Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P1: Is There Pairing Glue in Cuprate Superconductors?
Sponsoring Units: DCMPChair: C.C. Tsuei, IBM T.J. Watson Research Center
Room: Morial Convention Center LaLouisiane AB
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P1.00001: The Question of Pairing Glue in the Cuprate Superconductors Invited Speaker: The Hubbard and t-J models exhibit many of the properties which are observed in the cuprate superconductors. Thus they provide a framework for addressing the question of whether one should expect to find a ``pairing glue'' in the cuprtates. This question is basically a question regarding the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case one would not speak of a pairing glue. However, if the energies correspond to the spectrum seen in the dynamic spin susceptibility, then the interaction is retarded and one could speak of a spin-fluctuation glue which mediates the d-wave pairing interaction. We will review results from recent numerical calculations which provide insight into this question. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P1.00002: Strongly Correlated d-wave Superconductivity: a CDMFT Perspective Invited Speaker: We examine the impact of the proximity to a Mott insulating state on the superconduting properties of model system, (t-J and Hubbard) using cluster DMFT on a 2x2 plaquette. We study various observables such as the frequency and doping dependence of the order parameter, the one electron spectra, the optical conductivities, and the spin response. We compare the picture that emerges from these studies with the results of previous approaches such as the slave boson method, and with those of weak coupling approaches where the picture of superconductivity due to exchange of spin fluctuations is more established. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P1.00003: The Fluctuating Bond Model, a Glue for Cuprate Superconductivity? Invited Speaker: Twenty years of research have yet to produce a consensus on the origin of high temperature superconductivity (HTS). The mechanism of HTS - which originates in the CuO$_{2}$ plane, common to all HTS families - can be constrained by some key experimental facts regarding superconducting and pseudogap behaviors. Superconductivity, involving a $T_{c}$ of order $100 $K, exhibits an unusual $d$-wave superconducting gap, with Fermi liquid nodal excitations, and an anomalous doping- dependent oxygen isotope shift. A ``pseudogap,'' also with $d$-symmetry, leads to a dip in the density of states below a characteristic temperature scale $T^{\ast }$, which has a \textit{negative} isotope shift; we associate the pseudogap with the recently observed spatially inhomogeneous (nanometer- scale) C$_{4}$ symmetry breaking. The isotope shifts and other evidence imply a key role for oxygen vibrations, but conventional BCS single-phonon coupling is essentially forbidden by symmetry and by the on-site Coulomb interaction $U$. In a novel approach, we introduce a model based on a strong, local, nonlinear interaction between electrons within the Cu-O-Cu bond in the CuO$_{2}$ plane, and the oxygen vibrational degrees of freedom, termed the Fluctuating Bond Model (FBM) [D.M. Newns and C.C. Tsuei, Nature Physics \textbf {3}, 184 (2007)]. In mean field the model predicts a phase manifesting broken C$_{4}$ symmetry, with a $d$-type pseudogap, and an upper phase boundary in temperature, with a negative isotope shift, which we identify with $T^{\ast }$. An intrinsic $d$-wave pairing tendency is found, leading to a transition temperature dome and an anomalous isotope shift similar to that found experimentally. The softening in the oxygen vibrational frequency below $T_{c}$, seen in Raman and neutron spectra, has a natural explanation in the FBM. Recent \textit{ab initio} calculations have been implemented which provide microscopic support for the model. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P1.00004: The Normal State Pseudogap in Underdoped Cuprates: Precursor Pairing vs. Competing Order? Invited Speaker: The ``pseudogap'' in the normal state of underdoped High $T_c$ cuprates refers to a set phenomena associated with the loss of low energy spectral weight in various spectroscopies, starting at a temperature $T^\ast(x)$, much higher than $T_c(x)$ and with a completely different doping dependence. Understanding the unusual phenomena in the pseudogap regime, which lies in between the Mott insulator and the optimally doped superconductor, is an important challenge for any theory of high $T_c$ superconductivity. I will first review some of the main experimental facts about the pseudogap, focusing in particular on recent angle-resolved photoemission (ARPES) data. I will then critically examine which aspects of the data can be qualitatively understood in terms of theories of precursor pairing or those based on a competing order parameter. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P1.00005: Two-Dimensional Hubbard Model and High-Tc Superconductivity Invited Speaker: |
Session P2: Controlling Electron Spins: Propagation and Dynamics
Sponsoring Units: DCMPChair: Joseph Orenstein, University of California, Berkeley and Lawrence Berkeley National Laboratory
Room: Morial Convention Center LaLouisiane C
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P2.00001: An Exact SU(2) Symmetry and Persistent Spin Helix in a Spin-Orbit Coupled System Invited Speaker: Spin-orbit coupled systems generally break the spin rotation symmetry. However, for a model with equal Rashba and Dresselhauss coupling constant (the ReD model), and for the [110] Dresselhauss model, a new type of SU(2) spin rotation symmetry is discovered. This symmetry is robust against spin-independent disorder and interactions, and is generated by operators whose wavevector depends on the coupling strength. It renders the spin lifetime infinite at this wavevector, giving rise to a Persistent Spin Helix (PSH). We obtain the spin fluctuation dynamics at, and away, from the symmetry point, and suggest experiments to observe the PSH. Recent experimental efforts have already discovered signs of the Persistent Spin Helix. Although reaching the Rashba equal to Dresselhauss point is difficult, the Persistent Spin Helix manifests itself in a long-lived spin-density wave even away from this point, making its discovery especially suitable for optical techniques such as transient spin-grating spectroscopy. The SU(2) symmetric point allows one to obtain the exact analytic transport equations from the diffusive to the ballistic regime, which has previously been accessible only through numerical work. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P2.00002: Evidence for a persistent spin helix in a 2-dimensional electron gas Invited Speaker: Using time-resolved transient spin-grating spectroscopy we uncover strong evidence for the existence of a persistent spin helix (PSH) in a 2-dimensional electron gas. The PSH is a collective helical spin excitation that persists far beyond the lifetime of its individual constituent spins when the Rashba ($\alpha )$ and Dresselhaus ($\beta )$ spin-orbit coupling terms are comparable. The helix is predicted to have an infinite lifetime when they are exactly equal. These effects have great potential for application to spintronics where they would allow rapid gate control of the spin lifetime over several orders of magnitude in systems with both high electron density and high mobility. The transient spin-grating technique is ideally suited for study of the PSH as it can directly measure the dynamics of optically generated spin excitations of variable spatial periodicity. This is accomplished by interfering two non-collinear, orthogonally polarized pulses from a Ti:Sapphire laser at the surface of the sample. Through the optical orientation effect in GaAs, this generates a spin excitation which varies periodically in space between up and down spins at a wavelength determined by the angle between the interfering pulses. We tune the spin-orbit coupling in our GaAs based quantum wells through asymmetric modulation doping, which has allowed us to increase $\alpha $ to be comparable to $\beta $. In these systems we find that spin-gratings with periodicity comparable to that of the PSH can live several orders of magnitude longer than the individual spin lifetime as measured by time-resolved Faraday rotation. We study this over a wide range of parameter space, systematically varying doping asymmetry, well width, and disorder. We find that the lifetime of the PSH in these samples is ultimately limited by the spatial disorder in the Rashba strength, and by a novel relaxation mechanism based on phonon-induced Rashba coupling. Supported by DMSE office of BES-DOE, NSF, MARCO, ASEE and CNID. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P2.00003: Imaging the Drift and Diffusion of Optically- and Electrically-Injected Spins in Semiconductors Invited Speaker: We use methods for low-temperature scanning Kerr-rotation microscopy to directly image the drift, diffusion, and precession of spin-polarized electrons flowing laterally in GaAs. The 2-D images are used to explore spin generation and propagation resulting from both optically- as well as electrically-injected electron spins. The focused probe laser also allows to locally probe and spatially resolve the depolarization of an electron spin distribution by a small applied transverse magnetic field. The shape of these ``local Hanle effect" curves reveals important spin transport properties including the spin lifetime, drift velocity, mobility, and diffusion length [1]. The data can be accurately modeled using numerical solutions to the spin-drift- diffusion equations. Spatially-dependent asymmetries in the local Hanle effect data directly reveal the presence of additional effective magnetic fields due to spin-orbit coupling, and the dependence of these spin-orbit fields on the in-plane electron momentum {\bf k}. Using spin imaging and local Hanle effect measurements, we measure the drift and diffusion of electrically-injected spins in lateral spin transport devices having biased Fe/GaAs tunnel-barrier contacts [1], both within the charge current path as well as outside of the charge current path where only a pure spin current exists. A bias-dependent reversal of the injected spin polarization is directly observed, and we discuss how optical pumping methods can be used to measure (and tune) the spin dependent sensitivity of the epitaxial Fe/GaAs contacts when used as electrical spin detectors. [1] M. Furis {\it et al.}, New J. Phys. {\bf 9}, 347 (2007); X. Lou {\it et al.}, Nature Physics {\bf 3}, 197 (2007) [Preview Abstract] |
Session P3: Simple Views on Bulk Polymers: Symposium Honoring P G de Gennes
Sponsoring Units: DPOLYChair: Tom Witten, University of Chicago
Room: Morial Convention Center RO2 - RO3
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P3.00001: How Polymer Physics Was Born Invited Speaker: We review the physics of some of de Gennes' principal contributions to fundamental aspects of polymer solution science. These include critical phenomena views on interacting polymers, reptation, polyelectrolytes, and gels. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P3.00002: Polyelectrolyte Solutions Invited Speaker: Pierre-Gilles de Gennes once described polyelectrolytes as the ``least understood form of condensed matter''. In this talk, I will describe the state of the polyelectrolyte field before and after de Gennes' seminal contributions published 1976-1980. De Gennes clearly explained why electrostatic interactions only stretch the polyelectrolyte chains on intermediate scales in semidilute solution (between the electrostatic blob size and the correlation length) and why the scattering function has a peak corresponding to the correlation length (the distance to the next chain). Despite many other ideas being suggested since then, the simple de Gennes scaling picture of polyelectrolyte conformation in solution has stood the test of time. How that model is used today, including consequences for dynamics in polyelectrolyte solutions, and what questions remain, will clarify the importance of de Gennes' ideas. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P3.00003: Block Copolymers Invited Speaker: In 1969 (Rep. Prog. Phys. 32, 187(1969)) de Gennes proposed a simple way to understand and describe mesophase organization of block copolymers in what we now call the strong segregation limit. The following year he showed how self-consistent methods due to Edwards can be applied to understand subtle correlation effects in polymer melts (J. Physique 31, 235(1970)) and established theoretical framework for quantitative interpretation of scattering experiments and of what we call now weak segregation regime of block copolymers. This was the basis of the theory of order-disorder transition in multiblock copolymers that he published in 1980 (Faraday Transactions). In this pioneering work he also discussed the role of blocks' polydispersity. Eventually, as early as in 1977 de Gennes realized that block copolymers ``are most promising systems for deeper understanding of lyophilic phase diagrams'' (in Suppl. Solid State Phys Ed. Ehrenreich 1977) and this vision led not only to some very interesting theories of block copolymer solutions, but also to some very quantitative model of membranes and interfaces elasticity and to a very influential theory of microemulsions (J. Phys. Chem. 86, 2294(1982)). Since 1980 tens of thousands of papers dealing with block copolymers were published, but it should be stressed that in seventies it has been a very unknown area. Clearly de Gennes' visionary interest in block copolymers had, and has, a tremendous impact in the field. I will illustrate using several examples from very different areas how de Gennes' deep understanding of block copolymers and the theoretical methods he introduced influenced both theory and experiments and I will also show how they are relevant for many present and future industrial applications. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P3.00004: Percolation and Gelation Invited Speaker: The sol-Gel transition, as well as the vulcanization of Polymers chains, was described long ago by Flory, Stockmayer, Zimm within mean field approximation. This however had strong limitations because both excluded volume interactions and loops were neglected. An important progress was made when an analogy between percolation and gelation was made by de Gennes and Stauffer. We will discuss some recent experiments showing the relevance of percolation in the description of the sol-gel transition, as well as another important concept also introduced by de Gennes concerning the possibility of observation of classical exponents in the case of vulcanization. We will also consider briefly the influence of diffusion on aggregation properties. Our understanding of dynamical properties close to the sol-gel transition is lower than that of the static ones. Some analogies were given by de Gennes to describe various hydrodynamic limits, but the experimental results still lead to some discussions. Finally, we will mention some generalizations and open questions. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P3.00005: Liquid Crystalline Polymers and Networks -- orientation, molecular shape change, mechanics Invited Speaker: In a prescient paper of 1969, Pierre-Gilles de Gennes envisaged both liquid crystal polymers and elastomers. 10 years later, these systems were realised. After 25 years, monodomain elastomers were prepared and displayed phenomena he had predicted: rods incorporated into polymers induce liquid crystallinity in polymer melts and elastomers; orientational order causes shape changes in the back bones of such polymers; mechanical ramifications follow in networks, e.g. spontaneous elongations and contractions on changing order. The latter are proposed as the basis of micro-actuation and artificial muscles, both heat and light-driven. In 1969, de Gennes already described ideal networks heated through the nematic-isotropic transition losing all their order by mechanical relaxation. It is not obvious, but is true in theory and largely in experiment, even in highly non-ideal networks. He also envisaged that a cholesteric network, where there is a topological memory of chirality imprinted by crosslinking chains in a twisted state. Chirality cannot relax away on entering the isotropic phase, even in systems without molecular chirality (for instance those crosslinked in the presence of chiral solvent that is subsequently exchanged away). His chiral elastomers have found application as mechanically-tuneable, rubber lasers. De Gennes also constructed the first continuum elastic theories of nematic elastomers (1982), though distortions are generally very large. His elasticity has informed non-linear elasticity that works even at large amplitudes. I shall describe de Gennes' many contributions, and the current state of a field that has since yielded still more remarkable phenomena. [Preview Abstract] |
Session P4: Fluctuations and Rare Events in Physical, Chemical, and Biological Systems
Sponsoring Units: GSNPChair: Mark Dykman, Michigan State University
Room: Morial Convention Center 206
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P4.00001: Rare events and phase transitions in reaction diffusion systems Invited Speaker: I shall discuss a way to evaluate tails of the probability distribution functions in stochastic reaction-diffusion models. The method is based on the semi-classical treatment of a proper ``quantum'' field theory, which may be associated with reaction-diffusion systems. The same set of ideas may be applied to a classification of non-equilibrium phase transitions, taking place in these models. \newline \newline V. Elgart and A. Kamenev, Classification of phase transitions in reaction--diffusion systems, Phys. Rev. E 74, 041101 (2006). \newline V. Elgart and A. Kamenev, Rare Events Statistics in Reaction--Diffusion Systems, Phys. Rev. E. 70, 041106 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P4.00002: Switching and phase transitions in a parametrically-excited cold atom trap. Invited Speaker: Stochastic dynamics of cold atoms in a modulated magneto-optical atom trap was investigated. The studies focused on the phenomena related to switching between the parametrically excited period-2 states. The rates of single-atom activated transitions were analyzed. When the atom density was increased, there were observed Ising-class phase transitions where the symmetric population of period-2 states was spontaneously broken [1,2]. Anomalous fluctuations in the decay of the unstable state were investigated [3]. \newline \newline [1] Kihwan Kim, Myoung-Sun Heo, Ki-Hwan Lee, Kiyoub Jang, Heung-Ryoul Noh, Doochul Kim, and Wonho Jhe, Phys. Rev. Lett. \textbf{97}, 036104 (2006). \newline [2] Kihwan Kim,. Heung-Ryoul Noh, and Wonho Jhe, Phys. Rev. A, \textbf{71}, 033413 (2005). \newline [3] ``Transient and fluctuation behavior of atomic population at unstable state in parametrically driven magneto-optical trap,'' Myoung-Sun Heo, Yonghee Kim, Heung-Ryoul Noh, Mark Dykman and Wonho Jhe, in preparation. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P4.00003: Activation barrier scaling and switching path distribution in a micromechanical parametric oscillator Invited Speaker: Parametrically modulated systems develop multiple coexisting states under sufficiently large drive. In the presence of fluctuations, the system can occasionally overcome the activation barrier and switch from one state to the other, resulting in the phase of oscillation slipping by $\pi$. We study noise-induced switching in a parametrically-driven micromechanical torsional oscillator. Certain properties of the switching are generic to bistable systems, while others are specific to nonequilibrium systems that lack detailed balance. For instance, close to the bifurcation points, the activation barrier for switching is expected to display system-independent scaling. By measuring the rate of random transitions at different fluctuation intensities, we deduce the activation barrier as a function of frequency detuning from the bifurcation points and measure a critical exponent that is in good agreement with theoretical predictions. We also measure the escape trajectories followed by the oscillator, confirming the notion that they form narrow tubes in phase space centered at the most probable escape path. The uphill section of this path is found to be distinct from its time-reversed downhill section, an important property for systems far from thermal equilibrium. Near the saddle point the velocity is significantly diminished and the motion becomes diffusive, leading to strong broadening and increase in height of the probability distribution. Apart from fundamental interest, the sharp change in the oscillation amplitude near the subcritical bifurcation point can provide accurate determination of device parameters. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P4.00004: Spectral theory of extreme statistics in birth-death systems Invited Speaker: Statistics of rare events, or large deviations, in chemical reactions and systems of birth-death type have attracted a great deal of interest in many areas of science including cell biochemistry, astrochemistry, epidemiology, population biology, \textit{etc.} Large deviations become of vital importance when discrete (non-continuum) nature of a population of ``particles'' (molecules, bacteria, cells, animals or even humans) and stochastic character of interactions can drive the population to extinction. I will briefly review the novel \textit{spectral method} [1-3] for calculating the extreme statistics of a broad class of birth-death processes and reactions involving a single species. The spectral method combines the probability generating function formalism with the Sturm-Liouville theory of linear differential operators. It involves a controlled perturbative treatment based on a natural large parameter of the problem: the average number of particles/individuals in a stationary or metastable state. For extinction (the first passage) problems the method yields accurate results for the extinction statistics and for the quasi-stationary probability distribution, including the tails, of metastable states. I will demonstrate the power of the method on the example of a branching and annihilation reaction, $A \to\hspace{-2.8mm}\hspace{2mm}2A\,,\,2A \to\hspace{-2.8mm}\hspace{2mm} \emptyset$, representative of a rather general class of processes. \begin{enumerate} \item{M. Assaf and B. Meerson, Phys. Rev. Lett. \textbf{97}, 200602 (2006).} \item{M. Assaf and B. Meerson, Phys. Rev. E \textbf{74}, 041115 (2006).} \item{M. Assaf and B. Meerson, Phys. Rev. E \textbf{75}, 031122 (2007).} \end{enumerate} [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P4.00005: Strong Fluctuations and Cycling in Biological Systems Invited Speaker: In this talk I describe a mechanism for generating cycles in a large class of ``mesoscale'' biological populations (meaning populations composed of thousands to tens of thousands of units). Cycles are caused by a resonant amplification of the system dynamics triggered by internal noise. I will discuss this mechanism in the context of two classes of simple systems: ecological (e.g. predator-prey, host-pathogen) and biochemical (e.g. small gene regulation networks, modules of metabolic processes). [Predator-Prey Cycles from Resonant Amplification of Demographic Stochasticity, A. J. McKane and T. J. Newman, Physical Review Letters 94, 218102 (2005); Amplified Biochemical Oscillations in Cellular Systems, A. J. McKane, J. Nagy, T. J. Newman, and M. Stefanini, Journal of Statistical Physics 128, 165:191 (2007).] [Preview Abstract] |
Session P5: Quantum Information Meets Gravitation
Sponsoring Units: GQIChair: Matthew Leifer, Institute for Quantum Computing
Room: Morial Convention Center RO1
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P5.00001: Quantum Computational Complexity in the Presence of Closed Timelike Curves Invited Speaker: What are the consequences of modifying the laws of physics for the theory of computation? Considering this question in the context of quantum theory has led to a seemingly new class of computing devices known as quantum computers. In this talk I will discuss how modifying computation (in a quantum or classical context) to allow for closed timelike curves leads to a new model of computation. In particular I will discuss how such a model of computation with closed timlike curves can be formulated, whether it can be made robust to noise, and how recent results of Aaronson and Watrous show that this model is nothing more than the well studied complexity class PSPACE. Consequences of these results on foundational issues in quantum theory will also be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P5.00002: Black holes as mirrors: quantum information in random subsystems Invited Speaker: We study information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing, and assuming that the retriever has unlimited control over the emitted Hawking radiation. If the evaporation of the black hole has already proceeded past the ``half-way'' point, where half of the initial entropy has been radiated away, then additional quantum information deposited in the black hole is revealed in the Hawking radiation very rapidly. Information deposited prior to the half-way point remains concealed until the half-way point, and then emerges quickly. These conclusions hold because typical local quantum circuits are efficient encoders for quantum error-correcting codes that nearly achieve the capacity of the quantum erasure channel. Our estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis. This is joint work with Patrick Hayden. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P5.00003: Entanglement in non-inertial frames and curved spacetime Invited Speaker: The insight that the world is fundamentally quantum mechanical inspired the development of quantum information theory. However, the world is not only quantum but also relativistic, and indeed many implementations of quantum information tasks involve truly relativistic systems. In this talk I consider relativistic effects on entanglement in flat and curved spacetimes. I will emphasize the qualitative differences to a non-relativistic treatment, and demonstrate that a thorough understanding of quantum information theory requires taking relativity into account. The exploitation of such relativistic effects will likely play an increasing role in the future development of quantum information theory. The relevance of these results extends beyond pure quantum information theory, and applications to foundational questions in cosmology and black hole physics will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P5.00004: Spin-induced non-geodesic motion, Wigner rotation and EPR correlations of massive spin-1/2 particles in a gravitational field Invited Speaker: We investigate in a covariant manner, the spin-induced non-geodesic motion of massive spin-1/2 particles in an arbitrary gravitational field for trajectories that are initially geodesic when spin is ignored. Using the WKB approximation for the wave function in an arbitrary curved spacetime, we compute the $O\left( \hbar \right)$ correction to the Wigner rotation of the spin-1/2 particle, whose$O\left( 1 \right)$ contribution is zero on timelike geodesics. We consider specific examples in the Schwarzschild metric for motions in the equatorial plane for (i) particles falling in from spatial infinity with non-zero angular momentum and (ii) circular geodesic orbits. For the latter case we consider the Bell inequalities for a perfectly anti-correlated EPR entangled pair of spins as the separate qubits traverse the circular geodesic in opposite directions. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P5.00005: Quantum Graphity: a model of emergent locality Invited Speaker: Quantum graphity is a background independent condensed matter model for emergent locality, spatial geometry and matter in quantum gravity. The states of the system are given by bosonic degrees of freedom on a dynamical graph on N vertices (that is, changing in time). At high energy, the graph is the complete graph on N vertices and the physics is invariant under the full symmetric group acting on the vertices and highly non-local. We find evidence that the model has a phase, in which the ground state breaks the permutation symmetry to translations and discrete rotations. In this phase the system is ordered, low-dimensional and local. Consideration of the free energy associated with the dominant terms in the dynamics shows that the ground state is thermodynamically stable under local perturbations. The model gives rise to an emergent U(1) gauge theory in the ground state by the string-net condensation mechanism of Levin and Wen. We will reformulate the model in graph-theoretic terms and compare its dynamics to some common graph processes. [Preview Abstract] |
Session P6: Fluid Dynamics and Biology
Sponsoring Units: DFDChair: Silas Alben, Georgia Institute of Technology
Room: Morial Convention Center RO4
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P6.00001: Depolymerization-driven flow and the crawling of nematode sperm Invited Speaker: Cell crawling motility is integral in many biological and biomedical processes, such as wound healing, cancer metastasis, and morphogenesis. A complete understanding of the mechanisms by which cells crawl is still lacking, but it is known to entail at least three separate physical processes: (i) cytoskeletal extension at the front of the cell; (ii) adhesion to the substrate at the cell front and release at the rear; and (iii) advance of the cell body. In most cells, the cytoskeletal network is composed of actin. The mechanism by which force is generated to drive translocation of the cell body is still debated. Originally, this force was attributed to an actomyosin system similar to muscle. However, nematode sperm utilize a cytoskeleton composed of a network of Major Sperm Protein (MSP) that forms non-polar filaments for which molecular motors have not been identified. The motility of these cells still exhibits all three fundamental processes required for standard crawling motility. Experiments suggest that depolymerization of the cytoskeletal network is the force-producing mechanism for pulling up the rear. In this talk I will present a mechanical model that describes how depolymerization of the cytoskeleton can drive motility. This model accounts for both cytoskeletal displacements and cytsolic (the fluid component of the cell) flow. The model accurately fits in vitro data using nematode sperm extracts where depolymerization induces contraction of MSP polymer bundles. Application of this model to cell crawling produces testable predictions about how the size and shape of a cell affect crawling speed. Experiments using {\it Caenorhabditis elegans} sperm show good agreement with the model predictions. Interestingly, the model requires that cells are anisotropically elastic, being more stiff in the direction of motion than perpendicular to it. A simple physical picture can account for this anisotropy. The model also predicts that cell speed increases with anisotropy and with depolymerization rate. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P6.00002: Optimizing Low Reynolds Number Locomotion Invited Speaker: In this talk I will discuss optimal stroke patterns for low Reynolds number linked swimmers. We begin by optimizing stroke patterns for Purcell's 3-link swimmer modeled as a jointed chain of three slender links moving in an inertialess flow. The swimmer is optimized for efficiency and speed and we are able to attain significant increases is efficiency over those previously suggested by authors who only consider geometric design rather than kinematic criteria. We then go on to investigate uniflagellate and biflagellate organisms and compare the optimized results to biological data from spermatozoa and chlamydomonas. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P6.00003: Instabilities and pattern formation in active particle suspensions Invited Speaker: Suspensions of swimming microorganisms are characterized by complex dynamics involving strong fluctuations and large-scale correlated motions. These motions, which result from the many-body interactions between particles, are biologically relevant as they impact mean particle transport, mixing and diffusion, with possible consequences for nutrient uptake. Using direct numerical simulations, I first investigate aspects of the dynamics and microstructure in suspensions of interacting self-propelled rods at low Reynolds number. A detailed model is developed that accounts for hydrodynamic interactions based on slender-body theory. It is first shown that aligned suspensions of swimming particles are unstable as a result of hydrodynamic fluctuations. In spite of this instability, a local nematic order persists in the suspensions over short length scales and has a significant impact on the mean swimming speed. Consequences of the large-scale orientational disorder for particle dispersion are discussed and explained in the context of generalized Taylor dispersion theory. Dynamics in thin liquid films are also presented, and are characterized by a strong particle migration towards the interfaces. The results from direct numerical simulations are then complemented by a kinetic model, in which the dynamics are captured using a continuity equation for the particle configurations, coupled to a mean-field description of the flow arising from the active stress exerted by the particles on the fluid. Based on this model, the linear stability of both aligned and isotropic suspensions is revisited. In aligned suspensions, the instability observed in the simulations is predicted to occur at all wavelengths, a result that generalizes previous predictions by Ramaswamy et al. (2002). In isotropic suspensions, an instability for the active particle stress is also found to exist, in which shear stresses are eigenmodes and grow exponentially at long scales. Non-linear effects are also investigated using numerical simulations in two-dimensions. The results of the stability analysis are confirmed, and the long-time non-linear behavior is shown to be characterized by strong density fluctuations, which appear to be driven by the active stress instability. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P6.00004: Life in a drop of Ocean: microfluidic insights into microbial ecology Invited Speaker: Bacteria are the most abundant and successful form of life on Earth. Their physico-chemical interactions with their fluid environment are surprisingly complex and have enormous implications, which we can only hope to grasp if we learn to study microorganisms within realistic microenvironments. Microfluidics for the first time enables us to create microhabitats, including chemical and fluid mechanical landscapes, while visualizing bacterial behavior at a single-cell resolution. Here I focus on the application of microfluidics to gain insight in the life of marine bacteria. In their quest for nturients, marine bacteria often experience the Ocean as a desert, where rare and ephemeral nutrient patches represent transient resource oases. In this patchy seascape, swimming and chemotaxis represent critical assets, but effective patch utilization is constrained by energetic requirements. And then there are predators and viruses... These interactions form the basis of the 'microbial loop', the ensemble of microbial processes known to directly impact the productivity of marine ecosystems and the rates of carbon turnover in the Ocean. I will show how fundamental new insight on selected aspects of microbial life in a drop of Ocean can be achieved by a combination of microfluidic experiments and theoretical modeling. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P6.00005: Optimal flexibility in flapping appendages Invited Speaker: When oscillated in a fluid, appendages such as insect wings and fish fins can produce large thrust forces while undergoing considerable bending. Can we understand these bending patterns by comparing them with the patterns which produce maximum thrust, or a given thrust at maximum efficiency? We present a general model for how flexible surfaces produce vorticity and bend actively and passively in a fluid. We solve the model numerically, and discuss results for moderate deflections (relevant for large thrust), and for small deflections (relevant for high efficiency). We'll then consider how a fish-fin-like structure might be designed for optimal performance. [Preview Abstract] |
Session P7: Optical Lattices
Sponsoring Units: DAMOPChair: Charles Clark, National Institute of Standards and Technology, Gaithersburg
Room: Morial Convention Center RO5
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P7.00001: Repulsively bound atom pairs in an optical lattice. Invited Speaker: Three dimensional optical lattices represent an interesting environment for fundamental research with ultracold atoms. We have observed a novel kind of stable bound state of two atoms which is based on repulsion rather than attraction between the particles [1]. We will explain how these lattice-induced repulsively bound atom pairs come about and discuss their interesting properties. Ensembles of repulsively bound pairs are described by a Bose-Hubbard model and can be used to study strongly correlated condensed matter physics. [1] K. Winkler et al., Nature 441, 853 (2006).~ [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P7.00002: Critical velocity for superfluid flow across the BEC-BCS crossover Invited Speaker: Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is most robust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of dissipation when the velocity of a moving one-dimensional optical lattice was varied. The dependence of the critical velocity on lattice depth and on the inhomogeneous density profile was studied. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P7.00003: Towards Quantum Magnetism with Ultracold Quantum Gases in Optical Lattices Invited Speaker: Quantum mechanical superexchange interactions form the basis of quantum magnetism in strongly correlated electronic media and are believed to play a major role in high-Tc superconducting materials. We report on the first direct measurement of such superexchange interactions with ultracold atoms in optical lattices. After preparing a spin-mixture of ultracold atoms with the help of optical superlattices in an antiferromagnetically ordered state, we are able to observe a coherent superexchange mediated spin dynamics down to coupling energies as low as 5 Hz. Furthermore, it is shown how these superexchange interactions can be fully controlled in magnitude and sign. The prospects of using such superexchange interactions for the investigation of dynamical behaviour in quantum spin systems and for quantum information processing will be outlined in the talk. In addition we present results on the dynamical resolved co-tunnelling of repulsively bound atom pairs in optical superlattices and show how by using ``Coulomb-blockade'' type tunnelling resonance one can count atoms one by one to determine their number statistics in the lattice potential. Finally, latest results on ultracold Fermions and Bose-Fermi mixtures in optical lattices will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P7.00004: Probing and controlling quantum magnetism with ultra-cold atoms Invited Speaker: By loading spinor atoms in optical lattices it is now possible to experimentally implement quantum spin models. These systems allow the investigation of quantum magnetism ~ in strongly correlated systems within a controlled environment. In this talk we will describe a novel approach~to prepare, detect and control super-exchange interactions in ultra-cold~spinor atoms in optical superlattices [1]. Recently this approach was used for the first experimental realization of super-exchange interactions in ultra-cold atoms [2]. The many-body dynamics arising from the coherent coupling between singlet-triplet pairs in adjacent double-wells will be also discussed. ~In particular, we will describe how it can lead to the formation of frustrated spin states with a high degree of multi-particle entanglement. Finally, we will present an extension of this approach to prepare and detect d-wave pairing in an array of coupled plaquettes. \newline ~[1] A. M. Rey, V. Gritsev,I. Bloch, E. Demler, and M. D. Lukin, PRL 99, 140601 (2007). \newline ~[2] S. Trotzky, P. Cheinet, S. Folling, M. Feld, U. Schnorrberger, A.M. Rey, A. Polkovnikov, E. Demler, M. D. Lukin, and I. Bloch., submitted for publication. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P7.00005: Heteronuclear Molecules in a 3D Optical Lattice Invited Speaker: The production of ultracold polar molecular samples is a long standing goal of AMO and ultracold physics. The interest is motivated by novel quantum gases with anisotropic interactions between these polar molecules, by applications in quantum computation and simulation which benefit from the long-range interaction as well as by perspectives of measuring a T-violating permanent electric dipole moment of the electron in such a polar molecular system. While a lot of work has been done on direct cooling of polar molecules, a second proposed route to the production of ultracold polar molecules starts with ultracold atomic samples, where tremendous progress has been made in recent years, and assembles an ultracold atom mixture into weakly bound heteronuclear Feshbach molecules. These could then be transferred into the absolute internal molecular ground state using coherent Raman schemes. As a crucial step in this approach, we report on the first production of ultracold long-lived heteronuclear molecules. The molecules are associated from a quantum degenerate mixture of fermionic $^{40}$K and bosonic $^{87}$Rb atoms loaded into a 3D optical lattice. The binding energy of the heteronuclear molecules is precisely determined by rf spectroscopy and compared to a theoretical model based on a pseudopotential approach. We also characterize both the lifetime of the sample and the efficiency of rf association; comparison to the pseudopotential model results in excellent agreement. [Preview Abstract] |
Session P8: Glassy Dynamics
Sponsoring Units: DFDChair: Itai Cohen, Cornell University
Room: Morial Convention Center RO6
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P8.00001: Nonlinear Dynamics Near the Jamming Transition Edward J Banigan, David A Egolf How dynamical behaviors and static measures are related near the jamming transition remains an open question. In simulations of a two-dimensional sheared granular cell, we have calculated mathematical quantities that characterize the underlying nonlinear dynamics near the jamming transition. We find that the Lyapunov exponents and vectors characterizing the most important dynamical modes correlate well in space and time to localized events that alter the physical characteristics of the system. For example, the Lyapunov exponents and vectors highlight areas in which particles are involved in cooperative rearrangement or the formation or destruction of stress chains. In at least some cases, the behavior of the dynamical quantities appears to indicate future position or stress rearrangements. In addition, we report measurements of a dynamical time scale and a dynamical length scale that diverge as the system jams, suggesting an intriguing connection between the jamming transition and a transition between chaotic and non-chaotic dynamical states. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P8.00002: Probing Cooperative Motion in Super-Cooled Colloidal Suspensions Prasad Sarangapani, Y. Elaine Zhu The physics of the glass transition remains inadequately understood despite its broad technological relevance. The anomalous divergence of viscosity without apparent structural change as a liquid is cooled has been attributed to the existence of growing dynamic length scales of ``cooperatively rearranging regions'' (CRR). In this work, we use ultra-fast fluorescence correlation spectroscopy (FCS) combined with high-speed imaging to determine the CRR sizes by measuring single-particle dynamics of tracer nano-particle embedded in super-cooled ``hard-sphere'' colloidal suspensions. Fluorescent poly-(methyl methacrylate) (PMMA) tracer particles of radii ranging from $r=$0.1-0.4 $\mu $m, mixed with plain PMMA particles of radius, $r=$0.6 $\mu $m and bulk volume fraction, \textit{$\phi $ =} 0.38-0.58, serve as excellent probes for changes in the energy barrier landscape of the suspensions of increasing volume fraction and are sensitive to the creation and annihilation of icosahedral order in metastable colloidal fluids. We also find that the correlation length, determined by fluctuation-dissipation relations from the measured auto-correlation functions, shows a dramatic increase in the super-cooled regime until it diverges at \textit{$\phi $}=0.58. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P8.00003: Structural relaxation in sheared two-dimensional foams Matthias Mobius, Gijs Katgert, Martin van Hecke Athermal and disordered systems at rest, such as foams and granular media, are stuck in a meta-stable configuration. Upon shear the system unjams and complex vortex-like rearrangements ensue that are correlated in time and space. In our experiment we investigate what the typical time scales of these structural relaxations are as a function of the local shear rate in a two-dimensional, disordered foam that is linearly sheared. After an initial super-diffusive regime, the bubbles become diffusive at later times. This transition is reflected in the statistics of the bubble displacements, which are initially strongly correlated and non-Gaussian but eventually become Gaussian. We find that the relaxation time decreases with shear rate. For large shear rates the dependence follows a power law with an exponent significantly different from -1. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P8.00004: From spontaneous to induced dynamic fluctuations: Granular packings as an experimental probe Frederic Lechenault, Olivier Dauchot, Giulio Biroli, Jean-Philippe Bouchaud We track the motion of a horizontally vibrated amorphous assembly of bidisperse hard disks, for densities ranging across the jamming transition. The spatial extension of dynamical heterogeneities and the associated relaxation time are found to exhibit critical behavior. Moreover, a dynamical fluctuation inequality relating the dynamical susceptibility $\chi_4$ and the response of the dynamics to a change in density is tested. As the diffusion length is found to rescale these quantities, the dependencies of the inequality on length and time scales as well as density can be evaluated independently. Surprisingly, the lower bound is found to reproduce the non-monotonic behavior of $\chi_4$ in time, which reveals an intimate link between dynamical heterogeneity and marginal super diffusion. Finally, the bound is shown to be tight and to mimic the anomalous features of the dynamical susceptibility across the transition. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P8.00005: Influence of the microstructure on jammed packings of spheres Eric Corwin, Maxime Clusel, Alexander Siemens, Jasna Brujic Jammed matter is by definition impenetrable to light, such that
little is known about the geometry of jammed systems. Using
confocal microscopy to image an emulsion in 3D, we use the
enhanced fluorescence at the droplet contacts to determine the
contact network inside this model frictionless system. This
enables the experimental determination of the average
coordination number $ |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P8.00006: Mode-coupling and generalized mode-coupling theory: a diagrammatic approach Grzegorz Szamel We present a diagrammatic approach to the dynamics of interacting Brownian particles. Within this approach, the time-dependent density correlation function is represented by a series of diagrams with three and four leg vertices. We analyze the structure of this series and obtain a diagrammatic interpretation of reducible and irreducible memory functions. The one-loop self-consistent approximation for the latter function coincides with mode-coupling approximation for Brownian systems that was derived previously using a projection operator approach. Finally, we investigate the diagrammatic interpretation of a generalized mode-coupling theory. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P8.00007: Percolating Clusters in Systems of Gapped Rigid Rings. Christopher LaSota, Ariel Helfer We have examined the behavior of kinetically agitated collections of rigid rings with angular gaps in them. For small gap angles, large clusters form readily and are sufficiently tangled so that they may be raised vertically under gravitational stresses without decomposing. Using gravity as a stressor under semi-static conditions, we have measured average cluster size as a function of the gap angle and witness what appears to be a second order percolation phase transition. The critical gap angle depends somewhat on the relative thickness of the ring material compared to the ring diameter. Although friction is necessary for the formation of clusters, it appears that cluster formation is dominated by geometry effects. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P8.00008: Jamming transition in a temperature-sensitive 2D colloidal suspension Zexin Zhang, Daniel T. N. Chen, Arjun G. Yodh, Kevin B. Aptowicz, Piotr Habdas We experimentally investigate the jamming transition of a 2D colloidal system. The system consists of a bidisperse mixture of thermoresponsive microgel particles confined between two glass slides, with a thickness of roughly the diameter of the larger particle. The packing density of the system is tuned by changing the temperature. A range of packing densities, both below and above the jamming transition is studied. We use video microscopy and particle tracking techniques to characterize the motion of the particles. On approaching the jamming transition the motion becomes slower and more heterogeneous. We characterize the jamming transition in terms of both structure (pair correlation function) and dynamics (mean square displacement, non-Gaussian parameter, four-point susceptibility). To our knowledge this study provides the first experimental evidence for the jamming transition in a 2D colloidal system. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P8.00009: Exact Enumeration of Jammed States for Confined Hard Discs S.S. Ashwin, Richard K. Bowles Enumeration of jammed states of particle systems interacting with hard potentials such as hard discs and hard spheres is a long- standing problem which holds the key to understanding the nature of glassy dynamics and the question of the possibility of an ideal glass transition in these systems. A simple model consisting of hard discs (of diameter $\sigma$) trapped between two hard lines separated by a distance $H$ exhibits slow relaxation and heterogeneous dynamics characteristic of glassy systems. We map the locally jammed structures in this model to tiles and pose the problem of enumeration of jammed states for the case $H<2\sigma$, as a tiling problem on a subset of a plane. Further on applying constraints for collective jamming on the arrangement of the tiles, we exactly enumerate the entire jamming landscape of the system and explore how this landscape is connected to the thermodynamics and dynamics of the glassy system. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P8.00010: Heterogeneities in granular dynamics Anita Mehta The absence of Brownian motion in granular media is a source of much complexity; among these is the presence of heterogeneity, whether static or dynamic, within a given system. Such strong heterogeneities can exist as a function of depth in a box of grains; this is the system we study here. We present results from three-dimensional, cooperative and stochastic Monte Carlo shaking simulations of spheres on heterogeneous density fluctuations. These are justaposed with results obtained from a theoretical model of a column of grains under gravity; frustrations via competing local fields is included in our model, while the effect of gravity is to slow down the dynamics of successively deeper layers. The combined conclusions suggest that the dynamics of a real granular column can be divided into different phases -- {\it ballistic}, {\it logarithmic}, {\it activated} and {\it glassy} -- as a function of depth. The nature of the ground states and their retrieval, in the glassy phase, shows clear evidence of {\it intrinsic} states, which lie below a band of approximately degenerate ground states. In the other three phases, by contrast, the system jams into a state chosen randomly from this upper band of metastable states. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P8.00011: Confocal Microscopy of Shear-Induced Dynamics in Jammed Emulsions Joaquim Clara-Rahola, Eric R. Weeks Emulsions are liquid droplets suspended in a second continuous fluid. We study polydisperse decane-in-water emulsions at droplet volume fractions of about 0.8. At such concentrations emulsions are jammed and the system exhibits the properties of a solid. Droplet rearrangements due to Brownian motion are limited in this jammed material. Thus, to induce droplet displacements at length scales above a particle diameter, an oscillatory strain is applied. We use confocal microscopy to track the trajectories of the droplets in real time and space. By taking advantage of this technique we quantify the affine and non-affine motion of the droplets due to the shear. Moreover, we study elastic and plastic droplet reconfigurations as well as the spatial extent of the rearrangements when the droplet volume fraction and polydispersity are varied. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P8.00012: Vacancy diffusion in a triangular lattice model M. Jeng, M. Bowick, W. Krauth, J. M. Schwarz, X. Xing We study vacancy diffusion in the classical triangular lattice dimer model, subject to the kinetic constraint that dimers can only translate, but not rotate. A single vacancy---i.e. a monomer---in an otherwise fully packed lattice, is always localized in a tree-like structure. The distribution of tree sizes is asymptotically exponential and has an average of $8.16\pm 0.01$. A connected pair of monomers has a finite probability of being delocalized. When delocalized, the diffusion is anomalous: $\langle \vec{r}^2 \rangle \propto t^{\beta}$, with $\beta=0.46\pm 0.05$. The same diffusion law is also exhibited by clusters of three or four monomers. It is found that both swap motions (translations of dimers transverse to their axes) and glide motions (translations of dimers parallel to their axes) are essential for the large-scale diffusion of monomers. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P8.00013: Studying microscopic rearrangements in a sheared supercooled colloidal liquid Dandan Chen, Denis Semwogerere, Joaquim Clara-Rahola, Eric R. Weeks Shearing induces complex micro-structure changes inside an amorphous material, which is related to interesting phenomena like shear thicking and shear thinning. We use a colloidal suspension to simulate amorphous materials, and we study how shearing changes this structure using fast confocal microscopy. Many experiments and simulations have found macro-stress flucatuations in sheared dense jammed suspensions. However, the micro-rearrangements of the particles while being sheared are not very clear. We study the non-affine motion of the colloids, finding the particles move in groups, and characterize these groups for different shearing rates. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P8.00014: Temperature control of attractive interactions in colloids Peter Schall, Zhibing Hu Attractive colloidal systems have attracted increasing interest recently: They exhibit phase behavior with solid, liquid, and gas phases, and various metastable states, ranging from gel-like to glassy. These colloidal systems offer a convenient way to investigate important phenomena such as phase formation and kinetic arrest. The most prominent colloidal systems are colloid-polymer mixtures, in which the attractive strength is fixed by the concentration of the added polymer. We present a colloidal system that allows variation of the attractive potential with external control: a binary liquid solvent gives rise to temperature-dependant particle attractions close to the demixing temperature of the liquid mixture. This allows us to use temperature control to induce transitions from gas to liquid to solid, or to form metastable gel-like or glassy states. Variation of the heating rate allows us to investigate the kinetics of these transitions. In this talk, I will focus on a novel system, in which close index- and density matching of the solvent and the particles is possible; this enables us to study bulk processes with temperature control. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P8.00015: A thermodynamic equation of jamming Kevin Lu, H. Pirouz Kavehpour Materials ranging from sand to fire-retardant to toothpaste are considered fragile, able to exhibit both solid and fluid-like properties across the jamming transition. Guided by granular flow experiments, our equation of jammed states is path-dependent, definable at different athermal equilibrium states. The non-equilibrium thermodynamics based on a structural temperature incorporate physical ageing to address the non-exponential, non-Arrhenious relaxation of granular flows. In short, jamming is simply viewed as a thermodynamic transition that occurs to preserve a positive configurational entropy above absolute zero. Without any free parameters, the proposed equation-of-state governs the mechanism of shear-banding and the associated features of shear-softening and thickness-invariance. [Preview Abstract] |
Session P9: Microfluidic and Nanofluidic Devices
Sponsoring Units: DFDChair: Patrick Tabeling, City of Paris Industrial Physics and Chemistry Higher Educational Institution
Room: Morial Convention Center RO7
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P9.00001: Microfluidic Fabrication of Bio-compatible Vesicles by Self-assembly in Double Emulsions Ho Cheung Shum, Jinwoong Kim, Daeyeon Lee, David Weitz Vesicles are compartments surrounded by bilayered membranes of amphiphilic molecules such as diblock copolymers and phospholipids. To minimize the exposure of their hydrophobic part to water, amphiphilic molecules self-assemble into aggregates of different structures. When the hydrophobic to hydrophilic ratio is close to unity, amphiphiles self assemble into bilayers, which tend to fold themselves into vesicles. These vesicles are useful for encapsulating and transporting actives such as drugs, flavor, and fragrance. To solve the problems of low encapsulation efficiency and large vesicle size distributions afforded by traditional techniques to create vesicles, we engineer a novel route to generate vesicles using monodisperse double emulsions prepared in microfluidics as templates. The double emulsion-to-vesicle transition exhibits different behaviors depending on the properties of the amphiphilic molecules such as the hydrophobic-to-hydrophilic ratio. Using this technique, we have fabricated both bio-compatible diblock copolymer vesicles, also known as polymersomes, and also lipid vesicles with high encapsulation efficiency. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P9.00002: Glass Coating for PDMS Microfluidic Channels by Sol-Gel Methods Thao Do, Adam Abate, David Weitz Soft lithography in polydimethylsiloxane (PDMS) allows one to fabricate complex microfluidic devices easily and at low cost. However, PDMS swells in the presence of many organic solvents, which can significantly degrade the performance of PDMS microfluidic devices. We present a method to coat PDMS channels with a glass-like layer using sol-gel chemistry. As a demonstration of chemical resistance, we flow toluene and aqueous Rhodamine B through coated PDMS channels. Toluene is an organic solvent that significantly swells PDMS in a matter of seconds. Rhodamine B is an organic fluorescent molecule that leaches into PDMS and can therefore be used as a fluorescent probe. Indeed, the coating suppresses swelling of the channels when exposed to toluene; it also prevents leaching of Rhodamine B into PDMS channels. In addition, the channels can be functionalized with silanes to precisely control surface properties. We exploit the high chemical resistance and precise surface functionalization of the coating to produce both direct toluene-in-water and inverted water-in-toluene emulsions in coated, functionalized, PDMS microfluidic channels. This combines the ease of fabrication afforded by soft-lithography with the precision control afforded by sol-gel glass. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P9.00003: Fluid Flow and Heat Transfer in a Dual-wet Micro Heat Pipe Jin Zhang, Stephen Watson, Harris Wong Micro heat pipes have been used to cool micro electronic devices, but their heat transfer coefficients are low compared with those of conventional heat pipes. In this talk, a dual-wet pipe is proposed as a model to study heat transfer in micro heat pipes. The dual-wet pipe has a long and narrow cavity. The bottom-half of the horizontal pipe is made of a wetting material and holds a wetting liquid, whereas the top-half is made of a non-wetting material and is filled with the vapor. As one end of the pipe is heated, the liquid evaporates and increases the vapor pressure. The higher pressure drives the vapor to the cold end where the vapor condenses and releases the latent heat. The condensate moves along the bottom half of the pipe back to the hot end to complete the cycle. Hence, the heat pipe is driven by the difference in equilibrium vapor pressure between the hot and cold ends, and not by the liquid-vapor interfacial curvature as is commonly believed. Our analysis provides an explanation for the comparatively low effective thermal conductivity in micro heat pipes [1]. \newline [1] Zhang, Watson {\&} Wong, J. Fluid Mech. \textbf{589}, 1 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P9.00004: Measuring velocity profiles and nanoparticle interactions between 20 and 300 nm from surfaces Patrick Tabeling, Cedric Bouzigues The observation of flows at a nanometric scale is crucial for understanding phenomena involving interactions between liquids and solid surfaces, such as slippage and electro-osmosis. Here we report a new method based on nanoparticle imaging by total internal reflection fluorescence, allowing the first observation of water flows between 20 and 300 nm from surfaces. We probed the energy landscape, leading to first local measurements of the Debye length and surface/nanoparticle interactions; and provide an unambiguous determination with 10~nm accuracy of the slip length for different surfaces - wetting, non-wetting, hard, soft. These results represent an improvement of one order of magnitude compared to the state of the art. In addition to investigating locally energetic and electrostatic properties of the wall/liquid system, this Letter lays down the foundations of a technique that can foster the development of nanofluidics: Imaging of Nanoparticles for Energy landscape and Speed flow measurements (INES). [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P9.00005: Poisson-Nernst-Planck model of ion current rectification through a nanofluidic diode Dragos Constantin, Zuzanna Siwy We have investigated ion current rectification properties of a recently prepared bipolar nanofluidic diode. This device is based on a single conically shaped nanopore in a polymer film whose pore walls contain a sharp boundary between positively and negatively charged regions. A semiquantitative model that employs Poisson and Nernst-Planck equations predicts current-voltage curves as well as ionic concentrations and electric potential distributions in this system. We show that under certain conditions the rectification degree, defined as a ratio of currents recorded at the same voltage but opposite polarities, can reach values of over 1000 at a voltage range (-2V, +2V). The role of thickness and position of the transition zone on the ion current rectification is discussed as well. We also show that the rectification degree scales with the applied voltage. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P9.00006: Non-reflecting boundary conditions for fluctuating hydrodynamics of compressible fluids Rafael Delgado-Buscalioni, Anne Dejoan Many important phenomena in microfluidics involve propagation of fast sound waves. Computational modeling of such problems requires a way to evacuate the reflected waves out of the computational box. However, a way to construct open boundary conditions for Fluctuating Hydrodynamics (FH) is lacking in the literature. This work presents open boundary conditions for fluctuating hydrodynamics solvers based on the Navier-Stokes Landau-Lifshitz equations. The objectives are i) ensure robust non-reflecting boundary conditions and ii) keep thermodynamic consistency for total mass fluctuation, i.e. agreement with the grand canonical ensemble. We show that by ensuring the fluctuation-dissipation balance for the total mass, one also gets the correct equilibrium power spectra of local mass and momentum at each point of the computational box. We consider real compressible fluids (argon and water) under isothermal condition and present results for the equilibrium and several out-of-equilibrium states involving generation of sound waves. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P9.00007: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P9.00008: Liquid precursor films spreading on chemically patterned substrates Antonio Checco We study the spreading of nonvolatile liquid squalane on chemically patterned nanostripes by using non-contact Atomic Force Microscopy (NC-AFM). The substrates are octadecylthrichlorosilane(OTS)-coated silicon wafers chemically patterned on multiple length-scales using a combination of UV and AFM oxidative lithographies. This process allows us to locally convert the terminal methyl groups of the OTS surface (non-wettable) into carboxylic acid groups (wettable) without affecting considerably the substrate roughness ($<$ 0.3nm rms). The patterned regions are shaped as a network of large (mm-sized) wettable lines connected to smaller and smaller (nm-sized) lines. Liquid squalane spreads across this ``microfluidic network'' starting from the large lines eventually reaching the nanolines (50 to 500 nm-wide). NC-AFM is used to image the morphology of the liquid as it spreads across the nanolines. We find that the liquid thickness on the nanolines grows with time (up to $\sim $10 nm) according to a power-law with exponent $\sim $1. These preliminary results suggest that the spreading dynamics of laterally-confined liquids slightly differs, as expected, from the one of laterally homogeneous precursor films. We compare our findings to recent theoretical predictions of confined liquid flow and also discuss its relevance to nanofluidics. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P9.00009: Velocity Dependent Selectivity of Deterministic Lateral Displacement Arrays Jason Puchalla, Keith Morton, Robert Austin Deterministic lateral displacement (DLD) has been demonstrated as a promising microfluidic method to circumvent diffusive dispersion while separating small particles based on size. At low average flow velocity, steric repulsion and diffusion seem sufficient to describe particle behavior and array separation characteristics. However, at higher but still laminar flow velocities, particle behavior changes drastically. We have investigated this regime using a silicon DLD array. We present how the local disruption of fluid flow about a moving particle and the effects inertial forces can alter DLD behavior and can be exploited for selective sorting. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P9.00010: Fluctuation effects and evolution in bacterial populations on a chip Jaan Mannik, Juan E. Keymer, Cees Dekker Fluctuation effects are ubiquitous in physics. Relatively little is known what role these effects play in systems involving biological organisms. How do random fluctuations originating from the environment and from the biological organisms itself affect the population dynamics and evolution? Here, we address this question using an experimental approach where we grow a large number of independent E. coli populations on a microfluidic silicon chip designed to evolve the body size distribution. We provide the same environmental conditions for different populations and follow their evolution in real time measuring number of bacteria in different colonies. We analyze fluctuations in these numbers and how the body size distribution of bacteria changes. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P9.00011: Modelling colloidal dynamics in complex systems Christopher Smith, Colin Denniston We present a lattice Boltzmann method for dealing with solid moving boundaries in a fluid. A novel method is introduced to distribute a solid surface onto the fluid mesh. We show that for a single particle in a chute with Stokes flow, the quantitatively correct Stokes drag is obtained. Comparing two scenarios at the same Reynolds number, where the walls induce the flow or where the particle is moving, we show there is little discernible difference in the force measured. Next, we have a system with two particles and show we get quantitative agreement for the interaction between the two particles measured by our algorithm and the interaction expected according to the Rotne-Prager (RP) tensor or the Oseen tensor, in the regimes in which they are expected to be accurate. Moving away from irrotational flow, for a cylinder in a two dimensional chute the Reynolds number of the flow is increased further into the laminar region and we show the formation of eddies shedding off the solid surface. We incorporate this new algorithm into liquid crystals simulations to look at novel colloidal interactions through topological defects. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P9.00012: Experimental and Theoretical Studies of Electroosmotic Membrane Micropumps Zuli Xu, Jianying Miao, Ning Wang, Ping Sheng Electroosmotic (EO) effect means fluid flow (through a porous medium) induced by an applied electric field E. EO pumps have the advantages of no moving parts and easily-controlled accurate flow rate at low applied voltages. We have fabricated nano-channel EO membrane pumps using anodic aluminum oxide (AAO) as the template [1]. The diameter of the uniform-sized nanochannels can range from 60-300nm, with a membrane thickness of 30-100 microns. The EO effect is enhanced by coating the nano-channels with silica. By using de-ionized water, the nanopump performance is shown to agree reasonably well with the theoretical model, with factors such as the ratio of the double layer thickness to channel diameter, channel geometry, and treatment of the AAO membranes playing important roles. With silica coating to the nanochannels, the nanopump can produce a maximum pressure of 1 atm and a maximum flow rate of 86,000$\mu $L/min$\cdot $cm2 under an applied field of 0.94 V/$\mu $m. Besides DI water, the micropumps have also been tested to work well with salt, acid or base solution. [1] J.Y. Miao, Z.L. Xu, X.Y. Zhang, N. Wang, Z.Y. Yang, P. Sheng, submitted to Advanced Materials (Appeared online: 10.1002/adma.200700767). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P9.00013: Investigation and Characterisation of Resizeable Nanopores in an Elastomeric Membrane Geoff Willmott Experimental and theoretical work relating to the development of resizeable synthetic nanopores will be presented. The nanopores, which are roughly conical, are formed by puncturing a relatively thick ($\sim $250 $\mu $m) elastomeric membrane with an STM tip. The aperture can be closed and the size can be dynamically controlled by stretching the elastomer [1]. Use of this technology presents a collection of interesting physical problems, covering topics that include the failure and mechanical properties of the elastomer, flow of ionic current through the aperture and particle sensing using the resistive pulse technique. Synthetic nanopores have potential applications in many fields, but especially relating to nanoscale sensing and diagnostic devices, and replication of ion channels in living cells. [1] S. J. Sowerby, M. F. Broom, G. B. Petersen, Dynamically Resizable Nanometre-Scale Apertures for Molecular Sensing, Sensors and Actuators B: Chemical 123 (1), pp. 325-330 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P9.00014: The physics of densely-packed emulsions Donald M. Aubrecht, David F. Marran, Darren R. Link, David A. Weitz One strategy for microfluidic lab-on-a-chip applications is to use water droplets as tiny reaction vessels in a carrier stream of oil. As biochemical and cell-based experiments often require control over events that take place over a wide range of time scales, strategies need to be developed to ensure adequate timing without limiting droplet throughput. In general, longer time scales can be achieved by using longer channels or more densely packed droplets. Long channels become increasingly impractical at high throughputs for times exceeding tens of minutes, thus motivating work with densely packed droplets. Dense packing of droplets can be achieved by generating droplets on-chip, collecting them off-chip to allow the oil to drain, and re-injecting them back on-chip as a packed emulsion. This strategy is limited in that it only provides access to time scales in excess of hours. Moderate time scales can be accessed by removing carrier oil from the flow without removing the droplets. Here we present some of the physical principles governing how this can be implemented and discuss the flow of the resulting dense collections of droplets through microchannels. [Preview Abstract] |
Session P10: Superconducting Nanostructures I
Sponsoring Units: DCMPChair: Michael Bleiweiss, Naval Academy Prep School
Room: Morial Convention Center RO8
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P10.00001: Microwave Response in Short Superconducting Nanowires Robert Dinsmore, Myung-Ho Bae, Alexey Bezryadin Short superconducting nanowires, of length L$\sim $100 nm, have been exposed to microwave radiation (MWR) giving rise to phase slip centers that do not appear in DC measurements, without MWR. For frequencies in the 100-1000 MHz range multiple voltage jumps are observed indicating multiple phase slip centers. Each such voltage plateau is characterized by a small differential resistance, of the order of 10-20 Ohms. In this frequency range zero crossing plateaus were also observed. For frequencies from 2 to 9 GHz, MWR had a synchronization effect on the superconducting phase rotation. Resonances are observed for integer and half integer orders, similar to the original observation of Anderson and Dayem on much larger superconducting bridges [1]. [1] Anderson and Dayem, Phys. Rev. Lett. V.13, p.195 (1964) [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P10.00002: Resistance in One-Dimensional Superconducting Epitaxial Niobium Nanowires Timothy McArdle, Kevin Inderhees, Paul Welander, Allison Dove, James Eckstein Thermally activated phase slips cause resistance in one-dimensional superconducting wires near the critical temperature. However, this description of a thermally activated process is not able to explain additional resistance observed in extremely narrow nanowires well below Tc. We fabricate nanowires using electron beam lithography from single-crystal niobium films grown by ultra-high vacuum molecular beam epitaxy. Since the films are single crystal, the role of disorder is reduced and neither weak links nor grains are present. The films are 10 to 20 nm thick, have transition temperatures ranging from 7.2 to 9.2 K, and residual resistance ratios of 5 to 10, typical for ultra-thin single-crystal niobium films. The wires are 10 $\mu $m long and range in width from 35 to 200 nm. Transport measurements on the nanowires show two distinct regions of temperature dependence below Tc. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P10.00003: Magnetoresistance and Critical Current Oscillations in Superconducting NbSe$_{2}$ and NbN Nanowires U. Patel, Z.L. Xiao, J. Hua, R. Divan, U. Welp, W.K. Kwok Magnetoresistances and critical currents of superconducting NbSe$_{2}$ and NbN nanowires with cross-section dimensions from 300 nm to 2 um were studied as a function of magnetic field. Sample specific oscillations were found with respect to applied magnetic field. The oscillations were completely reproducible, symmetric with respect to the direction of the field and independent of the field sweep direction. They disappeared at high fields, temperatures or currents. The oscillations were periodic in nature with superposition of more than one frequency as revealed in Fast Fourier Transform of the oscillations. The frequencies of oscillations were independent of temperature and current. Details about the transport measurements of resistance and critical current and origin of such oscillations will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P10.00004: ``Giant'' strengthening of superconducting pairing in small metallic nanoparticles: high T$_{c}$ state Vladimir Kresin, Yurii Ovchinnikov, Baopeng Cao, Martin Jarrold The study focuses on metallic nanoclusters containing N$\sim $10$^{2}$-10$^{3}$ free carriers (e.g., Ga$_{56}$, Al$_{45}^{-})$. The delocalized cluster electrons form energy shells similar to those in atoms or nuclei. Under special conditions, superconducting pairing in such nanoclusters can become very strong, and they form a new family of high temperature superconductors. For realistic sets of parameters one can expect a high value of T$_{c}$ (150 K) as well as strong modification of the energy spectrum. In principle, it is possible to raise T$_{c}$ up to room temperature. Specific experiments aimed at detecting the phenomenon of pair correlation in nanoclusters can be proposed: spectroscopy, magnetic, and thermodynamic properties.Transition to the superconducting state of the cluster is accompanied by the peak in its heat capacity. The phenomenon is promising for the creation of high T$_{c}$ superconducting tunneling networks, and hence macroscopic superconductivity. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P10.00005: Evidence for High T$_{c}$ Superconducting Transitions in Isolated Al$_{45}^{-}$ and Al$_{47}^{-}$ Nanoclusters Martin Jarrold, Baopeng Cao, Colleen Neal, Anne Starace, Yurii Ovchinnikov, Vladimir Kresin Heat capacities measured for Al$_{45}^{-}$ and Al$_{47}^{-}$ nanoclusters have reproducible peaks at $\sim $ 200 K. The data were obtained using a multi-collision dissociation method [1] allowing us to perform measurements for isolated nanoclusters. The peaks are observed for selected Al clusters only. These peaks are consistent with theoretical predictions that some clusters with highly degenerate electronic states near the Fermi level will undergo a transition into a high T$_{c}$ superconducting state [2]. An analysis based on a theoretical treatment of pairing in Al$_{45}^{-}$ and Al$_{47}^{-}$ agrees well with the experimental data in both the value of the critical temperature and in the size and width of the peaks in the heat capacity. The observed value of T$_{c}$ exceeds those found in bulk systems. [1] G.Breaux, C.Neal, B.Cao, M.Jarrold, Phys. Rev. Lett. \textbf{94}, 173401 (2005) [2] V.Z.Kresin, Yu.Ovchinnikov, Phys. Rev. B \textbf{74}, 024514 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P10.00006: Quantum confinement effects on superconducting properties of Lead nanocrystals Herve Aubin, Helena Moreira, Benoit Mahler, Benoit Dubertret We developed a new chemical synthesis method for producing large quantities of monodispersed lead (Pb) nanocrystals. They are obtained from the alcohol reduction of a mixture of two lead carboxylates with alkyl chains of different lengths, dissolved in a high temperature solvent. The nanocrystals obtained are protected from oxydation and aggregation by long chain fatty acids and their diameter can be tuned to reach values as low as 10 nm. Our results suggest that monodispersed particules are obtained when nucleation and growth occur at distincts temperatures, possibly as a consequence of different reactivities of the two lead carboxylates used in the solution. Owing to the large quantities of monodispersed particles produced, thermodynamics studies as function of particles diameter become possible. In particular, we will present a study of the effect of quantum confinement on superconducting properties of these Pb particles through SQUID magnetometry measurements. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P10.00007: Superconductivity of nano-size Pb Islands studied by low-temperature scanning tunneling microscopy / spectroscopy Takahiro Nishio, Toshu An, Atsushi Nomura, Kousuke Miyachi, Toyoaki Eguchi, Yukio Hasegawa, Hideaki Sakata Nano-size superconducting materials, whose dimensions are comparable with or smaller than their coherent length / penetration depth, behave differently from bulk superconductors. By forming structures using lithographic methods various unique properties of mesoscopic superconductors have been elucidated. Since these studies, however, measure electrical conductance and / or magnetization, details inside the superconductors cannot be directly probed. In this study we investigated superconductivity inside superconductors by directly measuring the superconducting gaps over ultra thin Pb island structures using a LT-STM at 2.0 K [1]. The obtained tunneling spectra exhibit an increment of zero bias conductance (ZBC) with a magnetic field and its dependence on the lateral size of the islands. Moreover, from spatial mappings of ZBC, the island size dependence and spatial variation of superconductivity inside of each island are visualized. We found that the number of vortices piercing the islands before breakdown of superconductivity depends on the lateral size of the islands. Details of the size-dependent critical fields are discussed at the presentation. [1] Nishio et al., APL 88, 113115 '06, JJAP 46, L880 '07. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P10.00008: Flux jump in superconducting Pb networks at fractional numbers of the matching fields Takekazu Ishida, Yoshiaki Matsushima, Makoto Shimizu, Masahiko Hayashi, Hiromichi Ebisawa, Masaru Kato, Osamu Sato, Kazuo Sato, Tsutomu Yotsuya The extended Little-Parks effect of superconducting network is known as a periodic Tc variation as a function of magnetic field. Superconducting Pb honeycomb networks of matching field 0.106 G and triangular microhole lattice of Pb of matching field 0.425 G have been fabricated by the combined techniques of the electron beam lithography and a lift-off process of evaporated Pb films. The application of magnetic field corresponds to the vortex doping into networks. We measured the magnetization of the networks systematically by using a SQUID magnetometer. We found that flux jump appears rather periodically as a function of magnetic field. Flux jumps may be induced by a periodic decrease in the critical current density of the network. To the authors' knowledge, this is for the first time to observe the extended Little-Parks effect appeared in flux jumps. We also discuss the anomalous matching effect. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P10.00009: Switching Current Distributions of Superconducting Nanowires: Evidence for Individual Quantum Phase Slips Mitrabhanu Sahu, Myung-Ho Bae, Andrey Rogachev, David Pekker , Nayana Shah, Tzu-Chieh Wei, Paul Goldbart, Alexey Bezryadin Phase slip fluctuations cause premature stochastic switching of the state of a current biased quasi-one-dimensional nanowire from superconducting to normal at sub-critical currents. Here, we report on measurement of the distributions of switching currents performed on amorphous superconducting Mo$_{79}$Ge$_{21}$ nanowires over a range of temperatures. The measured widths of the switching current distributions are observed to increase with decreasing temperature. We explain this counterintuitive result by considering a microscopic stochastic model of heating caused by each phase slip event. The measured rates of escape from the superconducting state agree well with the predictions of the stochastic model under the assumption of phase slippage by thermal activation at relatively high temperatures and macroscopic quantum tunneling at sufficiently low temperatures. We identify and explore a region in which a \textit{single} quantum phase slip is all that is necessary to trigger switching from the superconducting to the normal state. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P10.00010: Theory of superconductive-resistive switching in nanowires due to heating by stochastic phase slips Nayana Shah, David Pekker, Paul Goldbart We study the stochastic dynamics of superconductive-to-resistive switching in hysteretic current-biased superconducting nanowires undergoing phase-slip fluctuations. We assume that the hysteresis is thermal in nature, and postulate that the mechanism for the switching is thermal runaway, i.e. rare sequences of stochastic phase slips, closely spaced in time, that heat the nanowire. Thus, via the master-equation formalism, we obtain the distribution of currents at which switching occurs. If switching were caused by single, thermally-activated phase-slip events then this distribution would narrow as the temperature is reduced. However, at higher temperatures we find that several phase-slip events are typically necessary for inducing switching, and this results in an initial broadening of the distribution upon cooling. Quite generally, we predict that at low temperatures thermal runaway is caused by a single phase-slip event. Thus, measurements of switching-current distributions in this regime are a direct probe of this basic collective process. In particular, this regime could yield observations of individual quantum phase slips in nanowires. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P10.00011: Anisotropic Superconductivity in MoGe - Permalloy Bilayers Goran Karapetrov, A. Belkin, V. Novosad, M. Iavarone, J. Fedor, A. Troncalli, J. Pearson, W.K. Kwok We studied the magneto-transport properties of superconductor-ferromagnet MoGe/Permalloy bilayers. The ferromagnet with stripe domain structure induces in-plane anisotropy in superconducting order parameter. Superconducting phase diagram shows that near the S-N phase boundary the superconductivity is localized in narrow mesoscopic channels just above the magnetic domain walls. By changing the in-plane direction of magnetic stripe domains it is possible to re-configure the direction of the superconducting channels and enable one to control the direction of the anisotropy axis in the superconductor. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P10.00012: Superconductor-Ferromagnet Bilayers: Influence of Magnetic Domain Structure on Vortex Dynamics Andrey Belkin, V. Novosad, M. Iavarone, J. Pearson, W.K. Kwok, G. Karapetrov We investigate the influence of orientation of stripe-like domain structure in ferromagnetic films on vortex dynamics in superconductor-ferromagnet bilayers. We measure transport properties in different external magnetic fields applied perpendicular to the surface of the bilayers. Parameters of superconductor-ferromagnet bilayers are such that domain period is much bigger than the superconducting coherence length but much smaller than the effective penetration depth. Prominent dissimilarity of critical currents of two studied configurations as well as pronounced commensurability effects are found. Diverse behavior of superconductor-ferromagnet bilayers with mutually orthogonal orientations of stripe domains is demonstrated by dependence of critical temperature on external magnetic field. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P10.00013: Topological Excitations in Superconducting Nanostripes: Resistive States and Noise Matt Bell, Andrei Sergeev, Vladimir Mitin, Aleksandr Verevkin We investigate competition between one- and two-dimensional topological excitations - phase slips and vortices - in formation of resistive states and noise generation in ultrathin superconducting NbN nanostripes in a wide temperature range below the mean-field transition temperature $T_{C0}$. The widths $w$ = 100 nm of our ultrathin NbN samples is substantially larger than the Ginzburg-Landau coherence length $\xi$ = 4nm and the fluctuation resistivity above $T_{C0}$ has a two-dimensional character. However, our data shows that the resistivity below $T_{C0}$ is produced by one-dimensional excitations, - thermally activated phase slip strips (PSSs) overlapping the sample cross-section. We determine the scaling phase diagram, which shows that even in wider samples the PSS contribution dominates over vortices in a substantial region of current/temperature variations [1]. The above fluctuations generated by topological excitations also provide a noise limit to superconducting detectors operating in a resistive state, e.g. for dark counts in single-photon detectors. [1] M. Bell et al., Phys. Rev. B 76, 094521 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P10.00014: Nonlinearities and Parametric Amplification of Superconducting Coplanar Waveguide Resonators David Haviland, Erik Thol\'en, Adem Ergul We have experimentally studied the nonlinear properties of superconducting coplanar stripline resonators fabricated from Al and Nb films with small transverse dimensions (gap size 1$\mu$m). Magnetic field penetration into the superconductor causes a current-dependant kinetic inductance, which gives an ideal Kerr nonlinearity. When the nonlinear oscillator is pumped very near its dynamic instability, it can be used to realize parametric amplification. We have achieved a gain of +22.4dB in a 5.8 GHz resonator cooled to 450 mK [E. Thol\'{e}n et. al. Appl. Phys. Lett. 90, 253509 (2007)]. Parametric deamplification or squeezing of a signal has also been verified with squeezing of 30 dB. The later effect is interesting because it can be used to generate squeezed vacuum states of the electromagnetic field. We have modeled the data using a theory developed by Yurke and Buks [J. Lightwave Technol. \textbf{24}, 5054 (2006)]. Excellent fit of the model to the measured data can be achieved over a wide range of pump power, and the strength of the nonlinear terms can be obtained with high accuracy. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P10.00015: Observation of Nonlocal Coherence Between Normal Metals Coupled by a Superconductor Paul Cadden-Zimansky, Jian Wei, Venkat Chandrasekhar In conventional superconductors the effective size of the constituent Cooper pairs can approach several hundred nanometers, a length scale accessible by nanolithographic techniques. By placing two normal metals on a superconductor within a coherence length of each other, it has been predicted that the quasiparticles in the separate metals can be coherently coupled by the Cooper pairs through the nonlocal processes of Elastic Cotunneling and Crossed Andreev Reflection. We present experimental observations of coherent, nonlocal thermoelectric effects between normal metals coupled to a superconductor by using a hybrid normal-superconductor interferometer. The sign of the observed thermoelectric voltage can be switched using an external field, indicating that the voltage is dynamically driven by a persistent current induced in the interferometer. [Preview Abstract] |
Session P11: Focus Session: MgB2-like: Disorder in Novel Superconductors
Sponsoring Units: DMPChair: Alex Gurevich, Florida State University
Room: Morial Convention Center RO9
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P11.00001: Penetration depth study of Li, C and Li$+$C doped MgB$_2$ single crystals Catalin Martin, Matthew Vannette, Ruslan Prozorov, J. Karpinski, N. Zhigadlo, R. Khasanov Magnetic penetration depth was studied in single crystals of MgB$_2$, pure and doped with Li, C and Li+C. At zero applied field London penetration depth is measured and superfluid density, n$_s$, can be evalutated. We analyze effect of the dopings on n$_s$, thus on the superconductiong gaps. In the vortex state, Campbell penetration depth is measured and it provides information about true critical current unaffected by the magnetic relaxation. In particular, penetration depth in vortex state becomes strongly irreversible, and we show that C doping enhances the irreversibility region, whereas the substitution with Li does not affect it. The results are discussed in terms of two-gap nature of MgB$_2$. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P11.00002: Nano-scale TiB$_{2}$ Precipitates in MgB$_{2}$ Superconducting Wire D.K. Finnemore, M.E. Tillman, P.C. Canfield, S.L. Bud'ko, Y.Q. Wu, M.J. Kramer , J.V. Marzik, M. Rindfleisch , S.T. Hannahs Superconducting MgB$_{2}$ wire containing uniformly dispersed, nano-scale TiB$_{2}$ precipitates has been produced using a plasma synthesis method to make the starting Ti doped B powder. A powder-in-tube method is used for making the wire. Sub-micrometer size MgB$_{2}$ grains are decorated with a random dispersal of 5 to 20 nm precipitates. The superconducting properties are reported for a family of powder-in-tube wires fabricated with pure boron, carbon doped boron, and titanium doped boron. The particle size of the doped boron powder, of 50 to 100 nm, permits low reaction temperatures and short reaction times. The best reaction temperatures are in the neighborhood of 700 C for reaction times of approximately 20 min. For the Ti doped, PIT wire, at T = 20 K, J$_{c}$ crosses 10,000 A/cm$^{2}$ at 1.8 T. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P11.00003: Properties of Carbon Doped MgB$_{2}$ Films by HPCVD Using TMB Wenqing Dai, R.H.T. Wilke, Ke Chen, Qi Li, Xiaoxing Xi Carbon-doping is an effective way to enhance the upper critical field of MgB$_{2}$. Our previous carbon-alloyed MgB$_{2}$ films using (MeCp)$_{2}$Mg as the carbon source show dramatically increased $H_{c2}^{\vert \vert}$ values to over 60 T at low temperatures. Structure analyses of these films indicate that only part of the carbon is doped into the MgB$_{2}$ lattice and the rest forms highly resistive foreign phases in the grain boundaries. To fabricate more homogeneously carbon doped thin films, gaseous trimethylboron (TMB) was used as the carbon source. The normal state resistivity of carbon doped films using TMB increases much more slowly with carbon concentration, demonstrating a better connection between the MgB$_{2}$ grains. However the relatively high growth temperature, required to decompose TMB, limits the film thickness in the original Hybrid Physical-Chemical Vapor Deposition (HPCVD) setup. A hot wire is then installed in the HPCVD system to help decompose TMB while the substrate and Mg bulk source are kept at relatively low temperature. Initial results of these films will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P11.00004: Inhomogeneous Nanoscale Disorder in Doped Magnesium Diboride Invited Speaker: Using TEM and STEM imaging and microanalysis, we have shown that doped MgB$_{2}$ has a variety of forms of disorder at a length scale of 5-50 nm. In [0001]-normal, carbon-doped HPCVD thin films, there is a thin, amorphous, C-rich layer which separates domains of lightly carbon-doped MgB$_{2}$. Carbon also causes significant spread in the local in-plane orientation and c-axis direction of the MgB$_{2}$. We also observed disorder in the orientation of small MgB$_{2}$ domains in oxygen-doped thin films grown by MBE. Both of types of films show dramatic enhancement in the upper critical magnetic field compared to pure MgB$_{2}$, with extrapolated $H_{c2}$(0 $K)$ reaching 65-70 T for some samples. The origin of this increase may be in the observed disorder, particularly in confinement of the superconducting MgB$_{2}$ domains by non-superconducting second phase layers. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P11.00005: The Ca and Yb Isotope Effect in Superconducting Ca and YbC$_{6}$ David Hinks, Dan Rosenmann, Helmut Claus The isotope effect ($\alpha $ = -$\partial $logT$_{c}$/$\partial $logM where M is the isotope mass and T$_{c}$ is the transition temperature) is a measure of the phonon contribution to the superconductivity. We have measured both the Ca and the Yb isotope effect in intercalated highly orientated pyrolytic graphic (HOPG) by vapor phase transport of the isotopes. We find a large and, within the experimental error, equal $\alpha $ for each element, 0.36(4) and 0.40(4) for Yb and Ca, respectively. The value for Ca is larger then theoretically predicted indicating a stronger electron-phonon coupling. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P11.00006: Crystal Structures and Physical Properties of One-Dimensional Borides $A_{1+\varepsilon }M_{4}$B$_{4}$ ($A$ = Ca, Y; La; $M$ = Fe, Co, Ru) Yukari Katsura, Hiraku Ogino, Yutaka Matsumura, Shigeru Horii, Jun-ichi Shimoyama, Kohji Kishio We investigated crystal structures and physical properties of La$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ (\textit{$\varepsilon $ }$\sim $ 0.06) and three new borides Y$_{1+\varepsilon }$Fe$_{4}$B$_{4}$(\textit{$\varepsilon $ }$\sim $ 0.16), CaCo$_{4}$B$_{4}$ and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$ (\textit{$\varepsilon $ }$\sim $ 0.13). Polycrystalline bulk samples were obtained by reacting metal boride precursors (FeB, CoB, RuB) with Ca, Y and La metals. Microstructure were analyzed using a SEM with an EDX. Crystal structures were analyzed through TEM studies and Rietveld analysis of powder XRD patterns. Electrical resistivity and magnetization measurements were carried out from 1.8 to 300 K. We found that all these compounds belong to the same structural family as RE$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ (RE = La-Tm): a tetragonal lattice composed of one-dimensional channels of FeB and single atomic chains of RE. Incommensurate structures along $c$-axis were observed in La$_{1+\varepsilon }$Fe$_{4}$B$_{4}$, Y$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$. The La$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ bulks exhibited type-II superconductivity below 6.0 K, although this might be due to the superconductivity of dirty $\beta $-La remained in the bulks. The other new borides did not show superconductivity down to 1.8 K. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P11.00007: Observation of a new phase in the Li-B system. Eduard Galstyan, Yuyi Xue, Yanyi Sun, Irene Rusakova, Ning Wang, Karen Mkhoyan, Ching-Wu Chu The simple compounds of boron and light elements have attracted interest since the unexpected discovery of high temperature phonon-coupled superconductivity (SC) in MgB$_{2}$. The numerous theoretical works show that the Li-B system has electronic features similar to those in MgB$_{2}$ and compounds in the system are expected to be superconducting. In fact, there were early reports of Li-B compounds. Although the small Li-size suggests a rich phase-diagram, the previous investigations discovered only one stable compound over a broad initial stoichiometry. The extreme reactivity and hydrolyses of Li in air may be partially responsible for this. We were able to synthesize a new Li-B pure phase that belongs to hexagonal symmetry through a new synthesis procedure. We report on the structural, magnetic, and electron energy loss spectra (EELS) analyses of this compound. Despite the similarity with the electronic structure of MgB$_{2}$, the Li-B system has not yet shown the existence of SC. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P11.00008: Superconductivity in heavily boron-doped single crystalline and nanocrystalline diamond thin films P. Achatz, C. Marcenat, E. Bustarret, T. Klein, O.A. Williams, J.A. Garrido, M. Stutzmann Our results show that the critical boron concentration $n_{c}$ for single crystalline diamond (scd) and nanocrystalline diamond (ncd) thin films is the same for the normal to superconducting and for the non-metal to metal transitions, on the order of $5 \times 10^{20}\ cm^{-3}$, in agreement with estimates derived from various theoretical approaches. In scd material, a variable range hopping behaviour was clearly observed on the insulating side of the transition, and, as expected, the characteristic temperature $T_{0}$ tended toward zero at the transition. On the metallic side, the zero temperature conductivity $\sigma_{0}$ scaled with $(n_{B}/n_{c} - 1)^{\nu}$ with $\nu \approx 1$. The critical temperature $T_{c}$ remained high in the vicinity of the metal-non metal transition, and it was rather found to scale with $(n_{B}/n_{c} - 1)^{1/2}$. These results led us to propose that the electron-phonon coupling parameter $\lambda$ remains large down to $n_{B}/n_{c} \approx 1.1$, and to examine the metal-insulator transition and the parameter set ($\lambda$, $\mu$) in terms of scaling laws. Low temperature magnetotransport measurements and the possible occurence of a superconductor-insulator transition in heavily boron-doped ncd will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P11.00009: Tracing the evolution of two energy gaps of MgB2 with increasing disorder Mauricio Escobar M., Yong-Jihn Kim Previously we have determined the phonon-mediated matrix elements for MgB2 using the two band model and the scattered states. We noticed that, in the dirty limit, where the resistance ratio is about 3, the impurity scattering does not reduce the matrix elements much due to the Cooper pair size effect, whereas, in the weak localization limit, where the resistance ratio is less than 3, all the matrix elements are decreasing significantly due to weak localization. Now we calculate numerically the matrix elements and solve the gap equations with increasing disorder. We determine when the two energy gaps merge into one common energy gap and compare our calculations with experiments. We also calculate the transition temperature, Tc, as a function of the resistance ratio. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P11.00010: Magnetic field-tuned superconductor-insulator transition in quenched-condensed ultrathin Be films Zuxin Ye, Wenhao Wu We quenched condensed ultrathin Be films onto glass slides inside a dilution refrigerator with the substrates held near 10 K. The films were first tuned from insulating to superconducting by increasing the thickness in fine steps. The thickness-tuned transition occurs at a normal state sheet resistance R$_{N} \quad \sim $ 13 k-Ohm measured at 10 K. For superconducting films of various thickness, the field-tuned superconductor-insulator transition was then investigated. Remarkably, the critical resistance of the field-tuned transition was found to be R$_{C}$ = h/4e$^{2}$, independent of the thickness for films of R$_{N}$ ranging from 11 to 6 k-Ohm, critical temperature T$_{C}$ ranging from 1.3 to 5.2 K, and critical field B$_{C}$ ranging from 1 to 8 T. This result is a strong evidence for a duality quantum phase transition from a vortex glass in the superconducting state to a Bose glass in the field-induced insulating state. For thicker films with R$_{N} \quad <$ 5 k-Ohm, the critical resistance no longer remained at h/4e$^{2}$ but was nearly equal to R$_{N}$. This observation suggested that these thick films were no longer in the vicinity of the quantum critical point. [Preview Abstract] |
Session P12: Skutterudites and Other Heavy Fermions
Sponsoring Units: GFCChair: John Wei, University of Toronto
Room: Morial Convention Center 203
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P12.00001: Metal-insulator and metal-metal behavior in filled-skutterudites Ling Yang, Shan-Wen Tsai Filled-skutterudite compounds are of the form $LT_4X_{12}$, where $L$ stands for Lanthanide, $T$ stands for transition metal and X stands for Pnictogen. They show many unusual properties, and are also important due to their thermoelectric properties. Here we focus on the metal-insulator and metal-metal behavior observed in $PrRu_4P_{12}$ and $PrOs_4P_{12}$ compounds. We propose a simple model where motion of the L atom inside the atomic cage made by the other atoms plays a crucial role. In this model, the hopping amplitude of the charge carriers depends on the positions of the L atoms in different minima inside the cage, and a phase transition occurs when the temperature is lowered. The nature of the transition depends on the density of carriers. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P12.00002: Andreev Spectroscopy Study of Multigap Pairing in PrOs$_{4}$Sb$_{12}$ C.S. Turel, J.Y.T. Wei, B. Djurkovic, J.B. Kycia, W.M. Yuhasz, R. Baumbach, M.B. Maple The recent discovery of superconductivity in the filled skutterudite heavy fermion material PrOs$_{4}$Sb$_{12}$ has generated widespread interest. Different experimental studies have reported various field-vs-temperature phase diagrams, with mixed evidence for nodes in the pairing gap. Some experiments have also indicated the presence of multiple gaps, suggesting that the pairing involves either multiple bands or multiple order parameters [1,2]. We present Andreev spectroscopy data, down to 80 mK and up to 2.5 T, taken using ballistic point contacts made with Pt-Ir tips on single crystals of Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$. We observed distinct spectral evidence for gap nodes. We also observed multiple spectral features arising from Ru-doping. We interpret the evolution of these spectral features within the scenario of multigap pairing. \\ $[1]$ G. Seyfarth et al. Phys. Rev. Lett. \textbf{97}, 236403 (2006). \\ $[2]$ R.W. Hill et al. cond-mat/0709.4265. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P12.00003: Defects and broken time-reversal symmetry in superconducting {\boldmath Pr(Os,Ru)$_{4}$Sb$_{12}$} D.E. MacLaughlin, Lei Shu, A.D. Hillier, O.O. Bernal, Y. Aoki, D. Kikuchi, H. Sato, Y. Tunashima, H. Sugawara, T.A. Sayles, T. Yanagisawa, W.M. Yuhasz, M.B. Maple Muon spin relaxation studies of a spontaneous local field $H_\mu$, previously observed in the superconducting state of PrOs$_4$Sb$_{12}$ and attributed to broken time-reversal symmetry, have been extended to Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ alloys. In flux-grown single crystals $H_\mu$ is strongly suppressed but remains observable for $x \le 0.2$. In powder samples prepared by solid state reaction, however, no field is observed for $x = 0.1$ or 0.2. Muon spin relaxation due to dynamic $^{141}$Pr nuclear spin fluctuations is also reduced in the powders. Both results can both be understood if the density of microscopic defects is smaller in the powders: defects increase muon spin relaxation by $^{141}$Pr spins, and supercurrents associated with defects are theoretically predicted to create spontaneous fields in TRS-breaking superconductors. Our results are strong experimental evidence for this prediction. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P12.00004: Multi-symmetry and Multi-band Superconductivity in Superconducting Filled-skutterudites. Robert W. Hill, Shiyan Li, M.B. Maple, Louis Taillefer Thermal conductivity measurements were performed on single crystal samples of the superconducting filled skutterudite compounds PrOs$_4$Sb$_{12}$ and PrRu$_4$Sb$_{12}$ both as a function of temperature and magnetic field applied perpendicular to the heat current. In zero magnetic field, the low temperature electronic thermal conductivity of PrRu$_4$Sb$_{12}$ is vanishingly small, consistent with a fully-gapped Fermi surface. For PrOs$_4$Sb$_ {12}$, however, we find clear evidence for residual electronic conduction as the temperature tends to zero Kelvin which is consistent with the presence of nodes in the superconducting energy gap. The field dependence of the electronic conductivity for both compounds shows a rapid rise immediately above H$_{c1}$ and significant structure over the entire vortex state. In the fully gapped superconductor PrRu$_4$Sb$_{12}$, this is interpreted in terms of multi-band effects. In PrOs$_4 $Sb$_{12}$, we consider the Doppler shift of nodal quasiparticles at low fields and multiband effects at higher fields. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P12.00005: High-field de Haas-van Alphen investigation of the filled skutterudite compound NdOs$_4$Sb$_{12}$ P.-C. Ho, J. Singleton, M.B. Maple, P. Goddard, T. Yanagisawa The filled skutterudite compound NdOs$_4$Sb$_{12}$ displays mean-field type ferromagnetism with a Curie temperature of 0.9~K. The apparent electronic specific heat coefficient $\gamma \sim$ 0.52~Jmol$^{-1}$K$^{-2}$ is very large. In order to search for possible heavy-fermion behavior, de Haas-van Alphen (dHvA) measurements were performed on NdOs$_4$Sb$_{12}$. Experiments for H//[100] show dHvA frequencies at 695$\pm$10, 950$\pm$10, and 2560$\pm$20 T with effective masses ranging from 1.8 to 2.9 $m_e$. Moreover, the angle dependence of dHvA frequencies is very similar to that of LaOs$_4$Sb$_{12}$. Both results suggests that heavy-fermion behavior is not primary responsible for the large $\gamma$. In this context, a theoretical model by Miyake et al. indicates that the tunneling motion of the rare earth ions between off-center sites may enhance the interaction between local phonons and neighboring conduction electrons in the filled skutterudites, resulting in a large $\gamma$. This may be relevant to the extra mode found at 15~K in ultrasound measurements on NdOs$_4$Sb$_{12}$, implying abnormal behavior of the phonon spectrum in this compound. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P12.00006: Non-Fermi liquid behavior in the filled skutterudite compound CeRu$_{4}$As$_{12}$ Ryan Baumbach, P.-C. Ho, T. Sayles, M.B. Maple, R. Wawryk, T. Cichorek, A. Pietraszko, Z. Henkie The filled skutterudite compounds of the form MT$_4$X$_{12}$ (M = alkali metal, alkaline earth, lanthanide, actinide, T = Fe, Ru, Os and X = P, As, Sb) exhibit a wealth of strongly correlated electron phenomena. The CeT$_4$X$_{12}$ subclass is interesting since it includes the only filled skutterudite known to show non - Fermi liquid (NFL) behavior, CeRu$_4$AS$_{12}$, in addition to various semiconductors where the gap size is correlated with the lattice constant. We present electrical resistivity $\rho$, specific heat C, and magnetic susceptibility $\chi$ measurements for the new compound, CeRu$_4$As$_{12}$, which reveal NFL T - dependences at low T, i.e., $\rho$(T) $\sim$ T$^{1.4}$ and weak power law or logarithmic divergences in C(T)/T and $\chi$(T). Measurements also show that the T - dependence of the thermoelectric power S(T) deviates from that in other Ce systems. The NFL behavior appears to be associated with a nonmagnetic or weakly magnetic ground state, as revealed by magnetization M(H,T) measurements. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P12.00007: Low-temperature thermal and transport properties of single-crystalline Ce$_{4}$Pt$_{12}$Sn$_{25}$ Nobuyuki Kurita, Han-Oh Lee, Yoshifumi Tokiwa, Eric Bauer, Joe Thompson, Zachary Fisk, Pei-chun Ho, M. Brian Maple, Roman Movshovich Low-temperature specific heat $C(T)$ and electrical resistivity \textit{$\rho $}($T)$ measurements have been performed on a flux-grown single-crystalline Ce$_{4}$Pt$_{12}$Sn$_{25 }$which has a body-centered cubic structure. As temperature decreased, $C(T)$ increased and showed a huge jump ($\Delta C$/$T\sim $43J/mole-K$^{2}$-Ce) at $T \quad \sim $ 0.2 K, probably due to a magnetic ordering. The entropy gain connected with the ordering reaches a half of $R$ln2 at the peak position and almost full $R$ln2 at 3 K, corresponding to the degeneracy of the fundamental crystal-field doublet. On the other hand, \textit{$\rho $}($T)$ showed metallic behavior and decreased rapidly around the ordering temperature with a clear kink. We will also discuss the results of external-field effect on this compound. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P12.00008: Magnetodynamics of Heavy Fermion YbCo$_{2}$Zn$_{20}$ and YbFe$_{2}$Zn$_{20}$ A.D. Christianson, E.A. Goremychkin, Michael M. Koza , J.L. Zarestky, C.H. Wang, A.I. Kolesnikov, N. Ni, S. Jia, E.D. Mun, S.L. Bud'ko, P.C. Canfield We have performed inelastic neutron scatting experiments on the recently discovered heavy Fermion systems YbCo$_{2}$Zn$_{20}$ and YbFe$_{2}$Zn$_{20}$. The magnetic excitation spectrum demonstrates that YbCo$_{2}$Zn$_{20}$ exhibits crystal field dynamics with an overall splitting 2.72 meV. Despite this, the splitting between the ground state and first excited state is large enough such that the large low temperature electronic specific heat is not due to crystal field entropy and hence YbCo$_{2}$Zn$_{20}$ is a truly heavy Fermion material. YbFe$_{2}$Zn$_{20}$ exhibits significantly different spin dynamics. At 200 K the scattering is quasielastic and as the temperature is lowered the spectral weight is progressively shifted to finite energy transfers. This temperature dependence is characteristic of an intermediate valence system with an unusually low Kondo temperature of $\sim $50 K. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P12.00009: High field de Haas-van Alphen measurements of RT$_{2}$Zn$_{20}$ (R=Yb and Lu,T=Fe, Co and Rh) N. Ni, S. Jia, N. Harrison , G.D. Samolyuk, S.L. Bud'ko, P.C. Canfield The de Haas--van Alphen (dHvA) effect in heavy fermion compounds YbFe$_{2}$Zn$_{20}$, YbCo$_{2}$Zn$_{20}$ and YbRh$_{2}$Zn$_{20}$ as well as the nonmagnetic compounds LuFe$_{2}$Zn$_{20}$, LuCo$_{2}$Zn$_{20}$ and LuRh$_{2}$Zn$_{20}$ have been observed in pulsed fields up to 55 T directed along [110] and in the temperature range 0.4K to 12K. The cyclotron effective masses of~YbT$_{2}$Zn$_{20}$ (T=Fe,Co and Rh) range from 1.8m$_{0}$ to 2.8m$_{0}$. Self-consistent tight binding linear muffin-tin orbital method in the atomic sphere approximation (TB-LMTO-ASA) has been used to construct Fermi surfaces of LuT$_{2}$Zn$_{20}$(T=Fe, Co and Rh). The calculated dHvA frequencies show good agreement with the experiments. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P12.00010: Induced Phase Separation in GdBaCo$_{2-x}$Fe$_{x}$O$_{5.5-\delta }$ by Fe Doping Yan-kun Tang, C.C. Almasan A detailed magnetic study has been carried out for the Fe doped GdBaCo$_{2-x}$Fe$_{x}$O$_{5.5-\delta }$ (x = 0, 0.02, 0.05, 0.10, and 0.20) cobaltites. Fe doping enhances the high temperature T ferromagnetism present in the Fe-free samples and it induces a second ferromagnetic order, which develops at a lower T. Also, the spin-state transition temperature shifts to higher T with increasing the Fe content. Phase separation in two magnetic phases, namely, Fe-rich and Fe-free regions, provides a phenomenological interpretation of these magnetic behaviors. The Fe-rich regions are responsible for the second ferromagnetic order present at lower T. The shift of the low temperature M(T) peak related to this second ferromagnetic order is due to the decrease in the oxygen content in the Fe-rich regions with increasing Fe doping and the presence of the antiferromagnetic Fe-Co superexchange interaction in these regions. The high temperature M(T) peak (corresponding to the Fe-free regions) and the spin-state transition temperature shift to higher temperatures with Fe doping because of the slight compression of the Fe-free regions with Fe doping. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P12.00011: Quantum Critical Phenomena in Ni$_{3}$Al$_{1-x}$Ga$_{x}$ Alloys M.H. Fang, J.H. Yang, Z.A. Xu, B. Chen, Y. Itoh, K. Yoshimura, Z.Q. Mao Considerable study has been devoted to quantum phase transitions (QPTs), which are believed to be a key concept for understanding the physics of strongly correlated electrons. In this talk we report on observation of quantum critical phenomena in Ni$_{3}$Al$_{1-x}$Ga$_{x}$ alloys. Ni$_{3}$Al is a ferromagnetic metal with $T_{c} =41.5K$. With Ga substitution for Al, $T_{c}$ and the spontaneous magnetic moment are gradually suppressed down to zero near the critical composition of $x_{c} \quad \sim $ 0.4. We found that near the critical composition the magnetization as a function of magnetic field M(H) and the magnetic susceptibility as a function of temperature \textit{$\chi $}($T)$ both obey the scaling laws theoretically expected for QPTs, i.e., $M(H) \quad \propto $ $H^{1/3}$ and \textit{$\chi $}$^{-1}\propto T^{4/3}$. In addition, we observed that near $x_{c}$ the derivative derived from the Arrott plots, i.e., \textit{$\gamma $=d(M}$^{2})/d(H/M)$ value$, $exhibits a remarkable peak at about 2-3 T. This peak enhances with decreasing temperature. In terms of a recent theory, we argue that \textit{$\gamma $} reflects characteristics of spin excitation spectrum near QPTs. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P12.00012: Physical properties of new ternary U compounds U$_{3}$Bi$_{4}$M$_{3}$ (M = Rh, Ni) Tomasz Klimczuk, Han-Oh Lee, Filip Ronning, Eric Bauer, Tomasz Durakiewicz, Heather Volz, Joe Thompson A common belief has been that only Sb and Sn can form the U$_{3}$X$_{4}$M$_{3}$ structure, but here we show that also Bi can stabilize the structure and two new U ternary compounds U$_{3}$Bi$_{4}$Ni$_{3}$ and U$_{3}$Bi$_{4}$Rh$_{3}$ can be grown as single crystals out of Bi flux. Both materials form in the same crystal structure as Ce$_{3}$Pt$_{3}$Bi$_{4}$, the well known Kondo insulator. Resistivity and photoemission spectroscopy measurements on U$_{3}$Bi$_{4}$Ni$_{3}$ indicate the presence of a charge gap, suggesting that it might be a Kondo insulator. However, the nonmagnetic reference compound Th$_{3}$Bi$_{4}$Ni$_{3}$ is a semiconductor, and, consequently, U$_{3}$Bi$_{4}$Ni$_{3}$ is rather a band insulator, similar to U$_{3}$Sb$_{4}$Ni$_{3}$. On the other hand, replacing Ni with Rh, which has one less electron than Ni, to form U$_{3}$Bi$_{4}$Rh$_{3}$ produces a metallic resistivity and modestly large electronic specific heat coefficient ($\gamma $(0.4K)=200 mJ/mol-U K$^{2}))$, characteristic of a Kondo-lattice system. Details of specific heat, magnetic susceptibility and electrical resistivity measurements on these single crystals will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P12.00013: Electronic and magnetic properties of single-crystalline UNi$_{0.5}$Sb$_{2}$. B. K. Davis, M. S. Torikachvili, E. D. Mun, J. C. Frederick, G. J. Miller, S. Thimmaiah, S. L. Bud'ko, P. C. Canfield, G. M. Schmiedeshoff We studied the electronic and magnetic properties of antiferromagnetic UNi$_{0.5}$Sb$_{2}$ (T$_{N} \quad \approx $ 161 K) by means of measurements of magnetic susceptibility, thermal expansion, and electrical resistivity ($\rho )$ at ambient pressure, and $\rho $ under hydrostatic pressures up to 20 kbar, in the temperature range from 1.9 to 300 K. The value of \textit{d$\rho $/dT} changes drastically from positive below $T_{N}$ to negative above it, reflecting the loss of spin-disorder scattering in the ordered phase. Two small features in the \textit{$\rho $ vs T} data centered near 40 and 85 K, which are quite hysteretic in temperature, correlate well in temperature with features in the magnetic susceptibility, and thermal expansion. The effect of pressure is to suppress the amplitude of the small features in \textit{$\rho $ vs T} at lower temperatures, and to raise $T_{N}$ at the rate of $\approx $ 0.76 K/kbar. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P12.00014: Tuning the ground state of CeNiGe$_{3}$ by applied magnetic field E.D. Mun, S.L. Bud'ko, P.C. Canfield We studied the thermal, magnetic and electrical transport properties of the ternary intermetallic system CeNiGe$_{3}$. In zero field, antiferromangeic order is observed below 5.1 K. In addition, at low temperatures, two pronounced metamagnetic transitions are also revealed. Applying magnetic fields along the magnetic easy direction leads to a suppression of the Neel temperature, with an apparent critical field $H_{c} \quad \sim $ 32.5 kOe. The $H-T$ phase diagram of CeNiGe$_{3}$ is consistent with a system that manifests field induced, quantum criticality. In the paramagnetic regime, above $H_{c}$ , the resistivity at low temperatures exhibits an unusual temperature dependence. These results make CeNiGe$_{3}$ appear to be most similar to the isostructural, YbNiSi$_{3}$ [1]. \newline [1] S. L. Bud'ko \textit{et al}, Phys. Rev. B 75, 094433(2007). [Preview Abstract] |
Session P13: Time-Dependent Density Functional Theory
Sponsoring Units: DCOMPChair: E.K.U. Gross, Freie Universitat Berlin
Room: Morial Convention Center 204
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P13.00001: Energy and charge transfer in photoexcited molecules - A challenge for TDDFT Espen Sagvolden, Filipp Furche We study charge-transfer excitations and radiationless energy transfer between two chromophores (parts of a molecule which are individually excitable). These reactions have widespread chemical significance, particularly to the design of organic solar cell panels and molecular switches and to photosynthesis. Time-Dependent DFT (TDDFT) offers a very favorable relationship between accuracy and calculational cost in many cases. Calculations are performed for the (2-pyridone)$_2$-dimer which is experimentally well-characterized [1]. TDDFT is compared to experiment and competing methods such as Time-Dependent Hartree-Fock, CI singles, and coupled-cluster methods. \newline [1] A. M\"uller, F. Talbot, and S. Leutwyler, J. Chem. Phys. \textbf{112}, 2836 (2002). [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P13.00002: Generator coordinates: a new road towards dynamics and excitations in DFT Klaus Capelle The generator-coordinate method is a flexible and powerful reformulation of the variational principle. Here we show that by introducing a generator coordinate in the Kohn-Sham equation of density-functional theory, excitation energies can be obtained from ground-state density functionals. Similarly, by introducing a generator coordinate in the equations of time-dependent DFT, memory effects can be built into any existing adiabatic exchange-correlation potential.(See J. Chem. Phys. 127, p. 124101 (2007) and J. Chem. Phys. 119, p. 1285 (2003).) [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P13.00003: Time-Dependent Transport Phenomena: Bound-State Oscillations and Pumping Stefan Kurth, Elham Khosravi, Gianluca Stefanucci, Angel Rubio, Eberhard K.U. Gross We present a description of transport based on the time evolution of the non-interacting time-dependent Schr\"odinger equation and develop a numerical algorithm for the time propagation which is suited for implementation of time-dependent density functional theory (TDDFT). The algorithm is used to study time-dependent transport phenomena such as electron pumping, transients and bound state oscillations. It has been shown recently [Phys. Rev. B {\bf 75}, 195115 (2007)] that the presence of at least two bound states in the biased electrode-device-electrode system of non interacting electrons leads to persistent oscillations in the total current whose amplitude depends on the history of the applied voltage and on the initial state. In the case of electron pumps driven by time-periodic gate voltages, the amplitude of these oscillations decays slowly with time. TDDFT results will be compared to those obtained for non-interacting electrons. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P13.00004: Time-dependent V-representability on lattice systems Yonghui Li, Carsten A. Ullrich We study the mapping between time-dependent densities and potentials on small lattices. As discovered recently by Baer (arXiv:0704.1787), there exist well-behaved time-dependent density functions on lattices which cannot be constructed from any real potential. However, one finds that such densities can always be reproduced by complex potentials. We analyze the breakdown of time-dependent V-representability on lattices and show that it is related to problems with the continuity equation which ultimately arise from discretization of the momentum operator. This imposes fundamental restrictions on practical numerical applications of TDDFT. In the continuum limit, time-dependent V-representability is restored. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P13.00005: TDDFT in Phase-Space Arun Rajam, Christian Gaun, Neepa Maitra We explore the possibility of a density-functional theory in phase-space, where the basic variable is a phase-space density rather than the usual coordinate-space density. In this way information about the momentum distribution is directly captured, rather than being hidden in the form of the exchange-correlation functionals, which often complicates the functional, making it hard to approximate. We give examples to motivate this approach, and discuss initial stages in the development of the functionals. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P13.00006: Linear and Non-Linear Optical Response using Real-Time Time-Dependent Density Functional Theory Y. Takimoto, F.D. Vila, J.J. Rehr We present an approach for the calculation of the frequency- dependent response of nano-scale organic molecules for non-linear optical (NLO) devices. These calculations are performed using an efficient implementation of real-time, time-dependent density functional theory (RT-TDDFT) \footnote{Y. Takimoto, F. D. Vila, and J. J. Rehr, J. Chem. Phys. {\bf 127}, 154114 (2007)}, and an adaptation of the SIESTA electronic structure code. This method yields frequency dependent nonlinear optical properties of large organic molecules, which have been difficult to obtain with frequency domain calculations. Here we discuss the efficiency of the method and compare the results against frequency-domain TDDFT methods and with experiment. Solvent effects on the NLO properties of photonic molecules are also briefly discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P13.00007: Time-dependent density-functional approach for exciton binding energies Volodymyr Turkowski, Carsten A. Ullrich We use TDDFT to study ultrafast electron dynamics and excitonic effects in insulators and semiconductors. Within a two-band approximation of the linearized semiconductor Bloch equations, we derive a TDDFT version of the Wannier equation for excitonic wave functions and binding energies. The TDDFT Wannier equation produces in principle the exact excitonic spectrum. However, this puts stringent requirements on the exchange-correlation (XC) kernel. We analyze various XC kernels that lead to bound excitonic states, and propose new model XC kernels designed to reproduce experimental exciton spectra. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P13.00008: Autoionizing Resonances in TDDFT Harshani Wijewardane, August Krueger, Gabriella Mullady, Neepa Maitra In an independent particle picture such as the Kohn-Sham system, bound states with an energy above one of the occupied orbital ionizations are truly bound. When interaction is accounted for, configuration coupling turns the bound state into an autoionizing resonance. In exact TDDFT, it is the exchange-correlation kernel that mixes the ionized and bound state, creating a Fano resonance profile. Although autoionization peaks arising from single excitations have been accurately captured with the available functional approximations, resonances arising from double excitations lying in the continuum are missing. By studying a simple model system, we uncover the features of the exact exchange-correlation kernel that are needed to capture the lifetimes and lineshapes of these resonances accurately. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P13.00009: Viscosity contribution to the impurity resistivity of metals by means of the current-density functional theory Vladimir U. Nazarov, Giovanni Vignale Within the time-dependent density functional theory formalism we relate the impurity resistivity $\rho$ of a metal to the friction coefficient $Q$ of the metal for the same impurity moving with the infinitesimally small velocity, i.e., $ \rho=n_i Q / n_e^2 \, (1), $ where $n_i$ and $n_e$ are the concentrations of the randomly distributed impurities and the valence electrons, respectively. While Eq.(1) occurs trivial within the single-particle theory with the scattering at the {\em statically} screened impurities, its general validity within the many-body theory with the {\em dynamical} exchange and correlation included presents a progress. We utilize results [1,2] on $Q$ of the electron liquid to put the electron-electron scattering contribution into the terms of the viscosity coefficients [3]. Calculations of the residual resistivity of aluminum as a function of the atomic number of the impurity are performed, improving the agreement with experiment compared to the single- particle theory [4]. \noindent [1].V. U. Nazarov, J. M. Pitarke, C. S. Kim, and Y. Takada, Phys. Rev. B {\bf 71}, 121106(R) (2005). \noindent [2].V. U. Nazarov, J. M. Pitarke, Y. Takada, G. Vignale, and Y.-C. Chang, Phys. Rev. B {\bf 76}, 205103 (2007). \noindent [3].G. Vignale, C. A. Ullrich, and S. Conti, Phys. Rev. Lett. {\bf 79}, 4878 (1997). \noindent [4].M. J. Puska and R. M. Nieminen, Phys. Rev. B {\bf 27}, 6121 (1983). [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P13.00010: Adiabatic connection fluctuation dissipation theorem density functionals beyond the random phase approximation Filipp Furche The random phase approximation (RPA) is an increasingly popular starting point for the construction of improved correlation energy functionals. As opposed to semi-local approximations, RPA-based functionals do not suffer from Coulomb self-interaction and naturally include van der Waals interactions; the price is higher computational cost. To compete with traditional correlated wavefunction methods, it is necessary to go beyond the bare RPA. I will analyze successes and failures of recent attempts to do so [1,2], and outline promising future directions. \newline [1] Z. Yan, J. P. Perdew, and S. Kurth, \textit{Phys. Rev. B} \textbf{61} (2000), 16430. \newline [2] F. Furche and T. Van Voorhis, \textit{J. Chem. Phys.} \textbf{122} (2005), 164106. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P13.00011: New Perspectives on the Fundamental Theorem of Density Functional Theory Viraht Sahni, Xiao-Yin Pan The fundamental theorem of time-dependent/time-independent DFT due to Runge-Gross(RG)/Hohenberg-Kohn(HK) proves the bijectivity between the density $ \rho({\bf r} t)/ \rho({\bf r})$ and the Hamiltonian ${\hat H}(t)/{\hat H}$ to within a function $C(t)$/constant $C$, and wave function $\Psi(t) /\Psi$. (Implicit in the RG theorem is that the initial condition $\Psi(t_{0})$ is fixed.) As such in DFT the wave function is considered solely a functional of the density. Since the density is gauge invariant, the wave function as a functional of the density is also gauge invariant. However, it is well known that the Hamiltonian and wave function are gauge variant. There is, therefore, an inherent inconsistency in the RG/HK theorem. We resolve this inconsistency of the theorem via a unitary or equivalently a gauge transformation. As a consequence we generalize the theorem to external potentials that include the momentum operator and a curl-free vector potential operator. The RG/HK theorems each then constitute a special case of this generalization. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P13.00012: Photo-excited dynamics of $\mathrm{CH_{2}N_{2}}$; time-dependent density functional theory Hosik Lee, Yoshiyuki Miyamoto A highly reactive organic molecule carbene has been an important subject in organic chemistry in several decades. The carbene which is formulated by $\mathrm{R_{1}R_{2}C:}$ shows high yield and lesser or no side products during its reaction [1]. By using ultra-fast (sub-pico second) laser flash photolysis(LFP) technique to its precursor diazirine or diazomethane, the highly reactive short-living ($\sim$100 fs) carbene can be conveniently prepared and used for production. In this study, photo-excited dynamics of diazirine and diazomethane will be shown within the scheme of the first- principles time-dependent density functional calculations. With quite good agreements to experimental photo-excitation spectra, our preliminary calculation results show different phases of molecular motion which hardly is achieved with thermal effect. Temperature-induced kinetic effect in the phto-excited dynamics also is discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P13.00013: Time dependent density functional study of enhanced field emission from carbon nanotubes Joseph Driscoll, Kalman Varga We have calculated the field emission current of carbon nanotubes in real-time and real-space using the Lagrange- function basis [1] combined with efficient time-propagating schemes. Experimental studies reported orders of magnitude increase of field emission current from Cesium deposited carbon nanotubes [2]. We have studied the increase of field emission current due to the deposition of different atoms (Cesium, Gold, Tungsten, etc.) on capped carbon nanotube tips. The theoretical results are in good agreement with the experimental findings. This work was supperted by NSF grant ECS 0622146. \newline [1] K. Varga, Z. Zhang, and S. T. Pantelides, Phys. Rev. Lett. 93, 176403 (2004). \newline [2] A. Wadhawan, R. E. Stallcup, and J. M. Perez, Appl. Phys. Lett., 78 108 (2001). [Preview Abstract] |
Session P14: High-Bandwidth Dynamic Atomic Force Microscopy
Sponsoring Units: DBP BPS FIAPChair: Brian Salzberg, University of Pennsylvania School of Medicine
Room: Morial Convention Center 205
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P14.00001: High-Bandwidth Atomic Force Microscopy Reveals A Mechanical spike Accompanying the Action Potential in mammalian Nerve Terminals Invited Speaker: Information transfer from neuron to neuron within nervous systems occurs when the action potential arrives at a nerve terminal and initiates the release of a chemical messenger (neurotransmitter). In the mammalian neurohypophysis (posterior pituitary), large and rapid changes in light scattering accompany secretion of transmitter-like neuropeptides. In the mouse, these intrinsic optical signals are intimately related to the arrival of the action potential (E-wave) and the release of arginine vasopressin and oxytocin (S-wave). We have used a high bandwidth (20 kHz) atomic force microscope (AFM) to demonstrate that these light scattering signals are associated with changes in nerve terminal volume, detected as nanometer-scale movements of a cantilever positioned on top of the neurohypophysis. The most rapid mechanical response, the ``spike'', has duration comparable to that of the action potential ($\sim $2 ms) and probably reflects an increase in terminal volume due to H$_{2}$O movement associated with Na$^{+}$-influx. Elementary calculations suggest that two H$_{2}$O molecules accompanying each Na$^{+}$-ion could account for the $\sim $0.5-1.0 {\AA} increase in the diameter of each terminal during the action potential. Distinguishable from the mechanical ``spike'', a slower mechanical event, the ``dip'', represents a decrease in nerve terminal volume, depends upon Ca$^{2+}$-entry, as well as on intra-terminal Ca$^{2+}$-transients, and appears to monitor events associated with secretion. A simple hypothesis is that this ``dip'' reflects the extrusion of the dense core granule that comprises the secretory products. These dynamic high bandwidth AFM recordings are the first to monitor mechanical events in nervous systems and may provide novel insights into the mechanism(s) by which excitation is coupled to secretion at nerve terminals. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P14.00002: Studying Chemical Reactions, One Bond at a Time, with Single Molecule AFM Techniques Invited Speaker: The mechanisms by which mechanical forces regulate the kinetics of a chemical reaction are unknown. In my lecture I will demonstrate how we use single molecule force-clamp spectroscopy and protein engineering to study the effect of force on the kinetics of thiol/disulfide exchange. Reduction of disulfide bond via the thiol/disulfide exchange chemical reaction is crucial in regulating protein function and is of common occurrence in mechanically stressed proteins. While reduction is thought to proceed through a substitution nucleophilic bimolecular (SN2) reaction, the role of a mechanical force in modulating this chemical reaction is unknown. We apply a constant stretching force to single engineered disulfide bonds and measure their rate of reduction by dithiothreitol (DTT). We find that while the reduction rate is linearly dependent on the concentration of DTT, it is exponentially dependent on the applied force, increasing 10-fold over a 300 pN range. This result predicts that the disulfide bond lengthens by 0.34 {\AA} at the transition state of the thiol/disulfide exchange reaction. In addition to DTT, we also study the reduction of the engineered disulfide bond by the E. coli enzyme thioredoxin (Trx). Thioredoxins are enzymes that catalyze disulfide bond reduction in all organisms. As before, we apply a mechanical force in the range of 25-450 pN to the engineered disulfide bond substrate and monitor the reduction of these bonds by individual enzymes. In sharp contrast with the data obtained with DTT, we now observe two alternative forms of the catalytic reaction, the first requiring a reorientation of the substrate disulfide bond, causing a shortening of the substrate polypeptide by 0.76$\pm$0.07 {\AA}, and the second elongating the substrate disulfide bond by 0.21$\pm$0.01 {\AA}. These results support the view that the Trx active site regulates the geometry of the participating sulfur atoms, with sub-{\AA}ngstr\"om precision, in order to achieve efficient catalysis. Single molecule atomic force microscopy (AFM) techniques, as shown here, can probe dynamic rearrangements within an enzyme's active site which cannot be resolved with any other current structural biological technique. Furthermore, our work at the single bond level directly demonstrates that thiol/disulfide exchange in proteins is a force-dependent chemical reaction. Our findings suggest that mechanical force plays a role in disulfide reduction in vivo, a property which has never been explored by traditional biochemistry. \newline \newline 1.-Wiita, A.P., Ainavarapu, S.R.K., Huang, H.H. and Julio M. Fernandez (2006) Force-dependent chemical kinetics of disulfide bond reduction observed with single molecule techniques. \textbf{Proc Natl Acad Sci} U S A. 103(19):7222-7 \newline 2.-Wiita, A.P., Perez-Jimenez, R., Walther, K.A., Gräter, F. Berne, B.J., Holmgren, A., Sanchez-Ruiz, J.M., and Fernandez, J.M. (2007) Probing the chemistry of thioredoxin catalysis with force. \textbf{Nature}, 450:124-7. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P14.00003: Imaging and Beyond with High Speed AFM. Invited Speaker: It is now possible to operate Atomic Force Microscopes (AFMs) at speeds of up to 6000 lines per second over scan ranges exceeding 10 microns. For a 100 x 100 pixel image this gives frame rates of 60 frames/second: faster than video rate. This has required small cantilevers, new scanners, new high voltage amplifiers, and a new scan control system. The small cantilevers are from SCL Sensor-Tech (Deutsch-Wagram, Austria). The new scanner is based on a sophisticated system of flexures that constrain the motion of each separate piezo stack to one dimension in a three-dimensional scanner. It has a scan range of 15 microns and a lowest resonance frequency of about 27 kHz. The new high voltage amplifier, built in collaboration with TechProject (Vienna, Austria), can deliver up to 8 amps over the entire output range from 0 to 150 volts with the challenge of having the piezo as a capacitive load. The new scan control system is built around a commercially available DAQ board in a Windows environment. One of the major challenges is now to move beyond imaging to Force-Volume imaging, which involves taking an array of force curves over a sample and then reconstructing a zero force image as well as a map of local mechanical properties. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P14.00004: AFM probes with integrated electrostatic actuators for fast, quantitative imaging and force spectroscopy Invited Speaker: In this talk, we summarize our efforts in developing novel AFM probes (FIRAT) with integrated sensing and actuation. These probes exploit recent advances in microscale sensor technology and open up the design space for AFM applications including fast imaging, quantitative material characterization and single molecular mechanics measurements. For fast imaging applications in air, probes with aluminum force sensing structures are surface micromachined on quartz substrates. Using 0.7-0.8$\mu $m thick, 40$\mu $m$\times $60$\mu $m clamped-clamped beams over 2.8$\mu $m of air gap, probes with resonance frequencies in the order of 1MHz and Q in the 5-15 range are obtained. These probes are actuated directly by electrostatic forces applied to the mechanical structure by rigid electrodes on the substrate shaped as optical diffraction gratings, enabling imaging bandwidths in the order of 100kHz. The integrated grating interferometer provides 10fm/$\surd $Hz level displacement sensitivity down to 3Hz. The surface micromachining approach used for probe fabrication lets one to precisely control the probe dynamics and overcome the difficulties associated with regular AFM cantilevers for applications such as time resolved interaction force (TRIF) measurements. Using FIRAT probes with over damped dynamics, clean TRIF signals are obtained while imaging the surface at regular speeds. This enables us to use a simple model to invert quantitative mechanical properties of a variety of polymers. For measurements on single molecules, membrane type FIRAT probes suitable for in liquid operation have been developed. These probes are made of dielectric materials with embedded actuation electrodes. Used only as actuators or both actuators and force sensors, these devices are shown to enable parallel force spectroscopy measurements. We also show that the spring constant of these probes can be electrically reduced to achieve higher force sensitivity while not affecting its noise performance and discuss the effect of hydrodynamic forces in these membrane type probes as compared to cantilever type probes for fast force spectroscopy measurements. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P14.00005: High-speed AFM for Studying Dynamic Biomolecular Processes Invited Speaker: Biological molecules show their vital activities only in aqueous solutions. It had been one of dreams in biological sciences to directly observe biological macromolecules (protein, DNA) at work under a physiological condition because such observation is straightforward to understanding their dynamic behaviors and functional mechanisms. Optical microscopy has no sufficient spatial resolution and electron microscopy is not applicable to in-liquid samples. Atomic force microscopy (AFM) can visualize molecules in liquids at high resolution but its imaging rate was too low to capture dynamic biological processes. This slow imaging rate is because AFM employs mechanical probes (cantilevers) and mechanical scanners to detect the sample height at each pixel. It is quite difficult to quickly move a mechanical device of macroscopic size with sub-nanometer accuracy without producing unwanted vibrations. It is also difficult to maintain the delicate contact between a probe tip and fragile samples. Two key techniques are required to realize high-speed AFM for biological research; fast feedback control to maintain a weak tip-sample interaction force and a technique to suppress mechanical vibrations of the scanner. Various efforts have been carried out in the past decade to materialize high-speed AFM. The current high-speed AFM can capture images on video at 30-60 frames/s for a scan range of 250nm and 100 scan lines, without significantly disturbing week biomolecular interaction. Our recent studies demonstrated that this new microscope can reveal biomolecular processes such as myosin V walking along actin tracks and association/dissociation dynamics of chaperonin GroEL-GroES that occurs in a negatively cooperative manner. The capacity of nanometer-scale visualization of dynamic processes in liquids will innovate on biological research. In addition, it will open a new way to study dynamic chemical/physical processes of various phenomena that occur at the liquid-solid interfaces. [Preview Abstract] |
Session P15: Focus Session: Superconducting Qubits II
Sponsoring Units: GQIChair: Frank Wilhelm, University of Waterloo
Room: Morial Convention Center 207
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P15.00001: Quantum non-demolition measurement of a superconducting two-level system Invited Speaker: In quantum mechanics, measurement can be understood as the interplay between extraction of information and disturbance of the state of the measured system. For projective measurements this disturbance is minimized: the post-measurement state is fully correlated to the indication of the detector. Quantum non-demolition (QND) detection is a strategy used to implement a projective measurement, which relies on a specific type of interaction between the measured system and the detector. In our experiments we apply these principles to the measurement of a superconducting flux qubit, which is an artificial two-level system built using mesoscopic Josephson junctions. Our detection method relies on probing the response of a hysteretic non-linear resonator coupled to the qubit. This setup allows for very efficient detection of the state of our system, with a measured contrast of 85{\%}. The large correlations between the results of two consecutive measurements demonstrate the QND nature of this method. This result establishes the validity of a QND strategy for projective measurement of superconducting qubits and has implications for quantum information processing. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P15.00002: Tunneling of a modulated oscillator: quantum interference in the classically forbidden region Michael Marthaler, Mark Dykman We describe a new coherent quantum effect in periodically modulated systems. It occurs in a modulated nonlinear oscillator and has no analog in two-level systems. The effect consists in oscillations and sign change, with the varying modulation frequency, of the tunnel splitting of the symmetric and antisymmetric time-periodic states. These states are formed by period-2 oscillator states, which classically have the same amplitudes and opposite phases. The effect is due to the wave function oscillations and the related interference in the classically forbidden region of the oscillator phase space. The tunnel splitting oscillations emerge already in the ``ground state'' of the oscillator Hamiltonian in the rotating frame. The WKB analysis in the rotating wave approximation is in excellent agreement with the numerical results. The tunnel splitting oscillations persist in the parameter range where the rotation wave approximation becomes inapplicable. The effect occurs in the parameter range accessible with currently available Josephson junction-based systems. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P15.00003: Dynamical tunneling in macroscopic systems Ioana Serban, Frank Wilhelm We investigate macroscopic dynamical quantum tunneling (MDQT) in the driven Duffing oscillator, characteristic for Josephson junction physics and nanomechanics. Under resonant conditions between stable coexisting states of such systems we calculate the tunneling rate. In macroscopic systems coupled to a heat bath, MDQT can be masked by driving-induced activation. We compare both processes, identify conditions under which tunneling can be detected with present day experimental means and suggest a protocol for its observation [1]. \\ {[1]} I. Serban and F. K. Wilhelm, Phys. Rev. Lett. 99, 137001 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P15.00004: Measurement induced heat bath and decay rates in circuit QED. Maxime Boissonneault, Jay Gambetta, Alexandre Blais In circuit QED, a superconducting qubit is fabricated inside a high quality superconducting coplanar resonator. This system allows for strong interaction of the artificial atom with the photon field [1]. In the dispersive regime, where the detuning between the qubit and the resonator frequency is large with respect to their coupling, the physics of this system is understood in terms of Lamb and Stark shifts. However, as the coupling strength or the number of photons in the resonator increases, this description breaks down. In this talk, we will explain that, when taking into account higher order corrections to the dispersive approximation, measurement photons act as a heat bath inducing incoherent relaxation and excitation of the qubit. We will discuss how this can decrease achievable signal-to-noise ratio and may reduce the QND aspect of the measurement. [1] A. Wallraff, et al., Nature 431, 162 (2004) [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P15.00005: Single shot readout in a circuit QED system Andrew Houck, Alexandre Blais, Steven Girvin, Robert Schoelkopf In the dispersive limit of circuit QED, photon transmission can be used for quantum non-demolition measurements of the state of a superconducting qubit. Here, we present an optimization of the measurement of a transmon qubit, including a new understanding of how measurement affects the rate of demolition. Both cavity and qubit parameters were optimized to maximize signal to noise without introducing substantial new channels for decoherence. Single shot readout fidelities of over 70\% have been acheived, and greater than 90\% fidelity should be possible with presently acheivable coherence times. This opens up the possiblity of observing quantum jumps in the state of the qubit. Work done in collaboration with the Yale circuit QED team. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P15.00006: Dissipation and cooling of a nanomechanical oscillator coupled to a Cooper pair box Rakesh Tiwari, D. Stroud We calculate the dynamics of a nanomechanical oscillator (NMO) coupled capacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger equation with two Lindblad operators. Both the NMO and the CPB are assumed dissipative. We show numerically that, if the CPB decay time is smaller than the NMO decay time, the coupled NMO will lose energy faster, and the coupled CPB more slowly, than the uncoupled NMO and CPB. We find that both of these effects are largest if $\hbar$ times the NMO frequency equals the energy splitting of the CPB. Thus we show that an NMO can be cooled to low temperatures much more efficiently by coupling the NMO to a CPB. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P15.00007: Microwave Reflectometry Measurements of Flux States of a dc SQUID Phase Qubit B. K. Cooper, R. M. Lewis, S. K. Dutta, T. A. Palomaki, Anthony Przybysz, H. Kwon, Hanhee Paik, J. R. Anderson, C. J. Lobb, F. C. Wellstood We examine microwave reflectometry readout of a dc SQUID phase qubit. Our device is a Nb/AlOx/Nb SQUID fabricated by Hypres with loop inductance of 1.3 nH and symmetric junction critical currents of approximately 5 $\mu $A. The SQUID is current and flux biased, with one junction used as the qubit and the other used to provide isolation. The isolation junction is shunted by a large capacitor to depress its plasma frequency to about 1.5 GHz. This frequency can be shifted by flux-induced circulating current in the SQUID loop, allowing us to determine which flux state we are in by making reflectometry measurements of the resonant behavior of the isolation junction. The utility of this measurement for qubit state readout is discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P15.00008: Measurements of a dc-SQUID phase qubit using rf-reflectometry R.M. Lewis, B.K. Cooper, S.K. Dutta, T. A. Palomaki, Hanhee Paik, A. Przybysz, H. Kwon, J. R. Anderson, C. J. Lobb, F. C. Wellstood We performed measurements of a $Nb/AlOx/Nb$ dc--SQUID phase qubit at 100 mK by monitoring the plasma frequency, $f_p$, of the readout/isolation junction. This qubit contains two Josephson junctions (JJ) separated by a 1.3 nH inductance; one JJ operates as a pure phase qubit, the second JJ isolates and reads out the qubit junction. When driving the isolation junction at 1.5 GHz, near $f_p$, current fluctuations in the SQUID loop cause fluctuations in $f_p$ which appear in the sidebands of the reflected microwave power. At 100 mK we find an effective flux noise, $S_{\Phi}^{0.5}$ of $50\ \mu \Phi_0 / Hz^{0.5}$ at 1 Hz. The measurement bandwidth is about 10 MHz, the upper limit being set by the $Q$ of the readout junction. We will discuss how the measurement performance depends on biasing parameters of each junction. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P15.00009: DC SQUID Phase Qubit with LC Filter Hyeokshin Kwon, A.J. Przybysz, Hanhee Paik, R.M. Lewis, T.A. Palomaki, S.K. Dutta, B.K. Cooper, J.R. Anderson, C.J. Lobb, F.C. Wellstood We investigate the use of an inductor-capacitor (LC) network to increase the isolation of a dc SQUID phase qubit from its current bias leads and thereby increase the dissipation time T$_{1}$ and coherence time T$_{2}$. One junction in the SQUID acts as an ideal phase qubit while the second junction and the SQUID loop inductance act as a broadband filter to isolate the first junction from the current bias leads. The LC-isolation network provides an additional isolation factor and allows flexibility in the choice of SQUID parameters. In addition to increasing the isolation from the leads, our design minimizes the effects of dielectric loss and two-level systems by using a relatively small Josephson junction, building the devices from Al/Al$_{2}$O$_{3}$/Al on sapphire, and only using insulating layers (SiN$_{x})$ in external capacitors for the phase qubit junction and LC network. *Funding provided by JQI, CNAM and the DOD. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P15.00010: Strong tunable coupling between a charge qubit and a phase qubit Aurelien Fay, Emile Hoskinson, Florent Lecocq, Laurent Levy, Frank Hekking, Wiebke Guichard, Olivier Buisson We have studied the quantum dynamics of a superconducting circuit based on a dc-SQUID coupled to a highly asymmetric Cooper pair transistor (ACPT). The dc-SQUID is a phase qubit controlled by a bias current and magnetic field. The ACPT is a charge qubit controlled by a bias current, magnetic flux and gate voltage. We have measured by microwave spectroscopy the lowest quantum levels of the coupled circuit as function of applied flux, bias current and gate voltage. Quantum state measurements of the phase and charge qubit are achieved by a nanosecond flux pulse applied to the dc-SQUID. Our circuit enables the independent manipulation of each qubit as well as the entanglement of the quantum states of the two circuits. We observe avoided level crossings between the two qubits when they are put in resonance. The coupling strength is measured over a large frequency range and varies from 100MHz to 1.3GHz. We succeed to realize a tunable coupling between the charge and the phase qubit. The measured tunable coupling strength is well explained by a combination of a capacitive and a Josephson coupling between the two qubits. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P15.00011: Superconducting phase qubit in a ``camel'' potential Emile Hoskinson, Florent Lecocq, Aur\'elien Fay, Nicolas Didier, Ralf Dolata, Alexander Zorin, Frank Hekking, Wiebke Guichard, Olivier Buisson A prototypical phase qubit consists of a single current biased Josephson junction, in which the dynamics of the phase across the junction is analogous to a quantum particle trapped in a quadratic-cubic potential. We demonstrate a phase qubit in a double barrier quadratic-quartic ``camel'' potential. This potential is formed by a 2-junction niobium circuit in a dc-SQUID configuration, with near zero current bias and flux bias close to half a flux quantum. Because of the symmetry of the potential, the qubit is predicted to be optimally insensitive to current fluctuations. We perform a nanosecond single shot measurement by applying a flux pulse which reduces the height of the two potential barriers, allowing the excited state of the qubit to escape by two independent paths to an adjacent flux state of the dc-SQUID. We find Rabi oscillation, Ramsey oscillation, and energy relaxation decay times on the order of 60 ns, 20 ns, and 100 ns, respectively. Via spectroscopy, we show that the effects of current noise are rendered negligible in this circuit. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P15.00012: Properties of a High-T$_c$ Intrinsic Phase Qubit X. Y. Jin, J. Lisenfeld, Y. Koval, A. V. Ustinov, P. M\"uller We discuss the properties of high-T$_c$ intrinsic phase qubits. An intrinsic phase qubit is a superconducting ring made of a Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystal, intercepted by two intrinsic Josephson junction stacks. As a stack consists of many intrinsic Josephson junctions, an intrinsic phase qubit can be regarded as a multi-junction system, i.e. a system of many degrees of freedom in phase space. However, I-V characteristics and switching current distributions of our samples show that an intrinsic phase qubit behaves like a system with only two degrees of freedom, independent of the number of junctions in the stacks, as long as the two stacks are uniform. Due to the large self-inductance, the potential of an intrinsic phase qubit has several minima. In order to perform quantum operations from a single well, a technique using low-frequency microwaves is presented. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P15.00013: ABSTRACT WITHDRAWN |
Session P16: Focus Session: Cytoskeletal Dynamics and Cell Motility I
Sponsoring Units: DBP DPOLY DFDChair: Greg Huber, University of Connecticut Health Center
Room: Morial Convention Center 208
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P16.00001: Actin Disassembly Mediated by Severing, Debranching, and Hydrolysis Invited Speaker: For cells to respond effectively to their environment, the actin cytoskeleton must both assemble and disassemble rapidly in the presence of external cues. A great deal of theory has been focused on assembly, but disassembly has so far received less attention. The talk will describe two theoretical treatments of actin disassembly resulting from debranching, severing, and ATP hydrolysis. 1) The dynamics of \textit{in vitro} actin polymerization caused by filament branching or severing. Via a combination of stochastic-growth simulation and analytic theory, we show that highly branched structures such as those found near the edges of cells cannot persist in steady state. Early in polymerization, highly branched structures form, but disassemble over time leaving very few branched filaments. This causes an overshoot in light scattering intensity as a function of time. Inclusion of the effects of ATP hydrolysis shows that hydrolysis causes an overshoot in the amount of polymerized actin which can be observed in pyrene fluorescence experiments. 2) The interaction between severing and annealing in disassembling a model lamellipodial actin network. The network is treated as a periodic array of crosslinked actin filaments which sever randomly. The lamellipodial actin density drops abruptly as a function of distance from the membrane in the absence of annealing. When annealing is included, the drop is more gradual, and at a critical value of the annealing rate the thickness becomes infinite. It is shown that lamellipodial disassembly is controlled by two characteristic times: the time that a single subunit remains in the network, and the time that it takes for actin polymerized at the membrane to move to the edge of the lamellipodium. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P16.00002: A Possible Role for a Viscous Fingering-Type Instability in Cell Motility Andrew Callan-Jones, Jean-Francois Joanny, Jacques Prost We present a novel flow instability that can arise in thin films of cytoskeletal fluids if the friction with the substrate on which the film lies is sufficiently strong. The motivation for this work are the experiments of Verkhovsky et al. (Verkhovsky et al, Curr. Biol., 9: 11-20 (1999)) in which flat, circular, stationary cell fragments on a substrate, containing only actin and myosin motors, can either spontaneously or under applied force change shape and start moving. In the stationary state in our model, actin polymerizes at the fragment edge and depolymerizes uniformly in the bulk. The initial velocity profile is radial and is imposed by mass conservation for constant polymer density. The radius of the fragment is fixed by conservation of total --- monomer and filamentous---actin. Performing a linear stability analysis of the actin velocity due to perturbations of the fragment boundary, we find that as the dimensionless parameter $\frac{\eta}{\xi R_0^2}\rightarrow 0$, where $\xi$ is the actin-substrate friction, $\eta$ is the viscosity, and $R_0$ is the initial fragment radius, the perturbed velocity obeys a Darcy Law, and combined with the force-free condition at the fragment boundary, this leads identically to a viscous fingering instability. This asymptotic limit should be achievable since $R_0$ can be tuned by making a fragment with enough actin. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P16.00003: The Stochastic Dynamics of Filopodial Growth Garegin A. Papoian, Yueheng Lan, Pavel Zhuravlev A filopodium is a cytoplasmic projection, exquisitely built and regulated, which extends from the leading edge of the migrating cell, exploring the cell's neighborhood. Commonly, filopodia grow and retract after their initiation, exhibiting rich dynamical behaviors. We model the growth of a filopodium based on a stochastic description which incorporates mechanical, physical and biochemical components. Our model provides a full stochastic treatment of the actin monomer diffusion and polymerization of each individual actin filament under stress of the fluctuating membrane. We have investigated the length distribution of individual filaments in a growing filopodium and studied how it depends on various physical parameters. The distribution of filament lengths turned out to be narrow, which we explained by the negative feedback created by the membrane load and monomeric G-actin gradient. We also discovered that filopodial growth is strongly diminished upon increasing retrograde flow, suggesting that regulating the retrograde flow rate would be a highly efficient way to control filopodial extension dynamics. The filopodial length increases as the membrane fluctuations decrease, which we attributed to the unequal loading of the mem- brane force among individual filaments, which, in turn, results in larger average polymerization rates. We also observed significant diffusional noise of G-actin monomers, which leads to smaller G-actin flux along the filopodial tube compared with the prediction using the diffusion equation. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P16.00004: Mechanics of Lamellipodia D. A. Quint, J. M. Schwarz The actin cytoskeleton is a morphologically-complex assembly of cross-linked F-actin filaments. The cytoskeleton provides rigidity for the cell within appropriate time scales so that it can change its shape to, for example, crawl along surfaces. In addition to cross-linking proteins, many other proteins are involved in the assembly of the actin cytoskeleton such as branching proteins, capping proteins, and severing proteins. Presumably these proteins work cooperatively toward the dynamic formation of rigidity. We will initially focus on the role of branching proteins. The F-actin filaments in lamellipodia---protrusions of the mobile edge of a crawling cell---have some overall orientation due to the branching. Branched filaments emerge at a 70 degree angle from the mother filament's growing end.$^1$ This overall orientation is modelled as an anisotropy in an effective medium theory determining the cytoskeleton's elasticity in the static regime. The potential for a splay rigid phase, in addition to a rigid phase, is also investigated. \\ $^1$T. M. Svitkina and G. G. Borisy, {\it J. Cell Biol.} {\bf 145}, 1009 (1999). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P16.00005: Assembly Mechanism of the Contractile Ring for Cytokinesis by Fission Yeast Dimitrios Vavylonis, Jian-Qiu Wu, Xiaolei Huang, Ben O'Shaughnessy, Thomas Pollard Animals and fungi assemble a contractile ring of actin filaments and the motor protein myosin to separate into individual daughter cells during cytokinesis. We studied the mechanism of contractile ring assembly in fission yeast with high time resolution confocal microscopy, computational image analysis methods, and numerical simulations. Approximately 63 nodes containing myosin, broadly distributed around the cell equator, assembled into a ring through stochastic motions, making many starts, stops, and changes of direction as they condense into a ring. Estimates of node friction coefficients from the mean square displacement of stationary nodes imply forces for node movement are greater than $\sim $ 4 pN, similarly to forces by a few molecular motors. Skeletonization and topology analysis of images of cells expressing fluorescent actin filament markers showed transient linear elements extending in all directions from myosin nodes and establishing connections among them. We propose a model with traction between nodes depending on transient connections established by stochastic search and capture (``search, capture, pull and release''). Numerical simulations of the model using parameter values obtained from experiment succesfully condense nodes into a continuous ring. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P16.00006: Nonlinear elasticity of composite networks of stiff biopolymers with flexible linkers Chase Broedersz, C. Storm, F.C. MacKintosh Motivated by recent experiments showing novel rheological properties of biopolymer networks, we develop an effective medium theory for rigid filaments cross-linked by flexible linkers. Specifically, we treat such a network as a collection of randomly oriented stiff polymers mechanically connected by highly compliant cross-linkers to an elastic continuum, which effectively represents the surrounding network. For cross-links with a finite compliance, we find a smooth cross-over between two distinct elastic regimes. Starting from a linear elastic regime dominated by cross-link elasticity, the network begins to stiffen significantly as the cross-links reach full compliance. We extend this model to a self-consistent one, in which the effective medium reflects the non-linear elastic properties of the cross-linked network. This model yields a cross-over to a nonlinear regime that is consistent with recent experimental studies of the cellular cytoskeletal polymer F-actinwith filamin cross-links$^1$. \par\vspace*{0.2cm} $1.$ \hspace*{0.25cm} ML Gardel, F Nakamura, J Hartwig JC Crocker, TP Stossel, DA Weitz, \textbf{103}, 1762 Proc. Nat. Ac. Sci. (2006). [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P16.00007: Effects of Osmotic Force and Torque on Microtubule Bundling and Pattern Formation Yongxing Guo, Yifeng Liu, Rudolf Oldenbourg, Jay Tang, James Valles We report the effect of Polyethylene Glycol (PEG, MW=35kd) on microtubule bundling and pattern formation. Without PEG, polymerizing microtubule (MT) solutions of a few mg/ml [1,2] can spontaneously form striated birefringence patterns through MT alignment, bundling and buckling in coordination. With PEG, bundles become more distinct and the birefringence pattern weakens. Using quantitative birefringence measurements, the average number of MTs in the cross section of a bundle induced by 1{\%} w/w PEG 35kd is determined to be around 26, with a wide spread in size. The amplitude of the buckling is reduced with increased PEG concentration. At sufficiently high PEG concentration ($\sim $0.5{\%} w/w), the pattern is totally suppressed and the sample contracts laterally during the development of a microtubule bundle network. We propose that the decrease of the buckling amplitude is due to the depletion of the dispersed MT network, which is essential for the pattern formation. We attribute the anisotropic contraction to an osmotic torque that drives bundles that cross to align. [1] Y. Liu, \textit{et al}., PNAS 103, 10654 (2006). [2] Y. Guo, \textit{et al}., PRL 98, 198103 (2007). [Supported by NASA (NNA04CC57G, NAG3-2882) and NSF (DMR 0405156)] [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P16.00008: Buckling and force propagation in intracellular microtubules Moumita Das, Alex J. Levine, F.C. MacKintosh Motivated by recent experiments [1] showing the buckling of microtubules in cells, we study theoretically the mechanical response of, and force propagation along elastic filaments embedded in a non-linear elastic medium. We find that embedded microtubules buckle when their compressive load exceeds a critical value $f_c$ which is two orders of magnitude larger than for an isolated MT as found earlier [1], and that the resulting deformation is restricted to a penetration depth that depends on both the non-linear material properties of the surrounding cytoskeleton, as well as the direct coupling of the microtubule to the cytoskeleton possibly through MT-associating proteins (MAPS). The deformation amplitude depends on the applied load $f > f_c$ as $(f-f_c)^{1/2}$. This work shows how the range of compressive force transmission by microtubules can be as large as tens of microns, and is governed by the mechanical coupling to the surrounding cytoskeleton. \newline References: \newline [1] CP Brangwynne et al., J. Cell Biology, 173, 733 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P16.00009: Hydrodynamic tether extrusion from ``gelly'' vesicles Karine Guevorkian, Sebastien Kremer, Francoise Brochard-Wyart Extrusion of cell tethers requires the detachment of the plasma membrane and can be used to probe the strength of membrane-cytoskeleton adhesion. We have studied the hydrodynamic extrusion of tethers from red blood cells [1] and developed a theoretical model based on permeation of lipids through the network of membrane proteins linked to the cytoskeleton [2]. Our aim here is to probe the model on biomimetic systems, namely lipid vesicles filled with artificial cytoskeleton made of synthetic or biological gels, where we can adjust the membrane-cytoskeleton coupling. The properties of tubes extruded from these ``gelly'' vesicles will be compared to simple vesicles on one hand, and to red blood cells or human carcinoid BON cells on the other. [1] N. Borghi et al, Biophys. J. 93 (2007) [2] F. Brochard-Wyart, et al, Proc. Natl. Acad. Sci. USA, 103 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P16.00010: Living Microlens Arrays Jessica Zimberlin, Patricia Wadsworth, Alfred Crosby Using the properties of living cells and early tissue formation, we define adaptable surface structures of three-dimensional, hexagonal arrays of microlenses. These ``living'' microlenses are achieved by growing a monolayer cell sheet on a thin film of polystyrene [PS] attached to a substrate of crosslinked poly(dimethyl siloxane) [PDMS] microwells. The contractile nature of the cells attached to the surface and the compliance of the PDMS surface geometry allows the PS thin film to buckle, forming arrays of convex microlenses. The curvature of the microlens structures is related to the strain applied by monolayer cell sheets to the PS surface. We use this measurement to differentiate the strains applied by two different cell types and relate these strains to differences in the intercellular coupling of the different cell types. We also show that by adding different chemical triggers to the system, the contractile nature of the cells changes, modifying the focal length of the microlenses. This design introduces a new paradigm for advanced materials and offers great promise for a range of applications. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P16.00011: Local viscoelasticity of the surfaces of individual Gram-negative bacterial cells measured using atomic force microscopy Virginia Vadillo-Rodriguez, Terry Beveridge, John Dutcher The cell wall of Gram-negative bacteria performs many important biological functions: it plays a structural role, it allows the selective movement of molecules across itself, and it allows for growth and division. These functions not only suggest that the cell wall is dynamic, but that its mechanical properties are very important. We have used a novel, AFM-based approach to probe the mechanical properties of single bacterial cells by applying a constant compressive force to the cell under physiological conditions while measuring the time-dependent displacement (creep) of the AFM tip due to the viscoelastic properties of the cell. For these experiments, we chose a representative Gram-negative bacterium, \textit{P. aeruginosa} PAO1, and we used AFM tips of different size and geometry. We find that the cell response is well described by a three element mechanical model with an effective cell spring constant $k$ and an effective time constant $\tau $ for the creep motion. Adding glutaraldehyde, which increases the covalent bonding of the cell surface, produced a significant increase in $k$ and a significant decrease in $\tau $. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P16.00012: Stall Force and Response of Lung Cilia Richard Superfine, David Hill, Vinay Swaminathan, E. Timothy O'Brien, Ric Boucher, Brian Button, Ashley Estes We report on the response of lung cilia to applied forces. We have applied magnetic forces to magnetic beads attached to individual human lung cilia in cell cultures. Our magnetic system is capable of generating large forces ($\sim $1nanoNewton on 1 micron beads) with a 3kHz bandwidth. We record the cilia beat motion using video microscopy to record beat frequency and amplitude as a function of applied force. We present three major findings. First, the stall force is approximately 150 pN. Second the frequency is unchanged by the application of forces up to the stall point. Third, the speed of the beat motion slows down according to the diminution of the beat amplitude while maintaining a constant frequency and the speed of the motion is the same whether the beat direction is in the same direction as the applied force or against the applied force. [Preview Abstract] |
Session P17: Quantum Fluids and Solids I
Sponsoring Units: DCMPChair: Garry Williams, University of California, Los Angeles
Room: Morial Convention Center 209
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P17.00001: Simulating the Melting Transition of Helium in Two Dimensions Keola Wierschem, Efstratios Manousakis We study the melting behavior of $^4$He in two dimensions with the path integral Monte Carlo method. Systems of helium atoms are simulated in a periodic cell designed to accommodate a triangular solid. We calculate the translational and orientational order parameters, as well as the defect fraction. Defects are defined as atoms with more or less than six neighbors; the nearest neighbor network is found through Delaunay triangulation. Two dimensional melting is a defect-mediated phase transition, thus, defects will proliferate as the solid is melted. Additionally, melting is expected to occur via a two-stage process, with transitions for both translational and orientational order. At high number density (0.0846 \AA$^{-2}$), we have seen a single transition (within the accuracy of our simulations). We are currently working to observe the melting transition at lower densities. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P17.00002: Lithium, magnesium and sodium $^{4}$He adsorption experiments performed below 1K. Eli Van Cleve, Peter Taborek, James Rutledge We have constructed a $^{3}$He optical cryostat and used a previously developed technique of Cryogenic Pulsed Laser Deposition(CPLD) $^{(1)}$, to deposit films of sodium, lithium and magnesium onto the surfaces of quartz crystal microbalances at cryogenic temperatures. The elements in the first and second column of the periodic table interact weakly with adsorbed helium. Theoretical calculations predict that helium will wet all the elements lighter than rubidium, but solid-like layers will not form, so liquid and superfluid films can exist at sub-monolayer coverage. We will present vapor pressure isotherms on Li, Mg and Na substrates in the temperature range 0.5K -1.3K and discuss the wetting and superfluid onset behavior. We will also present in-situ optical work function measurements of the metallic films, and discuss the relation between work function and wettability. $^{(1)}$ E. Van Cleve, P. Taborek, J.E. Rutledge JLTP online [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P17.00003: Third Sound Propagation in Superfluid $^4$He Films Adsorbed on Carbon Nanotube Bundles Sonny Vo, Tim Hsieh, John Schulman, Gary A. Williams We have observed the propagation of third sound waves in thin superfluid $^4$He films adsorbed on carbon nanotube bundles. The nanotubes are sprayed onto a plexiglass substrate, forming a tangle of interconnected bundles that is about 15 $\mu$m thick and 2.5 cm square, with an average bundle diameter of about 4 nm. A heater and bolometer at opposite corners allow detection of resonant third sound modes, and the third sound speed is deduced from the resonant frequencies. The helium adsorption is greatly affected by the surface tension forces generated by the high curvature of the nanotubes, and the film thickness on the tubes remains very thin compared to the thickness in flat regions of the cell. As helium is metered into the cell at 1.3 K the Kosterlitz-Thouless transition on the nanotubes is observed as the onset of the third sound signal, and then with increasing film thickness the third sound velocity decreases. The velocity appears to be dropping towards zero at a finite value of the film thickness on the tubes, in qualitative agreement with a surface-tension instability predicted for cylindrical geometries by Cole and Saam [Phys. Rev. Lett., \textbf{32}, 985 (1974)]. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P17.00004: Bose-Einstein Coherence in Two Dimensional Superfluid $^4$He Souleymane Diallo, Jonathan Pearce, Richard Azuah, Jon Taylor, Henry Glyde We present high-resolution measurements of the momentum distribution of atoms in liquid $^4$He films adsorbed in nanoporous MCM-41, with 45 {\AA} mean pore diameter. The measurements were performed at temperatures $T=0.3$ K and $T=2.3$ K and saturated vapor pressure (SVP) in the wavevectors range $24\le Q\le 29$ {\AA$^{-1}$} using the MARI time-of-flight (TOF) chopper spectrometer at the ISIS spallation neutron source. The main goal is to determine whether there is a Bose-Einstein condensate (or coherence) in a finite-size two dimensional (2D) Bose fluid at low temperatures. It is also to investigate the 2D-3D dimensional crossover in the condensate proprieties. We find clear evidence of a condensate parameter, $n_0$, at $T=0.3$ K in the films investigated. In the thinnest film ($\sim$ approximately one atomic layer thick), the observed condensate fraction is greater than but consistent with the bulk superfluid 4he value of 7.25\% within precision; i.e. $n_0=(9.34\pm3.84)$\%. As more $^4$He is adsorbed in the substrate pores, $n_0$ appears to decrease below the bulk value, possibly due to the disorder introduced by the confining media; i.e. $n_0=(2.45\pm2.54)$ \% near full pore filling. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P17.00005: Dynamics of one and two dimensional solid $^4$He adsorbed on nanotubes Bjorn Fak, Souleymane Diallo, Mark Adams, Oscar Vilches, Helmut Schober, Henry Glyde In a previous experiment[1], we showed that one dimensional (1D) solid helium can be created on the surface of nanotube bundles. Specifically, when $^4He$ is first adsorbed on nanotubes, it forms a 1D linear solid along the grooves between two nanotubes on the bundle surface with lattice parameter, a$_1$ = 3.40 $\pm$ 0.02 \AA. When more helium is added, 2D solid helium covers the whole bundle surface. We have now determined the vibrational dynamics of these 1D and 2D solids, the dynamic structure factor, $S(Q,\omega)$. From the inelastic intensity integrated over all $\omega$ we obtain the MS amplitude of vibration along 1D chain $\langle u^2\rangle = 0.28$ \AA$^2$or Lindemann ratio $\gamma = (\langle u^2\rangle)^{1/2}/a_1 = 0.15 $ which is less than the bulk solid value near melting. The vibrational density states (DOS) of the 2D solid shows a gap at $\omega\simeq$ 0.75 meV indicating a commensurate solid as found for $^3He$ and $^4He$ on graphite surfaces. In contrast the 1D DOS shows little or no gap and the DOS goes uniformly to zero as $\omega \rightarrow 0$. [1] Pearce et al. Phys. Rev. Lett. 95, 185302 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P17.00006: 100-fold reduction of 2D spin-polarized hydrogen gas's clock-shifts explained Kaden R.A. Hazzard, Erich J. Mueller Recent experiments have observed that when two-dimensional spin-polarized hydrogen is absorbed on a superfluid helium film, the density dependent shift of the 1S-2S spectral line (clock shift) is 100 times smaller than expected [1]. By studying the theory of interactions between hydrogen atoms and the helium surface, we show that helium-mediated hydrogen-hydrogen interactions dramatically reduce the clock shift. The mediated potential is sensitive to experimental parameters, such as temperature and $^3$He concentration. This explains another mysterious experimental result: we find that increasing $^3$He concentration increases the clock-shift, as observed. In contrast, the naive picture which neglects mediated interactions predicts the clock-shift to decrease with $^3$He concentration due to deconfinement of the hydrogen gas. [1] J. Ahokas, J. J\"arvinen, and S. Vasiliev, Phys. Rev. Lett. \textbf{98}, 43004 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P17.00007: Bound spin waves in the ferromagnetic layer of $^3$He on highly oriented graphite Lei Guo, Jinshan Zhang, C.M. Gould, H.M. Bozler The second monolayer of $^3$He on graphites such as Grafoil becomes highly ordered at millidegree temperatures. This system is a good model for nanoscale two-dimensional magnetism because of the large number of separated two-dimensional planes. Motivated by our interest in increasing the structural coherence of the graphite samples that we study, we have used exfoliated ZYX grade graphite as a substrate for our recent experiments. Much of the general picture of finite temperature ordering with ZYX is similar to Grafoil. However, as a byproduct of our increased structural coherence, we have observed several distinct resonances in the ordered spin system. This result is surprising because the structural size of platelets of graphite is not controlled. Nevertheless, the separation of the resonances is consistent with bound two-dimensional spin waves with length scales consistent with the average sizes of the graphite platelets. We will present our analysis of the temperature dependence of the spin wave modes. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P17.00008: Spin Pumping in Superfluid $^3$He in High Magnetic Field H. Kojima, K. Suzuki, Y. Aoki, A. Yamaguchi, H. Ishimoto The spin flow dynamics in superfluid $^3$He A$_1$ phase in magnetic field has been studied up to 13 tesla. The apparatus consists of a large reservoir of of A$_1$ phase in which a small enclosed chamber with a built-in differential pressure sensor is immersed. The chamber is connected to the reservoir via a superleak channel. The chamber is fabricated from Macor parts such that the residual heat leak is much reduced from those in our experiments. Our focus is on the measurement of relaxation of the induced pressure subsequent to either magnetically induced spin-polarized superflow or by electrostatic spin pumping. In general, both methods of measurement show that the relaxation time ($\tau$) of the induced pressure tends to vanish smoothly as the transition temperature T$_{c2}$ is approached. However, the observed dependence of $\tau$ on magnetic field is different. The measured $\tau$ by the field gradient method continues to increase up to 8 tesla. On the other hand, $\tau$ measured by the spin pumping method tends to saturate to a constant between 5 and 13 tesla. The discrepancy is unexpected and not yet understood. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P17.00009: Discovery of a New Excited Pair State in Superfluid $^{3}$He John P. Davis, Johannes Pollanen, Hyoungsoon Choi, James A. Sauls, William P. Halperin In superfluid $^{3}$He, the order parameter collective modes correspond to excited states of the $^{3}$He Cooper pairs and are classified by their total angular momentum, \textit{J = L + S}. Many of these modes with $J \le $ 2 have been experimentally observed through longitudinal sound measurements or NMR. As a result of coupling to the collective mode with $J $= 2 and $m_{J }=\pm $1 there is an enhanced restoring force for transverse sound in superfluid $^{3}$He-B. Previously, we have used the interference of transverse sound waves to study this collective mode. Recently we have discovered a new coupling to transverse sound near the pair-breaking threshold with the classic signatures of a collective mode. Application of a magnetic field results in circular acoustic birefringence and a new acoustic Faraday effect, from which we extract the corresponding Verdet constant. Selection rules for the coupling to transverse sound and acoustic birefringence require this mode to have $J \ge $ 4, suggesting that this mode is most likely the $J $= 4 ($m_{J }=\pm $1) mode resulting from an attractive $f$-wave pairing interaction in this $p$-wave superfluid. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P17.00010: Direct Sound Propagation in Superfluid $^{3}$He-A in 98\% Aerogel B. H. Moon, N. Masuhara, P. Bhupathi, M. Gonzalez, M. W. Meisel, Y. Lee, N. Mulders Liquid $^{3}$He impregnated in high porosity aerogel has been studied extensively in recent years since its unique structure provides static impurities in this system. The fragile nature of p-wave Cooper pairs against impurity was clearly demonstrated by the significant depression of the superfluid transition. The scattering off the aerogel also significantly modifies the low energy excitation by inducing impurity bound states inside the gap. Recent ultrasound attenuation measurements performed in the B-like phase of superfluid $^{3}$He in 98\% porosity aerogel revealed many interesting features and provided strong experimental evidence of gapless superfluidity. We conducted high frequency sound propagation measurements at 6.22 MHz in the A-like phase of superfluid $^{3}$He. The A-like phase is stabilized by magnetic fields (up to 4 kG) applied perpendicular to the direction of sound propagation. We present our preliminary results of ultrasound attenuation down to the zero temperature limit at 29 bar and the field dependent A-B transition identified by the jump in attenuation. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P17.00011: Effect of Global Anisotropy on Superfluid $^{3}$He in Compressed Aerogels P. Bhupathi, B. H. Moon, M. Gonzalez, Y. Lee, N. Mulders The importance of anisotropic scattering on the superfluid phases of $^{3}$He has been addressed recently and experiments using uniaxially distorted aerogel have been proposed in order to elucidate the influence of global anisotropy on the A-B transition $\footnote{C. L. Vicente {\it et al.}, Phys. Rev. B 72 094519 (2005).}$$^{,}$$\footnote{Kazushi Aoyama and Ryusuke Ikeda, Phys. Rev. B 73, 060504(R) (2006).}$. We performed high frequency transverse acoustic impedance measurements on superfluid $^{3}$He confined in 98{\%} porosity aerogel at 29 bar. The aerogel cylinder is compressed along the symmetry axis to generate global anisotropy. With 10{\%} axial compression, our measurements reveal that the A-like to B-like transition is absent on cooling down to $\approx $300 $\mu $K in the absence of magnetic field and in magnetic fields up to 3 kG. This behavior is in contrast to that in uncompressed aerogels, in which the supercooled A-like to B-like transitions have been identified by various experimental techniques. Our results are consistent with the theoretical prediction by Aoyama and Ikeda. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P17.00012: Low field NMR in aerogel-confined superfluid $^3$He Yuliang Du, H.M. Bozler, C.M. Gould The superfluid states of bulk liquid $^3$He were convincingly identified through their longitudinal and transverse NMR spectra. The order parameters of the superfluid phases of $^3$He confined within aerogel have generally been assumed to be identical to those in bulk liquid. While that identification has not been contradicted by experimental data, it has not yet been tested as carefully as in bulk. Fomin has suggested that the A-like phase in aerogel could be an axiplanar state, distinct from the bulk axial state. We have tested the identification by studying low-field NMR which is more sensitive to the distinction between the candidate states. Using the dc SQUID based NMR detection system developed in our laboratory over many years we have studied both longitudinal and transverse resonance spectra in 99.5\% porosity aerogel in magnetic fields of 1-4 mT, an order of magnitude lower than previous NMR work. Our work shows qualitative features similar to those found in higher magnetic fields. While we were unable to resolve the longitudinal resonance, transverse resonance measurements exhibit a characteristic field- and temperature-dependence. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P17.00013: Dynamics of Quantum Vortices Lara Thompson, Philip Stamp Quantized vortices exist in systems ranging from low-T magnets, to superfluids and superconductors; however, their dynamics remain controversial. Even the existence of a force acting transverse to the motion (like a Lorentz force) relative to thermal quasiparticles has been widely debated. Quite remarkably, it remains unresolved just what forces act on a quantum vortex. From an influence functional calculation, we show that the expected log divergent mass generalizes to a frequency dependent mass and damping, which, in time, manifest as memory dependent damping forces, acting both longitudinal and transverse to current motion. Because topological properties are involved, our results apply equally to quantum vortices in many different systems. For instance for vortices in insulating magnets, we are able to find the various forces, including those resulting from vortex-magnon interactions, and derive their dynamics. In contrast to superfluids and superconductors, an experimental test in insulating magnets should be possible using existing methods. [Preview Abstract] |
Session P18: Polymer Nanocomposites I
Sponsoring Units: DPOLYChair: Venkat Ganesan, University of Texas at Austin
Room: Morial Convention Center 210
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P18.00001: Disordered nanoparticle interfaces for defect-tolerance in the self-assembly of block-copolymers Kevin Yager, Alamgir Karim, Eric Amis Directed self-assembly is a promising route to controlling the nanostructure and surface properties of coatings. We describe a general and robust strategy for controlling the self-assembly of thin films by tuning the film-substrate interaction, using an inherently defective nanoparticle layer. These tunable surfaces exhibit hierarchical and controllable roughness via spin-coating conditions (20 nm silica nanoparticle solutions), and tunable surface energy via selective oxidation. Independent manipulation of these parameters enables control of self-assembled order for coatings cast on these tunable substrates. In particular, we demonstrate control of the orientation of lamellae in poly(deuterated-styrene-block-methyl methacrylate), with expression of the vertical lamellae orientation under certain conditions. Moreover we demonstrate that the lamellae orientation depends upon film thickness in a periodic manner in the range from 30 nm to 240 nm, which provides insights into the fundamental driving forces in this self-assembly. The proposed assembly orientations are compared with theory and validated by complementary neutron reflectivity and small-angle neutron scattering measurements. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P18.00002: Anisotropic Self-Assembly of Spherical Nanoparticles in Polymer Composites Pinar Akcora, Sanat K. Kumar, Yu Li, Brian Benicewicz, Linda S. Schadler, Devrim Acehan, Jack F. Douglas I will present our recent experimental findings on the organization of isotropic polymer grafted particles forming anisotropic three dimensional structures. Earlier studies have shown that particle shape and anisotropic particle interactions determine the self-assembly process. It has been also shown that isotropic particles can form string like colloidal assemblies within monolayers at two-dimensions but at high particle loadings. Here, I will present the three dimensional sheet structures formed by mixing spherical nanoparticles that are grafted uniformly with long polymers and dispersed in the same homopolymer matrices at relatively low loadings. The molecular origin of this anisotropic organization is the combined short ranged repulsive forces and longer ranged attraction interaction between the particles that is also supported through theory and numerical simulations. The self-assembly of isotropic nanoparticles into anisotropic structures within polymer melts has profound application potentials in improving the electrical and mechanical properties of composite materials. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P18.00003: Enthalpic Relaxation of Silica-Polyvinyl Acetate Nanocomposites. Samuel Amanuel, Sanford Sternstein While the effects of nanoparticle size and surface treatment on the glass transition temperature have received well-deserved attention, their effects on other physical parameters associated with the glass transition have received less interest. In order to understand how the incorporation of nanofillers affects the enthalpic relaxations associated with the glass transition, Differential Scanning Calorimeter (DSC) measurements were carried out on silica-polyvinyl acetate nanocomposites with respect to filler content, annealing temperature and annealing period. As expected, longer annealing periods below the glass transition temperature result in an increase of the subsequent enthalpic relaxations. However, the presence of filler substantially reduces the enthalpic relaxation relative to that of the neat polymer. Even after corrections to account for filler weight, the enthalpic relaxations still decrease monotonically with increasing filler content. The underlying enthalpic relaxations and the effects suppressed by the fillers are specific to the annealing temperature. These results suggest a significant alteration of the physical state of the matrix material by the presence of the filler particles. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P18.00004: Particle structuring in stretched soft/hard nanocomposite Yann Le Diagon, Stephanie Mallarino, Christian Fretigny The deformation mechanisms of nanocomposites made of disordered rigid inclusions imbedded in a soft matrix are rather complex as the local geometry and mechanical responses are very intricate. Atomic force microscopy (AFM) is used to analyze the surface of a model elastomer submitted to uniaxial traction. Since the sample contains monodisperse spherical rigid inclusions, images easily yield statistical data on the positions of the fillers. As expected, it is observed that the displacement field is affine at large scales. At short range, important deviations are observed. The 2D-structure factors present the characteristic ``butterfly'' patterns, similar to the neutron scattering patterns obtained on many deformed heterogeneous materials. We show that mechanical confinement of the inter-particular matrix regions must be taken into account in order to explain the results. Finally AFM images reveal higher order correlations: Fillers are observed to be arranged along lines which are roughly perpendicular to the stretching direction. Such a characteristic organization seems to be characteristic of the soft/hard disordered systems. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P18.00005: Particle inclusion effect on the rheological properties of polymeric materials Gregory Toepperwein, George Papakonstantopoulos, Juan de Pablo Recently developed algorithms have permitted the extraction of the primitive path and the entanglement length, Ne, from simulations of polymer melts. Experimental studies on the effect of the addition of nanoparticles to a polymer melt have revealed that nanoparticles can alter the plateau modulus and subsequently the entanglement length of the polymer. We use simulations to directly estimate the entanglement length of nanocomposite systems to study the effect of spherical and anisotropic nanoparticles on the entanglement length of a polymer matrix. In this work we present a systematic study of the effects of particle-polymer interactions, particle size, aspect ratio and volume fraction. Advanced Monte Carlo techniques involving chain connectivity algorithms are used to create statistically independent configurations. Attractive, neutral and repulsive polymer particle interactions are considered. Furthermore, we examine the effect of different assumptions on the calculation of Ne. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P18.00006: Dimensional Analysis of Percolation Theory: Applications to Polymer Composites Derrick Stevens, Torissa Hoffman, Russell Gorga, Laura Clark Percolation theory is well known to describe functional phenomena in polymer composites such as electrical conductivity, when combining a conducting particle and insulating matrix. Nanostructured composites can, however, present unique morphologies that are not easily described by the typical one, two, or three-dimensional viewpoint. One example is random mats of polymer/carbon nanotube (or silver nanoparticle) composite nanofibers. With this motivation, Monte Carlo simulations were developed to investigate various effects within such a fibrous geometry, including changes to the critical volume fraction due to the dimensionality: particle aspect ratio, relative size of particle and fiber (or film), sample size, continuous vs. porous structure. From these simulations a model was developed to predict the percolation threshold based on the dimensionality of the system. The results of these simulations and the derived model will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P18.00007: A hybrid particle-field (HPF) simulation method for polymer-nanoparticle composites Dominik Duechs, Scott Sides, Glenn Fredrickson A hybrid particle-field (HPF) simulation method for composite systems of polymers and sets of movable particles is presented. The particles are implemented as cavities in an otherwise flat density profile of the polymeric background (corresponding to overall incompressibility), enabling a common treatment of both components in the general framework of well-known self-consistent field theory (SCFT). The particle movements are controlled by force-biased and, where applicable, torque-biased propagation schemes. Particles and polymers interact via local contact interactions originating at the particle surfaces. Simulations with orientationally homogeneous spheres as well as with spheres possessing two distinct sets of interaction parameters on their two hemispheres are compared to experimental results. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P18.00008: Responsive Assemblies: Gold Nanoparticles with Mixed Ligands in Microphase Separated Block Copolymers Jinbo He, Elizabeth Glogowski, Qifang Li, Todd Emrick, Thomas Russell, Xuefa Li, Jin Wang A simple method for controlling the spatial distribution of gold nanoparticles in a polystryrene-\textit{block}-poly (2-vinylpyridine) (PS-$b$-P2VP) diblock copolymer is shown. By varying the ligand functionality of the gold nanoparticles, as well as the processing and annealing conditions, the distribution of gold nanoparticles in the microdomains of the diblock copolymer was controlled and altered. In addition, the presence of nanoparticles was also found to affect the diblock copolymer morphologies. Subsequent thermal annealing causes a coarsening of the nanoparticles, and a sequestration of the nanoparticles to the P2VP microdomain. Further heating leads to an expulsion of the particles from the microdomains, a modification of the interfacial interactions, and a reorientation of the copolymer morphology. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P18.00009: Dynamics of polystyrene/polystyrene-capped gold nanoparticle mixtures Peter Green, Hyunjoon Oh Studies of the chain dynamics and of the glass transition, T$_{g}$, of low molecular weight, unentangled, bulk mixtures of polystyrene (PS) with polystyrene-capped gold (AuPS$_{10})$ nanoparticles were performed using dielectric spectroscopy, capacitive scanning dilatometry and differential scanning calorimetry. Mixtures containing up to 5 weight percent of nanoparticles were examined; the nanoparticles were well distributed throughout all samples. A significant reduction of T$_{g}$ was observed with the addition of the AuPS$_{10}$ nanoparticles. The dielectric spectroscopy measurements reveal significantly decreases in $\alpha $ relaxation times in these nanocomposites compared to pure PS. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P18.00010: Magnetic Nanoparticle Dispersion in HOMO and Block Copolymer Films Russell Composto, Kohji Ohno, Vincent Ladmiral, Grant Smith, Dmitry Bedrov, Chen Xu Self-assembly in polymer films containing nanoparticles (NPs) can result in novel structures with attractive properties that depend on NP functionality. Here, magnetic iron oxide NPs (5nm) are dispersed in both poly(methyl methacrylate) (PMMA) and lamella forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films. The NPs are grafted with PMMA brushes ranging from 3k to 37k Da. Increasing brush length improves dispersion in PMMA in agreement with trends observed for potential of mean force between two nanoparticles as obtained from coarse-grained molecular dynamics simulations of equivalent systems. For the 3k brush, NPs disperse uniformly in PS-b-PMMA only at low loadings (1wt{\%}), locate in the PMMA domains, and slow down the perpendicular to parallel morphology transition. At 10 wt{\%}, the NPs form uniform sized aggregates ($\sim $22nm) and perturb the lamellar morphology. Increasing brush length leads to aggregation in the solution state and as a result large aggregates in the spin cast films. A correlation between the magnetic properties and aggregate size is observed. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P18.00011: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P18.00012: Novel Non-toxic Antifouling/Fouling Release Nanocomposite Materials Jason Fang Biofouling is a significant environmental problem. Traditional solutions to this problem have involved incorporation of toxic organometallic species into the paint. This approach while effective, is harmful to the environment. The resultant ban on the use of many of these coatings has created a need for alternative systems to control marine fouling. Silicones represent the only class of polymers currently used commercially, due to their inherently low surface energy, glass transition temperature, and modulus, combined with good chemical stability and ease of application. In this talk I will present our efforts to develop a new generation of practical, non-toxic coatings that combine antifouling/fouling release characteristics with good mechanical properties, ease of application and low cost. Specifically we have been focusing on a series of fouling release coatings based on PDMS-polyurea segmented copolymers and nanocomposites. The PDMS copolymers are much stronger than pure PDMS yet they exhibit fouling release performance comparable and, in some cases, better than pure PDMS. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P18.00013: Physical Characterization of Hierarchically Structured Nanocomposites Ross Behling, Eric Cochran In this contribution we present various aspects of the thermodynamics of self assembly in block copolymer / layered silicate nanocomposites (BCPLSs). Hierarchically structured BCPLSs were prepared using an \textit{in situ} atom transfer radical polymerization (ATRP) approach. The three part synthesis of the materials included an ion exchange functionalization of the clays, sonication during styrene (St) polymerization, and a block copolymer \textit{tert}-butyl acrylate (tBA) synthesis. Highly organized lamellar structures were formed with a periodicity of $\sim $200 nm, much larger than the $\sim $35 nm periods of bulk block copolymers of comparable molecular weights. The final material had two distinct glass transitions (Tg) 69\r{ }C for the tBA and 103\r{ }C for the St. This is a significant Tg enhancement for atactic tBA (Tg = 42\r{ }C) and is attributed to the chain extension which occurs in the confined geometry of the silicate sheets. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P18.00014: Conducting polymer nanofibers for high sensitivity detection of chemical analytes. Abhishek Kumar, Ignaty Leshchiner, Subhalakshmi Nagarajan, Ramaswamy Nagarajan, Jayant Kumar Possessing large surface area materials is vital for high sensitivity detection of analyte. We report a novel, inexpensive and simple technique to make high surface area sensing interfaces using electrospinning. Conducting polymers (CP) nanotubes were made by electrospinning a solution of a catalyst (ferric tosylate) along with poly (lactic acid), which is an environment friendly biodegradable polymer. Further vapor deposition polymerization of the monomer ethylenedioxy thiophene (EDOT) on the nanofiber surface yielded poly (EDOT) covered fibers. X-ray photo electron spectroscopy (XPS) study reveals the presence of PEDOT predominantly on the surface of nanofibers. Conducting nanotubes had been received by dissolving the polymer in the fiber core. By a similar technique we had covalently incorporated fluorescent dyes on the nanofiber surface. The materials obtained show promise as efficient sensing elements. UV-Vis characterization confirms the formation of PEDOT nanotubes and incorporation of chromophores on the fiber surface. The morphological characterization was carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P18.00015: Relaxation Dynamics of Nano Particles Embedded in a Soft Glassy Matrix Jaydeep Basu, Sunita Srivastava, Ajoy Kandar, Mrinmay Mukhopadhyay, Laurence Lurio, Sunil Sinha Using x-ray photon correlation spectroscopy, we have studied slow, wave vector and temperature dependent microrheology of nano particles embedded in glassy matrix with unique viscoelastic properties. The measurements were done for a polymer matrix (PMMA) using gold nanoparticles as probe. The intensity auto-correlation function exhibits a cross-over from compressed to stretched relaxation behavior on cooling from above the glass transition temperature (T$_{g})$ of PMMA. Although stretched exponential relaxation is expected in the glassy state one would expect simple exponential relaxation above the T$_{g}$. We also find that the relaxation time ($\tau)$, follows $\tau \sim $ q$^{-1}$ dependence indicating super-diffusive motion of nanoparticles. Interestingly, we have also observed subtle effects like length scale dependence of the stretching exponent. This points to the importance of the nanoparticles in modifying the viscoelastic property of the polymer matrix and highlights the strength of this technique in extracting their micro-rheological properties. [Preview Abstract] |
Session P20: Focus Session: Engineering Interfaces for New Materials I: Internal Interfaces
Sponsoring Units: DMPChair: Yue Qi, General Motors R&D
Room: Morial Convention Center 212
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P20.00001: Segregation Effects at Internal Interfaces in Alloys: Atom-Probe Tomographic Experiments and Simulations Invited Speaker: This talk first focuses on experimental studies of solute segregation effects on an atomic scale of solute segregation at grain boundaries (GBs) and heterophase interfaces employing atom-probe field-ion microscopy and three-dimensional atom-probe tomography; both instruments provide a spatial resolution of ca. 0.2 nm in direct space. It is demonstrated that the Gibbsian interfacial excess of solute at an internal interface depends on its five macroscopic degrees of freedom (DOFs), which is consistent with J. Cahn's local phase rules for GBs and heterophase interfaces. Experimental data is presented for GBs in metallic alloys (e.g. Fe-Si, Al-Sc-Mg, Ni-Al-Cr alloys), and metal silicide/silicon and indium arsenide heterophase interfaces. Secondly, atomic-scale simulations will be presented of GB segregation in binary metallic alloys described by embedded-atom method potentials employing Metropolis algorithm Monte Carlo simulations, which further demonstrate the intimate relationships between GB structure, on an atomic scale, and the Gibbsian interfacial excess of solute. It is also shown how the microscopic DOFs of a GB affect the Gibbsian interfacial excess of solute. Additionally, the results of atom-probe tomographic studies of segregation effects at heterophase interfaces between the gamma (f.c.c.) and gamma prime (L1$_{2}$ structure) heterophase interfaces in Ni-Al-Cr alloys are discussed and compared in detail with the results of lattice kinetic Monte Carlo (LKMC) simulations, which involves a vacancy mediated diffusion mechanism. The LKMC simulation allow us to explain the role of vacancy-solute binding energies on the observed concentration profiles of Ni, Al, and Cr between the gamma and gamma prime phases. These detailed experimental and simulation studies of segregation effects result in a relatively new atomistic picture of segregation at internal interfaces that differs from the conventional wisdoms found in the literature concerning segregation. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P20.00002: Interface structure and radiation damage resistance in Cu-Nb multilayer nanocomposites Michael Demkowicz, Richard Hoagland, John Hirth We use atomistic simulations to show that misfit dislocations in Cu-Nb interfaces can shift location between two adjacent planes by forming pairs of extended jogs, a mechanism that involves removal or insertion of atoms. Different jog combinations give rise to interface structures with unlike densities but nearly degenerate energies, making Cu-Nb interfaces virtually inexhaustible sinks for irradiation-induced point defects and catalysts for efficient Frenkel pair recombination. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P20.00003: Threshold Shear Stresses at Aluminum-Silicon interfaces Alice Noreyan, Yue Qi, Vesselin Stoilov The critical shear stress (CSS) was determined using molecular dynamics and MEAM potential for various Al/Si interfaces with different alignments (normal to the interface) and orientations (parallel to the interface). It was found that the primary influence parameter for CSS was the general crystallographic alignment of the interface. For all Al/Si interfaces the fracture under shear is mostly localized within 10 {\AA} in Al close to the interface. The critical shear stress of Al/Si interface is significantly lower than the critical tensile stress due to the partial stick-slip in sliding. In addition, there is not explicit relationship between shear and tensile critical stresses, which is dramatically different from isotropic materials, where the shear stress is about half of the tensile stress. The mis-orientation effects show great contrary in homogenous Al/Al interfaces and heterogeneous Al/Si interface: the mis-orientation can reduce the CSS at Al/Al interfaces by two orders of magnitude; while it has insignificant effect on CSS in Al/Si. Therefore, in general, introducing Si improves the strength of the interface (and the composite material in general) for different grain orientations. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P20.00004: The strength characterization of Al/Si interfaces with a hybrid nanoindentation/FEM method Shuman Xia, Yue Qi, Thomas A. Perry, Kyung-Suk Kim The mechanical property characterization of the reinforcement/matrix interface in a metal matrix composite (MMC) is entailed for tailoring the interface in the microstructure design of the composite. In this work we developed a hybrid method to characterize the interface strength of an MMC, combining a nanoindentation experiment and a finite element analysis. The nanoindentation experiment was carried out by indenting individual reinforcement particles on a free surface with a nanoindenter. The dependence of indentation response on the interface properties was systematically studied through the finite element analysis with cohesive zone modeling of the interface failure. The interface strength could then be extracted from the comparison between the experimental and FEM results. With this method, the shear strength of an Al/Si interface was measured approximately 240MPa which compares well with the lower bound of an atomistic simulation with a modified EAM potential. The intrinsic fracture toughness of the interface crack tip surrounded by densely populated dislocations was measured 0.25 J/m2. We also studied the effect of the strontium modification on the interface strength with this hybrid method. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P20.00005: Atomic Resolution Study of the Interfacial Bonding at Si$_3$N$_4$/CeO$_{2-\delta}$ Grain Boundaries Robert F. Klie, Weronika Walkosz, Serdar Ogut, A. Borisevich, Paul F. Becher, Steve J. Pennycook, Juan C. Idrobo Using a combination of atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si$_3$N$_4$ $(10 \overline{1}0)$ and CeO$_{2-\delta}$ inter-granular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different compared to the structure observed in a previous study. Our EELS experiments show that {\bf (i)} oxygen is present at the interface in direct contact with the terminating Si$_3$N$_4$ open-ring structures, {\bf (ii)} the Ce valence state changes from +3 to +4 in going from the interface into the IGF, and {\bf (iii)} while the N concentration decreases away from the Si$_3$N$_4$ grains into the IGF, the Si concentration remains uniform across the whole width of the IGF. Possible reasons for these observed structural and electronic variations at the interface and their implications for future studies on Si$_3$N$_4$/rare-earth oxide interfaces are briefly discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P20.00006: The Influences of different cathode materials on Tris-(8-Hydroxyquinoline)- Aluminum Doped with CsNO$_{3}$ in Organic Light emitting Devices Mei-Hsin Chen, Yin-Jui Lu, Chung-Chih Wu, Chih-I Wu This paper presents the investigations of interfacial interactions and electron-injection mechanisms between cesium nitrate (CsNO$_{3})$ and different cathode materials. By using ultraviolet and x-ray photoemission spectroscopy, the properties of electronic structures and the interfacial chemistry are studied. According to our results, there exists a phenomenon of electron exchange at the interface results in changes of Aluminum 2s core level binding energy by 1 eV when aluminum was deposited on CsNO$_{3.}$ This means electrons transfer from cathode materials to the surface of CsNO$_{3}$, forming a strong dipolar field at the interface and reduction of the electron injection barrier. But, in contract, there exists nearly no reaction between CsNO$_{3}$ and silver cathode. The evidences show that CsNO$_{3}$ is more effective only with aluminum cathode due to a reaction between Aluminum, Cesium and Nitrogen atoms. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P20.00007: Atomic-resolution study of cobalt valence and spin-state transitions in Ca$_{3}$Co$_{4}$O$_{9}$ using in-situ scanning transmission electron microscopy. Guang Yang, Yuan Zhao, Robert Klie The misfit-layered Ca$_{3}$Co$_{4}$O$_{9 }$(CCO) has been of great interest due to its high thermo-electric power and thermal stability. The CCO structure consists of five layers: three rock salt-type layers Ca$_{2}$CoO$_{3}$ are sandwiched between two CdI$_{2}$-type CoO$_{2}$ layers along the c-direction. The presence of different Co valence states is assumed to account for the thermal stability of CCO, and the abrupt changes of electrical resistivity at 420K is believed to be due to a Co spin-state transition. Here, we combine scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to study the atomic and electronic structure of CCO. Using atomic-column resolved EELS, the Co valence states in the different layers are quantified and significant charge transfer from CoO$_{2}^{ }$to Ca$_{2}$CoO$_{3}$ is measured. The effects of the potential spin-state transition at 420K on the local structure will be studied by in-situ heating experiments. We will show how atomic-resolution Z-contrast imaging in combination with EELS and in-situ experiments can be utilized to understand the effects of interfacial charge transfer and spin-state transitions in complex oxide materials. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P20.00008: The Classical Size Effect: Impact of Grain Boundaries on Resistivity in Encapsulated Cu Thin Films Tik Sun, Bo Yao, Andrew Warren, Kevin Coffey, Vineet Kumar, Katayun Barmak Surface and grain boundary electron scattering contribute significantly to resistivity as the dimensions of polycrystalline metallic conductors are reduced to, and below, the electron mean free path. A methodology is developed to independently evaluate surface and grain boundary scattering in encapsulated polycrystalline Cu thin films, with thicknesses in the range of 27-165 nm. The film resistivity, measured at both room temperature and at 4K, is compared for samples having different grain sizes (as determined from 400 to 1,500 grains per sample) and film thicknesses. The experimental data is compared to models of surface and grain boundary scattering in thin films. The resistivity contribution from grain boundary scattering is found to be significantly greater than that of surface scattering in Cu thin films which allows a quantitative measurement of the parameters for the Mayadas-Shatzkes model. It is also found that the Ta barrier layer prohibit grain growth which explains the higher resistivities observed in encapsulated Cu samples with Ta barrier layers. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P20.00009: Resistivity induced by electron-grain boundary scattering on thin gold films deposited on mica substrates under high vacuum Raul C. Munoz, Ricardo Henriquez, Pablo Leiva, Simon Oyarzun, Marco A. Suarez, Paula Manriquez, Simon Silva, German Kremer, Luis Moraga We report measurements of the room temperature resistivity and of the grain size distribution on a family of gold films deposited onto mica substrates under high vacuum. The films are of the same thickness (100 nm $\pm$ 10{\%}), deposited at the same rate (3 nm/min), varying both the temperature of the substrate and the annealing temperature (if any) between --170 $^{\circ}$C and +270 $^{\circ}$C. The grain distribution was measured with a Scanning Tunneling Microscope. The average grain size decreases from approximately 200 nm to a few nm, when the temperature of the substrate decreases from +270 $^{\circ}$C to --170 $^{\circ}$C during evaporation. The monotonic decrease in grain size leads to a monotonic increase in resistivity of almost one order of magnitude. The resistivity of the film evaporated with the substrate held at +270 $^{\circ}$C and annealed for one hour at +270 $^{\circ}$C after evaporation, exhibits a room temperature resistivity only a few percent larger than crystalline gold. Research funded by FONDECYT 1040723. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P20.00010: The effect of hydrogen content on intrinsic stress in nanocrystalline diamond (NCD) coatings Haibo Guo, Yue Qi, Xingcheng Xiao, Abhishek Kothari, Brian W. Sheldon The stress control is critical to ensure the reliability of nanocrystalline diamond (NCD) coatings. We found the intrinsic stress in NCD is tensile at deposition temperature above 700$^{\circ}$C. Decreasing the deposition temperature decreases this tensile stress, and eventually leads to compressive stresses. The stress evolution appears to be largely dictated by grain boundary formation and hydrogen incorporation, which involves absorption, desorption, and recombination kinetics on diamond surfaces. The competition between these reactions indicates that the hydrogen coverage at interfaces should increase with decreasing growth temperature. This is consistent with Raman spectra and elastic recoil detection. To understand hydrogen effects, density functional theory (DFT) is used to model the coalescence of two diamond grains that approach each other to form a grain boundary. The two surfaces exhibit attractive forces when hydrogen coverage is less than 75{\%}, and repulsive forces when all the surface bonds are hydrogen terminated (100{\%} hydrogen coverage). In this way, differences in the hydrogen coverage can explain the observed transition from tensile to compressive intrinsic stress as the growth temperature decreases. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P20.00011: Super Hard Cubic Phases of Period VI Transition Metal Nitrides: A First Principles Investigation S.V. Khare, S.K.R. Patil, N.M. Mangale, S. Marsillac We report a systematic study of mechanical and electronic properties of 32 cubic phases of nitrides of the transition metals M (M = Hf, Ta, W, Re, Os, Ir, Pt, Au), in zinc-blende, rocksalt, pyrite, and fluorite structure using \textit{ab initio} computations. Our results reveal that MN$_{2 }$(M = W, Re, Os, Ir, Pt, Au) in pyrite phase, have a bulk moduli greater than 330 GPa, MN$_{2 }$(M = Re, Os, Ir) in fluorite phase have a bulk moduli greater than 350 GPa and TaN in rocksalt phase has a bulk modulus of 380 GPa making them candidates for super hardness. Based on the bulk and shear modulus for stable phases, potential hard coating materials for cutting tools have been identified. The local density of states of all phases has been obtained and linked to mechanical stability. The high values of bulk moduli are attributed to strong bonding of transition metal d-orbitals with nitrogen p-orbitals. The trend in the bulk modulus is related to the valence electron density of these materials. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P20.00012: Coherent atomic motion in nano-crystal film Junjie Li, Xuan Wang, Shouhua Nie, Richard Clinite, Jianming Cao We report a theoretical study of the structural dynamics in metallic film in response to ultrafast laser heating. A two-dimensional model using a harmonic approximation is used to simulate the lattice thermal expansion dynamics in thin films. The results show that the surface shape and the orientations of nano-crystal grains are essential to determine the modes of lattice motions. Moreover, a large projection of coherent lattice oscillation in the in-plane direction is found, which was previously thought to be very small and was neglected in one-dimensional models. The simulation agrees well with our femtosecond electron diffraction measurements [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P20.00013: Tuning physical properties by assembling subnanometer inorganic and organic units Yong Zhang, P.A. Parilla, S.P. Ahrenkiel, A. Mascarenhas, Z. Islam, Y. Ren, P.L. Lee, M.J. McNevin, I. Naumov, H.X. Fu, X.Y. Huang, J. Li Designing inorganic-organic hybrid materials in a nanoscopic scale allows taking the full advantage of the two worlds, which has recently been demonstrated in a new family of hybrid crystalline materials that are the fully ordered assemblies of sub-nanometer scale inorganic units (e.g., few monolayer-thick slab, single atomic chain) and organic molecules[1].They have been shown to exhibit a number of unique properties that are not readily available in either of the components or their nanostructures: for instance, strongly enhanced exciton-polariton absorption and exciton binding energy[2], a massive bandgap blue shift ($\sim $ 2 eV) from that of the bulk inorganic semiconductor[3], and fine-tuning of thermal expansion and achieving zero-thermal expansio[4]. They have great potential for applications in areas including transparent conducting materials, thermoelectric materials, UV optoelectronic devices, because of their unusual electronic, vibrational and optical properties and the flexibility in tailoring the material properties adapting to the specific application requirements. .[1] X. H. Huang et al., JACS \textbf{125}, 7049 (03). [2] Y. Zhang et al., PRL \textbf{96}, 26405 (06). [3] B. Fluegel et al., PRB \textbf{70}, 205308 (04). [4] Y. Zhang et al., PRL \textbf{99}, 215901 (07). [Preview Abstract] |
Session P21: Focus Session: Fundamental Issues in Catalysis I
Sponsoring Units: DCPChair: Francisco Zaera, University of California, Riverside
Room: Morial Convention Center 213
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P21.00001: The role of hot electrons in catalysis science Invited Speaker: One long-standing observation in the field of heterogeneous catalysis is that the activity and selectivity in certain reactions is dramatically affected by the oxide onto which the metal nanoparticles are deposited, even though the oxide itself is not active in catalysis. Recently, studies which detected hot electron formation at metal surfaces helped to explain these curious findings. Pulse probe experiments have detected hot electron formation within femtoseconds when photons are incident on a metal surface. Experiments indicate that the mean free path of these hot electrons is on the order of 5 nm, which is in the range of the size of catalyst nanoparticles. Further studies indicate that exothermic catalytic reactions can also produce hot electrons readily, for example CO oxidation or the reaction of hydrogen and oxygen to form water. We have constructed a ``catalytic nanodiode'' in our laboratory whereby we carry out catalytic reactions at high and continuous turnover and, using a Schottky barrier, collect hot electrons. Simultaneous measurement of turnover frequency and hot electron current during CO oxidation has shown that the hot electron current and the turnover rate for the reaction are correlated. This implies that the catalytic activity at the oxide-metal interface in certain catalytic reactions is associated with the hot electron flow. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P21.00002: CO oxidation over noble metals: The continuum from ultrahigh vaccuum to atmospheric pressures Invited Speaker: Catalytic oxidation of CO has been investigated for many decades by numerous researchers and is considered to be one of the best understood catalytic reactions. Because of its importance in pollution control, fuel cells, etc., this reaction has received considerable attention for fundamental and practical reasons. Removal of CO from automobile exhaust is accomplished by catalytic converters using supported Pt-group metals of Pt, Pd and Rh catalysts. The catalytically removal of traces of CO from H$_{2}$ by Pt-group metals to the few ppm level is required for efficient operation in fuel cells. Efforts in our laboratory have addressed the adsorption of CO and the kinetics of CO-oxidation on single crystals and supported metal catalysts over a wide temperature (400--650 K) and pressure (1$\times $10$^{-7}\sim $500 Torr) range. Two active phases, CO-dominated and O-dominated, have been identified for which the mechanisms for CO catalytic oxidation are entirely different. The highly active phase formed in oxygen-rich reaction condition exhibits CO$_{2}$ formation rates several orders higher than the rates found for stoichiometric reaction conditions. This highly active surface was determined to consist of approximate one monolayer of surface oxygen using Auger spectroscopy and X-ray photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P21.00003: Gold atoms, chains and islands on oxide films: looking at orbitals and counting electrons. Invited Speaker: Low-temperature STM measurements combined with DFT calculations are employed to analyze the adsorption of gold on alumina/NiAl(110). The binding of Au monomers involves breaking of an oxide Al-O bond below the adatom and stabilizing the hence under-coordinated O ion by forming a new bond to an Al atom in the NiAl. The adsorption implies negative charging of the adatom. The linear arrangement of favorable binding sites induces the self-organization of Au atoms into chains. For every ad-chain, the number of electrons, in particular of transfer-electrons from the support, is determined by analyzing the node structure of its HOMO. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P21.00004: Theoretical Insights into C1 Surface Chemistry Invited Speaker: Reforming and partial oxidation of methane as well as other C1 fuels are important processes in the production of hydrogen and synthesis gas and will likely play important roles future energy strategies. Herein we use theory and simulation to examine the reactivity of methane, methanol and dimethyl ether with CO$_2$, H$_2$O, or O$_2$ over supported transition metals. We systematically probe the elementary C-H bond activation as well as the oxidation pathways involved in both reforming as the oxidation of methane and other C1 intermediates over well defined transition metal surfaces, metal alloys and metal nanoparticles. The calculations demonstrate well-established trends in C-H bond activation as the result of changes in the metal, the activating molecule (methane, methanol, and DME) as well as the reaction conditions. The reaction conditions ultimately dictate the surface coverage of carbon and oxygen which have important consequences on the surface reactivity. The theoretical and simulation results are compared with well defined experiments carried out at Berkeley over supported particles. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P21.00005: Atomic ordering periodicity and catalytic properties of nanoparticles Valeri Petkov Often nanosized particles of crystals are catalytically very active while the corresponding crystals are not. A typical example is gold. The enhanced catalytic performance of nanosized particles, however, does not come merely from their greatly enhanced surface-to-volume ratio. We would like to draw attention to the often overlooked fact that nanosized particles of crystals do not necessarily possess the periodic 3D structure of their bulk counterparts, and this too may impact their catalytic properties substantially. In particular, nanoparticles that do not have a periodic 3D structure may not come in a well-defined, faceted shape, i.e. may not be terminated by well-defined (usually high energy) atomic planes, as crystalline objects of the same size would be. Hence, nanoparticles may be catalytically more (or less) active than ``nanosized'' crystals. Results from recent structure studies (synchrotron XRD and computer simulations) on 1 -- 5 nm Ru, Au and Pt particles will be presented as evidence. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P21.00006: First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment Simone Piccinin, Catherine Stampfl, Matthias Scheffler By means of density-functional theory, together with concepts from atomistic thermodynamics, we present a theoretical procedure for describing the structure and stoichiometry of a binary alloy in contact with a surrounding gas phase environment. We apply the approach to the Ag-Cu alloy in an oxygen atmosphere, for which recent results report a superior selectivity for ethylene epoxidation compared to pure silver, the predominant catalyst for this reaction. We first show that the presence of oxygen leads to copper segregation to the surface. Then, considering the surface free energy as a function of the surface Cu composition, we construct the ``convex hull''. By including the dependence of the surface free energy of the oxygen chemical potential, we determine the phase diagram of the alloy as a function of temperature, pressure, and Cu surface content. We predict that for conditions typical of the epoxidation reaction, a number of structures can be present on the surface of the alloy including the clean silver surface, thin copper-oxide-like structures, and thick copper oxides. These findings are consistent with, and help explain the recent experimental results. We envisage this approach will be useful and generally applicable for the study of other alloys in contact with a gas or liquid phase. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P21.00007: The Cu/ZnO(0001) Surface under Oxidative and Reducing Conditions: A First-principles Study Katawut Chuasiripattana, Oliver Warschkow, Bernard Delley, Catherine Stampfl The Cu/ZnO(0001) surface is widely used as a catalyst for the production of H$_{2}$-gas from methanol and is thus of considerable relevance to the emergent hydrogen economy. A key to the further development of this catalyst system is a detailed atomic-scale understanding of the relation between surface structure and function versus environmental conditions such as copper content and state of surface oxidation. Towards this goal, we use density functional theory within the framework of ab initio atomistic thermodynamics to conduct a detailed survey of conceivable surface structures under variety of Cu exposures. This produces a surface phase diagram that reveals several distinct regimes of surface reconstruction under oxygen-rich and poor conditions. We correlate our findings with experimental studies, including recent scanning tunneling microscopy results by Dulub et al [1]. \newline References: \newline [1] O. Dulub, M. Batzill, and U. Diebold, Topics in Catalysis 36 (2005) 65. [Preview Abstract] |
Session P22: Focus Session: Organic Electronics: FETs II
Sponsoring Units: DMP DPOLYChair: Michael Chabinyc, Palo Alto Research Center
Room: Morial Convention Center 214
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P22.00001: High Performance Solution Processable TFTs Invited Speaker: Organic-based electronic devices offer the potential to significantly impact the functionality and pervasiveness of large-area electronics. We report on soluble acene-based organic thin film transistors (OTFTs) where the microstructure of as-cast films can be precisely controlled via interfacial chemistry. Chemically tailoring the source/drain contact interface is a novel route to self-patterning of soluble small molecule organic semiconductors and enables the growth of highly ordered regions along opposing contact edges which extend into the transistor channel. The unique film forming properties of soluble fluorinated anthradithiophenes allows us to fabricate high performance OTFTs, OTFT circuits, and to deterministically study the influence of the film microstructure on the electrical characteristics of devices. Most recently we have grown single crystals of soluble fluorinated anthradithiophenes by vapor transport method allowing us to probe deeper into their intrinsic properties and determine the potential and limitations of this promising family of oligomers for use in organic-based electronic devices. Co-Authors: O. D. Jurchescu$^{1,4}$, B. H. Hamadani$^{1}$, S. K. Park$^{4}$, D. A. Mourey$^{4}$, S. Subramanian$^{5}$, A. J. Moad$^{2}$, R. J. Kline$^{3}$, L. C. Teague$^{2}$, J. G. Kushmerick$^{2}$, L. J. Richter$^{2}$, T. N. Jackson$^{4}$, and J. E. Anthony$^{5}$ $^{1}$Semiconductor Electronics Division, $^{2}$Surface and Microanalysis Science Division, $^{3}$Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 $^{4}$Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802 $^{5}$Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055 [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P22.00002: Solution-Processible Thin Film Transistors Using Surface-modified BaTiO3/Polymer Nanocomposites as Gate Insulators Philseok Kim, Xiaohong Zhang, Peter Hotchkiss, Benoit Domerq, Simon Jones, Seth Marder, Bernard Kippelen, Joseph Perry Polymer/ceramic nanocomposites (NC) can exhibit high k and easily processible materials suitable for gate insulators in organic field-effect transistors (OFET). To obtain high k NCs, high volume fractions ($>$30 {\%}) of dielectric nanoparticles (NP) are needed. However, due to NP agglomeration at such high volume fractions, poor quality films with high leakage current are obtained. Recently, we have reported that phosphonic acids can strongly bind to BaTiO$_{3}$ (BT) NPs and provide enhanced dispersability of NPs in polymer hosts allowing increased volume loading. We report the use of phosphonic acid-modified BT NPs (30$\sim $50 nm) in poly(4-vinyl phenol) (PVP, k = 3.9) as gate insulators in OFET, which can be readily processed to high quality thin films by simple solution techniques. BT NPs modified with a phosphonic acid bearing a hydrophilic group afforded high quality NC thin films at high loading (up to 75 wt. {\%}) in PVP. Bottom-gate pentacene OFET devices were fabricated on the NC gate insulators. The improved film quality and increased capacitance density ($\sim $50 nF/cm$^{2}$, k $\sim $14) were reflected in a low threshold voltage ($\sim $1.1 V), a high on/off ratio ($\sim $2x10$^{5})$ and $\sim $10$^{5}$ fold decrease in leakage current as compared to that of unmodified BT. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P22.00003: Evolution of the Unoccupied States in Alikali metal doped Copper-Phthalocyanine Huanjun Ding, Kiwan Park, Yongli Gao The evolution of both the occupied and unoccupied states for Cs and Na-doped Copper-Phthalocyanine (CuPc) has been investigated with photoemission and inverse photoemission spectroscopy (IPES). From the IPES measurement, it is observed that, as the alkali metal doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts toward the Fermi level, and the shift becomes saturated when the LUMO edge is aligned with the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra. The evolution of the LUMO gives direct evidence for the origin of the doping-induced gap state in CuPc molecules. The intensity of the LUMO, as well as the gap state, for high Cs doping ratios clearly suggests that multiply charged CuPc species are formed in the doped film. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P22.00004: Soft X-Ray Emission and Absorption study of the Electronic Structure of the Organic Semiconductor Titanyl Phthalocyanine (TiO-Pc) Y. Zhang, S. Wang, A. DeMasi, L.F.J. Piper, K.E. Smith, J. Downes, A. Matsuura The electronic structure of thin films of the organic semiconductor titanyl phthalocyanine (TiO-Pc) has been investigated using synchrotron radiation-excited x-ray emission and absorption spectroscopies. The films were grown \textit{in-situ}, using organic molecular beam deposition. The C and N $K$-edge spectra display similarities with those from other metal-Pcs, while the O $K$-edge and Ti $L$-edge spectra support the premise that the titanyl species are spatially isolated. Good agreement is found between a calculation of the partial density of states and the measured spectra. The Ti $L$-edge spectra display marked differences with previous reports. Two energy-loss features are reported from resonant x-ray inelastic scattering of the Ti L-edge associated with Ti $3d$*-O $2p $and Ti $3d*$-N $2p$ charge transfer transitions. Our measurements will be discussed in the context of earlier soft x-ray studies of TiO-Pc, with particular attention paid to issues of contamination and beam damage. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P22.00005: Charge transport mechanisms in phthalocyanine thin films Corneliu Colesniuc, Amos Sharoni, Ivan K. Schuller Devices consisting of phthalocyanine thin films sandwiched between gold electrodes were fabricated by organic molecular beam deposition. Samples with different organic layer thickness were deposited on sapphire substrates in-situ, using a shadow mask and a mobile sample holder controlled manually. The structural asymmetry of the devices determined by the different metal-organic interfaces is reflected in the I-V curves at positive and negative voltages. The logarithmic scale I-V plots can be fitted with linear functions of different slopes corresponding to different conduction regimes. At low temperatures a transition from the ohmic regime to a slope two space charge limited conduction mechanism is followed at higher voltages by a high slope linear dependence that tends to saturate when the voltage reaches maximum values. At higher temperatures the intermediary space charge limited regime disappears and the transition is from ohmic to high slope space charge limited. Traps with different energy and energy distribution determine the different conduction regimes. Shallow traps located at discrete energy levels control the transport at intermediate voltages while exponentially distributed traps determine the high voltage behavior. Work supported by AFOSR-MURI. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P22.00006: Soft X-Ray Spectroscopic Studies of the Electronic Structure of Aluminum tris-8-hydroxyquinoline (Alq3) A. DeMasi, L.F.J. Piper, Y. Zhang, I. Reid, S. Wang, K.E. Smith, J. Downes, N. Peltekis, C. McGuinness, A. Matsuura The valence and core level electronic structure of the organic semiconductor aluminum tris-8-hydroxyquinoline (Alq$_{3})$ has been measured using synchrotron radiation-excited resonant x-ray emission spectroscopy (RXES), and x-ray photoelectron spectroscopy (XPS). Samples were in the form of thin films, grown \textit{in-situ} in an organic molecular beam deposition chamber attached to the spectrometer system. The films were found to be highly sensitive to photon induced beam damage, but this problem could be alleviated by continuous translation of the films during measurement. Our RXES measurements are compared to the results of density functional theory (DFT) calculations. The DFT calculated C, N and O partial densities of states are found to agree very well with the corresponding emission spectra. Our measurements will be discussed in the context of earlier soft x-ray studies of Alq$_{3}$, with particular attention paid to issues of beam damage. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P22.00007: High Carrier Density and High Hole Mobilities of Ion Gel Gated Polymer Thin-Film Transistors Jiyoul Lee, C. Daniel Frisbie, Timothy P. Lodge We report the comprehensive characterization of ion gel gated polymer thin-film transistors (IG-PTFTs), in which PQT-12 was used as the active layer and an ion gel comprising a polymer network swollen with an ionic liquid was used as the gate dielectric. The high capacitance of ion gels ($>$10 $\mu $F/cm2) can induce a very large hole density ($\sim $ 2 x 10$^{14}$ charges/cm$^{2})$ in the channel of polymer semiconductor layers in IG-PTFTs, leading to low operation voltages, high hole mobilities of $>$ 1 cm$^{2}$/V·s, and high ON currents. High ionic conductivities of ion gels ($>$ 1 mS/cm) enable fast response time ($\sim $ 1.5 ms at 80 {\%} ON/OFF) of IG-PTFTs. Temperature dependent measurements were carried out with IG-PTFTs. In the high temperature range (310 K $\sim $ 360 K), the device showed faster response time and little hysteresis due to increasing ionic conductivity with the operating temperature. At low temperature (20 K $\sim $ 185 K) where the ions are immobile, high ON currents between source and drain can be maintained with weak temperature dependence. Overall, the results demonstrate that the IG-PTFTs offer opportunities to probe transport of high 2-D charge carrier densities in semiconductors. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P22.00008: Ionic-doping-induced nonvolatile switching in conductive polymer/inorganic complex for nonvolatile memory Qianxi Lai, Yong Chen Organic nonvolatile memories have received extensive attention in recent years due to their low cost and highly scalability. We have studied the nonvolatile switching property of a metal/conductive polymer (MEHPPV)/metal system which is induced by ionic doping under electric field. The switching phenomenon have been observed both in devices doped by electrochemical doping in liquid solution (TBAI) and in devices integrated with a solid electrolyte (RbAg4I5) in the device structure. The device can be switched from its high-resistance state (OFF) into its low resistance state (ON) by a threshold voltage with appropriate polarity and vice versa. The switching on/off ratio is more than 3 orders of magnitude with a switching time as short as 1us and the switching is reversible and repeatable. The resistance change is attributed to the reversible p-type doping of MEH-PPV by injecting/extracting iodide anions into/from the conductive polymer under the voltage bias above certain threshold amplitude. The results of Capacitance-Voltage (CV) measurements also indicated the ion migration in the polymer under the electric field. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P22.00009: Chemical Vapor Sensing Using Dual Channel Hybrid Organic/Inorganic Field-Effect Transistors Shannon Lewis, Sebastian Schoefer, Deepak Sharma, Ananth Dodabalapur We have developed a field-effect chemical sensing device architecture in which two semiconducting channels are employed, one of which is exposed to the analyte and is chemically sensitive. The second channel (usually silicon) is used for signal transduction/amplification. Such sensors work can work in many device modes including one that can be described as a ``chemical memory mode''. For the chemically sensitive channel, several classes of materials can be employed including small molecule organic semiconductors, conjugated polymers, and inorganic oxides such as SnO$_{x}$. With organic semiconductor channels, it is possible to demonstrate charge trapping of volatile organic molecules with significant dipole moments such as ketones and alcohols. We will describe the physics of operation of such sensors in various modes and also outline how the selectivity/sensitivity can be enhanced by incorporating organic receptors. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P22.00010: Correlation of microstructure and magnetotransport in organic semiconductor spin valve structures Y. Liu, J. Gorham, T. Lee, H. Fairbrother, H. E. Katz, D. H. Reich, S. Waston, J. Borchers Magnetoelectronic devices based on organic semiconductors (OSC) hold promise due to the long spin relaxation time and the ability to tune relevant properties such as interface barriers. However, it is unclear to date whether magnetotransport effects observed in these systems is due to tunneling, or whether spin-coherent diffusive transport is also possible. We have studied magnetotransport in Co/OSC/Fe trilayer junctions, with 50 to 150 nm thick OSC layer, where tunneling would not be expected. Positive magnetoresistance (MR) is observed at T = 4.2 K for several OSCs and it persists up to T = 290 K for two systems: tris(8-hydroxyquinoline) Aluminum (III) (Alq3) and copper phthalocyanine (CuPc). In order to probe the origins of MR, we have done structural studies on Co/Alq3/Fe trilayer films by x-ray reflectivity and Auger depth profiling. The results indicate well-defined layers with modest interface roughness (3-5 nm) between the Alq3 and the surrounding FM layers. While these results rule out large-scale intermixing of Co or Fe into the OSC, they do not as yet rule out the existence of local defects, such as pinholes, in the OSC layers that could enable tunneling to occur. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P22.00011: EPR Studies of Highly Interconnected Nanostructured Polyaniline Network Oludurotimi O. Adetunji, N.-R. Chiou, N.P. Raju , A.J. Epstein We present temperature-dependent X-band electron paramagnetic resonance susceptibility and linewidth studies of nanostructured polyaniline doped with perchloric acid (PANN/HClO$_{4})$. From analysis of the EPR data we determine that network has both Pauli- and Curie-like susceptibility with ${\rm X}^{P}$ of $\sim $2 x 10$^{-5}$ emu/mole-2-ring repeat unit and a localized spin density of $\sim $ 1 spin per 400 2-ring repeat units and exhibits a Lorentzian-like lineshape. The EPR linewidth from 100 K to room temperature exhibits two different linear regimes, where the linewidth increases linearly with increase in temperature. We will discuss the role of Korringa relaxation in determining the high temperature linewidth. We will consider the roles of disorder, localization and interfiber contact within the nanostructure network. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P22.00012: Capacitance-voltage characterization of polythiophene-based field-effect transistors Behrang Hamadani, Iain McCulloch, Martin Heeney, David Gundlach We report on frequency-dependent capacitance-voltage characteristics of organic field-effect transistors based on (2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as the active polymer layer. The gate voltage and frequency behavior of the devices with the polymer spun on treated and untreated oxide gate dielectric are explored. The high quality of the devices (contact and channel properties) allows the use of traditional CV modeling to accurately describe the electrical characteristics of the intrinsic channel. The findings from this study provide new insight into charge trapping and transport in the field-accumulated channel of organic field-effect devices. [Preview Abstract] |
Session P23: Focus Session: Multifunctional Oxides: BiFeO3 and Thin Films
Sponsoring Units: DMP GMAGChair: Karin Rabe, Rutgers University
Room: Morial Convention Center 215
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P23.00001: Strain tunability and domain structures of epitaxial (001) BiFeO$_{3}$ thin films Invited Speaker: It was recently discovered that the spontaneous polarization ($P_{s})$ values determined in epitaxial BiFeO$_{3}$ thin films, $\sim $100 $\mu $C/cm$^{2}$, are over an order of magnitude higher than those previously measured in bulk samples. This raises a fundamental question: can the remanent polarization and other properties of BiFeO$_{3}$ be tuned by strain? We studied the strain dependence of remanent polarization and domain structures of BiFeO$_{3}$ through direct measurements on the \textit{same} epitaxial (001)$_{p}$ BiFeO$_{3}$ thin-film capacitors before and after releasing them from an underlying Si substrate. Our measurements reveal that: (1) the large $P_{s}$ of BiFeO$_{3}$ is indeed intrinsic; (2) the out-of-plane polarization ($P_{3})$ of (001)$_{p}$-oriented BiFeO$_{3}$ thin films has a strong strain dependence. These findings can be exploited in studying symmetry-dependent magnetoelectric coupling of BiFeO$_{3}$, where strain and/or symmetry play a role in the coupling because the direction of magnetic spin ordering is not parallel to that of ferroelectric polarization switching. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P23.00002: Local Electrical Conductivity of Multiferroic Domain Walls Qing He, Jan Seidel, Lane Martin, Ying-Hao Chu, Qian Zhan, Feng Wang, Ramamoorthy Ramesh There is an intense interest in magnetoelectric coupling between electric and magnetic due to its potential to the revolutionary of device architectures. Single-phase multiferroics - materials that show spontaneous magnetization and polarization simultaneously at ambient conditions – remain elusive as most systems (such as the manganites) exhibit multiferroicity only at low temperatures. Alternatively, multiferroics can be synthesized as a composite system, e.g. as a product property of a composite phase consisting of a magnetostrictive and a piezoelectric material. One multiferroic material, however, has played a key role in rejuvenating the field after a report of large ferroelectric polarization combined with interesting magnetic properties - BiFeO$_{3}$. Here we provide evidence of a unique property of single domain walls in multiferroic BiFeO$_{3}$. Unlike other multiferroic materials, e.g. PbTiO$_{3}$, on which the electronic properties of the domain walls are not significantly different from the domain area, we observe a finite electric conductivity at room temperature along such a wall using conductive atomic force microscopy. This intrinsic property of the domain wall is attributed to a changed crystallographic structure as revealed by high resolution transmission electron microscopy. Additionally, optical absorption measurements confirm a change in bandstructure at domain walls in BiFeO$_{3}$. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P23.00003: The multiferroic properties of Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$ compound Changqing Jin, J.L. Zhu, H.X. Yang, S.M. Feng, L.J. Wang, R.C. Yu, X.H. Wang, L.T. Li Dense Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$ ceramics of $R3c$ crystal structure were synthesized by solid state reaction under high pressure. TEM observations reveal superstructure characteristics in Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$. Magnetization as well as dielectric properties of Bi(Fe$_{1/2}$Cr$_{1/2})$O$_{3}$ ceramics were characterized over a broad temperature. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P23.00004: Optical coupling to spin waves in the cycloidal multiferroic BiFeO$_3$ Rogerio de Sousa, Joel E. Moore The magnon and optical phonon spectrum of an incommensurate multiferroic such as BiFeO$_3$ is considered in the framework of a phenomenological Landau theory. The resulting spin wave spectrum is quite distinct from commensurate substances due to soft mode anisotropy and magnon zone folding. The former allows electrical control of spin wave propagation via reorientation of the spontaneous ferroelectric moment. The latter gives rise to multiple magneto-dielectric resonances due to the coupling of optical phonons at zero wavevector to magnons at integer multiples of the cycloid wavevector. These results show that the optical response of a multiferroic reveals much more about its magnetic excitations than previously anticipated on the basis of simpler models. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P23.00005: Electric Field Controlled Magnetism in BiFeO$_{3}$/Ferromagnet Films M.B. Holcomb, Y.H. Chu, L.W. Martin, M. Gajek, J. Seidel, R. Ramesh, A. Scholl, A. Fraile-Rodriguez Electric field control of magnetism is a hot technological topic at the moment due to its potential to revolutionize today's devices. Magnetoelectric materials, those having both electric and magnetic order and the potential for coupling between the two, are a promising avenue to approach electric control. BiFeO$_{3}$, both a ferroelectric and an antiferromagnet, is the only single phase room temperature magnetoelectric that is currently known. In addition to other possibilities, its multiferroic nature has potential in the very active field of exchange bias, where an antiferromagnetic thin film pins the magnetic direction of an adjoining ferromagnetic layer. Since this antiferromagnet is electrically tunable, this coupling could allow electric-field control of the ferromagnetic magnetization. Direction determination of antiferromagnetic domains in BFO has recently been shown using linear and circular dichroism studies. Recently, this technique has been extended to look at the magnetic domains of a ferromagnetic grown on top of BFO. The clear magnetic changes induced by application of electric fields reveal the possibility of electric control. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P23.00006: Investigation of domain walls in multiferroic BiFeO$_{3}$ Jan Seidel, Lane Martin, Ying-Hao Chu, Qian Zhan, Qing He, Feng Wang, Ramamoorthy Ramesh, Axel Rother, Nicola Spaldin, Gustau Catalan, James Scott We present a thorough study of domain walls in multiferroic BiFeO$_{3}$ thin films using scanning probe methods, transmission electron microscopy, transport measurements coupled with theoretical studies. In rhombohedral BiFeO$_{3}$, three different domain wall orientations exist, namely 180, 109, and 71 degrees. We find that the domain configurations in thin films are strongly dependent on the processing conditions. Here we investigate electrical and structural properties of all three varieties. Atomic resolution TEM studies were used to reveal the structure across the domain walls, with a specific focus on 109$^{\circ}$. We observe that the changed crystallographic structure in domain walls gives rise to a change in local properties. From the investigation of individual domain walls we also infer their relation to changed macroscopic properties in thin films. This work is supported by the US DOE, ONR MURI, NSF Chemical Bonding Center program, and the Alexander von Humboldt Foundation. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P23.00007: Optical properties of BiFeO$_3$ thin films and single crystal R.C. Rai, X.S. Xu, T.V. Brinzari, J.L. Musfeldt, D.J. Singh, S. Lee, S-W. Cheong, Y.H. Chu, R. Ramesh, S. McGill BiFeO$_3$ has been studied using optical and magneto-optical spectroscopy and the results are compared with first-principle calculations. The optical gap in thin films (2.7 eV) is much larger than that in the single crystal (1.3 eV), the evidence that this system has a novel transition between strongly correlated and band insulator behavior. Both magnetic field and temperature suppress the excitation between the strongly hybridized valence levels and Fe d levels near 1.5 eV. The temperature and magnetic field effects appear to scale energetically suggesting spin-charge coupling. The high energy magneto-dielectric contrast shows a jump at 18 T corresponding to the transition from toroidal to homogeneous antiferromagnetic phase. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P23.00008: Magnetization manipulation in multiferroic devices. Martin Gajek, Lane Martin, Ying Hao Chu, Mark Huijben, Micky Barry, Ramamoorthy Ramesh Controlling magnetization by purely electrical means is a a central topic in spintronics. A very recent route towards this goal is to exploit the coupling between multiple ferroic orders which coexist in multiferroic materials. BiFeO3 (BFO) displays antiferromagnetic and ferroelectric orderings at room temperature and can thus be used as an electrically controllable pinning layer for a ferromagnetic electrode. Furthermore BFO remains ferroelectric down to 2nm and can therefore be integrated as a tunnel barrier in MTJ's. We will describe these two architecture schemes and report on our progresses towards the control of magnetization via the multiferroic layer in those structures. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P23.00009: Electronic and magnetic structures of double perovskite multifunctional La$_{2}$NiMnO$_{6}$ thin films Haizhong Guo, Jiandi Zhang, Arunava Gupta, M. Varela, S.J. Pennycook Electronic and magnetic structures of the ordered double perovskite La$_{2}$NiMnO$_{6}$ (LNMO) thin films grown by pulsed laser deposition have been studied by x-ray absorption spectroscopy (XAS) and magnetic circular dichroism spectroscopy (XMCD). Based upon the results of XMCD, we find that the primary ion valence states to be Mn$^{4+}$/Ni$^{2+}$, and the ferromagnetism resulting from Mn$^{4+}$ -O - Ni$^{2+}$ superexchange interaction. Additionally, we show that the LNMO samples contain some Mn$^{3+}$ and Ni$^{3+}$ Jahn-Teller ions caused by oxygen or cation related defects. The orbital and spin magnetic moments of the Mn 3$d $and Ni 3$d $also have been deduced from the magneto-optical sum rules and compared with magnetization measurements. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P23.00010: Influence of oxygen concentration on the magnetic properties of multifunctional La$_{2}$CoMnO$_{6}$ thin films Arunava Gupta, Haizhong Guo, Jiandi Zhang, M. Varela, S.J. Pennycook The dependence of the magnetic properties on oxygen concentration in epitaxial La$_{2}$CoMnO$_{6 }$(LCMO) thin films has been investigated grown by pulsed laser deposition (PLD). Using x-ray magnetic circular dichroism spectroscopy (XMCD) at Mn-$L_{2,3}$ and Co-$L_{2,3}$ edges, we have determined that the primary ion valence state is Mn$^{4+}$/Co$^{2+}$. Additionally, we see evidence of some low-spin Co$^{3+}$ ions, corresponding to the existence of a second ferromagnetic (FM) phases in our samples. The existence of oxygen vacancies induces the local vibronic Mn$^{3+}$-O-Co$^{3+}$ superexchange interactions in direct competition with the static FM Mn$^{4+}$-O-Co$^{2+}$ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions. [Preview Abstract] |
Session P24: Focus Session: Optical Properties of Nanostructures V: Plasmonics and Metallic Nanostructures
Sponsoring Units: DMPChair: Nicholas Fang, University of Illinois
Room: Morial Convention Center 216
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P24.00001: Calculations of second harmonic generation by nano-particles or holes William Schaich We are setting up finite-difference time-domain (fdtd) calculations of second harmonic generation (shg) by metallic nano-particles or at nano-holes in metallic films. This generation is driven by first order fields at the metal surfaces, using the phenomenological a and b parameters that have been used to describe shg at planar jellium-metal surfaces. Our interest is in locating and understanding the occurrence of resonances and hot-spots for shg. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P24.00002: Plasmonic bonding and anti-bonding forces in a bisphere Tsz Fai (Jack) Ng, C.T. Chan By exciting the surface plasmon resonance of a pair of nanoparticles using intense laser illumination, one can exert huge optical forces on the nanoparticles. We calculate such resonant optical force using a multiple scattering and Maxwell stress tensor formalism, which is ``exact'' within the classical electrodynamics. It is shown that the full electrodynamic calculation can give results that differ significantly from those obtained by the quasi-static approximation. As the pair of nanoparticles approach each other, the individual particle's plasmonic modes hybridize and split into bonding and anti-bonding modes, which induced attraction and repulsion respectively. At very small separations, the bonding (anti-bonding) modes are forced to curve downward (upward) in frequency significantly, resulting in the formation of a low frequency attractive (high frequency repulsive) band. Consequently, a low frequency laser illumination will induce strong attraction, promoting particle-clustering, and a high frequency illumination will induce strong repulsion, preventing particle aggregation. With high intensity, these resonant forces can dominate over the other relevant forces, including the van der Waals force, when the separation between the particles is several nanometers. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P24.00003: Surface-enhanced Raman Scattering from Virus-like Particle Crystals Christopher DuFort, Bogdan Dragnea Recently, a method for the encapsidation of gold nanoparticules by an icosahedral virus protein coat, termed a virus-like particle (VLP), has been developed. Of particular interest is in observing their spectroscopic properties upon arrangement into a three-dimensional crystal lattice. Here we present the surface-enhanced Raman scattering spectrum of such an assembly. This is made possible by the plasmonic coupling of adjacent gold nanoparticules when excited near their plasmon resonant frequency. To determine whether the SERS effect is arising from isolated hot spots or a large number of junctions acting in unison we employed scanning confocal Raman spectroscopy. This seems to indicate the latter, as a uniform Raman intensity is observed across entire crystals. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P24.00004: The effects of nanoparticle spacing on second-harmonic generation from gold nano-dimers Davon W. Ferrara, Kevin A. Tetz, Matthew D. McMahon, Richard F. Haglund, Jr. Second-harmonic generation (SHG) is an important signature of electron dynamics in nanoparticles (NPs) as well as a sensitive probe of surface effects. In the gap between closely spaced pairs of NPs, or nanodimers (NDs), localized electromagnetic field energy creates a hot spot that has been shown to affect SHG from asymmetric NDs. We will present new experimental results demonstrating the role that gap size and field localization plays in SHG from centrosymmetric arrays of gold NDs. Using standard electron-beam lithography techniques, NPs were made 20 nm in height with varying areal aspect ratios. In the ND arrays, symmetry forbids SHG in the forward direction, but not at larger angles. Our experiments indicate suppression in SHG intensity with decreasing gap size and evidence of stronger long range interactions between NPs with separation over 200 nm. Finite-difference time-domain simulations were also performed in order to correlate field localization with SHG. Our simulations show a strong dependence on the polarization of incident light. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P24.00005: Optical Imaging Properties of Metal Nanoparticle Chains David Citrin Chains of noncontacting noble-metal nanoparticles are known to support coupled plasmonic-electromagnetic modes known as plasmon polaritons, in which those polarized perpendicular to the chain axis exhibit group and phase velocity in opposite directions. This in turn has attracted interest in nanoparticle chains and arrays as left-handed materials for optical applications. In this contribution, I discuss recent work in my group on the imaging properties of nanoparticle chains. In particular, I present work that demonstrates theoretically the focusing properties of nanoparticle chains. I further discuss possible applications in near-field optics. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P24.00006: Plasmonic superfocusing on metallic tips for near-field optical imaging and spectroscopy Catalin C. Neacsu, Rob Olmon, Samuel Berweger, Alexandria Kappus, Friedrich Kirchner, Claus Ropers, Lax Saraf, Markus B. Raschke Realization of localized light sources through nonlocal excitation is important in the context of plasmon photonics, molecular sensing, and in particular near-field optical techniques. Here, the efficient conversion of propagating surface plasmons, launched on the shaft of a scanning probe tip, into localized plasmon at the apex provides a true nanoconfined light source. Focused ion beam milling is used to generate periodic surface nanostructures on the tip shaft that allow for tailoring the plasmon excitation. Using ultrashort visible and mid-IR transients the dynamics of the propagation and subsequent scattered emission is characterized. The strong field enhancement and spatial field confinement at the apex is demonstrated studying the coupling of the tip in near-field interaction with a flat sample surface. It is used in scattering near-field spectroscopic imaging (s-SNOM) to probe surface nanostructures~with spatial resolution down to 10 nm. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P24.00007: Novel properties of nanostructured metal particles and films Shengli Zou Using electromagnetic dynamics, we simulate the extinction spectra and enhanced electric fields of nanostructured metal nanoparticles and films. Their sensing and waveguide applications will be discussed. The sensing of molecules is achieved by the shift of the plasmon resonance wavelength of nanoparticles or enhanced Raman scattering or fluorescence signals, which are due to the enhanced local electric fields near or far away from the metal surface. The waveguide is achieved by the electromagnetic coupling in a one dimensional particle chain or a perforated metal film. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P24.00008: Resonance of metallic wire structures Lei Zhou, Y. Zhang, Siu Tat Chui Metallic wire structures form a common class of physical systems. We illustrate how to understand the general physics of the wire systems with a specific example, the split ring resonantor. We derived simple polynomial equations to determine the entire resonance spectra of split ring structures, which can be analytically solved in the limit of narrow wires. A resonance spectrum very similar to that of a straight wire is obtained. For double stacking split rings made with flat wires, we showed that the resonance frequency depends linearly on the ring-ring separation. In particular, we found that the wavelength of lowest resonance mode can be made as large as $10^5$ the geometrical size of the ring for realistic experimental conditions, whereas for current systems this ratio is of the order of 10. Finite-difference-time-domain simulations on realistic structures verified the analytic predictions. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P24.00009: Fabrication of Plasmonic Optical Probe and Its Characterization Seong Soo Choi, D.W. Kim, Vinayah Jha, O. Suwal, M.J. Park Recently, there have been tremendous interests about the nano-structured optical probe using surface plasmon polariton due to possible applications of the next generation local communication devices and nano-bio sensor technology. The nano-size metallic apertures such as metal-coated fiber probe and microfabricated pyramidal probe, have shortcomings of very low output intensities. With periodic groove or defects near the aperture on the microfabricated pyramidal probe, the huge output intensity enhancement has been reported [1]. In this talk, the fabrication with nano-flowers including metallic scattering centers around the nano-size aperture and its optical characterization of the pyramidal metallic probe will be presented. References: [1] The effect of groove shape on light transmission, S.S. Choi, etc, 3$^{rd}$ International Conference on Surface Plasmon Photonics, June 17-22, Dijon, France. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P24.00010: Inter-particle coupling and polarizability of silver nanoparticle dimers Ke Zhao, Claudia Troparevsky, Adolfo Eguiluz, Zhenyu Zhang Using a real-space implementation of density-functional theory, we investigate systematically the electronic/chemical coupling of two silver nanoparticles of varying sizes. The nanoparticles are allowed to approach each other along two distinct directions defined by, respectively, maximum and minimum static polarizabilities. We show that, as the inter-particle separation decreases, the static polarizability of the dimer shows signatures of crossover to a strong-coupling regime. Moreover, we analyze the connection between the crossover from weak to strong coupling regimes of the nanoparticles and the electron density overlap from the states contributed by the individual nanoparticles. The significance of these results will be discussed in connection with the prevailing theories of the electromagnetic enhancement of surface-enhanced Raman scattering. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P24.00011: Enhancement of Light Transmission through Bull's Eye Structures Sinan Selcuk, Daniel Arenas, David B. Tanner, Arthur F. Hebard Optical transmission of a single hole in a metal film surrounded by concentric surface grooves is shown to have an enhanced transmission. We fabricated these structures in silver films which are thin enough to let light through without a center hole. We have measured the optical transmission and reflection in visible to near infrared spectrum observing enhanced transmission scaling with groove periodicity. Opening a hole in the center gives rise to destructive interference between the light passing through the structure and the light passing through the hole. We will discuss the mechanisms behind light transmission for the bullseye structure for varying hole size, groove periodicity, groove depth and the metal thickness. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P24.00012: Fluorescence enhancement from nominally flat surfaces Shy-Hauh Guo, Hung-Chih Kan, Ray Phaneuf We report on experimental investigations of fluorescence enhancement from nominally flat silver and silicon substrates, in the presence of an oxide spacer layer. By varying the thickness of the oxide layer to change the spacing between fluorophores and the substrate, we find that the relative fluorescent intensity measured above Ag vs. Si substrates oscillates, indicating a resonant effect. We investigate the effect of nanoscale roughness on the observed fluorescent enhancement. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P24.00013: Anisotropic Light Scattering from Ferrofluids Corneliu Rablau, Prem Vaishnava, Ratna Naik, Gavin Lawes, Ron Tackett, C. Sudakar We have investigated the light scattering in DC magnetic fields from aqueous suspensions of Fe$_{3}$O$_{4}$ nanoparticles coated with tetra methyl ammonium hydroxide and $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles embedded in alginate hydrogel. For Fe$_{3}$O$_{4}$ ferrofluid, anomalous light scattering behavior was observed when light propagated both parallel and perpendicular to the magnetic fields. This behavior is attributed to the alignment and aggregation of the nanoparticles in chain-like structures. A very different light scattering behavior was observed for $\gamma $-Fe$_{2}$O$_{3}$ alginate sample where, under the similar conditions, the application of the magnetic field produced no structured change in scattering. We attribute this difference to the absence of chain-like structures and constrained mobility of iron nanoparticles in the alginate sample. The observation is in agreement with our relaxation and dissipative heating results$^{1}$ where both samples exhibited Neel relaxation but only the Fe$_{3}$O$_{4}$ ferrofluid showed Brownian relaxation. The results suggest that Brownian relaxation and nanoparticle mobility are important for producing non-linear light scattering in such systems. $^{1}$P.P. Vaishnava, R. Tackett, A. Dixit, C. Sudakar, R. Naik, and G. Lawes, J. Appl. Phys. \textbf{102}, 063914 (2007). [Preview Abstract] |
Session P25: Focus Session: DNA and Protein Analysis with Micro and Nanofluidics
Sponsoring Units: DPOLY DBPChair: Ron Larson, University of Michigan
Room: Morial Convention Center 217
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P25.00001: Learning from the Jersey Turnpike: Cell Lysis, Labeling and Washing with Microfluidic Metamaterials Invited Speaker: Directing objects across functional streamlines at low Reynolds number is difficult but important since this motion can be used to label, lyse, and analyze complex biological objects on-chip without cross-contamination. Here we use an asymmeteric post array to move cells across coflowing reagents and show on-chip, immunofluorescent labeling of platelets with washing and \emph{E.Coli} cell lysis with simultaneous separation of bacterial chromosome from the cell contents. Furthermore, we develop the concept of a microfluidic metamaterial by using the basic asymmetric post array as a building block for complex particle handling modes. These modular array elements could be of great use for developing robust techniques for on-chip, continuous flow manipulation and analysis of cells, large bio-particles, and functional beads. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P25.00002: DNA Docking with Functionalized Colloidal Probes Lu Zhang, Yingxi Elaine Zhu The docking of DNA with probe-functionalized microparticles remains inadequately understood, despite the significance of DNA hybridization-based technologies for high-throughput screening for genetic analysis and biomedical diagnostics. In this work, we employ fluorescence correlation spectroscopy (FCS) and confocal microscopy to examine DNA-colloid interaction and resulting conformational structures of DNA oligomers with oligonucleotide-functionalized colloidal probes, whose particle size varies from 100 nm to 3 um. We observe that the docking efficiency strongly depends on DNA length, colloid size and surface functionality. Optimal probe size and temperature are found for rapid hybridization. We conjecture that the resulting structure of DNA at the interface of colloidal probes is determined by both steric effects and DNA charge condensation. If time permits, we will discuss our recent work on the docking of elongated DNA with functional probes by imposed dielectrophoretic forces in the presence of AC fields. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P25.00003: Confinement spectroscopy: A novel approach to force spectroscopy Fredrik Persson, Pawel Utko, Walter Reisner, Anders Kristensen In physics DNA is readily described by its mechanical properties, neglecting its chemical composition. By studying variations in these properties conclusions can be drawn about the interaction between DNA and both its environment and also ligand binding to DNA. These investigations are normally performed by force spectroscopy using optical or magnetic tweezers associated with an elaborate experimental setup. We introduce confinement spectroscopy, as a complementary technique, where a continuously variable degree of spatial restriction is applied to the molecule in a fluidic funnel-like geometry. By driving the molecule along the funnel, an extension versus confinement curve is obtained. This curve contains not only information regarding the molecule elasticity, but also new details concerning the self- and surface-interactions of the molecule. It is also easily integrated into lab-on-a-chip devices. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P25.00004: Fluorescence microscopy studies of the DNA motion near voltage biased solid-state nanopores Kazuhiko Obana, Yoichi Nakamura, Kaya Kobayashi, Toshiyuki Mitsui A solid state nanopore in a Si-based thin insulating membrane works as a single DNA molecule sensing device that provides the information of the length and the folding configuration of the DNA by measuring ionic currents when the DNA translocates through the pore [1]. These nanopores may play a significant role in molecular electronics and rapid DNA sequencing. Now one of the issues related to this nanopore sensing technique is clogging the nanopores by DNA molecules because it significantly extends the DNA translocation time. To elucidate this issue, we use time-resolved fluorescence microscopy to observe the DNA motion near voltage biased nanopores. We will discuss the DNA motion near nanopores under the various applied voltages. [1]. J. Li M. Gershow, D. Stein, E. Brandin, and J.A. Golovchenko, Nature Materials 2: 611 (2003); T. Mitsui, D. Stein, Y.-R. Kim, D. Hoogerheide, and J.A. Golovchenko, Phys. Rev. Lett. 96: 036102-1 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P25.00005: Influence of polymer-pore interactions on translocation Tapio Ala-Nissila, Kaifu Luo, See-Chen Ying, Aniket Bhattacharya We investigate the influence of polymer-pore interactions on the translocation dynamics using Langevin dynamics simulations$^2$. An attractive interaction can greatly improve translocation probability. At the same time, it also increases translocation time slowly for weak attraction while exponential dependence is observed for strong attraction. For fixed driving force and chain length the histogram of translocation time has a transition from Gaussian distribution to long-tailed distribution with increasing attraction. Under a weak driving force and a strong attractive force, both the translocation time and the residence time in the pore show a non-monotonic behavior as a function of the chain length. Our simulations results are in good agreement with recent experimental data$^3$.\newline \newline $^1$ This work was supported by TransPoly Consortium grants.\newline $^2$ K. Luo, T. Ala-Nissila, S. C. Ying, and A. Bhattacharya, Phys. Rev. Lett. {\bf 99}, 148102 (2007).\newline $^3$ O. V. Krasilnikov {\em et al.}, Phys. Rev. Lett. {\bf 97}, 018301 (2006); A. Meller {\em et al.}, Proc. Natl. Acad. Sci. U.S.A. {\bf 97}, 1079 (2000). [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P25.00006: Nanofilters for high throughput DNA separation Nabil Laachi, Carmelo Declet, Christina Matson, Kevin Dorfman Nanofilters are a novel class of microfabricated devices for rapidly separating short, rigid DNA. The succession of alternating narrow slits ($\sim $50nm) and deep wells ($\sim $300nm) is used to trap the DNA, which then escape at a size-dependent rate. Experiments and near-equilibrium theoretical arguments both indicate that smaller DNA travel faster in a weak field, but the separation fails at around 100V/cm. We theoretically show that the speed and performance of the device can be enhanced using high fields of several hundred V/cm. Based on scaling arguments, the separation of short, rod-like DNA molecules at high fields occurs via ``torque-assisted escape,'' which originates from the non-uniform electric field at the slit entrance. The quadratic dependence of the torque on the molecule size indicates that larger molecules will now emerge first; under a high field, the device operates in a band-inverted manner. Brownian dynamics simulation results confirm the mobility increase with size, with a quasi-plateau at very large fields. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P25.00007: Rapid DNA Idetification by Dielectrophoresis of Nanocolloids Zachary Gagnon, Satyajyoti Senapati, Jason Gordon, Hsueh-Chia Chang Due to their size and number, dispersed oligo-functionalized nanocolloids can reduce the diffusion length/docking time and increase the sensitivity of ssDNA hybridization reactions by orders of magnitude compared to immobilized probes. We find that, for long target ssDNAs, their docked conformation is a sensitive function of the nanocolloid size, surface charge, functionalized probe density and number of docked DNAs per bead. Three distinct conformations (collapsed, stretched and condensed) are detected via independent light scattering, Zeta potential, dielectrophoresis (DEP) and electron micrograph techniques. By optimizing the hybridization conditions to produce a stretched conformation, we are able to significantly change the DEP cross-over frequency of hybridized beads, thus allowing rapid label-free detection of hybridization by simple impedance techniques down to pM concentrations. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P25.00008: The non-driven polymer translocation through a nanopore: relaxation and translocation are not decoupled Gary W. Slater, Michel G. Gauthier Most theoretical models describing the translocation of a polymer chain through a nanopore use the hypothesis that the polymer remains in an equilibrium random coil conformation during the process. In other words, models generally assume that the characteristic relaxation time of the chain is small enough compared to the translocation time that non-equilibrium polymer conformations can be ignored. We present Molecular Dynamics simulations that directly test this hypothesis by looking at the escape time of unbiased polymer chains starting with different initial conditions. We find that the chains are deformed for the systems studied, even though the translocation time is about 10 times larger than the relaxation time. Our most striking result is the observation that the last half of the chain escapes in less than 12\% of the total escape time, which implies that there is a large entropy-driven acceleration of the chain at the end of its escape from the channel. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P25.00009: Digital DNA: Physics of DNA in Nanopit Lattices Walter Reisner, Jonas Tegenfeldt, Niels Larsen, Henrik Flyvbjerg, Derek Stein, Anders Kristensen Controlling the on-chip organization and conformation of DNA is important for a number of interrelated nanotechnology disciplines. We introduce a new type of nanostructure consisting of a nanoslit with a built in spatial modulation of confinement created by arrays of embedded nanopits. Nanopits are square depressions in a 50-100 nm deep nanoslit with a width in the range of 100-500 nm and a depth of 100 nm. A DNA molecule placed in a nanopit lattice will spontaneously adopt a `digitized' conformation consisting of filled nanopits connected by fluctuating linkers. By adjusting the spacing, organization and placement of the nanopits it is possible to immobilize DNA at predetermined regions of device without additional chemical modification and achieve a high degree of control over local DNA conformation. We will present results from fluorescence microscopy experiments on the equilibrium behavior and dynamics of DNA in such structures and interpret these results in terms of a simple statistical mechanical model. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P25.00010: Dynamics of DNA molecules confined to slit-like nanofluidic channels Christine Meyer, Douwe Jan Bonthuis, Derek Stein, Cees Dekker We experimentally investigate the dynamics of DNA in confined spaces. This is not only important for a better understanding of the behavior of DNA as a biologically relevant molecule but also helps us to test general polymer dynamics models. Fluorescently stained DNA molecules are inserted into slit-like nanofluidic channels. The channel material is fused silica and the channels are fabricated using a bonding process. We take fluorescent images of the DNA in channels of different heights and measure the projected size of the DNA molecules fitted by an ellipse. Furthermore, we measure the relaxation times derived from the autocorrelation function of the size. If the channel height is smaller than twice the radius of gyration of the DNA molecules (R$_{g}$ = 700 nm) both parameters agree with the predictions of de Gennes. For even smaller channels with a height less than twice the persistence length of stained DNA (L$_{p}$ = 60 nm) the dynamics resemble the predictions made by Odijk for this regime. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P25.00011: Electrokinetic transport at a nanocapillary/microchannel interface Jarrod Schiffbauer, Kathleen Kelley , Boyd Edwards, Aaron Timperman Coupled electrokinetic transport phenomena play a central role in concentration polarization near the interface between a permselective nanocapillary membrane and a microchannel. Here the effects of ion concentration and potential distribution on transport through a finite-length nanocapillary are studied using both semi-empirical and fundamental models. The fundamental models are based on the coupled electrohydrodynamic transport equations for multiple charged species in aqueous solution. The semi-empirical models describe average species and fluid fluxes through the respective regions. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P25.00012: Water-encapsulated protein source for x-ray serial crystallography Daniel DePonte, U. Weierstall, R.B. Doak, J.H.C. Spence, D. Starodub, M. Hunter, P. Fromme, D. Shapiro A reliable source of micron size water droplets has been constructed for the purpose of delivering water-encapsulated protein for x-ray serial crystallography. A linear stream of droplets of negligible divergence is produced by accelerating a liquid water jet through a high pressure gradient.......[1] inside a converging gas nozzle. Using a co-flowing gas rather than the nozzle walls to squeeze the liquid jet to smaller diameter eliminates the problem of clogging that has thus far limited the minimum size of Rayleigh nozzle jets [2]. We examine the nozzle shape effects on the dripping-jetting transition and drop size. Supported by NSF award IDBR 0555845 and ARO award DAAD190010500. [1] Ganan-Calvo, A.M. and A. Barrero, \textit{A novel pneumatic technique to generate steady capillary microjets.} Journal of Aerosol Science, 1999. \textbf{30}(1): p. 117-125. [2] http://arxiv.org/abs/physics/0701129 [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P25.00013: Separation of DNA in nanoscale devices with alternating channel depth. Henry Lau, Elizabeth Strychalski, Harold Craighead, Lynden Archer The size-dependent separation of DNA using nanofabricated devices consisting of alternating deep and shallow regions have been the subject of numerous experimental and theoretical works. Recent Brownian dynamics simulations suggest that the separation of rigid-rod DNA can be effected at high electric fields without a loss of resolution (PRL, 2007, 98, 098106). To study the dynamics of DNA separation at high fields, electrophoresis experiments were carried out using DNA fragments up to 753 bp in size. As the transport mechanism of DNA fragments in gels has been shown to be a strong function of topology (Electrophoresis, 2004, 25, 1772), electrophoresis of branched rigid-rod DNA molecules was performed to investigate the effects of analyte architecture on mobility in nanofabricated devices. By comparing the mobility of branched and linear DNA molecules of identical total molecular weight, we exclude the influence of size and charge and focus on the effects of branch size and location, and overall analyte topology. Our results help to elucidate the electrophoretic migration mechanism of DNA molecules with complex architecture in sieving media with precisely-controlled internal structures. [Preview Abstract] |
Session P26: Focus Session: Quantum Control III
Sponsoring Units: DCPChair: Vlasta Bonacic-Koutecky, Humboldt-Universität zu Berlin
Room: Morial Convention Center 218
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P26.00001: Use of Ultrafast Molecular Dynamics and Optimal Control for Identifying Biomolecules Invited Speaker: With F.COURVOISIER,L.GUYON,V.BOUTOU, and M.ROTH,J. ROSLUND, H. RABITZ, Princeton University. The identification and discrimination of molecules that exhibit almost identical structures and spectra using fluorescence spectroscopy is considered quite difficult. In order to evaluate the capability of optimal control for discriminating between the optical emissions of nearly identical molecules, we developed a new approach called ``optimal dynamic discrimination (ODD). A proof of principle ODD experiment has been performed using Riboflavin (RBF) and Flavin Mononucleotide (FMN) as model system. We used a complex multipulse control field made of a pair of pulses (UV and IR). The UV part (400 nm) is optimally shaped using a control learning loop while the IR component (800 nm) is FT-limited (100 fs) and set at a definite time delay with respect to the UV pulse. Clear discrimination was observed for optimally shaped pulses, although the linear spectra from both molecules are virtually identical. A further experiment showed that, by using the optimal pulse shapes that maximize the fluorescence depletion in FMN and RBF in a differential manner, the concentration of both molecules could be retrieved while they were mixed in the same solution. The ODD demonstration sets out a promising path for future applications, as for example fluorescence microscopy where endogenous fluorescence spectra of many biomolecules overlap. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P26.00002: Spinning Tops in External Fields. From High Harmonic Generation to Control of Transport in the Nanoscale Invited Speaker: Nonadiabatic alignment is a coherent approach to control over the spatial properties of molecules, wherein a short, moderately- intense laser pulse is applied to populate a broad rotational wavepacket with fascinating properties. In the limit of small isolated molecules, nonadiabatic alignment has evolved in recent years into an active field of theoretical and experimental research with a rich variety of applications. Following a brief review of the essential physics underlying nonadiabatic alignment, we discuss one of these applications, namely the use of high harmonics generated from aligned molecules as a probe of the underlying electronic dynamics and rotational coherences. Next, we extend the alignment concept to dissipative media, including dense gases, solutions, and interfaces. We illustrate the application of rotational wavepackets as a probe of the dissipative properties of dense media and propose a means of disentangling population relaxation from decoherence effects via strong laser alignment. We extend alignment to control the torsional motions of polyatomic molecules, and apply torsional control in solutions to manipulate charge transfer events, suggesting a potential route to light controlled molecular switches. Turning to interfaces, we introduce a route to guided molecular assembly, wherein laser alignment is extended to induce long-range orientational order in molecular layers. Finally, we combine the nonadiabatic alignment concept with recent research on nanoplasmonics and on conductance via molecular junctions to develop an approach to optical control of transport in the nanoscale. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P26.00003: Enhancing vibrational selectivity and 2D IR spectroscopies with mid-IR pulse shaping Invited Speaker: We report on the capabilities of a new pulse shaper that operates directly in the mid-infrared. This shaper can adjust the phase and amplitudes of 500 frequency elements to generate complex time-domain pulses. In this talk, experiments will be reported using this shaper to coherently control the vibrational excitations of condensed phase molecules with adaptive learning feedback control. We will also report how this shaper can be used to collect two-dimensional infrared (2D IR) spectra by programming the pulse sequences. 2D IR spectroscopy via pulse shaping is extremely rapid, highly accurate, and more flexible than traditional means for collecting spectra. Taken together, mid-IR pulse shaping allows for new experiments in ground state coherent control and probing vibrations with unprecedented accuracy using new multidimensional spectroscopies. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P26.00004: Combined Dimensionality Reduction in Search and Detection Spaces via Diffusion Mapping Dmitri Romanov, Stanley Smith, John Brady, Ronald Coifman, Robert Levis Strong-field control settings involve highly nonlinear processes. Typically, both search and detection spaces are high-dimensional (with dimension $\sim $100 each). This poses considerable problems to analysis and interpretation of the process-related data. Here, we use the recently developed nonlinear statistical method of diffusion mapping to effectively reduce the combined dimensionality of the search and detection space and to sample essential patterns in the lower-dimensional representation. The diffusion maps are constructed and analyzed for the case study of maximizing integrated intensity in a second harmonic generation experiment. The use of a sampling set of 1000 random pulses in the diffusion mapping is sufficient for effective dimensionality reduction and for revealing the inherent structure of the process-related data. Extrapolation of the low-dimensional diffusion-space pattern helps indicate the area in the search space that is most amenable to effective optimization. The diffusion-mapping algorithm is sufficiently fast and robust that may make it a valuable preprocessing tool for optimal pulse searching. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P26.00005: Selective Rotational Manipulations of Close Molecular Species -- isotopes and isomers Yehiam Prior, Sharly Fleischer, Ilya Sh. Averbukh We experimentally demonstrate a new approach to selective excitation of close molecular species in mixtures. We apply two time delayed, ultrashort laser pulses where the first pulse rotationally excites both components in a binary mixture, and the second pulse de-excites one, while enhancing the excitation degree of the other. In our work we implemented this approach to molecular nitrogen, and study the cases of molecular isotopes and molecular spin isomers. The case of molecular isotopes is based on the mass difference between the molecular components which results in a slightly different revival period of the repetitive alignment that follows excitation by an ultrashort pulse. Following the revival process, one can distinguish between the isotopic components and selectively affect them. The case of spin isomers is more complicated since there are no differences in their mechanical or electrical properties. Here we utilize the symmetry and statistics of the specific molecular wavefunction and demonstrate highly selective ($\sim $18:1) excitation of Ortho/Para nitrogen. Numerical simulations agree very well with the observed results. Since this process is nonresonant and does not require any special conditions like temperature etc. this approach is general and can be applied to most symmetric molecules. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P26.00006: Anti-Correlated Pigment Fluctuations of Allophycocyanin for Highly Efficient Photosynthetic Light Harvesting in Cyanobacteria Andrew Moran, Rene Nome, Norbert Scherer The phycobiliprotein, allophycocyanin (APC), is an excellent model system for the study of light harvesting pigment interactions with a protein bath. This work investigates the relaxation of electronic excitations in APC with electric field-resolved transient grating and photon echo spectroscopies. Transient grating experiments observe a 35 fs internal conversion process between single exciton levels. Most importantly, our analysis shows that anti-correlated phycocyanobilin pigment energy level fluctuations cause the anti-diagonal orientation of the node in the measured dispersive photon echo spectrum. We believe this novel observation to reflect concerted protein bath fluctuations over the 2 nm length scale that separates the pigments. Consideration of the Forster energy transfer rate theory suggests that APC has evolved with this property to enhance its photosynthetic light harvesting efficiency. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P26.00007: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P26.00008: Understanding the relaxation of excited-state cis-1,3,5-hexatriene in order to augment the preferred pathway for control Michael Orozco, Kuo-Chun Tang, Roseanne Sension A study of the ground and excited-state relaxation of cis-1,3,5-hexatriene in various solvents and temperatures has been performed. The role solvent plays in the relaxation dynamics and relaxation pathways has been assessed and modeled to achieve a better understanding of the energy landscape. This information will be used to determine the preferred relaxation pathways and inform efforts to use sculpted UV pulses to influence the excited state dynamics through pump-dump interactions. Finally, further experiments are proposed wherein UV pulse-shaping will used to study and control other reactive systems. [Preview Abstract] |
Session P27: Focus Session: Low-dimensional Magnetism
Sponsoring Units: GMAGChair: Jian Shen, Oak Ridge National Laboratory
Room: Morial Convention Center 219
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P27.00001: Magnetic Stripe Phase at the Spin Reorientation Transition of an Ultrathin Magnetic Film Invited Speaker: Magnetic long-range order can not exist in an isotropic 2D Heisenberg system at any finite temperature, but could be established by adding a magnetic anisotropy to the system. In experiment, this topic has been addressed by investigating the so-called spin reorientation transition (SRT) in magnetic ultra-thin films, where the perpendicular magneto-crystalline anisotropy cancels the in-plane magnetic shape anisotropy. In this talk, we present our study on the SRT of Fe/Ni/Cu(100) system using Photoemission Electron Microscopy (PEEM) chamber at beam line 7.3.1.1 of the Advanced Light Source. We show that (1) a homogenous magnetic state is energetically unstable against the formation of magnetic stripe domains, (2) the stripe domain width decreases exponentially towards the SRT point, and (3) the Curie temperature of the film is reduced to result in a paramagnetic phase within a narrow thickness gap in the SRT region. Using magnetic interlayer coupling to simulate the effect of an external magnetic field, we further studied the SRT in Co/Cu/(Fe/Ni)/Cu(100) system and revealed a universal behavior of the stripe domain width. Moreover a new metastable bubble domain phase was observed near the SRT point in Fe/Ni film, which enriches the magnetic phase diagram of a 2D magnetic system. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P27.00002: Induced V magnetic polarization effects in V/Fe/V trilayers Rosa A. Lukaszew, Cesar Clavero, Yongseong Choi, Daniel Haskel, Cecilia Sanchez-Hanke, Brian Kirby, Mike Fitzsimmons Understanding magnetic interactions at interfaces between nonmagnetic and magnetic elements and in some cases magnetic polarization of non-magnetic elements at these interfaces is a challenging topic. In addition these phenomena are interesting also due to their potential application in magneto-optical (MO) data storage devices and sensors. In the case of Fe/V interfaces, a fundamental question is the effect of interdiffusion on the magnetic moments of the intervening elements at the interface. Here, we present a structural, morphological and magnetic study of a V/Fe/V trilayer. While no significant diffusion of V into the Fe film was observed with x ray reflectometry (XRR) and polarized neutrons reflectometry (PNR), x ray magnetic circular dichroism (XMCD) and x ray resonant magnetic scattering (XRMS) measurements reveal V magnetic polarization at both Fe/V interfaces. The magnetization profile in the Fe and V films was obtained by XRMS and will be compared with that obtained with PNR. Our results show the complementarity of these techniques to further clarify V polarization phenomena in Fe/V interfaces. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P27.00003: Structure and magnetism of Ni$_{50}$Mn$_{50}$ monolayers on Cu$_{3}$Au(001) Waldemar Macedo, Reinaldo Oliveira Jr., Maximiliano Martins, Manoel Pires The growth, structure and magnetism of equiatomic NiMn ultrathin films on Cu$_{3}$Au(100) and the magnetism of Fe/FeMn bilayers on this substrate were investigated. NiMn alloys in a concentration range around 50-50{\%} have an L1$_{0}$ structure with lattice constants a and c of 3,74 e 3,52 {\AA}, respectively. This NiMn-phase is antiferromagnetic (AFM), with high N\'{e}el temperature ($\sim $ 1070 K), being very interesting for ferromagnetic / antiferromagnet systems with exchange bias effect and, therefore, for magneto-electronic devices. Cu$_{3}$Au is an ordered fcc phase with lattice parameter of 3,75 {\AA}, a substrate with very good epitaxial relationship with L1$_{0}$ NiMn. The NiMn monolayers were grown by coevaporation, under molecular beam epitaxy conditions, and characterized in situ by RHEED, LEED, XPS, AES and MOKE. Structural analysis revealed the epitaxy and layer-by layer growth at room temperature. MOKE measurements suggest that the Fe/Ni$_{50}$Mn$_{50}$ bilayers present exchange bias, indicating that equiatomic NiMn films, as grown on Cu$_{3}$Au(100) at room temperature, is AFM, as expected for the L1$_{0}$. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P27.00004: Ab-initio simulation of magnetic exchange force mycroscopy of the antiferromagnetic Fe monolayer on W(001) Cesar Lazo, Stefan Heinze, Vasile Caciuc, Hendrik Hoelscher Magnetic exchange force microscopy (MExFM) is a promising new technique to perform magnetic imaging with atomic resolution by measuring the magnetic exchange force between a magnetically coated tip and a magnetic sample [1]. Here, we apply density functional theory using the full-potential linearized augmented plane wave (FP- LAPW) method to investigate the exchange forces on the antiferromagnetic monolayer Fe on W(001) [2]. We use an Fe cluster as a tip model and include relaxations of the cluster and the surface. Surprisingly, relaxation effects of tip and sample depend sensitively on the local magnetic configuration. Therefore, relaxations play a crucial role for the magnetic signal. In particular, the onset of the exchange forces is shifted to larger distances, which facilitates their experimental observation. Based on the calculated force-distance curves we simulate MExFM images which display a competition of chemical and magnetic forces. Our simulations can explain the experimentally observed magnetic contrast [3]. [1] U. Kaiser \emph{et al}., Nature 446, 522 (2007). [2] A. Kubetzka \emph{et al}., Phys. Rev. Lett. 94, 087204 (2005). [3] R. Schmidt, C. Lazo, \emph{et al}., submitted (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P27.00005: First-principles calculation of low-dimensional magnetic structures Invited Speaker: Magnetism in low dimensions is of great interest for fundamental and industrial research. Density functional calculations are important to provide clear physical insights for search, design and optimization of magnetic nanostructures that are essential in new technologies. We have recently performed systematic studies for search of giant magnetic anisotropy energies in single atom such as 3d on CuN, monatomic wires encompassing 3d-5d atoms, magnetic thin films such as 3d on Cu and Au. We will review the physics that governs the magnetic anisotropy and other phenomena driven by spin-orbit coupling. We will also discuss our recent results of spin dynamics in nanoentities. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P27.00006: Shape Anisotropy and Magnetization Modulation in Hexagonal Cobalt Nanowires Zuwei Liu, Paichun Chang, Chia Chi Chang, Gerd Bergmann, Jia G. Lu Ferromagnetic Co nanowires with diameter around 90 nm are synthesized via low voltage electrodeposition method. High resolution transmission electron microscopy and x-ray diffraction results show that the nanowires are uniform in size, and consist predominantly \textit{hcp} structure with the magnetocrystalline easy axis ($c$-axis) perpendicular to the wire axis. SQUID measurement illustrates the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the nanowire axis. The magnetic domain structures of individual, segmented or multiple nanowires are studied via magnetic force microscopy. It shows a strong dipole at the two ends of the nanowire, together with a spatial magnetization modulation along the nanowire with a period around 700 nm. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the nanowire axis. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P27.00007: Unique playground for complex magnetism: Fe monolayers on hexagonal transition-metal surfaces Stefan Heinze, Bjoern Hardrat, Paolo Ferriani, Ali Al-Zubi, Gustav Bihlmayer, Stefan Bluegel Recently, the complexity of magnetic order even in single monolayer (ML) thick magnetic films on non-magnetic substrates has been dramatically demonstrated by the discovery of a spin-spiral state for a Mn ML on W(110) [1] and a nanoscale magnetic structure for an Fe ML on Ir(111) [2]. Here, we use density functional theory calculations based on the full-potential linearized augmented plane wave method to systematically study the magnetic order of an Fe ML on hexagonal hcp (0001) and fcc (111) surfaces of $4d$- and $5d$-transition metals. We show that due to substrate $d$-band filling the exchange coupling changes gradually from antiferromagnetic (AFM) on Tc, Ru, Re, and Os to ferromagnetic (FM) on Rh, Ir, Pd, and Pt. On Ru and Re the AFM coupling leads to a non-collinear N\'eel ground state due to topological frustration of exchange interaction. On Ru, Rh and Ir, the nearest-neighbor exchange coupling is small and exchange beyond nearest-neighbors, higher order spin interactions, and anisotropic exchange interaction compete making these systems a playground for intriguing magnetic order. [1] M.~Bode {\sl et al.}, Nature {\bf 447}, 190 (2007). [2] K.~von Bergmann {\sl et al.}, PRL {\bf 96}, 167203 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P27.00008: Mean Field aspects of magnetic domain pattern evolution in ultrathin Fe/Cu(001) films Danilo Pescia, Alessandro Vindigni, Niculin Saratz, Oliver Portmann, Paolo Politi Ultrathin Fe/Cu(001) films are magnetized out of plane and represent an experimental counterpart of the 2D Dipolar Frustrated Ising Ferromagnet. Indeed, one of the most attractive feature of these films is the occurrence of a variety of magnetic domain patterns; the last few consist of superstructures of positive and negative magnetization which originate from the competition between long-ranged antiferromagnetic dipolar interaction and nearest-neighbor ferromagnetic exchange interaction. The experimental patterns are relatively free to change their characteristic length of modulation as well as their overall structure (striped, labyrinthine, etc...) when the temperature is varied. The Mean Field theory is able to reproduce the temperature dependence of some important experimental observables like the domain width and the inside-domain magnetization profile (obtained by SEMPA images). We report on some Mean Field predictions for the 2D Dipolar Frustrated Ising Ferromagnet in close relationship with quantitative analysis of experimental SEMPA images recorded on Fe/Cu(001) films. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P27.00009: Novel magnetism in gold and silver nanoclusters Weidong Luo, Stephen J. Pennycook, Sokrates T. Pantelides Ferromagnetic (FM) ordering in transition-metal systems (solids, surface layers, nanoparticles) arises from partially filled $d$ shells. Thus, recent observations of FM Au nanoclusters were unexpected, and an explanation has remained elusive. We report first-principles density-functional spin-polarized calculations for Au and Ag nanoclusters. We find that in highly symmetric Au nanoclusters, the highest-occupied molecular orbital (HOMO) is highly degenerate and partially filled by Au 6$s$ electrons with spins aligned according to Hund's rule. The nanoclusters behave like ``superatoms,'' with the spin-aligned electrons being itinerant on the outer shell of atoms. Similar results obtain for Ag nanoclusters. In contrast, the same kind of calculations for Pt nanoclusters find that FM ordering is controlled by the partially filled $d$ states in the usual way, and spin polarization generally occurs in many eigenstates of the Pt clusters. This research was sponsored in part by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by the McMinn Endowment at Vanderbilt University. Computations were performed at the National Energy Research Scientific Computing Center. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P27.00010: Non-Arrhenius Ferromagnetism In 1D Systems Luca Spadafora, Fausto Borgonovi, Luca Celardo, Bruno Goncalves Topological phase space disconnection has been recently found to be a general phenomenon in isolated anisotropic spin systems. This phenomenon sets a general framework to understand the emergence of ferromagnetism in finite magnetic systems. Here we study its relevance for finite systems in contact with a heat bath. The existence of this threshold, inducing extremely large magnetic reversal time, is shown to be able to determine metastable ferromagnetic behavior in finite samples. Also, it acts as a real energy barrier. Under suitable conditions the law for average reversal times can be obtained analytically and confirmed numerically. Consistent differences from the expected Arrhenius law of reversal times are shown for short range interacting spin systems. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P27.00011: Spin-orbit induced spin-spin coupling between electrons in coupled quantum dots Jianmin Sun, Suhas Gangadharaiah, Oleg Starykh We investigate spin-spin interaction between electrons localized in spatially separated quantum dots. We show that in the presence of single electron spin-orbit interaction (of Rashba type) in the dots and Coulomb interactions between electrons, a new anisotropic coupling of the van der Waals type between spins emerges. Unlike the standard exchange this coupling does not require overlap of the wavefunctions, and as a result becomes dominant for large distance between the dots.This ferromagnetic interaction is important in the Wigner crystal state where the exchange processes are severely suppressed. [Preview Abstract] |
Session P28: Superlattices and Nanostructures (Wires, Dots, etc.): Optical Properties I
Sponsoring Units: DCMPChair: Eric Stinaff, Ohio University
Room: Morial Convention Center 220
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P28.00001: Enhancement of optical sensitivity of quantum dots near metal-dielectric interface Prabath Hewageegana, Vadym Apalkov We study theoretically the enhancement of the incident light transmitted through the diffraction grating. We are interested in the mid-infrared frequency range, corresponding to the intraband absorption by quantum dots. We show that for the s-polarized light the enhancement is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating the enhancement of the light can be increased by a few orders of magnitude. The distribution of the transmitted light is highly nonuniform with very sharp peaks with the spatial width about 10 nm. Due to strongly inhomogeneous distribution of electromagnetic field the quantum dots should be placed at special points, i.e. hot spots, with large intensity of the field. We discuss the application of this effect to the quantum dot infrared photodetectors. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P28.00002: Surface Enhanced Infrared Absorption Spectroscopy (SEIRA) using Infrared resonant Au-nanoshell based substrates Janardan Kundu, Hui Wang, Fei Le, Peter Nordlander, Naomi Halas Enhancements of the molecular signals are known to occur in the mid-IR region when the molecules are in close proximity to rough metal surfaces. This phenomenon, known as surface enhanced infrared absorption (SEIRA), is complementary to surface enhanced Raman scattering (SERS) and can be used for biochemical sensing. However, designing substrates for SEIRA that are resonant in the mid-IR has proven challenging. One solution is to use metal nanoshells, plasmonic nanoparticles with a wide plasmon tunability range from the visible to the mid-IR. Here, we exploit this tunability property of nanoshells to fabricate nanoshell aggregates and nanoshell arrays as SEIRA substrates. Para-aminothiophenol (pMA) is used as a test molecule for studying SEIRA activity of these substrates. SEIRA enhancement factors are evaluated to be in the 10000 range for these substrates. These strong enhancements allow for sensing of biologically relevant molecules such as adenine. Spectral interpretation using SEIRA surface selection rule allows for insight into the molecule's preferred orientation on the nanoparticle surface. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P28.00003: Polarization memory of charged excitons in vertically coupled InAs/GaAs quantum dots Swati Ramanathan, Kushal Wijesundara, Mauricio Garrido, Eric Stinaff, Michael Scheibner, Allan Bracker, Dan Gammon Polarized photoluminescence of the InAs/GaAs coupled quantum dot system was studied, and circular polarization memory signatures of the neutral exciton, the positive trion and the negative trion are reported. Our samples are Stranski-Krastanow dots, vertically separated by a GaAs barrier. We obtain results for circular polarization memory that are consistent with previous polarization studies on single quantum dots, indicating that coupled dot systems have polarization signatures very similar to single dot systems. Due to their structure, our samples display hole level anticrossings. As the system shifts from one positive trion configuration to the other, a continuous change in circular polarization memory is observed. This change in polarization memory is explained by hole tunneling and exchange interactions. Identifying the two positive trion configurations as neutral exciton-like and positive trion-like provides a theoretical basis for understanding the circular polarization memory signature. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P28.00004: Spin interactions in a coupled InAs/GaAs quantum dot studied by polarization dependent photoluminescence Kushal C. Wijesundara, Mauricio Garrido, Swati Ramanathan, Eric Stinaff, Michael Scheibner, Allan Bracker, Dan Gammon Spins in a quantum dot molecule are of interest for possible quantum information and spintronic applications. By studying in detail the polarization dependent photoluminescence in the region where the ground state energy levels are in resonance and therefore behaving molecular-like we can gain insight into the various relevant interactions. Vertically stacked, self-assembled, InAs QDs grown by molecular beam epitaxy (MBE) on GaAs substrate were used in this study where the relative dot heights were controlled using a GaAs capping layer and the indium flush technique. The QDMs were embedded in a Schottky diode to control the electric field and selectively charge them. QDMs are brought into resonance which results in anticrossings at the positive trion states. The positive trion states demonstrated a high and low polarization for positive trion like, neutral exciton like, configurations respectively. Detailed polarization results show spin fine structure along with a continuous variation between the high and low values indicating an electric field tunable exchange interaction. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P28.00005: THz Charge Oscillations in a Modulation Doped Parabolic Quantum Well James Heyman, Laura Bell, Jeffery Rogers, Jeremy Zimmerman, Arthur Gossard We used ultrafast terahertz (THz) spectroscopy to observe THz-frequency electron oscillations in a modulation-doped InGaAs/AlGaAs parabolic quantum well. THz emission and absorption measurements yielded an electron subband spacing of 0.01eV, in agreement with sample design. Our study examined how the extrinsic electron density in the well influences THz emission efficiency, and we found no strong dependence. This indicates that THz emission in this structure arises from quantum beating of the photogenerated electrons. In contrast to a previously published report [1], we find that THz emission from the cold extrinsic electrons due to ultra-fast field screening plays at most a secondary role, even when the density of extrinsic electrons [$\sim $10$^{11}$ cm$^{-2}$] exceeds the density of photogenerated charge. This work was funded by the National Science Foundation under the NSF-RUI Program (DMR-0606181). \newline [1] R. Bratschitsch, et. al., APL 76, 3501 (2000). [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P28.00006: Scalable Single Photon Detector for Terahertz and Infrared Applications Bernard Matis, Dong Ho Wu Recent advancements in the research areas of quantum dot (QD) and single electron transistors (SET) open up an opportunity for the development of quantum dot detector, which can respond to a single photon over microwave to infrared (IR) frequencies. Currently, single photon detection is possible by means of the photomultiplier tube, but only for photons with wavelengths shorter than 1.5 $\mu$m. For the detection of photons with wavelengths longer than the IR, a bolometer is typically used. The sensitivity of a state-of-the-art bolometer, in terms of noise equivalent power (NEP), is 10$^{-17}$ W/Hz$^{1/2}$, which requires 10$^{5}$ photons to yield a detectable signal. In our research, similarly to Komiyama's work, we use QD and SET structures to develop single photon detectors, for which the NEP will be about 10$^{-22}$ W/Hz$^{1/2}$. This is five orders of magnitude more sensitive than the state-of-the-art bolometer, and sensitive enough to detect a single photon at microwave and IR frequencies. The small diameter of the QD and SET, about 200 - 250 nm, can increase the charging energy and therefore the operating temperature. In this presentation we will discuss the fabrication process of small quantum dots, which includes new lithographic methods in combination with e-beam lithography, and experimental problems and promises of our single photon detectors. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P28.00007: Terahertz Absorption of (In,Ga)As Quantum Post Nanostructures C.M. Morris, D. Stehr, D.G. Allen, J. He, H.J. Krenner, C. Pryor, P.M. Petroff, M.S. Sherwin Quantum posts (QPs) are a new kind of self-assembled semiconductor nanostructure created by vertical stacking of self-assembled InAs quantum dots into roughly cylindrical In rich regions embedded in a GaAs matrix.$^{2}$ These structures have potential applications for THz quantum information processing,$^{1}$ THz generation, and THz detection. For a single electron trapped in a 40 nm high QP, the orbital transition between the ground and first excited state is predicted to occur near 1 THz.$^{2}$ Voltage controlled electron loading of QPs is measured by capacitance-voltage spectroscopy. Terahertz absorption spectroscopy of electrons in quantum post samples is demonstrated as a function of electron loading. $^{1}$ M. S. Sherwin, A. Imamoglu and C. Montroy, PRA 60, 3508 (1999) $^{2}$ J. He et al, Nanoletters 7, 802 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P28.00008: Intrinsic optical bistability: the mechanism and features Victor Bondarenko Intrinsic optical bistability (IOB) in two-state quantum systems is investigated theoretically within the framework of density matrix formalism. The analytical relations governing the IOB are presented. The mechanism of the IOB is discovered. The IOB in the realm of time is considered within the framework of phase transitions. It is shown that the IOB in the realm of time represents cyclic process, and, thus, may be applied for description of evolution process. Numerical simulations are performed for two-state quantum dot (QD) systems. The obtained results may find applications for designing and exploiting all-optical components of QD-based optical switches and optical transistors. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P28.00009: Measurement of the separation dependence of the resonant energy transfer between CdSe nanocrystals Farbod Shafiei, Ricardo S. Decca An apparatus has been developed to study the separation dependence of interaction between two resonant groups of CdSe/ZnS quantum dots. A near-field scanning optical microscope (NSOM) is used to bring a group of mono-disperse 5.5 nm dots close (near-field range) to an 8.5 nm group of dots which are deposited on a solid immersion lens. The size of the small and large nanocrystals dots have been selected to make the excitonic ground state of the small dots coincide with the excited state of the large dots, as determined by photoluminescence and photoluminescence excitation experiments. Combination of spectral and positional filtering allows us to measure the interaction between a few quantum dots (with the ultimate goal of identifying the interaction between individual dots). The analysis of the separation-dependent photoluminescence signal from the two groups of quantum dots, yields the dipole-dipole and higher order (dipole-quadrupole) interaction terms. We expect that our results will improve the knowledge of the quantum states and decoherence processes in quantum dots. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P28.00010: Coherent coupling and energy transfer enhancement via multi-exciton levels in semiconductor nanocrystals Ameenah Al-Ahmadi, Sergio Ulloa The theory of coherent energy transfer (ET) in nanocrystal (NC) systems [1] is generalized for multiexciton levels. The relevant excitonic states in an isolated NC can be described by an effective four-level system, consisting of the ground level, two degenerate single exciton levels, and the biexciton level. We study the dynamics of a single donor-acceptor pair via the equation of motion for the density matrix of the system and consider analytical limits as well as numerical solutions. We show that the enhancement of the ET efficiency introduced by the biexciton levels is limited due to the coherent coupling of the exciton-biexciton levels in the donor-acceptor pair. The saturation of the ET rate in the donor-acceptor pair suggests a new mechanism to control the dipole-dipole coupling strength in NC systems, and we present here its dependence on structure parameters. \newline [1] A. N. Al-Ahmadi and S. E. Ulloa, Phys. Rev. B \textbf{70}, 201302(R) (2004). [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P28.00011: Luminescence excitation of InAs/GaAs coupled quantum dots Mauricio Garrido, Kushal C. Wijesundara, Swati Ramanathan, Eric A. Stinaff, Michael Scheibner, Allan S. Bracker, Dan Gammon An understanding of the excited states in coupled quantum dots is a necessary step in the road towards a coherent control of this system. Photoluminescence excitation studies were performed on an InAs/GaAs coupled quantum dot system embedded in a Schottky diode structure. The ground states of the positive trion, negative trion and neutral exciton are first clearly identified by their photoluminescence spectra in bias maps. Preliminary results are reported on the luminescence excitation spectra of these charge configurations; both near and far away from the region where molecule-like behavior is observed. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P28.00012: Record Extinction of a Laser Beam by a Single Quantum Dot Nick Vamivakas, Mete Atature, Jan Dreiser, Tunc Yilmaz, Antonio Badolato, Anna Swan, Bennett Goldberg, Atac Imamoglu, Selim Unlu The ability to efficiently couple far-field light to subwavelength light emitters is of critical importance for the rapidly growing field of nanophotonics. In this talk we present our recent work on the use of both index matched GaAs solid immersion lenses (SIL) and numerical aperture increasing lenses (NAIL) to improve far-field light coupling to and from single InAs$\backslash $GaAs quantum dots. The SIL$\backslash $NAIL leads to significant improvements in both non-resonant and resonant spectroscopic studies of single QDs. By incorporating a SIL$\backslash $NAIL in resonant scattering measurements we find that a single InAs QD can extinguish nearly 12{\%} of the exciting laser beam; a seven-fold improvement in extinction when compared to measurements made without a SIL$\backslash $NAIL. The strong extinction makes it possible to measure a typical QD extinction using a dc power-meter without the need for phase sensitive lock-in detection. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P28.00013: Quantum-mechanical description of Faraday rotation in a single quantum dot Yanjun Ma, Jeremy Levy Faraday rotation is one way to realize quantum non-demolition (QND) measurement of electron spin in a quantum dot. In the literature, it has been semiclassically modeled based on quantized electron spin states and classical electromagnetic fields. We have developed a fully quantum- mechanical model to describe Faraday rotation in single quantum dots, using an extension of the Jaynes-Cumming model which includes quantum Stokes operators. The intrinsic noise of Faraday rotation that results from the interaction between photon and electron is quantified under this model. Some effects, such as hyperfine interactions and transitions between off-resonant states such as light hole and conduction band electron states, and have not been included in our calculation. It is believed that these effects will affect the dynamics of spin and based on the current model, our calculation could be extended to examine the behavior of Faraday rotation with these effects included. This work was supported by NSF-DMR-0602846. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P28.00014: Optical Aharanov-Bohm oscillations in DMS type-II ZnMnTe/ZnSe quantum dots I.R. Sellers, V.R. Whiteside, M. Eginligil, W.-C. Chou, I. Khan, A. Petrou, A.O. Govorov, B.D. McCombe Low temperature magneto-photo luminescence studies of diluted magnetic semiconductor Zn(Mn)Te quantum dots (QDs) will be presented. As expected, the exchange interaction between the Mn spin and electric charge carriers results in a strong optical polarization of the luminescence at low temperature. However, in addition, the sample geometry for the structure, which consists of five Zn(Mn)Te QD layers separated by narrow ZnSe spacer layers, will be shown to be particularly suitable for the observation of the optical Aharanov-Bohm effect. This is illustrated by the presence of strong Aharanov-Bohm oscillations in the photoluminescence intensity. Finally, although the (ZnMn)Te system is known to be paramagnetic, at low temperatures the QD structures described display evidence of spontaneous magnetization at zero applied magnetic field both in the optical circular polarization degree and the magnetization. The origin of this behavior will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P28.00015: Robust Aharanov-Bohm oscillations at elevated temperatures in type-II ZnTe/ZnSe quantum dots V.R. Whiteside, I.R. Sellers, I.L. Kuskovsky, A.O. Govorov, B.D. McCombe The Zn(Te)Se material system is remarkable in that it is possible to study both Te-bound isoelectronic excitons and type-II ZnTe quantum dots (QDs) in the same sample. This is possible since with increasing tellurium deposition there is a clustering of the Te-atoms resulting in an evolution of Te isoelectronic centers, formed by Te-Se substitution, into ZnTe QD structures. The formation of columns of such QDs in multilayer superlattice structures has recently been shown to be particularly suitable for the observation of the optical Aharanov-Bohm effect. Here we present magneto-photoluminescence from such type-II ZnTe/ZnSe QDs that demonstrate large and persistent oscillations in \textit{both} the exciton energy \textit{and} intensity at high temperature indicating the formation of coherently rotating states. Furthermore, this high temperature Aharanov-Bohm effect is remarkably robust persisting until 180K despite significant quenching of the luminescence due to ionization of the type-II excitons. [Preview Abstract] |
Session P29: Focus Session: Carbon Nanotubes and Related Materials IX: Graphene Electronic Structure
Sponsoring Units: DMPChair: Eli Rotenberg, Lawrence Berkeley National Laboratory
Room: Morial Convention Center 221
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P29.00001: Veselago lens and p-n junctions in graphene. Invited Speaker: Both monolayer and bilayer graphene are gapless semiconductors. Their electrostatic modulation can be used to generate single and multiple p-n junctions. We have shown [V. Cheianov and V.I. Fal'ko - Phys. Rev. Lett. 97, 226801 (2006)] that p-n junctions in monolayer graphene are transparent for incident electrons. In particular, those electrons approaching the n-p interface in an almost perpendicular direction can cross it without reflection. Moreover, in graphene the transmission of charge through the n-p interface is quite similar to the refraction of electromagnetic waves at the interface where the refractive index inverts sign [J. Pendry - Nature 423, 22 (2003); J. Pendry - Phys. Rev. Lett. 85, 3966 (2000)]. This is because the electron dispersion in the conduction and valence bands in graphene is such that so that, after an electron crosses the n-p interface, from the n- to p-side, its wave vector becomes directed opposite to its velocity. As a result, n-p junctions in graphene possess intriguing and very promising transport properties: a single straight p-n interface can focus electrons [V. Cheianov, V.I. Fal'ko, B.L. Altshuler - Science 315, 1252 (2007)]. This situation is realised in the n-p junction with equal densities of carriers in the n- and p-regions. Also, we have shown that by varying the carrier density in, e.g., p-side of the junction the focus can be smeared into a pair of caustics meeting each other in a cusp, and calculated the characteristic interference pattern of electron waves in the vicinity of the cusp. Using the idea of fine-tuned focusing of electron flow by the p-n interface, we propose to use n-p-n junction In a bipolar graphene-based transistor to create Veselago lens and focused beam splitters for electrons. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P29.00002: Intraband Landau level transitions in monolayer graphene Zhigang Jiang, E.A. Henriksen, L.C. Tung, M.E. Schwartz, M. Takita, Y.-J. Wang, P. Kim, H.L. Stormer We study the cyclotron resonance of electrons and holes in monolayer graphene, via infrared transmission measurements in a magnetic field, $B$, up to 18 T. We find that, instead of having a single resonance energy as in a traditional two-dimensional system, a wide range of transitions between different sets of Landau levels (LLs) can be uniquely distinguished in monolayer graphene. We have observed intraband transitions between neighboring LLs up to $n=7$, where $n$ is the LL index. As expected from the unusual linear dispersion of the low-energy electronic band of monolayer graphene, we show that the corresponding energies of all observed LL transitions are proportional to $\sqrt{B}$. In addition, beyond such a simple linear dispersion, we find that the measured band velocity near the charge-neutral Dirac point ($E=0$) is $\sim$$12\%$ larger than that at higher energies. The LL transitions in the electron and hole bands of monolayer graphene show a considerable asymmetric behavior. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P29.00003: High Field Magnetoresistance of Graphene at the Dirac Point Joseph Checkelsky, Lu Li, N. P. Ong The longitudinal and Hall resistance of graphene near the charge neutral point have been studied down to low temperature (20 mK) in high magnetic field (20 T). At issue is the nature of the ground state in the vicinity of the Dirac point in high magnetic fields. In samples in which the offset voltage is small, we observe a highly unusual approach to an insulating state as the field increases. In samples with $\mu > 0.5$ T$^{-1}$ and $V_{0} < 3$ V, the resistance at the Dirac point R0 increases divergently to M$\Omega$ in fields of 14-20 T at temperatures T $<$ 2 K. This divergent behavior is suppressed in samples with large $V_{0}$. Surprisingly, this rise shows little temperature dependence below 2 K. The acute dependence on magnetic field and accompanying lack of activated behavior with temperature provides evidence for an unusual cross-over or transition to the insulating state. Implications for theoretical models including gapless edge modes and Quantum Hall Ferromagnetism will be discussed in the context of these results. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P29.00004: Effect of disorder on transport in a graphene p-n junction B. I. Shklovskii, M. M. Fogler, L. I. Glazman, D. S. Novikov We evaluate the resistance of a gate-tunable graphene ${p}$-${n}$ junction, in which the gradient of the carrier density is controlled by the gate voltage. Depending on this gradient and on the density of charged impurities, the junction resistance is dominated by either diffusive or ballistic contribution. We find the conditions for observing ballistic transport and show that in recent experiments they were satisfied at best marginally. We make suggestions how the disorder effects can be reduced. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P29.00005: Nonlinear screening and ballistic transport in a graphene p-n junction L. Matthew Zhang, M. M. Fogler Our theoretical work is devoted to a new class of graphene devices: lateral p-n junctions. Such structures have been recently realized experimentally by modulating the electron density in graphene samples with external gates. We study the charge density distribution, the electric field profile, and the resistance of such $p$-$n$ junctions. We show that the proper treatment of the electrostatic screening, including nonlinear effects, is crucial for obtaining the correct results for all these quantities. In particular, we show that the total electric field at the interface of the electron and hole regions is strongly enhanced due to limited screening capacity of Dirac quasiparticles. Accordingly, the junction resistance is significantly lower than estimated in previous theoretical literature. At the same time, our new theory enables us to achieve a closer agreement with the recent experiments. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P29.00006: Quantum Critical Scaling of Graphene Daniel E. Sheehy, Joerg Schmalian We show that the emergent relativistic symmetry of electrons in graphene near its quantum critical point (QCP) implies a crucial importance of the Coulomb interaction. We derive scaling laws, valid near the QCP, that dictate the nontrivial magnetic and charge response of interacting graphene. Our analysis yields numerous predictions for how the Coulomb interaction will be manifested in experimental observables such as the diamagnetic response and electronic compressibility. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P29.00007: Atomic Collapse and Quasi-Rydberg States in Graphene Andrey Shytov, Leonid Levitov, Mikhail Katsnelson We demonstrate that graphene opens a way to investigate in the laboratory a fundamental quantum relativistic phenomenon, that is, atomic collapse in a strong Coulomb electric field, long-sought for, but still inaccessible in high-energy experiments. We consider charged impurities in graphene and show that an impurity can host an infinite family of Rydberg-like resonance states of massless Dirac particles. Strong coupling of these states to the Dirac continuum via Klein tunneling leads to striking resonance effects with direct signatures in transport and local properties. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P29.00008: Delocalization of disordered massless Dirac fermions in graphene Kentaro Nomura, Mikito Koshino, Shinsei Ryu Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like low-energy excitations. Motivated by recent graphene transport experiments, we have undertaken a numerical study of the conductivity of disordered two-dimensional massless Dirac fermions. The beta function of the Dirac hamiltonian subject to a random scalar potential is computed numerically. Although it belongs to, from a symmetry standpoint, the two-dimensional symplectic class, the beta function monotonically increases with decreasing the dimensionless conductance. We also provide an argument based on the spectral flows under twisting boundary conditions, which shows that none of states of the massless Dirac Hamiltonian can be localized. K. Nomura, M. Koshino, S. Ryu, Phys. Rev. Lett. 99, 146806 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P29.00009: Strain induced phonon softening in graphene Mingyuan Huang, Hugen Yan, Dahua Song, Changyao Chen, Tony Heinz, James Hone We have developed a process to transfer single layer graphene from silicon dioxide to polydimethylsiloxane (PDMS). This allows the straightforward application of uniaxial strain by bending of the PDMS. Using this technique, we examine the Raman scattering spectra of graphene sheets under uniaxial strain. The spectra display significant strain-induced downshifts, as predicted by theory. The frequency shows up to 14 and 30 cm$^{-1}$ downshifts for the G mode and D$^{\ast }$ mode, respectively; the softening rate ($\Delta \omega $/$\omega )$ is about 1{\%} for both. This behavior is explained by the weakening of carbon-carbon bonds due to elongation, and is consistent with previous studies on carbon nanotubes. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P29.00010: Structural and Electronic Properties of Bilayer Epitaxial Graphene Gregory M. Rutter, Phillip N. First, Jason N. Crain, Joseph A. Stroscio Scanning tunneling microscopy (STM) and spectroscopy (STS) are used to study the structural and electronic properties of epitaxial graphene on SiC(0001) [1]. We address in this talk the surface morphology and stacking sequence of bilayer graphene. STM topographic images show that in the initial stages of growth, the surface morphology of graphene conforms to an underlying SiC interface reconstruction [1]. In bilayer epitaxial graphene, the top graphene layer forms a continuous sheet across steps separating adjoining terraces. A change in the apparent height between the two graphene basis atoms is observed as a function of tunneling bias. We model the relative heights based on a simple form for the local density of states in AB layer stacking (Bernal, as typical for bulk graphite) [2], and predict a smooth transition from imaging a single sublattice to imaging both sublattices. The experimentally-observed transition is consistent with Bernal stacking of the epitaxial bilayer, and an interlayer hopping energy of 0.4 eV. This work was supported in part by NSF grant ECS-0404084 and Dept. of Commerce/NIST grant 60NANB7D6166. [1] G. M. Rutter et al., Science \textbf{317,} 219 (2007); arXiv:0711.2523. [2] Z. F. Wang et al., Phys. Rev. B \textbf{75,} 085424 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P29.00011: Aharonov-Bohm effect and broken valley-degeneracy in graphene rings Patrik Recher, Bjoern Trauzettel, Adam Rycerz, Yaroslav Blanter, Carlo Beenakker, Alberto Morpurgo We analyze theoretically the electronic properties of Aharonov-Bohm rings made of graphene. We show that the combined effect of the ring confinement and applied magnetic flux offers a controllable way to lift the orbital degeneracy originating from the two valleys, even in the absence of intervalley scattering. The phenomenon has observable consequences on the persistent current circulating around the closed graphene ring, as well as on the ring conductance. We explicitly confirm this prediction analytically for a circular ring with a smooth boundary modelled by a space-dependent mass term in the Dirac equation. This model describes rings with zero or weak intervalley scattering so that the valley isospin is a good quantum number. The tunable breaking of the valley degeneracy by the flux allows for the controlled manipulation of valley isospins. We compare our analytical model to another type of ring with strong intervalley scattering. For the latter case, we study a ring of hexagonal form with lattice-terminated zigzag edges numerically. We find for the hexagonal ring that the orbital degeneracy can still be controlled via the flux, similar to the ring with the mass confinement. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P29.00012: Topological confinement in bilayer graphene Ivar Martin, Yaroslav Blanter, Alberto Morpurgo We study a new type of one-dimensional chiral states that can be created in bilayer graphene (BLG) by electrostatic lateral confinement. These states appear on the domain walls separating insulating regions experiencing the opposite gating polarity. While the states are similar to conventional solitonic zero-modes, their properties are defined by the unusual chiral BLG quasiparticles, from which they derive. The number of zero-mode branches is fixed by the topological vacuum charge of the insulating BLG state. We discuss how these chiral states can manifest experimentally, and emphasize their relevance for valleytronics. [Preview Abstract] |
Session P30: Electronic Properties of Graphene and Related Structures III
Sponsoring Units: DCMPChair: Antonio Castro Neto, Boston University
Room: Morial Convention Center 222
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P30.00001: Electric Field and Frequency Dependence of the Dielectric Storage, Loss, and Conductivity of Multi-Wall Carbon Nanotubes Rajratan Basu, Germano Iannacchione The dielectric storage ($\epsilon'$) and loss ($\epsilon''$) of the complex dielectric constant ($\epsilon*$) are reported for multi-wall carbon nanotubes (MWCNT) up to $10^5$~Hz as a function of ac-electric field amplitude $E_{rot}$ (in-phase and same frequency as the measurement) and $E_{ac}$ (fixed phase and frequency with respect to the measurement). A slow relaxation process (mode-1) is observed that increases in peak frequency with increasing $E_{rot}$ but is independent of $E_{ac}$. A fast relaxation process (mode-2) is also observed that is independent of $E_{rot}$ and shifts to higher frequency with increasing $E_{ac}$ (opposite to that seen for mode-1). A conductivity analysis of MWCNT reveals possible mechanisms for how $E_{rot}$ and $E_{ac}$ can effect the dielectric dissipation differently. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P30.00002: The genus g=2 problem- A solution of the Persistent current for the genus g=2 -An application to the Edge currents in Graphene David Schmeltzer We report the first solution of Persistent currents for genus $g=2$ Aharonov-Bohm coupled rings which form a character ``8'' structure. For two large coupled rings with equal fluxes, we found that the persistent current in the two coupled rings is equal to that in a single ring. For opposite fluxes the energy has a chaotic structure. This results are obtained within an extension of Dirac's second class constraints. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P30.00003: Potential barriers in graphene Nimrod Stander, David Goldhaber-Gordon, Benjamin Huard, Joey Sulpizio, Kathryn Todd, Bo Yang Graphene is a single sheet of graphite. Some of its remarkable electronic properties have been predicted over the past 6 decades, but only recently, the Geim group at Manchaster succeeded in fabricating graphene and measuring the Quantum Hall effect. These measurements agreed with earlier predictions by observing plateaus at half-integer values and triggered an immense theoretical and experimental effort. It was also predicted that the tunneling through a potential step in graphene is highly anisotropic, and occurs with probability 1 at normal incidence, due to the chiral nature of its quasiparticles. This behavior can be investigated in different potential configurations, such as pn junctions or npn barriers in graphene. In this talk, I will present our experimental work on electronic transport through a tunable potential barrier in top gated graphene devices. I will show that the experiments we have done, depend on the disorder and on the profile of the potential rise across the graphene sheet. Therefore, they can also be seen as as a tool to investigate scattering and screening properties in graphene. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P30.00004: Effects on the electron conductivity emerging from Majorana zero modes Miklos Gulacsi, Pasquale Sodano The link between single particle random Hamiltonians and non-linear sigma models (NLSM) comes about when setting up a generating function for the mean value taken by the product of Green's functions. Using the standard machinery of the replica trick we derived disorder averaged product of the retarded and advanced Green's functions from an effective NLSM describing low energy physics of graphene films with both disorder and defects. We then compute the electron transport properties and we find remarkable effects arising when the defect topology is such that the Majorana fields acquire zero modes. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P30.00005: BEC and Superfluidity of Magnetoexcitons in Graphene Oleg Berman, Godfrey Gumbs We propose the experimental observation of Bose-Einstein condensation (BEC) and superfluidity of quasi-magnetoexcitons in bilayer graphene. Electrons are in one layer and holes in another which are controlled by an applied gate voltage. We describe the dilute gas of magnetoexcitons with dipole- dipole repulsion in a strong magnetic field $B$ by a $4\times4$ matrix Hamiltonian. This Hamiltonian is mapped on to a scalar effective mass Hamiltonian for a dilute gas of dipolar excitons without an applied magnetic field. However, the magnetic field enters through a $B$-dependent effective mass for magnetoexcitons. Moreover, for $N$ excitons, we reduced the problem in a space with $2N\times 2$ dimensions into one with $N\times 2$ dimensions. This is accomplished by integrating over the coordinates of the relative motion of electron and hole. We will present the energy spectrum of collective excitations, the sound spectrum as well as the effective magnetic mass of magnetoexcitons in the strong magnetic field limit. The superfluid density $n_S$ and the temperature of the Kosterlitz-Thouless phase transition $T_c$ are shown to be increasing functions of the excitonic density $n$ but decreasing functions of $B$ and the interlayer separation $D$. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P30.00006: Fractionalization in a square-lattice model with time-reversal symmetry Marcel Franz, Conan Weeks, Babak Seradjeh We propose a two-dimensional time-reversal invariant system of essentially non-interacting electrons on a square lattice that exhibits configurations with fractional charges $\pm e/2$. These are vortex-like topological defects in the dimerization order parameter describing spatial modulation in the electron hopping amplitudes. Charge fractionalization occurs via a mechanism similar to that in graphene with the ``Kekule'' distortion and is established by a simple electron counting argument, analytical calculation within the effective low-energy theory, and by an exact numerical diagonalization of the lattice Hamiltonian. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P30.00007: Dirac Fermions in a Magnetic Forest Andres Concha Recent experimental progress have made it possible to study of a single layer of carbon atoms, i.e. $\it {graphene}$. This newly synthetized material seems to be a promising candidate for building up nano-devices as well as a useful experimental tool to study exotic properties of its low energy excitations. In this work we consider the effect of a vortex lattice in the adjacent superconductor on the transport properties of a graphene sheet. We show that the transport properties of graphene sheets of various geometries can be substantially altered and manipulated by changing the vortex lattice structure and the amount of magnetic flux. A relatively straightforward experimental test of our results is suggested. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P30.00008: Phonon-induced many-body renormalization of graphene electronic properties Wang-Kong Tse, Sankar Das Sarma In this talk, we present a many-body theory for the electron-phonon interaction effects on the electronic properties of graphene. We provide analytical results for the electron self-energy, spectral function, and band velocity renormalization due to phonon-mediated electron-electron interaction, showing that phonon-mediated electron-electron coupling has a large effect on the graphene band structure renormalization. Our analytic theory successfully captures the essential features of the observed graphene electron spectra in the angle-resolved photoemission spectroscopy (ARPES) experiments, predicting a kink at $\sim 200\mathrm{meV}$ below the Fermi level and a reduction of the band velocity by $\sim 10-20\%$ at the experimental doping level. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P30.00009: The effect of electronic correlations on Josephson current and proximity effect in SNS graphene junctions Annica Black-Schaffer, Sebastian Doniach Using the self-consistent tight-binding Bogoliubov-de Gennes (BdG) formalism, we investigate the proximity effect and current-phase relationship in SNS graphene Josephson junctions. Both short and long junctions are considered, as well as different doping levels of the graphene. For short junctions at zero doping in the uncorrelated regime our results agree with those found using the non self-consistent Dirac-BdG formalism [1]. We introduce electronic correlations in the Hamiltonian by including the intrinsic nearest-neighbor spin-singlet coupling present in $p \pi$-bonded planar organic molecules. We study the possibility of coupling this intrinsic $s$- or $d$-wave superconducting pairing [2] to the extrinsic $s$-wave order parameter induced by the metal electrodes. The intrinsic $d$-wave solution, favored in doped graphene, appears for longer doped junctions. For short junctions, the $s$-wave solution can occur, although the result is sensitive to the type of interface. We also report on the two different intrinsic superconducting states' influence on the supercurrent. \newline [1] M. Titov {\it et al.} PRB {\bf 74} 041401 (2006) \newline [2] A. Black-Schaffer {\it et al.} PRB {\bf 75} 134512 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P30.00010: Magnetoplasmons in Graphene Multilayers Godfrey Gumbs, Oleg Berman With the use of a massless Dirac-fermion band structure, we calculate the dielectric response function for magnetoplasmons in multi- layered graphene. The ambient quantizing magnetic field is perpendicular to the plane of the graphene layers which are embedded in a background dielectric medium. We carry out numerical calculations when only the highest valence band is populated and completely full at $T=0$ K. Transitions between the highest valence band and the lowest three conduction bands yield the magnetoplasmon dispersion relation between the plasmon excitation energy and the in-plane wave number. We analyze the instability of these modes by solving the dispersion equation in the complex frequency plane as well as examining the rate of transfer of energy between the layered structure and a charged particle current parallel to the graphene layers. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P30.00011: Strong coupling polarons in graphene Lucian Covaci, Mona Berciu Continued interest in electronic properties of graphene has prompted for the description of polaron formation in 2D honeycomb lattices. We employ the recently developed Momentum Average (MA) approximation to describe coupling to optical phonons in graphene. The extension of the original method to unit cells that have multiple sites makes possible a fast calculation of the electron self-energy in graphene. This method has been proved very successful with Holstein polarons in square lattices and thus allows us to obtain accurate non-perturbative results for the ground state energy, the effective mass and the spectral function in any electron-phonon coupling regimes. Both types of electron-phonon scattering (inter and intra band) can be easily solved within the MA approximation. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P30.00012: First-Principles Theoretical Analysis of Carbon Allotropes and Nanostructures Tejinder Singh, Michael J. Behr, Eray S. Aydil, Dimitrios Maroudas We analyze the various crystalline phases of C observed upon exposing carbon nanotubes to H2 plasmas, which produces an amorphous carbon matrix with carbon nanocrystalls embedded in it. Structural characterization with electron diffraction and high-resolution TEM yields three distinct crystalline phases of C consistent with a fcc lattice with lattice parameter a = 4.25 {\AA}, a bcc lattice with a = 3.0 {\AA}, and a diamond lattice with a = 3.57 {\AA}. Using first-principles density functional theory (DFT) calculations, we have analyzed the structure of several allotropes of pure carbon and we discuss our results in the context of the experimental findings. In addition, we consider the possibility of H incorporation in these C phases. According to our DFT calculations, incorporation at proper concentrations of H in interstitial sites of cubic phases of C provides interpretations for the experimentally observed crystalline C phases. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P30.00013: d+id'-wave Superconducting States in Graphene and Andreev Reflection Daoxin Yao, Yongjin Jiang, Erica W. Carlson, Han-Dong Chen, JiangPing Hu We study the d+id superconducting state in graphene through proximity effect. By introducing short range pairing (i.e. bond singlet pairing), we found that some effective, low energy pairing order parameter with symmetries will emerge. This is due to the Dirac cone structure in graphene at low energy. We focus on the d+id' pairing order between nearest neighbor sites on the honeycomb lattice of graphene. At low energy, the effective pairing has a p+ip-wave pairing component and a s-wave pairing component, which has a dramatic effect on band structure, Andreev reflection, differential conductance. For the energy gap, we found that it is dominated by p+ip-wave symmetry at low doping and saturated at high doping which is related to s-wave symmetry. The Andreev conductance spectra shows remarkable difference between d+id' wave and s-wave pairing case (including extended s-wave). We point out that an recent Andreev reflection experiment is consistent with our d+id' calculation. In the end, we discuss the possible ways to realize this d+id' superconducting state in graphene. Reference: Y. J. Jiang, D. X. Yao, E. W. Carlson, H.-D. Chen and J. P. Hu, arXiv:0710.3962 [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P30.00014: Magnetoplasmon excitations in graphene for filling factors lower than 6. Gerard Martinez, Yuri Bychkov Graphene is a monolayer of graphite with a band structure composed of two cones located at two inequivalent corners of the Brillouin zone at which conduction and valence bands merge. In contrast to conventional two dimensional electron gas, the dispersion relation obeys a Dirac law with an energy linear as a function of momentum which leads to a specific square root dependence of the Landau levels under an applied magnetic field. Because of this specific dispersion, electron-electron interactions are expected to play a significant role in the magneto-optical response of this system which should be analyzed in terms of magnetoplasmon excitations. In the frame of the Hartree-Fock approximation, the dispersion of these excitations have been calculated for all transitions corresponding to a filling factor lower than 6. It is found that each type of transitions, for wave vectors close to zero, displays a variation, as a function of the magnetic field, corresponding to a re-normalized Fermi velocity different for different types of transitions. The theoretical results will be compared to available experimental data. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P30.00015: Charge polarization and phonon renormalization at the K point of graphene Jose Gonzalez, Enrico Perfetto We study the renormalization of the dispersion of different phonon branches by low-energy electronic excitations at the K point of graphene. Among all the in-plane phonons, only the transverse optical modes have a nonvanishing coupling to electron-hole excitations, with a mild Kohn anomaly at the K point. We show, however, that the total charge polarization has a singular behavior and that the dispersion of the out-of-plane phonons undergoes therefore a strong renormalization, with a significant decrease of the phonon frequencies. This leads to an enhancement of the coupling between the two Dirac valleys in graphene, with the potential to open an instability in the spectrum of Dirac quasiparticles. [Preview Abstract] |
Session P31: Focus Session: Computational Nanoscience V: Mechanical Properties and General Methods
Sponsoring Units: DMP DCOMPChair: Murilo L. Tiago, Oak Ridge National Laboratory
Room: Morial Convention Center 223
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P31.00001: Materials under Extreme Conditions: Ultrahigh Strength under Shock Loading Invited Speaker: |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P31.00002: Effects of nanoscale structure on the mechanical properties of nanoporous silica. Daniel Lacks Porosity in materials can create materials with superior function, by reducing the weight and dielectric constant of the material -- e.g., in nature, porous bone has evolved to reduce the weight of the skeleton and thus minimize the energy required for animals to move. However, the introduction of porosity is not without a downside, as porosity generally compromises the mechanical behavior of a material, which could preclude its utility. We use molecular dynamics simulations, in conjunction with an experimental investigation, to show that nanoscale structuring can be used to make porous silica with mechanical properties that remain favorable as the porosotity increases. In particular, the elastic modulus scaling with density is much weaker in these materials than in conventional porous materials. Our simulations show that this scaling occurs because the nanoscale structure induces a change in the atomic level structure, where the new structure has a higher local stiffness. (Fan et al, Nature Materials 6, 418 (2007)). [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P31.00003: Phase-field simulations of martensite-martensite nanocomposites Mathieu Bouville, Rajeev Ahluwalia We study composites made of two martensite-forming materials, with different transition temperatures, $T_l < T_h$. The system remains austenitic at high temperature, and if the temperature is very low then the difference between the two materials is negligible. We therefore focus on intermediate temperatures, i.e.\ $T_l \le T < T_h$. Then only one material can transform to martensite~--- the other transformation may occur only if it is triggered by the martensite already formed (volume changes will then play a key role). We study the effect of reducing the size of the system, in particular how martensite can form in nanocomposites at temperatures at which no martensitic transformation can exist at the macroscopic scale. This work has relevance to multiferroics, where the phase transformation in one material (e.g.\ ferroelectric) triggers a transformation in another material, for instance magnetostrictive. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P31.00004: The dynamical activation-relaxation technique (DART): an on-the-fly kinetic Monte-Carlo algorithm Fadwa El-Mellouhi, Michel Cote, Laurent J. Lewis, Normand Mousseau We present DART, the dynamical activation-relaxation technique, that combines the activation-relaxation technique (ART nouveau) with a non-lattice KMC method that allows the on-the-fly identification of barriers and the full treatment of lattice deformations. Most KMC schemes rely on the use of a fixed list of events and barriers, which are drawn with the proper weight during the simulation. While this works well for a number of problems (such as metal-on-metal growth), it cannot be used for processes where the events may change with time. DART overcomes this limitation. ART nouveau has been used extensively for the study of activated mechanisms in different materials within both an empirical and an ab-initio description of the systems. In the DART implementation, KMC moves are based on a catalog of events constructed on-the-fly using ART. After each KMC move, this catalog is updated so as to take into account new environments that may appear. A topological description of the structure of the system at each moment allows the method to identify rapidly these new environments and to move forward efficiently. In this talk, we will describe the method and present the case of interstitial diffusion in Si. Our results are compared with previous molecular-dynamics and on-lattice KMC simulations. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P31.00005: Charge patching method for the calculation of electronic structure of organic semiconductors Nenad Vukmirovic, Lin-Wang Wang The electronic structure of organic semiconducting conjugated polymers and molecular crystals is essential in determining their optical and transport properties. Such organic semiconductor systems have potential applications for solar cells, field-effect transistors and luminescent devices. However, for such systems containing a large number of atoms, the direct calculations based on density functional theory (DFT) are often not feasible. Here, we present the development of the charge patching method for the calculations of organic systems, a method which was previously successful in treating inorganic semiconductor materials [1]. The results of the calculations for alkane and alkene chains using this approach yield the difference in Kohn-Sham wavefunction eigen energies of the order of only 30 meV compared with direct DFT calculations. Further investigations involving aromatic compounds, as well as elements such as sulfur, nitrogen, and oxygen, will be presented. [1] L.W. Wang, Phys. Rev. Lett. 88, 256402 (2002). [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P31.00006: Real-space grid representation of momentum and kinetic energy operators for electronic structure calculations. Domenico Ninno, Giovanni Cantele, Fabio Trani The development of computational methods based on real-space grids is contributing to the advances and understanding of nanoscale materials. Real-space grids methods, particularly within the pseudopotential density functional theory, have the advantage of producing highly structured matrices paving the way towards grid-based O(N) methods for both DFT total energy and molecular dynamics calculations. However, a limitation seems to be the lack of a basis set putting these approaches on a different ground with respect to standard methods. We prove that this limitation is only apparent showing that the position operator eigenkets are the natural basis set for the finite difference representation of momentum and kinetic energy operators. Some conceptual points and unpublished results related to the connection between the discrete and the continuum representations will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P31.00007: Single molecular NDR: A first principles study Ranjit Pati The demonstration of single molecule switch with a negative differential resistance (NDR) feature has drawn considerable attention in recent years. The NDR feature is described by a steady increase followed by a decrease in current with the increase in applied bias. Here we report a single molecular NDR in a strongly coupled metal-molecule junction, with peak to valley ratio (PVR) of 2.7 at both positive and negative bias. The bias dependent screening effect is explicitly included in our calculation through a rigorous self-consistent many body approach. The non equilibrium Green function approach is used to calculate the quantum transport. The origin of high PVR will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P31.00008: Electron correlations in molecular wires: e-e interactionion both in leads and bridge Yuri Dahnovsky, Vince Ortiz Molecular systems (molecules) with strong electron-electron interaction both in leads and a bridge are described in terms of the time-dependent electron Green functions. We prove that the Meir-Wingreen expression holds if one assumes that the bridge and lead electron subsystems are strongly separated. We develop a diagrammatic technique in a well determined, noncrossing cluster approximation. Within this approach, Dyson equations for various nonequilibrium Green functions are derived, and the validity conditions are found. In addition, we rigorously prove that despite strong electron-electron or electron-viration interaction in the systems with the finite number of quantum states, the Landauer-Buttiker expression for electric current is true. The ab initio electron propagator method is applied to the calculations of I-V characteristics in molecular electronic devices with the bridge composed from 1,4--benzene--dithiolate molecule. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P31.00009: Characterizing Capture-Zone Distributions: Generalized Wigner vs.\ Alternative Forms, and Experimental Fits T.L. Einstein, Alberto Pimpinelli In problems of growth of islands or of dots, it is often advantageous to consider the distribution of the areas of proximity (Voronoi) cells of nucleation sites, i.e.\ the capture zones (CZ). Extending results for terrace-width distributions on vicinal surfaces, \footnote{AP et al., PRL 95, 246101 (2005); TLE, Appl. Phys. A 87, 375 (2007)} we have shown that the (non-equilibrium, steady-state) CZ distribution is well described by the generalized Wigner expression $P_\varrho(s)=as^\varrho\exp(-bs^2)$ [with $a$ and $b$ being constants assuring normalization and unit mean, and $s$ here the CZ area divided by its mean], which accounts for a strikingly broad range of fluctuation phenomena. For CZ distributions we find that the single adjustable parameter $\varrho = (2/d)(i + 1)$, where $i$ is the size of the critical nucleus and $d$ (= 1 or 2) the spatial dimensionality.\footnote{A. Pimpinelli and T.L. Einstein, PRL 99, 226102 (2007).} We emphasize comparisons with other fitting expressions (esp.\ gamma functions), new applications to experimental data, and the generality of $P_\varrho(s)$. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P31.00010: Molecular simulations of capillary adhesion Shengfeng Cheng, Mark Robbins Adhesion due to capillary condensation is ubiquitous in nature, and dominates the adhesion force between particles in many experiments. Traditional models are based on continuum theory and may not describe nanoscale capillaries in microelectromechanical systems (MEMs) or at an atomic force microscope (AFM) tip. We employ molecular dynamics simulations to investigate the capillary adhesion between a nominally spherical tip and a flat substrate with a liquid bridge of fixed volume. The atomic scale roughness on the tip, contact angle and volume are varied. The adhesive force-distance curve and the separate contributions from Laplace pressure and surface tension are compared to continuum theory using independently measured parameters. Continuum theory provides a good description down to separations of a few molecular diameters. Atomic scale roughness affects the contact angle that enters the continuum theory and alters the adhesive force. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P31.00011: Molecular propellers and tunneling-driven motors Lela Vukovic, Boyang Wang, Petr Kral We design molecular propellers with carbon nanotube rotors and aromatic blades that allow selective pumping of hydrophobic and hydrophilic liquids [1]. Our molecular dynamics studies show that the pumping efficiency strongly depends on the chemistry of the liquid-blade interface. We also discuss several prototypes of highly efficient molecular motors driven by electron tunneling that could drive such rotary molecular machines [2]. These systems might pump liquids and provide motility at the nanoscale. [1] B. Wang and P. Kr\'al, Phys. Rev. Lett. 98, 266102 (2007). [2] L. Vukovic, B. Wang and P. Kr\'al, submitted. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P31.00012: An algebraic approach to computer program design and memory management James Raynolds, Lenore Mullin Beginning with an algebra of multi-dimensional arrays and following a set of reduction rules embodying a calculus of array indices, we translate (in a mechanizable way) from the high-level mathematics of any array-based problem and a machine specification to a mathematically-optimized implementation. Raynolds and Mullin introduced the name \it{Conformal Computing}\,${}^\star$\rm \ to describe this process that will be discussed in the context of data transforms such as the Fast Fourier, Wavelet Transforms and QR decomposition. We discuss the discovery that the access patterns of the Wavelet Transform form a sufficiently regular subset of those for our cache-optimized FFT so that we can be assured of achieving similar efficiency improvements to the Wavelet Transform as those that were found for the FFT. We present recent results in which careful attention to reproducible computational experiments in a dedicated/non-shared environment is demonstrated to be essential in order to optimally \it{measure} \rm the response of the \it{system} \rm (in this case the computer itself is the object of study) so as to be able to optimally tune the algorithm to the numerous cost functions associated with all of the elements of the memory/disk/network hierarchy. ${}^\star$ The name Conformal Computing is protected: \copyright ~2003, The Research Foundation, State University of New York. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P31.00013: Giant Hall Effect in Laterally Inhomogeneous 2D Electron Gas Hang Xie, Ping Sheng Giant Hall effect has been observed in non-magnetic granular metals at concentration close to the quantum percolation threshold [1], attributable to quantum interference effect. In this work we numerically simulate the Hall effect for 2D electron gas in a laterally inhomogeneous structure. At scales smaller than the electron dephasing length, we obtain the Hall coefficient of 2DEG by solving Schrodinger's equation, with 4 leads connected to the sample. It is shown that for special (laterally) nano-scaled structures, the Hall coefficient can be enhanced by at least 3 orders of magnitude. We have also simulated the effect of assembling such structures into a macroscopic sample, by solving the Laplace equation. \newline [1] X. X. Zhang, C. Wan, H. Liu, Z. Q. Li, P. Sheng and J. J. Lin, Phys. Rev. Lett. 86, 5562-65, (2001). [Preview Abstract] |
Session P32: Focus Session: Magnetic Media and Hard Magnetic Materials
Sponsoring Units: GMAG FIAPChair: Ping Liu, University of Texas at Arlington
Room: Morial Convention Center 225
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P32.00001: Magnetism of FePt nanoparticles and nanodot arrays. Invited Speaker: L1$_{0}$ structured FePt materials show great potential for magnetic data storage media applications.$^{1}$ The first part of this talk concerns the magnetism in chemically synthesized FePt nanoparticles. Discrete FePt nanoparticles with L1$_{0}$ structure have recently been realized by salt annealing, making it possible to study their size dependent magnetic properties.$^{2}$ We have discovered a strong reduction of magnetization with decreasing FePt particle size and an unusual temperature dependent magnetization that deviates from the Bloch's T$^{3/2}$ law at low temperatures. A model based on competing exchange interactions is proposed to explain the unusual behavior, considering explicitly the nanoparticle shape. FePt system has complicated exchange interactions, with interaction in the (100) plane being strongly ferromagnetic and inter-plane much weaker. The ferromagnetic and antiferromagnetic exchange interactions contribute differently at the nanoparticle surface and interior, leading to reduced ferromagnetic order at the surface terminated by certain facets. The model correctly explains the magnetization reduction with decreasing particle size, a surface paramagnetic phase as evidenced by Mossbauer spectroscopy and the unusual temperature dependent magnetization behaviors. The second part of this talk will report our recent efforts in developing ordered FePt nanodot arrays using self-assembled porous templates as evaporation masks. The arrays possess perpendicular anisotropy, large coercivity and extremely high density, all of which are desirable features for future data storage media. \newline $^{1}$S. Sun \textit{et al}., Science, 287, 1989 (2000). \newline $^{2}$C. Rong, \textit{et al.}, Adv. Mater. 18, 2984 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P32.00002: Permanent-Magnetic Fe-Pt Nanosparticles Ralph Skomski, D.J. Sellmyer Recently, it has become possible to produce magnetically stable small-scale single-phase [1] and hard-soft [2] Fe-Pt nanoparticles with potential applications in permanent magnetism. The coercivity is largely determined by the degree of ${L}$1$_{0}$ ordering and the presence of the soft phase, respectively, and affected by surface anisotropy. We model the coercivity of the particles as a function of composition, structure, and particle diameter. The smallest particles reverse coherently, with renormalized anisotropy constants, but with increasing size, micromagnetic corrections become important. The reversal modes in the two-phase particles are reminiscent of ${p} $-state wave functions in atomic physics [2] and well described by second-order perturbation theory. We also discuss extensions involving semihard phases, which may be created by substitutions or coatings using heavy transition metals, such as Pt and W. One example is hexagonal Co$_{1-x}$Pt$_{x}$, which exhibits a huge anisotropy per Pt atom and a substantial net anisotropy. - [1] C. B. Rong ${et al.}$, Adv. Mater. 18, 2984 (2006); R. Skomski ${et al.}$, JAP 103, in press (2008). - [2] J. E. Shield ${et al.}$, JAP 99, 08B508 (2006). - [3] R. Skomski, ${Simple Models of Magnetism}$, University Press, Oxford 2008. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P32.00003: FePt Nano-particles and Nano -wires Levent Colak, George Hadjipanayis In this work, we have studied the microstructure and magnetic properties of FePt nano-particles, nano-rods, and nano-wires synthesized by a modified chemical synthesis route described elsewhere$^{[1],[2]}$ The effect of synthesis parameters on the particle shape has been investigated for nanoparticles with sizes of 5-7 nm, for nano-rods and nano-wires with a diameter of 2-3 nm and a length of 20 and 100 nm, respectively. Low injection temperature for the iron precursor and usage of surfactants as reaction solvents resulted in 7 nm nano-cubes whereas a high heating rate to refluxing temperature and high injection temperature resulted in spherical shapes with 5 nm diameter. Nano-rods and nano-wires are formed by simply adjusting the relative ratios of surfactants to reaction solvents (oleic acid, oleyl amine and octadecene/benzylether) and the refluxing time. Transmission electron microscope (TEM) studies show that usage of high concentrations of oleyl amine and longer refluxing times induce nano-wire formation. HRTEM and magnetometry studies are currently in progress to investigate the development of particle morphology and microstructure during the synthesis and determine their influence on the magnetic properties. 1. C.Wang et. al. Angew. Chem. Int. Ed. 2007, 46,1-4. 2. M. Chen et. al. J. Am. Chem. Soc. 2007, 129, 6348-6349. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P32.00004: Nanostructure and Magnetic Properties $L$1$_{0}$ FePt Films with Additions of Au and Cu T. George, M. Yan, Y. Xu, R. Skomski, R. Kirby, D. J. Sellmyer Non-epitaxially grown $L$1$_{0}$ FePt:Au and FePt:Cu films have been fabricated and investigated.~ All films are initially deposited with the structure [FePt/X]$_{n}$ and have individual layer thicknesses from about 0.1 nm to 1 nm. The $L$1$_{0}$ phase is achieved by post-deposition annealing at temperatures from 500 to 600 \r{ }C for varying times.~ XRD and TEM show that Cu enters the $L$1$_{0}^{ }$lattice whereas Au segregates at the grain boundaries. Both types of films exhibit a decrease in $M_{s}$, due to magnetic dilution. The coercivity ($H_{c})$ increases and decreases with the addition of Au and Cu, respectively. These changes are due to reduced anisotropy (Cu) and to reduced inter-granular exchange coupling (Au). In the FePt:Au films, MFM shows a decrease in magnetic coherence length (L$_{M})$ from 90 to 74 nm and the $M-H$ slope $\alpha $ = (d$M$/d$H)_{Hc}$ decreases from 5.7 to 0.9 for Au contents from zero to 32 vol{\%}. A simple interaction model quantifies these trends by considering that interparticle exchange cooperatively enhances both $\alpha $ and L$_{M}$. In the FePt:Cu films, the addition of Cu yields a decrease in Curie temperature (574 K at 20 vol{\%}). Mean-field calculations qualitatively reproduce this decrease in $T_{c}$ but indicate deviations from random solid-solution behavior. - This research is supported by INSIC, NSF-MRSEC and NCMN. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P32.00005: Electric field assisted magnetization reversal in FePt films. Pavel Lukashev, Kirill Belashchenko, Renat Sabirianov We propose to use strain assisted reduction in anisotropy of FePt in order to make magnetization reversal easier in the writing of the magnetic storage devices. We performed first-principles calculations of the magnetocrystalline anisotropy of FePt under bi-axial stress using full-potential LAPW implemented in FLEUR code. Magnetocrystalline anisotropy decreases by 25{\%} with application of 1.5{\%} tensile biaxial strain. This is partially due to the reduction of the c/a ratio by about 1.5{\%} (calculated Poisson ratio is 0.33) in the tetragonal cell and partially due to the increase in volume by about 1.5{\%}. Biaxial strain can be obtained by placing piezoelectric film under FePt layer, and by applying electric field on the system. Modern ferroelectric systems can provide stress up to 2{\%}. Besides, we propose using thin ferroelectric films with asymmetric interfaces, which provides a simple way to generate bias field in the polarization reversal and related properties. The existence of the polar interfaces results in a different average polarization in the film upon reversal. As a result, the strain in the film depends on the direction of polarization. This asymmetric strain can be used do modulate magnetic properties. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P32.00006: Influence of interfacial non-magnetic materials on soft-hard bilayer interaction A. Zambano, H. Oguchi, I. Takeuchi, S. Lofland, J. Liu, D. Josell, L. Bendersky, Y. Liu, Z. Wang Among the factors that affect the hard magnet-soft magnet interaction, interfacial nonmagnetic impurities could play a significant role. Resembling 1-dimensional models, magnetic multilayer systems are simple tools to probe it. We have used the high-throughput approach [1] to study thickness gradient effects of a Cu nonmagnetic impurity layer on the interaction between a hard magnetic CoPt layer and a soft magnetic Fe layer. On single chips, multiple samples were grown by e-beam evaporation varying the impurity layer thickness (t$_{Cu})$. Magnetic hysteresis loops were taken by the magneto-optical Kerr effect, and the layer interaction was characterized by the nucleation field (H$_{N})$. H$_{N}$ vs. t$_{Cu}$ curves indicate that the hard-soft phase interaction is described by a RKKY oscillatory exchange coupling contribution plus a dipolar exponential one. We will discuss how the hard layer crystalline characteristics affect this behavior and how the interface nonmagnetic material can significantly alter the nature of the interaction. Such behavior can have a pronounced effect on hard-soft bulk nanocomposite magnets. ONR MURI N00014-05-1-0497. \newline [1] Zambano \textit{et al}., Phys. Rev. B \textbf{75}, 144429 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P32.00007: Depth dependence of anisotropy in graded Co/Pd multilayers S.M. Watson, J.E. Davies, K. Liu, G.T. Zimanyi , B.J. Kirby, J.A. Borchers As the magnetic recording industry looks beyond perpendicular recording [1] multilayered media such as exchange coupled composite [2] and graded media [3] have the potential for increasing storage density by combining low and high anisotropy materials. The soft layer reduces the required write field while the hard layer helps to maintain the thermal stability. Recent work has shown further enhancements when the anisotropy is gradually increased up to the hard layer anisotropy [3]. Grading the media in this manner is difficult to do experimentally. Equally difficult is accurately measuring the properties that make these materials unique, namely the depth dependence of the anisotropy. In this study we used polarized neutron reflectometry to measure the in-plane magnetization depth profile of graded Co/Pd multilayers with perpendicular-to-plane easy axis as a function of in-plane applied field. This technique allowed us to observe the depth-dependent response of the spins as they were pulled away from their easy axis, thus allowing us to determine the depth dependence of the anisotropy field. [1] M. Mallary, \textit{et al.} IEEE Trans. Magn. \textbf{38}, 1719 (2002). [2] R. Victora, \textit{et al.} IEEE Trans. Magn. \textbf{41}, 2828 (2005). [3] D. Suess, Appl. Phys. Lett. \textbf{89}, 189901 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P32.00008: Fast Reversal in Multilayer Exchange Spring Media Invited Speaker: Hard disk media that support ultra high densities require small grains in order to obtain high signal to noise ratios. The use of high coercive materials such as alloys in the L$_{1}$0 phase allow for thermally stable grains at grain diameters in the order of 4nm . However state of the art write heads produces too small fields to reverse these extremely hard magnetic grains. Recently composite media and exchange spring were proposed in order to decrease the write field requirements [1,2]. In exchange spring media an ultra hard magnetic storage layer is strongly exchange coupled to a softer magnetic nucleation host layer. The nucleation host decreases the switching field of the storage layer up to a factor of five without lowering the thermal stability of the entire structure. If the nucleation host is composed of multiple magnetic layers where the anisotropy increases from layer to layer it was shown that the resulting structure has a high thermal stability whereas at the same time the coercive field decreases with one over the total layer thickness [3]. Besides the previous results which were obtained in the quasi static limit, where the external field was applied slowly (several nanosecond) further surprising effects occur if the field rise time is in the order of several hundred picoseconds. These fast field rise times together with small damping constants in the media allow for precessional switching in composite media. It was demonstrated that precessional switching significantly lowers the coercive field [4] and also leads to ultra fast reversal modes [5]. We will present results on the reversal time of magnetic bilayers and magnetic trilayers in the precessional switching regime. Micromagnetic simulations show that a magnetic bilayer with a total thickness of 25 nm (hard layer anisotropy is $K_{1}$ = 1 MJ/m$^{3}$ ) can be reversed with a field pulse of 20 ps. Interestingly the reversal time increases to 0.5 ns as the field rise time is decreased from 0.1 ns to 0.01 ns. \newline \newline [1] R. H. Victora et al., IEEE Trans. Magn. 41, 2828 (2005). \newline [2] D. Suess et al. J. Magn. Magn. Mater, 290-291, 551 (2005). \newline [3] D. Suess, Appl. Phys. Lett. 89, 113105 (2006). \newline [4] B. Livshitz, et al Appl. Phys. Lett. 91, 182502 (2007) \newline [5] D. Suess, J. Magn. Magn. Mater., 41, 183 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P32.00009: Role of dipolar interactions in the determination of intrinsic switching field distributions in perpendicular recording media Yang Liu, Karin Dahmen, Andreas Berger The $\Delta H(M,\Delta M)$ method and its ability to determine the intrinsic switching field distributions of perpendicular recording media are numerically studied with the coupled hysteron model. It is found that the presence of dipolar interactions with strength of practical recording media enhances the reliability of the $\Delta H(M,\Delta M)$ method. The correlation between the fit quality measure and the deviation from redundancy measure indicates that the latter, which can be determined from experimental data alone, is a good predictor of the reliability. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P32.00010: Chiral electrical DW injection and single shot detection for ultra-high density data storage L. O'Brien, D.E. Read, D. Petit, A.-V. Jausovec, E.R. Lewis, H.T. Zeng, R.P. Cowburn Ultra high density data storage devices based on magnetic domain walls (DWs) propagating through Permalloy (Py) nanowires have recently been proposed [Cowburn et al., Science 2005]. Controlling the chirality (defined as the sense of rotation of the magnetic moments within the DW) of the DW is of vital importance for proper functioning of these devices. Chiral DW injection can be achieved using global magnetic fields; however, technological applications require this to be carried out using independent, localised fields in multiple wires simultaneously. Using the Oersted field from pulsed electrical currents passing through gold wires ($\sim $4$\mu $m wide, $\sim $200nm thick) fabricated at an angle over Py nanowires (100nm wide, 10nm thick) we inject transverse DWs. The chirality of the DWs is probed using spatially resolved MOKE measurements of their chirality dependent interaction with a cross-shaped trap. The results are consistent with chirally controlled DW injection. In addition, we are able to individually address four parallel wires and detect DW propagation using single shot MOKE measurements. Electrical readout was separately demonstrated by detecting the presence of single DWs at the end of a wire using Anisotropic Magnetoresistance (AMR) measurements. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P32.00011: Effect of Grain size on the Giant Intrinsic Coercivity of High-Energy Milled Sm(Co,Cu,Fe)5 Alloys Dilara Sultana, Alexandar Gabay, George Hadjipanayis The giant intrinsic magnetic hardness of Sm(Co,Cu)5 alloys have been known for a long time [1]. Previous studies suggested that this behavior is due to the crystal site disorder [2]. Our previous work has explained that the room-temperature intrinsic coercivity of 37 kOe after low-temperature aging is rather due to the intrinsic change in the Co atomic site occupation [3]. In this study, we investigated the effect of grain refinement through the high energy milling on the intrinsic coercivity of the Sm(Co,Cu,Fe)5 alloys. We have found that grain refinement does not affect the high coercivity of homogenized alloys, but strongly influences the onset of the giant coercivity during low-temperature aging. The microstructures of the samples are examined with TEM. [1] E.A. Nesbitt, R.H. Willens, R.C. Sherwood, E. Buehler, J.H. Wernick 1968 Appl.. Phys. Lett. 12, 361. [2] H. Oesterrier , F.T. Parker, M. Misroach 1979 J. Appl. Phys. 50, 4273. [3] A.M. Gabay, P. Larson, I.I. Manzin, G.C. Hadjipanayis 2005, J. Phys. D: Appl. Phys. 38, 1. [Preview Abstract] |
Session P33: Focus Session: Mostly Spin Injection in Si
Sponsoring Units: GMAG FIAP DMPChair: Darrell Schlom, Pennsylvania State University
Room: Morial Convention Center 224
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P33.00001: Generation, Modulation and Electrical Detection of Spin Currents in Silicon in a Lateral Transport Geometry Invited Speaker: The electron's spin angular momentum is one of several alternative state variables under consideration on the \textit{International Technology Roadmap for Semiconductors}. Electrical injection / transport of spin-polarized carriers is prerequisite for developing such an approach. While significant progress has been realized in GaAs, little has been made in Si. Electrical injection of spin-polarized electrons is demonstrated in Fe/Al2O3/Si (001) n-i-p structures by measuring the circular polarization of the electroluminescence (EL). The EL polarization tracks the Fe magnetization, confirming spin injection into the Si, and reflects Fe majority spin, consistent with the common delta{\_}1-symmetry of the Fe and Si bands. The Si spin polarization is $\sim $30{\%} at 5K, with significant polarization extending to at least 125K. These results are confirmed in Fe/Al2O3/Si/AlGaAs/GaAs quantum well structures -- the GaAs EL shows that spin transport occurs despite poor crystalline quality of Si epilayers on GaAs, the 0.3 eV Si/AlGaAs CB offset, and air exposure of the interfaces. Lateral transport structures and non-local detection techniques are used to create a spin current which flows separately from the spin-polarized charge current. This spin diffusion current is sensitive to the relative magnetizations of the injecting and detecting contacts, and can be modulated by a perpendicular magnetic field (Hanle effect) which causes precession in the transport channel. The generation of spin currents, coherent spin precession and electrical detection using magnetic tunnel barrier contacts and a simple lateral device geometry compatible with ``back-end'' silicon processing will facilitate development of silicon-based spintronic devices. \newline Refs: \textit{Nature Physics} \textbf{3}, 542 (2007); \textit{Appl. Phys. Lett} \textbf{91}, 212109 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P33.00002: Electrical Spin injection into Silicon: a comparison between Fe/Schottky and Fe/Al$_{2}$O$_{3}$ tunnel contacts G. Kioseoglou, A.T. Hanbicki, C.H. Li, P.E. Thompson, R. Goswami, G. Spanos, B.T. Jonker We have recently demonstrated successful electrical injection of spin-polarized electrons from an Fe film through an Al$_{2}$O$_{3}$ tunnel barrier into Si [1]. The spin polarization in the Si is $\sim $30{\%} at 5K, with significant polarization sustained to at least 125K. In this study we compare electrical spin injection from Fe into MBE grown Si n-i-p heterostructures using different tunnel barriers- a reversed biased Fe/Si Schottky contact and an Fe/Al$_{2}$O$_{3}$ barrier. For both types of structures the electroluminescence (EL) spectra are dominated by transverse acoustic and optical phonon emissions in the Si. The surface emitted circular polarization of the EL due to radiative recombination in the Si tracks the Fe magnetization, confirming that the spin-polarized electrons originate from the Fe for both types of samples. However, the polarization is lower for the Fe/Si contact than that of the Fe/Al$_{2}$O$_{3}$/Si system. Systematic TEM analysis has been performed to correlate the interface structure with the observed optical polarization, and reveals some Fe/Si intermixing which is absent in the Fe/ Al$_{2}$O$_{3}$/Si structure. [1] B.T. Jonker et al., Nature Physics \textbf{3}, 542 (2007). This work was supported by ONR and core programs at NRL. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P33.00003: Spin transport through n-type doped silicon using electrical methods H.-Jae Jang, Biqin Huang, Ian Appelbaum In this presentation, we report on all-electrical injection, transport, and detection of spin-polarized electrons through a 3um n-type Phosphorus-doped single-crystal silicon device. Using our hot-electron methods, we demonstrate both spin-valve behavior in an in-plane magnetic field and spin precession in a perpendicular magnetic field. Voltage spectroscopy reveals the effects of charge screening and band bending in the spin transport layer which are not evident in the operation of our previously-studied undoped silicon devices [1,2]. \newline References \newline [1] Ian Appelbaum et al. Nature 447, 295 (2007). \newline [2] Biqin Huang et al. Phys. Rev. Lett. 99, 177209 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P33.00004: Electrical injection and detection of spin-polarized carriers in silicon in a lateral transport geometry Olaf van 't Erve, Aubrey Hanbicki, Michael Holub, Connie Li, Chaffra Awo-affouda, Philip Thompson, Berend Jonker Significant progress has recently been made on spin injection into the technologically important semiconductor, Si, using vertical device structures. $^{1,2}$ We present the electrical injection, detection and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts. Fe/Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel. Non-local detection techniques show that the spin current detected after transport through the silicon is sensitive to the relative orientation of the magnetization of the injecting and detecting contacts. Hanle effect measurements demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the spin to precess and dephase in the transport channel. The realization of efficient electrical injection and detection using a tunnel barriers and a simple device geometry compatible with ``back-end'' Si processing should greatly facilitate development of Si-based spintronics. This work was supported by ONR and core NRL programs. 1. Jonker et. al., Nat. Phys. 3, 542 (2007) 2. Applebaum et. al., Nat. 447, 295 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P33.00005: Spin Transport in Silicon Invited Speaker: Silicon has been broadly viewed as the ideal material for spintronics due to its low atomic weight, lattice inversion symmetry, and near lack of nuclear spin, resulting in exceptionally long spin lifetime. Despite this appeal, however, the experimental difficulties of achieving coherent spin transport in silicon were overcome for the first time only recently, by using unique spin-polarized hot-electron injection and detection techniques. [1] Our subsequent observations of very long spin lifetimes and transit lengths [2] have impact on prospects for Silicon spintronics as the basis for a new paradigm of information processing. \newline \newline [1] Ian Appelbaum, Biqin Huang, and Douwe J. Monsma, ``Electronic measurement and control of spin transport in silicon,'' Nature 447, 295 (2007). \newline [2] Biqin Huang, Douwe J. Monsma, and Ian Appelbaum, ``Coherent spin transport through a 350-micron-thick silicon wafer,'' Phys. Rev. Lett. 99, 177209 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P33.00006: All-epitaxial heterostructure for tunneling spins into silicon Maitri Warusawithana, Darrell Schlom, Xianglin Ke, Peter Schiffer An all-epitaxial spin-tunnel structure has been constructed using molecular-beam epitaxy (MBE). The structure consists of an epitaxial layer of iron ($\sim $100 {\AA} thick) on commensurately strained SrTiO$_{3}$ ($\sim $20 {\AA} thick) on (100) Si. The thin SrTiO$_{3}$ layer serves simultaneously as a tunnel barrier for spin polarized currents and as a protective layer preventing the reaction between iron and the underlying silicon which would lead to the formation of an undesired iron silicide. While the iron film was grown in ultra high vacuum, the growth of the SrTiO$_{3}$ film on silicon was accomplished using molecular oxygen via a sequence of steps by which the formation of an interfacial amorphous silicon dioxide layer is kinetically suppressed. Magnetic measurements indicate strong magnetic anisotropy with the easy axis lying in the plane of the film and a curie temperature above 400 K. Electrical measurements probing spin injection and detection in microfabricated Fe-SrTiO$_{3}$-Si-SrTiO$_{3}$-Fe devices, where the ferromagnetic electrodes have different coercive fields due to size anisotropy, will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P33.00007: Tunneling Characteristics across Nano-Scale Metal Ferric Junction Lines into Doped Si Jian-Qing Wang, Keqiang Wang, Jiri Stehlik Tunneling properties were studied on nanofabricated metal ferric tunnel structures on phosphorus doped silicon by measuring $I-V$ characteristics and differential conductance versus bias over a wide temperature range between 80 K to 325 K. These properties were found to have very weak temperature dependences up to 250 K. Such temperature independencies in transport properties demonstrated tunneling characteristics from metal ferric nano-lines into Si via AlO$_{x}$ insulating barrier. Nanoscaled spin-dependent tunneling (STD) lines were patterned on doped Si with the injection contacts having the form of long strips with width and separation of 100 nm and several micron long patterned by e-beam lithography. The measured tunneling coefficient was nearly independent of the bias below 1.0 V, and abruptly increases above the threshold, indentifying such threshold as tunneling barrier height. The thermal transport of the active Si region demonstrated a direct correlation between thermal activation of deep levels (of 0.4 eV) in bulk Si and metal-semiconductor tunneling, revealing feasible mechanisms influencing the interfacial transport. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P33.00008: Tunnel magnetoresistance of spin tunnel contacts to silicon. R. Jansen, B.C. Min, R.S. Patel, S.P. Dash, M.P. de Jong For the development of silicon-based spintronic devices, careful design of the contacts between ferromagnet and semiconductor is crucial, as the resistance and potential energy landscape critically affects spin flow across the interface. One approach to engineer spin tunnel contacts to Si uses low work function materials, inserted between the ferromagnet (FM) and the insulator of FM/Al2O3/Si tunnel contacts [1]. Here we present another route to tune the properties of FM/Al2O3/Si contacts by exposure of the Si surface to Alkali metal atoms, such as Cs, prior to preparation of the tunnel barrier. This is surprisingly effective in reducing the Schottky barrier height, and we will present a series of measurements that elucidate the mechanism. Moreover, we show that the band bending near the contact can be inverted, leading to the formation of a two-dimensional electron gas observable in tunneling spectroscopy, and giving rise to novel tunnel magnetoresistance. [1] B.C. Min et al. Nature Materials 5, 817 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P33.00009: Spin blockade at semiconductor/ferromagnet junctions Massimiliano Di Ventra, Yuriy Pershin We study theoretically extraction of spin-polarized electrons at nonmagnetic semiconductor/perfect ferromagnet junctions. The outflow of majority-spin electrons from the semiconductor into the ferromagnet leaves a cloud of minority-spin electrons in the semiconductor region near the junction, forming a local spin-dipole configuration at the semiconductor/ferromagnet interface. This minority-spin cloud can limit the majority-spin current through the junction, creating a pronounced spin blockade at a critical current. We calculate the critical spin-blockade current in both planar and cylindrical geometries and discuss possible experimental tests of our predictions. [1] Yu. V. Pershin and M. Di Ventra, Phys. Rev. B \textbf{75}, 193301 (2007). [2] Yu. V. Pershin and M. Di Ventra, arXiv:0707.4475. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P33.00010: Spin injection and transport in graphene layers Wei-Hua Wang, Keyu Pi, Wenzhong Bao, Kathy McCreary, Wei Han, Jeanie C. N. Lau, Roland K. Kawakami Graphene is an intriguing material system for spintronics research. Due to its low atomic number and low spin-orbit coupling, graphene is an excellent candidate for spin transport. In our past study, we have demonstrated spin-polarized transport in mesoscopic graphite by two-probe spin-valve devices [1]. Recently, we further investigated this topic and fabricated non-local spin-valve devices consisting of single-layer and few-layer graphene. Ferromagnet (FM) and nonmagnetic electrodes are formed by using electron beam (e-beam) lithography and e-beam evaporation. Thin tunnel barriers consisting of magnesium oxide are inserted between graphene layers and FM electrodes to overcome conductivity mismatch and enhance spin injection efficiency. Atomic force microscopy and Auger spectroscopy are used to characterize their morphology and chemical composition. We performed magneto-transport using the Johnson-Silsbee geometry in a cryogenic environment and observed non-local spin signal up to room temperature. This unambiguously demonstrates the spin injection, transport and detection in graphene materials. [1] W.-H. Wang, \textit{et. al.}, Phys. Rev. B (Rapid Communications), in press. [Preview Abstract] |
Session P34: Tutorial for Authors and Referees of Physical Review
Room: Morial Convention Center 226
Wednesday, March 12, 2008 9:00AM - 10:30AM |
P34.00001: Tutorial for Authors and Referees Editors from Physical Review Letters and Physical Review will provide useful information and tips for our less experienced referees and authors. The information presented will be relevant to anyone who is looking to submit to or review manuscripts for any of the APS journals, or to anyone who would like to add to their knowledge and experience of the authoring and refereeing processes. Topics for discussion will include: (1) how to write good manuscripts and useful referee reports; (2) differences between manuscripts and referee reports for PRL and PR; (3) the roles of authors and referees in the review process; etc. Following a short presentation from the editors, there will be a moderated discussion of these and other topics. Questions from the audience will be most welcome. Refreshments will be served. [Preview Abstract] |
Session P35: Focus Session: Materials for Photovoltaics and Photocatalysis II
Sponsoring Units: DMPChair: Ulrike Diebold, Tulane University
Room: Morial Convention Center 227
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P35.00001: Multijunction solar cells for concentrator systems: prospects and challenges Invited Speaker: |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P35.00002: Photovoltaic properties of novel titanium oxide nanotubes Eugen Panaitescu, Christiaan Richter, Latika Menon Ultrafast synthesis of high aspect ratio titania nanotubes by anodization in chloride ions containing solutions has been reported and furthermore optimized by our group. We are presenting in this paper the results of relative measurements on photovoltaic properties of the chloride nanotubes samples sensitized with ruthenium dye N3, comparing them with samples obtained by other anodization methods, and with anatase nanopowders. Photoresponse parameters like short circuit current, open circuit voltage, maximum power and overall conversion efficiency have been measured under simulated solar radiation. Preliminary results on absolute measurements on dye sensitized solar cells employing these samples will also be presented. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P35.00003: Enhanced optical absorption by nanocavities inside titania Weiqiang Han, Lijun Wu, Robert Klie, Yimei zhu Titania, a wide band gap semiconductor, can generate powerful oxidants and reductants by absorbing photon energies. Titania has been extensively used in photoelectrochemical systems, such as dye-sensitized titania, a wide band gap semiconductor, can generate powerful oxidants and reductants by absorbing photon energies. To improve the photoreactivity of titania, several approaches, including doping and metal loading have been proposed. Nanocavities are isolated entities inside a solid and hence are very different from nanoporous, whose pores (often amorphous and irregular) connect together and open to the surface. Dense polyhedral nanocavities inside single-crystalline anatase titania nanorods were successfully synthesized by simply heating titanate nanorods. The size of the nanocavities is typically about 10 nm. The surfaces of the nanocavity polyhedron are determined to be the crystallographic low-index planes of the titania crystal. We found that these dense nanocavities significantly enhance the optical absorption coefficient of titania in the near-ultraviolet region, thereby providing a new approach to increasing the photoreactivity of the titania nanorods in the applications related to absorbing photons. This work is supported by the U. S. DOE under contract DE-AC02-98CH10886 and Laboratory Directed Research and Development Fund of Brookhaven National Laboratory (to W.H.). [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P35.00004: Charge Separation in layered Titanate Nanostructures: Effect of Ion Exchange induced Morphology Transformation O. Diwald, A. Riss, H. Grothe, J. Bernardi, E. Knoezinger Morphology changes induced by surface chemistry can provide important insights into photoexcitation processes on solids which are critical to photovoltaic and photocatalytic applications.$^{ }$We investigated charge separation processes on Na$_{2}$Ti$_{3}$O$_{7}$ nanowires and scrolled up H$_{2}$Ti$_{3}$O$_{7}$ nanotubes, two types of morphologies which by means of acid/base treatment can reversibly be transformed into each other. Some of the competitive processes photoexcited states undergo can be tracked by means of electron paramagnetic resonance and photoluminescence spectroscopy. A complementarity between efficient charge separation and radiative recombination of photoexcited states [1] was observed and clearly demonstrates the critical influence of morphology and interlayer composition on the photoelectronic properties of layered oxide nanostructures [2]. [1] Riss et al. Nano Lett. \textbf{2007, }7, 433-438. [2] Riss et al. Angew. Chem. Int Ed. \textbf{2007, a}nie.200703817, in press [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P35.00005: Bandgap Narrowing of Titanium Dioxides via Non-Compensated n-p Co-doping for Photocatalysis Wenguang Zhu, Baohua Gu, M. Parans Paranthaman, Gyula Eres, Zhenyu Zhang Titanium dioxide (TiO$_{2})$ is a promising photocatalyst for solar hydrogen production from water, yet its photocatalytic efficiency is limited by its intrinsic wide-bandgap nature. In this talk, we present a conceptually new and intuitive approach, termed non-compensated n-p co-doping, to narrow the bandgap of TiO$_{2}$. The validity of this approach has been demonstrated using first-principles calculations within density functional theory, showing that extra impurity bands are created in the gap region because of the non-compensated nature of the n-p co-doping, resulting in a narrowed bandgap around 2 eV. Moreover, the electrostatic attraction between the n and p dopants enhances their thermodynamic and kinetic solubility in the host semiconductors. Preliminary experimental results confirming the non-compensated n-p co-doping concept will also be presented, together with its applicability to other wide bandgap semiconductors. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P35.00006: The role of bond switches in light-induced defects in amorphous silicon Lucas Wagner, Jeffrey Grossman Amorphous silicon(a-Si) thin-film solar cells are promising materials for solar cells, but they suffer from the Staebler-Wronski effect, in which the efficiency degrades over the course of a few hours of light exposure. While there has been progress in mitigating this effect through sample preparation, there is still no clear microscopic explanation for the degradation. Using first principles density functional theory and highly accurate quantum Monte Carlo techniques, we investigate the simplest reaction in a-Si: a bond switch between two neighboring Si atoms. We find that these reactions can create defect states and can be light activated. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 10:12AM |
P35.00007: Time-domain ab initio studies of photoexcited electron's dynamics at chromophore-semiconductor interfaces Invited Speaker: |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P35.00008: First-principle study of the interfacial rehybridization in organic-inorganic composite photovoltaic devices Georgy Samsonidze, Filipe J. Ribeiro, Marvin L. Cohen, Steven G. Louie Composites of organic conjugated polymers and inorganic nanostructures offer cheap but at present inefficient photovoltaic materials. The efficiency of the photovoltaic device is critically dependent on charge transfer and orbital rehybridization at the donor-acceptor interface. In this work we investigate the P3HT/PCBM interface from density functional theory (DFT) based first-principles calculations. We find a strong rehybridization of the conduction band edge states suggesting an efficient route for exciton dissociation at the interface. Using many-body perturbation theory, we compute the quasiparticle corrections on top of the DFT results. These corrections are critical for accurate predictions and to reach agreement with experiment. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P35.00009: Pulsed optically detected magnetic resonance of intrinsic a-Si:H at low excitation power Sang-Yun Lee, Thomas Herring, Cungeng Yang, Heather Seipel, Christoph Boehme, Craig Taylor, Jian Hu, Feng Zhu, Arun Madan For more than 3 decades, there has been much effort devoted to the investigation of recombination processes in hydrogenated amorphous silicon (a-Si:H). Recently, low-temperature pulsed optically-detected magnetic-resonance (pODMR) studies have shown the presence of a variety of qualitatively different recombination mechanisms that influence the photoluminescence of this material [K. Lips, \textit{et. al.}, JOAM, \textbf{7}, 13 (2005).]. Here, we present similar experiments with comparatively low light excitation densities (60(10)mW/cm$^{2}$, 514nm, cw Ar$^{+}$ Laser). Qualitatively, our measurements confirm the presence of similar spin dependent recombination channels to those seen at high light excitation densities. However, due to the reduced densities of excess charge carriers, the dynamics of these processes are significantly slower. We attribute this behavior to the decreased transition probabilities at increased charge carrier separations. [Preview Abstract] |
Session P36: Focus Session: X-ray and Neutron Instrumentation and Science
Sponsoring Units: GIMSChair: George Srajer, Argonne National Laboratory
Room: Morial Convention Center 228
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P36.00001: Advances in Neutron Scattering Instrumentation at Oak Ridge National Laboratory Invited Speaker: Oak Ridge National Laboratory hosts two world-class neutron sources for condensed matter research. The High Flux Isotope Reactor (HFIR) has recently added a cold source to augment the existing high-intensity thermal beams, and the Spallation Neutron Source (SNS) is the most powerful pulsed spallation neutron source in the world. Investments in neutron scattering instrumentation have come to fruition in a current set of 9 high-performance scattering stations in the user program, covering a wide range of diffraction and spectroscopic techniques. Examples of current scientific results include reflectometry from bio-responsive polymer films and the spectroscopy of slow spin dynamics in frustrated magnetic systems. In 2008 an additional 6 instruments will start user operation or commissioning at the SNS, increasing the breadth and depth of scientific possibilities. Novel techniques and new devices in such areas as sample environment and neutron polarization are also under development. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P36.00002: Recent developments on polarized neutron scattering at NIST Wangchun Chen, Julie Borchers, Ross Erwin, James McIver, Thomas Gentile, Jeffrey Lynn, Gordon Jones $^{3}$He neutron spin filters (NSF) employ nuclear spin-polarized $^{3}$He gas, produced by optical pumping, and can be used to polarize or analyze neutron beams because of the strong spin dependence of the absorption cross section for neutrons by $^{3}$He. At the NIST center for Neutron Research, the polarized $^{3}$He NSF program has been developed to enhance the measurement capability in polarized neutron scattering. This technique has been applied in a number of neutron scattering instruments for user experiments and instrumentation development. Here we will discuss applications of the $^{3}$He NSF devices in polarized small-angle neutron scattering (SANS), polarized neutron reflectometry (PNR), and polarized triple-axis spectrometry (TAS). For these applications, we employ $^{3}$He NSFs as both neutron polarizers and neutron flippers that are achieved using the adiabatic fast passage nuclear magnetic resonance technique. We will present the results in each of these applications from magnetic nanoparticles on SANS, patterned magnetic thin films on PNR, and the multiferroelectric system on TAS. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P36.00003: Neutron detection in boron carbide/Si heterojunctions as functions of time constants and bias voltage Nina Hong, John Mullins, S. Adenwalla True solid-state neutron detectors have the potential to achieve high efficiencies at low mass, size and power. [1,2] Such detectors made from semiconducting boron carbide (BC) allow for neutron capture and charge collection in the same layer. Here we report neutron detection results from p-n heterojunction diodes of boron carbide on n-type Si. Neutron capture efficiency increases with time constant and reverse bias, from 0.15{\%} at 0 bias and short time constant to 0.46{\%} at 19 V and long time constant. Increasing reverse bias increases the depletion width in the BC layer, leading to a higher proportion of charge capture. The long time constants allow for the detection of charge capture in the BC-scope traces show charge capture times of $\sim $ 30 $\mu$s (as compared to $<$20 ns in Si). These results indicate that the BC layer is playing an active role in neutron detection, capturing neutrons as well as charge. [1] B. W. Robertson, S. Adenwalla, et al., APL, 80, 3644 (2002). [2] E. Day M. J. Diaz, and S. Adenwalla, J. Phys. D: Appl. Phys. \textbf{39, }2920 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P36.00004: The effect of Gd doping on the atomic and electronic structure of HfO$_{2}$ thin films. Ihor Ketsman, Andrei Sokolov, Kirill Belashchenko, Peter Dowben, Yaroslav Losovyj, Jinke Tang, Zhenjun Wang HfO$_{2}$ is a promising oxide for many applications, including high-$k$ gate dielectric for CMOS devices. In addition, Gd-doped HfO$_{2}$ could lead to a dilute magnetic semiconductor and provide an efficient neutron detection medium due to huge neutron absorption cross section of Gd. Gd-doped HfO$_{2 }$films deposited on both $p-$type and $n$-type silicon by PLD retain monoclinic phase at small doping levels, but can be stabilized in fluorite phase by increased doping [1]. At small doping levels, photoemission measurements indicate $n$-type character of the films as a result of overcompensation with oxygen vacancies. Depending on a doping level, the films form heterojunctions with good rectifying properties on $n$- or $p$-type silicon. Preliminary results show the potential ability of the formed diode structures to detect neutrons. [1] Ya.B.Losovyj, I.Ketsman et al.,APL, 91, 132908, (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P36.00005: Time-dependent small-angle X-ray and neutron scattering studies of solution-mediated nanoparticulate nucleation, growth and alignment. Andrew J. Allen, Vincent A. Hackley A remote-controlled, isothermal, circulating fluid flow cell has been developed for small-angle X-ray and neutron scattering (SAXS and SANS) studies of suspensions with online monitoring of flow rate, temperature or conditions, e.g., pH. Used with the small X-ray beams available at 3rd generation synchrotron sources and the nanometer-to-micrometer scale range accessible in ultrasmall-angle X-ray scattering studies, structural characteristics ranging from 1 nm to several micrometers can be measured, in situ and in real time, as a function of changing physical or chemical conditions. Used with time-resolved pinhole SAXS instruments, rapid reaction precursor phenomena at the nanoscale level can be similarly studied. Used in SANS studies, a neutron-adapted version of the flow cell allows real-time contrast variation techniques to further elucidate the structural evolution. Applications will be discussed for real-time studies of solution-mediated nanocrystalline ceramic oxide formation, and Au nanowire alignment in extensional flow. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P36.00006: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P36.00007: Dynamical reconstruction of the valence exciton in LiF Peter Abbamonte, Wei Ku, Tim Graber, James Reed, Serban Smadici, Abhay Shukla, Jean-Pascal Rueff We have used inelastic x-ray scattering, coupled with recently developed inversion techniques, to reconstruct the structure and dynamics of the valence exciton in the prototype alkali halide LiF. Our inversions, which yield resolutions $\Delta x = 0.533 \AA$ and $\Delta t = 20.67 as (2.067 \times 10^{-17} s)$, reveal that the exciton forms in less than $50 as$, oscillates with a period of $283 as$, and decays after approximately $5 fs$. It contains a pronounced $a/3$ internal periodicity, where $a = 4.027 \AA$ is the crystal lattice parameter, that changes little during the course of its life, indicating that this exciton lies very close to the Frenkel limit. Our results resolve a 70 year old contraversy about the valence exciton in alkali halides and, when compared to {\it ab initio} calculations, demonstrate a simplified theoretical approach to describing excitons in the limit of strong binding energy. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P36.00008: Core-hole propagator and resonant inelastic X-ray scattering: exact results within a Baym-Kadanoff-Keldysh approach Andrij Shvaika, Taras Mysakovych, James Freericks We solve for the core-hole propagator and the resonant inelastic X-ray scattering (RIXS) response functions in correlated materials by employing dynamical mean-field theory. We focus our attention on the spinless Falicov-Kimball model, where the problem can be solved exactly, and the system can be tuned to go through a Mott-Hubbard-like metal-insulator transition (the coupling with the core hole is also a Falicov-Kimball type of interaction). The core-hole propagator is expressed in terms of a continuous fermionic Toeplitz determinant defined only on the upper real-time branch of the Keldysh contour. We have derived exact large-time asymptotics for the core-hole propagator using the Wiener-Hopf finite sum equation technique which produces an efficient algorithm to obtain the density of states of the X-ray edge problem for any temperature and any interaction strength. We have also derived the two-particle vertices in a diagrammatic representation of the scattering processes (using a Baym-Kadanoff-Keldysh approach). As an example of this formalism, we show results for the L-edge contributions to RIXS. \textsl{This work was supported by Award No.~UKP2-2697-LV-06 of the U.S.~Civilian Research and Development Foundation.} [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P36.00009: Identification of Mn site in Pb(Zr,Ti)O$_{3}$ A. Boonchun, M.F. Smith, S. Rujirawat, B. Cherdhirunkorn, S. Limpijumnong The impurity Mn in PbTiO$_{3}$ and PbZrO$_{3}$ has been studied by mean of first-principles spin density functional theory.[1] It is found that the Mn atom energetically prefers to substitute on the Ti/Zr site over other sites (i.e., Pb site, O site or interstitial) under all equilibrium growth conditions. The calculations predict that a majority of Mn atoms substitute for Ti/Zr and have neutral-charge state each with a total electron spin of M$_{z}$ = 3/2. This prediction is supported by the combination of x-ray absorption near edge structure (XANES) experiment and first-principles simulation of the spectrum. [2] The measured XANES of the Mn-doped Pb(Ti,Zr)O$_ {3}$ within the concentration range of 0.5 - 2.0 at.\% yield the exact same features, indicating that the location of Mn in the crystal is independent of Mn concentration. The measured XANES is consistent with the partial density of states simulation of Mn atom on the Ti/Zr site and inconsistent with the simulations of Mn atom on other sites.\newline [1] A. Boonchun, M. F. Smith, B. Cherdhirunkorn, and S. Limpijumnong, J. Appl. Phys. 101, 043521 (2007). [2] S. Limpijumnong, S. Rujirawat, A. Boonchun, M.F. Smith, B. Cherdhirunkorn, Appl. Phys. Lett. 90, 103113 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P36.00010: Direct mapping of phonon dispersions in copper by x-ray thermal diffuse scattering. Ruqing Xu, Hawoong Hong, Tai Chiang X-ray Thermal Diffuse Scattering (TDS) arises from lattice vibrations. Measured TDS intensities can be used to extract phonon dispersion relations, as has been demonstrated in a number of systems. But most analysis methods so far have involved data fitting based on assumed atomic force constant models, and it is difficult to determine a priori the accuracy of the procedure. Methods of direct inversion, i.e., determining phonon frequencies directly from x-ray TDS data without a presumed model, have been proposed in prior work, but most of the schemes involve absolute intensity measurements, which are difficult especially in the presence of an unknown background. Here we report an improved approach, in which phonon frequencies are determined through the temperature dependence of TDS intensities at each point in reciprocal space. Polarization selection rules are employed to disentangle contributions from different phonon branches. Results taken from a simple model material, copper, will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P36.00011: Fractionation of uranium isotopes in minerals screened by gamma spectrometry. Jeffrey L. Geiger, Austin M. Baldwin, Charles C. Blatchley At least two groups have reported finding shifts in the ratio of U-235/U-238 for sandstone, black shale, and other sedimentary samples using precision ICP-MS. These shifts were tentatively attributed to a recently predicted isotope effect based on nuclear volume that causes fractionation for U$^{IV}$-U$^{VI}$ transitions. However, fractionation of high Z elements may be less likely an explanation than U-235 depletion induced by galactic cosmic ray neutrons. Isotope depletion in marine sediments could therefore be an indicator of changes in cosmic ray flux due to nearby supernovae, gamma-ray bursts, or longer term changes during the 62 million year cycle of the Sun above and below the galactic plane. We report using a less precise approach than ICP-MS, but one which can quickly screen samples to look for anomalies in isotope ratios, namely HPGe gamma ray spectrometry. Various levels of depletion were measured for uranium rich minerals, including brannerite, carnotite, and pitchblende, as well as coal and limestone samples. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P36.00012: Electronic Structure Study of Cerium Doped Scintillators Da Gao, Michael McIlwain The interest shown in recent years related to cerium doped lanthanum halides, such as LaCl3, LaBr3, and LaI3, is mainly due to their potential applicability as gamma ray scintillation detectors. We have performed a comprehensive theoretical study of these materials to better understand the scintillation process and define the nature of the self trapped exciton (STE) associated with thermally dependent scintillation process. The present work focuses on the study of the luminescence properties of cerium doped lanthanum halide scintillators from the point view of solid state band structure calculations. Our calculated band structures are in good agreement with experimental values. For example, LaCl$_{3}$ band gap is calculated to be 6.85 eV as compared to approximately 7.0 eV for the experimentally determined value. The theoretically calculated excitation spectra are also compared with the experimental spectra. We find that scintillation efficiencies are dependent on the location of the of Ce$^{3+}$ ground 4f and excited 5d levels with respect to the fundamental band gap of the host materials. [Preview Abstract] |
Session P37: Semiconductors III: Electronic and Optical Properties I
Sponsoring Units: FIAPChair: Willes H. Weber, The Physical Review
Room: Morial Convention Center 229
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P37.00001: VLab: A Service Oriented Architecture for Distributed First Principles Materials Computations Cesar da SIlva, Pedro da Silveira, Renata Wentzcovitch, Marlon Pierce, Gordon Erlebacher We present an overview of VLab, a system developed to handle execution of extensive workflows generated by first principles computations of thermoelastic properties of minerals. The multiplicity ($10^{2-3}$) of tasks derives from sampling of parameter space with variables such as pressure, temperature, strain, composition, etc. We review the algorithms of physical importance that define the system's requirements, its underlying service oriented architecture (SOA), and metadata. The system architecture emerges naturally. The SOA is a collection of web-services providing access to distributed computing nodes, workflow control, and monitoring services, and providing data analysis tools, visualization services, data bases, and authentication services. A usage view diagram is described. We also show snapshots taken from the actual operational procedure in VLab. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P37.00002: Atomistic Treatment of Electronic Transport: The Effect of Bandstructure Neophytos Neophytou, Abhijeet Paul, Gerhard Klimeck The effect of bandstructrue on the electronic transport properties of nanowire devices is investigated using the sp3d5s*-SO 20-orbital nearest neighbor tight-binding model with spin-orbit interactions (LCAO), self consistently coupled to a Poisson solver for treatment of the electrostatics. Under strong spatial and electrostatic confinement at the nanoscale, atomistic effects become important. Non-parabolicity, anisotropy, band coupling, and valley splitting are evident in the electronic structure of these devices and strongly influence their transport properties. In addition to that, potential variations at the nanoscale also have strong influence on the device performance. It is shown that both the transport and quantization effective masses are a sensitive function of the device geometry and behave differently in different wire orientations. Under such conditions, traditional effective mass methods are inadequate and in general fail to describe device characteristics. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P37.00003: Analytical modelling of symmetry breaking in extraordinary optoconductance K.A. Wieland, S.A. Solin The EOC effect\footnote{ K. Wieland et al., Appl. Phys. Lett., 88 (2006) 52105} is due to the differential mobilities of the photogenerated electrons and holes. EOC devices with symmetric leads have a antisymmetric positional dependence which, when the device is uniformly illuminated, leads to a minimization of the output voltage. Using a previously employed point charge model, we address two ways to break the symmetry and recover the output signal. The first is to impose uniform illumination on half the sample. This method has practical limitations as the device is miniaturized to the nanoscale. The second is asymmetric placement of the voltage probes. The crucial effect of the surface charge density is modeled in two ways - with constant charge density and by fitting experimental data. Utilizing this approach, optimal lead positions are found. For a 10mm by 2mm thin sample of GaAs with a Au shunt, the EOC reaches a calculated maximum of $\sim $ 600{\%} for lead positions x$_{1 }$= 5 mm and x$_{2}\sim $5 mm using the fit surface charge density model. However, the voltage lead positions for maximum EOC were found to not correlate with the voltage lead positions for maximum output voltage, making EOC a poor indicator of the suitability of the device as a nanosacale sensor. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P37.00004: Micro-spectroscopy of Ga$_{1-x}$Mn$_{x}$As and Ga$_{1-x}$Be$_{x}$As films with gradient doping and compensation Brian Chapler, R.C. Myers, D.D. Awschalom, M.C. Martin, K.S. Burch, D.N. Basov A detailed study into the problem of carrier induced magnetism in ferromagnetic III-Mn-V semiconductors is being carried out using micro-infrared measurement techniques. Infrared micro-spectroscopy and broad-band micro-ellipsometry measurements have been performed on films of Ga$_{1-x}$Mn$_{x}$As, x = 0.03, 0.16, as well as Ga$_{1-x}$Be$_{x}$As, x = 0.018. The films were prepared using non-rotated molecular beam epitaxial growth. The results of this growth are films with a varying As:Ga ratio across the sample. Using the above experimental techniques, measurements can be taken at specific locations along the As:Ga gradient, which have shown to cause systematic changes in spectra as a function of As:Ga ratio. These experiments provide a unique opportunity to study the effects of disorder, compensation, and doping in these samples. In addition, comparing Mn doped and Be doped films allows for distinguishing the effects of doping with magnetic versus non-magnetic impurities. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P37.00005: Slow light and anomalous pulse breakup near an exciton resonance in GaAs quantum wells Timothy Sweeney, Yan Guo, Susanta Sarkar, Hailin Wang We report experimental studies of optical pulse propagation near an exciton resonance in GaAs quantum wells. The spectral dependence of the group velocity reveals a sharp decrease in the group delay when the spectral position of the optical pulse is varied from below to above the exciton absorption line center. The decrease in the group delay occurs in a spectral range that is small compared with the exciton absorption linewidth. Pulse breakups are also observed when input optical pulses with relatively low intensities are slightly below the exciton absorption line center. Detailed nonlinear optical studies suggest that these surprising behaviors arise from coherent population oscillation and especially a sharp increase of the exciton decoherence rate from below to above the exciton absorption line center. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P37.00006: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P37.00007: Electronic properties of dilute Bismide alloys Rajeev Kini, Denis Karaiskaj, Ryan France, Aaron Ptak, Angelo Mascarenhas, Tom Tiedje The alloying of GaAs with small amounts of N or Bi results in a large reduction of the fundamental band gap, leading to the so called ``giant band gap bowing''. GaAs$_{1-x}$N$_{x }$has been the subject of intense investigation in recent years; however the lower mobility of the dilute nitride alloys limits its use for device applications. Bi incorporation is predicted to mainly perturb the valence band and does not significantly affect the electron mobility, thus promising better device performance. We report photoluminescence (PL) measurements of GaAs$_{1-x}$Bi$_{x}$ epilayers grown by MBE, carried about at different temperatures (4 - 300K), with above-band gap and selective excitation. Time-resolved PL measurements were also carried out to study the carrier lifetimes of the Bi-related states in these samples. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P37.00008: Effect of spin-orbit coupling on excitonic levels in layered chalcogenide-fluorides Andriy Zakutayev, Robert Kykyneshi, Joseph Kinney, David H. McIntyre, Guenter Schneider, Janet Tate BaCuChF (Ch=S,Se,Te) comprise a family of wide-bandgap p-type semiconductors. Due to their high transparency and conductivity, they have potential applications as components of transparent thin-film transistors, solar cells and light-emitting devices. Thin films of BaCuChF have been deposited on MgO by pulsed laser deposition (PLD). Solid solutions BaCuS$_{1-x}$Se$_{x}$TeF and BaCuSe$_{1-x}$Te$_{x}$ have been prepared by PLD of alternating thin BaCuChF layers. All films were deposited at elevated substrate temperatures. They are preferentially c-axis oriented, conductive and transparent in the visible part of the spectrum. Double excitonic peaks have been observed in the absorption spectrum of these films in the temperature range from 80 to 300K. The separation between the peaks in the doublet increases with the increase of atomic mass of the chalcogen. It also increases with the increase of the heavy chalcogen component $x$ in the solid solutions. This separation most likely is caused by the effect of spin-orbit coupling in the chalcogen atoms on excitonic levels in BaCuChF. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P37.00009: Photocurrent-induced transport of exciton energy in a single heterojunction quantum well Patrick Folkes, Yingmei Liu Excitons which coexist with a degenerate two-dimensional electron gas (2DEG) in the same quantum well subband have been observed in the photoluminescence (PL) from the recombination of electrons with localized photoexcited holes. Under pulsed photoexcitation at a critical applied voltage, the screening response of the 2DEG/exciton system to the appearance of a remote photocurrent filament in the 2DEG results in the existence of spatially direct and red-shifted indirect excitons in the photoexcitation region and the anomalously fast formation of in-plane spatially indirect excitons which are localized around the filament. Our data suggests the occurrence of a fast long range transport of exciton energy. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P37.00010: Two-dimensional hole systems in InSb and In$_{x}$Ga$_{1-x}$As quantum wells Chomani Gaspe, Madhavie Edirisooriya, Tetsuya Mishima, Michael Santos CMOS circuits require $p-$type transistors with high hole mobility, in addition to \textit{n{\-}}type transistors with high electron mobility. In zinc-blende semiconductors, a narrower band gap leads to smaller effective masses for electrons and holes. We have achieved room-temperature electron mobilities of 10,000 and 40,000 cm$^{2}$/Vs in quantum wells made of In$_{0.53}$Ga$_{0.47}$As and InSb, respectively. To achieve high hole mobilities, strain and confinement must be maximized. Both parameters increase the energy splitting between holes with light in-plane mass and those with heavy in-plane mass. We have observed a room-temperature hole mobility of 600 cm$^{2}$/Vs in InSb quantum wells with remotely Be-doped Al$_{x}$In$_{1-x}$Sb barriers grown on GaAs substrates by molecular beam epitaxy. We will discuss the effects of strain, structural parameters, and defect density on hole mobility in InSb and In$_{x}$Ga$_{1-x}$As quantum wells. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P37.00011: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P37.00012: Non-Markovian damping of Rabi oscillations in semiconductor quantum dots Luiz E. Oliveira, Dmitri Mogilevtsev, A.P. Nisovtsev, S. Kilin, S.B. Cavalcanti, H.S. Brandi A systematic investigation is performed on the damping of Rabi oscillations induced by an external electromagnetic field interacting with a two-level semiconductor system. We have considered a coherently driven two-level system coupled to a dephasing reservoir, and shown that in order to explain the dependence of the dephasing rate on the driving intensity, it is essential to consider the non-Markovian character of the reservoir. Moreover, we have demonstrated that intensity- dependent damping may be induced by various dephasing mechanisms due to stationary as well non-stationary effects caused by the coupling with the environment. Finally, present results [1] are able to explain a variety of experimental measurements [2-4] available in the literature. \newline References: 1. D. Mogilevtsev, A. P. Nisovtsev, S. Kilin, S. B. Cavalcanti, H. S. Brandi and L. E. Oliveira, Phys. Rev. Lett. (in press); 2. A. Zrenner et al., Nature 418, 612 (2002); 3. Q. Q. Wang et al., Phys. Rev. B 72, 035306 (2005); 4. B. Patton, U. Woggon, and W. Langbein, Phys. Rev. Lett. 95, 266401 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P37.00013: Nonlinear Terahertz pump-Terahertz probe Measurements of Semiconductor Carrier Dynamics Haidan Wen, Aaron Lindenberg A table-top terahertz (THz) source has been employed to study the nonlinear response of semiconductors to near-half-cycle femtosecond pulses in the THz regime. We report nonlinear field-induced changes in the far infrared absorption coefficient induced by THz pulses. The transmittance as the function of THz peak field was measured using a z-scan technique and it is observed that the absorption coefficient dramatically increases above a threshold field. Temperature-dependent pump-probe measurements indicate that free carriers are generated by the intense fields, as a result of field-induced tunneling and impact ionization processes. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P37.00014: Effect of Spin-Orbit Interaction on the Lattice Properties of Solids: Sb and Bi M. Cardona, L. E. Diaz-Sanchez, X. Gonze, R. K. Kremer, A. H. Romero, J. Serrano We present measurements of the specific heat ($C)$ vs. $T $for Bi and Sb and vs. isotopic mass for Sb. The measurements are compared with \textit{ab-initio }calculations performed with the ABINIT code, including spin-orbit (S-O) interaction. It is shown that the S-O interaction softens the lattice and thus increases the value of $C $at the low $T $maximum ($T_{m}$ \textit{$\sim $}8K for Bi, $T_{m}$ \textit{$\sim $}14K for Sb), improving agreement between theory and experiment. The effect of S-O interaction on other thermodynamic properties, such as the lattice parameter, $a_{0}$, and the cohesive energy, $E_{c}$, is also calculated. It is shown that this interaction decreases $E_{c}$ and, correspondingly, increases $a_{0}$. These effects are proportional to $c_{2}$\textit{$\lambda $} $^{2}+c_{3}$\textit{$\lambda $}$^{3}$, where \textit{$\lambda $ }is the S-O coupling constant, about twice as large for Bi as for Sb. [Preview Abstract] |
Session P38: Relaxor Ferroelectrics and Piezoelectrics
Sponsoring Units: DCMPChair: Nicola Spaldin, University of California, Santa Barbara
Room: Morial Convention Center 230
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P38.00001: Structure and Energetics of Bi(Zn,Ti)O$_{3}$ Tingting Qi, Ilya Grinberg, Andrew Rappe Lead-free piezoelectric materials are drawing more and more attention during recent years due to the environmental issues. Bi based materials are considered as quite promising alternative to Pb due to the 6s$^{2}$ ``inert pair'' electron configuration. Experiments have proved that BZT is an good analogue of PT with a high c/a ratio and a large cations' displacements with respect to oxygen cage centers. However, there has been no theoretical examination of this material. We apply DFT calculations for different B-cation arrangements of BZT' using a 40-atom supercell. The large supercell allows local structure information to be extracted from our computational results. We find extremely large ($\sim $1 {\AA}) Bi displacements as well as $\sim $0.5 {\AA} B-site displacements. Due to the low tolerance factor, large octahedral tilts are also present. The combination of large displacements and large octahedral tilts is in contrast to the Pb-based materials, where these structural motifs are mutually exclusive. The large ionic displacements give rise to a high polarization in the BZT material. This should lead to an extremely high Curie transition temperature (T$_{c})$, as the T$_{c}$ is proportional to the square of the polarization. Examination of the relaxed structures also shows that the ionic displacements from their ideal positions are inline with the Bond Valence Theory. Surprisingly, comparing the different B-cation arrangements, we find that rocksalt structure is not the most preferred contrary to the prediction of electrostatic model of atomic ordering. Although the rocksalt arrangement has a lower and favored Ewald energy, it's deviation from the ideal bond valence is high. This means that the energy of covalent bonding in the rocksalt structure is smaller than in the other structures. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P38.00002: Search for New High Performance Piezoelectrics P. Ganesh, Ronald E. Cohen In an effort to design new high performance piezoelectric materials, we have performed first-principles calculations to study the energetics of several ABO$_{3}$ type materials, with the `A' site being occupied by two types of atoms. The motivation comes from recent findings of a morphotropic-phase boundary, which gives rise to large electromechanical coupling, at high pressure and low temperatures in pure PbTiO$_{3}$[1]. This prompted us to substitute the `A' site with a smaller atom compared to Pb to apply ``chemical pressure'' and tune the morphotropic-phase boundary to lower pressures. We have discovered (Pb$_{1/2}$ Sn$_{1/2})$TiO$_{3}$, (Pb$_{1/2}$ Ge$_{1/2})$TiO$_{3}$ and (Sn$_{1/2}$ Ge$_{1/2})$TiO$_{3}$ to be promising new piezoelectric materials. (Pb$_{1/2}$ Sn$_{1/2})$TiO$_{3}$ shows lower energy monoclinic phases (space groups Cm with polarization along [xxz] and Pm with polarization along [x0z]) compared to the layered tetragonal phase (space group P4mm with polarization along [001]) while the remaining two compounds have a rhombohedral (space group R3m with polarization along [111]) ground-state compared to the tetragonal phase in the rock-salt pattern (space group I4mm with polarization along [001]).~Our results also suggest ease of polarization rotation and large electromechanical strain. [1] Zhigang Wu and Ronald E. Cohen, PRL, \textbf{95}, 037601, 2005 [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P38.00003: High-pressure x-ray diffraction study of Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-32{\%}PbTiO$_{3}$. Muhtar Ahart, Ronald E. Cohen, Russell J. Hemley Motivated to determine and understand PMN-32{\%}PT's behavior under pressure, we employed the angular dispersive x-ray diffraction methods (Advanced Photon Source, Argonne National Laboratory) to investigate PMN-32{\%}PT in a diamond anvil cell up to 15 GPa. The x-ray diffraction results show changes in Bragg peaks at 4 GPa which indicate that PMN-32{\%}PT undergoes a ferroelectric rhombohedral to a paraelectric cubic phase transition. In addition, we investigated the pressure dependence of domain structure of PMN-32{\%}PT up to 10 GPa; rhombohedral domains decrease with pressure and disappear above 4 GPa. These results are qualitatively consistent with earlier Raman study of B. Chaabane, {\{}Phys. Rev. B 70, 134114, 2004{\}}. We suggest a phase diagram for PMN-PT system which is slightly different from the one predicted by B. Chaabane et al. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P38.00004: Aging mechanisms in field cooled PMN-PT 12{\%} Matthew Delgado, Eugene Colla, Michael Weissman In the relaxor ferroelectric PMN-PT 12{\%} ((PbMg$_{1/3}$Nb$_{2/3}$O$_{3})_{0.88}$ (PbTiO$_{3})_{0.12})$ the dielectric susceptibility ($\varepsilon \prime $, $\varepsilon \prime \prime )$ and pyroelectric current I$_{P}$ were measured after different field-temperature histories. As previously reported [1], when the sample was cooled in zero field, standard spinglass-like aging of $\varepsilon \prime \prime $ was found in the glassy relaxor regime. Cooling in a DC electric field of 0.67 kV/cm drove a transition into a polarized ferroelectric-like state, which retained its polarization after the field was removed at low temperature. During 10 hours of subsequent zero-field aging at 160K, $\varepsilon \prime \prime $ showed almost no change. This suggests that the mechanisms responsible for glassy aging in the relaxor state are absent in the ferroelectric state. [1] L. Chao, et. al. \textbf{PRB} 74, 014105 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P38.00005: Hyper Piezoelectric Nano-Electromechanical Systems Dustin Kreft, Robert Blick, Seung-Hyub Baek, Chang-Beom Eom, V. Vaithyanathan, Darrell Schlom, Vladimir Aksyuk Piezoelectric materials are extremely important for applications in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS). Here, we will be presenting PMN-PT (Lead Magnesium Niobate-Lead Titanate) on SOI as a hyper piezoelectric material for NEMS devices. The main advantages are the extremely high piezoelectric coefficient, higher electromechanical coupling, and larger attainable bandwidth. This will lead to enhanced agility of NEMS/MEMS devices and lower energy consumption. The work will also have considerable impact for nanoscale mechanics. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P38.00006: Zone Boundary Soft Modes in Relaxor PMN Peter Gehring, Ian Swainson, Chris Stock, Guangyong Xu, Haosu Luo The lattice dynamics of PbMg$_{1/3}$Nb$_{2/3}$O$_3$, a prototypical relaxor, have been studied using neutron TOF techniques at the NIST Center for Neutron Research. Unusual ``columns'' of inelastic scattering are seen at the M and R-point zone boundaries at 300\,K that extend from 5\,meV to the elastic line. The columns weaken substantially upon heating to 600\,K, suggesting the existence of soft, zone boundary modes. This would imply a dynamical origin to the superlattice peaks previously observed via TEM and x-ray diffraction techniques. Preliminary neutron structure factor calculations indicate that the corresponding ionic displacements involve the Mg/Nb and Pb atoms. This picture is consistent with x-ray studies according to which the superlattice peaks result from $<$110$>$ correlated, anti- parallel Pb displacements [1]. The potential relationship between the columns and superlattice peaks is intriguing because the temperature dependence of the M-point superlattice peak tracks that of the soft, zone center mode [2], which is associated with the development of short-range ferroelectric correlations. [1] A. Tkachuk and H. Chen, AIP Conf. Proc., 677, p. 55 (2003); [2] P. M. Gehring, S. Wakimoto, Z.-G. Ye, and G. Shirane, Phys. Rev. Lett. 87, 277601 (2001). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P38.00007: Polar nanoregions and phase instability in relaxors Guangyong Xu, Jinsheng Wen, Christopher Stock, Peter Gehring Local polar clusters called polar nano-regions (PNR) are believed to play important roles in the electronic properties of relaxor ferroelectrics. Recent work has shown that the presence of PNRs in relaxor systems could also be related to the high piezoelectric response in these materials. Diffuse scattering studies on PNRs in PMN-xPT and PZN-xPT systems will be presented and their electric field response will be discussed. Our results suggest that there is an intrinsic phase instability in relaxor systems induced by the PNRs, which makes it easier for an (electric field) induced strain to develop. Possible scenarios for the further enhancement of the piezoelectric response near the morphotropic phase boundary (MPB) will also be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P38.00008: First-Principles Investigations of Pb Anti-Site Defects in PbZrO$_3$ and Pb(Zr, Ti)O$_3$ Perovskites Ricardo Kagimura, David J. Singh Lead zirconate (PZ) and lead zirconate titanate (PZT) have the perovskite type structure, ABO$_3$. Bivalent lead (Pb$^{+2}$) ions occupy the A site, while tetravalent titanium and zirconium (Zr$^{+4}$, Ti$^{+4}$) ions occupy the B site at random of the PZT solid solution. Also, lead can be tetravalent (Pb$^{+4}$), such as in PbO$_2$ structure. Recent experimental work has reported that tetravalent Pb ions can locate at the B site of the PZT perovskite forming a lead zirconate-titanate-plumbate solid solution. The experimental results suggest that, based on a PbZrO$_3$-PbTiO$_3$-PbPbO$_3$ ternary solution phase diagram [G. Suchaneck \emph{et al.}, Ferroelectrics {\bf 318}, 3 (2005)], the substitutional Pb atom prefers to occupy the Zr site instead of the Ti one. In this work, we report density functional supercell calculations for pure PbZrO$_3$ perovskite and for ordered Pb(Zr$_{1/2}$Ti$_{1/2}$)O$_3$ solid solution with different configurations for the Zr and Ti atoms. We investigate the anti-site defect energies and the effects on the electronic structure. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P38.00009: Stability of point defects in SrTiO$_{3}$ substrates during thin film deposition Emmanuel Towett, Shehnaz Jeddy, Gregg Janowski, Charles Brooks, Darryl Schlom, Mary Ellen Zvanut During deposition of multiferroic films on SrTiO$_{3}$, the substrate is subjected to ambients similar to those known to alter the oxygen vacancy concentration and conductivity. We have performed electron paramagnetic resonance (EPR) measurements on SrTiO$_{3}$ substrates at selected stages during film deposition as well as during controlled heat treatments. Fe$^{3+}$, Cr$^{3+}$ and an Fe-oxygen vacancy complex, Fe$^{3+}$V$_{o}$, were monitored during isochronal and isothermal vacuum (10$^{-6}$ Torr) and 1 atm O$_{2}$ heat treatments between 200 and 800 $^{\circ}$C. As expected, processing steps involving O$_{2}$ at 950 $^{\circ}$C reduces the concentration of Fe$^{3+}$V$_{o}$, consistent with the O$_{2}$ annealing study. Film deposition at 650 $^{\circ}$C, 5x10$^{-7}$ Torr with 10{\%} ozone returns the Fe$^{3+}$V$_{o}$ signal to the original intensity, consistent with vacuum treatments which follow 1 atm O$_{2}$ annealing. Surprisingly, isochronal and time-dependent vacuum and O$_{2}$ annealing produce the same trend for the Fe$^{3+}$V$_{o}$ signal for T $>$ 500 $^{\circ}$C. The results of all the studies suggest that electrostatic changes, as evidenced by variations in the intensity of Fe$^{3+}$ and Cr$^{3+}$, as well as oxygen vacancy migration can alter substrate characteristics during film growth. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P38.00010: Electron Beam Excitation Mechanism of Rare Earth Ions in LiNbO$_{3}$ S. Tafon Penn, V. Dierolf, G.S. Cargill III Many applications of ferroelectric materials in frequency conversion are based on precise engineering of the ferroelectric domains. Unfortunately, the quest for smaller feature sizes is slowed due to the lack of real-time local controls of the process. We explored the possibility to use as local probes RE ions in LiNbO$_{3}$ that exhibit in their excitation and emission spectra linear Stark shifts making them sensitive to local electric fields such as local charging and fields from defect dipoles. We studied the feasibility of this approach for the domain writing process induced by an e-beam in a SEM. A necessary first step is a good understanding of the excitation mechanisms of RE:LiNbO$_{3}$ that leads to CL under e-beam irradiation. We observe a pronounced saturation already for moderate e-beam currents indicating that not all RE ions can be excited. This excludes the direct excitation pathway as a major contributor towards the RE excitation. Our observations are explained by a defect trap mediated energy transfer between generated electron-hole pairs and the RE ions. Independent of the RE ion, we find that the energy transfer rate from the defect trap to the RE ion is less than (100$\mu $s)$^{-1}$. Under saturation, this small rate and the limited number of defect trap-RE ion pairs restricts the achievable CL intensity and the applicability of the proposed real-time probing scheme. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P38.00011: Raman study of the phonon modes in bismuth pyrochlores Daniel Arenas, Wei Qiu, Juan Nino, David Tanner, Lev Gasparov The Raman Spectra of the cubic bismuth pyrochlores Bi$_{3/2}$Zn$_{0.92}$Nb$_{1.5}$O$_{6.92}$, Bi$_{3/2}$ZnTa$_{3/2}$O$_{7}$, Bi$_{3/2}$MgNb$_{3/2}$O$_{7}$, and Bi$_{3/2}$MgTa$_{3/2}$O$_{7}$ were measured. The samples, in ceramic form, were measured from 50 to 1000 cm$^{-1}$ at room temperature. The Raman bands were tentatively assigned to specific vibrational modes. Overall, the Raman spectra were similar for all four samples and the number of modes was affected by the displacement disorder in the bismuth-based compounds. The results will also be compared to published infrared data to gain insight into these additional modes. Last, the existence of an 860 cm$^{-1}$ mode in BZN and BMN will be discussed as it suggests influence of the lone pair character on the displacement disorder. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P38.00012: A Low-Temperature Specific Heat Study of the Giant Dielectric Constant Materials C. P. Sun, J. Y. Lin, H. D. Yang, Jianjun Liu, Chun-gang Duan, W. N. Mei Low-temperature specific-heat study has been performed on the insulating giant dielectric constant material CaCu$_{3}$Ti$_{4}$O$_{12}$ and two related compounds, Bi$_{2/3}$Cu$_{3}$Ti$_{4}$O$_{12}$ and La$_{0.5}$Na$_{0.5}$Cu$_{3}$Ti$_{4}$O$_{12}$, from 0.6 to 10 K. From analyzing the specific heat data at very low-temperature range, 0.6 to 1.5 K, and moderately low-temperature range, 1.5 to 5 K, in addition to the expected Debye terms, we noticed significant contributions originated from the linear and Einstein terms, which we attributed as the manifestation of low-lying elementary excitations due to lattice vibrations occurred at the grain boundaries and induced by local defects. Together with the findings on electronic and mechanical properties, a phenomenological model is proposed to explain the high dielectric constant behaviors at both low and high frequency regions. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P38.00013: First Principles Investigation of Electronical and Lattice Dynamical Properties of High Dielectric Constant Material Na$_{1/2}$Bi$_{1/2}$Cu$_3$Ti$_4$O$_{12}$ Cihan Parlak, Resul Eryigit We will report the results of a first principles investigation of the electronic and lattice dynamical properties of so-called giant dielectric compound Na$_{1/2}$Bi$_{1/2}$Cu$_3$Ti$_4$O$_{12}$ which was found to have a very high 'extrinsic' as well as 'intrinsic' dielectric constant. The calculations have been carried out within the local spin density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The ground state is found to be antiferromagnetic direct-band gap semiconductor. Lattice dynamical properties, such as Born effective charge tensors, dielectric permittivity tensors, and phonon frequencies at the Brillouin zone center were calculated using density functional perturbation theory and found to be similar to more studied CaCu$_3$Ti$_4$O$_{12}$ and CdCu$_3$Ti$_4$O$_{12}$ compounds. The calculated electronic ($\epsilon_{\infty}\approx 11.5$) and static ($\epsilon_{0}\approx 150$) dielectric constants indicate that the observed high dielectric constant is extrinsic in origin. The main contribution to the static dielectric constant is found to be due to a low frequency (50 cm$^{-1}$) IR-active mode which has a large mode effective charge. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P38.00014: Transition temperature of martensitic transformations in hafnia and zirconia Xuhui Luo, A.A. Demkov Transition metal oxides find applications in ceramics, catalysis and semiconductor technology. In particular, hafnium dioxide or hafnia will succeed silica as a gate dielectric in advanced transistors. However, thermodynamic properties of thin hafnia films are not well understood, despite their technological importance. We use density functional theory to investigate the tetragonal to monoclinic phase transition in hafnia and zirconia. We find that unlike the case of the cubic to tetragonal transition, this phase transition is not driven by a soft mode. We use transition state theory to identify the minimum energy path (MEP) employing first principle calculations for hafnia and zirconia, sow that both transformations are martensitic, and obtain the transition barriers. Martensitic transformations include both the internal coordinate transformation and deformation of the cell lattice vectors (``strain and shuffle''), therefore the potential energy surface and MEP are function not only of the internal atomic coordinates but also of the unit cell lattice vectors. Considering the simplest case of uniform strain the transition temperatures we then relate the barrier height to the transition temperature. As a self-consistency check, assuming the equality of thermodynamics potentials of the tetragonal and monoclinic phases during the transition, and using the difference in the internal energy calculated from first principles we estimate the entropy change associated with the transition which is found in good agreement with that calculated form the phonon spectra. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P38.00015: Correlation effects in charge-density wave insulator BaBiO$_3$ Cesare Franchini, Martijn Marsman, Georg Kresse The negative-U nature of $\rm BaBiO_3$ leads to a charge-ordered insulating state in which pentavalent $\rm Bi^{5+}$ coexists with trivalent $\rm Bi^{3+}$. Despite the apparent absence of strong-correlation effects in $\rm BaBiO_3$ standard density functional (DFT) theory yields a much too small band gap of 0.14 eV. By means of an hybrid-DFT approach combined with self-consistent GW including vertex corrections we investigate the electronic, vibrational and dielectric properties of $\rm BaBiO_3$. We show that the inclusion of strong-correlation effects increases the band gap up to 1.2 eV, shifts the oxygen breathing modes upwards by $\approx$ 2 THz and reduces the dielectric constant by a factor of 3. The overall agreement with available experimental data is significantly improved. [Preview Abstract] |
Session P39: Applications of Complex Networks
Sponsoring Units: GSNPChair: Narayan Menon, University of Massachusetts, Amherst
Room: Morial Convention Center 231
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P39.00001: Effects of quenched randomness on predator-prey interactions in a stochastic Lotka-Volterra lattice model Uwe C. Tauber, Ulrich Dobramysl We study the influence of spatially varying reaction rates (i.e., quenched randomness) on a stochastic two-species Lotka-Volterra lattice model for predator-prey interactions using Monte Carlo simulations. The effects on the asymptotic population densities, transient oscillations, spatial distributions, and on traveling wave and invasion front speed velocities are investigated. We find that spatial variability in the predation rate yields an increase in the asymptotic population densities of {\em both} predators and prey. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P39.00002: Dynamical Clustering in Reaction-Dispersal Processes on Complex Networks Vincent David, Marc Timme, Theo Geisel, Dirk Brockmann We investigate nonlinear annihilation processes (e.g., $A+A\rightarrow \emptyset $) of particles that perform random walks on complex networks. In well mixed populations (mean field) this process exhibits $t^{-1}$ decay behavior in the total number of particles. Additional dispersal of particles adds a second time scale and drastically changes the decay behavior. Here we study these changes for two types of hopping processes. First, if particles independently select one of the possible exit channels at each node their exit rates are given by the sum of all outgoing weights such that the waiting times are degree-dependent. We compare this to the popular ansatz of a uniform waiting time process. Derived mean field equations show that for large numbers of particles per node both processes exhibit nearly identical relaxation properties. However, below a critical particle number the processes deviate not only from mean field predictions but, more importantly, by orders of magnitude from one another. We attribute this to dynamical clustering effects in the uniform waiting time model, that is absent in the independent channel dynamics. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P39.00003: Fluctuations and Food-web Structures in Individual-based Models of Biological Coevolution Per Arne Rikvold, Volkan Sevim We report very long kinetic Monte Carlo simulations of eco- systems generated by individual-based models of biological co-evolution, emphasizing the temporal fluctuations in community structure, diversity, and population sizes [1-3]. These multispecies coevolution models contain both producers that directly utilize an external resource, and consumers that must consume one or more other species for support. Time series of diversities and population sizes over tens of millions of generations display highly correlated fluctuations that give rise to power spectra of $1/f$ form. These model-intrinsic dynamic features correspond to large, correlated extinction events and similarly correlated bursts of new species, without the need for external catastrophic events. The communities generated by the evolution process take the form of simple food webs, whose species abundance distributions and degree distributions are consistent with data from real food webs.\newline [1] P. A. Rikvold, {\it J. Math. Biol\/} {\bf 55}, 653-677 (2007).\newline [2] P. A. Rikvold and V. Sevim, {\it Phys.\ Rev.\ E\/} {\bf 75}, 051920 (2007) (17 pages).\newline [3] P. A. Rikvold, arXiv:q-bio.PE/0609013. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P39.00004: Metabolic disease network and its implication for disease comorbidity Deok-Sun Lee, Zoltan Oltvai, Nicholas Christakis, Albert-Laszlo Barabasi Given that most diseases are the result of the breakdown of some cellular processes, a key aim of modern medicine is to establish the relationship between disease phenotypes and the various disruptions in the underlying cellular networks. Here we show that our current understanding of the structure of the human metabolic network can provide insight into potential relationships among often distinct disease phenotypes. Using the known enzyme-disease associations, we construct a human metabolic disease network in which nodes are diseases and two diseases are linked if the enzymes associated with them catalyze adjacent metabolic reactions. We find that the more connected a disease is, the higher is its prevalence and the chance that it is associated with a high mortality. The results indicate that the cellular network-level relationships between metabolic pathways and the associated disease provide insights into disease comorbidity, with potential important consequences on disease diagnosis and prevention. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P39.00005: The Human Phenotypic Disease Network Cesar Hidalgo, Nicholas Blumm, Albert-Laszlo Barabasi, Nicholas Christakis We study the evolution of patient illness using a network summarizing the disease associations extracted from 32 million Medicare claims recorded from 13 million elders using the ICD9-CM format. We find that the evolution of patients' illness is accurately described by a process in which once a patient develops a particular disease, subsequent disease are seen to occur among diseases lying close by in the network. In addition, we find that patients affected with diseases with high network connectivity are more likely to die during a follow-up period of eight years. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P39.00006: Bilateral interactions in disease dynamics - Decreasing epidemic thresholds with facilitated contact rates Alejandro Morales Gallardo, Dirk Brockmann, Theo Geisel Compartmental epidemiological models are very successful modeling paradigms in epidemiology. Typically, they are employed for quantitative assessments of key parameters such as the basic reproduction number $R_0$. These models rest on two key assumptions: 1.) a population is well mixed 2.) transmission is triggered by a population averaged contact rate. However, experimental evidence shows that contact rates vary substantially, and it has been hypothesized that this variability can change the dynamics of population relevant disease dynamics. However, for inhomogeneous populations the translation of distributed contact rates into effective disease transmission events is non-trivial. Transmission may either depend only on the contact rate of the transmitting individual alone (unilateral transmission), or on the contact rates of transmitting and receiving individual (bilateral transmission). In the SIS model we show that in either systems the endemic state of a disease can be stable for values of $R_0<1$ unlike homogeneous systems with a critical value $R_0=1$. Furthermore, bilateral contact dynamics entail parameter regimes in which a stable endemic state can cease to exist if the mean contact rate is increased, an unexpected effect absent in homogeneous populations. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P39.00007: Single species victory in a two-site, two-species model of population dispersion Jack Waddell, Len Sander, David Kessler We study the behavior of two species, differentiated only by their dispersal rates in an environment providing heterogeneous growth rates. Previous deterministic studies have shown that the slower-dispersal species always drives the faster species to extinction, while stochastic studies show that the opposite case can occur given small enough population and spatial heterogeneity. Other models of similar systems demonstrate the existence of an optimum dispersal rate, suggesting that distinguishing the species as faster or slower is insufficient. We here study the interface of these models for a small spatial system and determine the conditions of stability for a single species outcome. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P39.00008: Color Triads in Complex Networks: Uncovering Racial Segregation Patterns in US High Schools Julian Candia, Marta Gonzalez Introducing color subgraph analysis as a novel tool for characterizing complex network structures, we identify the basic racial patterns in a nationally representative sample of all public and private High Schools in the US. We apply this method on color triad subgraphs and obtain quantitative measurements on racial homophily effects, as well as on interracial mixing patterns. Strongest homophily phenomena are observed within the white student population, followed in decreasing order by black, hispanic and asian students. Racial reciprocity measurements reveal that white students tend to form triads in which they constitute a racial majority. Black-hispanic triads are also observed to be non-reciprocal, while black-asian and hispanic-asian triads show a stronger tendency towards symmetric ties. Racial preference measurements show a rather weak white-black affinity. Since both white and black triad majorities prefer a hispanic third party, hispanic students may play the role of a bridge between white and black students. In order to design better integration strategies, quantitative observations on homophily and interracial mixing patterns could be used to redefine school organizational features. Moreover, the color subgraph analysis method can be applied to a large variety of complex network systems on other interdisciplinary fields of science. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P39.00009: A Bayesian approach to network modularity Jake Hofman, Chris Wiggins We present an efficient, principled, and interpretable technique for inferring module assignments and identifying the optimal number of modules in a given network. Our approach is based on a probabilistic network model equivalent to an infinite-range spin-glass Potts model on the irregular lattice defined by a given network; the problem of identifying modules is then tantamount to inferring distributions over both the module assignments (i.e. spin states) and the model parameters (i.e. coupling constants) while also identifying the number of modules (i.e. number of occupied spin states) in the network. Using a variational approximation we derive a mean-field free energy, the minimization of which provides controlled approximations to the distributions of interest. We show how several existing methods for finding modules can be described as variant, special, or limiting cases of our work, and how related methods for complexity control -- identification of the true number of modules -- are outperformed. We apply the technique to synthetic and real networks and outline how the method naturally allows selection among competing network models. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P39.00010: Distribution of Node Characteristics in Complex Networks Juyong Park, A.-L. Barabasi Our enhanced ability to map the structure of various complex networks is accompanied by the capability to independently identify the functional characteristics of each node, leading to the observation that nodes with similar characteristics show tendencies to link to each other. Examples can be easily found in biological, technological, and social networks. Here we propose a tool to quantify the interplay between node properties and the structure of the underlying network. We show that when nodes in a network belong to two distinct classes, two independent parameters are needed. We find that the network structure limits the values of these parameters, requiring a phase diagram to uniquely characterize the configurations available to the system. The phase diagram shows independence from the network size, a finding that allows us to estimate its shape for large networks from their samples. We study biological and socioeconomic systems, finding that the proposed parameters have a strong discriminating power.~\footnote{J. Park and A.-L. Barab\'asi, \textit{Proc. Nat. Acad. Sci.} \textbf{104}, pp. 17916--17920 (2007)} [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P39.00011: The Modular Structure of Protein Networks Hern\'an D. Rozenfeld, Diego Rybski, Shlomo Havlin, Hern\'an A. Makse The evolution of the human protein homology network (H-PHN) has led to a complex network that exhibits a surprisingly high level of modularity. Topologically, the H-PHN presents well connected groups (conformed by proteins of similar aminoacid structure) and weak connectivities between the groups. Here, we perform an empirical study of the H-PHN to characterize the degree of modularity in terms of scale-invariant laws using recently introduced box covering algorithms. We find that the exponent that determines the scale-invariance of the modularity is unexpectedly higher than the box dimension of the network. In addition, we perform a percolation analysis that gives insight into the evolutionary process that led to the modular organization and dynamics of the present H-PHN. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P39.00012: Synchronization behavior in linear arrays of negative differential resistance circuit elements Huidong Xu, Stephen Teitsworth We study the electronic transport properties in a linear array of nonlinear circuit elements that exhibit negative differential resistance, and find that state-cluster synchronization emerges when there is heterogeneity in the element properties. This type of synchronization is associated with a non-uniform spatial distribution of total applied voltage across the array elements, as well as the formation of multiple stable branches in computed current-voltage curves for the entire array. Unlike most synchronizing systems studied previously [1], this system possesses coupling between elements that displays both positive and negative feedback depending on the state of each element. An empirical order parameter is defined which quantifies the degree of synchronization. We also find that the degree of synchronization is strongly dependent on the \textit{ramping rate} of the total applied voltage to the array, with complete synchronization observed in the limit of small ramping rate. This model provides a basis for describing related nonlinear phenomena in more complex electronic structures such as semiconductor superlattices [2]. [1] A. Pikovsky, M. Rosenbaum, and J\"{u}rgen Kurths, \textit{Synchronization: a universal concept in nonlinear sciences} (Cambridge University Press, Cambridge, 2001). [2] M. Rogozia, S. W. Teitsworth, H. T Grahn, and K. H. Ploog, Phys. Rev. B\textbf{65}, 205303 (2002). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P39.00013: Hierarchical, 4-connected Small-World Graph Bruno Goncalves, Stefan Boettcher A new sequences of graphs are introduced that mimic small-world properties. The graphs are recursively constructed but retain a fixed, regular degree. They consist of a one-dimensional lattice backbone overlayed by a hierarchical sequence of long-distance links in a pattern reminiscent of the tower-of-hanoi sequence. These 4-regular graphs are non-planar, have a diameter growing as 2$^{\sqrt{\log_{2}N^{2}}}$ (or as $\left[\log_{2}N\right]^{\alpha}$ with $\alpha\sim\sqrt{\log_{2}N^{2}}/\log_{2}\log_{2}N^{2}$), and a nontrivial phase transition T$_{c}>0$, for the Ising ferromagnet. These results suggest that these graphs are similar to small-world graphs with mean-field-like properties. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P39.00014: Exact Renormalization of Super-Diffusion on the Tower-of-Hanoi Network Stefan Boettcher, Bruno Goncalves We propose the Tower-of-Hanoi network as a hierarchical, small-world network possessing both, geometric and long-range links. Modeling diffusion via a random walk on this network provides a mean-square displacement with an exact, anomalous exponent $d_{w}=2-\ln(\phi)/\ln(2)=1.30576\ldots$. Here, $\phi=\left(1+\sqrt{5}\right)/2$ is the ``golden ratio'' that is intimately related to Fibonacci sequences. This may be the first solvable model with super-diffusion for any fractal structure. This appears to be also the first known instance of any physical exponent containing $\phi$. It originates from an unusual renormalization group fixed point with a subtle boundary layer. The connection between network geometry and the emergence of $\phi$ in this context is still elusive. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P39.00015: Scaling behavior of the non-affine deformation of random fiber networks Hamed Hatami-Marbini, Catalin Picu Random fiber networks exhibit non-affine deformation on multiple scales. This controls to a large extent their ``homogenized'' behavior on the macroscopic (system level) scale. It is currently believed that denser networks and networks in which the fibers have vanishing bending stiffness deform affinely. Here we show that these conclusions depend on the nature of the measure used to probe the non-affinity. If a strain based measure is used, it can be shown that all networks, irrespective of the axial or bending behavior of their fibers are non-affine. Furthermore, the non-affinity decreases with the observation scale, exhibiting a universal power law scaling. The behavior of dense and sparse networks is shown to be similar if a scale renormalization is applied. [Preview Abstract] |
Session P40: Surface Effects, Thermodynamics and Fabrication
Sponsoring Units: DCMPChair: David Singh, Oak Ridge National Laboratory
Room: Morial Convention Center 232
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P40.00001: Surface core-level shifts and atomic coordination at the W(320) surface Xubing Zhou, J.L. Erskine, O. Kizilkaya High resolution 4 $f_{7/2}$ Core-level photoemission spectra are reported from the W(110) and from the related vicinal W(320) surfaces. Curve fittings of the spectra permit tests of core-level binding-energy shift models that relate local atomic coordination to binding -energy differences associated with (for example) terrace and step-edge atoms. A well-resolved shoulder on the W(320) surface peak is attributed to step-edge atoms and contributions from surface atoms having higher atomic coordination are obtained from the curve-fitting exercises. The results are discussed in relation to prior core-level measurements, tight-binding models, and ab-initio calculations of core-level shifts for W(320). [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P40.00002: Secondary electron spectra of Au and Cu under bombardment by very low energy positrons S. Mukherjee, A.H. Weiss, M.P. Nadesalingam, P. Guagliardo, A. Sergeant, J. Williams Measurements of the secondary electron energy spectra resulting from very low energy positron bombardment of a polycrystalline Au and Cu (100) surfaces are presented. The low energy part of the secondary spectra contain significant contributions from two processes: 1. annihilation induced Auger electrons that have lost energy before leaving the surface and 2. secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions ($\sim $3eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P40.00003: Fermi-liquid effects in propagation of low frequency electromagnetic waves through thin metal films Natalya Zimbovskaya, Grigory Zimboskiy In the present work we theoretically analyze the contribution from a transverse Fermi-liquid collective mode to the transmission of electromagnetic waves through a thin film of a clean metal in the presence of a strong external magnetic field. We show that at the appropriate Fermi surface geometry the transverse Fermi-liquid wave may appear in conduction electrons liquid at frequencies $ \omega $ significantly smaller than the cyclotron frequency of charge carriers $ \Omega $ provided that the mean collision frequency $ \tau^{-1}$ is smaller than $\omega. $ Also, we show that in realistic metals size oscillations in the transmission coefficient associated with the Fermi-liquid mode may be observable in experiments. Under certain conditions these oscillations may predominate over the remaining size effects in the transmission coefficient. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P40.00004: Ground-state structure of the hydrogen double vacancy on Pd(111) Sungho Kim, Seong-Gon Kim, Steven Erwin We determine the ground-state structure of a double vacancy in a hydrogen monolayer on the Pd(111) surface. We represent the double vacancy as a triple vacancy containing one additional hydrogen atom. The potential-energy surface for a hydrogen atom moving in the triple vacancy is obtained by density-functional theory, and the wave function of the fully quantum hydrogen atom is obtained by solving the Schr\"odinger equation. We find that an H atom in a divacancy defect experiences significant quantum effects, and that the ground-state wave function is centered at the hcp site rather than the fcc site normally occupied by H atoms on Pd(111). Our results agree well with scanning tunneling microscopy images. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P40.00005: Band Mapping in Higher-Energy X-Ray Photoemission: Phonon Effects and Comparison to One-Step Theory Jan Minar, Lukasz Plucinski, Brian Sell, Juergen Braun, Hubert Ebert, Claus Schneider, Charles Fadley In view of the present interest in more bulk sensitive band mapping via x-ray photoemission, we have studied the temperature dependence of W(110) angle-resolved spectra excited at photon energies of 260, 870 eV, and 1254 eV and between 300K and 780K. Experimental results are compared to both a free-electron final-state model and theoretical one-step model calculations. At 300K, clear band dispersions can be observed in the data. The ratio between direct and non-direct transitions is approximately estimated from a Debye-Waller factor. One-step theoretical calculations reproduce well band dispersions and matrix element effects in the measured spectra at room temperature, but including phonon effects via complex phase shifts does not predict density-of-states related features observed in higher-temperature spectra. We will also discuss the implications of this work for future experiments on other materials and at even higher photon energies up to 10 keV. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P40.00006: Electronic Gr\"{u}neisen Parameter In Paramagnetic Nickel Shouhua Nie, Xuan Wang, Junjie Li, Richard Clinite, Mark Wartenbe, Jianming Cao We have conducted the first measurement of electronic Gr\"{u}neisen parameter \textit{$\gamma $}$_{e}$ in the paramagnetic state of ferromagnetic transition metal nickel by monitoring the laser-induced ultrafast stress dynamics using femtosecond electron diffraction. This method overcomes the restriction of traditional low-temperature methods and offers a unique path to study electronic thermal expansion in magnetic metals. Our measurement indicates that the local magnetic moment that persists in the paramagnetic state of nickel does not contribute significantly to electronic thermal expansion. This result would serve as an important test of current models regarding the magnetism in ferromagnetic transition metals. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P40.00007: Generalized Surface Thermodynamics of Solids with Application to Nucleation Robert Cammarata J.W. Gibbs formulated a general thermodynamics for surfaces in multicomponent fluid systems. For the case of solid-fluid surfaces, he restricted attention to single component solids. Attempts to generalize Gibbs' results for surfaces between multicomponent solids and fluids are problematic owing to the difficulty that the surface chemical potentials in the solid are generally not well defined, and therefore any expressions involving these chemical potentials will also not be well defined. A formulation of a general surface thermodynamics that can take into account capillary effects in systems involving interfaces between multicomponent solids and fluids while avoiding the aforementioned difficulties will be discussed that utilizes the concept of thermodynamic availability. It will be shown how this approach allows Gibbs-Thomson effects for finite size solids, an adsorption equation for a solid surface, and the thermodynamics of nucleation during solidification to be treated in a straightforward manner that avoids all references to ill defined surface quantities. In particular, a derivation will be presented that is the first to properly generalize Gibbs' analysis for the reversible work of nucleation to the case of solidification. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P40.00008: Controlling precipitate growth in aluminum rich alloys via externally applied stress Jack Franklin, Jennifer Lukes The material properties of metallic alloys are directly influenced by their microstructure. The final microstructure of bulk specimens is currently determined through specific heat treatments designed to control the homogenous precipitation of secondary phases from a saturated matrix. This talk will introduce a novel stress-based method of controlling the precipitation and directed growth of secondary phases to create desired microstructures on the surface of an aluminum-copper alloy. Microstructures obtained under different stress conditions will be presented and the mechanisms leading to their formation will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P40.00009: Bulk nanostructured alloys by large strain extrusion machining Wilfredo Moscoso, Christopher Saldana, Jon Madariaga, Ravi Shankar, Srinivasan Chandrasekar, Dale Compton Large strain extrusion machining (LSEM) is presented as a method of severe plastic deformation for the creation of bulk nanostructured materials in a wide range of metal alloys. This method combines inherent advantages afforded by large strain deformation in chip formation by machining, with simultaneous dimensional control of extrusion in a single step of deformation. Bulk nanostructured materials in the form of foils, plates, and bars of controlled dimensions are shown to result by appropriately controlling the geometric parameters of the deformation in large strain extrusion machining. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P40.00010: Engineering materials-design parameters of the Mg-Li Alloy System from ab initio calculations William Counts, Martin Friak, Dierk Raabe, Jorg Neugebauer Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. Fundamental physical properties (like formation energy and elastic constants) of 11 bcc magnesium-lithium alloys were calculated using density-functional theory (DFT) and compared with available experimental data. These DFT determined properties were in turn used to calculate engineering parameters like the bulk modulus/shear modulus (B/G) and Young's modulus/density (E/$\rho )$. From these engineering parameters, alloys with optimal mechanical properties need for a light weight structural material were identified. It was found that the stiffest bcc magnesium-lithium alloys contain about 70 at.{\%} magnesium while the most ductile alloys have 0-20 at.{\%} magnesium. In addition, the specific modulus for alloys with 70 at.{\%} magnesium was found to be equal to that of aluminum-magnesium alloys and slightly lower than that of aluminum-lithium alloys. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P40.00011: Testing the MacPherson-Srolovitz Theory in Simulations of 3D Grain Growth Anthony Rollett, Fatma Uyar, Seth Wilson, Jason Gruber, Sukbin Lee The theory by MacPherson and Srolovitz provides an exact prediction of the growth rate of individual cells or grains in a space-filling network (microstructure). Testing the predictions of the theory requires measurement of mean width and edge lengths where three cells meet at triple line junctions. This is most easily accomplished in networks that are discretized with a mesh. A Moving Finite Element (MFE) model was used to simulate the evolution (grain growth) over short times of a network discretized on a tetrahedral mesh and growth rates. Volumes, mean widths and edge lengths were measured. The growth rates measured from the simulation were found to be in very good agreement with the predictions of the MacPherson-Srolovitz theory. The results from similar measurements in Monte Carlo and Phase Field models of grain growth will also be reported. In this case, measurement of mean width and edge length is complicated by use of a regular grid to discretize the network on a set of points or voxels. A modified algorithm by Ohser and M\"ucklich is used to measure mean width. Edge length measurement along triple lines requires conversion of the voxel image to a surface mesh. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P40.00012: Phase-field Simulation for Analyzing Time Evolution of Grains of Precipitate in Phase Decomposition Processes of Magnetic Alloys Yoshiharu Kanegae The time evolution of the average grain size and the number of grains of precipitate in phase decomposition processes of magnetic alloys in two-dimensional systems was investigated using phase-field simulation. Specifically, the phase decomposition processes of Fe-Cu and Fe-Cr systems were studied. Chemical free energy from a thermodynamic database of phase diagrams was used and magnetic contribution was considered. In appropriate compositions, these systems show spinodal decomposition followed by Ostwald ripening. In the long-time region of these processes, the time evolution of the average grain size and the number of grains of precipitate was evaluated by the power of time. The exponent of the power of time of the average grain size \textit{$\alpha $} was \textit{$\alpha $}$\sim $1/3, consistent with the Lifshitz-Slyozov-Wagner theory. On the other hand, that of the number of grains \textit{$\beta $} was \textit{$\beta $}$\sim $-2/3, inconsistent with the theory, which predicts \textit{$\beta $}$\sim $-1. However, it was shown that in two-dimensional systems these results are reasonable. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P40.00013: Modeling Digestive and Ostwald Ripening of Nanocrystals Michael Tambasco, Sanat Kumar Ostwald and digestive ripening are two diametrically opposite phenomena that dramatically impact nanocrystal polydispersity. Ostwald ripening allows for large nanocrystals to grow at the expense of small ones, while digestive ripening involves the propagation of small nanocrystals at the cost of the larger. A detailed theoretical description of these two processes would aid in developing techniques that control nanocrystal size and polydispersity; however, there currently exists no a-priori means of describing the physics of these two processes. Here, we present an approach that is capable of characterizing both types of phenomena. We apply a mean field theory in order to model the role that ligands play in the ripening processes. We examine the effects of ligand concentration and chain length on the average size and size distribution of gold nanocrystals. We then employ potentials derived from the theoretical results in Monte Carlo simulations of the ripening processes in order to study the physics of the ripening phenomena. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P40.00014: Mean-field Theory of Multicomponent Phase Coarsening Ke-Gang Wang Study of phase coarsening in multicomponent systems is rare. Morral and Purdy developed a general theoretical frame for phase coarsening in n-component alloys. However, all work considered only the effect of solution thermodynamics, and ignored the kinetic effect from non-zero volume fraction. Therefore, all studies are valid only in the case of vanishing volume fraction. When $V_V$ is not zero, the interactions among precipitates exist. The diffusion screening length is used to describe these interactions, and it is found that the growth rate of particle depends on the volume fraction through diffusion screening length. A mean-field theory of multicomponent phase coarsening will be presented, which includes effects of both multicomponent thermodynamics and kinetics from nonzero volume fraction. [Preview Abstract] |
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