Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T1: Probes of Nanoscale Magnetism
Sponsoring Units: DCMPChair: Chris Hammel, Ohio State University
Room: Spirit of Pittsburgh Ballrom A
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T1.00001: Chiral magnetic order at surfaces driven by inversion asymmetry Invited Speaker: |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T1.00002: Ultrasensitive magnetometry and magnetic resonance imaging using cantilever detection Invited Speaker: Micromachined cantilevers make remarkable magnetometers for nanoscale measurements of magnetic materials and for magnetic resonance imaging (MRI). We present various applications of cantilever magnetometry at low temperature using cantilevers capable of attonewton force sensitivity. Small, unexpected magnetic effects can be seen, such as anomalous damping in magnetic field. A key application is magnetic resonance force microscopy (MRFM) of both electron and nuclear spins. In recent experiments with MRFM-based NMR imaging, 3D spatial resolution better than 10 nm was achieved for protons in individual virus particles. The achieved volumetric resolution represents an improvement of 100 million compared to the best conventional MRI. The microscope is sensitive enough to detect NMR signals from adsorbed layers of hydrocarbon contamination, hydrogen in multiwall carbon nanotubes and the phosphorus in DNA. Operating with a force noise on the order of 6 aN per root hertz with a magnetic tip that produces a field gradient in excess of 30 gauss per nanometer, the magnetic moment sensitivity is $\sim $0.2 Bohr magnetons. The corresponding field sensitivity is $\sim $3 nT per root hertz. To our knowledge, this combination of high field sensitivity and nanometer spatial resolution is unsurpassed by any other form of nanometer-scale magnetometry. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T1.00003: Ferromagnetic Resonance Imaging with Magnetic Resonance Force Microscopy Invited Speaker: Magnetic resonance force microscopy achieves very high resolution three-dimensional imaging capabilities of magnetic resonance imaging by taking advantage of very high sensitivity mechanical force detection. This enables non-contacting, microscopic studies and imaging of a broad range of materials. As a consequence of the strong interactions between spins, the assumptions underlying conventional MRI are not applicable to FMR imaging. However, using a new approach to localizing the resonant volume in an FMR measurement founded on the strong, nonuniform magnetic field of the micromagnetic probe tip, we have demonstrated scanned probe Ferromagnetic Resonance (FMR) imaging [1]. The scanned probe FMR images obtained in patterned ferromagnetic films are well explained by detailed numerical modeling. In addition to illuminating the mechanisms underlying localized FMR, the model provides the basis for submicron scanned probe FMR imaging of films and buried ferromagnetic elements. This work was supported by the U.S. Department of Energy through Grant No. DE-FG02-03ER46054. \newline \newline [1] ``Local Ferromagnetic Resonance Imaging with Magnetic Resonance Force Microscopy,'' Yu. Obukhov, D.V. Pelekhov, J. Kim, P. Banerjee, I. Martin, E. Nazaretski, R. Movshovich, S. An, T.J. Gramila, S. Batra, and P. C. Hammel, Phys. Rev. Lett. \textbf{100}(19), 197601 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T1.00004: Ultrafast magnetization dynamics of cobalt nanoparticles and individual ferromagnetic dots Invited Speaker: The ultrafast magnetization dynamics of magnetic materials can be investigated using femtosecond laser pulses to perform femtosecond magneto-optical Kerr and Faraday measurements [1]. In this talk, we will focus on the magnetization dynamics of cobalt nanoparticles which are either ferromagnetic or super-paramagnetic at room temperature and on the dynamics of individual ferromagnetic dots. In the first case (Co nanoparticles), we will demonstrate that the magnetization dynamics preceding the fluctuations over the anisotropy energy barrier is coherent but exhibits a strongly damped precession [2]. These results, which have been obtained with a three dimensional analysis of the magnetization vector [3] will be discussed in the context of the N\'{e}el-Brown models involving the gyromagnetic character of the magnetization. We will also examine the dynamics of self-organized supra-crystals of cobalt nanoparticles [4]. In the second case, we will present the ultrafast magnetization dynamics of individual ferromagnetic dots (CoPt$_{3}$, Permalloy, Nickel) made either by e-beam lithography or induced optically on thin films deposited on sapphire and glass substrates. The technique employed is the magneto-optical pump probe imaging (MOPPI) which allows performing time resolved magneto-optical Kerr images with with spatial and temporal resolutions of 300 nm and 150 fs [5]. The study of the demagnetization of the dots for different laser intensities shows that it is possible to write and read ultrafast monodomains on thin films. \\[3pt] [1] E. Beaurepaire, J.-C. Merle, A. Daunois, J.-Y. Bigot~ Phys. Rev. Lett., \textbf{76}, 4250 (1996) \\[0pt] [2] L.H.F. Andrade, A. Laraoui, M. Vomir, D. Muller, J.-P. Stoquert, C. Estourn\`{e}s, E.~Beaurepaire, J.-Y. Bigot Phys. Rev. Lett. \textbf{97}, 127401 (2006). \\[0pt] [3] M. Vomir, L. H.F. Andrade, L. Guidoni, E. Beaurepaire, J.-Y. Bigot Phys. Rev. Lett. \textbf{94}, 237601 (2005). \\[0pt] [4] I.~Lisiecki, V.~Halt\'{e}, C. Petit, M.-P.~Pileni, J.-Y.~Bigot Adv. Mater., \textbf{20}, 4176 (2008). \\[0pt] [5] A. Laraoui, M. Albrecht, J.-Y. Bigot Optic. Letters \textbf{32}, 936 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T1.00005: Magnetic soft X-ray microscopy: Towards imaging ultrafast spin dynamics on the nanoscale Invited Speaker: Modern magnetic microscopies are challenged with providing spatial resolution in the nanometer regime, a time resolution on a fs scale and elemental specificity to allow for studying multifunctional magnetic nanostructures and their ultrafast spin dynamics. Magnetic soft X-ray microscopy combines X-ray magnetic circular dichroism (X-MCD) as element specific magnetic contrast mechanism with high spatial and temporal resolution. Fresnel zone plates provide a spatial resolution down to currently $<$15nm [1] with current developments approaching the 10nm regime thus approaching fundamental magnetic length scales. Utilizing the inherent time structure of current synchrotron sources fast magnetization dynamics with 70ps time resolution, limited by the lengths of the electron bunches, can be performed with a stroboscopic pump-probe scheme. Soft x-ray microscopy at upcoming high brilliant fsec X-ray sources makes snapshot images of fsec spin dynamics feasible. In this talk I will present recent results on the study of the stochastical character in magnetization reversal and domain wall pinning [2] as well as on time resolved imaging of current induced resonant vortex core motion which allows to determine spin polarization of currents [3] \\[4pt] [1] D.-H. Kim, et al., J. Appl. Phys. 99, 08H303, (2006) \\[0pt] [2] M.-Y. Im et al, Adv. Mater 20 1750 (2008) \\[0pt] [3] S. Kasai et al. Phys Rev Lett (2008) accepted [Preview Abstract] |
Session T2: Electron, Exciton and Phonon Interactions in Nanoparticles
Sponsoring Units: DCMP DMPChair: Phillippe Guyot-Sionnest, University of Chicago
Room: Spirit of Pittsburgh Ballrom BC
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T2.00001: Mapping surface plasmons on a single metallic nanoparticle Invited Speaker: |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T2.00002: Controlled Crystallinity and Fundamental Coupling Interactions in Nanocrystals Invited Speaker: Metal and semiconductor nanocrystals show many unusual properties and functionalities, and can serve as model system to explore fundamental quantum and classical coupling interactions as well as building blocks of many practical applications. However, because of their small size, these nanoparticles typically exhibit different crystalline properties as compared with their bulk counterpart, and controlling crystallinity (and structural defects) within nanoparticles has posed significant technical challenges. In this talk, I will firstly apply silver metal nanoparticles as an example and present a novel chemical synthetic technique to achieve unprecedented crystallinity control at the nanoscale. This engineering of nanocrystallinity enables manipulation of intrinsic chemical functionalities, physical properties as well as nano-device performance [1]. For example, I will highlight that electron- phonon coupling constant can be significantly reduced by about four times and elastic modulus is increased $\sim $40{\%} in perfect single crystalline silver nanoparticles as compared with those in disordered twinned nanoparticles. One important application of metal nanoparticles is nanoscale sensors. I will thus demonstrate that performance of nanoparticles based molecular sensing devices can be optimized with three times improvement of \textit{figure}-\textit{of}-\textit{merit} if perfect single crystalline nanoparticles are applied. Lastly, I will present our related studies on semiconductor nanocrystals as well as their hybrid heterostructures. These discussions should offer important implications for our understanding of the fundamental properties at nanoscale and potential applications of metal nanoparticles. \\[4pt] [1] Yun Tang and Min Ouyang, Nature Materials, 6, 754, 2007. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T2.00003: New Developments in Nanocrystal Lasing: Type-II Nanostructures and ``Giant'' Quantum Dots Invited Speaker: Nanocrystal (NC) quantum dots show high photoluminescence quantum yields and size-dependent emission colors tunable through the quantum-confinement effect. Despite their favorable light-emitting properties, NCs are difficult to use in optical amplification. Because of almost exact balance between absorption and stimulated emission in nanoparticles excited with single excitons, optical gain can only occur due to NCs that contain at least two excitons. A resulting complication is fast optical-gain decay induced by nonradiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. In this talk, I will discuss two approaches for resolving the problem of ultrafast Auger recombination in NCs. In one approach, we utilize core/shell hetero-NCs engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a large, $\sim$100 meV transient Stark shift of the absorption spectrum with respect to the luminescence band. This effect breaks the exact balance between absorption and stimulated emission and allows us to demonstrate optical amplification in the single-exciton regime when Auger recombination is simply inactive. In another approach, we use recently developed ``giant'' dots that comprise a small emitting CdSe core overcoated with a thick shell (up to 20 monolayers) of a wider-gap CdS. These nanostructures produce a peculiar quasi-type-II localization regime, which develops as a result of a significant difference in effective volumes of the electron and hole wave functions. These structures show greatly suppressed Auger recombination, which allows us to realize broadband optical gain (extends over $>400$ meV) due to multiexcitons of various orders with an excitation threshold, which is at least a thousand times lower than in regular CdSe NCs. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T2.00004: Plasmon-enhanced Absorption, Modulation and Spontaneous Emission in Semiconductor Quantum Dots and Films Invited Speaker: Recent advances in plasmon dispersion and localization in quantum dot layers and semiconductor thin films have enabled study of several phenomena in coupled metal/semiconductor nanophotonic structures, including i) enhanced spontaneous emission in quantum dots, ii) all-optical modulation of plasmon propagation in quantum dot active media, and iii) enhanced absorption in plasmonic solar cells. Metal-dielectric plasmon waveguides with quantum dot active layers can serve as switching elements when the complex refractive index is actively modulated. We demonstrate all-plasmonic modulation in which the complex refractive index seen by a surface plasmon polariton at an infrared free-space wavelength of 1420 nm is modulated via interband excitation of the quantum dots at a visible wavelength of 514 nm. Metallic nanostructures can excite surface plasmons which can dramatically increase the optical path length in thin active photovoltaic layers to enhance overall photoabsorption, with potential for increased photovoltaic conversion efficiency, and new solar cell device designs. The strong mode localization of surface plasmon polaritons at metal-dielectric interfaces leads to strong absorption in very thin semiconductor films, enabling a dramatic (10-100X) reduction in the semiconductor absorber physical thickness needed to achieve optical thickness. Modal analysis in full wave simulation allows us to determine the fraction of power absorbed for both dielectric waveguide and plasmonic modes in a thin solar cell. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T2.00005: Exciton-plasmon interactions and energy transfer in nanoparticles Invited Speaker: Energy transfer between optically-excited nanocrystals coupled by the Coulomb interaction can be very efficient. The interaction of excitons and plasmons in nanocrystals leads to several effects: energy transfer between nanoparticles (NPs), electromagnetic enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, and interference and non-linear phenomena [1-3]. Using kinetic equations for excitons, we model exciton transport in a NW and explain the origin of the blue shift of exciton emission observed in the recent experiments on hybrid NW-NP assemblies [2]. We also model artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. Using superior optical properties of metal and semiconductor NPs, one can strongly enhance the efficiency of light harvesting [3]. An interaction between a discrete state of exciton and a continuum of plasmonic states can give rise to interference effects (Fano-like asymmetric resonances). These interference effects greatly enhance visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticle and molecules. In the nonlinear regime, the Fano effect becomes strongly amplified [4]. In conclusion, our theory explains present experimental results and also provides motivation for future experiments and applications. Potential applications of dynamical exciton-plasmon systems include sensors and light-harvesting. The above theoretical studies were performed in collaboration with several groups [1-4]. \\[4pt] [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters \textbf{6}, 984 (2006).\\[0pt] [2] J. Lee, P. Hernandez, J. Lee, A. Govorov, and N. Kotov, Nature Materials \textbf{6}, 291 (2007).\\[0pt] [3] A. O. Govorov and I. Carmeli, Nano Lett. \textbf{7}, 620 (2007); S. Mackowski, S. W\"{o}rmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Br\"{a}uchle, Nano Lett.~\textbf{8}, 558 (2008). \\[0pt] [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R.Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature \textbf{451}, 311 (2008). [Preview Abstract] |
Session T3: Keithley Award Session (GIMS)
Sponsoring Units: GIMSChair: James Matey, US Naval Academy
Room: 301/302
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T3.00001: Joseph F. Keithley Award Talk: Microwave Measurements of Mesoscopic Devices Invited Speaker: Typical measurements of mesoscopic devices at low temperatures suffer from annoyingly low speeds and the presence of excess low-frequency noise that can try the experimentalist's patience. Even though these devices are not well-matched to the fifty ohm world of microwaves, the ability to listen to signals coming from a cryogenic nanostructure with a wideband amplifier at gigahertz frequencies has proven quite beneficial. These techniques can be surprising precise and powerful, allowing access to high-speed dynamics, the collection of information from wideband signals such as noise, and an entry into the domain of quantum electrical signals. I will review some of our early experiments at Yale in this area, especially the development of the Radio-Frequency Single-Electron Transistor (RF-SET), which is still the most sensitive electrometer known. Today we find that microwave measurements are proving highly beneficial for solid-state quantum computing, which in turn is leading to a new wave of capabilities for generating and measuring microwave signals at the single photon level. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T3.00002: Near-Quantum-Limited SQUID Amplifier Invited Speaker: The SET (Single-Electron Transistor), which detects charge, is the dual of the SQUID (Superconducting QUantum Interference Device), which detects flux. In 1998, Schoelkopf and co-workers introduced the RFSET, which uses a resonance circuit to increase the frequency response to the 100-MHz range. The same year saw the introduction of the Microstrip SQUID Amplifier$^{1}$ (MSA) in which the input coil forms a microstrip with the SQUID washer, thereby extending the operating frequency to the gigahertz range. I briefly describe the theory of SQUID amplifiers involving a tuned input circuit with resonant frequency f. For an optimized SQUID at temperature T, the power gain and noise temperature are approximately G = f$_{p}$/$\pi $f and T$_{N}$ = 20T(f/f$_{p})$, respectively; f$_{p}$ is the plasma frequency of one of the Josephson junctions. Because the SQUID voltage and current noise are correlated, however, the optimum noise temperature is at a frequency below resonance. For a phase-preserving amplifier, T$_{N}$ = ($\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ + A)hf/k$_{B}$, where Caves' added noise number A = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ at the quantum limit. Simulations based on the quantum Langevin equation (QLE) suggest that the SQUID amplifier should attain A = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. We have measured the gain and noise of an MSA in which the resistive shunts of the junctions are coupled to cooling fins to reduce hot electron effects. The minimum value A = 1.1 $\pm $ 0.2 occurs at a frequency below resonance. On resonance, the value A = 1.5 $\pm $ 0.3 is close to the predictions of the QLE, suggesting that this model may fail to predict the cross-correlated noise term correctly. Indeed, recent work suggests that a fully quantum mechanical theory is required to account properly for this term$^{2}$. This work is in collaboration with D. Kinion and supported by DOE BES. $^{1}$M. Mueck, \textit{et al}., \textit{Appl. Phys. Lett.} \textbf{72}, 2885 (1998). $^{2}$A. Clerk, \textit{et al.,} http://arxiv.org/abs/0810.4729. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T3.00003: Optomechanics with microwave light Invited Speaker: Recently, superconducting circuits resonant at microwave frequencies have revolutionized the measurement of astrophysical detectors [1] and superconducting qubits [2]. In this talk, I will describe how we extend this technique to measuring and manipulating nanomechanical oscillators. By strongly coupling the motion of a nanomechanical oscillator to the resonance of the microwave circuit we create structures where the dominant dissipative force acting on the oscillator is the radiation pressure of microwave ``light'' [3]. These devices are ultrasensitive force detectors and they allow us to cool the oscillator towards its motional ground state. \\[4pt] [1] P. K. Day \emph{et al}., Nature \textbf{425}, 817 (2003).\\[0pt] [2] A. Wallraff \emph{et al}., Nature \textbf{431}, 162 (2004).\\[0pt] [3] J. D. Teufel, J. W. Harlow, C. A. Regal and K.~W. Lehnert, Phys. Rev. Lett., \textbf{101}, 197203 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T3.00004: Noise in Mesoscopic, Quantum, and Nano-Systems Invited Speaker: Studies of fluctuations and noise have been pursued for over a century. These allow investigation of basic physical concepts, dynamics of small systems, internal electronic structure of mesoscopic and nano systems, and quantum limits on detection. We review recent studies at Yale that illustrate these topics. We discuss quantum and classical noise in mesoscopic systems, noise in normal and superconducting tunnel junctions, higher moments of noise, and electrothermal fluctuations in superconducting nanosystems. We conclude by outlining a detector that utilizes many of these concepts, and should allow efficient detection and energy measurement of single microwave and higher-energy photons, at high count rates. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T3.00005: Nonlinear Dispersive Measurement with Superconducting Circuits Invited Speaker: Superconducting circuit elements can be used to form high quality factor harmonic and anharmonic oscillators. When coupled to a pseudospin system, these oscillators can be used for quantum state measurement. In the dispersive limit, the oscillator resonant frequency depends on the spin state. The case of a linear transmission line resonator coupled to a superconducting qubit was demonstrated by R. Schoelkopf and co-workers [1]. We will describe quantum measurement performed using a nonlinear resonator consisting of a Josephson tunnel junction shunted with a reactive impedance. As the Josephson oscillator is excited with an increasing number of photons, its resonant frequency progressively decreases. Under appropriate bias conditions, it is also possible to access a bifurcation where two dynamical states exist. We will show that with a nonlinear Josephson oscillator, it is possible to realize both analog and digital quantum state measurement with variable gain. We will discuss two protocols for accessing the nonlinear response of the junction, amplitude modulation and frequency modulation, and describe in detail two applications---superconducting qubit readout and high speed magnetometry of single molecule magnets. \\[4pt] [1] A. Wallraff et al, \textit{Physical Review Letters} \textbf{95}, 060501 (2005). [Preview Abstract] |
Session T4: Polariton Condensates
Sponsoring Units: DCMP DAMOPChair: Jacqueline Bloch, CNRS
Room: 306/307
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T4.00001: Theory of polariton condensation Invited Speaker: Lately, some novel experiments with planar optical microcavities make use of the mixing of excitons with photons to create composite bosons called polaritons that have a very light mass, and are thus a good candidate for high-temperature Bose condensation. Good evidence for spontaneous coherence has now been obtained.\footnote{J. Kasprzak, et al.Nature, 443, 409-415 (2006).} There are special issues to resolve\footnote{J. Keeling, F. M. Marchetti, M. H. Szymanska, P. B. Littlewood, Semiconductor Science and Technology, 22, R1-26 (2007).} considering the effects of low dimensionality, disorder, strong interactions, and especially strong decoherence associated with decay of the condensate into environmental photons\footnote{M. H. Szymanska, J. Keeling, P. B. Littlewood, Physical Review B 75, 195331 (2007).} --- since the condensate is a special kind of laser. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T4.00002: Bose-Einstein Condensation of Microcavity Polaritons in Harmonic Traps Invited Speaker: Polaritons in microcavities are a two-dimensional, weakly interacting boson gas, and their spatial distribution, momentum distribution, coherence properties, and excitation spectrum can all be observed. In several recent experiments, these observations are consistent with the interpretation of a quasiequilibrium Bose condensate of polaritons. In our experiments, we use a special stress geometry to create a harmonic potential for the polaritons in the plane of their motion, which is analogous to the traps used in experiments on BEC of cold atoms. Among other effects, we observe coherent light emission from the polariton condensate. Two questions arise: 1) Since the system emits coherent light, how can we distinguish it from a standard laser? 2) How do we distinguish it from some type of nonlinear amplification of the excitation light? In our experiments with a trapping potential, we can easily distinguish between these different effects. We demonstrate two transitions which occur in the same place in the same structure, one which is standard lasing and one which is polariton condensation. The quasiequilibrium polariton condensate in microcavities thus emerges as a new type of coherent light emitter. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T4.00003: Observation of Bogoliubov excitations in exciton-polariton condensates Invited Speaker: Particle-particle interaction and peculiar excitation spectra are keys for understanding BEC and superfluidity physics. A quantum field-theoretical formulation for a weakly interacting Bose condensed system was developed by Bogoliubov in 1947, which predicted the phonon-like excitation spectrum in the low- momentum regime. Exciton-polaritons in a semiconductor microcavity, which are elementary excitations created by strong coupling between quantum-well excitons and microcavity photons, were proposed as a new BEC candidate in solid-state systems. Recent experiments with exciton-polaritons have demonstrated several interesting signatures from the viewpoint of polariton condensation, such as quantum degeneracy at non-equilibrium conditions, the polariton-bunching effect at the condensation threshold, long spatial coherence and quantum degeneracy at equilibrium conditions. The particle-particle interaction and the Bogoliubov excitation spectrum are at the heart of BEC and superfluidity physics, but have only been studied theoretically for exciton-polaritons. In this talk, we report the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T4.00004: Quantum fluid dynamics and superfluid behaviour of polaritons in microcavities Invited Speaker: Achievement of polariton condensation in semiconductor microcavities [1,2] has opened the way to the study of new interesting phenomena related to the behaviour of non-equilibrium Bose particles in the quantum limit. In this talk we will see the formation of a coherent quantum state of polaritons created at a given momentum and at a given time using a combination of a continuous wave pump and a pulsed probe. This state is observed to persist in the cavity for a time much longer than the cavity lifetime [3]. Using this technique we are able to investigate the behaviour of a quantum state of polaritons with an extension of $\sim $ 20 $\mu $m moving a hundreds of microns within the cavity. One of the most striking effects of a moving polariton condensate is the observation of superfluid behaviour when crossing obstacles even at speeds only 100 times smaller than the speed of light [4]. Other interesting phenomena, which will be shown, are diffusion-less motion, due to the linearization of the polariton dispersion, and the formation of Cherenkov-like patterns for polaritons moving at supersonic velocities. \\[4pt] [1] J. Kasprzak et al. \textit{Nature} \textbf{443}, 409 (2006). \\[0pt] [2] R. Balili et al. \textit{Science} \textbf{316}, 1007 (2007). \\[0pt] [3] D. Ballarini et al. http://arxiv.org/abs/0807.3224 (arXiv:0807.3224) (2008). \\[0pt] [4] A. Amo et al. \textit{Nature}, forthcoming publication (2009). [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T4.00005: Theory of polariton condensation and superfluidity Invited Speaker: Exciton-polaritons in planar microcavities have two allowed spin projections on the axis of the structure, which is why they can be considered as a two-component weakly interacting Bose gas. The order parameter for BEC or the superfluid phase transition in this gas is a 2D vector analogous to the Jones vector of classical light. The build-up of the order parameter results is the build up of vector polarisation of light emitted by the polariton condensate. Recently observed appearance of the spontaneous vector polarisation stochastically changing from one experiment to another [1] manifests the spontaneous symmetry breaking in a polariton system and confirms observation of the polariton BEC at room temperature in bulk GaN microcavities. The spin dependence of polariton-polariton interactions in quantum well microcavities favours formation of linearly polarised superfluids. The sound velocity in such superfluids is polarisation-dependent [2]. We show that the static potential disorder provokes generation of vortices having a semi-integer topological charge [3]. These half-vortices are lowest energy excitations in two-component superfluids. They can be observed by polarisation-resolved near-field spectroscopy. We show also that the polarisation bistability in optically driven polariton condensates allows for optical excitation of the ``spin rings'' spreading in the real space [4] and paves way to fabrication of ``polariton neurons'' assembled in all-optical integrated circuits [5]. \\[3pt] [1] J. J. Baumberg, A. V. Kavokin, et al Phys. Rev. Lett. 101, 136409 (2008). \\[0pt] [2] I. A. Shelykh, et al, Phys. Rev. Lett. \textbf{97}, 066402 (2006). \\[0pt] [3] T. C. Liew, Yuri G. Rubo, and A. V. Kavokin, Phys. Rev. Lett. \textbf{101}, 187401 (2008). \\[0pt] [4] I. A. Shelykh, T. C. Liew, and A. V. Kavokin, Phys. Rev. Lett. 100, 116401 (2008). \\[0pt] [5] T. C. Liew, A. V. Kavokin, and I. A. Shelykh, Phys. Rev. Lett. 101, 016402 (2008). [Preview Abstract] |
Session T5: Industrial Biophysics
Sponsoring Units: FIAP DBPChair: Philip Wyatt
Room: 401/402
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T5.00001: Virus Characterization by FFF-MALS Assay Invited Speaker: Adequate biophysical characterization of influenza virions is important for vaccine development. The influenza virus vaccines are produced from the allantoic fluid of developing chicken embryos. The process of viral replication produces a heterogeneous mixture of infectious and non-infectious viral particles with varying states of aggregation. The study of the relative distribution and behavior of different subpopulations and their inter-correlation can assist in the development of a robust process for a live virus vaccine. This report describes a field flow fractionation and multiangle light scattering (FFF-MALS) method optimized for the analysis of size distribution and total particle counts. A method using a combination of asymmetric flow field-flow fractionation (AFFFF) and multiangle light scattering (MALS) techniques has been shown to improve the estimation of virus particle counts and the amount of aggregated virus in laboratory samples. The FFF-MALS method was compared with several other methods such as transmission electron microscopy (TEM), atomic force microscopy (AFM), size exclusion chromatography followed by MALS (SEC-MALS), quantitative reverse transcription polymerase chain reaction (RT Q-PCR), median tissue culture dose (TCID(50)), and the fluorescent focus assay (FFA). The correlation between the various methods for determining total particle counts, infectivity and size distribution is reported. The pros and cons of each of the analytical methods are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T5.00002: Determination of reversible protein equilibrium association coefficients using light scattering Invited Speaker: The characterization in solution of reversible protein associations as well as associations between proteins and small molecules is essential in many areas of science. Understanding cellular function or developing and formulating pharmaceuticals or other biologically active materials often requires quantitation of such associations. Most pharmaceuticals have functionality due solely to association with molecules within the body, and the discovery and accurate characterization of these associations is a key element for pharmaceutical development. Unfortunately, most methods used to measure associations of proteins require either immobilizing the protein on a surface (e.g. surface plasmon resonance), which potentially alters the protein characteristics, or require considerable time and effort and large quantities of sample (e.g. analytical ultracentrifugation, isothermal titration calorimetry). Light scattering based measurements of reversible association coefficients require much less sample and may be performed much more rapidly than other free solution techniques. In this talk I describe how static and dynamic light scattering may each independently be used to measure equilibrium association coefficients between proteins in free solution, and may also be used to observe and quantitate the association of small molecules with them. I present background theory for both static and dynamic light scattering measurements of equilibrium associations, and examples of measurements made of both model systems and of systems with commercial relevance in the pharmaceutical industry. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T5.00003: Novel medical imaging technologies for disease diagnosis and treatment Invited Speaker: New clinical approaches for disease diagnosis, treatment and monitoring will rely on the ability of simultaneously obtaining anatomical, functional and biological information. Medical imaging technologies in combination with targeted contrast agents play a key role in delivering with ever increasing temporal and spatial resolution structural and functional information about conditions and pathologies in cardiology, oncology and neurology fields among others. This presentation will review the clinical motivations and physics challenges in on-going developments of new medical imaging techniques and the associated contrast agents. Examples to be discussed are: \begin{itemize} \item The enrichment of computer tomography with spectral sensitivity for the diagnosis of vulnerable sclerotic plaque. \item Time of flight positron emission tomography for improved resolution in metabolic characterization of pathologies. \item Magnetic particle imaging -a novel imaging modality based on in-vivo measurement of the local concentration of iron oxide nano-particles - for blood perfusion measurement with better sensitivity, spatial resolution and 3D real time acquisition. \item Focused ultrasound for therapy delivery. \end{itemize} [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T5.00004: Measurement Frontiers in Molecular Biology Invited Speaker: Developments of molecular measurements and manipulations have long enabled forefront research in evolution, genetics, biological development and its dysfunction, and the impact of external factors on the behavior of cells. Measurement remains at the heart of exciting and challenging basic and applied problems in molecular and cell biology. Methods to precisely determine the identity and abundance of particular molecules amongst a complex mixture of similar and dissimilar types require the successful design and integration of multiple steps involving biochemical manipulations, separations, physical probing, and data processing. Accordingly, today's most powerful methods for characterizing life at the molecular level depend on coordinated advances in applied physics, biochemistry, chemistry, computer science, and engineering. This is well illustrated by recent approaches to the measurement of DNA, RNA, proteins, and intact cells. Such successes underlie well founded visions of how molecular biology can further assist in answering compelling scientific questions and in enabling the development of remarkable advances in human health. These visions, in turn, are motivating the interdisciplinary creation of even more comprehensive measurements. As a further and closely related consequence, they are motivating innovations in the conceptual and practical approaches to organizing and visualizing large, complex sets of interrelated experimental results and distilling from those data compelling, informative conclusions. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T5.00005: Biophysics at the Boundaries: The Next Problem Sets Invited Speaker: The interface between physics and biology is one of the fastest growing subfields of physics. As knowledge of such topics as cellular processes and complex ecological systems advances, researchers have found that progress in understanding these and other systems requires application of more quantitative approaches. Today, there is a growing demand for quantitative and computational skills in biological research and the commercialization of that research. The fragmented teaching of science in our universities still leaves biology outside the quantitative and mathematical culture that is the foundation of physics. This is particularly inopportune at a time when the needs for quantitative thinking about biological systems are exploding. More physicists should be encouraged to become active in research and development in the growing application fields of biophysics including molecular genetics, biomedical imaging, tissue generation and regeneration, drug development, prosthetics, neural and brain function, kinetics of nonequilibrium open biological systems, metabolic networks, biological transport processes, large-scale biochemical networks and stochastic processes in biochemical systems to name a few. In addition to moving into basic research in these areas, there is increasing opportunity for physicists in industry beginning with entrepreneurial roles in taking research results out of the laboratory and in the industries who perfect and market the inventions and developments that physicists produce. In this talk we will identify and discuss emerging opportunities for physicists in biophysical and biotechnological pursuits ranging from basic research through development of applications and commercialization of results. This will include discussion of the roles of physicists in non-traditional areas apart from academia such as patent law, financial analysis and regulatory science and the problem sets assigned in education and training that will enable future biophysicists to fill these roles. [Preview Abstract] |
Wednesday, March 18, 2009 5:30PM - 6:06PM |
T5.00006: Characterizing Protein Complexes with UV absorption, Light Scattering, and Refractive Index Detection. Invited Speaker: Many modern pharmaceuticals and naturally occurring biomolecules consist of complexes of proteins and polyethylene glycol or carbohydrates. In the case of vaccine development, these complexes are often used to induce or amplify immune responses. For protein therapeutics they are used to modify solubility and function, or to control the rate of degradation and elimination of a drug from the body. Characterizing the stoichiometry of these complexes is an important industrial problem that presents a formidable challenge to analytical instrument designers. Traditional analytical methods, such as using florescent tagging, chemical assays, and mass spectrometry perturb the system so dramatically that the complexes are often destroyed or uncontrollably modified by the measurement. A solution to this problem consists of fractionating the samples and then measuring the fractions using sequential non-invasive detectors that are sensitive to different components of the complex. We present results using UV absorption, which is primarily sensitive to the protein fraction, Light Scattering, which measures the total weight average molar mass, and Refractive Index detection, which measures the net concentration. We also present a solution of the problem inter-detector band-broadening problem that has heretofore made this approach impractical. Presented will be instrumentation and an analysis method that overcome these obstacles and make this technique a reliable and robust way of non-invasively characterizing these industrially important compounds. [Preview Abstract] |
Session T6: Panel Discussion: Preparation of Graduate Students for Careers in a Globalized World
Sponsoring Units: FIP FGSAChair: John W. Clark, Washington University in St. Louis
Room: 406
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T6.00001: Preparing for Change: Challenges and Opportunities in a Global World Invited Speaker: Our world is becoming increasingly global. This may sound like a clich\'{e}, yet it is true nonetheless, and poses unprecedented challenges for graduate education. For the new generation of researchers, teachers and professionals to be successful they must be prepared in more than the content area of their chosen field. They must also acquire proficiency in global awareness, cultural literacy, multicultural teamwork and language facility. These global skill sets form the basis for effective multicultural collaboration and will become increasingly important even for those who do not intend to study or work abroad. Knowledge has become more portable in the internet age; large data bases and reports can be accessed in real time from various locations around the globe; information is exchanged in multifaceted knowledge networks; collaborative research takes place within and outside of the traditional venue of the research university in the private sector, research institutes, and associations; research networks span multiple disciplines as progress invariably occurs at the intersection of previously discrete fields of inquiry. Global collaboration thus is no longer dependent on the physical proximity of collaborators but can take place anywhere any time. This then requires yet another set of skills, namely the ability to adapt to change, exhibit flexibility and transfer skills to a range of contexts and applications. Effective graduate education must address these realities and expose students to learning opportunities that will enable them to acquire these much needed global skills sets. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:30PM |
T6.00002: International Experiences for Graduate Students: Opportunities and Challenges Invited Speaker: Graduate students are often well aware that their physics careers may involve international partnerships at some point. Many students, however, wish to pursue international experiences during their graduate physics training. The speaker will discuss some of the international opportunities available to graduate students and provide insights into some potential challenges of engaging internationally. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:54PM |
T6.00003: Graduate studies in a globalized world Invited Speaker: In our days physics research, experiment and theory, is done in one way or an other in a framework of an international collaboration. As an experimental Medium Energy physicist, I will be talking about my experience in working within international collaborations for more than twelve years. I will go through a couple of questions graduates students should be asking: How is the work environment for a graduate student doing his or her research within these collaborations? What about language barriers? Can they be independent in their analysis? What will happen after getting their PhD.s? -- and more. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:18PM |
T6.00004: Physics Internationally From the Industrial Perspective Invited Speaker: Physicists traditionally get employed by academia, National Labs and industry. The investment of multi-national companies in R{\&}D and manufacturing operations globally has been accelerating owing to availability of trained human resources and the economy of operation. This has created tremendous opportunities for candidates with global experience as opposed to a highly localized education. In the last decade, the investments made by Asian academic institutions in education and research has seen a significant increase creating opportunities for Graduate students and researchers alike in parts of the world other than US and Europe, the traditional destinations for students and researchers over the last several decades. Many Asian universities are hiring a diverse faculty from all over the world as opposed to hiring from local talent pools. Many of the Asian countries are focusing on creating local hitech economies by fostering global entrepreneurship programs. In my talk I will discuss this globalization phenomenon with specific examples from both academia and industry. I will also discuss strategies for academic institutions in terms of making the appropriate modification to their programs to deal with this inevitable evolution. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:42PM |
T6.00005: I2CAM and ICAM: Physics Internationally Invited Speaker: The Institute for Complex Adaptive Matter (ICAM) through the National Science Foundation sponsored International Institute for Complex Adaptive Matter (I2CAM) has, since its formal inception in 2002, grown into a 60+ branch international scientific network devoted to the study of emergent phenomena in correlated electron matter, soft matter, and biological matter. We nucleate forefront research through a blend of discussion oriented workshops (at least 50{\%} of the time for discussion), exchange awards for junior scientists to initiate collaborations between two groups, travel awards for junior scientists to present research work or carry out brief research, and schools on topical subject matter. We also supplement our federal funding with contributions from each branch which support postdoctoral and senior scientist fellowships and unique science outreach activities such as an online science museum (The Emergent Universe). We have also outreach activities to universities with substantial numbers of underrepresented groups in the sciences and to outstanding science institutes in emerging nations. I will review what has worked well with ICAM/I2CAM, how we started and grew, and how we have inspired similar programs in other countries. (This research supported by NSF Grants DMR-0645461 and DMR-0456669). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 5:30PM |
T6.00006: Panel Discussion |
Session T7: Patterns on Thin Elastic Sheets
Sponsoring Units: GSNP DPOLYChair: Benny Davidovitch, University of Massachusetts
Room: 407
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T7.00001: The undulating shape of growing ribbons Invited Speaker: The undulating morphology of leaves and petals is now accepted as a consequence of differential growth of the underlying tissue. Various qualitative and quantitative aspects of the buckling patterns seen in both vascular and avascular leaves may thus be ascribed to the distribution of non-uniform growth in the lamina, and have been demonstrated in normal and mutant leaves, as well as in physical models thereof. To understand the different modalities that arise quantitatively, we construct a mathematical model for the stability of an initially flat or curved elastic ribbon with gradients in growth directly motivated by observations of kelp that are capable of phenotypic plasticity in different environments. Using a combination of analysis, numerical simulation, and experimental observations, we map out the phase space of possible shapes for these growing ribbons. In general, we find that as the relative growth strain is increased, the ribbon-like structure first switches to a catenoidal shape before developing undulating edges that can develop on the catenoid's edges. Our framework allows us to delineate the few macroscopic parameters that control the morphology of elongated leaves and flower petals and helps to explain the large variety of observed shapes. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T7.00002: The Mechanics of Non-Euclidean Plates in Synthetic and Natural Sheets Invited Speaker: Thin elastic flat plates attain non-trivial configurations when they are confined. I will show that plates with intrinsic non-Euclidean geometry attain multi scale three-dimensional configurations even when they are free of external loading. Such bodies do not have any stress-free configuration, thus current plate theories cannot properly describe their physics. I will present our recent experimental results and our theoretical model for the shaping principles of such plates. Finally, I will show how these principles are manifested during the growth of leaves. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T7.00003: Wrinkling patterns on floating elastic films Invited Speaker: A polymer sheet floating on the surface of a fluid is an ideal arena for studying elastic instabilities in thin sheets. In our experiments we use polystyrene sheets whose typical lateral size, $L \quad \sim $ 3 cm, and whose thickness, $t$ ranges from 30 to 300 nm, yielding aspect ratios $L/t$ of up to 10$^{6}$. In their unperturbed state, they lie on the surface of a pool of water, stretched flat by surface tension. We can then generate a rich variety of wrinkling patterns by perturbing the surface locally with capillary forces,\footnote{J. Huang et al., Science 317, 650 (2007).} or with controlled displacements at one or more points on the surface. I will review our understanding of the length scales that characterise these localised patterns. A simple experimental setting in which a multiplicity of these length scales come into play is a situation analogous to an Euler buckling experiment performed on the surface of a fluid. We push two sides of a rectangular sheet towards each other, creating a global pattern of parallel wrinkles whose wavelength is given by a balance between gravitational potential energy of the fluid and bending energy of the sheet. These wrinkles develop a cascade of fine structure at higher wavenumbers close to the uncompressed edges of the sheet. The length scale over which this cascade occurs is the capillary length, whereas the wavenumber at the edge of the sheet reflects a balance between bending energy and surface tension. We discuss the evidence that this is a fundamentally new type of elastic cascade, which proceeds to higher wavenumbers by smooth evolution of the wrinkles, rather than by discrete, sharply localised branching. Work done in collaboration with J. Huang, E. Cerda, B. Davidovitch, W.H. de Jeu, T.P. Russell, C. D. Santangelo [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T7.00004: Wrinkle to fold transitions: Stress relaxation in lipid monolayers and other elastic thin films Invited Speaker: Surfactants at air/water interfaces are often subjected to mechanical stresses as the interfaces they occupy are reduced in area. The most well characterized forms of stress relaxation in these systems are first order phase transitions. However, once chemical phase transitions have been exhausted, the monolayer undergoes global mechanical relaxations termed collapse. We have previously demonstrated that for lung surfactants, a mixture of lipids and proteins that coats the alveoli to reduce the work of breathing, collapse manifests itself as protrusions of folds into the subphase. These folds remain attached to the monolayer and reversibly reincorporated upon expansion. By studying different types of monolayers, we have shown that this folding transition in monolayers is not limited to lung surfactant films, but rather represents a much more general type of stress relaxation mechanism. Our study indicates that collapse modes are found most closely linked to in-plane rigidity. We characterize the rigidity of the monolayer by analyzing in-plane morphology on numerous length scales. More rigid monolayers collapse out-of-plane \textit{via} a hard elastic mode similar to an elastic membrane, with the folded state being the final collapse state, while softer monolayers relax in-plane by shearing. For the hard elastic mode of collapse, we have further demonstrated experimentally and theoretically that the folded state is preceded by a wrinkled state, and similar wrinkle to fold transitions has been observed in elastic thin films ranging from 2 nm to 10 $\mu $m in thickness of completely different chemical nature (lung surfactant lipid monolayers, gold nanoparticle trilayers, and polyester sheets). [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T7.00005: Nonlinear dynamics of wrinkle growth and pattern formation in stressed elastic thin films on viscoelastic substrates Invited Speaker: A stressed thin film on a soft substrate can develop complex wrinkle patterns. The onset of wrinkling and initial growth is well described by a linear perturbation analysis, and the equilibrium wrinkle patterns can be analyzed based on an energy approach. In between, the wrinkle pattern undergoes a growth and coarsening process with a peculiar dynamics. By using a proper scaling along with numerical simulations, this paper develops a quantitative understanding of the wrinkling dynamics from initial growth through coarsening toward equilibrium. By considering generally biaxial stresses and anisotropic elastic modulus of the film, we show that a rich variety of wrinkle patterns (e.g., labyrinth, orthogonal, parallel, zigzag, and checkerboard patterns) emerge as a result of the competition between the material anisotropy and the stress anisotropy. [Preview Abstract] |
Session T8: Onsager Prize, Nicholson Medal, Apker Award, Davisson-Germer Prize
Sponsoring Units: DCMPChair: Warren Pickett, University of California, Davis
Room: 414/415
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T8.00001: Nicholson Medal Talk: Hydrodynamic Turbulence Invited Speaker: This talk will be an introduction on hydrodynamic turbulence to a non-specialist audience. It will summarize the essential developments in the field, and only modest emphasis will be placed on the speaker's own work. Hydrodynamic turbulence may be said to have begun as a subject of scientific study with Osborne Reynolds' classical paper towards the end of the nineteenth century, but it is a subject of great interest in the 21st century. The intent is to highlight the principal accomplishments in a way that draws attention to their connections to other areas of physics, where possible. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T8.00002: Lar Onsager Prize Talk: An Exactly solvable model for Strontium Copper Borate: Mott Hubbard Physics on an Archimedean Lattices Invited Speaker: An exactly solvable model of spin half particles on a certain 2-dimensional frustrated lattice has been recently realized in the compound $SrCu_2(BO_3)_2$, and other similar systems have been found more recently. These systems appear to be ideal testing grounds for contemporary theoretical ideas on the role of correlations and frustration in Mott Hubbard systems. In this talk I will summarize the work on these systems emphasizing their role in testing key concepts. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T8.00003: LeRoy Apker Award Talk: Electronics at the Nanoscale: Graphene, Carbon Nanotubes, and Single-Molecule Devices Invited Speaker: Low-dimensional nanostructures are emerging as model systems for fundamental studies of quantum transport, as well as promising candidates for novel post-silicon electronic devices incorporating quantum size effects. Key examples of these include few-layer graphene, carbon nanotubes, polymer nanofibers, and even single molecules. In this talk, I will summarize my work combining experimental and computational tools to study, control, and apply molecular nanomaterials of low dimensionality -- using scanning probe microscopy techniques to study electronic phenomena in few-layer graphene and carbon nanotubes, as well as to elucidate the structure of biochemically-functionalized carbon nanotubes; using computer simulations to investigate key electronic properties of single-molecule transistors; and demonstrating a straightforward chemical technique by which samples of few-layer graphene can be etched along their crystallographic directions, potentially enabling the creation of a variety of new graphene-based nanostructures. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T8.00004: Davisson-Germer Prize Talk: Hydrogen storage in nanoporous materials Invited Speaker: To develop a hydrogen-based energy technology, several classes of materials are being considered to achieve the DOE targets for gravimetric and volumetric hydrogen densities for hydrogen storage, including liquids (e.g. ammonium borohydrides), clathrate structures, complex metal hydrides, nanostructured (e.g. carbon) an nanoporous materials. Fundamental studies are necessary to determine the ultimate hydrogen capacity of each system. Nanoporous Metal-organic Framework (MOF) materials are promising candidates for hydrogen storage because the chemical nature and size of their unit cell can be tailored to weakly attract and incorporate H$_{2}$ molecules, with good volumetric and mass density. In this talk, we consider the structure M$_{2}$(BDC)$_{2}$(TED), where M is a metal atom (Zn, Ni, Cu), BDC is benzenedicarboxylate and TED triethylenediamine, to determine the location and interaction of H$_{2}$ molecules within the MOF. These compounds are isostructural and crystallize in the tetragonal phase (space group P4/ncc), they construct 3D porous structures with relatively large pore size ($\sim $7-8 A\r{ }), pore volume ($\sim $0.63-0.84 cc/g) and BET surface area ($\sim $1500-1900 m$^{2}$/g). At high pressures (300-800 psi), the perturbation of the H-H stretching mode can be measured with IR absorption spectroscopy, showing a 35 cm$^{-1}$ redshift from the unperturbed ortho (4155 cm$^{-1}$ ) and para (4161 cm$^{-1}$ ) frequencies. Using a newly developed non empirical van der Waals DFT method vdW-DFT),\footnote{J.Y. Lee, D.H. Olson, L. Pan, T.J. Emge, J. Li, Adv. Func. Mater. 17, 1255 (2007)} it can be shown that the locus of the deepest H$_{2}$ binding positions lies within to types of narrow channels. The energies of the most stable binding sites, as well as the number of such binding sites, are consistent with the values obtained from experimental adsorption isotherms, and heat of adsorption) data.\footnote{M. Dion, H. Ryberg, E. Schroder, D. C. Langreth, B.I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004).} Importantly, the calculated shift of the H-H stretch is $\sim $-30 cm$^{-1}$ at the strongest binding points of the two channels, suggesting that the combination of IR and vdW-DFT gives a consistent and accurate picture of H$_{2}$ binding in MOF structures. These methods can therefore provide the fundamental information necessary to guide synthesis for improving H$_{2}$ uptake and release. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T8.00005: Davisson-Germer Prize Talk: Surface vibrations of adsorbates on Si(111): From small clusters to infinite lattices Invited Speaker: Organic functionalization of semiconductor surfaces is a growing research area that offers the possibility of molecular level control of surface features and tailored electronic properties. In this work, quantum chemical cluster calculations are used in conjunction with surface vibrational spectroscopy to determine the structures of functionalized Si(111) surfaces. Interestingly, the interpretation of these spectra even for simple adsorbates is not straightforward. In the limit of high coverage, most calculations using small cluster models lack the long range coupling of the real surface that is required to make definitive assignments. In order to understand the relationship between clusters and infinite periodic vibrations, we have investigated the geometries and harmonic vibrational frequencies of the methyl, acetylenyl, methylacetylenyl, hydrogen, deuterium and chlorine functionalized Si(111) surfaces. From a careful analysis of these systems, we have derived a technique where the collective vibrational modes corresponding to the vibrations of the infinite periodic system can be derived from relatively small cluster models. The calculated frequencies are in good agreement with available experimental values and yield novel insights about the coupling between low frequency adsorbate frequencies and surface phonons. The efficacy of this approach for surfaces of varying adsorbate coverage and the prediction of novel frequency shifts will be discussed along with more complex systems. [Preview Abstract] |
Session T9: Focus Session: Stochastic Processes in Biological Systems I
Sponsoring Units: GSNPChair: Uwe C. Tauber, Virginia Polytechnic Institute and State University
Room: 303
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T9.00001: Stochastic models of viral infection Invited Speaker: We develop biophysical models of viral infections from a stochastic process perspective. The entry of enveloped viruses is treated as a stochastic multiple receptor and coreceptor engagement process that can lead to membrane fusion or endocytosis. The probabilities of entry via fusion and endocytosis are computed as functions of the receptor/coreceptor engagement rates. Since membrane fusion and endocytosis entry pathways can lead to very different infection outcomes, we delineate the parameter regimes conducive to each entry pathway. After entry, viral material is biochemically processed and degraded as it is transported towards the nucleus. Productive infections occur only when the material reaches the nucleus in the proper biochemical state. Thus, entry into the nucleus in an infectious state requires the proper timing of the cytoplasmic transport process. We compute the productive infection probability and show its nonmonotonic dependence on both transport speeds and biochemical transformation rates. Our results carry subtle consequences on the dosage and efficacy of antivirals such as reverse transcription inhibitors. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T9.00002: Stochastic modeling of gene regulation by small RNAs Vlad Elgart, Tao Jia, Andrew Fenley, Rahul Kulkarni Recent research has uncovered several examples wherein post-transcriptional regulation by small RNAs plays an important role in critical cellular processes. We considered a stochastic model for regulation of target mRNAs by small RNAs. While the corresponding master equation is analytically intractable, application of the bursty synthesis approximation yields results for the steady-state protein probability distribution and the first moments. We compare our analytical results to stochastic simulation results using the Gillespie algorithm and to the results of linear noise approximation approach. The effects of transcriptional pulsing on protein steady-state expression are also explored within the same formalism. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T9.00003: Stochastic modeling of protein-based post-transcriptional regulation Tao Jia, Rahul Kulkarni Recent experiments have enabled monitoring gene expression in living cells at the level of single proteins. Data from such experiments for protein burst-size distribution and burst frequency can be used to obtain analytical expressions for the steady-state protein distribution across a population. We extend this analysis to the case of modulation of gene expression by binding/unbinding of a post-transcriptional regulatory protein. Closed-form analytical expressions and results from stochastic simulations will be presented. In the case that regulator binding results in complete repression of protein expression, the steady-state protein distribution has the same functional form as the unregulated case, once the mRNA degradation rate is appropriately renormalized. For the general case, wherein binding can result in partial repression or even activation of protein expression, we derive an analytical expression for the steady-state distribution which generalizes the result for the unregulated case. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T9.00004: Accurate fluctuation prediction in epidemics using stochastic model reduction Eric Forgoston, Ira Schwartz We consider a large-scale dynamical system with stochastic forcing and outline a general theory to reduce the dimension of the stochastic system. The general procedure employs a stochastic normal form coordinate transform and allows one to analytically derive both the stochastic center manifold and the reduced set of stochastic evolution equations. The transformation correctly projects both the dynamics and the noise onto the center manifold. We have applied the theory to a stochastic Susceptible-Exposed-Infected-Recovered (SEIR) epidemiological model. When compared with the original model, the reduced dynamical system accurately predicts fluctuations of disease outbreaks both in amplitude and phase. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T9.00005: Stochastic disease extinction in multistrain diseases with interacting strains Simone Bianco, Leah Shaw, Ira Schwartz The study of multistrain diseases, diseases with several coexisting strains, is a major challenge for mathematical biology. Examples of such diseases are influenza, HIV, dengue and ebola. In this work we present an agent-based model for multistrain diseases with strain interactions mediated by antibody-dependent enhancement. An individual infected with a strain develops antibodies which will protect him/her against all the strains. When the level of protection wanes, the presence of antibodies will enhance the infectiousness of the individual when an infection with a different strain occurs. This mechanism is called antibody-dependent enhancement (ADE). We use this model to investigate the role that fluctuations due to system size have on disease extinction paths and discuss how interactions mediated by ADE affect rates of disease fade-out. Finally, we discuss the effect that varying the number of strains has on disease extinction. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T9.00006: Controlling rare events: optimizing disease extinction with limited vaccine M. Khasin, M.I. Dykman In rare events such as switching between stable states or disease extinction the system has to overcome an effective barrier. The barrier height can be changed by applying a control field. The change is determined by the effective work of the field along the most probable trajectory followed in a rare event. In turn, the barrier change results in an exponentially strong change of the event rate. We study the optimal temporal shape of the control field with a constraint that the time- average field value and the sign of the field are fixed. An example is vaccination with a limited vaccine production rate or control by light intensity with a limited laser power. For a comparatively weak field, for a broad class of rare events, optimal control is accomplished by periodically applying $\delta$-like pulses. We show that the barrier change may display resonant dependence on the pulse period and is linear in the pulse area. For a stronger field, the dependence of the barrier change on the field amplitude becomes system-dependent. The results are applied to simple models of population dynamics. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T9.00007: Extinction Time Distribution in Stochastic Lotka-Volterra System Matthew Parker, Alex Kamenev The Lotka-Volterra model is one of the most basic problems in population dynamics. The mean-field solution to this problem predicts oscillatory evolution of two competing populations. However, an account of the discrete nature of agents inevitably results in the extinction of one or both species. We studied the distribution function of times required for such an extinction event to take place. We employed a combination of Monte-Carlo simulations and analytic techniques. As a result we achieved a complete understanding of the distribution function in the limiting cases of long and short extinction times. The long time tail is perfectly described by the lowest eigenvalue of the corresponding Fokker-Planck operator. Moreover, due to time scale separation, one may reduce the initial 2D operator to an effective 1D radial one. Remarkably, in the short time limit the Fokker-Planck approach fails, and one has to resort to the WKB treatment of the full evolution operator of the corresponding discrete stochastic problem. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T9.00008: Refined mean-field approaches to ``edge-effects'' in open TASEP's Jiajia Dong, Royce K.P. Zia, Beate Schmittmann We study the totally asymmetric simple exclusion process (TASEP) with a defect site, hopping rate $q <$ 1, at the edge of the system and particles occupying $\ell$ lattice sites. Using two different mean-field approximations, we analyze the behavior of the steady state current $J$ in the presence of the defect as a function of entry rate $\alpha $ and $q.$ In good agreement with Monte Carlo simulations, these two methods bring insight to understanding the significance of having one or a cluster of slow codons (unit of messenger RNA, template of protein synthesis) immediately after initiation during protein synthesis. Related work is published in Journal of Physics A, vol. 41 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T9.00009: Parallel Coupling of Symmetric and Asymmetric Exclusion Processes Konstantinos Tsekouras, Anatoly Kolomeisky A system consisting of a simple symmetric exclusion process (SSEP) and a totally asymmetric exclusion process (TASEP) coupled to each other at every site is constructed as a simplified model of a microtubule and the surrounding medium within the context of intracellular particle transport. Transitions between the channels are allowed at every site of both lattices. A cluster-based mean-field theory allows calculation of stationary phase diagrams, particle currents and densities for symmetric/asymmetric transition rates between the channels. It is shown that in general there are three stationary phases, similar to the case of a single-channel totally asymmetric exclusion process.Density profiles are identical in both channels if transition rates are symmetric, not so if they are asymmetric.At certain limiting values of the transition rates our theory provides exact solutions, so that the system can be described as a partially asymmetric exclusion process (PASEP). Extensive Monte Carlo simulations generally support theoretical predictions, although simulated stationary-state properties slightly deviate from calculated in the mean-field approximation. Dynamic properties and phase diagrams are discussed by analyzing symmetry requirements and constraints on the particle currents, as are possible implications for the problem of intracellular particle transport. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T9.00010: Anomalous diffusion and scaling in the dynamics of coupled stochastic processes. Golan Bel, Ilya Nemenman Stochastic processes are ubiquitous in nature, and multiple dynamical variables in the same physical system can be stochastic simultaneously. Common mathematical treatement of such cases limit the interactions among multiple stochastic variables to simple correlations. However, more complicated couplings are possible as well. For example, for many biochemical reactions, the rate (stochastic) of creation of one substance may depend on the presence of another one, itself stochastic variable. Here we present a theoretical study of one class of such coupled stochastic processes. We observe that, contrary to traditional modeling frameworks, even very weak coupling yields anomalous diffusion. Interestingly, the diffusion exponent cannot be predicted by simple scaling arguments, and anomalous scaling appears as well. Further, we show that even weak inhibitive coupling between the two processes may result in dynamics equivalent to that of the celebrated comb model, where the coupling between the two stochastic variables is so strong that one is able to diffuse only when the other is within a certain range. We compare the model to various mechanisms for generating anomalous diffusion and show that coarse-graining yields behavior equivalent to that of the non-ergodic continuous time random walk. We end with brief discussion of applications of the developed theory to biochemical systems. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T9.00011: Subdiffusion in the Internal Dynamics of Peptides Thomas Neusius, Jeremy C. Smith The internal dynamics of biopolymers is a topic of intense current research, both in experiment and theory. Recent experimental results have demonstrated the presence of internal subdiffusion in biopolymers at equilibrium. Molecular dynamics simulation of oligopeptide chains reveals configurational subdiffusion at equilibrium extending from 10$^{-12}$ to 10$^{-8}$~s. We examine the possible origins of the subdiffusion and demonstrate that it arises from the fractal-like structure of the accessible configurational space [PRL \textbf{100}, 188103 (2008)]. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T9.00012: Stochastic waiting times of complex biochemical reactions may exhibit universal behavior B. Munsky, G. Bel, N. Sinitsyn, I. Nemenman To model cell regulatory pathways, one must understand completion times of complex, multistep, often reversible biochemical reactions. As transient properties, these completion (first passage) times are typically unobtainable from stationary behavior, and their distributions are known only for simple homogeneous network topologies. Here, we derive explicit formulas for first passage time distributions of various biological models, such as multi-site phosphorylation, kinetic proofreading, and discrete walks along an inhomogeneous line, and others. In many cases, as system size grows, the system behavior frequently becomes simpler, approaching an unexpected universality. Under many conditions, this limiting behavior is deterministic, under others it is a memoryless Markovian dynamics, and the two results are separated by a phase transition. For example, below a critical parameter, the time to complete a given complex multistep reaction obeys a narrow gamma distribution, and above this threshold, waiting times are exponentially distributed. These findings suggest first that possibilities to coarse-grain cellular networks are immense, and second that the common practice of arbitrarily replacing unknown dynamics with ballistic motion or exponential waiting times may be justified in a wide array of circumstances. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T9.00013: Mechanisms of large length fluctuations during actin filament growth Matthew B. Smith, Dimitrios Vavylonis Prior TIRF microscopy experiments monitoring the growth of single actin filaments have indicated that the magnitude of growth rate fluctuations, characterized by a ``length diffusion coefficient'' $D$, is much larger than the value expected from a simple monomer-by-monomer polymerization process. Several theoretical studies have explored mini-catastrophes or oligomeric annealing and fragmentation as sources of enhanced fluctuations. We used numerical simulations and analytical theory to examine additional mechanisms that contribute to length fluctuations. Fluctuations caused by cooperative kinetics, in which the rate of monomer addition and/or subtraction depends on the type of nucleotide bound to neighboring actin subunits exhibit qualitatively distinct dependence on actin monomer concentration and on concentration of phosphate than those caused by ``short pauses.'' By comparing to analysis of experimental data, we show that the experimentally measured $D$ values can be distinguished from random noise. Further we propose experiments that will distinguish these sources of fluctuations. We relate our findings to other one-dimensional directed processes, such as in molecular motor walks. [Preview Abstract] |
Session T10: Focus Session: Optical Properties of Nanostructures V: Plasmonics and Metamaterials
Sponsoring Units: DMPChair: Markus Raschke, University of Washington
Room: 304
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T10.00001: Metal films with arrays of subwavelength holes show infrared spectral and plasmonic sensitivity to material in the holes James Coe, Joseph Heer, Katherine Cilwa, Marvin Malone, Lloyd Corwin Experiments and FDTD calculations show that infrared (IR) absorption spectroscopy and IR transmission resonances of metal films with arrays of subwavelength holes (mesh) are more sensitive to material in the holes as opposed to material on the front or back surface of the mesh. The basic optical physics of the transmission resonances will be discussed including determinations of front-back coupling and surface plasmon (SP) dispersion curves. Applications including enhanced IR absorption spectroscopy of nanocoatings, catalytic reactions, and individual dust particles in the 1-5 micron diameter range will be discussed. Finally, the interaction of an IR SP-mediated resonance with a vibration of molecules in a mesh coating will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T10.00002: Influence of the dielectric environment on periodic hole arrays Dimitrios Koukis, Daniel J. Arenas, Sinan Selcuk, David B. Tanner, Arthur F. Hebard, Sergei V. Shabanov The influence of various dielectric environments, surrounding periodic hole arrays in optically thick metal films, was studied experimentally. The transmittance (T) and reflectance (R) at nearly normal incidence, were measured using a microscope photometer in the near infrared region and Bruker IFS 113v in the mid-infrared region. The metal films are fabricated on substrates with different refractive indices for the two spectral ranges. The refractive index of the material on the other side, can be either that of air or of another dielectric. For large differences in refractive index, between the two sides, the spectra contains separate resonances. These resonances shift in frequency as the refractive indices match better and coincide with the existing ones in the case of good match, enhancing them in amplitude and linewidth. The experimental results are compared to theoretical predictions. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T10.00003: Controlling the polarization of light transmitted through metallic bilayers of subwavelength apertures Zsolt Marcet, J. Paster, H.B. Chan, D.W. Carr, J.E. Bower, R. Cirelli, F.P. Klemens, W.M. Mansfield, J.F. Miner, C.S. Pai, J.A. Taylor, I. Kravchenko Periodic apertures in a metal film exhibit extraordinarily high optical transmission at wavelengths where surface excitations are at resonance with the incident light. The evanescent fields created by these subwavelength structures can channel the optical energy to specific locations, resulting in strong and localized field enhancements. We fabricated two metal films patterned with subwavelength slit arrays in close proximity and tailored the evanescent field coupling between them to achieve new functionalities. At the resonant wavelength, TM polarized light is transmitted efficiently with its transmitted magnitude and phase being strongly coupled to the lateral shift between the two layers. Transmission of TE polarization is smaller by several orders of magnitude. We present both numerical simulations and experimental data to demonstrate that by exploiting these properties a novel polarization rotator can be manufactured using a planer process offering the potential to dramatically reduce system cost and size. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T10.00004: Perfect coupling of light to surface plasmons with ultra-narrow linewidths Maxim Sukharev, Paul Sievert, Tamar Seideman, John Ketterson We examine the coupling of electromagnetic waves entering a thin silver film that forms an oscillatory grating embedded between two otherwise uniform semi-infinite half spaces having identical dielectric constants. On reducing the grating period from the long wavelength limit we encounter signatures in the transmission, T, and reflection, R, coefficients associated with: 1) the symmetric surface mode, 2) the anti-symmetric surface mode, and 3) electromagnetic diffraction tangent to the grating; the first two can be regarded as generalized (plasmon) Woods anomalies while the third is the first-order conventional (electromagnetic) Woods anomaly. The energy density at the film surface is enhanced for wavelengths corresponding to these three anomalies, particularly for the antisymmetric plasmon mode in thinner films. When exciting with two waves entering from opposite directions we find that by adjusting the grating oscillation amplitude and fixing the relative phase of the incoming waves to be even or odd, T+R can be made to vanish for one or the other of the plasmon modes; this corresponds to perfect coupling (impedance matching) between the incoming light and these modes. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T10.00005: Plasmon Hybridization of a thin metallic film Tae-Ho Park, Peter Nordlander We investigate the surface plasmon modes of a thin metallic film using the Plasmon Hybridization method and solving Maxwell's equations. In the electrostatic limit, we show that the high energy plasmon mode is the antibonding mode in which surface charges are antisymmetrically distributed, and the low energy mode is the bonding mode in which surface charges are symmetrically distributed. In the thin film, secondary charges which are induced from the primitive plasmons on the other film surface play an important role to determine which plasmon mode has the higher or lower energy. Furthermore, we discuss how the secondary charges affect the propagation length of the surface plasmon by calculating the imaginary parts of the surface plasmon wave vectors. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T10.00006: Plasmon coupling in nanoparticle rings Stephan Link, Wei-Shun Chang, Liane Slaughter, Bishnu Kanal, Pramit Manna, Eugene Zubarev A surface plasmon is excited when the conduction band electrons of a metal oscillate coherently in phase with incoming excitation light. Plasmons can exist and propagate along structures that are smaller than the diffraction limit of light, the parameter which currently dictates the minimum size of optical interconnects. In addition to exploiting plasmons on continuous structures like thin films and nanowires for waveguiding, arrays of nanoparticles also pose potential for waveguiding. We have characterized the plasmon coupling of self-assembled rings of 40 nm gold nanoparticles functionalized with polystyrene using dark-field scattering microscopy and spectroscopy. Comparing images and spectra from the rings to those of single particles together with correlating images acquired by dark-field and SEM microscopy, we observe redshifted coupled plasmon modes that show a strong polarization dependence. In particular, segments of the ring aligned parallel to the axis of detected polarization display higher order longitudinal plasmon modes, similar to those observed for a long rod. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T10.00007: Evidence for Surface Plasmon Standing Waves in Ag Nanostructure Arrays Dominic Britti, Julia Heetderks, Hung-Chih Kan, Ray Phaneuf We report on simulations of the near field for arrays of Ag nanowires and nanocolumns excited by plane waves of light at normal incidence. The results show a systematic variation of the local electric field with spatial period and incident polarization, which is confirmed experimentally [1]. The distribution of the local field and the dependence of local field intensity versus spatial period and polarization indicate that the excitations in the Ag nanostructres are surface plasmon standing waves. \\[0pt] [1] S.H.Guo, J.J. Heetderks, H.-C. Kan and R.J. Phaneuf, Optics Express 16, 18417 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:30PM |
T10.00008: Transforming Light with Metamaterials Invited Speaker: Metamaterials are expected to open a gateway to unprecedented electromagnetic properties and functionality unattainable from naturally occurring materials, thus enabling a family of new ``meta-devices.'' We review this new emerging field and significant progress in developing metamaterials for the optical part of the spectrum. Specifically, we describe recently demonstrated artificial magnetism across the whole visible, negative-index in the optical range, and promising approaches along with challenges in realizing optical cloaking. A new paradigm of engineering space for light with transformation optics will be also discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T10.00009: Analytic LC model for plasmonic resonances in nano-structured split-ring resonators P. W. Kolb, T. D. Corrigan, A. B. Sushkov, H. D. Drew, D. C. Schamdel, R. J. Phaneuf We systematically investigate the plasmonic resonant behavior of metallic nano-structured split-ring resonators as a function of the size of the split gap, substrate permittivity, metal skin depth, and sample height. We performed simulations of the structures and examine the E-field and current density maps. We present a simple, analytic LC model to describe the lowest order resonance and its dependence on the aforementioned parameters. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T10.00010: Angular Dependence of Resonances from Rod Pairs and U-Shapes Timothy Corrigan, Paul Kolb, Andrei Sushkov, Dennis Drew, Dominic Britti, Raymond Phaneuf We examine and compare the angular dependence of electric and magnetic resonances from rod pairs and U-shapes made of Ag with the long arms placed both horizontally and vertically. We discuss the results in terms of photonic band structure effects. In addition, we observe that the splitting of the higher order mode for vertical U-shapes in the s-polarization behave as symmetric and anti-symmetric modes in which the modes red and blue shift, respectively, as observed previously. However, both horizontal U-shapes and rod pairs also show a split for the p-polarization in which both modes red shift. We discuss the reasons for this behavior using both experimental results and simulations. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T10.00011: Longitudinal elliptically polarized electromagnetic waves in anisotropic magnetoelectric split ring composites Wang Weihua, S.T. Chui, Lei Zhou, Z.F. Lin We study the propagation of plane eletromagnetic waves through different systems consisting of arrays of split rings of different orientations. We find a mode such that the electric field becomes elliptically polarized with a component in the \textbf{longitudinal} direction (i.e., parallel to the wave vector). Even though the group velocity and the wave vector are parallel, the Poynting vector possesses a component perpendicular to the wave vector. The speed of light can be real even when the product $\epsilon\mu$ is negative. Other novel properties are explored. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T10.00012: Natural negative refraction index in polycrystalline Fe and Ni at optical frequencies Adil-Gerai Kussow, Alkim Akyurtlu Analysis of the photon-magnon interactions in Fe and Ni, 3d transition-metal ferromagnetics, demonstrating the coupling between the incident light and high-frequency spin waves with energy ( 0.2 -- 0.35 ) eV is presented. As a consequence, these metals in their polycrystalline form with nanoscale grains are found to possess a negative refraction index at optical frequencies, close to the high-frequency ferromagnetic resonance. The effect is due to the coexistence of the spin wave mode with the plasmonic mode, and both modes are activated by the e.m. field of the light, with simultaneous permittivity and permeability responses within some frequency band. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T10.00013: Propagating and Localized Surface Waves in Metamaterial Stacks Ruwen Peng, Yongjun Bao, Zhaohui Tang, Feng Gao, Zhijian Zhang, Weihua Sun, Xin Wu, Mu Wang We demonstrate the interference effect between propagating and localized surface modes of electromagnetic wave in metamaterial stacks, which leads to a transmission extremum. When radiation is incident on a metal surface perforated with an array of ring-shaped subwavelength apertures, the phase difference between the propagating surface Bloch wave and the localized surface wave can be tailored by the geometrical parameters of the array so as to affect the shape of the transmission spectrum. Above the resonant frequency of the aperture, interference between the surface waves leads to a minimum in the transmission spectrum, whereas below it, the interference leads to a maximum. While in multiple metamaterial stacks with hole arrays, the coupling of surface electromagnetic wave yields a new resonant mode with increasing quality factor of the transmission peak. We suggest that these features provide flexibility in engineering surface wave-based all-optical devices. Reference: Y. J. Bao, R. W. Peng, D. J. Shu, Mu Wang, X. Lu, J. Shao, W. Lu,and N. B. Ming, Phys. Rev. Lett. (2008) 101, 087401. [Preview Abstract] |
Session T11: Focus Session: Transport Properties of Nanostructures V: Wires and Films
Sponsoring Units: DMPChair: Mark Hybertsen, Brookhaven National Laboratory
Room: 305
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T11.00001: Platform for Measurement of Phonon Scattering from the Surface of Silicon Nanostructures. J. P. Sullivan, T. A. Friedmann, E. S. Piekos, S. L. Shinde, J. R. Wendt We've created a micro-platform for measuring thermal phonon surface scattering in single-crystal Si nanostructures, specifically specular-to-diffuse surface scattering in the long phonon mean-free-path regime. The platform consists of three suspended co-linear monocrystalline Si islands with the center island resistively heated and connected to its neighbors by Si nanoligaments (one ligament straight, the other bent). The ligaments have a blade-like geometry with length, width, and depth of 1000 nm, 100 nm, and 2500 nm, respectively. Heat conducts from the center island across the ligaments in proportion to the ligament thermal conductance, which is lower for the bent ligament due to increased surface scattering. Monte Carlo simulations indicate that the heat flux differs between straight and bent nanoligaments by 10{\%} for diffuse (rough surface) phonon reflection and by almost 40{\%} for specular (smooth surface) reflection. Acknowledgment: DOE BES Div. of Mat. Sci. {\&} Eng. and LDRD (Sandia is operated by Sandia Corp. for the US DOE's NNSA, contract DE-AC04-94AL85000). [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T11.00002: Thermal Effects in Precision Nano-Electronics Construction Stephen Johnson, Douglas Strachan The development of high precision nano-electronics requires a detailed understanding of the non-equilibrium thermal effects during their construction and use. To better understand the dynamics of these nano-scale thermal effects, we investigate nanowires and nano-scale junctions of various dimensions with applied electric currents. During the application of current, significant joule heating occurs which induces the structures to evolve through thermo- and electromigration processes. A distinct change in the symmetry of these processes indicates that different mechanisms occur at the various stages of evolution. The results are compared to detailed thermal modeling of our structures and have implications on the development of a wide range of novel nanoscale devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T11.00003: First-principles parameter-free calculations of electron mobilities in silicon: phonon and Coulomb scattering Oscar D. Restrepo, Kalman Varga, Sokrates T. Pantelides Mobility is a key factor in charge transport since it describes how the motion of an electron is affected by an applied electric field. As such, it is an important element in the design of new devices. Mobilities are generally modeled using methods that suppress atomic-scale detail (effective mass theory or bulk energy bands for electron velocities, empirical deformation potentials, macroscopic roughness, etc). Parameter fitting to experimental data is needed. As new technologies require modeling of transport at the nanoscale and new materials are introduced, predictive parameter-free mobility modeling is needed. The main scattering mechanisms that limit mobilities are due to phonon, ionized impurities, and interface roughness. A first-principles calculation of mobilities limited by atomic scale roughness with atomic-scale detail was reported recently [1]. We report the development of parameter-free quantum-mechanical methods to calculate scattering rates and electron mobilities limited by phonon and ionized-impurity scattering in a self-consistent way. Results for n-doped silicon are in good agreement with experimental data. This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T11.00004: Reconstructing Fourier's law from disorder in quantum wires Massimiliano Di Ventra, Yonatan Dubi The validity of Fourier's law in nano-scale wires poses a fundamental theoretical challenge, with both scientific and technological implications. In this work, a novel theory of open quantum systems is used to study the local temperature and heat currents in metallic nanowires connected to leads at different temperatures. We show that for ballistic wires the local temperature is almost uniform along the wire and Fourier's law is invalid. By gradually increasing disorder, a uniform temperature gradient ensues inside the wire and the thermal current linearly relates to this local temperature gradient, in agreement with Fourier's law. Finally, we show that while disorder is responsible for the onset of Fourier's law, the non-equilibrium energy distribution function is determined solely by the heat baths. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T11.00005: A Single-Molecule Phonon Field-Effect Transistor Marcos Menezes, Brenda Moreira, Jordan Del Nero, Rodrigo Capaz Controlling phonons in the same way we control electrons in materials has been an old but elusive dream for physicists. In particular, it would be extremely desirable to control the thermal (phonon) flux between two reservoirs using a gate electric field, i.e., to construct a field-effect transistor for phonons. However, in most materials, electric fields do not couple strongly to lattice vibrations. Moreover, at the molecular and nano scale, in which the ballistic regime is dominant, thermal conductance of acoustic modes is universal, independent of the sound velocity. Therefore, modulating the sound velocity does not change the thermal conductance, thus making even more difficult the conception of such device. In this work, we propose a realizable architecture for a phonon field-effect transistor based on a single polar polymeric molecule placed between two reservoirs. An applied transverse electric field transforms the acoustic torsion mode into optical. For feasible temperatures and electric field magnitudes, this coupling can virtually suppress the contribution from this mode to the thermal conductance, therefore modulating the conductance by as much as 25{\%}. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T11.00006: Hypersonic Phononic Crystal Based on 2D Single Crystalline Nanoporous Alumina Akihiro Sato, Yan Pennec, Takashi Yanagishita, Bahram Djafari-Rouhani, Fytas George, Wolfgang Knoll, Hideki Masuda Periodic nanocomposite media consisted of alumina matrix and infiltrated cylindrical nanopores exhibit rich elastic wave propagation behaviour including the localization of phonons, anisotropic propagation and the formation of phononic band gaps at GHz frequencies. We have examined the translational symmetry dependence of dispersion relations on 2D single crystalline phononic crystals based on nanoporous alumina using Brillouin light scattering. The propagation of elastic waves is significantly different between native and filled with fluids alumina matrix. For the latter, the dispersion relations become independent of the propagation direction, as opposed to the native alumina scaffold. Theoretical band diagrams and the displacement fields describe well the experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T11.00007: Charge Carrier Confinement in a Nano-patterned Silicon Film Zheng Liu, Wenhui Duan, Feng Liu, Jian Wu Impurity scattering is becoming a critical problem in sub- micrometer MOSFET. One way to reduce the impurity scattering is by separating carriers from dopants, as used in the modulation- doping technique. From first-principles calculation, we find that by etching channels along (001) direction on the surface of a thin (110) silicon film, the hole states can be strongly confined in the film underneath the patterned layer. Therefore, by seletive doping in the top patterned layer, a modulation- doping-like effect can be achieved which is expected to greatly enhance the hole mobility. This effect arises from matching between carrier wavefunction orientation and quantum confinement direction determined by film and pattern geometry. It will be functional as long as the patterned feature size is within a few nanometers. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T11.00008: Magnetotransport of Bi nanowires: Evidence for surface carriers in bismuth. Tito Huber, Alla Nikolaeva, Leonid Konopko, Michael J. Graf Angle resolved photoemission spectroscopy studies (Hirahara et al, Phys. Rev. Lett. 97, 146803 (2006)) provide evidence of quantum-confined bulk-like states and surface states in ultrathin Bi films. Can these states be observed in electronic transport? We studied magnetotransport of trigonal Bi nanowires (30 nm $<$ diameter $<$ 200 nm) for fields up to 14 T. Bulklike states (M.R. Black et al, Phys. Rev. B68, 235417 (2003)) are identified because of its anisotropic Fermi surface and low effective mass. A two-dimensional behavior was expected of high-effective mass surface carriers; we found instead a three-dimensional behavior, with a rich spectrum of Landau levels in a nearly spherical Fermi surface. This behavior is related to the long penetration length of surface states in non-basal surfaces. On the basis of similarity of spectra, we show that recent observations of sharp peaks in the bulk Bi Nernst thermopower near the 9 T quantum limit, attributed to charge fractionalization (K. Behnia, L. Balicas and Y. Kopelevich, Science 317, 1729 (2007)), can be more plausibly interpreted in terms of quasiparticles that are based on surface states. Bismuth true quantum limit is 70 T. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T11.00009: Nanoscale Charge Transport in Realistic Organic Thin-Films: Beyond Variable-Range Hopping and Percolation Networks Geoffrey Hutchison, Marcus Hanwell, Xialing Chen, Aaron Crandall We are building up experimental and computational model systems for charge transport in nanoscale organic electronic devices. In particular, our combined approach is aimed at addressing questions as to the effect of impurities, traps, and other defects on electronic conductivity. Experimentally, we have designed thin films and monolayers to which we can controllably add known quantities of defects with known electronic properties. In tandem, we focus on a new Monte Carlo style simulation of charge transport in these imperfect devices with an aim to move beyond simple variable-range hopping models. Our goal is to establish all parameters for our simulations from first-principles calculations and detailed experimental results. I will describe initial results and comparisons with other organic electronic materials and existing charge transport models. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T11.00010: Computational Simulation of Charge Transport in Metal Terpyridine Monolayer FETs Marcus Hanwell, Geoffrey Hutchison Understanding the roles of charge traps and defects in electronic transport in organic materials is becoming increasingly important. Computational studies have been undertaken, using an agent-based Monte Carlo method, of the active region of a monolayer FET. Charge transport is assumed to be due to thermally activated, variable-range hopping between neighboring sites. This model system allows us to probe the role of charge traps/defects both computationally and experimentally. We do this by using multiple metal terpyridine complexes, each having known electronic structure. Using Marcus Theory and quantum calculations, the hopping rate between neighboring complexes can be predicted. Results from computational simulations of this system will be discussed, with special attention being paid to the results that can be experimentally verified, such as voltage-current curves. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T11.00011: Random telegraph signal and low frequency noise in molecular tunnel junctions Dominique Vuillaume, Nicolas Clement, David Guerin, Stephane Pleutin, David Cahen Monolayers of organic molecules present one of the main systems studied in molecular electronics. We report the observation and study of a low frequency noise and Random Telegraph Signal (RTS) in self-assembled alkyl chain junctions on silicon. The 2 levels of current can be clearly distinguished. With a sufficiently long recording time ($>$ 500 events), statistics can be performed on the current level and on the upper and lower times. The RTS amplitude is usually few {\%} of the average current and the process follows poissonian statistics. This RTS signal is also modulated by another RTS with a much longer time constant. This allowed us evaluation of the change of noise in the frequency domain from 1/f noise to Lorentzian like spectrum. In inorganic tunnel junctions, such signal can only be observed in sub-micrometric junctions whereas we observe it in almost millimetric junctions. This precludes mechanisms involving electron trapping / detrapping in single isolated trap. We propose several hypotheses leading to long-range fluctuations including molecular dynamics and relaxation processes. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T11.00012: Maximum energy transfer in nanoscale thermal radiation Soumyadipta Basu, Zhuomin Zhang Radiation heat transfer between closely spaced objects has received much attention lately because of the emerging applications of near-field thermophotovoltaics, thermal radiation scanning tunneling microscopy, and nanothermal manufacturing. The energy transfer in nanoscale radiation can exceed that of blackbody radiation by several orders of magnitude due to photon tunneling and the excitation of surface polaritons. An outstanding question remains as whether there exists an upper limit of near-field radiation for arbitrarily selected material properties. We examine the maximum radiative energy flux between two parallel plates separated by a vacuum gap from 0.1 and 100 nm distance. An upper bound is imposed to the parallel wavevector component in the analysis based on fluctuational electrodynamics. By assuming a frequency-independent dielectric function, we find that the maximum heat flux depends on the chosen complex permittivity and the distance. The determination of the achievable heat flux at nanometer distances will benefit future research and applications of near-field radiation for energy harvesting. [Preview Abstract] |
Session T12: Reaction Kinetics at Surfaces
Sponsoring Units: DMP DCMPChair: John Kitchin, Carnegie Mellon University
Room: 308
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T12.00001: Atomic structures and energetics of methanol and its reaction intermediates on the ZnO(0001) surface : A first-principles study Katawut Chuasiripattana, Oliver Warschkow, Bernard Delley, Cathy Stampfl Methanol (CH$_3$OH) is widely used in various chemical synthesis, and in particular it is predicted to be one of the next generation of renewable energy sources as a fuel for fuel cells. Industrially, methanol is mass produced by Al$_2$O$_3$-supported Cu/ZnO catalysts. However, the role of ZnO in the methanol systhesis is still unclear. To provide a better understanding of the mechanisms underlying this process, we present first-principles total- energy calculations of the methanol molecule and its reaction intermediates on the ZnO(0001) surface. A detailed characterization of atomic geometries and associated energetics is presented. The reaction intermediates we consider are CH$_3$O, CH$_2$O, CH$_3$, HCOOH, HCOO, HCO, H$_2$O, CO$_2$, CO, OH and H. These intermediate species are reported from experimental studies to be present during methanol decomposition on the ZnO(0001) surface. We also analyse the vibrational frequencies of each of the adsorbed fragments. The information obtained will be used for investigating the surface chemical reactions of associated with methanol synthesis over the ZnO surface. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T12.00002: Morphology of alloy catalysts in an oxidizing environment: Structure-sensitivity of ethylene epoxidation over Ag-Cu particles Simone Piccinin, Catherine Stampfl, Matthias Scheffler Ag-Cu alloys have been proposed as catalysts for ethylene epoxidation due to their superior selectivity compared to pure silver, the predominant catalyst for this reaction [1]. By means of density-functional theory and atomistic thermodynamics, we study the surface structure and morphology of Ag-Cu particles in thermodynamic equilibrium with an oxygen atmosphere. Contrary to the common assumption of the formation of a 2D surface alloy, we find that at temperatures and pressures of interest for practical applications the particles can display a variety of structures, including thin Cu-surface-oxides in coexistence with clean Ag. At variance with pure Ag in UHV, we find that under reactive conditions the (111) facet is not dominant. We identify different reaction pathways that will compete and/or synergetically interplay in the catalysis. In general, the reaction mechanism is structure-dependent and often the reaction does not proceed through the formation of stable intermediates, in contrast to clean Ag and the 2D alloy. Analyzing the competing reactions, we discuss how the addition of Cu improves the selectivity and stress the overall importance of accounting for the effect of ambient conditions. [1] S. Linic et al. J. Catal. 224, 148 (2004) [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T12.00003: Interaction of benzene and MMA vapors with TiO$_{2}$ surface: Relevance to EUV Lithography Boris Yakshinskiy, Shimon Zalkind, Robert Bartynski We characterize the surface processes that affect the reflectivity and lifetime of TiO$_{2}$ -- capped multilayer mirrors used in EUV lithography. Low energy electron beam is used to mimic excitations initiated by EUV radiation. Temperature programmed desorption, x-ray photoelectron spectroscopy, and low energy ion scattering are used to analyze the surface reactions. Carbon film growth on TiO$_{2}$ (011) crystalline surface is measured during electron bombardment in benzene or MMA vapor (model background gases) over a wide range of pressures and temperatures near 300 K. The surface temperature, gas pressure and electron energy are shown to be important for growth of carbon. The substrate temperature rise lowers the carbon accumulation rate. Increasing the vapor pressure over the surface enhances the carbon deposition, and variation of the electron energy shows a pronounce influence on the reaction rate. Low energy secondary electrons excited by EUV photons contribute substantially to carbon accumulation on clean TiO$_{2}$ cap layers. The steady-state coverage of the molecules on the surface and the cross-sections for electron-stimulated dissociation are key parameters for understanding and modeling the processes on the EUVL mirrors. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T12.00004: Selective Oxidation of Ammonia on RuO$_{2}$(110): a combined DFT and KMC study Sampyo Hong, Altaf Karim, Sergey Stolbov, Talat Rahman Motivated by the experiments of Wang et al [1] on the selective oxidation of ammonia on RuO$_{2}$(110), we have performed first principles electronic structure calculations based on the density functional theory (DFT) and the pseudopotential method to calculate the activation energy for the associated reaction processes, and used the DFT results in kinetic Monte Carlo (KMC) simulations of the reaction rates. We find the overall energy barriers for NH$_{3}$ + O $\to $ N + H$_{2}$O, N + N $\to $ N$_{2}$, and N + O $\to $ NO$_{ }$to be about 1.0, 0.6, and 0.56 eV, respectively. We also find, in agreement with experiment, intermediate products such as NH to be short-lived (i.e. not detectable in experiment). For a simple model of the surface dominated by the two end reactions above, our KMC simulations show indeed almost 100{\%} selectivity toward NO, in excellent agrement with experimental findings. For more realistic simulation, in which we include several intermediates and their reactions (over 20 processes), KMC simulations show about 60{\%} selectivity toward NO within the experimental O$_{2}$ pressure range. We compare our calculated reaction rates to those obtained in experiment and rationalize our results through details of the surface electronic structure. [1] Y. Wang, K. Jacobi, W.-D. Schoene, and G. Ertl, J. Phys. Chem. B 2005, 109, 7883-7893. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T12.00005: Surface Chemistry of Cyanogen on Copper Everett Lee, James Lallo, Erkan Ciftlikli, Sylive Rangan, Alexei Ermakov, B.J. Hinch The surface chemistry of cyanogen/cyanide species has been observed on Cu(100) crystal through Temperature Programmed Desorption, Helium Atom Scattering and soft X-ray Photoelectron Spectroscopy. Cyanogen (C2N2) dissociates on copper to form a mixture of cyanogen and cyanide (CN) species depending at cryogenic temperatures but pure cyanide at around ambient temperatures, leading to c(10x6) superstructure in two domains at saturation. Temperature Program Desorption spectra observed indicated that cyanide recombinatively desorbs as cyanogen at relatively high temperatures with no additional species observed. The desorption results were analysed using a variety of techniques in order to determine the activation energy of desorption (Ed) as well as its coverage dependency. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T12.00006: Identifying surface intermediates with TPD: Methylchlorosilanes on Cu(001) James Lallo, Everett Lee, B.J. Hinch, Dan Strongin Various methylchlorosilane molecules (SiH$_{x}$ Me$_{y}$ Cl$_{z}$ , x+y+z=4) were exposed to a Cu(001) surface. Dissociative adsorption was observed for several hydrogen containing species, at surface temperatures above 160K. The chemistry and thermal stability of the surface bound fragments were then studied as a means for understanding intermediates of the commercially important ``Direct Process,'' namely production of Cl2(CH3)2Si, from Si and CH3Cl, with a Cu catalyst.~ Temperature programmed desorption indicates that methyl groups are readily transferred between absorbed Si-containing species. A large fraction of Cl remains on the surface after observation of typically 2 or 3 distinct TPD features below 450K. By a comparison of~ the product species distributions to the parent species, we are then able to propose key intermediates common to many of the adsorption/desorption mechanisms. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T12.00007: Surface-catalyzed O2 adsorption on quantum thin films Jisun Kim, Alexander Khajetoorians, Wenguang Zhu, Zhenyu Zhang, Chih-Kang Shih Pure crystalline Pb is well known to be inert with respect to oxygen gas. By using scanning tunneling microscopy we demonstrate that the oxygen adsorption on Pb films is greatly increased by Cs adsorbates acting as catalysts. Our previous studies show that Cs atoms can be easily incorporated into the surface layer of thin Pb films grown on Si(111). In addition, Cs adsorbates are able to adjust the surface energy so as to initiate stable Pb nanoislands on Pb flat top mesas of unstable thickness. Because of this unique property of Cs adsorbates on originally inert Pb films, they are a natural choice of template to investigate surface catalysis of quantum thin films. Bare Pb films do not oxidize until much higher coverage (1000L), but in the presence of Cs we see adsorption of oxygen at much lower exposures of only a few Langmuir. The oxidation first occurs preferentially at sites of Cs adsorption. After oxygen clusters nucleate, oxidation of the Pb film increases in proportion to the amount of oxygen exposure. During this process the formation of oxygen clusters forces changes in the film morphology. First-principles density functional theory calculations of the O binding energies on the alloyed surface layers will be done and compared with experiment. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T12.00008: First principle studies of the oxygen reduction reaction on the CrN electrocatalyst Sergey Stolbov These fuel cells offer great advantages for various applications, but severe obstacles remain to their large scale implementation. The Pt-based catalysts, used in both electrodes make them unacceptably expensive. Furthermore, the low rate of the oxygen reduction reaction (ORR) on the Pt cathode significantly reduces efficiency of the device. In this work, ORR on alternative CrN electrocatalyst has been studied from first principles. It is found that, in contrast to Pt, oxygen molecules dissociate spontaneously on the CrN surfaces, which switch on simple 1-electron reduction mechanism. Based on the adsorption energy calculations the free energies of intermediates were obtained as functions of the electrode potential. This diagram explain a relatively high rate of ORR obtained experimentally [1] for this material. The effects of coverage and co-adsorbates have been also studied.\\[3pt] [1] H. Zhong, et al., Appl. Phys. Lett. 91, 163103 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T12.00009: Hydrogen Reduction at Room Temperature of Partially Oxidized Co Cluster Films Capped with Pd A. L. Cabrera, J. I. Avila, R. Trabol, C. Romero, M.J. Van Bael, P. Lievens Co clusters with mean size of 1.8 nm were deposited on sapphire substrates at 25$^{o}$C, 300$^{o}$C and 500$^{o}$C respectively. They were capped by a continuous 15 nm Pd film. Visible light transmission and reflection, in the range 400 to 900 nm, were measured when the samples were exposed to different hydrogen pressures up to 130 Torr. D.C. electrical resistance of the films was also measured as an independent property of the films to confirm hydrogen absorption by the samples. In all the samples the transmission and resistance of the films increased, reaching saturation at around 35-40 Torr hydrogen pressure. The relative change in the resistance of all Co cluster samples is smaller than the change in pure Pd films, indicating that hydrogen absorption is limited to the Pd capping layer only. We observed a significant decrease in the transmittance and the resistance during the first hydrogenation cycle of the sample prepared at 25$^{o}$C suggesting that a reduction of the partially oxidized Co clusters occurred at room temperature. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T12.00010: H$_{2}$ dissociation of H$_{2}$ on Co layers on Cu(111) from abinitio studies Duy Le, Sergey Stolbov, Talat Rahman Through first principles electronic calculations, based on the spin-polarized density functional theory using the generalized gradient approximation and the ultrasoft pseudopotential method in the plane wave representation, we studied the adsorption and the dissociation of H$_{2}$ on two Co layers grown on Cu(111). As H$_{2}$ approaches the surface with the H-H bond parallel to that surface, it dissociates at a distance of about 1.7? from the Co layer, and constituent H atoms proceed to occupy neighboring fcc and hcp sites. The ``adsorption'' energy barrier for H$_{2}$ is 0.14 eV and the ``adsorption'' energy is about 0.80eV. On the Co surface, H diffuses from an fcc site to an hcp site, or vise versa, with diffusion barriers of 0.17eV and 0.12eV respectively. We find no evidence of subsurface H. By analyzing the local electronic density of state, we establish, in agreement with suggestion from experiments [1], that the surface electronic states and magnetic moment of Co atoms depend very much on the H coverage. \\[3pt] [1] M. Sicot \textit{et al}, Phys. Rev. B \textbf{77}, 035417 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T12.00011: Structural and catalytic properties of thin CeO$_{2}$ films on TiO$_{2}$ substrate I.G. Batyrev, S.N. Rashkeev Structural properties of CeO$_{2}$(111) thin films on top of a TiO$_{2}$(110) substrate have been investigated by first-principles calculations. We found the special orientation of the ceria film relative to the rutile substrate that corresponds to a small (few percent) lattice mismatch between the two oxide surfaces. The positions of interfacial Ti atoms undergo some structural modulations during the relaxation process. Also, the relaxation of the interface resulted in the appearance of elongated Ce-O bonds at some sites of the O-terminated surface of CeO$_{2}$/TiO$_{2}$ films. These oxygen sites at the surface show low activation energy for loosing oxygen atoms and, therefore, they have higher catalytic activity in CO oxidation by the Mars-van Krevelen mechanism. We predict from the simulations that higher rate of the catalytic CO oxidation should have CeO$_{2}$(111)/TiO$_{2}$(110) films compared with CeO$_{2}$(111) surface, which may also be attributed to the elongation of Ce-O bonds at some sites of O-terminated film in comparison with a pure ceria surface. We discuss the role of oxygen vacancies in the CO oxidation at reduced films and investigate the interplay between migration of O bulk atoms to the surface O vacancy sites and the structural phase transition from the CeO$_{2 }$(111) (Ce$^{4+)}$ and Ce$_{2}$O$_{3 }$(0001) (Ce$^{3+})$ structures. This work was supported by the U. S. Department of Energy Contract DE-AC07-051D14517. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T12.00012: Adsorption and Dissociation of Molecular Hydrogen on the (0001) Surface of DHCP Americium Pratik Dholabhai, Asok Ray Hydrogen molecule adsorption on the (0001) surface of double hexagonal closed packed americium has been studied in detail within the framework of density functional theory. Weak molecular hydrogen adsorptions were observed. The most stable configuration corresponded to a Hor2 approach molecular adsorption at the one-fold top site where the molecule's approach is perpendicular to a lattice vector. Adsorption energies and adsorption geometries for different adsorption sites will be discussed. The change in work functions, magnetic moments, partial charges inside muffin-tins, difference charge density distributions and density of states for the bare Am slab and the Am slab after adsorption of the hydrogen molecule will be discussed. Reaction barrier for the dissociation of hydrogen molecule will be presented. The implications of adsorption on Am 5$f$ electron localization-delocalization will be summarized. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T12.00013: Atomistic Thermodynamic Approach for Determining Cu Catalyst Morphologies Under Reactive Water-Gas-Shift Conditions Nilay Inoglu, John Kitchin The water gas shift reactions (WGS) are widely used in several industrial processes and are catalyzed by Cu catalysts. In coal derived syngas, there are sulfur (S) impurities which can adsorb on the Cu surfaces and block the active sites of the catalyst and can result in catalyst deactivation. The sulfur adsorption will compete with other possible adsorbates such as CO, O, H and CO$_{2}$ making a quantitative estimate of the impact of sulfur on reactivity difficult. To determine the interactions between these possible adsorbates with Cu, the adsorption properties of these different adsorbates on low Miller index facets of Cu surfaces were studied in the limit where these surfaces are in thermodynamic equilibrium with the reactive environment. The effect of S adsorption on the surface electronic structure was significant and coverage dependent. In addition to a site-blocking deactivation mechanism, we found that sulfur modifies the surface electronic structure in a way that makes the Cu less reactive. Our results show that the adsorbates play a key role in defining the catalyst morphologies. We conclude that the presence of sulfur and other reactive adsorbates can impact the reactivity of the catalysts through several mechanisms including site-blocking, surface electronic structure modification and changes in the distribution and types of reactive sites through catalyst morphology changes. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T12.00014: The nonlocal correlation as the solution of the CO puzzle problem Predrag Lazic, Mojtaba Alaei, Nicolae Atodiresei, Vasile Caciuc, Radovan Brako, Stefan Bluegel For the last 20 years the Density Functional Theory (DFT) has been the standard approach for the calculation of chemisorption, adsorption, chemical reactions and electronic structure in general. Despite the great successes of the theory in predicting adsorption energies and other properties for many systems it turns out that the theory fails to predict correctly the adsorption site preference for the CO molecule on (111) surfaces of Pt, Cu and Rh, for example. The DFT calculations predict that the highly coordinated FCC (hollow) site adsorption is preferred over the top site adsorption, while experiments show unambiguously that CO adsorbs into the top site. Also, the calculated adsorption energies do not match well the experimental values. CO molecule chemisorption on these surfaces is a type of system in which one would expect DFT theory in its present implementation with semi-local (GGA) functionals to work well. We show that the inclusion of the nonlocal correlation into the DFT calculations of CO chemisorption through vdW-DF functional largely solves the discrepancies known as the CO puzzle problem. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T12.00015: ABSTRACT WITHDRAWN |
Session T13: Electronic Structure
Sponsoring Units: DCOMPChair: David Vanderbilt, Rutgers University
Room: 309
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T13.00001: Three-dimensional statistical reduction of the non-relativstic Schr\"{o}dinger equation for electrons with pair-wise Coulomb interactions Boyan Obreshkov Based on Ritz variational principle, we reduce in statistical fashion the non-relativistic $N$-body Schr\"{o}dinger equation for electrons with Coulomb interactions to a three-dimensional wave-equation for the motion of one electron with the residual $N-1$ electrons acting spectators of its motion [1]. As a consequence the Pauli's exclusion principle is interpreted as dynamical principle. Analytic solutions of the all electron quantal equations for the ground and excited states of the helium and lithium isoelectronic sequences will be represented and the comparison with the experimental measurements for the ground-state ionization potentials of atoms shown. \\[0pt] [1] B.~D.~Obreshkov , Phys.~Rev.~A {\bf 78}, 032503 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T13.00002: High Pressure Phases of Cu2O David Groh, Ravi Pandey, Miguel Blanco Copper Oxide's ambient phase is cubic. The copper atoms are in a simple face centered cubic packing sequence of hexagonal layers, with the oxygen atoms at alternating tetrahedral sites. As the pressure increases, cubic symmetry breaks. While the basic packing sequence is maintained, the distance between layers is no longer cubic -- a hexagonal structure results. At even further pressures, a major crystal structure change occurs to the CdI2 structure. The calculated phase change pressures and volumes tend to compare well with experiment. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T13.00003: $\bf{k}\cdot\bf{p}$ formalism within FLAPW method Tatsuya Shishidou, Tamio Oguchi We provide $\bf{k}\cdot\bf{p}$ formalism within the full-potential linearized augmented plane wave (FLAPW) method. Unlike the pure plane waves, the LAPW functions do not behave trivially in moving from $\bf{k}$ to $\bf{k}+\bf{q}$ and their incompleteness as a basis set should be taken into account. Derivatives of the sphere matching coefficients play the key role, for which we find a simple formula. Concrete formula for the $\bf{k}\cdot\bf{p}$ matrix elements is derived and numerically tested. Generalized second-order perturbation theory allowing for a degenerate case is presented and the literally-exact electronic band gradients and curvatures are accessible. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T13.00004: Electric polarization in a Chern insulator Sinisa Coh, David Vanderbilt We extend the Berry-phase concept of polarization to insulators having a non-zero value of the Chern invariant. The generalization to such Chern insulators requires special care because of the partial occupation of chiral edge states. We show how the integrated bulk current arising from an adiabatic evolution can be related to a difference of bulk polarizations. We also show how the surface charge can be related to the bulk polarization, but only with a knowledge of the wavevector at which the occupancy of the edge state is discontinuous. We conclude by presenting numerical calculations on a model Hamiltonian to provide additional support for our analytic arguments.\footnote{Preprint: arXiv:0810.4549} [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T13.00005: A new Laplacian representation for real-space calculations of general periodic and partially periodic systems Amir Natan, Ayelet Benjamini, Doron Naveh, Leeor Kronik, Murilo Tiago, Scott Beckman, James Chelikowsky We present a real-space method for electronic-structure calculations of systems with general full or partial periodicity. The method is based on the self-consistent solution of the Kohn-Sham equations, using first principles pseudopotentials, on a uniform three-dimensional non-Cartesian grid. Its efficacy derives from the introduction of a new generalized high-order finite-difference Laplacian that avoids the numerical evaluation of mixed derivative terms and results in a simple yet accurate finite difference operator. Our method is further extended to systems where periodicity is enforced only along some directions (e.g., surfaces), by setting up the correct electrostatic boundary conditions and by properly accounting for the ion-electron and ion-ion interactions. Our method enjoys the main advantages of real-space grid techniques over traditional plane-wave representations for density functional calculations, namely, improved scaling and easier implementation on parallel computers, as well as inherent immunity to spurious interactions brought about by artificial periodicity. We demonstrate its capabilities on bulk GaAs and Na for the fully periodic case and for a monolayer of Si-adsorbed polar nitrobenzene molecules for the partially periodic case. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T13.00006: Confinement effects on excitation energies and regioselectivity as probed by the Fukui function and the molecular electrostatic potential Alex Borgoo, David Tozer, Paul Geerlings, Frank De Proft When a molecule is placed as a guest inside a zeolite pore, its electronic structure will be altered, among others by the effect of the so-called ``confinement". It has been established that the compression of the molecular orbitals influences a system's reactivity. In this work we use a simple potential barrier method to quantify the importance of confinement effects on chemical reactivity. In the first part, excitation energies and molecular orbital energy gaps are evaluated for molecules placed in cavities of different sizes. Our results for ethylene and formaldehyde reveal an increase in excitation energy and the gap between the occupied and the unoccupied levels. In the case of the larger molecules naphthalene and anthracene, the HOMO-LUMO gap shows very little sensitivity to the confinement. To investigate the role of confinement effects on local aspects of chemical reactivity and on regioselectivity, we evaluated its effect on the Fukui function and the molecular electrostatic potential, reactivity indices that are central in the description of orbital and charge controlled reactions. The results indicate that confinement can influence the regioselectivity and that the reactivity of anions is expected to change, due to the artificial binding of the exess electron. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T13.00007: Maximally-localized Wannier functions for GW quasiparticles D. R. Hamann, David Vanderbilt Recent efforts carrying the GW many-body approximation to self-consistency have given improved electronic structure results.$^{1}$ Maximally-localized Wannier functions$^{2}$ formed from the quasiparticle wave functions$^{3}$ provide an efficient and highly accurate basis for interpolating the SCGW bands from a coarse Brillouin-zone mesh to symmetry lines. Since the MLWF's correspond to chemists' bond orbitals, they potentially also provide insight into the qualitative effects of the improved treatment of correlations in SCGW compared to LDA. We report results on SrTiO$_{3}$, solid Ar, and molecular CO. Band interpolation is accurate and effective for both solids. Small shifts in the degree of hybridization can be visualized for some of the SrTiO$_{3}$ and CO MLWF's. In Ar, individual conduction-band Bloch functions were found to have large differences between LDA and SCGW.$^{1}$ However, a manifold of 9 d and spd-hybrid MLWF's which proved to be the minimum necessary for the lower conduction bands showed minimal differences in the two cases. A fully-functional interface to the \textsc{WANNIER90} library within the SCGW-capable \textsc{ABINIT} code has been implemented and will be publicly available in the near future. 1. F. Bruneval \textit{et al}., Phys. Rev. B \textbf{74}, 045102 (2006). 2. N. Marzari and D. Vanderbilt, Phys. Rev. B \textbf{56}, 12 847 (1997). 3. M. van Schilfgaarde \textit{et al}., Phys. Rev. Lett. \textbf{96}, 226402 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T13.00008: High-performance computational condensed-matter physics in the cloud J.J. Rehr, L. Svec, J. P. Gardner, M. P. Prange We demonstrate the feasibility of high performance scientific computation in condensed-matter physics using {\it cloud computers} as an alternative to traditional computational tools. The availability of these large, virtualized pools of compute resources raises the possibility of a new compute paradigm for scientific research with many advantages. For research groups, cloud computing provides convenient access to reliable, high performance clusters and storage, without the need to purchase and maintain sophisticated hardware. For developers, virtualization allows scientific codes to be pre-installed on machine images, facilitating control over the computational environment. Detailed tests are presented for the parallelized versions of the electronic structure code SIESTA \footnote{J. Soler et al., J. Phys.: Condens. Matter {\bf 14}, 2745 (2002).} and for the x-ray spectroscopy code FEFF \footnote{A. Ankudinov et al., Phys. Rev. B {\bf 65}, 104107 (2002).} including CPU, network, and I/O performance, using the the Amazon EC2 Elastic Cloud. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T13.00009: A scalable algorithm for the computation of Hartree-Fock exchange Ivan Duchemin, Francois Gygi Electronic structure calculations based on hybrid density functionals require efficient algorithms for the computation of the Hartree-Fock exchange operator. The high computational cost of Hartree-Fock exchange currently limits the use of such functionals in large-scale First-Principles Molecular Dynamics applications. We present a scalable parallel algorithm for the computation of Hartree-Fock exchange in a plane-wave, pseudopotential framework, with applications to electronic structure calculations of liquid water and various nanostructures. Technical issues arising in the implementation of hybrid density functionals will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T13.00010: Compact representations of Kohn-Sham invariant subspaces Francois Gygi We present a method to compute a hierarchical approximate representation of the solutions of the Kohn-Sham equations. The approach is based on a recursive bisection algorithm and yields one-particle wavefunctions localized on domains of varying sizes. The accuracy of the representation is set {\em a priori} by specifying the maximum error in the norm of the approximate wavefunctions. Applications to the electronic structure of large systems are used to illustrate the data reduction achieved by this representation. The achievable data compression is similar to that obtained by truncating Maximally Localized Wannier Functions. Implications for the acceleration of electronic structure calculations and for the development of linear-scaling algorithms will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T13.00011: Improvements to the NRL Tight-Binding Model Michael Mehl, Dimitrios Papaconstantopoulos, Noam Bernstein, Daniel Finkenstadt, Stefano Curtarolo The original NRL Tight-Binding Method[1] has proven to be extremely successful in reproducing first-principles total energies and band structures for many elemental systems[2], and has been applied in computationally intensive molecular dynamics simulations[3]. When generalizing to multiple atom types, however, some difficulties arise because of the form of the interaction of the on-site matrix elements with the external environment. We discuss these difficulties, and describe a new version of the method which includes a proper two-center development of the on-site parameters[4,5], including applications of the method. [1] RE Cohen et al., Phys. Rev. B 50, 14694 (1994) [2] MJ Mehl and DA Papaconstantopoulos, Phys. Rev. B 54, 4519 (1996) [3] D Finkenstadt et al., Phys. Rev. B 74, 184118 (2006) [4] JL Mercer and MY Chou, Phys. Rev. B 49, 8506 (1994) [5] RE Cohen et al., Phys. Rev. B 56, 8575 (1997) [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T13.00012: Calculation of Correlation Functions using the Momentum Average Approximation Glen Goodvin, Mona Berciu The Momentum Average (MA) approximation has been successfully applied to a growing number of Hamiltonians involving electron-phonon coupling since its discovery only three years ago. This analytical non-perturbative approximation is exact in both the zero bandwidth and zero el-ph coupling limits, and by summing all of diagrams in the full diagrammatical expansion of the self-energy, albeit with approximations made on each of them, it gives highly accurate results over the entire parameter space. In this work we explore another significant generalization of the approximation by using MA to calculate correlation functions, here the optical conductivity of the Holstein polaron is used as a specific example. A comparison of the MA results against available numerical data again displays a high degree of accuracy for very minimal computational effort. Based on our previous generalizations of MA to systems with momentum-dependent el-ph couplings, we argue that MA could be used to study the linear response of an even broader class of problems. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T13.00013: Entanglement Spectrum of Topological Insulators F. D. M. Haldane Topological order in electronic systems leads to striking features in the ``entanglement spectrum'' which characterizes quantum entanglement between two halves of a system (FDMH, PRL 101,101504 (2008)). If the system is divided along a translationally-invariant boundary, the spectrum can be labeled by momentum parallel to it, and is gapless if topological order is present. The gapless spectrum is related to gapless edge states that appear at free edges of such systems. Topological insulators have non-trivial one-electron band structure properties; the entanglement spectrum of a non-interacting electron Slater determinant state is itself a non-interacting fermionic spectrum. This spectrum is gapped for ``trivial'' ordinary insulating band structures, but exhbits characteristic spectral flow and gapless modes when the band structure is topologically non-trivial. As a case study, the entanglement spectrum of the ``Haldane model'' of a Bloch electron band structure with a zero-field quantum Hall effect will be described in detail, and generalizations to spin-orbit-coupled topological insulators discussed. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T13.00014: Trends in electron correlation effects on $^{17}$O NMR chemical shifts in the alkaline earth oxides. Eric J. Walter, Daniel L. Pechkis, Henry Krakauer A recent study found that calculated GGA chemical shift $\delta_{iso}$(O) in CaO and other Ca oxides were in poor agreement with experiment. We have calculated $\delta_{iso}$(O) in the rock salt series MgO through BaO, using a range of methods from HF and DFT to highly accurate CCSD calculations. The crystal environments were modeled with finite quantum clusters. GGA results for 25-atom O$_{19}$X$_6$ embedded clusters are found to i) reproduce previous GGA results with periodic boundary conditions for Mg and Ca\footnote{Profeta et al.\ J. Am. Chem. Soc. {\bf 126}, 12628 (2004).}; ii) yield poor results for SrO as in CaO; iii) give good agreement with experiment for BaO. 7-atom OX$_6$ clusters were used to study trends with increasing levels of correlation. For MgO, correlated treatments result in only small changes compared to HF and DFT, as expected. Both CaO and SrO show large changes due to improved treatment of correlation, with the GGA-CCSD $\delta_{iso}$(O) difference similar to the GGA-Expt difference in the larger clusters. CCSD OX$_6$ results for BaO were inconclusive, from competing errors due to relativistic and correlation effects. We discuss our results in terms of the increasing covalency in XO crystals with heavier cations. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T13.00015: Efficient all-electron $GW$ calculations of complex semiconductors Christoph Friedrich, Arno Schindlmayr, Stefan Bl\"ugel The $GW$ approximation for the electronic self-energy yields quasiparticle band structures in very good agreement with experiment, but almost all implementations so far are based on the pseudopotential approach, which limits their range of applicability. We have developed an implementation (SPEX, http://www.flapw.de/spex/) within the all-electron full-potential linearized augmented-plane-wave (FLAPW) method. Within this method a large variety of materials can be treated, including d- and f-electron systems, oxides and magnetic systems. Our implementation employs a mixed product basis for the representation of wave-function products. A basis transformation to the eigenfunctions of the Coulomb potential allows a reduction of the basis-set size without compromising the accuracy, thus leading to a considerable speed-up in computation time. To demonstrate the efficiency of the implementation we present results for complex semiconductors. [Preview Abstract] |
Session T14: Disordered Systems, Glassy Dynamics, and Jamming I
Sponsoring Units: DFD GSNPChair: David Pine, New York University
Room: 315
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T14.00001: Shear Induced Structural Relaxation in a Supercooled Colloidal Liquid Dandan Chen, Denis Semwogerere, Eric R. Weeks The rheology of dense amorphous materials under large shear strain is not fully understood, partly due to the difficulty of directly viewing the microscopic details of such materials. We use a colloidal suspension to simulate amorphous materials, and study the shear-induced structural relaxation with fast confocal microscopy. We quantify the plastic rearrangements of the particles in two ways. First, we consider ``non-affine mobility'' by subtracting the global linear applied strain from the particle motion. Second, we examine ``local deformation'' by subtracting the local linear apparent strain (as measured from the particle motion). We find these measures of plasticity are spatially heterogeneous, with localized regions where many particles are strongly rearranging by these measures. We examine the shapes of these regions and find them to be essentially isotropic, with no alignment in any particular direction. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T14.00002: Complete Density Landscape for a Model Confined Liquid Ashwin Selvarajan Sampangiraj, Richard K. Bowles We enumerate the complete jamming landscape for a system of hard
discs with
diameter $\sigma$ confined between walls having a separation $h$,
such that
$1.866 |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T14.00003: Influence of Flow Quench Rate on the Internal Stress and Aging Dynamics of a Repulsive Colloidal Glass Chinedum Osuji, Ajay Singh Negi We investigate the dynamics of aging in a repulsive colloidal glass composed of charged clay particles in aqueous suspension. Dynamic rheological measurements show a power law evolution of the elastic modulus of the system with sample age, measured as time elapsed after the cessation of a rejuvenating shear flow. We show that the scaling exponent is dependent on the rate of flow cessation or the flow quench rate. Comparatively fast quenches lead to systems with a smaller elastic modulus and accelerated aging whereas slower quenches result in higher modulus but correspondingly less rapid aging. We apply a recently proposed technique to follow the dynamics of residual or internal elastic stresses immediately after the flow arrest and find striking parallels between the relaxation of these stresses and the aging of the system. These results indicate that the evolution of the slow dynamics is strongly coupled to the internal stress state of the system and point to the identification of the flow quench rate as a mechanical variable that characterizes the system's departure from equilibrium. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T14.00004: Hydrogen-bond network dynamics in sugar-based glasses Marcus Cicerone, Jerainne Johnson, Michael Pikal Hydrophilic organic glasses composed of sugars and polysaccharides are known to stabilize proteins against aggregation and chemical degradation. It has long been supposed that, due to the long timescales involved in protein aggregation and chemical degradation in the glass, $\alpha $ relaxation processes essentially control the rate of degradation. We have shown that, although there may be $>$ 7 orders of magnitude in time separation, $\beta $ relaxation processes can dominate in influencing both chemical and physical degradation. Also, it is apparent that these $\beta $ processes are closely related to dynamics of the hydrogen-bond network in these glasses. In this presentation we will briefly discuss the phenomenology of protein degradation in sugar-based glasses, and also present details of work on developing a fluorescent probe for use as a sensor for dynamics of the hydrogen-bond network in these glasses. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T14.00005: Density of states and soft modes of hard sphere colloidal glasses -- experimental observations. Antina Ghosh, Vijayakumar Chikkadi, Peter Schall, Jorge Kurchan, Daniel Bonn Recent theories and simulations have predicted the presence of soft modes due to which the DOS of glassy materials does not go to zero at zero frequency. We obtain DOS of colloidal hard sphere suspensions from experimental data. The displacement fields of hard sphere colloidal suspensions were studied for a range of volume fractions near the glass transition using confocal microscope. Normal mode frequencies are then computed from the time averaged correlation matrix. The density of vibrational states obtained from normal mode analysis indeed reveals an excess of low frequency anomalous modes in the system. To understand the nature of the modes we analyse the displacement vector fields at various frequencies. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T14.00006: XPCS Studies of Nanoparticle Motion within Glassy Polymer Melts H. Guo, G. Bourret, R. B. Lennox, M. Sutton, J. L. Hardon, R. L. Leheny We report x-ray photon correlation spectroscopy (XPCS) experiments to investigate the motion of nanoscale gold particles within polystyrene (PS) melts of molecular weight between 2K and 48K g/mol. The particles, with radius of approximately 2 nm, are dispersed in a highly dilute concentration (volume fraction 0.0004) and are functionalized with PS chains to stabilize them against aggregation. At high temperature, the observed motion is diffusive, with a rate that follows a Vogel-Fulcher temperature dependence. When the melts are quenched to lower temperature, the XPCS results indicate hyper-diffusive motion that can be modeled as strain in the melt in response to localized stress relaxation. These dynamics evolve with time following the quench, suggesting that they are coupled to aging of the polymer. Our observation of this hyper-diffusive motion among such a dilute concentration of stable nanoparticles indicates that the particles act as passive tracers and the motion is an intrinsic property of quenched melts. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T14.00007: Divergent four-point dynamic density correlation function of a glassy suspension Grzegorz Szamel, Elijah Flenner We use a diagrammatic formulation of the dynamics of interacting Brownian particles\footnote{G. Szamel, J. Chem. Phys. \textbf{127}, 084515 (2007).} to study a four-point dynamic density correlation function of a glassy colloidal suspension. We re-sum a class of diagrams which separate into two disconnected components upon cutting a single propagator. The resulting formula for the four-point correlation function can be expressed in terms of three-point functions closely related to the three-point susceptibility introduced by Biroli \textit{et al.}\footnote{G. Biroli \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 195701 (2006).} and the standard two-point correlation function. We numerically evaluate the four-point function and the associated dynamic correlation length. Both the amplitude of the four-point function and the correlation length diverge at the mode-coupling transition. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T14.00008: Self-Organized Criticality in Periodically-Sheared Sedimenting Suspensions Emmanouela Filippidi, Laurent Corte, Paul Chaikin, Laurence Ramos, David Pine Suspensions of non-colloidal particles under slow periodic strain can undergo a dynamical phase transition from an active fluctuating state to an absorbing steady state at a critical volume fraction In the case of density-mismatched particles, sedimentation and shear-induced diffusion drive the system towards a self-organized critical state. The lengthscales and timescales associated with the dynamics of the active particle clusters sustained near the critical point are shown to follow power-law behavior via simulation of activated random walkers. Finite-size effects and excluded volume interactions are explored for sedimenting and neutrally buoyant, mono- and bi-disperse suspensions both by simulation and experiment. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T14.00009: Heat transfer in model amorphous solids Vincenzo Vitelli, Ning Xu, Matthieu Wyart, Andrea Liu, Sidney Nagel We investigate heat transfer in model amorphous solids obtained from jammed packings of soft spheres. At the boson peak frequency, we find a sharp crossover from a weak-scattering regime, in which the energy diffusivity drops rapidly with frequency, to a strong-scattering regime, in which the diffusivity is nearly frequency-independent. We present a scaling analysis of how the crossover frequency shifts to zero as the system is decompressed towards the jamming transition. We show that the regime of flat diffusivity, invoked to explain the temperature dependence of the thermal conductivity of glasses, can arise from properties of the jamming transition. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T14.00010: Structural response of a colloidal glass to local forcing Kevin B. Aptowicz, Peter J. Yunker, Sean Gossin, Zexin Zhang, A. G. Yodh Video microscopy of glassy colloidal suspensions permits direct visualization of particle locations and trajectories, thereby providing an excellent experimental tool to aid our understanding of glasses and address current theories. We have conducted a series of experiments utilizing a bidisperse mixture of thermosensitive NIPA microgel spheres to study the structural response of a two-dimensional colloidal glass to point expansion. The packing fraction of the colloidal suspension is tuned from a liquid to a deeply jammed glass by varying the global temperature of the sample. Over this range of packing fractions, the response of the sample to point expansion is analyzed. In particular, an infrared laser tightly focused on the sample generates thermophoretic forces that lead to a point expansion in the colloidal glass. We track particle rearrangements and characterize the response as a function of packing fraction. These experiments take a step towards understanding the relationship between local structure and bulk properties of glass. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T14.00011: Relaxation processes in polystyrene melts and ultra-thin films A. Baljon, S. Williams, N. Balabaev, F. Paans, A. Lyulin By means of large-scale computer simulations we investigate relaxation processes in polystyrene melts and ultra-thin films. The local orientational mobility of the phenyl bonds is studied with the help of Legendre polynamials of the second-order P{\_}2(t). The spectral density of P{\_}2 (t) shows several distinctive peaks. They are caused by the large- scale motions of cooperative segments ($\alpha $ relaxation), smaller-scale structural dynamics ($\beta $ relaxation), and transient processes. Our simulations reveal that interfaces affect $\alpha -$ and $\beta $-relaxation processes differently. The most puzzling observation is a slight decrease in the structural relaxation time in the middle of the film, compared to that near the free surface. As expected, the $\alpha $-relaxation time is shorter near the free surface. The glass transition temperature, obtained from a plot of thickness versus temperature, decreases with decreasing film thickness, which is in agreement with an observed decrease in the $\alpha -$relaxation time. Surprisingly, the structural relaxation time is roughly the same for the bulk and for films. Our results will be compared with published experimental data. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T14.00012: Changes in Local Structure and Dynamic Heterogeneity in an Aging Glass Peter Yunker, Zexin Zhang, Kevin B. Aptowicz, Ahmed M. Alsayed, Arjun Yodh Recent works have shown a connection between structure and dynamical heterogeneity in glass$^{12}$. However, a connection between structure and aging dynamics remains elusive. To this end, we study aging in a bidisperse suspension of soft spheres. Micron-sized temperature-sensitive NIPA particles are employed in two-dimensions, and directly observed with video microscopy. After quenching from liquid to glass, the fraction of particles with crystalline order within the first coordination shell increases with time. Particles that undergo irreversible rearrangements$^{2}$, the aspect of dynamic heterogeneity most closely linked to structural relaxation, are identified. Particles with local crystalline order are observed to be very unlikely to irreversibly rearrange, and therefore more stable. This increase in stable particle configurations leads to the slowing of dynamics that is characteristic of aging. [1] A. Widmer-Cooper, H. Perry, P. Harrowell, and D. R. Reichman, Nat Phys \textbf{4}, 711 (2008) [2] K. Watanabe and H. Tanaka, Physical Review Letters \textbf{100} (2008) [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T14.00013: Investigation of electron beam induced changes in glassy Ge$_{x}$Se$_{1-x}$ thin films W. Zhou, G. Hoffman, H.O. Colijn, R.M. Reano, R. Sooryakumar, P. Boolchand Global structures in network glasses are characterized by their connectedness or mean co-ordination number. As the number of these cross-links within a covalent network increases by compositional tuning these systems steadily evolve from being underconstrained (floppy) to an overconstrained (rigid) solid. Recently (Appl Phys Lett 93, 041107 (2008)) we exploited electron beams to write nanoscale surface motifs in Ge$_{0.2}$Se$_{0.8}$ thin films that are at the special Ge in Se composition lying in the immediate vicinity of the floppy to rigid stiffness transition. In order to investigate the nature of the surface reliefs we have employed selected area transmission electron microscopy (TEM) to probe the electron beam induced structural changes to the film. Films of thicknesses less than 150 nm were deposited by pulsed laser deposition directly onto a carbon film on a mica substrate. The glass film and carbon layer were then lifted off onto copper grids for the TEM studies. Extension of the electron beam driven studies to other compositions, as well as the effect of multiple beam overwrites on the surface reliefs and trenches in several Ge$_{x}$Se$_{1-x}$ compositions will also be presented. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T14.00014: Periodic Lattices Near Isostaticity Anton Souslov, T.C. Lubensky Lattices in $d$ dimensions with with an average of $z=2d$ contacts per site are at the verge of mechanical stability and are called isostatic. Common isostatic lattices include the two-dimensional square and Kagome lattices as well as the three-dimensional cubic lattice with nearest-neighbor sites connected by central-force springs of spring constant $k$ and randomly packed spheres at random close packing at what is called point $J$. We calculate the phonon response functions and spectra of nearly isostatic square, cubic, and Kagome lattices in which springs of spring constant $k'$ connect next-nearest-neighbor sites. These lattices exhibit highly anisotropic modes at $k'=0$, among which are soft modes with one-dimensional dispersion in wavenumber, giving rise to a flat density of states as a function of frequency $\omega$. In the square lattice, these modes are shear acoustic phonons, whereas in the Kagome lattice, they are optical phonons. When $k'>0$, the low-energy modes crossover from acoustic phonons of the appropriate lattice symmetry for $\omega < \omega^* \sim\sqrt{k'}$ to the soft isostatic-like modes for $\omega>\omega^*$, and the density of states crosses over from Debye-like to flat. Static phonon response functions exhibit correlation lengths $\xi \sim 1/\sqrt{k'}$. We discuss the relation of these results to those for jammed systems near point $J$. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T14.00015: Soft modes and elasticity of nearly isostatic lattices: randomness and dissipation Xiaoming Mao, Tom Lubensky Isostatic periodic lattices, such as the square and kagome lattices in spatial dimension $d=2$, are systems at the onset of rigidity. They are marginally stable with coordination number $z=2d$, and they may exhibit a non-extensive number of soft modes that can be removed by adding an infinitesimal number of additional bonds. Randomly packed frictionless spheres at the jamming point J represent an important isostatic system that, because of its randomness, exhibits complexities beyond those of periodic systems. To study the effects of randomness on phonon response, propagation, and damping, we constructed model lattices near isostaticity by adding randomly distributed next-nearest and second-nearest neighbor bonds to the isostatic square and kagome lattices, respectively. We calculated a number of properties of these models using the CPA approximation and found them to resemble those of jammed solids near the point J. In particular, the phonon density of states crosses over from Debye-like at low frequency $\omega$ to the flat frequency-independent behavior of a one-dimensional systems at a characteristic frequency $\omega^*$ that scales as the density of additional random bonds $\Delta z$. The real and imaginary part of the effective random-bond spring constants become equal at $\omega^*$. We also identify a characteristic length that scales as $(\Delta z)^{-1}$. [Preview Abstract] |
Session T15: Fluidic Devices: Micro and Nano
Sponsoring Units: DFDChair: Thomas Cubaud, Stony Brook University
Room: 316
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T15.00001: Time evolution of distributive entropy in rectangular microchannel mixers Miron Kaufman, Petru Fodor Patterning ridges on the surface of microchannels has been found to be a viable strategy to induce mixing in straight channels, despite the characteristically small Reynolds numbers. In this work we evaluate the time evolution of the R\'{e}nyi entropy associated with the spatial distribution of tracers advected by an incompressible fluid moving in several straight rectangular channels: staggered herring bone [1], fractal surface patterning [2]. The steady state flow fields are obtained by solving the Navier -- Stokes and continuity equations using a finite element analysis package. The R\'{e}nyi entropy is then evaluated at different times using the spatial distribution of the tracers. The entropy increases with time as lnt with a slope approximately equal to unity. The slope quantifies the rate of distributive mixing. The rate of increase in the entropy is found to be independent of the Renyi beta parameter. This is qualitatively different than the distributive mixing in channels with moving walls [3] where the rate of distributive mixing changes with the beta parameter. We also study the dependence of the distributive entropy on the Reynolds number. [1] A.D. Stroock et al., Science 295, 647 (2002); [2] M. Camesasca, M. Kaufman, I. Manas-Zloczower, J. Micromech. Microeng. 16, 2298 (2006); [3] W. Wang, I. Manas-Zloczower, M. Kaufman, Chemical Engineering Communications, 192(4), 405-423 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T15.00002: Capillary absorption of metal nanodroplets by carbon nanotubes Shaun Hendy, Dmitri Schebachov We present a simple model that demonstrates the possibility of capillary absorption of non-wetting liquid nanoparticles by carbon nanotubes assisted by the action of the Laplace pressure due to the droplet surface tension. We test this model with molecular dynamics simulation and find excellent agreement with the theory, which shows that for a given nanotube radius, there is a critical size below which a metal droplet will be absorbed. We then consider the dynamics of capillary absorption using the steady-state flow model due to Marmur, which is based on the Lucas-Washburn model with the addition of a driving force due to the Laplace pressure of the droplet. We find an exact solution to Marmur's evolution equation for the height of the absorbed liquid column as a function of time, and show that this reproduces the dynamics observed in the simulations well. The simulations show that the flow of the metal exhibits a large degree of slippage at the tube walls, with slip lengths of up to 10nm. These findings suggest new methods for fabricating composite metal-CNT materials, and have implications for our understanding of the growth of CNTs from metal catalyst particles. The results also explain the recent observations of the absorption of Cu nanodroplets by carbon nanotubes. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T15.00003: Dynamic Pattern Formation In a Bubble-Generating Concentric Microfluidic Device Keng-hui Lin, Kuo-yuan Chung We observe rich spatiotemporal patterns of bubbles inside liquid droplets through a concentric microfluidic device made by two capillary tubes flown with gas and liquid respectively. When the gas pressure increases, the bubbles change from mondisperse, bidisperse to polydisperes. When the liquid flow rate to the gas flow rate is small, the bubble can not be stabilized inside the liquid droplet. The diameter of the bubbles can be scaled with the ratio of gas flow rate to the liquid flow rate. Our device offers different geometry to understand the bubble breakup in the microfluidic device. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T15.00004: Flow-Based Organization of Soft Matter in Three Dimensions Lian Leng, Siavash Aslanbeigi, Axel Guenther Flows of miscible and immiscible liquids through microchannel networks have been previously used to achieve spatial organization within one plane. However, extending this approach to three dimensions, an essential requirement to create synthetic bulk materials with a regular microstructure, is not straightforward. To our knowledge for the first time, we demonstrate microfluidic strategy for the three-dimensional organization of soft bulk materials. The approach is enabled by a massively scaled microfluidic architecture that distributes two miscible or immiscible fluid streams through an array of parallel channels. The soft-lithographic fabrication process was adapted to consistently define microfluidic channel networks in elastomer substrates that are only 500 microns thin; followed by subsequent bonding of up to ten such layers in the vertical direction. The chip was connected with fluidic inlets, completely immersed in water and continuously extruded the organized material at its exit. Upon leaving the chip, neighbouring fluid streams formed a hydrogel retaining the desired regular microstructure. The material microstructure was controlled by adjusting the flow rates of the interdiffusing fluid streams (e.g. aqueous alginate and calcium chloride solutions). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T15.00005: Viscous droplet deformation and breakup in microfluidic cross-flows Thomas Cubaud The dynamic response of translating high-viscosity droplets is experimentally investigated by means of a sharp increase of the flow velocity in a microchannel junction. The additional local injection of the continuous phase from symmetric side-channels into a square microchannel produces a broad range of time-dependent deformations and breakup. In particular, due to microscale wall confinement, the system displays a non-linear behavior with the initial droplet size. Deformations, relaxation times, and fragmentation processes are examined as a function of flow and fluids properties with a particular emphasis on the formation of slender viscous structures and spoon-like droplets, i.e., asymmetrical droplets. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T15.00006: Morphology of liquids spreading along open nanofluidic channels Antonio Checco Dynamic atomic force microscopy (AFM) in the non-contact regime is used to study the morphology of a non-volatile liquid (squalane) as it spreads along wettable nanostripes embedded in a non-wettable surface. AFM allows the direct observation of the microscopic contact line of spreading nanoliquids with unprecedented spatial resolution. Results show that the liquid profile depends on the amount of lateral confinement imposed by the nanostripes and it is truncated at the microscopic contact line in good qualitative agreement with classical mesoscale hydrodynamics. However, the width of the contact line is found to be significantly larger than expected theoretically. This behavior may originate from small chemical inhomogeneity of the patterned stripes as well as from thermal fluctuations of the contact line. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T15.00007: Separation of chiral objects by shear flow in microfluidic channels - Theory Henry Fu, Marcos, Thomas Powers, Roman Stocker Motivated by the desire to separate chiral molecules, we investigate the motion of helices in shear flow generated by a microfluidic channel. We present a model based on resistive force theory to show that hydrodynamic forces on a helix in shear flow produce a drift perperdicular to the shear plane. The drift depends on the sign of the shear rate and the chirality of the helix. Net drift results from preferential alignment with streamlines. For large ($>$ 1 micron), elongated particles, alignment is a consequence of the deterministic tumbling trajectories (Jeffery orbits) in shear flow. For smaller particles, we estimate the effect of Brownian rotational diffusion on chirality-sensitive drift. We deduce a lower size limit for separation of chiral objects by shear flow in microfluidic channels. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T15.00008: Separation of chiral objects by shear flow in microfluidic channels - Experiment Marcos, Henry Fu, Thomas Powers, Roman Stocker We use microfluidics to test the prediction that a helix in shear flow drifts across streamlines. We use the non-motile, helical-shaped bacterium Leptospira biflexa as our model chiral object. As the shear in the top and bottom halves of the microchannel has opposite sign, we predict and observe the bacteria in these two regions to drift in opposite directions. The magnitude of the separation is in good agreement with theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T15.00009: Tunable liquid optics: electrowetting-controlled liquid mirrors based on self-assembled Janus tiles Tom Krupenkin, Mike Bucaro, Paul Kolodner, Ashley Taylor, Alex Sidorenko, Joanna Aizenberg In this work we describe a tunable, high-reflectivity optofluidic device based on self-assembly of anisotropically-functionalized hexagonal micromirrors (Janus tiles) on the surface of an oil droplet to create a concave liquid mirror. The liquid mirror is deposited on a patterned transparent electrode that allows the focal length and axial position to be electrically controlled. The mirror is mechanically robust and retains its integrity even at high levels of vibrational excitation of the interface. The use of reflection instead of refraction overcomes the limited available refractive-index contrast between pairs of density-matched liquids, allowing stronger focusing than is possible for a liquid lens of the same geometry. This approach is compatible with optical instruments that could provide novel functionality - for example, a dynamic 3D projector; i.e., a light source which can scan an image onto a moving, non-planar focal surface. Janus tiles with complex optical properties can be manufactured using our approach, thus potentially enabling a wide range of novel optical elements. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T15.00010: A CMOS / Microfluidic Vesicle Based Lab-on-a-Chip Platform David Issadore, Thomas Franke, Keith Brown, Robert Westervelt Droplet based microfluidic systems have proved to be useful tools for performing lab-on-a-chip experiments. Our lab has designed CMOS / microfluidic chips to trap, move, merge, and separate droplets of water in oil using dielectrophoresis (DEP) [1]. Vesicles provide a robust container for cells, bacteria, viruses, fluorescent markers, and can withstand a wide range of chemistries, salinity, and pH. We present a platform for programmable chemical and biological experiments that traps, moves, and merges vesicles suspended in water using DEP on our hybrid chip. Vesicles are loaded with 1-4mM NaCl and rhodamine and are suspended in a 200mM glucose solution. We trap and move individual vesicles along programmable paths at speeds up to 70 micrometers/sec. Two vesicles may be brought together and merged into one when triggered with electric fields that are created by the chip. [1] TP Hunt, D Issadore, RM Westervelt - Lab on a Chip, 2008. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T15.00011: Designing actuated cilia pumping fluids in microchannels Alexander Alexeev, Julia Yeomans, Anna C. Balazs Using three-dimensional computational modeling, we examine the motion of actuated cilia in a fluid-filled microchannel. The cilia are modeled as deformable, elastic filaments, which are initially tilted with respect to the channel surface. A sinusoidal force normal to the microchannel wall is applied at the free ends of the tilted cilia and induces periodic oscillations of these flexible filaments. To capture the complex fluid-structure interactions among these filaments, the channel walls and the surrounding solution, we employ our hybrid computational approach that combines a lattice Boltzmann model for hydrodynamics of vicious fluids and a lattice spring model for the micromechanics of elastic solids. We find that the actuated cilia give rise to a unidirectional flow in the microchannel and by simply altering the frequency of the applied force, we can controllably switch the direction of the net flow. The findings suggest that beating elastic cilia could be harnessed to regulate the fluid streams in microfluidic devices. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T15.00012: A Deterministic Microfluidic Ratchet Kevin Loutherback, Jason Puchalla, Robert Austin, James Sturm We present a deterministic microfluidic ratchet where the trajectory of particles in a certain size range is not reversed when the sign of the driving force is reversed. This ratcheting effect is produced by employing triangular rather than the conventionally circular posts in a post array that selectively displaces particles transported through the array. The underlying mechanism of this method is shown to to be an asymmetric fluid velocity distribution through the gap between triangular posts that results in different critical particle sizes depending on the direction of the flow. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T15.00013: Topological Dependence of ds-DNA Confined in Nanoslits Po-keng Lin, Jen-Fang Chang, Cheng-Hung Wei, Pei-Kuen Wei, Y.-L. Chen Topological constraints are important for the DNA condensation in confinement, such as chromosome in the cell and bacteriophage DNA packaging. We investigated the topological dependence of the size, shape and diffusivity of $\lambda $-DNA confined in a nanoslit with height $h$ = 780 nm (\textbf{$\approx $ }bulk radius of gyration of $\lambda $-DNA) to strong confinement ($h$ = 20 nm $<<$ persistence length $p)$ are systematically investigated. Shape asphericity of both linear and circular DNA increases with decreasing $h$, which indicate the DNA become more anisotropic. Furthermore, we observed the transition from de Gennes to Odijk scaling in the measured extension and diffusivity when $h$ = Kuhn length $L_{k}$. Interestingly, the diffusivity of circular DNA is larger than linear DNA in the blob regime, but they are nearly equal in slits with $h \quad <<$ $L_{k}$. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T15.00014: Propagation modes of entropically trapped and extended DNA molecules Morten Mikkelsen, Walter Reisner, Henrik Flyvbjerg, Anders Kristensen Nanoconfinement is a powerful tool for controlling polymer conformation and dynamics in lab-on-a-chip type devices for the analysis of DNA and other biomolecules. We present a new device concept that combines confinement-based extension of DNA with the entropic trapping principle, leading to qualitatively new physics and applications. The device consists of a 50~nm slit channel with an array of transverse $100\times 100$~nm grooves, where the transport of DNA molecules perpendicular to the groove axis is investigated under pressure driven buffer flow. At low flow velocities the DNA remains trapped and extended in the nanogrooves while buffer circulates through the slit, enabling physical mapping of the DNA while performing real time buffer exchanges. For flow velocities above a molecular weight dependent escape threshold, we show that the molecule transport through the slit channel randomly alternates between two modes of propagation: A stepwise groove to groove hopping, called the 'sidewinder', and a continuous tumbling across the grooves, where the molecules feel the topology as an effective friction, called the 'tumbleweed'. The observed length dependence on the molecule velocity may lead to a novel separation methodology. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T15.00015: Multiplex selection and elution of aptamers using nanoporous sol-gel droplets and a microheater array Seung-min Park, Jiyoung Ahn, Minjoung Jo, Soyoun Kim, Dong-ki Lee, John Lis, Pangshun Zhu, Harold Craighead Aptamers are well-known protein capture reagents that bind to specific proteins and can be effective in inhibiting the protein's normal interactions. Here, we have described a process for selective binding and elution of aptimers from the nanoporous silicate sol-gel droplets within which target proteins are immobilized. These silicate sol-gel droplets are incorporated with polydimethylsiloxane (PDMS) microfluidic systems and individually addressable by electrical microheaters. These properties allow discrete protein -- nucleic acids interaction so that multiplexed selection is possible. It is shown that specific aptamers bind their respective protein targets and can be selectively eluted by micro-heating. Our microfluidic in vitro selection system improves selection efficiency, reducing the number of selection cycles needed to produce high affinity aptamers. We are also able to separate high-affinity nucleic acid species from a large random nucleic acid pool. The process is readily scalable to larger arrays of sol-gel-embedded proteins. [Preview Abstract] |
Session T16: Fermions in Optical Lattices I
Sponsoring Units: DAMOPChair: Henning Moritz, ETH Zurich
Room: 317
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T16.00001: Probing quasiparticle dispersion and order parameter symmetry of ultracold fermionic superfluids and density waves via lattice David Pekker, Rajdeep Sensarma, Eugene Demler We propose a pump-probe experiment for studying the properties of condensed states of ultracold fermionic atoms in optical lattices. The pump consists of periodic modulations of the optical lattice intensity in time and the probe of measuring either the momentum distribution function or the density-density correlation functions. We apply our scheme to probing d-wave superfluids and d-density waves. In both cases we show that the dispersion relation of quasi-particles can be extracted from the momentum distribution function, and the order-parameter symmetry can be extracted from the pattern of peaks and dips that form due to the interference of the excited quasiparticles in the appropriate density-density correlation function. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T16.00002: Relaxation of Double Occupancies in large U Hubbard Model Eugene Demler, Rajdeep Sensarma, David Pekker We study the relaxation rates of double occupancies in large U Fermionic Hubbard model both in the Mott insulating state and in the compressible state with holes. We find that the relaxation rate $\sim t exp(-U2/t2)$ in the insulating state and $\sim t exp(-U/t)$ in the compressible state. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T16.00003: Modulation of Optical Lattice Potential in a Fermionic Mott Insulator Rajdeep Sensarma, David Pekker, Eugene Demler We analyze the double occupancies produced in a Fermionic Mott insulator near half-filling by modulating the optical lattice potential. We relate the rate of production of doublons to the spectral function of a hole and a doublon in the background of the spins. In the high temperature (spin disordered) state, the hole (doublon) is completely incoherent and the rate of production of doublons is peaked around $\omega=U$ and decreases montonically upto twice the bandwidth on either side. We also derive a sum-rule for energy integrated response in this limit. At low temperatures (anti-ferromagnetically ordered state), the spin ordering leads to a coherent peak in the hole spectral function along with other broad features corresponding to shake-off of spin-waves. This shows up in the doublon production rate as a sharp edge at the lower end of the spectrum and oscillations as a function of perturbing frequency. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T16.00004: Equation of state and magnetic properties of the three-dimensional repulsive Hubbard model Chia-Chen Chang, Shiwei Zhang Motivated by recent advances in fermionic optical-lattice experiments, we report results from numerical simulations of the ground state properties of the three-dimensional single-band Hubbard model (with nearest-neighbor hopping and repulsive $s$-save onsite interaction). We focus on intermediate interaction strengths, $U/t<\sim 10$. The constrained-path auxiliary-field quantum Monte Carlo method is used, with a phaseless approximation to control the sign/phase problem. One-body finite size effects and shell effects are eliminated by implementing twist-averaged boundary conditions. The equation of state is determined accurately for several values of $U/t$. We study the nature of the ground state away from half-filling by examining the spin-spin and other correlation functions as a function of doping. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T16.00005: Phase diagram and Neel temperature of fermions in a three-dimensional optical lattice Charles Mathy, David Huse One of the most exciting prospects in the field of ultracold atoms is the experimental realization of an antiferromagnetic Mott state, in a system of fermions in a three-dimensional optical lattice. Experimentalists are currently wrestling with achieving the requisite ordering temperatures. We address the question of which regions of parameter space one should explore to find the highest Neel temperatures. To this end, we perform Hartree-Fock calculations and map out the magnetic phase diagram of two component fermions in a three-dimensional simple cubic lattice. We find that the superexchange and Neel temperature are maximized in a regime of intermediate coupling, where the system is no longer well described by a one-band Hubbard model. We also perform a perturbative expansion in a Wannier basis, and study the corrections to the Hubbard model in this region. We find that the largest correction is a Hund's rule ferromagnetic coupling. Finally, our calculations suggest that the Mott plateau would be large in the intermediate coupling regime, and therefore experimentally accessible. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T16.00006: Collisional cooling of ultra-cold atom ensembles using Feshbach resonances Ludwig Mathey, Eite Tiesinga, Paul Julienne, Charles Clark We propose a new type of cooling mechanism for ultra-cold fermionic atom ensembles, which capitalizes on the energy dependence of inelastic collisions in the presence of a Feshbach resonance. We first discuss the case of a single magnetic resonance, and find that the final temperature and the cooling rate is limited by the width of the resonance. A concrete example, based on a $p$-wave resonance of $^{40}$K, is given. We then improve upon this setup by using both a very sharp optical resonance and a very broad magnetic resonance and show that one can reach temperatures competitive to those created by current technologies. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T16.00007: Fulde-Ferrell-Larkin-Ovchinnikov-like pairing of attractively interacting fermions on a two-leg ladder geometry Fabian Heidrich-Meisner, Adrian Feiguin Recent experiments on spin-imbalanced ultracold Fermi gases at MIT and Rice have stimulated an active search for conditions that would allow for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state to be realized in ultracold atomic gases. Theoretical work indicates that FFLO-like pairing is favored in low-dimensions (see [1] and references therein). Indeed, in one-dimensional (1D) optical lattices, the FFLO pairing mechanism is a dominating feature and survives in the presence of a confining potential. Here we extend our previous study [1] to the case of a two-leg ladder geometry [2]. Experimentally, ladders can be realized as arrays of double wells. Using a numerically exact approach, the density matrix renormalization group method, we show that FFLO-like pairing is found in a large part of the respective phase diagram, with an order parameter with a much richer structure than in the strict 1D case. We further shed light on the effect of a harmonic potential as present in optical lattices and establish the emergence of two-dimensional physics and a novel phase separation scenario not encountered in 1D chains. \\[0pt] [1] A. E. Feiguin and F. Heidrich-Meisner, Phys. Rev. B 76, 220508(R) (2007)\\[0pt] [2] A. E. Feiguin and F. Heidrich-Meisner, preprint arXiv:0809.1539. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T16.00008: Complete Phase Diagram of the Attractive Hubbard Model with a Zeeman Field Yen Lee Loh, Nandini Trivedi We study the attractive Hubbard model on square and cubic lattices, using a variational mean-field theory in which the interaction is decoupled in six channels (spin, charge, and pairing). We present the phase diagram as a function of attraction U, chemical potential mu, and Zeeman (spin-exchange) field h, and also as a function of the numbers of up and down spins. We test our hypothesis that FFLO states have a larger range of stability in a lattice than in the continuum, especially in lower dimensions. We discuss the results in the context of ultracold fermions in optical lattices, as well as the implications for thin film superconductors in a parallel magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T16.00009: Orbital Analogue of the Quantum Anomalous Hall Effect in p-Band cold fermion Systems Congjun Wu We investigate the topological insulating states of the p-band systems in optical lattices induced by the on site orbital angular momentum polarization, which exhibit gapless edge modes in the absence of Landau levels. This effect arises from the energy-level splitting between the on site px+ipy and px-ipy orbitals by rotating each optical lattice site around its own center. At large rotation angular velocities, this model naturally reduces to two copies of Haldane's quantum Hall model. The distribution of the Berry curvature in momentum space and the quantized Chern numbers are calculated. The experimental realization of this state is feasible. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T16.00010: Anomalous expansion of attractively interacting Fermions in an optical lattice Takuya Kitagawa, Lucia Hackerm\"uller, Ulrich Schneider, Mar\'ia Moreno-Cardoner, Thorsten Best, Sebastian Will, Eugene Demler, Immanuel Bloch, Bel\'en Paredes We consider a two component Fermi mixture of ultracold atoms with attractive interactions in an optical lattice and in the presence of a parabolic potential. Using a high temperature expansion, we analyze the behavior of the system size when adiabatically increasing the interaction strength. We show that entropy conservation leads to an anomalous radius increase for large values of the interaction. We also find that the competition between entropy and the Hartree part of the attractive interaction makes the system reach a minimum size at a nonzero value of the interaction. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T16.00011: Strongly correlated Fermions in optical lattices: static and dynamic properties Ulrich Schneider, Lucia Hackerm\"uller, Thorsten Best, Sebastian Will, Simon Braun, Immanuel Bloch Fermionic atoms in optical lattices can serve as a model system for condensed matter physics: They implement the Hubbard model with high experimental control of the relevant parameters. We study static and dynamic properties of ultracold fermions in different regimes, varying from a metal to a band insulator in the non-interacting system and including complex metals and the Fermionic Mott Insulator for strongly repulsive systems. In the experiment, spin mixtures of fermionic $^{40}$K are loaded into a combination of a blue detuned three dimensional optical lattice and a red detuned dipole trap. This combination of optical potentials allows an independent control of lattice depth and harmonic confinement, thus enabling us to explore different regimes. In addition to the static properties we present measurements of the dynamic response of the system to changes of the external parameters. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T16.00012: Attractively Interacting Fermions in an Optical Lattice Lucia Hackermueller, Ulrich Schneider, Maria Moreno-Cardoner, Takuya Kitagawa, Thorsten Best, Sebastian Will, Simon Braun, Eugene Demler, Belen Paredes, Immanuel Bloch Mixtures of ultracold fermionic species in optical lattices can serve as a tool to test condensed matter physics models, a prominent example being the Fermi-Hubbard-Hamiltonian. We study a balanced spin mixture of $^{40}$K in $|F,m_F \rangle=|\frac{9}{2},-\frac{9}{2} \rangle$ and $|\frac{9}{2},-\frac{7}{2}\rangle$ in a three dimensional blue detuned optical lattice, where the interaction between the spin states can be tuned via a Feshbach resonance. Changing the scattering length allows us to go from non-interacting to attractive states and finally to a strongly paired system. For small attractive interactions, the cloud size shrinks upon increasing the interactions, but surprisingly we find a minimal cloud size for medium attractive interactions. For stronger interactions the cloud size increases again. This anomalous increase can be understood with a straightforward entropy argument. We compare the experimental data with the prediction of an exact calculation in the zero-tunneling limit and with a high temperature expansion theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T16.00013: Spin and Charge Dynamics in Atomic Fermions Loaded on Optical Lattice Masahiko Okumura, Hiroaki Onishi, Susumu Yamada, Masahiko Machida We study spin and charge dynamics of trapped two-component fermions loaded on an optical lattice by using the time dependent density matrix renormalization group (TDDMRG) method. The present target issue is dynamics of spin and charge in Mott state recently realized experimentally by [1]. Firstly, we simply shake a trapped potential superposed onto an optical lattice and observe the charge dynamics on the Mott state by using TDDMRG. Secondly, we do the same thing on a trapped potential which works only on a pseudo-spin species and observe the spin density dynamics. These results are compared with non-trapped case with an open boundary condition. Also, we compare one-dimensional chain like cases with those of n-legs square and triangular ladder systems. References [1] U. Schneider, L. Hackermuller, S. Will, Th. Best, I. Bloch, T. A. Costi, R. W. Helmes, D. Rasch, A. Rosch, arXiv:0809.1464. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T16.00014: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T16.00015: Multiband superfluidity and superfluid-insulator transition of strongly interacting fermions in an optical lattice Anton Burkov, Arun Paramekanti We study the multiband superfluid phase and superfluid-insulator transition of strongly interacting fermionic cold atoms in an optical lattice at a filling of two fermions per lattice site. Our mean-field approach includes both Hartree and pairing correlations and thus differs from previous approaches to this problem. We point out a very significant discrepancy between the mean-field theory result for the critical lattice depth of the superfluid-insulator transition and its experimental value. We show that this discrepancy is due to a very small superfluid stiffness of the paired fermion superfluid in a deep optical lattice. We also present new experimentally testable results on the modulated components of the superfluid order parameter, quasiparticle gap,and band occupation as a function of the lattice depth. [Preview Abstract] |
Session T17: Focus Session: Materials in Superconducting Qubits
Sponsoring Units: GQIChair: David Pappas, National Institute of Standards and Technology
Room: 318
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T17.00001: Noise and Dephasing from Surface Magnetic States in Superconducting Circuits Invited Speaker: Superconducting qubits are a leading candidate for scalable quantum information processing. In order to realize the full potential of these qubits, it is necessary to develop a more complete understanding of the microscopic physics that governs dissipation and dephasing of the quantum state. In the case of the Josephson phase and flux qubits, the dominant dephasing mechanism is an apparent low-frequency magnetic flux noise with a $1/f$ spectrum and a magnitude of several $\mu \Phi_0$/Hz$^ {1/2}$ at 1 Hz, where $\Phi_0 = h/2e$ is the magnetic flux quantum. Recent qubit results are compatible with the excess low-frequency noise measured by researchers at Berkeley more that 20 years ago in a series of experiments on SQUIDs cooled to millikelvin temperatures. The origin of this excess noise was never understood. Here we describe studies of flux noise and temperature- dependent magnetization in SQUIDs cooled to millikelvin temperatures. We observe that the flux threading the SQUIDs increases as $1/T$ as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data is compatible with the thermal polarization of unpaired surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin- glass freezing at low temperature. These results suggest a microscopic explanation for the universal $1/f$ flux noise in SQUIDs and superconducting qubits, and suggest that suitable surface treatments of the superconducting films will lower the density of magnetic states, leading to superconducting devices with lower noise and solid-state qubits with improved coherence times. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T17.00002: Susceptibility of Magnetic Surface States in Superconducting Circuits. Steven Sendelbach, David Hover, Robert McDermott, Michael Mueck Recent experiments indicate that there is a high density of unpaired spins residing on the surfaces of superconducting thin films used to implement SQUIDs and superconducting qubits. Fluctuations of these spins give rise to low frequency flux noise and dephasing of the qubit state. Realization of phase and flux qubits with improved dephasing times will require a deeper understanding of the microscopic physics that governs fluctuations of the surface spins. Here we describe experiments that probe the ac spin susceptibility of the surface magnetic states. The detector is a dc SQUID-based susceptometer optimized for the study of surface spins. We discuss the temperature and frequency dependence of the spin susceptibility, and relate these to interactions between spins, the distribution of spin relaxation times, and possible spin-glass freezing. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T17.00003: Reflectometry measurements of 1/f noise in SQUID phase qubits at mK temperatures B. K. Cooper, R. M. Lewis, B. S. Palmer, V. Zaretskey, A. J. Przybysz, H. Kwon, J. R. Anderson, C. J. Lobb, F. C. Wellstood We measure 1/f noise spectra in dc SQUID phase qubits using a microwave reflectometry technique. One of the SQUID junctions is shunted by a large capacitor, forming a microwave frequency resonator biased and driven to show nonlinear response, typically at 1.5 GHz. This nonlinearity means small current or flux fluctuations produce large changes in reflected phase which we can measure using homodyne detection. Measurements from aluminum qubits on sapphire are compared to previous measurements of 1/f flux noise in SQUIDs and a similarly designed Nb/AlOx/Nb on silicon dc SQUID qubit fabricated by Hypres; data was taken at temperatures ranging from 50 mK to 500 mK. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T17.00004: Crystalline Silicon Dielectrics for Superconducting Qubit Circuits David Hover, Weina Peng, Steven Sendelbach, Mark Eriksson, Robert McDermott Superconducting qubit energy relaxation times are limited by microwave loss induced by a continuum of two-level state (TLS) defects in the dielectric materials of the circuit. State-of-the-art phase qubit circuits employ a micron-scale Josephson junction shunted by an external capacitor. In this case, the qubit T$_1$ time is directly proportional to the quality factor (Q) of the capacitor dielectric. The amorphous capacitor dielectrics that have been used to date display intrinsic Q of order 10$^3$ to 10$^4$. Shunt capacitors with a Q of 10$^6$ are required to extend qubit T1 times well into the microsecond range. Crystalline dielectric materials are an attractive candidate for qubit capacitor dielectrics, due to the extremely low density of TLS defects. However, the robust integration of crystalline dielectrics with superconducting qubit circuits remains a challenge. Here we describe a novel approach to the realization of high-Q crystalline capacitor dielectrics for superconducting qubit circuits. The capacitor dielectric is a crystalline silicon nanomembrane. We discuss characterization of crystalline silicon capacitors with low-power microwave transport measurements at millikelvin temperatures. In addition, we report progress on integrating the crystalline capacitor process with Josephson qubit fabrication. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T17.00005: Testing of qubit materials and fabrication using superconducting resonators Shwetank Kumar, Matthias Steffen, David DiVincenzo, George Keefe, Mary Beth Rothwell, Matthew Farinelli, Jim Rozen, Frank Milliken, Mark Ketchen We will present the results of measurements made on superconducting resonators fabricated using different substrates and superconducting metals. Specifically, the quality factor of these resonators will be shown to be closely related to not only the purity of the substrates and metals used in the process but also to the details of the fabrication. We will demonstrate the change in quality factor of a bare resonator when subjected to the qubit process. Based on our measurements we propose that superconducting resonators may form a test bed for troubleshooting the fabrication process for minimizing the materials related dissipation in the qubits. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T17.00006: Optimizing silicon nitride for superconducting quantum circuits Hanhee Paik, Kevin Osborn Amorphous dielectrics are prevalent in lithographic circuits, but their presence can decohere superconducting qubits. We investigate the relationship between stoichiometry and low- temperature loss in silicon nitride dielectric films, where two- level system defects are unsaturated. The silicon nitride films are deposited by plasma-enhanced chemical vapor deposition at 300 degrees celsius, with silane and nitrogen as precursor gases. The precursor gas ratio is changed and film density, crystalline order, stress, and hydrogen incorporation are measured. Hydrogen, silicon and nitrogen content are monitored with FTIR spectroscopy. The loss is measured at low-field strengths at temperature of 30 mK with lumped-element superconducting resonators, where silicon nitride is used as the dielectric within a parallel-plate capacitor. Our data show that N-rich silicon nitride with a high concentration of nitrogen-hydrogen bonds exhibit a factor of 10 higher loss tangent than Si-rich films. The loss of the better film rivals other on-chip insulating techniques, and allows us to fabricate Josephson junctions next to silicon nitride with a negligible loss contribution from this dielectric. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T17.00007: Low-photon number studies of inductively-coupled superconducting resonators Moe Khalil, Hanhee Paik, Fred Wellstood, Kevin Osborn Quality factors near one million have been observed in on-chip superconducting resonators for many years, but new studies on resonators reveal much lower quality factors at low-photon numbers, perhaps due to the presence of anomalous two-level system defects. We have designed and fabricated four new aluminum thin-film resonator types near 6 GHz. They include a lumped-element resonator, a slot-line resonator, and two hybrids, both of which contain a slot line and either an inductor or a capacitor. The resonator types have a consistent line width and are fabricated on a sapphire substrate to facilitate the study of surface defects, such as two-level systems. We plan to compare their quality factors in an effort to better understand the loss mechanism associated with the surface. All the resonator types have inductive coupling to a coplanar waveguide with geometrical symmetry that can be used to construct useful Josephson junction resonators. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T17.00008: LC Filtered dc SQUID Phase Qubit with Low Dielectric Loss Hyeokshin Kwon, A. J. Przybysz, T. A. Palomaki, Hanhee Paik, K. D. Osborn, R. M. Lewis, B. K. Cooper, J. R. Anderson, C. J. Lobb, F. C. Wellstood We have investigated a dc SQUID phase qubit with LC filter, which has a relatively small ($\sim $4 $\mu $m$^{2})$ Al/AlO$_{x}$/Al Josephson junction shunted by an additional capacitor built using low-stress thin film SiN$_{x}$. The LC isolation provides an additional isolation factor at the junction plasma frequency and allows flexibility in the choice of SQUID parameters. We report Rabi oscillations with a 42 ns envelope decay time (T'), and a 32 ns energy relaxation time (T$_{1})$, consistent with a loss tangent of about 7 x 10$^{-4}$ in the loss-stress SiN$_{x}$. We also report on progress towards getting longer coherence times using a high-stress SiN$_{x}$ with a lower loss tangent. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T17.00009: Microwave response of vortices in Al and Re superconducting thin films C. Song, T.W. Heitmann, M.P. DeFeo, K. Yu, B.L.T. Plourde , R. McDermott , M. Neeley, J.M. Martinis Vortices trapped in superconducting microwave resonant circuits contribute excess loss and can result in substantial reductions in the quality factor. Thus, characterizing the microwave vortex response in superconducting thin films is important for the design of superconducting qubits, which are typically operated in small, but non-zero, magnetic fields. By cooling in fields of the order of 1 Gauss and below, we have characterized the magnetic field and frequency dependence of the microwave response of a small density of vortices in resonators fabricated from thin films of Re and Al. Above a certain threshold cooling field, vortices become trapped in the resonators and vortices in the Al resonators contribute greater loss and are influenced more strongly by flux creep effects than in the Re resonators. This different behavior can be described in the framework of a general vortex dynamics model related to the interplay between the vortex pinning in the films and the flux-flow viscosity. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T17.00010: One- and two-photon spectroscopy of a flux qubit coupled to a microscopic defect Adrian Lupascu, Patrice Bertet, Eduard Driessen, Kees Harmans, Hans Mooij We observed the dynamics of a superconducting flux qubit coupled to a microscopic defect. The presence of the defect is visible as an anticrossing in the spectroscopy of the flux qubit, as measured using one-photon excitation. We analyze the energy- level structure of the combined qubit-defect system using both one- and two- photon spectroscopy. The use of two-photon spectroscopy allows us to extract important additional information about the anharmonicity and coupling of the defect. We find that the system coupled to the qubit can be a two-level system, but not a harmonic oscillator. We consider two basic models, for a microscopic defect which is coupled to the qubit either magnetically or electrically respectively. We conclude that the large coupling constant, of approximately 200 MHz, can only be accounted for by electric coupling, and not by magnetic coupling. This shows that electrically coupled microscopic two- level systems are relevant to decoherence of superconducting flux qubits. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T17.00011: Probing anomalous two-level systems with a Cooper-pair box Invited Speaker: We have used an Al/AlO$_{x}$/Al Cooper-pair box (CPB) qubit to detect coupling to anomalous ``two-level'' quantum systems. By measuring the excitation spectrum and lifetime of the first excited state from 15 GHz to 50 GHz of the CPB at a temperature of 40 mK, one can identify anomalous levels and ascertain the magnitude of the quantum noise that is coupled to the qubit. It was found that the frequency of a distinct avoided level crossing depends on gate voltage and the size of the splitting depends on the effective Josephson energy. Both the gate voltage and Josephson energy dependence are consistent with coupling to extraneous charged two-level systems formed by point charges that can tunnel between two positions in the oxide of the Josephson junction. By fitting a model Hamiltonian to our data, we are able to extract microscopic information about the charge fluctuator such as the well asymmetry ($\sim$ 130 micro-eV), tunneling rate ($\sim$ 8 GHz) and a minimum hopping distance for the charge fluctuators ($\sim$ 0.8 Angstroms). [Preview Abstract] |
Session T18: Focus Session: Organic Photovoltaics and Other Photonic Devices
Sponsoring Units: DPOLYChair: Jeff Urban, Lawrence Berkeley National Laboratory
Room: 319
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T18.00001: Surface plasmon polariton assisted organic solar cells Invited Speaker: |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T18.00002: Harvesting Lost Photons: Minimizing Sub-Bandgap Losses in Organic Photovoltaic Devices by Up-conversion Clara Santato, Loranger Sebastian, Banville David, Rosei Federico, Perepichka Dmiytro We report on a novel approach to increase the efficiency of organic photovoltaic (OPV) cells in the near-infrared region of the solar spectrum by blending the organic semiconductors with rare-earth doped nanoparticles with up-converting photophysical properties. The approach consists in (i) synthesizing lanthanide-doped nanoparticles capable of efficient energy transfer of up-converted near-infrared (NIR) photon energy to conjugated polymers; (ii) assembling these nanoparticles, in blends with p-type polythiophenes and n-type fullerenes, in solution-processed OPV cells capable to harvest NIR photons. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T18.00003: In-situ X-ray characterization of thermal and solvent based annealing of thin P3HT and P3HT/PCBM films Tommy Hofmann, Htay Hlaing, Chang-Yong Nam, Charles Black, Benjamin Ocko We have studied the annealing of thin films of P3HT (polyhexylthiophene) and mixed P3HT and PCBM thin films using in-situ Grazing Incidence Angle x-ray scattering techniques at the National Synchrotron Light Source. The films, 50-200 nm thick, were prepared using spin coating from a volatile solution. Both thermal and solvent annealing techniques are well known to improve electrical properties yet the precise mechanism is not well understood. In our measurements, we have monitored the dependence of the diffraction peak positions and widths under a variety of different in-situ thermal and solvent conditions. A detailed comparisons between these methods provides new insight into how to improve the crystallinity beyond what can be obtained by thermal methods alone. This may eventually lead to better electrical properties in thin film organic photovoltaic devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T18.00004: Excited-State Dynamics at Organic Photovoltaic Heterojunctions by Pump-Probe Photoelectron Spectroscopy Gregory Dutton, Daniel Dougherty, Wei Jin, William Cullen, Janice Reutt-Robey, Steven Robey The critical process of charge separation in organic photovoltaic (OPV) devices is determined directly at the organic heterojunction, but these interfaces have been less extensively studied than organic/metal interfaces. We prepare model photovoltaic heterojunctions by deposition of ultrathin films of organic semiconductors on single-crystal metal substrates. The electronic structure of the component materials and their interfaces is determined with ultraviolet photoelectron spectroscopy (UPS) and two-photon photoemission (2PPE). The systems studied in this work involve phthalocyanines and analogs as donors and C$_{60}$ fullerene as acceptor. Time-resolved pump-probe experiments are applied to directly measure the excited state dynamics at these OPV heterojunctions. An ultrafast visible pump pulse selectively generates excitons in one material, followed by a time-delayed UV probe to interrogate the population of the acceptor charge transport level. Analysis of cross-correlations reveals the timescales of charge separation and recombination at the interface. Additionally, comparison will be made to structural and local spectroscopic studies of similar phthalocyanine/fullerene systems made by STM/STS. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T18.00005: The role of triplet excitons in enhancing polymer solar cell efficiency: a photo-induced absorption study Kai Yang, Suchi Guha Inclusion of heavy metal atoms in a polymer backbone allows transitions between the singlet and triplet manifolds. Interfacial dissociation of triplet excitons constitutes a viable mechanism for enhancing photovoltaic (PV) efficiencies in polymer heterojunction-based solar cells, which are now becoming feasible options for solar panels. The PV efficiency from polymer solar cells utilizing a ladder-type poly para phenylene polymer (PhLPPP) with trace quantity of Pd atoms and a fullerene derivative (PCBM) is almost 10 times more than its counterpart (MeLPPP) with no Pd atom. Evidence is presented for the formation of a weak ground-state charge-transfer complex (CTC) in the blended films of PhLPPP and PCBM, using photo- induced absorption (PIA) spectroscopy. Such complexes are not seen in the PIA spectrum of MeLPPP: PCBM blends. Possible mechanisms for the CTC state formation as well as the significance of this to the understanding and optimization of polymer blended solar cells will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T18.00006: Photophysics of charge-transfer excitons in thin films of $\pi$-conjugated polymers Demetra Psiachos, Sumit Mazumdar We develop a theory of the electronic structure and photophysics of interacting chains of $\pi$-conjugated polymers to understand the differences between solutions and films. While photoexcitation generates only the intrachain exciton in solutions, the optical exciton as well as weakly allowed charge-transfer excitons are generated in films. We show that a significant fraction of ultrafast photoinduced absorptions (PAs) in films originate from the lowest charge-transfer exciton. Using sophisticated many-body approaches that take into account high order configuration interaction, we have calculated the full wavelength-dependent PA spectra of pairs of interacting PPV oligomers. Good qualitative agreement is obtained with the experimental PA spectra of thin films of $\pi$-conjugated polymers. The origin of each individual PA is explained within our theory. Our work resolves long-standing controversies regarding the nature of the primary photoexcitations in films. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T18.00007: Photo-crosslinkable Polythiophenes for Efficient Thermally Stable Organic Photovoltaics Bumjoon Kim, Yoshi Miyamoto, Biwu Ma, Jean M.J. Frechet We report a new series of bromine-functionalized poly(3-hexylthiophene) (P3HT-Br) copolymers for use in solution processed organic photovoltaics (OPVs). P3HT-Br copolymers were synthesized from two different monomers, where the ratio of the monomers was carefully controlled to achieve a UV photo-crosslinkable layer while leaving the $\pi -\pi $ stacking feature of conjugated polymers unchanged. Photo-crosslinkable P3HT-Br was demonstrated as effective electron donors in OPVs. The crosslinking stabilizes P3HT-Br/PCBM blend morphology preventing the macro phase separation between two components, which lead to OPVs with remarkably enhanced thermal stability. The drastic improvement in thermal stabilities is further characterized by microscopy as well as grazing incidence X-ray scattering (GIXS). The use of these copolymers for solution processed efficient bilayer PVs is also described. Benefited from the little disturbance in $\pi -\pi $ stacking by crosslinkable units as evidenced in GIXS, P3HT-Br/PCBM bilayer device shows high power conversion efficiency at over 2.2{\%} and excellent thermal stability. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T18.00008: Temperature Dependence of Biexciton Decay and Intermolecular Hopping in Zinc Phthallocyanine Films. Christopher Ryan The femtosecond exciton dynamics of melt-pressed zinc phthalocyanine (ZnPC) films are studied in the temperature range of 90-400 K. In this range ZnPC goes through a transition from a crystalline solid to a liquid crystalline phase. For the entire temperature range, the excitons are shown to decay on the time scale of 10's of picoseconds, and these dynamics are nonlinear with respect to pump fluence. Such a behavior is well described by a biexciton recombination model under one dimensional diffusion constraints. The single exciton lifetime and the biexciton recombination crossection are extracted at all temperatures. From the latter, the exciton hopping time is calculated. The exciton hopping time decreases with temperature in the crystalline phase, but increases in the liquid crystalline phase. The role of temperature and structural order in the exciton hopping time will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T18.00009: White tandem OLED with carbon nanotube interlayer Alexios Papadimitratos, Raquel Ovalle Robles, Ray Baughman, Anvar Zakhidov White organic light emitting diodes (OLEDs) have become well recognized as an important candidate for future lighting and display applications. An existing idea to generate white color places R, G, B pixels in a side-by-side geometry. Also, white tandem OLEDs have been developed by vertically stacking in series multiple electroluminescent layers. However, such structures require a complex interfacial layer which is usually fabricated by strong dopants to form a p+/n+ interface. We have shown earlier that transparent carbon nanotubes (CNT) can be used as effective three dimensional charge injectors in polymer light emitting diodes[1] and OLEDs[2]. Now, we show that CNT can be used as an interlayer in two cell OLEDs with complimentary colors. We show that tandem devices with CNT interlayers, together with selective barriers and PEDOT:PSS coating can control the device color. In addition, the emission intensity can be controlled by independently tuning the driving voltage and current. In the case of overdoped p+/n+ interlayers we do not have this opportunity which is a great advantage of CNT injectors. We also compare the performance of multiwall CNTs vs. that of single wall CNTs in the tandem OLEDS. [1]R.H.Baughman et al.Science, 297,787-792(2002).[2]C.D.Williams et al.Appl. Phys. Lett. 93,183506(2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T18.00010: Tuning optical properties of blue-emitting polyfluorenes via hydrostatic pressure Keshab Paudel, Meera Chandrasekhar, Suchi Guha Polyfluorenes (PFs) represent a unique class of poly para- phenylene based blue-emitting polymers with intriguing structure-property relationships. Slight variations in the choice of functionalizing side chains result in dramatic differences in the inter- and intra-chain structures in PFs. Highlighting these differences are two prototypical PFs, poly (9,9-(di n, n-octyl) fluorene) (PF8) and poly (9,9-(di ethyl- hexyl) fluorene) (PF2/6). In addition to the nematic liquid crystal (n-LC) mesophase, PF8 is characterized by at least five structural phases. We present photoluminescence (PL) and Raman scattering studies of powder samples and thin films of PF8 under hydrostatic pressure. The powder sample was thermally annealed at 2GPa. The PL vibronics of the as-is powder sample red-shift at an average rate of 30 meV/GPa whereas the thermally annealed sample red-shifts at a higher rate of 50 meV/GPa, indicating a different crystalline mesophase for the annealed sample. The Huang-Rhys factor is found to increase with increasing pressures signaling a higher geometric relaxation of the electronic states. The Raman peaks harden with increasing pressures; the intra-ring C-C stretch frequency at 1600 cm $^{-1}$ has a pressure coefficient of 5 cm$^{-1} $/GPa and exhibits asymmetric line shapes at higher pressures. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T18.00011: SERS Characterization of Self-Assembled Monolayers Embedded on Plasmonic Nano-structure Masato Maitani, Douglas Ohlberg, Haeyoung Yoon, Ping Kao, Demirel Melik, Zhiyong Li, Duncan Stewart, Stanley Williams, Theresa Mayer, David Allara We discuss Raman spectroscopic analysis of self-assembled monolayers embedded in two different types of nano-structures capable of sustaining localized surface plasmon-surface plasmon polariton coupling via nanoscale gaps and curved surface features. Both structures cosnsist of metal-molecule-metal (M$^3$) junctions which can also allow charge transport through the molecular bridges. Our results indicate different electromagnetic and charge transport characteristics as a function of the top metal-molecule chemical interaction. We also report direct correlations between charge transport states, SERS response and inelastic vibrational scattering in selected M$^3$ molecular electronic device junctions. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T18.00012: Non-linear optics and local-field factors in liquid chloroform: A time-dependent density-functional theory study David A. Strubbe, Xavier Andrade, Angel Rubio, Steve G. Louie Chloroform is often used as a solvent and reference when measuring non-linear optical properties of organic molecules. We calculate directly the non-linear susceptibilities of liquid chloroform at optical frequencies, using molecular dynamics and the Sternheimer equation in time-dependent density-functional theory [X. Andrade et al., J. Chem. Phys. 126, 184106 (2007)]. We compare the results to those of chloroform in the gas and solid phases, and experimental values, and make an ab initio calculation of the local-field factors which are needed to extract molecular properties from liquid calculations and experimental measurements. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T18.00013: Time resolved photoluminescence studies of long lived emissive specie in F8BT:PFB blends Simon G\'elinas, Ian Howard, Richard Friend, Carlos Silva Type-II heterojunctions play a crucial role in organic optoelectronic devices. We use donor-acceptor polyfluorene blends as a model system to understand excited-state dynamics at heterojunctions. These interfacial excitations are intrachain singlet and triplet excitons, geminate polaron pairs, and exciplexes (interfacial charge-transfer excitons). Time-resolved photoluminescence (PL) spectra were taken at 10\,K and room temperature to investigate the interconversion dynamics of these species. We observe delayed PL with sub-linear excitation fluence dependence. This implies that delayed singlet exciton generation involves a bimolecular annihilation mechanism. By means of kinetic modeling, we propose triplet-triplet exciton annihilation as a regeneration route to singlet excitons, and subsequently to exciplexes. This points to a significant ($<15$\,\%) yield of triplet excitons after interfacial charge separation, and to the central role of these species on the interfacial dynamics. [Preview Abstract] |
Session T19: Theory and Simulation II
Sponsoring Units: DPOLYChair: Amalie Frischknecht, Sandia National Laboratories
Room: 320
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T19.00001: Theory of polyzwitterionic solutions Rajeev Kumar, Glenn Fredrickson Conformations of polyzwitterionic molecules in aqueous solutions are investigated using the variational method. We have carried out self-consistent calculations for the degree of counterion adsorption on the zwitterionic sites and the size of a single polyzwitterionic chain. These calculations are used to analyze the solubility of these molecules in water. Results for the effect of an asymmetric counterion adsorption, electrostatic interaction strength, salt concentration, solvent quality, specificity of the zwitterionic monomeric units and the added salt on the conformations of the polyzwitterionic chain will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T19.00002: Charge transport in conjugated polymers: a multiscale picture Victor Ruehle, James Kirkpatrick, Kurt Kremer, Denis Andrienko A framework to study charge transport in conjugated polymers using realistic morphologies is developed. First, the atomistic force field is refined using first-principles calculations. Systematic coarse graining is then performed to extend simulation times and system sizes accessible to molecular dynamics simulations. Material morphologies are generated using the coarse grained and atomistic models. Finally, the charge mobility is obtained using temperature activated hopping picture for charge transport [1]. The framework is tested on neutral and oxidized polypyrrole with different structural ordering [2]. \\[4pt] [1] J. Kirkpatrick, V. Marcon, J. Nelson, K. Kremer, D. Andrienko, Phys. Rev. Lett. 98, 227402 (2007)\\[0pt] [2] V. Ruehle, J. Kirkpatrick, K. Kremer, D. Andrienko, Phys. Stat. Solidi B, 245, 844 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T19.00003: First principles electronic properties investigation of polythienoacene and its derivatives Simon Pesant, Paul Boulanger, Guillaume Dumont, Michel C\^ot\'e The electronic properties of ladder-type polythiophene (polythienoacene) and its derivatives are studied using density functional theory. Upon an analysis of the variation of the band gap when comparing the non-ladder and the ladder-type polymers, a discrepancy is found between the thiophene and the pyrrole(nitrogen-substituted thiophene) polymer families. The polythienoacene has a larger band gap than the polythiophene whereas the opposite is found for the pyrrole polymers. Also, it is found that a simple alternation of the sulfur atom in polythienoacene structure by nitrogen or boron atoms can lead to small band gap polymers. The excitations of these polythienoacene's derivatives are investigated using time-dependent density functional theory. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T19.00004: Electronic structure and carrier transport in disordered conjugated polymers Nenad Vukmirovic, Lin-Wang Wang Thin films of realistic conjugated polymer materials contain both crystalline and amorphous regions, where the latter ones are less understood. This study was therefore focused on electronic structure and carrier transport in amorphous regions of polythiophene (PT) and poly(3-hexylthiophene) (P3HT). Atomic structures were obtained from classical molecular dynamics using a simulating annealing procedure and the charge patching method [1] was used to calculate the electronic structure. It was found that disorder in the electronic structure of P3HT comes from disorder in the conformation of individual chains, while in the case of PT there is an additional contribution due to disorder in electronic coupling among the chains [2]. The electron-phonon coupling matrix elements in P3HT were also calculated and carrier mobility due to phonon-assisted hopping was estimated. \\[0pt] [1] N. Vukmirovic and L.W. Wang, J. Chem. Phys. 128 121102 (2008).\\[0pt] [2] N. Vukmirovic and L.W. Wang, J. Phys. Chem. B, submitted. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T19.00005: \emph{Ab initio} Study of Diketo-Pyrrolo-Pyrrole Polymers for Photovolta\"ic Applications Simon L\'evesque, Jean Fr\'ed\'eric Laprade, Michel C\^ot\'e Using density functional theory with the hybrid functional B3LYP, we investigate the electrical and optical properties of polymers made with diketo-pyrrolo-pyrrole. It is found that the value of the band gap can be tuned by varying the number of thiophene units within the polymer. Band structure and time-dependent density functional theory results will also be presented. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T19.00006: First-principles investigation of PVDF and its copolymers V. Ranjan, Liping Yu, Marco Buongiorno Nardelli, J. Bernholc Recently, PVDF and its copolymers have generated significant interest due to their electroactive properties [1] and potential for ultra-high energy-storage applications [2]. In this talk, we present the results of first-principles calculations of stable phases and dielectric properties of different copolymers and terpolymers of PVDF at varying concentrations. Our results show that at very high concentrations of Chloro-trifluoroethylene (CTFE), PVDF/CTFE displays sharp transitions between non-polar ($\alpha$) and polar ($\beta$) phases. On the contrary, the same transitions in copolymers with trifluoroethylene (TrFE) and tetrafluoroethylene (TeFE) are not sharp and happen at lower concentrations. We discuss the interplay of copolymer admixture on the dielectric properties of PVDF and discuss the suitability of copolymers for energy storage and electroactive applications. [1] S. G. Lu et al., App. Phys. Lett. 93, 042905 (2008). [2] V. Ranjan et al., Phys. Rev. Lett. 99, 047801 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T19.00007: Adaptive Tempering Monte Carlo Study of Dense Polypyrrole Systems Yafei Dai, Estela Blaisten-Barojas A modified rigid-ion polarizable model potential of polypyrrole is developed with parameters fitted on multiple points of the electronic energy surface of pyrrole oligomers (n-Py) of different sizes calculated with a hybrid density functional approach [1]. Using this potential, systems containing 192 chains (4-Py) and 64 chains (12-Py) were structurally optimized with the Adaptive Tempering Monte Carlo algorithm [2]. Energetics and structure of these systems were studied as a function of density. Both systems have characteristics of a liquid for densities in the range 0.66 -- 1.09 g/cm$^3$ at T=300 K. The oligomer radius of gyration is insensitive to density changes. However, an orientational order parameter shows a sharp increase as a function of density indicating a tendency of the chains to stack forming regions of aligned chains for the denser systems. [1] Y. Dai, E. Blaisten-Barojas, J. Chem. Phys. 129, 164903 (2008); [2] X. Dong, E. Blaisten-Barojas, J. Comp. \& Theo. Nanoscience 3, 118 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T19.00008: Effect of the chain length and segment size of the free polymer on the interaction between two grafted monolayers in a good solvent Walter Chapman, Shekhar Jain, Valeriy Ginzburg, Prasanna Jog, Jeffrey Weinhold, Rakesh Srivastava The interaction between two polymer grafted surfaces is important in many applications, like nanocomposites, colloid- stabilization, and polymer alloys. In our previous work [Jain et. al., J. Chem. Phys. 128, 154910 (2008)], we showed that interfacial statistical associating fluid theory (iSAFT) density functional theory (DFT) successfully calculates the structure of the grafted polymer chains in the absence/presence of free polyatomic solvent. In the current work, we have applied iSAFT to calculate the force of interaction between two such grafted monolayers in implicit good solvent conditions. In particular, we have considered the case where the segment sizes of the free ($\sigma _{f})$ and grafted ($\sigma _{g})$ polymers are different. The interactions between the two monolayers in the absence of the free polymer is always repulsive. However, in the presence of free polymer, the force can be either purely repulsive or can have an attractive minimum depending upon the relative chain lengths of the free (N$_{f}$) and grafted polymers (N$_{g})$. The attractive minimum is observed only when the ratio, N$_{f} $/N$_{g}$, is greater than a critical value. We propose a scaling relation for this case, in agreement with self consistent field theory for ($\sigma_{f} = \sigma_{g}$). [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T19.00009: Interpolation between State Points in the Simulated Perturbation Contributions of Polymer Solutions Amir Vahid, Neil H. Gray, J. Richard Elliott Polymeric mixtures of hydrocarbons, alcohols have been simulated with discontinuous potential models to characterize the Helmholtz energy of the repulsive reference fluids (A0) along with the first and second order perturbation contributions (A1, A2) as functions of density and composition. Taken together, these terms generate a complete equation of state for the mixture, including temperature effects as well as density and composition. The specific hydrocarbons studied were methane, ethane, propane, n-butane, n-hexane, n-heptane, n-decane, and benzene. The specific alcohols were water, methanol, ethanol, n-propanol, and n-octanol. Unfortunately, a slight inconsistency was encountered when the trend observed for these small molecules was extrapolated to the long chain limit. Therefore, we extend the analysis to mixtures of n-alkanes, branched hydrocarbons, and aromatics with polymeric molecules of: n-alkanes, ethyl-styrenes, ethyl-propylenes, and isoprenes. The perturbation contributions can be accurately characterized by van der Waals mixing rules and compared with the MCSL SAFT and Guggenheim-Staverman theories. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T19.00010: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T19.00011: Simulation of Microheterogeneous Networks and Extraction of Segment Orientation Behavior from D-NMR Spectra Bernardo Aguilera-Mercado, Claude Cohen, Fernando Escobedo The degree of heterogeneity in the microstructure of end-linked elastomer networks has been shown to have a very strong impact on the network mechanical and elastic properties such as: ultimate strain, modulus, and toughness. Networks with crosslinks and chains inhomogeneously distributed are expected to exhibit heterogeneous segment orientation responses. The global segment orientation of systems with frozen inhomogeneities, and a significant amount of highly stretched chains at the unperturbed state, cannot be captured by measurements of the deuterium NMR spectra splits solely. Spectrum frequency splits quantify the segment orientation due to local excluded volume interactions only and do not account for the contributions arising from large end-to-end chain deformations. Long wings of the spectrum reflect the presence of strongly aligned segments ignored when one considers only the split. A new methodology based on the Maximum-Entropy method is proposed to find the probability density of an order parameter that describes the network segment orientation from which the global orientation behavior can be completely characterized. The methodology is validated with both molecular simulation and experimental data. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T19.00012: Cooperative dynamics in polymer melts: a comparison of theoretical predictions with Neutron Spin Echo experiments Marina Guenza We present a comparison between theoretical predictions of the Generalized Langevin Equation for Cooperative Dynamics (CDGLE) and Neutron Spin Echo data of dynamics structure factors for polyethylene melts. Experiments, peformed by Zamponi end coowrkers, cover an extended range of length- and time-scales providing a compelling test for the theoretical approach. Samples investigated include chains with increasing molecular weights, undergoing dynamics across the unentangled to entangled transition. Measured center-of- mass mean-square displacements display a crossover from subdiffusive to diffusive dynamics. The Generalized Langevin Equation for Cooperative Dynamics relates this anomalous diffusion to the presence of the interpolymer potential, which correlates the dynamics of a group of slowly diffusing molecules in a dynamically heterogeneous liquid. Theoretical predictions of the subdiffusive behavior, its crossover to free diffusion, and of the number of macromolecules undergoing cooperative motion are in quantitative agreement with experiments. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T19.00013: Equilibrium Partitioning of Polymers between Bulk Dilute Solution and Confining Pores Yanwei Wang, Flemming Y. Hansen, Guenther H. Peters, Ole Hassager We have developed a novel framework [1] for the description of the steric hindrance effect on polymers that are subject to confining geometries. The two main ingredients are (i) a new computational method, the Confinement Analysis from Bulk Structures (CABS) approach, which enables calculation of the equilibrium partition coefficient (pore-to-bulk concentration ratio) as a function of the confinement size solely based on snapshots of polymer configurations in bulk, and (ii) the definition of a new molecular size parameter, the steric exclusion radius, which permits collapsing all partition coefficient data for different polymers in the weak confinement regime onto a universal curve. Our latest development in extending the CABS method to cylindrical and spherical pores will be presented.\\[0pt][1] Wang et al. J. Chem. Phys. 128, 124904 (2008); 129, 074904 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T19.00014: Multivalent Nanoparticles: adsorption and organization of bidisperse polymer chains onto a solid interface Folusho Oyerokun, Richard Vaia, John Maguire, Barry Farmer Multivalent nanoparticles, i.e. nanoparticles with two or more ligands attached to their surfaces, are used in a variety of scientific and technological applications. The most common protocols for synthesizing these multivalent nanoparticles involves immersion of the particles into a solution containing the various ligands or into a solution containing an excess of one ligand to drive a partial (solvent mediated) exchange reaction with a previously bound ligand. Despite intense experimental activities, the dependence of the surface coverage on free ligand concentration and solvent quality is still poorly understood. This study addresses the thermodynamics of adsorption of bidisperse end-functionalized polymer chains in a good solvent onto a flat surface. At equilibrium, the absorbed chains form a bidisperse polymer brush in contact with the solution. The role of the degree of bidispersity, adsorption energy, solvent quality on monomer concentration profile, brush height and degree of penetration of free short and long chains into the brush layer will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T19.00015: Molecular simulation of crystal nucleation of n-alkane melts Peng Yi, Gregory Rutledge The homogeneous nucleation of a crystal phase is one of the most interesting phenomena of molecular fluids, yet the microscopic mechanism of which still remains poorly understood. It is even more a mystery in chain molecule systems because the chain connectivity could produce very different crystal nucleus conformations, which are important factors in determining the subsequent crystal growth process and the properties of the final product. In this work we report the results of molecular simulations of crystal nucleation of n-alkanes from the melt. A realistic united atom force field was employed. The crystal phase and melting behavior were first determined by ramping temperature in a set of molecular dynamics simulations. The adiabatic nucleation trajectory was then sampled using the Monte Carlo umbrella sampling technique. The surface energy of the crystal nuclei was calculated assuming a spherical nucleus model and compared with previous studies to validate our numerical definition of a crystal nucleus. We were also able to calculate the end and side surface free energies of a cylinder nucleus model from the simulation data without making further assumption. This method can be extended to study longer n-alkane molecules and the change of nucleus conformation as n increases. [Preview Abstract] |
Session T20: Focus Session: Magnetic Properties of Organic Semiconductors/ Surface Characterization of Organic Materials
Sponsoring Units: DPOLYChair: C.D. Frisbie, University of Minnesota
Room: 321
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T20.00001: Magnetoresistance and magnetic-field-effects in organic semiconductor devices. Invited Speaker: |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T20.00002: Photo-induced Magnetism and Spintronics in Organic Semiconductors Invited Speaker: Recent years have witnessed growing attention on manipulating spins in organic species. One of the interesting phenomena in organic-based magnets is controlling magnetic properties by optical stimulus, a property not exhibited in metallurgical magnets. Three classes of known phenomena and mechanism will be discussed: i) manipulation of number of spins by optically induced charge transfer in cyano-bimetallic complexes [1], ii) optical control of exchange coupling in Mn(TCNE)$_2$ compound [2], iii) light-induced change of magnetic anisotropy in the magnetic semiconductor V(TCNE)$_x$ [3]. The second part of this talk will be devoted to ongoing research on transferring spin polarized carriers through organic semiconductors. Recently, there have been lively activities as well as controversies on the application of organic semiconductors for transporting spin information. However, the understanding of spin injection and transport in organic semiconductors is still limited. We will address detailed mechanisms for spin injection and transport in organic semiconductor film of our rubrene (C$_{42}$H$_{28}$)-based spin valve and potential applications of organic-based spintronics. \\[4pt] [1] O. Sato, T. Iyoda, A. Fujishima, and K. Hashimoto, Science \textbf{272}, 704 (1996).\\[0pt] [2] D. A. Pejakovic', C. Kitamura, J. S. Miller, and A. J. Epstein, Phys. Rev. Lett. \textbf{88}, 057202 (2002).\\[0pt] [3] J. W. Yoo et al., Phys. Rev. Lett. \textbf{97}, 247205 (2006); \textbf{99}, 157205 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T20.00003: Magnetic properties of TCNQF4 reacted with Ni(cod)$_{2}$ Ian Terry, Kimberly Quinn, Marek Szablewski Recently it was reported that a room temperature ferromagnetic material (Tc$>$400K), Ni$_{2}$TCNQ, was synthesized by reacting the organic acceptor tetracyanoquinodimethane (TCNQ) with bis(1,5 cyclooctadiene) nickel (Ni(cod)$_{2})$[1]. In the present work we report the magnetic properties of a material which was synthesized following the same chemical route as that of Ni$_{2}$TCNQ, except tetrafluoro-tetracyanoquinodimethane (TCNQF4) was used instead of TCNQ. The new metal-organic compound shows qualitatively similar magnetic properties to Ni$_{2}$TCNQ, with ferromagnetic behavior being observed at room temperature. The specific magnetic properties can be described by assuming that there is both a paramagnetic and ferromagnetic phase in the material, with the ferromagnetic phase having a measured Curie temperature of about 620K, close to that of nickel. TEM and XRD data provide evidence for the existence of nickel nanoparticles within the material. We conclude that nickel nanoparticles are produced during the synthesis and are probably responsible for ferromagnetic properties observed at room temperature. 1.R. Jain \textit{et al}, Nature \textbf{445}, 291, (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T20.00004: Magnetic and surface studies of transition metal complexes for molecular spintronics Patrick Truitt, Raman Talwar, Ezekiel Johnston-Halperin, Norbani Abdullah, Carly Reed, Namrata Singh, Chandrani Chatterjee, Malcolm Chisholm We have synthesized organometallic complexes consisting of a transition metal ion chelated by amphiphilic ligands. This talk will focus on efforts to assess the suitability of these molecules for the creation of magnetically active monolayers via the Langmuir-Blodgett technique. The paramagnetic nature of the molecules is probed by SQUID magnetometry and EPR spectroscopy, demonstrating that the spin magnitude can be varied by chemical substitution of the transition metal ion. To study monolayer formation ability, the molecules are spread on a Langmuir trough and pressure-area isotherms are recorded under compression. Attempts to deposit monolayers onto substrates and to make electrical contact for transport measurements will also be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T20.00005: Magneto-Transport in Polyaniline Nanofiber Network K. Deniz Duman, N.-R Chiou, V.N. Prigodin, A.J. Epstein We report large magnetoresistance (up to 12{\%} at 8 T and 3 K) for polyaniline nanofiber network composed of nanofibers with an average diameter of about 80 nm. The polyaniline nanofiber networks were synthesized via chemical oxidative polymerization [1] and were studied at low and high electric and magnetic fields for temperatures 2 K- 250 K for their magneto-transport behavior. A transition from positive MR (temperatures 75 K and below) to negative MR (temperatures 100K and above) is observed. The MR may be explained by possible competing mechanisms; shrinkage of the hopping wavefunction and quantum interference effect in the applied magnetic field. It is also noted that applied electric field affects MR. In the positive MR regime an increase in MR is observed as the applied electric field decreases. Detailed results of various polyaniline nanofiber samples and possible mechanisms responsible for the magneto-transport behavior will be discussed.\\[0pt] [1] N.-R Chiou, A. J. Epstein, Adv. Mater. \textbf{17}, 1679 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T20.00006: Tuning the ionization energy of organic semiconductor films: The role of intramolecular polar bonds Invited Speaker: While an isolated individual molecule has only one ionization energy (IE), multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical conjugated organic compounds combined with first-principles calculations and electrostatic modeling reveal the existence of a surface dipole built into molecular layers. Its origin lies in intramolecular polar bonds (IPBs) of the individual molecules, and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the IE of one molecular species over up to 1 eV via control over layer morphology. Furthermore, mixing of differently terminated molecules (different IPBs) on a molecular length scale allows continuously tuning the IE of thin organic films between the limiting values of the two pure materials. Surface engineering of organic semiconductors via adjusting the polarity of intra-molecular bonds represents thus a viable alternative for controlling the energetics at organic/(in)organic interfaces. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T20.00007: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T20.00008: Perpendicular interaction between donor and accepter molecules on Au (111) U.G.E. Perera, R. Mishima, S-Wai Hla The capability to modify the electronic properties of materials by the interaction between donor and acceptor molecules plays a significant role in molecular electronics. Formation of molecular charge transfer complexes have been observed for different donor acceptor system in a lateral configuration. Here, we present the structural and electronic properties of decamethylmanganocene (Mn(C5Me5)2) and 7,7,8,8- tetracyanoquinodimethane (TCNQ ) molecules on a Au(111) surface at 4.6K using low temperature scanning tunneling microscopy (STM) to investigate the perpendicular interaction between the molecules. The molecular complexes were formed by depositing Mn(C5Me5)2 onto predeposited TCNQ on Au(111). The TCNQ formed a well ordered self-assembled clusters on Au(111) and Mn(C5Me5)2 adsorbed either on TCNQ layer or on bare Au(111) surface. Perpendicular interaction between the Mn(C5Me5)2 and TCNQ were determined by means of conductance tunneling spectroscopy. This work provides an important step for manipulating and tuning charge state of molecules using donor-acceptor molecular systems. The research is supported by United States Department of Energy BES grant number DE-FG02-02ER46012. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T20.00009: STM study of energy-storing photoactive organometallic molecules Jongweon Cho, Luis Berbil-Bautista, Niv Levy, Steve Meier, K. Peter C. Vollhardt, Michael F. Crommie (Fulvalene)tetracarbonyldiruthenium (FvRu$_{2}$(CO)$_{2})$ molecules store light energy through photoisomerization. UV illumination of molecules in solution or in the solid state results in a conformational change to a high-energy photoisomer. Upon mild heating the molecule reverses to its original structure, liberating $\sim $1.3 eV. Many potential future applications of this molecule involving light energy storage requires understanding its switching behavior in a device geometry, i.e., at a surface. We have investigated self-assembly and switching behavior of FvRu$_{2}$(CO)$_{2}$ molecules on Au(111) using scanning tunneling microscopy at cryogenic temperatures and we will report on these studies. [Preview Abstract] |
Session T21: Semiconductors: Mechanical and Dynamic Properties
Sponsoring Units: DCMPChair: Ben Larson, Oak Ridge National Laboratory
Room: 323
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T21.00001: 3D X-Ray Microscopy and Dislocation Dynamics Simulation Investigation of Deformation in Copper B. C. Larson, Jie Deng, Anter El-Azab, J. Z. Tischler We have combined submicron resolution 3D x-ray microscopy measurements at the Advanced Photon Source and discrete dislocation dynamics (DD) simulations to initiate fundamental investigations of deformation in metals. Half-micron resolution 3D x-ray microscopy measurements of local plastic rotation deformation were performed on initially dislocation free Cu single crystals that were compression deformed axially along [100] to strains varying from 1{\%} to 7.6{\%}. Accordingly, dislocation dynamics simulations of axial [100] deformation in fcc Cu were performed for strains ranging up to 1.6{\%}. The overlapping range of the measured and simulated strain magnitudes provides the first direct and quantitative link on mesoscopic length scales between first principles simulations of deformation and submicron resolution deformation measurements. Quantitative comparisons between the measured and simulated local lattice curvatures will be presented in graphical and statistical form. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T21.00002: Low speed fracture instabilities in a brittle crystal Noam Bernstein, James R. Kermode, Tristan Albaret, Dov Sherman, Peter Gumbsch, Michael C. Payne, G\'abor Cs\'anyi, Alessandro De Vita Brittle materials under mechanical load fail by nucleation and propagation of cracks, and these cracks show well known instabilities at high crack speeds. In this work we show that new instabilities caused by the atomic structure of the crack tip can occur at low crack speeds as well [1]. Using state of the art computer simulations, we find atomic rearrangements at a silicon crack tip on the (111) cleavage plane that occur preferentially on one side of the crack, but only at low crack speeds. Experiments using a novel technique for applying low tensile loads show that real silicon cracks form distinctive features on one side of the exposed crack surface. A mesoscopic model explains how the microscopic atomic rearrangements lead to the observed macroscopic features. We present extensive results on silicon and preliminary results on other brittle materials including sapphire, diamond, and silicon carbide. We conclude that even very brittle single-crystal materials can have a complex crack tip atomic structure, and that atomic scale rearrangements can lead to macropscopic changes in crack morphology. [1] J. R. Kermode {\it et al.}, Nature {\bf 455}, 1224 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T21.00003: Thermoelastic damping in micromechanical resonators Thomas Metcalf, Bradford Pate, Douglas Photiadis, Brian Houston The performance of micro- and nano-mechanical resonators as sensors, filters, and in other devices is determined by the quality factor, $Q$, which measures the fractional energy loss per oscillation cycle of the resonator. In any given resonator, several energy loss mechanisms are likely to be simultaneously present. However, for micro- and nano-scale resonators, the relative strengths and identity of these mechanisms is largely unknown. We measure the temperature dependence of $Q^{-1}$ of two resonant modes (460 kHz and 510 kHz) of a 1.5 $\mu$m thick silicon micromechanical plate resonator. In-situ ultra-high vacuum annealing lowers the background energy loss at 120 K to $Q^{-1}\leq5\times10^{-7}$. The $Q^{-1}$ increases with increasing temperature by different rates for the two modes, quantitatively agreeing with a modification of Zener's theory of thermoelastic damping. This provides strong evidence that thermoelasticity is the dominant energy loss mechanism in one resonant mode. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T21.00004: Ab initio guided design of bcc Mg-Li alloys for ultra light-weight applications Martin Fri{\' a}k, William Art Counts, Dierk Raabe, J{\" o}rg Neugebauer Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. In this paper, fundamental physical properties (like formation energies and elastic constants) of 11 bcc Mg-Li compounds are calculated using density-functional theory (DFT) and compared with available experimental data. These DFT-determined properties are in turn used to calculate engineering parameters like (i) specific Young's modulus (Y/$\rho )$ or (ii) bulk over shear modulus ratio (B/G) differentiating between brittle and ductile behavior. The engineering parameters are then used to identify alloys that have optimal mechanical properties needed for a light weight structural material. It was found that the stiffest bcc magnesium-lithium alloys contain about 70 at.{\%} Mg while the most ductile alloys have 0-20 at.{\%} Mg. The specific modulus for alloys with 70 at.{\%} Mg is equal to that of Al-Mg alloys. An Ashby map containing Y/$\rho $ vs. B/G shows that it is not possible to increase both Y/$\rho $ and B/G by changing only the composition or local order of a binary alloy (W. A. Counts, M. Fri\'{a}k, D. Raabe and J. Neugebauer, Acta Mater 57 (2009) 69-76). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T21.00005: Thermal Stability of Shape Transition in Strained Nano-Islands Cristiano Nisoli, Douglas Abrahams, Turab Lookman, Avadh Saxena Two dimensional Stranski-Krastanow strained islands are known to undergo a shape anisotropy transition as they grow in size, finally evolving toward nanowires. We investigate thermal stability of this process and find a phase transition both in temperature and, in simple cases, in growth. While our results are general, they can explain recent data on Erbium Silicide growth on vicinal Si surface. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T21.00006: Neutron Time of Flight phonon spectra of Cu$_{2}$O and Ag$_{2}$O powders Barry Winn, Mark Hagen, Steve Shapiro Negative thermal expansion materials cuprite (Cu$_{2}$O) and Ag$_{2}$O share the same structure (space group \textit{Pn3m}). Here, we report inelastic neutron time of flight measurements of room temperature powder samples of each system, using the Pharos chopper spectrometer at LANSCE, at up to 100 meV energy transfer. For Cu$_{2}$O, high energy optical phonons are observed between 60 and 80 meV, while for Ag$_{2}$O, these phonons are observed between 50 and 70 meV. Results are compared to previous work, and to recent neutron triple axis spectrometer results for Cu$_{2}$O, and their relevance to negative thermal expansion is discussed. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T21.00007: Large-Amplitude Anharmonic Decay of Coherent A1g phonon motion in Bismuth Stephen Fahy, Aaron Hurley, Donal O Donoghue, Eamonn Murray, David Prendergast, Tadashi Ogitsu, David Reis, David Fritz Large amplitude coherent motion of the A1g phonon in bismuth can be generated by ultrafast optical excitation. At low amplitude, the decay rate agrees with that observed in Raman scattering. At high levels of photoexcitation, the observed phonon damping is substantially increased, compared to low-amplitude motion. We present a classical simulation of the anharmonic decay of the phonon, including third-order anharmonic terms in the energy, calculated using density functional perturbation theory, coupling the A1g motion to modes throughout the Brillouin Zone. At low amplitude, the classical decay can be shown in perturbation theory to be almost identical in classical and quantum dynamics at room temperature, demonstrating the validity of a classical simulation of the dynamics. For very large A1g amplitude, the amplitude of motion in the final state modes is substantially increased over their thermal average values, leading to an increase in the decay rate of the A1g mode. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T21.00008: Phonon dispersion relations for unstrained Si$_{1-x}$Ge$_{x}$ via density functional theory Md Hossain, Jonathan Freund, Harley Johnson Phonon dispersions for the Si$_{1-x}$Ge$_{x}$ alloy system are computed using localized basis density functional theory. Including interactions up to third-nearest-neighbors and the effect of atomic randomness, phonon dispersion for the full Brillouin zone of a supercell containing 8 atoms is computed for 8 different compositions. Frequencies are found to be in excellent agreement with available experimental results for both crystalline Si and Ge. Results are compared with a 64-atom supercell calculation for optical frequencies to show any possible effect of supercell size on the phonon calculation. The atoms in the calculation are relaxed to a force tolerance of 0.0001eV/{\AA}, which is found to be important to correctly determine the dispersion near the $\Gamma $- and X-points of the Brillouin zone where q-convergence is harder to achieve. The highest optical phonon frequencies are observed to vary nonlinearly with composition, a fact not investigated before using computational methods. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T21.00009: Grating-Enhanced Response for current-driven coupled quantum wells Antonios Balassis, Godfrey Gumbs We have investigated the conditions necessary to achieve stronger plasmon instability leading to emission in the terahertz (THz) regime for semiconductor quantum wells (QWs). The surface response function is calculated for a bilayer two-dimensional electron gas (2DEG) system in the presence of a metal grating placed on the surface and which modulates the electron density. The 2DEG layers are coupled to surface plasmons arising from excitations of free carriers in the bulk region between the layers. A current is passed through one of the layers and is characterized by a drift velocity $v_D$. With the use of the surface response function, the plasmon dispersion equation is obtained as a function of frequency $\omega$, the in-plane wave vector ${\bf q}_{\parallel}=(q_x,q_y)$ and reciprocal lattice vector $nG$ where $n=0,\pm1,\pm2,\cdots$ and $G=2\pi/d$ with $d$ denoting the period of the grating. The dispersion equation, which yields the resonant frequencies, is solved in the complex $\omega$-plane for real wave vector ${\bf q}_{\parallel}$. It is ascertained that the imaginary part of $\omega$ is enhanced with decreasing $d$, and with increasing the doping density of the free carriers in the bulk medium for fixed grating period. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T21.00010: Where the reactive sites are in anatase nanoparticles? --Theoretical Investigation on (001) and (101) surfaces in anatase nanoparticles Hong Wang, James Lewis Recently, with the development of nanotechnology, the devices' size shrinks to nano-scale size where the surface properties play a role. Thus, it is required scientists to provide fundamental level understanding of anatase surfaces in nano-size anatase materials to improve its applications. In this work, applying DFT ab initio method, we investigate the fundamental properties of anatase (001) and (101) surfaces in anatase nanoparticles. By adopting different portions of (001) and (101) surfaces along with the size of nanoparticles, we analyze the geometric properties and energetic stabilities of nanoparticles. The electronic properties of these nanoparticles are also calculated in this work. The frontier orbitals located mostly in the (001) surfaces indicate these sizes are possibly reactive sizes in the external molecule adsorption reactions. To verify their activity, we add water molecules in different sits and different concentration on these nanoparticles. The results show that the sites where the frontier orbitals are localizing are very reactive for water adsorption. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T21.00011: Collective Excitations in Cylindrical Quantum Dots Chains Jimena Vergara, Angela Camacho We are interested in the study of collective excitations in quantum dot chains because these can be used to effectively transmit information at nano scale and to control spontaneous and stimulate electromagnetic emission in the quantum dots. [1] This work is centered in the study of semiconductor one-dimensional quantum dot arrays. Based on a tight-binding bandstructure calculation combined with a self consistent field approximation we obtain the dispersion relations and we analyze how the geometry of the dot affects the collective oscillation of charge and its propagation. We focus our study first on Coulomb interaction between charges as the main cause of the 1D plasmons neglecting tunneling to finally compare with the case where tunneling is allowed. We find out that Coulomb interaction plays an important role in these systems and that tunneling opens the energy spectrum permitting new excitations, which are good candidates to be used in nanometric devices. [1] A.V.Akimov, A.Mukherjee, C.L. Yu, D.E Chang, A.S.Zybrov, P.R. Hemmer, H Park and M.D Lukin, \textit{Generation of Single optical plasmons in metallic nanowires coupled to quantum dots}, Nature 450, 402 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T21.00012: Electrical characterization of MOVPE-grown InSb nanowires Henrik Nilsson, Philippe Caroff, Claes Thelander, Marcus Larsson, Lars-Erik Wernersson, Lars Samuelson, Hongqi Xu In bulk, InSb is a narrow band gap (E$_{g}$ = 170 meV) semiconductor with high electron mobility ($\mu _{n}$ = 77~000 cm$^{2}$/Vs) and is therefore of relevance for low power and high speed transistor applications. It also has a low electron effective mass (0.015m$_{e})$ and a very high electron g-factor $\vert $g$\vert $=51 which is of interest for studies of quantum and spin physics. InSb nanowires were grown by MOVPE from 40 nm Au aerosol seed particles deposited on a $<$111$>$B InAs substrate, where the growth was initiated by a 100 nm InAs segment. The InSb nanowires are untapered and free from stacking faults. The grown InSb nanowires were transferred to degenerately doped, SiO$_{2}$ capped, Si substrates. After locating the wires, Ti/Au contacts were made by electron beam lithography. Electrical measurements of the fabricated InSb nanowire devices were performed in the high bias, field-effect transistor (FET) regime at temperatures ranging from 300 K to 4.2 K as well as in the low bias, single-electron transistor (SET) regime at temperatures ranging from 4.2 K to 300 mK. In particular, effective electron g-factors and Kondo physics have been studied at low temperatures with the nanowire devices. [Preview Abstract] |
Session T22: Focus Session: GaMnAs
Sponsoring Units: GMAG DMP FIAPChair: Nitin Samarth, Penn State University
Room: 324
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T22.00001: Electric-field manipulation of magnetization vector direction Invited Speaker: Ferromagnetism and magnetization in Mn-doped III-V semiconductors can be manipulated by various means; by changing its carrier concentration by electric fields [1] or by spin- current flowing along with the electric current [2]. This material system is thus an excellent system to study the physics involved in manipulation of magnetism as well as exploring new ways to control magnetization. Here, we show that electrical control of magnetization direction can be done through manipulating electronically the magnetic anisotropy energies [3]. The basic idea behind the effort is to control the population of carriers on spin-split anisotropic valence bands that governs the magnetic anisotropy energies, which should result in change of the direction of magnetization. In order to measure the magnetic anisotropies under a gate that applies the electric-field to the ferromagnetic semiconductor channel, we used the planar Hall effect. Analyses showed that there are biaxial as well as uniaxial anisotropies. As the sheet carrier concentration is reduced by applying electric- field to the channel, the uniaxial anisotropy field reduced its magnitude and eventually changed its sign, whereas no significant change was apparent in the biaxial anisotropy field. From the electric-field dependent anisotropy fields, one can show that the angle of the magnetization direction in the absence of magnetic fields is modulated by electric-fields by 10 degrees. This opens up a new and unique opportunity for manipulating magnetization direction solely by electronic means, not resorting to magnetic-field, spin-current, mechanical stress, nor multiferroics. The conditions for switching the magnetization direction will also be discussed. The work was done together with D. Chiba, F. Matsukura, M. Sawicki, Y. Nishitani, and Y. Nakatani. \\[4pt] [1] H. Ohno, et al. Nature 408, 944 (2000). D. Chiba, et al. Science, 301, 943 (2003). D. Chiba, et al. Appl. Phys. Lett. 89, 162505 (2006). \\[0pt] [2] M. Yamanouchi, et al. Nature 428, 539 (2004). M. Yamanouchi, et al. Phys. Rev. Lett. 96, 096601 (2006). M. Yamanouchi, et al. Science 317, 1726, (2007). \\[0pt] [3] D. Chiba, et al. Nature 455, 515 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T22.00002: Bias-controlled ferromagnetism in quantum wells with Mn-delta doping Erika Dias Cabral, Rafal Oszwaldowski, Marco Boselli, Igor Zutic, Ivan da Cunha Lima Carrier-mediated magnetism in semiconductors shows important and potentially useful differences from magnetism in metals [1] such as light- or bias-controlled ferromagnetism [2-3]. Motivated by experiments reporting in GaAs quantum wells (QWs) with Mn-delta doping higher Curie temperatures (T$_{C})$ than in bulk (Ga,Mn)As [4], we explore theoretically the feasibility of bias-controlled ferromagnetism in QWs. We calculate self-consistently indirect Mn-Mn exchange interaction [5] and apply a Monte Carlo approach to calculate T$_{C}$. Our approach allows us to systematically study the effects of quantum confinement and the position of the Mn layer on magnetic ordering and T$_{C}$, beyond the mean field approximation, which we obtain as the limiting case. We compare our findings with the experimental results and suggest paths towards improved control of ferromagnetism. Supported by CNPq, FAPEMIG, FAPERJ, CAPES, US ONR, and NSF-ECCS Career. [1] I. Zutic et al., Rev. Mod. Phys. 76, 323 (2004). [2] S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997). [3] H. Ohno et al., Nature 409, 944 (2000). [4] A. M. Nazmul et al., Phys. Rev. Lett. 95, 017201 (2005). [5] M. A. Boselli et al., Phys. Rev. B 68, 085319 (2003). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T22.00003: STM studies of an atomic-scale gate electrode formed by a single charged vacancy in GaAs Donghun Lee, David Daughton, Jay Gupta Electric-field control of spin-spin interactions at the atomic level is desirable for the realization of spintronics and spin-based quantum computation. Here we demonstrate the realization of an atomic-scale gate electrode formed by a single charged vacancy on the GaAs(110) surface[1]. We can position these vacancies with atomic precision using the tip of a home-built, low temperature STM. Tunneling spectroscopy of single Mn acceptors is used to quantify the electrostatic field as a function of distance from the vacancy. Single Mn acceptors are formed by substituting Mn adatoms for Ga atoms in the first layer of the $p$-GaAs(110) surface[2]. Depending on the distance, the in-gap resonance of single Mn acceptors can shift as much as 200meV. Our data indicate that the electrostatic field decays according to a screened Coulomb potential. The charge state of the vacancy can be switched to neutral, as evidenced by the Mn resonance returning to its unperturbed position. Reversible control of the local electric field as well as charged states of defects in semiconductors can open new insights such as realizing an atomic-scale gate control and studying spin-spin interactions in semiconductors. http://www.physics.ohio-state.edu/{\$}$\backslash $sim {\$}jgupta [1] D. Lee and J.A. Gupta (in preparation) [2] D. Kitchen et al., Nature \textbf{442}, 436-439 (2006) [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T22.00004: Probing the nature of electronic state near the Fermi level in Ga$_{1-x}$Mn$_{x}$As with STM Anthony Richardella, Pedram Roushan, Shawn Mack, David Awschalom, Ali Yazdani We have studied the electronic states near the Fermi energy in GaMnAs/GaAs heterostructures as a function of doping across the metal-insulator transition. These measurements allow us to determine the position of E$_{F}$ with respect to the valance band edge and in gap states related to the Mn induced acceptor states. As the doping level increases we observe an increase in the density of states at the Fermi energy and map their spatial dependence. Statistical analysis of these measurements can be used to find a characteristic length scale associated with growth of bulk metallic behavior for these samples. In addition, our measurements indicated a suppression of the density of states near E$_{F}$ at all doping levels, consistent with that expected for correlation effects in doped semiconductors near the metal-insulator transition. We will discuss these findings and their relation with various theoretical models for electronic states in GaMnAs that are expected to mediate the magnetic interaction in this compound. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T22.00005: Scanning Tunneling Microscopy Studies of Mn acceptor levels in Ga$_{1-x}$Mn$_{x}$As Pedram Roushan, Anthony Richardella, Shawn Mack, David Awschalom, Ali Yazdani We have used a low temperature scanning tunneling microscope (STM) to perform studies of GaMnAs/GaAs heterostructures with various Mn dopant concentrations. The STM topography of the GaMnAs showed a variety of electronic structure modulations on the order of a few nm indicating the presence of a high level of disorder and compensation. These measurements show no indication of Mn clustering as the Mn concentration is increased. On both sides of the Metal-Insulator Transition (MIT), the differential conductance (dI/dV) measurements on Mn dopants showed a broad acceptor level above 100meV from the valence band edge. Furthermore, we have mapped in energy the spatial variations of these deep acceptor levels, and their distribution will be presented for all Mn concentrations studied. The effect of disorder and coulomb correlations in modifying the local density of states close to Fermi level will be discussed for insulating as well as metallic samples. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T22.00006: Theory of STM spectroscopy in Mn clusters on GaAs surfaces. Tor Olof Strandberg, Alan MacDonald, Carlo Canali Small numbers of Mn atoms can be manipulated into arbitrary spatial arrangements on the $<110>$ surface of GaAs by means of a novel STM atom-by-atom substitution technique, which enables the replacement of individual Ga atoms by Mn [1]. The tunneling differential conductance over an isolated Mn atom reveals a large and broad resonance in the GaAs energy gap. For a Mn pair placed less than 1 nm apart, the resonance splits into two peaks, whose spacing is thought to be related to the exchange-energy interaction between Mn ions. We report on theoretical results for the local density of states and the Mn acceptor-level splittings for a Mn dimer, based on a tight-binding model of Mn substitutions on the $<110>$ GaAs surface. We compare our model with previous work which does not account for the surface. We then derive an effective quantum spin Hamiltonian for the Mn cluster, based on a Chern number theory developed recently, which includes Berry phase effects [2]. We study the transition from surface to bulk for the substitutional Mn impurity in GaAs as well as Mn-Mn interactions at the surface and in bulk at various distances and along different crystalline directions. [1] D. Kitchen et al., Nature 442, 436 (2006). [2] C.M. Canali, A. Cehovin and A.H. MacDonald, Phys. Rev. Lett. 91, 046805 (2003) [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T22.00007: Strong Magnetic Circular Dichroism in Mn Delta-doped GaAs Nazmul Ahsan, Sanjukta Ghosh, Masaaki Tanaka Delta-doping of magnetic impurities ($i.e.$ Mn) in III-V semiconductors allows locally high concentration of magnetic moments. This can lead to systematic observation of fundamental properties of the system including the enhancement of the Curie temperature and magnetic anisotropy as a function of a wider range of Mn concentration[1]. The delta-doped Mn atoms in the MBE-grown GaAs-based heterostructures are abruptly confined as confirmed by high resolution transmission electron microscopy studies[1]. Here we study the magnetic circular dichroism (MCD) of 1 monolayer (ML) Mn delta-doped GaAs layer. The structure from the growth sequence is: GaAs substrate/GaAs-buffer/Al$_{0.9}$Ga$_{0.1}$As/Be-doped Al$_{0.3}$Ga$_{0.7}$As/1nm GaAs/1ML Mn/10nm GaAs cap. The sample was chemically etched to single out the 1nm GaAs/1ML Mn layer/10nm GaAs cap to measure MCD spectra in the transmission geometry. We observed strong MCD features even at 300K, indicating ferromagnetism with zinc-blende band structure. Ref.: [1] Nazmul \textit{et al}. Phys. Rev. B \textbf{67}, 241308 (2003); J. Crystal Growth, \textbf{251}, 303 (2003); Phys. Rev. Lett. \textbf{95}, 017201 (2005); Phys. Rev. B \textbf{77}, 155203 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T22.00008: Magneto-Optical Kerr Spectroscopy of Moderately Doped GaMnAs: Evidence for Interband Transitions C. Sun, J. Kono, Y.H. Cho, A.A. Belyanin, H. Munekata The role of impurity bands as well as the nature of free holes in carrier-mediated ferromagnetism in (III,Mn)V systems are still not well understood. Previous magneto-optical studies of GaMnAs have produced an array of conflicting results, especially in terms of the nature of optical transitions involved. Here, we have performed systematic magneto-optical Kerr spectroscopy studies of GaMnAs samples with different doping densities. The Kerr angle strongly depended on the photon energy, showing positive peaks at 1.7 eV and 3 eV and a negative peak at 2.5 eV. The 1.7 eV peak clearly shifts to higher energies with Mn doping from 1{\%} to 2.4{\%} and shifts to an even higher energy after annealing. We attribute these changes to the increased hole density and effective Mn content. A 30-band {\boldmath $k \cdot p$} model with exchange interaction is adopted to simulate the spectra. The excellent agreement between the experiment and calculation leads us to conclude that Kerr rotation in GaMnAs above the band gap is dominantly determined by interband transitions. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T22.00009: Below band gap Faraday and Kerr measurements in ferromagnetic GaMnAs Gheorghe Acbas, M.-H. Kim, J. Cerne, M. Cukr, V. Novak, T. Jungwirth, M.A. Scarpulla, O.D. Dubon, J. Sinova We have studied the Faraday and Kerr effects in a series of ferromagnetic GaMnAs films in the 115-1500 meV energy range. This provides a direct magneto-optical probe of the valence band, which is critical to understanding the ferromagnetic origin in this material. Previous magneto-optical studies probed the region around the band gap (1.5 eV) where numerous contributions to the optical response were inferred: ferromagnetic, paramagnetic, valence band, impurity bands, intra-d level or intra-gap defect levels. We compare our experimental results with predictions from mean field Zener model calculations. We find that the results are consistent with a picture in which the Fermi level resides inside the spin split valence band. Many-body band renormalization effects have to be included in order to model the results quantitatively. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T22.00010: Local Magnetic Characterization of Continuous (Ga,Mn)As Film using Mechanical Force Detection I. H. Lee, Yu. Obukhov, J. Kim, X. Li, N. Samarth, D. V. Pelekhov, P. C. Hammel We report on low temperature (T = 4.2 K) studies of the local spin dynamics in ferromagnetic samples using Ferromagnetic Resonance Force Microscopy (FMRFM) and probe-induced Magnetic Force Microscopy (MFM). Both techniques are based on sensitive mechanical detection of the dipolar magnetic interaction between a micromagnetic probe mounted on a flexible micro-cantilever and magnetic moments in the sample. The probe magnet not only detects the magnetic force, but also perturbs sample spin magnetization by adding the strongly inhomogeneous magnetic field. We demonstrate that the combination of FMRFM and probe-induced MFM can be used to extract and map local magnetic properties of a continuous (Ga,Mn)As film such as saturation magnetization and anisotropy field. These new approaches to scanned magnetic force imaging open the door to powerful new tools for spatially resolved studies of nanoscale magnetism and spin-based devices. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T22.00011: Simultaneous Optical Imaging and Electrical Control of Magnetization in (Ga,Mn)As M.E. Nowakowski, G.D. Fuchs, D.D. Awschalom, A. Balk, M.J. Wilson, N. Samarth Spin dependant phenomena in metals and semiconductors promises the development of low-power logic and memory devices based on electrical control of the magnetization. To realize this potential, precise visual information of magnetic domains is required to design and control electrical structures manipulated by the spin transfer torque. We present studies of magnetization behavior in micron-scale (Ga,Mn)As channels using a recently developed video-rate magneto-optical Kerr effect microscope. Measurements record real-time, diffraction-limited, surface magnetization information including magnetic switching and domain wall motion. The optical measurements are correlated with simultaneous electrical measurements to provide insight into pinning and magnetization transport in these structures. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T22.00012: Optical conductivity of diluted magnetic semiconductors: effects of dynamical screening Fedir Kyrychenko, Carsten A. Ullrich Most theoretical studies of transport and optical conductivity in diluted magnetic semiconductors like GaMnAs treat disorder and many-body effects within the simple relaxation time and static screening models. Here we present a more complete theory of transport in charge and spin disordered media that combines a multiband ${\bf k \cdot p}$ approach with a first-principles descriptions of disorder and electron-electron interaction through the memory function formalism and time-dependent density functional theory. We discuss the effects of dynamic screening and collective electron excitations on the charge and spin scattering off Coulomb impurities and fluctuations of localized spins and compare calculated values of optical conductivity in GaMnAs with experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T22.00013: Infrared probe of Ga$_{1-x}$Mn$_{x}$As films with controlled disorder and compensation Brian Chapler, R.C. Myers, S. Mack, D.D. Awschalom, M.C. Martin, A. Dattelbaum, K.S. Burch, D.N. Basov Arsenic antisite defects (As$_{Ga})$ formed due to low temperature growth conditions are a leading cause of disorder and compensation in Ga$_{1-x}$Mn$_{x}$As. Samples grown with gradient As:Ga growth condition for 0.005$<$x$<$ 0.16 have allowed for optimized As flux minimizing As$_{Ga}$. By studying samples at this optimized location via infrared spectroscopy, a new level of precision can be attained in exploring the electronic structure and other intrinsic properties of Ga$_{1-x}$Mn$_{x}$As samples. Using optical sum rule analysis of our experimentally determined optical conductivity ($\sigma _{1}(\omega ))$, we extract the free carrier band mass (m*) and find it to be several m$_{e}$. We also comment on the levels of interstitial Mn (Mn$_{i})$, finding for x $>$ 0.03 roughly 25{\%} of Mn resides at an interstitial location. Additionally, by probing positions along the As:Ga gradient we directly measure the effects of disorder and compensation on these samples. Systematic changes in $\sigma _{1}(\omega )$ as As$_{Ga}$ content is increased are reported, and the consequences of this on our understanding of the electronic structure of Ga$_{1-x}$Mn$_{x}$As are discussed. [Preview Abstract] |
Session T23: Quantum Hall Effect: Coherent Phenomena
Sponsoring Units: DCMP FIAPChair: Vladimir Goldman, SUNY at Stony Brook
Room: 325
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T23.00001: Low-field quantum Hall transport in an electron Fabry-Perot interferometer P.V. Lin, F.E. Camino, V.J. Goldman We report systematic experimental characterization of an interferometer device as a function of front-gate voltage at 10 mK. Application of front-gate voltage affects the constriction electron density, but the 2D bulk density remains unaffected. The low-field quantum Hall transport (filling $f > 4$) shows quantized plateaus in longitudinal resistance, while the Hall resistance is dominated by the low-density, low-filling constriction. This allows to determine independently both: the bulk and the constriction filling. At lower fields, when the quantum Hall plateaus fail to develop, we observe the bulk Shubnikov-de Haas oscillations in series corresponding to an integer number of the magnetoelectric subbands in the constriction. From a Fock-Darwin analysis, we obtain the constriction electron density as a function of the front-gate bias, and, extrapolating to the zero-field, the $B=0$ number of 1D electric subbands (conductance channels) resulting from the electron confinement in the constriction. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T23.00002: Oscillatory transport in electron Fabry-Perot interferometers F.E. Camino, P.V. Lin, V.J. Goldman We report experiments on GaAs/AlGaAs heterostructure interferometers in the integer QH regime with filling $f=1-4$. Etch trenches define the device, which consists of an electron island connected to the 2DES bulk via two wide constrictions. Front gates deposited in the trenches permit to fine tune the device. When tunneling occurs in the constrictions, electrons perform closed orbits around the island, producing an Aharonov- Bohm oscillatory signal in the conductance. On QH plateau transition between $f+1$ and $f$, we observe $f$ oscillations per flux $h/e$. In contrast, for all fillings, we observe one oscillation per back-gate charging period $e$. We also report a linear dependence of magnetic field period on front-gate voltage for three devices, with the slope inversely proportional to $f$. We attribute this behavior to self-consistent electrostatics of the electron island, and discuss the models of edge channel structure. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T23.00003: Universal dephasing in chiral one-dimensional electron systems Florian Marquardt, Clemens Neuenhahn The Green's function of a chiral interacting one-dimensional fermion system obeys a power-law decay at high energies, at zero temperature. Surprisingly, we find that the exponent is universal, i.e. independent of the interaction strength, for (almost) arbitrarily shaped interaction potentials. This has direct implications for the interference contrast in ballistic interferometers, e.g. the Mach-Zehnder interferometer composed of edge channels in the integer quantum Hall effect. Our result is obtained using a straightforward and physically transparent ``semiclassical'' approach to dephasing by electron- electron interactions. This approach is shown to coincide with the exact bosonization results in the high-energy regime of interest. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T23.00004: Determination of the coherence length in the Integer Quantum Hall Regime. F. Portier, P. Roulleau, P. Roche, A. Cavanna, G. Faini, U. Gennser, D. Mailly 0ne of the basic length scales limiting quantum effects in electrical conductors is the phase coherence length L$_{\varphi }$, the typical length on which an excitation looses its phase coherence via coupling to other degrees of freedom. In quasi-1D diffusive wires, due to electron-electron interactions, L$_{\varphi }$ was shown to scale as T$^{-1/3}$, as predicted by Altshuler-Aronov-Khmelnitsky. Surprisingly, little is known about L$_{\varphi }$ in the Integer Quantum Hall Regime (IQHE), where transport occurs through 1D chiral wires, localized on the edges of the sample. The number of these 'edge states' is equal to the filling factor (the number of electron per flux quantum flux). Chirality should prevent momentum conserving energy exchange processes, leading to a very long coherence length.We present an experiment where we have determined L$_{\varphi }$ in the IQHE at filling factor 2, by measuring the visibility of quantum interferences in an electronic Mach-Zhender interferometer. L$_{\varphi }$ shows a T$^{-1}$ dependence, proved to result from the coupling between the two neighbouring edge states: the thermal charge noise in one edge state blur the phase on the other edge state, leading to a finite coherence length proportional to T$^{-1}$. . [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T23.00005: Edge-State Velocity and Coherence in a Quantum Hall Fabry-Perot Interferometer Douglas McClure, Yiming Zhang, Eli Levenson-Falk, Charles Marcus, Loren Pfeiffer, Ken West We present finite-bias measurements of electronic Fabry-Perot interferometers in the integer quantum Hall regime. In devices large enough that Coulomb blockade is absent, checkerboard-like patterns of oscillations as a function of magnetic field and dc bias appear. Comparing our data to predictions for electromagnetic Aharonov-Bohm interference, we extract edge-state velocities over a range of magnetic fields, finding dependence consistent with a crossover from skipping orbits at low fields to $E\times B$ drift at high fields. Suppression of visibility observed at high bias and high field is quantitatively accounted for by including an energy-dependent dephasing rate. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T23.00006: Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers Yiming Zhang, Douglas McClure, Eli Levenson-Falk, Charles Marcus, Loren Pfeiffer, Ken West Two different types of resistance oscillations are observed in two electronic Fabry-Perot interferometers of different sizes. Measuring these oscillations as a function of magnetic field, gate voltage, or both, we observe three signatures that distinguish the two types. The oscillations observed in a $2~\mathrm{\mu m^2}$ device are understood to arise from Coulomb blockade, and those observed in an $18~\mathrm{\mu m^2} $ device from Aharonov-Bohm interference. This work clarifies, provides ways to distinguish, and demonstrates control over, the physical origins of resistance oscillations seen in electronic Fabry-Perot interferometers. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T23.00007: Fabry-Perot interferometer in Fractional Quantum Hall regime Aveek Bid, N. Ofek, M. Heiblum, Ady Stern, V. Umansky, D. Mahalu We have measured Aharonov-Bohm/Coulomb blockade oscillations in an electronic Fabry-Perot interferometer in the Fractional Quantum Hall regime. At $\nu $ = 2/5, when the inner channel is partially reflected (with the outer channel (1/3) being fully transmitted); the total transmission of the device oscillates as a function of magnetic field or modulation gate voltage. This is true also for $\nu $ =2, 3, 4 when the interference is of a partially reflected lower lying channel (with the other channels being either fully transmitted or fully reflected). However, in the outermost channel of all filling factors ($\nu $ = 1/3, 2/5, 1, 4/3, 2, 3, 4, 5) we do not see any oscillations as a function of B. This we interpret to be due to interplay between the magnetic field (which tries to modify the area of the compressible island inside the interferometer) and Coulomb energy (which prevents the density of quasiparticles within the island from building up indefinitely). The period of oscillations in modulation gate voltage in the inner channel of $\nu $ = 2/5 (partially partitioned) is found to be one-third of that observed in the second channels of the integer filling fractions which probably is an indication that the oscillations are due to the tunneling of quasiparticles of fractional charge 1/3. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T23.00008: Nonequilibrium Dephasing in an Electronic Mach-Zehnder Interferometer Seok-Chan Youn, Hyun-Woo Lee, H.-S. Sim We study nonequilibrium dephasing in an electronic Mach-Zehnder interferometer. We demonstrate that the shot noise at the beam splitter of the interferometer generates an ensemble of nonequilibrium electron density configurations and that electron interactions induce configuration-specific phase shifts of an interfering electron. The resulting dephasing exhibits two characteristic features, a lobe pattern in the visibility and phase jumps of $\pi $, in good agreement with experimental data. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T23.00009: Generating Excitonic Supercurrent in Quantum Hall Bilayers Jung-Jung Su, Tami Pereg-Barnea, Allan H. MacDonald Among the many examples of Bose condensation considered in physics, exciton condensation has maintained special interest because of controversy about condensate properties. Although ideal condensates can support an exciton supercurrent, it has not been clear how such a current could be induced or detected. We discuss the circuit conditions required to induce a steady-state counterflow superfluid. In addition, we will discuss interpretations of tunnel, drag and counterflow experiments in quantum Hall exciton condensates. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T23.00010: Density imbalance effect on the Coulomb drag upturn in an undoped electron-hole bilayer Christian Morath, John Seamons, John Reno, Mike Lilly A low-temperature upturn of the Coulomb drag resistivity measured in an undoped electron-hole bilayer (uEHBL) device, possibly manifesting from exciton formation or condensation, was recently observed. The effects of density imbalance on this upturn are examined. Measurements of drag as a function of temperature in a uEHBL with a 20 nm wide Al$_{.90}$Ga$_{.10}$As barrier layer at various density imbalances n$\ne $p are presented. The results show drag increasing as the density of either two dimensional system was reduced, both within and above the upturn temperature regime and with a significantly stronger dependence than the (np)$^{-3/2}$ predicted by the weak-coupling theory. A comparison of the data with numerical calculations of drag in the presence of electron-hole pairing fluctuations, which qualitatively reproduce the drag upturn behavior and easily accommodates density imbalance effects, is also presented. The calculations predict a peak in drag at matched densities, which is not reflected in the measurements. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T23.00011: Anyons in a weakly interacting system Gilad Rosenberg, Babak Seradjeh, Conan Weeks, Marcel Franz I will present our recent theoretical proposal for the realization and manipulation of anyons in a weakly interacting system. This system consists of a two-dimensional electron gas in the integer quantum Hall regime, adjacent to a type-II superconducting film with an artificial array of pinning sites. The anyons are realized in response to defects in the pinned vortex lattice and carry charge $\pm$e/2 and have an exchange phase $\pi$/4. We establish this result using a 2D continuum model of electrons in the magnetic field caused by the vortex lattice of the superconducting film. The charge of the defects is evaluated numerically, using the Aharonov-Casher exact solution (for $g=2$) for the ground state of a 2D system in magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T23.00012: Transport Measurements Between Superconductors and Semiconducting Structures Stephanie Law, Michael Vissers, Kevin Inderhees, Timothy McArdle, Allison Dove, Paul Goldbart, Nadya Mason, James Eckstein We report IV characteristics and differential resistance measurements on two dimensional electron gases and degenerately doped semiconductors in high magnetic fields. The samples are fabricated into Hall bars for measurement. Differential resistance and IV characteristics are then measured in a novel three terminal setup which allows us to measure voltage changes upstream of the current as well as conventional two and four terminal setups. The measurements are made at low temperatures in magnetic fields up to 7T both at fields where chiral edge states exist and at those where they do not exist. We are thus able to investigate the effect of chirality on transport. Samples are made both with high critical field superconducting contacts and with normal metal contacts, allowing us to compare the effect of injecting Cooper pairs into the structure. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T23.00013: An edge index for topological insulators Emil Prodan Topological insulators display dissipationless currents flowing at the edges of the samples. These currents are associated to chiral edge modes, whose existence is intrinsically linked to the topology of the electronic states of the bulk. The edge modes can be easily investigated when the edges are smooth and have a periodicity, but as soon as the periodicity is absent, the problem becomes un-traceable by purely theoretical means. In my talk I will exemplify the use of non-commutative calculus to explore the properties, especially the stability of the edge modes. For example, using such techniques one can give a fairly elementary proof that the edge modes in Chern insulators survive even for a rough (random) edge. Similarly, for the Spin-Hall effect, one can define an observable and its associated current whose conductance remains quantized during various deformations of the Hamiltonian system. It turns out that in all cases, the edge conductance is given by the index of a Fredholm operator, which provides a new topological invariant linked directly to the edge rather than the bulk. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T23.00014: Topological phases and topological surface states of three-dimensional time-reversal invariant superconductors Andreas Schnyder, Shinsei Ryu, Akira Furusaki, Andreas Ludwig We study topological phases of time-reversal invariant singlet superconductors in three spatial dimensions. In these particle-hole symmetric systems the topological phases are characterized by a winding number [1], similar to the $Z_2$ invariant of the $Z_2$ topological insulators. At a two-dimensional surface the topological properties of this quantum state manifest themselves through gapless surface states, that are robust against localization from random impurities respecting the discrete symmetries of the system. We construct a tight-binding model on the diamond lattice that realizes the topologically nontrivial phase and perform numerical studies of the winding number and the surface states of this model. \\[3pt] [1] A. P. Schnyder, S. Ryu, A. Furusaki, and A. W. W. Ludwig, arXiv:0803.2786 (PRB in press). [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T23.00015: Helical Metal Inside a Topological Band Insulator Yi Zhang, Ying Ran, Ashvin Vishwanath Topological defects, such as domain walls and vortices, have long fascinated physicists. A novel twist is added in quantum systems like the B-phase of superfluid helium He$_3$, where vortices are associated with low energy excitations in the cores. Similarly, cosmic strings may be tied to propagating fermion modes. Can analogous phenomena occur in crystalline solids that host a plethora of topological defects? Here we show that indeed dislocation lines are associated with one dimensional fermionic excitations in a `topological insulator', a novel band insulator believed to be realized in the bulk material Bi$_{0.9}$Sb$_{0.1}$. In contrast to fermionic excitations in a regular quantum wire, these modes are topologically protected like the helical edge states of the quantum spin-Hall insulator, and not scattered by disorder. Since dislocations are ubiquitous in real materials, these excitations could dominate spin and charge transport in topological insulators. Our results provide a novel route to creating a potentially ideal quantum wire in a bulk solid. [Preview Abstract] |
Session T24: Focus Session: Excitonic Effects in Nanotubes
Sponsoring Units: DMPChair: Oleg Prezhdo, University of Washington
Room: 326
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T24.00001: Time-Domain Ab Initio Studies of Photoinduced Electron-Phonon Dynamics in Carbon Nanostructures. Invited Speaker: The electron-phonon interactions in carbon nanotubes and nanoribbons determine the response times of optical switches and logic gates, the extent of heating and energy loss in nanowires and field-effect transistors, and even a superconductivity mechanism. We have developed state-of-the-art non-adiabatic molecular dynamics techniques and implemented them within time-dependent density functional theory in order to model the ultrafast photoinduced processes in carbon nanostructures at the atomistic level and in real time. Our ab initio studies directly mimic the experimental data and reveal many intriguing features of the excitation dynamics, including non-radiative fluorescence quenching, fast intrinsic intraband relaxation, phonon-induced component of fluorescence linewidths, the importance of defects, the dependence of the relaxation rates on the excitation energy and intensity, spin-orbit interaction and a detailed understanding of the role of active phonon modes. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T24.00002: Exciton Spectroscopy and Absorption Cross-section of Individual Single-Walled Carbon Nanotubes Invited Speaker: Semiconducting Single-Walled Carbon Nanotubes (SWNTs) display intrinsic exciton luminescence which is highly sensitive to the nanotubes environment. For instance single-molecule chemical reactions with individual SWNTs could be observed through the stepwise changes of the luminescence intensity within submicrometer segments of single nanotubes. Analysis of the step amplitudes revealed an exciton diffusion range of $\sim $90 nm. Each exciton thus visits approximately 10$^{4}$ atomic sites during its lifetime, providing highly efficient sensing of local chemical and physical perturbations [1]. SWNT luminescence decays are also sensitive to extrinsic factors. Using highly luminescent individual (6,5) SWNTs, time-resolved spectroscopy revealed however systematic biexponential luminescence decays, with short and long lifetimes around 45 and 250 ps. This intrinsic behavior is attributed to the band-edge exciton fine structure with a dark level lying a few meV below a bright one. Combining such time-resolved studies with cw luminescence ones, the absorption cross-section of individual SWNTs was determined. A mean value of $\sim $1.10$^{-17}$ cm$^{2}$ per carbon atom is obtained for (6, 5) tubes excited at their second optical transition [2]. This was further corroborated by independent photothermal heterodyne measurements. Because this highly sensitive method relies only on light absorption, it readily detects metallic nanotubes as well as the emissive semiconducting species in various environments and allowed recording for the first time images and absorption spectra of individual SWNTs of both types [3]. \\[4pt] [1] Cognet et al \textit{Science} \textbf{316}, 1465 (2007) \\[0pt] [2] Berciaud et al \textit{Phys.Rev.Lett.} \textbf{101}, 077402 (2008) \\[0pt] [3] Berciaud et al \textit{Nanoletters} \textbf{7}, 1203 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T24.00003: Exciton Distribution between the Bright and Dark States in Single Carbon Nanotubes Studied by Magneto-Photoluminescence Spectroscopy Ryusuke Matsunaga, Kazunari Matsuda, Yoshihiko Kanemitsu We have performed micro-photoluminescence (PL) spectroscopy for single carbon nanotubes under magnetic fields at various temperatures. Sharp PL spectra of single carbon nanotubes allow us to directly observe the dark exciton PL peak a few meV below the bright exciton PL peak due to the Aharonov-Bohm effect [1]. From the PL intensity ratio of the dark to the bright excitons under magnetic fields, we found that the non-equilibrium (non-Boltzmann) distribution occurs between the bright and dark states, because phonons cannot scatter excitons between the two states with different parities [2]. Furthermore, we discuss the diameter dependence of the exciton population of the bright and dark states in single carbon nanotubes. [1] R. Matsunaga, K. Matsuda, and Y. Kanemitsu, Phys. Rev. Lett. \textbf{101}, 147404 (2008). [2] V. Perebeinos, J. Tersoff, and Ph. Avouris, Nano Lett. \textbf{5}, 2495 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T24.00004: Chiral dependence of K-momentum exciton energies in carbon nanotubes P.M. Vora, X. Tu, M. Zheng, J.M. Kikkawa Fifteen of the sixteen excitons in the $E_{11}$ manifold of carbon nanotubes are nominally dark. Of these, the zero-momentum dark singlet has received the most experimental attention because it exhibits magnetic brightening. By contrast, here we focus on the $K$ and $K'$-momentum dark singlets. Absorptive ($X_{2})$ and emissive ($X_{1})$ sidebands appearing at $\sim $0.2 eV above and $\sim $0.13 eV below the bright exciton, respectively, have been interpreted in numerous ways in the literature. Recently, members of our group studied these sidebands in a (6,5) nanotube and concluded they could be used to energetically locate the $K$-momentum excitons (Torrens, et al, PRL 101, 157401 (2008)). Here we use a combination of experiment and theory to study $X_{1}$ and $X_{2}$ in at least ten samples that are highly purified in a single chirality and use these findings to study how the $K$-momentum exciton energy depends on chirality. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T24.00005: Controlling Optimal Excitation Wavelength of Energy Transfer from Photo-Excited Polymers to Single-Walled Carbon Nanotubes. Lain-Jong Li, Fuming Chen, Mingli Jia, Li Wei, Yuan Chen, M. B. Chan-Park, Andong Xia The optimal excitation wavelength for the energy transfer from aromatic polymers poly(9,9-dioctylfluoreny- 2,7-diyl) (PFO) to single-walled carbon nanotubes (SWNTs) is tunable in a wide wavelength range (from 388 to 480 nm) depending on the concentration of excess PFO polymers. The concentration governs the aggregation state and chain conformation of the polymers proximate to SWNT surfaces, which in turn alters the optimal excitation wavelength. This study suggests an exciting and convenient method of adjusting the desired optical wavelengths for the energy conversion, useful for polymer-SWNT composites in optoelectronic applications. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T24.00006: Exciton Radiative Lifetimes and Their Temperature Dependence in Single-Walled Carbon Nanotubes Yuhei Miyauchi, Ryusuke Matsunaga, Hideki Hirori, Kazunari Matsuda, Yoshihiko Kanemitsu We have investigated the radiative lifetimes of excitons in single-walled carbon nanotubes (SWNTs) from simultaneous measurements of the photoluminescence (PL) lifetimes [1] and the PL quantum yields. A high-quality sample of PFO dispersed-SWNTs was used for the PL measurements. The evaluated radiative lifetimes were ${\sim5-15}$ ns for SWNTs with diameters ${\sim0.8-1.1}$ nm at room temperature. The radiative lifetimes increased with the tube diameter. The exciton spatial coherence volume (length) was of the order ${10} ^{2}$ nm along the tube axis, as deduced from the radiative lifetimes. Furthermore, we discuss the dynamics of bright and dark excitons [2] from the temperature dependence of the radiative lifetime (10 to 300 K).\\[3pt] [1] H. Hirori, K. Matsuda, Y. Miyauchi, S. Maruyama, and Y. Kanemitsu, Phys. Rev. Lett. ${\bf 97}$, 257401 (2006). \\[0pt] [2] R. Matsunaga, K. Matsuda, and Y. Kanemitsu, Phys. Rev. Lett. ${\bf 101}$, 147404 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T24.00007: Effects of Exciton-Exciton Annihilation in Fluorescence of Individual SWCNTs Anni Siitonen, Sergei Bachilo, Dmitri Tsyboulski, R. Bruce Weisman Most studies of SWCNT exciton relaxation have been performed on bulk samples, with clear conclusions hampered by the variety of structural types, lengths, and aggregation states. To avoid such problems, we use near-IR fluorescence microscopy to study nearly pristine individual nanotubes with optically resolvable lengths. We find emission proportional to excitation at low intensities. But for stronger excitation, an increasingly sub-linear dependence is observed, due to exciton-exciton annihilation within single nanotubes. Since annihilation depends on exciton lifetime and mobility, these parameters can be studied by analyzing measured intensity dependences. We compare data on exciton excursion ranges and emission efficiency in individual SWCNTs to numerical simulations to quantify exciton lifetime and diffusion for a variety of (n,m) structures. Preliminary results yield lifetimes of a few nanoseconds for nearly pristine, highly emissive nanotubes and reveal some dependence of lifetime on nanotube diameter. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T24.00008: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T24.00009: Exciton Dynamics in (6,5) carbon nanotubes Andy Walsh, Jude Schneck, Alex Green, Mark Hersam, Sidney Redner, Lawrence Ziegler, Anna Swan Single color (E22) pump-probe data on a solution of (6,5) nanotubes reveal that use of pulses shorter than the dephasing time scale precludes the formation of multiple excitons on a single nanotube. Subsequent relaxation dynamics of the single exciton exhibits stretched exponential behavior, and data from low to saturation fluence, and over 3 order of magnitude of time delay, is described by the same model. The stretched exponential model with its implication of a distribution of decay rates is attributed to a distribution of length-dependent effective lifetimes due to end-quenching via diffusion. Results give values for the dipole moment, E22 dephasing time and E11 diffusion coefficient. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T24.00010: Exciton Dynamics in Individual Single-walled Carbon Nanotubes Marat Khafizov, Shujing Wang, Lisa J. Carlson, Todd D. Krauss, Ming Zhang Optical excitation of single-walled carbon nanotubes (SWNTs) results in strongly bound excitons. The dynamics and energetic pathways available to the exciton as it relaxes back to the ground state have recently received significant attention. We have performed transient absorption (TA) experiments on DNA-wrapped (6,5) SWNTs in the extremely low-excitation fluence regime. Excitation was provided by a Ti-sapphire oscillator whose output was focused into a highly nonlinear photonic crystal fiber generating a coherent, femtosecond white-light source. We found the recovery of the photobleach signal for excitons in the 1st and 2nd excited states (E$_{11}$and E$_{22})$ was governed by power-law dynamics. Interestingly, we also observed an induced absorption feature in the TA spectrum to the blue of the E$_{11}$ exciton that showed the same recovery dynamics as the photobleach signal, suggesting that they share a common origin. We will discuss the physical origins of the observed features in the TA spectrum in the context of current models of exciton states of the SWNT. [Preview Abstract] |
Session T25: Focus Session: Graphene XI: Scanning Probes I
Sponsoring Units: DMPChair: Eva Andrei, Rutgers University
Room: 327
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T25.00001: Scanning Kelvin Probe Study of Electric Field Effect Tuning of Graphene Work function Young-Jun Yu, Yue Zhao, Kwang S. Kim, Philip Kim We present the experimental work on the wok function variation of mono and bi-layer graphene device measured by scanning Kelvin probe microscopy (SKPM). Using the electric field effect (EFE), the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Mono and bi-layer graphene samples are deposited on a silicon oxide covered silicon substrate by a mechanical exfoliation method and electrical contacts are fabricated by electron beam lithography. The underlying silicon substrate is used as a back gate to tune the carrier concentration of the graphene. After subtracting off the large back ground signal originating from the electrostatic environment, we obtain the work function of graphene samples modulated by the gate voltage. The change of work function can be ascribed by the Fermi level shift due to the EFE induced carrier doping and well quantified by the electronic band structure of mono and bi-layer graphene. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T25.00002: STM Studies of Graphene Films Prepared by Sonication-Assisted Dispersion Elena Stolyarova, Kwang Taeg Rim, George Flynn We present STM (Scanning Tunneling Microscopy) studies of thin graphene films prepared by a spraying technique via direct~exfoliation of graphite in organic solvent (dimethylformamide). Our results show that these films are a patchwork of an unperturbed hexagonal graphene network mixed with heavily functionalized areas. The area of graphene-like spots does not exceed 10nm$^{2}$. Further, we discuss the influence of chemical reduction of these films on their atomic structure. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T25.00003: Moir\'{e} pattern for graphene on a graphite surface and its scanning tunneling spectroscopy Adina Luican, Guohong Li, Eva Andrei We report low temperature high magnetic field scanning tunneling microscopy and spectroscopy on a graphene layer rotated with respect to the orientation of a graphite substrate, which produces distinct Moir\'{e} patterns. Scanning tunneling spectroscopy in the rotated area reveals that the tunneling conductivity~has two pronounced peaks flanking the Dirac point. Comparison to a theoretical model [1] shows that the two peaks are the signature of rotated layers and their separation reflects the angle of rotation. We will discuss spatial variations of the tunneling spectra within the Moir\'{e} pattern and their dependence on magnetic field. [1] J\textit{. M. B. Lopes~dos~Santos, N. M. R. Peres, and A. H. Castro~Neto, Phys. Rev. Lett. 99, 256802 (2007)} [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T25.00004: Low temperature scanning probe imaging of electron transport in graphene nanostructures Jesse Berezovsky, Robert Westervelt Experiments on the transport properties of graphene over the last several years have revealed numerous unusual and fascinating results. These studies typically rely on lithographically patterned contacts and gates that can obscure effects arising due to spatially varying properties. Using a biased scanning probe tip, we can create a local gate or scattering potential and observe the resulting change in the transport properties of a graphene structure. Simulations show that this technique can image the fluctuating potential in a graphene sheet with a spatial resolution of tens of nanometers. By patterning the graphene into a nanowire or nanoconstriction, the local potential of the tip may be used to probe the dependence of the confinement-induced energy gap on the local atomic structure of the edges. These types of measurements provide a means for directly imaging the flow and behavior of electrons in graphene devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T25.00005: Imaging local potential and conductance variation around a tip gate in a graphene device with electrostatic force and scanning gate microscopes J.S. Chae, S. Jung, N.B. Zhitenev, J.A. Stroscio, Y. Kuk We fabricated graphene devices on a SiO$_{2}$ layer with 4-6 metallic contacts and a Si back gate. These devices revealed the well-known source-drain current versus gate-bias dependence with slight variation of the Dirac point upon gas adsorption on the graphene surfaces. They were inserted into an ultrahigh vacuum low temperature atomic force microscope (AFM), with which electrostatic force microscopy and scanning gate microscopy could be performed. The potential drops around the source and the drain contacts were carefully measured to estimate the barrier heights between the metallic contacts and the graphene. Using an AFM cantilever as a local gate, we measured the variation of source-drain current. This result showed some local variation, suggesting existence of electron and hole puddles. We also measured electron or hole scattering around defect states with nanometer resolution. These scanning probe microscopy results are compared with those performed by macroscopic transport measurement. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T25.00006: STM signatures for magnetic impurities in graphene Bruno Uchoa, L. Yang, S.-W. Tsai, N. M. R. Peres, A. H. Castro Neto Graphene is a two dimensional allotrope of carbon, whose elementary excitations are massless Dirac fermions which propagate ballistically in the submicron scale. With the adsorption of adatoms, such as transition metals, or simple molecules, the formation of local magnetic moments in graphene can be controlled by the application of a gate voltage [1], making graphene a potential candidate for spintronics. Unlike semiconductors, where the location of the magnetic impurities is random, in graphene the adatoms can be positioned by a scanning tunneling microscope (STM), allowing the construction of magnetic lattices. In this seminar, I will discuss the STM signatures for local magnetic impurities in graphene and promissing applications for spintronic devices. [1]B. Uchoa et al., PRL 101 026805 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T25.00007: Magnetic Oscillations in Scanning Tunneling Spectroscopy of Epitaxial Graphene on SiC Kevin D. Kubista, David L. Miller, Gregory M. Rutter, Ming Ruan, Walt A. de Heer, Phillip N. First, Joseph A. Stroscio Scanning tunneling microscopy (STM) and spectroscopy (STS) at a temperature of 4 K are used to study the electronic properties of epitaxial graphene on SiC in a magnetic field perpendicular to the graphene plane. While changing the magnetic field we observe Shubnikov de Haas-like magnetic oscillations in the tunneling conductance, dI/dV. The peak positions of these tunneling magnetic oscillations (TMO) vary periodically with inverse magnetic field, indicating they sample a constant cross-section of the graphene k-space. This new magnetic oscillation method can map extended parts of the electronic band structure of graphene as we vary the tunneling energy in the dI/dV measurement. This is in contrast to traditional magnetic oscillations which typically only probe the Fermi level. This work was supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T25.00008: Phonons, plasmons and impurities in Graphene probed with STM Victor Brar, Yunabo Zhang, Melissa Panlasigui, Caglar Girit, Alex Zettl, Michael Crommie It has been shown that the electronic structure of graphene is altered by interactions with plasmons, phonons and impurities. We probe such interactions at the atomic scale using scanning tunneling spectroscopy measurements on gated graphene flakes at 4.2K in an UHV environment. Our measurements show that collective excitations can be observed in the graphene tunneling spectra. By varying the voltage on the backgate of our devices, we are able to measure the charge density dependence of these features. We further probe atomic scale variations in the LDOS of graphene caused by impurities on the surface. We analyze our results in terms of graphene 2D electronic structure. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T25.00009: Kondo Effect for Massless Dirac Fermions in Graphene L. S. Mattos, C. R. Moon, P. B. Van Stockum, J. C. Randel, H. C. Manoharan, M. W. Sprinkle, C. Berger, W. A. de Heer, K. Sengupta, A. V. Balatsky We experimentally probe the addition of the spin degree of freedom to the local physics of graphene. This real spin degree of freedom is expected to become intertwined in various coherent scattering processes involving pseudospin and chirality intrinsic to single monolayers of graphene. A compendium of theoretical work on the interaction of localized spins with Dirac electrons predicts that low-temperature screening of localized magnetic moments by nodal quasiparticles gives rise to an unconventional Kondo effect, in which a critical exchange coupling can be controlled by charge carrier doping. We report the observation of this elusive Kondo ground state of Dirac particles, using scanning tunneling microscopy of adsorbed magnetic atoms on epitaxial graphene at low temperature. Tunneling spectroscopy and quasiparticle interference maps reveal chiral, linearly dispersing carriers that form sharp bimodal resonances around individual impurities, an effect traceable to Kondo screening of spins centered on two different graphene lattice sites. Using Fourier-transform scanning tunneling spectroscopy and concomitant measurements in a high magnetic field, we deduce the origin of these many-body ground states. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T25.00010: Atomic-scale scanning tunneling microscopy and spectroscopy studies of nanometer-sized graphene on the Si(111)-7x7 surface. Justin Koepke, Joseph Lyding We have used ultrahigh vacuum scanning tunneling microscopy to perform atomic-level studies of graphene on the Si(111)-7x7 surface. We used a dry contact transfer technique (DCT) developed by Albrecht and Lyding [1] to deposit mechanically exfoliated graphene in-situ [2] onto atomically clean Si(111)-7x7 surfaces. The DCT method deposits single, double, and thicker layers of atomically clean graphene. We observe varying degrees of transparency of the graphene monolayers and bilayers on the Si(111)-7x7 surface, where the substrate atomic structure is clearly seen through the graphene. We believe that the electronic structure of a graphene monolayer on the Si(111)-7x7 surface leads to the transparency of monolayers and bilayers, similar to the findings of Rutter, et al [3]. Room-temperature scanning tunneling spectroscopy (STS) measurements of the graphene monolayers and bilayers on the Si(111)-7x7 surface show predominantly metallic behavior. [1] P.M. Albrecht and J.W. Lyding, Appl. Phys. Lett. 83, 5029 (2003) [2] K.A. Ritter and J.W. Lyding, Nanotechnology 19, 015704 (2008) [3] G.M. Rutter, et al, Phys. Rev. B 76, 235416 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T25.00011: Atomic-scale studies of nanometer-sized graphene on III-V semiconductors using scanning tunneling microscopy. Kevin He, Justin Koepke, Joseph Lyding We utilize the Dry Contact Transfer (DCT) method [1] to deposit nanometer-sized, monolayer graphene flakes, \textit{in situ}, onto cleaved GaAs (110) and InAs (110) surfaces. The flakes were characterized using a homebuilt, room temperature, ultrahigh-vacuum scanning tunneling microscope. We report on the apparent electronic semi-transparency of the monolayer graphene flakes, such that the underlying III-V semiconductor lattice is revealed in our topographic images. This transparency is strongly dependent on the applied sample bias, similar to results seen on SiC (1000) for large sheets of graphene grown via thermal desorption [2]. \\[3pt] [1] P.M. Albrecht and J.W. Lyding, APL 83, 5029 (2003). \\[0pt] [2] G.M. Rutter et al, Phys. Rev. B 76, 235416 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T25.00012: Epitaxial Graphene on Co(0001) Probed by STM Measurements and First Principles Calculations Deborah Prezzi, Daejin Eom, Kwang T. Rim, Hui Zhou, Michael Lefenfeld, Colin Nuckolls, Mark Hybertsen, Tony Heinz, George Flynn Structural and electronic properties of finite-sized graphene patches on Co(0001) have been investigated through a combined experimental and theoretical characterization. The analysis of low-temperature scanning tunneling microscopy images establishes an atomically uniform epitaxial configuration of graphene on the Co surface in which a C atom is a-top the interface Co atom, in agreement with total energy calculations based on a density-functional theory (DFT) approach. Scanning tunneling spectroscopy measurements show that the electronic properties of the interface are significantly different from both the clean Co surface and isolated graphene, suggesting a strong electronic coupling at the interface. DFT calculations provide a detailed analysis of the spectrocopic features in terms of spin and site contributions and reveal the coupling between graphene p and Co d states. [Preview Abstract] |
Session T26: Focus Session: Graphene XII: Synthesis and Growth
Sponsoring Units: DMPChair: Taisuke Ohta, Sandia National Laboratories
Room: 328
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T26.00001: Evidence for graphene growth by C cluster attachment Elena Loginova, Norman C. Bartelt, Peter J. Feibelman, Kevin F. McCarty Until now the detailed mechanisms of graphene growth have not been experimentally determined, owing to limitations of the available experimental techniques. We study the epitaxial growth of graphene on Ru(0001) measuring simultaneously the growth rate of individual graphene islands and the local, absolute concentration of vapor-deposited, mobile carbon adatoms. We have learned what controls the nucleation and growth rate of graphene, and what species transport carbon over the metal surface. We find that the growth rate is limited by C-atom attachment, not by C-atom diffusion, and that the absolute value of the supersaturation required for appreciable growth rates is comparable to that required to nucleate new islands. Thus, a large barrier must exist for monomers to attach to the graphene step edge. The growth rate as a function of supersaturation is highly nonlinear. Such behavior can be explained if carbon clusters must form as precursors to carbon attachment. As experiment and theory reveal, this could arise from strong bonding of individual monomers to the metal substrate. We will discuss a model that explains and provides insight into the molecular processes by which graphene grows. This research is supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U. S. D. O. E. under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T26.00002: Large Scale Graphene Synthesized on Metal and Transferred to Insulators: Material and Electronic Properties Yong P. Chen, H. Cao, D. Pandey, I. Childres, D. Zemlyanov, V. Drachev, R. Reifenberger, Q. Yu, S. Siripongert, S. Pei, J. Lian, H. Li We report a systematic study of the material and electronic properties of large scale graphene films grown on metal then transferred to insulator substrates. Few-layer graphene films as large as several cm's in size are grown by cooling-induced surface segregation on Ni under ambient pressure (Q. Yu \textit{et al}., APL \textbf{93}, 113103 (2008)). The Ni is subsequently etched by acid and graphene film transferred on thin SiO$_{2 }$ on doped Si wafer. TEM and STM images show the expected graphitic lattice structure locally with atomic resolution. XPS and Raman spectroscopies further confirm the high quality of transferred graphene films. At larger scale, various SPM and optical imaging reveal non-uniform thickness and considerable height fluctuation, with the film consisting with domains ($\sim $1 $\mu $m in size) separated by elevated ridges. Using the doped Si as backgate, we observe moderate field effect in such transferred graphene films. Magnetotransport at variable temperatures show negative magnetoresistance at low magnetic field and characteristic features of weak localization in graphene, allowing us to extract information on carrier scattering in such large scale graphene. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T26.00003: Investigation of the early stages of graphene formation on 6H-SiC J. R. Skuza, C. Clavero, K. Yang, B. Wincheski, R. A. Lukaszew The predicted and/or observed unique properties of graphene have sparked tremendous research efforts to develop graphene-based ultra-high speed electronic and optical devices. The most promising technique to fabricate epitaxial graphene to date is via high temperature sublimation of atomic layers of Si from monocrystalline SiC substrates [1,2]. However, this approach leads to rough surfaces and little work has been done to investigate graphene nucleation during the early stages of growth. We have used atomic force microscopy, scanning electron microscopy, and Raman spectroscopy to investigate the early stages of graphene nucleation and surface evolution when annealing semi-insulating and n-type doped 6H-SiC substrates under low vacuum ($\sim $ 10$^{-3}$ Torr) and ultra-high vacuum (10$^{-9}$ Torr) regimes. Scaling laws applied to the surface evolution in these two cases will be compared. [1] I. Forbeaux \textit{et al.}, Phys. Rev. B \textbf{58}, 16396 (1998). [2] C. Berger \textit{et al.}, J. Phys. Chem. B \textbf{108}, 19912 (2004). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T26.00004: High-yield production of graphene sheets by chemical exfoliation of graphite Xiaohong An, Swastik Kar, Morris Washington, Saroj Nayak Graphene, a single atomic layer of graphite, has attracted vast interest recently owing to its perfect two-dimensional crystallographic nature, which have resulted in intensive investigations of fundamental physics and promising applications. Up to now, several techniques have been used to produce small areas of graphene, such as mechanical methods, exfoliation, epitaxial growth method and reduced graphene from graphene oxide. However, chemical approaches for high-yield production of graphene sheets is still absent. Here, we report that graphene dispersion produced by chemical exfoliation of graphite in solvent of 1-pyrenecarboxilic acid in water. We confirm the presence of monolayer graphene sheets by Scanning transmission electron microscopy and Raman spectroscopy. Large area of graphene sheets on SiO$_{2}$/Si substrate can be obtained by evaporating the graphene dispersion in oven and rinsing with methanol. We demonstrate the high-yield production of graphene sheets by optical microscopy and scanning electron microscopy. Electrical and other applications of graphene developed this way are currently being investigated. This new graphene processing of chemical exfoliation of graphite could lead to applications in future scalable graphene nano-electronics devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T26.00005: Scalable chemical vapor deposition of single- and few-layer graphene Lewis Gomez De Arco, Yi Zhang, Akshay Kumar, Chongwu Zhou We report the implementation of a simple and scalable method to prepare single and few-layer graphene films by chemical vapour deposition. Micro Raman spectroscopy analysis of the synthesized films revealed the presence of single and few-layer graphene domains throughout the substrate. Synthesized graphene films were recovered on Si/SiO$_{2}$ substrates where back-gated FETs were fabricated. Four-probe measurements revealed sheet resistance of $\sim $68 k$\Omega $/sq for the recovered films. I$_{DS}$-V$_{DS}$ and transfer characteristics indicate a weak p-type behavior in the films and weak modulation of the drain current by the gate bias. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T26.00006: Temperature-dependence of Epitaxial Graphene Formation on SiC(0001) Luxmi Luxmi, Nishtha Srivastava, Patrick Fisher, Randall Feenstra, Jakub Kedzierski, Yugang Sun, Gong Gu The formation of epitaxial graphene on SiC(0001) (the \textit{Si-face}) is studied using atomic force microscopy, Auger electron spectroscopy, low energy electron diffraction/microscopy, Raman spectroscopy, and electrical measurements. Starting from hydrogen-etched surfaces, graphene formation by vacuum annealing is observed to begin at about 1150\r{ }C, with the overall step-terrace arrangement of the H-etched surface being preserved but with significant roughness (pit formation) on the terraces. At temperatures near 1350\r{ }C, the surface morphology changes into relatively large flat terraces covered with several layers of graphene and containing a few large pits, with the terraces separated by step bunches. On the terraces the graphene thickness varies by typically $\pm $1 monolayer. At higher temperatures the graphene film is observed to buckle and break up, presumably due to thermal mismatch with the SiC. Field-effect mobilities as high as 4200 cm$^{2}$/Vs for few-layer graphene films are found. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T26.00007: Epitaxial Graphene Formation on SiC(000$\bar {1})$ Nishtha Srivastava, Luxmi Luxmi, Patrick Fisher, Randall Feenstra, Jakub Kedzierski, Yugang Sun, Gong Gu The formation of epitaxial graphene on SiC(000$\bar {1})$ (the \textit{C-face}) is studied using atomic force microscopy, spatially resolved Auger electron spectroscopy, low energy electron diffraction, Raman spectroscopy, and electrical measurements. Starting from hydrogen-etched surfaces, graphene formation by vacuum annealing is observed over the temperature range 1200-1400\r{ }C. Unlike the situation for the Si-face, it is found for the C-face that the initial graphene formation is three-dimensional. Micron-size islands with height of several nm are formed, with the graphene being \underline {thinner} on these islands than between the islands. At higher formation temperatures the graphene layer becomes relatively flat, and has typical thickness of $>$10 monolayers. Electron diffraction indicates rotational disorder, with $\pm $15\r{ }-oriented spots observed in addition to the known $\pm $2.2\r{ }-spots.$^{2}$ Field-effect mobilities as high as 4400 cm$^{2}$/Vs for multi-layer graphene films are found, with relatively good homogeneity over the wafer. $^{2}$J. Hass et al., Phys. Rev. Lett. 100, 125504 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T26.00008: Band-gap tuning through progressive oxidation of graphene Tanesh Bansal, Aditya Mohite, Bruce Alphenaar, Jacek Jasinski, Mahendra Sunkara Graphene has a high electron mobility at room temperature, making it attractive for device applications. Because graphene is a zero-gap semiconductor, it is challenging to modulate its conductance using a field effect gate. Oxidation of graphene opens up a band gap, transforming oxidized graphene into an insulator. However, theory also suggests that there are a range of stable oxidation states corresponding to different oxygen coverage on the surface. Here, we demonstrate that it is possible to tune the band-gap of oxidized graphene by varying the surface oxygen concentration. Commercially obtained KISH graphite was converted to graphite oxide by treatment with a mixture of sulfuric acid and nitric acid. Oxidized graphene sheets were dispersed on quartz substrates following sonication and centrifugation of the graphite oxide. Using photocurrent spectroscopy the energy gap of individual oxidized graphene flakes were observed to increase from 0.62 eV to 0.69 eV with increasing oxidation time. Band-gap measurements were correlated with the surface oxygen concentration using XPS, UPS and FTIR. \textit{ONR N00014-06-1-0228} [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T26.00009: Morphology and Electrical Characterization of Reduced Epitaxial Graphene Oxide Yike Hu, Xiaosong Wu, Michael Sprinkle, Nerasoa Madiomanana, Ming Ruan, Claire Berger, Walter de Heer We present results for on-chip oxidation of epitaxial graphene and sequential reduction of the insulating graphene oxide layers. In our previous work , we have used the Hummer's method to oxidize epitaxial graphene and used electron beam exposure and heat treatment to reduce the epitaxial graphene oxide (EGO) band gap by changing the degree of oxidation. Here we further explore various oxidation and reduction methods on epitaxial graphene. EGO is characterized by atomic force microscopy, low-energy electron diffraction, ellipsometry, and Raman Spectrometry. Mobility measurements of patterned structures are presented where epitaxial graphene layers pads are seamlessly connected to EGO ribbons. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T26.00010: Preparation of macroscopic graphene oxide membranes Zhengtang Luo, Ye Lu, Luke Somers, A.T. Charlie Johnson Graphene oxide membranes up to 2000 square micrometers in size can be synthesized with 90 {\%} yield in bulk quantities through a microwave assisted chemical method. Membranes are readily visualized on oxidized silicon substrate, which enables efficient fabrication of electronic devices and sensors. Field effect transistors made of the membrane show ambipolar behavior, and their conductivity is significantly higher than previously reported values. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T26.00011: Optical and diamagnetic anisotropy of graphene oxide A.L. Exarhos, P.M. Vora, Z. Lou, A.T. Johnson, J.M. Kikkawa We have recently shown that graphene oxide (GO) emits a broad photoluminescence (PL) band in both solid and aqueous preparations. The origin of this PL is not yet well understood, but for absorptive and emissive optical processes originating in the two dimensional GO plane, one expects an in-plane polarization. Studies of optical anisotropy can therefore help to clarify the origin of the PL. Here we use a method of optical nanomagnetometry (Torrens, et al, JACS 129, p. 252 (2007)) to extract these quantities, also determining the magnetic anisotropy. We find that when aqueous preparations of GO are placed in a magnetic field, diamagnetically induced alignment leads to marked linear polarization anisotropy of absorbance and photoluminescence. By taking six optical measurements at each magnetic field, we are able to extract the intrinsic polarization anisotropies of optical absorption and emission of GO flakes and to quantify the orbital diamagnetic anisotropy. We discuss how these quantities give insight into electronic delocalization in these systems. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T26.00012: Graphite oxide as a nanoscale dielectric Brian Standley, Anthony Mendez, Emma Schmidgall, Marc Bockrath Graphite oxide's ease of deposition and graphene-like properties when chemically reduced make it a promising electronic material. To complement this effort, we are studying graphite oxide as a potential dielectric for nanoscale devices. While unreduced graphite oxide is known to have a sheet resistance in the G$\Omega$ range, its out-of-plane conductivity has yet to be measured. We have fabricated ultrathin capacitors from graphite oxide sheets, and will present our efforts to measure its leakage current and breakdown electric field, thus providing an assessment of its potential as a gate insulator. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T26.00013: Diffusion and Self-alignment of Atomic Oxygens on the Graphene Surfaces: First Principles Calculations Takazumi Kawai, Yoshiyuki Miyamoto Graphene is attracting much attention for the application of nano-devices due to its interesting electronic properties, and its robustness. For the device applications, it is very important to know the behaviors of atmospheric molecules such as adsorption, diffusion, and desorption on the graphene surface since the reaction with such chemicals cause the significant change in the electronic properties at Fermi level and even break the $sp^2$ network. Here, the oxygen is one of the most important impurities that we want to know and control the behavior. In this paper, we performed density function calculations for the diffusion of atomic oxygens on a graphene sheet in a periodic boundary condition. The results for a single atomic oxygen in our calculations are consistent with the previous works with cluster models. However, the favorable adsorption site for the next oxygen atom and diffusion barriers are completely different from them. The atomic oxygens prefer to align along armchair direction but not zigzag one. We will further discuss the stability and diffusion of the next oxygen atom on the other side of the graphene. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T26.00014: Large area graphene growth on 6H-SiC(0001) L.I. Johansson, C. Virojanadara, M. Syv\"aj\"arvi, R. Yakimova, A.A. Zakharov, T. Balasubramanian Large area graphene growth on commercial Si-face on-axis 6H-SiC(0001) is demonstrated in this work. Samples were produced in a prototype of an inductively heated furnace. The growth was carried out in strongly isothermal conditions at a temperature of 2000 C and at an ambient argon pressure of 1 atm. The quality and thickness of the graphene layers grown, using this \textit{ex situ} method, were investigated using PES, ARPES), LEED as well as LEEM, PEEM micro-LEED and micro-PES at specifically defined small areas. Our results show that single layer graphene is formed over quite large areas on the sample but that two different domains can exist on some parts. A comparison with an \textit{in situ} graphene sample, prepared by resistive heating to 1275 C, was made. The results then obtained were similar to earlier findings [1-2] and showed that the size of the graphene flakes were very small compared to those obtained on the samples prepared with our \textit{ex situ }method. \\[3pt] [1]. T. Ohta, F. El Gabaly, A. Bostwick, J.L. McChesney, K.V. Emtsev, A.K. Schmid, Th. Seyller, K. Horn, E. Rotenberg, New. J. Phys. \textbf{10} 023034 (2008).\\[0pt] [2]. J.B. Hannon and R. M. Tromp, Phys. Rev. B\textbf{ 77} 241404 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T26.00015: Epitaxial graphene: Structure, growth and molecular interactions Andrew Wee, Wei Chen, Siew Wai Poon, Han Huang, Shi Chen, Dongchen Qi, Eng Soon Tok, Kian Ping Loh The discovery of graphene has opened up a new paradigm in nanoelectronics that could offer better performance than conventional semiconductor devices. We used \textit{in situ} scanning tunnelling microscopy (STM), synchrotron synchrotron radiation techniques and density functional theory (DFT) calculations to investigate the structure of the various reconstructions of 6H-SiC(0001) prior to its thermal decomposition to form epitaxial graphene (EG). Using Co-decoration technique coupled with STM, the evolution of EG was found to preferentially begin at SiC step edges and occurs with the loss of Si and breakdown of the C-rich ($\surd $6$\times \surd $6)$R$30\r{ } template, which provides the C source for graphene growth. The C-rich phase that forms at the interface acts as a buffer layer for graphene from the underlying bulk SiC. We show that the transition from monolayer to trilayer EG adopts a bottom-up growth mechanism. With increasing annealing temperature, the fluorescence yield of Si $K$-edge NEXAFS indicates an increase in disorder of Si atoms in the SiC substrate beneath the surface due to out-diffusion of Si atoms to the surface forming increased Si vacancies. We also show that EG thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, F4-TCNQ. [Preview Abstract] |
Session T27: Focus Session: Pulsed Laser Deposition of Electronic and Photonic Thin Films and Nanostructures
Sponsoring Units: FIAPChair: R. D. Vispute, University of Maryland, College Park
Room: 329
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T27.00001: Plasma Energetic in Pulsed Laser Deposition and Pulsed Electron Deposition Invited Speaker: Surface bombardment by energetic particles strongly affects thin film growth and allows surface processing under non-thermal equilibrium conditions. Deposition techniques enabling energy control can effectively manipulate the microstructure of the film and tune the resulting mechanical, electrical and optical properties. At the high power densities used for depositing stoichiometric films in the case pulsed ablation techniques such as Pulsed Laser Deposition (PLD) and Pulsed Electron Deposition (PED), the initial energetic s of the material flux are typically in the range of 100s of eV, much higher than the optimal values ($\leq$ 10 eV) required for high quality film growth. To overcome this problem and to facilitate particle energy transformation from the original as-ablated to the one optimal for film growth, one needs to carefully select the ablation conditions, conditions for material flux propagation through a process gas (or vacuum)and location of the growth surface (substrate) within this flux. In this talk, I will discuss the energetics of the propagating materials flux in the case of PLD and PED, and identify parameters that require critical control for realizing optimum thin film growth. As an example, growth optimization of epitaxial GaN films is provided. PED is complementary to PLD and exhibits an important ability to ablate materials that are transparent to laser wavelengths typically used in PLD. Some examples include wide band gap materials such as SiO2, Al2O3, MgO etc. Both PLD and PED features can be integrated within a single deposition module. PLD-PED systems enable in-situ deposition of a wide range of materials required for exploring the next generation of complex structures that incorporate metals, complex dielectrics, ferroelectrics, semiconductors and glasses. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T27.00002: Interface-controlled thin film growth of conjugated polymers via pulsed laser deposition R.K. Gupta, K. Ghosh, Suchi Guha Matrix-assisted pulsed laser evaporation, a derivative of pulsed laser deposition (PLD), is an alternative method of depositing polymer and biomaterial films that allows homogenous film coverage of high molecular weight organic materials for a layer-by-layer growth without any laser induced damage. Polyfluorene (PF) -based conjugated polymers have attracted considerable attention in blue-emitting displays. Di-octyl substituted polyfluorene (PF8), its copolymers, and thiophene- based polymers were deposited as thin films using matrix- assisted PLD by employing a KrF excimer laser. The optical and structural properties of these films are compared with spincoated films via Raman spectroscopy, absorption and photoluminescence. The Raman spectra of both PLD and spincoated films are similar indicating that the polymer films deposited via PLD maintain their molecular structure. We further discuss the application of interface-controlled PLD grown films in metal-insulator-semiconductor diodes and field-effect transistors. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T27.00003: Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$Epitaxial Thin Films on Silicon for Electronic and Photonic Applications Vera Smolyaninova, Grace Yong, Benjamin Hofmann, Rajeswari Kolagani, Yong Liang Thin films of rare-earth manganese oxides (manganites) are usually grown on oxide substrates. It is more challenging to grow thin films of these materials on technologically versatile silicon. Upon illumination with visible light, the resistivity of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$epitaxial thin films fabricated via PLD on oxide substrates decreases significantly in a wide temperature range due to the destruction of charge ordering. This makes Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$thin films attractive for potential photonic and opto-electronic device applications. Having in mind device applications, we have extended our studies to Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$epitaxial thin films grown on Si (001) with different buffer layers. The advantages of different buffer layer schemes on Si (001) will be discussed. Influence of deposition and annealing conditions on film photoresponse will be reported. Photoinduced and current induced effects in films grown on oxide substrates and on buffered Si substrates will be compared. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T27.00004: Photoinduced effects in Bi$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films with different oxygen content Grace Yong, Rajeswari Kolagani, Khim Karki, Benjamin Hofmann, Vera Smolyaninova Doped rare-earth manganese oxides (manganites) attract interest due to a variety of electronic, magnetic, and orbital states and their drastic response to application of modest external fields. A photoinduced insulator to conductor transition in thin films of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$ associated with melting of the charge ordering [1] is especially interesting for potential photonic and opto-electronic device applications. From this point of view it is important to know what factors influence the photoinduced effects. We have found that oxygen content of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin films significantly modify conductive, structural and photoinduced properties. The role of growing and annealing conditions will be discussed. The change in magnitude and lifetime of photoinduced changes in films with different oxygen content will be reported. The possible origin of these changes will be discussed. [1] V. N. Smolyaninova at al., Phys. Rev. B 76, 104423 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T27.00005: Strain-modulated Self-Assembly of Nanostructures within Epitaxial Thin-films via Pulsed Laser Ablation Amit Goyal, Sung-Hun Wee, Yanfei Gao, Claudia Cantoni, Karren More, Yuri Zuev, Junsoo Shin Nanocomposites comprising three-dimensionally (3D) ordered arrays of nanodots of one type of complex ceramic material embedded in another complex ceramic material are expected to exhibit novel physical properties, tunable by adjusting the overall composition, concentration, feature size and spatial ordering of the nanodots. Applications of such nanocomposites in the areas of multiferroics, photovoltaics, solid state lighting, ultra-high density storage and high temperature (high-Tc) superconductivity are of interest. A joint experimental, theoretical and computational study on achieving ordering via 3D self-assembly of nanodots of one complex ceramic material within another complex ceramic material, such as 3D self-assembly of insulating BaZrO3 (BZO) nanodots within high-Tc superconducting YBCO films, was performed. Vertically or horizontally ordered arrays (or simultaneous ordering in both directions) of BZO nanodots within superconducting films have been made possible via strain modulation between nanodots. Experimental results obtained for novel nanocomposites for other applications involving perovskite-spinel mixtures such as CoFe2O4-BaTiO3, CoFe2O4-BiFeO3, etc. will also be presented. Such materials with ``controlled self-assembly'' of nanostructures should find application in many areas. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T27.00006: Efficiency and Plume Dynamics for Mid-IR Laser Ablation of Cornea Aroshan Jayasinghe, Borislav Ivanov, M. Shane Hutson This paper reports ablation experiments on porcine corneal tissue using the Vanderbilt Mark III Free Electron Laser (FEL) and a tabletop Raman-shifted Alexandrite laser. These experiments were designed to test previous models that suggested wavelength and intensity dependent ablation mechanisms. In one test, we compare ablation efficiency and plume dynamics for two FEL wavelengths ($\lambda $=2.77, 6.45 $\mu $m) chosen such that different components of the tissue matrix act as the primary chromophore (water or protein respectively), while keeping the total absorption constant. We find small differences in ablation efficiency (with slightly more efficient ablation at 2.77 $\mu $m); no difference in shockwave propagation; and slightly more particulate matter in the plume at 6.45 $\mu $m. In a second test, we find that the Raman-shifted Alexandrite laser has similar ablation efficiency to the FEL in the 6-7 $\mu $m range -- despite a $\sim $500-fold higher intensity. Although these results do not confirm the previous model predictions, the findings do suggest that the Raman-shifted laser can be a viable alternative to the FEL for surgical applications. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:42PM |
T27.00007: Pulsed-Laser Deposition of ZnO Thin Films and Heterostructures for Device Applications Invited Speaker: ZnO is a wide bandgap semiconductor being explored for transparent electronics, UV light emitting diodes, spin-based devices and chemical sensors. In this talk, we will discuss recent progress and understanding for carrier doping and interface formation in epitaxial ZnO thin films grown by pulsed-laser deposition. One of the critical issues for device applications is the formation of low resistivity, high carrier density p-type ZnO material for minority carrier injection. The behavior of acceptor dopants within the ZnO and ZnMgO matrices will be described. Discussion will include stability of transport properties, stabilization of surfaces, and device characteristics. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T27.00008: Role of the top electrodes and their interfaces on the resistive switching behavior of epitaxial NiO thin films S.R. Lee, J.H. Bak, Y.D. Park, K. Char, D.C. Kim, S. Seo, X.S. Li, G.S. Park, R. Jung Initial I-V characteristics of resistive switching behavior have been investigated with epitaxial NiO films grown on (100) SrRuO$_3$ by using Al, Pt, and CaRuO$_3$ as the top electrodes. SRO/NiO/Al and SRO/NiO/Pt require an electroforming process for the initialization of the resistive switching, while SrRuO$_3 $/NiO/CaRuO$_3$ is initially in a low-resistance state. The temperature dependence of the initial I-V characteristics indicates that insulating layers exist at the NiO/Al and NiO/Pt interface, presumably broken by the electroforming process. On the other hand, SRO/NiO/Al does not show the resistive memory switching behavior despite the electroforming behavior. The resistive switching endurance is also distinct depending on the top electrodes. Our results suggest that the oxygen defects and their bonding energy with the top electrode metal play a critical role on the resistive switching behavior. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T27.00009: Effect of cation substitution on the resistive switching behavior in epitaxial NiO H.M. Kim, S.R. Lee, J.H. Bak, M.L. Jo, Y.D. Park, K. Char The resistive switching behavior of NiO has been extensively investigated due to the nonvolatile ReRAM device applications. In contrast to unipolar resistive switching of NiO grown on Pt, bipolar resistive switching is observed in NiO grown on SrRuO$_{3}$ (SRO). The unipolar switching has been explained by the formation and rupture of filamentary conduction with the Joule heating, while the bipolar switching is still controversial. Our previous study with epitaxial (epi) NiO, prepared under various growth conditions and electrodes, suggested that the oxygen defects at the NiO/top electrode (TE) interface may be responsible for the bipolar switching and TE may compensate the oxygen defects. In order to understand the role of the defect states at the interface on the resistive switching, 1-nm-thick epi-AlO$_{x}$ interlayer has been deposited on and under epi-NiO. The I-V characteristics have been investigated with an epi-CaRuO$_{3}$ (CRO) as TE, resulting in a clean interface with NiO. SRO/NiO/AlO$_{x}$/CRO shows poorer switching endurance in the less than 25{\%} of measured cells. However, SRO/AlO$_{x}$/NiO/CRO exhibits bipolar switching in the most of measured cells with better endurance. This may imply the different oxygen defect states of each interface of NiO. As an effort to investigate the defect states in bulk and their effect on the unipolar switching, the I-V characteristics of Al substitution in epi-NiO will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T27.00010: Ge Quantum Dot Formation on Si (100)-2x1 with Surface Electronic Excitation Ali Oguzer The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si (100)-2x1 grown by pulsed laser deposition is studied. The samples were first cleaned by using modified Shiraki method and then transferred into the deposition chamber. The vacuum system was then pumped down, baked for at least 12 hours, and the sample was then flashed to 1100 \r{ }C in order for the 2$\times $1 reconstruction to form. The experiment was conducted under a pressure $\sim $1x10$^{-10}$ Torr. A Q-switched Nd:YAG laser (wavelength $\lambda $ = 1064 nm, 10 Hz repetition rate) was used to ablate a Ge target. In-situ RHEED and STM and ex-situ AFM were used to study the morphology of the grown QD. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the surface morphology. Laser irradiation of the Si substrate is shown to decrease the roughness of films grown at a substrate temperature of $\sim $450 $^{o}$C. Electronic excitation also affected surface coverage ratio and cluster density and decreased the temperature required to form 3-dimensional quantum dots. Possible mechanisms involved will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T27.00011: Computer simulations of photochemistry controlled with subwavelength resolution Triet Nguyen, Alex Small A technique called Stimulated Emission Depletion (STED) has recently been developed to beat the diffraction limit in imaging. We propose to adapt this technique to control chemical reactions with nanoscale resolution. We simulated a process in which a series of laser pulses is applied at each site on a surface. The first pulse excites the molecules and the second pulse (with a TEM10 ``doughnut'' profile) then causes the excited molecules away from the node at the center to undergo stimulated emission and return to the ground state. The result is that the molecules at the center of the pulses (in a region of size $<<$lambda) remain in the excited state and can undergo chemical reactions. In this presentation, we will show results of computer simulations of this technique. We will show that even if the reaction rate constants are small, the application of several sequential pulses leads to a fractionation effect that compensates for low reaction rates. We will also show how the resolution of this technique depends on the intensities of the laser pulses used, and propose a few candidate molecules for experimental tests of this concept. [Preview Abstract] |
Session T28: Focus Session: Thermoelectric Materials: Nanostructures
Sponsoring Units: FIAP DMPChair: Donald Morelli, Michigan State University
Room: 330
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T28.00001: SiGe Nanocomposites Thermoelectrics: The Knowns and the Unknowns Invited Speaker: Silicon-germanium has been used in spacecraft for a long time to convert heat from radio isotope heat sources into electricity for deep space missions. Recently, we have reported significant improvement in the thermoelectric figure of merit of nanostructured Si$_x$Ge$_{1-x}$. The improvement in ZT comes mainly from reduced phonon thermal conductivity, while at the same time, maintaining the electron transport properties. These experimental successes, although providing strong support to the direction of using random nanostructures to improve thermoelectric performance, also call for a detailed understanding of thermoelectric transport in random bulk nanostructures. Careful examination of the spectral details of the electron and phonon transport reveals a significant deficiency in our current understanding, even for bulk materials. Different interfacial transport processes further complicate the picture. In this talk, we will discuss the current status of our understanding of thermoelectric transport in nanocomposites. In collaboration with A. Minnich, H. Lee, B. Muralidharan, and M.S. Dresselhaus, Massachusetts Institute of Technology, Cambridge, MA 02139; and X. W. Wang, G. Joshi, G. H. Zhu, Y. C. Lan, D. Z. Wang, and Z.F. Ren, Boston College, Chestnut Hills, MA 02467. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T28.00002: Thermoelectric Power of high concentration embedded Nano-particle Samples Mona Zebarjadi, Keivan Esfarjani, Ali Shakouri, Zhixi Bian, Je-Hyeong Bahk, Gehong Zeng, John Bowers, Hong Lu, Joshua Zide, Art Gossard High concentrations of embedded nano-particles inside thermoelectric elements are desirable because they can reduce the thermal conductivity. But they also affect the power-factor. Therefore they can enhance or suppress the figure of merit. We model the effect of such high concentrations on the power-factor using the coherent potential approximation. We optimize the power-factor versus nano-particle size, distribution and concentration. The analysis would help in designing nano-particle embedded matrices with high-performances. We characterize InGaAlAs samples with 3-10{\%} volume concentration of ErAs nano-particles and explain their properties such as the mobility and the Seebeck coefficient theoretically. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T28.00003: Transient Electrical and Thermal Characterization of InGaAlAs Thin Films with embedded ErAs Nanoparticles. Tela Favaloro, Rajeev Singh, James Christofferson, Younes Ezzahri, Zhixi Bian, Ali Shakouri, Gehong Zeng, Je-Hyeoung Bahk, John Bowers, Hong Lu, Arthur Gossard We developed a system for accurate high-temperature characterization of thermoelectric materials and devices. This system can be used for electrical measurements of thermoelectric properties and contains an integrated optical thermoreflectance imaging system is integrated into the thermostat for analysis of sample surface temperature profile resulting from the Peltier effect, Joule heating or external thermal excitation within the sample. Transient electrical and thermal measurements are useful to extract material diffusivity of each layer. We have performed high temperature transient analyses and thermal imaging of thin film devices optimized for direct figure of merit detection in the cross-plane direction. These devices consist of 25 micron thick samples of InGaAlAs films with embedded ErAs nanoparticles. Using the transient Harman technique, we determine the cross-plane figure of merit and electrical conductivity. Thermal imaging is used to ensure current injection uniformity across the device and to extract the Seebeck coefficient and thermal conductivity of the material. The experimental results and theoretical analysis are given. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T28.00004: Characterization and modeling of the randomly distributed ErAs nanoparticles in InGaAlAs semiconductors for thermoelectric power generation Je-Hyeong Bahk, Mona Zebarjadi, Zhixi Bian, Gehong Zeng, Ashok Ramu, Hong Lu, Ali Shakouri, Art Gossard, John Bowers We investigate temperature-dependent thermoelectric properties of the InGaAlAs semiconductors containing epitaxially embedded ErAs nanoparticles grown by Molecular Beam Epitaxy. Temperature-dependent Hall measurements and Seebeck coefficient measurements were performed for the materials with various Er concentrations and semiconductor compositions, and the results were analyzed using a theoretical modeling based on the ErAs nanoparticle's carrier scattering behaviors. In the analysis, the nanoparticles are modeled as charged spheres with Schottky barrier height at the interface with semiconductor, and the potential profile around a particle is used as perturbation for electron scattering. The particle scattering rate is calculated using both Born approximation and the partial wave method, respectively, and the two methods are compared to check the validity of Born approximation in various conditions. The theoretical calculation of mobility and Seebeck coefficient based on the modeling of particle scattering and other scattering mechanisms fits the measurement results, and we find that further enhancement of thermoelectric power factor is possible by optimizing the particle scattering in the materials. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T28.00005: Thermal and Thermoelectric Transport in Thin Films and Nanostructures B. L. Zink, R. Sultan, A. D. Avery Interest in increasing efficiency of energy generation continues to spur the development of new thermoelectric materials. Though bulk materials hold the most promise for large-scale energy generation, many groups continue to explore increasing the thermoelectric figure-of-merit by taking advantage of techniques for creating nanostructured materials such as multilayered thin films and nanowires. These systems could prove to have high figures-of-merit and be important for integrating energy harvesting and/or cooling with micro- or nanoscale devices ``on chip.'' Though many promising systems have been identified, measuring their fundamental thermal transport often remains a major challenge. In this talk, we briefly describe our recent advances in measuring in-plane thermal transport, thermopower and electrical conductivity on thin-films or nanolithographically patterned systems. Our technique allows great flexibility for studying the thermoelectric properties of a wide range of materials, from amorphous semiconductors to semi-metallic nanowires. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T28.00006: Nitride Metal-Semiconductor Superlattices for Solid State Thermionic Energy Conversion Robert Wortman, Jeremy Schroeder, Polina Burmistrova, Mona Zebarjadi, Zhixi Bian, Ali Shakouri, Timothy Sands A new class of thermoelectric materials based off of superlattices have been proposed that show a potential for enhanced thermoelectric performance$^{1,2}$. The increase of thermoelectric figure-of-merit ZT of these materials is due to both the energy filtering effect of the Schottky barriers as well as the reduced thermal conductivity that results from increased interface density. Our work has centered on the metal-semiconductor materials system of HfN-ScN. These are both high temperature materials (T$_{m} \quad >$ 2500C). They have the same rocksalt crystal structure and similar lattice constants, allowing epitaxial growth. We have grown superlattices of these materials via DC magnetron sputtering. Results from x-ray diffraction, and electrical and thermal tests will be presented. Their potential as thermoelectric energy conversion materials will be discussed. 1 G. D. Mahan et al, Phys. Rev. Lett., 80, 4016 (1998) 2 D. Vashaee et al, \textit{Phys. Rev. Lett}. 92, 106103 (2004) [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T28.00007: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T28.00008: Thermoelectric properties of nanoporous Ge Joo-Hyoung Lee, Jeffrey Grossman Recently, silicon nanowires were shown to have a thermoelectric (TE) figure of merit ($ZT$) two orders of magnitude larger than that of bulk Si \footnote{A. I. Hochbaum {\it et al.}, Nature {\bf 451}, 163 (2009); A. I. Boukai {\it et al.}, {\it ibid.}, {\bf 451}, 168 (2009)}. In addition, recent theoretical work predicted that Si with periodically arranged nanometer-sized pores (nanoporous Si) could result in a similar increase in $ZT$\footnote{J.-H. Lee {\it et al.}, Appl. Phys. Letts. {\bf 91}, 223110 (2007); J.-H. Lee {\it et al.}, Nano Lett. {\bf 8}, 3750 (2008)}. These results open an exciting new pathway towards efficient thermoelectrics based on standard semiconductor materials. In the present work, we extend our earlier calculations on silicon to explore the TE properties of nanoporous Ge. Specifically, we calculate the thermal and electrical conductivities, Seebeck coefficient and figure of merit of nanoporous Ge for a range of configurations using a combined {\it ab initio} electronic structure calculation and Boltzmann transport approach. The results show a substantial increase in $ZT$ compared with that of bulk Ge, as in the Si nanostructures. A detailed comparison between the TE properties of nanoporous Ge and Si will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T28.00009: Solution processable routes to nanostructured thermoelectric materials Joseph Feser, Robert Wang, Arun Majumdar, Jeffrey Urban The previous decade of research has shown that nanostructured thermoelectric materials can have superior performance compared to their bulk counterparts.. Often, the synthesis of nanostructured materials is performed by layer-by-layer methods, which hinders their ability to be produced as thick films. Here we show a scheme by which nanocrystals embedded in a thermoelectric matrix may be produced using solution processing. Using hydrazine chemistry, we prepare soluble precursors for Bi2X3 (X=S,Se,Te). Solutions containing those precursors are spun and drop-cast onto substrates, and their electrical and thermal properties are characterized. We show ongoing research to embed colloidal nanocrystals into a matrix made from the soluble precursors. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T28.00010: Self-supporting (Bi$_{0.11}$Sb$_{0.29 })$(Te$_{0.25}$,Se$_{0.41})$ nanowire arrays for thermoelectric microdevices Hatem El Matbouly, Timothy Sands, Kalapi Biswas Nanostructuring of thermoelectric material can lead to improved performance through suppression of the lattice contribution to thermal conductivity and enhancement of the power factor by quantum confinement or thermionic energy filtering. To take advantage of these effects in a Peltier microcooler or Seebeck generator, it is necessary to prepare nanostructure materials with leg lengths ranging from tens of microns to millimeters. We have developed a process for fabrication of thick, self-supporting (Bi$_{0.11}$,Sb$_{0.29})$(Te$_{0.25}$,Se$_{0.41})$ nanowire arrays using a novel branched porous anodic alumina template that can be removed completely by selective etching following electrodeposition of the thermoelectric material, resulting in 100-micron-thick nanostructured thermoelectric material without the parasitic thermal shunt that is associated with the template. The electrodeposition process allows composition modulation and grading, effects that are difficult to achieve by bulk synthesis. Bandgaps of the electrodeposited material range from 0.13 eV for Bi$_{2}$Te$_{3}$ to an optical gap of 0.52 eV measured for a (Bi,Sb)$_{2}$(Te,Se)$_{3}$ alloy, suggesting an operating tempurature range from below room temperature to $\sim $300\r{ }C. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T28.00011: Thermoelectric Properties of Nanostructured n-type Yb$_{x}$Co$_{4}$Sb$_{12 }$Bulk Jian Yang, Hui Wang, Yucheng Lan, Xiao Yan, Bo Yu, Xiaowei Wang, Gaohua Zhu, Dezhi Wang, Zhifeng Ren, Qing Hao, Gang Chen, Qinyu He, Mildred S. Dresselhaus Nanostructured single phase of Yb filled skutterudites CoSb$_{3}$ with a nominal composition of Yb$_{x}$Co$_{4}$Sb$_{12 }$(X = 0.3, 0.35, 0.4, and 0.5) have been synthesized by ball milling and direct current induced hot press. Thermoelectric properties including electrical conductivity, Seebeck coefficient, and thermal conductivity from room temperature to 550${^\circ}$ were measured and discussed. It was found that Yb$_{0.35}$Co$_{4}$Sb$_{12}$ has the optimal dimensionless figure of merit of 1.2 at 550${^\circ}$. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T28.00012: Thermoelectricity in Arrays of Thiolate Coated Au Nanoparticles E. Covington, F. Bohrer, E.T. Zellers, C. Kurdak We have developed a new technique to measure the thermopower of highly resistive films of thiolate coated Au nanoparticles. Using e-beam lithography, we fabricate two long parallel gold wires, spaced by 500 nm, on an insulating substrate and subsequently coat with a thin film of nanoparticles. The wires are used as electrodes for electronic conduction and heaters for thermopower measurements. We characterize the Joule heating in the wires using noise thermometry. To characterize the thermopower of the film, we excite one wire by an ac current with frequency $f$. Due to Joule heating, we establish a temperature difference between the two wires modulated with frequency 2$f$. We extract the thermopower by measuring the 2$f$ voltage signal between the wires using lock-in techniques.~We used this method on Au nanoparticles with 1-octanethiol (C8) ligands where the thermopower was less than 10 $\mu $V/K at room temperature. From the sign of the thermopower, we determined transport was mainly due to tunneling of electrons through the lowest unoccupied molecular orbital of the C8 molecule. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T28.00013: Molding Phonon Flow with Symmetry: Rational Design of Hypersonic Phononic Crystals Cheong Yang Koh, Edwin L. Thomas Phononic crystals structured at appropriate length scales allow control over the flow of phonons, leading to new possibilities in applications such as heat-management, sound isolation and even energy transfer and conversion. Symmetry provides a unified framework for the interpretation 1D to 3D phononic band structures, allowing utilization of a common set of principles for designing band structures of phononic crystals as well as actual purposeful defects such as waveguide location and boundary termination in finite devices. In this work, we explore the band structure properties of phononic crystals with non-symmorphic space groups, as well as those having quasi-crystalline approximants. We demonstrate gap opening abilities from both anti-crossing and Bragg scattering, as well as unique features like ``sticking'' bands. Symmetry concepts are also powerful means to tune the density of states of the structures. Importantly, we fabricate various theoretical designs and measure their experimental dispersion diagrams for comparison with theoretical calculation. This affords an elegant approach toward a design blueprint for fabricating phononic structures for applications such as opto-acoustic coupling. [Preview Abstract] |
Session T29: Focus Session: NSF's Research Experience for Undergraduates (REU) Program: Overview and Perspectives
Sponsoring Units: FEdChair: Catherine Mader, Hope College
Room: 333
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T29.00001: Physics NSF-REU Site Director Workshop: What Did We Learn and What Questions Remain? Invited Speaker: The Research Experience for Undergraduates program, traditionally funded by the National Science Foundation and by other agencies (including the Department of Defense), has been a great success. Every year, hundreds of students have the chance to participate in research activities at Universities and research entities other than their own. These extended experiences have helped the students develop confidence in their abilities as practicing physicists, train on state of the art instrumentation, work on communication skills through presentations, and frequently reach a publication milestone. The REU program's impact is manifold and complementary to the strong work done at the home institutions by faculty advisors and instructors. In this presentation we will discuss the current state of the program as determined by the recent (and first) meeting of the Physics REU site directors. In particular we will focus on the strengths of the program, shared good practices, data gathering, and some changes the new Steering Committee might implement. The intended audience should be current REU directors and faculty research supervisors; students who might be interested in the program; faculty at institutions who might be thinking of applying to become a site; and anyone attentive to undergraduate research in general. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T29.00002: Past participant's perspective on the Research Experience for Undergraduates program. Derek Padilla The Research Experience for Undergraduates (REU) programs funded by the National Science Foundation provide an outstanding opportunity for many students to participate in high-quality research at a facility other than their own institution. As a participant in the summer of 2006 I will attest to the success of a great REU program. This discussion will focus on my work and experience while in the program as well as the continuing relationship with my advisors and the host department as a whole. In collaborating with all levels of REU personnel we will improve the function of an already outstanding opportunity for undergraduate students for the coming years. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T29.00003: Recuiting minority students into your REU program David Ernst It is important for the field of physics that we draw our talent from the broadest pool. However, African Americans, Hispanic Americans, and Native Americans are substantially underrepresented in physics. Since summer research experiences are a useful tool to assist students in gaining entrance into graduate school, it is particularly important that underrepresented minority students are fully able to participate. The NSF REU program at Vanderbilt University is historically comprised of one half minority students. How this comes about and how we pro-actively recruit minority students will be described. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T29.00004: International Summer Research Program in Gravitational-Wave Physics operated by the University of Florida for the LIGO VIRGO Science Collaboration Guido Mueller, Bernard Whiting The NSF-funded Laser Interferometer Gravitational-Wave Observatory (LIGO), 20+ US universities and colleges, and their international partners in Europe, Australia, and Japan are operating a network of six large scale interferometers to detect gravitational waves from neutron star or small black hole mergers, supernovae, and other galactic or cosmological sources. The data analysis as well as R{\&}D activities for future gravitational-wave detectors are organized within the LIGO VIRGO Scientific Collaboration. This international frame provides the backdrop of our international summer research program. Our international partners offer currently up to 30 different research projects at 15 different institutions in 6 different countries on 3 different continents for US undergraduate students. Our primary target group are students from small US colleges who are active members of the LIGO Science Collaboration. In addition to the standard goal of exposing the students to cutting edge research, the students will also be exposed to a different culture (and usually language), and their research experience will help our small college groups to build-up or solidify their own research connection with our international partners. See http://www.phys.ufl.edu/ireu for more details. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T29.00005: REU in Physics at Kansas State University--- an Evolving Program Kristan Corwin, Bruce Glymour, Amy Lara, Larry Weaver, Dean Zollman The REU site in the Physics Department at Kansas State University, funded by NSF for 13 years between 1992 and 2007, originally focused on atomic collision physics. Now the theme has broadened to include laser-matter interactions on atomic and nanoscales, and an ethics component is incorporated. Students study how atoms and molecules interact with ultra-fast optical and x-ray pulses, reveal the structure of nanoparticle crystallization and gel formation with scattered laser light, and develop computer codes for atomic interactions in Bose-Einstein condensates and nanoparticle self-assembly from lattices to gels; some have traveled to Japan for neutrino experiments. The students we select come primarily from smaller colleges and universities in the Midwest where research opportunities are limited. Prof. Weaver, who has served as PI since 1992, facilitates their transition from a teaching to research environment through lectures and individual interactions. Our program is in a period of transition. While Prof. Weaver continues to be the ``impedance match'' between students and mentors, other leadership roles are gradually being assumed by a team of faculty members who strive to preserve the intimacy and excellence of the program. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T29.00006: Challenges and Opportunities in Interdisciplinary Materials Research Experiences for Undergraduates Yogesh Vohra, Thomas Nordlund The University of Alabama at Birmingham (UAB) offer a broad range of interdisciplinary materials research experiences to undergraduate students with diverse backgrounds in physics, chemistry, applied mathematics, and engineering. The research projects offered cover a broad range of topics including high pressure physics, microelectronic materials, nano-materials, laser materials, bioceramics and biopolymers, cell-biomaterials interactions, planetary materials, and computer simulation of materials. The students welcome the opportunity to work with an interdisciplinary team of basic science, engineering, and biomedical faculty but the challenge is in learning the key vocabulary for interdisciplinary collaborations, experimental tools, and working in an independent capacity. The career development workshops dealing with the graduate school application process and the entrepreneurial business activities were found to be most effective. The interdisciplinary university wide poster session helped student broaden their horizons in research careers. The synergy of the REU program with other concurrently running high school summer programs on UAB campus will also be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T29.00007: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T29.00008: The Assessment of the Impact of REU Programs on Student Classroom Performance Chris Hughes Supporters of undergraduate research claim that the research experience enhances the success of students in their classes and promotes their progress toward completing a science major. Since there are many other variables that can influence a student's progress through a curriculum, it is frequently difficult to compare students from undergraduate research programs with a suitable control group. At James Madison University, a significant number of chemistry and physics majors participate in summer REU programs on campus. However, since JMU is among the top 10{\%} of undergraduate institutions in the US in undergraduate physics enrollment, there are also a significant number who choose not to stay on campus for summer research. Using several years worth of data, we have determined the change in the GPAs of REU students (N=75) from the semester before the REU to the semester after the REU and compared these with the students who did not participate in summer research (N=663). We have found that the REU students' average GPA increased by a statistically significant amount while the non-REU students' average GPA was unchanged to within a standard deviation. We will also discuss other assessment methods used at JMU and some of the limitations in the interpretation of this study. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T29.00009: The REU Program From a PUI Perspective: Interdisciplinary Scientific Computation at Ohio Wesleyan University Brad Trees Running a research experience for undergraduates (REU) program at a predominantly undergraduate institution (PUI) has some unique aspects when compared to such a program at a doctoral/research university. Discussion will focus on the challenges of running an REU program at a PUI, as well as the potential benefits to undergraduate participants, faculty mentors, and the institution in general. The perspective for the discussion stems from experiences of the faculty mentors at Ohio Wesleyan's interdisciplinary REU program in scientific computation, which also includes a research experience for teachers component (NSF REU/RET Grant PHY-0648751). [Preview Abstract] |
Session T30: Focus Session: Excitations in Multiferroics
Sponsoring Units: DMP GMAGChair: Dimitri Argyriou, Helmholtz Center-Berlin
Room: 334
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T30.00001: Electromagnon spectra in multiferroic manganites $R$MnO$_3$ Invited Speaker: The magnetoelectric (ME) effect, in which electric polarization (magnetization) are controlled by magnetic (electric) field, have recently attracted intensive attention. One of the materials which exhibits large ME effects is the manganites $R$MnO$_3$. In this presentation I focus on {\em electromagnon} excitations (in a wide sense). In those manganites, cycloidal spin structures create ferroelectricity through spin-orbit couplings. Originally, electromagnon is defined as a collective excitation of spins in such a way that rotation of the cycloidal plane is driven by the {\em electrical field} of light coupled to the ferroelectric moment. Spectra for electrically excited magnons are indeed reported in TbMnO$_3$, GdMnO$_3$, (Eu,Y)MnO$_3$ and DyMnO$_3$. However, the selection rule observed in these compounds ($E^\omega \parallel a$, irrespective of the cycloidal planes) is inconsistent with the theoretical prediction. Alternatively, couplings between electric field and spins through spin-dependent local polarizations have been proposed. I show both one- and two-magnon excitation spectra, and clarify consistencies and discrepancies of the model with experimental data. Roles of phonons and orbital orders in these compounds are also discussed. \\\\ References: Y. Takahashi {\em et al.}, PRL{\bf 101}, 187201 (2008). Also, Katsura {\em et al.}, PRL{\bf98}, 027203 (2007); A. Pimenov {\em et al.}, Nature Phys. {\bf 2}, 97 (2006); R. Valdes Aguilar {\em et al.}, PRB{\bf 76}, 060404(R) (2007); Kida {\em et al.}, PRB{\bf 78}, 104414 (2008); R. Valdes Aguilar {\em et al.}, arXiv:0811.2966. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T30.00002: Selection rules for electromagnons in anharmonic cycloidal multiferroics Markku Stenberg, Rogerio de Sousa The coexistence of magnetic and ferroelectric phases in multiferroic materials gives rise to hybrid excitations with mixed magnetic and electric character. These excitations, so-called electromagnons, have been observed in several oxide materials, but their origin and optical selection rules are central questions that are not yet understood. We present a theory of electromagnon excitation in magnets with anharmonic cycloidal ground state. In contrast to previous theories, we show that multiple electromagnons are excited by light polarized along the cycloid plane as well as perpendicular to it. Our results allow the distinction between different magnetoelectric couplings and have important implications for the interpretation of optical experiments in rare earth manganites as well as other materials with anharmonic spiral magnetic order. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T30.00003: Low-magnetic field control of electromagnon N. Kida, S. Ishiwata, Y. Taguchi, R. Shimano, T. Arima, Y. Tokura Since the 1960s, there has been intense debate concerning the presence of the spin excitation driven by the electric field of light $E^\omega$ in ferroelectric magnets. This is recently revived by the observation of the absorption peak structure around 3 meV in a ferroelectric magnet TbMnO$_3$ [1]. As an origin of this excitation, the hybridized spin excitation with electric polarization (now called electromagnon) was considered [2]. However, this issue remains controversial by our THz spectroscopic studies on a family of $R$MnO$_3$ [3] ($R$ = Tb, Dy, and Gd$_{0.7}$Tb$_{0.3}$) in a variety of spin phases tuned by temperature and magnetic field. Here we report an optical investigation of the low-energy (2$-$10 meV) spin dynamics for other ferroelectric magnets, hexaferrite, by using THz time-domain spectroscopy. We find the signature of the genuine electromagnon at THz frequencies. As a manifestation of the strong magnetoelectric coupling inherent to the electromagnon, we demonstrate the low-magnetic field ($\sim$ 100 Oe) control of the optical constants at THz frequencies. \noindent [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [2] D. Senff {\it et al.}, PRL {\bf 98}, 137206 (2007). [3] N. Kida {\it et al.}, PRB {\bf 78}, 104414 (2008); Y. Takahashi {\it et al.}, PRL {\bf 101}, 187201 (2008); N. Kida {\it et al.}, J. Phys. Soc. Jpn. Dec. issue (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T30.00004: Theory of novel one-magnon excitation induced by electric fields in cycloidal spin magnets Shin Miyahara, Nobuo Furukawa We propose a new mechanism to induce a novel one-magnon excitation by electric-field component of light in cycloidal spin states. We calculated optical spectra in the ycloidal spin structures as observed in multiferroic perovskite manganites $R$MnO$_{3}$ where novel magnetic excitations induced by electric-field component of light are observed. When symmetric pairs of spins dependent electric polarizations are introduced, we have light absorptions at terahertz frequencies with one- and two-magnon excitations driven by electric-field components. Our results show that some parts of optical absorption peaks observed experimentally at terahertz frequencies are one-magnon excitation absorptions. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T30.00005: Origin of electromagnons in multiferroic manganites Invited Speaker: The interest in multiferroic materials has increased in the last few years due to the fundamental physics of strong interaction between ferroelectric and magnetic orders, as well as for the promise of novel applications in future electronics. From powerful symmetry arguments and with modeling of the microscopic coupling mechanism, these efforts have led to the discovery of a vast set of multiferroic compounds. An important recent step in this regard was the discovery of a new kind of magnetic excitation that couples strongly to light by acquiring electric dipole activity from the infrared active phonons, called electromagnon, which is a hybrid excitation of magnon and phonon character \footnote{A. Pimenov, et al. Nature Physics 2, 97 (2006)}$^{,}$\footnote{A.B. Sushkov, et al. PRL 98, 027202 (2007)}. These discoveries have highlighted the importance of the dynamical aspects of the magnetoelectric coupling. Even though a wide consensus has been reached regarding the origin of the static ferroelectric polarization, the mechanism of the magnetoelectric dynamic effect of electromagnons was not clear. In this talk a combination of theory and experiment is presented that clarifies the origin of the electromagnon excitations in RMnO$_3$. This model is based on symmetric exchange striction and takes into account the lattice and magnetic symmetry of this family of perovskite manganites. It reproduces the fact that the observed selection rule for electromagnons in RMnO$_3$ is independent of the spin plane\footnote{R. Vald\'{e}s Aguilar, et al. PRB 76, 060404 (2007)}$^{,}$\footnote{R. Vald\'{e}s Aguilar, et al. arxiv:0811.2966}. This result is due to the effective modulation of the exchange interaction between Mn spins induced by the electric field of light. The proposed mechanism is also related to the origin of static polarization in the E-phase of this RMnO$_3$ multiferroic family. The model and experiments carried out so far demonstrate that the symmetric exchange interaction is responsible for all the observed dynamical magnetoelectric effects, and opens a new avenue for study of these multiferroic compounds. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T30.00006: Infrared-active excitations related to the $R^{3+}$ ligand-field splitting in $R$Mn$_{2}$O$_{5 }(R$=Ho, Dy, Tb) A. A. Sirenko, S. M. O'Malley, T. D. Kang, K. H. Kahn, C. L. Carr, L. Mihaly, S. Park, S-W. Cheong Optical transitions between ligand-field split states of $R^{3+}$ ions in $R$Mn$_{2}$O$_{5}$ multiferroic single crystals have been studied at the phase transitions in the external magnetic field up to 13 T and uniaxial pressure up to 5 kbar. Spectra of the ligand field excitations change significantly in external magnetic field and correlate with the reversal of electric polarization induced by magnetic field. The oscillator strength and selection rules for ligand field excitations change with external uniaxial pressure. We discuss the connection between the ligand field on $R^{3+}$ with the magnetism and dielectric properties of this compounds. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T30.00007: THz magneto-optical study of multiferroic compound TbMnO$_3$ Urmas Nagel, D. H{\"u}vonen, T. R{\~o}{\~o}m, S.B. Kim, C.L. Zhang, S.-W. Cheong We present results of magneto-optical absorption measurements in the THz region on multiferroic TbMnO$_3$ in magnetic fields up to 12\,T. The temperature range of our studies covers the paramagnetic phase ($T>42$\,K), the collinear incommensurate spin density wave phase of Mn$^{3+}$ $S=2$ spins (28\,K$<42$\,K), the ferro-electric phase with incommensurate elliptic spin order ($T<28$\,K) and the phase where the magnetic moments of Tb are ordered, $T<7$\,K. It is known that in the FE phase the magnetic field ${\mathbf B_0}\parallel {\mathbf a}$ equal to 10.5\,T flops the electric polarization from ${\mathbf P}\parallel {\mathbf c}$ to ${\mathbf P}\parallel {\mathbf a}$. The polarization ${\mathbf P}$ is in the plane of elliptical spiral and perpendicular to spiral order vector, ${\mathbf k}\parallel {\mathbf b}$ in TbMnO$_3$. It is expected that the selection rule for the electric-dipole active spin excitation in the spiral phase changes from ${\mathbf E_1}\parallel {\mathbf a}$ to ${\mathbf E_1}\parallel {\mathbf c }$. Polarization-sensitive absorption measurements are performed to distinguish between magnetic- or electric-dipole active spin excitations, i.e. magnons or electromagnons, respectively, in the whole temperature and magnetic field range. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T30.00008: Magneto-optical THz study in multiferroic Ni$_3$V$_2$O$_8$ Dan H{\"u}vonen, U. Nagel, T. R{\~o}{\~o}m, N. Rogado, R. Cava We present results of absorption measurements in THz region, from 3 to 220\,cm$^{-1}$, in multiferroic Ni$_3$V$_2$O$_8$ in magnetic fields up to 12\,T and temperatures above 2\,K. Ni$_3$V$_2$O$_8$ is a magnetic insulator with Ni$^{2+}$ spin-1 ions arranged in a kagom\'{e} staircase lattice. The phase diagram of Ni$_3$V$_2$O$_8$ is complicated - ground state changes from paramagnetic (PM) to high temperature incommensurate (HTI) helical state, at 9.1\,K in zero field. In low temperature incommensurate (LTI) phase below 6.3\,K spontaneous electric polarization $\mathbf{P}$ appears along the $b$ axis. $\mathbf{P_b}$ can be suppressed by further cooling below 3.9\,K or by application of external magnetic field. Low energy optical excitations are discussed for all low-$T$ phases of Ni$_3$V$_2$O$_8$. Polarization sensitive absorption measurements are performed to distinguish between magnetoand electroactive spin excitations i.e. magnons and electromagnons. Different optical selection rules for magnon and electromagnon excitations enable us to search for evidence of the spin helix plane orientation and its changes at critical magnetic fields and temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T30.00009: Evolution of magnetic exchange interactions in the multiferroic Mn$_{1-x}$Fe$_{x}$WO$_{4}$ Jaime Fernandez-Baca, Feng Ye, Randy Fishman, Herb Mook, Yiming Qiu, R.P. Chaudhury, Y.-Q. Wang, B. Lorenz, C.W. Chu The experimental investigation of the spin dynamical properties in the multiferroic material Mn$_{1-x}$Fe$_{x}$WO$_{4}$ is essential to the understanding of the interplay between the magnetic and ferroelectric phenomena. We have systematically studied the low temperature magnetic excitations in the Fe-doped Mn$_{1-x}$Fe$_{x}$WO$_{4}$. The spin wave dispersion relations in the commensurate (CM) phase are well described by a model that accounts for the magnetic exchange coupling of up to nine nearest neighbors. Our results indicate that these magnetic systems are highly frustrated and the CM spin structures result from the competing interactions. The evolution of the spin dynamics reveals the role of the magnetic impurities and the influence of Fe-doping to the multiferroic properties are discussed. This work was partially supported by Division of Scientific User Facilities of the Office of Basic Energy Sciences, U.S. DOE. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T30.00010: Long-wavelength magnetic excitations in multiferroic BiFeO$_{3}$ D. Talbayev, J. G. Gigax, A. J. Taylor, Seongsu Lee, S.-W. Cheong Magnetic and lattice vibrations play a central role in the properties of multiferroics. This low-energy electrodynamics can help unravel the fundamental interactions between magnetic and lattice degrees of freedom. BiFeO$_{3}$ is a multiferroic material with robust room temperature ferroelectricity and antiferromagnetism and promising technological potential. The interaction between the ferroelectric and antiferromagnetic order parameters leads to the modification of the isotropic Heisenberg-antiferromagnet ground state that becomes an incommensurate cycloid with a very long period. The cycloidal magnetic structure results in a complex spectrum of zero-wavevector magnetic excitations; these magnetic modes were detected using Raman scattering. Here, we report a far-infrared spectroscopic study of a BiFeO$_{3}$ single crystal. We detected magnetic resonances at energies close to those reported in the Raman spectroscopy studies. The magnetic character of these excitations is supported by their characteristic temperature dependence. We will discuss our results in the context of possible electric-dipole activity of the observed resonances. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T30.00011: Low energy excitations in multiferroic Ca3CoMnO6 in the far infrared. Andrei Sushkov, H.D. Drew, Y.J. Choi, H.T. Yi, S. Lee, S.W. Cheong Ferroelctricity was recently discovered in Ca3CoMnO6 (Y.J.Choi et al., PRL 100 (2008) 047601) which is a quasi 1 D spin up-up- down-down system. We report the results of infrared (5-250 cm- 1) transmission study of multiferroic Ca3CoMnO6 as a function of temperature T (3-300~K) and magnetic field H (0-8 T). Two peaks are observed at and below T$_N=17$ K. Narrow peak at 35 cm-1 is observed at low T and is identified as the ground state feature. This feature is suppressed with raising T or in magnetic field and the other broad feature at 25 cm-1 emerges. Neither peak is split or shifted by magnetic field. We will discuss the possible origin of these two excitations. [Preview Abstract] |
Session T31: Focus Session: Magnetic, Electric, and Photo-Induced Magnetization Reversal
Sponsoring Units: DMP GMAGChair: Axel Enders, University of Nebraska
Room: 335
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T31.00001: Ultrafast magnetic imaging of nanostructures Invited Speaker: Today's technology advances into smaller and more complex structures for information processing. As structures get smaller, many processes of interest become faster as the propagation speed of excitations couple the length scale with the time scale. Microscopic techniques with a spatial resolution reaching the atomic level are being developed with very impressive success. On the other hand, time resolved techniques based on ultrafast laser systems allow us to explore processes on the femtosecond time scale. The focus of this talk is to unite the two worlds, the ultra-fast and the ultra-small. A powerful approach to time resolved microscopy is based on x-ray techniques. The wavelength of x-rays offers a spatial resolution in the nanometer range. Ultrafast x-ray techniques are currently being developed based on synchrotron sources as well as free electron lasers. The talk will demonstrate ultrafast microscopy techniques on imaging magnetization reversal dynamics in spin transfer devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T31.00002: Magnetization Dynamics of Magnetic Nano-Elements on Ultrafast Time Scales. J.P. Davis, J.A.J. Burgess, Z. Liu, R.D. Sydora, M.R. Freeman Recently, our group has systematically studied the switching of a \textit{single} permalloy nano-disk (160 nm diameter) between the vortex and quasi-single domain ground states as a function of applied bias field, including the observation of real-time switching at particular fields [1]. This was performed using the time-resolved magneto-optical Kerr effect (TR-MOKE), which allows dynamics to be studied on sub-nanosecond time scales. To accurately simulate the switching behavior, it was necessary to take into account the domed shape of the nano-disks, which result from the lift-off fabrication. Because of the sensitivity of the magnetization dynamics to the shape of the disks, we have begun fabrication of nano-disks using a shadow mask procedure [2] with a collimated deposition source under ultra-high vacuum (UHV) to produce high quality nano-elements. Magnetization dynamics of these UHV fabricated nano-disks will be discussed, with complementary scanning probe microscopy characterization of the disks.\\ 1. Z. Liu, R.D. Sydora and M.R. Freeman, PRB \textbf{77}, 174410 (2008).\\ 2. M.M. Deshmukh, D.C. Ralph, M. Thomas and J. Silcox, Appl. Phys. Lett. \textbf{75}, 1631 (1999). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T31.00003: Ultrafast switching of a nanomagnet by a combined in-plane and out-of-plane polarized spin-current pulse Oukjae Lee, V.S. Pribiag, P.M. Braganca, P.G. Gowtham, E.M. Ryan, D.C. Ralph, R.A. Buhrman For fast write operation of a spin-torque (ST) magnetic storage device, the exertion of a strong initial torque can switch the nanomagnet moment without the help of the thermal fluctuations. Use of an out-of-plane polarized reference layer can very quickly excite large free layer motion but reliable reversal requires precise ST pulse timing. The combination of strong in-plane and out-of-plane polarized spin currents can substantially relax this pulse-timing requirement. We have fabricated CPP spin-valve devices that incorporate both an out-of-plane polarizer, and an in-plane polarizer to quickly excite and reverse the moment of an in-plane polarized free layer. For pulse currents ranging between 100ps -- 10ns, the reversal speeds are notably faster and much less thermally distributed than for a conventional spin-valve with the same pulse current amplitude. We will discuss the details of the short-pulse behavior of these device structures and the optimization of this approach for high-speed magnetic memory. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T31.00004: Total angular momentum conservation in laser-induced femtosecond magnetism Guoping Zhang, Yihua Bai, Thomas F. George Spin momentum is not a classical quantity [1,2]. It is unclear how the conservation law affects spin momentum change in laser-induced femtosecond magnetization [3]. In solids, the rotational symmetry is lifted by the translational symmetry, and the spin and orbital momenta components of different total angular momenta mix to some extent. This mixing is the origin of the time-dependent total angular momentum in experiments. The remaining unmixed portion accounts for an extra spin change in three independent circularly-polarized laser experiments [4]. \\[0pt] [1] G. P. Zhang, Y. Bai, W. H\"ubner, G. Lefkidis, and T. F. George, J. Appl. Phys. {\bf 103}, 07B113 (2008). [2] G. P. Zhang and W. H\"ubner, Phys. Rev. Lett. {\bf 85}, 3025 (2000). [3] G. P. Zhang, Phys. Rev. Lett. {\bf 101}, 187203 (2008). [4] G. P. Zhang and T. F. George, Phys. Rev. B {\bf 78}, 052407 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T31.00005: Evolution of magnetic states in ferromagnetic nanorings in an applied azimuthal field Abby Goldman, Katherine Aidala, Tianyu Yang, Mark Tuominen Ferromagnetic nanorings form unique magnetic states that hold tremendous potential for maximizing data storage densities. One such state is the closed-flux vortex state, in which the magnetic field is completely enclosed within the ring, thus minimizing the magnetostatic energy, but also keeping the exchange energy low as adjacent magnetic moments are mostly aligned. A natural way to generate this state is through an external azimuthal field, as if from a current carrying wire passing through the center of the ring. We perform micromagnetic simulations to investigate the evolution of magnetic states in an external azimuthal field. For some applied current, the chirality of the ring will reverse, often into an intermediate state that evolves into a perfect vortex at higher current. Thin, wide rings have significantly lower switching currents than thick, narrow rings. We examine the dependence of the switching current and intermediate states on geometric properties such as the diameter, thickness, asymmetry and width of the ring. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T31.00006: Polarity reversal of magnetic vortex core by in-plane non-resonant pulsed magnetic field Xuemei Cheng, David Keavney, Kristen Buchanan Magnetic vortices have been of great interest because of their potential applications in non-volatile data storage. Recently, Van Waeyenberge et al. demonstrated vortex core reversal in permalloy squares using an in-plane r.f. excitation field with the frequency close to the translational-mode eigenfrequency. In this work we report polarity reversal of magnetic vortex core by non-resonant in-plane field pulses. The core polarity was first determined by watching the sense of vortex core gyration in a 6 micron permalloy disk imaged by time-resolved x-ray photoemission electron microscopy (TR-PEEM) with 1mT excitation field pulses. After the waveguide was pulsed at 5mT, we determined core polarity at 1mT again. We demonstrate that the core polarity can be switched back and forth by pulsing at 5mT. The micromagnetic simulations and TR-PEEM images confirm that when the core is displaced beyond 25{\%} of disk radius, distortions of the core region occur, promoting a transient domain state involving a complex cross-tie wall, and subsequent reversal of the core polarity. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T31.00007: Hybridization of quantum spin waves in small structures Valentyn Novosad, S.T. Chui, Sam Bader We apply the Holstein-Primakoff and Bogoliubov transformations to compute the spin wave states of small magnetic structures including the effect of the dipolar interaction. We found that as the film gets thicker, states with a significant q=0 component are hybridized with states with higher Fourier components. In the presence of a static magnetic field opposite to the magnetization direction, surface states that are responsible for magnetization reversal are coupled to the extended states. The response function is increased by an order of magnitude. This suggests an intriguing scenario for assisted switching of the magnetization with an additional external a.c. field. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T31.00008: Electric field-induced modification of magnetism in thin CoPd films. Mikhail Zhernenkov, Michael Fitzsimmons, Jerzy Chlistunoff, Jarek Majewski, Ioan Tudosa, Eric Fullerton Recently, M. Weisheit et al., [Science \textbf{315}, 349 (2007)] reported modification of magnetic properties of thin-film ferromagnets by applying a large electric field at the surface of a ferromagnet. For an applied voltage of -0.6 V, the coercivities of 20 {\AA} thick FePt and FePd films were changed by -4.5 and +1{\%}, respectively. Here, we report polarized neutron reflectometry measurements of the magnetization depth profile of a 180 {\AA} thick Co$_{50}$Pd$_{50}$ film immersed in an electrolyte as a function of applied electric field in an external magnetic field. The measurements were done at two values of applied electric potential of -0.6 volts and -1.2 volts and at zero volts (open-circuit potential). The applied magnetic field was 3 kG. We found a linear increase of the film magnetization with electric field. The change of magnetization occurred in the region of the film within 72 {\AA} of the electrolyte/Co$_{50}$Pd$_{50}$ interface. The magnetization of the top part of the layer is increased by 2{\%} (-0.6 V) and 3.6{\%} (-1.2 V) compared to the open circuit potential result. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T31.00009: Electric-field-driven spin resonance of a Mn dopant in GaAs V.R. Povilus, J.-M. Tang, M.E. Flatt\'e All-electric manipulation of the ground-state spin system of a Mn dopant in GaAs using a static electric field in combination with a transverse dynamic electric field requires careful positioning of two gates on the nanoscale[1]. Here we propose a method of efficiently controlling the spin of a Mn dopant using parallel static and dynamic electric fields, but adding a small static magnetic field. In a scalable geometry this would permit full control of the ground-state J=1 spin of a Mn dopant using a single electric gate. The energy states and eigenfunctions of the Mn dopant system under the influence of both an electric and magnetic field cannot be simply described using an effective electric-field-dependent g tensor, as would be done in g tensor modulation resonance[2]. However, the dynamical equations for the spin can be numerically solved and exhibits high-visibility Rabi oscillations. For example, with a static electric field of 500 V/cm, a dynamic electric field of 300 V/cm with frequency 9.1 GHz, and a magnetic field of 0.1 Tesla, all oriented in the [113] direction, the Rabi oscillation period is 2.7 ns.\\[0pt] [1] J.-M Tang, Jeremy Levy, and M. E. Flatt\'e, Phys. Rev. Lett. 97, 106803 (2006).\\[0pt] [2] Y. Kato, et al., Science 299, 1201 (2003). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T31.00010: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T31.00011: Photoinduced Magnetism of Ternary Transition Metal Prussian Blue Analogs D.M. Pajerowski, M.W. Meisel, J.E. Gardner, D.R. Talham The magnetism of Prussian blue analog materials (PBAs) can be tuned with external stimuli such as temperature, pressure, and light. Recently, novel effects have been seen in PBAs with substitutionally mixed ternary and quaternary transition metals, rather than the usual binary analogs. One noteworthy material we have studied is a Na$_{a}$Ni$_{1-x}$Co$_{x}$[Fe(CN)$_{6}$]$_{b}$$\cdot$nH$_{2}$O powder, which can show either a photoinduced increase or \emph{decrease} in magnetization depending upon the Ni substitution, the applied magnetic field, and the temperature. This result is the first example of a photoinduced decrease in magnetization while generating new spins via a charge transfer induced spin transition (CTIST) in a bulk material. Constrastingly, the photodecrease observed in PBA binary thin films has different microscopic origins [1-2]. Insight into the underlying mechanisms can be obtained by using mean field models, which qualitatively reproduce the experimental data. SQUID magnetometer, FT-IR, TEM, and EDS data will be presented.\newline [1] J.-H. Park, \emph{et al.}, Appl. Phys. Lett. \textbf{85}, 3797 (2004). \newline [2] F. A. Frye \emph{et al.}, Chem. Mater. \textbf{20}, 5706 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T31.00012: Bismuth in strong magnetic fields: unconventional Zeeman coupling and correlation effects Jason Alicea, Leon Balents Recent experiments on bismuth have uncovered remarkably rich magnetization structure at fields well beyond the regime in which all carriers are expected to reside in the lowest Landau level. Motivated by these findings, we start from a microscopic tight-binding model and derive a low-energy Hamiltonian for the holes and three Dirac electrons pockets in bismuth. We find that an unconventional electron Zeeman effect, overlooked previously, suppresses the quantum limit for the electrons dramatically, giving rise to the observed anomalous magnetization structure. We further study interaction effects near fields at which the 2nd Landau level for one electron pocket empties, where magnetization hysteresis was observed. Here we find instabilities towards both charge density wave and Wigner crystal phases, and propose that hysteresis arises from a first-order transition out of the latter. [Preview Abstract] |
Session T32: Focus Session: Gilbert Damping and Non-local Spin Injection
Sponsoring Units: GMAG DMP FIAPChair: Gerrit E. W. Bauer, Technical University Delft
Room: 336
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T32.00001: Anisotropic Damping in Single-Crystalline Ni/MgO(001) studied by the Time-Resolved Magneto-Optical Kerr effect (TR-MOKE) Kevin Smith, A. Lukaszew, J. Skuza, C. Clavero, K. Yang, A. Reilly, G. L\"{u}pke The damping behavior of uniform spin precession in single- crystalline Ni/MgO(001) of various thicknesses from t = 10 nm to t = 60 nm is investigated in the time domain using TR-MOKE over a wide range of external field parameters and temperatures. Planar measurements indicate that the effective Gilbert damping parameter, $\alpha_{\mathrm{eff}}$, is coupled to the magnetocrystalline anisotropy, as $\alpha_ {\mathrm{eff}}$ ranges from 0.05 near the hard axis to 0.10 near the easy axis. Previous experiments by other groups using FMR [1] and TR-MOKE on polycrystalline samples [2] have placed the intrinsic value of the damping at 0.045. When the field is applied normal to the film surface, $\alpha_{\mathrm {eff}}$ increases to as high as 0.3 when the angle of the magnetization, $\phi_{\mathrm{M}}$, is greater than 45 degrees out of plane. These results are discussed in terms of various models of extrinsic damping mechanisms, such as two magnon scattering. [1] S. Bhagat et al. Phys. Rev. B, \textbf{10} 179 (1974) [2] J. Walwoski et al. J. Phys. D: Appl. Phys. \textbf{41} 164016 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T32.00002: Scattering Theory of Gilbert Damping Arne Brataas, Yaroslav Tserkovnyak, Gerrit E.W. Bauer Magnetization relaxation is a collective many-body phenomenon that remains intriguing despite decades of theoretical and experimental investigations. It is important in topics of current interest since it determines the magnetization dynamics in magnetic memory devices and state-of-the-art magnetoelectronics experiments on current-induced magnetization dynamics [1]. We study the magnetization dynamics of a single domain ferromagnet in contact with a thermal bath by scattering theory. We recover the Landau-Lifshitz-Gilbert equation and express the Gilbert damping tensor in terms of the scattering matrix [2]. Dissipation of magnetic energy equals energy current pumped out of the system by the time-dependent magnetization, with separable spin-relaxation induced bulk and spin-pumping generated interface contributions. In linear response, our scattering theory for the Gilbert damping tensor is equivalent with the Kubo formalism [1] M. Stiles and J. Miltat, Top. Appl. Phys. 101, 225 (2006), and references therein. [2] A. Brataas, Y. Tserkovnyak, and G. E. W. Bauer, Phys. Rev. Lett. 101, 037207 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T32.00003: Physical damping processes in Co-Cr granular films. Sangita Kalarickal, Pavol Krivosik, Nan Mo, Carl Patton, Stella Wu Recent ferromagnetic resonance (FMR) results on metallic ferromagnetic alloy films have shown that a simple one parameter Gilbert damping description is inadequate for most systems. New FMR results have been obtained on Co-Cr granular films with the columnar microstructure amenable to perpendicular media applications. The nominal 17.3 GHz FMR field and linewidth vs. the out-of-plane field angle was measured and analyzed for a 16 nm thick granular film with a relatively low effective anisotropy field of 1 kOe and a nominal grain size of 8 nm or so. The analysis reveals a three component linewidth comprised of a small Gilbert term with an $\alpha $-value of 0.003 that is consistent with intrinsic processes, a large two magnon term that derives from the grain-to-grain anisotropy variations, and an inhomogeneity broadening term due to anisotropy dispersion and grain size variations. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T32.00004: Effect of rapid thermal annealing on ferromagnetic resonance line width of CoFeB thin films Yaping Zhang, Xin Fan, Weigang Wang, Xing Chen, Chaoying Ni, Rong Cao, John Xiao Magnetization dynamics has attracted much attention recently due to their implication on magnetic recording and storage applications. CoFeB material is one of most common magnetic layers used in MgO based magnetic tunnel junctions (MTJs). Unlike traditional thermal treatments, a giant Tunneling Magnetoresistance can be archived by rapid thermal annealing (RTA) [1]. We show the effect of RTA on magnetization dynamics. CoFeB thin film, subjected to RTA at 380 \r{ }C for various time, was investigated by ferromagnetic resonance (FMR) measurement. It is found that FMR linewidth reaches a minimum at 60 second annealing, after which the linewidth increases with annealing time. A clear trend of decreasing of uniaxial anisotropy and increasing of cubic anisotropy with annealing time indicates that competition between these anisotropies plays an important role in linewidth evolution. [1] WG. Wang et al., Appl. Phys. Lett. 92, 152501 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T32.00005: Calculation of intrinsic damping in half metals Chunsheng Liu, Claudia K.A. Mewes, Mairbek Chshiev, Tim Mewes, William H. Butler The extended Hueckel tight binding method in combination with Kambersky's torque correlation model [1] is used to calculate the precessional magnetization relaxation in half-metallic systems. In Kambersky's model damping is described by a combination of spin-flip excitations and orbital excitations. An analytical expression of the transition matrix element which represents scattering events within a single band (intraband) and between different bands (interband) respectively [2] can be obtained within the TB scheme, which enables a better understanding of the damping mechanisms in half-metallic structures. Due to the absence of spin-flip scattering in half-metallic systems, the Gilbert damping rate of half-metals is expected to be much smaller than that of metals. Using this approach we calculated the damping for different half-metallic structures. The minimum intrinisic relaxation rate $\lambda $ was calculated to be, 3.2 MHz, 1.1 MHz, 0.13 MHz, for the Heusler structures Co$_{2}$MnGe , Co$_{2}$MnSi and the Rutile structure CrO$_{2}$ respectively. The damping rates for these half-metallic materials are much lower than that of bcc Fe, as we anticipated from the analytical analysis. References: [1] V. Kambersky, Czech. J. Phys. B 26, 1366 (1976). [2] K. Gilmore, Y.U. Idzerda and M.D. Stiles, Phys. Rev. Lett. 99, 027204 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T32.00006: Unidirectional Damping in Exchange Biased Systems Matthew Bradford, Hwachol Lee, Eric Edwards, Zeenath Tadisina, Claudia Mewes, Subhadra Gupta, Tim Mewes We report on the investigation of the angular dependence of the damping parameter in thin film NiFe, exchange biased by a layer of FeMn. By using a broadband ferromagnetic resonance technique (shorted waveguide), the resonant field and linewidth were determined as a function of the in-plane angle and the microwave frequency. We find that the effective damping parameter, as extracted from the frequency dependent linewidth data, shows a unidirectional anisotropy, displaying a sinusoidal behavior with respect to the in-plane angle. The effective damping parameter is minimal when the field during FMR measurements is applied parallel to the exchange bias direction and maximal for antiparallel alignment. These experiments in conjunction with thickness dependent measurements suggest that uncompensated spins at the ferromagnet/antiferromagnet interface are responsible for the increased magnetization relaxation observed in these structures. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T32.00007: The origin of intrinsic Gilbert damping Mark C. Hickey, Jagadeesh S. Moodera The damping of magnetization, represented by the rate at which it relaxes to equilibrium, is successfully modeled as a phenomenological extension in the Landau-Lifschitz-Gilbert equation. This is the damping torque term known as Gilbert damping and its direction is given by the vector product of the magnetization and its time derivative. Here we derive the Gilbert term from first principles by a non-relativistic expansion of the Dirac equation. We find that the Gilbert term arises when one calculates the time evolution of the spin observable in the presence of the full spin-orbital coupling terms, while recognizing the relationship between the curl of the electric field and the time varying magnetic induction. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:30PM |
T32.00008: Theory of Magnetization Relaxation in Conducting Ferromagnets Invited Speaker: The advent of technologically promising spintronics devices has revived efforts to achieve a microscopic understanding of magnetization damping in magnetic metals and semiconductors. In absence of an electric current, magnetization relaxation is described by the Gilbert parameter $\alpha$, whose quantitative prediction relies on electronic structure calculations that treat disorder in an approximate fashion. We assess the reliability of these studies by using simple models where disorder may be treated exactly. Transport currents modify the magnetization damping. We associate this change with the non-adiabatic spin transfer torque, which is characterized by a dimensionless parameter $\beta$. We derive a concise analytical expression for $\beta$ that can be applied to real materials. We also discuss the counterparts of $\alpha$ and $\beta$ in out-of-equilibrium superconductors viewed as easy-plane ferromagnets in particle-hole space. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T32.00009: Nonlocal Spin Valves With Very Short Injector Detector Distances Andrew McCallum, Mark Johnson Nonlocal spin valves with a center to center distance of 42 nm between spin injector and detector have been fabricated. This distance is much less than the injector detector spacing in previously made nonlocal spin valves and is much shorter than the spin diffusion length of the Cu used as a channel. Nonlocal resistance changes of up to 2.6 $m\Omega$ were seen in these devices at room temperature. From this data it was determined that the average spin polarization of the ferromagnetic interfaces is between 4.3\% and 5.9\% at room temperature. The nonlocal resistance changes of these devices are much less sensitive to changes in temperature, as determined by measurements at liquid nitrogen temperature, than nonlocal spin valves with longer injector detector distances. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T32.00010: Temperature dependence of the non-local spin signal in Cu-based lateral spin-valves M.J. Erickson, C. Leighton, P.A. Crowell We report on measurements of the $T$ dependence of the non-local spin signal in lateral metallic spin valves, focusing on the limit of transparent ferromagnet (FM) / normal metal (Cu) interfaces. Devices with channel width 250 nm and contact widths $\approx $100 nm (Ni$_{80}$Fe$_{20}$ or Co) were fabricated using in-situ shadow masking. We employed high purity sources in UHV, enabling one-shot deposition with no air exposure of the interface. Multiple contact separations (250 -- 800 nm) were fabricated on a single substrate to facilitate measurement of the spin diffusion length ($\lambda _{s})$. NiFe/Cu devices with 250 nm contact separation show a maximum non-local transresistance of 420 $\mu \Omega $. Analysis of Hanle effect measurements yields spin lifetimes $\approx $ 8 ps at low $T$ which compare well to those extracted from the measured $\lambda _{s}$ (300 nm) and resistivity (1.5 $\mu \Omega $cm), demonstrating consistency of our analysis. We observe a qualitatively different $T$ dependence of the non-local signal depending on the relative sizes of the contact separation and $\lambda _{s}$. When the separation becomes comparable to $\lambda _{s}$ we observe a maximum in the non-local spin signal at 35 -- 85 K, with strongly thickness dependent magnitude. These measurements of spin lifetime, resistivity, and $\lambda _{s}$ vs $T$ allow a quantitative comparison with the conductivity mismatch model. Work supported by the NSF MRSEC program. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T32.00011: Bias-independent spin signals in a tunnel-junction-based non-local spin valve Xiaojun Wang, Han Zou, L.E. Ocola, R. Divan, Yi Ji A pure spin current can be generated in the non-magnetic component of a non-local spin valve (NLSV). It has been demonstrated recently that the pure spin current can be used for spin transfer torque and spin-Hall effects. A high spin current density is desirable for realizing these effects, and therefore a large d.c. bias current will be applied. It is essential to maintain high degree of spin polarization at a high bias current. It has been previously reported that the spin polarization decreases drastically in a tunnel-junction-based CoFe/Al/NiFe NLSV. The goal of this study is to investigate the dependence of spin signals upon a d.c. bias current in tunnel-junction-based Co/Cu/Co NLSV's. Submicron Co/Cu/Co NLSV's are fabricated by e-beam lithography combined with angle deposition. A layer of 2 nm Al$_{2}$O$_{3}$ is deposited at the Co/Cu interface to form a tunnel barrier. A spin signal $>$ 1m$\Omega $ is observed at room temperature (RT). A d.c. current up to 1.0mA is applied at both 4.2 K and RT. No change of spin signal is observed for an injection current density $>$ 10$^{6}$ A/cm$^{2}$. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T32.00012: Separating the Contributions of Spin Injection Efficiency and Spin Diffusion Length in Non Local Spin Valves Mikhail Erekhinsky, Felix Casanova, Amos Sharoni, Ivan K. Schuller Non Local Spin Valves (NLSV) are unique devices which permit studies of spin transport related phenomena at the nanoscale. Two important parameters determine the signal in NLSV: a) the effective polarization of injected current from the ferromagnet (FM) through the interface, and b) the spin diffusion length of the non-magnetic metal (NM). We performed a systematic study of the NLSV signal for different device lengths as a function of NM thickness, composition and temperature. By fitting the near-exponential decay of the signal with distance we can separate the effects of polarization and NM spin diffusion length. We will discuss the contributions of surface effects on NM spin diffusion length, and FM/NM interface on efficiency of injection. In addition, we show the importance of adjacent FM electrodes in a multi-terminal spintronics device. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T32.00013: Control of Spin Injection by DC Current in Transparent Lateral Spin Valves Amos Sharoni, Felix Casanova, Mikhail Erekhinsky, Ivan K. Schuller Lateral spin valves can be classified according to the type of interfaces (tunnel or transparent) between the ferromagnetic electrodes and the normal material. The tunnel barrier yields a large spin signal, but the maximum spin-polarized current was reported to decrease strongly with applied bias. In transparent interfaces the maximum current density through the interface is much larger, which is beneficiary for some applications, but the effect on spin signal was not measured. To address this issue, we prepared metallic lateral spin valves with excellent transparent interfaces. In addition, instead of AC lock-in techniques commonly used to measure these devices, we perform DC measurements, which enables us to measure directly the effects of the current direction and magnitude on the spin signal. We compared the injection of majority spins from the ferromagnet into a non-magnetic metal (NM) with the reversed process where minority carriers are left in the NM. We were also able to study the effect of joule heating and identified the origin of voltage backgrounds usually observed in these devices. [Preview Abstract] |
Session T33: Focus Session: Iron Pnictides and Other Novel Superconductors XI: Penetration Depth and Magnetic Anisotropy
Sponsoring Units: DMPChair: Myron Salomon, University of Texas at Dallas
Room: 403
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T33.00001: Temperature Dependence of the Penetration Depth of LaFePO from Scanning SQUID Susceptometry Cliffird Hicks, Thomas Lippman, Martin Huber, James Analytis, Jiun-Haw Chu, Ian Fisher, Kathryn Moler We use a scanning SQUID susceptometer to measure locally the temperature dependence of the penetration depth of the superconductor LaFePO. We observe a linear temperature dependence down to 850 mK, with a slope that varies with position on the sample. This is in contrast to recent measurements on the related iron arsenide family of superconductors, which indicate fully-gapped superconductivity. We also report on our ability to measure T$_c$ locally, and observe local regions with weak superconductivity at temperatures well above the dominant T$_c$. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T33.00002: Broadband microwave absorption of Fe-based superconductors Jake Bobowski, Pinder Dosanjh, James Day, Doug Bonn, Walter Hardy Preliminary microwave measurements of the penetration depth in single crystals of the hole-doped iron-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, using cavity perturbation and ac susceptometry, have been performed by our group. These results hint at a gap with nodes plus the presence of scattering. As a complement to these techniques, we have also undertaken broadband microwave absorption (i.e., surface resistance) measurements on the same pnictide samples. Our initial results of the microwave conductivity are consistent with a system possessing a finite density of states at low temperature due to impurity scattering which then evolves with temperature above about 5 K. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T33.00003: Lower Critical Fields and the Anisotropy in PrFeAsO$_{1-y}$ Single Crystals Ryuji Okazaki, Marcin Konczykowski, C.J. van der Beek, Tomonari Kato, Ken-ichiro Hashimoto, Masaaki Shimozawa, Hiroaki Shishido, Minoru Yamashita, Takasada Shibauchi, Motoyuki Ishikado, Shin-ichi Shamoto, Hijiri Kito, Akira Iyo, Hiroshi Eisaki, Yuji Matsuda By utilizing miniature Hall-sensor array, we evaluated the lower critical fields $H_{c1}$ in Fe-based oxipnictide PrFeAsO$_{1-y}$ single crystals for {\boldmath $H$}$\parallel c$ and {\boldmath $H$}$\parallel ab$-planes. The temperature dependence of $H_{c1}$ for {\boldmath $H$}$\parallel c$ is well scaled by the in-plane penetration depth and is consistent with a full-gap superconducting state. The anisotropy of penetration depths at low temperatures is estimated to be $\simeq$ 3, which is much smaller than that of coherence lengths. This indicates the multiband superconductivity, in which the active band for the superconductivity is more anisotropic. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T33.00004: Temperature Dependent Anisotropy of Oxypnictide Superconductors Studied by Torque Magnetometry Stephen Weyeneth, Roman Puzniak, Nikolai D. Zhigadlo, Sergiy Katrych, Zbigniew Bukowski, Janusz Karpinski, Urs Mosele, Stefan Kohout, Josef Roos, Hugo Keller Single crystals of different oxypnictide superconductors of the family ReFeAsO$_{1-x}$F$_{y}$ (Re $=$ Sm, Nd, Pr) with various carrier dopings and with masses $m\simeq100$ ng have been investigated by means of torque magnetometry. We present most recent data, obtained by using highly sensitive piezoresistive torque sensors from which the superconducting anisotropy parameter $\gamma$ and the in-plane magnetic penetration depth $\lambda_{ab}$ were extracted. As an important result $\gamma$ was found to increase strongly as the temperature is decreased from $T_c$ down to low temperatures. This unconventional temperature dependence of $\gamma$ is similar to that observed in the two-band superconductor MgB$_2$ and cannot be explained within the classical Ginzburg-Landau model. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T33.00005: Magnetic penetration depth and critical current density in aligned grains of NdFeAsO(F) Yuri L. Zuev, E. D. Specht, D. K. Christen, J. R. Thompson, R. Jin, B. C. Sales, A. Sefat, M. A. McGuire, D. G. Mandrus We have prepared a powder sample of NdFeAsOF, where most crystallite particles are aligned with their c-axis along a common direction. We have measured magnetic penetration depth and a critical current density in the basal-plane. The penetration depth shows a fully gapped superconducting state with two energy gaps, one roughly twice as large as the other. The in-plane critical current density is as high as several MA/cm$^{2}$ at low temperatures, zero field, which may be promising for applications. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T33.00006: Low temperature penetration depth of Fe-based superconductors James Day, Brad Ramshaw, Jake Bobowski, Pinder Dosanjh, Doug Bonn, Walter Hardy The recent discovery of superconductivity in layered iron-based pnictide compounds has generated levels of excitement comparable to the early days of the copper-based oxides. While connections may be drawn between the cuprates and the pnictides, there exist important differences between the two materials. We report high-precision microwave measurements of the penetration depth in single crystals of the hole-doped iron-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. Using cavity perturbation techniques in conjunction with ac susceptometry, we find that the low temperature penetration depth does not fit well to a simple s-wave model. Furthermore, our initial results suggest that the temperature dependence of the superfluid density is consistent with a gap with nodes plus the presence of scattering. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T33.00007: Penetration Depth and Quasiparticle Conductivity in Iron-based High-Temperature Superconductors Ken-ichiro Hashimoto, Takasada Shibauchi, Tomonari Kato, Kosuke Ikada, Ryuji Okazaki, Hiroaki Shishido, C.J. van der Beek, M. Konczykowski, Hijiri Kito, Akira Iyo, Hiroshi Eisaki, Motoyuki Ishikado, Shin-ichi Shamoto, H. Takeya, K. Hirata, Shigeru Kasahara, Takahito Terashima, Yuji Matsuda We measure the in-plane penetration depth and the quasiparticle conductivity in the newly discovered Fe-based high-temperature superconductors PrFeAsO$_{1-y}$[1] and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$[2], using a sensitive superconducting resonator. We show that the penetration depth exhibits a very flat behavior at low temperatures in both compounds, which indicates that the superconducting gap opens up all over the Fermi surface. The temperature dependence of the superfluid density is well fitted with the two gap model, suggesting the multi-gap nature of superconductivity in this system. Moreover, the observed large enhancement of the quasiparticle conductivity suggests a suppression of the quasiparticle scattering, reminiscent of the superconductors in strongly correlated electronic systems.[1] K. Hashimoto et al., arXiv:0806.3149., [2] K. Hashimoto et al., arXiv:0810.3506. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T33.00008: London Penetration Depth in Single Crystals of Nd(Fe,Co)As(O,F) Superconductors. H. Kim, C. Martin, R.T. Gordon, M.A. Tanatar, M.E. Tillman, S. Kim, S.L. Bud'ko, P.C. Canfield, R. Prozorov Iron arsenide superconductors, with the general formula RFeAsO$_ {1-x}$F$_x$ (R=Nd,Sm,Pr,Gd), exhibit the highest transition temperatures among the compounds of the pnictide family, in excess of 50 K. London penetration depth studies performed on single crystals, grown under high pressure with nominal fluorine content x=0.1, have revealed a nodeless superconducting state with a modestly anisotropic gap\footnote {C. Martin \textit{et al.}, arXiv:0807.0876}. Since doping is one of the most efficient ways to perturb the superconductivity, we explore here the evolution of the London penetration depth with doping, achieved by F substitution on O sites and by Co substitution on Fe sites. Notable differences in the superconducting transition temperatures for the two sets of crystals suggests the importance of impurity scattering for superconductivity in these compounds. Systematic variation of the London penetration depth with doping will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T33.00009: Co-concentration dependence of the London penetration depth in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ R.T. Gordon, C. Martin, H. Kim, M.A. Tanatar, N. Ni, S.L. Bud'ko, P.C. Canfield, J. Schmalian, R. Prozorov The in-plane London penetration depth, $\lambda_{ab}$(T), has been measured in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for several values of $x$ using a tunnel diode resonator (TDR) technique. The low temperature behavior of the variation of the penetration depth follows a clear power law behavior, $\Delta\lambda$(T) $\propto$ T$^n$, for all measured Co concentrations with n varying between 2 on the underdoped side to about 2.5 at the optimal doping. This non-exponential behavior in $\Delta\lambda$ indicates the existence of a significant number of superconducting quasiparticles, which is characteristic of a superconducting gap function possessing some type of nodal structure. The overal change in the superfluid density as a function of doping will be discussed. R. T. Gordon $\textit{et al.}$ arXiv:0810.2295 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T33.00010: Magnetic force microscopy study of a Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$ single crystal: Local penetration depth and flux pinning Lan Luan, Ophir M. Auslaender, Kathryn A. Moler, James G. Analytis, Jiun-Haw Chu, Ian R. Fisher We use a magnetic force microscope (MFM) to study both the penetration depth ($\lambda _{ab} )$ and flux pinning in the iron-pnictide superconductor Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$ by imaging and manipulating vortices. We observe the same regular signature from all vortices in a 50$\mu m$ x 50$\mu m$ field of view, implying little or no inhomogeneity of the superfluid density down to the sub-micron scale. Quantitative analysis of images of isolated vortices and of the Meissner repulsion of the magnetic tip from the sample gives the temperature-induced change of $\lambda _{ab} $ and an estimate for its absolute value. We detect no long-range order in the vortex positions, suggesting the absence of correlated pinning in the material. We measure the force required to depin individual vortices and the force required to drag them across the entire sample. This allows us to characterize the pinning potential in the material and its distribution as well as to set bounds on the local critical current. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T33.00011: Evidence for Two Energy Gaps in Superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Single Crystals and Breakdown of the Uemura Plot Cong Ren, Zhao-Sheng Wang, Hui-Qian Luo, Huan Yang, Lei Shan, Hai-Hu Wen We report a detailed investigation on the lower critical field $H_{c1}$ of the superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (FeAs-122) single crystals. A pronounced kink is observed on the $H_{c1}(T)$ curve, which is attributed to the existence of two superconducting gaps. By fitting the data $H_{c1}(T)$ to the two-gap BCS model in full temperature region, a small gap of $\Delta_a(0)=2.0\pm 0.3$ meV and a large gap of $\Delta_b(0) =8.9 \pm 0.4$ meV are obtained. The in-plane penetration depth $\lambda_{ab}(0)$ is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T33.00012: Anisotropic London Penetration depth in NdFeAsO$_{0.9}$F$_{0.1}$ and Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$. Catalin Martin, R.T. Gordon, M.A. Tanatar, H. Kim, M.E. Tillman, N. Ni, P.C. Canfield, V.G. Kogan, R. Prozorov, H. Lou, Z. Wang, H.H. Wen In and out of plane London penetration depth was measured in single crystals of NdFeAsO$_{0.9}$F$_{0.1}$ (Nd-1111) and Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$ (BaK-122) as a function of temperature by using an rf-resonator technique. In Nd-1111, penetration depth shows exponential behavior at low temperature, implying a fully-gapped Fermi surface. Superfluid density is best described in the full temperature range by a slightly anisotropic order parameter. In contrast, penetration depth of BaK-122 shows power-law temperature dependence ($\Delta\lambda(T)\propto T^{n}$, n$\approx$2) down to T$approx$0.02T$_{c}$ and anisotropy $\gamma_{\lambda}$ varies from 2 at T$_{c}$ to about 4 at 0.5 K. Possible symmetries of the gap consistent with such behavior and comparison with results from other techniques and with proposed theoretical models will be discussed. Also, the temperature dependence of the anisotropy $gamma_{\lambda}=\lambda_{c}/\lambda_{ab}$, for both compounds, will be compared to that of $\gamma_{H_{c2}}=H_{c2}^{c}/H_{c2}^ {ab}$, in connection with possible multiband superconductivity. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T33.00013: Muon Spin Relaxation Study of Ba(Fe$_{0.93}$Co$_{0.07})_{2}$As$_{2}$ Graeme Luke, A. Aczel, J.P. Carlo, T. Goko, T.J. Williams, N. Ni, S.L. Bud'ko, P.C. Canfield, Y.J. Uemura We have performed muon spin relaxation ($\mu $SR) measurements of superconducting Ba(Fe$_{0.93}$Co$_{0.07})_{2}$As$_{2}$. Zero field measurements show that static magnetism is absent for this Co concentration. In the mixed state, we observe a well-formed vortex lattice which results in an anisotropic $\mu $SR lineshape. We have fit the spectra to a microscopic model for the vortex state to obtain the magnetic field penetration depth. The penetration depth is about 2000{\AA} in low (200G) fields and increases with applied field. The temperature dependence of the superfluid density is well described by a power law; behaviour which is consistent with gap nodes, substantial gap anisotropy or multi-band superconductivity. Research at McMaster University is supported by NSERC and CIFAR. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T33.00014: Anisotropic superconducting properties of aligned Sm$_{0.95}$La$_{0.05}$FeAsO$_{0.85}$F$_{0.15}$ microcrystalline powder H.C. Ku, B.C. Chang, C.H. Hsu, Y.Y. Hsu, Z. Wei, K.Q. Ruan, X.G. Li The Sm$_{0.95}$La$_{0.05}$FeAsO$_{0.85}$F$_{0.15}$ compound is a quasi-2D layered superconductor with a superconducting transition temperature T$_c$ = 52 K. Due to the Fe spin-orbital related anisotropic exchange coupling (antiferromagnetic or ferromagnetic fluctuation), the tetragonal microcrystalline powder can be aligned at room temperature using the field- rotation method where the tetragonal $\it{ab}$-plane is parallel to the aligned magnetic field B$_{a}$ and $\it{c}$- axis along the rotation axis. Anisotropic superconducting properties with anisotropic diamagnetic ratio $\chi_{c}$/$\chi_ {ab}\sim$ 2.4 + 0.6 was observed from low field susceptibility $\chi$(T) and magnetization M(B$_{a}$). The anisotropic low- field phase diagram with the variation of lower critical field gives a zero-temperature penetration depth $\lambda_{c}$(0) = 280 nm and $\lambda_{ab}$(0) = 120 nm. The magnetic fluctuation used for powder alignment at 300 K may be related with the pairing mechanism of superconductivity at lower temperature. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T33.00015: Diminishing anisotropy of PrFeAsO$_{1-y}$ single crystal as temperature goes zero Daichi Kubota, Takekazu Ishida, Motoyuki Ishikado, Shin-ichi Shamoto, Hiroshi Eisaki, Hijiri Kito, Akira Iyo The magnetic torque of a high-quality PrFeAsO$_{1-y}$ single crystal has been investigated at temperatures from 10 K to 45 K in magnetic fields from 5 kG and 50 kG. The torque curves of the PrFeAsO$_{1-y}$ single crystal have been measured systematically. The superconducting anisotropy of PrFeAsO$_{1-y}$ is determined by analyzing the torque curves. The temperature and field dependence of $\gamma$ reflect that the Fe-oxypnictide superconductivity has the rather three- dimensional isotropic nature of Fe-As layer network. We consider that this is good evidence for the multi-band superconductivity in Fe-oxypnictide. We find that the superconducting anisotropy $\gamma$ in PrFeAsO$_{1-y}$ can be approximated by $\gamma = 1.08 + 0.0068 T$ in all fields by employing the Kogan model. We find it surprising to have such a low anisotropy parameter $\gamma$ even for a layered superconductor. Such a small anisotropic parameter in Fe-pnictides is preferential to ensure the intergrain connectivity and the resulting high critical current density $J_c$. [Preview Abstract] |
Session T34: Superconductivity: Vortex Statics and Dynamics
Sponsoring Units: DCMPChair: Wai Kwok, Argonne National Laboratory
Room: 404
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T34.00001: Manipulation of interlayer ``kinks'' in individual vortices in underdoped YBa$_{2}$Cu$_{3}$O$_{6+x}$ Ophir Auslaender, Lan Luan, Douglas Bonn, Ruixing Liang, Walter Hardy, Kathryn Moler We use magnetic force microscopy (MFM) to both image and manipulate individual vortex lines threading single crystalline YBa$_{2}$Cu$_{3}$O$_{6+x}$, a layered superconductor. We find that when we pull the top of a pinned vortex, it may not tilt smoothly, as in more isotropic superconductors [1]. In some cases, we observe a vortex breaking into discrete segments that can be described as short stacks of two-dimensional pancake vortices. This is similar to the ``kinked'' structure proposed by Benkraouda and Clem [2]. Quantitative analysis gives an estimate of the pinning force and the coupling between the pancake stacks. Our measurements highlight the discrete nature of stacks of pancake vortices in layered superconductors. \begin{enumerate} \item O. M. Auslaender, Lan Luan, E. W. J. Straver, J. E. Hoffman, N. C. Koshnick, E. Zeldov, D. A. Bonn, Ruixing Liang, W. N. Hardy and K. A. Moler, Nat. Phys., in press (2008). \item M. Benkraouda and J.R. Clem, Phys. Rev. B, \textbf{53}, 438 (1996). \end{enumerate} [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T34.00002: Moving flux quanta, driven by high density currents in low-impurity samples of V$_{3}$Si, LuNi$_{2}$B$_{2}$C, and NbSe$_{2}$ : Ordered flow and core-size effects S. Moraes, R.P. Khadka, A.A. Gapud, A.P. Reyes, L.L. Lumata, J.R. Thompson, D.K. Christen There is incomplete understanding about the dissipative motion of magnetic flux quanta in type II superconductors, especially under large Lorentz forces. This is mainly due to the technical challenges involving the application of large electric currents and the rarity of samples wherein flux quanta are relatively free to move -- i.e., samples with weak ``pinning'' -- which commonly make it impossible to observe dynamic phases. Progress towards overcoming these challenges is described, along with clear observations of flux-flow phases in high-quality samples of three ``low $T_{c}$'' superconductors, V$_{3}$Si, LuNi$_{2}$B$_{2}$C, and NbSe$_{2}$. Evidence of the rarely observed Bardeen-Stephen flux flow -- a highly ordered, collective motion of flux quanta in near-unison -- will be presented. These observations have also enabled an examination of a model by Kogan and Zelezhina [Kogan and Zhelezina, \textit{Phys Rev B} \textbf{71}, 134505 (2005)] predicting the effect of a field-dependent flux \textit{core size }on ordered flux flow, as will be discussed. \textit{Funded by the U. of South Alabama and by the Research Corporation. We thank P. Canfield and L. Delong for samples and helpful discussions. } [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T34.00003: Lattice slips in slow moving magnetic vortex lattices in NbSe$_2$ Michael Dreyer, Jonghee Lee, Hui Wang, Barry Barker We studied vortex matter in NbSe$_2$, a type II superconductor, at magnetic fields of 0.25 - 0.75 T and temperatures of 4.2 K. At these fields the vortices form an Abrikosov lattice. Due to a small residual resistance in our superconducting magnet the applied magnetic field slowly decayed, driving the vortex lattice. The velocity was low enough to allow acquiring highly resolved time series using a low temperature scanning tunneling microscopy (STM). From the images we where able to extract local variations in the lattice constant as well as time series of the average vortex position (path) and velocity. A more subtle observation where closed loops on the order of nanometer in diameter in the averaged path of the vortices. Although this was observed in more than one data series it was at first dismissed as an artifact. Later, similar loops where observed in simulations. The loops occurred when a lattice dislocation traveled through the vortex lattice. This observation, unexpected in the simulation, gave new credence to the previously observed loops in the data. Since the vortex lattice in NbSe$_2$ was in the Bragg glass phase we would expect locally ordered domains of several micrometer in diameter. It is conceivable that lattice slips occur at the domain boundaries and lead to the observed loops. The observation and simulation will be compared in detail. We would like to thank Eva Andrej and Helmut Berger for providing NbSe$_2$ samples. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T34.00004: Vortex interactions in superconducting weak-pinning channel ratchets K. Yu, T.W. Heitmann, C. Song, M.P. DeFeo, B.L.T. Plourde, M.B.S. Hesselberth, P.H. Kes We report on measurements of vortex ratchets fabricated from weak-pinning superconducting a-NbGe channels bounded by strong-pinning NbN banks with asymmetric sawtooth edges. This configuration for the vortex confinement potential results in an asymmetric response for the vortex dynamics in the channels. Interactions between vortices, both within a channel and between neighboring channels, have a substantial influence on the ratchet behavior, including certain regimes where the net vortex motion through the ratchet reverses direction. We discuss our measurements in terms of a model for describing the vortex interactions in the ratchet channels. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T34.00005: Flux-flow noise in a superconducting Corbino vortex ratchet channel T.W. Heitmann, K. Yu, C. Song, M.P. DeFeo, B.L.T. Plourde, M.B.S. Hesselberth, P.H. Kes We report measurements of vortex dynamics in a single nanofabricated weak-pinning ratchet channel of a-NbGe with strong-pinning NbN channel edges. The channel is arranged in a circle on a Corbino disk geometry with a radial bias current, thus eliminating the influence of edge barriers to vortex entry on the dynamics and resulting in closed circular orbits for the vortices. An asymmetric sawtooth shape for the channel walls produces a ratchet effect, resulting in large differences in the critical current for the two flow directions. The SQUID picovoltmeter that we have developed for resolving the flux-flow voltage provides a low noise floor such that we can resolve structure in the flux-flow noise, with substantial asymmetries for vortex motion in the two directions through the ratchet channel. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T34.00006: Unconventional Vortex Dynamics in Mesoscopic Superconducting Corbino Disks Vyacheslav R. Misko, Nansheng Lin, Francois Peeters The dynamics of vortex {\it shells}, driven by an external current $I_0$, is studied in a Corbino setup, in mesoscopic disks with two to six shells. The transition from a rigid-body rotation to a separate rotation of shells is analyzed as a function of $I_0$ and temperature $T$. The critical current $I_c$ has a remarkable nonmonotonous dependence on the applied magnetic field due to a dynamically induced structural transition [1]. Thermally activated externally driven flux motion in a disk with pinning centers explains the dynamically induced two-step melting transition observed in experiment [2]. We analyze different scenarios of the current- driven angular melting of shell configurations determined by the interplay between the gradient Lorentz force and the (in) commensurability between the number of vortices in adjacent shells. The inter- and intra-shell defects lead to unconventional dynamics of vortex shells [3]. [1] V.R. Misko and F.M. Peeters, Phys. Rev. B {\bf 74}, 174507 (2006). [2] D. Lopez et al., Phys. Rev. Lett. {\bf 82}, 1277 (1999). [3] V.R. Misko, N. Lin, and F.M. Peeters, unpublished (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T34.00007: Vortex dynamics simulation in high-T$_{c}$ superconductors with planar pinning arrays coexisting with point pinning sites Hidehiro Asai, Satoshi Watanabe The enhancement of critical currents $J_{c }$ by introducing artificial pinning sites is key issue in the application of high-T$_{c}$ superconductors. Planar defects such as twin boundary are well known as the possible candidates for tangible pinning sites. Recently, the improvement of $J_{c }$ has been reported in the samples having high-density planar defects. However, the pinning characteristics of planar defects, especially the change of the pinning efficiency in the presence of point pinning, are still unclear. We have studied the dynamics of vortices interacting with both planar pinning and point pinning sites using molecular dynamics simulation. We have fixed the pinning strength of planar pinning, and calculated the $J_{c}$ as a function of the point pinning strength $f_{p}$. With increasing $f_{p}$, $J_{c}$ changes from $J_{c}\mathbin{\lower.3ex\hbox{$\buildrel{\mathbin{\buildrel\scriptstyle.\hfill\over {\smash{\scriptstyle=}\vphantom{_{\scriptstyle x}}}}}\over {\hfill\smash{\scriptstyle\cdot}}$}} J_{pl}$ ($J_{pl}$: $J_{c}$ obtained in the system without point pinning) to $J_{c}\mathbin{\lower.3ex\hbox{$\buildrel{\mathbin{\buildrel\scriptstyle.\hfill\over {\smash{\scriptstyle=}\vphantom{_{\scriptstyle x}}}}}\over {\hfill\smash{\scriptstyle\cdot}}$}} J_{p}+\alpha $, ($J_{p}$: $J_{c}$ obtained in the system without planar pinning) and then to $J_{c}\mathbin{\lower.3ex\hbox{$\buildrel{\mathbin{\buildrel\scriptstyle.\hfill\over {\smash{\scriptstyle=}\vphantom{_{\scriptstyle x}}}}}\over {\hfill\smash{\scriptstyle\cdot}}$}} J_{p}$. This behavior corresponds to the appearance of the kink structure and the drastic change of $c$-axis correlation function of the vortices. We have also performed the similar calculation with different anisotropy parameters and observed that the stiffness of vortex line changes the $J_{c}$ behavior as a function of $f_{p}$. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T34.00008: The Peak Effect anomaly in low and high T$_{C}$ superconductors: stable and metaestables vortex lattice configurations Victoria Bekeris, Gabriela Pasquini, Diego Perez Daroca, Claudio Chiliotte, Gustavo Lozano Competing interactions in the vortex lattice (VL) of type II superconductors give rise to an order-disorder transition known as Peak Effect (PE) anomaly. The strong metastability related to the PE masks the stationary VL configurations (VLCs), both in low and in high T$_{C}$ superconductors. By means of linear \textit{ac} susceptibility experiments, that avoid VL reorganization, we explore quasi-static stable and metastable states, applying different shaking protocols before measurements. In low T$_{C }$ \textit{NbSe}$_{2}$ [1] crystals we identify T,H regions where stationary configurations are maximally ordered (Bragg Glass), fully disordered or where ordered and disordered stable phases coexist. In contrast, in high T$_{C}$ \textit{YBa}$_{2}$\textit{Cu}$_{3}O_{7-y}$ crystals, metaestability seems to dominate and different metaestable VLCs are accessed depending on the previous dynamic history, with no access to the lowest energy configuration. [1] G. Pasquini \textit{et al}. Phys. Rev. Lett. \textbf{100} 247003 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T34.00009: Dynamics of Josephson vortices mediated with pancake vortices Kazuto Hirata, Shuuichi Ooi, Takashi Mochiku Josephson vortices (JVs) flow very fast with a velocity up to tenth of the light velocity in Bi-2212. If the motion of JVs can be controlled, high-speed signal processing will be achieved. We have tried to apply symmetrical pinning centers in space and time-asymmetric input signals for controlling a dynamical behavior of JVs in Bi-2212. We have shown the periodic oscillations in JV flow-resistance against magnetic field in Bi-2212 (PRL89(2002)247002), which persist in wide range of temperature and magnetic field. Introducing pancake vortices (PVs) into JV system in Bi-2212, the JV flow-resistance abruptly decreases, because the JVs interact with the PVs strongly and the PVs are pinned in intrinsically existed pinning centers in Bi-2212. However, when the input current (signal) with two frequencies is applied, a finite dc voltage can be obtained even without the flow-resistance. This leads to a rectification effect in the JVs' motion by the PVs. The nonlinearity in I-V characteristics induces the rectification effect with time-asymmetric input signals of two harmonics. This may open a new application of HTSCs. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T34.00010: Exploring the Aging Effects in Pinned Vortex Lattices in Nb using Neutron Reflectometry Xi Wang, Helen Hanson, Xinsheng Ling, Brian Maranville We report the first experiment using neutron reflectometry to explore the aging effects in pinned vortex lattices in Nb. A striking prediction of the Bragg glass model is the existence of a pinned elastic Bragg glass solid matter in the vortex state of weakly disordered type-II superconductors. According to this model, the system is pinned in the random-manifold regime with local metastable states, yet at large length scale, the system remains elastic. This seemingly paradoxical property is predicted to lead to novel aging dynamics. This work was supported by a grant from DOE-BES. The experiments were carried out at NG-1- Advanced Neutron Diffractometer facility at NIST NCNR. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T34.00011: Measurement of Vortex Bragg Glass Structure Factor in Nb using Neutron Reflectometry Helen Hanson, Xi Wang, Xinsheng Ling, Brian Maranville One of the key predictions in the Bragg Glass model of weakly pinned vortex lattices is a power-law structure factor similar to that of a 2D solid. Previous attempts using SANS in Nb and HTSC have provided results that are consistent with the Bragg Glass model. Here we report the first experiment using neutron reflectometry to resolve S(Q) in a Nb single crystal. This work was supported by a grant from DOE-BES. The experiments were carried out at NG-1- Advanced Neutron Diffractometer facility at NIST NCNR. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T34.00012: Small-angle neutron scattering study of vortex matter in superconducting Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ M. R. Eskildsen, T. Blasius, A. I. Goldman, J. M. Densmore, C. D. Dewhurst, N. Ni, A. Kreyssig, S. L. Bud'ko, P. C. Canfield We present small-angle neutron scattering studies of the superconducting vortices Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. At all fields measured a ring of scattering was observed, indicating a highly disordered vortex configuration, and no discernable rocking curve could be measured. The field dependence of the magnitude of the scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry. An analysis of the scattered intensity due to the vortices shows a rapid decrease with increasing applied magnetic field, significantly exceeding what would be expected based on estimates of the upper critical field. These results are consistent with the existence of a vortex glass or Bragg glass phase in Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T34.00013: Pauli Paramagnetic Effects in the Mixed State of CeCoIn$_5$ Revealed by Small Angle Neutron Scattering P. Das, M.R. Eskildsen, L. DeBeer-Schmitt, J.S. White, E.M. Forgan, A.D. Bianchi, M. Kenzelmann, J.L. Gavilano, M. Zolliker, S. Gerber, J. Mesot, C. Wang, E.D. Bauer, J.L. Sarrao, C. Petrovi\'{c} We report on extensive small-angle neutron scattering measurements on the vortex lattice (VL) in the mixed state of CeCoIn$_5$ with the magnetic field ($H$) both parallel and perpendicular to the $c$-axis. We obtain the $H$- and temperature ($T$)-dependence of the form factor ($|F|^2$) - a measure of the field contrast in the mixed state. At low $T$, competition between Pauli paramagnetism and the antiparallel spin alignment of $d$-wave pairing gives ``magnetized" VL cores, causing $|F|^2$ to increase with $H$ [L. DeBeer-Schmitt {\em et al.}, Phys. Rev. Lett. {\bf 99}, 167001 (2007); A.D. Bianchi {\em et al.}, Science {\bf 319}, 177 (2008)]. However, $|F|^2$ falls again near to $H_{c2}$; the fall-off extends outside the proposed FFLO region and is believed to arise from expansion of the magnetized cores. This core expansion may also explain the sequence of VL phase transitions observed in this material. At higher $T$, we observe a crossover towards more conventional behavior. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T34.00014: Vortex phase diagram in weakly pinned Rh17S15 S. Ramakrishnan, H.R. Naren, Arumugam Tamizhavel, Arun Grover A vortex phase diagram of the strongly correlated superconductor Rh$_{17}$S$_{15}$ has been constructed via exploration of the anomalous variations in critical current density extracted from ac and dc magnetization measurements. The iso-field in-phase ac susceptiblity data reveal the presence of multiple steps at different fields. The dc magnetisation hysteresis loops show the presence of a very broad fishtail commencing deep inside the mixed state and lasting upto H$_{c2}$ . We have also analysed the scan rate dependence of the hysteresis width in the vibrating sample magnetometer data with a view to distinguish between the different possible order-disorder transformations in the flux-line lattice. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T34.00015: ABSTRACT WITHDRAWN |
Session T35: Focus Session: Iron Pnictides and Other Novel Superconductors XII: Phonons and Miscellaneous Experiment
Sponsoring Units: DMPChair: Pengcheng Dai, Oak Ridge National Laboratory
Room: 405
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T35.00001: Role of fluorine in the iron pnictides: phonon softening and effective hole-doping Jesse Noffsinger, Feliciano Giustino, Steven G. Louie, Marvin L. Cohen Using first principles techniques, we investigate the influence of fluorine doping on the electronic structure, lattice dynamics and electron-phonon coupling in LaFeAsO. We explicitly simulate the F-doping using a supercell model in order to explore properties not described by virtual crystal models. Our analysis reveals that local lattice relaxation accompanies the fluorine doping and modifies the lattice dynamics, in accord with recent experimental data. In addition, it is found that the charge density of the doped electrons cannot be described by simplified models of electron-doping in the two-dimensional Fe-plane. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T35.00002: Effects of Magnetic Ordering on Phonon Spectra in Iron-based Superconductors: First Principle Calculation and Theoretical Analysis Hiroki Nakamura, Masahiko Machida, Alfred Baron, Tatsuo Fukuda, Shinich Shamoto Recently, inelastic x-ray scattering measurements on single crystals of PrFeAsO$_y$ (y$\sim $0.2) have reported that phonons related with Fe-Fe and Fe-As bondings are significantly more softened than those obtained by the first principle calculations [1]. However, it is noted that any previous calculations do not include the magnetic degree of freedom. Therefore, we performed the phonon structure calculations by taking into account the magnetic structure in mother compounds. The magnetic calculations are in better agreement with the observed softening. We show the results and clarify the reason. \\[3pt] [1] T. Fukuda et al, J. Phys. Soc. Jpn. 77, 103715~ (2008). [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T35.00003: Cubic anharmonicity of the Fe-As bond of the iron-pnictides estimated from Raman spectroscopy. Mats Granath, Johan Bielecki, Joakim Holmlund, Chris Knee, Nan Lin Wang, Lars B\"orjesson We study the phonon spectrum of the iron-pnictide superconductors, (Ce,Nd)O$_{1-x}$F$_{x}$FeAs, using Raman spectroscopy. Based on the temperature dependent softening of the out of plane optical Fe-mode (B$_{1g}$ at $\Gamma$) we estimate the magnitude of the cubic anharmonicity of the Fe-As bond by calculating the self-energy due to phonon-phonon interactions. This also gives an estimate of the lattice expansion or contraction due to Fe isotope substitution which may in turn influence electronic properties. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T35.00004: Phonon Softening in PrFeAsO$_{1-y}$ (y$\sim $0.2) by Inelastic X-Ray Scattering Alfred Q.R. Baron, Tatsuo Fukuda, Shin-ichi Shamoto, Hiroshi Uchiyama, Jun-ichiro Mizuki, Hiroki Nakamura, Masahiko Machida, Motoyuki Ishikado, Masatoshi Arai, Hijiri Kito, Hiroshi Eisaki We present phonon dispersion measurements from single crystals of~PrFeAsO$_{1-y}$ with T$_{c}$ (onset) of 42 to 45 K made using inelastic x-ray scattering with 1.5 meV resolution at BL35XU of SPring-8. In agreement with our previous results on powders and crystals [1] we see pronounced softening of the in-plane Fe-As modes compared to phonon calculations using pseudopotential methods in the tetragonal (non-magnetic) structure. C-axis modes are somewhat harder. No strong changes in phonon spectra across T$_{c}$ were observed at the momentum transfers investigated. We also compare our results against calculations of phonons in the magnetic parent material. [1] Fukuda\textit{, et al}, J. Phys. Soc. Japan, \textbf{77} (2008), 103715. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T35.00005: CaFe$_{2}$As$_{2}$ Phonons via Inelastic X-ray Scattering and First Principles Calculations Steven Hahn, Ahmet Alatas, Bogdan Leu, Yongbin Lee, Ni Ni, Duck Young Chung, Iliya Todorov, Mercouri Kanatzidis, Ercan Alp, Paul Canfield, Alan Goldman, Robert McQueeney, Bruce Harmon In the iron pnictides, the sensitivity of the iron magnetic moment to the arsenic position suggests a strong relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high temperature tetragonal phase of CaFe$_{2}$As$_{2}$ using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab-initio calculations of the phonons. Spin polarized calculations imposing the antiferromagnetic order present in the low temperature orthorhombic phase dramatically improves agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T35.00006: Magnetic and lattice coupling in the AFe$_2$As$_2$ (A=Ca, Ba, and Sr) compounds Haifeng Li, David Vaknin, Jerel Zarestky, Wei Tian, Andreas Kreyssig, Ni Ni, Sergey Bu'dko, Paul Canfield, Robert McQueeney, Alan Goldman Systematic elastic and inelastic neutron scattering studies of the AFe$_2$As$_2$ (A=Ca, Ba, and Sr) compounds reveal some common and distinguished properties that may shed light on the nature of the coupling between the magnetic and lattice degrees of freedom. We find, that for all three samples, the structural and antiferromagnetic (AFM) transition temperatures coincide within the experimental uncertainty of measurements. We also find that the AFM propagation vector is unequivocally along the long a- orthorhombic axis for all three compounds. The coupling between the magnetic and chemical structure is in play below the transition down to the lowest temperatures. More results and discussion on the nature of the transitions will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T35.00007: Large electron-A$_{1g}$ phonon interaction in doped LaOFeAs: coupling with antiferromagnetism Felix Yndurain, Jose Soler We present first principles calculations of the atomic and the electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes non magnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the A$_{1g}$ phonon, the separation between the As and Fe planes induces a non-collinear arrangement of the Fe magnetic moments. This arrangement is anti parallel for interactions mediated by As, and perpendicular for Fe-Fe direct interactions, thus avoiding frustration. This coupling of magnetism with vibrations induces anharmonicities and an electron-phonon interaction much larger than in the pure paramagnetic case. We propose that such enhanced interactions play an essential role in superconducting compounds close to an atiferromagnetic phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T35.00008: Phonon and magnetic excitations in the novel BaFe1.8Co0.2As2 superconductor Dan Parshall, Konstantin Lokshin, Matthew Stone, Douglas Abernathy, Mark Lumsden, Andrew Christianson, David Mandrus, Athena Safa-Sefat, Takeshi Egami Phonon and magnetic excitations in the BaFe1.8Co0.2As2 superconductor single crystal were studied by inelastic neutron scattering using the ARCS time-of-flight spectrometer at the Spallation Neutron Source. Most of the phonon branches show a good agreement with the density functional theory calculations. However, the As-Raman vibrations along the c-axis demonstrate strong softening contrary to the flat behavior expected from the LDA calculations. The softening is strongest along the (0.5, 0.5, L) direction, by up to 4 meV. At the same time a sharp magnetic response was found along the same (0.5, 0.5, L) direction over a wide range of L-values at energy transfer of 10-25 meV. This dynamic magnetic responds indicates on a 2-D character of antiferromagnetic spin fluctuations in the superconducting phase, in strong contrast to the 3-D static antiferromagnetism in the undoped non-superconducting BaFe2As2. Thus, in Fe-As based superconductors magnetism shows strong sensitivity to the lattice, suggesting a possibility of spin-phonon coupling playing a role in superconductivity. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T35.00009: 3d crystal field excitations in iron pnictides studied by Raman spectroscopy Tao Zhou, Zhen Qin, Sean O'Malley, Kwok Lo, Chenglin Zhang, Sang-Wook Cheong We have measured the Raman spectra of LaFeAsO$_{1-x}$F$_{x}$ (x = 0, 0.1 and 0.33) as well as AFe$_{2}$As$_{2}$ (A = Ca, Sr, Ba) polycrystalline samples at different temperatures. We found that in addition to the phonon excitations at low frequency below 250 cm$^{-1}$, there are many strong excitations in the range between 250 cm$^{-1}$ and 1000 cm$^{-1}$. We attribute them to the crystal field excitations of Fe 3d electrons. The temperature and doping dependence of these excitations will be presented, and possible implications for the physics model of this new high temperature superconductor family will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T35.00010: Evidence for two gaps from specific heat in LiFeAs single crystals Fengyan Wei, Bing Lv, Feng Chen, Yuyi Xue, Arnold M. Guloy, Chingwu Chu The mechanism and the paring state of the newly discovered FeAs based superconductors are still open questions. We report here the specific heat of several single crystals of LiFeAs from 1.8K to 160K. The Cp anomaly around the transition temperature Tc = 17 K is clear, and a change of the slope d(Cp/T)/dT$^{2}$ is observed around 4 K. This down turn in the Cp/T vs. T$^{2}$ plot, however, is suppressed in both poly-crystal samples and under high fields. We attribute this feature to the second gap, and will compare the data with calculations. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T35.00011: Coexistence of superconductivity and ferromagnetism in BaFe$_{1.8}$Co$_{0.2}$As$_2$ Mario S. da Luz, R. K. Bollinger, J. J. Neumeier, A. Sefat, M. A. McGuire, R. Jin, B. C. Sales, D. Mandrus Thermal expansion and heat capacity measurements were performed on three single crystals of BaFe$_{1.8}$Co$_{0.2}$As$_2$ with superconducting transition temperatures $T_c$ = 16.5, 19, and 22 K . The thermal expansion coefficients $\mu_i$ (i = a and c axis) are highly anisotropic. Magnetization measurements reveal the presence of ferromagnetism at the same transition temperature as superconductivity in some of the samples. The ferromagnetism has a small moment on the order of 0.5x10$^{-3}$ $\mu_B$/Fe ($\mu_B$ is the Bohr magneton). Thus, two phases: superconductivity and magnetism coexist in some BaFe$_{1.8}$Co$_{0.2}$As$_2$ samples. This ferromagnetism could be associated with a canted (non- collinear) antiferromagnetic order. This material is based upon work supported by the U. S. Department of Energy (DE-FG02-07ER46269) and the National Science Foundation (DMR-050476 and DMR-0552458). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T35.00012: Comprehensive Characterization of Superconductivity in Co-doped BaFe$_{2}$As$_{2}$ Tsuyoshi Tamegai, Yasuyuki Nakajima, Toshihiro Taen We have grown high-quality single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ by self-flux method. Superconducting properties of these single crystals are characterized by measuring magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. A sharp drop of susceptibility is observed around 24 K for $x$=0.1. Irreversible magnetization shows fish-tail feature in a wide temperature range, indicating the presence of inhomogeneities in the crystal. The critical current density $J_{c}$ for $x$=0.1 is over 10$^{5}$ A/cm$^{2}$ at low temperatures up to 50 kOe. Upper critical field determined by resistive transition is anisotropic with anisotropic parameter $\sim $ 3.5. Hall effect measurements indicate that Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ is a multiband system with a dominant conduction by electron. Magneto-optical image for $x$=0.1 at 25 K reveals the presence of trapped vortices in a part of the crystal, which leads us to expect that bulk superconductivity above 25 K can be realized by fine tuning the Co-doping level. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T35.00013: Superconductivity in Sr(Fe,Ni)$_2$As$_2$ single crystals Nicholas Butch, Shanta Saha, Kevin Kirshenbaum, Johnpierre Paglione Iron pnictide compounds are the subject of intense research efforts because of their relatively high superconducting critical temperatures and the interplay of magnetic, structural, and superconducting phases found in these materials. Of the known superconducting iron pnictide compounds, those with the ThCr$_2$Si$_2$ structure appear to have the best chemical homogeneity. We present transport, magnetic, and specific heat measurements of Ni-substituted SrFe$_2$As$_2$ flux-grown single crystals. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T35.00014: Physical Properties in a 5-Band Spin Flucutuation Theory of Ferropnictides Greg Boyd, Siegfried Graser, Vivek Mishra, Peter Hirschfeld Within a 5 band spin fluctuation model for the ferropnictides, we give predictions for experimentally measurable quantities in the superconducting state. A BCS-RPA approach is used to examine the leading superconducting instabilities and determine the thermodynamically stable ground state. We then present results for superfluid density, nuclear magnetic relaxation, and Raman scattering. [Preview Abstract] |
Session T36: Frontiers in Electronic Structure Theory
Sponsoring Units: DCP DCOMPChair: Brian Landry, Harvard University
Room: 408
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T36.00001: New approach on calculating Green's functions in Full-Potential Multiple Scattering Methods. Aurelian Rusanu, Yang Wang, G. M. Stocks, John S. Faulkner The most common methods of computing Green's functions in modern full-potential multiple scattering applications rely on solving Schr\"{o}dinger (Dirac) equations for regular and irregular solutions of a single-site scatterer over an energy contour in the complex plane. While, for spherical potentials, the standard formulae for calculating the Green's function are numerically stable they often result in unphysical behavior for non spherical potentials, particularly close to the nucleus and for large angular momentum quantum numbers. Here we use a new analytical and numerical method that does not require calculation of the irregular solution, to which the numerical instability can be traced. The new approach results in the correct analytic behavior and numerical stability. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T36.00002: Local excitations in charge-transfer insulators: a super atom approach via Wannier functions Chi-Cheng Lee, Weiguo Yin, Wei Ku Local excitations in strongly correlated charge-transfer insulator are very often tied to the rich functionalities of these materials. However, these tightly bound local excitations prove to be difficult to calculate from first-principles. In particular, the strong local interactions render the typical first-principles perturbation approach (via diagrammatic Bethe-Salpeter equation) inapplicable to describe the multiplets. In this talk, our recent progress in evaluating the local excitations in NiO will be presented. Utilizing the gauge freedom of the Wannier functions, the oxygen (charge-transfer) degrees of freedom can be integrated into a ``super atom'', in which the strong local interactions can be incorporated on the equal footing as the strong coupling between the oxygen p- and Ni d- orbitals. Our results lead to good agreement with recent non-resonant inelastic X-ray scattering data [1] and the cluster calculation [2] for both q-dependence and excitation energies. Finally, extension to propagation of the local excitation will be addressed to include the dispersion in momentum space. [1] B. C. Larson et al, PRL 99, 026401 (2007) [2] M. W. Haverkort et al, PRL 99, 257401 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T36.00003: Self-healing diffusion quantum Monte Carlo algorithms: Theory and Applications F.A. Reboredo, P.R.C. Kent, M.L. Tiago, R.Q. Hood We present a method to obtain the fixed node ground state wave function from an importance sampling Diffusion Monte Carlo (DMC) run. The fixed node ground state wave-function is altered to obtain an improved trial wave-function for the next DMC run. The theory behind this approach will be discussed. Two iterative algorithms are presented and validated in a model system by direct comparison with full configuration interaction (CI) wave functions and energies. We find that the trial wave-function is systematically improved. The scalar product of the trial wave-function with the CI result converges to 1 even starting from wave-functions orthogonal to the CI ground state. Similarly, the DMC total energy and density converges to the CI result. In the optimization process we find an optimal non-interacting nodal potential of density-functional-like form. An extension to a model system with full Coulomb interactions demonstrates that we can obtain the exact Kohn-Sham effective potential from the DMC data. Subsequently we apply our method to real molecules such as benzene and find that we can improve the ground state energy as compared with the single determinant result even starting from random wave-functions. Results for other molecular systems and comparison with alternative methods will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T36.00004: Removal of residual nonspherical self-interaction error in LDA+$U$ Fei Zhou, Vidvuds Ozolins In the established LDA+$U$ method, the electron self-interaction, which is generally nonspherical and orbital-dependent, is removed in a mean-field way. This results in residual self-interaction errors, particularly pronounced for $f$-electrons. An alternative double counting scheme that modifies the exchange, not Hartree, energy of LDA is proposed as a remedy. We show that LDA+$U$ with our approach preserves the expected degeneracy of $f^1$ and $f^2$ states in free ions and the correct ground states in the PrO$_2$ solid. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T36.00005: The $f$-electron challenge: localized and itinerant states in lanthanide oxides united by $GW$@LDA+$U$ Matthias Scheffler, Hong Jiang, Ricardo I. Gomez-Abal, Patrick Rinke Understanding the physics of $f$-electron systems, characterized by the competition between itinerant (delocalized) and highly localized $f$-states, is regarded as a great challenge in condensed-matter physics today. As a first step towards a systematic \textit{ab initio} understanding of $f$-electron systems, we apply many-body perturbation theory in the $G_0W_0$ approach based on LDA+$U$ ground state calculations ($G_0W_0$@LDA+$U$) to a selected set of lanthanide oxides (CeO$_2$ and Ln$_2$O$_3$ (Ln=lanthanide series)). These compounds have important technological applications, in particular in catalysis and microelectronics. We demonstrate good agreement between the $G_0W_0$ density of states (DOS) and experimental spectra for CeO$_2$ and Ce$_2$O$_3$. For the whole Ln$_2$O$_3$ series $G_0W_0$@LDA+$U$ reproduces all main features found for the optical experimental band gaps. Inspection of the DOS reveals that the relative positions of the occupied and unoccupied $f$-states predicted by $G_0W_0$ confirm the experimental conjecture derived from phenomenological arguments. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T36.00006: Self-interaction correction in multiple scattering theory -- Application to transition metal oxide Markus Daene, Wolfram Hergert, Arthur Ernst, Martin Lueders, Zdzislawa Szotek, Walter Temmerman In this work we study the electronic structure of 3d-transition metal oxides as obtained with the self-interaction corrected-local spin density approximation method, implemented within multiple scattering theory. We briefly describe the formalism and discuss important technical issues of its implementation within the KKR band structure method.\\ We present results of such important properties as lattice constants, local magnetic moments, band gaps and discuss them in comparison with the LSD and the experimental values. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T36.00007: Computational Modeling of Actinide Complexes Pertinent to Environment Krishnan Balasubramanian We discuss computational projects relevant to actinide separation, complexes and the possibility of actinide sequestration by engineered mesoporous materials We have carried out computational studies on a number of actinide complexes in aqueous solution; as such complexes are of considerable importance in our understanding of behavior of actinide species in the environment and high level nuclear waste, especially experimental-theoretical collaboration on curium (III) complexes with multi-dentate ligands with Nitsche and coworkers at LBNL. Cu(III) complexes with phosphonic acid (PPA) were studied for assessing relative binding strengths of the two ligands with varying pH. Possible isomers of CmH2PPA2+ and CmHPPA+ complexes were computed both in the gas phase and aqueous solution and the results of spectra and geometry will be discussed. The effects of the aqueous solvent in the configuration preferences of CmH2PPA2+ and We have also studied aqueous complexes of U(VI), Np(VI) and Pu(VI) with OH-. We will discuss the results of out extensive \textit{ab initio} computations on the equilibrium structure, infrared spectra, and bonding characteristics of a variety of hydrated NpO2(CO3)$m ^{q-}$ complexes by considering the solvent as a polarizable dielectric continuum as well as the corresponding anhydrate complexes in the gas-phase. The work at CSU Eastbay was supported in part by Office of Basic Energy Sciences of DOE, and the work at LLNL was carried out under contract number W-7405-Eng-48. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T36.00008: The converse approach to NMR chemical shifts from first-principles: application to finite and infinite aromatic compounds T. Thonhauser, D. Ceresoli, N. Marzari We present first-principles, density-functional theory calculations of the NMR chemical shifts for polycyclic aromatic hydrocarbons, starting with benzene and increasing sizes up to the one- and two-dimensional infinite limits of graphene ribbons and sheets. Our calculations are performed using a combination of the recently developed theory of orbital magnetization in solids, and a novel approach to NMR calculations where chemical shifts are obtained from the derivative of the orbital magnetization with respect to a microscopic, localized magnetic dipole. Using these methods we study on equal footing the $^1$H and $^{13}$C shifts in benzene, pyrene, coronene, in naphthalene, anthracene, naphthacene, and pentacene, and finally in graphene, graphite, and an infinite graphene ribbon. Our results show very good agreement with experiments and allow us to characterize the trends for the chemical shifts as a function of system size. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T36.00009: NMR chemical shifts from first-principles using the converse approach in periodic boundary conditions Davide Ceresoli, Timo Thonhauser, Nicola Marzari An alternative, converse approach to the first-principles calculation of NMR shielding tensors can be formulated where NMR chemical shift are obtained from the derivative of the orbital magnetization with respect to the application of a microscopic, localized magnetic dipole. We apply here the modern theory of orbital magnetization to validate this formalism to the case of extended systems in periodic boundary conditions, finding very good agreement with established methods and experimental results. These results underscore the advantages of the converse approach over existing methods: (1) it can be applied to either isolated or periodic systems, (2) it avoids any linear response calculation, allowing to treat systems containing hundreds of atoms, and (3) it is not plagued by the gauge-origin problem. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T36.00010: MP2 and RPA applied to solid state systems Martijn Marsman, Andreas Grueneis, Judith Harl, Georg Kresse We present {\it ab initio} total energy calculations at the level of Hartree-Fock + 2nd-order M\o ller-Plesset perturbation theory (HF+MP2), and the random-phase-approximation within the framework of the adiabatic-fluctuation-dissipation-theorem (ACFDT-RPA), for extended systems under periodic boundary conditions, using plane wave basis sets. We characterize and compare the accuracy of these methods with respect to their description of the lattice constants, bulk moduli, and atomizations energies of several archetypical solid state systems. Furthermore we present calculations of HF+MP2 quasiparticle gaps and compare them to results obtained within the GW approximation to the electronic self-energy. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T36.00011: Bridging the size gap between density-functional and many-body perturbation theory Paolo Umari The calculation of quasi-particle spectra based on the GW approximation is extended to systems of hundreds of atoms and the calculation of empty states is avoided. This is achieved through an optimal strategy, based on the use of Wannier-like orbitals, for obtaining a basis for the polarization propagator. Then, a Lanczos chain approach permits to calculate the self-energy. Our method is validated by calculating the vertical ionization energies of the benzene molecule and the band structure of crystalline silicon. Its potentials are then demonstrated by addressing the quasi-particle spectrum of models of vitreous materials, as well as of large organic molecules. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T36.00012: Quasiparticle properties of DNA bases from GW calculations in a Wannier basis Xiaofeng Qian, Nicola Marzari, Paolo Umari The quasiparticle GW-Wannier (GWW) approach [1] has been recently developed to overcome the size limitations of conventional planewave GW calculations. By taking advantage of the localization properties of the maximally-localized Wannier functions and choosing a small set of polarization basis we reduce the number of Bloch wavefunctions products required for the evaluation of dynamical polarizabilities, and in turn greatly reduce memory requirements and computational efficiency. We apply GWW to study quasiparticle properties of different DNA bases and base-pairs, and solvation effects on the energy gap, demonstrating in the process the key advantages of this approach. [1] P. Umari,G. Stenuit, and S. Baroni, cond-mat/0811.1453 [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T36.00013: Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Leeor Kronik, Tamar Stein, Roi Baer We show how charge transfer excitations at molecular complexes can be calculated quantitatively using time-dependent density functional theory (DFT). Predictive power is obtained from range -separated hybrid functionals using non-empirical tuning of the range-splitting parameter. Excellent performance of this approach is obtained for a series of complexes composed of various aromatic donors and the tetracyanoethylene (TCNE) acceptor, paving the way to systematic non-empirical quantitative studies of charge-transfer excitations in real systems. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T36.00014: Transcorrelated method applied to solids: numerical assessment of the SCF effect Keitaro Sodeyama, Rei Sakuma, Shinji Tsuneyuki To calculate the electronic structures of solids including
electron correlation effects, we have developed the
transcorrelated (TC) method which was first proposed by Boys and
Handy. In the TC method, the wave function is represented by a
correlated wave function $F \Phi$, where $\Phi$ is a single
Slater determinant and $F$ is a Jastrow function,
$F=\exp[-\sum_{i |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T36.00015: Unique assignment of energy to atoms in a solid Min Yu, Dallas R. Trinkle, Richard M. Martin We propose a way to decompose the total energy in a material into the contribution associated with each of the atoms, using the first-principles energy density formalism [1]. Although the energy density function is non-unique up to a gauge transformation, it has been used to calculate surface energies by integrals over cells chosen by symmetry [1] or over Voronoi polyhedra [2]. Bader charge analysis [3] partitions space into regions with a unique intergrated energy for any system with no requirements of symmetry. We implement the energy density method in the Vienna ab initio simulation package (VASP [4]) for both US-PP and PAW. We calculate energies for the Si (111), GaAs (110) nonpolar and (111) polar surfaces; vacancies and interstitials in Si and Al; and O in Ti. [1] N. Chetty and Richard M. Martin, Phys. Rev. B 45, 6074 (1992). [2] K. Rapcewicz, et al., Phys. Rev. B 57, 7281-7291(1998). [3] R. F. W. Bader, Atoms in Molecules: A Quantum Theory (1990). [4] G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999). [Preview Abstract] |
Session T37: Focus Session: Structure and Dynamics of Interfacial Water III
Sponsoring Units: DCPChair: Kenneth Jordan, University of Pittsburgh
Room: 409
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T37.00001: Study of Water Adsorbed on the Cu(110) Surface using Scanning Tunneling Microscopy and Electron Stimulated Desorption Ion Angular Distribution Invited Speaker: The structure of water layer on the Cu(110) surface formed via hydrogen bonding has been investigated with scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), and density functional theory (DFT) calculations. STM results revealed 1D chain and 2D island growth at low temperature ($\sim $80 K) and at low coverage regime. ESDIAD results at low coverage indicate that there are two kinds of O-H bonds of water molecules that are pointing out of the surface plane. At higher coverage the two-dimensional islands of water prevail, eventually covering the whole surface at the saturation coverage. Dynamic changes in the structure of the water layer and the local O-H bond direction have also been investigated as a function of annealing temperature. Using DFT calculation, a model of the 1D chain structure will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T37.00002: Theoretical description of excited state dynamics in nanostructures Invited Speaker: There has been much progress in the synthesis and characterization of nanostructures however, there remain immense challenges in understanding their properties and interactions with external probes in order to realize their tremendous potential for applications (molecular electronics, nanoscale opto-electronic devices, light harvesting and emitting nanostructures). We will review the recent implementations of TDDFT to study the optical absorption of biological chromophores, one-dimensional polymers and layered materials. In particular we will show the effect of electron-hole attraction in those systems. Applications to the optical properties of solvated nanostructures as well as excited state dynamics in some organic molecules will be used as text cases to illustrate the performance of the approach. Work done in collaboration with A. Castro, M. Marques, X. Andrade, J.L Alonso, Pablo Echenique, L. Wirtz, A. Marini, M. Gruning, C. Rozzi, D. Varsano and E.K.U. Gross. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T37.00003: Interfacial water in electric field Alenka Luzar, D. Bratko, C.D. Daub As accessible experimental length scales become shorter, the modification of interfacial properties of water using electric field (electrowetting) must come to grips with novel effects existing at the nanoscale. I will briefly survey some of our recent progress we have made in understanding these effects using molecular simulations. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T37.00004: Molecular Simulation of Reverse Micelles Janamejaya Chowdhary, Branka Ladanyi Reverse micelles (RM) are surfactant assemblies containing a nanosized water pool dissolved in a hydrophobic solvent. Understanding their properties is crucial for insight into the effect of confinement on aqueous structure, dynamics as well as physical processes associated with solutes in confinement. We perform molecular dynamics simulations for the RM formed by the surfactant Aerosol-OT (AOT) in isooctane (2,2,4-trimethyl pentane) in order to study the effect of reverse micelle size on the aqueous phase. The structure of the RM is quantified in terms of the radial and pair density distributions. Dynamics are studied in terms of the mean squared displacements and various orientational time correlation functions in different parts of the RM so as to understand the effect of proximity to the interface on aqueous dynamics. Shape fluctuations of the RM are also analyzed. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T37.00005: Ti-O bond distance fluctuations at the hydrated rutile (110) surface Nitin Kumar, Paul Kent, David Cole, David Wesolowski, James Kubicki, Jorge Sofo We studied water on the rutile (110) surface using ab-initio molecular dynamics simulations in NVT ensemble at 280K, 300K, and 320K. Water adsorbs on the 5-fold coordinated titanium atoms or dissociates transferring a proton to a bridging-oxygen atom. The equilibrium between these configurations is dynamical and depends on temperature and water coverage. The titanium-oxygen bond distances at the surface can change as much as 12{\%} depending on the number of hydrogen atoms bonded to oxygen. Hydrogen bonds also affect this distance. A measurement of the Ti-O distance at the surface can be used to estimate the average degree of dissociation. In view of our simulation results, the experimental evidence, such as photoelectron diffraction, indicates a low degree of dissociation under dry conditions. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T37.00006: Interplay between hydrogen bonding and electron solvation on hydrated TiO2 surface Jin Zhao, Kenneth Jordan, Jinlong Yang, Hrvoje Petek At metal-oxide/protic-solvent interfaces, partially hydrated or ``wet electron'' states represent the lowest energy pathway for electron transfer.$^{1}$ Through a joint two-photo photoemission (2PP) experiment and density function theory (DFT) study, we identified the electronic states corresponding to the partially solvated or wet electron state in H$_{2}$O overlayers on rutile TiO$_{2}$(110) surface. We find that a network of dangling H atoms can stabilize photoexcited electrons, in so-called wet-electron states. The energies of the ``wet electron'' states correlate closely with the number and configuration of the dangling H atoms involved in stabilizing them. We also performed DFT calculations of H$_{2}$O and H covered anatase (101) surface. Comparing with rutile (110), anatase (101) surface accommodates weaker H$_{2}$O molecule-surface hydrogen bonding. Our calculated wet electron state on anatase (101) surface has a lower energy than on rutile. Moreover, the longer distance between the adjacent adsorbate sites and the lower binding energy of H$_{2}$O allow for greater freedom for the adsorbed molecules to undergo structural relaxation in solvation of injected electrons. These differences might contribute to the higher photocatalytic activity of anatase compared with rutile. [1] K. Onda B. Li, J. Zhao, K.D. Jordan, J. Yang and H. Petek, \textit{Science} \textbf{308}, 1154 (2005). [Preview Abstract] |
Session T38: Focus Session: The Transition State in Physics, Chemistry, and Astrophysics I
Sponsoring Units: DCPChair: Greg Ezra, Cornell University
Room: 410
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T38.00001: Transition State Theory for Higher-Rank Saddles Invited Speaker: Recent developments in transition state theory have lead to a geometric characterization of molecular reactions in phase space. Central to this new characterization is the existence of codimension-one surfaces in the energy shell; these invariant surfaces guide reacting trajectories through sections of no return, the transition states. The existence of codimension-one invariant surfaces has only been shown in the vicinity of rank-one saddles, i.e., near fixed points with one stable and one unstable direction in addition to neutrally stable directions. For higher-rank saddles, the current framework of geometric transition state theory has remained inapplicable. Here we describe a generalization of the theory to saddles of arbitrary rank. As an application, we describe the nonsequential ionization of helium atoms, a problem with a rank-two saddle. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T38.00002: Exploring Topographies and Dynamics on Many-Dimensional Landscapes Invited Speaker: A major challenge to understanding and using kinetics is finding the relationships between the topography of the many-dimensional potential energy landscape (or landscapes, for systems with multiple accessible electronic states) and the way that topography, local and large-scale, determines how systems change their structures, relax and anneal. One major difficulty is simply the complexity of the landscape; one is forced to work with small statistical samples of the surface; how should these samples best be chosen? What characteristics of the topography provide the most important information? Another: how does the nature of the interparticle forces determine the topography and hence the character of motion on the surface? And what are the most useful diagnostic tools to tell us about that behavior? We shall address these questions, more in terms of progress toward, rather than providing definitive answers. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T38.00003: Transition State Theory: The Phase Space Perspective Invited Speaker: Transition State Theory (TST), which is at the basis of chemical reactivity calculations, assumes that once reactants pass through the Transition State, they cannot return. This ``no-recrossing'' rule serves to define the TS and is a necessary assumption in TST. Conventional transition states always lead to overestimates of the reaction rate because each intersection of the trajectory with the TS counts as a reactive event. Enforcing this no-recrossing condition beyond two degrees of freedom has been the major obstacle to applying TST in multidimensional systems. We will explain the solution of this problem based on dynamical systems theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T38.00004: Phase Space Transition States for Deterministic Thermostats Gregory Ezra, Stephen Wiggins We describe the relation between the phase space structure of Hamiltonian and non-Hamiltonian deterministic thermostats. We show that phase space structures governing reaction dynamics in Hamiltonian systems, such as the transition state, map to the same type of phase space structures for the non-Hamiltonian isokinetic equations of motion for the thermostatted Hamiltonian. Our results establish a general theoretical framework for analyzing thermostat dynamics using concepts and methods developed in reaction rate theory. Numerical results are presented for the isokinetic thermostat. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T38.00005: Using invariant manifolds to classify chaotic transport pathways in mixed phase space Kevin Mitchell We describe how the topological structure of stable and unstable manifolds embedded within a chaotic phase space can be used to extract a symbolic classification of chaotic transport and escape pathways. We pay particular attention to phase spaces that contain a mixture of both chaos and regularity. For such systems, the dynamics in the vicinity of ``stable islands'' is known to be particularly troublesome to analyze. We describe a technique that utilizes the structure of invariant manifolds in the vicinity of such stable islands to extract a symbolic model for the islands' influence on the transport process. Though our analysis focuses on Hamiltonian systems of two degrees-of-freedom, we also discuss the extension of our technique to higher dimensional phase spaces. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T38.00006: Transition State Theory: Variational Formulation, Dynamical Corrections, and Error Estimates Eric Vanden-Eijnden Transition state theory (TST) is discussed from an original viewpoint: it is shown how to compute exactly the mean frequency of transition between two predefined sets which either partition phase space (as in TST) or are taken to be well separate metastable sets corresponding to long-lived conformation states (as necessary to obtain the actual transition rate constants between these states). Exact and approximate criterions for the optimal TST dividing surface with minimum recrossing rate are derived. Some issues about the definition and meaning of the free energy in the context of TST are also discussed. Finally precise error estimates for the numerical procedure to evaluate the transmission coefficient~$\kappa_S$ of the TST dividing surface are given, and it shown that the relative error on $\kappa_S$ scales as $1/\sqrt{\kappa_S}$ when $\kappa_S$ is small. This implies that dynamical corrections to the TST rate constant can be computed efficiently if and only if the TST dividing surface has a transmission coefficient~$\kappa_S$ which is not too small. In particular the TST dividing surface must be optimized upon (for otherwise $\kappa_S$ is generally very small), but this may not be sufficient to make the procedure numerically efficient (because the optimal dividing surface has maximum $\kappa_S$, but this coefficient may still be very small). [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T38.00007: The dynamics of a floppy molecule: a case study Xiaojian Mao Urea is a simple but interesting molecule. In the solid state it is known to be planar while in gas phase it is non-planar. This difference is attributed to the hydrogen bonding that is present in the solid state. Ab initio quantum calculations suggest that in the gas phase there exist two different non- planar minima: anti- and syn- respectively. In addition to these minima there also exist both rank-one and rank-two saddles separating these minima. In this talk I will discuss topology of the potential energy surface and its implication for the dynamics of the molecule. [Preview Abstract] |
Session T39: Focus Session: Physical Virology
Sponsoring Units: DBPChair: Alex Evilevitch, Lund University
Room: 411
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T39.00001: Stochastic modeling of virus capsid assembly pathways Invited Speaker: Virus capsids have become a key model system for understanding self-assembly due to their high complexity, robust and efficient assembly processes, and experimental tractability. Our ability to directly examine and manipulate capsid assembly kinetics in detail nonetheless remains limited, creating a need for computer models that can infer experimentally inaccessible features of the assembly process and explore the effects of hypothetical manipulations on assembly trajectories. We have developed novel algorithms for stochastic simulation of capsid assembly [1,2] that allow us to model capsid assembly over broad parameter spaces [3]. We apply these methods to study the nature of assembly pathway control in virus capsids as well as their sensitivity to assembly conditions and possible experimental interventions. \\[4pt] [1] F. Jamalyaria, R. Rohlfs, and R. Schwartz. J Comp Phys 204, 100 (2005). \\[0pt] [2] N. Misra and R. Schwartz. J Chem Phys 129, in press (2008). \\[0pt] [3] B. Sweeney, T. Zhang, and R. Schwartz. Biophys J 94, 772 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T39.00002: How viral capsids adapt to mismatched cargoes—identifying mechanisms of morphology control with simulations Oren Elrad During the replication of many viruses, hundreds to thousands of protein subunits assemble around the viral nucleic acid to form a protein shell called a capsid. Most viruses form one particular structure with astonishing fidelity; yet, recent experiments demonstrate that capsids can assemble with different sizes and morphologies to accommodate nucleic acids or other cargoes such as functionalized nanoparticles. In this talk, we will explore the mechanisms of simultaneous assembly and cargo encapsidation with a computational model that describes the assembly of icosahedral capsids around functionalized nanoparticles. With this model, we find parameter values for which subunits faithfully form empty capsids with a single morphology, but adaptively assemble into different icosahedral morphologies around nanoparticles with different diameters. Analyzing trajectories in which adaptation is or is not successful sheds light on the mechanisms by which capsid morphology may be controlled in vitro and in vivo, and suggests experiments to test these mechanisms. We compare the simulation results to recent experiments in which Brome Mosaic Virus capsid proteins assemble around functionalized nanoparticles, and describe how future experiments can test the model predictions. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T39.00003: The role of protein interactions in HIV-1 Capsid Shape and Stability: A Multiscale Analysis Vinod Krishna, Zhiyong Zhang, Wesley I. Sundquist, Chris P. Hill, Gregory A. Voth A coarse grained model of the HIV-1 CA dimer is constructed based on all-atom molecular dynamics simulations of the C-Terminal capsid dimer. Systematic approaches to identify coarse graining sites within the dimer are presented, and the relationship of the coarse grained model parameters to atomistic properties of the capsid discussed. Coarse grained representations of the capsid shell are constructed and their stability examined. The critical importance of an additional carboxyl-hexameric amino terminal interaction is demonstrated. It is shown that this interaction is responsible for generating the curvature of the capsid shell. It is demonstrated that variation of the strength of this interaction for different proteins in the lattice can cause formation of asymmetric, conical shaped closed capsid shells and it is proposed that variations in the structure of the additional carboxyl-amino terminal binding interface during self assembly are critical to capsid cone formation. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T39.00004: Mechanisms of viral capsid assembly around a polymer Aleksandr Kivenson, Michael Hagan We present a coarse-grained computational model inspired by the assembly of viral capsid proteins around nucleic acids or other polymers. Specifically, we simulate on a lattice the dynamical assembly of closed, hollow shells composed of several hundred to 1000 subunits, around a flexible polymer. As a function of capsid size, we determine the maximum polymer length that can be dynamically encapsidated and the polymer length around which assembly is most effective. The assembly process can often be described by three phases: nucleation, growth, and a completion phase in which any remaining polymer segments are captured. We find that the polymer can increase the rate of capsid growth by stabilizing the addition of new subunits and by enhancing the incoming flux of subunits. We determine the relative importance of these mechanisms as a function of parameter values, and make predictions for the dependencies of assembly rates and effectiveness on polymer length. These predictions can be tested with bulk experiments in which capsid proteins assemble around nucleic acids or other polymers; in addition, we will discuss how predictions for the polymer-length dependence of assembly rates during the growth phase can be tested with single molecule experiments. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T39.00005: Kinetics of human immunodeficiency virus budding and assembly Rui Zhang, Toan Nguyen Human immunodeficiency virus (HIV) belongs to a large family of RNA viruses, retroviruses. Unlike budding of regular enveloped viruses, retroviruses bud \emph{concurrently} with the assembly of retroviral capsids on the cell membrane. The kinetics of HIV (and other retroviruses) budding and assembly is therefore strongly affected by the elastic energy of the membrane and fundamentally different from regular viruses. The main result of this work shows that the kinetics is tunable from a fast budding process to a slow and effectively trapped partial budding process, by varying the attractive energy of retroviral proteins (call Gags), relative to the membrane elastic energy. When the Gag-Gag attraction is relatively high, the membrane elastic energy provides a kinetic barrier for the two pieces of the partial capsids to merge. This energy barrier determines the slowest step in the kinetics and the budding time. In the opposite limit, the membrane elastic energy provides not only a kinetic energy barrier, but a free energy barrier. The budding and assembly is effectively trapped at local free energy minimum, corresponding to a partially budded state. The time scale to escape from this metastable state is exponentially large. In both cases, our result fit with experimental measurements pretty well. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T39.00006: Non-equilibrium process of virus shell assembly Artem Levandovsky, Roya Zandi We study non-equilibrium process of a virus outer shell (capsid) formation as a process of self-assembly from identical protein subunits. We model shell growth by attachment of identical subunits resembling triangular prisms with subsequent relaxation of elastic energy. We elucidate the multitude of generic shapes pertinent to larger viruses. Our model predicts formation of not only the basic virus structures such as sphere or cylinder, but also much less explored irregular shapes of retroviruses, such as HIV. The conspicuous conic motif of HIV viruses is shown to appear as realization of one of the two intermediary substates arising as a destruction of symmetry in sphere-to-cylinder transition. The other substate characterized by highly irregular structures is also observed in this work and is consistent with recently reported experimental observations. We construct unified one-dimensional phase diagram that puts spherical, irregular, conical and cylindrical forms in a rather simple perspective of shapes governed by the spontaneous curvature of protein subunits. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T39.00007: Exploring the remarkable limits of continuum elastic theory to understand the nanomechanics of viruses Wouter Roos, Melissa Gibbons, William Klug, Gijs Wuite We report nanoindentation experiments by atomic force microscopy on capsids of the Hepatitis B Virus (HBV). HBV is investigated because its capsids can form in either a smaller T=3 or a bigger T=4 configuration, making it an ideal system to test the predictive power of continuum elastic theory to describe nanometre-sized objects. It is shown that for small, consecutive indentations the particles behave reversibly linear and no material fatigue occurs. For larger indentations the particles start to deform non-linearly. The experimental force response fits very well with finite element simulations on coarse grained models of HBV capsids. Furthermore, this also fits with thin shell simulations guided by the F\"{o}ppl- von K\'{a}rm\'{a}n (FvK) number (the dimensionless ratio of stretching and bending stiffness of a thin shell). Both the T=3 and T=4 morphology are very well described by the simulations and the capsid material turns out to have the same Young's modulus, as expected. The presented results demonstrate the surprising strength of continuum elastic theory to describe indentation of viral capsids. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T39.00008: to be determined by you Invited Speaker: |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T39.00009: Conformational changes of Gag HIV-1 on a lipid bilayer measured by neutron reflectivity provides insights into viral particle assembly H. Nanda, S.A.K. Datta, F. Heinrich, M. Loesche, A. Rein, S. Krueger Formation of the HIV-1 is mediated by the Gag polyprotein at the cytoplasmic membrane surface of the infected host cell. Studies suggest large conformational changes in the Gag protein may occur during self-assembly on the membrane [Current Biology, 1997 (7) p.729, J. Mol. Biol. 2007 (365) p. 812]. The one-dimensional profile of Gag bound to a lipid bilayer interface was determined at angstrom resolution by neutron reflectometry. This was done using a novel method for modeling reflectivity data by a Monte Carlo simulation technique. The results show conditions under which the Gag protein can be made to extend or stay compact on the membrane surface. Further atomic detail was obtained using atomistic models to fit the one-dimensional Gag structural data. This involved combining X-ray resolution structures of the ordered protein domains with conformational sampling of the flexible linker region. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T39.00010: Assembly of Viruses and the Pseudo Law of Mass Action Alexander Morozov, Robijn Bruinsma, Joseph Rudnick The self-assembly of the protein shell (``capsid'') of a virus is believed to obey the Law of Mass Action (LMA) despite the fact that viral assembly is not a reversible process. In this paper we examine a soluble model for irreversible capsid assembly, the ``Assembly-Line Model.'' We show that, in this model, viral assembly from a supersaturated solution is characterized by a shock front propagating in the assembly configuration space from small to large aggregate sizes. If this shock front is able to reach the size of an assembled capsid, then transient state develops characterized by a ``pseudo'' LMA. This pseudo LMA describes partitioning of capsid proteins between assembled capsids and a metastable, supersaturated solution of free proteins that decays logarithmically slowly. We show that the line energy of assembly intermediates is the key parameter that determines this metastable state. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T39.00011: A comparison of the elastic modulii of an animal and plant virus D. Gui, X. Chen, A.L.N. Rao, S. Gill, U. Mohideen, R. Zandi We have used Atomic Force Microscopy (AFM) to image single Sindbis and BMV viruses. The AFM was then used in force spectroscopy mode to measure their elastic Modulii. The similarities and differences will be discussed. [Preview Abstract] |
Session T40: Focus Session: Knots and Loops in Biomolecules
Sponsoring Units: DBPChair: Joanna Sulkowska, University of California, San Diego
Room: 412
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T40.00001: Dodging the crisis of folding proteins with knots Invited Speaker: Proteins with nontrivial topology, containing knots and slipknots, have the ability to fold to their native states without any additional external forces invoked. A mechanism is suggested for folding of these proteins, such as YibK and YbeA, which involves an intermediate configuration with a slipknot. It elucidates the role of topological barriers and backtracking during the folding event. It also illustrates that native contacts are sufficient to guarantee folding in around 1-2\% of the simulations, and how slipknot intermediates are needed to reduce the topological bottlenecks. As expected, simulations of proteins with similar structure but with knot removed fold much more efficiently, clearly demonstrating the origin of these topological barriers. Although these studies are based on a simple coarse-grained model, they are already able to extract some of the underlying principles governing folding in such complex topologies. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T40.00002: Size and Shape of Knotted Polymers Eric Rawdon We use numerical simulations to investigate how the chain length and topology of freely fluctuating knotted polymer rings affect their size and shape. In particular, we analyze different types of geometric containers that envelope polymer configurations and describe the similarities and differences between them. This work has been done in collaborations with Akos Dobay, John Kern, Kenneth Millett, Michael Piatek, Patrick Plunkett, and Andrzej Stasiak. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T40.00003: On the geometry of stiff knots Olivier Pierre-Louis We report on the geometry and mechanics of knotted strings. We focus on the situation where the string is stiff (it has a large bending rigidity), and thin (its width is much smaller than its length). We find that: (i) the equilibrium energy depends on the type of knot as the square of the bridge number; (ii) braid localization is a general feature of stiff strings entanglements; (iii) there is an upper bound for the multiplicity of the braids and contact points in the ground state. (iv) Finally, a general confinement inequality is used to derive an upper bound on the knot gyration radius. We shall also discuss the asymptotic behavior of the knot when the filament width is small, both in the presence and in the absence of torsion (twist) energy. We conjecture a universal ground state geometry for thin strings with torsion rigidity in the presence of a large twists. Ref: R. Gallotti, O. Pierre-Louis, Phys. Rev. E 75, 031801 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T40.00004: Conformation and Dynamics of Linear Chains, Circular and Partial Loops Jen-Fang Chang, Yeng-Long Chen Recent single molecule experiments have reported the diffusivity ratio between circular and linear DNA of the same molecular weight to be 1.3, between the ratio predict by renormalization group theory (1.45) and classical Kirkwood theory (1.18). In earlier light and neutron scattering measurements of synthetic polymers, the ratio has been reported to be around 1.1-1.2. Our work employs the Lattice-Boltzmann method with Brownian dynamics to examine the diffusivity ratio for a long chain (N=320). We also examined partially closed loops that are half-closed, quarter-closed, and eighth-closed with the same contour length. Surprisingly, we find that the loop with the smallest radiuses of gyration and the highest diffusivity is not the fully closed (circular) loop, but a partially-closed one. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T40.00005: Modeling the behavior of DNA-loop-extruding enzymes Elnaz A. Baum-Snow, John F. Marko Condensin proteins are large complexes belonging to a family of ATP hydrolyzing proteins known as SMC (Structural Maintenance of Chromosomes). Condensins are believed to play a vital role in chromosomal assembly and segregation in eukaryotic cells but the details of their function along chromatin are poorly understood. Here, we propose a model to describe the behavior of DNA-loop-inducing proteins, such as type I restriction enzymes, which we believe can be used to understand condensin's function. We assume an effective motor behavior for these enzymes in which the bias of the two dimer heads is to travel away from each other, which results in loop formation along the DNA lattice. Processivity causes the enzymes to stack on top of each other. We further discuss the results of theory and computer simulations for different values of motor bias and processivity. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:30PM |
T40.00006: Intricate knots in proteins: statistics, function and evolution Invited Speaker: Protein knots, mostly regarded as intriguing oddities, are gradually being recognized as significant structural motifs. These elusive knots are present in the backbone of only about 1 in 200 proteins. It is by and large unclear how these exceptional structures actually fold, and only recently, experiments and simulations have begun to shed some light on this issue. In this talk I will present an overview of these peculiar structures from the current version of the Protein Data Bank and discuss some particularly intriguing examples of this set as well as evolutionary and functional implications. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T40.00007: The Role of Entropic Effects on DNA Loop Formation David Wilson, Alexei Tkachenko, Todd Lillian, Noel Perkins, Jens Christian Meiners The formation of protein mediated DNA loops often regulates gene expression. Typically, a protein is simultaneously bound to two DNA operator sites. An example is the lactose repressor which binds to the Lac operon of \textit{E. coli}. We characterize the mechanics of this system by calculating the free energy cost of loop formation. We construct a Hamiltonian that describes the change in DNA bending energy due to linear perturbations about the looped and open states,~starting from a non-linear mechanical rod model that determines the shape and bending energy of the inter-operator DNA loop while capturing the intrinsic curvature and sequence-dependent elasticity of the DNA. The crystal structure of the LacI protein provides the boundary conditions for the DNA. We then calculate normal modes of the open and closed loops to account for the thermal fluctuations. The ratio of determinants of the two Hamiltonians yields the partition function, and the enthalphic and entropic cost of looping. This calculation goes beyond standard elastic energy models because it fully accounts for the substantial entropic differences between the two states. It also includes effects of sequence dependent curvature and stiffness and allows anisotropic variations in persistence length. From the free energy we then calculate the J-factor and ratio of loop lifetimes. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T40.00008: Helical growth trajectories in plant roots interacting with stiff barriers Sharon Gerbode, Roslyn Noar, Maria Harrison Plant roots successfully navigate heterogeneous soil environments with varying nutrient and water concentrations, as well as a variety of stiff obstacles. While it is thought that the ability of roots to penetrate into a stiff lower soil layer is important for soil erosion, little is known about how a root actually responds to a rigid interface. We have developed a laser sheet imaging technique for recording the 3D growth dynamics of plant roots interacting with stiff barriers. We find that a root encountering an angled interface does not grow in a straight line along the surface, but instead follows a helical trajectory. These experiments build on the pioneering studies of roots grown on a tilted 2D surface, which reported ``root waving,'' a similar curved pattern thought to be caused by the root's sensitivity to both gravity and the rigid surface on which it is grown. Our measurements extend these results to the more physiologically relevant case of 3D growth, where the spiral trajectory can be altered by tuning the relative strengths of the gravity and touch stimuli, providing some intuition for the physical mechanism driving it. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T40.00009: The Penrose-Hameroff Orchestrated Objective-Reduction Proposal for Human Consciousness is Not Biologically Feasible Jeffrey Reimers, Laura McKemmish, Noel Hush, Ross McKenzie, Alan Mark Penrose and Hameroff have argued that conventional models of brain function based solely on neural linear-computational elements cannot account for human consciousness, claiming instead that quantum-computation elements are required. Specifically, in their Orchestrated Objective Reduction (Orch OR) model it is postulated that microtubules act as quantum processing units, with individual tubulin dimers forming the computational elements. This model requires that tubulin is able to switch between alternative conformational states in a coherent manner, and that this process be rapid on the physiological time scale. Here the biological feasibility of the Orch OR proposal is examined in light of recent experimental studies on microtubule assembly and dynamics. It is shown that tubulins do not possess essential properties required for the Orch OR proposal, as originally proposed, to hold. Further, we extensively consider the likelihood of Fr\"{o}hlich condensates producing coherent motions in biological systems, a feature critical to Orch OR, and show that no biologically feasible reformation of the proposal could lead to the production of a quantum processor. Hence the Orch OR model is not a feasible explanation of the origin of consciousness. [Preview Abstract] |
Session T41: Ferroelectricity and Structural Phase Transitions
Sponsoring Units: DMP DCMPChair: Craig Fennie, Cornell University
Room: 413
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T41.00001: Strained enabled Ferroelectricity in CaTiO$_{3}$ Thin Films Probed by Nonlinear Optics and Scanning Probe Microscopy Eftihia Vlahos, Amit Kumar, Sava Denev, Charles Brooks, Darrell Schlom, Carl-Johan Eklund, Karin M. Rabe, Craig J. Fennie, Venkatraman Gopalan Calcium titanate, CaTiO$_{3}$ is not a ferroelectric in its bulk form. However, first principles calculations predict that biaxially tensile strained CaTiO$_{3}$ thin films should become ferroelectric.~ Here, we indeed confirm that strained CaTiO$_{3}$ films become ferroelectric with a Curie temperature of $\sim $125K. Optical second harmonic generation (SHG) measurements, polarization studies, and in-situ electric-field measurements for a number of films with different strain values will be presented: CaTiO$_{3}$/DyScO$_{3}$(110), CaTiO$_{3}$/SrTiO$_{3 }$(100),$_{ }$CaTiO$_{3}$/GdScO$_{3}$/NdGaO$_{3}$(110), CaTiO$_{3}$/LaSrAlO$_{3}$(001) as well as for a single crystal CaTiO$_{3}$. From these studies, we conclude that strained CaTiO$_{3}$ films are ferroelectric with a point group symmetry of \textit{mm2}, and show reversible domain switching characteristics under an electric field. We also present results of variable temperature piezoelectric force microscopy for imaging the polar domains in the ferroelectric phase. These results suggest that strain is a valuable tool for inducing polar, long range ferroelectric order in even non-polar ceramic materials such as CaTiO$_{3}$. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T41.00002: Potential and piezoelectric response imaging of 180$^{\circ}$ domain of atomically ordered clean surfaces of BaTiO$_{3}$ single crystals in UHV Yukio Watanabe, S. Kaku, D. Matsumoto, S.W. Cheong We report the electrostatic and piezoelectric properties of the clean, free surface of BaTiO$_{3}$ single crystal in ultra high vacuum (UHV) The topographic imaging by AFM confirmed that the surface is atomically wellordered exhibiting clear one-lattice-height atomic steps. The amplitude and the phase image of piezoelectric response microscopy (PFM) identified 180$^{\circ}$ domains. The electrostatic potential mapping by Kelvin force microscopy (KFM) of these domains revealed that the shapes of the domains agreed exactly with the PFM images, which confirms the correctness of the standard 180$^{\circ}$ domain theory and disagrees with closure domains. However, the potential difference of upward and downward domain is approx. 0.1V, which is 100 times smaller than the value estimated by the standard theory. Similar measurements with changing temperature across Curie temperature show that this result cannot be explained by the compensation of the spontaneous polarization by contamination or oxygen deficiency or ionic conduction). The present results suggest that an intrinsic electrostatic shielding mechanism exists for 180$^{\circ}$ domains, which is consistent with the reports of surface electron/hole layers [1]. \\[4pt] [1] Watanabe et al. \textit{PRL}\textbf{86}332(2001);\textit{Ferroelectr.}367, 23(2008) We acknowledge JSPS No.19340084. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T41.00003: Magnetic Color Symmetry of Lattice Rotations in a Non-magnetic Material Sava Denev, A. Kumar, M. D. Biegalski, H. W. Jang, C. M. Folkman, A. Vasudevarao, Y. Han, I. M. Reaney, S. Trolier-McKinstry, C.-B. Eom, D. G. Schlom, V. Gopalan Oxygen octahedral rotations are the most common phase transitions in perovskite crystal structures. Here we show that the color symmetry of such pure elastic distortions is isomorphic to magnetic point groups, which allows their probing through distinguishing polar versus magnetic symmetry. We demonstrate this isomorphism using nonlinear optical probing of the octahedral rotational transition in a compressively strained SrTiO$_3$ thin film that exhibits ferroelectric ($4mm$) and antiferrodistortive ($4'mm'$) phases evolving through independent phase transitions. The approach has broader applicability for probing materials with lattice rotations that can be mapped to color groups. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T41.00004: Spontaneous polarization and piezoelectricity in polar molecular crystals Ivo Borriello, Giovanni Cantele, Domenico Ninno, Giuseppe Iadonisi Molecular materials with a polar arrangement of the constituent dipoles are good candidates for exhibiting piezoelectric properties, directly related to the \mbox{strain-induced} polarization. The \mbox{metal-organic} molecular crystal (4-dimethylaminopyridyl)bis(acetylacetato)zinc(II) (ZNDA) has been investigated from first principles. The spontaneous polarization and the piezoelectric properties have been studied by means of the modern theory of polarization, focusing on the relation between the piezoelectric properties of the organic crystal and the electronic properties of the polar molecule. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T41.00005: Interfaces in ferroelastics: fringing fields, scaling, size and shape effects Turab Lookman We consider the problem of determining the elastic fields and microstructure within a transformable region of size L, in which structural transformations from a parent to a product phase with variants can occur, that is surrounded by a fixed parent matrix. By demanding mechanical equilibrium and strain compatibility at the parent-product interface, we demonstrate that for sufficiently coarse twins the width of the twinned product phase varies as sqrt(L), but this breaks down if L is small enough for the surface and bulk energies to compete. As L decreases further, a transition to a stable checkboard pattern containing the parent and product variants occurs and this subsequently disappears if L is too small. We relate our findings to the behavior of transformations in nanograins of NiTi within an amorphous matrix and nanoscale checkboard microstructure seen in inorganic spinels. Understanding how microstructure emerges from a given configuration of interfaces is a non-trivial task and our overarching theme is to study the interplay of orientations, decaying strain fields, length scaling of energy and dependence of transition on size and shape of a transformable region within a parent matrix. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T41.00006: Local Polarization Dynamics and Bias-Induced Phase Transitions in Ferroelectric Relaxors: Time-resolved Spectroscopy and Ergodic Gap Mapping S.V. Kalinin, B. Rodriguez, M.P. Nikiforov, N. Balke, S. Jesse, O.S. Ovchinnikov, A.A. Bokov, Z.-G. Ye Mesoscopic domain structure and dynamics in PMN-PT solis solutions is studied using spatially resolved time- and voltage spectroscopic imaging modes. For compositions close to the MPB, we observe the formation of classical ferroelectric domains with rough self-affine boundaries. In the ergodic phase (PMN and PMN-10PT), the formation of non-classical labyrinthine domain patterns characterized by a single characteristic length scale is observed. The (a) persistence of these patterns well above Tc and (b) the fact that cannot be switched by tip bias suggest that they can be attributed to the frozen polarization component. Spatial variability of polarization relaxation dynamics in PMN-10PT is studied. Local relaxation attributed to the reorientation of polar nanoregions was found to follow stretched exponential dependence, with $\beta \quad \approx $ 0.4, much larger than the macroscopic value determined from dielectric spectra ($\beta $ $\approx $ 0.09). The spatial inhomogeneity of relaxation time distribution with the presence of 100-200 nm ``fast'' and ``slow'' regions is observed. The results are analyzed to map the Vogel-Fulcher temperatures on the nanoscale. The applicability of this technique to map ``ergodic gap'' distribution on the surface is discussed. Research supported by the Division of Materials Science and Engineering, Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, which is managed by UT-Battelle, LLC. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T41.00007: Field Dependence of Glassy Freezing in a Relaxor Ferroelectric Matthew Delgado, Eugene Colla, Michael Weissman, Philip Griffin Multi-frequency susceptibility measurements on the cubic relaxor ferroelectric (PbMg$_{1/3}$Nb$_{2/3}$O$_{3})_{0.88}$(PbTiO$_{3})_{0.12}$ were performed at various DC electric field strengths applied along the [111] direction. The temperature-frequency dependences fit the Vogel-Fulcher form, allowing the extraction of a frequency-independent glassy freezing temperature. These Vogel-Fulcher temperatures showed significant reductions in applied fields, following an empirical Gabay-Toulouse form, similar to vector spinglasses. The magnitude of the sensitivity indicates that the glassy state is formed by interactions among the same entities that account for the susceptibility, i.e. the polar nanoregions. This interpretation is supported by data on a powder sample of PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ (PMN), with grains too small to support large-scale inter-nanoregion cooperativity, in which the Vogel-Fulcher behavior is lost [1]. [1] J. Carreaud et. al., Appl. Phys. Lett. 92, 242902 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T41.00008: The three characteristic Temperatures of Relaxor Dynamics and their Meaning Jean Toulouse In this report, we compare the temperature evolution of several physical properties of the relaxor systems PZN, PMN and KTN. We show that three rather than two characteristic temperatures can be identified, T$_{B}$, T* and T$_{f}$, and discuss their meaning in light of dielectric, Raman and neutron scattering experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T41.00009: Quantum Fluctuation of the Order Parameter in a Structural Phase Transition James L. Smith, S.M. Shapiro, K.A. Modic, J.C. Cooley, E.K.H. Salje, P.B. Littlewood, C.P. Opeil, J.C. Lashley Using a variety of microscopic and bulk-thermodynamic probes (e.g., elastic neutron scattering, inelastic x-ray scattering, specific heat, and pressure-dependent electrical transport), we provide evidence for the presence of a continuous martensitic transition in the binary AuZn system. In Au$_{0.52}$Zn$_{0.48}$ and AuZn, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at $Q$ = (1.33, 0.67, 0) at temperatures corresponding to each sample's martensitic transition temperature, $T_{M}$. The pressure dependence of the transition in each alloy, shows a low-temperature saturation of the order parameter (strain-shuffle) that leads to highly non-linear phase boundaries in temperature-pressure space and to superconductivity in the case of Au$_{0.52}$Zn$_{0.48}$. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T41.00010: First principles determination of phase transitions in magnetic shape memory alloys Tilmann Hickel, Matthe Uijttewaal, Joerg Neugebauer Magnetic shape memory alloys have recently attracted a lot of excitement, since they allow shape changes of more then 10\% with a frequency in the kHz regime. The fundamental origin of this property is related to a martensitic phase transition. The material system Ni$_2$MnGa is the most promising candidate for applications, but its operation temperatures and ductility still need to be improved. Hence, an extension of the currently very limited knowledge on the phase diagram is decisive. In order to identify the stable structures and their transitions we performed ab initio calculations of free energies for the austenite, the (modulated) pre-martensite and the unmodulated martensite. Quasiharmonic phonons and fixed-spin magnons are considered, employing density functional theory. Using this approach we were able to successfully describe the phase transition in detail, to reveal the involved delicate interplay of vibrational and magnetic excitations and to accurately determine the transition temperature. The methods are used to interpret the experimental findings and to make predictions for modified material compositions. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T41.00011: A fluctuation-based probe to criticality in structural transitions U. Chandni, Arindam Ghosh, H.S. Vijaya, S. Mohan Many natural phenomena, extending from biology to material science, involve slowly driven dissipative systems that are far from thermal equilibrium, triggered only by a slowly varying external field and to which the systems respond through scale-free avalanches in physical observables. In spite of decades of research, experiments are inconclusive whether these systems self organize to the critical state over a broad range of external field, or if there exists a unique critical point that is smudged by a wide critical zone. Here, through the higher order statistics of time dependent avalanches, or noise, in electrical resistivity during temperature-driven martensite transformation in thin nickel-titanium films, we demonstrate for the first time, the existence of a singular `global instability' or divergence of the correlation length as a function of temperature. These results not only establish a mapping of non-equilibrium first order phase transition and equilibrium critical phenomena, but perhaps also call for a re-evaluation of many existing experimental claims of self-organized criticality. References: 1. U. Chandni et.al, Appl. Phys. Lett. 92, 112110 (2008). 2. U. Chandni et. al, arxiv:0811.0102 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T41.00012: Structural phase transformations and dislocations within the Landau theory Roman Gr\"oger, Turab Lookman, Avadh Saxena We propose a two-dimensional model (Gr\"oger et al., PRB78:184101, 2008) that demonstrates how the Landau theory of first order phase transitions can be coupled with plasticity. It is based on Kr\"oner's continuum theory of dislocations that views each dislocation as a source of incompatibility between the components of the elastic strain tensor. This incompatibility then couples to the order parameter that is a local representation of the space group of the crystalline lattice. The order parameter field is obtained by minimizing the free energy and this provides both the stress fields and the Peach-Koehler forces on individual dislocations. The evolution of the dislocation density is then obtained by a Fokker-Planck equation. Updating the dislocation density results in a new estimate of the distribution of strain incompatibilities and this serves as an input to the subsequent minimization of the free energy. This self-consistent procedure thus allows for a simultaneous evolution of the order parameter texture and the density of dislocations. To develop a clear link between the microscopic and mesoscopic dislocation density, the crystal dislocations in individual discrete slip systems are restricted to glide in their well-defined slip planes. Upon cooling, the finite dislocation density gives rise to heterogeneous nucleation of the martensite and thus results in a shift of the transformation temperature. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T41.00013: Spontaneous phase transition of nano-sized boron nitride -- A quantum size effect Hongli Dang, Y.G. Shen, Sanwu Wang We report first-principles quantum-mechanical calculations that predict a novel phase transition of nano-sized boron nitride (BN) thin-films. When the thickness of the BN thin-film is below 1.4 nm, a spontaneous phase transition from the diamond-like structure to a graphite phase is predicted. The process would involve no energy barriers. When the thickness of BN increases, on the other hand, energy barriers for the phase transition would appear and gradually increase with the thickness. Calculations show that while the graphite structure has a lower total energy than the corresponding diamond-like structure for the BN thin-film with any thickness, the spontaneous phase transition would occur only when the size is small. We attribute this phenomenon to the quantum size effect. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T41.00014: High Pressure Studies of the Metal-Insulator Transition in Pure NiS2 Arnab Banerjee, Yejun Feng, Rafael Jaramillo, Thomas F. Rosenbaum Ni(S,Se)2 is a one of the few Mott-Hubbard systems where a structural phase transition does not preclude quantitative study of the localization of charge at the T = 0 metal-insulator transition. Using diamond anvil cell techniques, we study the corresponding behavior of pure NiS2 at its quantum critical point. We characterize the electronic, magnetic and structural behavior of this model system through a combination of transport and synchrotron scattering techniques, with a particular interest in the effects of disorder at a quantum phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T41.00015: Effect of pressure on the tetragonal distortion in TiH$_{2}$: a first-principles study R. de Coss, R. Quijano, D.J. Singh The transition metal dihydride TiH$_{2}$ present the fluorite structure (CaF$_{2})$ at high temperature but undergoes a tetragonal distortion with c/a$<$1 at low temperature. Early electronic band structure calculations have shown that TiH$_{2}$ in the cubic phase display a nearly flat double degenerated band at the Fermi level. Thus the low temperature tetragonal distortion has been associated to a Jahn-Teller effect. Nevertheless, recently we have show that the instability of fcc-TiH$_{2}$ is likely to be related with a van Hove singularity. In the present work, we have performed \textit{ab-initio} calculations of the electronic structure and the tetragonal distortion for TiH$_{2}$ under pressure (0-30 GPa). We found that the fcc-fct energy barrier and the tetragonal distortion increases with pressure. The evolution of the tetragonal distortion is analyzed in terms of the electronic band structure. This research was supported by Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grant No. 49985. [Preview Abstract] |
Session T45: Funding Opportunities at NSF
Room: 316
Wednesday, March 18, 2009 5:45PM - 6:45PM |
T45.00001: Opportunities for Funding at NSF Invited Speaker: Materials science, inter- and multi-disciplinary in nature, provides the bridge to many areas of fundamental and applied sciences such as biology, chemistry, physics, mathematics, computer sciences, and engineering. Strong links that may exist between materials science and other disciplines, such as biology or chemistry or physics, very often lead to novel applications and enable technologies of great benefit to our society. The Division of Materials Research (DMR) invested \$274.0 M in FY 2008 and is estimated to invest \$324.6 M in FY 2009 funding research and education as well as enabling tools & instrumentation for individual investigators, groups, centers, and national facilities. DMR programs cover a wide spectrum of materials research and education ranging from condensed matter and materials physics, solid-state and materials chemistry, multifunctional, hybrid, electronic, photonic, metallic, ceramic, polymeric, bio-materials, composites and nanostructures to list a few. New modes of funding, research opportunities and directions, such as the recent SOLAR solicitation, will be described. This Solar Energy Initiative launched jointly by three divisions, namely Chemistry, Materials Research and Mathematical Science is aimed at supporting truly interdisciplinary efforts that address the scientific challenges of highly efficient harvesting, conversion, and storage of solar energy. The goal of this new program is to create a new modality of linking the mathematical with the chemical and materials sciences to develop transformative paradigms based on the integrated expertise and synergy from three disciplinary communities. DMR is also seeking new ways to transform materials science and education, and make it more attractive as a career for bright, young women & men. A description will be given of several workshops held this year and planned for next year with this purpose in mind. Outreach programs that emphasize how the innovations resulting from materials research lead to a better quality of life and improved economic development for people all over the world will also be given. As science is becoming increasingly global, DMR is particularly interested in preparing students to be agile thinkers in this universal environment and in forging collaborations and cooperation among scientists and engineers around the world. Free movement of knowledge without any obstacles can only be achieved through a more coordinated approach for international collaboration. \\ \newline Following the presentation there will be a question-and-answer period. For additional information, visit the DMR Web page at www.nsf.gov/materials [Preview Abstract] |
Session T46: Funding Opportunities at DOE
Room: 315
Wednesday, March 18, 2009 6:15PM - 7:15PM |
T46.00001: Opportunities for Funding at DOE Invited Speaker: Dr. Harriet Kung will give an overview of the fundamental materials research activities at DOE Office of Basic Energy Sciences (BES). BES supports basic research in materials sciences, chemistry, geosciences, and biosciences, as well as the construction and operation of major scientific user facilities, including the nation's large synchrotron radiation-light sources, neutron-scattering facilities, electron beam microscopy centers, and nanoscale science research centers. The objective of the materials research program at BES is to support fundamental experimental and theoretical research that provides the knowledge base for the discovery and design of new materials with novel structure, functions, and properties. The main research elements of the program are: Condensed Matter and Materials Physics, Scattering and Instrumentation Sciences, and Materials Design, Discovery, and Synthesis. Each of the three program areas has a set of central program scope and goals, while there is close coupling and connection between the three elements. For example, in the condensed matter and materials physics—including activities in experimental condensed matter physics, theoretical condensed matter physics, materials behavior and radiation effects, and physical behavior of materials—research is supported to understand, design, and control materials properties and function. The activity emphasizes correlation effects, which can lead to the formation of new particles, new phases of matter, and unexpected phenomena. The theoretical efforts focus on the development of advanced theories and computational tools to treat large or complex systems. Information about the funding opportunities will be discussed in the presentation.\\ \newline After the presentation, there will be an informational session for BES program managers to meet with researchers interested in research opportunities in BES. Specific information about proposal submission and proposal evaluation will be provided. For additional information, visit the BES Web page at: http://www.sc.doe.gov/bes/bes.html [Preview Abstract] |
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