Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L1: John H. Dillon Medal Symposium in Honor of Venkat Ganesan
Sponsoring Units: DPOLYChair: Glenn Fredrickson, University of California, Santa Barbara
Room: Spirit of Pittsburgh Ballrom A
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L1.00001: Dillion Medal Prize Lecture Invited Speaker: Many aspects of polymer research have undergone a paradigm shift in the past decade, with an increased emphasis on technological applications which propose the use of materials and devices created by controlling matter from the atomic scales to the bulk commodity level. This talk will focus on multicomponent polymeric materials (block copolymers, rod-coil polymers and mixtures like polymer blends and polymer nanocomposites), which have played a central role in enabling this paradigm shift in the context of polymeric materials. In this talk, I will discuss our recent researches on developing simulation tools that can predict the structure, morphology and flow behavior of such multicomponent polymers. In contrast to conventional (``particle-based'') Monte Carlo and Molecular dynamics approaches, our methods work at a coarse-grained description of the system to predict the thermodynamics and dynamics of such multicomponent polymers. This talk will focus on an outline of the simulation strategies and present some results concerning both the equilibrium and dynamical properties of such materials. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L1.00002: Chain Stretching and Order-Disorder Transitions in Block Copolymer Monolayers and Multilayers Edward J. Kramer, Vindhya Mishra, Gila E. Stein, Karen E. Sohn, Sumi Hur, Glenn H. Fredrickson, Eric W. Cochran Both monolayers of block copolymer cylinders and spheres undergo order to disorder transitions (ODT) at temperatures well below those of the bulk. Monolayers of PS-b-P2VP cylinders undergo a ``nematic'' to ``isotropic'' transition at temperatures about 20 K below the bulk ODT while monolayers of PS-b-P2VP with P2VP spheres undergo a 2D crystal to hexatic transition at least 10 K below the bulk ODT. Bilayers of each structure disorder at temperatures well above that of the monolayers. While one is tempted to attribute all of the difference to the fact that ordered monolayers are quasi 2 dimensional while bilayers are not, an alternative explanation exists. In the cylinder monolayer the corona PS chains must stretch to fill a nearly square cross-section domain rather than a hexagonal one in the bulk, while the corona PS chains in a sphere monolayer must stretch to fill a hexagonal prism rather than an octahedron in the bulk. The more non-uniform stretching of the chains in the monolayer should increase its free energy and decrease its order-disorder temperature. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L1.00003: Free Energy Estimation in Field-Theoretic Simulations Glenn Fredrickson, Erin Lennon, Kirill Katsov A new technique is presented for computing absolute and relative free energies of polymeric fluids in the context of field-theoretic simulations. Complex Langevin sampling is combined with a thermodynamic integration scheme to provide access to free energies of homogeneous and inhomogeneous polymer phases. The scheme utilizes a harmonic crystal reference state whose free energy can be computed analytically. The method is demonstrated in the context of the order-disorder transition of diblock copolymer melts. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L1.00004: Directed Crystallization in polymer solutions Murugappan Muthukumar Theoretical considerations of amyloid fibrillization in protein solutions and polymer-mediated crystallization of nanoparticles will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L1.00005: The O$^{52}$ Network by Molecular Design: CECD Tetrablock Terpolymers Frank S. Bates, Michael Bluemle, Guillaume Fleury, Timothy Lodge Varying the length of poly(dimethylsiloxane) in poly(cyclohexylethylene-$b$-ethylene-$b$-cyclohexylethylene-$b$-dimethylsiloxane) (CECD) tetrablock terpolymers between 0 and 20{\%} produces the sequence of ordered phases: cylindrical-to-network-to-cylindrical. Small-angle X-ray scattering and transmission electron microscopy demonstrate \textit{Pnna }space group symmetry and a unique network morphology stabilized by the asymmetric molecular architecture and block interactions. These results establish a new design principle for the generation of triply periodic and multiply continuous nanostructured soft material. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L1.00006: Suppression of Segmental Relaxation as the Origin of Strain Hardening in Polymer Glasses Kenneth Schweizer, Kang Chen A nanometer scale dynamical theory is proposed for the post-yield large amplitude strain hardening phenomenon in polymer glasses. The physical picture is that external deformation induces anisotropic chain conformations which modifies interchain packing, resulting in density fluctuation suppression and intensification of localizing dynamical constraints and activation barriers. The strain amplitude dependence of the resulting stresses are well described by classic rubber elasticity form. However, the hardening stress is of interchain origin and arises primarily from prolongation of segmental relaxation, not single strand entropic elasticity. Theoretical predictions for the magnitude, temperature and deformation rate dependence of the hardening modulus are consistent with experiments and simulations. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L1.00007: Disappearance of high frequency modes in polymer dilute solution viscoelasticity Ronald Larson, Semant Jain We address the problem of the ``missing modes'' in the high frequency rheology of dilute polymer solutions. According to the Rouse-Zimm theory, the slow viscoelastic response of dilute polymers is dominated by the collective motion of the chain, as described by a bead-spring model. However, one expects this description to break down at high frequencies at which chain motion on scales too small to be represented by beads and springs should be evident; this motion should be controlled by rotations of individual backbone bonds of the polymer. The viscoelastic response produced by these ``local modes'' is observable in polymer melts; however, for dilute polymer solutions, the ``local modes'' are absent from viscoelastic spectra, as shown by Schrag and coworkers (Peterson, et al., J. Polym. Sci. B, 39:2860 (2001)). Here we address this problem by directly simulating single polymer chains using Brownian dynamics simulations, with realistic bending and torsional potentials. We show using these simulations that the ``missing modes'' result from barriers to bond rotation that make the chain ``dynamically rigid'' at high frequencies. As a result, the ``dynamical Kuhn length'' of the chain exceeds the static one, and the chain at high frequencies is not able to explore local conformations as fast as would be needed for their relaxation to contribute to the mechanical relaxation spectrum. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L1.00008: Polymer Conductivity through Particle Connectivity Yueh-Lin Loo To promote solution processability of conductive polymers, polymer acids, instead of small-molecule acids, are frequently used as dopants. Generally, the conductive polymer is synthesized in the presence of the polymer acid; sub-micron size particles that are electrostatically stabilized result during polymerization. We discovered that the molecular characteristics of the polymer acid have great implications on the structure of these conductive polymer particles. Templating the synthesis of the conductive polymer with a higher molecular weight polymer acid results in larger particles, and templating with a polymer acid having a larger molecular weight distribution results in a large size distribution in the particles. Because conduction in such conductive polymers is governed by how these particles pack, we show that the macroscopic conductivity of these films is dictated by a single parameter, i.e., the particle density, that is reducible from the various molecular characteristics of the polymer acid we explored. In the specific case of polyaniline that is doped with poly(2-acrylamido-2-methyl-1-propane sulfonic acid), the particles are structurally and chemically inhomogeneous. The conductive portions of the polymer preferentially segregate to the particle surface. Conduction in these materials are therefore mediated by the particle surface and conductivity thus scales superlinearly with particle surface area per unit film volume. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L1.00009: Modeling the Self-Assembly of Nanoparticle Amphiphiles Sanat Kumar, Athanassios Panagiotopoulos We demonstrate that spherical nanoparticles, uniformly grafted with macromolecules, robustly self-assemble into a range of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations both suggest that this self-assembly process reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body \textit{emergent property}. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L1.00010: Confinement Effects on Polymer Dynamics in Nanocomposites Ramanan Krishnamoorti, Tirtha Chatterjee, Mansour Abdulbaki, Madhusudan Tyagi The dynamics of polymers in systems with dispersed nanoparticles is studied using inelastic and quasi-elastic neutron scattering. In this study, the role of confinement between nanoparticles and the role of nanoparticle topology are examined by considering dispersions with spherical C60 buckeyballs, rod-like single walled carbon nanotubes and plate-like graphene. The polymers examined here include bisphenol A epicholorohydrin and bisphenol F epicholorohydrin. Significant changes in the dynamics of the polymer are observed and these will be examined in the context of mode coupling theories. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L1.00011: Long-time dynamics of chains in polymer nanocomposites Peter Green In polymer nanocomposites (PNCs), the presence of the nanoparticles has a marked effect on the dynamics and the T$_{g}$. In one limit, the chains become strongly attached to the particles, and two glass transitions, and bimodal dynamics, may be observed. In the other, where the chain/particle interactions are weak, the chain friction factor, z(T) can undergo significant changes, manifested in the translational diffusion and viscosity. In the polymethyl methacrylate (PMMA)/C$_{60}$ system, the dynamics slow down, accompanied by an increase in the glass transition. At the same time, the temperature dependence of the relaxations remains the same as pure PMMA. In polystyrene (PS)/Au-thiol capped PS ligands, the dynamics and the glass transition could be induced to increase or decrease, through manipulation of molecular parameters in the system. In this presentation, we propose a mechanism to describe translational diffusion and T$_{g}$ in PNC systems in which the polymer chain/nanoparticle interactions are weak. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L1.00012: Self-Assembly of Conjugated Rod-Coil Block Copolymers for Photovoltaic Applications R. A. Segalman, B.D. Olsen, Y. Tao, B. McCulloch The phase behavior of conjugated rod-coil block copolymers is significantly different from that of traditional block copolymers due to the interplay between liquid crystalline interactions of the rod blocks and microphase separation of the rods and coils. A universal phase diagram for rod-coil diblock copolymers depends on the strengths of the rod aligning interactions and the rod-coil repulsive interactions as well as the geometrical ratio of rod volume to coil and aspect ratios. In this talk, the experimental phase diagram of a weakly segregated model block copolymer will be compared to that predicted by self-consistent field theory. Conjugated rod-coil block copolymers with electron donating and accepting blocks are promising for photovoltaic applications. The self-assembly of poly(thiophene-b-acrylate perylene diimide) block copolymers as well as block copolymer-nanocrystal composites result in photovoltaic active layers with controllable degrees of order. We demonstrate that short range order on the nanoscale is beneficial to device performance. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L1.00013: Confinement Effects on Glassy-State Polymer Behavior in Thin Films, Nanocomposites, Tethered Nanoparticles, and Nanostructured Systems John Torkelson, Perla Rittigstein, Soyoung Kim, Rodney Priestley, Connie Roth, Manish Mundra Confinement of polymers at the nanoscale and even the microscale can lead to significant deviations in glass transition temperature, physical aging rate, and alpha-relaxation dynamics from bulk polymer behavior. Here we illustrate how model experiments involving several techniques applied to simple, thin polymer films help us to understand and predict qualitatively or semi-quantitatively the glassy-state response of more complex, confined systems, including nanocomposites, tethered nanospheres, nanostructured homopolymer films, and nanostructured systems consisting of more than one polymer component. We shall illustrate how the glass transition temperature can be altered by as much as 60 K and how physical aging can be nearly totally suppressed via confinement. The discovery of new confinement effects and implications for new applications of confined polymeric systems will be discussed. [Preview Abstract] |
Session L2: Buckley and Lilienfeld Prize
Sponsoring Units: DCMPChair: Warren Pickett, University of California, Davis
Room: Spirit of Pittsburgh Ballrom BC
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L2.00001: Oliver E. Buckley Prize Talk: Birth of tunnel magnetoresistance and its development Invited Speaker: Nowadays usually we use the word, tunnel magnetoresistance, but it required a long time to combine both words tunnel and magnetoresistance. The study of tunnel junction may originate p-n junction studied first around 1950. On the other hand, magnetoresistance effect was reported first in 1857 which was about 100 years earlier than the start of tunnel junction study. The research of tunnel magnetoresoistance has been mainly developed first for Al-oxide tunnel barrier junctions and made a big progress by the appearance of MgO barrier junctions for both basic research and applications. More recently Heusler electrode tunnel junctions exhibits a large TMR ratio up to about 750 {\%}. In my talk I will explain first the history of the study of tunnel junction and magnetoresistance effect. Then, I will focus on the Heusler electrode junctions and also application of tunnel magnetoresoistance junctions. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L2.00002: Oliver E. Buckly Prize Talk: Spin-dependent tunneling Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L2.00003: Oliver E. Buckley Prize Talk: Spin polarized tunneling and tunnel magnetoresistance -- Learning from the past and moving forward Invited Speaker: Electron tunneling phenomenon has contributed enormously to our understanding of various branches of physics over the years. The technique of spin polarized tunneling (SPT), sensing the spin polarization of tunneling electrons using a superconducting spin detector, discovered by Meservey and Tedrow in the early seventies has been successfully utilized over the years to understand many aspects of magnetism and superconductivity. Electrical spin injection/detection in a semiconductor is strongly believed to succeed through such an approach. The successful observation of a large change in tunnel current in magnetic tunnel junctions (MTJ) in the mid nineties has brought extreme activity in this field -- both from fundamental study as well as extensive application in mind (as sensors, nonvolatile memory devices, logic elements etc). From the early history of this field that led to the discovery of room temperature TMR effect to the observation of many novel phenomena to the exciting recent work on spin filtering, spin transport in semiconductors to toggling of the superconducting state with spin current will be highlighted and reviewed. Work done in collaboration with Drs. Meservey and Tedrow, PhD students, postdoctorals, as well as high school students and undergraduates. NSF, ONR, DARPA and KIST-MIT project funds supported the research over the years. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L2.00004: Oliver E. Buckley Prize Talk: Discovery and exploration of spin-dependent tunneling Invited Speaker: Experiments on thin-film superconductors in intense magnetic fields by R. Meservey and P. M. Tedrow led to the discovery of spin-polarized tunneling. Measurements of the critical magnetic of very thin aluminum films for temperatures down to 0.45K verified that spin-orbit scattering had to be included in the BCS description of the critical field. Theory predicted a first order transition at low temperature, and, although measurements of the shape of the resistive transition of the films strongly implied the existence of such a transition, magnetic field-dependent tunneling measurements of the energy gap of the aluminum were undertaken to observe directly the first-order nature of the transition. Splitting of the superconducting density of states by the applied magnetic field, i.e., spin-dependent tunneling, was observed in these measurements. Subsequent tunneling experiments demonstrated the spin polarization of tunnel currents from ferromagnets. The extension of these tunneling studies to include a wide range of superconductors and magnetic materials produced new qualitative and quantitative information about the behavior of conduction electron spins in such materials. Although experimental technique and theory have improved from these early times, there remain unanswered questions concerning electron tunneling into ferromagnets. An overview of these early experiments will be presented.\footnote{A review can be found in R. Meservey and P. M. Tedrow, Physics Reports \textbf{238}, 175 (1994).} [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L2.00005: Julius Edgar Lilienfeld Prize: The RG and me: love at first bite Invited Speaker: From the time I took the first bite out of that cut-off, I have been in love with the Renormalization Group, returning to it over and over again to apply it to classical and quantum problems, in clean as well as disordered systems. This talk, aimed at non-experts, introduces and illustrates the RG ideas, with a favorite application, understanding Landau's Fermi liquid. [Preview Abstract] |
Session L3: Fe-based Superconductors: Competing Orders
Sponsoring Units: DCMPChair: Hong Ding, Institute of Physics, CAS
Room: 301/302
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L3.00001: Competing orders and spin density wave instabilities in FeAs-based systems Invited Speaker: The discovery of superconductivity with T$_c$ up to 55 K in layered FeAs-based compounds has generated tremendous interest in the scientific community. Except for relatively high T$_c$, the Fe pnictides display many interesting properties. Among others, the presence of competing orders is one of the most intriguing phenomena. In the early stage of our study on the compounds, we identified a spin-density-wave (SDW) ordered state for the parent compound with a stripe (or collinear) type spin structure based on the transport, specific heat, optical spectroscopy measurements and the first- principle calculations. The proposed spin structure from a nesting of the Fermi surfaces is confirmed by subsequent neutron experiments. However, it could also be explained by a local superexchange picture. In this talk I shall focus on our recent optical data on single crystal samples, trying to address the debating issue about itinerant or localized approaches to the SDW order. We found that the undoped compounds are quite metallic with relatively high plasma frequencies above T$_{SDW}$. Upon entering the SDW ordered state, a large part of the Drude component is removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate is even more dramatically reduced. Those observations favor an itinerant description for the driving mechanism of SDW instability. Nevertheless, our experiments also indicate that Fe pnictides are not simple metals. A high energy gap-like feature is present even above T$_{SDW}$, which seems to be linked with the antiferromagnetic spin fluctuations. For the superconducting samples, a superconducting pairing energy gap is clearly observed in the far-infrared reflectance measurement. The Ferrell-Glover- Tinkham sum rule is satisfied at a low energy scale. Work done in collaboration with: G. F. Chen, J. L. Luo, Z. Fang, X. Dai, W. Z. Hu, J. Dong, G. Li, Z. Li, P. Dai, J. Lynn, H. Q. Yuang, J. Singleton. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L3.00002: Magnetic order close to superconductivity in the iron-based layered RFeAsO1?xFx (R = La, Ce) systems Invited Speaker: |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L3.00003: Competing Magnetic Interactions, Structural Phase Transition, and the Unprecedented Giant Coupling of Fe-spin State and the As-As Interactions in Iron-Pnictide Invited Speaker: From all-electron fixed-spin-moment calculations [1], we showed that the ferromagnetic and checkerboard antiferromagnetic ordering in LaOFeAs were not stable and the stripe Fe-spin configuration (i.e. SDW) was the only stable ground state. The main exchange interaction between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe SDW phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. We unravel surprisingly strong interactions between arsenic ions, the strength of which is controlled by the Fe-spin state in an unprecedented way [2]. Reducing the Fe-magnetic moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant reduction in the c-axis. For CaFe$_{2}$As$_{2}$ system, this reduction of c-axis with the loss of the Fe-moment is as large as 1.4 {\AA}, an unheard of giant coupling of local spin-state of an ion to its lattice. Since the calculated large c-reduction has been recently observed only under high-pressure, our results suggest that the iron magnetic moment should be present in Fe-pnictides at all times at ambient pressure. Implications of these findings on the mechanism of superconductivity in iron-pnictides will be discussed. \\[4pt] [1] T. Yildirim, Phys. Rev. Lett. 101, 057010 (2008) (arXiv:0804.2252). \\[0pt] [2] T. Yildirim, arXiv:0807.3936 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L3.00004: Superconductivity and Magnetism in LaO$_{1-x}$F$_{x}$FeAs Invited Speaker: Measuring $^{75}$As, $^{139}$La, and $^{57}$Fe Nuclear Magnetic Resonance (NMR) as well as $\mu $SR, transport and thermodynamic properties we have determined the phase diagram of LaO$_{1-x}$F$_{x}$AsFe superconductors [1-6]. In my talk, I will show experimental studies of the magnetic ordering [2, 5], properties of the superconducting state [1, 3, 5] and the normal state properties [1, 4, 6] in the superconducting regions of the phase diagram. While the temperature dependence of the London penetration as determined from $\mu $SR points to an isotropic s wave state [3], our early NMR data suggest singlet pairing and nodes of the order parameter [1]. Extending the NMR work to lower temperatures we find evidence for a deviation of the T$^{3}$ behaviour of the spin lattice relaxation, which would agree with the extended s-wave symmetry suggested in recent theoretical work. In the paramagnetic normal state, NMR on all three nuclei shows that the local electronic susceptibility rises with increasing temperature. This had led to suggest the presence of a pseudogap, which I will discuss in detail. The scaling of all NMR shifts with respect to the macroscopic susceptibility indicates that there is no apparent multiband effect through preferential hyperfine couplings. Relaxation measurements indicate a similar temperature-dependence for (T$_{1}$T)$^{-1}$, and suggest that the dynamical susceptibility changes uniformly in q space with varying temperature. The transport properties show some striking similarities to the findings in cuprates [6] and, finally, susceptibility [4] as well as NMR studies point to the antiferromagnetic fluctuations, whose relevance is also discussed in many theoretical models of the superconducting pairing mechanism. In collaboration with Hans-Joachim Grafe, Christian Hess, R\"udiger Klingeler, G\"unter Behr, Agnieszka Kondrat, Norman Leps, and Guillaume Lang, IFW Dresden; Hans-Henning Klauss, TU Dresden; and Hubertus Luetkens, PSI Villigen. \\[4pt] References: \\[0pt] [1] H.-J. Grafe et al., Phys. Rev. Lett. \textbf{101}, 047003 (2008) \\[0pt] [2] H.-H. Klauss et al., Phys. Rev. Lett. \textbf{101}, 077005 (2008) \\[0pt] [3] H. Luetkens et al., Phys- Rev. Lett. \textbf{101}, 097009 (2008) \\[0pt] [4] R. Klingeler et al., arXiv: 0808.0708 (2008) \\[0pt] [5] H. Luetkens et al., arXiv: 0806.3533 (2008) \\[0pt] [6] C. Hess et al., arXiv: 0811.1601 (2008) [Preview Abstract] |
Session L4: The Dynamics of Diversity in Astrophysics
Sponsoring Units: COMChair: Donna Stokes, University of Houston
Room: 306/307
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L4.00001: Probing Beyond Einstein: The Joint Dark Energy Mission Invited Speaker: The discovery of the acceleration of the expansion of the universe in 1998 represents perhaps the most profound challenge to our current understanding of physics and astronomy. The observation of acceleration requires either that more than 70\% of the contents of the universe be an exotic form of energy (the so-called ``dark energy'') or that there is a flaw in general relativity. The failure of present theories to convincingly explain the effect leads many experts to expect that elucidating the cause of the expansion will lead to fundamental breakthroughs that impact cosmology, astrophysics, and particle physics. The NASA/DOE Joint Dark Energy Mission (JDEM) will be the first of the Beyond Einstein probes. This mission will determine whether the acceleration of the expansion of the universe has varied over time in an attempt to determine the equation of state for dark energy or whether predictions from general relativity fail to adequately explain the acceleration. This talk will present the rationale for a space-based study of dark energy and the techniques likely to be used as part of JDEM. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L4.00002: Thermodynamics of the Universe Invited Speaker: Properties of fundamental particles are changed in hot and dense media. This fact helps to determine the thermodynamics of the universe from the changed properties of particles in the early universe. We study the behavior of light particles including neutrinos in thermal media to find out the details about the early universe. The neutrino oscillation and the magnetic moment of neutrino, however, do not seem to change significantly enough in thermal background to fully justify the big bang model. We have to look for other properties of neutrinos including entanglement to support the standard model. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L4.00003: Space Weather and Global Warming Invited Speaker: This presentation will give discussions of the broad topic of Space Weather, and of Global Warming (these have some associations, as well as differences). Both have the Sun as the major energy source; short-term differences in solar activity are the sources of space weather (which affects the entire solar system, not just our Earth), whereas global warming is a longer-term event which depends on both the Sun and on the lower regions of the Earth's atmosphere, and (more recently) human activities. In particular, the major gas of interest for global warming is the addition of carbon dioxide to the atmosphere by combustion of coal and oil. Means for decreasing the latter will be discussed, as well as of the effects of space weather on further human exploration of near-Earth and solar system environments. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L4.00004: The Accelerating Universe Invited Speaker: Several different types of observations indicate that the universe is accelerating at present, and was decelerating in the recent past. These observations and a model-independent analysis of the data will be discussed. The model-independent, or assumption-free, data analysis method will be applied to determine the expansion and acceleration rates of the universe as functions of redshift, independent of the contents of the universe and of a theory of gravity. A new model-independent function, the dark energy indicator, which provides a measure of deviations of the equation of state at different redshift from predictions in the standard model, will be presented and discussed. The data will be used to solve for the pressure, energy density, equation of state, and potential and kinetic energy densities of the dark energy as functions of redshift without assuming a model or functional form for the dark energy. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L4.00005: Participation and Research of Astronomers and Astrophysicists of Black African Descent Invited Speaker: |
Session L5: Competing Ground-States and Novel Excitations in Strongly Correlated Metals
Sponsoring Units: DCMPChair: Andrea Bianchi, Université de Montréal
Room: 401/402
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L5.00001: Quantum criticality in a cubic heavy fermion cage compound Invited Speaker: Matter at the absolute zero in temperature may reach a highly exotic state: Where two distinctly different ground states are separated by a second order phase transition the system is far from being frozen; it is undecided in which state to be and therefore undergoes strong collective quantum fluctuations. Quantum criticality describes these fluctuations, their extension to finite temperatures, and the resulting unconventional physical properties. Heavy fermion compounds have been much investigated in the past few years as model systems. An important recent finding is that in the tetragonal compound YbRh$_2$Si$_2$ [1] a new energy scale vanishes at the quantum critical point and is in addition to the second-order phase transition scale that governs the behavior of conventional quantum critical points [2,3]. New theoretical scenarios can account for this finding if 2-dimensional spin fluctuations are assumed [4]. Here similar behaviour of the new heavy fermion compound Ce$_3$Pd$_{20}$Si$_6$ [5] is discussed in which the cubic crystal structure and the highly symmetric local environment of the Ce atoms in molecular ``cages'' makes 2-dimensional spin fluctuations rather unlikely.\\[4pt] [1] For a review see Gegenwart et al., Nat. Phys. 4, 186 (2008).\\[0pt] [2] Paschen et al., Nature 432, 881 (2004).\\[0pt] [3] Gegenwart et al., Science 316, 90 (2007).\\[0pt] [4] Si et al., Nature 413, 804 (2001).\\[0pt] [5] Paschen et al., J. Magn. Magn. Mater 316, 90 (2007) and Refs. herein. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L5.00002: A New Route to Quantum Criticality in Yb$_{3}$Pt$_{4}$ Invited Speaker: The vanishing of magnetic order at a quantum critical point (QCP) is a central feature of virtually all classes of correlated electron systems, and may be accompanied by unconventional ordered states such as superconductivity, and by anomalous critical scattering. Some of the most detailed studies have focused on f-electron heavy electron compounds, and here the picture has emerged that magnetic order requires the formation of moments, provided by the divergence of the quasiparticle mass at the QCP. We combine specific heat, magnetization, and electrical resistivity measurements on the new compound Yb$_{3}$Pt$_{4}$ to argue that alternative routes to quantum criticality are also possible. The weakly first order antiferromagnetic transition in Yb$_{3}$Pt$_{4}$ can be tuned by field to a critical end point, which is extended to a quantum critical point at 1.62 T. Both the ordered and paramagnetic phases are Fermi liquids at low temperatures, but the quasiparticle mass does not diverge at the QCP. Instead, a divergence of the zero temperature susceptibility and the quasiparticle scattering is observed, controlled by a zero field fixed point and not the nearby QCP. We argue that Yb$_{3}$Pt$_{4} $ is the first example of a heavy electron systems where magnetic order occurs at the QCP due to increasingly strong quasiparticle interactions, much as is found in $^{3}$He, Stoner ferromagnets, and spin density wave systems. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L5.00003: Magnetism and the Fermi surface in heavy fermion metals Invited Speaker: With a plethora of different phases and quantum critical points, heavy fermion materials should reign supreme as the prototype for competing order, a major contemporary theme in condensed matter physics. One key feature that differentiates the types of magnetic phases/critical points is the presence or absence of Kondo screening. This singlet formation is dramatically manifested in the Fermi surface, which provides important experimental insight into the problem. The size of the Fermi surface therefore becomes an important issue. To provide a theoretical basis for the different types of magnetism, we have recently carried out asymptotically exact studies of the Kondo lattice model inside both the antiferromagnetic [1] and ferromagnetic [2] phases. A fundamental aspect of the approach is to map the magnetic Hamiltonian for the local f-moments onto a quantum nonlinear sigma model (QNLsM). The Kondo interaction results in an effective coupling between the QNLsM fields and the conduction electrons. Renormalization group analyses show that the Fermi surface in the corresponding ordered states is small (not incorporating the f-moments) for both the ferromagnetic and antiferromagnetic cases. These results are of relevance to a number of materials, including YbRh2Si2 and CeRu2Ge2, where experimental measurements of magnetotransport and de Haas van Alphen effects [3,4] have provided evidence for the small Fermi surface phases. The implications of our results for the heavy fermion quantum critical points will also be discussed.\\[4pt] [1] S. J. Yamamoto and Q. Si, PRL 99, 016401 (2007); Physica B 403, 1414 (2008); \\[0pt] [2] S. J. Yamamoto and Q. Si, to be published;\\[0pt] [3] S. Paschen et al, Nature 432, 881 (2004); H. Shishido et al, JPSJ 74, 1103 (2005);\\[0pt] [4] C. A. King and G. G. Lonzarich, Physica B 171, 161 (1991). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L5.00004: How Do Heavy Fermions Get Polarized And Die? Invited Speaker: In paramagnetic heavy fermion systems the f-spins dissolve into Kondo singlets and reappear within the Fermi volume, producing a ``large" Fermi surface populated by heavy quasiparticles. According to theory, when a very large magnetic field is applied to such a system the Kondo singlets are broken and the fully polarized bare f-spins vanish from the Fermi volume, leaving behind a ``small" Fermi surface populated by light quasiparticles. How the system passes from the low-field to the high-field limit is not clear. This talk will discuss recent transport and de Haas van Alphen studies of the archetypal heavy fermion systems $\rm CeRu_2Si_2$ [1] and $\rm YbRh_2Si_2$ [2], which are interpreted as showing that the f-electron disappears from the Fermi volume via two successive Lifshitz transitions: in the first transition a majority spin band sinks below the Fermi level, while in the second a new minority spin band appears at the Fermi level. While this interpretation is in accord with recent theoretical work of Kusminskiy et al. [3], it could be criticized on the grounds that only the first of the two postulated Lifshitz transitions have so far been observed.\\[4pt] References:\\[0pt] [1] R. Daou, C. Bergemann and S.R. Julian, ``Continuous evolution of the Fermi surface of $\rm CeRu_2Si_2$ across the metamagnetic transition,'' Physical Review Letters {\bf 96} (2006) 026401.\\[0pt] [2] P.M.C. Rourke, A. McCollam, G. Lapertot, G. Knebel, J. Flouquet and S.R. Julian, ``Magnetic field dependence of the $\rm YbRh_2Si_2$ Fermi surface,'' arXiv:0807.3970; accepted, Physical Review Letters.\\[0pt] [3] S. Viola-Kusminskiy, K.S.D. Beach, A.H. Castro Neto and D.K. Campbell, ``Mean-field study of the heavy fermion metamagnetic transition,'' Physical Review B {\bf 77} (2008) 094419. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L5.00005: Electron spin resonance in Kondo systems Invited Speaker: Well-defined electron spin resonance (ESR) lines have been detected recently in several heavy fermion compounds, in which ferromagnetic correlations appear to be present [1]. We first discuss [2] the theory of ESR for the Kondo impurity system at temperatures T$<<$T$_{K}$ (Kondo temperature), where the local spin ESR line has a width of order T$_{K}$ and is therefore unobservably broad. By contrast, in the Anderson lattice system in the Kondo regime the ESR linewidth is narrow, and gets broadened by spin lattice relaxation and quasiparticle interaction processes. We show [2] that the spin lattice induced ESR linewidth is greatly reduced by an effective mass factor. The quasiparticle induced linewidth is small in the Fermi liquid regime, proportional to max(T$^{2}$,B$^{2})$ (T=temperature, B=Zeeman energy). The total ESR linewidth is reduced by exchange narrowing induced by a ferromagnetic exchange interaction. This explains the available ESR data. \\[4pt] [1] C. Krellner et al., Phys. Rev. Lett. \textbf{100}, 066401 (2008) \\[0pt] [2] E. Abrahams and P. Woelfle, Phys. Rev. B\textbf{78}, 104423 (2008) [Preview Abstract] |
Session L6: Earth and Space Magneto-Fluid Dynamics
Sponsoring Units: DFDChair: John Hawley, University of Virginia
Room: 406
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L6.00001: Simulating Astrophysical Flows in Laboratory Experiments Invited Speaker: A laboratory plasma configuration which simulates astrophysical jets has been developed. The experimental geometry is arranged so that the jet is unaffected by walls and the experimental time scale is such that ideal magnetohydrodynamics is reasonably approximated. The jet evolves through a reproducible sequence consisting of formation, collimation, kink instability, and at sufficiently drive high currents, detachment. Diagnostics include high speed imaging, magnetic probing, spectroscopy, and interferometry. The collimated nature of the jet and of a related experiment simulating solar corona loops suggest that collimation is a ubiquitous feature of flux tubes having axial electric currents. This observation has motivated a model for the collimation mechanism. According to this model, pile-up of convected, frozen-in toroidal magnetic flux near the jet tip increases the toroidal magnetic flux density near the tip. This flux accumulation corresponds to an increase of the toroidal field near the tip so that the pinch force is increased, thereby collimating the jet. The model shows that plasma-filled coronal loops can be considered as resulting from two counter-propagating jets colliding head-on. Color-coded images of two colliding jets confirm this. The experiments have also motivated development of a dusty-plasma dynamo mechanism suitable for driving an actual astrophysical jet. This mechanism involves dust grains having a charge to mass ratio so small that their cyclotron frequency becomes comparable to the Kepler frequency. The resulting collisionless orbits spiral across magnetic field lines towards the central object and the accumulation of charged dust grains creates a radial electromotive force appropriate for driving an astrophysical jet. The spiral orbits are not described by magnetohydrodynamics but instead result from detailed considerations of canonical angular momentum in an axisymmetric Hamiltonian system. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L6.00002: Flows and jets around compact astrophysical objects Invited Speaker: |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L6.00003: Fluid Mechanics of the Geodynamo Invited Speaker: Fluid dynamical processes in the molten, iron-rich, electrically conducting core sustain Earth's magnetic field. Convection driven by secular cooling and chemical differentiation is the primary energy source for the geodynamo. Earth's rotation imparts helicity to the convection, which amplifies the geomagnetic field, balancing losses from Ohmic dissipation. Both the Ekman and Rossby numbers are very small in the outer core, so the convection is partly aligned with the planetary spin axis, which tends to orient the geomagnetic dipole axis in the north-south direction. The magnetic Reynolds number in the outer core is about 20 times the critical value for sustained dynamo action and the Reynolds number is about $10^7$, implying turbulent conditions. Fluctuations in the turbulence induce continuous changes in the geomagnetic field, including occasional polarity reversals. Geomagnetic polarity reversals have occurred about once every 250 kyr on average over the past 5 Myr, the last reversal occurred around 780 ka and there have been several long constant- polarity superchrons. The axial dipole collapses before a reversal, exposing the complex non-dipolar transition field, then the axial dipole is regenerated in the opposite polarity, the entire process lasting 10-20 kyr. Spontaneous polarity reversals have been observed in at least one liquid sodium dynamo experiment. Downward-extrapolated measurements from Earth-orbiting satellites reveal the axial dipole comes mostly from a few high-latitude concentrated flux spots on the core boundary. About 15\% of the core boundary has reversed-direction magnetic field, mostly in the southern hemisphere. Proliferation and growth of reversed flux regions are major reasons why the axial dipole is in decline, decreasing at 10 times its free decay rate and suggesting (to some) that the geomagnetic field may be in early stage of a polarity reversal. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L6.00004: Statistics and scaling in magnetohydrodynamic turbulence Invited Speaker: The nonlinear cascade of energy is one of the most prominent processes in turbulent systems. The associated self-similarity of two- point statistics leads to the appearance of inertial-range scaling laws, e.g. in the energy spectrum of turbulence. The scaling exponents that are observed in experiments or direct numerical simulations allow to verify the validity of cascade phenomenologies. Currently, controversial findings have led to a confusing situation in the phenomenological understanding of nonlinear inertial-range dynamics of magnetohydrodynamic turbulence which is discussed using recent results of direct numerical simulations. A new approach for investigating turbulent nonlinear dynamics which is based on the Lagrangian description of turbulence is also presented. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L6.00005: Turbulence in the interstellar and interplanetary medium Invited Speaker: |
Session L7: Mechanics of Biomolecular Systems I
Sponsoring Units: DBPChair: Jens-Christian Meiners, University of Michigan
Room: 407
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L7.00001: First-principles Calculation Of DNA Looping In Tethered Particle Experiments Invited Speaker: We show how to calculate the probability of DNA loop formation mediated by regulatory proteins such as Lac repressor, using a mathematical model of DNA elasticity. Our approach has new features enabling us to compute quantities directly observable in Tethered Particle Motion (TPM) experiments; e.g. it accounts for all the entropic forces present in such experiments. Our model has no free parameters; it characterizes DNA elasticity using information obtained in other kinds of experiments. It can compute both the ``looping J factor'' (or equivalently, looping free energy) for various DNA construct geometries and repressor concentrations, as well as the detailed probability density function of bead excursions. We also show how to extract the same quantities from recent experimental data on tethered particle motion, and compare to our model's predictions. In particular, we present a new method to correct observed data for finite camera shutter time. The model successfully reproduces the detailed distributions of bead excursion, including their surprising three-peak structure, without any fit parameters and without invoking any alternative conformation of the repressor tetramer. However, for short DNA loops (around 95 bp) the experiments show more looping than is predicted by the linear-elasticity model, echoing other recent experimental results. Because the experiments we study are done in vitro, this anomalously high looping cannot be rationalized as resulting from the presence of DNA-bending proteins or other cellular machinery. We also show that it is unlikely to be the result of a hypothetical ``open'' conformation of the repressor.\\[4pt] Ref: KB Towles et al, accepted for publication in Physical Biology. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L7.00002: Modulation of membrane mechanical properties by Sar1, a vesicle trafficking protein. Invited Speaker: The trafficking of cargo in cells involves dramatic changes in membrane shape and topology. Though trafficking is widely studied and the identities and interactions of the responsible proteins are well mapped, remarkably little is known about the mechanics involved. We focus on Sar1, the key regulator of the coat protein complex II (COPII) family that ferries newly synthesized proteins from the ER to the Golgi. Sar1 is the only member of the COPII coat that interacts directly with the ER lipid bilayer membrane. It has an amphipathic N-terminal helix; when Sar1 is GTP-bound, the helix is exposed and the hydrophobic hemi-cylinder can insert into the bilayer. To investigate whether Sar1 has a role beyond merely localizing the other COPII proteins, we directly measure the force involved in membrane deformation as a function of its presence or absence, using optically trapped microspheres to pull tethers from lipid membranes whose composition and large surface area mimic the composition and geometry of the ER. Tether measurements allow extraction of the membrane bending modulus, the parameter that governs the energetics of deformation. We find that the bending modulus measured in the presence of Sar1 with a non-hydrolyzable GTP analogue is half that measured without Sar1 or with Sar1-GDP. These results reveal a paradigm-altering insight into COPII trafficking: Sar1 actively alters the material properties of the membranes it binds to, lowering the energetic cost of curvature generation. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L7.00003: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L7.00004: ``Double Bubble'' and other trouble with DNA looping Invited Speaker: DNA looping is essential for such biological processes as regulation of gene expression and DNA packaging into nucleosomes. Classical theory of looping, based on the elastic description of DNA, was proposed more than two decades ago by Shimada and Yamakawa. However, a number of puzzles related to the problem remain largely unresolved to date. For instance, DNA loops in nature tend to be significantly shorter than the optimal once predicted theoretically, and the looping probability appears to be much larger. Even in vitro experiments conflict with each other and with the theory. In my talk, I will review a number of mechanisms which may be responsible for these discrepancies, and which add complexity to the overall problem. I will briefly discuss possible roles of bending-induced and protein-induced structural defects (such as kinks and bubbles), as well as effects of boundary constraints. I will then focus on two phenomena: the effect of sequence disorder, and the loop formation in a supercoiled DNA. The former results in the lack of self-averaging of looping probabilities. The supercoiling may explain the smaller optimal loop size observed in vivo. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L7.00005: Push or Pull? -- Cryo-Electron Microscopy of Microtubule's Dynamic Instability and Its Roles in the Kinetochore Invited Speaker: Microtubule is a biopolymer made up of alpha-beta-tubulin heterodimers. The tubulin dimers assemble head-to-tail as protofilaments and about 13 protofilaments interact laterally to form a hollow cylindrical structure which is the microtubule. As the major cytoskeleton in all eukaryotic cells, microtubules have the intrinsic property to switch stochastically between growth and shrinkage phases, a phenomenon termed as their dynamic instability. Microtubule's dynamic instability is closely related to the types of nucleotide (GTP or GDP) that binds to the beta-tubulin. We have biochemically trapped two types of assembly states of tubulin with GTP or GDP bound representing the polymerizing and depolymerizing ends of microtubules respectively. Using cryo-electron microscopy, we have elucidated the structures of these intermediate assemblies, showing that tubulin protofilaments demonstrate various curvatures and form different types of lateral interactions depending on the nucleotide states of tubulin and the temperature. Our work indicates that during the microtubule's dynamic cycle, tubulin undergoes various assembly states. These states, different from the straight microtubule, lend the highly dynamic and complicated behavior of microtubules. Our study of microtubule's interaction with certain kinetochore complexes suggests that the intermediate assemblies are responsible for specific mechanical forces that are required during the mitosis or meiosis. Our discoveries strongly suggest that a microtubule is a molecular machine rather than a simple cellular scaffold. [Preview Abstract] |
Session L8: Jamming at Nonzero Temperature and Stress
Sponsoring Units: DCMP GSNPChair: Vincenzo Vitelli, University of Pennsylvania
Room: 414/415
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L8.00001: Simple scaling of the glass transition temperature with pressure Invited Speaker: Zero-temperature packings of frictionless spheres have been used as a starting point for understanding granular materials, foams, colloids and even glass-forming liquids. Such packings exhibit a jamming transition, known as Point J, with increasing packing fraction. This symposium presents recent work that explores the implications of Point J for systems at nonzero temperature, shear stress, or friction. In this talk, I present results that push beyond zero temperature to explore the connection between Point J and the glass transition. We performed molecular dynamics simulations of several three-dimensional models of glass-forming liquids, and measured the relaxation time from the intermediate scattering function along several trajectories to the glass transition, such as lowering temperature at fixed packing fraction, or raising pressure at fixed temperature. Along each trajectory, we extrapolated the relaxation time using the form $\tau={\rm exp}(A/(T-T_0)^{\alpha})$ or $\tau={\rm exp}(A/(p^r-p^r_0)^{\alpha})$, depending on whether temperature or pressure was varied, where $p^r$ is the contribution to the pressure from repulsive forces, only. Here, $A$, $\alpha$, $T_0$ and $p^r_0$ are fit parameters. We find that $T_0$ is linear in the repulsive contribution to the pressure, $p^r$: $T_0=vp^r$. The fit parameter $v$ is approximately $0.035v_0=(0.37\sigma)^3$, independent of potential, where $v_0$ is the average volume per particle and $\sigma$ is the diameter of the particle. This linear scaling of $T_0$ with $p^r$ holds very well at low $p^r$, which corresponds to the vicinity of Point J in purely repulsive systems where jamming transition at T=0 exists. This suggests that Point J marks the onset of a nonzero value of the glass transition temperature, $T_0$. Experimental data for glycerol (K. Z. Win and N. Menon, Phys. Rev. E 73, 040501 (2006)) also show that $T_0$ is linear in pressure, with a prefactor of 0.04$\times$ the molecular volume. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L8.00002: Critical Scaling of Shear Viscosity At the Jamming Transition Invited Speaker: I review the assumptions behind a scaling theory of the jamming transition for shear driven non-equilibrium steady states of a granular medium. Scaling predictions are compared against data from numerical simulations for a simple two dimensional model of frictionless soft core interacting disks with overdamped dynamics. Methods are discussed to accurately measure the critical jamming density and the critical exponents describing the jamming transition. \\[4pt] Work carried out in collaboration with Peter Olsson, Department of Physics, Ume{\aa} University. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L8.00003: Rigidity vs. Glass transition of a granular system close to Jamming Invited Speaker: ``Jamming'' is associated with two rather different notions, not always well distinguished in the literature. One, the glass transition, is that of dynamical arrest and the divergence of the structural relaxation time, the second, the proper jamming transition, is the appearance of mechanical rigidity. Both may in principle be different. In the past few years we have investigated the dynamics of a bi-disperse monolayer of disks under two different mechanical forcing i.e. cyclic shear and horizontal vibrations. (i) In the first case, one observes the so-called cage effect: at short times, any given particle is trapped in a confined area by its neighbors until the particle has managed to leave its cage and is able to diffuse through the sample by successive cage jumps [1]. Such features are reminiscent of what is observed in colloidal suspension, super-cooled liquids or other glass formers, close to the glass transition. In the present case, we have shown that cage jumps organize in clusters which avalanche in a facilitation like process to build up long term dynamical heterogeneities [2,3]. (ii) In the second case, the quench protocol produces very dense configurations with structural relaxation time much larger than the experimental time scales. One observes that long-time correlations, accompanied by the growth of spatial correlations are maximal at a volume fraction , where a snapshot of the displacement field reveals the existence of a super-diffusive motion organized in channel currents meandering between blobs of blocked particles [4,5]. We will discuss these results focusing on the distinction between the glass and jamming transitions also underlying the key role of friction in granular media as opposed to other glass formers. \\[4pt] [1] G. Marty, O. Dauchot, PRL 94, 015701 (2005). \\[0pt] [2] O. Dauchot, G. Marty, G. Biroli, PRL 95, 265701 (2005). \\[0pt] [3] R. Candelier, O. Dauchot, G. Biroli, arXiv 0811.0201. \\[0pt] [4] F. Lechenault, O. Dauchot, G. Biroli and J. P. Bouchaud, EPL, 83 46003 (2008). \\[0pt] [5] F. Lechenault, O. Dauchot, G. Biroli and J. P. Bouchaud, EPL, 83 46002 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L8.00004: Measurement of growing dynamical length scale on approach to jamming in granular systems Invited Speaker: The flow of granular materials is of widespread practical and fundamental interest. One challenge to understanding and controlling behavior is that the response is nonlinear, with a forcing threshold below which the medium is static. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems brought close to jamming, where intermediate-time motion is correlated in the form of intermitted string-like swirls. Here this will be illustrated with experiments on air-driven beads, where jamming is approached by lowering the effective temperature, as well as by experiments on rapid heap flow, where jamming is approached as a function of depth from the free surface. Use of novel statistical quantities and optical spectroscopies reveal a growing dynamical length scale on approach to jamming. Collaborators: Adam Abate, Hiroaki Katsuragi, Aaron Keys, Sharon Glotzer. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L8.00005: Long-ranged anisotropic strain correlations in sheared amorphous solids Invited Speaker: |
Session L9: Focus Session: Systems Far from Equilibrium II
Sponsoring Units: GSNP DBPChair: Michel Pleimling, Virginia Polytechnic Institute and State University
Room: 303
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L9.00001: A Reacting Particles System arising from the Conserved Kuramoto-Sivashinsky Equation Invited Speaker: We study an interacting particles system arising from a mapping to the Conserved Kuramoto-Sivashinsky equation. Particles represent vanishing regions of diverging curvature, joined by arcs of a universal parabola; nearest particles are attracted to one another at a rate inversely proportional to their distance, and coalesce upon encounter. Although the model is deterministic, a coarse-grained representation yields a diffusion equation with negative coefficient: the build up of instabilities corresponds to the coalescence events. A preliminary analysis of the model correctly predicts the growth of the typical inter- particle gap with time, but fails to reproduce interesting structure of the probability distribution function for the gap observed in simulations, including a non-trivial power-law at small distances, and a faster than gaussian decay at large distances. At yet an higher level of abstraction, trails of coalescing events may too be viewed as ``particles'' that propagate ballistically at a speed proportional to the background density, and that annihilate upon encounter. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L9.00002: Noisy Transport in Reaction-Diffusion Systems with Quenched Disorder Andrew Missel, Karin Dahmen Reaction-diffusion (RD) models are useful tools for studying a wide variety of natural phenomena. The effects of quenched disorder in the reaction rates on RD models is not completely understood, especially in parameter regimes where internal noise or stochasticity is also important. In this talk, I will discuss an RD model in which both quenched disorder and stochasticity are important. I will show how ideas from the theory of hopping conduction in doped semiconductors and first passage percolation can be used to make predictions for a number of important transport-related features in the model: the infection time, or time needed for the population to traverse the system; the velocity of the front moving through the system; and the dynamic roughening of the coarse-grained front. I will also present the results of simulations of the model that largely confirm these analytical predictions. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L9.00003: Monte Carlo simulation and linear stability analysis of Turing pattern formation in noisy reaction-subdiffusion systems Keng-Hwee Chiam, Jiawei Chiu Subdiffusion is an important physical phenomenon observed in many systems. However, numerical techniques to study it, especially when coupled to noisy reactions, are lacking. In this talk, we develop an efficient Monte Carlo algorithm based on the Gillespie algorithm and the continuous-time random walk to simulate noisy reaction-subdiffusion systems. Using this algorithm, we investigate Turing pattern formation in the Schnakenberg model with subdiffusion. First, we show that, as the system becomes more subdiffusive, the homogeneous state becomes more difficult to destablize and Turing patterns form less easily. Second, we show that, as the number of particles in the system decreases, the magnitude of noise increases and again the Turing patterns form less easily. Third, we show that, as the system becomes more subdiffusive, the ratio between the two diffusive constants must be higher in order to observe Turing patterns. Finally, we also carry out linear stability analysis to validate the results obtained from our algorithm. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L9.00004: Continuous Time Random Walks with Internal Dynamics Stephan Eule, Rudolf Friedrich, Frank Jenko We formulate a generalized master equation for a class of Continuous Time Random Walks (CTRWs) in the presence of a presribed deterministic evolution between the successive transitions. This formulation is exemplified by means of a generalized advection-diffusion and a jump-diffusion scheme. Based on the generalized master equation the corresponding fractional evolution equations are presented. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L9.00005: Unusual finite-size crossover features in driven lattice gases George L. Daquila, Uwe C. Tauber We study the temporal scaling behavior of the autocorrelation function for the asymmetric exclusion process (ASEP) on a ring in one dimension as function of system size and hopping bias. We have performed extensive Monte Carlo simulations using standard and continuous time algorithms to extract the long-time asymptotic scaling behavior for some very large systems. Even for the totally asymmetric exclusion process (TASEP), the effective exponent for the temporal autocorrelation function displays an extremely slow crossover towards the asymptotic value 2/3, with unusual features. For the ASEP, the crossover time grows with increasing backwards hopping rates. In contrast, we observe standard crossover behavior and much shorter crossover times in two- and three-dimensional lattice gases. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L9.00006: Contact process with mobile disorder Ronald Dickman I study scaling properties of the absorbing-state phase transition in the one-dimensional contact process with mobile disorder via numerical simulation and the pair approximation. In this model, the dilution sites are permanently inactive but are free to diffuse, exchanging positions with the other sites, which host a basic contact process. Even though the disorder variables are not quenched, the critical behavior is drastically affected: the critical exponent $\delta$ and the ratio $\beta/nu_\perp$ are found to vary continuously with vacancy concentration and hopping rate. At the critical point, the mean lifetime $\tau$ scales with system size $L$ as $\tau \sim (\ln L)^\zeta $, rather than as a power law; the anomalous scaling of the lifetime is associated with fluctuations in the vacancy density. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L9.00007: Fluctuation ratios in reaction-diffusion systems Sven Dorosz, Michel Pleimling We study fluctuations in diffusion-limited reaction systems driven out of their stationary state. Using a numerically exact method, we investigate fluctuation ratios in various systems which differ by their level of violation of microscopic time reversibility. Studying a quantity that for an equilibrium system is related to the work done to the system, we observe that under certain conditions oscillations appear on top of an exponential behavior of transient fluctuation ratios. We argue that these oscillations encode properties of the probability currents in state space. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L9.00008: Work-fluctuation relations for a Brownian particle in an electromagnetic field J.I. Jim\'enez-Aquino, R.M. Velasco, F.J. Uribe In statistical physics of nano-sized systems out of equilibrium, a variety of theoretical approaches and experimental demonstrations to prove some work-fluctuation have been reported recently in the literature[1,5]. In this work , we discuss how some of those theoretical approaches can be extended to give a proof of some work-fluctuation relations for an electrically charged harmonic oscillator in the presence of an uniform electromagnetic field. The perspectives of our proposal will also be discussed. [1] R. van Zon and E. G. D. Cohen, Phys. Rev. E {\bf 67}, 046102 (2003). [2] C. Bustamante, J. Liphardt, F. Ritort, Phys. Today {\bf 58} (7), 43 (2005). [3] D. J. Evans, D. J. Searles, Adv. Phys. {\bf 51}, 1529 (2002). [4] G. M. Wang {\it et al}., Phys. Rev. Lett. {\bf 89}, 050601 (2002). [5] A. M. Jayannavar, M. Sahoo, Phys, Rev. E {\bf 75}, 032102 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L9.00009: Stochastic current switching behavior in semiconductor superlattices Stephen Teitsworth, Huidong Xu Numerical simulation results are presented for a discrete drift-diffusion model that describes electronic transport in weakly-coupled semiconductor superlattices under voltage bias and also includes shot noise in the tunneling currents. Sequential resonant tunneling between quantum wells is the primary conduction mechanism and noise terms are treated as delta-correlated in space and time. We study the response of this system to abrupt steps in applied voltage in a range for which the current-voltage characteristics exhibit bistability. The system switches from a metastable state to a stable state with a stochastically varying delay time, a process corresponding to relocation of charge density from one (critical) quantum well to an adjacent one. We find that the mean delay time $\tau$ varies as $\ln \tau \propto V - V_{th}^{3/2}$ where $V$ and $V_{th}$ denote, respectively, the system voltage and the voltage at the boundary of the bistability range [1]; $\tau$ also depends exponentially on the cross-sectional area of the superlattice. An effective one-dimensional potential energy is constructed for the charge density in the critical quantum well. We find that noise contributions of the quantum wells far from the critical well have a significant effect on the switching process. [1] O. A. Tretiakov and K. A. Matveev, Phys. Rev. B. \textbf{71}, 165326 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L9.00010: Fluctuation relations in a driven, nonlinear micromechanical torsional oscillator Corey Stambaugh, H.B. Chan Fluctuation relations in a periodically driven micromechanical oscillator are investigated. The system is first studied in a linear regime by applying a weak drive. The ratio of the work variance to the mean work is shown to be independent of the driving frequency, consistent with standard fluctuation relations for a steady state system near thermal equilibrium. When a strong drive is applied to the system the response becomes nonlinear and the system displays bistability. The work variance in this nonlinear system, driven far from equilibrium, is predicted to show a strong frequency dependence not seen in the linear case. For such bistable system the total variance has two contributing components. The first component, involving intrastate fluctuations about one stable attractor, is expected to scale with a power law dependence as the system approaches the bifurcation point where the occupied state disappears. The second component of the work variance for interstate fluctuations is shown to have a strong frequency dependence near the kinetic phase transition where the populations of the two states are comparable. The relationship between the work and variance is compared to previous results, the work variance in the linear regime, and to theory. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L9.00011: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L9.00012: Work distributions in the T=0 Random Field Ising Model Xavier Illa, Josep Maria Huguet, Eduard Vives The T=0 Random Field Ising Model is a prototype model for the study of collective phenomena in disordered systems. The model can be numerically studied from two different points of view: on the one hand, the exact ground state calculation provides an approach to the equilibrium phase diagram. On the other the use of a local relaxation dynamics based on single spin-flips provides a good framework for the understanding of avalanche dynamics and hysteresis, which is closer to experimental observations. In this sense, the model is a good workbench for the comparison of equilibrium and out-of-equilibrium trajectories. We perform a numerical study of the three-dimensional Random Iield Ising Model at T=0. We compare work distributions along metastable trajectories obtained with the single-spin flip dynamics with the distribution of the internal energy change along equilibrium trajectories. The goal is to investigate the possibility of extending the Crooks fluctuation theorem to zero temperature when, instead of the standard ensemble statistics, one considers the ensemble generated by the quenched disorder. We show that a simple extension of Crooks fails close to the disordered induced equilibrium phase transition due to the fact that work and internal energy distributions are very asymmetric. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L9.00013: Far-From-Equilibrium Measurements of Thermodynamic Length Edward Feng, Gavin Crooks Thermodynamic length is a path function that generalizes the notion of length to the surface of thermodynamic states. Here, we show how to measure thermodynamic length in far-from-equilibrium single molecule experiments using the work fluctuation relations. For these microscopic systems, it proves necessary to define the thermodynamic length in terms of the Fisher information. Consequently, the thermodynamic length can be directly related to the magnitude of fluctuations about equilibrium. The work fluctuation relations link the work and the free energy change during an external perturbation on a system. Using these results, we determine how to re-weight the probability of a trajectory to determine the equilibrium averages at an intermediate point of the protocol in which the system is out-of-equilibrium. This allows us to measure the thermodynamic length in single molecule experiments. [Preview Abstract] |
Session L10: Optical Properties of Nanostructures III: Quantum Dots, Nanowires, and Related Materials
Sponsoring Units: DMPChair: John McGuire, Los Alamos National Laboratory
Room: 304
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L10.00001: The effect of uniaxial stress on light emitted from GaN/AlN quantum dots Daniel Rich, Ofer Moshe, Benjamin Damilano, Jean Massies We have studied the effect of uniaxial stress on the optical polarization properties of GaN/AlN quantum dots (QDs) grown on Si(111) substrates. Microcracks form as a result of the thermal expansion coefficient mismatch between the GaN/AlN layers and the Si(111) substrate. We show that such microcracks serve as excellent stressors through which the strain tensor of the GaN/AlN QDs can be modified for studies of strain-induced changes in the optical properties using a spatially and temporally resolved probe, such as with cathodoluminescence (CL) imaging and spectroscopy. CL measurements of the ground- state excitonic transition of vertically stacked GaN/AlN quantum dots (QDs) exhibit an in-plane linear polarization anisotropy in close proximity to microcracks, consistent with the presence of uniaxial stress. The spatial dependence of the polarization anisotropy and CL decay time in varying proximity to the microcracks are studied as a function of temperature and excitation conditions in order to assess the influence of thermal stress variations on the oscillator strength between electrons and holes. Three-dimensional 6x6 k.p calculations of the QD eigenstates were performed to examine the influence of stress on the polarization-dependent momentum matrix element in varying proximity to the stressors. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L10.00002: Computational assessment of stimulated Raman adiabatic passage in embedded germanium nanocrystals Ceyhun Bulutay, Deniz Gunceler All-optical coherent population transfer is one of the key instruments of coherent control phenomena which is much needed for quantum information processing. One powerful scheme is the stimulated Raman adiabatic passage (STIRAP). It provides 100\% population transfer in a $\Lambda$ configuration achieved by two counter-intuitively ordered partially overlapping optical pulses which is robust against parameter variations. The major challenge is to achieve STIRAP in the solid-state. In this respect, Ge nanocrystals (NC) embedded in silica possess atomic-like states which is very convenient for coherent control schemes which is at the same time highly scalable and compatible with the mainstream solid-state technology. In this computational work, we provide a realistic assessment of STIRAP in a 4~nm Ge NC. The electronic structure and optical dipole matrix elements are computed using an atomistic pseudopotential Hamiltonian. A multi-level STIRAP scheme is implemented to incorporate the effects of large number of intermediate states which fall into the quasi-continuum part of the Ge NC states. Our extensive study over the parameter space provides working recipes for achieving STIRAP with the commonly available laser sources. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L10.00003: Optical Aharonov-Bohm Effect for type-II InAs Quantum Dots Vincent R. Whiteside, Ian R. Sellers, Bruce D. McCombe, Huiyun Liu The magnetic field dependence of the ground- and excited-state transitions in InAs quantum dots (QD), capped with a strain reducing layer of GaAs$_{0.76}$Sb$_{0.26}$, which results in a type-II band alignment, has been studied by magneto-photoluminescence (magneto-PL) spectroscopy. Oscillations in the PL intensity of both the excited state and ground state magneto-PL are observed. Similar oscillations in intensity have been previously attributed to the Optical Aharonov-Bohm (OAB) effect in other type-II QD systems. In the present case the ground state oscillations are only observed at low excitation power, for which only the ground state is occupied. At higher excitation power the excited state oscillations are more pronounced than the ground state oscillations. The room temperature emission wavelength of these QDs matches the optical telecommunications window at 1.55 $\mu $m, so the OAB effect in these structures may be useful in future devices. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L10.00004: Quasiparticle Gaps and Exciton Coulomb Energies in Si Nanoshells Kimberly Frey, Juan C. Idrobo, Serdar Ogut, Murilo L. Tiago, Fernando A. Reboredo Quasiparticle gaps and exciton Coulomb energies are calculated in Si nanoshells passivated by H at the inner and outer surfaces. We consider spherical nanoshells with inner radii $R_1$ up to 1 nm and outer radii $R_2$ up to 1.6 nm. Quasiparticle gaps are calculated using $\Delta$SCF and GW methods. While the single-band effective mass approximation predicts that the gap should depend only on the thickness $t=R_2 - R_1$ of the nanoshell, we find from first principles calculations that it depends on both $R_1$ and $R_2$. The dependences of the quasiparticle gap on $R_1$ and $R_2$ are mostly consistent with electrostatics of a charged metallic shell. We also find that the (unscreened) Coulomb energy in Si nanoshells has a somewhat unexpected size dependence at fixed outer radius $R_2$. Namely, the exciton Coulomb energy {\em decreases} as the nanoshell becomes more {\em confining}, contrary to what one would expect from quantum confinement effects. We show that this is a consequence of an increase in the average electron-hole distance, giving rise to reduced exciton Coulomb energies in spite of the reduction in the confining nanoshell volume. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L10.00005: Tunable Absorption Spectroscopy of Individual Nanowires Linyou Cao, Justin White, Joonshik Park, Mark Brongersma The optical properties of semiconductor nanowires have recently emerged as a major topic of research largely motivated by their potential for diverse optoelectronic applications. Here we present a combined experimental and theoretical investigation showing that the absorption individual germanium nanowire is tunable over a broad region from visible to near infrared. Close correspondence between the calculated and experimental results indicates structural resonance as mechanism for the tunable absorption. Similar tunable absorption expects to be also in other nanowires as the mechanism of structural resonance is universal. The large tunability of the light absorption, along with the well-acquired synthetic controllability of nanowires , open up a new pathway for the design of high-efficiency and broadband optoelectronic devices. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L10.00006: Ab-initio calculations of optical spectra of silicon nanowires Yuan Ping, Dario Rocca, Giulia Galli We present ab-initio calculations of absorption spectra of thin silicon nanowires (1-2 nm in diameter) and compare the results of different techniques. In particular we aim at assessing the ability of time dependent Density Functional Theory (TDDFT) to describe trends in the electronic properties of Si nanowires, by comparing results obtained within TDDFT with those of the Bethe-Salpeter Equation (BSE). We also discuss the numerical accuracy of both TDDFT and BSE calculations and the influence on computed spectra of several numerical parameters entering the calculations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L10.00007: Photoluminescence from core/multiple-shell GaAs/AlGAs Nanowires M.A. Fickenscher, S.D. Perera, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish We use photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE) to study the electronic structure of GaAs/AlGaAs core multi-shell NWs. Using Au-catalyst assisted MOCVD, a nominally 2~nm GaAs quantum well tube (QWT) with AlGaAs barriers is formed surrounding a central 50~nm GaAs nanowire core. PL measurements on single nanowires reveal a line at the expected exciton energy for the core and, in addition, several higher energy lines not observed in simple core/shell structures. PLE measurements suggest a coupling of the confined states in the QWT and the core states. A broad PLE response centered at 1.67~eV is suggestive of an AlGaAs shell concentration of approximately 12{\%}. We acknowledge the support of the NSF~(0701703 and 0806700) and the Australia Research Foundation. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L10.00008: Polarization Dynamics of Twin Free GaAs/AlGaAs Core-Shell Nanowires S. Perera, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish, X. Zhang, J. Zou We use polarized time-resolved photoluminescence to study exciton dynamics in GaAs/AlGaAs core-shell nanowires (NWs) at 20~K. By pumping the nanowire with lasers polarized parallel and perpendicular to the nanowire, the polarization dynamics reflect the exciton dipole distributions within the nanowires. The NWs were prepared by Au catalyzed MOCVD and excited by a pulsed titanium-sapphire laser at 798~nm. The polarization of the emitted PL was monitored at the exciton emission peak (1.515~eV) as a function of time after excitation by a polarized pulse. The diameter of the nanowire is much larger than the exciton Bohr radius so that the exciton dipoles are degenerate regardless of orientation; thus in thermal equilibrium the density of excitons parallel and perpendicular dipoles should be equal. At low excitation intensities we find that the excitons are created out of thermal equilibrium, but relax within several hundred picoseconds. At higher excitation powers, the exciton dipoles relax much more rapidly within a time. This suggests that exciton dipole relaxation is very sensitive to carrier-carrier scattering. We acknowledge the support of the NSF (0701703 and 0806700) as well as the Australia Research Foundation. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L10.00009: Polarization dependent photoluminescence studies of InP nanowires Lei Fang, Xianwei Zhao, Fengyuan Yang, Ezekiel Johnston-Halperin Control of the polarization anisotropy observed in measurements of single NWs has the potential to enable both fundamental studies of polarization-sensitive electronic states and potential applications in polarization-sensitive photodetectors. This anisotropy is caused by the large dielectric mismatch between the semiconductor nanowire and the environment (air), which suggests that with appropriate dielectric matching it is possible to minimize or eliminate polarization anisotropy. In order to explore this possibility, we measure the polarization dependence of ensembles of InP nanowires grown by pulsed laser deposition. The measured polarization response of these ensembles correlates well with the straightforward extension of previous models developed to describe single wire measurements. Further, initial studies involving coating InP nanowires with tantalum oxide, whose dielectric constant (5.76 to 8.41) is close to that of InP (9.61), reduces the polarization anisotropy by 20{\%}. These preliminary results will be presented and proposed strategies for more dramatic suppression will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L10.00010: Photocurrent spectroscopy of single InP nanowires A. Maharjan, H.E. Jackson, L.M. Smith, A. Kogan, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish We use photocurrent spectroscopy of single InP nanowires at room temperature to study nanowires having either zinc-blende (ZB) or wurtzite(WZ) crystal structures. Photolithography is used to fabricate the Ti/Al metal contact pads separated by 3 microns on several ZB or WZ InP nanowires. The metal- semiconductor-metal contacts are modeled based on thermionic emission and field emission theory. Analysis of the dark I-V characteristics of these devices determines important intrinsic properties including donor density, barrier heights and electrical conductivity. Current-voltage (I-V) photocurrent curves for a nanowire are obtained by broad illumination of the device from a Ti-Sapphire laser with energies ranging from 1.30 eV to 1.55 eV.The photocurrent at a given bias voltage is plotted as a function of photon energy to determine the band edge of given semiconductor nanowire. The photocurrent drops exponentially below the band edge reflecting Urbach's rule. We find that the energy band gaps of wurzite and zinc blende nanowires are 1.42 eV and 1.34 eV respectively at room temperature showing that the energy band gap of wurtzite structure is about $\sim$80 meV higher than zinc blende structure. Supported by the NSF (\# 0701703, \# 0806700 and \# 0804199) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L10.00011: Photoluminescence Study of Type II ZB-WZ InP nanowire homostructures K. Pemasiri, M. Montazeri, R. Gass, H.E. Jackson, L.M. Smith, J.M. Yarrison, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish, X. Zhang, J. Zou We use CW and time-resolved photoluminescence (TRPL) from single InP nanowires containing both wurtzite (WZ) and zincblende (ZB) crystalline phases to study the quantum confinement of excitons in a Type-II homostructure. We observe strong excitation power dependence, with a change in PL emission energy from the ZB edge to the WZ edge suggesting that the nanowires have mixed phases. TRPL shows a dramatic increase of recombination lifetime from 170 ps for excitons in the continuum above the conduction and valence band barriers to more than 8400 ps for electrons and holes which are strongly confined in quantum potential wells defined by monolayer-scale ZB sections in a predominantly WZ nanowire. Using detailed HR TEM measurements from a 600nm length of a single nanowire, we calculate a complete set of electrons and hole confined states using an eigenfunction expansion method. Analysis of the distribution of electron and hole confined states demonstrates that the observed energy-dependent dynamics are consistent with the type II nature of the confined electron and hole wavefunctions. Supported by the NSF ({\#}0701703 and {\#}0806700) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L10.00012: Scanning Photocurrent Imaging in CdS Nanosheets P. Kumar, A. Maharjan, M. Fickenscher, H.E. Jackson, L.M. Smith, A. Kogan, J.M. Yarrison-Rice, H. Rho, Y. Lee, Y.J. Choi, J. Choi, J.G. Park We study photocurrent from photoexcitation of charged carriers in a CdS nanosheet (NS) at room temperature. Photolithography followed by Ti/Al (20nm/200nm) metal evaporation was used to fabricate Schottky type contact pads separated by $\sim $4 microns across a $\sim $3$\mu $m wide single nanosheet. Dark current measurements and a model constructed using thermionic emission for the forward biased contact and thermionic field emission for the reverse biased contact is used to extract an intrinsic donor carrier density (N$_{d}$~$\approx $10$^{16}$~cm$^{-3})$, barrier height ($\phi _{b}\approx $~0.8-0.9eV) and depletion layer ($\sim $400nm) for each device. Spatial imaging of the photocurrent exhibits peak photocurrents near the reverse bias contact confirming the confinement of the electric field within the space charge region due to the applied bias voltage. Photogenerated electrons and holes are collected at forward and reverse biased contacts, respectively. Polarization analysis shows that the photocurrent is maximized for laser excitation polarized perpendicular to the c-axis of the nanosheet. Supporte by the NSF ({\#}0701703 and {\#}0806700), Korea Research Foundation and KIST. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L10.00013: Raman Stress Mapping of CdS Nanosheets M. Montazeri, J.M. Yarrison-Rice, L.M. Smith, H.E. Jackson, H. Rho, Y. Lee, Y.J. Choi, J. Choi, J.G. Park We present results of spatially-resolved room temperature second order Raman scattering measurements for single $\sim $3 micron wide CdS nanosheets. The sheets, grown by pulsed laser deposition using vapor-phase transport, are uniform in size and shape and exhibit hexagonal wurtzite structure. The orientation of the c-axis is determined by Raman polarization analysis. Spatially-resolved Raman scattering reveals a stress gradient across the nanosheets, with the 2LO phonon energy at the center of nanosheet being higher by $\sim $2~cm$^{-1}$ with respect to the edges which indicates that nanosheets are relaxed at the edges with a strain gradient toward the center. Support provided by NSF ({\#}0701703 and {\#}0806700), Korea Research Foundation and KIST. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L10.00014: Ultrafast spatially-resolved carrier dynamics in single CdSSe nanobelts Lars Gundlach, Piotr Piotrowiak A recently constructed Kerr-gated microscope was applied to spatially, temporally, and spectrally resolve the ultrafast non-linear excitation relaxation dynamics in single CdSSe nanobelts. Luminescence movies with a 100 fs frame resolution were constructed. The ability to spatially resolve the femtosecond dynamics in a single emitting object gives insights which would be impossible to obtain in an ensemble measurement. By applying the Kerr-gated microscope we are able to monitor the dynamics in a single nanobelt with a sufficient time resolution to reveal the different pathways that compete with the dissociation of multiple excitons. We will show that ensemble averaging methods give results that are complicated because of ensemble inhomogeneities. Indeed, already a different orientation of the nanoparticles with respect to the light-field leads to different dynamic response and difficult to interpret results. The onset of nonlinear behavior and the subsequent dynamics are particularly sensitive to even the most subtle inhomogeneities in composition and morphology and hence most difficult to study under the condition of ensemble averaging making time-resolved wide-field fluorescence microscopy a perfect aid in disentangling the complex response. [Preview Abstract] |
Session L11: Focus Session: Transport Properties of Nanostructures III: Molecular Junctions II
Sponsoring Units: DMPChair: Dan Ralph, Cornell University
Room: 305
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L11.00001: Electron Transport in Single Molecule Junctions: Stability, Electron-Phonon Interactions and Current-Induced Local Heating Invited Speaker: Understanding electron transport in a single molecule connected to two electrodes is as basic task in molecular electronics. A widely used approach is to attach the molecule with two linkers that can bind to the electrodes. Thiol is the most studied linker because of its well known capability to bind strongly to metal electrodes, such as Au, although several other linkers, such as isocyanide, amine, pyridine, carbon-carbon and carboxylic acid, have also been used to establish a molecule-electrode contact. It has been concluded that the linkers can play an important or even dominant role in the conductance and other electron transport properties of molecular junctions. Since the molecule-electrode contact is often the weakest link in a molecular junction, an important question that has not yet been well studied is: \textit{How stable is a molecular junction due to the finite lifetime of the linker-electrode bond?} Another important question is: \textit{How hot does a molecular junction get when passing a current through it?} In the present work, we investigate the stability and breakdown mechanism of a single molecule covalently attached to two gold electrodes via Au-S bonds. We report on an experimental study of current-induced local heating in single molecules covalently attached to two gold electrodes as a function of applied bias and molecular length We also discuss the related electron-phonon interactions in single molecule junctions. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L11.00002: Electron Transport and Thermoelectricity in Alkanethiol Molecular Junctions Yu-Chang Chen, Chun-Lan Ma, Diu Nghiem, Yu-Shen Liu We investigate the electron transport properties of alkanethiol molecules in the two- and three-terminal junctions by using first-principles approaches. We observe that novel states around the Fermi levels are introduced in the amino-substituted butanethiol junction. It leads to a sharp increase of the current owing to the resonant tunneling. We also describe a field-theoretic theory combined with first principles approaches to calculate the thermoelectricity. The dependence of the Seebeck coefficient on the biases, gate voltages, and temperatures is systematically investigated. Due to the novel states introduced by the amino-substituted butanethiol junction, the Seebeck coefficient could be easily controlled by using gate voltages and biases. When the temperature in one of the electrodes is set to zero, the Seebeck coefficient could vary pronouncedly with the temperature in the other electrode, and such dependence could be enhanced by varying gate voltages. At finite biases, we also find richer features in the Seebeck coefficient related to the density of states in the vicinity of the left and right Fermi levels. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L11.00003: Ab initio transport calculations of molecular wires with electron-phonon couplings Kenji Hirose, Nobuhiko Kobayashi Understanding of electron transport through nanostructures becomes important with the advancement of fabrication process to construct atomic-scale devices. Due to the drastic change of transport properties by contact conditions to electrodes in local electric fields, first-principles calculation approaches are indispensable to understand and characterize the transport properties of nanometer-scale molecular devices. Here we study the transport properties of molecular wires between metallic electrodes, especially focusing on the effects of contacts to electrodes and of the electron-phonon interactions. We use an ab initio calculation method based on the scattering waves, which are obtained by the recursion-transfer-matrix (RTM) method, combined with non-equilibrium Green's function (NEGF) method including the electron-phonon scatterings. We find that conductance shows exponential behaviors as a function of the length of molecular wires due to tunneling process determined by the HOMO-LUMO energy gap. From the voltage drop behaviors inside the molecular wires, we show that the contact resistances are dominant source for the bias drop and thus are related to local heating. We will present the electron-phonon coupling effects at contact on the inelastic scattering and discuss on the local heating and local temperature, comparing them with those of metallic atomic wires. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L11.00004: Transport properties and stability of molecular break junctions Nikolai Sergueev, Leonidas Tsetseris, Kalman Varga, Sokrates Pantelides The electrode-molecule interface in a break junction is known to be crucial to understand its electronic and transport properties. Using first-principles calculations we first probe a comprehensive set of mechanisms responsible for the stability of the prototype junction of a benzene-dithiol (BDT) between gold electrodes. We find that by pulling the electrodes apart the geometry of the molecule depends drastically on the electrode-surface morphology. We next report results of the quantum transport calculations for several stable junction configurations. The calculations are performed using the recently developed technique based on density functional theory and complex absorbing potentials[1]. The molecular junction is treated as a closed system with a set of complex potentials mimicking the source and the drain electrodes. We find that the conductance of the BDT molecule varies significantly within the different junction configurations. We will compare the results with recent experiments on BDT break junctions. [1]. K. Varga and S.T. Pantelides, PRL 98, 076804 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L11.00005: Electrical Conductance and Reversible Conductance Switching in Molecular Junctions Invited Speaker: A technology is demonstrated to fabricate reliable molecular metal-molecule-metal junctions with unprecedented device diameters up to 100 $\mu $m. The yield of these molecular junctions is close to unity. Stability investigations have shown a shelf life of years and no deterioration upon cycling. Key ingredients are the use of a conducting polymer layer (PEDOT:PSS) sandwiched between the self-assembled monolayer (SAM) and the top electrode to prevent electrical shorts, and processing in lithographically defined vertical interconnects (vias) to prevent both parasitic currents and interaction between the environment and the SAM [1--3]. Furthermore, a fully functional solid-state molecular electronic switch is manufactured by conventional processing techniques. The molecular switch is based on a monolayer of photochromic diarylethene molecules sandwiched between two electrodes. The monolayer reversibly switches the conductance by more than one order of magnitude between the two conductance states via optical addressing. This bidirectional conductance switch operates as an electronic ON/OFF switch and as a reprogrammable data storage unit that can be optically written and electronically read [4]. \\[4pt] [1]\textit{ Nature}, \textbf{441}, 69--72 (2006). \\[0pt] [2] \textit{Proc. Natl Acad. Sci USA, }\textbf{104}, 11161-11167 (2007). \\[0pt] [3] \textit{Nature} \textit{Nanotechn.}, \textbf{3}, December issue (2008) \\[0pt] [4] \textit{Adv. Mater. }\textbf{20}, 1467--1473. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L11.00006: Reversible, mechanically-activated switching in pyridine single molecule junctions Maria Kamenetska, Su Ying Quek, Michael L. Steigerwald, Hyoung Joon Choi, Steven G. Louie, Mark S. Hybertsen, J.B. Neaton, Latha Venkataraman We measured the conductance of single pyridine-terminated molecules by mechanically forming and breaking Au point contacts with a modified STM in a solution of molecules. Conductance traces recorded while stretching the junction reveal two distinct steps at different conductance, both due to the formation of a single molecule junction between gold electrodes. To better understand the origin of this bi-stable conductance signature, we devise a new method to experimentally determine the distance between the gold electrodes for any given molecular conductance. We find a clear correlation between the level of conductance and the distance between gold electrodes, with the lower conductance corresponding to a molecule fully stretched between the contacts and the higher conductance to a molecule bound at an angle. The dependence of conductance on metal-molecule contact geometry allows us to reversibly switch between conductance states by elongating and compressing the junction. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L11.00007: First-Principles Studies of Single-Molecule Junctions: Conductance and Mechanically-Controlled Switching Su Ying Quek, Hyoung Joon Choi, Steven G. Louie, Mark S. Hybertsen, Latha Venkataraman, J.B. Neaton We explore the conductance of amine- and pyridine-Au single-molecule junctions, in the context of recent experiments, with a density-functional theory (DFT)-based scattering state approach. Using a physically motivated self-energy correction, we compute conductance values in good agreement with experiment, in contrast to DFT values that are too large[1]. We investigate quantitatively conductance trends, and demonstrate, together with experiment, that reversible conductance switching can result from mechanically-induced changes in the metal-molecule contact geometry in pyridine-Au junctions. [1] Quek et al, Nano Lett 7, 3477 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L11.00008: Conductance of Molecular Wires Measured by STM- Break Junction Jonathan R. Widawsky, Maria Kamenetska, Adam C. Whalley, Jennifer E. Klare, Colin Nuckolls, Mark S. Hybertsen, Latha Venkataraman We present a comparison of the measured conductances of short molecular wires attached to gold electrodes in ambient conditions. The junctions are fabricated using a modified STM to repeatedly form and break Au point contacts, characterized by the quantum of conductance, in a solution of molecules. Specifically, we study how the conductance of three molecules -- 4,4'-diaminoazobenzene, 4,4'-diaminostilbene, and bis-(4-aminophenyl)acetylene -- depends on the voltage bias applied across the electrodes. In order to determine a statistically most-probable value of conductance, each measurement is obtained from data sets of approximately 10,000 individual conductance pull-out traces obtained over a few hours. In addition, we measure the conductance of solutions irradiated with ultraviolet light to induce photoisomerization of the azobenzene and stilbene from their \textit{trans} to the \textit{cis} configurations. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L11.00009: Molecular orbital theory of ballistic electron transport through molecules Matthias Ernzerhof, Philippe Rocheleau, Francois Goyer Electron transport through molecules occurs, for instance, in STM imaging and in conductance measurements on molecular electronic devices (MEDs). To model these phenomena, we use a non-Hermitian model Hamiltonian [1] for the description of open systems that exchange current density with their environment. We derive qualitative, molecular-orbital-based rules relating molecular structure and conductance. We show how side groups attached to molecular conductors [2] can completely suppress the conductance. We discuss interference effects in aromatic molecules [3] that can also inhibit electron transport. Rules are developed [1] for the prediction of Fano resonances. All these phenomena are explained with a molecular orbital theory [1,4] for molecules attached to macroscopic reservoirs. [1] F. Goyer, M. Ernzerhof, and M. Zhuang, JCP 126, 144104 (2007); M. Ernzerhof, JCP 127, 204709 (2007). [2] M. Ernzerhof, M. Zhuang, and P. Rocheleau, JCP 123, 134704 (2005); G. C. Solomon, D Q. Andrews, R P. Van Duyne, and M A. Ratner, JACS 130, 7788 (2008). [3] M. Ernzerhof, H. Bahmann, F. Goyer, M. Zhuang, and P. Rocheleau, JCTC 2, 1291 (2006); G. C. Solomon, D. Q. Andrews, R. P. Van Duyne, and M. A. Ratner, JCP 129, 054701 (2008). [4] B.T. Pickup, P.W. Fowler, CPL 459, 198 (2008); P. Rocheleau and M. Ernzerhof, JCP, submitted. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L11.00010: Quantum transport in Molecular Device Partha Pratim Pal, Brandon Johnson, Ranjit Pati Researchers have taken a lot of interest in designing electronic circuits using molecules ever since the pioneering work of Aviram and Ratner that showed that an organic molecule can be used as a rectifier. Organic molecules with their abundant availability, structural flexibility coupled with their versatile electronic properties are promising candidates for miniaturized electronic devices. To be able to predict the behavior of different organic molecules in an electronic circuit under applied bias, we need to have a detailed knowledge of the electronic structure of the molecule under the influence of the applied electric field. Thus first principles calculations are the best way to get to the root of this problem. In this talk, we report a new approach to model electron transport in single molecular junction, which gives results that match closely with the experimental counterparts. We believe this new approach would give a tremendous boost to the predictive capability of electronic properties of molecular devices. [Preview Abstract] |
Session L12: Focus Session: Photocatalysis and photovoltaic: Excitation, Trapping, and Transport of Charge Carriers at Surfaces and Interfaces
Sponsoring Units: DMP DCMPChair: Wladyslaw Walukiewicz, Lawrence Berkeley National Laboratory
Room: 308
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L12.00001: Polynuclear Metal Oxide Photocatalysts in Nanoporous Silica Scaffolds for Artificial Photosynthesis Invited Speaker: |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L12.00002: Understanding and Controlling Photovoltaic Effects in Complex Oxide Thin Films Steven Byrnes, Thomas Conry, Sourav Roger Basu, Lane Martin, Drew Paran, Varada Bal, Joel W. Ager, R. Ramesh Thin-film oxide heterostructures are a promising material system for large-scale photovoltaic energy conversion, as oxides can be cheap, abundant, stable, and highly light-absorbing. As a model system, we have investigated the room-temperature ferroelectric BiFeO$_3$ (BFO). Heteroepitaxial BFO films are grown by both metal-organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD), allowing for a wide range of control over thickness, composition, and ferroelectric domain structure. BFO has been measured to have a direct bandgap at 2.6~eV; moreover its bandgap and other material properties can be controlled by alloying and by modification of stoichiometry. In this work, we will demonstrate the photovoltaic properties of BFO thin films (100--1000~nm) grown heteroepitaxially on oxide bottom electrodes with transparent ITO top contacts. Electrical and external quantum efficiency measurements prove that the photovoltaic effect comes from a Schottky barrier between ITO and p-type BFO, but time-dependent and capacitance-voltage measurements show that ferroelectricity, ion motion, and/or trap states also play an important role in the electrostatics of the device. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L12.00003: Ultrafast Time-Resolved Spectroscopy of Photoinduced Electron Transfer in Novel Photovoltaic Devices L.M. Mier, A.R. Carter, T.L. Gustafson, A.J. Epstein We present work toward an understanding of the fundamental photophysics of photoinduced electon transfer between 9-anthracenecarboxylic acid (9-AC) and TiO$_2$ nanoparticles in order to apply the techniques to a novel photovoltaic device. The active layers of a proposed device consist of a broad-spectrum, metallo-organic absorber\footnote{M.H.Chisholm, et al., Inorg.Chem.\textbf{47}, 3415 (2008).} covalently bound through a carboxylic acid to a nano-porous TiO$_2$ structure. To study the electron transfer, a model compound, 9-AC, is covalently bound to TiO$_2$ nanoparticles. Ultrafast electron transfer from the excited 9-AC to the TiO$_2$ is observed within 50 fs using ultrafast broadband spectroscopy. Further evidence of this transfer is shown from quenching of the fluorescence of the 9-AC with increasing concentrations of TiO$_2$ with no effects on the lifetime of the fluorescence. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L12.00004: Photocatalytic Activity in Nitrogen-doped TiO$_{2}$ Nanowires De Nyago Tafen, Jin Wang, Nianqiang Wu, James P. Lewis We present a comprehensive experimental and theoretical study of the electronic properties and photocatalytic activity of nitrogen-doped anatase TiO$_{2}$ nanowires. UV-Vis spectra showed enhanced absorption in the visible light range for nitrogen doped nanowires compared to the plain sample. The nitrogen doped nanowires exhibit improved photocatalytic activity compared to the plain sample upon visible light irradiation while under exposure to UVA light the photocatalytic activity decreased after nitrogen doping. Furthermore, the incorporation of nitrogen introduces localized states in the band gap. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L12.00005: Interfacial charge dynamics studied by ultrafast electron diffraction Ryan Murdick, Ramanikalyan Raman, Yoshie Murooka, Richard Worhatch, Chong-Yu Ruan Of central importance to nanoscale device technology is the role of charge transfer at interfaces. Using ultrafast electron diffraction, which has recently emerged as a new technique in determining transient surface photovoltages with nanometer sensitivity (Murdick et al., PRB 77, 245329, 2008), we investigate the surface charge and space-charge dynamics at the Si/SiO2 interface. By varying the excitation wavelength, fluence, and pulse duration, we explore various pathways inducing electron tunneling through an insulating barrier to reach the surface states. We show that the surface states have relatively long lifetimes ($\sim $100 ps), but are rechargeable, thus ideal for serving as a charge pump for interfacial devices. Using the Si/SiO2 platform, we extend this diffractive potentiometry approach to study nanoparticle charging and molecular transport. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L12.00006: TiO2 nanowire sensitized by organic dyes for photovoltaic applications: influence of binding groups and molecular dimension Sheng Meng, Efthimios Kaxiras We investigate the electronic couplings including charge separation, injection, and recombination processes between a TiO2 nanowire and a set of organic dye sensitizers, based on the full time-dependent density functional theory treatment of electron excitation and atomic vibrations. For all the cases the highest occupied molecular orbital (HOMO) of dye molecules are found being located in the middle of the TiO2 bandgap and the lowest-unoccupied molecular orbital (LUMO) close to the TiO2 conduction band minimum, leading to enhanced visible light absorption and ultrafast electron injection into the TiO2 conduction band. The influences of the anchoring groups and molecular dimensions to the dye injection dynamics and electron-hole recombination process are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L12.00007: Analysis of Quasiparticle Energy Band Shifts Resulting from Introduction of Nitrogen into Titanates Wei Kang, Mark S. Hybertsen Titanium oxides and many titanates, such as rutile (TiO$_{2})$ and SrTiO$_{3}$, are promising in photo-catalysis for water splitting and photo-degradation of hazardous materials in the environment, although their large band gaps limit utilization of the solar spectrum to the UV region. Experiments show that introduction of nitrogen by various means can significantly affect the band gap. However, catalytic action also depends on individual conduction and valence band alignments. We address these issues by performing theoretical calculations of the energy spectrum for titanium oxides, titanates and various structures introducing nitrogen into the crystals using the GW method. In contrast to density functional theory approaches, the GW method generally leads to energy levels and band gaps that agree well with experiments. We here use this approach to illustrate the mechanism of band shifting in titanates due to the introduction of nitrogen, in particular the differences in correlation effects for nitrogen related energy bands in these materials. This work is supported by the DOE. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L12.00008: Nanocrystal-based Dyads for Solar to Electric Energy Conversion. Lei Wang, Mingyan Wu, David Waldeck We describe a new project which aims to develop a systematic and modular approach to creating a new generation of Gratzel-inspired solar energy conversion devices with the following novel advantages: the ability to capture the entire available range of solar irradiance by employing sets of linked nanoparticles, fabrication by self-assembly, enhanced robustness, and lowered cost through use of nanostructured, rather than molecular, charge transfer elements. The project team is designing, creating, and characterizing linked-nanoparticle dyads, which will act as the charge separation ``engine'' in new generation solar cells. By employing a mixture of dyads it should be possible to efficiently capture the entire solar spectrum. The proposed device architecture has two important advantages over existing solar conversion devices: It can be produced by a self-assembly process. Because of its modularity, each of its components (nanoparticles or organic linker) can be optimized separately. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L12.00009: Energy level alignment of zinc tetraphenylporphyrins derivatives adsorbed on wide band gap semiconductor oxides. Sylvie Rangan, Robert Allen Bartynski, Elena Galoppini Metalloporphyrins play an essential role in photosynthetic mechanisms and therefore are natural candidates for electron transfer mediator in dye sensitized solar cells (DSSCs). Among the possible metalloprophyrins, the zinc tetraphenylporphyrins (ZnTPP) derivatives have been found to have similar electron injection and charge recombination properties as the important standard ruthenium dye N3 for DSSCs, as well as reasonable performances using TiO$_{2}$ or ZnO as substrates. We have investigated the electronic structure, energy level alignment, and their changes with altered surface bonding geometries, using a selective functionalization with carboxylic anchoring groups of the meso-phenyl, of functionalized ZnTPP on single crystal TiO$_{2}$ and ZnO surfaces. Occupied and unoccupied electronic states were determined using direct (ultra-violet and x-ray) photoemission and inverse photoemission in the same ultra-high vacuum analysis chamber. Energy level alignment of the ZnTPP molecular orbitals with respect to the substrates band edges will be compared to the available literature. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L12.00010: Effects of Diffusion on Photocurrent Generation in Single-Walled Carbon Nanotube Films Christopher Merchant, Nina Markovic We have studied photocurrent generation in large carbon nanotube (CNT) films using electrodes with different spacings. We observe that the photocurrent depends strongly on the position of illumination, with maximum observed response occurring upon illumination at the electrode edges. The rate of change of the response decays exponentially, with the fastest response occurring for samples with the smallest electrode spacing. We show that the time response is due to charge carrier diffusion in low-mobility CNT films. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L12.00011: Efficient photocatalysis at pH7: in-situ formation of a water-splitting cobalt catalyst at electrode interfaces Invited Speaker: |
Session L13: Computational Methods: Quantum Monte Carlo
Sponsoring Units: DCOMPChair: Jeff Grossman, University of California, Berkeley
Room: 309
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L13.00001: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L13.00002: PIMC Simulation of Thermal Dissociation of Dipositronium Ilkka Kyl\"anp\"a\"a, Tapio Rantala Positronium is a hydrogen atom like pair of a positron and an electron, and correspondingly, dipositronium is a four-particle molecule formed by two positronium atoms. Stability of the dipositronium molecule was established by Hylleraas in 1947 [1], already, but not experimentally observed until recently [2]. This system of four light particles sets challenges for both theoretical and experimental consideration and in finite temperature, in particular. The experimental observations are based on the observation of positronium decay rate and the changes related to dipositronium formation or dissociation. The finite-temperature modeling of such light quantum particles has to done fully nonadiabatically, that we have accomplished with the Path-Integral Monte Carlo (PIMC) method for the thermal equilibrium. As the dissociation energy of dipositronium is about 0.4 eV, the recent observation of the thermal activation energy of about 0.07 eV was interpreted follow from the experiment related desorption process [2]. With our quantum statistical simulation we show, however, that the observed low energy obviously relates to the dissociation of the molecule, directly. [1] Hylleraas, E.A. and Ore A., Phys. Rev.71, 493 (1947). [2] Cassidy D.B. and Mills A.P. Jr., Nature 449, 195 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L13.00003: Convergence of the variational parameter without convergence of the energy in Quantum Monte Carlo (QMC) calculations using the Stochastic Gradient Approximation Daniel Nissenbaum, Hsin Lin, Bernardo Barbiellini, Arun Bansil To study the performance of the Stochastic Gradient Approximation (SGA) for variational Quantum Monte Carlo methods, we have considered lithium nano-clusters [1] described by Hartree-Fock wavefunctions multiplied by two-body Jastrow factors with a single variational parameter $b$. Even when the system size increases, we have shown the feasibility of obtaining an accurate value of $b$ that minimizes the energy without an explicit calculation of the energy itself. The present SGA algorithm is so efficient because an analytic gradient formula is used and because the statistical noise in the gradient is smaller than in the energy [2]. Interestingly, in this scheme the absolute value of the gradient is less important than the sign of the gradient. Work supported in part by U.S. DOE. \\ \mbox{[1] D. Nissenbaum {\em et al.}, Phys. Rev. B {\bf 76}, 033412 (2007).} \\ \mbox{[2] A. Harju, J. Low. Temp. Phys. {\bf 140}, 181 (2005).} [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L13.00004: Effective one-body potential fitted for many-body interactions associated with a Jastrow function: application to the quantum Monte Carlo calculations Naoto Umezawa, Brian Austin, William A. Lester, Jr An efficient method of optimizing a Slater determinant, $D$, in the Jastrow-Slater-type wave function, $FD$, is suggested. Here, the so-called transcorrelated Hamiltonian, $\frac{1}{F}{\cal H} F$, which is a similarity transformation of the usual Hamiltonian of an electronic system with respect to a Jastrow function $F$, is fitted to an effective Hamiltonian, ${\cal H}_{\rm eff} = \sum_{i}^{N} \left( -\frac{1}{2} \nabla^2_i + v({\mathbf r_i}) \right)$, in which all the electron-electron and electron-neucleus interactions are represented by a one-body potential, $v({\mathbf r})$. A single-particle Schr\"odinger equation is then solved by using $v({\mathbf r})$ to determine the orbitals, of which the Slater determinant consists. The obtained orbitals improve the atomic total energies in the variational Monte Carlo calculations compared to those given by the density-functional-based orbitals. Advantages of using the optimized orbitals in the diffusion Monte Carlo calculations are also discussed. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L13.00005: Improved Algorithm for Calculating Observables in Diffusion and Reptation Quantum Monte Carlo Jeremy McMinis, David Ceperley, Carlo Pierleoni By reformulating the calculation of observables using the Hellman-Feynman theorem we are able to reduce the bias on observables calculated in Diffusion and Reptation Monte Carlo. Unlike previous attempts [1], our technique assumes no knowledge about derivatives of the trial or exact ground state wavefunction with respect to the perturbation. We will outline the derivation of the operator and show examples for DMC by comparing to forward walking, and within RMC by showing faster convergence to the unbiased, ground state observable. [1]Assaraf and Caffarel, J. Chem. Phys. 119, 10536 (2003) [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L13.00006: Analysis of fixed-nodes errors in quantum Monte Carlo calculations of atoms and molecules Shuming Hu, Kevin Rasch, Minyi Zhu, Michal Bajdich, Lubos Mitas The accuracy of fixed-node QMC calculation is determined by the fermion nodes of the trial wavefunction. We analyze the fixed-node errors for a diverse set of atoms and molecules. In some cases, our simple wavefunctions have almost the exact nodes. But in other cases, it is very difficult to find the exact nodes even with wavefunctions of correlated many-body forms, such as extensive multi-reference expansions, pairing and backflow. We try to elucidate the impact of the size and extent of the basis sets as one of the factors influencing fixed-node biases. The testing systems also include transition metal atoms with all-electron and Ne-core pseudopotential calculations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L13.00007: Geometry of fermion nodes and its impact on many-body effects in quantum Monte Carlo Lubos Mitas Fermion nodes, which are zero sets of stationary fermionic many-body wavefunctions, play an important role in quantum Monte Carlo calculations. In the diffusion Monte Carlo method the so-called fixed-node approximation allows us to avoid well-known inefficiencies of the fermion sign problem and to use the method for large systems. Besides this practical importance fermion nodes are also related to spectral properties of second order differential operators and to several physical effects and quantities. In order to understand these relationships we study fermion nodal hypersurfaces, both their topologies and shapes, as determined by wavefunctions built from different types of correlations such as pairing orbitals and backflow many-body coordinates. We analyze the impact of particle interactions on the changes of nodal topologies and the conditions under which such changes can occur. We investigate impact of nodal topologies on properties of wavefunctions with periodic boundary conditions as well as relationship of the nodal surfaces to kinetic energy and some other quantities. We further attempt to elucidate the nodal properties on examples of exactly solvable models. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L13.00008: Hybrid DFT orbitals as a means of reduction of fixed-node errors in diffusion Monte Carlo simulations Jindrich Kolorenc, Shuming Hu, Lubos Mitas We explore possibilities to improve variational freedom of Slater-Jastrow trial wave function by using one-body orbitals from the hybrid density-functional theory (DFT) to construct its determinantal Slater part. Weight of the exact exchange term in the hybrid DFT functional serves as a variational parameter that is optimized with respect to the total energy calculated within the fixed-node diffusion Monte Carlo method. This approach is certainly less powerful than direct optimization of one-body orbitals within a basis-set expansion, but its modest computational requirements make it suitable for large-scale simulations of solids. Presented method will be illustrated on several materials with emphasis on transition-metal compounds. The weight of the exact exchange term optimized in this way can also serve as a guide for the hybrid DFT itself. For instance, it provides hints how the weight is changed/screened when a crystal is compressed. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L13.00009: Geometry optimization using Quantum Monte Carlo Lucas Wagner, Jeffrey Grossman There are many molecular and solid systems where correlation effects need to be treated very accurately to obtain correct geometries. Current density functionals often do not perform sufficiently well in excited states, weak-binding, and transition metal oxide systems. Quantum Monte Carlo (QMC) offers very accurate total energies due to explicit treatment of electron correlation, but its stochastic nature makes precise geometry optimization challenging. We present a method that uses noisy total energies to perform a stochastic series of line minimizations. This method is efficient for multiple degrees of freedom and is effective in both the excited state and when the trial function is relatively poor--two regimes in which forces in QMC have not been developed. Details of the approach will be presented as well as a number of applications. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L13.00010: Quantum Monte Carlo calculations of the energy-level alignment at organic-inorganic hybrid interfaces Zhigang Wu, Yosuke Kanai, Jeffrey Grossman Understanding interface properties of nano- and hybrid- materials at molecular level is of critical importance for fostering technological advancements. While the density functional theory (DFT) continues to be an important method for investigating opto-electronic and excitation properties of materials, the DFT calculations in some cases fail to provide an accurate description. One such difficult case is computing the energy-level alignment at a hybrid interface, composed of two distinct materials with very different electronic characteristics. In this work we present a quantum Monte Carlo approach to correct the Kohn-Sham (KS) level alignment, and we demonstrate this approach for hybrid interfaces between the silicon (001) surface and several organic molecules. Our calculations show that for some molecules there is a qualitative difference with the DFT-KS level alignment, completely changing the character of the heterojunctions formed. We will discuss its implication for understanding the opto-electronic behaviors of hybrid interfaces, along with some computational/theoretical challenges in extending this approach further. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L13.00011: QMC Study of Optical Switching of Azobenzene Molecule Rene Derian, Matus Dubecky, Lubos Mitas, Ivan Stich Optical Switching of photochromic azobenzene (AB) molecule via first excited singlet-state is studied. AB features two photoswitchable conformations, cis and trans with very different geometries and properties. Using QMC techniques we compute excitation/deexcitation ground-state -- first singlet- excited-state spectra of AB. By a careful QMC optimization of the ground/excited-state wave functions with up to 500 determinants chemical accuracy is obtained for the cis and trans conformers. Our QMC results are significantly superior to DFT results with proper spin symmetry (ROKS) and surpass also the available standard quantum chemistry results such as CAS SCF. These results open up the possibility of simulation of anchored AB opto-mechanical switches. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L13.00012: Multideterminant quantum Monte Carlo calculations of benzene dimers Richard G. Hennig, Kathleen A. Schwarz, Cyrus Umrigar, Julien Toulouse Benzene dimers represent the prototypical system for weak $\pi$-$\pi$ interactions that determine the bonding for various organic materials and carbon nanostructures. Several previous studies using coupled-cluster and quantum Monte Carlo methods have determined the binding energy of parallel, perpendicular and parallel-shifted configurations of the benzene dimer. Here we present multi-determinant variational and diffusion Monte Carlo calculations for the various benzene dimer configurations. The total energy of the benzene dimers depends strongly on basis set size, orbital coefficients and number of determinants in the trial wave function. The binding energy converges faster than the total energy with basis set size and number of determinants due to partial cancellation of errors. While orbital optimization lowers the total energy, the large number of orbital parameters and hence large computational cost limits orbital optimizations to wave functions with small basis sets and small numbers of determinants. In comparison the optimization of the Jastrow and determinant coefficients can efficiently converge the energy of benzene dimers and enables accurate predictions of the binding energies. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L13.00013: A Quantum Monte Carlo investigation of dispersion interactions in graphite Leonardo Spanu, Giulia Galli, Sandro Sorella We present a series of Quantum Monte Carlo (QMC) calculations of graphite, aimed at describing on the same footing the strong C-C covalent bonds and the weaker interlayer interactions. In particular, we carried out calculations of binding energies, bond lengths and compressibility by using the Variational Monte Carlo and Lattice Regularized Diffusion Monte Carlo techniques [1]. We use as a variational ansatz the Jastrow Antisymmetrical Wave function, including a pairing determinant and a Jastrow correlation factor [2]. Our results allow for a detailed analysis of dispersion forces between graphite layers, including their behavior at long distances, and yield a quantitative estimate of the layer binding energy. \vspace{0.3cm} \newline [1] Casula M. et al. Phys. Rev. Lett. 95 100201 (2005) \newline [2] Casula M. et al. J. Chem. Phys. 119, 6500 (2003) [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L13.00014: Auxiliary-Field Quantum Monte Carlo Studies of Pressure-Induced Phase Transitions in Silicon and MnO Wirawan Purwanto, Henry Krakauer, Eric Walter, Shiwei Zhang Accurate theoretical predictions across structural phase transitions are challenging, as they typically involve different electronic structures on the two sides of the transition. We use the phaseless auxiliary-field quantum Monte Carlo (AFQMC) method---which yields accurate many-body wave functions by means of importance sampled random walks in the space of Slater determinants---to calculate the equation of state near two phase transitions: in Si, from the diamond to metallic $\beta$-tin transition at $\sim 11$ GPa; and in MnO, the volume and magnetic moment collapse at $\sim 100$ GPa. The Si phase transition serves as a test case to study the accuracy of the AFQMC method; the calculated transition pressure is in good agreement with the experiment. Applications to the MnO phase transition will then be presented. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L13.00015: A Quantum Monte Carlo Study of Molecular Titanium Dihydride$^\dag$ Todd D. Beaudet, Jeongnim Kim, Kenneth Esler, Richard M. Martin Recently there has been interest in the possibility of reversibly storing molecular hydrogen on titanium decorated carbon-nanostructures$^1$. As part of our research$^2$ in this area, we present DFT and QMC results for molecular TiH$_2$ using pseudopotentals. We identify the low energy symmetry-classified states and compare with previous work$^{3,4}$, where there is not a consensus on the symmetry and geometry of the ground state. At the DFT level, the TiH$_2$ d-states are nearly decoupled from the molecular geometry so that several d-state orderings are very close in energy. In our work we use diffusion Monte Carlo with the fixed-node approximation where the symmetry and nodal structure are determined by a trial function constructed of molecular orbitals from DFT. We will also discuss progress on Ti-carbon systems pertaining to hydrogen adsorption. \\ \\ $^1$ E. Durgun \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 226102 (2006). \\ $^2$ T. D. Beaudet \textit{et al.}, J. Chem. Phys. \textbf{129}, 164711 (2008). \\ $^3$ J. A. Platts, J. Mol. Struct. \textbf{545}, 111 (2001). \\ $^4$ B. Ma, C. L. Collins, H. F. Schaefer, J. Am. Chem. Soc. \textbf{118}, 870 (1996). \\ $^\dag$ Supported by NSF DMR03-25939. [Preview Abstract] |
Session L14: Colloids IV: Colloids and Interfaces
Sponsoring Units: DFDChair: David Weitz, Harvard University
Room: 315
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L14.00001: The equilibrium colloidal crystal/colloidal liquid interface Eric R. Weeks, Jessica Hernandez-Guzman We use confocal microscopy to study an equilibrated crystal-liquid interface in a colloidal suspension. The surface shows spatial fluctuations due to capillary waves. Local measurements of the structure and dynamics near the rough surface reveal that the intrinsic surface, while meandering in space, is locally sharply defined. Examining different quantities finds slightly different widths of this intrinsic surface. In terms of the particle diameter $d$, this width is either $1.3d$ (based on structural information) or $2.4d$ (based on dynamics), both not much larger than the particle size. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L14.00002: Janus particles at the liquid-liquid interface Qian Chen, Stephen Anthony, Steve Granick Dipolar Janus particles (negatively charged on one side, positively charged on the other), deposited on PDMS droplets in water, are studied in real time by fluorescence and phase contrast microscopy. Crystals form, under some conditions with long-range hexagonal order, but this self-assembled structure depends strongly on particle size and ionic strength of the water phase. Their provocative translational and rotational dynamics is studied using single-particle tracking. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L14.00003: Short-time self-diffusion of nearly hard spheres at an oil-water interface Penger Tong, Yuan Peng, Wei Chen, Thomas Fischer, David Weitz Optical microscopy and multi-particle tracking are used to study hydrodynamic interactions of monodisperse polymethylmethacrylate (PMMA) spheres at a decalin-water interface. The short-time self-diffusion coefficient measured at low surface coverage has the form, $D^S_S(n) = \alpha D_0 (1- \beta n)$, where $n$ is the area fraction occupied by the particles and $D_0$ is the Stokes-Einstein diffusion coefficient in the bulk suspension of PMMA spheres in decalin. The measured values of $\alpha$ are found to be in good agreement with the numerical calculation for the drag coefficient of interfacial particles. The measured values of $\beta$ differ from that obtained for bulk suspensions, indicating that hydrodynamic interactions between the particles have interesting new features at the interface. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L14.00004: Observing the three-dimensional motion of colloids at an oil-water interface Ryan McGorty, David Kaz, Shankar Ghosh, V.N. Manoharan Our experimental system allows us to place micron-sized colloids at a flat oil-water interface.~ Using digital holographic microscopy we track the motion of particles at the interface in all three dimensions.~ Of particular interest is the out-of-plane motion of an adsorbed particle.~ I will present data of such motion and what it reveals regarding the energy and length scales of a particle attached to an interface.~ Introducing a laser tweezer and customized colloids (such as core-shell particles) into our experiment allows us to further investigate this system.~ [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L14.00005: Self-assembled Capillary Arrows Jean-Christophe Loudet, Bernard Pouligny Anisotropic particles adsorbed at a water-air interface are known to aggregate due to capillary interactions. We show that the packing configuration of a pair of prolate ellipsoids critically depends on their relative size and/or aspect ratio mismatch. While identical particles simply pack side-by-side, particles of slightly different sizes are observed to systematically self-assemble into characteristic \textit{arrows}, i.e. with a finite angle between their axes. The occurrence of such arrows cannot be explained within the far-field approximation of interacting polar quadrupoles. A numerical analysis is worked out which allows us to explore the near-field characteristics of the capillary interaction. Results clearly show the destabilization of the side-by-side configuration due to a size mismatch, in agreement with experimental observations. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L14.00006: Impact of Surfactant Sorption Kinetics on Microscale Tipstreaming Wingki Lee, Lynn Walker, Shelley Anna A microfluidic flow focusing system has been used to synthesize submicron sized droplets via a thread formation mode of drop breakup. This process utilizes the interaction of fluid motion and surfactant transport to draw out a thin thread, which then fragments into a stream of tiny droplets whose sizes are orders of magnitude smaller than the size of the device. In this work, we use a homologous series of C$_{n}$E$_{8}$ (n = 10, 12 and 14) surfactants to probe the impact of surfactant sorption kinetics on this process. To characterize the effects of these surfactants on the thread formation process, we measure the relevant timescales for the formation of a cone-like interface, the drawing and disintegration of a fine thread, and the period with which the process repeats. We then relate these timescales to the characteristic timescales for transport of surfactants to the oil-water interface. These measurements and simple scaling analyses suggest ways to extend the thread length and optimize the overall yield of submicron droplets. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L14.00007: Network Formation at the Air-Water Interface Aline Miller, Maria Simon Saenz de Samaniego A series of diacetylene end functionalised peptides have been designed to form beta-sheet rich monolayers at the air-water interface and their structure, rheological properties and ability to polymerize in response to UV light have been studied using a Langmuir trough and dilatational rheology. Surface pressure-area isotherms as well as compression-expansion cycles reveal all our peptide monolayers organise into the three distinct organisational states typically observed for surfactants at the air-water interface: gaseous (G), liquid expanded (LE) and liquid condensed (LC) and the limiting area per molecule suggests the alternating amphiphilic character of the peptide causes the molecule to orient with its long axes parallel to the air-water interface. The presence of the diacetylene group enhances surface activity and stability over time. Here we will discuss how peptide sequence, UV exposure strength and time, as well as peptide concentration (and hence organisation) influence the kinetics of network formation, and the morphology and mechanical properties of the final network formed. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L14.00008: Kinetically Controlled Adsorption to Freshly Formed Interfaces Nicolas Alvarez, Lynn Walker, Shelley Anna The coefficients of diffusion, adsorption, and desorption are fundamental properties of surfactant molecules and should be independent of the nature in which they are applied.~ However, the approaches currently used to obtain these parameters are highly context dependent and can lead to unphysical trends such as a concentration dependent diffusion coefficient and large mismatches between predicted and observed dynamic behavior. In pendant drop studies one is restricted to diffusion or mixed controlled adsorption at small concentrations, but in reality to get at the kinetic coefficients it would be more advantageous to probe the kinetic controlled regime. Recently it was shown that a characteristic length scale, R$_{D-K,}$ governs the transition from diffusion controlled adsorption to kinetically controlled adsorption for spherical interfaces. If the spherical interface has a radius smaller than R$_{D-K}$ the adsorption process is kinetically limited.~ This paper uses a micro-tensiometer to probe the adsorption dynamics to micron diameter spherical interfaces to test the transition from diffusion limited to kinetic limited adsorption. Using this method we measure kinetic adsorption constants directly. We also describe a new timescale for diffusion, which better describes the adsorption of surfactants onto spherical interfaces. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L14.00009: Structures formed by colloidal particles on a droplet at small particle number Jerome Fung, Ryan J. McGorty, Vinothan N. Manoharan We discuss 3D imaging studies of the self-assembled structures formed by small numbers ($N\sim$ 10) of micron-sized polymethylmethacrylate (PMMA) colloids pinned to the surface of a $\sim$10 micron oil droplet in an aqueous solution. In the low $N$ limit, these structures are governed by the interactions between the constituent colloidal particles on a given droplet. We prepare these droplets using a capillary microfluidic device. Since the droplets are not density matched to the continuous phase, we study them with a time-averaged zero gravity apparatus, based on a rotary stage. Specifically, we image the 3D structures formed by the colloidal particles on the droplets using digital holographic microscopy (DHM). DHM records the 2D interference patterns, or holograms, formed by light scattered from the colloidal particles and unscattered light. Subsequent analysis of the holograms, based on the Lorenz-Mie solution for light scattering by spheres, allows us to determine the 3D particle positions with time resolution limited by the camera frame rate. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L14.00010: Interfacial rheology in complex flow Jeffrey Martin, Steven Hudson Multiphase liquid systems are omnipresent in and essential to everyday life, e.g. foods, pharmaceutics, cosmetics, paints, oil recovery, etc. The morphology and stability of such systems depend on dynamic interfacial properties and processes. Typical methods utilized to measure such interfacial properties often employ drops that are much larger and flows that are much simpler than those encountered in typical processing applications. A microfluidic approach is utilized to measure dynamic structure and kinetics in multiphase systems with drop sizes comparable to those encountered in applications and flow complexity that is easily adjustable. The internal circulation and deformation of an aqueous droplet in clear mineral oil is measured using particle tracers and a detailed shape analysis, which is capable of measuring sub-micron deviations in drop shape. Deformation dynamics, detailed drop shape, interfacial tension, and internal circulation patterns and velocities are measured in Poiseuille and transient elongational flows. Flow kinematics are adjusted by varying the microchannel geometry, relative drop size, and drop height. The effects of confinement on interfacial dynamics and circulation patterns and velocities are also explored. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L14.00011: Reverse coffee-ring effect Byung Mook Weon, Lei Xu, Jung Ho Je, Yeukuang Hwu, Giorgio Margaritondo, David A. Weitz When a coffee drop dries on a solid surface, it commonly leaves a ring-like deposit along the edge, known as the coffee-ring effect. We present a reverse motion of particles in drying droplets, opposite to the coffee-ring effect. We reveal that the particle motion, initially toward the edge by the typical coffee-ring effect, is reversed to the droplet center owing to the capillary interaction generated by the droplet surface. The reverse coffee-ring effect always occurs whenever the capillary interaction prevails over the net outward force by the coffee- ring effect. The interaction predicts an inverse power-law time growth of moving distance from the edge, depending mostly on particle size and contact angle. The reverse coffee-ring effect may contribute to multiple ring formation by sweeping particles toward the center. We prove the mechanism with real-time optical, confocal, and X-ray microscopic observations of colloidal fluids. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L14.00012: Measurements of contact forces at the bottom of a droplet pile Hao Wang, T. Prisk, J. Zhou, A. Dinsmore We measure the contact forces at the bottom of a container of frictionless liquid droplets as a function of compression and of distance to the container wall. Glass cylinders are used to contain 20-micron-radius droplets of silicon oil; Brownian motion is not significant for this size. Reflection interference contrast microscopy is used since we are particularly interested in contacts with the bottom surface. By looking at the Newton's Ring interference pattern, we measure the deformation of each droplet, which arises from gravity and pressure from the whole pile transmitted through droplet contacts. We also measure the radius of each droplet and thereby obtain the vertical contact force. We vary the pile height to change the compressive stress and then measure the corresponding forces, probability distributions, and correlations of rearrangements. The results elucidate the roles that friction and confining walls play in granular systems and also shed light on force chains in bulk of the material. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L14.00013: Sticking colloids to liquid-liquid interfaces one by one David Kaz, Ryan McGorty, Shankar Ghosh, Vinothan Manoharan We investigate the dynamics of placing individual colloidal particles ($\sim $2 microns) onto a flat oil-water interface using optical tweezers. By monitoring the strength and position of the trap, we are able to measure the forces acting on a particle as it encounters the liquid-liquid interface. Digital holographic microscopy affords us three dimensional position information at high frame rates ($>$ 500fps), allowing us to probe short timescale behavior. We vary parameters such as particle surface chemistry, dissolved ion concentration, and pH in order to pursue questions about the nature of interface penetration dynamics. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L14.00014: Controlled Crystal Growth and Solid-Liquid Interface in temperature-sensitive colloidal systems Duc Nguyen, Zhibing Hu, Peter Schall We use temperature-sensitive colloidal NIPA systems to study crystal growth at the ``atomic scale''. By applying a temperature gradient we are able to control the growth of large colloidal single crystals. We visualize the nucleation of these crystals and solidification at the crystal-liquid interface in three dimensions by using confocal microscopy. Trajectories of particles on both the crystal and liquid side of an advancing interface are determined. These elucidate the mechanism of particle assembly at the interface of a growing crystal. At later stages of crystal growth, the interface becomes stationary, and we use the fluctuations of the stationary interface to determine the interface stiffness. Our data suggests a strong anisotropy of the interface tension. These microscopic observations provide unique insight into the mechanism of solidification. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L14.00015: A Surface Plasmon Resonance Investigation of How Water Meets a Hydrophobic Surface. Adele Poynor, Corey Shemelya By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low- density region forms near the surface. We have employed the surface-sensitive, quantum-optical technique of Surface Plasmon Resonance (SPR) to verify the existence of this region at the boundary. [Preview Abstract] |
Session L15: Pattern Formation and Nonlinear Dynamics
Sponsoring Units: DFDChair: Tom Solomon, Bucknell University
Room: 316
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L15.00001: Thermal Convection in the Presence of Internal Heating Gabriel Seiden, Stephan Weiss, Eberhard Bodenschatz Thermal convection in the presence of internal heat sources is an important mechanism of heat transfer in geophysics, particularly in planetary mantle convection. Carefully controlled laboratory studies of this mechanism are, however, scarce. We present experimental results on the effect of internal heating on Rayleigh-B\'{e}nard convection, where the heat sources are induced by IR absorption. The results are compared with available theoretical predictions. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L15.00002: Chaotic three particle dynamics in a viscous liquid filled rotating drum James E. Davidheiser, Eric R. Weeks, Phil Segre We conduct experiments to study the motions of three heavy spheres moving within a viscous liquid filled rotating cylindrical drum. Numerous works, in other geometries, demonstrated that assemblies of non-Brownian particles in viscous liquids have the potential to exhibit chaotic motion. We find that as the drum rotation rate $\omega$ is varied, there are several distinct periodic states as well as fully chaotic states. We track the motion of the spheres using a digital camera and custom particle tracking software. From our data, we characterize the chaotic trajectories as $\omega$ is varied. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L15.00003: Chaotic Dynamics of an Elastically Bouncing Dumbbell Colin Rees, Scott Franklin The dynamics of an elastically bouncing dumbbell is analogous to those of an ball bouncing on a sinusoidally oscillating surface with one important exception: the dumbbell's angular velocity, analogous to the surface's oscillation frequency, changes with each bounce, making the subsequent motion significantly more complicated. We investigate this dynamical system over a range of aspect ratios and initial energy, finding periodic, quasi-periodic and chaotic motions. As the initial energy is increased, the dumbbell can flip over and tumble. We find for large aspect ratios, however, narrow bands of energies well above this minimum where tumbling suddenly ceases. Because energy is conserved, the dynamics of a bounce are uniquely determined by the angle and angular velocity. The Lyapunov exponents of paths in this two dimensional phase space can be calculated, with the hope of identifying periodic islands within the chaotic sea. Finally, for certain parameters, the angle at each collision moves from its initial value in a subdiffusive manner, and we determine the characteristic exponents. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L15.00004: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L15.00005: Fixed point of a renormalization group approach for oscillator synchronization Tony Lee, Gil Refael, Michael Cross, Oleg Kogan, Jeffrey Rogers We apply a recently developed renormalization group method to a 1-dimensional chain of phase-coupled oscillators in the regime of weak randomness. The RG predicts how oscillators with randomly distributed frequencies and couplings form frequency-synchronized clusters. Although the RG was originally intended for strong randomness (distributions with long tails), we find good agreement with numerical simulations even in the regime of weak randomness. We also show analytically and numerically the existence of a stable fixed point in the functional RG space. At late stages of the RG, there is a universal approach to the fixed point regardless of the initial distributions of frequency and coupling. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L15.00006: Cell refinement and growing misorientations from a continuum dislocation density theory Yong Chen, Woosong Choi, Stefanos Papanikolaou, James P. Sethna, Surachate Limkumnerd At low temperatures, climb-free plastic deformation of crystals usually leads to the formation of cellular dislocation structures. Some experiments show fractal distributions of cell sizes; others show a single (non-fractal) characteristic cell size, but a scaling behavior of lengths and misorientations with external strain as the cellular structure refines. By adding an external growing stress field to a refined variant of our recently proposed wall-forming continuum dislocation dynamics theory \footnote{S. Limkumnerd and J. P. Sethna, Phys. Rev. Letters 96, 095503 (2006)}, we explore the formation and evolution of these cellular structures. We shall search both for the emergence of fractal geometries (in suitable experimental geometries) and for the emergence of scaling behaviors of misorientation angles and cell size distributions. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L15.00007: Dynamic self-assembly in far-from-equilibrium magnetic granular ensembles at the liquid/liquid interface Alexey Snezhko, Igor Aranson Magnetic particles suspended over an interface of two immiscible liquids and energized by a vertical alternating magnetic fields give rise to novel dynamic self-assembled structures (``pulsating magnetic stars,'' ``clams'') which are not accessible at the liquid/air interface. These novel structures is attributed to the interplay between surface waves, generated at the liquid/liquid interface by the collective response of magnetic microparticles to the alternating magnetic field, and hydrodynamic fields induced in the boundary layers of both liquids forming the interface. We show that while the onset of the dynamic self-assembly is controlled by the external driving magnetic field parameters the viscosity of the liquids forming the interface plays an essential role. Transition between different self-assembled structures with the parameters of the external excitations is observed. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L15.00008: Investigation on dynamics of colloidal particles with optically-controlled electrode patterns Kwan Hyoung Kang, Hyunjin Park, Horim Lee, Jiwoo Hong We investigated the dynamics of colloidal particles under ac electric fields. We used an optoelectronic substrate in which the conductivity of substrate can be changed optically. The shape of electrode pattern thus can be changed freely by controlling the optical pattern which is produced by a conventional projector. Interaction between particles showed a various patterns depending on applied electrical frequency, and rich dynamic characters are captured by dynamically changing the electrode pattern. Particle behaviors are in general governed by the balance between the dielectrophoresis and induced charge electroosmosis. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L15.00009: Memories in paste: their applications to control crack patterns Akio Nakahara, Yousuke Matsuo We experimentally find that a paste, i. e., a densely packed colloidal suspension with plasticity, has memories of external mechanical fields it suffered, such as flow and vibration. These memories are sustained as microscopically anisotropic network structures of colloidal particles. By drying these pastes, we find that the memories in pastes can be visualized as macroscopically anisotropic crack patterns. By using the memory effects of paste, we can imprint flow and vibration patterns into pastes to produce various crack patterns , such as lamellar, radial, ring, spiral, and so on [1]. [1] Physics Today 60 (2007), no. 9, p. 116. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L15.00010: Controlling chemical oscillations in heterogeneous BZ gels via mechanical strain. Victor Yashin, Krystyn J. Van Vliet, Anna C. Balazs We performed theoretical and computational studies to determine the effect of an applied mechanical strain on the dynamic behavior of heterogeneous polymer gels undergoing the oscillatory Belousov-Zhabotinsky (BZ) reaction. In these gels, the catalyst for the reaction is localized in specific patches within the polymer network and the BZ reaction only occurs within these BZ patches. We focused on a 1D model for the system, and considered two scenarios, in which the BZ reaction did or did not affect the degree of swelling within the gel. For gels having one and two BZ patches, we found that a longitudinal strain could induce transitions between the oscillatory and steady state regimes. For certain values of the BZ stoichiometric parameter $f$, these transitions could exhibit a hysteresis. In systems having two oscillating BZ patches, a strain could switch between the in-phase and out-of-phase synchronization of the oscillations. The ability to alter the dynamic behavior of BZ gels through mechanical deformations opens up the possibility of using these materials in novel chemo-mechanical sensors. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L15.00011: Rectification, Gating Voltage and Interchannel Communication of Nanoslot Arrays Due to Asymmetric Entrance Space Charge Polarization Gilad Yossifon, Yu-Chen Chang, Hsueh-Chia Chang A nanochannel array with homogenous surface charge and height (i.e. uniform electro-chemical potential) but with asymmetric channel separation at the entrances is shown to exhibit strong rectification and gating type I-V characteristics at large voltage. Unlike previous low-voltage Ohmic studies of ionic current rectification within nano-pores/channels, which is attributed to intrinsic non-uniformity of the electro-chemical potential along the nanochannel, the high-voltage rectification of asymmetric nanochannel array is due to asymmetric space-charge polarization and inter-channel communication at the entrances. This entrance polarization that controls the current flux at high voltages can overlap across nanochannels to render the array current very distinct from the collective current across isolated channels. [Preview Abstract] |
Session L16: Cold Gases: Spin, Rotation, and Reduced Dimension
Sponsoring Units: DAMOPChair: Ludwig Mathey, National Institute of Standards and Technology
Room: 317
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L16.00001: Spin Structure and Critical Nucleation Frequency of Fractionalized Vortices in 2D Topologically Ordered Superfluids of Cold Atoms Junliang Song, Fei Zhou We have studied spin structures of fluctuation-driven fractionalized vortices and topological spin order in 2D nematic superfluids of cold sodium atoms. Our Monte Carlo simulations suggest a softened $\pi$-spin disclination structure in a half-quantum vortex when spin correlations are short ranged; in addition, calculations indicate that a unique non-local topological spin order emerges simultaneously as cold atoms become a superfluid below a critical temperature. We have also estimated fluctuation-dependent critical frequencies for half-quantum vortex nucleation in rotating optical traps and discussed probing these excitations in experiments. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L16.00002: Vortex lattice locking in rotating two-component Bose-Einstein Condensates Ryan Barnett, Edward Chen, Mason Porter, Hans Peter Buchler, Gil Refael The vortex density of a rotating superfluid, divided by its particle mass, dictates a superfluid's angular velocity through the Feynman relation. To find how the Feynman relation applies to superfluid mixtures, we investigate a rotating two- component Bose Einstein condensate, composed of bosons with different masses. We find that in the case of sufficiently strong interspecies attraction, the vortex lattices of the two condensates lock and rotate at the drive frequency, while the superfluids themselves rotate at two different velocities, whose ratio is that of the particle masses of the two species. In this talk, I will characterize the vortex-locked state, establish its regime of stability, and find that it survives within a disk smaller than a critical radius, beyond which vortices become unbound. Finally, numerical solution of the coupled Gross-Pitaevskii equations in support of this will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L16.00003: Quantum fluctuations of a Bose-Josephson junction on a quasi-one-dimensional ring trap Nicolas Didier, Anna Minguzzi, Roberta Citro, Frank W.J. Hekking Ring traps for ultracold atomic gases are becoming experimentally feasible. We study the theory of quantum fluctuations of a Bose Einstein condensate confined to a quasi one-dimensional ring trap where a Josephson junction is realized with a localized barrier potential. We consider the situation where the transverse confinement of the trap is so tight that only longitudinal quasi-1D motion is allowed along the ring. The condensate is treated as a Luttinger liquid and the low energy properties are described within the bosonization formalism. For a very large barrier, we study the one-particle density-matrix including the correction due to the density fluctuations. Our analysis reveals different power law decays depending on the location of the probed points, i.e. whether they are chosen in the bulk or at the edge of the ring under consideration. This quasi-long range order could be probed using an interference measurement. In the Tonks-Girardeau limit, the density-density correlation function tends to the exact result obtained from the Bose-Fermi mapping. The momentum distribution is calculated and compared to the result for a very small barrier. Furthermore, for a barrier of finite height, within the renormalization group approach, we study how quantum fluctuations reduce the effective Josephson coupling energy. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L16.00004: Detecting statistics of quasiparticles using dynamical probes Claudia De Grandi, Roman Barankov, Anatoli Polkovnikov We study a time-dependent sine-Gordon model in the range of the coupling costant (Luttinger parameter) where the quasiparticles excitations change from massive bosons to free fermions. We find that, if we include the effects of finite temperature, the non-adiabatic response to slow perturbations is enhanced for the bosonic case and reduced for the fermionic one with respect to zero temperature. The signature of this \textit{bunching (anti-bunching)} behaviour can also be seen at zero temperature by analyzing the second order corrections of a perturbative approach in the number of quasiparticles created. This suggests the existence of a crossover (for the Luttinger parameter) that separates systems with bose-like statistics from systems with fermi-like statistics, and therefore time-dependent perturbations to the system can be used to probe the statistics of the quasiparticles. We show how this model is relevant for cold atoms experiments that realize splitting and merging of two one-dimensional bose gases. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L16.00005: Motion of an impurity in a one-dimensional quantum liquid Austen Lamacraft We consider the motion of an impurity particle in a general one-dimensional quantum fluid at zero temperature. The dispersion relation $\Omega(P)$ of the impurity is strongly affected by interactions with the fluid as the momentum approaches $\pm\pi\hbar n, \pm 3\pi\hbar n, \ldots$, where $n$ is the density. This behavior is caused by singular $\pm 2\pi\hbar n$ scattering processes and can be understood by analogy to the Kondo effect, both at strong and weak coupling, with the possibility of a quantum phase transition where $\Omega'(\pm \pi n)$ jumps to zero with increasing coupling. The low energy singularities in the impurity spectral function can be understood on the same footing. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L16.00006: Breakdown of Macroscopic Quantum Self Trapping in Coupled 1D Bose Gases Rafael Hipolito, Roman Barankov, Anatoli Polkovnikov Two coupled 3D Bose-Einstein condensates with a large population imbalance exhibit macroscopic quantum self-trapping if the ratio of interaction energy to the coupling energy between the two gases is above a critical value. Above the self trapping transition, one sees only small amplitude high frequency oscillations of the population difference. In the analogous case of 1D coupled gases we find similar behavior for short times, but quantum fluctuations destroy the self trapped state by production of particle pairs of opposite momenta through parametric resonance with the oscillations of the population imbalance observed at small times. We show that through this resonance it is possible, by choosing the parameters of the system appropriately, to produce an interacting 1D bose gas with sharp momentum distribution, and show the conditions that the system must satisfy in order to produce such a state. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L16.00007: The Mass of a Spin Vortex, in a Bose Einstein Condensate Ari Turner Ferromagnetic condensates can have both spin-current and charge-current vortices. A moving charge-vortex experiences the Magnus force, perpendicular to its motion, when it moves. This effective ``magnetic field" is so strong that it dominates the inertial term in Newton's law; therefore it is not possible to set a charge-vortex moving at an arbitrary speed relative to the condensate. As we will show, a spin-vortex \emph{can} move ``inertially" through a condensate and resists acceleration with a mass. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L16.00008: Unusual states of vortex matter in interacting multicomponent Bose-Einstein condensates Egor Babaev, Eskil Dahl, Asle Sudbo A striking property of a single-component superfluid under rotation, is that a broken symmetry in the order parameter results in a broken translational symmetry, a vortex lattice. If translational symmetry is restored, the phase of the order parameter disorders and the broken symmetry in the order parameter is restored. We show that for interacting mixtures of Bose-condensate (with a dissipationless intercomponent drag), new situations arise. A phase disordered nonsuperfluid component can break translational symmetry in response to rotation due to interaction with a superfluid component. In a different regime instead of a vortex lattice, the system forms a highly disordered tangle which constantly undergoes merger and reconnecting processes involving different types of vortices, with a breakdown of translational symmetry only in a statistical sense. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L16.00009: Normal modes of a ring-shaped BEC with vortices Sungjong Woo, Young-Kyun Kwon Recently, a ring-shaped BEC was realized experimentally at NIST and long lasting perpetual current was observed. Using Bogoliubov-de Gennes equations, we have analyzed dynamics of such a non-simply connected rotating condensate system with quantized vortices. Surface modes of a simply connected rotating BEC are known to be associated with driven vortices that can make interactions with the vortex lattice that exists due to the rotation. In our current work, it has been found that stable vortex dipoles or velocity dipoles that do not exist in a normal mode for a simply connected BEC, exist in a non-simply connected case generating ``inner'' surfave modes. The interactions of such inner surface modes with quantized vortices as well as the stability of the perpetual current related to the low lying excitations will be discussed. How the angular dispersion relation changes as a BEC makes a transition from a simply connected to a ring-shaped one will also be presented. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L16.00010: Hierarchy of Supercurrents in Multicomponent Atomic Josephson Vortices Vitaliy Kaurov We show that a quasi-1D long atomic Josephson junction [1,2] containing a mixture of BECs can sustain multi-component Josephson vortices (mJV). A new exact soliton solution is given to describe a stationary mJV in the general $N$-component case. Depending on system parameters (scattering lengths, tunneling strengths, and chemical potentials) Josephson supercurrents of different components form a hierarchy according to their intensity and proximity to phase slip. By tuning the parameters it is possible to turn off or on particular currents using the JV -- dark soliton interconversion effect [1,2]. Inside the mJV different components may circulate either in the same or opposite directions resulting in bulk super-counter-flow in the latter case. The weak tunneling limit can be described by a modified Sine-Gordon model. An approximate solution for mJV propagating along the junction is found for the two-component case. The degeneracy of stationary mJV with respect to co-flow or counter-flow configurations is lifted by the uniform motion of mJV. Which configuration is energetically preferable depends on the interspecies scattering length. [1] V. M. Kaurov and A. B. Kuklov, Phys. Rev. A 71, 011601(R) (2005). [2] V. M. Kaurov and A. B. Kuklov, Phys. Rev. A 73, 013627 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L16.00011: Rotating Bose-Einstein condensates at the phase transition point Mahir Hussein, Piet Van Isacker, Klaus Bartschat, Oleg Vorov Here we give analytic description of the phase transitions in the rotating Bose-Einstein condensate of weakly interacting atoms in a magnetic trap [1,2]. The analytic solution allows one to classify the instabilities in the condensates which occur when the rotational speed is increased [3]. In the case of predominantly repulsive interactions, the transition corresponds to the vortex entry the condensate. The transition to the Abrikosov state has follows if the rotational speed is increased further. In the case of predominantly attractive interactions, the transition corresponds to the escape of the condensate from the trap at the critical speed. [1] O. K. Vorov, P. Van Isacker, M. S. Hussein and K. Bartschat, Phys. Rev. Lett. 95, 230406 (2005). [2] O. K. Vorov, M. S. Hussein and P. Van Isacker, Phys. Rev. Lett. 90, 200402 (2003). [3] O. K. Vorov, P. Van Isacker, M. S. Hussein and K. Bartschat, to be submitted to Nature (2007). 1supported by NSF (USA), CEA (France) [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L16.00012: Strongly Interacting Quantum gases using spin-Coherent state Representation Radha Balakrishnan, Indubala Satija For strongly interacting boson gas, spin-coherent states representation may provide a useful description of the Bose-Einstein Condensate as it encodes fluctuations and depletion. We investigate the the non-linear evolution equation for the order parameter obtained using spin-coherent states. The equation is not of the GPE-type and exhibits local fluctuations and in the limit of small order parameter, it reduces to the GPE equation. We compare and contrast the quasi-particle excitation and the vortex excitations of this system with that of weakly interacting quantum gas described by GPE equation. For a variety of problems, implication of this description of quantum gases will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L16.00013: Dynamics of spinor condensates near point-group symmetric ground states Gil Refael, Ryan Barnett, Daniel Podolsky The mean-field ground state of spin-S BEC's often exhibits a high degree of symmetry, which only becomes apparent when considering the 2S reciprocal spin-states: coherent spin-states orthogonal to the ground state. Our presentation will concentrate on a description of the dynamics of spinor-condensates using these reciprocal states. First, we will present the resulting hydrodynamic Euler equations, which generalize Mermin-Ho relations to higher spin. Second, we will use the reciprocal states and their hidden point-group symmetry to construct the Goldstone and optical spin-wave modes of the spinor condensates. Finally, we will present a mapping between the spin-wave modes, and the wave functions of electrons in atoms, where the spherical symmetry is degraded by a crystal field. [Preview Abstract] |
Session L17: Superconducting Flux Qubits and Qubit Amplifiers and Readouts
Sponsoring Units: GQIChair: Joe Aumentado, National Institute of Standards and Technology
Room: 318
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L17.00001: Behavior of a Josephson Flux Qubit on a Sapphire Substrate Anthony Przybysz, E. Crowe, H. Kwon, B.K. Cooper, R.M. Lewis, B.S. Palmer, J.R. Anderson, C.J. Lobb, F.C. Wellstood We discuss the design, fabrication, and testing of a Nakamura- style [1] flux qubit. The device consists of a four-Josephson junction qubit loop that is directly coupled to a small dc SQUID, which is used for detection. The device was built on a sapphire substrate using electron beam lithography and double angle evaporation to form the Al/AlOx/Al tunnel junctions. A 200 nm thick layer of aluminum was deposited on the e-beam resist in order to counteract charging effects during the lithography. Three of the junctions in the qubit loop were 100 nm x 250 nm, and the fourth was 100 nm x 150 nm. The large junctions are the main contribution to the inductance of the qubit loop, and the smaller junction creates an energy splitting of 1-10 GHz between the two circulating current states. The SQUID junctions were 100 nm x 2000 nm, and the critical current of the detection SQUID was 240 nA. We present the results of ongoing measurements on the behavior of the device at 25 mK. This project was funded by the JQI, LPS, and CNAM. [1] F. Yoshihara, Y. Nakamura, et al.,``Decoherence of Flux Qubit Due to 1/f Flux Noise,'' PRL 97, 167001 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L17.00002: IBM's experimental quantum computing effort. Matthias Steffen, David DiVincenzo, Matthew Farinelli, George Keefe, Mark Ketchen, Shwetank Kumar, Frank Milliken, Mary Beth Rothwell, Jim Rozen We present our experimental quantum computing effort and discuss results on flux and phase qubits. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L17.00003: Tuning the gap of the superconducting flux qubit Arkady Fedorov, Floor Paauw, Kees Harmans, Hans Mooij Recent advances in experiments with the flux qubits include demonstration of single and two qubit quantum gates as well as a coupling between the flux qubit and a harmonic oscillator. It was also experimentally confirmed that the best coherence properties were achieved when the qubit was kept at the symmetry point. For these conditions the qubit's energy level splitting is minimal (the gap) and determined solely by the quantum tunnelling in the double-well potential. However, since the potential barrier and the gap of the conventional flux qubit are fully fixed by the fabrication, one needs to tune the qubit out of the symmetry point in order to bring it in resonance with another quantum system. We overcame this limitation by introducing the change in the qubit design and demonstrated the tuning of the gap over a range of several Gigahertz within a few nanoseconds. We believe this could be an important step toward the coupling of the flux qubit to another qubit or the quantum bus. This control also allows a more extensive study of the relaxation time of the qubit as a function of the gap size within a constant environment. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L17.00004: Ultra-strong coupling regime of cavity QED with flux qubits Jerome Bourassa, Alexandre Blais With improved dephasing rate and coupling strength, the transmon qubit has recently been used to reach the strong coupling regime of cavity QED [1,2]. With the transmon however, these improvements are done at the expense of lower anharmonicity compared to the Cooper-pair box. Here we present an alternative approach where a flux qubit is coupled to the transmission line. As was recently shown experimentally [1], very strong coupling can be obtained by directly connecting the qubit loop to the center conductor of the resonator whose local inductance is tuned to maximize the coupling. We will discuss how this system can be used to study the breakdown of the rotating-wave approximation and how the $\Lambda$-configuration of the energy levels of the flux qubit can be exploited. \\[3pt] [1] J. Koch et al, Phys. Rev. A, 2007, 76, 042319 (2007)\\[0pt] [2] J. A. Schreier et al, PRB 77, 180502 (2008)\\[0pt] [3] A. A. Abdumalikov, et al., PRB 78, 180502 (2008)\\[0pt] [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L17.00005: Amplitude spectroscopy of a superconducting artificial atom William Oliver, David Berns, Sergio Valenzuela, Mark Rudner, Leonid Levitov, Terry Orlando We introduce and demonstrate amplitude spectroscopy in a superconducting artificial atom [1]. A harmonic field at a fixed frequency drives the artificial atom through its energy-level avoided crossings. Spectroscopic information is obtained from the amplitude dependence of the system response. The resulting ``spectroscopy diamonds,'' regions of parameters space in which state transitions occur, exhibit quantum interference patterns and population inversion which serve as a fingerprint of the atom's energy spectrum. Using this approach, we determined the energy spectrum of a manifold of states with energies from $h $x 0.01 GHz to $h $x 120 GHz for a fixed driving frequency near only 0.16 GHz. The amplitude spectroscopy technique is complementary to frequency spectroscopy, providing a means to access, manipulate, and characterize quantum systems over broad bandwidths while using only a single drive frequency that may be orders of magnitude smaller than the energy scales being probed. [1] Berns et al., Nature 455, 51 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L17.00006: Phase tomography of a strongly driven superconducting artificial atom Mark Rudner, Andrei Shytov, Leonid Levitov, David Berns, William Oliver, Terry Orlando, Sergio Valezuela In a recent experiment [1], amplitude spectroscopy of a superconducting qubit was demonstrated by driving the system with a strong rf field through a manifold of states spanning energies up to 120 GHz. The interference between repeated Landau-Zener transitions in a qubit swept through an avoided level crossing results in Stueckelberg oscillations in qubit magnetization. The resulting oscillatory patterns are a hallmark of the coherent strongly-driven regime in qubits, quantum dots and other two-level systems. The two-dimensional Fourier transforms of these patterns are found to exhibit a family of one-dimensional curves in Fourier space [2], in agreement with experiment [1]. We interpret these images in terms of the time evolution of the quantum phase of the qubit state and show that they can be used to probe dephasing mechanisms in the qubit. [1] D. M. Berns et al., Nature 455, 51 (2008). [2] M. S. Rudner et al., Phys. Rev. Lett. 101, 190502 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L17.00007: Simultaneous cooling of an artificial atom and its neighboring quantum system Jianqiang You, Yu-xi Liu, Franco Nori We propose an approach for cooling both an artificial atom (e.g., a flux qubit) and its neighboring quantum system, the latter modeled by either a quantum two-level system or a quantum resonator. The flux qubit is cooled by manipulating its states, following an inverse process of state population inversion, and then the qubit is switched on to resonantly interact with the neighboring quantum system. By repeating these steps, the two subsystems can be simultaneously cooled. Our results show that this cooling is robust and effective, irrespective of the chosen quantum systems connected to the qubit. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L17.00008: Chirped nonlinear cavity for digital quantum state readout without switching Ofer Naaman, Jos\'e Aumentado, Lazar Friedland, Jonathan Wurtele, Irfan Siddiqi We observe a new phase-locking effect in a high-$Q$ cavity embedding a Josephson junction driven with a chirped microwave signal. Above a critical drive amplitude, the cavity phase-locks to the drive and its oscillation amplitude grows with time. Below threshold, the cavity dephases from the drive and its amplitude remains small. The transition to phase-locking is associated with a sharp threshold sensitive to the junction $I_0$, and can be used for digital detection of quantum states. This detector smoothly evolves into one oscillation state or the other without relying on any switching process. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L17.00009: Lumped-element microwave resonant circuit with a dc SQUID M.P. DeFeo, C. Song, T.W. Heitmann, K. Yu, B.L.T. Plourde, R. McDermott We have fabricated lumped-element microwave resonant circuits consisting of a dc SQUID shunted with a capacitor formed from superconducting layers. Adjusting the SQUID bias conditions changes its Josephson inductance, thus varying the resonant frequency. We discuss the possibility of time-domain monitoring of the oscillations in these circuits and their potential use in a new readout scheme for superconducting qubits. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L17.00010: Qubit decoherence due to a Josephson bifurcation amplifier trapped in one of Frank Wilhelm, Ioana Serban, Mark Dykman We investigate the relaxation of a superconducting flux qubit for the case when its detector, the Josepshon bifurcation amplifier, remains latched in one of its two (meta)stable states. We observe a qualitatively different behavior for the two different attractors, and interpret the result as the combined effect of the amplitude of the detector's response to external driving and the effective curvature of the detector's basins of attraction in a rotating frame, in the proximity of the stable points. We address the question of the proper version of detailed balance for the qubit. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L17.00011: Parametric Resonators for Quantum Information Applications C.M. Wilson, M. Sandberg, F. Persson, I. Hoi, G. Johansson, V. Shumeiko, P. Delsing, T. Duty We have fabricated and characterized tunable superconducting transmission line resonators. To change the resonance frequency, we modify the boundary condition at one end of the resonator through the tunable Josephson inductance of a SQUID. We demonstrate a large tuning range, high quality factors and that we can change the frequency of a few-photon field on a time scale orders of magnitude faster than the photon lifetime. When parametrically pumped at twice their resonance frequency, the devices can act as parametric amplifiers. When pumped strongly, a threshold is crossed where the resonators oscillate spontaneously. Within this regime of parametric oscillations, the devices can exist in a variety of dynamical states. We observe a rich pattern in the dynamics of switching between these states. We study the possibility of using this dynamical bifurcation for qubit readout. Finally, recent theoretical work has suggested that it may be easier to observe dynamical tunneling in this system than in the Duffing oscillator. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L17.00012: The effects of higher-harmonic mode coupling in quarter-wave SQUID parametric amplifiers Minhyea Lee, Lafe Spietz, Jose Aumentado Recent interest in quarter-wave SQUID-based parametric amplifiers has motivated concerns regarding the coupling of higher harmonic modes to the operating frequency mode. We will present experiments in which the harmonic mode coupling is attempted to measure. We will also discuss the effect of higher mode coupling on noise performance and gain, focusing on whether this mode coupling limits practical amplifier performance. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L17.00013: Nonlinear dissipative filters for measurement protection on superconducting qubits Pol Forn-Diaz, Raymond Schouten, Kees Harmans, Hans Mooij Measurements on superconducting qubits require the system to be well isolated from noise sources if its quantum state is not being accessed. This ensures that decoherence induced by the measurement apparatus is minimized. The need to have slow (sub-GHz) and fast (GHz) lines to measure and control the state of the qubit is difficult to combine with the requirement to attenuate the noise over a broad spectral range. To overcome this problem, we have built a new type of non-linear coaxial copper powder filter with a Josephson junction in its inside. The junction in the filter acts as a shorting switch. For low frequencies, the junction acts as a shortcut to ground, and high frequencies are absorbed in the metallic powder. The Josephson junction critical current is taken such that when sending a pulse to probe the measurement device (a DC SQUID in our case), the junction in the filter switches to the voltage state, thus reaching the SQUID to perform the measurement. A minimum noise suppression of 40 dB is obtained, while allowing ns pulses to be transported. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L17.00014: Development of a Microwave Resonator for Qubit Read-out Zaeill Kim, V. Zaretskey, K. D. Osborn, F. C. Wellstood, B. S. Palmer We have designed and fabricated a ``lumped-element'' thin-film superconducting Al microwave resonator on sapphire to be used to read out a Cooper-pair box. The resonator consists of a meandering inductor and an interdigitated capacitor coupled to a transmission line. At T=30 mK and on resonance at 5.578 GHz, the transmission through the transmission line decreases by 15 dB and the loaded quality factor is 60,000. We have studied the temperature dependence of our resonator at temperatures as high as 500 mK and compared it to the Mattis-Bardeen theory. Coupling of this resonator to a Cooper-pair box qubit will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L17.00015: Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift Andreas Fragner, Martin Goppl, Alexandre Blais, Andreas Wallraff Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4\% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states. [Preview Abstract] |
Session L18: Advanced Techniques in Acceleration and Coherent Radiation
Sponsoring Units: DPBRoom: 319
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L18.00001: Recent Advances in X-Ray Free Electron Lasers Invited Speaker: X-ray free electron lasers(FELs) are undergoing an exciting development. They will soon become a reality with the commissioning of the LCLS in early next year, soon followed by Spring 8 X-FEL and the European XFEL at Hamburg. Intense, coherent pulses of x-rays from these machines will permit exploration of the atomic world with spatial and temporal precisions hitherto not feasible. After these first generation x-ray FELs based on self-amplified spontaneous emission (SASE) in a single pass high-gain system, the next generation devices for higher performance and/or smaller and lower cost are under various stages of development. In the soft x-ray region, seeded FELs with high harmonic generated (HHG) laser input will produce coherent output in a shorter system. Drastic improvement in hard x-ray region is possible with new types of electron injectors producing electron beams with an order of magnitude smaller emittance. Hard x-ray FELs can be built with an order of magnitude smaller electron energy and thus with a significant savings in the cost. The Spring-8 FEL is a first step in this direction. Novel types of FELs are possible; ultra-short SASE providing sub-fs time-resolution and x-ray FEL oscillator (XFELO) providing ultra-narrow spectral resolution. The XFELO employs optical cavity formed by Bragg reflectors, delivering temporally and spatially coherent hard x-rays, with meV spectral resolution. Its peak spectral brightness is similar to but the average spectral brightness is about five orders of magnitudes higher than LCLS. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L18.00002: Novel Techniques for the Generation of Ultrashort Attosecond Pulses of Coherent Radiation Invited Speaker: Revealing the laws of Nature often require developing unique experimental tools. In this talk I will discuss new tools that will enable probing of matter with attosecond time resolution and Angstrom spatial resolution. Their emergence is largely due to the spectacular progress in development of x-ray free electron lasers and similar progress in generation of the ultra-short light pulses in optics. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L18.00003: Photonic Band-gap Materials for Particle Acceleration Invited Speaker: |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L18.00004: Emerging Long-Wavelength Coherent Light Sources---from the infrared to soft X-rays Invited Speaker: The theory and techniques of accelerator-based coherent light sources will be discussed. Application of these ideas to the design of user facilities from the IR to soft X-rays will be presented, including world-wide initiatives for next generation light source R{\&}D and construction at these longer wavelengths. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L18.00005: Atto-second Radiation via Higher Harmic generation in Laser-Plasma Interaction Invited Speaker: |
Session L19: Focus Session: Ionically Gated and Conventional OFETs and Related Devices
Sponsoring Units: DMP DPOLYChair: Jun Takeya, Osaka University
Room: 320
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L19.00001: All-Printed Low Voltage Operation Polymer Transistors and Circuits Based on Ion Gel Gate Dielectrics Yu Xia, Jeong Ho Cho, Mingjing Ha, Michael Renn, C. Frisbie A key challenge in the development of organic electronics lies in the realization of high quality devices with low cost. In this presentation, we demonstrate high performance polymer transistors and circuits with all components fabricated by a commercial aerosol jet printing technique. Printing saves the device manufacturing cost through its simple procedure, fast speed, high throughput and low waste of materials. Furthermore, by employing a specially designed ion gel as the gate dielectric material, ultra-high density carrier accumulation ($>$ 10$^{14}$ cm$^{-2})$ can be achieved in the transistor channel, which results in an exceptionally large transconductance of 10 $\mu $S/$\mu $m. Our typical transistors have mobility higher than 1cm$^{2}$/Vs and frequency response up to 10 kHz. Inverters, NAND and NOR logic circuits and ring oscillators have been realized as well, with low operation voltage, fast speed and high gain. In addition, the high polarizability of the gate dielectric allows us to print the gate electrode of each single transistor along with its source and drain electrodes at the same time in a coplanar architecture, which significantly simplifies the fabrication procedure. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L19.00002: Non-volatile Ferroelectric Poly(vinylidene fluoride-co-trifluoroethylene) Memory based on Single Crystalline Triisopropylsilylethynyl Pentacene Field Effect Transistor Seok Ju Kang, Insung Bae, Youn Jung Park, Tae Ho Park, Jinwoo Sung, Cheolmin Park We develop a new type of non-volatile ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) memory based on Organic Thin Film Transistor (OTFT) with single crystal Triisopropylsilylethynyl pentacene (TIPS-PEN) active layer. A bottom gate OTFT was fabricated with thin P(VDF-TrFE) film gate insulator on which 1D ribbon type single crystal TIPS-PEN grown via solvent exchange method was positioned between Au source and drain electrode. As consequences a memory device exhibits substantially stable source-drain current modulation with the hysteresis ON/OFF ratio larger than 10$^{3}$, superior to a ferroelectric P(VDF-TrFE) OTFT with vacuum evaporated pentacene layer. Data retention longer than 5x10$^{4}$ seconds was achieved in ambient condition by incorporating an interlayer between gate electrode and P(VDF-TrFE) thin film. The device is environmentally stable for more than 40 days without additional passivation. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L19.00003: Polymer Electrolyte Gated Inorganic Transistors Anoop Singh Dhoot, Casey Israel, Xavier Moya, Stuart Wimbush, Tim Benseman, Judith MacManus-Driscoll, John Cooper, Neil Mathur, Richard Friend Electric field-induced charge at the interface between gate dielectric and semiconductor is the basis of current semiconductor technology. We report that it is possible to use polymer electrolyte to gate inorganic materials, and to achieve field-induced `doping' equivalent to a full surface coverage of charged ions per unit cell area. Very high field-induced carrier densities, $\sim $10$^{15}$~cm$^{-2}$, in the transistor channel of La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ devices enable modulation of the Curie temperature of over 30~K. We have also used electrolyte gating of the superconductor YBa$_{2}$Cu$_{3}$O$_{7-x}$ to modulate the onset of superconductivity. This creates an exciting opportunity for use of the electrolyte as gate dielectric in a wide variety of inorganic materials to explore formerly inaccessible band-filling regimes without the need for chemical substitution and additional disorder. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L19.00004: Employing `Liquid Gap' Transistors to Examine the Mobility-Carrier Density Relation in Polymer and Single Crystal Organic Semiconductors Invited Speaker: It is generally known that the carrier mobility in organic semiconductors can depend on carrier density, but the precise relationship hinges on the degree of structural order and the dielectric polarizability at the organic/dielectric interface. We have fabricated both single crystal and polymer transistors using the PDMS stamp approach pioneered by Podzorov and Rogers [1], where we have replaced the usual `air gap' in these structures with liquids having different dielectric constants. This structure allows us to examine transport in single crystals and polymer semiconductors as a function of tunable dielectric constant and also charge density. We find striking differences in transport behavior for organic single crystals versus polymer semiconductor films using these liquid dielectric transistors. For organic single crystals such as rubrene, the carrier mobility does not seem to be a function of charge density but does strongly depend on the liquid dielectric constant, in keeping with previous results reported by Morpurgo [2] on the effects of dielectric polarizability. For polymer semiconductors, the effect of charge density is overwhelming; there is a strong increase in charge mobility with increasing carrier concentration, following a power law. These results are already largely known, but the `liquid gap' transistors provide a convenient testbed for examining these effects side-by-side for different materials in the same device. We will describe the device fabrication and the nature of our results, as well as discuss the origins of the very different behavior for single crystals versus polymer semiconductor films. 1) Sundar, V.C., \textit{et al}. \textit{Science} \textbf{303} (2004) 1643. 2) Hulea, I. N., \textit{et al. Nature Mater}. \textbf{5} (2006) 982. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L19.00005: Organic electrochemical transistors for sensing applications Fabio Cicoira, Sang Yoon Yang, Johm A. DeFranco, George G. Malliaras The application of organic semiconductor devices to chemical and biological sensors seems to be a great fit. A promising approach towards organic-based sensors involves the use of organic electrochemical transistors (OECTs). These devices can be operated in aqueous environment as efficient ion-to-electron converters, thus providing an interface between the worlds of biology and electronics and also a unique platform for the study of organic/organic and organic/metal interfaces in liquids. Using photolithography, surface engineering and micro fluidics we have developed several technique to fabricate OECTs having different geometries. This allows us to study the basic electronic properties and the sensing response of devices in order to understand their mechanism of operation. We studied how the dimensions of the transistors (in particular on the gate/channel area ratio) and the gate electrode material (metal or polymer) can be used to tune the device response for sensing of different species. The effect of the electrolyte on device response was evaluated studying transistors in aqueous electrolytes and ionic liquids. The detection limit of OECTs based sensors having different geometry, was analyzed for hydrogen peroxide, a species involved in glucose sensing. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L19.00006: Contact resistance and lifetime of organic thin film transistors Gvido Bratina, Andra\v{z} Petrovic We have used ed electric charge transport measurements coupled to Kelvin force probe microscopy of pentacene organic thin film transistors (OTFT's) to monitor the evolution of contact resistance as a function of time of exposure to ambient air. Or results demonstrate that exposure of OTFTs to ambient air for extended periods of time, results in two competitive mechanisms that are responsible for observed variation in drain-current. Initially, relatively fast oxygen doping through electronegativity-related creation of holes increases the carrier concentration and therefore increases the drain current. Slower, and persistent mechanism of water diffusion in the pentacene layer induces dipole-charge carrier interactions through the creation of energetic disorder. This results in long-term irreversible reduction of drain current. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L19.00007: Controlling charge carrier injection in solution processed pentacene transistors by molecular engineering of the electrodes Sangameshwar Rao Saudari, Cherie Kagan We present the device performance of pentacene transistors fabricated from a solution deposited precursor. The bottom-contact pentacene transistors are fabricated by spin-coating N-sulfinylacetamidopentacene precursor followed by thermal conversion of the precursor into pentacene. Hole mobilities $>$0.1 cm$^{2}$/Vs and I$_{on}$/I$_{off} \quad >$10$^{5}$ are repeatedly achieved by this process. The metal-semiconductor interface in organic transistors plays a very important role in charge carrier injection and the overall device performance. Here we have treated the metal surface with self-assembled monolayers having different head and tail chemistries prior to pentacene precursor deposition to tailor the interfacial electronic properties. We correlate monolayer chemistry with device contact resistance and threshold voltage. These studies are used to fabricate devices with high mobility, high I$_{on}$/I$_{off}$ and low subthreshold swing. Device hysteresis and stability issues will also be presented. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L19.00008: Infrared and Electro-Optic Properties of TIPS-Pentacene E.G. Bittle, J.W. Brill, J.E. Anthony We will discuss new measurements of the infrared and electro-optic properties of thin crystals of triisopropylsilylethynyl (TIPS) pentacene. As with THz studies of this class of materials, crystalline films were grown from saturated tetrahydrofuran solutions on a gold electrode mask. Square wave voltages were applied to the electrical contacts on the sample as well as a gate electrode below the sample, through an oxide dielectric. Changes in phonon frequencies were studied as functions of voltage, position between contacts, and frequency of applied voltage.~ The results are interpreted in terms of charge diffusion through the TIPS-pentacene crystals. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L19.00009: Crossover from recombination limited charge transport to mobility restricted charge transport in organic LED Vladimir Prigodin, Arthur Epstein The model of bipolar charge transport in organic semiconductors where the current solely is provided by e-h recombination (LEDs structures) is studied [1]. We have shown that depending on recombination rate there are two basic regimes of charge transport. For recombination rate above the critical value the current is space charge limited and the current as a function of recombination rate decreases with increasing the recombination rate. At recombination rate below the critical value the recombination takes place over the whole sample volume of sample and as a result the current is only contact limited. As a function of recombination rate the current increases with increasing recombination rate. Critical value for recombination rate depends on the thickness of sample, applied voltage and on both the hole and electron mobilities. [1] J.D. Bergeson \textit{et al.}, Phys. Rev. Lett. \textbf{100}, 067201 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L19.00010: Interface and bulk controlled charge transport in Pd/CuPc/Pd sandwich devices Corneliu Colesniuc, Ivan Schuller We present transport measurements in thin film devices of copper phthalocyanine sandwiched between palladium electrodes. The devices were grown in-situ using molecular beam deposition of phthalocyanine films with the thickness ranging between 30 and 600 nanometers. The I-V characteristics as a function of temperature and thickness exhibit two distinct regions - a low voltage Ohmic region with current proportional to the voltage and a high voltage region with a power law dependence. At low voltages the current shows an inverse power dependence on thickness, I$\sim $thickness $^{-n}$, with the exponent n $>$ 2, suggesting that both the interfaces and the film control the transport mechanism. The temperature dependence of the current does not show a clear activated behavior, supporting the same conclusion. On the other hand, at high voltages the power--law exponent of the I-V decreases at lower thickness for constant temperature implying also that both the interfaces and the film may control the transport. Work supported by AFOSR. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L19.00011: RC Transmission Line Characterization of Organic Semiconductors Daniel Lenski, Adrian Southard, Michael S. Fuhrer We have characterized thin films of organic semiconductors (pentacene and poly-3-hexylthiophene) using a 3-contact transmission line configuration, in which an AC voltage is applied between the thin film and the gate, and the phase and magnitude of the current are measured. We compare the results with those obtained from simultaneous DC measurement, and find good agreement in the sheet resistance in the ON state measured using the DC and transmission line techniques, indicating that the transmission line technique is useful for obtaining sheet resistance and mobility in the ON state. Near threshold, or at high frequencies and electric fields, we observe systematic deviations of the AC impedance from the DC values. We discuss these deviations in terms of the of frequency-dependent length scale probed by the transmission line technique, and how these measurements can shed light on the properties of the semiconductor materials as well as their interfaces with contacts and substrates. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L19.00012: Organic field effect transistors having single wall carbon nanotubes electrodes Fabio Cicoira, Carla M. Aguirre, Patrick Desjardins, Richard Martel Single Wall Carbon Nanotubes (SWCNTs) are of great interest as electrode materials in Organic Field Effect Transistors (OFETs). Thanks to their field emission properties, SWCNTs electrodes, in principle, are able to inject both electrons and holes into organics with low injection barriers, promoting tunneling injection. We well present recent result on the electrical properties of OFETs using \textit{hairy} SWCNTs electrodes (see Figure 1), where the CNTs are attached on the substrate by means of metallic Ti contact pads. Devices with SWCNTs electrodes show improved injection characteristics compared with devices using conventional metallic electrodes. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L19.00013: Electrospun tin oxide/poly(3-hexylthiophene) nanofiber p-n diodes Nicholas Pinto Electrospinning is a simple technique used to prepare nanofibers of various materials, organic and inorganic. Some advantages of this method is that the nanofibers are orders of magnitude longer than that obtained via conventional means and it is easy to isolate individual nanofibers. We have used this technique to make nanoribbons of $n$-doped tin oxide (SnO$_{2})$ and to make nanofibers of $p$-doped regio-regular poly(3hexylthiophene) (P3HT) in air and within seconds. Several $p-n$ junction nanodiodes were fabricated by crossing individual nanofibers of P3HT with individual nanoribbons of SnO$_{2}$ during the electrospinning process and electrically characterized them at room temperature. The SnO$_{2}$ nanoribbons were fabricated first by electrospinning a precursor of SnO$_{2}$ and then sintering them at 700 C to convert it to SnO$_{2}$ before crossing them with P3HT nanofibers. The devices show clear evidence of rectification in air and in vacuum with a turn-on voltage of $\sim $0.4 V and with rectification ratios $\sim $10. Exposure of the diode to UV light increases the on-state current, while removing the light restores the device to its original state making it suitable as a reusable UV light sensor as well. [Preview Abstract] |
Session L20: Liquid Crystalline Polymers and Anisotropic Particles
Sponsoring Units: DPOLYChair: Lei Zhu, University of Connecticut
Room: 321
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L20.00001: Dillon Symposium Break
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Tuesday, March 17, 2009 3:06PM - 3:18PM |
L20.00002: Patterns on the iridescent beetle, \textit{Chrysina gloriosa} Jung Ok Park, Vivek Sharma, Matija Crne, Mohan Srinivasarao The brilliant metallic color of a beetle \textit{Chrysina gloriosa} has been known to occur due to selective reflectance from a cholesteric structure on the exoskeleton. The surface also appears to have hexagonally packed structures. Crystallographic concepts and Voronoi analysis were used to determine the degree of order in different regions of the beetle. Along the hexagons in the Voronoi diagram, many clustered pentagons and heptagons were observed. Due to the surface curvature, the number of pentagons was found to be higher than the number of heptagons. The cells appear yellow in the center surrounded by a green region with a yellow edge. Confocal microscopy was used to image the underlying structure, which was found to consist of concentric arcs on a surface of a shallow cone. The observed structures resemble the defects on a cholesteric phase with a free surface, and provide an interesting explanation of structural color development in beetles, along with inspiration for the design of chiral photonic structures. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L20.00003: Frustrated self-assembly of dendron and dendrimer-based supramolecular liquid crystals Raffaele Mezzenga, Nadia Canilho, Janne Ruokolainen, Edis Kasemi, Dieter Schlueter, Won Bo Lee, Glenn Fredrickson A new inverted topological configuration is demonstrated both experimentally and theoretically for self-assembled dendron and dendrimer-based supramolecular liquid crystals in which the dendrons/dendrimers occupy the continuous domain and the ionically attached pendant chains are confined in discrete domains. All previous studies on dendrimer and dendron-based liquid crystals have reported normal liquid crystalline configurations in which the dendritic templates occupy discrete domains (in spherical or columnar phases) or continuous struts (in bicontinuous cubic phases), while the pendant chains occupy the continuous space-filling domain. These surprising results mandate a re-examination of the packing mechanisms for this important class of materials and open new routes to unique nanostructures of possible use in existing and emerging technologies. References: R. Mezzenga, J. Ruokolainen, N. Canilho, E. Kas\"{e}mi, D.A. Schl\"{u}ter, W.B. Lee, G. H. Fredrickson, \textit{Soft Matter}, in press (\textbf{DOI:} 10.1039/b814972k) [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L20.00004: Hydrogen-Bonding Assisted Supramolecular Self-Assembly of Double Discotic Supermolecues Jianjun Miao, Lei Zhu Symmetric double discotic supermolecules based on porphine (Py) and triphenylene (Tp), Py(Tp)$_{4}$, have been successfully synthesized via PyBOP-catalyzed amidization reactions. The Tp moieties had either C$_{5}$- or C$_{12}$-arms, and the spacer linking between the central Py and peripheral Tp was either C$_{6}$ or C$_{10}$. Thermal properties of these supermolecules were studied by differential scanning calorimetry, and self-assembled crystalline and/or liquid crystalline textures were confirmed by polarized optical microscopy. For samples with C$_{5}$-arms in Tp, only a crystal-melt transition was observed. X-ray diffraction (XRD) on shear-oriented samples showed that Py was crystalline and Tp formed columnar liquid crystal. For samples with C$_{12}$-arms in Tp, sequential crystal-liquid crystal-isotropic melt transitions were observed. XRD results indicated that the crystalline unit cells were orthorhombic for all samples, and amide hydrogen-bonding was responsible for their supramolecular self-assembly. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L20.00005: Glassy correlations in nematic elastomers Bing Lu, Paul Goldbart, Xiaoming Mao We address the physical properties of an isotropic melt or solution of nematogenic polymers that is then cross-linked beyond the vulcanization point. To do this, we construct a replica Landau theory involving a coupled pair of order- parameter fields: one describing vulcanization, the other describing local nematic order. Thermal nematic fluctuations, present at the time of cross-linking, are trapped by cross- linking into the vulcanized network. The resulting glassy nematic fluctuations are analyzed in the Gaussian approximation in two regimes. When the localization length is shorter than the thermal nematic correlation length, the nematic correlations are well captured as glassy correlations. In the opposite regime, fluctuations in the positions of the localized polymers partially wash out the glassy nematic correlations. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L20.00006: Orientation distribution and process modeling of thermotropic liquid crystalline copolyester (TLCP) injection-moldings Robert Bubeck, Jun Fang, Wesley Burghardt, Susan Burgard, Daniel Fischer The influence of melt processing conditions upon mechanical properties and degrees of compound molecular orientation have been thoroughly studied for a series of well-defined injection molded samples fabricated from VECTRA (TM) A950 and 4,4'-dihydroxy-a-methylstilbene TLCPs. Fracture and tensile data were correlated with processing conditions, orientation, and molecular weight. Mechanical properties for both TLCPs were found to follow a ``universal'' Anisotropy Factor (AF) associated with the bimodal orientation states in the plaques determined from 2-D WAXS. Surface orientations were globally surveyed using Attenuated Total Reflectance -- Fourier Transform Infrared (ATR-FTIR) spectroscopy and C K edge Near-Edge X-ray Absorption Fine Structure (NEXAFS). The results derived from the two spectroscopy techniques confirmed each other well. These results along with those from 2-D WAXS in transmission were compared with the results of process modeling using a commercial program, MOLDFLOW(TM). The agreement between model predictions and the measured orientation states was gratifyingly good. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L20.00007: In Situ X-ray Scattering Measurements and Polydomain Simulations of Molecular Orientation Development during Injection Molding of Liquid Crystalline Polymers Jun Fang, Wesley Burghardt, Robert Bubeck We report on a coordinated experimental/computational study of injection molding of commercial thermotropic LCPs. In situ synchrotron x-ray scattering, combined with a customized injection molding apparatus, is used to track development of molecular orientation during the mold filling process for a commercial LCP, Vectra A950, in two simple plaque mold geometries: square and T-shaped. Use of high brilliance undulator radiation at the Advanced Photon Source, coupled with a high speed CCD detector provides sufficient time resolution (~ 12 frames per second) to resolve the transient orientation dynamics during and following mold filling. In addition to in- situ scattering measurements, ex-situ 2-D wide angle X-ray scattering measurements are conducted on the same injection molded plaques. The experiments are complemented by process simulations performed using commercial mold filling software. A very close analogy between the Folgar-Tucker fiber orientation model and the Larson-Doi polydomain model for textured liquid crystalline polymers is exploited to allow for the first tests of Larson-Doi model predictions in injection molding processing. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L20.00008: Dynamics and rheology of active polar liquid crystalline films Luca Giomi, M. Cristina Marchetti, Tanniemola B. Liverpool I will discuss the dynamical and rheological properties of active polar liquid crystalline films. Like active nematic films, active polar films undergo a dynamical transitions to spontaneously flowing steady-states. Spontaneous flow in polar fluids is, however, always accompanied by strong concentration inhomogeneities or ``banding'' not seen in nematics. In addition, a spectacular property unique to polar active films is their ability to generate spontaneously oscillating and banded flows even at low activity. The oscillatory flows become increasingly complicated for strong polarity. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L20.00009: Influence of nanorods on the properties of polymeric materials Gregory N. Toepperwein, Robert A. Riggleman, Juan J. de Pablo Nanoscopic additions, such as metallic nanoparticles or carbon nanotubes, can dramatically impact the mechanical properties of polymeric materials, such as the plateau modulus, which is intimately related to the entanglement length of the polymer. To explore the connection between nanocomposite configurations and the dynamic mechanical effects that are difficult to probe experimentally, due to challenges associated with sample preparation and particle dispersion, we have performed extensive Molecular Dynamics and Monte Carlo simulations of polymer nanocomposites with nanoparticles whose size, shape, and concentration have been varied systematically. Calculations of the entanglement network through primitive path analysis of these composites have enabled us to connect nanorod effects on the entanglement network structure and density to the system's dynamic properties. The main outcome of our study is a better understanding of how inclusions alter entanglements and how those entanglements are magnified in macroscopic observables. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L20.00010: Interactions between rod-like cellulose nanocrystals and xylan derivatives: A light scattering study Jae Hyun Sim, Katrin Schwikal, Thomas Heinze, Shuping Dong, Maren Roman, Alan Esker Interactions between rod-like cellulose nanocrystals and 2-hydroxypropyl-trimethylammonium (HPMA) xylan were investigated by polarized (DLS) and depolarized dynamic light scattering (DDLS). Cellulose nanocrystals were prepared by the controlled hydrolysis of black spruce pulp. Binary rod-like cellulose nanocrystal/water and ternary HPMA xylan/rod-like cellulose nanocrystal/water systems with different concentrations of cellulose nanocrystals were probed. Translational and rotational diffusion coefficients of cellulose nanocrystals in water are (4.8 $\pm $ 0.4) $\times $ 10$^{-8}$ cm$^{2}$s$^{-1}$ and (526 $\pm $ 20) s$^{-1}$, respectively, and calculated lengths and diameters for nanocrystals are comparable to those of cellulose whiskers from cotton. At high cellulose nanocrystal concentrations, DDLS studies in ternary systems provide translational and rotational diffusion coefficients. However, at low cellulose nanocrystal concentrations, DDLS studies of ternary systems do not yield rotational diffusion coefficients. This behavior is attributed to bridging between polymer chains that causes non-linear deviation on standard decay rate ($\Gamma )$ versus scattering vector magnitude (q$^{2})$ plots. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L20.00011: Polymerization-Enhanced Alignment Order in Carbon Nanotube Composites Howard Wang, Yayong Liu, Naranyan Das, Kunlun Hong, Gyula Eres, David Urig Polymer nanocomposites containing vertically aligned carbon nanotubes (VACNTs) have been synthesized via vacuum infiltration of monomers into confined VACNT arrays followed by in situ polymerization. The alignment order of VACNTs before and after polymerization has been quantitatively assessed using small angle neutron and x-ray scattering. The trend of continuous variation of alignment order along the height of VACNTs remains unaltered whereas the degree of order is enhanced upon polymerization. Polymerization-enhanced alignment order may assist preparing better carbon nanotube composites. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L20.00012: Perylene diimide liquid crystals: A density functional study Jeverson Arantes, Matheus Lima, Adalberto Fazzio, Hongjun Xiang, Su-Huai Wei, Gustavo Dalpian Organic semiconductors (OSs) are getting each time more space in the field of electronic materials mainly due to its low manufacturing costs, the relative facility of manufacturing in the desired way. A model for the crystal structure of perylene diimide PPEEB has been proposed, partially based on experimental observations. In this structural model, we've performed an {\it ab initio} calculations on the electronic structure of this material. Due to the strongly localized nature of the Oxygen atoms on the side chains, is necessary to go beyond the standard LDA and GGA calculations. With the PBE0 approach, the electronic structure becomes in agreement with previous results. The tails of the molecular crystal not only is responsible for its structural conformation, but also can be used for tuning the electronic and optical properties of the material. [Preview Abstract] |
Session L21: Semiconductors: Atomic Structure and Lattices
Sponsoring Units: DCMP FIAPChair: David Snoke, University of Pittsburgh
Room: 323
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L21.00001: ABSTRACT HAS BEEN MOVED TO J23.00011 |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L21.00002: Bonding states of Cu atoms in superionic $\alpha$-CuI phase Hiroaki Oshihara, Kazuo Tsumuraya The fast migration of cations in solids is used for solid state battery. Therefore the mechanism of the migration is of importance for the development new material. The superionic conductor CuI is zinc blend-type structure at low temperature and fluorite-type at high temperature. So the bonding between Cu and I atoms has been considered to be a covalent bonding. The peak positions and the asymmetrical peaks in the pair distribution functions between the Cu-Cu and Cu-I components have been remained to be explained in an experiment and a computational studies. We investigate the electronic states of CuI using the planewave based density functional calculation. We evaluate the charge and bonding states with Bader decomposition method. The stability of the Cu-Cu pairs in the conductor will be discussed using their binding energies. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L21.00003: Non-equilibrium Phonons in CaWO$_{4}$: Issues for Phonon Mediated Particle Detectors Madeleine Msall, Timothy Head, Daniel Jumper The CRESST experiment looks for evidence of dark matter particles colliding with nuclei in CaWO$_{4}$, using cryogenic bolometers sensitive to energy deposition $\sim $ 10 keV with a few percent accuracy. Calibration of the energy deposited in the phonon system depends upon the details of the evolution of the non-equilibrium energy in the CaWO$_{4}$ absorber. Our phonon images sensitively measure variations in angular phonon flux, providing key information about the elastic constants and scattering rates that determine the energy evolution. Phonon pulses, created by focused photoexcitation of a 150 nm Cu film, are detected after propagation through 3 mm of CaWO$_{4}$. The 20 ns Ar-ion laser pulse creates a localized (10$^{-3}$ mm$^{2})$ source of 10-20 K blackbody phonons. The sample is at 2 K. Our images show that the elastic constants derived from ultrasonic velocities along high symmetry axes do not accurately predict the total phonon flux along non-symmetry directions. We present new data on the dependence of phonon flux on excitation level and discuss the influence of isotope and anharmonic decay on the shape of phonon pulses in these ultrapure samples. Thanks to J.P. Wolfe and the Frederick Seitz Materials Research Laboratory, Urbana, IL, for partial support of this work. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L21.00004: Ultrafast anisotropic strain in semiconductors measured by x-ray diffraction D.A. Walko, SooHeyong Lee, E.C. Landahl, D.A. Arms We have used time-resolved x-ray diffraction to probe the non-uniaxial properties of impulsive strains in ultrafast laser-excited III-V semiconductors. Transient shifts of x-ray rocking curves due to the strains are measured from three Bragg reflections whose scattering vectors range from perpendicular to the surface to nearly in plane. Time-dependent strain ellipsoids are then constructed, with a temporal resolution under $\sim$150 ps. We find that the strain consists not only of a longitudinal expansion along the surface normal, but it also includes slight compression along the transverse direction. We compare measurements for GaAs and InSb; their significant differences in electron diffusion rates allow us to distinguish between lattice and electronic effects. Supported by the U.S. Department of Energy. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L21.00005: Using DFT-based Cluster Expansions to Study Oxygen Adsorption on Platinum and Gold (111) Surfaces Spencer Miller We have studied oxygen adsorption on Platinum and Gold (111) surfaces using Density Functional Theory. We have addressed the limitation on the number of configurations we can consider through DFT through the use of two-dimensional cluster expansions on our DFT data, allowing for rapid energy calculations for any arbitrary surface. We have used the cluster expansion to study adsorption properties and phase behavior on the surface including simulated TPD experiments and atomistic thermodynamics phase diagrams which we compare to experimental behavior. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L21.00006: Can a re do of field theory numerically speaking help in I - E CDW curve data analysis? Andrew Beckwith Density wave physics has a plethora of current versus electrical field data from experimental measurements. The author presented in his PhD dissertation a way of giving a false vacuum interpretation of how current can be measured against the magnitude of an applied electric field in laboratory conditions. This article purports to re examine the feasibility of such modeling taking into account Dr. Fred Cooper's work on a time averaging scheme for phi to the fourth power field theory as could be applied to modified wash board potentials. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L21.00007: Temperature-composition phase diagrams of Gd-doped EuO and EuS Joonhee An, S. Barabash, K. Belashchenko We have computed the temperature-phase diagram of Eu$_{1-x}$Gd$_x$O alloys by combining density functional theory in the generalized-gradient approximation with Hubbard U correction on f-orbitals with the regular cluster expansion and Monte-Carlo approach. The cluster expansion fit has been performed with varying numbers of distinct cluster types until the formal cross-validation score is minimized. Our results indicate that (i) pair interactions are relatively stronger than other cluster types, (ii) the pair terms decay rapidly with distance up to 10 {\AA}, (iii) the pair terms are attractive for direct interactions between cations and repulsive for indirect interactions through anions, and (iv) the calculated convex hull is asymmetric about x=0.5, displaying more deep ground states in Eu-rich regions than in Gd-rich regions. The asymmetry of the convex hull may imply relative instability of Gd-rich compounds, as was shown by previously-reported experimental difficulties to make Gd-rich compounds. A comparison with a similar binary system - sulfur replacing oxygen - is made, showing that both oxides and sulphides are dominated by deformation interaction. The sulphides have a marginal tendency to phase-separate into pure compounds at low temperatures, whereas the oxides tend to order. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L21.00008: Low energy metastable states and immiscibility in (SiC)$_{1-X}$-(AlN)$_X$ Benjamin Burton, Axel van de Walle, Albert Davydov, Victor Vinograd A cluster expansion Hamiltonian was fit to VASP/PAW calculated supercell formation energies, $\Delta E_f$, and first principles based phase diagrams (miscibility gaps) were calculated for the wurtzite-structure pseudobinary system SiC$_{1-X}$AlN$_X$. An unusually wide range of $3 \alt \Delta E_f~ \alt 125$ kJ/mole MX (M= Al, Si; X= N, C) was calculated and all supercells with $\Delta E_f~ \alt 8$ kJ/mole exhibited characteristic (SiC)$_m$(AlN)$n$ crystallography, in which (SiC)$_m$ indicates m SiC-double layers $\bot$ to the hexagonal c-axis, and similarly for (AlN)$n$. The prediction of (SiC)$_m$(AlN)$n$ low-energy metastable states, may explain why one can synthesize SiC$_{1-X}$AlN$_X$ films, or single crystals of arbitrary bulk composition, in spite of the very strong tendency toward immiscibility. Specifically, one expects that metastable films or single crystals will be dominated by a disordered stacking of SiC- and AlN-double layers. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L21.00009: Alloy Stabilized Wurtzite Ground State Structures of Zinc-Blende Semiconducting Compounds Hongjun Xiang, Suhuai Wei, Shiyou Chen, Xingao Gong Although the ground state structures of zinc-blende (ZB) alloys have been extensively studied, the knowledge of the ground state structures of wurtzite (WZ) alloys remains incomplete. Here, the ground state structures of the A$_x$B$_{1-x}$C WZ alloys with $x=$0.25, 0.5, and 0.75 are revealed by a ground state search using the valence-force field model and density-functional theory total energy calculations. It is shown that the ground state WZ alloy always has a lower strain energy and formation enthalpy than the corresponding ZB alloy. Therefore, we propose that the WZ phase can be stabilized through alloying. This novel idea is supported by the fact that the WZ AlP$_{0.5}$Sb$_{0.5}$, AlP$_{0.75}$Sb$_{0.25}$, ZnS$_{0.5}$Te$_{0.5}$, and ZnS$_{0.75}$Te$_{0.25}$ alloys in the lowest energy structures are more stable than the corresponding ZB alloys. To our best knowledge, this is the first example where the alloy adopts a structure distinct from both parent phases. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L21.00010: First-principles thermodynamic theory of epitaxial alloys: Prediction of spontaneous rotation of epitaxial habits in InGaN and GaAsSb alloys Zhe Liu, Giancarlo Trimarchi, Alex Zunger A general-purpose method for calculating the stablest ground state structures and finite-temperature thermodynamics of AC-BC alloys grown \emph{epitaxially} on a coherent substrate is presented. In addition to the fact that such coherent epitaxy stabilizes certain ordered phases, we discovered that depending on the substrate and the film concentration there is a spontaneous rotation of the stablest film microstructure. This general behavior is revealed for both mixed-cation (In, Ga)N and mix-anion Ga(As, Sb) alloys on a variety of substrates. Such spontaneous rotation of the epitaxial habits can be understood by the tetragonal ratio $\eta\neq1$ of the corresponding bulk structure. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L21.00011: Atomistic origins of the phase transition mechanism in Ge$_2$Sb$_2$Te$_5$ Juarez L. F. Da Silva, Aron Walsh, Su-Huai Wei, Hosun Lee The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge$_2$Sb$_2$Te$_5$ has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the meta-stable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable-octahedron to high-energy-unstable tetrahedron sites close to the intrinsic vacancy regions. Due to the instability of the tetrahedra, the Ge atoms shift away from those sites, giving rise to the formation of local-ordered 4-fold motifs coupled with long-range structural disorder. The high figures of merit of Ge$_2$Sb$_2$Te$_5$ are achieved from the optimal combination of intrinsic vacancies provided by Sb2Te3 and the instability of the tetrahedron sites provided by GeTe. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L21.00012: First-principles theory of phase stability, solvus boundaries, and coherency strain in LAST (lead-antimony-silver-telluride) and in other doped AgSbTe$_{2}$ thermoelectric alloys Sergey V. Barabash, Vidvuds Ozolins, Chris Wolverton Bulk telluride alloys are promising thermoelectrics [e.g. the figure of merit (\textit{ZT}) of LAST (AgPb$_{m}$SbTe$_{2+m})$ alloys was reported$^{\ast }$ to exceed \textit{ZT}$\sim $2]. Recent theoretical examination$^{+}$ found that precipitation of ordered AgSbTe$_{2}$ phases in rocksalt PbTe likely contributes to the high \textit{ZT} of LAST, and predicted that the isoplethal PbTe-AgSbTe$_{2}$ phase diagram includes highly asymmetric miscibility gap. Here we generalize that analysis by first launching a search for \textit{unknown} (Ag,Pb,Sb)Te non-rocksalt phases (those deviating from the 1:1 cation:anion ratio), and second by presenting an extended analysis of the solubility limits for alloying AgSbTe$_{2}$ with PbTe and other tellurides. In particular, we find that the large asymmetry of the PbTe-AgSbTe$_{2}$ miscibility gap shares a common physical origin with the substitutional site preference for Pb in ordered AgSbTe$_{2}$, and that during coherent precipitation, the coherency strain increases the solubility limits in PbTe-AgSbTe$_{2}$ by a factor of $\sim $2 relative to the predicted$^{+}$ unstrained bulk values. $^{\ast }$K.F. Hsu \textit{et al.}, Science \textbf{303}, 818 (2004). $^{+}$S.V.Barabash \textit{et al.}, Phys.Rev.Lett. \textbf{101}, 155704 [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L21.00013: Crystal and electronic structure of quaternary chalcogenide semiconductors I$_{2}$-II-IV-VI$_{4}$ (I=Cu, Ag, II=Zn, Cd, IV=Ge, Sn and VI=S, Se, Te) Shiyou Chen, Xin Gao Gong, Aron Walsh, Su-Huai Wei Sequential cation mutation in zinc-blende chalcogenide semiconductors, from binary to ternary to quaternary compounds, is systematically studied using first-principles calculations. Several universal trends are found for the crystal and electronic structure of the ternary and two classes of quaternary chalcogenides. We find that (i) most I$_{2}$-II-IV-VI$_{4}$ compounds are more stable in the kesterite structure, rather than the widely-recognized stannite structure; (ii) Cu and Zn layers are easy to be randomized in kesterite Cu$_{2}$ZnSnS$_{4}$ and Cu$_{2}$ZnSnSe$_{4}$; (iii) the band gap decreases during the mutation; (iv) the band gap of Cu$_{2}$ZnSnSe$_{4}$ should be around 1.0 eV, not 1.5 eV as reported in previous absorption measurements. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L21.00014: The electronic structures and structural properties of the amorphous Ge$_{2}$Sb$_{2}$Te$_{5}$, a phase change memory material Eunae Cho, Jino Im, Jisoon Ihm, Seungwu Han Ge-Sb-Te compound is one of the most promising materials for phase change random access memory. Recently, Ge$_{2}$Sb$_{2}$Te$_{5}$ has been under intensive researches. However, there exists a critical discrepancy between experimental and theoretical observations. In experiment, the ideal glass following 8-$N$ rule has been observed. There are deviations from 8-$N$ rule for melt-quench structures obtained by molecular dynamics calculations. In this presentation, we compare the melt-quench structure with ideal glass. We theoretically obtained the ideal glass using Si-As-Se compounds with a higher covalency The amorphous structure of Si$_{2}$As$_{2}$Se$_{5}$ is obtained by the melt-quench process and the elements are replaced by Ge-Sb-Te. It is found that the resulting Ge$_{2}$Sb$_{2}$Te$_{5}$ structures satisfy the 8-$N$ rule and all Ge atoms are tetrahedrally coordinated. The total energy of the ideal glass is higher than that of the melt-quench structure, explaining why the ideal glass has not been observed in the MD simulations. The electronic structures are also compared between ideal glass, melt-quench structure, and crystalline phase. It is concluded that the electronic character of the melt-quench structure lies in between those of ideal glass and crystalline phase. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L21.00015: Molecular dynamics study on volume dependence of atomic and electronic structure in amorphous Ge$_2$Sb$_2$Te$_5$ Jino Im, Changwon Park, Jisoon Ihm, Eunae Cho, Seungwu Han In order to understand the confinement effect of the phase-change memory cell consisting of Ge$_2$Sb$_2$Te$_5$, we carry out first-principles molecular dynamics calculations with the simulation volume equal to or larger than the crystalline volume. The amorphous structures are obtained by rapidly quenching the liquid phase of Ge$_2$Sb$_2$Te$_5$. It is found that the energy gap increases monotonically with the simulation volume. Furthermore, the density of defect levels in the energy gap is reduced in the simulation using the large cell volume. The resistance drift of the phase-change memory cell is explained on the basis of the simulation results. [Preview Abstract] |
Session L22: Focus Session: Spin-Orbit Effects in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Roland Winkler, Northern Illinois University
Room: 324
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L22.00001: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L22.00002: Pure spin current pump in a quantum channel with both Rashba and Dresselhaus effects Chia-Hui Lin, Chi-Shung Tang, Yia-Chung Chang We demonstrate a spin pump to generate pure spin current of tunable intensity and polarization in the absence of charge current. The system under consideration is a two-dimensional electron gas (2DEG) that is present at the interface of a heterostructure due to modulation doping and has intrinsic static Rashba and Dresselhaus spin-orbit interactions. The pumping functionality is achieved by means of an ac gate voltage that modulates the Rashba constant dynamically in a local region of a quantum channel in which both the static Rashba and Dresselhaus spin-orbit interactions are taken into account. The spin-resolved Floquet scattering matrix formalism is applied to our system. Based on the Floquet theorem, this formalism provides an exact and nonperturbative solution to the time-periodic Schrodinger equation in the mesoscopic system. Because the time-dependent spin-orbit interaction couples two spin polarizations and all sidebands together, analytic expression for the sideband dispersion is not feasible. Thus, we determine the sideband dispersion relation numerically by solving the Schrodinger equation in a nearly complete basis with the spatial inhomogeneity handled by matching boundary conditions region by region. The Floquet scattering matrix gives a coherent solution that goes beyond the adiabatic regime. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L22.00003: Mesoscopic fluctuations in the spin-electric susceptibility due to Rashba spin-orbit interaction Mathias Duckheim, Daniel Loss Spin-orbit interaction enables the control of the spin with electric fields in non-magnetic semiconductors. The orbital transport processes generating the internal fields that are necessary for this control are typically described as classical diffusive drift. In contrast, when this orbital motion is phase coherent, typical mesoscopic effects occur not only in transport but also in the spin dynamics. We investigate mesoscopic fluctuations in the spin polarization generated by a static electric field and by Rashba spin-orbit interaction in a disordered 2D electron gas. In a diagrammatic approach we find that the out-of-plane polarization - while being zero for self-averaging systems - exhibits large sample-to- sample fluctuations which are shown to be within experimental reach. We evaluate the disorder-averaged variance of the susceptibility and find its dependence on magnetic field, spin-orbit interaction, dephasing, and chemical potential difference. [M. Duckheim and D. Loss, Phys. Rev. Lett.(in print), arXiv:0805.4143v1]. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L22.00004: Spin-orbit coupling effects and the angular dependence of the tunneling anisotropic magnetoresistance Alex Matos-Abiague, Martin Gmitra, Jaroslav Fabian We consider a tunnel junction in which one of the electrodes is ferromagnetic. Based on general properties and symmetry considerations, we develop a phenomenological model in which the anisotropy of the tunneling magnetoresistance with respect to the magnetization orientation of the ferromagnet originates from the spin-orbit interaction. The model reveals how the symmetry and angular dependence of the tunneling anisotropic magnetoresistance (TAMR) are determined by the form of the spin-orbit coupling field (SOCF), independently of the specific details of the system. We investigate the particularly important cases in which the SOCF is of Bychkov-Rashba and/or Dresselhaus type and obtain angular dependences which are in good agreement with available TAMR measurements. We also predict new forms of the angular dependence of the TAMR by exploring different geometric configurations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L22.00005: Magneto-transport in high mobility \textit{n}-InSb/InAlSb quantum wells W.R. Branford, A.M. Gilbertson, L.F. Cohen, M. Fearn, P.D. Buckle, L. Buckle The inherent large spin-orbit (SO) coupling InSb quantum wells
(QWs) is
expected to result in sensitive tunability of the Rashba effect with
electric field. The strength of the SO coupling can be extracted
from
measurements of weak anti-localisation (WAL) and from the beating of
Shubnikov-de Haas (SdH) oscillations [1].
We have investigated these phenomena and report magneto transport
measurements from a range of InSb/InAlSb QWs with varying carrier
density
$n$ and mobility \textit{$\mu $}. It is shown that the inherent
large Zeeman splitting
combined with inhomogeneous level broadening means that beating
in the SdH
oscillations in InSb QWs are rarely observed. However, her we
show that in
InSb/InAlSb QWs, $n$ can be modulated using a gate electric field
from
1.15$ |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L22.00006: Diffusive spin-charge dynamics in an external electric field Tudor-Dan Stanescu, Brandon Anderson, Victor Galitski We study the dynamics of a spin density injected into a two-dimensional electron system with generic spin-orbit interactions. We generalize the spin-charge diffusion equation formalism by including the effects of a uniform electric field. Within this approach, we study the coupling between spin and charge and we determine the charge (spin) profile induced by a non-uniform, periodic spin (charge) density in the presence of the external electric field. We determine the optimal range of parameters for observing the spin-charge coupling effects. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L22.00007: Spin-orbit control of magnetization and electrical detection of current-induced spin polarization. Mason Overby, Alex Chernyshov, Leonid Rokhinson, Xinyu Liu, Jacek Furdyna The success of future spintronic devices relies on the efficient control and detection of spin polarization. Extrinsically polarized currents, injected from ferromagnetic materials, can interact with magnetic domains and initiate domain rotation. Alternatively, spin polarization can be generated intrinsically via relativistic coupling of spin to the momentum of charge carriers, known as spin-orbit interaction (SO). While the use of SO for electrostatic control of polarization forms the basis of various theoretical device concepts, SO control of magnetization has not been realized experimentally. Here we demonstrate that magnetization can be reversibly manipulated by intrinsically polarized currents in ferromagnetic semiconductors with strong SO coupling. Magnetization direction is repeatedly switched between two orthogonal easy axes by SO effective magnetic field generated by the injection of unpolarized currents with densities $<$10$^{6}$ A/cm$^{2}$. We also show that current-induced SO field can be detected electrically. By monitoring magnetization direction in small external magnetic field we can measure both magnitude and direction of the SO field. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L22.00008: Antilocalization in low dimensional InSb/InAlSb systems R.L. Kallaher, J.J. Heremans, N. Goel, S.J. Chung, M.B. Santos Boundaries and a restricted phase space influence the spin coherence length in mesoscopic structures with strong spin-orbit coupling. We present mesoscopic transport experiments on the strongly spin-orbit coupled narrow gap semiconductor InSb. Low temperature magnetotransport measurements were performed on high mobility InSb/InAlSb two dimensional electron system (2DES) and quasi-1D wires fabricated from the 2DES. Antilocalization dominates the magnetoresistance in low applied magnetic fields; hence the magnetoresistance is sensitive to the electron spin and phase coherence lengths in the structures. Measurements of the low field magnetoresistance over temperature demonstrate that the antilocalization phenomena persists to temperatures above $\sim$20 K in the quasi-1D wires, whereas antilocalization is not observed above $\sim$15 K in the unpatterned 2DES. The extracted spin coherence lengths, obtained from fitting the magnetoresistance curves to localization theory, show only weak temperature dependence. Therefore, phase coherence appears to dominate the temperature dependence of antilocalization in the low dimensional InSb/InAlSb systems. (NSF DMR-0618235, DOE DE-FG02-08ER46532, NSF DMR-0520550) [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L22.00009: Full-zone spin-splitting for electrons and holes in bulk GaAs and GaSb Jun-Wei Luo, Gabriel Bester, Alex Zunger The spin-orbit interaction --- a fundamental electroweak force --- is equivalent to an effective magnetic field intrinsic to crystals, leading to band spin-splitting for certain k-points in sufficiently low-symmetry structures. This (Dresselhause) splitting has usually been calculated at restricted regions in the Brillouin-zone via small-wavevector approximations (e.g., ${\bf k\cdot p}$). We provide a full-zone description of the Dresselhaus splitting in zinc-blende semiconductors by using pseudopotentials, empirically corrected to rectify LDA errors by fitting GW results at a few firections. We find that (i) The largest spin-splitting occurs along the [210] direction, not the [110] direction as previously thought based on limited view of the Brillouin zone; (ii) The spin-splitting of the upper valence band VB1 is comparable to that of the next two valence bands VB2 and VB3. This has been previously overlooked due to the expectation that the largest spin-splitting will occur along the [110] direction; (iii) The spin-splitting pattern of each band is orthogonal to each other. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L22.00010: Controlling the persistent spin helix with strain induced spin-orbit coupling Luyi Yang, Jake Koralek, Joe Orenstein, Andrei Bernevig, Shoucheng Zhang, Shawn Mack, David Awschalom We use transient spin grating spectroscopy to study the persistent spin helix (PSH) state of the 2D electron gas. The PSH is a meta-stable helical spin density wave that emerges as a result of increased symmetry when the Rashba and Dresselhaus spin-orbit coupling terms are balanced, and which offers great promise as a means of controlling large ensembles of spins. We demonstrate that the spin-orbit symmetry, and the PSH dynamics, can be manipulated \textit{in-situ} by the application of uniaxial strain. This strain induces spin-orbit coupling with precisely the same symmetry as the Rashba term, allowing us to effectively tune the Rashba/Dresselhaus ratio in a single sample. This work is supported by DMSE office of BES-DOE, NSF, MARCO, ASEE and CNID. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L22.00011: The precessing persistent spin helix in a magnetic field Rundong Li, Jing Wang, Shoucheng Zhang While the spin-orbit interaction is useful for manipulating the electron spin, it could also cause spin decoherence. A Persistent Spin Helix (PSH) with infinite life time has been predicted [B. A. Bernevig et al., Phys. Rev. Lett. 97, 236601 (2006).] for 2D quantum wells with equal strength of Rashba and Dresselhaus spin-orbit coupling. This effect results from the spin SU(2) symmetry of electrons, which makes the spin density at a finite wave vector conserved. The PSH was later observed in the transient spin grating (TSG) experiment [C. P. Weber et al., Phys. Rev. Lett. 98, 076604 (2007).], where the spin density wave is pumped and its decay in the time domain is probed optically. In this work we propose measuring the PSH with an in-plane magnetic field and spin injection with alternating polarization. We derive and solve the drift-diffusion equation for the spin density and find that when the frequency of the spin injection is the same as the Larmor frequency, a great enhancement of the diffusion length and the amplitude of the spin oscillation should be observed, giving rise to a precessing PSH. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L22.00012: Theory of Electron Spin Relaxation in ZnO Nicholas Harmon, William Putikka, Robert Joynt Doped ZnO is a promising material for spintronics applications. For such applications, it is important to understand the spin dynamics and particulary the spin relaxation times of this II-VI semiconductor. The transverse spin lifetime T$_{2}^{\ast}$ has been measured by optical orientation experiments, and it shows a surprising non-monotonic behavior with temperature. We explain this behavior by invoking spin exchange between localized and extended states. Interestingly, the effects of spin-orbit coupling are by no means negligible, in spite of the relatively small valence band splitting. This is due to the wurtzite crystal structure of ZnO. Detailed analysis allows us to characterize the impurity binding energies and densities, showing for the first time that optical orientation experiments can actually be used as a characterization tool for semiconductor samples. \newline [1] N.J. Harmon, W.O. Putikka, and R. Joynt, cond-mat/0808.2913 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L22.00013: Effect of Induced Spin-orbit Coupling in Cold Atomic Gas Xiong-Jun Liu, Mario F. Borunda, Xin Liu, Jairo Sinova Spin-orbit (SO) coupling effect in semiconductors has emerged in the solid-state community as a very active field of research, fueled in part by the field of spintronics, e.g. spin current injection with spin Hall effect [1]. Recently, new schemes are developed to generate the SO interaction in cold atoms [2], which opens new possibilities in studying Spintronics in atomic systems. Here we shall report our recent proposal of SO coupling effects in Fermi atomic systems via optical method [3]. The induced SO coupling can be of the Dresselhaus and Rashba type with a Zeeman term. We show that the optically induced SO coupling can lead to a spin-dependent effective mass under proper condition, with one of them able to be tuned between positive and negative effective masses. As a direct observable we show that in the expansion dynamics of the atomic cloud the initial atomic cloud can split into two or four clouds depending on the effective mass regimes. Reference: [1] S. Murakami et al., Science 301, 1348 (2003); J. Sinova et al., Phys. Rev. Lett. 92, 126603 (2004). [2] X.-J. Liu et al., Phys. Rev. Lett. 98, 026602 (2007); S.-L. Zhu et al., ibid, 97, 240401 (2006); T. D. Stanescu et al., ibid, 99, 110403 (2007). [3] X.-J. Liu, M. F. Borunda, X. Liu, J. Sinova, submitted to PRL for publication, arxiv:0808.4137 (2008). [Preview Abstract] |
Session L23: Fractional Quantum Hall Effect, Bilayers
Sponsoring Units: DCMP FIAPChair: Raymond Ashoori, Massachusetts Institute of Technology
Room: 325
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L23.00001: Different Signatures of the Total Filling Factor 1 State Lars Tiemann, Youngsoo Yoon, Stefan Schmult, Maik Hauser, Werner Dietsche, Klaus von Klitzing Bringing two 2-dimensional electron systems in close proximity can yield a correlated state as the electrons will experience the presence of the neighboring system. At the individual filling factors of 1/2 this leads to a new double-layer ground state as positive and negative charges from opposite layers couple to excitons. Many remarkable properties were found such as vanishing Hall and longitudinal resistances in the counterflow configuration [1], a resonantly enhanced zero bias tunneling peak [2], and more recently, a critical DC tunneling current and vanishingly small interlayer resistances in DC measurements [3]. We will show how it is possible to combine the results of these three different measurements into a consistent picture. Under certain conditions it is possible to exceed the critical currents but still observe a minimum at total filling factor 1 in the counterflow configuration.\newline [1] M. Kellogg et al. PRL 93, 036801 (2004); E. Tutuc et al. PRL 93, 036802 (2004)\newline [2] I.B. Spielman et al., PRL 87, 036803 (2001)\newline [3] L. Tiemann et al., New Journal of Physics 10, 045018 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L23.00002: Valley polarization and the polarization mass of composite fermions around $\nu$ = 3/2 Medini Padmanabhan, Tayfun Gokmen, Mansour Shayegan In two-dimensional electron systems confined to AlAs quantum wells, composite fermions (CFs) around $\nu $ = 3/2 are known to possess a valley degree of freedom [1]. The relative occupation of the valleys can be controlled via the application of uniaxial, in-plane strain. In this study, we measure the strain needed to completely valley-polarize the various fractional quantum Hall states around $\nu $ = 3/2 as a function of density and compare our results to the theory explaining the complete spin-polarization of CFs in GaAs [2] . While the theory predicts it to be a constant, the energy needed for complete valley-polarization in units of the Coulomb energy is experimentally found to increase with increasing density. Translating this to the language of the 'polarization mass' for the CFs [2], we find an absence of the theoretically expected $\sqrt B$ dependence for the polarization mass. [1] N. C. Bishop \textit{et al.}, Phys. Rev. Lett. 98, 266404 (2007) [2] K. Park and J. K. Jain, Phys. Rev. Lett. 80, 4237 (1998) [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L23.00003: Clausius-Clayperon relation for onset of the coherent $\nu=1$ phase in bilayer quantum hall systems Yue Zou, Gil Refael, James Eisenstein, Ady Stern A bilayer system of two-dimensional electron gases in a perpendicular magnetic field exhibits extremely rich phenomena. At total filling factor $\nu = 1$, as one increases the layer separation, the bilayer system goes from an interlayer coherent exciton condensed state to an incoherent phase of two decoupled composite fermion Fermi liquids. Many question still remain as to the nature of the transition between these two phases. Recent experiments investigated the phase boundary as a function of both in plane magnetic field and density imbalance. We compare these experimental results, e.g., the curvature of the phase boundary, with respect to the interlayer density imbalance, with a theoretical calculation based on the assumption that there is a direct first order transition between the two phases. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L23.00004: Quantum Hall Bilayer in a Periodic Potential Ganpathy Murthy, Jianmin Sun, Herbert Fertig Disorder is known to be central to the $\nu=1$ bilayer[1,2]. We study the bilayer $\nu=1$ system in a periodic potential, which mimics the nonperturbative effects of disorder by creating frozen-in Hall currents. The coupling to the potential is through the Pontryagin density of the pseudospin. We find the spinwave modes and quantize the theory to account for the effects of quantum fluctuations. 1. H. A. Fertig and G. Murthy, Phys. Rev. Lett. {\bf 95}, 156802 (2005). 2. H. A. Fertig and G. Murthy, Sol. St. Commun. {\bf 140}, 83 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L23.00005: Piezoresistance and Metal Insulator Transition of Composite Fermions at $\nu$ = 3/2 Tayfun Gokmen, Medini Padmanabhan, Mansour Shayegan In the composite fermion (CF) picture, at the Landau level filling factor $\nu$ = 3/2 the particle-flux CF quasi-particles are analogous to electrons at zero perpendicular magnetic field. Here we report piezoresistance measurements of CFs at $\nu$ = 3/2 in AlAs quantum wells. In this system, the electrons occupy two conduction band valleys with elliptical Fermi contours, and the valley occupation of electrons can be controlled via the application of uniaxial, in-plane strain. The system's response to strain at $\nu$ = 3/2 is qualitatively very similar to that of the electrons at zero perpendicular magnetic field, and consistent with the picture of CFs with a valley degree of freedom. Temperature dependent studies also show that CFs, like their counterpart electrons, go thorough a metal-insulator transition as they become valley polarized. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L23.00006: Composite Fermion signature in the single particle spectrum of the fractional quantum Hall system Ray Ashoori, Oliver Dial, Loren Pfeiffer, Ken West Using time domain capacitance spectroscopy we measure the single particle spectrum of the fractional quantum Hall system. The very high energy resolution of the techique (limited ultimately only by sample temperature) allows us to uncover the existence of new spectral features. Among these is a sharp line that crosses the Fermi level at filling factor $\nu=1/2$. The structure is consistent with the composite Fermion Landau fan, allowing measurement of the composite Fermion mass. Observation of this feature allows us to study the composite Fermion mass as a function of magnetic field and filling fraction. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L23.00007: Quasiparticles in the tunneling spectrum of the fractional quantum Hall system Oliver Dial, Raymond Ashoori, Loren Pfeiffer, Ken West Despite the central role that the tunneling (or single-particle) particle density of states (TDOS) plays in our theories of many-body systems, it has proven a difficult quantity to access experimentally in two dimensional electron systems. We have developed a technique, time domain capacitance spectroscopy, which allows measurement of the TDOS over a range of 30 meV centered about the Fermi surface, revealing the detailed and beautiful structure present in these systems far from the Fermi energy. With increased sample quality and higher magnetic fields, we see the emergence of the fractional quantum Hall effect in the TDOS along with negative compressibility and chemical potential jumps associated with several fractions. Most strikingly, we also observe a number of new, sharp quasiparticle lines far from the Fermi energy. The dependence of the quasiparticle energies on density allows us to identify different fractional quasiparticles as well as estimate emergent effective quasiparticle masses. These high energy spectral features shed new light on the highly correlated fractional ground state, as well as the nature of the state near $\nu=1$. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L23.00008: Quantum Hall States at filling $\nu=\frac{2}{k+2}$ Waheb Bishara, Gregory Fiete, Chetan Nayak We study the $\nu=\frac{2}{k+2}$ quantum Hall states which are particle-hole conjugates of the $\nu=\frac{k}{k+2}$ Read-Rezayi states. We find that equilibration between the different modes at the edge of such a state leads to an emergent SU(2)$_k$ algebra in the counter-propagating neutral sector. Heat flow along the edges of these states will be in the opposite direction of charge flow. In the $k=3$ case, which may be relevant to $\nu=2+\frac{2}{5}$, the thermal Hall conductance and the exponents associated with quasiparticle and electron tunneling distinguish this state from competing states such as the hierarchy/Jain state. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L23.00009: Phase diagram for bilayer quantum Hall effect at total filling $\nu_T=5$ Chuntai Shi, Shivakumar Jolad, Nicolas Regnault, Jainendra Jain There has been much interest in bilayer quantum Hall systems at total filling $\nu_T=1$, which exhibit excitonic superfluidity at small separations and two uncoupled composite fermion Fermi seas at large separations. We evaluate the phase diagram of the bilayer quantum Hall effect at total filling $\nu_T=5$, neglecting interlayer tunneling and spin fluctuations, which is expected to be a bilayer excitonic superfluid at small separations and two uncoupled 5/2 FQHE states at large separations. Based on a combination of variational and exact diagonalization (for up to 20 electrons) studies, we estimate that the transition between these states occurs at a layer separation of approximately one magnetic length,independent of the individual layer thickness. The composite fermion Fermi sea is not found to be relevant for any parameters. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L23.00010: How universal is the fractional quantum Hall edge physics? Shivakumar Jolad, Diptiman Sen, Jainendra Jain We report on study of edge excitations of fractional quantum Hall states at filling factors $\nu=1/3$ and $\nu=2/5$. By considering the restricted basis of composite fermion wave functions, which is very accurate for the low energy eigenstates, we are able to diagonalize systems with up to 54 particles which allows us to make extrapolations to the thermodynamic limit. In a model with neutralizing positive jellium background disk at a distance d, we find that edge reconstruction is generic, occurring even when the electron and the background disks coincide. We also test the postulated form for the electron field operator of the effective field theory approach for the fractional edge and find it to be inconsistent with our microscopic calculations. Implications of our results for the observed non-universality of the edge exponent will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L23.00011: A universal molecular description for the spectra of bosons and fermions in the lowest Landau level Constantine Yannouleas, Uzi Landman We show that both the {\it yrast\/} and {\it excited\/} states in the LLL spectra of small systems can be expressed as linear superpositions of appropriate rovibrational molecular trial functions, akin to the rotating-electron-molecule functions introduced earlier.\footnote{C. Yannouleas and U. Landman, Phys. Rev. B {\bf 66}, 115315 (2002); Rep. Prog. Phys. {\bf 70}, 2067 (2007).} Thus the nature of strong correlations in the lowest Landau level reflects the spontaneous emergence of intrinsic point-group symmetries associated with rotations and vibrations of molecules of localized particles arranged in concentric polygonal-ring configurations. The present molecular picture is valid for both bosons and fermions. We stress its validity and superiority for {\it low\/} (as well as high) angular momenta, where ``quantum-fluid'' trial functions of a markedly different nature (including Laughlin, composite-fermion, and Pfaffian ones) have been assumed\footnote{R.B. Laughlin, Phys. Rev. Lett. 50, 1395 (1983); J.K. Jain, {\it Composite Fermions\/} (Cambridge University Press, 2007); N.R. Cooper, arXiv:0810.4398v1.} to apply. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L23.00012: Origin of chiral p-wave pairing in even-denominator fraction quantum Hall effect Yuan-Ming Lu, Yue Yu, Ziqiang Wang We show that gauge field fluctuations in the composite fermion field theory can be exactly integrated out using a non-unitary transformation. An instantaneous statistical interaction is induced which makes the Fermi sea unstable to chiral p-wave pairing. We show that the paired state is a Moore-Read Pfaffian and discuss the effects of Coulomb interaction in connection to even-denominator fractional quantum Hall effect. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L23.00013: Pinning mode of integer quantum Hall Wigner crystal of skyrmions Han Zhu, G. Sambandamurthy, Y.P. Chen, P.-H. Jiang, L.W. Engel, D.C. Tsui, L.N. Pfeiffer, K.W. West Just away from integer Landau level (LL) filling factors $\nu$, the dilute quasi-particles/holes at the partially filled LL form an integer-quantum-Hall Wigner crystal, which exhibits microwave pinning mode resonances [1]. Due to electron-electron interaction, it was predicted that the elementary excitation around $\nu = 1$ is not a single spin flip, but a larger-scale spin texture, known as a skyrmion [2]. We have compared the pinning mode resonances [1] of integer quantum Hall Wigner crystals formed in the partly filled LL just away from $\nu = 1$ and $\nu = 2$, in the presence of an in-plane magnetic field. As an in-plane field is applied, the peak frequencies of the resonances near $\nu = 1$ increase, while the peak frequencies below $\nu = 2$ show neligible dependence on in-plane field. We interpret this observation as due to a skyrmion crystal phase around $\nu = 1$ and a single-hole Wigner crystal phase below $\nu = 2$. The in-plane field increases the Zeeman gap and causes shrinking of the skyrmion size toward single spin flips. [1] Yong P. Chen et al., Phys. Rev. Lett. 91, 016801 (2003). [2] S. L. Sondhi et al., Phys. Rev. B 47, 16 419 (1993); L. Brey et al., Phys. Rev. Lett. 75, 2562 (1995). [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L23.00014: Observation of a Fractional Quantum Hall State at $\nu=1/4$ in a Single Wide GaAs Quantum Well Dwight R. Luhman, W. Pan, D.C. Tsui, L.N. Pfeiffer, K.W. Baldwin, K.W. West We have preformed low temperature ($T\sim35$ mK) transport measurements using a 50 nm high-quality GaAs quantum well with an electron density of $n_e=2.55\times10^{11}$ cm$^{-2}$ and a mobility of $\mu \sim 10^{7}$ cm$^2$/Vs. Magnetic fields up to $B=45$ T were used to reach filling factor $\nu=1/4$. With the sample situated perpendicular to the applied magnetic field, the diagonal resistance displays a kink at $\nu=1/4$. When the sample is tilted to an angle of $\theta=20.3^{\circ}$, a clear minimum in the diagonal resistance and plateau in the Hall resistance are present at $\nu=1/4$ indicating a fractional quantum Hall state at $\nu=1/4$ in this sample. Several possibilities regarding the origin of this state will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L23.00015: Charge-density wave formation in interacting two-dimensional electronic systems with Landau level mixing Peter Smith, Malcolm Kennett Anisotropic transport in half-filled Landau levels has been explained in terms of charge-density wave (CDW) formation. We use the Hartree-Fock approximation to study the influence of electron-electron interactions and Landau level mixing on the formation of CDWs in two-dimensional electron and hole systems. For the situation of two nearly degenerate eigenstates, we construct a Landau free-energy theory appropriate for competing order parameters that allows for both striped and triangular CDW formation. We find the possibility of coexisting CDW ordering in the two states, along with possible hysteretic behaviour. This physics might be realized using an external parameter such as spin-orbit coupling to tune states into near degeneracy. [Preview Abstract] |
Session L24: Focus Session: Electronic Properties of Nanotubes
Sponsoring Units: DMPChair: Sergei Tretiak, Los Alamos National Laboratory
Room: 326
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L24.00001: Strongly Correlated Electron Phenomena in Carbon Nanotubes Invited Speaker: In this talk I will discuss our recent results demonstrating strongly correlated electron behavior in ultra-clean carbon nanotube quantum dots. Specifically, we have observed one-dimensional (1D) Wigner crystal behavior of dilute holes in semiconducting nanotubes, finding three distinct regimes of spin and valley quantum number ordering as the charge density and axial magnetic field are varied. The boundaries between the regimes in density and magnetic field are well-described by the theory of Levitov and Tsvelik for a narrow-gap Luttinger liquid. In the second part of the talk I will present results showing that the electrons in nominally metallic nanotubes comprise a 1D Mott insulator. This indicates that carbon nanotubes are never truly metallic, in agreement with theoretical predictions that account for Umklapp scattering at half-filling due to electron-electron interactions. Using inelastic cotunneling spectroscopy, we also observe neutral electronic excitations within the gap, yielding an additional signature of the Mott insulating state. Our results demonstrate nanotubes' promise for studying a variety of tunable correlated electron phenomena in 1D. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L24.00002: Relaxation and dephasing in a two-electron $^{13}$C nanotube double quantum dot Hugh Churchill, Ferdinand Kuemmeth, Jennifer Harlow, Andrew Bestwick, Emmanuel Rashba, Karsten Flensberg, Carolyn Stwertka, Thiti Taychatanapat, Susan Watson, Charles Marcus We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron $^{13}$C nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time, $T_1$, first decreases with parallel magnetic field then goes through a minimum in a field of 1.4 T. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which suppresses hyperfine mediated relaxation and enhances relaxation due to soft phonons. The inhomogeneous dephasing time, $T_2^*$, is consistent with previous data on hyperfine coupling strength in $^{13}$C nanotubes. This work was supported by the National Science Foundation under grant no.~NIRT 0210736 and the GRFP, ARO/iARPA, the Department of Defense, and Harvard's Center for Nanoscale Systems. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L24.00003: Electrical evidence for the encapsulation of C$_{60}$ inside a carbon nanotube: Random telegraph signal and hysteric current-voltage characteristics Yung Woo Park, Han Young Yu, Dong Su Lee, Ursula Dettlaff-Weglikowska, Siegmar Roth We present electrical evidence for the encapsulation of C$_{60}$'s inside a carbon nanotube: random telegraph signals (RTSs), and hysteric current-voltage characteristics. The RTSs is ascribed to the instability of the quantum harmonic oscillations of C$_{60}$'s. RTSs are smeared out at a temperature which is consistent with the energy level of the vibrational quantum mediated by the van der Waals binding between the carbon nanotube and C$_{60}$'s. In addition, hysteric behavior in cyclic current-voltage characteristics is explained by the longitudinal motion and resettlement of the C$_{60}$'s with the modulation of the size of the quantum dot mediated by the C$_{60}$ insertion. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L24.00004: Breakdown of the Wigner-Mattis theorem in semiconductor carbon-nanotube quantum dots Massimo Rontani, Andrea Secchi, Franca Manghi The Wigner-Mattis theorem states the ground state of two bound electrons, in the absence of the magnetic field, is always a spin-singlet. We predict the opposite result --a triplet- for two electrons in a quantum dot defined in a semiconductor carbon nanotube. The claim is supported by extensive many-body calculations based on the accurate configuration interaction code DONRODRIGO (www.s3.infm.t/donrodrigo). The crux of the matter is the peculiar two-valley structure of low-energy states, which encodes a pseudo-spin degree of freedom. The spin polarization of the ground state corresponds to a pseudo-spin singlet, which is selected by the inter-valley short-range Coulomb interaction. Single-electron excitation spectra and STM wave function images may validate this scenario, as shown by our numerical simulations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L24.00005: Casimir Interactions Between Scatterers on Carbon Nanotubes Dina Zhabinskaya, Jesse Kinder, E. J. Mele We study the interactions between two short-range scatterers in metallic carbon nanotubes as a Casimir problem. In the massless Dirac Hamiltonian for the electrons, a defect can be represented by a scattering potential with a pseudospin polarization. Sublattice-asymmetric and bond-centered potentials may lead to small momentum backscattering, depending on the chiral angle of the nanotube. Quasibound states formed between two defects determine the forces at the boundaries. We develop a force operator approach within the Dirac model to calculate the forces on two square well potentials of finite width, and take the limit of sharp and strong scatterers to study the Casimir forces mediated by the fermions. For the special case of two identical scatterers we recover the conventional one-dimensional attractive Casimir force. For the general problem with inequivalent scatterers we find that the magnitude and sign of this force depends on the relative pseudospin polarizations of the two defect potentials. We will also discuss the effects intervalley scattering on the Casimir interactions between defects. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L24.00006: Water confined in carbon nanotubes: Magnetic response and proton chemical shieldings Patrick Huang, Eric Schwegler, Giulia Galli Carbon nanotubes (CNT) provide a well-defined environment for the study of confined water, whose behavior can differ markedly from bulk water. The application of nuclear magnetic resonance (NMR) to probe the local water structure and dynamics in these cases is hindered by ambiguities in the interpretation of the NMR spectra. We employ linear response theory to evaluate the $^1$H chemical shieldings of liquid water in semiconducting CNTs, where the electronic structure is derived from density functional theory with periodic boundary conditions. The shieldings are sampled from trajectories generated via first-principles molecular dynamics simulations at ambient conditions, for water in CNTs with diameters $d=11$~\AA\ and $14.9$~\AA\@. We find a large ($\sim -23$~ppm) {\em upfield} shift relative to bulk liquid water, which is a consequence of strongly anisotropic magnetic fields induced in the CNT by the applied magnetic field. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L24.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L24.00008: Effect of spin-orbit interactions on the static polarizability of single-wall carbon nanotubes* G. S. Diniz, S. E. Ulloa The electronic structure of carbon nanotubes (CNTs) is known to exhibit different metallic or insulating behavior as different chiral vectors are considered. Application of external electric fields and the presence of spin-orbit interaction (SOI) result in the Rashba effect modifying the level structure of CNTs, strongly coupling spin and orbital degrees of freedom, as demonstrated in recent experiments [1]. In this work we calculate the static long-wavelength limit of the dielectric response of different single-wall CNTs when subjected to uniform external electric fields both along and across the longitudinal direction (parallel to the nanotube axis). Our calculation uses a $\pi$-band tight-binding formalism, considers Rashba and intrinsic SOI, and utilizes the random phase approximation to evaluate $\epsilon(q\rightarrow 0, \omega=0)$ [2]. We use parameters from the literature and find that the metallic-to-semiconductor transition induced by the intrinsic SOI is suppressed as the Rashba field is taken into account. We further show that this behavior has a clear signature on measurable quantities, such as the static polarizability. We discuss its dependence on nanotube size and chirality and propose possible nanoprobe experiments to study this phenomenon. [1] F. Kuemmeth \emph{et al.}, Nature 452, 448-452 (2008). [2] M. L. Cohen \emph{et al.}, Phys. Rev. B 52, 8541 (1995). *Supported by Fulbright, CAPES and NSF-DMR MWN. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L24.00009: Low-Temperature Studies of Electrostatic Doping and Phonon Renormalization in Individual Single-Walled Carbon Nanotubes Gugang Chen, Tereza Paronyan, Gamini Sumanasekera, Elena Pigos, Avetik Harutyunyan Electrostatic doping of carbon nanotubes (CNTs) induced by applied external gate voltage (V$_{g})$ in the field-effect transistor (FET) configuration allows controllable variation of charge density and the Fermi level in nanotubes. Raman scattering from a CNT is very sensitive to doping and its interaction with the surrounding environment. Recently, the peculiarities of G-mode as a function of gate induced carrier at room temperature have been reported in [1] [Nature Nanotechnology 2, 725 (2007)]. In the present work, we used Raman scattering to study the p- and n- electrostatic doping of CNT as a function of V$_{g}$ in a wide range of temperature from 6.8 K to 300 K. Virtually symmetric blue-shifting of the G-mode for both p- and n- doping of CNTs has been observed. We found that the prehistory of the CNT measurement as well as environment interaction such as gas adsorption on CNT plays an important role on the observed phenomena. Explanation of our findings will be discussed based on phonon energy renormalization of carbon nanotubes [1] due to the carrier density variation during electrostatic doping. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L24.00010: Field Enhancement inside Carbon Nanotubes Hong Zhang, Yoshiyuki Miyamoto There are many investigations on photo-excitations with polarization vector parallel to tube axis (parallel-polarization), compared to it, few studys on those with polarization vector perpendicular to tube axis (cross-polarization) are reported because of early theoretical consideration on electric-field-depolarization effect of nanotubes with cross-polarization [1]. Using dynamical consideration and TDDFT analysis, we herein present the influence of external electric field perpendicular to the axis of semiconductor carbon nanotube (CNT). By adjusting frequency of applied E-field in corresponding wavelength of light from 800nm to 591 nm, the total E-field inside carbon nanotubes has been found to show great change depending on the frequency; incomplete screening and strong enhancement even without including the excitonic effect [2]. The enhancement comes from increase of oscillating amplitude of electron cloud with resonant frequency given by the applied E-field. Also the numerical stability and the satisfaction of energy conservation rule with application of dynamical E-field were numerically checked [3]. This finding should be taken into account in interpreting a measurement of optical response of molecules being encapsulated in CNTs. [1]. H. Ajiki and T. Ando, Physica B \textbf{201}, 349 (1994) [2]. S. Uryu and T. Ando, Phys. Rev. B \textbf{76}, 115420 (2007) [3]. Y. Miyamoto and H. Zhang, Phys. Rev. B \textbf{77}, 165123 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L24.00011: Quasi-particle Energy Gap of Metal-coated Carbon Nanotubes Yu Zhou, Li Chen, Yiming Zhang, Swastik Kar, Pulickel Ajayan, Saroj Nayak We have studied the electronic structures of metal-coated carbon nanotubes using density functional theory and many body corrections based on GW approximation (GWA). In particular, we will present energy band gap variation as a function of number of metal atoms on the nanotube surface. Our results are compared with recent experiments. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L24.00012: Microwave Rectification by Carbon Nanotube Schottky Diodes Enrique Cobas, Steven Anlage, Michael Fuhrer We report the fabrication and electrical characterization of carbon nanotube Schottky diodes (CNT-SDs) via photolithography on high-frequency-compatible substrates using dissimilar contacts of chromium and platinum. The diodes are well-described by the ideal diode equation (n = 1.0). DC and low-frequency behavior is compared to a model of a diode in series with a resistor. The diodes rectify microwave signals beyond 18GHz and produce dc currents of hundreds of nanoamperes. The frequency and voltage dependence is used to estimate the junction capacitance of 1aF and an intrinsic device cut-off frequency of 400GHz. \\[3pt] [1] Cobas, E. and Fuhrer, M., Applied Physics Letters 93, 043120 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L24.00013: Microwave Irradiation Induced Effects to Single-walled Carbon Nanotube Thin Films Lu Wang, Yao Xiong, Ziran Wu, Liwei Chen, Hao Xin Carbon nanotubes have been considered as potential building blocks for nano-scale circuits in virtue of their unique mechanical and electrical properties. However, one of the biggest obstacles for massive production of nanotube circuits is the difficulty of separating semiconducting tubes from metallic tubes or vice versa. In this work, a convenient method which may be potentially employed to selectively remove metallic tubes using microwave induced breakdown is proposed and investigated. Carbon nanotube thin films deposited on glass and quartz substrates are placed in a commercial microwave oven and heated for up to several minutes. The radial breathing mode in Raman spectra on the nanotube samples before and after the microwave irradiation suggests that the metallic-to-semiconducting ratios are reduced by around 20{\%}. Meanwhile, because in the thin film samples most of the nanotubes are entangled, smaller diameter nanotubes (both metallic and semiconducting) tend to be affected more. THz transmission measurements of these thin films are also performed before and after microwave irradiation. The significant increase of transmission after the microwave irradiation process confirms the loss of metallic tubes. [Preview Abstract] |
Session L25: Focus Session: Probing and Modifying Materials with Lasers II
Sponsoring Units: DMPChair: Craig Arnold, Princeton University
Room: 327
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L25.00001: Modeling of Laser Material Interactions Invited Speaker: Irradiation of a substrate by laser light initiates the complex chemical and physical process of ablation where large amounts of material are removed. Ablation has been successfully used in techniques such as nanolithography and LASIK surgery, however a fundamental understanding of the process is necessary in order to further optimize and develop applications. To accurately describe the ablation phenomenon, a model must take into account the multitude of events which occur when a laser irradiates a target including electronic excitation, bond cleavage, desorption of small molecules, ongoing chemical reactions, propagation of stress waves, and bulk ejection of material. A coarse grained molecular dynamics (MD) protocol with an embedded Monte Carlo (MC) scheme has been developed which effectively addresses each of these events during the simulation. Using the simulation technique, thermal and chemical excitation channels are separately studied with a model polymethyl methacrylate system. The effects of the irradiation parameters and reaction pathways on the process dynamics are investigated. The mechanism of ablation for thermal processes is governed by a critical number of bond breaks following the deposition of energy. For the case where an absorbed photon directly causes a bond scission, ablation occurs following the rapid chemical decomposition of material. The study provides insight into the influence of thermal and chemical processes in polymethyl methacrylate and facilitates greater understanding of the complex nature of polymer ablation. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L25.00002: Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation Leonid Zhigilei, Elodie Leveugle There are a number of applications that utilize the ability of laser ablation of a volatile matrix to entrain, eject and, if needed, deposit large macromolecules to a substrate with minimum chemical modification. In particular, the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique is used in fabrication of ultra-thin organic films for optoelectronic, biomedical, and chemical sensor applications. In this presentation we report the results of a computational investigation of the mechanisms of molecular ejection in MAPLE. Coarse-grained molecular dynamics simulations are performed for polymer concentrations up to 6 wt.{\%}. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected as parts of polymer-matrix clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. An internal release of matrix vapor in the overheated droplets is shown to be capable of pushing polymer molecules to the outskirts of the droplets, forming ``molecular balloons'' in which polymer-rich surface layers enclose the volatile matrix material. The results of the simulations explain some of the complex morphologies observed in polymer films deposited in MAPLE and conventional polymer ablation/deposition experiments. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L25.00003: Mechanism of Resonant Mid-Infrared Laser Ablation of Polystyrene Richard Haglund, Stephen Johnson, Daniel Bubb We investigated the mechanism of resonant-infrared laser ablation of polymers using polystyrene as a model material. The ablation laser was a picosecond mid-infrared free-electron laser tuned to mid-IR laser wavelengths that are resonant with specific vibrational modes of the polystyrene target. Time-resolved plume imaging combined with etch-depth measurements and finite-element calculations indicate that a blowoff model fits the experimentally measured etch depths and plume images, provided one accounts for moderate shielding of the surface by the ablation plume. The finite-element model includes the temperature-dependent absorption coefficient and specific heat that dramatically change the material properties above the glass-transition temperature. Ablation begins after a thin surface layer of the material is superheated to temperatures exceeding 1000 C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area at the target surface. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L25.00004: The influence of thermal confinement and temperature-dependent absorption on resonant infrared ablation of frozen aqueous and alcohol targets Daniel Bubb, Stephen Johnson, Richard Haglund We investigated the mechanism of matrix-assisted resonant infrared laser ablation in frozen aqueous and methanol solutions of polymer, by performing plume shadowgraphy and ablation yield measurements. A picosecond, tunable free-electron laser was used for ablation at two wavelengths, one (2940 nm) that was resonant with the --OH stretch in both water and methanol, and the other (3450 nm) that is resonant with the --CH stretch in methanol. The plume images showed gross similarities, differing only in the time required for the shockwave to appear and in the velocity of the shock front. Typically, 15-25 \textit{$\mu $}s after the ablation laser pulse arrives the primary material ejection commences and lasts for hundreds of \textit{$\mu $}s. In all three cases, the ablation plume appears to consist entirely of vapor with no droplets or solid particles. The ablation yield is either linear or quadratic in fluence. This dependence can be understood if we consider thermal diffusion in the targets and the temperature dependence of the absorption coefficient. . [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L25.00005: VUV excimer laser-materials interactions with fluorocarbon polymers Tom Dickinson, Sharon George, Steve Langford Particle emission from transparent wide bandgap materials at laser fluences below the threshold for optical breakdown can provide important insight on interactions at the higher fluences employed for surface modification, machining, and laser ablation deposition. We present recent studies of ion and neutral molecule emission from polytetrafluorethylene [(C$_{2}$F$_{4})_{N}$---PTFE---Teflon{\textregistered}] and polyvinylidene fluoride [(CH$_{2}$CF$_{2})_{N}$---PVDF] during nanosecond pulsed 157-nm excimer laser irradiation. The chemical and electrical properties of these materials play important roles in many technologies. In PTFE, the primary mechanism for material removal involves bond scission along the backbone of the polymer. In PVDF, HF emission is accompanied by carbonization of the irradiated region. High-energy positive and negative ions are observed from both materials under 157-nm irradiation. We describe critical measurements that reveal the physics and chemistry underlying these processes. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L25.00006: Depth Profiling of Polymer Composites by Ultrafast Laser Ablation Christopher Young, Clive Clayton, Jon Longtin Past work has shown femtosecond laser ablation to be an athermal process at low fluences in polymer systems. The ablation rate in this low fluence regime is very low, allowing for micro-scale removal of material. We have taken advantage of this fact to perform shallow depth profiling ablation on carbon fiber reinforced polymer (CFRP) composites. Neat composite and resin samples were studied to establish reference ablation profiles. These profiles and the effects of the heterogeneous distribution of carbon fibers were observed through confocal laser profilometry and optical and scanning electron microscopy. Weathered materials that have been subjected to accelerated tests in artificial sunlight or water conditions were ablated to determine the correlation between exposure and change in ablation characteristics. Preliminary Raman and micro-ATR analysis performed before and after ablation shows no chemical changes indicative of thermal effects. The low-volume-ablation property was utilized in an attempt to expose the sizing-matrix interphase for analysis. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L25.00007: Laser Interactions with Vertically-Aligned Carbon Nanotube Arrays David Geohegan, Chris Rouleau, Alex Puretzky, Jeremy Jackson, Norbert Thonnard, Ilia Ivanov, Karren More Femtosecond and nanosecond laser interactions with vertically aligned carbon nanotube arrays (VANTAs) have been studied in vacuum and background gases. As-grown VANTAs were synthesized by chemical vapor deposition onto Fe/Al-coated Si wafers to typical heights of 10-20 microns. The forests of aligned nanotubes grow from catalyst nanoparticles anchored to the substrate and typically have a disordered layer of nanotubes at their top surfaces. Laser interactions in vacuum, inert, and oxidizing atmospheres were compared. The nanotubes were found to form periodic surface structures in response to repeated laser pulses. Patterning of the nanotube arrays with scanning beams was studied. Damage to the nanotubes was studied by Raman spectroscopy and high-resolution TEM. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L25.00008: Temperature-dependent energy-transfer between electrons and phonons in nickel around Curie temperature and its relation to ultra-fast demagnetization Xuan Wang, Shouhua Nie, Junjie Li, Rick Clinite, Jim Cao We report on the use of Ultrafast Electron Diffraction (UED) to study the ultrafast dynamics of nickel induced by fs-laser excitation. Particularly, we have observed a significant increase of electron-phonon coupling time when the pre-set sample base-temperature is reduced across its Curie temperature. This implies a strong quenching of magnetic ordering (ultrafast demagnetization), which serves as an extra energy reservoir other than lattice to relax the electron energy. By modeling the energy transfer among these three systems, we conclude that ultrafast demagnetization happens in a time-scale even shorter than electron-phonon coupling and one temperature for both electron and magnetic ordering, as suggested by several former studies, works very well from energy point of view. Our results also support the former observed ultrafast demagnetization in itinerant ferromagnets, which happens in sub-ps time scale. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L25.00009: Ultrafast Time Resolved Reflection High-Energy Electron Diffraction Study of Laser-Matter Interactions for Silicon Hyuk Park, J.M. Zuo We report a study of silicon surface using ultrafast time-resolved reflection high energy electron diffraction (RHEED) based on the pump-probe approach. The probe beam is an electron pulse generated by a femtosecond laser, accelerated to 30 kV and focused by a magnetic lens. Using this probe, we investigated the pulse laser interaction with silicon by monitoring the electron diffraction pattern recorded in the glancing angle geometry. We observed transient angle-dependent electron beam deflection from silicon surfaces. We show that the electron beam deflection comes the change in surface potential and charge produced by laser-matter interaction. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L25.00010: Ultrafast Photoinduced Structural Dynamics in Graphite Zhibin Lin, Roland Allen In this work, we employ the density-functional-based tight-binding approach of Frauenheim and coworkers in simulations employing semiclassical electron-radiation-ion dynamics (SERID), which treats the coupled dynamics of electrons and ions during the nonadiabatic processes induced by laser irradiation. A series of computer simulations are performed in studies of graphite responding to femtosecond-scale laser pulses of various intensities and durations. It is found that the contraction of the interlayer distance in graphite happens shortly after optical excitation. Transient changes in the interlayer distance are related to changes in the interlayer bonding due to ultrafast changes in the populations of the electronic states. Results of these computational studies are compared with recent pump-probe experimental data on graphite. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L25.00011: Photo-Induced Structural Dynamics of Graphite Studied by Ultrafast Electron Crystallography. Ramani K. Raman, Ryan A. Murdick, Yoshie Murooka, Chong-Yu Ruan The graphite to diamond conversion is believed to involve the rhombohedral phase of graphite as an intermediate state. Using ultrafast electron crystallography, we have observed the formation of transient interlayer \textit{sp}$^{3}$ bonds in graphite beyond a threshold fluence, leading to a non-thermal structural change (Raman, R. K. et al$.$ Phys. Rev. Lett. 101, 077401 (2008)). This transient rebonding towards diamondization is likely driven by a compressive Coulomb stress created by the photoinduced charge separation following the initial E$_{2g}$ phonon excitation that alters the layering symmetry within graphite. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L25.00012: Photo-irradiation effect on charge-ordered states in strongly correlated electron systems Yu Kanamori Photo-induced phenomenon has attracted much attention in the research field of strongly correlated electron systems. Recently, the photo-induced effects have been examined experimentally in a charge-ordered (CO) insulating phase associated with the antiferromagnetic (AF) order in perovskite manganites, and changes in the charge and magnetic properties are observed. It is believed that strong coupling between itinerant electrons and localized spins plays a key role on the phenomena. We investigate photo-induced effects in correlated electron systems where conduction electrons couple with localized spins. In particular, the photo-induced phenomena in a CO insulator associated with AF order are examined. Several transient spectra are calculated numerically in the double exchange model. We find that in the photo-excited states, finite spectral weights in the optical absorption spectra appear inside of the insulating gap, and its intensity increases with increasing time. In the one-particle excitation spectra, the in-gap band appears by the photo-irradiation, and its width becomes broad with time evolution. These electronic-structure changes are correlated with the time evolution of the localized spin correlation. These results indicate that the spin degree of freedom plays an important role on the photo-excited states. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L25.00013: Optical, Mechanical, and Opto-Mechanical Switching of Anchored Dithioazobenzene Bridges Ivan Stich, Robert Turansky, Martin Konopka, Nikos Doltsinis, Dominik Marx The ability of anchored photochromic molecules to perform optically, mechanically, and opto-mechanically driven switching cycles is studied using electronic structure methods. As a model for such nanoscale devices we have simulated molecular switches consisting of single dithioazobenzene bridges between gold tips. Purely optical switching cycles are hindered by mechanical effects due to the tips. However, the possibility to perform both combined opto-mechanical and purely mechanical two- way switching is predicted. The simulations help to elucidate also the role played by mechanical and chemical effects due to anchoring. Possible experimental realization of such devices will also be mentioned. [Preview Abstract] |
Session L26: Focus Session: Graphene VI: Phonons and Raman Spectroscopy
Sponsoring Units: DMPChair: Kostya Novoselov, Manchester University
Room: 328
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L26.00001: Photovoltaic Hall Effect in Dirac systems -- Application to Graphene Takashi Oka, Hideo Aoki We theoretically propose to irradiate electron systems with massless Dirac dispersion with circularly polarized light, for which we predict that the photo-irradiation can induce a dc Hall effect in the absence of static, uniform magnetic fields. The effect bears a geometric origin, traced back to the non-adiabatic phase (Aharonov-Anandan phase) which is acquired by the motion of $k$-points in the Brilliouin zone when they encircle the Dirac cones. The Kubo formula for linear responses is extended to the nonlinear effects via the Floquet formalism for strong ac fields, which is used to calculate the \textit{photo-induced Berry curvature}. The irradiation induces a dynamical gap at the Dirac point which gives rise to a universal ac Wannier-Stark ladder in Dirac systems observable in the density of states. We further use the Keldysh + Floquet method to analyze finite graphene systems, which confirms the existence of photovoltaic dc Hall effect. The required strength of the circularly polarized light to observe these effects is estimated to be O($10^7$eV/m), which is within an accessible range for present laser sources. (arXiv:0807.4767) [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L26.00002: Thermal conductivity and thermal rectification in graphene nanoribbons: a molecular dynamics study Jiuning Hu, Xiulin Ruan, Zhigang Jiang, Yong Chen We have used molecular dynamics based on the Brenner potential to calculate the thermal conductivity of graphene nanoribbons. For symmetrical nanoribbons, the calculated thermal conductivity is the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have considerable larger thermal conductivity than that of nanoribbons with armchair edges. For asymmetric nanoribbons, we have found considerable thermal rectification. For example, for a 6nm-long triangular shaped nanoribbon, the thermal conductivity from the wider to the narrower end is nearly 2.5 times that from the narrower to the wider end. Furthermore, the thermal rectification can be significantly enhanced by increasing the size of the asymmetrical nanoribbon. Such rectification effects can be useful in nanoscale thermal management. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:30PM |
L26.00003: Raman scattering in bilayer graphene: probing phonons, electrons and electron-phonon interactions Invited Speaker: The application of resonance Raman spectroscopy to study the electronic, vibrational and electron-phonon interaction properties in bilayer graphene will be presented. From the dependence of the second-order Raman bands on the laser excitation energy, we obtain experimental values for the Slonczewski-Weiss-McClure band parameters of bilayer graphene. We will discuss in detail the effect of each one of the tight band parameters on the electronic structure, showing that bilayer graphene has a larger electron-hole asymmetry compared to graphite. We will also present experimental results for the phonon dispersion relations near the K point, showing a strong Kohn anomaly for one of the phonon branches. In a gating experiment, the change in Fermi level of bilayer graphene gives rise to a symmetry breaking, allowing the observation of both the symmetric (S) and anti-symmetric (AS) phonon modes. The dependence of the energy and damping of these phonons modes on the Fermi level energy is explained in terms of distinct couplings of the S and AS phonons with intra- and inter-band electron-hole transitions. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L26.00004: Phonon dispersion of graphene revisited Silvia Viola Kusminskiy, David Campbell, Antonio Castro Neto We calculate the phonon spectrum for a graphene sheet resulting from the model proposed by T. Lenosky {\it et al.} (Nature {\bf 355}, 333 (1992)) for the free energy of the lattice. This model takes into account not only the usual bond bending and stretching terms, but captures the possible misalignements of the $p_z$ orbitals. We compare our results with previous models used in the literature. We analyze the effect of anharmonic terms. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L26.00005: Giant nonadiabatic effects in layer metals: Raman spectra of intercalated graphite and doped graphene explained. A. Marco Saitta, Michele Lazzeri, Matteo Calandra, Francesco Mauri The occurrence of nonadiabatic effects in the vibrational properties of metals has been predicted since the 1960s [1], but hardly confirmed experimentally. We report the first fully ab initio calculations of nonadiabatic frequencies of a number of conventional (hcp Ti and Mg), layered metals (MgB2, CaC6, other intercalated graphites) [2] and doped graphene [3]. Nonadiabatic effects can be spectacularly large (up to 30\% of the phonon frequencies), but they can only be experimentally observed in the Raman spectra of layered compounds. In layered metals nonadiabatic effects are crucial to explaining the observed Raman shifts and linewidths. Moreover, we show that those quantities can be used to extract the electron momentum-relaxation time. [1] S. Engelsberg and J.R. Schrieffer, Phys. Rev. 131, 993 (1962). [2] A.M. Saitta et al., Phys. Rev. Lett. 100, 226401 (2008). [3] M. Lazzeri and F. Mauri, Phys. Rev. Lett. 97 , 266407 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L26.00006: Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers Stephane Berciaud, Sunmin Ryu, Louis Brus, Tony Heinz The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, which were suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G, D, and 2D phonon modes. The G-mode phonons exhibit low energies (1580 cm$^{-1})$ and broad widths (14 cm$^{-1})$ compared to the response for samples supported on the SiO$_{2}$--covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2$\times $10$^{11 }$cm$^{-2}$ for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show very little local disorder, based on the very low Raman D mode intensity. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L26.00007: Micro Raman spectroscopy of graphene Hall Bars in the QHE regime Sebastian Remi, Constanze Metzger, Anna Swan, Bennett B. Goldberg One of the most intriguing aspects in the physics of graphene are new types of quantum Hall effects which differ significantly from observations on conventional 2DEG samples. Furthermore the Raman spectrum of graphene shows charge carrier dependent Kohn anomalies. So far transport and optical measurements on graphene haven't been combined to explore the behavior of the Raman features dependent on the Landau levels. We present our latest measurements of Raman scattering on graphene single and bilayers in the Quantum Hall regime. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L26.00008: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L26.00009: Raman Scattering from Pt Island-Decorated Graphene Awnish Gupta, Humberto Gutierrez, Peter Eklund We performed microRaman studies of decorated $n$-Graphene Layers ($n$GLs). Nano-islands (NI; dia$\sim $5-10 nm) of Pt were created by deposition on the $n$GL with gaps between the NI in the range of few nm. When the NI were present, we observed D and D' Raman bands as well as splitting of the G-band into G$^{+}$ and G$^{-}$ (most pronounced for 1GL). The observations may be related to graphene ``confined'' in the interstitial spaces between NIs. The D and D' bands show the following properties: (1) Intensity of D and D' relative to G band decreases with increasing number of layers $n $in the $n$GL. (2) Peak frequencies, $\omega _{D}$ decreases linearly with 1/$n $while $\omega _{D\mbox{'} }$remains constant. (3) Linewidth $\Gamma _{D}$ decreases linearly with 1/$n$ , while $\Gamma _{D\mbox{'}}$ increases linearly with 1/$n$. Our results will be discussed in terms of results theoretically predicted by zone folding (Jishi \textit{et al}). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L26.00010: Raman Spectroscopy of the 2D Mode in Free-Standing Graphene Monolayers Janina Maultzsch, Stephane Berciaud, Louis Brus, Tony Heinz Raman spectroscopy of the 2D (or G') mode is a critical tool for the analysis of graphene mono- and multilayers. This symmetry-allowed overtone mode permits one to probe zone-edge phonons by Raman spectroscopy. Moreover, its doubly-resonant electronic character$^{1}$ renders it readily observable and encodes in it information about the electronic structure of the graphene sample. In particular, the 2D mode provides a clear signature of the thickness of mono- and multilayer graphene films$^{2}$. In this paper, we present a detailed study of the properties of the 2D for a pristine, free-standing graphene monolayer prepared by mechanical exfoliation over a trench structure. In contrast to the behavior of monolayers of graphene on substrates, for this pristine graphene sample, we observe a positively skewed line shape. The linewidth is also somewhat reduced compared to that observed for graphene supported on a substrate. Further, the 2D mode in the free-standing graphene films exhibits a slightly stronger dispersion with energy of the pump photons than for supported monolayers. We discuss our findings within the framework of double resonance theory, taking into account the intrinsically undoped nature of the free-standing graphene samples. $^{1}$J. Maultzsch \textit{et al.}, Phys. Rev. B \textbf{70}, 155403 (2004) $^{2}$A. Ferrari \textit{et al.} Phys. Rev. Lett. \textbf{97}, 187401 (2006) [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L26.00011: Seeing and counting graphene layers by elastic light scattering C. Casiraghi, A. Hartschuh, E. Lidorikis, H. Qian, H. Harutyunyan, T. Gokus, K. S. Novoselov, A. C. Ferrari Raman scattering has recently emerged as a viable and nondestructive technique for the identification of graphene, its doping, edges and amount of defects [1-3]. However, the Raman scattered photons are a minority compared to those elastically scattered. Here we show that large graphene layers can be mapped and identified in a few minutes by elastic scattering. We report an extensive investigation of graphene on silicon/silicon oxide substrate by monochromatic and white-light elastic scattering and the theoretical understanding of the experimental data [4]. Maps of the scattered light are obtained by raster scanning the sample with a piezoelectric stage. We show that the image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. [1] A. C. Ferrari et al., PRL 97, 187401 (2006) [2] S. Pisana et al., Nature Materials 6, 198 (2007) [3] C. Casiraghi et al., APL 91, 233108 (2007) [4] C. Casiraghi et al., Nano Letters 7, 2711 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L26.00012: Polarized micro Raman spectroscopy of bilayer graphene Hyerim Moon, Duhee Yoon, Young-Woo Son, Hyeonsik Cheong The frequency of Raman 2$D$ band of the graphite depends on the excitation laser energy. This phenomenon is explained with double resonance Raman process. In polarized micro-Raman spectroscopy of single layer graphene, Raman $G$ band ($\sim $1586 cm$^{-1})$ is isotropic, and 2$D$ band ($\sim $2686 cm$^{-1})$ strongly depends on relative polarizations of the incident and scattered photons. This strong polarization dependence originates from inhomogeneous optical absorption and emission mediated by resonant electron-phonon interaction. In bi-layer graphene, Raman 2$D$ band can be decomposed into four Lorenztian peaks which can be interpreted in terms of the four transition paths in the double resonance Raman process. We investigated the polarization dependence of each Lorenztian peak in the Raman 2$D$ band of bi-layer graphene for different excitation laser energies. Strong polarization dependence of the Raman 2$D$ band, similar to the case of single layer graphene, is observed. The excitation energy dependence of the polarized Raman scattering is analyzed in terms of the band structure of bi-layer graphene. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L26.00013: Interference effect on Raman spectrum of single layer graphene on SiO$_{2}$/Si Duhee Yoon, Hyerim Moon, Young-Woo Son, Jin Sik Choi, Bae Ho Park, Young Hun Cha, Young Dong Kim, Hyeonsik Cheong We studied the dependence of the Raman spectrum of graphene on the thickness of the SiO$_{2}$ layer. We prepared the single layer graphene samples with the various SiO$_{2}$ layer thicknesses ($\sim $240 to $\sim $388 nm) using micro-mechanical cleavage from the natural graphite and observed a strong variation of the Raman spectrum as a function of the thickness of SiO$_{2}$ layer. It is found that the intensity of Raman signal is strongly influenced by the interference due to multiple reflections in the graphene and SiO$_{2}$ layers. The Raman enhancement factor was calculated by taking into account the interference effect. The model calculation fits well with the experimental data. The interference also affects the Raman intensity ratio of the 2$D$ band to the $G$ band due to the difference in the wavelength of these signals. Moreover, we calculated the Raman enhancement factor as a function of the SiO$_{2}$ thickness and the excitation laser wavelength. [Preview Abstract] |
Session L27: Focus Session: Graphene Device and Applications II
Sponsoring Units: FIAPChair: Marcus Freitag, IBM
Room: 329
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L27.00001: Graphene and chemically modified graphene sensors Invited Speaker: Molecular sensing via sp$^{2}$-bonded carbon nanomaterials is a promising research area in both nanoscience and nanotechnology. In general these materials are thermally and chemically stable, come in a variety of different geometries (spheres, tubes, and sheets), and are process compatible with conventional micro-lithographic techniques. In this talk we focus on atomically thin sheets of sp$^{2}$-bonded carbon, known as graphene, and discuss their sensing properties when exposed to chemical vapors. The remarkable physical properties of graphene-- from near ballistic electron conduction to ultra high stiffness ($>$ 5 times that of steel)-- make it a unique system to study both electronic and mechanical transduction modes. Finally, we demonstrate the utility of graphene-based films is greatly expanded after chemical functionalization. In this regard, chemically modified graphene (CMG) is emerging as a material system whose properties are complementary to nominally pure graphene for sensing applications. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L27.00002: Intrinsic Response of Graphene Vapor Sensors Ye Lu, Yaping Dan, Nicholas Kybert, Charlie Johnson Ye Lu$^{1}$, Yaping Dan$^{1}$, Nicholas J. Kybert$^{2}$, A. T. Charlie Johnson$^{1}$, $^{1}$\textit{University of Pennsylvania, USA }$^{2 }$\textit{University of Warwick, UK.}Graphene is a purely two-dimensional material that has extremely favorable chemical sensor properties. It is known, however, that conventional nanolithographic processing typically leaves a resist residue on the graphene surface, whose impact on the sensor characteristics of the system has not yet been determined. Here we show that the contamination layer both degrades the electronic properties of the graphene and masks graphene's intrinsic sensor responses. The contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. We demonstrate a cleaning process that verifiably removes the contamination on the device structure and allows the intrinsic chemical responses of graphene to be measured. Additionally, methods of functionalizing clean graphene device as high quality chemical vapor sensor are explored. Funding: JSTO DTRA and the Army Research Office Grant {\#}W911NF-06-1-0462, NSF-NSEC/NBIC DMR-0425780, REU program of the Laboratory for Research on the Structure of Matter (N.J.K.). [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L27.00003: Single-layer Graphene Motion and Mass Sensors with Electrical Readout. Changyao Chen, Sami Rosenblatt, Kirill Bolotin, Horst Stormer, Philip Kim, Tony Heinz, James Hone We report for the first time the implementation of graphene electromechanical resonators that can detect their own motion. Suspended single-layer graphene field-effect transistors allow for electrical detection of the resonances while functioning as heterodyne mixers in a manner analogous to the operation of a radio receiver. Mechanical resonances occur in the 10-100 MHz range, can be lithographically-tailored, are tunable by tens of MHz, and have quality factors up to 200 while operated in vacuum at room temperature. Furthermore, by analyzing the frequency response of the resonators, we succeed in weighing both the pristine single-layer graphene and with a layer of organic material deposited on. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 4:06PM |
L27.00004: Electronic and magnetic functions of nanographene-based host-guest system Invited Speaker: The electronic structure of nanographene having open edges crucially depends on its edge shape. According to theoretical predictions, nanographene has nonbonding $\pi $-electron state (edge state) localized in zigzag edges. We investigated the electronic structure of graphene edges, the magnetism of the edge-state spins in nanographene and the effect of host-guest interaction on the magnetism. For magnetic investigations, we employed nanoporous activated carbon fiber (ACF) having a 3D disordered network of nanographite domains, each of which is a stack of 3-4 nanographene sheets. STM/STS investigations of hydrogen-terminated graphene edges confirm the presence of edge states around zigzag edges, in good agreement with theoretical works. The feature of the edge state depends on the detailed geometry of the edge structures. The magnetism of nanographene in ACF has a ferrimagnetism feature with a net magnetic moment, for which the cooperation of ferromagnetic intra-zigzag-edge and ferromagntic/antiferromagnetic inter-zigzag-edge interactions is responsible. Heat-treatment, which induces an insulator-metal transition, brings about spin glass state of the edge-state spins in the vicinity of the transition. Physisorption of guest species such as water, organic molecules, rare gas in the ACF nanopores generates a high-spin/low-spin magnetic switching phenomenon, in which a discontinuous reduction of the magnetic moment takes place. This is explained in terms of the strengthening of the inter-graphene-sheet antiferromagnetic interaction, which is induced by the mechanical compression of nanographite domains by the condensed guest molecules. The magnetic oxygen molecules physisorbed in the nanopores work seriously to decrease the magnetoresistance in ACF as a consequence of the interaction between the oxygen molecule spins and edge-state spins. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L27.00005: Investigation of Molecular Functionalizing Agents for Graphene Device Optimization. Damon Farmer, Yu-Ming Lin, Ali Afzali-Ardakani, Phaedon Avouris Due to its linear dispersion relation and the predicted chiral nature of its quasiparticles, graphene has become a material of intense experimental and theoretical investigation. There has been rapid progress in the fabrication and understanding of graphene devices. However, many key issues still need to be addressed in order to fully exploit graphene for technological applications. Here, we identify stable molecular compounds that can be used as dopants and functionalizing agents on graphene. As dopants, these compounds are used to both modify the potential profile in the channel region of graphene devices, and reduce parasitic resistances in these devices. As functionalizing agents, these compounds serve as nucleation sites for the uniform growth of thin high-$\kappa $ gate dielectrics, allowing for enhanced capacitive coupling with the graphene channel. The characteristics of graphene devices employing these molecular compounds will be presented, and problems associated with the implementation of these molecules in graphene devices will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L27.00006: Fabrication and Chemical Doping of Carbon-based Transparent Electrodes George Tulevski, Ali Afzali The use of carbon-based materials (carbon nanotubes and graphene) as transparent electrodes has attracted enormous interest due to their high conductivities, transparency and potential as a lower cost alternative to traditional transparent electrode materials (i.e. Indium Tin Oxide). This talk will focus on using solution processes to suspend both carbon nanotubes and graphene flakes in solution and the fabrication of transparent electrodes from these solutions. Solutions were prepared using both surfactants and organic solvents, followed by purification to remove large aggregates and impurities. A variety of chemical dopants were then employed including metal salts and small organic molecules. The sheet resistance of the resultant films can be significantly reduced with chemical doping. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L27.00007: Atomic Layer Deposition (ALD) methods for fabricating graphene devices - theory and experiments Yvette Hancock, Samiul Haque, Maarit Karppinen, Asta K\"arkk\"ainen, Leo K\"arkk\"ainen, Reijo Lehteneimi, Pirjo Pasanen, Juho Per\"al\"a, Elina Sahramo, Karri Saloriutta ALD has the potential to be a well controlled method for coating and patterning graphene structures and making new generation devices. We have investigated the ALD of 5 nm thin coatings of high-k dielectric Al2O3 onto graphene, and have determined the selectivity of the chemical specific deposition, for example, to the edges or starting from defect sites. Experimentally, we see an affinity for Al2O3 to coat the edges of graphene, which is also supported by our ab initio calculations. The affinity of the Al2O3 coating with the edges of graphene allows us to make a mask, which could then be used to fabricate graphene nanoribbons of widths less than 50nm that are also gated. [Preview Abstract] |
Session L28: Focus Session: Thermoelectric Materials: Tellurides
Sponsoring Units: DMP FIAPChair: Stefan Zollner, IBM
Room: 330
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L28.00001: Enhancement of the thermoelectric figure of merit by distortions of the dispersion relation Invited Speaker: A doubling of the thermoelectric figure of merit (zT) of p-type PbTe above 700 K has been recently demonstrated (1) in thallium-doped material. The effect comes about because an electronic energy level of the Tl atoms resonates with the valence band of PbTe. This creates an excess density of states, g(E), at a specific energy about 60 meV below the valence band edge, which in turn gives a thermoelectric power at that carrier concentration about three times higher than that of similarly-doped p-type PbTe. In this talk, we will review the mechanisms by which this distortion of the g(E) function, from the normal E$^{1/2}$ form valid for parabolic bands in three dimensions into a spike-like function, increases the thermoelectric power and thus zT. We further derive a set of criteria for the excess g(E) to improve zT. We will discuss the applicability of this approach to other electronic levels in PbTe first, and then describe the more general quest for such energy levels in other thermoelectric semiconductors. (1) J. P. Heremans et al., Science \textbf{321} 554 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L28.00002: Electronic Inhomogeneity in PbTe-based High Performance Thermoelectric Materials Observed by NMR E.M. Levin, K. Schmidt-Rohr, B.A. Cook, M.G. Kanatzidis Effects of composition and synthesis conditions on the local structure and charge carrier concentration in Ag$_{x}$Sb$_{y}$Pb$_{18}$Te$_{20}$ (LAST-18) thermoelectric (TE) materials have been studied by $^{125}$Te and $^{207}$Pb nuclear magnetic resonance (NMR) with magic-angle spinning. The high-resolution $^{125}$Te NMR spectra show that most Sb and Ag is not part of Sb$_{2}$Te$_{3}$, AgSbTe$_{2}$, or Ag$_{2}$Te inclusions.\textbf{ }Biexponential NMR spin-lattice (T$_{1})$ relaxation as well as Knight shifts of $^{125}$Te and $^{207}$Pb NMR signals show that many LAST-18 materials contain two phases of similar composition but with free electron concentrations that differ by more than an order of magnitude, i.e. these materials are electronically inhomogeneous. The NMR data were calibrated against Hall- and Seebeck-effect measurements to give the charge carrier concentrations in the two phases. This electronic inhomogeneity may result in the appearance of potential barriers inside TE materials, similar to those observed for semiconductor-semiconductor or metal-semiconductor junctions. Such barriers may affect thermopower, electrical, and thermal conductivity of TE materials. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L28.00003: Thermodynamic Properties of Pb and Ag-Sb Based Chalcogenides: A First-Principles Study Yi Zhang, Xuezhi Ke, Changfeng Chen, Jihui Yang, Paul R. C. Kent The Pb and Ag-Sb based chalcogenide compounds have received considerable interest for their potential applications in thermoelectric devices. Their low thermal conductivity plays a key role in producing the high figure of merit (ZT) that is critical for applications. We performed a series of first- principles calculations on several Pb and Ag-Sb based chalcogenide compounds to understand their lattice dynamics. The direct force method and density functional theory calculations were used to obtain the phonon dispersion and density of states. The phonon softening processes with the volume change were carefully evaluated. Moreover, we employed the quasiharmonic approximation to calculate the thermodynamic functions. The calculated results are in good agreement with available experimental data and provide insights for understanding the physical properties. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L28.00004: Link between changes in \textit{ZT} and microstructure in AgSbTe$_{2}$ Peter Sharma, Joshua Sugar, Douglas Medlin The best thermoelectric alloys have complex microstructures. For example, the LAST alloys, (AgSbTe$_{2})_{1-x}$(PbTe)$_{x}$, possess \textit{ZT}$\sim $1.5-2 but have a great variety of inclusions with different chemistry at different length scales. How does microstructure affect thermoelectric efficiency? Since the phase diagram of this and most quaternary alloys is poorly known, transport properties have not been systematically connected to microstructure. We are attacking this problem by studying the simple ternary alloy AgSbTe$_{2}$, a component of the LAST system, in order to show how thermoelectric transport changes with a known, controlled microstructure. AgSbTe$_{2}$ forms within the well-studied Ag$_{2}$Te-Sb$_{2}$Te$_{3}$ pseudobinary phase diagram. We have found that Sb-rich AgSbTe$_{2}$ is composed of Sb$_{2}$Te$_{3}$ precipitates embedded in a homogeneous rocksalt Ag$_{16}$Sb$_{30}$Te$_{54}$ matrix. The precipitates are plate-like and crystallographically aligned along their close packed planes parallel to that of the matrix. The size of these Sb$_{2}$Te$_{3}$ plates can be tuned from the nanometer to micron scale. In this work, the formation and growth of precipitates over a wide length scale is linked to changes in thermoelectric properties for the first time. This study is useful for understanding the complexity of LAST, or any bulk thermoelectric where second phase precipitation occurs. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L28.00005: Microstructure and Nucleation Mechanism for Nanoprecipitates in PbTe-AgSbTe$_2$ Xuezhi Ke, Changfeng Chen, Jihui Yang, Lijun Wu, Juan Zhou, Qian Li, Yimei Zhu, Paul R.C. Kent Many recent advances in thermoelectric (TE) materials are attributed to their nanoscale constituents. Determination of the nanocomposite structures has represented a major experimental and computational challenge and eluded previous attempts. Here we present the first atomically resolved structures of high performance TE material PbTe-AgSbTe$_2$ by transmission electron microscopy imaging and density functional theory calculations. The results establish an accurate structural characterization for PbTe-AgSbTe$_2$ and identify the interplay of electric dipolar interactions and strain fields as the driving mechanism for nanoprecipitate nucleation and aggregation, which provides key insights for understanding a broad class of complex nanocomposite materials. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L28.00006: Galvanomagnetic and Thermomagnetic Properties of Ag$_{(1-x)}$Na$_{x}$SbTe$_{2}$ Alloys Michele Nielsen, Vladimir Jovovic, Christopher Jaworski, Joseph Heremans Group I-V-VI$_{2}$ alloys have intrinsically low thermal conductivity$^{1}$ on the order of 0.65 W/mK due to Umklapp phonon-phonon scattering. Combined with the high valence band density of states in AbSbTe$_{2}$, this makes this material system ideal for thermoelectric applications up to 416 K, where AgSbTe$_{2}$ undergoes a crystallographic phase transition. The partial substitution of Na for Ag is expected to address this problem. We synthesize bulk Ag$_{(1-x)}$Na$_{x}$SbTe$_{2}$ alloys and measure the evolution of the phase transition as a function of Na concentration x. The thermoelectric and galvanomagnetic properties of the alloys are also studied: based on the measurement of resistivity, Seebeck, Nernst and Hall coefficients we calculate mobilities, Fermi energies and partial carrier concentrations of holes and electrons. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L28.00007: Electronic and Thermal Properties of Cubic Ge-Sb-Te Compounds Donald Morelli, Kevin Zhou The ternary rocksalt structure compound Ge$_{4}$SbTe$_{5}$ is unusual because most members of the Ge-Sb-Te family form along the tie-line of the binary compounds GeTe and Sb$_{2}$Te$_{3}$, and thus do not possess the 1:1 cation:anion ratio necessary to present themselves in a cubic structure. The TE properties of these (GeTe)$_{x}$(Sb$_{2}$Te$_{3})_{1-x}$ compounds, while interesting in their own right, are no better than those of commercially available materials. Ge$_{4}$SbTe$_{5}$ and its relatives, with equal numbers of atoms on the cation and anion sites, form stably in the cubic rocksalt structure. For TE applications a cubic compound is advantageous because there is no issue regarding anisotropy of the thermoelectric properties. We have fabricated bulk samples of Ge$_{4}$SbTe$_{5}$ and related compounds, characterized their crystal structure, and measured some of their thermal and electronic properties. Results of isoelectronic substitution of Se on the Te site and Sn on the Ge site will be reported. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L28.00008: Thermoelectric Figure-of-merit in Bulk p-type PbTe Bo Yu, Hui Wang, Bed Poudel, Kenneth McEnaney, Gang Chen, Zhifeng Ren Lead telluride and its related chalcogenide alloys have been well studied for decades. With various achievements in not only bulk by also in thin films, quantum dots, superlattices, nanowires, etc., they always come up as one of the best thermoelectric materials for middle-range temperature applications. Recently, thallium was reported as a good candidate for band structure engineering in p-type lead telluride ingot system to largely enhance the thermoelectric power factor and hence the dimensionless figure-of-merit (ZT). Here we used mechanical alloying as the approach for large-scale production and achieved ZT value of 1.05 at 300 \r{ }C. The details will be presented in this talk. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L28.00009: Modular assembly of binary nanocrystal composite solids for high-efficiency thermoelectric power generation Dong-Kyun Ko, Christopher Murray Despite the numerous advantages of semiconductor nanostructures, doping nanometer size crystals show difficulties not found in conventional bulk systems. Especially for thermoelectric applications, it is critical to control the number of carriers that are available in semiconductors in order to maximize the figure of merit. Here, we report modular assembly of binary composite nanocrystals, as an effective bottom-up design tool, to create a new family of artificial solids with a prescribed set of doping levels. Silver telluride (Ag2Te) nanocrystals, which can act as dopants, are introduced in lead telluride (PbTe) nanocrystal assemblies in order to modify the carrier concentration until an optimum power factor is realized. This study focuses on electronic and thermoelectric characterization of these binary composite solids. Hall measurement and field effect transistor characteristics were investigated in order to identify the carrier type, mobility, and concentration. Temperature dependence of low-bias conductivity was also characterized to gain a better understanding of electronic conduction. Finally, Seebeck voltage was measured with varying PbTe to Ag2Te nanocrystal concentration ratios in order to investigate the Seebeck coefficient as a function of carrier concentration. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L28.00010: Galvanomagnetic and thermomagnetic properties of thallium doped PbSnTe and PbSeTe Vladimir Jovovic, Joseph Heremans Thallium acts as a resonant level in PbTe, so that PbTe:Tl shows a significant improvement of thermoelectric properties due to an increase in thermopower as compared to that of similarly Na-doped PbTe [2]. Further improvements in zT are expected from a reduction of the thermal conductivity by alloy scattering in Pb$_{1-x-y}$ Tl$_{y}$Sn$_{x}$Te and Pb$_{1-y}$Tl$_{y}$Te$_{1-x}$Se$_{x}$ alloys. However, the band structure of PbTe is sensitive to alloying with Sn and Se, and thus the location of the Tl level with respect to the valence band can change with x. In this study, we investigate the effects that band structure modifications have on the enhancement of thermopower. Thermoelectric properties of Pb$_{1-x-y}$ Tl$_{y}$Sn$_{x}$Te and Pb$_{1-y}$Tl$_{y}$Te$_{1-x}$Se$_{x}$ alloys with y=0.01-0.04 and x=0-0.3 are measured in directions longitudinal and transverse to magnetic fields in the range of -1.5 to 1.5T. We report zero field values of electrical resistivity, thermopower, Hall coefficient and adiabatic Nernst-Ettinghausen coefficient as measured in temperature range 80-420K. From these we calculate carrier density and mobility and the density of states effective masses and Fermi energies. [2] J.P. Heremans et al., Science 321, 554 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L28.00011: Antimony: a Dual Donor in Lead Telluride Christopher Jaworski, Janusz Tobola, Joseph Heremans Band structure calculations indicate the formation of an antimony impurity level just above the Fermi level for Pb$_{1-x}$Sb$_{x}$Te and just below the Fermi level for PbSb$_{x}$Te$_{1-x}$.~ For experimental verification, we prepare bulk samples of Pb$_{1-x}$Sb$_{x}$Te and PbSb$_{x}$Te$_{1-x}$ (x = 0.25, 0.5, 1{\%}).~~ Electrical resistivity, Seebeck, Hall and transverse Nernst-Ettingshausen coefficients of the crystals have been measured in the temperature range 2-580 Kelvin. Thermal conductivity data was measured in the range 80-800 Kelvin.~We confirm the ability of antimony to take the place of a lead atom and dope PbTe n-type or take the place of a tellurium atom and dope PbTe p-type. Antimony, however, is not as efficient an acceptor in p-type material as it is a donor in n-type material. The Fermi levels are calculated using experimental data and will be reported here. Also, a phase transition is experimentally observed at 500 K in p-type PbSb$_{x}$Te$_{1-x}$.~ [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L28.00012: Enhancement of Thermoelectric Figure-of-Merit by a Nanostructure Approach Zhifeng Ren, Bed Poudel, Yi Ma, Yucheng Lan, Xiaowei Wang, Giri Joshi, Gaohua Zhu, Jian Yang, Bo Yu, Xiao Yan, Hui Wang, Dezhi Wang, Qing Hao, Hohyun Lee, Austin Minnich, Andrew Muto, Daryoosh Vashaee, Mildred Dresselhaus, Gang Chen The dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, the nanostructure approach has been successfully applied to a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient. [Preview Abstract] |
Session L29: Focus Session: Incorporating Computational Physics into Teaching
Sponsoring Units: FEd DCOMPChair: James Belak, Lawrence Livermore National Laboratory
Room: 333
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L29.00001: One Lattice Gauge Theorist's Perspective on Important Skills and Concepts for Computational Physics Courses Invited Speaker: Lattice Gauge Theory employs a number of numerical and statistical techniques including: sparse matrix inversion, Monte Carlo methods, higher order numerical integration schemes, resampling methods such as jackknife and bootstrap, and parameter estimation from correlated data. Many of these techniques can be taught to undergraduates in contexts more easily understood than a lattice gauge theory simulation. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L29.00002: Tips and Tools for Teaching Quantum Mechanics Guangtian Zhu, Chandralekha Singh Learning quantum mechanics is challenging -- students usually struggle to master the basic concepts, even though they may perform well on solving quantitative problems. Our group is investigating the difficulties that upper-level students have in learning quantum mechanics. To help improve student understanding of quantum concepts, we are designing quantum interactive learning tutorials (QuILTs) and tools for peer-instruction. Many of the tutorials employ computer simulations to help students visualize and develop better intuition about quantum phenomena. We will discuss the common students' difficulties, share the material we have developed and evaluated to make the quantum mechanics class engaging and useful, and show ways to bridge the gap between quantitative and conceptual aspects of quantum mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L29.00003: Building a Digital Library: Theory, Computation, and Education Wolfgang Christian Over the past dozen years the Open Source Physics (OSP) project has produced some of the most widely used interactive curricular materials for the teaching of introductory and advanced physics courses. These materials are based on Java applets called Physlets and on new OSP programs and applications. In this talk we will outline the pedagogical and technical features of these programs and describe our current effort to create and distribute our material using the comPADRE National Science Digital Library. Open Source Physics collection is available on the comPADRE website at http://www.compadre.org/osp/ [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L29.00004: Introducing scientific computation from high school to college: the case of Modellus V\'Itor Teodoro, Rui Neves The development of computational tools and methods has changed the way science is done. This change, however, is far from being accomplished on high school and college curricula, where computers are mainly used for showing text, images and animations. Most curricula do not consider the use of computational scientific tools, particularly tools where students can manipulate and build mathematical models, as an integral part of the learning experiences all students must have. In this paper, we discuss how Modellus, a freely available software tool (created in Java and available for all operating systems) can be used to support curricula where students from the age of 12 to college years can be introduced to scientific computation. We will also show how such a wide range of learners and their teachers can use Modellus to implement simple numerical methods and interactive animations based on those methods to explore advanced mathematical and physical reasoning. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L29.00005: DIYModeling: a place for students and faculty to build their own game-quality simulations to enhance learning. Bryndol Sones, Frank Wattenberg DIYModeling (Do it Yourself Modeling) aims to improve both the quality of learning in the STEM disciplines and the extent to which the very best STEM learning reaches all students by leveraging the power of game- quality modeling and simulation. It builds on earlier work by many people using platforms like Java, Flash and game quality simulations like the Federation of American Scientists' Immune Attack. DIYModeling adds a new element that enables students and faculty to build their own game-quality simulations by specifying the underlying scientific and mathematical models without getting into the details of programming. The DIYModeling team is a consortium of math and basic science faculty from six universities teamed up with the software development company Tietronix Software (an 8a certified company), which does contract work for NASA to build complex software systems including game-quality immersive simulations. The goal of the program is to enable curriculum developers and students to develop game- quality, three-dimensional immersive simulations with educational benefit. Current applications under development include a first-person shooter game environment for use in data collection and statistical analysis, orbital mechanics in executing the Hohlman transfer, and solar power generation. Some pilot tests are planned for use in the spring semester. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L29.00006: Computational Physics at Haverford College Peter Love We will describe two new physics courses at Haverford College: Physics/CS 304, Computational Physics, an upper level elective for Physics, CS and Math Majors, and Physics 412, Research in Theoretical and Computational Physics. These courses are designed to extend students experience of physics using computation. They are also part of an interdisciplinary Concentration in Computational Science mounted jointly by the departments of Computer Science, Economics, Biology Chemistry and Mathematics. These courses make extensive use of Python, Scipy , Numpy and Visual Python, and include extensive independent projects. We will describe some results obtained and lessons learned. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L29.00007: Developing Computational Physics in Nigeria Godfrey Akpojotor, Emmanuel Enukpere, Famous Akpojotor, Sunny Ojobor Computer based instruction is permeating the educational curricula of many countries oweing to the realization that computational physics which involves computer modeling, enhances the teaching/learning process when combined with theory and experiment. For the students, it gives them more insight and understanding in the learning process and thereby equips them with scientific and computing skills to excel in the industrial and commercial environments as well as at the Masters and doctoral levels. And for the teachers, among others benefits, the availability of open access sites on both instructional and evaluation materials can improve their performances. With a growing population of students and new challenges to meet developmental goals, this paper examine the challenges and prospects of current drive to develop Computational physics as a university undergraduate programme or as a choice of specialized modules or laboratories within the mainstream physics programme in Nigeria institutions. In particular, the current effort of the Nigerian Computational Physics Working Group to design computational physics programmes to meet the developmental goals of the country is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L29.00008: EJS enhances traditional learning in freshman mechanics Amy Bug, Jen Trinh Easy Java Simulations (EJS) is a freeware authoring tool (part of the Open Source Physics project) [1]. EJS allows not only physics teachers, but students as well, to produce nonproprietary, platform-independent simulations with both numerical and graphical output. We report on the use of EJS as a helpful tool in a physics course for majors. In particular, EJS allows a student to conceptualize tough introductory material such as kinematics in polar coordinates and conservation of momentum with mass-transfer. The construction of an EJS simulation gives the student an environment in which to surmount both conceptual and mathematical roadbloacks to learning. [1] http://www.opensourcephysics.org/ [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L29.00009: Computational Labs Using VPython Complement Conventional Labs in Online and Regular Physics Classes Martina E. Bachlechner Fairmont State University has developed online physics classes for the high-school teaching certificate based on the text book Matter and Interaction by Chabay and Sherwood. This lead to using computational VPython labs also in the traditional class room setting to complement conventional labs. The computational modeling process has proven to provide an excellent basis for the subsequent conventional lab and allows for a concrete experience of the difference between behavior according to a model and realistic behavior. Observations in the regular class room setting feed back into the development of the online classes. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L29.00010: A New Graduate Minor Program in Computational Science Lyle Long This talk will discuss the need for graduate educational programs in computational science. Due to the continued increase in computer power, algorithms, and software the need for students trained in computational science has increased dramatically. Theoretical and experimental methods are still important, but there is an enormous need for students who understand numerical methods, programming, parallel computing, and software engineering. A new Graduate Minor program has been developed at Penn State and is now available to all graduate students (http://www.csci.psu.edu). The Ph.D. students are required to take two core courses (out of three possible), attend two seminar series, and choose two additional courses (which are often in their major). This is an extremely popular program, with 17 students graduating in the first two years and 75 more currently enrolled (primarily from various engineering and science departments). This program allows students to pursue a traditional M.S. or Ph.D. degree, but also acquire knowledge in computational science and receive credit for it. We believe this is a good alternative, rather than including this material as additional requirements to the traditional programs or developing new M.S. or Ph.D. programs in Computational Science. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L29.00011: Material and Virtual Workspaces in Physics Research Chad Wickman, Christina Haas, Peter Palffy-Muhoray A growing body of research has examined the potential for computer-based tools to improve the quality and scope of physics education. Yet, few studies have investigated how experienced scientists deploy those tools in the conduct and communication of their work. Based on a study of text production in liquid crystal physics, I will discuss how specific applications, like LabVIEW, mediate the practice of experimental research. Findings suggest that experimentation involves a complex negotiation of material and virtual constraints and that, as a result, a concept of scientific literacy must account for the processes through which scientists visualize, display, and characterize their objects of study symbolically and textually. This approach, in examining the relationship between the material and virtual in a modern scientific workplace, ultimately offers insight into education that prepares students to undertake and communicate research in dynamic, multimedia laboratory environments. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L29.00012: Singular Function Integration in Computational Physics Javier Hasbun In teaching computational methods in the undergraduate physics curriculum, standard integration approaches taught include the rectangular, trapezoidal, Simpson, Romberg, and others. Over time, these techniques have proven to be invaluable and students are encouraged to employ the most efficient method that is expected to perform best when applied to a given problem. However, some physics research applications require techniques that can handle singularities. While decreasing the step size in traditional approaches is an alternative, this may not always work and repetitive processes make this route even more inefficient. Here, I present two existing integration rules designed to handle singular integrals. I compare them to traditional rules as well as to the exact analytic results. I suggest that it is perhaps time to include such approaches in the undergraduate computational physics course. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L29.00013: N-body, parallel simulation using a Barnes-Hut algorithm: performance versus accuracy Norman Chonacky, Brian Dobbins The Barnes-Hut method facilitates prioritizing two-body interactions in an N-body system according to their likely significance in calculating the system's dynamics. In particular, it allows a consistent segregation of two-body interactions into those that should be treated by direct calculation versus those that can be aggregated in subsets and then treated by mean-field approximations. In this paper we describe the principles of the Barnes-Hut method, its use in parallelized N-body simulations, and the performance/accuracy trade-offs it presents. We present the latter in the context of results from simulation cases: N-bodies interacting via a gravitational potential, and N-bodies interacting via a Lennard- Jones potential. These should be available in the near future to operate as part of the ``Bootable Cluster CD'' parallel computation environment of the National Computational Science Institute of the Shodor Educational Foundation. [Preview Abstract] |
Session L30: Focus Session: Multiferroics I
Sponsoring Units: DMP GMAGChair: Craig Fennie, Cornell University
Room: 334
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L30.00001: Ferroelectric magnets: a Conical Spiral and an Ising Chain Invited Speaker: Multiferroics, where magnetic order with broken inversion symmetry accompanies the occurrence of ferroelectric polarization, can show remarkable tunability of dielectric properties by applied magnetic fields, such as reversibly flipping of ferroelectric polarization or a drastic change of dielectric constant with fields. It turns out that the origin of ferroelectricity in most of new multiferroics is spiral magnetism that tends to produce uniform lattice distortions, i.e., ferroelectric distortions, through antisymmetric exchange coupling. Among such materials, spinel CoCr$_{2}$O$_{4}$, exhibiting a conical-spiral spin order, is unique in that it exhibits spontaneous magnetization as well as electric polarization. We have studied the detailed switching behavior of magnetoelectric domains, characterized by polarization, magnetization, and magnetic wave vector, under variation of temperature and applied magnetic fields. New aspects that we have discovered from this study include [1] polarization reversal at the magnetic lock-in transition temperature (T$_{L})$ with thermal variation or with isothermal variation of magnetic fields (without changing their direction) at a temperature near T$_{L}$, [2] surprisingly, this polarization reversal happens without change in spin rotation direction in spiral spins. We also present newly-discovered multiferroicity in a ``collinear'' chain magnet of Ca$_{3}$(Co,Mn)$_{2}$O$_{6}$. In the Ising chain magnet, ``symmetric'' exchange coupling seems responsible for the multiferroicity. We show that in Ca$_{3}$Co$_{2-x}$Mn$_{x}$O$_{6}$ (x$\approx $1), the ferroelectricity originates from the coexistence of the alternating order of magnetic ions with different charges and the collinear up-up-down-down spin order in the frustrated spin chain. The system also exhibits magnetic freezing transitions that affect the temperature dependence of the electric polarization magnitude. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L30.00002: Magnetic-filed-induced polarization in the square-lattice antiferromagnetic Ba$_{2}$CoGe$_{2}$O$_{7}$ Hee Taek Yi, Young Jai Choi, Seongsu Lee, Sang-Wook Cheong We have discovered the appearance of ferroelectricity below the Neel temperature of 6.7 K in the square-lattice antiferromagnetic Ba$_{2}$CoGe$_{2}$O$_{7}$ single crystal, grown by using a floating zone technique. The ferroelectric polarization aligns along the tetragonal $c$ axis, but is very small in magnitude. However, the magnitude of polarization increases remarkably and the polarization direction smoothly rotates away from the $c$ axis with increasing magnetic fields along the $c$ axis. This change of polarization and the associated change of dielectric constant with fields are monotonic without going through any phase transition. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L30.00003: Electric polarization reversal under high magnetic field in square lattice antiferromagnet Ba$_{2}$CoGe$_{2}$O$_{7}$ Jae Wook Kim, S.H. Chun, S.H. Kim, Kee Hoon Kim, Y. Jo, L. Balicas, Y.J. Choi, S.-W. Cheong, F. Balakirev, N. Harrison Recently, Ba$_{2}$CoGe$_{2}$O$_{7}$ was found to develop electric polarization ($P)$ below $T_{N}$=6.7 K [1]. Interestingly, $P$ along the $a$-axis increases linearly, crossing zero at $H$=0 when magnetic field ($H)$ is applied along the $c$-axis. To investigate the linear $H$-dependence of $P$ further, we measured $P$ dielectric constant (\textit{$\varepsilon $}), and magnetization ($M)$ under high $H$ up to 45 T. On application of high $H$, $P$ increases linearly up to $H\sim $15 T but suddenly decreases to a constant negative value. A peak in \textit{$\varepsilon $} is found at the $P$-reversal point which is suppressed with increasing $H$ to lower temperature with a concomitant sharpening up to $H\sim $36 T at $T$=0.6 K Furthermore, $M(H)$ curves below $T_{N}$ show saturation above the $P$-reversal magnetic field, indicating that the negative $P$ state is due to the fully ordered spin configuration. This phenomenon is similar to the case of multiferroic BiMn$_{2}$O$_{5}$, in which $P$reversal is driven by a spin-flop crossover [2]. However, in Ba$_{2}$CoGe$_{2}$O$_{7}$, $P$-reversal does not accompany a $H$ induced magnetic phase transition. We discuss possible mechanisms for this unique magnetoelectric behavior and suggest possible quantum phase transition behavior. [1] H. Yi \textit{et al.}, Appl. Phys. Lett. 91, 212904 (2008). [2] Jae Wook Kim \textit{et al.}, arXiv:0810.1907. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L30.00004: Magnetic Frustration and Magnetoelectric Coupling in CoSe$_2$O$_5$ Brent Melot, Ram Seshadri, Ambesh Dixit, Gavin Lawes, Emmanuelle Suard We present structural and magnetic measurements on CoSe$_2$O$_5$, a compound with one dimensional chains of irregular edge-shared CoO$_6$ octahedra, separated by Se$_2$O$_5$ units. Below 8.5\,K low-field magnetic susceptibility and heat capacity measurements show long range antiferromagnetic order develops. The magnetic structure of this ordered state has been determined by neutron diffraction to consist of moments aligned antiparallel along the length of the chain as well as antiparallel to neighboring chains. The magnetic ordering becomes more complex when the compound is cooled under strong fields, with highly non-linear $M-H$ behavior below the ordering temperature. Measurement of the dielectric constant and pyrocurrent has also shown exposure to fields larger than 3\,T results in a spontaneous electric polarization of the order of 1.5\,$\mu$C m$^{-2}$ which develops below the magnetic transition temperature. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L30.00005: Ferroelectricity from magnetic order Invited Speaker: Magnetic insulators with competing exchange interactions can give rise to strong fluctuations and qualitatively new ground states. The proximity of such systems to quantum critical points can lead to strong cross-coupling between magnetic long-range order and the chemical lattice. Case in point is a new class of multiferroic materials in which the magnetic and ferroelectric order parameters are directly coupled, and a magnetic field can suppress or switch the electric polarization [1]. Our neutron measurements reveal that ferroelectricity is induced by magnetic order and emerges only if the magnetic structure creates a polar axis [2-5]. Our experiments prove that the onset of ferroelectricity is described by a magneto-electric Landau theory that seems to apply for a wide range of multiferroic materials [6]. The spin dynamics and the field-temperature phase diagram of the ordered phases provide evidence that competing ground states are essential for ferroelectricity. The magneto-electric coupling, however, arises from relatively small interactions that are currently under intense investigation. \\[4pt] [1] T. Kimura et al, Nature 426, 6962 (2003).\\[0pt] [2] G. Lawes et al, Phys. Rev. Lett. 95, 087205 (2005).\\[0pt] [3] M. Kenzelmann et al, Phys. Rev. Lett. 95, 087206 (2005).\\[0pt] [4] M. Kenzelmann et al, Phys. Rev. B 74, 014429 (2006).\\[0pt] [5] M. Kenzelmann et al, Phys. Rev. Lett. 98, 267205 (2007).\\[0pt] [6] A.B. Harris, Phys. Rev. B 76, 054447 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L30.00006: Q-domains in Multiferroic CoCr$_2$O$_4$ Thomas A. Kaplan In spinel CoCr$_2$O$_4$, the observed spin state at low temperature is, approximately, a ``ferrimagnetic spiral"\footnote{N. Menyuk et al., J. de Physique \textbf{25}, 528 (1964)}, given by\footnote {D. H. Lyons et al., Phys. Rev. \textbf{126},540 (1962)} $\mathbf{S}_{n\nu} =\sin\theta_\nu[\hat{x}\cos(\mathbf{Q} \cdot\mathbf{R}_{n\nu}+\gamma_\nu)+ \hat{y}\sin(\mathbf{Q} \cdot\mathbf{R}_{n\nu}+\gamma_\nu)] +\cos\theta_\nu\hat{z}$ . $\nu=1\cdots6$ goes over the six magnetic fcc sublattices, $\mathbf{R}_{n\nu}$ are the positions of the magnetic ions, $\hat{z}$ = [001] crystallographic direction, $\theta_\nu$ are the cone half-angles on which the spins lie, and $\gamma_\nu$ are the phases of the 6 conical spirals, all with wave vector $\mathbf{Q}$ in the [110] direction. This yields magnetization $\mathbf{M},^1$ and, via the Katsura et al mechanism\footnote{H. Katsura et al., Phys. Rev. Lett. \textbf {95}, 057205 (2005)}, electric polarization~$\mathbf{P} $.\footnote{Y. Yamasaki et al., Phys. Rev. Lett. \textbf{96}, 207204 (2006)} Equivalent $\mathbf{Q}$'s, e.g. $\pm\mathbf{Q} $, with associated $\mathbf {M}$'s and $\mathbf{P}$'s, are expected to give degenerate states, ``$\mathbf{Q-M-P}$ domains"; poling in electric and magnetic fields selects a single such domain. Reversal of magnetic field then leads to $\mathbf{P}$ reversal$^{4,}$\footnote{Y. J. Choi et al, submitted for publication} and $\mathbf{Q}$ reversal$^5$. But $\mathbf{Q}\rightarrow -\mathbf{Q}$ in the equation above does not appear to give a degenerate state. I show, via the Heisenberg model and the Generalized Luttinger-Tisza method used in the prediction of the spin state,$^2$ that $\gamma_\nu\rightarrow- \gamma_\nu$ on $\mathbf{Q}$ reversal, making manifest the $\mathbf{Q} \rightarrow-\mathbf {Q}$ degeneracy. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L30.00007: Ginzburg-Landau theory for the conical cycloid state in multiferroics: Applications to CoCr$_{2}$O$_{4}$ Chuanwei Zhang, Sumanta Tewari, John Toner, Sankar Das Sarma We show that the cycloidal magnetic order of a multiferroic can arise in the absence of spin and lattice anisotropies, e.g., in a cubic material, and this explains the occurrence of such a state in CoCr$_{2}$O$_{4}$. We discuss the case when this order coexists with ferromagnetism in a so-called ``conical cycloid'' state and show that a direct transition to this state from the ferromagnet is necessarily first order. On quite general grounds, the reversal of the direction of the uniform magnetization in this state can lead to the reversal of the electric polarization as well without the need to invoke ``toroidal moment'' as the order parameter. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L30.00008: Spin-driven ferroelectricity in triangular lattice antiferromagnets $A$CrO$_2$ ($A$ = Cu, Ag, Li, or Na) Shinichiro Seki, Yoshinori Onose, Yoshinori Tokura The correlation between the dielectric and magnetic properties is investigated on the triangular-lattice antiferromagnets $A$CrO$_2$ ($A=$ Cu, Ag, Li, or Na) with the 120-degree spiral spin structure. For the $A=$ Cu and Ag compounds with delafossite structure, the ferroelectric polarization emerges with the spiral spin order, implying the strong coupling between the ferroelectricity and the spiral spin structure. For the $A=$ Li and Na compounds with ordered rock salt structure, on the other hand, no spontaneous polarization is discerned, while the clear anomaly in dielectric constant can be observed upon the transition to the spiral-spin ordered state. This feature can be ascribed to the possible antiferroelectric state induced by the alternate stacking of the Cr-spin sheet with opposite vector spin chirality. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L30.00009: Magnetoelectric coupling in a triangular lattice antiferromagnet CuCrO$_{2}$ Kenta Kimura, Hiroyuki Nakamura, Kenya Ohgushi, Tsuyoshi Kimura A triangular lattice antiferromagnet CuCrO$_{2}$ shows an out- of-plane 120\r{ } spin structure. Recently, ferroelectricity in the 120\r{ } phase has been reported for polycrystalline samples. However, no anisotropic information (e.g. direction of polarization) has been provided for lack of single-crystal measurement [1]. Therefore, we grew single crystals of CuCrO$_{2}$ and investigated their magnetic and ferroelectric properties [2]. We found that CuCrO$_{2}$ undergoes two successive magnetic phase transitions ($T_{N2} \quad \approx $ 24.2 K and $T_{N1} \quad \approx $ 23.6 K), probably into a collinear antiferromagnetic phase and then the out-of-plane 120$^{\circ}$ phase. Ferroelectric polarization within the triangular lattice plane appears only in the 120$^{\circ}$ phase. In the talk, we also discuss the magnetoelectric properties. [1] S. Seki et al., \textit{Phys. Rev. Lett.} \textbf{101}, 067204 (2008). [2] K. Kimura et al., \textit{Phys. Rev. B }\textbf{78}, 140401(R) (2008). [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L30.00010: Synthesis and properties of PbTi$_{1-x}$Ni$_{x}$O$_{3}$. Larry Buroker, Somaditya Sen, Marija Gajdardziska-Josifovska, Ying Zou, Shishir Ray, Mark Wiliamsen, Prasenjit Guptasarma Magnetoelectrics are a class of multiferroic materials with magnetic and ferroelectric properties in the same phase. These have been a subject of intense investigation due to their fascinating physical properties, and the potential for new devices. We examine here the question of whether the successful substitution of a magnetic ion into a traditional ferroelectric lattice can result in a new magnetoelectric phase. Using a sol gel technique employing metal-ion chelate complexes, we have synthesized phase pure nanoparticulate samples of PbTi$_{1-x}$Ni$_{x}$O$_{3}$ for 0$<$x$<$0.3. We report our studies of crystal structure refinement, magnetic and dielectric properties 0.3$<$T$<$300 Kelvin, microstructure studies using High Resolution TEM, optical properties and vibrational spectroscopy in this new system. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L30.00011: Impurity effects in multiferroic componds Trinanjan Datta We investigate the effect of impurities in multiferroic systems. Using an equation of motion approach for the spin dynamics of the host multiferroic compound we find that the amplitude of the spin components of the material are affected by the impurities. We model the impurities as a two-level system and focus on the regime where the impurity spins relax slowly. When the impurity strength is weak the host spins oscillate with no decay and the electric polarization survives. However as the impurity strength is increased the host spin components get damped. This in turn causes the ferroelectricity to be destroyed. Since polarization in multiferroic materials is driven by magnetic ordering we conclude that the presence of impurities is detrimental to multiferroicity. [Preview Abstract] |
Session L31: Focus Session: Spin Transport and Exchange Bias in Nanostructures
Sponsoring Units: GMAG DMP FIAPChair: Samir Lounis, Forschungszentrum Jülich
Room: 335
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L31.00001: Spin Injection and Accumulation in Metallic Lateral Spin Valves with Transparent Contacts Invited Speaker: Creation and control of spin currents is a key ingredient in spintronics, which has as a goal the use of both the spin and charge of the electron. Ferromagnetic (FM)/non-magnetic (NM) lateral spin valves are powerful devices that decouple a pure spin current from an electrical current by using a non-local geometry. We will review previous works to show how the FM/NM interface and materials control in an essential way the generation and manipulation of a spin current in non-local spin valves (NLSV). For this reason, we have studied the electrical spin injection and spin accumulation in metallic NLSV with transparent interfaces as a function of important experimental parameters such as injection current direction and magnitude, temperature, materials, and thickness. Using injected DC currents we find that the spin injection is perfectly symmetric when injecting current \textit{from} the FM or \textit{into} the FM, causing exactly the opposite spin accumulation in the NM. This provides means for a pure electrical manipulation of the spin current polarity. The change in spin accumulation with increasing injected current is produced by a temperature raise of the device due to Joule heating and confirmed by independent spin accumulation measurements as a function of temperature. Comparing experimentally measured spin accumulation in NMs with a spin-diffusion model allows us to identify the effect of surfaces on the spin diffusion length and injection efficiency, and the effect of FM electrodes on spin accumulation. These experiments have important implications for the physics of NLSV and for the development of devices based on these phenomena. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L31.00002: Large Room-Temperature Resistive Switching Behavior in Spinel Structured Nanoparticle Compacts. Tae Hee Kim, Eun Young Jang, Nyun Jong Lee, Jung-Tak Jang, Jin-Sil Choi, Jinwoo Cheon, Kyung-Jin Lee Here we report an abrupt and large bipolar switching behavior in the form of nanoparticle assembly consisting of an infinite number of monodispersed magnetic oxide single-crystallines. In the assembly of magnetite nanoparticles with size below 10 nm, we observed a room temperature current-voltage hysteresis with an abrupt and large bipolar resistive switching (switching ratio of $\sim $ 2000 {\%}). We also found that such switching behaviors can be general phenomena for nanoparticle assemblies: not limited to magnetites but also consistently observed for other kinds of spinel structured nanoparticles with different compositions. Such a huge switching phenomenon it has never been observed before in bulk powders, particularly at room temperature. Our results showed clearly that the new I-V hysteresis is dependent on the nanoparticle size, and arises from interparticle contacts. In an effort to understand and interpret the origin of the bipolar reversible switching behavior, a new theoretical model was suggested in this work. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L31.00003: Self-Assembly and Tunneling Magneto Resistance of Magnetic Nanoparticle Superlattices Chaitanya Lekshmi Indira, Concetta Nobile, Raffaella Bounsanti, Eliana D'Amone, Davide Cozzoli, Giuseppe Maruccio Template assisted self-assembly of magnetic oxide nanostructures into systematically ordered superlattices in presence of magnetic field can offer controlled interfaces and useful properties for the fabrication of magnetically engineered tunnel junctions with application in high performance magnetic random access memories. In our work we employ magnetite, an important class of half-metallic material showing super paramagnetic behavior close to room temperature and valuable coercivity at low temperatures, as nanoparticles. The self-assembly of nanoparticle superlattices on metallic non-magnetic substrates is demonstrated. Further, enhanced spin-dependent electron transport and tunneling magneto resistance in devices with crossbar geometry is discussed. \begin{enumerate} \item M. P. Pileni, J. Phys. D: Appl. Phys. 41, 134002 (2008). \item Z. M. Liao et al. Nano Lett. 6, 1087 (2006). \item K. Yakushiji et al. Nat. Mater. 4, 57 (2005). \end{enumerate} [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L31.00004: Conductive atomic force microscopy measurements of nanopillar magnetic tunnel junctions E. R. Evarts, C. Hogg, J. A. Bain, S. A. Majetich Magnetic tunnel junctions have been studied extensively for their magnetoresistance and potential uses in magnetic logic and data storage devices, but little is known about how their performance will scale with size. Here we examined the electronic behavior of 12 nm diameter magnetic tunnel junctions fabricated by a novel nanomasking process. Scanning electron microscopy images indicated feature diameter of 12 nm, and atomic force microscopy showed a height of 5 nm suggesting that unmasked regions have been milled on average to the oxide barrier layer, and areas should have the remnants of the free layer exposed with no remaining nanoparticle. Electrical contact was made to individual nanopillars using a doped-diamond-coated atomic force microscopy probe with a 40 nm radius of curvature at the tip. Off pillar we observed a resistance of 8.1 x 10$^{5}$ $\Omega $, while on pillar we found a resistance of 2.85 x 10$^{6} \quad \Omega $. Based on the RA product for this film, 120 $\Omega -\mu $m$^{2}$, a 12 nm diameter cylinder with perfect contact would have a resistance of 1.06 x 10$^{6} \quad \Omega $. The larger experimental value is consistent with a smaller contact area due to damaging the pillar during the ion milling process. The magnetoresistance characteristics of these magnetic tunnel junctions will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L31.00005: Spin filtering in transport through single-molecule magnet Mn12 Kyungwha Park, Salvador Barraza-Lopez, Jaime Ferrer, Victor Garcia-Suarez We investigate electronic transport through a single-molecule magnet Mn12 in a two-terminal set up using the Green's function method in conjunction with density-functional theory. Our transport calculation will provide crucial information on the effect of interfaces and molecular geometry on transport, and complement theoretical studies based on many-body model Hamiltonians. We consider a single-molecule magnet Mn12 bridged between Au electrodes via thiol group and alkane chains such that its magnetic easy axis is normal to the transport direction. The electrodes are treated semi-infinite and the transport calculation is performed self-consistently within density-functional theory. We present a spin-filtering effect through Mn12 and coupling strength of the Mn12 and electrodes. We also discuss the effect of additional electron correlations on the transport. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L31.00006: Antiferromagnetic exchange coupling measurements on single Co clusters W. Wernsdorfer, D. LeRoy, C. Portemont, A. Brenac, R. Morel, L. Notin, D. Mailly We report on single-cluster measurements of the angular dependence of the low-temperature ferromagnetic core magnetization switching field in exchange-coupled Co/CoO core-shell clusters (4 nm) using a micro-bridge DC superconducting quantum interference device ($\mu$-SQUID). It is observed that the coupling with the antiferromagnetic shell induces modification in the switching field for clusters with intrinsic uniaxial anisotropy depending on the direction of the magnetic field applied during the cooling. Using a modified Stoner-Wohlfarth model, it is shown that the core interacts with two weakly coupled and asymmetrical antiferromagnetic sublattices. Ref.: C. Portemont, R. Morel, W. Wernsdorfer, D. Mailly, A. Brenac, and L. Notin, Phys. Rev. B 78, 144415 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L31.00007: The exchange bias effect in Ni/NiO and NiO nanoparticles Angela Kou, Mikhail Feygenson, Lauren Kreno, Jonathan Patete, Amanda Tiano, Fen Zhang, Stanislaus Wong, Meigan Aronson We used magnetic measurements, X-ray diffraction, and HRTEM to study the exchange bias field in Ni/NiO and NiO nanoparticles made by a modified wet chemistry method. We oxidized re-dispersed powders of bare Ni nanoparticles in air at 400$^{o}$C and 900$^{o}$C. HRTEM showed that annealing at 900$^{o}$C of bare Ni nanoparticles led to the formation of exceptionally high quality NiO nanoparticles, resembling perfect bulk-like crystalline order. To our knowledge, there are no reports of NiO particles of such quality in the literature. The loop shift was 1000 Oe at 300K for the NiO nanoparticles, while it was only 120 Oe at 10K for the Ni/NiO nanoparticles. The difference is explained by the different origins of the loop shift in Ni/NiO and NiO nanoparticles. In Ni/NiO nanoparticles, the loop shift is associated with exchange interactions between ferromagnetic Ni and antiferromagnetic NiO. In NiO nanoparticles, however, the origin of the shift is an uneven number of ferromagnetic sublattices present in NiO nanoparticles, which interact differently with an applied magnetic field (Kodama, 1999). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L31.00008: Correlation between bias fields and magnetoresistance in CoPt biased FeNi/Ta/FeNi GMR heterosystems Yi Wang, S. Sahoo, W. Echtenkamp, Ch. Binek Exchange coupled magnetic hard layer (HL)/ soft layer (SL) thin films show SL biasing in close analogy to conventional exchange bias systems with antiferromagnetic pinning.$^{1}$ Here we study CoPt(35nm)/FeNi450nm/Ta(d)/FeNi450nm heterostructures with d between 0.7 and 5nm. The CoPt films have in-plane magnetic anisotropy and pin the adjacent FeNi SL films. The latter are exchange coupled from top via Ta spacer layers with FeNi in a GMR-type architecture. We use AGFM and SQUID magnetometry to study the FeNi magnetization reversal with (CoPt) and without (vacuum) pinning layer proximity. The two minor FeNi hysteresis loops of the GMR trilayer reveal different biasing effects due to the distinct exchange interaction at the respective interfaces. The FeNi/CoPt coupling is systematically tuned via a series of set fields which allow partial demagnetization of the pinning layer. Moreover we study the correlation between the overall and minor magnetization reversals and the corresponding magnetoresistance effects for various temperature between T=20 and 400K. $^{1}$Ch. Binek, S. Polisetty, Xi He and A. Berger, Phys. Rev. Lett. \textbf{96}, 067201 (2006). Financial support by NSF through Career DMR-0547887, MRSEC DMR-0820521 and the NRI. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L31.00009: From Exchange Bias to Magnetic Memory Karine Chesnel, Steve Kevan, Eric Fullerton, Matt Carey, Jeff kortright, Brian Wilcken, Joseph Nelson A better understanding and control of magnetic domain morphology and reversal processes in magnetic thin films is useful in the realm of perpendicular magnetic recording technology. We found the possibility to create magnetic domain memory in thin ferromagnetic films by inducing a spatially varying exchange coupling interactions. We evidenced this phenomenon in a perpendicular exchange bias film made of [Co/Pd] IrMn mutilayers. Our coherent X-ray magnetic scattering speckle correlation study shows that the film exhibits no memory at room temperature but acquires a very high degree of magnetic memory, above 95{\%} with subsequent field cycling when the sample is zero field cooled below the blocking temperature of the IrMn layers (T$<$275K). [1]. We present here the memory's dependency with magnetic field, temperature, and cooling conditions. We also discuss the spatial dependency by analyzing finely the local speckle correlation as a function of scattering angle, thus indicating variations of memory with different spatial scales in the domain pattern. [1] Chesnel et al, PRB, 78, 132409 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L31.00010: Detection of Bottom Electrode Oxidation in Magnetic Tunnel Junctions via Exchange Bias Effect Wei Chen, Nam Dao, Kevin West, David Kirkwood, Jiwei Lu, Stuart Wolf The oxidation of the bottom ferromagnetic (FM) electrode in a magnetic tunnel junction (MTJ) is detrimental for high tunneling magnetoresistance (TMR). This has long been a tricky problem for the fabrication of MTJs. We propose a method to detect such oxidation by measuring the exchange bias effect from the CoO/FM system if the FM surface is oxidized and CoO is formed. Along with the moderate exchange bias even more significant training effect and increased FM coercivity are observed at low temperature that depend on the oxidation level. All of these effects help in the detection of the FM surface oxidation. MTJs with MgO and vanadium oxide as tunnel barrier candidates are tested by this technique with the purpose of optimizing the barrier quality for best TMR performance. [Preview Abstract] |
Session L32: Focus Session: Nanostructured Manganites, Thin Films and Others
Sponsoring Units: GMAG DMPChair: Hariharan Srikanth, University of South Florida
Room: 336
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L32.00001: Growth and characterization of the La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ nanowires Jugdersuren Battogtokh, Sungmu Kang, Robert S. DiPietro, Donald Heiman, Andrew C. Buechele, Ian L. Pegg, John Philip Conventional electrospinning method provides a simple approach to synthesis polymer nanowires. In this work, we report the growth, structural characterization, and magnetic properties of half-metallic, ferromagnetic La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) alloy nanowires that are first time grown on Si/SiO$_{2 }$substrates by the electrospinning method. Electrospun nanowires are annealed in an ultra-high purity argon-hydrogen gas mixture. Uniform, continuous, high aspect ratio LSMO nanowires with diameters in the range of 60--300 nm and lengths up to 500 $\mu $m are grown. The temperature dependent magnetization behavior of LSMO nanowires shows ferromagnetic behavior, and symmetric hysteresis loops are observed with magnetic fields applied to the substrate at 10 K and 300 K. Finally, we will discuss the spin dependent electrical transport properties of the single LSMO nanowire. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L32.00002: Resonant soft x-ray scattering from La$_{\mbox{1-x}}$Sr$_{\mbox{x}}$MnO$_{\mbox{3}}$ quantum wire arrays Shuai Wang, Serban Smadici, James Lee, Michael Odlyzko, Xiaofang Zhai, James Eckstein, Amish Shah, Jian-Min Zuo, Peter Abbamonte, Anand Bhattacharya Any finite sized, patterned system with an energy gap is expected to have elementary excitations that are characteristic of its boundary. To test this idea we have fabricated large arrays ($>60000$ elements) of colossal magnetoresistance- phase $\mbox{La}_{\mbox{2/3}} \mbox{Sr}_{\mbox{1/3}} \mbox{MnO}_{\mbox{3}}$ quantum wires. These wires are 80 nm in width so have properties that are dominated by edge effects. We used resonant soft x-ray scattering (RSXS) and SQUID magnetometry to study their magnetic properties. We found that patterning lowers the Curie temperature and suppresses the degree of magnetization. RSXS studies show diffraction maxima from the wire period, as well as temperature-dependent diffuse scattering. We will discuss these results in the context of combined structural and magnetic disorder. Funding \#: DOE grants DE-FG02-07ER46453 and DE-FG02-06ER46285 [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L32.00003: Structure driven collapse of charge ordering in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ nanoparticles A.K. Raychaudhuri, Tapati Sarkar, E. Bozin, T. Proffen, T. Chatterji, S. Billinge High resolution X-Ray and neutron diffraction had been used to show that size reduction below a certain size ( $<$ 150nm) can lead to a collapse of the charge and orbitally ordered as well as the Antiferromagnetic ground state of the half doped manganite La$_{0.5}$Ca$_{0.5}$MnO$_{3 .}$This leads to a ferromagnetic ground state. We show that the phenomena is linked to the structural changes that accompany the size reduction. The low temperature (T$\sim $15K) structure of the nanocrystals is significantly different from that of the bulk. The structure of the nanoparticles shows a distortion albeit different from that seen in the bulk which is driven by the Jahn -- Teller distortion. The Rietveld analysis along with analysis of the Pair Distribution Function data show that there are differences in the way the MnO$_{6}$ octahedra are distorted in the bulk and the nanocrystals. We find that in the nanocrystals the structural distortion sets in at room temperature and shows very little variation on cooling. The Bragg peak of the ferromagnetic order in the nanoparticles was found to have the same indexes and approximately same d -- spacing as that seen in ferromagnetic La$_{0.67}$Ca$_{0.33}$MnO$_{3}$. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L32.00004: Jahn-Teller contribution to the magneto-optical response of ferromagnetic manganite thin films Gervasi Herranz, David Hrabovsky, Jose Manuel Caicedo, Ingrid Canero-Infante, Florencio Sanchez, Josep Fontcuberta We report on the temperature dependence of the magneto-optical response in the visible spectrum of ferromagnetic manganite thin films measured in transverse Kerr geometry. We show that this response is dominated by the usual magneto-optical Kerr effects for all temperatures except for a narrow window around the Curie temperature (T$_{C})$. Remarkably, the magneto-optical response of these manganite films does not die out near the ferromagnetic transition, in spite of the vanishing Kerr effect at those temperatures. On the contrary, the transverse Kerr response is hugely enhanced near T$_{C}$ and follows the same temperature dependence as the colossal magnetoresistance. We attribute these remarkable phenomena to the magnetic field-induced suppression of Jahn-Teller dynamical charge localization around T$_{C}$. Thus, the peculiar optical response of manganite films comes from the intricate physics of these strongly correlated electronic systems. We argue that the methodology we use is demonstrated to be very useful to understand the nature of some structural and electronic transitions driven by magnetic/electric fields or by temperature in other complex oxides. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L32.00005: Observation of Ferromagnetic Resonance in SrRuO$_{3 }$Using the Time-Resolved Magneto-Optical Kerr Effect M.C. Langner, C.L.S. Kantner, Y.H. Chu, L.W. Martin, R. Ramesh, J. Orenstein We report the observation of ferromagnetic resonance (FMR) in strontium ruthenate using the time-resolved magneto-optical Kerr effect. The FMR oscillations in the time-domain appear in response to a sudden, optically induced change in the direction of easy-axis anistropy. The large FMR frequency, 250 GHz, and damping parameter are consistent with strong spin-orbit coupling. The damping coefficient, as well as other parameters associated with the magnetization dynamics, have a non-monotonic temperature dependence, suggestive of a link to the anomalous Hall effect. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L32.00006: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L32.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L32.00008: Swiching Spectroscopy Piezoresponse Force Microscopy study of Domain Wall hysteresis at the nanoscale: Mapping lattice and defect pinning effects Vasudeva Rao Aravind, Samrat Choudhury, Yulan Li, Katyayini Seal, Stephen Jesse, Anna Morozovska, Eugene Eliseev, Long-Qing Chen, Sergei Kalinin, Venkatraman Gopalan In this work,~Using scanning probe microscopy with $\sim $10 nanometer resolution along with theoretical modeling, we demonstrate the role of 180$^{o}$ ferroelectric domain wall as an intrinsic defect that lowers coercive fields in its vicinity by an order of magnitude. The interaction of ferroelectric 180$^{o}$ domain wall with a strongly inhomogeneous electric field of biased scanning probe microscope tip is analyzed within decoupling approximation allowing for the spatial redistribution of polarization caused by the biased probe using continuous Landau-Ginzburg-Devonshire theory. Theoretical calculations predict that equilibrium shape of the initially flat domain wall boundary bends, attracts or repels from the probe apex. The bending of the wall and its depolarization electric field facilitates tip induced domain nucleation. The experiments and theory are compared quantitatively, to show that lattice friction as well as lattice pinning play important role in the domain wall softening behavior. \underline {Acknowledgements:} [1] National Science Foundation, [2] Center for Nanophase Materials Sciences, Oak Ridge National Laboratory. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L32.00009: Controlling Orthorhombic Domain Orientations in Epitaxial LaPrCaMnO Thin Films John Budai, T. Zac Ward, Jon Tischler, Jian Shen Microstructural effects such as strain and domain formation are known to influence the physical properties of transition metal oxide materials. For epitaxial films, lattice mismatch with the substrate can be used to investigate the effects due to in-plane biaxial tensile or compressive strain. Using synchrotron x-ray diffraction at the Advanced Photon Source, we have investigated the temperature-dependent lattice parameters and orthorhombic domain orientations for distorted perovskite LaPrCaMnO thin films grown on several different substrates (SrTiO3, LaAlO3, SrLaGaO4, NdGaO3). We find that structural phase transition in the substrate can have a large effect on the film. More generally, we find that tensile and compressive stresses generate different orthorhombic domain orientations and can be used to control the microstructure of the LPCMO films . [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L32.00010: Effect of the substrate on the orbital phase transition in a manganite thin film under magnetic field Y. Wakabayashi, H. Sagayama, T. Arima, Y. Nakamura, Y. Ogimoto, K. Miyano, H. Sawa Thin films of strongly correlated materials are studied intensively because of their potential of device application. Those materials in bulk form show various fascinating properties such as metal-insulator transition. However, clear phase transitions are often suppressed under the strain from the substrates. We have studied Nd$_{0.5}$Sr$_{0.5}$MnO$_3$ thin film on SrTiO$_3$ (011) substrate, which is a unique film that has clear orbital-ordering (OO) transition, by x-ray scattering under magnetic field up to 8T. As reported earlier [1], this system show three phase transitions, paramagnetic (PM), ferromagnetic (F), A-type OO (A) to CE-type OO (CE) with cooling in zero field, and at F-A transition temperature (170K), the symmetry lowers and twin occurs. The phase sequence was changed to PM, F to CE above 4T, and above this field, considerable amount of the FM phase remains down to 10K. This field induced phase separation is attributable to the martensitic accommodation strain at the domain boundary. [1]Y.W. et al., Phys. Rev. Lett. {\bf 96} 017202 (2006), J.Phys.Soc.Jpn. {\bf 77}, 014712 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L32.00011: Coherent Long Range Lateral Charge Ordering in strained Epitaxial Oxide Film Structures. Jong-Woo Kim, Philip Ryan, Jak Chakhalian, Mikhail Kareev, Jian Liu, Steve May, Anand Bhattacharya, John Freeland The quality of ordered oxide films has reached the level whereby epitaxial superlattice structures can now be achieved by both pulsed laser deposition (PLD) and ozone assisted molecular beam epitaxial (MBE) growth. Engineering each layer coupled with compressive and tensile strain with the explicit aim of controlling and or enhancing the macroscopic electrical and magnetic ordering is a considered aim of ordered oxide film growth. The question how highly strained films structurally respond to such stress is examined by synchrotron diffraction. Both LaSrMnO films grown on STO(001) by MBE and PLD grown LaNiAlO films on both LAO(001) and STO(001) have revealed coherent lateral ordering dependent upon film disorder, substrate mismatch induced strain and even dislocations induced by the substrate step morphology. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L32.00012: Dember effect induced photovoltage in perovskite \textit{p}-\textit{n} heterojunctions Kui-juan Jin, Kun Zhao, Hui-bin Lu, Leng Liao, Guo-zhen Yang An unusual and rather large transient lateral photovoltage (LPV) has been observed in La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrNb$_{0.01}$Ti$_{0.99}$O$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/Si heterojunctions under the nonuniform irradiation of pulsed laser. The irreversible LPVs on both sides of a $p-n $ junction challenge the well established model for LPV in conventional semiconductor $p-n$ junctions, which can be well explained by Dember effect. Much larger LPV is observed in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/Si than that in La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrNb$_{0.01}$Ti$_{0.99}$O$_{3}$. Similar results measured from both substrates of SrNb$_{0.01}$Ti$_{0.99}$O$_{3}$ and Si also support such a Dember effect. Much larger LPVs in heterojunctions than those in simple samples (SrNb$_{0.01}$Ti$_{0.99}$O$_{3}$ or Si) suggest a potential application of Dember effect in heterostructures. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L32.00013: Ellipsometric Study of the Electronic Band Structure of CrO$_2$ across the Ferromagnetic Transition M.K. Stewart, B. Chapler, M.M. Qazilbash, A.A. Schafgans, D.N. Basov, K. Chetry, A. Gupta, T. Tiwald We present a detailed study of the optical properties of half metallic CrO$_2$ at temperatures below, at, and above the Curie temperature. The films were grown epitaxially on (100) and (110) oriented TiO$_2$ substrates by chemical vapor deposition. Using variable angle spectroscopic ellipsometry we have extracted the optical conductivity of the films in the range from 0.06 to 6 eV. Our data reveal an important anisotropy in the films which is in good agreement with what is know about their crystal structure. The main features of the conductivity spectra in the ferromagnetic state are consistent with existing band structure calculations. However, we observe no temperature dependence of these features across the ferromagnetic transition, posing questions about the electronic structure of the material. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L32.00014: Spins and Twins: Correlation between Crystallographic and Magnetic Domains at Co/NiO(001) Interfaces Hendrik Ohldag, Elke Arenholz, Gerrit van der Laan Using soft x-ray spectromicroscopy we show that NiO(001) exhibits a crystallographic and magnetic domain structure near the surface identical to that of the bulk. Upon Co deposition a perpendicular coupling between the Ni and Co moments is observed that persists even after formation of uncompensated Ni spins at the interface through annealing. The chemical composition at the interface alters its crystallographic structure and leads to a reorientation of the Ni moments from the h112i to the h110i direction. We show that this reorientation is driven by changes in the magnetocrystalline anisotropy rather than exchange coupling mediated by residual uncompensated spins. [Preview Abstract] |
Session L33: Superconductivity: Electronic Structure I
Sponsoring Units: DCMPChair: Mike Bleiweiss, Naval Academy Preparatory School
Room: 403
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L33.00001: IR Hall measurements in overdoped $Pr_{2-x}Ce_{x}CuO_{4}$: evidence for magnon induced current-vertex corrections Gregory S. Jenkins, Don C. Schmadel, R.L. Greene, H.D. Drew, P. Fournier, H. Kontani In overdoped $Pr_{2-x}Ce_{x}CuO_{4}$, the dc Hall coefficient achieves its expected value $R_{H} \propto 1+x$ consistent with the large hole-like Fermi surface observed in ARPES, but only at low temperatures. As temperature is raised, the dc Hall coefficient falls off and becomes negative at a temperature that increases with $x$. We have measured the IR Hall angle of two overdoped $Pr_{2-x}Ce_{x}CuO_{4}$ samples at sufficiently low optical excitation energies (below 10meV) to directly probe the Fermi-surface properties. The observed large deviations from the classical result correspond to the addition of electron-like contributions to $\sigma_{xy}$, even at T=0, due to the finite frequency. Results of a model developed by H. Kontani of the low frequency IR Hall response which incorporates current-vertex corrections induced by magnon scattering are directly compared to the data. The model fully captures the salient features of the measured Hall response as a function of doping, temperature, and frequency. These results demonstrate that the anomalous Hall effect in the cuprates is a consequence of current vertex corrections to $\sigma_{xy}$. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L33.00002: Fermi surface reconstruction in e-doped cuprates: IR Hall measurements in underdoped $Pr_{2-x}Ce_{x}CuO_{4}$ D.C. Schmadel, G.S. Jenkins, H.D. Drew, R.L. Greene, P. Fournier The complex IR Hall angle is measured in PCCO at doping levels ranging from 10\% to 15\% at low optical excitation energy (10 meV). A precipitous decrease in Hall mass with a decrease in doping level in the underdoped regime is strong evidence of Fermi surface reconstruction and pocket formation, an observation consistent with ARPES and optical spectroscopy measurements. The data over the entire underdoped region is consistent with the predicted IR Hall response based upon ARPES data and Boltzmann theory. The temperature dependence of the Hall mass indicate a gradual roll-over from small pockets to the large unreconstructed Fermi surface expected in overdoped PCCO. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L33.00003: Evidence for Fermi surface reconstruction in h-doped cuprates: IR Hall measurements in underdoped $La_{2-x}Sr_{x}CuO_{4}$ H.D. Drew, G.S. Jenkins, D.C. Schmadel, R.L. Greene, Ichiro Tsukada We measure the IR Hall angle in $La_{2-x}Sr_{x}CuO_{4}$ as a function of doping ranging from 7\% to 16.5\%. The optimally doped sample is shown to be consistent with ARPES measurements. However, large deviations at low doping between the Hall mass and the IR Hall response predicted by the measured Fermi arcs in ARPES experiments is observed. The rapid decrease in Hall mass with underdoping is a hallmark signature of Fermi surface reconstruction exhibited in systems which are well known to fractionalize into Fermi pockets (underdoped PCCO). Comparisons with Fermi surface models will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L33.00004: Quantum Oscillations in the Specific Heat of Ultraclean YBCO in 45T magnetic fields Scott Riggs, Jon Betts, Suchitra Sebastian, Neil Harrison, Albert Migliori, Greg Boebinger, Ruixing Liang, Walter Hardy, Doug Bonn We report specific heat measurements of Ortho-II YBCO in a magnetic field as large as 45T to directly compare the superconducting state with the normal state at low temperatures. This thermodynamic measurement of the electronic density of states determines the total number of carrier pockets in the two-dimensional Fermi surface of Ortho-II YBCO. These measurements also reveal quantum oscillations in the specific heat that provide a bulk measurement of the quasiparticle density of states in the d-wave mixed state. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L33.00005: De Haas-Van Alphen Experiments on BaNi$_2$P$_2$ Taichi Terashima, Motoi Kimata, Hidetaka Satsukawa, Atsushi Harada, Kaori Hazama, Motoharu Imai, Shinya Uji, Hijiri Kito, Akira Iyo, Hiroshi Eisaki, Hisatomo Harima We have observed de Haas-van Alphen (dHvA) oscillations in BaNi$_2$P$_2$, which is isostructural with BaFe$_2$As$_2$ and becomes superconducting below $T_c$ ~ 3 K without doping [T. Mine et al., Solid State Commun. 147, 111 (2008)]. It is a suitable compound to study how differnet electronic structures are between iron and nickel-based superconductors. The single crystals used in the study were obtained by high- pressure synthesis. dHvA frequencies up to 8 kT were observed, and their sizes and angular dependences can be explained very well by a band-structure calculation. Effective masses are two to three times largere than the corresponding band masses, suggesting moderate mass enhancement due to electron-phonon and electron-electron interactions. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L33.00006: de Haas van Alphen Effect in Strongly Interacting Systems Lara Thompson, P.C.E. Stamp We present calculations of de Haas van Alphen (dHvA) oscillations for strongly interacting systems, for (1) systems near a quantum phase transition (QPT); and/or (2) 2D and quasi-2D systems. The standard Lifshitz-Kosevich (LK) results are then inapplicable. Near a QPT, the electronic interaction scale goes to zero, giving strong corrections to LK. In 2D, LK breaks down entirely in the presence of interactions. Recently, dHvA oscillations in high Tc systems have been measured, but their form does not yet rule out non-Fermi liquid behaviour. We calculate the expected magnetization response assuming various Fermi reconstruction scenarios. The response depends crucially on the inter-plane couplings, and we find deviations from LK if the reconstruction is interaction-driven. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L33.00007: Interpreting the Gap Signatures in the Raman Spectra of Hg-1201. James Storey, Jeffery Tallon Recently, peaks in the B1g and B2g Raman spectra of the Hg-1201 high-Tc cuprate superconductor have been interpreted in terms of two gaps. These are i) a gap near the Brillouin zone boundary that decreases monotonically with doping, and ii) a gap near the zone-diagonals that follows the dome-shaped doping dependence of the superconducting transition temperature. The former has come to be interpreted as the pseudogap and the latter the superconducting gap. However, this dome shaped superconducting gap contradicts other measurements (specific heat and infra-red spectroscopy) which suggest a simple monotonic decrease with doping. By performing calculations in which the doping dependence of the Fermi arcs is taken into account, we show that the apparent Raman B2g gap is modified by the pseudogap and that the actual superconducting gap deduced from Raman data is in fact consistent with the monotonic decrease seen in these other spectroscopies. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L33.00008: Gap structure and mean-field $T_c$ in HTS cuprates Jeffery Tallon, James Storey, John Loram We show that phase and amplitude fluctuations set in simultaneously in high-Tc cuprates and determine the mean-field transition temperature $T_c^{mf}$ which is found to increase substantially above $T_c$ in optimal and underdoped cuprates (by up to 60 or 70K). We find $\Delta/k_BT_c^{mf}$=2.5, little more than the weak-coupling BCS $d$-wave value. On the other hand the pseudogap $T^*$ has a distinct doping dependence from $T_c^{mf}$ and correlates with the pseudogap energy $E_g$. The gap structure for $\Delta$ and $E_g$ are characterised and shown to be distinct. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L33.00009: The existence of substantial Ca 3\textit{d} derived states at \textit{E}$_{F}$ in Ca-intercalated graphite superconductor CaC$_{6}$ Hiroyuki Okazaki, Rikiya Yoshida, Keisuke Iwai, Kengo Noami, Takayuki Muro, Tetsuya Nakamura, Takanori Wakita, Yuji Muraoka, Masaaki Hirai, Fumiaki Tomioka, Yoshihiko Takano, Asami Takenaka, Masahiro Toyoda, Tamio Oguchi, Takayoshi Yokoya We have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor CaC$_{6 }(T_{c}$ = 11.2 K). The valence band spectrum shows six main structures that correspond to those of calculated DOS that predicts large Ca 3$d$ contribution at the Fermi level ($E_{F})$. The Ca 2$p$ core level spectrum has a very large asymmetric line shape, suggesting the existence of Ca 3$d$ derived conduction electrons at Ca sites. These results provide spectroscopic evidence for the existence of Ca 3$d$ electrons at $E_{F}$, which probably play a crucial role for the unusually superconductivity. The electronic structure of CaC$_{6}$ is compared with the electronic structures of other graphite intercalation compounds, providing deeper understanding of the superconductivity of CaC$_{6}$. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L33.00010: Orbital excitation in Sr$_{2}$CuO$_{2}$Cl$_{2}$ resonant inelastic x-ray scattering at the Cu K pre-edge Jungho Kim, D. S. Ellis, T. Gog, D. Casa, Young-June Kim $d-d$ excitations has attracted much attention due to its fundamental importance in elucidating electronic structure. However, experimental study of these excitations is difficult, since direct optical transition is dipole forbidden. We show that the Cu \underline {1}\underline {\textit{s}}-3$d$ intermediate state, which can be reached via electric quadrupole operator, provides an excellent high-resolution means for studying $d-d$ excitations in cuprates, and complements other well established techniques. Since quadrupole operator is sensitive to the symmetry of the intermediate state, considerable information on the symmetry of $d-d$ excitation can be gained by exploiting this resonance. We have carried out comprehensive angle resolved x-ray absorption spectroscopy experiment, which clearly demonstrates the quadrupole nature of the absorption. We find that our RIXS spectra at this quadrupole resonance exhibit a broad excitation centered at 2 eV. We suggest that the scattering angle dependence of the quadrupole resonance agrees well with the calculated polarization dependence of quadrupole matrix element and information on the symmetry of $d-d$ excitation can be obtained. Our analysis show that the quadrupole resonance at 2eV is consistent with the excitation involving 3$d_{yz, zx}$ symmetry. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L33.00011: Quantum oscillation experiments on YBa$_2$Cu$_3$O$_{6.60}$ John Singleton, Ross McDonald, Susan Cox, Shiliang Li, Pengcheng Dai Pulsed magnetic fields of up to 75~T and temperatures down to 0.40~K have been used to study single crystals of YBa$_2$Cu$_3$O$_{6.60}$. The samples are measured using a MHz technique that is sensitive to small changes in penetration depth in the superconducting state, and to changes in the skin depth in the normal state. Two series of magnetic quantum oscillations are observed, periodic in inverse field; the frequencies are $590\pm 20$~T and $1990\pm 40$~T. This suggests that the predicted large Fermi surface is broken into smaller pockets due to nesting. These findings are discussed in the context of other recent observations of quantum oscillations in the cuprates, and a magnetically-mediated mechanism for superconductivity, driven by the topological mapping of the d-wave Cooper-pair wavefunction onto the antiferromagnetic fluctuations (due to Fermi-surface nesting) that are observed across the whole cuprate phase diagram [R.D. McDonald et al., J. Phys.: Condens. Matter, in press (2008)]. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L33.00012: Five Principles of Photoemission of High Temperature Superconducting Cuperates Deduced from the Dipolon Theory Ram Sharma In the process of the theoritical explanation of the observed photoemission result of high temperature superconducting cuprates we have been able to derive by means of the dipolon theory [1,2] five principles of photoemission in various situations of doping, temperature and Fermi level crossing.These five principles interrelate the peak-dip-hump phenomenon,the kink structure (including the predicted high energy kinks [3]) in the quasiparticale energy dispersion and electron velocities at different energies with respect to the charactristic dipolon excitations in the system.Details of the five principles will be presented. The theory contains Mott renormalization and all important and necessary electron-electron correlations. \\[3pt] [1] R.R. Sharma, phys. rev.{\bf63}, 054506 (2001)\\[0pt] [2] R.R. Sharma, Physica {\bf C439},47 (2006).\\[0pt] [3] R.R. Sharma,Physica C{\bf468}190 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L33.00013: Circular dichroism in angle-resolved photoemission spectrum as a technique to probe symmetry breaking in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Matti Lindroos, Ville Arpiainen, Arun Bansil We report first-principles computations of the ARPES response with circularly polarized light in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Comparison with available experimental results shows that the measured circular dichroism is reproduced within one-step computations, which include the effects of the ARPES matrix element. Employing the average orthorhombic crystal structure our study clearly establishes the sensitivity of the dichroism to experimental and structural details, and indicates that the detection of time-reversal symmetry breaking via ARPES using circularly polarized light will be complicated by the masking effects of the lattice distortions. Due to the surface sensitivity of ARPES dichroic measurements can be used to study the symmetry and quality of single crystal surfaces. \\[4pt] [1] V. Arpiainen, V. Zalobotnyy, A. A. Kordyuk, S. V. Borisenko, and M. Lindroos, Phys. Rev. B {\bf 77}, 024520 (2008) \\[0pt] [2] V. Arpiainen, A. Bansil, and M. Lindroos, Submitted to Phys. Rev. Lett. (2008) [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L33.00014: Using photoemission spectroscopy to probe a strongly interacting Fermi gas John Stewart, John Gaebler, Deborah Jin We use photoemission spectroscopy to directly probe the elementary excitations and energy dispersion of a strongly interacting Fermi gas of atoms. In these photoemission experiments, an rf photon ejects an atom from our strongly interacting system via a spin-flip transition to a weakly interacting state. This new measurement technique for ultracold atom gases, like photoemission spectroscopy for electronic materials, directly probes low energy excitations and thus can reveal excitation gaps and/or pseudogaps. We observe a back-bending of the excitation spectrum consistent with a BCS-like dispersion curve. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L33.00015: Ab initio calculation of core-valence-valence Auger spectra in closed shell systems. Gian Paolo Brivio, Guido Fratesi, Mario Italo Trioni, Simona Ugenti, Enrico Perfetto, Michele Cini We propose an ab initio method to evaluate the core-valence-valence Auger spectrum of systems with filled valence bands. The method is based on the Cini-Sawatzky theory, and aims at estimating the parameters by first-principles calculations in the framework of DFT. Photoemission energies and the interaction energy for the two holes in the final state are evaluated by performing DFT simulations for the system with varied population of electronic levels. Transition matrix elements are taken from atomic results. The approach takes into account the non spherical density of states of the emitting atom, spin-orbit interaction in core and valence, and non quadratic terms in the total energy expansion with respect to fractional occupation numbers. It is tested on two benchmark systems, Zn and Cu metals, leading in both cases to $L$23$M$45$M$45 Auger peaks within 2 eV from the experimental ones. Especially problematic is the evaluation of the hole-hole interaction for systems with broad valence bands: our method underestimates its value in Cu, while we obtain excellent results for this quantity in Zn. [Preview Abstract] |
Session L34: Focus Session: Hybrid Magnetic-Superconducting Systems
Sponsoring Units: DMP GMAGChair: Yvan Bruynseraede, Katholieke Universiteit Leuven
Room: 404
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L34.00001: Geometry and magnetic-state induced phenomena in S/F nanohybrids: unusual flux pinning effects and bistable superconductivity Invited Speaker: Superconducting/Ferromagnetic (S/F) hybrids exhibit a plethora of induced effects and novel physical properties, due to the interplay between the competing S and F orders. We will show a few examples of those, in a series of experiments on a simple hybrid system: a S thin film with an array of F nanodots. Changing the array geometry, the nanodots size or their magnetic-state allows to investigate a large variety of physical phenomena. We will focus on two of them: flux pinning effects and stray-magnetic-field induced manipulation of superconductivity. We will firstly consider \textit{geometry} induced effects; in particular, we will compare the pinning properties of periodic, quasiperiodic, and fractal arrays [1]. Secondly, we will discuss the effects induced by particular nanodot magnetic-states. We will show experiments on the interaction between flux quanta and nanodot magnetic vortices, which can be used to obtain switchable flux pinning potentials [2]. Finally, we will describe an experiment in which the magnetic reversal events of the nanodot magnetic vortices are imprinted into the transport properties of a superconducting thin film [3]. This yields a very unusual hysteretic magnetoresistance. This effect is induced by the stray magnetic fields from the nanodots, which drive the superconducting-to-normal transition of the hybrid depending on the magnetic history. \\[4pt] [1] J.E. Villegas \textit{et al}., Phys. Rev. Lett. 97, 027002 (2006). \\[0pt] [2] J.E. Villegas \textit{et al., }Phys. Rev. B 77\textbf{,} 134510 (2008). \\[0pt] [3] J.E. Villegas \textit{et al.}, Phys. Rev. Lett. 99, 227001 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L34.00002: Vortex ratchet effect induced by different magnetic configurations in magnetic superconducting hybrids. D. Perez de Lara, F.J. Castano, B.G. Ng, R.K. Dumas, E.M. Gonzalez, Kai Liu, C.A. Ross, Ivan K. Schuller, J.L. Vicent We have used E-Beam Lithography to prepare hybrid systems consisting of arrays of nanometric Ni rings (elliptical and circular)covered by a superconducting Nb film. These nanometric rings were characterized by a First Order Reversal Curve method to realize the onion and vortex magnetic state at remanence. The transport properties of the superconducting Nb film were measured in the mixed state by applying a magnetic field(H) perpendicular to the sample. Classical pinning matching effects of very high order were observed in resistance vs H, which vary with the magnetic state of Ni rings. Interestingly, a ratchet effect characterized by a dc output voltage produced by an applied ac current is found. Moreover, the ratchet effect is drastically modified by the remanent magnetic state of the Ni rings. The systematic and origin of the ratchet effect will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L34.00003: Antivortex complexes and intrinsic ratchet dynamics in superconductors with progressive magnetic topology Andras Libal, Milorad Milosevic, Francois Peeters, W. Gillijns, A.V. Silhanek, V.V. Moshchalkov Theoretically and experimentally, we analyze characteristic properties of a superconducting (Sc) film deposited on parallel arrays of ferromagnetic (Fm) dots with gradually increasing diameter in a periodic saw-tooth manner. Due to their perpendicular magnetization, dots induce vortex-antivortex molecules in the sample, with number of constituent (anti) vortices growing with magnet size. Resulting gradient of antivortex density between the dots predetermines local nucleation of superconductivity in the sample as a function of applied external field and temperature. In applied drive however, antivortices act collectively in an asymmetric potential of the dots and pinned vortices, and exhibit unique ratchet dynamics intrinsic to the Sc-Fm hybrids. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L34.00004: Vortex dynamics and vortex lattice reconfiguration in superconducting-magnetic hybrids. Jose L. Vicent, David Perez de Lara, Alejandro Alija, Elvira M. Gonzalez, Jose I. Martin, Maria Velez, Jose V. Anguita Amorphous superconducting films (Mo$_{3}$Si) have been grown on top of array of nanometric magnets. These periodic magnetic centers have been fabricated on Si substrates by Electron Beam Lithography and sputtering techniques. In the mixed state the competition between the intrinsic and random pinning potential of the superconducting film and the artificial induced periodic pinning potential governs the vortex lattice behavior. Close to critical temperature, the periodic potentials could overcome the random potentials, then the vortex lattice dynamics shows effects which are related with the array dimension and symmetry. We will show in these hybrid systems enhancements of matching effects between the vortex lattice and the array unit cell, and different vortex lattice configurations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L34.00005: Scanning Tunneling Spectroscopy Study of Proximity Effect in Bilayer Manganite/Cuprate Thin Films I. Fridman, J.Y.T. Wei, L. Gunawan, G.A. Botton Recent work has suggested novel proximity and spin diffusion effects in ferromagnet/superconductor heterostructures composed of transition-metal perovskites. We have performed scanning tunneling spectroscopy (STS) on La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bilayer thin films. Films were epitaxially grown on $<$001$>$ SrTiO$_{3}$ substrates using pulsed laser deposition with either the LCMO or YBCO layer on top. The STS data taken at 4.2 K were analyzed for spectral signatures of a pairing gap on the LCMO layer and spin diffusion in the YBCO layer, and to determine the length scale of the proximity effect and the role played by magnetic domain walls. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L34.00006: Quantum limit of the triplet proximity effect in half-metal - superconductor junctions Joern N. Kupferschmidt, B. B\'eri, C.W.J. Beenakker, P.W. Brouwer We apply the scattering matrix approach to the triplet proximity effect in superconductor-half metal structures. We find that for junctions that do not mix different orbital modes, the zero bias Andreev conductance vanishes, while the zero bias Josephson current is nonzero. We illustrate this finding on a ballistic half-metal/superconductor (HS) and superconductor/half-metal/superconductor (SHS) junction with translation invariance along the interfaces, and on HS and SHS systems where transport through the half-metallic region takes place through a single conducting channel. Our calculations for these physically single mode setups – single mode point contacts and chaotic quantum dots with single mode contacts – illustrate the main strength of the scattering matrix approach: it allows for studying systems in the quantum mechanical limit, which is inaccessible for quasiclassical Green's function methods, the main theoretical tool in previous works on the triplet proximity effect. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L34.00007: Re-entrant resistance in mesoscopic superconductor-ferromagnet-superconductor structures Madalina Colci, Martin Stehno, Dale Van Harlingen We report measurements of the resistance as a function of temperature and magnetization alignment in hybrid structures consisting of superconducting electrodes connected by two ferromagnetic nanowires separated by less than a superconducting coherence length. It has been predicted that such structures could exhibit a supercurrent due to Cooper pair splitting and coherent transport through the ferromagnets. Although we have not observed a zero-voltage supercurrent, we find that as the temperature is lowered below the critical temperature of the superconductor, the resistance of the structure shows a minimum and then rises, suggestive of re-entrant behavior. The resistance of the antiparallel alignment of the magnetization of ferromagnetic wires is found to be lower than in the parallel case just below Tc but becomes distinctly larger than in the parallel case at the lowest temperature. We discuss possible explanations and implications of this result. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L34.00008: Ferromagnets without inversion symmetry - room for superconductivity? Andriy Nevidomskyy, Jacob Linder, Asle Sudb{\o} Motivated by the recent discoveries of ferromagnetic and non-centrosymmetric superconductors, we present a mean-field theory [1] for a superconductor that \textit{both} lacks inversion symmetry and displays ferromagnetism, a scenario which is believed to be realized in UIr under applied pressure [2]. We study the interplay between the order parameters to clarify how superconductivity is affected by the presence of ferromagnetism and spin-orbit coupling. We find that the spin-orbit coupling seems to enhance both ferromagnetism and superconductivity in both singlet and triplet channels. We discuss our results in the context of the heavy fermion superconductor UIr and analyze possible symmetries of the order parameter. \\[3pt] [1] J. Linder, A. H. Nevidomskyy, and A. Sudb{\o}, Phys. Rev. B {\bf 78}, 172502 (2008). \\[0pt] [2] T. Akazawa et al., J. Phys. Cond. Mat. {\bf 16}, L29 (2004); J. Phys. Soc. Jpn. {\bf 73}, 3129 (2004). [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L34.00009: Inhomogeneous superconducting states and Umklapp processes in ferromagnet/superconductor nanostructures M.G. Khusainov, N.G. Fazleev, Yu.N. Proshin A new boundary-value problem is derived for a ferromagnetic metal/superconductor (F/S) nanostructure assuming that superconductivity in a structure is characterized by a superposition of the BCS pairing with zero total momentum in the S layers and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing with nonzero 3d coherent momentum \textbf{k} in the FM layers. The processes of mutual transformation between the BCS and FFLO pairs at the S/FM boundary are shown to be the Umklapp processes during which the excess coherent pair momentum k becomes confined in the F layer. As a result, significantly new boundary conditions are introduced for Eilenberger equations. The interplay between the BCS and 3D FFLO states is explored in F/S bilayer, F/S/F trilayer, and F/S superlattice. Two $\pi $ phase superconducting states with electron-electron repulsion in the F layers are predicted in F/S/F trilayers. Two $\pi $ phase magnetic states (0$\pi $ and $\pi \pi )$ are also predicted in F/S superlattices. Theoretical results are used to explain the data on measurements of Tc in Gd/La superlattice. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L34.00010: Splitting of a Cooper pair by a pair of Majorana bound states Johan Nilsson Majorana fermions are spatially localized superpositions of electron and hole excitations in the middle of a superconducting energy gap. These unusual particles have been predicted to occur at the interface between a magnetic and a superconducting electrode, in contact with a topological insulator (such as a BiSb crystal or a HgTe quantum well). A single qubit can be encoded nonlocally in a pair of spatially separated Majorana fermions. We have discovered a unique experimental signature of the nonlocality, involving the injection of an electron into one bound state followed by the emission of a hole by the other bound state (equivalent to the splitting of a Cooper pair over the two states). [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L34.00011: Sign reversal of ac Josephson current in a ferromagnetic Josephson junction Shin-ichi Hikino, Michiyasu Mori, Saburo Takahashi, Sadamichi Maekawa It is known that in a superconductor/insulator/superconductor (SIS) junction, when a finite voltage is applied, the Josephson current shows a logarithmic divergence, i.e., the so-called Riedel peak(RP) at the gap voltage, $V$=2$\Delta $/e, ($\Delta $ is a superconducting gap). In a double barrier Josephson junction such as SXS junction, on the other hand, the voltage dependence of $I_{c}$ has not been investigated so far, where X is a normal metal (N) or a ferromagnet (F). We study the voltage dependence of Josephson critical current ($I_{c})$ in a variety of SXS junctions. In a SNS junction, $I_{c }$shows the RP at the gap voltage similar to a SIS junction. On the other hand, in a SFS junction, $I_{c}$ shows a damped oscillation with the alternation of sign as a function of thickness ($d)$ of F due to 0-$\pi $ transition. The RP exhibits a strong dependence on $d$, and changes its sign. It is predicted that the RP disappears at the 0-$\pi $ transition in the SFS junction. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L34.00012: Josephson junctions with a synthetic antiferromagnetic interlayer Mazin A. Khasawneh, William P. Pratt Jr., Norman O. Birge We have measured the critical current of Josephson junctions of the form Nb/Co/Ru/Co/Nb, where the two Co layers are exchange-coupled antiferromagnetically by the thin (0.6 nm) Ru interlayer. The antiferromagnetic coupling causes nearly complete cancellation of the intrinsic magnetic flux produced by the Co domains, and allows us to study large-area junctions with total Co thicknesses ranging from 2 to 20 nm -- four times thicker than in previous studies of Nb/Co/Nb Josephson junctions [1]. The dependence of the critical current on an in-plane external magnetic field results in a nearly perfect Fraunhofer pattern, due to the intrinsic flux cancellation. The junctions were fabricated by sputtering the S/F/N/F/S multilayer onto a Si substrate, followed by subtractive patterning by photolithography and ion milling into circular junctions ranging in diameter from 10-80 microns. The critical current density of the junctions decays exponentially with Co thickness, with a characteristic decay length of $\xi _{F}$ = 2.2 nm. There is no sign of a crossover to a slower decay at large Co thicknesses, which, if observed, might be a signature of the predicted long-range spin triplet state [2]. [1] Robinson et al., Phys. Rev. Lett. 68, 177003, 2006. [2] Bergeret et al., Rev. Mod. Phys. 77, 1321, 2005. [Work Supported by US DOE under grant DE-FG02-06ER4634] [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L34.00013: SFS Josephson Junctions using PdNi alloy Trupti Khaire, William P. Pratt, Jr., Norman O. Birge We have studied the variation of critical current in Superconductor/Ferromagnet/Superconductor (S/F/S) Josephson Junctions as a function of ferromagnet thickness (d$_{F})$ using a weakly ferromagnetic alloy, Pd$_{82}$Ni$_{12}$. The critical current density oscillates and decays over five orders of magnitude as d$_{F}$ is increased from 32 to 100 nm. These oscillations are indicative of 0-$\pi $ transitions in S/F/S junctions. We find the characteristic length of oscillation ($\xi _{F2} )$ to be 4.3 $\pm $ 0.1 nm and the characteristic length of decay ($\xi _{F1} )$ to be 7.9 $\pm $ 0.4 nm. Earlier studies [1] using a similar PdNi alloy in S/I/F/S junctions found $\xi _{F1} \approx \xi _{F2} \approx 2.8{\kern 1pt}nm$, however, those measurements were performed for d$_{F}$ between 4.5 and 14 nm. In our experiment, $\xi _{F1} >\xi _{F2} $, indicating that our samples are in the regime $E_{ex} \tau >\hbar $ [2, 3], where E$_{ex}$ is the exchange energy and $\tau $ is the mean free time between electron collisions in the ferromagnet. In spite of covering this wide range, we see no evidence of a crossover to a slower decay, which, if present, would be indicative of long-range spin triplet correlations [4]. [1] T. Kontos et al.,Phys. Rev. Lett. 89, 137007 (2002). [2] F. S. Bergeret, et al., Phys. Rev. B, 64, 134506 (2001) [3] Kashuba, et al., Phys. Rev. B. 75, 132502 (2007). [4] F.S. Bergeret, et al., Rev. Mod. Phys. 77, 1321 (2005). [This work is supported by US-DOE grant, DE-FG02-06ER46341.] [Preview Abstract] |
Session L35: Density Functional Theory: Methods
Sponsoring Units: DCOMPChair: Steven Louie, University of California, Berkeley
Room: 405
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L35.00001: Improvement of C$_{60}$'s calculated electron-phonon coupling using hybrid functional Jonathan Laflamme Janssen, Michel C\^ot\'e, Steven G. Louie, Marvin L. Cohen Superconductivity in doped C$_{60}$ crystals is generally admitted to be phonon mediated. However, the electron-phonon coupling calculated within density functional theory do not agree with measured values, in contrast to others phonon related properties. This discrepancy hasn't been understood yet. Up to now, only calculations using the local density approximation for the exchange-correlation functional (LDA) were performed. In this study, we demonstrate that using exact-exchange functionals increases the calculated couplings, bringing it closer to experiment, while others properties, such as geometry and phonon frequencies, are little affected. We investigate how such an improvement is possible while little change is seen on more commonly calculated properties. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L35.00002: Complex band structure of a metallic nanowire under plane-wave nonlocal pseudopotential Hamiltonian and non equilibrium quantum transport calculations Maia G. Vergniory, Lin-Wang Wang We present ab initio calculations of the complex band structure of a copper and gold nanowire with a nonlocal plane-wave pseudopotential Hamiltonians. This new method allows us to calculate the evanescent states exactly using plane waves of any metallic electrode. The calculation of the evanescent states is important for quantum transport calculations when the transmisson energy is close to a band structure minimun. Nonlocal pseudopotential effects are introduced using the Kleinman-Bylander implementation. Using the method in Ref[1], where the quantum transport is calculated by means of the exact scattering states using plane waves basis set, we have calculated the nonequilibrium transmission coefficient and conductivities of a di-thiol-benzene (DBT) and other molecules connected by two Cu or Au nano wires. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L35.00003: Towards switchable carbon nanotube interconnects Nicolas Poilvert, Nicola Marzari Carbon nanotubes have attracted much attention both in theory, computation and experiment for the past fifteen years. Despite synthetic challenges those unique quasi-one dimensional systems remain one of the preferred components of future electronic devices beyond silicon technology. We explore here the use of organic addends to functionalize carbon nanotubes and tailor and engineer their conducting properties. Once functionalized metallic nanotubes can become insulators if sp$^2$ to sp$^3$ rehybridization takes place. We explore here with first-principles calculations a number of organic addends that could lead to switchable rehybridization, allowing for real-time on/off control of the conductance. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L35.00004: Hybrid Density Functional Studies of $\delta $-Pu Raymond Atta-Fynn, Asok Ray Hybrid density functionals, which replaces a fraction of a density functional theory exchange with exact Hartree-Fock (HF) exchange, have been used to study the structural, magnetic, and electronic properties of $\delta $-Pu. The fractions of exact Hartree-Fock exchange used were 25{\%}, 40{\%}, and 55{\%}. Compared to the pure PBE functional, the lattice constants expanded with respect to the experimental value when the PBE-HF hybrid functionals were applied. For pure PBE and hybrids functionals with HF exchange amounts of 25{\%} and 40{\%}, the ground state structure was anti-ferromagnetic, while for 55{\%} HF contribution the ground state was non-magnetic. The $5f$ electrons tend to exhibit slight delocalization or itinerancy for the pure PBE functional and well-defined localization for the hybrid functionals, with the degree of 5$f$ electron localization increasing with the amount of HF exchange. Overall, the performance of the hybrid density functionals do not seem superior to pure density functionals for $\delta $ --Pu. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L35.00005: Application of Hybrid Functionals to Semiconductor Surfaces Manish Jain, James Chelikowsky, Steven Louie Hybrid functionals within Generalized Kohn Sham formalism have been shown to give good band gaps for semiconductors and small band gap insulators, when a particular mixing of nonlocal exchange is included. We explore the use of hybrid functionals for semiconductor surfaces. Semiconductor surfaces often have multiple band gaps - the bulk-state gaps and surface-state gaps - which can be quite different. This study examines the applicability of hybrid functionals for such systems. In particular, we focus on diamond and silicon and their (100) and (111) surfaces, where surface states exist and are known to have lower band gaps. We employ the hybrid functionals - PBE0, HSE and B3LYP - to examine the structural and electronic properties of these surfaces. \\[0pt] This work was supported by NSF under DMR-0551195 and DMR07-05941, and the US DOE under DE-FG02-06ER46286, DE-FG02-06ER15760 and DE-AC02-05CH11231. Computer time was provided by NERSC and Teragrid. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L35.00006: An LDA+$U$ study of the photoemission spectra of ground state phase of americium and curium Md Islam, Asok Ray We have investigated the photoemission spectra and other ground state properties such as equilibrium volume and bulk modulus of dhcp americium and the density of states and magnetic properties of dhcp curium using LDA+$U$ method. Our calculations show that spin polarized americium is energetically favorable but spin degenerate configuration produces experimental quantities much better than that calculated using spin polarized configuration. The DOS calculated using LDA+$U$ with both non-magnetic and spin polarized configurations is compared and the non-magnetic DOS is shown to be in good agreement with experimental photoemission spectra when U=4.5 eV. In spin polarized case, $U$ is observed to increase the splitting between occupied and unoccupied bands by enhancing Stoner parameter. The results are shown to be in good agreement with that calculated using dynamical mean field theory for these two heavy actinides. For curium, exchange interaction appears to play the dominant role in its magnetic stability. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L35.00007: Tackling localized $d$-states: a systematic investigation by \textit{GW}@LDA+\textit{U} Hong Jiang, Ricardo I. Gomez-Abal, Patrick Rinke, Matthias Scheffler First-principles modeling of systems with localized $d$-states is currently a great challenge in condensed matter physics. Density-functional theory (DFT) in the standard local-density approximation (LDA) proves to be problematic. This can be partly overcome by including local Hubbard $U$ corrections (LDA+$U$), but itinerant states are still treated on the LDA level. Many-body perturbation theory in the $GW$ approach offers both a quasiparticle perspective (appropriate for itinerant states) and an exact treatment of exchange (appropriate for localized states), and is therefore promising for these systems. Here we present a systematic investigation of the $G_0W_0$ method based on LDA+$U$ ($G_0W_0@$LDA+$U$) for a series of prototype systems: 1) ZnS with semicore $d$-states, 2) ScN and TiO$_2$ with empty $d$-states and 3) late transition metal oxides (MnO, FeO, CoO and NiO) with partially occupied $d$-states. We show that for ZnS, ScN and TiO$_2$, the $G_0W_0$ band gap only weakly depends on $U$, but for the other transition metal oxides the dependence on $U$ is as strong as in LDA+$U$. These different trends can be understood in terms of changes in the hybridization and screening. Our work demonstrates that $G_0W_0@$LDA+$U$ with ``physical'' values of $U$ provides a balanced and accurate description of both localized and itinerant states. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L35.00008: LDA+DMFT Charge Self-consistency Applied to Yb Valence Transition Erik Ylvisaker, Warren Pickett, Andrew McMahan, Jan Kunes Elemental ytterbium metal is known to undergo a gradual transition from a divalent $spd^2f^{14}$ state to a trivalent $spd^3f^{13}$ state in a pressure range of 0 to 34 GPa. We present LDA+DMFT studies of this transition, comparing three impurity solvers (Hirsch-Fye QMC, continuous time QMC and Hubbard I) with each other and with experimental data. All Yb states of interest are kept; no downfolding to a minimum basis is done. This application of DMFT (especially the QMC solvers) to the correlated f-orbitals gives reasonable agreement with the experimental transition. However, the neglect charge self-consistency is questionable for a valence transition where the concentration of valence electrons changes. Therefore we generalize the procedure and compare and contrast LDA+DMFT results with and without charge self-consistency for Yb using the Hubbard I impurity solver. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L35.00009: Computation of phonon spectra from density-functional perturbation theory in the projector augmented-wave approach Marc Torrent, Francois Jollet, Christophe Audouze, Xavier Gonze The density-functional perturbation theory expressions have been derived within the projector augmented-wave formalism (PAW) and compared to those found in the ultrasoft pseudopotential framework~[1]. They have been recently implemented in the \textsc{abinit} package~[2] in the case of perturbations of the atomic-displacement type. We summarize the key points of this implementation: The variational and non-variational forms of the 2nd-order total energy changes are detailed. The resolution of the variational principle by a generalized Sternheimer equation is explained (the 1st-order wave-function change is found with a band-by-band conjugate gradient algorithm). We focus on some difficulties: metallic electronic occupations, response to incommensurate perturbations of periodic systems {\ldots} Results on pure compounds are presented; a comparison with results from pseudopotentials approach is performed in order to highlight the effect of the PAW methodology and its accuracy. [1] Audouze, Jollet, Torrent and Gonze. Phys. Rev. B \textbf{73}, 235101 (2006); \textbf{78}, 035105 (2008) [2] \textit{http://www.abinit.org}. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L35.00010: eliminating the V-representability problem via coarse-graining Paul Lammert The mathematical foundations of density functional theory remain in an incomplete state, with old nagging problems and questions. I propose a coarse-grained approach to eliminating the V-representability problem and present results on differentiability of the Lieb functional. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L35.00011: Density Functional Theory and Semiclassical Methods Peter Elliott, Donghyung Lee, Attila Cangi, Kieron Burke In this work we explore the relationship between semiclassical methods and density functional theory. There is a rich history between the two, for example, the gradient expansion approximation (GEA), on top of which all common generalized gradient approximations (GGAs) are built, may be derived semiclassically. However methods like Thomas-Fermi and the GEA miss important contributions from quantum oscillations, such as shell structure. In Ref. [1] we showed why these are missing and how, for a simple system, one could derive them . This led to approximations to the density and kinetic energy density which were non-local functionals of just the external potential. Interpreting these in the context of DFT, allows us to understand current approximations and improve them. In fact the potential scaling (re-)introduced in Ref. [1] can be used to derive new exact conditions on the universal functional of DFT and its components. This talk will discuss both the development of potential functionals and how they can be used to understand DFT.\\[0pt] [1] P. Elliott, D. Lee, A. Cangi, and K. Burke, Phys. Rev. Lett. {\bf 100}, 256406 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L35.00012: Correlation-Kinetic Contributions in the Mapping to Model Noninteracting Fermion and Boson Systems Xiaoyin Pan, Viraht Sahni In the mapping from a system of electrons in an external field $\vec{\cal{F}}^{ext} = - \nabla v(\vec{r})$ to one of noninteracting fermions or bosons in their ground state with equivalent density $\rho (\vec{r})$, electron correlations due to the Pauli principle, Coulomb repulsion, and Correlation-Kinetic effects must be accounted for. Via Quantal Density Functional Theory$^{\dag}$ (QDFT), it is proved that the contributions due to the Pauli principle and Coulomb repulsion to either mapping are the same. The application to atoms of the QDFT mapping to the model fermion system shows the Correlation-Kinetic energy contribution to be a very small fraction of the electron-interaction energy. In contrast, the same application of the QDFT mapping to the model boson system shows the corresponding Correlation-Kinetic energy to be a substantial fraction of the electron-interaction energy. Thus, whereas Correlation-Kinetic effects are insignificant in the mapping to the fermionic system, they play a significant role in the mapping to the model system of bosons. \\ $^{\dag}$Quantal Density Functional Theory, Springer-Verlag, 2004 [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L35.00013: DFT-MD simulations of shocked Xenon Rudolph J. Magyar, Thomas R. Mattsson Xenon is not only a technologically important element used in laser technologies, jet propulsion and dental anesthesia, but it is also arguably the simplest material in which to study the metal-insulator transition at high pressure. Because of its closed shell electronic configuration, Xenon is often assumed to be chemically inert, interacting almost entirely through the van der Waals interaction, and at liquid density, is typically modeled well using Leonard-Jones potentials. However, such modeling has a limited range of validity as Xenon is known to form compounds at normal conditions and likely exhibits considerably more chemistry at higher densities when hybridization of occupied orbitals becomes significant. In this talk, we present DFT-MD simulations of shocked liquid Xenon with the goal of developing an improved equation of state. The relative importance of the van der Waals interaction compared to other Coulomb interactions is considered, and estimates of the relative accuracy of various density functionals are quantified. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L35.00014: Evaluation of Magnetic Moments using Bader Analysis Tomoharu Shikauchi, Kazuo Tsumuraya Evaluation of the magnetic moments in solids is crucial in the computational physics. The moments have been calculated by an atomic sphere approximation or a Voronoi polyhedron approximation. There has been a method to partition the space with the minimum electron charge density surface, called zero flux planes, around each atom. The space is called Bader region. We apply the method to calculate the local magnetic moments of each atom depending on their circumstance using the first principle electronic structure calculation. We obtain the moments from the Bader charges using the up-spin charge and the down-spin charges. We apply the validity of this scheme to the analyses of the spin moments in Fe-N compounds, fcc Fe, and bcc Fe crystals and compare them with the experimental values. For Fe$_4$N, the difference of the moments between Fe(I) and Fe(II) atoms has been larger than that of the Voronoi method and is better agreement with the experimental values than the Voronoi method. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L35.00015: Layered dilute magnetic semiconductors: A dynamical mean field study Majid Nili, Unjong Yu, Juana Moreno, Mark Jarrell We study ferromagnetism of layered dilute magnetic semiconductors within the Dynamical Mean Field Approximation. Our approach includes the spin-orbit coupling in the host compound and the interaction between the magnetic ions and the itinerant carriers using a modified double-exchange coupling. We simulate heterostructures with different distributions of magnetic ions: uniform doping, delta-doping in one single layer and delta-doping in two layers. We investigate the magnetic properties by changing the hole filling as well as the magnetic doping and the position of the doped layers in the heterostructure. We also include the on-site attraction potential between the magnetic ions and charge carriers to show its effect on the formation of the impurity band. We find that the ferromagnetic transition temperature and other properties strongly depend on the distribution of magnetic ions. [Preview Abstract] |
Session L36: Panel Discussion: APS Publishing: Present and Future
Room: 408
Tuesday, March 17, 2009 2:30PM - 4:00PM |
L36.00001: Panel Discussion: APS Publishing, Present and Future This panel discussion will consist of a set of short presentations covering issues relevant to publication in the APS journals, including the concept of ``significant'' new physics, followed by questions and comments from the audience. The editors will comment on the identification of ``significance'' and its role in the editorial process, past, present, and future. There will be a brief ``State of the Journals'' address by APS Editor-in-Chief, Gene Sprouse, and statements of key issues facing the journals, by their editors. There will also be an introduction to the new APS publication Physics by its Editor, David Voss. The questions and comments section will be moderated by the Editor-in-Chief. [Preview Abstract] |
Session L37: Focus Session: Spectroscopic Probes of Biomolecular Structure and Function III
Sponsoring Units: DCPChair: David Pratt, University of Pittsburgh
Room: 409
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L37.00001: UV Resonance Raman Discovery of Gibbs Free Energy Landscape for Protein Alpha Helix Folding Invited Speaker: We developed a powerful method to follow the evolution of secondary structure in the amide peptide bonds of peptides and proteins. UV Raman excitation into these $\sim $200 nm electronic transitions results in the enhancement of the amide vibrations of the peptide backbone. In our most recent studies we reassigned the amide III region and found a particular band (the amide III$_{3}$ band) which reports selectively on the Ramachandran $\Psi $ angle and the state of peptide bond hydrogen bonding. We demonstrate that this band is Raman scattered independently by each peptide bond with insignificant coupling between peptide bonds. We also show that isotope editing of a peptide bond (by replacing the C$_{\alpha }$- H with C$_{\alpha }$- D) allows us to determine the frequency of an individual peptide bond within a peptide or protein which gives us its $\Psi $ angle. Consideration of the Boltzmann equilibria allows us to determine the $\Psi $ angle energy landscape which connects secondary structure conformations. The $\Psi $ angle coordinate is the most important reaction coordinate required to enable the understanding of the mechanism(s) of protein folding. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L37.00002: IR/UV laser spectroscopy of secondary structures in isolated peptide chains: an original insight onto the non-covalent interactions that shape proteins Invited Speaker: Protein shape and flexibility is governed by non-covalent interactions that need to be accurately described by molecular structure simulations. However, the computational methods adapted for large molecules like proteins have difficulties to reproduce precisely these non-covalent interactions and cannot reach the level of structural details required to address many biological relevant issues. Special efforts are currently done to document these interactions by carrying out experiments on simple model systems\footnote{J.-P. Schermann, Spectroscopy and modeling of biomolecular building blocks. (Elsevier, 2007).} in order to help the development of reliable DFT-D calculations\footnote{P. Jurecka, J. Cerny, P. Hobza, and D. R. Salahub, J. Comput. Chem. \textbf{28} (2), 555 (2007); S. Grimme, J. Comput. Chem. \textbf{27} (15), 1787 (2006).} and polarisable force fields explorations.\footnote{N. Gresh, G. A. Cisneros, T. A. Darden, and J. P. Piquemal, Journal of Chemical Theory and Computation \textbf{3} (6), 1960 (2007).} The selectivity of the IR/UV double resonance spectroscopy of gas phase isolated peptides (less than 10 residues) laser desorbed and cooled in a molecular beam enables us to discriminate the spectral signature of the several H-bonds shaping the molecular conformation. This can be carried out for each conformation detected in the molecular beam, providing a precise IR fingerprint of the intramolecular \textbf{hydrogen bond network} of these molecules\footnote{W. Chin, F. Piuzzi, I. Dimicoli, and M. Mons, Phys. Chem. Chem. Phys. \textbf{8}, 1 (2006); W. Chin, M. Mons, J.-P. Dognon, F. Piuzzi, B. Tardivel, and I. Dimicoli, Phys. Chem. Chem. Phys. \textbf{6}, 2700 (2004); E. Gloaguen, F. Pagliarulo, V. Brenner, W. Chin, F. Piuzzi, B. Tardivel, and M. Mons, PCCP \textbf{9}, 4491 (2007).}$^{\mbox{ and references therein}}$ These IR frequencies are directly compared to the calculated frequencies of selected conformations of the isolated molecule for assignment purpose. Once the experimentally observed conformations are known, the accuracy and the predictability of several computational methods can then be assessed through their ability to provide structures for each conformation that are both geometrically and energetically in accordance with the experimental results. In addition to H-bonds, other weaker non-covalent interactions such as NH-$\pi $, $\pi -\pi $ or $\pi $-CH$_{3}$ are also at play in these systems. They can indeed impact the H-bond network in a measurable way, which makes the investigation of these computationally challenging weak forces also accessible to this powerful experimental technique. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L37.00003: Inhibition of urinary calculi -- a spectroscopic study Felicia Manciu, Jayesh Govani, William Durrer, Layra Reza, Luis Pinales We present multi-technique spectroscopic investigations by Raman, infrared absorption, X-ray photoelectron spectroscopy (XPS), and photoluminescence on the effects of the herb \textit{Rotula Aquatica Lour (RAL)} on the growth of synthetically prepared Mg-based calculi of similar composition to common urinary calculi. Three samples were prepared; one MgPO$_{4}$-based standard and two others, separately incorporating 1 wt.{\%}~and 2 wt.{\%} \textit{RAL} herbal extract. Raman and infrared data show a newberyite structure for the crystals without and with inhibitor. XPS revealed the unexpected presence of Zn and a significant increase in Mg in the samples with \textit{RAL} inhibitor. The presence of metallic Zn may contribute to the inhibition process by initiating rapid stone formation. XPS and Raman results also suggest another mechanism of inhibition by revealing evidence for Mg-O bonding between the plant extract and the phosphate units of urinary calculus. Similarity between our photoluminescence measurements and those of \textit{in vivo} \textit{chlorophyll a} provides additional evidence of Mg-related inhibition. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L37.00004: Application of a Newly Built Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectrometer to Study Biomolecules in the Gas Phase Ryan Bird, David Pratt, Justin Neill, Brooks Pate Chirped-pulse Fourier Transform Microwave (CP-FTMW) spectroscopy is an exciting new technique that makes possible the recording of the complete microwave spectrum of a gas phase sample using a single 1 $\mu$s pulse.\footnote[2]{G.~G.~Brown et al. \textit{J.~Mol.~Spectrosc.} \textbf{238}, 200-212 (2006).} In this report, we will describe the recent introduction of a laser ablation nozzle for the study of small biomolecules using this technique. Potential applications to samples such as nucleic acid base pairs and small polypeptides will also be described. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L37.00005: Optical Conformational Transition Pathways of DsRed, Elucidated by Polarization-Modulated Fourier Imaging Correlation Spectroscopy Andrew Marcus, Eric Senning, Geoffrey Lott, Michael Fink This work presents a novel `phase-selective' approach to fluorescence fluctuation spectroscopy that simultaneously determines the joint probability distributions and two-dimensional spectral densities of protein conformational transitions, and nanometer center-of-mass displacements. Fourier imaging correlation spectroscopy (FICS) combines polarization- and intensity-modulated photo-excitation with phase-sensitive signal detection to monitor the collective coordinate fluctuations from a large population of fluorescent molecules (N $\sim $ 106). FICS is based on the principle that fluctuations of partially averaged molecular coordinates can be monitored through variations of an optical signal phase. Experiments are performed on DsRed, a tetrameric complex of fluorescent protein subunits, derived from a reef-building coral. Thermally induced conformational transitions of the DsRed complex lead to fluctuations in the optical dipolar coupling between adjacent chromophore sites. An analysis of polarization-resolved FICS fluctuation data, in terms of two-dimensional spectra and joint probability distributions, provides detailed information about cooperative `transition pathways' between distinct dipole-coupled DsRed conformations. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L37.00006: Insights on the Structural Details of Endonuclease EcoRI-DNA Complexes by Electron Spin Resonance Jessica Sarver Pulsed electron spin resonance (ESR) was used to probe the binding specificity of EcoRI, a restriction endonuclease. Using site-directed spin labeling, a nitroxide side chain was incorporated into the protein, enabling the use of ESR to study structural details of EcoRI. Distance measurements were performed on EcoRI mutants when bound to varying sequences of DNA using the Double Electron-Electron Resonance experiment. These distances demonstrated that the average structure in the arm regions of EcoRI, thought to play a major role in binding specificity, is the same when the protein binds to different sequences of DNA. Also, it was determined that the arms exhibit higher flexibility when bound to sequences other than the specific sequence due to the larger distance distributions acquired from these spin labeled complexes. Molecular dynamics (MD) simulations were performed on the spin-label-modified specific EcoRI-DNA crystal structure to model the average nitroxide orientation. The distance distributions from MD were found to be narrower than experiment, indicating the need for a more rigorous sampling of the nitroxide conformers in silico. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L37.00007: 1D and 2D-IR spectroscopy of blended polymer-porphyrin thin films Aaron Massari, Audrey Eigner, Patrick Konold One and two-dimensional IR spectroscopies are used to study the static and dynamic environments that form when ruthenium(II)tetraphenylporphyrin carbonyl is blended with regioregular poly(3-hexylthiophene). The 1D-IR spectra of the Ru-bound CO symmetric stretch indicate the development of several inhomogeneously broadened microenvironments as the concentration of porphyrin is increased. Transmission electron microscopy is used to characterize the blended films, which show evidence of phase-segregation. By correlating the degree of separation with the relative proportions of each component of the 1D-IR spectrum, we identify the IR spectra corresponding to the free and aggregated porphyrin-CO stretches. 2D-IR vibrational echo spectroscopy is then used to measure the ultrafast dynamics that are present in the polymer and porphyrin phases. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L37.00008: Microtubule-associated-protein (MAP) Tau Regulates the Number of Protofilaments in Microtubules: A Synchrotron X-ray Scattering Study Myung Chul Choi, Uri Raviv, Herbert Miller, Michelle Gaylord, Erkan Kiris, Donovan Ventimiglia, Daniel Needleman, Mahn Won Kim, Leslie Wilson, Stuart Feinstein, Cyrus Safinya Microtubules (MTs), 25 nm protein nanotubes, are a major filamentous element of the cytoskeleton involved in intracellular trafficking and cell division. MAP tau regulates tubulin assembly and MT stability. Dysfunction of tau has been correlated with numerous neurodegenerative diseases. We describe our recent findings about the effects of six naturally occurring central nervous system (CNS) tau isoforms on the assembly structure of taxol-stabilized MTs, using synchrotron small angle x-ray scattering (SAXS). Most significantly, we found that tau, at low binding density, regulates the distribution of protofilament numbers in MTs. DOE DE-FG02-06ER46314, NSF DMR-0803103, NIHI RO1-NS35010 and NS13560. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L37.00009: ESR distance measures show that Cu$^{2+}$ coordinates to Histidine 114 in the EcoRI-DNA complex Zhongyu Yang Metal coordination of EcoRI is intimately connected to its function of cleaving viral DNA. In order to gain insight into this process pulsed ESR distance measurement methodology was extended to the case of copper-copper and copper-nitroxide in proteins. This talk will discuss the first results on Cu$^{2+}$-Cu$^{2+}$ and Cu$^{2+}$-nitroxide distances measured in proteins by pulsed ESR. A triangulation procedure that involved the measurement of several long range distances (2-4 nm) demonstrates that copper ions bind to histidines 114 in EcoRI. The combination of ESR distance measurement and molecular dynamics simulations show distinct differences in the coordination of Cu$^{2+}$ versus the natural cofactor Mg$^{2+}$. This difference translates into the differences in roles of the metal ions - Mg$^{2+}$ catalyzes cleavage while Cu$^{2+}$ binding without cleavage of DNA. [Preview Abstract] |
Session L38: Focus Session: Theory of Electron Transport Through Molecules III
Sponsoring Units: DCPChair: Kieron Burke, University of California, Irvine
Room: 410
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L38.00001: Statistical mechanics of non-equilibrium steady state systems Invited Speaker: One of the important classes of non-equilibrium systems is the systems, which are maintained in non-equilibrium steady state by the contact with several external macroscopic reservoirs. These systems are ubiquitous and their theoretical description has been a challenging fundamental scientific problem for many years. They are also of significant practical interest for various nanotechnological and biological applications, such as quantum contacts, molecular motors, nanowires, and molecular junctions. There is no unique theoretical approach to wide variety of non-equilibrium steady state systems. General theoretical description of non-equilibrium steady states has not been developed yet and many fundamental theoretical questions are yet to be answered. For example, how to include many-particle correlation effects into theoretical treatment, is there exist a general variational principle for non-equilibrium steady state, do we enforce by the choice of a particular theoretical treatment a specific non-equilibrium steady state which is not the same as the real system would establish under the same boundary conditions, do we have a unique steady state in a system of non-equilibrium interacting particles for given boundary conditions? In my talk, I will review these questions and their relevance to electron transport through molecules. I will also give account of our recent computational and theoretical work on non-equilibrium quantum transport through molecular nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L38.00002: Vibronic effects in single molecule conductance Invited Speaker: Recent experimental advances have allowed to study the conductance properties of single-molecule junctions and revealed a wealth of intriguing transport phenomena. An important aspect that distinguishes nanoscale molecular conductors from mesoscopic devices is the influence of the nuclear degrees of freedom of the molecular bridge. Due to the small size of molecules, the charging of the molecular bridge is often accompanied by significant changes of the nuclear geometry that indicate strong coupling between electronic and nuclear (in particular vibrational) degrees of freedom. In this contribution, the effect of electron-vibrational (vibronic) coupling on the transport properties of single molecule junctions is studied. The study is based on a combination of first-principles electronic structure calculations to characterize the system and different transport methods including inelastic scattering theory, master equations and nonequilibrium Green's function theory. The basic mechanisms of vibrationally coupled electron transport are analyzed for a generic model of a molecular junction as well as benzenealkanethiolates between gold electrodes. The results show that vibronic coupling can have a significant effect on the conductance of molecular junctions. It manifests itself in pronounced structures in the current-voltage characteristics. Moreover, the current-induced excitation of vibrational modes mays result in a significant deviation of the vibrational degrees of freedom from their equilibrium distribution. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L38.00003: Controlling Current Flow Through Molecules With Electric Fields Emanating From Nearby Molecules: Theory and Experiment G. Kirczenow, P. G. Piva, R. A. Wolkow We show that electrical conduction through molecules can be strongly modulated by electric fields of nearby polar molecules. We study 1D organic heterostructures consisting of contiguous lines of CF$_3$- and OCH$_3$-styrene molecules on H-terminated Si(100). For suitable alignment of the OCH$_3$ groups in the molecular chain, their combined electric fields are shown by density functional calculations to give rise to potential profiles along the OCH$_3$-styrene chain that result in strongly enhanced conduction through molecules near the CF$_3$-styrene/OCH$_3$-styrene heterojunction for moderately low negative substrate bias, as is observed by STM. Under similar bias, dipoles associated with CF$_3$ groups are found in both theory and experiment to depress transport in the underlying Si. Under positive substrate bias, simulations suggest that the structural and electrostatic properties of CF$_3$-styrene molecules may lead to more sharply localized conduction enhancement near the heterojunction. Thus choice of substituents, their attachment site on the host styrene molecules on Si and the orientations of the molecular dipoles and multipoles provide a means of differentially tuning transport on the molecular scale. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L38.00004: Effects of Dephasing on DNA Sequencing via Transverse Electronic Transport Matt Krems, Yuriy Pershin, Michael Zwolak, Massimiliano Di Ventra We study theoretically the effects of dephasing on DNA sequencing in a nanopore via transverse electronic transport. To do this, we couple classical molecular dynamics simulations with transport calculations using scattering theory. Previous studies, which did not include dephasing, have shown that by measuring the transverse current of a particular base multiple times, one can get distributions of currents for each base that are distinguishable. We introduce a dephasing parameter into transport calculations to simulate the effects of the ions and other fluctuations. These effects lower the overall magnitude of the current, but have little effect on the current distributions themselves. The results of this work further implicate that distinguishing DNA bases via transverse electronic transport has potential as a sequencing tool. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L38.00005: Single-Electron Transistors made by chemical patterning of silicon dioxide substrates and selective deposition of gold nanoparticles Ulas Coskun, Henok Mebrahtu, Thom LaBean, Gleb Finkelstein We describe a method to pattern SiO$_2$ surfaces with colloidal gold nanoparticles by e-beam lithography and selective nanoparticle deposition. The method allows us to deposit nanoparticles in different shapes, including long continuous lines just one nanoparticle wide. We contact the pre-positioned nanoparticles with metal leads to form Single Electron Transistors. The Coulomb blockade pattern surprisingly does not show the parasitic ``offset charges'' at low temperatures, indicating relatively little surface contamination. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L38.00006: Contact Transparency of Nanotube-Molecule-Nanotube Junctions S. H. Ke, H. U. Baranger, Weitao Yang The transparency of contacts between conjugated molecules and metallic single-walled carbon nanotubes is investigated using a single-particle Green's function method which combines a Landauer approach with \textit{ab initio }density functional theory. We find that the overall conjugation required for good contact transparency is broken by connecting through a six-member ring on the tube. Full conjugation achieved by an all-carbon contact through a five-member ring leads to near perfect contact transparency for different conjugated molecular bridges. [Phys. Rev. Lett. \textbf{99, }146802 (2007)] [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L38.00007: Quantum many-body effects on the electric and thermoelectric response of molecular heterojunctions Justin Bergfield, Charles Stafford A semi-empirical $\pi$-electron Hamiltonian (extended Hubbard model) is used to model the electronic degrees of freedom most relevant for transport in a heterojunction consisting of a conjugated organic molecule coupled to two (or more) metallic electrodes. With an appropriate choice of parameters, the {\em complete spectrum of electronic excitations} of the molecule up to 8--10eV can be accurately described,$^1$ which is essential to accurately model transport far from equilibrium. The electric and thermoelectric response of the junction is calculated within a many-body theory of transport based on nonequilibrium Green's functions. For benzenedithiol-Au junctions, the parameters characterizing the lead-molecule coupling (tunneling width and chemical potential offset) are determined by comparison to linear-response measurements of conductance and thermopower. The nonlinear transport can then be predicted: the differential conductance as a function of gate and bias voltages exhibits clear signatures of charge quantization and resonant tunneling through excited states, with an irregular ``molecular diamond'' structure analogous to the regular Coulomb diamonds observed in quantum dot transport experiments. Several other small conjugated organic molecules are also investigated. $^1$C.\ W.\ M.\ Castelton and W.\ Barford, J.\ Chem.\ Phys.\ {\bf 117}, 3570 (2002). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L38.00008: Current induced local heating and heat transport in single molecular bridge junction Yoshihiro Asai Current induced local heating will be discussed theoretically. Both electric conductance and heat conductance of electronic and phonon origins are calculated in a microscopic way, including inelastic scattering effects due to electron-phonon couplings. [1] Based on the self-consistent solution for an alkanethiol molecule bridging gold electrodes, we found that the effective temperature T$_{eff}$ due to the local heating is largely reduced by the heat conductance, which releases the Joule heat out of the molecule. All these calculations are made in a fully microscopic way without introduction of the phenomenological phonon diffusion effect used in literature. Theoretical voltage dependence of T$_{eff}$ agrees nicely with an experiment. [2] Ref.) [1]Y. Asai, Phys. Rev. B78, 045434 (2008). [2] Z.Huang et al, Nano Letters, 6, 1240 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L38.00009: Fabrication and characterization of vertically aligned and interconnected nickel oxide Nanowalls Latha Kumari, Wenzhi Li, Charles H. Vannoy, Roger M. Leblanc , Dezhi Wang Vertically aligned and well interconnected NiO nanowalls were fabricated on Ni foil by a two step hydrothermal route. The as-prepared nickel hydroxide was converted to NiO by further heat treatment. The NiO nanowalls are typically 15 nm thick and around 1-1.5 $\mu $m wide. The NiO nanowalls have cubic crystal structure with their growth plane along the [111] direction. The NiO nanowalls show an optical band gap of about 3.8 eV and exhibit broad photoluminescence emission band centered at around 390 nm. The present synthesis technique supports the growth of well aligned 2D nanostructures with large surface area for possible applications in nanoscale devices. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L38.00010: Conductance switching in organic monolayers Luis Agapito, Sabri Alkis, Jeffrey Krause, Hai-Ping Cheng Self-assembled monolayers of some organic molecules, such as the bipyridyl-dinitro (BPDN) [1], present conductance switching (toggling between ON and OFF states). The switching happens upon crossing fixed threshold values in the applied bias voltage and the device can ``remember'' its previous state; thus, they have potential value as electronic memory devices. We use density-functional theory to elucidate the atomistic origins of this phenomenon. Extensive geometry relaxations revealed two adsorption states; namely, an atop and a hollow adsorption geometry. The electronic structure of both adsorption states were further recalculated using localized basis functions and the electrical currents through these devices were estimated within the Landauer approximation. The atop state shows a higher current than the hollow state, which matches the ON and OFF conductance states observed experimentally. We attribute the conductance switching to fluctuations in the adsorption geometry of the monolayers. Ab initio calculations can help us to understand the atomistic causes of the memory effect, which is essential for having a systematic approach to theory-guided molecular synthesis. Supported by DOE grant DE-FG02-02ER45995. [1] A. S. Blum, et al., Nature Materials 4, 167 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L38.00011: ABSTRACT WITHDRAWN |
Session L39: Focus Session: Theories and Simulations for Biomolecular Dynamics in Cell-like Environments
Sponsoring Units: DBPChair: Margaret Cheung, University of Houston
Room: 411
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L39.00001: Biochemistry on a leash: Confinement as a regulatory mechanism for bimolecular reaction rates Daniel Reeves, Keith Cheveralls, Jane Kondev We describe two mechanisms by which confinement regulates diffusion-limited bimolecular reaction rates. The first mechanism, illustrated by the actin capping protein formin, uses a flexible polymer to tether ligand binding sites, which serve as intermediaries, to the reactive site. The second mechanism uses a potential (e.g. hard wall potential), to constrain the motion of a ligand receptor within a confining volume. We analyze both mechanisms theoretically, using a combination of analytic and numerical techniques, to obtain the steady state binding kinetics. We explore how the reaction rates are regulated by parameters of the model such as the length of the polymer tether, and use our findings to explain the key features of the formin system. Finally, we suggest other systems, both synthetic and biological, in which these mechanisms for regulating bimolecular reactions might be at play. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 3:18PM |
L39.00002: Crowded, cell-like environment induces shape changes in aspherical protein Invited Speaker: How the crowded environment inside cells affects the structures of proteins with aspherical shapes is a vital question because many proteins and protein--protein complexes \textit{in vivo }adopt anisotropic shapes. Here we address this question by combining computational and experimental studies of a football-shaped protein (i.e. \textit{Borrelia burgdorferi }VlsE) under crowded, cell-like conditions. The results show that macromolecular crowding affects protein-folding dynamics as well as overall protein shape. In crowded milieus, distinct conformational changes in VlsE are accompanied by secondary structure alterations that lead to exposure of a hidden antigenic region. Our work demonstrates the malleability of ``native'' proteins and implies that crowding-induced shape changes may be important for protein function and malfunction \textit{in vivo}. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L39.00003: Molecular dynamics simulation study of multimerization of the Mms6 protein from Magnetospirillum magneticum strain AMB-1 Monica Lamm, Rastko Sknepnek, Lijun Wang, Marit Nilsen-Hamilton In order to optimize their search for nutrients, magnetotactic bacteria have developed an ability to align themselves to Earth's magnetic field. This is achieved by forming a chain of vesicles containing magnetite superparamagnetic nanoparticles with sizes of the order of 50nm. The presence of the small protein Mms6 plays an important role in the successful in vitro growth of magnetite nanoparticles, although the mechanism of this process is not understood. Preliminary experiments on Mms6 in solution indicate that the protein forms multimers of variable sizes, depending on the salt concentration. Using an intermediate level coarse grained model for Mms6 we investigated the formation of these multimers as a function of temperature and salt concentration. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 4:06PM |
L39.00004: The Packing of Flexible Screws and the Self-Limited Assembly of Biopolymer Bundles Invited Speaker: Living cells rely heavily on assemblies of filamentous proteins, such as F-actin and microtubules, to perform a variety of tasks, ranging from adhesion and locomotion to cell division and intracellular transport. In the dynamic cellular environment, the efficiency of these tasks is crucially dependent on the robust assembly and disassembly of rope-like bundles of filamentous molecules. Recent {\it in vitro} studies of F-actin assembly [Lai {\it et al.}, Phys. Rev. Lett. (2007)] suggest that bundle formation may take place as an equilibrium process, with a thermodynamically-preferred bundle diameter. Within the context of a generalized elastic model of filament packings, we explore the possibility that limited-bundle growth is directly linked with the intrinsic chiral structure of biological filaments themselves. The hexagonal packing of biopolymers leads to the build up of chiral stress, or torque, that generically induces the formation of twisting filament bundles of finite size. We demonstrate that the underlying elasticity of the bundle--i.e. whether hexagonal-solid or hexagonal-columnar--plays a key role in dictating both the thermodynamics (i.e. disperse, bundled or bulk aggregation) and structure (i.e. size and twist) of ``self-braiding" aggregates of helical filaments. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L39.00005: Simulation of signal transduction in model multiprotein systems Julius Su To simulate the dynamics of multiprotein machines, I have developed a method called multiconformer Brownian dynamics (mcBD). In this method, proteins rotate and translate via Brownian motion while their conformations are varied among a prestored set of structures on a simplified energy landscape, taking into account inter-protein interactions. As an example, I build a simple model of a G-protein coupled receptor/G-protein complex, and show that ligand binding causes conformational shifts, which induce GDP to leave, GTP to bind, and the complex to dissociate. The two proteins couple their fast fluctuations together into large-scale coordinated functional motions, resulting in signal transduction. I vary the shapes, electrostatics, and energy landscapes of the proteins independently and examine the impact this has on the system's function. In one result, increasing the binding between proteins improves the fidelity of communication, but at the expense of overall switching frequency. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L39.00006: Implicit solvent model for linear-scaling first-principles electronic structure calculations Hatem H. Helal, Mike Payne, Arash A. Mostofi Density functional theory (DFT) enables first-principles calculations that exhibit cubic scaling of the computational time required with respect to the number of atoms in the system. This presents an unavoidable difficulty when first-principles accuracy is needed for the study of large-scale biological systems. The ONETEP program reformulates DFT so that the required computational effort scales only linearly with system size, recently demonstrated for up to 32,000 atoms on 64 cores.\footnote{N.~D.~M.~Hine, P.~D.~Haynes, A.~A.~Mostofi, C.-K.~Skylaris and M.~C.~Payne, submitted to \emph{J.~Chem.~Phys.} (2008).} Further complicating DFT based studies of biomolecular systems is the need for an accurate representation of the electrostatic environment. Rather than introducing explicit solvent molecules into the system, which would be computationally prohibitive, we present our recent efforts to integrate an implicit solvent model\footnote{D.~A. Scherlis \emph{et al.}, \emph{J.~Chem.~Phys.} \textbf{124}, 074103 (2006).} with ONETEP in order to study systems in solution consisting of many thousands of atoms. We report preliminary results of our methodology with a study of the DNA nucleosome core particle. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L39.00007: Using stochastic dynamics to validate runtimes of protein simulations Stephen D. Hicks, Christopher L. Henley We use short molecular dynamics simulations ($\sim$200 cpu-hr, using NAMD) of individual bonds between capsid proteins to microscopically determine coarse-grained elastic parameters of entire virus capsids. In particular, we treat each protein (or for larger proteins, each domain) as a rigid body described by a 6-vector of translational and orientational degrees of freedom, $x_i(t)$. We then model the evolution of the relative positions as an overdamped random walk, $\dot x_i(t) = -\Gamma_{ij}K_{jk}(x_k(t)-\bar x_k) + \zeta_i(t)$, where $\zeta_i(t)$ are random variables satisfying $\langle\zeta_i(t)\zeta_j(t')\rangle = 2\Gamma_{ij}T\delta(t-t')$. Our goal is to determine the stiffness matrix $K_{ij}$, but this requires long-time data to measure accurately. We therefore measure the noise matrix $2\Gamma_{ij}T$, which depends on much shorter timescales, and compute the relaxation times by diagonalizing $\Gamma^{\frac12}K\Gamma^{\frac12}$. Although we use biologically relevant configurations in each simulation, we have taken the domains out of their full context by simulating one pair at a time, and therefore external stresses are missing, which we measure from the drift and compensate for in subsequent simulations. Finally, we apply this technique to the HIV capsid protein. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L39.00008: A symplectic integration method for elastic filaments Tony Ladd, Gaurav Misra Elastic rods are a ubiquitous coarse-grained model of semi-flexible biopolymers such as DNA, actin, and microtubules. The Worm-Like Chain (WLC) is the standard numerical model for semi-flexible polymers, but it is only a linearized approximation to the dynamics of an elastic rod, valid for small deflections; typically the torsional motion is neglected as well. In the standard finite-difference and finite-element formulations of an elastic rod, the continuum equations of motion are discretized in space and time, but it is then difficult to ensure that the Hamiltonian structure of the exact equations is preserved. Here we discretize the Hamiltonian itself, expressed as a line integral over the contour of the filament. This discrete representation of the continuum filament can then be integrated by one of the explicit symplectic integrators frequently used in molecular dynamics. The model systematically approximates the continuum partial differential equations, but has the same level of computational complexity as molecular dynamics and is constraint free. Numerical tests show that the algorithm is much more stable than a finite-difference formulation and can be used for high aspect ratio filaments, such as actin. We present numerical results for the deterministic and stochastic motion of single filaments. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L39.00009: Biomolecular Structure Determination with Divide and Concur Yoav Kallus, Veit Elser Divide and concur ($D-C$) is a general computational approach, designed for the solution of highly frustrated problems. Recently applied to the problems of disk packing, the kissing number problem, and 3-SAT, it was competitive or outperformed special-purpose methods.\footnote{S. Gravel and V. Elser, Phys. Rev. E 78, 036706 (2008)} We present a method for applying the $D-C$ framework to the problem of biomolecular structure determination. From a list of geometric constraints on groups of atoms in the molecule, we construct a deterministic iterative map that efficiently searches for structures simultaneously satisfying all constraints. As our method eschews an energy function and its minimization to focus on geometric constraints, it can very naturally integrate with the geometric constraints due to chemistry and physics, experimental constraints due to NMR data or many other experimental or biological hints. We present some results of our method. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L39.00010: Adaptive anisotropic network model: generating transition pathways of supramolecular system Zheng Yang, Ivet Bahar Generating of functional transition pathways of biomolecular systems is often complicated. This task becomes even more challenging in exploring systems of the order of megadaltons. Coarse-grained models that lend themselves to analytical solutions appear to be the only possible means of approaching such cases. We introduce a new method, \textit{adaptive anisotropic network model} ($a$ANM) for exploring functional transitions, based on the elastic network models, which have been widely used to describe the collective dynamics of biomolecular systems. Application to bacterial chaperonin GroEL highlights the utility of the methodology. Comparisons with experimental data and results from action minimization algorithm support the utility of $a$ANM as a computationally efficient, yet physically plausible, tool for unraveling potential transition pathways sampled by large complexes/assemblies and assessing the critical inter-residue interactions formed/broken near the transition state(s), most of which involve conserved residues. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L39.00011: Generic Coarse-Grained Model for Protein Folding and Aggregation Tristan Bereau, Markus Deserno The complexity involved in protein structure is not only due to the rich variety of amino acids, but also the inherent weak interactions, comparable to thermal energy, and important cooperative phenomena. This presents a challenge in atomistic simulations, as it is associated with high-dimensionality and ruggedness of the energy landscape as well as long equilibration times. We have recently developed a coarse-grained (CG) implicit solvent peptide model which has been designed to reproduce key consequences of the abovementioned weak interactions. Its intermediate level of resolution, four beads per amino acid, allows for accurate sampling of local conformations by designing a force field that relies on simple interactions. A realistic ratio of $\alpha$-helix to $\beta$-sheet content is achieved by mimicking a nearest-neighbor dipole interaction. We tune the model in order to fold helical proteins while systematically comparing the structure with NMR data. Very good agreement is achieved for proteins that have simple tertiary structures. We further probe the effects of cooperativity between amino acids by looking at peptide aggregation, where hydrophobic peptide fragments cooperatively form large-scale $\beta$-sheet structures. The model is able to reproduce features from atomistic simulations on a qualitative basis. [Preview Abstract] |
Tuesday, March 17, 2009 5:30PM - 5:42PM |
L39.00012: Computational Investigation of Conformational Changes in Proteins upon Adsorption Sumit Sharma, Gaurav Anand, Georges Belfort, Sanat K. Kumar Amyloidogenic diseases, such as, Alzheimer's, are caused by adsorption and aggregation of partially unfolded proteins. Protein adsorption is often accompanied by conformational rearrangements, which are thought to affect many properties such as their adhesion strength to the surface, biological activity, and aggregation tendency. Experiments have shown that many proteins, upon adsorption to hydrophobic surfaces, undergo a helix to sheet or random coil secondary structural rearrangement. To better understand the equilibrium structural complexities of this phenomenon, we have performed Monte Carlo (MC) simulations and Single Chain Mean Field calculations of adsorption of different proteins, modeled as lattice chains, to study the adsorption behavior and equilibrium protein conformations at different temperatures, protein concentration and surface hydrophobicity. Free energy and entropic effects on adsorption have been studied by determining density of states using Weighted Histogram Analysis Method. Conformational transitions of proteins on surfaces will be discussed as a function of surface hydrophobicity. [Preview Abstract] |
Tuesday, March 17, 2009 5:42PM - 5:54PM |
L39.00013: Role of van der Waals interactions for the intrinsic stability of polyalanine helices Alexandre Tkatchenko, Volker Blum, Joel Ireta, Matthias Scheffler The helical motif is an ubiquitous conformation adopted by aminoacid residues in a protein structure and helix formation is the simplest example of the protein folding process. How stable is the folded peptide helix in comparison to a random coil structure? What are the interactions responsible for stabilizing the helical conformation? Answering these questions has thus a direct implication for understanding protein folding. In this work we use density functional theory (DFT) augmented with a non-empirical correction for van der Waals (vdW) forces to study the stability of alanine polypeptide helices \textit{in vacuo}. We find a large stabilization of the native helical forms when vdW correction is used. It amounts to 121\%, 157\% and 83\% on top of the Perdew-Burke-Ernzerhof (PBE) functional in the case of infinite $\alpha$, $\pi$ and 3$_{10}$ helices, respectively. Thus, the experimentally observed $\alpha$ helix is significantly stabilized by vdW forces both over the fully extended and the 3$_{10}$ conformations. Our findings also suggest an explanation to the remarkable stability of gas-phase alanine helices up to high temperatures [M. Kohtani \textit{et al.} JACS 126, 7420 (2004)]. [Preview Abstract] |
Session L40: Nucleic Acids: Structure, Function and Dynamics
Sponsoring Units: DBPChair: Michael Poirier, Ohio State University
Room: 412
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L40.00001: Histone octamer acetylation affects the free energy of nucleosome formation Alex Mooney, Mridula Manohar, Annick Edon, Robin Nakkula, Jennifer Ottesen, Michael Poirier Nucleosomes, histone octamer-DNA complexes, form the fundamental repeating units of eukaryotic chromatin. Numerous post-translational modifications of histone octamers are found \emph{in vivo} and are known to play roles in gene regulation and DNA repair, but the molecular functions of these modifications are not well understood. In this study we consider the effects of acetylating histone protein H3 residues Lys$^{115}$ and Lys$^{122}$. These modifications reduce the positive surface charge of the histone octamer at contact points with the negatively charged DNA phosphate backbone and add steric bulk in the dyad region. We report results from competitive reconstitutions that show the free energy of nucleosome formation between wild-type and modified histone octamer binding to a strong nucleosome positioning sequence is reduced. These results suggest that these modifications may be involved in nucleosome assembly and disassembly. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L40.00002: Density fluctuations in confined and non-confined DNA Junhan Pan, Chunda Zhou, Robert Riehn DNA stretching in quasi one-dimensional nanochannels is an emerging technique for the analysis of genomic-sized DNA molecules. For formulating an optimal measurement strategy, the thermal fluctuations of confined molecules are of crucial importance. While previous measurements have concentrated only on the end-to-end length, we present here an experimental study of density fluctuations within the molecule, and find a good agreement with a model similar to a oscillator chain. We further discuss how such a model leads to a natural interpretation of the interesting intramolecular collapse of DNA that we recently under application of a.c. electric fields at frequencies of a few hundred Hertz. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L40.00003: Direct Observation of Multiple Pathways of DNA Stacking Using Single-Molecule AFM Ching-Hwa Kiang, Wei-Hung Chen During DNA interactions, single-stranded DNA (ssDNA) is often stretched and stabilized by coupling with ssDNA binding proteins to serve as an intermediate state. The conformational and energetic changes of stretched DNA are of great interest because of their relevance in biological functions. Direct manipulation of DNA has yielded much of the information about the mechanical properties of DNA without the complication of interacting molecules. Stretching ssDNA has provided direct measurement of the base stacking mechanics and energetics. For example, polydA has been shown to have two transitions during overstretching. Here we showed direct observation of two overstretching pathways during the second transition. We have observed ``hopping'' between these two pathways during constant-force measurements. We will discuss the implications of such transition and its significance in biological functions. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L40.00004: Stabilities of Constituent Hairpins Determine Whether RNA Folds via Ordered and/or Parallel Mechanisms Samuel Cho, Devarajan Thirumalai RNA molecules are increasing becoming seen as a set of critical players in numerous cellular processes, and a firm understanding of their folding energy landscapes is essential for understanding how they carry out their functions. While it might seem natural to assume that the simplicity of RNA molecules, with only four possible bases, dictate that they must fold via simple mechanisms, experiments continue to point to complex folding energy landscapes involving parallel mechanisms. In our present study, we address how even simple RNA molecules can give rise to very complex folding mechanisms. We begin by making the argument that the complexity observed for RNA folding are in fact fully expected because the lack of variability of the RNA subunits leads to a lack of specificity for folding to the native basin. To illustrate our point, we performed coarse-grained simulations of three RNA pseudoknots and a tRNA molecule, which each are all relatively simple RNA molecules that contain at least two hairpins. From our simulations, we find that the main determinant for how these RNA molecules fold is largely dependent on the relative stabilities of their constituent hairpins. Ordered mechanisms arise if the stabilities of the constituent hairpins are sufficiently dissimilar and parallel folding mechanisms occur if the stabilities of the hairpins are similar. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L40.00005: Structural basis of pathway-dependent force profiles in DNA Daniel Roe, Anne Chaka Since DNA must bend and/or stretch to perform many of its functions, it is important to understand the mechanical properties of DNA. Single molecule experiments have been able to study the response of DNA to applied forces. One interesting result of such studies is that at high loading rates a greater force is required to stretch DNA when pulling from the 3' ends as opposed to the 5' ends. While these experiments provide valuable insights into the stability of DNA, it is often difficult to relate the results to specific structural changes. We have used molecular dynamics simulation methods to study the structure and dynamics of DNA under a tensile load. Simulations were performed on a variety of fully solvated DNA sequences up to 30 base-pairs in length, and were conducted under both non-equilibrium and equilibrium conditions. Different stretched DNA structures are observed depending on whether pulling occurs from the 5' ends or 3' ends. Detailed analysis of these structures provides a direct structural explanation of the observed difference between 3' and 5' pulling. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L40.00006: Folding Kinetics of Riboswitch Transcriptional Terminators Benjamin Sauerwine, Michael Widom Riboswitches control the expression of genes in bacteria by halting gene transcription or allowing it to proceed based on the presence of ligands in solution. A key feature of every riboswitch is a transcriptional terminator in which the messenger RNA folds into a secondary structure with the stem-loop structure of a hairpin. Through kinetic Monte Carlo simulation we show that terminators have been naturally selected to fold with high reliability on the time-scale of gene transcription. This efficient folding behavior is preserved among two classes of riboswitch and among two species of bacteria. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L40.00007: First Principles Study of Nuclear Quadrupole Interactions in Single and Double Chain DNA and Solid Nucleobases T.P. Das, R.H. Pink, S.R. Badu, Archana Dubey, R.H. Scheicher, H.P. Saha, Lee Chow, M.B. Huang Nuclear Quadrupole Interactions (NQI) of $^{17}$O, $^{14}$N and $^{2}$H nuclei have been studied for free nucleobases and nucleobases in single strand and double strand DNA and in solid state. Our first-principles investigations were carried out using the Gaussian 2003 set of programs to implement the Hartree-Fock procedure combined with many-body effects included using many-body perturbation theory. As expected for NQI in general, many-body effects are found to be small. Results will be presented for the quadrupole coupling constants (e$^{2}$qQ) and asymmetry parameters ($\eta)$ for the nucleobases in the various environments. Trends in e$^{2}$qQ and $\eta $ in the different environments will be discussed. In the case of the solid nucleobases, comparisons will be made with available experimental data [1] for $^{17}$O nuclei.\\[3pt] [1] Gang Wu et al., J. Am. Chem. Soc. 124, 1768 (2002) [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L40.00008: Optical control of DNA radio-sensitivity Ramin Abolfath We explore the manipulation of the radio-sensitivity of the DNA molecules driven by the spin blockade mechanism of diffusive free radicals. We propose a mechanism which uses the simultaneous application of circularly polarized light and an external magnetic field to control the polarization of the free radicals and create an $S=1$ electron-hole spin excitation (exciton) on DNA molecules. It allows us to manipulate and partially suppress the damage induced by ionizing radiation. We deploy an {\em ab-initio} molecular dynamics model to calculate the characteristic parameters of the light needed for optical transitions and investigate the effect of spin-injection on the formation of a free energy barrier in diffusion controlled chemical reaction pathways that controls radiation-induced DNA damage. As a specific example, we present the numerical results calculated for a nucleotide-base, e.g., Guanine, in the presence of an OH free radical. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L40.00009: Acetylated histone H3 increases nucleosome dissociation Marek Simon, Mridula Manohar, Jennifer Ottesen, Michael Poirier Chromatin's basic unit structure is the nucleosome, i.e. genomic DNA wrapped around a particular class of proteins -- histones -- which due to their physical hindrance, block vital biological processes, such as DNA repair, DNA replication, and RNA transcription. Histone post-translational modifications, which are known to exist \textit{in vivo}, are hypothesized to regulate these biological processes by directly altering DNA-histone interactions and thus nucleosome structure and stability. Using magnetic tweezers technique we studied the acetylation of histone H3 in the dyad region, i.e. at K115 and K122, on reconstituted arrays of nucleosomes under constant external force. Based on the measured increase in the probability of dissociation of modified nucleosomes, we infer that this double modification could facilitate histone chaperone mediated nucleosome disassembly \textit{in vivo}. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L40.00010: First Principles Study of Muonium Trapping and Associated Magnetic Hyperfine Interactions in Nucleobases in Single and Double Chain DNA and Solid Nucleobases S.R. Badu, R.H. Pink, Archana Dubey, R.H. Scheicher, H.P. Saha, K. Nagamine, E. Torikai, Lee Chow, M.B. Huang, T.P. Das The trapping of muonium (Mu) and muon hyperfine interactions (HFI) are studied for free nucleobases and nucleobases in single and double strand DNA and in solid nucleobases. For our investigations we have utilized the Hartree-Fock procedure with many-body effects included using many-body perturbation theory. Results for the muon magnetic contact and dipolar HFI will be presented for the various environments. The trends among the different environments is rather different from those for the nuclear quadrupole interactions in the corresponding systems because of the differences in geometry of the Mu trapping sites in the various systems. Quantitative comparison will be made between our theoretical results and experimentally measured\footnote{Penny L Hubbard et al., J. Phys. Chem. A108, 9302 (2004).} muon HFI properties in the solid nucleobases. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L40.00011: Modeling nucleic acid structure in the presence of single-stranded binding proteins Robert Forties, Ralf Bundschuh There are many important proteins which bind single-stranded nucleic acids, such as the nucleocapsid protein in HIV, the RecA DNA repair protein in bacteria, and all proteins involved in mRNA splicing and translation. We extend the Vienna Package for quantitatively modeling the secondary structure of nucleic acids to include proteins which bind to unpaired portions of the nucleic acid. All parameters needed to model nucleic acid secondary structures in the absence of proteins have been previously measured. This leaves the footprint and sequence dependent binding affinity of the protein as adjustable parameters of our model. Using this model we are able to predict the probability of the protein binding at any position in the nucleic acid sequence, the impact of the protein on nucleic acid base pairing, the end-to-end distance distribution for the nucleic acid, and FRET distributions for fluorophores attached to the nucleic acid. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L40.00012: A theoretical investigation of the interconversion between B and Z-DNA using the Adaptively Biased and Steered Molecular Dynamics methods Mahmoud Moradi, Chistopher Roland, Volodymyr Babin, Celeste Sagui The transition between right-handed B-DNA and left-handed Z-DNA in an implicit solvent environment was investigated via the free energy landscape of DNA as a function of the collective variables of handedness and radius of gyration, using the recently developed Adaptively Biased Molecular Dynamics (ABMD) method. The ABMD method, which belongs to the general category of umbrella sampling methods with a time-dependent potential, allows for an efficient and accurate estimation of the free energy barriers associated with the transition, especially when combined with multiple-walker and umbrella correction runs. The ABMD results are compared to those obtained using the Steered Molecular Dynamics (SMD) method. The implication of all these free energy results on the microscopics of the B to Z-DNA transition is to be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L40.00013: End-monomer dynamics in semiflexible polymers Michael Hinczewski, Xaver Schlagberger, Michael Rubinstein, Oleg Krichevsky, Roland Netz Precise experimental observations over the last few years of end-monomer dynamics in the diffusion of double-stranded DNA have given conflicting results: one study indicated an unexpected Rouse-like scaling of the mean squared displacement (MSD) $\langle r^2(t) \rangle \sim t^{1/2}$ at intermediate times, corresponding to fluctuations at length scales larger than the persistence length but smaller than the coil size; another study claimed the more conventional Zimm scaling $\langle r^2(t)\rangle \sim t^{2/3}$ in the same time range. Spurred by this experimental controversy, we investigate the end-monomer dynamics of semiflexible polymers through Brownian hydrodynamic simulations, an improved dynamic mean-field theory, and a heuristic scaling argument [1]. Both theory and simulation point to a novel intermediate dynamical regime where the effective local exponent of the end-monomer MSD, $\alpha(t) = d\log\langle r^2(t) \rangle /d\log t$, drops below the Zimm value of 2/3 for sufficiently long chains. This deviation increases with chain length (though it does not reach the Rouse limit of 1/2), and is related to hydrodynamic effects in the slow crossover from dynamics on length scales smaller than the persistence length to dynamics on larger scales. [1] arXiv:0809.0667v1, Macromolecules {\it in press} (2008). [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L40.00014: Hydration effect on optical property of a DNA fiber: A first-principles study Takenori Yamamoto, Tsuyoshi Uda, Takahisa Ohno We present a first-principles study for salvation effects on properties of a deoxyribonucleic acid (DNA) double helix fiber. The first-principles electronic structure and the molecular dynamics simulations reveal that the electronic structure of the DNA fiber is varied by the hydration amount and the deformation. The electrostatic interaction in the DNA fiber is screened by the hydration water. The screened electrostatic interaction determines the electronic structure of the DNA fiber, while the electronic structure of the water is determined by its polarized change as the result of the electrostatic response. We show that the optical conductivity is influenced by the hydration and the deformation, and that our findings agree with other theoretical results and experimental observations. In conclusion, we really stress that the solvation must be carefully taken account for simulating electronic structures and properties of DNA's. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L40.00015: Supercoil Formation During DNA Melting Mehmet Sayar, Baris Avsaroglu, Alkan Kabakcioglu Supercoil formation plays a key role in determining the structure-function relationship in DNA. Biological and technological processes, such as protein synthesis, polymerase chain reaction, and microarrays relys on separation of the two strands in DNA, which is coupled to the unwinding of the supercoiled structure. This problem has been studied theoretically via Peyrard-Bishop and Poland-Scheraga type models, which include a simple representation of the DNA structural properties. In recent years, computational models, which provide a more realtistic representaion of DNA molecule, have been used to study the melting behavior of short DNA chains. Here, we will present a new coarse-grained model of DNA which is capable of simulating sufficiently long DNA chains for studying the supercoil formation during melting, without sacrificing the local structural properties. Our coarse-grained model successfully reproduces the local geometry of the DNA molecule, such as the 3'-5' directionality, major-minor groove structure, and the helical pitch. We will present our initial results on the dynamics of supercoiling during DNA melting. [Preview Abstract] |
Session L41: Atom-Photon Interactions
Sponsoring Units: DAMOPChair: Pierre Meystre, University of Arizona
Room: 413
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L41.00001: Few electron systems in a strong laser pulse Nicholas Vence, Predrag Krstic, Robert Harrison We propose a numerical procedure for investigating the dynamics of a one electron wave function in a strong, sub-femtosecond laser field. The non-perturbative time evolution method does not rely on an eigenfunction basis set but uses the multiresolution techniques for spatial discretization as described in [Harrison et. al., J. Chem. Phys. 121, 2866 (2004)]. The time propagation is done by the chin-chen time splitting method [Chin Chen, J. Chem. Phys. 114, 7338 (2001)]. The excitation and ionization cross-sections for the hydrogen atom, the oxygen ion and the hydrogen molecular ion could serve as a benchmark for future calculations and experiments due to the well controlled accuracy inherent in this numerical scheme. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L41.00002: Theoretical study of electron momentum distribution in He tunneling ionization Li Guo, Shengsheng Han, Jingyun Fan, Jing Chen The electron scattering amplitude given by the S-matrix theory, up to the third order, can be written as $S_{fi} =-i\int_{-\infty }^\infty dt\left\langle {\psi _{A_f } (p,t)} \right|V_A (t)\left| {\varphi _0 } \right\rangle -i\int_{-\infty }^\infty dt\int_{-\infty }^\infty d{t}'\left\langle {\psi _{A_f } (p,t)} \right|VG_A (t,{t}')V_A ({t}')\left| {\varphi _0 } \right\rangle -i\int_{-\infty }^\infty dt\int_{-\infty }^\infty d{t}'\int_{-\infty }^\infty d{t}''\left\langle {\psi _{A_f } (p,t)} \right|VG_A (t,{t}')VG_A ({t}',{t}'')V_A (t)\left| {\varphi _0 } \right\rangle $, with the first term describing the direct ATI process (that the initial state directly scatters to the final state), the second term describing that the electron initially scatters to the intermediate states via the laser-electron interaction (V$_{A})$ and then scatters to the final state via the electron-ion interaction (V) (a rescattering ATI process), and the third term describing that the electron scatters to the final state via two cascaded ATI processes. All scattering processes are physically indistinguishable, so we assume that all divergence parts in scattering processes (corresponding to the forward scattering) can be absorbed into the first term and exclude them in the calculation, where the divergence is due to the long range Coulomb interaction ($V=-$1$/r)$. We apply this method to study the He ionization and our theoretical results are qualitatively consistent with recent experimental observations. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L41.00003: Modulation of molecular high harmonic generation by electron de Broglie wave interference Jing Chen, Jingyun Fan In the intense laser field, the amplitude for the $n^{th}$-order high harmonic generation (HHG) of a two-center molecule using the modified Lewenstein model is written as $S(n)\propto \sum\limits_{l,m} {\vert (e^{i\vec {k}'\cdot \vec {R}/2}-e^{-i\vec {k}'\cdot \vec {R}/2})\Phi _i (\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {k}} ')\vert ^2J_l (-\frac{\vec {k}\cdot \vec {A}_0 }{\omega })J_m (\frac{\vec {k}\cdot \vec {A}_0 }{\omega })} $, where $m $and $l $are number of photons that the electron absorbs/emits at ionization /recombination and are restricted by the energy conservation, $n=(m\pm 1)+l$. The electron's kinetic energy is related to photon number $m$ and molecular ionization potential $I_p $ as $\vec {k}^{\mbox{2}}\mbox{/2 }=\mbox{ (}m\mbox{ }\pm 1)\omega -I_p $. $\vec {k}'$ is parallel to $\vec {k}$with $\vec {k}'^2/2=\vec {k}^2/2+I_p $ due to the bound potential acceleration effect in the recapture. $J_l ()$ is $l^{th}$ order Bessel function and $\Phi _{i}(k)$ is the amplitude of electron momentum state (Fourier transformation of the atomic wavefunction \textit{$\varphi $}$_{i}$ in the LCAO-MO approximation). Clearly MHHG at each order ($n^{th})$ is contributed by a number of momentum states, being a summation of interferences $(e^{i\vec {k}'\cdot \vec {R}/2}-e^{-i\vec {k}'\cdot \vec {R}/2})$ weighed by state probabilities which is affected by laser parameters. We numerically evaluate the MHHG spectra for different laser intensities and various alignment angles, the results are consistent with recent experimental observations. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L41.00004: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L41.00005: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L41.00006: Entangling photons by means of the nonlinear response of quantum wells to an ultrashort pulse Mikhail Erementchouk, Michael Leuenberger Polarization-entangled photons can be produced from semiconductor bulk crystals made of CuCl through resonant hyperparametric scattering off the bound biexciton state with a yield exceeding $10^{-5}$, much higher than yields $<10^{-9}$ achieved with bulk nonlinear crystals. Here we show a different method to produce pairs of entangled photons in the short time response of a quantum well excited by a short intense pulse. At the time scales, where the biexciton effect is not yet pronounced, the Pauli exclusion principle is responsible for many-body correlations among excitons, giving rise to the production of entangled photons with a yield of around $10^ {-2}$. We make use of a quantum-field theoretical two-particle density matrix to calculate the entanglement for arbitrary emission angles of the entangled pairs of photons. At the time scales, where the heavy-light hole splitting is resolved, the resonances corresponding to different two-exciton states are developing, so that a simple kinematic theory can be presented, which relates the states of the outgoing photons with the respective two-exciton states. We study remarkably strong nontrivial dependence of entanglement on the emission angles of the entangled photons and on the ellipticity parameters of the incident photons. We show that the emitted entangled 2-photon states are always in a triplet state. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L41.00007: Two and three dimensional study of the hydrogen molecular ion H2+ confined between double boxes of spheroidal and spherical geometries. Martin Molinar, German Campoy Considering first a two-dimensional system, we study the hydrogen molecular ion confined in the space between two ellipses, and then we consider its confinement in the space between two prolate spheroids. In the Born -- Oppenheimer approximation, we solve numerically the Schrodinger's equation for the above mentioned cases, using an algorithm that allows us to calculate the energies for different given values of the confinement parameters. We also consider the confinement in the region limited by two concentric circumferences and in the three-dimensional case, in the region between two concentric spherical shells. In the last two cases we use the variational method in order to estimate the energy of the ground state. Some properties of the system as the pressure exerted by the confinement, the polarizability in the approximations of Kirkwood and Buckingham and the energies of the vibrational states are calculated. The behavior of the internuclear separation is analyzed for all the geometries considered. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L41.00008: Dipole in a Magnetic Field, Work, and Quantum Spin Robert J. Deissler Place an atom in a nonuniform static external magnetic field and, because of the interaction between the atom's magnetic moment and the magnetic field gradient, the atom will accelerate. An important and fundamental question, which has been neglected in the literature, is whether or not the magnetic field is doing work on the atom. It is shown that, while the magnetic field does no work on the electron-orbital contribution to the magnetic moment (the source of translational kinetic energy being the atom's internal energy), whether or not it does work on the electron-spin contribution to the magnetic moment depends on whether the electron has an intrinsic rotational kinetic energy associated with its spin. If the electron does have a rotational kinetic energy, which is shown to be consistent with the Dirac equation, the acceleration of a silver atom in a Stern-Gerlach experiment or the emission of a photon from an electron spin-flip can be explained without requiring the magnetic field to do work. A classical dipole (a spinning charged ball) is also studied. For details please refer to R.J. Deissler, Phys. Rev. E. {\bf 77}, 036609 (2008). A link to this paper, as well as other information, may be found at http://deissler.us/. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L41.00009: Casimir force measurements between a gold sphere and a rectangular corrugated Silicon plate Yiliang Bao, Jie Zou, H.B. Chan The Casimir force is the interaction that results from quantum fluctuations of electromagnetic fields in vacuum and strongly depends on the shape of the boundaries that confines the electromagnetic fields. Most previous experiments involve simple geometries such as plate-sphere, two parallel plates or two cylinders, where the pair-wise summation of two-body interactions is still valid. To demonstrate the strong shape dependence of the Casimir force, we choose one of the interacting surfaces to be an array of trenches with widths ranging from 200 nm to 500 nm. Both high-aspect-ratio trenches with depth of 1 um and shallow trenches with depth of 100 nm are fabricated. The force gradient on these structures is measured with a micromechanical torsional oscillator for the separations between 150 nm and 500 nm. We observe deviations from both the pair-wise additive approximation and the proximity force approximation. The observed deviation, however, is smaller than the calculated values for perfectly conducting surfaces, possibly due to the interplay between finite conductivity and geometry effects. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L41.00010: Distance dependence of contact potential in cylindrical-plane Casimir force measurements Qun Wei, Kevin Miller, Diego Dalvit, Roberto Onofrio We report on the status of an experiment aimed at measuring the Casimir force in cylinder-plane geometry. In order to characterize the apparatus, we have first performed small distance electrostatic calibrations. This has allowed us to better identify various general issues on the measurement of the Casimir force, such as the distance dependence of the contact potential, and the delicate assessment of the absolute distance. The determination at all distances of the contact potential $V_{0}$ is particularly crucial since its distance dependence can affect the entire data analysis procedure. We also carried on the measurements of $V_{0}$ in sphere-plane and plane-plane geometries for comparison. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L41.00011: Optical binding force acting on two optically trapped particles H.D. Ou-yang, Ming-Tzo Wei In addition to common optical manipulation setups such as an optical tweezers, the radiation forces generated by a laser can also induce chain-like arrangements of $\mu $m-sized dielectric spheres through coherent multiple scattering, through a process known as optical binding (OB). Although the forces generated through OB are on the order of piconewtons, they are still sufficient to overcome other relevant interactions in the suspension such as Van Der Waals and gravitational forces and Brownian fluctuations. The OB force oscillates from attractive to repulsive as function of interparticle separation; as observed in theoretical models and optical fields found in systems such as counter propagating lasers in dual-beam optical-fibers. Using a dual optical tweezers setup, we have measured the inter-particle OB force from two 1.5 micron diameter polystyrene particles in suspension as a function of their separation by holding them in separate optical traps. Using a calibration scheme, we have isolated the OB force from the background of hydrodynamic and Brownian forces. Using experimental measurements and theoretical predictions, we also proved that by changing the respective polarizations between parallel and perpendicular orientations of the two traps, the OB force was the only force acting on the particles. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L41.00012: Reflection of Various Types of Waves by Layered Media Sergiy Mokhov, Boris Zeldovich The one-dimensional wave equation describing propagation and reflection of waves in a layered medium is transformed into an exact first-order system for the amplitudes of coupled counter-propagating waves. Any choice of such amplitudes, out of continuous multitude of them, allows one to get an accurate numerical solution of the reflection problem. We discuss relative advantages of particular choices of amplitude. We also introduce the notion of reflection strength $S$ of a plane wave by a nonabsorbing layer, which is related to the reflection intensity $R$ by $R=\tanh ^2S$. We show that the total reflection strength by a sequence of elements is bounded above by the sum of the constituent strengths, and bounded below by their difference. Reflection strength is discussed for propagating acoustic waves and quantum mechanical waves. We show that the standard Fresnel reflection may be understood in terms of the variable $S$ as a sum or difference of two contributions, one due to a discontinuity in impedance and the other due to a speed discontinuity. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L41.00013: Gravitational Redshift and Deflection of Slow Light Justin Dressel, S. Rajeev, John Howell, Andrew Jordan We explore the nature of the classical propagation of light through media with strong frequency-dependent dispersion in the presence of a gravitational field. In the weak field limit, gravity causes a redshift of the optical frequency, which the slow-light medium converts into a spatially-varying index of refraction. This results in the bending of a light ray in the medium. We further propose experimental techniques to amplify and detect the phenomenon using weak value measurements. Independent heuristic and rigorous derivations of this effect are given. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L41.00014: Control of a mechanical resonator mode by cavity-enhanced light scattering Ako Chijioke, John Lawall Reaching the quantum regime of a mechanical resonator is facilitated by using a resonator with a small mass and high frequency. On the other hand, optical interferometry fails if the dimensions of the resonator are not significantly larger than the optical wavelength. Here we discuss and demonstrate an alternative optical technique employing scattering losses within an optical cavity to sense the motion of a resonator that can have dimensions well below the optical wavelength. We place a wavelength-scale mechanical resonator at the waist of a high-finesse optical cavity, lock the cavity to a resonance, and monitor the transmission. As the resonator vibrates, it modulates the cavity loss and thereby the transmitted power. We calibrate the sensitivity to resonator position by means of a known static displacement. We then sense the thermal motion of the resonator, and employ active feedback to cool, heat, and stiffen the mechanical mode. [Preview Abstract] |
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