Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y1: Fractional Quantum Hall Effect: Spin Effects and Broken-translational-symmetry States
Sponsoring Units: DCMPChair: James Eisenstein, California Institute of Technology
Room: Colorado Convention Center Four Seasons 2-3
Friday, March 9, 2007 11:15AM - 11:51AM |
Y1.00001: Low frequency spin dynamics in a quantum Hall canted antiferromagnet Invited Speaker: In quantum Hall (QH) systems, Coulomb interactions combined with the macroscopic degeneracy of Landau levels (LLs) drive the electron system into strongly correlated phases as illustrated by the series of fractional QH effects and may also lead to various forms of broken symmetry dictated by the LL filing factor $\nu $. When two layers of such electron systems are closely separated by a thin tunnel barrier, the addition of interlayer interactions and the layer degree of freedom brings about even richer electronic phases, opening up possibilities for different classes of symmetry breaking. In particular, at total filling factor $\nu _{T}$ = 2, where the two of the four lowest LLs split by the Zeeman and interlayer tunnel couplings are occupied, the competing degrees of freedom due to the layer and spin are predicted to lead to rich magnetic phases. Here we present results of resistively detected nuclear spin relaxation measurements in closely separated electron systems that reveal strong low-frequency spin fluctuations in the QH regime at $\nu _{T}$ = 2 [1]. As the temperature is decreased, the spin fluctuations, manifested by a sharp enhancement of the nuclear spin-lattice relaxation rate 1/$T_{1}$, continue to grow down to the lowest temperature of 66 mK. The observed divergent behavior of 1/$T_{1}$ signals a gapless spin excitation mode (i.e., a Goldstone mode) and is a hallmark of the theoretically predicted canted antiferromagnetic order. Our data demonstrate the realization of a two-dimensional system with broken planar spin rotational symmetry, in which fluctuations do not freeze out when approaching the zero temperature limit. [1] N. Kumada, K. Muraki, and Y. Hirayama, Science \textbf{313}, 329 (2006). [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y1.00002: Density dependent anisotropic phases in a two-dimensional hole system Invited Speaker: Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors $\nu $=7/2, $\nu $=11/2, and $\nu $=13/2 at low temperature. In stark contrast, the transport at $\nu $=9/2 is \textit{isotropic} for all temperatures. Our results for a 2D hole system differ substantially from 2D electron transport where no anisotropy has been observed at $\nu $=7/2, and the strongest anisotropy occurs at $\nu $=9/2. Isotropic hole transport at $\nu $=13/2, 11/2 and 7/2 is restored for sufficiently low 2D densities. The density dependence of the observed anisotropies suggests that strong spin-orbit coupling in the hole system contributes to the unusual transport behavior. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y1.00003: Spectroscopy of quasiparticle excitations in quantum Hall fluids Invited Speaker: Quantum Hall fluids support low-energy excitation modes that are linked to remarkable behaviors that emerge from fundamental interactions in two-dimensions. Inelastic light scattering methods at very low temperatures (below 1 Kelvin) offer unique experimental venues to study excitations in the charge and spin degrees of freedom of the fluids. This talk presents an overview of recent results. The focus is on low-lying excitations that express distinct quantum phases of the electron liquids. The experiments offer insights on translational symmetry, on magnetoroton excitations and on quasiparticle energy level structure. The excitations are probed in diverse states of the electron liquids to provide insights on quasiparticle properties and on the phase transformations between quantum fluid states. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y1.00004: Microwave Spectroscopy of Wigner crystals in 2DES and Bilayer Systems: Many-body correlation in electronic quantum solids Invited Speaker: It is generally known that in high quality two dimensional electron systems (2DES, similarly for 2D hole systems and bilayer systems) under sufficiently large perpendicular magnetic field $B$, the quantum Hall (QH) states terminate into an electronic solid --- a Wigner crystal (WC) pinned by disorder. After a brief review of solid phases in QH systems (including several recently discovered ones [1]) as known from microwave spectroscopy (measuring a characteristic pinning mode resonance of the solid), I will discuss two of our experiments that highlight the importance of many-body quantum correlation in the high-$B$ WC. In one experiment [2], we measured the \textit{melting} temperature ($T_c$) of the high-$B$ WC at many different $B$ and densities $n$ and in multiple 2DES samples. The data show unambiguously that in a given sample, $T_c$ is controlled by Landau filling $\nu$=$nh/eB$ instead of by $n$. This demonstrates the quantum nature of the high-$B$ WC and that its melting is dependent on many-body quantum correlation (via $\nu$). Such behavior contrasts with any other known solids (in particular, a classical electron solid), whose $T_c$ are determined by $n$. In addition, we found that stronger pinning disorder in samples with tighter vertical confinement led to an enhancement of $T_c$. In another experiment [3], we studied \textit{bilayer} WC (BWC) in bilayer hole systems (in low inter-layer tunneling limit). We found that in samples with a bilayer exciton condensate (BEC) QH state at $\nu$=1, the pinning mode frequency of the BWC ($\nu$$\ll$1) is systematically enhanced from what would be expected from two classically interacting single-layer WC. The enhancement decreases with increasing effective layer separation and is not observed for samples without the $\nu$=1 state. We suggest that our results give evidence for a pseudospin (layer index) ferromagnetic BWC, which possesses interlayer quantum correlation and long range in-plane phase coherence similar to that in the $\nu$=1 BEC state and can experience enhanced pinning [4] in the presence of interlayer spatial correlation of disorder. [1] Yong P.~Chen \textit{et al}., Phys.~Rev.~Lett. \textbf{93}, 206805 (2004); Phys.~Rev.~Lett. \textbf{91}, 016801 (2003); [2] Yong P.~Chen \textit{et al}., Nature Physics \textbf{2}, 452 (2006); [3] Z. Wang \textit{et al}, submitted; [4] Yong P.~Chen, Phys.~Rev.~B \textbf{73}, 115314 (2006). [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y1.00005: Interweaving of Spin and Pseudospin in Bilayer Quantum Hall Systems Invited Speaker: In a bilayer quantum Hall system, the layer index may effectively act like a two-valued degree freedom that is analogous to the spin of the electron. Near filling factor $\nu=1$ this pseudospin is thought to lock into a linear combination of the two possible values of the layer index, yielding an interlayer coherent state analogous to an easy-plane ferromagnet. Such systems possess excitations known as merons, vortex-like objects in which the pseudospin tilts out of the plane near their cores. In quantum Hall systems these are charged objects, and can be injected into the groundstate by doping away from $\nu=1$, yielding a pseudospin textured state. However, recent experiments [1] have suggested that charged excitations may tilt the {\it real} electron spin away from its most polarized state. In this work [2] we study the possibility of simultaneously producing both spin and pseudospin textures in a quantum Hall bilayer near $\nu=1$. Our Hartree-Fock calculations demonstrate that the groundstate generically forms a textured crystal, and that for appropriate choices of Zeeman coupling, interlayer tunneling, interlayer separation, and interlayer bias, the texture can be present in both the spin and pseudospin degrees of freedom. Such states spontaneously break the real rotational spin symmetry and possess a gapless spin wave mode. The possible relevance of this to enhanced NMR relaxation rates observed recently in experiment is discussed. \hfil\break [1] I. Spielman et al., PRL {\bf 94}, 076803 (2005); N. Kumada et al., PRL {\bf 94}, 096802 (2005). \hfil\break [2] J. Bourassa, B. Roostaei, R. C\^ot\'e, H.A. Fertig, and K. Mullen, PRB {\bf 74}, 195320 (2006). [Preview Abstract] |
Session Y2: Collective Motions of Living and Nonliving Self-Propelled Particles
Sponsoring Units: DBPChair: Frank Moss, University of Missouri-St. Louis
Room: Colorado Convention Center Four Seasons 4
Friday, March 9, 2007 11:15AM - 11:51AM |
Y2.00001: Swarming by Nature and by Design Invited Speaker: The cohesive movement of a biological population is a commonly observed natural phenomenon. With the advent of platforms of unmanned vehicles, this occurrence is attracting renewed interest from the engineering community. This talk will review recent research results on modeling and analysis of biological swarms with some connection to the design ideas for efficient algorithms to control groups of autonomous agents. For biological models we consider two kinds of systems: driven particle systems based on force laws and continuum models based on kinematic and dynamic rules. Both models involve long-rage social attraction and short range dispersal and yield patterns involving clumping, mill vortices, and surface-tension-like effects. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y2.00002: Direct and Indirect Mechanisms for Collective Behavior in the Spatial Dynamics of Plankton Invited Speaker: Plankton are the dark matter of life in the sea. Though they are poorly understood and usually unseen, plankton dominate the biological dynamics that ultimately determine characteristics important to humans ranging from sustainable fish harvests to rates of carbon sequestration. Through a variety of social, sensory and biophysical mechanisms, plankton display collective behaviors that profoundly alter ecological systems. These collective behaviors include formation of large, coherent social groups (e.g. swarms and schools); alteration of water's mechanical properties (e.g. viscosity) and motion (e.g. bioconvection); and induction of self-organized spatial heterogeneity. In this talk, I will describe recent individual-level observations of collective plankton behaviors. I will develop mathematical descriptions that link some of these behaviors to spatio-temporal patterns in plankton populations. Finally, I will outline some important unsolved problems in plankton ecology that can be addressed using analytical and computational approaches. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y2.00003: Predicting the growth of fractal particle agglomeration networks with graph theoretical methods. Invited Speaker: We study an electromechanical system [J. Jun, A. Hubler, \textit{PNAS} \textbf{102}, 536 (2005); J. Jun, Ph.D. thesis, UIUC (2004)], where conducting particles self-organize into dendritic patterns under the influence of an electric field for the purpose of collecting and transporting charge. The system forms stable open-loop networks with many reproducible statistical quantities, such as the number of termini and the number of branch points, but the final topology of the network is sensitive to the initial conditions of the particles. Small differences in the initial configuration may lead to very different stationary states. We present robust and reliable ensemble prediction algorithms for the growth of such fractal charge transportation networks. These predictors may lead to the discovery of common properties and serve a prototype to predict fractal growth in other areas, including neural systems; blood vessel systems, river networks, and dielectric break through. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y2.00004: Swarming Behavior of Particle-Like Waves in Excitable Media Invited Speaker: Unstable waves in the photosensitive Belousov-Zhabotinsky reaction are stabilized by global feedback, and the motion of these waves is controlled by imposing excitability gradients that are regulated by a secondary feedback loop. We describe studies of these particle-like waves interacting with one another via realistic excitability potentials. Simulations and experiments with increasing numbers of mutually coupled waves have demonstrated very complex swarming behavior. Measures for characterizing the behavior, such as the average velocity and group size, will be discussed. We will also describe experiments and simulations of stabilized waves navigating excitability landscapes. Of particular interest is the interaction of a swarm with various obstacles as it navigates through the medium. [E. Mihaliuk, T. Sakurai, F. Chirila, and K. Showalter, Phys. Rev. E 65, 65602 (2002); T. Sakurai, E. Mihaliuk, F. Chirila, and K. Showalter, Science 296, 2009-2012 (2002); V. S. Zykov and K. Showalter, Phys. Rev. Lett. 94, 068302 (2005).] [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y2.00005: Chemically Powered Nanomotors Invited Speaker: Molecular motors play important roles in transport in biological systems. These molecular machines are powered by chemical energy and operate in the regime of low Reynolds number hydrodynamics. Recently a class of simple inorganic molecular motors has been constructed and studied experimentally [1,2]. These motors are bimetallic rods, one end of which is chemically active. The talk will describe simple mesoscopic models for the motion of such nanomotors. The motor consists of two linked spheres, one of which catalyzes the conversion between two chemical species. The chemical species interact differently with the the two spheres in the dimer. The nano-dimer motor is solvated by a molecules treated at a mesoscopic level whose evolution is governed by multi-particle collision dynamics. The dynamics conserves mass, momentum and energy so that coupling between the nanomotor and the hydrodynamic modes of the solvent is treated correctly. The simulations allow one to explore the mechanisms of the chemically powered motion and the effects of fluctuations on the motor dynamics. \newline \newline [1] W. F. Paxton, et al., ``Catalytic Nanomotors: Autonomous Movement of Striped Nanorods,'' J. Am. Chem. Soc. (JACS), 126 (41), 13424 (2004). \newline [2] S. Fournier-Bidoz, et al. ``Synthetic Self-Propelled Nanorotors,'' Chem. Commun., (4), 441 (2005). [Preview Abstract] |
Session Y3: Exotic electronic structures of complex materials and phases
Sponsoring Units: DCMPChair: Richard Martin, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center Korbel 2A-3A
Friday, March 9, 2007 11:15AM - 11:51AM |
Y3.00001: The electronic structure of a liquid Pb film Invited Speaker: Our understanding of the electronic structure of condensed matter in the liquid phase is far from complete. We used angle-resolved photoemission spectroscopy (ARPES) in order to study the evolution of the electronic bands, the Fermi surface and the spectral function of a lead monolayer on Cu(111) as the film went through its melting transition at 568 K [1]. The crystalline copper substrate provides the reciprocal lattice vectors, absent in the liquid state, that are needed in ARPES for wave-number conservation in the excitation process, and the well-resolved copper bands serve as an important reference frame for identifying the dramatic changes in the lead states. Electron spectra and momentum distribution maps of the liquid film reveal three important features: the persistence of a Fermi surface, the filling of band gaps, and the localization of the wave functions upon melting. Distinct coherence lengths for different sheets of the Fermi surface are found, indicating a strong dependence of the localization lengths on the character of the constituent atomic wave functions. \newline \newline [1] F. Baumberger, W. Auw\"arter, T. Greber, J. Osterwalder, Science 306, 2221 (2004). [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y3.00002: Thin-Film Electronic Structure: Beyond the ``Particle in a Box" Model Invited Speaker: The quantization of electronic states in thin metallic films is now well-established, having been observed in a number of systems including films on metal as well as semiconductor substrates. The impact of this quantization on the films' physical properties has been demonstrated in several studies, including the dependence on thickness of films' thermal stabilities, work functions, and superconductivity transition temperatures. In the simplest model, the electrons are confined to the film by the substrate and vacuum interfaces, which work as ``mirrors'' to reflect the electrons back into the film, resulting in discrete standing-wave states. In this picture, the substrate forms a reflecting barrier due to a mismatch of electronic structures between it and the overlayer, and the main result is the formation of discrete energy subbands. In this talk, photoemission results will be presented from a variety of thin-film systems that show more interesting electronic structures due to interactions with the substrate and interface. The systems studied highlight different effects, including interfacial scattering and diffraction, hybridization of film and substrate states, and the formation of a composite quantum well from a thin metallic film on a semiconductor. In the latter case, the semiconductor depletion region forms part of the system via coherent coupling between film and substrate electronic states. In collaboration with S. J. Tang, N. J. Speer, D. Ricci, M. Upton, L. Basile, S.-L. Chang, Y.-R. Lee, and T.-C. Chiang. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y3.00003: Ultrahigh-resolution photoemission from superconductors and strongly-correlated materials Invited Speaker: I would like to introduce the ultra-high resolution photoemission study of several superconductors and strongly correlated materials using UV-laser-photoemission [1]. The laser-PES system has the performance of about 360-$\mu $eV resolution at about 3 K at present. We will also introduce the newly developed laser-PES system that has the higher performance. Recently, it is found that the diamond becomes superconductor when the boron is doped heavily. We observed the superconducting gap and several phonon structures in the ultra-high resolution photoemission spectra of the doped diamond [2]. We would like to introduce several superconductors including organic materials. It is found, for the first time, as a transition metal compound that LiV$_{2}$O$_{4}$ is a heavy Fermion material. We have directly observed a sharp peak structure at 4 meV just above $E_{F}$ in $d$-electron heavy Fermion LiV$_{2}$O$_{4}$ [3]. The spectral shape and the temperature dependence of the peak structure are very similar to those of the Kondo resonance in conventional $f$-electron heavy Fermion compounds. The present result can be well described within Kondo scenario. I also show several ARPES results on superconductors and strongly-correlated materials. \newline \newline [1] T. Kiss, et al., Phys. Rev. Lett. \textbf{94}, 057001 (2005). \newline [2] Ishizaka et al., unpublished \newline [3]A. Shimoyamada,et al., H. Ueda, Y. Ueda, S. Shin, Phys. Rev. Lett. \textbf{96}, 026403 (2006). [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y3.00004: The spectral function and quasiparticle dynamics of graphene thin films Invited Speaker: Graphene, a single layer of carbon atoms arranged in a simple honeycomb lattice, is the building block of graphite, fullerenes, and carbon nanotubes and has fascinating electronic properties deriving from the effectively massless, relativistic behavior of its charge carriers. The study of many-body interactions among these carriers is of interest owing to their contribution to superconductivity in these systems. I will report synthesis of graphene thin films (1-4 layers) grown on SiC and the evolution of their band structure using angle-resolved photoemission spectroscopy (ARPES). We determined the spectral function for graphene as a function of doping, which encodes the many-body interactions among the quasiparticles in the system---namely the charge and vibrational excitations. Our measurements show that the bands around the Dirac crossing point are heavily renormalized by electron-electron, electron-plasmon, and electron-phonon coupling, showing that these interactions must be considered on an equal footing in attempts to understand the quasiparticle dynamics in graphene and related systems. At very high doping (comparable to graphite intercalation compounds (GICs)) renormalization of the carrier mass near $E_{F}$ becomes significant for electrons moving in certain directions, supporting the importance of electron-phonon coupling in superconductivity in GICs. [Preview Abstract] |
Session Y4: Assembly and Organization in Polymeric Systems
Sponsoring Units: DPOLYChair: Jack Douglas, National Institute of Standards and Technology
Room: Colorado Convention Center Korbel 2B-3B
Friday, March 9, 2007 11:15AM - 11:51AM |
Y4.00001: Self Organization via Frontal Polymerization Invited Speaker: There are three modes of frontal polymerization: Isothermal, Photo and Thermal Isothermal frontal polymerization (IFP) is a directional polymerization that utilizes the Norish-Trommsdorff effect, to produce optical gradient materials. When a solution of methyl methacrylate and thermal initiator contacts a polymer seed (a small piece of poly(methyl methacrylate), a viscous region is formed in which the polymerization rate is faster than in the bulk solution. PhotoFP is driven by a continuous input of light. Thermal frontal polymerization is the propagation of a localized reaction zone through the coupling of thermal transport with the Arrhenius dependence of the kinetics of an exothermic polymerization. We will examine IFP and its use in making Gradient Optical Materials (GRIN) and our work on elucidating the mechanism. We will consider how thermal frontal polymerization can be used rapid rapid repair, making gradient materials and to study interesting nonlinear modes of thermal front propagation. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y4.00002: Structural Analysis and Properties of Supramolecular Assemblies Invited Speaker: Supramolecular and particle assembly is controlled by the shape and interaction among the constituents, and there is great interest involving asymmetric systems. Here we explore physical rules and behavior governing the assembly of anisotropic molecules and particles. These are wedge and conical shaped amphiphilic molecules and asymmetric particles. Diffraction and microscopy are used to determine the assembled structures (such as tubes and capsules) and to establish relationships between structures in systems that undergo transitions from one ordered state to another. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y4.00003: Controlling Self-Assembly in Thin Block Copolymer Films: From Model Systems to Applications Invited Speaker: We discuss recent experiments and computer simulations dealing with the influence of external fields (electric fields, surface fields) on the spontaneous structure formation in block copolymer melts and block copolymer solutions. We demonstrate the aligning effect of electric fields on non-cubic block copolymer microphases and discuss the microscopic processes responsible for the macroscopic effects. Recent findings include the possibility to tune the nanoscopic characteristic spacing via the strength of the external field. In thin films, on the other hand, the presence of surfaces and the geometrical constraints of finite film thickness dominate the resulting structures. We shall include some applications of these structures, e.g. in the area of responsive membranes and organic light harvesting devices. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y4.00004: pH and Solubility Effects as Control Mechanisms for Vesicle Interfaces Invited Speaker: It was shown some time ago that in diblock copolymer vesicles the corona chains are segregated by length with short block inside and long blocks outside. More recently, it was shown that a triblock copolymer consisting of a long polystyrene segment with short segments of poly(acrylic acid) and poly(-4-vinyl pyridine) at each end could be converted into vesicles which, depending on the pH during preparation, could have either PAA or P4VP on the outside. For example a low pH, at which the 4VP is protonated, leads to P4VP on the outside because those chains become longer than the nonionic PAA. Furthermore, inversion of the vesicles from PAA outside to P4VP outside is possible by dropping the pH of the solution. It is now shown that under extreme pH conditions in dimethylformamide, the inside and outside interfaces are behaving exactly opposite to what it is expected. For example, at very low pH (3), at which the P4VP should be ionic and therefore longer than PAA, it is found that the PAA is on the outer interface of the vesicle, while the P4VP is inside. The reason for such behavior can be found in the solubilities and coil dimensions of the quaternized P4VP, which is poorly soluble in DMF at low pH and also the poor solubility of the poly(sodium acrylate) in DMF at high pH. The poor solubilities overwhelm the coil expansion accompanying ionization at moderate pH. This solubility effect thus provides us with an added mechanism for controlling the interior and exterior interfaces of vesicles, in addition to the block length and the simple pH effect. This work is based on the MSc Thesis of Renata Vyhnalkova, McGill University, 2005. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y4.00005: Nanomechanics of Bone: Nanogranular Friction and Heterogeneity Invited Speaker: Bone is a complex nanostructured biocomposite material composed of approximately 60 wt percent carbonated apatite mineralites (10s of nanometers in length and width, 3-5 nm in thickness) that permeate in and around type I collagen fibrils. Here, we present results from studies investigating the ultrastructural origins of the strength and toughness of bone, which is critical for its proper physiological function. A combination of dual instrumented nanoindentation, three-dimensional elastic-plastic finite element analysis (FEA) using a Mohr-Coulomb cohesive-frictional strength criterion, and angle of repose measurements was employed. Our results suggest that nanogranular friction between mineral particles is responsible for increased yield resistance in compression relative to tension and that cohesion originates from within the organic matrix itself, rather than organic-mineral bonding. Nanomechanical heterogeneity is also expected to influence elasticity, damage, fracture and remodeling of bone. Here, we quantify the spatial distribution of nanomechanical properties at the length scale of individual collagen fibrils using atomic force microscope-based nanoindentation. Our results show elaborate patterns of stiffness which do not correlate with topography, and hence are attributed to organic-inorganic compositional variations and nanoscale porosity. We propose a new energy dissipation mechanism arising from nanomechanical heterogeneity which offers a graceful means for ductility enhancement, damage evolution, and toughening. This hypothesis is supported by FEA which incorporate the nanoscale experimental data and predict markedly different biomechanical properties compared to a uniform material, through nonuniform inelastic deformation over larger areas and increased energy dissipation. The fundamental concepts discovered here are applicable to a broad class of biological materials and may serve as a design consideration for biologically-inspired materials technologies. [Preview Abstract] |
Session Y5: Fluctuating Random Solids and their universal properties
Sponsoring Units: GSNP DCMPChair: Horacio Castillo, Ohio University
Room: Colorado Convention Center Korbel 1A-1B
Friday, March 9, 2007 11:15AM - 11:51AM |
Y5.00001: Nonaffine deformations in random solid media Invited Speaker: The elastic properties of materials which are inhomogeneous on mesoscopic length scales is a subject of broad interest in soft matter physics. Example systems include stiff polymer or biopolymer gels, foams, emulsions, grain packs, and microstructured solids. These diverse systems share the common feature that their linear elastic response is highly non-uniform, or ``non-affine'' at intermediate length-scales. I will present a general theoretical framework for interpreting the non-affine component of the linear elastic response of inhomogeneous materials. I will outline the connection between measured correlation functions and internal quantities such as correlation lengths, internal stress fields, and the degree of local elastic heterogeneity. I will show that the simplest 2-point correlation function gives misleading results in 2 dimensions, and I will propose better functions to measure. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y5.00002: Incompressibility, fluctuations, and elasticity in random solids Invited Speaker: Rubbers and elastomers are usually characterized by two common properties: entropic elasticity and incompressibility. At short length-scales, these systems behave as incompressible liquids. Nevertheless, macroscopic shear deformations reduce the entropy of the polymer network, and therefore cost an elastic free energy that is proportional to temperature. In this talk I shall discuss the role of incompressibility in the elasticity of rubbery materials, and its interplay with the long wave-length fluctuations. Rubbers gain shear rigidity through the vulcanization transition, a second-order phase transition driven by cross-link density and closely related to percolation. The scaling of shear modulus as a critical phenomenon sensitively depends on the incompressibility. We have recently discovered that the vulcanization theory naturally exhibits two universality classes: phantom systems and incompressible systems. Each class exhibits distinct scaling exponent for the shear modulus near the transition. Incompressibility also crucially affects the nonlinear elasticity of rubbery materials. As we have shown recently, a subtle interplay between incompressibility and long wave-length fluctuations leads to a qualitative modification of the stress-strain relation predicted by the classical theory. To leading order, this mechanism provides a simple and generic explanation for the peak structure of Mooney-Rivlin stress-strain relation, and shows good agreement with experiments. It also leads to the prediction of a phonon correlation function that depends on the strain deformation. If time permits, I will also address incompressibility and fluctuations in liquid crystalline elastomers. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y5.00003: Floppy modes and non-affine deformations in biopolymer networks Invited Speaker: Fibrous materials are ubiquitous in nature. They form the cytoskeleton of cells and are essential components of the extracellular matrix. Its building blocks are stiff protein filaments and a myriad of associated crosslinking proteins. The interplay between the elasticity of the biopolymers and the binding and elastic properties of the crosslinkers lead to a variety of network architectures [1]. We review recent advances in understanding the elastic properties of these networks in terms of ``floppy modes'' [2], which are the relevant low-energy excitations characterizing non-affine deformations. This approach might very well serve as a novel paradigm to understand the elasticity of microstructured materials. The theoretical concepts are applied to recent experimental studies of F-actin networks crosslinked with fascin. [1] C. Heussinger and E. Frey, Stiff Polymers, Foams and Fiber Networks, Phys. Rev. Lett. 96, 017802 (2006). [2] C. Heussinger and E. Frey, Floppy Modes and Non-Affine Deformations in Random Fiber Networks, Phys. Rev. Lett. 97, 105501 (2006) [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y5.00004: Molecular dynamics studies of rigidity in solids Invited Speaker: We have used molecular dynamics (MD) to study the elastic properties of systems of particles randomly and permanently crosslinked to each other as function of crosslink density $p$. At zero temperature, such systems generically lose the ability to withstand shear at a rigidity percolation point, $p_r$, that is (at least for particles interacting through central forces) different from the geometric percolation point $p_c$. At finite temperatures there is an entropy-generated component of the shear modulus $G(p,T)$ that remains finite for all $p > p_c$ and which vanishes with a characteristic power law $G(p,T)\sim (p-p_c)^t$. Our simulations in both two and three dimensions seem to indicate that $t$ is model-independent and, within our error bars, the same as the exponent that describes the behavior of a disordered network of conductors near its percolation point. [Preview Abstract] |
Session Y6: Towards Medium Temperature Proton Conductors for Fuel Cell Applications
Sponsoring Units: FIAPChair: Ali Sayir, NASA Glenn Research Center, Cleveland, Ohio
Room: Colorado Convention Center 207
Friday, March 9, 2007 11:15AM - 11:51AM |
Y6.00001: On the question of ``dry'' proton motion in ionic liquids and plastic crystals Invited Speaker: In supercooled water containing 0.01MHCl, the ionic conductivity at -32$^{o}$C is more than an order of magnitude higher than would be predicted from its fluidity. The developing tetrahedral order and the associated high vib-librational anharmonicity, permits efficient ``dry'' proton ``hopping'' transfer of protons between favorable sites. Reproducing this transport mechanism in non-aqueous (and preferably also solid) phases is a leading aim of current research. We report progress in this direction substituting water by spinning protonated cations such as NH$_{4}^{+}$ and CH$_{3}$NH$_{3}^{+}$, and anions such as HSO$_{4}^{-}$, HPO$_{3}$F$^{- }$and H$_{2}$PO$_{4}^{-}$ for the water molecules, studying both liquid and plastic crystal phases. We use pulsed field gradient NMR to distinguish proton motion from host $^{31}$P species motion, and use double quantum techniques to study $^{1}$H{\ldots}$^{31}$P separation kinetics. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y6.00002: Polymic protic salt membranes, a new approach to the Holy Grail of a solid state proton conductor Invited Speaker: Electrons are readily transported in solids through the electronic conduction band in metals and semiconductors, but ion conduction is not as simple. Most proton conductors are aqueous solutions. Water plays two roles: i) water ionizes dissolved acids (and bases), and ionic conductivity results from the diffusion of protons and anions, a vehicular mechanism; ii) water also has an accessible~``proton hopping path'' (proton transport via hydrogen bonding and rotations) resulting in higher solution conductivity than by diffusion of ions alone. Some liquids, like phosphoric acid, have been know to conduct only protons with no co-transport of other species, but until recently these have been few in number. Some solids, like polyguanine, conduct only protons, but there have been even fewer reports of these. A proton transfer salt is an equimolar mixture of an acid and a base that internally transfer a proton. Recently, a number of proton transfer salts in the liquid state have been found that can transport proton without water, even at temperatures well above the boiling point of water. Whether a vehicular or hopping transport mechanism operates for these liquid proton transfer salts is under study. Vehicular transport is not possible in a solid membrane made for proton transfer salt formed from a solid polymer with one moiety (e.g., base) covalently fixed into the polymer and with the other moiety (e.g., acid) electrostatically bound after proton transfer. Synthetic strategies and characterization of solid proton conducting membranes, including solid protic transfer salt membranes, will be presented. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y6.00003: Polybenzimidazole-phosphoric acid complexes and proton conducting membranes Invited Speaker: Polybenzimidazole (PBI) polymers are excellent candidates for PEM fuel cell membranes capable of operating at temperatures up to 200$^{\circ}$C. The ability to operate at high temperatures provides benefits such as faster electrode kinetics and greater tolerance to impurities in the fuel stream. In addition, PBI membranes doped with phosphoric acid can operate efficiently without the need for external humidification and the related engineering hardware to monitor and control the hydration levels in the membrane. A new sol-gel process was developed to produce PBI membranes loaded with high levels of phosphoric acid. This process uses polyphosphoric acid as the condensing agent for the polymerization and the membrane casting solvent. Upon hydrolysis of the solvent to phosphoric acid, a sol-gel transition occurs to provide membranes with an attractive set of properties. PBI membranes are currently being investigated as candidates for portable, stationary, and transportation PEM fuel cell applications. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y6.00004: Protonic Conducting Ceramic for 300-400 \r{ }C Invited Speaker: Combining high protonic conductivity with thermodynamic stability is considered to be a key problem for high temperature protonic conducting ceramic (HTPC) membranes for electrochemical applications. The objective was to gain an understanding of the relationship of crystallite size, grain boundaries and defect chemistry on proton conduction and thermodynamic stability. We developed an analytical method using concurrent techniques of high-resolution transmission electron microscopy, impedance spectroscopy and nuclear microprobe to reveal spatial distribution of hydrogen. In our recent work, we shown that high-density of defects exist in the microstructure can reduce the level of proton incorporation. The results showed that hydrogen is concentrated at the grain boundaries where the hydrogen mobility is low. Perovskite structure BaCe$_{1-x}$Y$_{x}$O$_{3-\delta }$ (BCY) and BaZr$_{1-x}$Y$_{x}$O$_{3-\delta }$ films were deposited using pulsed laser deposition system on porous structures to provide mechanical strength. The ease with which the stoichiometry of a multi-component system can be maintained in the deposited films using pulsed laser deposition approach offered a significant advantage over other conventional film deposition techniques. Impedance spectroscopy was used to investigate protonic conductivity of high-density BaCe$_{1-x}$Y$_{x}$O$_{3-\delta }$ (BCY) and BaZr$_{1-x}$Y$_{x}$O$_{3-\delta }$ (BZY) films (2 - 5 $\mu $m). The crystallite size, grain boundaries and defect chemistry were characterized by XRD, SEM, TEM and HRTEM which showed the adverse effect of grain and domain boundaries. Variation of the process parameters, in particular of the substrate temperature, induced changes in the microstructure of the films and in their conductivity. Columnar grains enclosing reduced density of defects were seen to yield the best proton conductivities. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y6.00005: Understanding microstructure-induced limitations of hydrogen transport in high temperature proton conductors: can nuclear microanalysis give an answer? Invited Speaker: High temperature protonic conductors (HTPC) are envisioned as electrolytes for fuel cells working at intermediate temperature (400\r{ }C -- 600\r{ }C) to complement Y:ZrO$_{2}$ electrolytes operating at 800\r{ }C -- 1000\r{ }C. The most mature HTPC are doped perovskites (ABO$_{3})$ where tetravalent cation B is partially substituted by a trivalent one. Protons can be introduced in the lattice as point defects corresponding to hydroxyl groups on oxygen ion sites. In the temperature region of interest for technological applications, lattice vibrations allow the diffusion of protons by jumping and reorientation of O-H bonds (hoping mechanism). BaCeO$_{3}$ or SrCeO$_{3}$-based perovskites doped with a rare earth are the most widely studied compounds. However the proton conductance of these ceramics and their chemical stability are lower than the calculated values on single crystals and not sufficient to fulfill technological requirements. In most cases, the reasons for these discrepancies lie in uncontrolled microstructures with inter- and intra-granular defects that act as barriers for hydrogen diffusion but are preferential paths for chemical degradation by hydrolysis or carbonatation. Despite this crucial point, very few efforts are devoted to the optimization of microstructure of HTPC. Microstructure induced limitations are usually evidenced via impedance measurements which enable determination of respective contributions of bulk and grain boundaries to overall conductivity. Further information on hydrogen transport relevant for improvement of microstructure design requires local methods for hydrogen concentration measurement. Nuclear microanalysis, based on the use of MeV light ions microbeam, meets this demand. According to the chosen technique, nuclear reaction, elastic recoil or forward coincident scattering, the nuclear microprobe gives 2D-3D quantitative information on hydrogen distribution and diffusion within microstructure and enables to identify barriers and short-circuits. [Preview Abstract] |
Session Y7: Nonequilibrium Thermodynamics
Sponsoring Units: DBPChair: Felix Ritort, Universitat de Barcelona
Room: Colorado Convention Center Korbel 4A-4B
Friday, March 9, 2007 11:15AM - 11:51AM |
Y7.00001: Exactly solvable models illustrating nonequilibrium work relations Invited Speaker: Nonequilibrium work relations establish a connection between the work performed when driving a system away from thermal equilibrium, and the free energy difference between two equilibrium states of the system. I will discuss several exactly solvable model systems that illustrate these relations. While these examples represent idealized systems, and can be analyzed at the level of undergraduate mechanics, they nevertheless provide insight into subtle and sometimes counter-intuitive aspects of nonequilibrium work relations. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y7.00002: Invited Speaker: |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y7.00003: Invited Speaker: |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y7.00004: Stochastic Thermodynamics: Theory and Experiments Invited Speaker: Stochastic thermodynamics provides a framework for describing small systems embedded in a heat bath and externally driven to non-equilibrium. Examples are colloidal particles in time-dependent optical traps, single biomolecules manipulated by optical tweezers or AFM tips, and motor proteins driven by ATP excess. A first-law like energy balance allows to identify applied work and dissipated heat on the level of a single stochastic trajectory. Total entropy production includes not only this heat but also changes in entropy associated with the state of the small system. Within such a framework, exact results like an integral fluctuation theorem for total entropy production valid for any initial state, any time-dependent driving and any length of trajectories can be proven [1]. These results hold both for mechanically driven systems modelled by over-damped Langevin equations and chemically driven (biochemical) reaction networks [2]. These theoretical predictions have been illustrated and tested with experiments on a colloidal particle pushed by a periodically modulated laser towards a surface [3]. Key elements of this framework like a stochastic entropy can also be applied to athermal systems as experiments on an optically driven defect center in diamond show [4,5]. For mechanically driven non-equilibrium steady states, the violation of the fluctuation-dissipation theorem can be quantified as an additive term directly related to broken detailed balance (rather than a multiplicative effective temperature) [6]. Integrated over time, a generalized Einstein relation appears. If velocities are measured with respect to the local mean velocity, the usual form of the FDT holds even in non-equilibrium. [1] U. Seifert, Phys. Rev. Lett. 95: 040602/1-4, 2005. [2] T. Schmiedl and U. Seifert, cond-mat/0605080. [3] V. Blickle, T. Speck, L. Helden, U. Seifert, and C. Bechinger, Phys. Rev. Lett. 96: 070603/1-4, 2006. [4] S. Schuler, T. Speck, C. Tietz, J. Wrachtrup, and U. Seifert, Phys. Rev. Lett. 94: 180602/1-4, 2005. [5] C. Tietz, S. Schuler, T. Speck, U. Seifert, and J. Wrachtrup, Phys. Rev. Lett. 97: 050602/1-4, 2006. [6] T. Speck and U. Seifert, Europhys. Lett. 74: 391-396, 2006. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y7.00005: Invited Speaker: |
Session Y8: Superconductivity: Theory
Sponsoring Units: DCMPChair: Alexei Koshelev, Argonne National Laboratory
Room: Colorado Convention Center Korbel 1C
Friday, March 9, 2007 11:15AM - 11:27AM |
Y8.00001: Impurity scattering interference in high-T$_{c}$ superconductors Chung-Pin Chou, Ting Kuo Lee, Noboru Fukushima Recent STM measurements have observed many inhomogeneous patterns of the local density of state (LDOS) on the surface of high-$T_{c}$ cuprates. In particular, for Bi2212 crystals, well defined interference patterns in the momentum space has been seen at low bias voltage. And recently, for the underdoped sample, it has been observed that the spatially ``checkerboard'' LDOS modulations appear at higher energies. By using a simple impurity scattering potential with BCS Hamiltonian, we describe all LDOS features in Bi2212 materials in terms of quasiparticle scattering interference. We are able to obtain all these features seen by STM experiments in both real- and momentum- space. Our results also show that the observed ``checkerboard'' patterns are dispersionless. Additionally, by using variational Monte Carlo method, we show that the impurity scattering are greatly suppressed due to the presence of strong correlations. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y8.00002: Identifying Collective Modes in $d_{x^2-y^2}$-wave Superconductors via Impurities Roy Nyberg, Dirk Morr, Enrico Rossi We demonstrate that magnetic impurities can be employed to identify the nature of collective modes in the cuprate superconductors. Specifically, we show that a magnetic impurity in an external magnetic field pins an antiferromagnetic collective mode, thus inducing to a local spin-density wave (SDW), i.e., a magnetic droplet. Using a scattering ${\hat T}$-matrix formalism, we find that the presence of such a droplet significantly changes the local electronic structure of the $d_{x^2-y^2}$-wave superconductor. In particular, it suppresses the local density of states (LDOS) inside the droplet on the energy scale of the superconducting gap without inducing an impurity states inside the gap. Moreover, the spin-resolved LDOS exhibits characteristic differences on the two sublattices of the antifferomagnetic droplet. This effect, together with the spatial dependence of the LDOS provides inside into the characteristic momentum of the mode as well as its correlation length. Since these features are absent for other collective modes such as phonons or charge-density waves, our study provides future experiments with the possibility to identify the nature of collective modes. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y8.00003: Identifying Collective Modes in $d_{x^2-y^2}$-wave superconductors via Impurities: II Dirk Morr, Roy Nyberg, Enrico Rossi In the preceeding talk, we demonstrated that magnetic impurities can be employed to identify the nature of collective modes in the cuprate superconductors. In particular we showed that a magnetic impurity in an external magnetic field pins an antiferromagnetic spin mode and induces to a local magnetic droplet. This droplet in turn changes the local electronic structure of the $d_{x^2-y^2}$-wave superconductor. Using a non self-consistent ${\hat T}$-matrix formalism, we identified several characteristic features in the local density of states (LDOS) that arise from the presence of the magnetic droplet. The question naturally arises whether the suppression of the superconducting order parameter (SCOP) in the droplet will alter our conclusions. To investigate this question, we employed a Bogoliubov de Gennes formalism that allows us to self-consistently compute the spatial form of the SCOP. Our results are two-fold. First, we find that the SCOP is significantly changed from its bulk value only in the center of the droplet, and that it recovers the bulk value within a few lattice spacings from the center of the droplet. Second, the suppression of the SCOP only leads to small quantitative changes in the LDOS. Hence our conclusions obtained within the ${\hat T}$-matrix formalism remain unchanged. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y8.00004: Dispersing and non-dispersing peaks and two-energy scales in AC-Arpes in underdoped cuprates Belen Valenzuela, Elena Bascones Recent experiments have shown that the structure in Autocorrelation (AC) Arpes maps compare well with the one observed in Fourier Transform Scanning Tunneling Spectroscopy (FT-STS). In particular dispersing peaks (consistent with the octet model due to scattering induced interference) are observed at low energies in the superconducting state and non-dipersing ones are seen in the pseudogap state and at higher energies in the superconducting state of underdoped cuprates. We have computed the AC-Arpes using the Yang, Rice and Zhang (YRZ) model for the pseudogap. This model assumes that pseudogap and superconductivity compete below a critical doping and has been succesfully used (cond-mat/0611154) to explain the two energy scales found in Raman and ARPES experiments below Tc. We will show that the computed AC-ARPES compares well with the experimental results. The pseudogap is characterized by non-dispersing peaks. In underdoped superconducting cuprates with pseudogap scale larger than the superconducting order parameter, dispersing peaks, associated to the superconducting order parameter appear at low energies and non-dispersing ones, related to the pseudogap, at higher energies. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y8.00005: From the BCS equations to the Anisotropic Superconductivity equations. Jose Samuel Millan, Luis Antonio Perez, Chumin Wang Since the discovery of cuprate superconductors, many new correlated electronic models have been proposed in order to understand their substantially different features, such as high transition temperature (Tc) at an optimal doping, quasi two-dimensional behavior, d-symmetry superconducting order parameter, less influence of the isotope effect, and a power-law behavior of the superconducting specific heat. Recently, we have studied a two-dimensional generalized Hubbard model, in which a second-neighbor correlated hopping is included in addition to the on-site and nearest-neighbor repulsions [1]. This model has the advantage to be able to give some insights on all these new features within the BCS formalism. In this work, we report a unified description of s-, p-, and d-wave superconductivities, in which the experimental power-law behavior of anisotropic superconducting specific heat can be nicely reproduced [2]. [1] J.S. Mill\'{a}n, L.A. P\'{e}rez, and C. Wang, Phys. Lett. A \textbf{335}, 505 (2005). [2] J.S. Mill\'{a}n, L.A. P\'{e}rez, and C. Wang, Proceedings of AIP \textbf{850}, 563 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y8.00006: ABSTRACT HAS BEEN MOVED TO C1 |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y8.00007: Tuning effective interactions in high-$T_c$ cuprates via apical oxygen atoms: New realization from the first-principles Wannier function approach Weiguo Yin, Wei Ku Based on a novel first-principles Wannier function approach, the low-energy effective Hamiltonian for high-$T_c$ cuprates is derived. The apical oxygen atoms are found to significantly modify the mobility and distribution of the Zhang-Rice singlets in the CuO$_2$ plane, by tuning the effective hopping parameters $t^\prime$ and $t^{\prime\prime}$, and local chemical potential, $\mu_{\mathrm{eff}}$. Most remarkably, $V_{\mathrm{eff}}$, an additional effective repulsion (de- pairing) between neighboring doped holes, is found to be significantly tuned by a ``vacuum fluctuation'' mechanism inherited from the correlated multiband nature of the cuprates. Our results identify the apical oxygen states as the main material dependence of these systems and provide a microscopic avenue to the understanding of recent spectroscopic imaging STM data [K. McElroy \textsl{et al.}, Science \textbf{309}, 1048 (2005)]. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y8.00008: Realistic Model of Cuprate High-Energy Pseudogaps J.C. Phillips Cuprates become metallic only when doped, much like semiconductor impurity bands. The unique properties of the cuprates are the result of self-organization of the dopants to form off-lattice filamentary networks (``pearls on strings''). The internal structure of these glassy networks is optimized by maximizing their dielectric screening of internal ionic fields. At low energies ARPES peaks in energy and momentum distributions give similar quasiparticle dispersions, but Lanzara has identified a spectral domain between 0.3 eV and 0.8 eV where the two distributions yield orthogonal dispersion relations. I explain this quasiparticle bifurcation with my model, which also explains: why the cuprate phase diagram exhibits an intermediate phase (IP), and only the IP is superconductive; chemical trends in Tcmax (R), where R is the average number of Pauling resonating bonds; the unique architectonic properties of the CuO2 planes; the nature of the glassy Davis 3 nm nanodomains, and the glassy Davis dopant sites; the two Ando lines in the planar resistivity occurring at the pseudogap transition temperature T* and at optimal doping; the Shen Fermi arcs that evolve with doping, whose angular strength ratio has a step-function at optimal doping; a similar step-function jump in the relaxation of spectral holes at 1.5 eV; all the Lanzara angular isotopic trends observed across the phase diagram by ARPES, and the diamagnetic anomalies associated with the pseudogap, with onset temperatures To as large as 2Tcmax. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y8.00009: High-T$_{c}$ superconductivity originates in BaO or similar planes, not in cuprate-planes. John D. Dow CuO$_{2}$ planes are not needed for high-T$_{c}$ superconductivity, as demonstrated by Sr$_{2}$YRuO$_{6}$ and Ba$_{2}$YRuO$_{6}$, weakly doped on Ru sites with Cu but having \textit{no cuprate-planes}. These materials have onsets of superconductivity at 49K and 93K, respectively. We have shown that the related Cu-Ru materials Gd$_{2-z}$Ce$_{z}$Sr$_{2}$Cu$_{2}$RuO$_{10}$ and GdSr$_{2}$Cu$_{2}$RuO$_{8}$ do not superconduct in their cuprate planes, which are magnetic, but in their SrO layers (with onset T$_{c }\approx $ 45K) [1]. The claims that the cuprate-planes superconduct are based on a one-point unconfirmed jump in the Bell Labs data that was supposedly confirmed by Jorgensen [2], although \textit{Jorgensen's data actually} \textit{contradict the Bell datum}. In all the materials we have studied, and even in YBa$_{2}$Cu$_{3}$O$_{7}$, the superconductivity occurs in the bulk in layers that do not contain Cu, namely \textit{in BaO layers of} YBa$_{2}$Cu$_{3}$O$_{7}$, which have $s$-wave character, not $d$-wave character [3]. \newline [1] J. D. Dow \textit{et al.,} J. Vac. Sci. Technol. \textbf{B 24}, 1977 (2006). \newline [2] J. D. Jorgensen, Phys. Today, 34 (\textbf{June}, 1991). \newline [3] D. R. Harshman \textit{et al}., Phys. Rev. \textbf{B 69}, 174505 (2004). [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y8.00010: Ba$_2$YRuO$_6$: High-T$_c$ superconductivity without CuO$_2$ planes. Hermann Azemtsa Donfack Doped Ba$_2$YRuO$_6$ begins superconducting at 93~K, although it has no cuprate-planes. It does have Cu as a dopant, but superconducts with so little Cu (1\%) [S. M. Rao et al., J. Crystal Growth {\bf 235,} 271 (2002)] that it is clearly not a cuprate-plane superconductor. This means that CuO$_2$ planes are not needed for high-T$_c$ superconductivity. It also means that all theories of high-T$_c$ superconductivity based on cuprate-planes superconducting are incorrect, or else that there are at least two theories of high-T$_c$ superconductivity, not just one: one for cuprate-plane materials, and one for ruthenates. (It is our opinion that there is just one theory of high-T$_c$ superconductivity, and that it involves superconductivity in the BaO or similar layers.) In doped Sr$_2$YRuO$_6$, a sister compound of Ba$_2$YRuO$_6$ that begins superconducting at 49~K, the superconductivity is clearly in the (SrO)$_2$ layers, not in the Cu-doped YRuO$_4$ layers, which contain magnetic fields of order 3~kG in zero applied field, and so are unlikely layers to superconduct [J. D. Dow and D. R. Harshman, J. Low Temp. Phys. {\bf 131,} 483 (2003)]. Unlike doped Ba$_2 $YRuO$_6$, doped Ba$_2$GdRuO$_6$ does not superconduct, because Cooper pairs in the BaO layer are disrupted by the magnetic ion Gd. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y8.00011: Nodeless Ground State Symmetry of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Dale R. Harshman, Anthony T. Fiory, John D. Dow Muon spin rotation measurements were conducted on a single-crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (T$_{c}$ = 91.3 K) as a function of temperature and magnetic field [1]. By correctly accounting for the temperature-activated fluxon depinning and disorder known to exist in these and other high-quality YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ crystals grown today, the underlying ground state symmetry was found to be nodeless, consistent with s-wave (or extended s-wave) pairing. This result is in agreement with earlier data acquired on samples with strongly pinned vortices. The best fit was obtained assuming the two-fluid model. Analysis of the data assuming a d-wave gap function proved to be extremely unsatisfactory. In fact, the probability that the d-wave model tested gives a better fit than the two-fluid model is less than 4 $\times $ 10$^{-6}$. By ignoring the effects of fluxon depinning, others have erroneous claimed evidence of d-wave pairing. This result effectively negates that possibility. \begin{enumerate} \item D. R. Harshman et el., Phys. Rev. B \textbf{69}, 174505 (2004). \end{enumerate} [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y8.00012: Superconductivity in the ordered limit Vladimir Cvetkovic A novel mechanism for superconductivity is proposed based on the duality in the quantum elasticity. Using a charge crystal as a starting point, these superconductors can be viewed as liquid crystal phases of charge in sense that the broken translational symmetries are restored by the Bose condensation of dislocation defects. Although the crystalline correlations are lost at long distance, the order (and the shear rigidity of the solid) persists at scales large comparable to the lattice spacing. This leads to a host of unconventional properties predicted for this `ordered' superconductor: Meissner effect with oscillating currents, overscreening of Coulomb force, long-range topological order, and the presence of a new excitation in the dynamical electric response. The origin of this excitation lies in the short range shear rigidity, i.e., transient order of a solid. Therefore, an experiment designed to measure the presence of the predicted excitations in the cuprate superconductors could be used to unambiguously (dis)prove the existence of fluctuating stripes. [Preview Abstract] |
Session Y9: Superconductivity: Josephson Junctions, Proximity Effect & Squids II
Sponsoring Units: DMPChair: Wan Kyu Park, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center Korbel 1D
Friday, March 9, 2007 11:15AM - 11:27AM |
Y9.00001: Microwave characterization of Josephson junctions arrays for Coulomb blockade Vladimir Manucharyan, Michael Metcalfe, R. Vijay, Etienne Boaknin, Michel Devoret Coulomb Blockade of a single Josephson junction leads to oscillations in time of the voltage across the junction when a DC electrical current is applied. Because the frequency $f$ of these so-called Bloch oscillations is related to the current $I$ only through the charge $2e$ of a Cooper pair, $f = I/2e$, the phenomenon can be utilized to build a primary standard of electrical current. However, to reach the regime of Coulomb blockade, the current must be applied to the junction through the leads with electromagnetic impedance exceeding the quantum of resistance for Cooper pairs ($6.5$ k$\Omega$) at frequencies $\omega_c \approx \hbar/E_c$, $E_c$ being the charging energy of the junction. For typical parameters $\omega_c$ lies in the microwave domain, where electromagnetic impedances tend to be of the order of the free space impedance ($377$ $\Omega$). This fundamental mismatch in the impedance turns the realization of a current standard based on Bloch oscillations into a very challenging problem. Our proposal is to use kinetic impedance of a superconductor, namely, the Josephson inductance. We have fabricated arrays of large Josephson junctions which are superconducting at DC and characterized them at microwave frequencies. Our results indicate that these arrays are capable of beating the impedance quantum and we are setting up an experiment on electrical current metrology. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y9.00002: Low-frequency Critical Current Fluctuation Measurements in Nb/AlOx/Nb Junctions Shawn Pottorf, Vijay Patel, J. E. Lukens We have measured the low frequency critical current noise in Nb/AlO$\mathrm{_{x}}$/Nb Josephson junctions used for qubits in quantum computation circuits. Low frequency current noise measurements were made using a bridge circuit with a SQUID null detector. The current noise spectra density showed a 1/$f$ component at low frequencies for both an unshunted junction biased near 6 mV and a shunted junction biased near $\sim 7$ $\mu$V. In both cases this corresponded to critical current fluctuations with a spectral density at 1 Hz of $2.2 \cdot 10^{-24}$ $\mathrm{A^{2}/Hz}$. Our measured value of critical current fluctuations is roughly two orders of magnitude less than the average of various technologies reported by Van Harlingen et al. (2004). [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y9.00003: Microscopic Model of 1/f Noise in Josephson Junctions Magdalena Constantin, Clare Yu, John Martinis We present a simple microscopic model to show how fluctuating two-level systems in the Josephson junction tunnel barrier can modify the potential energy of the barrier and produce critical current noise spectra as well as charge noise. We find $1/f$ critical current and charge noise at low frequencies. Our values are in good quantitative agreement with recent experimental measurements of noise in Al/AlO$_{x}$/Al Josephson junctions. We also investigate the sensitivity of the critical current noise on the nonuniformity of the tunnel barrier. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y9.00004: High-frequency spectroscopy and emission properties of a single-Cooper pair transistor Pierre-Marie Billangeon, Fr\'ed\'eric Pierre, H\'el\`ene Bouchiat, Richard Deblock We have characterized the high-frequency properties of a single-Cooper pair transistor (SCPT), by a capacitive coupling with a Josephson junction. We have alternately used the Josephson junction (JJ) as a high-frequency generator by using the AC Josephson effect, and as a high-frequency detector by using the photo-assisted tunneling current. We have been able to induce transitions between the first energy levels of a SCPT, thanks to the high-energy photons emitted by the AC Josephson effect. This allowed us to perform a high-frequency spectroscopy of a SCPT(10-200 GHz). Moreover, as the emitted photons can have an frequency higher than the superconducting gap of the island, we can not only induce the transfer of Cooper pairs, but also quasiparticles, allowing us to control the poisoning of the SCPT. Then, we used the Squid geometry of the Josephson junction in order to tune its Josephson energy to zero, and use it as a high-frequency detector. We have been able to detect different kinds of high-frequency emission process associated with the coherent transfer of Cooper pairs through the SCPT (AC Josephson effect), and the resonant transfer of Cooper pairs, already characterized in transport measurements by Joyez et al. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y9.00005: Using a High-Q Josephson Resonator as a Non-Dissipative RF-SQUID J.A. Strong, K.D. Osborn, A.J. Sirois, R.W. Simmonds Superconducting Quantum Interference Devices (SQUIDs) have been used for years to measure small magnetic fields. Such devices measure the DC voltage across a Josephson junction as a function of magnetic flux. It is well known, however, that a voltage-biased Josephson junction radiates energy. This is problematic for many superconducting quantum device applications including readout methods for superconducting quantum bits and SET's. Here, we examine a newly developed Josephson junction resonator as a new breed of SQUID, wherein the resonator's resonant frequency (instead of the junction's voltage) is measured as a function of magnetic flux. In this way, the Josephson junction is kept perpetually in the super- current state, with zero DC voltage and therefore no Josephson radiation. We examine issues of sensitivity, noise, and read- out speed. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y9.00006: Cooper-pair Tunneling in a High Impedance Environment. M. A. Castellanos-Beltran, K. W. Lehnert Coulomb blockade of current and coherent oscillations in the voltage across a small tunnel junction can only be observed if the junction is embedded in the appropriate electromagnetic environment. The demanding condition is an environment with an impedance large compared to the quantum of resistance ($R_Q=6.5$~k$\Omega$) at all relevant frequencies (DC - 20~GHz). We will show results from two different experiments that characterize the impedance of one-dimensional Josephson junction arrays. First, by using the array to bias a SQUID made from small area junctions, we show the behavior of this system can only be explained if the array creates an environment with an impedance several times $R_Q$. Second, we measure the speed at which microwave signals propagate through coplanar waveguides whose inner conductors are formed from an array of Josephson junctions. We find that these waveguides behave as LC transmission lines with wave impedances of several k$\Omega$ and wave speeds less than 1\% of the speed of light in free space. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y9.00007: Adiabatic Perturbing a Bloch Transistor by Microwave Irradiation: Inversion of Coulomb Oscillation Watson Kuo, Saxon Liou, Y.W. Suen, W.H. Hsieh, C.S. Wu, C.D. Chen We experimentally studied the switching current and DC current-voltage (IV) characteristics of a Bloch transistor irradiated with microwaves of frequency from several GHz up to 18GHz. The photon energy is well below the level spacing of the two-level quantum states so that the Bloch transistor is perturbed in the adiabatic regime. Phase-charge duality in a Josephson junction is clearly seen in the switching current distribution as a function of gate voltage. The reduction of switching current due to photon excitation is significant when the phase fluctuation is small. In particular, an inversion of Coulomb oscillation of switching current is observed at a higher microwave power level. When the microwave frequency is below 7GHz, the IV characteristics of the Bloch transistor evolve from being superconductor-like to being blockade-like as the microwave power level increases, and the zero-bias resistance R$_{0}$ shows a Coulomb oscillation accordingly: when R$_{0}$ is maximal, the switching current is also maximal, opposite to that without microwave irradiation. As the microwave power level increases further, Shapiro steps in IV characteristics are observed. The step height can be analyzed using a model for an ac voltage source applied to a single Josephson junction. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y9.00008: Superconductor-normal metal contact conductance of a graph node Vladimir Lukic, Elisabeth Nicol We study the conductance of a superconductor-normal metal (SN) contact with the topology of a graph node. We derive the extension of the Blonder-Tinkham-Klapwijk (BTK) equations using the boundary conditions for a wavefunction at a graph node, and show that in the appropriate limit they reduce to the standard BTK formula for an SN contact. Qualitatively new conductance features arise from crossed Andreev reflection and interference of partially reflected waves from different graph legs, and we demonstrate their importance by using the Landauer method to rederive the formula for conductance. The relevance of these effects to experiment will be discussed. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y9.00009: Singular length dependence of critical current in superconductor/normal-metal/superconductor bridges Alex Levchenko, Alex Kamenev, Leonid Glazman We examine the dependence of the critical Josephson current on the length $L$ of the normal bridge $N$ between two bulk superconductors. This dependence turns out to be nonanalytic at small $L$. The nonanalyticity originates from the contribution of extended quasiparticle states with energies well above the superconducting gap. This should be contrasted with the more familiar contribution to the Josephson current coming from Andreev bound states localized in the normal region at energies below the gap. We also have preliminary results on the ac Josephson effect above the critical temperature $T_{c}$ where we have studied the influence of the superconducting fluctuations on the current noise. It turns out that the current noise acquires singular in $T-T_c$ correction, which is peaked at the Josephson frequency. This correction originates from the fluctuating ac Josephson current. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y9.00010: Interaction of breathers with moving vortices in a Josephson junction ladder Devin Edwards, Ken Segall, Juan Mazo Josephson junction arrays offer an important method to experimentally study nonlinear dynamics. We have studied Nb-AlOx-Nb Josephson junction ladder arrays with 24 periods which support two distinct nonlinear modes. The first is a discrete breather, which is a spatially localized excitation that does not propagate through the ladder. The second is a propagating two pi phase shift, or moving vortex. Both modes have been observed independently in our arrays. Predictions have been made regarding the dynamics of the interactions of these two modes, but these have not been verified experimentally. One such prediction is that under some conditions the breather will `pin' the propagating vortices and prevent them from passing beyond the breather. We will present theoretical simulations and recent experiments attempting to observe their interaction. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y9.00011: Fluxon ratchet dynamics in a Josephson junction array Kenneth Segall, Adam Dioguardi, Nikhil Fernandes, Ushnish Ray, Juan Mazo, Fernando Naranjo We present theoretical and experimental work on the ratchet dynamics of fluxons in an array of Josephson junctions. Fluxons trapped in a parallel array of Josephson junctions upon cooldown experience a potential determined by the junction critical currents and the cell inductances. By varying these quantities in an asymmetric way, the potential can be made ratchet. We probe the dynamics of the fluxon with switching current measurements, which allow determination of the transition rate of the fluxon from its pinned state to a running state. We find two temperature regimes, both experimentally and in simulations. At low temperatures, the fluxon behaves like a single particle and undergoes thermal activation. At intermediate temperatures, the fluxon undergoes diffusion for several periods and then jumps to the running state. The dynamics in this region cannot be explained with a single particle picture. We have observed two temperature-dependent crossovers in the direction of transport in this temperature region. We present temperature-dependent measurements, comparisons with simulation, and possible interpretations of the crossovers. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y9.00012: Manipulation of half-integer flux quanta C.J.M. Verwijs, Ariando, J.A. Boschker, H. Hilgenkamp Because of the different pairing symmetry in high-T$_{c}$ cuprates ($d$-wave) and low-T$_{c}$ superconductors ($s$-wave) it is possible to realize hybrid superconducting rings with a built-in $\pi $-phase shift. These rings have a twofold degenerate groundstate characterized by spontaneously generated circulating currents corresponding to a half flux quantum. We have realized such rings by connecting the cuprate YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ to the low-T$_{c}$ superconductor Nb via ramp-type Josephson junctions. We present the integration of $\pi $-rings in a superconducting quantum electronics device, a toggle flip-flop, in which the fractional flux polarity is toggled by applying single flux quantum pulses. We will also present experiments in which the half flux quanta at the discontinuities of 0-$\pi $-0 Josephson corner junctions are manipulated. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y9.00013: Vortex flow characteristics of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Long Intrinsic Josephson Junctions Kazuo Kadowaki, Kohei Kawamata, Yuimaru Kubo, Kazuki Fukui, Takashi Yamamoto, Itsuhiro Kakeya Intrinsic Josephson junction characters in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$, where the junction length $L$ is much longer than the Josephson length \textit{$\lambda $}$_{J}$=\textit{$\gamma $s}, have been studied from view point of Josephson vortex dynamics, since they are expected to be released from the strong geometrical constraint, which confines Josephson vortices into a square box, and as a result they are free to move in the two dimensional channels. We fabricated such long junctions with $L$=20-40 $\mu $m and have measured $c$-axis transport characteristics. We have found that Josephson flow resistance $R_{f}$ suddenly suppressed at the intermediate field region of 1-2 T, then begins to reappear gradually as field is increased. The sharp lock-in transition as a function of angle appearing below it where the periodic oscillation of $R_{f}$ is seen, becomes immediately broad and a round peak above it, where the periodic oscillation of $R_{f}$ is no longer observable. It seems that the transition field becomes lower as $L$ is increased so that in short junctions it cannot be observed in a field region up to 6 T. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y9.00014: Optimization of HighTc Josephson nanojunctions by Monte Carlo simulations M. Sirena, N. Bergeal, J. Lesueur, G. Faini, R. Bernard, J. Briatico, D. Crete The fabrication of YBCO JJ by ion damage is the best method that allows closed packed JJ series within the nanoscale and that could operate at high temperature. However, the strong variation of the JJ's critical current with temperature (T) and the increase of dispersion for high irradiation dose are still important issues for several applications. Reproducible HTc JJ have been produced combining electron beam lithography and ion beam irradiation, whose characteristics can be adjusted on a wide range of T. To further improve the homogeneity of planar JJ and optimize their behaviour, we have studied its lateral ion damage distribution (LDD) for different ions and incident energies using Monte Carlo simulations. The LDD was used to calculate the transition temperature (Tc') of the irradiated zone and its resistance as a function of T. Dispersion in the irradiation mask's size was introduced as the source of the JJ's in-homogeneity. The simulations results reproduce quite well the observed dispersion of the irradiated JJ. A linear behaviour of the JJ's Tc' dispersion with basically the LDD width was found, independent of the incident ions mass, its energy, the films thickness, etc. By choosing the appropriate parameters is possible to increase the JJ homogeneity, reducing the LDD width. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y9.00015: Fiske and size-independent resonances in $I-V$ characteristics of micron-sized Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals Itsuhiro Kakeya, Yuimaru Kubo, Masashi Kohri, Kazuki Fukui, Kohei Kawamata, Takashi Yamamoto, Kazuo Kadowaki We have investigated the $c$-axis transport properties of micron-size Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals fabricated by the focused ion beam method under magnetic field parallel to the $ab$-plane. It was found that periodic current steps in current-voltage ($I-V$) characteristics, whose features are similar to the Fiske step known in a single Josephson junction. We also found another current step with non- oscillating field dependence in low voltage region. Since the voltage of this step does not depend on the sample size unlikely to the Fiske step, it is considered that the step is attributed to an intrinsic phase excitation of Bi2212. [Preview Abstract] |
Session Y12: Focus Session: Spin Relaxation and Dynamics
Sponsoring Units: GMAG DMP FIAPChair: Sayantani Ghosh, University of Cailfornia, Merced
Room: Colorado Convention Center Korbel 3C
Friday, March 9, 2007 11:15AM - 11:27AM |
Y12.00001: Spontaneous polarization in classical spin transport far from equilibrium Tobias Reichenbach, Thomas Franosch, Erwin Frey Spintronic devices like the Datta-Das spin-field effect transistor aim to exploit the spin of electrons for information processing. Based on an exclusion process, we present a classical model for the spin transport in such systems [1]. Particles move unidirectionally on a one-dimensional lattice under the constraint of Pauli's exclusion principle. Analyzing the emerging non-equilibrium steady state, we find that it can be tuned by controlling the particle fluxes at the boundaries. In particular, a spontaneous polarization may occur at a certain spatial position and, upon changing the fluxes at the boundaries, be driven in or out of the system. We derive the shape of the density profiles as well as resulting phase diagrams analytically by a mean-field approximation and a continuum limit. \newline \newline [1] T. Reichenbach, T. Franosch, E. Frey, Phys. Rev. Lett. 97, 050603 (2006) [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y12.00002: Ultrafast hole-spin dynamics in bulk GaAs Hans Christian Schneider, Michael Krauss This talk presents theoretical results on hole-spin dynamics in bulk GaAs after optical excitation. The coupled dynamics of spin and orbital angular momentum is determined by solving dynamical Boltzmann equations for carrier-carrier scattering, which include the effect of spin-orbit coupling on the level of a 4-band Luttinger Hamiltonian. Hole-spin relaxation takes place in two stages. In the first regime, on a timescale of a few hundred femtoseconds, pure momentum scattering dominates the dynamics and the anisotropic contributions to the orbital angular momentum, which are created by the optical excitation, are evened out. In the second regime, on a timescale of a few picoseconds, energy relaxation dominates. The hole-spin dynamics can be approximated by a different relaxation time for each of the two regimes. The fast spin relaxation-time in the first regime is in agreement with experimental results for heavy-hole spin relaxation. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y12.00003: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y12.00004: Time Resolved Probe of Spin and Carrier Lifetimes in InSb Based Quantum Wells and Films Rajeev N. Kini, K. Nontapot, M. Frazier, B. Spencer, J.J. Heremans, G.A. Khodaparast, N. Goel, S.J. Chung, M.B. Santos Semiconductor quantum wells (QWs) based on InSb have potential applications in the rapidly growing field of spintronics. Due the large Bychkov-Rashba and Dresselhaus spin-orbit interaction terms, g factor, and the high electron mobility of InSb, it may be possible to realize a number of novel high speed, spin-sensitive electronic, and optoelectronic devices. A recent measurement has indicated a value of $\sim $ 0.5ps for the room temperature spin lifetime in InSb/AlInSb QWs (Litvinenko \textit{et al}, New Journal of Physics, 8, 49, (2006)). Here we present our measurements of the spin and carrier relaxations in InSb based QWs with symmetric and asymmetric doping profile and n-type InSb films. We use pump probe and magneto-optical Kerr effect (MOKE) spectroscopy to study the dynamics at different excitation wavelengths, power densities, and temperatures. We observe relaxations which last longer than 5ps and our measurements can provide important information regarding the relaxation mechanisms in this narrow gap system. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y12.00005: Spin relaxations in 2D electron gas determined by the memory in the carrier dynamics. Eugene Sherman, Mikhail Glazov The effects of long memory, in carrier dynamics in a magnetic field, on spin polarization evolution in 2D electron gas are investigated qualitatively and quantitatively. As examples we consider (i) systems with random Rashba-type SO coupling and (ii) quantum wells with rigid short-range scatterers (antidotes) and regular Dresselhaus SO coupling. In both cases the spin dynamics is strongly non-Markovian. In the system with the random SO coupling the time dependence of the spin polarization shows Gaussian rather than exponential behavior with the cusps corresponding to the electron revolutions. The relaxation speeds up with the increase of the magnetic field. In the system with antidotes scattering, the spin polarization shows a long-tail behavior with the relaxation rate determined by inelastic electron-phonon and electron-electron collisions and demonstrates unusual field dependence. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y12.00006: Spin dynamics in a two-dimensional electron system with generic spin-orbit coupling Tudor Stanescu, Victor Galitski We study the dynamics of a spin density injected into a two-dimensional electron system with generic spin-orbit interactions. We derive the general spin-charge coupled diffusion equation for a model that includes Rashba as well as linear and cubic Dresselhaus spin-orbit interaction terms. We analyze in detail two regimes: The first regime corresponds to negligible spin-charge coupling and is characterized by standard charge diffusion, decoupled from the spin dynamics. The interplay between different spin-orbit interaction terms leads to interesting phenomena such as a finite momentum enhancement of the spin relaxation time, real space oscillatory dynamics, and anisotropic transport. In the second regime, we include the effects of the spin-charge coupling. The coupling depends quadratically on the overall strength of the spin-orbit interaction and, in addition, it strongly depends on the relative magnitude of the various interaction terms. It is shown that the spin-charge coupling leads to an enhancement of the effective charge diffusion coefficient. In the presence of a boundary, spin-charge coupling leads to a current-induced spin accumulation. In the strong spin-charge coupling limit, we find that the relaxation rates formally become complex and the spin/charge dynamics is characterized by real time oscillations, qualitatively similar to those observed in spin-grating experiments [Weber et al., Nature 437, 1330 (2005)]. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y12.00007: Electrically Detected Hahn Echoes in Si:P Hans Huebl, Benno Grolik, Andre R. Stegner, Martin Stutzmann, Martin S. Brandt Several proposals discuss the realization of quantum computation with the help of the spin degree of freedom in semiconductors, including implementations using the nuclear or electronic spin of phosphorus donors. Recently, Stegner et al. [1] showed in ensemble measurements that the electron spin state of donors can be read out electrically by investigating the current transient after the application of a microwave pulse and by observing Rabi oscillations. In this contribution we show that this technique can be extended to detect Hahn echoes which allow to determine the $T_2^*$ time of the specific spin-to-charge transfer system used. We observe $T_2^* \approx 1.2\rm{\mu}$s at 6~K under illumination with a tungsten lamp. This time is considerably shorter than $T_2$ characteristic of isolated P in Si, most likely due to the interaction with the Si-SiO$_2$ interface and photoexcited carriers. The experiments show that dynamic constants specific to the actual spin read-out technique used can be determined by pulsed electrically detected magnetic resonance. [1] A. R. Stegner et al., Nature Physics doi:10.1038/nphys465 [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y12.00008: Control and measurements of spin and carrier dynamics in InAs films Kanokwan Nontapot, Rajeev Kini, Giti Khodaparast, Louis Guido, Roger Welser In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors. We report control and measurements of spin and carrier relaxations in InAs (100) and InAs (111) films with doping densities of $\sim $ 3x10$^{12}$ cm$^{-2}$ (mobility $\sim $20,000 cm$^{2}$/Vs at 77 K) and 2.0x10$^{12}$ cm$^{-2}$ (mobility $\sim $33,00 cm$^{2}$/Vs at 77 K) at room temperature and 77K, respectively. We use standard pump-probe and magneto-optical Kerr effect (MOKE) spectroscopy at different excitation wavelengths, power densities, and temperatures. Spin relaxations in these structures demonstrate dynamics which is different from the carrier relaxations under the same experimental conditions. We explain our results using the Elliot-Yafet picture which is considered to be the dominant relaxation process in narrow gap semiconductors. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y12.00009: Evolution of coherently controlled charge and spin currents injected by optical pulses. R. Abrarov, A. Najmaie, E. Ya. Sherman, J. E. Sipe We consider dynamics of coherently controlled currents injected by short (100 fs) optical pulses with frequencies $\omega $ and 2$\omega $ in multiple GaAs/AlGaAs quantum wells. Our approach is based on the series expansion of the carrier and current densities in an appropriate set of basis functions. The role of space-charge effects (long-range Coulomb interaction between electrons and holes) and current-conserving and non-conserving collisions of the carriers on the dynamics of the quantities observable in pump-and-probe experiments is investigated. We show that under certain conditions, dependent on the relaxations rates and the band structure effects, displacement of electrons and holes from the initial positions can be finite even on a long time scale of the order of few picoseconds. Due to the skew scattering arising during electron-hole collisions, injected charge (spin) currents drive the spin (charge) currents which can be observed experimentally. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y12.00010: Bias-Dependent Electron Spin Lifetimes in n-Type GaAs and the Role of Donor Impact Ionization M. Furis, D.L. Smith, S.A. Crooker, J.L. Reno We present a study of electron spin lifetimes $\tau_s$ in n-type GaAs as a function of applied lateral electrical bias [1]. Using the magneto- optical Kerr effect, $\tau_s$ is obtained from Hanle depolarization measurements on n-GaAs epilayers doped near the metal-insulation transition ($n_e\sim 0.4 - 5.0$ x $10^{16} $ cm$^{-3}$) . Below 10 K, we observe that applied electric fields larger than 10 V/cm result in a large and sudden decrease of $\tau_s$. This collapse results from impact ionization of the cold donor-bound electrons into free electron states that are subject to Dyakonov-Perel spin relaxation. The effect is less dramatic at higher temperatures and at higher doping concentrations above the metal- insulator transition, where most electrons are delocalized even in the absence of an applied electrical bias. The collapse of $\tau_s$ is shown to strongly influence lateral spin transport in the impact ionization regime. [1] M. Furis, D. L. Smith, S. A. Crooker, and J. L. Reno, Appl. Phys. Lett. \textbf{89}, 102102 (2006) [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y12.00011: Spin precession and spin relaxation in semiconductors Dimitrie Culcer, Roland Winkler In order to achieve a lasting spin polarization a proper understanding of the mechanisms leading to spin polarization decay is critical. We present a general theory for spin polarization decay due to the interplay of spin precession and momentum scattering that is applicable to both spin-1/2 electrons and spin-3/2 holes and that allows us to identify and characterize a wide range of qualitatively different regimes [1]. The spin polarization of ballistic carriers is reduced by spin dephasing, which is characterized by a non-exponential time dependence and results in an incomplete decay of the spin polarization. For weak momentum scattering or fast spin precession, the spin relaxation time is proportional to the momentum relaxation time. For strong momentum scattering and slow spin precession we recover the D'yakonov-Perel result that the spin relaxation time is inversely proportional to the momentum relaxation time. \newline \newline [1] D. Culcer and R. Winkler, cond-mat/0610779. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y12.00012: Spin relaxation of electrons in bulk CdTe Daniel Sprinzl, Petra Nahalkova, Jan Kunc, Petr Maly, Petr Horodysky, Roman Grill, Eduard Belas, Jan Franc, Petr Nemec We report on the measurements of the spin relaxation time $T_{1}$ of photo-excited electrons in bulk CdTe. The carrier dynamics were investigated by transient absorption experiments using 80 fs circularly polarized laser pulses at sample temperatures from 20 to 300 K. We studied both p and n type doped CdTe samples, which were prepared in the form of thin platelets from the crystals grown by the modified Bridgman method. The obtained results are compared with the spin relaxation times reported for other semiconductors with the same crystal structure (e.g., GaAs [1]). Finally, the relative contributions of the D'yakonov-Perel, Elliott-Yafet, Bir-Aronov-Pikus, and other mechanisms to the measured spin relaxation times in CdTe are discussed. This work was supported by the Grant Agency of the Czech Republic (grant 202/03/H003), by the Ministry of Education of the Czech Republic in the framework of the research centre LC510 and the research plan MSM 0021620834. [1] J. M. Kikkawa and D. D. Awschalom, Phys. Rev. Lett. 80, 4313 (1998). [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y12.00013: Suppression of nuclear polarization in photon-irradiated GaAs M.R. Fitzsimmons, B.J. Kirby, F. Trouw, P.A. Crowell, C. Adelmann, S.D. Flexner, C.J. Palmstrom , M. Erickson, J.A. Borchers, C.F. Majkrzak, W. Chen, T.R. Gentile, R. Pynn We measured the spin dependence of polarized neutron beams reflected by a GaAs sample at 20 K in a magnetic field of 250 Oe applied along the sample's surface normal. Neutron data were acquired with left and right circularly polarized light (0.25 W/cm$^{2}$ and 808 nm) illuminating the sample. These conditions yielded nuclear polarization in the same sample (before and after the neutron experiment) of several percent as evidenced by a shift of a peak in the optical Hanle curve. The neutron data exhibit a correlation with light polarization and thus nuclear polarization. Quantitative analysis of the spin dependence of the polarized neutron reflectivities indicates nuclei within 50 nm of the sample's surface are not polarized, and then nuclear polarization increases to a small value in the bulk. We attribute suppression of nuclear polarization near the sample's surface to the electric field in the depletion layer that inhibits binding of spin polarized carriers to donor sites and to the electric field gradient at the nuclei (induced by the electric field in the depletion layer) that depolarizes nuclei with quadrupole moments such as Ga and As. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y12.00014: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y12.00015: Electric field dependence of the electron g-factor for a Si donor in bulk GaAs A. De, Craig E. Pryor, Michael E. Flatt\'e Modulation of the electron g-factor by an applied electric field may be used to coherently manipulate spins for quantum information processing. We present numerical calculations of the electric field dependence of the $g$-factor of an electron bound to a silicon donor in bulk GaAs. The calculations were carried out using 8-band $k.p$ theory in the envelope function approximation, which is implemented using finite differences on a real-space grid. The binding energy of the Si impurity in GaAs was fit to experimental data by adding a central cell correction to the donor site. Electrically modulating the impurity's $g$-factor is possible as the electric field modifies the binding energy for the Si donor. In our calculations, it is seen that the variation in $g$ is nearly quadratic as a function of electric field (up to 0.2 mV/nm) and for $E=0$, ${d^2g} / {dE^2}=1.2~ \rm (mV/nm)^{-2}$. The largest variation in $g$ is obtained when the applied magnetic field and electric fields are in the same direction. The proposed scheme provides a realizable alternative to quantum information processing using quantum dots. [Preview Abstract] |
Session Y13: Focus Session: Charge/Orbital Order in Complex Oxides
Sponsoring Units: DMP GMAGChair: Igor Solovyev, National Institute for Materials Science
Room: Colorado Convention Center Korbel 4C
Friday, March 9, 2007 11:15AM - 11:27AM |
Y13.00001: Commensurate and incommensurate charge order in Fe$_2$OBO$_3$ Manuel Angst, Raphael Hermann, Jong-Woo Kim, Peter Khalifah, Brian C. Sales, David G. Mandrus Charge order CO in the form of a Wigner crystal had been proposed by Attfield {\em et al}. [Nature {\bf 396}, 655 (1998)] based on measurements, particularly M\"{o}ssbauer spectroscopy, on polycrystalline Fe$_2$OBO$_3$, but no superstructure due to the CO had been detected. We have grown the first single crystals of pure Fe$_2$OBO$_3$, and resistivity and thermal analysis indicate not one, but two transitions associated with CO. To elucidate the nature of these two transitions a synchrotron study was performed. At low $T$ a superstructure corresponding to a doubling of the $a$ axis was observed for the first time. The phase between the two phase transitions, in contrast, exhibits an incommensurate modulation with propagation vector ($\frac{1}{2}$,$0$,$\tau$), $\tau$ increasing with $T$ towards $\frac{1}{2}$. Resonances in the energy-dependence of the scattered intensity around the Fe $K$ edge suggest that the modulations in both phases are indeed associated with CO. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y13.00002: Optical study of CuIr2S4 and MgTi2O4: experimental support for orbital-Peierls transitions Nan Lin Wang, Jun Zhou, Guanghan Cao, Jing Shi In metals with low-dimensional electronic structure, Fermi surface instability can drive a system into a symmetry-breaking insulating state. However, such instability is not expected to develop in a three-dimensional (3D) system. Recently two types of highly exotic orderings were discovered by Radaelli et al. in two spinel compounds: an octamer ordering in CuIr2S4 and a helical ordering in MgTi2O4. In both cases, sharp metal-insulator transitions (MIT) and spin-dimerizations associated with the structural distortions occur simultaneously. It was suggested that the orbital degree of freedom plays a key role in such transition: the ordering of the orbitals makes the electrons travel exclusively along certain chains which effectively reduces the dimension from 3D to 1D, then is susceptible to Peierls instability at low T. Here we present optical measurement on spinels CuIr2S4 and MgTi2O4, and show that the spectral change across the MIT can be well understood from the picture of 1D Peierls transitions driven by the $d_{xy}$ orbital ordering in CuIr2S4, and the $d_{yz}$ and $d_{zx}$ orbital orderings in MgTi2O4, respectively, proposed by Khomskii et al. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y13.00003: Orbital ordering in spinel AV$_{2}$O$_{4}$ (A=Mn,Fe) T. Katsufuji, T. Suzuki, M. Katsumura, H. Takei, K. Taniguchi, T. Arima In spinel vanadates, AV$_{2}$O$_{4}$ (A:divalent ion), there is an orbital degree of freedom in the V$^{3+}$ ion, which contains two $d$ electrons in the triply degenerate $t_{2g}$ orbital. We measured the x-ray diffraction of the single crystal of MnV$_{2}$O$_{4}$, which exhibits simultaneous ferrimagnetic and a structural phase transition at 57 K, and found that a diamond-glide symmetry is broken in the ferrimagnetic tetragonal phase in this compound, indicating the antiferro-type ordering of the V $t_{2g}$ orbitals. This orbital ordering can be explained by the enhancement of the antiferro-orbital interaction caused by the ferromagnetic alignment of the V spins in the ferrimagnetic phase. We also measured the x-ray diffraction of the FeV$_{2}$O$_{4}$ single crystal, and found that a different type of orbital ordering from that of MnV$_{2}$O$_{4}$ occurs in this compound. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y13.00004: Magnetic fluctuations and orbital orderings in ferrimagnetic spinels Jae-Ho Chung, Jung Hwa Kim, Seung-Hun Lee, Taku S. Sato, Takuro Katsufuji We report our inelastic neutron scattering studies of two related spinels, AB$_2$O$_4$ (A = Mn, B = Mn and V), with e$_g$ and t$_{2g}$ orbital degeneracy, respectively. Both systems undergo noncollinear triangular ferrimagnetic orderings at low temperatures, where the lattice symmetries are tetragonal. Characteristics of the phase transitions, however, are different. In the case of Mn$_3$O$_4$, the tetragonal distortion with $c > a$ exists below T = 1443 K, and upon cooling a noncollinear ferrimagnetic ordering occurs at 42 K, followed by two more magnetic transitions into incommensurate (40 K) and commensurate cell doublings (34 K). Those magnetic phases exhibit magnetocapacitance. In the case of MnV$_2$O$_4$, on the other hand, a collinear ferrimagnetic ordering occurs at 65 K, followed by the tetragonal distortion with $c < a$ and a noncollinear ordering at 58 K. Our single crystal inelastic neutron scattering data show magnetic excitations up to 20 meV for Mn$_3$O$_4$ and up to 40 meV for MnV$_2$O$_4$. We have performed linear spin wave calculations to obtain their effective Hamiltonians by comparing the calculated dispersions of spin waves to the observed ones. The implications of the spin Hamiltonians to their orbital states, and the polarization of spin waves will be discussed. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:39PM |
Y13.00005: Electronic structure of complex oxides using DMFT Invited Speaker: |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y13.00006: Orbital ordering in striped phases of La$_{1-x}$Ca$_x$MnO$_3$: GGA+U versus GGA results Nadia Binggeli, Giancarlo Trimarchi We have investigated the orbital ordering in La$_{1-x}$Ca$_x$MnO$_3$, with x=2/3 and 1/2, within the generalized gradient approximation (GGA) of density functional theory and within the GGA plus onsite Coulomb interaction (GGA+U) approach. The calculations were performed using the pseudopotential plane-waves scheme and include structural relaxation. In the half-doped case, similar results are obtained for the orbital and atomic structure within the two approaches, with structural properties consistent with experiment. For the two-third-doped system, instead, the results differ between the two approaches. Only the GGA+U yields an insulating striped phase with structural properties in agreement with experiment. Our results show the importance of the cooperative effects of the Jahn-Teller distortion, strain modulation, and electronic localization in stabilizing the orbital-ordered striped phases, and provide new insight into the atomic-scale structure of the orbital order in La$_{1/3}$Ca$_{2/3}$MnO$_3$. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y13.00007: Direct evidence of charge inhomogeneity during the commensurate-incommensurate phase transition in charge ordered La$_{1-x}$Ca$_{x}$MnO$_{3}$ J. Tao, M. Varela, S. J. Pennycook, J. M. Zuo Commensurate-incommensurate (C-IC) phase transitions occurring in charge ordered (CO) manganites have been measured extensively by neutron scattering, x-ray diffraction and electron diffraction. However, these measurements only provide an average picture of the CO transition. Direct observations at the nano-scale are lacking, and the exact mechanism underlying the CO phase transitions remains unknown. Here, we report our \textit{in-situ} electron microscopic studies of the CO C-IC phase transition in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$. Scanning an electron probe about 1.7 nm in size over the sample, the local CO structures are recorded in the electron nano-diffraction patterns and the super-reflections associated with the CO phase are used to study the local effective doping. The mapping of the CO phase clearly shows inhomogeneous patterns of phase separation during the phase transition. The C-IC phase transition is interpreted by the change of the distribution of electrons/holes in La$_{1-x}$Ca$_{x}$MnO$_{3}$. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y13.00008: Competing Ferromagnetic and Charge-Ordered States in Realistic Models for Manganites: the Origin of the CMR Effect Cengiz \c{S}en, Gonzalo Alvarez, Elbio Dagotto The one-orbital model for manganites with cooperative phonons has been investigated via large-scale Monte Carlo (MC) simulations. Focusing on electronic density $n=0.75$, a regime of competition between ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects were observed. These effects appear even in the clean limit, but only by fine tuning parameters, while adding quenched disorder makes the CMR effects robust. The CMR magnitude is much larger than recently reported when randomly distributed polarons form the competing insulator. The crucial role of the superexchange coupling $J_{\rm AF}$ is discussed. The appearance of CMR effects is shown to be associated with the development of correlations among polarons above the spin ordering temperatures, in agreement with early neutron scattering investigations. These polarons tend to form small regions resembling the charge and spin arrangement of the low-temperature CO insulating state. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y13.00009: Fe-doping-induced charge-orbital ordering in manganese oxides Hideaki Sakai, Kiminori Ito, Yoshinori Tokura We investigated Fe-doping effects on a ferromagnetic metallic crystal, (La$_{0.7}$Pr$_{0.3}$)$_{0.65}$Ca$_{0.35}$MnO$_3$, which locates near the phase boundary to the charge-orbital ordered insulator. It was found that the competing charge-orbital ordering correlation is induced by substituting a small amount of Fe atoms for Mn ones. Such a tendency nicely contrasts with the impurity-induced ferromagnetic metallic phase appearing in the charge-orbital ordered manganites, for example, Cr-doped Nd$_{0.5}$Ca$_{0.5} $MnO$_3$ as intensively investigated for past years. Furthermore, we observed glassy magnetotransport properties as well as diffuse insulator-metal transition, such as magnetic- field annealing effects and long-time relaxation, like a ``relaxor ferromagnet". [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y13.00010: Dirty Peierls transition to stripe phase in manganites Susan Cox, Jason Lashley, Edward Rosten, Anthony Williams, John Singleton, Peter Littlewood The nature of the phase transitions in La$_{0.48}$Ca$_{0.52} $MnO$_3$ and Pr$_{0.48}$Ca$_{0.52}$MnO$_3$ has been probed using heat capacity and magnetisation measurements. The phase transition associated with the onset of the stripe phase has been identified as a second order transition which can be very well fitted by the model of a Peierls transition in a disordered system (a `dirty' Peierls transition). We demonstrate that this model can also be applied to other systems (such as alpha- Uranium) in which Peierls transitions occur. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y13.00011: Weak charge disproportion and leading mechanisms in half-doped manganites. Dmitri Volja, Wei Ku, Wei-Guo Yin The puzzling very weak charge disproportionation found in half- doped manganites such as La$_{1/2}$Ca$_{1/2}$MnO$_3$ is reconciled with the well-accepted Mn$^{3+}$/Mn$^{4+}$ picture of charge and orbital orders via our novel first-principles Wannier function analysis. The strong electron itinerancy is found to delocalize the ``Mn$^{3+} $'' Wannier states significantly, producing remarkable charge leaking into the ``Mn$^{4+}$'' sites. Thus, it is necessary to distinguish for this charge-transfer system actual charge from the occupation number. Finally, a realistic low-energy effective Hamiltonian is derived, revealing the interesting role of electron-electron interactions in the charge and orbital channels, which can be applied to other doping regions including the CMR phase. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y13.00012: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y13.00013: Experimental and theoretical determination of the anisotropic anomalous scattering tensor at the Mn K edge in LaMnO$_{3}$ Joaquin Garcia, Gloria Subias, M.C. Sanchez, J. Herrero-Martin, Keisuke Hatada, C.R. Natoli, S. Di Matteo, C. Mazzoli, J. Blasco A resonant x-ray scattering (RXS) study of (h 0 0), (0 k 0) and (0 0 l) forbidden reflections (h, k, l odd) at the Mn K- edge of LaMnO$_{3 }$was performed between 10 and 300 K. We observed strong resonant peaks at the three reflections. The azimuth angle dependence of all these reflections showed a characteristic sine evolution of $\pi $-period. The energy dependence of the intensity for (h 0 0) and (0 k 0) reflections was identical while different structures were observed for (0 0 l) reflections. We did not observe any change either on the resonance intensity or on the line shape when crossing the N\'{e}el temperature T$_{N}\sim $ 140 K. The energy, azimuth angle and polarization dependences of the three reflections are originated by off-diagonal terms of the scattering tensor. The theoretical analysis show that the principal axes of the anomalous scattering tensor depends on the photon energy. Moreover, resonant scattering in LaMnO3 is not a probe of d-orbital ordering. [Preview Abstract] |
Session Y14: Focus Session: Magnetic Nanostructures II
Sponsoring Units: DMP GMAGChair: Alejandra Lukaszew, University of Toledo
Room: Colorado Convention Center Korbel 4E
Friday, March 9, 2007 11:15AM - 11:51AM |
Y14.00001: InAs quantum well Hall devices for room-temperature detection of magnetic biomolecular labels Invited Speaker: The integration of micro- and nanoscale magnetics with molecular biology promises novel applications in fundamental studies of molecular interactions as well as in bioanalysis and biomedical functions. The implementation of this concept requires detection of biomolecular labels in the form of superparamagnetic micro/nano beads, ideally with single bead sensitivity. In this talk we will present our work on the development of miniaturized Hall sensors for detection of such beads. The devices, with Hall cross widths of $\sim $1 $\mu $m and $\sim $250 nm, were fabricated from InAs/AlSb quantum well semiconductor heterostructures. Their room-temperature characteristics were examined by Hall effect and electronic noise measurements. In the low frequency range, from 20 Hz to 1.6 kHz, devices have the noise-equivalent magnetic moment sensitivities of order 10$^{6}\mu _B /\sqrt {\mbox{Hz}} $ and 10$^{5}\mu _B /\sqrt {\mbox{Hz}} $ respectively. The sensitivity of the latter reaches the 10$^{4}\mu _B /\sqrt {\mbox{Hz}} $ range above $\sim $ 1 kHz. By using a phase-sensitive measurement technique and micron-sized Hall crosses we achieved detection of a single 1.2 $\mu $m diameter bead with a signal to noise ratio (S/N) of $\sim $ 33.3 dB, as well as detection of six 250 nm beads with S/N of $\sim $ 2.3 dB per bead. Our results from the micro-Hall susceptibility measurement on a single microbead can be explained quantitatively as due to the magnetic response of an ensemble of non-interacting magnetic nanoparticles with broad distribution of magnetic moments. The work demonstrates the efficacy of InAs quantum well Hall devices for applications in high sensitivity magnetic biomolecular detection. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y14.00002: Superparamagnetic bead assembly via biomolecular recognition and detection using micro Hall sensor Pradeep Manandhar, Goran Mihajlovic, Stephen von Molnar, Peng Xiong, Keita Ohtani, Hideo Ohno, Mark Field, Gerard J. Sullivan Specific binding of biological molecules onto a selectively functionalized area is a necessary key step in biological sensing with a solid state device. Here we present our results on the directed self-assembly of streptavidin coated superparamagnetic micron and submicron sized beads onto selectively biotinylated solid-state surfaces with organic molecular templates. Large-scale high-yield assembly of 130nm strepdavidin-coated beads onto biotinylated micro-patterns with little nonspecific binding is realized using molecular template generated by micro-contact printing. Specific binding of similar beads onto a semiconductor micro-Hall cross can be realized using high-spatial registry functionalization with dip-pen nanolithography, and the binding can be detected using phase sensitive Hall magnetometry with InAs quantum well micro-Hall sensors.\footnote{G. Mihajlovi{\ae} et al., APL 87, 112502 (2005).} [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y14.00003: In-vitro heating with Polyethylene Glycol Coated Magnetic Nanoparticles Srinivasan Balakrishnan, M.J. Bonder, D. Gallo, G. C. Hadjipanayis Magnetic nanoparticles synthesized with a biocompatible polymer coating are under investigation for future detection and treatment of cancer. In this study we investigate the heating characteristics of Fe based nanoparticles coated with polyethylene glycol. Structural characterization indicates a variation in the composition with polymer concentration. The x-ray analyses show that samples become increasingly amorphous as evidenced by the broad amorphous-like peak superimposed on the alpha iron (110) peak. The magnetization curves indicate that all samples are soft ferromagnets with the coercivity dependent on mean particle size. When the nanoparticle suspension is subjected to a 4Oe, 500 kHz AC magnetic field there is a steep rise in temperature reaching an equilibrium temperature for all cases. Correlating the equilibrium temperature with the static magnetic properties shows that the temperature is linearly dependent on the saturation magnetization and inversely proportional to the coercivity of the particle. Work supported by NSF DMR-0302544 . [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y14.00004: Ferromagnetic Gd$_{100-x}$Fe$_{x}$ (x = 4 - 40) Nanostructures D. Schmitter, J. Goertzen, G. Shelburne, T. M. Pekarek, J. E. Shield, P. M. Shand, D. Haskel, D. L. Leslie-Pelecky Iron in concentrations as small as 4 at. {\%} (where the distance between Fe atoms is more than three lattice spacings) produces ferromagnetic behavior at temperatures up to 50 K above the Gd Curie temperature T$_{C}$ in Gd$_{100-x}$Fe$_{x}$ nanostructures. X-ray diffraction and XAFS show that Gd$_{100-x}$Fe$_{x}$ nanostructures made by inert-gas-condensation and melt-spinning have nanoscale hcp Gd grains with Gd-Fe grain boundaries. Magnetization and XMCD measurements indicate that, above the bulk Gd T$_{C, }$Fe atoms polarize Gd atoms and produce ferromagnetic behavior with coercivities on the order of 50-100 Oe. The coercivity decreases as the temperature decreases toward the Gd T$_{C, }$which we attribute to random anisotropy averaging produced by ordering of the hcp-Gd grains. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y14.00005: Geometrically frustrated honeycomb and ladder lattices of nanoscale ferromagnetic islands Jie Li, Xianglin Ke, Ruifang Wang, William McConville, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied arrays of interacting single-domain ferromagnetic islands which are arranged on lattices such that the interactions between the islands are frustrated by the geometry of the arrays. While previous studies in our group [1] have focused on a frustrated square lattice, we now report results on lattices with the honeycomb geometry and with a topologically equivalent ladder geometry in which the islands meet in vertices of three islands. Each permalloy island measures approximately 80nm by 220nm with a thickness 25nm, and is evenly spaced with lattice spacing ranging from 225nm to 425nm for honeycomb lattice and from 320nm to 880nm for ladder lattice. Magnetic force microscopy measurements of the arrays after demagnetization demonstrate that the interactions between the islands are frustrated and that the correlations between islands decrease with increasing spacing of the islands. A detailed analysis of the correlations between the islands will be presented. This research has been supported by the Army Research Office. [1] R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi, and P. Schiffer, Nature 439, 303 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y14.00006: Field annealing study of a frustrated interacting nanomagnet array X. Ke, J. Li, W. McConville, R. Wang, C. Nisoli, P. Lammert, V. Crespi, P. Schiffer Lithographically patterned ferromagnetic nano-islands provide an ideal model to explore the physics of frustrated `spin ice' materials due to the competition of dipole interaction between elements [1]. Since the energy scales are large compared to thermal energies, field annealing is crucial to obtaining a low-energy demagnetized state among the interacting islands. We have studied various field annealing protocols to demagnetize the array by rotating the sample in a time-varying magnetic field. We find that reversing the field direction while stepping down the field magnitude is needed to successfully demagnetize the array. The annealing can also be tuned by varying the field step size, especially for field magnitudes near the coercive field of the array. The competition of dipole interaction with external field and dipole field of neighboring elements will be discussed. \newline [1] R. Wang, et al, Nature \textbf{439}, 303 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y14.00007: Magnetic domain wall phases in perpendicularly magnetized ultrathin films. Nidal Abu-Libdeh, David Venus At low temperature, the ground state of a perpendicularly magnetized ultrathin film is the stripe domain state with an equilibrium density (n) of domain walls that respond to the applied magnetic field with an equilibrium magnetic susceptibility$_{ }(\sim $1/n). The change in domain density with temperature involves the creation/annihilation of domain walls. We propose a simple relaxation model in which the domain wall creation/annihilation is an activated process. The model predicts a non-equilibrium domain density and thus an effective susceptibility that depends on the time scale of the measurements. We have measured the ac-magnetic susceptibility of perpendicularly magnetized ultrathin Fe films on a 2 ML Ni/W(110) substrate, as a function of temperature while changing the temperature at different heating rate (R) between 0.03K/s and 1K/s. In the low temperature range, the model calculations provide a consistent explanation of the measured susceptibility. In the high temperature range, the susceptibility measured with low heating rates (0.03K/s -- 0.1K/s), deviates from the calculation due to an increase of the high temperature half-width of the susceptibility. This has been tentatively interpreted as a phase change from the stripe domain phase to the tetragonal phase in which the domains have no preferred direction. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y14.00008: Magnetization Reversal in Europium Sulfide Nanocrystals James Dickerson, Marcela Redigolo, Dmitry Koktysh, Sandra Rosenthal, Zheng Gai, Lan Gao, Jian Shen We report the observation of the reversal in the magnetization hysteresis curve of europium sulfide nanocrystals. This phenomenon was investigated through the temperature-dependent magnetization of two classes of nanomaterials, nanocrystalline (2.0 nm \textit{$\le $ }d$_{NCs}$\textit{ $\le $ }100 nm), and quantum-confined (d$_{NCs}$\textit{ $\le $ }2.0 nm), where d$_{NCs}$ is the diameter of the nanomaterial. The effect of the size of the nanomaterial on the magnetization is attributed to the competition between the magnetic properties of strained surface atoms and unstrained core atoms. Superconducting quantum interference device (SQUID) probed the magnetic response. Electron microscopy and X-ray diffraction spectroscopy revealed the crystallinity and monodispersivity of the nanomaterials. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y14.00009: Magnetism and $^{13}$C NMR relaxation of nanodiamond powder E.M. Levin, S.L. Bud'ko, X.W. Fang, W.E. Straszheim, R.W. McCallum, K. Schmidt-Rohr The magnetization, $^{13}$C NMR relaxation, and composition of commercial nanodiamonds with an average grain diameter of 4 nm have been studied. The magnetization contains several contributions due to (1) the diamagnetic effect of core and valence electrons of carbon, (2) ferromagnetic-like and (3) superparamagnetic contributions from Fe-bearing particles detected in nanodiamonds, and (4) a paramagnetic contribution from unpaired electrons. The spin concentration obtained from the paramagnetic susceptibility is 2.2$\times $10$^{20}$ spins/g. At 300 K, nanodiamond powder shows ferromagnetic magnetization of 0.01 emu/g. $^{13}$C NMR spectra and relaxation times should be unaffected by the ferromagnetic particles with so small magnetization. Thus, a reduction of $^{13}$C NMR $T_{1}$ relaxation times by orders of magnitude compared to microdiamond can be explained by unpaired electrons in the nanodiamond grains. The origins of unpaired electrons and ferromagnetism in nanodiamond powder and other carbon-based materials are discussed in view of our results. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y14.00010: Magnetic Endohedral Metallofullerenes with Floppy Interiors Meichun Qian, Shiv Khanna, Mark Knickelbein Investigations on the electronic structure and magnetic properties of a free Gd$_{3}$N and Gd$_{3}$N@C$_{80}$ have been carried out using to examine the stability and the electronic and magnetic properties of the endohedral species. Using a synergistic approach combining Stern-Gerlach experiments in beams and first principles electronic structure studies, it is demonstrated that an isolated Gd$_{3}$N has a ground state spin moment of 23 $\mu $B followed by a non-collinear state of 17.2 $\mu $B only 88 meV above the ground state. The large moment is largely due to localized f-electrons. As a Gd$_{3}$N is embedded inside a C$_{80}$ cage, the localized f-electrons maintain the magnetic character while the hybridization between the s,d states of isolated Gd$_{3}$N and p-states of C$_{80}$ leads to a strongly bound motif with an interaction energy of 13.63 eV and a large HOMO-LUMO gap of 1.48 eV. Gd$_{3}$N@C$_{80}$ is further shown to possess two isomers corresponding to the location of the N atom on either side of the Gd$_{3}$ triangle with an appreciable electric dipole moment and a low barrier of 91 meV for transition between them offering potential for a fluctuating dipole. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y14.00011: Can Carbon Be Ferromagnetic - X Rays Can Give The Answer Hendrik Ohldag, Tolek Tyliszczak, Roland H{\"o}hne, Daniel Spemann, Pablo Esquinazi, Magda Ungureneau, Tilman Butz While conventional wisdom says that magnetic materials have to contain some metallic atoms [1], the confirmation of intrinsic magnetic order in pure metal free carbon represents an ultimate and general scientific breakthrough because of the fundamental importance of carbon as an elemental building block of organic as well as inorganic matter. The common controversy raised across all disciplines is whether the magnetism of carbon is intrinsic or induced by other elements. We address this controversy by providing clear experimental evidence that metal free carbon can be ferromagnetic at room temperature using dichroism x-ray absoprtion spectro-microscopy. For this purpose we acquired x-ray microsopy images of magnetic structures on a thin carbon film that have been produced by irradiation with a focussed 2.25MeV proton beam [2]. \newline \newline [1] F. Palacio, Nature 413, p. 690 (2001) \newline [2] P. Esquinazi et al., PRL 91, p 227201 (2003) [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y14.00012: Magnetic Properies of Ni Nanoparticles Used for Carbon Nanofiber Synthesis K. D. Sorge, O. Malkina, C. Finkel, J. D. Fowlkes, P. D. Rack, K. L. Klein, A. V. Melechko, M. L. Simpson Magnetic properties of Ni catalyst particles used for vertically-aligned carbon nanofiber (VACNF) synthesis are investigated. Ni thin films are deposited on Si wafers by sputter-depositing to thicknesses of 2--10 nm. The VACNFs are then grown in a Plasma-Enhanced Chemical Vapor Deposition (PECVD) chamber with NH$_3$ and C$_2$H$_2$ at relative flow rates of 80/40 sccm, respectively, a pressure of 3 Torr, and a temperature of 700$^{\circ}$C. The catalyst particles, after nanofiber growth, are 10--150 nm in diameter. Magnetic properties are investigated by SQUID magnetometry in applied magnetic fields of $|H| < 10$ kOe and temperatures $T = 5$--300 K. The catalyst particles are ferromagnetic with low coercivity and remanence. The ferromagnetic properties are thermally stable up to room temperature in all but the smallest particle sizes. Saturation magnetization is much less than would be expected from the deposited quantity of Ni metal. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y14.00013: Moving magnetic nanoparticles through soft-hard magnetic composite system Hemachander Subramanian, Jong Han An important requirement during the design of a nano-electromechanical system is the ability to move a nanoparticle from one point to another in a predictable way. Through simulations, we demonstrate that soft-hard magnetic stuctures can help us move nanoparticles predictably. We simulated a 2-D system, in which the exchange-coupled soft-magnetic magnetization is frustrated with the boundary condition set by a hard magnetic array and rotating external field. We consider a geometry with three-fold degenerate magnetic local minima and show that the hysteretic transitions are manipulated by an external field. Due to the reduced interfacial energy from weak demagnetization energy in the composite magnets and magnetic hysteresis, the energy landscape can be manipulated in a well-defined and predictable manner. We apply this idea to control the movement of a magnetic particle placed on a non-magnetic layer on top of the structure. We are interested in extending this simple, preliminary study to include complex geometries. We expect that complex geometrical constraints would lead to interesting orbits of nanoparticles in these systems. [Preview Abstract] |
Session Y17: Focus Session: Multiscale Modeling in Polymeric Materials
Sponsoring Units: DPOLY DCOMPChair: Michael Rubinstein, University of North Carolina, Chapel Hill
Room: Colorado Convention Center 102
Friday, March 9, 2007 11:15AM - 11:51AM |
Y17.00001: Molecular Dynamics Simulations of Layer-by-Layer Assembly of Charged Macromolecules Invited Speaker: Molecular dynamics simulations of electrostatic assembly of multilayers of flexible polyelectrolytes and charged nanoparticles at a charged surface were performed. The multilayer build-up was achieved through sequential adsorption of oppositely charged macromolecules in a layer-by-layer fashion from dilute solutions. The steady state multilayer growth proceeds through a charge reversal of the adsorbed polymeric film, which leads to a linear increase in the polymer surface coverage after completion of the first few deposition steps. Moreover, substantial intermixing between chains adsorbed during different deposition steps is observed. This intermixing is consistent with the observed requirement for several deposition steps to transpire for completion of a single layer. However, despite chain intermixing, there are almost perfect periodic oscillations of the density difference between monomers belonging to positively and negatively charged macromolecules in the adsorbed film. Weakly charged chains show higher polymer surface coverage than strongly charged ones. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y17.00002: Dynamics of melts consisting of circular and linear polymers Michael Lang, Michael Rubinstein Recent experimental results indicate that small contaminations of linear polymers with 0.1\% volume fraction or less in ring polymer melts lead to dramatic changes in the rheology of the melt. These volume fractions are clearly below overlap concentration of the linear species. Thus, the experimental observations cannot be explained by a simple picture based on a percolating cluster of linear chains penetrating rings. The goal of our computer simulation studies of comparable systems is to solve this puzzle. We use the bond fluctuation method on a lattice as introduced by Carmesin and Kremer (Macromolecules 21, 2819-2823 (1988)) to model homopolymer melts ranging from 32 to 1024 monomers per chain. The volume fraction of linear polymer is varied from 1/16 to zero. We simulate small melts of 16384 monomers in order to have access to the long-time behavior of the samples. We will present and discuss simulation data on diffusion, ring and linear polymer conformations, mobility and contact statistics of different samples with varying volume fraction of linear polymer. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y17.00003: Numerical Advances in Field Theoretic Simulations of Polymers Erin M. Lennon, Kirill Katsov, Hector D. Ceniceros, Carlos J. Garcia-Cervera, Glenn H. Fredrickson Field theoretic simulations have been very successful in predicting the mesoscopic behavior of polymer self-assembly in the mean-field limit. Nevertheless, studies beyond this approximation have been hindered by the numerical complexities of simulating a fluctuating field theory with a complex Hamiltonian. To address these problems, we have developed a suite of highly efficient numerical methods to study practically arbitrary polymer systems. Moreover, we further propose a thermodynamic integration technique suitable for determining the free energy of such field-based fluctuating systems. Using the standard diblock copolymer as our model system, we quantitatively investigate the effect of fluctuations on the order-disorder transition. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y17.00004: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y17.00005: Multiscale modeling of self-assembling polymer solutions Dmitry Bedrov, Grant Smith, Ben Hanson We have applied a multiscale modeling approach to study complex self-assembling systems such as micellar solutions and biopolymer networks. In our multiscale modeling approach we start with quantum chemistry calculations to parameterize accurate fully atomistic force fields, perform extensive atomistic explicit solvent molecular dynamics simulations on model self-assembling building blocks in solutions to understand structure and thermodynamics at atomistic scale, parameterize coarse-grained implicit solvent models, conduct extensive implicit solvent model simulations to study the self-assembly, and use a self consistent field approach to predict the system morphology. Specifically, we applied this methodology to self-assembly of triblock (PEO-PPO-PEO, Pluronic) micelles as well as to network formation of polypeptide (leucine)-modified hyaluronic acid in aqueous solutions. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y17.00006: Coarse grained model of polymer dynamics R.C. Picu, A. Rakshit A coarse grained representation of the structure and dynamics of polymer melts is developed. In the coarse grained model an entanglement segment is represented as a blob with a single degree of freedom. The inner blobs of the chain are constrained to perform random walks along the primitive path, while the chain end blobs have three degrees of freedom and interact through a non-bonded potential with all other blobs of the system. The primitive path is initially calibrated from the fine, atomistic system and then evolves as dictated by the kinematics of the end blobs. The inter-blob bonded and non-bonded potentials are calibrated from the fine model to reproduce the chain structure statistics. The coarse system is evolved with Brownian dynamics using a monomeric friction which is computed from the fine system. Constraint release is introduced and is dictated by an error indicator that monitors the neighborhood of a given primitive path segment during the simulation. The only parameter which is not necessarily (but it may be) calibrated from the fine model is the characteristic entanglement segment length. The coarse model predictions are compared with full atomistic simulation results. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y17.00007: Multiscale Simulation of polyethylene oxide: Combined United Atom and Coarse-Grained Modeling Praveen Depa, Janna Maranas We present combined united atom [UA] and coarse-grained [CG] molecular dynamics simulations of polyethylene oxide [PEO]. A hybrid region is used to connect united atom and coarse-grained regions, allowing them to occupy different spatial locations in the same simulation box. The presence of a hybrid region allows for a smooth and continuous change in the description of an interacting particle from a coarse-grained bead to a united atom and vice versa. This multiscale simulation is tested with united atom PEO and a coarse-grained model of PEO developed in our group. In the coarse-grained model each CG bead consists of six united atoms and the interactions between them are parameterized to accurately represent static properties, defined by the UA simulations and verified by experiments. Employing the hybrid region in the multiscale simulation works well, as evidenced by good agreement between static properties from the multiscale simulation with those from all UA and all CG simulations. We investigate the performance of the multiscale simulation when addressing dynamics properties, by comparing to UA and CG simulations, and experiments. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y17.00008: Structure and Evolution of Ordered Domains in Deeply Quenched Polyethylene Melt Naida Lacevic, Laurence Fried, Richard Gee Solidification of polymeric materials strongly depends on how the melt is cooled below its crystallization temperature. If cooling is at moderate rates, the most common and well understood mechanism is via nucleation and growth of spherulites, but special cases exist where crystallization is preceded by a pre-transition state induced by density fluctuations. Such multi-step crystallization scenarios are suggested by many experiments, and recent theoretical and simulation work. In this special case, the melt is quenched into a metastable region and the transition from the disordered phase to an ordered phase is uniform in contrast to the classical nucleation and growth mechanism. In this study we investigate this continuous transition. We have conducted the largest and longest timescale simulations of polyethylene to date. These elucidate the initial separation of a metastable, mesomorphic phase. Via energetic and geometric analyses, we have examined the structure of mesomorphic domains and the dynamics of their formation and evolution, including atomic scale details of molecular addition to ordered domains, as well as particle dynamics in the system, including high mobility jumps in the ordered domains at wavelengths matching the monomer spacing. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y17.00009: Simulating the Oxidation of Polypropylene Using a Reactive Forcefield Joanne Budzien, Aidan Thompson Oxidation of organic materials is a problem for seals and membranes in many environments. Any particular instance of a chemical reaction is fast with purely local effects. Over longer times, the cumulative effect of many reactions results in large changes in average stress, strain, and other macroscopic properties of the sample. We have had some success in developing constitutive models for rubber, which connect the topology of the network with the observed stress during aging under strain. As the next phase of the project, we have performed simulations using a reactive forcefield to examine the atomistic changes during oxidation. Results will be presented for polypropylene and compared with experimental data for labeled samples undergoing oxidation. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y17.00010: Rheological properties of polymer melts in confined shear flow from dynamic Monte Carlo simulations. John Dorgan The viscoelastic properties of dense polymer melts in shear flow are examined using dynamic Monte Carlo simulation for plate spacings less than 10 times the molecular radius of gyration. The coarse graining methodology employed consists of the cooperative motion algorithm of Pakula and a derived biasing technique based on previous studies of Binder and Baushnagel. For relatively large plate spacings and slow flows, a uniform linear velocity profile is obtainable. Use of the Kramers form for entropic springs allows the calculation of stress in the simulation providing a means for exploring rheological properties including viscosity and normal stress differences. Results are in excellent agreement with well-established experimental facts; a shear thinning viscosity is obtained, the first normal stress difference increases with shear rate, and the first normal stress coefficient decreases with shear rate. Evidence of entanglements are present for longer chain lengths. For fast flows, the linear velocity profile is lost and shear banding is observed. A non-monotonic stress with shear rate is found in conjunction with the shear banding and mechanistically this is attributable to a cohesive failure with an excess of chain ends being found at the slip plane. Results for variable plate spacings shed some insight into novel confinement effects that are being exploited in emerging areas of nanotechnology. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y17.00011: Flow-deformed conformations of entangled polymers as persistent random walks Yitzhak Shnidman Modeling interfacial phenomena in polymer fluids requires resolution of chain conformations on the Kuhn length scale. If the chains are at thermodynamic equilibrium, or undergo flow deformation in the unentangled regime, this is accomplished by representing chain conformations as Wiener (uncorrelated) random walks. When \textit{entangled} chains are deformed by flow, stretching and orientation of chain \textit{strands} between successive entanglements entails \textit{inertial}, as well as diffusive, aspects in the anisotropic propagation model for strand conformation. This is best captured by a \textit{persistent} (correlated) random walk at constant speed, which is a second-order Markov process governed by the initial probabilities and the scattering rates for the velocities. We present here a generalized Green-Kubo relation linking these parameters to the second moment of the strand's end-to-end distance. The latter evolves according to an approximate differential equation coupling local flow deformation rate with strand stretching and orientation, which relax on distinct time scales [G. Marrucci and G. Ianniruberto, \textit{Phil. Trans. R. Soc. Lond. A} \textbf{361}, 677 (2003)]. The proposed relation provides a cornerstone for a new \textit{entangled} version of our dynamic self-consistent field theory, that thus far has been limited to unentangled inhomogeneous polymer fluids [M. Mihajlovic, T. S. Lo, and Y. Shnidman, \textit{Phys. Rev. E} \textbf{72}, 041801 (2005)]. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y17.00012: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y17.00013: DNA-Particle Hydrodynamic Interactions In Microchannels Yeng-Long Chen The hydrodynamic contribution to the dynamics of DNA - particle mixtures in microfluidic channels is important for phenomena such as DNA/particle aggregation and DNA conformation change. Recent simulation and experimental works have successfully showed the importance of hydrodynamic contributions to the dynamics of DNA migration away from microchannel walls in a pressure-driven flow. In this work, we investigate the interplay between particle and DNA hydrodynamics by employing the lattice Boltzmann method (LBM) and coarse-grained Brownian dynamics. In addition, particle-DNA hard core interactions also affect the conformation and the dynamics of the flexible DNA molecule. The roles of particle / DNA size asymmetry and DNA and particle concentrations on the dynamic properties of the solution are systematically investigated. [Preview Abstract] |
Session Y18: Polymer Blends
Sponsoring Units: DPOLYChair: Valeriy Ginzburg, Dow Chemical
Room: Colorado Convention Center 103
Friday, March 9, 2007 11:15AM - 11:27AM |
Y18.00001: Thermodynamic Properties of A/B/A-C Polymer Blends from SANS and USANS Nisita Wanakule, Megan Robertson, David Lohse, Nitash Balsara We have studied the thermodynamic properties of blends of two homopolymers and a block copolymer (A/B/A-C blends) by small angle neutron scattering (SANS) and mean field theories. The binary A-C interactions demonstrate an upper solution critical temperature, B-C interactions show a lower solution critical temperature, and A-B interactions exhibit entropic behavior. We have demonstrated that as little as 1\% of the A-C block copolymer is adequate for stabilization of 50/50 A/B mixtures. It is however, difficult to quantify the structure of blends with very low copolymer concentrations alone because the periodic length scales obtained are in the 100s of nanometers. We are thus embarking on ultra-low angle SANS (USANS) experiments to elucidate the properties of these systems. A comparison of SANS and USANS data obtained from these blends will be presented at the meeting. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y18.00002: Kinetic Studies of Pressure-Quenched A/B/A-C Polymer Blends Alisyn Nedoma, Megan Robertson, David Lohse, Nitash Balsara The pressure-dependence of the thermodynamics of multicomponent A/B/A-C polymer blends was studied using small angle neutron scattering (SANS) and mean field theories. At atmospheric pressure, the A/C and A/B Flory-Huggins chi parameters are positive and decrease with increasing temperature while the B/C chi parameter is negative at room temperature and it increases with increasing temperature. Under ambient conditions a microphase separated state exists for low temperatures, a disordered micelle phase for intermediate temperatures, and a macrophase separated state for high temperatures. Surprisingly, we find that at elevated pressures the system is homogeneous over a wide range of temperatures and pressures. Starting from the homogeneous phase, we will perform time-resolved SANS experiments after a reverse pressure quench on our A/B/A-C blend to study the kinetics of macrophase and microphase separation. The results of these experiments will be presented at the meeting. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y18.00003: Effects of deuterium labeling at PS/PMMA interfaces studied with resonant soft x-ray reflectivity H. Ade, C. Wang, S. E. Harton, B. Watts, T. Araki The interfacial widths of PS/PMMA and deuterated-PS/PMMA bilayer interfaces were analyzed using resonant soft x-ray reflectivity (RSoXR). The PS and dPS utilized had the same molecular weight and poly-dispersity. Identical sample preparation and film thicknesses were used, respectively. The PS/PMMA bilayer width was consistently smaller than the dPS/PMMA width for a number of different thickness combinations. This is unexpected, based on previously reported bulk Flory-Huggins parameters for PS/PMMA and dPS/PMMA, respectively. Based on these bulk values, self consistent field theory asserts that the width of a PS/PMMA interface would be greater than that of a dPS/PMMA interface. In contrast, Harton \textit{et al. }[1] have recently reported the strong preferential segregation of dPS to a dPS:PS/PMMA interface. The RSoXR results are qualitatively consistent with the results of Harton \textit{et al} and confirm that the thermodynamic properties of PS/PMMA interfaces are substantially different from the properties in the bulk. [1] S. E. Harton\textit{ et al.}, Macromolecules \textbf{39}, 1639 (2006). [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y18.00004: Ion solvation and its effects on polymer blend miscibility Zhen-Gang Wang We study the solvation of small ions in a binary blend of two homopolymers having different dielectric constants. The preferential solvation of ions by the high dielectric constant component results in decreased miscibility of the two polymers. We first consider the solvation of a single ion in the mixture and its effect on the local composition change using the Flory-Huggins-de Gennes free energy for the polymer blend. This single-ion information is then used to construct a bulk free energy valid for small ion concentrations, from which the shift in the spinodal and coexistence curve is calculated. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y18.00005: Thermoreversible bond formation in multi-component polymer blends Richard Elliott, Glenn Fredrickson We investigate theoretically thermoreversible bond formation and phase transitions in a system composed of end-linking, difunctional linear polymers. The two chemically distinct polymer species are allowed to bond heterogeneously so that only linear chains with alternating block sequences form. Interactions between dissimilar segments are described with Flory-Huggins contact potential, whereas the amount of reversible bonding is controlled with a single parameter of binding affinity. We use a mean-field approach to describe the onset of meso-scale ordering as a function of the binding affinity. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y18.00006: Modeling of Crystallization and Phase Separation in Binary Blends Driven by Photopolymerization Pankaj Rathi, Thein Kyu Polymerization of a reactive component in a binary blend raises the molecular weight of the reactive species which drives phase transformation involving phase-separation, mesophase ordering in liquid crystals or crystallization. We demonstrate a novel phenomenon of photo-polymerization induced crystallization in a blend of a crystalline component and reacting monomer. Blending a crystalline substance with a solvent or monomer lowers melting temperature of the crystalline component. When photopolymerization is carried out at an isotropic phase temperature above the melting point of the blend, the depressed melting point curve of the crystalline component shifts above the reaction temperature and triggers crystallization. To predict this phenomenon, a theoretical model has been developed. Phase diagrams are calculated from the free energy description of a blend containing crystalline constituent. Snapshots indicating the movement of melting point curve and the upper critical solution temperature with polymerization reaction are demonstrated. Pattern formation dynamics are calculated to demonstrate the crystal growth driven by photopolymerization. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y18.00007: Shear-induced Crystallization of and Polypropylene/Poly(Ethylene-co-Octene) Blends Xia Dong, Kun Meng, Chenggui Zhang, Tongchen Sun, Charles C. Han, Jianhua Dong Isothermal crystallization under shear in a blend of isotactic polypropylene (iPP) and poly (ethylene-co-octene) (PEOc) was investigated by in-situ optical microscopy and shear hot stage under various thermal and shear histories. Shish-kebab crystal structures were observed under shear in phase separated iPP/PEOc blends. Very long cylindrites can form under shear, with length scale much longer than the dimension of the liquid-liquid phase separated domains under the applied shear conditions. The cylindrites appear to grow through crystallizable domains, as well as through non-crystallizable ones. All evidences points to the exacted that the nuclei (`shish') came from the orientation of the entangle network chains instead of pull out chain bundles. The shear rate and the shear time have different effects on the formation of the cylindrites after liquid-liquid phase separation. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y18.00008: Nanoparticles as Blend Compatibilizers: Layered Silicates and Fullerenes Jitendra Sharma, Romesh Patel, Liang Xu, Ramanan Krishnamoorti Blending of polymers is an excellent way to achieve designer materials that combine specific properties of individual polymers to obtain end product with improved properties. However, processing of such materials many times is confounded by the undesirable large scale phase separation. Recent research has focused on utilizing micron-sized additives like carbon black(1) and glass beads(2) etc., to reduce the interfacial energy and promote mixing. Despite industry-scale success of the method, features like high particle loadings and the subsequent increase in the density of the material is undesirable. Replacement of micron-sized particles with nanoparticles (NPs) and the resultant increase in the interfacial surface to volume ratio potentially helps resolve this issue. We present results from our investigation of the role of two different classes of NPs namely, layered silicates and fullerenes (covering a range of three dimensional architecture and aspect ratio), on the compatibilization of two immiscible polymers of PS and PMMA as probed by AFM. \begin{enumerate} \item C. Calberg et al., \textit{Appl. Phys. } \textbf{1999}, $32$, 1517. \item H. Tanaka et al., \textit{Phys Rev Lett.} \textbf{1994}, $72$, 2581. \end{enumerate} [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y18.00009: Compatibilization of Polymer Blends via Reactive Processing with Telechelic Copolymers Earl Ashcraft, Mark Dadmun Blends of polystyrene and polyisoprene are reactively compatibilized using telechelic polystyrene and polyisoprene additives. A blend of PS/PI and telechelic polymers is melt mixed at 180\r{ }C, resulting in the in-situ formation of multi-block copolymers during mixing by the reaction of telechelics at the interface, which compatibilize the phase-separated homopolymer blend. The conversion of telechelic polymer into copolymer is quantified using GPC with fluorescence detection. In this project, the effect of using high and low reactivity end-groups is examined. The effect of the telechelic molecular weight on copolymer formation and blend properties is also examined. The results show that the anhydride/amine pair yields approximately 30{\%} - 50{\%} conversion after 10 minutes of mixing, with no further conversion after this time period. The epoxy/COOH pair shows much slower conversion. The stability of the blend morphology is quantified by determining the coarsening constant of the phase-separated blend from SEM. Preliminary DMA data suggests that the multi-block copolymers improve the interfacial adhesion of the blend, supporting the GPC and SEM results. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y18.00010: The Effect of Copolymer Composition on the Dynamics of Random Copolymers of Styrene and Methylmethacrylate in a PMMA Matrix: A Neutron Reflectivity Study Sudesh Kamath, Mark Dadmun, William Hamilton, Michael Arlen The effect of copolymer composition on their dynamics in a homopolymer matrix have been studied using specular neutron reflectivity (NR). Four random copolymers containing 50, 54, 67 and 80 $\%$ MMA were studied at 10 $\%$ wt loading in d-PMMA. The mutual and tracer diffusion coefficients, the effective friction coefficients, and the relaxation times for these copolymers were determined. The results demonstrate that copolymer composition has a significant impact on their dynamics. Analysis of the friction factor using the Lodge-Mcleish model indicates that the local composition around a copolymer is richer in Styrene than the model predictions. We attribute this to the fact that the model uses only chain-connectivity to calculate the self-concentration and ignores contributions due to thermodynamic interactions between the two blend components. Our data indicate that the local environment around a copolymer is richer in styrene. This is in agreement with our simulation results and indicates that the styrene monomers in the copolymer aggregate together to minimize contact with the PMMA matrix. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y18.00011: Nucleation in Polymer Blends Edward Feng The existing theories for nucleation in polymer blends fail to capture the results of recent experiments by Balsara and coworkers. Part of the explanation for this disagreement is the thermodynamic nature of these theories which does not account for kinetic effects. In order to study effect of dynamics on nucleation, we consider a kinetic Ising model in which the total magnetization is constant, the simplest realistic model for liquid-liquid phase separation. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y18.00012: Poly(ethylene oxide) Dynamics in Blends with Poly(vinyl acetate) Junshu Zhao, Mark Ediger $^{2}$H NMR relaxation measurements were performed to study the segmental dynamics of perdeuteropoly(ethylene oxide) (d$_{4}$PEO) in miscible blends with poly(vinyl acetate) (PVAc). For two compositions (2{\%} and 50{\%} d$_{4}$PEO), spin-lattice relaxation times were measured in a temperature range well above T$_{g}$. Over the temperature range studied, the segmental dynamics of the PEO component in 2{\%} PEO/PVAc blend is almost the same as that in 3{\%} PEO/PMMA blend reported by Lutz et al. (\textit{Macromolecules 2003, 36, 1724-1730}), even though the T$_{g}$ of PVAc is 100K lower than that of PMMA. In the 2{\%} d$_{4}$PEO blend, the segmental dynamics of PEO is 9 orders of magnitude faster than PVAc segmental dynamics at the blend T$_{g}$. For the two compositions studied, segmental dynamics of the PEO component can be well described by L-M model with a self-concentration of 0.3. By comparing the segmental correlation times of the PEO component with viscoelastic shift factors reported by Urakawa et al. (\textit{J. Non-Crystalline Solids 352, 2006 5042-5049}), it is found that the temperature dependence of the segmental dynamics of the PEO component is weaker than the temperature dependence of the PEO terminal dynamics. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y18.00013: Viscosity of ``Nanoparticle''/Polymer Mixtures John G. Curro, Amalie L. Frischknecht Recently it has been observed by Tuteja et al. (Macromolecules 38, 8000 (2005)) that when single crosslinked polystyrene macromolecules (nanoparticles) are added to sufficiently high molecular weight linear polystyrene melts, the viscosity of the mixture decreases in apparent contradiction to Einstein's law of viscosity of dilute suspensions. We propose that this unexpected behavior can be understood by considering the system as a miscible polymer blend rather than a suspension. An approximate formulation for the blend viscosity is developed based on an additive mixing rule using the Rouse and reptation models for the linear melt at low and high molecular weights respectively. This theory predicts that upon addition of crosslinked macromolecules, the mixture viscosity increases (decreases) for low (high) molecular weights of the linear chain component in qualitative agreement with experiment. Moreover, the effect of the molecular weight of the crosslinked macromolecules on the mixture viscosity was in accordance with the data of Tuteja and coworkers. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y18.00014: T$_{g}$ in Polymer/Oligomer Athermal Blends Wei Zheng, Sindee Simon The glass transition behavior of poly($\alpha $-methyl styrene), its tridecamer, and athermal blends with its hexamer is investigated using differential scanning calorimetry. The blends are prepared to exhibit approximately the same T$_{g}$ as the tridecamer but with varying molecular weight distributions. The glass transition of the blends is found to become broader than that of the pure materials. However, the absolute heat capacity of the blends maintains unchanged from its components indicating that the blends are athermal mixtures. The broadening of the Tg is evaluated in the context of recent models describing this behavior. In addition, we examine the ability of the TNM model to describe the rate dependence of T$_{g}$ and aging effects. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y18.00015: Surface-induced structure formation of polymer dispersed liquid crystals on chemically gradient substrate Jun Wang, Jianfeng Xia, Suck Won Hong, Feng Qiu, Zhiqun Lin We investigate that the domains of polymer dispersed liquid crystals (PDLC) thin film can be directed into ordered structures by a chemically gradient substrate. A unique phase separation kinetics was observed. The pattern on the substrate was successfully transferred to the PDLC film, resulting in alternating LC rich phase and polymer rich phase as confirmed by confocal Raman measurements. This simple yet novel approach enables the organization of micrometer size LC domains in thin polymer matrix without photo irradiation. [Preview Abstract] |
Session Y19: Surfaces, Interfaces, and Colloids III
Sponsoring Units: DCPChair: Nancy Levinger, Colorado State University
Room: Colorado Convention Center 104
Friday, March 9, 2007 11:15AM - 11:27AM |
Y19.00001: Surfactant Adsorption at the Air-Liquid and Hydrophobic Solid-Liquid Interfaces: Unraveling the Mechanism for Superspreading Makonnen Payne, Charles Maldarelli, Alexander Couzis In this paper we report our findings with regard to the synergistic interactions between polyethylene oxide surfactants of the general structure CiEj and compare the behavior to a known super wetting surfactant. Pendant drop tensiometry experiments and sessile drop contact angle measurements on hydrophobic surfaces were conducted on combinations of CiEj surfactants with 1-dodecanol. We found that a number of combinations were capable of reducing significantly the air-liquid tension, however only systems that exhibited the propensity to form extended liquid crystalline phases, as shown by the combination of cross-polarized microscopy, cryo-TEM, and light scattering experiments, were able to improve on the wetting performance of the these systems. We have also conducted the parallel experiment focused on the surfactant adsorption at the hydrophobic solid-liquid interface. Using in-situ infrared internal reflection spectroscopy and complimentary sum-frequency generation spectroscopy, we are able to dynamically interrogate the surfactant adsorption kinetics and interfacial water structure evolution at the hydrophobic solid-liquid interface. We will relate these findings to gain insight into the molecular requirements for superspreading. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y19.00002: Interrogating the strength of the bond between salt and water: a combined DFT and MP2 study Bo Li, Angelos Michaelides, Matthias Scheffler The interaction of water with salt (NaCl) is of widespread importance and of considerable general interest. Although numerous theoretical studies have been reported [1], none has provided a convincingly reliable estimate of the strength of the bond between water and a flat NaCl surface, such as NaCl(001). Moreover, the computed adsorption energies predicted by DFT vary from 0.2-0.7 eV/H$_{2}$O depending on the choice of exchange-correlation functional [2]. Here, we address this issue through an extensive series of periodic Hartree-Fock and post-Hartree Fock [M{\o}ller-Plesset perturbation (MP2) and coupled cluster (CCSD(T))] calculations. Periodic Hartree-Fock calculations have been performed for H$_{2}$O molecules adsorbed on NaCl slabs, and by evaluating the local dependence of the correlation contribution to the adsorption energy with respect to cluster size, we obtain accurate MP2 and CCSD(T) estimates of the H$_{2}$O adsorption energy on NaCl(001). Our computed adsorption energy, which is around 0.6 eV/H$_{2}$O, comes close to the experimental value [3] and is at the upper end of the range predicted by DFT. [1] A. Verdaguer, G. M. Sacha, H. Bluhm, and M. Salmeron, Chem. Rev. 106, 1478 (2006). [2] B. Li, A. Michaelides, and M. Scheffler, in preparation. [3] L. W. Bruch, A. Glebov, J. P. Toennies, and H. Weiss, J. Chem. Phys. 103, 5109 (1995). [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y19.00003: Cluster Morphology and Aggregation Kinetics in Dense Aerosols Rajan Dhaubhadel, Amitabha Chakrabarti, Christopher Sorensen We studied the cluster morphology and kinetics of an aggregating aerosol system using the small angle light scattering technique. Exploding a mixture of a hydrocarbon gas and oxygen in closed chamber generated a system of aggregating soot particles. The soot particles started as individual monomers, ca. 38 nm radius, grew to bigger clusters with time and finally stopped evolving after spanning a network across the whole system volume. The gelled clusters showed a hybrid morphology with a lower fractal dimension at length scales of a micron or smaller and a higher fractal dimension at length scales greater than a micron. The study of the kinetics of the aggregating system showed that the system gelled when the homogeneity \textit{$\lambda $} attained a value 0.4 or higher. The aggregation kernel $K$ appearing in SE was also determined using the light scattering data.. The observed data indicated a slight increase in $K$ value when the system was denser. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y19.00004: Adsorption of Water, Methanol and Toluene on the Surface of Soot Particulate Matter Mohsen Yeganeh, Shawn Dougal, Bernard Silbernagel, El-Mekki El-Malki Soot particles are byproducts of incomplete hydrocarbon combustion. The adsorption of water and organic molecules on the soot surfaces is of technological and environmental importance. We have applied a newly developed technique of SFG spectroscopy for high surface area materials [1] to investigate the adsorption of water, methanol, and toluene on the surface of standard soot particles from the National Institute of Standards and Technology (NIST). Adaptations of standard SFG procedures are required to compensate for the high refractive index of these carbonaceous materials. We find that adsorption of water, methanol, and toluene on the soot surface is reversible at room temperature. We also find that UV radiation modifies the surface composition of these soot particles. The effect of UV radiation on the adsorption, as well as the kinetics and thermodynamics of methanol adsorption/desorption will also be discussed. [1] Mohsen S. Yeganeh, Shawn M. Dougal, and Bernard G. Silbernagel, Langmuir \textbf{22}, 637 (2006) [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y19.00005: Drops Bouncing on a Superhydrophobic Surface Frank van Swol Low solid interfacial energy and fractally rough surface topography confer to Lotus plants superhydrophobic (SH) properties like high contact angles, rolling and bouncing of liquid droplets, and self-cleaning of particle contaminants. By exploiting the porous fractal structure of a novel, synthetic SH surface it is possible to produce a synthetic lotus effect. This is relevant to realizing self-cleaning properties for particle concentration, and the slippery nature of the coating can be used to enhance the performance of fluidic devices. Here we report on molecular dynamics (MD) and some classical density functional theory calculations that provide valuable insight into the conditions needed to cause liquid droplets to form and bounce on a surface. The MD results report on the details of the droplets impacting surfaces of varying wettability, ranging from complete wetting to complete drying. For the SH surfaces we present results for the contact time, between droplet and surface, as a function of impact velocity, droplet size and surface friction. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y19.00006: Pair Interaction of microparticles at oil-water interfaces Chuan Zeng, Anthony Dinsmore We confined microparticles at fluid-fluid interfaces and measured the interaction between them. Aggregates of colloidal particles were observed, suggesting an attractive capillary force at long range, which cannot be explained by gravity. We report measurements of the interaction between carboxylate-modified polystyrene spheres (radius $\sim $ 1 micron) at the oil-water interface using image analysis and particle tracking. The interaction between two isolated spheres was measured from particle trajectories and analyzed through the Markovian Dynamics Extrapolation method developed by J.C. Crocker and D.G. Grier. Different choices of oil (1,1,1-trifluoroheptan, silicone oil, decahydronapthalene, etc.) and various sample geometries were explored. The role of surfactants will be discussed. We acknowledge support from NASA through the Fluid Physics program (NRA 02-OBPR-03-C). [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y19.00007: Selective crystal nucleation at a soft organic template. Sumit Kewalramani, Kyungil Kim, Guennadi Evmenenko, Pulak Dutta Langmuir monolayers floating on supersaturated aqueous solutions can guide the growth of oriented inorganic crystals.$^{1}$ Although, oriented crystal growth under such monolayers has been observed for a variety of inorganic species, barium fluoride and strontium fluoride are the only systems where an epitaxial match between the organic and inorganic lattices is directly observed.$^{2}$ We have studied different growth stages of this model system by varying the subphase conditions. We find that, in the earliest nucleation stages, strained epitaxial thin films of barium fluoride and barium fluoride chloride can be grown under the same organic structures. We will also discuss late crystal growth stages, specifically orientation/misorientation effects in thick films, which are governed by reorganization of preformed crystals. 1. S. Mann, Biomineralization Principles and Concepts in Bioinorganic Materials Chemistry, Oxford University Press, Oxford, 2001. 2. J. Kmetko et al. Phys. Rev. Lett. \textbf{89}, 186102-1 (2002). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y19.00008: A Light Scattering Study of Solvent Solidification in Colloidal Suspensions Melissa Spannuth, Simon Mochrie, John Wettlaufer Intermolecular forces between a solid and a foreign substrate can lead to the formation of a thin film of liquid between the two surfaces at temperatures below the solid's bulk melting temperature. These forces also result in fluid flow towards colder regions of the interface when a temperature gradient is present. When the fluid component of a colloidal suspension solidifies, the particles become encased within the ice both individually and in clusters. Near the melting temperature, a premelted layer bathes the particles, providing a restricted, but finite volume of liquid that facilitates Brownian motion through the background solid. We will present the results of our X-ray Photon Correlation Spectroscopy experiments by which we can characterize the dynamics of the particles in a solidifying colloidal suspension and thereby extract information about the amount of premelted liquid. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y19.00009: Ion-size effect at the surface of silica hydrosol Aleksey Tikhonov The forces of electrical imaging strongly polarize the surface of colloidal silica. I used X-ray scattering to study the ion-size effect in the adsorbed 2-nm-thick compact layer of alkali ions at the surface of concentrated solutions of 5-nm, 7-nm, and 22-nm particles, stabilized either by NaOH or a mixture of NaOH and CsOH, with the total bulk concentration of alkali ions ranging from 0.1- to 0.7-mol/L. The observed structure of the compact layer is almost independent of the size of the particles and concentration of alkali base in the sol; it can be described by a two-layer model, i.e., a $\sim $ 6 - 8-Angstrom-thick layer of directly adsorbed hydrated alkali ions with a surface concentration $\sim 3\times 10^{18}$ m$^{-2}$, and a $\sim $ 13-Angstrom-thick layer with a surface concentration of sodium ions $\sim 8\times 10^{18}$ m$^{-2}$. In cesium-enriched sols, Cs$^{+}$ ions preferentially adsorb in the first layer replacing Na$^{+}$; their density in the second layer does not depend on the presence of cesium in the sol. The difference in the adsorption of Cs$^{+}$ and Na$^{+}$ ions can be explained by the ion-size-dependent term in the electrostatic Gibbs energy equation derived earlier by others. Brookhaven National Laboratory is supported by U.S.D.O.E., Contract No. DEAC0298CH10886. X19C is partially supported through funding from the ChemMatCARS National Synchrotron Resource and the University of Chicago. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y19.00010: Steric effects on the dynamics of electrolytes Mustafa Sabri Kilic, Martin Bazant, Armand Ajdari The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatics. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt. In this talk, we adopt a simple model for steric effects with only one parameter (the effective ion size), and we analyze the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. This effect is able to predict the reversal of AC electro-osmotic flow at high voltage and high frequency, as well as the increase of capacitance with temperature in molten salts. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y19.00011: Transport of colloids in porous medium. Hsiang-Ku Lin, Roya Zandi, Leonid P. Pryadko Pathogenic microorganisms such as bacteria and viruses in groundwater cause over one million illnesses per year in the United States. Despite the considerable research, the transport of microorganisms (colloids) in porous media is not well understood. In the reported work, we present a phenomenological filtration model that describes transport of colloids and the dynamics of colloid deposition and release at the attachment sites. The model has a soliton-like solution for the filtering front separating ``clean'' anterior and ``dirty'' posterior regions. The computed breakthrough curves and time-dependent deposition curves are in good agreement with experimental measurements. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y19.00012: Charge Inversion by Electrostatic Complexation: Molecular Dynamics Simulations Jordi Faraudo, Alex Travesset Ions near interfaces play an important role in many biological and physico-chemical processes and exhibit a fascinating diverse range of phenomena. A relevant example is charge inversion, where interfacial charges attract counterions in excess of their own nominal charge, thus leading to an inversion of the sign of the interfacial charge. In this work, we argue that in the case of amphiphilic interfaces, charge inversion can be generated by {\em complexation}, that is, electrostatic complexes containing several counterions bound to amphiphilic molecules. The formation of these complexes require the presence at the interface of groups with conformational degrees of freedom with many electronegative atoms. We illustrate this mechanism by analyzing all atomic molecular dynamics simulations of a DMPA (Dimirystoil-Phosphatidic acid) phospholipid monolayer in contact with divalent counterions. The results are found to be in agreement with recent experimental results on Langmuir monolayers. We also discuss the implications for biological systems, as Phosphatidic acid is emerging as a key signaling phospholipid. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y19.00013: Microscale Curveballs: Translational and Rotational Coupling of Colloids Stephen Anthony, Minsu Kim, Steve Granick Optically anisotropic MOON particles (modulated optical nanoparticles) allow the simultaneous measurement of translation and rotation for individual particles. Through chemical modification, these particles can be made hydrophilic, amphiphilic, or hydrophobic. Among those three, the boundary condition exhibits varying amounts of slip, which registers in the value of the rotational diffusion constant. Additionally, for the amphiphilic case, the translational and rotational motions are no longer independent of each other, exhibiting coupling due to the asymmetric hydrodynamic drag. Understanding these processes is fundamental to particle dynamics, with implications to kinetically limited processes such as the self-assembly of multi-unit proteins. Addressing the influence of these varied boundary conditions, this study presents single-particle tracking of micron-sized spherical colloids suspended in deionized water, tracked and quantified using home-written algorithms. [Preview Abstract] |
Session Y26: Focus Session: Charge Transport in Nanostructures V
Sponsoring Units: DCPChair: Massimillimo Di Ventra, University of California, San Diego
Room: Colorado Convention Center 205
Friday, March 9, 2007 11:15AM - 11:51AM |
Y26.00001: What are `molecular wires' and how might we use them? Invited Speaker: Through-bond tunneling is generally greatly enhanced over through-space tunneling, so organic `wires' can connect electrodes over distances with $>$nS conductance over distances of several nm. Very small decay lengths (or even non-exponential decay) suggest that mechanisms other than tunneling can contribute to transport. Wires made with electroactive (reducible/oxidizable) molecules can be gated electrochemically, and can be switched without any gate at all if the field owing to the applied bias is large enough, giving rise to switching and NDR. Wires that reconcile the competing requirements of flexibility (so they can be properly bonded) with high conductance will be useful elements in nanoscale devices and sensors. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y26.00002: Molecular conductance from density functional theory calculations Daniel Kosov, Zhenyu Li We will present a plane-wave/pseudopotential implementation of a method to calculate the electron transport properties of nanostructures. We performed density functional theory based electron transport calculations of amine, dithiocarboxylate and dithiiocarbamate anchored junctions. We demonstrated that the stronger molecule-electrode coupling associated with the conjugated dithiocarbamate linker broadens transmission resonances near the Fermi energy. The conductance enhancement factor is as large as 25 is predicted for dithiocarbame anchored junctions. A microscopic origin of the experimentally observed current amplification by dithiocarboxylate anchoring groups is established. We calculated the conductance traces for amine and thiol anchored junctions as the molecules are pulled by the STM tip from the Au electrode. Our calculations show that the stretching of the thiol anchored junction during its formation is accompanied by significant electrode geometry distortion. Oppositely, the electrode for the amine terminated junction remains intact when the junction is stretched by the STM tip. Z. Li and D. Kosov, J.Phys.:Cond.Matt. 18 (2006) p.1347; J.Phys.Chem B 110 (2006) p.19116, ibid p.9893 [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y26.00003: Exploring conductance switching properties of molecular scale devices - a computational approach. Barry D. Dunietz, Alexander Prociuk, Mousumi Das, Trilisa Perrine A computational approach is used and developed to study electron transport through molecular scale devices. The study identifies and provides insight into mechanisms underlying transport switching properties. These systems include: 1. Spin-dependent electronic transport through a Porphyrin ring Ligating an Fe(II) atom, 2. Contact geometry and orientation effects of conjugated molecular transistors and 3. Chemical sensors with focus on metal recognition properties recently exhibited only for certain short peptide chains. The research also involves developing new models and methods to describe electron conductance through single molecular systems. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y26.00004: Measurement of Single Molecule Conductance Joshua Hihath, Xiulan Li, Fang Chen, Nongjian Tao Understanding the electron transport properties of single molecules is a basic requirement for achieving molecular-scale electronic devices reliable enough for everyday use. To help elucidate the transport mechanisms involved in various single metal-molecule-metal junctions we have performed measurements while applying an electrochemical gate, changing the environmental temperature, the chemical binding at the contacts, the bias, and the molecular length in an STM break junction system. Using this system and these handles to help elucidate conduction mechanisms we have explored several molecular systems, including simple alkane chains, conjugated redox molecules, and biologically relevant molecules such as DNA and proteins, in all cases we have gained some insight into the transport capabilities of each metal-molecule system. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y26.00005: Statistical Analysis of Electronic Transport in Alkanethiol Molecular Devices with Nanowell Structures. Hyunwook Song, Takhee Lee, Nak-Jin Choi, Hyoyoung Lee We study charge transport through nanoscale molecular monolayers. For example, alkanethiol [CH$_{3}$(CH$_{2})_{n-1}$SH] self-assembled monolayer (SAM) is one of the most standard molecular systems that has been extensively investigated, and is very useful as a control in molecular devices because their structure and configuration have been sufficiently well-characterized. Reported here is a statistical analysis of electronic transport in alkanethiol SAM using different length alkanethiols. Particularly, we fabricated more than 6,000 molecular devices with nanowell structure, a vertical type of metal-molecule-metal junctions with nanometer scale junction diameter. We investigated transport properties such as temperature-variable current-voltage I(V,T) characteristics from these mass-fabricated devices. Based on the extensive I(V,T) data, a statistical analysis of transport characteristics in alkanethiol SAM will be presented. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y26.00006: Electronic transport of low concentrations of P3HT molecules across nanogap source-drain electrodes. Jeff Worne, Behrang Hamadani, Douglas Natelson Poly 3-hexothiophene (P3HT) is a widely studied, versatile material used in organic electronics. Little is known, however, about the electronic transport properties of individual or small groups of P3HT molecules. Initial experiments suggest that the behavior of low concentrations of molecules differ significantly from bulk P3HT. We have fabricated nanoscale P3HT transistors using electromigrated nanogap structures as source-drain electrodes and the underlying silicon/SiO$_{2}$ substrate as a gate. We present preliminary transport data on these devices as a function of temperature and electrode material. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y26.00007: Adsorption and conductance of BDT on the Au(111) surface A. Fazzio, Renato B. Pontes, Frederico D. Novaes, Antonio J.R. da Silva Molecular electronics is a new trend in the science and technology at the nanometer-scale. A prototypical system where transport properties have been widely studied both experimentally and theoretically is benzene-1,4-dithiolate (BDT) on Au(111). We present, using Total Energy Density Functional Theory calculations, a detailed study of such a system for different surface coverages and sites, and we find that except at high enough coverages, the BDT lowest energy configuration has the molecule almost lying down on the Au surface. We also find that when the BDT is bridging two Au(111) leads, this lying down configuration still has the lowest energy when compared to the standing up configuration (by approximately 0.4 eV). We have also calculated, using a DFT-based non-equilibrium Green's Function formalism, the conductance for a variety of BDT configurations, including how they vary as a function of the separation between the leads. We find that due to resonant features in the conductance, it can vary significantly depending on the distance between the leads. We also calculate the total energy, forces and conductance for a variety of BDT configurations for different separation between leads. We thank the Brazilian agencies FAPESP and CNPq, and CENAPAD-SP for computer time. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y26.00008: Ab-Initio calculations of electron transport properties of Si-Porphyrin-Si devices Filipe J. Ribeiro, Wenchang Lu, Jerry Bernholc We present results of numerical calculations of the electronic transport properties of devices consisting of porphyrin molecules connected to Si(100) leads. Our calculations are based on ab-initio ultrasoft pseudopotentials and the generalized gradient approximation (GGA) to the exchange and correlation energy functional. Transport properties were calculated using a non-equilibrium Green's function method in a basis of optimally localized orbitals. We studied three different molecules: H2-, Zn-, and Ni-porphyrins. The somewhat different alignments of the HOMO and LUMO levels of the molecules with the top of the valence and bottom of the conduction bands of the Si leads has strong implications on the I-V characteristics of the devices. In particular, the turn-on voltages are different for the different molecules. In addition, if the Si leads are heavily doped n-type, regions of negative differential resistance exist in all three systems. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y26.00009: Role of dephasing and surface states in Si based molecular electronics Hassan Raza We study the effect of an unpaired dangling bond (DB) on occupied molecular orbital conduction through a styrene molecule bonded to a n++ H:Si(100)-(2$\times $1) surface. For molecules relatively far from the DB, we find good agreement with the reported experiment using a model that accounts for the electrostatic contribution of the DB, provided we include some dephasing due to low energy phonon modes. However, for molecules within 10{\AA} to the DB, we have to include electronic contribution as well along with higher dephasing to explain the transport features. Apart from this, we study the electronic band structure of unpaired and paired DBs, DB wires and clusters on H:Si(100)-(2$\times $1) surface using Extended H\"{u}ckel Theory (EHT) and report their effect on the Si band gap. An unpaired DB introduces a near-midgap state, whereas a paired DB leads to $\pi $ and $\pi $* states. The unpaired and paired DB wires introduce states in similar fashion however with larger dispersion. Furthermore, different DB clusters exhibit states that can be interpreted as superposition of states due to unpaired and paired DBs. (cond-mat/0607226,0611417) [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y26.00010: Time-Resolved, Single Molecule Spectroelectrochemistry of Conjugated Polymers in Contact with ITO John Grey, Rodrigo Palacios, Wei-Shun Chang, William Miller, Allen Bard, Paul Barbara Time-resolved, single molecule spectroelectrochemistry was used to study excited-state interfacial electron transfer between single conjugated polymer (MEH-PPV) molecules (possessing about 200 redox sites) and an indium tin oxide (ITO) electrode. Decay kinetics and emission yields were obtained while cycling the electrode potential in the range of -.5V to +.5V (Silver wire), which lies between the reduction (-1.5 eV) and oxidation potentials (0.8 eV) of the ground state. At +0.5 V, the emission intensities and average lifetimes were observed to increase about 20{\%} whereas at -0.5 V both values decrease by the same amount. Several possible origins of the potential-induced intensity modulation are proposed. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y26.00011: Renormalized Couplings and the Insulator and Metallic Behavior of Double-Stranded DNA Efta Yudiarsah, Sergio E. Ulloa Electronic transport in double-stranded DNA is studied using a ladder model in a tight-binding Hamiltonian, withRealistic on- site energies [1] and hopping constants [2]. The effect of DNA molecules coupling to leads is studied on periodic poly (dG)-poly(dC) sequences with an embedded TGGGGT defect group. The differential conductance features diminish gradually and vanish at small coupling. The influence of counter-ions, local fields, and interaction with phonons can renormalize the hopping constants; we study the role of increasing intra-strand hopping on $\lambda$-phage DNA sequences. Increasing coupling results in the electronic transport of $\lambda$-sequences to change from insulator to metallic. Differential conductance $dI/dV$ at low bias is vanishingly small for bare hopping constants found in the literature [2], and increases rapidly if they are enhanced by more than 5 times. Even at large uniform intra-chain coupling (1 eV), $dI/dV$ drops drastically at low bias for sequences longer than 300 base pairs. Electron-phonon interactions are also considered. The diagonal (local) interaction results in polaronic effects while the non-diagonal terms yield phonon- assisted hopping. [1] S. Roche, Phys. Rev. Lett. 91, 108101 (2003). [2] A. A. Voityuk \emph{et al.}, J. Chem. Phys. 114, 5614 (2001). [Preview Abstract] |
Session Y27: Nanowire Sensors and Oscillators
Sponsoring Units: DCMPChair: Masa Ishigami, University of Maryland
Room: Colorado Convention Center 301
Friday, March 9, 2007 11:15AM - 11:27AM |
Y27.00001: Effect of Chamber Pressure on the Vibrational Properties of Micro- and Nano-Cantilevers Gayatri Keskar, Jay Gaillard, Jonathan Taylor, Malcolm Skove, Apparao Rao We have studied the nonlinear dynamics of micro- and nano-cantilevers under varying ambient conditions using the Harmonic Detection of Resonance technique (HDR)$^{1}$. In our studies, a cantilever is either microstructure shaped like a diving board, or a cantilevered MWNT. In this work, we report the dependence of the amplitude and Q of a silicon microcantilever (300 $\mu$m long, 35 $\mu$m wide, 2 $\mu$m thick) on ambient pressure. An environment of air at a pressure of 10$^{-3}$ Torr gives a high quality factor of $\sim$7000. The response of higher harmonics of the ac voltage that drives the cantilever is also observed with varying chamber pressure. An investigation of the influence of ac and dc voltages on sensitivity shows very good agreement with a model calculation. The shift in the resonant frequency of cantilevers under different environments such as helium, air and argon at different pressures will be discussed. \textbf{References:} 1. J. Gaillard, M. J. Skove, R. Ciocan, and A. M. Rao, Rev. Sci. Instrum. 77, 073907 (2006). \textbf{Contact Info: arao@clemson.edu} [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y27.00002: Chemical and bio- sensor chips based on nanowires and carbon nanotube Chongwu Zhou, Fumiaki Ishikawa Significant effort has been devoted to the study of sensors using one-dimensional structured nanomaterials. Integration with other functionalities or combinational use of such sensors is a promising direction with which highly sophisticated functionalities can be realized. In this talk, we will present the use of the integrated/combined nano-chemical/bio sensors for more sensitive, precise, and selective sensing. We integrated micromachined heater into the chemical sensor based on In2O3 nanowire (NW), and the sensing experiments at the elevated temperatures demonstrated the detection of ethanol, which is undetectable at room temperature. Furthermore, combinational use of NW and carbon nanotube (CNT) chemical sensors combined with the micromachined heater were demonstrated toward the construction of electronic nose system. In addition, manufacturable multiplexed biosensor chips based on In2O3 NW, CNT, and aligned CNT were successfully fabricated with a highly generic strategy that can be extended to other one-dimensional materials. These sensors chips can be used as the platform for multiplexed sensing combined with selective functionalization. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y27.00003: Detection of adsorbed monolayers on suspended singlewalled carbon nanotubes. Zenghui Wang, Jiang Wei, Wei Chen, Andrew Jones, Oscar Vilches, David Cobden Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two.~ This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed.~ The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance.~ We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen.~ The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time.~ The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important.~ We have made suitable nanotube devices and will report on our progress in detecting monolayers on them. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y27.00004: Modeling of the Nonlinear Response of a Microcantilever: Understanding Higher Order Harmonics and Resonances J.D. Taylor, Jay Galliard, Malcolm Skove, Apparao Rao The most promising and readily scalable detection scheme for micro and nanocantilevers is electrostatic excitation and capacitive detection. This method has proven difficult to implement because of a large parasitic capacitance which masks the dynamic signal from the cantilever. Fortunately, the cantilever response exhibits several higher order harmonics and resonant peaks that can be exploited to avoid the parasitic capacitance and dramatically improve the signal to noise ratio. In this report a theoretical model of the cantilever response is presented that explains these higher order harmonics and resonances by considering nonlinear effects. Also, an experiment is presented in which the response of a microcantilever is measured simultaneously using laser reflectometry and capacitive techniques in order to separate the effects of current modulation and mechanical motion. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y27.00005: Sequence Dependent Single Stranded DNA-Single Walled Carbon Nanotube Interactions and Their Applications in Detection of Gaseous Analytes Samuel Khamis, Michelle Chen, Robert Johnson, A.T. Charlie Johnson Recently there has been great interest in sensing strategies based on the use of non-covalent means to tune the chemical affinity of single walled carbon nanotube field effect transistors (SWNT FET's). The combination of single-stranded DNA (ssDNA) and SWNT FET's is particularly intriguing because of their chemical compatibility and diverse chemical recognition properties. We have demonstrated the utility of such devices for vapor sensing$^{1}$, and report here on results involving more than a dozen different ssDNA sequences. ssDNA/SWNT based sensors are sensitive to ppms of said gases, with response and recovery times on the scale of seconds. In tests involving a gas panel that includes explosives, neuro-toxins, and disease defining compounds, sensor response is specific to particular sequences of ssDNA. Given the extremely large number of different ssDNA sequences available, this observation opens up possibility of creating a large number of sensors with widely varying response characteristics, as required for an ``electronic nose'' system for the detection and classification of vapor mixtures. $^{1}$ C. Staii, M. Chen, A. Gelperin, and A.T. Johnson, Nano Lett. 2005, 5, 1774-1778 This work supported by the JSTO DTRA and the Army Research Office Grant {\#} W911NF-06-1-0462 [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y27.00006: Fabrication and characterization of individual nanotube based nanoelectrodes for chemical and biological sensing Kyungsuk Yum, Hanna Cho, Jie Hu, Min-Feng Yu We present the fabrication and characterization of individual nanotube based high aspect ratio nanoelectrodes for chemical and biological sensing. The nanoelectrodes are fabricated by coating nanotubes with metal and, subsequently, with thin insulating layers, and cutting the end of the nanotubes. This process yields ring-shaped nanoelectrodes with total structural diameter of $\sim $ 100 nm, including insulating layers, and length up to $\sim $ 30 $\mu $m. The nanoelectrodes are characterized by cyclic voltammetry (CV), and the structure of the nanoelectrodes is examined in transmission electron microscope (TEM). The nanoelectrodes show steady-state voltammetric current responses and good insulation of the side wall of nanoelectrodes. These high aspect ratio nanoelectrodes will open up a new opportunity for electrochemical sensing in microscale environments, e.g. probing local intracellular environments without damaging cells, with high temporal and spatial resolution. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y27.00007: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y27.00008: Transmission line impedance of carbon nanotube thin films for chemical sensing applications. G. Esen, M. S. Fuhrer, J. H. Chen, M. Ishigami, E. D. Williams We measure the resistance and frequency-dependent (50 Hz - 20 KHz) gate capacitance of carbon nanotube (CNT) thin films as a function of DC gate bias under ambient conditions, in ultra-high vacuum, and under low-pressure (10$^{-6}$ torr) gaseous environments of water, acetone, and argon. We have analyzed our results by modeling the CNT film as an RC transmission line. We show that changes in the measured capacitance as a function of gate bias and analyte pressure can be explained by the changes in the CNT film resistivity alone; the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption to within the resolution of our measurement. We also show that the resistance of the CNT film is enormously sensitive to exposure to low pressures ($<$ 10$^{-6}$ Torr) of analytes. This research was supported by the Laboratory for Physical Sciences and the U.S. Army Research Laboratory MICRA Program. MI received support from the Director of Central Intelligence Postdoctoral Fellowship program. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y27.00009: Harmonic Detection of $\omega _{0}$/n Superharmonics in Microcantilevers via Electrostatic Actuation/ Detection J. Gaillard, J. Taylor, G. Keskar, M. Skove, A. Rao In nonlinear dynamics, mechanical motion can be made up of a complicated mixture of vibrations. In resonating structures, nonlinearities are ubiquitous and more often than not are undesirable. On the other hand, nonlinear dynamics and chaos in electrostatic microelectro-mechanical systems (MEMS) has been shown to be useful for various applications, including secure communications, MEMS filters, and scanning force microscopy. Exploiting these dynamics opens the door for nanoelectro-mechanical systems (NEMS) by providing signals with higher quality factors and better signal-to-background ratios. In cantilever-based MEMS, the nonlinear dynamics usually stem from harmonically forced excitation in which only the second superharmonic has been theorized or detected. Here we measure the nonlinearly modulated charge on a silicon microcantilever up to the seventh superharmonic of the fundamental resonant mode via electrostatic actuation/detection. In agreement with experiment, simulated results reveal that the time dependence of the modulated charge due to the cantilever's motion carries a set of harmonics for each superharmonic of order $\omega $0/n. We propose that using a system of harmonics and modes of the cantilever increases applications for cantilevered and doubly-clamped microbeams, and that exploiting the nonlinearities in the modulated charge provides a valuable tool for the study of the dynamics in electrostatic transduction. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y27.00010: Electromechanical Response of Single-wall Carbon Nanotubes to Torsional Strain in a Self-Contained Device A. R. Hall, M. R. Falvo, R. Superfine, S. Washburn The response of single-wall carbon nanotube transport properties to applied shear strain has been measured. The strain is applied in a self-contained nanoelectromechanical device. We find that the measured resistance of an individual nanotube can increase or decrease depending on initial band structure, and that this change is approximately proportional to the applied strain. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y27.00011: Kinetics of Desorption of Oxygen Sharvil Desai, Gamini Sumanasekera, Chaminda Jayasinghe, David Mast The kinetics of desorption of oxygen was studied by measuring in-situ thermopower of the Single Walled Carbon Nanotube samples subjected to (a) post synthesis acid treatment by refluxing (b) high temperature annealing at 10$^{-7}$ Torr (c) plasma (Ar, O$_{2}$, H$_{2})$ treatment using an inductively coupled plasma reactor. Raman Spectroscopy and X-Ray Photoluminescence Spectroscopy were used to identify wall defects and other disorders created due to each treatment on the nanotubes. Also we have estimated binding energy of oxygen with the carbon in all cases. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y27.00012: High-Performance ZnO Nanowire FETs Pai-chun Chang, Zhiyong Fan, Chung-Jen Chien, Jia Grace Lu Zinc oxide (ZnO) nanowires have attracted tremendous interest due to their remarkable physical properties and versatile applications in electronic devices, such as logic circuit, UV emitter and photodetector, as well as chemical sensor. In the previous research report, ZnO nanowire configured as field effect transistor (FET) shows an electron mobility ranging from 3 to 80 cm$^{2}$/V$\cdot $s without surface treatments. In order to optimize the performance of devices, it is crucial to improve the carrier transconductance and mobility in the nanochannel. In our work, single-crystalline ZnO nanowires were synthesized via a catalyst-assisted chemical vapor deposition method. Temperature dependent photoluminescence measurements demonstrate the evolution of peak intensities resulted from different radiative mechanisms. The sharp peak with its full-width half maximum of 3.6meV at 12 K and the absence of other bound-exciton lines indicate that the as-grown nanowires are of high crystal and optical qualities. Following CMOS compatible procedures to passivate surface defect states and also to reduce chemisorption processes, the nanowire FETs exhibit orders of magnitude improvement in the on/off ratio, sub-threshold swing and field effect mobility. Remarkable mobility exceeding 4000~cm$^{2}$/ V$\cdot $s was estimated. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y27.00013: N-doped carbon nanotubes and their behavior as ammonia sensors Antonio J.R. da Silva, Mariana Rossi, Frederico D. Novaes, A. Fazzio CNx nanotubes can display a measurable variation in resistance upon exposure to ammonia. We present a microscopic model for the origin of these variations. We studied, using Total Energy DFT calculations, a (5,5) CNT containing pyridine-like N atoms replacing C atoms, and how the NH$_{3}$ molecule binds to these sites. We also investigate how these defects affect the charge transport properties using a Non-Equilibrium Greens Function formalism. We initially studied a defect composed by a vacancy surrounded by 3 pyridine-like rings. The most stable adsorption configuration for the ammonia molecule adsorbed close to this defect is dissociative, with an amino group (NH$_{2})$ fragment bound to one of the nitrogens and a H atom bound to another. This configuration leads to an increase in the conductance and cannot, therefore, explain the increase of resistance that has been experimentally observed. We then investigate a variety of other configurations in order to propose possible causes for the resistance increase. We find that a divacancy surrounded by 4 pyridine-like defects is the most stable N-defect, instead of the previously proposed one. The ammonia also dissociates into NH$_{2}$ and H. Moreover, the calculated change in conductance after the NH$_{3 }$dissociation has the correct trend when compared to the experimental results. We acknowledge FAPESP, CNPq and CENAPAD-SP. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y27.00014: First principles study of hydrogen adsorption on carbon nanowires. Alejandro Tapia, Luis Aguilera, Gabriel Murrieta, Romeo de Coss Recently has been reported a new type of one-dimensional carbon structures. Carbon nanowires formed by a linear carbon-atom chain inside an armchair (5,5) carbon nanotube has been observed using high-resolution transmission electron microscopy. In the present work we have studied the changes in the electronic structure of a carbon nanowires and (5,5) single-walled carbon nanotubes (SWCN) when a hydrogen atom is adsorbed. We used the Density Functional Theory and the calculations where performed by the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure, density of states (LDOS), and the local orbital population. We found charge transfer from the nanotube to the linear chain and the hydrogen atom, the electronic character of the chain and nanotube sub-systems in chain@SWCN is the same that in the corresponding isolated systems, chain or SWCN. But the hydrogen adsorption produced changes in the atomic estructure and the electronic properties. This research was supported by PRIORI-UADY under Grant No. FING-05-004 and Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grants No. 43830-F and 49985-J. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y27.00015: Hydrogen Physisorption Properties of Single-Walled Carbon Nanotubes Studied by Soft-X-Ray Spectroscopy. J. Zhong, S. Barcelo, J.-W. Chiou, C. L. Dong, C. L. Chang, W.-F. Pong, Y. Y. Chen, S. Mao, Z. Y. Wu, J.-H. Guo Single-walled carbon nanotubes (SWNT) for nano gas sensors becomes a subject of active research. Different mechanisms for interaction of gas and SWNTs were reported. The electrical resistance was reported to change in a semiconductor SWNT when exposed to gases. The mechanism was interpreted to be collisions between gas molecules and tube wall. The major experimental effect is transport response to inert gases. It is said that deformation from collisions can change the electronic properties of SWNTs. So far there is few study to verify the mechanism and thus further investigation is needed. We have performed soft-x-ray absorption experiment to show the in-situ interaction between SWNTs and gas molecules under ambient temperature and pressure. The gas adsorption caused changes in electronic structure of SWNTs can be recovered by evacuation of gas. The collision of gas molecules to SWNTs can be responsible to the pressure-dependent spectral change. The raising gas pressure up to 10 torr induces deformation of SWNTs thus decreases the conductance, and further increase of pressure will recover the deformation due to inner tube collisions. [Preview Abstract] |
Session Y31: Synthesis, Growth and Processing of Complex Materials
Sponsoring Units: DCMPChair: Michael Mehl, Naval Research Laboratory
Room: Colorado Convention Center 401
Friday, March 9, 2007 11:15AM - 11:27AM |
Y31.00001: Magnetic Resonance Studies of Crystalline Ge$_{2}$Sb$_{2}$Te$_{5 }$and Sb$_{2}$Te$_{3}$ David Bobela, P. Craig Taylor Recent technological applications of some chalcogenide materials, which are materials containing a group VI atom, have prompted studies of the local atomic structure of the amorphous phase. In particular, Ge$_{2}$Sb$_{2}$Te$_{5}$ has been employed as the prototypical phase-change memory material, since its structure can be rapidly switched from the crystalline phase to the amorphous phase with high fidelity. The metastability in the local bonding structure that produces this phase-change phenomenon is not yet understood. Magnetic resonance methods can be used to study local order as a function of the stoichiometry and phase of the material. As a starting point in understanding the magnetic resonance data for amorphous Ge$_{2}$Sb$_{2}$Te$_{5}$, we have used nuclear magnetic resonance and nuclear quadrupole resonance techniques to study crystalline Ge$_{2}$Sb$_{2}$Te$_{5}$ and Sb$_{2}$Te$_{3}$. We present estimates of the quadrupole coupling constants, and the asymmetry parameters of the electric field gradient for the $^{121}$Sb nuclei. The relationship between these parameters and the local atomic structure of crystalline Ge$_{2}$Sb$_{2}$Te$_{5}$ will be discussed. In particular, the Sb sites appear to depart from axial symmetry, a conclusion that is difficult to obtain from standard scattering experiments. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y31.00002: Terahertz Transmission Through Quasiperiodic Arrays of Subwavelength Apertures Tatsunosuke Matsui, Amit Agrawal, Ajay Nahata, Z. Valy Vardeny The extraordinary light transmission (EOT) through optically thick metallic films perforated with two-dimensional (2D) subwavelength hole array was originally explained in terms of resonant coupling to surface plasmon polariton (SPP) modes via grating coupling; and thus numerous EOT studies have been done on periodic hole arrays. Here we demonstrate terahertz EOT through 2D quasicrystalline array of subwavelength apertures that are not Bravais lattices, but, nevertheless of which structure factor shows discrete Fourier transform components. We found that such patterns also exhibit strong EOT bands indicating that SPP interaction also occur in quasiperiodic structures. This approach dramatically expands potential design parameters for aperture arrays and opens up exciting new avenues for optoelectronic devices, especially in the THz spectral range. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y31.00003: Terahertz Transmission Through Aperiodic Arrays of Subwavelength Apertures Amit Agrawal, Tatsunosuke Matsui, Valy Vardeny, Ajay Nahata The recent surge of interest in the field of surface plasmon polaritons (SPPs) resulted in part following the initial demonstration of extraordinary optical transmission (EOT) through periodic arrays of subwavelength apertures. Ensuing experimental and theoretical studies on these and associated structures were focused primarily on \textit{periodic} structures. In this submission, we demonstrate that aperture \textit{periodicity} is not crucial for obtaining strong EOT resonances. We demonstrate this phenomenon by measuring the EOT of \textit{aperiodic} array of subwavelength apertures fabricated on free standing metal films in the terahertz spectral range. We observed sharp resonances in the transmission spectra at frequencies matching the aperture array `structure factor'. The \textit{aperiodic} structures were designed using a general numerical approach for patterns in which no associated geometrical tiling rules exist. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y31.00004: Micro-Raman and FTIR Studies of Synthetic and Natural Apatites A. Antonakos, E. Liarokapis, Th. Leventouri B-type synthetic carbonate hydroxyapatite (CHAp), natural carbonate fluorapatite (CFAp) and silicon-substituted hydroxyapatite (SiHAp), have been studied by using micro-Raman and infrared (IR) spectroscopy. It was found that while B-type carbonate substitution predominates in carbonate apatites (CAps), A-type is also detected in all types of apatites. B-type carbonate substitution causes a broadening of the v$_{1}$ P-O stretching mode that is associated with the atomic disorder within the phosphate tetrahedron. An $\sim $15 cm$^{-1}$ shift of the v$_{3c}$ P-O antisymmetric IR mode was observed upon heat-treatment of the CFAp to drive the carbonate off. This shift indicates that the P-O bond lengths on the mirror plane increase when carbonate leaves the apatite structure suggesting that carbonate substitutes on the mirror plane of the phosphate tetrahedra. The present results support the substitution mechanism proposed on the basis of neutron powder diffraction studies of the same samples. The intensity ratios of the v$_{2}$ IR CO$_{3}$ and v$_{1}$ PO$_{4}$ bands in samples with various carbonate contents provide a calibration curve for estimating the degree of carbonization of CAps. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y31.00005: Propagation of Electromagnetic Waves in a One Dimensional Photonic Crystal with DPS/DNG Layers Joseph Shahbazian, Aram Karakashian Although materials having positive permittivity and permeability (DPS) are well known, recently electromagnetic materials with negative permittivity and permeability (DNG) have been given much attention. Wave propagation in a double negative medium and also photonic crystals has been studied analytically and experimentally. The material parameters are complex and frequency dependent to account for both dispersion and absorption. The real part of the corresponding index of refraction can be negative only in narrow frequency bands. Here we have studied theoretically the propagation of electromagnetic waves in a one dimensional photonic crystal composed of alternating layers of DNG and DPS layers. We find that this type of photonic crystal in the visible wave range exhibits negative refraction in a wider frequency range. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y31.00006: NMR Studies of Ba8AlxGe46-x and Ba8GaxGe46-x Clathrates Gou Weiping, Yang Li, Ji Chi, V. Goruganti, K. D. D. Rathnayaka, Joseph H. Ross, Jr. We have prepared a series of clathrates of the general form, Ba$_{8}$Al$_{x}$Ge$_{46-x}$. X-ray measurements indicate that Al occupies predominantly the 24$k$ site, similar to previous observations for the Ga analogs. $^{27}$ NMR measurements using wide-line and MAS spectrometers indicate two different local Al sites, one with a relatively small quadrupole splitting, attributed to the 24$k$ site, and one with a larger quadrupole splitting attributed to Al adjacent to a vacancy. We also discuss the results of $ab$-$initio$ calculations supporting these conclusions. $T$-dependent NMR relaxation results show roughly Korringa-type behavior for both series of compounds for $x <$ 16, characteristic of heavily-doped semiconducting materials. The lowest-carrier density Ga$_{16}$ sample shows changes in $T_{1}$ and Knight shift characteristic of incipient localization at low temperatures, however analysis indicates that the carriers remain in the regular band rather than forming an impurity band. For Ga$_{x}$ clathrates synthesized from excess Ga, the NMR $T_{1}$ changes abruptly, indicative of a change in band-edge symmetry for $p$-type material. This work was supported by the Robert A. Welch Foundation (grant A-1526), and the National Science Foundation (DMR-0103455). [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y31.00007: Thermodynamics of type-I and type-II Si clathrates: a computer simulation study Caetano Miranda, Alex Antonelli Few years ago a guest free type-II clathrate of silicon was obtained. This new form of elemental silicon is remarkably stable up to high temperatures. It is now clear that in order to devise new synthesis routes for these materials a better understanding of their thermodynamic properties is highly desirable. In this work, we present a computational study, employing the isobaric Monte Carlo method, of the thermodynamic properties at zero pressure of different phases of silicon, namely, both clathrate forms, $\rm Si_{46}$ (type-I) and $\rm Si_{34}$ (type-II), diamond crystalline silicon, liquid silicon, and amorphous silicon (\textit{a}-Si). The Gibbs free energies, calculated by the reversible scaling method, allow us to determine the stable and metastable relations between these various phases. We have found the melting point of Si$_{46}$ and Si$_{34}$ clathrate structures to be at $1482\pm25$~K and $1522\pm25$~K, respectively. Our result for the melting point of Si$_{34}$ is in good agreement with the experimental value of 1473~K. The results also indicate that both clathrate forms are more stable than \textit{a}-Si for any temperature up to their melting point. Based on our findings we discuss the feasibility of routes for the growth of these materials, such as solid-phase epitaxy and liquid-phase epitaxy. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y31.00008: Dielectric spectroscopy of pulsed laser deposited type Pb(Zr,Ti)O$_{3}$ (PZT)/CoFe$_{2}$O$_{4}$ (CFO) and CFO/ PZT multilayers thin films. Nora Ortega, N. Baskaran, R. Katiyar, I. Takeuchi Mutiferroics are a novel class of next generation multifunctional materials. Intensive research is being pursued towards the development of materials with high magnetoelectric (ME) coupling. In case of composite thin films, ferromagnetic (FM) columns epitaxially grown in ferroelectric (FE) matrix, exhibit large ME coefficient compared to multilayers structures. From the studies it appears that the distribution of FM phase in FE matrix plays an important role to obtain high ME effect. In the present work we fabricated composite thin films of FE Pb(Zr,Ti)O$_{3}$ (PZT) and FM CoFe$_{2}$O$_{4}$ (CFO) multilayers with 3,5, and 9 layers PZT/CFO (PC) and CFO/PZT (CP) by pulsed laser deposition. We have investigated the effect of reversing the order of FE and FM layers in the multilayer configuration on the properties. The TEM and XPS depth profile of the films, showed that the layer structure was not maintained. The dielectric constant of PC and CP multilayers showed strong frequency dispersion. Reversing the multilayers configuration from CP to PC resulted in increasing the remanent polarization. The observed dielectric relaxation has been explained by Maxwell-wagner type contributions. The ME effect of multilayer films will be discussed. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y31.00009: Measurement of the DC resistance of thin film semiconductor-gas systems: comparison to the effective medium approximation J. Garner, B. Carrico, J. Saredy, J. Tracy, N. Patel DC resistance measurements have been made of a thin film semiconductor (indium tin oxide) that has been exposed to various gases (acetylene, methane, and sulfur dioxide). Our motivation is to use the time-dependent resistance as a means of identifying unknown gases. Three transport models were used to compare to the measurements: a simple resistor network model, a dynamic Maxwell-Garnett theory (DMGT), and a dynamic effective medium approximation (DEMA). The latter two theories are \textit{dynamic} because the volume fractions of the constituents of the thin film-gas composite vary with time during the gas adsorption process. The resistor network model gives the general trend of the data. This novel application of the DMGT and DEMA yield results that are nearly identical to each other and that are in good agreement with our measurements. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y31.00010: Functionally graded, nanocrystalline, multiphase, B- and C-based superhard coatings Charles Blatchley, Fereydoon Namavar, Eric Tobin, John Adams, Michael Graham Candidate ceramic coatings for bearing applications must meet several criteria, such as hardness, to limit abrasive damage. Adhesion is essential to prevent destructive three-body wear. Toughness helps avoid brittle fracture and coating failure. Finally, temperatures during deposition must not damage the substrate. We report fabrication and testing of functionally-graded, nanocrystalline, multiphase Ti/BN coatings by ion beam assisted deposition (IBAD), with these properties. Hardness was measured $>$ 42 GPa. Structural grading transitions from metallic to covalent bonding through the film, controlling constituents (TiN, TiB$_{2}$, B$_{4}$C, BN) to optimize adhesion, internal stress, hardness, and wear resistance. Pin-on-disk wear testing for 5 million cycles at 1 GPa contact stress, showed no wear, to potentially extend lifetimes by orders of magnitude in industrial or biomedical applications. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y31.00011: Photoemission Studies on Bulk Amorphous Steels Michael Buettner, B. Simon Mun, Peter Oelhafen, Petra Reinke The recent availability of bulk amorphous metals (BMGs) promises interesting new applications for the near future based on the superior structural, physical, and chemical properties of such materials compared to conventional steels. In order to shed light on local atomic and electronic structure photoemission studies have been performed on BMGs containing Fe, Cr, Mo, C, B, and Er. Progressing from ternary to penternary alloys we studied changes in the local electronic environment in a systematic manner by means of core-level and valence band spectroscopy. Step-by-step ion irradiation revealed differences in surface and bulk elemental composition and bonding. In particular the bonding state of B changes from oxide (surface) to boride (bulk). Iron exhibits the most prominent spectral changes of all the metal constituents, showing significantly higher core-level binding energies in penternary alloys than in ternary and quaternary compounds. Investigations of the Fe 3s multiplet splitting indicates a dependence of the splitting energy on the abundance of erbium in the alloy. Future experiments will focus on bulk material properties by preparing alloy surfaces in vacuum and addressing the constituents' chemical environment in more detail. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y31.00012: Nanoscale Order in Marginal and Bulk Amorphous Metal Alloys Paul Voyles, William Stratton, Jinwoo Hwang, Joseph Hamann, Hongbo Cao, John Perepezko, Y. Austin Chang Using fluctuation electron microscopy (FEM), we have shown that both an Al-based marginal amorphous Al alloy, Al$_{88}$Y$_{7}$Fe$_{5}$, and a Zr-based bulk amorphous alloy, Zr$_{54}$Cu$_{38}$Al$_{8}$, have significant nanoscale structural order at a length scale of $\sim $1.5 nm. In both cases, that order can be reduced by annealing below $T_{g}$. In the Al alloy, this order is associated with proto-crystalline clusters formed in the liquid or during the rapid quench. The size and/or density of these clusters can also be modified by small additions of Cu and Ti. The nature of the structure in the Zr alloy has not been determined, but our results show that structural relaxation on aging, which in bulk metallic glasses is commonly understood in terms of redistribution of free volume in the form of atom-size voids, involves rearrangments of groups of $\sim $10-100 atoms. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y31.00013: Strain localization in disordered materials and implications for constitutive laws for sliding interfaces Mary Lisa Manning, James S. Langer, Jean M. Carlson We investigate localization in a model for disordered materials such as granular particles or plastically deforming asperities that are sheared between sliding interfaces. We model the dynamics of configurations of particles that are more susceptible to deformation under shear stress using Shear Transformation Zone (STZ) theory. STZ theory has recently been modified to include effective disorder temperature dynamics, which provides a mechanism for strain softening and shear localization because as the material is strained it becomes more disordered and a more disordered material is more susceptible to deformation. The STZ effective temperature model is investigated in a simple shear geometry by numerically integrating the equations of motion and analyzing their generalized stability. We find that a material with a spatially homogeneous initial effective temperature distribution (corresponding to a slowly quenched sample) supports homogeneous flow, but one with a finite spatial perturbation in the disorder (corresponding to a rapidly quenched sample) develops a shear band, which leads to a dramatic reduction in the yield stress and a modified constitutive law for the interface. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y31.00014: A comparison between defects created by light soaking and tritium decay in amorphous silicon Tong Ju, Stefan Zukotynski, Nazir Kherani, P. Craig Taylor, Paul Stradins We compare two ways to create defects in a-Si:H, namely decay of bonded tritium and irradiation with visible light. Tritium decays to He$^{3}$, emitting a beta particle and an antineutrino. In tritium doped a-Si:H samples each beta decay of tritium bonded to silicon will create a defect by converting a bonded tritium to an interstitial helium, leaving behind a silicon dangling bond. We track these defects using electron spin resonance (ESR). We have kept the sample in liquid nitrogen for two years. After two years the defect density increases without saturation to a value of 7x10$^{19}$/cm$^{3}$. In the second experiment, we have kept the sample in liquid nitrogen irradiated with white light of intensity about 100mW/cm$^{2}$. After about 6 months, the spin density increased to about 9x10$^{17}$/cm$^{3}$ with no evidence of saturation. In the tritiated sample the increase in the defect density is proportional to the time, t, while in the light-soaked sample the increase is approximately proportional to t$^{1/3}$. This difference in behavior will be discussed. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y31.00015: Effective Mass Density of Fluid-Solid Composites Jun Mei, Zhengyou Liu, Weijia Wen, Ping Sheng We show through rigorous derivation and experimental support that the dynamic effective mass density of an inhomogeneous mixture, used in the prediction of wave velocities in the long wavelength limit, can differ from the static version---the volume average of the component mass densities. The physical reason for this difference is explained. The dynamic mass density expression, first derived by Berryman more than two decades ago, is shown to give a closer correspondence between the acoustic and electromagnetic metamaterials by allowing for negative mass densities at frequencies around resonances. [Preview Abstract] |
Session Y32: Mott Phases and Novel Quantum Systems
Sponsoring Units: DAMOPChair: Dan Haxton, University of Colorado / JILA
Room: Colorado Convention Center 402
Friday, March 9, 2007 11:15AM - 11:27AM |
Y32.00001: Mott phases and superfluid-insulator transition of spin-3 bosonic atoms in an optical lattice Jean-Sebastien Bernier, Krishnendu Sengupta, Yong Baek Kim We present a theoretical study of the Mott phases and superfluid-insulator transition of spin-3 bosonic atoms with dipolar interactions in an optical lattice. We present the various broken symmetry states obtained and discuss the application of our results to Chromium atoms in optical traps. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y32.00002: Criterion for bosonic superfluidity in an optical lattice Roberto Diener, Qi Zhou, Hui Zhai, Tin-Lun (Jason) Ho We show that the current method of determining superfluidity in optical lattices based on a visibly ``sharp" bosonic momentum distribution $n({\bf k})$ can be misleading, for even a {\em normal} Bose gas can have a similarly ``sharp'' $n({\bf k})$. We show that superfluidity can be detected from the so-called ``visibility'' $(v)$ of $n({\bf k})$ $-$ that $v$ must be 1 within $O(N^{-2/3})$, where $N$ is the number of bosons. Many current experiments, however, have interpreted states with $v<1$ as superfluid. Such states are in fact normal, reflecting strong temperature effects in the system. These normal states, however, allow one to explore the physics in the quantum critical regime. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y32.00003: Theory of Clock-Shift Density-Profile Measurements and Applications Kaden Hazzard, Erich Mueller ``Clock shifts'' -- shifts in spectroscopic line energies proportional to the local density -- provide a spectroscopic way of measuring spatial density profiles. By measuring the absorption of a probe laser at various frequencies, one can obtain a histogram of the number of particles with the associated local densities. Campbell \textit{et al.} (\textit{Science} \textbf{313}, 649 (2006)) have recently imaged the ``wedding-cake structure'' of the Mott insulator phase in trapped ultracold bosons in optical lattices, using this technique. We develop a theory of clock-shifts for these systems, making comparisons with experiment. We find qualitative agreement with experiment using the simplest strong-coupling mean-field theory with a local density approximation, and a harmonic, isotropic trap model. We show which embellishments to these approximations are best at accounting for the experimental data. We propose applications of this technique to cooling, thermometry, and measurement of correlations, giving an elementary theory of each. Finally, we highlight a \textit{superfluid} shell structure that can be particularly apparent in these experiments. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y32.00004: Synchronization versus dephasing in the pairing dynamics of cold fermions Roman Barankov, Leonid Levitov Motivated by recent experiments on degenerate Fermi gases with time-dependent interaction~[1,2], we consider the time dynamics of BCS-paired fermions with switchable interaction. Several new regimes [3] of dissipationless coupled dynamics of the collective BCS mode and individual fermion pair states are identified and explored. The system can exhibit synchronized evolution in which all pair states are fully phased-locked, trasnforming to a Landau-damped dephased behavior upon variation of coupling strength. At the synchronization-dephasing transition the amplitude of long-time persistent oscillations vanishes. A second transition is found in the dephased regime, at which the long-time asymptotic constant value of pairing amplitude vanishes. Using a combination of numerical and analytical methods we establish a continuous (type II) character of both transitions. We also propose an experiment which could probe these new dynamical states. \newline [1] C.~A.~Regal, M.~Greiner, and D.~S.~Jin, Phys. Rev. Lett. {\bf 92}, 040403 (2004). \newline [2] M.~W.~Zwierlein, C.~A.~Stan, C.~H.~Schunck, S.~M.~F.~Raupach, A.~J.~Kerman, and W.~Ketterle, Phys. Rev. Lett. {\bf 92}, 120403 (2004). \newline [3] R.~A.~Barankov and L.~S.~Levitov, Phys. Rev. Lett. {\bf 96}, 230403 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y32.00005: Quantum phase transitions of polar molecules in bilayer systems Daw-Wei Wang We investigate the quantum phase transitions of bosonic polar molecules in a two-dimensional double layer system. We show that an interlayer bound state of dipoles (dimers) can be formed when the dipole strength is above a critical value, leading to a zero energy resonance in the interlayer $s$-wave scattering channel. In the positive detuning side of the resonance, the strong {\it repulsive} interlayer pseudo-potential can drive the system into a maximally entangled state, where the wavefunction is a superposition of two states that have all molecules in one of the two layers and none in the other. We critically discuss how the zero-energy resonance, dimer states and the maximally entangled state can be measured in time-of-flight experiments. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y32.00006: Quantum phase diagram of Polar Molecules in 1D Double Wire Systems Chi-Ming Chang, Daw-Wei Wang We study the quantum phase transitions of fermionic polar molecules loaded in a double wire potential. By tuning the magnitude and direction of external electric field we observed many interesting quantum phases in different parameter range, including an easy-plane spin density wave, a triplet superconducting phase, and a truly long range order of easy-axis ferromagnetic phase in strong interacting regime. We also discuss how these exotic quantum phases can be measured in the existing experimental techniques. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y32.00007: Hidden order in one dimensional Bose insulators Emanuele Dalla Torre, Erez Berg, Ehud Altman We investigate the phase diagram of spinless bosons with long range ($\propto 1/r^3$) repulsive interactions, relevant to ultracold polarized atoms or molecules, using DMRG. Between the two conventional insulating phases, the Mott and density wave phases, we find a new phase possessing hidden order revealed by non local string correlations analogous to those characterizing the Haldane gapped phase of integer spin chains. We develop a mean field theory that describes the low energy excitations in all three insulating phases. This is used to calculate the absorption spectrum due to oscillatory lattice modulation. We predict a sharp resonance in the spectrum due to a collective excitation of the new phase that would provide clear evidence for the existence of this phase. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y32.00008: Phase-locking transition of coupled low-dimensional superfluids Ludwig Mathey, Anatoli Polkovnikov, Antonio Castro Neto We study the phase-locking transition of two coupled low-dimensional superfluids, either two-dimensional superfluids at finite temperature, or one-dimensional superfluids at zero temperature. We find that these superfluids have a strong tendency to phase-lock. The phase-locking is accompanied by a sizeable increase of the transition temperature ($T_{KT}$ in 2D) of the resulting double-layer superfluid, which suggests a plausible way of observing the Kibble-Zurek mechanism in two-dimensional cold atom systems by rapidly changing the ratio $T/T_{KT}$ varying the tunneling rate between the superfluids. When the two superfluids interact with each other, which is the case for polar condensates or for radio frequency induced double well potentials, further phases can be realized. We also extend the discussion to more than two coupled superfluids. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y32.00009: Quantum quenches in a spinor condensate Austen Lamacraft We discuss the ordering of a spin-1 condensate when quenched from its paramagnetic phase to its ferromagnetic phase by reducing magnetic field. We first elucidate the nature of the equilibrium quantum phase transition, which has a multicritical point when the magnetization in the direction of the field vanishes. Quenching rapidly through this transition reveals XY ordering either at a specific wavevector, or the `light-cone' correlations familiar from relativistic theories, depending on the endpoint of the quench. For a quench proceeding at a finite rate the ordering scale is governed by the Kibble-Zurek mechanism. The creation of vortices through growth of the magnetization fluctuations is also discussed. The long time dynamics again depends on the endpoint, conserving the order parameter in zero field, but not at finite field, with differing exponents for the coarsening of magnetic order. The results are discussed in the light of a recent experiment by Sadler \emph{et al.} [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y32.00010: Absorption of microwave radiation by an array of vortices in a p-wave superfluid of fermionic cold atoms Eytan Grosfeld, Nigel Cooper, Ady Stern, Roni Ilan We propose an experiment to identify the weak-pairing (``non-abelian'') phase of a two-dimensional $p$-wave superfluid of cold atoms by microwave absorption. We consider transitions between two atomic hyperfine states, where atoms in the ground state form a rotated $p$-wave superfluid, and atoms in the excited state are subject to a rotation and a periodic potential. We focus our calculations on absorption originating from zero energy Majorana states present in cores of vortices of the weakly- paired superfluid, and identify van-Hove type singularities in the absorption spectrum. The absorption peaks are unique to the weak-pairing phase, and their appearance in the spectrum may serve as a demonstration of the phase transition into the weak pairing phase. We discuss how these results can be extended to three dimensional superfluids, and explore extensions of non-abelian statistics to multiple two-dimensional layers. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y32.00011: Time-dependent electromagnetic wave dynamics in ultracold, high-density Rb vapor M. D. Havey, S. Balik, C. I. Sukenik, D. V. Kupriyanov, I. M. Sokolov Recent experiments and theoretical results on light localization in condensed samples show that diffusive transport is strongly suppressed and that a regime of anomalous diffusion develops dynamically. Proximity of the light localization threshold can be detected through time evolution of either forward or diffusely scattered light. We report in this paper experimental and theoretical results on time-dependent light scattering in the spectral vicinity of the F = 2 - F$'$ = 3, and the F = 1 - F$'$ = 0 optical transitions in dense, ultracold atomic $^{87}Rb$ samples formed in an optical dipole trap. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y32.00012: Correlated Wavefunction for the Li Atom Frank Harris Accurate wavefunctions are extremely valuable as tools for gaining understanding of quantum systems. Here we use a wavefunction explicitly containing all the interparticle distances to obtain a highly precise description of the ground state of the Li atom. In contrast to the widely used Hylleraas approach (in which the interelectron distances enter the wavefunction only as integer powers), our wavefunction exhibits an exponential dependence on all the interparticle distances, with a spatial dependence (before imposing spin and symmetry restrictions) of the form $\sum_{n=1}^N c_n \exp(-w_1r_1-w_2r_2-w_3r_3-u_1r_{23} -u_2r_{13}-u_3r_{12})$, where $r_i$ are electron-nuclear distances, $r_{ij}$ are electron-electron distances, and $w_i$ and $u_i$ are parameters. When the nonlinear parameters are carefully optimized (a nontrivial task), this type of basis causes a far more rapid convergence (with $N$) than the Hylleraas basis. We will survey the results we have obtained and compare with other studies of the Li atom. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y32.00013: Diabolic Topology , Berry Phase and Optical Phase Shifts of Light in Mobius-Type Strips Radha Balakrishnan, Indubala Satija We compute the optical phase shifts between the left and the right-circularly polarized light after it traverses non-planar cyclic loops describing the boundary curves of twisted strips. These optical results expressed in quantum-mechanical language in fact illustrate the phenomenon of Berry phase. Equation for the evolution of the fields described by Fermi-Walker formulation is mapped to Schroedinger equation with Hamiltonian whose eigenvalues are equal to the $\pm kappa$ where $\kappa$ is the curvature of the path. The inflexion points underlying the twisted strips manifest as the diabolic crossings of the quantum Hamiltonian. For the Mobius loops, the critical width where the diabolic geometry resides also correspond to the characteristic width where the optical phase shift is minimal. [Preview Abstract] |
Session Y33: Decoherence and Quantum Control
Sponsoring Units: GQIChair: Ivan Deutsch, University of New Mexico
Room: Colorado Convention Center 403
Friday, March 9, 2007 11:15AM - 11:27AM |
Y33.00001: Single spin decoherence by general spin chains Cheng-Yan Lai, Pochung Chen Spin decoherence induced by a spin bath has recently been the subject of interest in the field of quantum computation and spintronics. Unlike the spin-boson model, the resulting decoherence depends crucially on the nature of the spin bath and its coupling to the central spin. In this work we investigate the decoherence of a central spin which is coupled non-uniformly to a spin chain by means of the time-dependent density matrix renormalization group technique. Using this technique the coupling between the central spin and the spin chain can take any form, in contrast to the typical uniform or on-site coupling taken in the literature. We have studied the resulting spin decoherence induced by spin chains in the Ising, XY, XXZ, and Heisenberg universality classes. Connection between the decoherence the quantum phase transition of the spin chain is discussed. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y33.00002: Dynamical decoupling induced renormalization of the non-Markovian dynamics Pochung Chen In this work we develop a numerical framework to investigate the renormalization of the non-Markovian dynamics of an open quantum system to which dynamical decoupling is applied. We utilize a non-Markovian master equation which is derived from the non-Markovian quantum trajectories formalism. It contains incoherent Markovian dynamics and coherent Schr\"odinger dynamics as its limiting cases and is capable of capture the transition between them. We have performed comprehensive simulations for the cases in which the system is either driven by the Ornstein-Uhlenbeck noise or or is described by the spin-boson model. The renormalized dynamics under bang-bang control and continuous dynamical decoupling are simulated. Our results indicate that the renormalization of the non- Markovian dynamics depends crucially on the spectral density of the environment and the envelop of the decoupling pulses. The framework developed in this work hence provides an unified approach to investigate the efficiency of realistic decoupling pulses. This work also opens a way to further optimize the decoupling via pulse shaping. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y33.00003: Coherence oscillations produced by non-Gaussian quantum noise Florian Marquardt, Izhar Neder, Benjamin Abel, Mordehai Heiblum The usual models for dissipative environments involve a bath of harmonic oscillators, producing Gaussian fluctuations. However, modern experiments on dephasing in qubits and electronic interferometers indicate strong coupling to non-Gaussian quantum noise. Most strikingly, the coherence (interference contrast) may oscillate as a function of time and other control parameters. We present the theory behind a recent ``controlled dephasing'' experiment involving an electronic Mach-Zehnder interferometer strongly coupled to the non-Gaussian shot noise of a detector edge channel [cond-mat/0610634,cond-mat/ 0611372], as well as applications to qubits dephased by shot noise or two-level fluctuators. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y33.00004: Optimal Control of Large Spin Systems Seth Merkel, Souma Chaudhury, Andrew Silberfarb, Tobias Herr, Ivan Deutsch, Poul Jessen A quantum system is said to be controllable if the accessible Hamiltonians (as a Lie algebra) generate all unitary operators on Hilbert space. Optimal quantum state control seeks a time-dependent sequence of Hamiltonians that maximize the fidelity with an arbitrary target state given a fixed initial state. We consider optimal control of the spin of a cesium atom restricted to its F=3 ground state hyperfine manifold, with a Hilbert space of dimension 2F+1=7. Control is implemented through time varying magnetic fields in two orthogonal directions along with a quadratic AC-Stark shift created by an off-resonant laser probe. The optimization is performed under several constraints, most importantly a temporal limitation determined by dephasing due to photon scattering and parameter inhomogeneity. The fidelity of state preparation is verified through both a full density matrix simulation and reconstruction from experimental data. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y33.00005: The effect of qubit-qubit exchange interaction on qubit relaxation rates Diu Nghiem, Robert Joynt In this report we extend an exactly soluble model of one-qubit decoherence to two qubits with exchange interactions. Closed-form expressions for the transfer matrix can be obtained, but contrary to the single-qubit case, the matrix must be diagonalized numerically. We compute the single- and two-qubit relaxation rates. In the first approximation, the two-qubit rates can be obtained by additivity of single-qubit rates. Interactions and the resulting entanglement modified this, but their effect is surprisingly small. These results suggest that N-qubit decoherence rates scale linearly with N. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y33.00006: Resilient Quantum Computation in Correlated Environments: A Quantum Phase Transition Perspective Eduardo Novais, Eduardo R. Mucciolo, Harold U. Baranger The `threshold theorem' is a central result in the theory of quantum error correction. It was derived initially for a stochastic error model, but relentless effort has been dedicated to including correlated errors. Here, we demonstrate that a large class of correlated error models is reduced to the simple stochastic model in the asymptotic limit of large number of qubits or long time. Thus, in order to prove the resilience of the quantum information for these models, we can fall back on the traditional derivation of the threshold theorem. Because the conditions for this fall back have clear parallels with the theory of quantum phase transitions, we rephrase the threshold theorem as a dimensional criterion: (1) For systems above their ``critical dimension'', the traditional proof of resilience is valid, and there are two regimes, or phases, as a function of the coupling with the environment. (2) However, when the system is below its ``critical dimension'', correlations produce large corrections, and it is not possible to prove resilience by our arguments. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y33.00007: Linked cluster expansion of a qubit decoherence Semion Saikin, Wang Yao, Lu Sham We present a theoretical approach to study evolution of a qubit affected by a coupling with a spin bath. The procedure based on a linked cluster decomposition of system and bath dynamics. Unlike previous studies the approach allows exact evaluation of terms of each perturbative order in the exponent contributing to qubit decoherence and phase fluctuations. The procedure has a simple diagrammatic representation. We have utilized the theory to evaluate decoherence of a localized electron spin subject to an interaction with a nuclear spin bath. The novel results we report on are effects of nuclear spin clusters on electrons spin decoherence beyond pair correlation models and also control for dissipation processes in a spin environment. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y33.00008: Collective decoherence of nuclear spin clusters Leonid Fedichkin, Arkady Fedorov The problem of dipole–dipole decoherence of nuclear spins is considered for strongly entangled spin clusters. We consider the pure dephasing part of the dipole–dipole interaction which can be classically interpreted as a random field fluctuating along the quantization axes. Due to the long (but finite) range nature of dipole–dipole interaction this field is expected to be partially correlated at the sites of different spins in the cluster. Consequently our results show that the dynamics of the entangled spin cluster can be described as the decoherence due to interaction with a composite bath consisting of fully correlated and uncorrelated parts. The correlated term causes the slower decay of coherence at larger times. The decoherence rate scales up as a square root of the number of spins, giving the linear scaling of the resulting error. Our theory is consistent with recent experiments reported on decoherence of correlated spin clusters. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y33.00009: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y33.00010: Experimental demonstration of stimulated polarization wave in a chain of nuclear spins Jae-Seung Lee, Travis Adams, Anatoly Khitrin A one-dimensional Ising chain irradiated by weak resonant transverse field is the simplest model of quantum amplifier [Phys.\ Rev.\ A 71, 062338 (2005)]. The quantum state of the chain is stationary when all the qubits (spins) are in the same state. However, when the first qubit is flipped, it triggers a stimulated wave of flipped qubits, propagating through the chain. Such ``quantum domino" dynamics induces huge change in the total polarization, a macroscopic observable. Here we present the experimental demonstration of this quantum amplification process on a four-qubit system by using nuclear magnetic resonance technique. The physical system is a linear chain of four $^{13}$C nuclear spins in a molecule of fully $^ {13}$C-labeled sodium butyrate dissolved in D$_2$O. The pseudopure ground state (with all spins up) is prepared by multi-frequency partial saturation. The wave of flipped spins has been clearly observed when the first spin of the chain is flipped. We define a coefficient of amplification as the relative enhancement of the total polarization change. In our experimental system, the measured coefficient of amplification is about 3. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y33.00011: Initial quantum corrections, quantum process tomography, and physicality of not completely positive maps Kavan Modi, Cesar Rodriguez, Aik-Meng Kuah, Anil Shaji, E. C. George Sudarshan We make the connection between initial quantum correlations and not completely positive maps. Though this has been suggested in literature for some time now, our arguments are supported by explicit calculations. In the process we will discuss our work in relation with quantum process tomography. We are especially interested in recent experiments that yielded so called non-physical results. We will offer new interpretations of these experiments and show that the results do make sense in the end. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y33.00012: Zero Discord leads to Completely Positive Maps Cesar Rodriguez, Kavan Modi, Aik-meng Kuah, Anil Shaji, E.C.George Sudarshan The stochastic evolution of a quantum system can be expressed by a dynamical map that acts as a superoperator on a density matrix. If all eigenvalues of this map are positive, the map is said to be completely positive. If the dynamical map comes from the reduced unitary evolution of a bipartite system, the map depends on the correlations, and can have negative eigenvalues. Quantum discord is a measure of the quantumness of a correlation. A state with zero discord has the properties that the only correlations that it has are equivalent to the classical conditional probability. We prove that states with zero quantum discord always lead to completely positive maps. The connection with the proper preparation of states for experiments is made. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y33.00013: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y33.00014: Refocusing of a Qubit System Coupled to an Oscillator. Gregory D. Quiroz, Leonid P. Pryadko Within an NMR-like approach to coherent control, we analyze the performance of ``soft'' refocusing pulses and pulse sequences in protecting the coherence of a qubit system coupled to a quantum oscillator. We focus on the effects of the oscillator excitation and heating and associated deterioration of qubits' fidelity. These effects cannot be addressed in the conventional master equation formalism with the bath assumed in thermal equilibrium. Analytically, we construct the effective Hamiltonian of the controlled qubit plus oscillator system to quadratic order of the Magnus expansion in powers of the couplings. The qubit error operators and the terms responsible for the oscillator excitation are thus identified explicitly. These terms dominate the oscillator evolution when it is close to resonance with the qubit(s). The corresponding single- and few-qubit simulations show continuously increasing average oscillator energy accompanied by deteriorating qubit fidelity. The magnitude of the oscillator frequency bias needed to arrest this run-away effect is smaller for second-order refocusing sequences, where the order of the sequence is the number of suppressed terms in the effective Hamiltonian of the qubit system, with the oscillator operators replaced by $c$-numbers. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y33.00015: Real-time dynamics of dissipative quantum systems Michael Zwolak, Gil Refael Dissipation in quantum systems is the source of a number of interesting and important phenomena. In quantum computing, for instance, environmental memory can have a significant effect on the operation of solid state devices and on error correction. In condensed matter, strong dissipation can cause phase transitions, as in the ubiquitous spin-boson model. In an effort to create a generic computational method for studying real-time non-Markovian and strongly dissipative dynamics, we have examined the construction of master equations containing memory. We have found this approach lacking because most of the physics beyond weak coupling is contained within the memory kernel of the master equation. Therefore, the majority of the effort in solving for the dynamics goes into the calculation of a system specific memory kernel. We discuss these issues as well as a potential solution based on the use of ancillary systems which represent part of the environment. [Preview Abstract] |
Session Y34: Properties of Biological Membranes
Sponsoring Units: DBPChair: Dobrin Bossev, Indiana University
Room: Colorado Convention Center 404
Friday, March 9, 2007 11:15AM - 11:27AM |
Y34.00001: Bending Elasticity of Bio-Membranes Studied by Neutron Spin-Echo Zheng Yi, Dobrin Bossev We have used neutron spin echo (NSE) spectroscopy to study the effects of the unsaturated double bond and the hydrocarbon chain length on the bending elasticity of lipid membranes. The bending elasticity \textit{$\kappa $} of bilayer vesicles made of 1,2-Dioleoyl-\textit{sn}-Glycero-3-phosphocholine(18:1 PC), has been measured in the fluid (L$_{\alpha })$ phase in different temperatures. When lipid bilayers made of DOPC are in fluid phase, the temperature effect on bending elasticity is minimal. The bending elasticities of 14:1 PC and 16:1 PC were measured in fluid phase in 30\r{ }C. We found that the lipid bilayers with longer chains have higher bending elasticities. Our data confirms that the stiffening of lipid bilayers increases with increasing chain length of the lipid molecules. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y34.00002: Using Neutron Spectroscopy to Study Collective Dynamics of Biological and Model Membrane Systems Maikel Rheinstadter Only recently, it has become possible to study collective dynamics of planar lipid bilayers using neutron spectroscopy techniques. By combining different neutron scattering techniques, namely three-axis, backscattering and spin-echo spectroscopy, we present measurements of short and long wavelength collective fluctuations in biomimetic and biological membranes in a large range in momentum and energy transfer, covering time scales from about 0.1ps to almost 1$\mu $s and length scales from 3{\AA} to about 0.1$\mu $m [1-4]. The measurements offer a large window of length and time scales to test and refine theoretical models of dynamics of biomimetic and biological membranes. The objective of this project is to establish dynamics-function relationships in artificial and biological membranes to relate in particular the collective dynamics, i.e., phonons, to key functions of the membranes, as, e.g., transport processes within and across the bilayers. M.C. Rheinst\"{a}dter, C. Ollinger, G. Fragneto, F. Demmel, T. Salditt, \textit{Phys. Rev. Lett.} \textbf{93}, 108107 (2004).$^{2}$ Maikel C. Rheinst\"{a}dter, Tilo Seydel, Franz Demmel, Tim Salditt, \textit{Phys. Rev. E} \textbf{71}, 061908 (2005).$^{3}$ Maikel C. Rheinst\"{a}dter, Wolfgang H\"{a}u{\ss}ler, Tim Salditt, \textit{Phys. Rev. Lett.} \textbf{97}, 048103 (2006).$^{4}$ Maikel C. Rheinst\"{a}dter, Tilo Seydel, Tim Salditt, submitted to PRE, cond-mat/0607514. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y34.00003: Damping of the thermal undulations of bio-membranes Dobrin Bossev, Zheng Yi In this work we discuss the damping mechanisms of the thermal undulation of lipid membranes. In the past, we have attempted to determine the bending elasticity of bio membranes by neutron spin-echo spectroscopy (NSE) as a function of the temperature, molecular structure of the phospholipids, ionic strength of the surrounding aqueous environment, and presence of cholesterol. NSE is ideal for studies of the thermal undulations of the biomembranes because it probes the short correlation times (0.01--100 ns) and length scales (10--100 {\AA}) that are characteristic for the biomembrane undulations. The bending modulus of elasticity is obtained through analysis of the intermediate scattering function I(Q,t) using Zilman-Granek theory, which considers the solution viscosity as the only damping mechanism for the thermal undulations. As a result the absolute k values are about an order of magnitude greater than those measured by other methods and predicted by simulations. Here we report measurements in water/glycerol mixtures in attempt to modify the bulk viscosity and to clarify the contribution of the different energy dissipation mechanisms. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y34.00004: Interaction Forces and Mechanics of Cellular Membranes using Novel Atomic Force Microscopy Probes Benjamin Almquist, Nicholas Melosh In order to probe the nature of nanostructure-membrane interfaces, we have developed an AFM probe platform that can quantitatively measure the interaction forces between specifically functionalized layers and the cell membrane. This platform consists of a cantilever with a post-style tip that ends in a hetero-metallic layer. This metallic layer can be selectively functionalized with various molecules of interest. Once functionalized, the layer is inserted into the hydrophobic region of the cell membrane. By varying the molecular species and examining the associated penetration and extraction forces, we will be able to correlate the molecule-membrane interaction forces to the molecular structure. This, in turn, will allow us to determine the role of molecular size, hydrophobicity, and disorder. In addition, the effects of functional layer thickness and post geometry will be examined. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y34.00005: Measuring Surface Potential of Zwitterionic Lipid Bilayers with Atomic Force Microscope Yi Yang, Kathryn Mayer, Jason Hafner Electrostatic potential was measured near supported zwitterionic lipid bilayer membrane surfaces with atomic force microscope. In our recent work, two methods were developed to measure the surface charge density of the membrane surface, Fluid electric force microscopy (FEFM) which creates a two- dimensional map of a surface charge density with a corresponding topographic map simultaneously and quantitative measurement method which based on tip-sample force curve analysis.Both FEFM and tip-sample force curve analysis showed that the surface of a DOPC (dioleoylphosphatidylcholine) lipid bilayer carries a negative electrostatic potential.This is an interesting and surprising result, for the head group of DOPC is carrying zero net charge over a broad range of pH where both the choline and phosphate groups are ionized.Two sources are proposed to explain the origin of this negative charge.The bilayers could carry a net charge density due to the counterions from the electrolyte binding to the lipid head groups.Alternatively, the dipole density in the DOPC lipid head group layer could cause an effective surface potential outside the membrane region.To study the source of this negative potential, Charge densities of supported DOPC bilayers under different ion concentrations were measured and compared with both of these two charge mechanisms. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y34.00006: Molecular Organization and Dynamics of Cholesterol Nanodomains in Fluid Lipid Bilayers Kwan Cheng, Brian Cannon, Qing Zhu, Mark Vaughn, Juyang Huang The molecular organization and dynamics of cholesterol nanodomains in lipid bilayers containing phospholipid (PL) and cholesterol (CHOL) were examined using FTIR, time-resolved fluorescence and surface-acting cholesterol oxidase enzyme (COD). In binary PL/CHOL system, abrupt changes in the PL C=O frequency, fluorescence lifetime and rotation rate of chain labeled PL, and the rate of cholesterol oxidation by COD were observed at $\sim $ 40 mole{\%} of CHO. For ternary PL$_{1}$/PL$_{2}$/CHOL system composed of two dissimilar PL's of different chain lengths or headgroup sizes, abrupt changes at PL$_{1}$/PL$_{2} \quad \sim $ 2 were found. The above critical lipid compositions agree favorably with the theoretical compositions predicted by the lipid superlattice model, suggesting that PL of different structures and CHOL can form regularly distributed, or superlattice-like, nanodomains at the polar headgroup and the acyl chain levels, respectively. The feasibility of the coexistence of headgroup and acyl chain nanodomains was demonstrated by a spacing filling model and MD simulations. We speculate that lipid superlattice domains may play an important role in the regulation of protein/lipid interaction in cells. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y34.00007: Temperature and Composition Dependent Phase Behavior in Two ``Raft-Like'' Ternary Membrane Mixtures: DPPC/DLPC/Cholesterol and DPPC/DOPC/Cholesterol Jeffrey Buboltz, Geoffrey Siegel, Matthew Schutzer, Krystle Williams, Charles Bwalya, Santiago Reyes For the last several years, so-called ``lipid-raft'' membrane domains have been the subject of intense research activity. As part of this effort, we have been carrying out experiments based on Probe-Partitioning FRET, a technique specifically designed to map out both phase boundaries and tie lines in artificial membrane mixtures. Specifically, we have studied two cholesterol-rich ternary mixtures, DPPC/DLPC/Cholesterol and DPPC/DOPC/Cholesterol, that mimic lipid-raft phase behavior. By studying more than 3000 independently prepared samples, we have gained insight into the general features (i.e., both temperature and composition dependence) that characterize the phase behavior in these two ternary systems. As we work toward extending our studies to other raft-like ternary mixtures, we are also adapting a different, purely thermodynamic technique (Equilibrium Surface Pressure Analysis) for the purpose of corroborating tie line patterns inferred from PP-FRET. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y34.00008: On the interactions between neutral lipid bilayers Oscar Calvo, Marian Manciu, Eli Ruckenstein The stability of many colloids is thought as a balance between attractive van der Waals interactions and double layer repulsive forces. However, the latter does not exist for neutral lipid bilayers, for which the repulsive forces are supposed to be provided by a combination between hydration forces and Helfrich forces, due to the suppression of the thermal undulation, when two bilayers approach each other. Hydration forces are related to the structuring of water near surfaces, which is likely to be decreased by the thermal undulations. Helfrich forces have a longer range than the attractive forces and cannot lead by themselves to a stable minimum. We will show that the polarization model for the hydration forces combined with a statistical treatment for undulating bilayers might explain the interactions between neutral lipid bilayers. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y34.00009: Effect of dipolar moments in domain sizes of lipid bilayers and monolayers Alex Travesset Lipid domains are found in systems such as multi-component bilayer membranes and single component monolayers at the air water interface. It was shown by McConnell and collaborators that in monolayers the size of the domains results from balancing the line tension, which favors the formation of a large singular single circular domain, against the electrostatic cost of assembling the dipolar moments of the lipids. In this talk, I will generalize this argument to include effects of ionic strenght, dielectric discontinuities (or image charges) and the polarizability of the dipoles and extend the results to bilayer membranes. I will finish with a discussion on the experimental implications of the calculations. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y34.00010: Non-equilibrium dynamics of heterogeneous lipid membranes Mikko Haataja, Jun Fan, Maria Sammalkorpi Plasma membranes surrounding mammalian cells play a key role in regulating the exchange of information and matter between the cells and their surroundings. The unique properties of these membranes arise from the interactions between amphiphilic lipid molecules, sterols (incl. cholesterol), and proteins. It has been proposed that the plasma membrane displays dynamic heterogeneities (lipid rafts) in the local lipid composition. While such rafts have not yet been observed directly in vivo, there is ample indirect evidence that supports their existence. From a fundamental biophysical perspective, processes which may control the aggregation and stability of these rafts are poorly understood at the moment. Here, we address this issue by introducing a continuum model for the local lipid composition which incorporates non-equilibrium aspects of lipid recycling to and from the membrane. We show that recycling leads to coherent structures with a characteristic size which depends on both the recycling rate and the tendency of the components to phase separate in the absence of recycling. We argue that incorporating non-equilibrium effects is crucial in understanding the biophysical properties of the plasma membrane. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y34.00011: Mechanisms of protein transduction domains: HIV TAT and ANTP penetratin as prototypical cases Abhijit Mishra, Nathan Schmidt, Vernita Gordon, Gerard Wong Biologically active molecules such as proteins and oligonucleotides can be transduced across cell membranes with high efficiency when covalently linked to a Protein Transduction Domain (PTD), such as the TAT domain in the HIV virus and ANTP from the fruitfly. All PTDs have a high content of basic amino acids resulting in a net positive charge. Electrostatic interactions between cationic PTDs and the negatively charged phospholipids that constitute the plasma membrane are likely to be responsible for peptide uptake, but no detailed structural studies exist. We examined membrane structures induced by the cationic TAT domain and those induced by other cationic polypeptides as a function of membrane composition using synchrotron x-ray scattering. We find that both the TAT PTD and ANTP generate negative Gaussian curvature, which is necessary for pore formation, and produce a bicontinuous Pn3m double diamond cubic phase. A general mechanism is proposed. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y34.00012: Molecular Insights into Phospholipid -- NSAID Interactions Mohan Babu Boggara, Ramanan Krishnamoorti Non steroidal anti inflammatory drugs (NSAIDs) e.g. Aspirin and Ibuprofen, with chronic usage cause gastro intestinal (GI) toxicity. It has been shown experimentally that NSAIDs pre-associated with phospholipids reduce the GI toxicity and also increase the therapeutic activity of these drugs compared to the unmodified ones. Using all atomistic simulations and two different methodologies, we studied the partitioning behavior of~ two model NSAIDs (Aspirin and Ibuprofen) as a function of pH and drug loading. The results from two methodologies are consistent in describing the equilibrium drug distribution in the bilayers. Additionally, the heterogeneity in density and polarity of the bilayer in the normal direction along with the fact that NSAIDs are amphiphilic (all of them have a carboxylic acid group and a non-polar part consisting of aromatic moieties), indicate that the diffusion mechanism in the bilayer is far different compared to the same in a bulk medium. This study summarizes the various effects of NSAIDs and their behavior inside the lipid bilayer both as a function of pH and drug concentration. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y34.00013: Non-equilibrium Lipid Distributions in a Simulated Three-Species Biomembrane Andrew P. Paradis, Susan R. McKay, Samuel T. Hess Cellular biomembranes are in continual states of flux, yet theoretical models of biomembranes have primarily focused on equilibrium behavior, where constituent species interact but are not driven. This study examines the complex phase behavior of a three-species biomembrane driven out of equilibrium through frequent, simulated endo- and exo-cytosis events. The three species, representing unsaturated lipids, saturated lipids, and cholesterol, move and interact on a two-dimensional triangular lattice, simulated using a Metropolis algorithm. Two types of phase behavior are specifically investigated and discussed: cholesterol super-lattice structures and phase separation of saturated and unsaturated lipids, both as functions of cholesterol mole fraction and temperature. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y34.00014: Simulations of the Pore Structures for a M2GlyR Derived Channel Forming Peptide in Different Membrane Environments A. Al-Rawi, A. Herrera, J. Tomich, T. Rahman As part of an effort to develop a peptide-based compound suitable for clinical use as a channel replacement therapeutic for treating channelopathies such as cystic fibrosis, we present a reductionist model that appears to grasp the characteristics of ion channeling peptides. In particular we present the observed changes in the functional characteristics of NK$_{4}$-M2GlyR p22 (KKKKPARVGLGITTVLTMTTQS), a M2 GlyR derived channel forming peptide. Starting with a structure determined by multidimensional NMR (800 MHz) in SDS, a potential from CHARMM force-field was used to relax the structure of NK$_{4}$-M2GlyR p22. Following the relaxation, numerous pore structures were generated for the symmetric five-helix assembly with geometries varying from cylindrical to conical. As it is difficult \textit{a priori} to assign accurately the orientation of the hydrophilic portion of M2GlyR derived amphipath towards the inside of the pore, we tilted and rotated the helical structure by five different angles about the backbone axis before forming the pore. Energy minimization of the channel was performed in vacuum, in phosphotidylcholine (POPC) membrane, and 60{\%} POPC 30{\%} phosphotidylethanolamine (POPE) in order to determine the effect of the environment surrounding on the structure on its energy minimization. We will present the various pore assemblies, in the different membrane environments, used to predict the most probably membrane bound structure. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y34.00015: The Effects of Polyunsaturated Lipid Components on bilayer Structure Y. Pramudya, A. Kiss, Lam T. Nguyen, J. Yuan, Linda S. Hirst Polyunsaturated fatty acids (PUFAs), such as DHA (Docosahexanoic Acid) and AA (Alphalinoleic Acid) have been the focus of much research attention in recent years, due to their apparent health benefits and effects on cell physiology. They are found in a variety of biological membranes and have been implicated with lipid raft formation and possible function, particularly in the retinal rod cells and the central nervous system. In this work lipid bilayer structure has been investigated in lipid mixtures, incorporating polyunsaturated fatty acid moieties. The structural effects of increasing concentrations of both symmetric and asymmetric PUFA materials on the bilayer structure are investigated via synchrotron x-ray diffraction on solution samples. We observe bilayer spacings to increase with the percentage of unsaturated fatty acid lipid in the membrane, whilst the degree of ordering significantly decreases. In fact above 20{\%} of fatty acid, well defined bilayers are no longer observed to form. Evidence of phase separation can be clearly seen from these x-ray results and in combination with AFM measurements. [Preview Abstract] |
Session Y35: DNA/RNA in vivo
Sponsoring Units: DBPChair: John Bechoefer, Simon Fraser University
Room: Colorado Convention Center 405
Friday, March 9, 2007 11:15AM - 11:27AM |
Y35.00001: Designability as a Selection Force? An Analysis of the Yeast Cell Cycle Dynamics. Yigal Nochomovitz, Surya Ganguli, Hao Li The concept of designability may play a role in the evolution of biological phenotypes. We define ``designability'' generally as the number of genotypes that encode a particular phenotype. For networks, the designability of a dynamical phenotype is the number of topologies that encode a particular ordering of dynamical states. By analyzing ensembles of simplified models of topologies and dynamics (Nochomovitz, Y.D. {\&} Li, H. PNAS 103, 2006.) we have begun to explore the validity of the designability hypothesis at an abstract level. We have discovered from these exploratory studies that certain dynamical signals are highly designable, indicating that some dynamical signals can be realized by many different topological connections. To test the designability hypothesis on a real biological system, we analyze the dynamics of the budding yeast cell cycle. We compute the designability of the yeast cell cycle phenotype and the designabilities of $\sim $ 1000 weakly perturbed variants of the yeast cell cycle phenotype. A comparison of the designability of the true yeast cell cycle phenotype with the designabilities of the pool of perturbed phenotypes reveals that the designability of the budding yeast cell cycle dynamics is near-optimal. This finding provides some evidence for the hypothesis that designability, as an ``entropic'' force, may couple with the traditional fitness landscape to influence the evolution of biological phenotypes. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y35.00002: Fine tuning by miRNAs in development Peter McHale, Erel Levine, Herbert Levine The unique role played by microRNA in a developing embryo is a topic of much current research interest. One possibility is that microRNA diffuse within a developing tissue, acting as communicators between different cells. Here we pursue this possibility in two different contexts. The first case occurs when the transcription profiles of the microRNA and its target are spatially anticorrelated, as for example is the case in the iab4-Ubx system in fly. Conversely, in the second context the two transcription profiles are correlated in space, as may be the case for the mir10-Hoxb4 system in mouse. In each context we identify a major function for a mobile miRNA. In the first, miRNA serve to induce an all-or-nothing response of the mRNA profile to its morphogen by generating a sharp boundary between domains of high and (ultimately) low target expression. In the second, miRNA amplify polarity in the target expression pattern by removing residual mRNAs. Importantly, our model predicts that these two functions require very different type of diffusion. While our results are highly quantitative, we propose ways of realizing them in experiments, taking into account limitations of standard experimental techniques. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y35.00003: Modeling the dynamics of the nucleosome at various levels. Alexey Onufriev, Andrew Fenley, Jory Zmuda-Ruscio, David Adams The primary level of DNA compaction in eukaryotic organisms is the nucleosome, yet details of its dynamics are not fully understood. While the whole nucleosome must be highly stable, protective of its genetic material, at the same time its tightly wrapped DNA should be highly accessible, easily revealing its information content. A combination of atom-level classical molecular dynamics and a course-grained continuum description provide insights into the functioning of the system. In particular, the nucleosomal DNA appears to be considerably more flexible than what can be expected based on its canonical persistence length. A coarse-grained electrostatic model of the nucleosome explains how its stability can be modulated with small environmental changes as well as post-translational modifications. Implications for the nucleosome assembly process in \textit{vivo} are discussed. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y35.00004: How frog embryos replicate their DNA reliably John Bechhoefer, Brandon Marshall Frog embryos contain three billion base pairs of DNA. In early embryos (cycles 2-12), DNA replication is extremely rapid, about 20 min., and the entire cell cycle lasts only 25 min., meaning that mitosis (cell division) takes place in about 5 min. In this stripped-down cell cycle, there are no efficient checkpoints to prevent the cell from dividing before its DNA has finished replication - a disastrous scenario. Even worse, the many origins of replication are laid down stochastically and are also initiated stochastically throughout the replication process. Despite the very tight time constraints and despite the randomness introduced by origin stochasticity, replication is extremely reliable, with cell division failing no more than once in 10,000 tries. We discuss a recent model of DNA replication that is drawn from condensed-matter theories of 1d nucleation and growth. Using our model, we discuss different strategies of replication: should one initiate all origins as early as possible, or is it better to hold back and initiate some later on? Using concepts from extreme-value statistics, we derive the distribution of replication times given a particular scenario for the initiation of origins. We show that the experimentally observed initiation strategy for frog embryos meets the reliability constraint and is close to the one that requires the fewest resources of a cell. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y35.00005: Modeling the Forced Extension of Nicked DNA Alexander Balaeff, Stephen Craig, David Beratan The design and study of DNA-based nanodevices has been a topic of considerable interest in the last decade. While the applications of classical continuous DNA structures have been thoroughly studied, nicked DNA structures, i.e., ones that contains breaks (``nicks'') in one or both DNA backbone chains, have received much less attention. Recently, Kersey et al. (JACS, 2004) reported the force spectroscopy of long DNA chains with periodic nicks, self-assembled from short DNA oligomers. We attempt to model the experimental force-extension profiles in a series of steered molecular dynamics simulations. The simulated all-atom model of a basic unit of the long self-assembled chain, a 16bp-long DNA segment with a nick in the middle of one strand, is extended by applying either a constant force or a moving harmonic potential to the DNA ends. The computed force-extension profiles are compared to those for a non-nicked DNA; the dynamics of structural changes in the nicked DNA during the forced extension is discussed. A theoretical framework is established to link the extension and rupture in the simulated basic unit to the corresponding events in the long self-assembled chain. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y35.00006: Single-Molecule Studies of the Temperature Dependence of Viral DNA Packaging Motors Michael White, Dorian Raymer, Peter Rickgauer, Derek Fuller, Shelley Grimes, Paul Jardine, Dwight Anderson, Doug Smith A key step in the assembly of many viruses is the packaging of dsDNA into a preformed capsid by the action of a portal molecular motor complex. ~We have developed methods for directly measuring viral DNA translocation at the single molecule level using optical tweezers and applied these methods to study bacteriophages $\Phi $29, lambda, and T4. Our previous measurements with $\Phi $29 were performed at room temperature. ~Here we report that the rate of DNA translocation is strongly temperature dependent. ~Preliminary measurements indicate that the motor velocity increases $\sim $2-fold, to $\sim $250-300 bp/s when the temperature is increased from $\sim $20 to 30 degrees C. ~As the viral packaging motors are enzymes that catalyze ATP hydrolysis, such a trend with increasing temperature is to be expected, at least up to the point where the motor complex is thermally dissociated or denatured. ~However, the detailed form of the temperature dependence is difficult to quantify using standard bulk assay methods. ~We have installed a heating/cooling system in our optical tweezers instrument that allows us to precisely control the temperature in our sample chamber. ~This system allows us to systematically study the temperature dependence of the DNA translocation rate. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y35.00007: Identifying Dyads and their conservation in Drosphila. Debasis Dan Core promoter regions in Drosophila are enriched with binding sites like TATA, Inr, DPE, MTE, etc. They have very strict spacing between each other in promoters where they occur together. For example, in Drosophila melanogaster TATA-Inr has a spacing of 25-30 bp. Our aim in this work is to identify all such pair of motifs having strict positional constraint in the core promoters of all Drosophila species. We discover how these motifs and the spacing between them evolve within Drosophila species. For this we analyze 700 bp upstream and 300 bp downstream of TSS in D. melanogaster and the corresponding orthologous region in other Drosophila species. For each species, this 1000 bp region is searched for statistically over-represented compound words of the form W1N{L}W2, where L is the spacing between words W1 and W2. These compound words are systematically clustered for further analysis. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y35.00008: Relating Promoter Sequences to the Proteins that Bind to Them: A Comparison Study. Kimberly Glass Chromatin Immunoprecipitation (ChIP-on-ChIP) microarray data reveals that the proteins H3K9dimethyl and RNA-Polymerase II are exclusive regarding their binding to the promoter region of genes. When comparing the base pair sequences of the promoters that bind to Pol2 versus H3K9, striking differences appear. The mononucleotides have fundamentally different behaviors in each group. In addition, motifs that cluster before the transcriptional start site also generally have a strong enrichment in one group compared to the other. Using this knowledge a model can be developed that allows one to calculate a probability that a promoter will bind to either H3K9 or Pol2 based on its base pair sequence. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y35.00009: Studying Codon Usage: From sequence to function Terry Hwa, Stefan Klumpp, Jiajia Dong Protein coding sequences exhibit strong variances in the use of codons. Highly expressed genes such as those encoding ribosomal proteins use codons corresponding to the highly abundant tRNAs (``optimized codons''). High expression of heterologous genes also requires codon optimization, but even the codon usage of very weakly expressed genes tends to be far from random. To understand this biased choice of codon usage, we develop a theory based on the concept of ``ribosomal load.'' Ribosome is the key limiting commodity for rapidly growing organisms so that the use of ``non-optimal'' codons in any gene prolongs the translational elongation time, thus reducing the effective ribosome concentration. This presents a fitness cost, the magnitude of which depends on the amount of that protein being translated. We formulated and solved an evolution equation based on the above ingredients. This provides a quantitative relation between codon usage and protein abundance, which is found to be in good agreement with the available data for E.coli. This result suggests a convenient way to quantitatively predict protein abundances based on genome sequence data. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y35.00010: A Model of Codon Usage Bias Morten Kloster, Chao Tang The genetic code is degenerate; most amino acids can be encoded by from two to as many as six different codons. While one might expect these codons to be used with equal frequency, this turns out not to be the case---not only are some codons favored over others, but their usage can vary significantly between different genes in the same organism. Known causes of codon bias include differences in mutation rates as well as selection pressure related to the expression level of a gene, but the standard analysis methods can explain only a fraction of the observed codon usage variation. We here introduce an explicit model of codon usage bias, inspired by statistical physics. Combining this model with a maximum likelihood approach, we are able to clearly identify up to four different sources of bias in various genomes. We have applied the algorithm to Saccharomyces cerevisiae as well as 325 bacterial genomes, and in most cases our model explains essentially all observed variance. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y35.00011: Single Molecule Study of Metalloregulatory Protein-DNA Interactions Susanta Sarkar, Jaime Benitez, Zhengxi Huang, Qi Wang, Peng Chen Control of metal concentrations is essential for living body. Metalloregulatory proteins respond to metal concentrations by regulating transcriptions of metal resistance genes via protein-DNA interactions. It is thus necessary to understand interactions of metalloregulatory proteins with DNA. Ensemble measurements provide average behavior of a vast number of biomolecules. In contrast, single molecule spectroscopy can track single molecules individually and elucidate dynamics of processes of short time scales and intermediate structures not revealed by ensemble measurements. Here we present single molecule study of interactions between PbrR691, a MerR-family metalloregulatory protein and DNA. We presume that the dynamics of protein/DNA conformational changes and interactions are important for the transcription regulation and kinetics of these dynamic processes can provide useful information about the mechanisms of these metalloregulatory proteins. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y35.00012: A plausible model for the digital response of p53 to DNA damage Gustavo Stolovitzky, Lan Ma, John Wagner, J. Jeremy Rice, Hu Wenwei, Arnold Levine The single-cell response of p53 to ionizing radiation (IR) is such that the number of oscillations of p53 shows dependence on the radiation dose. We present a model of this phenomenon. In our model, double strand break (DSB) sites induced by IR interact with a limiting pool of DNA repair proteins, forming complexes that are sensed by ATM, a protein kinase that activates p53 once phosphorylated by DNA damage. The ATM sensing module switches on or off the downstream p53-mdm2 negative feedback loop. Our simulations show that by assuming stochasticity in the initial number of DSBs and the DNA repair process, p53 and Mdm2 exhibit a coordinated oscillatory dynamics upon IR stimulation in single cells, with a stochastic number of oscillations whose mean increases with IR dose. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y35.00013: Transcription factor binding energy vs. biological function M. Djordjevic, E. Grotewold Transcription factors (TFs) are proteins that bind to DNA and regulate expression of genes. Identification of transcription factor binding sites within the regulatory segments of genomic DNA is an important step towards understanding of gene regulatory networks. Recent theoretical advances that we developed [1,2], allow us to infer TF-DNA interaction parameters from in-vitro selection experiments [3]. We use more than 6000 binding sequences [3], assembled under controlled conditions, to obtain protein-DNA interaction parameters for a mammalian TF with up to now unprecedented accuracy. Can one accurately identify biologically functional TF binding sites (i.e. the binding sites that regulate gene expression), even with the best possible protein-DNA interaction parameters? To address this issue we i) compare our prediction of protein binding with gene expression data, ii) use evolutionary comparison between related mammalian genomes. Our results strongly suggest that in a genome there exists a large number of randomly occurring high energy binding sites that are not biologically functional. [1] M Djordjevic, submitted to\textit{ Biomol. Eng.} [2] M. Djordjevic and A. M. Sengupta, \textit{Phys. Biol.} \textbf{3:} 13, 2006. [3] E. Roulet et al., \textit{Nature Biotech.} \textbf{20}: 831, 2002. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y35.00014: Dynamics of DNA bending/unbending in complex with DNA-bending protein IHF Anjum Ansari, Paula Vivas, Serguei Kuznetsov Kinetics of conformational changes in proteins and DNA that lead to precise recognition of specific DNA binding sites are difficult to observe with the limited time-resolution of stop-flow and single-molecule techniques. Here we use a $\sim $10 ns laser T-jump apparatus to probe the kinetics of a $\sim $35-bp DNA substrate bound to \textit{E. coli} Integration Host Factor (IHF) and end-labeled with a FRET pair. These T-jump measurements, in combination with stop-flow, provide the first direct observation of the DNA bending/unbending kinetics in a protein-DNA complex (Sugimura and Crothers, PNAS, in press; Kuznetsov et al., PNAS, in press). The rates and activation energy of DNA bending are similar to that of a single A:T base pair opening inside uncomplexed DNA, suggesting that spontaneous thermal disruption in base-pairing nucleated at an A:T site may be sufficient to overcome the free energy barrier needed to partially bend/kink DNA. An unusual salt dependence of the binding affinity observed previously for IHF/DNA complex, and explained in terms of DNA binding coupled with disruption of a network of salt bridges within the protein (Holbrook et al., 2001, JMB, \textbf{310}, 379), is reflected in the salt dependence of the observed bending rates. These results suggest that salt-dependent protein conformational changes may be playing a role in the DNA bending process. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y35.00015: HMGB binding to DNA: comparisons between single and double box motifs Micah J. McCauley, Jeff Zimmerman, L. James Maher III, Mark C. Williams High Mobility Group B (HMGB) proteins contain two HMG box domains known to bind non-sequence specifically into the DNA minor groove, slightly intercalating base pairs and producing a strong bend in the DNA backbone. These proteins are believed to alter DNA elasticity, making DNA more accessible for transcription in vivo. To probe the effects of HMG proteins on DNA elasticity, we use optical tweezers to measure the forces required to stretch single DNA molecules, alone and in the presence of HMGB proteins at varying solution conditions. Experiments quantify the binding constant of HMGB to DNA, as well as changes in the flexibility and stability of the double helix. Previous results from a protein fragment containing a single HMG box suggested significant flexibility changes in the double helix but did not show helix stabilization, while a double box protein from rat HMGB--1 appears to significantly stabilize the DNA helix. [Preview Abstract] |
Session Y38: Focus Session: Emerging Research Devices and Materials for the Microelectronics Industry III
Sponsoring Units: FIAPChair: Alex Demkov, University of Texas at Austin
Room: Colorado Convention Center 501
Friday, March 9, 2007 11:15AM - 11:27AM |
Y38.00001: High Mobility InSb Quantum Well with Dislocation Filtering Buffer Layer Grown on GaAs (001) Substrates Madhavie Edirisooriya, Tetsuya Mishima, Michael Santos A small electron mass makes InSb quantum wells (QWs) with Al$_{x}$In$_{1-x}$Sb barriers attractive for field-effect transistors, mesoscopic magnetoresistors, and ballistic transport devices. The large spin-orbit effects in InSb make InSb QW structures attractive for spin transport devices. The electron mobility of an InSb QW with an Al$_{x}$In$_{1-x}$Sb buffer layer is partly limited by scattering caused by crystalline defects that arise from the large lattice mismatch (14.6{\%}) between the epilayers and the GaAs (001) substrate. Our transmission electron microscopy measurements show that Al$_{x}$In$_{1-x}$Sb/Al$_{y}$In$_{1-y}$Sb interfaces reduce the concentration of threading dislocations. We observed electron mobilities of 38,000 cm$^{2}$/Vs and 121,000 cm$^{2}$/Vs at 300K and 77K, respectively, in an InSb QW grown on a 1.5$\mu $m thick Al$_{x}$In$_{1-x}$Sb buffer layer with two Al$_{y}$In$_{1-y}$Sb interlayers. These values are 16{\%} and 75{\%} higher at 300K and 77K, respectively, than observed in a structure without interlayers. The improved mobility is apparent in studies of geometrical magnetoresistance in devices with channels that are short and wide. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y38.00002: Large-Scale Array of Pristine Carbon Nanotube Transistors Minbaek Lee, Jiwoon Im, Byung Yang Lee, Sung Myung, Juwan Kang, Seunghun Hong 1D nanostructures such as carbon nanotubes (CNTs) have attracted tremendous attention due to their possible applications including transistors, chemical or biological sensor, etc. However, a lack of a massive manufacturing method for such devices has been an obstacle to their practical applications. Herein, we report a strategy for large scale assembly of CNT-based devices. In this strategy, inert molecular patterns were used to guide the adsorption of CNTs onto bare surfaces to form large scale integrated devices. Using this method, we demonstrated the wafer-scale fabrication of devices based on single-, double-, or multi- walled CNTs on virtually general substrates including SiO$_{2}$, Si, Al, Au, etc. Moreover, we also performed extensive analysis regarding the uniformity of fabricated CNT devices and the yield of this method. Importantly, since our method relies only on conventional semiconductor processing facilities, it is readily accessible for current semiconductor industry and should open up immediate applications such as sensors, FETs, and interconnectors. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y38.00003: Photocurrent Measurements of Carbon Nanotube PN Junctions Nathaniel Gabor, Zhaohui Zhong, Ken Bosnick, Jiwoong Park, Paul McEuen Gated p-n junctions in semiconducting nanotubes have recently drawn much attention for their electronic and optoelectronic characteristics [1,2,3]. We investigate the photocurrent response at a nanotube gated p-n junction using a focused laser illumination source. We find that the photocurrent at zero source-drain bias increases linearly with optical power for the component of light along the length of the nanotube. Scanned photocurrent imaging demonstrates that carrier generation occurs primarily between the p- and n- type segments of the device. Measurements in an optical cryostat down to 4K reveal large photoresponse and step-like structure in the reverse bias photocurrent. These results show that nanotube p-n junctions are highly sensitive, nanoscale photodetectors. [1] J.U. Lee et al, App. Phys. Lett. \textbf{85}, 145 (2004). [2] J.U. Lee, App. Phys. Lett. \textbf{87}, 073101 (2005). [3] K. Bosnick et al, App. Phys. Lett. \textbf{89}, 163121 (2006). [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y38.00004: Carbon nanotube and oxide nanobelt FETs: fabrication, characterization and applications Invited Speaker: High-performance field effect transistors (FETs) based on single-wall carbon nanotubes (SWNTs) and oxide nanobelts were fabricated and characterized. The SWNT-FETs were constructed via molecular template-directed assembly of HiPCO tubes onto pre-patterned metal electrodes on a Si/SiO$_{2}$ substrate. The devices exhibit operating characteristics comparable to state-of-the-art CNT FETs, and the process is amenable to large-scale functional CNT circuit assembly. Importantly, the integration of hydrophobic self-assembled organic monolayers in the device structure eliminates the primary source of gating hysteresis in SWNT-FETs, which leads to hysteresis-free FET operation while exposing unmodified nanotube surfaces to ambient air$^{[1]}$. Individual oxide (SnO$_{2}$ and ZnO) nanobelt FETs with multi-terminal contacts were fabricated via conventional lithography. Simultaneous two-terminal and four-terminal measurements enabled direct correlation of the FET characteristics with the nature of the contacts. Low-resistance ohmic contacts on the nanobelts result in high-performance n-channel depletion mode FETs with well-defined linear and saturation regimes, and ``on/off'' ratio as high as 10$^{7}$ at ambient conditions$^{[2]}$. Intrinsic values of the carrier concentration and effective mobility for the nanobelts were consequently obtained. Channel-limited SnO$_{2}$ nanobelt devices show significant modification of the FET characteristics when exposed to gas flows containing 0.2-2{\%} H$_{2}$ at room temperature. The gas sensitivity and response were carefully evaluated$^{[3]}$. The effort to utilize the channel-limited nanobelt FETs for protein detection will be discussed. $^{[1] }$S.A. McGill et al., APL \textbf{89}, 163123 (2006). $^{[2] }$Y. Cheng et al., APL \textbf{89}, 093114 (2006). $^{[3] }$L.L. Fields et al., APL \textbf{88}, 263102 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y38.00005: Self-assembly of Epitaxial Monolayers for Vacuum Wafer Bonding. Igor Altfeder, Biqin Huang, Ian Appelbaum, Barry Walker Self-assembled epitaxial metal monolayers can be used for hetero-integration of mismatched semiconductors, leading to simultaneously low interfacial resistance and high optical transparency. Lattice-mismatched wafers of Si(100) and Si(111) were bonded at room temperature in situ after vacuum deposition of a single atomic layer of Ag on them. The interfacial resistance was measured to be 3.9$\times$ 10$^{-4}$ ohm$\cdot$ cm$^ {2}$ and the optical transmission of the interface at 2500 nm is approximately 98\%. We discuss the important role of electron confinement in ultrathin Ag layers as a possible contributor to the bonding energy. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y38.00006: Identifying Read/Write Speeds for Field-Induced Interfacial Resistive Switching. Stephen Tsui, Nilanjan Das, Yaqi Wang, Yuyi Xue, C.W. Chu Efforts continue to explore new phenomena that may allow for next generation nonvolatile memory technology. Much attention has been drawn to the field-induced resistive switch occurring at the interface between a metal electrode and perovskite oxide. The switch between high (off) and low (on) resistance states is controlled by the polarity of applied voltage pulsing. Characterization of Ag-Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ interfaces via impedance spectroscopy shows that the resistances above 10$^{6}$ Hz are the same at the on and off states, which limits the reading speed to far slower than the applied switching pulses, or device write speed at the order of 10$^{7}$ Hz. We deduce that the switching interface is percolative in nature and that small local rearrangement of defect structures may play a major role. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y38.00007: Evidence for segregation of Te in ``phase-change" thin chalcogenide Ge-Sb-Te films C. Cabral, Jr., L.. Krusin-Elbaum, K. N. Chen, M. Copel, J. Bruley, V. R. Deline The novel chalcogenide phase-change materials are promising candidates for new technologies such as nonvolatile memories and programmable switches in 3D integration and planar logic. They are typically thin Ge-Sb-Te (GST) films, where a thermally induced amorphous-to-crystalline phase transformation can be fast and reversible, with the corresponding large swing in resistance values between the two stable structural states. Here we report on the structural evolution of GST films during thermal cycling and demonstrate using high-resolution (0.5~nm focused probe STEM) scans that Te segregates to the grain boundaries at fairly low temperatures. We show that diffusion of Te along grain boundaries results in its pileup at the free surface and interaction with Ti in adhesion layers in device- compatible stacks. This is consistent with impeded grain growth and with post-crystallization stress release. This motion may impact the ultimate life-cycle of phase-change based devices and should guide the optimal GST material design. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y38.00008: Characterizing oxide surfaces for successful interfacial resistive switching Nilanjan Das, Stephen Tsui, Waqi Wang, Yuyi Xue, C.W. Chu Resistive switching has been observed in many oxide-metal interfaces upon application of electric pulses. However, the mechanisms behind the phenomenon and the conditions for obtaining a successful switch are still matters of debate. It has been suggested that local defect rearrangement plays a role in the switching, which suggests that a defect-rich interface is required. There has also been indication that the local application of an electric field greater than some threshold is enough to induce a switch. We attempt to differentiate between these two scenarios by measuring samples with different surface treatments using a needle electrode method. Ac measurements have also been made to characterize the difference between switching and non-switching samples. The results suggest that the switching interface is a percolative layer. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y38.00009: Band alignments and electron transport in metal/epi-Sc$_{2}$O$_{3}$/Si (111) structures studied by BEEM and Internal Photoemission W. Cai, S.E. Stone, J.P. Pelz, L.F. Edge, D.G. Schlom Recently, Internal Photemission (Int-PE) has been used to study band alignments between Si and amorphous rare-earth/transition metal oxide films, of interest as possible high dielectric gate insulators for future MOS electronic devices [1,2]. Surprisingly, a variety of these oxide films were found to have nearly the same band alignments and band gap, and also ``tailing'' conduction band (CB) states extending $\sim $1 eV below the primary CB. We have applied Ballistic Electron Emission Microscopy (BEEM) and Int-PE to 20 nm-thick epitaxial Sc$_{2}$O$_{3}$ film grown at 700 $^{o}$C on Si(111), to study electron transport through these ``tail'' states and to estimate oxide fixed charge. These tail states are found to form a ``robust'' CB that supports elastic electron transport even against an applied electric field, with a $\sim $1.1 eV CB offset at the Si interface. Al/epi-Sc$_{2}$O$_{3}$/Si structures were $\sim $1000 times leakier than those made with Pt, consistent with the lower electron tunneling barrier expected for the lower work function Al. The measured dependence of the BEEM threshold voltage on metal bias suggests $\sim $0.2 C/cm$^{3}$ fixed negative oxide charge with a 250 $^{o}$C anneal before Pt deposition and no post-metallization anneal. Work supported by SRC. [1] V. V. Afanas'ev\textit{ et al}., Appl. Phys. Lett. \textbf{85}, 5917 (2004). [2] V. V. Afanas'ev\textit{ et al}., Appl. Phys. Lett. \textbf{88}, 032104 (2006). [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y38.00010: First Principles Study of Strain Effects on the Electronic Properties in Silicon Nanowires Xihong Peng, Sanat Kumar, Saroj Nayak Silicon nanowires have drawn much attention in the past decades due to their potential applications in many fields, such as optoelectronics, micro- and nano- electronics. The study of size dependence on the band gap of silicon wires have been addressed both using theoretical methods and experimental techniques. In parallel, industry routinely applies strain to engineer the electronic properties in bulk Si. In present work, using first principles density functional theory we have studied the uniaxial strain effects on the electronic properties in Si nanowires with lateral dimension up to 5 nm. We discovered that the strain effects on the band gap display qualitatively new trends for the nanowires smaller than $\sim $5 nm. In Si bulk, indirect band gap decreases linearly with hydrostatic compression, while the band gap is increasing with uniaxial compressive strain for wires smaller than 2 nm. In the intermediate size range 2$\sim $5 nm, the band gap decreases both with compressive and tensile strains, exhibiting an approximately parabolic behavior. Finally we will present our results of strain effect on the effective masses of electrons and holes in nanowires that may have immense impact on future nanoelectronics devices. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y38.00011: Theoretical study of the insulator/insulator interface: band alignment at the SiO$_{2}$/HfO$_{2}$ junction Onise Sharia, Alexander A. Demkov Hafnia has emerged as a front runner for replacing silica as a gate oxide in CMOS technology. One of the problems which still remains outstanding is finding a p-type gate metal for hafnia. Thus the problem of band alignment at the hafnia/metal and hafnia/Si interfaces has recently received significant attention. However, it is worth noting that during the deposition of hafnia on a silicon substrate a thin layer of silica is always created. And the band alignment between silica and hafnia can dramatically change the overall alignment across the gate stack. In this presentation we will discuss the band alignment at the SiO$_{2}$/HfO$_{2}$ interface. As we shall show it can be significantly different from the simple Schotky limit. We perform \textit{ab-initio s}tudies of the interface using density functional theory in the local density approximation. We construct several atomic level models of the interface which connect hafnia to silica \textit{via} an oxygen plane as required by the electron count rule that ensures the absence of electronic states in the gap. The models differ by the interfacial oxygen coordination, HfO$_{2}$ phases, and strain, and are fully relaxed. All interfaces can be categorized by the interfacial oxygen average coordination number. The calculated valence band offset varies from 1.0 eV to -2.0 eV and most strongly depends on the average coordination of the interface oxygen. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y38.00012: Extremely Broadband Semiconductor Optical Amplifiers Gagik Shmavonyan Extremely broadband InGaAsP/InP ridge bent-waveguide semiconductor optical amplifiers with seven non-identical multiple quantum wells were designed and fabricated on InP substrate. The emission spectra of bent-waveguide semiconductor optical amplifiers at different injection current levels were experimentally studied. To achieve the broadband characteristics, a sequence of non-identical multiple quantum wells were designed. When designing a broadband semiconductor optical amplifier using a non-identical MQW structure, factors such as QW transition energy, number and sequence of different QWs, the thickness of the separate confinement heterostructure layer, the selection of the dominant carrier, the ability of the QW to trap the 2D carrier, the uniformity of the 2D carrier within the QWs, etc. must be taken into account. Using appropriate non-identical MQW structure of SOA allows us to achieve broad emission of semiconductor optical amplifiers. The bandwidth of these semiconductor optical amplifiers is 400 nm, which cover from 1250 to 1650 nm, the range for low-loss window of optical fibers. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y38.00013: Interband Cascade Laser ($\lambda $ = 3.7 $\mu $m) Operating cw to Thermoelectric Cooler Range D. Larrabee, J.A. Nolde, W.W. Bewley, C.L. Canedy, C.S. Kim, M. Kim, I. Vurgaftman, J.R. Meyer In the mid-infrared, a significant range of wavelengths, from 3.1 to 3.8 $\mu $m, is currently inaccessible to cw semiconductor lasers operating at ambient temperature. The most promising device design for reaching this range is the interband cascade laser (ICL), based on a type-II ``W'' quantum well active region. Here we present results of ICLs fabricated in narrow ridges, which improves both the lateral heat dissipation and the beam quality compared with broad-area lasers. For example, a five-stage ICL with 12-$\mu $m ridge width and Au electroplating for improved epitaxial-side-up heat sinking operates cw to a maximum temperature of 257 K, where the emission wavelength is 3.7 $\mu $m. The device emits 100 mW per facet for cw operation at 80 K, 54 mW at 200 K, and 10 mW at 250 K. The beam quality is within twice the diffraction limit for injection currents up to 14 times the lasing threshold. [Preview Abstract] |
Session Y39: Focus Session: Physics & Technology of III-V Semiconductors in Infrared & THz Imaging II
Sponsoring Units: FIAPChair: Nancy Haegel, Naval Postgraduate School
Room: Colorado Convention Center 502
Friday, March 9, 2007 11:15AM - 11:51AM |
Y39.00001: Passive THz Imaging with Superconducting NbN microbolometer Arrays Invited Speaker: Passive THz imaging applications indoors require temperature difference resolution well below 1 K and integration times down to 0.1 ms. Recently we have shown that such resolution, approaching the photon noise limit, can be achieved using an antenna-coupled superconducting microwire bolometer with about 10 K transition temperature. The bolometer signal is read out with a low-noise room-temperature amplifier, thus eliminating the need for SQUID amplifiers. The readout method utilizes electro-thermal feedback at the $I-V$ curve minimum of a voltage-biased bolometer. At this working point, the very high power gain of the bolometer makes noise matching of the readout to the detector straightforward. The readout amplifier can be used with transition bolometers and calorimeters operating even at mK temperatures. We are presently developing a video-rate THz imager for concealed weapon detection, utilizing conical scanning and a 128-pixel NbN bolometer array, cooled down to 4 K with a pulse-tube cryocooler. We will characterize the bolometer arrays and the readout electrically and compare the results with the theory. We will also present the design of the system and results of preliminary imaging experiments. The work is done in collaboration between VTT, Millilab and NIST. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y39.00002: Short-period InAs/GaSb superlattices for mid-infrared photodetectors. H.J. Haugan, F. Szmulowicz, G.J. Brown, B. Ullrich, S.R. Munshi, J.C. Wickett, D.W. Stokes Using a newly developed envelope function approximation model that includes interface effects, several InAs/GaSb type-II superlattices (SLs) were designed for uncooled mid-infrared detector applications. The 4 micron cutoff could be achieved with several SL designs. Superlattices with shorter-periods have larger intervalence band separations than larger-ones, which could increase the optical signal and reduce the detector noise, thus making room temperature operation possible. To test these possibilities, several short-period SLs were grown by molecular-beam epitaxy and their optical properties with reducing SL period were studied by band-edge absorption, photoconductivity and photoluminescence measurements. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y39.00003: Demonstration of interface-scattering-limited electron mobilities in InAs/GaSb superlattices F. Szmulowicz, S. Elhamri, H. Haugan, G. Brown, W. Mitchel In-plane transport in InAs/GaSb type-II superlattices (SLs) is a sensitive indicator of SL growth quality and of the eventual performance of devices made from these materials. The in-plane mobility of electrons that move predominantly in the InAs layer is affected by a number of intrinsic and extrinsic scattering mechanisms, including interface roughness scattering (IRS). The hallmark of classic IRS-limited transport in SLs and quantum wells is the sixth power dependence of mobility on layer width. While IRS-limited transport was demonstrated in a number of SL and quantum well systems, it has never been demonstrated in the important InAs/GaSb SL material. We performed temperature dependent Hall effect measurements on a series of InAs/GaSb SLs with a fixed GaSb layer width and a variable InAs layer width, $d$. The low temperature (10K) in-plane electron mobilities, $\mu $, as a function $d$ behave as $\mu \propto d^{6.20}$, which follows the classic sixth power dependence expected from theory. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y39.00004: GaAs based antenna-coupled terahertz detector operating at 300 K Sangwoo Kim, Jeramy Zimmerman, Paolo Focardi, Dong Ho Wu, Arthur C. Gossard, Mark S. Sherwin A Terahertz detector which consists of twin-slot antennas, coplanar waveguides, and a GaAs Metal Semiconductor-Field-Effect-Transistor (MESFET) has been developed. This talk will present design, fabrication, and recent measurements of our detector. As Terahertz photons are coupled into the antenna, an oscillating electric field is formed across the two gates of the GaAs MESFET. Then the oscillating electric field excites collective motion of the electron plasma in the active area, thereby changing the source-to-drain resistance of the transistor. The impedances of the antenna and the transistor were matched in order to maximize the power coupling efficiency. Our device is designed to have electronics-limited response time ($\sim $1 ns), broadband ($\sim $0.5 THz, HWHM) response, low Noise Equivalent Power (NEP) ($\sim $10$^{-10}$ Watt/(Hz)$^{1/2})$, responsivity of $\sim $1000 V/W, and ability to operate at room temperature. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y39.00005: Near-Infrared In$_{0.53}$Ga$_{0.47}$As / AlAs$_{0.56}$Sb$_{0.44}$ Quantum Cascade Detectors Fabrizio Giorgetta, Esther Baumann, Christian Manz, Quankui Yang, Klaus Koehler, Daniel Hofstetter Quantum cascade detectors (QCDs) are a promising approach for photovoltaic electro-optical detectors in the infrared. They are based on intersubband transition, which makes them intrinsically fast due to the short unipolar relaxation times. Furthermore, no dark current noise occurs in QCDs because of the biasless operation. So far, QCDs with operating wavelengths down to 5 $\mu $m were demonstrated. For shorter wavelengths, a material system with a large conduction band discontinuity $\Delta $E$_{c}$ is required. A suitable choice is In$_{0.53}$Ga$_{0.47}$As / AlAs$_{0.56}$Sb$_{0.44}$ lattice matched to InP, with $\Delta $E$_{c}$=1.6 eV. We therefore present three InGaAs / AlAsSb QCDs detecting down to 2 $\mu $m. The exact well and barrier widths were determined by a self-consistent Schroedinger-Poisson solver and the samples were then grown by molecular beam epitaxy. The spectral room temperature responsivity of the three samples peaks at 2.34 $\mu $m (R$_{max}$=23 mA/W), 2.37 $\mu $m (16 mA/W), and 2.03 $\mu $m (4.3 mA/W). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y39.00006: Quantum efficiency of an InAs/GaSb type-II superlattice photodiode Shin Mou, Jian Li, Shun Lien Chuang We present the experimentally measured and theoretically modeled quantum efficiency of an $n$-on-$p$ InAs/GaSb superlattice photodiode with a cutoff wavelength of 7.5 microns. The model is based on an analytical photocurrent solution with all the parameters obtained by measurements. For instance, the optical absorption coefficient was obtained from transmission measurements and the transport parameters were obtained from electron beam induced current (EBIC) experiments. With an absorptive layer 1.15 micrometer thick, we show the space charge region contributes most of the photocurrent for this specific structure. We found a design consisting of a ternary InAs/InGaSb superlattice absorptive region with an improved absorption coefficient gives us 50 {\%} larger external quantum efficiency with a similar cutoff wavelength. This shows that engineering the quantum structure could optimize the quantum efficiency. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y39.00007: Molecular Beam Epitaxy Grown Long Wavelength Infrared HgCdTe on Compliant Si Substrates Priyalal Wijewarnasuriya, Yuanpin Chen, Gregory Brill, Nibir Dhar, Michael Carmody Large format, low cost, reliable and high performance infrared focal plane arrays (IRFPA) are essential for the Army's Third Generation IR Imaging Technology. Bulk-grown Cd$_{0.7}$Zn$_{\sim 0.3}$Te (CZT) substrates are the natural choice for HgCdTe epitaxy since it is lattice matched to HgCdTe alloy. However, a lack of large area CZT substrates, high production costs, and more importantly, the difference in thermal expansion coefficients between CZT substrates and silicon readout integrated circuits are some of the inherent drawbacks of CZT substrates. Consequently, Hg$_{1-x}$Cd$_{x}$Te detectors fabricated on silicon substrates are an attractive alternative generating considerable interest. Recent developments in the MBE Chalcogenide buffer layer growth technology on Si substrates has revolutionized the HgCdTe research and offered a new dimension to HgCdTe-based IR technology. We have fabricated large format 256x256 pixels with 40 microns pitch on LW-MBE-HgCdTe material grown on compliant CdSeTe/Si and CdTe/Si substrates. This paper will present data on 256x256 FPA and single device performance. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y39.00008: Monolithic Mid-Infrared Photonic Integration of a Quantum Cascade Laser and a Passive Semiconductor Waveguide Kale J. Franz, Claire Gmachl, Kuen-Ting Shiu, Stephen R. Forrest On-chip integration of a quantum cascade (QC) laser with a passive semiconductor waveguide is reported. We use conventional semiconductor processing techniques to fabricate a QC laser directly coupled to a passive waveguide, where the unbiased active region and the lower cladding layers of the QC laser are used as the waveguide. The QC architecture is compatible with this technique due to the different allowed optical transitions when the QC structure is biased (for laser operation) and unbiased (for the passive waveguide). Thus the same epitaxial layers are made to emit light under bias and not absorb that same light when unbiased. The waveguide portion of the structure is fabricated by using a selective etch to remove the top InP cladding layers above the QC active core. We find the effective refractive index contrast provided by the etch step between the laser and waveguide to be sufficient for providing optical feedback for the laser. For our structure, we calculate a laser-waveguide ``facet'' reflectivity of about 3{\%} based on a 25{\%} increase in threshold current density compared to the same laser structure with two cleaved facets. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y39.00009: A THz niche for AlP/GaP quantum wells M. Goiran, J. Galibert, J. L\'eotin, V.V. Rylkov, M. Semtsiv, O. Bierwagen, W.T. Masselink The development of THz spectroscopy and imaging based on intersubband transitions in Quantum Cascade Lasers (QCL) is precluded to date in the wavelength range 20-60 $\mu $m (15-5 THz) because of the reststrahl band of currently used GaAs alloy materials. One option to overcome this limitation is to use AlP/GaP Quantum wells grown on a GaP substrate, but until recently the intersubband structure of AlP quantum wells was unknown, because the X-conduction band structure of AlP was not established. We report on subband energy spectrum of electrons in AlP quantum wells as the outcome of recent effective mass measurements and valley-degeneracy, including the effect of strain caused by lattice mismatch between AlP and GaP [1]. We show that depending on the well thickness, the ground state subband has X$_{z}$ symmetry for well thickness shorter than 5nm and X$_{xy}$ symmetry for larger thickness. The knowledge of subband parameters in AlP/GaP quantum wells allows the design of both QCLs and QW detectors, taking into account the unique multi-valley subband structure of AlP quantum wells. [1] M.P. Semtsiv et al. Phys. Rev. B \textbf{74}, 041303(R) (2006) [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y39.00010: In-plane integration of quantum-cascade lasers with resonant intersubband nonlinearities Alexey Belyanin, Feng Xie, Venkata R. Chaganti, Don Smith We have recently demonstrated that the active region of a quantum cascade laser can be integrated with a cascade of intersubband transitions designed for the intracavity nonlinear frequency conversion of laser light. Integration has been implemented in the growth direction by vertically stacking the nonlinear region and active laser stages. While this approach is easy to realize, it has limited ability to control frequency and conversion efficiency of the nonlinear signal. Here we propose in-plane integration strategy, in which the laser is divided into two separately contacted and biased sections along the cavity length. One section operates as a laser active medium while another section serves as a nonlinear element. The sections share the same layer structure but may have very different set of electron states depending on the applied biases. We show that such schemes turn out to be surprisingly flexible in implementing various optical nonlinearities. The proposed approach enables convenient and broad tuning by applied voltage and extends the room-temperature operation range of quantum cascade lasers to very short (2.5-4 $\mu $m) or very long (THz) wavelengths. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y39.00011: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y39.00012: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y39.00013: Propagation of Guided Modes in Curved Nanoribbon Waveguides Zhuo Ye, Xinhua Hu, Ming Li, Kai-Ming Ho, Peidong Yang We develop a planewave-based transfer matrix method in curvilinear coordinates to study the guided modes in curved nanoribbon waveguides. The problem of a curved structure is transformed into an equivalent one of a straight structure with spatially-dependent tensors of dielectric constant and magnetic permeability. The dispersion curves, mode profiles, and self-transmission of guided modes are obtained for the curved waveguides. Oscillations in the self-transmission as a function of wavelength are found to increase in amplitude as the bend becomes sharper. The period of the oscillations decreases with increase in size of the bending region. We show that curved sections can result in strong oscillations in the transmission spectrum similar to the recent experimental results in [M. Law, D. J. Sirbuly, J. C. Johnson, J. Goldberger, R. J. Saykally, P. Yang, Science\textbf{ 305}, 1269(2004)]. [Preview Abstract] |
Session Y40: Semiconductor Spectroscopy
Sponsoring Units: FIAPChair: Hans-Peter Wagner, University of Cincinnati
Room: Colorado Convention Center 503
Friday, March 9, 2007 11:15AM - 11:27AM |
Y40.00001: Stress-Induced Shifts of the Photoluminescence and Raman Peaks in Al$_{x}$Ga$_{1-x}$As Observed as a Function of Al Composition Grady White, Albert Paul, Kris Bertness In Al$_{x}$Ga$_{1-x}$As, both photoluminescence (PL) and Raman peak positions are strongly sensitive to x, a fact that has made PL a primary tool for monitoring composition in the optoelectronics industry. However, the peak positions also depend upon stress. Because layered thin film systems inherently experience residual stresses, use of PL or Raman measurements without compensation for existing stresses limits the accuracy of composition determination. We present results of both PL and Raman measurements of Al$_{x}$Ga$_{1-x}$As films as a function of biaxial tensile stress for 0 $\leq $ x $\leq $ 0.9, comparing the sensitivity of the stress-induced shifts with the composition-induced shifts and, thereby, providing an estimate of the uncertainties associated with composition determination. We also discuss an upper limit to the composition sensitivity of the phonon deformation potentials of Al$_{x}$Ga$_{1-x}$As. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y40.00002: Ballistic carrier injection induced electroluminescence of InAs quantum dots in a hot-electron metal-base transistor Wei Yi, Venkatesh Narayanamurti, Joshua Zide, Seth Bank, Arthur Gossard Utilizing hot electrons ballistically injected by a tunnel junction over the Schottky barrier into a semiconductor collector, ballistic electron emission microscopy (http://www.deas.harvard.edu/venky/pdffiles/29.pdf) BEEM characterizes carrier filtration through buried interfaces with nanometer spatial resolution. Under forward collector bias, interband light emission may occur in a heterostructure collector by injection of minority carriers with sub-bandgap kinetic energies. Such a concept, ballistic electron emission luminescence, is tested using a hot-electron metal-base transistor, the solid-state prototype of BEEM. The heterostructure collector with embedded InAs quantum dots (QDs) is grown on p-GaAs substrate. Either majority carriers (holes) or minority carriers (electrons) are injected into the collector under different collector bias. Light emission from InAs QDs, InAs wetting layer, and bulk GaAs are observed in concert with minority carrier injection. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y40.00003: Time-resolved x-ray diffraction studies of the thermal properties of AlGaAs Y.M. Sheu, S.H. Lee, D.M. Fritz, J.K. Wahlstrand, M. Reason, R.S. Goldman, D.A. Reis We report on studies of thermal properties of epitaxial AlGaAs on GaAs using an ultrafast laser-excitation and x-ray probing technique. Femtosecond laser pulses were used to excite the material at the buried interface. Heat is generated by electron-hole recombination in the GaAs substrate and diffuses into the AlGaAs film. Following the initial heating the film and substrate cool as heat diffuses into the bulk. Time resolved X-ray diffraction (TRXD) is sensitive to precise~ lattice movement and can resolve the time evolution of the material as it reaches thermal equilibrium. We show that TRXD has the potential to study thermal properties such as the thermal conductivity and thermal boundary resistance in hermoelectric materials. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y40.00004: Polariton Condensation in CdTe Microcavities: the phase diagram. Marzena Szymanska, Jacek Kasprzak, Jonathan Keeling, Francesca Marchetti, Regis Andre, Peter Littlewood, Daniel Le Si Dang The first realisation of a polariton condensate has been very recently achieved in a CdTe microcavity [Kasprzak et al., Nature 443, 409 (2006)]. The direct comparisons with theoretical calculations reveal crucial information about the nature of the condensed phase for such composite light-matter particles. In particular, we compare the phase boundaries obtained experimentally, for different values of detuning and cryostat temperature, with those evaluated theoretically using a model which takes into account features of microcavity polaritons such as their reduced dimensionality, their internal composite structure, the disorder in the quantum wells, the polariton-polariton interaction, and their finite lifetime. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y40.00005: The parametric oscillation threshold of semiconductor microcavities Michiel Wouters, Iacopo Carusotto Since its first experimental observation a few years ago, triply resonant optical parametric oscillation (OPO) in continuously pumped semiconductor microcavities in the strong coupling regime has attracted a lot of attention not only in view of applications to integrated nonlinear optics devices, but also from the point of view of fundamental physics. A most important property is in both cases the behaviour of the OPO around the threshold: because of the third-order nature of the excitonic nonlinearity, a much richer variety of behaviours can be observed as a consequence of the interplay of parametric oscillation and optical bistability effects. In the present talk, we classify the different behaviours in terms of the continuous or discontinuous switching to the OPO state, akin to respectively a second or a first order phase transition, and we characterize the properties of the quantum fluctuations as the threshold is approached as a function of the pump, signal and idler frequencies. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y40.00006: Single-photon nonlinearity of a semiconductor quantum dot in a cavity Frank Bello, Daniele Sanvitto, Fabrice Laussy, Paolo Guimaraes, David Whittaker, Maurice Skolnick, A. Tahraoui, P.W. Fry, M. Hopkinson A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED).In the weak coupling regime, where losses exceed the interaction energy between atoms and the cavity mode, irreversible decay of the excitations occurs. Strong coupling, which corresponds to the reversible exchange of energy between the atom and the mode opens up a much wider range of CQED phenomena. We present evidence depicting non-linear effects for a quantum dot embedded within a semiconducting micropillar cavity. Emission spectra show transitions from the strong to weak coupling regime between the n=2 and n=1 photon states. Transforming from a system with weak coupling and low pumping to one with higher pumping, lessens the conditions needed for strong coupling due to the increased number of photons inside the cavity. Good agreement with theoretical calculations using the Jaynes-Cummings ladder model is also shown, along with corresponding transition rates. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y40.00007: Phase coherent photorefractive effect in ZnMgSe/ZnSe quantum wells using ultrashort light pulses Pradeep Bajracharya, Amin Kabir, Hans - Peter Wagner We report on an efficient exciton resonant phase coherent photorefractive (PCP) effect in ZnMgSe/ZnSe single quantum wells (QWs) using ultrashort light pulses that do not overlap in time. The diffraction efficiency as well as the electron grating dynamics that is responsible the PCP effect is studied in a four-wave-mixing (FWM) configuration using 90 and 30 fs pulses. Experiments with significantly reduced repetition rate of subsequent pulse pairs reveal an electron grating lifetime of greater 10 $\mu $s at 55 K. For spectrally broad 30 fs pulses when both excitons and electron-hole-pairs are excited in the QW the PCP signal is strongly reduced. The PCP signal reappears at higher pulse energies when ZnMgSe barrier excitons are additionally excited. The observation of PCP at incident light intensities as low as 100$\mu $Wcm$^{-2}$ has potential for optical coherence imaging. This work is supported by the National Science Foundation (DMR 0305076). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y40.00008: In-plane optical anisotropy in self-assembled Ge quantum dots induced by interfacial chemical bonds Chih-Ming Wei, Tzung-Te Chen, Yang-Fang Chen In-plane optical anisotropy has been observed in self-assembled Ge quantum dots (QDs). It is found that the photoluminescence (PL) spectrum polarized along $[110]$ exhibits different features compared to that corresponding to $[1\overline 1 0]$. Besides, the polarized PL spectrum is able to reveal the detailed fine structure much more pronounced than that in unpolarized spectrum. It is shown that the observed optical anisotropy is a result of the inherent property of the type-II band alignment of Ge QDs embedded in Si matrix. The light emission arises from the recombination of electrons and holes across the interface, and it thus reflects the anisotropic nature of the interfacial chemical bonds. The predicted results according to our proposed mechanism have been successfully tested for SiGe/Si multiple quantum wells as well as superlattices. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y40.00009: Superradiance Of Quantum Dots Michael Scheibner, Thomas Schmidt, Lukas Worschech, Alfred Forchel, Gerd Bacher, Thorsten Passow, Detlef Hommel Quantum dots (QDs) may be considered to form a coupled quantum system if they interact with a common radiation field. By its nature the range of this coupling mechanism is on the order of the radiation wavelength and its signature is a modified radiation rate. Here we analyze the decay time of the photoluminescence emitted from a single layer of self assembled CdSe/ZnSe QDs [1]. We find that the decay time depends on the number of QDs. For example, under (quasi-) resonant excitation conditions the decay time increases as QDs are removed from the sample by etching mesas. This indicates that in the as grown sample QDs radiate cooperatively. The range of this interaction is shown to be at least 150 nm. [1] Scheibner, et al. `Superradiance of Quantum Dots', submitted for publication [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y40.00010: Excitonic effects in optical absorption spectra of CdTe. Kalum Palandage, Gayanath Fernando, Rampi Ramprasad We have used a first principles, quasiparticle, self-consistent GW and similar approximations to predict the electronic structure of various nanocrystals with high accuracy. In this preliminary stage, we applied the method to a selection of different classes of materials including alkali metals, Transition metals and semiconductors. It was observed that the self-consistency improves the agreement with experiment. Our goal is to analyze dynamical signatures of excitons and multi-excitons in CdTe nanocrystals using several self-consistent approximations. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y40.00011: Photoluminescence properties of CdTe/CdSe core-shell type-II quantum dots Chun-Hsiung Wang, Tzung-Te Chen, Kee-Wee Chen, Yang-Fang Chen We report investigations on the optical properties of type-II CdTe/CdSe core-shell quantum dots. By varying the core size, we provide an elegant way to verify that the detected emission signal indeed arises from type-II band alignment. The photoluminescence (PL) peak energy increases with a third root of the excitation power. Both of the PL peak energy and linewidth exhibit unique temperature dependence. All these observations can be rationalized by the band bending effect resulting from the spatially separated photo-excited carriers in a type-II band alignment. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y40.00012: The Virtual Scanning Tunneling Microscope: A Novel Probe Technique for Imaging Two-Dimensional Electron Systems Adam Sciambi, Kathryn Todd, David Goldhaber-Gordon, Seth Bank, Arthur Gossard We propose a novel probe technique, the virtual scanning tunneling microscope (VSTM), which could provide both spatial and spectroscopic information about two-dimensional electron systems (2DESs) in semiconductor heterostructures. The VSTM's innovation is the addition of a second 'probe' 2DES separated by a low barrier from the sample 2DES below. Simulations show that a positively-biased tip held above the sample surface can diminish the interlayer barrier and induce tunable tunneling between the two 2DESs. If the tip is scanned, the tunneling region will follow below, acting as a virtual tip while screening the true tip from the sample 2DES. This probe technique is motivated by interesting local 2DES physics that can only be studied indirectly because of the depth of 2DESs; we describe a range of predicted spatially-organized phases of 2D electrons, and transport properties of ErAs self-assembled quantum dots, which could be accessed with this new probe. We also present preliminary experimental results from a GaAs/AlGaAs bilayer 2DES sample, supporting the results of the simulation. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y40.00013: A non-contact system for spatial mapping of carrier lifetimes Jamiyanaa Dashdorj, Reuben Collins, Steven Johnston, Bhushan Sopori A novel approach to spatially resolved, non-contact, carrier lifetime mapping using resonance-coupled photoconductivity decay (RCPCD) was developed. Here, a pulsed laser excites a sample and the resulting transient is detected using a resonantly coupled antenna. The key to the present system is a patch antenna, which operates at 425 MHz and was designed using the EM simulation software, HFSS. By providing a uniform, high sensitivity detection area, the antenna only requires tuning to be performed once, after initial placement of the sample. Using this antenna, a fully computerized system has been developed for scanning samples, acquiring transients, and automatically extracting lifetimes. The present resolution is 0.25 mm, although there is no real limit on this. The new system was successfully applied to the evaluation of polycrystalline silicon wafers. In addition, effects of surface passivation on lifetime was determined. This work was supported by NREL Award {\#} KXEA-3-33607-17. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y40.00014: Origin of a Localized Vibrational Mode in a GaSb Substrate With a MBE-grown ZnTe Epilayer A. K. Ramdas, Hyunjung Kim, E. Tarhan, G. Chen, M. Dean Sciacca, R. L. Gunshor In the infrared spectrum of a MBE-grown ZnTe epilayer grown on GaSb, a localized vibrational mode (LVM) is observed with a remarkable fine structure. On the basis of the Zn and Te deposited on the GaSb substrate during the MBE growth of ZnTe, it is deduced that $^{64}$Zn, replacing Sb substitutionally as an anti-site impurity, is responsible for the LVM. The fine structure can then be interpreted in terms of the infrared active modes of a XY$_{4}$ quasimolecule, X$\equiv$$^{64}$Zn and Y$\equiv$$^{69}$Ga and $^{71}$Ga occupying the nearest neighbor sites, reflecting all the possible combinations and permutations as well as their natural isotopic abundance. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y40.00015: Valley-splitting in strained Silicon quantum wells on a miscut substrate using tight-binding model. Neerav Kharche, Marta Prada, Timothy Boykin, Gerhard Klimeck The splitting of the two-fold degeneracy of conduction-band valleys in bi-axially strained SiGe/Si/SiGe quantum wells (QWs) grown on (001) and $2^o$ miscut substrates is computed as a function of magnetic field using semi-empirical nearest-neighbour sp$^3$d$^5$s$^*$ tight-binding model in NEMO-3D. Unlike flat QWs, miscut QWs show two degenerate valleys centred at $(k_x,k_y)=(\pm k_x^0,0)$. Interaction between these two valleys due to confinement in the lateral (perpendicular to growth) dimension, leads to valley splitting (VS) in miscut quantum wells. VS in miscut QWs is suppressed by at least two orders of magnitude as compared to flat QWs. Numerical calculations with perfect step ordering underestimate experimentally observed VS. Simulation of experimental non-idealities such as step-roughness and alloy-disorder raise computed VS to the experimentally observed values. [Preview Abstract] |
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