Li, Tomasz Durakiewicz, John J. Joyce, Kevin S. Graham, John L. Sarrao, Eric D. Bauer, Clifford G. Olson The electronic structure of heavy-fermion superconductor CeIrIn$_{5}$ is investigated at $\sim $20K by high-resolution angle-resolved photoemission (ARPES). The low energy ARPES spectra indicate a kink near the Fermi surface within the~energy scale of~the order of~20meV.~Existence of a kink may suggest~coupling of electron~to~a collective boson mode of unknown origin. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V29.00009: Interplay of magnetism and screening in the Kondo Lattice Piers Coleman, Andriy Nevidomskyy An increasing body of experimental evidence suggests that frustration and the Kondo effect have complimentary roles that act together to either reduce, or completely eliminate magnetic order in heavy electron systems[1]. I will review our attempts to explore the joint effects of frustration and Kondo effect in the Kondo Heisenberg model, using the large N Schwinger boson approach[2]. These results will be discussed in the context of recent doping experiments on YbRh$_2$Si$_2$ [3], where an intermediate spin liquid appears to develop between the antiferromagnet and the large Fermi surface metal. \\ $\hbox{[1]}$ S. Nakatsuji, Y. Machida et al, Phys. Rev. Lett. 96, 087204 (2006).\\ $\hbox{[2]}$ J. Rech, P. Coleman, O. Parcollet and G. Zarand, Phys. Rev. Lett. 96, 016601 (2006).\\ $\hbox{[3]}$ T. Westerkamp, P. Gegenwart, C. Krellner et al., Physica B - Condens. Matter 403, 1236-1238 (2008). [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V29.00010: Electronic band structure of Pr-based filled skutterudite antimonides J.W. Allen, B.J. Kim, Ravi S. Singh, O. Krupin, J.D. Denlinger, R.E. Baumbach, M.B. Maple Filled skutterudites exhibit a wide range of strongly correlated electron phenomena including heavy fermion, superconductivity, non-Fermi liquid and quantum critical behaviors. Knowledge of the electronic structures of these materials is almost entirely from band calculations and dHvA studies. We present the first angle-resolved photoemission spectroscopy (ARPES) measurements of the filled skutterudites antimonides: PrOs$_{4}$Sb$_{12}$ and PrRu$_{4}$Sb$_{12}$. Band dispersions and Fermi surface maps of these three-dimensional materials will be discussed and compared to available LDA band structure calculations. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V29.00011: High frequency thermal transport for the 2d Hubbard model Louis-Francois Arsenault, Syed Hassan, Andre-Marie Tremblay In order to calculate thermal transport coefficients of correlated systems when the Boltzmann equation is not applicable, Shastry [1] has developed a new theoretical approach. Although, in this theory, quantities such as the thermopower depend only upon the one particle Green's function, vertex corrections are included. The price to be paid is that only the high frequency limit is accessible. This may be adequate for aforementioned transport coefficients. Results for the triangular lattice t-J model and the 1d Hubbard model are already in the literature but there are no results for the 2d Hubbard model, the prototype of correlated electron systems. We thus applied the Shastry approach to the 2d Hubbard model using quantum cluster approaches that include CDMFT + exact diagonalization, Bethe's lattice DMFT + CTQMC and CDMFT + CTQMC. Results were obtained for the thermopower as a function of temperature (T), chemical potential (mu), and band structure. Since infinite frequency is reached differently in the t-J and in the Hubbard model, our results enable us to assess the degree to which infinite frequency is related to experimental results on DC transport. [1] B. Sriram Shastry, Phys. Rev.~ B 73, 085117 (2006)~ [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V29.00012: Time evolution of excited state in the system with first-order metal-insulator transition W. Koshibae, N. Furukawa, N. Nagaosa We have studied numerically the relaxation process
in the system with a first-order metal-insulator transition
using the double-exchange model:
$H=-t\sum_{ |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V29.00013: Chiral spin states in the pyrochlore Heisenberg magnet: Fermionic mean-field theory and variational Monte Carlo calculations Junghoon Kim, Jung Hoon Han Fermionic mean-field theory and variational Monte Carlo calculations are employed to shed light on the possible uniform ground states of the Heisenberg model on the pyrochlore lattice. Among the various flux configurations, we find the chiral spin states carrying $\pm$ $\pi$/2 flux through each triangular face to be the most stable both within the mean-field theory and the projected wave- function studies. Properties of the spin-spin correlation function and the chirality order parameter are calculated for the projected wave functions. Meanfield band structures are examined. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V29.00014: Spectral properties of orbital polarons in Mott insulators Krzysztof Wohlfeld, Maria Daghofer, Andrzej M. Ole\'s, Peter Horsch Since orbital symmetry is lower than SU(2), superexchange in Mott insulators with orbital degrees of freedom is typically not Heisenberg-like and hole propagation is highly nontrivial [1]. We investigate cases with Ising-like superexchange, where the hole cannot propagate by its coupling to spin fluctuations. We find that the usually neglected three-site hopping determines hole motion [2]. One realization of Ising superexchange is the Falicov-Kimball model, where only electrons with one orbital flavor can move, and the other ones are localized --- then a hole inserted into the Mott insulator either moves via three-site hopping processes, or remains trapped in a small polaron. In another case of Ising exchange, a class of $t_{2g}$ or $e_g$ orbital systems, renormalized three-site hopping leads to one-dimensional hole propagation, with its direction determined by the orbital flavor of the hole. \\[3pt] [1] J. van den Brink {\it et al.\/}, Phys. Rev. Lett. \textbf{85}, 5174 (2000). \\[0pt] [2] M. Daghofer {\it et al.\/}, Phys. Rev. Lett. \textbf{100}, 066403 (2008). [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V29.00015: Spin conservation and Fermi liquid near a Pomeranchuk transition Andrey Chubukov, Dmitrii Maslov We analyze system behavior near a Pomeranchuk instability in terms of Fermi liquid theory. We argue that the original assumption that a single Landau parameter approaches $-1$ at a Pomeranchuk transition, while others are non-critical is incorrect in $D \leq 3$. We show that, near the transition, a system enters into a novel regime in which all other Landau components increase and eventually diverge at the critical point. We demonstrate that in this novel regime the relation between the Landau function and the full vertex is different from that in a conventional Fermi liquid theory -- the proportionality factor no longer contains the running effective mass and has the same constant value as at the boundary between a conventional Fermi liquid and the novel Fermi liquid behavior. We show how to restore spin conservation near a Pomeranchuk transition and discuss extra features specific to Pomeranchuk instabilities in the spin channel. [Preview Abstract] |
Session V30: Focus Session: Multiferroic Properties of Oxide Films
Sponsoring Units: DMP GMAGChair: Jacabo Santamaria, Universidad Complutense de Madrid
Room: 334
Thursday, March 19, 2009 8:00AM - 8:36AM |
V30.00001: Understanding Magnetism in Multiferroics Invited Speaker: Multiferroics are interesting materials not only because of their exciting order parameters, but for the potential for parameter coupling. In order to understand the magnetoelectric coupling, the individual order parameters must first be understood. BiFeO3 (BFO), a room temperature ferroelectric and an antiferromagnet, is an excellent model system for understanding the coupling between ferroelectricity and magnetism. A combination of in-plane and out-of-plane piezoresponse force microscopy (PFM) allows 3D mapping of the ferroelectric polarization directions in micron-sized regions of the films. The magnetic order of BFO was obtained by using x-ray linear dichroism images using a photoelectron emission microscope (PEEM). When compared with our dichroism models, angle and temperature dependent absorption measurements allow decoupling and direction determination of the two order parameters, ferroelectric and magnetic, contributing to the photoemission signal. These studies reveal a strain-driven reduction in magnetic symmetry in thin films, leading to the formation of a preferred magnetic axis as opposed to the observed easy plane for bulk films. This reduction along with the previous proof of FE-AFM coupling allows electrical control of its magnetic axis. This electrical BFO control has a strong effect on ferromagnets even at room temperature. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V30.00002: Infrared and Raman spectroscopy of the magneto-electric couping in BiFeO$_3$ R.P.S.M. Lobo, R. Schleck, R.L. Moreira, P. Rovillain, M. Cazayous, D. Lebeugle, D. Colson We measured the phonon spectra of BiFeO$_3$ single crystals utilizing infrared spectroscopy and Raman scattering. The data was taken from 4 K to 300 K using a fine temperature step. Small accidents observed in the temperature dependence of phonon frequencies, in particular the lowest $E$ mode, have corresponding features in the electromagnon response. High temperature data, up to 1200 K, on ceramics (infrared) or single crystals (Raman) also show phonon frequency renormalization at the N\'eel temperature. Our results reinforce a picture where the ferroelastic character of BiFeO$_3$ plays an important role. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V30.00003: Ferroelectric Size-effect on BiFeO$_{3}$ Films in Ultra-high Vacuum Peter Maksymovych, Stephen Jesse, Sergey Lisenkov, Laurent Bellaiche, Nina Balke, Mark Huijben, Ramamoorthy Ramesh, Arthur P. Baddorf, Sergei V. Kalinin The ferroelectric size effect is a highly pursued and controversial topic encompassing the scaling of polar distortion, domain structure and switchable polarization. We have studied epitaxial BiFeO$_{3}$ films using ultrahigh vacuum piezoresponse force microscopy. The films in vacuum develop out-of-plane polarization domains which, based on their lateral dimensions, drastically violate the Kittel law. From the analysis of the piezoresponse amplitude, we have established that the presence of a topographic island induces in-plane rotation of the polarization vector by 90$^{o}$. These findings are analyzed using a first-principles based model Hamiltonian approach. We have also obtained stable and reproducible piezoresponse hysteresis loops on the 2 nm films. Experiments conducted at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, sponsored by the Division of Scientific User Facilities, U.S. Department of Energy. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V30.00004: Electrical conduction at domain walls in multiferroic BiFeO3 Jan Seidel, Lane Martin, Qing He, Qian Zhan, Ying-Hao Chu, Axel Rother, Michael Hawkridge, Peter Maksymovych, Pu Yu, Martin Gajek, Nina Balke, Sergei Kalinin, Sybille Gemming, Feng Wang, Gustau Catal\'an, James Scott, Nicola Spaldin, Joseph Orenstein, Ramamoorthy Ramesh We report the observation of room temperature electronic conductivity at ferroelectric domain walls in BiFeO3. The origin and nature of the observed conductivity is probed using a combination of conductive atomic force microscopy, high resolution transmission electron microscopy and first-principles density functional computations. We show that a structurally driven change in both the electrostatic potential and local electronic structure (i.e., a decrease in band gap) at the domain wall leads to the observed electrical conductivity. We estimate the conductivity in the wall to be several orders of magnitude higher than for the bulk material. Additionally we demonstrate the potential for device applications of such conducting nanoscale features. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V30.00005: Ferroelastic domain formation in epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ thin films Tim Fister, Dillon Fong, Jeffrey Eastman, Paul Fuoss, Kavaipatti Balasubramaniam, Paul Salvador Epitaxial La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) thin films are known to form domains to reduce substrate-induced strain. For instance, on cubic SrTiO$_{3}$ (100), LSMO thin films can have up to four rhombohedrally-strained variants. These individual strain states can distort the MnO$_{6}$ octahedra and lead to unique electrical and magnetic properties. We have used synchrotron x-ray diffuse scattering to probe the in- and out-of-plane domain structure of a 5 nm LSMO film grown on SrTiO$_{3}$ (100). Satellites are present near integer order and half-order peaks that result from octahedral tilting in the coherent LSMO film. By analyzing the amplitude, position, and anisotropy of the satellite peaks for multiple half-order peaks, we obtain a robust measure of the dimensions of the domains, the ordering of the variants, and the strain state of each variant. Implications of the domain structure on magnetic properties will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V30.00006: Development of All Oxide Exchange Bias Systems David Kirkwood, Yonghang Pei, Nam Dao, Jiwei Lu, Stuart Wolf Multiferroic materials exhibit multiply states of order which are often coupled. Bizmuth Ferrite (BFO$_{3})$ is a room temperature antiferromagnetic, ferroelectric materials, where electrical control of magnetism and vice versa has been established. Combining BFO$_{3}$ with ferromagnetic oxides such as Magnetite (Fe$_{3}$O$_{4})$ or Lanthanum Strontium Manganate (L$_{.7}$S$_{.3}$MO) could yield interesting system with electrically controllable exchange bias. We have used a novel deposition tool employing two pulsed electron beam sources (PEBS) to deposit epitaxial layers of BFO$_{3}$, LSMO, and Fe$_{3}$O$_{4}$ onto STO, LAO, and MgO substrates. We are in the process of making bilayers of these materials and examining the quality and influence of the oxide interface on the development and system control of the exchange bias. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V30.00007: Growth and characterization of multiferroic BiMnO$_{3}$ thin films Amlan Biswas, G. Singh-Bhalla, Chelsey Morien, Hyoung Jeen Jeen, Patrick Mickel, Sefaatin Tongay, Julia Neff, A. F. Hebard BiMnO$_{3}$ is a rare single phase, multiferroic compound which displays both ferromagnetic and ferroelectric properties. However, it is complicated to grow thin films of BiMnO$_{3 }$due to the volatility of bismuth and substrate induced strain. We have grown thin films of BiMnO$_{3}$ on SrTiO$_{3}$ (100) substrates using pulsed laser deposition. These films have a ferromagnetic $T_{C}$ of about 95 K and electric polarization vs. electric field curves have confirmed their ferroelectric properties. The structure and chemical composition of these thin films have been characterized using x-ray diffraction, atomic force microscopy, scanning electron microscopy, and Auger electron spectroscopy. We will present evidence of the sensitivity of the multiferroic properties of BiMnO$_{3}$ thin films to the growth conditions and substrate induced strain. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V30.00008: Multiferroicity in half-doped manganites Sanjeev Kumar, Gianluca Giovannetti, Jeroen van den Brink, Silvia Picozzi Using a joint approach of density functional theory and model calculations, we focus on unconventional physical mechanisms leading to multiferroicity in a prototypical half-doped manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$. We focus on the strong competition between two exotic charge-orbital-spin ordered states. These are, (1) the charge and orbital ordered zig-zag spin state, also known as the CE-state, and (2) a spin-dimer Zener-polaron state. Both these states respect the inversion symmetry of the lattice and hence can not be ferroelectric. We identify a single variational parameter in terms of the coherent rotation of spin-dimers which interpolates between these two ordered states. It is shown that the true groundstate could be intermediate between these two and can break the inversion symmetry of the lattice. Using DFT calculations we show that the groundstate is indeed ferroelectric with a polarization up to few $\mu C/cm^2$. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V30.00009: Element specific magnetic moments of Ni and Mn in multiferroic Bi$_2$NiMnO$_6$ film grown on SrTiO$_3$ substrate Vemuru Krishnamurthy, Dave Keavney, David Singh, A. Venimadhav, Q. Li X-ray magnetic circular dichroism (XMCD) and x-ray absorption spectroscopy (XAS) at Ni L$_{2,3}$ edges and at Mn L$_{2,3}$ edges have been performed at 4.5 K and higher temperatures in a multiferroic thin film grown on SrTiO$_3$(001) substrate. These spectra show that Ni is in a divalent state and Mn is in a tetravalent state. The total magnetic moment of Mn is found to be about 2.8 $\mu_B$. The total magnetic moment at the Ni site is strongly reduced from the 2.0 $\mu_B$ expected for divalent Ni. We have also detected a small orbital magnetic moment at both Mn and Ni sites. We suggest that the weaker crystal fields at the Ni and Mn sites in the thin film give rise to an orbital moment. These results will be compared with the predictions of local spin density calculations. Supported by US Dept. of Energy. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V30.00010: Field Modulated Intrinsic Positive Exchange Bias in Novel Ferrite Ru$_{0.25}$Cr$_{0.75}$O$_{2}$ near the Compensation Point Kevin G. West, Nam Dao, Jiwei Lu, Stuart A. Wolf In some ferrimagnetic materials systems a compensation point is observed where the opposing sublattice magnetizations are equal and opposite resulting in a zero net magnetization. The resulting magnetization decreases below zero at temperatures below T$_{c}$ and then increase to zero at T$_{c}$. We observe this type of unusual ferrimagnetic behavior in the Ru$_{0.25}$Cr$_{0.75}$O$_{2}$ system. In addition, near the compensation point we observe positive exchange bias that can be modulated using an external applied magnetic field. Possible mechanisms will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V30.00011: Polarization Coupling in Ferroelectric Multilayers as a Function of Interface Charge Concentration Mahmut Okatan, Joseph Mantese, Pamir Alpay Intriguing properties of multilayered and graded ferroelectrics follow from the electrostatic and electromechanical interactions. The strength of the interlayer coupling depends on the concentration of interfacial defects with short-range local electrostatic fields. Defects may locally relax polarization differences and thus reduce the commensurate bound charge concentration at the interlayer interfaces. In this talk, we develop a theoretical analysis based on non-linear thermodynamics coupled with basic electrostatic relations to understand the role of charge compensation at the interlayer interfaces. The results show multilayered ferroelectrics with systematic variations in the composition may display a colossal dielectric response depending upon the interlayer electrostatic interactions. It is expected that other properties such as the pyroelectric and piezoelectric response will yield concomitant increases through the dielectric permittivity. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V30.00012: Magnetoelectric effects in SrRuO$_{3}$/BaTiO$_{3}$ heterostructures: A First Principles Study M.K. Niranjan, J.D. Burton, S.S. Jaswal, E.Y. Tsymbal, J.P. Velev Ferroelectric materials in combination with ferromagnets have emerged as structures in which strong magnetoelectric coupling may exist originating from unconventional physical mechanisms. The use of oxides such as SrRuO$_{3}$ as a metal electrode has been found to be of fundamental importance for the realization of ferroelectric films with critical thicknesses down to three unit cells. Here we present a study of SrRuO$_{3}$/BaTiO$_{3}$ heterostructures within the framework of density functional theory. This heterostructure is interesting since SrRuO$_{3}$ is a weak ferromagnetic oxide metal and hence, when used as an electrode on BaTiO$_{3}$, presents the possibility of coupling between electric and magnetic order parameters. In particular we study the magnetoelectric (ME) effect at the interface of SrRuO$_{3}$/BaTiO$_{3}$ by treating SrRuO$_{3}$ as spin polarized metal. We find that magnetic properties at the interface are affected as the ferroelectric polarization in the BaTiO$_{3}$ is reversed. We discuss the origins of ME effect and compare them with previously proposed ME coupling mechanisms in Fe/BaTiO$_{3}$, Fe$_{3}$O$_{4}$/BaTiO$_{3, }$and SrRuO$_{3}$/SrTiO$_{3}$ heterostructures$^{1, 2}$. $^{1}$Niranjan et al., Phys. Rev. B, \textbf{78}, 140405 (2008); $^{2}$Rondinelli et al., Nat. Nanotechnology, \textbf{3,} 46, (2008) [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V30.00013: Surface Magnetoelectric Effects from First Principles. Chun-Gang Duan, Ce-Wen Nan, Sitaram S. Jaswal, Evgeny Y. Tsymbal A magnetoelectric effect allows affecting magnetic properties of materials by applying electric fields which may be interesting for potential technological applications such as electrically controlled magnetic data storage. We use density functional calculations to reveal and elucidate magnetoelectric effects due to an electric field applied to ferromagnetic metal surfaces.$^{1}$ We find that the surface magnetoelectric effect originates from spin-dependent screening of the electric field and leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. If the ferromagnet is a half-metal the screening charge is formed entirely by a single conducting spin channel which leads to the surface magnetoelectric coefficient being the universal constant \textit{$\mu $}$_{B}$/\textit{ec}$^{2} \quad \approx $ 6.44$\times $10$^{-14}$ \textit{Gcm}$^{2}$/$V$. This is in an excellent agreement with our first-principles calculation result for the half-metal CrO$_{2}$. These results are of considerable interest in the area of electrically-controlled magnetism and magnetoelectric phenomena. 1. C.-G. Duan et al., \textit{Phys. Rev. Lett.} 101, 137201 (2008). [Preview Abstract] |
Session V31: Focus Session: Frustration Theory and Modeling
Sponsoring Units: GMAGChair: Taner Yildirim, NIST Center for Neutron Research
Room: 335
Thursday, March 19, 2009 8:00AM - 8:12AM |
V31.00001: Rotational symmetry breaking in Heisenberg model on triangular lattice Ryo Tamura, Naoki Kawashima We study a finite-temperature phase transition in the two-dimensional classical Heisenberg model on a triangular lattice with a ferromagnetic nearest-neighbor interaction $J_1$ and an antiferromagnetic third-nearest-neighbor interaction $J_3$ using Monte Carlo simulation. Apart from a trivial degeneracy corresponding to O(3) spin rotations, the ground state for $J_3\ne 0$ has a threefold degeneracy corresponding to 120 degree lattice rotations. We find that this model exhibits a phase transition with breaking of the three-fold symmetry when $J3=J1/3$ and that the transition is of the first order. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V31.00002: Finite-size scaling in frustrated Heisenberg models Sasha Chernyshev, Steven White Numerical studies of the 2D frustrated antiferromagnets are hindered by the absence of large-scale quantum Monte Carlo methods and by large finite-size effects in other methods. Using the effective $\sigma$-model we demonstrate that the most significant finite-size effects can be eliminated by an appropriate choice of the cluster aspect ratio, allowing for much more precise estimates of observables already in small systems. We show that such a ``magic'' aspect ratio depends on the boundary conditions, with a simple and convenient choice being precisely the geometry optimal for the DMRG method. Combining the improved DMRG accuracy with the use of non-traditional clusters for rapidly converging finite-size scaling, we study the ordering in the square- and triangular-lattice Heisenberg models. We demonstrate the vanishing of the leading finite-size effect $\sim O(1/L)$ in the order parameter $M$ for the sequence of clusters with the ``magic'' aspect ratio ($L_x/L_y$ close to 2), in agreement with the effective $\sigma$-model. We determine the thermodynamic limit of $M$ for the square lattice with an error comparable to quantum Monte Carlo. For the triangular lattice, we verify the existence of three-sublattice magnetic order, and estimate the order parameter to be $M = 0.205(15)$. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V31.00003: Classical and quantum dimers on the star lattice John Fjaerestad We show that dimer coverings on the star lattice (aka the 3-12, Fisher, expanded kagome or triangle-honeycomb lattice) have $Z_2$ arrow and pseudo-spin representations analogous to those for the kagome lattice, and use these to construct an exactly solvable quantum dimer model (QDM) with a Rokhsar-Kivelson (RK) ground state. This QDM, first discussed by Moessner and Sondhi from a different point of view, is the star-lattice analogue of a kagome-lattice QDM analyzed by Misguich et al. We discuss various properties of the classical equal-weight dimer model on the star lattice, most of which are related to those of the RK state. Using both the arrow representation and the fermionic path integral formulation of the Pfaffian method, we calculate the number of dimer coverings, dimer occupation probabilities, and dimer, vison, and monomer correlation functions. The results show unusual features similar to those of dimers on the kagome lattice. We also discuss some generalizations to general Fisher lattices and their ``reduced'' lattices (the kagome, squagome, and triangular-kagome lattice being examples of the latter). Ref.: J. O. Fjaerestad, arXiv:0811.3789 [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V31.00004: Spatially anisotropic Heisenberg kagome antiferromagnet Invited Speaker: We study the quasi-one-dimensional limit of the spin-1/2 quantum Heisenberg antiferromagnet on the kagome lattice. The lattice is divided into antiferromagnetic spin-chains (exchange $J$) that are weakly coupled via intermediate ``dangling'' spins (exchange $J'$). Using one-dimensional bosonization, renormalization group methods, and current algebra techniques the ground state is determined in the limit $J' \ll J$. We find that the dangling spins and chain spins form a spiral with $O(1)$ and $O(J'/J)$ static moments, respectively, atop of which the chain spins exhibit a smaller $O[(J'/J)^2]$ antiferromagnetically ordered component along the axis perpendicular to the spiral plane. We describe similarities and differences of our findings with other recent studies, based on semi-classical and large-N approaches. Critical comparison of quasi-one-dimensional kagome antiferromagnet with other quasi-one-dimensional models will be presented as well. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V31.00005: Ordered States on the Kagome Antiferromagnetic Heisenberg Model Simeng Yan, Steven White We numerically study the spin 1/2 Kagome antiferromagnetic Heisenberg Model with DMRG techniques. Recently, Singh and Huse proposed a dimerized ground state with a 36 site unit cell. To test this proposal, we have simulated the system on clusters which favor this order. If the order was not found, this would disprove the proposal. However, the results do show the proposed order. The strength of the dimerization on the pinwheels is surprisingly strong, with $<$S $\cdot $ S$>$ taking values of -0.7J on the strong bonds and -0.1J on the weak. We also have studied the system on clusters with a cylindrical geometry to test for the presence of the order. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V31.00006: Universality Classes of Dimerized Bond-Disordered Quantum Spin Models Jonas Gustafsson, Daoxin Yao, Erica Carlson, Anders Sandvik We study the dimerized bond disordered S=1/2 Heisenberg models on the square lattice. Each spin belongs to one strong bond (a dimer) by introducing strong and weak couplings, $J_s$, $J_w$. By means of quantum Monte Carlo simulations, we find two different universality classes for the random dimer model and the random plaquette model. The change of universality class may be associated with the cancellation of Berry phase. Furthermore, we study the dilution effect by setting some strong bonds to 0. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V31.00007: Emergent multipolar spin correlations in a fluctuating spiral - The frustrated ferromagnetic S=1/2 Heisenberg chain in a magnetic field, Andreas Lauchli, Julien Sudan, Andreas Luscher We present the phase diagram of the frustrated ferromagnetic $S=1/2$ Heisenberg $J_1$-$J_2$ chain in a magnetic field, obtained by large scale exact diagonalizations and density matrix renormalization group simulations. A vector chirally ordered state, metamagnetic behavior and a sequence of spin-multipolar Luttinger liquid phases up to hexadecupolar kind are found. We provide numerical evidence for a novel locking mechanism, which can drive spiral states towards spin-multipolar phases, such as quadrupolar or octupolar phases. Our results also shed new light on previously discovered spin-multipolar phases in two-dimensional $S=1/2$ quantum magnets in a magnetic field. We conclude by presenting numerical results on the dynamical spin structure factor in the various phases which are valuable in identifying multipolar phases in experiments. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V31.00008: Dynamically dominant excitations of string solutions in the antiferromagnetic Heisenberg chain in magnetic fields Masanori Kohno We investigate behaviors of dynamical structure factors in the spin-1/2 antiferromagnetic Heisenberg chain in magnetic fields, using Bethe-ansatz solutions. We uncover a well-defined continuum in $S^{+-}(k,\omega)$, which comes from 2-string solutions in the Bethe ansatz. It continuously connects the des Cloizeaux-Pearson mode in the zero-field limit and the bound state of overturned spins from the ferromagnetic state near the saturation field. Also, we give a natural interpretation to particles in magnetic fields, psinon and antipsinon, as those carrying fractional quantum numbers $S^z$=+1/2 and -1/2, respectively. We argue that not only psinons and antipsinons but also particles representing strings play important roles for dynamical properties of the antiferromagnetic Heisenberg chain in magnetic fields. We confirm the relevance of the present results to real materials through comparisons with experimental results. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V31.00009: Interplay between interaction and (un)correlated disorder in Heisenberg spin-1/2 chains Frieda Dukesz, Marina Zilbergerts, Lea Santos We consider a Heisenberg spin-1/2 chain and study the interplay between the Ising interaction and on-site disorder, while keeping the hopping amplitude constant. Disorder is characterized by both: uncorrelated and long-range correlated random on-site energies. The level of delocalization, quantified by the number of principal components, is largest in clean systems with non-interacting particles. However, in the presence of uncorrelated disorder, delocalization becomes maximum for a non-zero value of the interaction amplitude. The inclusion of long-range correlated disorder may further extend two-particle states, but the effect decreases with the number of excitations and strength of the interaction, and may even be reversed, as shown for half-filled chains. Quantum correlations, determined by a global entanglement measure, present similar behavior, but the largest value appears for clean systems with interacting particles. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V31.00010: Algebraic spin liquid in an exactly solvable spin model Hong Yao, Shou-Cheng Zhang, Steven Kivelson We have introduced an exactly solvable quantum spin-3/2 model on the square lattice. Its ground state is a spin liquid with half integer spin per unit cell. The fermionic excitations are gapless with a linear dispersion, while the topological ``vison'' excitations are gapped. Moreover, the massless Dirac fermions are stable against any small perturbations with time reversal symmetry. Thus, this model is, to the best of our knowledge, the first exactly solvable model whose ground state is an ``algebraic spin liquid'' with half integer spin per unit cell. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V31.00011: Extended supersolid phase of frustrated hard-core bosons on a triangular lattice Fa Wang, Frank Pollmann, Ashvin Vishwanath We study a model of hard-core bosons with frustrated nearest-neighbor hopping ($t$) and repulsion ($V$) on the triangular lattice. We argue for a supersolid ground state in the large repulsion ($V\gg|t|$) limit where a dimer representation applies, by constructing a unitary mapping to the well understood unfrustrated hopping case. This generalized `Marshall sign rule' allows us to establish the precise nature of the supersolid order by utilizing a recently proposed dimer variational wavefunction, whose correlations can be efficiently calculated using the Grassmann approach. By continuity, a supersolid is predicted over the wide parameter range, $V>-2t>0$. This also establishes a simple phase diagram for the triangular lattice spin 1/2 XXZ antiferromagnet. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V31.00012: A $\Gamma $-matrix generalization of the Kitaev model Hsiang-Hsuan Hung, Congjun Wu, Daniel Arovas We extend the Kitaev model defined for the Pauli-matrices to the Clifford algebra $\Gamma $-matrices by taking the 4$\times $4 representation as an example. In a 2D decorated square lattice, the ground state spontaneously breaks time-reversal symmetry and exhibits a topological phase transition. The topologically non-trivial phase carries gapless chiral edge modes along the sample boundary. In the 3D diamond lattice, the ground states can exhibit gapless 3D Dirac cone- like excitations and gapped topological insulating states. The generalizations to even higher rank $\Gamma $-matrices are also discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V31.00013: Dzyloshinskii-Moriya interactions in valence bond systems II Mayra Tovar, Kumar Raman, Kirill Shtengel We investigate the effect of Dzyaloshinskii-Moriya interactions on the low temperature magnetic susceptibility for a system whose low energy physics is dominated by short-range valence bonds (singlets). Our general perturbative approach is applied to specific models expected to be in this class, including the Shastry-Sutherland model of the spin-dimer compound SrCu$_2 $(BO$_3$)$_2$ and the antiferromagnetic Heisenberg model of the recently discovered $S=1/2$ kagome compound ZnCu$_3$(OH)$_6 $Cl$_2$. The central result is that a short-ranged valence bond phase, when perturbed with Dzyaloshinskii-Moriya interactions, will remain time-reversal symmetric in the absence of a magnetic field but the susceptibility will be nonzero in the zero temperature limit. Applied to ZnCu$_3$(OH)$_6$Cl$_2$, this model provides an avenue for reconciling experimental results, such as the lack of magnetic order and lack of any sign of a spin gap, with known theoretical facts about the kagome Heisenberg antiferromagnet. [Preview Abstract] |
Session V32: Focus Session: Vortex and Domain Wall Dynamics
Sponsoring Units: GMAG DMP FIAPChair: Ilya Krivorotov, Univerisity of California Irvine
Room: 336
Thursday, March 19, 2009 8:00AM - 8:12AM |
V32.00001: Dynamics of Exchange-Biased Magnetic Vortices T. Y. Chen, M. K. Chan, P. A. Crowell We have studied magnetization dynamics in micron-sized circular disks composed of ferromagnetic (FM)-antiferromagnetic (AFM) bilayers. The patterned samples of FeMn/NiFe are field-cooled (FC) or zero-field cooled (ZFC) from above the blocking temperature to room temperature. Time-resolved Kerr microscopy measurements show that the vortex gyrotropic mode fluctuates in frequency as the vortex core is displaced by a static in-plane magnetic field. The average gyrotropic frequency and the magnitude of its fluctuations, which are due to pinning of the vortex core, are larger than in single layer FM films. The enhancement of the gyrotropic frequency is largest in the ZFC samples, in which the effective field due to exchange coupling is expected to enhance pinning of the vortex core at the center of the disk. We find, however, that micromagnetic simulations incorporating uniform or vortex-like exchange-bias fields do not explain our results quantitatively. We interpret this discrepancy as a consequence of randomly orientated AFM domains, which are comparable in size to the vortex core. This work was supported by NSF and the Univ. of Minnesota Graduate School. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V32.00002: Ferromagnetic resonance force spectroscopy of a magnetic vortex G. de Loubens, O. Klein, A. Riegler, F. Lochner, G. Schmidt, L.W. Molenkamp, H. Hurdequint, F. Boust, N. Vukadinovic, A.N. Slavin Due to its nanometer size (of the order the exchange length), probing the high frequency dynamics of a magnetic vortex core is an experimental challenge. Precessional dynamics of the magnetization of individual nano-disks of NiMnSb perpendicularly magnetized is measured in a wide range of bias magnetic fields using a magnetic resonance force microscope (MRFM). A full dynamic phase diagram, demonstrating excitation of a Kittel-type dipolar mode in the saturated disks and the gyrotropic mode of vortex core rotation in the vortex-state unsaturated disks, is established. Switching of the vortex core polarity in a negative (anti-parallel to core) bias magnetic field is registered dynamically. Analytic theory and micromagnetic simulations provide a quantitative description of the experimental results. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V32.00003: Spin motive electric field driven by magnetic vortex motion Jun-ichiro Ohe, Stewart E. Barnes, Sadamichi Maekawa The current-induced magnetization dynamics realized in spintronics devices involve both of charge and spin degrees of freedom. Recently, it has been pointed out that the magnetization dynamics induces an effective electric field acting on the conduction electrons through the spin Berry phase. The effective electric field, or a ``spin motive electric field,'' was investigated for a simple one-dimensional domain wall. It is difficult to estimate analytically this effective electric field in actual systems, because the magnetization dynamics obeys the non-linear Landau-Lifshitz equation. In this report, we describe numerical studies of the spin motive electric field induced by the dynamics of a vortex core. The vortex structure can be realized in a Permalloy disc. It is known that the magnetic vortex core shows a resonant motion when the oscillating magnetic field is applied. The direction of the core is switched rapidly by applying a pulsed such magnetic field. During the core motion, we obtain an electric field near the core. The direction of the electric field is perpendicular to the direction of the core motion. We also obtain the electric field driven by spin waves which are excited by the core switching. We propose an experimental setup for measuring the electric field. The calculated voltage is large enough to measure. We show that the voltage induced by core switching is quite large. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V32.00004: Thickness and field dependence of the driven dynamic mode-splitting of magnetic vortices Kristen Buchanan We have explored the effects of increased driving field amplitude on the dynamics of magnetic vortices using a microwave reflection technique and found that the vortex translational eigenmode profile first takes on a distorted shape and then splits into two well-defined peaks as the field is increased [1]. Here we examine the thickness and field dependence of this mode-splitting phenomenon via measurements of lithographically patterned micron-sized Permalloy ellipses with thicknesses of 20, 40, and 60 nm. The experimental results will be compared to numerical calculations that incorporate a critical velocity parameter and provide new insight into the origin of the observed vortex dynamic mode splitting. Acknowledgments: Thank you to Marcos Grimsditch, Frank Fradin, Sam Bader, and Val Novosad for stimulating discussions. [1] Buchanan et al. PRL 99,267201 (2007). [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V32.00005: Time-resolved X-ray Microscopy of Magnetic Antivortex Dynamics Markus Bolte, Thomas Kamionka, Michael Martens, Bernd Guede, Guido Meier, Kang Wei Chou, Tolek Tyliszczak, Michael Curcic, Bartel Van Waeyenberge, Hermann Stoll The study of magnetic singularities, vortices and antivortices, has recently intensified as they have been suggested as non-volatile data storage elements or for spin-wave logic applications. Magnetic antivortices occur during the switching process of their topological counterparts, the vortices [1], as well as in crosstie-domain walls and special geometries [2]. Understanding the dynamics of antivortices [3] is therefore fundamental for gaining a detailed knowledge necessary to design new spintronic applications. Here we show by time-resolved X-ray microscopy experiments that magnetic antivortices indeed gyrate when excited by alternating currents, in agreement with theoretical models and micromagnetic simulations [3], albeit with much lower efficiency than their topological counterparts, the vortices. [1] B. Van Waeyenberge et al., Nature \textbf{444}, 461 (2006). [2] K.Shigeto et al., APL \textbf{80}, 4190, (2002). [3] A. Drews et al., PRB \textbf{77}, 094413 (2008); B. Krueger et al., JAP \textbf{103}, 07A501 (2008). [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V32.00006: High frequency spin dynamics in soft magnetic dots in biased vortex state: precise probing and nature of the eigenmodes Farkhad Aliev, Juan Francisco Sierra, Ahmad Awad, Gleb Kakazei, Dong-Soo Han, Sang-Koong Kim, Konstantin Guslienko, Bojan Ilic, Vitali Metlushko Regular arrays of soft magnetic dots in the vortex state are being considered as a potentially new high-density nonvolatile recording media characterized by two binary properties: chirality and polarity of magnetic vortex core. Here we unambiguously demonstrate the existence of two distinct dynamic vortex (stable and metastable) regimes with qualitatively different spin wave eigenmodes. We find that dynamic response in the metastable vortex state qualitatively changes with relative orientation of the driving rf and bias magnetic fields. These findings, supported by numerical simulations, open new possibilities for development of magnetic devices with precise control over the magnetization switching process. They also underscore importance of understanding of dynamic response in different nanostructured materials with vortices in confined and stratified conditions. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V32.00007: Spin-Torque Ferromagnetic Resonance Spectroscopy of Permalloy Nanowires Carl Boone, Jordan Katine, Jeff Childress, Jian Zhu, Xiao Cheng, Ilya Krivorotov We develop a technique for studies of spectral properties of spin waves excited by spin transfer torque in metallic ferromagnetic nanowires, and apply this technique to measure frequencies and damping constants of several low-energy quantized spin wave modes in permalloy nanowires of rectangular cross section. Our measurements demonstrate that the spin wave spectrum of nanowires as narrow as 100 nm is well described by an analytic theory of dipole-exchange spin waves in thin ferromagnetic strips. Geometric quantization of the spin wave spectrum in nanowires significantly reduces the phase space for magnon-magnon scattering leading to opening and closing of discrete scattering channels as a function of magnetic field. These scattering channels manifest themselves as peaks in plots of spin wave damping versus magnetic field. In particular, we observe damping enhancement of the lowest energy spin wave mode at the values of magnetic field corresponding to three-magnon confluence processes in which two lowest energy magnons merge into a single higher-energy mode magnon. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V32.00008: Effects of disorder and temperature on vortex domain wall dynamics Hongki Min, Michael Donahue, Mark Stiles Domain wall motion, whether driven by applied magnetic fields or electrical current, can be strongly affected by sample irregularities. Using micromagnetic simulations and a collective coordinate approach, we study the dynamics of domain wall motion driven by a spin-polarized current or an external magnetic field in the presence of extrinsic random potential at finite temperatures. We compare these calculations and discuss the region of validity of the approximations in the simple model. Information about the strength of the random potential is taken from recent magnetic resonance experiments. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V32.00009: Electron Drift Speed And Current-Induced Drive Torques On A Domain Wall Luc Berger It has become fashionable to describe [1] current-induced torques on a DW in terms of an electron drift speed u = - P*j*mu$_B$/e*M where mu$_B$ is the Bohr magneton and M the saturation magnetization. While appropriate for adiabatic torques, this quantity u is misleading and not the best choice in the case of non-adiabatic torques. For example, it leads [2] to beta not equal to alpha, where beta represents the intensity of the non-adiabatic torque, and alpha is the damping parameter. By writing equations of motion for conduction- electron spins in a moving frame where the electron gas is at rest, we find [3] a direct relation between damping and non- adiabatic torques. The correct electron drift speed turns out to be the speed of the frame, and is v = P*j/(n*q) where n and q are the carrier density and charge. It is related to the ordinary Hall constant R$_0$ by v ~ P*R$_0$*j. After substituting v for u in the expression of the non-adiabatic torque, we find that beta = alpha holds now. Because v is larger than u in Permalloy, it can explain better the large current-induced DW speeds found [4] experimentally. In materials where R$_0 > 0$ and the carriers are dominantly hole-like, v and u have opposite signs, leading to different predictions for the sense of DW motion. We discuss examples of such materials. 1. G. Tatara and H. Kohno, Phys. Rev. Lett. 92, 086601 (2004). 2. H. Kohno et al., J. Phys. Soc. Japan, 75, 113706 (2006). 3. L. Berger, Phys. Rev. B 75, 174401 (2007). 4. M. Hayashi et al., Phys. Rev. Lett. 98, 037204 (2007). [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V32.00010: Universal Electromotive Force Induced by Domain Wall Motion Qian Niu, Shengyuan Yang, Geoffrey Beach, Carl Knutson, Di Xiao, Maxim Tsoi, James Erskine The electromotive force induced by a moving magnetic domain wall in a nanostrip has been calculated theoretically and detected experimentally. It is found that the emf depends only on the domain wall transformation frequency through a universal Josephson type relation, which is closely related to the topological nature of the domain wall. Our experimental measurements confirm the theoretical prediction. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V32.00011: Motion of a vortex domain wall in a rough nanowire Paula Mellado, David Clarke, Oleg Tchernyshyov The motion of a vortex domain wall in a ferromagnetic nanowire under the influence of an applied magnetic field has been recently cast in the language of collective coordinates [1]. The theory, taking into account the two softest modes of the domain wall, works well below and immediately above Walker's breakdown [2] and can be extended to include the influence of spin current. Here we examine the motion of a vortex domain wall in a wire with rough edges. Integrating out the transverse coordinate yields an effective one-dimensional problem of a massive particle moving in a viscous medium. The edge roughness translates into a combination of a random pinning potential and a random Zeeman force. We calculate the average velocity of the domain wall and the probability of passing a wire of specified length as a function of the applied magnetic field. [1] O. A. Tretiakov \textit{et al.,} Phys. Rev. Lett. \textbf {100,} 127204 (2008). [2] D. J. Clarke et al., Phys. Rev. B \textbf{78,} 134412 (2008). The work was supported in part by the NSF Grant DMR-05204291. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V32.00012: Domain pinning and disorder in Fe/Gd magnetic multilayers. Jyoti Mohanty, Ashish Tripathi, Erik Shipton, Keith Chan, SangSoo Kim, Ian McNulty, Eric Fullerton, Oleg Shpyrko We study the evolution of magnetic domains and effect of pinning centers in thin film magnetic systems as a function of magnetic field, temperature, and dopants to identify the role the disorder in formation and stability of the domains in these systems. We have studied Fe/Gd multilayer exhibiting ordered stripes due to perpendicular magnetic anisotropy (PMA). Samples are well characterized using Polar Kerr effect and Vibrating sample magnetometry. Magnetic Force Microscopy (MFM) measurements show out-of-plane magnetized stripe domains. We study the effects of field pinning of the local magnetic structure of these systems through their magnetization hysteresis loops and their temperature driven dynamics. Using element sensitivity and depth resolution of resonant magnetic x-ray coherent scattering technique we investigate the magnetic domain structure and intermittent switching dynamics. Comparison of the magnetic speckles (in momentum space) provides information on correlation between the magnetic structures (in real space). We will present the X-ray Coherent Speckle Metrology approach to study of Barkhausen noise spectrum as a function of the applied magnetic field, and will discuss extension of this study to Tb-doped Fe/Gd magnetic films, which would induce strong PMA. [Preview Abstract] |
Session V33: Superconductivity: Complex Order Parameter
Sponsoring Units: DCMPChair: Suk Bum Chung, Stanford University
Room: 403
Thursday, March 19, 2009 8:00AM - 8:12AM |
V33.00001: Topological Superconductivity and Superfluidity Taylor Hughes, Xiao-Liang Qi, S. Raghu, Shou-Cheng Zhang We construct time reversal invariant topological superconductors and superfluids in two and three dimensions. These states have a full pairing gap in the bulk, gapless counter-propagating Majorana states at the boundary, and a pair of Majorana zero modes associated with each vortex. The superfluid $^3$He B-phase provides a physical realization of the topological superfluidity, with experimentally measurable surface states protected by time- reversal symmetry. We show that the time reversal symmetry naturally emerges as a supersymmetry, which changes the parity of the fermion number associated with each time-reversal invariant vortex and connects each vortex with its superpartner. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V33.00002: Observing Majorana Zero Modes in a $p_x+ip_y$ Superconductor at High Temperature by Tunneling Spectroscopy Yaacov E. Kraus, Assa Auerbach, Herbert A. Fertig, Steven H. Simon Directly observing a zero energy Majorana state in the vortex core of a chiral superconductor by tunneling spectroscopy requires energy resolution better than the spacing between core states $\Delta^2_0/\epsilon_F$. We show\footnote{arXiv:0811.2557, Phys. Rev. Lett. in press} that nevertheless, its existence can be clearly detected by comparing the temperature broadened tunneling conductance of a vortex with that of an antivortex even at temperatures $T \gg \Delta^2_0/\epsilon_F$. The Bogoliubov-de-Gennes (BdG) equation of a $p_x+ip_y$ superconductor is solved numerically on a sphere with vortex-antivortex pair at the poles. The robustness of the exponentially Majorana mode energy is verified by including a moderate white noise potential. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V33.00003: Frequency dependence of the intrinsic Hall conductivity in a chiral $p+ip$ superconductor with impurities Pavel Nagornykh, Roman Lutchyn, Victor Yakovenko We calculate frequency dependence of the intrinsic Hall conductivity induced by impurity scattering in a chiral $p_x+ip_y$ superconductor. We find that, at large frequencies compared to the superconducting gap ($\Omega\gg\Delta$), the real part of the intrinsic Hall conductivity at zero temperature is proportional to $\Delta/\Omega^3\log(\Omega/2\Delta)$. Using our results for the Hall conductivity, we estimate the Kerr angle and compare it with the experimental data on $\rm Sr_2RuO_4$ by Xia et al., Phys. Rev. Lett. 97, 167002 (2006). [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V33.00004: Electromagnetic response of time reversal invariant triplet superconductors Rahul Roy, Catherine Kallin, John Berlinsky We study the effective action of time reversal invariant triplet superconductors in two and three dimensions and obtain the electromagnetic response. The B-phase of Helium 3 and its two dimensional analog are topologically non-trivial phases. The two dimensional triplet superconductor may be regarded as two copies of a chiral $p_x + i p_y $ superconductor. We discuss signatures of the non-trivial topology in the effective action and in the electromagnetic response. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V33.00005: Microscopic analysis of the stability of half-quantum vortices in $p_x + ip_y$ superfluids in an annular geometry. Victor Vakaryuk We present a microscopic analysis of the thermodynamic stability of a half-quantum vortex (HQV) in $p_x + i p_y$ variant of equal-spin-pairing state which, under suitable conditions, is believed to be realized in $\rm Sr_2RuO_4$ and $^3$He-A. Our approach is based on a description of the HQV in terms of a BCS-like wave function with a spin-dependent boost. Stability criterion is found by comparing energies of half- and full-quantum vortices with appropriate account taken of Fermi liquid corrections. While we confirm earlier phenomenological findings by Suk Bum Chung et al.~(2007) for the stability of the HQV in the annular geometry, we also predict a novel feature that the HQV, if exists, should be accompanied by a non-zero spin polarization of the system. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V33.00006: Josephson tunneling studies of odd-parity superconductivity in Sr$_2$RuO$_4$ Ronald Myers, Ying Liu, D. Fobes, Z. Mao, H. Yaguchi, Y. Maeno In the recent phase-sensitive work that provided the most unambiguous evidence for odd-parity superconductivity in Sr$_2$RuO$_4$, we used Au$_{0.5}$In$_{0.5}$ as the s-wave superconducting counter electrode in the SQUID structure. However, Au$_{0.5}$In$_{0.5}$ has a T$_c$ less than that of Sr$_2$RuO$_4$, making detection of the pairing symmetry near the T$_c$ of Sr$_2$RuO$_4$ inaccessible to the phase sensitive measurements. To go beyond this limit and open up possibilities of several other experiments involving Josephson tunneling into Sr$_2$RuO$_4$, we seek an alternative material system with an s-wave superconductor of T$_c$ $>$ 1.5K that would exhibit Josephson coupling with Sr$_2$RuO$_4$. An Ag/Pb/Ag trilayer has been chosen for this purpose. Ag/Pb/Ag-Sr$_2$RuO$_4$ tunneling devices were prepared that showed a suppressed superconducting gap feature of Sr$_2$RuO$_4$, suggesting the presence of superconductivity at the polished ac face of a Sr$_2$RuO$_4$ crystal. However, no Josephson coupling between Ag/Pb/Ag and Sr$_2$RuO$_4$ was detected. More experiments are currently underway, and new results will be presented. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V33.00007: Measurements of superconducting energy gap in individual Ru islands embedded in Sr$_{2}$RuO$_{4}$ Yiqun Ying, B. Clouser, R. Myers, N. Staley, Ying Liu, D. Fobes, Z.Q. Mao, Y. Xin, L. Allard We report our tunneling measurements on individual single-crystalline Ru islands embedded in a bulk Sr$_{2}$RuO$_{4}$ single crystal. Tunneling junctions were prepared on large (micron size) and small (submicron size) Ru islands by fabricating tunneling windows using quartz filaments as shadow masks. Our measurements revealed the presence of an energy gap below a temperature close to the $T_{c}$ of bulk Ru. In the zero temperature limit, the gap was found to be 0.07 meV for large Ru islands, consistent with our measurements on bulk polycrystalline Ru. However, in small Ru islands a gap of 0.1 meV, clearly larger than that seen in large Ru islands, was observed. The difference in energy gap may reflect difference in pairing state in Ru islands of different sizes. Above the $T_{c}$ of Ru but below the $T_{c}$ of Sr$_{2}$RuO$_{4}$, we detected no proximity induced energy gap. This observation is unexpected as our Z-contrasted transmission electron microscope study showed that the interface between a Ru island and Sr$_{2}$RuO$_{4}$ is atomically sharp, which appears to rule out the suppression of the proximity effect by disorder. We argue that these observations are associated with chiral p-wave superconductivity in Sr$_{2}$RuO$_{4}$. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V33.00008: Magnetic response of Sr$_{2}$RuO$_{4}$ nanocrystals: search for chiral currents and fractional vortices D. J. Bahr, M. J. A. Stoutimore, R. Budakian, D. J. Van Harlingen, Y. Maeno The ruthenate superconductor Sr$_{2}$RuO$_{4}$ may have a chiral order parameter of the form p$_{x}\pm $ip$_{y,}$, making it a candidate for nucleation of excitations with non-Abelian statistics that could enable topologically protected quantum computing. To test this scenario, we have measured the magnetic response of Sr$_{2}$RuO$_{4}$ nanocrystals to search for spontaneous chiral currents and half-integer vortices. Each nanocrystal (1$\mu $m x 1$\mu $m x 0.5$\mu $m) was extracted from a large single crystal with bulk transition temperature in the range 1.2K-1.4K. We then glued it into the pickup loop of a flux transformer or a gradiometer that is inductively coupled to a dc-SQUID magnetometer. We have observed the diamagnetic screening of the crystal and the entry of discrete vortices in an applied magnetic field. We report on our search for spontaneously generated currents, chiral domain dynamics and the nucleation of half-integer vortices, which we should be sensitive to due to the small size of the crystal. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V33.00009: Investigating magnetic order in Sr$_{2}$RuO$_{4}$ using cantilever torque magnetometry Raffi Budakian, Joonho Jang, Dale Van Harlingen, Yoshi Maeno Following the initial $\mu $SR and NMR studies suggesting that Sr$_{2}$RuO$_{4}$ is a spin-triplet superconductor, there has been a great deal of interest to understand the nature of the order parameter. Although Sr$_{2}$RuO$_{4}$ is similar in structure to the layered high-Tc cuprate superconductors, it is thought to possess chiral p$_{x}\pm $ip$_{y}$ pairing symmetry. The complex order parameter can give rise to a rich variety of new correlated states, such as domains having orbital order that possess a net magnetic moment and half-integer vortices with zero-energy modes. We have applied ultrasensitive cantilever torque magnetometry to measure the magnetic moment, susceptibility, and vortex entry into micron-size Sr$_{2}$RuO$_{4}$ in search of magnetic moments generated by chiral edge currents and fractional vortices. This talk will present recent data in this effort. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V33.00010: Orbital magnetic moment in the chiral p-wave superconductor Sr2RuO4 James Annett, Karol Wysokinski, Balazs Gyorffy The existence and magnitude of a bulk orbital angular momentum of the condensate chiral a phase in superfluid helium-3 is a longstanding matter of controversy. The analogous problem in a chiral p-wave superconducting material is the existence of a finite orbital magnetic moment in the bulk. In Sr2RuO4 the existence of such an orbital moment is strongly suggested by experimental evidence for spontaneously time reversal symmetry breaking (TRSB) in the superconducting state, but the theories disagree on the expected magnitude of this moment. We show that a non-zero orbital magnetization density arises natually in a realistic band model for Sr2RuO4, and its temperature dependence is qualitatively similar to those of the muSr and Kerr effect experimental results. The simplest model which leads to the orbital moment requires at minimum two degenerate atomic orbitals per Ru, which correspond to the Ru d xz and d yz states. This is in contrast to the theories of orbital angular momentum in the isotropic superfluid 3-He, or models of orbital moment in Sr2RuO4 which assume only a single band at the Fermi level. The implications of this surprising result are explored. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V33.00011: Absence of superconductivity in the half-filled anisotropic triangular lattice Hubbard model Hongtao Li, R.T. Clay, S. Mazumdar The superconducting $\kappa$-(BEDT-TTF)$_2$X salts, with one hole per molecular site and strong dimerization are widely thought to have an effective $\frac{1}{2}$-filled band. The presence of antiferromagnetism (AFM) near superconductivity (SC) in their pressure-temperature phase diagram has led to the suggestion suggest that the SC can be explained within an anisotropic triangular lattice $\frac{1}{2}$-filled band Hubbard Hamiltonian. In this model increasing frustration suppresses the AFM transition, and it has been suggested that d-wave SC appears near the metal/AFM interface. We performed exact diagonalizations on a 16-site periodic anisotropic triangular lattice and determined the full phase diagram. We confirm the Mott metal-insulator transition and AFM, change of the AFM wavevector for large anisotropy, and the presence of a non-magnetic insulating phase. We do not find any hint of long range superconducting correlations. In our results the Hubbard $U$ always suppresses the superconducting pair-pair correlations over their non-interacting value. We conclude that the Hubbard model is too simple to explain the SC in organic charge-transfer solids. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V33.00012: Nuclear spin-lattice relaxation rate as a link between antiferromagnetism and superconductivity in organic conductors Claude Bourbonnais, Abdelouahab Sedeki The interdependence of antiferromagnetism and superconductivity in the Bechgaard salts series of organic conductors is examined in the light of the anomalous temperature dependence of the nuclear spin-lattice relaxation rate. We use the renormalization group approach to the electron gas model to demonstrate that the metallic state anomaly of the nuclear relaxation rate found in the Bechgaard salts and the mechanism of passage from antiferromagnetism to superconductivity can be both described within a unified framework. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V33.00013: New details in the phase diagram of $\lambda\textnormal{-}(\textnormal{BETS})_2\textnormal{Ga}\textnormal{Cl}_4$ made by advancing the art of RF penetration depth measurements in pulsed fields using a tunnel diode oscillator William A. Coniglio, Laurel E. Winter, Kyuil Cho, Braunen E. Smith, C.C. Agosta, L.K. Montgomery We report improvements to the Tunnel Diode Oscillator method of measuring the penetration depth of a superconductor at RF frequencies above 100 MHz. Optimizations to the circuit for high frequency and pulsed fields are briefly discussed as well as a digital demodulation technique for rendering the oscillation frequency with accuracy suitably better than the stability of the oscillator itself. Using a 390 MHz oscillator, we measured the penetration depth of $\lambda\textnormal{-}(\textnormal{BETS})_2\textnormal{Ga}\textnormal{Cl}_4$ with the magnetic field oriented parallel to the conducting planes of the sample using fields up to 21\,T and temperatures from 400\,mK to 5.5\,K. Our new data crunching techniques have allowed us to resolve two phase transitions between the superconducting and normal states as well as a third transition that appears at low temperature as an enhancement to the upper critical field. We explore the properties of the phase diagram in two samples. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V33.00014: Upper Critical Field and Phase Diagram Studies for $\lambda$-(BETS)$_2$GaCl$_4$ Laurel E. Winter, William A. Coniglio, Kyuil Cho, Braunen E. Smith, C.C. Agosta, L.K. Montgomery The upper critical fields for the highly anisotropic organic superconductor $\lambda\textnormal{-}(\textnormal{BETS})_2\textnormal{Ga}\textnormal{Cl}_4$ have been studied by measuring the in-plane RF penetration depth with a tunnel diode oscillator technique in pulsed fields. At zero field we found a $T_c$ of 5 K. With the field perpendicular to the conducting layers we extrapolate $H_{c2,T=0}$ to 2.8 T and with the field parallel $H_{c2,T=0}$ is 11 T. With the field applied parallel to the conducting layers, for $T > 0.5T_c$ the $H_{c2}$ follows the superconducting gap function $\sqrt{1-\frac{T}{T_c}}$, then saturates below $0.5 T_c$. Below 0.35$T_c$ we see a clear enhancement of 1.5 T in $H_{c2}$ and in addition there is a second phase line at a lower field than $H_{c2}$. These features are both characteristic of the FFLO state. We will discuss this second phase line in relation to the Pauli Limit as calculated in a semi-empirical method and compare our phase diagram to previous results. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V33.00015: Mesoscopic conductance oscillations in superconducting nanoparticle films Al-Amin Dhirani, Brian Lam Recent advances in nanoparticle synthesis yield control over key nanoparticle characteristics such as structure and chemical composition. This in turn enables fabrication of nanostructured materials with novel and controlled properties. We have found that superconducting 100 nm niobium nanoparticles can be sintered to make porous macroscopic films that routinely exhibit conductance oscillations as a function of bias voltage and magnetic field. We speculate the effect is related to electron-hole interference previously observed at interfaces between disordered normal materials and superconductor electrodes. Our results show that the oscillations in the present system are associated with nanoparticle state (superconducting vs. normal) and ubiquitous elastic scattering at length scales dictated by nanoparticle size. Robust observation of this mesoscopic interference phenomenon in a disordered, macroscopic system is remarkable. It is enabled by the present approach's ability to balance conterveiling considerations: sufficient disorder to induce elastic scattering and restricted averaging to limit dephasing. [Preview Abstract] |
Session V34: Superconductivity: Spectroscopy
Sponsoring Units: DCMPChair: David Tanner, University of Florida
Room: 404
Thursday, March 19, 2009 8:00AM - 8:12AM |
V34.00001: Comparing the three characteristic electronic excitations in the pseudogap state of underdoped Bi$_{2}$Sr$_{2}$Ca$_{0.8}$Dy$_{0.2}$Cu$_{2}$O$_{8+\delta }$ K. Fujita, Jhinhwan Lee, C. K. Kim, A. Schmidt, H. Eisaki, S. Uchida, J. C. Davis We investigate the quasiparticle interference processes as a function of temperature for heavily underdoped Bi$_{2}$Sr$_{2}$Ca$_{0.8}$Dy$_{0.2}$Cu$_{2}$O$_{8 }$ (T$_{c}$=42K). We demonstrate that three types of electronic excitations exist in the pseudogap phase: (1) metallic excitations on the Fermi Arc, (2) the Bogoliubov quasiparticle excitations of what appears to be a phase incoherent d-wave superconductor in the confined area in momentum space (Jhinhwan Lee \textit{et al} (2009)) and (3) the high energy pseudogap excitations seen in the anti-nodal region outside the $\surd $2X$\surd $2 Brillouin zone (Y. Kohsaka \textit{et al}. Nature 454, 1072 (2008 ). We discuss the relationship of these three components of the electronic structure to the thermodynamic and transport characterization of this phase. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V34.00002: Quasiparticle interference above and below $T_c$ in underdoped cuprates Aakash Pushp, Colin Parker, Abhay Pasupathy, Kenjiro Gomes, Shimpei Ono, Yoichi Ando, Jinsheng Wen, Zhijun Xu, Genda Gu, Ali Yazdani There is considerable debate over the evolution of quasi-particle excitations between the superconducting and pseudogap phases in the underdoped cuprates. In the superconducting phase, dispersive real space modulations are observed [1], which can be explained by quasi-particle interference (QPI), whose location in momentum space is consistent with ARPES. What should happen to these modulations in the pseudogap state, where ARPES indicates a finite arc of gapless Fermi surface? We will present STM data from underdoped $\textrm{Bi}_2\textrm{Sr}_2\textrm{CaCu}_2\textrm{O}_{8+x}$ that investigates the nature of this change and its connection with the non-dispersive features seen above $T_c$ [2]. [1] Kohsaka $\textit{et al. Nature } \textbf{454}$, 1072 (2008). [2] Vershinin $\textit{et al. Science } \textbf{305}$, 1993 (2004). [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V34.00003: Extinction of quasiparticle interference in underdoped cuprates with coexisting order Brian Andersen, Peter Hirschfeld Recent scanning tunnelling spectroscopy measurements [Y. Koksaka et al., Nature 454, 1072 (2008)] have shown that dispersing quasiparticle interference peaks in Fourier transformed conductance maps disappear as the bias voltage exceeds a certain threshold corresponding to the coincidence of the contour of constant quasiparticle energy with the antiferromagnetic zone boundary. Here we argue that this is caused by quasistatic short-range coexisting order present in the d-wave superconducting phase, and that the most likely origin of this order is disorder-induced incommensurate antiferromagnetism. We show explicitly how the peaks are extinguished in the related situation with coexisting long-range antiferromagnetic order, and discuss the connection with the realistic disordered case. Since it is the localized quasiparticle interference peaks rather than the underlying antinodal states themselves which are destroyed at a critical bias, our proposal resolves a conflict between scanning tunneling spectroscopy and photoemission regarding the nature of these states. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V34.00004: Quasi-particle interference and vortex ``checkerboard'' in Bi$_2$Sr$_2$CaCu$_2$O$_y$ T. Hanaguri, Y. Kohsaka, T. Tamegai, H. Takagi Relationship between the ``checkerboard'' electronic-state modulation in a vortex core~[1] and the quasi-particle interference effect has been studied using STM/STS in optimally- doped Bi$_2$Sr$_2$CaCu$_2$O$_y$. We found that the vortex- induced signals in Fourier-transform spectroscopic images appear in the close vicinity to some of the ``octet'' scattering vectors for the quasi-particle interference~[2], suggesting that the vortex ``checkerboard'' is associated with the Fermi momentum. Conductance spectrum taken at the center of the vortex core shows a sharp peak at a low energy ($\sim$ meV) in the empty state. We argue the possible relationship between these observations and the quantum-limit nature of the vortex core. [1] J. E. Hoffman {\it et al.}, Science {\bf 295}, 466 (2002). [2] K. McElroy {\it et al.}, Nature {\bf 422}, 592 (2003). [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V34.00005: Quasiparticle scattering from impurities in the cuprates E.A. Nowadnick, I.M. Vishik, B. Moritz, W.S. Lee, Z.X. Shen, T.P. Devereaux, K. Tanaka Scanning tunneling spectroscopy (STS) measurements have shown that the local density of states in the cuprate superconductors is spatially inhomogeneous. Fourier-transformed STS has been used to investigate the mixing of momentum space eigenstates of the superconducting quasiparticles in the presence of this inhomogeneity, and has observed the extinction of the quasiparticle peaks upon approaching the antinodal region of the Fermi surface. We present calculations of momentum dependent quasiparticle scattering from impurity sites. Our results demonstrate that the quasiparticle extinction observed in FT-STS can be interpreted as resulting from the momentum dependence of the quasiparticle scattering rather than the absence of the quasiparticle itself. This interpretation agrees with recent ARPES measurements that observe quasiparticle peaks over the entire Fermi surface. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V34.00006: Echolocation of Scatterers by Quasiparticles in Cuprate Superconductors Sumiran Pujari, Christopher Henley How much can STM techniques tell us about the realization of disorder in a particular sample under study? We propose a new method of STM-data analysis which allows for the determination of the position and strength of impurities/scatterers. Furthermore, for cuprates, it can potentially be used to distinguish if the scatterer is ``ordinary'' or ``anomalous"~\footnote{T. S. Nunner et al, Phys. Rev. B, \textbf{73}, 104511 (2006)}, i.e. part of the pairing potential. The method relies on quasiparticle interference \footnote{Q. Wang and D.-H.~Lee, Phys. Rev. B \textbf{67}, 020511 (2003)} as observed in cuprates$^3$. As for much of the STM phenomenology in cuprates$^{1-3}$, our starting point is the existence of well-defined Bogoliubov quasiparticles defined by a quadratic phenomenological Hamiltonian with intrinsic disorder. By \emph{Energy} ``Fourier-Transform''ing the measured local density of states (LDOS) spectrum from a single point, one can extract the ``echo'' time that a quasiparticle takes to go to and return from a nearby scatterer; doing this at several points in a local patch allows a ``sonar''-like echolocation of the scatterer. This method is complementary to Fourier-Transform Scanning Tunneling Spectroscopy \footnote{K. McElroy et al, Nature, \textbf{422}, 592 (2003)} wherein \emph{Space} Fourier transforms of LDOS data yield the quasiparticle dispersion. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V34.00007: Interference of nematic quantum critical quasiparticles: a route to the octet model Eun-Ah Kim, Michael Lawler Given the presence of glassiness and inhomogenaity in cuprate superconductors, the capability of quasiparticle interference (QPI) in inferring momentum space electronic structure from real space local density of states(LDOS) images is surprising. Particularly, the simplicity of the QPI image, a set of well defined dispersing peaks is striking. Regarding the nature of QPI peaks, the ``octet model'' was based on the observation that the peak positions are determined by the eight tips of the ``banana'' shaped qp equal energy contours. However, a key open question has the mechanism for the accumulation of coherence at the tips. Here we show that nematic quantum critical fluctuations, combined with the known extreme velocity anisotropy, provide a natural mechanism for the accumulation of coherence at those special points [1]. Our results raise the intriguing question of whether the nematic fluctuations provide the unique mechanism for such a phenomenon.\\[4pt] [1] E.-A. Kim and M. J. Lawler, arXiv:0811.2242. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V34.00008: Tunneling-mediated Impurity Resonances in Bilayer Cuprate Superconductors Degang Zhang, Chin-Sen Ting We have studied tunneling-mediated local density of states (LDOS) of the surface layer of a bilayer cuprate, where a Zn impurity is located on the second Cu-O layer. When the tunneling strength between two Cu-O layers is larger than a critical value, the LDOS on the site just above the Zn impurity first exhibits a resonant peak near the Fermi surface. The larger the tunneling strength, the stronger the resonant peak. It is also shown that the height of the resonant peak oscillates decreasingly with the distance from the site just above the Zn impurity. The location of the resonant peak in the surface LDOS depends on doping, energy gap, and the tunneling strength, and has an opposite bias voltage to that on its nearest neighboring sites. The results could be tested by the STM experiments and be used to further understand the electronic properties of high temperature superconductors. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V34.00009: Spin Filtering and Dephasing through an Aluminum Nanoparticle Felipe Tijiwa Birk, Christopher Malec, Dragomir Davidovic Measurements of spin-polarized current through a single Al nanoparticle in weak tunnel contact with two ferromagnets will be discussed. As a function of the bias voltage across the particle, spin polarized current saturates within the first few discrete energy levels above the ground state. The saturation is related to the energy dependence of the spin relaxation time T$_{1}$, from which we find that T$_{1}$ is about microsecond for the lowest excited state. Spin polarized current is extremely sensitive with respect to the direction of the applied magnetic field relative to magnetization. The discrete levels filter the spin of transmitted electrons along the direction specified by the applied magnetic field, explaining the directional dependence both qualitatively and quantitatively. In zero magnetic field, the filtering direction is determined by the field of the environment, making spin-filtering a new technique to study electron spin-dephasing in single metallic particles and other quantum dots. The spin-dephasing time in the nanoparticle at 4.2K is T$_{2 }>$8ns. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V34.00010: Possible competing order-induced Fermi arcs and self-consistent gap evolution with temperature in cuprate superconductors G.P. Lockhart, A.D. Beyer, M.L. Teague, B.-L. Yu, J.C.F. Wang, N.-C. Yeh We explore, via numerical simulations, the possibility that competing orders (CO's) induce both the pseudogap (PG) and Fermi arc phenomena in cuprate superconductors. We find that both phenomena occur in hole-type cuprates if (1) a CO arises below a PG temperature T*, which is greater than the superconducting transition temperature, T$_{C}$, and (2) the periodic wave-vector of the CO, \textbf{Q}, is parallel to the Cu-O bonding direction. In contrast, neither phenomena is observed in electron-type cuprates because T*$<$T$_{C}$, but we find evidence that the CO scenario may explain the so-called non-monotonic d-wave gap observed in electron-type cuprates for T$<$T$_{C}$ if \textbf{Q }is parallel to the nodal direction, as in the case of commensurate spin density waves. Finally, we consider a candidate model for self-consistently calculating the superconducting and CO energy gaps as a function of temperature and doping in the hole-type cuprates, as well as estimating the value of T*. Ref.: B.-L. Yu, \textit{et.al. }[arxiv:0804.4028]. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V34.00011: How does the gap change at $T_c$ in underdoped cuprates? Colin Parker, Aakash Pushp, Abhay Pasupathy, Kenjiro Gomes, Shimpei Ono, Yoichi Ando, Jinsheng Wen, Zhijun Xu, Genda Gu, Ali Yazdani Many measurements on underdoped cuprates have shown a gap that persists up to room temperature. This raises an important question: what happens at $T_c$ in order to cause the loss of perfect conductivity? In ARPES, the nature of the gap changes from d-wave below $T_c$ to Fermi arcs above $T_c$. However, ARPES necessarily averages over significant nanoscale disorder. We will present detailed STM spectroscopy on underdoped $\textrm{Bi}_2\textrm{Sr}_2\textrm{CaCu}_2\textrm{O}_{8+x}$ from both single points and areal averages. By using a local probe we avoid averaging over the disorder. We have performed lattice tracking spectroscopy on identical atomic sites [1] and indentical grids of points [2] for a range of temperatures both below and above $T_c$. Unlike overdoped samples, the STM spectrum in underdoped cuprates shows two energy scales [1]. We will compare our data to models based on ARPES, with emphasis on the difference between the superconducting and pseudogap phases. [1] Gomes \textit{et al., Nature} \textbf{447}, 569 (2007) [2] Pasupathy \textit{et al., Science} \textbf{320}, 196 (2008) [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V34.00012: The Effect Of Spontaneous Magnetization On The Reliability Of The Value For The Spin Polarization As Fitted From Ferromagnet/Superconductor Point Contact Data Paul J. Dolan, Jr., Charles W. Smith The generalized BTK model for charge transport in a ferromagnet/superconductor point contact can be used to estimate the spin polarization in a ferromagnet. However, even when these measurements are carried out in zero applied magnetic field, there can be a substantial field in the active region of the contact due to the spontaneous magnetization of the ferromagnet itself. We estimate the effect of spontaneous magnetization on the reliability of the values of the spin polarization parameter for various ranges in contact transparency, inelastic scattering and temperature. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V34.00013: Coherence factor effects in the antisymmetrized LDOS correlators Marianna Maltseva, P. Coleman Recent scanning tunneling experiments on underdoped cuprates by Hanaguri et al [1] show the appearance of coherence factor effects. Unlike conventional observables, we show that the tunneling density of states in a superconductor does not have a well defined coherence factor. However, by extracting the component that is either even, or odd in the bias voltage, we show that these separate components have well-defined coherence factors. These results are used to understand the appearance of coherence factor effects in the antisymmetrized local density of states correlators in recent scanning tunneling experiments. \\[3pt] [1] T. Hanaguri, Y. Kohsaka, M. Ono, M. Maltseva, P. Coleman, I. Yamada, M. Azuma, M. Takano, K. Ohishi and H. Takagi, to be published (2009). [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V34.00014: Visualizing electronic segregation in lightly-doped Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ Yuhki Kohsaka, Tetsuo Hanaguri, Masaki Azuma, Mikio Takano, J. C. Seamus Davis, Hidenori Takagi We report spectroscopic imaging on evolution of the electronic state in a lightly-doped cuprate superconductor Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ across the metal-insulator critical doping. We find nm-scale electronic segregation between regions breaking and showing the lattice symmetry. The former shows C$_2$ symmetry characterized by the unidirectional nano-domains and the V-shaped pseudogap found in superconducting samples [1] while the latter shows C$_4$ symmetry and wider U-shaped gap prominent in non-superconducting samples. This indicates that the local symmetry breaking is inherent in the electronic states created inside the Mott gap by hole doping. We also discuss spectra in C$_2$/C$_4$ domains and superconducting/insulating samples. \par ~ \par \noindent [1] Y. Kohsaka et al., Science 315, 1380 (2007), Nature 454, 1072 (2008). [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V34.00015: Search for Orbital-Current Effects in Y$_{2}$Ba$_{4}$Cu$_{7}$O$_{15-\delta}$ using $^{89}$Y NMR Simon Str\"assle, Josef Roos, Mihael Mali, Hugo Keller, Takashi Ohno Recent efforts at explaining the exotic electronic properties of cuprates by involving orbital currents attracted a lot of attention. Here we present $^{89}$Y NMR measurements on an oriented $\mathrm{Y_{2}Ba_{4}Cu_{7}O_{15-\delta}}$ powder sample to search for the possible orbital-current phase. The temperature behavior of the $^{89}$Y line width and the spin-lattice relaxation rate in the normal-conducting phase were investigated in the normal-conducting state of the compound. The study provides upper limits for a static magnetic field and the amplitude of a fluctuating magnetic field at the Y site of $\alt 0.15mT$ and $\alt 0.7mT$, respectively. These values provide significant constraints on possible static or quasi-static orbital currents. [Preview Abstract] |
Session V35: Focus Session: Iron Pnictides and Other Novel Superconductors XIII: Pairing Symmetry, Theory and Experiment
Sponsoring Units: DMPChair: Igor Mazin, Naval Research Laboratory
Room: 405
Thursday, March 19, 2009 8:00AM - 8:12AM |
V35.00001: On the magnetic fluctuations and unconventional superconducting pairing in iron pnictides Junhua Zhang, Rastko Sknepnek, Joerg Schmalian We explore the role played by spin and density fluctuations in the FeAs based superconductors using the fluctuation-exchange (FLEX) approach. We calculate the superconducting transition temperature and the fluctuation induced pairing gap. In order to compare with experiments, we evaluate the change of the penetration depth with temperature and the evolution of the order parameter. Because of the multi-band feature in this type of material, the interplay between intra- and inter-band fluctuations gives rise to rich physics. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V35.00002: Signatures of the s+ superconducting gap in electronic Raman Scattering and optical conductivity of Fe-based superconductors Ilya Eremin, Andrey V. Chubukov, Maxim M. Korshunov We analyze the consequences of the extended s-wave symmetry of the superconducting gap, proposed recently in Fe-based superconductors, for the electronic pair-breaking Raman scattering and optical conductivity. We calculate conductivity and Raman intensity for elastic scattering and find that an extended s-wave superconducting gap gives rise to several specific features in optical and Raman response functions. In particular, we find that, for the A1g symmetry of the incoming light, there will be a resonant collective mode in the Raman response function at an energy $\omega <$2$\Delta $. The latter is as a hallmark of the s+ superconductivity. Furthermore, the Cooper-pair weakening due to strong inter-band impurity scattering shifts the 2$\Delta $ features towards higher energies in both Raman scattering and optical conductivity. We argue that these features are present in the experimental data for iron-based superconductors. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V35.00003: Experiments for probing the macroscopic quantum coherence and pairing state in FeAs-based superconductors C.C. Tsuei, Ching-Tzu Chen, M.B. Ketchen, J.R. Rozen, Z.A. Ren, Z.X. Zhao, X.H. Chen The recent discovery of superconductivity in FeAs-based pnictides has added another member to the growing family of high-temperature superconductors. In sharp contrast to cuprates, the new superconductor has a $T_c$ up to $55$ K despite the absence of strong electronic correlation. Furthermore, its physical properties are best described by the multi-orbital electronic band structure. It is therefore of great interest to explore the consequence of these novel characteristics on the nature of Cooper pairing in the new FeAs superconductors. In this talk, we will present the design and implementation of experiments which probe the pairing state and the macroscopic quantum coherence across the interface between a pnictide and a conventional s-wave superconductor. Preliminary results will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V35.00004: Limits on the Superconducting Order Parameter in NdFeAsO$_{1-x}$F$_y$ and SmFeAsO$_{1-x}$ from Scanning SQUID Microscopy Thomas Lippman, Clifford Hicks, Martin Huber, Zhi-An Ren, Zhong-Xian Zhao, Kathryn Moler As a test of the symmetry of the order parameter of the ferric oxyarsenide family of superconductors RFeAsO$_{1-x}$F$_y$, where R is a rare earth, we perform scanning SQUID microscopy on dense polycrystalline samples of NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$. Dominant $d$- or $p$-wave orders, for example, would result in direction-dependent phase shifts in tunneling. In well-coupled polycrystalline samples, these phase shifts would result in spontaneous orbital currents and magnetization in the superconducting state. We do not find any spontaneous currents in NdFeAsO$_{0.94}$F$_{0.06}$ or SmFeAsO$_{0.85}$, ruling out order parameters with direction-dependent phase shifts in tunneling. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V35.00005: Effect of disorder on sign reversing s-wave pair state for Fe-pnictides Vivek Mishra, G. Boyd, S. Graser, P. J. Hirschfeld In this work, we consider a phenomenological model for sign changing s-wave order parameters appropriate for the ferropnictide superconductors. We consider both magnetic and non-magnetic impurities within the Born approximation, and study the effects of this disorder on transport properties in superconducting state. We compare our results with experimental data for Fe-pnictides, where such a sign-changing s-wave state may be realized. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V35.00006: Phase fluctuations in a magnetic exchange model of iron-based superconductors F.J. Burnell, B. Andrei Bernevig, Meera M. Parish We analyze the phase fluctuations in a two band model of iron- based superconductors. Multi-band superconductors have a sound- like collective excitation (Leggett mode) due to phase oscillations between different condensates. We calculate the spectrum of this mode in a short-range exchange model of iron pnictide superconductors. Unlike previously studied systems, in this model the superconducting order parameters of the particle and hole Fermi surfaces naturally have opposite sign. We describe the consequences of this for the Leggett mode and outline possible experimental signatures of this sign difference between the two superconducting gaps. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V35.00007: Near degeneracy of several pairing channels in a multi-orbital model for the Fe-pnictides Thomas Maier, Siegfried Graser, Peter Hirschfeld, Douglas Scalapino The experimental evidence regarding the gap structure in different iron pnictide superconductors is currently conflicting. In addition, weak-coupling approaches to the pairing problem in multi-orbital models of the iron pnictides have predicted a wide variety of superconducting ground states. We argue here that this controversy is naturally explained by the near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we will present results for the spin susceptibility and the pairing symmetry within a five-band random phase approximation model. We will discuss the robustness of these results for different dopings, interaction strengths, and variations in the band structure, in the light of recent experiments. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V35.00008: Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides B. Andrei Bernevig, Kangjun Seo, Jiangping Hu We study the pairing symmetry of a two orbital $J_1-J_2$ model for FeAs layers in oxypnictides. We show that the mixture of an intra-orbital unconventional $s_{x^2y^2}\sim \cos(k_x)\cos(k_y)$ pairing symmetry and a small $d_{x^2-y^2}\sim \cos(k_x)-\cos (k_y)$ component is favored in a large part of $J_1-J_2$ phase diagram. A pure $ s_{x^2y^2}$ pairing state is favored for $J_2>>J_1$. The signs of the $d_{x^2-y^2}$ order parameters in the two different orbitals are opposite. While a small $d_{xy} \sim \sin(k_x)\sin(k_y)$ inter-orbital pairing coexists in the above phases, the intra-orbital $d_{xy}$ pairing is not favored even for large $J_2$. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V35.00009: Self-consistent calculations of the local density of states of FeAs superconductor/ferromagnet bilayers Nayoung Lee, Han-Yong Choi We study the local density of states (LDOS) of the superconductor/ferromagnet (S/F) bilayers using the self-consistent Bogoliubov-de Gennes equation, where the S is modeled in terms of the $s\pi $ pairing state. The $s\pi $ pairing is an s-wave pairing state with an internal $\pi $ phase~ between the two condensates of a two band superconductor which seems relevant for the FeAs superconductors. We calculate the pairing and magnetic order parameters self-consistently to obtain LDOS as a function of the energy and position of S/F bilayers. The results will be discussed in terms of the interplay between the internal and external $\pi $ phases. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V35.00010: Possible time-reversal symmetry breaking pairing state in FeAs superconductors Wei-Cheng Lee, Shou-Cheng Zhang, Congjun Wu We investigate the competition between the extended s-wave and d-wave pairing order parameters in the iron-based superconductors. Because of the frustrating pairing interactions among the electron and the hole Fermi pockets, a time reversal symmetry breaking $s+$i$d$ pairing state could be favored. We analyze this pairing state within the Ginzburg-Landau theory, and explore the experimental consequences. In such a state, spatial inhomogeneity induces supercurrent near a non-magnetic impurity and the corners of a square sample. The resonance mode between the s and d-wave order parameters can be detected through the B$_{1g}$-Raman spectroscopy. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V35.00011: Spin fluctuation mediated extended s-wave pairing from multiple Fermi surfaces in iron pnictide superconductors Kazuhiko Kuroki, Seiichiro Onari, Ryotaro Arita, Hidetomo Usui, Yukio Tanaka, Hiroshi Kontani, Hideo Aoki For the superconducting iron pnictides, we have constructed a minimal model, where all the five Fe d bands turn out to be involved[1]. The model is used to investigate the origin of superconductivity with a five-band random-phase approximation for solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realize basically an extended s-wave pairing, but that the gap function is in fact a matrix with significant off-diagonal elements. [1] K. Kuroki, S. Onari, R. Arita, H. Usui, Y. Tanaka, H. Kontani, and H. Aoki, Phys. Rev. Lett. \textbf{101}, 087004 (2008). [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V35.00012: Spin fluctuation dynamics and multiband superconductivity in iron pnictides Valentin Stanev, Jian Kang, Zlatko Tesanovic Multiband superconductivity, involving resonant pair scattering between different bands, has emerged as a likely possibility for the iron pnictides. In this scenario the gap changes sign between the hole and the electron Fermi surfaces (separated by wave-vector M). In the quest to distinguish this extended s- from an ordinary s- wave state, it is essential to use experiments that have momentum space resolution and can probe momenta of order M\footnote{ V. Stanev, J. Kang, and Z. Tesanovic, Phys. Rev. B \textbf{78}, 184509 (2008).}. We study the fluctuation dynamics of these superconducting states, as well as d- and p-wave states. The coupling between spin fluctuations and the quasiparticles of the superconducting state leads to damping of the former. The gap structure leaves a signature in the form of this damping. This can be used to diagnose the order parameter in spin sensitive experiments. We also discuss the case of coexistence of superconductivity and spin-density wave. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V35.00013: Emergent symmetry of superconductivity and antiferromagnetism in the iron pnictides Yong Baek Kim, Daniel Podolsky, Hae-Young Kee We show the presence of an emergent symmetry that unifies superconductivity and magnetism in the iron pnictides. We discuss the expected experimental consequences of this symmetry. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V35.00014: Two-band Model for Unconventional Superconductivity in Iron-Based Superconductors Yan Chen The recent discovery of FeAs superconductors with high Tc has triggered intensive efforts to explore magnetism and superconductivity in this family of materials. We adopt an effective model Hamiltonian including a two-band tight-binding term as well as a spin-spin exchange interactions term. The first term can fit approximately the multi-band dispersions near electron/hole Fermi-pockets while the second term may lead to the appropriate superconducting pairing symmetry as well as the SDW state at low doping. Different pairing symmetry candidates of FeAs materials are evaluated for various model parameters. In particular, we study the local electronic structure at vortex cores in the system subject to an external magnetic field. The relevance to recent STM measurements will be discussed. Moreover, we calculate the tunneling conductance of various superconducting junctions, including the Josephson currents and the Andreev reflection, between the FeAs materials modeled by a two-band superconducting state and various systems include normal metal, a conventional s-wave superconductor, d-wave superconductor, or same FeAs materials. The Josephson current can be decomposed into two parts, an interband and an intraband components. Distinct interference effects can be used to distinguish the pairing symmetry of FeAs system. [Preview Abstract] |
Session V37: Chemical Dynamics and Molecular Spectroscopy
Sponsoring Units: DCPChair: Alan Aspuru-Guzik, Harvard University
Room: 409
Thursday, March 19, 2009 8:00AM - 8:12AM |
V37.00001: Laser-excitation of molecular systems within stochastic time-dependent current-density-functional theory Heiko Appel, Massimiliano Di Ventra In this talk we investigate the excited electron dynamics of molecular systems due to laser excitation. The system dynamics is described within the recently proposed stochastic time-dependent current-density-functional theory [1,2]. \\[2mm] $[1]$ Massimiliano Di Ventra and Roberto D'Agosta, Phys. Rev. Lett. \textbf{98}, 226403 (2007).\\ $[2]$ Roberto D'Agosta and Massimiliano Di Ventra, Phys. Rev. B \textbf{78}, 165105 (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V37.00002: Nonlinear Response Functions in Model Dissipative Anharmonic Systems Mohammad Sahrapour, Nancy Makri We report the results of simulations of third order response functions ($R^{(3)}(\tau _3 ,0,\tau _1 )=\mbox{Tr}\left\{ {\hat {\alpha }(\tau _3 )[\hat {\alpha },[\hat {\alpha },[\hat {\alpha }(\tau _1 ),\rho _0 ]]]} \right\}$ where $\hat {\alpha }$ is the polarizability) for harmonic, Morse, and anharmonic model systems in a linearly dissipative environment. These simulations are carried out via the iterative path integral methodology developed earlier in our group which delivers efficient, numerically exact long time quantum dynamics. We find that even minor anharmonicity in the potential qualitatively changes the response function; rotating the pattern seen by 45$^{o}$ in the $\tau _1 -\tau _3 $ plane. We also observe that modulations in the $\tau _3 $ direction increase in frequency as we go to a more anharmonic potential. As the temperature is increased, these modulations also appear in the $\tau _1 $ direction. It is also found that asymmetry in the potential, at least at temperatures considered here, does not have a significant effect. Finally, in all three systems we notice that decay in the $\tau _3 $ direction is faster than in the $\tau _1 $ direction. The observed sensitivity of the response function to anharmonicities in the potential can be exploited to construct more accurate molecular potentials once the appropriate non-linear spectroscopic experiments have been performed. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V37.00003: Hamiltonian Monodromy: Unexpected behavior of quasi-linear molecules, atoms in traps and of hydrogen in crossed fields J.B. Delos, C. Schleif, D. Sadovskii, G. Dhont, B. Zhilinskii A system exhibits monodromy if we take the system around a closed loop in its parameter space, and we find that the system does not come back to its original state. Many systems have this property, including quasi-linear molecules, atoms in a trap or a hydrogen atom in crossed fields. Using classical perturbation theory, Sadovskii and Cushman predicted the presence of monodromy in perpendicular fields. It shows up as a defect in the lattice of quantum states. When the fields are tilted from perpendicular, these lattice defects undergo a series of bifurcations. Atoms in a trap can display a newly discovered dynamical manifestation of monodromy. This phenomenon will also occur with oriented dipolar molecules in fields or with quasilinear molecules. (Supported by NSF and Region Nord--Pas-de-Calais) [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V37.00004: On the internal photorelaxation mechanism of DNA Eric Bittner We propose a model for the photo-deactivation mechanism for DNA based upon accurate quantum chemical and molecular dynamical evaluations of model Watson/Crick nucleoside pairs and stacked pairs. Our results corroborate recent ultrafast experimental studies on DNA oligonucleotides and suggest that following photo-excitation to a local $\pi-\pi^*$ state, the excitation is rapidly delocalized over several (3-4) bases on an ultrafast time-scale. However, this delocalized state is unstable with respect to the motions of the protons involved in hydrogen-bonding between Watson/Crick pairs and rapidly re-localizes to a charge-transfer state on a longer time-scale ranging from 10 to 100 ps. This state, too, is unstable and relaxes via a conical intersection with the ground state near the geometry of the enol- and imino-tautomeric form. We suggest that this internal deactivation mechanism is responsible for the intrinsic photostability of DNA. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V37.00005: Tunneling Splitting in the Rotationally Resolved Electronic Spectrum of 1,3-Benzodioxole Jessica A. Thomas, Leonardo Alvarez-Valtierra, David W. Pratt Gas phase rotationally resolved electronic spectra were collected for the origin and several vibronic transitions of 1,3- benzodioxole. For each band, an autocorrelation program identified the presence of two overlapping spectra which were each fit using a least-squares algorithm to determine the rotational constants. Interpretation of these constants and how they change from one band to the next gives information about the physical causes of this splitting, which will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V37.00006: Vacuum Ultraviolet Absorption of Supercritical Water David Bartels, Ireneusz Janik, Timothy Marin The first continuum $\mathop X\limits^\sim \to \mathop A\limits^\sim $ peak in the gas phase absorption spectrum of H$_{2}$O (maximum at 7.4 eV) , is ascribed to promotion of a nonbonding valence electron to a dissociative excited state which is an admixture of antibonding sigma and 3s Rydberg orbitals. Due to the large spatial extent of this orbital the$\mathop X\limits^\sim \to \mathop A\limits^\sim $ transition is strongly perturbed by the local environment. In liquid water, this peak is notably broadened and shifted to the blue, with an absorption maximum of 8.3 eV at room temperature. An obvious question is how this transition changes as a function of the water density in the supercritical regime as the system transitions from liquid to gas. As density decreases, most water molecules will be located at an interface between large clusters and voids. We will describe an experiment to measure the VUV absorption of the $\mathop X\limits^\sim \to \mathop A\limits^\sim $ transition in supercritical water (T$>$374 \r{ }C, P$>$220 bar ) where the density can be tuned continuously. Experimentally this requires over six orders of magnitude detector dynamic range in the vacuum UV and a short path (ca. 1 micron) high temperature/pressure cell with sapphire windows. This has not been accomplished in any previous experiment. Results will be discussed in the light of recent ab initio calculations of the spectrum in large and small clusters. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V37.00007: Vibrational Coupling Pathways in the $\nu _{3}$ CH Stretch Fundamental Region of Methanol as Revealed by Coherence-Detected FTMW-IR Spectroscopy Sylvestre Twagirayezu, David S. Perry, Justin L. Neill, Matt T. Muckle, Brooks H. Pate Rotational state-selection on the methanol E species transitions 2$_{0}\leftarrow $3$_{-1}$ and 2$_{1}\leftarrow $3$_{0}$ is used to record the infrared (IR) spectra of the connected rotational levels of jet-cooled methanol, CH$_{3}$OH and CH$_{3}$OD. The observed spectra of CH$_{3}$OH contain twelve interacting vibrational bands in the interval 2755-2855 cm$^{-1}$, whereas a direct state count gives 14 vibrations in this interval grouped into 6 tiers by coupling order. The isotope dependence of the spectra confirms that the dominant coupling pathway is a third order coupling of the CH stretch to a combination of the COH bend and an HCH bend. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V37.00008: Ab Initio Torsion-Wag Surface for the Ethyl Radical Ram S. Bhatta, David S. Perry The torsion-wag potential of the ethyl radical has a 6-fold barrier to internal rotation and the minimum energy path involves deviations of the CH$_{2}$ wag angle of 6 to 11 degrees on either side of planar. Partially optimized 2-dimensional surfaces were calculated at the B3LYP, MP2, and CCSD(T) levels with 6-311++G(d,p) and 6-311++G(3df, 2p) basis sets and they were fit to a function containing a polynomial in the wag angle $\tau $ and trigonometric functions of the torsional angle $\alpha $. Comparison is made with the corresponding surfaces for CH$_{3}$NH$_{2}$ and CH$_{3}$OH$_{2}^{+}$. Unlike CH$_{3}$CH$_{2}^{\bullet }$, both have a substantial barrier to inversion. The dominant torsion-wag coupling term in all three cases has the form $\tau $cos3$\alpha $. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V37.00009: Terahertz Investigations of Extraordinarily Efficient Conduction in a Redox Active Ionic Liquid. Verner Thorsmolle, Jan Brauer, Guido Rothenberger, Daibin Kuang, Shaik Zakeeruddin, Michael Gr\"atzel, Jacques Moser Iodine added to iodide-based ionic liquids leads to extraordinarily efficient charge transport, vastly exceeding expectancy for such viscous systems. Using terahertz time-domain spectroscopy, in conjunction with dc conductivity and viscosity measurements we unravel the conductivity pathways in 1-methyl-3-propylimidazolium iodide melts. Applying low temperatures, we demonstrate for the first time conduction entirely due to a Grotthus bond-exchange mechanism at iodine concentrations higher than 3.9 M. The terahertz and transport results are reconciled in a model providing a quantitative description of the conduction by physical diffusion and the Grotthus bond-exchange process. These novel results are of great importance for the fundamental understanding of conduction in molten salts and for applications where ionic liquids are used as charge-transporting media such as in batteries and dye-sensitized solar cells. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V37.00010: Investigation of the Order-Disorder Transition in the Hybrid Inorganic-Organic System [(CH$_{3})_{2}$NH$_{2}$]Zn(HCOO)$_{3}$ by means of $^{1}$H NMR T. Besara, P. Jain, A.P. Reyes, P.L. Kuhns, N.S. Dalal, H.W. Kroto, A.K. Cheetham [(CH$_{3})_{2}$NH$_{2}$]Zn(HCOO)$_{3}$, a hybrid ABX$_{3}$ perovskite, with A=(CH$_{3})_{2}$NH$_{2}$, B=Zn and X=HCOO, undergoes a paraelectric-antiferroelectric transition around 156 K. Synchrotron studies indicate that hydrogen bonding between the H-atoms in the NH$_{2}$ group and O-atoms from the formate group is involved. The dimethylamine cation is disordered with nitrogen existing in three different positions, but not known whether statically or dynamically. We have investigated it by means of spin-lattice relaxation time, T$_{1}$, using proton NMR. We find that the cation is dynamically disordered and that the transition involves its slowing down. Evidence is seen for tunneling of the CH$_{3}$ groups, and for the compound becoming a glass, with the cation displaying several metastable equilibrium geometries (T$_{1}$ trajectories). [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V37.00011: High Resolution Cavity Ringdown Spectroscopy of Jet-Cooled Reactive Intermediates Gabriel Just, Patrick Rupper, Linsen Pei, Terry Miller Alkyl peroxy radicals long have been well known to be key intermediates in atmospheric chemistry as well as in low temperature combustion. For the last several years, our group has generated a data set for these radicals using room temperature cavity ringdown spectroscopy. We have recently extended our investigations of the peroxy radicals to obtain a high resolution data set of spectra under jet cooled conditions using a quasi-Fourier-transform-limited laser source and a supersonic slit jet discharge expansion.Over the last few years, we have developed our capability to obtain narrow-bandwidth, near infrared (NIR) radiation for performing high resolution cavity ringdown spectroscopy using the post-amplification of a Ti:Sa ring laser. The NIR light can be generated by either stimulated Raman shifting or by difference frequency mixing by combining the second harmonic of a Nd:YAG laser with our post- amplified light in a BBO crystal.Using this apparatus, we have observed isomer and conformer specific spectra of the following species : methyl peroxy, CH$_3$O$_2$, ethyl peroxy, C$_2$H$_5$O$_2$, propyl peroxy, C$_3$H$_7$O$_2$, and phenyl peroxy, C$_6$H$_5$O$_2$ . These spectra show rotationally resolved structure with a temperature of $\sim$15 K as well as other structure attributable to spin-rotation interactions, tunneling splittings etc. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V37.00012: Rotationally Resolved $\tilde{A}^2$A$_1$ - $\tilde{X}^2$E Electronic Spectra of Symmetric Methoxy Radicals: CH$_3$O and CD$_3$O Ming-Wei Chen, Dmitry Melnik, Jinjun Liu, Terry A. Miller Methoxy radical has attracted spectroscopic interest for more than twenty years. Microwave measurements of CH$_3$O and CD$_3$O with precision on the kHz scale have determined the $\tilde{X}^2$E parameters. Jet-cooled laser induced fluorescence (LIF) spectra have also been observed by our group with high-resolution ($\Delta\nu\sim$250 MHz) and high-accuracy ($\Delta\sigma\sim$50 MHz), for the 3$^2_0$ and 6$^1_0$ bands of the $\tilde{A}^2$A$_1$ - $\tilde{X}^2$E$_{3/2}$ electronic transition. Since the ground state component E$_{1/2}$ is $\sim$60 cm$^{-1}$ energetically higher than the E$_{3/2}$ spin component, the $\tilde{X}^2$E$_{1/2}$ state is not thermally populated in a jet-cooled environment. However, our complementary stimulated emission pumping (SEP) experiment with the same resolution and accuracy as the LIF work directly interrogates the $\tilde{X}^2$E$_{1/2}$ level of CH$_3$O and CD$_3$O by depleting the fluorescence from the $\tilde{A}^2$A$_1$ 3$^2$ levels. The global analysis of the microwave, LIF, and SEP data breaks correlations in the microwave data and provides better determinations for the $\tilde{X}$ and $\tilde{A}$ states' parameters. Comparison of the values for CH$_3$O, $^{13}$CH$_3$O, and CD$_3$O allows us to separate first-order from second-order electronic and vibrational contributions based upon the isotopic dependencies of the effective ground state parameters. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V37.00013: The Photoprotective Properties of Adenine: Time-resolved Photoelectron Spectroscopy at different excitation wavelengths Susanne Ullrich, Nick L. Evans, William M. Potter, Amanda N. Brouillette The UV photostability of biomolecules is determined by their excited state electronic relaxation mechanisms. To be effective, these mechanisms must operate on ultrafast timescales in order to dominate over competing photochemical processes that potentially lead to destruction of the biomolecule. Femtosecond time-resolved photoelectron spectroscopy (TRPES) provides unique capabilities for studying photoinduced processes in small polyatomic molecules. Changes in the PES, observed as the delay between the pump and probe pulses is scanned, can be associated with electronic configurational changes during the relaxation process. Analysis based on ionization correlations allows us to extract the electronic character of the excited states in addition to their lifetimes. Details of the experimental setup and technique will be presented in this talk as well as our initial results on the deactivation pathways in the DNA base adenine following excitation by wavelengths between 245-266 nm. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V37.00014: Observation of the $\tilde{A}$ -- $\tilde{X}$ Electronic Transitions of Cyclopentyl and Cyclohexyl Peroxy Radicals Via Cavity Ringdown Spectroscopy Rabi Chhantyal Pun, Phillip Thomas, Terry Miller Organic peroxy radicals are important intermediates in combustion chemistry. These molecules, formed from the addition of oxygen to alkyl radicals, are pivotal species in many atmospheric processes. We have previously targeted the $\tilde{A}$-$\tilde{X}$ transitions of straight and branched aliphatic peroxy radicals. This research has now been extended towards cyclic systems starting with cyclopentyl peroxy (C$_5$H$_9$O$_2$) and cyclohexyl peroxy (C$_6$H$_{11}$O$_2$), which are predicted by $ab initio$ and DFT calculations to have four and two low-lying conformers respectively. Both have conformers which differ on the orientation of the O-O bond with respect to the cyclic carbon skeleton namely, $cis$- and $gauche$-, with C$_6$H$_{11}$O$_2$ also having conformers with axial and equatorial placement on the ring. We observe strong bands for both peroxies in the near-IR which are favorably assigned as the origin and the O-O stretch in agreement with the calculations. We have also obtained the spectrum of C$_6$D$_{11}$O$_2$ which facilitates the assignments of the weaker vibrational structure in C$_6$H$_{11}$O$_2$. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V37.00015: Dynamic light scattering in an aqueous solution of 3-methylpyridine Dimitry Ivanov, Anna Trubetskaya, Andrei Kostko, Mikhail Anisimov, Jan Sengers We report a set of dynamic light scattering experiments in an aqueous solution of 3-methylpyridine. The dynamic correlation functions appear to exhibit two modes: one associated with a normal diffusion process and another one with network relaxation. The observed correlations seem to be associated with long-living nonequilibrium structures. To obtain further insight into this phenomenon we have made systematic studies of the nature of the observed dynamics as a function of time and concentration. [Preview Abstract] |
Session V38: Focus Session: The Transition State in Physics, Chemistry, and Astrophysics II
Sponsoring Units: DCPChair: K. Srihari, Indian Institute of Technology
Room: 410
Thursday, March 19, 2009 8:00AM - 8:36AM |
V38.00001: Quantum Transition State Theory Invited Speaker: The main idea of Wigner's transition state theory (TST) is to compute reaction rates from the flux through a dividing surface placed between reactants and products. In order not to overestimate the rate the dividing surface needs to have the no- recrossing property, i.e. reactive trajectories cross the dividing surface exactly once, and nonreactive trajectories do not cross it at all. The long standing problem of how to construct such a diving surface for multi-degree-of-freedom systems was solved only recently using ideas from dynamical systems theory. Here a normal form allows for a local decoupling of the classical dynamics which leads to the explicit construction of the phase space structures that govern the reaction dynamics through transition states. The dividing surface is spanned by a normally hyperbolic manifold which is the mathematical manifestation of the transition state as an unstable invariant subsystem of one degree of freedom less than the full system. The mere existence of a quantum version of TST is discussed controversially in the literature. The key isssue is the presence of quantum mechanical tunneling which prohibits the existence of a local theory analogous to the classical case. Various approaches have been devloped to overcome this problem by propagating quantum wavefunctions through the transition state region. These approaches have in common that they are computationally very expensive which seriously limits their applicability. In contrast the approach by Roman Schubert, Stephen Wiggins and myself is local in nature. A quantum normal form allows us to locally decouple the quantum dynamics to any desired order in Planck's constant. This yields not only the location of the scattering and resonance wavefunctions relative to the classical phase space structures, but also leads to very efficient algorithms to compute cumulative reaction probabilities and Gamov-Siegert resonances which are the quantum imprints of the transition state. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V38.00002: Transition States in a Noisy Environment Invited Speaker: |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V38.00003: Statistical Theory of Asteroid Escape Rates Charles Jaffe Transition states in phase space are identified and shown to regulate the rate of escape of asteroids temporarily captured in circumplanetary orbits. The transition states, similar to those occurring in chemical reaction dynamics, are then used to develop a statistical semianalytical theory for the rate of escape of asteroids temporarily captured by Mars. Theory and numerical simulations are found to agree to better than 1\%. These calculations suggest that further development of transition state theory in celestial mechanics, as an alternative to large-scale numerical simulations, will be a fruitful approach to mass transport calculations. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 10:00AM |
V38.00004: Exploring remnants of invariants buried in a deep potential well in chemical reactions Invited Speaker: How the reacting system climbs through saddles from one basin to another on potential energy surface has been one of the most intriguing subjects not only in chemistry but also physics and biology. This decade significant progress has been achieved in establishing the concept of the so-called transition state (TS), that is, a hypersurface of co-dimension one through which the system passes through only once from one basin to another [1-3]. However, there exist still open problems to be resolved; 1) how the no-return TS ceases or bifurcates as the energy increases [4], 2) how the stable/unstable invariant manifolds emanating from the normally hyperbolic invariant manifold \textit{wander} in deep potential wells in many-degrees of freedom (dofs) systems [5] or how one can generalize the \textit{remnant of invariant manifolds} [6] to many-dofs systems, 3) how one can generalize the concept of no-return TS besides the region of first-rank saddles. Related to the problem 2), most of all the chemical reaction theories assume that all of the available energy redistributes \textit{statistically} through the dofs of system in the reactant well before the reaction takes place. It is implicitly expected that the ratio of the measure occupied by tori in phase space to that of the ambient space decreases exponentially as the dimensionality of the system increases. Here we present a novel technique to scrutinize the remnant of invariants buried in chaos in many-degrees of freedom systems [7]. This is regarded as the remnants of a destroyed invariant manifold that may dominate the transport in phase space even at high energy regions where most of all tori vanish. We demonstrate the potentiality of our technique for HCN isomerization, where the conventional procedure based on a finite order truncation in the coordinate transformation of canonical perturbation theory prevent us from detecting remnants of invariants. \\[4pt] [1] T. Komatsuzaki \textit{et al.}, \textit{J. Chem. Phys.} \textbf{105}, 10838(1996); \textit{ibid}. \textbf{110}, 9160 (1999) \\[0pt] [2] W.S. Koon \textit{et al.}, \textit{Chaos} \textbf{10}, 427 (2000) \\[0pt] [3] T. Uzer \textit{et al.}, \textit{Nonlinearity} \textbf{15}, 957(2002); H. Waalkens\textit{ et al.}, \textit{Nonlinearity} \textbf{21}, 1 (2008) \\[0pt] [4] C.-B. Li\textit{ et al.}, \textit{Phys. Rev. Lett. }\textbf{97}, 028302 (2006) \\[0pt] [5] R. B. Shirts \textit{et al.}, \textit{J. Chem. Phys. }\textbf{77}, 5204 (1982) \\[0pt] [6] C. Jaff\'e\textit{ et al.}, \textit{Phys. Rev. A} \textbf{60}, 3833 (1999) \\[0pt] [7] H. Teramoto \textit{et al.} \textit{J. Chem. Phys. }\textbf{129} 094302~(2008); \textit{Phys. Rev. E} \textbf{78}, 017202 (2008) [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:36AM |
V38.00005: Intramolecular energy transfer, driving mechanisms, and reaction rates for collective motions of clusters Invited Speaker: Conformational transitions of molecules, clusters, and biopolymers are typically large-amplitude collective motions that involve a large number of degrees of freedom in a coherent manner. One of the major challenges in modern molecular science is to understand the general mechanism for such collective motions. This talk highlights a novel dynamical mechanism for conformational transitions of atomic clusters in terms of intramolecular energy transfer and the driving forces for large-amplitude collective motions. First, we introduce a method of hyperspherical mode analysis, which generally classifies the (3n-6) internal modes of an arbitrary n-atom molecule into three gyration-radius modes, three twisting modes, and (3n-12) shearing modes. This hyperspherical mode analysis reveals that the gyration-radius modes are the primary collective modes that need to be activated in order for the system to achieve large-amplitude conformational transitions. Moreover, it illustrates how the twisting modes and the shearing modes critically initiate and trigger the conformational transitions by inducing the essential driving forces that activate the gyration-radius modes via mode coupling. Finally, we characterize this driving mechanism for conformational transitions from the viewpoint of the phase space geometry of the low-dimensional dynamical system of the gyration-radius modes. This reduced phase space geometry clearly accounts for the origin of non-statistical reaction rate processes of the clusters. The present method of hyperspherical mode analysis as well as the driving mechanism for collective motions could be widely applicable to conformational dynamics of complex molecular systems. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V38.00006: Collective coherent control: Synchronization of polarization in ferroelectric PbTiO$_{3}$ by shaped THz fields Tingting Qi, Young-Han Shin, Ka-Lo Yeh, Nelson Keith, Rappe Andrew Coherent optical control over ultrafast molecular behavior including chemical reactions has been explored in recent years, spurred by the application of optimal control theory and related methods and by the development of femtosecond pulse shaping techniques through which complex optical waveforms have been crafted and optimized to induce specified molecular responses. Here we propose and model theoretically the extension of coherent control to collective structural change. We show that properly shaped terahertz fields, resonant with selected lattice vibrational frequencies, could be used to move ions in ferroelectric crystals from their positions in an initial domain orientation along well defined collective microscopic paths into the positions they occupy in a new domain orientation. Collective coherent control will enable direct observation of fast highly nonlinear material responses and far-from-equilibrium structures that can be harnessed in electro-optic devices and non-volatile computer memory. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V38.00007: Classical-Quantum correspondence in isomerization dynamics: quantum eigenstates and classical Arnol'd web S. Keshavamurthy Recently, there has been a renaissance of sorts in chemical dynamics with researchers critically examining the validity of the two pillars of reaction rate theory - transition state theory and the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Since both theories have classical dynamics at their foundation, advances in our understanding of nonlinear dynamics and continuing efforts to characterize the phase space structure of systems with three or more degrees of freedom are beginning to yield crucial mechanistic insights into the dynamics. This talk focuses on a mechanistic understanding of the deviations from RRKM theory for a model isomerization problem with three degrees of freedom. Several studies have established that such systems are prime candidates for observing non-RRKM behavior\footnote{D. M. Leitner, Int. J. Quant. Chem. {\bf 75}, 523 (1999).}. The model is inspired, and generalized, from a much earlier study by De Leon and Berne\footnote{N. De Leon and B. J. Berne, J. Chem. Phys. {\bf 75}, 3495 (1981).}. We try to answer two of the questions posed in this early work by studying the intramolecular vibrational energy flow in the system from both classical and quantum viewpoints. Using a wavelet-based local frequency analysis it is possible to construct a useful representation of the classical phase space (Arnol'd web) highlighting the important dynamical structures. Insights into the dynamics originate from the various nonlinear resonances and phase space traps which potentially result in quantum eigenstates of varying degree of localization\footnote{D. M. Leitner and M. Gruebele, Mol. Phys. {\bf 106}, 433 (2008).}. [Preview Abstract] |
Session V39: Biological Networks and Systems Biology
Sponsoring Units: DBPChair: Ilya Nemenman, Los Alamos National Laboratory
Room: 411
Thursday, March 19, 2009 8:00AM - 8:12AM |
V39.00001: Boolean modeling of collective effects in complex networks Johannes Norrell, Joshua Socolar Complex systems are often modeled as Boolean networks in attempts to capture their logical structure and reveal its dynamical consequences. Approximating the dynamics of continuous variables by discrete values and Boolean logic gates may, however, introduce dynamical possibilities that are not accessible to the original system. We show that large random networks of variables coupled through continuous transfer functions often fail to exhibit the complex dynamics of corresponding Boolean models in the disordered (chaotic) regime, even when each individual function appears to be a good candidate for Boolean idealization. A simple criterion identifies continuous systems that exhibit the full dynamical range of their Boolean counterparts. Transfer functions inferred from the literature on transcriptional regulation of genes do not satisfy the criterion. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V39.00002: A Network Model For Sea Urchin Development Xinwei Gong, Xianrui Cheng, Joshua Socolar The sea urchin embryo developmental gene regulatory network has been a subject of experimental study for decades. While current knowledge of the network is incomplete, boolean network models with autonomous updating can reveal dynamical features of the known network. Our analysis of such a model based on the network provided by Davidson et al [http://sugp.caltech.edu/endomes/index.html] shows that, with the suggested initial inputs and certain sets of logic functions that are consistent with the known regulatory relations, a 3-cell system settles into an attractor that corresponds to the 3 different cell fates expected for the organism. The attractor is not sensitive to modest variations in the time delay parameters. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V39.00003: Achieving optimal growth: lessons from simple metabolic modules Sidhartha Goyal, Thomas Chen, Ned Wingreen Metabolism is a universal property of living organisms. While the metabolic network itself has been well characterized, the logic of its regulation remains largely mysterious. Recent work has shown that growth rates of microorganisms, including the bacterium \textit{Escherichia coli}, correlate well with optimal growth rates predicted by flux-balance analysis (FBA), a constraint-based computational method. How difficult is it for cells to achieve optimal growth? Our analysis of representative metabolic modules drawn from real metabolism shows that, in all cases, simple feedback inhibition allows nearly optimal growth. Indeed, product-feedback inhibition is found in every biosynthetic pathway and constitutes about 80{\%} of metabolic regulation. However, we find that product-feedback systems designed to approach optimal growth necessarily produce large pool sizes of metabolites, with potentially detrimental effects on cells via toxicity and osmotic imbalance. Interestingly, the sizes of metabolite pools can be strongly restricted if the feedback inhibition is ultrasensitive ($i.e.$ with high Hill coefficient). The need for ultrasensitive mechanisms to limit pool sizes may therefore explain some of the ubiquitous, puzzling complexity found in metabolic feedback regulation at both the transcriptional and post-transcriptional levels. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V39.00004: Bifurcation in stochastic differential equations with delayed feedback Gaudreault Mathieu, Jorge Vinals The bifurcation diagram of a model nonlinear Langevin equation with delayed feedback is obtained numerically. This model relates to a common motif in genetic regulatory networks, and we study the effect of fluctuating parameters on the bifurcation diagram of the network. We observe both direct and oscillatory bifurcations in different ranges of model parameters. Below threshold, the stationary distribution function $p(x)$ is a delta function at the trivial state $x=0$. Above threshold, $p(x) \sim x^{\alpha}$ at small $x$, with $\alpha = -1$ at threshold, and monotonously increasing with the value of the control parameter above threshold. Unlike the case without delayed feedback, the bifurcation threshold is shifted by fluctuations by an amount that scales linearly with the noise intensity. With numerical information about time delayed correlations, we derive an analytic expression for $p(x) $ which is in good agreement with the numerical results. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V39.00005: Constraints imposed by nonfunctional protein-protein interactions on gene expression and proteome size Jingshan Zhang, Sergei Maslov, Eugene Shakhnovich Crowded intracellular environments present a challenge for proteins to form functional specific complexes while reducing nonfunctional interactions with promiscuous nonfunctional partners. Here we show how nonfunctional interactions limit the proteome diversity and the average concentration of co-expressed and co-localized proteins. We use yeast compartments to verify our hypothesis that the yeast proteome has evolved to operate closely to the upper limit of its size, while keeping individual protein concentrations sufficiently low to reduce nonfunctional interactions. These findings have implication for conceptual understanding of intracellular compartmentalization, multicellularity, and differentiation. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V39.00006: Effect of TNF autocrine signaling on dosage-dependent NF-kappaB response to lipopolysaccharide stimulation Jaewook Joo, Bryan Carson, Cathy Branda, Jens Poschet We will present the dosage-dependent characteristics of NF-kappaB translocation patterns from single macrophages stimulated by E. Coli lipopolysacchride. The NF-kappaB translocation patterns in single cells are found to be quite heterogeneous: The patterns are more heterogeneous with low dosage stimulation than with high dosage stimulation. For low dosage stimulation, most of cells take a rising pattern and we demonstrate that it is due to the TNFalpha autocrine signaling effect. The above results are predicted and explained by a computational model, and corroborated and verified by a single cell fluorescence imaging technique. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V39.00007: Gain control in molecular signaling without feedback Ilya Nemenman Statistical properties of environments experienced by biological systems in the real world change, and this requires adaptation to achieve a high fidelity information transmission in cellular networks. One form of such adaptive response is gain control. When the mean response of a signaling system is matched to the mean value of its signal, rescaling the gain allows to respond to signals with different variances without saturation and by utilizing the entire available dynamic range of the response. Here we argue that a certain simple mechanism of gain control, understood well in the context of systems neuroscience, translates to molecular signaling systems as well. The mechanism allows to transmit more than one bit (on or off) of information about the signal independently of the signal variance. The mechanism does not require additional molecular circuitry beyond that already present in many molecular systems, and, in particular, it does not depend on existence of feedback loops. This analysis provides a plausible explanation for certain structural aspects of cellular networks. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V39.00008: Noise in Random Boolean Networks Tiago Peixoto, Barbara Drossel We investigate the effect of noise on Random Boolean Networks. Noise is implemented as a probability $p$ that a node does not obey its deterministic update rule. We define two order parameters, the long-time average of the Hamming distance between a network with and without noise, and the average frozenness, which is a measure of the extent to which a node prefers one of the two Boolean states. We evaluate both order parameters as function of the noise strength, finding a smooth transition from deterministic ($p=0$) to fully stochastic ($p=1/2$) dynamics for networks with $K\le2$, and a first order transition at $p=0$ for $K>2$. Most of the results obtained by computer simulation are also derived analytically. The average Hamming distance can be evaluated using the annealed approximation. In order to obtain the distribution of frozenness as function of the noise strength, more sophisticated self-consistent calculations had to be performed. This distribution is a collection of delta peaks for $K=1$, and it has a fractal sructure for $K>1$, approaching a continuous distribution in the limit $K\gg1$. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V39.00009: The effect of negative autoregulation on eukaryotic gene expression Dmitry Nevozhay, Rhys Adams, Kevin Murphy, Kresimir Josic, G\'abor Bal\'azsi Negative autoregulation is a frequent motif in gene regulatory networks, which has been studied extensively in prokaryotes. Nevertheless, some effects of negative feedback on gene expression in eukaryotic transcriptional networks remain unknown. We studied how the strength of negative feedback regulation affects the characteristics of gene expression in yeast cells carrying synthetic transcriptional cascades. We observed a drastic reduction of gene expression noise and a change in the shape of the dose-response curve. We explained these experimentally observed effects by stochastic simulations and a simple set of algebraic equations. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V39.00010: Cell stimulation with optically manipulated microsources. Holger Kress, Jin-Gyu Park, Cecile Mejean, Jason Forster, Jason Park, Spencer Walse, Dianqing Wu, Orion Weiner, Tarek Fahmy, Eric Dufresne Many cells can sense spatial and temporal heterogeneities in concentrations of soluble molecules. The cellular signal transduction which forms the basis of this ability consists of signaling cascades and loops whose length and time scales are largely unknown. The systematic investigation of these networks requires control over the chemical microenvironment of cells. We present a novel technique to create molecular concentration patterns that are chemically, spatially and temporally flexible. Our approach uses optically manipulated colloidal particles which act as microsources of soluble molecules. This technique for flexible cell stimulation is combined with quantitative live cell microscopy measurements of cellular responses. We demonstrate the method by inducing chemotaxis in neutrophils. We quantify the intracellular calcium release, actin distribution, shape and motility of single cells. The possibility for quantitative stimulus-response measurements on single cells makes this method applicable to a wide range of systems biology studies. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V39.00011: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V39.00012: Determining Regulatory Networks Governing the Differentiation of Embryonic Stem Cells to Pancreatic Lineage Ipsita Banerjee Knowledge of pathways governing cellular differentiation to specific phenotype will enable generation of desired cell fates by careful alteration of the governing network by adequate manipulation of the cellular environment. With this aim, we have developed a novel method to reconstruct the underlying regulatory architecture of a differentiating cell population from discrete temporal gene expression data. We utilize an inherent feature of biological networks, that of sparsity, in formulating the network reconstruction problem as a bi-level mixed-integer programming problem. The formulation optimizes the network topology at the upper level and the network connectivity strength at the lower level. The method is first validated by in\textit{-silico} data, before applying it to the complex system of embryonic stem (ES) cell differentiation. This formulation enables efficient identification of the underlying network topology which could accurately predict steps necessary for directing differentiation to subsequent stages. Concurrent experimental verification demonstrated excellent agreement with model prediction. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V39.00013: Stabilizing Motifs in Autonomous Boolean Networks and the Yeast Cell Cycle Oscillator Volkan Sevim, Xinwei Gong, Joshua Socolar Synchronously updated Boolean networks are widely used to model gene regulation. Some properties of these model networks are known to be artifacts of the clocking in the update scheme. Autonomous updating is a less artificial scheme that allows one to introduce small timing perturbations and study stability of the attractors. We argue that the stabilization of a limit cycle in an autonomous Boolean network requires a combination of motifs such as feed-forward loops and auto-repressive links that can correct small fluctuations in the timing of switching events. A recently published model of the transcriptional cell-cycle oscillator in yeast contains the motifs necessary for stability under autonomous updating [1]. \newline \newline [1] D.~A. Orlando, et al. \newblock \emph{Nature (London)}, 453\penalty0 (7197):\penalty0 944--947, 2008. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V39.00014: The Transition Pathway from Nonspecific to Specific Complex of DNA with a DNA-Bending Protein Serguei Kuznetsov, Paula Vivas, Yogambigai Velmurugu, Anjum Ansari Integration host factor (IHF) from \textit{E. coli} is a DNA-bending protein that recognizes and binds to its specific sites primarily by the indirect read-out mechanism, in which sequence-dependent DNA dynamics and flexibility play an important role. The crystal structure of IHF bound to a 35-bp long cognate site H' indicates that the DNA is kinked at two sites separated by $\sim $9 bp, resulting in a ``U-turn'' bend of the DNA. To probe the DNA bending dynamics, we use a laser T-jump, and time-resolved FRET on end-labeled DNA substrates. Our results show that DNA bending occurs on the same time-scales as thermal disruption of single base-pairs in B-DNA, suggesting that spontaneous kinking may be the rate-limiting step. To test this hypothesis, we modified the DNA at the site of the kinks by introducing (i) a nick in the sugar-phosphate backbone, and (ii) mismatches to create internal loops. For each of these substrates, the 4-20 fold increase in the binding affinity is reflected in a corresponding increase in the bending rates. Furthermore, the DNA bending rates are independent of the salt concentration. These results indicate that in the transition state ensemble the DNA is kinked, but specific protein-DNA interactions involving ion release have not occurred. [Preview Abstract] |
Session V40: Mechanics of Biomolecular Systems II
Sponsoring Units: DBPChair: Andrew Rutenberg, Dalhousie University
Room: 412
Thursday, March 19, 2009 8:00AM - 8:12AM |
V40.00001: Non-monotonic wave dispersion in one-dimensional spiral track of cardiac cells Tae Yun Kim, Okyu Kwon, Kyoung J. Lee Alternans, periodic cardiac beat-to-beat alternation, is an important precursor to cardiac fibrillation. The underlying mechanism for this phenomenon has been discussed mostly based on the electro-chemical kinetics of constituent cells (myocytes) that comprise the heart system. An important unexplored aspect of this phenomenon is the role of wave dispersion that reflects the cell-to-cell coupling as well as the local kinetic properties. A recent modeling study in fact suggests that a non-monotonic wave dispersion can be a source for alternans. We have designed and built very long ($\sim $ 15 cm) in vitro quasi-one dimensional tracks of rat ventricular cells for elucidating the instability responsible for the transition to alternans. One dimensional tracks are favorable, since it excludes the effect of local wave curvature. Systematic investigations based on S1-S2 stimulation protocols are carried out and here we present some preliminary evidence of non-monotonicity in cardiac wave dispersion. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V40.00002: Periodic reversals allow bacteria to swarm Yilin Wu, Dale Kaiser, Yi Jiang, Mark Alber Many bacteria can rapidly traverse surfaces from which they are extracting nutrient for growth. They generate flat, spreading colonies, called swarms because they resemble swarms of insects. We seek to understand how members of any dense swarm track their neighbors while interfering minimally with the motion of others'. We choose myxobacteria as our model system. Individual myxobacteria cells regularly reverse their gliding directions. With a cell- and behavior-based computational model, we show that reversals of gliding direction are essential for swarming and that reversals increase the outflow of cells across the edge of the swarm. We also find that the reversal period predicted to maximize the outflow of cells is the same (within the errors of measurement) as the period observed in experiments with normal myxobacteria cells. This coincidence suggests that the circuit regulating reversals evolved to its current sensitivity under selection for growth achieved by swarming. Our work suggests a crucial componentin the general behavioral algorithm governing bacterial swarming. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V40.00003: Do Recurved Sensory Organs in Drosophila Form Through a Turing-Type Bifurcation? Huifeng Zhu We study the recurved bristles on {\it Drosophila} wing margin of wild-type and mutant.The expression levels of the {\it achaete-scute} complex protein determine the epidermal or neural fate of a pro-neural cell. In wide-type flies, the development ends in a state where a recurved bristle grows out nearly every fifth cell. Recent experiments have shown that the frequency of recurved bristles can be changed by adjusting the mean concentrations of the zinc-finger transcription factor {\it Senseless} and the micro\_RNA miR-9a. With reduced levels of miR-9a, mutant flies grow regular organization of recurved bristles, but with a lower periodicity. We argue that the characteristics of bristle organization are signatures of a Turing-type bifurcation which emerges from a uniform background in reaction-diffusion process, in continua. In contract, fly wing margin consists of a discrete array of cells with possible cross-species interactions. Further, proteins do not diffuse between cells. We argue that the intracellular actions can play the role of diffusion in a discrete cell array. However, the analogs of diffusion coefficients can be positive or negative. Intracellular actions should give a conserved cell number periodicity. We introduce a simple model to study pattern formation in such cellular arrays based on intracellular actions. Also, we observe that periodicity both in length and cell numbers from different group of flies. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V40.00004: Sisyphus at the Nanoscale: Bacterial Topotaxis in a Microfabricated Environment Guillaume Lambert, Peter Galajda, David Liao, Robert H. Austin The ballistic-like motion of self-propelled particles at low-Reynolds number can be exploited to influence their direction of motion. In particular, it has been demonstrated that by using the right topology (in this case a micro-fabricated array of funnel-like asymmetrical barriers), {\it E.coli} bacteria can be ``pumped'' between two adjacent regions (Galajda 2007, Wan 2008). We built upon this idea and developed a micro-habitat array in which chemotaxis and topotaxis --nutrient- and topology-driven motion, respectively-- are in opposition, leading to an inherently unstable environment in which a bacterium is constantly pushed away from the fitness landscape's summit in a Sisyphean fashion. Surprisingly, we find that the bacterial population as a whole is able to overcome the rectifying array. An in-depth microscopic analysis of the swimmer's motion is used to quantify the strategies adopted by the bacteria. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V40.00005: Microrheology of swimming bacteria suspension. Andrey Sokolov, Igor Aronson We study rheology of suspension of swimming bacteria Bacillus Subtilis at high concentrations. Experiments were performed in a free standing fluid film contained in a transparent chamber with adjustable Oxygen/Nitrogen ratio. The swimming velocity of bacteria is controlled by the concentration of dissolved Oxygen: it reduces to zero when Oxygen is completely replaced by Nitrogen. Macroscopic flow in a film is produced by oscillations and rotations of magnetic particles by rotating external magnetic field. To extract the effective viscosity, we measured macroscopic velocity field generated by the particles using PIV of fluorescent markers seeded to the film. We discovered that viscosity of bacterial suspension is increasing with decreasing swimming speed of bacteria due lack of Oxygen. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V40.00006: Microrheology of Actin Network Depends on Probe Size, Surface Chemistry and Depletion Effect Jun He, Jay Tang Microrheological properties of F-actin were measured by video particle tracking using beads with different size and surface chemistry. We found that the mean square displacements of probe particles scale with bead diameter with an exponent of about -0.45 instead of -1. This scaling behavior results in the measured shear moduli of F-actin network varying with the probe size. The main features of our data can be accounted for by the probe surface stickiness and the opposing depletion effect, both of which are confirmed by confocal imaging of beads in the actin network. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V40.00007: Mechanics of biomimetic systems propelled by actin comet tails Hyeran Kang, Dhananjay Tambe, Vivek Shenoy, Jay Tang The motility of intracellular bacterial pathogens such as \textit{Listeria monocytogenes} is driven by filamentous actin comet tails in a variety of trajectories. Here, we present the \textit{in vitro} study on the actin-based movements using spherical beads of different sizes coated with VCA protein, a partial domain of N-Wasp, in platelet extracts. Long term two-dimensional trajectories of the spherical beads motility show characteristic difference than those observed for bacteria, which have both elongated shape and asymmetric expression of the polymerization inducing enzyme. The trajectories also vary sensitively with the bead size and shape. These results provide a useful test to our new analytical model including the rotation of the bead relative to the tail. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V40.00008: Generalized rules for the optimization of elastic network models Timothy Lezon, Eran Eyal, Ivet Bahar Elastic network models (ENMs) are widely employed for approximating the coarse-grained equilibrium dynamics of proteins using only a few parameters. An area of current focus is improving the predictive accuracy of ENMs by fine-tuning their force constants to fit specific systems. Here we introduce a set of general rules for assigning ENM force constants to residue pairs. Using a novel method, we construct ENMs that optimally reproduce experimental residue covariances from NMR models of 68 proteins. We analyze the optimal interactions in terms of amino acid types, pair distances and local protein structures to identify key factors in determining the effective spring constants. When applied to several unrelated globular proteins, our method shows an improved correlation with experiment over a standard ENM. We discuss the physical interpretation of our findings as well as its implications in the fields of protein folding and dynamics. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V40.00009: Pulling helices inside bacteria: imperfect helices and rings Andrew Rutenberg, Jun Allard We study steady-state configurations of intrinsically-straight elastic filaments constrained within rod-shaped bacteria that have applied forces distributed along their length. Perfect steady-state helices result from axial or azimuthal forces applied at filament ends, however azimuthal forces are required for the small pitches observed for MreB filaments within bacteria. Helix-like configurations can result from distributed forces, including co-existence between rings and imperfect helices. Levels of expression and/or bundling of the polymeric protein could mediate this co-existence. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V40.00010: Bending rigidity of type I collagen homotrimer fibrils Sejin Han, Sergey Leikin, Wolfgang Losert Normal type I collagen is an $\alpha $1(I)$_{2}\alpha $2(I) heterotrimeric triple helix, but $\alpha $1(I)$_{3}$ homotrimers are also found in fetal tissues and various pathological conditions, e.g., causing bone fragility and reducing tendon tensile strength. It remains unclear whether homotrimers alter mechanical properties of individual fibrils or affect tissues by altering their organization at a higher level. To address this question, we investigated how homotrimers affect fibril bending rigidity. Homotrimer fibrils have been shown to be more loosely packed so that we expected them to be more susceptible to bending. However, homotrimer fibrils were more rigid despite being thinner and more hydrated. To quantify fibril rigidity, we analyzed their shape by Fourier decomposition, determined the correlation function for the direction along each fibril, and calculated the distribution of local fibril curvature. The estimated persistence length of homotrimer fibrils was 3 $\sim $ 10 times longer than for heterotrimer fibrils, indicating much higher bending rigidity of homotrimer fibrils. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V40.00011: Surface manipulation of protein filaments Laurent Kreplak, Douglas Staple, Marko Loparic, Hans-Juergen Kreuzer Within mammalian tissues, cells move by actively remodeling a dense network of collagen fibrils. In order to study this situation, we analyze the force response of two types of filamentous protein structures, desmin intermediate filaments 12 nm in diameter and collagen fibrils 80 nm in diameter. Both types of filaments were adsorbed at a solid-liquid interface and locally moved with an AFM tip at constant velocity against surface friction in the interfacial plane. In the case of collagen fibrils, that have an extensibility below 30{\%} extension, we observed that microns long fibrils could be moved by the tip and deformed into shapes that could not be explain by the linear elastic theory for a stiff rod. In the case of desmin filaments that can be stretched up to 3.5 times there length, we observed local stretching of the filaments and discreet steps in the torsional force measured with the cantilever. In order to describe both types of filaments' behaviors, we described the protein filaments as a chain of beads of mass $m$ linked together by a mass-less polymer linker. By solving the Newtonian equations of motions for the coupled beads in the presence of a point load and a viscous drag due to the surface-filament interactions we were able to reproduced our experimental data and extract information on friction. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V40.00012: Multiscale Mechanics of Fibrin Polymer Andre Brown, Rustem Litvinov, Dennis Discher, Prashant Purohit, John Weisel Blood clots and thrombi consist primarily of fibrin, a branched, open mesh of polymeric fibers made of protein monomers, with a remarkable and unexplained extensibility and elasticity. Understanding the origin of fibrin mechanics may ultimately be significant for modulating thrombosis and bleeding. We propose that the unique mechanical properties of fibrin are based on its ability to undergo concerted structural rearrangements at the network, fiber, and molecular levels. Stretching of a whole fibrin clot is followed by clot shrinkage, fiber alignment and bundling, and extension of the constituent fibrin molecules. We develop constitutive models that integrate these quantitative observations and suggest that fibrin extensibility and elasticity are largely manifestations of protein unfolding. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V40.00013: Cardiomyocytes beat best on a matrix with heart-like elasticity -- Molecular mechanics of the changes Christine Carag, Adam Engler, Dennis Discher Cardiomyocytes derived from embryos beat spontaneously in culture, but it is shown here with a series of flexible substrates that matrices which mimic the elasticity of the developing heart are optimal for 1-Hz beating, for transmitting contractile work to the matrix, and for promoting actomyosin striation. On hard matrices that mechanically mimic a post-infarct fibrotic scar, cells overstrain themselves, lack striated myofibrils and stop beating; on very soft matrices, cells preserve contractile beating for days in culture but do very little work. Optimal matrix leads to a strain match between cell and matrix, and suggests dynamic differences in intracellular protein structures. A novel `cysteine shotgun' method of labeling the in situ proteome reveals differences in assembly or conformation of several abundant cytoskeletal proteins, including vimentin, filamin and myosin. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V40.00014: Dynamic Viscoelasticity of Individual Bacterial Cells Virginia Vadillo-Rodriguez, John Dutcher We have used an AFM-based approach to probe the mechanical properties of single bacterial cells (gram-negative \textit{Escherichia coli }K12) by applying a constant compressive force to the cell under fluid conditions while measuring the time-dependent displacement (creep) of a colloidal AFM tip due to the viscoelastic properties of the cell. We observed that the cells exhibited a viscoelastic solid-like behavior with retarded elasticity, i.e. both an instantaneous and a delayed elastic deformation, which is well described by a three-parameter mechanical model. Using the best fit parameter values, we have calculated the dynamic viscoelastic behavior of the cells over a wide range of frequencies based on a numerical time-frequency transform technique and we have compared the calculated behavior with that measured experimentally. Comparison of the results obtained for \textit{E. coli }with previously reported data on the mechanical properties of others gram-negative cells and their isolated surface layers suggests that the elastic component of the cell viscoelastic response is dominated by the properties of the peptidoglycan layer, whereas the viscous component likely arises from the liquid-like character of the cell membranes. [Preview Abstract] |
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