Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y1: Charge Order in Cuprates
Sponsoring Units: DCMPChair: Peter Hirschfeld, University of Florida
Room: LACC 152
Friday, March 25, 2005 11:15AM - 11:51AM |
Y1.00001: Local Ordering in the Pseudogap Regime of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Invited Speaker: The underdoped cuprate superconductors are anything but normal materials at temperatures above the superconducting transition. Various exotic electronic states have been proposed to explain their abnormal behavior in this regime, and in particular the pseudogap that appears in the density of states. In this talk, I will report on our recent STM measurements which show that the electronic states inside the pseudogap form dispersionless standing wave patterns in real-space.\footnote{M. Vershinin, S. Misra, S. Ono. Y. Abe, Y. Ando and A. Yazdani, {\it Science} {\bf 303}, 1995 (2004).} The dispersionless nature of the patterns indicates they are the signature of some form of local electronic ordering. How this local ordering relates to the physics of the pseudogap, however, is a topic of intense ongoing investigation. We will present these results and more recent work which addresses the nature of the pseudogap state through examining the response of the standing wave patterns to defects. \newline \newline This work was supported by NSF grants DMR-98-75565 and DMR-03- 1529632, U.S. Department of Energy grant DEFG-02-91ER4539 through the Frederick Seitz Materials Research Laboratory, and Office of Naval Research grant N000140110071. \newline \newline [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y1.00002: Electronic Crystal State in a Hole Doped Cuprate Invited Speaker: In cuprate superconductors, there are four ground states depending on the doping level namely undoped Mott insulator, pseudogap phase, superconductor, and normal metal. Among these phases, the pseudogap phase in the lightly doped regime is rich in peculiar phenomena which may be closely related to the strong correlation effects in cuprates. We have carried out scanning tunneling microscopy/spectroscopy (STM/STS) on Ca$_{2-x} $Na$_x$CuO$_2$Cl$_2$ single crystals ($x=0.08\sim0.12$) to visualize the spatial variation of the electronic states in the pseudogap phase. This compound has a simple crystal structure with only one crystallographic Cu site and provides high quality cleaved surfaces of lightly doped regime. We found 'checkerboard' local density of states (LDOS) modulation which is characterized by the tiling of the commensurate $4a \times 4a$ electronic entity. Each $4a \times 4a$ unit cell carries intricate internal structures which include $4a/3 \times4a/3$ LDOS modulation. The modulation amplitudes of these components are strongly energy dependent but the periodicities are energy independent, both inside and outside of the pseudogap. This suggests that the pseudogap phase is associated with the electronic order or electronic crystal which manifests itself as the observed `checkerboard'. \\ Reference: T. Hanaguri {\it et al.}, Nature {\bf 430}, 1001 (2004). [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y1.00003: Doping Evolution and Charge Ordering in Ca2-xNaxCuO2Cl2 from ARPES Invited Speaker: The evolution of Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ from Mott insulator to superconductor was studied using ARPES, where we tracked both the doping dependence of the electronic structure and the chemical potential. Our work reveals failures in the conventional quasiparticle theory, including the broad lineshapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wavevector with the behavior of the chemical potential. Comparisons of the lineshape and temperature dependence of the lower Hubbard band states to other states in the valence band and core levels indicate that lattice relaxation effects have a strong influence on the broadened lineshapes of the parent insulator, suggesting that the photoholes form polaron states. In addition, we have observed signatures of the unusual 4a$_{0} \times 4a_{0}$ checkerboard charge-ordered state recently observed by STM. Our results show a striking dichotomy between the real and momentum space probes, with well-nested but extremely weak antinodal sections of Fermi surface coexisting with sharp nodal QP-like excitations. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y1.00004: Cooper pair density-wave and dual Hofstadter butterfly in underdoped cuprates Invited Speaker: An XY-type model of quantum phase fluctuations in a correlated 2D lattice $d$-wave superconductor is introduced and studied$^{2}$. The model is based on the QED$_3$ effective theory of high temperature superconductors and is geared toward describing not only the long distance but also the {\em intermediate} lengthscale physics of underdoped cuprates. In particular, I elucidate the dynamical origin and investigate specific features of the Cooper pair density-wave, which I propose as the state behind the periodic modulation in the local tunneling density of states (DOS) discovered in recent STM experiments. I illustrate how Mott-Hubbard correlations near half-filling suppress superfluid density and favor an incompressible state which breaks translational symmetry of the underlying atomic lattice. The formation of the Cooper pair density-wave in such a quantum-fluctuating superconductor can be recast as an Abrikosov-Hofstadter problem in a type-II {\em dual} superconductor, with the role of dual magnetic flux per plaquette, $f$, played by the electron density: $f=(1-x)/2$, where $x$ is the doping. The Abrikosov-Hofstadter problem generates the sequence of doping ``magic fractions'' at which $f$ exhibits high degree of commensurability with the CuO$_2$ lattice. The resulting Abrikosov-Hofstadter lattice of dual vortices translates into the periodic modulation of the gap function and the electron density. The energetics of various dual vortex arrays and their signatures in the single-particle local tunneling DOS are studied in detail. A 4$\times$4 checkerboard modulation pattern naturally arises as an energetically favored ground state at and near $x=1/8$ and produces local DOS in good agreement with experiments.\par \noindent $^2$Z. Te\v sanovi\' c, Phys. Rev. Lett. {\bf 93}, 217004 (2004); A. Melikyan and Z. Te\v sanovi\' c, cond-mat/0408344. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 2:15PM |
Y1.00005: A dichotomy between nodal and antinodal excitations and competing order in high temperature superconductors Invited Speaker: ARPES and STM reveal that the electronic excitations near the nodes and antinodes have very different characteristics. The nodal excitations are quasiparticle-like and the nature of antinodal excitations depends on temperature, doping, and other material details. This nodal-antinodal dichotomy is manifested most dramatically in the recent recent STM experiment that reveal a checkerboard order in the Na doped Ca$_2$CuO$_2$Cl$_2$. The constraint of this dichotomy on theories will be discussed. [Preview Abstract] |
Session Y2: Kondo Physics and Spin Control in Nanostructure Optics and Transport
Sponsoring Units: DCMPChair: Mikhail Raikh, University of Utah
Room: LACC 151
Friday, March 25, 2005 11:15AM - 11:51AM |
Y2.00001: Kondo effect in a many-electron quantum ring Invited Speaker: The Kondo effect is investigated in a many-electron quantum ring as a function of magnetic field. For fields applied perpendicular to the plane of the ring a modulation of the Kondo effect with the Aharonov-Bohm period is observed. This effect is discussed in terms of the energy spectrum of the ring and the parametrically changing tunnel coupling. In addition, we use gate voltages to modify the ground state spin of the ring. The observed splitting of the Kondo related zero bias anomaly in this configuration is tuned with an in-plane magnetic field. It has been shown that the dot-lead coupling can be determined quantitatively for quantum dots connected to three terminals. The Kondo effect is studied in a three-terminal quantum ring. By measuring the currents through the differently biased terminals it can be determined which lead has Kondo correlations with the dot and which does not. The possibility to probe the density of the dot in the Kondo regime using a three-terminal configuration is discussed. This work was done in collaboration with A. Fuhrer, R. Leturcq, and T. Ihn. A. Fuhrer, et al., Phys. Rev. Lett. 93, 176803 (2004), cond-mat/0406247 [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y2.00002: Transport Spectroscopy of Coupled Quantum Dots in Conditions of the Kondo Effect Invited Speaker: We develop electron transport theory for novel devices [1,2], which are interesting in the context of correlated electrons physics. The device proposed in Ref. [1] is designed for an observation of a non-Fermi-liquid behavior of itinerant electrons. The device measured in Ref. [2] may serve a similar purpose, and also may become important for quantum computing.\\ In the case of Ref. [1], our theory [3] provides a strategy for tuning to the non-Fermi-liquid fixed point -- a quantum critical point in the space of device parameters. We explore the corresponding quantum phase transition, and make explicit predictions for the behavior of differential conductance in the vicinity of the quantum critical point. \\ Motivated by the measurements [2], we developed a theory of conductance of Kondo quantum dots coupled by the RKKY interaction [4]. Investigation of the differential conductance at fixed interaction strength may allow one to distinguish between the possible ground states of the system. Transition between the ground states is achieved by tuning the interaction strength; the nature of the transition (which includes a possibility of a non-Fermi-liquid point) can be extracted from the temperature dependence of the linear conductance.\\ This research is supported by NSF grants DMR02-37296 and EIA02- 10736.\\ 1. Y. Oreg and D. Goldhaber-Gordon, Phys. Rev. Lett. {\bf 90}, p. 136602 (2003). \\ 2. N.J. Craig J.M. Taylor, E.A. Lester, C.M. Marcus, M.P. Hanson, and A.C. Gossard, Science {\bf 304}, 565 (2004).\\ 3. M.G. Vavilov and L.I. Glazman, preprint cond-mat/0404366.\\ 4. M. Pustilnik, L. Borda, L.I. Glazman, and J. von Delft, Phys. Rev. {\bf B 69}, 115316 (2004). [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y2.00003: Electrical control of the spin-flip rate of an exciton in a semiconductor Invited Speaker: At the heart of the Kondo effect is a tunneling process in which a localised electron is exchanged with an electron in a Fermi sea. This process can flip the spin of the localised electron. We present here a novel application of this concept to an exciton, an electron-hole complex, in a quantum dot. By determining the temporal emission characteristics of a single self-assembled quantum dot, we show that the exciton spin can be reversed through an electron exchange with a Fermi sea in a neighboring n-doped layer. A very significant point is that the exciton spin flip completely changes the radiative properties of the exciton, either from dark to bright or from bright to dark. We can control the rate of the spin flip to be either much larger or much smaller than the radiative recombination rate of the bright exction simply with the voltage applied to the gate of our device. Calculations based on the Anderson Hamiltonian give excellent agreement with the experimental results. Our work has important consequences in two areas. First, the effect corresponds to the high temperature Kondo regime, motivating the possibility of observing a Kondo exciton in a semiconductor nanostructure for the first time. Secondly, the effect offers a way of manipulating the dark exciton and therefore a means of exploiting its long lifetime and long spin coherence time in quantum information processing. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y2.00004: Kondo Effect and Controlled Spin-Entanglement in Coupled Quantum Dots Invited Speaker: Semiconductor double-quantum dots represent an ideal system for studying the novel spin physics of localized spins. On each quantum dot when the number of electrons is odd and the net spin is 1/2, a strong coupling of this localized spin to conducting electrons in the leads gives rise to Kondo correlation. On the other hand, in the coupled double-quantum-dot if the inter-dot antiferromagnetic interaction is strong, the two spins can form a correlated spin-singlet state, quenching the Kondo effect. This competition between Kondo and antiferromagnetic correlation is studied in a controlled manner by tuning the inter-dot tunnel coupling. Increasing the inter-dot tunneling, we observe a continuous transition from a single-peaked to a double-peaked Kondo resonance in the differential conductance. On the double-peaked side, the differential conductance becomes suppressed at zero source-drain bias. The observed strong suppression of the differential conductance at zero bias provides direct evidence signaling the formation of an entangled spin-singlet state. This evidence for entanglement and the tunability of our devices bode well for quantum computation applications. [Preview Abstract] |
Session Y3: Quantum Spin Liquids
Sponsoring Units: DCMPChair: Douglas Scalapino, University of California-Santa Barbara
Room: LACC 515B
Friday, March 25, 2005 11:15AM - 11:51AM |
Y3.00001: Numerical studies of Spin Liquid Phases Invited Speaker: After discussing the early 90' classification of the different phases of Quantum Antiferromagnets, I will explain why exact diagonalizations have been a useful tool in the quest of Quantum Spin Liquids. Contrary to superficial thinking, signatures of symmetry breakings appear very clearly in exact spectra of small samples of quantum spins. Spin Liquids are states which do not break any simple symmetry: neither spin-rotational symmetry nor lattice symmetry. They are characterized by topological degeneracies (which may be helpful for the realization of quantum bits, with very low rates of decoherence), and generically gapful excitations. The first supposed-to be Quantum Spin Liquid has been discovered in 1998 by Misguich and coll. in the frustrated 4-spin ring exchange model on the triangular lattice. Following this first break-through, simpler models exhibiting well defined Quantum Spin Liquids have been produced. After a rapid review of these cases, I will show some results that may be characteristic of new Quantum Critical Phase Transitions between non colinear N\'eel magnets [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y3.00002: Two Spin Liquid phases in the anisotropic triangular Heisenberg model Invited Speaker: Recently there have been rather clean experimental realizations of the quantum spin 1/2 Heisenberg Hamiltonian on a 2D triangular lattice geometry in systems like $Cs_2 Cu Cl_4$ and organic compounds like $k-(ET)_2 Cu_2 (CN)_3$. These materials are nearly two dimensional and are characterized by an anisotropic antiferromagnetic superexchange. The strength of the spatial anisotropy can increase quantum fluctuations and can destabilize the magnetically ordered state leading to non conventional spin liquid phases. In order to understand these interesting phenomena we have studied, by Quantum Monte Carlo methods, the triangular lattice Heisenberg model as a function of the strength of this anisotropy, represented by the ratio $r$ between the intra-chain nearest neighbor coupling $J'$ and the inter-chain one $J$. We have found evidence of two spin liquid regions, well represented by projected BCS wave functions[1,2] of the type proposed by P. W. Anderson at the early stages of High temperature superconductivity [3]. The first spin liquid phase is stable for small values of the coupling $r \alt 0.6$ and appears gapless and fractionalized, whereas the second one is a more conventional spin liquid, very similar to the one realized in the quantum dimer model in the triangular lattice[4]. It is characterized by a spin gap and a finite correlation length, and appears energetically favored in the region $0.6 \alt r \alt 0.9$. The various phases are in good agreement with the experimental findings and supports the existence of spin liquid phases in 2D quantum spin-half systems. %%%%%%%%%%%%%%%%%% \vspace{1cm} \begin{description} \item{[1]} L. Capriotti F. Becca A. Parola and S. Sorella , Phys. Rev. Letters {\bf 87}, 097201 (2001). \item{[2]} S. Yunoki and S. Sorella Phys. Rev. Letters {\bf 92}, 15003 (2004). \item{[3]} P. W. Anderson, Science {\bf 235}, 1186 (1987). \item{[4]} P. Fendley, R. Moessner, and S. L. Sondhi Phys. Rev. B {\bf 66}, 214513 (2002). \end{description} [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y3.00003: Quantum Spin Liquids in XY models with Ring Exchange Invited Speaker: Many promising candidate Hamiltonians have been proposed recently as harboring emergent quantum spin liquid states. Regardless, convincing examples of the state are still lacking in large-scale quantum Monte Carlo simulations of microscopic spin models, due in part to the negative sign problem which inhibits studies of antiferromagnetic spins on frustrating lattices. However, recently several unfrustrated spin models have been studied, with results that suggest that emergent spin liquid states can exist there. One of these is the square lattice $S=1/2$ XY model with ring exchange, tractable by quantum Monte Carlo without the sign problem [1]. The basic Hamiltonian is purported to harbor an isolated spin liquid point with emergent $U(1)$ gauge symmetry and spinons [2]. Using suggestions from analytical theory, we attempt to stabilize an extended region of spin liquid around this critical point by adding terms to the Hamiltonian, and increasing the dimensionality of the lattice. However, such modifications produce no spin liquid state. We therefore explore a version of the Hamiltonian on the kagome lattice, which with a particular diagonal interaction is exactly soluble analytically, and is argued to be in a stable spin liquid state with $Z_2$ gauge symmetry [3]. The Monte Carlo is able to simulate directly all parameter regions of this Hamiltonian to test this claim, and in addition is able to explore the adjacent superfluid and insulating phases and respective phase transitions. [1] Sandvik, Daul, Singh and Scalapino, Phys Rev. Lett. 89 247201 (2002). [2] Senthil et al., Science 303 1490 (2004). [3] Balents, Fisher and Girvin, Phys. Rev. B 65, 224412 (2002); Sheng and Balents, cond-mat/0408639. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y3.00004: Spin liquid aspects of the two dimensional Heisenberg antiferromagnet Invited Speaker: Since it was established that the 2D quantum (S=1/2) Heisenberg antiferromagnet on a square lattice develop long range order at T=0, albeit with only 60\% of the classical moment, it was believed that the excitaitons of this model should be classical spin waves only weakly renormalised by quantum fluctuations. Through neutron scattering investigations of CFTD, an excellend physicsl realisation of the model system, we have recently discovered i) a moderate deviation from the spin wave prediction for the zone boundary energies and ii) a huge, 50\%, deviation in intensity at Q=(pi,0). We interpret this as signature of valence bond type correlations in the quantum fluctuating part of the ground state. Although the valence bond state lacks long range order, which must be introduced through a variational approach, our results suggest that it may actually be a better starting point for approaches to understand spin fluctuations upon hole-doping as in the high temperature superconducting cuprates. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 2:15PM |
Y3.00005: The quasiparticle spectrum termination in a quantum spin liquid Invited Speaker: Igor Zaliznyak The crossover from a single quasi-particle to a spin-continuum response was recently observed in the spin dynamics of the Haldane-chain antiferromagnet CsNiCl3 [1,2]. It can be understood as a manifestation, in the particular case of the quantum spin liquid, of the peculiar property of the quantum Bose liquids, the quasiparticle spectrum termination point. The spectrum termination was first predicted for the superfluid helium-4 [3], where it was extensively studied both theoretically and experimentally. The quantum-spin-liquid (QSL) state of the two-dimensional (2D) S=1/2 Heisenberg antiferromagnet (HAFM) is of particular interest, as it may be relevant to the type of high-temperature superconductivity found in the cuprates. An organo-metallic material piperazinium hexachlorodicuprate (PHCC) is among the best known examples of the 2D QSL [4]. The spin excitations in this material have spectral gap of about 1 meV above which they follow a nearly 2D- isotropic dispersion with a bandwidth slightly larger than the gap. Recent experiments indicate that a quasiparticle spectrum termination point also exists in the excitation spectrum of the 2D quantum spin liquid existing in PHCC [5]. It signals the failure of the Bose-quasiparticle description in an extended region of the system's phase space. REFERENCES [1] I. A. Zaliznyak, S.-H. Lee, in Y. Zhu (Ed.), Modern Techniques for Characterizing Magnetic Materials, Kluwer Academic, New York (2005). [2] I. A. Zaliznyak, S.-H. Lee and S. V. Petrov, Phys. Rev. Lett. 87, 017202 (2001); Phys. Rev. Lett. 91, 039902 (2003). [3] Landau \& Lifshitz, Course of Theoretical Physics (Statistical Physics, Part 2, by Lifshitz, E. M. \& Pitaevskii, L. P.) Vol. 9, 125-139 (Pergamon Press, Oxford, 1981). [4] Stone, M. B., Zaliznyak, I., Reich, D. H., and Broholm, C., Phys. Rev. B 64, 144405 (2001). [5] M. Stone, I. A. Zaliznyak, et. al., in preparation (2004). [Preview Abstract] |
Session Y4: Polymer Dynamics and Rheology
Sponsoring Units: DPOLYChair: Kalman Migler, NIST
Room: LACC 515 A
Friday, March 25, 2005 11:15AM - 11:51AM |
Y4.00001: Nonlinear Flow Behavior of Model Branched Polymers Invited Speaker: Stress relaxation dynamics of entangled long-chain branched (LCB) polymers are investigated using model symmetric stars, asymmetric stars, and multiarm (pom-pom), polymer liquids subject to small-amplitude oscillatory shear and nonlinear step and steady shear deformations. This talk focuses on the effect of molecular architecture on chain stretching, tube dilation, convective constraint release, and interfacial slip processes. It is shown that even small degrees of arm length asymmetry leads to large differences in the nonlinear relaxation dynamics of branched molecules. Specifically, while symmetric stars manifest greater strain softening characteristics and much lower degrees of chain extension than entangled linear chains, asymmetric stars and pom-poms with even small levels of arm length asymmetry display nonlinear rheological properties that are in most respects identical to those of entangled linear polymers with comparable terminal times. Consequences of these observations on tube-based constitutive models for branched polymers are discussed. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y4.00002: How Nanoparticles Impact Phase Evolution in Polymer Blend Films Invited Speaker: Polymer films containing nanoparticles (NP) are of technological interest because their unique mechanical, electrical and optical properties. Here, we present a systematic study of phase separation dynamics in polymer blend films containing mobile NP. Films (650nm) of PMMA:SAN (50:50) containing methyl-terminated silica NP provide a model system to investigate wetting and phase separation dynamics. Concurrent with PMMA wetting, NP segregate to the air and substrate interfaces. The NP also preferentially partition into the PMMA rich domains. During the intermediate stage, the correlation length between domains, $\xi $, scales as $t^{1/3}$ for neat blends as well as those with 2 and 5 wt.{\%} NP. As NP concentration increases, $\xi $ increases more slowly, consistent with a coalescence model that reflects the increase in PMMA viscosity due to the NP. The effect of PMMA-grafted silica NP on phase separation and wetting depends on the length of the grafted chain. In some cases, the early stage morphology is preserved for long-times and film rupture is prevented. Selected studies of Ag-nanoparticle formation in homopolymer films will also be presented. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y4.00003: Time-dependent structure of polymer brushes Invited Speaker: The time dependence of polymer brush structure may be viewed at a number of levels: One level is the construction time for a brush from solution of polymers that self-assemble on a surface by attachment at one end only to form the brush; another concerns the relaxation of brushes towards equilibrium after they have been sheared, a property of importance for lubricating brushes. The talk will consider progress at both of these levels, especially the effect on the first level of varying the brush-end sticking energy, and the effect on the second level of shear of mutually-compressed sliding brushes. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y4.00004: Phonons in Soft Microstructures Invited Speaker: George Fytas Among the inelastic scattering methods only Brillouin light scattering spectroscopy possesses sufficient resolution to record the rich phonon spectrum S(q,\"{y}) of soft microstructures. Ordered systems with spacing qd=0(1) (q being the scattering photon wavevector) and elastic constants varying in space control the propagation of several high frequency excitations. Besides the acoustic phonon of the effective medium, ``Bragg'' modes due to the interaction of q with the phonon wavevector k and the reciprocal lattice vector G, mixed phonons of the phononic band structure and vibrational eigenfrequencies can contribute to the S(q,\"{y}) of ordered polymer and colloidal systems. We present rich phonon dispersion relations in polystyrene opals, ordered diblock copolymers and interference lithography patterned single crystalline polymer films. Theoretical calculations of dispersion relations were performed to explain the nature of the observed propagation modes. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 2:15PM |
Y4.00005: Turbulent drag reduction by additives Invited Speaker: Turbulent drag reduction by additives Daniel Bonn ENS/LPS Paris, France and WZI Amsterdam, The netherlands A spectacular reduction of energy losses in turbulent flows can be achieved by the addition of small amounts of certain polymers or surfactants. I will discuss a few of our recent experiments that shed some light on the different possible mechanisms of drag reduction. Polymer drag reduction is believed to be due to the large elongational viscosity of the polymer solution; this stabilizes the turbulent boundary layer, leading to less turbulent energy generation, and hence less dissipation. For flexible polymers, indeed a correlation between drag reduction and elongational viscosity can be found. We show however that solutions of rigid polymers that exhibit no measurable elastic effects, drag reduction is as efficient as for flexible polymer solutions with an elevated elongational viscosity. This drag reduction can be attributed to the existence of a spatial viscosity gradient in the boundary layer. For surfactants, we show that drag reduction is due to an effective wall slip. Due to the wall slip, the bulk of the turbulent system does not experience a large wall friction. Since it is the wall friction that generates the turbulence, this naturally leads to a smaller dissipation. [Preview Abstract] |
Session Y5: Parallel and High-Throughput Experimentation for the Physical Sciences: Methods, Applications and Technology Development
Sponsoring Units: FIAPChair: Eric Amis, NIST
Room: LACC 502B
Friday, March 25, 2005 11:15AM - 11:51AM |
Y5.00001: Combinatorial method for materials screening and device optimization of thin layer opto-electronic devices Invited Speaker: A rapid materials screening and a combinatorial development of thin film multilayer electro-optical devices is essential for a fast research and development progress and the implementation of device structures into commercial products. A well-established and reliable film preparation technique within the required nanometer film thickness range is the vapor deposition. The presented combinatorial methods are based on the preparation of linear or steps gradient and the preparation of sectors of material combinations or device structures by using mask movements in combination with a substrate positioning. Both of the two principles are combined to obtain an infinite number of possible libraries with different complexity, which may differ in each sector by the layer thickness, the sequence of layers, and the material selection. In addition by simultaneous evaporation using two or more sources and varying the deposition rate material compositions can also be created. In this presentation we represent an overview of our work on the combinatorial material screening and combinatorial optimization of multilayer thin film organic electro-optical devices prepared by vapor deposition. This article covers results on organic light emitting devices (OLEDs), organic solar cells, the orientation behavior of formanisotropic functional molecules on alignment layers, and the in-situ preparation and orientation of rod like and thermally stable aromatic polyimides. \newline \newline In collaboration with Markus B\"ate, Christian Neuber, and Mukundan Thelakkat, Universit\"at Bayreuth, Makromolekulare Chemie I [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y5.00002: Sensor Platforms for High-Throughput Analysis of Materials Libraries Invited Speaker: The multidimensional nature of the interactions between the composition, process parameters, and end-use conditions of functional materials result in significant materials development challenges. To accelerate discovery and optimization of materials, combinatorial and high-throughput methods combine parallel materials synthesis with automated materials screening. Rapid materials characterization on microscale remains the key challenge. In this presentation, we will discuss our applications of microfabricated sensors for high-throughput characterization of combinatorial functional materials. Microfabricated sensors are an attractive addition to the infrastructure of analytical instruments for combinatorial materials science. Using several examples from our labs, we will demonstrate attractiveness of microfabricated sensors and importance of a proper selection of transduction principles. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y5.00003: Gradient and High-Throughput Methods for the Accelerated Development of Nanomaterials and Nanometrology Invited Speaker: Increasingly, new materials are highly tailored towards specific applications, are formulated from many components, and exhibit behavior governed by a multitude of physical, chemical and processing factors. Accordingly, the discovery and optimization of materials are met by considerable challenges inherent to the understanding of large, complex parameter spaces. In this respect, combinatorial and high-throughput (C{\&}HT) approaches are advantageous, since they present the ability to rapidly assess materials properties over large parameter ranges. The NIST Combinatorial Methods Center (NCMC, see www.nist.gov/combi) specializes in the development of quantitative C{\&}HT measurement methods for materials research. In large part, the NCMC concentrates on continuous gradient (CG) combinatorial methods, which involve the fabrication and HT measurement of systems that gradually vary parameters over a single specimen, and which offer an alternative to the (often costly) robotics-driven C{\&}HT paradigm used by the pharmaceutical industry. CG techniques are particularly suited for materials science since they naturally produce thorough maps (e.g. continuous phase diagrams) that relate materials properties to chemical, compositional, physical and processing parameters. This presentation focuses on NCMC research applied to the advancement of polymer-based nanotechnology. Topics to be discussed include CG techniques for the design and optimization of self-assembled systems, ultra-thin films, and intelligent surfaces; and HT methods for measuring thin film morphology and mechanical properties. In addition, the application of CG methods to the advancement of nanometrology, specifically scanned probe microscopy, will be discussed. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y5.00004: Combinatorial thin film methodology for rapid investigation of composition-structure-property relationship in complex multifunctional materials Invited Speaker: We have developed thin film composition spread techniques to map compositional phase diagrams of multicomponent functional materials. Various combinatorial thin film deposition techniques including pulsed laser deposition, co-sputtering, and electron-beam deposition are used to fabricate binary and ternary composition spreads of metal oxide systems and metallic alloy systems. A variety of rapid characterization tools are employed to track physical properties of the materials as a function of sweeping composition changes. They include scanning SQUID microscopy, scanning microwave microscopy, and x-ray microdiffraction. High-throughput mapping of phase diagrams allow identification of new compounds as well as rapid delineation of composition-structure-property relationships. One of our emphases in these studies is to systematically look for structural phase transitions/boundaries in phase diagrams since these regions often display onset or peaking of ferroic properties such as ferroelectricity and magnetism. Mapping these active physical properties as a function of composition is an integral part of understanding the underlying physical mechanisms of the properties. To date, we have applied our methodology to investigation of a number of multifunctional materials including ferromagnetic shape memory alloys and magnetoelectric materials. We have demonstrated the utility of composition spreads as compact integrated devices where continuously changing physical parameters are used as a basis for functionally broadband detector arrays. The role of informatics in the high-throughput experimentation will also be discussed. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 2:15PM |
Y5.00005: Invited Speaker: |
Session Y13: Photoemission Studies of Superconductors
Sponsoring Units: DCMPChair: Daniel Dessau, University of Colorado
Room: LACC 402B
Friday, March 25, 2005 11:15AM - 11:27AM |
Y13.00001: Fine Details of the Electronic Excitations in the Near-Nodal Region of the Fermi Surface in Bi 2212 T. Valla, T.E. Kidd, J. Rameau, P.D. Johnson, G.D. Gu Recent high resolution ARPES experiments on high quality Bi$_2 $Sr$_2$CaCu$_2$O$_{8+\delta}$ samples have uncovered extremely coherent excitations in the near-nodal region of the Fermi surface. The width of these excitations at low energies and at low temperature seems to be essentially resolution limited. This observation has enabled a more detailed insight into the intrinsic properties of these excitations. Some of the ``fine features" that have not been previously resolved, will be discussed. This work was supported by the DOE under contract number DE- AC02-98CH10886. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y13.00002: Doping dependence of the band structure of Bi2212 Adam Kaminski, Stephan Rosenkranz, Mike Norman, Helen Fretwell, Utpal Chaterjee, Juan Carlos Campuzano, Mohit Randeria, Helene Raffy, Z. Z. Lee, Kazuo Kadowaki We present careful study of doping dependence of the band structure of Bi2212 high temperature superconductor. Our study reveals many fine details that were not noticed before. We use this data to understand the spectrum of collective excitations present in this and other cuprate materials. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y13.00003: Quasiparticle Dynamics of Bi$_2$Sr$_2$CaCu$_2$O$_8$ from High Momentum Resolution Laser-Based ARPES Jake Koralek, Fraser Douglas, Nick Plumb, Zhe Sun, Margaret Murnane, Henry Kapteyn, Steven Cundiff, Daniel Dessau, Y. Aiura, K. Oka, H. Eisaki We present the first laser based angle resolved photoemission (ARPES) study of the High Tc superconductor Bi-2Sr-2CaCu-2O-8. This new technique uses 6 eV photons from the 4th harmonic of a high repetition rate Ti:Sapphire oscillator. The new light source offers increased count rate and momentum resolution, while reducing surface sensitivity and background relative to traditional ARPES light sources. In addition to reproducing BSCCO features seen in previous ARPES experiments (d-wave superconducting gap, dispersion kinks, etc.) these improvements have allowed us to make much stronger statements about the nature of the quasiparticle dynamics both in the normal and superconducting states. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y13.00004: Competing Degrees of Freedom in High Temperature Superconductor – New Insights from Angle Resolved Photoemission Spectroscopy G.-H. Gweon, S. Y. Zhou, J. Graf, M. C. Watson, T. Sasagawa, H. Takagi, D.-H. Lee, A. Lanzara A fundamental issue for the high temperature superconducting cuprates is how various degrees of freedom, e.g.~lattice and spin, compete or cooperate as basic ingredients of the superconductivity. In this talk, I will address this issue based on the line shape analysis of angle resolved photoemission data obtained on the optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ high temperature superconductor. The momentum, energy, temperature and oxygen isotope mass dependence of the single particle spectral function will be analyzed. The important role of the lattice that we can learn primarily through the oxygen isotope mass dependence will be emphasized. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y13.00005: Unusual spectral weight transfer in high temperature Bi-superconductors Jeff Graf, Gey-Hong Gweon, Shuyun Zhou, Eli Rotenberg, Hiroshi Eisaki, Alessandra Lanzara High resolution angle resolved photoemission spectroscopy (ARPES) studies of the electronic structure of single layer (Bi$_{2}$Sr$_{2}$CuO$_{6+\delta })$ and double layer (Bi$_{2}$Sr$_{2}$CaCuO$_{8+\delta })$ Bi-compound are reported. A detailed study of the quasiparticle spectral function as a function of momentum, temperature and doping is presented. Unusual spectral weight transfer is observed in both systems and discussed in terms of a strong interplay between the electron-lattice and electron-electron interaction in these materials. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y13.00006: Doping Dependent Electronic Structure of KxC60 Monolayers Studied by Angle Resolved Photoemission W.L. Yang, V. Brouet, X.J. Zhou, Z. Hussain, Z.-X. Shen KxC60 (1$<$x$<$6) shows complicated but interesting phase transitions between insulators and metals, in contrast to a rigid band picture where the system always has a partially filled conduction band. The study on the novel doping behavior is thus of great interest but seriously intervened by lattice structure for bulk materials. Through angle resolved photoelectron spectroscopy, we were able to study systematically the electronic structure evolution of KxC60 monolayers which does not show bulk-like lattice structure change. Unexpectedly, our data strongly suggest the existence of phase separation between K3C60 and K4C60 in monolayers, accompanied by metal-insulator transition. We also reveal that the electron-phonon coupling manifests itself in different ways for the metallic and insulating phases. Further, the long standing problem on the shifting insulating peak with doping could be expected by a polaronic scenario for insulating phases. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y13.00007: Evolution of Electronic Structure with Doping in (La$_{2-x}$Sr$_x$)CuO$_4$ X. J. Zhou, T. Yoshida, W. L. Yang, V. Brouet, Seiki Komiya, Yoichi Ando, F. Zhou, J. W. Xiong, W. X. Ti, Z. X. Zhao, T. Sasagawa, T. Kakishita, H. Eisaki, S. Uchida, A. Fujimori, Z. Hussain, Z.-X. Shen High resolution angle-resolved photoemission measurements have been carried out on (La$_{2-x}$Sr$_x$)CuO$_4$ (LSCO) over the entire doping range (x=0$\sim$0.3). Systematic evolution of the energy band, Fermi surface, and many-body effect with doping will be discussed. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y13.00008: Evolution of the electronic structure from the insulator to the superconductor in Bi2212 cuprates Kiyohisa Tanaka, Teppei Yoshida, Kyle Michael Shen, Donghui Lu, Wei-Sheng Lee, Hajime Yagi, Atsushi Fujimori, Zhi-Xun Shen, . Risdiana, Takenori Fujii, Ichiro Terasaki We have performed angle-resolved photoemission study of Bi$_2 $Sr$_{2-x}$La$_x$CaCu$_2$O$_{8+\delta}$ (Bi2212) cuprates from the lightly-doped to the underdoped region, which can be achieved by a recent progress in sample synthesis. We found that the lower Hubbard band (LHB) of the parent insulator is gradually shifted upward with hole doping without changing the global dispersions, which indicates a shift of the chemical potential with hole doping. This behavior is analogous to that in Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ (Na-CCOC) but is different from La$_{2-x}$Sr$_x$CuO$_4$ (LSCO), where the LHB stays well below the chemical potential and does not move in the underdoped region. The details of the electronic structure for different families of cuprates will also be compared from the viewpoint of doping dependence of the quasiparticle band and the $k_{F}$ position and the observed difference will be discussed. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y13.00009: APRES study of the Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10}$ and HgBa$_2$CuO$_4$ Wei-Sheng Lee, Donghui Lu, Wanli Yang, Xingjiang Zhou, Kyle M. Shen, Guichuan Yu, Martin Greven, Chengtian Lin, Jun-ichi Shimoyama, Zhi-Xun Shen The wide variation of the critical temperature of the high Tc superconductors is an important unresolved issue. For example, the optimal Tc of the mercury-based superconductor is 97 K, while that of the lanthanum-based superconductor is only 38K. In this study, we used the Angle Resolved Photoemission Spectroscopy (ARPES) to explore the electronic state of the Bi2223 (Tc = 110K) and Hg1201 (Tc = 94K) system; both of them have a very high Tc compared to other high Tc superconductors. Comparisons of their electronic structures to other high Tc superconductors with lower critical temperature will discussed. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y13.00010: ARPES study on four layered superconductor compound Ba2Ca3Cu4O8(O,F)2 (F0234) Yulin Chen, Wanli Yang, Xingjiang Zhou, Donghui Lu, Akira Iyo, Hiroshi Eisaki, Zahid Hussain, Zhixun Shen An interesting phenomenon found in several families of cuprate superconductors is that with the increase of the number of CuO$_{2}$ layers (that are believed to be responsible for the superconducting phenomenon) within a unit cell of the crystal, the T$_{c}$ increase first with the layer number n when n$<$=3, then decrease when n$>$3 and reach the maximum at n=3. To understand this, we use the Angular Resolved Photoemission Spectroscopy (ARPES) to study the four layered cuprate superconductor F0234 and compare the results with results from compounds with less layers. We found that the electronic band structure of the four layered system exhibits clear difference from the previously studied cuprate superconductors with less layers. These results provide new insights on the nature of cuprate physics. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y13.00011: Bosonic Mode Coupling in the Single-Layer Bi-Cuprate, Bi2201 Worawat Meevasna, Nicholas Ingle, Xingjiang Zhou, Donghui Lu, Felix Baumberger, Wanli Yang, Kyle Shen, Wei-Sheng Lee, Juren Shi, Hiroshi Eisaki, Zhixun Shen In the study of the high-Tc superconductors, one of the recent highlights is the observation of the electron self-energy renormalization effect in the form of a ``kink'' in the dispersion. This kink is interpreted as the signature of a collective bosonic mode coupling to the electrons. In our study,high-resolution-angle-resolved-photoemission-spectroscopy data is collected with good statistics to reveal the fine structure in the electron self-energy of the single-layer Bi-cuprate, Pb0.55Bi1.5Sr1.6La0.4CuO6. The fine structure will be discussed. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y13.00012: Elastic forward scattering in superconducting cuprates Lingyin Zhu, T. Dahm, Peter Hirschfeld, Douglas Scalapino We investigate the effect of elastic forward scattering on the ARPES spectrum of cuprates superconductors. The single particle elastic scattering rate at the antinodal point is shown to be suppressed upon the opening of the gap due to the cancellation between the normal and anomalous scattering processes, but is much larger at the nodal point. We further point out that the spectra for $\bf k$ points away from the Fermi level possess a dispersing quasiparticle peak and a rounded square root-like peak at the local gap edge ($\omega=\Delta_k$). While both of these features evolve with the concentrations and effective ranges of defects, the latter provides a possible direct measure of the superconducting order parameter away from the Fermi surface, and may thus be used to determine the range of the pair interaction. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y13.00013: Low-lying excitations of a strongly correlated superconducting state: role of correlation in the ARPES experiments for cuprates Seiji Yunoki, Elbio Dagotto, Sandro Sorella Motivated by recent photoemission experiments on cuprates, the low-lying excitations of a strongly correlated superconducting state are studied numerically. It is observed that along the nodal direction these low-lying one-particle excitations show a linear momentum dependence for a wide range of excitation energies and, thus, they do not present a kink-like structure. The nodal Fermi velocity $v_{\rm F}$, as well as other observables, are systematically evaluated directly from the calculated dispersions, and they are found to compare well with experiments. It is argued that the parameter dependence of $v_{\rm F}$ is quantitatively explained by a simple picture of a renormalized Fermi velocity. The intimate correlation between quasi particle weight and the superconducting ordering is also discussed. [1] S. Yunoki, E. Dagotto, and S. Sorella, preprint (2004). [Preview Abstract] |
Friday, March 25, 2005 1:51PM - 2:03PM |
Y13.00014: On the band structure of bond-centered stripes Mats Granath We study the band structure of a stripe ordered system through mean-field treatments of both single and three band models for the Cu-O plane of the cuprate superconductors. The results suggest that the striking difference between low-energy nodal and anti-nodal excitations seen by ARPES [PRL 92, 187001 (2004)] in underdoped LSCO may be related to the parity of states living on bond-centered stripes and the corresponding coupling to the particular charge sensitive Cu-O bond-stretching phonon. [Preview Abstract] |
Friday, March 25, 2005 2:03PM - 2:15PM |
Y13.00015: ARPES in the cuprate superconductors -- a mean field study Tiago C. Ribeiro, Xiao-Gang Wen Angle resolved photo-emission spectroscopy (ARPES) experiments have played a key role in disclosing information about the cuprates. Intriguing features observed by ARPES in hole underdoped materials include the development of spectral weight inside the Mott gap, the nodal dispersion ``kink'' and the Fermi arcs. We show that a new mean field approach to the $tt't''J$ model, which describes doped Mott insulators in terms of hole-like quasiparticles and spin-charge separated excitations, provides a framework to account for the observed evolution of the electron spectral function from the undoped insulator to the overdoped Fermi metal. [Preview Abstract] |
Session Y14: Focus Session: Supercritical Carbon Dioxide Processing
Sponsoring Units: FIAPChair: Shubhra Gangopadhyay, U of Missouri
Room: LACC 403 B
Friday, March 25, 2005 11:15AM - 11:51AM |
Y14.00001: Supercritical Fluid Technology for Interconnect Fabrication Invited Speaker: Supercritical fluids (SCFs) offer a unique technology platform for semiconductor devices. The absence of surface tension, favorable transport properties and densities that approach those of liquids provide a means for solution-based processes in an environment that behaves much like a gas. These attributes are ideally suited for executing materials chemistries within the smallest device features. This talk will describe the motivation for SCF-based processes and survey potential applications including the deposition of metals including Cu, Ru, and Co, and the etching of metal oxides. Particular attention will be paid to the rapid and efficient preparation of ordered, mesoporous ultra-low dielectric constant (ULK) organosilicate films by the three dimensional replication of structured organic templates in supercritical carbon dioxide. These ULK films are sufficiently robust to survive CMP. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y14.00002: Development of Supercritical Processing in BEOL Cleans Invited Speaker: Supercritical CO$_{2}$-based (SC-CO$_{2})$ processing permits rapid mass transport, high penetration capability into pores and narrow features, low reagent concentrations, and an environmentally benign alternative to current back end cleans---key issues in future semiconductor technology nodes. Because of these assets, supercritical processing is a candidate for a number of semiconductor cleans applications such as plasma damage repair, pore sealing, photoresist/BARC removal and metal deposition. Plasma processing of porous low-k films often remove surface layers and hydrophobic functional groups (creating silanol (Si-OH) species), densify near-surface layers, and increase dielectric constants. Further, this damaged region is vulnerable to additional thickness losses during wet etch. Supercritical silylation reactions (e.g., SC-CO$_{2}$ + hexamethyldisilazane (HMDS)) have been shown to react with post-plasma silanols replacing some fraction of the lost hydrophobic groups, thus, repairing the dielectric constant. We have examined the depth of reactant penetration into the low-k and the fraction of silylated silanols as functions of temperature, pressure, pore size, and silylating agent (e.g., HMDS and alkylchlorosilanes). In addition, we have detailed the effect of heat treatments on pre- and post-silylated silanols, and the impact of these heat treatments on film dielectric constants. Recent efforts have shown that supercritical silylation reactions protect plasma-damaged regions from wet etch losses and prevent intrusion of metal species into low-k pores. We have examined damage protection as functions of ashing conditions, silylating agents, and wet etch conditions. Pore sealing has been studied as functions of silylating agent, heat treatment, and penetrant species. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y14.00003: Supercritical Fluid Research Capabilities and Collaborations at Los Alamos National Laboratory Kirk Hollis, Jerry Barton, Craig Taylor, Laurie Williams, Melvin Carter The supercritical fluids facility (SCRUB) at Los Alamos National Laboratory is a unique research center with a history of successful collaboration with the industrial sector. This presentation will focus on one of these collaborations with DuPont EKC Technology in the continuing development of Supercritical Carbon DiOxide Resist Removal (SCORR). This research has studied co-solvents that effectively remove photoresist from integrated circuit wafers. The technical capabilities of the SCRUB include small-scale equipment, 1-20 ml, up to industrialized pilot scale processes. Initial work focused on the phase behavior of binary mixtures using a small volume (20ml) view cell. A series of SCCO$_{2}$ /co-solvent phase diagrams were investigated over ranges of pressure, temperature and concentration variables. Mixtures of interest were scaled to a 500ml vessel to test removal effectiveness, keeping the conditions (pressure / temperature) within the single phase region during the pulsing process of SCORR. Results from these tests are used to finalize conditions for pilot scale test on 200mm wafers in a 10L vessel. Selected results from all steps will be presented to emphasize the R{\&}D process. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y14.00004: SCORR - Supercritical Carbon Dioxide Resist Removal Craig Taylor, Jerry Barton, Kirk Hollis, Jim Rubin, Laurie Williams Supercritical Carbon diOxide Resist Removal, SCORR, is an emerging technology that may enable the further miniaturization of photolithography processes in industry. SCORR utilizes the physical properties of supercritical fluids (SCFs) to remove coatings, residues, and particles from the high-aspect-ratio and small minimal dimension structures required in modern integrated circuits. In the SCORR process, it is the diffusion of the SCF into the polymer and the resultant swelling that is utilized to debond materials from the surface. By ``tuning'' with pressure, temperature and modifier the SCF is more readily absorbed into the photoresist. This plasticizes the polymer and lowers its glass transition temperature maximizing polymer swelling. Subsequent pressure ``pulsing'', forces a rapid volume expansion effectively debonding the polymer from the substrate. Small amounts of modifier may be added to act at the polymer/substrate interface to further enhance debonding. To optimize the swelling and pulsing steps, a single supercritical phase needs to be maintained. The minimal interfacial tension of SCFs results in a surface boundary layer flow condition that allows the fluid to then remove extremely small particles and residue layers. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y14.00005: Supercritical CO2 for Resist Development: Towards an All-Dry Lithography Process Nelson Felix, Yu (Jessie) Mao, Karen Gleason, Christopher Ober The increased use of organic solvents, halogenated solvents, and water in manufacturing and processing of ICs has led to a need of environmentally responsible and energy efficient processes. Alternatives to the previously mentioned solvents have been researched and one possibility is supercritical carbon dioxide (scCO$_{2})$. It has been well established that by adding a small volume of polar solvents (also called co-solvents) to an scCO$_{2}$ mixture, the solubility of a polar solute in scCO$_{2}$ can dramatically increase. This fact makes scCO$_{2}$ technology extremely attractive as an alternative solvent for lithography. Various resist platforms were tested to demonstrate their ability to be developed in CO$_{2}$. These include copolymers of functionalized methacrylates or phenolics with perfluorooctyl methacrylates, homopolymers of functionalized methacrylates, and novel small molecule resists. Films were either deposited via spin-coating or hot filament chemical vapor deposition (Gleason Group, MIT). Depending on the resist system, features on the order of 100 nm were obtained after development in scCO$_{2}$. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y14.00006: Effect of Ionic Conductivity of Elevated Pressure TMAHCO3 /MeOH/CO2 Mixtures on Cleaning Efficiency Galit Levitin, Dennis W. Hess Carbon-dioxide (CO$_{2\_}$based elevated pressure cleaning mixtures are being considered as environmentally benign alternatives for photoresist and plasma etch residue removal in integrated circuit and microelectronic device manufacture. Despite many attractive features, CO$_{2}$ is non- polar and has little solvating power for photoresist or inorganic materials. Therefore, addition of polar modifiers is necessary. Addition of tetramethylammonium bicarbonate (TMAHCO$_{3})$ in methanol to CO$_{2}$ at elevated pressure and temperature (3000 psi, and 70\r{ }C) efficiently removes photoresist and post plasma etch residues. Our previous studies of the phase behavior of various cleaning mixtures have demonstrated that the cleaning phase state significantly affects cleaning effectiveness. In this work, measurements of ionic conductivity of elevated pressure cleaning solutions was studied to gain insight into the chemical-physical properties of TMAHCO$_ {3}$ /methanol/CO$_{2}$ mixtures, and the resulting effect on residue removal efficiency. Ionic conductivity of cleaning mixture gives information on both the behavior of ions in the mixture and the solvation properties of the fluid medium. This work discusses the characterization of the TMAHCO$_{3}$ /methanol-modified fluids by measuring the ionic conductivity as a function of concentration and mole fraction of CO$_{2}$ at various temperatures and pressures. The ionic conductivity displays a strong dependence on the dielectric properties of the fluid medium. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y14.00007: Atomic Layer Deposition of High-k Dielectrics Using Supercritical CO$_2$ Rajesh Shende, Jorge Lubguban, Maslina Othman, Shubhra Gangopadhyay Atomic layer deposition (ALD) of high-$\kappa $dielectric was performed in supercritical CO$_{2}$ (SCCO$_{2})$, using a two-step reaction sequence.~~ In step one, tetraethoxy silane (TEOS) precursor was injected in SCCO$_{2}$ at 80-100\r{ }C and 50 MPa pressure to obtain a chemisorbed surface monolayer, which was then oxidized into SiO$_{2 }$using peroxide entrained in SCCO$_{2}$. ALD process was controlled by estimating precursor solubility and its mass transport with respect to the density of SCCO$_{2, }$and~correlating these parameters with precursor injection volume. In the ALD process, 7 pulses of precursor were used anticipating deposition of one atomic layer in each of~the pulses. The thickness of the SiO$_{2}$ atomic layers deposited using SCCO$_{2}$ was measured by variable angle spectroscopic ellipsometry (VASE), and the $C-V$ measurements were also performed. The result obtained using VASE indicates that there were 7 monolayers of SiO$_{2}$ with total thickness of 35 \textit{{\AA}}, and the dielectric constant of the deposited layers was 4.0$\pm $0.1. Our initial findings clearly demonstrate that SCCO$_{2}$ is capable of atomic layer deposition of high quality dielectric films at very low process temperatures preventing interface reaction.~~~More research is in progress to achieve ALD of HfO$_{2}$ and TiO$_{2}$ in SCCO$_{2}$. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y14.00008: Supercritical CO2/Co-solvents Extraction of Porogen and Surfactant to Obtain Jorge Lubguban, Arnold Lubguban, Maslina Othman, Rajesh Shende, Shubhra Gangopadhyay A method of pore generation by supercritical CO$_{2}$ (SCCO$_{2})$/co-solvents extraction for the preparation of nanoporous organosilicate thin films for ultralow dielectric constant materials is investigated. A nanohybrid film was prepared from poly (propylene glycol) (PPG) and poly(methylsilsesquioxane) (PMSSQ) whereby the PPG porogen are entrapped within the crosslinked PMSSQ matrix. Another set of thin films was produced by liquid crystal templating whereby non-ionic (polyoxyethylene 10 stearyl ether) (Brij76) and ionic (cetyltrimethylammonium bromide) (CTAB) surfactant were used as sacrificial templates in a tetraethoxy silane (TEOS) and methyltrimethoxy silane (MTMS) based matrix. These two types of films were treated with SCCO$_{2}$/co-solvents to remove porogen and surfactant templates. As a comparison, porous structures generated by thermal decomposition were also evaluated. It is found that SCCO$_{2} $/co-solvents treatment produced closely comparable results with thermal decomposition. The results were evident from Fourier Transform Infrared (FT- IR) spectroscopy and optical constants data obtained from variable angle spectroscopic ellipsometry (VASE). [Preview Abstract] |
Session Y15: Biosensors and Hybrid Biodevices
Sponsoring Units: FIAPChair: Pritraj Mohanty, Boston University
Room: LACC 405
Friday, March 25, 2005 11:15AM - 11:27AM |
Y15.00001: Direct microscopic observation of localized protein bindings in topographically patterned lipid rafts Tae-Young Yoon, Cherlhyung Jeong, Sin-Doo Lee, Joon Heon Kim, Myung Chul Choi, Mahn Won Kim Signal transductions through binding of ligands to cell membrane receptors are the most fundamental way of cell-to-cell communications in multicellular organisms. Important classes of the cell membrane receptors are predominantly concentrated at the phase-separated domains of membranes, the lipid rafts. By localizing lipid rafts at predetermined sites on membranes, cells control the distribution of the ligand bindings to membrane receptors, thereby manipulating the position and intensity of the signal transductions. Thus, prescribed localization of lipid rafts in model membranes could become an important biomimetic methodology of studying cell-to-cell signaling and its engineering in laboratory environments. Here, we demonstrate that topographical nano structures incorporated in supported membranes control the organization processes of lipid rafts; formation, growth, and clustering, by generating elastic energy barriers. We made direct microscopic observations of localized protein bindings in topographically patterned lipid rafts that were prepared by micro-fabrication and nano-corrugation technologies. This topographical concept of controlling the distribution of ligand-membrane receptor binding processes, not disrupting the integral structure of lipid membranes, should provide a viable platform to study human diseases and drug delivery systems. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y15.00002: Physical evidence for a glue holding mineralized collagen fibrils together in bone* P. Hansma, G.E. Fantner, J.K. Kindt, P. Thurner, M.M. Finch, P. Turner, G. Schitter, B. Erickson, Z. Schriock, L.S. Golde, E. Strong, S.F. Udwin Evidence from Atomic Force Microscope indentation, pulling and imaging, and macroscopic testing and enzymatic digestion, suggests that collagen fibrils and mineral plates are not the only components of bone with mechanical roles. A ``glue'' appears to bind mineralized collagen fibrils together. Order of magnitude calculations show that less than 1{\%} by weight of this ``glue'' profoundly affects bone fracture resistance, as it involves a remarkable natural toughening and strengthening system: sacrificial bonds and hidden length. This system dissipates large amounts of work against entropic forces while stretching out the hidden length that is exposed when sacrificial bonds break. This appears to occur when mineralized collagen fibrils are torn apart or slid against each other during bone fracture. In bone, this system depends on multivalent positive ions such as calcium ions, which allows us to follow its influence up to macroscopic fracture testing levels. Many bone matrix proteoglycans and glycoproteins have negatively charged groups at physiological pHs that could be bound together into sacrificial bonds by multivalent positive ions, and are thus natural candidates for this ``glue.'' We cannot rule out a possible involvement of nonfibrillar collagen. Precisely which candidates are involved is yet to be determined. *NSF MRL DMR00-80034, NIH GM65354, NASA BiMAT URETI NCC-1-02037 (00000532), Veeco, USARL ARO DAAD19-03-D-0004 [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y15.00003: Induced electrostatic interactions between atomic force microscope tip and iridovirus Sergei Lyuksyutov, Olga Mayevska, Pavel Paramonov, Shane Juhl, Richard A. Vaia Electrostatic nanolithography based on atomic force microscopy (AFMEN) is implemented to study interactions between an AFM tip and iridovirus assembly on a conductive substrate. Iridovirus is composed of proteins. Certain sequences of natural amino-acids arranged into a helix and folded in ternary structural pattern to form capsomers. Those are the building blocks of the virion capsid. AFMEN implemented on the iridovirus culture deposited on the surface of Au (111). In this proof-in-principle experiment the culture was deluted in water, poured on the gold surface and dried for 24 hours. An AFM tip was manipulated above the iridovirus surface drawing a square of 2$\times $2 $\mu $m$^{2}$ so the virus attracted toward the tip forming piles on the otherwise uniform surface. The robust implementation of AM-AFMEN$^{ }$permits marking an individual virus in air under ambient humidity. Insight of structural protein changes with field magnitude, the threshold character of activation energy of protein re- arrangement, electrostriction, and the influence of filed-induced water condensation are discussed. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y15.00004: The Micro-patterned Device for Cell Attachment using New Extracellular Matrix S.K. Chae, C. Kim, D.S. Na, J.W. Lee, B.K. Ju, C.H. Lee, H. Lee, S.U. Kim, C.N. Hwang, Y.S. Jung, S.H. Lee In this study, we tried to culture mouse P19EC (embryonal carcinoma) stem cell on a new ECM substrate, and we found the new source of the bio-compatible ECM material from zebrafish. The new ECM material is composed primarily of polysaccharide and cross-linked by a N-linked saccharide. We integrated the new ECM material into microwalls of a micro-patterned microdevice for cell attachment. The result of cell adhesion and proliferation on the new ECM was compared with those of other ECM substrates. The surface morphology of the new ECM substrate was investigated using atomic force microscope and the surface properties like electrostatic potential and wettability were measured by contact angle, respectively. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y15.00005: DFT Study of Amino Acids on Si Surface for Hybrid Organic-Semiconductor and Protein-Semiconductor Structures Guilluame Dupont, Charles Musgrave We have used DFT to investigate the formation of hybrid organic- inorganic interfaces between amino acids and silicon surfaces. Not only do amino acids provide a large library of novel organic attachment chemistries to semiconductors, but the reactivity of their side chains also serve as models for the local interactions involved in forming protein- semiconductor interfaces. The amino acid common group is found to react through several low energy pathways, with OH dissociation of the carboxyl group being the most kinetically favorable. Consequently, reaction of amino acids through the amine and carboxyl functionalities is not expected to be selective. The reactivity of many of the amino acids are similar to those of their simpler organic analogues, although we have found several cases which display unique, and possibly useful properties not exhibited by organic functionalities previously considered. Of special interest are attachments of particular amino acid side chains which create quantum mechanical resonances between the amino acid and the semiconductor substrate that may find application in molecular electronics. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y15.00006: Biotin chemisorption on clean and hydroxylated Si-SiC(001) surfaces Yosuke Kanai, Giancarlo Cicero, Giulia Galli, Annabella Selloni, Roberto Car In recent years, there have been substantial experimental efforts toward achieving nanoscale functionalization of semiconductor surfaces. One of the main motivations of such experiments is biosensing application. Biotin is a leading candidate for such functionalization because of its strong, unmatched affinity to specific proteins such as Streptavidin. On the other hand, silicon carbide (SiC) has emerged as a promising biocompatible material that may be employed in new biomedical devices. Using Density Functional Theory, we have carried out a theoretical investigation of the structural and electronic properties of biotin after chemisorption on both the clean and hydroxylated Si-SiC(001). We find that, upon chemisorption, Biotin retains the electronic properties responsible for its strong affinity to proteins. While the electronic states of the hydroxylated surface undergo negligible changes in the presence of biotin, those of the clean surface are substantially affected by the presence of the molecule. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y15.00007: Vibrational modes of viruses and virus-based nano-assemblies Alexander A. Balandin, Vladimir A. Fonoberov Viruses have been proposed as biological templates for fabricating identical nanostructures. Various metallic and semiconductor nanowires and nanotubes were fabricated using cylindrical viruses such as tobacco mosaic virus (TMV) and M13 bacteriophage. The knowledge of the phonon (vibrational) modes of the viruses used for nano-templating is important for monitoring the chemical assembly process and understanding carrier transport properties in resulting nanostructures. In this paper we present results of our investigation of the low-frequency phonon spectra of TMV and M13 viruses immersed in air and water [1]. It is demonstrated that the vibrational modes of inorganic nanowires and nanotubes undergo strong modifications when the organic virus is present inside the nanotube. [1] V.A. Fonoberov and A.A. Balandin, Phys. Stat. Solidi B \textbf{241}, R67 (2004); A.A. Balandin and V.A. Fonoberov, J. Biomed. Nanotechnol. \textbf{1}, in press (2005); see also at http://ndl.ee.ucr.edu [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y15.00008: Single-Walled Carbon Nanotube Transporter for Gene Delivery Pu-Chun Ke, Qi Lu, Jessica Moore, Rahul Rao, Katherine Freedman, Apparao Rao Recent studies have shown great promises in integrating nanomaterials in biomedicine. To explore the feasibility of using single-walled carbon nanotubes (SWNTs) as transporters for gene delivery, we have investigated the binding of SWNTs and RNA polymer poly(rU), and the diffusion and the translocation of the SWNT-poly(rU) complexes. Through single-molecule fluorescence imaging, we have found that the pi- stacking dominates the hydrophobic interactions between the carbon rings on tubes and the nitrogenous bases of RNA. Our diffusion study has further demonstrated the feasibility of tracking the motion of water soluble SWNT-poly(rU) complexes. The uptake of SWNT-poly(rU) by breast cancer cells MCF7 was observed using confocal scanning fluorescence microscopy. It was evident that the complexes could penetrate through cell membrane into cytoplasm and cell nucleus. Our cell culture, MTS assay, and radioisotope labeling showed the negligible cytotoxicity of surface modified SWNTs with RNA polymer and amino acids in cell growth medium. These studies have paved the way for gene transfection using SWNTs as transporters. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y15.00009: Biomolecular Recognition with Functionalized Silicon Nanowires Yu Chen, Agnes Kalinowski, Shyam Erramilli, Pritiraj Mohanty Nanotechnology has the potential to lead to novel techniques for ultra-sensitive biomolecular recognition. We report preliminary results on biomolecular recognition by the measurement of conductance change of bio-functionalized nanowires. The change is primarily due to the contribution of surface states to the conductance, which for larger sensors is dominated by volume effects. The fractional change is greatest for the smallest sensors, due to the increased surface-to-volume ratio. Our silicon nanowires are fabricated from SOI wafer by electron beam lithography, which provides highly controllable nanowire sensors in comparison to other nanoelectronic approaches. We detect ultra-sensitive conductance change at nanoampere-level currents in functionalized nanowires with APTES-modified surface. This work is supported by Department of Defense (CDMRP). [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y15.00010: Optical measurement of DNA torsional modulus under various stretching forces Jaehyuck Choi Optical measurement of DNA torsional modulus under various stretching forces Jaehyuck Choi[1], Kai Zhao[2] Y.-H. Lo[1] [1] Department of Electrical and Computer Engineering, [2] Department of Physics University of California at San Diego, La Jolla, California 92093-0407 We have measured the torsional spring modulus of a double stranded-DNA by applying an external torque around the axis of a vertically stretched DNA molecule. We observed that the torsional modulus of the DNA increases with stretching force. This result supports the hypothesis that an applied stretching force may raise the intrinsic torsional modulus of ds-DNA via elastic coupling between twisting and stretching. This further verifies that the torsional modulus value (C = 46.5 +/- 10 pN nm2) of a ds-DNA investigated under Brownian torque (no external force and torque) could be the pure intrinsic value without contribution from other effects such as stretching, bending, or buckling of DNA chains. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y15.00011: Nonlinear Analysis of Nanomechanical Biosensors in Viscous Fluids Agnes Kalinowski, Shyam Erramilli, Pritiraj Mohanty Current analytical techniques, such as thin beam approximations, inviscid models and finite element simulations fail to accurately model the behavior of nanomechanical structures such as singly- and doubly-clamped beams, coupled linear structures and beam arrays in viscous fluid. We report the nonlinear analysis of elastic nanomechanical structures in highly viscous, laminar flow fluids. The change in resonant frequency of these nanomechanical structures resulting from surrounding fluid dynamics is compared with fluid-structure analysis using the finite element method. This work is supported by the Department of Defense (CDMRP) and the National Science Foundation. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y15.00012: Detection of single magnetic bead using InAs micro-Hall sensors for biological applications Goran Mihajlovic, Peng Xiong, Stephan von Molnar, Keita Ohtani, Hideo Ohno, Mark Field, Gerard J. Sullivan We have fabricated and characterized micro-Hall sensors from InAs/AlSb quantum well heterostructures containing a two-dimensional electron gas. The sensors exhibit room temperature field sensitivities as high as 600 $\Omega $/T, mobilities $>$2$\times $10$^{4}$ cm$^{2}$/V$\cdot $s and low 1/f noise which result in an average field resolution down to the sub- gauss range. Measurements were carried out at temperatures below 150 K on a single submicron superparamagnetic bead (d$\sim $0.9 $\mu $m) that are intended to be used as magnetic labels in biological applications [1]. The magnetization showed expected Langevin behavior as a function of applied field with good signal to noise ratio, demonstrating good potential for the sensors to be used as a detection tool in biological applications. We have also measured the magnetic hysteresis for a single ferromagnetic Ni nanowire (d $\sim $ 200nm) using the device. Our ongoing efforts to demonstrate room temperature operation and to develop biocompatible detection schemes utilizing the micro-Hall sensors will be presented. This work was supported by NSF NIRT Grant ECS-0210332 [1] Q. A. Pankhurst et al., J. Phys. D \textbf{36 }R167 (2003) . [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y15.00013: Microring Resonators for Biochemical Sensing Ayca Yalcin, John C. Aldridge, Ketul C. Popat, Tejal A. Desai, Nabil Chbouki, M. Selim Unlu, Bennett B. Goldberg Resonant microcavities have recently become popular for research in optical biosensor applications. Whispering Gallery Mode (WGM) microresonators are preferred as active sensing surfaces due to their high-Q values which provide measurable shifts in mode frequencies as surface characteristics change. In this study, the active sensing surface is a microring resonator vertically coupled to waveguides and mounted to a flow cell. Resonant transmission at a specific wavelength is measured as a function of time as flow solution concentrations are varied, and high sensitivity to surface refractive index changes and repeatability are demonstrated. To investigate Avidin-Biotin binding, surfaces are modified with silane and Avidin, followed by Biotinylated Lectin flow. Surface characterization performed by ellipsometry and XPS shows successful deposition of discrete layers. Preliminary results show detection of binding and near complete regeneration of the sensing surface, indicating a high potential for sensitive and selective biosensor applications of microring resonators. [Preview Abstract] |
Friday, March 25, 2005 1:51PM - 2:03PM |
Y15.00014: A Combinatorial Computational Approach to Optimizing Drug Delivery from Passive Hydrogels S\'ebastien Casault, Gary W. Slater We introduce a new computational approach to design passive drug delivery systems based on porous materials such as hydrogels. The approach uses four tools: a method to establish the exact release pattern from all possible loading sites inside a given hydrogel; a method to generate a large number of hydrogel structures to be tested numerically; a method to compute the loading pattern that would provide the best release curves for a given hydrogel structure; and an optimization method that leads to the selection and design of optimal hydrogel structures. Using this novel approach, we show that non-trivial release curves can be obtained by generating a multitude of random structures. Strategies to generalize this approach to other systems will be discussed. [Preview Abstract] |
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Y15.00015: What is nano to cells \& the body? Effects of shape Dennis Discher Viruses protect, target, and deliver active agents to cells and generally have quasi-spherical and filamentous morphologies. Diblock copolymer amphiphiles can assemble in water into similar shapes, namely vesicles (or polymersomes) and worm-like micelles, that prove especially robust. Controlled release polymersomes were prepared with the hydrolysable block copolymers poly(ethyleneglycol)--poly(lactic acid) (PEG-PLA) and PEG--poly(caprolactone) (PEG-PCL). When blended with non-degradable diblocks, release reflects a highly quantized process in which any given vesicle is either intact, retaining its encapsulant, or the vesicle is porated and slowly disasembing. In vivo studies demonstrate the stealthiness of polymersomes, while emerging tests of these vesicles in cell culture demonstrate great promise for controlled release of drugs and oligonucleotides into various cell types. Similar diblock copolymers are being studied as with the vesicles, although the worm micelle formers have more symmetric proportions initially. The goal is to exploit these micelles for fluid transport and delivery of the many hydrophobic drugs to cells. In vitro studies demonstrate the degradation kinetics as well as the great flexibility and targetability of these self-assembled micelles. Surprisingly, initial in vivo studies show that microns-long worm micelles circulate in the blood stream for days longer than even the longest circulating 100 nm PEGylated vesicles. [Preview Abstract] |
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Y15.00016: The Role of Nanobiotechnology in the Study of Dystrophin and B-Dystroglycan in Membrane Stability of Aging Skeletal Muscles Ashok Vaseashta, Olivia Boskovic, Allison Webb Duchene muscular dystrophy (DMD) is one of nine types of muscular dystrophy, a group of genetic degenerative diseases, primarily affecting voluntary muscles, caused by absence of dystrophin. New experiments on mice with DMD has shown that gene therapy can reverse some symptoms of the disease. The ultimate goal of gene therapy for muscle diseases is improvement of strength and function, which will require treatment in multiple muscles simultaneously. A major limitation to gene therapy until now has been that no one had found a method by which a new gene could be delivered to all the muscles of an adult animal. Recent utilization of nanotechnology to life sciences has shown exciting promises in a wide range of disciplines, showing advances in the ability to manipulate, fabricate and alter tiny subjects at the nanometer scale. In the present investigation, we have employed such techniques to study single motors such as myosin and kinesin, as well elastic proteins viz. titin and nebulin, muscle filaments, cytoskeletal filaments, and receptors in cellular membranes and cellular organelles viz. myofibril, ribosome, and chromatin. Application of AFM to images and measures the elastic properties of single monomeric and oligomeric protein, genetically engineered titin, and nebulin molecules will be presented. [Preview Abstract] |
Session Y16: Superconducting Single and Coupled Qubits
Sponsoring Units: DCMPChair: Guido Burkard, University of Basel
Room: LACC 404A
Friday, March 25, 2005 11:15AM - 11:27AM |
Y16.00001: Measurement Crosstalk in the Josephson Phase Qubit R. McDermott, K.B. Cooper, M. Steffen, M. Ansmann, J.M. Martinis, K. Osborn, K. Cicak, S. Oh, D.P. Pappas, R.W. Simmonds In order to accurately assess the fidelilty of quantum gates, or to perform quantum state tomography and thereby definitively prove entanglement, it is necessary to measure the states of all qubits in the system (wordwise readout). In multi-qubit circuits with fixed couplings – a common architecture for superconducting qubits – realization of this goal is complicated by measurement crosstalk: the measurement of one qubit perturbs the states of the other qubits, destroying information about quantum correlations. For the flux-biased Josephson phase qubit, the measurement of a $|1\rangle$ state implies a tunneling transition between local minima of the qubit potential. The resulting time-varying voltage across the measured qubit junction couples a transient current to other qubits, which can induce transitions between the qubit $|0\rangle$ and $|1 \rangle$ states. We present a semiclassical model which quantitatively accounts for the observed measurement crosstalk in our circuit, and describe how fast, simultaneous state measurement can circumvent this problem. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y16.00002: Flux Qubits and Readout Device with Two Independent Flux Lines B.L.T. Plourde, T.L. Robertson, T. Hime, P.A. Reichardt, C.-E. Wu, John Clarke Circuits involving multiple flux qubits require an architecture which is scalable. In particular, the flux bias must be settable for each element individually. We report measurements on two superconducting flux qubits coupled to a readout Superconducting QUantum Interference Device (SQUID). The devices were fabricated with Al-AlOx-Al tunnel junctions using electron-beam lithography. Two on-chip flux bias lines allowed independent flux control of any two of the three elements. By rastering the currents in these two flux lines, we observed the modulation of the SQUID critical current due to the applied flux as well as the changing screening currents in the two qubits. These results are illustrated by a two-dimensional qubit flux map. By varying the flux bias currents to move along a line of constant flux applied to the SQUID, we could measure the qubit transitions while maintaining a fixed SQUID sensitivity. When combined with variable qubit-qubit coupling based on the circulating currents in the readout SQUID, this architecture should be scalable to many qubits and SQUIDs on a single chip. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y16.00003: Quantum Coherence in a Superconducting Flux Qubit T. Hime, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, C.-E. Wu, John Clarke We report observations of quantum coherence in a superconducting flux qubit. As the flux applied to the qubit was swept through the degeneracy point, $(n+1/2)\Phi_0$, we could resolve the change in qubit screening flux produced by the reversal of the qubit circulating current. By applying microwave radiation to the qubit, we observed resonant excitation when the qubit level splitting matched the energy of the microwave photons, corresponding to a change in the qubit screening flux. We varied the microwave frequency and mapped out the dispersion of the excited state transition which fit well to the expected hyperbolic dependence. With high-resolution spectroscopy, we measured anomalous structure and splittings on the excited state line, which may correspond to coupling to defect states in the junction tunnel barriers. We performed coherent manipulation of the qubit state by applying microwave pulses of fixed amplitude and frequency, but variable width. This resulted in Rabi oscillations with a Rabi frequency which scaled linearly with the amplitude of the microwave pulses. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y16.00004: Measurements of Dephasing in Superconducting Flux Qubits C.-E. Wu, T. Hime, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, John Clarke The time over which a superposition of qubit states maintains phase coherence is an important figure of merit for a qubit. One technique for measuring this dephasing time is the Ramsey fringe, consisting of two $\pi/2$ pulses detuned from resonance. Varying the time between the pulses produces a damped oscillatory fringe, with the frequency equal to the detuning from resonance and the decay time given by the dephasing time. The dephasing time can also be extracted from measurements of the spectroscopic linewidths for different excitation amplitudes. We report measurements using both techniques in a superconducting flux qubit, giving dephasing times of the order of 10 ns. We present the variation of the dephasing time with various parameters, such as qubit level splitting, readout SQUID operating point, and temperature. We compare our results with expected levels of low frequency noise in the qubit environment and discuss possible methods for enhancing the coherence, including spin echo pulse sequences. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y16.00005: Measurements of Relaxation in Superconducting Flux Qubits P.A. Reichardt, T. Hime, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, John Clarke The exchange of energy between a qubit and its environment leads to the decay of an excited state, characterized by the inelastic relaxation time. This relaxation is determined by the noise in the qubit environment at the level splitting frequency. We have measured the relaxation time in a superconducting flux qubit by exciting the qubit with a resonant microwave pulse, and varying the time following the excitation before the qubit is read out with a Superconducting QUantum Interference Device (SQUID). We present the variation of the relaxation time with various parameters, such as the qubit level splitting, the readout SQUID operating point, measurement repetition time, and temperature. We compare our results with expected levels of noise in the qubit environment and discuss possible sources of the qubit relaxation. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y16.00006: Spectroscopic study of energy levels and transition rates in large area flux qubit Jaan Mannik, Douglas Bennett, Vladimir Kuznetsov, Vijay Patel, Wei Chen, James Lukens The level structure and transition rates between fluxoid states of a large area flux qubit have been studied using microwave spectroscopy and resonant tunneling spectroscopy. This qubit uses an rf-SQUID in a gradiometer configuration and has independent, \textit{in situ}, controls for the barrier height between fluxoid wells and the relative positions of levels in different wells. This design makes it well suited for the study of decoherence mechanisms that adversely affect the operation of flux as well as phase qubits based on fluxoid states. This work was supported in part by NSF and by AFOSR and ARDA through a DURINT program. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y16.00007: Junction Resonances in Josephson Phase Qubits John Martinis, R. McDermott, K.B. Cooper, M. Steffen, M. Ansmann, K. Osborn, K. Cicak, S. Oh, D.P. Pappas, R.W. Simmonds Careful spectroscopic measurements of Josephson phase qubits have revealed avoided crossings characteristic of the qubit interacting with a set of two-level resonators. As these extra splittings degrade the quality of the qubit, it is extremely important to characterize and understand their origin. We will present data from a variety of qubits that show these resonators have an amplitude probability distribution which scales inversely with splitting magnitude up to a certain splitting size. This data can be explained with a model based on two-level critical-current fluctuators in the junction. This model is also reasonably consistent with previous measurements of 1/f (low frequency) critical-current noise. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y16.00008: Time-Domain Investigation of Coupled Superconducting Qubits Andrew Berkley, Sergei Govorkov, Murray Thom, Brock Wilson We report on measurements of two coupled rf SQUID phase qubits. With both qubits initially in their ground states, the first is excited with a pi pulse. Being initially detuned they are brought into resonance with a coupling pulse that allows the qubits to be put into a maximally entangled state. By varying the length of the interaction time we are able to perform a full logical SWAP gate operation. The coherence times and Rabi amplitudes measured on these qubits are small when compared to quantum error correction limits. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y16.00009: Energy Relaxation in Josephson Phase Qubits Raymond Simmonds, K. Cicak, S. Oh, K. Osborn, J. A. Strong, D. P. Pappas, M. Ansmann, K. B. Cooper, R. McDermott, M. Steffen, John M. Martinis The characteristic energy relaxation time $T_1$ of any ``isolated" quantum system depends on the degree of isolation, or the strength with which a number of ``environmental" degrees of freedom couple to that system. In order for superconducting based quantum bits to be a viable technology for quantum computing, these systems must be sufficiently isolated from their environment to enable them to undergo relatively undisturbed quantum evolutions. Additionally, they must be controlled precisely in order to perform quantum operations. This operation time is ultimately limited by the single qubit $T_1$ time. Recently, we have investigated a number of different Josephson phase qubit design geometries in order to identify what, where, and how environmental degrees of freedom couple to individual qubits, reducing $T_1$. These measurements will help to improve the operation of future coupled phase qubit systems. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y16.00010: Decay of Rabi Oscillations in Josephson Charge Qubits. Roman Lutchyn, Leonid Glazman, Anatoly Larkin We analyze the decay of Rabi oscillations in a charge qubit consisting from a Cooper pair box connected by a Josephson junction to a finite-size superconductor.\footnote{A. Blais \emph{et. al.}, cond-mat 0402216.; A. Wallraff \emph{et. al.}, Nature (London) 431,162 (2004)} The dominant mechanisms of the decay are still debated, and we concentrate on the contribution of quasiparticles to the decay rate. Passing of a quasiparticle through the Josephson junction tunes the qubit away from the charge degeneracy, thus spoiling the Rabi oscillations. We find the temperature dependence of the quasiparticle contribution to the decay rate. This dependence has three distinct regimes defined by two temperature scales, $T^*_{1,2}=\Delta/\ln(\Delta/\delta_{1,2})$, where $\Delta$ is the superconducting gap, and $\delta_{1,2}$ are the one-electron level spacings in the electrodes forming the qubit. Temperatures $T^*_{1,2}$ characterize the appearance of thermally excited quasiparticles in the parts of qubit; typically $T^*_{1,2}\ll E_c, E_j$, where $E_c$ and $E_j$ are the charging and Josephson energies, respectively. [Preview Abstract] |
Session Y17: Spin Dynamics in Semiconducting Materials and Quantum Dots
Sponsoring Units: DCMPChair: Janica Whitaker, Naval Research Lab
Room: LACC 404B
Friday, March 25, 2005 11:15AM - 11:27AM |
Y17.00001: Phonon Decoherence of a Double Quantum Dot Charge Qubit Eduardo R. Mucciolo, Serguei Vorojtsov, Harold U. Baranger The decoherence of a lateral double quantum dot charge qubit due to coupling to piezoelectric acoustic phonons has been investigated within the Born-Markov approximation. After including appropriate form factors, we have found that phonon decoherence rates are one to two orders of magnitude weaker than earlier predictions based on the spin-boson model. We have calculated the dependence of the Q-factor on lattice temperature, quantum dot size, and interdot coupling. Our results suggest that mechanisms other than phonon decoherence play a more significant role in current experimental setups. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y17.00002: Fidelity of spin ensemble memory for mesoscopic quantum bits V.V. Dobrovitski, J.M. Taylor, M.D. Lukin Development of techniques for coherently manipulating electron spins in quantum dots is important for future applications in spintronics and in quantum information processing. In this work we study the quantum memory protocol suggested recently [1] for storage and retrieval of the electron spin states in the lattice nuclear spins. We report detailed studies of this technique in the presence of imperfections, such as the incomplete polarization of the nuclear spins and the spread in the hyperfine couplings between the electron and the nuclei. We numerically simulate the memory protocol by solving the time-dependent Schr\"odinger equation for the system comprising the electron spin and the bath spins [2]. We find that the memory operation is robust with respect to these relalistic imperfections and that high fidelity operation is possible with realistic values of nuclear spin polarization. This work was supported by the NSA, ARDA and ARO.\\ 1. J. M. Taylor, C. M. Marcus, and M. D. Lukin, Phys. Rev. Lett. {\bf 90}, 206803 (2003).\\ 2. V. V. Dobrovitski and H. A. De Raedt, Phys. Rev. E {\bf 67}, 056702 (2003) [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y17.00003: Controlling decoherence due to nuclear spins in III-V compounds: Which price do we pay? Rogerio de Sousa, Neil Shenvi, K. Birgitta Whaley Nuclear spins of the host lattice are the dominant source of decoherence in semiconductor donor and quantum dot spin qubits. There are two channels for nuclear induced decoherence: (1) Loss of visibility arising from the non-secular hyperfine coupling; (2) Spectral diffusion arising from the combined effect of inter-nuclear dipolar coupling and the secular hyperfine term. We performed numerical calculations to show that application of a moderate static magnetic field ($\sim$ 2 Tesla) is enough to suppress mechanism (1) within the $10^{-4}$ criteria of quantum error correction. On the other hand a much greater overhead is required to control mechanism (2). We consider the Carr-Purcell-Meiboom-Gill sequence as a means to control (2) and provide a realistic assessment of the required overhead in number of qubit $\pi$-pulses. We show that the required rate of $\pi$-pulsing is proportional to the nuclear spin quantum number squared, showing that robust coherent manipulation in the large spin environments characteristic of the III-V compounds is possible without resorting to nuclear spin polarization. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y17.00004: Electron Spin Dynamics of Hyperfine Interaction in a Quantum Dot Channgxue Deng, Xuedong Hu We investigate spin dynamics of electrons in a quantum dot interacting with nuclei through the inhomogeneous hyperfine coupling. The problem has been studied previously using either perturbation theory (fails at zero nuclear spin polarization) or treating the system semi classically (Markovian approximation). In this paper we study the system systematically with a large N (effective number of nuclear spins in the dot) expansion technique which is valid at arbitrary nuclear polarization and external magnetic field. The coherent oscillations and the decoherence, represented by the poles and the branch cuts respectively, are treated in a unified way within this non-perturbative method. Our calculations reproduce previous results of highly polarized nuclear spin configuration. On the other hand we find some new features of the real time dynamics for the unpolarized nuclear spins which has not been obtained. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y17.00005: Electron-Nuclear Spin Transfer in Triple Quantum Dot Networks Marta Prada, Ryan Toonen, Robert Blick, Paul Harrison We investigate the conductance spectra of coupled quantum dots to study systematically the nuclear spin relaxation of delta- and y-junction networks and observe spin blockade dependence on the electronic configurations. We derive the conductance using the Beenakker approach generalised to an array of quantum dots where we consider the nuclear spin transfer to electrons by hyperfine coupling. This allows us to predict the relevant memory effects on the different electronic states by studying the evolution of the single electron resonances in presence of nuclear spin relaxation. We find that the gradual depolarisation of the nuclear system is imprinted in the conductance spectra of the multidot system. Our calculations of the temporal evolution of the conductance resonance reveal that spin blockade can be lifted by hyperfine coupling. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y17.00006: Large Magnetic Field Gradients for Crystal Lattice Quantum Computing Jonathan Goldman, Thaddeus Ladd, Charles Santori, Shinichi Koseki, Glenn Solomon, Bingyang Zhang, Yoshinori Matsumoto, Fumiko Yamaguchi, Yoshihisa Yamamoto A quantum computer using nuclear spins in a crystal lattice requires a method for addressing individual quantum bits. This identification can be achieved with a spatially varying magnetic field. Spins at different lattice sites can have distinguishable Zeeman frequencies allowing initialization, logic operations, and measurements to be performed through radio frequency (rf) pulse techniques. Here, we present magnet designs that have gradients between 1 and 20 G/Angstrom, which are necessary to realize quantum computation with particular crystals. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y17.00007: Experimental and Theoretical Limits on Pulse Quality in Silicon NMR Experiments Rona Ramos, Kenneth MacLean, Yanqun Dong, Dale Li, Anatoly Dementyev, Sean Barrett Previous NMR experiments on Silicon involving multiple pulses showed long lived spin echoes [A.E. Dementyev, D. Li, K. MacLean, S.E. Barrett, Phys. Rev. B, 68, 153302 (2003)], an anomalous behavior that disagrees with conventional NMR theory. The application of several pulses to a many spin system warrants the understanding of pulse quality in fine detail. A series of experiments to improve the pulse characteristics and to approach the limit of delta function, spatially homogeneous pulses were performed. These experiments and detailed calculations of the pulse fields involved will be discussed, as well as the implications in understanding the anomalous long lived behavior of previous experiments. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y17.00008: Quantum and Classical Spin Dynamics in Silicon NMR Yanqun Dong, Rona Ramos, Kenneth MacLean, Anatoly Dementyev, Dale Li, Sean Barrett Recent Si29 NMR measurements [A.E. Dementyev, D. Li, K. MacLean, S.E. Barrett, Phys. Rev. B, 68, 153302(2003)] revealed several surprises, such as unexpectedly long tails and even-odd asymmetry in CPMG data. To understand these phenomena, we implemented a series of simulations with increasing levels of sophistication designed to reflect the actual experimental conditions (e.g. including H1 field inhomogeneity, finite pulse duration, etc.). I will present the simulations, compare them with our data and discuss the implications. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y17.00009: Puzzles in Silicon NMR: Pushing the limits of pulse quality Dale Li, Anatoly Dementyev, Yanqun Dong, Rona Ramos, Sean Barrett Recent observations of anomalously long-lived spin echoes in Silicon are still not understood [A.E. Dementyev, D.Li, K. MacLean, S.E. Barrett, Phys. Rev. B 68, 153302 (2003).]. This talk will discuss our most recent experimental efforts to understand the basis of this effect. In particular, detailed studies of the effect of pulse quality (including spatial uniformity across our many-spin system) will be reported. [Preview Abstract] |
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Y17.00010: Relaxation of the electron spin in a quantum dot due to interaction with nuclear spin bath Khaled Al-Hassanieh, V.V. Dobrovitski, E. Dagotto, B.N. Harmon Understanding the dynamics of electron spins in semiconducting nanostructures is important for novel applications in spintronics and in quantum information processing. An electron spin in a quantum dot is strongly affected by its interaction with the environmental degrees of freedom, in particular, with the nuclear spins. In this work we study the longitudinal relaxation of the electron spin component $S^z$ by the nuclear spin bath for different applied fields and initial polarizations of the bath. We numerically simulate the motion of the compound system (the electron spin plus the bath) by explicitly solving the corresponding time-dependent Schr\"odinger equation using the method described in [1]. Typically, $S^z$ exhibits a pronounced oscillation with subsequent saturation; at high fields, several such oscillations are observed. We compare our numerical results with the earlier analytical predictions [2], and discuss the agreements and differences.\\[4pt] [1] V. V. Dobrovitski and H. A. De Raedt, Phys. Rev. E {\bf 67}, 056702 (2003)\\[0pt] [2] A. V. Khaetskii, D. Loss, and L. Glazman, Phys. Rev. Lett. {\bf 88}, 186802 (2002); I. A. Merkulov, E. L. Efros, and M. Rosen, Phys. Rev. B {\bf 65}, 205309 (2002). [Preview Abstract] |
Session Y21: Focus Session: Multiscale Analysis in Biology: Computation
Sponsoring Units: DBPChair: Robert Eisenberg, Rush Medical Center
Room: LACC 409A
Friday, March 25, 2005 11:15AM - 11:51AM |
Y21.00001: Hierarchy of computational electronics approaches for multiscale charge transport simulation Invited Speaker: Semiconductor device simulation has been developed over several decades resulting in a hierarchy of computational electronics tools and approaches that are uniquely powerful for treating self-consistently the transport of charged particles. These methodologies, originally conceived for electronic transport in solid state systems, can be extended to other fields, particularly ionic transport in aqueous solutions, as found in biological systems. This talk will outline the hierarchy and the most important features of these transport model and discuss the new challenges encountered in treating general molecular systems as devices. Emphasis will be placed on discussing the approaches which are more suitable for future practical applications for engineering and design of molecular systems and devices, including multi-scale problems. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y21.00002: Multiscale Modeling of Heterogeneous Lipid Bilayers Roland Faller, Sandra Bennun-Serrano, Allison Dickey The first line of defense for a cell against intrusive molecules is the membrane which must be resilient to prevent unwanted molecules from passing through as a change in the intracellular ion balance could be detrimental. Experimentally, it has been shown that as chain length and concentration of alcohols near a membrane increase, the area per lipid expands, increasing the likelihood of permeation. Additionally, there is evidence for pattern formation in cell membranes due to the presence of various lipids. These patterns or rafts are believed to play important roles in cell signaling. Here, we use MD to study the interactions between alcohols and pure lipid bilayers as well as pattern formation in mixed membranes using atomistic and coarse-grained models. We characterize the effect of alcohol chain-length and concentration on the lipid bilayer through area per head group, order parameter, and density profile. We also examine the effects of lipid-alcohol interactions on membrane curvature with the CG model and find satisfactory system representation. We use a mixture of DLPC and DSPC as model system for phase separation. Different concentrations and temperatures are used to reproduce phase transitions. We obtain agreement with experiments for area per lipid head group and deuterium order parameter. At high DSPC concentrations phase separation into a gel and liquid state is found. Simulations confirm that increasing DLPC concentrations lower the transition temperature. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:39PM |
Y21.00003: MultiScale, Renomalization, and Biology Invited Speaker: Is biology renormalizable and amenable to multiscale simulation methods? Multiscale simulations provide a comprehensive computational model, spanning many length and time scales, starting with the characteristic molecular assemblies, and connecting them to meso scale and finally macro scale material properties. A hierarchy of overlapping calculations in space and time are performed. Parameters are input to each calculation based on the calculations from a smaller length and time scale and checked with calculations at a larger length scale. In this lecture examples will include the properties of proteins in an ultracentrifuge and the elastic properties of DNA from the atomic to the material length scales. These problems have an explicit coupling of length-scales from the microscopic correlations between molecules in the assembly to the shape of the resulting assembly to the continuum elastic properties. The results are tested against experiments and checked for consistency with known thermodynamic and structural data. The question of the renormalizablility of biology is shown to be linked to intrinsic complexity in space and time. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y21.00004: Flexible lipid bilayers in implicit solvent Grace Brannigan, Peter Phillips, Frank Brown A minimalist simulation model for lipid bilayers is presented. Each lipid is represented by a flexible chain of beads in implicit solvent. The hydrophobic effect is mimicked through an intermolecular pair potential localized at the ``water''/hydrocarbon tail interface. This potential guarantees realistic interfacial tensions for lipids in a bilayer geometry. Lipids self assemble into bilayer structures that display fluidity and elastic properties consistent with experimental model membrane systems. Varying molecular flexibility allows for tuning of elastic moduli and area/molecule over a range of values seen in experimental systems. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y21.00005: Simulation of lipid bilayers using coarse grained methods Mark Stevens There are many important biological processes involving lipid bilayers on times scales beyond that accessible by atomistic simulations. We have developed coarse-grained, bead-spring models of lipid molecules to treat membrane fusion, domain formation and the general physical characteristics of lipid bilayers. A key aspect of these coarse-grained models is that the liquid nature of a bilayer is explicitly present in the simulations; the lipids diffuse far beyond their neighbors in contrast to atomistic simulations. With these models self-assembly into a bilayer starting from a random configuration of lipids and solvent is readily simulated. We have performed extensive simulations to characterize these lipid models in single component lipid bilayers. For a variety of tail lengths, the area per lipid as a function of temperature has been calculated; the liquid-gel transition has been characterized. Models have been developed for a variety of lipids including double bonds in the lipid tails. Simulation results will be presented for fusion and domain formation. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y21.00006: Efficient tunable generic model for self-assembling fluid bilayer membranes Markus Deserno, Ira R. Cooke, Kurt Kremer We present a new model for the simulation of generic lipid bilayers in the mesoscopic regime (between a few nanometers and many tens of nanometers), which is very robust, versatile, and extremely efficient, since it avoids the need for an embedding solvent. Based entirely on simple pair potentials, it features a wide region of unassisted self assembly into fluid bilayers without the need for careful parameter tuning. The resulting membranes display the correct continuum elastic behavior with bending constants in the experimentally relevant range. It can be readily used to study events like bilayer fusion, bilayer melting, lipid mixtures, rafts, and protein-bilayer interactions. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y21.00007: Membrane Geometry and Forces Eric Peterson, Rob Phillips, Paul Wiggins, Feng Feng, William Klug Recent advances in cryo-electron microscopy have enabled biologists to reconstruct detailed three-dimensional structures of cellular-scale biological systems via tomography. In particular, tomography can be exploited to capture lipid bilayer membrane conformations of membrane bound organelles and membrane-mediated cellular functions such as transport, mobility and viral budding. We propose to exploit the observed membrane geometry to measure the forces applied on submicron length scales by the protein machines responsible for cellular function. We discuss the computational technique and present preliminary in vitro experimental results for liposomes. [Preview Abstract] |
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Y21.00008: On the translocation of stiff chains Roya Zandi, William Gelbart, Joseph Rudnick, David Reguera We study the dynamics of the passage of a stiff chain through a pore into a cell containing particles that bind reversibly to it. Using Brownian Molecular Dynamics simulations we investigate the mean-first-passage time as a function of the length of the chain inside, for different concentrations of binding particles. As a consequence of the interactions with these particles, the chain experiences a net force along its length whose calculated value from the simulations accounts for the velocity at which it enters the cell. This force can in turn be obtained from the solution of a generalized diffusion equation incorporating an effective Langmuir adsorption free energy for the chain plus binding particles. These results suggest a role of binding particles in the translocation process which is in general quite different from that of a Brownian ratchet. Furthermore, non-equilibrium effects contribute significantly to the dynamics, \emph{e.g.}, the chain often enters the cell faster than particle binding can be saturated, resulting in a force several times smaller than the equilibrium value. [Preview Abstract] |
Session Y25: Fullerenes and Nanographite
Sponsoring Units: DCOMP DMPChair: Tunna Baruah, Howard University
Room: LACC 501A
Friday, March 25, 2005 11:15AM - 11:27AM |
Y25.00001: Electronic transport, structure, and energetics of endohedral Gd@C82 metallofullerene and nanopeapods L. Senapati, J. Schrier, K. B. Whaley Electronic structure and transport properties of the fullerene C$_{82 }$and the metallofullerene Gd@C$_{82}$ are investigated with density functional theory and the Landauer-Buttiker formalism for transport. The metal binding site for Gd is found to differ from that in Sc, Y, and La metallofullerenes, and is located on the C$_{2}$ symmetry axis, adjacent to the C-C double bond. Insertion of Gd into C$_{82 }$causes a slight deformation of the carbon structure in the vicinity of the Gd atoms. Significant overlap of the electron distribution is found between Gd and the C$_{82}$ cage, with the transferred Gd electron density localized mainly on the nearest carbon atoms. This charge localization reduces some of the conducting channels for the transport, resulting in a reduction in the conductivity of the Gd@C$_{82}$ species relative to the empty C$_{82}$ molecule. The electron transport across the metallofullerene is found to be relatively insensitive to the spin of the transported electrons relative to that of the Gd atom. In addition, we present results of transport calculations when the optimized Gd@C$_{82 }$is inserted into a carbon nanotube to form a nanopeapod structure. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y25.00002: Modification of electronic properties of nanographite due to chemical treatments Dmitriy Dikin, Sasha Stankovich, Xinqi Chen, Richard Piner, Rodney Ruoff, Oleksandr Chernyashevskyy, SonBinh Nguyen Electronic and magnetic properties of nanostructured graphite platelets and individual graphene sheets are highly anisotropic and exhibit high sensitivity to adsorption, doping and intercalation. The magnetization results obtained at room temperature for pristine graphite, graphite oxide, and chemically or thermally reduced graphite oxide show direct correlation between oxidation and reduction treatments. Diamagnetic response for graphite oxides is significantly decreased compared to the pristine material, and is restored back in chemically and thermally reduced samples, so the electronic properties can be modified in a controlled manner. At low temperatures all of the measured samples exhibit paramagnetic behaviour, which dominates for thermally treated graphite oxide. SEM and AFM are also used in our study to analyse the electronic and chemical modifications of nanographite. We gratefully acknowledge the \textit{NASA University Research, Engineering and Technology Institute on Bio Inspired Materials (BIMat)} under award No. NCC-1-02037. SEM was done at the Electron Probe Instrumentation Center at Northwestern University. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y25.00003: Theoretical calculations on the mechanism of local polymerization and depolymerization of C$_{60}$ Shigeru Tsukamoto, Masato Nakaya, Tomonobu Nakayama, Masakazu Aono A reversible chemical reaction between C$_{60}$ molecules, polymerization/depolymerization, has been realized and controlled using an STM tip: the polymerization and depolymerization occur at negative and positive sample bias conditions, respectively. Using a first-principles calculation method, we investigated the mechanism responsible for the experimental results. The electronic structures were calculated for polymerized and depolymerized structures of C$_{60}$ dimers under negatively and positively ionized conditions as well as neutral condition. When a couple of C$_{60}$ is negatively ionized, the added electron occupies the initially unoccupied bonding orbital between C$_{60}$ molecules. Then the activation barrier of a polymerization is reduced. When a C$_{60}$ dimer is positively ionized, the bonding orbital energetically approaches to Fermi level and the activation barrier of depolymerization process is reduced. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y25.00004: First principles investigation of one monolayer of C60 on h-BN/Ni(111) Jingguang Che, Hai-Ping Cheng The geometric and electronic structures of a monolayer of C60 on a monolayer h-BN on Ni(111) surface are studied by first principles calculations. The interaction between ions and electrons is described by the projector-argumented wave method. The most stable structure is found to be N on the top and B on the fcc site of Ni(111). The structure in which a hexagon of the C60 molecular is parallel to the substrate is 0.1eV in energy more favorable than that of a pentagon parallel to the substrate. For the most stable adsorption sites of C60 on h-BN/Ni (111), the distance between the bottom hexagon of C60 and the h- BN/Ni substrate is 3.6A. The calculated results show that the energy differences for different orientations are all smaller than 0.2eV. No magnetic moment is found for C60 monolayer. The calculated electronic structures confirm that the h-BN/Ni(111) may be a good insulator platform to study the electronic structures of C60 ultrathin films, since only a weak interaction and a few charge transfer exist between C60 and h-BN/Ni(111). [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y25.00005: The structural and electronic properties of a K-doped C60 monolayer of on h-BN/Ni(111) Hai-Ping Cheng, Jingguang Che The structural and electronic properties of a K-doped C60 monolayer are investigated using first principles calculations based on density functional theory. The K-doped C60 monolayer is deposited on an insulator h-BN monolayer on a Ni(111) surface. The interaction between ions and electrons is described by the projector-argumented wave method (PAW). It is found that K atoms are incorporated into the interstitial sites of the C60 monolayer with a distance of 2.9$\sim$3.0A to the h-BN/Ni substrate, while the distance of a hexagon face of C60 to the substrate is 3.6A. The calculated results indicate that with K doping the electron occupation on bands that are derived from the C60 lowest unoccupied molecular orbital (LUMO) is enhanced. For doping of four K atoms per C60 molecule, the LUMO is almost half- filled. Detailed analysis and comparison with an un-doped C60 monolayer on the same substrate and with pure metal surfaces will be presented. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y25.00006: Theoretical Study of Stabilization of Fullerene-like Silicon Cages Ajit Hira, Nichole Moya-Leyba, Daniel Bulnes We extend our work on fullerenes$^{1}$, by exploring the stabilization of fullerene-like silicon cages through intercalation of carbon atoms. \textit{Ab initio} theoretical techniques are used to derive the physical and chemical properties of various (Si$_{60})_{m}$C$_{n }$systems (m = 1-3, n =1-10). The first phase of our investigation focuses on endohedral and exohedral complexes of a single Si$_{60 }$with$_{ }$C$_{n }$clusters. Electron correlation effects are incorporated using both Many Body Perturbation Theory (MBPT) and Density Functional Theories (DFT). The second phase of the investigation examines the interactions of the fullerene-like silicon "super molecules" with the small carbon clusters. The properties discussed will include bondlengths, ground-state energies, optimum absorbate distances, dissociation channels, and dissociation energies are presented. Possibilities exist for applications in silicon-based electronics at the nano scale. \begin{enumerate} \item A. S. Hira and A. K. Ray, Phys. Rev. \textbf{A 52}, 141(1995); \textbf{A 54}, 2205(1996). \end{enumerate} [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y25.00007: Theoretical Study of Na3-C60 and Na4-C60 Clusters: Pathways to Nanoscale Contacts Daniel Bulnes, Nichole Moya-Leyba, Erica Velarde, Ajit Hira We continue our interest in fullerene-alkali complexes$^{1-2}$ by investigating the physical and chemical properties of Na$_{3}$-C$_{60 }$and Na$_{4}$-C$_{60}$ systems. Five categories of adsorption sites for the alkali atoms on the fullerene molecule, namely fivefold, threefold, midbond-long, midbond-short and top, are considered. Electron correlation effects, using both Many Body Perturbation Theory (MBPT) and Density Functional Theories (DFT) are incorporated in the calculations for binding energies and optimal intermolecular bondlengths. For these complexes, various properties including bondlengths, ground-state energies, optimum absorbate distances, dissociation channels, and dissociation energies are presented. Possibility of tunneling between different sites is investigated. We also examine implications for the fabrication of nanoscale contacts, and for the study of dynamical systems involving C$_{60}$. \begin{enumerate} \item A. S. Hira and A. K. Ray, Phys. Rev. \textbf{A 52}, 141(1995); \textbf{A 54}, 2205(1996). \item A. S. Hira, Billy Terrazas, Erica Velarde and Desirae Vigil, ``An \textit{Ab Initio} Theoretical Study of Alkali-C$_{60}$ and Alkali-(C$_{60})_{n}$ Clusters,'' Bull. Am. Phys. Soc. \textbf{49}, 599 (March 2004). \end{enumerate} [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y25.00008: Hyperfine coupling of endohedral fullerene Sc@C82 Seung Mi Lee, B.J. Herbert, J.C. Green, D. Nguyen-Manh, A. Ardavan, J. van Tol, A.P. Horsfield, G.W. Morley, K. Porfyrakis, D.G. Pettifor, G.A.D. Briggs The hyperfine coupling of the endohedral metallofullerene, Sc@C$_{82}$, which is a candidate qubit for quantum computing, has been investigated theoretically and experimentally. Using density functional theory (DFT), we have systematically studied the molecular structures and energetics of nine isomers of C$_{82 }$and Sc@C$_{82}$ fullerenes. DFT predicts that the most stable isomer has $C_{2v}$ symmetry with the Sc atom lying off-centre along the $C_{2 }$ symmetry axis and forming partially covalent bonds with a carbon hexagonal ring of the fullerene cage. The hyperfine couplings between the unpaired electron spin and the Sc and C atoms have been calculated and compared to the electron spin resonance (ESR) spectra. The experimental isotropic hyperfine coupling constants confirm the $C_{2v}$ symmetry predicted for the ground state isomer. Furthermore, the calculated anisotropy of the hyperfine coupling tensor is in good agreement with low temperature experimental measurements. www.nanotech.org [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y25.00009: Evidence of nanosegregation and Jahn-Teller effect in Na$_2$C$_{60}$ Katalin Kamar\'as, Gy\"ongyi Klupp, P\'eter Matus, L\'aszlo F. Kiss, S\'andor Pekker, Dario Quintavalle, Andr\'as J\'anossy, Norbert M. Nemes, Craig M. Brown, Juscelino Leao Na$_2$C$_{60}$ is the only known solid fulleride salt containing the divalent fulleride ion C$_{60}^{2-}$. Calculations predict a Jahn-Teller distortion of this ion, similar to the A$_4$C$_{60}$ compounds, to which they are related by electron-hole symmetry. However, by combining various experimental methods, we found that divalent ions exist only above 450 K in solids with composition Na$_2$C$_{60}$; at room temperature and below, methods sensitive to molecular symmetry and charge (infrared absorption, ESR, NMR) detect at least two phases, most probably C$_{60}$ and Na$_3$C$_{60}$. We explain our data by a model where nanosegregated regions of the size 3-30 nm with different Na concentration coexist. The concentration gradient disappears at higher temperature by diffusion of sodium, observed by neutron scattering. High temperature infrared spectra show evidence of a uniaxial (D$_{3d}$/D$_{5d}$) distortion of the fullerene balls. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y25.00010: Fermi surfaces of pure and K-doped C$_{60}$ monolayer on Cu(111) surfaces studied by high resolution angle-resolved photoemission spectroscopy K.-D. Tsuei, C.-M. Cheng, C.-C. Wang, J.-Y. Yuh The electronic structure of C$_{60}$ on Cu(111) surfaces has been studied by high-resolution photoemission. The LEED pattern for one monolayer C$_{60}$ on a Cu(111) surface shows a sharp (4x4) pattern. In photoemission the LUMO-derived band is a partially filled by charge transfer from Cu surface and the spectrum is similar to that of gas phase C$_{60}^{-}$. The satellite peaks of LUMO indicate strong electron-phonon coupling. Comparing to spectra of K doped films the undoped 1 ML film was found to have nearly half-filled LUMO or ``K$_{3}$C$_{60}$,'' in contrast to 0.8e found in a recent calculation. [1] The Fermi surface was mapped by angle-resolved photoemission. It shows a clear electron-like pocket near the zone center, unlike the hole-like Fermi surface in ``K$_{3}$C$_{60}$'' on Ag(111), [2] due to different C$_{60}$ orientation. In the case of Cu the Fermi surface near the zone center accounts for only 0.1 electrons. Near the K-point at least one hole-like Fermi surface can be observed. The detailed Fermi surface will be discussed. [1] L.-L. Wang and H.-P. Cheng, Phys. Rev. B 69, 045404 (2004). [2] W. L. Yang et al., Science 300, 303 (2003). [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y25.00011: Understanding the Dynamics of Kr@C$_{60}$ : A Far-Infrared Vibrational Study S. Brown, J. Cao, J.L. Musfeldt, N. Dragoe, A. Revcolevski, Y. Yokoyama, S. Ito, A. Takeda, T. Miyazaki, H. Takagi, H. Shimotani, F. Cimpoesu, K. Kitazawa The C$_{60}$ molecule has remarkable ability to encapsulate atoms inside the hollow carbon cage. Here, we report a high-resolution far infrared spectral investigation of endohedral Kr@C$_{60}$. Unexpected softening of the T$_{1u}$(1) mode ($\sim$528 cm$^{-1}$) and sharp peak at $\sim$600 cm$^{-1}$ have been observed throughout the temperature range of investigation. The response of Kr@C$_{60}$ is compared to that of pristine C$_{60} $, and the spectral differences are discussed in terms of local symmetry and prospects of Kr@C$_{60}$ as a building block for organic superconductors. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y25.00012: Negative Differential Resistance and Current Rectification in C$_{60}$ Multilayers Michael Grobis, Andre Wachowiak, Ryan Yamachika, Michael Crommie Electronic components exhibiting negative differential resistance (NDR) and current rectification (CR) play a crucial role in modern electronic devices. Though most such devices are based on semiconducting technology, several molecular systems have been recently shown to exhibit NDR and CR that involve very different mechanisms. This talk will focus on NDR and CR behavior seen in our scanning tunneling spectroscopy studies of C$_{60}$ multilayers on metal surfaces. The NDR mechanism observed here appears to differ from those seen in previous studies and is consistent with the behavior expected from a bias-dependent tunneling barrier height. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y25.00013: Doping dependence of C$_{60}$ monolayers studied by scanning tunneling microscopy Ryan Yamachika, Andre Wachowiak, Michael Grobis, Michael Crommie The electronic properties of C$_{60}$ compounds can be tuned by charge-doping them with alkali impurities. This results in an interplay between molecular charge transfer, Coulomb repulsion, phonon coupling, and nearest neighbor interactions. Here we present a scanning tunneling microscopy/spectroscopy study of K doped C$_{60}$ monolayers on Au(111). We find that the morphology and electronic structure of C$_{60}$ monolayers change significantly with doping level. In addition to LUMO/LUMO+1 shifts, we observe strong variations in the low-energy local density of states. [Preview Abstract] |
Session Y27: Focus Session: Carbon Nanotubes: Spectroscopies
Sponsoring Units: DMPChair: David Luzzi, University of Pennsylvania
Room: LACC 501 C
Friday, March 25, 2005 11:15AM - 11:27AM |
Y27.00001: Atomically-resolved Imaging of Electrical Bearkdown of Multi-walled Carbon Nanotubes Willi Mickelson, Tom Yuzvinsky, Gavi Begtrup, Adam Fennimore, Andras Kis, Steve Konsek, B.C. Regan, Shaul Aloni, Alex Zettl We report on the controllable electrical breakdown of multi- walled carbon nanotubes (MWCNTs) in air. We see current steps that correspond to the removal of individual walls. This is confirmed by high resolution transmission electron microscopy (HRTEM) of MWCNTs over thin membranes. Using this technique we able to determine how the atomic structure changes the transport through MWCNTs. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y27.00002: Imaging carbon nanotubes by scanning electron microscopy Dmitriy Dikin, Richard Piner, Rodney Ruoff Scanning electron microscopy (SEM) is used as a primary tool for imaging of nanostructures, including carbon nanotubes. Recent developments of the SEM technique have opened new capabilities for sample analysis at the nanoscale with potential industrial and metrological applications. We will discuss selective imaging of single-walled carbon nanotubes on insulators. The role of the electron beam parameters, effect of surrounding media and substrate, and carbon nanotubes properties on their appearance as seen by an SEM are analysed. The comparison of scanning electron microscopy with atomic force microscopy images will also be presented. Our results support some of the commonly accepted opinions about why such images appear as they do, but mostly contradict them. At the same time our discovery opens new analytical possibilities for applications of an SEM. Funding for this work has been provided by a NASA/MSFC Phase II SBIR, Contract No. NAS8-02102, through a subcontract from Lytec, LLC., and by the NSF NIRT Program, Grant No. 0304506. SEM was done at the Electron Probe Instrumentation Centre at Northwestern University. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y27.00003: The spin-probe ESR study of single-walled carbon nanotubes at low temperatures Amarjot Dhami, Shrivalli Bhat, Ajay Sharma, S.V. Bhat Single walled carbon nano-tubes (SWNTs) are ESR silent unless they contain ferromagnetic impurities or conduction electrons. A technique that has been widely used to study ESR silent systems is the so-called spin probe ESR, where a free radical, typically a nitroxide radical, is dilutly doped into the system and ESR studied. In this work, we have incorporated the nitroxide radical TEMPO (2.2,6.6-tetramethyl-1-piperidinyloxyl) inside the SWNTs. The ESR spectra were recorded from 5K to 300 K using an X band EPR spectrometer (Bruker ER 200D). The main result of this work is change in dynamics of the probe molecule at 230 K indicated by the appearance of a narrow triplet signal. The ESR measurements were also done on TEMPO in methanol for the comparative studies in the same temperature range, and in the latter observations, no change in spectra was seen around 230K. The origin of the appearance of this narrow triplet at 230 K indicates the possibility of change in some intrinsic property of the carbon nanotube around 230 K, which has been reflected in the motional characteristics of the spin probe. Thus, our studies show that spin probes, which have, till been used widely in biological samples and polymers, can be used for the further studies of carbon nanotubes to investigate their physical properties. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y27.00004: Thermal Conductivity of Single Wall Carbon Nanotube (SWNT) Epoxy Composites M.B. Bryning, D.E. Milkie, M.F. Islam, J.M Kikkawa, A.G. Yodh Pristine SWNTs were dispersed in epoxy to create composites. Our dispersion procedure achieved fairly uniform SWNT distributions without introducing stabilizing agents that could affect composite performance. We measured thermal and electrical conductivity of the resulting composites. Thermal conductivity increased continuously with increasing SWNT concentration, exhibiting an enhancement of about 30{\%} at a volume fraction of $\sim $0.01. In similar composites made with surfactant stabilized SWNTs, the thermal enhancement was weaker; volume fractions of $\sim $0.03 were needed to achieve a 30{\%} increase. We contrast these findings with electrical measurements on the same samples, which show a percolation-like behavior with a sharp insulator-conductor transition. This work has been partially supported by the NSF through MRSEC grant DMR-0079909 and DMR-0203378, by DARPA/ONR grant N00015-01-1- 0831, and by NASA through grant NAG8-2172. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y27.00005: Formation of long-lived image states around carbon nanotubes. Mikhail Zamkov, N. Woody, B. Shan, H. Chakraborty, Z. Chan, U. Thumm, P. Richard The long-lived electronic states with wave functions enclosing a carbon nanotube (NT) were observed in two-color time-resolved photoemission experiments. These cylindrical ``electronic rings'' constitute a new class of surface image states due to their quantized angular motion. The electron rotation about the axis of the nanotube gives rise to a centrifugal force that virtually detaches the electron charge-cloud from the tube's body. This isolation results in enhanced lifetimes that were measured to be one order of magnitude longer than those of image states forming above metal surfaces. The experiments were performed using the high-purity ($>$ 95{\%}) samples of individual multi walled NTs. Our current efforts are aimed towards the observation of these ``exotic'' states around the isolated double walled NTs as well as in bundles of single walled NTs. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y27.00006: Collective electronic excitations in single wall carbon nanotubes Ricardo Perez, William Que Collective electronic excitations in single wall carbon nanotubes are studied theoretically, using a tight-binding model. Our results suggest that a new theoretical explanation is possible for the controversial nondispersive modes discovered in the electron energy-loss spectroscopy experiment of Pichler et al. on bulk carbon nanotube samples. These modes have been attributed by Pichler et al. to interband excitations between localized states polarized perpendicular to the nanotube axis. This interpretation has been challenged by a theorist Bose [2] who attributed the modes to optical plasmons carrying nonzero angular momenta. Both interpretations suffer from difficulties. We find that the nondispersive modes could be due to collective electronic modes in chiral carbon nanotubes, while the observed dispersive mode should be due to collective electronic modes in armchair and zigzag carbon nanotubes. Momentum-dependent electron energy-loss experiments on individual carbon nanotubes should be able to confirm or disprove this interpretation decisively. [1] T. Pichler et al., PRL 80, 4729 (1998). [2] S. M. Bose, Phys. Lett. A 289, 255 (2001). [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y27.00007: Accurate determination of the atomic structure of multiwalled carbon nanotubes by electron diffraction Zejian Liu, Lu-Chang Qin We report a new method that allows direct, systematic and accurate determination of the atomic structure of multiwalled carbon nanotubes by analyzing the scattering intensities on the non-equatorial layer lines in the electron diffraction pattern. As an example to illustrate the method, we will present complete structure determination of a quadruple-walled carbon nanotube. It was determined that the chiral indices of the four shells were: [32,1], [26,24], [39,25], and [64,2]. All are semiconducting in this case. We also found that the inter-tubular distance varied from 0.36 nm to 0.5 nm with a mean value of 0.42 nm. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y27.00008: Telling the chiral indices of carbon nanotubes from electron diffraction Lu-Chang Qin, Zejian Liu We report a newly developed one-step method that allows direct reading of the chiral indices [u,v], which specify the perimeter vector of a carbon nanotube, from an electron diffraction pattern of the nanotube. We will present a systematic procedure for the actual use of this method. With the chiral indices of a carbon nanotube determined, its atomic structure such as diameter and helicity and its electronic properties such as being metallic or semiconducting are readily obtained. We have examined a large number of individual single-walled carbon nanotubes to study the distribution of the atomic structure of nanotubes in a sample material produced by arc-discharge. Some selected examples will be presented to illustrate the applications of this method. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y27.00009: Pressure effect on multiwalled carbon nanotubes (MWCNT) up to 25 GPa probed by X-ray Raman scattering Malcolm Nicol, Ravhi Kumar, Andrew Cornelius, Michael Pravica, Michael Hu X-ray Raman scattering (XRS) is a sensitive bulk probe that yields information about the local structure of systems and compounds in the soft x-ray regime. The availability of high energy synchrotron radiation has allowed combining this method with different sample environmental conditions such as extreme pressures and temperatures. Here we present the C k-edge measurements under pressure to investigate the bonding changes of multi walled carbon nanotubes (MWCNT) up to 25 GPa. The results show that the MWCNT transform to a superhard phase similar to diamond around 15 GPa. Further compression results in a structural collapse leading to an amorphous state. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y27.00010: The Neutron Scattering Length Density of Carbon Nanotubes Howard Wang, Gregory Christopherson, Tom Xu, Derek L. Ho, Erik K. Hobbie Contrast matching experiments have been carried out to obtain the neutron scattering length density (SLD) of carbon nanotubes (CNT). The quantitative variation of SLDs of CNTs from various sources and with various processing histories is related to their structure characteristics and elemental impurity. The neutron SLD is particularly sensitive to the inclusion of hydrogen in CNT samples. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y27.00011: Single-wall nanohorn structure and distribution of incorporated materials Alan Maigne, Alexandre Gloter, Kumiko Ajima, Katsuyuki Murata, Masako Yudasaka, Christian Colliex, Sumio Iijima Single-wall carbon nanohorns (SWNHs) are unique spherical-aggregates of single-wall carbon quasi-nanotubes. So far, the observable area has been limited to the aggregate surfaces. We studied core-region structure with TEM using thickness measurement method, EELS, and EDS, and found that carbon density was uniform over the whole aggregate. This result allows to modelize the core-region and to clarify previous models of SWNHs. We used same tools to investigate the incorporation of materials such as fullerenes or platinium compounds. We found that particles can even be incorporated in the core-region and that their distribution in the aggregate depends on their concentration. The information available with these models should be useful in the study of SWNH applications to, for example, drug delivery system. [Preview Abstract] |
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Y27.00012: Drugs incorporation in single-wall carbon nanohorns Kumiko Ajima, Masako Yudasaka, Alan Maigne, Tatsuya Murakami, Kiyotaka Shiba, Sumio Iijima To apply single-wall carbon nanohorns (SWNHs) as drug carriers in drug delivery systems, we have been studying the drug incorporation inside SWNHs. We incorporated several drugs in SWNHs (DR@SWNHs) at room temperature by liquid-phase methods, such as Nano-Titration and Nano-Precipitation. Whether DR was inside or outside of SWNHs was judged by X-ray diffraction, and the elements of DR inside SWNHs were identified by energy dispersive X-ray spectroscopy. The release of DR in phosphate-buffered saline was measured with atomic absorption spectroscopy, which indicated slow and fast release processes. The biological integrity of the released DR was studied with cultured cells. These results indicate that SWNHs will be useful for drug carriers. [Preview Abstract] |
Session Y30: Solid Amorphous Polymers
Sponsoring Units: DPOLYChair: Greg McKenna, Texas Tech
Room: LACC 505
Friday, March 25, 2005 11:15AM - 11:27AM |
Y30.00001: Effective Medium Theory of the Translation-Rotation Paradox for Probe Diffusion in Glass Grigori Medvedev, James Caruthers Whereas classic Stokes-Einstein and Debye (SED) theories state that the ratio of translational and rotational diffusion coefficients for a particle in a viscous fluid is a constant, studies of the probe diffusion in supercooled liquids have shown that these diffusion coefficients have essentially different temperature dependencies. The dynamic heterogeneity of the medium near glass transition has been postulated to be the source of the apparent violation of the SED prediction in the numerical simulation by Ediger and co-workers; however these authors did not deal with the rotational diffusion explicitly, estimating it as a simple average over all spatial locations. We expand the Zwanzig theory of diffusion in a dynamically disordered medium to include both translational and rotational diffusion. Our main result is that there exists a range of fractions of ``fast'' domains in which the translational diffusion is already above its percolation threshold, and thus enhanced, whereas the rotational diffusion is still below its percolation threshold, and thus controlled by the ``slow'' domains. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y30.00002: A Comparison of Techniques for Analyzing Dielectric Relaxation Spectra Containing DC Conductivity Chad Snyder Because of its versatility and broad frequency range, dielectric relaxation spectroscopy (DRS) continues to find new applications in materials science. The one limitation of the technique is that DC conductivity ($\sigma _{DC}$) can overwhelm the dielectric loss spectra ($\varepsilon ^{\prime \prime}(\omega )$), making meaningful analysis difficult. As a result of this difficulty, a number of methods have been developed over the years to estimate $\sigma _{DC}$ and thereby "remove" it from $\varepsilon ^{\prime \prime }(\omega )$. A comparison is made of the benefits and limitations of several of these methods, including graphical methods based on complex plane analysis, curve fitting, and numerical Kramers-Kronig methods. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y30.00003: Evolution of entanglements in crazing of glassy polymers Robert S. Hoy, Mark O. Robbins Highly entangled polymer glasses often fail via crazing [1]. The polymer expands by a large factor $\lambda$ from an initial dense state to a craze network of fibrils and voids. The value of $\lambda$ is found to correlate with the chemical distance between entanglements in both experiments [1] and simulations [2], indicating that the entanglements act like chemical bonds in limiting the expansion. We have applied the primitive path analysis method developed by Everaers et. al. [3] to follow the real space structure of entanglements in model polymer glasses during crazing. A wide range of initial states corresponding to melts with different Kuhn lengths and entanglement lengths was studied. In each case the primitive paths deform affinely and the number of entanglements remains constant during craze formation. Straining the craze past $\lambda$ leads to a gradual reduction in the number of entanglements, and ultimately to craze fracture.\newline 1. E. Kramer and L. L. Berger, Adv. Polym. Sci. {\bf 91/92}, 1 (1990).\newline 2. J. Rottler and M. O. Robbins, Phys. Rev. E {\bf 68}, 011801 (2003).\newline 3. R. Everaers, {\it et al.}, Science {\bf 203}, 823 (2004). [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y30.00004: Heat Capacity of Poly(vinylmethylether) in the Presence and Absence of Water M. Pyda, K. Van Durme, B. Wunderlich, B. Van Mele Heat capacity, $C_{p}$, of poly(vinylmethylether), PVME, without and with water has been measured from 5-375 K, using adiabatic and differential scanning calorimetry. The PVME has a glass transition at 248 K. At lower temperatures, $C_{p}$ of the solid is linked to group and skeletal vibrations. The skeletal vibrations are described by a Tarasov equation with three characteristic temperatures. The experimental $C_{p}$ agrees to better than $\pm $3{\%}. The $C_{p}$ of the liquid is 72.36 + 0.136~$T$ [J/(K mol)] and compares well with results estimated from constituent groups of other polymers using the ATHAS Data Bank. The calculated solid and liquid $C_{p}$ was used as baseline for the analysis of amorphous PVME with different thermal history, to follow the crystallization, melting, mixing, and demixing within the PVME-water system. Also, knowing $C_{p}$ of the solid and liquid, the enthalpy, entropy, and Gibbs function of glassy and liquid PVME was calculated.. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y30.00005: Dynamics of phenylene rings in engineering thermoplastics. A quasielastic neutron scattering study Arantxa Arbe, Silvia Arrese-Igor, Iban Quintana, Angel Alegria, Juan Colmenero, Bernhard Frick By using quasielastic neutron scattering we have contributed to the long-standing question of the molecular motions behind the secondary relaxations in engineering thermoplastics. The phenylene Hydrogen motions in polyethersulfone and bisphenol-A polysulfone, polycarbonate and phenoxy have been studied by combining time of flight and backscattering techniques in the time range from about 0.1 ps to 1ns. After vibrations and fast processes below 2 ps, the Hydrogen motions reveal relatively fast oscillations and 180$^{\circ}$ flips of the rings in all cases. Polycarbonate phenylene groups additionally show rotations of about 90$^{\circ}$. Relationships among the microscopically observed motions and the secondary relaxations as followed by mechanical and/or dielectric spectroscopy are established. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y30.00006: Chemical Vapor Deposition of Polybenzoxazole Precursors Mitchell Anthamatten, Xichong Chen Polybenzoxazoles are well known for their outstanding thermal and mechanical properties. We will describe a new solventless, chemical vapor deposition approach to fabricating films of high strength polybenzoxazoles. Advantages of such an approach are: 1) control of film thickness, 2) conformal coating onto substrates, and 3) potential improvement of in-plane mechanical properties. Our process involves the vacuum evaporation of two monomers: a hydroxyl-functionalized diamine (DHB) and a dianhydride (PMDA). The resulting films appear uniform and contain poly(amic acid) linkages. FTIR experiments show that the poly(amic acid) converts to a polyimide material upon curing at 150 to 200 degrees C. At higher temperatures there is evidence that benzoxazole linkages form, though, interestingly, we also observe a concomitant decrease in the film's mass. Our current studies aim at understanding these high-temperature processes and how they impact the polymer's crystallinity and mechanical properties. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y30.00007: Depletion Effect on Self-Organization of atactic Polymer Chain Segments in Microcells Zhaoqu Wang, Kaiyi Liu, Bo Che, Gi Xue Glass transitions for atactic polystyrene and poly(methyl methacrylate) prepared in nano-cells by microemulsion polymerization in the presence of non-solvents were measured by DSC experiments. An increase in $T_{g }$was observed for these polymers. The first DSC scan for the virgin polymers indicated $T_{g}$ at 112 $^{o}$C and 138 $^{o}$C for$ a$-PS and $a$-PMMA, respectively. While the re-precipitated samples show $T_{g}$ at 105 $^{o}$C and 125 $^{o}$C, respectively. Solid-state NMR relaxation and wide angle X-ray diffraction experiments indicated that the virgin polymer powders were in compact and partially self-organized states. The re-precipitated $a$-PS and $a$-PMMA did not show any self-organization under the same experimental conditions, although there are no changes in molecular weight or tacticity of the polymer chains. A depletion-interaction phenomenon was understood to provide entropic force for the self-organization of polymer chains inside the walls of the microemulsion cells during polymerization. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y30.00008: Structure Effect on the Crosslinking Behavior of Bismaleimides Jung Park, Sung Jang The molecular structure effect on the cure characteristics of some bismaleimides (BMIs) was investigated. Four different types of BMI resins were synthesized via addition reaction of diglycidyl ether of bisphenol A with N-(3-carboxy phenyl) maleimide or N-(4-carboxy phenyl) maleimide. The change in the chemical structure was confirmed by FTIR and NMR. Kinetic parameters were determined from dynamic and isothermal differential scanning calorimetry (DSC). The maximum conversion as a function of temperature was compared with the viscosity change during cure. Both DSC and chemorheology studies show that the overall curing rate is faster for meta substituted BMIs than para substituted ones. At a fixed heating rate in the chemorheology study, viscosity started to increase at higher temperature for meta substituted BMIs, compared with para substituted BMIs. The asymptotic final viscosities were higher for shorter chains due to their resultant higher crosslinking densities. The substituent position showed much greater effect on the cure kinetics than the chain length within the experimental range of this study. The activation energies of the cure reactions were obtained from both DSC and viscosity measurements. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y30.00009: Frontal photopolymerization and applications in complex fabrication Joao Cabral, Steven D. Hudson, Christopher Harrison, Jack Douglas The exposure of a photopolymerizable liquid to light may leads to a propagating wavefront of network formation that invades the unpolymerized material from the illuminated surface. We theoretically describe this light-driven frontal photo-polymerization (FPP) process, which is the basis of many commercially important fabrication methods, in terms an order parameter phi(x,t) characterizing the extent of monomer-to-polymer conversion, the temporally and spatially evolving optical attenuation T(x,t) of the medium, and the height h(t) of the resulting solidified material. The non-trivial aspects of this frontal polymerization process derive from the coupling of miu(x,t) and the growing non-uniform network h(x,t) and we consider limiting situations in which the optical attenuation increases (`photodarkening') or decreases (`photobleaching') in time. Since FPP fabrication of complex three-dimensional structures containing components having different material characteristics would greatly extend the practical utility of this method, we explore the influence of nanoparticle (silica, titania, and multi-wall carbon nanotube) additives on FPP front propagation. We also characterize the influence of temperature on the kinetics of FPP since this factor can often be controlled in practice. Our results are utilized in the fabrication of complex structures and in particular, of microfluidic masters and devices. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y30.00010: Chain Length Dependence of the Thermodynamic Properties of Linear and Cyclic Alkanes and Polymers Sindee Simon, Dinghai Huang, Gregory McKenna The specific heat capacity was measured with step-scan DSC for linear alkanes from pentane to nonadecane, for several cyclic alkanes, for linear and cyclic polyethylenes, and for a linear and a cyclic polystyrene. For the linear alkanes, the specific heat capacity in the equilibrium liquid state decreases as chain length increases; above a carbon number N of 10 (decane) the specific heat asymptotes to a constant value. For the cyclic alkanes, the heat capacity in the equilibrium liquid state is lower than that of the corresponding linear chains and increases with increasing chain length. At high enough molecular weights, the heat capacities of cyclic and linear molecules are expected to be equal, and this is found to be the case for the polyethylenes and polystyrenes studied. The thermal expansion coefficients and the specific volumes of the linear and cyclic alkanes are also calculated from literature data and compared with the trends in the specific heats. [Preview Abstract] |
Session Y31: Polymer Melts and Solutions
Sponsoring Units: DPOLYChair: Howard Wang, Michigan Tech University
Room: LACC 503
Friday, March 25, 2005 11:15AM - 11:27AM |
Y31.00001: Shear SANS Study of Entangled Polymer Solutions Howard Wang, Lionel Porcar, Derek L. Ho, Prashant Tapadia, Shi-Qing Wang, Michael Olechnowicz, Roderic Quirk In situ small angle neutron scattering and rheological measurements have been carried out on entangled polybutadiene solutions in simple shear flow in a Couette shear cell (SANS) and cone/plate cell (rheology). Stable shear flow has been achieved over a range of shear rates covering the stress plateau, at which polymer chains are expected to partially disentangle [1]. Shear-induced structure was recorded in both the flow-vorticity and gradient-vorticity planes. The data appeared to imply the absence of ensemble averaged large chain deformation in steady state, whereas geometrical scaling analysis suggested that chain stretching could occur locally. [1] Tapadia, P.; Wang, S. Q. Macromolecules, 37, 9083 (2004). [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y31.00002: Molecular Weight Dependence of the Viscosity of Polyethylene Macrocycles Jian Wang, Gregory McKenna, Diego Benitez, Irina Gorodetskaya, Robert Grubbs The Grubbs group at Caltech is using a new method of synthesis for the production of macrocycles that avoids the problems of sample contamination with long chain linear molecules. They have produced macrocyclic polyethylenes having molecular weights as high as 400,000 g/mole. At Texas Tech University rheological measurements are being made using both the Plazek Magnetic Bearing Torsional Creep Apparatus and a Paar Physica controlled stress rheometer. Because the samples are polydisperse, with M$_{w}$/M$_{n}$ values of 1.6-1-9, we here report only the results of the viscosity as a function of molecular weight. Surprisingly, unlike reports in the literature for polystyrene rings or polybutadiene rings, the viscosities of the polyethylene rings are larger than those for equivalent molecular weight linear polyethylenes and the viscosity increases approximately exponentially with weight average molecular weight. Further results will be reported at the meeting. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y31.00003: Thermorheological Complexity in Polystyrene Melt Yn-Hwang Lin Using the successful description of creep compliance$ J(t)$ of nearly monodisperse polystyrene melts$^{1}$ in terms of the extended reptation theory$^{2}$ (ERT) in the rubber(like)-fluid region as the\textit{ reference frame} in time, the analysis of the glassy-relaxation process$ A_{G}u_{G}(t)$ that occurs in the short-time region of $J(t)$ in terms of a stretched exponential form incorporated into ERT reveals that the temperature dependence of the $A_{G}u_{G}(t)$ process being stronger in a simple manner than that of the \textit{entropy-derived} ERT processes accounts fully for the uneven thermorheological complexity in the $J(t)$. The results being displayed in the modulus $G(t)$ form, it is shown that at $T_{g}$, the contribution from$ A_{G}u_{G}(t)$ to $G(t)$ at the time scale corresponding to the highest Rouse-Mooney normal mode greatly exceeds that derived from entropy, indicating vitrification at the Rouse-segmental level. At the same time the Rouse-Mooney normal modes provide an internal yardstick for estimating the length scale of the polymer at $T_{g}$, giving 3 nm for polystyrene. Based on the obtained results, the basic mechanism for the thermorheological complexity is analysed, showing that the break-down of Stoke-Einstein relation in glass-forming liquids, such as OTP, should occur for a similar reason. Ref: (1) D. J. Plazek, \textit{J. Phys. Chem.} \textbf{1965}, $69$, 3480; \textit{J. Polym. Sci. A}-2 \textbf{1968}, $6$, 621. (2) Y.-H. Lin, \textit{Macromolecules} \textbf{1984}, $17$, 2846; \textbf{1986}, $19$, 159; 168; \textbf{1987}, $20$, 885; \textbf{1999}, $32$, 181; \textit{Polymer Viscoelasticity}:\textit{ Basics, Molecular Theories and Experiments}; World Scientific: Singapore, 2003. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y31.00004: Nonlinear hydrodynamic description of non-Newtonian fluids Harald Pleiner, Mario Liu, Helmut R. Brand We review conventional constitutive equations for non-Newtonian fluids from a hydrodynamic point of view. Using general thermodynamic and symmetry arguments and applying valid physical principles we describe viscoelasticity by setting up nonlinear dynamic equations either for a relaxing (Eulerian) strain tensor or for a transient orientational order parameter tensor. This covers the usual non-Newtonian effects, like shear thinning, strain hardening, stress overshoot, normal stress differences and non exponential stress relaxation. In both cases an effective dynamic equation for the stress tensor can be derived approximately and compared with conventional non-Newtonian rheological models. It is more general in structure than those, comprises most, restricts some, and discards a few of them. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y31.00005: Mechanical Hole Burning Spectroscopy of Branched and Linear Polymers Xiangfu Shi, Gregory McKenna We have developed a mechanical spectral hole burning (MSHB) scheme that is analogous to non-resonant dielectric spectral hole burning (DSHB). DSHB experiments have been performed close to the glass temperature and interpreted in terms of dynamic heterogeneity. Here we find that holes are burned far above the glass temperature and in the terminal regimes for a branched polymer melt and a polymer solution. The results suggest that MSHB is a potentially powerful tool with which to examine dynamics of complex fluids. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y31.00006: Stress relaxation of polymer networks containing low concentrations of dangling chains and star shaped polymers Daniel A. Vega, Leopoldo R. G\'omez, Marcelo A. Villar, Enrique M. Vall\'{e}s We analyze the influence of low concentrations of star and dangling polymer chains on the stress relaxation process of model polymer networks. Model PDMS networks with well defined structure were obtained by the hydrosylilation reaction, based on the addition of hydrogen silanes from a trifunctional cross- linker to end vinyl groups of $\alpha - \omega$ polydimethylsiloxane chains. Rheological characterization was carried out in a rotational rheometer by dynamic and stress relaxation tests. Viscoelastic properties of the networks depend strongly on the molecular weight of the stars or pendant chains. It was found that a modified Pearson-Helfand model provides a very good fit to the behavior of these networks. This model incorporates the effect of higher Rouse modes on the arm retraction [Milner and McLeish, Macromolecules, 1997] and the potential for arm retraction originally proposed by Doi and Kuzuu. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y31.00007: Stress Relaxation of 1,4-polyisoprene T- and Y-shaped Star Polymers Jung Hun Lee, Lynden A. Archer We discuss the stress relaxation of 1,4-polyisoprene T- and Y-shaped star polymers in order to identify the effect of branch-point on the relaxation dynamics. The effects of arm length asymmetry and the position of branch-point on the stress relaxation are investigated by varying molecular architecture from T-shaped to Y-shaped. We observed the measured loss modulus more sensitively reflects a transition in the governing dynamics from star-like to linear-like according to the given architectures. This finding suggests a possibility to describe the dynamic transition by the constituent structural terms. A simple relationship between the branch-point motion and the given architecture is discussed based on the widely used microscopic model frame for the branched polymers. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y31.00008: When does a molecule become a polymer? Yifu Ding, Alexander Kisliuk, Vladimir Novikov, Alexei Sokolov The molecular weight (MW) dependences of chain, segmental and fast dynamics have been studied for a few polymers. These properties (except chain dynamics) appear to have similar MW dependence that they all saturate when chain approaches Gaussian coil behavior. Chain dynamics in PDMS reaches asymptotic Rouse prediction at the same MW. We demonstrate that the difference in the MW dependence for various polymers does not correlate to either the difference in the Kuhn length or Me. We introduce an additional parameter, mR (MW for each random step) that might be important for characterizing chain statistics and the MW dependence of many physical properties. The most intriguing result is that the MW dependence of the fast dynamics, elastic property and fragility observed in PS is opposite to the one observed in PIB. We speculate that the difference in symmetry of the structural unit is responsible for the opposite behavior. Based on this idea, predictions for the dependence of the fast dynamics and fragility in polymers on MW and tacticity are formulated. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y31.00009: Pressure effects on Solutions of Diblock Copolymers: Small Angle Neutron Study Dvora Perahia, Gang Cheng The effects of pressure on the shape of the micelles of Polystyrene-b-Polyisoprene (PS-PI) in decane were studied as a function of temperature and concentration using small angle neutron scattering (SANS). Decane is a preferential solvent for the PI block, thus driving the association of the polystyrene groups. These diblocks self-assemble in decane to form star-like spherical micelles with a swollen polystyrene core and a diffuse polyisoprene corona. All the solutions studied exhibit a critical micelle temperature, i.e. temperatures under which most of the polymer molecules are within the aggregates. In solutions consisting of none-interacting micelles, with increasing pressure, increasing the pressure up to 2000 PSI, results in changes of the shape of the micelles from spherical to elongated objects. At higher concentrations, the inter-micellar interactions increase significantly. Changing the pressure affects the miscibility of the different components and as such the structure of the basic aggregates. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y31.00010: Brownian Dynamics Study on the Dynamics of Asymmetric and Symmetric Star-Branched Polymers in Dilute Solutions Yong Lak Joo, Yongmin Lee It has been known that the dynamical behavior of branched polymers can significantly be different from linear polymers even in dilute solutions. The goal of this investigation is to understand detailed configuration and stretching dynamics of polymers with complex branching topologies in dilute solutions under various flows. In particular, we are interested in the transient stress response and conformational hysteresis of star-branched polymers in shear and extensional flows. Using Brownian dynamics simulations of bead-spring models, systems such as dilute solutions of Y-branched and star-branched polymers under flows are investigated. Studies on polymers with asymmetric arms reveal that the initial transient response is governed by both the number of arms and the shortest arm, whereas the stress response at intermediate strains becomes dominated by the longest arm. The results obtained from stress-conformation hysteresis simulations indicate that rapid extension followed by retarded relaxation of short arms in star-branched molecules. Finally, excluded-volume interactions are incorporated through the Lennard-Jones potential, and are applied to the study of the dynamics of branched polymers under confinement. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y31.00011: Diffusive Transport in Hydroxypropylcellulose:Water Kiril A. Streletzky, George D.J. Phillies, Robert O'Connell, Paul Whitford, Helen Hanson A systematic analysis of mode structure of diffusive relaxations in solutions of neutral polymer of 1MDa hydroxypropylcellulose (HPC) is presented. Experimental techniques that have been applied in our studies include static light scattering, dynamic light scattering, optical probe diffusion, and viscometry. In the optical probe diffusion method we monitor the translational diffusion of dilute monodisperse spheres through aqueous polymer solutions over a range of temperatures, distances, and time scales. Based on the data from these extensive studies we conclude that: 1) HPC solutions have a characteristic length of 50-70nm that matches the polymer’s hydrodynamic radius; 2) the diffusion rate of optical probes through aqueous polymer solutions is not determined by the macroscopic viscosity of the solutions; 3) probe and polymer relaxations are not generally the same; 4) the apparent viscometric crossover near 6g/L is confirmed by the optical probe behavior. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y31.00012: Multiple Light Scattering Probes of Polyurethane Foam Structure Weijun Zhou, Dwight Latham, Anne Leugers The cellular structure of polyurethane foams is probed quantitatively by investigating the multiple scattering characteristics of foams with near IR wavelength light. In the limit of strong multiple scattering regime, the propagation of light can be approximated as a diffusion process. Static diffusive transmission (T{\%}) of foams showed a rather interesting relationship with foam thickness (L) as T $\sim $ 1/L. This is consistent with what one expects from diffusion theory. The transport mean free path of light diffusion was determined to correlate directly with foam cell size. But surprisingly, the transport mean free path of light within polyurethane foam was found to be smaller than cell size, suggesting a strong intra-cellular scattering mechanism. Applications of multiple light scattering techniques to dynamic foaming process will also be discussed. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y31.00013: Scaling laws for polymer chains using mesoscopic simulations Vasileios Symeonidis, Bruce Caswell, George Karniadakis Several types of bead-spring representations of chains are used to model simulations of dilute polymer solutions by dissipative particle dynamics (DPD). The static exponent scaling law is used to detect the prevalence of phantom collisions. For the models used, values range between 1/2 and 3/5, the Flory value for self-avoiding walks. Chain behavior under steady shear shows no similar scaling law, but different stretching response for finitely and infinitely extensible springs. The effect of temperature is also investigated for a chain of five, ten and twenty monomers. [Preview Abstract] |
Session Y34: Surfaces and Interfaces II
Sponsoring Units: DCPChair: Andrew Rappe, University of Pennsylvania
Room: LACC 511A
Friday, March 25, 2005 11:15AM - 11:27AM |
Y34.00001: Density Functional Approach to Investigate the Stability of Nanobubbles Gota Kikugawa, Shu Takagi, Yoichiro Matsumoto Recently, some experimental results showing the existence of nanobubbles which are bubbles of tens or hundreds of nanometer diameter in water have been reported. However, it is generally considered that such tiny bubbles are inherentlyunstable because of a strong effect of surface tension. Therefore, the stability of the nanobubbles are regarded as an unresolved problem. In order to investigate the stability mechanism, our approach is based on a microscopic standpoint. Especially, we introduced the density functional theory of classical systems to take into account the microscopic information on gas-- liquid interface and to execute the stability analyses in the microscopic system. The density functional approach has many advantages in the stability analysis of the two-phase microscopic system compared with the molecular dynamics approach. First, natural external conditions such as the grand canonical ensemble can be imposed. Second, the free energy evaluation which is required in the stability analyses can be performed directly. In this study, we focused on the effect of impurities in water on the stability of the nanobubbles. In the experimental system, electrolytes or surfactants are usually used in the literature, and we consider that the effect of the impurities is crucial. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y34.00002: Surface X-ray Scattering Observation of High-Density CO Monolayers on Pt(111) A. Menzel, Y.V. Tolmachev, V. Komanicky, A.V. Tkachuk, Y.S. Chu, H. You We report on surface x-ray scattering studies of high-density long-range ordered CO adsorbates on Pt(111) in equilibrium with gas phase at near-atmospheric CO partial pressures. We adsorbed CO with an electrochemical method and transferred the sample into a controlled gas atmosphere. At CO partial pressure $P_{\mathrm{CO}} \sim 1\,\mathrm{atm}$ we found long-range ordered $\left(2\! \times\!2\right)$-3CO, at $P_{\mathrm{CO}} < 0.5\,\mathrm{atm}$ we found $(\sqrt{19}\!\times\!\sqrt{19})\!R23.4^{\circ}$-13CO. These structures do not depend on the electrochemically controlled initial coverage, but are only a function of CO partial pressure and temperature. Based on our x-ray studies, we propose a phase diagram of CO monolayers on Pt(111) under near atmospheric pressure of CO. We will discuss the parallel behavior of CO monolayers in high-pressure atmospheres and those in electrochemical environment. However, there is disagreement between our results and those of recent studies of CO adsorbed from the gas phase, where $(\sqrt{19}\!\times\!\sqrt{19})$ has been reported for $P_{\mathrm{CO}}=1\,\mathrm{atm}$ and room temperature [Longwitz \emph{et al.}\ JPCB \textbf{108} 14497 (2004)]. Similar differences have been reported for studies in UHV when comparing gas adsorbed CO structures and electrochemically prepared structures studied \emph{in vacuo} [Zurawski \emph{et al.}\ JPC \textbf{94} 2076 (1990)]. We will address plausible causes for these discrepancies. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y34.00003: Probing and modeling orbital-specific contributions to chemisorption using DFT Sara E. Mason, Ilya Grinberg, Andrew M. Rappe We present a DFT study of atomic and molecular chemisorption spanning a variety of transition metals, facets and adsorption geometries. We cast our results in a physical model for chemisorption that takes into account which metal and adsorbate orbitals interact to form chemisorption bonds. We use DFT chemisorption energies, orbital overlaps and changes in charge density induced by adsorption to track how the metal and adsorbate states interact as a function of metal identity, surface facet, adsorption geometry and strain. We introduce strain to the adsorption systems as a probe that causes relatively small, but geometrically specific, changes to electronic structure of the metal. By taking into account orbital specific contributions to chemisrotpion, our model is able to reproduce subtle differences in chemisorption present in our DFT results and offers a means for predicting adsorption energies on different metal surfaces at different sites and in different states of strain. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y34.00004: Electronic structure and bonding properties of potassium (K) on graphite under external electric field. Alejandro Tapia, Romeo de Coss, Gabriel Canto The effect of an external electric field on the potassium (K) adsorption on the graphite surface, are studied by means of first-principles total-energy calculations. The results were obtained with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. The structural parameters, bonding properties, and electronic structure of the K-graphite system are studied in the triangular $(2\times 2)$ overlayer phase as a function of the external electric field magnitude. We find an important change in the K-graphite bonding as a consequence of the charge transfer from the adatom towards the substrate induced by the electric field. The results are discussed in the light of the experimental observed difussion of K into graphite induced by external electric fields. This work was supported by Consejo Nacional de Ciencia y Tecnolog\'ia (CONACYT, M\'exico) under Grants No. 43830-F and No. 44831-F. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y34.00005: Chemical properties of Au nanoparticles on oxides Ruqian Wu Using the density functional DMol, VASP and FLAPW approaches, we studied the electronic and chemical properties of Au nano-particles on oxide substrates with O vacancies. Au clusters of more than 5 atoms are found to be unstable on MgO(001). Many configurations are explored for Au clusters on TiO$_{2}$(110) and SiO$_{2}$(0001). We found interesting size and shape dependence of their chemical properties, characterized by the density of states, HUMO/LUMO features and core level shifts. For substrates, we found that even SiO$_{2}$ monolayer displays a sizable gap on Mo(110). [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y34.00006: Quantum Delocalization and Hydrogen Adsorption on Pd (111) Surface Seong-Gon Kim, Sungho Kim, Steven C. Erwin The quantum delocalization of hydrogen atoms near the hydrogen atom vacancies on a palladium (111) surface is studied using density functional theory (DFT). Our quantum delocalization model elucidates the puzzle presented by a recent experiment [T.~Mitsui, \textit{et al}, Nature \textbf{422}, 705 (2003)]. In this experiment, Salmeron and his coworkers reported, contrary to conventional belief, that two-vacancy sites are inactive and that aggregates of three or more hydrogen vacancies are required for efficient hydrogen molecule dissociation and adsorption on a palladium (111) surface. Our total energy calculation shows that one or two hydrogen atoms are delocalized over three available adsorption sites. We found that delocalized vacancies provide a very compelling mechanism to explain the ineffectiveness of two-vacancy sites. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y34.00007: Density functional theory study of GaSb(100) surface reconstruction Jeffery Houze, Sungho Kim, Seong-Gon Kim We use density functional theory to study the (100) surface of GaSb semiconductors. 2x2 reconstructions of the Ga and Sb terminating surfaces revealed the following patterns. The Ga terminating surface exhibits a pattern alternating between dimerization and buckling. The Sb terminating surface exhibits only dimerization with each pair alternating in height. Simulations of 2x3 reconstructions will be presented to indicate possibility of more complex patterns. Surface and reconstruction energies will be given in support of number of fixed layers used for simulating bulk, and number of free layers needed to adequately simulate the reconstructions. Charge density plots will also be presented to explain interaction between surface atoms. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y34.00008: Hydrogen Adsorption on Stepped Surfaces of Metal Nickel Huiqiu Deng, Haixia Xiao, Wangyu Hu The chemisorption of gas molecules on transition-metals' surfaces has been an attractive field due to its importance in understanding the mechanisms of catalytic reactions. In the present paper the adsorption of hydrogen on nickel stepped surfaces (210), (211), (311), (410), (511), (977) and the low-index surfaces are studied with the embedded-atom model (EAM) many-body potentials which are based on density functional theory. The stable adsorption sites of hydrogen atoms are determined by the adsorption energies. The calculated results show that there exist more active adsorption sites near the steps. It is found that the stepped surfaces affect the adsorption properties of hydrogen seriously. The dissociative adsorption pathways of hydrogen molecules on the different nickel surfaces are also investigated. The results calculated in the present paper are in good agreement with the available experiment data and other theoretical values. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y34.00009: Adsorption Isotherm studies of Methyl Bromide adsorbed on Magnesium Oxide Teresa Burns, Michael Sprung, John Larese Understanding the interaction of polar molecules with ionic surfaces is technologically very important. Using high precision, volumetric adsorption isotherms the layering properties of methyl bromide on the MgO(100) surface were examined between 164 K and 179 K. Methyl bromide (Triple point = 179.49K) is found to exhibit two layering transitions within this temperature interval. Thermodynamic quantities derived from this study including the layering transition temperatures, the 2D compressibility, layer enthalpy and entropy of adsorption, and the isosteric enthalpy of adsorption will be presented. Comparisons with the adsorption properties of methyl chloride and methyl iodide will also be included. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y34.00010: The Role of the Ferroelectric Nature of a Substrate on its Ability to absorb Carbon Dioxide E. Ramos-Moore, Jurgen Baier, A. L. Cabrera The CO$_{2}$ adsorption on potassium niobate (KNbO$_{3})$ and potassium tantalate (KTaO$_{3})$ was studied in order to compared a ferroelectric and a parelectric surface towards CO$_{2}$ adsorption. Both oxides has Perovskite structure but when the niobate is ferroelectric at room temperature the tantalate is not. Characterization of the oxide powders was made by XRD, SEM and Micro-Raman spectroscopy. The adsorption of carbon dioxide was performed by TPD using the same system were BET surface area was obtained. TPD results will be displayed and discuss in the presentation. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y34.00011: Vibrational Spectroscopy of Alcohol/ Silica Interfaces Weitao Liu, Luning Zhang, Y. Ron Shen Sum-frequency vibrational spectroscopy was used to probe alcohol/silica interfaces in order to study how methanol, ethanol, and 1- and 2-propanol molecules orient at the interfaces. It was found that in the CH stretch range, the symmetric CH$_{3}$ peaks of all alcohols decreased comparing with the air/liquid interface, while the spectra of ethanol and propanol exhibited prominent antisymmetric CH$_{3}$ peaks. The results can be understood by knowing that the first interfacial monolayer of alcohol molecules must adsorb with their OH terminal on the hydrophilic silica and the CH$_{3}$ terminal toward liquid, but the adjacent less ordered alcohol monolayer should have the reverse orientation. Because the CH$_{3}$ groups of the two monolayers are oppositely oriented, their symmetric stretch modes tend to cancel each other in SFG, but the antisymmetric stretch modes generally do not. In the case of methanol, the antisymmetric stretch is intrinsically weak in the sum-frequency spectra, making the overall spectra very weak. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y34.00012: From cluster to bulk: Size dependent energetics of silica and silica-water interaction Yao He, Chao Cao, Yin-Xia Wan, Hai-Ping Cheng We present our computational studies on the energetics of clusters that consist of H2O and SiO2 using first-principles Born-Oppenheimer molecular dynamics method. Cohesive energy and hydration energy of both pure (or dry) and hydroxylated (or wet) ring-structured clusters are investigated as functions of system size. We have found clear trends in both energy values as the cluster size increases. Energetics of a small silica rod that contains 108 atoms is also obtained as a middle reference point for size evolution. The interaction of the nano-rod with water molecular has been investigated thoroughly at various sites. Results from cluster calculations are compared with one from bulk quartz and cristobalite calculations. (Acknowledgement: This work is supported by NSF/ITR grant). [Preview Abstract] |
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Y34.00013: Nonlinear Optical Probe of Buried Interface of Organic Thin Films Susan Dounce, Minchul Yang, Tom Rockey, Hai-Lung Dai Nanometer-thick thin films of small organic molecules have found many recent technology applications in organic semiconducting devices where the interfacial layer structure presumably greatly affects the film electrical properties. In this presentation we show that by using a combination of nonlinear optical and surface science techniques we can determine the intrinsic structure within the molecular films including the orientation and alignment of the molecules at the film/substrate interface. It is demonstrated by using optical second harmonic generation that the pyridine molecules in the interfacial layer underneath the film are found to align along the [11bar0] direction of the Ag(110) surface with a small tilt angle (11 degrees) from the surface norm. This interfacial ordering is found to have a notable effect in inducing crystallization at the heterogeneous boundary of the amorphous molecular film. [Preview Abstract] |
Session Y35: Materials
Sponsoring Units: DCPChair: Mark Ratner, Northwestern University
Room: LACC 511B
Friday, March 25, 2005 11:15AM - 11:27AM |
Y35.00001: Polymeric CO: A new class of High Energy Density Material Magnus Lipp, William Evans, Hyunchae Cynn, Bruce Baer, Ken Visbeck, Choong-Shik Yoo Covalently bonded extended phases of molecular solids made of first- and second-row elements at high pressures are a new class of material with advanced optical, mechanical and energetic properties. The existence of such extended solids has recently been demonstrated using diamond anvil cells in several systems, including N2, CO2, and CO. However, the microscopic quantities produced at the formidable high-pressure/temperature conditions have limited the characterization of their predicted novel properties including high-energy content. Here we present the first experimental evidence that these extended low-Z solids are indeed high energy density materials via milligram-scale high-pressure synthesis, recovery and characterization of polymeric CO (p-CO).~ This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y35.00002: Identification of intrinsic and extrinsic transitions in pentacene single crystals by optical methods (*) Rui He, X. Chi, Aron Pinczuk, D.V. Lang, A.P. Ramirez Pentacene single crystals with different purity levels are studied by using photoluminescence and resonant Raman scattering. Two luminescence bands are identified as intrinsic optical emissions by recombination of free excitons and self- trapped excitons. The luminescence bands observed below 1.6eV are attributed to the extrinsic optical emissions. Raman scattering from the pentacene crystals exhibits resonance enhancements with the two intrinsic bands. Temperature dependence of the luminescence from the pentacene crystals reveals that optical emission from the self-trapped excitons are sensitive to the sample qualities between 50 and 100K, and its intensity can be quenched by extrinsic traps. \newline \newline (*) Supported primarily by the Nanoscale Science and Engineering Initiative of the NSF under NSF Award Number CHE-0117752 and by the NYSTAR, and by a research grant of the W. M. Keck Foundation. We thank Mark Hybertsen, I. Dujovne, and C. F. Hirjibehedin for their helpful discussions. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y35.00003: Photophysical properties of hemicyanine dyes in zeolite film grown on glass plate Doseok Kim, T. K. Shim, M. H. Lee, H. S. Kim, K. B. Yoon Hemicyanine dye molecules were put into the pores of zeolite (ZSM-5) crystals self-assembled on a glass substrate, and their photophysical properties were investigated by using time-- resolved fluorescence methods. The dye molecules were found to be aligned normal to the plate along the vertical channel of the pore and oriented unidirectionally. The photoluminescence (PL) decay lifetime of the dye molecules in zeolite pore measured by the Time-Correlated Single Photon Counting (TCSPC) method was several nanoseconds, which is much longer compared to that of the same dye in solution phase. This can be interpreted as the restricted twisted intramolecular charge transfer of the molecules confined in zeolite pores. We compared the photophysical properties (steady-state and time- resolved PL) from the molecules in various zeolite crystals with different structural and dielectric properties. This zeolite film can potentially be used as a matrix for investigating the optical and photophysical properties of a molecule in controlled environment. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y35.00004: Mass Spectrometric Analyses of Two Isomeric Forms of Functionalized Pentacene Molecules Svitlana Shcherbyna, Diethard Bohme, Vladimir Baranov, John Anthony Functionalized Pentacene (FP) derivatives are one of the most interesting semiconducting organic materials for use in molecular electronics. We report a comprehensive Mass Spectrometic (MS) study of two isomeric forms of FP molecules that have recently gained considerable attention due to their high charge carrier mobility in organic thin film transistors (OTFT). Both compounds were investigated in an Electrospray Ion Source Triple Quadruple Mass Spectrometer (ESI-MS) fitted with an Ion-Molecule Reactor instead of a Collision Cell. Attempts were made to distinguish the symmetric and asymmetric isomers of FP employing differences in the rates of ion-molecule reactions, ion mobilities and Collisional Induced Dissociation (CID) patterns. In addition, the Matrix-Assisted Laser Desorption/Ionization technique (MALDI-MS) and Ion Mobility Spectrometry (IMS) were employed to study different ionization and ion transport methods prior to mass separation. IMS was able to separate the isomers only marginally. Strong ion-molecule association was observed during MALDI ionization, particularly for the symmetric FP isomer, under the reduced pressure of the source. The results of these measurements and their interpretation will be presented. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y35.00005: Mass Transport and the Ion Beam Sculpting of Solid State Nano-scale pores Toshiyuki Mitsui, Derek Stein, Jene Golovchenko We report on AFM and TEM studies of mass transport during the formation of isolated nano-scale pores formed in silicon nitride membranes with the aid of surface ion beam exposure.~This ion beam sculpting process allows the fabrication of nanopores with nanometer scale control for use in single molecule detection experiments. The nano-scale pores generally close under ion beam exposure and we show from AFM studies that excess material is transported to the vicinity of the pore position and accumulates over regions surrounding the pore that extend hundreds of nanometers from it.~TEM images of multiple pore arrays formed by Ion Beam Sculpting indicate proximity effects associated with the surface diffusion and membrane stress effects leading to pore formation. In addition we discuss the dramatic role of accumulated surface charge during ion beam exposure on surface atomic transport in the formation of these nano-scale pore structures. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y35.00006: Neutron and Thermodynamic Investigation of n-Butane Adsorption on MgO(100) Surfaces Sami Chanaa, Rick Cook, Tom Arnold, T. Ramirez-Cuesta, S. Clarke, John Larese The adsorption properties of thin films normal butane films adsorbed on the MgO (100) surface were investigated using neutron scattering and volumetric isotherm techniques. A series of high-resolution, adsorption isotherm measurements were performed between 145K and 200K using an automated volumetric adsorption apparatus. These data were used to determine both the two-dimensional isothermal compressibility and the isosteric heat of adsorption and to identify regions where phase transitions might occur. Evidence is found for the presence of two layering transitions. Using neutron diffraction. We find that at low temperatures monolayer butane forms a commensurate solid phase that is commensurate with the underlying MgO (100) surface. Inelastic neutron spectroscopy was used to gain some preliminary indications of the dynamical response of the butane solid phase. Comparisons of the neutron spectra with calculations of the molecular-vibrational modes will be made. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y35.00007: Scattering-type near-field infrared microscopy of selforganized nanodomains of diblock copolymers Markus B. Raschke, Leopoldo Molina, Dong Ha Kim, Wolfgang Knoll, Karsten Hinrichs The expansion of scattering-type scanning near-field optical microscopy ({\em s}-SNOM) into the infrared spectral region provides the ability to achieve all-optical resolution down to the several nanometer range in combination with the chemical sensitivity of infrared spectroscopy. Here, we have performed a nanometer scale surface analysis and identification of domains formed by phase separation of the diblock copolymers polystyrene-{\em b}-polyvinylpyridine (PS-b-P2VP) and polystyrene-{\em b}-polyethyleneoxide (PS-b-PEO). This has been achieved by means of non-interferometric IR-scattering detection based on epi-illumination of sharp Au-coated cantilever tips in a noncontact atomic force microscopy configuration. Contrast is obtained probing characteristic differences in the C--H stretch vibrational resonances between the different polymer constituents and a spatial resolution down to 10 nm has been made possible. The mechanism of the near-field tip-sample coupling by vibrational resonances responsible for the imaging contrast has been deduced and can be modelled based on the dielectric functions of the polymer compounds measured by spectroscopic ellipsometry. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y35.00008: Quantum-mechanical process in the gas-surface reaction and PESs Taizo Sasaki The reaction between a surface and a molecule is investigated by using the one-dimensional model potential-energy surfaces (PESs) with multi components, which represents the electron transfer effect. The motion of the molecule is treated quantum mechanically, and the sticking probabilities are calculated for various types of the PES model. For the O$_2$-Al(111) reaction, the initial sticking probability ($S_0$) with the S-shape has been found experimentally[1]. The present study exhibits that such a behavior can be seen only in the case with a small electron-transfer matrix and a slowly-varying potential. On the basis of the results, it is concluded that $S_0$ in the observed reaction should be attributed not to the diabatic effect, but to the tunneling through the energy barrier which appears in the adiabatic approximation to PES.¥¥ [1] L. \"{O}sterlund, I. Zor\'{i}c, and B. Kasemo, Phys. Rev. B \textbf{55}, 15452 (1997). [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y35.00009: Study of Binding Energies of Halogen Molecules in Solid State M.M. Aryal, D.R. Mishra, D.D. Paudyal, S. Byahut, N.B. Maharjan, R.H. Scheicher, Junho Jeong, T.P. Das As part of a program for first-principles understanding of the binding of molecules in molecular solids, we have investigated the binding energies of the halogen molecules in chlorine, bromine and iodine. For studying the electronic structures of these solid-state systems, we have employed the Hartree-Fock Cluster Procedure together with Many-Body Perturbation Theory, the latter allowing direct inclusion of Van der Waals interactions between the molecules which are found to be crucial for the stability of these molecular solids since one-electron effects in the intermolecular interactions are found to be repulsive, opposing the binding of these molecules. The VDW effect is seen to increase systematically from Cl to I, as expected form polarizability considerations. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y35.00010: High-Field Transient Electron Paramagnetic Resonance of Excited Triplet States M. Bortolus, J. van Tol, M. Prato, A.-L. Maniero The zero-field splitting of the excited triplet states of organic molecules often is of the order of 1 GHz or less, and transient EPR at X-band is generally sufficient for the determination of the zero-field splitting and kinetic parameters in these type of molecules. However, information on the g-factor and g-anisotropy cannot be obtained at conventional EPR frequencies, and interpretations of the data in terms of electronic structure are mostly limited to symmetry considerations. On the other hand information of the g-anisotropy provides additional clues with respect to electronic structure, while a direct comparison with radical-ion forms of the molecules becomes possible. Experimental data of transient EPR at 240 GHz will be presented for a variety of methano-fullerenes and pyrrolidino-fullerenes, either embedded in a polymer of in partially oriented liquid crystals. It will be shown that transient EPR at these very high frequencies can accurately determine the g-anisotropy and in some cases the orientation of the g-tensor with respect to the ZFS tensor. The information gained will be discussed in relation to the electronic structure of the lowest excited triplet state of these molecules. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y35.00011: Measurement of the Born-Oppenheimer Potential in a Superprotonic Conductor Using Deep Inelastic Neutron Scattering George Reiter, Dirar Homouz, Juergen Eckert, Robert Blinc, Phil Platzman The momentum distribution of the proton in the superionic conductor Rb3H(SO4)2 is measured using Deep Inelastic Neutron Scattering with the Vesuvio Instrument at ISIS.This material is well suited to a single particle interpretation of the momentum distribution, and the results have been used to extract a direct measurement of the full 3-D Born-Oppenheimer potential surface at 10K, 70K and 102K. We find that there is significant variation of the potential surface in the direction of the bond as the temperature is varied, with the proton becoming more confined in this direction at higher temperatures [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y35.00012: Interaction of oxygen with oxide-supported gold films Su Ying Quek, Joydeep Bhattacharjee, Umesh Waghmare, Efthimios Kaxiras Whilst bulk gold is known to be inert, oxide-supported gold nanoparticles have been found to be highly effective in the catalysis of low temperature oxidation of carbon monoxide. This remarkable activity has been attributed to effects of charge transfer to and from the gold nanoparticles, quantum size effects, reduced co-ordination of gold atoms, as well as interface effects. However, the relative importance of such effects is still controversial. In this work, we seek to distinguish these effects and understand their relative importance, using model systems involving oxide-supported gold films. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y35.00013: Pulse chirping in coherent control via stimulated Raman scattering Svetlana Malinovskaya Selective excitation of molecular vibrations with implementation of ultrashort chirped laser pulses is considered in stimulated Raman spectroscopy. An effective control is demonstrated via adiabatic pulse interaction with molecules. A model of two, two-level systems interacting with transform-limited pump and chirped Stokes pulse is used. The goal of creating a desired coherent superposition of states is analyzed within a dressed state picture. [Preview Abstract] |
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Y35.00014: Fluorescence dynamics of microsphere-adsorbed sunscreens R. Krishnan, T. Nordlund Sunscreens are generally oily substances which are prepared in organic solvents, emulsions or dispersions with micro- or nanoparticles. These molecules adsorb to and integrate into skin cells. In order to understand the photophysical properties of the sunscreen, we compare steady-state and time-resolved fluorescence in organic solvent of varying dielectric constant $\varepsilon $ and adsorbed to polystyrene microspheres and dispersed in water. Steady-state fluorescence is highest and average fluorescence lifetime longest in toluene, the solvent of lowest $\varepsilon $. However, there is no uniform dependence on $\varepsilon $. Sunscreens PABA and padimate-O show complex emission spectra. Microsphere-adsorbed sunscreens exhibit highly non-exponential decay, illustrative of multiple environments of the adsorbed molecule. The heterogeneous fluorescence dynamics likely characterizes sunscreen adsorbed to cells. [Preview Abstract] |
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Y35.00015: Cyclodextrin-templated Fluorescent Anisotropic Structure Jong Seung Park, James N. Wilson, Uwe H.F. Bunz, Mohan Srinivasarao $\gamma -$Cyclodextrin ($\gamma $-CD) consisting of eight glucose units has a large cavity with diameters of 9.5 {\AA}. This large cavity size allows the inclusion of molecules and the proximity of the molecules can lead to electronic interaction among them. In this presentation, we describe the complex formation of linear and planar molecule acetylene dye with $\gamma $-CD. When mixed with $\gamma $-CD in high enough concentrations, the dye with CD forms a liquid that is anisotropic in nature, as evidenced by observations using a polarized light microscope. Fluorescent properties of these complexes were examined to probe the nature of the complex formed in aqueous solutions. Wide angle X-ray scattering (WAXD) and differential scanning calorimetry (DSC) were also used to characterize the complex formed. [Preview Abstract] |
Session Y42: Magnetotransport
Sponsoring Units: GMAGChair: Adnan Rebei, Seagate Technology
Room: LACC 150B
Friday, March 25, 2005 11:15AM - 11:27AM |
Y42.00001: Magnetoresistance Measurements in Nanoconstricted Nickel Wires Zachary Keane, Lam Yu, Douglas Natelson Nanoscale constrictions between two ferromagnetic electrodes have been the subject of much recent interest because of reports of ballistic magnetoresistance (BMR). Substantial controversy exists regarding the size and mechanism of this effect. We report preliminary measurements of the electron transport across Ni wires with nanoscale constrictions and tunnel junctions. Test structures are fabricated using a combination of e-beam lithography and electromigration. Sample geometries are chosen to allow independent control of electrode magnetizations. Measurements are performed from room temperature to cryogenic temperatures, at magnetic fields ranging up to 9 T. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y42.00002: Ballistic anisotropic magnetoresistance in nanowires Julian Velev, Renat Sabirianov, Sitaram Jaswal, Evgeny Tsymbal We have performed an \textit{ab-initio} study of the ballistic conductance of very thin ferromagnetic nanowires for magnetization parallel and perpendicular to the axis of the wire. We find that there can be a significant difference in the resistance for the two orientations of the magnetization giving rise to an appreciable ballistic anisotropic magnetoresistance (BAMR).This effect is similar to the AMR observed in the bulk systems. BAMR is due to the change in the number of bands crossing the Fermi energy produced by the spin-orbit interactions. The spin-orbit interactions lift the degeneracy of $d$-type bands for magnetization parallel but not perpendicular to the wire axis. This can cause a change in conductance if the degenerate levels are close to the Fermi energy. We find that BAMR can be either positive or negative. Similar ballistic magnetoresistance effect can be achieved by mechanical deformation caused, for example, by strain or magnetostriction. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y42.00003: Electronic structure and Magnetoresistance of Ni nanocontacts J.J. Palacios, D. Jacob, J. Fernandez-Rossier We address the fundamental question of whether magneto-resistance (MR) of atomic-sized contacts of Nickel is very large because of the formation of a domain wall (DW) at the neck. Using both {\em ab initio} and model Hamiltonian transport calculations we find [1] that, as in the case of non-magnetic electrodes, transport in Ni nanocontacts depends very much on the orbital nature of the electrons. Our results are in agreement with several experiments in both average value of the conductance [2,3] and the fact that MR can be either positive or negative [3]. Contrary to existing claims [4], DW scattering does {\em not} account for large MR in Ni nanocontacts. \newline \newline [1] J.J. Palacios, D. Jacob, J. Fernandez-Rossier, condmat/0406249\newline [2] C. Untiedt {\em et al.}, Phys. Rev. B {\bf 69}, 081401 (R) (2004)\newline [3] M. Viret {\em et al.}, Phys. Rev. B {\bf 66}, 220401 (2002)\newline [4] N. Garc\'ia {\em et al.},Phys. Rev. Lett. {\bf 82}, 2923 1999. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y42.00004: A magnetic-field-effect transistor and spin transport. R.N. Gurzhi, A.N. Kalinenko, A.I. Kopeliovich, A.V. Yanovsky, E.N. Bogachek, Uzi Landman A magnetic-field-effect transistor is proposed that generates a spin-polarized current and exhibits a giant negative magnetoresistance. The device consists of a nonmagnetic conducting channel (wire or strip) wrapped, or sandwiched, by a grounded magnetic shell. The process underlying the operation of the device is the withdrawal of one of the spin components from the channel, and its dissipation through the grounded boundaries of the magnetic shell, resulting in a spin-polarized current in the nonmagnetic channel. The device may generate an almost fully spin-polarized current, and a giant negative magnetoresistance effect is predicted. \footnote{R.N. Gurzhi et al., Appl. Phys. Lett. {\bf 83}, 4577 (2003).} [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y42.00005: Spin Filtering Of Hot Holes in a Metallic Ferromagnet Tamalika Banerjee, Ehtsham Haq, J.C. Lodder, Ronald Jansen Spin-transport of non-equilibrium holes in ferromagnetic metals with energy below the Fermi level has been investigated using Ballistic Hole Magnetic Microscopy. Using a semiconductor/ferromagnet hybrid structure, we show that a thin ferromagnetic film acts as an efficient spin-filter for holes. Unpolarized hot holes injected from an STM tip after transmission through a ferromagnetic stack (NiFe/Au/Co) are collected in the valence band of p-type Si. The hole attenuation length for Co is found to be short and increases from 0.6 to 1.0 nm in the energy range 0.8 to 2 eV below the Fermi level. For a NiFe/Au/Co trilayer, the hole transmission is clearly spin-dependent with a surprisingly large magnetocurrent of 130{\%}. The large spin dependence of the hole transmission as well as the increase of the attenuation length with energy cannot be explained by the phase space available for inelastic decay of hot holes. We discuss other factors to explain such a large spin asymmetry. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y42.00006: Hot-Electron Spin-Transport Through Tantalum Films and Interfaces Huseyin Gokcan, Ronald Jansen, Cock Lodder The spin-valve transistor (SVT) is a hybrid semiconductor/ferromagnet device based on the spin-dependent transmission of hot electrons across a metallic spin-valve sandwiched between a semiconductor emitter and collector. The transistors have the configuration Si / NM / FM / NM / FM / NM / Si and contain ferromagnetic (FM) materials such as Co, NiFe, and various non-magnetic (NM) materials in the spin-valve base. SVT's with 7 nm Au as the NM spacer exhibit a magnetocurrent (MC) of 350{\%}. Interestingly, we find a sharp drop of the MC to only 9{\%} when Au is replaced by 6 nm Ta as the spacer. We also find that Ta has a short hot-electron attenuation length of about 1.5 nm, attributed to the existence of partially empty d-bands. The strong reduction in MC for SVT's with Ta spacer is surprising, since it is known that the large MC in the SVT results from the spin-dependence of the bulk attenuation lengths for majority and minority spins in ferromagnets. We consider spin-dependent scattering at the Ta/FM interfaces and a short spin relaxation time in bulk Ta as possible reasons. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y42.00007: Domain Wall Resistance in Ferromagnetic Wires Renat Sabirianov, John Burton, Sitaram Jaswal, Evgeny Tsymbal We present first-principles studies of the domain wall (DW) resistance in atomic-size cobalt and nickel ferromagnetic wires. Several types of domain walls are considered: Bloch, Neel and linear DW. The DWs are modeled using a constrained geometry with a fixed width of the non-collinear region allowing the electronic degrees of freedom to relax. Electronic structure calculations are performed using a tight-binding LMTO method in real space for monatomic wires and wires of the 4-fold-symmetry. The electronic transport properties are calculated using the Landauer-Buttiker approach. We find that DW resistance decreases very rapidly, on the scale of a few interatomic layers, with increasing DW width for both Bloch and Neel walls. The largest magnetoresistance value of about 250{\%} is predicted for an abrupt DW in a monatomic Co wire. The density of states and the conductance of the Co wires display energy gaps in one spin channel making the magnetoresistance of an abrupt DW for these energies infinitely large. For the abrupt domain wall the large magnetoresistance is observed in some cases even if the ferromagnetic wire has conductance in both spin channels. This work is supported by National Science Foundation and Nebraska Research Initiative [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y42.00008: Effect of Inserted Ni layers on Magnetoresistance of Co/Cu Multilayers Kenji Tanahashi, Takahiro Inomata The effect of the inserted Ni layers on the magnetoresistance(MR) of Co/Cu multilayers has been investigated. It was found that the insertion of Ni layer enhanced the MR ratio of the multilayers. The effect was investigated on sputter deposited Co(1nm)/Cu(2.3nm) multilayers which give one of the maximum MR ratios. The degree of the effect depends on the thickness of Ni layer, and the prominent effect was observed only when the thickness of Ni layer is 0.1-0.2 nm. The symmetric and asymmetric two types of insertion of Ni layer were carried out to see the effect of the interface on the magnetoresistance, since an interface plays a significant role in scattering of conduction electrons. MR ratio is strongly affected by the thickness of Ni layer itself and the total thickness of Co and Ni layers between two Cu layers. The two types of insertion of Ni layer yield little difference in MR ratio, showing that MR effect depends more on the electrical structure which the total thickness of ferromagnetic Co and Ni layers forms than on the morphological change or the interdiffusion at the interfaces due to the insertion of Ni layers. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y42.00009: Anomalous magnetic after-effect near orientation transition in epitaxial [Fe/Cr(100)]10 multilayers Raul Villar, Ruben Guerrero, Rainer Schad, Jose Luis Martinez, Farkhad Aliev Transport and magnetic properties near a field induced orientation transition (OT) from easy to hard axis in magnetization of antiferromagnetically coupled epitaxial [Fe/Cr(100)]$_{10}$ multilayers have been studied down to 1.7K. It was found that in the vicinity of the OT both the amplitude of the magnetic susceptibility anomaly and the relaxation rate of the magnetoresistance change dramatically below a few K. These observations, together with the previously observed strong reduction of zero field magnetic losses at temperatures below 5-7K [1] indicate a qualitative transformation of the magnetic dynamics of antiferromagnetically coupled Fe/Cr multilayers at very low temperatures. [1] F.G. Aliev, et al., Phys. Rev. Lett., v.88, p.187201 (2002). [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y42.00010: Observation of spin-bottleneck due to spin-charge separation in a superconductor Brigitte Leridon, Jerome Lesueur, Marco Aprili Quasiparticles were injected from a ferromagnet ($Ni_{0.8}Fe_{0.2}$) through a tunnel junction into a conventional superconductor (Nb), while the density-of-states of the superconductor was measured through a second tunnel junction with a paramagnet. No significant decrease of the superconductive gap was observed while a noticeable heating of the quasiparticles of the superconductor was measured. A similar experiment performed with current injected from a paramagnet showed no such effect. This can be interpreted in terms of an enhanced recombination time for the spin-polarized quasiparticles. Estimations give for the intrinsic recombination time a value of about $10^{-9}$ s at 2 K, while some measurements of the spin relaxation time in superconducting Nb give about $10^{-8}$ s at the corresponding temperature (Vier, Phys. Lett. A 98, 283, 1983). The existence of intrinsic spin-charge separation in a superconductor (see Kivelson Phys. Rev. B 41, 11693, 1990) is indeed responsible for a decrease of the spin-orbit coupling matrix coefficients leading to infinite spin-relaxation time at zero temperature (as calculated by Yafet, Phys. Lett. A 98, 287, 1983). In our experiment, it ensures a large spin-relaxation time, thus blocking the recombination process and acting as a "spin-bottleneck". Due to the geometry of the experiment, the pure spin excitations are spatially separated from the charge and are able to thermalize above the gap edge. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y42.00011: Determination of spin diffusion length of gold utilizing lateral spin valves Yi Ji, Axel Hoffmann, Sam Jiang, Sam Bader Spin transport in lateral metallic structures is an intriguing, emerging area, which offers rich opportunities to explore complex spintronic devices. We demonstrate spin injection, diffusion, and detection in a series of lateral spin valves. A 220-nm wide and 20-nm thick gold wire is defined by e-beam lithography on a SiN substrate. Two permalloy (Py) electrodes, an injector and a detector, are subsequently overlaid on the Au wire. The charge current and spin current are separated by connecting current drain and voltage ground to opposite ends of the Au wire. A charge current that passes through the Py injector into the Au induces a spin accumulation, resulting in a split of the chemical potentials for the spin-up and spin-down electrons in the Au. The injected spins diffuse away on both sides of the injector, resulting in a spin current towards the detector even without a charge current. Depending on the relative orientations of the accumulated spin and the detector spin, a voltage contrast can be observed at the detector/Au interface. This separation of charge and spin currents results in a large percentage value of the spin signals, which rules out the possibilities of spurious effects such as AMR. By changing the injector-detector separation, we observe an exponential decay of the spin signals, and thus determine a spin diffusion length of 63 nm in Au at 10 K. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y42.00012: Topological effects on conduction electrons in ferromagnetic rings Adnan Rebei, Eugen Simanek Using two examples, I will show possible measurable effects of \textit{global }geometry on spin accumulations in ring geometries as a function of the diameter of the nano-ring. Because of the spin character of the conduction electrons and the magnetization in transition metals, we have two possible inequivalent spin structures on a ring which are given by the first Cohomology group of the ring. In the first example, the magnetization is assumed to be everywhere the same with respect to the normal to the ring, i.e., the magnetization has a 360 degrees periodicity. In the second example, the magnetization is assumed to have a 720 degrees periodicity and topologically equivalent to a Mobius band. In both examples the magnetization is locally the same but differ globally. We will argue that as the size of the ring becomes smaller (i.e., around 10 nm or less), the topologically non-trivial spin structure should become more stable. This topological property can manifest in e.g. resistance differential between the two examples. This latter physical effect is expected to be present in addition to the Berry phase. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y42.00013: Induced spin currents in alkali films Funing Song, Gerd Bergmann, Doug Garrett Sandwiches of FeK and FeCs are prepared at helium temperature and under ultra-high vacuum. The mean free path within these sandwiches can exceed the film thickness by a factor of five. This implies almost perfect specular reflection of the electrons at the interfaces. Furthermore, the experiments suggest that the specular reflections for spin-up and spin-down electrons are different at the Fe interface, resulting in a spin current in the alkali films. In order to detect this current, dilute Pb impurities are condensed on top of the free surface of the alkali films. Strong spin-orbit scatterers, such as Pb, introduce an Anomalous Hall Effect in the presence of a spin current, which can be easily detected through straightforward Hall measurements. The results of the AHE experiments clearly indicate the existence of a local spin current. [Preview Abstract] |
Friday, March 25, 2005 1:51PM - 2:03PM |
Y42.00014: Leads as Self-Energies in Nonequilibrium Spin Transport Michael Johnson, Fredrick Michael Open quantum systems consist of semi-infinite leads which transport electronic spin or charge to and from a device. We show here that within the nonequilibrium Green's function technique for continuum systems, the leads can be replaced by simple c-number self-energies. Our approach is a reformulation of a continuum calculation developed by T.E. Feuchtwang and connects this method with calculations on a lattice. While the choice of internal boundary conditions gives a limited variability to the self-energies, a particular choice greatly simplifies calculations in two and three dimensions. We close with an example from spin transport. [Preview Abstract] |
Friday, March 25, 2005 2:03PM - 2:15PM |
Y42.00015: Spin current in an asymmetric quantum well: 2x2 effective Hamiltonian vs multi-band model A. A. Kiselev, K. W. Kim We have conducted an analysis of the problem of electron spin currents in two-dimensional systems with structural asymmetry. It is shown that an attempt to derive this quantity based exclusively on the 2x2 effective Hamiltonian is internally inconsistent and actually misses the dominant contribution to the spin current. In short, this happens because "effective" (i.e., obtained via partial diagonalization of multi-band Hamiltonian and truncation) forms of spin and velocity operators are not sufficient, alone, to define the effective form of their superposition. Proper approach should start with the multi-band Hamiltonian explicitly incorporating spin-orbit-split bands and define an operator of the spin current on a complete system, not on one of its effective subspaces. Understandably, this also resolves ambiguities, associated with the order of spin and velocity operators in the superposition, that are inherently present in the 2x2 approach. [Preview Abstract] |
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Y42.00016: Electrical transport in oxidized iron thin films Jolanta Stankiewicz, Juan Bartolom\'{e} , F\'{e}lix Jimen\'{e}z-Villacorta, Carlos Prieto We report results of electrical resistivity, Hall effect and anisotropic magnetoresistance (AMR) measurements performed on thin films ($\sim 50 $ nm) of oxidized iron in the temperature range from 10 to 300 K, and in magnetic fields up to 0.6 T. The films were sputtered on naturally oxidized Si(100) substrates whose deposition temperature $T_s$ can be varied between 170 and 300 K. Samples were oxidized at room temperature for 30 minutes, and subsequently capped with a SiO$_2$ layer in order to prevent further oxidation. By changing $T_s$ during the growth process we can control the size and shape of crystalline grains in these films. We find that the residual resistivity, carrier concentration, coercive field $H_c$ as well as the magnetoresistance of the films are strongly affected by growth conditions, particularly by the substrate temperature. All measured samples are metallic with a room temperature electron concentration of about $2\times 10^{21}$ cm$^{-3}$. As $T_s$ increases, the resistivity at $T$=10 K decreases from $\approx$ 400 $\mu\Omega$cm (for $T_s$=170 K) to $\approx$ 40 $\mu\Omega$cm (for $T_S$=300 K). The low-temperature coercive field, obtained from AMR data, also decreases from $\approx 500$ to $\approx 300$ Oe as $T_s$ increases. At $T$=300 K, $H_c \approx 100$ Oe for all values of $T_s$. AMR values increase with increasing $T_s$. We find the room-temperature planar Hall effect ratio of about 15\% in the films grown at $T_s\geq$ 250 K. [Preview Abstract] |
Session Y43: New Techniques and Applications
Sponsoring Units: GMAGChair: Sam Lofland, Rowan University
Room: LACC 150C
Friday, March 25, 2005 11:15AM - 11:27AM |
Y43.00001: Enhanced Magnetostrictively Transduced SAW Devices - Measurements \& Applications Noble C. Woo, Jonathan R. Petrie, R. Bruce van Dover Surface acoustic wave (SAW) transducers can be made with magnetic materials using magnetostriction as a means of electromechanical coupling. Unlike conventional piezoelectrically transduced SAW devices, the magnetically transduced SAWs do not require an exotic single-crystal substrate or high temperature processing, and therefore may be easily integrated into Si-based integrated circuits. These devices have many potential applications, biosensors being one of the most promising. When the substrate between a transmitter and a detector transducer is functionalized with specific bio-receptors, a binding event will affect propagation of the SAW wave that can be detected with simple electronics. In previous MTSAW devices, the magnetomechanical coupling was found to be poor, resulting in insufficient signal amplitude. To obtain better performance, we are studying the use of alternative magnetostrictive materials including an amorphous CoFeTaZr alloy, CoNbZr, and compositions in the Terfernol family (TbFe2, etc.). We are using combinatorial materials science (continuous composition spread approach) to identify optimum alloy compositions. The devices are also being redesigned to yield better performance. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y43.00002: Permeability and Permittivity Spectra of Ferromagnetic Composites at High Frequencies Xiaokai Zhang, Erik Pearson, Karl Unruh, John Xiao Composites with submicron and nanometer-sized ferromagnetic particles embedded in a dielectric matrix are very promising for a variety of high frequency applications. In particular, these materials offer the potential for exhibiting a high permeability and permittivity, as well as low power losses at operating frequencies. In order to study the effect of the size, shape, and volume fraction of the ferromagnetic component on the microwave properties of these composites, we have prepared a series of samples with varying particle sizes, shapes, and volume ratios in a polymeric matrix. The magnetic and dielectric properties of these samples have been measured in the frequency range from 100 kHz to 18 GHz and compared with the effective medium theory. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y43.00003: Magnetic anisotropy probed by momentum-resolved EELS Yasuo Ito, Yoshie Murooka, Russell Cook, Michel Van Veenendaal, Nanda Menon, Dean Miller We present an investigation of the temperature induced MLD in microcrystalline antiferromagnetic $\alpha $-Fe$_{2}$O$_{3}$ (hematite) particles, using the STEM-based momentum-resolved EELS [1] with a nanometer-scale electron probe in a 120kV TEM/STEM with the energy resolution lower than 0.7 eV. The focused incident beam was parallel to the c-axis of the (001) oriented microcrystalline. The difference between spectra obtained below and above 263K agrees with the calculated MLD spectrum based on the atomic multiplet calculation for the octahedrally coordinated Fe$^{3+}$ ion in $\alpha $-Fe$_{2}$O$_{3}$. The present result is also consistent with the result obtained by the synchrotron X- ray MLD experiment on a single crystal $\alpha $-Fe$_{2}$O$_{3}$ [2]. [1] J. Yuan, N.K. Menon, J. Appl. Phys. 81, (1997) 5087 [2] P. Kuiper et al., Phys. Rev. Lett. 70 (1993), 1549. [3] This work is supported by the US Department of Education, State of Illinois under HECA, NIU URA program, and work at Argonne, carried out in the Electron Microscopy Center, is supported by the U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under Contract {\#}W-31-109-ENG-38. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y43.00004: Neutron Diffuse Reflectometry of Magnetic Thin Films with a 3He Analyzer Wangchun Chen, Kevin O'Donovan, Julie Borchers, Philippe Mangin, Charles Majkrzak, Thomas Gentile Polarized neutron reflectometry (PNR) is a powerful probe that characterizes the magnetization depth profile and magnetic domains in magnetic thin films. Although the conventionally used supermirrors are well-matched for specular PNR, they have limited angular acceptance and hence are impractical for complete characterization of the magnetic off-specular scattering where polarization analysis for diffusely reflected neutrons is required. Polarized $^{3}$He gas, produced by optical pumping, can be used to polarize or analyze neutron beams because of the strong spin dependence of the neutron absorption cross section for $^{3}$He. Here we report efficient polarization analysis of diffusely reflected neutrons in a reflectometry geometry using a polarized $^{3}$He analyzer in conjunction with a position-sensitive detector (PSD). We obtained spin-resolved two-dimensional Q$_{x}$-Q$_{z}$ reciprocal space maps for a patterned array of Co antidots in both the saturated and the demagnetized states. The preliminary results for a patterned amorphous bilayer, Gd$_{40}$Fe$_{60}$/ Tb$_{55}$Fe$_{45}$, measured with a $^{3}$He analyzer and a PSD will also be discussed. Using the spin exchange optical pumping method we have achieved record high $^{3}$He polarizations of 76{\%} on the neutron beam line where we measured an initial analyzing efficiency of 0.97 and a neutron transmission for the desired spin state of 0.45. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y43.00005: Lithographically defined microdisks as alternative magnetic carriers for biomedical applications V. Novosad, F.Y. Fradin, K. Buchanan, K. Yu. Guslienko, V. Yefremenko, S.D. Bader, A.J. Rosengart, S.G. Guy Magnetic particles can be used as a transduction mechanism for target-directed delivery, manipulation, detection and functional control of attached single bio-molecules or cells. Here we will demonstrate how thin film growth techniques combined with traditional mask-transfer lithography can offer a new concept of magnetic nanoparticles with superior properties for bio-medical applications. The experimental and micromagnetic data for small (from hundred nanometers to few microns) microdisks will be reported. The microdisks have zero remanence due to formation of magnetic vortices in each particle, which results in independent behavior of the assembly of disks, e.g. no agglomeration. A very high Ms (up to 20 kOe) can be achieved~by applying relatively small magnetic fields. Furthermore, the static and dynamic properties of the microdisks can be tailored by choosing the appropriate geometrical aspect-ratio. We will also demonstrate that the resonant collective behavior of spins offers additional possibilities for detection by measurement of the reflected complex impedance of a coplanar micro-waveguide.~ [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y43.00006: Scanning element-specific magnetic microscopy at low temperatures Andrew Cady, D. Haskel, J. C. Lang, G. Srajer, P. Chupas, R. Osborn, J. F. Mitchell, S. Park, S-W. Cheong We have developed a low-temperature element-specific magnetic microscopy tool at beamline 4-ID-D at the Advanced Photon Source. The setup provides a means to measure localized ($\sim$1 sq. micron) magnetic behavior in materials at low temperature ($>$10K) under a moderate applied field ($<$0.8T). We demonstrate the potential of this apparatus by presenting results from two experiments. The first experiment shows the technique’s ability to measure paramagnetic-to-ferromagnetic transition temperatures in micron sized regions of a sample and correlate them with small chemical inhomogeneities. The second experimental result illustrates how the technique can be used to correlate magnetic and chemical domains in a multiferroic system. Work at the Advanced Photon Source was supported by the DOE, Office of Basic Sciences, under contract no. W-31-109-Eng-38. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y43.00007: Ferromagnetic Resonance Force Microscopy (FMRFM): Magnetostatic modes in the presence of a localized tip field Radovan Urban, Alexander Putilin, Philip Wigen, Michael Cross, Michael Roukes Magnetostatic modes of Yttrium Iron Garnet (YIG) films have been investigated by ferromagnetic resonance force microscopy (FMRFM). A probe magnet on the tip of a compliant cantilever introduces inhomogeneity in the internal field of the YIG sample. The local variation of this internal field creates a perturbation upon the sample's magnetostatic modes analogous to either a potential barrier or well, depending on the mutual orientation of the external dc and the tip field. For antiparallel orientation of the external and tip field, localization of the magnetostatic modes can be observed. This allows one to extract local magnetic properties of ferromagnetic samples in spite of the strong spin-spin interaction. The lateral resolution of localized modes scales with size of the probe magnet. Data from our experiments is in excellent agreement with the theoretical predictions based upon the Landau-Lifshitz-Gilbert equation of motion and Maxwell's equations. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y43.00008: Scanning Hall Probe Microscopy (SHPM) using Quartz Crystal AFM Feedback Munir Dede, Koray Urkmen, Ahmet Oral Scanning Hall Probe Microscopy (SHPM)[1] is a quantitative and non-invasive technique for imaging localized surface magnetic field fluctuations such as ferromagnetic domains with high spatial and magnetic field resolution of $\sim $50nm {\&} 7mG/(Hz)$^{0.5}$ at room temperature. In the SHPM technique, Scanning Tunneling Microscope(STM)[1] or Atomic Force Microscope(AFM)[2] feedback is usually used for bringing the Hall sensor into close proximity of the sample. In the latter, the Hall probe has to be integrated with an AFM cantilever in a complicated microfabrication process. In this work, we have eliminated the difficult cantilever-Hall probe integration process; a Hall sensor is simply glued at the end of a 32,768 Hz Quartz crystal, which is used as force sensor. The sensor assembly is dithered at the resonance frequency and the quartz force sensor output is detected with a Lock-in. SHPM electronics is modified to detect AFM topography and the phase, along with the magnetic field image. NIST MIRS ( Hard Disk ) sample is imaged with the Quartz Crystal AFM feedback and the performance is found to be comparable with the SHPM using STM feedback. Quartz Crystal AFM feedback offers a very simple sensor fabrication and operation in SHPM. This method eliminates the necessity of conducting samples for SHPM. [1] A. Oral \textit{et. al.} Appl. Phys. Lett., 69, 1324 (1996) [2] A.J. Brook \textit{et. al.} Appl. Phys. Lett. 82, 3538 (2003) [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y43.00009: A study of the magnetic hysteresis of a single magnetic element using a sensitive microcantilever torque magnetometer L. Gao, L. Yuan, K.H.P. Kim, S.H. Liou, M.D. Chabot, D.H. Min, J.M. Moreland A sensitive microcantilever torque magnetometer (MTM) has been developed for measuring the magnetic properties of nanomagnets. The sensitivity of the technique at room temperature in air was demonstrated by the successful measurement of the magnetization switching of a single micronmeter sized Ni$_{80}$Fe$_{20 }$element. The permalloy dot has a size of 1.5 $\mu $m in diameter and 30 nm in thickness with a moment of 5.5 $\times $ 10$^{-11}$ emu. The sensitivity of our current MTM instrument can be estimated as 10$^{-12}$ emu, which is about three orders better than commercial SQUID. The hysteresis loop indicates that the switching process of the element is dominated by the domain wall propagation through the element gradually. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y43.00010: The Born Oppenheimer-Potential for the Hydrogen Bond in the H5O2+ Complex: Comparison of Theories and Experiment Nina Verdal, Bruce Hudson, George Reiter The Born-Oppenheimer potential surface for motion of the central H atom in the hydrogen bond in the H$_{5}$O$_{2}^{+}$ ion is of considerable interest. This quantity has been elusive experimentally and computations are of unknown reliability. By measuring the momentum distribution of the protons in the centrosymmetric aquonium perchlorate, H$_{5}$O$_{2}^{+}$ ClO$_{4}^{-}$, and subtracting the ice-like contribution from the non-hydrogen bonded protons, we are able to measure the momentum distribution of the proton in the bond. This distribution is calculated for several ab-initio treatments of the potential surface and compared with the measurements as a means of distinguishing between these methods. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y43.00011: Ordered and Disordered Magnetic States in Mott insulators: Insights from Spectral Density Functional Theory Quan Yin, Sergej Savrasov, Gabriel Kotliar Self-consistent many body + electronic structure calculations using dynamical mean field based spectral density functional method are presented to study various magnetic states in several Mott-Hubbard insulators such as classical transition metal oxides MnO, FeO, CoO, NiO, as well as FeS. Solution of the Anderson impurity problem in these strongly correlated systems will be approximated by using the Hubbard I method. Comparison will be made for the electronic spectral functions and magnetic moments in both ordered and local moment regimes. The results will be contrasted to less rigorous LDA+U calculations. Prospects for theoretical predictions of the Neel temperatures within this method will be discussed. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y43.00012: Renormalization group method for weakly-coupled quantum chains: application to the spin one-half Heisenberg model Samuel Moukouri The Kato-Bloch perturbation formalism is used to present a density-matrix renormalization-group (DMRG) method for strongly anisotropic two-dimensional systems. This method is used to study Heisenberg chains weakly coupled by the transverse couplings $J_{\perp}$ and $J_{d}$ (along the diagonals). An extensive comparison of the renormalization group and quantum Monte Carlo results for parameters where the simulations by the latter method are possible shows a very good agreement between the two methods. It is found, by analyzing ground state energies and spin-spin correlation functions, that there is a transition between two ordered magnetic states. When $J_{d}/J_ {\perp} \alt 0.5$, the ground state displays a N\'eel order. When $J_{d}/J_{\perp} \agt 0.5$, a collinear magnetic ground state in which interchain spin correlations are ferromagnetic becomes stable. In the vicinity of the transition point, $J_ {d}/J_{\perp} \approx 0.5$, the ground state is disordered. But, the nature of this disordered ground state is unclear. While the numerical data seem to show that the chains are disconnected, the possibility of a genuine disordered two- dimensional state, hidden by finite size effects, cannot be excluded. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y43.00013: Method of measuring anhysteretic magnetization and magnetostriction in ferromagnetic thin films Peter Finkel, Edward Garrity We have developed a new method of the stress dependant anhysteretic magnetization measurements in a thin--film ferromagnetic materials. This is achieved by combining conventional vibrating sample magnetometer with the specially designed loading fixture allowing to apply uniaxial stresses on thin film and wires. The anhysteretic magnetization is obtained by ac demagnetization of the sample and measuring the subsequent magnetization at a given dc magnetic field. Also we have developed a contactless method for determining uniaxial stress in thin anisotropic ferromagnetic samples based on monitoring velocity of a flexural wave using time of flight measurements. The method has been demonstrated for membrane thickness down to 100$\mu $m and stresses up to 1 GPa. Estimated accuracy of this method is better than 0.2{\%}. [Preview Abstract] |
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Y43.00014: Sensitive Measurement of Parallel and Transverse Susceptibility by Alternating Gradient Magnetometry Mladen Barbic, Mary Brady Reversible susceptibility tensor measurements reveal important information about the switching fields and anisotropies of magnetic materials. We show that a simple reconfiguration of an alternating gradient magnetometer can be used to measure both reversible parallel and transverse susceptibilities with high sensitivity. It is demonstrated that positioning the sample off-axis with respect to the magnetometer gradient field coils results in a signal at twice the frequency of the gradient field that is directly proportional to the reversible susceptibility. Offsetting the sample along the x-axis results in a sensor signal proportional to the reversible parallel susceptibility, while rotating the sample holder by 90 degrees and offsetting it along the y-axis results in a sensor signal proportional to the reversible transverse susceptibility. Examples of reversible parallel and transverse susceptibility measurements of aligned nanoparticle systems will be demonstrated. [Preview Abstract] |
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