Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N1: Quantum Electronics in Silicon
Sponsoring Units: DCMPChair: Xuedong Hu, State University of New York at Buffalo
Room: Colorado Convention Center Four Seasons 2-3
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N1.00001: Spin Coherence in Silicon Quantum Structures Invited Speaker: |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N1.00002: Spectroscopy of Few-Electron Quantum Dots in Silicon Invited Speaker: Valley states in Si offer interesting opportunities for new physics through their interaction with spin and orbital states. At the same time, such valleys could in principle create undesired decoherence pathways in quantum devices. Recent advances have allowed new experiments studying these effects. Here we report measurements of few-electron quantum dots in Si quantum wells. Both the Kondo and Fano effects are found in this system. Valley states, if their splitting is small, would produce additional peaks in the non-linear conductance - a feature not observed in the experiments. We propose that their absence is due to enhanced valley splitting in Si quantum dots compared with quantum wells (where measurements have long shown very small splitting). We experimentally confirm such large valley splitting in Si nanostructures by performing measurements of Si quantum point contacts. We find valley splittings of order 1 meV, comparable to the largest predicted theoretical values, and much larger than numerous experiments, by ourselves and others, on laterally unconfined 2DEGs. We offer an explanation based on the role of steps and disorder at the quantum well interface. Finally, building on this understanding of the role of disorder, we discuss recent advances in silicon membranes that offer new ways to create quantum wells with lower disorder. Si membranes with thicknesses as thin as one hundred nanometers and lateral widths as large as a centimeter have been achieved. We discuss their application as hosts for quantum wells and as an enabling technology for the formation of Si/SiO2/Si multilayers in which all Si layers are single crystal. Work performed in collaboration with L.M. McGuire, C. Simmons, N. Shaji, K.A. Slinker, S. Goswami, L.J. Klein, W. Peng, M.M. Roberts, J.O. Chu, R. Joynt, M. Friesen, S.N. Coppersmith, R. Blick, M.G.Lagally, and D.E. Savage. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N1.00003: Single-dopant spectroscopy in triple-gate nano MOSFETs Invited Speaker: In this talk we discuss the physics of transport through a single dopant atom in semiconductor matrix to which we have spectroscopic access in a prototype silicon MOSFET. These FinFETs are three dimensional nano-scale devices consisting of a lithographically defined Si nanowire surrounded by a gate. They are fabricated on a Si-on-insulator substrate and have an active region as small as 50x60x35nm$^3$. The electronic states of the dopant appear as resonances in the low temperature conductance at energies below the conduction band edge. We can set the charge on the dopant by means of the gate electrode and observe the single and doubly charged donor state which is under magnetic field successively being occupied by a spin-up and then a spin-down electron. The binding energy of the neutral $D^0$ state is consistent with that of an arsenic donor. The $D^-$ state with two electrons shows a reduced charging energy compared to bulk Si due to the electrostatic coupling with electrodes. The level spectrum of the dopant exhibits a large separation of the ground state from excited states but is not bulk-like. This is also due to the close proximity to the gate which leads to a strong electric field and the formation of a second well close to the interface that overlaps with the donor well. The manipulation of the dopant wavefunction by an electric field (Stark effect) is a key element in Si quantum electronics, {\em e.g.} the solid-state quantum computer. We discuss the level spectrum of this gated $D^0$ system for different field strengths up to 50 MV/m and relate it to theory. At these high fields the charge still remains localized but shows a strongly altered level spectrum.\\ Recent references: H. Sellier {\em et al.}, Transport Spectroscopy of a Single Dopant in a Gated Silicon Nanowire, PRL 97, 206805 (2006) and H. Sellier {\em et al.}, Sub-threshold channels at the edges of nanoscale triple-gate silicon transistors, cond-mat/0603430 [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N1.00004: Coherent electron spin transport and fault-tolerant semiconductor-based quantum computer architectures. Invited Speaker: The recent progress in single atom fabrication techniques for discrete gated donor systems in semiconductors offer new opportunities for coherent quantum technology applications. We review a new scheme for coherent electron spin transport by adiabatic passage (CTAP) particularly suited to atomic and solid-state systems. In a semiconductor implementation, CTAP based transport is a highly robust mechanism for shuttling electron spin states coherently along pathways defined by ionised donors spaced 20-30 nm apart. Such novel discrete transport of electrons may lead to new applications in semiconductor technology, however, as a transport mechanism for spin-encoded quantum information it is an essential development for the successful design of a strongly scalable quantum computer architecture. Using phosphorous donor electron spins in silicon as a model system, the tunnelling rates, transfer times, and effects of decoherence are calculated. The introduction of electron spin transport leads to a scalable 2D quantum computer architecture for Si:P with spatially separated interaction, storage and readout regions and incorporates non-nearest-neighbour interactions between qubits. The transport rails which provide these non-local interactions, also provide alternative pathways to avoid non-functioning regions. The fault-tolerant operation of such an architecture using CTAP for qubit transport is considered in detail. [Preview Abstract] |
Session N2: Progress in Superconducting Quantum Computing
Sponsoring Units: GQI DCMPChair: Robert Schoelkopf, Yale University
Room: Colorado Convention Center Four Seasons 4
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N2.00001: Superconducting qubits on the way to a quantum processor Invited Speaker: Experimental research on supeconducting qubits has seen an enormous progress in recent years. About 10 years after its first theoretical proposals, experiments have demonstrated the necessary building blocks for the exploration of quantum information along several avenues: Single qubit-rotations, long coherence times, high-fidelity nondemolition readout, two-qubit interactions and gates, coupling to delocalized qubit modes. With this progress, analogies to other qubit candidates such as magnetic resonance systems, atomic, and optical systems are evident, but we also see the specific strengths of superconducting qubits play out - in situ tunable strong qubit-qubit coupling, strong coupling between qubits and the quantized electromagnetic field, strong intrinsic nonlinearity, and the possibility to fabricate large circuits. Most of these achievements will be discussed later in the session. I will give an introduction to superconducting qubits in the perspective of quantum information processing [1] accessible to outsiders in the field. I will put the different elements of the session in the perspective of an actual scalable architecture which allows for fault-tolerant quantum computation [1,2]. In order to make further progress in direction, the fidelities of quantum operations need to be improved. I will discuss the crucial topic of understanding and further supressing noise from material defects in these systems, which can influence both the phase and bit-flip error rate [3,4]. I will show, how optimal control theory can help to find fast and high-fidelity shaped pulses for superconducting qubits, even though they, other than spin 1/2 systems, have relatively close leakage levels outside the qubit manyfold [5,6]. This technique also allows to optimize pulses in the presence of telegraph noise [6]. Finally, I will describe how the strong nonlinearity of Josephson circuit can be used for the generation of single microwave photons [7] and lead to a nonlinear generalization of cavity quantum electrodynamics effects [8].\newline \newline [1] M.R. Geller, E.J. Pritchett, A.T. Sornborger, and F.K. Wilhelm quant-ph/0603224 \newline [2] A.G. Fowler, W. Thompson, Z. Yan, A.H. Majedi, and F.K. Wilhelm, in preparation\newline [3] R. de Sousa, K.B. Whaley, F.K. Wilhelm, and J. von Delft, Phys. Rev. Lett 95, 247006 (2005)\newline [5] A.K. Sporl, T. Schulte-Herbrueggen, S.J. Glaser, V. Bergholm, M.J. Storcz, J. Ferber, and F.K. Wilhelm quant-ph/0504202\newline [6] P. Rebentrost, I. Serban, T. Schulte-Herbrueggen, and F.K. Wilhelm, in preparation\newline [7] M. Mariantoni, M.J. Storcz, F.K. Wilhelm, W.D. Oliver, A. Emmert, A. Marx, R. Gross, H. Christ, and E. Solano, cond-mat/0509737\newline [8] I. Serban, E. Solano, F.K. Wilhelm, cond-mat/0606734. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N2.00002: Solid State Qubits with Current-Controlled Coupling Invited Speaker: The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum computing algorithms. We have demonstrated such control with two, three-junction flux qubits coupled together via their mutual inductances and via the dc SQUID (Superconducting Quantum Interference Device) that reads out their magnetic flux states. The flux in each qubit was controlled by an on-chip loop, and the chip was surrounded by a superconducting cavity that eliminates fluctuations in the ambient magnetic field. By applying microwave radiation to the device, we observed resonant absorption in each of the qubits when the level splitting in the qubit matched the energy of the microwave photons. With the qubits biased at the same frequency, the interaction produced an avoided crossing in their energy spectrum. At the avoided crossing transitions to the first excited state were suppressed and transitions to the second excited state enhanced, indicating formation of singlet and triplet states in the coupled-qubit system. The observed peak amplitudes were consistent with calculated matrix elements. When both qubits were biased at their degeneracy points, a level repulsion was observed in the energy spectrum. A bias current applied to the SQUID in the zero-voltage state prior to measurement induced a change in its dynamic inductance, reducing the coupling energy controllably to zero and even reversing its sign. The dependence of the splitting on the bias current was in good agreement with predictions. This work was performed in collaboration with P.A. Reichardt, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, A.V. Ustinov, and John Clarke, and supported by NSF, AFOSR, ARO and ARDA. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N2.00003: Measurement and Generation of Single Photons in a Circuit Invited Speaker: I will describe the measurement and generation of single photons in a circuit quantum electrodynamics system. A one-dimensional transmission line cavity realizing well-defined microwave linear photon modes is coupled to a Cooper-pair box qubit. The qubit-photon coupling is exploited to realize a quantum non-demolition measurement of the qubit state by the photons, resulting in high visibility and long coherence times. The reverse measurement can also be performed: the qubit can used to measure the number of photons in the cavity. In this case, the qubit transition is resolved into separate spectral lines for each photon number, leading to a photon statistics analyzer. The same interaction can also be used to convert qubit states into a flying qubit consisting of superpositions of photon states, and to generate single microwave photons on demand, enabling a full range of quantum optics experiments. Work done in collaboration with D.I. Schuster, A. Wallraff, A. Blais, J. Schreier, L. Frunzio, J.A. Gambetta, J. Koch, J. Majer, B. Johnson, J. Chow, T. Yu, M. Devoret, S.M. Girvin, R.J. Schoelkopf. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N2.00004: Direct Measurement of the Entanglement of Two Superconducting Qubits via State Tomography. Invited Speaker: The Josephson phase qubit can be thought of as an electrical ``atom'' whose resonance frequency can be tuned via an external control bias. Owing to its potential compatibility with conventional integrated circuit fabrication techniques, this system is a promising candidate for a scalable architecture for a quantum computer. Currently, the critical path towards a real device consists of understanding all sources of decoherence that destroy the fragile quantum states. Recently, dielectric loss was identified as the main source of decoherence for phase qubits. By employing techniques to minimize dielectric loss we improved the performance of our quantum bit, which enabled us to show quantum-mechanical entanglement between two phase qubits and identify the generation of a Bell state with a fidelity of up to 0.87, still limited by decoherence effects. We detail the experiment and outline further progress on reducing dielectric loss, leading to an improvement of the measured energy relaxation time by a factor of five. We also identified other insulating materials, which should improve the energy relaxation time by an additional factor of two, resulting in overall coherence times of about one microsecond. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N2.00005: Flux qubits: quantum nondemolition readout and controlled-not gate Invited Speaker: Superconducting flux qubits have of a loop with three Josephson junctions, biased at about half a flux quantum. Basic states have opposite persistent currents, readout is by inductive coupling to a SQUID magnetometer. The following results have been obtained in a bias flux regime where the qubit energy states closely resemble the current states. Coherence was significantly lower than for the best samples. A dispersive method for readout was developed, where the inductance of the SQUID is measured rather than the critical current. The SQUID together with an on-chip capacitance forms a nonlinear oscillator where the resonant frequency depends on the flux in the SQUID, in turn influenced by the qubit. For high driving, two oscillation modes exist with low and high amplitude with a hysteretic transition. A short microwave pulse is applied and the probability that the oscillator switches to the high-amplitude mode is determined. This readout method yields a fidelity of 87{\%} without any corrections for relaxation. We have performed series of two consecutive measurements on a qubit in various superposition states and correlations between the outcomes were determined. Between the first measurement and the second a Rabi pulse was applied. Results were consistent with fully projective measurement, with a quantum nondemolition fidelity of 88{\%} without corrections. We have also studied a system of two permanently coupled flux qubits. For each qubit, the energy splitting is shifted by the other qubit to plus or minus 200 MHz. When a suitable pulse is applied to a target qubit, it acts as a pi-pulse when the control qubit is in one state, and does nothing in the opposite case. This controlled-not operation that consists of a single microwave pulse has been performed for arbitrary superposition states of the two qubits. We have determined the phase reliability of the operation as well as its amplitude response. [Preview Abstract] |
Session N3: Frontiers in Ultra-cold Gases in Optical Lattices
Sponsoring Units: DAMOPChair: Alexander Fetter, Stanford University
Room: Colorado Convention Center Korbel 2A-3A
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N3.00001: Evidence for Superfluidity of Ultracold Fermions in an Optical Lattice Invited Speaker: The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behavior. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N3.00002: Interacting Fermions and Bose-Fermi mixtures in Optical Lattices Invited Speaker: When fermionic atoms are placed in the focus of several interfering laser beams, they feel a periodic potential and behave very much like electrons in a solid. However, the properties of this synthetic material can be changed at will. Here, we report on the realization of a strongly interacting Fermi gas in a 3D optical lattice, which opens the way to study condensed matter physics with light and atoms. For instance, the Fermi surface of the fermions in the lattice can be imaged and a dynamical transition from a band insulating state to a conductive state observed. The focus of the talk we will be on the behaviour of the system with changing interaction. By accessing a Feshbach resonance, we have formed molecules in the optical lattice and studied interaction induced coupling between the lowest Bloch bands as well as low-dimensional interacting Fermi gases. Finally, experiments with an interacting Bose-Fermi mixture in the lattice will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N3.00003: Doing Solid State Physics With Cold Atoms Invited Speaker: At present, there are worldwide efforts to emulate strongly correlated electronic systems using cold atoms in optical lattices. We shall discuss the necessary conditions for the success of this effort, how close current experiments are in meeting these conditions, and the new type of novel ``solid state'' phenomena unique to lattice quantum gases. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N3.00004: 2-D Lattices at JILA Invited Speaker: I will discuss recent developments in our work on 2-D optical lattices at JILA, in particular the interaction of vortices with a lattice, and the effects of finite-T fluctuations. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N3.00005: Normal State of a Polarized Fermi Gas at Unitarity Invited Speaker: I will discuss the Fermi gas at unitarity and at T=0 by assuming that, at high polarizations, it is a normal Fermi liquid composed of weakly interacting quasiparticles associated with the minority spin atoms. I will show that a quantum Monte Carlo approach can be used to calculate their effective mass and binding energy, as well as the full equation of state of the normal phase as a function of the concentration of minority atoms. We predict a first order phase transition from normal to superfluid at a concentration of 0.44 corresponding, in the presence of harmonic trapping, to a critical polarization of 77 per cent. I will discuss radii and the density profiles of both spin components in the trap and our prediction that the frequency of the spin dipole mode will be increased by a factor of 1.23 due to interactions. [Preview Abstract] |
Session N4: Novel Approaches Aimed at Rational Design of Functional Polymeric Materials
Sponsoring Units: DPOLYChair: Jan Genzer, North Carolina State University
Room: Colorado Convention Center Korbel 2B-3B
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N4.00001: Conformation -- Dependent Design of Copolymer Sequences Invited Speaker: In 1998 we have formulated the idea of design of copolymer sequences in order to achieve required functional properties of single macromolecules in the globular state. This approach was inspired by unique sophisticated functional performance of globular proteins-enzymes. In the present talk the review of the recent advances in this field will be given. A robust free-radical polymerization procedure of synthesis of copolymer macromolecules with core-shell structure in the globular state will be described. Like globular proteins, such macromolecules do not precipitate in poor solvents. Next problem is to design a catalytic activity for these molecularly dispersed objects. To this end we investigate amphiphilic substrates and catalytically active groups which possess surface activity and, thus, an authomatically attracted by globular surfaces. We show that the resulting non-specific catalytic effect can be very significant. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N4.00002: Novel Antimicrobial Materials Invited Speaker: Many peptides, such as the Magainins and Defensins, are amphiphilic in nature and known to fold into specific conformations responsible for their antimicrobial membrane activity. Recently, facially amphiphilic peptides built from $\beta$-amino acids have been shown to mimic both the structures as well as the biological function of natural antimicrobial peptides. The design of simple polymers and oligomers that mimic the complex structures and remarkable biological properties of proteins is an important endeavor and would provide attractive alternatives to the difficult synthesis of natural peptides. We have designed a series of facially amphiphilic molecules that capture the essential physical and biological properties of antimicrobial peptides, but are easy to prepare from inexpensive monomers. They have potent activity (single micromolar) and are active against a broad spectrum of bacteria including gram-positive and gram-negative as well as antibiotic resistant strains. They act directly on the phospholipid membrane so that molecule-membrane interacts are critically important to understand. We will discuss our recent insight on this topic including calorimetry, SAXS, and vesicle leakage data. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N4.00003: Biosynthetic Polypeptides as Templates in Materials Design Invited Speaker: Biosynthetic routes to protein-based polymeric materials offer important opportunities for the production of well-defined macromolecular templates, owing to the control of sequence and molecular weight inherent in the biosynthesis of proteins. In particular, the biosynthesis of polypeptides with controlled presentation of functional groups in multiple positions, coupled with their subsequent chemical modification with biologically relevant ligands, will permit the production of well-defined, bioactive macromolecules that may provide insight into biological binding events in which multivalent binding is important. Modification of the well-defined macromolecules with ligands such as saccharides has application in the study of events such as toxin neutralization and mediation of the immune and inflammatory responses. In this work, alanine-rich polypeptides of both random coil and helical conformations, equipped with glutamic acid residues to impart chemical versatility, have been produced via biosynthetic strategies. Analysis via spectroscopic and calorimetric methods indicates that the polypeptides adopt helical, beta-sheet, or random-coil conformations that can be controlled with variations in temperature, pH, and salt concentration; the conformational behavior of the polypeptides is not compromised upon chemical modification with saccharides. The binding of these macromolecules to bacterial toxins has been characterized via immunochemical and spectroscopic methods; results indicate that specific architectural features of the glycopolymer scaffold cause changes in the binding of these molecules to multivalent receptors. Given the chemical flexibility in the design of such scaffolds, they can be modified with many different moieties in addition to saccharides, so multiple opportunities exist for their application in areas where control of active side chains is important, such as in biomaterials, electronic devices, and bioinorganic structures. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N4.00004: Polyvalent Recognition of Biopolymers:The Design of Potent Inhibitors of Anthrax Toxin Invited Speaker: Polyvalency -- the simultaneous binding of multiple ligands on one entity to multiple receptors on another -- is a phenomenon that is ubiquitous in nature. We are using a biomimetic approach, inspired by polyvalency, to design potent inhibitors of anthrax toxin. Since the major symptoms and death from anthrax are due primarily to the action of anthrax toxin, the toxin is a prime target for therapeutic intervention. We describe the design of potent polyvalent anthrax toxin inhibitors, and will discuss the role of pattern matching in polyvalent recognition. Pattern-matched polyvalent inhibitors can neutralize anthrax toxin in vivo, and may enable the successful treatment of anthrax during the later stages of the disease, when antibiotic treatment is ineffective. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N4.00005: Recent Advances in the Synthesis of Polymeric Nanostructured Materials. Invited Speaker: In designing polymeric materials for use in nanotechnology it rapidly becomes apparent that control over all aspects of polymer structure (molecular weight, polydispersity, number and position of functional groups, architecture, etc.) is required if these materials are to be used successfully to create nanostructures in the sub-50 nm size regime. Equally important to the rapid introduction and incorporation of these materials into devices is the development of robust and simple techniques for their synthesis. This presentation will detail recent advances in living polymerizations, Click chemistry and self-assembly strategies for the rapid and efficient synthesis of multi-functional polymeric nanostructures in applications ranging from microelectronics/storage devices to in vivo drug delivery and diagnostics. [Preview Abstract] |
Session N5: The Generation of Magnetic Fields in the Cosmos and the Role of Turbulence
Sponsoring Units: DFDChair: Annick Pouquet, National Center for Atmospheric Research
Room: Colorado Convention Center Korbel 1A-1B
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N5.00001: Laboratory models of the Earth's outer core Invited Speaker: We construct liquid sodium experiments as models of the Earth's core. Key to understanding these several experimental devices is knowing how turbulence is effected by rotation and magnetic fields. In the approach to the planetary regeme, several remarkable behaviors appear [1]. As rotation and magnetic fields add some measure of elasticity to the flows, several types of driven planetary modes are observed depending on the force balances involved. Ordering the Coriolis, Lorentz, and inertial forces is key to understanding the complicated states observed. While these experiments are undertaken in part to understand the geodynamo, they have led to a number of different first observations, including the magneto-rotational instability [2] and inertial waves in spherical Couette flow. These different approaches to using laboratory experiments are opening up a new direction to understanding the dynamics of the Earth's outer core, other Planetary interiors, and a host of astrophysical objects. [1] W.L. Shew and D.P. Lathrop, ``Liquid sodium model of geophysical core convection,'' Phys. Earth and Planetary Interiors, 153, 136-149 (2005). [2] D.R. Sisan, N. Mujica, W.A. Tillotson, Y.-M. Huang, W.Dorland, A.B. Hassam, T.M. Antonsen, and D.P. Lathrop, ``Experimental Observation and Characterization of the Magnetorotational Instability,'' Phys. Rev. Lett. 93, 114502 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N5.00002: Making Magnetic Fields on Cosmic Scales Invited Speaker: Magnetic fields are observed very early in the evolution of structure of the universe. It is not known how or when these fields were created. I will discuss the various theories of the field origin and the fluid mechanical issues that arise. Small scale fields of observable amplitudes are relatively easy to create on short time-scales by turbulent flows or compact objects. The central issue is the the creation of the observed long scale coherence in the field. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N5.00003: Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium Invited Speaker: Industrial dynamos routinely generate currents and magnetic fields from mechanical motions. In these devices, pioneered by Siemens, the path of the electrical currents and the geometry of the rotors are completely prescribed. As it cannot be the case for planets and stars, experiments aimed at studying dynamos in the laboratory have evolved towards relaxing these constraints. Solid rotor experiments showed that a dynamo state could be reached with prescribed motions but currents free to self-organize. A landmark was reached in 2000, when the experiments in Riga and Karlsruhe showed that fluid dynamos could be generated by organizing favourable sodium flows, the electrical currents being again free to self-organize. For these experiments, the self-sustained dynamo fields had simple time dynamics (a steady field in Karlsruhe and an oscillatory field in Riga). No further dynamical regimes where reached. We report the observation of dynamo action in swirling flows for which turbulence is fully developed. The flows are generated in the gap between counter-rotating impellers (the von Karman Sodium experiment -VKS). Dynamo action is reached at magnetic Reynolds number Rm$\approx $30. When the impellers are rotating at equal rates, the dynamo field is statistically steady, although the rms fluctuation level is of the order of the mean amplitude. For impellers rotated at different speeds, a variety of dynamical regimes are observed, including magnetic field reversals. We will describe and discuss the features of these dynamos, including the nature of the bifuraction, the scaling of the self-sustained fields, the excess mechanical power, etc. Some regimes have geomorphic characteristics, while others may be relevant in the astrophysical context. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N5.00004: Dynamo action in penetrative Boussinesq convection Invited Speaker: Dynamo action in any highly turbulent, electrically-conducting fluid medium is plausible. Dynamo amplification of the magnetic fields on the scales of the velocity patterns might be expected if the effects of diffusion and packing of fields are not too drastic. An interesting question is whether magnetic fields can be generated on scales {\sl larger} than the velocity scale. We investigate the generation of magnetic fields in a Boussinesq convecting layer, and examine the effects of including a convectively-stable layer of fluid below, of rotation, and of adding a forced shear. We examine the efficiency of the dynamo and the relative production of small-scale and large-scale magnetic fields. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N5.00005: Scale interactions in MHD turbulence and dynamo action Invited Speaker: In recent years the increase in computing power, as well as the development of subgrid models for magnetohydrodynamic (MHD) turbulence has allowed the study of a numerically almost unexplored territory in MHD flows: the regime of low magnetic Prandtl number ($P_M$). This regime is of particular importance since several astrophysical and geophysical problems are characterized by $P_M<1$, as for example in the liquid core of planets such as the Earth, or in the convection zone of stars as the Sun. Liquid metals used in dynamo experiments to generate magnetic fields are also characterized by $P_M<1$. In this talk we will review some studies of dynamo action and MHD turbulence for $P_M \leq 1$, down to $P_M \sim 5 \times 10^{-3}$. In particular, we will focus on cases where a large scale flow is present and turbulence develops as the result of an instability. Interactions between scales will be discussed, and evidence of non-local interactions involving disparate scales in simulations of MHD turbulence with resolutions up to $1536^3$ grid points will be presented. The implications of these results for universality will be briefly discussed. [Preview Abstract] |
Session N6: Condensed Matter Physics at Synchrotron Facilities: History as Prologue to the Future
Sponsoring Units: FHP DPBChair: David Moncton, Massachusetts Institute of Technology
Room: Colorado Convention Center 207
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N6.00001: Soft X-Ray Science -- From Photon Drought to X-Ray Lasers Invited Speaker: Soft x-ray science, loosely defined as research with 200-2000 eV photons, has come a long way over the last 30 years. This talk highlights some of the scientific developments and gives a glimpse of the future. Today, high-intensity soft x-rays are available with meV spectral resolution, picosecond pulse lengths and nanoscale spot sizes. Their tunable energy and polarization allows the control of electronic core-to-valence transitions that provide access to the fundamental charge and spin properties of valence electrons in matter. Large resonant cross sections associated with absorption edge resonances provide sensitivity to small numbers of atoms, as encountered in nanostructures, ultra-thin films, interfacial layers and surfaces. Presently, the most advanced experiments use sophisticated spectro-microscopy and lensless coherent imaging techniques with nanoscale spatial and picosecond temporal resolution. On the horizon are experiments with soft x-ray lasers which, among other things, will provide femtosecond snapshots of matter. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N6.00002: Inelastic X-ray Scattering Invited Speaker: This presentation is devoted to review the Inelastic X-ray Scattering (IXS) method to study atomic density fluctuations. The IXS as a complement to neutrons has been suggested for many years now with a first attempt dating back to the eighties. Only the advent of Hard X-ray third generation synchrotron light sources has allowed the establishment of IXS as a powerful routine technique for condensed matter studies. It has enabled important breakthroughs in our understanding of phonon-like excitations in disordered materials and matter at extreme conditions. The very small gauge volume and possible future advances in instrumentation allow to expect further developments in phonon microscopy. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N6.00003: Surface Structure as a Foundation of Nanotechnology Invited Speaker: The three generations of synchrotron sources achieved to date, parasitic, dedicated and undulator-based, have each time revolutionized the field of X-ray diffraction. Surface structure determination, demonstrated (but very difficult) already with Coolidge tube sources, benefited from the enormous flux gain in the first generation, such as SSRL. Dedicated 2nd-generation sources, such as NSLS, allowed in-situ surface preparation and reliable steady beams to be available when a surface was ready to measure. Third generation sources, such as APS, enormously improved the brightness, hence coherence, and thus allowed access to the surfaces of nanoparticles. This talk will illustrate how these technological advances led to two significant scientific breakthroughs. The concept of crystal truncation rods (CTR) led to new views of how the surface is a modification of, but still an extension of the bulk crystal structure. The development of lensless coherent x-ray diffraction (CXD) imaging has allowed access to the structure of nanocrystalline materials by three-dimensional phase mapping of the particle interiors. The structural principles of these new nano materials are being investigated at present using these new methods. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N6.00004: Magnetic X-Ray Scattering Invited Speaker: The 1980s saw the convergence of the development of synchrotron sources; the development of techniques to grow new materials layer by layer, and the realization that x-rays could probe the magnetic properties of materials. In addition to magnetic x-ray scattering, most magneto-optical effects have been extended from the visible to the soft x-ray region. Because of the tunability of both the energy and the polarization, synchrotron sources are element and site specific probes, and there are large resonant enhancements in the scattering or absorption cross sections at atomic absorption edges. Synchrotron radiation is routinely used to study the magnetic polarization of different components of a material and to separate their spin and orbital angular momentum densities. In addition synchrotron radiation can be used to determine the interplay between the atomic, orbital and magnetic ordering in materials. The history and current trends in magnetic x-ray scattering will be reviewed. Future trends include further development of the spectroscopic aspects of magnetic scattering and probing magnetism on smaller and smaller length scales and at shorter and shorter time scales. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N6.00005: The Use of Coherent X-Ray Beams to Study the Dynamics of Soft Condensed Matter Systems Invited Speaker: The study of slow dynamics in soft condensed matter systems has been of interest for many years. One of the most powerful techniques for studying dynamics at these time scales has been Dynamical Light Scattering (DLS). However, it was recognized over twenty years ago that a similar application of X-rays in order to achieve shorter length scales and avoid problems of multiple and stray particle scattering, could open up whole new areas of research. The advent of the high-brilliance third generation synchrotron X-ray sources over a decade ago made it possible for the first time to deliver an intense beam of highly coherent X-rays, enabling many new applications of X-ray scattering, some of which will be discussed. In particular, the technique of X-ray Photon Correlation Spectroscopy (XPCS), the X-ray analog of DLS, has now become an exciting new research area with applications primarily in soft condensed matter. In this talk, we shall trace the development of the use of coherent X-ray beams from the early demonstrations at the NSLS, ESRF and APS synchrotron light sources to current applications which include the study of dynamical fluctuations in colloids and polymers and in particular the study of surface fluctuations in liquid films and membranes. We shall show how XPCS has yielded interesting new results on these systems difficult if not impossible to obtain by other techniques. \newline \newline I wish to acknowledge collaborations with Hyunjung Kim, Larry Lurio, Zhang Jiang, Christian Gutt, Metin Tolan, Tuana Ghaderi, Jyotsana Lal, Simon Mochrie, Miriam Rafailovich, Jonathan Sokolov, Chinhua Li, Tadanori Koga, Xuesong Jiao, Suresh Narayanan. [Preview Abstract] |
Session N7: Nonlinear Rheology and Dynamics of Soft Materials
Sponsoring Units: DCMPChair: David Weitz, Harvard University
Room: Colorado Convention Center Korbel 4A-4B
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N7.00001: Elasticity of Crosslinked Biopolymer Networks Invited Speaker: Crosslinked networks of biopolymers exhibit an enormous variety of nonlinear elastic behaviors depending on the rigidity of constituent polymers and the geometry and topology of the network. This talk will present a brief review of the general theory of nonlinear elasticity. It will then discuss the phenomenon of strain stiffening in networks of semiflexible polymers and present a theory [1] of this phenomena based on the nonlinear force-extension curve of these polymers and the simplifying assumption of affine response. The nonlinear stress-strain curves predicted by this theory agree remarkably well with experiments on a number of different polymer networks. Limitations and extensions of the simple theory including extensions to nonaffine behavior will also be discussed. \newline \newline [1] Storm, Cornelis, Jennifer J. Pastore, Jennifer J., Fred C. MacKintosh, Fred C., T.C. Lubensky, T.C., and Paul A. Janmey, Paul A., \textit{Nature} \textbf{435}, 191-194 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N7.00002: Strain-Rate Frequency Superposition (SRFS) - A rheological probe of structural relaxation in soft materials Invited Speaker: The rheological properties of soft materials such as concentrated suspensions, emulsions, or foams often exhibit surprisingly universal linear and nonlinear features. Here we show that their linear and nonlinear viscoelastic responses can be unified in a single picture by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new approach to oscillatory rheology, which keeps the strain rate amplitude fixed as the oscillation frequency is varied. This allows for a detailed study of the effects of strain rate on the structural relaxation of soft materials. Our data exhibits a characteristic scaling, which isolates the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques. Our approach is reminiscent of a technique called time-temperature superposition (TTS), where rheological curves measured at different temperatures are shifted onto a single master curve that reflects the viscoelastic behavior in a dramatically extended range of frequencies. By analogy, we call our approach strain-rate frequency superposition (SRFS). Our experimental results show that nonlinear viscoelastic measurements contain useful information on the slow relaxation dynamics of soft materials. The data indicates that the yielding behavior of soft materials directly probes the structural relaxation process itself, shifted towards higher frequencies by an applied strain rate. This suggests that SRFS will provide new insight into the physical mechanisms that govern the viscoelastic response of a wide range of soft materials. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N7.00003: Probing the nonlinear response of soft materials by active microrheology Invited Speaker: In passive microrheology, the linear viscoelastic properties of complex fluids are inferred from the Brownian motion of colloidal tracer particles. Active (but gentle) forcing may also be used to obtain such linear-response information. More significant forcing may drive the material significantly out of equilibrium, thus potentially providing a window into the nonlinear response properties of the material. In leaving the linear-response regime, however, the theoretical underpinning for passive microrheology is lost, and a variety of issues arise. Most generally, what exactly can be measured, and how can such measurements be interpreted? Using a model system (a large colloidal probe pulled through a dilute suspension of small bath particles), we examine the different sources of stress upon the probe particle (e.g. direct probe-bath collisions vs. microstructural deformations within the bulk suspension) and discuss their analog in the corresponding macro- rheological measurement (or lack thereof). Several crucial issues emerge for the interpretation of nonlinear microrheology: 1) how to interpret the inhomogeneous and non-viscometric nature of the deformation field around the probe, 2) the distinction between of direct and bulk stresses and their deconvolution, and 3) the (Lagrangian) time-dependent nature of the stress histories experienced by material elements as they advect past the probe. Having identified these issues, we discuss several adaptations of the basic technique/interpretation, both to more faithfully recover bulk rheology as well as to measure properties inaccessible to macro- rheology. While we specifically discuss a model colloidal suspension, we ultimately envision a technique capable of measuring the nonlinear rheology of general materials. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N7.00004: Nonlinear structural dynamics in metal nanowires Invited Speaker: Most atoms in a metal nanowire are surface atoms with low coordination number. Classically, surface effects are expected to dominate their stability and structural dynamics, leading in particular to wire break-up due to the Rayleigh instability. On the other hand, long gold [1] and silver [2] nanocylinders have recently been observed using transmission electron microscopy, pointing to the presence of an additional stabilizing mechanism. Evidence of electron-shell filling effects [3] have been found in conductance histograms for various metals, suggesting that this mechanism comes from the transverse confinement of the electrons within the nanowire. Using the nanoscale free-electron model, a continuum model of the structural dynamics of simple-metal nanowires, I will discuss how the interplay of surface and electron-shell effects explains the stability and long lifetimes of nanowires, and favors the formation of kinks connecting cylindrical segments of the wire. A rich dynamics involving kink interactions and kink/antikink pair-creation and annihilation is uncovered, and is shown to explain the observed step-by-step thinning mechanism of Au nanowires [4]. \newline \newline [1] Y. Kondo et al., Science 289, 606 (2000). \newline [2] V. Rodrigues et al., Phys. Rev. Lett. 85, 4124 (2000). \newline [3] A. I. Yanson et al., Phys. Rev. Lett. 84, 5832 (2000). \newline [4] Y. Oshima et al., J. Electron Microsc. 52, 49 (2003). [Preview Abstract] |
Session N8: Focus Session: Novel superconductors V: Borides, Organics and Others
Sponsoring Units: DMPChair: David Mandrus, Oak Ridge National Laboratory
Room: Colorado Convention Center Korbel 1C
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N8.00001: Engineering superconductors with ab initio methods: the example of LiB Invited Speaker: The identification of novel crystal structures is a fundamental step for predicting new stable compounds in alloys. While performing ab initio data mining of intermetallic compounds [1], we discover a new family of layered metal borides [2], of which MgB$_2$ is one particular element (the new phases are called Metal Sandwich (MS)). Thermodynamic stability and electronic properties of these MS phases are investigated in details, leading to the prediction of a hypothetical novel superconductor MS-LiB [2,3]. Calculations show that the MS phases in the Li-B system exhibit electronic features similar to those of MgB$_2$ [2,3] and CaC$_6$ [4]. Although the predicted critical temperature of LiB is lower than that of MgB$_2$ (references [4] and [5] for MS2-LiB and MS1-LiB, respectively), the peculiarities of MS-LiB in terms of electronic structure, layer arrangements and doping capabilities allow a lot of freedom in the search for higher $T_c$ systems [5]. We acknowledge the San Diego Supercomputer Center for computational resources. \newline \newline [1] S. Curtarolo et al., Phys. Rev. Lett. {\bf 91}, 135503 (2003). \newline [2] A.N. Kolmogorov, S. Curtarolo, Phys. Rev. B {\bf 73}, 180501(R) (2006). \newline [3] A.N. Kolmogorov, S. Curtarolo, in press, Phys. Rev B {\bf 74} (2006), condmat/0607654. \newline [4] A.Y. Liu, I.I. Mazin, cond-mat/0610057. \newline [5] M. Calandra, A.N. Kolmogorov, S. Curtarolo, submitted (2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N8.00002: Electronic structure of MS2-LiB under hydrostatic pressure. Edgar Martinez-Guerra, Romeo de Coss, Aleksey N. Kolmogorov, Stefano Curtarolo Recently ab initio calculations have found that the Li-B phase equilibrium diagram has two new phases: MS1 and MS2 [A. Kolmogorov and S. Curtarolo, Phys. Rev. B \textbf{73}, 180501R (2006)]. These two phases are stable enough to compete against known phases. These lithium borides exhibit electronic features similar to those in magnesium diboride and they are expected to be superconductors. In this work, we have studied the structural and electronic properties of the MS2-LiB system under pressure. The calculations were performed using the SIESTA code, with the GGA exchange-correlation functional in the PBE form. We have used numerical atomic orbitals as the basis set for the valence wavefunctions employing a double $\zeta $-polarized basis. We present a detailed analysis of the band structure, Fermi surface, and orbital populations as a function of the hydrostatic pressure. In particular, we focus on the behavior of the $\sigma $- and $\pi $-bands derived from the boron $p$-states. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N8.00003: Combining the advantages of superconducting MgB2 and CaC6 in one material: suggestions from first-principles calculations Amy Liu, Igor Mazin We show that a recently predicted layered phase of lithium monoboride, Li$_2$B$_2$, combines the key mechanism for strong electron-phonon coupling in MgB$_2$ (i.e., interaction of covalent B $\sigma$ bands with B bond-stretching modes) with the dominant coupling mechanism in CaC$_6$ (i.e., interaction of free-electron-like interlayer states with soft intercalant modes). Yet, surprisingly, the electron-phonon coupling in Li$_{2}$B$_{2}$ is calculated to be weaker than in either MgB$_2$ or CaC$_6$. This is due to the accidental absence of B $\pi$ states at the Fermi level in Li$_{2} $B$_{2}$. In MgB$_2$, the $\pi$ electrons play an indirect but important role in strengthening the coupling of $\sigma$ electrons. We discuss the use of doping to restore $\pi$ electrons at the Fermi level, which would enhance the coupling and the superconducting $T_c$. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N8.00004: The Structural and Physical Properties of the Vacancy Ordered LiBC Phases Bora Kalkan, Ebru Gungor, Engin Ozdas The prediction of superconductivity on the hole doped LixBC system [1] has triggered to particular interest on the synthesis of non-stoichiometric LiBC compounds. However, isolation of a non-stoichiometric phase of the LiBC have not been succeed as a single phase, yet. All of the experimental studies exhibited non-superconductivity in the disordered Li$_{x}$BC phases. Contrary to the disordered Li$_{x}$BC phases synthesized in the literature [2], non-stoichiometric Li vacancy ordered phases were obtained in this work. Additionally, the structural analysis with Rietveld refinement in a series of samples identified the stages of the intercalation of Li between the BC layers. The effect of stoichiometry on the physical properties of ordered Li$_{x}$BC phase was investigated at low temperatures. [1] Rosner H.et al., PRL 88, 12, 2002. [2] Fogg A.M.et al.JACS,128, 10043, 2006. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N8.00005: The Pressure Effect on the Electronic Structure of the Ordered LiBC Ebru Gungor, Engin Ozdas In this study, the effect of the higher pressures (0-100GPa) on the electronic structure was investigated for an ordered structure of Li$_{x}$BC phase. And also, the stoichiometric effect was examined by the first principles calculations in terms of the metallic behaviour for the range of 0 $\le $ x $\le $ 1. It was observed that the density of states near the Fermi level decreases depending on the pressure and the energy gap above the Fermi level contracts for the higher pressure values for especially Li$_{0.5}$BC compound predicted as a superconductor [1-3]. DOS is extremely sensitive to the Li stoichiometry and the unit cell volume. The pressure has the different effect on the electronic structure of Li$_{1.0}$BC system behaviour for the same pressure range by contrast with the nonstoichiometric LiBC. \newline [1] Rosner H.et al., PRL 88, 12, 2002. \newline [2] Singh P.P. et al., Solid State Comm., 124,25-28, 2002. \newline [3] Dewhurst J.K. et al., PRB 68, 020504(R), 2003. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N8.00006: Mechanism of Superconductivity in Boron-Doped Diamond Feliciano Giustino, Jonathan R. Yates, Ivo Souza, Marvin L. Cohen, Steven G. Louie The recent discovery of superconductivity in boron-doped diamond above liquid helium temperature has attracted considerable interest. Theoretical investigations indicate that the superconducting pairing in this material is of the conventional phonon-mediated type. However, the nature of the phonon mechanism involved and the role of the dopants are still controversial issues. In order to elucidate such issues we performed first-principles calculations of the electron-phonon interaction in boron-doped diamond, considering a virtual crystal model and a supercell model which explicitly includes the boron atoms. For each model we calculated the Eliashberg functions with high accuracy by sampling the corresponding Brillouin zone with a million of inequivalent $k$-points. We found that the localized vibrational modes associated with the boron atoms provide a significant contribution to the electron-phonon coupling strength and that superconductivity in diamond is crucially linked to the breaking of the lattice periodicity induced by the doping. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N8.00007: Critical fields, vortex melting and the irreversibility line in quasi 2D organic superconductors. Braunen Smith, Kyuil Cho, Charles Agosta We have measured the critical field and aspects of the vortex structure in anisotropic organic superconductors using pulsed and dc fields up to 50 and 45 T respectively and at temperatures down to 55 mK. In all cases we measured the penetration depth using the tunnel diode oscillator technique. When the sample is oriented with the conducting planes parallel to the applied magnetic field, we have found that the irreversibility line does not extrapolate to the high fields predicted by Mola et. al. [1] based on their measurements and the use of the Tinkham equation. We also find that many signatures of the vortex system, such as jumps, melting and hysteresis are absent in this parallel sample orientation. In addition, when using a pulsed field apparatus we have consistently measured lower critical fields than we find from the use of dc field apparatus. We assume this is due to a time constant associated with the vortices entering and leaving the sample, but not all of our data supports this claim. [1] M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett. \textbf{86}, 2130 (2001). [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N8.00008: Unconventional Metallic States of the Superconducting Layered Organic Charge Transfer Salts Eddy Yusuf, B. J. Powell, R. H. McKenzie We show, by analyzing previously published nuclear magnetic resonance data, that there are large antiferromagnetic (AF) fluctuations above 50 K and a pseudogap below 50 K in the metallic state of $\kappa-$(ET)$_2$Cu[N(CN)$_2$]Br. We discuss the relationships between the metallic state, the AF Mott insulating state, and the unconventional superconducting state. The AF correlation length is found to be $3.5 \pm 2.5$ lattice constants at $T=50$ K; this places the material between the isotropic triangular lattice and the square lattice We show that the low temperature regime of the metallic state of $\kappa-$(ET)$_2$Cu[N(CN)$_2$]Br is not a renormalized Fermi liquid, as has been previously thought. We argue that a pseudogap is responsible for the loss of the density of states in the spin degrees of freedom, seen in NMR data, while that probes of the charge degrees of freedom have a Fermi liquid character in these materials. We compare and contrast our phenomenological description with the predictions of dynamical mean field theory (DMFT) and the resonating valence bond (RVB) theory. Similar spin fluctuations and pseudogap are also found in $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Cl, fully deuteratred $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br, and $\kappa$-(ET)$_2$Cu(NCS)$_2$ suggesting common physics in these salts. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N8.00009: Specific heat study of the effect of cooling rate on the superconducting and normal states of $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br Antony Carrington, O.J. Taylor, C.M.J. Andrew, R.W. Giannetta, T. Olheiser, J. Schlueter It is well known that the $T_c$ of $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br is dependent on the rate it is cooled in the temperature range 80-60~K. One interpretation of this effect is that rapid cooling introduces disorder which suppresses $T_c$ because of its unconventional nature. Here we present a specific heat study of this effect in both hydrogenated and deuterated samples. We find that not only does $T_c$ depend on the cooling rate, but that the normal-state Sommerfeld coefficient, $\gamma$, is strongly suppressed (by up to a factor 2) with rapid cooling. The data indicate that rapid cooling induces macroscopic phase separation between an insulating and metallic / superconducting phase at low temperature. The field dependence of $\gamma$ for the deuterated sample is highly unusual. As the field is increased it initially increases in a conventional way then suddenly collapses to a small value. We interpret this as evidence for a field induced superconductor-insulator transition. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N8.00010: Low temperature penetration depth of $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br J.D. Fletcher, A. Carrington, R.W. Giannetta, J. Schlueter Several experimental results have suggested that the quasi-2D organic metal $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br is host to some form of unconventional superconductivity. The presence of gap nodes in the superconducting order parameter should be detectable through power law behavior in the penetration depth at low temperature. The most accurate measurements of the temperature dependent penetration depth to date show a fractional power law, $\lambda\propto T^{1.5}$. However, these measurements were not performed at sufficiently low temperatures to determine whether this was due to the combination of gap nodes and the effects of impurity scattering, or due to an intrinsic form of exotic pair excitation. Using a radio frequency (rf) tunnel diode technique in a dilution fridge we have extended these measurements to T~$\sim$ 75~mK ($\sim$0.006~$T_c$). Special care has been taken to eliminate heating effects at these temperatures due to the presence of the small applied rf field. Data at the lowest temperature are more consistent with a nodal state in the presence of impurities. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N8.00011: Mott Transition, Antiferromagnetism, and d-wave Superconductivity in Two-Dimensional Organic Conductors A.-M.S. Tremblay, Bumsoo Kyung We study the Mott transition, antiferromagnetism and superconductivity in layered organic conductors using Cellular Dynamical Mean Field Theory for the frustrated Hubbard model. A $d$-wave superconducting phase appears between an antiferromagnetic insulator and a metal for $t^{\prime }/t=0.3-0.7 $, or between a nonmagnetic Mott insulator (spin liquid) and a metal for $t^{\prime }/t\geq 0.8$, in agreement with experiments on layered organic conductors including $\kappa $-(ET)$_{2}$Cu$_{2}$(CN)$_{3}$. These phases are separated by a strong first order transition. The phase diagram gives much insight into the mechanism for d-wave superconductivity. Two predictions are made. \newline \newline B. Kyung and A.-M.S. Tremblay, Phys. Rev. Lett. \textbf{97}, 046402 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N8.00012: Comparative Studies of Quasi-One-Dimensional Superconductivity in Sc$_{5}$Ir$_{4}$Si$_{10}$ and Lu$_{5}$Ir$_{4}$Si$_{10}$ Tsuyoshi Tamegai, Guoji Li Compounds with a formula $R_{5}T_{4}X_{10}$ ($R$=Sc, Y, rare earth elements, $T$=Co, Ir, Rh, Os, $X$=Si, Ge) crystallize in Sc$_{5}$Co$_{4}$Si$_{10}$--type structure with Sc-Si chains running along the $c$-axis. Some of them show superconductivity with relatively high transition temperatures and coexistence of superconductivity and charge-density wave. We have grown high quality single crystals of Sc$_{5}$Ir$_{4}$Si$_{10 }$and Lu$_{5}$Ir$_{4}$Si$_{10}$ using the floating-zone method. Thus obtained crystals show superior properties compared with polycrystalline materials, such as higher $T_{c}$ and $H_{c2}$. Anisotropic superconducting properties in these crystals are studied in detail. The upper critical field shows clear anisotropy, with $H_{c2}^{c }>H_{c2}^{ab}$, consistent with the quasi-one-dimensional crystal structure. Both compounds have modest anisotropies with $\gamma (=H_{c2}^{c }$/$H_{c2}^{ab}$ )=2.3 for Sc$_{5}$Ir$_{4}$Si$_{10}$ and $\gamma $=1.6 for Lu$_{5}$Ir$_{4}$Si$_{10}$. Magnetic penetration depths in Sc$_{5}$Ir$_{4}$Si$_{10}$ ($\lambda _{c}$= 900 A and $\lambda _{ab}$ =2100 A) estimated from the magnetic field dependence of the equilibrium magnetization confirm quasi-one-dimensional nature of the superconducting state. [Preview Abstract] |
Session N9: Superconductivity: Oxide Photoemission II
Sponsoring Units: DMPChair: Michael Fluss, Lawrence Livermore National Laboratory
Room: Colorado Convention Center Korbel 1D
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N9.00001: Theory of Photoemission Line Shape in High Temperature Superconducter $Bi_2Sr_2CaCu_2O_{8+\delta}$ via Dipolon Mediated Electron-Electron Pairing Mechanism Ram Sharma Observed photoemission (PE) line shape near ($\pi$,0) in superconductor BISCCO containing a peak, a dip and a broad feature has been explained naturally by dipolon pairing mechanism using temperature dependent expressions [1,2] including all necessary and important electron correlations. The calculated positions of the peak, dip and broad feature agree well with the observed results.The theory predicts the possibility of observing dipolon side bands in PE. Contributions not only from nearest $Cu-O_2$ layer but also from all other layers and rows are important. The peak in PE is due to excitation solely of a quasiparticle; the broad feature and dip are due to the excitations of quasiparticles with concomitant O(1,1) and O(1,2) dipolon excitations. In normal state PE [3] of UD, Op and OD samples we have identified these dipolon excitations. Matrix element effects have been considered. \newline \newline [1] R.R. Sharma, Phy. Rev. {\bf B 63}, 054506 (2001). \newline [2] R. R. Sharma, Physica {\bf C 439}, 47 (2006). \newline [3] Y.-D. Chuang, Phy. Rev. {\bf B 69}, 094515 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N9.00002: Van Hove singularity crossing in overdoped superconducting cuprates - effect on superconducting and normal-state properties Jeffery L. Tallon, James Storey, Tobias Fellmeth, Grant Williams We show from modelling the entropy, superfluid density and thermoelectric power using a rigid ARPES-derived dispersion that the crossing of the van Hove singularity (vHs) occurs in Bi$_2$Sr$_2$CaCu$_2$O$_8$ in the heavily overdoped region at a hole concentration of p=0.22 holes per Cu. This concurs with recent results from ARPES measurements. The impact of the vHs crossing on the thermodynamic properties, the Knight shift and susceptibility, transport, optical and superconducting properties is described and shown to account for much of the overall doping dependence of these properties. The only exotic feature that needs to be introduced is the pseudogap showing that much of the physics of HTS materials is conventional. These insights help us to see that the ``lost entropy'' of 1kB per doped hole, previously associated with the pseudogap, is just associated with the DOS asymmetry arising from the proximity of the vHs. Remarkable is the insensitivity of the doping dependence of Tc to the singularity in the DOS. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N9.00003: Distinct Fermi-momentum dependent energy gaps in deeply underdoped Bi2212. K. Tanaka, W.S. Lee, D.H. Lu, A. Fujimori, T. Fujii, * Risdiana, I. Terasaki, K. Fujita, M. Ishikado, S. Uchida, D.J. Scalapino, T.P. Devereaux, Z. Hussain, Z.-X. Shen Our recent angle-resolved photoemission spectroscopy study of deeply underdoped cuprate superconductors Bi2Sr2(Ca,R)Cu2O8 (R = Y or Dy) (Bi2212) suggested the presence of two distinct energy gaps exhibiting different doping dependences [1]. One gap, associated with the antinodal region where no coherent peak is observed, increases with underdoping - a behavior known for more than a decade and considered as the general behavior of the gap in the underdoped regime. The other gap, associated with the near nodal regime where a coherent peak can be observed in energy distribution curves (EDCs), does not increase with less doping - a behavior not seen in the single particle spectra before. The theoretical implications of these findings and temperature dependence of the spectra will be discussed. [1] Science, in press. (http://www.sciencemag.org/cgi/content/abstract/1133411) [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N9.00004: Detail Properties of Band Renormalization Effect of the Bi$_{2}$Sr$_{2}$Ca$_{1}$Cu$_{2}$O$_{8+\delta }$ Wei-Sheng Lee, Worawat Meevesana, Donghui Lu, Steve Johnston, Tomas Devereaux, Hiroshi Eisaki, Zhi-Xun Shen Since the observation of a kink in the nodal dispersion and the peak-dip-hump structure at antinodal region, the band renormalization effects in High-Tc superconducting cuprates have drawn lots of attention in the field of High-Tc superconductivity. Despite of the consensus that these renormalization structures are induced by coupling to some bosonic mode(s), data with much improved quality are needed to clarify the origin of the mode and gain further insight into its relation to the High-Tc superconductivity. In this work, detailed temperature and doping dependence of the band renormalization effect in Bi2212 system are studied via ARPES spectra with improved data quality. Our data suggest that there are multiple bosonic modes coupled to the electrons exhibiting a rich superconductivity and doping induced phenomena. Simulations based on electron-phonon interaction will also be discussed in comparison to our data. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N9.00005: Evidence for two energy scales in the superconducting state of optimally doped (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+\delta}$ Takeshi Kondo, Tsunehiro Takeuchi, Syunsuke Tsuda, Shik Shin, Adam Kaminski We use angle-resolved photoemission spectroscopy (ARPES) to investigate the properties of energy gap(s) in the optimally doped (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+ \delta}$ (Bi2201). We find significant differences in the momentum- and temperature- dependence of the pseudogap and supercondcuting gap suggesting that these gaps have two separate energy scales. The ARPES spectra slightly off the node have a sharp peak with a small gap below $T_c$, which closes at $T_c$. Around the antinode, the broad spectra with a large energy gap of $\sim$40meV are observed above and below $T_c$. The spectral shape and the gap size around the antinod are almost unchanged across $T_c$, indicating that the pseudogap state coexists with superconducting state below $T_c$, and it dominates the character of ARPES spectra around antinode. We speculate that the pseudogap state competes with superconductivity by diminishing spectral weight in the superconducting antinode. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N9.00006: Laser-ARPES studies of BSCCO-BASED cuprate superconductors J.F. Douglas, J.D. Koralek, Z. Sun, N.C. Plumb, Q. Wang, T.J. Reber, J.D. Griffith, Y. Aiura, K. Oka, H. Eisaki, D.S. Dessau, T.P. Devereaux, S.S. Johnson Utilizing 6 eV and 7 eV laser light, we have performed high-resolution ANGLE RESOLVED PHOTOEMISSION studies of the BSCCO family of superconductors. This higher resolution, in both energy and momentum, has allowed the observation of interesting new doping- and temperature-dependent features in the nodal and near nodal dispersions in these materials. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N9.00007: VUV Laser-Based ARPES on Electronic Structure of High Temperature Superconductors Xingjiang Zhou, Guodong Liu, Wentao Zhang, Haiyun Liu, Lin Zhao, Jianqiao Meng, Xiaoli Dong, Z.Y. Xu, G.L. Wang, H.B. Zhang, Y. Zhou, C.T. Chen, Y. Zhu, G.C. Zhang, X.H. Wen, G.D. Gu The last decade has witnessed a dramatic improvement of the angle-resolved photoemission spectroscopy (ARPES) technique in terms of energy and momentum resolution. This in turn has resulted in a number of new findings on the electronic structure of high-Tc cuprate superconductors, particularly the identification of many-body effects. In this talk, we will present results on the electronic structure of high-Tc superconductors measured from our new ARPES system based on VUV laser, realized by frequency doubling from a non-linear optical crystal KBBF. The laser-based ARPES system has ultra-high energy resolution, high photon flux, and a potential to enhance bulk sensitivity. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N9.00008: Protected nodes and the collapse of the Fermi arcs in high $T_{c}$ cuprates Amit Kanigel, U. Chatterjee, M. Randeria, M.R. Norman, S. Souma, M. Shi, Z.Z. Li, H. Raffy, J.C. Campuzano Angle resolved photoemission studies on underdoped samples of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ reveal that the superconducting gap's magnitude and anisotropy remain unchanged up to $T_{c}$. Above $T_{c}$, the nodes of the d-wave gap abruptly expand into finite length Fermi arcs. As this change occurs within the resistive width of the transition, we argue that the Fermi arcs are not simply thermally broadened nodes, but rather a unique signature of the pseudogap phase. This is in contrast to BCS theory, which predicts a gap with fixed anisotropy that changes with temperature and disappears above $T_{c}$. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N9.00009: The Shrinking ``Fermi Arc'' in Cuprates Lijun Zhu, C. M. Varma The angle-resolved photoemission spectroscopy (ARPES) on cuprates in the pseudogap region reveal an extraordinary topological transition in which the ground state changes from one with a usual Fermi surface to one with four Fermi points. We argue that such a state is not possible without some symmetry breaking which allows interference between one-particle basis states which is normally forbidden. We also show that the experimental results are quantitatively given without any free parameters by a theory and discuss the implications of the results. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N9.00010: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N9.00011: Nature of the quasiparticle remnant in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) A. Bansil, S. Sahrakorpi, R.S. Markiewicz, M. Lindroos, V. Arpiainen, X.J. Zhou, T. Yoshida, W.L. Yang, T. Kakeshita, H. Eisaki, S. Uchida, A. Fujimori, Z. Hussain, Z.-X. Shen Angle resolved photoemission (ARPES) studies in LSCO have revealed a remarkable state of affairs in that the observed Fermi surface maps are in excellent accord with the LDA calculations even in the highly underdoped regime. Here we demonstrate that the agreement with gross band dispersion persists to quite high energy scales of several hundred meV's. For example, even in the 3\%-doped sample, the position and shape of the van Hove singularity is found to be in accord with LDA predictions. Signatures of strong correlation physics are manifest however through the suppression of spectral weight near the Fermi energy particularly in the underdoped system. In this way, even though the gross dispersion is virtually unrenormalized, there is a strong renormalization of the spectral weight. Work supported in part by the USDOE. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N9.00012: Universal spectral weight transfer in high temperature superconductors Jeff Graf, Gey-Hong Gweon, Kyle McElroy, Shuyun Zhou, Chris Jozwiak, Eli Rotenberg, Andreas Bill, T. Sasagawa, H. Eisaki, S. Uchida, H. Takagi, Dung-Hai Lee, Alessandra Lanzara High resolution angle resolved photoemission spectroscopy (ARPES) studies of the electronic structure of several cuprate families, over the entire phase diagram, from undoped to highly overdoped regime are reported. A detailed study of the one-electron dynamics as a function of momentum, temperature and doping is presented. A universal spectral weight transfer is observed for all systems and discussed in terms of a strong interplay between the electron-lattice and electron-electron interaction in these materials. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N9.00013: Spectral properties of the Hubbard-Holstein model and comparison to ARPES experiments in the copper oxides Brian Moritz, Alexandru Macridin, Ehsan Khatami, Francois Vernay, Thomas Maier, Thomas P. Devereaux, Mark Jarrell We employ a dynamical cluster Quantum Monte Carlo technique to study the two-dimensional Hubbard-Holstein model over a range of fillings, electron-phonon interaction strengths, and phonon frequencies. Previous investigations of the two-dimensional Hubbard model using these techniques have revealed many of the same features as the cuprate superconductors including strong antiferromagnetic correlations and $d$-wave superconductivity. We have modified the QMC algorithm to treat a continuous phonon field and take advantage of the long time scales associated with the phonon dynamics to offset the computational expense associated with sampling the relatively large configuration space. The Maximum Entropy method is employed to calculate the real frequency spectrum which we compare and contrast with recent angle-resolved photoemission (ARPES) experiments. We discuss qualitative and quantitative results in the context of features that seem to be universal to the copper oxides. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N9.00014: Intermediate Phase in the Superconducting Cuprates. Takeshi Egami It has long been speculated that upon doping the Mott-Hubbard insulator may go through an intermediate phase before becoming a Fermi-liquid metal. If there is such a phase its structure may be intimately connected to the mechanism of the pseudogap and superconductivity. The only well-defined and popular option has been the spin-charge stripe phase, but the strongly one-dimensional nature of the stripe phase is at odds with the highly two-dimensional CuO$_{2}$ plane. We propose a superlattice of $2\sqrt 2 \times 2\sqrt 2 $ in the $a-b$ plane as an alternative candidate for the intermediate phase. In this phase the Mott-Hubbard states and the Fermi-liquid phase coexist in different Brillouin sub-zones. The presence of such a phase is consistent with the recent results of the ARPES at a high energy scale (J. Graf, \textit{et al.}, \textit{cond-mat/}0607319), dispersion of Cu-O bond-stretching phonon mode in YBCO, pulsed neutron PDF analysis of LSCO, and our recent observation of the supperlattice peaks in YBa$_{2}$Cu$_{4}$O$_{8}$ single crystal by x-ray diffraction. The intensity of the superlattice peaks in YBa$_{2}$Cu$_{4}$O$_{8}$ decreases below 250K. This is an unusual behavior for the ordering peak, suggesting the interplay with superconductivity. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N9.00015: Dual character of the electronic structure in YBa$_2$Cu$_4$O$_{8}$: conduction bands of CuO$_2$ planes and CuO chains A. Kaminski, T. Kondo, R. Khasanov, J. Karpinski, S.M. Kazakov, N.D. Zhigadlo, T. Ohta, H.M. Fretwell, A.D. Palczewski, J.D. Koll, J. Mesot, E. Rotenberg, H. Keller We use microprobe Angle-Resolved Photoemission Spectroscopy ($\mu$ARPES) to separately investigate the electronic properties of CuO$_{2}$ planes and CuO chains in the high temperature superconductor, YBa$_2$Cu$_4$O$_{8}$. In the CuO$_2$ planes, a two dimensional (2D) electronic structure with nearly momentum independent bilayer splitting is observed. The splitting energy is 150 meV at ($\pi$,0), almost 50\% larger than in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and the electron scattering at the Fermi level in the bonding band is about 1.5 times stronger than in the antibonding band. The CuO chains have a quasi one dimensional (1D) electronic structure. We observe two 1D bands separated by $\sim$ 550meV: a conducting band and an insulating band with an energy gap of $\sim$240meV. We find that the conduction electrons are well confined within the planes and chains with a non-trivial hybridization. [Preview Abstract] |
Session N10: Metal-Insulator Transition II
Sponsoring Units: DMPChair: Erik Helgren, University of California, Berkeley
Room: Colorado Convention Center Korbel 1E
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N10.00001: Infrared study of the metal-insulator transition regime in vanadium dioxide M. M. Qazilbash, B. G. Chae, H. T. Kim, D. N. Basov Vanadium dioxide (VO$_{2})$ undergoes a metal-insulator transition at T$_{c} \quad \approx $ 340 K. The transition region of a VO$_{2}$ film has been studied with infrared ellipsometry and near-normal incidence reflectance between 40 cm$^{-1}$ and 5000 cm$^{-1}$. The measured optical constants are compared to calculations based on effective medium theories. The anomalies in the frequency and temperature dependence of the optical constants will be presented. The implications of the data for the mechanism of the metal-insulator transition will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N10.00002: Metal-Insulator Transition in Ca$_{1-x}$Na$_{x}$IrO$_{3}$ with Post-Perovskite Structure Kenya Ohgushi, Hirotada Gotou, Takehiko Yagi, Yoko Kiuchi, Fumiko Sakai, Yutaka Ueda We developed a novel solid solution Ca$_{1-x}$Na$_{x}$IrO$_{3}$ (0 $< x \quad <$ 0.37) with the post-perovskite structure [1, 2]. Upon carrier doping into the $S$ = 1/2 antiferromagnetic Mott insulator CaIrO$_{3}$, the magnetic long-range order is gradually destabilized, culminating in a paramagnetic state at $x \quad >$ 0.30, with simultaneous change from the insulating to metallic behavior. The temperature dependence of the resistivity for metallic samples exhibits several characteristic features: (1) the $T^{\alpha }$ dependence with \textit{$\alpha $} $\sim $ 1.2 in the metallic range, (2) the ln$T$ dependence in the weak-localization regime, and (3) the positive magnetoresistance violating the Kohler's rule. These results indicate the anomalous metallic state caused by the strong electron correlation effect is realized on the verge of the Mott transition. [1] Nobuyoshi Miyajima, Kenya Ohgushi, Masaki Ichihara, and Takehiko Yagi, Geophys. Res. Lett. \textbf{33}, L12302 (2006). [2] K. Ohgushi, H. Gotou, T. Yagi, Y. Kiuchi, F. Sakai, and Y. Ueda, submitted. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N10.00003: Metal-insulating phase transition in YBa$_2$Cu$_3$O$_{6+x}$ by First-Principles Alessio Filippetti The basic chemistry of YBa$_2$Cu$_3$O$_{6+x}$ represents an hystorical challenge for first-principles approaches, due to the well known difficulty of standard local-spin density functional theories (such as LSDA or GGA) in describing the correct spin-polarized S=1/2 ground-state of Cu (II) ion. Here we employ the pseudo-SIC approach, which is based on an approximate form of self-interaction corrected (SIC) Kohn-Sham Equations and works well in both Mott-insultaing (i.e. x=0) and metallic limit (x=1), to describe the effect of oxygen-doping on the electronic properties of YBa$_2$Cu$_3$O$_{6+x}$. Our results give a sound description of the order-disorder as well as magnetic-non magnetic phase competitions. We show that the phase transition from the antiferromagnetic insulating to the paramagnetic metal is mainly governed by the ordering of doping oxygens in Cu(I)-O-Cu(I) chains, which in turn, subtly affects the chemistry of Cu(II)-O$_2$ planes through a non trivial pattern of p-d couplings. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N10.00004: Quantum Monte Carlo Study of an Interaction-Driven Band Insulator to Metal Transition Norman Paris, Karim Bouadim, Frederic Hebert, George Batrouni, Richard Scalettar We study the transitions from band insulator to metal to Mott insulator in the ionic Hubbard model on a two dimensional square lattice using determinant Quantum Monte Carlo. Evaluation of the temperature dependence of the conductivity demonstrates that the metallic region extends for a finite range of interaction values. The Mott phase at strong coupling is accompanied by antiferromagnetic (AF) order. Inclusion of these intersite correlations changes the phase diagram qualitatively compared to dynamical mean field theory. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N10.00005: Metal to Nonmagnetic-Insulator Transition in LiVS$_{2}$ Naoyuki Katayama, Minoru Nohara, Masaya Uchida, Hidenori Takagi LiVS$_{2}$ has been reported to exhibit a first order magnetic transition with a drastic decrease in susceptibility at about 310 K$^{[1]}$. In order to clarify the nature of this transition, we performed resistivity, magnetic susceptibility, and electron diffraction measurement for LiVS$_{2}$. The resistivity in LiVS$_{2}$ revealed a metal to insulator (MI) transition at $T_{c} \quad \sim $ 310 K. In the insulating state below $T_{c}$, we observed $\sqrt 3 a_0 \times \sqrt 3 a_0 $superstructure in the electron diffraction, indicating a formation of vanadium trimers in the \textit{ab} plane. Together with the drastic decrease in susceptibility at $T_{c}$, we propose a formation of trimer singlet state below $T_{c}$ for LiVS$_{2}$. Although this ground state is analogous to that observed in the isostructural and isoelectronic oxide LiVO$_{2}^{[2]}$, the MI transition is unique to LiVS$_{2}$. [1] D. W. Murphy \textit{et al.}; Inorg. Chem. \textbf{15} (1976) 17. [2] W. Tian \textit{et al.}; Mater. Res. Bull. \textbf{39} (2004) 1319. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N10.00006: Use of Abrikosov-Gorkov Density of State to Extract Spin Polarization at the Metal-Insulator Transition R. V. A. Srivastava, W. Teizer We have discovered and applied an analytical solution of the Abrikosov-Gorkov $^{1}$ density of states (DOS), describing superconductors with impurities, to extract the spin-polarization of 3-dimensional amorphous (a-) Gd$_{x}$Si$_{1-x}$ in the quantum critical regime (QCR) of a magnetic field tunable metal-insulator transition (MIT). The analysis of the experimental spin-polarized (SP) tunneling conductance of an Al/Al$_{2}$O$_{3}$/a-Gd$_{x}$Si$_{1-x}$ planar tunnel junction at T=25mK in parallel magnetic field H$\le $3.0T indicates a larger polarization near the MIT of a-Gd$_{x}$Si$_{1-x}$ (x=0.14) as compared to previous work $^{2}$, where a SP Bardeen-Cooper-Schrieffer DOS $^{3}$ was used. We will present polarization values at different applied magnetic fields in the QCR. \newline $^{1 }$A. A. Abrikosov and L. P. Gor'kov, Zh. Eksperim. I Teor. Fiz. 39, 1781 (1960). A.A. Abrikosov and L.P. Gorkov, Zh. Eksp. Teor. Fiz. 39, 866 (1961). Soviet Phys.---JETP 12, 1243 (1961). \newline $^{2}$ W. Teizer, F. Hellman, and R. C. Dynes. Phys. Rev. B 67, 121102 (2003). \newline $^{3}$ J. Bardeen, L. N. Cooper, and J. R. Schrieffer, Phys. Rev. 108, 1175 (1957). [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N10.00007: X-ray absorption spectroscopy studies of Vanadium dioxide thin films across the metal-insulator phase transition boundary Dmitry Ruzmetov, Shriram Ramanathan, Sanjaya D. Senanayake X-ray absorption (XAS) and photoemission (XPS) spectroscopy of the V 2p edges and O 1s edge was performed on VO$_{2}$ thin films synthesized by RF sputtering at various conditions. Distinct changes of the electronic structure depending on the film quality, whether the sample is above or below the metal insulator transition (MIT) temperature, and thermal history of the sample are observed. The spacing between 3d$_{\pi }$ and 3d$_{\sigma }$ band peaks probed by O- edge XAS decreases by 0.8eV with concurrent peak broadening for the sample sputtered at lower substrate temperature and consequently having more polycrystalline and disordered character. There is a similar tendency in the V 2p$_{3/2}$ and 2p$_{1/2}$ edges, i.e. the convergence of the doublets for the disordered sample. The temperature dependence of the XAS V and O edges including repeated crossing of the MIT has been studied. The reversible switches of the 3d$_{\pi }$ and 3d$_{\sigma }$ band peak widths in the O-edge on different sides of the MIT are measured while the peak separation remains the same. The abruptness of the band structure transformation at MIT suggests that the band width changes are determined by the VO$_{2}$ MIT phase rather than gradual evolution with temperature. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N10.00008: Mott-Hubbard Scenario for the Metal-Insulator Transition in the Two Dimensional Electron Gas Ping Sun We exam the experimental observations of the metal-insulator transition in Si-MOSFET and GaAs quantum well. We find that the observed critical behaviors in the magneto transport experiments can be understood within the Mott-Hubbard scenario. Disorder, while playing an important role in both metallic and insulating phases, does not affect the universal critical behaviors. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N10.00009: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N10.00010: 2D Wigner crystal: metal to insulator transition via self doping Sergey Pankov, Vladimir Dobrosavljevic We consider a scenario of metal to insulator transition in the 2D Wigner crystal. The Wigner crystal is modeled as a two band (bands represent the site and interstitial orbitals) Hubbard model. It is found that the transition is unstable to the electron self doping, resembling conceptually the liquid-solid transition in Helium III. The self doping is shown to stabilize the metallic phase, pushing the transition to lower electron densities. The implication of the self doping to the compressibility, phase separation and transport properties of the Wigner crystal is discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N10.00011: Disappearance of the metal-like behavior in GaAs two-dimensional holes below 30mK. Jian Huang, Jian-Sheng Xia, D. C. Tsui, L.N. Pfeiffer, K.W. West The T-dependence of the resistivity of two-dimensional holes are observed to exhibit two qualitatively different characteristics for a fixed carrier density at temperatures below 100mK. In this putatively metallic regime of the so-called metal-insulator transition, the sign of the derivative of the resistivity with respect to temperature changes from being positive (d$\rho $/dT$>$0) to negative (d$\rho $/dT$<$0) when the temperature is lowered below 30 mK and the resistivity continuously rises with cooling down to 1mK, suggesting a crossover from being metal-like to insulator-like. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N10.00012: Metal-insulator transition and domains in suspended VO2 nanobeams Jiang Wei, Wei Chen, Zenghui Wang, David Cobden VO$_{2}$ undergoes a metal-insulator transition (MIT) around 67$^{\circ}$C. We investigate the transition in suspended crystalline nanobeams of VO$_{2}$. The nanobeams are grown by vapor phase deposition on SiO$_{2}$ substrates and contacted by electron beam lithography with chromium-gold metallisation. After suspending them by selectively etching away the substrate, the resulting nanobeams are firmly clamped at the contacts. Under some conditions the MIT occurs suddenly throughout the entire beam, associated with a single hysteretic conductance jump. This is in contrast with the behavior of nanobeams attached to the substrate in which alternating metallic and insulating domains form during the transition. Under other conditions, a single metallic domain forms and grows gradually as temperature is increased. At room temperature the longer beams are buckled, and on warming they unbuckle when the MIT occurs. When a force is applied to bend a suspended nanobeam, alternating domain patterns form in the bent region reflecting the strain field. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N10.00013: Physical properties of VO$_{2}$ and V$_{2}$O$_{3}$ nanowires. Wei Chen, Jiang Wei, David Cobden Both VO$_{2}$ and V$_{2}$O$_{3}$ show dramatic metal-insulator transitions, whose manifestations on the nanoscale are not known. We investigate techniques to differentiate and pattern the metallic and insulating domains in small VO$_{2}$ crystals and nanowires grown by vapor phase deposition. For instance, it has been reported that insulating VO$_{2}$ can be metallized by electron beam exposure and by hydrogenation. We attempt to distinguish the domains by scanning probe techniques, including topography and electric force microscopy, and observe a pinning effect of the domains by oscillating strain variations when the nanowire is attached to a substrate. When the strain is released by etching, the pinning is removed. The VO$_{2}$ crystals can be converted to V$_{2}$O$_{3}$ crystals by reducing in hydrogen and annealing. By patterning the V$_{2}$O$_{3}$ on the nanoscale we aim to realize strongly correlated quantum dots. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N10.00014: Orientation Studies of Recrystallized Vanadium Dioxide Felipe Rivera, Mike Clemens, Laurel Burk, Robert Davis, Richard Vanfleet Crystalline films and isolated vanadium dioxide particles were obtained through thermal annealing of amorphous vanadium dioxide thin films on silicon dioxide. Vanadium dioxide undergoes an insulator to metal transition near 66 $^{\circ}$C. Orientation Imaging Microscopy (OIM) was used to study the phase and orientation of the crystals formed, as well as to differentiate from different vanadium oxide crystal structures. Kikuchi patterns for the tetragonal phase of vanadium dioxide were used for indexing as the Kikuchi patterns for the two phases are indistinguishable by OIM. There is a preferred orientation for the growth of these crystals with the c axis of the tetragonal phase parallel to the plane of the specimen. Resistance and Capacitance measurements on these films are being performed to study the electronic chracteristics of this phase transition. The results of this study will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N10.00015: Electron Dynamics in Transition Metal Granular Films N.E. Massa, J.C. Denardin, L.M. Socolovsky, M. Knobel, X.X. Zhang Near normal incidence reflectivity spectra of transition metal $\sim $500 nm thick cosputtered granular films on SiO$_{2}$ subtracts were measured from 30 to 11000 cm$^{-1}$ and at temperatures from 30 to 490 K. The reflectivity for Co$_{0.85}$(SO$_{2})_{0.15}$ has a frequency and temperature behavior according to conducting metal oxides. The electron scattering rate denotes an unique relaxation time characteristic of a single type of carriers and has a very strong temperature dependence due to strong electron-phonon interactions. Using small polaron fits we individualize these as related to glass stretching vibrational modes. The optical conductivity of Ni$_{0.61}$(SO$_{2})_{0.39}$, undergoing a metal-insulator transition at $\sim $77 K, has a Drude mode (freer carriers) and a mid-IR band (mid-infrared ``carriers''). This last disorder related strong resonance drives the phase transition by localization decreasing in magnitude as the temperature is lowered and points to a double relaxation process (two different scattering mechanisms). On the other hand, Co$_{0.51}$(SO$_{2})_{0.49}$ has an insulator reflectivity in which a distinctive band at $\sim $1450cm$^{-1}$ originates in electron promotion, localization, and defect induced quasiparticle formation. [Preview Abstract] |
Session N11: Structural and Magnetic Phase Transitions
Sponsoring Units: DCMPChair: Craig Fennie, Argonne National Laboratory
Room: Colorado Convention Center Korbel 1F
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N11.00001: Ti-V alloy phase diagram: DFT-based cluster approach Michael R. Fellinger, John W. Wilkins, Dallas R. Trinkle V is an important alloying element for Ti since it stabilizes the high-temperature bcc phase of Ti at lower temperatures, while minimizing the formation of intermetallic compounds [1]. Theoretical determination of the Ti-V phase diagram must take into account structures based on both bcc and hcp lattices over the full V concentration range. With the cluster expansion formalism for alloys [2], DFT calculations determine the energies of all structures necessary for constructing the cluster expansion for the energetics of the alloy. Preliminary results indicate the DFT-based cluster approach determines solubility limits and transformation temperatures that are in good agreement with the experimental phase diagram [3]. \newline [1] M. J. Donachie, Jr., \textit{Titanium: A Technical Guide}. ASM International: Metals Park, OH (1988). [2] D. de Fontaine, Solid State Phys. \textbf{47}, 33 (1994). [3] T. B. Massalski, et al., \textit{Binary Alloy Phase Diagrams}, Vol. 2. ASM International: Metals Park, OH (1986). [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N11.00002: Phase Transition to Modulated Cubic Phase in AuZn alloy S. M. Shapiro, J. C. Lashley, W. Ratcliff, D. J. Thoma, J. L. Smith AuZn is believed to undergo a martensitic transition from a high-temperature cubic phase to a reported low-temperature trigonal phase around 60K. Specific heat studies revealed that the transition is second-order for the 50-50 alloy, and first-order for off-stoichiometric compositions. Elastic neutron diffraction studies on a single crystal of the 50-50 alloy showed the surprising result that the martensitic transformation is suppressed and new Bragg peaks appear at q$_{o }$= (1/3,1/3,0), in a continuous manner. The low-temperature structure can be viewed as cubic with a 3-fold increase in the cell along the [110] direction, similar to what has been observed in premartensitic phases. Attempts to measure the phonon dispersion curves using inelastic x-ray scattering will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N11.00003: Structural Studies on First-Order Phase Transitions inRPd$_{2}$Ga$_{2}$ Robin Macaluso, L. Chapon, E. Goremychkin, R. Osborn, J. Mitchell, B. Rainford We have investigated the structure of RPd$_{2}$Ga$_{2}$ (R = La, Ce) compounds by neutron powder diffraction. For the first time, a first-order structural transition is observed at T$_{t}$ = 70 K and T$_{t}$ = 125 K for CePd$_{2}$Ga$_{2}$ and LaPd$_{2}$Ga$_{2}$, respectively. The high-temperature structure (T $>$ T$_{t})$ for both compounds is the tetragonal CaBe$_{2}$Ge$_{2}$ type with lattice parameters of a = 4.4791(4) {\AA} and c = 9.83732(17) {\AA} and a = 4.83185(19) {\AA} and c = 10.7548(5) {\AA} for LaPd2Ga2 and CePd2Ga2 at 305 K, respectively. Below T$_{t }$ the symmetry of both structures descends to an orthorhombic space group, \textit{Pmmn}. Lattice parameters at 2 K are $a $= 6.07032(3) {\AA}; $b $= 12.90921(6) {\AA}; $c $= 9.87899(5) {\AA} and $a$ = 6.3996(4), $b$ = 11.9508(8), $c$ = 9.9291(7) for LaPd$_{2}$Ga$_{2}$ and CePd$_{2}$Ga$_{2}$. In this talk, evidence for the order of the structural transition will be presented and the low and high temperature structures will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N11.00004: High pressure phases of alkali ternary borohydrides Ravhi Kumar, Andrew Cornelius Insitu synchrotron x-ray diffraction experiments were carried out on MBH$_{4}$ (M = K and Rb) borohydrides at high pressures up to 26 GPa using diamond anvil cells. KBH$_{4}$ undergoes a structural transition at 4 GPa to a tetragonal phase from cubic and then to an orthorhombic phase around 7 GPa which are very similar to NaBH$_{4}$ investigated earlier [1]. However, RbBH$_{4}$ shows, a direct transition from the ambient cubic to an orthorhombic phase at 2.9 GPa, followed by a monoclinic phase at 8 GPa. Complementary high pressure Raman experiments, support the transitions observed in the diffraction experiments. The results will be presented in detail. [1]. Ravhi S. Kumar and Andrew L. Cornelius, Appl.Phys.Lett., 87,261916 (2005) This work is supported in part by the U.S. Department of Energy (DOE) under Award Number DE-FG36 05GO85028. HPCAT is supported by DOE-BES, DOE-NNSA,NSF, and the W.M. Keck Foundation. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N11.00005: First-principles study of the Jahn-Teller distortion in transition metal dihydrides. Ramiro Quijano, Romeo de Coss The transition metal dihydrides TiH$_{2}$ and ZrH$_{2}$ present the fluorite structure (CaF$_{2})$ at high temperature but undergoes a tetragonal distortion with c/a$<$1 at low temperature. Early electronic band structure calculations have shown that TiH$_{2}$ and ZrH$_{2}$ in the cubic phase display a very flat band at the Fermi level. Thus the low temperature tetragonal distortion has been associated to a Jahn-Teller effect. Previous total energy calculations of the tetragonal distortion for TiH$_{2}$ within the Density Functional Theory (DFT), find that the ground state correspond to a tetragonal structure with c/a$>$1, in contradiction with the experimental observation (c/a$<$1). In the present work, we have performed full-potential LAPW calculations using the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA) for the exchange correlation functional energy. Special attention was paid to the convergence of the total-energy calculations, since in TiH$_{2}$ the energy differences for a tetragonal distortion at constant volume are only fractions of 1 mRy. We find that the ground state of TiH$_{2}$ and ZrH$_{2}$ corresponds to a tetragonal distorted fluorite structure with c/a$<$1, in agreement with the experimental observations. The same behavior is predicted for HfH$_{2}$. The electronic band structure of the three systems is analyzed in the context of the Jahn-Teller effect. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N11.00006: Phonon-based Mechanisms for Structural Transformations in Pu Turab Lookman, Avadh Saxena, Robert Albers Plutonium undergoes a number of transformations that involve changes in unit cell shape and numbers of atoms. We focus on the fcc to monoclinic $\delta \rightarrow \alpha^{\prime}$ transformation and suggest displacive mechanisms that can describe the transformation. We also explore signatures of the mechanisms in phonon dispersion curves and present a phase diagram in paramter space. The implications for the $\beta \rightarrow \alpha$ transformation, assuming a displacive mechanism, will also be explored. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N11.00007: Classical and quantum formalism of mode-based atomic-scale descriptions of lattice distortions K.H. Ahn, Jichan Moon We present classical and quantum mechanical atomic scale theory of lattice distortions using atomic scale modes and their constraint equations. The formalism is demonstrated for a 1-dimensional chain and a 2-dimensional square lattice. We comment on the application of the mode-based atomic-scale approach for systems with strong anharmonicity and structural phase transitions. \newline \newline [1] K. H. Ahn, T. Lookman, A. Saxena, and A. R. Bishop, Phys. Rev. B 68, 092101 (2003); \newline [2] K. H. Ahn, T. Lookman, and A. R. Bishop, Nature 428, 401 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N11.00008: Homogeneous nucleation of Lennard-Jones liquids Hui Wang, Harvey Gould, W. Klein We investigate the homogeneous nucleation of a Lennard-Jones liquid as a function of the degree of supercooling. The umbrella sampling method, with the size of the largest cluster as the order parameter, is used to compute the free energy cost of the nucleating droplets. Their saddle point nature is verified using an intervention technique. For moderate supercooling the nucleating droplets are found to be compact and spherical, with the core having fcc symmetry and the surface bcc. As the system is quenched near the spinodal, which corresponds to the vanishing of the free energy barrier, the nucleating droplets become more diffuse and anisotropic with no well defined core or surface. The environment of these nucleating droplets form randomly stacked planes, which is consistent with the spinodal nucleation picture. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N11.00009: Time-domain thermoreflectance of FeRh across magnetic/structural phase transition D.A. Walko, J. Wang, D.G. Cahill, J.-U. Thiele, E.E. Fullerton As FeRh is heated above $\sim100^{\circ}$ C, an antiferromagnetic to ferromagnetic phase transition is accompanied by an abrupt expansion in its lattice parameter of $\sim$0.5\%. This first-order phase transition has a large temperature hysteresis (often $>20^{\circ}$). Ultrafast laser pulses have been shown to heat thin FeRh films through the phase transitions on a picosecond time scale. We have used time-domain thermoreflectance (TDTR) in the temperature range 35 to 160$^{\circ}$ C to study FeRh films grown on MgO substrates. The TDTR measurements were performed with a mode-locked Ti:sapphire laser; the sample was slightly heated with the near-infrared pump beam, and small changes in the sample's reflectivity were observed with the delayed probe beam using lock-in detection. We used TDTR to measure the thermal conductance of the film/substrate interface. Additionally, the small temperature excursions produced in TDTR allowed us to observe the transient behavior of FeRh at various temperatures across the phase transition. The reflectivity is affected by fast changes in the film's optical properties and in its thickness, and we discuss the effects of hysteresis on the measurement. Work supported by U.S. Department of Energy. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N11.00010: Thermodynamic and transport anisotropic properties of RVSb3 crystals Athena S. Sefat, Sergey L. Bud'ko, Paul C. Canfield The RVSb3 series (R= La, Nd, Sm, Gd - Dy) offers the possibility of studying the magnetic ordering in materials with a single, crystallographically unique, rare-earth site. The anisotropic magnetization M(H, T), resistivity, and heat capacity C(T) results, on flux-grown crystals, will be presented. All of the compounds are metallic, and all, with the exceptions of non-magnetic LaVSb3 and ferromagnetic CeVSb3, show features typical of antiferromagnetic order below 10 K. For CeVSb3, the easy axis of magnetization is parallel to c in the ordered state, whereas for the antiferromagnetic RVSb3 members of Pr, Nd, Tb, and Dy, the crystalline fields confine the spins close to the a-axis. Given that CeVSb3 is a rare example of a Ce based ferromagnet, we measured the pressure dependence of Tc up to 10 kbar and found it to increase at a rate of 0.14 K/kbar. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N11.00011: X-ray resonant magnetic scattering study of the spin-flop transition in Gd$_5$Ge$_4$ Lizhi Tan, Shibabrata Nandi, Andreas Kreyssig, Shuang Jia, Alan Goldman, Robert McQueeney, Paul Canfield, Jonathan Lang, Zahirul Islam, Thomas Lograsso, Deborah Schlagel, Vitalij Pecharsky, Karl Gschneidner Gd$_5$Ge$_4$ crystallizes in the orthorhombic space group Pnma. Below 127~K the Gd moments order antiferromagnetically in a layered structure with a magnetic unit cell same as the chemical unit cell. The ferromagnetic Gd-rich slabs are stacked antiferromagnetically along b-axis The magnetic moments are primarily aligned along the c-axis. X-ray resonant magnetic scattering was used to study a fully reversible spin-flop transition in a single crystal Gd$_5$Ge$_4$ and to elucidate details of the magnetic structure in the spin-flop phase. The Gd moments at the three Wyckoff sites flop from c-axis antiferromagnetically aligned to a-axis antiferromagnetically aligned in a critical field $H_{sf} = 9$ kOe applied along c-axis at $T = 10$ K. The magnetic space group changes from $Pnm'a$ to $Pn'm'a'$ at all three sites. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N11.00012: Pressure-Dependent Magnetization Studies of Two Rare Earth-Based Intermetallic Systems R.P. Guertin, E.S. Choi, B. Andraka, C.R. Rotundu, W. McCallum, Y. Janssen Pressure dependent magnetization studies have been performed on two rare earth-based ternary intermetallic systems, R$_{2}$CoIn$_{8}$, where R=Gd, Dy and Pr, and Pr$_{6}$Ni$_{2}$Si$_{3}$, the n=2 member of the Pr$_{(n+1)(n+2)}$Ni$_{n(n+1)+2}$Si$_{n(n+1)}$ family. The pressure dependence of the magnetization was measured for 2$<$T$<$300 K, 0$<$H$<$ 9 T and hydrostatic pressures 0$\le $P$\le $8 kbar using a vibrating sample magnetometer. For the R$_{2}$CoIn$_{8}$ system, R=Dy and Gd order antiferromagnetically at T$_{N}$=17.0 and 34.5 K, respectively and dT$_{N}$/dP=+0.1 K/kbar for R=Dy and +0.4 K/kbar for R=Gd. Pr$_{2}$CoIn$_{8}$ is a van Vleck paramagnet, indicating a crystalline electric field (CEF) singlet ground state. For Pr$_{6}$Ni$_{2}$Si$_{3}$, the Curie temperature (T$_{C}$= 35.0 K) and the saturation magnetization (1.35 $\mu _{B}$/Pr) decrease non-linearly with increasing pressure, consistent with a pressure-induced increase in the CEF splitting. The Pr$^{3+}$ ground state is presumably a singlet, as the local Pr symmetry is very low. Preliminary high field VSM data suggest that a CEF level crossing occurs at 10.5 T where magnetization increases sharply to above 3.0 $\mu _{B}$/Pr. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N11.00013: The equivalency between hydrostatic pressure and Si doping in the giant magnetocaloric compound Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ studied by X-Ray Magnetic Circular Dichroism. Y.C. Tseng, D. Haskel, J. Lang, S. Sinogeikin, Ya. Mudryk, V.K. Pecharsky, K. Gschneidner Jr. The effect of pressure (P$\le $150 kbar) upon the magnetic properties of giant magnetocaloric material Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ (x=0.125, 0.5) was investigated by x-ray magnetic circular dichroism measurements in a diamond anvil cell. The ferromagnetic Curie temperature, Tc, increases linearly with pressure albeit with different slopes dTc/dP for x=0.125 and 0.5. This pressure dependence of Tc, together with a discontinuity in Tc (P) at $\sim $274K, are also observed in the x-T phase diagram. The equivalency of pressure and Si demonstrates that the magnetic properties in this class of materials are controlled by volume, and not by preferential substitution of Si/Ge at certain lattice sites. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N11.00014: Fluctuation induced first-order phase transitions in a dipolar Ising ferromagnetic slab Rafael M. Fernandes, Harry Westfahl Jr. We investigate the size effects on the magnetic phase diagram of an Ising ferromagnetic slab with finite width and finite thickness in which two interactions compete: the short-range strong exchange interaction and the long-range weak dipolar one. We show that the homogeneous ordered state is unstable towards the formation of a modulated phase and that thermal fluctuations induce a first-order Brazovskii transition. By considering striped and bubble modulated configurations, we show that the first has a lower energy and a higher spinodal limit and that in the most stable ordered phase the order parameter is modulated along the limited direction but uniform along the unlimited one. This effect is shown to be a consequence of finite lengths and of Dirichlet boundary conditions in a system with competing interactions. Applications of this model to the domain structure of thin films are discussed, specially for the case of MnAs:GaAs thin films, for which qualitative behaviors of the number of domains and of the mean value of modulation as functions of the temperature are outlined. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N11.00015: Hidden zero-temperature bicritical point in the 2D anisotropic Heisenberg model: Monte Carlo simulations and novel finite-size scaling Chenggang Zhou, David Landau, Thomas Schulthess The bicritical point in the phase diagram of the 2D anisotropic Heisenberg antiferromagnet in a field has not been fully resolved by Monte Carlo simulations. A recent study [Phys. Rev. B \textbf{72}, 064443 (2005)] showed an upper bound for the bicritical temperature. By performing quite detailed Monte Carlo simulations near the apparent spin-flop line, we found this system was governed by a single-spin Hamiltonian, which terminates the renormalization group flow of a finite-size 2D nonlinear $\sigma $ model. Using a novel finite-size scaling analysis, we confirm that the bicritical point in two dimensions is Heisenberg-like and occurs at T=0. Thus, the uncertainty in the phase diagram is completely removed [Phys. Rev. B \textbf{74}, 064407 (2006)]. [Preview Abstract] |
Session N12: Focus Session: Diluted Magnetic Semiconductors I
Sponsoring Units: GMAG DMP FIAPChair: Ramin Abolfath, State University of New York at Buffalo
Room: Colorado Convention Center Korbel 3C
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N12.00001: Effects of co-doping on ferromagnetism in (Zn,Cr)Te. Invited Speaker: Room-temperature ferromagnetism in semiconductors has emerged as one of the most challenging topics in today's materials science and technology. Indeed, enormous research activities have so far been directed towards developing ferromagnetic semiconductors with high transition temperatures. Despite many reports claiming high-temperature ferromagnetism for a broad class of diluted magnetic semiconductors, their intrinsic nature has sometimes been controversial[1], with a lack of elaborated analysis of structural and electronic properties. Among them, Cr-doped ZnTe has been regarded as one of the promising materials of room-temperature ferromagnetism because its intrinsic nature was confirmed through magnetic circular dichroism (MCD) measurement[2]. In this presentation, we report the effect of co-doping of charge impurities on ferromagnetic properties in this material. It was found that ferromagnetism was suppressed in (Zn,Cr)Te co-doped with nitrogen (N) as an acceptor impurity[3] and was enhanced in a crystal co-doped with iodine (I) as a donor impurity[4]. In particular, the apparent Curie temperature $T_{C}$ of Zn$_{1-x}$Cr$_{x}$Te with a Cr composition of $x$ = 0.05 increased up to 300K at maximum due to I-doping, compared to $T_{C}\sim $30K in the undoped crystal. In the structural and compositional analysis using TEM/EDS, it was revealed that the origin of this remarkable effect of the co-doping was the variation of Cr distribution in the crystals; the Cr distribution was strongly inhomogeneous in I-doped crystals with higher $T_{C}$, in contrast to an almost uniform distribution in undoped or N-doped crystals with lower $T_{C}$ or being paramagnetic. In the crystals of inhomogeneous distribution, Cr-rich regions with a typical size of several ten nanometers formed in the Cr-poor matrix act as ferromagnetic nanoclusters, resulting in an apparent ferromagnetic behavior of the whole crystal. These variation of the Cr uniformity can be linked to a change in the Cr charge state due to the co-doping, which is considered to affect the aggregation energy of Cr ions in the host compound ZnTe[5]. These findings will open a new way to control the formation of magnetic nanoclusters in the semiconductor matrix and ferromagnetic properties by manipulating the charge state of magnetic impurities. \newline [1] see, $e.g.$ C. Liu \textit{et al.}, J. Mater. Sci.: Mater. Electron. \textbf{16}, 555 (2005), S. A. Chambers \textit{et al.}, Mater. Today \textbf{9}, 28 (2006). \newline [2] H. Saito \textit{et al.}, Phys. Rev. Lett. \textbf{90}, 207202 (2003). \newline [3] N. Ozaki \textit{et al.}, Appl. Phys. Lett. \textbf{87}, 192116 (2005). \newline [4] N. Ozaki\textit{ et al.}, Phys. Rev. Lett. \textbf{97}, 037201 (2006). \newline [5] T. Dietl, Nature Mater. \textbf{5}, 673 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N12.00002: Reentrant ferromagnetism and its stability in magnetic semiconductors Igor Zutic, Steven Erwin, Andre Petukhov The magnetization of a ferromagnetic material normally decays monotonically with increasing temperature. Here we demonstrate theoretically the possibility of quite different behavior: reentrant ferromagnetism in semiconductors [1]. Reentrant magnetism can arise in semiconductors because as the temperature rises, the resulting higher concentration of thermally excited carriers can enhance the exchange coupling between magnetic impurities. This opens the possibility of materials exhibiting a transition from the low-temperature paramagnetic phase, in which carriers are frozen out, to a ferromagnetic phase at higher temperature. Thus, in the absence of other ferromagnetic mechanisms there will be two critical temperatures, T$_{c1}$ $<$ T$_{c2}$, describing para-to-ferromagnetic and ferro-to-paramagnetic transitions, respectively. Here we determine the phase diagram and the stability of reentrant ferromagnetism within a self-consistent description in which the spin-splitting in both carrier bands is included [2]. We discuss the implications of our findings for transport measurements in magnetic semiconductors, and suggest several candidate materials in which reentrant ferromagnetism might be observable. [1] I. \v{Z}uti\'c, A. Petukhov, S. C. Erwin, preprint. [2] I. \v{Z}uti\'c, J. Fabian, S. C. Erwin, Phys. Rev. Lett. 97, 026602 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N12.00003: Ferromagnetic transition temperature of a two-band model for diluted magnetic semiconductors Florentin Popescu, Yucel Yildirim, Gonzalo Alvarez, Cengiz Sen, Adriana Moreo, Elbio Dagotto Within dynamical-mean field theory we investigate the ferromagnetic transition temperature ($T_{c}$) of a two-band model for diluted magnetic semiconductors in a large range of coupling constants, hopping parameters, and carrier densities [1]. We reveal that $T_{c}$ is optimized at all fillings when both impurity bands fully overlap in the same energy interval, namely when the exchange couplings and bandwidths are equal. The optimal $T_{c}$ is found to be about twice larger than the maximal value obtained in the one-band model. Within a one-band model we also discuss the influence of the Coulomb attractive potential by acceptors on the critical ferromagnetic temperature [2]. \newline [1]. F. Popescu, Y. Yildirim, G. Alvarez, A. Moreo, and E. Dagotto, Phys. Rev. B, 73 (2006), 075206. \newline [2]. F. Popescu, C. Sen, E. Dagotto, and A. Moreo, in preparation. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N12.00004: Magnetic properties of the two-impurity Anderson model for a semiconductor host Nejat Bulut, Yoshihiro Tomoda, Kazuo Tanikawa, Saburo Takahashi, Sadamichi Maekawa We study the nature of the magnetic correlations in the two-impurity Anderson model for a semiconductor host using the quantum Monte Carlo technique and the Hartree-Fock approximation. We find that the impurity spins exhibit ferromagnetic correlations with a range which can be much more enhanced than in a half-filled metallic band. In particular, the range is longest when the Fermi level is located above the top of the valence band and decreases as the impurity bound state becomes occupied. In addition, we investigate the magnetic correlations between the impurity moments and the host electronic spins. Comparisons with the photoemission and optical absorption experiments suggest that this model captures the basic electronic structure of Ga$_{1-x} $Mn$_x$As, the prototypical dilute magnetic semiconductor (DMS). These numerical results might also be useful for synthesizing DMS or dilute-oxide ferromagnets with higher Curie temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N12.00005: Phase Diagram of the Disordered RKKY Model in Dilute Magnetic Semiconductors Invited Speaker: We consider ferromagnetism in spatially randomly located magnetic moments, as in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect exchange RKKY interaction. We obtain, via Monte Carlo calculations, the magnetic phase diagram as a function of the impurity moment density $n_{i}$ and the relative carrier concentration $n_{c}/n_{i}$. As evidenced by the diverging ferromagnetic correlation length and magnetic susceptibility, the boundary between ferromagnetic and nonferromagnetic phases constitutes a line of zero temperature critical points which can be viewed as a magnetic percolation transition. In the dilute limit, we find that bulk ferromagnetism vanishes for $n_{c}/n_{i} > 0.1$. We also incorporate the local antiferromagnetic direct superexchange interaction between nearest neighbor impurities and examine the impact of a damping factor in the RKKY range function. This work has been done in collaboration with Sankar Das Sarma at the University of Maryland and supported by the US-ONR and NSF. \newline \newline [1] D.J. Priour, Jr. and S. Das Sarma, Phys. Rev. Lett., {\bf 97}, 127201 (2006). \newline [2] D.J. Priour, Jr. and S. Das Sarma, Phys. Rev. B, {\bf 73}, 165203 (2006). \newline [3] D.J. Priour, Jr., E.H. Hwang, and S. Das Sarma, Phys. Rev. Lett. {\bf 95}, 037201 (2005). \newline [4] S. Das Sarma, E.H. Hwang, and D.J. Priour, Jr., Phys. Rev. B {\bf 70}, 161203 (2004). \newline [5] D.J. Priour, Jr., E.H. Hwang, and S. Das Sarma, Phys. Rev. Lett. {\bf 92}, 117201 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N12.00006: Absence of Self-Averaging in Disordered Heisenberg Models Donald Priour, Jr., Sankar Das Sarma With the aid of direct large-scale Monte Carlo simulations, we find a lack of self-averaging near the Curie temperature $T_ {c}$ for classical ferromagnetic Heisenberg models on disordered three dimensional lattices. Our calculations encompass a wide range of system sizes, generally systems with between $10^{3}$ and $10^{5}$ magnetic moments, and we have in general found the extent of the violation of self-averaging to be very stable throughout this range of sizes. In contradiction to the Harris Criterion, which predicts self-averaging to be intact for disordered Heisenberg models, we find the degree of violation of self-averaging (as extrapolated to the bulk limit) to rise monotonically with increasing disorder strength; even small amounts of disorder yield a nonzero, albeit weak, violation of self- averaging. We examine various bond and site disordered Heisenberg models, and we also consider strongly disordered RKKY models for dilute magnetic semiconductors, where we find a marked violation of self-averaging. This work has been supported by the US-ONR and NSF. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N12.00007: The Mobility Edge in Disordered Ferromagnetic Doped Semiconductors Erik Nielsen, R. N. Bhatt While the clearest example of ferromagnetism in doped semiconductors is seen in diluted magnetic semiconductors such as Ga$_{1-x}$Mn$_x$As,\footnote{H. Ohno, Science 281, 951 (1998)} under certain conditions, semiconductors doped with non-magnetic impurities may also exhibit ferromagnetic ground states.\footnote{Erik Nielsen and R. N. Bhatt, APS March Meeting 2006.} We present numerical results of the nature of single particle states in such a positionally disordered three-dimensional system with a maximally spin-polarized ground state using a realistic potential for hydrogenic centers.\footnote{R. N. Bhatt and T. M. Rice, Physical Review B 23, 1920 (1981).} In particular, we identify the mobility edges, which mark the energies at which single particle states become delocalized, and whose location relative to the Fermi energy determine electronic transport in the system. We describe the dependence of the mobility edges on impurity density and potential, and discuss the variation of conductivity with impurity and carrier density. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N12.00008: Exchange interactions in Mn-doped ScN Aditi Herwadkar, Walter R. L. Lambrecht We present a computational study of the exchange interactions in Mn-doped ScN. First, we test to what extent the Heisenberg Hamiltonian applies to large spin rotations. We calculate them instead from a non-collinear calculation with a small rotation. This shows that the exchange interactions obtained from a collinear energy difference (antiferromagnetic- ferromagnetic) would be overestimated by about 30\%. Next, we calculate the exchange interactions using the linear response multiple scattering theory approach of Liechtenstein et al. We find $\sum_j J_{0j}$ for a nearest neighbor pair is the same as before, but we now find this to be a sum over many long-range interactions. The actual near neighbor interactions are an order of magnitude smaller. Finally, we study various special quasirandom structures with 256 and 432 atoms per cell with concentrations of 3-10 \%. These calculations indicate that with randomness, many of the long-range interactions become negative and thus lead us to believe that Mn-doped ScN is not a ferromagnetic semiconductor but a spin-glass. These calculations do not yet include the effects of n-type doping,which might add ferromagnetic couplings for long-range neighbors. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N12.00009: Coulomb attraction and defects in dilute magnetic semiconductors P. L. Reis, M.A. Majidi, J. Moreno, R.S. Fishman, M. Jarrell Employing the dynamical mean-field approximation we study the phase diagram of a double-exchange model that includes interactions between the holes and the local magnetic moments and also the negatively charged ions. We calculate the ferromagnetic transition temperature, magnetization and susceptibility for a range of parameters and compare the results of a single band model with a four-band model which properly includes the heavy and light bands. The inclusion of the Coulomb attraction allows a better comparison with experiments by reducing the values of the exchange coupling needed to support a ferromagnetic transition. For small or intermediate exchange couplings the Coulomb attraction increases the transition temperature. We will also study a model where additional non-magnetic defects are included in the Hamiltonian. In the presence of these defects the ferromagnetic transition is expected to be rapidly suppressed. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N12.00010: Numerical Study of Magnetically doped III-V Zinc-Blende-Type Semiconductors Yucel Yildirim, Gonzalo Alvarez, Adriana Moreo Using a newly developed real-space Hamiltonian for zinc-blende dilute magnetic III-V semiconductors we study the properties of a variety of materials. The hopping parameters are functions of the tabulated Luttinger parameters of the III-V parent compounds, and the dispersion of the heavy-hole, light-hole, and split-off bands is reproduced next to the top of the valence band in the undoped case. The exchange interaction parameter $J$ is obtained from measurements; thus, there are no free parameters in the model. The spin-orbit interaction is considered as well as the randomness in the doping. Unbiased Monte Carlo techniques are applied to clusters containing about 300 III-type ions. After successfully reproducing experimental results for (Ga,Mn)As [1], here we present a comprehensive study of the magnetization vs. temperature, the density of states, and the charge distribution for Mn doped GaSb, GaP, GaN, AlP, and InAs. We also present the calculated Curie temperatures for a variety of Mn concentrations and hole compensations. \newline \newline [1] Y. Yildirim, G. Alvarez, A. Moreo, and E. Dagotto, preprint October 2006 [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N12.00011: Complexities in diluted magnetic semiconductors-a theoretical perspective from ab-initio electronic structure calculations Biplab Sanyal, Diana Iusan, Olle Eriksson Diluted magnetic semiconductors (DMS), the essential materials for semiconductor spintronics, show a variety of complex properties, e.g., defect-mediated (ferro/antiferro)magnetic interactions and the disorder leading to magnetic percolation effects. Using the ab-initio Korringa-Kohn-Rostoker-Coherent-Potential-Approximation, the magnetic pair exchange parameters of a Heisenberg model have been calculated for Mn doped ZnO and half-Heusler NiTiSn hosts followed by the calculation of transition temperatures using Monte-Carlo simulations. Zinc vacancies and nitrogen substituting oxygen atoms lead to ferromagnetic interactions in Mn doped ZNO while in a defect free case, the interaction between Mn atoms is antiferromagnetic. The calculated critical temperatures are low ($\sim $35 K) due to the short-ranged exchange interactions and low defect concentration. In the other case, Mn doped NiTiSn shows a high critical temperature ($\sim $300 K) for 22 {\%} Mn concentration. Below 3{\%} Mn, there is no magnetic long range order as the magnetic percolation is not established. The results are in good agreement with experiments. [Preview Abstract] |
Session N13: Focus Session: Multiferroics IV
Sponsoring Units: DMP GMAGChair: Claude Ederer, Columbia University
Room: Colorado Convention Center Korbel 4C
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N13.00001: Multiferroic Domain Dynamics and Phase Transitions in Strained SrTiO$_3$ Films S. Denev, A. Vasuvaderao, A. Kumar, M. Biegalski, Y. Li, L.-Q. Chen, S. Trolier-McKinstry, D. Schlom, V. Gopalan SrTiO$_3$ is a material that is not normally ferroelectric or multiferroic at any temperature. However, epitaxial biaxial strain in thin film form can induce multiferroicity in strained SrTiO$_3$ (J.H.Haeni \textit{et al.},Nature \textbf{430},758 (2004)). We have demonstrated multiferroicity in strained SrTiO$_3$ films on scandate substrates, with the presence of two independent order parameters, a polar ferroelectric polarization vector, and an axial antiferrodistortive rotation vector. Using Optical Second Harmonic Generation (SHG), we have distinguished these axial and polar properties, tracked the ferroelectric and antiferrodistortive phase transitions as a function of temperature, and determined the point group symmetry of the ferroelectric and multiferroic phases. For the first time, we have shown direct imaging of ferroelectric domains and revealed the mechanism of coupled switching of ferroelectric-ferroelastic domains under electric fields using piezoelectric force microscopy combined with phase field simulations. (Phys. Rev. Lett., Accepted, in print (2006)). These studies have broader relevance to multiferroics with coupled polar and axial order parameters, such as ferroelectric antiferromagnets. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N13.00002: Broadband IR Spectroscopy of Multiferroic BiFeO$_{3}$ J. Seidel, C.L.S. Kantner, Y.-H. Chu, L. Yang, Z. Schlesinger, D. Viehland, J. Orenstein, R. Ramesh BiFeO$_{3}$ (BFO) is a multiferroic material in which both the ferroelectric and antiferromagnetic ordering is present at room temperature. In order to investigate the dynamics of the coupling between order parameters, optical spectroscopy measurements were made on both a single crystal and epitaxially grown thin film samples. Measurements were made from 3-30cm$^{-1}$ using time domain terahertz spectroscopy, and from 20-700 cm$^{-1}$ with FTIR reflectivity. We report on the spectral weight and damping of modes in BFO in the spectral region where antiferromagnetic resonance is typically observed in orthoferrites. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N13.00003: Electrodynamics of a multiferroic perovskite manganite in terahertz frequency range N. Kida, Y. Ikebe, R. Shimano, Y. Yamasaki, T. Arima, Y. Tokura There is a growing interest for the study of the magnetoelectric effect, as stimulated by the observation of a magnetic control of the ferroelectric polarization in perovskite manganites. Recently, a broad peak structure was observed in TbMnO$_3$ and GdMnO$_3$ in terahertz (THz) frequency range [1]. The spin-wave excitation driven by ac electric field, which is referred to as electromagnon, was proposed as an origin of this structure. However, detailed characteristics, especially, the role of the rare-earth ion and the relationship between the complex dielectric constant $\tilde\epsilon$ in THz and low ($\sim$kHz) frequency range were not clarified yet. Here we used the THz time-domain spectroscopy to directly extract $\tilde\epsilon(\omega)$ (1.2-- 4.5 meV) of a multiferroic perovskite manganite and discuss the origin of the ferroelectricity, as manifested by a gigantic response of the low-frequency $\tilde\epsilon(\omega)$ with temperature and magnetic fields. [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N13.00004: Magnetoelectric Emission in a Magnetic Ferroelectric Er-doped (Ba,Sr)TiO$_3$ Yoshi-Aki Shimada, Masakazu Matsubara, Yoshio Kaneko, Jing-Ping He, Yoshinori Tokura In the system where both space-inversion ($\mathcal{I}$) and time-reversal ($\mathcal{R}$) symmetries are broken simultaneously, a nonreciprocal optical phenomenon termed the optical magnetoelectric (OME) effect is expected to show up. As the crystal that has neither $\mathcal{I}$ nor $\mathcal{R}$ symmetry, we have investigated an Er$^{3+}$-doped ferroelectric (Ba,Sr)TiO$_3$ single crystal under magnetic field in which the luminescent magnetic Er$^{3+}$ ion occupies a noncentrosymmetric site. The ${\boldmath k}$-directional dichroism derived from the OME effect was verified in the $^{4}I_{13/2} {\rightarrow}^{4}I_{15/2}$ emission by the reversal of magnetic field and spontaneous polarization. The observed nonreciprocity $\Delta I/I \sim 0.5$ \% at 3000 Oe implies the possibility of the application of the OME effect to the function of an optical isolator. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N13.00005: Colossal magnon-phonon coupling in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$ Rolando Valdes Aguilar, A.B. Sushkov, H.D. Drew, C. Zhang, S-W. Cheong The temperature dependence of the far infrared (IR) transmission spectra (1-30 meV) of multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$ has been measured. This system is chosen to correspond to TbMnO$_3$, but without the magnetism of the rare earth ion. We find a spectacular transfer of spectral weight from the lowest frequency IR active phonon to a magnetic excitation, at lower frequencies, when light is polarized parallel to the static polarization \textbf{P}$_a$. We also observe the electromagnon at a frequency of 2.5 meV, with the same selection rule. The electromagnon produces the observed increase in the dielectric constant as the system enters the ferroelectric phase. The observations of large spectral weight transfer and of the electromagnon selection rule, are not consistent with the model of the electrodynamic response of helical magnets proposed by Katsura, et al\footnote{Katsura, et al. CondMat (2006)}. We compare and contrast these results to other multiferroic manganites. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N13.00006: Giant magnetoelastic effect in multiferroic Ba$_{0.6}$Sr$_{1.4}$Zn$_{2}$Fe$_{12}$O$_{22}$ Diyar Talbayev, Richard D. Averitt, Antoinette J. Taylor, Tsuyoshi Kimura Dynamical studies of multiferroic materials help unravel the fundamental interactions between various degrees of freedom and answer technological questions such as achievable switching speeds in multiferroic-based memory elements. We report the results of the ultrafast optical study of multiferroic Ba$_{0.6}$Sr$_{1.4}$Zn$_{2}$Fe$_{12}$O$_{22}$, which reveals a giant magnetoelastic effect in the material. The compound exhibits a hexagonal crystal structure and a helical magnetic ground state below $\sim $ 330 K. In applied magnetic field, the hexaferrite undergoes a series of magnetic phase transitions and develops ferroelectric polarization. The magnetoelastic effect is detected via the measurement of the speed of sound in the crystal as a function of magnetic field. The oscillation in the optically induced transient reflectivity resulting from the propagating coherent-phonon strain pulse allows us to measure the field-induced changes in the speed of sound and the corresponding dramatic changes in the elastic stiffness. The dependence of the exchange interaction on the distance between Fe ions gives rise to the observed magnetoelasticity. Our results indicate a route towards the magnetically modulated transducers and piezoelectric devices. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N13.00007: Far-infrared transmission spectroscopy studies of HoMn$_{2}$O$_{5}$ single crystals at the commensurate-incommensurate phase transition A.A. Sirenko , S. Park , S. M. O' Malley , G. L. Carr , S-W. Cheong Spectra of the low-frequency IR-active excitations in HoMn$_{2}$O$_{5}$ multiferroic single crystals have been studied using synchrotron radiation based far-infrared transmission spectroscopy at U12IR beamline of NSLS-BNL in the frequency range between 8.5 and 105 cm$^{-1}$. Both preferable polarization of IR-active excitations along crystallographic directions of HoMn$_{2}$O$_{5}$ and temperature variation of their oscillator strength reveal strong changes at the commensurate-incommensurate phase transition at $T_{3 }$= 19 K. Transmission spectra are interpreted in terms of the electromagnon, magnon, and crystal-field splitting excitations. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N13.00008: Electromagnons in multiferroic RMn$_{2}$O$_{5}$ compounds Andrei Sushkov, Rolando Valdes Aguilar, Dennis Drew, Soonyong Park, Sang-Wook Cheong Electromagnons, or magnons with electric dipole activity, were observed so far only in some multiferroic RMnO$_{3}$ and RMn$_{2}$O$_{5}$ compounds. Electromagnons in these two systems have essentially different properties. We try to take a systematic look at electromagnons in the whole RMn$_{2}$O$_{5}$ family. In this talk, we report the results of detailed infrared study$^{\ast }$ of YMn$_{2}$O$_{5}$ and TbMn$_{2}$O$_{5 }$ as well as some results on other RMn$_{2}$O$_{5}$ compounds. We found that electromagnons in the RMn$_{2}$O$_{5}$ family is a property of the manganese sublattices. The electromagnon spectrum consists of a set of well defined far infrared (3-80 cm$^{-1})$ modes which are just slightly broader than uncoupled magnons. No obvious changes in the phonon spectrum have been observed. Polarization of electromagnons is in agreement with the dominating symmetric exchange. * A. B. Sushkov, R. Valdes Aguilar, S. Park, S-W. Cheong, and H. D. Drew, cond-mat/ 0608707. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N13.00009: Magnons and electromagnons in multiferroic materials Maxim Mostovoy The interest in studying excitations in frustrated magnets lies in their unusual nature and strong effect on frustrated ordering. The coupled spin-lattice dynamics in frustrated magnets, in which magnetic ordering breaks inversion symmetry and induces electric polarization, was recently studied in optical absorption and neutron scattering experiments. I will present a theory of magnetic excitations coupled to polar phonon modes (electromagnons) in multiferroic materials showing incommensurate magnetic orders, e.g. RMnO$_{3}$, Ni$_{3}$V$_{2}$O$_{8}$, and MnWO$_{4}$, and discuss the evolution of the excitation spectrum at the transition from the paraelectric sinusoidal to the ferroelectric spiral state. The incommensurate orders give rise to a multi-band structure of magnetic excitations, while the magnetoelectric coupling makes possible to excite magnons by oscillating electric field. Even for weak coupling the probability of electro-excitation of magnons is relatively large. Furthermore, the polarization dependence of the optical absorption makes possible to discriminate between the electromagnon and antiferromagnetic resonance peaks. I will also discuss electromagnons in a different class of multiferroics, such as the RMn$_{2}$O$_{5}$ compounds. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N13.00010: Theory of electrical control of spin waves in multiferroic materials Rogerio de Sousa, Joel Moore We consider the question of electrical generation, control, and detection of magnons in thin films of multiferroic BiFeO$_{3}$. This material possesses simultaneous ferroelectric and antiferromagnetic order, with Dzyaloshinskii-Moriya and other magnetoelectric couplings. The spectrum for the coupled spin and polarization waves is found to be extremely anisotropic, allowing the control of spin waves via electrical switching of the direction of the spontaneous polarization vector. Electrical generation and detection of spin waves is optimal at the anticrossings of the polarization and magnetic branches, where the excitations have electromagnon character. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N13.00011: Temperature-dependent Raman spectroscopy of multiferroic TbMn$_2$O$_5$ J. R. Simpson, A. R. Hight Walker, R. Valdes Aguilar, A. B. Sushkov, H. D. Drew, S. Park, S.-W. Cheong Multiferroic materials that display coupling between order parameters, \textit{e.g.}, magnetic and dielectric, stimulate fundamental interest and provide the potential for applications in novel multifunctional devices. The multiferroic manganite TbMn$_2$O$_5$ exhibits non-collinear magnetic order and a strong magnetoelectric coupling effect. The recent observation\footnote{R. Vald\'{e}s Aguilar \textit{et al.}, Phys. Rev. B \textbf{74}, 184404 (2006).} of infrared (IR) phonon modes correlated with magnetic and dielectric phase transitions suggests a complementary Raman study may provide important information regarding the nature of coupling in these systems. We present Raman measurements of single-crystal TbMn$_2$O$_5$ in a collinear backscattering configuration as a function of temperature ($4-300\,$K) and polarization along various crystallographic axes. Additionally, we compare the temperature dependence of Raman active phonons with the activation of an IR forbidden mode in the low-temperature ferroelectric state. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N13.00012: Magnetic properties and electronic structure of doped multiferroic Y$_{x}$A$_{1-x}$MnO$_{3}$(A=Ca, Sr, Ba) J. Y. Juang, C. C. Hsieh, T. Y. Cheng, J. M. Lee, J. M. Chen, J.-Y. Lin, K. H. Wu, T. M. Uen, Y. S. Gou We report the magnetic properties, X-ray absorption spectroscopy (XAS) on a series of doped multiferroic materials Y$_{x}$A$_{1-x}$MnO$_{3 }$(A=Ca, Sr, Ba). YMnO$_{3}$ when doped by alkaline-earth metal with various ionic sizes, display dramatic changes in magnetic properties as compared with the parent compound. For Ca-doped sample, the antiferromagnetic (AFM) phase transition appears to take place at a much lower temperature (30 K) as compared to that of undoped one (42 K), which could be Mn-rich. On the other hand, when doped with ions of larger size such as Ba and Sr, the AFM temperature decreased only slightly to around 38 K but with significantly smeared transition. By comparing the XAS results to standard manganese oxide powder, YMnO$_{3}$ exhibits the dominant Mn$^{+3}$ characteristics obtained from the standard Mn$_{2}$O$_{3}$ powder. Although, the undoped-YMnO$_{3}$ and Sr-, Ba-doped YMnO$_{3}$ exhibited very similar electronic structure as revealed in the XAS data, the XAS of Ca-doped sample, again, is very different from that of YMnO$_{3}$. It is surprising to observe that Ca-doping has resulted in most significant modifications in the magnetic property and electronic structure of YMnO$_{3}$, since Ca$^{+2}$ is having exactly the same ionic size as that of Y$^{+3}$ and is expected to cause minimal distortion on the lattice. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N13.00013: Multiferroic Behavior in Barium Hexaferrite Probed with Optical Second Harmonic Generation Eftihia Vlahos, Sava Denev, Venkatraman Gopalan, Tsuyoshi Kimura Barium hexaferrite $Ba_{0.5} Sr_{1.5} Zn_2 Fe_{12} O_{22} $ is a very promising material, which exhibits significant magnetoelectric (ME) effect, i.e., the generation of electric polarization/magnetization by the application of magnetic/electric) field. Optical second harmonic generation (SHG) in the reflection geometry was used to determine the magnetic point group symmetries and phase transitions of the sample versus temperature, and variable magnetic field. Simultaneous measurements of magnetocapacitance, and ME current as functions of temperature and applied magnetic field are performed and correlated with SHG measurements. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N13.00014: Observation of local non-centrosymmetry in weakly ferroelectric YCrO3 Anna Llobet, Kannadka Ramesha, Thomas Proffen, C.R. Serrao, C.N.R. Rao Using high resolution neutron powder diffraction we have determined the average and local structure of YCRO in order to explain the recently reported ferroelectric character YCRO. Unlike other ferroelectric systems, YCRO has been found to have a centrosymmetric crystal structure which is inconsistent with the development of electric polarization because it requires atomic off-centering. We have characterized the different length scales existent in YCRO and found that, although the average crystallographic structure above and below the ferroelectric transition is orthorhombic and centrosymmetric ($Pnma$), in the ferroelectric state YCRO is locally non-centrosymmetric and Cr is displaced about 0.01~{\AA} from its position along \textit{z} direction. We conclude that the local character of the Cr off-centering and the small value of the displacement observed could explain the weak ferroelectric behavior. This new concept of ``local non-centrosymmetry'' might be of great importance for the understanding of unusual properties of other multifunctional materials as well. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N13.00015: Computational study of the ferromagnetic and ferroelectric properties of (Bi$_{2}$Mn NiO$_{6})_{4}$. Leonard Kleinman, B.R. Sahu, Adrian Ciucivara Using the VASP PAW code with the GGA and including spin-orbit coupling and allowing for non collinear magnetization, we performed electronic structure calculations for the multiferroic crystal, (Bi$_{2}$Mn NiO$_{6})_{4}$. The lattice angle and lattice constants are in excellent agreement with experiment. The magnetization is 4.94 $\mu _{B}$. The polarization, for which there is no experimental value, is 16.84 $\mu $Ccm$^{-2}$. Inverting the positions of all the atoms we iterated to convergence. The magnetization did not change and the total energy was also unchanged. Thus we conclude that the polarization and magnetization are uncoupled and (Bi$_{2}$Mn NiO$_{6})_{4}$ is unlikely to have any device applications. [Preview Abstract] |
Session N14: Focus Session: Current Induced Magnetization Dynamics and Spin Transfer
Sponsoring Units: GMAG DMP FIAPChair: Tingyong Chen, The Johns Hopkins University
Room: Colorado Convention Center Korbel 4D
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N14.00001: Temperature rise due to Joule heating in a spin transfer torque nano-pillar structure Chun-Yeol You, Seung-Seok Ha, Hyun-Woo Lee Considering that the spin-transfer-torque-induced magnetization dynamics in a nano-pillar structure usually requires a large current density of $10^{11}$ A/m$^2$, it is desired to have an accurate estimation of the temperature rise caused by the current-induced Joule heating. We investigate the current- induced heating effect in the nano-pillar by analytical and numerical methods. We employ the Green's function method to obtain analytic solution of the heat conduction equation. With proper approximations, we derive a simple analytic relation that expresses the temperature in term of the current density, the geometry of the nano-pillar, and material properties. The validity of the analytic expression is confirmed by the comparis on with commercial finite element method software. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N14.00002: Current-Hysteretic Low Frequency Oscillations in Spin-Transfer Nanocontacts Matthew Pufall, William Rippard, Michael Schneider, Thomas Silva, Stephen Russek We have observed spin-transfer-driven large amplitude, current hysteretic, low frequency ($<$ 500 MHz) oscillations in nanocontacts made to spin valve structures. The oscillations occur only for small ($<$50 Oe) in-plane applied fields, but persist in fields up to several kOe for out of plane fields. The frequency of oscillation is typically far below the uniform-mode ferromagnetic resonance frequency, and is only a weak function of applied field. Hysteresis in the presence/absence of the oscillations is observed with dc current, with oscillations first appearing at high currents with increasing current, but persisting to lower currents upon decreasing the current. We suggest that these observations are consistent with dynamics of a vortex-like state in the vicinity of the contact, one nucleated by the Oersted fields generated by the dc current, and with dynamics driven by the spin transfer torque. The electrical oscillation amplitudes are large, with the largest amplitudes approaching 1 mV, and are narrowband, with many devices exhibiting sub-megahertz linewidths. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N14.00003: Thermal effects in spin torque switching Michael Schneider, Mattheew Pufall, William Rippard, Stephen Russek, Jordan Katine We compare low temperature device behavior with room temperature behavior. We find agreement between our low temperature critical current measurements and Slonczewski theory$^{1}$. In addition, we find that the values extrapolated from the low temperature measurements were robust with respect to device size. At room temperature we find substantial variation in the hysteretic region from device to device for devices of the same nominal size. While this is not expected, it has been attributed to thermal effects having a strong influence on the response of the freelayer to applied field as well as the coercivity$^{2}$. We find that by reducing the temperature, and thus any thermal fluctuations, the device to device variations are drastically reduced. While we did observe indications of non-single domain behavior at 5 K, it is noteworthy that these did not seem to affect the critical switching current. Thus, we conclude that the room temperature device to device variations in the quasi-static switching behavior is dominated by thermal effects. 1. J. C. Slonczewski, J. Magn. Magn. Mater. \textbf{159,} L1-L7 (1996) 2. D. Lacour, J. A. Katine, N. Smith, M. J. Carey, and J. R. Childress, \textbf{85,} 4681-4683 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N14.00004: Nonlocal magnetization dynamics Invited Speaker: Recently, nonlocal properties of ferromagnetic dynamics in magnetic nanostructures, such as damping sensitive to the Ohmic environment and spin-wave transfer between exchange-decoupled ferromagnets, have attracted a considerable interest [1]. It is also becoming clear [2] that nonlocal dynamic effects are important for understanding intrinsic properties such as magnetic damping, noise, and spin-transfer torques in inhomogeneous ferromagnets, with consequences for phenomena ranging from spin- wave propagation and domain-wall motion to current-driven instabilities in the bulk. I will present a self-consistent mean-field approach for treating these properties in a unified and rather general fashion. \newline \newline [1] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, Rev. Mod. Phys. 77, 1375 (2005) \newline [2] Y. Tserkovnyak, H. J. Skadsem, A. Brataas, and G. E. W. Bauer, Phys. Rev. B 74, 144405 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N14.00005: Precession damping in itinerant ferromagnets Keith Gilmore, Mark Stiles, Yves Idzerda The damping of excited magnetic states has long been understood at a phenomenological level through the Landau-Lifshitz-Gilbert equation. Increased interest in nanoscale devices, the behavior of which can be strongly dependent on the damping, demands a more thorough understanding of the relaxation process. While magnetic alloy systems are used in most applications, we consider the simpler 3d transition metals (iron, nickel, and cobalt) in order to understand the most basic processes involved in damping before approaching the more complicated mechanisms expected in alloys. Resonance experiments for Co and Ni indicate low and high temperature regions for which the damping parameter is roughly proportional and inversely proportional, respectively, to the scattering time. We report and numerically test a model that produces both of these behaviors. As with all previous work, the calculations presented are given in terms of an unknown electronic scattering time. To make meaningful comparisons between the calculated and measured damping parameters we evaluate an expression for the conductivity, derived by similar methods, and compare also to transport experiments. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N14.00006: Fast Pulse Measurements and Temperature Variation of Enhanced Magnetic Damping of Spin-Transfer Excitation E. M. Ryan, A. G. F. Garcia, P. M. Braganca, G. D. Fuchs, N. C. Emley, J. C. Read, E. Tan, D. C. Ralph, R. A. Buhrman, J. A. Katine Recently, light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase the damping parameter $\alpha $ by several orders of magnitude [1]. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.004 um$^{2}$ Py/Cu/Py nanopillar spin valves with 0 and 2{\%} Tb in the free layer, and measured critical currents across a range of temperatures from 4.2 K to 295 K. We find that the critical currents for reversibly switching the free layer, generally expected to be proportional to $\alpha $, are several times larger on average in the 2{\%} Tb samples than in pure Py samples, and increase linearly with decreasing temperature. We will also discuss FMR data, and data for switching with fast pulses from 1 to 100 nsec at both 150 K and room temperature, along with matching simulations that allows us to extract $\alpha $ and other spin-torque parameters [2]. These results suggest one approach for controllably reducing the negative impact of spin-torque effects on nanoscale spin valve and read head sensors, and achieving a deeper understanding of these spin-torque devices. [1] W. Bailey, P. Kabos, F. Mancoff, and S. E. Russek, IEEE Trans. Magn. 37, 1749 (2001). [2] P. M. Braganca, et al. Appl. Phys. Lett. 87, 112507 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N14.00007: Modeling study on the self-consistent feedback between inhomogeneous magnetization and the spin torque Kyung-Jin Lee, Bernard Dieny The Slonczewski's spin torque terms were originally suggested within the context of homogeneous magnetic domain. Micromagnetic [1] and experimental [2] studies have revealed that the magnetizations excited by the spin torque could be inhomogeneous. Therefore we have to find a way of correcting the Slonczewski's terms in describing the magnetization dynamics. We show the self-consistent model to numerically solve the equations of motion of local magnetization and spin accumulation. The self-consistent model enables us to consider the feedback between inhomogeneous magnetization and the spin torque. We found the feedback is crucial in the magnetization dynamics induced by the spin torque. We will show the computational evidence of the importance of the feedback for the current-induced magnetic excitation in a single Co layer and a spin valve structure. [1] K. J. Lee et al. Nat. Mat. 3, 877 (2004); Appl. Phys. Lett. 88, 132506 (2006), [2] Y. Acremann et al. Phys. Rev. Lett. 96, 217201 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N14.00008: Planar Spin-Transfer Device with a Dynamic Polarizer. Yaroslaw Bazaliy, Debo Olaosebikan, Barbara Jones In planar nano-magnetic devices magnetization direction is kept close to a given plane by the large easy-plane magnetic anisotropy (e.g. by shape anisotropy in a thin film). In conventional micromagnetics it is known that in this case the magnetization motion is effectively in-plane with only one angle required for its description, and can become overdamped even for small values of the Gilbert damping. We extend the equations of the effective in-plane dynamics to include the spin-transfer torques. The simplifications achieved in the overdamped regime allow us to study systems with several dynamic magnetic pieces (``free layers''). A transition from a spin-transfer device with a static polarizer to a device with two equivalent magnets is observed: when the size difference between the magnets is less than critical, the device does not exhibit switching, but goes directly into the ``windmill'' precession state. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N14.00009: Role of Spin dependent Inelastic Scattering in Spin Torque Devices Sayeef Salahuddin, Supriyo Datta Spin torque devices are commonly modeled by looking only at the spin dependent transmission and reflection at the tunnel oxide-ferromagnet interface. Here, we describe a different approach where, in addition to barrier dependent phenomena, an inelastic spin-flip scattering is included at the interface. We show that such scattering events may have significant influence on the device behavior, specifically on the magnitude of TMR and on the efficiency of spin torque. We shall show that recent experiments provide evidence for this prediction. Our transport model is based on Non Equilibrium Green's Function (NEGF) formulation where the scattering is included through a self energy matrix. We also discuss$^{1}$ how the spin flip scattering may help to reduce the switching current necessary to flip the magnetization in penta-layer spin torque devices, a phenomenon demonstrated in recent experiments$^{2}$. \begin{enumerate} \item S.Salahuddin and S. Datta, Appl. Phys. Lett., 89,153504, 2006. \item G. D. Fuchs et. al., Appl. Phys. Lett. 86, 152509, 2005;M. Huai et. al., Appl. Phys. Lett., 87, 222510, 2005~;H. Meng et. al., Appl. Phys. Lett. 88, 082504, 2006. \end{enumerate} [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N14.00010: Spin-polarized scanning tunneling microscope and the Kondo effect Kelly Patton, Stefan Kettemann We calculate the tunneling current between a spin-polarized scanning tunneling microscope (SP-STM) and a Kondo impurity on a metallic substrate, including the effects of the spin-polarization of the SP-STM on the adsorbate. This spin-polarization breaks the spin symmetry of the Kondo system, similar to an applied magnetic field, which leads to a splitting of the Abrikosov-Suhl-Kondo resonance. The amount of splitting is controlled by the strength of the coupling between the impurity and the SP-STM tip. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N14.00011: Absence of persistent spin transport Noah Bray-Ali, Zohar Nussinov, Alexander Balatsky A system that is in its ground state does not transport charge. Spin transport also does not occur. We extend Bohm's argument for the absence of persistent charge transport to show the absence of persistent spin transport. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N14.00012: Effect of Annealing on Extraordinary Hall Effects in Sputtered Granular Cu$_{80}$Co$_{20}$ Thin Films Nam H. Wang, Jian-Qing Wang This work explores the microstructure dependence of extraordinary Hall effect (EHE) in Cu$_{80}$Co$_{20}$ granular thin films. Upon annealing, the Cu-Co films showed anomalous microstructure evolution into two-particle distribution, as evident in measured magnetic susceptibility versus temperature, showing existence of double peak structures in magnetic blocking for annealing temperature above 300 $^{o}$C. Such unusual nanostructure directly affected the magneto-transport properties, most noticeable in the extraordinary Hall effect (EHE). The measured EHE was compared with Co-Ag films series, with more uniform particle distribution, which was shown to inversely scale with the scattering length and average particle sizes. Such scaling relationship was absent in Cu-Co films. It was concluded that the EHE in Cu-Co annealed films primarily depends on the population of smaller-sized particles. This was evident in independence of EHE saturation field on the annealing temperature. The gradual decrease of EHE with the annealing is a result of two combined effects. The initial linear decrease below 250 $^{o}$C in EHE is a result of interface change of the Co particles in Cu matrix. As the larger particles began to emerge, further decline in EHE is due to the reducing smaller particle population, while the larger particles do not contribute significantly to the EHE. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N14.00013: Evidence for a disorder-dependent localization correction to the anomalous Hall conductance of ultrathin Fe films Rajiv Misra, Arthur F. Hebard We present an experimental study of quantum corrections to the conductivity tensor of thin ferromagnetic films when the disorder is systematically varied. Using the sheet resistance as a measure of disorder, \textit{in situ} magnetotransport studies were performed on a series of thin iron films deposited onto sapphire substrates having sheet resistance $R_{0 }\equiv $~$R_{xx}$(5K) varying over the range 140~$\Omega $ (60{\AA}) to 6250 $\Omega $ ($<$20 {\AA}). For temperatures $T$~$<$~20 K, a logarithmic temperature dependence of the longitudinal $R_{xx}$ and anomalous Hall resistances $R^{AH}_{xy}$ is observed. In the low disorder limit (R$_{0}$ $<$~150~$\Omega )$, we find that relative changes in the anomalous Hall conductivity $\delta $\textit{$\sigma $}$^{AH}_{xy}$/\textit{$\sigma $}$^{AH}_{xy }$exhibit a temperature independent behavior implying that there are no quantum corrections to \textit{$\sigma $}$^{AH}_{xy}$. As disorder increases, a finite logarithmic temperature dependence to $\delta $\textit{$\sigma $}$^{AH}_{xy}$/\textit{$\sigma $}$^{AH}_{xy}$ appears and then evolves toward a universal weak localization correction defined by the equality $\delta $\textit{$\sigma $}$^{AH}_{xy}$/\textit{$\sigma $}$^{AH}_{xy}=-\delta R^{AH}_{xy}$/$R^{AH}_{xy}$ [1]. Thus with increasing disorder, we see a crossover from a region where there are no quantum corrections to \textit{$\sigma $}$^{AH}_{xy}$ to a region dominated by weak localization corrections. These results for iron, where spin is carried by itinerant electrons, will be compared with data on thin films of gadolinium, a localized moment system. [1]. Mitra P. et al. \textit{cond-mat 0606215} (2006) [Preview Abstract] |
Session N15: Focus Session: Quantum Dimers and Bose Einstein Condensation
Sponsoring Units: GMAGChair: Matthew Stone, Oak Ridge National Laboratory
Room: Colorado Convention Center Korbel 4E
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N15.00001: Engineering exotic phases for topologically protected quantum computation: Emulating quantum dimer models A.F. Albuquerque, H.G. Katzgraber, M. Troyer, G. Blatter Motivated by recent interest in engineering topologically ordered phases for achieving fault-tolerant quantum computation, we analyze an implementation of a quantum dimer model on the triangular lattice using an array of Josephson junctions [L.~B.~Ioffe {\it et al.}, {\it Nature} {\bf 415}, 503 (2002)]. Using the numerical Contractor Renormalization (CORE) technique, we are able to derive in an unbiased way an effective Hamiltonian describing the low-energy physics of the underlying Bose Hubbard model on the Josephson junction lattice. Our results show that resonances and interactions with three or more dimers have to be included in order to obtain an optimal set of junction capacitances and currents. We discuss the effects of these higher-order terms on the topological dimer liquid phase which is required for fault-tolerant quantum computation. Attempts to suppress higher-order dimer operators can only be attained if the junction's capacitances and currents are far beyond values obtainable with current technology. An alternative implementation based on cold-atoms loaded into optical lattices is also considered, but in this case the absence of sizable interactions is a major obstacle. Our results suggest that the emulation of topological phases in quantum devices can only be a viable approach if special attention is paid to the design and engineering limits. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N15.00002: Devil's staircases, quantum dimer models, and stripe formation in strong coupling models of quantum frustration. Kumar Raman, Stefanos Papanikolaou, Eduardo Fradkin We construct a two-dimensional microscopic model of interacting quantum dimers that displays an infinite number of periodic striped phases in its T=0 phase diagram. The phases form an incomplete devil's staircase and the period becomes arbitrarily large as the staircase is traversed. The Hamiltonian has purely short-range interactions, does not break any symmetries, and is generic in that it does not involve the fine tuning of a large number of parameters. Our model, a quantum mechanical analog of the Pokrovsky-Talapov model of fluctuating domain walls in two dimensional classical statistical mechanics, provides a mechanism by which striped phases with periods large compared to the lattice spacing can, in principle, form in frustrated quantum magnetic systems with only short-ranged interactions and no explicitly broken symmetries. Please see cond-mat/0611390 for more details. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N15.00003: Quantum Phase Transition of Heisenberg Antiferromagnet with Four-Spin Ring Exchange Daoxin Yao, Valeri N. Kotov, Antonio H. Castro Neto, David K. Campbell We discuss the S=1/2 Heisenberg antiferromagnet model on a square lattice with nearest-neighbor (J) and plaquette (K) exchanges, which exhibits a quantum phase transition from a spontaneously dimerized phase to N\'eel order at a critical coupling (K/J). We calculate the triplon spectrum starting from the Valence Bond Solid phase and show that good agreement with recent Monte Carlo data (A. W. Sandvik) can be achieved. The quantum phase transition is signaled by vanishing of the triplon gap at the N\'eel vector. We find that strong quantum fluctuations of the dimer background are present, especially near the critical point, which signals a tendency towards restoration of lattice symmetry as evidenced by the strong reduction of the dimerization. Even though within our method full restoration of symmetry is impossible to achieve, the above features are consistent with a critical point exhibiting ``Deconfined quantum criticality,'' of which the present model is believed to be the first example. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N15.00004: Phase separation and crystallization in doped quantum dimer models Stefanos Papanikolaou, Erik Luijten, Eduardo Fradkin By employing analytical methods and Monte Carlo simulations, we study generalized doped quantum dimer models with exact ground-state wavefunction amplitudes that are given by the weights of generic two-dimensional classical partition functions. We derive the phase diagram of these models in the coupling-density plane. At low doping, liquid and solid phases are separated by continuous transitions, but beyond a tricritical point the transition becomes first order. We focus on the properties of high-doping regions where solid-liquid phase coexistence is observed and analyze the fate of these regions under the introduction of additional interactions. 1. S. Papanikolaou, E. Luijten and E. Fradkin, cond-mat/0607316, 2. S. Papanikolaou, E. Luijten and E. Fradkin (in preparation). [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N15.00005: Exotic Phases of Hard-core Bosons with Correlated Hopping Kai P. Schmidt, Julien Dorier, Andreas Laeuchli, Frederic Mila We investigate the interplay of correlated hopping and repulsive interactions for hard-core bosons on a square lattice using quantum Monte Carlo simulations. The first part of this contribution is on a remarkable low-density pairing phase. While for non interacting hard-core bosons the effective attraction induced by the correlated hopping leads to phase separation at low density, we show that a nearest-neighbor repulsion suppresses phase separation, leading to a low-density pairing phase with no single particle Bose-Einstein condensation but long-range two-particle correlations, signalling a condensation of pairs. The second part of this contribution targets at the physics at higher densities. Here, the most important question is the possible existence of a supersolid phase in such a model with correlated hopping. \newline [1] K.P. Schmidt, J. Dorier, A. Laeuchli, and F. Mila, Phys. Rev. B 74, 174508 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N15.00006: Bose-Einstein Condensation in quasi-one dimensional Ladder materials Thierry Giamarchi, Edmond Orignac, Roberta Citro Various magnetic systems are made of assemblies of dimers. In such systems it was proposed [1] that a magnetic field can induce a quantum phase transition in the universality class of a Bose-Einstein condensation. Such a transition has been, by now, observed in a variety of dimer systems. Recently, systems such as BPCB [2], where the dimers are organized in quasi-one dimensional ladders have been investigated. This compounds offers an interesting crossover between one dimensional and three dimensional behavior. We build on [1] and [3] to analyze various physical properties of such a system both in the one- and three-dimensional regimes. We focus in particular on the NMR relaxation time properties.\newline \newline [1] T. Giamarchi and A. M. Tsvelik Phys. Rev. B {\bf 59} 11398 (1999). \newline [2] B. C. Watson et al. Phys. Rev. Lett {\bf 86} 5168 (2001).\newline [3] R. Chitra and T. Giamarchi Phys. Rev. B {\bf 55} 5816 (1997) [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N15.00007: Excitations from a Bose-Einstein condensate of magnons in coupled spin ladders. Andrey Zheludev, Ovidue Garlea, Takatsugu Masuda, Hirotaka Manaka, Louis-Pierre Regnault, Jae-Ho Chung, Yiming Qiu, Klaus Habicht, Klaus Kiefer The weakly coupled quasi-one-dimensional spin ladder compound (CH$_3$)$_2$CHNH$_3$CuCl$_3$ is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in a reference 3D quantum magnet. However, for the upper two massive magnon branches the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N15.00008: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N15.00009: Magnetic Excitation Spectrum of Doped and Undoped Spin Ladders B. Lake, S. Notbohm, D. A. Tennant, R. I. Bewley, C. D. Frost, P. Manuel, R. S. Eccleston, K. P. Schmidt, G. S. Uhrig, P. Ribeiro, C. Hess, C. Sekar, R. Klingeler, G. Krabbes, G. Behr, B. Buchner We present inelastic neutron scattering measurements of three spin-ladder compounds. All are based on two-dimensional copper oxide layers where the copper ions form two-leg, spin-1/2, spin-ladders. Strong antiferromagnetic interactions couple the spin moments along the legs and rungs of the ladder, weaker higher order four-spin ring exchange terms exist, while interladder coupling is weak and frustrated. Two-leg spin-1/2 spin-ladders are characterized by gapped, well-defined one-magnon excitations and multi-magnon continuum excitations. CaCu$_{2}$O$_{3}$ has a weak rung interaction (compared to the leg) and a strong ring interaction which act to drive the system gapless and quantum critical. La$_{4}$Sr$_{10}$Cu$_{24}$O$_{41}$ has a stronger rung and is gapped. Both compounds are undoped and their magnetic excitation spectrum is compared to theoretical models. The third material, Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$, is similar to La$_{4}$Sr$_{10}$Cu$_{24}$O$_{41}$ but with holes on the ladder. The holes give rise to significant changes in the excitations which are discussed and compared to theory and the excitation spectrum of high-T$_{c}$ superconductors. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N15.00010: A low-field disordered-free-moment phase in site-diluted spin-gap antiferromagnets Rong Yu, Tommaso Roscilde, Stephan Haas Site dilution of spin-gapped antiferromagnets leads to localized free moments, which can order antiferromagnetically in two and higher dimensions. A very important question of high experimental relevance is: what is the response of the diluted system to an applied magnetic field? This is a very complicated problem since the exponentially decaying interactions between the free moments introduce a large variety of energy scales, which respond differently to the field. Here we show how a weak magnetic field drives this order-by-disorder state into a novel \emph{disordered-free-moment} phase, characterized by the formation of local singlets between neighboring moments and by localized moments aligned antiparallel to the field. This disordered phase is characterized by the absence of a gap, as it is the case in a Bose glass. The associated field-driven quantum phase transition is consistent with the universality of a superfluid-to-Bose-glass transition. The robustness of the disordered-free-moment phase and its prominent features, in particular a series of \emph{pseudo}-plateaus in the magnetization curve, makes it accessible and relevant to experiments. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N15.00011: SP (N) Treatment of Frustrated Spin Dimer Systems in Magnetic Field Marianna Maltseva, Rebecca Flint, Piers Coleman We present a Schwinger boson treatment of a frustrated bilayer dimer spin system using a reformulation of the SP (N) approach to frustrated spin systems. Unlike previous SP (N) approaches[1], our starting model is composed uniquely of SP (N) spin generators, which permits a more symmetric treatment of antiferromagnetic and ferromagnetic bonds. We apply our methods to model the spin condensation process that occurs in $Ba CuSi_{2}O_{6}$. One of the issues of particular interest is the dependence of the interlayer order-from-disorder effects[4] on the applied magnetic field, and the interesting possibility that these couplings vanish at the critical field[2,3]. [1] S. Sachdev, N. Read, International Journal of Modern Physics B 5, 219 (1991). [2] S. E. Sebastian, N. Harrison, C. D. Batista, L. Balicas, M. Jaime, P. A. Sharma, N. Kawashima, I. R. Fisher, Nature 441, pp 617-620 (2006). [3] C. D. Batista, J. Schmalian, N. Kawashima, S. E. Sebastian, N. Harrison, M. Jaime, I. R. Fisher, cond-mat/0608703. [4] M. Maltseva, P. Coleman, Phys. Rev. B 72, 174415-9 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N15.00012: Complex 2D Oxide BaCuSi$_2$O$_6$: A NMR Study R. Stern, S. Kr\"{a}mer, M. Horvatic, C. Berthier, I. Heinmaa, E. Joon, T. Kimura, S.E. Sebastian, I.R. Fisher BaCuSi$_2$O$_6$ is a quasi-2D oxide composed of Cu$_2$Si$_4$O$_{12}$ layers where Cu$^{2+}$ ions are arranged in well-separated dimers perpendicular to 2D layers. It has a singlet ground state in zero magnetic field, with a large gap to the lowest excited triplet states. Magnetic fields in excess of $H_{c1}$ $\sim$ 23.5 T close the gap, cooling in $H \leq H_{c1}$ results in a state characterized by long-range magnetic order, the nature of which has not been determined yet. We present nuclear magnetic resonance (NMR) measurements of $^{29}$Si and $^{63,65}$Cu on single crystals of BaCuSi$_2$O$_6$ below as well as above $H_{c1}$. Our results prove that the system is less symmetric and more complicated than initially supposed. In the ``normal'' phase we confirmed an IC character of the phase below 100 K. Unexpectedly, two copper sites having strongly different spin polarizations have been observed, which can be associated by the presence of two different gaps and J (J$_1$ and J$_2$) values in the system. Analysis of these data provides a quantitative measure for the size of the perturbation of the ``ideal'' Hamiltonian, helping to decide whether a BEC-type description is still possible. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N15.00013: Magnons dispersion and anisotropies in SrCu$_2$(BO$_3$)$_2$ Olivier Cepas, Y. F. Cheng, P.W. Leung, T. Ziman We study the dispersion of the lowest excited states in the 2d Shastry-Sutherland system, SrCu$_2$(BO$_3$)$_2$, including all relevant Dzyaloshinskii-Moriya interactions. We can reduce the complexity of the general Hamiltonian to a new simpler model at zero-field that is obtained by transformations of the spin operators. The resulting Hamiltonian is studied by means of exact numerical diagonalization on a 32-site cluster and the couplings are extracted. The Dzyaloshinskii-Moriya interactions affect the dispersion of the magnons (triplet states) to linear-order because they partially lift the frustration of the lattice. We argue that earlier perturbative techniques have overestimated the dispersion and missed the dominant interactions responsible for the dispersion. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N15.00014: Perturbation Effects in the Spin-Singlet State of the Two-Dimensional System SrCu$_2$(BO$_3$)$_2$ Adam Aczel, Graeme Luke, Greg MacDougall, Jose Rodriguez, Chris Wiebe, Hanna Dabkowska, Yasutomo Uemura, Peter Russo, Andrei Savici, Hiroshi Kageyama SrCu$_2$(BO$_3$)$_2$ is a quasi-two dimensional spin system with a spin-singlet ground state. This system has attracted much interest recently due to its relevance to the two-dimensional Shastry-Sutherland model. We have performed $\mu$SR studies on single crystals of SrCu$_2$(BO$_3$)$_2$. We observe two different muon sites which we associate with muons located adjacent to the two inequivalent O sites in the system. One site, presumed to be located in the Cu-O-Cu superexchange path, exhibits a large increase in the Knight shift with decreasing temperature which is unaffected by the singlet formation, indicating that the muon has locally broken at least one spin singlet bond and created some quasi-free spins. Further evidence of this phenomenon is provided by examining our ZF-$\mu$SR data, as we observe a large increase in relaxation for the site with the large Knight shift. This is in contrast to the weak, practically temperature-independent relaxation expected in a singlet state. In this talk, I will present TF, ZF, and LF-$\mu$SR data describing this system and explain this perturbation effect in more detail. In addition, results for single crystals doped on the Cu and Sr sites will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N15.00015: Signatures of phonon splitting in the infrared spectra of a quantum magnet SrCu$_2$(BO$_3$)$_2$ S.V. Dordevic, C.C. Homes, T. R{\~o}{\~o}m, D. H{\"u}vonen, U. Nagel, A. Gozar, G. Blumberg, A. LaForge, D.N. Basov, N. Drichko, M. Dressel, H. Kageyama Infrared spectroscopy studies of SrCu$_2$(BO$_3$)$_2$ have been performed along both the in-plane and c-axis crystallographic directions. The reflectance will be reported over a broad range of frequencies (from about 30 cm$^{-1}$ to 20,000 cm$^{-1}$) and temperatures (from 4.2 K to 300 K). In the in-plane spectra we observe a new feature developing at 443 cm$^{-1}$ (55 meV) below about 20 K. Detail temperature, magnetic field and polarization dependence of this feature will be reported. All the results point toward close relation of 443 cm$^{-1}$ mode with the development of singlet ground state in SrCu$_2$(BO$_3$)$_2$. [Preview Abstract] |
Session N16: Focus Session: Spin and Magnetization Dynamics
Sponsoring Units: GMAG DCOMP DMPChair: Oleg Tchernyshyov, Johns Hopkins University
Room: Colorado Convention Center Korbel 4F
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N16.00001: {\it Ab-initio} calculation of electron-phonon coupling for spin relaxation in metals. Miguel Pruneda, Ivo Souza Spin-electronic devices have motivated an important effort in understanding the mechanisms for spin-relaxation, because the operation of such devices requires long spin-diffusion lenghts. Two main factors contribute to spin relaxation: (i) spin-orbit interaction, which mixes the spin-up and spin-down components of the electronic wavefunction, and (ii) electron scattering from defects or phonons. In metals, the phonon-mediated Elliot-Yafet mechanism is believed to be dominant. Realistic calculations are computationally demanding,\footnote{J. Fabian and S. Das Sarma, {\it Phys. Rev. Lett.} {\bf 83}, 1211 (1999).} requiring an accurate description of the electronic states near the Fermi surface and their coupling to the lattice (phonons). Here we use a Density Functional Perturbation Theory implementation to calculate from first-principles the electron-phonon interaction in systems with spin-orbit coupling. Combined with recently-developed Wannier-interpolation methods for sampling efficiently the Brillouin zone, this will allow for a fully {\it ab-initio} calculation of the spin relaxation in metals. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N16.00002: Gilbert damping and spin Coulomb drag in a magnetized electron liquid with spin-orbit interaction. Ewelina Hankiewicz, Giovanni Vignale, Yaroslav Tserkovnyak We present a microscopic calculation of the Gilbert damping constant for the magnetization of a two-dimensional spin- polarized electron liquid in the presence of intrinsic spin- orbit interaction. First we show that the Gilbert constant can be expressed in terms of the auto-correlation function of the spin-orbit induced torque. Then we specialize to the case of the Rashba spin-orbit interaction and we show that the Gilbert constant in this model is related to the spin-channel conductivity. This allows us to study the Gilbert damping constant in different physical regimes, characterized by different orderings of the relevant energy scales -- spin-orbit coupling, Zeeman coupling, disorder, $e-e$ interaction, spin precession frequency -- and to discuss its behavior in various limits. Particular attention is paid to interaction effects, which enter the spin conductivity via the spin Coulomb drag coefficient. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N16.00003: Wannier interpolation study of the Elliot-Yafet spin relaxation in metals Eric Roman, Ivo Souza, Jonathan Yates Energy states of nonmagnetic metals may be chosen to be purely spin up and down in the absence of spin-orbit coupling. Spin-orbit coupling mixes the two states by a small amount $b^2$. A spin-conserving interaction (e.g. electron-phonon) causes transitions between the two states, and flips the electron's spin. Some insight into this Elliot-Yafet spin relaxation mechanism can be obtained by averaging $b^2$ over the Fermi surface. In trivalent metals, such as aluminum, $b^2\ll 1$ almost everywhere on the Fermi surface, except at small ``hot spot'' regions. \footnote{J. Fabian and S. Das Sarma, Phys. Rev. Lett. {\bf 81}, 5624 (1998).} Although the small regions of large $b^2$ dominate the spin relaxation process, they are difficult to capture numerically. We describe a Wannier interpolation strategy \footnote{X. Wang, J. Yates, I. Souza, and D. Vanderbilt, Phys.\ Rev.\ B, in press (cond-mat/0608257).} to compute $\langle b^2\rangle$. We validate it by performing {\it ab initio} calculations on aluminum, finding good agreement with previous results.$^1$ We also discuss interpolating {\it ab initio} electron-phonon matrix elements to compute the spin relaxation rate. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N16.00004: Berry phase Chern number spin Hamiltonians for nanomagnets using DFT techniques Invited Speaker: We will present a formalism capable of describing the low-energy spin dynamics of ferromagnetic metal nanoclusters consisting of up to a few tens of atoms[1]. Our procedure is based on a quantum action with a single magnetization-orientation degree of freedom corresponding to the direction of the Kohn-Sham spin- density functional theory wave-function. Besides the magnetic anisotropy energy functional, the action contains a Berry phase term arising when the fast electronic degrees of freedom are integrated out. The associated Berry curvature has a nontrivial dependence on magnetization orientation when spin-orbit interactions are included; its average over all magnetization directions is a topological invariant known as Chern number, which can only be a multiple of half integers. From the magnetic anisotropy energy and Berry curvature functionals, it is possible to construct an effective quantum Hamiltonian for the nanomagnet, in terms of a single giant-spin degree of freedom whose magnitude is equal to the Berry phase Chern number. We illustrate this procedure by computing within DFT the anisotropy energy and Berry curvature for small clusters of transition metal atoms, from which we extract the corresponding spin Hamiltonians. We show that the Berry phase term can profoundly alter the dynamics of the spin degree of freedom. Our approach can address the spin dynamics of small nanomagnets, which is now accessible experimentally in STM-engineered magnetic clusters[2]. [1] C.M. Canali, A. Cehovin and A.H. MacDonald, Phys. Rev. Lett. {\bf 91}, 046805 (2003); [2] C.F. Hirjibehedin et al., Science {\bf 312}, 1021 (2006); D. Kitchen et al., Nature {\bf 442}, 436 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N16.00005: Gauge fields, the Berry phase, motive forces and the dynamics of domain walls etc. Stewart Barnes, Jun-ichi Ieda, Sadamichi Maekawa The theory of the dynamics of domain walls and spin valves is described within the Stoner model. Using principally domain walls as examples, to be outlined are issues which arise from the requirements of energy conservation and the nature of relaxation within such a simple model. While they are not currently common currency for those working in this field, emphasized are the importance of certain vector potentials which reflect angular momentum transfer and energy conservation and which lie beyond the traditional single electron approach to this simplest model. The (majority/minority electron) spin derived forces $\vec f^\pm_s$ which arise from such dynamics are given by \begin{equation} \vec f^\pm_s = - \frac{\hbar}{2}\frac{ \partial \vec A^\pm_s}{\partial t} - \vec \nabla_{\vec r} \varphi_s^\pm. \label{force} \end{equation} where the vector potential $\vec A^\pm_s$, introduced here, reflects the Berry phase and corresponds to a ``no name'' non-conservative spin forces. The {\it usual\/} ``Stern-Gerlach'' forces correspond to the second term. This and a second gauge field $\vec A^t_s$ are required if the dynamical version of the Stoner theory is to conserve energy and angular momentum. The effects are {\it not\/} small and have significant experimental consequences and device applications. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N16.00006: Topology of composite domain walls in magnetic nanostrips O. Tchernyshyov, O. Tretiakov, Ya. B. Bazaliy, D. Clarke We discuss the internal structure of domain walls in thin magnetic nanostrips of submicron width. The walls are composite objects made from elementary topological defects. These defects are characterized by two topological charges: the O(2) vortex winding number [1] and the O(3) skyrmion number. The defects are ordinary vortices and antivortices in the bulk and fractional vortices with half-integer winding numbers at the edge. Topology and energetics restrict the allowed compositions of a domain wall to a halfvortex and an antihalfvortex (a transverse wall) or a vortex and two antihalfvortices (a vortex wall). We present a variational model [2] that reproduces quite well the major features of a vortex wall. Despite the apparent complexity, the wall has a rigid structure. Its main degrees of freedom are the location of the vortex core and the out-of-plane magnetization of the core, which is related to the skyrmion number of the vortex. [1] O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. \textbf{95}, 197204 (2005). [2] H. Youk \textit{et al.}, J. Appl. Phys. \textbf{99}, 08B101 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N16.00007: Dissipative dynamics of composite domain walls in magnetic nanostrips O. Tretiakov, Ya. B. Bazaliy, O. Tchernyshyov We describe the dynamics of domain walls in thin magnetic nanostrips of submicron width under the action of magnetic field. Once the fast precession of magnetization is averaged out, the dynamics reduces to purely dissipative motion where the system follows the direction of the local energy gradient (Glauber's model A) [1]. We then apply the method of collective coordinates [2] to our variational model of the domain wall [3] reducing the dynamics to the evolution of two collective coordinates (the location of the vortex core). In weak magnetic fields the wall moves steadily. The calculated velocity is in good agreement with the results of numerical simulations (no adjustable parameters were used). In higher fields the steady motion breaks down and acquires an oscillatory character caused by periodic creation and annihilation of topological defects comprising the domain wall [3]. Numerical simulations uncover at least two different modes of oscillation. [1] C. J. Garc\'{\i}a-Cervera and W. E, J. Appl. Phys. \textbf{90}, 370 (2001). [2] A. S\'anchez and A. R. Bishop, SIAM Rev. {\bf 40}, 579 (1998). [3] Preceding talk by O. Tchernyshyov. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N16.00008: First-principles laser-driven magnetic switching scenario in NiO Georgios Lefkidis, Wolfgang H\"ubner The dispersionless discrete intragap d-character levels of the (001) surface and the bulk of NiO can be selectively addressed by laser pulses and thus serve as intermediate levels for a Lambda-based all-optical magnetic switching scenario [1]. To this goal the existence of spin-mixing terms in the Hamiltonian of the system is essential, in our case it is the spin-orbit coupling term in combination with a static external magnetic field. We compute from first principles the aforementioned intragap levels with high-level correlated quantum chemistry on a doubly embedded cluster model [2] and we propagate the population in time under the influence of the laser field. The polarization, duration, shape and geometrical dependences on the laser pulse as well as the influence of the static magnetic field are shown, and the importance of going beyond the electric dipole approximation is discussed. \newline \newline [1] R. G\'{o}mez-Abal, O. Ney, K. Satitkovitchai and W. H\"{u}bner, Phys. Rev. Lett. 92, 227402 (2004) \newline [2] G. Lefkidis and W. H\"{u}bner, Phys. Rev. Lett. 95, 77401 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N16.00009: Atomistic simulations of domain wall dynamics in magnetic wires Maria Stamenova, Tchavdar Todorov, Stefano Sanvito The dynamical interplay between the conduction electrons and magnetization in mesoscopic magnetic structures generates interesting new physics. For instance, there is the possibility of a domain wall (DW) motion, driven by a spin-polarized electron flux. Here we address computationally the reverse phenomenon, namely, the generation of an electromotive force (emf) by the motion of a domain wall. We describe a one-dimensional magnetic wire within the \textit{s-d} model, where conduction electrons are locally exchange coupled to classical magnetizations. For this closed quantum-classical spin-polarized system we have developed an Ehrenfest Molecular Dynamics simulation, which allows us to study the spatial and temporal evolution of any observables, characterizing the system. We have studied the motion of DWs in magnetic field as function of their thicknesses (ranging from the physical limit of one atomic spacing to two orders of magnitude thicker). For all of those we have systematically found charge redistribution along the wire, governed by the DW motion, which is a signiture of an emf. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N16.00010: Magnetization induced by an acoustic wave Simon Kos, Peter Littlewood, Darryl Smith We predict that in a semiconductor with a Rashba-type spin-orbit coupling to strain, an acoustic wave will induce a wave of magnetization. We study the effect in the ballistic and diffusive regime, and we estimate its magnitude. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N16.00011: Effects of surface waves on crystals of molecular magnets: Semi-classical approach. Carlos Calero, Eugene Chudnovsky The effect of surface waves on the spin-state of a molecular magnet is theoretically investigated. As it was recently noted, the anisotropy axis of a molecular magnet is locally defined, so that its direction is modified by local distortions of the lattice. Therefore, its spin-Hamiltonian must be generally written as $\mathcal{H} = \exp[-\imath{\bf S}\cdot \delta {\bf {\phi}} ]\mathcal{H}_A \exp[\imath {\bf S}\cdot \delta {\bf {\phi}}] + \mathcal{H}_Z $, where $\delta {\bf \phi} = \frac{1}{2}\nabla \times {\bf u}({\bf r})$ is the angle of the local rotation induced by the displacement field ${\bf u}({\bf r})$, $\mathcal{H}_A$ is the anisotropy Hamiltonian and $\mathcal{H}_Z$ is the Zeeman term. Based on this idea we obtain the Hamiltonian describing the interaction between spin and the distortion of the lattice produced by the surface waves. We then analyze the spin-dynamics of a single nanomagnet by employing a semi-classical approach: the displacement field ${\bf u}({\bf r})$ is treated as a continuous classical field, whereas the spin-state of the nanomagnet is described quantum-mechanically. Analytical formulas for the spin-dynamics are given for certain geometrical arrangements. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N16.00012: Ferromagnetism and current-controlled magnetization of nanomagnets with giant magnetic anisotropy Bang-Gui Liu Because the giant uniaxial magnetic anisotropy is advantageous in keeping the spins stable for practical applications in information processing and storage, we study ferromagnetism of nanomagnets with giant uniaxial magnetic anisotropy and how to control their magnetization by injecting a spin-polarized current. The giant anisotropy leads to a barrier for reversing a spin. We use kinetic Monte Carlo method to simulate the spin dynamics. We obtain the experimental ferromagnetism and its temperature dependence with experimental parameters. The ferromagnetism is formed because the nanomagnets are limited in space and the experimental duration is finite in time. Furthermore, we design a special nanomagnet and study its magnetization reversal under applied spin-polarized currents. We observe a hysteresis loop against the current. Starting from whatever value, the magnetization can be controlled by the spin-polarized current. Y Li and B-G Liu: Phys. Rev. Lett. 96, 217201 (2006); Phys. Rev. B 73, 174418 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N16.00013: Strain and Stress in nano-structure spintronics devices due to spin transfer torque. Hao Yu, Jun-Ming Liu There have been many interests of the effect of magnetization reversal induced by current in spintronics, namely, spin transfer or spin torque effect, firstly predicted by Slonczewski and Berger in 1996. Because of the conservation of angular momentum in the spin transfer process, an additional lattice angular momentum has to be brought to balance the redundant angular momentum of the spin transfer torque. The lattice angular momentum introduces strain and stress to the nano structure of a spintronics device. In this theoretical work, we calculate the strain and stress tensors due to spin transfer in two kinds of structure: a giant magnetoresisteance (GMR) sandwich structure and a ferromagnetic nanowire. When high-density current (above some threshold value) is through them and then the named spin transfer effect occurs, the strain and stress in both longitudinal and transverse direction of the structure appear. We obtain the relationship between the strain tensors and the spin polarized current density, and sketch the diagram of the strain of the nano structures. The stain and stress produced by the spin transfer torque may introduce destructive force in spintronics devices. [Preview Abstract] |
Session N17: Adsorption Phenomena
Sponsoring Units: DPOLYChair: Hide Yokoyama, Advanced Industrial Science and Technology, Japan
Room: Colorado Convention Center 102
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N17.00001: Adsorption of polymers on colloid particles Dadong Yan, Shuang Yang, Charles C. Han, An-Chang Shi The adsorption of homopolymers on spherical particles with a strong attractive potential has been studied within the self-consistent field theory. The particles are immersed in concentrated polymer solutions and the structure of the adsorbed polymer layer has been examined as a function of the particle size, focusing on the average loop and tail length at different bulk concentrations and solvent qualities. The scaling relationship between the average tail/loop length and the degree of polymerization has also been investigated. It is found that the average loop length is insensitive to the particle size, while the average tail length depends strongly on the particle size. In particular, tails become longer for smaller particles or larger surface curvatures. It is argued that this size effect may provide a mechanism for the excess entanglements induced by adding nanoparticles to polymer solutions. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N17.00002: Colloidal Lithography and Particle Decoration Metrology Steven Hudson, Thuy Chastek, Barry Bauer The self-assembled organization of particles depends on the symmetry of their interactions, and strides are being made in producing nanoparticles of controlled shape and functionalization. Here we use particle adsorption to control and detect surface modification, particle symmetry and shape. We have studied ways to achieve a random sequential adsorption of polystyrene (PS) spheres to make asymmetric particles on charged polyelectrolyte films. After coating the spheres with an oppositely charged layer and releasing them from the charged film, they had a small charged patch on their surface. This provided sufficient area to associate with a single oppositely charged particle of comparable size, and resulted in the controlled formation of asymmetric doublets. Additionally, a strategy that added oppositely charged particles, which were smaller than the charged surface patch on the PS spheres, was used to measure the size of the exposed charged area. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N17.00003: Adsorption and Fractionation of RAFT-polymerized PS-b-PMMA Block Copolymers for 2D Liquid Chromatography Junwon Han, Chang Y. Ryu, Ho-Cheol Kim, Greg Breyta, Hiroshi Ito Polymer adsorption in nanoporous silica is important for the advancement of polymer separation and purification techniques. In particular, we will demonstrate how understanding of the polymer nanopore adsorption can be applied for (1) a quantitative analysis of block copolymers using the adsorption-based interaction chromatography and (2) a large scale fractionation of block copolymer using a simple gravity column of silica gel. Our target polymers for the analysis and fraction are polystyrene-block-poly(methyl methacrylate) diblock copolymers (PS-b-PMMA) synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. PMMA precursors with phenyldithiobenzoate end group are used as a macromolecular chain transfer agent for the RAFT polymerization, and the contents of PS and PMMA homopolymers in the RAFT PS-b-PMMA block copolymers have been quantitatively analyzed by a solvent gradient interaction chromatography technique. Specifically, we have employed both bare silica and C18-bonded silica columns for the 2-dimensional chromatography analysis and large scale fractionation of the block copolymers in terms of their chemical heterogeneity. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N17.00004: Direct Fluorescence Measurements of Polymer Surface Diffusion and Intramolecular Rearrangements Janet Wong, Liang Hong, Sung Chul Bae, Steve Granick A picture is emerging, based on few-molecule fluorescence spectroscopy, of polymer surface dynamics at the solid-liquid interface. Here we describe experiments using fluorescence correlation spectroscopy (FCS) and F\"{o}rster Resonance Energy Transfer (FRET) of polystyrene (PS) and polyethylene oxide (PEO) adsorbed from good solvent. In-plane translational diffusion of these polymers was measured as a function of molecular weight and surface coverage. We show the surface diffusion (D) decreases with molecular weight in a power law fashion with exponent equal to -3/2 in the regime of dilute surface coverage. The surface coverage ($\Gamma )$ effect on D is even more intriguing, with an initial increase with $\Gamma $and then decreases after a critical $\Gamma $ is achieved. Exploring the hypothesis that the change in D reflects chain conformational change as $\Gamma $ increases, experiments are underway that employ FRET to quantify the chain end-to-end separation. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N17.00005: Brownian diffusion close to polymer brushes Benoit Loppinet, Emma Filippidi, Vassilik Michailidou, George Fytas, Juergen Ruehe Brownian diffusion of diluted colloidal particles of different sizes was investigated by evanescent wave dynamic light scattering in the vicinity of polystyrene polymer brushes grafted to a glass surface. The particles concentration profiles, resolved from the penetration depth dependence of the scattered intensities, evidenced an excluded region close to the glass hard wall with a characteristic size increasing with the brushes grafting density. The dynamic of large hard spheres particles (R=120nm), excluded from the brushes, was slowed down though slightly faster than the hard wall case. Smaller polystyrene microgels particles (R=16 nm and 42nm) that partially penetrated the brushes, presented a very slowed down dynamics, much more so for the smaller particles, reminiscent of size exclusion type of mechanism. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N17.00006: Displacer Effects on Pre-adsorbed Polystyrenes In Nanoporous Silica Chang Y. Ryu, Chansu Kim, Joel Batson, Sanat Kumar The addition of low molecular weight displacers has been used to probe the nature of adsorbed polymer chains on surfaces, and we have employed the displacers to understand the adsorption of polystyrene (PS) onto nanopores of silica particles in cyclohexane. When the radius of gyration (Rg) of PS is smaller than the pore radius (Rp) of the nanoporous silica, the displacement behavior of PS on nanopore surfaces is in quantitative agreement with that of PS on flat surfaces. However, when Rg of PS is larger than Rp of nanopores, the addition of displacers after preadsorbing PS in nanopores has increased the surface access of PS by a factor as large as 100{\%} -200{\%}, depending on the relative size ratio of Rg/Rp. On the contrary, when the displacers are mixed with cylcohexane prior to the PS adsorption in small nanopores, the surface access of PS is monotonically dependent of the composition of displacers. This suggests that the larger PS chains adsorbed in smaller pores are kinetically entrapped with severely limited mobility, and the addition of displacers will facilitate the diffusion of PS in nanopores by inducing a weaker surface binding and swelling of the congested PS chains in nanopores. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N17.00007: Theory of the adsorption of polymers onto chemically non-uniform surfaces with applications to the polymer adsorption onto the mixed brushes. Alexander Chervanyov, Gert Heinrich By developing the self-consistent perturbation expansion we theoretically study the adsorption of polymers onto the chemically non-uniform planar surfaces. The present theory deals with both regularly and randomly patterned surfaces having the position-dependent affinity for polymers. We predict that chemical non-uniformity of the surface dramatically enhances the overall affinity of this surface for polymers. The corresponding effective adsorption potential is calculated as a function of the periodicity of the surface-to- the size of polymer ratio and the excluded volume parameter. The obtained results are applied to the study of the adsorption of polymers onto the selective binary brushes that assume different morphologies. As a main result of the study, we demonstrate that the reversible switching from random to `ripple' microphase of the binary brush results in the significant enhancement of the adsorption ability of this brush. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N17.00008: Effect of Silane Sizing on Polymer-Glass Adhesion Moshe Gottlieb, Haim Dvir Glass slides sililated with organofunctional silanes were used to study polymer-glass interaction strenth. The extent of surface coverage, surface properties and topology were experimentally determined for the different silane treatments. For the different polymers were deposited on the silane treated glass the strength of polymer interaction with the silane treated glass was investigated using contact-mode Atomic Force Microscopy and the adsorbed layer thickness was determined optically. Typically, for each polymer a characteristic layer thickness was measured irrespective of the silane treatment or strength of adhesion. Adhesion strength was attributed mainly to van der Waals interactions with no indications of large scale covalent bonding between the polymer and the surface. The interaction strength and affinity of the polymer to the surface is dominated by hydrophobic/hydrophilic interactions and hydrogen bonds between grafted side groups and the functional groups of the silane treatment. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N17.00009: Dynamic Self-Assembly of Polymers from a Sphere-on-Flat Geometry Zhiqun Lin, Suck Won Hong, Jun Xu Self-assembly of micro- and nano-scale materials to form ordered structures promises new opportunities for developing miniaturized electronic, optoelectronic, and magnetic devices. In this regard, several elegant methods based upon self-assembly have emerged, for example, self-directed self-assembly and electrostatic self-assembly. Dynamic self-assembly of nonvolatile solutes via irreversible solvent evaporation has been recognized as an extremely simple route to intriguing structures. However, these dissipative structures are often randomly organized without controlled regularity. In this presentation, we will show a simple, one-step technique to produce well-ordered structures (e.g., concentric rings) consisting of polymers with unprecedented regularity by allowing a drop of polymer solution to evaporate in a sphere-on-flat geometry. This technique, which dispenses with the need for lithography and external fields, is fast, cost-effective and robust. As such, it represents a powerful strategy for creating highly structured, multifunctional materials and devices. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N17.00010: Monte Carlo Simulations of the Selective Adsorption of Heteropolymers on Heterogeneous Surfaces Jesse Ziebarth, Jennifer Williams, Yongmei Wang Lattice Monte Carlo simulations are used to study the selective adsorption of self-avoiding walk heteropolymers on heterogeneous surfaces near the critical adsorption point, the point at which polymer chains just become adsorbed to a surface. The critical adsorption point, determined as the polymer-surface interaction energy for which adsorption is least dependent on chain length, is identified for several different copolymer chain sequences on several surfaces with different site distributions. Selective adsorption is defined as the ability of a surface to more strongly adsorb chains with a certain sequence over chains with other chain sequences. It is found that highly patchy and alternating surfaces are able to selectively adsorb blocky and alternating chains, respectively, while surfaces with a random distribution of sites do not selectively adsorb any chains. Additionally, it is shown that adsorption is most selective for low chain concentrations and relatively weak adsorption energies. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N17.00011: The breaking of chiral symmetry using long-range electrostatic forces Kevin Kohlstedt, Francisco Solis, Graziano Vernizzi, Monica Olvera de la Cruz Surface charge heterogeneities result in the adsorption of oppositely charged amphiphilic molecules along charged fibers. The competition of this two-component system between electrostatic interactions, favoring ionic structures, and the net incompatibility of the co-assembled species, favoring macroscopic segregation, leads to local segregation and the formation of periodic patterns along the surface. We analyze the symmetry and size of the surface patterns on the surface of cylindrical structures. Lamellar patterns are arranged into helical structures along the cylinder, breaking the chiral symmetry. We also describe the critical transition between periodic patterns and macroscopic segregation. The characteristic domain size $L_0$ jumps discontinuously to infinity, resulting in macroscopic phase segregation of the components, at the critical salt concentration $\kappa_c$. The dependence of $\kappa_c$ on the helical pitch angle $\theta$ of the lamellar is shown. Our results suggest a new physical method to separate patterned ionic fibers with different pitch angles by modifying the salt concentration. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N17.00012: Activated Desorption of Water from a Polymer Surface Carolina C. Ilie, P.A. Jacobson, I.N. Yakovkin, L.G. Rosa, Matt Poulsen, D. Sahadeva Reddy, J.M. Takacs, S. Ducharme, Peter A. Dowben We studied water adsorption and desorption on the dipole ordered polymer poly(methylvinylidene cyanide) PMVC. The polymer has a distinct bulk absorbed water phase. The absorption of water is believed to distort the polymer chain placement. The kinetic parameters are obtained from thermal desorption spectra. Arrhenius plots yield the activation energy and the order of desorption process is determined from the best linear fit in the Arrhenius plots. Unusual angular dependence in thermal desorption is also observed. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N17.00013: Understanding Polymer Adhesion: First-principles calculations of the adsorption of organic molecules onto Si surfaces Karen Johnston, Risto M. Nieminen The adhesion of plastics to ceramics is important for many industrial and technological appliations. It is therefore essential to understand the underlying structure and bonding of the polymer and the surface. The aim of this research is to improve plastic adhesion using a multiscale approach. The first step involves the use of density functional calculations to understand the atomic-scale structure and bonding of polymers on surfaces. The plastic of interest is mainly composed of the polymer bisphenol-A-polycarbonate (BPA-PC). The BPA-PC monomer consists of two phenol groups, one propane group and a carbonic acid group. First-principles calculations of the adsorption of these molecules onto the Si(001)-(2$\times$1) dimer surface will be presented. Finally, the incorporation of first-principles data into a coarse-graining method will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N17.00014: Quartz Microbalance Measurement of Adsorption Potential Well-Depths Ryan Foltz, Rafael Garcia Changes in the resonant frequency of a quartz crystal microbalance (QCM),can be used to measure film thicknesses on the order of 0.1 monolayer or less that are adsorbed on the microbalance's electrode surfaces. The well-depth of the adsorption potential for molecules on a flat surface is a key parameter for determining the wetting transition temperature for molecules on that surface. However, it is a difficult quantity to predict with precision using theoretical models. We will examine the viability of using the adsorption on the QCM at low pressures to determine the well-depth of the adsorption potential for for nitrous oxide and other polar molecules on flat surfaces. We will compare our data with available theoretical predictions. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N17.00015: Phonon-induced Anisotropy in Dispersion Forces on a Metallic Substrate Je-Luen Li It is known that surfactant micelles spontaneously adsorb on gold (111) surfaces with orientational order dictated by the gold crystal structure. All this happens despite the screening effects of delocalized electron clouds in metallic systems. To understand the van der Waals forces that provide organization on metallic substrates, we describe a formalism wherein the dielectric response acquires directional dependence through phonon dispersion relations related to the crystal structure. In metals, ionic screening is enhanced along certain directions and a crystalline metallic substrate generates both torque and attraction on geometrically asymmetric objects. Numerical calculations show that the anisotropic van der Waals force will orient a dielectric rod-like micelle on a gold (111) surface. [Preview Abstract] |
Session N18: Surfaces, Interfaces, and Colloids I
Chair: Gil Nathanson, University of WisconsinRoom: Colorado Convention Center 103
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N18.00001: Interfacial interactions of alkane and ether molecules tethered to mesoporous MCM-41 using quasielastic neutron scattering Edward Kintzel, Kenneth Herwig, Michelle Kidder, Phillip Britt, A.C. Buchanan, III, Alan Chaffee The motion of 1,3-diphenylpropane (DPP, $\equiv $Si-O-C$_{6}$H$_{4}$(CH$_{2})_{3}$C$_{6}$H$_{5})$ and phenethyl phenyl ether (PPE, $\equiv $Si-O-C$_{6}$H$_{4}$(CH$_{2})_{2}$OC$_{6}$H$_{5}$ and $\equiv $Si-O-C$_{6}$H$_{4}$O(CH$_{2})_{2}$C$_{6}$H$_{5})$ tethered to the interior pore surface of MCM-41 silica was investigated using quasielastic neutron scattering. Measurements of the elastic intensity were carried out in the temperature range 50-380 K to probe the changes in dynamics between DPP and PPE as well as investigate the role of ether oxygen location in the PPE isomers. Full quasielastic scans over an energy range of $\pm $ 17 $\mu $eV were performed at temperatures of 240 K, 280 K, and 320 K. Analysis of the quasielastic data elicits the role that temperature has on the motion of these molecules tethered within the pores. The two PPE isomers exhibit much different dynamics illustrating the importance of hydrogen bonding to the silica surface. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N18.00002: Thermodynamic Investigation of thin n-Heptane films adsorbed on Magnesium Oxide (100) surfaces David Fernandez-Canoto, John Larese The thermodynamic properties of thin films of \textit{n-Heptane} on MgO (100) were measured using high resolution adsorption isotherms between 205 K and 275 K. Heats of adsorption were derived for first and second layer adsorbed on substrate and yield 42.38 $\pm $ 0.98 kJ/mol and 40.58 $\pm $ 0.46 kJ/mol, respectively. The isothermal compressibility was determined as a function of temperature and used to identify two possible phase transitions at 246.2 $\pm $ 1.5 K for the first and at 250.7 $\pm $ 0.8 K for the second layer respectively. These results will be compared to previous thermodynamic and neutron diffraction measurements of other shorter chained alkanes. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N18.00003: Structure of Iron Oxide Water Interface: a combined CTR and DFT study Sanjit Ghose, Peter Eng, Cynthia Lo, Kunaljit Tanwar, Thomas Trainor, Glenn Waychunas Chemical reactions at the mineral and water interface play an important role in many natural and technological processes, from controlling the fate and transport of environmental contaminants, and biological availability and geochemical cycling of iron to the electronic device fabrications. The interface structure and composition of a mineral, dictates the chemical interactions that take place between the mineral and its environment, and also play a pivotal role in nanoparticle growth process. Therefore, we have conducted a systematic investigation of the solid-solution interface structure of two of the most common and stable phases of iron oxides systems: hematite ($\alpha $-Fe$_{2}$O$_{3})$ (1-102) and goethite ($\alpha $-FeOOH) (100) -- using the crystal truncation rod (CTR) diffraction technique. The distribution and form of surface hydroxyl groups at the interface are described using combined CTR and \textit{ab initio} density functional theory (DFT) calculations. Goethite (100) interface structure is determined to be a relaxed double hydroxyl termination with the presence of two semi-ordered water layers exposing two types of hydroxyl groups at the surface and the hematite (1-102) interface structures show vacancies in the near surface metal occupancies and different distributions of surface hydroxyl groups. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N18.00004: Structure and dynamics of fluorinated alkanes on silicon dioxide surfaces Mesfin Tsige Despite their great promise in various applications, the structure and dynamics of fluorinated alkanes at interfaces is still an open question. In particular, the knowledge from both theoretical and experimental perspectives is very limited when it comes to understanding the interface between these systems and a solid substrate. Molecular dynamics simulations based on the All Atom OPLS model are used to predict the equilibrium structure and dynamics of short fluorinated alkanes on both amorphous and crystalline silicon dioxide surfaces. In order to understand the effect of layer-layer interaction on the ordering of chains in a given layer, the thickness of the liquid film is increased layer-by-layer from monolayer to multilayers. Results for structural and dynamics of the liquid films near the silicon dioxide surfaces will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N18.00005: Delayering of Intermediate-Length Alkanes Adsorbed on Solid Surfaces H. Taub, M. Bai, A. Diama, K. Knorr, U.G. Volkmann, F.Y. Hansen We have recently discovered that a film of the intermediate-length alkane, dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) does not completely wet SiO$_{2}$ and highly oriented graphite surfaces on a nanometer length scale.\footnote{M. Bai \textit{et al}., cond-mat/0611497.} In a narrow temperature range near the bulk melting point $T_{b}$, we observe a single layer of C32 molecules oriented with their long axis perpendicular to the surface. On heating just above $T_{b}$, these molecules undergo a delayering transition to three-dimensional droplets that remain present up to their evaporation point. Here we report noncontact Atomic Force Microscopy and synchrotron x-ray measurements indicating that a similar delayering transition occurs for films of other intermediate-length alkanes: C24, C25, C30, and C36 deposited from solution onto a SiO$_{2}$ surface. These results raise a number of interesting questions including whether the delayering transition is driven by conformational changes in the molecules and what implications the nonwetting behavior may have for lubricating nanoscale devices. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N18.00006: Linear surfactant-mediated spreading of nanodroplets: molecular dynamics simulation Hye-Young Kim, Yong Qin, Kristen Fichthorn We utilized molecular dynamics simulations to probe surfactant-mediated spreading of nanodroplets on a solid surface. We find that the spreading speed is strongly influenced by the attraction of the hydrophobic surfactant tail to the solid surface. When this attraction is sufficiently strong, surfactant molecules partition to the liquid-solid interface and can lead to an inhomogeneous distribution of surfactant over the liquid-vapor interface, which could drive the Marangoni convection. The result also shows that the surfactant molecules can assemble into micelles. The repulsion between micelles leads to break-off and migration of the micelles from the liquid-solid to the gas-solid interface and spreading is facilitated in this way. Our model system contains features that have been connected with superspreading in experimental studies and provides insight into the workings of a successful surfactant. REF: J. Chem. Phys. 125, 174708 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N18.00007: State-resolved inelastic and reactive scattering dynamics of gases with liquid surfaces Bradford Perkins, Alexander Zolot, Paul Dagdigian, David Nesbitt Energy transfer dynamics between gas and liquid surfaces are investigated by colliding a molecular beam of CO$_{2}$ with low vapor pressure liquids in vacuum. Nascent quantum states of CO$_{2}$ are probed via direct infrared absorption of the $\nu_{3}$ asymmetric stretch with a Pb-salt diode laser. The high spectral resolution ($\sim $20 MHz) of the laser provides the means to characterize the translational, rotational, vibrational, and angular distributions of the scattered CO$_{2}$. Experiments have probed an array of collision energies, incident and final scattering angles, liquids, and surface temperatures. In each case, multi-channel dynamics have been observed and characterized as trapping-desorption (TD) and impulsive scattering (IS). Rotational and translational distributions show considerable excitation above the surface temperature (T$_{S})$, while the vibrational distributions remain colder than T$_{S}$. Similar experiments have probed the HF(v,J) product from reactive scattering of fluorine atoms with a hydrocarbon surface. Both the inelastic and reactive scattering distributions are well-characterized by a two-temperature model where T$_{TD} \quad \sim $ T$_{S}$ and T$_{IS} \quad >$ T$_{S}$. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N18.00008: Dielectric Response of Thin Surface Water Layer Feng Chen, Jason Shulman, Rafael Longoria, Stephen Tsui, Yuyi Xue, C. W. Chu Recently, we have reported a negative dielectric constant ($\varepsilon $') for various nano-particle assemblies (urea- coated Ba$_{0.8}$Rb$_{0.4}$TiO(C$_{2}$O$_{4}$)$_{2}$ and Al$_2 $O$_3$ {\it et al}). There is a close correlation between the zero-frequency electrical conductivity and the occurrence of the negative $\varepsilon $'. The large surface area of the nano- assemblies and moisture level play an important role. To determine the surface water effect on the negative $\varepsilon $', we carried out different dielectric measurements (time and frequency domain with different bias voltages) for differently prepared surfaces with controlled humidities. We will present our data and analysis for surface water ranging from a few layers to ~100 $\mu$m. The contribution of water decomposition by electrolysis to $\varepsilon $' will also be evaluated. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N18.00009: Lateral Adhesion Balance (LAB) -- a Novel Surface Characterization Technique Rafael Tadmor Drop lateral adhesion to a surface and the condition for drop sliding along a surface are key issues in many disciplines including biophysics, environmental science, fluid dynamics and agriculture. Yet, to date, except for the tilt stage method, which is extremely limited in range of forces, there is no systematic experimental instrumentation to measure the forces required for drop sliding. We present a new instrument that uses centrifugal forces to slide any drop along a surface. Beyond extending the range of measurable drop-surface interaction, the instrument enables decoupling of some parameters that are bound to be coupled with the simple tilt stage method. Specifically the tilt stage method has two variables varying at the same time: the lateral and normal forces. This violates a fundamental principle of experimental science which leads to obscure understanding of surface characteristics. The LAB avoids this problem. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N18.00010: How Water Meets a Hydrophobic Surface: Reluctantly and with Fluctuations Adele Poynor, Liang Hong, Steve Granick, Ian Robinson, Paul Fenter, Zhan Zhang By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low- density region forms near the surface. To investigate the existence of this layer, we have employed three surface sensitive techniques, time-resolved phase-modulated ellipsometry, surface plasmon resonance, and X-ray reflectivity. Both ellipsometry and X-ray reflectivity provide strong evidence for the low-density layer and illuminate unexpected temporal behavior. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N18.00011: Studies of dynamical layering in adsorbed alkane films by molecular dynamics simulations and quasielastic neutron scattering. F.Y. Hansen, P. Soza, A. Diama, H. Taub, U.G. Volkmann From experiments using a surface apparatus it is known that alkane fluids confined between two surfaces exhibit a layered structure at the molecular level. This static layering has motivated us to consider the possibility that the individual molecular layers in fluid alkane films adsorbed on a solid surface also exhibit different dynamical properties. Here we report molecular dynamics (MD) simulations of the diffusive motion in layers of tetracosane molecules ($\rm C_{24}H_{50}$) (C24) adsorbed on graphite in time scales from 1 to 100 ps and from 1 to 4 ns and compare the results with high--energy-- resolution quasielastic neutron scattering spectra that probe motions on these time scales. The MD simulations are set up to answer the questions: a) is interlayer diffusion of C24 molecules significant on these time scales? b) are the diffusive motions in the layers different? and c) what is the nature of the diffusive motions observed in the high--energy--resolution quasielastic neutron scattering experiments using the Disk Chopper Spectrometer (1-100 ps) and the High Flux Backscattering Spectrometer (1-4 ns) at NIST? [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N18.00012: Contact Angle Measurements by AFM on Droplets of Intermediate-Length Alkanes Adsorbed on SiO$_{2}$ Surfaces M. Bai, H. Taub, K. Knorr, U.G. Volkmann, F.Y. Hansen We have recently discovered that films of intermediate-length alkanes ($n$-C$_{n}$H$_{2n+2}$; 24 $<$ n $<$ 40) do not completely wet a SiO$_{2}$ surface on a nanometer length scale [2]. In a narrow temperature range near the bulk melting point $T_{b}$, we observe a single layer of molecules oriented with their long axis perpendicular to the surface. On heating just above $T_{b}$, these molecules undergo a delayering transition to three-dimensional droplets that remain present up to their evaporation point. Here we report measurements by noncontact Atomic Force Microscopy of the contact angle of these droplets for a film of hexatriacontane ($n$-C$_{36}$H$_{74}$ or C36). Our preliminary measurements indicate that there is a weak maximum in the contact angle at $\sim \quad T_{b}$ + 3 \r{ }C. Further measurements are planned to investigate whether the weak maximum in the contact angle is consistent with the droplets supporting a surface freezing effect as at the bulk fluid/air interface. $^{2}$M. Bai, K. Knorr, M. J. Simpson, S. Trogisch, H. Taub, S. N. Ehrlich, H. Mo, U. G. Volkmann, F. Y. Hansen, cond-mat/0611497. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N18.00013: Adsorption and Growth Studies of CF$_{4}$ on CF$_{3}$Cl - Covered HOPG Petros Thomas, Daniel Velazquez, George Hess We have studied the adsorption and growth of CF$_{4}$ on a CF$_{3}$Cl-covered graphite surface from 60 K to 105 K, using infrared reflection absorption spectroscopy (IRAS) supplemented by ellipsometry. For the monolayer liquid phase of CF$_{3}$Cl, the CF$_{4}$ initially mixes/dissolves in the CF$_{3}$Cl layer and then continuously replaces the CF$_{3}$Cl on the surface. However, there remains a trace of CF$_{3}$Cl even after a number of layers of CF$_{4}$ are deposited on the surface. The orientation of the residual CF$_{3}$Cl is different from the orientation of the original CF$_{3}$Cl monolayer. For the monolayer solid phases of CF$_{3}$Cl, the CF$_{4}$ adsorbs on top of the CF$_{3}$Cl layer, with little or no solubility. With two layers of CF$_{3}$Cl on the graphite, CF$_{4}$ displaces one of the layers and adsorbs on top of the remaining layer. [Preview Abstract] |
Session N19: Focus Session: Frontiers in Electronic Structure Theory III
Sponsoring Units: DCP DCOMPChair: Peter Gill, Australian National University
Room: Colorado Convention Center 104
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N19.00001: A New View of the Kondo Effect from an Ab Initio Embedded Configuration Interaction Theory Invited Speaker: Over the past decade, we have been developing an ab initio theory to describe localized correlated many-electron states in condensed matter. This theory embeds a correlated quantum chemistry description into surroundings described by periodic density functional theory (DFT). Recent technical advances in the theory include: (i) implementation of ultrasoft pseudopotentials (USPPs) in a consistent manner across all levels of theory (periodic DFT, CASSCF, and CI), (ii) self-consistent updates of the density of the total system, thereby allowing a fully-self-consistent embedding operator, and (iii) a multi-reference singles and double excitation CI (MRSDCI) treatment of electron correlation in the embedded region. Our current embedded configuration interaction (ECI) theory is now more efficient (via USPPs), less approximate (by use of self-consistent embedding potentials), as well as more accurate (via MRSDCI) than earlier versions that were based either on many-body perturbation theory or valence CI/CASSCF wavefunctions. The current version is now being used to study a variety of systems/phenomena where DFT is known to fail, due to either neglect of many-body effects or self-interaction artifacts. Time permitting, more than one example will be given of how the embedding theory is able to give a \textit{qualitatively (as well as quantitatively)} different view of these systems/phenomena. We will focus on the Kondo effect, a long standing problem in condensed matter physics, which has not had a first principles solution until now. The Kondo effect refers to the observation of an anomalous resistivity minimum at low temperatures for materials containing magnetic transition metal impurities in nonmagnetic host metals. We will show that the ECI theory is able to capture the physics and offer a new view of this phenomenon, while periodic DFT and finite cluster quantum chemistry calculations do not. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N19.00002: First-principles calculations of nanoscale capacitors at finite bias potential Invited Speaker: When the thickness of an oxide film is reduced to few unit cells, its dielectric properties (which are relevant, e.g., for nonvolatile ferroelectric memories and as gate oxides in MOSFET transistors) start to deviate from those predicted by macroscopic models, and cannot be disentangled from the metallic or semiconducting contacts. One particularly important issue related to interfacial effects is the ``dielectric dead layer'', which plagues the performance of thin-film perovskite capacitors by substantially reducing the effective permittivity ($\kappa$) of the active high-$\kappa$ material. The microscopic origins of this reduced permittivity, and in particular whether it stems from defects or from the fundamental properties of a metal/insulator interface, are not well understood. To address this problem from first principles, we will first show how the macroscopic polarization (and the coupling to an external field) can be rigorously defined for a periodic metal-insulator heterostructure, by using techniques and ideas borrowed from Wannier-function theory [1]. We will then demonstrate our new method by calculating the dielectric properties of realistic SrRuO$_3$/SrTiO$_3$/SrRuO$_3$ nanocapacitors [2]. In particular, we demonstrate the existence of an intrinsic dielectric dead layer and analyze its origin by extracting the ionic and electronic contributions to the electrostatic screening. We establish a correspondence between the dead layer and the hardening of the collective SrTiO$_3$ zone-center polar modes, and determine the influence of the electrode by repeating our calculations for Pt/SrTiO$_3$/Pt capacitors. Our results provide practical guidelines for minimizing the deleterious effects of the dielectric dead layer in nanoscale devices. \begin{itemize} \item[{[1]}] \underline{M. Stengel} and N. A. Spaldin, {\em Origin of the dielectric dead layer in nanoscale capacitors}, Nature (London) {\bf 443}, 679 (2006).\\ \item[{[2]}] \underline{M. Stengel} and N. A. Spaldin, {\em Ab-initio theory of metal-insulator interfaces in a finite electric field}, cond-mat/0511042 (2005). \\ \end{itemize} [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N19.00003: \textit{Ab Initio} Quantum Simulations of Liquid Water John Gergely, David Ceperley, Francois Gygi Some recent efforts at simulating liquid water have employed ``\textit{ab initio}'' molecular dynamics (AIMD) methods with forces from a version of density functional theory (DFT)\footnote{E. Schwegler, J.C. Grossman, F. Gygi, G. Galli, J. Chem. Phys \textbf{121}, 5400 (2004).} and, in some cases, imaginary-time path integrals (PI) to study quantum effects of the protons. Although AIMD methods have met with many successes, errors introduced by the approximations and choices of simulation parameters are not fully understood. We report on path integral Monte Carlo (PIMC) studies of liquid water using DFT energies that provide quantitative benchmarks for PI-AIMD work. Specifically, we present convergence studies of the path integrals and address whether the Trotter number can be reduced by improving the form of the (approximate) action. Also, we assess $1)$ whether typical AIMD simulations are sufficiently converged in simulation time, i.e., if there is reason to suspect that nonergodic behavior in PI-AIMD methods leads to poor convergence, and $2)$ the relative efficiency of the methods. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N19.00004: Adaptive multilevel Finite Element Method for Solving the Electronic Schr\"{o}dinger Equation Eric Bylaska, Mike Holst, John Weare It is widely appreciated that to use computational methods for the design of materials encompassing a wide assortment of elements from the Periodic Table, highly efficient methods based as closely as possible on accurate quantum mechanics are needed. We have developed an O(N) ab initio molecular dynamics method based on an adaptive multilevel finite element first principles solver with an efficacious implementation of hybrid functionals . The matrix representations of the discrete Hamiltonian operator in the finite element basis are always sparse due to the local support nature of finite element basis functions. As a result, application of the Hamiltonian operator to a discrete function has complexity which is linear in the number of discretization points. This development also makes use of completely unstructured simplex meshes that have the advantage of giving resolution of the near singular features around atomic nuclei using minimal computational resources. Various aspects of the implementation and computational efficiencies will be discussed. This method has been applied to several systems including excitons in quartz, transition metal dimers, and aqueous complexes. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N19.00005: GW calculations of large model structures Paolo Umari, Stefano Baroni We introduce a novel approach for performing first-principles GW calculations of large model structures. A description of the valence and conduction manifolds in terms of non-orthogonal generalized Wannier functions permits to minimize the dimension of the basis set required for describing the space of single electron transitions. This dimension scales linearly with the size of the system. Then a space-time approach is used to calculate the self-energy operator in the space of Kohn-Sham eigenstates. Ultrasoft pseudopotentials are straightforwardly implemented within this scheme.We validate our approach by calculating the vertical ionization energies of small molecules and find excellent agreement with the experiment. Then we shows its potentiality by addressing a model structure of vitreous silica. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N19.00006: Toward an accurate and practical description of Xe/Cu(111) physisorption Garold Murdachaew, Stefano de Gironcoli, Patrick Huang, Emily Carter, Giacinto Scoles The physisorption of rare gases on metal surfaces has often been described by density functional theory. However, standard DFT has shown very limited success due to its well-known shortcomings when applied to weak interactions. A possible approach which at least includes the relevant missing physics is to use a blend of ``corrected" DFT coupled with a damped-dispersion interaction. Alternatively, one may model the surface by a cluster since it is possible to apply highly accurate quantum chemical methods to small clusters. Unfortunately, cluster model approximations do not give a good description of the physisorption process on the surface. In particular, the site preference of Xe/Cu(111) physisorption as given by cluster models is qualitatively incorrect. For this reason, an approach which better simulates the surface is required. Some recent results obtained using the embedded cluster approach of E. A. Carter, P. Huang, and coworkers [P. Huang and E. A. Carter, J. Chem. Phys. {\bf 125}, 084102 (2006)] will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N19.00007: Boundary Conditions for States with Maximally Broken Time-Reversal Symmetry Roger Haydock, C.M.M. Nex For non-crystalline materials, electronic states can only be calculated for finite clusters, and the results are sensitive to the boundary conditions. States which go to zero on the boundary have infinite life-times, appropriate for isolated clusters, but not for macroscopic materials whose states have finite life-times. Instead, we chose a boundary condition for which the states have minimal life-times, in other words, one for which the states have maximally broken time-reversal symmetry. This approach is tested for a variety of systems and compared with its close relative, the maximum entropy approximation. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N19.00008: Importance of second neighborhood ensembles on PdAu bimetallic surfaces Dingwang Yuan, Ruqian Wu, Xingao Gong Atomic configurations of two or three Pd substituents on the Au(111) and Au(001) surface are investigated using the first-principles pseudopotential plane wave approach. Pd atoms are found to form second neighborhoods on PdAu surfaces. The Pd-d band becomes narrow and well below the Fermi level, very different from those in a Pd film or bulk Pd. Yet the surface Pd atoms are still active and serve as independent attractive centers towards adsorbates. Through studies of example reactions such as CO oxidation, ethylene dehydrogenation and vinyl acetate synthesis, we demonstrate the importance of special ensembles in catalyzing reactions by confining reactants in a small region. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N19.00009: Dielectric function by FLAPW method. Tatsuya Shishidou, Tamio Oguchi Response functions, which describe how electrons respond to external fields, are the central quantity in solid state physics. Many physical properties, such as optical spectra, phonon spectra, dielectric constant, magnetic and structural instabilities, and so on, are accessible if one can calculate the corresponding response function. Moreover, the response functions play important role in the application of many-body perturbation theory. In this paper, we present a way to calculate dynamical inverse dielectric function $\varepsilon^{-1}(r,r',\omega)$ within the framework of the all-electron full-potential linearized augmented plane wave (FLAPW) method. We work with the random phase approximation (RPA) instead of the plasmon pole approximation. Local field effects are taken into account. Details of our method and implementation will be given, focusing on its efficiency and the treatment of the Coulomb singularity at $\Gamma$ point. Calculations for semiconductors, ferromagnetic $3d$ transition metals, and insulating antiferromagnetic transition-metal oxides will be presented and compared with available experiments and theories. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N19.00010: State-of-the Art Procedure for the Calculation of Quartic Force Fields: Application to HO$_{2}^{+}$ Timothy Lee, Xinchuan Huang In the 1990's, ab initio methods began to yield quartic force fields for use in the calculation of ro-vibrational spectra with an accuracy that previously had been unimaginable. The main reason for this advance was the development of efficient computer programs for calculating singles and doubles coupled-cluster energies that included an estimate for connected triple excitations, denoted CCSD(T). Thus the advent of CCSD(T) quartic force fields computed with large one-particle basis sets changed the paradigm for the ab initio calculation of ro-vibrational spectra. Small correction terms have now been successfully incorporated into these procedures, including core-correlation, scalar relativistic, and others. Previously, we investigated procedures where all of these correction terms are appended in one way or another to a base calculation. In the current work, we develop a new procedure where most of these correction terms are included from the beginning, while still minimizing the overall computational cost. Our new procedure is detailed and its application to the lowest triplet and singlet states of HO$_{2}^{+}$ presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N19.00011: A Correct, Density Functional Description of Semiconductors D. Bagayoko, G. Zhao, L. Franklin, H. Jon The profusely reported inability of some density functional calculations to describe correctly the band gaps of semiconductors has been ascribed to the derivative discontinuity of the exchange correlation energy, the self-interaction associated with approximate potentials, and other factors, i.e., pd repulsion in the case of wurtzite InN. From 1998 to present, we have studied several semiconductors with local density approximation (LDA) and generalized gradient approximation (GGA) potentials. Upon applying the Bagayoko, Zhao, and Williams (BZW) method to the implementation of the linear combination of atomic orbital (LCAO) formalism, we have obtained band gaps and electron effective masses in excellent agreement with experiment for BaTiO$_{3}$, GaN, GaAs Si, Ge, 3C-SiC, 4H-SiC, ZnSe, ZnO, carbon nanotubes, InN, and AlAs among others. This ab-initio method avoids a basis set and variational effect inherently associated with LCAO calculations -- irrespective of the selected potential. We present a summary of the BZW method and of the aforementioned results, including \textit{the correct description of low-lying conduction bands as verified by agreements with measured optical transition energies and dielectric functions.} These results clearly point to an urgent need to revisit (a) the above presumed causes of reported failures of DFT and (b) computational methods suffering from the identified and well-defined basis set and variational effect. [Preview Abstract] |
Session N20: Dielectrics
Sponsoring Units: DCMPChair: Fred Walker, Yale University
Room: Colorado Convention Center 105
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N20.00001: The mechanism of Fermi level pinning/unpinning at high k Oxide/GaAs interface M.L. Huang, W.C. Lee, P. Chang, T.D. Lin, Y.J. Lee, M. Hong, J. Kwo Unpinning of Fermi level at oxide/GaAs interface is the one of the key issues of realizing GaAs-based III-V metal-oxide-semiconductor field-effect-transistors (MOSFETs) for high-speed and high power applications due to inherent advantages of high electron mobility, semi-insulating substrates, and high breakdown fields. In this study several important high dielectric constant materials, Al$_{2}$O$_{3}$, HfO$_{2}$, Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$ and Y$_{2}$O$_{3}$, were \textit{in-situ} deposited on GaAs(001), and exhibited the different Fermi level pinning/unpinning behavior of current-capacitance (C-V) characteristics. In order to correlate the relationship between the oxide/GaAs interfacial structure and their electrical behavior,\textit{ in-situ} XPS analysis was conducted shortly after nano high $\kappa $ oxides were deposited on GaAs. Our studies suggest that Fermi level unpinning in the oxide/GaAs hetero-structure is attributed to the exclusion of the As-As and the As-O bonding during the initial interfacial formation. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N20.00002: MOS Ge Diodes Based on High $\kappa $ Gate Dielectrics Grown by MBE and ALD Kun Yu Lee, W.C. Lee, T.D. Lin, C.S. Lee, Y.C. Chang, Y.J. Lee, M.L. Huang, Y.D. Wu, M. Hong, J. Kwo Germanium-based CMOS technology is gaining importance due to its high carrier mobility. In this work high $\kappa $ gate-dielectrics, Al$_{2}$O$_{3}$, HfO$_{2}$, Y$_{2}$O$_{3}$ and Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$ grown by MBE and ALD were investigated as passivation layers on n type Ge(100). Thermal stability of the MOS diodes was examined after various anneals. Prior to dielectric depositions surface pretreatments were applied to reduce the unwanted GeO$_{x}$ interfacial layer, and to improve electrical properties. Frequency dispersion of C-V curves was reduced by using a 350$^{o}$C preclean process, compared to the sample without precleaning. The leakage current density of ALD grown HfO$_{2}$ (6.8nm) is 4.6×10$^{-6 }$A/cm$^{2}$ with $\kappa $ of 10.5. The improved CV curve was attributed to less GeO$_{x}$ formed at substrate and oxide interface, as confirmed by XPS analysis. However, with higher cleaning temperature over 400$^{o}$C, the CV curves showed additional inversion capacitance, possibly due to minority carriers from defect states near the interface. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N20.00003: Inelastic Electron Tunneling Spectroscopy of Silicon Based MOS Diode with High Permittivity Gate Dielectrics Syuanlong You Inelastic electron tunneling spectroscopy (IETS) has been known as a powerful technique for detecting the molecular vibrations in the spectra. This technique was also applied to the study of the silicon MOS to reveal the information of electrode phonons, dielectric phonons, chemical bonding, and trap states in MOS structure. In this work IET spectra of silicon MOS diode with SiO$_2$, high $\kappa$ \quad HfO$_2$, and YDH (HfO$_2$ doped with Y$_2$O$_3$) as gate dielectrics were investigated. The gate bias dependence of the IET spectrum enables us to ascribe the vibration mode adjacent to the metal gate interface, or to the silicon substrate interface. We show variations of the IET spectrum with respect to Y$_2$O$_3$ doping and annealing conditions of the dielectrics, and compare with reported data of infrared, Raman, and XPS. We also present the changes in IET spectra as induced by electrical stress that eventually leads to soft-breakdown in the dielectrics. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N20.00004: MBE and ALD grown High k Dielectrics Gate Stacks on GaN Y.C. Chang, K.Y. Lee, W.C. Lee, T.D. Lin, Y.J. Lee, M.L. Huang, M. Hong, J. Kwo, Y.H. Wang III-nitride compound semiconductors are attractive for high-temperature and high-power MOSFET applications due to their intrinsic properties of wide band gap, high breakdown field, and high saturation velocity under high fields. In this work GaN-based high k MOS diodes were fabricated using MBE-grown Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$, MBE-grown HfO$_{2}$ and ALD-grown HfO$_{2}$ as the gate dielectrics with dielectric constants of 14.7, 17.4 and 16.5, respectively. All MOS diodes exhibited low leakage ($<$10$^{-6}$ A/cm$^{2}$ at V$_{fb}$+1) and well behaved capacitance-voltage curves with a low interfacial density of states of $\sim $10$^{11}$ cm$^{-2}$eV$^{-1}$. Energy-band diagrams of the MOS structures have been determined by extracting valance-band offset ($\Delta $E$_{V})$ from HR-XPS and with the bandgaps of the oxides. For example, the ALD-grown HfO$_{2}$-GaN at the interfaces gave approximately $\Delta $E$_{C}$ and $\Delta $E$_{V}$ of 1.2 eV and 1.1 eV, respectively. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N20.00005: Spectroscopic Study of Band Alignment in Alternative High-k MOS Dielectric Stacks E. Bersch, S. Rangan, E. Garfunkel, R.A. Bartynski The study of high-k dielectrics and metal gate electrodes is critical to next generation MOSFETs. We have measured the band offsets of alternative MOS stacks using photoemission and inverse photoemission in the same chamber as well as synchrotron photoemission. At Rutgers, we have measured the valence and conduction band densities of states (DOS) and edges with UV photoemission and inverse photoemission, respectively, in situ. Using synchrotron photoemission we have measured the core level positions as well as the valence band DOS of clean and metallized dielectric/Si systems. The measurement of the chemical shifts of the core levels upon metallization enables us to evaluate the conduction band offset at the metal/dielectric interface. For Hf(x)Si(1-x)O(2), we find the conduction band offset (CBO) does not change as x is varied from 1 to 0.8, but the valence band offset increases by 0.4 eV. Titanium, aluminum and ruthenium were chosen as gate metals because of their prospective use as low and high workfunction metals in dual metal gate CMOS devices. We measured the CBO for the Ti, Al and Ru/Hf(x)Si(1-x)O(2) interfaces and found barriers involving Ti and Ru to be in good agreement with the interface gap state model, whereas the barrier involving Al deviated substantially from it due to the formation of an AlO(X) layer at the interface. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N20.00006: First-principles study of direct electron tunneling through ultra-thin SiO$_{2}$ layers Joongoo Kang, K.J. Chang, Y.-H. Kim As the size of metal-oxide-semiconductor devices is scaled down to the sub-10-nm regime, the thickness of SiO$_{2}$ insulating layers reaches the range of 1-2 nm. Then, gate leakage current is unavoidable due to direct tunneling of electrons. In this work, we study the electron tunneling current through thin gate oxide layers for various Si(100)/SiO$_{2}$ interface models, which have different oxide thicknesses and crystal phases. We use a combined approach of the local-density-functional approximation and the matrix Green's function method. We test oxide layers in the $\alpha $-quartz, tridymite, and amorphous structures, which are sandwiched between two Si(100) electrodes. We find that Si induced gap states result from a decay of the silicon valence (conduction) band wave functions into the oxide region. The gate leakage current between two p+ Si electrodes is exponentially reduced as the oxide thickness increases, with the almost same decay rate of -1 decade/0.2 nm, regardless of the structure of oxide layers. We also find that the gate leakage current is affected by introducing interface roughness and oxygen vacancies in the oxide. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N20.00007: Search for Sub lattice Disorder in CaCu$_{3}$Ti$_{4}$O$_{12}$ Kevin Stone, Jae-Hyuk Her, Peter Stephens, Jonathan Hanson, Haiding Mo, Christie Nelson, Lijun Wu, Yimei Zhu One of the proposed mechanisms for the Internal Barrier Dielectric Capacitance believed to be responsible for the giant dielectric response of the perovskite material CaCu$_{3}$Ti$_{4}$O$_{12}$ is that of sub lattice disorder on the Ca and Cu sites.~ Such disorder should have measurable affects on both the intensity and shape of the Bragg peaks of different symmetries.~ We investigate the possible existence of such disorder through high resolution charge density maps, based on a large dataset of x-ray and electron diffraction integrated intensity measurements, and, separately, peak shape measurements on a restricted set of reflections. Supported by the U.S. Department of Energy. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N20.00008: Extrinsic Mechanisms for the Giant Dielectric Constant in CaCu$_{3}$Ti$_{4}$O$_{12}$: A Low-Temperature Specific-Heat Study C.P. Sun, H.D. Yang, Jianjun Liu, W.N. Mei, J.-Y. Lin, Chun-gang Duan Low-temperature specific-heat study has been performed on the insulating giant dielectric constant material CaCu$_{3}$Ti$_{4}$O$_{12}$. Analyzing the specific heat data in the very low-temperature range (0.6 to 1.5 K) and moderately low-temperature range (1.5 to 5 K), we noticed significant contributions originated from the linear and Einstein terms, we attributed as the low-lying elementary excitations due to lattice vibrations occurred at the grain boundaries and induced by local defects. These findings correlate well with the core-shell model deduced from the earlier experiments, and offer explanation to the extrinsic mechanisms of the giant dielectric constants at both low (DC to 10$^{5}$ Hz) and high frequency (10$^{6}$ to 10$^{9 }$Hz) regions. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N20.00009: Fabrication and properties of TiO$_{2}$ cluster films Xiaohui Wei, Y.F Xu, Z Sun, Ralph Skomski, D.J Sellmyer Recently much interest has been directed toward nanostructured pure and doped semiconductors for their interesting dielectric, optical and magnetic properties and potential applications in spintronic devices. In this study a gas-condensation cluster-deposition system was used to make cluster-assembled films. Ti and TiO$_{2 }$targets were used to prepare Ti and TiO$_{2}$ cluster assembled films. The Ti and TiO$_{2 }$clusters were examined by TEM to determine their size and size distribution. Films were scanned by AFM to see their surface morphology and examined by XRD to see their structural evolution with annealing temperature and annealing time. TEM revealed that the sizes of the Ti and TiO$_{2}$ clusters are 9 and 20 nm respectively. After annealing at 400 $^{o}$C for an hour in oxygen, the Ti cluster films transform into pure polycrystalline rutile, which has a high dielectric constant, whereas the TiO$_{2 }$cluster films remain a mixture of anatase and some rutile even after 900$^{ o}$C annealing. The optical, magnetic and dielectric properties of the films will also be discussed. This work is sponsored by NSF-MRSEC,ONR and NCMN. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N20.00010: Second-harmonic generation measurements of porous low-k dielectric materials Joanna Atkin, Daohua Song, Robert Laibowitz, Eduard Cartier, Thomas Shaw, Robert Rosenberg, Tony F. Heinz Low-k dielectric materials based on porous carbon-doped oxides, with relative dielectric constants as low as 2.1, are widely used in the microelectronics industry. Knowledge of these materials' basic electronic properties, such as energy gaps, barrier heights, and trap states, is essential for developing an understanding of their electrical leakage and stability characteristics. In this paper, we present the results of measurements of optical second-harmonic generation (SHG) from thin films of the low-k material deposited on silicon. SHG measurements at low laser fluence probe the nature of interfacial trap states. At higher fluence, multiphoton charge injection is produced by the femtosecond laser pulses and yields a time dependence of the SHG signal. Analysis of these measurements provides information about barrier heights. The results of these non-contact optical measurements will be compared with C-V characterization of the dielectric films. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N20.00011: First-principles calculations for the elastic properties of superhard TiN/Si$_{3}$N$_{4}$ superlattices Sanwu Wang, Y.G. Shen, S.T. Pantelides We report first-principles density-functional calculations for the atomic structures, the electronic properties, and the elastic properties of superlattices containing nano-scale crystalline TiN and thin layer of silicon nitride. We found that the elastic properties (bulk modulus, shear modulus, and elastic constacts) are strongly dependent on the size of the components. Superlattices with TiN thickness smaller than 2.5 nm have far smaller values of bulk and shear moduli than bulk crystalline TiN, while $\sim $3 nm TiN can make the superlattice have the elastic properties close to those of crystalline TiN. The results are helpful for optimization of the component size to achieve high values of both elastic properties and hardness. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N20.00012: ABSTRACT WITHDRAWN |
Session N21: Computational Methods: Multiscale Modeling
Sponsoring Units: DCOMPChair: Brian Good, NASA Glenn
Room: Colorado Convention Center 106
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N21.00001: The Improvement of Length Scaling in the Hyperdynamics Method Soo Young Kim, Arthur Voter Many important physical phenomena, such as film growth, bulk diffusion, radiation damage annealing, dislocation climb and catalysis, require both long time scale and large length scale molecular dynamics, where conventional molecular dynamics methods are not applicable due to the computational costs. The hyperdynamics method has enabled us to perform molecular dynamics for a longer time scale. However, this method is limited in length scale because the overall computational speedups achieved by the current bias potential methodologies decrease rapidly with the size of the system. To overcome this disadvantage, we are designing new bias potential methodologies to maintain the overall computational speedup in larger systems. We calculate the hyperdynamics rates and the overall speedups with the current and new approaches and discuss the fundamental aspects of both approaches. The early results show that these new methods are promising for reaching greater time and length scales simultaneously. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N21.00002: Finding the minimum-energy atomic configuration in large multi-atom structures: Genetic Algorithm versus the Virtual-Atom Approach Mayeul d'Avezac, Alex Zunger In many problems in molecular and solid state structures one needs to determine the energy-minimizing decoration of sites by different atom-types (i.~e.\emph{configuration}). The sheer size of this configurational space can be horrendous even if the underlying lattice-type is known. The ab-initio total-energy surface for different (relaxed) configurations can often be parameterized by a spin-like Hamiltonian (\emph{Cluster-Expansion}) with discrete spin -variables denoting the type of atom occupying each site. We compare two search strategies for the energy-minimizing configuration: (i) A discrete-variable genetic-algorithm approach( S. V. Dudiy and A. Zunger, PRL {\bf 97}, 046401 (2006) ) and (ii) a continuous-variable approach (M. Wang et al, J. Am. Chem. Soc. {\bf 128}, 3228 (2006) ) where the discrete-spin functional is mapped onto a continuous-spin functional (\emph{virtual atoms}) and the search is guided by local gradients with respect to each spin. We compare their efficiency at locating the ground-state configurations of fcc Au-Pd Alloy in terms of number of calls to the functional. We show that a GA approach with diversity-enhancing constraints and reciprocal-space mating easily outperforms the VA approach. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N21.00003: The global space-group optimization approach to crystal structure prediction Giancarlo Trimarchi, Alex Zunger We present the global space-group optimization (GSGO) approach to the prediction of both the lattice structure and the atomic configuration of a crystalline solid. The GSGO method is based on an evolutionary algorithm within which a population of crystal structures is evolved substituting the highest total-energy structures with new ones. The search is performed directly on the atomic positions and the unit-cell vectors, after a similarity transformation is applied to bring structures of different unit-cell shapes to a common basis. Following this transformation, we can define a crossover operation that treats on the same footing structures with different unit-cell shapes. Newly generated structures are fully relaxed to the closest local total-energy minimum. Starting from random unit-cell vectors and atomic positions, and using the VASP code, the GSGO procedure found for Si, GaAs, SiC the correct lattice structure and configuration. In the case of Au$_{8}$Pd$_{4}$, the search retrieved the correct underlying lattice type (fcc), but energetically closely spaced ($\sim 2$ meV/atom) alloy configurations were not resolved. The GSGO approach opens the way to predicting unsuspected structures, using, in the cases noted above, an order of $\leq 100$ total-energy {\em ab initio} calculations. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N21.00004: Quantifying cluster expansions in multicomponent systems:\ Precise expansions from noisy databases Alejandro Diaz-Ortiz, Helmut Dosch, Ralf Drautz We have performed a systematic analysis of the ubiquitous numerical errors contained in the databases used in cluster expansions of multicomponent alloys. Our results underscore the importance of numerical noise on the effective cluster interactions and on the selection mechanisms. The relevance of the size of and the information contained in the input database is highlighted. It is shown that cross-validatory approaches by themselves can produce unphysical expansions characterized by non-negligible, long-ranged coefficients. A selection criterion that combines both forecasting ability and the physical limiting behavior for the expansion is proposed. Expansions performed under this criterion exhibit the remarkable property of noise filtering. We illustrate our findings on bcc-based Fe-Co alloys. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N21.00005: Diffusion, coarsening and plasticity in alloys using the phase field crystal model Pak Yuen Chan, Jonathan Dantzig, Nigel Goldenfeld The phase field crystal model describes materials at the nanoscale on diffusive time scales, and can capture elasticity, crystallography, dislocation and grain boundary dynamics, as well as solidification processes. Here we present the extension to binary alloys, taking into account vacancies. We show how the model can be applied to technologically important phenomena, such as diffusion, grain coarsening and plasticity. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N21.00006: Rapid First-Principles Design Estimates of Alloy Order-Disorder Temperatures Teck L. Tan, Nikolai Zarkevich, D.D. Johnson From DFT calculations, we propose a rapid, mean-field estimate for order-disorder temperatures T$_{c}$ and phase diagrams via cluster expansion Hamiltonians $H=\sum_i V_{i} \phi_{i}$, where $V_{i}$ and $\phi_{i}$ are, respectively, the $i$-th cluster interaction and correlation function. We discuss when the estimate is valid and confirm its accuracy via Monte Carlo simulation. As the cost of Monte Carlo (MC) increases with number and size of clusters, such rapid estimates are desirable both for design and to limit the T and composition range needed for MC. We show two broad classes of systems as determined by $V_{i}$ in which T$_{c}$ is given accurately by (i) $\Delta H_{d-o}/\Delta S_{d-o}$ or (ii) $\Phi \Delta H_{d-o}$, where $\Delta H_{d-o}$ and $\Delta S_{d-o}$ are the enthalpy and entropy differences between fully disordered and ordered phases, respectively, and $\Phi $ is a lattice-topology dependent constant. With no finite-T intermediate phases, phase boundaries are found analytically by T$_{c} (x-x_{s})=\eta^{2} (x-x_{s})$T$_{c} (x_{s})$, where $\eta $ ($0\le \eta \le 1$) is the long-range order parameter and ($x-x_{s}$) is deviation from stiochiometry, $x_{s}$, found rapidly by CE ground-state analysis, and T$_{c} (x_{s})$ is from (i) or (ii). We exemplify results for several alloys in each class. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N21.00007: Molecular Dynamics Simulations of Interface Failure Martina E. Bachlechner, Deng Cao, Robert H. Leonard, Eli T. Owens, Wm. Trevor Swan, III, Samuel C. Ducatman The mechanical integrity of silicon/silicon nitride interfaces is of great importance in their applications in micro electronics and solar cells. Large-scale molecular dynamics simulations are an excellent tool to study mechanical and structural failure of interfaces subjected to externally applied stresses and strains. When pulling the system parallel to the interface, cracks in silicon nitride and slip and pit formation in silicon are typical failure mechanisms. Hypervelocity impact perpendicular to the interface plane leads to structural transformation and delamination at the interface. Influence of system temperature, strain rate, impact velocity, and system size on type and characteristics of failure will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N21.00008: Spatial Stratification of Order As Used in Failure Analysis Robert H. Leonard, Martina E. Bachlechner Silicon nitride deposited on silicon substrates has application in dielectric layers for microelectronics as well as in photovoltaics. During production and operation of components involving silicon/silicon nitride interfaces, stresses and strains can build up at various temperatures resulting in component failure. Using molecular dynamics simulations the influence of temperature and rate of externally applied strain on silicon/silicon nitride interfaces has been analyzed. The primary purpose of this research is to understand the mechanisms leading to the failure of these films. Analyses involving bond lengths and angles have been developed to gain insight into these mechanisms. Methods for stratifying bond lengths and bond angles into unique sub-populations on the basis of spatial orientation have been developed, and have given much insight to how the material behaves, particularly with regards to the Poisson effect. Possible extensions of this stratification method to primitive rings will also be examined. In combination with experimental observations, this analysis will deepen our understanding of the structural properties of silicon/silicon nitride interfaces. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N21.00009: Non-equilibrium Molecular Dynamics Study of the Thermal Resistance at the Interface Between Two Materials John Lyver IV, Estela Blaisten-Barojas Two different crystalline systems comprised of atoms interacting through Lennard-Jones (LJ) potentials were set in contact. The thermal conduction through such solid-solid interface was studied as a function of temperature and relative materials parameters, where the species differ in mass, hard-core atomic diameter and well depth. The computational setup simulated a solid sample with two different materials separated at a central interface. A non-equilibrium Molecular Dynamics approach was taken to calculate the Kapitza thermal resistance across the interface and its dependence on the two species and LJ parameters. It is found that the Kapitza resistance decreases as a function of temperature for mostly all combinations of the two materials LJ parameters. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N21.00010: Graded-Sequence-of-Approximations: Quantum Mechanical Forces for Molecular Dynamics Keith Runge, DeCarlos E. Taylor, V. V. Karasiev, S. B. Trickey, Frank E. Harris The rate-limiting step in the multiscale simulation of materials, biomolecular, and other complex systems is quite generally the generation of the quantum mechanical (QM) forces in the chemically active region. A sequence of approximations involving both QM and classical approximations is used to reduce the computational intensity of the problem. More computationally intensive approximations of greater accuracy, are used at infrequent simulation steps to recalibrate forces from the less intensive calculations, which have lesser accuracy. The graded-sequence-of-approximations technique is illustrated in a particularly demanding case in which we have used a published classical potential for silica with QM forces generated by a quantum chemical technique independently trained to reproduce relevant coupled-cluster forces. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N21.00011: Incoporating Existing Large Applications in the PUPIL System: Amber Sam Trickey, Juan Torras Costa, Gustavo de Miranda Seabra, Adrian Roitberg, E. Deumens PUPIL (Program for User Package Interfacing and Linking)$^1$ inter-operates existing codes for multi-threaded, multi-scale quantum and classical mechanical simulations via JAVA, XML, JAVA, a C++ library, and minimally intrusive wrappers for each code. An architectural challenge for PUPIL is support of modules from a multi-scale QM-MD suite with much internal coupling. We have succeeded with the AMBER suite MD module (Sander), with Gaussian03 for QM. Our demonstration study is the decomposition of Angelis' salt with explict water. A variable quantum zone (solute and first solvation cell) was used, with the remaining waters via TIP3P. Sander calculated the Potential of Mean Force for the reaction through umbrella sampling, with the QM forces from Gaussian. We summarize PUPIL architecture and implementation aspects, report efficiency and overhead measures, and discuss the computed results. $^1$J.~Torras, E.~Deumens and S.B.~Trickey, J. Computer Aided Mat. Des. {\bf 13}, 201 (2006); J.~Torras et al. Comp. Phys. Comm. 2006 [accepted] [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N21.00012: Concepts of Multi-Scale Modeling A. Mallik, K. Runge, J.W. Dufty The approximate representation of a quantum solid as an equivalent composite semi-classical solid is considered. In the classical bulk domain this potential energy is represented by potentials constructed to give the same structure and elastic properties as the underlying quantum solid. In a small local quantum domain the potential is determined from a detailed quantum calculation of the electronic structure.The features of this problem are the representation of the classical domain by potentials focused on reproducing the specific quantum response being studied, development of `pseudo-atoms' for a realistic treatment of charge, and inclusion of polarization effects on the quantum domain due to its distant bulk environment [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N21.00013: Quasi-continuum orbital-free density-functional theory (QC-OFDFT) Vikram Gavini, Kaushik Bhattacharya, Michael Ortiz Density-functional theory has provided insights into various materials properties in the recent decade. However, its computational complexity has made other aspects, especially those involving defects, beyond reach. Here, we present a seamless coarse-graining scheme for orbital-free density-functional theory (OFDFT), that enables the study of multi-million atom clusters with no spurious physics and at no significant loss of accuracy. The key ideas are (i) a real-space formulation, (ii) a nested finite-element implementation of the formulation and (iii) a systematic means of adaptive coarse-graining retaining full resolution where necessary and coarsening elsewhere with no patches, assumptions or structure. Fully-resolved OFDFT and finite lattice-elasticity are obtained as special limits of this scheme. This methodological development has enabled OFDFT calculations on large systems, which have revealed interesting physics and phenomena that have not been observed to date. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N21.00014: First-passage Monte Carlo for simulations of alloy microstructure Aleksandar Donev, Vasily Bulatov, Tomas Oppelstrup, Malvin Kalos, George Gilmer, Babak Sadigh We unveil a principally new Monte Carlo algorithm for simulations of multiple diffusing particles of finite dimensions that coalesce or annihilate on collisions. The algorithm is derived from the theory of first-passage processes and a time-dependent Green's function formalism. The new method circumvents the need for long and tedious diffusion hops by which the particles find each other in space. At the same time, the algorithm is exact and its computational efficiency is astonishing. The new algorithm is generally applicable in 1d, 2d, 3d, ... and to a wide variety of important physical situations, including nucleation, growth and coarsening of alloy particles, interstitial and vacancy clusters after quench or under irradiation. We will present simulation of multi-million particle ensembles covering over 10 decades of time of microstructural evolution. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N21.00015: Adaptive Resolution in Molecular Dynamics Simulations Matej Praprotnik, Luigi Delle Site, Kurt Kremer, Silvina Matysiak, Cecilia Clementi For the study of complex synthetic and biological molecular systems by computer simulations one is still restricted to simple model systems or to by far too small time scales. To overcome this problem multiscale techniques are being developed. However in almost all cases, the regions treated at different level of resolution are kept fixed and do not allow for a free exchange. We here give a basic theoretical framework for an efficient and flexible coupling of the different regimes. The approach leads to a concept, which can be seen as a geometry induced phase transition and to a counterpart of the equipartition theorem for fractional degrees of freedom. The efficiency of the presented approach is illustrated on two numerical examples, i.e., the molecular dynamics simulations of bulk water and a generic polymer in a solvent. [Preview Abstract] |
Session N22: Focus Session: Jamming I
Sponsoring Units: GSNPChair: Jennifer Schwarz, Syracuse University
Room: Colorado Convention Center 108
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N22.00001: Discontinuous jamming transition in driven foam Invited Speaker: Aqueous foam (gas bubbles with liquid walls) is a surprising substance. Every molecule in foam is in a fluid state, either liquid or gas. Yet, the entire foam holds its shape as a solid would. In fact, when subjected to an applied strain at a slow enough strain rate, the initial response of the foam is the same as an elastic solid. On the other hand, under sufficiently large stress or strain, the foam can flow in a fashion similar to a fluid. This is similar to plastic flow that occurs in many ``molecular'' solids. In this talk, we will focus on experimental studies of the transition from solid behavior to flowing behavior, with an emphasis on to what degree this ``jamming'' transition is analogous to a ``real'' phase transition. We will focus on recent results using a model, two-dimensional foam: bubble rafts. Bubble rafts are a single layer of bubbles on the surface of water. By focusing on a two-dimensional system, it is relatively easy to track individual bubbles and gain insight into the connection between bubble dynamics (the mesoscopic scale) and the response of the entire foam (macroscopic scale). We will focus on recent measurements of a \textit{discontinuous } transition from solid to fluid like behavior in the bubble raft. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N22.00002: Dynamical heterogeneity at the jamming transition Invited Speaker: We investigate the dynamics of a variety of soft materials close to the jamming transition, including strongly attractive colloidal gels, concentrated surfactant phases, and charged platelets (Laponite). By using novel time- and space-resolved light scattering techniques, we show that, quite generally, the dynamics of these systems are strongly hetergogeneous both in time and space, suggesting that they relax through discrete rearrangement events. Surprisingly, we find that each event affects a volume much larger that the size of the system's constituent (particles or clusters). This finding is in stark contrast with simulations and experiments on supercooled fluids, where spatial correlations of the dynamics extend over a few particles at most. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N22.00003: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N22.00004: A Statistical Ensemble for Soft Granular Matter Silke Henkes, Corey O'Hern, Bulbul Chakraborty Work on packings of soft spheres (PRE \textbf{68}, 011306 (2003)) has shown the existence of a Jamming transition and has highlighted the need for a general statistical framework to describe granular packings. This work presents an extension of the formalism proposed by Edwards (Physica A \textbf{157}, 1080 (1989)) to packings of soft particles. We base our analysis on a height formalism developed in two dimensions (PRL \textbf{88}, 115505 (2002)) to extract a topological invariant $\Gamma$, the trace of the global stress tensor, which is conserved under internal rearrangements of the system. Upon assuming a flat measure in $\Gamma$-space, we can derive a canonical distribution of the local $\Gamma$-values in a grain packing. We then check the predictions of this ensemble against distributions of mechanically stable packings of frictionless disks obtained from computer simulations. Work supported by NSF-DMR 0549762. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N22.00005: Some Packings Are More Equal Than Others Leo Silbert Computer simulations of packings of frictionless and frictional monodisperse spheres are discussed in the context of the jamming transition. Power-law scalings in several quantities characterising the packings are identified with distance from the jamming transition point, over several orders of magnitude in the particle friction coefficient. It is also noted that the `critical' values of the coordination number and packing fraction scale with the friction coefficient. How friction modifies the structural and dynamical properties of the packings are also discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N22.00006: Isostatic Frictional Packings: Topology and Response Functions David Wu Frictionless disks and spheres are known to spontaneously organize into isostatic contact networks with minimal coordination number under common loadings such as gravity or compression. The isostatic character of such networks has been associated with the force-chain character and constitutive properties of the macroscopic assembly. However, for non-spherical or frictional grains, the conditions for an isostatic network are no longer spontaneously satisfied, most notably due to the indeterminacy associated with frictional contacts. Here I show the existence of a general isostatic limit of frictional packings of general shape grains similar to the case of frictionless disks. I discuss the consequences for force response functions and relationship to experiments showing the onset of network failure at low coordination numbers. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N22.00007: Growing length scale for dynamical heterogeneity in an air-driven granular system near jamming Aaron Keys, Sharon Glotzer, Adam Abate, Douglas Durian Anomalous behaviour known as ``spatially heterogeneous dynamics'' (SHD) has been observed in supercooled liquids, dense colloids, and, more recently, in confined granular packings. Dynamics in these systems may be governed by proximity to a generic ``jamming transition,'' beyond which rearrangements cease and the viscosity diverges. However, the universality of this jamming hypothesis has not yet been tested in terms of variation in the hallmark dynamical heterogeneities as a function of control parameter. Here, we report measurement of SHD in systems of air-driven granular beads, as a simultaneous function of both density and effective temperature. On approach to jamming, the dynamics are found to become progressively slower and more heterogeneous. The measured dynamical time and length scales appear to diverge, and can be modeled both by mode-coupling theory and by the Vogel Tammann-Fulcher (VFT) equation, in quantitative analogy with glass-forming liquids. The Vogel temperature arising from the VFT fit, which corresponds to an ideal glass transition temperature in liquids, coincides with point-J, the volume fraction corresponding to a random close-packed structure. Our findings provide a significant step forward in the quest for a unified theory of ``jamming'' in disparate systems. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N22.00008: Jamming with attractive interactions Corey O'Hern, Gregg Lois, Jerzy Blawzdziewicz We numerically study the effects of cohesion on granular solids using a minimal model relevant to various experimental settings. The inclusion of a small amount of attraction between contacting grains is shown to significantly alter even the qualitative features of both the attainable mechanically stable packings and their material response. The structure of the jammed packings formed using energy minimization techniques varies from dilute and heterogeneous gel-like states with large void spaces to dense and homogeneous packings reminiscent of the random close packed state. The mechanical response exhibits stability under tension and a much greater sensitivity to plastic events produced by non-affine grain motion. In elastic regions the values of the moduli depend on geometric features of the packing. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N22.00009: Shear-Induced crystallization in jammed systems Daniel Lacks, Nathan Duff Simulations are used to address the effects of oscillating shear strain on jammed systems composed of spherical particles. The simulations show that shear oscillations with amplitudes of more than a few percent lead to substantial crystallization of the system. To ensure that the conclusions are independent of the simulation methodology, a range of simulations are carried out that use both molecular dynamics and athermal dynamics methods, soft and hard potentials, potentials with and without attractive forces, and systems with and without surrounding walls. The extent of crystallization is monitored primarily by the Q6 order parameter, but also in some simulations by the potential energy and the radial distribution function, and by direct visual inspection. A mechanism is proposed for shear-induced crystallization of jammed systems, based on fold catastrophes of the free energy landscape. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N22.00010: Heterogeneity of the structural relaxation of jammed state in particle-filled elastomers Xiaorong Wang, Christopher Robertson The Payne effect is a low-strain hysteretic softening in particle-filled elastomers which we recognize as part of jamming physics [1-2]. We find that in particle-filled elastomers aging at a fixed oscillatory strain $\gamma _{a}$ produces a spectral hole in the loss modulus vs strain spectrum which is localized near the aging strain [3]. Sequential aging at two strains reveals that when $\gamma _{a1}>\gamma _{a2}$ the resulting dynamic spectra appear to be a combination of that aged at $\gamma _{a1 }$and $\gamma _{a2}$; whereas for $\gamma _{a1}<\gamma _{a2}$, the resulting dynamic spectra only reflect the characteristic hole burning of the second strain after holding at $\gamma _{a2}$. This remarkable behavior of particle-filled elastomers suggests that structural relaxations in jammed state are heterogeneous and aging at a fixed strain $\gamma_{a}$ only affects part of the relaxation spectra. \newline \newline [1] \textit{Phys. Rev. E}, \textbf{2005}, 72 (3), 031406; \newline [2] \textit{Phys. Rev. Lett.}, \textbf{2005}, 95, 075703; \newline [3] \textit{Europhys. Lett.}, \textbf{2006}, 76(2) 278. [Preview Abstract] |
Session N24: Organic LEDS and Light Emission
Sponsoring Units: DPOLY DMPChair: Eric Lin, National Institute of Standards and Technology
Room: Colorado Convention Center 201
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N24.00001: Green polariton photoluminescence in organic microcavities containing the red-emitting phosphor PtOEP Stephane Kena-Cohen, Stephen R. Forrest Green upper and lower branch polariton photoluminescence (PL) is observed in microcavities containing the red-emitting organic phosphor 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphyrin platinum(II) (PtOEP). This PL is attributed to cavity polariton states formed by coupling to the Q(0,0) transition of PtOEP (fluorescence). The PL spectra mirror the polariton dispersion obtained from angle-resolved reflectivity measurements. The increased fluorescence intensity compared to the neat film case is due to the reduced lifetime of the polariton states. This PL is also accompanied by strong red emission (phosphorescence) due to intersystem crossing (ISC) to the triplet state of PtOEP. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N24.00002: Morphology, structure and photoluminescence properties of thin films of a conjugated polymer poly(2,5-dinonyl para phenyleneethynylene) Craig Szymanski, Yunfei Jiang, Jasson McNille, Dvora Perahia, Uwe H. F. Bunz Poly(para phenyleneethynylenes) (PPE), are inherently semiconductor and their electro-optical characteristics depends among other factors on their association mode. PPE molecules were trapped on the surface from molecular solutions, aggregates and gels. When cast from toluene solutions bellow the critical micellar concentration (CMC) of the polymers, small clusters with an average diameter of the molecular length were observed. When cast from gel phases, a supra molecular structure that consists of well defined associating rods were detected. All of these association modes emit at 460nm. Further emissions are detected as the structure evolves from collapsed single molecules to supramolecular structure where for small non interacting rods, feature at 511 in observed and for associating rods it shifts to 550nm. Further studies are currently underway to correlate the fluoresce patterns with the structural features. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N24.00003: Photoemission study of tris(8-hydroxyquinoline) aluminum/aluminum oxide/tris(8-hydroxyquinoline) aluminum interface Huanjun Ding, Serkan Zorba, Yongli Gao, Liping Ma, Yang Yang The evolution of the interface electronic structure of a sandwich structure involving aluminum oxide and tris(8-hydroxyquinoline) aluminum (Alq), i.e. (Alq/AlO$_{x}$/Alq), has been investigated with photoemission spectroscopy. Strong chemical reactions have been observed due to aluminum deposition onto the Alq substrate. The subsequent oxygen exposure releases some of the Alq molecules from the interaction with aluminum. Finally, the deposition of the top Alq layer leads to an asymmetry in the electronic energy level alignment with respect to the AlO$_{x}$ interlayer. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N24.00004: Hybrid Organic/Inorganic Semiconductor Structures: Efficient F\"orster Energy Coupling and Prospective Optical Devices Invited Speaker: Hybrid organic/inorganic semiconductor structures offer the prospect of combining the favourable electrical and optical characteristics of each to achieve desirable new device functions. They also provide a test bed to study the exciton state interactions between these distinct semiconductor varieties and the resultant energy transfer processes that occur between them. In this talk I will describe work on conjugated polymer/GaN structures for which efficient non-radiative F\"orster transfer can be achieved from the inorganic quantum well excitons to the organic (polymer) excitons. I will also discuss the prospects for optical devices based on this and related hybrid systems. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N24.00005: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N24.00006: Luminescence from single colloidal nanocrystals embedded in organic light emitting devices. August Dorn, Hao Huang, Vladimir Bulovic, Moungi Bawendi The photophysical properties of individual CdSe/ZnS (core/shell) nanocrystals embedded in the active layers of electrically driven organic light emitting devices (OLEDs) were investigated at room temperature. Emission from the same nanocrystals was recorded under laser illumination and when the OLED was driven electrically. For both types of excitation we observed blinking and spectral diffusion, key signatures of single quantum dot fluorescence. Enhanced electroluminescence from the organics at the sites of nanocrystals suggests the formation of current channels through the quantum dots. This hypothesis is supported by atomic force microscopy studies of the organic layers. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N24.00007: Charge injection and Raman scattering studies from polyfluorene-based light-emitting diodes M. Arif, S. Guha Efficient and well balanced injection of charge carriers and transport capabilities are of particular importance for high luminescence efficiency in organic light-emitting diodes. Polyfluorene (PF) conjugated polymers have received widespread attention due to their strong blue emission, high charge mobility and excellent chemical and thermal stability which creates great prospect for optoelectronic device applications. Although ethyl-hexyl substituted PF (PF2/6) has a high level of molecular disorder, charge injection in single layer polymer devices can be described very well by space-charge-limited conduction for a discrete set of trap levels. This is attributed to the nature of ordering in the polymer. PFs are characterized by a number of Raman-active peaks originating from C-H bending and C-C stretching type motion. We further analyze our working devices using Raman scattering in the presence of photogenerated carriers. The Raman intensities in the 1000-1250 cm$^{-1}$ corresponding to a C-H bend-type motion quench in the presence of carriers with increasing fields. This effect most probably arises due to the interaction of phonons and free carriers. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N24.00008: Multi-walled carbon nanotube sheets as transparent anodes in organic light-emitting diodes Christopher Williams, Raquel Ovalle Robles, Mei Zhang, Sergey Li, Ray Baughman, Anvar Zakhidov Carbon nanotubes have emerged as useful components for next-generation electronic devices. We have investigated one such area by producing organic light-emitting which use transparent multi-walled carbon nanotube sheets as a replacement for indium tin oxide (ITO). These sheets offer high optical transparency with the additional advantage of being very flexible with no loss in conductivity, making them ideal candidates for devices built on plastic substrates. We have produced devices on both high quality display glass and plastic substrates and have observed bright emission with efficiencies which are comparable to those obtained from ITO-based devices. We also present results for devices which combine ITO and nanotube sheets. Such devices take advantage of the planar conductivity of the ITO and improved injection from the nanotube sheets. We show improved efficiency in plastic devices which use this bilayer anode structure. Our results also demonstrate the importance of using a planarization layer on top of the carbon nanotube sheet to eliminate sources of leakage current. Lastly, we propose alternative device architectures such as transparent and inverted structures. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N24.00009: Conformations in di-octyl substituted polyfluorene: a combined theoretical and experimental Raman scattering study C. Volz, M. Arif, S. Guha The structural properties of polyfluorenes (PF) are extremely sensitive to the choice of functionalizing side chains. Di- octyl substituted PF (PF8) adopts metastable structures that depend upon the thermal history and choice of solvents used in film forming conditions. We present a detailed study of the changes in the backbone and side chain morphology in PF8, induced by the various crystallographic phases, using Raman scattering techniques. The vibrational frequencies and intensities of fluorene oligomers are calculated using hybrid density-functional theory with a 3-21G* basis set. The alkyl side chains are modeled as limiting conformations: all \emph {anti}, \emph{anti-gauche-gauche}, and end \emph{gauche} representations. The calculated vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the $\beta$ phase, which is known to originate in regions of enhanced chain planarity, as a direct consequence of the alkyl side chain conformation. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N24.00010: Influence of Environment on the Electronic Structure of Polyfuorenes Elizabeth M. Lupton, Feng Liu The influence of the structure of polyfluorene molecules on their emissive characteristics, as utilized in polymer LEDs, can be characterized using first principles methods. Here we concentrate on how factors such as external restrictions, structural and chemical defects, and constraints caused by side groups can affect the electronic structure of polyfluorenes, in particular the extent of conjugation along the backbone. Using Car -- Parrinello Molecular Dynamics simulations, where the electronic structure is calculated according to Density Functional Theory `on the fly' for a molecular dynamics trajectory, we systematically investigate how curving the backbone combined with torsional rotation between repeat units, as well as ketone defects, can affect the electronic structure. This demonstrates the way in which restrictions imposed by the environment could ultimately affect the light emitting properties of the polymer. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N24.00011: NEXAFS measurements of chain alignment in order polyfluorene thin films Xiaosong Liu, Hyeunseok Cheun, Frank Galbrecht, F. J. Himpsel, Ullrich Scherf, Michael Winokur Carbon K-edge near edge X-ray absorption fine structure (NEXAFS) has been used to characterize the uniaxial surface chain alignment within the top surface (2-3 nm) of poly[bis(2-ethyl)hexylfluorene] thin films spin-cast atop rubbed polyimide templating substrates before and after thermal annealing. The film thicknesses range from approximately 25 to 130 nm. In the thinnest films appreciable chain alignment extends through to the top surface prior to annealing. Thermal annealing produces comparatively high levels of surface chain alignment in all film thicknesses despite a drop in the dichroic ratios, as measured by polarized optical absorption spectroscopy, in the thickest films. These data support a model that exhibits a graded morphology in which the top and bottom surfaces exhibit planar, uniaxial alignment while the film interior is less well aligned and includes a proportion of homeotropic alignment. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N24.00012: Optical studies of platinum-containing conjugated polymers Minghong Tong, Alessio Gambetti, Tomer Drori, Zeev Vardeny 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 intrachain heavy metal Pt atom increases the spin-orbit coupling, and this influences both the intersystem crossing time, T$_{ic}$, and the phosphorescence emission strength. From the ps transient pump-probe photomodulation spectroscopy and emission dynamic measurements using the up-conversion technique, we found that T$_{ic}$ for these polymers is of order of few ps; whereas the phosphorescence lifetime is of order of few microseconds. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N24.00013: Crystalline PTCDA waveguides grown by organic molecular beam deposition V.R. Gangilenka, J. Markus, H. Schmitzer, H.P. Wagner During the past few decades molecular organic semiconductors have become interesting candidates for optical and opto-electronic device applications such as organic light emitting diodes (OLEDs). Of many organic materials 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) is one of the most intensively investigated organic molecule in literature. However, so far studies on PTCDA optical waveguides are very limited. We investigate various PTCDA waveguide structures by m-line spectroscopy. The waveguides are fabricated by organic molecular beam deposition (OMBD) under high vacuum. The waveguides are deposited on a pyrex substrate or on a rutile prism with an index matching film of aluminum-quinoline (Alq3). The effective indices of refraction of observed TE and TM modes are consistent with classical electromagnetic theory. The calculated thicknesses of the waveguides under investigation are compared with reflection measurements on the same samples. [Preview Abstract] |
Session N25: Focus Session: Biopolymers I: Mechanical Properties
Sponsoring Units: DPOLY DBPChair: Ting Xu, University of California, Berkeley
Room: Colorado Convention Center 203
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N25.00001: Probing Polarization Dynamics and Energy Dissipation in Ferroelectric Polymers on the Nanoscale Invited Speaker: Ferroelectric polymers are emerging as prominent materials for ultrasonic actuators, gate materials for non-volatile ferroelectric memories, and energy storage. The nature of ferroelectricity in polymers is significantly different from that in inorganic perovskites, resulting in significant interest to elementary mechanism of switching and the role of local microstructure. In this talk, I briefly delineate Piezoresponse Force Microscopy and Spectroscopy as applied for characterization of Langmuir-Blodgett ferroelectric PVDF polymer films. The slow polarization switching in PVDF can be attributed to the grain-by grain switching mechanism. Recent advances in PFM probing of polarization dynamics and electromechanical energy dissipation are discussed. In particular, switching spectroscopy PFM is used to probe the spatial variability of switching behavior and role of grain boundaries on switching. Local energy dissipation imaging through the changes of the Q-factor of electrically driven cantilever in contact with the surface is developed to study energy losses in the ferroelectric switching processes. In collaboration with Brian J. Rodriguez and Stephen Jesse, Materials Sciences and Technology Division and The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory; Jihee Kim and Steven Ducharme, Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience University of Nebraska, Lincoln. \newline \newline Research was supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Materials Sciences and Engineering (SVK, BJR, and SJ) and user proposal of The Center for Nanophase Materials Sciences (JK and SD) and was performed at Oak Ridge National Laboratory which is operated by UT-Battelle, LLC. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N25.00002: Synchrotron X-ray Diffraction Study on the Effect of the Tau protein on the Mechanical Properties of Microtubules Myung Chul Choi, Uri Raviv, Herbert Miller, Michelle Massie, Youli Li, Leslie Wilson, Stuart Feinstein, Mahn Won Kim, Cyrus Safinya Microtubules (MTs) are 25 nm protein nanotubes used as tracks for intracellular trafficking of biomolecules, for example, those involved in transmitting signals between neurons. In neurons, MTs are long-lived both in axons and dendrites. A distinct member of microtubule-associated-proteins (MAPs) regulates microtubule assembly, although the mechanisms of regulation resulting from different tau isoforms remains to be fully elucidated. Incorrectly phosphorylated MAP tau is implicated in a large number of neurodegenerative diseases where altered tau-MT interactions and MT depolymerization and tangles of taus lead to detrimental consequences for neuronal survival. We will describe our recent finding on the effect of tau isoforms on the mechanical properties of MTs, probed by synchrotron X-ray diffraction. Supported by NSF DMR-0503347, DOE DE-FG02-06ER46314, and NIH GM59288. M.C.Choi received partial support from the Korean Foundation Grant KRF-2005-2214-C00202. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N25.00003: Time-resolved studies of actin organization by multivalent ions and actin-binding proteins Ghee Hwee Lai, Kirstin Purdy, James R. Bartles, Gerard Chee Lai Wong Actin is one of the principal components in the eukaryotic cytoskeleton, the architecture of which is highly regulated for a wide range of biological functions. In the presence of multivalent salts or actin-binding proteins, it is known that F-actin can organize into bundles or networks. In this work, we use time-resolved confocal microscopy to study the dynamics of actin bundle growth induced by multivalent ions and by espin, a prototypical actin binding protein that is known to induce bundles. For divalent ion induced bundles, we observe a rapid lateral saturation followed by longitudinal growth of bundles, in sharp contrast to the bundling mechanism of espin, which favors finite length bundles. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N25.00004: Elastic Behavior of Composite Actin and Microtubule Networks Yi-Chia Lin, Gijsje Koenderink, Frederick Mackintosh, David Weitz We explore the non-linear shearing behavior of composite actin and microtubule networks. Large bending rigid microtubules are used as a probe of the deformation mode of cross-linked actin networks. For a sparsely cross-linked actin network that deforms non-affinely, adding microtubules can drive the system back to affine by suppressing local rearrangements of actin filaments. It applies to both permanently rigid cross-linker, such as scruin, and flexible cross-linker, such as filamin. This experiment also shows that filamin cross-linked actin networks are deforming in an affine manner. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N25.00005: Mechanics of actin networks crosslinked with mutant human $\alpha$-actinin-4 Sabine Volkmer, Daniel Blair, Karen Kasza, David Weitz Globular actin can be polymerized {\em in vitro} to form F-actin in the presence of various binding proteins. These networks often exhibit dramatic nonlinear rheological response to imposed strains. We study the rheological properties of F-actin networks crosslinked with human $\alpha$-actinin-4. A single genetic mutation of the $\alpha$-actinin-4 protein is associated with focal and segmented glomerulosclerosis (FSGS), a genetic disorder which leads to renal failure. Mechanically, the mutant crosslinker has an increased binding strength compared to the wild type. We will show that human $\alpha$-actinin-4, displays a unique stiffening response. Moreover, we also demonstrate that a single point mutation dramatically effects the inherent relaxation time of the crosslinked network. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N25.00006: Viscoelastic properties of Ionomer Melt Monojoy Goswami, Sanat Kumar Viscoelastic prperties of a model telechelic ionomer, i.e., a melt of non-polar polymers with a charge at each chain end along with neutralizing counterions, have been examined using molecular dynamics simulation. Equlibrium calculation of the loss modulus $G^{\prime\prime}(\omega)$ and storage modulus $G^\prime(\omega)$ shows plateau at lower temperatures when the systems are not relaxed. In this situation the specific heat ($C_v$) peak corresponds to the self-assembly of the system, at lower temperatures the specific heat begins to plateau. Similarities of the dynamic features found for telechelic melts with those observed in glass-forming liquids and entangled polymers have been shown. Furthremore, using an athermal 'probe', the properties of these materials is being distinctly classified as 'strong' glass or physical gels. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N25.00007: Controlling the Properties of Thermoreversible Protein Hydrogels Hui Yan, Alberto Saiani, Aline Miller In this work we have explored the potential of using self-assembling protein molecules as the basic unit for novel biomaterials for biomedical applications. Here we will show how thermo-reversible fibrillar hydrogels can be formed from an aqueous solution of hen egg white lysozyme by adding the reductant dithiothreitol. The elastic modulus of the hydrogels formed has been examined and micro differential scanning calorimetry experiments confirmed that the hydrogels were thermally reversible and that gelation and melting occurs through a solid-liquid like first order transition. Infra-red and transmission electron microscopy studies of very dilute samples revealed the presence of beta-sheet rich fibrils that were 4--6 nm in diameter and 1micron in length. These fibrils self-assemble along their long axes to form larger fibers that become physically entangled to form the 3D network observed in both cryoSEM and small angle neutron scattering studies. We will also demonstrate that we can control and manipulate gel properties by varying the protein concentration, reductant concentration and ionic strength of the matrix. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N25.00008: Electrospinning of Hyaluronic acid (HA) and HA/Gelatin Blends Aihua He, Junxing Li, Charles Han, Dufei Fang, Benjamin Hsiao, Benjamin Chu It was found that the processability of HA solution with high viscosity had been improved greatly by using a DMF-water solvent mixture or/and by adding gelatin(GE) into the HA solution. Nano-fibrous membranes with different average fiber diameters and different HA/GE compositions could be obtained. Measurements on viscosity indicated that the HA solution in DMF-water mixed solvent still showed high viscosity. The decrease in surface tension contributed to the fiber formation of HA and HA/GE by electrospinning. Therefore, this study not only provided a novel and simpler way to electrospin the natural polyanion HA solution, but also provided the fundamental physical insight and solution to this spinning difficulty. The HA-GE nanofibrous membranes at different HA/GE compositions are expected to be useful in the biomedical field as novel scaffolds for many applications. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N25.00009: Rheology and lubricity of hyaluronic acid Jing Liang, Wendy E. Krause The polyelectrolyte hyaluronic acid (HA, hyaluronan) is an important component in synovial fluid ($i.e$., the fluid that lubricates our freely moving joints). Its presence results in highly viscoelastic solutions. In comparison to healthy synovial fluid, diseased fluid has a reduced viscosity and loss of lubricity. In osteoarthritis the reduction in viscosity results from a decline in both the molecular weight and concentration of HA. In our investigation, we attempt to correlate the rheological properties of HA solutions to changes in lubrication and wear. A nanoindenter will be used to evaluate the coefficient of friction and wear properties between the nanoindenter tip and ultrahigh molecular weight polyethylene in both the presence and absence of a thin film of HA solution. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N25.00010: Physical Control of Stem Cells via Matrix Elasticity Florian Rehfeldt, Dennis Discher Most of our cells reside in soft tissue, but it has only become clear over the last decade that substrate elasticity exerts a major influence on cell motility, contractility, and overall cell function. The mechanical properties of the matrix can even direct the differentiation of human adult stem cells as reported by our group recently (Engler et al. Cell 2006). Basically, the greater the resistance to matrix deformation, the larger the force with which the cell pulls on the matrix, driving the assembly of cytoskeleton and adhesions. For a deeper understanding of the molecular mechanisms of force generation and transduction, various biophysical and biochemical tools must be combined with well-defined extracellular matrix (ECM) models. Past studies have been conducted mostly with synthetic and uncharged polyacrylamide (PA) gel matrices, motivating more bio-relevant gel models. We have developed such a biocompatible hydrogel system of widely and finely tunable elasticity using hyaluronic acid (HA), which is ubiquitous in development and in particular adult tissues. The effective Young's modulus $E$ of these negatively charged hydrogels measured by AFM can be finely tuned by variation of cross-linker and HA concentration yielding a stiffness of 0.1 kPa to 150 kPa. $E$ scales with the concentration of HA to the power of $n$=2.6 and is a biphasic function of cross-linker concentration. We will describe the influence of these unique gels on stem cell differentiation. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N25.00011: Elasticity of Short DNA Molecules: Quantitative Agreement Between Theory and Experiment Yeonee Seol, Jinyu Li, Philip Nelson, Thomas Perkins, M. D. Betterton Single-molecule experiments have yielded new insight into the mechanical behavior of individual DNA molecules and protein-DNA interactions. Single-molecule force experiments require a model to deduce the polymer's intrinsic contour length ($L)$ from measurements of force and extension. To date, the worm-like chain model (WLC) provides the best description of DNA elasticity. This theory requires parameters, the contour length $L$ and the persistence length $p$. Using both theory and experiment, we studied the elasticity of dsDNA as function of $L$ using the classic WLC solution, for $L$ between 632 nm and 7.03 microns. When the elasticity data were analyzed using the classic WLC, the fit value of $p$ depended $L$. Therefore we developed the finite worm-like chain solution (FWLC) by including the finite length of the chain and bead rotation. After incorporating these two corrections, our FWLC solution was used to predict elasticity curves and to analyze experimental data. The FWLC provides a single theoretical framework in which to analyze single molecule experiments over a broad range of experimentally accessible DNA lengths, including both short and very long molecules. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N25.00012: Tube Radius in Entangled Networks of Semiflexible Polymers Hauke Hinsch, Jan Wilhelm, Erwin Frey The mechanical properties of the cytoskeleton play an important role in many cellular functions like locomotion or adhesion. One of the cytoskeleton's dominant constituents is a network structure composed of the semiflexible polymer F-Actin. To connect the single polymer properties to the macroscopic behavior of the network, a single polymer is considered to be constrained to a tube established by neighboring filaments. Here we focus on the tube's diameter in entangled networks. While scaling laws for the tube diameter are well established, the absolute value is still under debate and different theoretical concepts and experimental measurements exist. We present a new approach to the problem and have conducted extensive computer simulations to check the validity of our assumptions. A model of independent rods is used to describe the confinement of a single semi-flexible polymer in the network environment. A self-consistency approach allows us then to derive an absolute tube radius for the network as a function of several parameters and compare our results to experimental measurements. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N25.00013: Stretching and bending in cross-linked biopolymer networks Claus Heussinger, Erwin Frey The elastic response of cross-linked biopolymer networks is usually interpreted in terms of affine stretching models, adopted from the theory of rubber-elasticity valid for flexible polymer gels. Unlike flexible polymers, however, stiff polymers have a highly anisotropic elastic response, where the low-energy elastic excitations are actually of bending nature. As a consequence, similar to springs connected in series, one would expect the softer bending mode to dominate the elastic energy rather than the stiff stretching mode. We propose a theory that, unlike recent affine models, properly accounts for the soft bending response of stiff polymers. It allows calculating the macroscopic elastic moduli starting from a microscopic characterization of the (non-affine) deformation field. The calculated scaling properties for the shear modulus are in excellent agreement with the results of recent simulations obtained in simple two-dimensional model networks, and can also be applied to rationalize bulk rheological data in reconstituted actin networks. [Preview Abstract] |
Session N26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections IV
Sponsoring Units: DCPChair: Anna Krylov, University of Southern California
Room: Colorado Convention Center 205
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N26.00001: Coherent 2D Spectroscopy and Control of Molecular Complexes Invited Speaker: Coherent two-dimensional femtosecond spectroscopy is used to investigate electronic couplings within molecular complexes. Third-order optical response functions are measured in a non-collinear three-pulse photon echo geometry with heterodyne signal detection. In combination with suitable simulations this allows recovering the delocalization of excited-state wavefunctions, their coupling, and the corresponding energy transport pathways, with nanometer spatial and femtosecond temporal resolution. Examples of multichromophoric systems are the FMO and the LH3 light-harvesting complexes from green sulfur bacteria and purple bacteria, respectively, for which energy transfer processes have been determined. Additional challenges arise if one is interested in the spectroscopy of photochemical rather than photophysical processes in molecular complexes: The product yields attained by a single femtosecond laser pulse are often very small, and hence time-dependent signals are hard to measure with good signal-to-noise ratio. In the context of coherent control, this implies that bond-breaking photochemistry in liquids is still difficult despite the many successes of optimal control in gas-phase photodissociation. In a novel accumulative scheme, macroscopic amounts of stable photoproducts are generated in an optimal fashion and with high product detection sensitivity. In connection with time-resolved spectroscopy, the accumulative scheme furthermore provides kinetic information on the pathways of low-efficiency chemical reaction channels. This was applied to investigate the photoconversion of green fluorescent protein. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N26.00002: Watching the electronic motions driven by a conical intersection Invited Speaker: In chemistry, the fastest electronic rearrangements proceed through ``conical intersections'' between electronic potential energy surfaces. With sufficiently short pulses, the electronic motion can be isolated by polarized excitation of aligned electronic wavepackets at a conical intersection. Polarized femtosecond probing reveals signatures of electronic wavepacket motion (due to the energy gaps) and of electron transfer between orbitals (due to the couplings) driven by the conical intersection. After exciting a D$_{4h}$ symmetry silicon naphthalocyanine molecule onto a Jahn-Teller conical intersection in the first excited state, electronic motions cause a $\sim $100 fs drop in the pump-probe polarization anisotropy. The polarized vibrational modulations of the signal can be used to deduce the symmetry and stabilization energies for each vibration. The initial decay of the polarization anisotropy can be quantitatively predicted from these vibrational parameters. Both coupling and energy gap variations are important on the $\sim $100 fs timescale. A 1 meV stabilization drives electrons from orbital to orbital in 100 fs, and the theory indicates that a chemically reactive conical intersection with 1000x greater stabilization energy could cause electronic equilibration within 2 fs. We have recently carried out experiments on a nominally D$_{2h}$ symmetry free-base naphthalocyanine for which the splitting between $x$ and $y$ polarized transitions is not resolved in the linear spectrum. For this molecule, the anisotropy also decays on a similar timescale and exhibits damped modulations whose origin (vibrational or electronic) has not yet been determined. The role of the central protons and nominal D$_{2h}$ symmetry in the electronic dynamics will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N26.00003: Electronic relaxation dynamics in (water)$_{n}^{-}$ ($n$=25-100) and (CH$_{3}$OH)$_{n}^{-}$ ($n\sim $140-530) clusters via femtosecond photoelectron imaging Aster Kammrath, Graham Griffin, Jan Verlet, Art Bragg, Daniel Neumark Large clusters of (H$_{2}$O)$_{n}^{-}$ ($n$=25-50), (D$_{2}$O)$_{n}^{-}$ ($n$=25-100) and (CH$_{3}$OH)$_{n}^{-}$ ($n\sim $140-530) are studied with femtosecond time-resolved photoelectron imaging. For all three systems, the excess electron is promoted to an excited state with a pump laser pulse at 1.55 eV. Subsequent dynamics are monitored by observing photoelectrons detached after a variable delay with a probe pulse at 3.1 eV. For all three systems the excess electron is seen to decay via internal conversion back to the ground state with lifetimes of 190-130 fs for (H$_{2}$O)$_{n}^{-}$, 360-150 fs for (D$_{2}$O)$_{n}^{-}$ and 260-170 fs for (CH$_{3}$OH)$_{n}^{-}$. For all three systems, lifetime of the excited state decreases with increasing cluster size and is found to vary linearly with 1/$n$. Extrapolation to the bulk yields lifetimes of 54$\pm $30 fs for H$_{2}$O, 72$\pm $22 fs for D$_{2}$O and $\sim $150 fs for CH$_{3}$OH. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N26.00004: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N26.00005: Quantum dynamics and photochemistry of negative ions via photoelectron imaging and photofragment spectroscopy. Andrei Sanov, Emily Grumbling, Richard Mabbs, Terefe Habteyes, Kostya Pichugin, Luis Velarde Photochemistry of molecular and cluster anions is studied using photoelectron imaging and photofragment spectroscopy. Photoelectron imaging is used to observe interference effects in electron photoemission and monitor the transformations of the electronic structure in chemical reactions. The transformations of electronic energy levels and the corresponding wavefunctions are studied in the solvent and reaction-coordinate domains. Time-resolved results reflect the electron emission dynamics, establishment of the reaction product electronic identity and provide dynamical tests of the anion electronic potentials and exit-channel interactions in chemical reactions. Photofragment spectroscopy in the solvent domain reveals solvent-enabled and solvent-controlled bond dissociation and ion-molecule association reactions in anionic environments, including state-crossings and Renner-Teller interactions. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N26.00006: Conformationally controlled chemistry: Excited state dynamics dictate ground state dissociation Arthur Suits, Myung-Hwa Kim, Lei Shen, Bailin Zhang, Hongli Tao, Todd Martinez Ion imaging results show distinct photodissociation dynamics for propanal cations initially prepared in either the \textit{cis-} or \textit{gauche-} conformation, even though these differ only slightly in energy and there is a small barrier between them. The product kinetic energy distributions for the H elimination channels are bimodal, and the two peaks are readily assigned to propanoyl cation + H and hydroxyallyl cation + H. Ab initio multiple spawning dynamical calculations show that distinct ultrafast dynamics in the excited state leads to internal conversion to the ground state in isolated regions of the potential surface for the two conformers, and from these distinct regions, conformer interconversion does not effectively compete with dissociation. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N26.00007: Conformer-selected photodissociation: Ab Initio Multiple Spawning Dynamics of Excited Propanal Cation Hongli Tao, Todd Martinez Recent experiments have shown that pure cis and gauche propanal cations can be prepared using REMPI (Resonance Enhanced Multiphoton Ionization Spectroscopy). The H elimination pathway which results when these conformer-selected cations are photoexcited was found to depend on the conformer (cis vs. gauche). This dependence is very surprising since the interconversion barrier of the two conformers is small compared to the excited energy. We use the ab initio multiple spawning (AIMS) method developed in our group to model the conformer-specific photodissociation and to elucidate its origins. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N26.00008: A Numerical Study of Pulse-Shape Control of Non-Adiabatic Electron Excitation in the Strong-Field Regime Stanley Smith, Xiaosong Li, Alexei Markevitch, Dmitri Romanov, H. Bernhard Schlegel, Robert Levis The electron optical response of several molecular monocations to short strong-field laser pulses was studied using time-dependent Hartree-Fock theory. In addition to the carrier frequency and maximum amplitude (up to 3.75 x 10$^{13}$ W/cm$^{2})$, the short pulses were characterized by pulse shape parameters: the amplitude profile (trapezoidal and gaussian) and the carrier phase shift. The electron response was traced by the evolution of the excited states occupation numbers and by the instantaneous dipole moment of the molecule. In the molecular monocations studied, butadiene, naphthalene, and anthracene, we observed significant modifications in the dipole moment response and in the corresponding excited state spectra, controlled by intensity, frequency, phase, and shape of the laser pulse. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N26.00009: Adaptive Control Goal Selection for Strong-Field Dissociative Ionization of Polyatomic Molecules Dmitri Romanov (1,3), Huyen Tran (2,3), Robert Levis (2,3) In many settings (for instance, in strong-field mass-spectral sensing technologies) improving control efficiency is more important than achieving specific control goals. In this case, control goals may be adaptively formulated in the process of a strong-field experiment. To determine the pairs of fragment ions in a mass spectrum that are most susceptible to control by adaptive optimization of the laser pulse shapes in the strong-field regime, a statistical method is proposed that is based on covariance analysis of the mass spectral fragmentation patterns generated by a set of random shaped pulses. As a test, the method was applied to fragmentation of a large organic molecule dimethylmethylphosphonate, (CH$_{3}$O)-PO-(OCH$_{3})$-(CH$_{3})$. All possible pairs of the ionized fragments in \textit{tof} mass spectrum were ranked by the value of their correlation coefficients ranging from +1 to --1. A genetic-algorithm based adaptive control was then used to optimize the ion peak ratios in these pairs. Convincingly, the pairs of fragment ions that have higher negative covariances possess a correspondingly higher degree of controllability, while the pairs that have higher positive covariances possess correspondingly lower controllability. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N26.00010: Probing Strong-Field Electron-Nuclear Dynamics of Polyatomic Molecules Using Proton Motion Robert Levis (1,3), Alexei Markevitch (1,3), Dmitri Romanov (2,3), Stanley Smith (1,3) Protons ejected from a large polyatomic molecule during its Coulomb explosion can carry information about the dynamics of explosion and pre-explosion processes related to specific molecular structure. To extract this information, the proton kinetic energy distributions were derived from the shape or the time-of-flight proton peak for three structure-related molecules, anthracene, octohydroanthracene, and anthraquinone, subjected to intense 800 nm, 60 fs laser pulses. The kinetic energy distributions are found to be markedly molecular-specific, providing insight into similarities and differences in the nonadiabatic electron-nuclear dynamics in these molecules during the laser pulse. In particular, analysis of the proton energy distributions reveals molecular specificity of non-adiabatic charge localization and field-mediated restructuring of polyatomic molecules polarized by strong laser fields. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N26.00011: Toward Coherent Control of Cis-Stilbene Photodynamics Jason Quenneville, Todd J. Martinez Stilbene can undergo photoisomerization between its cis and trans isomers. Non-radiative quenching of excited state population to the ground state can occur at a twisted and pyramidalized conical intersection of S$_{0}$ and S$_{1}$ that is remarkably similar to that found for ethylene. In addition, photo-excited cis-stilbene can undergo a cyclization reaction giving 4a,4b-dihydrophenanthrene. Here, population decay occurs through a conical intersection in the cis-stilbene configuration. Both competing reaction pathways give appreciable reactant recovery. The product branching ratios can be directly related to the location of the conical intersections in nuclear coordinate space and also, more specifically, to the wavepacket dynamics in the nonadiabatic region. A significant effort is currently underway at Los Alamos to achieve coherent control of photo-excited cis-stilbene. The goal will be to design a shaped femtosecond laser pulse that will control photo-product yield. More importantly, we hope to gain an understanding of the important features of the optimized electric field and thus insight into the prospects for more complicated materials. We will detail the potential surfaces of photo-excited cis-stilbene, the initial S$_{1}$ dynamics, as well as opportunities for quantum control. [Preview Abstract] |
Session N27: Focus Session: Computational Nanoscience IV - Nanoparticles
Sponsoring Units: DMP DCOMPChair: Talat Rahman, University of Central Florida
Room: Colorado Convention Center 301
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N27.00001: Evolution of Magnetism from Atoms to Crystals Invited Speaker: The existence of spontaneous magnetization in metallic systems is an intriguing problem because of the extensive technological applications of magnetic phenomena and an incomplete theory of its fundamental mechanisms. Clusters of metallic atoms are important in this respect as they serve as a bridge between the atomic limit and the bulk, and they can form a basis for understanding the emergence of magnetization as a function of size. In solids, ferromagnetism is understood in terms of the exchange interaction and the formation of distinct energy bands for the majority spin and minority spin channels. In clusters, energy bands are replaced with dellocalized electronic orbitals, whose properties are affected by the finite size and the presence of a surface. Therefore, the size and shape of a cluster play important role in its magnetic properties. Indeed, direct measurements have indicated a strong dependence of magnetic moment with the size of the cluster, especially in iron clusters but also nickel and cobalt. Taking advantage of recent developments in computational methods for the electronic structure of nanosystems, we can now investigate in greater detail the magnetic properties of metallic clusters containing several hundreds of atoms and understand the role of size and shape. This analysis is based on first-principles density-functional theory, within the generalized gradient approximation. Numerical calculations were done in clusters containing up to 400 atoms (iron, nickel, and cobalt). Calculations are done using the PARSEC code ( www.ices.utexas.edu/parsec ). We also discuss some of the recently developed capabilities of PARSEC. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N27.00002: Electronic and structural properties of binary Pt-Ni nanoclusters Luis A. P\'erez, Ignacio L. Garz\'on The lowest energy structures of binary (PtNi$_{3})_{n}$, (Pt$_{3}$Ni)$_{n}$, and (PtNi)$_{m}$ nanoclusters, with n=3-10 and m=3-20, modeled by the many-body Gupta potential, were obtained by using a genetic-symbiotic algorithm. These structures were further relaxed with DFT-GGA. In agreement with the experimental evidence, segregation is observed in these clusters, where the Ni atoms are mainly found in the cluster core and the Pt atoms on the cluster surface. Furthermore, it has been experimentally found that the (Pt$_{3}$Ni)$_{n}$ nanoalloys present a higher catalytic activity for the N$_{2}$O + H$_{2}$ reaction at low temperatures than the other compositions [1], while the contrary trend is observed in the case of the oxidation of carbon monoxide in the presence of hydrogen, where the (PtNi$_{3})_{n}$ nanoparticles present a higher catalytic activity than the other ones. In order to understand these tendencies in the catalytic activity, we performed an analysis of the surface electronic structure of the bimetallic Pt-Ni nanoclusters with the mentioned compositions, by means of first-principles density functional calculations. Acknowledgments: This work was supported by CONACyT No. 43414-F. [1] Arenas-Alatorre J, Avalos-Borja M, Diaz G J. Phys. Chem. B \textbf{109}, 2371 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N27.00003: First principles studies of the geometric and electronic structure of nanoalloy Ag$_{27}$Cu$_{7}$. M. Alcantara Ortigoza, T.S. Rahman We present first-principles calculations of the structure and electronic density of states (DOS) for the perfect core-shell Ag$_{27}$Cu$_{7}$ nanocluster. Our results show an expansion of 0.4 A in the \textit{diameter} of the cluster compared with previous results$^{\ast }$. From the projected DOS we conclude that the 34-atom cluster has only 2 non-equivalent Cu atoms (core) and 4 non-equivalent Ag atoms (shell), confirming that this finite-size structure has D$_{5}$h symmetry. The HOMO-LUMO gap is found to be 0.77 eV, in agreement with previous results$^{\ast }$. Comparing with Ag bulk, the valence band centroid of Ag$_{27}$Cu$_{7}$ presents shift of $\sim $1.0 eV towards the Fermi energy, but a 0.5 eV shift away from it, compared with Cu bulk. The total DOS of the structure as a whole does not present valence band narrowing when compared to the bulk of either species. Individual Ag atoms show band narrowing, a positive centroid shift to lower binding energies, and a very small enhancement of the DOS at the top of the band. Electronic states of Cu atoms are greatly concentrated in two sharp peaks in the top region of their valence band. In the bottom of the band, however, copper and silver atoms hybridize in spite of their short d-wavefunctions. Charge density plots give some insight about the hybridization of electronic states between atoms. $^{ \ast }$G. Rossi et al. Phys. Rev. Lett. \textbf{93,} 105503 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N27.00004: Bond Stiffening in Small Clusters, and its Consequences Shobhana Narasimhan, Raghani Pushpa, Umesh Waghmare We have used density functional theory and density functional perturbation theory to compute the interatomic force constant tensors for small clusters of Si, Sn and Pb; these results have important implications for the size dependence of the elastic and thermal properties of nanosized objects. We find a clear sequence of relationships: as the size of the cluster is decreased, bonds get shorter and stiffer and vibrational frequencies higher; however the behaviour relative to the bulk depends on the coordination number of the latter. Though all the clusters we have studied are softer than the corresponding bulk, vibrational amplitudes may be enhanced or damped relative to the bulk values, and vary non-monotonically with size. Scaling relations connect results for varying sizes and different elements. These results also provide a framework for understanding recent results showing that, surprisingly, some clusters have melting temperatures that are much higher than that of the corresponding bulk material. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N27.00005: Structural Properties of Small Pd Clusters Jos\'e Rogan, Griselda Garc{\'i}a, Juan Alejandro Valdivia, Ricardo Ram{\'i}rez, Miguel Kiwi The properties of small Pd clusters ($2\leq N \leq 21$) are computed by means of the most common phenomenological many body potentials, and also by {\it ab initio} methods. The lowest energy configuration is found by means of an unbiased search using computational space annealing (CSA). Satisfactory agreement between the results of the several methods implemented is achieved. Of special interest is the fact that different phenomenological potentials yield the same symmetry group for the lowest energy cluster geometries. Moreover, they are in general compatible with {\it ab initio} results both of our own and other already published data. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N27.00006: Efficient method to calculate total energies of large nanoclusters Min Yu, Rampi Ramprasad, Gayanath W. Fernando, Richard M. Martin We present a computationally efficient method to calculate total energies of very large nanoclusters based on first principles electronic structure techniques. The total energy of a cluster with well-defined facets can be separated into surface, edge, and corner energies, each a function of the chemical potentials, in addition to bulk contributions. Using density functional calculations we have verified that this separation describes the total energies of fcc $Cu$ and zincblende $CdSe$ polyhedral clusters with up to $256$ atoms. The calculated energies are then used to estimate the shapes of stable structures for large polyhedral nanoclusters. For sufficiently large clusters, only the surface and bulk terms survive. This method has been shown to be applicable to stoichiometric as well as non-stoichiometric clusters, containing polar or non-polar surfaces, and we are in the process of calculating the energies of various surfaces using total energy and energy density methods in order to predict equilibrium shapes of clusters as a function of size and chemical potentials. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N27.00007: First principles study of coulombic correlation effect on lithium doped zinc oxide nanocrystals Hyunwook Kwak, Murilo L. Tiago, James R. Chelikowsky We examine the role of quantum confinement for impurities in zinc oxide nanocrystals. The electronic gap between the highest occupied level and the lowest unoccupied level for these systems will be larger than the band gap of bulk ZnO crystal owing to quantum confinement. We also expect quantum confinement to enhance correlation effects from on-site coulombic interactions, which will occur for lithium doped zinc oxide nanocrystals. We investigate the ionization energy for lithium impurities in ZnO nanocrystals and characterize the properties of the impurity levels. We assess the validity of arguments from recent experimental studies in which lithium impurities are expected to form shallow donors and acceptors in ZnO. We use a real-space ab initio pseudopotential method to obtain the ground state properties of an isolated nanocrystal. We use a rotationally invariant ``LDA+U'' scheme to model the on-site coulombic interaction of the zinc d-levels. The Hubbard U potential is rescaled for each nanocrystal using the static dielectric constant to reflect the reduced screening in a nanocrystal. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N27.00008: Excitonic effects and optical properties of passivated CdSe clusters Marie Lopez del Puerto, Murilo Tiago, James Chelikowsky We calculate the optical properties of a series of passivated non-stoichiometric CdSe clusters using two first-principles approaches: time-dependent density functional theory within the local density approximation, and many-body methods, based on computing the self-energy in the GW approximation and solving the Bethe-Salpeter equation for optical excitations. We analyze the character of optical excitations leading to the first low-energy peak in the absorption cross-section of these clusters. Within time-dependent density functional theory, we find that the lowest-energy excitation is mostly a single-level to single-level transition. In contrast, many-body methods predict a strong mixture of several different transitions, which is a signature of excitonic effects. We also find that the majority of the clusters have a series of dark transitions before the first bright transition. This may explain the long radiative lifetimes observed experimentally for these clusters. Reference: PRL 97, 096401 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N27.00009: Structure and Dynamics of Silicon Carbide Clusters: A Tight-Binding Adaptive Monte Carlo Application Anthony Patrick, Xiao Dong, Estela Blaisten-Barojas, Thomas Allison, Anwar Hasmy A tight-binding parametrization for silicon carbide nanoclusters was developed based on the electronic energy surface of small clusters calculated within the generalized gradient approximation of density functional theory. This parametrization includes s and p angular momentum symmetries and parameters for the on-site, hopping and overlap matrix elements. With the aid of these new parameters, the global minima of silicon carbide clusters in the range of 10-30 atoms were discovered with the adaptive Monte Carlo Method [1]. The ATMC optimization process is fast and drives the system across configuration space very effectively reaching the global minimum in a small number of tempering events. Growth sequence, stability patterns, and temperature behavior were also obtained. [1] X. Dong and E. Blaisten-Barojas, J. of Comp. {\&} Theor. Nanoscience, 3, 118-127 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N27.00010: First-principles studies of isomerization processes of silicon clusters Leonidas Tsetseris, George Hadjisavvas, Sokrates Pantelides Nanoclusters typically exhibit a large number of isomers, often with strikingly different structural and electronic properties. Controlled growth and use of these ultrasmall particles depends, therefore, on an understanding of the atomic-scale details of inter-isomer conversions. Here we use first-principles calculations to study the isomerization kinetics of silicon clusters. Based on the results on activation energies, we infer a classification scheme for the complex phase of isomers in domains which are delineated by bond-breaking events at the outer cluster shells. Our findings are consistent with experimental measurements and they have implications for theoretical searches of low-energy cluster structures. We also present results on hydrogenation and oxidation kinetics and we discuss their relevance for pristine and functionalized silicon clusters. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N27.00011: Investigation of the structure and properties of vacancies in Si and Ge nano-crystals by \textit{ab initio} methods Scott Beckman, James Chelikowsky The production of nano-scale devices requires the ability to selectively dope nano-structures either n-type or p-type. The functionality of such devices demands that the dopant species remains in the nano-structure, and not diffuse into neighboring regions or to surfaces. The diffusivity of impurities in a crystal depends explicitly upon the self-diffusion of the host species. Understanding this requires understanding the modes of self-diffusion, and the mobility of intrinsic defects in the host crystal. Here we investigate the structure and properties of vacancies in Si and Ge nano-crystals. Using a real space pseudopotential method we study the energy of vacancies within 2 nm diameter crystals. It is observed that vacancies are naturally pulled toward the surfaces; however, in highly symmetric crystals, it is possible to trap vacancies in the center of the crystal. Once a vacancy is within 0.4 nm of the surface a bucking effect occurs, which indicates that a surface reaction will probably act to pull the vacancy to the surface. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N27.00012: Self-purification in semiconductor nanostructures Gustavo Dalpian, James R. Chelikowsky Doping semiconductors is an important process in order to develop functional devices with them. This suggests that, when dealing with semiconductor nanostructures, they should also be doped in order to broaden their possible applications. Experimentally this shows to be a very difficult task. ``Self-purification'' mechanisms are often claimed to make this task even more difficult, as the distance a defect or impurity must move to reach the surface of a nanocrystal is very small. Kinetic effects like this are usually invoked in order to explain this difficulty. Here we show that self-purification can be explained through energetic arguments and is an intrinsic property of defects in semiconductor nanocrystals. We find the formation energies of defects increases as the size of the nanocrystal decreases. This is due to the pinning of the impurity levels as the size of the nanocrystal decreases and experimental evidences support our argumentation. We analyze the case of Mn-doped CdSe nanocrystals and compare our results to experimental findings, proposing ways to improve their dopability. [Preview Abstract] |
Session N28: Focus Session: Graphene II
Sponsoring Units: DMPChair: Zhihong Chen, IBM - Watson
Room: Colorado Convention Center 302
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N28.00001: Superconducting junctions in graphene Invited Speaker: Graphene, a single sheet of graphite, is a two-dimensional material which has been long-studied theoretically, but only recently become available to experimentalists. Recent experiments have shown that the electronic properties of graphene are even more remarkable than previously thought. In my talk I will describe the fabrication and characterization of graphene devices, and introduce their basic electronic properties. I will then focus on our recent experiments where we study induced superconductivity in graphene, an observation which elucidates on the quantum coherent properties of electrons in this novel two-dimensional electron gas. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N28.00002: Mesoscopic Electron Transport in Nanostructured Graphene Barbaros Oezyilmaz, D. Efetov, K. Bolotin, M. Y. Han, P. Jarillo-Herero, P. Kim We present experimental results on low energy electric transport studies in mesoscopic graphene quantum devices. Graphene sheets have been fabricated by means of micromechanical exfoliation. Subsequently we define mesoscopic Aharonov-Bohm (AB) rings. The electron interference in such ring shaped graphene ribbons is controlled using a perpendicular magnetic field. We will discuss magnetoresistance oscillations obtained on AB rings with ring width of $\sim $ 50 nm and ring diameters ranging from 300 nm to 3000 nm as a function of both temperature and carrier density. In addition, we present our efforts on locally controlling the carrier density in graphene sheets. The latter are patterned into ribbons of $\sim $ 100nm width and contacted in a first step with source and drain electrodes. In a second step multiple lithographically-patterned electrostatic local top gates are aligned to each device. We will discuss transport measurements as a function of local gate voltages. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N28.00003: Observation of Proximity Effect and Multiple Andreev Reflections in Graphene/Superconductor Junctions Xu Du, Ivan Skachko, Eva Y. Andrei Graphene, a single atomic layer of graphite, has attracted much interest recently both for its unique physical properties and for its potential in electronics applications. Due to the combined effects of a linear energy-momentum dispersion and internal degrees of freedom (pseudo-spin) associated with the honeycomb lattice, the low energy excitations in graphene are expected to behave like massless relativistic fermions. This leads to many novel and unusual physical properties. We will present experimental studies on a gate controlled superconductor/graphene hybrid device. Electric field dependent superconducting proximity effect and multiple Andreev reflections will be discussed. Results obtained for junctions fabricated on graphene and on multi-layer graphite films will be compared. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N28.00004: Quantum Transport in Single and Bi-Layer Graphene Coupled to Superconducting Electrodes F. Miao, S. Wijeratne, U. Coskun, Y. Zhang, C. N. Lau Graphene, the two dimensional honeycomb lattice of carbon atoms, has attracted significant attention in recent years, due to its unique electrical properties. Here we present experimental studies of single and bi-layer graphenes coupled to superconducting electrodes. At low temperatures the devices display signatures of ballistic electrical transport, and the minimum conductivity varies between 6.5 and 20k$\Omega $. When the electrodes become superconducting, we observe gate-tunable low-bias conductance peaks, which are attributed to multiple Andreev reflections. Latest experimental results will be discussed in terms of various theoretical models. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N28.00005: Low Field Electronic Transport Properties and Scattering Mechanisms of Graphene Y.-W. Tan, Y. Zhang, M. Han, J. A. Jaszczak, P. Kim, H. L. Stormer We report low magnetic field transport properties of fourteen graphene devices with a wide spread of mobilities. The minimum conductivity at the Dirac point lies in the range $2 - 12e^ {2}/h$ with an average of $\sim 8e^2/h$. When comparing the conductivity versus density (n) curves with theoretical models, we find that high mobility samples show the features characteristic of short range scattering and low mobility samples show the features characteristic of long range scatterering. Samples exhibiting different scattering mechanisms also show different weak localization behaviors. Samples having short range disorders show total suppression of weak localization (WL), whereas samples with long range scatterering show the conventional WL peak with a reduction in amplitude. Devices in between these two limits have a sharp, narrow WL peak. Under moderate disorder scattering, we find inelastic electron-electron scattering to be the major cause of phase decoherence, and the phase coherence length has a n$^{1/4}$ dependence. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N28.00006: Controlling the Electronic Structure of Bilayer Graphene Taisuke Ohta, Aaron Bostwick, Jessica McChesney, Thomas Seyller, Karsten Horn, Eli Rotenberg Carbon-based materials such as carbon nanotubes, graphite intercalation compounds, fullerenes, and ultrathin graphite films exhibit many exotic phenomena such as superconductivity and an anomalous quantum Hall effect. These findings have caused renewed interest in the electronic structure of ultrathin layers of graphene: a single honeycomb carbon layer that is the building block for these materials. There is a strong motivation to incorporate graphene multilayers into atomic-scale devices, spurred on by rapid progress in their fabrication and manipulation. We have synthesized bilayer graphene thin films deposited on insulating silicon carbide and characterized their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands [1]. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic scale electronic devices. [1] T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science, 313, 951 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N28.00007: Superconductivity in metal coated graphene Bruno Uchoa, Antonio Castro Neto Graphene, a single atomic layer of graphite, is a two dimensional (2D) zero gap insulator with a high electronic mobility between nearest neighbor carbon sites. The unique electronic properties of graphene, from the semi-metallic behavior to the observation of an anomalous quantum Hall effect and a zero field quantized minimum of conductivity derive from the relativistic nature of its quasiparticles. By doping graphene, it behaves in several aspects as a conventional Fermi liquid, where electrons may form Cooper pairs by coupling with a bosonic mode. In this talk, we develop a mean-field phenomenology of superconductivity in a honeycomb lattice. We predict the possibility of two distinct phases, a singlet s-wave phase and a novel p+ip wave phase in the singlet channel. At half filling, the p+ip phase is gapless and superconductivity is a hidden order. We propose a few possible sources of Cooper pairing instability in graphene coated with alkaline and transition metals, and similar low dimensional graphene based devices. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N28.00008: Tunneling and Josephson coupling studies of n-layer graphene Conor Puls, Neal Staley, Haohua Wang, Jeremy Forster, Kelly McCarthy, Ben Clouser, Ying Liu We investigate planar tunnel and superconductor-graphene-superconductor (SGS) junctions involving n-layer graphene. We fabricate our devices using an ultrathin quartz filament as a shadow mask over mechanically exfoliated graphene as an alternative to lithographic procedures so as to avoid possible contamination in a wet lithography process. Our tunnel junctions use Al$_2$O$_3$ as the tunnel barrier and Pb or Au as the counter-electrode. We observed a reduction of density of states in the n-layer graphene and the superconducting energy gap of Pb when Pb was an electrode. Results from work on SGS junctions and other atomically thin materials such as NbSe$_2$ will also be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N28.00009: RPA Theory of Carrier Correlations in Graphene Yafis Barlas, Tami Pereg-Barnea, Marco Polini, Allan MacDonald We have applied RPA theory to examine some consequences of electronic correlations in doped and undoped graphene. The full wavevector and complex-frequency dependent polarization bubble was evaluated using dimensional regularization in the absence of doping and adding the appropriate carrier-scattering and Pauli-blocking corrections in doped systems. We have evaluated the RPA correlation energy as a function of charge and spin density by integrating the dynamically screened Coulomb interaction along the imaginary axis and from this have extracted the compressibility, and the spin and valley susceptibilties. We have also evaluated the frequency and wavevector dependent self-energy and used this to extract the doping dependence of velocity renormalization and the quasiparticle spectral weight at the Fermi energy. The accuracy of RPA theory applied to graphene will be critically discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N28.00010: Localization and Polarization in Graphene Systems with Edges Jason Hill, Hongki Min, Tami Pereg-Barnea, Nikolai Sinitsyn, Allan MacDonald The properties of localized states (especially states localized at the edge of ribbons) will be presented for various graphene systems. The orientation dependence of the properties will be discussed. Methods for devising appropriate boundary conditions for Dirac ribbons will be reviewed. Localization at zero field due to finite size effects, applied magnetic fields, and spin-orbit coupling will be discussed. Tendencies toward true spin and pseudospin polarizations in graphene ribbons will also be examined. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N28.00011: Impact of substrate-graphene interaction on transport properties of graphene Jianhao Chen, Masa Ishigami, Elba-Gomar Nadal, Ellen Williams The silicon oxide substrate has nanoscale corrugations and charge traps, which influence the electronic propreties of graphene. We modify the substrate-graphene interaction by functionalizing the oxide. We are able to modify the chemical adhesive force as demonstrated by changes in the yield of graphene with different self-assembled monolayers prior to mechanical cleavage of graphite. We will discuss the impact of oxide functionalization on the transport properties of graphene. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N28.00012: Electrostatic Driving of Graphite Resonators Arend van der Zande, Scott Bunch, Scott Verbridge, Ian Frank, David Tanenbaum, Jeevak Parpia, Harold Craighead, Paul McEuen We fabricate nanoelectromechanical graphite resonators and drive the resonators electrostatically. Graphite sheets are suspended over trenches in SiO$_{2}$ and contacted to electrodes. Mechanical vibrations of the graphite sheets are actuated by applying a radio frequency voltage relative to a doped silicon back-gate, and the resonance frequency is tuned by varying an additional DC gate voltage. Mechanical vibrations are detected optically by laser interferometry. We detect the thermal motion of the resonators, and use the equipartition theorem to calibrate the amplitude of motion. For example, a 5nm graphite sheet at room temperature has thermal motion on resonance of 200 fm/ Hz$^{1/2}$, and shows a driven linear response in displacement up to 6 nm, comparable to the thickness of the resonator. The unusually small mass, electrically active material and reasonable dynamic range indicate that graphite resonators would make excellent force and charge sensors. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N28.00013: Absence of Wigner Crystallization in Graphene Hari Dahal, Yogesh Joglekar, Kevin Bedell, Alexander Balatsky We study the possibility of Wigner crystal phase as a ground state in graphene. The Wigner crystal phase results when the ratio of potential to kinetic energy is much higher than one, and generally, the particle density serves as a tuning parameter. Our calculation shows that potential and kinetic energy have the same density dependence resulting in a density independent ratio of energies. Moreover, kinetic energy is higher than potential energy; which rules out the possibility of Wigner crystallization. The physical reason of this observation can be traced back to the linear dispersion of carriers of Graphene, the Dirac ferminos. cond-mat/0609440; Physical Review B (to be published) [Preview Abstract] |
Session N29: Focus Session: Carbon Nanotubes: Synthesis
Sponsoring Units: DMPChair: John Cumings, University of Maryland
Room: Colorado Convention Center 303
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N29.00001: HRTEM studies of various carbon nanotubes Invited Speaker: Among various synthesis methods of carbon nanotubes (CNT), catalytic CVD method has gained its majority. There are several reasons for the popularity of CCVD method, which is the simplicity of the instrumental setup, ease of varying the CNT growth condition, and relatively simple control over the number of layers. We have been working with the CCVD method for producing high quality nanotubes, and by finding the right synthesis condition, succeeded in obtaining highly homogeneous double-wall carbon nanotubes (DWCNT). In the heat-treated DWCNT sample, we were able to find various coalesced tubes with interesting morphologies. In the present talk, the process of producing high quality DWCNT and its morphological changes by various treatments will be shown. In collaboration with Hiroyuki Muramatsu, Yoong Ahm Kim, and Morinobu Endo, Shinshu University and the Endo Lab Team. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N29.00002: Growth Characteristics of Long, Gas Flow Aligned Carbon Nanotubes (CNT) by Chemical Vapor Deposition (CVD) Alfonso Reina, Mario Hofmann, David Zhu, Jing Kong An investigation is undertaken to elucidate differences in the growth of long, gas flow aligned nanotubes and short, randomly oriented nanotubes. The synthesis is carried under ambient pressure ethanol and methane CVD from iron-based nanoparticles on SiO$_2$ substrates. The two CNT morphologies are controlled by tuning chemical parameters such as catalyst pre-treatment. The growth of long, aligned CNTs presents two main differences over the growth of short, randomly aligned CNTs. The growth lifetime is significantly higher (15 minutes vs. 10 seconds) and alignment with the gas flow occurs only when a nanotube achieves a length of 40 $\mu$m, suggesting that short growth lifetimes limit not only the length of CNTs but also their alignment with the gas flow. Finally, a series of studies were done to explore possible mechanisms of CNT growth death. Differences in the nanoparticles catalytic lifetime and/or CNT growth rate are the most probable factors determining alignment. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N29.00003: Distribution and stability of Carbon in Fe-C nanoparticles. Neha Awasthi, Aiqin Jiang, Aleksey Kolmogorov, Wahyu Setyawan, Kim Bolton, Stefano Curtarolo Catalytic Chemical Vapor deposition (CVD) method is widely used to produce carbon nanotubes. To improve our understanding of the CVD growth mechanism, we focus on the thermodynamics and the phase stability of catalyst Fe-C nanoparticles. Using \textit{ab initio} methods and classical molecular dynamics simulations, we investigate 1) the diffusion and solubility of carbon atoms in nanoparticles by calculating the distribution of carbon atoms inside the clusters, 2) the formation and stability of carbides at nanoscale, and 3) the effect of substrates on such structures. We address the implications of these results on NT growth, and give possible strategies to mitigate the problems. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N29.00004: Challenges for Growth of Smallest Diameter Single-Walled Carbon Nanotubes by Catalytic Method Oleg Kuznetsov, Elena Mora, Toshio Tokune, Stefano Curtarolo, Kim Bolton, Avetik Harutyunyan We investigate the viability of formation of very small diameter ($<$ 0.5nm) freestanding SWNTs by CVD based on concept of carbon diffusion through the catalyst particle, originated from the vapor-liquid-solid growth mechanism. We found that the decrease of particle size required for nucleation of small diameter tubes results in a significant increase of catalytic decomposition temperature of hydrocarbon and, accordingly, the temperature required for nucleation and growth of nanotubes. However, high temperature increases the mobility of particles and endorses their agglomeration with formation of bigger particles, as well as leads to deactivation of catalyst by formation of intermetallic compounds with support material. The results of Raman spectroscopy, (n,m) assignments of the grown tubes and TEM studies for the smallest diameter tubes are presented. Performed \textit{ab-inito} and molecular dynamics simulations qualitatively explain the experimental finding based on size dependent carbon solubility of catalyst, by analyzing supported nanocatalyst-carbon binary phase diagram. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N29.00005: Growth of carbon nanotubes by the pyrolysis of thiophene Gaohui Du, Wenzhi Li Branched carbon nanotubes have been reported and produced by the pyrolysis of metallocene-thiophence mixture. In our experiments, we prepared the carbon nanotubes (CNTs) by the pyrolysis of thiophene as the carbon source over cobalt catalysts. The lengths of carbon nanotubes can reach 0.5-1 mm for the growth time of 15 min. The effects of flow rate and temperature on the growth of CNTs have been investigated. The branched carbon nanotubes were also found in the experiments, showing Y-junction or T-junction, even connecting each other to form a web. The growth mechanism of the branched CNTs was studied using transmission electron microscopy. The electron transportation properties along these branched CNTs are under investigation. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N29.00006: Initial Stage of Growth of Single-Wall Carbon Nanotubes: Modeling and Simulations I. Chaudhuri, M. Yu, C. S. Jayanthi, S. Y. Wu Through a careful modeling of interactions, collisions, and the catalytic behavior, one can obtain important information about the initial stage of growth of single-wall carbon nanotubes (SWCNTs), where a state-of-the-art semi-empirical Hamiltonian [Phys. Rev. B,\textbf{ 74}, 155408 (2006)] is used to model the interaction between carbon atoms. The metal catalyst forming a supersaturated metal-alloy droplet is represented by a jellium, and the effect of collisions between the carbon atoms and the catalyst is captured by charge transfers between the jellium and the carbon. Starting from carbon clusters in different initial configurations ($e.g$., random structures, cage structures, bulk-cut spherical clusters, \textit{etc}.), we anneal them to different temperatures. These simulations are performed with clusters placed in the jellium as well as in vacuum. We find that, in the presence of jellium, and for an optimal charge transfer of $\sim \quad 0.2 e,$ open cage structures (and some elongated cage structures) are formed, which may be viewed as precursors to the growth of SWCNTs. We will also discuss the implications of a spherical boundary on the nucleation of a SWCNT. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N29.00007: Universal Template Technique for Patterned Growth of Carbon Nanotubes Ying Chen, Hua Chen, Jun Yu, Bill Li, Vince Creig, James Williams A new template technique has been developed to help patterned growth of carbon nanotubes on Si surface without predeposition of metal catalysts. Focused ion beam (FIB) milling system was used to create nanosized patterns on Si wafer surface as the template. Under a controlled pyrolysis of iron phthalocyanine at 1000 $^{o}$C, carbon nanotubes only nucleate and grow in the template. The selective growth is due to the special surface morphology and crystalline structure created by FIB. This template technique can be used to help patterned growth of other nanotubes and nanowires on any substrates. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N29.00008: Synthesis of Narrow Chirality Distributions of Single-Walled Carbon Nanotubes using Catalyst Particle Templates Produced by Nanosphere Lithography Noureddine Tayebi, Joseph Lyding We report a simple and inexpensive technique based on nanosphere lithography [1], which allows for the fabrication of periodically-spaced and monodispersed metal particles from which the chemical-vapor-deposition synthesis of single-walled carbon nanotubes (SWNTs) is achieved. We have controlled the diameter of these metal particles, and thus that of the SWNTs, from 1 nm down to 0.7 nm, with an interparticle spacing varying from 50 nm down to 5 nm. Raman spectroscopy analysis reveals that a narrow chirality distribution is achieved. We are currently confirming the chirality results using fluorescence spectroscopy and scanning tunneling microscopy. Transmission electron microscopy analysis reveals that the 0.7 nm particles are crystallographically identical, which could be the origin of such a narrow distribution. Furthermore, the current technique was used to grow aligned SWNTs on single-crystal quartz substrates [2]. [1] J. C. Hulteen et al., J Vac Sci Technol A, 13, 1553 (1995) [2] C. Kocabas et al., J Am Chem Soc, 128, 4540 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N29.00009: Synthesis and characterization of dense, vertically-aligned carbon nanotube forests from 10nm colloidal iron oxide nanoparticles David Hutchison, Brendan Turner, Richard Vanfleet, Robert Davis, Brian Woodfield, Juliana Boerio-Goates We report growth of vertically-aligned carbon nanotubes (VACNTs) on alumina using 10nm iron oxide nanoparticles dried from a colloid. VACNTs were grown by chemical vapor deposition using ethylene, hydrogen and argon, and found to be dense forests with height, number of walls, and density dependent on the catalyst concentration. Comparison between VACNTs produced from nanoparticles and those from more traditional sputtered or evaporated iron films will be made. The forests have been characterized by Raman, TEM, and SEM, and the iron catalyst particles by AFM and TEM. Growth directly from pre-prepared nanoparticles of uniform size offers insight into how the catalyst particles seed carbon nanotube growth and is easier to prepare and faster than iron film deposition by sputtering. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N29.00010: Synthesis and Structure of Carbon Nanotube Y-junctions Bimal Pandey, Wenzhi Li The effect of catalyst and carbon source on the synthesis and structure of carbon nanotube Y-junctions (CNTYs) using chemical vapor deposition has been investigated. Three different nitrates, including cobalt nitrate, calcium nitrate, and magnesium nitrate, are used as catalyst precursors and thiophene (C$_{4}$H$_{4}$S) is used as carbon source to synthesize CNTYs. CNTYs with straight branches are synthesized by using mixture of cobalt/magnesium nitrates or cobalt/calcium nitrates while individual cobalt nitrate, magnesium nitrate, calcium nitrate, or mixture of magnesium/calcium nitrates doesn't grow any CNTYs, indicating that cobalt/magnesium or cobalt/calcium facilitates the formation of CNTYs. Experimental result shows that the diameter and yield of CNTYs are affected by the ratio of cobalt/magnesium or cobalt/calcium nitrates. In addition, carbon sources such as methane (CH$_{4})$ and acetylene (C$_{2}$H$_{2})$ have also been used as carbon source to grow CNTYs. It is found that linear nanotubes rather than CNTYs can be formed. The result shows both the catalyst and the carbon source affect the formation of CNTYs. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N29.00011: Melting and Premelting of Carbon Nanotubes Kaiwang Zhang, G. Malcolm Stocks, Jianxin Zhong We report results of molecular dynamics simulations of melting and premelting of single-walled carbon nanotubes (SWNTs). We found that the traditional critical Lindemann parameter for melting of bulk crystals is not valid for SWNTs. Using the much smaller critical Lindemann parameter developed for melting of nanoparticles as a criterion, we show that the melting temperature of perfect SWNTs is 4800K. We further show that Stone-Wales defects in a SWNT significantly reduce the melting temperature of atoms close the defects, resulting in premelting of SWNTs around the defects at 2600K. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N29.00012: Direct growth of carbon nanotubes on BaTiO$_3$ thin films for ferroelectric field effect devices Patrycja Paruch, Agham-Bayam Posadas, Charles H. Ahn, Paul L. McEuen Carbon nanotube field effect transistors have been extensively investigated using a variety of gate dielectrics. We propose instead to use {\it ferroelectric} (FE) field effect, replacing the dielectric by a thin FE film on a conducting substrate. The remanent FE polarization can provide reversible, locally-controlled, and non-volatile electronic doping of up to $\sim$ 5 x 10$^{14}$ charges/cm$^2$, over 10 times greater than that available with SiO$_2$ at breakdown fields. However, many FE materials cannot withstand the high temperatures and reducing atmosphere required for CNT growth. We subjected different perovskite FEs to CNT growth conditions, and from subsequent local and macroscopic measurements of their polarization we have identified BaTiO$_3$ as a good device material. Single walled CNTs grown on BaTiO$_3$ were characterized using the Nb:SrTiO$_3$ substrate as a gate electrode. The effects of FE polarization on the CNT electronic properties are currently being studied. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N29.00013: Replicating carbon nanotubes with molybdenum chalcogenide nanowires Teng Yang, David Tomanek, Igor Popov, Gotthard Seifert In an attempt to design chemically stable and easily separable one-dimensional conductors, we performed {\em ab initio} Density Functional calculations for Mo$_6$S$_{6-x}$I$_x$ nanowires with a varying concentration of iodine. Such Chevrel like systems have been synthesized before, but had necessitated alkali counter-ions for stabilization. The backbone of our nanowires consists of Mo$_6$ octahedra structures, covered by I and S atoms. We find the stoichiometry with $x=2$ to be preferred on energy grounds. Our results suggest these nanowires to be not only structurally rigid, but also to be rather easily separable. The electronic structure of these nanowires strongly resembles that of semi-metallic carbon nanotubes, with two crossing bands giving rise to a constant density of states, flanked by a pair of van Hove singularities near the Fermi level. Since the semi-metallic nature of these nanowires is robust, these systems may offer a viable alternative to carbon nanotubes, where conductivity strongly depends on chirality. [Preview Abstract] |
Session N30: Liquid Crystals: Experiments
Sponsoring Units: DFDChair: Noel Clark, University of Colorado at Boulder
Room: Colorado Convention Center 304
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N30.00001: Alignment of Liquid Crystals by Nano- and Micro -Scale Topographic Patterns Made by Nanoimprint Lithography Youngwoo Yi, Michi Nakata, Alexander Martin, Noel Clark, Vaibhav Khire, Christopher Bowman Topographic patterns prepared using nanoimprint lithography have great potential in the alignment of liquid crystals (LCs) because their preparation is a parallel and low cost process. Nano- and micro- scale topographic patterns are made by stamping a mold on liquid material, which is then cured by UV light illumination. Such topographically patterned plates are used as one of the window of LC cells. Depolarized transmission light microscopy shows that nematic liquid crystals are aligned uniformly along the lines on the linear patterns and aligned either homeotropically or with bistable planar states in patterns of squares, depending on their scale. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N30.00002: Absence of uniform nematic phase for thin 8CB films Ergys Subashi, Rafael Garcia Certain thermotropic liquid crystal films exhibit a strange phenomenon in which two different thicknesses coexist side-by-side on solid surfaces. For 5CB these two film thicknesses appear to correspond to two different phases: nematic and isotropic. The coexistence persists for a temperature range which depends on the initial thickness of the film. A similar phenomenon is present in films that have a smecticA phase such as 8CB. For these films just below the nematic to isotropic transition temperature, we observe a coexistence region very similar to that observed previously for 5CB. We also report new experiments which show that for 8CB films on silicon, near the smecticA to nematic transition temperature, there is a coexistence region that is strikingly different from the one observed near the nematic to isotropic transition. Furthermore, there is a new phenomenon for 8CB films thinner than a certain critical thickness d*: as the temperature increases or decreases the film goes from smecticA to isotropic and back with no intermediate uniform nematic phase. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N30.00003: Electrically controlled negative refraction in a uniaxial nematic liquid crystal Oleg D. Lavrentovich, Oleg P. Pishnyak We demonstrate the phenomenon of electrically controlled negative refraction at the interface between an isotropic material and a uniaxial nematic liquid crystal, in which the optic axis makes a large angle (40-60 degrees) with the interface and is switched by a modest (few volts) electric field. Depending on the applied voltage, the refracted beam is either on the opposite side of the interface normal as compared to the incident beam (positive refraction) or on the same side (negative refraction) . [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N30.00004: Effect of the Functionalization Compound on the Magnetic Nanoparticle -- Smectic-A Liquid Crystal Interaction: A Phenomenological Model Luz J. Martinez-Miranda, Lynn K. Kurihara We present the details of a phenomenological model that explains how the functionalization or surface compound affects the way that a magnetic nanoparticle interacts with a liquid crystal molecule. This model considers the surface interaction between the functionalization compound and the liquid crystal, and the relative size of the liquid crystal compared to the size of the nanoparticle. These two properties can aid or hinder in the effects of the nanoparticle on the orientation of the liquid crystal, specifically on the magnetic field effects of the nanoparticle in the reorientation of the liquid crystal. Comparisons with experimental data will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N30.00005: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N30.00006: Broadening the Smectic C* Sub-Phases by Chiral and Achiral Doping J. Kirchhoff, L.S. Hirst Materials exhibiting the liquid crystal smectic (Sm) C* sub-phases have been a topic of great interest for several years and recent work has shown that by mixing chiral and achiral dopants with these materials it is possible to broaden the SmC* subphases significantly, in particular, the intermediate phases (ferrielectric) which typically have a very narrow temp. range [1]. Mixtures of smectics with chiral or achiral dopants were studied to investigate phase stability in the SmC* sub-phases. As dopant conc. is increased, both phase width and the temp. range over which the transition occurs is broadened. These effects have been measured via calorimetry (DSC), and optical microscopy. Electro-optical characteristics of mixtures as a function of dopant conc. were also studied to investigate the effects of phase and transition broadening on material properties as it is not yet clear what role the addition of chiral dopants will play. By controlling phase widths, we hope to expand the possibilities, through carefully formulated mixtures, of generating commercially interesting materials in these phases. \newline \newline [1] S. Jaradat et al. J. MATER. CHEM. \textbf{16}, 3753, (2006) [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N30.00007: Use of the chiral smectic A liquid crystal electroclinic effect for sensitive measurement of enantiomeric excess Duong Nguyen, Lior Eshdat, Arthur Klittnick, Renfan Shao, David Walba, Noel Clark We present here a procedure which is capable of detecting an enantiomeric excess (ee) as low as $10^{-4}$\% using the electroclinic effect. The electroclinic effect is a field induced effect on the optic axis of the SmA phase of chiral molecules in which the tilt angle $\theta$ is linear with the electric field E. Thus, varying the voltage across a cell with planar alignment of a sample of unknown ee in SmA phase induces varying orientation of the director. This in turn induces varying intensity of a laser beam passing through the sample. The signal from the sensor detecting this beam is fed to a lock-in amplifier for low-noise measurement of the beam's intensity variation, from which we can extract even a small ee of the sample. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N30.00008: Topological Defect Structure and Annihilation in High-Polarization Freely Suspended Films Chenhui Zhu, Apichart Pattanaporkratana, Joseph Maclennan, Noel Clark The texture of freely suspended liquid crystal SmC* films of a high polarization material C7 [4-(3-methyl-2-chloropentanoyloxy)-4'-heptyloxybiphenyl] is studied using polarized light microscopy. In particular, we focus on c-director defects with topological strength --1 found in the chessboard texture. Due to the competition between the elastic energy and the electrostatic energy of polarization splay, the c-director field near the defect core consists of four domains with homogenous orientation of the c-director inside each domain. The boundaries between domains are sharp and the c-director orientation jumps by 90 degrees at each boundary. We will present experimental and theoretical studies of the structure of these polarization-stabilized discontinuities. We will also present studies of the annihilation dynamics of +1 and --1 pairs of defects on this high-P material film and compare them to those on low-P material films. [1] E. Demikhov, Europhys. Lett. \textbf{25} (4), 259 (1994). [2] E. Demikhov and H. Stegemeyer, Liq. Cry. \textbf{18}, 37 (1995). [3] Ch. Bahr and G. Heppke, Phys. Chem. \textbf{91}, 925 (1987). [4] D. R. Link, N. Chattham, J.E. Maclennan, and N.A. Clark, Phys. Rev. E \textbf{71}, 021704 (2005). This work is supported by NSF MRSEC Grant DMR0213918. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N30.00009: Interactions between Structurally Chiral Islands on Freely-Suspended Smectic C films Apichart Pattanaporkratana, Cheol Park , Joseph Maclennan , Noel Clark Islands on a freely-suspended Smectic C film, circular regions of greater thickness than the surrounding film area, have strong tangential boundary conditions of the c-director (the projection of the molecular long axis onto the plane of the film) at their edges. These islands form dipolar structures, with an s = +1 topological defect inside and an s = -1 defect nearby on the background film. Unlike in 2D nematics, the c- director field on Smectic C islands does not have reflection symmetry, and we see both left and right-handed islands on the film. Islands with the same handedness form chain-like structures with topological dipoles pointing in the same direction and along the chain (these have been reported in the literature). Here we describe the interaction between left and right-handed islands, where the topological dipoles point in opposite directions and form a quadrupolar structure. The two - 1 defects are half way between the islands and offset from the line joining them. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N30.00010: A New Bistable SmA Display Mode Hui-Yu Chen, Renfan Shao, Eva Korblova, David Walba, Wei Lee, Noel A. Clark In the traditional SmA display, crossed polarizers are absent and one can switch a light transparent state to an opaque light scattering state by using laser addressing or electric addressing. Such displays are bright, but of only moderate contrast ratio. Here, we present a new operation mode for a SmA display using two sets of electrodes, with one to induce homeotropic orientation and the other having an in-plane structure to induce planar orientation. This switching with crossed polarizer and analyzer enables a high contrast, bistable electro-optical effect. This SmA display mode exhibits a high contrast ratio (2500:1) for non-striped ITO pattern cells, prefect bistability, and reasonably fast switching (a few ms). These characteristics may enable potential applications on e-paper. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N30.00011: The frequency-dependent electrooptic response of the electroclinic effect in deVries SmA materials Christopher D. Jones, Ute Dawin, Frank Giesselmann, Noel Clark, Per Rudquist It is well established that electroclinic switching in standard SmA* materials relates to a reorientation of the molecules in a plane normal to the layers, and thus there is no corresponding change in birefringence due to reorientation about a cone, as is the case in the SmC* phase. When the electrooptic response is then analyzed via lock-in amplifier, the signal at the driving frequency is strong, while the second harmonic response, is non-existent [1]. Using this method we have investigated deVries materials W530 and TSiKN65, and show that there is a frequency-dependent second order response -- implying an electroclinic switching that corresponds to a change in birefringence, suggesting a reorientation of the molecule about a cone. We will present our findings and a model for the type of electroclinic switching that occurs in these two materials. Work supported by NSF MRSEC Grant DMR-0213918 and The Swedish Foundation for Strategic Research 2002/0388. [1] W. Kuczynski, et. al., \textit{Ferroelectics}, \underline {244}, [491]/191, (2000) [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N30.00012: Effect of Polarization on the Interactions of SmC* Islands on Freely-Suspended Films Cheol Park , Apichart Pattanaporkratana, Joseph Maclennan , Noel Clark We generate islands, circular regions of greater thickness than the surrounding film area, on freely-suspended SmC* films by applying air jets to the film. The c-director field (the projection of the molecular long axis onto the plane of the film) is strongly tangential at the edges of the islands, leading to the formation of s = +1 topological defects inside the islands and s = -1 defects nearby on the background film. Islands interact via these topological defects, with a short- range repulsion and a long-range dipolar attraction governing their stability and leading to their organization in chain-like structures with an equilibrium island separation. We use optical tweezers to measure the force between a pair of islands as a function of their separation. As we vary the enantiomeric excess and hence the polarization of the liquid crystal, the force scales as the polarization squared and the shape of the force curve changes. Simulations based on a simple model of film elasticity and polarization explain our experimental results fairly well. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N30.00013: Combination of separate smectic-C-alpha phases in binary mixtures Zengqiang Liu, Bradley McCoy, Suntao Wang, Ron Pindak, Wolfgang Caliebe, Philippe Barois, Paulo Fernanders, H.T. Nguyen, C.S. Hsu, C.C. Huang Applying resonant x-ray diffraction, we showed that the two previously considered separate smectic-$C^*_{\alpha}$ phases were combined into a single phase in a series of binary mixtures. The pitch evolved continuously and smoothly, contradicting a previously successful theoretical model. New phase sequences found in our studies can be explained by a newer model. The comparison indicates long-range interactions exist in the smectics. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N30.00014: Evidence of Broken Reciprocity in Cholesteric Liquid Crystals Nithya Venkataraman, Michele Moreira, Bahman Taheri, Peter Palffy-Muhoray Reciprocity of scattering of a plane incident wave is predicated on bounded scattering media with symmetric and linear permittivity, conductivity and permeability. In chiral media, such as cholesteric liquid crystals, the dielectric tensor is asymmetric due the presence of odd powers of the wave vector resulting from nonlocality and broken inversion symmetry. Evidence of non-reciprocity has been found in optically active crystals by Bennet [1] and in stacks of cholesteric and nematic liquid crystal cells [2]. Here we present transmittance and reflectance data for cholesteric cells with different pitches having overlapping but distinct reflection bands. We relate our results to simple analytic descriptions of the materials properties and of propagating modes and assess them in light of the requirements for reciprocity. 1. P.J. Bennett, S. Dhanjal, Yu. P. Svirko and N. I. Zheludev, \textit{Opt. Lett}. \textbf{21}, 1955 (1996) 2. J. Hwang; M.H. Song; B. Park; S. Nishimura; T. Toyooka; J.W. Wu; Y. Takanishi; K. Ishikawa; H. Takezoe, \textit{Nat. Mat.} \textbf{4}, 383 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N30.00015: Lasing Thresholds of Obliquely Pumped Cholesteric Liquid Crystal Lasers Michele Moreira, Bahman Taheri, Peter Palffy-Muhoray, Vladimir Belyakov Cholesteric liquid crystals (CLCs) in the helical cholesteric phase are 1D photonic band gap materials. Mirrorless lasing has been achieved in dye doped CLC systems. An area of great interest is the reduction of the lasing threshold for optically pumped CLCs. The effects of sample thickness and dye concentrations have been investigated and optimized for CLC systems [1]. However, the pump beam geometry has remained unchanged in most experiments. Numerical calculations of the DFB lasing threshold as function of the angle of incidence have been recently carried out for helical cholesterics [2]. The results predict that the lasing threshold is reduced at oblique incidence for critical angles, and the minimum threshold occurs at the first critical angle closest to the band edge. We have carried out measurements of the lasing thresholds of a variety of cholesteric samples as function of the angle of incidence. The samples were pumped by both ns and ps sources. We present our experimental results, and compare these with predictions of theory. [1]. W. Cao et al, \textit{Mol. Cryst and Liq. Cryst.}, \textbf{429}, 101 (2005). [2] V.A. Belyakov, \textit{Mol. Cryst and Liq. Cryst.}, \textbf{453}, 43 (2006). [Preview Abstract] |
Session N31: Focus Session: Carbon Nanotubes: Sensors and Adsorption
Sponsoring Units: DMPChair: Masa Ishigami, University of Maryland
Room: Colorado Convention Center 401
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N31.00001: Raman Spectroscopy in Single-Wall and Double-Wall Carbon Nanotube Systems Doped with H$_2$SO$_4$ Eduardo Barros, Antonio Souza Filho, Yoong-Ahm Kim, Hiroyuki Muramatsu, Takuya Hayashi, Riichiro Saito, Moribonu Endo, Mildred Dresselhaus In this work we performed Raman experiments on a mixture of Single-wall and Double-wall carbon nanotubes for different relative concentrations and using different laser energies. Two sets of samples were analyized, one which was exposed to $H_2SO_4$ for 5$~s$ and one which is pristine. The $H_2SO_4$ is known to act as an acceptor for the electrons of graphitic materials. The effect of hole doping on the vibrational and electronic properties of the double and single-wall carbon nanotubes is probed using Resonant Raman scattering with different excitation energies probing different nanotubes. For selected excitation energies, it is possible to probe, at the same time, the inner and outer walls of double-wall nanotubes. The lineshape of the G$'$ band double-wall nanotubes is discussed in terms of the interlayer interaction and the effect of single-wall nanotube contaminants and the H$_2$SO$_4$ doping to the G$'$ band is studied. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N31.00002: Evidence of a possible phase transition in ethane adsorbed on purified HiPco Nanotubes Murat Bulut, Dinesh Rawat, Aldo Migone We conducted adsorption measurements for ethane on purified HiPco single-walled carbon nanotubes for coverages in the first layer. In order to obtain the binding energy of ethane, we measured three low-coverage isotherms for temperatures between 220 K and 240 K. The value that we determined, 308 meV, is 1.7 times larger than that obtained for the binding energy of ethane on planar graphite. We measured eight full isotherms between 103 K and 170 K. Evidence of a phase transition in the adsorbed films was investigated by studying temperature dependence of the height of the isotherm substep corresponding to the adsorption of ethane molecules on the external surface of the SWNTs. There is a significant difference in the size of the substeps measured below and above 110 K. This difference suggests that a possible structural phase transition is occurring in the adsorbed film. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N31.00003: First Principles Study of Metal Adatom Adsorption on Carbon Nanotubes Kevin T. Chan, Jeffrey B. Neaton, Marvin L. Cohen Recent experiments observed bias-induced mass transport between indium nanoparticles on a carbon nanotube. Ab initio studies later suggested that electromigration in this case can be explained by charge transfer from indium adatoms to the nanotube and small diffusion barriers, and that defects can serve as nucleation sites for nanoparticle formation. Here we use ab initio calculations to explore adhesion, diffusion, and the possibility of mass transport on nanotube surfaces for other metallic species. We will present calculations examining binding energy, binding site, diffusion barriers, and charge transfer for a range of metal adsorbates on surfaces, defects, and vacancies of carbon nanotubes and sheets. Adatom coverage dependence and the role of curvature effects will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N31.00004: Accessible surface area in filter deposited, single wall carbon nanotube films Rajib K. Das, Bo Liu, Ryan M. Walczak, Andrew G. Rinzler A convenient process for fabricating thin, homogeneous, transparent films of single wall carbon nanotubes (SWNTs), with potential application as electrodes in organic light emitting diodes and photovoltaic devices, was described in 2004[1]. Among the advantages claimed for such films was the high surface area contact afforded by the nanotubes. Recent measurement of the density for such films shows them to possess nearly 60{\%} of the crystallographic hexagonal close pack density for nominally 1.36 nm diameter SWNTs. Such dense packing does not bode well for infiltration of the films with viscous electro-active polymers by e.g. spin coating. Hence, while the films in principle possess large surface areas, it is not necessarily accessible. To improve this circumstance we have developed a simple, controllable method for modifying the open porosity of the films. We will describe the method and its characterization by imaging, sheet resistance and electrochemical capacitance measurements. [1] Z. Wu et al. Science 305, 1273 (2004) [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N31.00005: Gas Adsorption on Carbon Nanohorns Aggregates Jeff Wagner, M. Mercedes Calbi We evaluate a simple model of adsorption to predict the possible adsorbed phases of gases on a triangular array of tubes as a function of the distance between the tubes and the external pressure. Specific results are presented for the cases of Ne and CF$_{4}$. In addition, Grand Canonical Monte Carlo simulations are performed for particular choices of the aggregates' geometry. Adsorption isotherms and isosteric heats of adsorption as a function of coverage are obtained. Comparison with experimental results allows us to obtain information on the structure of the aggregates that is then use to predict the adsorption behavior of other gases. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N31.00006: Kinetic Selectivity and Competitive Adsorption on Carbon Nanotube Bundles Jared Burde, M. Mercedes Calbi We investigate the kinetics of adsorption of a binary mixture on one-dimensional chains by means of Kinetic Monte Carlo simulations. A competition based on both the binding energies and the adsorption rates is demonstrated. Since the species with smaller binding adsorb faster, it is shown that before reaching equilibrium, that species is the favored one contrary to what eventually happens in equilibrium. Therefore, the weaker binding species can initially reach coverages larger than its equilibrium value, providing evidence of a kinetic selective process that can be exploited for gas separation purposes. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N31.00007: Scaffolding Carbon Nanotubes into Single-Molecule Circuitry Invited Speaker: As electronic devices shrink to the nanometer scale, the relative importance of individual chemical bonds becomes larger and larger. Single-walled carbon nanotubes (SWNTs) represent an extreme limit of this rule, as the modification of a single lattice site can dramatically change chemical activity and electronic properties. This presentation will focus on single-site experimentation in which we find, create, and alter point defects in SWNTs. Due to the correspondence between chemical and electronic properties, changes in SWNT device conductance reveal these chemical processes happening in real-time and allow the SWNT sidewall to be deterministically broken, reformed, and conjugated to target species. We routinely functionalize pristine, defect-free SWNTs at one, two, or more sites, and have demonstrated three-terminal devices in which a single-molecule attachment controls the electronic response. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N31.00008: Supported Lipid Bilayer/Carbon Nanotube Hybrids Xinjian Zhou, Jose Moran-Mirabal, Harold Craighead, Paul McEuen We form supported lipid bilayers on single-walled carbon nanotubes and use this hybrid structure to probe the properties of lipid membranes and their functional constituents. We first demonstrate membrane continuity and lipid diffusion over the nanotube. A membrane-bound tetanus toxin protein, on the other hand, sees the nanotube as a diffusion barrier whose strength depends on the diameter of the nanotube. Finally, we present results on the electrical detection of specific binding of streptavidin to biotinylated lipids with nanotube field effect transistors. Possible techniques to extract dynamic information about the protein binding events will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N31.00009: Carbon nanotube biosensors strongly affected by the biosensitivity of quasi reference electrodes Ethan Minot, Anne Janssens, Iddo Heller, Dirk Heering, Cees Dekker, Serge Lemay Semiconducting carbon nanotubes are extremely sensitive to their electrostatic environment. This property can be utilized to build sensors in liquid environments that detect bio-molecule adsorption in real time via changes in device conductivity. Control of the liquid potential is critical for operation of these sensors, yet nearly all carbon nanotube sensors operating in liquid have employed bare Pt wire to control the liquid potential. We show that the interface voltage between Pt and an electrolyte solution changes by tens of mV upon protein adsorption. This quasi reference electrode biosensitivity can easily mask signals associated with protein adsorption around the carbon nanotube. We demonstrate stable control of the liquid potential using a standard reference electrode, and report signals due entirely to protein adsorption around individual semiconducting NTs. These improved measurements allow us to quantify and differentiate the mechanisms of protein sensing by carbon nanotube devices. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N31.00010: Theoretical Investigation on Alcohol Sensing of Glycine-Coated Carbon Nanotubes Tao Jiang, Gary Kussow, Young-Kyun Kwon It has been observed that single walled carbon nanotube field effect transistors (SWNT-FET) coated with glycine can be used as alcohol sensors. The original semiconducting glycine-coated SWNT-FET have been changed to be metalic in the presence of alcohol. Using {\em ab initio} density functional theory, we compute the structural and electronic properties of carbon nanotubes coated with glycine in the absence or in the presence of alcohol (Isopropanol) to investigate alcohol sensing mechanism. To demonstrate specificity of such glycine-coated SWNT-FETs on alcohol, we also study those properties in the presence of other molecules, such as acetone and water. Furthermore, we investigate the effect of an external fields on glycine-coated SWNT with IPA, and indentify the gate-electric-field screening in SWNT-FET to be a major role for alcohol sensing. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N31.00011: Detection of Individual Gas Molecules Absorbed on Graphene Andre Geim, Kostya Novoselov, Fred Schedin, Sergey Morozov, Da Jiang, Ernie Hill The ultimate aspiration of any detection method is to achieve such a level of sensitivity that individual quanta of a measured value can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason for this is fluctuations due to thermal motion of charges and defects which lead to intrinsic noise exceeding the sought-after signal from individual molecules usually by many orders of magnitude. We describe micron-size sensors made from graphene, which are able to detect individual events when gas molecules attach to graphene's surface at room temperature. The absorbed molecules change the local carrier concentration by one electron, which leads to clear step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material, which makes it a promising candidate not only for ultra-sensitive chemical detectors but also for other applications where local probes sensitive to external charge or magnetic field are required. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N31.00012: Elucidating the mechanism of bio-sensing with carbon nanotube devices in solution. Iddo Heller, Anne Janssens, Ethan Minot, Hendrik Heering, Serge Lemay, Cees Dekker We address the mechanism of electronic sensing with carbon nanotube field-effect transistors in solution. It has been demonstrated that the electrostatic interaction of proteins with single-walled carbon nanotube (SWNT) devices can strongly modulate the transport properties. The exact nature of this electrostatic interaction however remains ill-defined. Recent reports suggest that protein adsorption at the metal-SWNT contact interface plays a more dominant role than adsorption along the bulk of the SWNT. We will report on scanned probe experiments that demonstrate that sensing is not only localized at the contacts. Furthermore, through protein adsorption experiments we show that the effect of either bulk or contact adsorption can dominate depending on the electrolyte gate potential. Because protein adsorption at bulk and contacts can have opposite effects on device conductivity, the two mechanisms can even cancel each other out. This makes it crucial to carefully choose the operating gate potential for protein sensing experiments where the device conductivity is monitored over time. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N31.00013: Light-assisted oxidation of carbon nanohorns for enhancing bio-compatibility Minfang Zhang, Masako Yudasaka, Kumiko Ajima, Sumio Iijima Single-wall carbon nanohorn (SWNH) has a structure similar to single-wall carbon nanotubes but with a larger diameters (2-5 nm) and shorter lengths (40-50 nm), and about 2000 of them assembled to form a spherical aggregate with a diameter of about 100 nm. For various applications of SWNHs, the chemical modification is the crucial issue. To chemically modify SWNHs, the --COOH groups at the holes edges are useful. We show in this report that --COOH groups are formed abundantly when SWNHs were oxidized with H$_{2}$O$_{2}$ under the light irradiation (Xe lamp). SWNHox thus obtained well dispersed by themselves in water, which was even more enhanced by attaching proteins to SWNHox. The modification with proteins effectively enhance the bio-compatibility of SWNHox, which was confirmed through in vitro assay using the mammalian cells. [Preview Abstract] |
Session N32: Casimir Forces, Precision Measurements, and Fundamental AMO Interactions
Sponsoring Units: DAMOPChair: Tim Gay, University of Nebraska
Room: Colorado Convention Center 402
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N32.00001: An Electron EDM Search Using Trapped Molecular Ions Laura Sinclair, John Bohn, Aaron Leanhardt, Edmund Meyer, Russell Stutz, Eric Cornell A sample of trapped molecular ions offers unique possibilities to search for a permanent electron electric dipole moment (EDM). Specifically, we plan to perform this search using the unpaired electron spins in the $^3\Delta_1$ state of trapped HfF$^+$ molecular ions. Ions are easy to trap which will provide the long coherence times necessary to measure the small energy differences associated with an electron EDM. Additionally, the internal electric fields in polarized diatomic molecules can exceed $10^{10}$~V/cm, which will amplify any EDM induced energy splittings. We have created HfF$^+$ ions in a supersonic expansion jet by ablating a Hf target with a pulsed Nd:YAG laser in a He + 1\%SF$_6$ environment. The chemical reaction $\rm{Hf}^+ + \rm{SF}_6 \longrightarrow \rm{HfF}^+ + \rm{SF}_5$ is exothermic and proceeds rapidly. The He buffer gas in the expansion cools the molecular translational, vibrational, and rotational degrees of freedom to $\sim 10$~K. We have measured these temperatures via laser induced fluorescence spectroscopy on known neutral Hf atomic lines and newly identified neutral HfF molecular lines, and are currently searching for the unknown HfF$^+$ electronic transitions. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N32.00002: Testing local position invariance with four Cesium primary frequency standards and four NIST Hydrogen masers Neil Ashby, Thomas Heavner, Steven Jefferts, Thomas Parker In General Relativity, Local Position Invariance (LPI) implies that if atomic clocks of different structure are placed together and syntonized at a particular location, they will remain syntonized while they move through a variable gravitational potential. In this work we compare four active Hydrogen masers located at the National Institute of Standards and Technology (NIST) with Cesium fountain primary frequency standards at NIST, Physikalische-Technische Bundesanstalt (PTB, Germany), Bureau National de M\'etrologie Syst\`emes de R\'ef\'erence Temps Espace (BNM-SYRTE, France) and Istitute Nazionale di Ricerca Metrologica (INRM, ITALY). For the NIST fountain, comparisons have been conducted for six years, while comparisons with fountains at PTB, BNM-SYRTE, and INRM have been reliably conducted for almost three years. During this time the sun's gravitational potential $\Phi$ changes due to earth's orbital eccentricity $e$, with an amplitude given by $\Delta \Phi/c^2 \approx GM_{\odot}e/(a c^2) \approx 1.66 \times 10^{-10}$, where $a$ is the earth's orbital semimajor axis. The Cs-H maser comparisons show no correlation with variations in the solar potential, within an uncertainty that is about 30 times smaller than the previous most sensitive comparisons. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N32.00003: Absolute optical frequency measurements of Cs two-photon transitions with a femtosecond frequency comb Vela L. Mbele, Jason E. Stalnaker, Vladislav Gerginov, Tara Fortier, Carol E. Tanner, Scott A. Diddams, Leo Hollberg We study by direct excitation with a mode-locked femtosecond optical frequency comb, multiple transitions in Cs atoms in a vapor cell at room temperature. We improve by up to two orders of magnitude the uncertainties in the absolute optical frequency and hyperfine structure of the $6s$ $^2 {\rm S}_{1/2} \rightarrow 8s ^2 {\rm S}_{1/2},9s ^2 {\rm S}_{1/2}$, and $7d ^2 {\rm D}_{3/2,5/2}$ transitions in $^{133}{\rm Cs}$. Cesium is one of the well studied heavy atoms, with atomic structure calculations on the order of 1\%, and has provided a fertile testbed for fundamental tests of atomic theory and QED. This work reports on a simple and novel experimental approach that allows simultaneous recording of multiple transition frequencies. Atoms in a vapor cell at room temperature have a broad Doppler velocity distribution which allow selective excitation by discrete modes of a mode-locked femtosecond comb. This, in turn, results in stepwise multiphoton resonant transitions in the atoms. We model the collected spectra using a standard 2$\gamma$ formula and use least square fitting routines to extract improved values of absolute optical frequencies and coupling constants. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N32.00004: Rb Magnetic Resonance Near Coated Glass Surfaces in an Inhomogeneous Field Kaifeng Zhao, M. Schaden, Z. Wu Evanescent waves are used to measure the rf magnetic resonance signal of Rb spin polarization near Pyrex glass surfaces coated with anti-relaxation coatings in an inhomogeneous magnetic field. The signal shows an asymmetric line shape, with one side having approximately Lorentzian profile and the other side being inhomogeneously broadened. The origin of this asymmetry is due to the diffusion of spins. We studied its dependence on buffer gas pressure, cell thickness, field gradient and rf amplitude modulation rate. A theoretical model is developed to understand this line shape. Interesting characteristics of atom- surface interaction, such as dwell time, collision relaxation rate and de-phasing on the surface, can be estimated by fitting the measured line shape with the calculated one. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N32.00005: Nuclear spin relaxation of $^{129}$Xe due to persistent xenon dimers B. Saam, B.N. Berry-Pusey, B.C. Anger, G. Laicher An understanding of longitudinal relaxation mechanisms (characterized by the time $T_1$) that limit both achievable polarization and sample storage time is critically important to applications of hyperpolarized noble gases. We have measured $T_1$ for $^{129}$Xe in Xe-N$_2$ mixtures at densities $< 0.5 $~amagats in a magnetic field of 8.0 T. The intrinsic relaxation in this regime is due to fluctuations in the intramolecular spin- rotation (SR) and chemical-shift-anisotropy (CSA) interactions, mediated by the formation of $^{129}$Xe-Xe persistent dimers. Our results$^*$ are consistent with previous work done in one case at much lower applied fields where the CSA interaction is negligible and in another case at much higher gas densities where transient Xe dimers mediate the interactions. The 8.0-T field suppresses the persistent-dimer mechanism: we have measured $T_1 > 25$~h at 8.0~T for $^{129}$Xe at room temperature. These data also yield a maximum possible low-field $T_1$ for pure xenon gas at room temperature of $5.45\pm 0.2 $~h.\\ {\rm $^*$B.N.\ Berry-Pusey, {\it et al.}, Phys.\ Rev.\ A {\bf 74}, no.\ 6 (in press).} [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N32.00006: Geometric Weakening of the Casimir Interaction Liviu Mateescu, Martin Schaden We examine the dependence of the Casimir interaction between separate (metallic) bodies on their geometry. From a semi-classical point of view it depends strongly on whether the dominant periodic orbits are stable or unstable and on the number of focal points. We give a very simple semiclassical argument for the theorem [1] that mirror-symmetric periodically corrugated metallic surfaces always attract. Although counter-intuitive because the Van DerWaals interactions between individual pairs of atoms are attractive at long range, we argue that this need not be the case for multi-atom interactions. Semi-classical methods are used to determine the shape of surfaces with minimal Casimir interaction. [1] O.Kenneth, I. Klich, Phys.Rev.Lett. 97, 160401 (2006)~ [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N32.00007: Casimir force measurements between a sphere and a surface with high-aspect ratio, nanoscale channel arrays Yiliang Bao, Ho Bun Chan The Casimir force is a quantum effect that strongly depends on the shape of the boundaries that confines the electromagnetic fields. So far the majority of experiments have concentrated on the simple arrangement of plate-sphere or two parallel plates. Demonstrating the strong shape dependence of the Casimir force would require other geometries with interactions that deviate significantly from the pair-wise summation of two-body potentials. Here we present measurements of the Casimir force between a gold-coated sphere and a silicon plate with an array of nanoscale, high-aspect-ratio rectangular trenches. A micromachined torsional oscillator acts as the force transducer which allows us to measure the interactions between the surfaces at high sensitivity. Channels with widths ranging from 200 nm to 500 nm and depth of 1 um are fabricated on a silicon substrate. We will compare the Casimir interaction between the sphere and these trench arrays with different aspect ratios. Such measurements might open up new possibilities to manipulate the Casimir force by tailoring the shape of the interacting surfaces. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N32.00008: Measurements of the Casimir force in fluids Jeremy Munday, Federico Capasso Confinement of the quantum fluctuations of electromagnetic fields between two grounded, conducting surfaces gives rise to an attractive force first predicted by H. B. G. Casimir. During the past decade, there have been many experimental demonstrations of this force between two metal surfaces in vacuum. While high precision experiments have been performed for this case, few experiments have been done between metallized or dielectric objects in fluids. For this situation, a more general formalism was developed by Lifshitz. If materials are chosen with suitable dielectric response functions, repulsive quantum electrodynamical (QED) forces can also arise. We will discuss experimental results using an atomic force microscope (AFM) to measure the interaction force between a metallized sphere and a plate, made of either metal or dielectric, in fluid. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N32.00009: Casimir force measurements between metal and high-$T_c$ superconductor surfaces Mark B. Romanowsky, Jeremy N. Munday, Richard Schalek, Federico Capasso, Qiang Li, Genda Gu It is well known that the strength of the Casimir force between two objects is controlled by the dielectric properties (or optical conductivity) of the objects. Nearly all precision measurements of Casimir forces to date are between two metals. Here we report measurements of the Casimir force between a metal-coated sphere and a plate made of the high-$T_c$ cuprate superconductor BSCCO-2212, using an atomic force microscope at room temperature. BSCCO has dielectric properties substantially different from metals and indeed most materials, displaying extreme anisotropy in dc and optical conductivity, as well as a ``strange metal'' normal state. The force between metal and BSCCO is compared to the force measured between two metals. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N32.00010: The Casimir force on transparent conductors Limor Spector, Jeremy Munday, Federico Capasso, Nicholas Geisse, Kevin Kit Parker The Casimir force arises from quantum fluctuations of electromagnetic fields in vacuum and is dependent on the dielectric properties of the interacting materials. This force can have a profound impact on the functionality of systems operating on the micro- and nanoscale. As nanotechnology continues to evolve, the ever-present Casimir force will have to be carefully considered during the design stage. Eliminating or greatly reducing this force could be of tremendous importance. To this end, we have performed Casimir force measurements using atomic force microscopy (AFM) between metals (gold and palladium) and transparent conductors (e.g. indium tin oxide). Due to the transparence of these materials, it is expected that the electromagnetic modes will be less well confined, and the Casimir force will be reduced. Experimental results of such studies will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N32.00011: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N32.00012: Adiabatic change assisted Rabi transitions between Adiabatic change assisted Rabi transitions between decoupled quantum states Xingxiang Zhou, Ari Mizel A periodic perturbation such as a laser field cannot induce transitions between two decoupled states because the transition matrix element vanishes. However, if in addition some system parameters are varied adiabatically, such transitions are possible via the adiabatic change induced excitations to other states. We study such transitions between two decoupled states and show that full amplitude transfer can be achieved. The resulting physics can be understood in terms of the rotation of an effective spin 1/2 in the two-state subspace, but with a rotation angle dependent on the path traversed by the system in the parameter space only. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N32.00013: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N32.00014: Direct Dissociative Recombination of NO$_2$ Daniel J. Haxton, Chris H. Greene We provide estimates for direct dissociative recombination (DR) rates for collisions of NO$_2^+$ + $e^-$ via the 2 $^2\Pi$ and $^2\Phi$ states of the neutral. No calculations or measurements of this rate exist in the literature, despite the fact that NO$_2$ is an important constituent of the atmosphere and DR of the cation may play a role in its atmospheric chemistry. Little is known about the potential energy surfaces of neutral NO$_2$ at energies near the ionization threshold. However, preliminary calculations suggest that the 2 $^2\Pi$ and $^2\Phi$ states may intersect the ground state potential energy curve of the neutral near its Franck-Condon region. R-matrix calculations are employed to obtain the widths of these states, and the direct DR rate is extracted by employing the multidimensional reflection principle along with the formalism of O'Malley. The calculated rates may help to elucidate whether the direct or indirect mechanism plays a larger role in DR of NO$_2^+$. This work was supported in part by the DOE Office of Science. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N32.00015: Neutralization/Ionization of Si Scattered from Adsorbate Sites Xiaojian Chen, Zdenek Sroubek, Jory Yarmoff In low energy ion scattering, ion-surface charge exchange strongly depends on the surface electronic structure and the ionization level of the projectile. Si has an ionization level that overlaps the center of the surface conduction band and is intermediate in energy to that of alkali ions and noble gas ions, which are the projectiles traditionally used. The scattering of Si thus provides new pathways for ion-surface charge exchange. A considerable fraction of the low energy Si$^{+}$ ions backscattered from submonolayers of Cs deposited onto Al(100) are found to be emitted as positive or negative ions. The negative ions result from simple resonant charge transfer (RCT) into the electron affinity level. The formation of Si$^{+}$, however, is in contrast to the expected complete neutralization due to the overlap with the surface bands. It is proposed that valence electron RCT enhanced by the interaction of the Si ionization level with the Cs 5p level is responsible for the ion formation. Positive ions were also produced in Si scattered from I adatoms on Al(100), presumably by a similar mechanism. The ion fractions are smaller than those for scattering from Cs, which suggests that electron tunneling from the occupied I chemisorption states provides an additional neutralization channel. [Preview Abstract] |
Session N33: Quantum Measurement
Sponsoring Units: GQIChair: Girish S. Agarwal, Oklahoma State University
Room: Colorado Convention Center 403
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N33.00001: Simple multi-parameter unitary estimation Sergio Boixo, Rolando Somma We consider multi-parameter estimation of a general unitary operation acting on a set of qubits. We show a simple quantum circuit that estimates operators at the optimal Heisenberg limit, i.e., achieving a sensitivity for determining the parameters that scales as $1/N$, where $N$ is the number of times the unknown unitary is applied. The circuit makes use of one extra qubit (ancilla) which is initially prepared in a pure state, while the system qubits are initially prepared in the totally mixed state. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N33.00002: Quantum State Detection through Repetitive Mapping D. B. Hume, T. Rosenband, J. C. Bergquist, D. J. Wineland State detection plays an important role in quantum information processing and quantum-limited metrology. In some cases the quantum system of interest can only be detected with poor efficiency. One approach to overcoming this limitation is to couple the primary quantum system to an ancillary quantum system used for measurement [1]. The measurement process consists of mapping the primary state to the ancilla followed by ancilla detection. If this can be done without affecting the projected populations of the primary system, the measurement may be repeated. In this case, detection fidelity can be significantly higher than both the fidelity of state transfer and the intrinsic measurement fidelity of the ancillary system. Using two ions as the primary and ancillary systems ($^{27}$Al$^{+ }$and $^{9}$Be$^{+}$ respectively) held in a harmonic trap, we demonstrate near unit fidelity measurement despite imperfect information transfer and ancilla detection. \newline \newline [1] P.O. Schmidt, et. al. Science 309 749 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N33.00003: Examination of the Charge Quantum in a Single-Electron Pump Mark W. Keller, Neil M. Zimmerman, Ali L. Eichenberger In single-electron tunneling (SET) circuits, charge moves in discrete quanta that are generally assumed to carry a charge of exactly $e$, the free electron charge. To the extent that this is true, SET devices have an important role to play in fundamental metrology by providing a solid-state current source that is directly linked to a fundamental constant of nature. But is the SET charge quantum in fact exactly $e$? We discuss why this is not a trivial question and present an experimental answer to the question: by placing a known number of SET charge quanta onto a known capacitor, and by measuring the resulting voltage across the capacitor using a Josephson voltage standard, we compare the SET charge quantum to $e$. We find that the SET charge quantum is equal to $e$ within a relative standard uncertainty of 1 part in $10^6$, a constraint that is $\sim 100$ times smaller than the best previous result. This measurement is expected to reach an uncertainty $\sim 3$ parts in $10^7$ in the near future, at which point it will also give useful information on possible corrections to the Josephson constant, $K_J = 2e/h$. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N33.00004: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N33.00005: Radio frequency operation of a quantum point contact charge detector Madhu Thalakulam, A.J. Rimberg, L.N. Pfeiffer, K. W. West Quantum point contact (QPC) charge detectors are sensitive electrometers, and their ease of fabrication and integration into semiconductor-based qubit systems makes them an attractive candidate as a readout device for spin or charge based qubits in quantum dots. Nevertheless, QPC performance to date has been limited by relatively low operational speeds and 1/f noise. Here we report the operation of a QPC charge sensor realized in an GaAs/AlGaAs two dimensional electron gas at radio- frequencies (RF-QPC), in a mode analogous to rf operation of the single electron transistor [1]. For a typical QPC detector coupled to a quantum dot (QD), a charge oscillation of one electron in the QD corresponds to a change in the QPC conductance of 1-3 percent. We simulate these operating conditions by applying a small ac voltage to the QPC gate to cause a similar change in the zero bias QPC conductance. When operated this way the signal to noise ratio of the RF-QPC is about 30dB, which corresponds to a charge sensitivity of about $7x10^{-4}e/\sqrt{Hz}$ referred to the dot charge. The operational characteristics of the RF-QPC at 4.2K also will be discussed. [1] R. J. Schoelkopf et al., Science \b {280}, 1238–1242 (1998). [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N33.00006: Radio-frequency measurement of an asymmetric single electron transistor Zhongqing Ji, Weiwei Xue, A.J. Rimberg Since the invention of the radio-frequency single-electron transistor (RF-SET) by Schoelkopf et al.,[1] most measurements have focused on the symmetric single electron transistor. It has been shown, however, that the symmetric SET has a rather low measurement efficiency in its normal working regime.[2][3] Recently, it has been pointed out that an asymmetric SET can be considerably more efficient than a symmetric SET as a quantum amplifier. In this case the measurement efficiency of the asymmetric SET becomes similar to that of the quantum point contact (QPC) detector which can approach the quantum limit. We investigate the asymmetric SET by fabricating Al/AlO$_{x}$ SETs with junction areas 40x40 nm$^{2 }$and 40x80nm$^{2}$ and total resistance of about 25k$\Omega $. The results of RF and DC characterization of such asymmetric SETs will be discussed. [1] R. J. Schoelkopf, P. Wahlgren, A. A. Kozhevnikov, P. Delsing, D. E. Prober, Science, \textbf{280}, 1242 (1998). [2] A. N. Korotkov, Phys. Rev. B, \textbf{63}, 085312 (2001); \textbf{63}, 115403 (2001). [3] D. Mozyrsky, I. Martin, and M. B. Hastings, Phys. Rev. Lett., \textbf{92}, 018303 (2004). [4] S. A. Gurvitz and G. P. Berman, Phys. Rev. B, \textbf{72 }, 073303(2005). [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N33.00007: Spin Measurement in Quantum Electro-Mechanical Systems Dian Wahyu Utami, Jason Twamley, Hsi-Sheng Goan, Gerard J. Milburn Interests in spin measurement in solid state nanostructure has been growing in the last few years. The measurement of spin is particularly important in the realization of spin based solid state quantum computer proposals. Here we present our study on spin detection via a quantum electromechanical shuttle system using the example of an endohedral N@C60 that is placed in a magnetic gradient generated by a nearby nanomagnet. Using quantum optics methods, the currents across the system are found to be different for each of the different spin orientations. This is due to the different directional forces produced as a result of the interaction between each of the spin orientation to the magnetic gradient. The resulting force affects the steady state position of the island and thus modifies the system's conductance. We investigate the feasibility of the application of the system as a single spin measurement by looking at the current noise spectral density and investigating the measurement time required to distinguish the two currents for each of the spin states. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N33.00008: Laser cooling of diffraction limited size micromirrors Constanze Metzger, Ivan Favero, Khaled Karrai The prospect of realizing entangled quantum states between macroscopic objects and photons [1] has recently stimulated interest in laser-cooling schemes of macroscopic mechanical resonators [2-5]. We describe passive optical cooling of the Brownian motion of a cantilevered micromirror. Since the cantilever forms one mirror of a confocal Fabry-P\'{e}rot cavity, its mirror end has to be of the size of the optical wavelenght in order to ensure high reflectivity. In our setup, the mirror's size is 2.4$\mu $m and hence in the range of the diffraction limit for 1.3$\mu $m laser light. With its weigth of 11pg it represents the smallest mass cooled so far. The optically induced excitation regime was also explored, opening a path to optically driving nanostructures with high frequency resonances. [1] Marshall et al., Phys. Rev. Lett. \textbf{91}, 130401 (2003). [2] Metzger and Karrai, Nature \textbf{432}, 1002 (2004). [3] Gigan et al., Nature \textbf{444}, 67 (2006). [4] Arcizet et al., Nature \textbf{444}, 71 (2006). [5] Kleckner and Bouwmeester, Nature \textbf{444}, 75 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N33.00009: ABSTRACT HAS BEEN MOVED TO J32.00013 |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N33.00010: Quantum Undemolition: Undoing quantum measurement by erasing information Alexander Korotkov, Andrew Jordan Extensive research into controllable quantum systems and detectors has led to a reexamination of the very nature of quantum measurement in a condensed matter context. Quantum detectors used in recent experiments naturally give rise to weak quantum measurements, where the detector output is not perfectly correlated with the state of the measured system. According to textbook quantum measurements, wavefunction collapse of an unknown state is essentially an irreversible process; the measurement record is indelible. Contrary to this conventional wisdom, we will demonstrate how to undo a weak quantum measurement, showing that quantum information is written in pencil, not pen. The undoing procedure has a finite probability of success, and it is accompanied by a clear experimental indication of whether or not the undoing has been successful. Our proposed phenomenon can be experimentally realized using quantum dot (charge) or superconducting (phase) qubits. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N33.00011: ABSTRACT HAS BEEN MOVED TO U33.00015 |
Session N34: Focus Session: Brownian Motors in Physics, Chemistry, and Biology
Sponsoring Units: DBP GSNPChair: Dean Astumian, University of Maine
Room: Colorado Convention Center 404
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N34.00001: Brownian Heat Engines -- from Leidenfrost Droplets to Nanowire Thermoelectrics Invited Speaker: A Brownian heat engine is a system that rectifies the flow of Brownian particles to transform local temperature variations into directed motion (work). In the context of electronics, this is the principle of thermoelectric energy conversion. For a long time it was thought that Brownian heat engines (and thermoelectric devices) are inherently irreversible and would therefore necessarily fall short of the Carnot limit for the energy conversion efficiency. I will introduce the concept of a Brownian heat engine, and will discuss how quantum energy-filtering can in fact be used to design a Carnot efficient, Brownian heat engine [1]. I will then present two experimental systems. The first, heat-propelled Leidenfrost droplets [2], is not really `Brownian' but nevertheless a very entertaining and illustrative ratchet heat engine. The second is our experimental effort to demonstrate a near-Carnot efficient thermal-to-electric energy converter [3] based on a quantum dot embedded into a heterostructure nanowire [4]. The physics behind this novel thermoelectric system, and the status of experiments will be discussed. \newline \newline [1] T. E. Humphrey, R. Newbury, R. P. Taylor, H. Linke, \textit{Phys. Rev. Lett.} \textbf{89}, 116801 (2002). \newline [2] H. Linke\textit{ et al.}, \textit{Phys. Rev. Lett.} \textbf{96}, 154502 (2006). \newline [3] M. O'Dwyer, T. E. Humphrey, H. Linke, \textit{Nanotechnology} \textbf{17}, S338 (2006). \newline [4] M. T. Bj\"{o}rk\textit{ et al.}, \textit{Nano Letters} \textbf{2}, 87 (2002). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N34.00002: Study of single flagellar propulsion with optical tweezers Suddhashil Chattopadhyay, Xiao-Lun Wu Various theoretical models predict propulsion by the bacterial flagellum. Use of these models to calculate dynamical quantities of bacterial swimming are commonplace. However, direct verification of the various mathematical approaches has been difficult due to the lack of precise experimental data, which has been challenging to obtain. In this work we perform measurements on swimming bacterium which posses a single polar flagellum. Swimming with a single flagellum allows simpler parametrization as compared to a flagellar bundle. Bacteria are stably trapped in the bulk fluid (away from a surface) and perpendicular to the trapping axis with the aid of an imposed flow. This approach avoids hydrodynamic effects due to wall proximity, which were observed in previous measurements. The optical trap allows all dynamical quantities of a swimming bacterium to be determined. Flagellar dimensions are obtained by fluorescent imaging to obtain all pertinent information, required to put different theoretical models to test. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N34.00003: The stochastic dynamics of filopodial growth Yueheng Lan, Garegin Papoian We build stochastic models for filopodial growth and retraction that combine mechanical and spatiotemporal signaling components to elucidate the mechanisms of filopodia dynamics. We explicitly model the tip signaling and diffusion process while the membrane and retrograde flow are modeled implicitly. The results are compared with experiments to verify the model effectivness. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N34.00004: Swimming movements of filaments in a linearly viscoelastic medium Henry Fu, Thomas Powers, Charles Wolgemuth Motivated by the swimming of sperm in the non-Newtonian fluids of the female mammalian reproductive tract, we examine beating filaments in a linearly viscoelastic medium. The forces exerted by the medium are incorporated via a resistive force theory approriate for a Maxwell fluid, in which the force per unit length acting on a filament relaxes to the force per unit length exerted by a purely viscous fluid. We calculate the shapes of beating patterns of filaments with prescribed driving forces in two models: 1) an elastic passive filament forced from one end; 2) a simplified sliding-filament model for sperm flagellum with active internal sliding forces. We note that in a linearly viscoelastic model, for prescribed beating patterns, swimming velocity is the same in viscoelastic and viscous fluids, and there is a simple relation between the power dissipated in each fluid. In contrast, for prescribed driving forces, beating patterns may be different in viscoelastic and viscous fluids leading to changes in swimming velocities and power dissipated. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N34.00005: Fusion versus endocytosis: the stochastic entry of enveloped viruses Tom Chou Viral infection requires the binding of receptors on the target cell membrane to glycoproteins, or ``spikes,'' on the virus membrane. Fusion peptides that make up part of these spikes on the viral membrane may then be triggered by pH changes or binding of additional coreceptors. Thus, binding of virus envelope proteins to cell surface receptors not only initiates the viral adhesion and the wrapping process necessary for internalization, but also starts the direct fusion process. Both fusion and internalization may be viable pathways for some viruses, under appropriate conditions. We develop a stochastic model for viral entry that incorporates both receptor mediated fusion and endocytosis. The relative probabilities of fusion and endocytosis of a virus particle initially nonspecifically adsorbed on the host cell membrane are computed as functions of receptor concentration, binding strength, and number of spikes. We find the parameter regimes where each pathway is expected to arise and discuss possible experimental tuning of these parameters. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N34.00006: Exact results for random deposition-driven ratcheting Maria-Rita D'Orsogna, Tom Chou We consider the discrete translocation of a polymer through a pore, across a wall, driven by the irreversible, random sequential adsorption of particles on one side of the pore. Although the kinetics of the wall motion and the deposition are coupled, we find the exact steady state distribution for the gap between the wall and the nearest deposited particle. From this exact result, the mean translocation velocity and variance are constructed. We explicitly show that translocation is faster and less variable when the adsorbing particles are smaller. The relative efficiencies of ratcheting using different sized deposition particles are also defined and compared. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N34.00007: Molecular motors driven by asymmetric nucleation Amit Lakhanpal, Tom Chou We study a one dimensional model of asymmetric nucleation where the phase boundaries are coupled to a load particle. Sites on the one-dimensional lattice are either empty or filled. Empty sites get filled faster if the is a filled site immediately preceding it. This model has applicability to nucleation problems where the substrate is directional. Examples include nucleation of proteins on filamentary substrates such as nucleic acids and microtubules. The hydrolysis of ATP or GTP in microfilaments such as RecA has been proposed as a mechanism of moving Halliday junctions, and can also be described qualitatively by our model. Using Monte Carlo simulations, we find mean velocities and of a load particle as function of the nucleation rates and the asymmetry parameter. Our results are compared with simple mean field approximations. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N34.00008: Free boundaries and confinement in driven diffusive systems Pak-Wing Fok, Sarah Nowak, Tom Chou We study the dynamics of a load wall confining an asymmetric exclusion process with Langmuir kinetics. Results from Monte Carlo simulations and mean field approximations are compared. We find that the mean position of the wall depends not only on the load on the wall and the injection, adsorption, and desorption rates, but also on the intrinsic fluctuations of the wall. Our results are discussed in the context of nonequilirium phases of the system, fluctuating boundary layers, and particle densities in the lab frame versus the frame of the fluctuating wall. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N34.00009: Slow axonal transport: Neurofilaments switch between distinct mobile and stationary states during their transport along axons Peter Jung, Niraj Trivedi, Lei Wang, Anthoni Brown According to the stop-and-go hypothesis of slow axonal transport, cytoskeletal and cytosolic proteins are transported along axons at fast rates but the average velocity of movement is slow because the movements are infrequent and bidirectional. To test whether this hypothesis can explain the kinetics of slow axonal transport in vivo, we have developed a stochastic model of neurofilament (NF) transport in axons based on tracking of single NF molecules. Based on this model, we propose that NFs in vivo move in both, anterograde and retrograde directions along cytoskeletal tracks switching between mobile and a stationary states. To verify the proposed stationary state we have developed a novel pulse-escape fluorescence photoactivation technique. We find that on average, the NFs spent 92{\%} of their time in the stationary state and 97{\%} of their time pausing. We speculate that the relative proportion of the time that NFs spend in the stationary state may be a principal determinant of their transport rate and distribution along axons, and a potential target of mechanisms that lead to abnormal NF accumulations in disease. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N34.00010: A Geometric Mechanism for Asymmetric Diffusion and Membrane Rectification Robert Shaw, Norman Packard Biological membranes commonly conduct ions freely in one direction while clogging in the other. Existing theories emphasize electrostatic binding of blocking ions in pores as a mechanism for rectification. Here we show that rectification can have a purely geometric origin, based on the interaction of shapes of diffusing particles and pore geometry. The two possibilities can be experimentally distinguished. Blocker binding based on confinement in a potential well will have a strong Arrhenius temperature dependence, whereas ``geometric binding'' will have a much smaller dependence on temperature. We present both Hamiltonian and Brownian-based computer simulations which demonstrate this effect. A rectifying membrane can maintain different concentrations on either side, resulting in a long-lived metastable state. We derive a dynamic equation of state describing the decay of this metastable system. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N34.00011: Untying molecular friction knots Serdal Kirmizialtin, Dmitrii Makarov Molecular knots tied in individual polymer strands have fascinated researchers from many fields. Recently, laser tweezers have been used to tie knots in individual DNA and protein molecules and to observe their dynamics. Unlike their macroscopic counterparts, knots in tensioned polymer strands undergo rapid diffusion caused by thermal fluctuations. Here, we use computer simulations to study the dynamics of a ``friction knot'' joining a pair of polymer strands. While a friction knot splicing two ropes is jammed when the ropes are pulled apart, molecular friction knots eventually become undone by thermal motion. We show that depending on the knot type and on the polymer structure, a friction knot between polymer strands can be strong (the time $\tau$ the knot stays tied increases with the force \textit{F} applied to separate the strands) or weak ($\tau$ decreases with increasing \textit{F}). We further propose a simple model explaining these behaviors. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N34.00012: Exact Solutions of Burnt-Bridge Models for Molecular Motor Transport Alexander Morozov, Ekaterina Pronina, Anatoly Kolomeisky, Maxim Artyomov Transport of molecular motors, stimulated by interactions with specific links between consecutive binding sites (called ``bridges''), is investigated theoretically by analyzing discrete-state stochastic ``burnt-bridge'' models. When an unbiased diffusing particle crosses the bridge, the link can be destroyed (``burned'') with a probability $p$, creating a biased directed motion for the particle. It is shown that for probability of burning $p=1$ the system can be mapped into one-dimensional single-particle hopping model along the periodic infinite lattice that allows one to calculate exactly all dynamic properties. For general case of $p<1$ a new theoretical method is developed, and dynamic properties are computed explicitly. Discrete-time and continuous-time dynamics, periodic and random distribution of bridges and different burning dynamics are analyzed and compared. Theoretical predictions are supported by extensive Monte Carlo computer simulations. Theoretical results are applied for analysis of the experiments on collagenase motor proteins. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N34.00013: Molecular Dynamics simulation of Buttiker-Landauer Refrigerator Ronald Benjamin, Ryoichi Kawai A position dependent temperature profile in presence of a periodic potential leads to directed current of Brownian particles, commonly known as Buttiker-Landauer ratchet. Onsager symmetry tells us that inhomogeneous temperature profile can be generated by reversing the Buttiker-Landauer ratchet. When Brownian particles driven by a constant external force cross over the potential barrier, they carry heat from one side to the other. Hence, starting with uniform temperature the flow of Brownian particles induces inhomogeneous temperature profile. We investigate this phenomenon using first principles molecular dynamics simulations as well as the phenomenologial Langevin equation. [Preview Abstract] |
Session N35: Focus Session: Time Resolved Structural Investigations on Protein Folding and Function
Sponsoring Units: DBP DCPChair: Aihua Xie, Oklahoma State University
Room: Colorado Convention Center 405
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N35.00001: Correlating folding and signaling in a photoreceptor by single molecule measurements and energy landscape calculations Invited Speaker: Receptor activation is a fundamental process in biological signaling. We study the structural changes during activation of photoactive yellow protein (PYP). This is triggered by photoisomerization of the p-coumaric acid (pCA) chromophore of PYP, which converts the initial pG state into the activated pB state. Mechanical unfolding of Cys-linked PYP multimers probed by atomic force microscopy (AFM) in the presence and absence of illumination reveals that the core of the protein is extended by 3 nm and destabilized by 30 percent in pB. These results establish a generally applicable single molecule approach for mapping functional conformational changes to selected regions of a protein and indicate that stimulus-induced partial protein unfolding can be employed as a signaling mechanism. Comparative measurements, Jarzynski-Hummer-Szabo analysis of the data, and steered MD simulations of two double-Cys PYP mutants reveal strong anisotropy in the unfolding mechanism along the two axes defined by the Cys residues. Unfolding along one axis exhibits a transition-state-like feature where six hydrogen bonds break simultaneously. The other axis displays an unpeaked force profile reflecting a non-cooperative transition, challenging the notion that cooperative unfolding is a universal feature in protein stability. MD simulations with a coarse-grained protein model show that the folding of pG is two-state, consistent with experimental observations. In contrast, the folding free energy surface of a coarse-grained model of pB involves an on-pathway partially unfolded intermediate that closely matches experimental data. The results reveal that interactions between the pCA and its binding pocket can switch the energy landscape for PYP from two- to three-state folding, and show how this can be exploited to trigger large functionally important protein conformational changes. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N35.00002: Spectroscopic probes of enzyme-ligand interaction dynamics Christopher Cheatum, Jigar Bandaria, Samrat Dutta, Sarah Hill, Amnon Kohen Formate dehydrogenase catalyzes the NAD-dependent oxidation of formate to carbon dioxide. The intrinsic chemical step involves hydride transfer from formate to the nicotinamide ring of NAD. As with several other NAD-dependent dehydrogenases, kinetic measurements suggest that thermal fluctuations of the enzyme are important in the hydride-transfer reaction. We have measured the dynamics of enzyme-inhibitor interactions in binary and ternary complexes of formate dehydrogenase with pseudohalides using infrared photon-echo spectroscopy. The pseudohalides are excellent vibrational chromophores that are known to be sensitive reporters of interactions with their local environments. They are also excellent inhibitors for formate dehydrogenase. Our measurements reveal significant differences in the dynamics of the different binary and ternary complexes. By comparing and contrasting the dynamics for different complexes we gain insight into the active-site components that make the most important contributions to the observed dynamics. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N35.00003: Shallow Free Energy Landscapes Remodelled by Ligand Binding Troy Messina, David Talaga, Emilio Gallichio, Ronald Levy Glucose/galactose binding protein (GGBP) functions as part of a larger system of proteins for molecular recognition and signalling in enteric bacteria. Here we report on the thermodynamics of conformational equilibrium distributions of GGBP 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 GGBP has with cytoplasmic membrane proteins. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N35.00004: High-throughput biophysics of functional tuning in photoactive yellow protein Wouter Hoff, Andrew Philip, George Papadantonakis The relationship between the structure of a protein and its function is a central unresolved problem in biology. We use photoactive yellow protein (PYP) to develop quantitative high-throughput methods to study this problem. PYP is a small bacterial photoreceptor with rhodopsin-like photochemistry based on its p-coumaric acid (pCA) chromophore. The absorbance maximum and pKa of the pCA in the active site of native PYP are shifted from 400 nm and 9.0 in water to 446 nm and 2.8 in the protein. Thus, PYP offers a unique model system to probe protein-ligand interactions. Here we show that high-throughput microscale methods can be used for quantitative biophysical studies of the absorbance spectrum PYP, its fluorescence quantum yield, apparent pKa of the pCA, protein stability against chemical denaturation, and kinetics of the last PYP photocycle step. A wide range of properties was observed among the mutants, and structural features that tune functional properties were identified. These results open the way for high-throughput quantitative biophysical studies of PYP. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N35.00005: Probing protein dynamics using Fluorescence Resonance Energy Transfer with donors of different lifetimes Weiqun Peng, Tania Chakrabarty, Paul Goldbart, Paul Selvin Fluorescence resonance energy transfer (FRET), using nanosecond dyes, and its derivative, Lanthanide-based resonance energy transfer (LRET), using millisecond-lifetime lanthanide chelates, are methods to measure distances on the 2-10 nm length-scale. It has been found that in certain systems energy transfer efficiency E for FRET and LRET measurements can be dramatically different [Chakrabarty et al., PNAS, 99: 6011-6016 (2002)]. Here we develop a theoretical model that shows that the dramatic difference can be explained by the presence of intrinsic dynamics of the system. Furthermore, we quantitatively investigate how information about the time-scale and distance-scale associated with the intrinsic dynamics can be inferred, by comparison of FRET and LRET results. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 10:00AM |
N35.00006: Advanced Infrared Spectroscopy for Time-Resolved Structural Investigation of Protein Structure and Function Invited Speaker: The human genome encodes approximately 30,000 different proteins. A single mutation at a critical site of one protein can cause serious diseases, such as cardiac failure and cancer. This illustrates the significant role of protein structures in protein functions. In order to obtain a fundamental understanding of protein structure-function relation, we must develop and employ both physical theories and experimental techniques. In my talk, I will report both experimental and computational studies on vibrational structural markers for advanced infrared spectroscopy, slaved protein structural dynamics, and ``electrostatic epicenter'' model as a general mechanism for activation of receptor proteins in cell signaling. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N35.00007: Channel noise reduction due to gating charge effects Gerhard Schmid, Igor Goychuk, Peter H\"anggi We investigate the influence of gating charge effects on the channel noise-induced spontaneous spiking activity of excitable membrane patches [1] within a stochastic Hodgkin-Huxley model [2]. The random switching of the channel gates between an open and a closed configuration is always connected with movement of gating charge within the cell membrane. At the beginning of an action potential the gating current is opposite to the direction of the ion current through the membrane. Therefore, the excitability is expected to become reduced due to the influence of gating current. Our study revealed that while the deterministic modelling with gating charge effects does not differ dramatically from the original Hodgkin-Huxley model for the standard set of parameters, the corresponding stochastic model which takes into account the channel noise -- i.e. the fluctuations of the number of open ion channels -- does behave very differently for intermediate-to-large membrane patch sizes. A main finding is that spontaneous spiking activity becomes drastically reduced [1]. \newline \noindent [1] G. Schmid, I. Goychuk, and P. H\"anggi, Phys. Biol., in press (2006); (arXiv:abs/q-bio.NC/0611040). \newline [2] G. Schmid, I. Goychuk, and P. H\"anggi, Europhys. Lett. {\bf 56}, 22 (2001) [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N35.00008: A Molecular Dynamics-Decorated Finite Element Method (MDeFEM) Framework for Simulating the Gating of Mechanosensitive Channels Xi Chen, Yuye Tang, Guoxin Cao, Jejoong Yoo, Arun Yethiraj, Qiang Cui The gating pathways of mechanosensitive channels of large conductance (MscL) are studied using the finite element method. The phenomenological model treats transmembrane helices as elastic rods and the lipid membrane as an elastic sheet of finite thickness. The interactions between various continuum components are derived from atomistic energy calculations. The structural variations along the gating pathway are consistent with previous analyses based on structural models and biased molecular-dynamics simulations. Upon membrane bending, there is notable and nonmonotonic variation in the pore radius. This emphasizes that the gating behavior of MscL depends critically on the form of the mechanical perturbation. Compared to popular all-atom simulations, the MDeFEM framework offers a unique alternative to bridge detailed intermolecular interactions and biological processes occurring at large spatial and timescales. It is envisioned that such a hierarchical multiscale framework will find great value in the study of a variety of biological processes involving complex mechanical deformations such as muscle contraction and mechanotransduction. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N35.00009: Develop vibrational structural makers for probing the protonation state and hydrogen bonding interactions of tyrosine in proteins and their functional intermediates Anupama Thubagere, Beining Nie, Edward Manda, Aihua Xie Proteins are dynamic in nature. In order to understand how a protein performs its function based on laws of physics, it is critical to probe and investigate functionally important structural transitions of the protein. Time-resolved infrared spectroscopy offers excellent time resolution (picoseconds to seconds), and contains extensive structural information. The real challenge is how to extract structural information from time-resolved infrared data. We will report computational methods for developing vibrational structural markers of tyrosine. Using density function theory (DFT) based first principle computational studies combined with experimental data, we found that it is possible to unambiguously determine if the phenolic ring in Tyrosine is neutral or negatively charged based on the frequency of one ring vibrational mode. In addition, we show that it possible to determine the number and nature of hydrogen bonding interactions of a phenolic group in proteins using a combination of C-O stretching and O-H stretching frequencies (2D vibrational spectroscopy). [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N35.00010: Correlated Fluorescence Parameters of Single Molecules Claudiu Gradinaru, David Chandler, Carl Hayden A novel detection system is used in a confocal optical microscope for measuring correlated fluorescence lifetimes and spectra. Fluorescence photons emitted from a sample are imaged through a dispersive optical system onto a time- and position-sensitive detector. For each photon the apparatus records the wavelength, the emission time relative to the laser excitation pulse and the absolute detection time, so that correlations among all the fluorescence properties are maintained. A histogram over many photons can generate a full fluorescence spectrum and a correlated decay plot at every pixel in a fluorescence image. The complex data structure allows mapping the time-dependent distribution of multiple fluorescent species in a sample and enables monitoring the dynamics of single molecules on a time scale that spans from picoseconds to minutes. Unique correlations between intensity, spectrum and lifetime prove useful for tracking changes in the nanoenvironment of fluorescent probes. The detection method also provides a more complete description of the fluorescence resonance energy transfer (FRET) than conventional microscopy techniques, as demonstrated by single-pair FRET experiments between dyes spaced apart by short peptides and by dsDNA chains. [Preview Abstract] |
Session N38: Focus Session: Advances in Scanned Probe Microscopy II: Force Methods
Sponsoring Units: GIMSChair: Andreas Heinrich, IBM-Almaden
Room: Colorado Convention Center 501
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N38.00001: AFM/STM with sub-Angstrom modulation. Invited Speaker: Atomic manipulation of single atoms and molecules by scanning probe microscopy enables the assembly of structures at the single-atom scale - the ultimate lower size limit. However, it has been difficult to answer the simple question: How much force does it take to manipulate atoms and molecules on surfaces? To address this question, we combine scanning tunneling microscopy and frequency modulated atomic force microscopy. To enable simultaneous detection of the tunneling current and frequency shift we utilize the q-plus sensor design, in which a metallic STM tip is mounted on a cantilever made from a quartz tuning fork. The instrument operates in ultra-high vacuum at liquid helium temperature. High mechanical stability together with a stiff cantilever design, which avoids snap to contact between sample and tip, allows us to use very small modulation amplitudes of 25 pm normal to the surface. To detect such a small amplitude with a piezoelectric cantilever requires a low-temperature preamplifier stage. Mapping the frequency shift at different heights above the sample surface allows us to calculate the vertical forces acting between tip and surface. This data is then used to determine the full 3D interaction potential between the tip and a single adsorbate on a clean metallic surface by integrating the forces normal to the surface. A small amplitude is essential to achieve 10 pm resolution in all spatial directions necessary to discriminate between long range and short rage forces. With this method we are able to determine the vertical and lateral forces that are required to move individual cobalt (Co) atoms and carbon monoxide (CO) molecules across a copper (111) surface. The lateral forces, which are responsible for moving the adsorbates, are one to two orders of magnitudes smaller than the forces that act in conventional atomic force microscopy with atomic resolution. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N38.00002: Atomically-resolved surface imaging by low temperature atomic force microscopy using a quartz resonator Yukio Hasegawa, Toshu An, Takahiro Nishio, Toyoaki Eguchi, M. Ono, Kotone Akiyama We have developed a frequency-modulation atomic force microscope (FM-AFM) using a length-extension quartz resonator as a force sensor. Atomically-resolved images of the Si(111) 7x7 surface were obtained with the AFM in UHV both at room temperature [1] and 5 K. The high resonance frequency ($\sim $1 MHz) of the resonator improves the sensitivity to its deflection. Its self-sensing property eliminates the cumbersome optical alignment, which is usually required in conventional AFMs, and thus it can be easily installed into a low temperature system. The high stiffness of the resonator enables us to operate with a very small oscillation amplitude; less than 0.1nm, and thus to detect a short-range force effectively, such as a covalent bonding force, which is crucial for the highly resolved imaging. For the probe tip, a tungsten wire was attached at the end of the resonator and sharpened by focused ion beam. The native oxide layer covering the tip was removed by \textit{in-situ} field ion microscopy. [1] T. An, T. Eguchi, K. Akiyama and Y. Hasegawa, APL \textbf{87}, 133114 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N38.00003: Imaging of electronic defect states in SiO2 and HfSiOx films with sub-nanometer spatial resolution by two-way Single Electron Tunneling Force Mircroscopy J.P. Johnson, N. Zheng, C.C. Williams Electronic defects in dielectric materials are currently in sharp focus, for nano-technology and quantum information processing. A novel technique has been developed for imaging these states with sub-nanometer spatial resolution. It can be applied to completely non-conducting dielectric films, in contrast to the STM. The method is based on force detected single electron tunneling events to and from the defect states [1-3]. The exponential dependence of the tunneling rate on tip-sample gap provides the same spatial resolution as STM. An oscillating AFM tip is scanned at constant height above the sample surface. A voltage waveform, synchronous with the tip motion is applied. When the tip is above an accessible state, individual electrons shuttle between tip and state with the applied voltage (300 Hz). The two-way tunneling causes a detectable change in tip resonance. Images of SiO2 and HfSiOx films show a repeatable, random array of individual ``point-like'' defect states, some with sub-nanometer width. Spectroscopic measurements of the defect energy are also performed by this approach. The new method and an analysis of the defects in SiO2 and HfSiOx will be presented. [1] E Bussman et al., Appl. Phys. Lett. 85, 2538 (2004) [2] E Bussman and CC Williams, Appl. Phys. Lett. 88, 263108 (2006) [3] E Bussman et al., Nano Lett. 6, 2577 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N38.00004: Detection of Embedded nanostructures by Electrostatic Force Microscopy Zonghai Hu, Yuanzhen Chen, Michael Fischbein, Robin Havener, Marija Drndic Non-destructive imaging of embedded structures with high lateral resolution is of great technological interest. Scanning probe microscopy is generally thought to be sensitive only to surfaces. We report that electrostatic force microscopy (EFM) can be used to study electrostatic inhomogeneities hundreds of nanometers below a uniform sample surface with sub-micron lateral resolution. The sub-surface material can be in liquid phase. Our experimental and simulation results show that the EFM signal depends on many factors such as the distance between the tip and the sample, the depth, dielectric constants, and the carrier density of the embedded inhomogeneities. Potential applications of this technique will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N38.00005: Sinc or Sine? The Band Excitation Method and Energy Dissipation Measurements by SPM Stephen Jesse, Sergei Kalinin Quantitative energy dissipation measurements in force-based SPM is the key to understanding fundamental mechanisms of energy transformations on the nanoscale, molecular, and atomic levels. To date, these measurements are invariably based on either phase and amplitude detection in constant frequency mode, or as amplitude detection in frequency-tracking mode. The analysis in both cases implicitly assumes that amplitude is inversely proportional to the Q-factor and is not applicable when the driving force is position dependent, as is the case for virtually all SPM measurements. All current SPM methods sample only a single frequency in the Fourier domain of the system. Thus, only two out of three parameters (amplitude, resonance, and Q) can be determined independently. Here, we developed and implemented a new approach for SPM detection based on the excitation and detection of a signal having a finite amplitude over a selected region in the Fourier domain and allows simultaneous determination of all three parameters. This band excitation method allows acquisition of the local spectral response at a 10ms/pixel rate, compatible with fast imaging, and is illustrated for electromechanical and mechanical imaging and force-distance spectroscopy. The BE method thus represents a new paradigm in SPM, beyond traditional single-frequency excitation. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N38.00006: Spring constant calibration of AFM cantilevers with a piezolever transfer standard D. Hurley, E. Langlois, G. Shaw, J. Kramar, J. Pratt Accurate determination of forces in the AFM requires knowledge of the cantilever spring constant $k_{c}$. We describe a method to measure $k_{c}$ traceable to SI units. The transfer standard was a commercial piezoresistive cantilever (``piezolever'') calibrated by the NIST electrostatic force balance (EFB). The active piezolever device eliminates the need to measure the optical lever sensitivity. The method does not depend on cantilever geometry and determines $k_{c}$ under loading conditions. The calibrated piezolever was used to measure cantilevers with nominal values of $k_{c}$ from 0.2 to 40 N/m. Measured values differed by as much as 300 \% from the nominal values. Values of $k_{c}$ were also obtained with four other methods: thermal noise, geometric (Sader), nanoindentation loading, and direct EFB loading. Differences between the direct EFB and piezolever results ranged from 15-20 \% for the stiffest cantilevers to $<$1 \% for the most compliant. Experimental issues critical to accurate measurements with each method will be discussed. Methods will also be compared in terms of implementation in other laboratories. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N38.00007: High Speed Scanning Property Measurements David Shuman, Ramesh Nath, Ramamoorthy Ramesh, Bryan Huey Atomic Force Microscopy (AFM) is a ubiquitous surface science tool, but the slow speed of standard equipment remains a continuing limitation for widespread application. A novel AFM variation is reported here for High-Speed Scanning Property Mapping (HS-SPM), uniquely allowing full-frame nanoscale-resolution image acquisition in $<$3 seconds with tip speeds $>$1 cm/sec. Using off-the-shelf commercial equipment, the method combines acoustic and AFM concepts: the sensitivity of AFM-cantilever contact resonances to materials properties, and conversely the insensitivity of these resonances to contact force variations due to rapidly raster scanning an AFM probe. The method is applicable to a broad range of materials and properties, as demonstrated by mechanical property maps of bacterial membrane fragments and integrated circuits; magnetic property maps of domains in magnetic hard drives; and movies of ferroelectric domain reading and writing with sub-second frame rates for dynamic domain nucleation and growth studies. HS-SPM thereby provides a novel yet off-the-shelf solution for both significantly enhanced throughput in nanoscale materials property mapping, as well as dynamic surface studies with previously inaccessible time constants. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N38.00008: Atomic Force Microscope Tip for Dielectrophoresis J.A. Aguilar, T.P. Hunt, A.C. Bleszynski, R.M. Westervelt Bottom-up fabrication of nanoscale structures has long been an aspiration of the nanotechnology community. We have designed and built an AFM tip with coaxial electrodes that produces a very high field gradient. Dielectrophoresis (DEP) in a strong localized, RF electric field is useful for manipulating nanoparticles in a fluid, performing electric force microscopy, and reading and writing data on ferroelectric materials. The capabilities of an AFM allow imaging the sample before and after manipulation with DEP, and the sharpness of the AFM tip gives high spatial resolution. The inner electrode is formed by a doped Si AFM tip, which is insulated from the grounded outer shield by a thin thermal Si oxide layer. The field lines escape through a small hole in the outer shield cut at the tip's point by a focused ion beam. Currently, the tip diameter is about 300 nm; this can easily be made smaller. This sets the stage for experimentation on the actual manipulation of nanoscale particles with coaxial AFM tips. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N38.00009: Cantilever mean deflection: average tip-sample force in tapping mode spectroscopy F. Michael Serry In tapping mode AFM spectroscopy, tip-sample interaction is nearly always studied in cantilever amplitude and phase. Theory shows that the \textit{mean} deflection is another quantity that carries a wealth of information about the interaction (1). However, mean deflection remains largely unexplored in experiments. One historic reason is tapping mode was invented to avoid relying on static (mean) deflection of cantilever in contact mode AFM. Mean deflection is easier to measure with softer cantilevers, and becomes more important with smaller amplitudes. We present mean deflection data which often contain features with no readily decipherable counterparts in amplitude or phase; validate some theoretical results; and possibly contradict others. The data (vs. mean tip-sample separation) provide a direct, intuitive, experimental proof that phase follows the polarity of average tip-sample force (2). However, the slope of this data does not always follow that of the phase. Average force often plateaus as mean separation reduces and even approaches zero, which may help explain why similarly high-quality images are frequently possible across a range of amplitude setpoint values. (1) A. San Paulo, R. Garcia, Phys Rev B (64), p193411, 2001. (2) R. Garcia, A. San Paulo, Phys Rev B (60), p4961, 1999. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N38.00010: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N38.00011: Monotonic and fatigue tests of amorphous silicon nanostructures using atomic force microscope Churamani Gaire, D.-X. Ye, T.-M. Lu, G.-C. Wang, C. R. Picu The plastic deformation and the failure properties of a-Si slanted nanostructures (one- and two-armed) fixed at one end to the substrate, grown by oblique angle physical vapor deposition, have been studied with the use of AFM. Monotonic loading/unloading tests were carried out to determine the elastic and plastic failure properties. We also developed the fatigue test methodology suitable for nanoscale specimens with the use of AFM. The AFM was used for imaging (to locate) as well as for loading the structures in monotonic bending and force (stress) controlled cyclic loading/unloading mode until the specimen failed completely. A novel way was used to identify the failure of the specimens during the fatigue test. The post-test analysis of the failure surface was done through SEM imaging. The possible inhibition of the brittleness of the a-Si samples with the reduction of the size and damage evolution during fatigue test on the nanoscale specimens will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N38.00012: Enhanced compositional sensitivity in atomic force microscopy by the excitation of the first two flexural modes Ricardo Garcia, Nicolas F. Martinez, Shivprasad Patil, Jose R. Lozano We demonstrate that the compositional sensitivity of an atomic force microscope is enhanced by the simultaneous excitation of its first two normal eigenmodes$^{1-2}$. The coupling of those modes by the non-linear probe-surface interactions enables to map compositional changes in several conjugated molecular materials with a phase shift sensitivity that is about two orders of magnitude higher than the one achieved in amplitude modulation atomic force microscopy. \begin{enumerate} \item T.R. Rodriguez and R. Garcia, Appl. Phys. Lett. 84, 449 (2004) \item N.F. Martinez, S. Patil, J.R. Lozano and R. Garcia, Appl. Phys. Lett. 89, 153115 (2006) \end{enumerate} [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N38.00013: Systematic Variations in Apparent Topographic Height as Measured by Non-contact Atomic Force Microscopy Deng-Sung Lin, T.-C. Chiang, K.M. Yang, J.Y. Chung, M.F. Hsieh, S.S. Ferng A flat Si(100) surface is prepared with neighboring n- and p- doped regions. The contact potential difference between the tip and the two well-defined regions of similar material is utilized to examine the effects and interplay of essential tip- sample forces in atomic force microscopy. Measurements with a frequency-modulated non-contact atomic force microscope (nc- AFM) show large apparent topographic height variations across the differently doped regions. The height differences depend on the bias polarity, bias voltage, radius, and conducting state of the tip. The functional relationships are well explained by integrated model calculations. These findings provide a coherence scenario of nc-AFM operation under these essential forces and facilitate quantitative understanding of the systematic errors in surface topographic height measurement commonly performed in nanoscience. [Preview Abstract] |
Session N39: Focus Session: Emerging Research Devices and Materials for the Microelectronics Industry I
Sponsoring Units: FIAPChair: Jean Heremans, Virginia Polytechnic Institute and State University
Room: Colorado Convention Center 502
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N39.00001: Silicide Nanowires for Low-Resistance CMOS Transistor Contacts. Invited Speaker: Transition metal (TM) silicide nanowires are used as contacts for modern CMOS transistors. (Our smallest wires are $\sim$20 nm thick and $\sim$50 nm wide.) While much research on thick TM silicides was conducted long ago, materials perform differently at the nanoscale. For example, the usual phase transformation sequences (e.g., Ni, Ni2Si, NiSi, NiSi2) for the reaction of thick metal films on Si no longer apply to nanostructures, because the surface and interface energies compete with the bulk energy of a given crystal structure. Therefore, a NiSi film will agglomerate into hemispherical droplets of NiSi by annealing before it reaches the lowest-energy (NiSi2) crystalline structure. These dynamics can be tuned by addition of impurities (such as Pt in Ni). The Si surface preparation is also a more important factor for nanowires than for silicidation of thick TM films. Ni nanowires formed on Si surfaces that were cleaned and amorphized by sputtering with Ar ions have a tendency to form NiSi2 pyramids (``spikes'') even at moderate temperatures ($\sim$400$^{\circ}$C), while similar Ni films formed on atomically clean or hydrogen-terminated Si form uniform NiSi nanowires. Another issue affecting TM silicides is the barrier height between the silicide contact and the silicon transistor. For most TM silicides, the Fermi level of the silicide is aligned with the center of the Si band gap. Therefore, silicide contacts experience Schottky barrier heights of around 0.5 eV for both n-type and p-type Si. The resulting contact resistance becomes a significant term for the overall resistance of modern CMOS transistors. Lowering this contact resistance is an important goal in CMOS research. New materials are under investigation (for example PtSi, which has a barrier height of only 0.3 eV to p-type Si). This talk will describe recent results, with special emphasis on characterization techniques and electrical testing useful for the development of silicide nanowires for CMOS contacts. \newline \newline In collaboration with: P. Grudowski, D. Jawarani, R. Garcia, M.L. Kottke, R.B. Gregory, X.-D. Wang, D. Theodore, P. Fejes, W.J. Taylor, B.Y. Nguyen, C. Capasso, M. Raymond, D. Denning, K. Chang, R. Noble, M. Jahanbani, S. Bolton, P. Crabtree, D. Goedeke, M. Rossow, M. Chowdhury, H. Desjardins, A.Thean. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N39.00002: Atomistic pseudopotential simulation of nanometer sized CMOS devices Lin-Wang Wang, Jun-Wei Luo, Shu-Shen Li, Jian-Bai Xia When the size of a CMOS is shrunk to 10-20 nm, quantum mechanical effects such as individual quantum levels, quantum tunneling, and single impurity fluctuations exhibit themselves. We have developed a method to calculate the electronic structures and I-V curves of million atom CMOS devices using atomistic pseudopotential method. The electronic structure is described by the empirical pseudopotentil. The Hamilotnian is solved using the linear combination of bulk band (LCBB) [1] method in which the electron wavefunction is expanded by a set of bulk Bloch functions. Approximated formulas are developed to describe the carrier occupation and the electron current in a source-drain biased nonequilibrium system. The electrostatic potential is calculated selconsistently by solving the Poisson equation with a given boundary condition and the occupied carried density. We will present the differences between the quantum mechanical results and the traditional semiclassical results in carrier charge density, electron current, turn-on gate potential and short channel effects. [1] L.W. Wang, A. Zunger, Phys. Rev. B 59, 15806 (1999). [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N39.00003: Interfacial engineering and electrical properties of Hf oxide films on InGaAs Lyudmila Goncharova, Ozgur Celik, Eric Garfunkel, Torgny Gustafsson, Niti Goel, Safak Sayan, Wilman Tsai The low chemical stability and poor electrical quality of native oxides, leading to Fermi level pinning, has so far prevented the fabrication of competitive InGaAs{\-}based$^{ }$devices, and made integration of InGaAs with high-$\kappa $ dielectric films a viable option. We have used atomic layer deposition to grow ultra-thin HfO$_{2}$ films on InGaAs with several surface passivation and investigated their interfacial and electrical characteristics after Al gate metal deposition using medium energy ion scattering (MEIS), x-ray photoemission (XPS), high-resolution transmission electron microscopy and electrical measurement. Structures with very thin or no interfacial oxide layer were achieved, as measured both by MEIS and XPS. Surprisingly S-passivated samples revealed that the S-containing layer does not stay at the InGaAs/HfO$_{2}$ interface but floats on top of the HfO$_{2}$ layer during deposition. Interfacial layer formation or Hf diffusion into the substrate was observed after annealing of un-passivated InGaAs devices. Electrical measurements reveal no strong change of capacitance equivalent thickness after the HfO$_{2}$ stack is annealed, although a decrease in C-V stretch out as well as in hysteresis for un-passivated capacitors is observed. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N39.00004: A theoretical investigation of selected silicides and germanides Manish Niranjan, Len Kleinman, Alexander A. Demkov A germanium channel field effect transistor (FET) is being considered for the next generation CMOS technology. New material system requires re-development of most elements of the device. Low resistance contacts compatible with a self aligned process have to be developed based on metal germanides, and germanides with low n- and p-type Schottky barriers (for the use in NMOS and PMOS devices) to germanium channel need to be identified. We report a comprehensive theoretical study within the framework of density functional theory of several germanides (NiGe, PtGe, YGe, Y$_{5}$Ge$_{3,}$ HfGe and HfGe$_{2})$ and compare them with monosilicides of Pt and Ni. We report bulk electronic structure and elastic properties. However, we focus on the surface properties important to thin films such as surface energy, work function and Schottky barrier height. We are able to identify thermodynamic stability fields for surface terminations resulting in work functions consistent with the low Schottky barrier requirement. Several interface structure were also considered which afford a direct evaluation of the barrier height. Germanides have complex phase diagrams, and we find \textit{ab-initio} calculations extremely useful in providing fundamental understanding of the structure-property relations between the crystal structure, chemical composition and atomic structure of the alloy/semiconductor interface on one hand and the Schottky barrier height on the other hand. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N39.00005: Strain-based self assembly of nanostructures for non-destructive large-scale integration E. V. Moiseeva, Y. M. Senousy, C. K. Harnett New types of curved nanostructures, departing from the plane of the substrate yet integrated with microscale contact pads, may be formed by using a strain-based assembly method. This process relies on the strain mismatch between thin films in a bilayer (in our case, metal/insulator or two different metals). By incorporating conducting and insulating materials, this method will be able to integrate active electromechanical micro- and nanostructures into microdevices, such as steerable antenna arrays, thermal nanoactuators, strain-sensitive inductors, electromagnetically resonant metamaterials, and bistable nanomechanical switches. ``Top-down'' lithography and the highly selective XeF$_{2 }$silicon dry etching process are used to obtain our released structures. The strain-based assembly technique requires no alignment step for combining nanostructures with large features, including electrical contacts and other interconnects with the outside world. We will discuss the prospects and limits for obtaining smaller thickness dimensions and lateral dimensions through electron beam lithography. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N39.00006: Electronic Structure and Carrier Mobility in Strain-Engineered Nanostructures Decai Yu, Yu Zhang, Ji Zang, Feng Liu Strain engineering is a major driving force to continue the performance scaling of silicon devices. However, currently strain engineering is confined in planar hetero-structures. It is anticipated that future generation of devices may employ nanostructures and new quantum principles. Here, we present theoretical studies of strain engineered nanostructures for potential device applications. Combining first-principles and finite element calculations, we analyze the electronic band structure and carrier mobility in SiGe nanotubes and Si nanomembranes that are strain-modulated by Ge quantum dots. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N39.00007: Experimental and Theoretical Investigation of Si(001)/Si (110) Junctions Adrian Ciucivara, Sachin Joshi, B.R. Sahu, Sanjay Banerjee, Leonard Kleinman We have observed large current asymmetries in Si(001)/Si(110) junctions where both sides are identical in all respects, except orientation. With a 280 atom GGA supercell calculation using the VASP ultrasoft pseudopotential code, we have obtained an adhesion energy of 110 meV/{\AA}$^{2}$. The covalent and dangling bonds at the interface are displayed. The Si(110) potential averaged over a (110) interior unit cell was found to be 85 meV more negative (positive for holes) than the Si(001). This offset was used in a device simulator to simulate the behavior of the junction. Qualitative agreement with the experimental I-V characteristics was obtained. We will discuss possible errors introduced in the offset by the GGA energy gap error. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N39.00008: Electron Transport in SiGe single-electron transistors Ian Gelfand, Jian Liu, Ya-Hong Xie, Marc Kastner It is expected that electron spins in two dimensional electron gasses (2DEGs) in SiGe heterostructures will have longer spin coherence times than GaAs 2DEGs. We have fabricated quantum point contacts and single electron transistors in this material system using palladium Schottky gates. We find that these gates can deplete the 2DEG with negligible leakage if the area of the gates is minimized, as shown by previous workers.$^{1}$ 1 K.A. Slinker et. al., New Journal of Physics 7 246 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N39.00009: Raman Spectroscopy Study of Uniaxial Strained SOI with SiGe Junctions Yan Du, Mehmet Ozturk, Veena Misra, Johnson Kasim, Zexiang Shen In bulk PMOSFETs, selective epitaxial Si$_{1-x}$Ge$_{x}$ junctions have been used to introduce strain into the channel for mobility enhancement purposes. Freescale has applied this idea to SOI wafers where they demonstrated that mobility enhancement is prominent for 400A partially depleted SOI PMOSFETs. But the scaling capability of this technology for very thin SOI wafers needs to be verified. In this report, we studied the impacts of body thickness and recessing on thin SOI films down to 200A. UV-raman data confirms that even without recessing, except for silicon consumed during RCA cleaning step, epitaxial Si$_{0.5}$Ge$_{0.5}$ still introduces a certain amount of strain into the channel. This is beneficial for fully depleted SOI applications, in which the ultra thin body presents challenges for RIE etching. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N39.00010: Low-temperature transport in Ga-implanted wires in Si S.J. Robinson, J.R. Tucker, T. Schenkel, T.-C. Shen Focused ion beams (FIBs) have potential applications in maskless device fabrication. Specifically, the narrow beam diameter and large scan range allow for the possibility of creating nanoscale interconnects (which must be highly conductive and ohmic) or even quantum devices. Here we report a low-temperature ($<$ 20 K) transport study of Ga wires created by both conventional implantation and a commercial FIB system. We find that the FIB wires yield nonlinear sheet resistances that are much higher than those of the conventional wires, which remain metallic and ohmic below 1 K. In addition, although both types of wires have positive magnetoresistance, the FIB wires show a much greater magnetic effect. The apparent conduction mechanism in our FIB wires is variable-range hopping, which transitions into Efros--Shklovskii transport and yields a Coulomb gap in $I$--$V$ measurements as the temperature is lowered. The effects of dose and annealing in conductivity will be discussed in the context of lattice defects, Ga clustering, and solid solubility. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N39.00011: On the Origins of Non-Exponential, Pulse Bias Induced, Capacitance Transients in Semiconductor PN Junctions Walter R. Buchwald, Peter J. Drevinsky, Christian P. Morath This work presents an analytical investigation of the temporal dependence of the pulse bias induced capacitance transient associated with an abrupt semiconductor PN junction. With minimal assumptions, Poisson's equation is used to derive a general equation for the capacitance transient which reduces to the expected exponential form only at low defect concentrations. The effect of this non-exponential transient on deep level transient spectroscopy experiments is investigated. It is shown that with increasing defect concentration, shifts in DLTS peak height maxima with respect to temperature are expected. Simulations also reveal that under certain conditions, deep level transient spectroscopy peak heights can have different magnitudes even though the defects producing the peaks have identical defect concentrations. The experimental conditions over which this general, non-exponential form, can be replaced by the purely exponential approximation are also reported. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N39.00012: Tuning of plasmonic resonance via modification of the shape factor of silver pillars embedded in nanopatterned silicon Jeffrey Shainline, Jimmy Xu We present a study of the feasibility of achieving ``complete resonance'' [1] in a system where silicon-on-insulator (SOI) is coupled to silver nanopillars. The SOI structure is nanopatterned using reactive ion etching through an anodized aluminium oxide etch mask to contain a periodic array of pores with radius 50nm and pitch 100nm. The resultant pores are filled with silver. The plasmonic response of the silver is studied. Attempts are made to achieve complete resonance by 1) tuning the dielectric environment of the silicon by injecting optical gain (tuning the imaginary part of the dielectric function) and by 2) modifying the shape factor of the embedded silver pillars by changing the shape of the pores in the template used for the etching. In this talk the theoretical elements will be briefly reviewed and results of recent experiments will be presented. [1] A. Smuk and N. Lawandy, Appl. Phys. B 84, 125 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N39.00013: Electro-Optic Films for Transmission-Mode Wavelength Demultiplexing Applications Kashma Rai, Anna Fox, Adam Fontecchio Applications for a thin-film switchable wavelength sensing device include spectral detection of telecom signals as well as chemical or biological sample identification through absorption or emission spectroscopy. The proposed device consists of configurations of holographically formed polymer dispersed liquid crystal (H-PDLC) thin film gratings for transmission-mode spectral filtering. H- PDLC films have the unique ability to selectively transmit a particular wavelength as a function of bias applied across the film. The initial configuration includes a serial wavelength sensing device formed by stacking layers of H-PDLC films. A second configuration includes parallel sensing of the spectral content by fabrication of an H-PDLC grid within a single film. The films fabricated for this study were made of a thiolene based monomer syrup with grating notches formed in the near infrared. Results of both switchable wavelength sensing systems are compared and evaluated. [Preview Abstract] |
Session N40: Semiconductors: Magneto-optics and Dynamics
Sponsoring Units: FIAPChair: David Reitze, University of Florida
Room: Colorado Convention Center 503
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N40.00001: Cyclotron Dephasing Times in the Quantum Hall Regime X. Wang, D. J. Hilton, L. Ren, D. M. Mittleman, J. Kono, J. L. Reno, M. M. Fogler We have used time-domain THz spectroscopy to study a high-mobility GaAs two-dimensional electron gas in quantizing magnetic fields. We observe very long-lived (up to $\sim $30 ps) coherent cyclotron oscillations, which can be viewed as the free induction decay of a coherent superposition between the lowest-unfilled Landau level and the highest-filled Landau level induced by the incident coherent THz pulse. From the frequency and decay time of these oscillations, we can directly determine the cyclotron mass and dephasing time. The magnetic field dependence of the dephasing time shows an oscillatory behavior, which can be due to the filling factor dependent dielectric screening of long-range potentials. The temperature dependence of the dephasing time at a fixed magnetic field shows three pronounced regions where different scattering potentials dominate. The two high temperature regions have standard interpretations while the lowest temperature range has not been observed before, to our knowledge. A sharp increase in dephasing time is observed below $\sim $ 5 K, which could be due to strong screening enhanced by the spin splitting of Landau levels. We provide detailed theoretical calculations to explain these observations. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N40.00002: Theory of Cyclotron Resonance in Si,Ge,Zn and Cd Shigeji Fujita, Robert Simion, Rohit Singh, Rohit Singh, Seichi Watanabe, Salvador Godoy A quantum theory is developed for the cyclotron resonance (CR) in Si and Ge by introducing the concept of the Cyclotronic Planes (CP) in which the conduction electrons (``electrons'',``holes'') complete circulations. The angular dependent CR peaks for heavy ``holes'' are analyzed, using the Dresselhaus-Kip-Kittel (DKK) formula: $\omega=(\omega_{t}^{2}cos^2\theta + \omega_{t}\omega_{l}sin^2 \theta)$, $\omega_{t}\equiv eB/m_{t}$, $\omega_{l}\equiv eB/m_ {l}$.The Fermi surfaces for Si(Ge) are spheroids oriented along $\left\langle 100\right\rangle$ axes with the transverse mass $m_{t}=0.46(0.29)m$ and the longitudinal mass $m_{l}=1.03(0.78) m$.The fluted energy surfaces used by DKK were avoided.The angular-independent CR peaks for light ``holes'' in Ge(Si) arise from the spherical Fermi surface with the effective mass $m_{l} =1.03(0.78)m$ with the CP $\left\{100\right\}$. The reason why there are light and heavy ``holes'' with the same CP in $\left\{100\right\}$ is explained by decomposing the fcc lattice in two sets of sublattices. The theory is extended to treat the CR peaks in Zn and Cd, both hexagonal-close-packed metals. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N40.00003: Infrared longitudinal and Hall conductivity of SrRuO$_3$ and Ga$_{1-x}$Mn$_x$As films obtained by magneto-polarization measurements M.-H. Kim, G. Acbas, M.-H. Yang, J. Cerne, I. Ohkubo, H. Christen, D. Mandrus, M.A. Scarpulla, O.D. Dubon, Z. Schlesinger By measuring the changes in the polarization of transmitted (Faraday effect) and reflected (Kerr effect) mid-infrared (MIR: 115-366 meV) light induced by an external magnetic field, we determine the complete complex magneto-conductivity tensor in SrRuO$_3$ and Ga$_{1-x}$Mn$_x$As films. Thick film transmission and reflection equations are used to convert the measured complex Faraday and Kerr angles into the MIR complex longitudinal conductivity $\sigma_{xx}$ and the complex transverse (Hall) conductivity $\sigma_{xy}$. The resulting $\sigma_{xx}$ is consistent with the values obtained from conventional transmittance and reflectance measurements on these films, as well as the results from Kramers-Kronig analysis of reflectance measurements on similar films. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N40.00004: Time-resolved Faraday rotation measurements of spin relaxation in InGaAs/GaAs quantum dots Jennifer Robb, Ye Chen, Adam Timmons, Oleg Shchekin, Dennis Deppe, Kimberley Hall We report measurements of room temperature spin dynamics in InGaAs quantum dots using time-resolved differential transmission and Faraday rotation techniques. We observe an enhancement of the electron spin lifetime by an order of magnitude for direct optical pumping of the quantum dot ground state compared to optical pumping of the GaAs barriers. These findings indicate that the optical excitation conditions can have a critical influence on the spin kinetics, a result which may account for the wide variation of spin lifetimes reported to date. The observed enhancement in spin lifetime is attributed to the reduction of phonon-mediated spin-flip scattering. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N40.00005: Measurement of the GaSb surface band bending potential from the magnetotransport characteristics of GaSb-InAs-AlSb quantum wells Patrick Folkes, Godfrey Gumbs, Wen Xu, M. Taysing-Lara Low-temperature magnetotransport measurements on GaSb/InAs/AlSb coupled quantum well structures with a GaSb cap layer and self-consistent calculations of their electronic structure have led to the determination of the Fermi level at the surface, E$_{FS}$, of undoped molecular-beam-epitaxy-grown GaSb. E$_{FS}$ is pinned around 0.2 eV above the top of the GaSb valence band when the GaSb cap layer is width is greater than 900 {\AA}. For smaller GaSb cap widths, E$_{FS}$ decreases with the GaSb width. The heterostructures' Fermi level is determined by bulk donor defects in the AlSb layer adjacent to the InAs quantum well. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N40.00006: Magnetoexcitons in InSb Quantum Wells William Gempel, Taroshani Kasturiarachchi, Ryan Doezema, Madhavie Edirisooriya, Tetsuya Mishima, Michael Santos, Gary Sanders, Christopher Stanton We report on an experimental and theoretical study of excitons in strained InSb quantum wells with Al$_{x}$In$_{1-x}$Sb barriers. Perpendicular magnetic fields of 0$<$B$<$8T are applied in far infrared transmission measurements at a temperature of 4.2K. We observe rich spectra of excitonic transitions between hole and electron subbands in square and parabolic wells. Strain and confinement lift the degeneracy between the light and heavy holes. These data are in the large B regime since the electron effective mass is small (0.014m$_{o})$ and the dielectric constant is large (18) in InSb. Spin splitting is well resolved due to the large g-factor (-51) for electrons in InSb. We use a modified Bauer-Ando theory to identify the magnetoexciton transitions and to explain observed anti-crossing behavior. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N40.00007: Time-resolved infrared spectroscopy of a DMS InGa(Mn)As film H. Zhang, D.H. Reitze, C.J. Stanton, D.B. Tanner, G. Khodaparast, J. Kono, H. Munekata, G.L. Carr We report a two-color, time-resolved, differential absorption study of the dilute magnetic semiconductor InGa(Mn)As. We varied the temperatures above and below the nominal Curie point of 50 K. The sample is an (In0.53Ga0.47)0.87Mn0.13As layer grown on InGaAs buffer layers on InP. The study was performed using near-IR pulses from a Ti:sapphire laser to photoexcite the sample and pulsed far-infrared synchrotron radiation (at beamline U12IR of the NSLS) to probe the relaxation process. Our time resolution is $\sim $200 ps. A two-step decay process was observed, with the initial decay in the few ns range followed by a much slower decay lasting $\sim $100 ns or longer. The relaxation time for the faster decay was found to increase as the temperature decreased below 60 K. The slower decay may be due to thermal relaxation of the sample into its surroundings. Details of the photo-induced far-infrared spectra, as well as the dependence on photoexcitation fluence and wavelength, will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N40.00008: Effects of a parallel magnetic field on the Fermi-energy in two-dimensional electron systems Y.D. Jho, X. Wang, D.H. Reitze, J. Kono, S.A. Crooker, C. Kadow, A.C. Gossard, M. Semtsiv, W.T. Masselink The transport properties in 2DEG systems are determined by the electrons near the Fermi energy, and whether those electrons are localized or delocalized determines the character of the material, be it metallic or insulating. Here, we investigate whether there is any optical process that is sensitive to the conductivity change of the electrons near the Fermi energy, and report results of correlated experimental studies of interband optical spectra and transport properties in a two-dimensional electron gas placed in an in-plane magnetic field up to 45 T. Our magneto-absorption experiments show an initial red shift of the Fermi energy with increasing magnetic field, while photoluminescence strength at the Fermi-energy edge singularity reveals a quenching behavior only at low temperature along with resistivity increment as the multiple electron-electron scattering is being suppressed. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N40.00009: Magnetic field induced Raman excitations in Zn$_{1-x}$Cr$_{x}$Te, Cd$_{1-x}$Cr$_{x}$Te and Cd$_{1-x}$Cr$_{x}$Se X. Lu, S. Tsoi, I. Miotkowski, S. Rodriguez, A.K. Ramdas, H. Alawadhi, T.M. Pekarek Raman electron paramagnetic resonance(Raman-EPR) of the transitions due to the $\Delta $m$_{s}=\pm $1 spin-flip of the 3d electrons of Cr$^{+}$ in Zn$_{1-x}$Cr$_{x}$Te and Cd$_{1-x}$Cr$_{x}$Te are observed at $\omega _{PM}$=g(Cr$^{+})\mu _{B}$B, g(Cr$^{+})$=2.0041$\pm $ 0.0095 and 2.0039 $\pm $ 0.0093, respectively. Raman lines appear at $\omega _{LO}\pm $ n$\omega _{PM}$, n=1,2 and 3, resulting from the strong Fr\"{O}hlich interaction with LO phonon. The intensity of $\omega _{PM}$ can be enhanced through the photo-generation process Cr$^{2+}\to $ Cr$^{+}$; photoluminescence spectra reveal signatures of excitons bound to Cr$^{+}$ acceptors in Zn$_{1-x}$Cr$_{x}$Te. The resonance profile of $\omega _{PM }$shows that the strong resonant enhancement is mediated via an exciton bound to a neutral acceptor. Spin flip Raman scattering (SFRS) at $\omega _{SFR}$ from donor-bound electrons in Cd$_{1-x}$Cr$_{x}$Se, as well as in pure CdSe, are observed, in turn yielding the s-d exchange energy. The magnetization of Cd$_{1-x}$Cr$_{x}$Se is intermediate between van Vleck and a B$_{2}$ Brillouin paramagnetism. The linear dependence of the s-d exchange energy as a function of magnetization yields the s-d exchange constant in Cd$_{1-x}$Cr$_{x}$Se, $\alpha $N$_{0}$=(213.7 $\pm $13) meV. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N40.00010: Aharonov-Bohm oscillations in Type-II ZnSe$_{1-x}$Te$_{x}$ excitons. I.R. Sellers, V.R. Whiteside, B.D. McCombe, I.L. Kuskovsky, W. MacDonald, A.O. Govorov The luminescence of ZnSe$_{1-x}$Te$_{x }$has been studied extensively and it is generally accepted that the emission of this system is dominated by excitons bound to isoelectronic centers [1]. We will show that when these isoelectronic centers group into large clusters they appear to form type-II quantum dot-like structures (QDs). In such structures the hole is strongly localised within the dot while the electron is localised outside through coulombic attraction. These localised excitons are analogous to the hydrogen atom with the electron orbiting the confined hole in a `ring-like' geometry. We will show that as a result of this behaviour we observe Aharonov-Bohm (AB) oscillations in the magneto-photoluminescence of excitons bound to QDs formed with various configurations of isoelectronic centers (atoms, clusters) [2]. The behaviour of QD-like structures of different energy will be presented and it will be shown that in the case of QDs formed by larger Te-clusters the AB oscillations are preserved up to 180K. Finally, we will also present the optically detected resonance for these structures, in which a FIR laser is used to probe resonances related to the atomic-like higher order shells of the bound excitons. [1] I. L. Kuskovsky et al. Phys. Rev. B. (2001) [2] I. L. Kuskovsky et al. Cond-mat/0606752 [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N40.00011: NMR Probing Spin Excitations in the Ring-Like Structure of a Two-Subband System Xinchang Zhang, Gavin Scott, Hongwen Jiang Resistively detected nuclear magnetic resonance (NMR) is observed inside the ring-like structure, with a quantized Hall conductance of 6$e^2/h$, in the phase diagram of a two subband electron system. The NMR signal persists up to 400 mK and is absent in other states with the same quantized Hall conductance. The nuclear spin-lattice relaxation time, $T_1$, is found to decrease rapidly towards the ring center. These observations are consistent with the assertion of the ring-like region being a ferromagnetic state that is accompanied by collective spin excitations. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N40.00012: The dynamics of Bloch electrons in 2D hexagonal systems M. J. Rave, W. C. Kerr We investigate the semiclassical equations of motion (EOM) for Bloch electrons in a 2D hexagonal lattice. When a system has a non-zero Berry curvature, there are additional terms in these EOM beyond the familiar group velocity and classical electric and magnetic force terms. We investigate the existence and consequences of these terms for Bloch electrons in a 2D hexagonal tight-binding model, parameterized so that it can represent either graphene or a single layer of boron nitride (h-BN). We calculate these terms in the vicinity of a K point in the first Brillouin Zone of h-BN using perturbation theory carried to sufficiently high order to give a non-constant Berry curvature. These terms can be written in terms of momentum operator matrix elements, which we estimate using the tight-binding wave functions. We show the effects of these terms on electron trajectories in the presence of external fields. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N40.00013: Optical phonon spectra of CdS crosslinked sulfonate polystyrene nanocomposites Jayesh Govani, Felicia Manciu, S. Ortiz-Col\'on, Matthew Espe, Ronald Ziolo We have used IR transmission and FT-Raman spectroscopy to study optically active phonon modes of CdS nanoparticles synthesized in sulfonated polystyrene resin and obtained information about the morphology, crystallinity, and surface interactions. The dominant feature in the far-infrared region of CdS/polystyrene nanocomposites spectra is a sharp peak centered at 255 cm$^{-1}$, which could be assigned to the transversal optical mode at the L edge of the Brillouin zone of CdS nanoparticles. Also, this vibrational line, based on theoretical core-shell model calculation, could be attributed to the presence of a very thin CdS shell layer. HRTEM images of the CdS nanocomposites show CdS nanoparticles of about 2.5 nm aligned in rows or strings on the polymer surface. Amorphous CdS is also present and may be seen surrounding the nanocrystalline regions. Complementary solid state $^{113}$Cd NMR analysis will be presented as well. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N40.00014: Phonon-assisted coherent control of injected carriers in indirect bandgap semiconductors Julien Rioux, Fred Nastos, John E. Sipe Charge and spin currents can be generated in direct semiconductors by quantum interference between one- and two-photon absorption\footnote{M.J. Stevens, R.D.R. Bhat, A. Najmaie, H.M. van Driel, J.E. Sipe and A.L. Smirl, in \emph{Optics of Semiconductors and Their Nanostructures}, edited by H. Kalt and M. Hetterich (Springer, Berlin, 2004), vol. 146 of Springer Series in Solid-State Sciences, p. 209.}. For semiconductors such as Si and Ge, optical injection of carriers over the indirect bandgap must be assisted by momentum transfer from phonon scattering. We consider the optical properties for such 1+2 photon processes in the presence of the electron-phonon interaction. The latter is modelled by acoustic deformation potential. Indirect transitions involve double Brillouin zone integrations, which are computed by a linearized tetrahedron method. We compare our results to those for bulk GaAs. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N40.00015: \textit{Ab Initio} XAS Debye-Waller Factors Beyond the Harmonic Approximation Fernando Vila, H. H. Rossner, H. J. Krappe, J. J. Rehr We introduce an \textit{ab initio} approach to calculate the temperature dependent vibrational effects in x-ray absorption spectra beyond the harmonic approximation. Instead of relying on empirical models, we apply electronic structure theory to determine the dynamical matrix of the system, from which the appropriate vibrational densities of state can be obtained using a Lanczos recursion algorithm [2]. By combining thermodynamic perturbation theory and the quasi-harmonic approximation we obtain x-ray absorption fine structure (XAFS) cumulants such as the mean square relative displacement (2nd cumulant), the thermal expansion (first cumulant), the asymmetry of the distribution (third cumulant) and the perpendicular motion contribution to the DW factor. Other quantities of interest such as mean square atomic displacements are also discussed. {[2]}H.J. Krappe and H.H. Rossner, Phys. Rev. B\textbf{70}, 104102 (2004). [Preview Abstract] |
Session N42: Focus Session: STM of Surface-Based Nanostructures
Sponsoring Units: DMPChair: Arthur Smith, Ohio University
Room: Colorado Convention Center 505
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N42.00001: Roles in Modulation of Molecular Structures on Metal Surfaces Invited Speaker: We studied the adsorption of organic molecules, their growth behavior, and their physical properties on silver and gold surfaces at the single molecule or sub-molecular scale by using low-temperature scanning tunneling microscopes. Combined with low energy electron diffraction and first-principles density functional theory calculations, the key parameters in modulating molecular structures on metals are analyzed. It is found that the alkyl chains of quinacridone derivatives (QA) determine the orientation of molecular overlayers on an Ag(110) substrate. The interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to specific positions at the silver substrate, determining the molecular orientation and preferred adsorption site. However, the intermolecular arrangement can be adjusted via the length of attached alkyl chains. We are thus able to fabricate uniform QA films with very well controlled physical properties. Furthermore, by thermal and chemical control, we are able to self-assemble three dimensional molecular nanostructures, e.g. ordered PTCDA structures exclusively on flat Ag(111) facets, or DMe-DCNQI structures exclusively on stepped Ag(221) facets. It is demonstrated that bonding, the key factor for selectivity, occurs via the end-atoms, while the molecule's mid-region arches away from the substrate. Theoretical results, obtained by high-level theory, are consistent with the experimental observations, which have previously been interpreted in terms of bonding through the mid-region. \newline \newline In collaboration with D.X. Shi, S.X. Du, W. Ji, Z.T. Deng, L. Gao, Institute of Physics, and X. Lin, Chinese Academy of Sciences, China; C. Seidel and H. Fuchs, Universit\"at M\"unster, Germany; W.A. Hofer, The University of Liverpool, Britain; and S. T. Pantelides, Vanderbilt University, USA. \newline \newline [1] D.X. Shi et al., Phys. Rev. Lett. 96, 226101(2006). \newline [2] S.X. Du et al., Phys. Rev. Lett. 96, 226101(2006). \newline [3] L. Gao et al., Phys. Rev. B 73, 075424(2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N42.00002: Molecular organization: the role of substrate interaction Martin Oehzelt, Leonhard Grill, Stephen Berkebile, Georg Koller, Falko P. Netzer, Michael G. Ramsey The influence of the surface chemistry is studied for para-sexiphenyl (6P) grown on clean, partly and fully oxygen reconstructed Cu(110). LT-STM, NEXAFS and ARUPS measurements were carried out to determine the exact geometry (orientation and registry) and the nature of the bond of the molecules to the substrate. On clean copper and on the partly reconstructed surface the molecules lie flat on the surface, having a similar strong bond due to backdonation, but are adsorbed in opposite directions: [1-10] and [001], respectively. On the full reconstructed surface the 6P molecules are pointing in [001] direction and their aromatic planes have a significant tilt angle to the substrate arising from the weak bond to the surface and adopting the surface corrugation. Because of the flexibility of the Van der Waals interaction, dominating the growth on this surface, the resulting layer is similar to a 6P(20-3) bulk crystal plane but modified due to the constraints of commensurability. This increase in the tilt angle exhibits a new stress release mechanism unknown and inaccessible for inorganic heteroepitaxy. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N42.00003: Self-assembly of methanethiol on cluster arrays of Co/Au(111) Georgi Nenchev, Bogdan Diaconescu, Karsten Pohl Self-assembly on strained metallic interfaces is an attractive option for growing highly ordered multi-functional nanopatterns. We present a Variable Temperature STM and Auger Electron Spectroscopy study of selective adsorption of sulfur-terminated CH$_{3}$SH molecules on the lattice of Co clusters on Au(111). We investigate the growth of a uniform network of Co on the reconstructed Au(111) surface, the temperature evolution of the island height and the termination, and the onset of surface alloying. Further we will show the evolution of morphology of the CH$_{3}$SH film on Au (111) as a function of coverage and temperature, and the importance of the herringbone reconstruction for the SAM formation and orientation. Successful combination and control of these two processes leads to the creation of an ordered, stable patterned Co/CH$_{3}$SH heterostructure with nanometer-sized unit cell. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N42.00004: An STM Study of Nucleation and Growth of Co Nanostructures on Stepped Cu(775) Nader Zaki, Denis Potapenko, Richard Osgood, Jr., Peter Johnson We conduct an STM study of nucleation and growth of Co nanostructures on stepped Cu(775) surface. This surface has a relatively narrow terrace width of 1.4nm, which should allow a different growth mode than on previously studied step-edge growth of Co bilayer nanoislands on Cu(111). Growth of other metals on narrow stepped surfaces is known to favor step-edge nanowire formation. On the bare Cu(775) surface, STM imaging at 300K is blurred by Cu-atom surface diffusion; low-coverage Co deposition modifies this behavior by step pining. The effects of deposition rate and substrate temperature are investigated, and specific conditions for Co nanowire growth and stability will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N42.00005: Size dependent superconductivity of nano-sized Pb islands studied by low temperature scanning tunneling spectroscopy under magnetic field Takahiro Nishio, Masanori Ono, Toyoaki Eguchi, Hideaki Sakata, Yukio Hasegawa We performed scanning tunneling microscopy/spectroscopy at low temperature ($<$2 K) on atomically-flat nano-sized Pb islands formed on the Si(111)-7x7 substrate. The measured tunneling spectra revealed that the superconducting gap does not dependent on the sites in a single Pb island but depends on the lateral size of islands. These are consistent qualitatively with the results of a theoretical calculation which includes the fluctuation of superconductivity[1]. We also investigated superconductivity of Pb islands under magnetic fields up to 2.1 T. The superconducting gaps were still observed above the critical magnetic field of bulk Pb. In addition, the obtained spectra showed the decrease in the conductance at zero bias voltage when the island size is small. The conductance decrease can be explained with Gor'kov equations on superconducting spheres whose size is smaller than the coherence length. [1] T. Nishio \textit{et al}., APL \textbf{88}, 113115 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N42.00006: STM and LEEM Observations of Pb Growth on W(110) Shirley Chiang, Donell Hoffman We have recently used both scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) in a combined UHV system to study the growth of Pb on W(110). As seen in previous studies, Stranski-Krastanov growth occurs. STM images show rows of Pb islands. A critical mass of Pb is required before condensation of clusters into stacks occurs at room temperature, while at 200C the Pb immediately formed distinguishable stacked islands. After the completion of the first monolayer, LEEM observations of this system show the development of 3D Pb crystallites, with the island density depending strongly on temperature. For Pb deposition at a substrate temperature of 200C, the islands grow together and form larger islands with quasi-hexagonal sides. Annealing the Pb crystallites causes them to merge and reshape while maintaining long-range order. The crystallites melt at the usual Pb melting temperature of 323C, resulting in small, round, disordered islands. Upon recooling, the islands develop a hexagonal shape. They desorb from the surface at approximately 425C. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N42.00007: STM investigation of quantum size effect on adsorption and reactivity of different gases and alkali metals on thin Pb films Alexander Khajetoorians, Shengyong Qin, Murat Ozer, Chih-Kang Shih Recent work has shown that the Quantum Size Effect (QSE) plays a critical role in the catalytic behavior in reactivity. More specifically, the presence of quantum well states in thin metal systems can have profound effects on surface reactivity. Epitaxial thin Pb films on Si(111) are well known to exhibit pronounced QSE manifested by the phase matching of the Fermi wavelength and the layer thickness, giving rise to bilayer oscillation as well as a re-entrant quantum beats of longer periodicity. Such quantum oscillation phenomena have been observed in preferred film thickness, the location of quantum well states, as well as superconductivity. This work focuses on studies of adsorption and surface reactivity of different gases (hydrogen, oxygen and carbon monoxide) and alkali metal on thin Pb films grown on Si(111) surface. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N42.00008: Automated Tracking of Nanometer-Scale Feature Evolution Using an STM Russell Lake, Adam Dean, Niru Maheswaranathan, Chad Sosolik Time-resolved measurements of vacancy pits and adatom islands on monatomic metallic surfaces (e.g. Ag(111) [1]) have provided valuable insight into the underlying atomic diffusion processes that drive dynamics at nanometer length scales. Utilizing our variable temperature scanning tunneling microscope or STM, we are extending this probing method to more complex systems, such as the AuCu and NiAl alloys. To increase the rate of successful data acquisition for these measurements, we have developed automated tracking routines that allow for the continuous monitoring of evolving surface features with minimal operator involvement. Post-acquisition image analysis is further enhanced utilizing feature detection algorithms. Current proof-of-concept results spanning several hours of acquisition time on single crystal metal surfaces are presented. [1] K. Morgenstern et al., Phys. Rev. B 63, 045412 (2001). [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N42.00009: Chemical Identification in the Cu$_3$Au (100) Surface Using STM and DFT Rodrigo B. Capaz, Luis G. Dias, Alexandre A. Leit\~ao, Ralf-Peter Blum, Horst Niehus, Carlos A. Achete We describe the structure, energetics and electronic structure of the Cu$_3$Au (100) surface using a combination of scanning tunneling microscopy (STM) and first-principles calculations based on density functional theory (DFT). Our calculations show that the CuAu termination is the one with lower surface energy, in agreement with experiments. The well-known surface atomic rippling is also well reproduced by the calculations. Atomically-resolved STM images show an interesting voltage dependence, showing both types of atoms in the surface unit cell for lower voltages but just one type for higher voltages. Comparisons with theoretically-simulated STM images and cross- sectional electronic density profiles allows for an unambiguous assignment of Au atoms as the one appearing in higher voltage images, thus providing chemical identification at the surface. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N42.00010: STM characterization of a graphitized SiC(0001)surface Victor Brar, Yuanbo Zhang, Yossi Yayon, Taisuke Ohta, Jessica McChesney, Eli Rotenberg, Mike Crommie The two-dimensional electron gas in a single graphene sheet exhibits unique properties due the cone-shaped electron band structure near the Fermi energy. Recently the growth of a single layer of graphene on SiC(0001) has been demonstrated, opening new possibilities for fabricating large scale graphene-based devices. We have performed scanning tunneling microscopy and spectroscopy of single and bi-layer graphene films on SiC(0001). Atomically resolved topographs and dI/dV maps show clear differences between the single and bi-layer surfaces at different length scales. We have characterized the energy dependence and spatial distribution of the electron local density of states in these single and bi-layer films. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N42.00011: Calibrating the Bending of Molecule Adsorbed Nanoscale Si Cantilevers with a Modified Stoney Formula Ji Zang, Feng Liu Fundamental understanding of mechanical bending of molecule adsorbed nanoscale thin films is of both scientific and technological importance. Our current understanding, however, is limited within macroscopic analysis that neglects the atomic details of film structure and surface effects. Here, we report atomistic simulation and theoretical analysis of bending of freestanding nanometer-thick silicon (Si) films induced by adsorption of hydrogen and acetylene molecules. It reveals the dominant role of atomic surface structure and surface stress in governing their bending behavior. We show that the bending curvature of molecule adsorbed Si nanofilm does not follow the classical Stoney formula, and we develop a modified Stoney formula by taking into account of the effects arising from atomic surface reconstruction and surface stress. Our findings suggest that re-calibration has to be made in detecting trace amount of molecules by nanoscale Si mechanochemical sensors. [Preview Abstract] |
Session N43: Focus Session: Physics of Thermoelectric Materials and Phenomena III
Sponsoring Units: FIAP DMPChair: Ctirad Uher, University of Michigan
Room: Colorado Convention Center 506
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N43.00001: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N43.00002: Thermoelectric and Thermomagnetic Properties of Nanostructured Lead Chalcogenides. Suraj Joottu Thiagarajan, Troy Pyles, Ramachandra R. Revur, Vladimir Jovovic, Joseph D. West, Joseph P. Heremans, Suvankar Sengupta, J. Richard Schorr We report results of a study of the thermoelectric and thermomagnetic properties of nanostructured PbTe/PbSe alloys. By the four-coefficients method, the carrier density, effective mass, mobility and scattering parameter are obtained which could then be used for optimizing the material. The samples show higher thermopower than bulk. This enhancement is due to the energy filtering induced by change of the scattering parameter, which in turn, could be due to the electronic density of states altered by the quantum dot inclusions. This is a promising lead to high performance nanostructured thermoelectric materials manufacturable in bulk quantities. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N43.00003: Transport and magnetic properties of dilute rare-earth-PbSe alloys V. Jovovic, S. Joottu-Thiagarajan, J. West, J. P. Heremans, T. Story, Z. Golacki, W. Paszkowicz , V. Osinniy An increase in the density of states is predicted [1] to increase the thermoelectric (TE) figure of merit, and could be induced by doping TE materials with rare-earth elements. This was attempted here: the galvanomagnetic and thermomagnetic properties of dilute alloys of PbSe and Ce, Pr, Nd, Eu, Gd and Yb were measured from 80 to 380K; magnetic susceptibilities were measured from 4 to 120K. The density of states effective mass, the relaxation time, and the carrier density and mobility are calculated from measurements of the electrical conductivity and the Hall, Seebeck and transverse Nernst-Ettingshausen coefficients. The Eu, Gd, Nd and Yb-alloyed samples are paramagnetic; the concentrations of rare-earth atoms are determined from fitting a Curie-Weiss law. The magnetic behavior of the Ce and Pr-alloyed samples is different. Ce, Pr, Nd, Gd and Yb act as donors with efficiencies that will be reported. Alloying with divalent Eu does not affect carrier density but increases the energy gap. This work suggests that the 4f orbitals preserve their atomic-like localized character and exhibit only weak sp-f hybridization. 1 G. D. Mahan and J. O. Sofo, Proc. Natl. Acad. Sci. USA 93 7436 (1996) [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N43.00004: Synthesis \& Properties of Nano-Composite Thermoelectric Materials Invited Speaker: PbTe nanocrystals have been grown in our labs by chemical vapor deposition. These materials grow in size selective regions exhibiting very high yield and have size distributions of around 100 nm to 1000 nm. These nano-materials are incorporated into a bulk matrix, making a composite material in hopes of achieving a higher thermoelectric performance due to the increased phonon scattering that the nano-materials are expected to exhibit, as well as potential for enhancement of their Seebeck coefficient. Some of the advantages as well as the challenges will be discussed. These nanocomposites give a new level of potential control as a tuning parameter with which to vary the materials' thermoelectric properties. In addition, Bi$_{2}$Te$_{3}$, another state of the art thermoelectric material and skutterudites (CoSb$_{3})$ have been synthesized as nanomaterials using hydrothermal techniques. A brief discussion of the synthesis techniques, the characterization techniques and highlights of several systems of materials will be presented. \newline \newline In collaboration with Xiaohua Ji, Jian He, Bo Zhang, Nick Gothard, and Paola Alboni, Dept. of Physics, Clemson University. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N43.00005: Neutron scattering study of phonon dynamics on cage compounds C.H. Lee, H. Yoshizawa, I. Hase, H. Sugawara, M.A. Avila, T. Takabatake, H. Sato Filled skutterudite compounds have attracted great attention due to their potential as thermoelectric devices. In particular, their low lattice thermal conductivity is advantageous to achieve high thermoelectric performance. To improve the performance further, it is important to clarify the origin of their low lattice thermal conductivity. Previous studies suggest that the low thermal conductivity is a consequence of free vibration of rare-earth atoms in large lattice cages, which is so called rattling effect. To confirm the hypothesis, we have studied phonon dynamics of CeRu$_4$Sb$_{12}$ by neutron scattering using single crystal samples at JRR-3M reactor of JAERI in Tokai. As results, we have found optical phonons associated with large vibration of Ce atoms at relatively low energy of $E$=6meV, which show an anticrossing with acoustic phonons. According to the analysis based on a Born-von K'{a}rm'{a}n force model, the longitudinal force constants of the nearest Ce-Sb and Ce-Ru are both estimated to be 0.025 mdyn/A, while that of the nearest Ru-Sb shows a large value of 1.4 mdyn/A, indicating that the Ce atoms are bound very weakly with surrounding rigid RuSb$_6$-octahedron cages. We will discuss that the origin of the low lattice thermal conductivity can be intensive Umklapp scattering originating from low-lying optical phonons. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N43.00006: Superconductivity of Ba8Si46-xGax clathrates Yang Li, Ruihong Zhang, Ning Chen, Xingqiao Ma, Guohui Cao, Z.P. Luo, C.R. Hu, Joseph H. Ross, Jr. We have presented a combined experimental and theoretical study of the effect of Gallium substitution on the superconductivity of the type I clathrate Ba8Si46-xGax. In Ga-doped clathrates, the Ga state is found to be strongly hybridized with the cage conduction-band state. Ga substitution results in a shift toward to a lower energy, a decrease of density of states at Fermi level, a lowering of the carrier concentration and a breakage of integrity of the sp3 hybridized networks. These play key roles in the suppression of superconductivity. For Ba8Si40Ga6, the onset of the superconducting transition occurs at Tc=3.3 K. The investigation of the magnetic superconducting state shows that Ba8Si40Ga6 is a type II superconductor. The critical magnetic fields were measured to be Hc1=35 Oe and Hc2=8.5 kOe. Our estimate of the lectron-phonon coupling reveals that Ba8Si40Ga6 is a moderate phonon-mediated BCS superconductor. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N43.00007: Thermoelectric Properties of RE$_{3}$Ru$_{4}$Ge$_{13}$ compounds (RE = Y, Dy, Ho, Lu) D. Morelli, H. Kong, X. Shi, C. Uher Rare earth based compounds have been suggested as ideal thermoelectric materials due to the potential existence of sharp features in their electron density of states. One such series of compounds is of the form R$_{3}$Ru$_{4}$Ge$_{13}$. These materials crystallize in the cubic structure Pm3n and are known to exhibit a variety of interesting properties, including magnetic ordering, superconductivity, and anomalous semiconductor-like resistivity. These compounds can be considered as variants of the cubic structure compound RERuGe$_{3}$ (RE$_{4}$Ru$_{4}$Ge$_{12})$ in which one RE atom is replaced by a germanium atom. This ``extra'' Ge atom can reside on either the Ge or RE site, and the site disorder combined with the complex unit cell of these compounds suggests inherent low lattice thermal conductivity. In order to survey the potential of these materials as thermoelectrics we have synthesized several member compounds of this family. Results on Seebeck coefficient, resistivity, Hall coefficient, and thermal conductivity as a function of temperature will be reported. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N43.00008: The thermoelectric properties of Bi nanowires. Role of quantum size and surface effects. Tito Huber, Albina Nikolaeva, Dmitri Gitsu, Leonid Konopko, Michael Graf Because of the increased density of states arising from one-dimensional confinement, it is anticipated that bismuth quantum wires will exhibit superior thermoelectric properties. Recently, angle-resolved photoemission spectroscopy (ARPES) studies have shown that Bi supports surface states that have not been considered in current models of quantum confinement. Studies of the Fermi surface, employing the Shubnikov-de Haas (SdH) method, in arrays of 30- to 80-nm bismuth nanowires partially corroborates ARPES findings. Assuming diffusive conditions, the impact of the excess surface carriers on the thermopower is to effectively make it smaller than that of bulk Bi, in agreement with measurements reported in the literature. We will report the result of experiments designed to decrease the concentration of surface carriers.. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N43.00009: Thermoelectric properties of SiGe nanoparticle composites Ming Tang, Hohyun Lee, Asegun Henry, Ronggui Yang, Dezhi Wang, Jean-Pierre Fleurial, Pawan Gogna, Gang Chen, Zhifeng Ren, Mildred Dresselhaus Prior theoretical and experimental proof of principle studies on quantum well superlattice and quantum wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. We have shown that nanostructural composites exhibit nanostructures and properties that show great promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. We demonstrate that we can achieve (1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and (2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N43.00010: Synthesis and Characterization of Nanocomposite Chalcogenides Joshua Martin, W. Zhang, L. Chen, G.S. Nolas Recent results on thermoelectric properties of superlattice and nano-scale materials have energized the search for high-performance thermoelectric materials. Thermoelectric technology requires materials in large quantities therefore new approaches are needed to incorporate nano-scale enhancement into bulk materials. We present a technique to synthesize dense bulk nanocomposites reproducibly, and investigate their structural and transport properties. Transport properties demonstrate a strong dependence on porosity, highlighting the necessity of dense nanocomposites for thermoelectric applications. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N43.00011: Time-Resolved Diffraction Studies of Nanoscale Thermal Transport Matt Highland, Bryan Gundrum, Yee Kan Koh, Victor Elarde, James J. Coleman, David G. Cahill, Don Walko, Eric Landahl One of the major considerations in fabricating devices on ever smaller length scales is thermal management in nanometer sized structures. Studying thermal transport requires a temperature measurement accurate on short time scales and sensitive to temperature changes in nanoscale structures. Time Resolved X-ray Diffraction (TRXD) utilizes 100ps x-ray pulses as a probe of lattice expansion and ultrafast laser pulses as a pump for the measurement of optically excited materials. Thermal expansion due to laser heating can therefore be used to study thermal transport in thin films. Reported here are (TRXD) measurements of the temperature rise in (InAs)$_{x}$(GaAs)$_{1-x}$ thin films due to laser heating. These results are compared with continuum model predictions of temperature rise based on film parameters measured independently using Time Domain Thermal Reflectance. This comparison shows a continuum model is inadequate in predicting the thermal behavior these films on short time scales and is indicative of complex transport phenomenon. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N43.00012: Thermoelectric Properties of Heterostructure InAs/InP Nanowires Eric Hoffmann, Ann Persson, Henrick Nilsson, Linus Fr\"oberg, Lars Samuelson, Heiner Linke InAs nanowires with an embedded quantum dot defined by an InP double-barrier structure offer quantum confinement in all three spatial directions. Using a global gate, the Fermi energy can be tuned relative to the dot's density of states. Furthermore, at temperatures below $\sim $10 K, phonon states freeze out, decoupling phonons and electrons, and electron temperatures can be controlled independent of the lattice temperatures. Due to this control, nanowires at low temperatures lend themselves to detailed investigations of the dependence of thermoelectric effects on a strongly modulated density of states. This is of interest, because such systems have been predicted to be able to convert thermal energy to electrical energy at very high efficiency. We report on experiments where a temperature gradient in the electron gas is created along a single nanowire by heating the metallic lead at one end of the nanowire using an ac heating current. The resulting temperature gradient creates a thermovoltage across the nanowire whose sign and magnitude can be tuned by adjusting the Fermi energy relative to the discrete energy levels in the quantum dot. We find that the thermovoltage depends nonlinearly on the temperature differential at surprisingly small temperature gradients. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N43.00013: Round-robin measurement survey for Seebeck coefficient standard reference material Nathan Lowhorn, Winnie Wong-Ng, Makoto Otani, Martin Green, Thanh Tran Full characterization of a thermoelectric material requires measurement of the electrical resistivity, thermal conductivity, and Seebeck coefficient. While standard materials exist or have existed for the first two properties, a Seebeck coefficient standard material with moderate or high values does not exist. In an effort to expedite research efforts in this field, we have initiated a project to develop a Seebeck coefficient, or thermopower, standard reference material. To this end, we have conducted a round-robin measurement survey of candidate standard materials. Both rounds of the survey have been completed, and the results and methodology will be presented. [Preview Abstract] |
Session N44: Focus Session: Optical Properties of Nanowires and Nanocrystals
Sponsoring Units: DMPChair: Greg Salamo, University of Arkansas
Room: Colorado Convention Center 507
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N44.00001: Electronic and optical properties of single-walled GaN nanotubes from first principles Sohrab Ismail-Beigi There is current experimental interest in fabricating GaN nanotubes for possible optoelectronics/luminescence applications. To date, {\it ab initio} studies of these potentially interesting systems have used ground-state density functional theory which has well-known shortcomings when used to predict electronic excitations. We report on our {\it ab initio} predictions of the electronic and optical properties of single- walled GaN nanotubes using electronic Green's functions within the GW-Bethe Salpeter Equation formalism. We present results the nanotube band structures, optical spectra, excitonic states, and likely luminescence properties. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N44.00002: Optical Second-Harmonic Generation from Single GaN Nanowires. J.P. Long, B.S. Simpkins, D.J. Rowenhorst, P.E. Pehrsson The nonlinear optical response of nanostructured materials is of interest because of the need for active elements in nanophotonic applications, and because the nonlinear response can provide information about the nanostructure itself. Here we report measurements of second-harmonic generation (SHG) from individual GaN nanowires (NWs) based on far-field optical microscopy. By correlating the polarization behavior of the SHG signal from each NW with its orientation as determined with electron backscattered diffraction, we show that far-field methods can provide a flexible approach for distinguishing the crystallographic orientations of wurtzite NWs lying on a substrate. Analysis is based on the quasi-static approximation, which assumes that a NW's transverse dimension (75 nm) is less than the relevant wavelengths and thus permits treating the optical electric-fields as spatially uniform. This approach proves sufficient to explain the main SHG polarization features of these NWs, once one accounts for internal depolarization effects for both the excitation and SH electric fields, and for the collection-aperture of the microscope objective. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N44.00003: Non-Equilibrium Exciton Spin Dynamics in Resonantly Pumped Single Core-Shell GaAs-AlGaAs Nanowires Thang B. Hoang, L.V. Titova, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, A.O. Govorov, Y. Kim, H.J. Joyce, H.H. Tan, C. Jagadish We use spatially-resolved photoluminescence (PL) imaging in combination with polarized resonant excitation to investigate the non-equilibrium exciton spin states in single core-shell GaAs-AlGaAs nanowires ($\sim $40 nm core diameter) at low temperature. The large dielectric mismatch between the nanowire and the vacuum results in a strong polarization of excitonic dipoles in the nanowire. This leads to strong polarization of both exciton excitation and emission along the nanowire. Resonant excitation shows two resonances at 1-LO and 2-LO phonons of GaAs and a third resonance likely from electronic states of the AlGaAs. More interestingly, we observe that the polarization of the PL emission is strongly enhanced as the excitation energy comes closer to resonance with the exciton emission. This strong polarization enhancement indicates that resonant excitation creates non-equilibrium exciton spin distributions near resonance. Rate equation modeling allows us to estimate the spin relaxation times which range from $\sim $5ps at high energies to $\sim $50ps at energies close to resonance. Financial support for this work was provided by the University of Cincinnati, Ohio University and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N44.00004: Photoluminescence dynamics of single InP nanowires L.V. Titova, A. Mishra, Thang B. Hoang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish We use time-resolved photoluminescence (PL) spectroscopy to study exciton dynamics in single InP nanowires prepared by catalyst-assisted vapor-liquid-solid growth. In contrast to other III-V materials like GaAs, InP has a lower surface recombination velocity, which should result in longer excitonic lifetimes and higher quantum efficiencies. Indeed, the InP nanowires exhibit emission lifetimes ranging from 80 ps to 2 ns compared to $<$80 ps lifetimes observed for GaAs/AlGaAs nanowires. The large variation in the lifetimes from nanowire to nanowire may be the result of structural inhomogeneities and defects that act as nonradiative recombination centers, thus limiting the excitonic lifetimes. In addition, we have observed changes in the recombination dynamics for single InP nanowires as a function of energy. On the high energy side of the PL peak, the recombination rate is rapid ($\sim $50 ps), while on the low energy side it is significantly slower (up to 1 ns) due to the spectral diffusion of carriers. Preliminary polarization measurements show rapid depolarization of the PL during the $\sim $75 ps emission risetime due to spin scattering of excitons. Financial support for this work was provided by the University of Cincinnati and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N44.00005: Low temperature photoluminescence of single InP nanowires A. Mishra, D. Shereen, Thang B. Hoang, L.V. Titova, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish We investigate low-temperature optical emission properties of a number of single InP nanowires prepared by catalyst-assisted vapor-liquid-solid growth. Photoluminescence (PL) spectra of the nanowires display a broad (full width at half maximum of 27 $\pm $ 3 meV) peak centered at 1.414 $\pm $ 0.008 eV, often accompanied by a broader lower energy shoulder at $\sim $ 1.378 $\pm $ 0.008 eV. The variability in energy of the main peak, which may correspond to bandgap emission, may be explained by structural and compositional variations and non-uniformities of the nanowires. The origin of the lower energy emission is not clear but is likely related to the defect states. We find the bandgap emission as well as excitation of InP nanowires to be strongly linearly polarized along the nanowire axis with a degree of polarization which varies significantly (45{\%} - 85{\%}) from wire to wire. We anticipate that polarization-sensitive single nanowire excitation spectroscopy will reveal information about spin dynamics in these nanostructures. Financial support for this work was provided by the University of Cincinnati and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N44.00006: Temperature dependence of photoluminescence from single InP nanowires D. Shereen, A. Mishra, Thang B. Hoang, L.V. Titova, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish Temperature-dependent micro-photoluminescence (PL) measurements have been carried out to study electronic states of InP nanowires prepared by catalyst-assisted vapor-liquid-solid growth. Low temperature PL spectra of the nanowires show a broad (full width at half maximum of 27 $\pm $ 3 meV) near bandgap peak centered at 1.414 $\pm $ 0.008 eV, in some cases accompanied by a weak defect-related lower energy shoulder at $\sim $ 1.378 $\pm $0.008 eV. Unlike the emission from GaAs/AlGaAs nanowires, which quenches at temperature 120 K due to the presence of large number of non-radiative centers, the PL from the single InP nanowires persists up to room temperature. With increasing temperature, the emission broadens and redshifts. The emission intensity is nearly constant at low temperatures, and begins to quench at 50K. We compare this data with data obtained from epitaxial InP layers grown under similar conditions. Strong linear polarization of the nanowire emission in the direction of nanowire axis is observed in the entire temperature region (8 K -- 270 K). Financial support for this work was provided by the University of Cincinnati and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N44.00007: Synthesis and Raman Scattering from DMS Zn1-xMnxS Nanowires Jian Wu, Humberto Gutierrez, Peter Eklund Diluted Magnetic Semiconductor (DMS) have attracted a lot of attention in the field of Spintronics. Here, we report on our progress to grow DMS Zn1-xMnxS (0$\le $x$<$0.6) nanowires using the vapor-liquid-solid growth mechanism and CVD source techniques based on sublimation of ZnS and MnCl2 powder. Ar/H2 carrier gas was passed over ZnS and MnCl2 maintained at specific temperatures to control the Zn/Mn ratio in the stream. The Zn/Mn concentration also is found to determine the structure, i.e., wurtzite vs zinc blende. HRTEM measurements show that the nanowires are single crystal and the composition was determined by EDS. Raman scattering was performed to study the vibrational properties of these nanowires vs. x. With increasing x, the TO band disappears quickly and cannot be observed; the LO band, however, is found insensitive to x. Between the TO and LO bands, there are three additional peaks appear. Two bands associated with zone edge modes (LA branch) are observed; one upshifts and the other downshifts with x. When possible, our Raman data on nanowires will be compared to that of the bulk. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N44.00008: Optical imaging of oscillating nanowires Dan Hessman, Monica Lexholm, Kimberly Dick, Sara Ghatnekar-Nilsson, Lars Samuelson Systematic investigations of the mechanical properties of semiconductor nanowires require new measurement techniques. In this paper, we present a stroboscopic imaging technique using an optical microscope, capable of tracking the bending and oscillation of a nanowire in space and time. Due to the ideal shape of the nanowires, their position within an image may be determined with a precision given by the signal-to-noise ratio rather than by the optical resolution. We demonstrate an accuracy below 1 nm, more than two orders of magnitude better than the diffraction limit. Temporal information is obtained stroboscopically using a pulsed LED as a light source. The time-resolution is given by the width of the light pulses which in our experiments is below 100 ns. The nanowires are electrostatically bent by applying a voltage between the nanowire and a nearby W-needle. By applying voltage pulses we induce damped oscillations and by applying a sinus voltage we drive the nanowire at varying frequency. In both cases we get resonance frequencies of a few MHz for nanowires about 100 nm in diameter and 5 $\mu$m long. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N44.00009: Optical Absorption in Small Diameter Si Nanowires Kofi Adu, Gugang Chen, Humberto Guti\'errez, Peter Eklund Optical absorption spectra on 3 sets of Si nanowires with most probable diameter 3.5 nm, 5.5 nm and 9 nm are presented. In the optical absorption, apart from the direct gap absorption at E$_{\Gamma 1} \quad \sim $ 3.4 eV and E$_{\Gamma 2 }\sim $ 4.2 eV, we observed two additional strong absorption bands near 1.5 eV and $\sim $2.5 eV. Interestingly, these lower energy features are not expected on the basis of the bulk dielectric function of Si. They are observed experimentally to increase in intensity with decreasing nanowire diameter. It is also interesting that the 1.5eV peak does not shift with decreasing wire diameter. This behavior leads us to tentatively assign this structure to SiO2:Si interface states. On the other hand, the $\sim $2.5 eV absorption band exhibits a systematic blue shift with decreasing diameter. Many of the features in the experimental absorption spectrum will be explained on the basis of dielectric function calculations carried out in the discrete dipole approximation (DDA). The DDA model results are shown to depend on the nanowire diameter/length and the thickness of amorphous SiO2 shell. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N44.00010: Short-wavelength visible light emission from silicon nanocrystals Xiaodong Pi, Rick Liptak, Stephen Campbell, Uwe Kortshagen Si is the material of choice for modern microelectronics but, as an indirect-bandgap semiconductor, it is not an efficient light emitter. An electrically pumped Si laser would present a breakthrough for optoelectronic integration that may enable optical interconnect to make computers faster. Si light emitting diodes may revolutionize solid-state lighting and displays because of the low cost and environmental friendliness of Si. One of the most challenging problems of Si-based lighting and displays is the lack of a reliable and efficient full visible spectrum emission. Si nanocrystals (Si-NCs) have so far been the most promising form of Si to emit light. Most of the synthesis approaches of Si-NCs, however, only lead to red light emission. Our recent work on Si-NCs synthesized by non-thermal plasmas has focused on extending their light emission into the short-wavelength range. Firstly, the process of oxidation-etching-oxidation of Si-NCs is investigated. This process causes the size of Si-NCs to decrease, leading to shorter wavelength light emission from Si-NCs. Yellow or green photoluminescence (PL) has been observed from initially oxidized red light emitting Si-NCs after HF vapour etching and atmospheric oxidation. The intensity of PL from Si-NCs, however, decreases by a factor up to 100. It is found that HF etching restructures the surface of Si-NCs. This leads to a decrease in the incorporation of O during subsequent oxidation, which finally results in silicon suboxide SiO$_{1.9}$. Such an understoichiometry indicates a high density of defects such as Si dangling bonds at the Si-NC/oxide interface. Therefore, the PL efficiency is extremely low for short-wavelength light emitting Si-NCs obtained by the process of oxidation-etching-oxidation. Secondly, an integrated two-stage plasma system is employed to achieve the light emission from Si-NCs in the full visible spectrum range. Red-light-emitting Si-NCs are produced in the first stage by the plasma decomposition of SiH$_{4}$. In the second stage a tetrafluoromethane (CF$_{4})$-based plasma etches Si-NCs and at the same time passivates them with carbon and fluorine. After the two-stage process Si-NCs emit light in the short-wavelength region from yellow to blue. We find that a self-limited oxidation process blueshifts the light emission until saturation is reached. Significantly, relatively high quantum yields of short-wavelength light emission from Si-NCs are obtained in spite of oxidation. It is interesting to note that Si-NCs treated by CF$_{4}$-based plasma are hydrophilic while those without CF$_{4}$-based plasma treatment are hydrophobic. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N44.00011: Dynamical and Optical Properties of Si and Ge Nanocrystals Kelly Knutsen, Matt Beard, P.R. Yu, Qing Song, Wyatt Metzger, Art Nozik, Randy Ellingson Si nanocrystals exhibit the unusual property of having a high photoluminescence quantum yield as well as a long first exciton lifetime. This implies that the decay rates for the non-radiative channels have decreased compared to the bulk. We explore this phenomenon by first characterizing the optical properties of the Si nanocrystals by measuring their linear absorption and photoluminescence spectra as a function of nanocrystal size, which show an expected shift to the blue for the transition onset with decreased particle size. The nanoparticles exhibit indirect transition characteristics, and emit roughly 1eV to the red of the absorption onset. We also employ time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy to investigate the Auger dynamics of the single and biexcitons. Initial results for 9nm Si nanoparticles show that the biexciton lifetime is roughly 200 ps and the single exciton lifetime is greater than 200 microseconds. The size dependence on the single and biexciton lifetimes, as well as the potential presence of multiple exciton generation (MEG) in these materials will be presented. Initial optical studies of Ge nanocrystal charge carrier dynamics will also be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N44.00012: Visible light absorption and photodarkening in Te-modified TiO$_{2}$ nanocrystals Steven Phillips, Ian James, Bret Hess Applications of titanium dioxide nanocrystals in solar cells and solar photocatalysis are limited by the lack of visible light absorption. We have created TiO$_{2}$ nanocrystals modified by tellurium, which causes absorption in the visible. In TiO$_{2}$:Te nanocrystals annealed between 300 and 600 C, light exposure quickly causes the visible absorption to increase until the sample is reddish-brown. The presence of Te stabilizes the anatase structure, while the rutile phase is found in undoped nanocrystals. We discuss possible mechanisms for the photosensitivity, and explore whether this visible absorption is useful in photocatalysis. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N44.00013: Density Matrix Approach for Valence Band Optical Properties M.P. Prange, J.J. Rehr We present an extension of the {\it ab initio} real-space multiple-scattering (RSMS) theory currently used for core-level spectra (e.g. EELS, XAS and NRIXS) to calculations of the valence band optical response. The method is based on RSMS calculations of the occupied and unoccupied density matrices and transition matrix elements between the two resulting in an efficient way to calculate various optical constants in aperiodic materials. In contrast to bandstructure or basis-set methods, the calculation can be applied to a large class of materials including both insulators and metals up to the nanoscale. By combining the method with the RSMS approach for core level response, we obtain an approach applicable for spectra from the far IR to x-rays. Results are compared with experiment and with other theoretical techniques. Possible extensions are also discussed. [Preview Abstract] |
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