Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H1: Single-Ion Magnetic Moments in Semiconductors
Sponsoring Units: DCMPChair: Michael Flatte, University of Iowa
Room: Morial Convention Center LaLouisiane AB
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H1.00001: Single-ion magnetic moments in semiconductors Invited Speaker: The ability to manipulate single spins in a solid-state environment provides a new pathway to study the fundamental physics of magnetism and may lead to the development of nanoscale spin-based quantum devices. Single magnetic ions bound with valence holes in semiconductors are a good candidate for building scalable quantum spin systems that can be directly integrated into conventional charge-based electronics. The highly extended hole wave function is susceptible to external non-magnetic control fields, such as electric or strain fields. The spin-orbit coupling between the orbital and spin character of the bound hole state permits indirect manipulation of the spin states of the magnetic ions. It is also possible to make use of the spin-dependent local density of states to probe the spin state of individual magnetic ions. I will discuss our theoretical understanding on the anisotropic shape of the bound hole states and the resulting anisotropic magnetic interaction. The hole wave function can be significantly altered by a strain field, which in turn influences the magnetic interaction. The magnetic interaction is found to be sensitive to the local changes in the hole binding energy as well. Our results pave the way for electrical manipulation of single ion spins in semiconductors and also shed light on bulk magnetic properties of very dilute magnetic semiconductors. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H1.00002: Atom-by-Atom Substitution of Mn in GaAs and Visualization of their Hole-Mediated Interactions Invited Speaker: |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H1.00003: Zero-field optical manipulation of magnetic ions in semiconductors Invited Speaker: For coherent spin information processing, spin coherence times must be long enough to perform multiple state operations, thus requiring a balance between gating time and spin lifetime. Because single magnetic spins in semiconductors can be strongly coupled to both itinerant carriers and to other magnetic ions, these interactions can be rapidly manipulated optically and electrically. We show that small numbers of magnetic spins in III-V GaAs quantum wells can be polarized by optical spin injection without the need for applied magnetic fields, and exhibit unusually long coherence times \footnote{R. C. Myers, M. H. Mikkelsen, J.-M. Tang, M. E. Flatt\'{e}, A. C. Gossard, and D. D. Awschalom, \textit{submitted} (2007).}. Mn ions provide acceptor states within the bandgap of GaAs \footnote{J. Schneider, U. Kaufmann, W. Wilkening, M. Baeumler, and F. K\"ohl, \textit{Phys. Rev. Lett.} \textbf{59}, 240 (1987).}, enabling optical readout and control of the magnetic ions in a manner distinct from paramagnetic II-VI materials. Spin polarized electrons created within the quantum well dynamically orient the Mn spins in a manner analogous to dynamic nuclear polarization, generating a dynamic exchange splitting of the magnetic spins. The Mn ions are manipulated at zero field solely by changing the excitation helicity or energy. Ion spin lifetimes increase sharply as the concentration is reduced exhibiting T$_{2}^{\ast}$ times exceeding 6 ns at the lowest doping, longer than is typically observed in other magnetic semiconductors. These results indicate that hole-mediated Mn-Mn interactions dominate the decoherence, and suggest that long lifetimes may be expected for single Mn spins in GaAs. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H1.00004: The wave function of a single-ion magnetic moment in GaAs Invited Speaker: |
Session H2: New Developments in HTSC III
Sponsoring Units: DCMPChair: Eduardo Fradkin, University of Illinois at Urbana-Champaign
Room: Morial Convention Center LaLouisiane C
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H2.00001: The role of charge order in the mechanism of high Tc Invited Speaker: |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H2.00002: From high T$_c$ superconductivity to quantum spin liquids Invited Speaker: The underdoped cuprates exhibit many unusual properties going under the name pseudogap. These observations lend support to the notion that high Tc superconductivity is intimately tied to doping of a Mott insulator. Building on earlier work on the quantum spin liquid, the resonating valence bond (RVB) idea of Anderson provides an adequate physical understanding of the pseudogap. The problem can be formulated as a gauge theory even though many details are beyond the powers of current calculational tools. Part of the difficulty is that the pseudogap phenomenology occurs only at finite temperature where precise statements about excitations and possible emergent gauge fields cannot be made. Meanwhile the problem of the quantum spin liquid is a simpler version of the high Tc problem where significant progress has been made recently. It is understood that the existence of a matter field can lead to deconfinement of the U(1) gauge theory, and novel new particles such as fermionic spinons which carry spin 1/2 and no charge, and gapless gauge bosons can emerge in a new critical state at low temperatures. Two experimental systems, the organic compound and the Kagome lattice, have emerged as promising examples of a spin liquid. I shall argue that these may be described by a spinon Fermi surface and Dirac spinons coupled to a U(1) gauge field, respectively. Further experimental tests will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H2.00003: On the pseudogap in high temperature superconductors Invited Speaker: The ``pseudogap,'' a suppression of the density of states observed in at least some high temperature copper-oxide superconductors at carrier concentrations lower than that which maximizes the superconducting transition temperature, presents a long-standing and still unresolved problem in condensed matter physics. Basic questions including whether it is a signature of a new phase of matter or a consequence of thermal or quantal disordering of a superconducting or spin density wave state, remain unresolved. This talk will summarize the present status of the problem, including what is known about the form of the low temperature gap function (one gap or two), the role of thermal scattering in the formation of ``fermi arcs'' and the significance of recent recent high-field quantum oscillation experiments. \newline \newline References: \newline ``Gapless pairing and the Fermi arc in the cuprates'' A. V. Chubukov, M. R. Norman, A. J. Millis, and E. Abrahams Phys. Rev. {\bf B76}, 180501 (2007). \newline ``Antiphase Stripe Order as the Origin of Electron Pockets Observed in 1/8-Hole- Doped Cuprates,'' A. J. Millis, M. Norman, Phys. Rev. B in press (2007) (cond-mat/07090106) \newline ``Gaps and Our Understanding'' A. J. Millis Science {\bf 314}: 1888-1889 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H2.00004: Superconductivity, quasi-particle dynamics and strong-coupling physics Invited Speaker: I discuss the problem of superconductivity from the perspective of pairing mediated by a boson and compare with the situation of ``no-glue'' superconductivity in a strongly correlated material. This is done for Tl-doped PbTe, where recent experiments support superconductivity due to quantum valence (charge Kondo) fluctuations, and for the doped Mott insulator SrCu$_{2}$(BO$_{3})_{2}$, where d-wave superconductivity emerges due to a reorganization of a valence bond crystal state. I discuss the implications of these results for high temperature superconductivity in the cuprates. [Preview Abstract] |
Session H3: Polymer Physics Prize
Sponsoring Units: DPOLYChair: Mark Ediger, University of Wisconsin-Madison
Room: Morial Convention Center RO2 - RO3
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H3.00001: Polymer Prize Talk: Segmental Dynamics in Polymers : From Cold Melts to Aging and Stressed Glasses Invited Speaker: Polymers are excellent glass formers. In the cold molten state they exhibit chemically-specific and strongly non-Arrenhius segmental relaxation which sets the time scale for the generic chain scale dynamics. In the amorphous solid or plastic state the temperature dependence of the alpha relaxation time changes, physical aging emerges, and a rich mechanical response occurs characterized by the dynamic yielding, strain softening and strain hardening processes. We have developed a statistical mechanical theory of activated segmental relaxation in cold melts by combining and extending methods of mode coupling, dynamic density functional and activated hopping theories. The approach is built on the concept of a confining nonequilibrium free energy which quantifies local dynamical constraints and the barrier hopping process. The localizing consequences of interchain caging forces are quantified by the amplitude of nanometer scale density fluctuations (compressibility) and backbone stiffness. Predictions for the kinetic glass and dynamic crossover temperatures, dynamic fragility, and thermal dependence of the segmental relaxation time are consistent with experiments. The theory has been generalized to treat alpha relaxation, physical aging, and nonlinear mechanical properties in the glass. The structural component of density fluctuations become (partially) frozen resulting in a crossover to Arrenhius relaxation. Physical aging is modeled based on a kinetic equation for collective density fluctuations. At intermediate time scales the relaxation time (shear modulus) grows as a power law (logarithmic) function of aging time with a temperature dependent exponent. Applied stress weakens dynamical constraints thereby accelerating relaxation and softening the elastic modulus. A constitutive equation has been constructed from which the temperature dependent dynamic yielding and mechanical response under constant strain rate, constant stress (creep), and other modes of deformation can be calculated. This work was done in collaboration with Drs. Kang Chen and Erica J. Saltzman. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H3.00002: Application of Integral Equation Theory to Polymers in the Condensed State Invited Speaker: Large scale computer simulations involving many chains can be used to model the physical properties and structure of amorphous polymers, however, such simulations are computationally intensive. An alternative is to employ the approximate, but computationally less demanding, Polymer Reference Interaction Site Model or PRISM theory. PRISM theory is an extension to polymers of liquid state, integral equation theories originally developed for atomic and small molecule liquids. From a computational standpoint, PRISM theory can be viewed as a self-consistent formalism to map the statistical mechanics of the difficult many-chain system to a much simpler, single chain problem. Depending on the model, the statistics of the single chain can be solved analytically or through a single chain Monte Carlo simulation. Solution of PRISM theory yields the intermolecular and intramolecular radial distribution functions characterizing the packing of the polymer chains in the condensed liquid. These distribution functions can then be employed to compute the thermodynamic properties of the polymer system. Various applications of PRISM theory to polymer liquids, blends and copolymers will be discussed and comparisons will be made with exact MD simulations and scattering experiments. Finally, a more accurate approach will be discussed that involves mapping the many chain system to an equivalent two-chain problem. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H3.00003: Complex Fluid Microstructure, Rheology and Glass Transitions:~ Effect of Continuous Phase Molecular Weight Invited Speaker: The mechanical properties of suspensions depend dramatically on the suspension microstructure. Microstructure in turn depends on the nature of particle interaction potentials.~ For those systems that are thermally activated there will be an equilibrium microstructure and thus equilibrium transport properties.~ One of the model systems used to understand the links between interactions, microstructure and transport properties is that of hard spheres suspended in a Newtonian continuous phase.~ This model system can be studied experimentally and direct comparisons made with model predictions.~ With the increase in particle volume fraction, if the particles cannot crystallize, the suspension forms a glass where long range self diffusion is essentially eliminated.~ The approach to the glass transition has been studied experimentally and agreement with models is strong.~ In this talk we discuss what changes as the continuous phase takes on a granularity where the continuous phase molecules have substantial degrees of freedom. In particular, we investigate the mechanics and microstructure of hard sphere suspensions in polymer melts.~ The particles are composed of silica while we use polyethelene glycols of different weights ranging from small degrees of polymerization through the entanglement molecular weight.~ These studies are motivated by a desire to understand properties of polymer nanocomposites where the role of particle/polymer segment interactions is poorly understood but the state of particle dispersion is key to composite properties.~ In this talk I explore equilibrium and nonequilibrium phases of this system and compare with extant theoretical approaches. ~ [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H3.00004: Dynamics of fluids in complex environments Invited Speaker: |
Tuesday, March 11, 2008 10:24AM - 11:00AM |
H3.00005: The Theta Point Of Long Flexible Polymer Chains: When Does It Exist? Invited Speaker: The standard description of the conformation of a long flexible polymer coil in dilute solution implies a swollen state under good solvent conditions, while deterioration of solvent quality (by decrease of temperature) causes a (gradual) chain collapse below the Theta point. At the Theta point, the chain follows Gaussian statistics, apart from logarithmic corrections. Monte Carlo simulations of the bond fluctuation model will be discussed that provide evidence for a second scenario, where the chain experiences a first order transition from the swollen state to a dense solid phase, provided the range of effective attractive interactions is sufficiently short. This scenario then implies that in solution at finite concentration no vapor-liquid-like phase separation occurs. The analogy between this prediction and the behavior of some colloidal dispersions is discussed. [Preview Abstract] |
Session H4: Selected Applications Using Materials Science
Sponsoring Units: DMPChair: Yvan Bruynseraede, Katholieke Universiteit Leuven
Room: Morial Convention Center 206
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H4.00001: Integrated Functionality: Nanosensors Invited Speaker: Integrated nanosensors are nanostructured systems in which several sensors of different types have been integrated on a single platform including those sensitive to optical, magnetic, chemical, or biological stimuli [1,2]. Nanoparticle-based detector systems rely on the development of nanoparticles as sensing species. I discuss state-of-the-art nanosensors which are based on various advanced materials: nanoshells and nanorice, gold nanoparticles with attached fluorescent dyes, nanopores, carbon nanotubes, neuroelectronic hybrid systems, semiconductor nanocrystals and quantum-dot quantum wells (QDQW's). Phonon-assisted optical processes in semiconductor nanocrystals and QDQW's are highly sensitive to their shape and geometry i. a. due to the non-adiabaticity of the exciton-phonon systems. This sensitivity opens new perspectives for applications of quantum dots in optical sensing. For example, the simultaneous consideration of the tetrahedral shape of a CdS/HgS/CdS QDQW, interface optical phonons, and non-adiabatic phonon-assisted transitions allows for control of the photoluminescence of a QDQW [3]. Quantum-dot-based systems are also considered as examples of communication with nanodevices, which is a prerequisite of their integration. Nanosensors allow for building a new class of integrated devices, which provide the elemental base for ``intelligent sensors'' capable of data processing, storage and analysis. The development of integrated nanosensors could have many applications in several fields such as process monitoring, robotics, environmental, medical, consumer, homeland security{\ldots} [1] I. K. Schuller, http://nanosensors.ucsd.edu/Introduction.htm. [2] J. T. Devreese and Y. Bruynseraede, Integrated Nanosensors. \textit{McGraw-Hill 2008 Yearbook of Science {\&} Technology, }McGraw-Hill, 2008. [3] V. A. Fonoberov, E. P. Pokatilov, V. M. Fomin, and J. T. Devreese, \textit{Phys. Rev. Lett.} \textbf{92}, 127402 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H4.00002: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H4.00003: Nanodevice sensors measured with rf- and microwave reflectometry Invited Speaker: Nanodevices can be extremely sensitive sensors, but they typically operate at low temperatures and have high impedances. This makes it hard to measure these devices at high frequencies, however this problem can be overcome by impedance matching these devices with resonant circuits using rf and microwave techniques. We will review a number of nanodevices probed with rf and microwave methods, both dissipative and non dissipative nanodevices can be measured in this way. Typical examples of dissipative devices are single electron transistors, quantum point contacts and scanning tunneling probes. The change in dissipation of the device will translate into a magnitude change of the reflected signal. Non-dissipative devices like parametric capacitances or inductances of superconducting circuits give rise to a shift in the resonance frequency of the resonant circuits which results in a phase shift of the reflected signal. Using these methods drastically increases the operation frequency and often also the sensitivity of the measured quantity. The non dissipative devices also have very low back-action and can potentially approach the limits set by quantum mechanics. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H4.00004: Detection of Explosive Materials Invited Speaker: High explosives present a challenge for detection methods because of their range of physical properties, which range from volatile liquids to nonvolatile solids. They share the common feature of possessing both oxidizing and reducing chemical properties within a single molecule or an intimate chemical mixture. Our research group has been focused on the synthesis of new luminescent polymers, which undergo electron transfer quenching by a variety of organic high explosives, such as TNT, RDX, and PETN. The application to imaging trace explosive particle residues will be described. Density functional calculations show an excellent correlation between the sensor response and the lowest unoccupied molecular orbital of the explosive analyte. For volatile high explosives, such as organic peroxides (e.g. TATP), vapor sensors based on chemically sensitive transistors containing different metal phthalocyanines have been explored. The mechanism of current response in these films has been shown to be a result of surface Lewis acid-base chemistry or redox catalysis at the metal centers. The link between surface chemistry and electronic resonse has led to a simple peroxide specific vapor sensor array. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 11:00AM |
H4.00005: Materials Informatics: Using machine learning techniques with large amounts of ab-initio computed or experimental data Invited Speaker: Machine learning techniques can be applied to large amounts experimental or computed materials data in order to identify the underlying factors that determine a target property. While the use of experimental data is complicated by the fact that it is mostly non-standardized in property or structure databases, experimental data still tends to be richer in information than computed data. One problem that can be addressed with machine learning techniques is the prediction of structure. By using structure prototype as a mathematical descriptor, and constructing its correlation in chemical spaces through machine learning techniques, it is possible to create a highly effective structure prediction method. Previously, we demonstrated that by simply applying maximum entropy ideas to a large experimental structure database of binary metals, it was possible to suggest a short list of candidate structures for new compounds which contains the proper ground state with very high probability [Ref]. This list of probable structures can then be computed with ab initio energy methods. We have now extended this method to multi-component and non-metal systems by prototyping the $\approx $ 100,000 structure records in the International Crytallographic Structure Database, and a similar accuracy of prediction is achieved in these high component spaces. We believe that such a machine learning approach solves the crystal structure prediction for many practical purposes. Machine learning techniques can also be used to point at likely errors in experimental structure databases and I will give some examples of this. In the long-term computed data is more likely to form the input for machine learning techniques as it is well defined and obtained under controlled conditions. Using high-throughput ab-initio computing techniques we have determined the structure and energy for several thousand compounds and have begun to data mine this information for property models relevant to energy generation and storage. [Preview Abstract] |
Session H5: CeMIn5 (115 ) Heavy Electron Materials: A Rosetta Stone for the Kondo Lattice?
Sponsoring Units: DCMPChair: Zachary Fisk, University of California, Irvine
Room: Morial Convention Center RO1
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H5.00001: Angle-resolved Photoemission of CeCoIn$_5$: Detailed Comparison to LDA and LDA+DMFT Invited Speaker: Highly-automated photon-dependent angle resolved photoemission spectroscopy (ARPES) in the energy range of 80-200 eV has been used to characterize the three dimensional (3D) Fermi surface (FS) topology and electronic band structure of cleaved single crystals of CeCoIn$_5$. The sample temperature of $\approx26$K is well below the lattice coherence onset temperature of $\approx45$K found in a recent ``two fluid'' analysis of transport data. Detailed comparison of ARPES FS contours to LDA calculations for the Ce 4f electrons treated as itinerant or confined to the core reveals remarkable agreement to fine topological details of the f-core calculations. Also in agreement to the f-core calculations is the experimental absence of extra electron-like contours predicted in the f-itinerant calculation, originating from $\alpha$ and $\beta$ bands re-entrant below $E_F$ along Z-A. Finally, the areas enclosed by FS contours for the $\alpha$ and $\beta$ bands are significantly smaller than are found in very low temperature CeCoIn$_5$ de Haas van Alphen data that agrees generally with the f-itinerant calculation. It is concluded that clear signatures of coherence in the transport data can develop at temperatures for which the f-electrons are not yet included in the FS. In this connection, comparison will also be made to recent T-dependent LDA+DMFT calculations for CeIrIn$_5$. This work was done in collaboration with J. D. Denlinger, Feng Wang, R. S. Singh, K. Rossnagel, S. Elgazzar, P. M. Oppeneer, V. S. Zapf and M. B. Maple, and was supported by the U.S. DOE (DE-AC03-76SF00098 at the ALS, DE-FG02-07ER46379 at UM for current work, DE FG02-04ER-46105 at UCSD), by the U.S. NSF (DMR-03-02825 at UM for initial work, DMR-03-35173 at UCSD) and by the Swedish Research Council (VR) and the European Commission (JRC-ITU). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H5.00002: Modeling the Localized to Itinerant Electronic Transition in the Heavy Fermion System CeIrIn5 Invited Speaker: Within the ab-inition calculation we adress the crossover from localized to itinerant state of a heavy fermion material CeIrIn5. The temperature evolution of the one electron spectra and the optical conductivity is predicted from first principles. The buildup of coherence in the form of a dispersive many body feature is followed in detail and its effects on the conduction electrons of the material is revealed. We find multiple hybridization gaps and link them to the crystal structure of the material. Our theoretical approach explains the multiple peak structures observed in optical experiments and the sensitivity of CeIrIn5 to substitutions of the transition metal element and may provide a microscopic basis for the more phenomenological descriptions currently used to interpret experiments in heavy fermion systems. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H5.00003: Andreev reflection in heavy fermions and the superconducting order parameter in CeCoIn$_{5}$ Invited Speaker: Andreev reflection, a retro-reflection of an incoming electron as a hole at a normal-metal/superconductor interface, is well understood in conventional superconductors. For heavy-fermion superconductors, the microscopic consequences of the heavy electronic mass remain an open question. According to the Blonder-Tinkham-Klapwijk (BTK) theory, no Andreev process is allowed because of the large mismatch in the Fermi velocities [1]. However, conductance enhancement due to Andreev reflection, albeit reduced, has been frequently observed in heavy-fermion superconductors [2]. In this talk, I will present such conductance spectra obtained along three different crystallographic orientations of the heavy-fermion superconductor CeCoIn$_{5}$ [3]. Qualitative analysis using the extended BTK model shows the first spectroscopic evidence for $d_{\mbox{x}^2-\mbox{y}^2} $-wave (instead of $d_{xy}$-wave) symmetry, resolving the controversy over the node locations. In order to explain the reduced Andreev signal and the conductance asymmetry, both commonly observed in heavy-fermion superconductors, we propose a conductance model based on the two-fluid phenomenology [4] and an assumed energy-dependent density of states. I will discuss the significance of this model and possible clues for developing it into a microscopic theory. [1] G. E. Blonder, M. Tinkham, and T. M. Klapwijk, Phys. Rev. B \textbf{25}, 4515 (1982); G. E. Blonder and M. Tinkham, \textit{ibid}. \textbf{27}, 112 (1983). [2] Yu. G. Naidyuk and I. K. Yanson, J. Phys.: Condens. Matter \textbf{10}, 8905 (1998). [3] W. K. Park \textit{et al}., arXiv:0709.1246 (submitted to Phys. Rev. Lett.). [4] S. Nakatsuji, D. Pines, and Z. Fisk, Phys. Rev. Lett. \textbf{92}, 016401 (2004). This work is done in collaboration with L. H. Greene, H. Stalzer, J. L. Sarrao, J. D. Thompson, Z. Fisk, and P. Canfield. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H5.00004: Universal behavior in heavy electron materials Invited Speaker: I describe improvements in the phenomenological two-fluid theory that enable one to reconcile the thermal and magnetic measurements of the collective deconfinement of localized spins in Kondo lattice materials that begins at T* and leads to the formation of a new quantum state of matter - the heavy electron Kondo Liquid (KL). The resulting phenomenological KL density of states provides a good fit to the theoretical results for Ir 115 and its experimental measurement in Knight shift and Hall effect anomalies, tunneling experiments, and Raman scattering in the 115 materials. I discuss the relationship between T* and the single ion Kondo temperature and present an updated version of the Doniach diagram for heavy electron materials. [Preview Abstract] |
Session H6: Understanding Hurricanes and Severe Storms: Patterns, Prediction and Mitigation
Sponsoring Units: FPSChair: Andrew Post-Zwicker, Princeton Plasma Physics Laboratory
Room: Morial Convention Center RO4
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H6.00001: Understanding Severe Hurricanes Invited Speaker: Hurricanes are complex phenomena, whose understanding involves many facets, of which my presentation will provide an overall flavor and review. Understanding the physical hurricane involves a complex amalgam of fluid dynamics, thermodynamics and scale interactions. The basic structure is one of a fluid vortex, which dictates everything from the characteristic spiral shape to the clear eye region. Energetically, once formed a hurricane is a self sustaining heat engine, one that extracts energy from the enthalpy difference between the warm ocean surface and the cold upper atmosphere, and one that will continue its merry way until it is destroyed by some external influence (such as landfall). Hurricanes also are a response to the global climate in which they develop and can feed back to influence and perhaps even change that climate. For example a series of hurricanes moving into the higher latitudes in the Pacific can set off a train of events that are still affecting European weather a year later. From a societal perspective they are the most dangerous and deadly of all natural atmospheric systems, capable of causing widespread destruction and long-term disruptions to entire societies. The damage wreaked by Katrina in New Orleans provides a canonical example, but this was by no means the worse cyclone in history. Even lesser damage on a small island nation can be much more catastrophic and exceed their entire gross domestic product. This capacity for disruption arises from three main mechanisms: the high surface winds, the response of the ocean to these winds, and the intense rainfall. These have widely different contributions in different storms: the extended region of high winds and particularly the storm-surge response were dominant factors in Katrina; whereas the $>$10,000 deaths by Hurricane Mitch arose entirely from rainfall and the associated flooding and landslides. Societal response to this danger involves complex interplays of warning, communication culture, previous experiences and perceptions, interplays that are neither well understood nor adequately predictable. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H6.00002: Tornadoes and Severe Thunderstorms: Physical Understanding and Climate Questions Invited Speaker: Severe thunderstorms (those that produce large hail, high winds and/or tornadoes) are of importance because of the threat to life and property they pose. This talk will review our understanding of the physical processes that lead to them and their distribution in time and space. The basic approach follows that of weather forecasting, focusing on the atmospheric ``ingredients'' in the environment necessary to produce severe thunderstorms and tornadoes, particularly the thermodynamic state of the atmosphere and the organizing effects of vertical wind shear that leads to the most severe storms. We will look at the challenges of reconciling our limited reporting databases of events and our physical expectations derived from the distribution of those environmental conditions. Consistent (and inconsistent) aspects of the various databases around the world will be discussed with their implications for what we can and cannot say about the basic physical processes. Of particular interest is the record from the United States. Some simple efforts to deal with the spatial and temporal inhomogeneities in the observational record will be developed with the limits that are implied on our ability to detect past changes. Finally, the talk will close with a discussion of the possible effects of anthropogenic global warming on severe thunderstorms, particularly in the United States. Global climate model studies of this problem are very recent, with the first peer-reviewed results appearing in 2007. The limitations of the climate models and possible scenarios for the future will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H6.00003: Factors Influencing Hurricane Surges along the Louisiana-Mississippi Coast Invited Speaker: The effects of recent hurricanes along U.S. Coastlines, along with expected future sea level rise and the potential for increased storm activity all point to a critical need for improved methods for estimating coastal hazards and associated risks. Many of the models used today to assess hazards and risk incorporate considerable empiricism in their formulation. Unfortunately, most of the empirical evidence is drawn from small to moderate storm events and cannot be effectively extrapolated to extreme storms such as Hurricane Katrina. This presentation will critique the state of the art in hurricane surge prediction, including the adequacy of numerical models, coefficients within these models, and the wind fields utilized to force them. Once the predictive system and its physical basis are introduced and discussed, a methodology will be described for utilizing information from such a system to estimate risk for coastal areas, including the effect of uncertainties in both the modeling system and storm climate,. Using the methodology introduced here, maps of estimated storm surge levels for selected recurrence intervals in the New Orleans area will be presented along with a comparison to some previously derived values to provide perspective. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H6.00004: Wetland Loss and Restoration Options in Southern Louisiana Invited Speaker: Wetlands are productive landscape features of the broad Mississippi River Delta system. In addition to their ecological services of providing habitats for a variety of species including juvenile commercial and recreational fish, they provide a valuable wave reduction role during severe storm events characterized by elevated water levels and high waves. Currently, these wetlands are stressed by a combination of natural and human-related forces resulting in rapid loss rates. Although many factors contribute to wetland loss rates, the single greatest factor is the shunting of river borne sediments offshore into deep water. Navigational interests benefit greatly from the present fixed location of the main navigation channel at Southwest pass with its terminus at the edge of the continental shelf such that the sediment load is discharged into deep water. The Mississippi River Delta region is subsiding at up to more than 10 times the Eustatic rate of sea level rise and thus the wetland and barrier island systems require these sediments for maintenance and growth. With the increasing scarcity and costs of energy, it is highly desirable that wetland restoration be done using natural forces to the degree possible. Absent legal issues, a pragmatic approach could be to identify those areas where progress can be made with realistic investments of economic and energy resources and to accept that areas with less benefit per investment will continue to degrade. The paper will review various options and discuss obstacles and opportunities. [Preview Abstract] |
Session H7: Complex Active Biomaterials: Mechanics and Microrheology
Sponsoring Units: DBP DPOLYChair: John Crocker, University of Pennsylvania
Room: Morial Convention Center RO5
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H7.00001: Non-equilibrium mechanics of motor-driven cytoskeletal polymer networks Invited Speaker: Cells both actively generate and sensitively react to forces using their mechanical framework, the cytoskeleton, which is a non-equilibrium, composite material including polymers and motor proteins. We have measured the dynamics and mechanical properties of a simple three-component model system, consisting of myosin II, actin filaments, and crosslinkers. Stresses arising from motor activity control network mechanics: both increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoleastic response of the network in an ATP-dependent manner. We have quantified the mechanical properties as well as the active fluctuations in these networks by a combination of passive and active microrheology. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H7.00002: Non-equilibrium mechanics and dynamics of active gels and living cells Invited Speaker: Much like the bones in our bodies, the \textit{cytoskeleton} consisting of filamentous proteins largely determines the mechanical response and stability of cells. Such important cellular processes as locomotion, cell division, and mechanosensing are largely governed by complex networks of cytoskeletal biopolymers and associated proteins that cross-link these and/or generate forces within the network. In addition to their important role in cell mechanics, cytoskeletal biopolymers have also provided new insights and challenges for polymer physics and rheology. In the cell, however, these polymer networks or gels are far from equilibrium in a way unique to biology: they are subject to active internal force generation by molecular motors. We describe recent theoretical and experimental results on active in vitro networks that demonstrate significant stiffening and non-equilibrium fluctuations due to motor activity [1]. We show how this activity leads generically to a colored, 1/$\omega ^{2}$ spectrum of force fluctuations, which can account for surprisingly Brownian-like motion in elastic networks. We also discuss how the fluctuations of individual cytoskeletal filaments can be used to probe both mechanical properties and non-equilibrium activity in living cells [2]. \par [1] D Mizuno, C Tardin, CF Schmidt, FC MacKintosh, \textit{Science}, 315:370 (2007). \par [2] CP Brangwynne, FC MacKintosh, DA Weitz, \textit{PNAS}, 104:16128 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H7.00003: Cytoskeletal mechanics: Structure and Dynamics Invited Speaker: The actin cytoskeleton, a dynamic network of semiflexible filaments and associated regulatory proteins, is responsible for the extraordinary viscoelastic properties of cells. Especially for cellular motility the controlled self assembly to defined structures and the dynamic reorganization on different time scales are of outstanding importance. A prominent example for the controlled self assembly are actin bundles: in many cytoskeletal processes cells rely on the tight control of the structural and mechanical properties of the actin bundles. Using an\textit{ in vitro} model system we show that size control relies on a mismatch between the helical structure of individual actin filaments and the packing symmetry within bundles. While such self assembled structure may evoke the picture of a static network the contrary is the case: the cytoskeleton is highly dynamic and a constant remodeling takes place in vivo. Such dynamic reorganization of the cytoskeleton relies on the non-static nature of single actin/ABP bonds. Here, we study the thermal and forced unbinding events of individual ABP in such \textit{in vitro} networks. The binding kinetics of the transient crosslinkers determines the mechanical response of such networks -- in the linear as well in the non-linear regime. These effects are important prerequisites for the high adaptability of cells and at the same time might be the molecular mechanism employed by them for mechanosensing. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H7.00004: Microrheology in Active Cytoskeletal Networks Invited Speaker: The mechanics of the in vivo cytoskeleton is controlled in part by the details of its non-equilibrium steady-state. In this ``active'' material, molecular motors (e.g. myosin) exert transient contractile stresses on the F-actin filament network, driving it into a particular non-equilibrium state. Since microrheology traditionally relies of the linear response properties of the soft materials in thermal equilibrium, this departure from equilibrium has profound implications for the interpretation of microrheological data from the interior of living cells and in vitro active networks. In active networks, such as the in vitro systems of Mizuno et al. [Science 315 (5810) pp. 370-373 (2007).] and in living cells, the underlying theoretical foundation of the interpretation of microrheology -- the Fluctuation-Dissipation theorem -- does not apply. New ideas are needed. In this talk, I review microrheology, and then discuss a new theoretical interpretation of microrheology in active (i.e. molecular motor driven) networks. To develop this new theory, I introduce a motor-driven, two-fluid model of the active network and background (aqueous) solvent. Using this model and knowledge of the statistical properties of the molecular-motor induced forces, I calculate the non-equilibrium fluctuation spectrum expected for one- and two-particle microrheology in the driven system. I then compare these results to the data of Mizuno et al.. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 11:00AM |
H7.00005: Force fluctuations and polymerization dynamics of intracellular microtubules Invited Speaker: Microtubules are dynamic biopolymers within the cytoskeleton of living cells. They play a central role in many biological processes including cell division, migration, and cargo transport. Microtubules are significantly more rigid than other cytoskeletal biopolymers, such as actin filaments, and are insensitive to thermal fluctuations on cellular length scales. However, we show that intracellular microtubules exhibit bending amplitudes with a surprisingly thermal-like wavevector dependence, but with an apparent persistence length about 100 times smaller than that measured \textit{in vitro}. By studying the time-dependent bending fluctuations of individual filaments, we find that the thermal-like bends are fluctuating significantly only on short length scales, while they are frozen-in on longer length scales [1], reminiscent of non-ergodic behavior seen in systems far from equilibrium. Long wavelength bends are suppressed by the surrounding elastic cytoskeleton, which confines bending to short length scales on the order of a few microns [2]. These short wavelength bending fluctuations naturally cause fluctuations in the orientation of the microtubule tip. Tip fluctuations result in a persistent random walk trajectory of microtubule growth, but with a small non-equilibrium persistence length, explaining the origin of quenched thermal-like bends. These results suggest that intracellular motor activity has a highly fluctuating character that dominates over thermal fluctuations, with important consequences for fundamental biological processes. \newline \newline [1] CP Brangwynne, FC MacKintosh, DA Weitz, \textit{PNAS}, 104:16128 (2007). \newline [2] CP Brangwynne, FC MacKintosh, S Kumar, NA Geisse, J Talbot, L. Mahadevan, KK Parker, DE Ingber, DA Weitz, \textit{JCB}, 173:733 (2006). [Preview Abstract] |
Session H8: Focus Session: Glassy Dynamics in Colloids
Sponsoring Units: DFDChair: David Weitz, Harvard University
Room: Morial Convention Center RO6
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H8.00001: Are colloidal and molecular glass formation related? Invited Speaker: Understanding why and how a glass is formed on a microscopic level remains an outstanding problem in condensed matter physics. A molecular glass is normally formed by cooling of a liquid. Upon entering the supercooled state, the structural dynamics slows down dramatically and eventually the liquid enters the non- equilibrium glassy state. On route towards the glass, the behaviour shows a range of highly general, near universal characteristics, such as stretched exponential behaviour of dynamic correlation functions and cooperative dynamics. Such generalities exist even though molecular glasses can be formed from liquids encompassing a wide range of molecular structures and interactions. Glass formation also occurs in altogether very different systems. One of the most interesting, both from a fundamental and an applications point of view, is that of colloidal suspensions. The high degree of control that can be achieved regarding colloidal particle size, shape and interactions makes this a fantastic model system in learning about glass-formation. We know that a range of properties observed during dynamic arrest in molecular systems for decreasing temperature are indeed mirrored in the arrest of a colloidal suspension upon increase of particle volume fraction. However, the richness in phenomenology observed for liquids has generally not been observed for colloids. We will discuss to what extent colloidal glass formation can be viewed as equivalent to molecular glass formation and present recent experimental work that suggests a remarkably direct connection. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H8.00002: Applications of patchwork dynamics for glassy systems Creighton Thomas, Alan Middleton, Olivia White We present work on ``patchwork dynamics'' as a technique for studying the nonequilibrium properties of glassy systems. In patchwork dynamics, we replace local Monte Carlo simulations, which require exponentially long times to equilibrate at a given length scale, with exact equilibration on patches at a given length scale, which can be done rapidly in models such as the 2D Ising spin glass and disordered dimer models. We have demonstrated some interesting applications of patchwork dynamics to such systems: 1) as a heuristic ground state algorithm for the 2D Ising spin glass on a torus (for which there are no known fast exact algorithms) and the 3D Ising spin glass; 2) as a method to study aging effects, persistence, and memory in 2D and 3D Ising spin glasses; 3) as a sampling procedure to study the nonequilibrium properties of disordered dimer models at finite temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H8.00003: Aging of a Binary Colloidal Glass Jennifer M. Lynch, Gianguido C. Cianci, Eric R. Weeks After having undergone a glass transition, a glass is in a non-equilibrium state, and its properties depend on the time elapsed since vitrification. We study this phenomenon, known as aging. In particular, we study a colloidal suspension consisting of micron-sized particles in a liquid --- a good model system for studying the glass transition. In this system, the glass transition is approached by increasing the particle concentration, instead of decreasing the temperature. We observe samples composed of particles of two sizes ($d_{1} = 1.0\mu m$ and $d_{2} = 2.0\mu m$) using fast laser scanning confocal microscopy, which yields real-time, three-dimensional movies deep inside the colloidal glass. We then analyze the trajectories of several thousand particles as the glassy suspension ages. Specifically, we look at how the size, motion and structural organization of the particles relate to the overall aging of the glass. We find that areas richer in small particles are more mobile and therefore contribute more to the structural changes found in aging glasses. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H8.00004: Structural signatures of dynamical heterogeneity in supercooled liquids Heidi Perry, David Reichman The underlying mechanism of the transition from liquid to glass is a long-standing open question in condensed matter physics. One long sought after clue to understanding the glass transition is a link between the structure and dynamics of a vitrifying fluid. The dynamics of a supercooled liquid near the glass transition have proven to be collective and heterogeneous, with the length scale of the dynamic regions increasing as the glass transition temperature is approached. Using computer simulations and a normal mode analysis, we demonstrate a link between the structural properties of a super cooled liquids and the collective dynamical regions. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H8.00005: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H8.00006: Fluctuations in the aging regime of a polymer glass Azita Parsaeian, Horacio E. Castillo We perform numerical simulations to investigate the fluctuations in the aging regime of a system of polymers which are interacting via the Lennard-Jones potential. We characterize how the fluctuations evolve by studying (i) probability distributions of local observables such as individual particle displacements $\Delta x$ and intermediate scattering functions $C_r$ associated with small regions and (ii) dynamic correlation functions such as the four-point density correlation $g_4({\bf r}, t, t_w)$. We find that, similar to small molecule glasses, the probability distributions of local observables approximately collapse when the global two-time correlation $C_{\mbox{global}}(t,t_w)$ is held fixed. We test for universality by comparing the probability distributions in the small molecule glass with those in the polymer glass. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H8.00007: Equipartition theorem in glasses and liquids Valentin A. Levashov, Takeshi Egami , Rachel S. Aga , James R. Morris In glasses and liquids phonons have very short life-time, whereas the total potential energy is not linear with temperature, but follows the T**(3/5) law. Thus it may appear that atomic vibrations in liquids cannot be described by the harmonic oscillator model that follows the equipartition theorem for the kinetic energy and potential energy. We show that the description of the nearest neighbor oscillation in terms of the atomic level stresses indeed provide such a description. The model was tested for various pair-wise potentials, including the Lennard-Jones potential, the Johnson potentials, and only the repulsive part of the Johnson potential. In all cases each of the local elastic energies of the six independent components of the stress tensor is equal to kT/4, thus the total potential energy is equal to (3/2)kT. Thus this model provides the basis for discussing the thermodynamic properties of glasses and liquids based on atomistic excitations. An example of this model leading to the description of the glass transition temperature in metallic glasses is discussed [1]. \newline [1] T. Egami, et al., Phys. Rev. B 76, 024203 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H8.00008: Exceptionally Stable Organic Glasses with Low Enthalpy and High Kinetic Stability Prepared by Vapor Deposition Kenneth L. Kearns, Stephen F. Swallen, M.D. Ediger, Ye Sun, Tian Wu, Lian Yu Vapor deposition can be used to prepare glasses of 1,3,5-(tris)naphthylbenzene (TNB) and indomethacin (IMC) that are much more stable than those created by cooling from the liquid. By controlling the temperature of the substrate and the deposition rate, the stability of the glass can be tuned. Glasses can be deposited with enthalpies as much as 10 J/g lower than the glass made by cooling the liquid. Vapor-deposited glasses can also be superheated well above the conventional T$_{g}$. The slow evolution from the low energy glass to the supercooled liquid is observed and can take tens of hours to evolve at temperatures near T$_{g }$+ 5 K. Trends in stability support an enhanced surface dynamics mechanism where we estimate the dynamics in the top 1 nm to be about 7 orders of magnitude faster than the bulk at T$_{g}$ -- 25 K. Vapor deposition has also allowed us to progress more than 40{\%} towards the bottom of the amorphous potential energy landscape. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H8.00009: Magnetic Analogies for the Dynamics of Glass Forming Liquids Jacob Stevenson, Peter Wolynes We present a direct mapping between the dynamics of glass forming liquids and a general random field / random coupling Ising model using the replica effective potential approach. Using the overlap between two structural states of a supercooled liquid we construct a constrained overlap free energy that can be mapped directly onto that of an Ising Hamiltonian. For a Lennard-Jones glass the fluctuations and mean values of the random fields and interactions place it within the universality group of the random field Ising magnet, not the Ising spin glass. This corresponds with the explanation for a random first order transition. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H8.00010: Direct imaging of particle dynamics in attractive colloidal glasses Piotr Habdas, Andrzej Latka, Yilong Han, Ahmed Alsayed, Arjun G. Yodh We use confocal and fluorescent microscopy to study the dynamics of glassy colloidal suspensions. The suspensions are composed of PMMA colloidal particles in density and index-of-refraction matched liquid and stained with a fluorescent rhodamine dye. A controllable depletion attraction is induced between hard-sphere PMMA particles by adding different amounts of polystyrene polymer to the suspension. Our dynamical measurements focus on jumps experienced by PMMA particles that escape the cage formed by its neighbors. We track these particles over time and correlate particle fluctuations with its changes in average position. We find that as the strength of the attractive potential increases, and the system enters an ``attractive liquid'' phase, the number of jumping particles increases. We calculate the distribution of particle jump sizes, time between jumps, and spatial distribution of particle jumps; these observations are compared to predictions of molecular dynamics simulations. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H8.00011: Shear-induced ordering and vitrification of concentrated emulsions Jung-Ren Huang, Thomas G. Mason Using time resolved light scattering, we investigate the degree of droplet deformation and ordering within concentrated oil-in-water emulsions subjected to oscillatory shear between parallel glass plates. We create uniform microscale droplets between the plates by rupturing a premixed emulsion of larger droplets at a fixed strain amplitude and frequency. Subsequently, by independently adjusting the strain amplitude and frequency and recording videos of the dynamic scattering pattern, we examine how the instantaneous applied shear and prior shear history influence the positional structure of the droplets. We also explore how the ordering of the emulsion droplets depends on the oil volume fraction, both above and below that associated with maximally random jamming of uniform hard spheres. The short-range stabilizing repulsion between oil droplets enforces ordering in the shear direction; yet, by contrast to sheared colloidal hard spheres, the deformability of the oil droplets allows concentrated emulsions to un-jam at sufficiently high shear rates. We propose a real-space model, based on the form factor of ellipsoidal droplets and structure factor of ordered, jammed, and un-jammed configurations, that is consistent with the observed light scattering patterns. This technique can be used to transform the structure of a uniform emulsion between ordered and disordered droplet configurations. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H8.00012: Accentuated shear thinning of soft sphere suspensions Hans M. Wyss, Johan Mattsson, Alberto Fernandez-Nieves, Giovanni Romeo, Melaku Muluneh, Zhibing Hu, David A. Weitz Suspensions of soft colloidal particles exhibit highly unusual rheological behaviors; surprisingly, despite the importance of these materials in a wide range of applications, the underlying physical mechanisms remain poorly understood. Experiments show that suspensions of soft particles exhibit a highly pronounced shear thinning; this decrease in viscosity with increasing shear rate far exceeds the shear thinning observed in suspensions of solid particles. We use soft microgel particles as a model system to elucidate this behavior. Our experimental system allows us to study the mechanical behavior both macrosopically and locally, at the scale of the colloidal particles themselves. We combine data obtained at different length scales to arrive at a simple picture of the observed accentuated shear thinning. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H8.00013: Investigation of Rejuvenation and Overaging in Glassy Energy Landscapes Mya Warren, Joerg Rottler Many glassy systems experience a change in their aging dynamics under the influence of mechanical load. It has long been known that large stresses can cause an apparent decrease in relaxation times (rejuvenation) in polymer glasses, but in colloidal glasses an increase (overaging) has also been observed depending on the strain amplitude. The conditions under which rejuvenation or overaging occur are not yet fully understood. Additionally, there is still considerable controversy over the nature of the resultant states. In order to gain intuition on these outstanding questions, we investigate the aging dynamics under load though stochastic simulations of the Soft Glassy Rheology (SGR) model. For both stress controlled and strain controlled loading, the SGR model exhibits clear regions of overaging and rejuvenation in a parameter space defined by the noise temperature, the quench history, and the strain. Additionally, results show that the states produced under loading are distinct from those that would naturally be visited during aging, and this has effects on the subsequent aging trajectory. Results from the energy landscape picture are compared to pertinent molecular dynamics studies. [Preview Abstract] |
Session H9: Instabilities and Fluid Dynamics
Sponsoring Units: DFDChair: Jerzy Blawzdziewicz, Yale University
Room: Morial Convention Center RO7
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H9.00001: Low Frequency Oscillations in the Upper Atmosphere Sudip Sen No definitive theory exists which explains the origin of various low frequency oscillations observed in the ionosphere. Various authors, over the course of time, have put forward various explanations of this important phenomenon. Most recently it has been proposed that the spatial transverse shear in the parallel flow destabilizes many low frequency oscillations and this may be the origin of low frequency oscillations in the ionosphere [V V Gavrishchaka et al., Phys. Rev. Lett. {\bf 80}, 728 (1998) and Phys. Rev. Lett. {\bf 85}, 4285 (2000)]. In this article we review the various theories proposed till date to explain the origin of low frequency oscillations. We address the most recent theories in more detail. We show that the recent proposition of the spatial transverse shear might excite many instabilities may not be so obvious. Parallel flow curvature when taken into account might actually act to stabilize various instabilities [S. Sen et al. Phys. Rev. Lett. {\bf 88}, 185001 (2002)]. This article therefore concludes while much work has been done on the ionospheric oscillations much more work possibly remains to be done in this important area of space physics. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H9.00002: Capillary-controlled instability in immiscible, parallel flow in porous media Thomas Ramstad, Alex Hansen When two immiscible fluids flow in parallel in a strongly wetted porous medium the global interface separating them tend to be kept in place by local capillary barriers. However, above a certain threshold in the flow rate, the separating interface may become unstable and mobilized. We study this instability theoretically by using a two-dimensional network as a model for porous media in a flow regime where capillary forces cannot be neglected. It is found that a boundary zone with a sharp saturation profile occurs between the regions originally saturated with either a wetting or a non-wetting phase. This zone has a well defined width and moves with constant speed towards the non-wetting region. In the opposite direction, a current of non-wetting bubbles is set up, but wetting bubbles into the non-wetting region are absent. This behavior is genuinely different from shear-induced Kelvin-Helmholtz instabilities. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H9.00003: Stability of multi-layer Hele-Shaw flows with and without diffusion Prabir Daripa In this talk, we will provide some results in the context of multi-layer Hele-Shaw flows. We will address issues related to collective effects of individually unstable interfaces on the overall stability of multi-layer Hele-Shaw flows in the presence of interfacial surface tensions. We will also discuss complications in the analysis resulting from making, in the above set-up, individual layers also unstable. We obtain some sufficient conditions for suppressing instability of two-layer flows by introducing arbitrary number of constant viscosity fluid layers in between. For stabilization purposes, this condition allows selection of fluids in internal layers based on interfacial surface tensions and viscosities of fluids. Time permitting, we also examine the effects of species diffusion on the stability of the three-layer Hele-Shaw flows. This has relevance to enhanced oil recovery by polymer flooding. Analytically, we will prove the diffusive slow-down of unstable waves. It will be shown that a strong enough diffusion can almost stabilize the flow, though the magnitude of this diffusion coefficient required to completely stabilize the flow will depend on the magnitude of interfacial viscosity jumps and the viscosity gradient of the basic viscous profile of the internal layer. This work has been done in collaboration with Gelu Pasa. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H9.00004: Meandering instability of a rivulet on a partially wetting incline Adrian Daerr, Laurent Limat It is common to observe small rivulets in sinks or on window-panes which follow sinuous paths (stationary or not) instead of flowing down along the direction of steepest slope. A laboratory experiment shows that these meandering rivulets exist only for certain ranges of the control parameters (flow rate and substrate inclination). The geometrical properties of the resulting paths can be understood in terms of force balances between inertia, capillarity and contact line pinning. The nature of the instability, i.e. why the straight rivulet becomes unstable, however remains unclear. We study the rivulet near the onset for meandering to understand the role of noise and surface defects. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H9.00005: A quasi 2-D molecular dynamics study of the initiation and evolution of the Kelvin-Helmholtz instability Kyle Caspersen, Robert Rudd, David Richards, Jim Glosli, John Gunnels, Fredrick Streitz Typically hydrodynamic phenomena are modeled with continuum mechanics via integration of the Navier-Stokes (NS) equation or a closely related variant. However, as fluids are studied at smaller and smaller length scales atomistic effects can, and will, ultimately dominate; furthermore, even at micron scales it is not clear that the NS equation provides a complete description of the fluid, e.g. due to the initiation of instabilities at the molecular scale in initially quiescent fluids. To assess the effect of atomistic behavior on one particular hydrodynamic phenomenon--the Kelvin-Helmholtz instability--we have performed a very large molecular dynamics simulation of molten metals undergoing shear flow. Nine billion copper and aluminum atoms were sheared at a speed of 2000 m/sec for a total simulated time of more than a nanosecond. We present here results showing the initiation of the instabilities, the crossover to hydrodynamics, and the evolution and scaling behavior of the KH instability in a quasi 2-D geometry. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H9.00006: A fully 3-D molecular dynamics study of the initiation of the Kelvin-Helmholtz instability Robert E. Rudd, K.J. Caspersen, D.F. Richards, J.N. Glosli, J.A. Gunnels, F.H. Streitz The modeling of hydrodynamic phenomena has almost exclusively been the purview of continuum mechanics, specifically, through the use of the Navier-Stokes equation and closely related variants. Nevertheless, at the smallest length scales, where atomistic effects become important, it is not clear that this continuum approach provides a complete description of fluid behavior. To understand the effects of atomistics, we have performed a 62.5-billion-atom, fully 3-D molecular dynamics simulation of a cubic micron of molten copper and aluminum. The shear flow at 2 km/s exhibits complex phenomena associated with a Kelvin-Helmholtz (KH) instability. In this presentation we will discuss the initiation and early evolution of the KH instability, focusing specifically on the effects of full atomistic resolution. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H9.00007: Bubble Pinch-off at High Pressures J.C. Burton, P. Taborek Previously we have studied the pinch-off of conventional air bubbles in water [1]. For inviscid fluids, the shrinking of the neck radius of the bubble can be described by a power-law in time with an exponent close to 1/2. As the density of the interior gas is increased, instabilities are expected to occur in the liquid/gas interface [2]. We present high-speed videos and numerical simulations of the pinch-off of high-pressure gaseous bubbles in and exterior inviscid fluid. The density ratio between the exterior fluid and interior gas is D=$\rho_{ext}/\rho_{int}$. In the simple case of small D$\sim$0.001, the pinch-off is similar to that of a water drop pinching-off in air, while at large D$\sim$1000, the pinch-off is that of an air bubble in water. By using sulfur hexafluoride as a working gas, we are able to span a wide ranging of density ratios simply by increasing the pressure of the gas. A high-pressure ($\sim$30 atm) chamber with optical access through sapphire windows was constructed in order to view the pinch-off. The numerical simulations are performed assuming perfectly inviscid fluids using boundary-integral techniques. Instabilities in the interface are seen for intermediate density ratios. Comparisons between experimental and numerical results will be discussed. \newline [1] J.C. Burton, R. Waldrep, and P. Taborek. Phys. Rev. Lett. 94, 184502, (2005). \newline [2] D. Leppinen and J.R. Lister. Phys. Fluids 15, 568, (2003). [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H9.00008: First order phase transition in the height of a meniscus in a tapered capillary Michael Pettersen, Etienne Rolley When a fluid rises in a capillary of non-uniform cross section, additional terms arise in the balance of capillary forces, compared to the case of a capillary of uniform cross section, due to the changing area of the meniscus. Recently, it has been pointed out that this can lead to a first order phase transition, resulting in a discontinuous jump in the equilibrium position of the meniscus. We present the results of an experiment using isopropanol and silicone oil in cones with apex upwards of different opening angles. The cone is slowly lowered into the liquid using a translation stage. We have measured the capillary rise in this geometry, and observed the predicted phase transition. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H9.00009: Luminescence from Laser-Induced Bubbles in Water-Glycerol Mixtures: Effect of Viscosity Erin Englert, Allison McCarn, Gary A. Williams We have studied the luminescence emitted from collapsing laser-induced bubbles in water-glycerol mixtures, as a function of the mixture concentration and applied hydrostatic pressure. The primary effect of increasing the glycerol concentration is to increase the viscoslty of the fluid. We find that the pulse duration of the luminescence increases by more than a factor of two as the concentration increases up to 33\% by volume, where the viscosity is nearly four time that of pure water. At higher concentrations the pulse duration remains nearly unchanged, until no luminescence can be observed at concentrations above 60\% (viscosity greater than 15 times that of water). The pulse duration further increases with applied pressures up to 8 bars, similar to that seen earlier in pure water. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H9.00010: Hysteretic and Chaotic Dynamics of Viscous Drops in Creeping Flows with Rotation Yuan-Nan Young, Jerzy Blawzdziewicz, Vittorio Cristini, Roy Goodman It has been shown in our previous publication (Blawzdziewicz et al 2003) that high-viscosity drops in two dimensional linear creeping flows with a nonzero vorticity component may have two stable stationary states. One state corresponds to a nearly spherical, compact drop stabilized primarily by rotation, and the other to an elongated drop stabilized primarily by capillary forces. Here we explore consequences of the drop bistability for the dynamics of highly viscous drops. Using both boundary-integral simulations and small-deformation theory we show that a quasi-static change of the flow vorticity gives rise to a hysteretic response of the drop shape, with rapid changes between the compact and elongated solutions at critical values of the vorticity. In flows with sinusoidal temporal variation of the vorticity we find chaotic drop dynamics in response to the periodic forcing. A cascade of period-doubling bifurcations is found to be directly responsible for the transition to chaos. In random flows we obtain a bimodal drop- length distribution. Some analogies with the dynamics of macromolecules and vesicles are pointed out. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H9.00011: A Temporal Period Doubling Route to Spatiotemporal Chaos in a System of Amplitude Equations for the Nematic Electroconvection Iuliana Oprea, Gerhard Dangelmayr We analyze the transition from periodic solutions to spatiotemporal chaos in a system of four globally coupled Ginzburg Landau equations describing the dynamics of instabilities in the electroconvection of nematic liquid crystals, in the weakly nonlinear regime. If spatial variations are ignored, these equations reduce to the normal form for a Hopf bifurcation with O(2) x O(2) symmetry. Coexistence of low dimensional and extensive spatiotemporal chaotic patterns, as well as a temporal period doubling route to spatiotemporal chaos, corresponding to a period doubling cascade towards a chaotic attractor in the normal form, are also identified and discussed, for values of the parameters including experimentally measured values of the nematic I52. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H9.00012: Angular momentum transport in complex fluids Xiaoyu Zheng, Peter Palffy-Muhoray, Michael Shelley When dyes are dissolved in nematic liquid crystals, the light intensity required for the optical Freedericksz transition can be dramatically decreased. This is due to the torque exerted by the dye on the liquid crystal. The dye molecules absorb light energy and rotate; torque balance is mediated by angular momentum transport from the cell walls via shear flow generated by the rotation [1]. We present a model which accounts for the transport of angular momentum caused by singular vortices present in these complex fluids. The singular vortices generate flow, and are transported by the flow which they generate. For simple fluids, the distribution of vorticity satisfies the biharmonic equation in the Stokes limit, which can be solved analytically. In the case of the non-Newtonian fluids, such as liquid crystals, Leslie-Ericksen continuum theory is used to describe the interactions between the rod-like molecules. [1] P. Palffy-Muhoray, T. Kosa and Weinan E, ``Brownian Motors in the Photoalignment of Liquid Crystals'', \textit{Appl. Phys. A} \textbf{75}, 293-300 (2002). [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H9.00013: Particle Dynamics in Bi-Disperse Liquid Fluidized Beds Phil Segre, Gary L. Hunter, James Davidheiser, Elizabeth Baker We study particle velocities and concentration profiles of mixtures of $2$ different sized particles in concentrated liquid fluidized beds. For binary systems of particles of the same density, we find that there is always a complete phase separation in the bed. The larger particles occupy a zone in the lower part of the bed, and the smaller ones a zone in the upper part. For binary systems of particles of {\it different} density materials, conditions are found where the binary particles are either fully separated, partially mixed together, and at a single point called the inversion point, fully mixed into a one phase state. Results will be presented on the phase diagrams of several binary suspensions as well as the properties of the velocity fluctuation magnitudes and spatial correlation lengths. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H9.00014: Polygonal hydraulic jump on microtextured surfaces Emilie Dressaire, Laurent Courbin, Jerome Crest, Howard A. Stone Fluid motion can be drastically influenced by the nature of boundaries. For instance, we have shown recently \footnote{L. Courbin, E. Denieul, E. Dressaire, M. Roper, A. Ajdari and H.A. Stone, Nature~Mater. \textbf{6}, 661 (2007)} that a substrate with a regular array of micron-size posts can cause partially wetting fluids to take on polygonal shapes. Here, we report on the hydraulic jump that occurs when a water jet impinges a topographically patterned surface, i.e. an array of micron-size posts arranged on square or hexagonal lattice. By varying the topographic features (shape and height of the posts, lattice distance) and the jet properties (size of the nozzle, flow rate), we obtain a variety of stable shapes including hexagons, eight corner stars and circles. We rationalize our results by taking into account a fluid velocity that depends on the orientation of the lattice. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H9.00015: Enstrophy-constrained stability analysis of beta-plane Kolmogorov flow with drag Yue-Kin Tsang, William Young For forced two-dimensional flows, energy injected at a certain wavenumber is redistributed to both larger and smaller wavenumbers. This results in a constraint on the time evolution of the difference between the energy and enstrophy. By incorporating this constraint in an energy stability analysis of Kolmogorov flow on a beta-plane with drag, we establish an extended region in the parameter space of beta and the drag coefficient where the flow is stable to arbitrary perturbations. Complementary to this nonlinear stability result, linear instability theory is used to determine the part of the parameter space where the flow is unstable to infinitesimal perturbations. We also find that the most unstable mode in the linear stability analysis has a discontinuous change in structure as beta decreases below a certain value. Results from numerical simulations spanning the parameter space support the theoretical predictions. [Preview Abstract] |
Session H10: Josephson Effects
Sponsoring Units: DCMPChair: Chris Lobb, University of Maryland
Room: Morial Convention Center RO8
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H10.00001: Suppression of macroscopic quantum tunneling in a large Josephson junction coupled to a resonator Brad Trees, Joshua Schiffrin, Yaser Helal, Brian Siller We calculated the zero-temperature macroscopic quantum tunneling
rate of a current-biased Josephson junction weakly coupled to a
resonator. We allow for the effects of environmental dissipation
on both the junction and the resonator, and we consider both
cases of weak and strong junction damping. We find that coupling
to the resonator has a suppressive effect on the junction's
tunneling: the stronger the coupling strength between the
junction and resonator, the greater the reduction of the
tunneling rate. Including damping to the junction also
suppresses tunneling, but damping the resonator partially {\it
counteracts} the suppression provided directly by the
junction-resonator coupling. Modeling the dependence of the
junction-resonator coupling on the resonator's frequency
$\omega_R$ in a power law fashion $U_{int}\propto(\omega_{R})^n$,
we find that for $0 |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H10.00002: Identifying the Odd-Frequency Superconducting State by a Field-Induced Josephson Effect Jacob Linder, Takehito Yokoyama, Asle Sudbo The prevalent symmetry in known superconductors may be described as odd under exchange of spin coordinates, and even under an exchange of spatial or time coordinates of the electrons constituting the Cooper pair. However, other types of pairing are also permitted by the governing Pauli principle. Among these is the so-called odd-frequency pairing state, which has been predicted to arise both in N/S and F/S proximity systems. Extending the possible pairing states compatible with the Pauli-principle will likely have impact on a wide range of sub-disciplines in physics, ranging from astrophysics to extremely compressed quantum liquids. Recent experiments report that such an odd-frequency superconducting bulk state may be realized in certain heavy-fermion compounds. While the Josephson current normally only flows between superconductors with the same symmetries with respect to frequency, we demonstrate that an exchange field may induce a current between diffusive even- and odd-frequency superconductors. This suggests a way to identify the possible existence of bulk odd-frequency superconductors. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H10.00003: Tunable current-phase relation in double-dot Josephson junctions Jens Koch, Karyn Le Hur The current-phase relation $I(\varphi)$ for a Josephson junction contains information about the microscopic nature of the Cooper pair transfer. In particular, junctions more complicated than the single tunnel junction exhibit characteristic non-sinusoidal forms. Here, we investigate the Josephson effect in a superconducting double dot device, similar to the devices studied experimentally by Y.\ A.\ Pashkin et al.\ [1] and E.\ Bibow et al.\ [2]. In the vicinity of a charge degeneracy line, the system reduces to a two-level system equivalent to a charge qubit. In this regime, we find that the interplay between sequential tunneling and cotunneling of Cooper pairs leads to a strongly non-sinusoidal current- phase relation, tunable via gate electrodes. We propose the measurement of $I(\varphi)$ in a SQUID configuration, analyze the implications of flux noise, and compare our results to different types of Josephson junctions such as single-dot systems and microbridges. [1] Y.\ A.\ Pashkin et al., Nature (London) \textbf{421} (2003), 823 [2] E.\ Bibow, P.\ Lafarge, L.\ L\'{e}vy, Phys.\ Rev.\ Lett.\ \textbf{88} (2002), 017003 [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H10.00004: Spontaneous spin accumulation in singlet-triplet Josephson junctions Krishnendu Sengupta, Victor Yakovenko We show that the Andreev bound states in Josephson junctions between singlet $s$-wave and triplet $p$-wave superconductors carry a net magnetic moment. This magnetic moment depends on the relative phase between the superconductors constituting the junction and changes sign when the relative phase shifts by $\pi$. We estimate the net magnetization of such junctions and suggest several realistic experiments to detect this magnetization. We also discuss possible magnetization flip in such junctions in the presence of an applied bias voltage. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H10.00005: Small Josephson junctions in asymmetric SQUIDs. Dan Sullivan, Tauno Palomaki, Mark Gubrud, Michael Dreyer, Barry Barker, James Anderson, Chris Lobb, Fred Wellstood Ultra-small Josephson junctions are known to be susceptible to quantum fluctuations in the phase difference across the junction, resulting in an effective suppression of the critical current. We have investigated a method for stabilizing this phase difference by shunting a small junction (with a critical current I$_{01} \quad \approx $ 1 nA) with an additional capacitance and incorporating the junction in a dc SQUID loop. The second junction in the Al/AlO$_{x}$/Al SQUID has a much larger critical current (I$_{02}$ $\approx $ 1 $\mu $A ), producing a SQUID that is highly asymmetric. Our results show that the SQUID inductively couples the phase differences of the large and small junctions, leading to reduced phase fluctuations, and thus allowing accurate measurement of the small junction's critical current at millikelvin temperatures. This work was supported by the National Science Foundation and the Laboratory for Physical Sciences. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H10.00006: Scanninng Josephson Tunneling Microscopy of Single Crystal Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ from a Conventional Superconducting Tip Hikari Kimura, Richard Barber, Shimpei Ono, Yoichi Ando, Robert Dynes Using a scanning tunneling microscope with superconducting Pb-coated tips (S-STM), we have observed the thermally fluctuated Josephson Effect between the tip and conventional superconductors. Such STM-based Josephson junctions are a powerful tool that can directly probe the phase of the superconducting condensate via the Josephson Effect as well as characterize the quasiparticle spectrum, both on a nanometer length scale. In this talk we present data from Josephson junctions formed between the S-STM tips and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals. These results clearly show c-axis Josephson tunneling between a conventional superconductor and both overdoped and optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. Josephson measurements at various surface locations indicate an inhomogeneous structure of the I$_{C}$R$_{N}$ product in overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. These local I$_{C}$R$_{N}$ data of the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ are related to the local superconducting gap. This work is supported by DOE Grant No. DE-FG02-05ER46194. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H10.00007: Thermal Management in Large Bi2212 Mesas used for Terahertz Sources C. Kurter, K. E. Gray, Q. Li, L. Ozyuzer, A. E. Koshelev, T. Yamamoto, K. Kadowaki, U. Welp We report the intrinsic tunneling characteristics of 300x100x1 $\mu $m$^{3}$ mesas on Bi2212 single crystals that have recently shown high-power emission at terahertz frequencies due to the ac Josephson effect. Despite the large mesa volumes compared to those of others, there is an accessible range of voltages for which self-heating does not exceed T$_{c}$ and significant terahertz emission can be observed. We use a model of the current-voltage curve, I(V), based on (1) the low-current normal-state c-axis resistance of the mesa and (2) a temperature increase proportional to power, P=IV. We find that the local temperatures along the nonlinear I(V) are consistent with the observed unpolarized thermal radiation from the mesa, thus verifying the model. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H10.00008: Quantized energy levels in quantum and classical regimes in current-biased intrinsic Josephson junction Myung-Ho Bae, Mitrabhanu Sahu, Hu-Jong Lee, Alexey Bezryadin The multiphoton transitions between quantized energy levels in the current-biased Bi$_{2}$Sr$_{2}$SrCaCu$_{2}$O$_{8+x}$ intrinsic Josephson junctions (IJJs) in the quantum and classical regimes are studied through the switching current distributions. The system shows the saturation behavior of the switching current distributions near $T^{\ast }\sim $0.8 K, which is the crossover temperature between classical and quantum nature of the system. We observe the multiphoton transitions between quantized energy levels in the quantum regime, which is manifested by the enhancement of the escape rate in the microwave radiation with frequencies of 12-16 GHz. This enhancement behavior keeps even in the classical regime and is washed out near T$\sim $2 K, of which thermal energy corresponds to the energy level spacing at the switching currents. This means that the existence of the quantized energy levels even in the classical regime of IJJs, due to the relatively large plasma frequency in the IJJs. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H10.00009: Direct observation of a sin(2$\phi )$ component in the current-phase relation of superconductor-ferromagnet-superconductor (SFS) Josephson junctions M.J.A. Stoutimore, A.Yu. Rusanov, D.J. Bahr, V.A. Oboznov, V.V. Bolginov, A.N. Rossolenko, V.V. Ryazanov, D.J. Van Harlingen We present direct measurements of the current-phase relation (CPR) of SFS Josephson junctions in an rf-SQUID geometry. The junctions are fabricated from Nb-Cu$_{47}$Ni$_{53}$-Nb trilayers with a junction area of 2x2 $\mu $m$^{2}$ and a CuNi thickness of 7 nm. By measuring the magnetic flux through the rf-SQUID as a function of applied current, we observe transitions between an ordinary 0-Josephson junction state and a $\pi $-junction state characterized by a phase difference of $\pi $ in the ground state occurring at temperatures between 1.5 K and 3.5 K. Near this temperature crossover, we observe period-doubling of the CPR indicating the existence of a term proportional to sin(2$\phi )$. Work is underway to determine if this signifies an intrinsic second-order tunneling mechanism or is the result of junction inhomogeneities. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H10.00010: Sub-Gap Currents in Nb/Al/AlOx/Nb Josephson Junctions and Their Dependence on the Method of Barrier Formation Paul B. Welander, Tim J. McArdle, Stephanie Law, James N. Eckstein Josephson tunnel-junctions have been fabricated using two different methods of barrier formation. Both types of devices start with single-crystal Nb/Al bi-layers grown by molecular beam epitaxy on A-plane sapphire. It is found that complete wetting of the Nb layer is achieved with 20 nm of Al evaporated at room temperature. The barrier is then formed either by thermal oxidation of the Al surface in molecular oxygen (the well-known process developed by Gurvitch et al.) or by co-depositing Al in an oxygen background of about 5 micro-torr. A Nb counter-electrode is deposited in situ by evaporation at room temperature. Josephson junctions fabricated from these multi-layers exhibit Fiske resonances and a reduced gap voltage due to the relatively thick Al layer. For devices tested at 4 K, the co-deposition process yields junctions that show a sub-gap current in agreement with theory and no measurable shunt conductance. In contrast, those devices with barriers formed by thermal oxidation show a small shunt conductance in addition to the predicted sub-gap current. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H10.00011: Ferromagnetic Josephson Junctions Trupti Khaire, William Pratt, Norman Birge Superconducting correlations cannot penetrate into a ferromagnet over a large distance due to the pair breaking effect of the exchange field. Ferromagnet/Superconductor (F/S) systems are often studied using weak ferromagnetic alloys with smaller exchange energy and correspondingly larger penetration depth. We are studying S/F/S junctions using the weak ferromagnetic alloy, CuNi [1]. The samples are fabricated by sputtering the S/F/S trilayer onto a Si substrate; they are subsequently patterned using trilayer photolithography and ion milling to obtain pillars of 50 micron diameter. Measurements performed on these pillars at 4K show the Josephson effect with the expected modulation of the critical current as a function of applied magnetic field. Because spin-flip scattering and spin-orbit scattering are strong in weak ferromagnetic alloys such as CuNi, there is an incentive to work with strong ferromagnets. The results from our CuNi data confirm the robustness of the sample fabrication technique and pave the way to future studies of Josephson junctions with strong ferromagnets such as Ni. [1] V. A. Oboznov, Phys. Rev. Lett. 96, 197003 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H10.00012: I-V Characteristics vs. Spatial Dissipation Maps in YBCO Grain Boundary on Bicrystal Substrates Chuhee Kwon, Megumi Yamamoto, Samuel Pottish, Timothy Haugan, Paul Barnes Grain boundary (GB) properties of YBCO films on SrTiO3 bicrystal substrates with 24 degree misorientations are examined by transport and scanning laser microscopy (SLM) techniques. Thermoelectric SLM clearly shows the location of grain boundaries, and variable temperature SLM confirms that GB has lower Tc. A series of I-V measured in superconducting states exhibit clear step-like features identified in earlier papers as sub-gap structures. The low temperature SLM shows a close relation between the step-like features and the local dissipation pattern in GB. We believe that the activation of Fiske steps is responsible for the step-like I-V, and SLM images show the spatial pattern of the self-excited resonance in GB. We will also discuss how Ca-doping and nanoparticle additions on YBCO affect the junction properties. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H10.00013: Vibronic Effects in Superconducting Niobium Nanowires Brandon Donehoo, Zhenting Dai, Alexei Marchenkov Research of superconducting transport through microscopic objects with intrinsic vibrational degrees of freedom is a frontier research avenue. Here we report on experimental studies of a few-atom niobium nanowires prepared in a mechanically-controlled break junction set-up. We present evidence for the resonant interaction between the ac Josephson effect and the mechanical motion of atoms in niobium dimer nanowires at frequencies up to about 8~THz. This is application-rich, but a largely unexplored frequency range (``terahertz gap''), which interrogates the lowest frequency vibrational modes of complex organic and biological molecules. We also discuss supercondicting transport and noise in niobium nanowires with oxygen contamination. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H10.00014: Fluxon dynamics in a Josephson junction parallel array Nikhil Fernandes, Kenneth Segall, Ushnish Ray, Adam Dioguardi We present experimental measurements on the dynamics of fluxons in an array of Josephson junctions. Fluxons trapped in a parallel array of Josephson junctions upon cooldown experience a potential determined by the junction critical currents and the cell inductances. We probe the dynamics of the fluxon with switching current measurements, which allow determination of the transition rate of the fluxon from its pinned state to a running state. The transition to the running state is initiated by thermal activation at temperatures higher than the quantum crossover temperature for the junctions. Below the crossover, we observe an abrupt change in the critical force needed to move the fluxon. Quantum tunneling of the fluxons is a possible explanation for this observation. We present the data, numerical simulations, and a discussion of the results. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H10.00015: Field-dependence of interlayer tunnelling in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ Timothy Benseman, John Cooper, Geetha Balakrishnan Micron-scale `mesa' structures fabricated on the surface of single crystals of strongly anisotropic high-temperature superconducting (HTS) compounds form stacks of `intrinsic Josephson junctions' connected in series. Studying the current-voltage (I-V) characteristics of HTS mesas is now an established technique for obtaining important information regarding the electronic density of states (DoS) in these compounds, such as the magnitude $\Delta$ of the superconducting energy gap, and its symmetry in $k$-space. We have fabricated mesas on the HTS compound Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ (Bi-2212) and studied these at a range of hole-doping levels, temperatures, and applied magnetic fields. Of particular interest is the field-dependent behaviour of the I-V characteristic at bias voltages much less than the sum-gap voltage 2$\Delta $/e, corresponding to quasiparticles near the gap nodes. We compare our results with predictions for the field-dependent DoS made by Volovik [1] in which the local energy is assumed to be Doppler shifted by the local superfluid velocity. We also discuss features seen in our tunnelling characteristics at voltages above 2$\Delta $/e, which may correspond to strong-coupling effects in Bi-2212. [1] G. E. Volovik, JETP Lett., 58: 469-473, 1993. [Preview Abstract] |
Session H11: Focus Session: MgB2-like: Computational Design of Novel Superconductors
Sponsoring Units: DMPChair: Igor Mazin, Naval Research Laboratory
Room: Morial Convention Center RO9
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H11.00001: Quasi-two-dimensional electronic states in a dense three-dimensional Li-Be alloy Richard G. Hennig, Ji Feng, Roald Hoffmann, N.W. Ashcroft High pressure can affect electronic structure and crystal packing, and in some cases even induce compound formation between elements that do not bond under ambient conditions. Our computational study for the Li-Be system shows that the reactivity of Li and Be is fundamentally altered by pressure. These two lightest of all metallic elements are immiscible at ambient conditions. Using structure search methods we discover four stoichiometric Li$_x$Be$_{1-x}$ compounds that are stable over a range of pressures. The electronic density of states of one of them displays a remarkable step-like feature and plateau at the bottom of the valence band, which is typical of a quasi-2D electronic structure and rather unexpected in a 3D crystal environment. We attribute this feature to large size differences between the ionic cores of Li and Be. Under increased pressure, the Li cores start to overlap and thereby expel valence electrons into quasi-two-dimensional layers characterized by delocalized free-particle-like states in the vicinity of Be ions. These alloys are also interesting from the perspective of superconductivity. The T$_c$ in the LiBe alloy is expected to be substantially higher than that of the component elements due to the increased density of states at the Fermi energy compared particularly to elemental Be. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H11.00002: First-principles search for potential high temperature superconductors in the Mg-B-A (A=alkaline metal) system with high boron content Roman Chepulskyy, Igor Mazin, Stefano Curtarolo Possible superconductivity at 50K was recently reported [1] in the Mg-B-A (A=Cs, Rb, Ba) system. Although attempts to reproduce this finding have been unsuccessful so far [2], if a stable or metastable phase could be found by a first principle search similar to that in Ref. 3, this would have lent credibility to the experimental finding [1] and outline possible further directions. The results of Ref. 1 suggest that (a) the superconducting phase is not similar to MgB$_2$ (B site has cubic or similarly high symmetry) and (b) boron content is higher than in MgB$_2$. We report first-principles study of the thermodynamics of alkali and alkaline earth doping in the boron- rich part of the Mg-B phase diagram (MgB$_n$ with $n>2$), searching for a phase that could explain the results of Ref. [1]. [1] A.V. Palnichenko {\it et al.}, JETP Letters \textbf{86}, 272 (2007). [2] R K Singh {\it et al} http://arxiv.org/abs/0709.4001v1. [3] S. Curtarolo {\it et al}, Calphad \textbf{29}, 163 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H11.00003: Engineering superconductors with ab initio methods: ternary metal borides Aleksey Kolmogorov, Matteo Calandra, Stefano Curtarolo We have performed a targeted search for novel superconducting ternary borides starting from the recently identified class of binary metal sandwich (MS) structures [1]. Our {\it ab initio} calculations suggest that a theoretically-devised MS lithium monoboride gains in stability when alloyed with electron-rich metals [2,3]. In an effort to pre-select compounds with the strongest electron-phonon coupling we evaluate the softening of the in-plane boron phonon mode in a large class of metal borides. Our results reveal interesting general trends for the frequency of the in-plane boron phonons as a function of the boron-boron bond length and the valence of the metal [4]. Research supported by ONR and NSF. [1] PRB 73, 180501(R) (2006) [2] PRB 74, 224507 (2006). [3] PRB 75, 144506 (2007) [4] A.N. Kolmogorov, M. Calandra, S. Curtarolo, submitted to PRB. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H11.00004: Electron-Phonon interaction in hexagonal layered compounds Lilia Boeri, Matteo Giantomassi, Giovanni B. Bachelet, Ole Krogh Andersen The discovery of superconductivity in MgB$_2$ has initiated a thorough search for new electron-phonon ($e-ph$) superconductors, particulary among hexagonal layered compounds. In this talk I will describe, using {\em ab-initio} methods, the factors that determine the electron-phonon properties of two classes of recently discovered superconductors, namely alkali-earth intercalated graphites (highest T$_c$ 15.1 K for CaC$_6$)[1] and metal-intercalated ternary compounds MaAlSi. [2] \newline References: \newline [1] J. S. Kim, L. Boeri, R. K. Kremer, and F. S. Razavi Phys. Rev. B 74, 214513 (2006), Phys. Rev. Lett. 96, 217002 (2006), and Phys. Rev. Lett. 027001 (2007). \newline L. Boeri, G.B. Bachelet, M. Giantomassi, O.K. Andersen, Phys. Rev. B 76, 064510 (2007). \newline [2] M. Giantomassi, L. Boeri, and G. B. Bachelet, Phys. Rev. B 72, 224512 (2005) [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H11.00005: Structure and Superconductivity of Calcium under Pressure Z. P. Yin, F. Gygi, W. E. Pickett The structure, phonon spectrum and electron phonon coupling of Ca under pressure is studied by first principle calculations. Experimentally, Ca at room temperature is simple cubic (SC) at pressure between 30 GPa and 109 GPa and goes to unknown structure above 109GPa. Its superconducting T$_c$ increases significantly in the SC phase, increasing to 23 K at 109 GPa (25 K at 161 GPa). Linear response calculations reveal that SC Ca is horribly unstable in the corresponding pressure range (at T=0). Ab initio molecule dynamics calculations on a 4x4x4 supercell find the SC phase is distorted into a four-atom bcc structure that is dynamically stable in the 40-110 GPa range. At even higher pressure this bcc structure becomes dynamically unstable (imaginary frequencies). T. Ishikawa {\it et al.} (private communication) proposed an orthorhombic structure around 120 GPa. We confirmed this structure with minor differences of internal parameters. The theoretical x-ray diffraction (XRD) pattern of this structure has very good match with the experimental XRD pattern of the Ca V phase. Our linear response calculations further confirm that it is dynamically stable, and indications of strong electron-phonon coupling in this phase will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H11.00006: Constraints on $T_c$ for superconductivity in heavily boron-doped diamond Jonathan E. Moussa, Marvin L. Cohen Calculations of electron-phonon coupling are performed for boron-doped diamond structures without electronically compensating defects over a wide range of boron concentration. The effects of boron substitutional disorder are incorporated through the use of randomly generated supercells, leading to a disorder-broadened distribution of results. After averaging over disorder, this study predicts a maximum bulk $T_c$ near 55 K for boron concentrations between $20\% - 30\%$, assuming the validity of the simple structural model used and a Coulomb pseudopotential of $\mu^* = 0.12$. Considering only the largest electron-phonon coupling values of the distribution, superconductivity may still percolate through the material at higher temperatures, up to 80 K, through the regions of large coupling. A synthesis path is proposed to experimentally access this class of materials. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H11.00007: Superconductivity in high-pressure solids Invited Speaker: The structural principle behind the unusual features in the high-pressure phases of simple alkali elements is reviewed. It is shown that there exists a pressure regime in which the elemental solids are likely to adopt a layer structure. There are two novel characteristics associated with this structure type. The system tends to be at the proximity of phonon and electronic instabilities. The combined effect is a significant enhancement of electron-phonon coupling, resulting in a superconducting state. We demonstrate this empirical observation with selected examples including a recently predicted novel structure of high pressure SnH$_4$ which shows very high superconducting critical temperature. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H11.00008: Layered Structures Favor Superconductivity in Compressed Solid SiH$_{4}$ Hai-Qing Lin, Xiao-Jia Chen, Jiang-Long Wang, Viktor V. Struzhkin, Ho-kwang Mao The electronic and lattice-dynamics properties of compressed solid SiH$_{4}$ have been calculated over the pressure regime up to 300 GPa with density functional theory. We find that the structures having layered network with eight-fold SiH$_{8}$ coordination favor the metallization and superconductivity. The layered $Cmca$ SiH$_{4}$ is predicted to have a superconducting transition temperature of 75 K at 70 GPa thus opening new possibilities for exploring high temperature superconductivity in the hydrogen-rich system. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H11.00009: First-Principles Study of Superconductivity in boron-doped SiC Jesse Noffsinger, Feliciano Giustino, Steven Louie, Marvin Cohen The discovery of superconductivity in materials such as intercalated graphite, alkali-doped fullerenes, and boron-doped diamond has drawn significant interest to carbon-based superconductors. Recent experiments indicate that boron-doped cubic SiC may superconduct above 1 K [1]. We investigate the superconductivity in cubic SiC using a first-principles approach. We describe the electronic structure within density functional theory and the lattice dynamics within density functional perturbation theory. The electron-phonon interaction matrix elements are calculated via a recently developed method based on Wannier functions [2]. The boron doping is accounted for by a virtual crystal approximation. In addition to the coupling of Fermi surface electronic states to optical phonon modes, there appears to be a non-negligible contribution to the electron-phonon coupling arising from acoustic phonons. Superconductivity is discussed by analyzing the similarities and the differences with respect to the closely related boron-doped diamond. [1] Z-A. Ren et. al, \textit{private communication}. [2] F. Giustino et. al, Phys. Rev. B 76, 165108 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H11.00010: First principles study of Al and C-doped MgB$_2$: evolution of two gaps and critical temperature Omar De la Pe\~na-Seaman, Romeo de Coss, Rolf Heid, Klaus-Peter Bohnen We have studied the electron-phonon and superconducting properties of the Mg$_{1-x}$Al$_x$B$_2$ and MgB$_{2(1-x)}$C$_{2x}$ alloys within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloys. For both systems, the Eliashberg spectral function ($\alpha^{2}F(\omega)$) and the electron-phonon coupling parameter ($\lambda$) have been calculated in the two band model ($\sigma$,$\pi$) for several concentrations until $x(\mathrm{Al})=0.55$ and $x(\mathrm{C})=0.175$. Using the calculated $\alpha^{2}_{ij}F(\omega)$ and a diagonal expression for the Coulomb pseudopotential matrix, $\mu^{*}$, we solved numerically the Eliashberg gap equations in the two band model without interband scattering. We reproduce the experimental decreasing behavior of $\Delta_{\sigma}(x)$, $\Delta_{\pi}(x)$, and $T_{c}(x)$ for both alloy systems. The role of the interband scattering in the observed behavior of the superconducting gaps and $T_c$ in the Al- and C-MgB$_2$ alloys is discussed. This research was supported by Consejo Nacional de Ciencia y Tecnolog\'ia (Conacyt) under Grant No. 43830-F. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H11.00011: Effects of Quenched Random Gap Inhomogeneities on the Specific Heat of a Model High-$T_c$ Superconductor David Stroud, Daniel Valdez-Balderas In many cuprate superconductors, scanning tunneling microscopy experiments show that the energy gap has substantial quenched random spatial variations. We have calculated how such gap variations affect the specific heat $C_V$ in a model for the most anisotropic of these materials. The model is based on a Ginzburg-Landau free energy functional in which position- dependent coefficients are used to model quenched inhomogeneity. Using Monte Carlo simulations, we evaluate $C_V$ for different disorder strengths. Near optimal doping, we find that quenched gap disorder substantially broadens the specific heat anomaly near the phase ordering transition $T_c$, compared to that due to thermal fluctuations alone. But for strongly underdoped samples, in which $T_c$ is greatly separated from the pseudogap temperature $T_{c0}$, disorder only slightly increases the broadening beyond the already substantial amount due to thermal fluctuations. We compare these results to recent experiments. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H11.00012: The temperature dependent gap edge in strong-coupling superconductors David G. Walmsley, XueHeng Zheng Using the theory of Eliashberg and Nambu for strong coupling superconductors we have calculated the gap function for a model superconductor and a selection of real superconductors including the elements Al, Sn, Tl, Nb, In, Pb, and Hg and one alloy, Bi$_2 $Tl. We have determined the temperature dependent gap edge in each and found that in materials with weak electron-phonon coupling ($\lambda<0.8$)it is single-valued but in materials with intermediate coupling ($0.8\leq\lambda\leq1.2$) the gap edge is double-valued whereas in materials with strong coupling ($\lambda>1.20$) not only is the gap edge double-valued but it also departs significantly from the BCS form and develops a shoulder-like structure which may in some cases denote a gap edge exceeding the $T=0$ value. These computational results support the insights obtained by Leavens in an analytic consideration of the general problem. Both the shoulder and double value arise from a common origin seated in the form of the gap function in strongly coupled materials at finite temperatures. From the calculated gap function we can determine the densities of states in the materials and the form of the tunneling current-voltage characteristics for junctions with these materials as electrodes. By way of illustration results are shown for the contrasting cases of Sn ($\lambda=0.74$) and Hg ($\lambda=1.63$). The reported results are distinct in several ways from BCS predictions. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H11.00013: A DFT study of the $UCoGe$ magnetic superconductor Pablo de la Mora, Oracio Navarro Recently Huy \textit{et al.} (PRL 99, 067006) found that $UCoGe$ is a superconductor coexisting with magnetism. Electronic structure calculations were performed on this compound using the WIEN2k package, results show that its magnetism is anisotropic, with the easy magnetic axis in the $c$-direction. The magnetic moment of the U atom is quite small, but there is a large moment in the Co atom which is in disagreement the experimental result of Huy \textit{et al.}. These results contrast with the isostructural magnetic superconductor $URhGe$, in this latter compound the U-atom magnetic moment is relatively large; while for the Rh atom it is small. The main contribution at $E_F$ is due to U-5f and Co-3d, Ge has little contribution. There are many similarities with the $MgB_2$ superconductor, there are many bands at $E_F$ and $UCoGe$ has a distorted $MgB_2$ crystalline structure, but the superconductivity mechanism seems to be of different origin. [Preview Abstract] |
Session H12: Lattice and Magnetic Properties of Multiferroics
Sponsoring Units: DCMPChair: Andrei Sushkov, University of Maryland
Room: Morial Convention Center 203
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H12.00001: Infrared phonon dynamics of multiferroic BiFeO$_3$ single crystal R.P.S.M. Lobo, R.L. Moreira, D. Lebeugle, D. Colson We discuss the first infrared reflectivity measurement on a BiFeO$_3$ single crystal between 5 K and room temperature. The 9 predicted \textit{ab}-plane $E$ phonon modes are fully and unambiguously determined. The frequencies of the 4 $A_1$ \textit{c}-axis phonons are found. These results settle issues between theory and data on ceramics. Our findings show that the softening of the lowest frequency $E$ mode is responsible for the temperature dependence of the dielectric constant, indicating that the ferroelectric transition in BiFeO$_3$ is soft-mode driven. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H12.00002: Electromagnons and Multiple Phase Transitions in BiFeO$_{3}$ Multiferroic System Mariola Ramirez, A. Kumar, S. Denev, J. Ihlefeld, D. Schlom, Venkatraman Gopalan, R. Rai, X. Xiu, J. Musfeldt, S. Lee, S. Cheong, R. Ramesh, J. Seidel, E. Chiu, J. Orestein Magnetoelectrics (ME) multiferroics has recently become an exciting research area due to its potential technological applications. Of special relevance is the case of Bismuth ferrite, BiFeO$_{3}$ (BFO) where multiferroicity coexist at room temperature. In this work, the Fe-Fe exchange interaction effects on the optical properties of BiFeO$_{3}$ are analyzed by using both, linear and non linear spectroscopy as a function of temperature. Two and three magnons Raman scattering as well as detectable \textit{electromagnons} in the second harmonic generation (SHG) signal are reported. Temperature studies up to 750 K reveals a cascade of phase transitions associated to different dynamic reorientations in the magnetic subsystem. These transitions were detectable by several optical methods including linear absorption, Raman spectroscopy and SHG due to the strong electric dipole coupling found between electromagnetic radiation and spin waves in BFO. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H12.00003: Temperature studies of multiferroic TbMnO$_3$ with resonant Raman scattering Ilka Mahns, M. Bastjan, B. Schulz, S. Mueller, A. Rusydi, M. Ruebhausen, N. Aliouane, D.N. Argyriou, H. Barath, M. Kim, S.L. Cooper Using temperature dependent resonant Raman scattering with different excitation energies from the ultraviolet (UV) to near infrared (NIR), we have investigated the complex interplay between the orbital, structural and magnetic ordering in the multiferroic material TbMnO$_3$. Depending on the scattering geometry, the magnetic transition at the N\'{e}el temperature at 41 K or the ferroelectric transition at T$_f$ = 28 K is observed. In resonance studies with an incident frequency of 1.91 eV, the Jahn-Teller mode shows a strong softening below T$_N$. In the low frequency spectra, a quasielastic response is identified. The results give information about the electron-phonon coupling and the correlations between electronic and structural degrees of freedom that contribute to the multiferroic behavior in TbMnO$_3$. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H12.00004: Temperature- and field-dependent inelastic light scattering studies of TbMnO$_{3}$ Harini Barath, Minjung Kim, S.L. Cooper, M. Rubhausen, D.N. Argyriou TbMnO$_{3}$ is one of several multiferroic materials that have coexisting magnetic and electric orders that are strongly coupled. Because it is exquisitely sensitive both to structural order and magnetic degrees of freedom, field-dependent inelastic light scattering measurements are ideally suited to studying magnetoelastic coupling and multiferroic phases in materials such as TbMnO$_{3}$.~ By carefully examining the temperature- and field-dependent evolution of new magneto-elastic modes in various phases of TbMnO$_{3}$, our study reveals several new features of the IC-C transition in TbMnO$_{3}$, including the co-existence of distinct structural phases in the intermediate field regime around the IC-C phase transition and evidence for dynamical fluctuations of the IC and C phases outside the established phase boundaries. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H12.00005: Pressure tuned phonon mode splitting in magnetic frustrated spinel ZnCr$_{2}$O$_{4}$ Tao Zhou, Zhen Qin, Zhenxian Liu, Chenglin Zhang, Sang-Wook Cheong ZnCr$_{2}$O$_{4}$ has cubic spinel structure. Below 390 K, the geometrically frustrated magnet enters a paramagnetic state. Below 12.5 K, it undergoes a first-order phase transitions, resulting into an antiferromagnetic order and a structural distortion simultaneously. An IR-active phonon related to the Cr$^{3+}$ ion's motion undergoes a splitting at 12.5 K. This transition is explained as a spin-Peierls like transition. However, the exact cause and effect in such a transition is not clear. Is it because the lattice undergoes transition first, spin just follows, or is it spins' interaction that forces the lattice to undergo changes? Pressure can provide a crucial service in clarifying this issue, since pressure can change spin and lattice interactions in different ways, it can differentiate these two scenarios. We have measured the infrared absorption spectra of ZnCr$_{2}$O$_{4}$ under pressure. Our data shows that Tc, at which the spin-Peiers like transition occurs and the phonon at about 370 cm$^{-1}$ starts to show the splitting, increases from its ambient pressure value of 12.5 K to about 15.8 K at 1 GPa. This provides an important clue for the exact nature of this transition. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H12.00006: Calculation of the Order Parameter and the Damping Constant in the Ferroelectric Phase for NaNO$_{2}$ Ali Kiraci, Huseyin Karacali, Hamit Yurtseven The temperature dependence of the order parameter and the damping constant is calculated in the ferroelectric phase in the range of 27 to 162 $^{o}$C close to the phase transition (T$_{c}$=436 K) for NaNO$_{2}$. The values of the order parameter calculated from the molecular field theory, are used to evaluate the damping constant as a function of temperature on the basis of the soft phonon-hard phonon coupling model for NaNO$_{2}$ in the ferroelectric phase. By representing the damping constant calculated at various temperatures in terms of an Arrhenius plot, the activation energy is computed for this crystal in the ferroelectric phase. Our calculated order parameter agrees with the measured one and also the damping constant predicts the critical behaviour exhibited by the NaNO$_{2}$ crystal near the transition temperature in the ferroelectric phase. From the values of the activation energies obtained here, the mechanism of an order-disorder transition which involves the orientation of the NO$_{2}^{-}$ ions is investigated for NaNO$_{2}$. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H12.00007: Pressure Dependence of the Phonon Modes of Hexagonal-RMnO3 Peng Gao, Trevor A. Tyson, Zhenxian Liu, Sung Baek Kim, Sang-Wook Cheong We present high pressure IR measurements of the phonon spectra of HoMnO3 and YMnO3. Measurements were conducted over the pressure range ambient to $\sim $20 GPa. No phase changes were observed over this broad range of hydrostatic pressures. A strong non-linear variation of frequency with pressure is observed suggesting saturation at higher pressures. A discussion of the effect of hydrostatic pressure on the ferroelectric properties of these systems will be given based on comparisons with density functional calculations. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H12.00008: Probing Spin-Lattice Correlations in Hexagonal RMnO3 Multiferroics Trevor Tyson, Zhiqiang Chen, Sungback Kim, Sang-Wook Cheong The hexagonal multiferroic system RMnO3 is known to exhibit strong spin-lattice correlations based on bulk thermal expansion measurements. Enhanced correlations at the spin ordering temperatures are observed. In this work, we examine the local structure about the R and Mn sites in order to determine the changes in atomic interactions which coincide with the spin alignments. Measurements over a abroad range of temperatures are presented and estimates of the changes in atomic bond distances are given. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H12.00009: Giant magneto-elastic coupling in multiferroic hexagonal manganites Seongsu Lee, A. Pirogov, M.S. Kang, K.-H. Jang, M. Yonemura, T. Kamiyama, S.-W. Cheong, F. Gozzo, Namsu Shin, H. Kimura, Y. Noda, J.-G. Park In order to investigate a possible structural change of RMnO$_3 $ at the magnetic transition temperature, we have carried out high-resolution structural studies using neutron diffraction. Here we show that the hexagonal manganites RMnO$_3$ undergo an isostructural transition at T$_N$ with unusually large atomic displacements: two orders of magnitude larger than those seen in any other ordinary materials, resulting in a uniquely strong magneto-elastic coupling. For the first time, we could follow the exact atomic displacements of all the atoms in the unit cell as a function of temperature and found consistency with theoretical predictions based on group theories. We argue that this gigantic magneto-elastic coupling of RMnO$_3$ arises from geometrical frustration, and holds the key to the recently observed magnetoelectric phenomenon in this intriguing class of materials. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H12.00010: Anomalous low-temperature magnetic ordering and spin-phonon coupling in BiFeO$_{3}$ thin films Manoj Singh, Ram Katiyar, W. Prellier, H.M. Jang Low-temperature magnetic properties and Raman spectra of epitaxal BiFeO$_{3}$ (BFO) thin films grown on (111) SrTiO$_{3}$ substrates have been studied. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves showed a large discrepancy beginning at a characteristic temperature which did depend on the magnetic-field strength, suggesting a spin-glass-like behavior of the epitaxial BFO film with R3c symmetry. For all three major A$_{1}$-symmetry Raman modes (138, 170, and 214 cm$^{-1})$, there was a good linear correlation between the mode-frequency softening and the square of the in-plane magnetization in the temperature range between 80 and 300 K. These observations were ascribed to the spin-phonon coupling below the N\'{e}el temperature (T$_{N }$= 643 K). [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H12.00011: Magneto-dielectric study of multiferroicity in biferroic YCrO$_{3}$ Relja Vasic, Haidong D. Zhou, Chris R. Wiebe, James S. Brooks Dielectric measurements are used to characterize multiferroicity in the doped monoclinic ferroelectric oxides $Y_{1-x}$\textit{Ca}$_{x}$\textit{MnO}$_{3}$ ($x$ = 0, 0.15, 0.3). The focus of this study is on the effects of the magnetic field and Ca doping on the temperature- magnetic field dependent dielectric response of polycrystalline samples of rare earth chromates YCrO$_{3}$. YCrO$_{3}$ shows dielectric relaxation around 150 K related to the weak ferromagnetic ordering in system. Dipolar response is activated following Arrhenius formalism in the frequency range of 0.01-100 kHz, but it is independent on magnitude of magnetic field. As frequency increases dielectric peaks become broader and smaller. Dielectric constant relaxation behavior and magnetic phase transitions are not coupled through lattice distortions in this ferroelectric ferromagnetic system. YCrO$_{3}$ is an example of system with dipolar response in magnetic field without coupling with magnetic structure. There is strong dispersion of Debye relaxation peaks but absence of influence of magnetic field on ferroelectric system YCrO$_{3}$. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H12.00012: Multiferroicity in the spin-1/2 quantum matter of LiCu$_{2}$O$_{2}$ Andrivo Rusydi, Ilka Mahns, Sonja Mueller, Michael Ruebhausen, S. Park, Y.J. Choi, C.L. Zhang, S.-W. Cheong, Serban Smadici, Peter Abbamonte, Martin von Zimmermann, George Sawatzky Multiferroicity in LiCu$_{2}$O$_{2}$ single crystals is studied using resonant soft x-ray magnetic scattering, hard x-ray diffraction, heat capacity, magnetic susceptibility, and electrical polarization. Two magnetic transitions are found at 24.6 K (\textit{T}$_{1}$ and 23.2 K (\textit{T}$_{2}$. Our data are consistent with a sinusoidal spin structure at \textit{T}$_{2}$ $<$ \textit{T} $<$ \textit{T}$_{1}$ and with a helicoidal spin structure at \textit{T} $<$ \textit{T}$_{2}$ giving rise to ferroelectricity. Surprisingly, above \textit{T}$_{2}$ the correlation lengths of the spin structures increase as the temperature increases with dramatic changes of $\sim $42{\%} occurring along the \textit{c} -axis. Our results demonstrate the intimate connection between frustration and coupling between electronic and magnetic polarizations in LiCu$_{2}$O$_{2}$. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H12.00013: Magnetically-induced ferroelectric polarization in a molecule-based quantum magnet Vivien Zapf, Frederik Fabris, Michel Kenzelmann, Fedor Balakirev, Ying Chen, Colin Broholm Ferroelectricity coupled to antiferromagnetic (AFM) order has been observed in the organic S=1/2 chain compound CDC (CuCl$_{2}$ 2(CH$_{3})_{2}$SO). For magnetic fields along the orthorhombic c-axis, AFM order occurs below T$_{N}$ = 0.93 K and H $\sim $ 4 T. A spin-flop transition above H$_{sf}$ = 0.35 T leads to a magnetically ordered state that breaks inversion symmetry along the b-axis for 0.35 T $<$ H $<$ 4 T. Measurements of the pyroelectric effect and the dielectric constant along b indicate ferroelectricity occurring in this same region of HT phase space with the spin polarization closely tracking the magnetic order parameter. The ferroelectric polarization is observed without electrically poling the material, and polarization switching can be observed by consecutive field sweeps in the same direction. While the magnetically-induced ferroelectricity in CDC is far from practical temperatures and fields, it nevertheless demonstrates that this phenomena can occur in a whole new class of compounds. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H12.00014: Spin Dynamics and Spin-flop transition in Magnetoelectric Effect LiMnPO$_{4}$ J. Li, W. Tian, Y. Chen, J.L. Zarestky, D. Vaknin, J.W. Lynn Neutron scattering techniques were used to study~the magnetic phase transition and spin dynamics in single crystal~LiMnPO$_{4}$ both with and without magnetic field.~~Elastic scattering confirmed that~LiMnPO$_{4}$ has a collinear antiferromagnetic ground state with~moments along $a$-axis in zero-field. The temperature dependent~order parameter, calculated from the integrated intensity of the (010)~magnetic reflection, was fit to a power law equation, yielding a~transition temperature $T_{N}$ = 33.8 K. By applying magnetic field along the a-axis, the moments rotate from a-axis to the c-axis at a critical field of 3.5 Tesla at 5 K.~ The field dependent (100) and (001) intensities indicate a complicated intermediate state between the ground state and the spin-flop state. The critical field increased from 3.5 Tesla at 5 K to 4.5 Tesla near the transition $T_{N}$. Spin-wave dispersion curves along the three principal axes were measured in the antiferromagnetic state at 4.5 K in zero magnetic field and were analyzed using a 3D Heisenberg~model. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H12.00015: Magnetic Excitations in LiCoPO$_{4}$ Wei Tian, Jiying Li, Jeff Lynn, Jerel Zarestky, David Vaknin LiCoPO$_{4}$ continues to attract much attention due to its exceptionally large magnetoelectric (ME) effect coefficient and the observed weak ferromagnetism and ME ``butterfly loop'' anomaly. To gain insight into the microscopic magnetic interactions in LiCoPO$_{4}$, inelastic neutron scattering experiments were performed in the antiferromagnetic phase at T = 8 K (T$_{N} \approx $ 21.8 K). Weak dispersion was detected in the magnetic excitation spectra along the three crystallographic axes measured around the (0 1 0) magnetic reflection. A gap of $\sim$ 4.7 meV was observed below T$_{N}$ that vanished above T$_{N}$. We analyze the data within a linear spin-wave approximation by explicitly including single-ion anisotropy terms in the Heisenberg spin Hamiltonian. The magnitude of the single-ion anisotropy is found to be comparable to the strongest nearest-neighbor magnetic interaction suggesting that the Co$^{2+}$ single ion anisotropy plays an important role in the spin dynamics of LiCoPO$_{4}$. [Preview Abstract] |
Session H13: Focus Session: Simulations of Matter at Extreme Conditions III: Classical MD, Potentials, and Energetic Materials
Sponsoring Units: DCOMP GSCCMChair: Carter White, Naval Research Laboratory
Room: Morial Convention Center 204
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H13.00001: Theoretical Approach for Developing Accurate Potentials for Molecular Dynamics Simulations: Thermoelastic Response of Aluminum J.M. Winey, A. Kubota, Y.M. Gupta To achieve the correct thermoelastic response of solids in classical simulations, a new approach is presented for developing accurate interatomic potentials. In this approach, the potentials are fitted to values for the atomic volume and the second- and third-order elastic constants at T = 0K by extrapolating the room temperature values, using classical thermo-mechanical relations. This procedure avoids the low- temperature quantum regime, enabling recovery of the correct response in classical simulations to higher temperature. As an example of this approach, an EAM potential was developed for aluminum. Results using this potential provide consistently better agreement with thermoelastic data at higher temperature compared to previous EAM potentials. Our approach is applicable to the development of other types of potentials as well and is amenable to incorporating the results of first principles calculations performed using the classically extrapolated volume for T = 0K. Work supported by DOE. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H13.00002: Petaflop simulations of shock-induced particulate ejection from copper Timothy C. Germann, James E. Hammerberg, Guy Dimonte We present the results of several large-scale, Non-Equilibrium Molecular Dynamics (NEMD) simulations of shock-induced surface instability development. We consider single crystal Cu described by an embedded atom method (EAM) potential and driven by a shock wave along the [111] crystallographic direction, impinging upon a roughened Cu/vacuum or Cu/Ne interface. The initial temperature is 300K and the NEMD simulation cell is a quasi-2D $2.23\mu m \times 5.67 \mu m$ slab geometry, 1.5 nm thick in the (periodic) third dimension. The first third of the sample length ($1.89 \mu m$) is occupied by Cu ($5.3 \times 10^{8}$ atoms), and the remainder either empty vacuum or Ne gas at a pressure of 0.67 MPa ($1.95 \times 10^{8}$ atoms). The Cu/Ne (or Cu/vacuum) interface has an initial perturbation with average amplitude 30 nm and dominant wavelength of $0.74 \mu m$. A shock wave is created by driving the free end of the Cu slab at a fixed particle velocity $u_p =$ 2.0 to 3.5 km/s. Single-mode and multi-mode interfaces were considered using 212,992 CPUs of the LLNL BlueGene/L supercomputer for times approaching 1 ns. At the higher particle velocities, the Cu release state is in the fluid-solid mixed phase. We discuss the evolution of the density and velocity distributions of the ejected mass, the modes of particle breakup, and comparisons to source theories of ejecta formation and Richtmyer-Meshkov instabilities. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H13.00003: Interatomic bond-order potentials for molecular dynamics simulations of materials at extreme conditions Romain Perriot, Mikalai Budzevich, Ivan Oleynik Molecular dynamics (MD) is a powerful research tool for studying materials at extreme conditions. At the heart of MD are the interatomic potentials, whose quality in describing a variety of chemical effects, including bond-breaking and bond-making, plays a decisive role in delivering meaningful results. We have performed extensive MD simulations of shock compression of covalently bonded materials, such as diamond and silicon, and found that REBO interatomic potential for diamond and EDIP potential for Si have substantial deficiencies at large pressures and temperatures in spite of the fact that the near equilibrium properties of both diamond and silicon are well reproduced. We are addressing this outstanding issue by developing analytic bond-order potentials (BOPs) specifically for the simulation of covalently bonded materials at extreme conditions. These BOPs include explicit analytic expressions for both the $\sigma $ and $\pi $ bonds. We will discuss important steps of BOP construction which includes devising a fist-principle database of fundamental materials properties, fitting this database by the orthogonal tight-binding, and devising the analytic BOPs using the direct link between TB and analytic BOPs via the bond orders. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H13.00004: Interatomic Potentials for Large-Scale Simulations of High-Pressure, High-Temperature Phenomena Invited Speaker: The use of large-scale atomistic simulations in the study of high-compression, high strain-rate phenomena has dramatically increased in the last decade. Most of this type of simulations utilize classical empirical or semi-empirical potentials to describe the inter-atomic interactions. The regime of validity of most of these potentials is however often limited to a narrow region of the pressure-temperature phase diagram. In the development of accurate inter-atomic potentials for material simulations at high-pressures or temperatures, a high degree of transferability is desirable without resorting to fitting everywhere in phase space. We will review two popular cluster functional models: the embedded-atom-method (EAM) and the modified embedded-atom-method (MEAM). The embedded atom method provides a very good description of metallic properties at a low computational cost and has become the workhorse of large-scale atomistic simulations of metallic systems. The modified embedded-atom-method is an improvement of EAM which includes the effect of angular bonding. We outline inherent limitations of these models and present a systematic approach to improving their transferability and predictive accuracy at high pressures and/or temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H13.00005: Ultrafast semi-metallic layer formation in detonating nitromethane Evan Reed, M. Riad Manaa, Laurence Fried, Kurt Glaesemann, John Joannopoulos We present the first quantum molecular dynamics simulations behind a detonation front (up to 0.2 ns) of the explosive nitromethane (CH$_3$NO$_2$) represented by the density-functional-based tight-binding method (DFTB). This simulation is enabled by our recently developed multi-scale shock wave molecular dynamics technique (MSST) that opens the door to longer duration simulations by several orders of magnitude. The electronic density of states around the Fermi energy initially increases as metastable material states are produced but then later decreases, perhaps unexpectedly. These changes indicate that the shock front is characterized by an increase in optical thickness and conductivity followed by a reduction around 100 picoseconds behind the front. We find that a significant population of intermediate metastable molecules are charged and charged species play an important role in the density of states evolution. The transient transformation to a semi-metallic state can be understood within the Anderson picture of metallization. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H13.00006: Reactive Molecular Dynamics Studies of Thermal Induced Chemistry in HMX Jason Quenneville, Timothy Germann, Thomas Sewell, Edward Kober Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemistry induced by intense heating (`cook-off') of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers$^{1}$ at CalTech and has shown promise in predicting the chemistry in a variety of systems, including RDX and TATB under either shock compression or intense heat. In the current work, we investigate the effect of initial equilibration temperature (1000 to 1500 K), volumetric compression, crystal polymorph ($\beta $ and $\delta )$, and system size (ranging from 150 to 1200 molecules) on the reaction rate and reaction products. Finally, we will compare these results with those from our previous work on TATB. $^{1}$A. C. T. Van Duin, \textit{et al}, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001). [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H13.00007: Anisotropic Constitutive Relationships in Energetic Materials: TATB Mikalai M. Budzevich, Aaron Landerville, Mike Conroy, Ivan I. Oleynik, Carter T. White One of the principal thrusts in energetic materials (EM) research is the acquisition of accurate equations of state (EOS) for various important classes of EMs. In the past, both theoretical and experimental studies concentrated on hydrostatic EOS. However, these isotropic EOS still need to be expanded to include anisotropic materials response, including uniaxial compression which are more relevant to shock initiation of detonation. To this end, we performed first-principles density functional calculations of the EOS for TATB, including uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] crystallographic directions. Equilibrium properties, such as lattice parameters and elastic constants, as well as the hydrostatic EOS were calculated and compared with experimental results. Finally, we discuss the possible relationship between shear stresses induced by the uniaxial compression of TATB and the relative shock sensitivities of different crystallographic directions. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H13.00008: First-Principles Reactive Molecular Dynamics of Chemistry in Detonating Energetic Materials Aaron Landerville, Ivan I. Oleynik, Mortko A. Kozhushner, Carter T. White We investigated the initial chemistry of shock compressed energetic materials that results from inter-molecular collisions behind the shock wave front by performing first-principles MD simulations of bimolecular collisions for PETN and RDX with different crystallographic orientations and velocities. For each orientation, we determined the threshold collision velocity for reaction, the reaction timescales, and the products of decomposition. We find that the calculated threshold velocities lie within the range of typical particle flow velocities in detonating materials. Owing to the extremely short reaction timescales ($\sim $ 10$^{-13 }$s), these initial chemical events are largely driven by the direct collision dynamics, instead of temperature. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H13.00009: Reactive MD simulations of anisotropic response of PETN under high-rate shear deformation Xu Peng, Sergey Zybin, Aidan P. Thompson, William A. Goddard Several experiments have indicated that the shock sensitivity of single crystal energetic materials can depend on the crystallographic direction. We develop a compress-and-shear modeling approach to study the mechanisms of anisotropic shock sensitivity using the ReaxFF reactive molecular dynamics. ReaxFF is a first-principles based force field capable to reproduce the quantum chemical energies of the reactants, products, intermediates and transition states with functional forms suitable for large-scale molecular dynamics simulations of chemical reactions under extreme conditions. In this presentation we will discuss the results of high-rate shear simulations of uniaxially compressed PETN. We found noticeable differences in the physical and chemical responses of PETN for different combinations of the slip system and compression direction. The simulation results agree well with the experimental shock-initiation sensitivity data and Dick's steric hindrance theory. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H13.00010: First-Principles Constitutive Relationships in PETN and HMX under Hydrostatic and Uniaxial Compressions Sergey V. Zybin, Michael W. Conroy, Ivan I. Oleynik, Carter T. White The physical mechanisms leading to shock-induced detonation at the atomic level are ultimately related to energetic materials response to uniaxial compression at the shock front. Due to the intrinsic anisotropy of the constitutive relationships, a description of the compressed state should be extended beyond hydrostatic equations of state that are frequently used for analysis of precursor states of EMs. In this presentation, we will discuss the results of first-principles density functional theory calculations of both hydrostatic and uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] directions applied to the energetic materials PETN-I and $\beta $-HMX. A comparison will be made of available experimental data with calculated physical properties such as unit cell geometry, isothermal equations of state, and elastic constants. The presentation will focus on the anisotropic nature of the constitutive relationships in molecular crystals under uniaxial compression. The behavior of the shear stress projections on available slip systems upon uniaxial strain and their possible relationship to experimental shock-initiation sensitivity data will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H13.00011: Reactive molecular dynamics simulations of shocked PETN Joanne Budzien, Aidan P. Thompson, Sergey V. Zybin We have performed molecular dynamics simulations of PETN crystals subjected to shock along the [100] direction. Using the reactive forcefield, ReaxFF, and the molecular dynamics code, GRASP, allows us to track the chemical reactions that occur as both a function of time and position. By simulating larger systems, we can observe the formation of both primary and secondary products to make comparisons with experiments. Composition profiles of these products will be shown along with profiles of stress, temperature, and potential energy. [Preview Abstract] |
Session H14: Cold Gases: One Dimensional Phenomena
Sponsoring Units: DAMOPChair: Robert Cherng, Harvard University
Room: Morial Convention Center 205
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H14.00001: Feshbach physics in a one-dimensional optical lattice Nicolai Nygaard, Rune Piil, Klaus M{\O}lmer We consider a pair of atoms in a one-dimensional optical lattice interacting via a Feshbach resonance. Using a two-channel description of the resonance, we derive the analytic form of the Fano scattering resonance inside the continuum band and the discrete bound states outside the band. We suggest experiments to probe and utilize the special properties of the system, which arise from the continuum having an upper edge. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H14.00002: Pairing states of a polarized Fermi gas trapped in a one-dimensional optical lattice Adrian Feiguin, Fabian Heidrich-Meisner We study the properties of a one-dimensional (1D) gas of fermions trapped in a lattice by means of the density matrix renormalization group method, focusing on the case of unequal spin populations, and strong attractive interaction. In the low density regime, the system phase-separates into a well defined superconducting core and a fully polarized metallic cloud surrounding it. We argue that the superconducting phase corresponds to a 1D analogue of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, with a quasi-condensate of tightly bound bosonic pairs with a finite center-of-mass momentum that scales linearly with the magnetization. In the large density limit, the system allows for four phases: in the core, we either find a Fock state of localized pairs or a metallic shell with free spin-down fermions moving in a fully filled background of spin-up fermions. As the magnetization increases, the Fock state disappears to give room for a metallic phase, with a partially polarized superconducting FFLO shell and a fully polarized metallic cloud surrounding the core. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H14.00003: The FFLO Phase in Imbalanced Spin Fermions in 1-d Miriam Huntley, Richard Scalettar, George Batrouni, Valery Rosseau Pairing in imbalanced spin populations has lately been the focus of many experimental and theoretical studies. Different mechanisms have been proposed to explain how pairing takes place between the two fermionic species. In this talk we will present exact Quantum Monte Carlo simulations of imbalanced spin populations in one dimension. We will first discuss the case where no confining potential is imposed. Here we have found that the pair-pair correlation function develops oscillations at nonzero polarizations, corresponding to a pair momentum distribution that, instead of being peaked at zero, is peaked at a momentum equal to the difference in the Fermi momenta. This indicates that the pairing mechanism is described by the Fulde Fuerrel Larkin Ovchinnikov (FFLO) picture, and not by other proposed phases which require zero-momentum pairing. When a confining potential is included in the simulations, we find that the FFLO oscillations are still present. In addition, we show that the local polarization displays a dip in the center of the trap, similar to experimental observations performed with 3-dimensional optical traps. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H14.00004: One dimensional trapped fermions with attractive contact interactions Michele Casula, David Ceperley Recent advancements in cold atomic physics allow creation of optical lattices which reproduce well defined model Hamiltonians. This opens the route to resolve the phases of strongly correlated systems by carrying out experiments with trapped cold atoms. In this work, we study the properties of one dimensional trapped spin 1/2 fermions with attractive contact interactions by means of exact quantum Monte Carlo techniques. According to the local density approximation (LDA), such a system is expected to show phase separation between a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state and either fully polarized or fully paired outer shells.$[1]$ Here we show how the size and temperature affect the LDA phase boundaries. Mapping out the dependence on the number of particles, temperature, and interaction strength is extremely useful to benchmark experiments where cold atoms are trapped in arrays of cigar-shaped tubes, and understand whether the related setup will be able to detect the signatures of the FFLO state. \newline [1] G. Orso, Phys. Rev. Lett. 98, 070402 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H14.00005: Quantum Monte Carlo study of the visibility of one-dimensional Bose-Fermi mixtures Christopher Varney, Valery Rousseau, Richard Scalettar The study of ultra-cold, optically trapped atoms has opened new vistas in the physics of correlated quantum systems. Much attention has now turned to mixtures of bosonic and fermionic atoms. A central puzzle is the disagreement between the experimental observation of a reduced bosonic visibility ${\cal V}_b$, and quantum Monte Carlo (QMC) calculations which show ${\cal V}_b$ increasing. In this talk, we present new QMC simulations which evaluate the density profiles and ${\cal V}_b$ of mixtures of bosons and fermions in one-dimensional optical lattices. We resolve the discrepancy between theory and experiment by identifying parameter regimes where ${\cal V}_b$ is reduced, and where it is increased. We present a simple qualitative picture of the different response to the fermion admixture in terms of the superfluid and Mott insulating domains before and after the fermions are included. Finally, we show that ${\cal V}_b$ exhibits kinks which are tied to the domain evolution present in the pure case, and also additional structure arising from the formation of composite boson-fermion particles. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H14.00006: Molecular superfluid phase in one-dimensional multicomponent fermionic cold atoms Guillaume Roux, Sylvain Capponi, Philippe Lecheminant, Patrick Azaria, Edouard Boulat, Steven R. White We study a simple model of $N$-component fermions with contact interactions which describes fermionic atoms with $N = 2F + 1$ hyperfine states loaded into a one-dimensional optical lattice. We show by means of analytical and numerical approaches that, for attractive interaction, a quasi-long-range molecular superfluid phase emerges at low density. In such a phase, the pairing instability is strongly suppressed and the leading instability is formed from bound-states made of $N$ fermions. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H14.00007: Spin and charge velocities of one-dimensional boson-fermion mixture Shi-Jian Gu, Junpeng Cao, Hai-Qing Lin We study the ground state and elementary excitations of a one-dimensional mixture of scalar bosons and spin-1/2 fermions with repulsive delta-function interaction by the Bethe-ansatz method. Though the ground state properties are dramatically changed once bosons are mixed into fermions, the spin and charge excitations still feature linear dispersions, and their velocities satisfy a simple universal relation. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H14.00008: The ground state properties of two-component ultra-cold Fermi gas in hard wall confinement Bo-Bo Wei, Jun-Peng Cao, Shi-Jian Gu, Hai-Qing Lin We investigate the ground state properties of a one-dimensional two-component ultra-cold Fermi gas subjected to a hard wall trap. The explicit form of the wave function is obtained by solving the Bethe Ansatz equations numerically. Then we obtain the one-body density matrix of the system for different interaction strengths. Results of the momentum distribution of the atoms, which are obtained from the Fourier transform of the one-body density matrix, are reported for different interaction strengths. This interacting system may be experimentally accessible using ultra-cold atoms. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H14.00009: Quantum Phase Transition between (Luttinger) Liquid and Gas of Cold Molecules Dima Feldman, Kam Tuen Law We consider cold polar molecules confined in a helical optical lattice similar to those used in holographic microfabrication. An external electric field polarizes molecules along the axis of the helix. The large-distance inter-molecular dipolar interaction is attractive but the short-scale interaction is repulsive due to geometric constraints and thus prevents collapse. The interaction strength depends on the electric field. We show that a zero-temperature liquid-gas transition occurs at a critical field. It can be observed under experimentally accessible conditions. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H14.00010: Dynamics of excitations in a one-dimensional Bose liquid Maxim Khodas, Michael Pustilnik, Alex Kamenev, Leonid I. Glazman We studied the dynamical structure factor $S(q,\omega)$ of interacting bosons in one-dimension. The sharp resonant peak $S(q,\omega) \propto \delta(\omega - \epsilon(q))$ as predicted by the Bogolubov theory is transformed into a power law singularity, $S(q,\omega) \propto (\omega - \epsilon(q))^{-\mu(q)}$ due to the strong quantum fluctuations. The corresponding momentum dependent exponent $\mu(q)$ is evaluated using the Lieb-Liniger model. The full momentum dependence $\mu(q)$ has been found in the strongly interaction regime using the Fermi Bose mapping. For the large momentum $q$ the different method allows us to express the exponent through the Luttinger liquid parameters. The two results agree in their common region of applicability. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H14.00011: Probing the excitations of a one dimensional topological Bose insulator Emanuele G. Dalla Torre, Erez Berg, Ehud Altman We investigate the dynamic response of a system of ultracold dipolar atoms or molecules in the one dimensional Haldane Bose insulator phase. This phase, which was recently predicted theoretically [1], is characterized by non-local string order and its elementary excitations are domain walls in this order. We compute experimentally relevant response functions and we derive asymptotically exact expressions near the quantum critical points separating the Haldane insulator from the conventional Mott and density wave insulators. In particular, we predict a narrow absorption peak in Bragg spectroscopy experiments, due to the excitation of a single domain wall in the string order. \newline [1] E.G. Dalla Torre, E. Berg, E. Altman, {\it Phys. Rev Lett.} {\bf 97}, 260401 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H14.00012: Adiabatic loading of a one dimensional system of interacting bosons into an optical lattice Claudia De Grandi, Anatoli Polkovnikov We study excitations from the ground state of a system of bosons confined to one dimension (1D) and interacting via contact repulsion, while loading them adiabatically into an optical lattice. In particular, we analyze the dependence of the density of created excitations on the ramp rate $\delta$. We find that for strongly interacting commensurate bosons this density scales as $\sqrt{\delta}$, while for weakly interacting bosons it scales as $\delta^{2}$. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H14.00013: Bosonic Mixtures in one-dimensional optical lattices I. Ludwig Mathey, Anzi Hu, Ippei Danshita, Carl Williams, Charles Clark We study binary bosonic mixtures of ultra-cold atoms, confined to one dimension in an optical lattice, for various densities. Within a Luttinger liquid description, using renormalization group equations at second order, we determine both ordered and quasi-ordered phases of these mixtures. At commensurate filling, e.g. unit-filling and half-filling, we find different types of localized phases, whereas for densities that are incommensurate to the lattice, but equal for the two species, we find the behavior of a spin gap transition. We discuss the properties of the various phases, and how they could be detected in experiment. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H14.00014: Bosonic Mixtures in one-dimensional optical lattices II Anzi Hu, Ludwig Mathey, Ippei Danshita, Charles Clark, Carl Williams Using a Time-evolving Block Decimation (TEBD) approach, we study the quantum phases of bosonic mixtures in one dimension. With this numerical approach we determine how various phases appear for both finite and infinite systems. In particular, we relate the different phases of this system to the microscopic parameters of the underlying Bose-Hubbard model and calculate the correlation functions that characterize the phases in different parameter regions. Furthermore, for a finite system, we discuss the effects of a global trap on the state of the system. [Preview Abstract] |
Session H15: Focus Session: Superconducting Qubits I
Sponsoring Units: GQIChair: Britton Plourde, Syracuse University
Room: Morial Convention Center 207
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H15.00001: Designing quantum-information-processing superconducting qubit circuits that exhibit lasing and other atomic-physics-like phenomena on a chip Invited Speaker: Superconducting (SC) circuits can behave like atoms making transitions between a few energy levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip. This talk overviews a few of our theoretical studies on SC circuits and quantum information processing (QIP) including: SC qubits for single photon generation and for lasing; controllable couplings among qubits; how to increase the coherence time of qubits using a capacitor in parallel to one of the qubit junctions; hybrid circuits involving both charge and flux qubits; testing Bell's inequality in SC circuits; generation of GHZ states; quantum tomography in SC circuits; preparation of macroscopic quantum superposition states of a cavity field via coupling to a SC qubit; generation of nonclassical photon states using a SC qubit in a microcavity; scalable quantum computing with SC qubits; and information processing with SC qubits in a microwave field. Controllable couplings between qubits can be achieved either directly or indirectly. This can be done with and without coupler circuits, and with and without data-buses like EM fields in cavities (e.g., we will describe both the variable-frequency magnetic flux approach and also a generalized double-resonance approach that we introduced). It is also possible to ``turn a quantum bug into a feature'' by using microscopic defects as qubits, and the macroscopic junction as a controller of it. We have also studied ways to implement radically different approaches to QIP by using ``cluster states'' in SC circuits. For a general overview of this field, see, J.Q. You and F. Nori, Phys. Today 58 (11), 42 (2005) [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H15.00002: A Tunable Coupling Architecture For Josephson Phase Qubits Radoslaw Bialczak, M. Ansmann, M. Hofheinz, E. Lucero, R. McDermott, M. Neeley, A.D. O'Connell, H. Wang, A. Cleland, J. Martinis Previous coupled-qubit experiments with Josephson phase qubits have used a fixed coupling scheme. However, in order to create high-fidelity multi-qubit gates, a tunable coupling scheme is needed. Fixed coupling schemes cannot be used because single-qubit operations on a coupled-qubit system cannot be performed with high fidelity due to the errors induced by always-on coupling. Fixed coupling also allows for crosstalk between coupled qubits during measurement. We show how to implement a tunable-coupling architecture for Josephson phase qubits using simple linear elements. This architecture can be used to vary the interaction strength from fully-off to fully-on allowing us to get around the problems inherent with the use of a fixed coupling scheme. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H15.00003: High-fidelity gates in Josephson phase qubits Erik Lucero, Markus Ansmann, Radoslaw Bialczak, Max Hofheinz, Nadav Katz, Mattthew Neeley, Aaron O'Connell, Haohua Wang, Andrew Cleland, John Martinis Complex algorithms for a quantum computer will require error correction, which calls for logic gates with fidelity below a fault tolerant threshold. We present significant progress towards this goal with our detailed measurements of gate fidelity. We carefully separate out gate and measurement error and construct a complete error budget to demonstrate single qubit gate fidelities of 0.98, limited by energy relaxation. We introduce a new metrology tool `a Ramsey interference error filter' that can measure the excited two-state population down to 10$^{-4}$, a magnitude near the fault tolerant threshold. This measurement demonstrates that our quantum system remains in the two-state qubit manifold during our single qubit operations. This precision and accuracy is made possible by custom control electronics that can create arbitrarily shaped microwave pulses. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H15.00004: Recent progress towards violating the CHSH Bell inequality in Josephson phase qubits Markus Ansmann, Radoslaw Bialczak, Max Hofheinz, Nadav Katz, Erik Lucero, Matthew Neeley, Aaron O'Connell, Haohua Wang, Andrew Cleland, John Martinis Improvements of gate fidelity and control electronics for Josephson phase qubits have provided the foundation for revisiting Bell's inequality. The violation of Bell's inequality is the primary argument against the possible existence of a hidden-variable-theory as an alternative to quantum mechanics. It also serves as a convincing demonstration that a given system behaves in a truly non-classical way. The most widely accepted form of Bell's inequality follows closely along a correlation measurement proposed by Clauser, Horne, Shimony and Holt (CHSH) in 1969. Here we present our latest attempt to implement the CHSH Bell test using Josephson phase qubits. The nature of this experiment places high demands -- compared to the current state of the art in solid state qubits -- on qubit performance measures such as the energy relaxation time T1, the decoherence time T2, single and two qubit gate fidelities, and measurement fidelities. We will examine these demands with respect to the number of fronts we have improved upon in our system. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H15.00005: Analysis of Bell inequality violation in superconducting phase qubits Abraham G. Kofman, Alexander N. Korotkov We analyze conditions for violation of the Bell inequalities, focusing on experiments with Josephson phase qubits. In the ideal case we discuss all possible situations of maximum violation, but mainly focus on two important types of optimal qubit-measurement directions in the pseudospin space, lying within either horizontal or vertical planes. Only the vertical type remains optimal in presence of local measurement errors, while in the case of local decoherence of qubits either the horizontal or vertical configuration is optimal. Besides local measurement errors and decoherence, we also discuss the effect of measurement crosstalk, which affects both the classical inequality and the quantum result. In particular, we propose a version of the Bell inequality which is insensitive to the crosstalk. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H15.00006: High Fidelity Universal Quantum Gates in Superconducting Qubit Systems Using Non-adiabatic Rapid Passage Frank Gaitan, Ran Li Recent theoretical work\footnote{R. Li, M. Hoover, and F. Gaitan, Quant. Info. Comp. \textbf{7}, 594 (2007).} has suggested that a class of non-adiabatic rapid passage sweeps first realized experimentally in NMR systems in 1991\footnote{J. W. Zwanziger, S. P. Rucker, and G. C. Chingas, Phys.\ Rev.\ A \textbf{43}, 3232 (1991).}, and which generate controllable quantum interference effects\footnote{F. Gaitan, Phys.\ Rev.\ A \textbf{68}, 052314 (2003).}$^{,}$\footnote{J. W. Zwanziger, U. Werner-Zwanziger, and F. Gaitan, Chem.\ Phys.\ Lett.\ \textbf{375}, 429 (2003).}, can be used to produce a high fidelity universal set of quantum gates. We show how this class of sweeps can be implemented in both superconducting charge and flux qubit systems. We discuss the current challenges facing the use of these sweeps to produce a universal set of high fidelity quantum gates in superconducting qubit systems. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H15.00007: Improving dc SQUID Phase Qubit Lifetimes through Increased Isolation from Bias Leads Anthony Przybysz, Tauno Palomaki, Sudeep Dutta, Fred Wellstood, Rupert Lewis, Hanhee Paik, Hyeokshin Kwon, Ben Cooper, Kaushik Mitra, Bob Anderson, Alex Dragt, Chris Lobb The dc SQUID phase qubit has been plagued by a relatively short coherence time, T$_{2}$, and relaxation time, T$_{1}$ (tens of ns). By using a sapphire substrate and small (15 $\mu $m$^{2}$ or less) Al/AlO$_{x}$/Al junctions the performance of the qubit has improved to the point where the impedance of the bias leads is the main source of dissipation and decoherence in the device. We identify the main circuit parameters that effect the isolation (junction capacitance, loop inductance, etc.), and present designs to improve the qubits isolation from the bias leads. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H15.00008: Anomalous Avoided Level Crossings in a Cooper-Pair Box Spectrum Zaeill Kim, V. Zaretskey, Y. Yoon, J.F. Schneiderman, M.D. Shaw, P.M. Echternach, F.C. Wellstood, B.S. Palmer We have used a radio-frequency superconducting single-electron transistor to measure the detailed spectrum of an Al/AlO$_{\mbox {x}}$/Al Cooper-pair box (CPB) qubit. The CPB had a charging energy $E_{C}/k_{B}$ = 0.58 K and a Josephson energy $E_{J}/k_{B} $, which can be tuned by an external magnetic flux, between 0.1 and 1 K. From 15 to 50 GHz we have found four anomalous avoided level crossings in the excited state spectrum of the CPB. We note the splitting size has a strong dependence on the Josephson energy and the location of the splitting depends on the gate voltage of the CPB, evidence that the CPB is coupled to other quantum systems that are charged. We compare our results to a model Hamiltonian that describes a charge fluctuator coupled to a CPB and extract fit parameters that provide microscopic information about the charge fluctuators. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H15.00009: Detailed Study of the Excited State Lifetime of a Cooper-Pair Box Vitaley Zaretskey, Z. Kim, Y. Yoon, J. F. Schneiderman, M. D. Shaw, P. M. Echternach, F. C. Wellstood, B. S. Palmer We have used a radio-frequency superconducting single-electron transistor (rf-SET) to measure the lifetime of the excited state ($T_{1}$) of an Al/AlO$_{\mbox{x}}$/Al Cooper-pair box (CPB) qubit with a charging energy $E_{C}/k_{B}$ = 0.58 K. We measured the lifetime by continuously measuring the decay of the qubit from a mixed state. By effectively decreasing $E_{J}/k_{B}$ from 1 K to 0.1 K we could increase $T_{1}$ from 50 ns to 5 $\mu$s which indicates that charge is the dominant noise source. Additionally we noted that the decay rate as a function of transition frequency had several narrow peaks in the range 15 to 50 GHz. These peaks correlated with the locations of anomalous avoided level crossings we observed in the excited state spectrum of the CPB\footnote{\textbf{Anomalous Avoided Level Crossings in a Cooper-Pair Box Spectrum}, ZAEILL KIM \emph{et al}., BAPS (March 2008)}, suggesting that interaction with microstates is a source of dissipation for these qubits. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H15.00010: Direct Observation of Quasiparticle Tunneling Rates in a Pair of Superconducting Charge Qubits M.D. Shaw, B. Palmer, P. Delsing, P.M. Echternach We directly measure quasiparticle tunneling rates in the time domain for a pair of superconducting charge qubits based on the single Cooper-pair box. We discuss the dependence of these rates on a variety of experimental parameters, such as RF power, microwave power, magnetic flux, sample temperature, and gate voltage. Measurements are performed using RF reflectometry to measure the quantum capacitance of each device. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H15.00011: Robust optimal quantum gates for Josephson charge qubits Simone Montangero, Tommaso Calarco, Rosario Fazio Quantum optimal control theory allows to design accurate quantum gates. We employ it to design high-fidelity two-bit gates for Josephson charge qubits in the presence of both leakage and noise. Our protocol considerably increases the fidelity of the gate and, more important, it is quite robust in the disruptive presence of 1/f noise. The improvement in the gate performances discussed in this work (errors $ \sim 10^{-3} \div 10^{-4}$ in realistic cases) allows to cross the fault tolerance threshold. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H15.00012: Suppression of decoherence due to classical noise by dynamical decoupling Lukasz Cywinski, Roman M. Lutchyn, Cody P. Nave, Sankar Das Sarma We consider a pure dephasing model in the context of superconducting qubits. We show that the coherence time $T_2$ can be significantly increased by the application of a series of $\pi$ pulses. The most well known example, spin echo, a one pulse sequence, removes inhomogeneous broadening. However, $T_2$ can be further increased by applying more pulses. We discuss the experimental implications of various pulse sequences in the context of classical $1/f^{\alpha}$ noise in a Cooper-pair box qubit. We show that a recently proposed coherence-restoring pulse sequence [1, 2], discovered in the context of the spin-boson model, is optimal in certain regimes of parameter space. [1] G. S. Uhrig Phys. Rev. Lett. 98, 100504 (2007) \newline [2] B. Lee, W. M. Witzel, S. Das Sarma, arXiv:0710.1416 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H15.00013: The effect of dynamical decoupling in the case of a single fluctuator coupled to a qubit Cody Nave, Roman Lutchyn, Lukasz Cywinski, Sankar Das Sarma We consider the role of dynamical decoupling in the case of a single classical fluctuator coupled to a qubit which is operated in a pure dephasing regime. We study the effect of various pulse sequences on the decoherence time for both weakly and strongly coupled fluctuators described by random telegraph noise (RTN). For a strongly coupled two-level system, the application of multiple pulses leads to a large enhancement of qubit coherence time. By theoretically comparing various dynamic decoupling schemes, we conclude that the Car-Purcell-Meiblum-Gill (CPMG) pulse sequence, well-known in NMR spectroscopy and recently discussed in the context of electron spin qubits in semiconductors [1], is the most optimal coherence-restoring scheme for the single fluctuator problem of relevance to superconducting qubits. We also find that for a large number of applied pulses the Gaussian approximation for the noise reproduces the exact results even in the strongly coupled regime. \newline [1] W. M. Witzel and S. Das Sarma, Phys. Rev. Lett. 98, 077601 (2007). [Preview Abstract] |
Session H16: Focus Session: Biochip Physics I
Sponsoring Units: DBP DFDChair: Peter Kiesel, Palo Alto Research Center
Room: Morial Convention Center 208
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H16.00001: Size Scaling of Protein Sensitivity on the BioCD Kevin O'Brien, Ming Zhao, Xuefang Wang, David Nolte We investigate size scaling of the surface-height sensitivity of spinning-disk interferometry (SDI) implemented on the in-line-quadrature BioCD as a function of laser focal radius. The in-line-quadrature BioCD consists of a silicon wafer with a 120 nm layer of silicon dioxide that creates a quadrature condition between the incident and reflected light. When a laser beam is focused on the BioCD, proteins printed on the silicon dioxide substrate create a phase shift leading to quadrature interference, which is detectable in the far field as an intensity shift. The purpose of this scaling experiment is to determine the practical and fundamental limits on the sensitivity of the BioCD, and how those limits change as a function of the size of the focal spot. We imaged a single 100 micron wide protein spot with focal spot sizes of 1, 5 and 10 microns and observe a square-root scaling as a function of the number of pixels per protein spot. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H16.00002: Detection Limits of Captured Protein on the BioCD David Nolte, Xuefang Wang, Kevin O'Brien, Ming Zhao The BioCD is an interferometric biosensor that detects protein captured by antibody arrays. The sensor readout is performed on a spinning disc using a common-path interferometric configuration that is stable and sensitive to sub-monolayer coverage of captured protein. Protein is detected using phase quadrature that converts phase to intensity modulation using local generation of signal and reference to lock the relative phase of the waves. The purpose for spinning is to move far from 1/f noise to achieve high surface mass sensitivity. Several different classes of the BioCD have been developed, differentiated by the means of generating the phase-locked reference. These include the microdiffraction (MD) class, the phase contrast (PC) class, the adaptive optical (AO) class and the in-line (IL) class of BioCD. Of these different quadrature classes, the in-line BioCD has the highest sensitivity with a detection sensitivity of 0.25 pg/mm. The minimum detectable mass is set by simple scaling relations. The metrology limit is set by surface roughness combined with repositioning offset between pre- and post-incubation scans. Optimal sensitivity is achieved by critical sampling of protein spots in radial arrays. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H16.00003: Docetaxel-loaded Nanohorn-streptavidin-antibody for Anti-cancer Drug Delivery Jianxun Xu, Masako Yudasaka, Minfang Zhang, Sumio Iijima Single wall carbon canohorn (SWNH) is a new kind of nano-carbon tubule having horn-like structure at its tip. The tube diameters are 2 to 5 nm, and about 2,000 SWNHs assemble to form a spherical aggregate. SWNH is an attractive candidate for drug delivery, especially promising to carry anticancer drug, many of which are not water soluble and highly toxic. We incorporated Docetaxel (Doc), an anticancer drug used for stomach cancer and others, into hydrogen peroxide treated SWNH (SWNHox). By using carboxylic groups on SWNHox, we attached amine-PEO3-biotin, and then streptavidin to biotin. The streptavidin moiety on SWNH makes it easy to attach some other biotinylated molecules, thus we introduced a cancer targeting ligand, anti-tumor associated glycoprotein, to the SWNH system. Due to the targeting effect of the antibody, the cells were effectively killed when they were incubated with the Doc SWNHox-streptavidin-andtibody system. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H16.00004: Detection limits and scalability of miniaturized antibody assays in real-world applications Invited Speaker: |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H16.00005: InAs quantum well $\mu$-Hall sensors for magnetic biosensing Khaled Aledealat, S. Hira, K. Chen, G. Mihajlovic, P. Xiong, G. Strouse, P.B. Chase, S. von Molnar, M. Field, G. Sullivan Magnetic sensing is potentially a sensitive and rapid technique for monitoring DNA-DNA and protein-DNA interactions. Here we present an effort on the noise characterization and selective biofunctionalization of InAs $\mu $-Hall sensors for magnetic detection of DNA hybridization. Room-temperature noise measurements were performed in the frequency range from 20 Hz to 104 kHz. The noise equivalent magnetic moment resolutions were estimated to be $\sim $10$^{6} \quad \mu _{B}$/$\sqrt {Hz} $ and $\sim $10$^{4} \quad \mu _{B}$/$\sqrt {Hz} $ at 92 Hz and 23 kHz respectively. The active region of the InAs $\mu $-Hall device was covered with sputter-deposited SiO$_{2}$ and Au pads were patterned on top of some of the Hall crosses. Thiolated ssDNA were assembled on the Au pads and the rest of the device platform was passivated with PEG-silane. Biotinylated and fluorescently-tagged complementary ssDNA were labeled with commercial streptavidin-coated 350 nm superparamagnetic beads, which were found to assemble selectively onto the Au pads through DNA hybridization using laser scanning confocal microscopy. This work was supported by NIH NIGMS GM079592. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H16.00006: Ligand-receptor binding kinetics in surface-plasmon resonance devices: A Monte Carlo simulation study Matthew T. Raum, Manoj Gopalakrishnan, Kim Forsten-Williams, Uwe C. Tauber We use lattice Monte-Carlo simulations to probe the kinetics of ligand-receptor association and dissociation. Simulations were run under conditions approximating the geometric configuration of surface plasmon resonance devices. These conditions include viscous flow of ligands over a surface of receptors which is achieved by using a spatially varying biased random walk. Our simulations allow for the occurrence of multiple rebinding events which result in strong deviations from the standard mean-field rate equation approximation. Our simulations also allow us to test improved theoretical predictions for the binding dynamics and to determine their range of applicability. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H16.00007: Comparative study of different DNA chip preparation methods by means of Surface Plasmon Resonance Yannick Sartenaer, Ryuji Hara, Haruma Kawaguchi, Paul A. Thiry Recently, we demonstrated that SFG vibrational spectroscopy allows the detection of the specific recognition between the two molecules of a model ligand-protein biosensor. Moreover, we studied by this technique, the formation of thiolated single stranded DNA (ssDNA) monolayers immobilized on metallic substrates which are the basis for various biotechnology applications. Before going further into monitoring the hybridisation process in DNA based sensors, it is important to identify a preparation method providing good quality DNA chips with respect to the recognition process. Therefore, we performed investigations by Surface Plasmon Resonance (SPR). Practically, we used four different methods of chip preparation on gold surfaces and we measured the amount of deposited molecules when the sensor is exposed to a target DNA solution. By this way, we monitored for each case the sensitivity and the selectivity of the sensor by comparing the hybridisation of complementary and non complementary target ssDNA, respectively. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H16.00008: Nanoscale Building Blocks for Biosensor Development Invited Speaker: The development of new technologies based on nano- and microscale phenomenon is important and significant for many reasons. One of the most prominent of these is biological sensors for the diagnosis of diseases, detection of environmental toxins, and drug discovery. Research in our group focuses on the microscopic and spectroscopic analysis of the optical properties of nanostructures and their integration with microfluidic devices with applications in biological sciences. In this talk, we will show results for an optical sensor based on localized surface plasmon resonance spectroscopy. It will be demonstrated that this nanoparticle based sensor can be used to detect a variety of ligands, including a biomarker for Alzheimer's disease. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H16.00009: Predictive Model for Label-free Electrical Detection of Bio-molecules Pradeep Nair, Muhammad Alam Biosensors based on MOSFETs, silicon nanowires, and carbon nanotube nanocomposites \textit{promise }highly sensitive, dynamic, label-free, electrical detection of bio-molecules with potential applications in genomics and proteomics. Although tremendous improvements in sensitivity have been reported in electrical detection of bio-molecules, many aspects of experimentally observed sensor response (S) are unexplained within the theoretical frameworks of kinetic response or electrolyte screening. In this paper, we combine analytic solutions of Poisson-Boltzmann and reaction-diffusion equations to show that the electrostatic screening within an ionic environment limits the response of nanobiosensor such that $S\left( t \right)\sim c_1 \left( {\ln \left( {\rho _0 } \right)-\frac{\ln \left( {I_0 } \right)}{2}+\frac{\ln \left( t \right)}{D_F }+\left[ {pH} \right]} \right)+c_2 $ where $c_i $ are geometry-dependent constants, $\rho _0 $ is the concentration of target molecules, $I_0 $ the salt concentration, and $D_F $ the fractal dimension of sensor surface. Our analysis provides a coherent theoretical interpretation of wide variety of puzzling experimental data that have so far defied intuitive explanation and have important implications for the design and optimization of nanoscale biosensors. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H16.00010: Hybrid CMOS/Microfluidic Dielectrophoresis and Magnetic Manipulator Chip David Issadore, Thomas P. Hunt, Keith A. Brown, R.M. Westervelt We present hybrid CMOS/microfluidic chips that combine the biocompatibility of microfluidics with the programmability of CMOS integrated circuits (ICs). The chips use a two-dimensional array of RF-electrode pixels that use dielectrophoresis (DEP) to simultaneously and independently control the location of many objects, including biological cells and chemical droplets [1]. We highlight our next generation of CMOS/microfluidic chips that combine a two-dimensional array of high voltage (50 V) RF pixels to produce large DEP forces, a microelectromagnetic matrix [2] that can independently trap and move magnetic beads, and integrated temperature sensors. We show the design, fabrication, and testing of the hybrid chips as well as ongoing work to interface and package the chips for robust biological and chemical experiments. [1] T.P. Hunt, D. Issadore, R.M. Westervelt, Lab Chip, 2008, DOI: 10.1039/b710928h. [2] H. Lee, A.M. Purdon and R.M. Westervelt, Appl. Phys. Lett. 85, 1063 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H16.00011: Hybrid CMOS/Microfluidic Chip Applications Keith A. Brown, David Issadore, Thomas P. Hunt, R.M. Westervelt We present our continuing work on hybrid CMOS/microfluidics systems that enable programmable experiments on single biological cells and picoliter chemistry. A 128x256 array of 10x10 micron RF-electrode pixels in the integrated circuit (IC) allows positioning of cell-sized objects using dielectrophoresis in a microfluidic chamber observed using a fluorescence microscope[1]. The fluid environment in the chamber is controlled through external piping, integrating the hybrid chip into a complete microfluidic system. We demonstrate the use of this integrated circuit as a cell-sorting stage. Applications and prototypical experiments with relevance to biologically motivated research will be presented. We highlight single cell experiments made possible by the ability to move, combine and separate picoliter droplets using computer control with video feedback. \newline [1] Thomas P. Hunt, et al. Lab Chip, 2008, DOI: 10.1039/b710928h [Preview Abstract] |
Session H17: Proteins: Structure and Function
Sponsoring Units: DBPChair: Andrea Markelz, State University of New York at Buffalo
Room: Morial Convention Center 209
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H17.00001: Linking enzyme conformational dynamics to catalytic function with single-molecule FRET Yan-Wen Tan, Jeffrey A. Hanson, Karl Duderstadt, Sucharita Bhattacharyya, Haw Yang Many enzymes endure sizable conformational remodeling on a timescale comparable to their catalytic cycle. These conformational dynamics may be critical to the enzymes' catalytic function. In adenylate kinase (AK) from \textit{E. coli}, this involves a large-amplitude rearrangement of the enzyme's lid domain. We use high-resolution single-molecule FRET developed in our laboratory to measure AK's domain movements on its catalytic timescale. We utilize maximum entropy-based methods to remove photon-counting noise from raw data, so that the enzyme's entire conformational distribution can be quantitatively recovered without a presumed model. Multiple sequence alignment suggests regularities between the conserved residues and their structural-functional roles. Armed with precise single-molecule FRET dynamics measurements and comprehensive bulk kinetic studies of the mechanism, we were able to quantitatively correlate AK's stochastic lid dynamics with its deterministic catalytic rates. Implications on the structure-function conservation and protein engineering will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H17.00002: Concentration-dependent Cu(II) binding to prion protein Miroslav Hodak, Wenchang Lu, Jerry Bernholc The prion protein plays a causative role in several neurodegenerative diseases, including mad cow disease in cattle and Creutzfeldt-Jakob disease in humans. The normal function of the prion protein is unknown, but it has been linked to its ability to bind copper ions. Experimental evidence suggests that copper can be bound in three distinct modes depending on its concentration, but only one of those binding modes has been fully characterized experimentally. Using a newly developed hybrid DFT/DFT method [1], which combines Kohn-Sham DFT with orbital-free DFT, we have examined all the binding modes and obtained their detailed binding geometries and copper ion binding energies. Our results also provide explanation for experiments, which have found that when the copper concentration increases the copper binding mode changes, surprisingly, from a stronger to a weaker one. Overall, our results indicate that prion protein can function as a copper buffer. 1. Hodak, Lu, Bernholc, JCP, in press. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H17.00003: Solvable model of mechanical unfolding of proteins Oleg Vorov, Dennis Livesay, Donald Jacobs We present exact analytical results describing single-molecule experiments on mechanical unfolding of proteins within a realistic model [1]. The corresponding relation between the extension at a given temperature of the macromolecule and the applied force is derived [2]. The configuration partition function is calculated exactly for a distance constraint protein model that describes the beta-hairpin to coil transition. The resulting extension-force curve is derived, and the results agree with the data from the single-molecule pulling experiments. \newline [1] O.K.Vorov, A.Y.Istomin, D.R.Livesay, D.J.Jacobs, subm. to Phys.Rev.Lett., 2007. [2] O.K.Vorov, D.R.Livesay, D.J.Jacobs, to be subm. to Science, 2007, in preparation. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H17.00004: Dynamical Transition in polypeptides Yunfen He, Andrea Markelz Two of the possible causes for the so called dynamical transition (the rapid increase in flexibility for biomolecules at $\sim $ 200 K) are: thermally activated side chain diffusive motions with hydration dependent activation energies; or a glass transition in the biological water directly adjacent to the biomolecule. If the transition is strictly due to side chain activation, it should not depend on protein structure. Previously we demonstrated that the dynamical transition remains after tertiary structure was removed using THz time domain dielectric spectroscopy (0.2 -2.0 THz, 0.5-5ps). Here measurements on polyalanine as a function of chain length show that the dynamical transition does not occur for peptide length shorter than 5. However, the transition is observed for 5 mer and higher. Structural and simulation studies indicate that the 5 mer transiently occupies structured forms [1,2]. These results suggest that A) the dynamical transition is not due to thermally activated side chain motion and B) secondary structure is necessary for the dynamical transition. Secondary structure possibly induces sufficient ordering in the adjacent water to result in a fragile to strong glass transition resulting in increased protein flexibility [3]. [1] KAH Wildman et al. Solid State Nucl. Magn. Reson. 24 (2003) 94-109. [2] Yuguang Mu,et al. Proteins 58, (2005) 45-52. [3] S.H. Chen et al. PNAS (2006) 9012--9016. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H17.00005: Dynamically stable beta-sheets in Cu-initiated misfolding of $\alpha$-synuclein Francis Rose, Miroslav Hodak, Jerry Bernholc The human protein $\alpha$-synuclein has been implicated as a central constituent in multiple neurodegenerative diseases. In Parkinson disease it is even thought to be the causative link. $\alpha$-synuclein can be stimulated to aggregate into deleterious fibrillar structures by mutation, metal binding, and agitation. In particular, Cu$^{2+}$ has been found in high concentrations in neural tissues of Parkinson sufferers. We propose a scenario involving the metal ion Cu$^{2+}$ as the misfolding $\beta$-sheet initiator of fibrillogenesis. A model fragment of the metal-bound protein was investigated using DFT to obtain conformational details of the energetically favorable geometries. Feasible $\beta$-sheet structures incorporating the DFT geometries were explored using heuristic $\beta$-sheet guidelines and inverse kinematics. The resulting structures were tested for dynamic stability by simulating the fully solvated protein by classical MD constrained by the DFT geometries. Our results indicate that dynamically stable structures exist and that the metal binding is directly responsible for initiating misfolding. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H17.00006: Dynamics of Lysozyme in Trehalose solutions Pavan Ghatty, Edward C. Uberbacher Anhydrobiosis in Tardigrades and Nematodes has been a topic of constant interest and intrigue in the scientific community. An increase in the concentration of Trehalose has been attributed to the ability of some organisms to survive extreme conditions of temperature, pressure and pH. Although there exist many experimental studies attributing this effect to Trehalose, the molecular details governing the interaction between Trehalose and proteins remains unclear. We have conducted a 20ns study of Lysozyme in varying concentrations of Trehalose in water. Strong and weak hydrogen bonds and hydrophobic interactions between water, Trehalose and protein seem to dictate the interactions in the system. We have observed a hydrogen bonded network of Trehalose around the protein entrapping a layer of water between itself and protein. Lysozyme remains in a near-native conformation throughout the simulation giving hints on the ability of Trehalose in preserving the structure of protiens. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H17.00007: Spectral and Hydration Dependence of Protein Dynamical Transition Ferdinand Lipps, J. R. Knab, Jing Yin Chen, Yunfen He, A. G. Markelz The protein dynamical transition, a rapid increase in flexibility at $\sim $ 200K, is hydration dependent suggesting that the transition may in fact be due to a transition in the surrounding water. Previously we have shown that the terahertz dielectric response is sensitive to the dynamical transition using terahertz time domain spectroscopy [1]. The broadband technique allows the determination of what motions are affected by the transition, that is whether long time scale motions such as side chain rotations, or faster vibrational motions. Here we examine both the frequency and hydration dependence of the protein dynamical transition for hydrated myoglobin powder for the 0.2 -- 2.0 THz and 80-295 K ranges. The transition is observed in both the real and imaginary parts of the dielectric response. Our earlier measurements of solutions did not show a transition in the real part of the permittivity, likely due to bulk solvent dominating the index. There is a strong frequency dependence with hydration. While a slight transition is observed at frequencies higher than 1 THz which is nearly hydration independent, for frequencies below 1 THz the strength of the transition rapidly increases with hydration. [1] A. G. Markelz, J. R. Knab, Jing Yin Chen, Yunfen He, Chem. Phys. Lett. \textbf{442}, 413 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H17.00008: Vibrational Dynamics of Heme Model Compounds Alexander Barabanschikov, Timothy Sage, Minoru Kubo, Paul Champion, Jiyong Zhao, Wolfgang Sturhahn, Ercan Alp Synchrotron- and laser-based measurements supported by DFT calculations identify vibrational modes of the iron atom in Fe(P)(Cl) and Fe(P)(Br). These compounds are large enough to capture many essential aspects of heme geometry and vibrations. On the other hand, porphine models are small enough to simplify the vibrational spectrum and enable accurate analysis using DFT methods. Nuclear resonance vibrational spectroscopy (NRVS) and femtosecond coherence spectroscopy (FCS) provide a rare opportunity to identify both doming and Fe-halide stretching components of the reaction coordinate with confidence. Correlation analysis between 4-coordinate and 5-coordinate compounds suggests significant mixing between Fe-ligand and heme modes. Comparison with the larger model Fe(III)(OEP)(Cl) reveals the effect of peripheral group substitutions. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H17.00009: Ligand-Modulated Free Energy Landscapes of Glucose/Galactose Binding Protein Troy Messina Glucose/galactose binding protein (GBP) functions as part of a larger system of proteins for molecular recognition and signaling in enteric bacteria. Here we report on the thermodynamics of conformational equilibrium distributions of GBP from both time-resolved fluorescence experiments and computational umbrella sampling molecular dynamics analyzed by the weighted histogram analysis method (WHAM). Three conformations appear at zero glucose concentration and systematically transition to three conformations at high glucose concentration. Fluorescence anisotropy correlations, fluorescent lifetimes, thermodynamics, computational structure minimization and molecular dynamics, and previous work were used to identify the three components as open, closed, and twisted conformations of the protein. The existence of three states at all glucose concentrations indicates that the protein continuously fluctuates about its conformational state space via thermodynamically driven state transitions, and the glucose biases the populations by reorganizing the free energy profile. These results and their implications are discussed in terms specific and non-specific interactions GBP has with cytoplasmic membrane proteins. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H17.00010: Ligand Binding Kinetics in Myoglobin and Solvent Relaxation at High Pressure Alfons Schulte, Silki Arora, SangHoon Park Pressure is increasingly used as a variable to examine protein structure-function relationships, since it is crucial for chemical equilibria, reaction rates, and protein conformational states. We investigate pressure effects for the prototype reaction of ligand binding to myoglobin over a wide dynamic range in time and temperature. The distribution of rebinding rates is evaluated from kinetic absorption measurements of CO and O$_{2}$ binding to (horse) myoglobin at variable pressure (0.1 - 190 MPa) and temperature (180 - 300 K) in aqueous and 75 {\%} glycerol/buffer solutions. The data demonstrate that pressure significantly affects the amplitudes (not just the rates) of the component processes. The amplitude of the geminate process increases with pressure corresponding to a smaller escape fraction of ligands into the solvent and a smaller inner barrier. Solvent relaxation rates at variable pressure are determince independently from specific heat spectroscopy. We discuss the role of solvent dynamics, hydration shell, and internal protein cavities in the binding reaction. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H17.00011: Electronic Structure and the Magnetic Hyperfine Interactions in Heme Unit of Metmyoglobin N.B. Maharjan, S.R. Badu, Archana Dubey, R.H. Scheicher, R.H. Pink, Lee Chow, A. Schulte, H.P. Saha, T.P. Das The $^{14}$N and $^{57m}$Fe hyperfine interactions in the heme unit of metmyoglobin are available experimentally by electron-nuclear double resonance (ENDOR) and Mossbauer spectroscopic techniques. We have carried out electronic structure investigations on the heme system including the H$_{2}$O and proximal imidazole ligands by the first-principles Hartree-Fock procedure and studied the magnetic hyperfine and nuclear quadrupole coupling constants for the $^{57m}$Fe nucleus and all the six $^{14}$N nuclei on the four pyrrole and imidazole ligands as well as the $^{17}$O nucleus on the H$_{2}$O ligand. Comparison will be made with available experimental data [1, 2] and earlier theoretical investigations [3] by the approximate self-consistent charge Extended Huckel procedure. Results will also be presented for the optical frequencies and intensities from transitions between ligand-like and iron d-like states and the Fe-N$_{\varepsilon }$ vibrational frequency [1] G. Lang, Q. Rev. Biophys. \underline {3}, 1 (1970) [2] C.P. Scholes, R.A. Isaacson and G Feher, Biochim. Biophys. Acta \underline {263},448(1972) [3] S.K. Mun, Jane C. Chang and T.P. Das J. Am. Chem. Soc. \underline {101}, 5562(1979) [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H17.00012: Hartree-Fock Investigation of Electronic Structure and Associated Properties of Heme Unit in Deoxyhemoglobin S.R. Badu, Archana Dubey, K. Ramani Lata, R.H. Scheicher, R.H. Pink, A. Schulte, Lee Chow, H.P. Saha, K. Nagamine, T.P. Das Using the Hartree-Fock-Roothaan procedure and the most recent version of the Gaussian set of programs we have studied the electronic structure of the heme unit including the imidazole ligand of iron from the proximal histidine using x-ray data for the positions of all the atoms except the hydrogen. The positions of the latter have been obtained through energy optimization. The results obtained from the calculated electronic structure for the magnetic and electronic quadrupole hyperfine interactions of $^{57m}$Fe and $^{14}$N nuclei will be discussed. Comparison will be made with available experimental data and earlier theoretical investigations [1]. Results will also be presented for the proximal Fe-N$_{\varepsilon }$ vibrational frequency and the frequencies and intensities of optical transitions between ligand like states and d-like states of Fe [1]. K Ramani Lata PhD Thesis SUNY Albany (1993) (Unpublished) [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H17.00013: The Electronic Investigation of Singlet and Triplet States of Oxyhemoglobin by Hartree-Fock Procedure and Associated Hyperfine Interaction. Archana Dubey, S.R. Badu, R.H. Scheicher, N. Sahoo, R.H. Pink, A. Schulte, H.P. Saha, Lee Chow, K. Nagamine, T.P. Das The observation of paramagnetic susceptibility [1] in Oxy-Hb from measurements over a broad temperature range has stimulated interest in the occurrence of a low-lying excited triplet state close to the ground singlet state of Oxy-Hb. An earlier theoretical investigation [2] has shown the existence of such a triplet state providing support to the interpretation of the susceptibility data [1]. Support for the low-lying excited triplet state has been augmented recently [3] from microscopic relaxation rate measurements for muon attached to the heme group of Oxy-Hb. We are studying by first principles Hartree-Fock procedure the energies and the electronic wave functions of the ground and triplet states and the quantitative theoretical prediction of muon magnetic hyperfine interaction in room temperature $\mu $SR measurements on Oxy-Hb. Results will be presented for hyperfine interactions of muon and other nuclei in Oxy-Hb [1] M.Cerdonio etal. Proc. Nat. Acad. Sci USA \underline {75}, 4916(1978). [2] Zalek S. Herman and Gilda H Loew JACS \underline {102}, 1815(1980).[ 3] K. Nagamine etal Proc. Jpn. Acad.Ser.B 83,120(2007). [Preview Abstract] |
Session H18: Block Copolymers in Solution and Blends
Sponsoring Units: DPOLYChair: Ryan Hayward, University of Massachusetts-Amherst
Room: Morial Convention Center 210
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H18.00001: Polymer Physics Prize Break
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Tuesday, March 11, 2008 8:36AM - 8:48AM |
H18.00002: Competitive Adsorption, Exchange and Binding of Polymers and Proteins at the Oil/Water Interface Daniel Carvajal, Kenneth Shull, Igal Szleifer Drop Shape Analysis (DSA) of pendant drops was used to study competitive adsorption, exchange kinetics and binding of macromolecules at the oil/water interface. Amphiphilic diblock and triblock copolymers were dissolved in the oil drop phase, while proteins were added to the water subphase. By using DSA to monitor the interfacial tension of the system, we were able to gather data on how the polymers and proteins are behaving and interacting at the oil/water interface. Some polymer systems were found to fully inhibit both specific and non-specific adsorption of proteins to the interface. Adding biological receptors to these polymers allowed us to study the specific binding of proteins to polymers located at the interface. In other systems, proteins were able to penetrate the amphiphilic block copolymer layer and reach the interface. The dynamics of exchange and competitive adsorption in these polymer/protein systems were also studied. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H18.00003: Interfacial Properties of Semifluorinated Alkane Diblock Copolymers Flint Pierce, Dvora Perahia, Mesfin Tsige, Oleg Borodin, Gary Grest The surface interaction of semifluorinated alkane diblock (SFAs) copolymers with water and normal alkanes are studied using explicit atom molecular dynamics (MD) simulations. At the diblock/air interface, the surface is dominated by fluorinated groups as a result of their low surface tension, and these groups reside at the interface for longer periods of time than the hydrogenated groups. Fluorinating even a single end group on an otherwise hydrogenated chain results in low surface tensions, close to that of perfluoroalkanes and far from normal alkanes. For the interface with water, results for the rate of water uptake by alkanes, perfluoroalkanes, and SFAs will be presented. Additionally, we report the interfacial surface tensions and equilibrium density profiles for these samples, focusing on the prevalence of fluorinated and hydrogenated segments at each interface. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04- 94AL85000. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H18.00004: Amphiphilic copolymer assemblies formed by interfacial instabilities of oil-in-water emulsions Jintao Zhu, Ryan C. Hayward Self-assembly of amphiphiles into discrete nano-objects is of fundamental interest and is important for applications including encapsulation and drug delivery. We will describe a new method by which amphiphilic block copolymers can be controllably assembled into hierarchically-structured microparticles and various micellar morphologies. We first form oil-in-water emulsion droplets, where the dispersed phase consists of a volatile organic solvent containing a dilute concentration of amphiphilic polystyrene-block-poly(ethylene oxide) diblock copolymer. Upon extraction of the organic solvent, the droplets undergo interfacial instabilities which lead to formation of microparticles with budding vesicle, foam-like, or dendritic structures, or micelles with spherical, cylindrical, or more complicated morphologies. We will discuss how the structures of these assemblies can be tuned, and the opportunities that they present for encapsulation of hydrophobic species. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H18.00005: Helical cylinders or multicompartment cylinders through the solution assembly of charged block copolymers with multivalent organic counterions Darrin Pochan, Sheng Zhong, Honggang Cui, Zhiyun Chen, Karen Wooley By manipulating the interaction of charged block copolymer hydrophilic corona blocks with multivalent organic counterions, and controlling the kinetics of block copolymer solution self-assembly, desired micelle geometries can be formed. Specifically, polyacrylic acid-b-polymethylacrylate-b-polystyrene amphiphilic triblock copolymers were studied in water/THF solvent mixtures with organic multiamines as counterions. By manipulating block copolymer and solvent composition, different micelle geometries were formed. However, by altering the chemical structure and/or concentration of the multiamine counterions, as well as the kinetic pathway through which the molecules are assembled, complex nanostructures were formed. An example of nanostructure from kinetic control includes spherical micelles that can be controllably assembled into 1-d multicompartment cylinders. Examples of nanostructure from control of the type and amount of multivalent organic counterion added are helical cylinder superstructures many micrometers in length. The system has been investigated by means of cryogenic transmission electron microscopy (cryo-TEM) and small angle neutron scattering (SANS). [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H18.00006: Spotted Polymersomes and Striped Worms - a theoretical analysis of lateral segregation of diblock copolymers Wouter G. Ellenbroek, David A. Christian, Aiwei Tian, Andrea J. Liu, Tobias Baumgart, Dennis E. Discher Lipids and amphiphilic block copolymers are both known to assemble into vesicle and worm-like micelle morphologies, but only mixtures of lipids in vesicles have been directly seen to phase separate into meso-scale lateral domains. Here we show direct visualization of meso-scale spots in tough polymersomes and micron-length stripes in stable worms that result from strong lateral segregation of polyanionic and neutral diblock copolymers. We present a model for understanding the crucial role of calcium ions on segregation behavior, which incorporates counterion condensation and ``crosslinking'' (ion bridging). We find a tendency towards segregation near the isoelectric point as a result of competition among counterion entropy, repulsion due to the net charge, and attraction due to crosslinking. These results portend new classes of robust membranes and cylinders that exhibit lateral patterns at the meso-scale. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H18.00007: Pathways of Spontaneous Vesicle Formation of ABA Amphiphilic Molecules in Selective Solvent. Wei Jiang, Hongbo Du We study the pathways of spontaneous vesicle formation of the ABA amphiphilic molecules in selective solvent by Monte Carlo simulation. Simulation results reveal that the pathway for the spontaneous vesicle formation of the amphiphilic molecules in selective solvent depends strongly on the annealing speed. We can not use one pathway model to describe the vesicle formation even though for the same system and condition. Hydrophobic molecules diffusing in spherical micelles and oblate membrane closing may coexist for the vesicle formation. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H18.00008: Effects of depletion interactions on block copolymer micelles Sayeed Abbas, Timothy P. Lodge Block copolymer micelles exhibit two levels of hierarchical self-assembly: the process of micellization itself, and the ordering of these micelles onto a lattice. By a combination of small angle x-ray scattering and neutron scattering, we show that both levels of self-assembly are affected when non-adsorbing homopolymer is added to the solutions. The phenomena are analogous to depletion interactions in colloid/polymer mixtures. We have chosen poly(styrene-$b$-isoprene) micelles dissolved in diethyl phthalate as the model system. To these solutions polystyrene homopolymer was added. The effects strongly depend on the molecular weight and concentration of the added homopolymer. We find an induced attraction between micelles at moderate micelle concentrations, and a preference for fcc over bcc lattices in more concentrated solutions. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H18.00009: Influence of Electric Fields on the Phase Behavior of Concentrated Block Copolymer Solutions Kristin Schmidt, Heiko Schoberth, Alexander B\"oker We investigate the influence of the electric field on the phase behavior of diblock copolymer concentrated solutions using synchrotron SAXS. We find a significant dependence of the characteristic domain spacing on the electric field strength. For lamellae aligned parallel to the electric field direction we observe that the lamellar spacing decreases with increasing field strength, while for perpendicularly oriented lamellae the domain spacing increases. We also find that the electric field can induce an order-order transition if the block copolymer has a composition close to the predicted phase boundary. Due to the lower free energy of aligned anisotropic microdomain structures parallel to the electric field, we can induce a transition from the metastable hexagonally perforated lamellar phase to the lamellar phase without perforations by applying strong electric fields. Similarly an isotropic cubic gyroid phase, which is stable in the absence of, but cannot be aligned by, the field, transforms to aligned cylinders when a strong electric field is applied. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H18.00010: Well Ordered Melts from Low Molar Mass Pluronic Copolymers Blended with Poly (acrylic acid): Effect of Homopolymer Molar Mass Vikram Daga, Vijay Tirumala, Alvin Romang, Eric Lin, James Watkins The use of short chain block copolymer melts as nanostructured templates is often limited by their low segregation strength ($\chi N)$. Since increasing molar mass to strengthen segregation also increases the interdomain spacing, it is more desirable to increase the segment-segment interaction parameter, $\chi $ to produce strong segregation. We have recently shown that block copolymer melts with a molar mass less than 15 kg/mol undergo disorder-to-order transition without a significant increase in interdomain spacing when blended with a selectively associating homopolymer, due to an apparent increase in effective $\chi $. Here, we study the effect of homopolymer molar mass on the segregation of a disordered poly (oxyethylene-oxypropylene-oxyethylene) copolymer melt that forms lamellar microstructure in the ordered phase. Based on small-angle scattering measurements, we find that the melts remain ordered over a broad range of homopolymer chain lengths, ranging up to ten times that of the copolymer. This approach has many implications for the use of commodity block copolymer surfactants as inexpensive nanostructured templates for commercial applications. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H18.00011: Novel Characterization of Critical Micelle Concentrations of Block Copolymers and Gradient Copolymers in Homopolymer Robert Sandoval, Daniel Williams, Christopher Wong, Jungki Kim, John Torkelson Here we demonstrate a new method based on the intrinsic fluorescence of styrene-containing block copolymers and gradient copolymers to determine the critical micelle concentrations (cmcs) of copolymers present at low levels in homopolymer. The method relies on the fact that the when styrene/methyl methacrylate (S/MMA) block copolymers and gradient copolymers are well dispersed in a glassy homopolymer such as poly(methyl methacrylate) (PMMA), only monomer fluorescence and no excimer fluorescence is observed from the copolymer. When micelle formation occurs, then excimer fluorescence is present. With this simple method, we have found that gradient copolymers yield much lower cmc values (about an order of magnitude smaller) than comparable block copolymers of similar molecular weight and overall composition. We are extending these studies to consider the effects of block copolymer molecular weight and composition as well as homopolymer molecular weight on the cmc values. While these parameters have received heavy consideration from a theoretical standpoint, little experimental work has focused on these issues. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H18.00012: The influence of macromolecular architecture on the micellization in block copolymer/homopolymer blends E. Pavlopoulou, K. Chrissopoulou, S.H. Anastasiadis, G. Portale, W. Bras, H. Iatrou, S. Pispas, N. Hadjichristidis We investigate the micellar formation and micelle characteristics of block copolymers of varying architecture within homopolymer matrices. A series of symmetric (polyisoprene)$_{n}$(polystyrene)$_{n}$ (I$_{n}$S$_{n}$) miktoarm star block copolymers, with n identical pairs of arms, and a series of (polyisoprene)$_{2}$(polystyrene), I$_{2}$S, graft copolymers with constant total MW and varying composition, $f_{PS}$, are added to a low MW PI homopolymer matrix and the blends are investigated by small-angle X-ray and light scattering as a function of copolymer concentration and n or $f_{PS}$. The functionality of the junction point of the copolymer does not influence the characteristics of the I$_{n}$S$_{n}$ micelles, while $f_{PS}$ controls the behavior of the I$_{2}$S grafts. A simple thermodynamic model is developed that describes theoretically the micellization of A$_{n}$B$_{n}$ copolymers within B homopolymers and its predictions agree very well with the experimental data both qualitatively and quantitatively. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H18.00013: Phase Behavior and Dimensional Scaling of Symmetric Block Copolymer-Homopolymers Ternary Blends in Thin Films Guoliang Liu, Mark Stoykovich, Shengxiang Ji, Paul Nealey We have studied the phase behavior and dimensional scaling of symmetric ternary blends of PS-$b$-PMMA block copolymers and the respective PS and PMMA homopolymers in thin films. Below the order disorder transition (ODT) temperature, the symmetric ternary blends form lamellae, microemulsion and macrophase separated phases as a function of $\chi $N, $\alpha $ (ratio of degree of polymerization of homopolymers to that of the block copolymer), and \textit{$\phi $}$_{{\rm H}}$ (volume fraction of homopolymers). The phase transition compositions from lamellae to microemulsion and from microemulsion to macrophase separation depend weakly on $\chi $N and $\alpha $ in the range of 12.7$\le \chi $N$\le $37.6 and 0.20$\le \alpha \le $0.99. The dimensions of swollen lamellae and microemulsion ($L_{B})$ can be determined as a function of \textit{$\phi $}$_{{\rm H} }$ and $\alpha $, explicitly, $L_{B}=L_{o}$/(1-\textit{$\phi $}$_{{\rm H}})^{\beta }$, where $L_{o }$is the natural bulk period of pure block copolymer, and\textit{ $\beta $} is a parameter depending strongly on $\alpha $. [Preview Abstract] |
Session H19: Molecules on Surface: Structure
Sponsoring Units: DCMPChair: Karsten Pohl, University of New Hampshire
Room: Morial Convention Center 211
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H19.00001: Competition between intermolecular and substrate interactions in a multicomponent lattice gas model Qiang Liu, John Weeks Recent experiments have shown that acridine-9-carboxylic acid (ACA) molecules form a dense phase consisting of chain-like structures on Ag(111) with alternating orientations along the chain direction that permit the formation of strong hydrogen bonds. Despite the anisotropy in intermolecular interactions that leads to chain formation, molecular boundaries along and normal to the chair direction have very similar thermodynamic properties and fluctuations. We introduce a multicomponent lattice gas model where molecules with different orientations are treated as different species with different intermolecular interactions as well as different interactions with the substrate. This a generalization of the familiar Blume-Emery-Griffiths model of a binary lattice gas but in a region of parameter space not usually explored. We argue that the novel domain shapes, boundary fluctuations and phase densities seen in experiment arise from a competition between favorable anisotropic pair interactions in the chain phase and less favorable substrate interactions due to the different orientations. Detailed results of Monte Carlo simulations of this model and analytic work using mean field and pair approximation theories will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H19.00002: Van der Waals interactions at the molecule-metal interface: PTCDA on Ag(111) Lorenz Romaner, Dmitrii Nabok, Peter Puschnig, Egbert Zojer, Claudia Ambrosch-Draxl A detailed understanding of the processes governing the adsorption of molecules on metallic surfaces is of major interest for the field of molecular electronics. In this context, the molecule 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) has been extensively studied on a variety of metallic surfaces and has so gained fundamental, academic importance. The theoretical description of the adsorption is, however, still controversial as standard density functional theory (DFT) does not include van der Waals interactions, which, for PTCDA and many other molecules, yields the dominant binding contribution. We present DFT calculations of PTCDA adsorbed on Ag(111) where a recently developed exchange-correlation functional was adopted to include van der Waals interactions. Adsorption energy and distances, molecular distortions, charge rearrangements and orbital occupancies are discussed in detail. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H19.00003: Tilt Angle Determination in Thin Films with Anisotropic Molecules Thomas Gredig, Ge Liu, Ivan K. Schuller Many electronic properties of organic semiconductors depend critically on their physical and chemical arrangement of atoms. Here, a method is described to extract information about the molecular tilt angle and to determine the center electron density of anisotropic molecular thin films by means of specular x-ray diffraction. Thin films of phthalocyanine (Pc), an anisotropic molecule with a metal ion in the center, have been deposited in an organic molecular beam deposition system and studied with high-resolution x-ray diffraction. In particular, two isomorphous molecules, H$_{2}$Pc and CuPc, are compared experimentally and then studied with numerical simulations. The results show that the intensity distribution of the diffraction peaks belonging to the same series of lattice planes provides important structural information such as the molecular tilt angle or the center electron density of the anisotropic molecule. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H19.00004: The role of defects and phonons in O$_2$ adsorption on Cu(100) Matti Alatalo, Antti Puisto We present the results of an extensive {\it ab initio} study for O$_2$ adsorption on the Cu(100) surface. The calculated potential energy surfaces and first principles molecular dynamics trajectories reveal that on the clean surface without defects the dissociation probablility is low at small translational energies of the incoming molecule wheras at higher energies the molecules dissociate directly without a barrier. Of the different defects, steps turn out to be less reactive than expected and adatoms rather lower than enhance the reactivity. In contrast, vacancies act as dissociative centers, which locally enhance the reactivity. This result is in agreement with the molecular beam surface scattering experiments which show an increase at the oxygen sticking probability at low energies. We also discuss the role of phonons in O$_2$ dissociation, showing that the open space involved at certain phonon modes lowers the dissociation barrier, an effect analogous to the vacancy induced dissociation. Our results clearly demonstrate the importance of including both defects and surface dynamics in a realistic description of the adsorption process. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H19.00005: Vibrational spectroscopy and \textit{ab initio} dynamics of the O-induced \textit{added-row} reconstructed Cu(110) surface. Talat S. Rahman, M. Alcantara Ortigoza, R. Heid, K.P. Bohnen, K. Brueggemann, H. Ibach It is known that O$_{2}$ molecules dissociatively adsorb on Cu(110) and, upon subsequent annealing, Cu and O atoms catenate along the [100] direction arranging themselves in a striped periodic super-grating, depending on O coverage and annealing temperature. It has been proposed that stress along the [110] direction in the regions that locally hold a (2x1)O added-row structure causes the formation of the stripes. Our electron energy loss spectroscopy (HREELS) and density functional perturbation theory calculations show however that the Rayleigh wave softens along the [110] direction, providing no indication that stress relief drives the formation of the stripes. Nonetheless, our calculations show also the stiffening of an in-plane mode which is peeled off above the bulk band and signifies strong O-induced intralayer force constant stiffening. Furthermore, HREELS at the $\Gamma $-point of the striped phase shows different degrees of softening of the O vertical mode, suggesting a stress relief gradient from the center of the stripes to the edges. Support to this interpretation is provided by our calculated phonon dispersion along the [110] direction of the O(3x1) added-row structure in which the O vertical mode appears softened with respect to that in the O(2x1) structure. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H19.00006: Insights from surface stress calculations on the structure of c(2$\times $2)-N overlayer on Cu(001) Sampyo Hong, Talat Rahman We present results of calculated changes in surface stress for N overlayers on Cu(001), in an effort to discriminate between several proposed structural models. Our calculations of the surface geometry and electronic structure are based on the density functional theory in the generalized gradient approximation and the pseudopotential method. We find that a c(2$\times $2) N overlayer causes a large change in the surface stress ($\approx $ 5 N/m) on Cu(001) turning it from tensile to compressive. We also perform calculations for several stress relief models to find that the so-called ``rumpling'' and ``clock reconstruction'' models fail to relieve the N induced stress. On the other hand, formation of strips of clean Cu(001) areas, aligned along the $\langle $100$\rangle $ direction, and trench-like steps of Cu atoms, along the $\langle $110$\rangle $ direction on Cu(001), relieve the induced stress most effectively, in agreement with predictions from experiments. We consider the implications of these results on surface phonon dispersion curves for unreconstructed c(2x2)-N/Cu(001), which are in good agreement with experiment. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H19.00007: First principles calculations of the vibrational dynamics of c(2x2)-CO on Ag(001). M. Alcantara Ortigoza, T.S. Rahman, R. Heid, K.P. Bohnen The reaction pathway of CO oxidation on Ag surfaces is still a subject of debate because of the complicated chemistry of O and the possibility that contaminants stabilize CO. Indeed, at $\sim $150 K, the dissociative O$_{2}$ adsorption is scarcely triggered while the CO adsorption on clean Ag(001) is hardly stable. The nature of the CO adsorption is thus by itself a matter of discussion and the characteristic energy losses for exciting the phonon modes introduced by the adsorbed species have an uncertain assignment. We present an \textit{ab initio} study of the structure and phonon dispersion of a c(2x2), atop, CO overlayer on Ag(001). Comparison with a similar study of c(2x2) CO on Cu(001) indicates that CO chemisorbs on Ag(001) despite the low binding energy. The frequency of the C-O stretch mode at the $\Gamma $-point is in excellent agreement with HREELS measurements and is reduced on Ag(001) almost as much as on Cu(001). The weak Ag-CO bond is reflected in the low frequency of the rest of the CO modes. Yet, in the Ag-CO stretch, the Ag surface atoms are strongly coupled, as in the case of CO on Cu(001). Likewise, the CO frustrated translation mode couples to the substrate in the vicinity of the $\Gamma $-point but, unlike that on Cu(001), the CO frustrated rotation mode on Ag(001) couples to the substrate inside the surface Brillouin zone. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H19.00008: Oxygen-Induced Reconstructions on the $\beta-$Si$_3$N$_4$ $(10\overline{1}0)$ Surfaces Weronika Walkosz, Juan Carlos Idrobo, Serdar Ogut Motivated by recent electron microscopy studies at the Si$_3 $N$_4$/rare-earth oxide (REO) interfaces, we present first principles calculations for the preferred bonding sites and configurations of oxygen on various terminations of the $\beta- $Si$_3$N$_4$ $(10\overline{1}0)$ surface as a function of coverage and surface stoichiometry. We predict that oxygen induces various surface reconstructions, and it has a strong tendency to replace N on the surface. The structural stability of most low-energy surfaces is driven by the tendency of Si to saturate its dangling bonds and of oxygen to bridge two Si atoms similar to the bonding in SiO$_2$. The present {\em ab initio} results resolve the discrepancy between the experimental observations at the Si$_3$N$_4$/REO interfaces and previous theoretical studies\footnote{Juan C. Idrobo {\em et al.}, Phys. Rev. B {\bf 72}, 241301(R) (2005).} for bare surfaces regarding the lowest energy surface termination. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H19.00009: Insight into water molecules bonding on 4d metal surfaces Javier Carrasco, Angelos Michaelides, Matthias Scheffler Water-metal interactions are of capital importance to a wide variety of phenomena in materials science, catalysis, corrosion, electrochemistry, etc. Here we address the nature of the bond between water molecules and metal surfaces through a careful systematic study. Specifically, the bonding of isolated water molecules to a series of close-packed transition metal surfaces - Ru(0001), Rh(111), Pd(111) and Ag(111) - has been examined in detail with density functional theory (DFT). Aiming to understand the origin behind energetic and structural trends along the 4d series we employ a range of analysis tools, such as decomposition of the density of states, electron density differences, electronic reactivity function and inspection of individual Kohn-Sham orbitals. The results obtained allow us to rationalize the bonding between water and transition metal surfaces as a balance of covalent and electrostatic interactions. A frontier orbital scheme based on so-called two-center four-electron interactions between molecular orbitals of water and d band states of the surface proves incisive in understanding these systems. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H19.00010: Water uptake on thin film MgO using ambient pressure XPS John T. Newberg, David E. Starr, Erin Mysak, Susumu Yamamoto, Anders Nilsson, Hendrik Bluhm Understanding the molecular level interactions of water with metal oxide surfaces is important in both industrial processing and environmental chemistry. MgO(100) is one of the most widely studied metal oxide surfaces due to its simple rock salt cubic structure. However, whether water adsorbs dissociatively (hydroxylation) or molecularly (thin film wetting) remains unanswered. We have characterized the uptake of water on 7ML MgO(100) on Ag(100) at RT using ambient pressure XPS. Surface compositions were measured in-situ under water vapor pressures ranging up to 1 Torr. Our results indicate that initial hydroxylation occurs at low pressures mostly at Mg(2+) sites up to about 0.1 mTorr. At higher coverages both hydroxylation and thin film wetting continues up through 1 Torr. We will also discuss results from: 1. water uptake under higher/lower effective vapor pressures, 2. additional MgO(100) film thickness, and 3. water uptake on MgO(111)/Ag(111) surfaces. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H19.00011: Surface-induced solid-liquid phase transitions in ultra-thin water films at T $>$ 0 $^{\circ}$C Animesh Chakraborty, Andrew Gellman, Layton Baker, Estebahn Broitman We report here the measurements of both the adsorption isotherms and the dissipation in ultra-thin films of water adsorbed on the surfaces of SiO2 . The measurements were made in a small high vacuum chamber in which we have mounted a QCM. The chamber was evacuated to $\sim$10-8 Torr and then filled with water vapor at pressures ranging from 10-3 -- 40 Torr (the vapor pressure of water at room temperature is $\sim$22 Torr). In addition the temperature of the apparatus can be varied in the range 10 -- 60$^{\circ}$C. This is sufficient to measure the adsorption isotherm and to probe the phase of adsorbed water films over the range of conditions. Recently published work studying the adsorption of water on the SiO2 layer formed on Si single crystals has suggested that the phase of the water at temperatures well above 0$^{\circ}$C is actually that of a solid, ice-like structure rather than liquid water [1]. That work is based on the comparison of the vibrational spectrum of thin water films with those of liquid water and ice. In our study we are using the QCM to investigate the possibility of formation of Ice-like structures on SiO2. \newline [1] Asay, D. B. and Kim, S.H., Evolution of the Adsorbed Water Layer Structure on Silicon Oxide at Room Temperature. J. Phys. Chem. B. 2005, 109, 16760-16763 [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H19.00012: Relaxation of the (111) Surface of $\delta $-Pu and Effects of Atomic Adsorption: An \textit{Ab Initio} Study Raymond Atta-Fynn, Asok Ray The full-potential all-electron linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method has been employed to study the relaxation of the $\delta $-Pu (111) surface and the consequent effects for atomic adsorption of C, N and O atoms on this surface. The surface was modeled by a 5-layer slab with a (2$\times $2) surface unit cell. Upon relaxation of the slab, the interlayer separation between the surface and the subsurface layers expanded by 7.1{\%} with respect to the bulk interlayer separation while the separation between the subsurface and central layers expanded by 0.4{\%}. The hollow fcc adsorption site was found to be the most stable site for C and N with chemisorption energies of 6.420 eV and 6.549 eV respectively, while the hollow hcp adsorption site was found to be the most stable site for O with a chemisorption energy of 7.858 eV. The adsorbate-induced changes in the surface properties, namely the Pu magnetic moments, work function, and electronic structure will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H19.00013: Probing Adsorption and Thin-Film Growth with Optical Reflectance. Krish Kotru, Roger Tobin Optical reflectance provides a simple, inexpensive, noninvasive and sensitive method of monitoring adsorption and thin-film growth on metals in real time. Conduction electron scattering from isolated surface impurities decreases the near-surface electrical conductivity and causes a drop in reflectance. For chemisorbed species, the reflectance decreases monotonically and then saturates with completion of the adsorbed monolayer. Our probe, based on a modified laser pointer, can follow adsorption of oxygen and carbon monoxide on Cu(100) in ultrahigh vacuum with submonolayer sensitivity. We also monitor the deposition of Fe and Cu thin films on the same surface, and for layer-by-layer growth expect to see an oscillatory variation of reflectance in which local maxima correspond to the completion of each atomic layer. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H19.00014: A Phenomenological Study of Adsorption on fcc (335) Terraces Alain Phares, David Grumbine, Jr., Francis Wunderlich The thermodynamic equilibrium model of adsorption on fcc (112) terraces (Langmuir 22, 7646, 2006) is extended to adsorption on fcc (335). There is preferential adsorption on one of the steps, and the difference, $U$, between adsorbate-substrate interaction energy on this step,$V_s$, and that on the rest of the terrace, $V_b$, is not necessarily zero. First-neighbor adsorbate-adsorbate interaction energy $V$ is usually repulsive, and we consider $V<0$, account for arbitrary second-neighbor interaction energy $W$, and allow the chemical potential energy per particle in the gas $\mu'$ to vary. The relatively low temperature 3-D energy phase diagram is obtained in terms of $u = -U/V$, $v=-\mu/V$, and $w = -W/V$, where $\mu = \mu' + V_b$. Based on this diagram, the experimental observation of just one or more phases allows one to predict the ranges or, ultimately, all of the values of $u$, $v$, and $w$. As the model is phenomenological and does not require the knowledge of the chemical composition of the substrate and the adsorbates, it can be applied to adsorption on a wide variety of fcc (335) terraces. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H19.00015: The Nature of the Dissociation Sites of Hydrogen Franck Rose, Mous Tatarkhanov, Evgeni Fomin, D. Frank Ogletree, Miquel Salmeron Previously, we have demonstrated [1] that pairs of neighboring monoatomic hydrogen vacancies (HVs) on Pd(111) are totally unreactive toward H$_{2}$ dissociative adsorption and that active sites for the catalytic reaction can only be constituted of ensembles of at least three aggregated HVs. Thanks to LT-STM atomic observations, we show that this new description of H$_{2}$ dissociative adsorption onto transition metal surfaces is not an exotic particular catalytic case relevant only to Pd(111) and close-packed faces of fcc metals, but that it also applies to close-packed faces of hcp metals such as Ru(001) [2, 3]. Close to saturation of 1 ML, HVs were observed either as single entities or forming transient aggregations. Vacancies diffuse and aggregate to form active sites for the dissociative adsorption of H$_{2}$. We have found that H$_{2}$ dissociation takes place only on Ru sites where the metal atom is not bound to any H atom [3]. Such active sites are formed when at least 3 HVs aggregate by thermal diffusion. Sites formed by single HV or pairs of adjoining HVs were found to be unreactive toward H$_{2}$. [1] T. Mitsui, et al., \textit{Nature} \textbf{422} 705 (2003) [2] M. Tatarkhanov, et al., \textit{Surf. Sci.,} \textit{in Press }(2007) [3] F. Rose, et al., \textit{J. Phys. Chem. C,} \textit{Accepted} (2007) [Preview Abstract] |
Session H20: Focus Session: Assembly of Nanowires and Related Structures
Sponsoring Units: DMPChair: Ken Shih, University of Texas at Austin
Room: Morial Convention Center 212
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H20.00001: Formation and Properties of CdS-Ag$_2$S Nanorod Superlattices Denis Demchenko, Richard Robinson, Bryce Sadtler, Lin-Wang Wang, A. Paul Alivisatos, Can Erdonmez The mechanism of formation of recently fabricated$^1$ ordered CdS-Ag$_2$S nanorod superlattices is explained and their elastic and electronic properties are predicted theoretically. We show that diffusion-limited growth of Ag$_2$S islands in CdS nanorods partially contributes to the observed ordering, but cannot account for the full extent of the ordering alone. The valence force field (VFF) model results for the nanostructure show significant repulsion between Ag$_2$S segments due to strain created by the lattice mismatch between the two materials. This suggests that the interplay between the chemical interface energy and strain drives the spontaneous pattern formation. A first principles calculation of the energy levels in the superlattice shows a nested band alignment. The nanorod superlattice therefore corresponds electronically to a sequence of quantum wells of Ag$_2$S separated by barriers of CdS. The minibands formed in such superlattices make them desirable for applications in the solar cells. 1. R. D. Robinson, B. Sadtler, D. O. Demchenko, C. K. Erdonmez, L.-W. Wang, and A. P. Alivisatos, {\em Science} {\bf 317}, 355 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H20.00002: Transport studies on ultrathin silicide nanowires Violeta Iancu, Changgan Zeng, Stephen Jesse, Arthur Baddorf, Hanno Weitering Minute amounts of yttrium deposited on a silicon (100) surface assemble into ultrathin YSi$_{2}$ nanowires. Ultrathin nanowires receive a great deal of interest due to their possible uses as interconnects in nano-electronic devices or as nano-electrodes to measure e.g. the transverse current across DNA molecules in nanofluidic channels. Here we present electrical conductance measurements of a nanowire bridged by macroscopic electrodes and the characterization of the nanowire/electrode contact by scanning probe microscopy. The stability of the nanowires after exposure to air, water, and a KCl solution is also addressed for future use in nanofluidics. Research was conducted in part at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. DOE. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H20.00003: Copper Incorporation into ZnO Nanowires Susie Eustis, Douglas Meier, Babak Nikoobakht The applications of zinc oxide (ZnO) nanowires (NWs) in devices are promising due to the optical, mechanical and electrical properties of these one-dimensional structures, but current uses are limited by the ability to produce high quality nanowires at desired locations. Copper is an attractive catalyst for generating zinc oxide nanowires due to the long length and high density of ZnO NWs produced. However, defects due to impurities, oxygen deficiencies, and structural defects lead to decreased optical and electrical transport. Photoluminescence (PL) microscopy found that ZnO NWs grown by high temperature evaporation on a bulk copper substrate display the expected band gap emission at 380nm. A larger visible emission is also observed in the PL spectrum around 520nm due to defect states. High-resolution transmission electron microscopy (HR-TEM) shows the ZnO nanowires are single crystalline with hexagonal structure. The metallic tip shows a polycrystalline structure in HR-TEM images. Energy dispersive x-ray spectroscopy (EDS) mapping and auger electron spectroscopy (AES) clearly show copper throughout the length of the nanowire, which is most likely responsible for the strong deep trap emission from these nanowires. AES also finds large amounts of oxygen on the surface of these NWs, a further contributor to defect emission states. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H20.00004: Self-organization of atom wires on vicinal surfaces Invited Speaker: Self-organization is possibly the best way to produce nanostructures in large quantities. This also holds for the ultimate 1D system, atom wires; they can be self-assembled in large arrays on vicinal Si surfaces. Such atom wire systems often show intriguing electronic properties such as competing charge density waves and spin-orbit split one-dimensional bands. However, because of their low dimensionality, these wires also frequently show profound thermodynamic fluctuations that limit their \textit{structural} uniformity and have a large influence on their electronic properties. Therefore, in this talk I will focus on structural fluctuations in Ga atom wires self-organized on the Si(112) surface. In these atom wires, strain-relieving adatom vacancies self-organize into meandering vacancy lines (VLs) similar to the well-known nx2 superstructures for Ge on Si(100). The average spacing between these line defects can be experimentally controlled continuously by adjusting the chemical potential $\mu $ of the Ga adatoms. Significant VL correlations are discovered in STM experiments that cannot be captured within a mean field analysis. These structural flucuations are well described by a new lattice model that combines Density Functional Theory (DFT) calculations for perfectly ordered structures with the fluctuating disorder seen in experiment, and the experimental control parameter $\mu $. This hybrid approach of lattice modeling and DFT can be applied to other examples of line defects in hetero-epitaxy, especially in cases where correlation effects are significant and a mean field approach is not valid. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H20.00005: Growth and Properties of Self-Aligned MgO Nanowires Elena Cimpoiasu, Robert F. Klie, Ryan A. Munden, Mark A. Reed A simple VLS route was used to produce self-aligned MgO nanowires on both polished crystalline (c-axis sapphire) and ceramic (alumina) surfaces. Growth on alumina produces vertically-aligned, very thin nanowires, indicating enhanced growth at the liquid-solid interface. Growth on polished sapphire results in faceted MgO nanowires which are perpendicular to the $r$-plane of sapphire and show evidence of competing vapor-solid growth mechanism. The difference in the morphology and structure of the nanowires grown using the two different substrates clearly illustrates the affect of substrate on the growth process. \textit{This work was partially supported by DARPA, by the Department of Homeland Security, and by the National Science Foundation.} [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H20.00006: Novel Nanocomposites for Energy and Electromagnetic Applications via 3D self-assembly Amit Goyal, Sung-Hun Wee, Yanfei Gao, Junsoo Shin, Karren More, Yuri Zuev, Claudia Cantoni, Jianxin Zhong, Malcolm Stocks Nanocomposites comprising ordered 3D arrays of nanodots of one type of ceramic material coherently embedded in another ceramic matrix comprise are expected to exhibit novel physical properties tunable by adjusting the overall composition, concentration, feature size and spatial ordering of the nanodots. Wide-ranging applications such as photovoltaics, solid state lighting, ultra-high density storage and high temperature superconductivity are of interest. We report here on a joint experimental, theoretical and computational study on achieving 3D ordering via 3D self-assembly of nanodots of a complex ceramic material within another complex ceramic material, such as 3D self-assembly of BaZrO$_{3}$ nanodots in REBCO superconducting films. Examples will also be given for other ceramic and metal/ceramic systems. In all cases 3D self-assembly was obtained in epitaxial thick films grown via pulsed laser ablation on single-crystal or single-crystal-like substrates. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H20.00007: Terrace-width Distributions on Vicinal Surfaces: Effective Attraction Between Noninteracting Touching Steps Rajesh Sathiyanarayanan, Ajmi BHadj Hammouda, T.L. Einstein Terrace-width distributions (TWDs) characterize equilibrium as well as non-equilibrium morphology of vicinal surfaces. Using the terrace-step-kink (TSK) model, we apply Monte Carlo simulations (both Metropolis and kinetic) to study TWDs. Steps interact via inverse-square entropic and elastic interactions. Steps which cannot touch each other (except at corners) have monatomic height; their configurations, correspond to the worldlines of fermions in 1D. The associated TWDs are well described by the generalized Wigner surmise. The fit parameter ($\varrho$) is directly related to the dimensionless energetic interaction strength ($\tilde{A}$).\footnote{Hailu Gebremariam et al., Phys. Rev. B {\bf 69}(2004)125404} If steps are allowed to touch, one can find some double (or greater) height steps. For closely spaced steps, this can alter the TWD considerably. We simulated the TWD of steps with $\tilde{A}=0$ but touching allowed. Our results indicate an effective attraction between steps, as reflected by a value of $\varrho$ significantly less than 2 (the $\tilde{A}=0$ value with touching forbidden). As expected, this effective attraction becomes weaker as the terraces become wider; we discuss the crossover behavior. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H20.00008: Study of Fe atomic chains on Pt(997) vicinal surface Elijah Ayieta, Ya.B Losovyj, Cheng Ruihua The vicinal substrates form the promising templates for low cost effective bottom-up fabrications of nanostructures. A variety of one-dimensional atomic chains can be synthesized on the stepped surfaces. The electronic structure of a vicinal surface plays a significant role in determining the physical properties of atomic chains on stepped surface as well as the surface morphology. The Pt(997) surface is cut $6.5^o$ of Pt(111)surface forming terraces width of 2nm. The surface of the substrate is then characterized using scanning tunneling microscopy STM and angle resolved photoemission spectroscopy from synchrotron radiation source. The data shows that the surface has uniform steps with no reconstructions. Electron confinement is observed with wave vector perpendicular to the step direction. Fe atomic chains were then carefully deposited on the surface of Pt(997) and then finally characterized The exchange splitting of Fe 3$d$ bands is estimated according to the photoemission spectroscopy data. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H20.00009: Thermoelectric properties of Bi$_{2}$Te$_{3}$ films and nanowire arrays Cheng Lung Chen, Yang Yuan Chen, P. C. Lee, C. T. Chen, S. R. Harutyunyan, S. J. Lai, C. D. Chen, S. J. Lin The n-type Bi$_{2}$Te$_{3}$ nanowire arrays with diameter $\sim $120 nm and thin films with thickness $\sim $10 $\mu $m have been fabricated by electrochemical deposition from nitric acid bath, containing bismuth nitrate and tellurium dioxide. Extensive characterizations of the morphology, structure, and composition of the films and nanowires were performed by means of SEM, XRD, EDS, and TEM. The films have nanocrystalline structure whereas the nanowires are single crystallines. The influence of microstructure on thermoelectric properties was investigated by comparison charge carrier transport in two mutually perpendicular crystallographic directions. The measurements of Seebeck coefficient and electrical resistivity were carried out in temperature region of 180 to 300 K. The highest value of $\sigma $S$^{2}$ = 840 $\mu $W/m-K$^{2}$ was obtained at 285 K for film. The electrical resistivity of an individual Bi$_{2}$Te$_{3}$ nanowire is $\sim $4.3 $\mu \Omega $-m using e-beam lithography technique. Based on the Seebeck coefficient obtained from nanowire arrays, the calculated value of $\sigma $S$^{2}$ for single nanowire is $\sim $1000 $\mu $Wm$^{-1}$K$^{-2}$ which is higher than that of the film. These results may help in designing processes for thermoelectric microdevices. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H20.00010: Highly Aligned Epitaxial Nanorods with a Checkerboard Pattern in Oxide Films S. Park, Y. Horibe, T. Asada, N. Lee, S-W. Cheong, L.S. Wielunski, T. Gustafsson, P.L. Bonanno, S.M. O'Malley, A.A. Sirenko, A. Kazimirov, T. Tanimura One of the central challenges of nano-science is fabrication of nano-scale structures with well-controlled architectures using planar thin-film technology. Herein, we report our discovery of a periodic nanometer-scale self-assembly in spinel films, fabricated by manipulating spontaneous phase separation and substrate strain. The films consist of two types of chemically-distinct nanorods with mutually coherent interfaces, perfectly aligned along the film growth direction. This unique three dimensional epitaxy process contrasts with a typical behavior in conventional growth of highly lattice-mismatched films, and thus provides an important route for film fabrication of nano-structured arrays with periodically varied electronic and magnetic properties. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H20.00011: Photo-Induced Nanowire Formation on Polarity Patterned Ferroelectric Domains -- Wavelength Dependence Yang Sun, Robert Nemanich This research is focused on the bottom-up growth of nanostructures on periodically polled (congruent) lithium niobate. The formation of silver nanowires has been reported through a photo-induced reaction of UV exposed lithium niobate immersed in an aqueous silver nitrate solution. The metallic wires assemble predominantly at the domain walls of the periodically polled lithium niobate. In this study the process has been studied with Hg lamp excitation using a set of wavelength filters. Depending on the wavelength, the process can exhibit deposition over the whole surface or predominantly at the domain boundaries as previously described. The research has been extended to explore the photo-induced deposition of copper using an aqueous copper sulfate solution The process of copper wire formation on lithium niobate is occasionally observed, which suggests a dependence on surface preparation. When the thickness of the solution layer is reduced and the intensity of the UV light is increased, the lithium niobate surface is essentially uniformly covered with nanosized dots. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H20.00012: Low melting metal-induced synthesis of multi-component III-V semiconducting nanowires. Romaneh Jalilian, Zhiqiang chen, Gamini Sumanasekera Semiconductor multi-component alloys provide a natural means of tuning the band gap and other parameters to optimize and extend the application of semiconductor electronic/optoelectronic devices. In this study, multi-component nanowires have been synthesized in vapor phase by laser ablation of solid targets consisting of initial bulk materials. Growth of nano-crystals is believed to seed from low melting metallic droplets generated from laser bombardment and heating of the target. Curved tips have been observed at one end of the nanowires which contain same elemental components as the body of the nanowires. This technique is proven to be a successful approach to eliminate the need for external catalyst which can have detrimental consequences affecting the performance of an optoelectronic device. As-synthesized nanowires were characterized using TEM, SEM, XRD, EDS, photo luminescence, Vis-IR absorption and Raman spectroscopy. Results of transport properties of individual nanowires will also be presented. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H20.00013: Si nanowires on Ag(110): an experimental and electronic structure studies A. Kara, B. Ealet, C. Leandri, H. Oughaddou, G. LeLay, B. Aufray Si nanowires images on Ag(110) were obtained using Scanning Tunneling Microscopy. Wires with tailored width and very large lengths were observed with the wires growing along the open channels of Ag(110). For those wires 5 lattice constants wide, we observed 4 distinct bright spots across the wire with a periodicity of 2 times the Ag nearest neighbor distance along the wire. From the theoretical side, we propose several structures of Si on Ag(110) where Si atoms occupy either single or double layers, with the second layer may be formed by single or pair of atoms; and the first layer may be commensurate or incommensurate with the substrate. Comparison between calculated and observed STM images will be presented and a detailed analysis of the electronic structure of all the proposed structures will be discussed. [Preview Abstract] |
Session H21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials III
Sponsoring Units: DCPChair: Roberto Rodrigues, Virginia Commonwealth University
Room: Morial Convention Center 213
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H21.00001: Probing electronic and magnetic properties of atomic and molecular clusters with sharp tips Invited Speaker: Probing magnetic and transport properties on a local basis with the tip of a scanning tunneling microscope (STM) allows establishing close links with the exciting field of magnetic read and write processes.~ Some examples of applications of this approach to magnetic nanostructures will be shown. First of all, the fundamental properties of Co nanoclusters, on metal surfaces will be presented. These clusters have been probed by low temperature dI/dV spectroscopy (STS). It is found that occupied electronic surface states of the Co clusters are sensitive to the crystallographic stacking and furthermore exhibit a downward energy shift as the cluster size decreases. \textit{Ab initio} calculations confirm that the observed shift is due to the size dependent mesoscopic relaxation in the clusters. When a magnetic tip is used in low temperature spin polarized (SP)-STM experiment, it is possible to reveal spin polarized feature in the local density of states. For example, one is able to identify two magnetization states of the nanometer Co clusters, corresponding to the parallel or antiparallel configuration with respect to the tip polarization. Progress in the emerging field of spintronics strongly relies on the fundamental understanding of electron/spin transport and magnetic phenomena in reduced dimensions, down to the extreme limit of individual molecules, or even single atoms where sizeable quantum effects are expected. Electronic and magnetic properties of Co atoms and metal-based molecular magnets adsorbed on magnetic nano-islands or on non magnetic surfaces will be presented. On the example of Co-phtalocyanines prepared in UHV at 4.6 K, it will be shown that dI/dV characteristics are representative of both, the nature of the molecule and also its interaction with the substrate. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H21.00002: First Principles Theory of Supported Clusters with Complex Magnetic Order. Invited Speaker: It is demonstrated that the magnetic interactions can be drastically different for nano-sized systems compared to those of bulk or surfaces. In a real-space formalism we have developed a technique to calculate non-collinear magnetization structures and hence exchange interactions. Our results for magnetic Cr, Mn and Fe clusters supported on a Cu(111) surface show that the magnetic ordering as a rule is non-collinear and can not always be described using a simple Heisenberg Hamiltonian. We argue that the use of \textit{ab initio} calculations allowing for non-collinear coupling between atomic spins constitute an efficient and reliable way of analyzing nano-sized magnets. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H21.00003: Magnetic properties and stability of metalloinorganic clusters Roberto Robles, Shiv Khanna Theoretical studies on the structure, stability, electronic structure and magnetic properties of binary clusters Si$_n$TM$_m$ (n=1-8, TM=Cr,Mn) have been carried out within a density functional formalism using the generalized gradient approximation. The stability of the clusters as a function size is analyzed in terms of several criteria, like the progression in bonding energy and HOMO-LUMO gap, with the ultimate objective of identifying the simple rules that can guide the search of stable species. The magnetic properties of the clusters are investigated by considering different ferromagnetic and antiferromagnetic arrangements of the local spin moments and optimizing the geometry and the spin state to determine the ground state including possible isomers. The possible use of these clusters as building blocks of cluster assemblies is discussed, and finally, the interest of these assemblies in the design of materials which could be used in the field of spintronics is briefly considered. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H21.00004: Intermediate valence, local antiferromagnetic coupling and the Kondo effect in ytterbium organometallic molecules C.H. Booth, W.W. Lukens, M.D. Walter, D. Kazhdan, R.A. Andersen, E.D. Bauer, L. Maron, O. Eisenstein Studying magnetic ions coupling to aromatic rings in organometallic molecules provides an analogous route to studying the Kondo effect in nanoscale systems. We extend the number of molecules displaying such effects in their magnetism and x-ray absorption spectroscopy from cerocene [Ce(C$_8$H$_8$)$_2$] and Cp*$_2$Yb(bipy) [Cp*=pentamethylcyclopentadienyl, bipy=bipyridine) to a collection of Cp*$_2$Yb($L$) molecules, where $L$ is one of various bipyridyl or diazadiene ligands. Clear trends are observed in both the magnetic susceptibility and the Yb valence that indicate changes in the fundamental temperature scale. CASSCF calculations indicate the intermediate valence is primarily due to a configuration interaction between the open-shell $f^{13}\pi^{*1}$ and the closed-shell $f^{14}\pi^{*0}$ spin-singlet states, in direct analogy to the Kondo effect in intermetallic systems. These studies increase the range of molecular species where such properties are observed, and point toward understanding the ubiquity of such effects and their involvement in fundamental bonding and magnetism in organometallic molecules. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H21.00005: The structure, stability, and magnetic properties of Au(111)/NiO(111) interface: density functional theory study K.L. Yao, Y.L. Li, Z.L. Liu We studied the electronic structure of Au(111)/NiO(111) interface in accordance with the two models of NiO(111) surface. The work of adhesion, the spin magnetic moment, the stability and the electronic properties of the Au(111)/NiO(111) interface were calculated by density functional theory (DFT). The calculated results of Au(111)/NiO(111) interface were then compared with non-polar Au(100)/NiO(100) interface. At the same time, the total density of states (DOS) of Au(111)/NiO(111) interface corresponding to the two models were also calculated. The calculations reveal that the Ni-terminated and the oxidized interfaces have antiferromagnetic properties, while the O-terminated interface exhibits ferromagnetic properties. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H21.00006: Investigating the Molecular Level Details of Catalytic Oxidation Reactions Invited Speaker: Gas-phase cluster reactivity studies are providing significant insight into the molecular level mechanisms of oxidation reactions occurring on catalytic surfaces. Our experimental approach, employing tandem mass spectrometry, uses mass selected metal oxide clusters to model specific catalytic active sites. This technique enables investigation of the influence of factors such as size, stoichiometry, charge state, and elemental composition on the reactivity of catalytic materials. Particular emphasis is on identifying species with enhanced activity for the selective oxidation of simple hydrocarbons and atmospheric pollutants. Recent findings pertain to the kinetics of ethylene oxidation in the presence of vanadium oxides and the oxidation of carbon monoxide in the presence of gold and iron. Through a combination of experiments and theoretical calculations we establish structure-reactivity relationships and propose general reaction mechanisms for these catalytic processes. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H21.00007: Effect of ligand on the geometric and electronic structure of Au$_{13}$ cluster Ghazal Shafai, Sampyo Hong, Talat Rahman, Massimo Bertino We have carried out calculations based on the density functional theory in the projector augmented wave scheme (PAW) and the pseudopotential approach, to examine the effect of the ligand on the geometric and electronic structure of Au$_{13}$ cluster. We find \textit{bare} Au$_{13}$ to form a flat flake, in agreement with previous theoretical calculations. This structure is lower in energy by 2.60 eV in comparison with the well ordered icosahedron geometry. Our results show, however, that the \textit{Au}$_{13}$\textit{ cluster covered with ligands of phosphine (PH}$_{3}$\textit{) forms a stable spherical structure} \textit{(icosahedron) }in agreement with the experiment [1] which is by 0.08 eV lower in energy when compared to the flat-flake complex. If the phosphine is replaced by H, the spherical structure is no longer stable, but it still maintains a 3 dimensional form, signifying the effect of the ligand in stabilization of the structure. We observe a narrow d-band for flat-flake gold atoms in the complex, while in the icosahedron structure the d-band is wider. We also find a stronger overlap between the p orbitals of the P atom with d orbitals of gold atoms in the icosahedron complex. [1] M. F. Bertino et. al. Phys. Chem. B Lett. 110, 21416 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H21.00008: Superheating, Melting and Precursors to Melting in Metal Nanoparticles Dmitri Schebarchov, Shaun Hendy We have investigated precursors to melting in metal nanoparticles using molecular dynamics in the microcanonical ensemble. At the onset of solid-liquid phase coexistence, we find first-order transitions in clusters with non-melting facets (i.e. facets that are not wet by the melt such as Pb (111)), and continuous transitions otherwise. In sufficiently small clusters however, we find that static solid-liquid coexistence is unstable. Further, the size at which the instability arises, and even the melting temperature, depends on the ability of the melt to wet the solid facets of the cluster. In particles with non-melting facets we show that the melting temperature can exceed that of the bulk material. Finally, we also discuss a range of solid-solid transitions that have been observed to occur during solid-liquid phase coexistence, some of which are again driven by the preference of the melt to wet certain crystal facets. [Preview Abstract] |
Session H22: Electrically and Optically Active Polymers
Sponsoring Units: DPOLYChair: Wesley Burghardt, Northwestern University
Room: Morial Convention Center 214
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H22.00001: Polymer Physics Prize Break
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Tuesday, March 11, 2008 8:36AM - 8:48AM |
H22.00002: Periodic Polymers for Technology Edwin Thomas \textit{Periodic} polymeric materials comprised of solid polymer and air have interesting interactions with electromagnetic and mechanical waves giving rise to complex dispersion relations including zero density of states (band gaps). The key concern for photonic materials is dielectric contrast, whereas for phononic materials, it is density contrast and relative speed of longitudinal and transverse waves in the two media that is important. The creation of a material with a dual band gap, that is a complete band gap for light and a complete band gap for sound would allow strong coupling in localized defect regions. Progress in this area requires the ability to design and model targeted geometries and excellent control of structure fabrication. Top-down, and bottom-up approaches, involving interference lithography and self assembly respectively are demonstrating good success in fabricating the requisite structures and creating desired properties for photonics and phononics. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H22.00003: First-principles investigation of high energy density in PVDF copolymers V. Ranjan, Liping Lu, M. Buongiorno Nardelli, J. Bernholc PVDF and its copolymers exhibit excellent electromechanical properties and in the case of PVDF-CTFE also a very high energy density [1]. We have investigated the phase diagram of these systems and can quantitatively explain the observed energy density of PVDF-CTFE as due to a para to ferroelectric phase transition in a disordered, multidomain structure [2]. Our results show that pure PVDF prefers the $\alpha$ phase at zero field. Electric field lowers the free energy of the $\beta$ phase, resulting in a structural phase transition at a sufficiently high field. Copolymer admixture lowers the critical field and eventually leads to an energetic preference for the $\beta$ phase even at zero field. For PVDF-CTFE with CTFE content below 17 \%, the $\alpha$ phase is still preferred and the field-induced phase transformation reversibly stores large amounts of energy. For PVDF-TeFE, the total energy difference between the two phases is much smaller, resulting in substantially smaller energy density. [1] B. Chu et al., Science 313, 334 (2006). [2] V. Ranjan et al., PRL 99, 047801 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H22.00004: High and Stable Light Induced Birefringence from Spacer-Free Dye-Polyelectrolyte Liquid Crystal Complexes Qian Zhang, C. Geraldine Bazuin, Christopher J. Barrett, Christian Pellerin Azo materials are promising in photonic applications due to the well-known photoisomerization of azo groups, which, for example, allows efficient inscription of gratings using light induced birefringence (LIB). The incorporation of liquid crystal (LC) character in these materials can be desirable to improve LIB properties, such as in side chain liquid crystal polymers (SCLCPs). However, SCLCPs are costly, and flexible alkyl spacers tend to diminish LIB properties. Here, we present LC azo materials obtained by simple ionic complexation procedures involving commercially available (or easily synthesized) dyes and oppositely charged polyelectrolytes; for example methyl orange (MO) and methylated poly(4-vinyl pyridine) (PVPMe). The latter complex, which possesses neither flexible spacer nor tail, has a single-layer smectic A-like structure until degradation and provides exceptionally high and stable LIB properties. These materials can be inscribed with surface relief gratings. Moreover, we have successfully obtained a photoresponsive electrospun mat from a solution of the MO/PVPMe complex mixed with poly(ethylene oxide). [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H22.00005: Reducing radiation-induced conductivity in polymeric dielectrics by small molecule electron traps Robert J. Klein, John L. Schroeder, Shannon M. Lacy, Michael E. Belcher, Phillip J. Cole, Joseph L. Lenhart Polymeric dielectrics, when exposed to ionizing radiation, undergo the formation of electron-hole pairs and consequently exhibit radiation-induced conductivity (RIC), severely limiting the insulating capability of polymeric dielectrics used in ionizing environments. RIC can be significantly reduced by the incorporation of small-molecule traps: in the appropriate concentration range, small molecules consisting of aromatic rings and strongly electron-withdrawing groups can reduce RIC by more than 95 {\%} in poly(ethylene terephthalate) films. The dopant structure is critical: the addition of one nitro group, the strongest electron-withdrawing substituent, leads to $>$ 98 {\%} RIC reduction when placed on fluorenone, pyrene, acenaphthene, and anthracene cores. Other substituents, such as cyano or amino, improve RIC reduction over the isolated cores, but not as effectively as the nitro group. The electron-withdrawing capability of each substituent side group can be quantified using the Hammett parameter. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H22.00006: Dynamics of acoustic phonons in exciton self-trapping in a quasi-one-dimensional system F.X. Morrissey, S.L. Dexheimer The localization of electronic excitations via electron-lattice interactions is an important fundamental process in molecular-based electronic materials. In our previous work, we directly time-resolved the electronic and vibrational dynamics of the exciton self-trapping process in the quasi-one-dimensional mixed-valence metal-halide linear chain (MX) complexes [Pt(en)$_{2}$][Pt(en)$_{2}$X$_{2}$], (X = Cl, Br, I) using femtosecond coherent phonon techniques. In this work, we present transient absorption measurements on PtBr(en) at low temperature that reveal a large amplitude, strongly damped oscillatory component at a frequency of 11 cm$^{-1}$ that is consistent with the generation of a coherent acoustic wave associated with the formation of the localized lattice deformation that stabilizes the self-trapped state. Comparison with models for polaron formation provides an estimate of the spatial extent of the local deformation of $\sim $ 5 unit cells. This work is supported by the NSF under grant DMR-0305403. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H22.00007: THz time domain spectroscopy of low-frequency vibrations in a quasi-one-dimensional system A. Bandyopadhyay, S.L. Dexheimer The mixed-valence halide-bridged transition metal linear chain (MX) complexes are prototypical quasi-one-dimensional systems, with a charge density wave ground state and localized electronic excitations analogous to those of conjugated organic polymers. In this work, we present studies of the low-frequency infrared-active vibrational modes of the MX complex [Pt(en)$_{2}$][Pt(en)$_{2}$I$_{2}$](PF$_{6})_{4}$ (en = ethylenediamine, C$_{2}$H$_{8}$N$_{2})$ in the frequency range 0.3 - 3 THz using terahertz time-domain spectroscopic techniques. Distinct polarization-dependent complex refractive indices are observed in single-crystal samples of this highly anisotropic material. The measurements reveal a strong absorption at a frequency of 2.24 THz (75 cm$^{-1})$ polarized along the chain axis, which we assign to the infrared-active $\nu _{3}$ vibrational mode, involving relative motion of the mixed-valence ions in the charge density wave structure. This work is supported by NSF grant DMR-0706407. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H22.00008: Optical studies of Pt-rich $\pi $--conjugated Polymers Tomer Drori, M. Tong, A. Gambetti, S. Singh, C. Yang, Z. V. Vardeny, S. Tretiak We have used a variety of steady state and ultrafast spectroscopies for studying the photophysics of platinum-containing conjugated polymers, which have potential applications as the active layer of light-emitting diodes. The heavy metal Pt atom that is incorporated in the polymer chain dramatically increases the spin-orbit coupling, and this influences both the intersystem crossing time, T$_{ISC}$, and the phosphorescence emission intensity. The Pt-polymers were newly synthesized, where the intrachain Pt atom was incorporated into the polymer either in each (Pt-1) or in every three (Pt-3) monomer units. We will discuss an interesting effect for the photoexcited triplets, which dramatically influence the phosphorescence spectral shape vs. temperature. We also observed the existence of circular polarization memory of the phosphorescence emission in Pt-1 polymers, in which the platinum atoms are separated by only one phenyl ring; but not in Pt-3. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H22.00009: Electronic Structure of Photo-degraded Polypropylene Ultrathin Films Orhan Kizilkaya, Pingheng Zhou, Eizi Morikawa The structural degradation induced by synchrotron radiation in polypropylene ultrathin films has been investigated by ultraviolet photoemission spectroscopy (UPS) and molecular orbital (MO) calculations. The UPS results of pristine and degraded films show very good agreement with the calculated density of states obtained from model MO calculations. The UPS results of the degraded films show a new peak appearance as the highest molecular state in the photoemission spectrum. Model MO calculations and UPS results correlate the new peak to the generation of double bond conjugation. This pi (double) bond generation has also been proved with the near edge X-ray absorption fine structure (NEXAFS) measurements. The pre-edge feature in the C-1S NEXAFS spectrum was augmented after the film exposed to white light emitted from synchrotron radiation. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H22.00010: Molecular Dynamics Simulation of Highly Rigid Polymers in Dilute Solutions Sabina Maskey, Flint Pierce, Dvora Perahia, Gary Grest The dynamics of highly rigid polymers control their degree of conjugation and hence their electro-optical characteristics. Molecular dynamics (MD) simulations have been used to study the conformation of a dilute solution of dinonyl \textit{para}-polyphenyleneethylene (PPE) in toluene, which is a good solvent for the backbone of the polymer. The goal of this study is to identify the factors that affect the conformation of a single chain. PPEs in solutions and at interfaces form a rich variety of optically active structures from micelles to gels, whereas the conformation of a single chain controls their optical response. Experimental studies have shown that the conformation of PPE has been affected by the nature of the solvent, the degree of polymerization as well as the nature of the side chains. The degree of cooperatively of the solvent with the polymer as well as the relative conformation of the aromatic groups with in the backbone, as obtained from MD studies will be discussed, together with effects of varying the molecular weight and the nature of the side chains. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H22.00011: Ab-initio calculations of quasiparticle and excitonic properties of low band gap, polythiophene-based polymers Filipe Ribeiro, Georgy Samsonidze, Steven Louie, Marvin Cohen Electron donor polythiophene-derived polymers coupled with electron acceptor C60 compounds are the basis for the state-of-the-art organic photovoltaic (OPV) technology. However, with an incident photon to converted electron efficiency of only 5\%, OPV cells are not yet competitive with conventional inorganic semiconductor technology. One of the limitations is the relatively high energy gap of polythiophene which precludes the absorption of infrared photons. In this work, using the GW approximation and solving the Bethe-Salpeter equation, we compare results of the quasiparticle and excitonic properties of thiophene-, vinylene- and cyanovinylene-based copolymers with lower energy band gaps than polythiophene. The energy band alignments of the polymers and the C60 molecule are also discussed. [Preview Abstract] |
Session H23: Focus Session: Probing and Modifying Materials with Lasers I
Sponsoring Units: DMPChair: Craig Arnold, Princeton University
Room: Morial Convention Center 215
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H23.00001: Vibrational energy on surfaces: Ultrafast flash-thermal conductance of molecular monolayers Invited Speaker: Vibrational energy flow through molecules remains a perennial problem in chemical physics. Usually vibrational energy dynamics are viewed through the lens of time-dependent level populations. This is natural because lasers naturally pump and probe vibrational transitions, but it is also useful to think of vibrational energy as being conducted from one location in a molecule to another. We have developed a new technique where energy is driven into a specific part of molecules adsorbed on a metal surface, and ultrafast nonlinear coherent vibrational spectroscopy is used to watch the energy arrive at another part. This technique is the analog of a flash thermal conductance apparatus, except it probes energy flow with angstrom spatial and femtosecond temporal resolution. Specific examples to be presented include energy flow along alkane chains, and energy flow into substituted benzenes. Ref: Z. Wang, J. A. Carter, A. Lagutchev, Y. K. Koh, N.-H. Seong, D. G. Cahill, and D. D. Dlott, Ultrafast flash thermal conductance of molecular chains, Science 317, 787-790 (2007). \newline \newline This material is based upon work supported by the National Science Foundation under award DMR 0504038 and the Air Force Office of Scientific Research under award FA9550-06-1-0235. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H23.00002: Mechanism of Resonant Infrared Laser Ablation of Polystyrene. Stephen Johnson, Richard Haglund, Daniel Bubb, Kannatessen Appavoo Although the ablation of intact polymers by resonant infrared (IR) laser irradiation has been demonstrated, the mechanism has remained mysterious. This is partly because the IR excitation of complex polymer materials is poorly understood, and partly because most of the experiments have been conducted with a tunable infrared free-electron laser (FEL) with an unusual micropulse-macropulse temporal structure. We have fully characterized the resonant and non-resonant IR ablation of polystyrene (PS) at several IR wavelengths. The energy input at each wavelength was ascertained by convoluting the temperature-corrected lineshape functions of individual C-H and C-C bonds with the spectral profile of the picosecond FEL micropulses. Data from ablation rate, ablation depth, time-resolved photoacoustic and photothermal measurements and nanosecond pulsed-laser shadowgraphy were fed into a simple finite-element model of energy deposition and relaxation. The data and model are consistent with a steady-state ablation mechanism, modified by plume shielding late in the microseconds-long FEL macropulse. Thus the mechanism of the resonant IR laser ablation process is apparently connected primarily with the bond-selective absorption leading to relatively shallow absorption depths and a high local density of vibrational excitation. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H23.00003: Resonant Infrared Matrix Assisted Pulsed Laser Deposition of Polymers: Improving the Morphology of As-Deposited Films Daniel Bubb, Michael Papantonakis, Brian Collins, Elijah Brookes, Joshua Wood, Ullas Gurudas Resonant infrared matrix assisted pulsed laser deposition has been used to deposit thin films of PMMA, a widely used industrial polymer. This technique is similar to conventional pulsed laser deposition, except that the polymer to be deposited is dissolved in a solvent and the solution is frozen before ablation in a vacuum chamber. The laser wavelength is absorbed by a vibrational band in the frozen matrix. The polymer lands on the substrate to form a film, while the solvent is pumped away. Our preliminary results show that the surface roughness of the as-deposited films depends strongly on the differential solubility radius, as defined by Hansen solubility parameters of the solvent and the solubility radius of the polymer. Our results will be compared with computational and experimental studies of the same polymer using a KrF (248 nm) laser. The ejection mechanism will be discussed as well as the implications of these results for the deposition of smooth high quality films. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H23.00004: Laser Ablation Electrospray Ionization: A Molecular Probe for Biological Tissues Peter Nemes, Alexis A. Barton, Yue Li, Akos Vertes Interaction of light and matter has long served as the basis of probing and modifying physical and chemical properties of materials. Recent biomedical applications focus on the mid-infrared (mid-IR) region to couple the laser energy into samples through absorption by the native water. For example, mass spectrometry (MS), relying on atmospheric pressure mid-IR matrix-assisted laser desorption ionization, takes advantage of the small amount of ions in the laser plume. In mid-IR laser ablation, owing to the recoil pressure buildup in the sample, most of the material is expelled in the form of neutral molecules, clusters, and particulates. To enhance ion production, we intercept this plume with a cloud of charged droplets to post-ionize them for MS. As a result, laser ablation electrospray ionization (LAESI) can directly probe the molecular makeup of water rich targets with superior ion yield and dramatically extended mass range (up to 66,500 amu). LAESI also enables two and three dimensional imaging of live tissues. Fast imaging of the plume-plume interaction reveals the mechanistic aspects of LAESI. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H23.00005: Deposition of functional nanoparticle thin films by resonant infrared laser ablation. Richard Haglund, Stephen Johnson, Hee K. Park, Kannatessen Appavoo We have deposited thin films containing functional nanoparticles, using tunable infrared light from a picosecond free-electron laser (FEL). Thin films of the green light-emitting molecule Alq$_{3}$ were first deposited by resonant infrared laser ablation at 6.68~$\mu $m, targeting the C=C ring mode of the Alq$_{3}$. TiO$_{2}$ nanoparticles 50-100~nm diameter were then suspended in a water matrix, frozen, and transferred by resonant infrared laser ablation at 2.94~$\mu $m through a shadow mask onto the Alq$_{3}$ film. Photoluminescence was substantially enhanced in the regions of the film covered by the TiO$_{2}$ nanoparticles. In a second experiment, gold nanoparticles with diameters in the range of 50-100~nm were suspended in the conducting polymer and anti-static coating material PEDOT:PSS, which was diluted by mixing with N-methyl pyrrolidinone (NMP). The gold nanoparticle concentration was 8-10{\%} by weight. The mixture was frozen and then ablated by tuning the FEL to 3.47~$\mu $m, the C-H stretch mode of NMP. Optical spectroscopy of the thin film deposited by resonant infrared laser ablation exhibited the surface-plasmon resonance characteristic of the Au nanoparticles. These experiments illustrate the versatility of matrix-assisted resonant infrared laser ablation as a technique for depositing thin films containing functionalized nanoparticles. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H23.00006: Photochemical ablation of Polytetrafluoroethylene (PTFE) under 157-nm irradiation Sharon R. John, Steven C. Langford, J. Thomas Dickinson We report time- and mass-resolved measurements on neutral molecular particles emitted from polytetrafluoroethylene during exposure to 157-nm laser radiation at fluences where relatively rapid etching is observed. By comparing the time-of-flight signals over a range of masses, we conclude that (CF$_{2})_{N}$ fragments for N=1-6 are emitted directly from the surface in substantial quantities. In contrast, the monomer (N=2) is the principal product during irradiation at 248 nm, due to a thermal decomposition mechanism. The time-of-flight signals of all the (CF$_{2})_{N}$ fragments show fast components with kinetic energies of $\sim $0.6 eV, indicating a non-thermal mechanism. These high kinetic energies are consistent with photochemical scission of the polymer backbone, where a part of the excitation energy is delivered to the fragment as kinetic energy. Although clean etching is observed under these conditions, the great majority of the mass removed appears as much larger fragments with a size distribution of 10 nm to 1$\mu $m. The time-of-flight signals also show a slow component. We present a collisional model to explain the slowing down of neutrals molecules created by photochemical scission. Intense electron, positive and negative ion emissions are also observed. Their formation and emission mechanism will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H23.00007: Rational Design of Two-Photon Absorbing Photochromic Materials for Optical Switching and Data Storage I.A. Mikhailov, K.D. Belfield, A.E. Masunov Diarylethenes are able to undergo light-induced transition from the open to closed ring isomer (photocyclization) accompanied by the change in optical properties (photochromism). This ability holds a great promise for photonic applications, including optical data storage and ultrafast optical switching. Photocyclization initiated by absorption of two photons could drastically increase the density of these devices. However, attachment of fluorene substituent to diarylethene to increase two-photon absorbing cross-section led to the loss of photochromic activity. Analysis of the Kohn-Sham orbitals reveals that the relaxation of the lowest excited state of diarylethene fragment leads to photocyclization, while the occupied level of the chromophore substituent generates an excited state below the photoreactive one. To design the molecular switch active in two-photon regime we suggest stabilizing the highest occupied orbital, which can be accomplished by fluorination of the chromophore. We applied time dependent Density Functional Theory to predict potential energy surfaces of excited states and two-photon absorbing profiles. The obtained results are in agreement with the qualitative orbital description. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H23.00008: Toward molecular switches and biochemical detectors employing adaptive femtosecond-scale laser pulses Roland Allen, Petra Sauer The following topics will be discussed: (1) Photoisomerization of azobenzene, with nuclear motion allowing extra electronic transitions for pulse durations > about 50 fs. (2) Photoinduced ring-opening in a model dithienylethene. (3) Response of dipicolinic acid to femtosecond-scale laser pulses, including excited states and nuclear motion. Although real applications (such as molecular switches and biochemical detectors) will involve adaptive techniques -- with femtosecond-scale laser pulses whose durations, photon energies, fluences, shapes, etc. are tailored for specific applications -- as well as larger systems, one needs an understanding of the rich interplay of electronic and nuclear dynamics to guide more empirical approaches. This understanding can be obtained through detailed computational studies of the kind reported here. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H23.00009: Ultrafast carrier-phonon dynamics under intense optical excitation of GaAs Amlan Basak, M. Hase, M. Kitajima, Hrvoje Petek We report the response of n-doped GaAs (n$_{d}$=2x10$^{18}$ cm$^{-3})$ when excited to an e-h pair density n$_{exc}\sim $10$^{19}$-10$^{20}$ cm$^{-3}$ with a 10 fs laser pulse centered at 400 nm. The experiment is performed in reflective electro optic sampling geometry. Coherent LO phonon oscillation is excited through both ultrafast screening of the depletion field and the deformation potential scattering. The time domain signal contains near-instantaneous transient electronic response as well as coupled plasmon-phonon oscillation. The amplitude of reflectivity is sub-linear possibly indicating saturation of the screening effect with excited carrier density. Fourier Transform analysis shows the bare LO phonon and the lower branch of the LO phonon-plasmon coupled modes (L-). With increasing photocarrier density, the LO phonon response is essentially unaffected, while the L- peak red shifts to the TO phonon limit. Time windowed FT analysis reveals complex carrier density dependent spectral evolution. The coupled carrier-phonon dynamics are discussed in the context of deformation potential scattering and high field transport. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H23.00010: Effect of Hot Electron Pressure in Ultrafast Laser Interaction with Metals Zhibin Lin, Leonid Zhigilei Ultrafast laser irradiation can transiently bring a metal into a highly nonequilibrium state in which the electron temperature can reach thousands of Kelvin while the lattice remains cold. Under these conditions the thermal pressure from the hot conduction electrons can play an important role in defining the initial relaxation dynamics of the irradiated target. In this work, a description of the hot electron pressure due to the presence of the excited electrons is incorporated into a continuum-atomic computational model combining the molecular dynamic method with the two temperature model. Computer simulations employing this approach are performed for Al, Au, and Ni metal films and bulk targets. The effect of the hot electron pressure on the generations of acoustic phonons in the laser-irradiated metal film will be discussed and compared with existing experimental data. The relative contributions of the hot electron pressure and thermoelastic stresses due to the lattice heating to the dynamics of the irradiated target and spallation/ablation process are discussed based on the results of the computer simulations. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H23.00011: Coherent LO phonon self-energy renormalization under high photoexcited carrier densities in Si Anca-Monia Constantinescu, Muneaki Hase, Masahiro Kitajima, Hrvoje Petek The study of hot carrier-phonon interaction dynamics is motivated by their influence on optical and electrical properties of semiconductors. Following high-density (10$^{19}$--10$^{20}$ carriers/cm$^{3})$ photoexcitation of Si(001) with 10 fs duration 400 nm laser pulses, the complex self-energy (i.e. frequency and decay rate) of coherent LO phonon (\textbf{\textit{k}}$\approx $0) renormalize due to deformation potential interaction with the photogenerated non-equilibrium plasma. We evaluate the time dependent LO phonon frequency and dephasing time by analyzing the transient electro-optic reflectivity of variously doped Si(100). We measure the coherent LO phonon mode oscillations in the transient reflectivity over a delay time of 6 ps between pump and probe pulses. Varying the pump power from 50 to 5 mW, we observe that the electronic softening of the lattice (i.e. LO phonon frequency change) and the quasi-exponential dephasing time of the phonon depend on the initial photoexcited carrier density. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H23.00012: Mechanistic study of negative ion emission from single crystal alkali halide surfaces due to pulsed UV laser irradiation J. T. Dickinson, Kenichi Kimura, S. C. Langford We report on extensive measurements of negative alkali ion emission from four alkali halides during exposure to 248-nm pulsed excimer laser radiation at fluences well below the threshold for optical breakdown. A detailed study on the emissions from single crystal KCl shows no evidence for negative halide ions, suggesting that negative alkali ions are not formed by electron attachment to thermally emitted neutral particles. Furthermore, the KCl surface charges positively during laser irradiation (due to electron emission from defects), which would hinder direct emission of negative ions from the surface. We present strong evidence for a negative ion formation mechanism involving double electron attachment to singly charged positive alkali ions. Extension of these measurements to single crystal KBr and to other dielectric materials confirm this mechanism. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H23.00013: Observation of Saturable and Reverse Saturable Absoption in Silver Nanodots Ullas Gurudas, Daniel Bubb, Thomas Lippert, Sebastian Heroith Saturable absorption (SA) and reverse saturable absorption (RSA) were observed in Ag nanodots prepared by pulsed laser deposition. The Real [Re $\chi ^{(3)}$] and Imaginary [Im $\chi ^{(3)}$] part of the third order nonlinearity of these films are measured as and respectively, using Z-scan technique. The decrease of absorption under strong optical illumination results in a negative Im $\chi ^{(3)}$ at the photon energy used. At higher input irradiance RSA becomes dominant. The transformation from SA to RSA suggests that another nonlinear process takes place and become dominant. To evaluate the recovery time of these nonlinear processes and get an idea about the underlying mechanism, we conducted a degenerate pump-probe experiment with 25 psec, 532 nm laser pulses. The increased $\chi ^{(3)}$ and fast response time of the Ag nanoparticles can be used for optical pulse compressor, optical switching, laser pulse narrowing and protecting optical sensors from intense laser pulses. [Preview Abstract] |
Session H24: Focus Session: Optical Properties of Nanostructures III: Functional Nanowires
Sponsoring Units: DMPChair: Ramamurthy Ramprasad, University of Connecticut
Room: Morial Convention Center 216
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H24.00001: Polarized Photouminescence from Single Wurtzite and Zincblende InP Nanowires A. Mishra, L.V. Titova, T.B. Hoang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, Y. Kim, H.J. Joyce, Q. Gao, H.H. Tan, C. Jagadish We use polarized photoluminescence spectroscopy of single InP nanowires to compare the optical properties of vapor-liquid-solid growth of single zincblende (ZB) and wurtzite (W) nanowires. Since ZB and W nanowires have different symmetries and selection rules, their optical properties should also be different. The emission from single W nanowires is observed to be $\sim $80 meV higher than for ZB nanowires. Low temperature polarization measurement shows that ZB nanowires are strongly polarized along the nanowire axis, while the W nanowires are polarized perpendicular to the NW axis. The temperature dependence of the ZB and W NW emissions are compared with a bulk InP epilayer. Apart from the 80meV shift in bandgap, the temperature dependencies are similar. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H24.00002: Investigation of the Electronic Structure of GaAs/AlGaAs Core Multi-Shell Nanowires S. Perera, M.A. Fickenscher, T.B. Hoang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Q. Gao, Y. Kim, H.H. Tan, C. Jagadish We use photoluminescence and PL excitation spectroscopy to study the electronic structure of GaAs/Al$_{x}$Ga$_{x}$As core multi-shell NWs. Using Au-catalyst assisted MOCVD, a 10 nm GaAs quantum well tube (QWT) with AlGaAs barriers is formed surrounding a central $\sim $50 nm GaAs core. With resonant excitation at 780 nm emission is seen from both the core as well as the QWT. The QWT emits in a narrow intense peak $\sim $22 meV above the exciton emission from the core suggesting quantum confinement in a 10 nm quantum well. This QWT emission exhibits a $\sim $600 ps recombination lifetime, while the core decays in $\sim $1 ns. Preliminary PLE measurements exhibit possible excited state structure of this novel quantum-confined nanostructure. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H24.00003: Photoluminescence Dynamics of GaAs/AlGaAs Core-Shell Nanowires H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish, M.A. Fickenscher, S. Perera, T.B. Hoang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, X. Zhang, J. Zou We use time-resolved PL spectroscopy to study the exciton dynamics of GaAs/AlGaAs core-shell nanowires (NWs) at 20 K. NWs were prepared by Au catalyst-assisted MOCVD. PL emission from single NWs exhibits an excitonic peak at $\sim $1.515~eV. The exciton lifetime depends on the morphology and crystallographic defect density of the GaAs core, which are in turn dependent upon the growth conditions. Nanowires cores grown at higher temperatures (450~~C) give short exciton lifetimes ($<$100~ps). Reducing defects within the nanowire (twinning) or at the interface should increase exciton lifetime and improve luminescence efficiency. Indeed, twin-free minimally tapered nanowires achieved using a low growth temperature (375~~C), exhibit high quantum efficiency with an exciton lifetime approaching 1.6~ns at 20 K. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H24.00004: Ultrafast dynamics in semiconductor nanowires Rohit Prasankumar, Sukgeun Choi, George Wang, Samuel Picraux, Antoinette Taylor Semiconductor nanowires (NW) have recently attracted much interest due to their novel electronic and optical properties along with their potential for device applications in areas including nanoscale lasers and thermoelectrics. However, the further development and optimization of NW-based devices will depend critically on an understanding of carrier relaxation in these unique nanostructures. Here, we present the first all-optical time-resolved measurements of carrier dynamics in free standing semiconductor nanowires. Optical pump-probe measurements on GaN NW reveal a rapid transfer ($<$500 fs) of photoexcited carriers into states responsible for deleterious yellow luminescence, which can be modified by varying the growth and annealing temperatures. Polarization, angle, and wavelength-resolved measurements on vertically aligned Ge NW allow us to independently measure electron and hole dynamics parallel and perpendicular to the NW axis. Carriers propagating parallel to the NW axis have significantly longer lifetimes, clearly demonstrating the influence of two dimensional confinement on carrier dynamics in semiconductor nanowires. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H24.00005: Optical Switching of Porphyrin-Coated Silicon Nanowire Field Effect Transistors Vincent Bouchiat, Clemens Winkelmann, Irina Ionica, Xavier Chevalier, Christophe Bucher, Guy Royal We study [1] porphyrin coated silicon nanowire field effect transistors, which display a large, stable and reproducible conductance increase upon illumination. The efficiency and the kinetics of the optical switching are studied as a function of gate voltage, illumination wavelength and temperature. The decay kinetics from the high- to the low-conductance state is governed by charge recombination via tunneling, with a rate depending on the state of the SiNW-FET. The comparison to porphyrin sensitized Carbon Nanotube FETs allows to distinguish the environment- and molecule-dependent photoconversion process from the charge-to-current transducing effect of the semiconducting channel. The spectral dependence of the photoconductance agree with the UV-visible absorption spectrum of the isolated molecule [1] C. Winkelmann et al, Nano Lett, vol. 7 , p. 1454 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H24.00006: Spectral dependence of thermal radiation from metallic nanowires on wire geometry Yat-Yin Au, Helgi Skuli Skulason, Snorri Ingvarsson, Levente J. Klein, Hendrik F. Hamann We report polarization-sensitive thermal radiation measurements of individual platinum nanoheaters (nanowires) as a function of their length, width and temperature. The heaters confine lateral extent of the resistively heated area to dimensions smaller or comparable to the emission wavelengths within our sensitivity range (2-5 $\mu$m). Spectra taken by Fourier Transform Infrared Spectrometry reveal strong suppression of radiation polarized perpendicular to the heater long axis as the heater width shrinks, while at the same time radiation polarized along the long axis of the heater approaches a constant value, resulting in highly polarized emission for heaters with very narrow width. We also observe a $\lambda/2$-like resonance that we believe is associated with surface plasmon oscillations across the heater width. These findings have important implications for nanoscale thermal light generation. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H24.00007: Anisotropic plasmon excitation and dispersion of Ag nanowires on Cu(110) I. Senevirathne, Asoka Sekharan, Richard Kurtz, Phillip Sprunger Epitaxial Ag nanowires have been found to self-assemble on Cu(110) exceeding 1.2 ML. The plasmon excitation and dispersion of these nanowires have been characterized by low-energy reflection EELS. Previous STM images reveal that the Ag nanowires are approximately 2 nm ($\sim $12 nm) in height (width). However, the nanowires orientate with the long axis parallel to the [\={ }110] substrate direction and posses an anisotropic morphology. EELS reveals that the Ag plasmon excitation of 3.7 eV at the zone-center and is nearly dispersionless perpendicular to the nanowire direction. However, parallel to the Ag nanowires, EELS shows a slight red-shift of the plasmon at q = 0 and disperses to higher energy with increasing momentum transfer. These results will be discussed in light of recent ARPES band-structure measurements, electronic calculations, and anisotropic optical measurements of the Ag nanowires. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H24.00008: Enhanced Raman Scattering Near the Tip of Semiconducting Nanowires Qiujie Lu, Jian Wu, Awnish Gupta, Peter Eklund Results of polarized microRaman scattering experiments are presented on individual $\sim $20$\mu $m long crystalline GaP zinc-blende nanowires (NWs) as a function of the probe beam position along the wire. The probe beam had a spot size of $\sim $0.7$\mu $m. The NWs were characterized by TEM lattice images and selected area diffraction (SAD) patterns. We found enhanced LO and TO phonon scattering near the tip of the nanowire, i.e., the scattering is at least a factor of 5x stronger at the tip than observed at distances many microns away from the tip. The polarized scattering patterns I($\theta )$, where $\theta $ is the angle between the incident electric field and the NW axis, also change as the probe beam approached the tip of the nanowires. The effects observed here should be general and apply to other semiconducting nanowire systems as well. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H24.00009: Polarized Rayleigh Back Scattering from Individual GaP Nanowires Jian Wu, G. Chen, Qiujie Lu, P.C. Eklund Results of polarized Rayleigh back-scattering studies are reported for individual $\sim $20 $\mu $m long crystalline GaP Nanowires (NWs) using 514.5 nm excitation. The NWs were supported on Transmission Electron Microscope (TEM) grids. The diameters of the NWs were determined by TEM. Positions of characteristic LO, TO phonon Raman bands were found to agree with bulk GaP. The Rayleigh back-scattering intensity polar pattern I($\theta )$ was measured at room temperature, where $\theta $ is the angle between the incident electric field and the NW axis. The scattered radiation was polarized parallel to the incident electric field. For small NW diameter (d$\sim $70) nm, we observed $\sim $ cos$^{4}\theta $ polar patterns. With increasing NW diameter above 100 nm, the polar scattering patterns rotate by 90\r{ } with respect to those seen in small diameter NWs and then they broaden to a circle. Our experimental data will be compared to the calculated Rayleigh back-scattering efficiency calculated via the Discrete Dipole Approximation (DDA). Our DDA calculations show that the polar patterns are sensitive to both the diameter and the NW length. Although the calculated polar patterns qualitatively support our data, improvement in the modeling is still needed. This work is supported by NSF NIRT, grant DMR-0304178. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H24.00010: Non-Linear Raman Scattering from Semiconducting GaP Nanowires A. Gupta, Jian Wu, P.C. Eklund Results of polarized micro-Raman scattering from LO and TO phonons in individual GaP nanowires (NWs) with different diameter and length are reported. The NW diameters were determined by Atomic Force Microscope (AFM) and length was measured by Scanning Electron Microscope (SEM). NWs with the same growth direction but variable length were prepared by cutting $\sim $40 $\mu $m long wires into segments using a Focused Ion Beam. The polar plots of the back scattered intensity I$_{TO,LO}(\theta )$ from these segments were collected, where $\theta $ is the angle between the incident electric field and the NW axis. Interestingly, the shapes of these polar patterns depend on both the length and diameter of the NWs. The Raman scattering intensities for short wires (i.e., L$<$1$\mu $m) also exhibit a non-linear dependence on the incident laser power I$_{0}$. The non-linearity increases with decreasing NW length and behaves as $\sim $I$_{0}^{1.5}$ for the shortest wires measured so far (i.e., L$\sim $500 nm). Our results strong suggest strong enhancement in the internal electric field via antenna effects. This work is supported by NSF NIRT, grant DMR-0304178. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H24.00011: Optical Antenna Effect in Semiconducting Nanowires P.C. Eklund, Jian Wu, G. Chen, Qiujie Lu, H.R. Gutierrez, Qihua Xiong, M.E. Pellen, J.S. Petko, D. Werner Using Raman scattering, we have observed strong optical antenna effects which we identify with internal standing wave photon modes of the wire. The antenna effects were probed in individual GaP NWs whose diameters are in the range 40$<$d$<$300 nm. The data and our calculations show that the nature of the backscattered light is critically dependent on the interplay between a photon confinement effect and bulk Raman scattering. At small diameter, d$<$65 nm, the NWs are found to act like a nearly perfect dipole antenna and the bulk Raman selection rules are masked leading to a polarized scattering intensity function I$_{R }\sim $ cos$^{4}\theta $. For larger wires, many other different polar patterns are found. Underscoring the importance of this work is the realization that a fundamental understanding of the ``optical antenna effect'' in semiconducting NWs is essential to the analysis of all electro-optic effects in small diameter filaments. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H24.00012: Spatially-Resolved Photoluminescence Mapping of Single CdS Nanosheets M. Fickensher, T.B. Hoang, L.V. Titova, A. Mishra, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, H. Rho, K.-Y. Lee, Y.-J. Choi, K.J. Choi, J.-G. Park We present results of spatially-resolved low temperature photoluminescence of single 5 micron wide CdS nanosheets. The sheets, grown by pulsed laser deposition using vapor-phase transport, are uniform in size and shape and exhibit a hexagonal wurtzite structure. The orientation of the c-axis determined by PL polarization analysis and HR TEM varies from sheet to sheet. The spatially-resolved PL reveals spectral variation across the sheet, with A-like excitons at the edges showing a spectral peak at 2.547eV, and B like excitons at the center showing a peak at 2.563eV. Exciton lifetimes of $\sim $200 ps are observed, which are significantly longer than CdS nanowires of identical diameter, but shorter than measured in bulk CdS. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H24.00013: Anomalous Photoluminescence in CdSe Quantum Dot Solids at High Pressure due to Non-uniform Stress. Sebastien Hamel, Christian Grant, Jonathan Crowhurst, Andrew Williamson, Nathalia Zaitseva The application of static high pressure provides a means to precisely control and investigate many fundamental and unique nanoparticle properties. CdSe is a model quantum dot (QD) system whose behavior under high pressure has been extensively studied; however, the effect of non-uniform stresses on this system has not been fully appreciated. In order to model these experiments and account for the behavior of the photoluminescence we carried out electronic structure simulations of wurtzite CdSe QDs with a core diameter of 2 to 5 nm using the Semi-Empirical Pseudopotential Method (SEPM). These calculations guide the interpretation of the photoluminescence data obtained from CdSe QD solids in different stress environments varying from purely uniform to highly non-uniform. Small deviations from a uniform stress distribution are found to profoundly affect the electronic properties of this system. In non-uniform stress environments, we observe a pronounced photoluminescence energy flattening above 3 GPa. The importance of this effect must be considered when investigating other potentially pressure mediated phenomena. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H24.00014: Dynamics of Photo-Excited Carriers in Single InP Nanowires Under High Excitation Density L.V. Titova, T.B. Hoang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, Y. Kim, H.J. Joyce, Q. Gao, H.H. Tan, C. Jagadish The dynamics of photo-excited carriers in single InP nanowires at low temperature is investigated using time-resolved photoluminescence spectroscopy. Under highly intensity excitation, the photoluminescence spectrum from a single nanowire shows a broad emission band at early times after the excitation pulse indicating the presence of a degenerate, high density electron-hole plasma. At later times ($>$ 600 ps) when the density of carriers decreases, the emission spectrum becomes narrower and converges toward the free exciton emission band. The lifetime of free excitons in a single nanowire is measured to be close to the lifetime of excitons in high quality InP epilayers, indicating the relative insensitive of the carriers to the InP nanowire surface. These results indicate that significant state filling and band gap renormalization occur in single InP nanowires. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H24.00015: Near Infrared Photoresponse in Annealed CdSe Nanocrystal Films Liwei Liu, Paul Stokes, Artem E. Masunov, Saiful I. Khondaker We found unexpected near infrared (NIR) photo response in CdSe nanocrystal superlattice film annealed above 400 C in air. The current voltage characteristic measured in a planer device geometry show a large increase in NIR current over dark current. The calculated external quantum efficiency of the device is up to 10.6 {\%} at -5V and the responsivity is 0.7A/W obtained under 1.32 $\mu $W IR irradiation. UV-VIS absorption of annealed CdSe shows the redshifting and broadening of exciton peak and a decrease of band gap as the annealing temperature is increased. TEM image show that CdSe nanocrystals have been melted to fuse to different size distribution nanoparticles during annealing. We discuss possible reason for this unexpected behavior. [Preview Abstract] |
Session H25: Adsorption of Organics on Surfaces
Sponsoring Units: DPOLYChair: Kookheon Char, Seoul National University
Room: Morial Convention Center 217
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H25.00001: Polymer Physics Prize Break
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Tuesday, March 11, 2008 8:36AM - 8:48AM |
H25.00002: Examining the air-water interfacial activity of beta-peptides using molecular simulation and experiment. Clark A. Miller, Juan J. de Pablo Amphiphilic beta-peptides (oligomers of beta-amino acids) are predicted to adsorb at the air-water interface using computer simulation and verified using experiments. Molecular dynamics simulations are used to calculate the free energy of adsorption for different degrees of amphiphilicity and different display of hydrophilic groups. Adsorption of selected peptides is examined by measuring the surface tension of a solution of beta-peptides at different concentrations and agreement with simulation results is found. Simulations show that 2/3 hydrophobic residues lead to favorable adsorption at the air-water interface while 1/3 hydrophobic residues is unfavorable. We analyze changes in the conformational properties and angle with the interface to understand the manner of adsorption. We further investigate the behavior of multiple peptides at the interface using computer simulation and determine the surface pressure and peptide-peptide interactions at the interface. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H25.00003: Why are hyperactive ice-binding-proteins so active? Ido Braslavsky, Yeliz Celik, Natalya Pertaya, Young Eun Choi, Maya Bar, Peter L. Davies Ice binding proteins (IBPs), also called `antifreeze proteins' or `ice structuring proteins', are a class of proteins that protect organisms from freezing injury. These proteins have many applications in medicine and agriculture, and as a platform for future biotechnology applications. One of the interesting questions in this field focuses on the hyperactivity of some IBPs. Ice binding proteins can be classified in two groups: moderate ones that can depress the freezing point up to $\sim $1.0 $^{\circ}$C and hyperactive ones that can depress the freezing point several-fold further even at lower concentrations. It has been suggested that the hyperactivity of IBPs stem from the fact that they block growth out of specific ice surfaces, more specifically the basal planes of ice. Here we show experimental results based on fluorescence microscopy, highlighting the differences between moderate IBPs and hyperactive IBPs. These include direct evidence for basal plane affinity of hyperactive IBPs, the effects of IBPs on growth-melt behavior of ice and the dynamics of their interaction with ice. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H25.00004: Probing (bio)-organic monolayers at the metal/air and metal/liquid interface by sum-frequency generation spectroscopy Francesca Cecchet, Dan Lis, Yves Caudano, Christophe Silien, Alaa Adin Mani, Paul Thiry, Andr\'e Peremans In the present work, ordered monolayers of thiols prepared by self-assembly (SAM), and of lipids obtained by the Langmuir-Blodgett technique (LB), have been studied by sum frequency generation spectroscopy (SFG) at the metal/air and metal/liquid interface, in different sets of polarizations. This study is focused on the determination of the molecular orientation (i.e. the tilt angle and the twist angle of the axis and of the plane of the molecular groups, respectively) and on the analysis of the interactions occurring within the layers or with outer target molecules. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H25.00005: Flow Induced Growth of Striped Alkane Monolayers. M. Bai, H. Taub, A. Diama, K. Knorr, U. G. Volkmann, F. Y. Hansen We report our observation of the growth of striped monolayer phases of alkanes when deposited from a solution under flow. AFM measurements show that the structure and morphology of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films grown from solution depend sensitively on the flow direction over a SiO$_{2}$-coated Si(100) substrate. The C32 film exhibits one or two layers adjacent to the SiO$_{2}$ surface in which the molecules are oriented with their long axis parallel to the interface followed by a striped monolayer of perpendicularly oriented molecules. The stripes form along the direction of solution flow with typical dimensions of a few micrometers wide and a few hundred micrometers long, depending on the solution concentration. A striped morphology is also observed for C29 grown under similar conditions. Grazing incident-angle x-ray diffraction measurements indicate that the C32 stripes are crystalline and can be indexed by a rectangular unit cell. We offer some speculations on the origin of the striped morphology. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H25.00006: Molecular dynamics studies of the structure and dynamics of ``perpendicular'' layers of \textit{n}-alkane molecules adsorbed on a solid substrate F.Y. Hansen, P. Soza, H. Taub, U.G. Volkmann Both AFM and ellipsometry studies of $n$-alkane films adsorbed on a solid substrate from a solution have shown interesting wetting and de-layering phenomena$^{2}$. It was found that on top of one or two ``parallel'' layers of molecules, where the long axis of the molecules is parallel to the surface, ``perpendicular'' layers of molecules are formed with the long axis of the molecules perpendicular to the surface. MD simulations of layers of tetracosane, $n$-C$_{24}$H$_{50,}$ molecules are set up to answer the following questions about the ``perpendicular'' layers: a) Is the melting transition driven by \textit{gauche} defect creations in the alkane chains like in the parallel layers?, b) can a rotator phase be identified?, c) is there a lateral translational mobility of the chains prior to melting?, and d) what is the mechanism driving the wetting and de-layering transitions in the films? $^{2}$H. Mo et al. Chem. Phys. Lett. \textbf{377}, 99 (2003) [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H25.00007: Structure and Phase Transitions of Vapor-Deposited C32 Films V. del Campo, E.A. Cisternas, I. Vergara, T. Corrales, U.G. Volkmann, M. Bai, S.-K. Wang, H. Taub, H. Mo, S.N. Ehrlich We have compared the structure, topography, and phase transitions of dotriacontane films ($n$-C$_{32}$H$_{66}$ or C32) that have been vapor-deposited onto a SiO$_{2}$-coated Si(100) wafer with those that have been deposited from solution. X-ray reflectivity measurements indicate that the as-deposited films differ in their morphology but share the following structural features at room temperature: adjacent to the substrate there is a nearly complete bilayer in which the molecules are oriented with their long axis parallel to the surface. Above the parallel film are partial layers of molecules oriented perpendicular to the surface. After a heating cycle above the bulk C32 melting point ($T_{b})$, AFM images of all films show the presence of 3D mesa-shaped bulk particles. On a second heating, AFM reveals the same succession of phase transitions for both film types in which a perpendicular monolayer spreads outward from the mesa-shaped particles below $T_{b}$ followed by a delayering transition to a 3D fluid droplets just above $T_{b}$.$^{2}$ $^{2}$M. Bai \textit{et al}., Europhys. Lett. \textbf{79}, 26003 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H25.00008: Thermodynamic Studies of n-Octane Thin Films Adsorbed on Magnesium Oxide(100) David Fernandez-Canoto, J.Z. Larese Thermodynamic properties of $n-$octane adsorbed on the MgO(100) surface were investigated using high-resolution adsorption isotherms in the temperature range of 225 K to 295 K. Two distinct adsorption steps were observed in all isotherms. The average area occupied by an $n$-octane molecule was estimated to be 139.1 {\AA}$^{2}$. The temperature variation of the two dimensional compressibility was used to identify phase transitions near 265.9 K and 271.4 K for the first and second layers, respectively. COMPASS force field has been used to calculate the minimum energy configuration of a single $n$-octane molecule sited on the MgO (100) facet. Calculations suggest that the most likely configuration for the adsorbed molecule is with the carbon backbone parallel to the (100) plane, and with the center of mass atop the Mg$^{2+}$ site. U.S. DOE, Materials Science Division under contract No. DE-AC05-00OR22725 with ORNL operated by UT-Battelle, LLC, and the NSF under grant DMR-0412231. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H25.00009: X-ray Atomic-Scale Analysis of Self-Assembled Monolayer Growth on Silicon J.C. Lin, J. Kellar, J. Kim, N. Yoder, K. Bevan, S. Datta, S. Nguyen, M. Hersam, M. Bedzyk Organic functionalization of silicon is of interest for applications ranging from biosensing to molecular electronics. The efficiency of molecular devices heavily depends on the ordering of the structure. Traditionally spectroscopy is used to characterize bonding, but often the overall structure can be ambiguous. Our strategy is to combine a compliment of techniques, including AFM, XPS, XRR(X-ray reflectivity), XSW(X-ray standing wave), XRF(X-ray fluorescence), and DFT(Density functional theory) to determine the atomic scale molecular configuration and packing density of Self-Assembled Monolayers (SAMs) grown on H-passivated Silicon. Our periodic DFT study of 4-bromo-phenyl-acetylene (BPA) predicts that the local packing density can affect the Br height by as much as 2 angstrom. XSW, which is used to measure the 3D Br distribution shows that the local structure is unchanged when the average SAM coverage is increased. This indicates the type of 2D island nucleation growth process being observed. Comparison between 4-bromostyrene (BrSty) and BPA SAMs provides direct evidence that the double bond root of the BPA contributes to a stiffer configuration than the single bond root. With the aromatic rings in the structure for conducting electrons, BrSty and BPA molecules are a starting point for future molecular electronic designs with more complex molecules. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H25.00010: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H25.00011: STM/S of Polydiacetylene Nanowires on Gold and Graphite Lili Wang, Rajiv Giridharagopal, Kevin Kelly The structural and electronic properties of 10,12--pentacosadiynoic acid (PCDA) monolayer films and polydiacetylene (PDA) nanowires on Au islands on a HOPG substrate have been studied using STM. Our results indicate that PCDA monolayer films can be formed on both HOPG and 1-2 monolayer (ML) Au islands, but arrange in small domains due to the dense Au clusters. The arrangement of PDA nanowires exhibits differences in density, length and height between Au and HOPG areas due to differing chemical and electronic interactions, which play an important role in the charge transfer between conducting polymers and electrodes in commercial devices. STM-tip induced nanowire cutting, desorption and polymerization is also observed, with the surrounding PCDA molecules restoring the packing nearly instantaneously. This implies that the interaction with Au clusters is not strong enough to weaken the intramolecular interactions that produce the reordering cascade effect, although it strongly influences the arrangement of the nanowires. Furthermore, the local work function and dI/dV images indicate electronic structure differences between PCDA monolayer films on 1 ML Au islands and those on 2 ML islands, and between PDA nanowires on HOPG and those across Au islands. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H25.00012: Thermodynamic and Neutron Scattering Investigation of Ethylene Wetting on MgO (100) Andi Barbour, Craig Brown, J. Z. Larese The adsorption properties of a molecular film on a solid substrate are governed by the relative strength of the molecule-substrate versus molecule-molecule interaction. The wetting properties of ethylene (C$_2$H$_4$) molecular thin films on graphite are of fundamental interest because the number of observed adlayers increases as the isothermal temperature increases with T$\leq$104K (bulk triple point). In adsorbate/substrate systems like C$_2$H$_4$/graphite, it is accepted that triple point wetting occurs. For our studies, we employed MgO nanocubes because they represent a prototypical metal oxide with a wide variety of technological uses including catalyst support. Of particular interest are wetting/layering transitions and the changes that take place in the neighborhood of the bulk triple point. We report our experimental investigation of the adsorption behavior of evidence C$_2$H$_4$ on MgO (100) using high-precision adsorption isotherms and neutron diffraction and scattering. We demonstrate the dominate role that molecule-molecule interaction plays in the wetting phenomena by comparing the behavior of ethylene on graphite and MgO. U.S. Department of Energy (DE-AC05-00OR22725) at ORNL managed and operated by UT-Battelle, LLC, and the NSF (DMR-0412231). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H25.00013: Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions Priya Sony, Peter Puschnig, Dmitrii Nabok, Claudia Ambrosch-Draxl We present \textit{ab-initio} calculations to study the interface energetics of a weakly adsorbed organic molecule on metallic surfaces, which serve as model interfaces relevant for organic electronics. Thereby we focus on the role of the exchange-correlation potential and, in particular, the van der Waals interaction. To this extent the thiophene molecule is relaxed on clean Cu(110) and Cu(110)-(2x1)O, and the adsorption energy corresponding to various positions and orientations of the molecule is calculated on the search for the most favorable adsorption site. The molecule is found to be more strongly bound on the clean Cu(110) surface with an adsorption energy of $-0.50$~eV, as compared to $-0.30$~eV on Cu(110)-(2x1)O. Nonlocal correlations, i.e., the van der Waals interaction is found to be solely reponsible for the binding in such weakly bound systems, while the commonly used generalized gradient approximations not only underestimate the adsorption energyy but also provide the wrong physical picture for the binding. The adsorption of thiophene lowers the work function of the metallic substrate due to the formation of surface dipoles while no sizeable charge transfer is found. [Preview Abstract] |
Session H26: Focus Session: Photophysics of Cold Molecules IV
Sponsoring Units: DCPChair: Jochen Kuepper, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Room: Morial Convention Center 218
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H26.00001: Reactions of cold trapped anions Invited Speaker: Interactions of negative ions with small organic molecules represent model systems for the investigation of reaction dynamics in few-body systems. Their corrugated potential energy landscape, originating in long-range attractive and short-range repulsive forces, requires the coupling of different degrees of freedom for reactions to occur. We have adopted two complementary approaches to study anion-molecule reaction dynamics. Using velocity map imaging in combination with crossed beams at low energy we study the differential cross section of negative ion reactions. For nucleophilic substitution reactions we have observed several distinct reaction mechanisms when varying the collision energy [1]. Total reaction rate measurements, which we carry out in the box-shaped potential of a 22pole ion trap [2], have revealed unexpected temperature-dependences for proton transfer and for cluster stabilisation at low temperatures. In addition, laser-induced photodetachment is studied in the trap to obtain absolute destruction cross sections for negative ions in light fields [3]. These results are relevant for the understanding of the negative ion abundances in interstellar molecular clouds. \newline [1] J. Mikosch et al., Science (in press) \newline [2] J. Mikosch et al., Phys. Rev. Lett. 98, 223001 (2007) \newline [3] S. Trippel et al., Phys. Rev. Lett. 97, 193003 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H26.00002: Cold reactive collisions between laser-cooled ions and velocity-selected neutral molecules Martin Bell, Stefan Willitsch, Alexander Gingell, Simon Procter, Timothy Softley The recent development of a range of techniques for producing ``cold'' molecules at very low translational temperatures T $<$ 1 K in the gas phase has provided the opportunity for studying molecular collisions in a new physical regime. We report a new experimental method to study reactive collisions between ions and neutral molecules at very low temperatures which allows for tunable collision energies and a variety of chemically diverse reaction partners. Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of cold polar molecules with a facility to produce strongly ordered samples of laser-cooled ions in an ion trap, usually referred to as Coulomb crystals. Despite the low fluxes of neutral molecules obtained from the quadrupole-guide, the strong localization and long trapping times of the ions allows chemical reactions to be studied at the single-particle level. In a proof-of-principle experiment, we have studied the chemical reaction between translationally cold CH$_3$F molecules and laser-cooled Ca$^+$ ions in a collision energy range corresponding to 1-10 K. The characteristics of our cold-molecule sources and the performance of the new technique as well as perspectives for further developments will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H26.00003: Demonstration of a three-dimensional trap for state-selected Rydberg atoms Stephen Hogan, Frederic Merkt Recent progress in the development of methods by which to decelerate and manipulate the translational motion of Rydberg atoms and molecules in the gas phase using static and time-varying inhomogeneous electric fields has led to the experimental realization of Rydberg atom optics elements including a lens, a mirror and a two-dimensional trap. These experiments exploit the very large electric dipole moments associated with Rydberg Stark states, and have demonstrated the possibility to stop a seeded, pulsed, supersonic beam of atomic hydrogen traveling with an initial velocity of 700~ms$^{-1}$ within 3~mm and $5~\mu$s using electric fields of only a few kVcm$^{-1}$.\\ With the goal of achieving complete control of a cloud of Rydberg atoms or molecules in three-dimensions, we have recently designed and constructed a three-dimensional electrostatic trap for these particles. The design of this trap will be presented along with the results of a series of experiments in which we have used the trap to confine, in three dimensions, a cloud of atomic hydrogen Rydberg atoms in states with principal quantum numbers around $n=30$. The dynamics of the Rydberg atoms in the trap have been investigated by pulsed field ionization and imaging techniques. Under favorable conditions, trapping times on the order of 150~$\mu$s have been observed, corresponding to the radiative lifetimes of the excited states. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H26.00004: Measuring the role of alignment in a molecule optical lens Simon M. Purcell, Peter F. Barker Far off-resonant pulsed lasers have been used to deflect and focus molecules via the optical dipole force, which is proportional the effective polarisability of the species [1]. Molecules have an anisotropic polarisability, which in the presence of an intense linearly polarised optical field (10$^{12}$ Wcm$^{-2})$ causes the molecule to align with the field polarisation vector. This alignment occurs due to the creation of pendular states which are a superposition of the field free rotational states of the molecule[2]. This alignment of the molecule with the electric field can result in a higher effective polarisability leading to an increased dipole force that can be used to tailor the properties of molecular optical elements. Using this property, we are studying how the field polarisation can be used to modify the focal length of the molecule optical lens, created by a focused laser beam. We will present calculations of this process and a comparison with our experiments on cold (3 K) carbon disulphide molecules focused by a Nd:YAG laser beam. [1] H.S Chung, B.S Zhao, S.H. Lee \textit{et al., }J. Chem. Phys \textbf{114,} 8293 (2001) [2] B. Friedrich, D. Herschbach, J. Phys. Chem. \textbf{99,} 15686 (1995) [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H26.00005: Cold and ultracold polar molecules Invited Speaker: Study of ultracold molecules promises important benefits such as novel control of chemical reactions and molecular collisions, precision measurement of fundamental physical properties, and new methods for quantum information processing and quantum simulations. We undertake two approaches aimed to produce cold, polar molecular samples. In the first approach, we work directly with ground-state polar molecules such as hydroxyl radicals (OH) or formaldehyde molecules (H$_{2}$CO). After Stark deceleration through an inhomogeneously distributed electric field, OH molecules are loaded into a magnetic trap at a density $\sim $3 x 10$^{5}$ cm$^{-3}$ and temperature of 50 mK. An important advantage of magnetically trapping OH molecules is the freedom in applying an external electric field without significantly affecting the trap dynamics. The open geometry of the trap will enable experimental studies of cold, dipolar collisions subject to an external electric field. We will report our latest progress towards this goal. In the second approach we explore the possibility of producing ultracold polar molecules via association of two different atoms from ultracold atom gas mixtures near quantum degeneracy. Specifically, an interspecies Feshbach resonance between bosonic $^{87}$Rb and fermionic $^{40}$K permits efficient creations of heteronuclear Feshbach molecules. Subsequent optical spectroscopy reveals promising paths to efficiently transfer populations from the weakly bound to more deeply bound states. Progress on the production of these ultracold fermionic polar molecules will be reported. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H26.00006: Experimental demonstration of electrostatic surface guiding for cold polar molecules Yong Xia, Yaling Yin, Jianping Yin We demonstrate an electrostatic surface guiding for cold polar molecules over a long distance of 44.5 cm on a substrate by using a hollow electrostatic field, which is generated by two parallel charged wires and a grounded metal-plate. We measure the transverse spatial distribution of the guided supersonic D$_{2}$O (including CH$_{3}$Br) molecular beam and its longitudinal velocity one, and study the dependence of the relative guiding efficiency and the transverse temperature of the guided molecular beam on the guiding voltage, also perform Mote-Carlo simulations and theoretical studies for the molecular guiding process, and our guiding scheme has some potential applications in molecule optics, such as molecular-beam splitter, integrated molecular optics, etc. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H26.00007: Rotational Spectra of Methane in Helium-4 Robert Zillich, Birgitta Whaley We extend correlated basis function (CBF) theory, in combination with diffusion Monte Carlo simulations, to spherical top molecules solvated in superfluid $^4$He droplets. Similarly to our previous CBF work on linear molecules, the rotational excitations of a spherical top molecule are renormalized by a self energy which contains the $^4$He density modulation around the molecule as coupling. Due to the high symmetry of this density in the case of solvated spherical molecules the rotation-$^4$He coupling turns out to be weak, and the corresponding reduction of the effective rotational constant is small. Furthermore, unlike for linear molecules, for spherical top molecules the symmetry of the gas phase rotational spectrum is not preserved. Instead, for excitations of total angular momentum $J\ge 2$, we find that the self energy induces a splitting of the rotational energies. We present results for the rotational spectrum of solvated CH$_4$ and CD$_4$ for several available He-CH$_4$ potential energy surfaces, and compare with experimental results measured recently. We propose to measure spectra of partially substituted methane (e.g. CDH$_3$) to investigate the effect of symmetry breaking on the rotation-$^4$He coupling strength. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H26.00008: Water containing molecular complexes studied by superfluid helium droplet spectroscopy Susumu Kuma, Mikhail Slipchenko, Takamasa Momose, Andrey Vilesov Superfluid helium droplets offer an ideal environment for spectroscopic studies of molecular complexes by virtue of the controllable aggregation process of embedded molecules and its weak interaction as a matrix medium. Here, we report the infrared spectroscopy of Ar-H$_{2}$O, N$_{2}$-H$_{2}$O, and O$_{2}$-H$_{2}$O complexes picked up in He droplets. The observed spectra in the anti-symmetric stretching vibrational region ($\nu_{3}$) of water around 3750 cm$^{-1}$ indicated that the water molecule in complexes rotates nearly freely in Ar-H$_{2}$O and O$_{2}$- H$_{2}$O, while not in N$_{2}$-H$_{2}$O. The spectra of Ar-H$_{2}$O and O$_{2}$- H$_{2}$O exhibited the splitting of the rotational lines, which is due to the anisotropy of their intermolecular potential. We have analyzed the observed splittings in the spectra to determine the intermolecular potentials of Ar-H$_{2}$O and O$_{2}$- H$_{2}$O in helium droplets. These results are compared with the corresponding potentials previously studied in both experimentally and theoretically. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 11:00AM |
H26.00009: Microwave spectroscopy of doped helium clusters and doped helium droplets Invited Speaker: High resolution microwave and infrared spectroscopy of small to medium sized doped helium clusters, e.g. He$_{N}$-OCS with $N$ from 2 to 70, has given detailed insights into how superfluidity, a bulk phase property, evolves from the microscopic scale. Some of the most significant findings were oscillatory behavior of cluster rotational constant $B$ with number of helium atoms, $N$, and the observation of very narrow lines (15 kHz in microwave and 0.001 cm$^{-1}$ in the infrared region), even for the largest $N$. How can this be reconciled with the broad (up to several GHz wide) lines of rotational and ro-vibrational transitions of molecular dopants in helium droplets? Microwave experiments of molecular dopants embedded in helium nanodroplets can help answer this question. We have measured the pure tunneling inversion transition of ammonia in helium droplets at about 20.7 GHz. A complex line shape, consisting of a sharp (15 MHz wide) line on top of a broad background (1.5 GHz wide) was observed. The line shape could be simulated by assuming identical energy sublevels of the initial and final state of the transition. This provides direct evidence for the existence of an energy level substructure of molecular states in doped helium droplets. Microwave rotational transitions of carbonylsulfide, OCS, in helium droplets show increase in line width with increasing rotational quantum number $J$ and, in some cases, prominent fine-structures. Some of these features can be interpreted in terms of droplet size distribution. [Preview Abstract] |
Session H27: Correlated Electrons: Heavy Fermions and Exotic Magnets
Sponsoring Units: DCMPChair: Johnpierre Paglione, University of Maryland
Room: Morial Convention Center 219
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H27.00001: Magnetism and metallicity in FeSb2-xTex Cedomir Petrovic, Rongwei Hu, Vesna Mitrovic Single crystals of FeSb2-xTex (0$\le $x$\le $1) were grown by molten metallic flux technique. A rich variety of electronic and magnetic ground states will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H27.00002: Phase transitions in $R_{5}$NiPb$_{3}$ ($R$=Ce,Nd,Gd) V. Goruganti, K. D. D. Rathnyayaka, Joseph H. Ross, Jr. We report magnetic and thermodynamic measurements for recently-synthesized $R_{5}$NiPb$_{3}$ ($R$=Ce,Nd,Gd) (hexagonal Hf$_{5}$CuSn$_{3}$-type structure), as well as non-magnetic La-based analogs. High-temperature Curie-Weiss fits yield effective moments of 2.43, 3.70 and 9 $\mu_{B}$ for Ce$_{5}$NiPb$_{3}$, Nd$_{5}$NiPb$_{3}$ and Gd$_{5}$NiPb$_{3}$ respectively. These are close to the $R^{3+}$ ionic moments, showing that Ni is nonmagnetic in all cases. For Ce$_{5}$NiPb$_{3}$ a peak seen in both the magnetization and specific heat at 48 K indicates an apparent ferromagnetic transition at that temperature, which is also confirmed by field dependent heat capacity and a positive Curie-Weiss temperature. Nd$_{5}$NiPb$_{3}$ exhibits two magnetic transitions, an antiferromagnetic transition at 42 K and an apparently weak ferromagnetic canting transition at 8 K. Ce$_{5}$NiPb$_{3}$ shows a kink in both the magnetization and specific heat at 68 K indicates a ferro- or ferrimagnetic transition at that temperature, which is also confirmed by a positive Curie-Weiss temperature. For this material, ZFC and FC measurements show irreversibility at transition temperature. For Ce and Nd samples $M$-$H$ curves show metamagnetism at low temperatures. We will compare the results with the non magnetic analog La$_{5}$NiPb$_{3}$. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H27.00003: Universal heat transport in the heavy-fermion superconductor CeIrIn5 Hamideh Shakeripour, M.A. Tanatar, C. Petrovic, Louis Taillefer In superconductors with nodes in the gap, zero-energy quasiparticles give rise to a residual linear term in the thermal conductivity at T=0. When the quasiparticle density of states is linear in energy, this term is universal, in the sense that it does not depend on impurity concentration. Such universal heat transport has been observed in cuprates and ruthenates but never in heavy-fermion superconductors. Here we show that heat transport in the heavy-fermion superconductor CeIrIn5 is unchanged by the doping of La impurities. This universal transport confirms the presence of a line node in the gap [1]. \newline [1] H. Shakeripour et al., Phys. Rev. Lett. 99, 187004 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H27.00004: Fermi Surface Investigation of Cd doped CeCoIn5 Cigdem Capan, Luis Balicas, Younjung Jo, Roy Goodrich, Andrea Bianchi, John Ditusa, Zachary Fisk CeCoIn5 is a 2.3K superconductor member of the 115 family of heavy fermion compounds that have attracted much attention due to the competing magnetism and superconductivity. The superconductivity in CeCoIn5 emerges from a metallic state that exhibits strong deviations from the Fermi Liquid theory, with the presence of a field-tuned antiferromagnetic quantum critical point near the upper critical field. Recently it was shown that Cd substitution can tune the ground state from superconducting to antiferromagnetic. One important question is how the shape of the Fermi surface influences the stability of the ground state. We will present de-Haas-van-Alphen results in Cd doped LnCoIn5 (Ln=La,Ce) and discuss the origin of magnetic order in this system. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H27.00005: Two fluid analysis of La and Cd doped CeIrIn$_{5}$. Nicholas Berry, Andrea Bianchi, Zachary Fisk The heavy fermion superconductor CeIrIn$_{5}$ has been shown to have a ground state dependence on Cd doping which induces antiferromagnetism while suppressing superconductivity. This system provides an interesting regime in which to study the onset of the heavy fermion superconducting and antiferromagnetic ground states as well as the interplay between the different electronic interactions at low temperatures. The Two Fluid Model has quantified the energy scales in the related La doped CeCoIn5. We attempted a two fluid analysis on specific heat and susceptibility measurements of La and Cd doped CeIrIn$_{5}$ single crystals. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H27.00006: Anisotropic effect of Cd-doping on superconducting phase in CeCoIn$_{5}$ at high fields Yoshifumi Tokiwa, Roman Movshovich, Filip Ronning, Eric Bauer, Andrea Bianchi Unconventional superconductor CeCoIn$_{5}$ at high magnetic field displays first order superconducting (SC) transition, and an additional high field-low temperature SC phase (previously proposed to be an inhomogeneous superconducting FFLO state). We have studied Cd-doping effect on the FFLO state and the first order SC transition by measuring specific heat $C$(T) of CeCo(In$_{1-x}$Cd$_{x})_{5}$ (x=0.01, 0.02 and 0.03) at low temperatures and high fields. Our data show that the FFLO state is already destroyed by 1{\%} Cd-doping. The effect of Cd doping on the first order SC transition is anisotropic. The cross-over temperature $T_{0}$, where the superconducting transition changes its nature from first to second order, decreases rapidly with increasing doping for $H$//[100] and disappears already at x=0.02, while it remains rather temperature-independent for $H$//[001] up to x=0.03. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H27.00007: Angular Dependent Magnetic Properties of CeCoIn$_{5}$ at Low Temperatures J.-H. Park, R.L. Stillwell, T.P. Murphy, E.C. Palm, S.W. Tozer, J.C. Cooley The heavy-fermion compound CeCoIn$_{5}$ exhibits a superconducting transition at 2.3 K. As an unconventional superconductor, many unusual physical properties of the compound have been actively studied. In particular, evidence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in this compound was reported first by Radovan and co-workers.$^{\dag }$ At the lowest temperature ($\sim $ 20 mK), the FFLO state of CeCoIn$_{5}$ was observed in fields between 10 and 11.7 T when the \textit{ab}-plane of the compound was placed parallel to the external magnetic field. In addition, at these low temperatures, the angular dependent peak effect was observed and interpreted as a crossover between Abrikosov and Josephson vortex lattices.$^{\ddag }$ Further experimental studies of the low temperature ($>$ 12 mK) magnetic properties of CeCoIn$_{5}$, performed in various sample orientations with respect to magnetic field will be presented. $^{\dag }$H.A. Radovan, et al., Nature \textbf{425} (2003) 51. $^{\ddag }$H.A. Radovan, et al., Philosophical Magazine \textbf{86} (2006) 3569. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H27.00008: Evolution of the FFLO state of CeCo(In$_{1-x}$Hg$_{x})_{5}$ Roman Movshovich, Yoshifumi Tokiwa, Andrea Bianchi, Eric Bauer Specific heat C(T) of the unconventional superconductor CeCoIn$_{5 }$displays an anomaly within the superconducting (SC) state, indicating an additional SC phase in the high field and low temperature corner of the SC state of the H-T phase diagram. This SC state was previously proposed to be an inhomogeneous superconducting FFLO state. We have performed specific heat measurements of very low Hg-doped samples with x=0.001, 0.002 and 0.003, in order to study the evolution of the proposed FFLO state in CeCoIn$_{5}$. In a sample with x=0.001, C(T) still shows the additional transition inside the SC state, but it is broadened. The transition temperature is slightly higher than that of pure compound. C(T) of a higher Hg-doped sample with x=0.003 displays a very shallow and broad hump around the same temperature. Given that only 20{\%} of the nominal Hg concentration goes into the sample, the FFLO state appears to be extremely sensitive to the presence of impurities. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H27.00009: Supression of superconductivity in CeRh$_{1-x}$Co$_{x}$In$_5$ by incommensurate antiferromagnetic order Seiko Ohira-Kawamura, Hiroaki Shishido, Akari Yoshida, Ryuji Okazaki, Hazuki Kawano-Furukawa, Takasada Shibauchi, Hisatomo Harima, Yuji Matsuda CeCoIn$_5$ shows superconductivity (SC) below 2.3~K, while the SC is highly suppressed in a related system CeRhIn$_5$, which exhibits the antiferromagnetic (AF) order below 3.8~K\@. Then the mixed compound CeRh$_{1-x}$Co$_{x}$In$_5$ is expected to have a complex $x$-$T$ phase diagram where the SC and magnetism coexist. In order to understand the relationship between the SC and the antiferromagnetism in CeRh$_{1-x}$Co$_{x}$In$_5$, we have performed neutron diffraction measurements on this system for various $x$\@. A commensurate (C) AF order with a propagating vector {\boldmath $q$}$_c=(1/2, 1/2, 1/2)$ is observed for $0.3 \le x \le 0.6$, coexisting with the SC\@. However, they are strongly suppressed at $x \sim 0.3$, and an incommensurate (IC) AF order with {\boldmath $q$}$_I=(1/2, 1/2, 0.298)$ simultaneously appears. We interpret that there is no intrinsic coexistence between the IC- and C-AF orders and that the SC competes with the IC-AF order but coexists with the C-AF one. These results imply that particular areas on the Fermi surface nested by {\boldmath $q$}$_I$ play an active role in forming the superconducting state in CeCoIn$_5$. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H27.00010: Chiral d-wave superconductivity in the heavy fermion compound $CeIrIn_5$ Aditya Raghavan, Kazumi Maki, Stephan Haas A recent experiment by Shakeripour et al indicates that the superconductivity in heavy fermion compound $CeIrIn_5$ is very different from the one in $CeCoIn_5$. We have re-examined their data and concluded that it belongs to the chiral d-wave SC or $\Delta(\vec{k})\sim e^{\pm i\phi}\cos(ck_z)$. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H27.00011: Fabrication of artificial heavy fermion superlattices by the molecular beam epitaxy Hiroaki Shishido, Tomonari Kato, Manabu Izaki, Takasada Shibauchi, Yuji Matsuda, Takahito Terashima We have grown artificial superlattices of CeIn$_3$ ($m$) / LaIn$_3$ ($n$), in which $m$-layers of heavy-fermion antiferromagnet CeIn$_3$ and $n$-layers of a non-magnetic isostructual compound LaIn$_3$ are stacked alternately, by a molecular beam epitaxy. Growth process were monitored by reflection high energy electron diffraction (RHEED). Sharp streak pattern of RHEED indicates the epitaxial growth of thin films. Satellite peaks observed in a X-ray diffraction pattern also indicates the superlattice structure. By reducing the thickness of CeIn$_3$, we observe a suppression of antiferromagnetic order and an enhancement of effective mass inferred from the resistivity coefficient, which both imply new `dimensional tuning' towards a quantum critical point. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H27.00012: Antiferromagnetism in the Kondo lattice system Ce(Ni$_{0.25}$In$_{1.75})$. W.H. Lee, H.H. Wu, H.H. Sung, K.J. Syu, Y.Y. Chen The pseudobinary compound Ce(Ni$_{0.25}$In$_{1.75})$, which crystallizes in a hexagonal AlB$_{2}$ type structure with space group P6/mmm, exhibits antiferromagnetic ordering below the temperature 3.9 K, as revealed in the magnetic susceptibility, electrical resistivity and low-temperature specific-heat data. A ln(T) dependence is seen in the high temperature region for the magnetic contribution to the resistivity $\rho _{m}$, which is one of the characteristic features of dense Kondo systems. The magnetic entropy S$_{m}$(T$_{N})$ associated with the magnetic structure of Ce(Ni$_{0.25}$In$_{1.75)})$ is found to be only 62{\%} of Rln(2), corresponding to a reduction of 38{\%} of the cerium moment. This large magnetic reduction may be able to be attributed to Kondo effect. The heat capacity C(T) of Ce(Ni$_{0.25}$In$_{1.75})$, in the paramagnetic state at temperatures between 8 and 20 K, can be fitted to the expression C$_{n}$ = $\gamma $T + $\beta $T$^{3}$ by a least squares analysis, which yields the value $\gamma $ = 123 mJ/mol K$^{2}$ and $\beta $ = 1.2 mJ/mol K$^{4}$, the latter value corresponding to the Debye temperature $\Theta _{D}$ = 169 K. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H27.00013: Angle dependent quasiparticle weights in correlated metals Pouyan Ghaemi, Senthil Todadri, Piers Coleman The variation in the quasiparticle weight $Z$ on moving around the fermi surface in correlated metals is studied theoretically. Our primary example is a heavy Fermi liquid treated within the standard hybridization mean field theory. The most dramatic variation in the quasiparticle weight happens in situations where the hybridization vanishes along certain directions in momentum space. Such a ``hybridization node" is demonstrated for a simplified model of a Cerium-based cubic heavy electron metal. We show that the quasiparticle weight varies from almost unity in some directions, to values approaching zero in others. This is accompanied by a similar variation in the quasiparticle effective mass. Some consequences of such hybridization nodes and the associated angle dependence are explored. Comparisons to somewhat similar phenomena in the normal metallic state of the cuprate materials are discussed. A phenomenological picture of the pseudogap state in the cuprates with a large Fermi surface with a severely anisotropic spectral weight is explored. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H27.00014: Magnetic properties of RFe$_2$Zn$_{20}$ (R = Y, Gd - Lu): $4f$ moments embedded in strongly correlated electron host Shuang Jia, Ni Ni, A. Safa-Sefat, S.L. Bud'ko, P.C. Canfield, Hyunjin Ko The RFe$_2$Zn$_{20}$ series of compounds manifest varied magnetic properties, from near ferromagnetism (R = Y, Lu), to enhanced ferromagnetic (FM) ordering of local moments (R = Gd to Tm), to heavy Fermi ground state (R = Yb). Thermodynamic and transport measurement results reveal that these varied magnetic states can be understood in the framework of $4f$ moments embedded in a nearly ferromagnetic Fermi liquid. In such a highly polarizable electronic host, the different type of 4f moments, null moments for Y$^{3+}$ and Lu$^{3+}$, pure spin- contributed Gd$^{3+}$, spin-orbital-coupling contributed Tb$^ {3+}$ to Tm$^{3+}$, and the hybridized Yb ions, correlated with the itinerant electrons, lead to this magnetic versatility. For local moment members (R = Gd to Tm), the Curie temperatures roughly scale with the de Gennes parameter, indicating negligible crystal electric field (CEF) effect on the magnetic ordering, although the CEF on the $4f$ local moments leads to anisotropic FM ground state for R = Tb to Tm. For hybridized moment member (R = Yb), the Kondo temperature seems to be enhanced, associated with the electronic host. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H27.00015: Low-Energy Magnetic Excitations in the Itinerant Quantum Ferromagnet ZrZn$_2$ Stephen Hayden, Edward Yelland High resolution de Haas-van Alphen measurements offer a powerful method to probe the low energy magnetic excitations in ferromagnets. Here we report measurements of the temperature dependence of the exchange splitting $\Delta(T)$ in the weak itinerant ferromagnet ZrZn$_2$ (Curie temperature $T_c=28$\,K) using the de Haas-van Alphen (dHvA) effect. Quantitative comparison with the magnetic moment $M(T)$ shows that longitudinal or `Stoner' excitations dominate, in strong contrast with observations on metallic ferromagnets like Fe and Ni, where spin-waves dominate at low $T$. We ascribe the difference to the proximity of a quantum critical point (QCP) in ZrZn$_2$. The results are discussed in terms of a phenomenological Ginzburg-Landau model which includes effects of thermal fluctuations and enhanced fluctuations due to the nearby quantum critical point. We find that the model accurately predicts both the rate of collapse of $M(T)$ with $T$ and the relative reduction of $\Delta(T)$ and $M(T)$. We suggest a picture for the evolving nature of magnetic excitations within the ferromagnetic state as the QCP is approached. [Preview Abstract] |
Session H28: Photonic Crystals
Sponsoring Units: DCMPChair: Mark Sherwin, University of California, Santa Barbara
Room: Morial Convention Center 220
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H28.00001: Moving the band gaps and changing the transmission of magnetic photonic crystals Shiyang Liu, Junjie Du, Zhifang Lin, Siu Tat Chui We classify different types of the photonic band gaps (PBGs) of two dimensional magnetic photonic crystals (MPCs) consisting of arrays of magnetic cylinders and study the different tunability (by an external static magnetic field) of these PBGs. One type of the band gaps comes from infinitely degenerate flat bands and is closely related to those in the study of plasmonics. We calcualte the transmission of the PBG's and found excellent agreement with the results of the photonic band structure calculation. Positional disorder of the lattice structure affects the different types of PBGs differently. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H28.00002: Probing states with macroscopic circulations in magnetic photonic crystals Siu Tat Chui, Zhifang Lin We predict that when light is reflected off a magnetic photonic crystal (MPC) there is a grazing component that is {\bf parallel} to the surface; the magnitude of this component can be changed by an external field. The direction of this parallel component is reversed (dotted line) as the direction of the magnetization is reversed. This provides a way to probe states with macroscopic circulations inside the MPC. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H28.00003: Experimental investigation of photonic gaps in optical Thue-Morse multilayers fabricated using nanostructured thin films Matthew Hawkeye, Michael Brett One-dimensional Thue-Morse (TM) multilayers are realized by stacking together layers of different refractive index according to simple rules. The result is a deterministically generated aperiodic system representing an intermediate stage between a periodic medium and a random one. This work focuses on the formation of photonic band gap regions at multiple frequencies relating to local positional correlations in the TM structure. TM multilayers are realized in the visible and near-IR spectral regions using glancing angle deposition (GLAD), a single-step nanofabrication technique providing control over the internal columnar structure of a deposited thin film. The effective refractive index of the deposited layer is tuned by controlling the columnar structure leading to great flexibility over the choice of refractive index in the experiment. Using GLAD, TM multilayers are fabricated out of titanium dioxide by varying the density of the columnar structures. The resulting photonic gaps are characterized using transmittance and reflectance spectroscopy and compared with the results of transfer matrix simulations. The creation of gaps in different generations of the TM system will also be examined. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H28.00004: Effective medium theory of photonic crystals W.T. Lu, S. Zhang, Y.J. Huang, S. Sridhar We develop an effective medium theory for photonic crystals including negative index metamaterials. This theory is based on field summation within the unit cell. The unit cell is determined by the surface termination. The orientation of the surface breaks the field summation symmetry. This theory is self-consistent. The effective permittivity and permeability tensors will give the exact dispersion relation obtained from the band structure calculation. For waves incident into multilayered structures, our theory gives exact transmittance and reflectance for any wavelengths. For interface with periodic surface structures, our theory gives very accurate results for wavelength down to being comparable with the lattice spacing. By properly taking into account the multiple Bloch modes inside the photonic crystal, our theory can be made to give exact Bragg coefficients. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H28.00005: Refraction at a photonic crystal surface: exact characterization Prabasaj Paul Refraction at plane air-photonic crystal surfaces in a class of photonic crystals is studied. The class of photonic crystals has particularly simple band-structure and Bloch wave solutions, which make the evaluation of exact reflection and transmission coefficients relatively simple. New analytical results are presented, and the effects of variation in surface location and orientation are explored. A close look is taken at two important issues -- negative refraction, and the validity of the Rayleigh hypothesis. The results obtained are consistent with those in existing literature. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H28.00006: Quantum interference near a photonic band edge beyond the weak field approximation Paul M. Alsing, David A. Cardimona, Danhong H. Huang We investigate spontaneous emission and quantum interference effects involving a three level atom in the vicinity of a photonic band edge, beyond the weak driving field approximation. We consider two different three-level atoms, each subject to a probe field from the ground state, and each embedded within a different photonic crystal (PhC). The first atom has the two excited states separated by a dipole transition in the optical frequency range, with this frequency being close to the surrounding PhC's band edge. The probe field couples the ground state and the highest excited state, and is well outside the PhC bandgap. If a coupling field is applied between the two upper levels, Electromagnetically Induced Transparency (EIT) may occur, depending on the position of the band edge. The second atom has the two upper levels each dipole-coupled to the ground state, and close enough that the emissions from each can coherently interfere. This atom is embedded within a PhC whose band edge lies near the lower of the two excited states, and a probe field is applied that lies just beyond this band edge. This atom exhibits a quantum interference phenomenon related to EIT called Field-Induced Transparency (FIT), again depending on the position of the band edge relative to the lower excited state. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H28.00007: Microwave transmission measurements through wire array photonic crystals Graeme Dewar, Nathan Souther, Michael Johnson We have measured the microwave transmission between 12.4 and 18.0 GHz through wire arrays formed into two dimensional square lattices. One array made of copper wire 0.16 mm in radius consisted of five rows by 21 columns having a lattice constant of 5.15 mm. This array exhibited a pass band above 15 GHz, in good agreement with the calculated plasma frequency found from an expression for the permittivity$^{1}$ derived in the long wavelength limit. A second array was made with wire of radius 18 microns and lattice constant 0.8 mm. This array was filled with dielectric loaded with powdered magnetite. A sample of this metamaterial 5.8 mm thick and with no externally applied magnetic field exhibited a pass band above 16 GHz. Implications for creating metamaterials with a negative index of refraction from wire arrays embedded in a magnetic host will be discussed. \newline $^{1}$G. Dewar, in \textit{Complex Mediums III: Beyond Linear Isotropic} \textit{Dielectrics}, Akhlesh Lakhtakai, Graeme Dewar, Martin W. McCall, Editors, Proceedings of SPIE Vol. 4806, 156-166 (2002). [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H28.00008: Tailoring Self-Assembled Metallic Photonic Crystals for Modified Thermal Emission Sang Eon Han, Andreas Stein, David Norris Photonic crystals are solids that are periodically structured on an optical length scale. Previous work has shown that specific photonic crystal structures can lead to changes in the thermal emission spectra of a material. This may allow elimination of unwanted heat from emission sources, such as tungsten filaments in conventional light bulbs, or lead to new materials for thermophotovoltaics. Here, we study the possibility that metallic photonic crystals obtained via self-assembly can modify thermal emission. These structures, known as inverse opals, are easy to fabricate. However, experiments on tungsten inverse opals suggest that they also have strong optical absorption. In this case, the light does not interact sufficiently with the periodicity of the crystal and modification of thermal emission does not occur. We consider the origin of this effect and show theoretically how to tailor both absorption and surface coupling in experimentally realizable metallic inverse opals. Calculations for tailored inverse opals made from tungsten, molybdenum, and tantalum show that their optical properties can be similar to or even better than the tungsten woodpile structure, which has previously shown modified thermal emission. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H28.00009: Integrated lenses for enhanced coupling into terahertz photonic crystal slab waveguides Cristo Yee, Stephen Parham, Mark Sherwin The fundamental property of a photonic crystal (PC), its optical band gap, can be exploited by the introduction of defects that allow the existence of spatially localized states within the optical band gap. A PC waveguide consist of a line of defects in which the localized states will coalesce to form bands that allow the transmission of light otherwise prohibited. Coupling light directly from a source into the waveguide is restricted by the impedance mismatch of the PC waveguide and the surrounding media. In this work we use integrated lenses to enhance coupling of light into PC waveguides. PC slabs with lattices constants ranging from 56 to 64 microns were fabricated with Reactive Ion Etching on a high-resistivity Si wafer. An narrow band tunable source was employed to measure the transmission trough the waveguides. The results are compared with a full 3D FDTD calculations. This work was supported by NSF under grant CCF0507295 and CONACYT-UCMEXUS. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H28.00010: Modeling Conformal Growth in Photonic Crystals and Comparing to Experiment Andrew Brzezinski, Ying-Chieh Chen, Pierre Wiltzius, Paul Braun Conformal growth, e.g. atomic layer deposition (ALD), of materials such as silicon and TiO$_{2}$ on three dimensional (3D) templates is important for making photonic crystals. However, reliable calculations of optical properties as a function of the conformal growth, such as the optical band structure, are hampered by difficultly in accurately assessing a deposited material's spatial distribution. A widely used approximation ignores ``pinch off'' of precursor gas and assumes complete template infilling. Another approximation results in non-uniform growth velocity by employing iso-intensity surfaces of the 3D interference pattern used to create the template. We have developed an accurate model of conformal growth in arbitrary 3D periodic structures, allowing for arbitrary surface orientation. Results are compared with the above approximations and with experimentally fabricated photonic crystals. We use an SU8 polymer template created by 4-beam interference lithography, onto which various amounts of TiO$_{2}$ are grown by ALD. Characterization is performed by analysis of cross-sectional scanning electron micrographs and by solid angle resolved optical spectroscopy. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H28.00011: The couple between modes of planar wave guides and the evanescent fields produced in the total internal reflection. Raul Garcia-Llamas, Jorge Gaspar-Armenta, Ramon Munguia-Arvayo The coupling between the modes of planar wave guides and the evanescent fields produced in the total internal reflection (TIR) system is studied theoretically. The planar guides are assumed as a semi-infinite inhomogeneous periodic medium (IM) with modulation only in the $y$-direction and period $a$, which is perpendicular to the propagation ($z$-axis) of the modes. This medium is separated by a vacuum (VA) gap, of uniform thickness $d_{2}$, from a semi-infinite homogeneous dielectric medium (HM). Then, two interfaces are found, one at z = 0 between HM/VA and other at $z=d_{2}$ between VA/IM. A transverse magnetic electromagnetic plane wave with wavelength \textit{$\lambda $}$_{0}$, is impinging the VA/IM interface and its wave vector, in the $z-y$ plane, is doing an angle \textit{$\theta $}$_{i}$ with the $z$-axes. The solution of the electromagnetic diffracted field in the IM is a multimodal expansion as proposed by Burckhardt [J. Opt. Soc. Am. \textbf{56 }(1966) pp. 1502]. Following the approach used by Glass and Maradudin [Phys. Rev. B \textbf{29} (1984) pp. 321] a matrix equation for the amplitudes of the diffracted field is found. Numerical results of the near field intensity are presented. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H28.00012: Negative bi-refraction of acoustic waves in sonic crystals Yan-Feng Chen Optical birefringence and dichroism are classical and important effects originating from two independent polarizations of optical waves in anisotropic crystals. However, it is impossible for acoustic waves in the fluid to show such a birefringence because only the longitudinal mode exists. The emergence of an artificial sonic crystal (SC) has significantly broadened the range of acoustic materials in nature that can give rise to acoustic bandgaps and be used to control the propagation of acoustic waves. Recently, negative refraction has attracted a lot of attention and has been demonstrated in both left-handed materials and photonic crystals. Similar to left-handed materials and photonic crystals, negative refractions have also been found in SCs. Here we report the acoustic negative-birefraction phenomenon in a two-dimensional SC, even with the same frequency and the same `polarization' state. By means of this feature, double focusing images of a point source have been realized. This birefraction concept may be extended to other periodic systems corresponding to other forms of waves, for example, electron for semiconductors, photon for photonic crystals, and plasmon for plasmonic crystals, showing great impacts on both fundamental physics and device applications. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H28.00013: Random high-Q cavities in disordered photonic crystal waveguides Frank Vollmer, Juraj Topolancik We demonstrate experimentally that structural perturbations imposed on highly dispersive photonic crystal-based waveguides give rise to spectral features that bear signatures of Anderson localization. Sharp resonances with effective Q's of over 30 000 are found in scattering spectra of disordered waveguides. The resonances are observed in a $\sim $20-nm bandwidth centered at the cutoff of slowly guided Bloch modes. The origin of the spectral features can be explained by the interference of coherently scattered electromagnetic waves which results in the formation of a narrow impurity (or localization) band populated with spectrally distinct quasistates. http://webmac.rowland.org/rjf/vollmer/index.php [Preview Abstract] |
Session H29: Focus Session: Carbon Nanotubes and Related Materials VI: Graphene Transport
Sponsoring Units: DMPChair: Alessandra Lanzara, University of California, Berkeley
Room: Morial Convention Center 221
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H29.00001: Binary memory switching in zigzag-edge graphene nanostrips Daniel Gunlycke, Denis A. Areshkin, John W. Mintmire, Junwen Li, Carter T. White Owing to a peculiar boundary condition, the ground state of any hydrogen-terminated zigzag-edge graphene nanostrip is predicted to exhibit spin-polarized edge states. We capture this physics in a model which has a Hamiltonian that consists of tight-binding terms and terms describing the potential from a spin-dependent scalar field. The model is solved in the presence of a ballistic current passing through the nanostrip. Studying the grand canonical potential for the system, it is shown that the spin-polarized state collapses above a certain bias. Below that threshold there is a bi-stable regime which could be exploited in a possible memory device. The memory could be both set and cleared through the bias and be read by measuring the current through the device. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H29.00002: Graphene and Graphite Nanoribbons: phonon-scattering limit of conductivity from tight-binding Daniel Finkenstadt, Gary Pennington, Michael Mehl To understand nanoribbons of graphene, and multilayers of such ribbons, we developed an ab initio parametrized fit to Carbon and Hydrogen chemical data, out to arbitrary neighbor interactions, including relaxations [\emph{Phys. Rev. B} \textbf{76}, 121405R (2007)]. Our computed band structure shows a decrease in the armchair edge band gap when ribbons are multilayered. Further, the well-known three-family behavior of armchair bangaps is confirmed and shown here to apply also to the drift velocity of charge carriers, which can have $\pm 20$\% deviation from the ideal Fermi velocity. Boltzmann carrier transport simulations from calculated phonon spectra also show a familial dependence of conductance, peak field-effect mobility and ``on'' conductance that increase linearly with ribbon width. We will also discuss phonon-limited scattering of charge carriers in graphene multilayers and the temperature dependence of transport. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H29.00003: Spin transport in rough graphene nanoribbons Inanc Adagideli, Michael Wimmer, Savas Berber, David Tomanek, Klaus Richter We investigate spin conductance in zigzag graphene nanoribbons and propose a spin injection method based only on graphene. Combining density functional theory with tight-binding transport calculations, we find that nanoribbons with asymmetrically shaped edges show a non-zero spin conductance and can be used for spin injection. Furthermore, we show that nanoribbons with rough edges exhibit mesoscopic spin conductance fluctuations with a universal value of $\mathrm{rms} G_{\mathrm{s}}\approx 0.4 e/4\pi$. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H29.00004: Electron Transport in Graphitic Nanostructures Invited Speaker: Local control of the electrostatic potential in graphene nanostructures can provide a new insight into Dirac fermions in confined geometries in electric and magnetic fields. In this presentation, we report electronic transport measurements in patterned locally gated graphene nanoconstrictions and locally gated single walled carbon nanotubes with tunable transmission and bipolar heterojunctions. We observe various unusual transport phenomena, such as energy gap formation in confined graphene structures and series of fractional quantum Hall conductance plateaus at high magnetic fields as the local charge density is varied in the graphene heterojunction regions.~These observed results can be explained in terms of equilibration of chiral edge states at the heterojunction interfaces, indicating charge polarity dependence of quantum Hall edge state equilibration. ~ [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H29.00005: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H29.00006: Coulomb blockade effects in graphene nanoribbons Fernando Sols, Francisco Guinea, Antonio Castro Neto We propose that recent transport experiments revealing the existence of an energy gap in graphene nanoribbons may be understood in terms of Coulomb blockade. Electron interactions play a decisive role at the quantum dots which form due to the presence of necks arising from the roughness of the graphene edge. With the average transmission as the only fitting parameter, our theory shows good agreement with the experimental data. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H29.00007: Distinct Transport Properties of Mono-Layer and Bi-Layer Graphene Nanoribbons Yu-Ming Lin, Zhihong Chen, Phaedon Avouris Graphene holds promise for future electronic applications owing to its exceptional carrier mobility and a Dirac-like, massless dispersion relation for charge carriers. Here we report on experimental studies of electrical transport properties of graphene nano-ribbon devices. Graphene nano-ribbons devices, consisting of mono-layer and bi-layer graphene, were fabricated by e-beam lithography and plasma etching process. These nano-ribbon devices exhibit size-dependent transport properties due to quantum confinement at low temperatures. We observed distinct transport behaviors for mono-layer and bi-layer devices, and this may be related to a tunable bandgap that can be induced in bi-layer graphene. In particular, in bi-layer devices, we observed a significantly lower noise level than that of single-layer graphene. These findings provide insight into the intrinsic noise mechanisms in graphene layers and also elucidate the impact of the coupling between the two layers in the bi-layer graphene on transport properties. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H29.00008: Transverse Field Effect in Graphene Nanoribbons Kathryn Todd, Hung-tao Chou, David Goldhaber-Gordon We describe transport measurements on graphene nanoribbon devices with separately addressable side gates. Applying the same voltage to both side gates allows us to resolve the Dirac points in the nanoribbon and in the 2-dimensional graphene leads. In conjunction with the side gates, a back gate allows us to separately tune the nanoribbon and the leads between p-type and n-type. Source-drain measurements illustrate the importance of charging effects in these short nanoribbons. Applying opposing voltages to the two side gates allows us to test predictions about the effect of a transverse electric field in graphene nanoribbons. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H29.00009: High-Bias Electronic Transport in Graphene Ribbon Devices Melinda Han, Inanc Meric, Kin Fai Mak, St\'ephane Berciaud, Tony Heinz, Ken Shepard, Philip Kim We present experimental studies of electronic transport in graphene under high electric fields. Graphene ribbon devices with varying widths and lengths are fabricated from mechanically exfoliated single-layer graphene sheets using electron beam lithography followed by oxygen plasma etching. Conductance measurements show a tendency of current saturation under high source-drain bias. In addition, we employ micro-Raman spectroscopy simultaneously with transport measurement on the current carrying device in vacuum. We observe an enhancement of the G-band anti-Stokes/Stokes intensity ratio, where the optical phonon temperature is estimated to be over $\sim $500K before device failure. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H29.00010: Graphene zigzag ribbons: hidden multiferroic order and transport J. Fernandez-Rossier, F. Munoz-Rojas, Juan Jose Palacios A new type of electronic phase with coexisting magnetic and ferroelectric order is predicted for graphene ribbons with zigzag edges [1]. The electronic structure of the system is described with a mean field Hubbard model that yields results very similar to those of density functional calculations[2,3]. Without further approximations, the mean field theory is recasted in terms of a BCS wave function for electron-hole pairs in the edge bands. The BCS coherence present in each spin-channel is related to spin-resolved electric polarization. Although the total electric polarization vanishes, due to an internal phase locking of the BCS state, strong magneto-electric effects are expected in this system. We explore these by spin polarized transport across finite length zig-zag ribbons, connected to non-magnetic electrodes [4]. We demonstrate that such system can present very large changes in resistance due to application of lateral electric field that modify the magnetic structure [2,4]. [1] J. Fernandez-Rossier, arXiv:0710.3484 [2] Y. Son, M.L. Cohen, and S. G. Louie, Nature *444*, 347 (2006) [3] J. Fernandez-Rossier and J. J. Palacios, Phys. Rev. Lett. *99*, 177204 (07) [4] J. Fernandez-Rossier, F. Munoz-Rojas, J. J. Palacios, in preparation [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H29.00011: Rough edges in graphene Vladimir Cvetkovic, Zlatko Tesanovic Recent progress in fabrication of graphene and the understanding of its properties make the graphene a strong contender for a building block of future electronic devices. We analyze the effects of the scattering off rough edges on the transport in graphene nanoribbons. The confinement of the Dirac particles in a nanoribbon is achieved either by means of a large mass term in the Hamiltonian or by imposing boundary conditions appropriate for graphene sheets on the quantum mechanical wave-functions. Variations in the nanoribbon width lead to a nontrivial lateral channel-mixing and provide important limitations to quantum transport. We present a perturbative solution for the problem and derive experimentally measurable conductivity up to the leading order in the case of a nanoribbon with such randomly varying width. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H29.00012: Ab initio study of edge functionalization effects on electronic transport through graphene nanoribbons Amir Farajian, Narjes Gorjizadeh, Yoshiyuki Kawazoe We investigate quantum transport through graphene nanoribbons with and without hydrogen saturation. Both armchair-edged and zigzag-edged ribbons with and without hydrogen saturation are considered. For calculating transport properties, we make use of ab initio electronic structure calculations followed by Green's function implementation of the Landauer's formalism. The calculated conductance characteristics show significant dependence on the edge functionalization, i.e., whether or not the armchair and zigzag nanoribbons are saturated by hydrogen atoms. The effects of the carrier spin orientations are also discussed. These results are useful in interpreting the experimental data, and in using functionalized graphene nanoribbons for nanoelectronics and sensor applications. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H29.00013: Thermoelectric effects in ballistic graphene ribbons. Eduard Bogachek, Igor Romanovsky, Uzi Landman A theoretical analysis of electrical and thermal transport in ballistic graphene strips and carbon bilayers, connected to electrodes, is presented. Gate voltage and temperature dependences of thermoelectric coefficients and thermoconductance for different ratios of strip lengths and widths are studied both in the linear and nonlinear regime (finite applied voltage and temperature differences). Violation of the Onsager relation between the Peltier and thermopower coefficients in the nonlinear regime is considered. Results obtained for carbon bilayers are compared with those in single layers. The effect of transverse voltage applied between layers on the thermoelectric transport in carbon bilayers is investigated. [Preview Abstract] |
Session H30: Electronic Properties of Graphene and Related Structures II
Sponsoring Units: DCMPChair: Giovanni Cantele, Istituto Nazionale per la Fisica della Materia
Room: Morial Convention Center 222
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H30.00001: Electronic structure of graphene in the presence of disorder Alexander Kemper, Manoj Srivastava, Hai Ping Cheng Graphene, a single layer of the carbon structure graphite, has a number of interesting electronic properties. To aid in the understanding of these properties we have performed first-principles calculations of single graphene layers in the presence of disorder of various forms, including single and double vacancies, Stone-Wales defects, and metallic dopants. We report the effects of defects and dopants on the charge density and electronic density of states. Furthermore, we discuss energetics of these systems and defect-induced spin-states. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H30.00002: Density of states in graphene with charged impurities Ben Yu-Kuang Hu, E. H. Hwang, S. Das Sarma We discuss the density of states of graphene in the presence of charged screened impurity scattering. The density of states is obtained from the imaginary part of the single-particle Green's function, which is evaluated in the Born and the self-consistent Born approximations, and the screened Coulomb impurity potentials are evaluated within the random phase approximation. The density of states in the presence of impurities is typically larger than that of clean graphene at any given energy. In particular, the density of states at the Dirac point, which is zero in a clean sample, becomes non-zero, with a magnitude that is given by an expression akin to that for the BCS superconducting gap. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H30.00003: Topological Frustration in an Alkali-Graphene-Halogen System Youjian Tang, Vincent Crespi We theoretically studied a system with alkali and halogen adsorbed to opposing sides of a graphene sheet, their mutual interactions then being modulated and constrained by the interposing presence of the sheet. Charge transfer from alkali to halogen generates a substantial dipole moment and large local electric field. Trends with respect to electron affinity, ionization energy, areal density, and the character of the bounding layer (i.e. BN versus graphene) will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H30.00004: Energy gaps and Stark effects in boron nitride nanoribbons Cheol-Hwan Park, Steven G. Louie Graphene nanoribbons, which have been recently synthesized, are regarded as promising candidate materials for nanoscale electronics. It is expected that boron nitride nanoribbons may be produced in a similar way. Notwithstanding their structural similarity, the electronic properties of boron nitride nanoribbons are qualitatively different from those of graphene nanoribbons. Here, we present first-principles calculations of the electronic properties of boron nitride nanoribbons with widths up to 10 nm both without any external potential or under a transverse electric field. The results show a rich set of behaviors and promise for possible applications of boron nitride nanoribbons in nanoscale electronics. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H30.00005: Band Engineering in C/BN Super-stripes Jeffrey Mullen, Marco Buongiorno-Nardelli Using electronic structure calculations from first principles, we have studied the electronic characteristics of graphene/BN sheets in a planar ``super-striped'' geometry. Similarly to Hydrogen-terminated graphene nanoribbons, also C/BN super-stripes show a variation of band gaps associated with the stripe size. Moreover, the bonding with BN introduces confinement effects that can be potentially exploited to enhance the electronic transport properties of these systems. We have characterized these effects by evaluating the band offsets and the electrostatic potential profile across the super-stripe structures. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H30.00006: Selective nesting and sensing of molecules on optimally modified material surfaces John Russell, Boyang Wang, Petr Kral We develop a methodology of optimal modifications of material surfaces allowing us to design selective nesting sites for inorganic, organic and biological molecules [1]. The idea is to modify material surfaces by atomic dopants and charged ligands in such a way that the created local electric fields form selective Coulombic traps for the adsorbed molecules. We demonstrate this methodology by molecular dynamics simulations of short peptides docked in nesting sites designed on graphene sheets substitutionally doped with B and N atoms. We show that the same approach can be used to selectively dock proteins in water solvent on graphene layers modified by short charged ligands. As a practical application of this methodology, we design and model chemical sensors that could detect the selectively nested molecules. The detection is realized by evaluating the change of the electrical conductivity of the modified graphene sheets upon the docking of the molecules. [1] B. Wang and P. Kral, Small, 3, 580, (2007). [2] J. Russell, B. Wang and P. Kral, in preparation. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H30.00007: Anisotropic Electron-Phonon Coupling on Graphene-Derived Fermi Surface in CaC$_{6}$ Tonica Valla, J. Camacho, Z-H. Pan, A.V. Fedorov, A.C. Walters, C.A. Howard, M. Ellerby Superconductivity in graphite intercalated compounds had been studied for more than 40 years and it is still not fully understood, despite the recent progress and the discovery of relatively high Tc superconductivity in CaC$_{6}$ and YbC$_{6}$. Initially, even unconventional mechanisms, such as excitonic and plasmonic pairing were considered, but recent studies now suggest that superconductivity in graphite intercalated compounds is more conventional and that the electron-phonon coupling is responsible for pairing. However, it is still not clear whether the graphene-derived electronic states and vibrations or the intercalant-derived ones play more important role, or if some particular combination of graphene-intercalant states and vibrations dominates the coupling. Here, we present angle-resolved photoemission studies of electronic structure in CaC$_{6}$. We find that the electron-phonon coupling on the graphene-derived Fermi surface is very strong and anisotropic, reflecting the interaction of graphene-derived states with high-frequency graphene-derived vibrations. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H30.00008: Fractional statistics of topological defects in graphene and related structures Babak Seradjeh, Marcel Franz We show that fractional charges bound to topological defects in the recently proposed time-reversal-invariant models of spinless fermions on the honeycomb lattice with Kekule distortion and on the $\pi$-flux square lattice with Peierls distortion obey fractional statistics. The continuum effective low-energy description is given in terms of a `doubled' level-2 Chern-Simons field theory, which is parity and time-reversal invariant and implies two species of semions (particles with statistical angle $\pm\pi/2$) labeled by a new emergent quantum number that we identify as the fermion axial charge. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H30.00009: Irrational vs. rational charge and statistics in graphene-like system Chang-Yu Hou, Claudio Chamon, Roman Jackiw, Christopher Mudry, So-Young Pi, Andreas P. Schnyder Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. We will show that quasiparticle excitations with irrational charge and irrational exchange statistics exist in tight-biding systems, such as graphene-like structure, described, in the continuum approximation, by the Dirac equation in (2+1)-dimensional space and time. These excitations can be deconfined at zero temperature, but when they are, the charge re-rationalizes to the value 1/2 and the exchange statistics to that of ``quartons'' (half-semions). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H30.00010: Internal mobility edge in doped graphene: frustration in a renormalized lattice Gerardo Naumis We show that an internal localization mobility edge can appear around the Fermi energy in graphene by introducing impurities in the split-band regimen, or by producing vacancies in the lattice. The edge appears at the center of the spectrum and not at the band edges, in contrast with the usual picture of localization. Such result is explained by showing that the bipartite nature of lattice allows to renormalize the Hamiltonian, and the internal edge appears because of frustration effects in the renormalized lattice [1]. The size in energy of the spectral region with localized states is similar in value to that observed in narrow gap semiconductors. [1] G.G. Naumis, Phys. Rev. B 76, 153403 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H30.00011: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H30.00012: Possible bandgap opening in graphene due to deposited Au nanoparticles: First-principles calculations Ronaldo J.C. Batista, Sabrina S. Carara, Helio Chacham We perform first-principles calculations to investigate electronic and structural properties of graphene with a layer of deposited Au nanoparticles. We consider Au$_{38}$ nanoparticles that can be either covered with methylthiol molecules, or not. We also consider that the nanoparticles are arranged in a hexagonal lattice, and we focus on the effect of net charge, applied electric fields, and molecular coverage on the electronic structure of the graphene+nanoparticles system. We find that covered nanoparticles interact weakly with graphene, and that the main effect of the nanoparticles on the electronic structure of graphene is a doping effect that can be modified with the application of an electric field perpendicular to the graphene plane. The system is always metallic, without the opening of a bandgap, even if the nanoparticles are charged: neither the Coulomb potential nor the weak nanoparticle-graphene interaction is able to break the graphene sublattice symmetry. In contrast, in the case of deposited bare (non-covered) Au nanoparticles, there is a relatively strong interaction between low-coordinated Au atoms and graphene carbon atoms beneath. This leads to a symmetry breaking of the graphene sublattices and to the opening of a small bandgap of a few tens of meV. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H30.00013: Nonequilibrium-induced metal-superconductor quantum phase transition in graphene So Takei, Yong Baek Kim We study the effects of dissipation and time-independent nonequilibrium drive on an open superconducting graphite monolayer, or graphene. In particular, we investigate how dissipation and nonequilibrium effects modify the semi-metal-BCS quantum phase transition that occurs at half-filling in equilibrium graphene with attractive interactions. Our system consists of a graphene sheet sandwiched by two semi-infinite three-dimensional Fermi liquid reservoirs, which act both as a particle pump/sink and a source of decoherence. A steady-state charge current is established in the system by equilibrating the two reservoirs at different, but constant, chemical potentials.The nonequilibrium BCS superconductivity in graphene is formulated using the Keldysh path integral formalism, and we obtain generalized gap and number density equations valid for both zero and finite voltages. The behaviour of the gap is discussed as a function of both attractive interaction strength and filling for various graphene-reservoir couplings and voltages. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H30.00014: Detecting single graphene layer by using fluorescence from high-speed Ar$^{7+}$ ion Yoshiyuki Miyamoto, Hong Zhang A highly-charged-ion interacting with graphite causes structural change in nano-scales [1]. While when the ion's kinetic energy reaches few MeVs, the induced is not the structural change but electronic excitation. An experiment [2] showed fluorescence from Ar$^{7+}$ ions penetrating through carbon foil with kinetic energy of 2 MeV. Motivated by this experiment, we tested interaction between an Ar$^{7+}$ ion and a graphene sheet by the time-dependent density functional approach, and found that the electronic excitation in the Ar$^ {7+}$ ion is also the case even when the incident kinetic energy is 500 KeV and the target thickness is only mono-atomic layer. This simulation suggests the possibility of detecting a suspended mono-atomic layer of graphene [3] by monitoring fluorescence from the penetrated Ar$^{7+}$ ions. We will discuss its importance for analyzing bombardment of solids by highly charged, high-speed ions and possible experiments according to the present result. References: [1] T. Meguro, et al., Appl. Phys. Lett {\bf 79}, 3866 (2001). [2] S. Bashkin, H. Oona, E. Veje, Phys, Rev. A{\bf25}, 417 (1982). [3] J. Mayer et al., Nature (London), {\bf 446}, 60 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H30.00015: Charge response function and a novel plasmon mode in graphene Eugene Mishchenko, Abdel-Khalek Farid, Suhas Gangadharaiah Polarizability of non-interacting 2D Dirac electrons has a inverse square root singularity at the boundary of electron-hole excitations. The screening of this singularity by long-range electron-electron interactions is usually treated within the random phase approximation. The latter is exact only in the limit of N->infinity, where N is the ``color'' degeneracy. We find that the ladder-type vertex corrections become crucial close to the threshold. The strong singularity in the ladder series arises due to the long range interaction between electrons which move almost collinearly to the external momentum. The series is summed up analytically yielding a non- perturbative result: the density and spin response functions acquire non-zero imaginary part in an additional frequency range $ q u < \omega < q v$. The reversal of the sign of the electron polarizability in this new domain gives rise to a sharp plasmonic mode which is absent in the conventional RPA. [Preview Abstract] |
Session H31: Focus Session: Computational Nanoscience III: Ferroelectrics, Surfaces, and Water
Sponsoring Units: DMP DCOMPChair: Eric Schwegler, Lawrence Livermore National Laboratory
Room: Morial Convention Center 223
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H31.00001: Properties of Ferroelectric Nanostructures Invited Speaker: Ferroelectric nanostructures (FENs) such as thin films, nanowires and nanodots are receiving a lot of attention due to their potential for technological applications and to the rich variety of underlying physics. Interestingly, properties of FENs can substantially deviate from their bulk counterpart due to their sensitivity to many factors. Examples of such factors are the electrical boundary conditions (associated with the full, partial or non-existent screening of polarization-induced surface charges) and mechanical boundary conditions (arising from the lattice mismatch between the FEN and its substrate). \newline Here, we developed and used computational schemes to predict many properties in various FENs, as well as, to provide atomistic insight to their complex phenomena. In particular, we will show the striking following features and reveal their origins: \begin{itemize} \item The interplay between electrical boundary conditions, mechanical boundary conditions and growth direction results in the appearance of novel dipole patterns and new low-symmetry phases possessing superior dielectric properties in ferroelectric dots, wires and films [1,2]. \item FENs can exhibit dielectric anomalies, such as a {\it negative} dielectric susceptibility [3]. \item Nanobubbles can form in ferroelectric films under an external electric field [4]. \item An homogeneous electric field can be used to control the chirality of vortex structures in asymmetric ferroelectric dots, via the creation of original intermediate states [5]. \end{itemize} [1] I. Ponomareva {\it et al.}, Phys. Rev. B {\textbf 72}, 214118 (2005). \newline [2] I. Ponomareva and L. Bellaiche, Phys. Rev. B {\textbf 74}, 064102 (2006). \newline [3] I. Ponomareva {\it et al.}, to be published in Phys. Rev. Lett. (2007). \newline [4] B.-K. Lai {\it et al.}, Phys. Rev. Lett. \textbf{96}, 137602 (2006). \newline [5] S. Prosandeev {\it et al.}, submitted (2007). \newline These works have been done in collaboration with L. Bellaiche, I. Kornev, B.-K. Lai, I.I. Naumov, R. Resta and S. Prosandeev. Some computations were made possible thanks to the MRI Grants 0421099 and 0722625 from NSF. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H31.00002: Internal electric field effects in ferroelectric nanostructures Byounghak Lee, Zhengji Zhao, Lin-Wang Wang A ground state dipole moment of ferroelectric nanostructures is a long-standing problem. A permanent dipole moment can alter optical and electronic properties of nanostructures, causing state localizations and electron hole separations. A dipole moment in the bulk can have a self-screening effect. In a nanostructure, the different shape of the system can result in an effective screening different from the bulk. In ferroelectric nanostructures, this can lead to a shape dependent total dipole moment, different from the volumetric result derived from its bulk value. Direct ab initio theoretical study on the nanostructure dipole moment is scarce due to the large computational cost. We present a first principles study of this problem using a recently developed Linear Scaling 3-Dimensional Fragment method. We investigated shape and size dependence of the surface and bulk contributions of the electric dipole moment in various ferroelectric nanorods. We compared our results with experiments and provided an insightful physical picture by analyzing the ab initio numerical results with a classical dielectric model. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H31.00003: Nanowettability by saline solutions in electric field Alenka Luzar, C.D. Daub, D. Bratko Molecular simulations of nanosized aqueous droplets and films next to apolar surfaces show a remarkable sensitivity of water contact angles on the applied electric field polarity and direction relative to the liquid/solid interface. We explain the effect by analyzing the influence of the field on interfacial hydrogen bonding which in turn affects the interfacial tensions. When electric field is applied on the aqueous film in the direction perpendicular to the confining hydrophobic surfaces, the competition between field-induced alignment and orientational preference of interfacial water molecules relative to the surfaces results in asymmetric wettability of opposing surfaces (Janus interface). The observed anisotropy in droplet or film wetting is a new nanoscale phenomenon that has so far been elusive as, in current experimental setups, surface molecules represent a very low fraction of the total number of molecules, affected by the field. We discuss amplification of these effects in saline solutions. The work gives basic understanding of field charge effects that can modulate local hydrophilicity of engineered and biological interfaces, as well as surface manipulation in nanofluidic devices. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H31.00004: Electric Field Control of Structure, Dimensionality and Reactivity of Gold Nanoclusters Supported on Thin Films of MgO/Ag(100) Bokwon Yoon, Uzi Landman External electric field control and manipulations of the structural stability, dimensionality, and chemical reactivity of gold nanoclusters deposited on MgO films grown on an Ag(100) substrate, are introduced and illustrated with the use of first-principles electronic structure calculations. Field-controlled interfacial charging and field-induced structural dimensionality crossover are predicted; These structural changes are accompanied by variations of the chemical reactivity of the adsorbed gold nanostructures. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H31.00005: Adsorption of water on anatase TiO$_2$ nanoparticles: the role of a wire of undercoordinated Ti sites. M. Posternak, A. Baldereschi, B. Delley Presence of hydroxyl groups is known to be an important element for the initial integration of titania--coated implants in natural tissues. The existence of OH$^-$ radicals due to water dissociation is indeed responsible for major changes in their surface reactivity, and depends in particular on the preparation of the surface. The stable phase for nanoparticle--sized microstructures is anatase, and crystallites are expected to expose different surface terminations and edges. In this work, we study water adsorption on a model system, consisting of an edge along the $[1\,1\,\overline{1}]$ direction, at the intersection of two major (101) surfaces. Using the DMol$^3$ approach\footnote{B. Delley, J. Chem. Phys. \textbf{113}, 7756 (2000).} within DFT, we find that water dissociative adsorption occurs at the four--fold coordinated Ti atoms present on this edge, in contrast with the case of the unreactive (101) surface. These results provide evidence of an increased chemical activity of such binding sites for adsorption of atoms and molecules. The mechanisms of the hydroxylation process are discussed in terms of the geometric arrangement, coordination numbers, and acidic/basic character of the relevant Ti and O sites. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H31.00006: Characterization of Anatase Nanoparticles Using Tight Bonding DFT Simulation Hong Wang, Ning Ma, Hao Wang, James P. Lewis The structure and electronic properties of anatase nanoparticles with size ranging from 1.5nm to 2.4 nm have been presented in this paper under ab initio Density Functional Theory computational method. Based on the relaxed structures obtained in our calculation, we propose that the portion of surface in the whole structure effects the geometric configuration of anatase nanoparticles. However, as long as the nanoparticles grow bigger, the affection decreases obviously. The analysis of the frontier orbitals of the nanoparticles in our work make us believe that the frontier orbitals (so called HOMOs) are mostly localized on some corner positions of the whole structure. These corner positions are constisted of 4-coordinated Ti atom lack of bridge oxygen atoms. When we adsorb water molecule on different positions on the facets of the smallest particle (TiO$_{2})_{48}$, it turns out the corner position where the frontier orbitals are mostly localized are most energetic favorable adsorption position for water molecule. These special corner positions will likely act as high attractive spots for the external molecules existing in the nanoparticles' environment. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H31.00007: Single oxide overlayers grown on top of another oxide: Their stability, interaction with metal nanoparticles, and contribution to catalytic activity Sergey Rashkeev Oxides are widely used as catalysts as well as supports for catalytically active metal nanoparticles. The catalytic activity of the system depends on many different factors such as anchoring, sintering, decomposition, and diffusion of metal atoms/clusters on the oxide surface. Here we use a combination of first-principles density-functional calculations and molecular dynamics simulations to investigate how all of these factors may change when the surface of the oxide support is modified by an additional single overlayer of another oxide positioned on it. In particular, we found that deposited monolayer oxide films may show instabilities that result in formation of strong anchoring sites for metal atoms/clusters. Also, an atomic-scale roughness introduced in such a way may slow down the surface diffusion processes and inhibit nanocluster growth/sintering. For example, a single layer of SiO2 on a TiO2 substrate may significantly increase the stability of Au nanoparticles and the efficiency of the catalytic CO oxidation. The author thanks Dr. Steven Overbury (Oak Ridge National Laboratory) for attracting his attention to this problem and INL Laboratory Directed Research and Development program and the U. S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07-051D14517 for financial support. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H31.00008: Role of hydrogen on catalytic reduction of nitric oxide on selected transition metal surfaces Faisal Mehmood, Anand U. Nilekar, Manos Mavrikakis Self-consistent periodic DFT-GGA calculations are used to investigate the NO reduction reaction in presence of atomic H on seven close-packed transition metal surfaces namely, Cu(111), Ag(111), Pd(111), Pt(111), Rh(111), Ir(111), and Ru(0001). The chemisorption of atomic (N, O, H) and molecular (NO, NH, OH, HNO, NOH, HNOH, H$_{2}$NO) reaction intermediates has been systematically studied on each metal surface and the preferred sites and binding energies are determined for a 1/4 ML surface coverage. The activation energy barriers for the relevant set of reactions have also been calculated. Based on these results, the potential energy surfaces (PESs) for direct and H-assisted NO reduction reaction on all the metal surfaces have been constructed. These PESs are used for elucidating the trends for various elementary steps involved in the NO reduction reaction, across the periodic table. These PESs indicate that on (111) surfaces of Cu, Ag, Pd and Pt, H assisted NO reduction is quite favorable by either NOH or HNO reaction intermediate. For remaining three close-packed metal surfaces, i.e. Rh, Ir and Ru we do not find significant change due to presence of H. We also find that the presence of extra H may contribute in formation of HNOH or H$_{2}$NO on the surface that cause barrier to reduce even further. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H31.00009: First-principles study of surface stresses induced by target-receptor interactions on a cantilever sensor Varadharajan Srinivasan, Giancarlo Cicero, Jeffrey C. Grossman Nanoscale cantilevers have shown great promise as ultrasensitive, low-power chemical sensors based on the surface stresses induced by interactions between the target species and the receptor coating layer. However, the basic mechanism of these induced stresses is yet to be fully understood, and it is therefore of great fundamental and practical interest to elucidate their electronic and structural origins via the weak interactions that lead to cantilever deflection. An example of such a device is the Au-SiN$_{x}$ cantilever sensor using functionalized long-chain alkanethiols as a coating layer. Even though the target-receptor interactions are often weak, the induced stresses are quite sensitive to the chemistry of the interacting species. Taking water molecules as a model target and $\omega $-hydroxy and $\omega $-carboxy alkanethiols as the receptor layer on a Au(111) surface, we use first-principles surface stress calculations to provide a detailed atomistic-level understanding of the various contributions leading to the deflection of a cantilever. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H31.00010: Atomic motion and electronic structure of alkanethiol monolayer covered gold surfaces Sabri Alkis, Hai-Ping Cheng, Jeffrey Krause Self-assembled alkanethiol monolayers are subjects of great interest because of potential applications in future nano-electronics. In this talk, we report our recent studies of the motions of Au atoms on alkanethiol monolayers using molecular dynamics in conjunction with first-principles calculations. Guided by accurate quantum mechanical calculations, we have calibrated the interactions between Au atoms and monolayers for classical simulations. We then investigate the motions of Au atoms as a function of coverage and temperature. Simulations with improved potential parameters show a good agreement with experimental observations. In addition, we discuss the electronic structure and charge transfer at the interface between the molecular monolayer and gold (111) surfaces. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H31.00011: Comparison between GGA+U and LSDA-GGA on CuN/Cu(100) Pushpa Raghani, Chiung-Yuan Lin, Barbara Jones It has become possible to study atomic magnetism by the Scanning Tunneling Microscope. The spin of a magnetic atom is often screened when it is adsorbed on a metal surface. However, a CuN layer reduces this screening and the atomic spins become detectable by STM. We have applied DFT to calculate the charge transfer, binding energies of the CuN layer on a Cu(100) surface. Within DFT, we use two different techniques for exchange correlation interaction: GGA+U and LSDA-GGA. Then we will calculate the atomic spins for Mn on CuN/Cu(100) using these two techniques. We will compare and discuss the results. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H31.00012: Formation and reactions of hydronium species in silica I.G. Batyrev, L. Tsetseris, S.T. Pantelides Water-related impurities in silica-based glasses are known to affect the properties of the network and corresponding devices in many types of ways. Here, we use first-principles calculations to highlight the special role of one of these species, hydronium (H$_{3}$O). We elucidate the atomic-scale details of formation and reactions of H$_{3}$O molecules in amorphous SiO$_{2}$. We find that the attachment of a migrating H$^{+}$ on a water molecule is an exothermic reaction with an energy gain of 0.4 eV and activation energy of only 0.6 eV. We present results on pertinent features in the vibrational spectra of silica that support the presence of H$_{3}$O, and we describe the role of H$_{3}$O as a passivant of oxide defects, like oxygen vacancies and non-bridging oxygen atoms, and the atomic-scale details of H$_{3}$O-mediated diffusion of H species in SiO$_{2}$. The results bear on the formation and dynamics of defects in electronic devices and the physical properties of hygroscopic silica-based glasses. This work was supported in part by the AFOSR and the US Navy. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H31.00013: Hydrogen Bonds and the Vibrational Modes of Water at Interfaces: ab-initio Molecular Dynamics meets Neutron Scattering Nitin Kumar, Sanghamitra Neogi, Paul Kent, Andrei Bandura, James Kubicki, David Wesolowski, Jorge Sofo We study the vibrational density of states (VDOS) of a thin water layer on the rutile (110) surface. The VDOS is obtained from the velocity-velocity autocorrelation function calculated from trajectories of large scale ab-initio molecular dynamics simulations. The rutile surface induces a shift to lower frequencies of the stretching modes with respect to pure water. The water vapor surface shows a peak at the vibrational frequency of free hydroxyls. Overall, the average stretching mode vibrational frequency increases with decreasing hydrogen bonding density. This density depends strongly on temperature. The water dissociation percentage at the surface can be correlated with the ratio between the weights of the stretching and the bending modes. Our results are in good agreement with inelastic neutron scattering measurements done on wet titania nanoparticles. [Preview Abstract] |
Session H32: Focus Session: Magnetic Imaging
Sponsoring Units: GMAG DMPChair: Sergei Urazhdin, West Virgina University
Room: Morial Convention Center 225
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H32.00001: Measuring Spin Dependent Hot Electron Transport Using Spin-Polarized Ballistic Electron Emission Microscopy Vincent LaBella, Andrew Stollenwerk, John Garramone, Evan Spadafora, Ilona Sitnitsky, Joseph Abel Spin polarized ballistic electron transport has been studied in Fe/Si(001) Schottky diodes using ballistic electron emission microscopy. Spin dependent scattering of polarized ballistic electrons injected from an Fe coated Au tip into the Fe films has been shown to affect the BEEM current. The spin dependent attenuation lengths were determined by measuring this effect with Fe thickness and found to be 1.8~$\pm$~0.2~nm for the minority spin electrons and 2.5~$\pm$~0.3~nm for the majority spin electrons at a tip bias of 1.5~eV. In addition, the attenuation lengths were measured as a function of tip bias, which indicated that the Fe/Si(001) interface band structure has an effect on the hot electron transport through the diode. Applications of the SP-BEEM technique to other systems will also be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H32.00002: Magnetic properties of single and bilayer MnGa on GaN(0001) investigated by spin-polarized STM Y. Qi, G. Sun, M. Weinert, L. Li We investigated the magnetic properties of ultrathin GaMn layers grown on GaN(0001) by spin-polarized scanning tunneling microscopy (SP-STM) using an Fe coated W tip. The GaN films are grown by plasma-assisted MBE on 6H-SiC(0001), and exhibit a metallic pseudo-1x1 surface structure, consisting of 2.3 ML Ga on top of the Ga-terminated GaN. At room temperature, Mn deposition on this surface resulted in the formation of GaMn islands, with the second layer islands begin to nucleate before the first layer is completed. With a Fe coated W tip, contrast between odd and even layers is observed, indicating layered antiferromagnetic magnetization of the GaMn layers. When Mn is deposited on the pseudo (1x1) at 200 C, a GaMn (3x3) structure is observed. First principles calculations show that Mn substitution of Ga leads to virtual bound states with bandwidth of 1.5 eV, indicating significant Mn-Ga interactions. As a result, top layer Ga atoms form covalent-like bonds. The Mn and the librated Ga atoms from the ``1x1'' form the (3x3) structure, with the adatom on the T$_{4}$ site. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H32.00003: Localized spectroscopic and topographic studies of heterostructures of OSE/M (OSE: organic semiconductor, M: metal) using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). C. R. Hughes, M. L. Teague, S. Mitrovic, N. C. Yeh We employ STM with AFM to study the charge transport and domain structures of OSE/M heterostructures fabricated under differing growth conditions [OSE: sublimated tris(8-hydroxyquinoline) aluminum (Alq$_{3})$, M: paramagnetic Au or ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO)]. Specifically, using STM in the point contact mode we are able to determine the work function of the heterostructures by measuring the differential conductance versus bias voltage. In addition, we can compare the Alq$_{3}$ resistivity variations for heterostructures prepared under different Alq$_{3}$ annealing conditions and with Au or LCMO as the metal. In contrast, using STM in the tunneling mode we can determine the ballistic charge transport length by varying the Alq$_{3}$ thicknesses in the OSE/M heterostructures. Moreover, conductance maps for biased voltages above the Alq$_{3}$ band-gap provide spatially resolved information for the local conductance channel and the surface quality of the Alq$_{3}$ film, the latter further compared with the surface morphology taken with AFM. This work was supported by NSF under the Center for Science and Engineering of Materials at Caltech. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H32.00004: Current-induced magnetization switching with a spin-polarized scanning tunneling microscope Invited Speaker: In present data storage applications magnetic nanostructures are switched by external magnetic fields. Due to their non-local character, however, cross-talk between adjacent nanomagnets may occur. An elegant method to circumvent this problem is magnetization switching by spin-polarized currents, as observed in GMR\,[1] as well as in TMR\,[2] studies. However, the layered structures of these devices do not provide any insight to the details of the spatial distribution of the switching processes. Spin-polarized scanning tunneling microscopy (SP-STM) is a well-established tool to reveal the magnetic structure of surfaces at spatial resolution down to the atomic scale. Besides, SP-STM takes advantage of a perfect TMR junction consisting of an isolating vacuum barrier separating two magnetic electrodes, which are represented by the foremost tip atom and the sample. Our experiments demonstrate that SP-STM serves as a tool to manipulate the switching behavior of uniaxial superparamagnetic nanoislands\,[3]. Furthermore, we show how SP-STM can be used to switch the magnetization of quasistable magnetic nanoislands at low temperature ($T=31$\,K). Besides its scientific relevance to investigate the details of current-induced magnetization switching (CIMS), this technique opens perspectives for future data storage technologies based on SP-STM. \par \noindent [1] J.~A.~Katine \textit{et al.}, Phys.~Rev.~Lett. \textbf{84}, 3149 (2000). \par [2] Y.~Liu \textit{et al.}, Appl.~Phys.~Lett. \textbf{82}, 2871 (2003). \par [3] S.~Krause \textit{et al.}, Science \textbf{317}, 1537 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H32.00005: Bloch line `crystallization' as intrinsic pinning mechanism in ferrimagnetic YIG films John Neal, Milorad Milosevic, Simon Bending, Irina Grigorieva, Alexander Grigorenko The present intense drive to develop current-switched magnetic storage media has lead to a renewed interest in ferrimagnetic garnet films which, for several decades, were the focus of devices exploiting manipulation of magnetic `bubbles'. In such uniaxial materials, the appearance of Bloch lines in structured domain walls strongly influences their \textit{dynamic} properties in an applied magnetic field. Here we show that the \textit{static} magnetic properties of garnet films can also be profoundly influenced due to \textit{crystallization} of Bloch lines into a square lattice along adjacent domain walls. This rigid lattice \textit{intrinsically} pins domain walls and suppresses the expected expansion/contraction of magnetic domains in an applied field. Even in the pinned regime, ultra-sensitive scanning Hall probe measurements reveal the \textit{nanoscale motion} of magnetic blocks in the walls comprising an integer number of Bloch-lines. Although the estimated displacements ($\sim $2-25 nm) are very much smaller than the domain period, we observe highly correlated motion across many domain walls, driven by the strongly interacting Bloch line lattice. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H32.00006: Evolution of magnetic domain reversal with temperature in Co/Pt multilayers observed by magneto-optical Kerr imaging X.P. Xie, X.W. Zhao, J.W. Knepper, F.Y. Yang, R. Sooryakumar The nucleation and evolution of magnetic domain structures with temperature and magnetic field in Co(4 {\AA})/Pt($t_{Pt})$ multilayers with perpendicular anisotropy have been studied by magneto-optical imaging techniques. Relatively large Pt layer thicknesses t$_{Pt}$ = 43 {\AA} and 63 {\AA} are chosen for this study because the interlayer coupling strength in the multilayers varies from weak at room temperature to strong at low temperature. Kerr imaging during the magnetization reversal processes shows the transformation of domain patterns with temperature, which correlates directly with enhancement of interlayer exchange coupling with decreasing temperature, as well as the conversion from domain- wall-propagation dominant reversal at room temperature to nucleation-dominant reversal at low temperatures. The enhanced interlayer coupling at low temperatures leads to the entire multilayer switching as a single ferromagnet; while at higher temperatures, when the interlayer coupling weakens, quasi-independent layer-by-layer magnetic reversal is observed. The transformation from propagation- to nucleation-dominant magnetic reversal can be understood by the competition between activation energies for domain nucleation and propagation, Zeeman energy and thermal energy. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H32.00007: Adding depth sensitivity to photoelectron microscopy using the standing wave/wedge method Florian Kronast, Alexander Keiser, Carsten Wiemann, Ruslan Ovsyannikov, Andrea Locatelli, Daniel Buergler, Reinert Schreiber, See-Hun Yang, Hermann Duerr, Claus Schneider, Charles Fadley Photoelectron microscopy (PEEM) is by now a well-established technique for studying many types of multilayer or multicomponent structure, including samples of relevance to spintronics, semiconductor technology, and polymer-based materials. The lateral resolution in such microscopes is typically 20 nm, but with the prospect of going down to ca. 1 nm in the near future. However, resolution perpendicular to the surface is not inherent in PEEM measurements, and we here discuss a novel method for providing this at sub-nm resolution, by exciting the photoelectrons with a standing wave created by soft x-ray reflection from a multilayer substrate, and growing one layer of the sample in a wedge form. This standing wave/wedge method has been demonstrated for the first time in measurements with a PEEM located at BESSY in Berlin. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H32.00008: Study of the correlation between structural and magnetic properties of MnAs/Si S. Hegde, J. Kwon, E. Fraser, H. Luo, D.H. Lee, C.R. Wie Ferromagnetic MnAs, has been widely studied because of its ferromagnetic properties and structural compatibility with conventional semiconductors. Magnetic properties of MnAs grown on Si(001) vary depending upon the growth conditions. To understand the variations, we carried out experiments using X-ray diffraction, atomic force microscopy (AFM) and magnetic force microscopy (MFM), together with magnetization measurements. For this study, MnAs was grown by molecular beam epitaxy (MBE) on Si(001) and Si(111) substrates. The surface structure of MnAs is correlated with the magnetic properties. For samples with no in-plane anisotropy, both AFM and X-ray diffraction measurements show the coexistence of MnAs with orthogonal orientations. The magnetic domains are very different from those observed in MnAs grown on GaAs (001). Significant differences in surface morphology are observed between MnAs layers grown on Si(001) and Si(111) because of the different orientations of MnAs . [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H32.00009: Magnetic domains in Nd$_{2}$Fe$_{14}$B on various length scales A. Kreyssig, R. Prozorov, C. Dewhurst, P.C. Canfield, R.W. McCallum, A.I. Goldman Detailed knowledge about the structure of ferromagnetic domains provides a link between microscopic physics and macroscopic magnetic response. In the well known, and already widely used compound, Nd$_{2}$Fe$_{14}$B, the dimension, shape and arrangement of magnetic domains are still in discussion due to lack of suitable methods to study magnetic domain structures in the bulk and due to the geometric complexity observed on the surface. Here, we demonstrate that domain patterns revealed by quantitative Kerr and Faraday microscopy, exist well below the surface as detected by small angle neutron scattering. At room temperature, the easy-axis magnetic anisotropy yields very complex structures of domains on various length scales. In contrast, the cone-like magnetic anisotropy below 135~K reduces the complexity of the domain arrangement to a more regular and anisotropic structure of much larger domains. As a consequence the bulk magnetization increases due to the significant volume reduction of the domain walls. -- The support by U.S. DOE (DE-AC02-07CH11358), DFG (SFB463) and the Alfred P. Sloan foundation is acknowledged. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H32.00010: Collective Dynamics and Slow Relaxation of Charge/Spin Density Wave domains in Antiferromagnetic Chromium Oleg Shpyrko, Eric Isaacs, Jonathan Logan, Hyekyung Kim, Martin Holt, Michael Sprung, Zhonghou Cai, Alec Sandy We present coherent x-ray diffraction and x-ray microscopy measurements of slow fluctuations and relaxation of charge- and spin-density wave domains in antiferromagnetic Chromium. Intensity fluctuations of the coherent x-ray speckle of incomensurate charge density wave satellite, combined with time-resolved x-ray microscopy measurements, reveal the collective nature of the uncharacteristically slow domain wall and phase defect fluctuation as well as non-equilibrium charge- and spin-density wavevector relaxation. The observed dynamics of pinned charge- and spin-density wave condensate in Chromium is similar to other examples of elastic media in presence of quenched disorder, ranging from dynamics of vortex lattices in disordered superconductors and sliding friction to snow avalanches and earthquakes. A particularly interesting analogy is dynamics of soft matter undergoing jamming transition, which show similar compressed exponential relaxation behavior. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H32.00011: Color properties, hydrogen bonding and magnetic interactions in (TBA)$_{3}$[Ni(NCS)$_{5}$] T. V. Brinzari, O. A. -I. Swader, J. L. Musfeldt, C. Tian, M. -H. Whangbo, J. A. Schlueter We investigated the optical and vibrational properties of (TBA)$_{3}$[Ni(NCS)$_{5}$] a pentacoordinate Ni compound, and compared the results with the more traditional hexacoordinate analog (TEA)$_{4}$[Ni(NCS)$_{6}$]. Based upon electronic structure calculations, color properties of this high spin complex can be understood in terms of the crystal field splitting of the d-orbitals and their strong hybridization with the ligands. Temperature dependent vibrational studies show an additional splitting and softening of some of the modes at low temperature, which indicates enhanced hydrogen bonding between sulfur centers and organic ligands at low temperature as well as weak structural phase transitions. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H32.00012: Ferrofluid Photonic Dipole Contours Michael Snyder, Jonathan Frederick Understanding magnetic fields is important to facilitate magnetic applications in diverse fields in industry, commerce, and space exploration to name a few. Large electromagnets can move heavy loads of metal. Magnetic materials attached to credit cards allow for fast, accurate business transactions. And the Earth's magnetic field gives us the colorful auroras observed near the north and south poles. Magnetic fields are not visible, and therefore often hard to understand or characterize. This investigation describes and demonstrates a novel technique for the visualization of magnetic fields. Two ferrofluid Hele-Shaw cells have been constructed to facilitate the imaging of magnetic field lines [1,2,3,4]. We deduce that magnetically induced photonic band gap arrays similar to electrostatic liquid crystal operation are responsible for the photographed images and seek to mathematically prove the images are of exact dipole nature. We also note by comparison that our photographs are very similar to solar magnetic Heliosphere photographs. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H32.00013: Magnetic Reversal Time in Open Long Range Systems Fausto Borgonovi, Luca Celardo, Bruno Goncalves, Luca Spadafora Topological phase space disconnection has been recently found to be a general phenomenon in isolated anisotropic spin systems. It sets a general framework to understand the emergence of ferromagnetism in finite magnetic systems starting from microscopic models without phenomenological on-site barriers. Here we study its relevance for finite systems with long range interacting potential in contact with a thermal bath. We show that, even in this case, the induced magnetic reversal time is exponentially large in the number of spins, thus determining {\it stable} (to any experimental observation time) ferromagnetic behavior. Moreover, the explicit temperature dependence of the magnetic reversal time obtained from the microcanonical results, is found to be in good agreement with numerical simulations. Also, a simple and suggestive expression, indicating the Topological Energy Threshold at which the disconnection occurs, as a real energy barrier for many body systems, is obtained analytically for low temperature. [Preview Abstract] |
Session H33: Focus Session: Optical Properties of Magnetic Semiconductors
Sponsoring Units: GMAG FIAP DMPChair: Jim Allen, University of California, Santa Barbara
Room: Morial Convention Center 224
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H33.00001: Full band structure calculations of optical spin injection in Si and CdSe Julien Rioux, Fred Nastos, John E. Sipe We present a theoretical study of optical electron spin injection (optical orientation) in the bulk semiconductors Si and CdSe from direct optical excitation with circularly polarized light.\footnote{F. Nastos, J. Rioux, M. Strimas-Mackey, B.S. Mendoza, and J.E. Sipe, Phys. Rev. B \textbf{76}, 205113 (2007).} To describe excitation at energies significantly above the band edge, we use full-zone band structures from pseudopotential calculations. For Si, we find that there can be up to 30\% spin polarization from direct transitions. The relatively low symmetry of wurtzite CdSe leads to an orientation dependent spin injection, which can be up to 100\% polarized at the band edge. Averaging over crystal orientation gives a 50\% spin polarization for band edge excitation. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H33.00002: Ultrafast optical injection of magnetization in non-magnetic semiconductors F. Nastos, R. Newson, H. M. van Driel, J. E. Sipe We discuss the optical injection of magnetization into a nonmagnetic semiconductor by absorption of circularly polarized light. A microscopic approach, based on Fermi's golden rule and $\mathbf{k}\cdot\mathbf{p}$ band models, is used to quantify the magnetization-injection rate in GaAs. We find that under usual experiment conditions, relevant to optical orientation, the magnetization-injection rate of holes is approximately 20 times larger than it is for electrons, which reflects the large hole magnetic moment. We then turn to the ultrafast excitation regime and explore the possibility that the injected magnetization can radiate a detectable THz field. Using a phenomenological approach for the magnetization relaxation dynamics, we predict that the THz field from optical orientation is at the limit of current THz detection technology. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H33.00003: Direct and non-demolition optical detection of pure spin currents in semiconductors R.-B. Liu, J. Wang, B.-F. Zhu We put forward a scheme of direct and non-demolition measurement of a pure spin current in a direct-gap semiconductor by a polarized light beam, which may be view as a ``photon spin current'' [1]. The effective coupling between the ``hoton spin current'' and the electron spin current is realized via the spin-orbit coupling in valence bands, but involves neither Rashba effect from structure inversion asymmetry nor Dresselhaus effect from bulk inversion asymmetry. Thus a pure spin current, though bearing no net magnetization, induces Voigt and Faraday rotation of a polarized light beam. For the pure spin current studied in Ref. [2], a light beam, if oblique instead of normal incident, would present Voigt and Faraday rotation in the order of 1 millionth rad in the center region of the sample where the spin current flows without spin accumulation. [1] J. Wang, B. F. Zhu, and R. B. Liu, cond-mat/0708.0881. [2] Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, Science 306, 1910 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H33.00004: Theory of Optical Manipulation of Electron Spins in Semiconductors Invited Speaker: |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H33.00005: Carrier Dynamics in Microdisk Photonic Molecules Felix M. Mendoza, Roberto C. Myers, Greg Calusine, Arthur C. Gossard, David D. Awschalom, Xia Li, B.J. Cooley, Nitin Samarth Semiconductor microcavities offer unique means of controlling light-matter interactions, which may be important in optical communications and for quantum information processing schemes. The cavities under study here are coupled microdisks that behave like ``photonic molecules'' (PMs) with bonding and antibonding states for the confined photon modes. We study different PM geometries consisting of laterally coupled GaAs/GaAlAs microdisks of both circular and elliptical shape. Steady-state photoluminescence measurements reveal bonding and antibonding modes with distinct polarization characteristics. Additionally, we present direct time-resolved spectroscopy of the carrier and spin dynamics in these structures. The combination of static and dynamic spectroscopies is used to explore the evolution of spin coherence in photonic molecule structures. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H33.00006: Ultrafast Coherent Precession of Interfacial Electron Spins in Fe/AlGaAs (001) Yichun Fan, Haibin Zhao, Gunter Lupke, Aubrey Hanbicki, C. Li, Berend Jonker Magnetic second harmonic generation (MSHG) is used to selectively study the spin dynamics of the interface magnetization for Fe films on AlGaAs(001) in both the dc and time-resolved domain. The interface spin precession is compared with the bulk Fe precession obtained by MOKE. We find: (a) the coherent precession of the interface magnetization is largely non-collinear to the bulk Fe electron spins even at the picosecond time scale; (b) higher frequency spin precession occurs at the interface than in the bulk; (c) the phase of the interface precession is opposite to that of the bulk at low fields; and (d) the interface and bulk precession exhibit different hysteretic behavior. Model calculations of the field dependence of precession frequencies show a large out-of-plane magnetic anisotropy and a large ratio of in plane uniaxial to cubic anisotropy at the interface, attributed to its tetragonal structure and Fe-As bonding. These may account for the significant reduction in exchange coupling between the interface and bulk magnetization. These effects are studied as a function of Fe film thickness and the corresponding evolution of the anisotropy, as well as for the Fe/Si(001) and Fe/Al2O3 interfaces. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H33.00007: Spin galvanic effect between subband of InGaAs/AlGaAs 2DEG Junfeng Dai, Xiaodong Cui We report the electric current induced by optically injected spin current on the vicinity of a crossbar shaped InGaAs/AlGaAs two dimensional electronic gas (2DEG) without external electric or magnetic field at room temperature. The electric current is either both in-ward or both out-ward flowing through the crossbar, while the total Hall voltage or current remains negligibly. The geometric size, temperature and external magnetic field dependence of the electric current have been examined and the corresponding mechanism has been discussed. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H33.00008: Manipulating nonlinear optical response from electron spins in a 2D electron gas via exciton injection Shannon O'Leary, Hailin Wang The well-known robustness of electron spin coherences in semiconductors has stimulated intense interest in the use of electron spins in semiconductors for spintronics, quantum information processing, and coherent nonlinear optics. Of special importance to these efforts is the understanding and the manipulation of nonlinear optical processes of electron spins. Here, we report experimental studies of coherent nonlinear optical processes of electron spins in a modulation-doped CdTe semiconductor quantum well. These studies elucidate the important roles of trions and excitons and the underlying manybody interactions in the nonlinear optical process. By exploiting a two-color three-pulse pump-control-probe technique, we demonstrate that nonlinear optical responses of electron spins can be effectively manipulated through the injection of an exciton population at an appropriate time. The manipulation of the nonlinear response takes place without electron spin rotation, providing a new approach for the control and applications of electron spins in semiconductors. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H33.00009: Rate equation modeling of semiconductor spin-polarized lasers and diodes Christian Gothgen, Athos Petrou, Igor Zutic Optically or electrically pumped spin-polarized carriers into semiconductor lasers can provide important advantages as compared to the conventional lasers in which the carriers are unpolarized. Motivated by recent experiments in spin-polarized lasers which demonstrate the feasibility of polarization modulation and threshold current reduction [1,2], we model these structures using rate equations. Our approach allows a direct comparison of the analytical and numerical results applied to the steady-state laser response. In the absence of material gain, our findings describe the behavior of spin-polarized diodes. We calculate the dependence of threshold reduction on the degree of pumped spin polarization and suggest how a change in the spin polarization could provide several useful device functionalities. \newline [1] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \newline [2] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H33.00010: Theory of spin-polarized semiconductor lasers Rafal Oszwaldowski, Christian Gothgen, Igor Zutic In semiconductor systems spin-polarized electrons couple to photons with definite angular momentum. This effect is the basis for numerous existing and proposed devices [1]. Quantum-well based Vertical Cavity Surface Emitting Lasers (VCSELs) take advantage of this phenomenon to produce circularly-polarized light by using either optical or electrical pumping [2]. We describe the VCSEL system employing Semiconductor Bloch Equations. We include the influence of spin-orbit coupling and the dependence of dipole matrix elements on carrier's wavevectors. We reduce this description to an effective four-level model, incorporating such effects as different spin lifetimes for electrons/holes and laser-cavity birefringence. Applying this approach to a spin-polarized system, we calculate the threshold current, the polarization of the emitted light and other relevant quantities. [1] I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007); J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H33.00011: Disorder and many-body effects in transport and optical conductivities of diluted magnetic semiconductors Fedir Kyrychenko, Carsten A. Ullrich The nature of itinerant carriers in diluted magnetic semiconductors like GaMnAs is a subject of intense current debate. The valence-band picture has been widely used, but recent experimental results suggest that the carriers reside in impurity bands. Theoretical results have not been fully conclusive. Most studies within the valence-band picture treat band structures in detail, while disorder and many-body effects are only treated with simple relaxation time and static screening models. We present a more complete theory for electron dynamics in DMSs, combining a multiband ${\bf k\cdot p}$ approach with first-principle descriptions of disorder and many- body effects through the memory function formalism and time- dependent density functional theory. This allows us to capture dynamic screening and collective electronic excitations of the itinerant carriers as well as correlations between impurities. We calculate transport properties and optical conductivities for GaMnAs and compare with experimental results. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H33.00012: Theory for Beats Observed in Optical Orientation Experiments on n-GaAs Nicholas Harmon, William Putikka, Robert Joynt At low temperatures in n-type semiconductors optical orientation experiments can create two types of spin states: itinerant states in the conduction band and localized states on the donor sites. These two states will in general have differnt gyromagnetic ratios, and will thus precess at different frequencies when the system is put into a magnetic field. Strong evidence for two types of spins would be to see beats in optical orientation experiments. We have developed a model in the context of coupled Bloch equations to describe this phenomenon. Unpublished data by Awschalom and Kikkawa on n-GaAs at a temperature of 5 K and in a magnetic field of 6 T show beats. We apply our model to this data and interpret it in terms of spins in a Landau level formed from the conduction band states and localized spins on the donor sites. [Preview Abstract] |
Session H35: Focus Session: Negative Index Materials III
Sponsoring Units: FIAPChair: Igor Smolyaninov, BAE Systems
Room: Morial Convention Center 227
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H35.00001: Plasmonic metamaterials with tuneable optical properties Invited Speaker: Negative refraction in metamaterials has recently attracted significant attention due to its possible numerous applications in high-resolution imaging and photolithography with the so-called ``perfect lenses,'' for electromagnetic shielding (invisibility cloak), optical signal manipulation, etc. Among various realizations of negative index materials, plasmonic nanostructures play a prominent role as they allow negative refraction properties to be engineered in the visible and near infrared spectral ranges. The coupling of light to plasmonic modes, that are collective electronic excitations in metallic nanostructures, provides the possibility to confine the electromagnetic field on the sub-wavelength scale and manipulate it with high precision to achieve the desired mode dispersion and, thus, reflection, absorption and transmission properties of the nanostructures. In this talk we will discuss various pathways to control dispersion of the electromagnetic waves in plasmonic metamaterials, including plasmon polaritonic crystals and plasmonic nanorod arrays, and the approaches to active tuneability of their optical properties using optical and electric control signals. Both approaches take advantage of the very high sensitivity of surface plasmon mode dispersion on the refractive index of the dielectric adjacent to metallic nanostructure. Hybridization of plasmonic nanostructures with molecular species exhibiting nonlinear optical response allows the development of metamaterials with high effective nonlinear susceptibility due to the electromagnetic field enhancement related to plasmonic excitations. Signal and control light are then coupled to plasmonic modes that strongly interact via nonlinearity introduced by the hybridization. Concurrently, the use of electro-optically active dielectrics incorporated into plasmonic nanostructures provides the route to control optical signals electronically. Plasmonic metamaterials with tuneable optical properties can be used to control negative refraction and electromagnetic field propagation in various applications in nanophotonics, optoelectronics and optical communications. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H35.00002: Tunable plasmonic negative index nanostructures and nanolenses in optical domain E.V. Ponizovskaya, A.M. Bratkovsky We have designed by means with the use of FDTD method a metamaterial, which is a stack of metallic films with periodic hole arrays separated by dielectric layers (usually called fishnet, FN) to have negative index at IR frequencies. Optical modulation of the effective refractive properties of a FN Ag/Si/Ag metamaterial structure in the near-IR range has been confirmed experimentally [1]. Pump excitation of the amorphous Si layer was found to be responsible for the observed modulation of the effective refractive index [1]. We discuss the use of gain material to compensate the losses. Arrays of metallic nanoparticles or holes support individual and collective plasmonic excitations that contribute to surface enhanced Raman scattering (SERS), anomalous transparency, negative index, and subwavelength resolution in various metamaterials [2]. Using the FDTD and a boundary integral method we design 2D plasmonic nanolenses with thousand-fold field enhancement factor that can be used for single-molecule SERS detection. [1] E. Kim, et al., Appl. Phys. Lett. 91, 173105 (2007) [2] E.V. Ponizovskaya, A.M.Bratkovsky, Appl.Phys. A 87, 161 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H35.00003: Novel acoustic surface plasmons on Cu(111) Karsten Pohl, Bogdan Diaconescu, Luca Vattuone, Mario Rocca The recent discovery of a fundamentally new sound-like plasmon on a bare metal surface of beryllium introduced a new research direction in the area of plasmonics [1]. While conventional surface plasmons are optical modes and have a finite excitation energy of a few eV, the novel acoustic mode can be excited with very low energies of a few meV. This allows, in principle, for a coupling with visible light for signal processing and advanced microscopies as well as new catalysts on metallic surfaces. In order to show that this novel excitation is a general phenomenon on closed-packed noble metal surfaces, as predicted by our theoretical collaborators [2], we have measured the dispersion of the acoustic surface plasmon on Cu(111) by electron energy-loss spectroscopy for a parallel momentum-transfer range from 0 to 0.15 1/{\AA}. We can report that the dispersion is indeed linear (acoustic) with a slope (sound velocity) in good agreement with theory [2], and energy values that extend up to 500 meV. We will discuss the lifetime (decay length) of acoustic surface plasmons.\newline [1] B. Diaconescu, K.Pohl, L. Vattuone, et al., Nature 448, 57 (2007).\newline [2] V.M. Silkin, J.M. Pitarke, et al. Phys. Rev. B 72, 115435 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H35.00004: Thickness dependent plasmon excitation and damping in metallic thin films Zhe Yuan, Shiwu Gao We present a theoretical study of collective plasmon excitation and lifetime in metallic thin films using a jellium model [1, 2]. The excitation spectra are calculated with linear response theory and time-dependent local density approximation. The plasmon energy dispersion follows qualitatively the classical electrodynamical model. For ultrathin films with a few atomic layers, the collective plasmon resonances evolve into single particle transitions at small momenta. The plasmon linewidth due to Landau damping is found to depend exponentially on the film thickness. Quantum oscillations are found in ultrathin films with a period that is about three times longer than the universal period $\lambda_F/2$ observed in many other quantities. This long period results from the dynamical Friedel oscillations in the collective excitation normal to the films. [1] Z. Yuan and S. Gao, Phys. Rev. B 73, 155411 (2006). [2] Z. Yuan and S. Gao, Surf. Sci. in press. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H35.00005: Surface Plasmon Polariton Amplification at Telecommunication Frequencies. Muralidhar Ambati, Sunghyun Nam, Dentcho Genov, Erick Ulin-Avila, Xiang Zhang Active plasmonics describes the interaction between an active medium and surface plasmons, and it offers a foundation for fundamental studies and an opportunity to expand surface plasmon based applications. In order to overcome the challenges posed by surface plasmons - primarily the metal losses - recent studies have focused extensively on surface plasmon amplification; however, there has been very limited headway from the experimental front. We present an experimental evidence of the amplification of long range surface plasmon polaritons (SPPs) by stimulated emission at telecom frequencies. We design SPP waveguides - thin gold metal strips - embedded in a gain medium, erbium doped phosphate glass. We confirm SPP amplification by showing an increase in the propagation length of surface plasmons in both pulsed and continuous modes. We present the design, fabrication and measurements of the gold SPP waveguides in erbium doped glass. Such structures will be suitable as integrated coupling devices as well as for the study of plasmon-exciton interface in cavity quantum electrodynamics. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H35.00006: Novel electromagnetic effective medium based on nanocoaxes Krzysztof Kempa, Xiwen Wang, Zhifeng Ren, Michael J. Naughton A thin film of an opaque material, pierced with an array of subwavelength coaxial nanowaveguides, decomposes an incident electromagnetic wave into spatially discrete wave components, propagates these components without frequency cut-off through the film, and reassembles them on the far side. The propagation of these wave components is fully controlled by the physical properties of the waveguides and their geometrical distribution in the array. This allows for an exceptional degree of control over the electromagnetic response of this effective medium, with numerous potential applications, including metamaterial functionality enabled in the visible frequency range. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H35.00007: Controlling surface plasmons and local field by two-dimensional arrays of metallic nano-bottles Hei Iu, Daniel H.C. Ong, Jones T.K. Wan, Jia Li In recent years, studies of surface plasmon polaritons (SPPs) have been intensive. It is of great interest to control SPPs with great precision and flexibility. In this talk, we present our recent work on SPPs manipulation by using two-dimensional arrays of bottle-shaped, metallic cavities. We propose that by tuning the geometry of such ``nano-bottle'' contained in a two-dimensional nano-scale array it is possible to control the resonance frequencies and near field patterns of different SPP modes. The dispersion relations are not sensitive to the sizes and depths of the nano-bottles, but depends strongly on the polarization In particular, by using different polarizations, it is observed that different types of SPPs, either propagating or localized, can be excited independently. Moreover, we attempt to control the local field by closing up the aperture of the nano-bottle. We have found that the local field slowly moves up from the bottom to the neck of bottle by increasing its depth. In addition, the field intensity can be fine-tuned by controlling the topology of the bottleneck, for example, a smaller and thinner neck leads to stronger field intensity. As a result, we believe these nano-bottle arrays are good candidates for making high sensitivity chemical and biological sensors. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H35.00008: Inverse Mapping Structures onto Transparency Kim F. Ferris, Xin Sun, Paul A. Whitney Composite materials have continued to make a number of improvements in physical properties (mechanical moduli), but lag behind in optical responses such as transparency. The hybrid nature of the composite material, particle and host matrix, divides light scattering issues into particle size regimes, where the particle size d $>>$ lambda and approximations such as anomalous dispersion have proven useful, and d$<<$ lambda where more exacting methods are necessary. The real-life difference between the `design' particle size and the practical particle size distribution often finds contributions to light scattering losses from both regimes. Using a ceramic-polymer composite as a case example, we have used black box optimization methods to examine the practical bounds for each regime and to assess design rules. These guidelines suggest limitations for particle morphologies and the optical properties of the component materials. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H35.00009: A method for determining refractive indices of epilayers of multi-structure active layer Gagik Shmavonyan The refractive indices of bulk and epitaxially grown materials differ. Besides, it is very difficult to experimentally determine and theoretically calculate the refractive indices of compound semiconductor epitaxial layers of quantum hetero-structures, especially the refractive indices of active layer of multilayer compound semiconductor devices. For that reason the precise determination of the refractive indices of epilayers is actual. A new method for the determination of the refractive indices of epitaxially grown compound semiconductor materials of multilayer heterostructure is suggested. This method is a combination of experimental measurements and theoretical calculations. The equipment for the fulfillment of the suggested method is also elaborated. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H35.00010: Electromagnetic field distributions of perfect and imperfect cloaks Tai Hang Fung, Lai Lai Leung, Kin Wah Yu In this work, based on the ideal cloaking model proposed by Pendry et al. [1], we calculate, by first-principles approach, the electric field distributions of ideal cloaks (both spherical and cylindrical) subject to different external electric field sources. The results show that the external electric field of an ideal cloak remains unperturbed and the field inside the cloaking region vanishes, thus verifying that Pendry et al.'s model is indeed perfect. We then extend the investigation to imperfect cloaks, whose permittivity tensors can be perturbed due to dispersion or loss, by solving the appropriate boundary-value problems [2]. The resultant electric field distributions can become nonzero inside the cloaking regions as expected. We further evaluate the visibility of the imperfect cloak against the perturbation. We find small visibility under appropriate conditions. \newline [1] J. B. Pendry, D. Schurig, and D. R. Smith, Science 312, 1780 (2006). \newline [2] L. Dong, J. P. Huang, K. W. Yu and G. Q. Gu, Eur. Phys. J. B 48, 439 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H35.00011: Design of cloaking metamaterials using spectral representation theory Lai Lai Leung, Tai Hang Fung, Kin Wah Yu Controlling the propagation of electromagnetic (EM) waves, for instance in cloaking problem, has become an important topic in nanophotonics. So far, following the cloaking model proposed by Pendry et al. [1], the experimental realization was only limited to the microwave region [2]. Since practical application lies in the visible range, we have extended the investigation to that region by utilizing nanocomposites with reference to the material parameters proposed by Pendry et al. and Shalaev et al. [3]. The calculations can be made much simpler by invoking the spectral representation theory [4]. The loss and dispersion effects, as well as the propagation of EM waves are assessed for the designed cloaking models in order to investigate the cloaking performance. Further analyses show that our models can accomplish the desired cloaking effect in the visible range. Moreover, the loss and dispersion effects are found to be small and acceptable.\newline [1] J. B. Pendry, D. Schurig, and D. R. Smith, Science 312, 1780 (2006). \newline [2] D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, D. R. Smith Science 314, 5801 (2006). \newline [3] Wenshan Cai, Uday K. Chettiar, Alexander V. Kildishev and Vladimir M. Shalaev, Nature photonics 1 (2007). \newline [4] L. Dong, Mikko Karttunen, K. W. Yu, Phys. Rev. E 72, 016613 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H35.00012: Nonlinear Cerenkov radiation in a two-dimensional nonlinear photonic crystal waveguide. S.N. Zhu, Y. Zhang, Z. Yan, Z. Qi, G. Zhao We present a new type of quasi-phase-matched Cerenkov radiation generation from a two- dimensional nonlinear photonic crystal waveguide: a hexagonally poled LiTaO3 waveguide. The waveguide was fabricated by field poling followed by proton exchange technique. The fundamental source was a LD-pumped, 90-ns pulsed Q-switch double wavelength Nd:YAG laser at 1064-$\mu $m and 1319-$\mu $m. The pulse repetition rates was 8-kHz. When the fundamental beams at 1064-$\mu $m and 1319-$\mu $m were collinearly focused into the waveguide and propagated along its x-axis, multiple radiation spots at red, yellow, green with different propagation directions and radiation angles are simultaneously exhibited from such a hexagonally poled waveguide. Scattering involved erenkov arc is also observed. These frequency conversion processes were realized by guided-to-radiated mode interaction. Phase-matching for these processes in the waveguide was automatically achieved by a quasi-phase-matched Cerenkov configuration. [Preview Abstract] |
Session H36: Focus Session: Advances in Scanned Probe Microscopy II: Force Methods
Sponsoring Units: GIMSChair: Andreas Heinrich, IBM
Room: Morial Convention Center 228
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H36.00001: Multi-dimensional Scanning Probe Microscopy Invited Speaker: Since the first demonstration of true atomic resolution with a scanning force microscope in 1995 a large variety of samples have been imaged with atomic resolution. In 2000 Giessibl obtained atomic resolution with a (macroscopic) tuning fork operated with small oscillation amplitudes that correspond to the length of the inter-atomic interaction potentials. However, tuning fork sensors have a series of disadvantages that can all be overcome by optimized micro-fabricated cantilevers operated at similarly small or even smaller oscillation amplitudes. With their small mass and high mechanical quality factor, particularly after appropriate annealing in UHV force sensitivities that are orders of magnitude better than that of tuning forks and also far better than that of the best NEMS sensors is obtained. Recently Kawai et al and Sugimoto et al obtained atomic resolution with cantilevers with a operated in higher oscillation modes with sub-nanometer amplitudes. In order to make use of the excellent force sensitivity we developed a Fabry-Perot type interferometer optical sensor that maps flexural and torsional cantilever oscillations with fm/sqrt(Hz) deflection sensitivity. This deflection sensor allows the use of sub-{\AA} cantilever oscillation amplitudes and thus the direct measurement of atomic interaction force gradients and, -- in principle, the use of advanced tunnelling spectroscopy techniques that are well-established in the field of scanning tunnelling microscopy. The simultaneous mapping of flexural and torsional oscillation modes further allows the measurement of vertical and lateral tip-sample interaction forces and corresponding atomic scale energy loss processes. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H36.00002: A Low Temperature Scanning Force Microscope with a Vertical Cantilever and Interferometric Detection Scheme Jeehoon Kim, T.L. Williams, Sang Lin Chu, Hasan Korre, Max Chalfin, J.E. Hoffman We have developed a fiber-optic interferometry system with a vertical cantilever for scanning force microscopy. A lens, mounted on a Pan-type walker, was used to collect the interference signal in the cavity between the cantilever and the single mode fiber. ~This vertical geometry has several advantages: (1) it is directly sensitive to lateral forces; (2) low spring constant vertical cantilevers may allow increased force sensitivity by solving the ``snap-in'' problem that ~occurs with soft horizontal cantilevers. ~We have sharpened vertical cantilevers by focused ion beam (FIB), achieving a tip radius of 20 nm. ~We will show test results of a magnetic force microscope (MFM) with this vertical cantilever system. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H36.00003: New Interpretation of Non-contact Atomic Force Microscopy Images of Dihydride Si(001) Surface Based on Simulation Akira Masago, Satoshi Watanabe, Katsunori Tagami, Masaru Tsukada Hydride Si(001) surfaces have attracted attention as a substrate of organic semiconductor devices. In non-contact atomic force microscopy (NC-AFM) observation of dihydride Si(001) surface, 1x1 and 2x1 images were observed depending on the preset frequency shift value [1]. For both images, bright spots were assigned to hydrogen atoms. Recently, we have developed a simulator on the basis of the density-functional based tight-binding method, and have simulated NC-AFM image of the dihydride Si(001) surface. As a result, we obtained frequency shift images with the 1x1 and 2x1 periodicities, which agree well with the experiments. Surprisingly, we have found that the bright spots of the 2x1 image do not correspond to hydrogen atoms. Each spot corresponds to the bridge site, where the sum of the attractive forces from two nearby hydrogen atoms becomes large. \newline [1] S. Morita, Y. Sugawara, Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 4857. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H36.00004: Real-time detection and reduction of probe-loss in atomic force microscopy Pranav Agarwal, Tathagata De, Murti Salapaka In this presentation, a real-time methodology is developed to determine regions of dynamic atomic force microscopy based image where the cantilever fails to be an effective probe of the sample. Probe-loss is more pronounced during high speed imaging operations. A quantitative measure called reliability index is proposed as diagnostic measure for determining probe- loss. It is experimentally demonstrated that probe-loss affected portion of the image can be unambiguously identified by a signal termed the reliability index, that can be determined in real-time. The reliability index signal, apart from indicating the probe-loss affected regions, can be used to minimize such regions of the image. A PI controller with adjustable gains has been implemented on FPGA (Field programmable gate array), which uses reliability index signal to switch. It is experimentally demonstrated that by using such a scheme, probe-loss areas can be reduced by a factor of 4, suggesting a possible increase in imaging bandwidth by the same factor. Improvement in on-sample performance has also been observed. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H36.00005: Evaluation of sensitivity and selectivity of piezoresistive cantilever-array sensors Genki Yoshikawa, Hans-Peter Lang, Urs Staufer, Peter Vettiger, Toshio Sakurai, Christoph Gerber Microfabricated cantilever-array sensors have attracted much attention in recent years due to their real-time detection of low concentration of molecules. Since the piezoresistive cantilever-array sensors do not require a bulky and expensive optical read-out system, they possess many advantages compared with optical read-out cantilever-array sensors. They can be miniaturized and integrated into a match-box sized device. In this study, we present the piezoresistive cantilever-array sensor system and evaluate its sensitivity and selectivity using various vapors of molecules, including alkane molecules with different chain length from 5 (n-pentane) to 12 (n-dodecane). Piezoresistive cantilevers were coated with different polymers (PVP, PAAM, PEI, and PVA) using an inkjet spotter. Each cantilever has a reference cantilever, constituting a Wheatstone-bridge. Each vapor was mixed with a constant nitrogen gas flow and introduced into the measurement chamber. According to the principle component analysis of data obtained, each molecule can be clearly distinguished from others. We also confirmed that this piezoresistive cantilever-array sensor system has sub-ppm sensitivity. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H36.00006: Observation of locally excited ferromagnetic resonance via magnetic resonance force microscopy Evgueni Nazaretski, Denis Pelekhov, Ivar Martin, Peter C. Hammel, Roman Movshovich Magnetic resonance force microscopy spectra of a 50 nm thick permalloy film were measured as a function of the probe-sample distance and the angle between the film plane and the direction of the externally applied magnetic field. At larger angles the multiple resonance modes were observed at small probe-sample distances. Micromagnetic simulations which include the inhomogeneous magnetic field of the probe tip reveal the \textit{localized} nature of the exited resonance modes, opening a way to spatially resolved ferromagnetic resonance measurements in a continuous ferromagnetic media. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H36.00007: Characterization of the High Coercivity Magnetic Probe Tips for Magnetic Resonance Force Microscopy I.H. Lee, J. Kim, Yu Obukhov, P. Banerjee, D.V. Pelekhov, P.C. Hammel Magnetic Resonance Force Microscopy on ferromagnetic systems calls for high coercivity probe magnets needed for exciting localized Ferromagnetic Resonance (FMR) modes in the sample under investigations. We have characterized high coercivity Sm2Co17 MRFM probes fabricated by Focused Ion Beam (FIB) micro machining and mounted on a commercial Si cantilever (characteristic dimension 1$\mu$m x 1$\mu$m x 1$\mu$m). We report vibrating cantilever magnetometry measurements of probe coercivity. Low temperature (4 K) probe coercivity as high as 1 T has been observed. Probe characteristics have also been deduced from deconvolution of MFM data obtained on 5.3 4$\mu$m diameter permalloy dots. We also discuss energy dissipation in the micromechanical cantilever when the probe magnet approaches a ferromagnetic sample. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H36.00008: Fabrication Challenges in Producing Magnet-tipped Cantilevers for Magnetic Resonance Force Microscopy Steven A. Hickman, Sean R. Garner, Lee E. Harrell, Jeremy C. Ong, Seppe Kuehn, John A. Marohn Magnetic resonance force microscopy (MRFM) is a technique that may allow MR imaging of single molecules -- an extremely exciting prospect. To date we have demonstrated MRFM sensitivity of $\sim$10$^5$ proton spins. By making improved magnetic tips and increasing force sensitivity, it may be possible to achieve single-proton sensitivity necessary for molecular imaging. In MRFM the force exerted on the cantilever, per spin, is proportional to the field gradient from the cantilever's magnetic tip. Achieving single proton sensitivity thus requires dramatically reducing magnet size. We have developed an e-beam lithography process for batch fabricating nanoscale magnets on silicon cantilevers. With these sized magnets we will still require attonewton force sensitivity. Research by our group has shown that surface induced dissipation is a major noise source. We believe this can be minimized by producing magnets overhanging the cantilever end. As proof of concept, we will show a 50-nm overhanging cobalt magnet made by a process involving KOH etching, as well as preliminary work on making overhanging magnets by dry fabrication methods. Our current challenge appears to be preventing the formation of metal silicides. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H36.00009: Fabricating overhanging magnets for use in magnetic resonance force microscopy using a XeF2 isotropic etch. Sarah Wright, Steven Hickman, John Marohn Pushing magnetic resonance force microscopy towards single proton sensitivity demands meeting the nanofabrication challenge of producing an attonewton-sensitivity cantilever with a magnetic tip whose diameter is 50 nm or less. At the same time, the cantilever should also experience low force noise (and force gradient noise) near the surface of technologically interesting samples. Ideally then, the magnetic tip would overhang the leading edge of the cantilever -- to increase the signal created by the magnet while simultaneously minimizing the noise created by the rest of the cantilever interacting with the surface. We will show that the isotropic etchant XeF$_{2}$ can be used to underetch a single crystal silicon cantilever to create an overhanging magnet. This etch is a controllable etch process with high selectivity to metals that can be used not only to produce magnetic resonance force microscopy cantilevers, but other overhanging metallic structures as well. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H36.00010: Bimodal AFM imaging of individual protein molecules with sub-pico Newton force sensitivity. Nicolas F. Martinez, Shiva Patil, Jose R. Lozano, Ricardo Garcia The capability of atomic force microscopes (AFM) to generate atomic or nanoscale resolution images of surfaces has deeply transformed the study of materials. However, high resolution imaging of biological systems has proved more difficult than obtaining atomic resolution images of crystalline surfaces. In many cases, the forces exerted by the tip on the molecules (1-10 nN) either displace them laterally or break the noncovalent bonds that hold the biomolecules together. Here, we apply a force microscope concept based on the simultaneous excitation of the first two flexural modes of the cantilever (bimodal excitation). The coupling of the modes generated by the tip-molecule forces enables imaging under the application of forces (around 35 pN) which are smaller than those needed to break non-covalent bonds. With this instrument we have resolved the intramolecular structure of antibodies in monomer and pentameric forms. Furthermore, the instrument has a force sensitivity of 0.2 pN which enables the identification of compositional changes along the protein fragments. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H36.00011: Rheological Measurements by AFM of the Formation of Polymer Nanofibers Mehdi Yazdanpanah, Mahdi Hosseini, Santosh pabba, Scott Berry, Vladimir Dobrokhotov, Abdelilah Safir, Robert Keynton, Robert Cohn Polymer fiber can be formed by pulling a thread of polymeric liquid if the fiber solidifies before it breaks up by capillary thinning. Fiber diameter is well correlated with a processing parameter that is a simple function of viscosity, surface tension and evaporation rate. The fundamental material parameters can also be determined with the same AFM setup. The usual problem with tapered AFM tips, of liquids wetting unstably up the tapered AFM tip and even onto the cantilever, is resolved by the use of long cylindrical tips of constant diameter. We recently demonstrated a method of growing Ag-Ga nanowires onto AFM tips at room temperature. These constant diameter nanowires are shown to give clearly measurable force-distance curves when inserted through the surface of a liquid, which provides clean measurements of surface tension, contact angle, and evaporation rate, while shear viscosity is determined through cantilever Q-damping as a function of insertion distance into the liquid. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H36.00012: Micromechanical force detectors for measuring magnetization at high magnetic fields and the magnetic~ response of Ba3Cr2O8. K. Ninios, Y. J. Jo, L. Balicas, A. Aczel, G. M. Luke, H. B. Chan We report magnetization measurements of Ba3Cr2O8 using micromechanical faraday balance magnetometers. The magnetometers consist of a movable polysilicon plate (500 by 500 micrometers) supported~ by four springs 2.75 micrometers above a fixed electrode. When small samples of the magnetic material are placed at the center of the movable plate, the natural gradient of the field creates a force on the sample that changes the capacitance between the plate and electrode, while the response to magnetic torque is minimized. The absolute magnetization of the sample can be determined provided that the magnetic field gradient is known. The device is used to measure the magnetization of a small sample of Ba3Cr2O8 with mass of 1 microgram. At high fields, our measurements reveal an asymmetric dome like structure in the temperature-magnetic field phase diagram, possibly related to the Bose-Einstein condensation of spin triplet degrees of freedom. [Preview Abstract] |
Session H37: Optical Properties of Semiconductors (mostly oxides)
Sponsoring Units: FIAPChair: Matthias Schubert, University of Nebraska at Lincoln
Room: Morial Convention Center 229
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H37.00001: Density-functional theory study of the effects of atomic doping on the band edges of monoclinic WO$_{3}$ Muhammad N. Huda, Yanfa Yan, Su-Huai Wei, Mowafak M. Al-Jassim The effects of impurities in room temperature monoclinic WO$_{3}$ were studied using the local density approximation to density-functional theory. Our main focus is on nitrogen impurity in WO$_{3}$, where both substitutional and interstitial dopings were considered. We have also considered doping with transition-metal atoms and some co-doping approaches in WO$_{3}$. We find that, in general, band gap reduction was a common result due to the formation of impurity bands in the band gap. Also, the changes of band-edge positions, valence-band maxima and conduction-band minima, were found to depend on the electronic properties of the foreign atom and their concentration. Our results, therefore, provide guidance for making WO$_{3}$ a suitable candidate for photo-electrodes for hydrogen generation by water splitting. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H37.00002: Materials for Transparent Electronics: Ab initio calculation of wide bandgap semiconductor interfaces Skye Dorsett, Guenter Schneider Materials used in transparent electronics (TE) must be transparent in the visible portion of the electromagnetic spectrum which requires the use of wide bandgap semiconductors as contacts and rectifiers as well as passivation and barrier-shaping layers. Of particular importance are the source and drain contacts of transparent thin-film transistors (TTFT). The contact characteristics at the interface between the channel material (e.g. ZnO, SnO$_2$) and the contact material (commonly Indium Tin Oxide) are determined by the band offset which can be estimated from a heterojunction model based on material properties alone. The development of new materials for TE greatly benefits from estimates of interface properties but for most materials which hold promise for use in TE (e.g. indium gallium zinc oxide) the relevant material parameters such as work function, electron affinity and in particular the charge neutrality level are not known. To close this gap we report ab-initio density functional theory calculations of band offsets for wide bandgap semiconductors which are commonly used or hold promise for use in TTFT. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H37.00003: \textit{Ab initio} calculations of the dielectric functions of semiconductors including the electron-hole interactions via LASTO method HyeJung Kim, Yia-Chung Chang We calculate dielectric functions of semiconductors including the electron-hole interactions within the $\it ab$ $\it initio$ framework. The Bethe-Salpeter equation is constructed using a full-potential linear augmented-Slater-type orbital (LASTO) method [1, 2]. Using the LASTO method allows us to compute optical matrix element for large number of k points efficiently. Due to requirements of a dense k-point mesh, we use quasi-minimum residual (QMR) method to solve the equation. The inclusion of the electron-hole interactions both shifts the peak positions and changes peak heights of the imaginary part of the dielectric functions, resulting in better agreement with experimental data than the spectra obtained without including the electron-hole interactions. The calculated dielectric functions are compared to experimental data of ZnSe, CdSe, CdTe, InP, InAs, AlAs and GaN. The self-energy correctoins are described by an empirical tight-binding formula. [1] J. W. Davenport, Phys. Rev. B 29, 2896 (1984) [2] Y.-C. Chang, R.B.James, and J.W.Davenport, Phys. Rev. B 73, 035211 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H37.00004: Structural and optical properties of a transparent conductor oxide: Nb:In$_{2}$O$_{3}$ O. Lozano, P.V. Chinta, P.V. Wadekar, L.H. Chu, H.W. Seo, Q.Y. Chen, X.M. Wang, D. Wijesundera, L.W. Tu, N.J. Ho, W.K. Chu Thin films of niobium-doped indium oxide, Nb:In2O3, have been deposited on YSZ(001) and MgO(111) substrates by magnetron sputtering at 450\r{ }C. The transparent semiconducting films obtained on YSZ(001) were epitaxial, but when deposited under the same condition on MgO(111), the film qualities worsen upon Nb doping. The structural and optical properties in relation to the Nb content and the general growth conditions were studied by Rutherford backscattering, ion channeling, optical absorption spectroscopy, x-ray diffraction, and atomic force microscopy. The magneto-transport behaviors will also be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H37.00005: Nature of the bandgap in In$_{2}$O$_{3}$ revealed by first-principles calculations and X-ray spectroscopy Aron Walsh, Juarez L.F. Da Silva, Su-Huai Wei, Christoph K\"orber, Andreas Klein, L.F.J. Piper, Alex DeMasi, K.E. Smith, G. Panaccione, P. Torelli, D.J. Payne, A. Bourlange, R.G. Egdell The origin of weak absorption 1 eV below the onset of strong optical absorption in In2O3 has previously been attributed to the presence of an indirect fundamental bandgap or surface band bending. We demonstrate conclusively that this is not the case. Through the application of bulk and surface sensitive X-ray spectroscopic techniques, we reveal that the valence band edge is found much closer to the bottom of the conduction band than expected on the basis of the widely quoted bandgap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit small dispersion and the conduction band minimum is positioned at $\Gamma $; however, direct optical transitions give minimal dipole intensity until 0.8 eV below the valence band maximum. Our results set an upper limit on the fundamental bandgap of 2.9eV. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H37.00006: Raman scattering properties of $\mathrm{SnO}_x$ Ralf Meyer, Cedrik Meier, Axel Lorke Oxidic semiconductors like $\mathrm{ZnO}_x$ and $\mathrm{SnO}_x$ have recently attracted a lot of attention as possible optical materials for novel technological applications. Results from Raman scattering experiments at $\mathrm{SnO}_{1.5}$ nanoparticles are presented which show strong differences compared to the Raman spectra of bulk $\mathrm{SnO}_2$. In order to understand these differences, ab-initio calculations of the Raman scattering properties of bulk $\mathrm{SnO}_2$ and $\mathrm{SnO}_{1.5}$ have been performed. Raman spectra derived from these calculations compare qualitatively well with the experimental findings. From this, it is concluded that the differences in the experiments are an effect of the bulk materials. An analysis of the nature of the calculated Raman active vibrational modes makes it possible to draw further conclusions on the reasons behind the differences between the stochiometric $\mathrm{SnO}_2$ and the understochimetric $\mathrm{SnO}_{1.5}$. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H37.00007: Studies of electronic relaxation and coherent control in sensitized semiconductor surfaces. Victor Batista, Luis Rego This talk addresses the feasibility of using sequences of unitary pulses for coherent-control of quantum dynamical phenomena, including superexchange hole tunneling in sensitized TiO$_{2}$ surfaces and control of tunneling and decoherence in archetype model systems. The proposed dynamical decoupling scenario is based on the repetitive application of unitary pulses, affecting the interference phenomena between wave-packet components. The pulses affect the overall relaxation dynamics without collapsing the coherent-quantum evolution of the system. It is shown that both bound-to-bound state tunneling and bound-to-continuum tunneling processes can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not collapse the unitary evolution or affect the potential energy tunneling-barriers. The reported results are therefore particularly relevant to the understanding of coherent optical manipulation of electronic excitations in semiconductor devices where performance is limited by quantum tunneling and decoherence. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H37.00008: Electron-phonon interaction and charge carrier mass enhancement in electron doped alkali earth titanate semiconductors Dook van Mechelen, Dirk van der Marel, Claudio Grimaldi, Peter Armitage, Alexey Kuzmenko, Hans Hagemann, Nicolas Reyren, Rolf Lortz, Igor Mazin We have studied the electron-phonon coupling in electron doped SrTiO3 for which the carrier concentration ranges from a dilute gas of polarons to a polaron liquid. Here we report a comprehensive THz, infrared and optical study together with DC conductivity, Hall effect and specific heat measurements. Our THz spectra at 7 K show the presence of a very narrow ($<$ 2 meV) Drude peak, the spectral weight of which shows approximately a factor of three enhancement of the band mass for all carrier concentrations. The missing spectral weight is regained in a broad `mid-infrared' band which originates from electron-phonon coupling. Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, a $\sim$4. Specific heat measurements below 4 K show the mass enhancement to be about eight times the band mass for all carrier concentrations. The ostensible discrepancy with the optical mass is interpreted together with the temperature dependence of the Hall constant, the optical spectral weight and the dc scattering rate within the framework of a polaron liquid. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H37.00009: Non-resonant inelastic x-ray scattering spectra of lithiated titanium oxides for battery applications Kenneth Nagle, Mali Balasubramanian, Christopher Johnson, Gerald Seidler, Ilias Belharouak Although lithium-ion batteries now see widespread use, there remain considerable questions concerning the basic solid state chemistry of both electrodes. Improved understanding of the local electronic structure, particularly the mechanism of charge transfer upon insertion and removal of lithium, could lead to innovation in battery design and improved performance. We present non-resonant inelastic x-ray scattering (NRIXS) spectra from 2p initial states in titanium; these spectra are among the first recorded for such states in a transition metal. These spectra were obtained using the lower energy resolution inelastic x-ray scattering (LERIX) spectrometer, which is capable of making simultaneous measurements at nineteen values of momentum transfer. We demonstrate the ability to obtain soft x-ray absorption-like information using a bulk-sensitive, hard x-ray technique. In addition, at high momentum transfer NRIXS provides information about non-dipole transitions that are inaccessible by soft x-ray spectroscopic methods. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H37.00010: New green phosphor Ba$_{3}$Si$_{6}$O$_{12}$N$_{2}$:Eu for white LED: crystal structure and optical properties Masayoshi Mikami, Kyota Uheda, Satoshi Shimooka, Hiroyuki Imura, Naoto Kijima A new oxynitride, Ba$_{3}$Si$_{6}$O$_{12}$N$_{2}$, has been synthesized. The crystal structure has been successfully determined by close collaboration between experiment and first-principles band calculation based on density functional theory. This compound doped with Eu exhibits intense green photoluminescence with high color purity under near-ultraviolet to blue light excitation. It has much less thermal quenching than other green phosphor (Ba,Sr,Eu)$_{2}$SiO$_{4}$. Hence (Ba,Eu)$_{3}$Si$_{6}$O$_{12}$N$_{2}$ appears promising green phosphor for white LED backlight for display. The atomic/electronic structure is discussed in comparison with a similar oxynitride Ba$_{3}$Si$_{6}$O$_{9}$N$_{4}$, which could not become efficient phosphor by doping Eu due to strong thermal quenching at room temperature. The optical properties of these compounds have been interpreted from theoretical and crystallographic viewpoint. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H37.00011: Optical properties of group-II oxides -- excitons and absorption in MgO, ZnO and CdO Andr\'e Schleife, Claudia R\"odl, Frank Fuchs, Friedhelm Bechstedt ZnO is a material that has been very attractive for researchers for many decades by now. However, recently also alloys and heterostructures with other group-II oxides are becoming more and more interesting. Together with MgO or CdO the tuning of electronic and optical properties becomes possible with potential applications for optoelectronic devices in the blue or UV spectral region. For the three materials we study the influence of excitonic effects on the dielectric function in the region of interband transitions and on the electron-hole binding near the absorption edge by solving the Bethe-Salpeter equation. As starting point we compute the electronic structure using a GGA+$U$ approach. The $U$ is chosen to widely reproduce more sophisticated HSE03+$GW$ calculations. We combine two efficient approaches to calculate the spectrum and bound excitonic states, using a large number of ${\bf k}$-points in combination with hybrid ${\bf k}$-point meshes to ensure convergence. We find good agreement of our challenging \emph{ab initio} calculations with experimental absorption spectra as well as values for the binding energies. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H37.00012: Mechanistic model for the influence of Ag thickness on the electrical and optical properties of ZnO/Ag/ZnO nanoscale multilayers Terry Alford, Hauk Han, N. Theodore In this study, we have engineered TCO structures with greatly improved electrical conductivity by introduction of a thin layer of Ag. Results show that carrier concentration, mobility, and conductivity increase with Ag thickness. The electrical conductivity decreased from 1 $\Omega $-cm to 8$\times $10$^{-5} \quad \Omega $-cm for a 14 nm Ag middle layer, by increasing carrier concentration and mobility. AFM and TEM results indicate that low Ag thickness results in Ag island formation. The optical transmittance of the composite structure decreases when compared to a single ZnO layer of comparable thickness. However, we demonstrate that an optimum Ag thickness exists (12 nm) to fulfill the conductivity and transmittance requirements for optoelectronic devices. Moreover the optical band-gap of ZnO/Ag/ZnO composite multilayer films decreases with increasing the Ag thickness and also increases with carrier concentration. Based on these results, we propose a mechanistic model for the influence of Ag thickness on the electrical and optical properties of this system. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H37.00013: Soft X-Ray Spectroscopic studies of Intrinsic Quantum Well States, Shallow Core Level Hybridization, and Valence Band Structure in CdO and InN L.F.J. Piper, L. Colakerol, A. DeMasi, T.D. Moustakas, K.E. Smith, J. Zuniga-P\'erez, V. Munoz-Sanjos\'e, Alexei Fedorov, T. Veal, C. McConville InN and CdO are post-transition metal compounds that display significant metal-ligand shallow core level hybridization [1], and have recently been discovered to posses intrinsic quantum well states in electron accumulation layers near their surfaces [2]. We report here new synchrotron-based soft x-ray spectroscopic measurements of the electronic structure of CdO and InN single crystal thin films. Resonant x-ray emission spectroscopy has been employed to study the detailed valence band and shallow core level electronic structure, while high resolution angle-resolved photoemission spectroscopy was used to measure quantized electron subbands at the near-surface of both InN and CdO. [1]. L .F. J. Piper \textit{et al}., Phys. Rev. B (2007) \textit{in press; }[2]. L. Colakerol \textit{et al}., Phys. Rev. Lett. 97, 237601, (2006) [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H37.00014: Probing defects in ZnO nanostructures by Photoluminescence and Positron Annihilation Spectroscopy Manoranjan Ghosh, A.K. Raychaudhuri, S.K. Chaudhuri, Dipankar Das We have investigated defect related emission in the blue green region (2.2 eV -- 2.5 eV) of ZnO nanostructures having spherical (5 nm-15 nm) as well as those with hexagonal platelet and rod like morphologies (20nm-100 nm), synthesized by solvo-thermal route. This emission show anomalous size dependence. Emission energy near 2.2 eV, shifts to higher energy (2.5 eV) for increase in size beyond 20nm when shape of the nanostructures changes. This change in photoluminescence has a close correlation with the size (and shape) induced change in the positron trapping rate which is directly proportional to the defect concentration. The trapping rates show non-monotonous dependence on size. It increases initially as the size increases (5nm-15nm) and then decreases as the size increases beyond 20nm. While increase of the trapping rate on size reduction is expected due to accumulation of more defects at the surface, the initial dependence of the trapping rate on the size (below 20nm) is anomalous. The data are explained by the presence of defects like Zn vacancy and confinement due to size reduction. [Preview Abstract] |
Session H39: Student Prize Session followed by Focus Session: Friction
Sponsoring Units: GSNPChair: Cristina Marchetti, University of Syracuse
Room: Morial Convention Center 231
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H39.00001: Restricted Defect Dynamics in Colloidal Peanut Crystals Sharon Gerbode, Stephanie Lee, Bettina John, Angie Wolfgang, Chakesha Liddell, Fernando Escobedo, Itai Cohen We report that monolayers of hard peanut-shaped colloidal particles consisting of two connected spherical lobes order into a crystalline phase at high area fractions. In this ``lobe- close-packed'' (LCP) crystal, the peanut particle lobes occupy triangular lattice sites, much like close-packed spheres, while the connections between lobe pairs are randomly oriented, uniformly populating the three crystalline directions of the underlying lattice. Using optical microscopy, we directly observe defect nucleation and dynamics in sheared LCP crystals. We find that many particle configurations form obstacles blocking dislocation glide. Consequently, in stark contrast to colloidal monolayers of close-packed spheres, single dislocation pair nucleation is not the only significant energetic barrier to relieving an imposed shear strain. Dislocation propagation beyond such obstructions can proceed only through additional mechanisms such as dislocation reactions. We discuss the implications of such restricted defect mobility for the plasticity of LCP crystals. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H39.00002: Elastic Theory of Defects in Toroidal Crystals Luca Giomi, Mark Bowick Crystalline assemblages of identical sub-units packed together and elastically bent in the form of a torus have been found in the past ten years in a variety of systems of surprisingly different nature, such as viral capsids, self-assembled monolayers and carbon nanomaterials. We investigate the structural properties of toroidal crystals and we provide a unified description based on the elastic theory of defects in curved geometries. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H39.00003: Hopping Conduction and Bacteria: Transport Properties of Disordered Reaction-Diffusion Systems Andrew Missel, Karin Dahmen Reaction-diffusion (RD) systems are used to model everything from the formation of animal coat patterns to the spread of genes in a population to the seasonal variation of plankton density in the ocean. In all of these problems, disorder plays a large role, but determining its effects on transport properties in RD systems has been a challenge. We present here both analytical and numerical studies of a particular disordered RD system consisting of particles which are allowed to diffuse and compete for resources ($2A\to A$) with spatially homogeneous rates, reproduce ($A\to2A$) in certain areas (``oases''), and die ($A\to0$) everywhere else (the ``desert''). In the low oasis density regime, transport is mediated through rare ``hopping events'' in which a small number of particles diffuse through the desert from one oasis to another; the situation is mathematically analogous to hopping conduction in doped semiconductors, and this analogy, along with some ideas from first passage percolation theory, allows us to make some quantitative predictions about the transport properties of the system on a large scale. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H39.00004: Hard Discs on the Hyperbolic Plane Carl Modes, Randall Kamien We examine a simple hard disc fluid with no long range interactions on the two dimensional space of constant negative Gaussian curvature, the hyperbolic plane. This geometry provides a natural mechanism by which global crystalline order is frustrated, allowing us to construct a tractable model of disordered monodisperse hard discs. We extend free area theory and the virial expansion to this regime, deriving the equation of state for the system, and compare its predictions with simulation near an isostatic packing in the curved space. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H39.00005: Experimental Observation of Quantized Vortex Reconnection and Turbulence in Superfluid Helium Matthew Paoletti, Katepalli Sreenivasan, Daniel Lathrop We present experimental studies of the first direct visualization of reconnecting quantum vortices and the decay of superfluid turbulence in $^{4}$He. Micron-sized solid hydrogen particles are used for particle tracking. The cores of the superfluid vortices trap the hydrogen particles, thereby allowing direct visualization of the dynamics of the line-like defects. We generate superfluid turbulence by driving a thermal counterflow. After pulsing the counterflow, the system relaxes through a cascade of reconnection events. We examine the dynamics of pairs of particles trapped on reconnecting vortices and observe that these particles separate as power laws in time with a scaling exponent distributed about the predicted value of $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern- .15em\lower.25ex\hbox{$\scriptstyle 2$} $. We show that reconnection leads to power-law tails in the velocity probability distribution function, which is in stark contrast to the Gaussian tails that are ubiquitous in classical turbulence and thermal motion. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H39.00006: Slippery or sticky boundary conditions: control of wrinkling in metal-capped thin polymer films by selective adhesion to substrates Hugues Vandeparre, Julien Leopoldes, Christophe Poulard, Sylvain Desprez, Cyprien Gay, Gwennaelle Derue, Pascal Damman As demonstrated by countless studies, sheets are more easily bent than stretched. Under planar forces, a thin sheet will thus deform out of plane by forming wrinkles. Surface buckling or wrinkling can be generated in various systems such as rigid thin films supported on elastomers, or gels that could be swollen or dried. Here, we describe an original and simple method to control the spatial layout of wrinkles in polymer/metal bilayer systems. To generate surfaces with a tailor-made buckling pattern, we have tuned the boundary conditions at the polymer-substrate interface by using chemically patterned substrates with highly contrasted surface free energies ($\gamma )$, easily produced by microcontact printing of alkanethiols on gold substrates. In addition, to explain our original variant of the experiments, we will also expand the existing mechanisms and models described in the literature to take into account the adhesion at the polymer-substrate interface. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H39.00007: Shrinky Dinks : Dynamic Shape Transformations Ajay Gopinathan, Michelle Khine, Arnold Kim Biaxially oriented polystyrene thermoplastic sheets (shrinky dinks) have been recently used by one of us (Khine, Lab on a Chip, 2008) as a template for rapid and non-photolithographic microfluidic pattern generation. This method utilizes the shrinkage properties of the shrinky dinks upon heating to generate microscale structures. During the heating process the sheets show a variety of non-trivial three dimensional intermediate structures before returning to a shrunken flat state upon completion of the process. We show that these structures arise due to the imposition of a non-uniform spatial metric on the sheet which in turn is governed by the dynamic temperature gradients generated in the sheet. Our results allow us to quantitatively describe the dynamic sequence of structures generated and suggest routes to the design and fabrication of different structures in a controllable fashion. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H39.00008: The Roll of Friction in the Mechanical Failure Properties of a Polymer Particulate Composite Donald Wiegand, Brett Reddingius, Kevin Ellis, Claire Leppard The mechanical failure properties of a composite containing an organic crystalline particulate and a polymer-plastizer binder have been investigated as a function of hydraulic pressure between 0.1 and 138 MPa. The results indicate that in a low pressure range between about 0.1 and 7.0 MPa crack processes are important in failure. The pressure dependence of the compressive strength is attributed to coulomb friction between surfaces of closed cracks$^{+}$, and from the observed linear increase of the strength with pressure a friction coefficient is obtained. Fiction coefficients can also be obtained from the ratio of compressive to tensile strength and in addition from the angle which the failure plane makes with the direction of the applied stress. both at 0.1 MPa$^{+}$. The friction coefficients obtained from these three separate observations are in agreement and this is taken as strong evidence for the importance of this friction in determining strength and mechanical failure for this composite. $^{+}$Dienes, J. K., 1983. J. Geophysical Research 88: 1173 -- 1179. Zuo, Q. H. and Dienes, J. K., 2002. LA-13962-MS. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H39.00009: Force-driven transport in entropy barriers Kevin Dorfman, Nabil Laachi, Martin Kenward, Ehud Yariv We consider theoretically the transport of a point-sized Brownian particle in a two-dimensional channel with a slowly varying, periodic cross-section. Such channels are associated with the concept of an ``entropy barrier," where the change in the number of available ``states" for the Brownian particle governs the transport process. Using generalized Taylor-Aris dispersion theory and long-wavelength asymptotics, we exactly compute the mean particle velocity and effective diffusivity (dispersivity) for two cases: electrophoretic transport in an insulating channel and motion under the influence of a constant force. At the same time, we arrive at rational definitions for the concept of an entropy barrier as a function of the driving force. The agreement between simulations and the exact calculation for a constant force is excellent and represents a significant advance over existing models of the transport process. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H39.00010: Friction and wear of nanocrystalline diamond coatings. Abhishek Kothari, Brian Sheldon, XingCheng Xiao, Kyung-Suk Kim, Leo Lev Nanocrystalline diamond films were tested for friction and wear response using a pin on disc tribometer. COF, starting with 0.7 successively drops to 0.1 as the wear progresses. Understanding this friction and wear response is important to minimize wear of these materials as tool coatings. SEM images at the periphery of wear tracks indicate the presence of ring cracks which are due to stress at the circle of contact exceeding the tensile strength. Effects of engineering the stress on the wear have been verified experimentally. Estimation of wear rate in these coatings is of high importance. AFM was used to obtain topography information on wear tracks of the film successively after 2K-200K pin revolutions in the tribometer. It is noted that peak heights of the asperities were decreasing with wear. Image analysis of the topographical evolution of the successive wear tracks could provide an estimation of the wear rate. This analysis also indicates that the distribution of asperity contact size shifts towards larger size with successive wear of the film. Previous studies of Krim, Hurtado and Kim revealed that the frictional stresses of individual asperities are dependent on the asperity contact sizes -- the larger the asperity contact size, the lower the frictional stresses. The micromechanics model of asperity friction explains well the decreasing of coefficient of friction with progression of wear. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H39.00011: Ultra-high vacuum cryotribology of diamond and diamond-like films Matthew Aggleton, P. Taborek We have used a sliding block tribometer (described in J.C. Burton, P. Taborek, and J.E. Rutledge, TRIBOLOGY LETTERS 23, 131, 2006) to measure the temperature dependence of the kinetic friction coefficient of single crystal diamond on various types of CVD diamond films including microcrystalline diamond, nanocrystalline diamond, and diamond-like carbon. We have also studied various other solid and fluid lubricants. These measurements have been performed in ultra-high vacuum and at temperatures ranging from 6 to 300 Kelvin. Although microcrystalline diamond has a low friction coefficient in air; in vacuum, the friction coefficient rises to approximately 0.7 and is independent of temperature. Nanocrystalline diamond is a much better tribological material in vacuum, particularly for T$>$240K. Near 240K there is a reversible transition to a higher friction state. We will discuss the correlation between the tribological properties and the material properties such as sp2/sp3 ratio, hydrogen content, and grain size. This work is supported by Extreme Friction: MURI AFOSR {\#} FA9550-04-1-0381 [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H39.00012: Quartz Crystal Microbalance Studies of Temperature Rise in a Sliding Contact Jacqueline Krim, Benjamin Dawson, Matthew Walker, Cherno Jaye, Douglas Irving, Donald Brenner The exact relation between temperature rise in a sliding contact and frictional energy dissipation is of great technological importance, but poorly understood at a fundamental level. Temperature rise is presumably due to frictional heating that results from phononic and electronic excitations, but efforts to relate temperature rise to friction and sliding velocity have proven very difficult. We have performed QCM studies of adsorbed Krypton monolayers, and also joint QCM-STM studies, to examine temperature rise associated with friction in two well characterized geometries. In the first, we utilized the static phase diagram of two-dimensional Kr adsorbed on graphite as compared to the dynamic phase diagram (with the Kr layer sliding) to determine temperature rise. In the second study, we have recorded frequency shift data for a QCM with indium electrodes in contact with an STM tip while increasing the sliding speed to a point where melting is indicated. A temperature rise on the order of 10 (40-100) degrees is observed in the first (second) geometry. Comparisons to theory yield plausible fit parameters. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H39.00013: Combined ab initio and classical molecular dynamics simulations of the tribological properties of rare gas monolayers sliding on metal surfaces Maria Clelia Righi, Mauro Ferrario Progress in the ability of understanding tribological properties in adsorbed film systems is of paramount importance to unravel fundamental issues in the emerging field of nanoscale science and technology. Many extensive studies have used a quartz-crystal microbalance (QCM) to measure the friction between adsorbed rare gas monolayers and metal substrates. In this work, we report a theoretical investigation of the tribological behavior of different rare gas-metal adsorbate systems, namely, Ar, Kr, Xe on Cu(111), and Xe on Ag(111), based on combined \textit{ab~initio} and classical molecular dynamics simulations. The frictional properties are analyzed in details as a function of system temperature, presence of interlayer defects, and load. The numerical simulations suggest that the simultaneous presence of thermal effects and of interlayer defects, lowering significantly the activation energy barrier, causes a considerable reduction of the static friction threshold. An unexpected dependence on load is also predicted. In particular, we show that friction of anticorrugating systems can be dramatically decreased by applying an external load [1]. The counterintuitive behavior that deviates from the macroscopic Amonton law is dictated by quantum mechanical effects that induce a transformation from anticorrugation to corrugation in the near-surface region. [1] M. C. Righi and M. Ferrario, Phys. Rev. Lett. \textbf{99}, 176101 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H39.00014: Solid-fluid transitions at high sliding rates at Al/Al interfaces J.E. Hammerberg, B.L. Holian, R. Ravelo, T.C. Germann Large scale NonEquilibrium Molecular Dynamics (NEMD) simulations (1.4 $10^{6}$ atoms) for single crystal Al have shown a transition as a function of sliding velocity from a defective solid phase to a fluid phase beyond a critical velocity, $v_{c}$, which depends very nearly linearly with the homologous temperature $T/T_{m}$ where $T_{m}$ is the melting temperature and $T$ is the sample temperature far from the interface. Above $v_{c}$, a Couette flow pattern develops with a slope which is independent of velocity. We discuss the properties of this transition and the power law dependence of the frictional force with velocity observed in this regime. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H39.00015: ABSTRACT HAS BEEN MOVED TO SESSION H8 |
Session H40: Focus Session: Earth and Planetary Materials I
Sponsoring Units: DMP DCOMPChair: Robert Liebermann, COMPRES
Room: Morial Convention Center 232
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H40.00001: Elasticity of (Mg,Fe)O through the spin transition of iron in the lower mantle Invited Speaker: Recently, the important question of spin-pairing transitions of iron from high-spin (HS) to low-spin (LS) states in ferropericlase (Ref. 1 and references therein) affecting the lower mantle's density and seismic-wave velocities has been recognized (2,3). Since knowledge of this deep and inaccessible region is derived largely from seismic data, it is essential to determine the influence of the spin transition on elastic wave velocities at lower-mantle pressures. Here we discuss the results of measurements of the elastic tensor of (Mg0.94Fe0.06)O up to 60 GPa using impulsive stimulated light scattering. We find that all tensor elements soften substantially through the HS to LS transition, and that the softening occurs over an extended pressure range from 40 GPa to at least 60 GPa at room temperature. By invoking a simple thermodynamic description (4) of the transition we can compare our results to literature compression data (2,5) obtained from material with the higher iron concentrations likely to be found in ferropericlase in the lower mantle. The agreement is good and thus suggests that the thermodynamic description is reasonable. This in turn allows us to predict the effect of high temperature on the transition; we find that as temperature is increased the transition region is extended (see also Ref. 6) and the magnitude of the softening decreases. We conclude that although the spin transition in (Mg,Fe)O is too broad to produce an abrupt seismic discontinuity in the lower mantle, the transition will produce a correlated negative anomaly for both compressional and shear velocities that extends throughout most if not all of the lower mantle. 1. J. Badro, et al., Science, 300, 789 (2003). 2. J. F. Lin, et al., Nature, 436, 377 (2005). 3. J. F. Lin, et al., Geophys. Res. Lett., 33, L22304 (2006). 4. T. Tsuchiya, R. M. Wentzcovitch, C. R. S. da Silva, S. de Gironcoli, Phys. Rev. Lett., 96, 198501 (2006). 5. Y. Fei, et al, Geophys. Res. Lett. 34, L17307 (2007). 6. J.F. Lin, et al., Science, 317, 1740 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H40.00002: Elasticity of ferropericlase at Earth's lower mantle conditions Renata Wentzcovitch, Joao Justo, Zhongqing Wu, Cesar da Silva The thermoelastic properties of ferropericlase Mg$_{1-x}$Fe$_{x}$O (x = 0.1875) across the iron high-to-low spin crossover at lower mantle conditions have been investigated by combining first principles calculations with a thermodynamics model of this system. At room temperature the transition is somewhat sharp and the effect on the bulk modulus is quite dramatic. Along a typical geotherm the transition should occur across most of the lower mantle with a noticeable bulk modulus reduction in the mid lower mantle. This transition should also alter noticeably the magnitude of velocity heterogeneities caused by lateral temperature changes. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H40.00003: Inter-site Partitioning of Iron in Wadsleyite at High Pressures Rose Perea, Boris Kiefer (Mg$_{1-x}$,Fe$_{x})_{2}$SiO$_{4}$ -- wadsleyite is thought to be the most abundant metal in the upper part of the Earth's transition zone (410 -520 km depth). Wadsleyite contains three crystallographically non-equivalent octahedral sites, M1, M2, and M3. Experimentally, it has been observed that the M2 site is depleted in iron relative to the M1 and M3 sites. This asymmetric partitioning may affect the melting temperature and the density of iron bearing wadsleyite and influence the transport of mass, momentum, and energy across the transition zone. We performed LDA and GGA first-principle calculations of ferrous iron substitutions in the three crystallographically distinct octahedral sites: M1, M2, and M3. At low pressures we find, as expected, that iron in wadsleyite adopts its high spin state. The enthalpy differences from our high-spin calculations are consistent with the experimental observations that the M2 site is depleted in iron and independent of the magnetic moment of iron. This finding indicates that the inter-octahedral site partitioning of iron is due to the presence of the d-orbitals. If this finding can be corroborated our results will affect the understanding of the partitioning of other divalent transition metals and geochemical trace elements in wadsleyite, the most abundant mineral in the upper part of the Earth's transition zone. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H40.00004: First principles studies on several major phase transitions in Earth upper mantle Yonggang Yu, Zhongqing Wu, Renata Wentzcovitch First principles quasi-harmonic free energy calculations have been conducted to study several important phase transitions in mantle minerals with compositions Mg2SiO4 and MgSiO3 under realistic Earth's mantle conditions. We find encouraging and similar level of agreement with experiments for thermodynamic and vibrational properties and phase transition boundaries in~all cases where comparisons between results and data are possible. We also find systematic trends: 1) equations of state and thermodynamic properties of single crystalline phases are best described by the LDA; 2) phase boundaries are bracketed by LDA and GGA results, with GGA offering the upper bound transition pressure and being somewhat closer to the experimental ones. A summary of these results is offered for forsterite, wadsleite, ringwoodite, periclase, perovskite, post-perovskite, low clino-, and high clino-enstatite, and garnet, as well as~phase transitions between them. In overall these results can supplement high-PT experimental data on these minerals. (Research supported by NSF/EAR 013533, 0230319, 0635990, and NSF/ITR 0428774 (VLab), and Minnesota Supercomputing Institute.) [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H40.00005: The Phase Diagram of Portlandite, Ca(OH)$_{2}$ at Pressures up to 40 GPa Boris Kiefer, Megan Lockwood Hydroxides have attracted significant scientific interest over the past decades. They have been used as analogues to further our understanding of hydrogen bonding in complex materials. Several experimental observations suggest that portlandite undergoes reversible solid state amorphization at high pressures but the cause for this transition and its appearance in portlandite remains unknown. We performed static (0 K) first-principle calculations based on the GGA within the framework of Density-Functional-Theory in order to explore the phase diagram of portlandite for pressures up to 40 GPa. All structures were fully relaxed to determine the ground state of portlandite. We find that a split-site model for the hydrogen array is energetically preferred consistent with neutron diffraction experiments. At a pressure of $\sim $4.5 GPa we observe a phase transition from trigonal to monoclinic symmetry in agreement with previous studies. At all higher pressures we find that the monoclinic structure is the ground state of portlandite. However, we identified several energetically comparable structures. This indicates that the potential energy surface of portlandite has a surprisingly complex structure. The interplay of these structures suggest that the solid state amorphization is driven by non-hydrostatic stress and allows to rationalize experimentally observed differences between portlandite powders and single-crystals. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H40.00006: High PT elasticity within the quasiharmonic approximation with relaxed thermal stresses Pierre Carrier, Joao F. Justo, Renata M. Wentzcovitch We describe in detail a method to compute high PT elasticity within the quasiharmonic approximation (QHA). This approach differs from the usual formulation used to compute the statically constrained high PT elastic constants by including corrections due to deviatoric thermal stresses. The formulation is general and valid for crystals with up to triclinic symmetry. We use perovskite and post-perovskite phases of MgSiO3 to exemplify the use of the method to calculate elasticity and crystal structures at high PT. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H40.00007: Quantum Monte Carlo Study of Elastic Instability in Stishovite K.P. Driver, R.E. Cohen, P.L. Rios, M.D. Towler, R.J. Needs, J.W. Wilkins Stishovite is a octahedrally coordinated polymorph of silica which is stable at pressures within Earth's lower mantle (10 GPa). Elastic properties of stishovite are important for explaining seismic structure and it serves as a model system for other six-coordinated silicates. Near 50 GPa, stishovite transforms to the $\rm CaCl_{2}$-type structure due to an instability in the elastic shear modulus. The instability was first predicted by density functional theory (DFT) calculations and later confirmed by Raman spectroscopy and X-ray diffraction. Quantum Monte Carlo accurately predicts elastic constants and benchmarks previous DFT results on the stishovite elastic instability. Over the pressure range of 0 to 50 GPa, QMC shows the elastic shear modulus softens from 270 to 0 GPa in agreement with previous DFT and experimental results. Computations were performed at NERSC. Funding provided by the NSF (EAR-0530282, EAR-0310139) and the DOE (DE-FG02-99ER45795). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H40.00008: Pressure dependence of lattice anharmonicity and phonon lifetime in MgO: a first-principles calculation and implications for lattice thermal conductivity Xiaoli Tang, Jianjun Dong We report a recent first principles calculation of harmonic and anharmonic lattice dynamics of MgO. The 2$^{nd}$ order harmonic and 3$^{rd}$ order anharmonic interatomic interaction terms are computed explicitly, and their pressure dependences are discussed. The phonon mode Gr\"{u}neisen parameters derived based on our calculated 3$^{rd}$ lattice anharmonicity are in good agreement with those estimated using the finite difference method. The phonon lifetime due to lattice anharmonicity is calculated based on the single mode excitation approximation (SMEA). We have further estimated the isotope effect on phonon lifetime within the random mass disorder approximation. The implications for lattice thermal conductivity at high pressure are discussed based on a simple kinetic transport theory. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H40.00009: Pure Iron Compressed and Heated to Extreme Conditions Arkady Mikhaylushkin, Sergei Simak, Leonid Dubrovinsky, Natalia Dubrovinskaia, Borje Johansson, Igor Abrikosov The results of a first-principles study supported by the temperature-quenched laser-heated diamond anvil-cell experiments on the high-pressure high-temperature structural behavior of pure iron are reported. We show that in contrast to the widely accepted picture, the face-centered cubic (fcc) phase becomes as stable as the hexagonal-close-packed (hcp) phase at pressures around 300–360 GPa and temperatures around 5000–6000 K. Our temperature-quenched experiments indicate that the fcc phase of iron can exist in the pressure-temperature region above 160 GPa and 3700 K, respectively. This, in particular, means that the actual structure of the Earth's core may be a complex phase with a large number of stacking faults. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H40.00010: First-principles study of MgSiO$_{3}$ at core-mantle boundary conditions Siu-Chung Sung, Jones Tsz-Kai Wan Perovskite MgSiO$_{3}$ is an important mineral in geoscience studies. It plays a crucial role in the understanding of geophysical and geochemical activities taken place in the Earth's interior. In this talk, we report our recent work on First-principles molecular dynamics (FPMD) simulations of solid MgSiO$_{3}$ perovskite and post-perovskite, and molten MgSiO$_{3}$ at core-mantle boundary (CMB) conditions. The equations of state are determined at pressures up to 200 GPa and temperatures up to 6000K. The post-perovskite phase is found to be favoured over the perovskite at pressures above 102 GPa at zero temperature. Melting of MgSiO$_{3}$ has been observed by heating both perovskite and post-perovskite at high temperatures ($\sim$6000 K). The melting curve and electronic structures of solid and molten MgSiO$_{3}$ are also presented. Our simulated results thus provide useful constraints on structure and phase stability of MgSiO$_{3}$, which is the key to the understanding of deep-earth phenomena, such as the D$''$ discontinuity and seismic anisotropies in D$''$ layer. More importantly, the phase transformation of MgSiO$_{3}$ studied in this work provides insights into other aspects of geosciences like chemical heterogeneity and mantle convection, which may lead to a better model of the Earth's evolution. [Preview Abstract] |
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