Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W1: Quantum Coherence in Superconducting Devices
Sponsoring Units: DCMPChair: Moses Chan, Pennsylvania State University
Room: Colorado Convention Center Four Seasons 2-3
Thursday, March 8, 2007 2:30PM - 3:06PM |
W1.00001: Flux Qubits: Coupling and Decoherence Invited Speaker: The principles of the three-junction flux qubit are briefly reviewed. We investigated two such qubits coupled together via their mutual inductance and via the dc SQUID (Superconducting Quantum Interference Device) that reads out their magnetic flux states. On-chip flux lines enabled us to bias the two qubits individually. Microwave spectroscopy revealed that the energy splittings of the symmetric and antisymmetric states of the two qubits at their respective degeneracy points were remarkably close, 8.872 GHz and 8.990 GHz. At the double degeneracy point, the energy difference between the first and second excited states of the coupled qubits was enhanced by level repulsion as predicted. We performed time domain measurements on the individual qubits and on excited states of the coupled qubits, including Rabi oscillations, flux echoes, Ramsey fringes and measurements of the relaxation time, and also determined the linewidths of the individual peaks. These measurements enable us to compare the relaxation and decoherence times of the individual and coupled qubits. For example, at the double degeneracy point of the coupled qubits, the decoherence rate determined by flux echoes is equal to the sum of the rates in the separate qubits. Sources of decoherence are discussed, and estimates given of the various known contributions including those of the biasing and measurement circuitry. This work was performed in collaboration with T. Hime, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, A.V. Ustinov and C.-E. Wu. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W1.00002: Mach-Zehnder Interferometry and Microwave-Induced Cooling in Persistent-Current Qubits Invited Speaker: Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the energy-level avoided crossings. The resulting Landau-Zener transitions mediate a rich array of quantum-coherent phenomena as a function of the driving amplitude and frequency. In this talk, we present three such demonstrations of quantum coherence in a strongly-driven niobium persistent-current qubit. The first is Mach-Zehnder-type interferometry [1], for which we observe quantum interference fringes for 1-50 photon transitions. The second is a new operating regime exhibiting coherent quasi-classical dynamics [2], for which the MZ quantum interference persists even for driving frequencies smaller than the resonance linewidth. The third is microwave-induced cooling [3], for which we achieve effective qubit temperatures < 3 mK, a factor 10x-100x lower than the dilution refrigerator ambient temperature. These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state qubit modalities. In addition to our interest in these techniques for fundamental studies of quantum coherence in strongly-driven solid-state systems, we anticipate they will find application to nonadiabatic qubit control and state-preparation methods for quantum information science and technology. [1] W.D. Oliver, Y. Yu, J.C. Lee, et al., Science 310, 1653 (2005). [2] D.M. Berns, W.D. Oliver, S.O. Valenzuela et al., PRL 97, 150502 (2006). [3] S.O. Valenzuela, W.D. Oliver, D.M. Berns, et al., Science (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W1.00003: Vacuum Rabi oscillations observed in a flux qubit LC-oscillator system Invited Speaker: Superconducting circuit containing Josephson junctions is one of the promising candidates as a quantum bit (qubit) which is an essential ingredient for quantum computation [1]. A three-junction flux qubit [2] is one of such candidates. On the basis of fundamental qubit operations [3,4], the cavity QED like experiments are possible on a superconductor chip by replacing an atom with a flux qubit, and a high-Q cavity with a superconducting LC-circuit. By measuring qubit state just after the resonant interaction with the LC harmonic oscillator, we have succeeded in time domain experiment of vacuum Rabi oscillations, exchange of a single energy quantum, in a superconducting flux qubit LC harmonic oscillator system [5]. The observed vacuum Rabi frequency 140 MHz is roughly 2800 times larger than that of Rydberg atom coupled to a single photon in a high-Q cavity [6]. This is a direct evidence that strong coupling condition can be rather easily established in the case of macroscopic superconducting quantum circuit. We are also considering this quantum LC oscillator as a quantum information bus by sharing it with many flux qubits, then spatially separated qubits can be controlled coherently by a set of microwave pulses. [1] F. Wilhelm and K. Semba, in Physical Realizations of Quantum Computing: Are the DiVincenzo Criteria Fulfilled in 2004?, (World Scientific; April, 2006) [2] J. E. Mooij \textit{et al.}, Science \textbf{285}, 1036 (1999). [3] T. Kutsuzawa\textit{ et al.}, Appl. Phys. Lett. \textbf{87}, 073501 (2005). [4] S. Saito\textit{ et al.}, Phys. Rev. Lett. \textbf{96}, 107001 (2006). [5] J. Johansson\textit{ et al.}, Phys. Rev. Lett. \textbf{93}, 127006 (2006). [6] J. M. Raimond, M. Brune, and S. Haroche, Rev. Mod. Phys.\textbf{ 73}, 565 (2001). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W1.00004: Controllable coupling of superconducting flux qubits Invited Speaker: As a first step, by making use of conventional niobium technology, we have implemented controllable flux coupling between two qubit prototypes (in our case single junction interferometers) by using a third one as the coupler. The fabricated qubit prototypes operate in the hysteretic mode, where the screening parameter $>$1, which provides double degenerate state for an external flux equal to half a flux quantum. The coupler parameters were chosen so that it operates in the non-hysteretic mode with a screening parameter of 0.9. The coupling amplitude is proportional to the derivative of the coupler's current-flux relation. By changing the coupler's magnetic flux, we have shown ferromagnetic as well as anti-ferromagnetic coupling between the interferometers. In particular, we have demonstrated that the coupling could also be switched off. As the next step of our investigation we implemented similar ideas in to our Al shadow-evaporation technology. Recently, we have also demonstrated a tuneable coupling between three junctions persistent current qubits in the quantum regime. A possible combination of Al and Nb technologies is discussed. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W1.00005: Quantum Dynamics of a d-wave Josephson Junction Invited Speaker: Thilo Bauch $^{1}$, Floriana Lombardi $^{1}$, Tobias Lindstr\"{o}m $^{2}$, Francesco Tafuri $^{3}$, Giacomo Rotoli $^{4}$, Per Delsing $^{1}$, Tord Claeson $^{1}$ 1 Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience, MC2, Chalmers University of Technology, S-412 96 G\"{o}teborg, Sweden. 2 National Physical Laboratory, Queens Road, Teddington, Middlesex TW11 0LW, UK. 3 Istituto Nazionale per la Fisica della Materia-Dipartimento Ingegneria dell'Informazione, Seconda Universita di Napoli, Aversa (CE), Italy. 4 Dipartimento di Ingegneria Meccanica, Energetica e Gestionale, Universita of L'Aquila, Localita Monteluco, L'Aquila, Italy. We present direct observation of macroscopic quantum properties in an all high critical temperature superconductor d-wave Josephson junction. Although dissipation caused by low energy excitations is expected to strongly suppress quantum effects we demonstrate macroscopic quantum tunneling [1] and energy level quantization [2] in our d-wave Josephson junction. The results clearly indicate that the role of dissipation mechanisms in high temperature superconductors has to be revised, and may also have consequences for a new class of solid state ``quiet'' quantum bit with superior coherence time. We show that the dynamics of the YBCO grain boundary Josephson junctions fabricated on a STO substrate are strongly affected by their environment. As a first approximation we model the environment by the stray capacitance and stray inductance of the junction electrodes. The total system consisting of the junction and stray elements has two degrees of freedom resulting in two characteristic resonance frequencies. Both frequencies have to be considered to describe the quantum mechanical behavior of the Josephson circuit. [1] T. Bauch et al, Phys. Rev. Lett. 94, 087003 (2005). [2] T. Bauch et al, Science 311, 57 (2006). [Preview Abstract] |
Session W2: Molecular Motors (Biophysical Society Symposium)
Sponsoring Units: DBPChair: Carlos Catalano, University of Washington
Room: Colorado Convention Center Four Seasons 4
Thursday, March 8, 2007 2:30PM - 3:06PM |
W2.00001: Pathway of Force Production by the Kinesin-Microtubule ATPase Invited Speaker: Kinesin is the smallest of the molecular motors, consisting of a dimer of motor domains that interact with microtubules and ATP to generate motion towards the plus ends of microtubules for fast axonal transport of membranous organelles. It operates via an alternating site ATPase pathway in which the binding of ATP to one motor domain stimulates the release of ADP from the neighboring domain as the motor walks ``hand over hand'' along the microtubule surface. This alternating site pathway is accomplished in part due to strain that distinguishes the leading from the lagging motor domains when both are bound to the microtubule. This strain leads to a weak nucleotide binding state in the leading motor and a strong nucleotide binding state in the lagging motor. The ATPase activity is linked to alternating weak and strong nucleotide binding states that are coupled to association and dissociation at the microtubule surface to produce a force for forward motion. Strain in the leading motor domain appears to be due to the disruption of the ``neck linker'' in the leading motor. Release of the trailing motor domain from the microtubule surface is the rate-limiting step and, by relaxing the tension, allows the leading domain to bind ATP and continue the cycle and forward motion. Although many of the rate constants for steps in this pathway are known, details regarding the structural and thermodynamic basis for the coupling of ATP hydrolysis to force production remain to be established. I will review our current understanding and describe some of our early attempts to resolve intermediates during movement using single molecule fluorescence methods. \newline \newline In collaboration with Tim Scholz and Bernhard Brenner, Hannover Medical School. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W2.00002: Invited Speaker: |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W2.00003: Optical tweezers studies of viral DNA packaging: Motor function and DNA confinement in Bacteriophages phi29, lambda, and T4 Invited Speaker: In the assembly of many viruses a powerful molecular motor translocates the genome into a pre-assembled capsid. We use optical tweezers to directly measure translocation of a single DNA molecule into the viral capsid. Improved techniques allow us to measure initiation and early stages of packaging. With phi29 the DNA terminal protein was found to cause large variations in the starting point of packaging. Removal of this protein results in terminal initiation, permitting more accurate assessment of motor function and DNA confinement forces. We investigated the role of electrostatic repulsion by varying ionic screening of the DNA. The observed trends are in accord with those theoretically expected considering counter-ion competition; however the forces are larger than expected in comparison with recent theories and DNA ejection measurements. We have recently succeeded in extending our methods to study two other phages: lambda and T4. These systems have unique structural and functional features, presenting an opportunity for comparative studies in this family of molecular motors. Initial measurements show that lambda and T4 translocate DNA several times faster than the phi29 motor, but are more sensitive to applied load. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W2.00004: The Viral DNA Packaging Motor of Bacteriophage Lambda Invited Speaker: Terminase enzymes are common to both eukaryotic and prokaryotic double-stranded DNA viruses. These enzymes, which serve as molecular motors that selectively ``package'' viral DNA into a pre-formed procapsid structure, are among the most powerful biological motors characterized to date. Bacteriophage lambda terminase is a heteroligomer composed of gpA and gpNu1 subunits. The smaller gpNu1 subunit is required for specific recognition of viral DNA, a process that is modulated by ATP. The gpA subunit possesses site-specific nuclease and helicase activities that ``mature'' the viral genome prior to packaging. The subunit further possesses a DNA translocase activity that is central to the packaging motor complex. Discrete ATPase sites in gpA modulate the DNA maturation reactions and fuel the DNA packaging reaction. Kinetic characterization of lambda terminase indicates significant interaction between the multiple catalytic sites of the enzyme and has led to a minimal kinetic model describing the assembly of a catalytically-competent packaging motor complex. Biophysical studies demonstrate that purified lambda terminase forms a homogenous, heterotrimeric structure consisting of one gpA subunit in association with two gpNu1 proteins. Four heterotrimers further assemble into a ring-like structure of sufficient size to encircle duplex DNA. The ensemble of data suggests that the ring tetramer represents the biologically relevant, catalytically-competent motor complex responsible for genome processing and packaging reactions. We present a model for the functional DNA packaging motor complex that finds general utility in our global understanding of the enzymology of virus assembly. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W2.00005: Structural dynamics of myosin V: characterization of the one-head bound intermediate Invited Speaker: Myosin V transports cargo along actin filaments by walking hand over hand. Although this basic model is supported by numerous studies, little is known about the intermediate that occurs when only one of the two heads is bound to actin. Here we use submillisecond darkfield imaging of gold nanoparticle labeled myosin V to directly observe the free head as it releases from the actin filament, diffuses forward, and rebinds. The released head rotates freely about the lever arm junction, a trait which likely facilitates travel through crowded actin meshworks. Free head rebinding occurs more rapidly when one of the six calmodulins bound to the lever arm is replaced with the light chain LC1sa. Our data suggest that strong rebinding and phosphate release occur rapidly, but that the lever arm swing is thwarted by intramolecular strain. The effect of light chain composition on free head rebinding kinetics suggests a potentially elegant means of modulating filament switching and processivity in a tissue-specific manner. [Preview Abstract] |
Session W3: Magnetotransport in Organic Conductors and Semiconductors
Sponsoring Units: DCMPChair: Zeev Vardeny, University of Utah
Room: Colorado Convention Center Korbel 2A-3A
Thursday, March 8, 2007 2:30PM - 3:06PM |
W3.00001: Spins and Organic Materials: The Spin-dependent OLED Invited Speaker: |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W3.00002: Spin correlations in organic semiconductors Invited Speaker: Organic semiconductors differ from their inorganic counterparts by large exchange interactions and weak spin orbit coupling. As a result, parallel and anti-parallel spin configurations are highly non-degenerate and spectroscopically well-defined. Whereas singlet excitons are highly emissive, triplet excitons generally decay non-radiatively. Addition of heavy metal centres to the polymer backbone induces localized spin-orbit coupling, which can activate radiative triplet decay through phosphorescence [1]. By tuning the concentration of these triplet acceptors to match the diffusion length of the triplets (which exceeds that of the singlets), triplets can be harvested radiatively without significantly affecting the actual triplet formation pathway through intersystem crossing of the singlet [2]. Using this technique we can study the interconversion between spin states of exciton precursors (charge carrier pairs) as a function of time, temperature, and electric and magnetic fields [3]. We find that the probability of a spin change occurring in the exciton precursor state is extremely small, which suggests that the primary recombination pathway in organic light-emitting diodes is governed by spin statistics [3]. Phosphorescence spectroscopy of organic semiconductors has a number of immediate applications. Stimulated emission competes with intersystem crossing required for triplet generation so that a phosphorescent polymer laser acts as a highly non-degenerate all-optical excitonic switch [4]. Singlet-triplet mixing in metallorganics with strong spin-orbit coupling also provides a versatile method for ultrafast luminescence based molecular thermometry [5,6]. \newline \newline [1] Lupton et al., \textit{Phys. Rev. Lett.} \textbf{89}, 167401 (2002). \newline [2] Reufer et al., \textit{Phys. Rev. B Rapid} (in press 2006). \newline [3] Reufer et al., \textit{Nature Mat.} \textbf{4}, 340 (2005). \newline [4] Reufer et al., \textit{Appl. Phys. Lett.} \textbf{89}, 141111 (2006). \newline [5] Stehr et al., \textit{Adv. Mater.} \textbf{16}, 2170 (2004). \newline [6] Balouchev et al., \textit{US Patent} \textbf{7097354} (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W3.00003: Characterization and Application of Large Magnetoresistance in Organic Semiconductors Invited Speaker: Recent years have seen a surge in interest in magnetoresistive and spintronic properties of organic semiconductors, whereas this field was previously almost exclusively concerned with their electrooptical properties. We report on the extensive experimental characterization of a recently discovered large and intriguing magnetoresistive effect in organic light- emitting diodes that reaches up to 10\% at room temperature for magnetic fields, B = 10mT. This magnetoresistive effect is therefore amongst the largest of any bulk material. The study includes a range of materials that show greatly different chemical structure, mobility, hyperfine and spin-orbit coupling strength. We show that the applied magnetic field affects the carrier transport inside the bulk semiconductor. By demonstrating that the effect is critically altered by the presence of strong spin- orbit coupling and that it does not occur in fullerene devices, we prove that the transport in organics sensitively depends on spin-dynamics induced by hyperfine interaction with the hydrogen protons. We discuss a possible relation between organic magnetoresistance and other magnetic field effects in organics that were known long before its discovery. As a possible mechanism we describe how Pauli's principle restricts carrier hopping between singly occupied sites near the Fermi level. However, spin-mixing by the hyperfine interaction may partially lift this restriction. Since the devices we describe can be manufactured cheaply they hold promise for applications where large numbers of magnetoresistive devices are needed, such as magnetic random- access-memory (MRAM); and applications related to organic light- emitting diode displays such as touch screens where the position of a magnetic stylus is detected (patent pending). We will show a video of a simple demonstrator device. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W3.00004: Spin-Orbital Coupling and Magnetoresistance Tuning in Organic Semiconductors Invited Speaker: Magnetoresistance can be readily obtained from non magnetic organic semiconductors in light-emitting diodes. Tuning this novel magnetoresistance is an important issue in using an external magnetic field to control optoelectronic response in organic semiconductors. The experimental results indicate that weak-spin-orbital coupling materials exhibit much more significant magnetoresistance as compared to strong-spin-orbital coupling molecules. We find that uniformly mixing strong-spin-orbital-coupling fac-tris (2-phenylpyridinato) iridium [Ir(ppy)$_{3}$] molecules and weak-spin-orbital-coupling poly(N-vinyl carbazole) (PVK) leads to a concentration-dependent magnetoresistance. There are three possible processes, namely intermolecular spin-orbital interaction, energy transfer, and charge transport, that can contribute to the concentration-dependent magnetoresistance. The magnetic field-dependent electroluminescence shows that an intermolecular spin-orbital interaction is formed in the PVK+Ir(ppy)$_{3}$ mixture. This intermolecular spin-orbital interaction modifies the singlet/triplet exciton ratio, changing further charge injection when the space charge carriers from exciton dissociation are considered. Based on this intermolecular spin-orbital interaction effects, metal electrode-dependent spin-orbital coupling and magnetoresistance have been demonstrated. This presentation will discuss the effects of spin-orbital coupling on magnetoresistance tuning through exciton dissociation and exciton-charge reaction in organic light-emitting diodes through controlling energy transfer and bipolar injection. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W3.00005: Low Field, Large Magnetoresistance in Nonmagnetic Organic Semiconductors Invited Speaker: Transport in various thin-film organic semiconductors has been shown to have an anomalously high sensitivity to low magnetic fields at room temperature (RT). Early experiments on polydiacetylene single crystals and poly(phenylenevinylene)s revealed increases in photoconductivity of a few percent at RT.\footnote{E.L. Frankevich, et al., Mol.\ Cryst.\ Liq.\ Cryst.\textbf{175}, 41 (1989); E.L. Frankevich, et al., Phys.\ Rev.\ B \textbf{46}, 9320 (1992).} Further magnetotransport studies showed larger effects in $\pi$-conjugated backbone polymers and small molecules.\footnote{\"{O}. Mermer, et al., Phys.\ Rev.\ B \textbf{72}, 205202 (2005).} We report magnetoresistance (MR) for semiconducting oligomer and nonconjugated polymer materials in addition to small molecule and conjugated backbone polymer materials. For example, films of the light emitters poly(N-vinylcarbazole) and Alq$_{3}$ each have an MR response greater than 5\% at an unusually low magnetic field of 100 Oe $(\mu_{B}H \sim \mbox{0.0006 meV})$ at an unusually high temperature of 300 K $(k_{B}T \sim \mbox{26 meV})$. Increasing the spin-orbit coupling in Alq$_{3}$ films by doping with the phosphorescent sensitizers Ir(ppy)$_{3}$ or PtOEP strongly suppresses the MR signal. MR in thin films of the oligomer $\alpha $-sexithiophene can be negative, similar to the behavior of other organic semiconductors, or positive depending on the temperature, layer thickness, or applied voltage. We have developed a model, termed Magnetoresistance by the Interconversion of Singlets and Triplets (MIST), accounting for this anomalous MR.\footnote{V.N. Prigodin, et al., Synth.\ Met.\textbf{156}, 757 (2006).} At zero field, the singlet and triplet e-h pair states are degenerate and the states can readily interconvert due to hyperfine interaction. Finite magnetic fields lift triplet degeneracy which affects the hyperfine interconversion of e-h pairs between singlet and triplet states. By changing the carrier recombination the MIST mechanism gives rise to a space-charge-limited current that depends on magnetic field, producing MR. [Preview Abstract] |
Session W4: Computational Challenges in Simulations of Macromolecular Assemblies
Sponsoring Units: DPOLY DCOMPChair: Grant Smith, University of Utah
Room: Colorado Convention Center Korbel 2B-3B
Thursday, March 8, 2007 2:30PM - 3:06PM |
W4.00001: Confined Self-Assembly of Block Copolymers Invited Speaker: Spontaneous formation of ordered structures from amphiphilic molecules has attracted tremendous attentions in the last decades. Among the many different amphiphilic systems, block copolymers with their rich phase behavior and ordering transitions have become a paradigm for the study of structural self-assembly. In a physically confined environment, structural frustration, confinement-induced entropy loss and surface interactions can strongly influence the molecular organization. In particular, it is possible that confinement can lead to unusual morphologies which are not accessible in the bulk, thus providing opportunities to engineer novel structures. For confined asymmetric diblock copolymers, a rich variety of novel morphologies, ranging from helices to toroids to complex networks, is expected. The complexity of the possible structures presents computational challenges in simulations of macromolecular assemblies under confinement. We have used a combination of simulated annealing method and self-consistent field theory to exam the self-assembly of block copolymers confined in different geometries. A generic structural evolution path is obtained for the confined systems. The study demonstrates that confined self-assembly of amphiphilic molecules provides a robust method to produce nanoscopic structures which are not accessible in the bulk phases. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W4.00002: Simulation of driven self assembly of complex polymeric systems across multiple length scales Invited Speaker: The self assembly of macromolecular systems is often driven or facilitated by the application of external fields, including flow, voltage, or confinement. The structures that arise when external fields are applied often depend on the history of the sample, and it is therefore important to develop theoretical and computational methods capable of describing the order formation process across multiple length and time scales. Over the past several years we have developed several new classes of multiscale modeling techniques for study of the structure and properties of polymeric materials under external fields, including confinement or flows. For systems at equilibrium, these systems permit precise calculation of the free energy. For systems beyond equilibrium, these methods include the effects of fluctuating hydrodynamic interactions (for dilute and semidilute systems) and the effects of constraining molecules (for concentrated melts). These models and methods can be used to investigate the equilibrium structure and relaxation of a variety of fluids, including solutions of biological macromolecules. The usefulness and limitations of our proposed approach will be discussed in the context of three applications. The first application is concerned with the elongation and presentation of long DNA molecules in nanofluidic channels. A multiscale model, that includes fluctuating hydrodynamic interactions, has been used to design a gene mapping device and to interpret experimental data pertaining to the structure and dynamics of confined chromosome-length DNA. The second application is concerned with the study of liquid-crystal based biosensors. A multiscale model has been used to design a liquid-crystal based device in which nanoscale particles suspended in a liquid crystal self assemble into highly regular structures, including chains, upon exposure to proteins or virions. The third application focuses on the formation of ordered block copolymer structures on nanopatterned substrates. Results from mesoscopic multiple length and time scale simulations will be presented to explain the effects of surfaces and different types of confining walls on the free energy of a variety of morphologies. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W4.00003: Chemistry Unified Language Interface: a Novel Toolkit for Hybrid Macromolecular Models Invited Speaker: In our everyday life, we appreciate the enormous diversity of industrial formulation in many applications, from personal and health care, to plastics and coatings, to novel nanotechnology. In all of these cases the systems are compound, containing solvents mixtures, polymers, surfactants, actives and colloids or fillers. CULGI (Chemistry Unified Language Interface) is a library of many modeling functions and approaches, encompassing molecular, mesoscopic and macroscopic chemical modeling and machine learning. We discuss several of the important theoretical challenges that we face: (a) the lack of unification in classification and description of common chemical entities, such as molecules, colloids and surfactants, (b) the curse of loss of thermodynamic accuracy in large scale molecular dynamics, and (c), most importantly: how to derive interaction models and parameters for real mixed systems of industrial relevance on a coarse-grained level. We illustrate the challenges by some resent results relevant to industrial formulation and application: the in situ phase formation of polyolefin blends, the lyotropic phase structure of concentrated surfactant ? polyacid mixtures, and dissolution rates of sparingly soluble drugs from polymer stabilized suspensions. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W4.00004: Modeling Microcapsule-Substrate Interactions: Repairing Damages Surfaces and Separating Damaged Cells. Invited Speaker: We model two different scenarios that involve capturing the behavior of macromolecular assemblies. In the first study, we model the rolling motion of a fluid-driven, particle-filled microcapsule along a heterogeneous, adhesive substrate to determine how the release of the encapsulated nanoparticles can be harnessed to repair damage on the underlying surface. We integrate the lattice Boltzmann model for hydrodynamics and the lattice spring model for the micromechanics of elastic solids to capture the interactions between the elastic shell of the microcapsule and the surrounding fluids. A Brownian dynamics model is used to simulate the release of nanoparticles from the capsule and their diffusion into the surrounding solution. We focus on a substrate that contains a damaged region (e.g., a crack or eroded surface coating), which prevents the otherwise mobile capsule from rolling along the surface. We isolate conditions where nanoparticles released from the arrested capsule can repair the damage and thereby enable the capsules to again move along the substrate. Through these studies, we establish guidelines for designing particle-filled microcapsules that perform a ``repair and go'' function and thus, can be utilized to repair damage in microchannels and microfluidic devices. In the second study, we extend the above model of fluid-filled, elastic spheres rolling on substrates to three dimensions and thereby demonstrate a useful method for separating cells or microcapules by their compliance. In particular, we examine the fluid-driven motion of these capsules over a hard adhesive surface that contains soft stripes or a weakly adhesive surface that contains ``sticky'' stripes. As a result of their inherently different interactions with the heterogeneous substrate, particles with dissimilar stiffness are dispersed to distinct lateral locations on the surface. Since mechanically and chemically patterned surfaces can be readily fabricated through soft lithography and can easily be incorporated into microfluidic devices, our results point to a facile method for carrying out continuous ``on the fly'' separation processes. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W4.00005: Non-equilibrium dynamics at polymer surfaces and interfaces Invited Speaker: The ability to predict the rheological and flow properties of polymer blends and other multi-phase systems relies critically on the ability to understand how the interfacial dynamics affect the overall properties. Continuum theories tend to break down in the near interfacial region since they do not account for local molecular orientation and other related effects. We have shown, using MD simulations, that the behavior at interfaces tends to be controlled by the stress transfer between the two phases and this is dependent on the local chain structure and conformation at the interface. Studies on nanoparticles added to polymer blends also illustrates the importance of controlling the interfacial region. To account for all these effects we have formulated a dynamic self-consistent field theory that couples local chain conformation to a constitutive equation that describes the rheological properties of the system. We illustrate this approach by studying the effect of slip at polymer/polymer interfaces. [Preview Abstract] |
Session W5: Crystallography Without Crystals
Sponsoring Units: FIAPChair: Abbas Ourmazd, University of Wisconsin-Milwaukee
Room: Colorado Convention Center Korbel 1A-1B
Thursday, March 8, 2007 2:30PM - 3:06PM |
W5.00001: Crystallography without Crystals: An Overview Invited Speaker: Protein X-ray crystallography, an ``outgrowth of physics,'' is now the mainstay of biology, biochemistry, and the pharmaceutical industry. However, roughly 40{\%} of biological molecules do not crystallize. And although more than half a million proteins have been sequenced, the structure of less than 40,000 has been determined. By obviating the need for purification and crystallization, the ability to determine the structure of individual biological molecules would constitute a fundamental breakthrough. The confluence of four developments has generated intense interest in achieving this by short-pulse X-ray scattering: \begin{enumerate} \item The advent of algorithms capable of ``solving the phase problem'' with practical demonstrations in astronomy, high-energy electron diffraction, and protein crystallography [1,2,3]. \item Development of sophisticated techniques for determining the relative orientation of electron microscope \textit{images} of biological entities such as cells and large macromolecules [4]. \item Development of techniques for producing beams of hydrated proteins [3,5]. \item The promise of ultra-bright, short pulses of X-rays from X-ray Free Electron Lasers (XFELs) under construction in the US, Europe, and Japan. \end{enumerate} I will describe how these and other key developments have brought the prospect of single-molecule structure determination ``tantalizingly close,'' perhaps even closer than generally realized in the literature. \newline [1] J. R. Fienup, Appl. Opt. \textbf{21}, 2758 (1982). \newline [2] J. Miao et al. PNAS \textbf{98}, 6641 (2001). \newline [3] J.C.H. Spence et al. Acta Cryst. \textbf{A61}, 237 (2005) \newline [4] J. Frank, \textit{Three-Dimensional Electron Microscopy of Macromolecular Assemblies} (OUP Press, 2006) \newline [5] J.B. Fenn, J. Biomolecular Techniques \textbf{13}, 101 (2002). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W5.00002: New X-Ray Optics and Sources for Single-particle Crystallography Invited Speaker: With the continued development of extremely bright x-ray sources, the minimum size of objects suitable for detailed study with x-ray scattering now approaches that of single molecules and other nanoparticles. To date, x-ray scattering is a tool which explores the statistical structural properties of an ensemble of particles. While this has been an extremely powerful approach to understanding the structural properties of materials and structure/property relationships, important details are often difficult to extract because of averaging. In particular, when looking at collections of nanoparticles, the only information available is small angle x-ray scattering which yields the general shape of the particles. Combining the current statistical information with detailed scattering from individual particles would greatly reduce the difficulty of extracting the important structural information. This talk will discuss the status of x-ray sources and optics, and explore the feasiblity and challenges of applying them to real-world crystallographic studies of single nanoparticles. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W5.00003: Serial Crystallography Invited Speaker: To solve proteins which cannot be crystallized we have devised an aerodynamic focussing, monodispered droplet beam, which runs in single file across a synchrotron X-ray beam (LBNL Advanced Light Source) as it freezes in vacuum. The aim is to obtain a charge-density map of the protein at 0.7nm resolution , sufficient to locate alpha-helices. Water is removed before the proteins, coated by a thin ice-jacket, are aligned by the dipole moment induced by a 100 W NIR polarized fiber laser. All three orthogonal beams (proteins, X-rays, laser) intersect in a 10 micron diameter volume, and run continuously without sychronization. Elliptical polarization aligns molecular axes in direction but not sense. Data is collected continuously until adequate statistics are achieved before rotating the polarization to a new orientation. Details of the adiabatic laser-alignment and damping processes will be given. Misalignment is shown to be proportional to temperature and inversely to laser power and molecular volume. Polarizability tensor calculations for proteins will be discussed. Preliminary X-ray results (without laser) will be shown. Iterative methods for solving the phase problem will be demonstrated using experimental soft X-ray diffraction patterns from non-crystalline particles. Detailed simulations of the continuous diffraction patterns from a moving stream of partially aligned large hydrated molecules will be shown, and inverted to density maps using the Fienup-Gerchberg-Saxton algorithm. From this the exposure time and resolution may be estimated for tomographic reconstruction. Experimental comparisons of Rayleigh, electrospray and aerodynamic focussing droplet beam sources will also be described. The research team includes B.Doak, U. Weierstall, D. Shapiro (LBNL), D. Deponte, P. Fromme, D. Starodub, G. Hembree and H. Chapman (LLNL). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W5.00004: Electron Densities from Diffraction Patterns of Randomly Oriented Molecules Invited Speaker: A diffraction pattern from a single biological molecule would consist of a more or less continuous intensity distribution rather than Bragg spots. Successive diffraction patterns from molecules of the same protein in a molecular beam would each represent the intensity diffracted by a single molecule in a random orientation. Each diffraction pattern may in fact be regarded as an Ewald sphere passing through a random part of the molecular reciprocal space containing the origin. In principle, these portions can be ``patched together'' to extract the 3-dimensional diffracted intensity distribution of the molecule. The problem is to identify each measured pixel of a 2-dimensional diffraction pattern produced by a molecule of unknown orientation with a unique position in the 3D reciprocal space representing the Fourier Transform of the molecule's electron density. Methods for identifying relative molecular orientations from a set of projected images have been developed for 3D electron microscopy [1]. However, the absence of phase information in diffraction patterns causes additional difficulties, including ``Friedel pair'' ambiguities. Nevertheless, inspired by 3D electron microscopy, we have explored two different approaches to this problem: the method of common lines [2]; and a projection matching method [3]. We will describe the extent to which such approaches, combined with an iterative phase recovery algorithm [4] may be expected to yield the electron density distribution, and hence the structure of individual biological molecules. \newline \newline [1] J. Frank, Three-Dimensional Electron Microscopy of Macromolecular Assemblies, Oxford University Press, 2006. \newline [2] A. B. Goncharov \textit{et al.}, Sov. Phys. Crystallogr. \textbf{32}, 504 (1987). \newline [3] P. Penczek \textit{et al.}, Ultramicroscopy \textbf{40}, 33 (1994). [4] e.g. J. R. Fienup, Appl. Optics \textbf{21}, 2758 (1982) [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W5.00005: Crypto-tomography: the data assembly challenge in single-molecule diffraction Invited Speaker: In the absence of a molecular alignment mechanism, the diffraction patterns collected in single-molecule XFEL experiments will sample randomly oriented, 2D slices of a 3D data set. The signal to noise ratio in the individual slices will be so low that the relative orientations of any two will be poorly determined. This talk describes a new strategy for data assembly, where the relationships among multiple slices are determined collectively. [Preview Abstract] |
Session W6: Keithley Award
Sponsoring Units: GIMSChair: Stan Tozer, National High Magnetic Field Laboratory, Florida State University
Room: Colorado Convention Center 207
Thursday, March 8, 2007 2:30PM - 3:06PM |
W6.00001: Keithley Award Talk Invited Speaker: Superconducting transition-edge sensors (TES) operated at temperatures below 1 K are a sensitive tool for the detection of electromagnetic radiation from microwaves through gamma rays, and for the measurement of the energy of particle interactions and nuclear decays. They have evolved beyond the research and development phase, and they are being used in applications as diverse as astronomy, nuclear and particle physics, and materials science. The low noise, low source impedance, and low operating temperature of superconducting quantum interference devices (SQUIDs) make them the preamplifier of choice for TES devices. In order to realize their full potential, it has been necessary to develop arrays of thousands of SQUID-coupled transition-edge sensors. Due to constraints on cryogenic wiring and circuit complexity, SQUID multiplexing is necessary to realize these advances. In this talk, I will describe the development of large arrays of TES detectors integrated with multiplexed SQUID amplifiers. SQUID multiplexers use an orthogonal basis set (usually time- or frequency-division) to encode the signal from many input channels into a single wire. I will discuss both fundamental limits and practical issues of implementation, including bandwidth-limiting filters, power dissipation, and crosstalk. I will highlight work done by our group at the National Institute of Standards and Technology to develop time-division multiplexed arrays of thousands of SQUID amplifiers, and collaborations with different groups to integrate them into large arrays of TES sensors for a variety of applications. I will also discuss future trends, including the development of microwave techniques to read out even larger arrays of SQUID amplifiers in high-Q resonant circuits. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W6.00002: Cold Probes of the Hot Universe Invited Speaker: High-resolution x-ray spectroscopy is becoming a powerful tool for studying the hot (1 - 100 MK) and dynamic universe. The grating spectrometers on the XMM and Chandra satellites have sparked a new era in x-ray astronomy, but there is need to deploy instrumentation that can provide higher spectral resolution with high throughput in the Fe-K band (around 6 keV) and for extended sources. These new spectrometers will be based on arrays of microcalorimeters operated at 0.1 K and below. A microcalorimeter measures a small amount of heat in a weakly heatsunk thermal mass by sensing a temperature change in the presence of thermodynamically unavoidable temperature fluctuations. Low temperature operation is required in order to minimize this thermal noise and to reduce the heat capacity. The most advanced microcalorimeter technology to date is based on using a temperature-dependent resistance for the thermometer element, either a semiconductor thermistor or a superconducting transition-edge sensor (TES). At Goddard, we have been developing microcalorimeters for x-ray astrophysics since our pioneering work in 1984, and we have pursued both silicon and TES technology, and optimizations for different telescopes and energy bands. In our latest TES design, we have achieved a resolution of 2.5 eV at 6 keV. I will review the microcalorimeter research at Goddard and will discuss prospects for getting such an instrument deployed in orbit. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W6.00003: The Cosmic Microwave Background Invited Speaker: Measurements of the Cosmic Microwave Background (CMB) provide our earliest direct information about the evolving Universe. This talk will begin with a summary of how the CMB was produced and why it is important. The focus will than shift to the nature of the experimental challenge of extracting Cosmological information from the CMB. Examples will be given of technology development in small-scale experiments leading to major space missions which produce definitive data sets. The millimeter-wave spectral range of the signals corresponds to the crossover between coherent (radio) techniques and bolometric (optical) techniques. These challenges have stimulated enormous development of bolometric detectors, which are used to measure both the spectrum and the anisotropy of the CMB. The next generation of CMB experiments will require a new generation of bolometric detectors in large format arrays. This year, the Keithly Prize is given to Kent Irwin for ideas that have made this next step possible. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W6.00004: Dark Matter Invited Speaker: |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W6.00005: Quantum Calorimeter Gamma-ray Detectors: New Tools for Non-proliferation Invited Speaker: High resolution $\gamma$-ray spectroscopy is an important tool for non-destructive analysis of nuclear materials and is often used by safeguards inspectors to help verify the inventories of nuclear materials held around the world. The energy spectrum of photons emitted from isotopes of uranium or plutonium in the $40-1000$ keV energy range give unique signatures that, if accurately measured, give inspectors important information about the age and enrichment of the material and therefore its intended purpose. In this talk I will describe recent work by a team of researchers from the Unversity of Denver, the National Institute of Standards and Technology, and Los Alamos National Laboratory on $\gamma$-ray spectrometers with more than an order of magnitude improvement in energy resolution over standard techniques. The heart of this improved tool for non-proliferation is a microcalorimeter $\gamma$-ray detector that combines a micromachined thermal isolation structure with a bulk absorber and a highly sensitive superconducting transition-edge thermometer optimized for operation well below $1$ K. In the last several months, we have assembled and tested arrays of these microcalorimeters, with many detector pixels on a single chip. When read out with SQUID multiplexers, these arrays dramatically increase the speed of data collection, allowing ultra-high resolution $\gamma$-ray spectra to be acquired in roughly the same time needed for traditional detector technologies. In addition to presenting high-resolution $\gamma$-ray spectra of nuclear materials such as plutonium, I will describe the physics of the microcalorimeter, which ranges from the lifetime of quasiparticles in bulk superconductors to the thermal properties of glue. [Preview Abstract] |
Session W7: Novel Phenomena in Quantum Fluid He-3
Sponsoring Units: DCMPChair: Gary Ihas, University of Florida
Room: Colorado Convention Center Korbel 4A-4B
Thursday, March 8, 2007 2:30PM - 3:06PM |
W7.00001: Transport in highly spin-polarized normal liquid 3He Invited Speaker: Normal liquid Helium 3 is an ideal system to study the role of correlations in fermions physics. It is characterized by strong interactions between particles, the range of which is comparable to the inter-atomic distance. As such, it represents an intermediate case of complexity, halfway between the electronic systems and the ultra-cold Fermi gases. In particular, transport in degenerate Helium 3 involves not only s-wave scattering, but also partial waves with non-zero orbital angular momentum. Studying the polarization dependence of transport allows to directly probe this fact. We will report on transport experiments in highly spin-polarized, degenerate, liquid 3He, obtained by melting spin polarized solid 3He and rapidly cooling the resulting liquid down to about 60 mK. While the polarization dependence of viscosity is unexpectedly close to that predicted for a free fermion gas, the thermal conductivity increases much less with polarization than expected in that case. We will discuss the possible reasons for this difference. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W7.00002: Magnetic Relaxation and Minority Spin Condensate in Spin-Polarized Superfluid $^3$He A$_1$ Invited Speaker: The magnetic relaxation phenomena in superfluid $^3$He A$_1$ phase are studied using a magnetic fountain pressure detector in which a large reservoir is connected to a small sensor chamber through two superleak channels of height 18 $\mu$m. Superflow in simultaneous mass/spin current is driven by an externally applied magnetic field. Measurements of the relaxation of the induced fountain pressure are carried out under a variety of conditions including pressure(3 - 29 bar), temperature, static field(up to 8 T) and $^4$He(5 monolayers) coverage. The relaxation of the fountain pressure arises from the time dependent spin density in the sensor chamber. The observed relaxation time $\tau$ varies from 80 s near the upper transition temperature, T$_{c1}$, to less than 0.1 s near the lower transition temperature, T$_{c2}$. The measured relaxation rate increases starting near the middle of A$_1$ phase and more rapidly as the T$_{c2}$ is approached. The $^4$He coverage is observed not to affect the measured spin relaxation rate and this indicates that the relaxation is a bulk liquid effect. The rapid increase in relaxation rate is interpreted in terms of the Leggett-Takagi$^1$ mechanism of intrinsic spin relaxation arising from a small but increasing presence of minority spin pair condensate$^2$(with pair magnetic moment aligned in the opposite direction to the applied field) in A$_1$ phase as T$_{c2}$ is approached. It is concluded that the conventional view of the superfluid A$_1$ phase being composed of condensate pairs with magnetic moment aligned strictly along the applied field is inadequate. \newline $^1$ A.J. Leggett and S. Takagi, Ann. Phys. \textbf{106}, 79(1977). \newline $^2$ H. Monien and L. Tewordt, J. Low Temp. Phys. \textbf{60}, 323(1985). [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W7.00003: Specific Heat of Superfluid $^{3}$He and Andreev Bound States Invited Speaker: The specific heat at the normal to superfluid transition gives a clear thermodynamic signature for onset of the superfluid state marked by a discontinuity which has been accurately determined by Greywall[1]. We have measured the effect of a silver surface on the specific heat at this transition and we have found a temperature dependent suppression of the specific heat in the superfluid state which we have studied as a function of temperature and pressure[2]. This result can be understood in terms of the contribution to the Free energy from surface Andreev bound states which have a range of half of a superfluid coherence length. For the case of very large surface-to-volume ratio, as can be achieved with high porosity silica aerogel, the superfluid transition is suppressed. We have measured the specific heat anomaly at the transition temperature for this case[3] and interpret our measurements in terms of scattering theory. At the lowest temperatures a band of Andreev surface bound states dominate the specific heat of the superfluid $^{3}$He/aerogel system. This work was performed in collaboration with H. Choi, J.P. Davis, and J. Pollanen at Northwestern University, supported by the NSF grant DMR-0244099. [1] D.S. Greywall, Phys. Rev. B. 33, 7520, (1986). [2] H. Choi, J.P. Davis, J. Pollanen, and W.P. Halperin, Phys. Rev. Lett. 96, 125301 (2006). [3] H. Choi, K. Yawata, T.M. Haard, J.P. Davis, G. Gervais, N. Mulders, P. Sharma, J.A. Sauls, and W.P. Halperin, Phys. Rev. Lett. 93, 145301 (2004). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W7.00004: Surface Andreev Bound States of $^3$He-B by Transverse Acoustic Impedance Measurements Invited Speaker: Complex transverse acoustic impedance of the superfluid $^3$He was measured at the frequencies of 10 to 80 MHz from 6 up to 25 bar by a CW bridge method. The observed temperature dependence of it was well explained by the quasi-classical theory with random $S$-matrix model for a diffusive surface. The impedance was influenced by pair breaking and by quasi-particle density of states at the surface, which was drastically modified from the bulk one by the formation of surface Andreev bound states. In B phase, an additional gap in SDOS opened between the upper energy edge $\Delta^*$ of the surface Andreev bound states band and the bulk energy gap $\Delta$. Temperature dependence of $\Delta^*$ was measured and was about 30\% smaller than theoretical values. In A phase, flat and gapless SDOS was confirmed experimentally for the first time. It is demonstrated that the present spectroscopic method is a good tool to investigate the surface microscopic state, which has not been possible for the charge neutral {\it P}-wave superfluid. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W7.00005: Pure quantum turbulence in superfluid $^{3}$He. Invited Speaker: Turbulence in classical fluids has far reaching technological implications but is poorly understood. A better understanding might be gained from studying turbulence in quantum systems. In a pure superfluid (at low temperatures), there is no viscosity and vortex lines are quantised. Quantum turbulence consists of a tangle of quantised vortex lines which interact via their self induced flow. We have recently developed techniques for detecting quantum turbulence in superfluid $^{3}$He-B in the low temperature limit. We find that the transition to turbulence from a moving grid occurs by the entanglement of emitted vortex rings. At low grid velocities, ballistic vortex rings are emitted which become entangled at higher grid velocities leading to turbulence. The quantum turbulence decays in a manner very similar to classical turbulence, suggesting that energy is transferred down a broad range of length scales. The decay mechanism, in the absence of any normal fluid, is a very interesting, and currently unsolved, theoretical problem. Experimentally, it appears that the quantum of circulation plays the role of viscosity in governing the decay rate. We hope to gain more detailed information by measuring fluctuations in the turbulent flow field. We discuss current and future experiments. [Preview Abstract] |
Session W8: Superconductivity Theory: Phonons and Other Mechanisms
Sponsoring Units: DMPChair: Nandini Trivedi, The Ohio State University
Room: Colorado Convention Center Korbel 1C
Thursday, March 8, 2007 2:30PM - 3:06PM |
W8.00001: Anti-Jahn-Teller effect and d-wave phonon mechanism in cuprates Invited Speaker: Undoped cuprates are Jahn-Teller (JT) Materials [1] and Mott AF insulators [2]. When holes are doped, the octahedrons or pyramids elongated by the JT effect shrink. We call such distortion against the JT effect ``\textbf{\textit{anti-Jahn-Teller effect}}''[3]. By the interplay of the \textit{anti-Jahn-Teller effect }and \textit{Mott physics}, the two multiplets, the Zhang-Rice singlet and the Hund's coupling triplet, become nearly degenerate, and thus the hole-carriers in the underdoped regime form a metallic state, by taking the two multiplets alternately in the presence of the local AF order without destroying it [3,4]. On the basis of this two-component K-S model with small Fermi surfaces, the mechanism of superconductivity is discussed by the interplay of the electron-phonon interactions and local AF order [5,3]. It is shown that (1) the phase difference of wave functions between up- and down-spin carriers leads to the gap of dx$^{2}$-y$^{2}$ symmetry, and (2) the calculated concentration dependences of Tc and of isotope effects for LSCO are consistent with recent experimental results including anomalous isotope effects [6]. Finally the origin of the high-energy pseudogap is discussed based on the K-S model [3]. \newline \newline [1] J.G. Bednorz and K.A. Mueller, \textit{Z. Phys}. \textbf{B64}, 189 (1986). \newline [2] P.W. Anderson, \textit{Science} \textbf{235}, 1196 (1987). \newline [3] H. Kamimura \textit{et al}., \textit{in Theory of Copper Oxide Superconductors} (Springer, Heidelberg, 2005). \newline [4] H. Kamimura and Y. Suwa, \textit{J. Phys. Soc. Jpn.}\textbf{ 62,} 3368 (1993). \newline [5] H. Kamimura \textit{et al.} \textit{Phys. Rev. Lett.}\textbf{ 77,} 723 (1996). \newline [6] A.R. Bishop \textit{et al}., \textit{J. Supercond}., to be published (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W8.00002: Effects of Inhomogeneous Magnetic Correlations on the Penetration Depth in d-Wave Superconductors William Atkinson The influence of static magnetic correlations on the temperature-dependent superfluid density $\rho_s(T)$ is calculated for $d$-wave superconductors. In self-consistent calculations, itinerant holes form incommensurate spin density waves (SDW) which coexist with superconductivity. In the clean limit, the density of states is gapped, and $\rho_s(T\ll T_c)$ is exponentially activated. In inhomogeneously-doped cases, the SDW are disordered and both the density of states and $\rho_s(T)$ obtain forms indistinguishable from those in dirty but pure $d$-wave superconductors, in accordance with experiments. We conclude that the observed collapse of $\rho_s$ at $x\approx 0.35$ in underdoped $\backslash$YBCO may plausibly be attributed to the coexistence of SDW and superconductivity. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W8.00003: Frustrated metallicity in the quasi-one-dimensional conductor PrBa$_2$Cu$_4$O$_8$ Alessandro Narduzzo, Araz Enayati-Rad, Florence Rullier-Albenque, Shigeru Horii, Nigel Hussey We have investigated the ground state of the extremely anisotropic quasi-one-dimensional metal PrBa$_2$Cu$_4$O$_8$ ($t_ {b}^2:t_{a}^2: t_{c}^2 \sim 4000: 2: 1$), the non- superconducting analogue of the high-T$_c$ cuprate YBa$_2$Cu$_4 $O$_8$, as a function of disorder content, introduced either through atomic-site substitution or electron irradiation. A common single disorder threshold is found to drive interchain and in-chain resistivities into a low temperature regime where they display $d\rho/dT<0$. The survival of a large magnetoresistance of orbital origin reveals the itinerancy of the electronic system not to be suppressed by the presence of disorder. We propose an interpretative scenario based on a microscopic fragmentation of the metallic chains, though in contrast to many previous theoretical proposals, coherent hopping between chains appears to remain a relevant perturbation within the disordered system. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W8.00004: Critical Number of Fermions in Anisotropic QED$_{3}$; Application to the Pseudogap State of High-T$_c$ Cuprates Andres Concha, Valentin Stanev, Zlatko Tesanovic The low-energy physics of d-wave superconductors is marked by the presence of four nodal points where the gap function vanishes. This nodal structure can be used as the basis of an effective theory for fermionic quasiparticles, which turns out to be an incarnation of a two-dimensional quantum electrodynamics (QED$_3$), where the gauge field encodes quantum fluctuations in the phase of the gap function. The theory predicts the Fermi surface of the pseudogap state which is confined to the four nodal points and contains an intrinsic anisotropy reflecting the difference between the gap and Fermi velocities. In this context, the emergence of an antiferromagnetic order can be described as the dynamical generation of mass due to the phenomenon of spontaneous chiral symmetry breaking. Mass generation occurs when the number of fermionic species in the theory is less than some critical number N$_{c}$, the actual value of which is still much debated. Based on simple physical arguments we find that N$_{c}$ does depend on anisotropy and, more surprisingly, different regimes emerge depending on the ratio between the Fermi and gauge field velocities. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W8.00005: Properties of the ``quasi-particles'' at a nodal nematic quantum critical point Eun-Ah Kim, Paul Oreto, Steve Kivelson, Eduardo Fradkin We study the properties of a $d-$wave superconductor in the vicinity of a quantum critical transition to a nematic (or $d+s$ superconducting) phase. Most interactions have little effect on nodal quasiparticles, due to the limitted phase space available for scattering. The few critical modes that do couple effectively (such as the phase fluctuations treated in the context of QED3) produce a renormalized (fixed point) dispersion that is isotropic (pseudo-Lorentz invariant). This contrasts with the extreme anisotropy found in ARPES experiments on cuprate superconductors, which is often considerably larger even than anticipated from mean-field considerations based on the magnitude of $\Delta_0/E_F$ We find quantum fluctuations near a nodal nematic quantum critical point strongly enhance the dispersion anisotropy, and are efficient inelastic scatterers. The quantum field theory which describes the nodal nematic critical point is non-Lorentz invariant and the nodal quasiparticles display clear non-Fermi liquid behavior. The fermion spectral function displays nontrivial structure, which can be compared with those measured by ARPES in cuperate superconductors. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W8.00006: Excitons in QED$_3$ and spin response in a phase-fluctuating $d$-wave superconductor Babak Seradjeh, Igor Herbut We study the particle-hole exciton mode in the QED$_3$ theory of a phase-fluctuating $d$-wave superconductor in ladder approximation. We derive a Schr\"odinger-like equation for the exciton bound state and determine the conditions for its existence. We find the dispersion of this mode below the particle-hole continuum and compare our results with the resonance observed in neutron scattering measurements in cuprates. See http://www.physics.ubc.ca/~babak/march07/ for a list of references on this work. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W8.00007: Effects of inhomogeneities and thermal fluctuations on the spectral function of a model d-wave superconductor Daniel Valdez-Balderas, David Stroud We compute the spectral function of a model for high-temperature superconductors, at both zero and finite temperatures $T$. The model consists of a two-dimensional BCS Hamiltonian with $d$-wave symmetry, which has a spatially varying, thermally fluctuating, complex gap $\Delta$. Thermal fluctuations are governed by a Ginzburg-Landau free energy functional. We assume that a fraction $c_{\beta}$ of the superconductor area has a large $\Delta$ ($\beta$ regions), while the rest has a smaller $\Delta$ ($\alpha$ regions). $\alpha$ and $\beta$ regions are randomly distributed in space. We find that the inhomogeneous gap distribution of $\Delta$ affects the spectral function primarily near $\mathbf k = (\pi,0)$. For $c_{\beta}\simeq 0.5$, a split band appears if the difference between the gap magnitudes in the $\alpha$ and $\beta$ regions is sufficiently large; otherwise, the band is only broadened. Thermal fluctuations also affect the spectral function most strongly near $\mathbf k = (\pi,0)$, where the peaks that are sharp and high at zero temperature become reduced, widened, and shifted toward smaller energies as $T$ increases through the Kosterlitz-Thouless transition temperature. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W8.00008: Enhanced superconductivity due to inhomogeneous bond order in a doped Mott insulator Jun Liu, Joerg Schmalian, Nandini Trivedi At half filling, the ground state of SrCu2(BO3)2, a half filled Mott insulator on the Shastry-Sutherland lattice, is exactly described by a valence bond wave function. Using a resonating valence bond wavefunction for the doped system, that includes the correct limit at half-filling, we find that the doped quantum magnet shows long ranged superconducting order. The superconductivity is boosted by the spontaneous emergence of a checker board pattern of the pairing strength on the bonds. We further find a strong asymmetry between hole and electron doping. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W8.00009: Mott transition in Kagom{\'e} lattice Hubbard model Takuma Ohashi, Norio Kawakami, Hirokazu Tsunetsugu We investigate the Mott transition in the Kagom{\'e} lattice Hubbard model using the cellular dynamical mean field theory. The calculation of the double occupancy, the density of states, the static and dynamical spin correlation functions demonstrates that the system undergoes the first-order Mott transition at the Hubbard interaction $U/W \sim 1.4$ ($W$: bandwidth). In the metallic phase close to the Mott transition, we find the strong renormalization of three distinct bands, giving rise to the formation of heavy quasiparticles with strong frustrated interactions. It is elucidated that the quasiparticle states exhibit anomalous behavior in the temperature-dependent spin correlation functions. We also find a dramatic change of the dominant spin fluctuations around the Mott transition. The spin fluctuations in the insulating phase favor down to the lowest temperature a spatial spin configuration in which antiferromagnetic correlations are strong only in one chain direction but almost vanishing in the others. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W8.00010: Spin susceptibility representation of the pairing interaction for the two-dimensional Hubbard model Thomas Maier, Mark Jarrell, Douglas Scalapino We will discuss recent dynamic cluster quantum Monte Carlo studies of the effective pairing interaction responsible for d-wave pairing in the doped two-dimensional Hubbard model with an on-site Coulomb interaction U equal to the bandwidth. Motivated by earlier studies that show that the dominant contribution to the pairing interaction comes from the spin S=1 channel, we study a simple spin susceptibility representation of the particle-particle irreducible vertex. We find that with an effective temperature dependent coupling $\bar{U}(T)$ and the numerically calculated spin susceptibility $\chi(K-K')$, the d-wave pairing interaction is well approximated by $\frac{3}{2}\bar{U}^2(T) \chi(K-K')$. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W8.00011: Electron-Phonon Interaction and Antiferromagnetic Correlations Giorgio Sangiovanni, Olle Gunnarsson, Erik Koch, Claudio Castellani, Massimo Capone Recent experiments suggesting sizeable lattice effects in the cuprates raisethe issue of the the role of electron-phonon (e-ph) interaction in strongly correlated systems. By means of Dynamical Mean-Field Theory, we show that, in the Hubbard-Holstein model, antiferromagnetic (AF) correlations strongly enhance the effects of the e-ph coupling with respect to the paramagnetic phase,even though the net effect of the Coulomb interaction is a moderate suppression of the e-ph interaction. Doping weakens the AF correlations and reduces the effects of the e-ph, leading to a scenario in which the tendency to polaron formation is weakened by doping, in agreement with the experimental results [1]. \newline \newline [1] G. Sangiovanni {\it et al.}, Phys. Rev. Lett. {\bf 97}, 046404 (2006) [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W8.00012: High Energy Kinks in the Cuprates R.S. Markiewicz, S. Sahrakorpi, A. Bansil Tunneling studies in conventional superconductors are well known to reveal details of the electron-phonon interaction responsible for pairing. Similar features--low energy kinks in the 40-70 meV range--have also been observed in the cuprates, but their origin and possible role in pairing have been hotly debated. Recently, even higher energy kinks above 200 meV have been reported in the ARPES spectra of several cuprates. In this connection we discuss the roles of electron-plasmon as well as electron-magnon effects and show that collective modes in the charge and spin channels in the cuprates yield band renormalizations at low energies and anomalous features in band dispersion at higher energies, which are in substantial accord with experimental results. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W8.00013: ABSTRACT HAS BEEN MOVED TO P8.00003 |
Session W9: Superconductivity: Optical and Raman Spectroscopy
Sponsoring Units: DMPChair: David Tanner, University of Florida
Room: Colorado Convention Center Korbel 1D
Thursday, March 8, 2007 2:30PM - 2:42PM |
W9.00001: Evolution of the Spin Susceptibility of High-$T_c$ Superconductors. Thomas Timusk, Jungseek Hwang, Ewald Schachinger, Jules Carbotte We demonstrate that a new tool, a model independent numerical Eliashberg inversion of the optical self-energy, based on maximum entropy considerations can be used to extract the magnetic excitation spectra of high-transition-temperature superconductors. In Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ we explicitly show that the magnetic mode that dominates the self-energy at low temperatures directly evolves out of a smooth transfer of spectral weight to the mode from the continuum just above it. This redistribution starts already at 200 K in optimally doped materials but is much weaker in overdoped samples. This provides evidence for the magnetic origin of the superconductivity and presents a challenge to theories of the spin susceptibility and to neutron scattering experiments in high-temperature superconductors. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W9.00002: Intraband Optical Spectral Weight in the presence of a van Hove singularity: application to Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Frank Marsiglio, Fabrizio Carbone, Alexey Kuzmenko, Dirk van der Marel While the Kubo sum rule is often applied to the entire optical spectral weight to learn about the bare plasma frequency, the so-called ``Kubo single band sum rule'' is used to determine the optical spectral weight corresponding to intraband transitions in the valence band. We use a tight binding band with further than nearest neighbour hopping to explore the range of superconductivity-induced changes that are possible. We find that changes of both signs can occur within a conventional BCS framework. Using a band structure determined for Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, we find that a straightforward BCS calculation of the optical spectral weight cannot account for the experimental observations. Including a scattering rate collapse phenomenologically, however, gives a very good description of the change in optical spectral weight at $T_c$ as a function of doping. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W9.00003: Hall conductivity spectral weight in electron and hole doped cuprates H. D. Drew The optical conductivity quasiparticle spectral weight in optimally electron and hole doped cuprates is suppressed to about 25{\%} of value predicted by band stucture.$^{1}$ This suppression is due to Mott-Hubbard correlations caused by strong coulomb interactions. These correlations inter the longitudinal conductivity and the Hall conductivity differently. We have investigated the Fermi-liquid like behavior of the electron doped Pr$_{1.82}$Ce$_{0.18}$CuO$_{4}$ (slightly overdoped) and optimally hole doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O $_{8+\delta }$ using a spectral weight analysis of the Hall conductivity. In both materials the Hall conductivity spectral weight was found to be suppressed to about 10{\%} of the band value. This more substantial suppression of the Hall spectral weight addresses fundamental questions concerning the effects of Mott and antiferromagnetic correlations on the transport properties of strongly correlated materials. $^{1 }$ A. J. Millis, \textit{et. al.}, Phys. Rev. B72, 224517 (2005). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W9.00004: High energy scales in electronic self-energy imaged by optical conductivity E.J. Nicol, J. Hwang, T. Timusk, A. Knigavko, J.P. Carbotte We use a new technique to directly extract an estimate of the quasiparticle self-energy from the optical conductivity which can be easily related to both theory and angle-resolved photoemission spectroscopy (ARPES) experiments. In the high $T_c$ cuprate Bi-2212 we find evidence for a new high energy scale at 900 meV in addition to the two previously well known ones at roughly 50 and 400 meV. The intermediate scale at 400 meV has been recently seen in ARPES as a large kink which optics finds to be weaker and shifted. In YBCO, the three energy scales are shifted to lower energy relative to Bi-2212 and we observe the emergence of a possible fourth high energy feature at 600 meV. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W9.00005: Optical and thermodynamic properties of Hg-1201 E. van Heumen, R. Lortz, F. Carbone, A.B. Kuzmenko, D. van der Marel, X. Zhao, G. Yu, Y. Cho, N. Barisic, M. Greven, C.C. Homes, S.V. Dordevic We present optical spectra and specific heat measurements of the optimally doped, single layer cuprate superconductor HgBa$_{2}$CuO$_{4}$ (T$_{c}$ = 97 K). Optical spectra have been obtained with a high temperature resolution allowing us to track small changes in the integrated spectral weight. We find that the low frequency spectral weight shows an extra increase when the system enters the superconducting state, indicating that the kinetic energy of the charge carriers decreases in the superconducting state. This is consistent with our earlier observations on other optimally doped cuprates$^{1,2}$. From specific heat measurements on the same sample we estimate the change in internal energy and compare this with the estimated changes of the kinetic energy. We find a change in internal energy $\Delta $U$\approx $0.1 meV per copper and $\Delta $W$\approx $0.1 meV per copper for the kinetic energy. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W9.00006: Optical conductivity of single plane cuprate superconductor HgBa$_2$CuO$_4$ R.P.S.M. Lobo, N. Bontemps, J. Hwang, J. Yang, T. Timusk, D. Colson, A. Forget We investigated the ab-plane infrared and visible spectra of a HgBa$_2$CuO$_4$ single crystal close to optimal doping ($T_c = 90$~K) from 100 to 40000 cm$^{-1}$. Data as a function of temperature (down to 30 K) was limited to frequencies below 10000 cm$^{-1}$. The low frequency scattering rate has a linear frequency dependence. Under 120~K a supplementary small drop below 1000 cm$^{-1}$ suggests the presence of a pseudogap. This is the same frequency at which the optical conductivity shows a clear loss of spectral weight in the superconducting state. The low frequency effective mass is temperature dependent and increases from 1.5 at room temperature to 2.5 just above $T_c$. We will compare our results to other single plane cuprates. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W9.00007: Visible Pump-THz Probe Spectroscopy of the Undoped Cuprate Sr$_2$CuCl$_2$O$_2$ Jesse Petersen, J. Steven Dodge, Ruixing Liang We present experimental results on the mobility of photoexcited carriers in Sr$_2$CuCl$_2$O$_2$, an undoped cuprate. We use ultrafast laser pulses to excite photocarriers in the antiferromagnetic insulating lattice. We then probe the low-frequency dynamical conductivity of this nonequilibrium state with time-domain terahertz spectroscopy. Our current measurements place an upper bound on the mobility that is consistent with Hall mobility measurements.\footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)} [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W9.00008: Small Magnetic Fields Arrest the Josephson Plasma Resonance in La$_{2-x}$Sr$_{x}$CuO$_{4 }$for x=1/8 Alexander Schafgans, Andrew LaForge, Sasa Dordevic, Seiki Komiya, Yoichi Ando, Dimitri Basov We report on a study of the far infrared interlayer response in a La$_{2-x}$Sr$_{x}$CuO$_{4}$ (La214) crystal at the x=1/8 doping. A magnetic field up to 8 Tesla, applied perpendicular to the CuO$_{2}$ planes, is found to completely suppress the Josephson plasma resonance (JPR) in sharp contrast to the mild suppression of the JPR if the field is applied along the planes. We suggest that this anomalous sensitivity of the JPR feature to modest fields for H$\vert $c-axis is due to the interaction of in-plane charge inhomogeneities with vortices that form in the CuO$_{2}$ planes. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W9.00009: Order parameter of the paired hole states in Sr$_{14}$Cu$_{24}$O$_{41}$ studied by optics and UV-resonance Raman scattering Andrivo Rusydi, B. Schulz, R. Rauer, I. Mahns, H. Eisaki, Y. Fujimaki, S. Uchida, P. Abbamonte, M. R\"ubhausen The order parameter of the paired hole states of hole Wigner crystal (HC) in the self-doped spin (S)=1/2 two-leg ladders of Sr$_{14}$Cu$_{24}$O$_{41}$ (SCO) is studied with optics and UV-resonance Raman scattering. We observe a pair breaking excitation of the holes (2$\Delta _{h-h})$ at 200 meV which can be attributed to the rungs of the ladders. The intensity of the 2$\Delta _{h-h}$ peak as function of temperature matches very well with the formation of HC at about 250 K ($T_{HC})$. The energy of 2$\Delta _{h-h}$ is temperature independent, even at its transition temperature indicating a remaining short range order with a strongly decreased volume fraction. The order parameter of the paired hole states develops in a non-mean-field fashion and 2$\Delta _{h-h }$/k$_{b}$T$_{HC}$ is about 11, i.e. in the strong coupling limit. Our optics studies also show low- and high-spin transitions along the legs and rungs below 130 K. Our measurements confirm theoretical predictions of the existence of the paired hole states outlining the strong local pairing of the holes. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W9.00010: Ultrafast Observation of the Coexistence of Antiferromagnetism and Superconductivity in the High-T$_{c}$ Superconductor Tl-2223 Elbert E.M. Chia, Jian-Xin Zhu, D. Talbayev, A.J. Taylor, R.D. Averitt, In-Sun Jo, Kyu-Hwan Oh, Sung-Ik Lee In high-T$_{c}$ superconductors (HTSC), it is commonly believed that Cooper pairing occurs via antiferromagnetic spin fluctuations. These spin fluctuations can be shown to exist if the competing ground state to the superconducting (SC) state is antiferromagnetism (AFM). It reveals itself when, for example, the SC state is destroyed or suppressed using an externally applied magnetic field. Ultrafast spectroscopy has been widely used in probing the relaxation dynamics of photoexcited quasiparticles in correlated electron systems, and in particular, cuprate HTSCs. However, no such measurements have been taken for the regime where AFM and SC might possibly coexist. We report the first ultrafast relaxation measurements in such a coexistence phase in the HTSC Tl$_{2}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{y}$. Without applying any external magnetic field, we see a qualitative change in the relaxation dynamics below $\sim $40 K, which is suggestive of an entry into the AFM+SC coexistence phase. To quantitatively explain our data, we combined a coupled model describing the time-evolution of quasiparticles and high-frequency phonons in the presence of a gap in the density of states, and a mean field model that gives rise to a decrease in the SC gap as one enters the coexistence state. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W9.00011: Electron-phonon coupling in SrTi$_{1-x}$Nb$_{x}$O$_{3}$ D. van Mechelen, P. Armitage, C. Grimaldi, A. Kuzmenko, J. Teyssier, D. van der Marel One of the major questions in the physics of high temperature is, to what extent electron-phonon coupling is important for the transport anomalies and for the superconductivity itself. One of the difficulties in addressing this issue for the cuprates, is the complexity of these materials, which are doped Mott-insulators, anti-ferromagnetic, striped, etcetera. In order to separate out the electron-phonon coupling we have studied the perovskite SrTi$_{1-x}$Nb$_{x}$O$_{3}$ with 0.0002 $<$ x $<$ 0.02. The lowest unoccupied bands of pristine SrTiO$_{3 }$are Ti 3d bands of t$_{2g}$ character, which become occupied with electrons upon substituting Nb for Ti. Here we report THz, infrared and optical spectra, DC resistivity and Hall effect. The infrared spectra at 7 K reveal a very narrow (less than 2 meV) Drude peak, the spectral weight of which reveals a strong mass-enhancement (m*/m$\sim $10) at the lowest carrier concentration, which diminishes gradually to m*/m$\sim $2 as we increase the carrier concentration. The mass enhancement, the doping dependence thereof, and several other features in the infrared spectra, indicate that electron-phonon coupling is strong. The trend as a function of doping furthermore suggests the evolution from a phonon-renormalized Fermi-liquid toward a gas of small polarons in the limit of x$\to $0. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W9.00012: First demonstration of a superconducting detector cooled by solid-state refrigerators. N.A. Miller$^{1,2}$, J.A. Beall$^{1}$, D.J. Benford$^{3}$, T.C. Chen$^{4}$, J.A. Chervenak$^{3}$, W.D. Duncan$^{1}$, F. Finkbeiner$^{5}$, G.C. Hilton$^{1}$, K.D. Irwin$^{1}$, S.H. Moseley$^{3}$, G.C. O'Neil$^{1,2}$, D.R. Schmidt$^{1}$, L.R. Vale$^{1}$, R.F. Silverberg$^{3}$, J.N. Ullom$^{1}$ We have successfully cooled a Transition-Edge Sensor (TES) using solid-state refrigerators based on Normal metal/Insulator/Superconductor (NIS) tunnel junctions. The cooling mechanism is the preferential tunneling of the highest energy (hottest) electrons through the biased NIS junctions. We describe the cooling performance, temperature noise, and energy resolution of the NIS-cooled TES. In particular, we show that the NIS refrigerators introduce no detectable noise in the TES operation. NIS refrigerators can cool from temperatures near 0.3 K to below 0.1 K. Combining a pumped $^{3}$He system with NIS refrigerators provides a compact, lightweight alternative to adiabatic demagnetization refrigerators and dilution refrigerators. Bath temperatures near 0.1 K are desirable for state-of-the-art sensors for astronomy and materials analysis, as well as for a wide range of basic science applications. $^{1}$National Institute of Standards and Technology (NIST) -- Boulder $^{2}$University of Colorado at Boulder $^{3}$NASA/Goddard Space Flight Center $^{4}$Global Science and Technology $^{5}$SSAI [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W9.00013: Fast, Single-photon Detection with Superconducting Niobium Nanowires Anthony Annunziata, Aviad Frydman, Michael Rooks, Daniel Prober We present recent measurements of the quantum efficiency, counting rate, and dark count rate for single-photon detectors based on superconducting niobium nanowires at 337 nm and 470 nm wavelengths for several detector geometries. From this data it is shown that the reset time (and therefore the single-photon counting rate) of these detectors is not dependent on the kinetic inductance of the niobium wire, as is the case for other detectors of this type made from niobium-nitride. The counting rate approaches 1 GHz even for very large area (100 $\mu $m$^{2})$ detectors. A phenomenological model of detection is presented that suggests the ability to resolve the number of photons absorbed during individual detection events. Preliminary data is shown that supports this assertion. These detectors have a variety of potential applications ranging from VLSI circuit diagnostics to quantum communication and single molecule spectroscopy. This work is supported by NSF and IBM research. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W9.00014: Antenna-Coupled Superconducting Bolometers for Studying Dynamics with Terahertz Spectroscopy Daniel Santavicca, Matthew Reese, Alan True, Charlie Schmuttenmaer, Daniel Prober We report microwave and terahertz characterizations of antenna-coupled hot electron bolometers designed for laboratory-based terahertz spectroscopy. The active element is a superconducting niobium microbridge, and the incident signal is coupled to the microbridge by a planar double-dipole antenna. These devices combine sub-nanosecond response with high sensitivity and the ability to operate below saturation when viewing a room temperature background. The optimum small signal responsivity is 4.4 x 10$^{4}$ V/W, obtained at a bath temperature T$_{b}\approx $ 0.9T$_{c}$. The corresponding saturation power is 7 nW. The saturation power increases and the responsivity decreases as the bath temperature is lowered. The measured noise equivalent power is 2.0 x 10$^{-14}$ W/(Hz)$^{1/2}$, near the predicted thermal fluctuation limit. The unique combination of speed and sensitivity demonstrated by these detectors will enable new measurements of dynamic processes in the far-infrared on millisecond to nanosecond timescales. [Preview Abstract] |
Session W11: Low-dimensional Systems; Theory
Sponsoring Units: DCMPChair: Kirill Shtengel, University of California, Riverside
Room: Colorado Convention Center Korbel 1F
Thursday, March 8, 2007 2:30PM - 2:42PM |
W11.00001: Pseudogap in a Magnetic Field: A Case of Quantum Vortex Liquid in a Corelated d-wave Superconductor Zlatko Tesanovic Recent experiments by the Princeton group provide a clear indication of enhanced superconducting fluctuations in the pseudogap state of underdoped cuprates. In addition to the giant Nernst effect, which testifies to the thermal vortex excitations, strong diamagnetic and ``vortex liquid-type'' responses at very low temperatures point to predominance of {\em quantum} phase fluctuations throughout the pseudogap regime. This is of much importance: while {\em all} superconductors exhibit thermal phase fluctuations, the quantum phase disorder -- leading to a non-superconducting ground state despite robust local pairing correlations -- is decidedly outside the reach of any BCS-like, weak-coupling style theory. I will introduce general aspects of the theory of quantum disordered, strongly correlated d-wave superconductors and will use the theory to sketch the phenomenology of quantum vortex-liquid in heavily underdoped cuprates. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W11.00002: Hidden charge 2e boson in doped Mott insulators philip phillips, Robert Leigh, Ting-Pong choy We construct the low energy theory of a doped Mott insulator, such as the high-temperature superconductors, by explicitly integrating over the degrees of freedom far away from the chemical potential. For either hole or electron doping, integration of the high energy scale generates a charge 2e bosonic field that accounts for dynamical spectral weight transfer across the Mott gap. The bosonic field represents non-projective physics as its vanishing leads to the standard projected result, namely the $t-J$ model. A mean-field solution reveals that the free energy acquires a lower value in the presence of the bosonic field than when it is absent. Moreover, we find that at mean-field boson condensation defeats d-wave superconducting order in the vicinity of half-filling. The relationship between boson condensation and local-pairing in the pseudogap phase of the cuprates is explored. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W11.00003: Metal-Insulator Transition Revisited for Cold Atoms in Non-Abelian Gauge Potentials Indubala Satija, Daniel Dakin, Charles Clark We discuss the possibility of realizing metal-insulator transitions with ultracold atoms in two-dimensional optical lattices in the presence of artificial gauge potentials. Such transitions have been extensively studied for magnetic fields corresponding to Abelian gauges; they occur when the magnetic flux penetrating the lattice plaquette is an irrational multiple of the magnetic flux quantum. Here we present the first study of these transitions for non-Abelian $U(2)$ gauge fields. In contrast to the Abelian case, the spectrum and localization transition in the non-Abelian case is strongly influenced by atomic momenta. In addition to determining the localization boundary, the momentum fragments the spectrum. Other key characteristics of the non-Abelian case include the absence of localization for certain states and satellite fringes around the Bragg peaks in the momentum distribution and an interesting possibility that the transition can be tuned by the atomic momenta. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W11.00004: Anomalous superfluidity in two dimensional spin-1 systems Shailesh Chandrasekharan We study thermodynamics of a strongly coupled lattice gauge system in $2+1$ dimensions. The partition function of our model can be written elegantly as a statistical mechanics of dimers and loops. The model is invariant under an $SO(3) \times U(1)$ symmetry which makes it interesting in describing phase transitions in spin-1 systems. At low temperatures, we find evidence for superfluidity in the $U(1)$ symmetry sector while the $SO(3)$ symmetry remains unbroken. The finite temperature phase transition belongs to the Kosterlitz-Thouless universality class, but the superfluid density jump $\rho(T_c)$ at the critical temperature Tc is anomalously higher than the normal value of $2Tc/\pi$. We show that by adding an $SO(3)$ symmetry breaking term to the model, one can obtain a variety of superfluid density jumps, including the normal jump and four times the normal jump that arises in the presence of half vortices. We believe the presence of spin causes the anomalous superfluid behavior seen. Our results may be of interest to researchers studying superfluidity in spin-1 systems. Our work is published in PRL 97, 182001 (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W11.00005: Superconductivity in a two-orbital Hubbard model on a square lattice with Hund's rule coupling Katsunori Kubo It has been recognized that orbital degree of freedom plays important roles in determination of physical properties, in particular magnetism, of several materials. In recent years, effects of orbital degree of freedom on superconductivity have also been discussed. In this study, we investigate possible superconducting states of a two-orbital Hubbard model on a square lattice by applying fluctuation exchange approximation. We can classify superconducting states by spin states, orbital states, and representations of tetragonal symmetry. In particular, we focus on superconducting states peculiar to multi-orbital systems, such as a spin-triplet orbital-singlet s-wave state. We show that the Hund's rule coupling stabilizes such an exotic state. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W11.00006: Phase separation and electron pairing in repulsive Hubbard clusters Armen Kocharian, Gayanath Fernando, Tun Wang, Kalum Palandage, Jim Davenport The exact numerical diagonalization in ensemble of small Hubbard clusters reveal pairing fluctuations, Bose condensation and charge-spin separation in the ground state and finite temperatures [1,2]. The phase diagram off half filling strongly suggests existence of subsequent transitions from electron pairing into unsaturated ferromagnetic and saturated ferromagnetic Mott- Hubbard like insulators driven by electron repulsion. Rigorous criteria for occurrence of corresponding quantum critical points and crossover temperatures are formulated. The phase diagram for ${2\times 4}$-site clusters illustrates how these features are scaled with cluster size. The phase separation and electron pairing monitored by magnetic field and electron doping surprisingly resemble phase diagrams in family of doped high T$_c$ cuprates. [1] Phys. Rev. B 74, 024511, [2] (2006) cond-mat/0608579 (2006) [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W11.00007: Ferromagnetism in the one-band Hubbard Model on a triangular lattice Shi-quan Su, Zhong-bing Huang, Rui Fan, Hai-qing Lin We investigated numerically the existence of ferromagnetic phase in the one-band Hubbard model on a triangular lattice. By studying the spin susceptibilty, we found the model exhibits ferromagnetic properties when the density of electrons is low. Auxillary Field Quantum Monte Carlo (AFQMC) and Constrained Path Monte Carlo (CPMC) data are used to present the system behaviors including spin susceptibility, pair correlation, when the parameters of the model change. We found that these behaviors are related to the ferromagnetism of the model. These results can be viewed as evidences to support a route to metallic ferromagnetism in the one-band Hubbard models. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W11.00008: Novel Electronic Properties of the Hubbard Model on a Frustrated Triangular Lattice Bumsoo Kyung We study novel electronic properties of the Hubbard model on a triangular lattice using the cellular dynamical mean-field theory. The interplay of strong geometric frustration and electron correlations causes a Mott transition at the Hubbard interaction $U/t=10.5$ and an unusual suppression of low energy spin excitations. Doping of a triangular Mott insulator leads to a quasiparticle peak (no pseudogap) at the Fermi surface and to an unexpected increase of low energy spin excitations, in stark contrast to the unfrustrated square lattice case. The present results give much insight into strongly frustrated electronic systems. A few predictions are made. cond-mat/0608202 [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W11.00009: Luttinger Liquid Kink Trinanjan Datta, Erica W. Carlson, Jiangping Hu We consider a spin rotation invariant Luttinger Liquid at finite temperature and show the existence of a kink in the effective dispersion, as determined by the frequency-dependent peak in the momentum distribution curve (MDC). The MDC is defined by considering the single hole spectral function $A^<(\vec{k},\omega)$ as a function of $\vec{k}$ at a fixed frequency $\omega$. When the charge velocity is greater than the spin velocity, $v_c > v_s$, the high frequency dispersion is linear in $\vec{k}$ and follows $v_c$, while the low frequency dispersion (which is also linear in $\vec{k}$) follows some average of the two velocities. The energy scale of the crossover between the two velocities defines a kink, $E_{\rm kink}$. Since the Luttinger Liquid is quantum critical, $E_{\rm kink}$ scales with the temperature. The kink energy is also affected by the interaction strength, and the strength of the kink is controlled by the ratio of the spin and charge velocities. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W11.00010: Heisenberg spin chains with four-spin couplings Alexios Klironomos, Julia Meyer, Toshiya Hikihara, Konstantin Matveev We obtain and analyze the phase diagram of the zigzag Heisenberg spin chain including four-spin interactions arising from ring exchange processes. We perform exact diagonalization of chains up to 24 sites. In addition to the expected ferromagnetic, antiferromagnetic and dimer phases, the phase diagram contains a region of partial spin polarization as well as a region occupied by novel phase(s). [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W11.00011: Phase diagram of the one dimensional Hubbard-Holstein Model at 1/2 and 1/4 filling Rahul Hardikar, Torsten Clay We present a detailed study of the phase diagram of the Hubbard Holstein model at 1/2 filling and 1/4 filling, including finite-frequency quantum phonons within the numerically exact Stochastic Series Expansion quantum Monte Carlo method. In one dimension at 1/2 filling, the electron-phonon (e-ph) coupling gives a Peierls charge density wave, while Hubbard onsite $U$ promotes antiferromagnetic correlations and a Mott insulating state. Our previous study revealed a third Intermediate phase when the electron-electron and e-ph interaction are closely balanced. We show here from direct calculations of charge and spin susceptibilities that (i) as the e-ph coupling strength is increased first a spin gap transition and then the Peierls transition occurs, (ii) transitions between Mott/Intermediate and Intermediate/Peierls states are of the Kosterlitz-Thouless type, (iii) for large $U$, the two transitions merge into a single fist order transition. Our data is consistent with a renormalization of the Luttinger Liquid exponent $K_\rho$, which gives a slightly larger intermediate region as determined from susceptibilities than in previous calculations of $K_\rho$. At 1/4 filling we find a very similar phase diagram. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W11.00012: Deformation of SU(4) singlet spin-orbital state due to Hund's rule coupling Hiroaki Onishi, Takashi Hotta It has been widely recognized that the interplay of spin and orbital degrees of freedom plays a crucial role in the emergence of novel magnetism in strongly correlated systems. In this context, a one-dimensional spin-orbital model with the highest SU(4) symmetry has been one of the subjects of much interests from a theoretical viewpoint, and the critical behavior of the SU(4) singlet ground state has been clarified. However, in a more realistic situation, the Hund's rule coupling should break the SU(4) symmetry. In the present work, by exploiting a density-matrix renormalization group method, we investigate a one-dimensional spin-orbital model in which the SU(4) symmetry is broken down to SU(2)$_{\rm spin}$$\times$U(1)$_{\rm orbital}$ due to the Hund's rule coupling ($J$). At $J=0$, spin and orbital correlations coincide with each other with a peak at $q=\pi/2$, indicating the SU(4) singlet state with a four-site periodicity. On the other hand, with increasing $J$, the peak position of orbital correlation changes to $q=\pi$, while that of spin correlation remains at $q=\pi/2$. We will discuss in detail how the SU(4) singlet state is deformed by the Hund's rule coupling. [Preview Abstract] |
Session W12: Focus Session: Spin Dependent Tunneling II
Sponsoring Units: GMAG DMP FIAPChair: Athos Petrou, State University of New York at Buffalo
Room: Colorado Convention Center Korbel 3C
Thursday, March 8, 2007 2:30PM - 2:42PM |
W12.00001: Spin polarization of electrons in 2D structures due to reflection from a barrier V. Teodorescu, R. Winkler In two-dimensional semiconductor structures Rashba spin-orbit coupling can orient the electron's spin in two opposite directions perpendicular to the direction of motion. We analyze here the possibility to change the spin polarization of an electron beam which is reflected from a barrier in the 2D plane. In general, an unpolarized incident beam gives rise to three reflected beams with different polarizations [1]. We give the orbital and spin parts of the current densities inside and outside of the interference zones. Also we estimate for an initially unpolarized (or partially polarized) electron beam the change of the degree of polarization due to multiple reflections between two parallel barriers in a ballistic regime using realistic material parameters. [1] A.~O.\ Govorov et al., Phys.\ Rev.\ B 70, 245310 (2004) [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W12.00002: Spin-dependent properties of Fe/MgO/GaAs heterostructures Y. Li, Y. Chye, Y. Chiang, J. Stephens, D. Awschalom, R. Kawakami Developing efficient spin injectors and spin detectors is an important goal for semiconductor-based spintronics. Recently Jiang et. al.'s work using CoFe/MgO tunnel spin injectors showed significantly enhanced spin injection efficiency into GaAs due to a spin filtering effect of the MgO layer [a]. Using molecular beam epitaxy (MBE) deposition, we have successfully grown atomically flat MgO films on GaAs(001) epilayers. Below 2 nm thickness, the MgO films are found to be single crystalline. The spin-dependent properties of a Fe/MgO/GaAs heterostructure are investigated by time-resolved Faraday rotation (TRFR) to measure ferromagnetic proximity polarization (FPP) across MgO [b]. It is seen that a very small amount of MgO (less than 0.5 nm thickness) enhances the FPP significantly. We are investigating the FPP dependence on MgO thickness by scanning the optical beams across an MgO wedge. A systematic study on MgO thickness dependence will be presented and the mechanism of indirect FPP across MgO will be discussed. Supported by NSF, ONR, and CNID. (a) X. Jiang, et al., Phys. Rev. Lett. \textbf{94, }056601 (2005). (b) R. J. Epstein, et al., Phys. Rev. B \textbf{65, }121202 (2002) [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W12.00003: Magnetic field dependence of a resonant tunneling diode based in the GaMnAs/AlGaAs material system. Edward Likovich, Kasey Russell, Wei Yi, Venkatesh Narayanamurti, Keh-Chiang Ku, Nitin Samarth A resonant tunneling diode was fabricated with magnetic GaMnAs emitter and quantum well regions and a nonmagnetic p-GaAs collector.~ At 4K, below the Curie temperature for GaMnAs, negative differential resistance (NDR) associated with resonant tunneling of holes was observed.~ Both the magnitude of NDR as well as its associated bias were found to be dependent on magnetic field.~ If the device bias is held constant and the magnetic field is swept, our device exhibits either positive or negative tunneling magetoresistance (TMR) up to several tens of percent, depending on the device bias. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W12.00004: Electron Tunneling across EuS / InAs Heterojunctions R.L. Kallaher, Peng Xiong, Stephan von Moln\'{a}r, Mark Field, Gerard J. Sullivan The tunneling properties of the heterojunction formed between the ferromagnetic semiconductor EuS and the non-magnetic semiconductor InAs are investigated to explore the feasibility of injecting spin polarized electrons into a two dimensional electron gas. Below the ferromagnetic transition temperature, T$_{c}$, of EuS the barrier height of the heterojunction follows a Brillouin function with S=7/2, demonstrating that the transport is dominated by the large, $\sim$0.5 eV, Zeeman splitting of the conduction band in EuS.\footnote{J. Trbovic et al., Applied Physics Letters, 87, 82101 (2005).} At temperatures above T$_{c}$ the zero-bias conductance of EuS / InAs heterojunctions show two separate regimes, each having an exponential temperature dependence, indicating that other scattering mechanisms are present in the barrier in addition to magnetic fluctuation effects seen in Schottky barriers formed between EuS and metals.\footnote{W.A. Thompson et al., Physical Review Letters, 26, 1308 (1971).} [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W12.00005: Digital magneto resistance in magnetic MOBILEs Christian Ertler, Jaroslav Fabian Resonant tunneling structures comprising magnetic semiconductor layers are promising for realizing efficient spin filters and detectors [1]. Recently [2], we showed that a paramagnetic MOBILE (Monostable-Bistable Transition Logic Element), which consists of two serial connected resonant tunneling diodes (RTDs), the nonmagnetic load and the driver with a paramagnetic quantum well (QW), exhibits digital magneto resistance (DMR): a continuous change of the external magnetic field above a threshold value leads to a discrete jump of the output voltage from low to high. We have also proposed a nonvolatile ferromagnetic MOBILE, where the driver-RTD comprises a ferromagnetic emitter and QW. We show that DMR is realized by changing the relative orientation of the magnetizations above a threshold angle. In the low voltage regime the driver IV can be changed from ohmic to negative differential resistance behavior. Since conventional MOBILEs have been demonstrated to work up to 100 GHz the proposed device might be useful for performing very fast detections of magnetic signals. [1] I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] C. Ertler and J. Fabian, Appl. Phys. Lett. 89, 193507 (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W12.00006: Reversing the sign of the spin-polarized current across a Fe/GaAs tunnel barrier at finite voltage bias S.A. Crooker, X. Lou, C. Adelmann, E.S. Garlid, J. Zhang, S.M. Reddy, S.D. Flexner, C.J. Palmstrom, P.A. Crowell As a function of the voltage bias across a Fe/GaAs Schottky tunnel barrier, we measure the sign and magnitude of the electrically-injected electron spin polarization in the semiconductor, $P_{GaAs}$, using magneto-optical Kerr rotation at 10 K. Both images and Hanle depolarization curves reveal that the sign of $P_{GaAs}$ inverts when sweeping from small reverse bias (electrons flowing into GaAs) to small forward bias (electrons flowing into Fe), as expected from linear response. More strikingly, $P_{GaAs}$ inverts sign again at higher bias across the Fe/GaAs barrier. This crossover bias ($|V_{cross}| < 0.1$ V in the structures studied) is sample-dependent, and can occur under either forward- or reverse-bias conditions, depending on sample. These data concur with all-electrical measurements of $P_ {GaAs}$ in lateral spin transport devices having a source, drain, and a third `non-local' detection electrode. Models to describe these data will be discussed. We further show that, when Fe/GaAs tunnel barriers are employed as electrical spin {\it detectors}, both the sign and magnitude of the detection sensitivity can be tuned with applied bias on the detector. This work is supported by the Los Alamos LDRD and NSF MRSEC programs, and ONR. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W12.00007: Spin extraction theory and its spintronics applications Hanan Dery, Lu J. Sham Extraction of electrons from a semiconductor to a ferromagnet as well as the case of injection in the reverse direction may be formulated as a scattering theory. However, the presence of bound states at the interface arising out of an inhomogeneous doping on the semiconductor side must be taken into account in the scattering theory. Inclusion of the interface states yields an explanation of a recent result of spin imaging measurement which contradicts the current understanding of spin extraction (S. A. Crooker \textit{et al.}, Science \textbf{309}, 2191 (2005)). A particular consequence of our theory is a proposed electrically controlled spin-switch in which a non-magnetic back-gate monitors the spin polarization in a semiconductor. The switch also utilizes a ferromagnet to filter either of the spin species depending on the gate bias. Based on these ideas (and if time allows), we will also present a semiconductor spintronics prototype of a reprogrammable, universal \textbf {logic} gate which does not require magnetic fields throughout its operation. (See also, cond-mat/0609045) [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W12.00008: Spin-dependent tunneling properties in GaMnAs-based magnetic tunnel transistors Yosuke Mizuno, Shinobu Ohya, Pham Nam Hai, Masaaki Tanaka III-V-based ferromagnetic-semiconductor heterostructures comprising GaMnAs are hopeful candidates for future spintronic devices. Thus far, only two-terminal devices have mainly been studied. Meanwhile, GaMnAs-based `three-terminal' magnetic tunnel transistors (MTTs) have a potential to add novel functions to integrated circuits. We prepared MTT structures composed of GaMnAs (30 nm)/ AlAs (2 nm)/ GaMnAs (30 nm)/ GaAs:Be (30 nm; 1*10$^{17}$cm$^{-3})$ on $p$-GaAs(001) substrates using molecular-beam epitaxy (MBE). The $V_{EB}$ dependence of $I_{C}$, $I_{E}$, and $I_{B}$ shows that the current transfer ratio \textit{$\alpha $} (= $I_{C }$/$ I_{E})$ is 0.8-0.95; this is much higher than 0.03, the maximum value reported in metal-based MTTs. The current gain \textit{$\beta $} (= $I_{C }$/$ I_{B})$ is of the order of 10, which means that GaMnAs-based MTTs have current amplifiability. The $V_{EC}$ dependence of the tunneling magnetoresistance (TMR) ratio differed significantly from that observed in single-barrier magnetic tunnel junctions (MTJs). This work was partly supported by PRESTO / SORST of JST, Grant-in-Aids for Scientific Research, IT-RR2002 of MEXT, and Kurata-Memorial Hitachi Sci. {\&} Tech. Foundation. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W12.00009: Resonant tunneling effect and tunneling magnetoresistance in ferromagnetic-semiconductor quantum heterostructures Shinobu Ohya, Pham Nam Hai, Yosuke Mizuno, Masaaki Tanaka Ferromagnetic-semiconductor quantum heterostructures are expected to realize novel functions by combining the resonant tunneling effect and the tunneling magnetoresistance (TMR). However, there are no reports on the clear observation of the resonant tunneling effect and TMR associated with it in these structures. We fabricated the GaMnAs quantum-well (QW) double-barrier heterostructures composed of GaMnAs(20 nm)/AlGaAs(4nm)/GaMnAs(d=3.8-20 nm)/ AlAs(4nm)/GaAs:Be on p-GaAs (001) substrates using molecular-beam epitaxy (MBE). The dI/dV-V characteristics and bias dependence of TMR measured at 2.6 K clearly show oscillatory behaviors in the negative bias region where holes are injected from the GaAs:Be layer to the GaMnAs QW. With increasing d, the peaks of these oscillations shift to smaller voltages and the period becomes short, which indicates that they are induced by the resonant tunneling effect. This work was partly supported by PRESTO/SORST of JST, Grant-in-Aids for Scientific Research, IT Program of RR2002 of MEXT, and Kurata-Memorial Hitachi Science {\&} Technology Foundation. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W12.00010: Exchange Splitting and 100\% Spin Polarization in Monolayer level EuO Films Tiffany Santos, Jagadeesh Moodera, Ezana Negusse, Yves Idzerda The exchange splitting of the conduction band in an ultrathin film of ferromagnetic EuO just 2.5 nm thick has been determined for the first time using tunneling techniques. In a Al/EuO/Y tunnel junction, a huge drop in junction resistance versus temperature was observed below the EuO Tc=69K, resulting from an exchange splitting of 0.3 eV, which corresponds to a spin filter efficiency of 98\% ! Furthermore, substantial tunnel magnetoresistance = 280\% has been observed in Cu/EuO/Gd quasi-magnetic tunnel junctions. From these observations, it appears that EuO is approaching its theoretical spin polarization P of 100\%. Whereas previously, a value of only 30\% was obtained using the Meservey-Tedrow technique of directly measuring P. This drastic improvement occurred after examining the chemical and magnetic properties of EuO at the monolayer level and its interfacial properties with metals, using SQUID magnetometry, XAS, XMCD and XRS. With the right combination of interface materials and deposition parameters, one can have a 1nm EuO film with a high moment of $>$7 $\mu_{B}$. With this high spin filter efficiency and its compatibility with Si, the EuO spin filter shows promise for injecting highly-polarized spins into Si-based semiconductors. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W12.00011: Room Temperature Tunneling Characteristics through SDT Nanoscaled Lines into n-Doped Si Yu Zhang, Nam H. Kim, Jian-Qing Wang, Jim Daughton Nanoscaled spin-dependent tunneling (SDT) lines were patterned on n-doped Si layer and studied for tunneling characteristics from ferromagnetic nano-lines through an AlO$_{2}$ insulating barrier into the semiconductor. The functional magnetic layering was deposited on doped Si with phosphorus (n-type) having resistivity of 0.006-0.02 Ohm-cm. The configuration of the SDT film is 1.5 nm AlO$_{x}$ / 4 nm NiFeCo / 1 nm FeCo / 15 nm Cu / 15 nm CrSi / 10 nm Si$_{3}$N$_{4}$ as spin injection contact. The patterned lines with line width and separation of 100 nm were produced using e-beam lithography. The tunneling characteristics versus temperature (80 to 300 K) were measured by wire bonding and with assistance of ohmic contacts of heavily doped regions. The tunneling studied through the barrier between layered-magnetic metals and semiconductor clearly showed the electronic transport as ballistic tunneling, showing weakly dependence on the temperature. This is qualitatively different similarly scaled-up SDT line-structures with 2 micron gap distance. In the later configuration, the electronic transport was observed to be mainly thermal emission dominant process at elevated temperatures, with characteristic activation energy in agreement with the impurity level. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W12.00012: Measuring Spin Dependent Hot Electron Transport in Fe/Si(001) Schottky Diodes Andrew Stollenwerk, Michael Krause, John Garramone, Evan Spadafora, Vincent LaBella Devices that utilize the spin degree of freedom rely on transport of electron spin through materials and material interfaces.~ Further\textbf{ }knowledge of spin-polarized electron transport can aid in the development of spintronic devices.~ To this end, we developed a novel technique; spin polarized ballistic electron emission microscopy (SP-BEEM). This technique has been utilized to study the spin dependent transport properties in Fe/Si(001) Schottky diodes.~ The energetic dependence of the spin dependent attenuation lengths was measured.~ Most interestingly, it was found that the interface band structure played a prominent role in this dependence.~~ [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W12.00013: Spin-Valve Photo-Transistor Biqin Huang, Igor Altfeder, Ian Appelbaum The Spin-Valve Photo-Transistor is a semiconductor-ferromagnetic metal multilayer-semiconductor transistor operated by photo- exciting hot electrons in the emitter semiconductor into a Schottky collector. We have realized this device using a vacuum- bonded float-zone Si/multilayer/n-InP structure. To distinguish the emitter interband-excited component of collector current from base/collector internal photoemission, we use a lockin spectroscopy sensitive only to the magnetocurrent. Our experimental results indicate a pathway to improve the magnetocurrent of a related device, the Spin- Valve Photo-Diode, by increasing the fraction of hot electron current that travels through both layers of the ferromagnetic spin-valve. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W12.00014: Influence of Spin-Orbit Interactions on Point Contact Andreev Reflection Ion Garate, Allan MacDonald In PCAR (point contact Andreev reflection) the I(V) characteristics of an interface between a singlet superconductor and a ferromagnetic metal is used (1)(2) to probe the degree of spin-polarization near the Fermi energy of the ferromagnet. Motivated by recent PCAR studies (3)(4) of (III,Mn)V ferromagnetic semiconductors, in which the spin-orbit interaction scale is comparable to the exchange energy scale, we report on a theoretical study the effect of spin-orbit interactions on the quasiparticle current through a ferromagnet-superconductor interface. Our theoretical analysis generalizes the Blonder-Tinkham-Klapwijk model results commonly used to interpret PCAR experiments. We find that PCAR provides a good qualitative measure of Fermi energy spin-polarization, even when the quasiparticle bands are strongly spin-orbit coupled.\\ (1) R.J. Soulen \emph{et al.}, Science \textbf{282},85 (1998)\\ (2) S.K. Upadhyay \emph{et al.}, Phys. Rev Lett. \textbf{81}, 3247 (1998)\\ (3) J.G. Braden \emph{et al.}, Phys. Rev. Lett. \textbf{91},56602 (2003)\\ (4) R.P. Pangulury \emph{et al.}, Appl. Phys. Lett. \textbf{84},4947 (2004) [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W12.00015: Zero-Bias Conductance Peak in Al/AlOx/Sc Tunnel Junctions Sheng-Shiuan Yeh, Juhn-Jong Lin We have fabricated a series of Al/AlO$_{x}$/Sc tunnel junctions and measured the differential conductances at low temperatures. 25-nm thick Al (99.999{\%}) stripes were first thermally evaporated onto a glass substrate, followed by glow discharge under an O$_{2}$ atmosphere, to form a thin insulating AlO$_{x}$ layer. Subsequently, a 60-nm thick Sc (99.99{\%}) film was thermally evaporated across the oxidized Al stripes to form tunnel junctions of 1 mm $\times $ 1 mm. Lock-in techniques were used to measure the differential conductances dI/dV(G) of the junctions. Zero-bias conductance peaks were found in all the tunnel junctions. In particular, the magnitudes of the zero-bias conductance peaks reveal a -ln$T$ dependence below about 30 K, which could be attributed to the electron-magnetic impurities interactions according to the theory of Appelbaum. However, the magnetic field has only a small effect on the conductance peaks. An asymmetric term in G(V) was observed, which is strongly temperature dependent and magnetic-field insensitive. Possible explanations will be discussed. [Preview Abstract] |
Session W13: Focus Session: Thin Films and Superlattices
Sponsoring Units: DMP GMAGChair: Manuel Bibes, Universite Paris Sud
Room: Colorado Convention Center Korbel 4C
Thursday, March 8, 2007 2:30PM - 3:06PM |
W13.00001: Fabrication and Funcntionality of Complex Oxide Superlattices Invited Speaker: In this talk, we focus on three-constituent magnetic oxide superlattice (`tricolor' superlattice), where the asymmetric stacking like $\cdot \cdot$ ABCABC $\cdot \cdot$ artificially breaks the space-inversion symmetry. The purpose of the study is to realize artificial polar ferromagnets. The polar/noncentrosymmetric magnets, represented by multiferroic compounds, recently attract a considerable interest, because we can expect novel phenomena, such as magneto-electric (ME) effect, or magnetization-induced second harmonic generation (MSHG). In the artificial superlattice, the gigantic MSHG was first realized with a `tricolor' consisting of a ferromagnet La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ and band insulators LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ [H. Yamada,\textit{ et al.}, APL \textbf{81}, 4793 (2002), Y. Ogawa, \textit{et al.}, PRL \textbf{90}, 217403 (2003)]. From the temperature dependence of MSHG in this superlattice, we found that the MSHG originate from the interface magnetism. By utilizing the MSHG as a probe for interface magnetism, we optimized the various oxide interfaces, leading to the discoveries of huge tunneling magnetoresistance in a junction with engineered interfaces [H. Yamada \textit{et al.}, Science \textbf{305}, 646 (2004)], or enhanced optical ME effect in the `tricolor' superlattice composed of LaMnO$_{3}$, SrMnO$_{3}$ and LAO. In those functionalities, crucial roles are played by the interface effects characteristic of correlated electron oxides, such as charge transfer or orbital-state-mediated magnetism [H. Yamada, et al., APL\textbf{89}, 052506 (2006)]. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W13.00002: Physical properties of epitaxial n-type La$_{0.7}$Ce$_{0.3}$MnO$_{3}$ films. Hsiung Chou, C. Y. Wu, C. B. Wu Electronic doped Colossal Magnetoresistance (CMR) Materials have been long for spintronic applications. Since the n-type CMR material can exist only in a metastable state rather than in a thermodynamic equilibrium state [1], \textit{in situ} growth of epitaxial films is regarded as an efficient way for forming single n-type CMR films. In this study, La$_{0.7}$Ce$_{0.3}$MnO$_{3}$ (LCeMO) films were grown on LaAlO$_{3}$ and SrTiO$_{3}$ substrates at various growth conditions. Earlier reports of LCeMO films exhibits a single T$_{p}$, the metal-insulator like transition temperature, at 250K. It is found in our studies, all films shows higher T$_{p}$ and T$_{C}$ between 260 and 300K. Hall measurements indicate, only specific growth conditions within a narrow growth window can approach to the metastable state and results in n-type LCeMO films. Because the Ce and La are next to each other in the periodic table, it is not easy to identify the composition of films by either the energy dispersion spectrum or the Rutherford backscattering spectrum. We are unable to tell the exact compositions of our films. The phase separation that usually occurred when the films approaches to a thermodynamic equilibrium state does not happen for our films. The detail of our results will be reported in the presentation. Reference [1]: H. Chou, C. B. Wu, S. G. Hsu, and C. Y. Wu, Phys. Rev. B \textbf{74}, 174405 (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W13.00003: Influence of $B$-site cationic ordering on the magnetic properties of La$_{2}$NiMnO$_{6}$ epitaxial thin films. A. Venimadhav, D.A. Tenne, M.J. Wilson, P. Schiffer, Qi Li, J.H. Lee , D.G. Schlom, X.X. Xi Monoclinic (0 0 1)-oriented La$_{2}$NiMnO$_{6}$ thin films were grown on (0 0 1) SrTiO$_{3}$ and (0 0 1) LAST substrates by pulsed-laser deposition. The crystal structures, magnetic properties, and the Raman spectrum have been studied for films with different growth conditions. The magnetic properties were found to be very sensitive to the growth conditions and to the substrate. Analysis of the magnetization, x-ray diffraction, and the Raman spectroscopy measurements demonstrate that the $B$-site cationic ordering, which is sensitive to both the growth conditions and the lattice mismatch with the substrate, affects the magnetic properties. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W13.00004: Structural and Magnetic Characterization of Fe-doped La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ Films Oscar Luis Arnache Olmos, Axel Hoffmann, Doris A. Girata Lozano We have investigated pure and $^{57}$Fe-doped La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ thin films, which were prepared via high O$_{2}$-pressure (500~mTorr) by magnetron DC sputtering on (100)~LaAlO$_{3}$, (100)~SrTiO$_{3}$ and (100)~MgO substrates. The $^{57}$Fe-doped samples contained 1\% and 3\% $^{57}$Fe per Mn. The structural and magnetic properties of the films and targets were characterized using X-ray diffraction(XRD) and reflectivity, M\"{o}ssbauer spectroscopy and magnetometry measurements. XRD shows that films are single phase and epitaxially oriented, and have negligible structural changes upon $^{57}$Fe-doping. The M\"{o}ssbauer spectra measured at room temperature exhibit one doublet with an isomer shift of 0.320$\pm$0.003~mms$^{-1}$, indicating the presence of the Fe$^{3+}$ ion at room temperature in the sample, which is a typical value of the high-spin of Fe$^{3+}$ with octahedral coordination. The quadrupole splitting value was 0.210$\pm$0.006~mms$^{-1}$. This clearly indicates that Fe is incorporated into the structure by substituting Mn. We will furthermore discuss the influence of $^{57}$Fe-doping on magnetic and magnetotransport properties. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W13.00005: Electrical Transport and Magnetic Behavior of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ Film Grown by Chemical Solution Deposition Technique M. Jain, Q.X. Jia, F. Ronning, T. Park, J.D. Thompson Rare-earth manganites {\{}R$_{1-x}$A$_{x}$MnO$_{3}$ (R = rare earth, e.g. La, Pr, A = alkaline earth metal, e. g. Ca, Sr){\}} have attracted much interest because of their rich phenomena like colossal magnetoresistance and charge ordering (CO). For certain values of $x$, close to 0.5, these compounds undergo a first-order CO transition. The CO state is characterized by the real space ordering of Mn$^{3+}$/Mn$^{4+}$ in the mixed valent manganite. The CO in bulk Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ has been widely studied, however, there are very limited studies on thin films of this material. In this work, we have systematically studied the magnetic and electrical behaviors of the Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (PCMO) thin films grown by chemical solution deposition technique on LaAlO$_{3}$ substrates. The ground state in PCMO was found to be a charge-ordered antiferromagnetic insulator. The CO transition at 235 K was observed. With the application of magnetic field the resistivity of the film decreased at low temperatures. Detailed magnetic and electrical properties of these films will be presented. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W13.00006: Characterization of Anisotropy in Manganite (LPCMO) Thin Films G. Singh-Bhalla, S. Tongay, T. Dhakal, R. Rairigh, A. Biswas, A.F. Hebard Resistance measurements on thin films of strongly correlated electronic materials that have anisotropic properties due to atomic layering and/or substrate induced strain are primarily sensitive to in-plane conduction paths and therefore fail to capture any information about perpendicular transport. We present an experimental technique in which the films under investigation, pulse laser deposited (La$_{1-y}$Pr$_{y})_{5/8}$Ca$_{3/8}$MnO$_{3}$ (LPCMO) with thicknesses in the range 300-900 {\AA}, comprise the base electrodes of trilayer capacitor structures, thus allowing the simultaneous characterization of dc transport (resistance) in the parallel direction and ac transport (capacitance) in the perpendicular direction. For a given film, we find two distinct direction-dependent insulator-metal percolation transitions reflecting the competition between insulating and ferromagnetic metallic phases. With increasing thickness, the temperature difference between these transitions decreases. This decrease occurs because the presence of a strain-stabilized ferromagnetic metal phase at the LPCMO/substrate (NdGaO$_{3})$ has less of an effect on transport as the thickness increases and the LPCMO manifests isotropic bulk behavior. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W13.00007: Plasmon Enhancement of Photoinduced Resistivity Changes in Bi$_{1-x}$Ca$_{x}$MnO$_{3 }$ Thin Films Vera Smolyaninova, E. Talanova, Rajeswari Kolagani, G. Yong, R. Kennedy, M. Steger, K. Wall Doped rare-earth manganese oxides (manganites) exhibit a wide variety of physical phenomena due to complex interplay of electronic, magnetic, orbital, and structural degrees of freedom and their sensitivity to external fields. A photoinduced insulator to conductor transition in charge-ordered manganites is especially interesting from the point of view of creating photonic devices. Thin films of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ exhibit large photoinduced resistivity changes associated with melting of the charge ordering by visible light [1]. We have found a considerable increase of the photoinduced resistivity changes in the Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin film after depositing metal nanoparticles on the surface. This increase can be explained by enhancement of local electromagnetic field in the vicinity of the gold nanoparticle due to the plasmon resonance. The changes in lifetime of the photoinduced state will be reported, and the possible origin of these effects will be discussed. [1] V. N. Smolyaninova at al., Appl. Phys. Lett. 86, 071922 (2005). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W13.00008: Colossal Magneto Resistive (CMR) Bolometric Sensors as a Total Energy Monitor for the Linac Coherent Light Source (LCLS) Free Electron Laser (FEL) G. Yong, R. Kolagani, R. Mundle, A. Davidson, Y. Liang, O. Drury, E. Ables, S. Friedrich We are developing a CMR based bolometric x-ray detector as a total energy monitor for the LCLS FEL to be built at the Stanford Linear Accelerator Center. The FEL will produce $\sim $200 femtosecond pulses in the energy range 0.8 to 8 keV with 10$^{12}$ photons per pulse. The bolometer is designed to measure the total energy of each laser pulse with repeatability below 1{\%} and an absolute accuracy below 10{\%}. The detector is fabricated using epitaxial thin films of Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ grown on Si by Pulsed Laser Deposition. An epitaxial buffer layer of SrTiO$_{3}$ is used as a chemical barrier and an additional template layer of Bi$_{4}$Ti$_{3}$O$_{12}$ is employed to facilitate lattice match with the Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ layer. We have been able to obtain good quality epitaxial thin films by this approach. Prototype detectors have been fabricated photolithographically, and are operated in a pulse tube refrigerator at temperatures between 100 to 150 K. Initial tests with a thermal heater pulse show that the film properties are sufficient for detector functionality. We will discuss the details of material optimization, characteristics of the sensor material such as the temperature coefficient of resistance and 1/f noise, and projections of the detector response under photon illumination. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W13.00009: Observing Metal-Insulator Transitions in Spatially-Confined Perovskite Manganite Thin Films T. Zac Ward, H.Y. Zhai, J.X. Ma, Kenji Fuchigami, E.Ward Plummer, Jian Shen Transition metal oxides (TMO) exhibit a strong spin-charge-lattice interaction that can lead to electronic phase separation (PS). This phenomenon carries a number of fascinating electronic and magnetic phases while maintaining a single crystalline structure. To better understand the nature of phase transition involving the coexistence of multiple phases, we have fabricated La$_{5/8-x}$Pr$_{x}$Ca$_{3/8}$MnO$_{3}$ (LPCMO) wires from single crystal LPCMO thin films using optical and E-beam lithographic techniques. These wires display giant and ultrasharp steps with varying temperature and magnetic field near the metal-insulator transition, which is believed to be a direct consequence of the influence of spatial confinement on percolative transport in these structures. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W13.00010: Low-Electric-Field Tuned Mobile Carrier Density and Heat Conduction in SrMnO$_3$ Joshua Cohn, Corneliu Chiorescu, John Neumeier Nominally undoped CaMnO$_3$ (CMO) and SrMnO$_3$ (SMO), both G-type antiferromagnets with orthorhombic and cubic structure, respectively, exhibit very different low-temperature thermal conductivities ($\kappa$), with $\kappa^{CMO}\gg\kappa^{SMO}$. These compounds are lightly electron doped due to oxygen vacancies, with $n\sim 10^{18}-10^{19}$~cm$^{-3}$ at room temperature. Measurements of the electrical conductivity and Hall coefficient indicate that the low-temperature mobile carrier density in SMO is larger by four orders of magnitude than that of CMO, suggesting that the disparity in $\kappa$ values reflects enhanced phonon-electron scattering in the former compound. We will report results of thermal conductivity measurements on SMO designed to test this hypothesis by using applied electric fields to vary the mobile carrier density at fixed temperature. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W13.00011: Low-Electric-Field Tuned Impurity Conduction in Antiferromagnetic Manganites Corneliu Chiorescu, Joshua Cohn, John Neumeier Transport measurements for temperatures in the range $4.2~{\rm K}\leq T\leq 300$~K are reported for the semiconducting, antiferromagnetic manganites SrMnO$_3$ and CaMnO$_3$. At low $T$ where impurity conduction predominates, the electrical conductivity and Hall coefficient are found to be strongly electric-field dependent. For SrMnO$_3$, the mobile carrier density is continuously tunable over a range of more than three orders of magnitude in electric fields $F\leq 50$~V/cm. The conductivity and carrier density scale with field $\propto\exp(\sqrt{F})$, indicating Poole-Frenkel field-assisted ionization of bound carriers. The binding energy for SrMnO$_3$ ($\delta \sim 3.5$~meV) implies that electrons are ionized to more mobile states within the energy gap, rather than to the conduction band. This small energy scale correlates with the low-temperature onset of a small ferromagnetic moment in this compound, suggesting that bound electrons form ferromagnetic polarons. Strong electron correlation effects are suggested by the electric-field dependent Hall mobility. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W13.00012: Voltage-induced Metal-Insulator Transitions in Perovskite Oxide Thin Films Doped with Strongly Correlelated Electrons Yudi Wang, Soo Gil Kim, I-Wei Chen We have observed a reversible metal-insulator transition in perovskite oxide thin films that can be controlled by charge trapping pumped by a bipolar voltage bias. In the as-fabricated state, the thin film is metallic with a very low resistance comparable to that of the metallic bottom electrode, showing decreasing resistance with decreasing temperature. This metallic state switches to a high-resistance state after applying a voltage bias: such state is non-ohmic showing a negative temperature dependence of resistance. Switching at essentially the same voltage bias was observed down to 2K. The metal-insulator transition is attributed to charge trapping that disorders the energy of correlated electron states in the conduction band. By increasing the amount of charge trapped, which increases the disorder relative to the band width, increasingly more insulating states with a stronger temperature dependence of resistivity are accessed. This metal-insulator transition provides a platform to engineer new nonvolatile memory that does not require heat (as in phase transition) or dielectric breakdown (as in most other oxide resistance devices). [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W13.00013: Variable electron correlation in high-quality MBE- and PLD-grown SrRuO$_{3}$ thin films. Wolter Siemons, Gertjan Koster, Hideki Yamamoto, Arturas Vailionis, Theodore Geballe, Dave Blank, Malcolm Beasley We show that systematic variations in the degree of correlation can occur within SrRuO$_{3}$ as a function of disorder/off-stoichiometry. In particular, we find that one source of disorder can be controlled in SrRuO$_{3}$ thin films by varying the deposition conditions or the deposition technique. Specifically, we clearly demonstrate that variation of vacancies on the ruthenium site gives rise to a variation in correlated behavior as seen in the photoemission spectra (XPS and UPS),. Moreover, the transport properties of our samples are clearly linked to their photoemission spectra, and independently the crystal unit cell parameters. SrRuO$_{3}$ appears to be a system where these effects can be studied in a more systematic fashion, usually not easily accessible, but we suspect that the underlying physics is generic in complex oxidesWork supported by the DoE BES and EPRI. [Preview Abstract] |
Session W14: Focus Session: Nanomagnetic Oxides
Sponsoring Units: GMAG DMPChair: Christian Binek, University of Nebraska
Room: Colorado Convention Center Korbel 4D
Thursday, March 8, 2007 2:30PM - 2:42PM |
W14.00001: Magnetic Properties of Epitaxial Cr/Cr$_{2}$O$_{3}$/ Cr Multilayers Tathagata Mukherjee, Sarbeswar Sahoo, Christian Binek We study Cr/Cr$_{2}$O$_{3}$/Cr trilayer structures grown by Molecular Beam Epitaxy on (111) oriented Al$_{2}$O$_{3 }$substrates. X-ray diffraction reveals perfect single crystalline (110) Cr and stoichiometric single crystalline Cr$_{2}$O$_{3}$ (111) films. Both, Cr and Cr$_{2}$O$_{3 }$order antiferromagnetically with bulk N\'{e}el temperatures of 311 and 307K, respectively. Cr is an itinerant antiferromagnet where the antiferromagnetic (AF) order establishes as an incommensurate spin density wave. Cr$_{2}$O$_{3 }$in contrast is an AF insulator with localized magnetic moments where magnetoelectric and piezomagnetic effects are both symmetry allowed. Its insulating, magnetoelectric and piezoelectric properties make Cr$_{2}$O$_{3}$ an interesting material for extrinsically controlled tunnel barriers in TMR type structures. The lattice mismatch of $\sim $1.2{\%} at the Cr -- Cr$_{2}$O$_{3}$ interface creates a strong stress induced piezomagnetic moment revealed by SQUID measurements. The interaction between the piezomoment and the spin distribution at the Cr- interface gives rise to a rich scenario of magnetic proximity effects which we study by SQUID magnetometry, magneto-optical Kerr effect and electrical transport measurements. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W14.00002: Piezomagnetism in Epitaxial Cr$_{2}$O$_{3}$ Thin Films Yi Wang, Sarbeswar Sahoo, Christian Binek Recently, the magnetoelectric material Cr$_{2}$O$_{3}$ attracted renewed interest due to its potential for future spintronics applications which can be realized by novel magnetic thin film heterostructures [1]. Here we study thin films of Cr$_{2}$O$_{3}$ (111) on c-Al$_{2}$O$_{3}$ (111) substrate which are grown by thermal evaporation of Cr metal in an O$_{2}$ atmosphere. X-ray diffraction data reveal stoichiometric epitaxially grown Cr$_{2}$O$_{3}$ (111) films. Owing to a lattice mismatch of $\sim $4{\%} at the interface between the Al$_{2}$O$_{3 }$substrate and the film we observe a strong stress induced piezomagnetic moment in the Cr$_{2}$O$_{3}$ film. We measure the temperature dependence of this piezomoment by Superconducting Quantum Interference Device (SQUID) magnetometry and Kerr rotation. The presence of high inherent stress, a significant piezomagnetic moment and the possibility to realize high electric fields makes our Cr$_{2}$O$_{3}$ thin films ideal candidates for the challenging quest of the symmetry allowed but hitherto undiscovered piezomagnetoelectric effect. [1] Ch. Binek, B. Doudin, J. Phys. Condens. Matter\textbf{ 17}, L39 (2005). \newline [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W14.00003: Magnetoelectric thin films for electrically controlled exchange bias in spintronic applications Xi He, Sarbeswar Sahoo, Srinivas Polisetty, Yi Wang, Tathagata Mukherjee, Christian Binek Epitaxial (111) oriented thin films of magnetoelectric (ME) Cr2O3 are grown by MBE. These films are the key component of Cr2O3(111)/(Co/Pt)3 heterostructures allowing for electrically controlled exchange bias (EB) and novel spintronic applications [1]. Pure voltage control of magnetic configurations in TMR-type devices is proposed as an alternative to current-induced switching. Basic effects of electrically controlled EB and its ME switching are studied by magnetometry and polar Kerr rotation. Exchange coupling between the ME antiferromagnet Cr2O3 and the ferromagnetic CoPt multilayer gives rise to perpendicular EB. The latter is controlled by axial electric fields inducing excess magnetization at the interface which controlls the EB field. Recently, the sign of the EB field has been tuned via a field cooling procedure where a Cr2O3(111) bulk/(Co/Pt)3 system is exposed to either parallel or antiparallel axial magnetic and electric fields [2]. Here we study this ME switching effect in an all thin film heterostructure. [1] Ch.Binek, B.Doudin, J. Phys. Condens. Matter 17, L39 (2005). [2] P.Borisov et al., Phys. Rev. Lett. 94, 117203 (2005). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W14.00004: Coupling Magnetism to Electricity in Multiferroic Heterostructures Invited Speaker: Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc.) in thin film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric (FE) and ferromagnetic oxides as the background, we are now exploring such materials, as epitaxial thin films as well as nanostructures. Specifically, we are studying the role of thin film growth, heteroepitaxy and processing on the basic properties as well as magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling. What is the importance of this work? Antiferromagnets (AFM) are pervasive in the recording industry. They are used as exchange biasing layers in MTJ's etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet (and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W14.00005: Coercivity and Nano-structure in Magnetic Spinel Mg(Mn,Fe)$_{2}$O$_{4}$ Chenglin Zhang, S. Yeo, S.-W. Cheong We discovered that the micro-to-nano-structure of Mg(Mn,Fe)$_{2}$O$_{4}$ drastically changes with different thermal treatment. This extraordinary structural evolution is associated with spinodal chemical decomposition associate with the Jahn-Teller structural distortions around Mn ions. The magnetic properties of the polycrystalline Mg(Mn,Fe)$_{2}$O$_{4}$ vary with the structural progress. Particularly, the Curie temperature and magnetic coervcivity considerably change with the structural evolution. The significantly-enhanced coercivity in the system with elongated nanostuructre stems from the large shape anisotropy of the nanostructure. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W14.00006: Magnetic Properties of Cobalt-Ferrite Nanoparticles Prepared by a Sol-Gel Synthesis Technique Thomas Ekiert, Karl Unruh, E. Carpenter, K. Pettigrew, J. Long, D. Rolison Cobalt-ferrite nanoparticles have been prepared as highly porous aerogels using a sol-gel technique and characterized by XRD, TEM, and nitrogen-sorption porosimetry measurements. The XRD patterns for calcined Co-ferrite aerogels corresponded to a cubic structure with a lattice parameter near that of bulk Co-ferrite and a particle size of about 6 nm. TEM images indicated a similar particle size and a morphology similar to that of silica aerogels. The magnetic properties of these materials have been studied from 5 K to 340 K. Hysteresis loop measurements indicated that the coercivity and saturation magnetization of these materials evolves from nearly 19 kOe and 56 emu/g at 5 K to less than 10 Oe and 40 emu/g at 340 K. ZFC magnetization curves displayed a broad maximum that smoothly varied between about 300 K in an applied field of 100 Oe to about 180 K in a 10 kOe field. These measurements have been interpreted in terms of a distribution of effective particle sizes arising from a distribution in interparticle interactions. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W14.00007: Investigations of a New Diluted Magnetic Oxide with Room Temperature Ferromagnetism in Co-doped HfO$_{2}$ Y.H. Chang, W.C. Lee, M.L. Huang, S.F. Lee, Y.L. Soo, M. Hong, J. Kwo The structural, chemical, and magnetic properties of HfO$_{2}$ epitaxial films $\sim $100nm thick grown on YSZ at varying growth temperature have been systematically investigated. Nearly cobalt cluster-free films with RT ferromagnetic behaviors can be obtained via low T growth. \textit{In-situ} XPS analysis during growth indicated the formation of metallic cobalt at the initial growth stage under a low O$_{2}$ partial pressure $\sim $10$^{-10 }$Torr, and that the metallic cobalt can be mostly eliminated by raising the pressure to 10$^{-7}$ Torr. In conjunction with EXAFS local structural analysis and post annealing experiments, we infer that cobalt ions in low T grown films are located at interstitial site and appeared to be stable after being annealed in O$_{2}$ at 350$^{\circ}$C. Further Hall measurements are now underway to measure the carrier concentration, and to elucidate their role to the apparent ferromagnetism. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W14.00008: Magnetocaloric effect (MCE) in Nickel Ferrite nanoparticles J. Gass, M.B. Morales, N.A. Frey, M.J. Miner, S. Srinath, H. Srikanth We report on the magneto caloric effect (MCE) in a Nickel ferrite (NiFe2O4) nanoparticle system. The nanoparticles were synthesized using chemical co-precipitation. Extensive characterization of structural and magnetic properties was done using XRD, TEM, DC and AC magnetization, and transverse susceptibility. The change in entropy was calculated using the thermodynamic Maxwell relation from the family of M-H curves taken at different temperatures. Maximum entropy change in nanoparticle systems is influenced by particle size, anisotropy, and collective dipolar behavior. While the MCE is not as large as that reported in bulk systems, there are advantages as ferrite nanoparticles are easily produced and the operational temperature is tuned by the average particle size. In our studies, we observed a sharp peak in M-T curves at around 60K in addition to the blocking transition which occurs at 120 K. This results in a larger entropy change in comparison with the MCE results on other reported ferrite nanoparticles. The origin of this anomalous MCE is analyzed in the context of surface anisotropy and other possible contributions in the NiFe2O4 system. Work supported by NSF through grant CTS-0408933. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W14.00009: Verwey transition and magnetic irreversibility in nano-crystalline magnetite produced by magnetotactic bacteria T. Prozorov, R. Prozorov, T.J. Williams, D.A. Bazylinski, S.K. Mallapragada, B. Narasimhan Magnetic properties of 50 nm magnetite nanocrystals from different strains of magnetotactic bacteria are compared to high quality single crystal. It is found that the Verwey transition depends mostly on the particle shape. It is sharpest and occurs at a temperature approaching bulk values in elongated nanoparticles from MV-1 bacteria. This result contradicts previous reports of the Verwey temperature reduction in nanoparticles. Magnetic irreversibility below the Verwey transition shows thermal-history dependence and, in nanoparticles, is strongly influenced by the interparticle interactions. Collected data are analyzed in terms of the interplay between crystalline and shape anisotropies as well as collective behavior of the nanoparticles. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W14.00010: Magnetic properties of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ ferrofluids M.B. Morales, J. Gass, S.L. Morrow, H. Srikanth We study in detail the magnetic properties of ferrofluids consisting of $\sim $0.02 g/cm$^{3}$ volume concentration of surfacted Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles suspended in two solvents, hexane and dodecane, with different room temperature viscosities. DC and AC magnetization measurements were done using a Physical Properties Measurement System (PPMS). Hysteresis loops of Fe$_{3}$O$_{4}$ in both liquids at different temperatures show that the particles are superparamagnetic having low coercivities even at low temperature. CoFe$_{2}$O$_{4}$-based ferrofluids, on the other hand, have a wide range of grain sizes and show high coercivity of 10 kOe at low temperature. From the ZFC-FC curves, the blocking temperature was determined to be 76 K and 223 K for Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ in dodecane, respectively. To probe the dynamic relaxation effects, temperature-dependent complex AC susceptibility of all the ferrofluids were measured at frequencies of 100 Hz and 10 kHz. From these data sets, the relaxation contributions due to Neel and Brownian mechanisms were identified. We will also report on systematic magnetic measurements and analysis of ferrofluids with different nanoparticle concentrations. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W14.00011: Quantum Monte-Carlo Study of Mn and Mn-oxide clusters. Hiori Kino, Lucas K. Wagner, Lubos Mitas Many molecules and clusters of Mn and Mn-oxide have not only interesting physical properties but also can be found in enzymes as important components in biochemical reactions. The electronic structure calculations of these systems are difficult and, for example, choice of exchange-correlation functionals in Density Functional Theory can significantly influence both ground state geometries and spin-state predictions. Therefore, highly accurate calculation is very desirable for these systems. Experimentally, it is established that the Mn dimer is a van der Waals system with weak binding, however, the spin multiplicity has not been settled unambiguously with possibilities covering a range from singlet, triplet, etc, up to 2S+1=11. On the other hand, Mn$_n$O$_n$ molecules are quite well understood as being a high-spin system, but their geometries depend on the exchange-correlation functionals. We will present our recent results from the fixed-node quantum Monte Carlo calculations of these systems. We will also report on recent progress in modeling the [4Mn-4O-Ca] cluster structural prototypes for the oxygen evolving center in green plants Photosystem II. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W14.00012: Magnetic Isomerization of Chromium Clusters Wei Jiang, Forrest Payne, Louis Bloomfield We have used the Stern-Gerlach deflection technique to study magnetic properties of chromium clusters of different sizes (N=20-133) at different temperatures (T=60K-120K) and magnetic fields. Though chromium bulk is antiferromagnetic, we observed that nearly all these clusters are magnetic. And the deflection profiles of them suggest that two or more magnetic isomers exist in the beam, which have significantly different magnetic moments. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W14.00013: Simulation of interacting nanoparticles with random anisotropy axes Julio F. Fernandez, Juan J. Alonso We report Monte Carlo simulation results for the collective behavior of single-domain nanoparticles with randomly oriented easy magnetization axes. Such randomness may follow from a random orientation of the crystalline axes within each nanoparticle. Dipole-dipole interactions, as well as nearest neighbor exchange interactions of various strengths are taken into account. We report on the effect random anisotropy has on long range order as well as on magnetic relaxation at low temperature. [Preview Abstract] |
Session W15: Magnetic Cooperative Phenomena
Sponsoring Units: GMAGChair: Chris Leighton, University of Minnesota
Room: Colorado Convention Center Korbel 4E
Thursday, March 8, 2007 2:30PM - 2:42PM |
W15.00001: Ultrafast time-resolved magneto-optical Kerr study of coherent spin waves in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films Yuhang Ren, Daimian Wang, Roberto Merlin, Adyam Venimadhav, Qi Li We report on ultrafast time-resolved optical measurements on La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) thin films. The differential polarization shows coherent spin wave excitations in LCMO. We determine the magnetic anisotropy and spin stiffness constants from the magnetic-field dependence of the frequency of the spin waves. The gyromagnetic factor, g $\sim $ 2, is consistent with the value determined for the 3d electrons of Mn ions. Values of the magnetic bulk anisotropies for all the LCMO samples are in good agreement with those from other techniques [1-2]. Moreover, we observed a high-order spin wave mode in LCMO and its frequency shows strong sample dependence. The results are explained by a possible existence of nanoscale ferromagnetic clusters in LCMO. 1. J. O'Donnel, M. S. Rzhowski, J. N. Eckstein, and I. Bozovic, Appl. Phys. Lett. 72, 1775 (1998). 2. C. M. Xiong, J. R. Sun, and B. G. Shen, Solid State Commun. 134, 465 (2005). [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W15.00002: Spin band gap and emerging low energy excitations in a doped cobaltate perovskite Andrei Savici, Igor Zaliznyak, Genda Gu, Ying Chen, Vasile Garlea We investigated spin dynamics in half doped cobaltate La$_{1.5}$Sr$_{0.5}$CoO$_{4}$, using inelastic neutron scattering. At high temperatures we observe a broad band of continuum antiferromagnetic dynamic spin correlations, extending to energies above 20 meV. As temperature decreases below spin ordering transition (T$_{SO}$ = 30K), a single coherent magnon mode emerges at low energies (bandwith ~ 15 meV), separated by an energy band gap from a broad continuum scattering residing at high energies $20 \le E \le 30$ meV. This points to an emerging 2D-XY behavior of spins, consistent with the strong planar anisotropy of this material. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W15.00003: Multi phonon resonance Raman scattering, spin phonon coupling and lattice dynamics of antiferromagnetic NdFeO$_{3}$ Manoj K. Singh, Ram S. Katiyar The Raman-active phonons in the orthorhombic NdFeO$_{3}$ single crystals were studied by means of polarized Raman scattering and lattice dynamics computations (LDC). The A$_{g}$-symmetry zone-center phonons were distinguished from the B$_{1g}$ eigenmodes by performing polarized Raman scattering experiments using two parallel polarization configurations, X$\prime $(ZZ)X$\prime $ and Z(X$\prime $X$\prime )$Z. Observed phonon spectra at 100, 300, 347 cm$^{-1}$ showed anomalous temperature dependence in the range of the magnetic spin reorientation temperature (100 --200K) indicating strong spin -phonon coupling. The anomalously shaped observed phonon between 500 to 1500 cm$^{{\-}1}_{, }$observed in the A$_{g}$-symmetry X$\prime $(ZZ)X$\prime $ and B$_{1g}$ spectrum was attributed to a multi-phonon scattering caused either by multiple combination of B$_{1g}$ or by A$_{1g }$phonons. With the help of lattice dynamics calculations, we were able to assign most of the observed Raman-active modes, including B$_{2g}$ and B$_{3g}$-symmetry phonons. The LDC results indicated that among the sixteen force constants employed, the force constant corresponding to the stretching vibration between the central Fe cation and the axial oxygen atom in a FeO$_{6}$ octahedron unit had the largest value. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W15.00004: Dynamic Spin Correlations in a Pyrochlore Slab Goran Gasparovic, S.-H. Lee, C. Broholm, R. J. Cava We use neutron scattering to study the spin dynamics of Ba$_2$Sn$_2$ZnGa$_3$Cr$_7$O$_{22}$, an $S=3/2$ strongly frustrated, disorder free, quasi-two-dimensional spin system based on a kagom\'{e} sandwich. Low temperature spin freezing, with timescale dependent onset and magnitude, is observed. Relaxation rate distribution displays $\omega/T$ scaling, characteristic of quantum critical regime, and anomalously broadens below 11 K. Wavevector dependence of the excitation spectrum supports the notion that the dynamics is driven by the interaction of spin simplexes, which carry no dipole moment. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W15.00005: Magnetic Phase Transition and Spin Dynamics in Magnetoelectric Effect LiMnPO$_{4}$ J. Li, W. Tain, J.L. Zarestky, D. Vaknin, Y. Chen, J.W. Lynn Elastic and inelastic neutron scattering techniques were used to study the magnetic phase transition and spin dynamics in single crystal LiMnPO$_{4}$. In this mulitiferroic, antiferromagnetism coexists with induced ferroelectricity below the Neel temperature. Elastic neutron scattering technique confirmed that LiMnPO$_{4}$ has a collinear antiferromagnetic ground state with magnetic moments oriented along the $a$-axis. The temperature dependent order parameter, calculated from the integrated intensity of the (010)~magnetic reflection, was fit to a power law equation, yielding a transition temperature $T_{N}$ = 33.7 K and a critical exponent of \textit{$\beta $} = 0.114. Above the Neel temperature, in the paramagnetic phase, unusually strong magnetic fluctuations were observed to temperatures as high as 60 K. The correlation lengths in the MnO layer and between the layers were also determined. The spin-wave dispersion curves along $a-$ and $b-$axis were measured in the antiferromagnetic state at 4.5 K and the system was shown to be quasi-2D by analyzing the dispersion curves using a 3D Heisenberg model from linear spin wave theory. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W15.00006: Inelastic neutron scattering study of the Verwey transition in YBaCo$_2$O$_5$ S. Chang, P. Karen, M.P. Hehlen, F.R. Trouw, R.J. McQueeney A new class of oxides with the chemical formula $R$Ba$M_2$O$_5$ ($R$~=~rare-earth, $M$~=~transition metal) are based on the perovskite structure with a doubled unit cell and a layer of oxygen vacancies. The structure consists of pyramids of five-coordinated $M$-sites, and more importantly the $M$-site is mixed valent in the stoichiometric formula unit (with an average valence of +2.5). Here, we present results of inelastic neutron scattering experiments on polycrystalline YBaCo$_2$O$_5$, which orders antiferromagnetically at $T_{\mathrm{N}}$~=~330~K and is accompanied by a tetragonal to orthorhombic distortion. Below $T_{\mathrm{V}}$~=~220~K, orbital/charge ordering occurs as well as a change in the Co$^{2+}$ spin state from low to high spin. A dramatic change in the spin wave spectrum at $T_{\mathrm{V}}$, as well as a large damping of zone-boundary spin waves above $T_{\mathrm{V}}$ are similar to those observed in YBaFe$_2$O$_5$ and are discussed in terms of a coupling of magnetism and valence fluctuations. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W15.00007: Neutron scattering study of vanadium spinel MnV$_{2}$O$_{4}$. Ovidiu Garlea, Stephen Nagler, Rongying Jin, David Mandrus, Bertrand Roessli, Martha Miller, Arthur Schultz, Christopher Frost, Doug Abernathy We report the results of elastic and inelastic neutron scattering studies carried out on single-crystals of MnV$_{2}$O$_{4}$. In this spinel system the octahedral site is occupied by the V$^{3+}$ ion, having two 3d electrons in threefold $t_{2g}$ levels, and the tetrahedral site is occupied by the Mn$^{2+}$ (3d$^{5})$ with no orbital degrees of freedom. MnV$_{2}$O$_{4}$ was found to exhibit a ferrimagnetic ordering at approximately 56 K. There is a structural phase transition from a cubic to a tetragonal phase at about 53K, reportedly associated with a spin reorientation process [1]. The inelastic measurements at low temperatures reveal the presence of an energy gap in spin-wave spectrum of 1.6 meV.~ The gap decreases with increasing temperature, and vanishes as the temperature is increased above the structural transition. Linear spin wave theory is used to evaluate the exchange interactions in this material. \newline [1] R. Plumier and M. Sougi, Solid State Commun. 64, 53 (1987) [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W15.00008: Spin wave dispersion in the helical spin ordered system SrFeO$_3$ and CaFeO$_3$ C. Ulrich, G. Khaliullin, V. Damljanovic, M. Reehuis, A. Maljuk, A. Ivanov, K. Schmalzl, Ch. Niedermayer, K. Hradil, B. Keimer One of the most interesting problems in condensed matter physics is the metal-insulator (MI) transition driven by strong electron correlations. The cubic perovskites SrFeO$_3$ and CaFeO$_3$ are isoelectronic to the manganite system (t$^3_{2g}$ e$^1_{g}$) and exhibit colossal magneto resistance effects. But in contrast, the ferrates show a helical instead of a collinear spin structure. Furthermore, perfectly cubic SrFeO$_{3}$ shows no charge order and is metallic whereas pseudocubic CaFeO$_{3}$ shows a MI-transition at the charge ordering transition of 290 K. Therefore, both compounds are right at the borderline between itinerant and strongly correlated systems. We have determined the static and dynamic spin properties by neutron scattering. The extracted parameters in the spin Hamiltonian are a big step towards the understanding of the mechanisms behind the helical spin order and the charge order in the ferrates. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W15.00009: Magnetic and Physical Properties of RCuGe V. Goruganti, Weiping Gou, Ji Chi, K. D. D. Rathnayaka, Joseph H. Ross, Jr. , Y. Oner We report magnetic, transport and thermodynamic measurements for AlB$_{2}$-type CeCuGe, NdCuGe and GdCuGe. In NdCuGe we observe a peak at 3K in specific heat and susceptibility corresponding to an AF transition. The magnon contribution to the specific heat is fitted very well and supports a CEF scheme, for which we found a first excited state 66 K from the ground state. GdCuGe exibits an AF transition at 18 K, with a peak in susceptibility and a kink in resistivity. For CeCuGe, ferromagnetic with $T_{c}$=10 K, the specific-heat peak is broadened and shifted to higher temperatures as the applied magnetic field increased. At high temperatures the magnetization obeys a Curie-Weiss law and the estimated magnetic moment agrees with the rare-earth free ion moment. To better understand the magnetism of these materials we also report local probe NMR measurements. Lineshape, and Knight shift measurements, show that coupling machanism is dominated by $J$ coupling, and from relaxation measurements we obtain a measure of the spin dynamics. This work was supported by the Robert A. Welch Foundation (grant A-1526). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W15.00010: Magnetism in GdFe2Zn20 Vladimir Antropov The magnetic structure formation and the origin of the exchange couplings in GdFe2Zn20 have been studied using the standard band-structure methods at T=0K and the ab-initio spin dynamics in the paramagnetic case. At low temperatures the relative importance of the different channels of the magnetic coupling have been analyzed. At high temperatures the soft spin deviation modes responsible for the observed high temperature of the magnetic phase transition [1] have been identified. Several strong magnetic short range order parameters have been found in this material. \newline [1] S. Jia, S.L. Bud'ko, G.D. Samolyuk, P.C. Canfield. cond-mat/0606615 [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W15.00011: Microemulsion phases in one and two dimensional magnetic models with long-range interactions Erik Nielsen, R. N. Bhatt, David Huse Spivak and Kivelson\footnote{B. Spivak and S. A.Kivelson, Physical Review B, 70 155114 (2004)} have proposed that the first order phase transition between the Wigner crystal and Fermi liquid phases of the interacting electron gas in two dimensions is pre-empted by a series of microemulsion phases characterized by phase separation on the mesoscopic scale, which may be responsible for the anomalous conductivity. We have studied analogous classical magnetic models in one and two dimensions. In particular, we present an exact analytical solution of a one dimensional classical ferromagnetic Ising spin chain frustrated by a long range antiferromagnetic interaction, which clearly exhibits such phase separation in which the mesoscale varies continuously with applied magnetic field. We describe these phases in the 1D model and consider extensions to stripe and bubble phases in two dimensions.\footnote{K. Ng and D. Vanderbilt, Physical Review B, 52 2177 (1995)} [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W15.00012: The spin glass transition of the three dimensional Heisenberg spin glass. Sergio Perez-Gaviro, Isabel Campos, Maria Cotallo-Aban, Victor Martin-Mayor, Alfonso Tarancon We present our results[1] about the study of the critical properties of the three dimensions Edwards-Anderson model with Heisenberg spins, by means of Monte Carlo simulation and Finite Size Scaling analysis. A mixture of Heath Bath and Overrelaxation algorithms allowed us to thermalize lattices of size up to L=32. We have found a finite temperature transition where both the spin glass and the chiral glass orderings develop. The presence of logarithmic corrections suggests that the phase transition is of Kosterlitz-Thouless type, although we may not exclude a lower critical dimension barely smaller than three. \newline \newline [1] I. Campos, M. Cotallo-Aban, V. Martin-Mayor, S. Perez-Gaviro and A. Tarancon, Phys. Rev. Lett. in press (cond-mat/0605327). [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W15.00013: Impurity induced order and moment distribution in quantum antiferromagnets Sebastian Eggert, Olav Syljuasen, Fabrizio Anfuso, Markus Andres, Stellan Ostlund The local response to a uniform field around vacancies in antiferromagnetic lattice models is analyzed by using a combination of spin wave calculations, an effective continuous theory, and quantum Monte Carlo simulations. The impurity breaks the sublattice symmetry and causes a tilting of the order parameter towards the field over a large range. This corresponds to an induced antiferromagnetic order {\it parallel} to the applied field, which is understood in quantitative detail. The relevance for NMR and susceptibility measurements will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W15.00014: ABSTRACT WITHDRAWN |
Session W16: Focus Session: Theory of Magnetism: Traditional and Novel Magnets
Sponsoring Units: GMAG DCOMP DMPChair: Yaroslaw Bazaliy, University of South Carolina
Room: Colorado Convention Center Korbel 4F
Thursday, March 8, 2007 2:30PM - 2:42PM |
W16.00001: Exact Diagonalization studies of frustrated AFM Heisenberg polytopes Ioannis Rousochatzakis, Andreas Laeuchli, Frederic Mila We explore the low energy physics of the AFM $s=1/2$ Heisenberg model on a number of frustrated magnetic molecule systems using exact diagonalization (ED). Particular emphasis is given to molecules with spins occupying the vertices of symmetric polyhedra. To this end, we have extended the standard ED technique in order to exploit the full point group (permutation) symmetry, thus including higher than one-dimensional irreducible representations. Apart from classifying the energy spectra according to both spin and permutation symmetries, our method provides the exact level degeneracies. In particular, for large frustrated polytopes, we find the existence of an accordingly large number of low-lying singlets below the first triplet, similarly to the case of frustrated 2D magnets. We also study the properties of the local spectral density functions, in view of interpreting recent neutron scattering experiments in Fe$_{30}$, one of the biggest AFM frustrated molecule available (comprising 30 spins 5/2 mounted on the vertices of a icosidodecahedron). [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W16.00002: Berry-Phase Oscillations of the Kondo Effect in Single-Molecule Magnets Michael N. Leuenberger, Eduardo R. Mucciolo We show that it is possible to topologically induce or quench the Kondo resonance in the conductance of a single-molecule magnet ($S>1/2$) strongly coupled to metallic leads. This can be achieved by applying a magnetic field perpendicular to the molecule easy axis and works for both full- and half-integer spin cases. The effect is caused by the Berry-phase interference between two quantum tunneling paths of the molecule's spin. We have calculated the renormalized Berry-phase oscillations of the Kondo peaks as a function of the transverse magnetic field as well as the conductance of the molecule by means of the poor man's scaling method. We propose to use a new variety of the single-molecule magnet Ni$_4$ for the experimental observation of this phenomenon. \\ Reference: Phys. Rev. Lett. 97, 126601 (2006). [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W16.00003: Single-ion and exchange anisotropy in high-symmetry tetramer single molecule magnets Dmitri Efremov, Richard Klemm We study the effects of single-ion and both symmetric and antisymmetric exchange anisotropy in equal-spin $s_1$ tetramer single molecule magnets exhibiting the molecular group symmetries $g=C_{4h}$, $D_{4h}$, $C_{4v}$, $S_4$, $D_{2d}$, and $T_d$. The near-neighbor and next-nearest-neighbor isotropic exchange interactions are $J$ and $J'$, respectively. From the vector basis used to diagonalize the general quadratic spin-spin interaction Hamiltonian ${\cal H}$ for each site and site pairs, we impose the symmetries characteristic of each $g$ upon ${\cal H}$. Using our exact, compact forms for the four-spin single-ion matrix elements, we calculate the eigenstate energies to first order in the anisotropy interactions. Type I tetramers with $J'-J>0$ act as two dimers with maximal pair quantum numbers $s_{13}=s_{24}=2s_1$ at low temperature $T$. Type II tetramers with $J'-J<0$ are frustrated, with minimal low-$T$ pair quantum numbers. For both Type-I and Type-II antiferromagnetic tetramers, we calculate the first-order level-crossing inductions analytically. Accurate Hartree expressions for the thermodynamics, electron paramagnetic resonance (EPR) and inelastic neutron scattering cross-section are given. An EPR procedure to extract the effective microscopic parameters is provided. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W16.00004: Double-Exchange Model for Molecule-Based Magnets Serkan Erdin, Michel van Veenendaal We report a detailed study of a model proposed for the molecule-based magnets, which is similar to the double-exchange mechanism. The model is applied to a two-dimensional periodic complex made of a transition metal and an organic molecule in which the electronic structure is described by effective $d$ orbitals of the transition metal ion at infinite Hund's coupling limit and the lowest unoccupied molecular orbital of the organic molecule. Depending on the average electron density of the organic molecules and various superexchange couplings between metal ions' core spins, magnetic states of the complex are investigated. In Monte-Carlo calculations for a model Hamiltonian, as a function of electron density on the organic molecule, the average magnetization and critical magnetic ordering temperatures are determined. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W16.00005: Stripes and hysteresis in thin film ferromagnets David Clarke, Oleg Tretiakov, Oleg Tchernyshyov Recent experimental studies have focused on the magnetic behavior of thin materials that have strong out-of-plane anisotropy. We study the behavior of such systems near the reorientation phase transition (RPT), the point at which the dipolar interaction overcomes the internal anisotropy to force the magnetization to lie in the plane. Previous studies have classified canted, polarized, and stripe domain regions of the thermodynamic phase diagram, but have not found the boundaries of metastability necessary for an understanding of observed hysteresis curves. We complete the anisotropy-applied field phase diagram near the RPT by including metastability boundaries and find the hysteresis loops characteristic of the model using analytic and numerical techniques. The system displays a line of second order transitions from a canted phase to a spin density wave (SDW) phase, and first order transitions from the SDW phase to a striped phase. We show the existence of a liquid-gas like critical point beyond which the SDW and striped phases are indistinguishable. The phase diagram is universal for thin ferromagnetic materials up to a rescaling of the applied field and effective anisotropy by a characteristic value proportional to the square of the ratio of the thickness to the exchange length. The hysteresis loops found match behavior observed in experiments. This work was supported in part by NSF Grant DMR-0520491 [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W16.00006: Stray Fields and Metastable Magnetization Configurations in Thin Films Adebanjo Oriade, Siu-Tat Chui An important aspect of the utility of magnetic tunnel junctions and the giant magneto-resistive effect devices is reversal of the magnetization of a thin film. In these devices, found in hard disk drive read heads and magneto-resistive random access memory technology, robust control of magnetization in thin films is necessary. We study, via Monte-Carlo simulations, the nature of metastable (intermediate) magnetization states in thin films and their connection to failure in the reversal process. These metastable states usually show up as plateaus in the hysteresis loop close to the switching field. The net magnetization of the film in this state is much less than the saturation magnetization. Details of the magnetization configuration in, and during reversal of, these metastable states are presented. Two mechanisms for failure are described. (1) Strong stray fields that exist during the reversal of these metastable states will affect other elements within as much as $1\mu m$ from the longest edge of an $0.2\mu m \times 1\mu m \times 50 \AA$ film. (2) Turning field off whilst the film is in a metastable state results in relaxation into a paramagnetic state, useless for application. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W16.00007: 2d order ferromagnetic resonance in nanoparticles and the dating of archaeological ceramics Derek Walton Ferromagnetic resonance is almost exclusively explored experimentally in 1$^{st}$ order where one photon decays into a single magnon, necessarily of the uniform or magnetostatic modes. In 2d order where the photon creates two magnons of equal and opposite wave-vector, it is well-known that details of the magnon spectrum become significant. An important consideration is the cut-off in the dispersion relations for magnons whose wavelength exceeds twice the scale of the particle. I will discuss the use of this property to selectively magnetize or demagnetize assemblies of single domain grains. This permits rather sensitive dating of ancient ceramics, and accurate determination of grain size distributions [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W16.00008: First-principles Calculation of the Single Impurity Surface Kondo Resonance Chiung-Yuan Lin, Antonio Castro Neto, Barbara Jones We have performed first-principles calculation of the surface and bulk wavefunctions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co [1]. By analyzing the calculated hybridization energies, we have calculated the Kondo temperature to remarkable accuracy. We find the bulk states dominate the contribution to the Kondo temperature, in agreement with a recent experiment [2]. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope on this system and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances ($<$ 6 angstroms). Our results indicate the need for a new formulation of the problem at larger distances. [1] C.-Y. Lin, A. H. Castro Neto, and B. A. Jones, Phys. Rev. Lett. \underline {97}, 156102 (2006). [2] N. Knorr, M. A. Schneider, L. Diekhoner, P. Wahl, and K. Kern, Phys. Rev. Lett. \underline {88}, 096804 (2002). [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W16.00009: Shot Noise in the SU(4) Kondo regime Vitushinskiy Pavel, Le Hur Karyn, Clerk Aashish It has recently been shown that shot noise is a direct probe of interparticle interactions which characterize the Fermi liquid fixed point of the standard Kondo model. We now examine the transport properties of the systems which are known to exhibit an unusual SU(4) Kondo correlated liquid behaviour at low temperatures. It was shown using T-matrix approach that conductance in this regime has unexpected linear in eV corrections, as dictated by the low-energy SU(4) Fermi-liquid fixed point. We confirm this result by the microscopic calculation of backscattering current using Keldysh formalism. The SU(4) symmetry in turn affects the current shot noise and thus leads to renormalized value of the effective charge. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W16.00010: Exact solution of SU(4) non-equilibrium Kondo model at the Toulouse point. Solomon Duki, Harsh Mathur SU(4) symmetry in quantum dots has become a growing interest in both semiconductor quantum dots and carbon nanotube quantum dots[1]. We investigate theoretically the properties of an SU(4) Kondo model out of equilibrium by solving the problem exactly at a special point in the parameter space. The solution reveals that, in contrast to the SU(2) model, there are two more excitations in the system other than the charge and spin excitations. We investigate the differential conductance for arbitrary voltage bias. \newline \newline [1] P. Jarillo-Herrero, J. Kong, H.S.J. van der Zant, C. Dekker, L.P. Kouwenhoven and S. De Franceschi, http://www.nature.com/openurl?url\_ver=Z39.88-2004\&rft\_val\_fmt=info:ofi/fmt:kev:mtx:journal\&rft.genre=journal\&rft. volume=434\&rft.spage=484 \&rft.date=2005 (Nature) 434, 484, (2005). [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W16.00011: Spatially dependent Kondo effect in Quantum Corrals Enrico Rossi, Dirk K. Morr We study the Kondo screening of a single magnetic impurity placed inside a quantum corral consisting of non-magnetic impurities on the surface of a metallic host system. We show that the spatial structure of the corral's eigenmodes leads to a spatially dependent Kondo effect whose signatures are experimentally measurable spatial variations of the Kondo temperature, $T_K$, and of the critical Kondo coupling, $J_{cr}$. Moreover we find that the screening of the magnetic impurity is accompanied by the formation of multiple Kondo resonances with characteristic spatial patterns that provide further experimental signatures of the spatially dependent Kondo effect. Our results demonstrate that quantum corrals provide new possibilities to manipulate and explore the Kondo effect. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W16.00012: Propagators in Position, Momentum, and Spin Variables Bailey Hsu, Jean-Francois Van Huele Propagators describe the evolution of quantum dynamical systems. Their expression depends on the dimension of the system, on the environment, and on the boundaries. Specific techniques have been developed to calculate the propagators for different functional forms of the potential, which in the case of spinless particles depend on position and momentum. Particles with spin interacting with magnetic fields in Stern-Gerlach-like systems lead to propagators involving position and spin variables. Spin-orbit-like systems combine momentum and spin variables. We investigate the applicability of methods found in the literature to systems that exhibit different combinations of momentum, position, and spin variables. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W16.00013: Size Distribution of Superparamagnetic Particles Determined by Magnetic Sedimentation Jean-Francois Berret We report on the use of magnetic sedimentation as a means to determine the size distribution of dispersed magnetic particles. The particles investigated here are i) single anionic and cationic nanoparticles of diameter D $\sim $ 7 nm and ii) nanoparticle clusters resulting from electrostatic complexation with polyelectrolytes and polyelectrolyte-neutral copolymers. A theoretical expression of the sedimentation concentration profiles at the steady state is proposed and it is found to describe accurately the experimental data. When compared to dynamic light scattering, vibrating sample magnetometry and cryogenic transmission electron microscopy, magnetic sedimentation exhibits a unique property : it provides the core size and core size distribution of nanoparticle aggregates [Preview Abstract] |
Session W17: Dynamics and Glass Transition Phenomena in Thin Polymer Films
Sponsoring Units: DPOLYChair: Christopher Stafford, National Institute of Standards and Technology
Room: Colorado Convention Center 102
Thursday, March 8, 2007 2:30PM - 2:42PM |
W17.00001: Effect of Confinement on the Relaxation Dynamics in an Antiplasticized Polymer Melt Robert Riggleman, Juan de Pablo We have developed a coarse grained model which exhibits antiplasticization in a polymer melt. Using molecular simulations, we have characterized our model and investigated the effect of antiplasticization on the relaxation dynamics of the polymer in both bulk and free-standing thin film geometries. In the bulk, we show that antiplasticization reduces the size of the cooperatively rearranging regions (CRRs) and leads to a weaker temperature dependence of the relaxation times of the system. We also show that it decreases the fragility of the material. Upon confinement, we find that the CRRs in the pure polymer film are strongly heterogeneous, and more cooperative near the free surfaces, leading to a large decrease in the glass-transition temperature (Tg). In contrast, the antiplasticized film shows a homogeneous distribution of the CRRs, which eliminates the effects of the free surface, and causes little change in the material properties upon confinement, including Tg, offering an explanation of recent experimental results. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W17.00002: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W17.00003: A New Look at Polymer Films and the Glass Transition Jane Lipson, Scott Milner The effect on the glass transition (Tg) of a polymer in going from bulk to film is still not well understood. Among the outstanding issues is the influence of a free surface relative to that of a hydrogen bonding substrate. We have developed a model able to capture the shift in Tg as a function of distance from two interfaces, accounting for the potentially different impacts of air and of a silicon oxide surface. While most of the literature reports Tg as a function of total film thickness, in ongoing work the Torkelson group has used labeling methods to probe the Tg of slices of controlled thickness within a polystyrene (PS) film.[1] In separate studies they have changed the thickness of a labeled surface layer with the underlayer held constant, the thickness of the underlayer with that of the surface layer held constant, and the thickness of the whole (labeled) film. Polystyrene is unusual in having relatively weak interactions with the substrate. Here we both apply our model to the PS film data and investigate the impact of changing from PS to a polymer able to hydrogen bond to the silicon oxide surface. [1] Ellison, CJ; Torkelson, JM Nature Mat. 2, 695 (2003). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W17.00004: The Glass Transition of Miscible Binary Polymer-Polymer Thin Films Peter Green, Brian Besancon, Christopher Soles Studies of the glass transition temperatures, T$_{g}$, of completely miscible thin film blends of tetramethyl bisphenol-A polycarbonate (TMPC) and deuterated polystyrene (dPS), supported by SiO$_{x}$/Si, were examined using spectroscopic ellipsometry (SE) and incoherent elastic neutron scattering (INS). While both sets of measurements independently reveal that T$_{g}$ exhibits qualitatively similar trends with film thickness, h, there were important quantitative differences, which depended on composition. The T$_{g}$s measured by INS were consistently larger than those determined by SE for PS weight fractions $\phi$$>$0.1. These observations are rationalized in terms of theory based on the notion of a self- concentration and reveal evidence of heterogeneous component behavior in these miscible polymer-polymer systems. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W17.00005: Molecular-dynamics simulations of thin films with a free surface Simone Peter, Hendrik Meyer, Joerg Baschnagel We present results [1,2] from molecular-dynamics simulations for a model of non-entangled short polymer chains in a free standing and a supported film geometry. We investigate the influence of confinement on static and dynamic properties of the melt. We find that the relaxation at the surfaces is faster in comparison to the bulk. We perform a layer-resolved analysis of the dynamics and show that it is possible to associate a gradient in critical temperatures Tc(y) with the gradient in the relaxation dynamics. This finding is in qualitative agreement with experimental results on supported polystyrene (PS) films [Ellison et al, Nat. Mater. 2, 695 (2003)]. Furthermore we show that the y-dependence of Tc(y) can be expressed in terms of the depression of Tc(h), the global Tc for a film of thickness h, if we assume that Tc(h) is the arithmetic mean of Tc(y) and parameterize the depression of Tc(h) by Tc(h)=Tc/(1+h0/h), a formula suggested by Herminghaus et al [Eur. Phys. J E 5, 531 (2001)] for the reduction of the glass transition temperature in supported PS films. We demonstrate the validity of this formula by comparing our simulation results to results from other simulations and experiments. \newline [1] S. Peter, H. Meyer and J. Baschnagel, J. Polym. Sci. B, 44, 2951 (2006) \newline [2] S. Peter, H. Meyer, J. Baschnagel and R, Seemann, J. Phys: Condens. Matter (2007) [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W17.00006: Dewetting of Thin Polymer Films Elie Raphael, Thomas Vilmin We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W17.00007: Hole growth in free-standing block copolymer films: does lamellar structure imitate a support? Matthew J. Farrar, Andrew B. Croll, Kari Dalnoki-Veress We will discuss how the lamellar structure of a symmetric polystyrene-poly (methyl methacrylate) diblock co-polymer can affect the hole formation of free-standing films. It is found that ordered films (with lamellae aligned parallel to the film surface) exhibit a dramatically enhanced stability over disordered films. This stability is shown to be directly related to the lamellar structure through atomic force microscopy and optical microscopy. Secondly we note how the rim structure of the holes in these two experiments is extremely different. In particular, the steep rims observed in the ordered samples show a striking similarity to holes grown in supported films, which is difficult to reconcile with current theory. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W17.00008: Confinement and interfacial effects on the alpha relaxation dynamics of thin polymer films Rodney Priestley, Linda Broadbelt, Koji Fukao, John Torkelson It is now known that both the glass transition temperature ($T_{g})$ and the alpha relaxation dynamics can deviate substantially from the bulk with confinement of polymer films. Using a novel fluorescence multilayer method, it has been established that the deviations in $T_{g}$ observed with decreasing film thickness for polystyrene and poly(methyl methacrylate) films supported on silica is a result of interfacial effects. Since the glass transition is related to the cooperative segmental (alpha) relaxation dynamics, it must be that these dynamics at the interfaces are different from the bulk. However, because of experimental limitations no direct measurement of the complete alpha relaxation time distribution at the interfaces has been conducted. Here we present results of a novel dielectric multilayer method that allows for such measurements. Our results indicate that both the alpha relaxation time distribution and the average alpha relaxation time are affected by the interfaces for polystyrene films. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W17.00009: Free Volume behavior and Structure of Polymer Thin Film Seisuke Ata, Toshiaki Ougizawa, Makoto Muramatsu, Toshiyuki Ohdaira, Ryoichi Suzuki, Toshitaka Oka, Kenji Ito, Yoshinori Kobayashi Free volume hole size (Fv) of polystyrene thin films were measured by positron annihilation lifetime spectroscopy (PALS) by using slow positron beam. Though glass transition temperature was decreased in very thin film(less than 2Rg), decrease of Fv was observed concurrently. But these were inconsistent from the general recognition of relationship between Fv and glass transition temperature. In addition, increase of thermal expansion coefficient of Fv and decrease of generation temperature of positronium bubble which indicated decline of cohesive force of polymer chains, were observed. From these results, it was inferred that the physical properties change by thinning of polymer film was induced by change of polymer stracture due to the reduce of polymer chain entanglement. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W17.00010: Effect of Confinement in Ultrathin Films on Translational Diffusion in Polymers near the Glass Transition Manish K. Mundra, John M. Torkelson A novel, extremely simple fluorescence-based multilayer method, based on breakthrough time assuming Fickian diffusion, has been developed to measure the impact of nanoscale confinement near free surfaces or polymer-substrate interfaces on translational diffusion in amorphous polymers near the glass transition temperature, Tg. With decreasing polystyrene film thickness, two dye molecules, Disperse Red 1 and decacyclene, exhibit reductions in their diffusion coefficients associated with translational diffusion away from the silica substrate interface and toward the free surface of the film. The effect of confinement on translational diffusion is much greater for the smaller dye, Disperse Red 1, with as much as an order of magnitude reduction in its translational diffusion coefficient being observed in the most confined film relative to its value in bulk polymer. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W17.00011: Hindering Cooperative Segmental Dynamics at the Free Surface of Polystyrene: The Impact of Narrow Immiscible Interfaces in Polymer Multilayer Films Connie B. Roth, John M. Torkelson The glass transition temperature (Tg) of individual layers within multilayer films of different polymers are measured to investigate how competing free surface and substrate interactions are mediated by the narrow interfaces between the immiscible polymer layers. We demonstrate that the enhanced mobility at the free surface of polystyrene (PS), as characterized by the -32 K Tg reduction observed in a 14-nm thick PS surface layer on bulk PS, can be virtually eliminated by placing a 14-nm thick PS surface layer on bulk poly(methyl methacrylate) (PMMA) and bulk poly(2-vinyl pyridine) (P2VP). We explore the extent to which the interfacial widths and layer thicknesses affect the propagation of the enhanced cooperative segmental dynamics at the free surface of PS into the film. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W17.00012: Dynamics of Polymer Melts Confined by Smooth Walls: Crossover from Non-entangled to Entangled Regime Qi Liao, Yijie Li, Dongshan Wei, Xigao Jin, Charles Han We present the results of molecular dynamics simulations of polymer films confined by smooth walls. Simulations were performed for a wide range of chain lengths covering both non-entangled and entangled polymer dynamics, as well as film thickness crossing over the order of chain size to the bulk state. The simulation results for chain size dependence on the film thickness are compared with the prediction of the scaling model. By measuring the correlation function of the end-to-end vectors, we have determined that relaxation time of polymer chains in the different entanglement state confined in the film. It is shown that there is a minimum of relaxation time with the decrease of the film thickness in the entangled regime, which may be caused by the confinement-induced disentanglement. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W17.00013: Surface Dynamics of Polymer Brushes in the Melt State: An XPCS Study Mark D. Foster, Bulent Akgun, Gokce Ugur, William J. Brittain, Suresh Narayanan, Heeju Lee, Sanghoon Song, Hyunjung Kim, Zhang Jiang, Sunil K. Sinha The suppression of long-wavelength modes of surface fluctuations on a molten polymer brush has been demonstrated using direct measurements of dynamics for the first time. The surface dynamics of densely grafted polystyrene brushes of reasonably monodisperse chains were investigated by X-ray photon correlation spectroscopy. Within the range of time and length scale investigated, 0.2 s to 1000 s, and 200 nm to 5 um, there were no detectable dynamics on the brush surfaces, even 130 C above the polymer bulk glass transition temperature. A comparably thick film of untethered chains has a q-dependent surface relaxation time of the order of 30 s, indicating that the tethering of the chains alters the surface relaxation rate by at least 3 orders of magnitude. Such a suppression of long wavelength fluctuations on the surface of a molten brush was predicted by Frederickson and co-workers. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W17.00014: Dynamics from Buried Polymer-Polymer Interfaces in Thin Films Jyotsana Lal, Xuesong Hu, Zhang Jiang, Sunil K. Sinha, Suresh Narayanan, Alec R. Sandy, Xuesong Jiao, Laurence B. Lurio We report a further development of X-ray photon correlation spectrocopy (XPCS) in order to probe capillary wave dynamics at a buried polymer interface of a bilayer. The bi-layer was chosen so that the critical angle for total external reflection for the top layer is smaller than that for the bottom layer. When x-rays are incident below the critical angle of the top layer only the structure and dynamics of the top layer are probed. When x-rays are incident above the critical angle of the top layer but below that of the bottom layer, a standing wave is set up. The phase of this standing wave can be adjusted to have a maxima at the polymer-polymer interface and simultaneously a node at the polymer-air interface. Consequently, one can isolate the static scattering and XPCS from the buried layer. Results on a system consisting of a 100 nm polystyrene (PS) film on top of an 100 nm polybromostyrene (PBrS) film, supported on a Si substrate will be reported. The dynamics are consistent with a low-viscosity mixed layer between the PS and PBrS. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W17.00015: Dynamics of complicated phase behavior in ultrathin film of polymer blend by in situ AFM Tongfei Shi, Yonggui Liao, Lijia An The stability of polymer films is important for many applications. There are many reports on dewetting of polymer films, but most of them focused on dewetting behavior of single-component polymer films. In the phase-separated regions, reports suggest the formation of bilayer via phase separation, followed by the dewetting of the upper layer, which we call phase separation/bilayer-dewetting mechanism. Recently, it was theoretically predicted that a film of a binary miscible polymer blend can dewet on a solid substrate. In this work, we present first direct experimental observations of this dewetting dynamics, driven by a composition fluctuation, of ultrathin films of PMMA/SAN blend in miscible region by in situ AFM, and then the dynamics of the complicated dewetting and phase behavior of this ultrathin film in two-phase region, which is not the phase separation/bilayer-dewetting mechanism, but the dewetting of the film is followed by the phase separation in the droplets, coupling with the wetting of the substrate by the PMMA extracted by the strong attractive interaction between PMMA and the substrate, which we call dewetting-phase separation/wetting mechanism. [Preview Abstract] |
Session W18: Theoretical Methods and Algorithms
Sponsoring Units: DCPChair: Anne Chaka, National Institute of Standards and Technology
Room: Colorado Convention Center 103
Thursday, March 8, 2007 2:30PM - 2:42PM |
W18.00001: Quantum Entanglement and Electron Correlation in Molecular Systems Hefeng Wang, Sabre Kais We study the relation between quantum entanglement and electron correlation in quantum chemistry calculations. We prove that the Hartree-Fock (HF) wave function does not violate Bell's inequality, thus is not entangled while the configuration interaction (CI) wave function is entangled since it violates Bell's inequality. Entanglement is related to electron correlation and might be used as an alternative measure of the electron correlation in quantum chemistry calculations. As an example we show the calculations of entanglement for the H$_2$ molecule and how it correlates with the traditional electron correlation, which is the difference between the exact and the HF energies. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W18.00002: Infinite, periodic systems in external fields -- an efficient, theoretical method Michael Springborg The response of periodic systems to external electric fields is a challenging theoretical problem. We show how the vector potential approach yields a numerically efficient treatment of the combined electronic and nuclear response to a finite static field. Our method is based on a self-consistent reformulation of the charge flow term in the single particle Hamiltonian. Careful numerical implementation yields a treatment whose computational needs are only marginally larger than those of a conventional field-free calculation. To prove the method we have performed model calculations for a qusi-one-dimensional (polymeric) system. The model contains all essential elements of an ab initio Kohn-Sham or Hartree-Fock Hamiltonian but allows for extensive testing. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W18.00003: Quantum master equation in phase space: Application to the Brownian motion in a periodic potential William Coffey, Yuri Kalmykov, Sergey Titov, Bernard Mulligan The quantum Brownian motion of a particle in a periodic potential $V(x)=-V_0 \cos (x/x_0 )$ is treated using the master equation for the time evolution of the Wigner distribution function $W(x,p,t)$ in phase space $(x,p)$. Explicit equations for the diffusion coefficients of the master equation for this dissipative quantum system are derived. The dynamic structure factor and longest relaxation time are evaluated by using matrix continued fractions. The longest relaxation time so obtained is compared with the quantum-mechanical escape rate formula. The matrix continued fraction solution agrees well with the analytical solution of the corresponding Kramers turnover problem. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W18.00004: Scrutinizing concepts in chemical kinetics: Sensitivity analysis and mean-field approximation Hakim Meskine, Karsten Reuter, Matthias Scheffler, Horia Metiu We present kinetic Monte Carlo (kMC) simulations for the CO oxidation reaction at RuO$_{2}$(110), based on rate constants determined by density-functional theory and transition-state theory. The composition and structure of the catalyst surface are computed in reactive environments ranging from ultra-high vacuum (UHV) to technologically relevant conditions (pressures of several atmospheres and elevated temperatures). This setup enables us to scrutinize frequently employed concepts in the modeling of chemical kinetics: Sensitivity analyses are performed to identify the rate determining steps under the different environmental conditions. While helpful under UHV conditions, this analysis proves to be of little use for catalytically relevant environments, since then a larger number of elementary processes contributes equally to the total rate of product formation. We also check on the mean-field approximation employed in phenomenological microkinetics by comparing rate equations based on the same first-principles rate constants to the kMC simulations, where the spatial distribution of the chemicals at the catalyst surface is explicitly considered. The rate equation activities are found to be in serious error, even failing to identify the correct dominant reaction mechanism. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W18.00005: A new mixed quantum/semiclassical propagation methodology Steven Schwartz We present a new propagation algorithm for the evolution of a highly quantum subsystem coupled to a more classical like bath. The quantum system is treated exactly, while the bath is evolved with a frozen Gaussian evolution. An evolution operator correction scheme we recently developed is then applied to compute the coupling between the quantum systems and the semiclassical bath. The scheme is applied to test problems and found to be accurate and not significantly more difficult to implement than standard classical molecular dynamics. The approach also admits the possibility of higher order correction to obtain exact quantum results. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W18.00006: Improved Polarizabilities and Dissociation in DFT: Vignale-Kohn Revisited Neepa Maitra, Meta van Faassen We develop a novel approach to the problem of polarizabilities and dissociation in electric fields from the static limit of the Vignale-Kohn (VK) functional. The VK response potential, extracted from the longitudinal component of the VK vector potential has ground-state properties that notably improve over VK response and over usual (semi-)local functionals. The VK density response is not the ground-state response in the corresponding field. Cases where VK density response yields poor polarizabilities, eg the H2 chain, work well in our approach. This is the first density functional method that correctly dissociates open-shell fragments in a field. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W18.00007: What decides if a smarter army can win a battle? Linda Shanahan, Surajit Sen We study the kinetics associated with a ``war'' in which an attacking army attempts to win over a spatially dispersed defender on a 2D lattice. The levels of engagement are comparable in our study. The conflicting parties can annihilate, win or lose at any given site depending upon certain preselected rules of engagement. The attacker possesses higher intelligence, which is manifested through the moves of the attackers. We show that an ``intelligent'' attacker with subcritical number of attackers cannot win in the engagement. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W18.00008: Foundations for Cooperating with Control Noise in the Manipulation of Quantum Dynamics Feng Shuang, Herschel Rabitz, Mark Dykman This work develops the theoretical foundations for the ability of a control field to cooperate with noise in the manipulation of quantum dynamics. The noise enters as run-to-run variations in the control amplitudes, phases and frequencies with the observation being an ensemble average over many runs as is commonly done in the laboratory. Weak field perturbation theory is developed to show that noise in the amplitude and frequency components of the control field can enhance the process of population transfer in a multilevel ladder system. The analytical results in this paper support the point that under suitable conditions an optimal field can cooperate with noise to improve the control outcome. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W18.00009: Enthalpy of molecular solids beyond the harmonic approximation: application to hydrogen storage Nikolai Zarkevich, D.D. Johnson With low potential energy barriers, the harmonic approximation for phonon modes can be invalid [1]. Molecular solids are composed of strongly-bonded molecules held together by relatively weak intermolecular forces. As intermolecular interactions are usually not harmonic, a new theoretical approach is needed to obtain enthalpies of molecular solids at finite temperature. We develop such a theory for molecular solids and liquids, and apply it to obtain enthalpy differences between various phases from the first principles. We also calculate Gibbs free energy, and show that a phase diagram (e.g., a van't Hoff plot) can be constructed as a graphical solution of the Gibbs equation. To exemplify important applications, we consider materials and reactions for the high-capacity hydrogen storage. [1] Phys.Rev.Lett.97, 119601 (2006). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W18.00010: Realistic simulations of low temperature Cu(100) growth: extending time and length scales by parallel temperature-accelerated dynamics Y. Shim, J.G. Amar, A.F. Voter, B.P. Uberuaga The temperature-accelerated dynamics (TAD) method is a powerful tool for carrying out non-equilibrium simulations of systems with infrequent events over extended time-scales. However, due to the serial nature of the method the computation time scales as $N^2 - N^3$, where $N$ is the number of atoms. As a result, TAD simulations have been limited to relatively small system sizes. By combining temperature-accelerated dynamics with a recently proposed parallel synchronous sublattice algorithm, we are able to simulate the dynamic evolution of systems over much larger length as well as longer time scales. In particular, we find that the computational time using our parallel accelerated dynamics method scales as $log(N)$ or better. Preliminary results for the growing film morphology in low-temperature Cu(100) growth will also be presented. These include the observation of a wide variety of defect configurations. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W18.00011: Electrostatics in Periodic-boundary Conditions and Real-space Corrections Ismaila Dabo, nicola Marzari We address periodic-image errors arising from the use of periodic-boundary conditions to describe systems that do not exhibit full three-dimensional periodicity. We show that the difference between the Coulomb potential calculated by Fourier transforms and the exact potential can be characterized analytically. Based on this observation, we present an efficient real-space method to correct periodic-image errors in plane-wave calculations. Comparing the method with existing schemes, we show that it is particularly advantageous for studying systems exhibiting one- or two-dimensional periodicity. As an application, we consider the vibrational properties of CO adsorbed on charged platinum surfaces. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W18.00012: Quantum instanton evaluation of the kinetic isotope effects Jiri Vanicek, William H. Miller The quantum instanton approximation is used to compute kinetic isotope effects for intramolecular hydrogen transfer in \textit{cis}-1,3-pentadiene. Due to the importance of skeleton motions, this system with 13 atoms is a simple prototype for hydrogen transfer in enzymatic reactions. The calculation is carried out using thermodynamic integration with respect to the mass of the isotopes and a path integral Monte Carlo evaluation of relevant thermodynamic quantities. Efficient ``virial'' estimators are derived for the logarithmic derivatives of the partition function and the delta-delta correlation functions. These estimators require significantly fewer Monte Carlo samples since their statistical error does not increase with the number of discrete time slices in the path integral. The calculation treats all 39 degrees of freedom quantum-mechanically and uses an empirical valence bond potential based on a modified general AMBER force field. The importance of quantum effects due to the skeleton motion is demonstrated by comparison with a mixed quantum-classical calculation. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W18.00013: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W18.00014: Novel mechanism of dissipation in synthetic rotary motors Corina Barbu, Vincent Crespi We study novel mechanisms of dissipation in nanoscale and molecular-scale motors. In traditional treatments of such systems, the background degrees of freedom are integrated out into a thermal bath, and the rotator is coupled directly to this bath via phenomenological terms such as viscous damping or Langevin forces. We have investigated a situation in which one degree of freedom is pulled out from the thermal bath and into the explicit equations of motion, interposed between the bath and the motor. We describe a regime in which the deceleration of an unpowered rotor follows a universal \textit{power law}, rather than a standard exponential decay. [Preview Abstract] |
Session W19: Ultrafast Dynamics using X-rays and Electrons
Sponsoring Units: DCPChair: Phil Bucksbaum, Stanford University
Room: Colorado Convention Center 104
Thursday, March 8, 2007 2:30PM - 3:06PM |
W19.00001: Rydberg electrons spy conformational dynamics of hot molecules Invited Speaker: The observation of structural dynamics of flexible molecules at high temperatures is arguably one of the most challenging problems of molecular dynamics. We succeeded in observing conformational dynamics by using electrons in Rydberg orbits as spies of the molecular structure. The time-resolved photoionization from the Rydberg states, providing a purely electronic spectrum that serves to characterize the molecular structure, allows us to follow the molecular motions in real time. The internal rotation about carbon-carbon bonds affords the unsaturated hydrocarbon chain molecules N,N-dimethyl-2-butanamine (DM2BA) and N,N-dimethyl-3-hexanamine (DM3HA) an opportunity to assume multiple conformeric structures. We explore the equilibrium compositions and the dynamics of transitions between such structures. An ultrashort laser pulse rapidly increases the molecule's internal energy and changes the potential energy landscape. The molecules respond by adjusting their shape, i.e. by converting between conformeric molecular structures. For DM2BA at a total internal energy of 1.79 eV, the time constants for interconversion between conformers are 19 ps and 66 ps, respectively. In DM3HA, the respective time constants are 23 and 41 ps. Comparison with a calculated conformational energy landscape reveals the conformeric forms of DM2BA involved in the molecular shape transformation. Thus, for the first time a time-resolved and quantitative view of the conformational dynamics of a flexible hydrocarbon chain at high temperature is revealed. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W19.00002: Attosecond Electron Interferometry Invited Speaker: Attosecond optical pulse generation is electron interferometry. Quantum mechanical tunnelling in an intense laser field splits the electron. After tunnelling, one component of the electron wave function is accelerated away from the ion by the laser field, but returns once the field reverses its sign. The other component remains bound to the ion. These two paths form the two arms of the interferometer. When the two components of the electron wave function overlap, they interfere. The interference leads to an oscillating dipole that produces attosecond optical pulses and simultaneously images molecular orbitals. Interferometry allows sub-wavelength changes in the length of one arm to be measured relative to another. Using rotational wave packets, we show that the high harmonics are very sensitivity to very small molecular motion and local fields. We adapt transient grating spectroscopy with two or more grating elements to observe phase changes to the high harmonics as we rotate a molecule. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W19.00003: Ultrafast Transient Absorption and Photoelectron Spectroscopy with High Order Harmonics Invited Speaker: Laser-produced high order harmonics are used to probe chemical dynamics of atoms and molecules on femtosecond timescales. Two basic methods are developed, ultrafast transient absorption and photoelectron spectroscopy. The high order harmonics are produced with an 800 nm Ti:sapphire laser focused into a capillary or rare gas jet. Both inner shell core levels and outer shell valence states are investigated. The transient absorption of xenon ions produced by high field ionization of neutral xenon atoms is probed by core level spectroscopy. The alignment of the vacancy created in forming the ion is measured as a function of pump-probe delay by promotion of an inner d electron to the vacancy in the outer shell. Small molecules are excited to repulsive dissociative states and individual harmonics are used to obtain time-resolved photoelectron spectra. A wave packet on the dissociative state of bromine molecules is detected, as well as the production of atoms at longer time delays. By the use of velocity map imaging, the angular distributions of outgoing photoelectrons are analyzed. In a new experimental system, carrier-envelope phase-stabilized few-cycle pulses will be used to create attosecond pulses of the high order harmonics, to study electronic-time-scale processes in atoms and molecules. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W19.00004: Observation of Intra-molecular Dynamics using High-Harmonic Generation as a Probe Nick Wagner, Xibin Zhou, Wen Li, Robynne Hooper, Margaret Murnane, Henry Kapteyn We report two observations of intramolecular dynamics using electrons rescattered during the process of high-order harmonic generation. In the first experiment, we excite coherent vibrations in SF$_{6}$ using impulsive Raman scattering. A second, more-intense pulse generates high-order harmonics from the excited molecules, at wavelengths of 20-50nm. The harmonic yield is observed to oscillate, at frequencies corresponding to all the Raman-active modes of SF$_{6}$, with an asymmetric breathing mode most visible. This is in contrast to conventional Raman spectroscopy where only the symmetric breathing mode of the molecule is easily observed. The data also show evidence of relaxation dynamics following impulsive excitation of the molecule. Our results indicate that harmonic generation is a very sensitive probe of vibrational dynamics, yielding more information simultaneously than conventional ultrafast spectroscopies. In our second experiment, we dissociate CF$_{3}$I with a 266nm pulse, and monitor the dissociation by probing high harmonic emission from the intact and dissociated molecule. Since the de Broglie wavelength of the recolliding electron is on the order of interatomic distances ($\sim $1.5{\AA}), small changes in the shape of the molecule lead to large changes in the high harmonic yield. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W19.00005: Ultrafast x-ray pulses emitted from a liquid mercury laser target Christopher Laperle, Christian Reich, Brian Ahr, Xiaodi Li, Frank Benesch-Lee, Christoph Rose-Petruck We report the generation of ultrashort, hard x-ray pulses from a liquid mercury target irradiated by 5-kHz laser pulses. The new x-ray source is designed for time-resolved x-ray absorption spectroscopy as well as imaging applications. This marks the first laser-driven plasma x-ray source that continuously recycles the target material, facilitating maintenance-free operation. Theoretical calculations show mercury targets emit shorter x-ray pulses than targets of lighter elements under identical illumination and x-ray detection conditions. The plasma-physical properties of mercury are very well suited for sub-50-fs hard x-ray pulse generation. The x-ray emission properties of this source have been simulated by a combination of particle-in-cell (PIC) and Monte-Carlo (MC) calculations of the laser target interaction and the resulting electron dynamics. All calculations were performed for p-polarized, 100-fs, 800-nm laser pulses with an incidence angle of 45 degrees. The calculated x-ray yields are in good agreement with the measured emission spectra. The simulated x-ray pulses have a width of 60 fs (fwhm), as short as the driving laser pulse width. Applications of laser-pump x-ray probe measurements are presented. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W19.00006: Control and interrogation of electronic dynamics by above-threshold ionization Mark Abel, Thomas Pfeifer, Phil Nagel, Daniel Neumark, Stephen Leone While direct interrogation of coherent nuclear dynamics has been possible for some time, only recently have the motions of valence- and core-level electrons become experimentally accessible. This access is provided by strong-field physics, through the application of high harmonic generation to ultrafast x-ray pulse synthesis. We show that another phenomenon from strong-field physics, above-threshold ionization (ATI), can yield information about electronic states and electronic dynamics without using subfemtosecond x-ray pulses. In particular, quantum beating in Xe atoms and in a 1-dimensional argon atom model show that electronic motion can be excited and interrogated in a pump-probe ATI experiment. Measurements in molecular gases show that this technique is also applicable to ro-vibrational dynamics. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W19.00007: Vibrational Modulation of High Harmonic Generation in SF$_{6}$ Zachary Walters, Stefano Tonzani, Chris H. Greene In a recent experiment performed at JILA (N. Wagner et al, PNAS {\bf 103} 13279, 2006), a molecule is hit by two pulses: the first stimulates Raman-active vibrations while the second generates high-order harmonics. The harmonic intensity oscillates as a function of delay time between the two pulses, with oscillation frequencies equal to those of the Raman-active modes. We interpret this oscillation as a form of quantum interference between neighboring vibrational states of the molecule. Nonzero derivatives of the ionization and recombination amplitudes with respect to nuclear coordinates give the molecule some amplitude to change vibrational states during the high harmonic process. We present a theoretical description of vibrational high harmonic modulation and compare with the experimental results of Wagner {\em et al.} [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W19.00008: Strong-field ionization of Xe probed by femtosecond high-order harmonic absorption spectroscopy Zhi-Heng Loh, Munira Khalil, Raoul E. Correa, Robin Santra, Stephen R. Leone Recent experiments on strong-field ionization of atoms and ions have led to conflicting conclusions regarding the existence of orbital alignment in the ionized species. Using table-top, femtosecond high-order harmonic absorption spectroscopy, we have measured the alignment of Xe$^{+}$ formed via strong-field ionization. High-order harmonics generated by focusing an intense 800 nm pulse into a Ne-filled capillary are spatially overlapped with an optical pump pulse in a sample gas cell before they are spectrally dispersed in an extreme ultraviolet spectrometer. Probing the transition from the 4$d$ core level to the $^{2}P_{3/2}$ state of Xe$^{+}$ at 55.4 eV yields a polarization anisotropy of 0.12 $\pm $ 0.01, in good agreement with the theoretical value of 0.1. This result suggests that strong-field ionization exclusively populates the $m_{J}=\pm $1/2 sub-levels in the Xe$^{+} \quad ^{2}P_{3/2}$ state. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W19.00009: Using high-order harmonics with momentum imaging techniques to study atomic and molecular dynamics Arvinder Sandhu, Etienne Gagnon, Ariel Paul, Margaret Murnane, Henry Kapteyn Laser-generated high-order harmonics provide a source of extreme-ultraviolet radiation with unique capabilities for probing atomic and molecular dynamics. Here we present the first studies that employ high harmonics in conjunction with coincidence momentum imaging (COLTRIMS) techniques for studies of molecular dynamics. We generate pulse at $\sim $ 43 eV photon energy by upconverting intense ($>$ 10$^{14 }$Wcm$^{-2})$ 25 fs laser pulses in an argon filled waveguide. These photons illuminate a cold molecular beam of CO, CO$_{2}$ or N$_{2}$, with the ion and electron products from ionization/dissociation detected using time-and-position resolved detectors. We obtain count rates as high as 0.25 per harmonic pulse, sufficient for a variety of studies. By employing pump-probe techniques, we can launch molecules into highly excited states near the molecular double-ionization threshold, from where the dynamics unfold along different channels. We also employ field-free impulsive molecular alignment to demonstrate for the first time the use of single photon excitation to obtain ion and electron angular distributions in the lab frame. [Preview Abstract] |
Session W21: General Theory: Electronic Structure and Interactions
Sponsoring Units: DCOMPChair: Richard Scalettar, University of California, Davis
Room: Colorado Convention Center 106
Thursday, March 8, 2007 2:30PM - 2:42PM |
W21.00001: Spectral-product representations of atomic and molecular Hamiltonians P.W. Langhoff, R.J. Hinde, J.D. Mills, J.A. Boatz An alternative approach to {\it ab initio} computational studies of the electronic structure of matter is described. Antisymmetry restrictions are enforced subsequent to construction of the Hamiltonian matrix for an atom or molecule in an orthonormal spectral-product basis. Transformation to a permutation-symmetry representation obtained from the eigenstates of the aggregate electron antisymmetrizer enforces the requirements of the Pauli principle, and eliminates the unphysical (non-Pauli) states spanned by the product basis. Results identical with conventional use of prior basis-state antisymmetry are obtained in applications to many-electrons atoms. For polyatomic molecules, the development accommodates incorporation of fragment information in the form of Hermitian matrix representatives of atomic and diatomic operators which include the non-local effects of overall electron antisymmetry, providing a new exact atomic-pair representation of polyatomic Hamiltonian matrices. Illustrative applications to the well-known low-lying doublet and quartet states in the H$_{3}$ molecule demonstrate that the eigensurfaces of the antisymmetrizer can anticipate the structures of the more familiar energy surfaces, including seams of conical intersection. The calculated energy surfaces are found to be in good agreement with corresponding accurate values obtained from valence-bond and higher-level computational procedures. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W21.00002: A Quantum Monte Carlo study of Molecular Hydrogen adsorbed on Benzene Todd D. Beaudet, Jeongnim Kim, Michele Casula, Richard M. Martin, Simone Chiesa Many prospective hydrogen storage systems contain carbon scaffolding comprised of benzene-like structural units. The binding energy of H$_2$ with these benzene-like rings is below the $\sim$ 0.2-0.4 eV/H$_2$ target necessary for reversible adsorption$^1$. Here we study the hydrogen-benzene system using quantum Monte Carlo (MC) methods suitable for resolving small energy differences. Potential energy curves are calculated using correlated umbrella sampling and variational MC. Reptation MC calculations are also in progress. A Jastrow correlated geminal wave function previously applied to benzene$^2$ is compared to Slater-Jastrows with orbitals derived from the PBE and B3LYP density functionals. We compare our results to previous work$^3$ and discuss our progress on larger systems that may have the desired binding affinity. \\ \\ $[1]$ R. C. Lochan and M. Head-Gordon, Phys. Chem. Chem. Phys. 8, 1357 (2006). \\ $[2]$ M. Casula, C. Attaccalite, and S. Sorella, J. Chem. Phys. 121, 7110 (2004). \\ $[3]$ S. Hamel and M. C\^ot\'e, J. Chem. Phys. 121, 12618 (2004). [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W21.00003: Quantum Monte Carlo study of equilibrium phase stability of crystalline FeO J. Kolorenc, L. Mitas, A. Kollias, K. Esler, R. E. Cohen We investigate phase stability of crystalline FeO at experimental equilibrium volume by means of several approaches. It is well known that energy ordering of the B1 (rocksalt) and the inverse B8 (NiAs) structures is reversed in standard DFT methods when compared to experiment. Therefore, we consider more advanced DFT-based band structure techniques, such as hybrid exchange-correlation functionals and LDA+U, and use the corresponding set of orbitals as an input for quantum Monte Carlo wave functions. We compare the results from these calculations and discuss the reasons for the DFT difficulties in describing the correct ground state of this solid. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W21.00004: Continuum Mechanics of An Inhomogeneous System Jianmin Tao, Giovanni Vignale Starting from the hydrodynamical form of the Heisenberg equations of motion, we develop the continuum mechanics of inhomogeneous quantum many-body systems subject to weak time-dependent external potentials. The formalism allows excitation energies and transition currents to be obtained from the solution of an eigenvalue problem. First, we express the noninteracting kinetic part of the stress tensor in terms of the ground-state density and the transition current exactly, while leaving the correlation part treated approximately with the assumption of local isotropy of the correlation hole. Then we treat the linear response of the exchange part employing the first-order perturbation theory. The resulting eigenvalue problem is a fourth-order differential equation. This theory is exact for one-electron systems and expected to be accurate for many-electron systems. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W21.00005: Ground state spin of a Fermi system with random interactions Vladimir Zelevinsky Consider a small fermionic system in a spherically symmetric field (external or self-consistent). As an example, atomic nuclei or atoms in a trap can be taken. Let the particles interact through all possible randomly selected but rotationally invariant two-body interactions. The random interaction amplitudes $V_{L}$ for the channels with all possible angular momenta $L$ of the pair are taken from an ensemble symmetric with respect to the sign of the amplitudes. As a statistical result of many realizations of the ensemble, in spite of the fact that the states with total angular momentum zero appear with a small multiplicity among all many-body states of the system, the system prefers the ground state spin zero with large probability. The probability of the maximum possible spin is also enhanced compared to pure statistical expectations. We discuss underlying physics in relation to ideas of quantum chaos and geometric chaoticity of angular momentum coupling in mesoscopic systems. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W21.00006: Numerical Linked-Cluster Algorithms for Quantum Lattice Models Tyler Bryant, Marcos Rigol, Rajiv R. P. Singh We discuss recently introduced Numerical Linked-Cluster (NLC) Algorithms that allow one to obtain temperature dependent properties of quantum lattice models, in the thermodynamic limit, from exact diagonalization of finite clusters. We present studies of thermodynamic observables for spin models on square, triangular, and {\it kagom\'e} lattices. Results for several choices of clusters and extrapolations methods, that accelerate the convergence of NLC, are presented. We also include a comparison of NLC results with those obtained from exact analytical expressions (where available), High Temperature Expansions (HTE), exact diagonalization (ED) of finite periodic systems, and quantum Monte-Carlo (QMC) simulations. For many models and properties NLC results are substantially more accurate than HTE and ED. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W21.00007: A study of a nonuniform, non-interacting electron gas in two dimensions. Michael Koivisto, M. J. Stott The study of an impurity in a two dimensional, non-interacting electron gas (Zaremba, et. al., Phys. Rev. Lett. 90, 046801 (2003)) shows significant simplifications over the three dimensional case. Linear response theory appears to have a wide range of validity even when the potential is sufficiently attractive to bind an electron. We have carried out an investigation of this two dimensional case, and studied both the electron density and energy associated with the impurity for cases of both attractive and repulsive potential. The significance of the results for density functional theory for a two dimensional system of fermions is investigated. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W21.00008: Study of two-dimensional interacting electrons under the renormalized-ring-diagram approximation Xin-Zhong Yan, C. S. Ting The renormalized-ring-diagram approximation (RRDA) is an important approximation for investigating interacting electrons system. It is a challenge to understand the theoretical behavior of a two dimensional electron gas (TDEG) under this approximation. So far, this has never been done in the strong- coupling regime. With a super-high-efficient numerical algorithm, we self-consistently solve the integral equations for the electron Green’s function under RRDA in a TDEG with long-range Coulomb interactions from weak to strong couplings. In our numerical calculation, the equations are solved at the imaginary Matsubara frequency, so we avoid dealing with singularities in the Green's function with real frequency. Our momentum convolution is computed with the Fourier transform into real space, so reducing the two-dimensional calculation to a one-dimensional one. By so doing, the momentum-integral can be performed precisely. The obtained ground-state energy is found to be in excellent agreement with that of the Monte Carlo simulation. We will also present the numerical results of the self-energy, the effective mass, the distribution function, and the renormalization factor of the Green's function for the coupling constants in a wide range. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W21.00009: The Holstein polaron: coupling to multiple phonon branches Mona Berciu, Lucian Covaci We extend a recently developed approach, the Momentum Average approximation, to study polaron properties when the electron couples to two or more phonon branches through Holstein-like terms. The efficient numerical procedure we propose for obtaining the Green's function within this approximation allows the accurate calculation of physical properties for a wide range of parameters and in any dimension. Our results are exact in limiting cases of very weak and very strong couplings, and accurate in the intermediate regime. This is demonstrated by studying the sum rules of the spectral function, the first 6 of which are satisfied exactly. We present results for the polaron ground state energy, quasiparticle weight, average number of phonons in the ground state and effective mass, as well as spectral functions. These are all readily calculated for a wide range of momenta and a wide range of couplings. An ansatz allowing efficient generalizations to more phonon branches will also be presented. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W21.00010: The Green's Funciton of the Holstein Polaron Glen Goodvin, Mona Berciu, George Sawatzky I will present a novel, highly efficient yet accurate analytical approximation for the Green's function of a Holstein polaron. It is obtained by summing all of the self-energy diagrams, but with each self-energy diagram averaged over the momenta of its free propagators. The result becomes exact for both zero bandwidth and for zero electron-phonon coupling, and is accurate everywhere in the parameter space. The resulting Green's function satisfies exactly the first six spectral weight sum rules. All higher sum rules are satisfied with great accuracy, becoming asymptotically exact for coupling both much larger and much smaller than the free particle bandwidth. Comparison with existing numerical data also confirms this accuracy. I will then use this approximation to analyze in detail the redistribution of the spectral weight as the coupling strength varies. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W21.00011: A uniform algebraically-based approach to computational physics and efficient programming James Raynolds, Lenore Mullin We present an approach to computational physics in which a common formalism is used both to express the physical problem as well as to describe the underlying details of how computation is realized on arbitrary multiprocessor/memory computer architectures. This formalism is the embodiment of a generalized algebra of multi-dimensional arrays (A Mathematics of Arrays) and an efficient computational implementation is obtained through the composition of of array indices (the psi-calculus) of algorithms defined using matrices, tensors, and arrays in general. The power of this approach arises from the fact that multiple computational steps (e.g. Fourier Transform followed by convolution, etc.) can be algebraically composed and reduced to an simplified expression (i.e. Operational Normal Form), that when directly translated into computer code, can be mathematically proven to be the most efficient implementation with the least number of temporary variables, etc. This approach will be illustrated in the context of a cache-optimized FFT that outperforms or is competitive with established library routines: ESSL, FFTW, IMSL, NAG. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W21.00012: Transverse Quantum Noise in Landau-Zener Transition. Bogdan Dobrescu, Valery Pokrovsky, Deqiang Sun We derive master equation for quantum noise in Landau-Zener transition starting from microscopic Hamiltonian, obtain general analytical solution of the master equation and analyze most important limiting cases. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W21.00013: Finite Size Scaling with Gaussian Basis Sets Sabre Kais, Winton Moy, Pablo Serra We have developed the finite size scaling method, which is based on taking the number of elements in a complete basis set as the size of the system,to calculate the critical parameters for a given quantum system using Gaussian basis sets. We studied the Yukawa potential and Helium-like systems by expanding the system with a Gaussian basis. The finite size scaling approach was then used with the ab initio methods to find the critical parameters of atomic and molecular systems. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W21.00014: Data Parallel Real Symmetric Eigensolver for Approximate Eigen-Solutions in SCF Yihua Bai, Guoping Zhang Solving large real symmetric eigenvalue problems is a demanding and time consuming task in electronic structure calculations. For example, when using the Su-Schrieffer-Heeger(SSH) model to study the fundamental properties of trans-polyacetylene (trans- PA), as well as many other materials, the size of Hamiltonian matrix increases with the chain length of the material and can become very large. A data parallel divide-and-conquered eigensolver has been developed for eigen-decomposition of real symmetric matrices with strong locality properties like those generated from trans- PA, that is, matrix elements with larger magnitudes are closer to the diagonal. This eigensolver computes the approximate eigen-solutions of real symmetric matrices to user prescribed accuracy tolerance. Performance tests show that this new implementation scales up well and is extremely efficient for the computation of electronic spectrum of trans-PA compared to traditional dense eigensolvers. In some cases, the savings is order of magnitude, with the potential of saving significant amount of computation time in iterative methods like SCF. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W21.00015: Conductance and transmission times of electrons and electromagnetic wave packets Pedro Pereyra, Herbert Simajuntak We study the conductance and transmission times of electrons and electromagnetic wave packets through semiconductor superlattices and optical superlattices, respectively. We follow the space- time evolution (described by the Schr\"odinger or Maxwell equations) of Gaussian packets with centroid at resonance, in a gap or opaque region and at the edge between the gap and the allow band. The time spent by the wave packets inside the potential region, or the optical structures, agrees extremely well with the phase time predictions and the superluminal experimantal results. [Preview Abstract] |
Session W22: Applications to Networks and Organized Systems
Sponsoring Units: GSNPChair: John Rundle, University of California, Davis
Room: Colorado Convention Center 108
Thursday, March 8, 2007 2:30PM - 2:42PM |
W22.00001: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W22.00002: Predicting large events in power-law distributed avalanches: implications for earthquake forecast. Osvanny Ramos, Ernesto Altshuler, Knut Jorgen Maloy It is a common idea that power law distributed avalanches are inherently unpredictable. It mostly comes from the concept of Self-organized criticality. Nevertheless, we have found in classical simulations and experiments where the slowly addition of energy drives the system into a state of power law distributed avalanches, clear signs of both long and short term prediction. The simulations consist of a more realistic modification of the Olami-Feder-Christensen earthquake model where criticality and periodicity coexist. The experiment shows a clear power law behaviour for almost three decades in the avalanche size distribution of moving grains in a ``sandpile'' setup. Both systems display characteristic waiting times between large avalanches, and their internal structure suffer continuous variation preceding a large event. Monitoring those variations it is possible to predict large avalanches in a system if the slope of its pdf is larger than 1 in absolute value. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W22.00003: Space-Time Clustering and Correlations of Earthquakes James Holliday, John Rundle, Donald Turcotte, William Klein, Kristy Tiampo, Andrea Donnellan Earthquake occurrence in nature is thought to result from correlated elastic stresses, leading to clustering in space and time. We show that occurrence of major earthquakes correlates with time intervals when fluctuations in small earthquakes are suppressed relative to the long term average and estimate a probability of less than 1\% that this coincidence is due to random clustering. Furthermore, we show that an order parameter can be defined to characterize these fluctuations and that a generalized Ginzburg criterion can be established to measuring the relative importance of fluctuations in the parameter. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W22.00004: Numerical simulations of the 2-dimensional Robin-Hood model Gabriel Cwilich, Perry Fox, Fredy Zypman, Sergey Buldyrev The Robin Hood, or Zaitsev model [1] has been successfully used to model depinning of interfaces, friction, dislocation motion and flux creep, because it is one of the simplest extremal models for self-organized criticallity Until now, its properties have been well understood theoretically in one dimension and its scaling laws numerically verified. It is important to extend the range of validity of these laws into higher dimensions, to find precise values for the scaling exponents, and to investigate how they depend on the details of the model (like anisotropy). The case of two dimensions is of particular importance when studying surface friction [2]. Here, we numerically evaluate high precision scaling exponents for the avalanche size distribution, the avalanche fractal dimension, and the Levy flight-like distribution of the jumps between extremal active sites. [1] S.I. Zaitsev , Physica \textbf{A 189}, 411 (1992). [2] S. Buldyrev, J. Ferrante and F. Zypman Phys. Rev E (accepted) [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W22.00005: Self-organized criticality of elastic networks Mykyta V. Chubynsky, M.-A. Bri\`ere, Normand Mousseau We consider a model of elastic network self-organization inspired by studies of covalent glasses [1,2]. In the model, networks self-organize by avoiding stress whenever possible, but otherwise are random. Instead of a single rigidity percolation transition, with percolation always absent below a certain bond concentration and always present above, we find that the percolating rigid cluster exists with a probability between 0 and 1 in a finite range of bond concentrations, the {\it intermediate phase}. A power-law distribution of non-percolating cluster sizes, normally observed at a single critical point in percolation transitions, is seen everywhere in the intermediate phase. There is also a finite probability of percolation appearing and disappearing upon the application of a microscopic perturbation (addition or removal of a single bond). These properties indicate that in this phase the network maintains itself in a critical state on the verge of rigidity, a signature of self-organized criticality, but in a system at equilibrium.\\ \noindent [1] M.V. Chubynsky, M.-A. Bri\`ere and N. Mousseau, Phys. Rev. E 74, 016116 (2006)\\ \noindent [2] M.-A. Bri\`ere, M.V. Chubynsky and N. Mousseau, cond-mat/0610557 [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W22.00006: Transport in Weighted Networks: Partition into Superhighways and Roads Zhenhua Wu, Lidia A. Braunstein, Shlomo Havlin, H. Eugene Stanley Transport in weighted networks is dominated by the minimum spanning tree (MST), the tree connecting all nodes with the minimum total weight. We find that the MST can be partitioned into two distinct components, having significantly different transport properties, characterized by centrality—the number of times a node (or link) is used by transport paths. One component, superhighways, is the infinite incipient percolation cluster, for which we find that nodes (or links) with high centrality dominate. For the other component, roads, which includes the remaining nodes, low centrality nodes dominate. We find also that the distribution of the centrality for the infinite incipient percolation cluster satisfies a power law, with an exponent smaller than that for the entire MST. The significance of this finding is that one can improve significantly the global transport by improving a tiny fraction of the network, the superhighways. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W22.00007: Geometric properties of minimal-cost spanning trees Tom Jackson, N. Read The minimal-cost spanning tree (MST) problem is one of the oldest combinatorial optimization problems of computer science: given a graph with a unique cost associated with each edge, the MST is the subset of edges that will connect all vertices of the graph to each other at lowest total cost. While the MST is easy to compute (i.e., of polynomial complexity), it is of interest both intrinsically and as a heuristic approximation to harder questions in optimization, such as the Steiner tree and Traveling Salesman problems. Using techniques of statistical field theory, we study the random MST where the edge costs are independent, identically distributed, random variables. We develop a mean field theory by solving the MST exactly on the Bethe lattice using the relationship between bond percolation and Kruskal's greedy algorithm for the MST. These considerations carry over to finite dimensional lattices and the field theory for percolation in $6- \epsilon$ dimensions. From this we find that the critical dimension $d_c = 6$ for the MST problem, contrary to the result $d_c = 8$ previously suggested by Newman and Stein. Finally we calculate to order $\epsilon$ the Hausdorff (fractal) dimension of the unique path on the MST connecting two widely separated points. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W22.00008: Ridge Network of Crumpled Paper Christian Andresen, Alex Hansen, Jean Schmittbuhl The work presented has investigated the network formed by the complete sets of ridges from samples of crumpled paper. Sheets of paper were crumpled, and their height profiles measured by a laser profilometer. From these data lines of high curvature were identified as ridges. Intersections between ridges were considered as nodes, and the ridges as links between these nodes. The emerging networks have been investigated using network theory. Properties such as the degree distribution, degree correlation and clustering coefficient are reported. These are compared to comparable random networks and networks formed by the Voronoi diagrams. Spatial properties such as the ridge length, domain area and vertex distributions have also been investigated. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W22.00009: Physics of curling ribbons Anna M. Klales, Buddhapriya Chakrabarti, Vincenzo Vitelli, L. Mahadevan, Vinothan Manoharan Curling decorative ribbons by dragging it past one's thumb and the blade of a scissor is a well known technique used frequently. However a quantitative understanding of this apparently simple phenomenon is still lacking. We present results from recent experimental and theoretical investigations of this problem. Using the insights gained from this we propose a method of generating novel shapes by differential stretching and subsequent selective stress relief for thin sheets. We discuss the implications of this mechanism for the formation of ribbon like structures in biological systems. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W22.00010: Griffiths singularities and algebraic order in the exact solution of an Ising model on a modular network Michael Hinczewski We use an exact renormalization-group transformation to study the Ising model on a modular network composed of tightly-knit clusters with a scale-free distribution of cluster sizes. By varying the ratio $K/J$ of inter-cluster to intra-cluster interaction strengths, we obtain an unusual phase diagram: at high temperatures or small $K/J$ the system exhibits a disordered phase with a Griffiths-like singularity in the free energy as a function of magnetic field, due to the effects of rare large clusters. As the temperature is lowered, true long-range order is not seen, but there is a transition to an algebraically ordered phase, with thermodynamic characteristics reminiscent of the XY model, but in a different universality class. The transition is infinite-order at small $K/J$, and becomes second-order above a threshold value $(K/J)^\ast$. We investigate the nature of pair correlations in the model, allowing us to test recent predictions on the relationship between network topology and correlations in cooperative systems. Despite the absence of magnetization in the low-temperature phase, we find that a subset of spin pairs (vanishingly small in the thermodynamic limit) remains strongly correlated at arbitrarily large distances. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W22.00011: Nonstationary Increments, Scaling Distributions, and Variable Diffusion Processes in Financial Markets Gemunu Gunaratne, Kevin Bassler, Joeseph Mccauley Arguably the most important problem in quantitative finance is to understand the nature of stochastic processes that underlie market dynamics. One aspect of the solution to this problem involves determining characteristics of the distribution of fluctuations in returns. Empirical studies conducted over the last decade have reported that they are non-Gaussian, scale in time, and have power-law (or fat) tails. However, these studies implicitly assume that the underlying process has stationary increments. We explicitly show that this assumption is not valid for the Euro-Dollar exchange rate between 1999-2004. In addition, we find that fluctuations in returns of the exchange rate are uncorrelated and scale as power-laws for certain time intervals during each day. This behavior is consistent with a diffusive process with a diffusion coefficient that depends both on the time and the price change. Within scaling regions, we find that sliding interval methods can generate fat-tailed distributions as an artifact. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W22.00012: Parameter Inference in the Ornstein-Uhlenbeck Process Paul Mullowney, Satish Iyengar The Ornstein-Uhlenbeck process has been proposed as a model for the spontaneous activity of a neuron. In this model, the firing of the neuron corresponds to the first passage of the process to a constant boundary, or threshold. While the Laplace transform of the first passage time distribution is available, the real density has not been obtained in any tractable form. We address the problem of estimating the parameters of the process when the only available data from a neuron are the interspike intervals, or the times between firings. In particular, we give an algorithm for computing maximum likelihood estimates (MLEs) and their corresponding confidence regions for three of the five model parameters by numerically inverting the Laplace transform. We also provide an analysis on the reliability of the estimates and their confidence regions when simulated data is used to generate the first passage sample. [Preview Abstract] |
Session W23: Metals: Alloys and Compounds
Sponsoring Units: DCMPChair: Stefano Curtarolo, Duke University
Room: Colorado Convention Center 110
Thursday, March 8, 2007 2:30PM - 2:42PM |
W23.00001: Anomalous electronic correlations in ground state momentum density of Al$_{97}$Li$_3$ B. Barbiellini, J. Kwiatkowska, S. Kaprzyk, A. Bansil, H. Kawata, N. Shiotani We report high resolution Compton scattering measurements on an Al$_{97}$Li$_3$ disordered alloy single crystal for momentum transfer along the [100], [110] and [111] symmetry directions [1]. The results are interpreted via corresponding KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation) first principles computations. By comparing spectra for Al$_{97}$Li$_3 $ and Al, we show that the momentum density in the alloy differs significantly from the predictions of the conventional Fermi liquid picture and that the ground state of Al is modified anomalously by the addition of Li. Work supported in part by the USDOE.\\ \mbox{[1]} J. Kwiatkowska, B. Barbiellini, S. Kaprzyk, A. Bansil, H. Kawata, and N. Shiotani, Phys. Rev. Lett. 96 (2006) 186403. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W23.00002: Pseudo-gap Observed at Martensite Transition in Ni$_{2}$MnGa Single Crystal C.P. Opeil, J.C. Lashley, R.K. Schulze, B. Mihaila, W.L. Hults, J.L. Smith, P.S. Riseborough, L. Manosa, A. Planes Specific heat and coefficient of linear thermal expansion coefficient measurements show that the ferromagnetic shape-memory alloy Ni$_{2}$MnGa single crystal exhibits a pre-martensitic transition (PMT) at T = 214 K and a MT at 196 K. Lee \textit{et al.}\footnote{Y. Lee, J. Y. Rhee, B. N. Harmon, \textit{Physical Review B} \textbf{66}, 054424 (2002).} predicts that magnetically-tuned Fermi-surface nesting at \textbf{q} = (2$\pi $/3a) (1,1,0) is responsible for phonon softening at the PMT. On the basis of temperature dependant angle resolved photoemission (ARPES) measurements, we show that a pseudo-gap opens at 0.3 eV below the Fermi energy at the martensitic transition (MT) thus providing further evidence that the Fermi surface is nested at the MT and is only partially nested at the PMT. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W23.00003: First-principles electronic structure of shape-memory alloy Ni$_2$MnGa Bogdan Mihaila, Alice Acatrinei, Christopher D. Taylor, Cyril P. Opeil, Llu\'is Ma\~nosa The Ni$_2$MnGa memory-shape alloy undergoes both a feromagnetic phase trasition (T$_C$~$\sim$~380~K) and a martensitic transformation (MT, T$_M$~$\sim$~175~K) upon cooling. In addition, the MT is preceeded by a premartensitic (pre-MT) phase transition corresponding to a micromodulated structure accompanied by phonon softening presumably related to Fermi-surface nesting and strong electron-phonon coupling. Here, we report results of a comparison study of first-principles electronic structure calculations and recent angle-resolved photoemission measurements, at temperatures T=219~K, in close proximity with the pre-MT, and T=173~K, in the martensite phase. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W23.00004: Electronic Structure Theory of the Structural Transformation of Shape Memory NiTi Nicholas Hatcher, Oleg Kontsevoi, Arthur Freeman The unique property of displacive phase transformations has created much interest in the study of martensitic materials. Among them, NiTi finds a wide range of applications due to its shape memory behavior; however, the detailed mechanism of its structural evolution during the martensitic transformation is not fully understood. We have investigated the transition paths between the B2, R, B19, and B19' phases using the highly-precise FLAPW method\footnote{Wimmer,\,Krakauer,\,Weinert, \,and\,Freeman,\,Phys.\,Rev.\,B,\,{\bf 24},\,864\,(1981)} to identify the governing processes of the transformation through calculations of the total energy, electronic structure, elastic moduli, and shear energetics. For example, we show the role of topological shifts of the Fermi surface and band structure evolution with the changing monoclinic angle of the B19' and R phases. In addition, the magnetism and magnetic susceptibility of the phases are investigated using both fixed spin moment and Stoner enhancement calculations. To date, we find the B2, B19, and B19' phases to be Stoner enhanced paramagnets. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W23.00005: Possible metastable rhombohedral states of the bcc transition metals Michael Mehl, Daniel Finkenstadt The energy $E(c/a)$ for a bcc element stretched along its [001] axis (the Bain path) has a minimum at c/a = 1, a maximum at c/a = $\sqrt2$, and an elastically unstable\footnote{M. J. Mehl, A. Aguayo, L. L. Boyer, and R. De Coss, {\em Phys. Rev. B} {\bf 70}, 014105 (2004).} local minimum at c/a $>~\sqrt2$. A rhombohedral strain is an alternative method of connecting the bcc and fcc structures. The primitive lattice keeps $R\overline3m$ symmetry, with the angle $\alpha$ changing from 109.4$^\circ$ (bcc), to 90$^\circ$ (simple cubic), to 60$^\circ$ (fcc). We studied this path for the non-magnetic bcc transition metals (V, Nb, Mo, Ta, and W) using both a full-potential LAPW and PAW VASP. Except for Ta, the energy $E(\alpha)$ has a local {\em maximum} at $\alpha=60^\circ$, with local minima near 55$^\circ$ and 70$^\circ$, the later having lower energy. We studied the elastic stability of the 70$^\circ$ minimum structure. Only W is elastically stable in this structure, with the smallest eigenvalue of the elastic tensor at 4~GPa, while the other three elements are unstable. We discuss the possibility that Tungsten is actually metastable in this structure. We also consider the possible epitaxial growth of this structure. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W23.00006: The pressure-induced bcc-hcp structural transformation in iron Babak Sadigh, Micheal Surh We study within the framework of the spin density-functional theory the microscopic mechanism of the bcc-to-hcp martensitic transformation in iron from first principles. We investigate the correlation of the pressure-induced structural instability of the bcc phase with the disappearance of ferromagnetism in this system. We show that our calculations can shed new light on recent novel shock experiments in single-crystalline iron. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W23.00007: Prediction of a metastable cubic phase for the transition metals with hcp ground state. Romeo de Coss, Aaron Aguayo, Gabriel Murrieta The discovery of a metastable phase for a given material is interesting because corresponds to a new bonding and new properties are expected. The calculation of the total-energy along the Bain path is frequently used as a method to find tetragonal metastable states. However, a local minimum in the tetragonal distortion is not a definitive proof of a metastable state, and the elastic stability needs to be evaluated. In a previous work, using the elastic stability criteria for a cubic structure, we have shown that the transition metals with hcp ground state; Ti, Zr, and Hf have a fcc metastable phase [Aguayo, G. Murrieta, and R. de Coss, Phys. Rev. B \textbf{65}, 092106 (2002)]. That result is interesting since the fcc crystal structure does not appear in the current pressure-temperature phase diagram of these metals, and support the experimental observations of fcc Ti and Zr in thin films. In the present work, we extend the elastic stability study of the fcc structure to the non-magnetic transition metals with hcp ground state; Sc, Ti, Y, Zr, Tc, Ru, Hf, Re, and Os. We find that all the metals involved in this study have a metastable fcc structure. From these results, substrates on which the fcc structure of these metals could be growth epitaxially are predicted. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W23.00008: Anharmonic Phonons in Vanadium Alloys and Compounds. Olivier Delaire, Max Kresch, Matthew Lucas, Rebecca Stevens, Jorge Munoz, Brent Fultz Using inelastic neutron scattering, we investigated the temperature-dependence of the phonon density of states (DOS) of pure BCC vanadium and vanadium alloys V-6.25{\%}X, with X a transition metal solute, as well as the A15 compounds V3Si and V3Ge. Phonons in pure vanadium exhibit an anomalous stiffening with increasing temperature up to 1000C. The addition of 6.25{\%} impurities in solid solution changes this behavior. Solutes to the right of vanadium in the periodic table induce a reversal to the expected quasiharmonic softening with thermal expansion, solutes to the left increase the stiffening. V3Si and V3Ge also exhibit strong phonon stiffenings up to 500C. Anharmonic couplings arising from phonon-phonon or electron-phonon interactions are used to explain the departure from the quasiharmonic behavior. Using differential scanning calorimetry, we measured the heat capacity of vanadium and its alloys up to 1400C, and related it to the temperature-dependent phonon DOS. We compare our findings to theoretical studies of the effect of phonon-phonon and electron-phonon couplings on the heat capacity. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W23.00009: Ab-initio calculations of phonon properties for the Nb-Mo alloy using the virtual crystal approximation O. de la Pe\~{n}a-Seaman, R. de Coss, R. Heid, K.-P. Bohnen We have studied the structural, electronic, and lattice dynamic properties of the Nb$_{1-x}$Mo$_x$ alloy within the framework of the density functional perturbation theory, using the mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. The calculations were performed for both LDA and GGA $xc$-functional approximations. The structural parameters were optimized via the total-energy method for the whole range of Mo-concentration (0$\leq$x$\leq$1). The electronic properties were analyzed in terms of the electronic topological transitions (ETT) in the Fermi surface. The calculated phonon dispersion curves for the different Mo-concentrations are in very good agreement with experimental data reported in the literature. We find that LDA results are in general higher in frequency than GGA. The evolution of the Kohn anomaly in the $\Gamma$–H direction observed experimentally in the Nb-Mo alloy is well reproduced by the VCA calculations. Thus, we have shown that the virtual crystal approximation as implemented in the present work is useful for the study of random intermetallic alloys where the alloying elements are adjacent in the periodic table. This research was supported by CONACYT, Mexico under Grant No. 43830-F. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W23.00010: Electronic Gr\"{u}neisen Parameter in the Non-equilibrium Regime Jincheng Wang, Chunlei Guo The Gr\"{u}neisen parameter is a fundamental parameter characterizing the relationship between thermal expansion and specific heat of a solid. Conventionally, electronic Gr\"{u}neisen parameter\textit{ $\gamma $}$_{e}$ is measured in a solid by minimizing the lattice contribution to thermal expansion at low temperatures, and the value of \textit{$\gamma $}$_{e}$ is believed to be a constant. In this report, we perform pump-probe experiments using surface plasmon as the probe technique to resolve the dynamics of acoustic phonons that are impulsively excited in Ag film using fs laser pulses. Our study shows that the conventional value of electronic Gruneisen parameter is not necessarily valid in thermal non-equilibrium distribution immediately following ultrafast pulse excitation. A revised \textit{$\gamma $}$_{e}$ is proposed here to precisely take into account the role of electron pressure in driving acoustic phonons in solids. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W23.00011: A new intermetallic prototype? Verifying new structure predictions in CdPt and PtPd Gus Hart A recent data-mining approach to predicting the structure of intermetallic compounds has suggested that the CdPt and PtPd systems harbor a new crystal structure. Unlike other such predictions, this one is unique. The structure, never seen in any other fcc- based intermetallic, contains only four atoms per unit cell. Furthermore, this structure is the only one of this small size, except L1$_2$, that cannot be characterized as a sequential stacking of planes containing only A or B atoms. The stability of such a structure is tested using an exhaustive search of a cluster-expansion Hamiltonian. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W23.00012: Novel ground states in mixed bcc/fcc, high-/low-spin Fe-(Ni,Pd,Pt) from first-principles Sergey V. Barabash, Roman V. Chepulskii$^3$, Volker Blum$^4$, Alex Zunger Among the observed ordered phases of Fe-X (X=Ni,Pd,Pt), some structures are curiously absent: L1$_2$ Fe$_3$Pt exists, but Fe$_3$Pd and Fe$_3$Ni do not; L1$_0$ FePt and FePd exist, but the stability of L1$_0$ FeNi is debated. Furthermore, the recently measured short range order (SRO) in Fe-rich Fe-Ni is at odds with L1$_2$-type SRO. Theory has been hindered by the appearance of geometric (bcc/fcc) and magnetic (high-spin [HS]/low-spin [LS]) bi-stabilities. We address such bi-stabilities by performing first-principles mixed-basis cluster expansion with added geometric and magnetic ``filters,'' separating structures according to HS/LS and ``degree'' of fcc-ness and bcc-ness. We performed separate fcc HS and (for Fe-Pd) bcc HS cluster expansions. We predict that {\em at low temperatures}: (i) New ordered structures exist, including fcc Pt$_8$Ti-type FeX$_8$ and the (100) superlattices Fe$_2$Pd and Fe$_2$Pt; (ii) {\em All } fcc Fe$_3$X compounds are unstable w.r.t. bcc Fe + L1$_0$ FeX (iii) Fcc FePd is unstable w.r.t. bcc Fe + (100) Fe$_2$Pd superlattice, while L1$_0$ FeNi is, in fact, stable w.r.t. bcc Fe + L1$_2$ FeNi$_3$. At high T, (iv) The SRO in Fe-rich Fe-Ni is governed by (100) superlattice ordering. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W23.00013: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W23.00014: The Role of Displacement Short Range Order in the Determination of Higher Order Correlation Yevgeniy Puzyrev, Don Nicholson, Malcolm Stocks, Gene Ice Scattering experiments are routinely used to measure pair correlation in solid solutions. Only in special cases can model structures that reproduce the pair correlation be used to determine higher order correlations. However, the requirement that measured displacement short range order also be reproduced by the model further constrains the possible higher order correlation. The determination of further restrictions on higher order correlation relies on the use of a model for atomic displacement in terms of local environment. We illustrate this procedure for a 1-d lattice-liquid1using a simple displacement model and for NiFe using ab initio calculations to relate atomic displacements to local environment. \newline (1) Frank H. Stillenger and Salvatore Torquanto, J. Phys. Chem. B 108 19589 (2004). [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W23.00015: Short-range atomic ordering in magnetostrictive Fe-Ga alloys Yingzhou Du, Robert McQueeney, Mianliang Huang, Thomas Lograsso, Deborah Schlagel, Sung Chang, Douglas Robinson Fe$_{1-x}$Ga$_{x}$ alloys are known for having large magnetostriction (MS) while maintaining good ductility. Further improvement of the MS properties appears to be limited by Ga ordering. As Ga composition is increased, the MS coefficient is observed to peak, and then drop rapidly, when the system crosses over from a solid solution to an ordered alloy. X-ray diffuse scattering was used to study the Ga short-range order (SRO) as a function of composition and heat treatment. The data indicate the development of DO$_{3}$-type SRO with increasing Ga composition in the BCC solid solution. For the slow-cooled samples, the correlation length and area of the SRO scattering both increase dramatically beyond the peak in the MS at x = 0.18, indicating that the development of Ga clusters beyond a certain size limits the MS . The trends for the quenched samples are similar, but not as clear as the slow-cooled ones. The results indicate that nanoscale sized Ga clusters act to enhance the magnetoelastic coupling. [Preview Abstract] |
Session W24: Microphysical Properties of Block Copolymer Aggregates, Going Beyond Structure
Sponsoring Units: DPOLYChair: Steve Hudson, National Institute of Standards and Technology
Room: Colorado Convention Center 201
Thursday, March 8, 2007 2:30PM - 3:06PM |
W24.00001: Solvated Block Copolymers as a Novel Class of Electroactive Nanostructured Polymers Invited Speaker: Microphase-ordered block copolymers serve as model systems to elucidate the potential of molecular self-assembly and organic templates to fabricate functionalized polymer materials. Both aspects are related to the incorporation of secondary species such as low-molar-mass compounds or nanoparticles within copolymer matrices. Since the resulting properties of such functionalized copolymers depend on the morphology of the blend or composite, the nonrandom distribution of such inclusions within the copolymer matrix must be understood. Using a self-consistent field theoretical approach, we first evaluate the segregation and interfacial excess of low-molar-mass and nanoscale species in ordered triblock copolymers as functions of block selectivity and inclusion size. The predictions are found to agree with the morphology observed in a model triblock copolymer/nanoparticle composite, suggesting a wide correspondence in the structure-forming effect of molecular and nanoscale inclusions that will have implications in the design of functional nanostructured polymers such as conformable electroactive actuators. Such responsive materials, stimulated by electric fields, are required for emergent technologies such as microrobotics, micro air vehicles and responsive prosthetics. High actuation strains ($>$50{\%}) are currently afforded by dielectric elastomers at relatively high electric fields ($>$50 V/$\mu $m). In this work, we demonstrate that incorporation of a low-volatility solvent into a triblock copolymer yields physical networks that exhibit excellent displacement under an external field. Ultrahigh actuation strains ($>$200{\%}) accompanied by low cyclic hysteresis are realized at significantly reduced electric fields ($<$40 V/$\mu $m). Use of nanostructured polymers whose properties can be tailored by varying copolymer characteristics or blend composition represents an innovative and tunable avenue to reduced-field actuation for advanced engineering, biomimetic and biomedical applications. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W24.00002: Self-Assembled Micro-Phase Separated Semi-Permeable Membranes. Dale Handlin, Scott Trenor, Carl Willis Anionic polymerization of block copolymers affords superior microstructure control and thus morphology management through precise control of block structures. Numerous researchers have studied sulfonated styrenic block copolymers as semi-permeable membranes for various applications. Traditionally, the styrenic endblocks of triblock copolymers are sulfonated, as was first practiced by Kraton polymers in the late 1960's. Once endblock sulfonated polymers are hydrated, water plasticizes the endblocks which results in a significant degradation in the mechanical properties. We have recently designed new sulfonated block copolymer structures that have continuous ionic phases and retain their strength when fully hydrated. We have used known structure-property relationships to tailor the properties of the sulfonated polymer membranes via polymer structure, placement of sulfonic acid functionality, and membrane formation techniques. This ability to control membrane morphology allows us to decouple the degree of sulfonation from the amount of water swelling as well as the water swelling from the transport properties. The initial characterization and the ability to manipulate the mechanical and transport properties of these polymers will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W24.00003: Toward structure-property relationships in block copolymer electrolytes Enrique Gomez, Mohit Singh, Vincent Chen, Nitash Balsara Polymer membranes with high ionic conductivity are important for applications such as solid-state batteries and fuel cells. These polymer electrolytes must have a high modulus to prevent the catastrophic formation of dendrites. However, current approaches rely on poly(ethylene oxide) (PEO)/lithium-salt mixtures whose conductivity is inversely proportional to their modulus. Our strategy is to decouple the mechanical and ionic transport properties by utilizing PEO-based block copolymers comprising of soft, nanoscale conducting channels in a hard, non-conducting glassy matrix. In order to determine the role of structure on the ionic conductivity of these materials, we perform various transmission electron microscopy (TEM) experiments. Three-dimensional reconstructions provide important structural information regarding the manner in which the conductive phase percolates through the copolymer electrolyte. Energy-filtered electron microscopy allows for the direct imaging of lithium. Current efforts are focused on using these TEM experiments to determine the structure-property relationships of block copolymer battery electrolytes. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W24.00004: Phase Behavior of Block Copolymers containing Poly(vinyl pyridine) by Coordination with Metal Chloride Dong Hyun Lee, Hwang Yong Kim, Jin Kon Kim, Du Yeol Ryu, June Huh We studied, via small angle X-ray scattering, rheology, and transmission electron microscopy, the change of the domain spacing (D) and the order-to-disorder transition temperature (T$_{ODT})$ with the amount of cadmium chloride (CdCl$_{2})$ for polystyrene-\textit{block}-poly(2-vinyl pyridine) copolymers (PS-P2VP) and polystyrene-\textit{block}-poly(4-vinyl pyridine) copolymers (PS-P4VP). With increasing amount of CdCl$_{2}$, both D and T$_{ODT}$ of PS-P2VP increased greatly. On the other hand, with increasing amount of CdCl$_{2}$, D of PS-P4VP decreased, whereas T$_{ODT}$ of PS-P4VP increased dramatically. These results are due to different types of the coordination between CdCl$_{2}$ and nitrogen atoms in the 2-position of pyridine ring (intra-chain coordination) in PS-P2VP, compared with nitrogen atoms in the 4-position (inter-chain coordination) in PS-P4VP. This work was supported by Creative Research Initiative Program supported by KOSEF [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W24.00005: Brownian Dynamics Simulation of ABA Block Copolymer in Selective Solvent: Kinetics of HEX Cylinders to BCC Spheres Transition Minghai Li, Yongsheng Liu, Rama Bansil A Brownian Dynamics simulation was performed on 200 bead spring chains of triblock copolymer, A$_{10}$B$_{10}$A$_{10}$. The repulsive interactions of A monomers (in good solvent) are modeled by the Weeks-Chandler-Anderson potential. The poor solvent attraction of the B monomers is described by a Lennard-Jones (LJ) potential. We have determined the phase diagram of 30{\%} ABA block copolymer in a selective solvent for the A block. At temperature T=1 (in units of $\varepsilon $/k$_{B}$, where $\varepsilon $ is the well depth of the LJ interaction potential and k$_{B}$ the Boltzmann constant) the equilibrium state is HEX cylinder; at T=1.5 the system is in BCC spheres. We follow the time evolution of the HEX to BCC transition by jumping from T=1 to 1.5. The Fourier transform is calculated at each time-step in the simulation and compared to time-resolved small angle x-ray scattering data from triblock copolymer solution (Kraton G1650 in mineral oil). The simulation is also compared with a calculation based on a geometric model of coupled anisotropic fluctuations to describe the transition from HEX cylinders to BCC spheres. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W24.00006: Symmetric Diblock Copolymers in Nanopores: Monte Carlo Simulations and Strong-Stretching Theory Qiang Wang We have performed lattice Monte Carlo simulations to study the self-assembled morphology of symmetric diblock copolymers in nanopores. The pore diameter and surface preference are systematically varied to examine their effects on the chain conformations, structures of various morphologies and their phase transition. Various ensemble-averaged profiles and quantities are used to provide detailed information about the system. The simulation results are also compared with the predictions of a strong-stretching theory commonly used in the literature. Such comparisons reveal the deficiencies of this theory in describing the morphologies under cylindrical confinement, and call for further theoretical studies using more accurate formalisms. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W24.00007: Self-assembly of a diblock a copolymer melt absorbed in porous materials. Panagiotis Maniadis, Ioannis Tsimpanogiannis, Edward Kober Self-consistent field theory is used to study the self-assembly of a diblock copolymer melt absorbed in a porous material. We find that self-assembly is affected when the characteristic length scales of the porous material are of the same order as the polymer ratio of gyration (Rg). When the porous size is much larger than Rg, then the polymer self-assembly is affected only locally close to the contact with the pore surface. Interesting new morphologies appear when the size of the pores and the distance between them is comparable to the diblock characteristic lengths. In this case the polymer structure changes according to the constrains from the porous. We will present results for two types of regular pores arrangement a) checkers board and b) staggered lattice. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W24.00008: Effective Control of Pore Size in the Block Copolymer by Matrix Crosslinking E. Kim, C. Shin, D.Y. Ryu, J. Bang, C. Hawker, T. Russell Thin films of block copolymer with nano-sized morphology have great attention for their potential use. Especially, the control of block copolymer pore size in the cylindrical microdomain has been studied by many research groups. Previously, pores of PS-b-PMMA thin films with diameters as low as 3nm only via crosslinking by ozone the matrix surrounding the cylindrical microdomains are reported, in which the diameters of the pores are found to increase by increasing ozone exposure with the pore size limitation, only to 8 nm. The main objective of our study is to find out the condition of controlling the pore size widely as well as tunability by using BCB crosslinking unit. PS-b-PMMA copolymers were synthesized with reactive benzocyclobutene (BCB) functionality which is randomly incorporated into the PS backbone, having PMMA volume fraction of $\sim$0.3 with various BCB amount from 3\% to 16\%. We investigated the optimal thermal annealing condition, time dependence to get the ordered cylindrical nanostructure with controlled size, where nanostructures of block copolymer are oriented normal to the substrate due to balanced interfacial interaction on the surface. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W24.00009: Superelastic materials based on multigraft copolymers U. Staudinger, R. Weidisch, Y. Zhu, S. P. Gido, D. Uhrig, J. W. Mays, M. Klueppel, G. Heinrich PI-PS-multigraft copolymers with tri- tetra- and hexafunctional PS branches have been studied to investigate the influence of molecular architecture on morphological and tensile properties and to find novel material concepts. The materials form nanostructures ranging from spheres to cylinders to lamellae. Thus these materials show excellent transparency. Mechanical properties are strongly depending on functionality of the graft copolymer and on the number of branch points per molecule. Increasing functionality and a larger number of branch points cause a distinct increase in tensile strength due to enhanced physical crosslinking in such multigrafts. Tetra- and hexafunctional multigrafts show surprising high strain at break values up to 1550 {\%} and excellent elasticity far exceeding that of commercial elastomers. Simulation of hysteresis behavior revealed that the deformation mechanism can be explained by the flocculation model based on filled elastomers. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W24.00010: Mesoscopic Archimedian Tiling Patterns in ABC Star-Shaped Terpolymers Atsushi Takano, Kenichi Hayashida, Tomonari Dotera, Yushu Matsushita Microphase-separated structures formed by ABC star-shaped terpolymers were investigated by transmission electron microscopy (TEM), electron tomography (3D-TEM), and small-angle X-ray scattering (SAXS). The samples are composed of polystyrene (S), polyisoprene (I) and poly(2-vinylpyridine) (P), their volume ratios of I:S:P are 1:1:X, where 0.2$<$X$<$4.9. From morphological observations by TEM, it was found that ISP stars show characteristic cylindrical structures when X is within the range 0.7$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $X$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $1.9. By the careful investigation of the ISP stars by TEM and 3D-TEM, it was confirmed that the cross-sections of cylindrical structures of four samples show two-dimensional tiling patterns consisting of regular polygons, that is, [6.6.6], [4.8.8], [3.3.4.3.4], and [4.6.12], which are families of the Archimedian tiling patterns. Furthermore the SAXS patterns of four samples are quite consistent with structural observation of TEM. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W24.00011: Hierarchical Structures of a Multiblock Copolymer Melt Weihua Li, An-Chang Shi Hierarchical structures of a multiblock copolymer melt are investigated using real-space self-consistent mean-field theory. The polymer, $A(BC)_nBA$, is composed of three species A, B, and C. The parameter n indicates the number of short BC blocks with equal lengths. Hierarchical lamellar structures with parallel double periodicity have been observed in very recent experiments done by Masuda, et al. in this type of multiblock copolymer melts. These heirachical structures are reproduced in our one-dimensional calculations. We locate the transitions between hierarchical lamellar phase and single lamellar phase as the composition $f_A$ is varied for two types of hierarchical lamellae with five and seven thin layers, respectively. In addition, we explore hierarchical cylindrical structures using two-dimensional calculations. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W24.00012: Whispering Gallery Modes in Highly Hexagonal Symmetric Structures of Three Dimensional SBA-1 Mesoporous Silica Chih-Wei Chen, Yang-Fang Chen An interesting optical resonant mode, called whispering gallery mode (WGM), has been discovered, inside the three dimensional highly hexagonal symmetry of SBA-1. The hexagonal structure provides a suitable environment for the light wave to circulate around due to multiple total internal reflection at the resonator's boundary and generates the resonant states. By means of the Fourier transform infrared transmittance, we observed the optical WGMs in mesoporous silica SBA-1 decaoctahedron for the first time. Based on the hexagonal total internal reflecting model, the observed eigenmodes can be explained quite well. We also discovered that under the condition of WGMs, the absorption of CO$_{2}$ and H$_{2}$O molecules can be greatly enhanced. [Preview Abstract] |
Session W25: Polymer Melts and Solutions
Sponsoring Units: DPOLYChair: Kyusoon Shin, Seoul National University
Room: Colorado Convention Center 203
Thursday, March 8, 2007 2:30PM - 2:42PM |
W25.00001: Dynamic Light Scattering Studies of Light Absorbing Solutions Thomas Seery, Maria DeMesa Dynamic light scattering (DLS) is a powerful tool for characterizing polymers in solution and is especially useful for observing polymer-polymer interactions. The ease of sample preparation and \textit{in situ}, non-perturbative nature are powerful advantages to a technique capable of probing 9 decades of dynamic behavior in a single measurement. However, application of scattering methods is greatly restricted when the sample absorbs light at the scattering wavelength. Absorption leads to sample heating, deflection of the probing laser beam and convection in otherwise quiescent solutions. Convective flow contributes an oscillating component to the smoothly decaying correlation functions that are normally obtained from DLS. These effects have limited the application of DLS in studies of conducting polymers, nanotubes, heme containing proteins and noble metal nanoparticles. We have a theoretical model that provides broad agreement with the behavior observed for light absorbing solutions of polyaniline, carbon nanotubes, cytochrome-C and silica coated gold nanoparticles and we have a physical model system that produces oscillating correlation functions using non-absorbing scatterers and absorbing dyes. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W25.00002: Small Angle Neutron Scattering Study of Oligo(ethylene glycol) Grafted Polystyrene in Aqueous Solutions G. Cheng, Y.B. Melnichenko, G.D. Wignall, F. Hua, K. Hong, P.F. Britt, J.W. Mays Considerable efforts have been made to understand the cooperative interactions of living organisms using synthetic polymers. A typical example is poly(ethylene glycol) (PEG) in water where two fundamental interactions (hydrophobic and hydrophilic) are present and control its conformation and phase behavior. By manipulation these interactions in a systematic way, one gains better understanding of many phenomena of water soluble macromolecules. To this end, polystyrene grafted with short EG side chains have been synthesized by nitroxide-mediated radical polymerization and temperature induced phase separation and conformation changes of (EG)$_{4}$ densely grafted polystyrene in aqueous solutions have been investigated by Small Angle Neutron Scattering (SANS) and Dynamic Light Scattering (DLS). While large clusters are detected by DLS in the solutions, SANS data indicate individual polymer assumes a prolate ellipsoid. With increasing temperature, the attractive interaction between ellipsoids increases while slight intra-chain contraction occurs. The particle however stays the same near the LCST and collapse into a sphere at temperatures far above the LCST. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W25.00003: Steady state structure factor and stress in sheared semi-dilute polymer solutions Prasanth Jose, Grzegorz Szamel We use Brownian dynamics simulations to elucidate the contributions of intra and inter chain correlations to anisotropic scattering patterns observed in light scattering experiments on sheared semi-dilute polymer solutions (Wu et al., Phys. Rev. Lett. 66, 2408 (1991)). We find that while in equilibrium solutions the contributions to structure factor from intra and inter chain correlations cancel each other almost completely, under shear these contributions are modified in different ways resulting in incomplete cancellation and the formation of anisotropic scattering patterns. We also investigate the contribution of intra and inter chain correlations to shear stress and viscosity, and to the shear stress relaxation. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W25.00004: Structure of polydisperse star branched polymers grown by diffusion Guillermo Ramirez-Santiago, Carlos I. Mendoza We present a numerical algorithm to construct polydisperse star branched polymers in two and three dimensions whose morphology is fully determined by diffusion. We analyze the monomer-monomer correlation function to calculate the fractal dimension of the structures. In addition, we carry out a finite-size analysis to determine the scaling properties of the radius of gyration. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W25.00005: Modeling the liquid-solid transition in saturated triglycerides C.B. Hanna, D.A. Pink, A.J. MacDonald, K. Thillainadarajah, R. Corkery, D. Rousseau Corkery \textit{et al.} have proposed that the high-temperature state of the triglyceride trilaurin (TL) is a Y-conformer, in which the three hydrocarbon chains are dynamically twisted with an average angle of $\sim $120\r{ } between them. Using computer simulations, we first show that the high-temperature state is indeed the Y conformation. We then develop a theory of the liquid-solid transition of this system, in which TL molecules are in a chair (h) conformation, with extended, possibly all-trans, chains at low-temperatures, and are in a Y conformation in the liquid phase at temperatures higher than the transition temperature, T*$\approx $319K. We map this ``h-Y model'' onto an Ising model in a temperature-dependent field, perform a mean-field approximation, and calculate the transition enthalpy, which is in good agreement with experiment. We also predict the temperature-dependence of the 1132 cm$^{-1}$ Raman band. Our results support the proposal that the liquid state is made up of molecules in the Y conformation. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W25.00006: Kinetics of Anionic Polymerization of Polybutadienyl Lithium in Benzene: An Osmotic Effect on Propagation Process. Hiroshi Watanabe The anionic polymerization (propagation) kinetics of polybutadienyllithium (PBLi) in benzene was examined with H-NMR. The PBLi chains formed aggregates with an average aggregation number f = 4 through their Li ends. The residual monomer fraction y(t) did not rigorously exhibit the single-exponential decay expected for the conventional propagation mechanism through the dissociated chains, suggesting a competing propagation mechanism through the transiently fused 2f-mer aggregates (detected with Li NMR): This fusion-aided propagation should have been osmotically suppressed on a decrease of y (increase of the PB concentration) to give the deviation from the conventional behavior. The propagation became slower in the presence of chemically inert, neutral PB chains that just tuned the osmotic environment for the aggregates, lending support to molecular picture. Furthermore, the y(t) data in the absence/ presence of the neutral PB chains were semi-quantitatively described by a simple model considering the competition of the propagation through the fused 2f-mer aggregates and dissociated chains. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W25.00007: Conformational studies of conjugated polymers substituted with different side chains Yunfei Jiang, Uwe H. F. Bunz, Dvora Perahia The conformation of conjugated poly(\textit{para}phenylene ethylene) (PPE), in different complex fluids states including molecular solutions, aggregates and gels, has been studied by Nuclear Magnetic Resonance and Molecular Mechanics simulations. PPEs are inherently semiconductors. Their electro-optical response depends on their degree of conjugation, determined by the degree of torque applied on the backbone by the substituents and the association mode of the polymer. Two types of substituents were compared: a nine carbons linear chain nonyl group and a space occupying tri-\textit{iso}propylsilyloxy (TIPS). In molecular solution the side chains are fully flexible assuming random conformations while in any aggregation mode a well-ordered stretched out conformation due to the dense packing between polymeric molecules, is detected. TIPS substituted PPEs results in a non correlated configuration in all of the association modes. These findings are consistent with our independent fluorescence studies of this system. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W25.00008: High-Pressure Vibrational Spectroscopy of Polymers E.D. Emmons, R.G. Kraus, J.S. Thompson, A.M. Covington Polymers are often subjected to extreme conditions of high pressure and temperature in shock compression experiments and in their use as binders in high explosives. We have begun a program to examine polymeric materials at high-pressures in a diamond anvil cell using infrared and Raman vibrational spectroscopies. There is a significant lack of measurements of basic spectroscopic data on scientifically and technologically interesting polymeric materials at high pressure. Data for different materials, including pressure-dependent FTIR absorption spectroscopy of poly (methyl methacrylate) (PMMA) will be presented. The data were analyzed to determine mode Gr\"uneisen parameters and vibrational anharmonicities. Such measurements are useful for interpreting experimental studies of shock compression of polymers as well as benchmarking theoretical models of the behavior of polymers under pressure. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W25.00009: Conformational Heat Capacity of Liquid Biodegradable Polymers in the Absence and Presence Water Marek Pyda, Elzbieta Nowak-Pyda The conformational heat capacity of biodegradable polymers such as amorphous poly(lactic acid) PLA and starch with and without water have been evaluated from a fit of experimental data to a one-dimensional Ising-like model for two discrete states, characterized by parameters linked to stiffness, cooperativity, and degeneracy. For the starch-water system the additional changes in the conformational heat capacity arise from the interaction of the carbohydrate chains with water. The liquid heat capacities at constant pressure$ C_{p}$, of amorphous PLA and partially liquid state of starch, starch-water have been computed as the sum of vibrational, external, and conformational contributions. The vibrational contribution was calculated as the heat capacity arising from group and skeletal vibrations. The external contribution was estimated from experimental data of the thermal expansivity and compressibility in the liquid state. The experimental liquid $C_{p}$ agrees with these calculations to better than $\pm $3{\%}. The calculated liquid $C_{p}$ with the solid $C_{p}$ was employed in the quantitative thermal analysis of the experimental $C_{p }$of biodegradable polymer PLA, starch, and starch-water. Supported by European Union, grant (MIRG-CT-2006-036558), Cargill Dow LLC [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W25.00010: Diffusion in Polypropylene Melts: Role of Stereochemistry. Ernst von Meerwall, Numan Waheed, Wayne Mattice We have performed numerical simulations and experiments at 180$^{o}$C to study the effect of stereochemical composition on the diffusion (D) of linear polypropylene melts of moderate polydispersity. The coarse-grained Monte-Carlo (MC) simulations were based on the rotational isomeric state model and repulsive Lennard-Jones potentials. For the pulsed-gradient $^{1}$H NMR diffusion measurements the three specimens used had probabilities of meso diad P$_{m}$ = 0.02, 0.23, and 0.89. The conversion factor between MC steps and real time was obtained by comparison with the measured D; no dependence on stereochemistry was evident. Using a molecular-weight (M)-scaling known from earlier work on n-alkanes, results were normalized to a common M after accounting for differences in experimental polydispersity. Results agreed closely with the monodisperse simulations. D at high P$_{m}$ was found to be several times faster than at low P$_{m}$, but the simulation also showed a maximum in D at P$_{m}$ near 0.75, an effect attributed to quenched randomness. Likely for similar reasons the experimental D-distribution for the P$_{m}$ = 0.89 sample greatly exceeded that expected from the known polydispersity. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W25.00011: An examination of the whipping instability of viscoelastic jets in electrospinning. Pradipto Bhattacharyya, Jian Yu, Gregory Rutledge, Gareth McKinley The whipping instability of a viscoelastic jet is examined for a number of different test fluids. The whipping motion of the jet during electrospinning is generally conjectured to initiate the large drawdown required to generate polymeric nanofibers with diameters less than 100 nanometers. The phenomenon plays a dominant role in determining the characteristic strain rate at which a fluid filament is deformed during the drawdown. In order to study the whipping motion, a laser beam is passed through the whipping region such that the beam is intercepted by the jet at intervals of time. We analyze and compare both mechanically-forced and natural (unforced) jets. The beam intersection events are manifested as regular drops in the detector voltage and subsequent analysis of the detector signals combined with high-speed digital videomicroscopy provides information on the periodicity of the jet's motion. It is demonstrated that the dynamics transition from a periodic to more complex behavior depending on the viscoelastic nature of the fluid used. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W25.00012: The role of extensional stress in the formation of electrospun fibers Jian Yu, Sergey Fridrikh, Gregory Rutledge Electrospinning is a process that employs electrostatic forces to form polymer continuous fibers. Although the process of making electrospun fibers is easy to implement, many polymer solutions are not readily electrospun into uniform fibers. Here we present a study on the role of fluid elasticity in the formation of fibers from polymer solution by electrospinning. For some dilute polymer solutions without elasticity, the electrospinning jet breaks up into droplets due to the Rayleigh instability driven by surface tension. On the other hand, solutions with some degree of elasticity can generate a stress that retards the growth of Rayleigh instability, forming a ``beads-on-string'' structure. In the extreme case, where a large stress on the jet can suppress the Rayleigh instability completely, uniform fibers are obtained. We calculate a theoretical critical stress on the jet required for complete suppression of the instability. This calculated value is in agreement to experimental observation. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W25.00013: First observation of the first-order transition in ultra-filtration of flexible linear polymer chains Chi Wu, Fan Jin Using a special double-layer membrane to avoid interaction among flow fields generated by different pores, we have, \textit{for the first time}, observed the predicted discontinuous first-order transition in ultra-filtration of flexible linear polymer chains. Namely, the chain could pass through a pore much smaller than its unperturbed radius only when the flow rate is higher than a certain value. When only one chain and one pore considered in theory, such a threshold is surprisingly independent of both the chain length and the pore size. Our results reveals that for a membrane with many pores and at a microscopic flow rate (q) lower than the threshold, the inevitable blocking of some pores by longer non-stretched coiled chains increases q in those non-blocked pores because the macroscopic flow rate (Q) is a constant. Long chains have two populations, coiled and stretched, in a real ultra-filtration experiment when q is lower than the threshold. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W25.00014: First-order Conformation Transition of Single Polyelectrolyte Molecules in Aqueous Solutions Shengqin Wang, Jiang Zhao The molecular conformation of a weak polyelectrolyte, poly 2- vinylpyrindine (P2VP), as a function of charge density and electrostatic screening was studied by single molecule fluorescence techniques. By fluorescence correlation spectroscopy (FCS), the diffusivity of the P2VP in solution was studied at single molecule level. It was found that the diffusion coefficient (therefore the hydrodynamic radius) of P2VP experienced a first-order transition at varying pH values (charge density), while it underwent a continuous transition at different electrostatic screening. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W25.00015: Influence of the solvent size on the behavior in polymer solution Lijia An, Yunqi Li, Tongfei Shi The effects of solvent size on properties of homopolymer solution have been investigated by Monte Carlo Simulation. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains ($R_{g})$ is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of $R_{g}$ on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules. [Preview Abstract] |
Session W26: Focus Session: Charge Transport in Nanostructures III
Sponsoring Units: DCPChair: Mark Ratner, Northwestern University
Room: Colorado Convention Center 205
Thursday, March 8, 2007 2:30PM - 3:06PM |
W26.00001: Magneto-Resistance of Nanoscale Molecular Devices Invited Speaker: Affecting the current through a molecular or a nano-scale junction is usually done by a combination of bias and gate voltages. Magnetic fields are less studied because nano-devices can capture only low values of the magnetic flux. Here, I review our recent theoretical work on the use of magnetic fields as gates for such junctions. Several plausible model systems of such devices will be presented, such as the quantum corral, carbon nanotubes and polycyclic aromatic hydrocarbon molecules. Despite the similarly between gating properties of the magnetic and electric fields, we find that there are also striking differences. This will be illustrated for a multi-terminal device, where the polarity of the magnetic field plays a key role, and with respect to inelastic effects, where the conductance as a function of the gate voltage broadens upon coupling to phonons while it actually narrows considerably in response to a magnetic field. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W26.00002: Shot Noise in Single-Molecule Transistors Zachary Keane, Douglas Natelson While single-molecule electronic devices have been studied extensively, both experimentally and theoretically, a detailed understanding of the physics of charge transport through molecules is still lacking. Recent experiments have shown that it is feasible to measure shot noise in mechanically fabricated single-molecule transistors. Shot noise is a particularly interesting measurement in that it has the potential to reveal details about the correlations between electrons as they cross a molecule. In devices known to exhibit strong correlated-electron effects (e.g. in the Kondo regime), shot noise measurements could provide useful guidance to theorists as they attempt to develop working models for electron transport. We present preliminary results of noise measurements in three-terminal single-molecule devices fabricated by electromigration. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W26.00003: Magnetoconductance of molecularly linked Au nanoparticle arrays near the metal-insulator transition Al-Amin Dhirani, Jeff Dunford, Brian Statt Magnetoconductance of 1,4-butanedithiol-linked Au nanoparticle films reveal features consistent with ``weak localization'' (coherent backscattering). Elastic, inelastic, and spin-orbit-scattering time scales extracted using a theoretical model are consistent with those found in other studies on granular Au films, and in particular, reveal that elastic-scattering time scales are comparable to those required for an electron to traverse a nanoparticle. The latter result is consistent with non-Arrhenius conductance vs temperature data. Together, the data suggest that scattering within clusters of molecularly linked nanoparticles plays a critical role in hopping-electron transport in films near a percolationdriven metal-insulator transition. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W26.00004: Metal to Insulator Transition in Films of Molecularly Linked Gold Nanoparticles Amir Zabet-Khosousi, Al-Amin Dhirani Self-assembled structures comprising nanoparticles (NPs) and molecular linkers exhibit remarkable electronic behaviours ranging from insulating to metallic. These behaviours can be controlled via chemical synthesis and choice of linker molecules. However, charge transport through these structures is not well understood. Here, we report a metal-insulator transition (MIT) in films of alkanedithiol (C$_{n}$S$_{2})$-linked gold NPs, as the length of linkers ($n)$ is systematically varied. Our results provide strong evidence for a MIT occurring at $n$ = 5. We describe these results in a context of a Mott-Hubbard model. We find that all insulating samples ($n \quad \ge $ 5) exhibit a universal scaling behaviour $R \sim $ exp[($T_{0}$/$T)^{p}$] (where $R$ is resistance, $T$ is temperature, $T_{0}$ is a fitting parameter and $p$ = 0.65), and all metallic samples ($n \le $ 5) exhibit weaker $R$--$T$ dependencies than bulk gold. We discuss these observations in terms of competitive thermally-activated processes and strong $T$-independent elastic scattering, respectively. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W26.00005: Theoretical study of molecule mediated spin-polarized electron tunneling between magnetic materials Haiying He, Ravindra Pandey, Shashi Karna There has been a recent interest in organic molecule-mediated spin-polarized electron transport with a potential application in molecular-scale spintronics. In this presentation, we present the results of a theoretical study on the spin-dependent electron tunneling via a self-assembled monolayer of $\sigma $-bonded bicyclo[2.2.2]octane-1,4-dithiol on Ni(111). Comparison with a similar study involving $\pi $-conjugated molecules, suggests that the magnitude of the tunnel current and the spin-dependent current are strongly influenced by the nature of chemical bonds in the molecular structure. It gives further understanding of the role of the organic molecules on the spin-polarization of electron transport and provides a basic guideline in choice of molecules in this respect. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W26.00006: Influence of Correlated Hybridization on the Conductance of Molecular Transistors Jong-Chin Lin, Frithjof Anders, Daniel Cox We study the spin-1/2 single-channel Anderson impurity model with correlated (occupancy dependent) hybridization for molecular transistors using the numerical renormalization-group method. Correlated hybridization can induce nonuniversal deviations in the normalized zero-bias conductance and, for some parameters, modestly enhance the spin polarization of currents in applied magnetic field. Correlated hybridization can also explain a gate-voltage dependence to the Kondo scale similar to what has been observed in recent experiments. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W26.00007: Path integral simulations of quantized conductance in nanowires John Shumway, Matthew Gilbert Theoretical studies of spin and charge transport in nanostructure often include interactions perturbatively or at a mean-field level. In some cases it is desirable to have a fully quantum many-body method to describe the interacting system: such is the case when investigating spin ordering near the ``0.7-structure'' in quantum point contacts or for simulating systems with strong polaronic effects. We have developed a new path-integral quantum Monte Carlo (QMC) approach to transport. Previous QMC simulations have been valued for accurately treating electronic correlation in quantum dot spectroscopy---this work now opens up many new opportunities for simulating quantum transport. We show simulation data demonstrating how current-current correlation functions in the Kubo formalism lead to quantization of conductance in GaAs nanowires. This new, finite-temperature, many-body computation technique should have many uses in the study of quantum wires and molecular electronics. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W26.00008: Ab-initio study of transport in the Coulomb-blockade regime Haitao Wang, Osamu Hino, Garnet Chan Here we report a new ab-initio model for molecular conductance in the Coulomb blockade regime using unrestricted Hartree-Fock theory within the non-equilibrium Greens function (NEGF) formalism. We demonstrate calculations on recent experimentally studied transition metal complexes, studying the effect of gating on current and the corresponding Coulomb blockade effects. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W26.00009: Non-Markovian Transport of Charges in Solid-State Quantum Dots e. Ying-Tsan Tang, Yueh-Nan Chen, Brandes Tobias, Der-San Chuu The population dynamics of an electron in a double-dot system coupled to reservoirs is theoretically investigated. Basically, as we put an extra single dot that is strongly coupled to the extended reservoir, it would be possible for experimentalists to realize their modification of coupling strength; therefore we could properly control the memorial effect between system and reservoir by extra coupling. Throughout this study, we effectively change the decay of the entire system. Moreover, the exact results for non-Markovian couplings to both phonon and electron reservoirs with structured tunneling density of states are obtained, which contains the coherent states created by the distance of double dot embedding in the same system as well as the energy shift caused by purely electron-phonon coupling. Eventually, the relaxation dynamics of the Zeno or Anti-Zeno effect reveals insight into the defined decay rate. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W26.00010: Controlled production and electronic characterization of defects in carbon nanotubes Brett Goldsmith, Vaikunth Khalap, Alexander Kane, Philip Collins Electrochemical functionalization of nanotubes allows fine control of the number of functionalized sites on a nanotube down to the limit of single, point functionalizations in otherwise pristine devices. This presentation will describe the local and 2-terminal electronic properties of the resulting devices. Point-functionalized devices exhibit spatially-localized resistance as mapped by scanning Kelvin probe microscopy and local gate sensitivity associated with the chemical disorder. The findings are reinforced by further attachment of specific chemical markers visible to electron microscopy. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W26.00011: Electrons, holes, and electron-hole junctions in carbon nanotubes Invited Speaker: Carbon nanotubes possess an unusual band structure consisting of symmetric electron and hole subbands separated by a gap determined by the nanotube's chirality, diameter and any external perturbations. Here, we study the properties of both electrons and holes in these one-dimensional subbands. Capacitance measurements are used to directly probe the van Have singularities in the density of states and the energies of the electron and hole subbbands[1]. Electrical[2] and photocurrent measurements are employed to investigate the properties of nanotube p-n junctions. These measurements directly yield the nanotube bandgap and show fascinating step-like behavior in the reverse-bias region. Finally, measurements of p-n-p nanotube quantum dots are presented where the bandgap is tuned to zero by an external magnetic field. These experiments illustrate just a few of the exciting opportunities available in electron-hole nanotube devices. [1] S. Ilani, L.A. Donev, M. Kinderman, and P.L. McEuen, Nature Physics 2, 687 (2006). [2] K. Bosnick, N. Gabor, and P. L. McEuen; Appl. Phys. Lett. 89, 163121 (2006) [Preview Abstract] |
Session W27: Focus Session: Computational Nanoscience IX - Nanowire, Rods & SAMs
Sponsoring Units: DMP DCOMPChair: Tunna Baruah, University of Texas at El Paso
Room: Colorado Convention Center 301
Thursday, March 8, 2007 2:30PM - 2:42PM |
W27.00001: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W27.00002: Conductance of telescoped double wall nanotubes calculated with ADF program package Ryo Tamura In double wall nanotubes (DWNTs), the interlayer current is negligible compared to the intra-layer current. When the inner tube is partially extracted (telescoped) from the outer tube, however, the total current must flow between the layers so that the interlayer interaction drastically influences the conductance. Here the interlayer bonds can be considered as weak covalent bonds rather than van der Waals bonds since they are anisotropic and their number per atom is limited. In this presentation, the transfer integrals between the layers are calculated by ADF program package and their effects on the conductance in the telescoped DWNTs are investigated. They are compared with our previous results. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W27.00003: Origin of Giant Piezoresistance in Pristine $<$111$>$-Si nanowires Juexian Cao, Ruqian Wu It was found recently that silicon nanowires possess an unusually large piezoresistive coefficient, 350 times higher compared with Si bulk. Using first principles density functional calculations, we demonstrated that this stems from the strain-induced change in band ordering of surface states. The pristine $<$111$>$-Si nanowire is metallic under ambient condition but the mobility of the carrier is extremely small due to the strong localization. The compression shrinks the surface shell and hence shifts the itinerant state across the Femi level, which consequently leads a surge in conductance. The effective masses of those two bands differ by a factor of 100, a number that can roughly account the experimental data. Since the key bands for transport are surface states, the surface modification plays a vital role on the piezoresistance effects, as observed experimentally. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W27.00004: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W27.00005: The structure and stability of thin H-passivated $<$112$>$ silicon nanowires Ning Lu, Cristian Ciobanu, Tzu-liang Chan, Cai-zhuang Wang, Kai-ming Ho, Feng-Chuan Chuang Recent experiments on the synthesis on monocrystalline nanowires reveal that their axis can only have a limited number of crystalline orientations. Among these orientations, $<$112$>$ is the highest Miller-index wire axis and generates a rectangular cross-sectional shape. Using a combination between genetic algorithm search and density functional theory calculations, we determine the precise shape of the wire cross-section that corresponds to the lowest formation energy per silicon atom. We analyze the deviations of the cross-sectional shape from the Wulff shape, and show how the shape of the nanowires evolves as a function of cross-sectional area and the chemical potential of hydrogen. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W27.00006: Small World Carbon Nanomaterials: Density Functional Theory Simulations Jeremy Yancey, Mark Novotny, Steven Gwaltney The possible existence of small, pure carbon molecules based on physical small-world networks is addressed using density functional theory calculations. A ring of atoms with one or more small-world connections between pairs of non-nearest-neighbor sites was chosen for the network topology. The small-world connections are made with and without additional carbon atoms placed along the link. The energy per atom of these small-world carbon systems is compared with benchmark carbon clusters such as the C$_{20}$ ring, bowl, and cage isomers, the C$_{60}$ Buckyball, monocyclic pure carbon rings ranging from C$_{4}$ to C$_{60}$, bare linear carbon chains ranging from C$_{2}$ to C$_{48}$, fullerenes ranging from C$_{20}$ to C$_{60}$, and various all-carbon graphitic fragments. The energy per atom results for these materials provides an indication that some of these pure-carbon small-world nanomaterials are reasonable for real world synthesis. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W27.00007: Electronic properties of 1D LaB$_{6}$ rods G. P. Li, W. N. Mei, Jing Lu, R. F. Sabirianov, C. L. Cheung, X. C. Zeng Metal hexa-borides have varieties of interesting properties and were utilized frequently in technological applications: e.g. LaB$_{6}$ is known to have extremely low work function, thus is used as one of the most popular electron emitter. Our project is initiated by the experimental findings that LaB$_{6}$ nano-rods generated stronger electric current than in the bulk case. Thus we focus on the band structure calculations of quasi-1D nano-rods with various widths and breadths for the purpose of studying the relationship between work function and rod shapes. Our samples consist of up to ten unit cells, i.e. n\textbf{\textit{a}} X m\textbf{\textit{b}} (\textbf{\textit{a}} and \textbf{\textit{b}} are lattice vectors and n X m $\le $ 10). To accomplish our calculations, we applied GGA density functional theory with all electron and relativistic effect included. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W27.00008: Measuring Order and the Debye-Waller Factor for Porous Arrays Forrest Kaatz, Adhemar Bultheel, Takeshi Egami We derive methods that explain how to quantify the amount of order in ``ordered'' and ``highly ordered'' porous arrays. Ordered arrays from bee honeycomb and several from the general field of nanoscience are compared. Accurate measures of the order in porous arrays are made using the discrete radial distribution function (RDF) and the Debye-Waller Factor (DWF) from 2-D discrete Fourier transforms calculated from the real-space data using MATLAB routines. Nanoporous anodized aluminum oxide, hexagonal arrays from functional materials, hexagonal arrays from nanosphere lithography, and arrays from block copolymer lithography (all taken from the literature) are compared to two-dimensional model systems. The DWF is normalized to the first harmonic and depends on N, the number of peaks in the fit for these finite arrays. We optimize N to the classical model for the DWF as a fit to reciprocal space \textbf{\textit{K}}$^{2}$. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W27.00009: Molecular Simulations of Liquid/Vapor Phase Equilibria for Single Component and Binary Mixtures of Nanoparticles Mark Horsch, Pieter In't Veld, Jermey Lechman, Gary Grest Self-assembly of nano and colloidal particles into ordered structures is an important technological challenge for the design of future materials and devices. One promising self-assembly technique is the evaporation of nanoparticles suspended in droplets. However, it is difficult to experimentally observe the self-assembly process in the evaporating droplet. Computer simulation provides an avenue with which to address and directly observe the self-assembly of model nano and colloidal particles within the droplet provided an efficient model can be developed. Here we present the liquid-vapor phase envelopes for model particles as a function of particle size. We compare the liquid/vapor phase envelopes and the computational efficiency for several different models including composite particles comprised of Lennard-Jones (LJ) atoms and particles interacting via integrated LJ potentials. Results for binary mixtures of nanoparticles in a solvent of LJ atoms will also be presented. These studies provide a framework for the size range of particles that can be addressed by each model. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W27.00010: Structural and Electronic Properties of Aromatic Isocyanide Self-Assembled Monolayers on Au(111) Surface Yan Li, Giulia Galli The search for molecular assemblies with interesting transport properties for molecular electronic devices is an active field of research. Isocyanide self-assembled monolayers (SAMs) have received some attention lately, as they may provide a better $\pi$-network for electron transport than other molecular SAMs such as benzenethiols. We have studied the structural and electronic properties of the interface between a gold surface and an aromatic isocyanide SAM, using density-functional theory in the GGA-PBE approximation. Our calculations predict a herringbone arrangement at high coverage, instead of the conventional structure with $(\sqrt{3}\times\sqrt{3})R30^{\circ}$ periodicity. The most favorable geometry is however found at low coverage, where the interaction between molecules is negligible and the barriers between differently tilted geometries are small compared to room temperature. These results explain the disordered patterns recently observed in room temperature STM measurements and point at possible difficulties in using isocyanide SAMs for molecular devices. Our calculations also give insight into the alignment of the molecular energy levels with respect to the Fermi energy of the metal substrate, and the charge redistribution at the interface, which provide essential guide for understanding and predicting transport properties of these SAMs, in case ordering can be achieved. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W27.00011: Computer Simulation of Ligated Nanoparticle Assembly from Solution Flint Pierce, Amit Chakrabarti, Chris Sorensen Nanoparticles are becoming increasingly important for the design of novel materials in a wide range of new applications. Ligation of these particles by chemical species provides a means to stabilize them into useful assemblies. It is essential to have a clear physical picture of the way these particles interact. To this end, we are investigating systems of metal nanoparticles ligated with alkyl chains. Our approach is three-fold. First, we are simulating (Monte Carlo) systems of ligated nanoparticles, including all chain/particle interactions in order to develop a model potential. Second, we are simulating (molecular dynamics) systems of these particles interacting via this model potential, varying the alkyl chain length, solvent, core material, and particle volume fraction. Finally, for comparison we are simulating these systems using theoretically derived potentials found in the literature. Initial results indicate a range of morphologies, from fractal aggregates to crystallites, depending on the temperature and potentials involved. Our goal is to provide a guide to researchers in choosing materials and assembly conditions that will lead to desired assembly properties. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W27.00012: The electronic and structural properties of the self-assembled monolayer of Au-benzene-1,4-dithiol-Au molecules T.-H. Lu, Y.-H. Tang, M.-H. Tsai The electronic and structural properties of the self-assembled monolayer of Au-benzene-1,4-dithiol-Au molecules are calculated by first-principles calculation methods. The Au-S bond length obtained is 2.20{\AA}, which is about 6.8{\%} smaller than the sum of their covalent radii of 2.36 {\AA}. The Au-S-C bond angle obtained is 98.9$^{0}$, which is within the range of known bond angles of S, e.g. $\angle $FSF=98.2$^{0}$ for$^{ }$SF$_{2}$, $\angle $ClSCl =103$^{0 }$for SCl$_{2}$ and $\angle $CSH =96.4$^{0 }$for CH$_{3}$CH$_{5}$-SH. The Au 5d band is dominantly located at -2.3 eV below the Fermi level, E$_{F}$, with a sharp peak in the partial density of states (PDOS). The PDOS's also show that the highest-occupied-molecular-orbital band contains S 3p, C 2p and Au 5d hybridized states, while the lowest-unoccupied-molecular-orbital band contains S 3p, C 2p, Au 6s and Au 5d hybridized states. The dominant Au 6s states are located at about -1.5eV and 1.3eV relative to E$_{F}$. The present result shows that Au 5d states, which are usually ignored in previous theoretical studies, play an important role in the S-Au bonding and contribute significantly to the transport property of the molecule. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W27.00013: Auger electron-hole scattering leads to efficient $P\rightarrow S$ electronic relaxation in self-assembled (In,Ga)As/GaAs quantum dots Gustavo A. Narvaez, Gabriel Bester, Alex Zunger We have applied our pseudopotential approach to predict Auger-type relaxation mechanisms in million-atom quantum dots. The electronic structure of (In,Ga)As/GaAs self-assembled quantum dots shows an excited P state about $30$-$50\;{\rm meV}$ above the lowest excited S state. Measured P-to-S relaxation times for electron-hole exciton range from $2$-$10\;{\rm ps}$. Because the P-to-S energy spacing is comparable to the energy of an optical phonon, it has been argued that polaron relaxation is reponsible for the fast observed relaxation. Here, we show that in the presence of a hole, Auger electron-hole scattering---decay of the electron from P to S accompanied by an energy conserving hole excitation---leads to a fast, {\rm ps}-scale decay without invoking polaron relaxation. To this end, we calculate the P-to-S decay lifetime $\tau(P\rightarrow S)$ of electrons in lens-shaped (In,Ga)As/GaAs dots due to Auger electron-hole scattering. We find that this Auger-type relaxation mechanism leads to $\tau(P\rightarrow S)\sim 1$-$7\;{\rm ps}$ for dots of different size, in agreement with available data. [Preview Abstract] |
Session W28: Focus Session: Carbon Nanotubes: Transport and Thin Films
Sponsoring Units: DMPChair: Yuri Zuev, Columbia University
Room: Colorado Convention Center 302
Thursday, March 8, 2007 2:30PM - 2:42PM |
W28.00001: Hall Effect and Magnetoresistance of Single-walled Carbon Nanotubes S. H. Jhang, S. H. Lee, U. Dettlaff, D. S. Lee, S. Roth, Y. W. Park, C. Strunk We report Hall coefficient and magnetoresistance measurements on films and networks of single-walled carbon nanotubes (SWNTs). Four different types of SWNTs are prepared as films; Purified SWNTs synthesized either by HiPCO (High-Pressure CO Conversion) process or by laser ablation method (laser SWNTs), and HiPCO and laser SWNTs chemically treated by SOCl$_{2}$. SOCl$_{2}$-modified SWNTs show higher conductivity due to doping effect. The measured Hall voltages are linear for all samples in fields up to 6 T. The carrier density of SWNTs is determined to be $\sim $10$^{22}$ cm$^{-3}$ for HiPCO and SOCl$_{2}$-modified SWNTs, and $\sim $10$^{21}$ cm$^{-3 }$for laser SWNTs. Considering that theoretically predicted carrier density of metallic SWNT is $\sim $10$^{22}$ cm$^{-3}$ and that of semiconducting SWNT is $\sim $10$^{20}$ cm$^{-3}$, the difference in carrier density between HiPCO and laser SWNTs can be originated from the difference in the ratio of metallic and semiconducting SWNTs in both films. While Hall coefficient is positive in the whole temperature range of 1.4 - 300 K for HiPCO and SOCl$_{2}$-modified SWNTs, the Hall coefficient of laser SWNTs interestingly shows a sign change around at T = 15 K. The magnetoresistance of SWNTs studied in high magnetic fields up to 33 T, and in a temperature range of 0.4 - 300 K will be also presented. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W28.00002: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W28.00003: Energy Anomaly and Polarizability of Carbon Nanotubes Dmitry Novikov, Leonid Levitov Electron properties of carbon nanotubes can change qualitatively by applying a strong perpendicular electric field. In metallic tubes the sign of Fermi velocity can be reversed by a sufficiently strong field, while in semiconducting tubes the effective mass can change sign. Changes in the spectrum manifest themselves in a breakup of the Fermi surface and in the energy gap suppression, respectively. The effect is controlled by the field inside the tube which is screened due to the polarization induced on the tube. The theory of screening is linked to the chiral anomaly for 1D fermions that reveals universality and scale invariance of the response dominated by $\pi$ electrons. [Phys. Rev. Lett. {\bf 96}, 036402 (2006)] [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W28.00004: Massive Integration of Nanotube/Nanowire-based Devices Invited Speaker: Although nanotube (NT)/nanowire (NW)-based devices are drawing an attention as next generation device architecture, a lack of massive assembly method has been holding back their practical applications. In this talk, we will present a massive integration strategy of NT/NW-based devices, where surface molecular patterns guide the `selective assembly' and `alignment' of various NTs/NWs (e.g. carbon NT, vanadium oxide NWs, etc) on virtually general substrates (e.g. Au, silicon oxide, Si, Al, etc). Interestingly, NT/NW adsorbed on solid substrates exhibit `2-dimensional sliding motions' on the substrates to form desired device structures. This strategy is named here as `surface-programmed assembly' in the sense that entire assembly process can be programmed by surface molecular patterns. This process is utilized for wafer-scale fabrication of NT/NW-based devices such as high-performance transistors and bio-sensors. Importantly, since entire processing steps can be done using only conventional microfabrication facilities without any high-temperature processing, the strategy is readily accessible for conventional device industry and may open up new `NT/NW-silicon hybrid device industry' in the future. [REF: Nature 425, 26 (2003); Nature Nanotechnology 1, 66 (2006)] [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W28.00005: Quantum capacitance and gate coupling in NT array field effect devices Slava V. Rotkin, John A. Rogers Modern electronics may essentially benefit on a new approach to fabricating nanotube field-effect thin film transistors (NT-TFT) that consist of parallel NT arrays with high uniformity of inter-tube spacing and orientation of neighbor NTs. The large density of NTs per channel area allows to improve such device characteristics as drain current and transconductance. Here we address theoretically the issue of a quantum capacitance as a unique feature of the single-wall NT material and its role in transport in the dense or sparse array NT-TFTs. We present a complete electrostatic model joined with the quantum theory of the NT response to investigate the capacitance coupling to the backgate and/or the top gate for the broad range of the TFT device geometry. We vary the NT density, uniformity of the array, dielectric substrate and the top-gate dielectric to study the factors possibly limiting performance of the NT-array TFT. We found that due to the effect of strong electrostatic coupling between neighbor NTs in the TFT channel the device characteristics, such as gate coupling and the conductance related to the latter, are robust even to significant deviations from an ideal geometry. This is discussed with respect to the problem of fabrication of devices with good uniformity at the system level despite of some disorder at the subsystem level of discrete elements. [Supported by NSF-0403489] [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W28.00006: Self-assembled Monolayers of Carbon Nanotubes and Their Properties Vladimir Samuilov, Jaseung Koo, Jean Galibert, Vitaly Ksenevich, Nikolaj Poklonski Electronic and thermal transport properties of carbon nanotubes are of particular interest due to their potential use as components in nano electronics applications. Applications of the \textit{individual} nanotubes are progressing rapidly. However, the electrical and thermal conductivity transport properties of the 2-D layers still fall far short of the properties of the individual carbon nanotubes. We have developed a new method of self-assembling of carbon nanotubes (CNT) into high-density 2-D arrays without prior functionalization based on modified Langmuir-Blodgett technique. The method shows several major advantages over the conventional method of CNT monolayers formation. The electrical, thermal conductivity and magneto-transport properties of the monolayers (arrays) of multi-wall and single-wall carbon nanotubes in the temperature range 1.8-300K and in magnetic fields up to 35 T have been tested. Gas sensing properties of the self-assembled arrays of CNTs are discussed. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W28.00007: Towards an optimal nanotube dispersion for transparent conductive coatings Matthew Garrett, Ilia Ivanov, Bin Zhao, Alex Puretzky, David Geohegan Thin films of carbon nanotubes have been investigated as a potential material for transparent conductive coatings. There is a range of transmission and resistance that must be met to make the film useful for technological applications, down to 10 $\Omega $/Square at over 80{\%}T. When nanotubes in solution are made into thin films, the electro-optical properties of the film is dependent on the method of dispersion of the tubes used to make the film, in addition to the quality of the starting material used to make the dispersion. At 90{\%}T, the method of dispersion can cause nearly a factor of ten difference, 15000 $\Omega $/Square, in the resulting film's resistance. Aggregates can cause scattering from the film, detracting from its transmission. The length and purity of the tubes affects the overall resistance of the film. The extent of tube bundling also plays an important role in the electro-optical properties of these films. Methods of quantifying the nature of tubes in solution can yield much insight into the quality of the film which will result from the solution. We have shown how a thorough characterization of the tubes during dispersion as well as after deposition is helpful in determining how to achieve the desirable attributes of a transparent conductive film of nanotubes. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W28.00008: Ambient Formation of Aligned Carbon Nanotube Networks Marcus Lay, Pornnipa Vichchulada Nano-scale electronic materials will play a roll of great significance in electronic devices of the near future. Carbon nanotubes (CNTs), in particular, show great technological promise. Yet, major obstacles to the incorporation of CNTs into practical electronic devices remain; one such challenge is the lack of a method to form ordered constructs of individual carbon nanotubes on a large scale. 2-dimensional networks of CNTs show potential as a method of circumventing the difficulties associates with lack of control over the physical and electrical properties of individual CNTs; for a random distribution of CNTs, the density of the network is the major factor controlling device properties, as fluctuations in characteristics of individual CNTs become less important. Therefore, a 2-D network composed of a mixture of metallic and semiconducting CNTs behaves as a semiconductor above the percolation threshold for semiconducting nanotubes. A novel method of creating ordered arrays of purified CNTs has been developed to attain a higher level of control over reproducibility in CNT-based applications. This method uses unidirectional airflow to order CNTs in aqueous suspension and deposit them on a prepared surface. The result is an electrically continuous array of highly aligned CNTs. These ordered arrays of CNTs exhibit electrical conductivity over macroscopic lengths (up to 3$''$), and have shown promise in field-effect transistor (FET) applications. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W28.00009: Geometry-dependent resistivity scaling in single-walled carbon nanotube films Ashkan Behnam, Ant Ural We study the resistivity scaling in transparent and conductive carbon nanotube films as a function of nanotube and device parameters. First, we observe experimentally that the nanotube film resistivity exhibits an inverse power law dependence on device width below a critical width of 2 microns. We then use Monte Carlo simulations to model this behavior and to study the effect of four parameters, namely tube-tube contact to nanotube resistance ratio, nanotube density, length, and alignment on resistivity and its scaling with device width. We observe stronger resistivity scaling with device width when the transport characteristics in the film are dominated by tube-tube contact resistance, or when the nanotube density, length, or alignment is increased. We also observe that, near the percolation threshold, the resistivity of the nanotube film exhibits an inverse power law dependence on all of these parameters, which is a distinct signature of percolating conduction. However, the strength of resistivity scaling for each parameter is different. We explain these observations, which are in agreement with experimental work, by simple physical and geometrical arguments. Nanoscale study of percolating transport mechanisms in nanotube films is essential for understanding and characterizing their performance in submicron devices. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W28.00010: Modeling of Nanotube Network Semiconductors Meg Noah, Young-Kyun Kwon Novel modeling techniques are used to characterize the structural, electronic and optical properties of nanocomposite network semiconductors. Ab initio computations of the structural properties of ensembles of nanotubes on a variety of substrates are presented. We use Monte Carlo and percolation simulations to predict manufacturing success rates given semiconductor design and processing constraints. The performance impact of flattening of nanotubes and topological defects on nanotubes will also be presented. The purpose of this study is to assist experimentalists and to stream-line and optimize nanomanufacturing. Our research focuses on the fundamental understanding of nanostructured materials and their application to molecular electronic devices. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W28.00011: Microwave Conductivity of Single Wall Carbon Nanotube Arrays C. Highstrete, E.A. Shaner, Mark Lee, F.E. Jones, P.M. Dentinger, A.A. Talin We have measured the microwave conductivity spectra of carbon nanotube (CNT) parallel arrays from room temperature to 4K. Single wall CNTs were assembled by AC dielectrophoresis into parallel arrays of individual CNTs and ropes spanning the electrodes of coplanar waveguides (CPWs). The CPW complex reflection and transmission coefficients were measured from 0.1 to 50 GHz. Measurements of identical bare CPWs were utilized to calculate the frequency dependent complex conductivity and power dissipation of the CNT arrays and provide estimates of these quantities for individual CNTs in this configuration. Small loss due to the CNT arrays is consistently measured and increases with frequency. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W28.00012: Kohn anomalies and non-adiabaticity in doped carbon nanotubes A. Marco Saitta, Nicolas Caudal, Michele Lazzeri, Francesco Mauri The tangential vibrational modes of metallic single-walled carbon nanotubes (SWNTs) are thought to be characterized by Kohn anomalies resulting from the combination of their intrinsic one-dimensional nature and a significant electron- phonon coupling. These properties are modified by the doping- induced tuning of the Fermi energy level $\epsilon_F$, obtained through the intercalation of SWNTs with alkali atoms or the application of a gate potential. We present a Density- Functional Theory (DFT) study of the vibrational properties of a (n,n) metallic SWNT as a function of electronic doping. For such study, we use, as in standard DFT calculations of vibrational properties, the Born-Oppenheimer (BO) approximation, but we also use time-dependent perturbation theory to explore non-adiabatic effects beyond this approximation. We compare our results with existing measurements and suggest features to be explored in future experiments. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W28.00013: Radiation Protection Materials for Space Missions and Industries Ram Tripathi NASA has a new vision for space exploration in the 21st Century encompassing a broad range of human and robotic missions including missions to Moon, Mars and beyond. Exposure from the hazards of severe space radiation in deep space long duration missions is ``the show stopper.'' The great cost of added radiation shielding is a potential limiting factor in deep space missions. In the enabling technology, we have developed methodology and concomitant technology for optimized shield design over multi-segmented missions involving multiple work and living areas in the transport and duty phase of space missions. The total shield mass over all pieces of equipment and habitats is optimized subject to career dose and dose rate constraints. Studies have been made for various missions. Current technology is adequate for low earth orbit missions. Revolutionary materials need to be developed for career astronauts and deep space missions. The details of this new technology and its impact on space missions and other technologies will be discussed. [Preview Abstract] |
Session W29: Focus Session: Graphene V
Sponsoring Units: DMPChair: Kirill Bolotin, Columbia University
Room: Colorado Convention Center 303
Thursday, March 8, 2007 2:30PM - 2:42PM |
W29.00001: Scanning tunneling microscopy of graphene field effect transistors Masa Ishigami, Jianhao Chen, Ellen Williams We have investigated the electronic properties of graphene field effect transistors at atomic scale using scanning tunneling microscopy. We find that photoresist, required by conventional electron beam lithography, binds to graphene and leaves residues with thickness of approximately 1 nm. We will present the procedure necessary to eliminate this residue and report our results of scanning tunneling microscopy and spectroscopy performed on graphene. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W29.00002: Scanning Probe Microscopy studies of 2D and quasi-2D graphene crystal structures. Elena Polyakova, Yuanbo Zhang, Melinda Han, Philip Kim, George Flynn In these studies we utilize a variety of Scanning Probe techniques to observe evolution of material properties as a result of transitions from 3D to 2D crystal structures. Graphite is an ideal candidate for these studies as its stand alone 2D crystal (graphene) and quasi-2D films are conductive, stable, and chemically inert under ambient conditions. These crystals can be easily deposited on an oxidized silicon wafer, and the number of atomic layers can be precisely counted. Specific examples will be given to relate local and mesoscopic properties of these crystals as a function of the number of graphene monolayers forming the crystal. The role of the interaction between the substrate and graphene films will be considered. The finite thickness of crystals allows us to examine defects formed not only on the surface of the film but also below the topmost layer. Attenuation of corrugation in Scanning Tunneling images by overlayers of graphene is described. Compatibility of graphene films with atomic-scale electronics will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W29.00003: Imaging Electronic Interference Effects in Ultrathin Epitaxial Graphite Jason Crain, Gregory Rutter, Joseph Stroscio, Tianbo Li, Phillip First We have used scanning tunneling microscopy and spectroscopy at 4K to investigate local fluctuations in the electronic structure of ultrathin epitaxial graphite grown on SiC. Spectroscopic maps of the density of states for two- and three-layer films reveal spatial modulations that fluctuate with energy. These maps show short range root three by root three ordering reminiscent of Bloch wave interference observed in finite carbon nanotube segments [1]. Additional long range fluctuations have a characteristic length scale that may be related to the underlying structure of the SiC interface.\newline [1] S. G. Lemay et al., Nature 412, 617 (2001). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W29.00004: Instabilities of correlated electrons on the honeycomb lattice Carsten Honerkamp Motivated by the possible relevance to graphene we analyze instabilities of electrons on a honeycomb lattice, interacting by Hubbard and longer-ranged interactions. Using a functional renormalization group scheme which takes into account the wavevector-dependence of the interactions throughout the Brillouin zone, we detect the leading ordering tendencies at low temperatures. Near half band filling and for dominant onsite repulsion, a critical minimal interaction strength is required for an instability toward antiferromagnetic order, in support of a previous large-$N$ work of Herbut [Phys. Rev. Lett. 97, 146401 (2006)] which focused on the Dirac points. We also present results for longer-ranged interactions and away from half band filling. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W29.00005: Fermi Liquid Theory of a Fermi Ring Tobias Stauber, Nuno Peres, Francisco Guinea, Antonio Castro Neto We study the effect of electron-electron interactions in the electronic properties of a biased graphene bilayer. This system is a semiconductor with conduction and valence bands characterized by an unusual ``mexican-hat'' dispersion. We focus on the metallic regime where the chemical potential lies in the ``mexican-hat'' in the conduction band, leading to a topologically non-trivial Fermi surface in the shape of a ring. We show that due to the unusual topology of the Fermi surface electron-electron interactions are greatly enhanced. We discuss the possibility of an instability towards a ferromagnetic phase due to this enhancement. We compute the electronic polarization function in the random phase approximation and show that, while at low energies the system behaves as a Fermi liquid (albeit with peculiar Friedel oscillations), at high frequencies it shows a highly anomalous response when compare to ordinary metals. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W29.00006: Electronic Excitations in Bilayer Graphene J.H. Ho, Y.H. Lai, M.F. Lin The $\pi$-electronic excitations are studied for the AA- and AB-stacked bilayer graphene within the linear self-consistent-field approach. They are strongly affected by the stacking sequence, the interlayer atomic interactions, the interlayer Coulomb interactions, and the magnitude of the transferred momentum. However, they hardly depend on the direction of the transferred momentum and the temperature. There are three low-frequency plasmon modes in the AA-stacked system but not the AB-stacked system. The AA- and AB-stacked plasmons exhibit the similar $\pi$ plasmons. The first low-frequency plasmon behaves as a acoustic plasmon, and the others belong to optical plasmons. The bilayer graphene quite differ from the monolayer graphene and the AB-stacked bulk graphite, such as the low-frequency plasmons and the small-momentum $\pi$ plasmons. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W29.00007: Correlation Energy of Graphene Valeri Kotov, A. H. Castro Neto We discuss the ground state energy of an electron gas on a honeycomb lattice (graphene), where the quasiparticle spectrum has Dirac structure, i.e. linear energy-momentum relation. The correlation energy, due to electron-electron interactions, is calculated in the two-loop approximation, which is the first correction to the Hartree-Fock energy. The possibility of inhomogeneous states is discussed. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W29.00008: Electromechanical instabilities of suspended carbon nanotubes - multi mode excitations Magnus Jonsson, Leonid Gorelik, Robert Shekhter, Mats Jonson We have theoretically investigated electromechanical instabilities of suspended carbon nanotubes when using an STM-tip to probe the suspended part of the tube. A coupling between the vibrational modes of the nanotube and tunneling electrons may lead to a pumping of energy into the mechanical subsystem, resulting in large amplitude vibrations of the CNT. This effect is related to the ''shuttle instability'' and changes the transport properties of the system. In the present study, instability of different bending modes have been investigated. We show that, with respect to the instability, different modes can be treated independently in the limit of weak electromechanical coupling. Also, we show that excitations of different modes are controlled by their vibration frequency and tunneling rates. Tunneling rates of the order of the frequency are found optimal for an instability to occur. Hence, a selective excitation of a single mode is possible. We analyze the limit cycle behavior in this case. Another scenario is simultaneous excitation of several modes, leading to a complex behavior in stationary regime. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W29.00009: Electronic properties of graphene multilayers Francisco Guinea, Antonio H. Castro Neto, Nuno M.R. Peres The electronic structure, screening properties, and charge distribution in stacks of graphene layers is studied. We analyze: i) The stability of Dirac points as function of the ordering of the stack and the number of layers, ii) The existence of surface bands at the top and bottom layers for some stack orderings, iii) The appearance of gaps induced by inhomogeneous charge distributions, and iv) The charge induced by external electric fields. We find that electronic bands with linear, Dirac like, dispersion exist in stacks with the Bernal stacking and an odd number of layers, and for rhombohedral stacking. In tha last case, a dispersionless surface band is also formed. In the presence of interlayer hopping, the dielectric response of a stack with the Bernal ordering favors the formation of a charge density wave with periodicity equal to twice the interlayer spacing. In doped stacks, the charge will accumulate at the surfaces, and present an even-odd modulation. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W29.00010: Local density of states of graphene with diagonal and off-diagonal disorder N.M.R. Peres, Joao Ricardo Santos, F. Klironomos, Shan-Wen Tsai, J.M.B. Lopes dos Santos, A.H. Castro Neto We study the effect of diagonal and off-diagonal disorder in the local density of states of a graphene sheet. The exact Green's functions for graphene in the presence of a local potential and in the presence of a modification of the local hopping parameter are given. A discussion of the resonances induced by disorder in the local density of states is provided. We obtain the exact Green's function for a vacancy as a limiting procedure applied to the Green's functions with either diagonal or off-diagonal disorder. The exact Green's function in the presence of both local and off-diagonal disorder is given. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W29.00011: Moire patterns in graphene bilayers: electronic structure Joao Lopes dos Santos, Nuno Peres, Antonio Castro Neto, Eduardo Castro Moire patterns, resulting from a small angle rotation of the top layer have been observed in graphite [1]. We consider a similar situation in a graphene bilayer. We determine the angles for which the resulting structure is periodic and study its symmetries. We develop a general formalism for the calculation of the electronic properties at low energies (close to the Dirac points of the uncoupled bilayers) and for small rotation angles, based on a continuum approximation for the uncoupled layers. We discuss the resulting electronic structure and possible consequences for transport properties. [1] Z. Rong and P Kuiper, Phys. Rev B. 48, 17427, (1993) [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W29.00012: Experimental signatures of topological defects in graphene Andrew Iyengar, Herbert Fertig, Luis Brey We study the electronic structure of graphene with topological defects, in which some of the hexagonal plaquettes of the honeycomb lattice are replaced by pentagons or heptagons. Our tight-binding calculations show that the local electronic density of states becomes particle-hole asymmetric in the vicinity of such defects. This provides a means of experimentally distinguishing so-called ``plastic'' curvature from elastic deformation. We evaluate various analytic approaches to these defects and discuss their effects on scattering and transport. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W29.00013: Direct Observation of Interface States between Single Layer Graphene and SiC* Gregory Rutter, Tianbo Li, Phillip First, Jason Crain, Emily Jarvis, Nathan Guisinger, Mark Stiles, Joesph Stroscio Graphite films grown on carbon-terminated SiC exhibit coherent transport properties that suggest potential for novel nanoelectronics applications [1]. However, for films grown on silicon-terminated SiC the coherence is greatly reduced, suggesting that the interface electronic structure influences the transport [1]. We have investigated the interface structure and electronic states that form in single layer graphene grown on silicon terminated SiC, using scanning tunneling microscopy and spectroscopy measurements at 4 K. Imaging a single graphene layer reveals features of both the graphite structure and the~SiC~interface. Which structure dominates is observed to be a function of the imaging bias. Sharp peaks in the density of states were found over SiC interface features, which correspond to the onset voltages observed in topography measurements. A comparison of experimental and theoretical findings will be discussed including relevance to transport measurements. *This work is supported in part by the Office of Naval Research and NSF. [1] C. Berger et al., Science 312, 1191 (2006); J. Phys. Chem. B 108, 19912 (2004). [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W29.00014: Graphene-based Silica Composite Thin Films Supinda Watcharotone, Dmitriy Dikin, Sasha Stankovich, Richard Piner, Geoffrey Dommett, Inhwa Jung, Guennadi Evmenenko, Rodney Ruoff, Shang-En Wu, Shu-Fang Chen, Chuan-Pu Liu Very thin, smooth, transparent, and electrically conductive silica films with embedded graphene-based sheets were fabricated via the sol-gel route. Individual `graphene oxide' sheets exfoliated in water were incorporated into silica sols. Composite films were formed by spin coating and rendered conductive by treatment with hydrazine, followed by curing at 400\r{ }C under nitrogen flow. The films were studied by SEM, AFM, TEM, X-ray reflectivity, XPS, UV-Vis spectroscopy, and the electrical conductivity was measured. Transparent and conductive thin silica composite films approximately 30 nm thick were fabricated on glass and silicon substrates, opening up new possibilities for making glassy materials with moderate conductivity and high optical transparency. Support from NASA ({\#}NCC-1-02037) through the University Research, Engineering and Technology Institute on Bio-inspired Materials and the NSF ({\#}CMS-0510212) is appreciated. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W29.00015: Preparation and transport studies of single layer graphite oxide and graphene films Song Han, Scott Gilje, Richard Kaner, Kang Wang Single sheet graphite oxide films are synthesized by intercalation and exfoliation routes of graphite. Because of its layered structure, graphite can readily be intercalated using alkali metals. Such method opens up the possibility of synthesizing ultra-thin layers of graphite by reducing the graphite oxide films. The as-synthesized graphite oxide films are deposited on SiO$_{2}$/Si substrates. Ebeam lithography is used to fabricate graphite oxide Field Effect Transistors (FETs). The transport properties of these devices are studied before and after the reduction of graphite oxide films. [Preview Abstract] |
Session W30: Pattern Formation and Nonlinear Dynamics
Sponsoring Units: DFDChair: Carsten Beta, Max Planck Society for the Advancement of Science
Room: Colorado Convention Center 304
Thursday, March 8, 2007 2:30PM - 2:42PM |
W30.00001: Optically-Induced Spatial Forcing in Rayleigh-Benard Convection Gabriel Seiden, Stephan Weiss, Eberhard Bodenschatz Spatial forcing of spatially extended pattern forming systems has received little attention over the past years. Here we report experimental results on optically forced Rayleigh-Benard (isotropic system) and inclined layer convection (anisotropic system). These include a mapping of the phase space as a function of forcing periodicity and forcing strength. A comparison of the observed patterns with the predictions from Ginzburg-Landau theories is made. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W30.00002: Apparatus for Real-Time Acoustic Imaging of Rayleigh-B\`{e}nard Convection Kerry Kuehn, Jonathan Polfer, Joanna Furno Shadowgraph visualization of the Rayleigh-B\`{e}nard instability in optically transparent fluids has enabled comparison between theoretical and experimental work on a well defined nonlinear system. Rayleigh-B\`{e}nard convection in liquid metals, however, remains largely unexplored owing primarily to the difficulty of imaging flow patterns in opaque fluids. We have designed and built an apparatus for high-resolution real-time imaging of convective flow patterns in optically opaque fluids which takes advantage of recent advances in two-dimensional ultrasound transducer array technology. The experimental apparatus employs a modified version of a commercially available two-dimensional ultrasound camera, similar to those now employed in non-destructive testing of solids. Images of convection patterns are generated by observing the lateral variation of the temperature dependent speed of sound \textit{via} refraction of acoustic plane waves passing vertically through the fluid layer. The apparatus has been utilized to observe straight rolls in transparent 5 cSt. silicone oil. Thus far, we have not observed stable convection rolls in liquid mercury. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W30.00003: Tension kills ... harmonic moments in viscous fingering Matthew Thrasher, Alexander Leshchiner, Harry L. Swinney, Mark B. Mineev-Weinstein We measure the displacement of oil by air between two horizontal, closely-spaced glass plates to track the evolution of harmonic moments, which are integrals of integer powers of $z = x+iy$ over the oil domain. Richardson's theory (1972) predicts that the harmonic moments should be \emph{time invariant in the absence of surface tension}. When we extend the theory to include surface tension, the harmonic moments are predicted to \emph{decay in time because of surface tension}. From measurements of the time decay of the harmonic moments, we obtain a value for the surface tension within 5\% of the accepted value. To obtain such precise agreement, the effect of silicone oil wetting the glass plates must be included. Our results implicitly validate Richardson's theory and directly demonstrate that a full description of interface dynamics in terms of harmonic moments is physically realizable and robust. In addition, a novel growth method using feedback produces nearly n-fold symmetric bubbles. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W30.00004: The early stages of sidebranching in dendritic crystal growth. Andrew Dougherty, Thomas Nunnally We report an experimental study of the early stages of sidebranching in the dendritic crystal growth of NH$_4$Cl crystals in aqueous solution. In steady state, the growing dendrites are characterized by a smooth, nearly parabolic tip. A short distance behind the tip, sidebranches begin to emerge. We characterize the growth of the sidebranches by an envelope $A(z)$, where $z$ is the distance behind the tip. We consider two basic models. In one model, the smooth tip is unstable, and the sidebranches result from the selective amplification of microscopic noise. In this model, $A(z)$ depends on the amplitude of the noise $\bar{S}$ and on various materials parameters. In the second model, the dendrite tip grows in an oscillating mode, with sidebranches emerging like waves in its wake. We have observed no strong oscillations, but very small amplitude ones can not be ruled out. Given the finite experimental resolution, no measurement of the tip region can be completely free of contamination from early sidebranches. We will discuss this and other experimental challenges that need to be overcome before we can understand the origin of sidebranches. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W30.00005: Pattern selection in columnar joints Lucas Goehring, Stephen Morris The pattern of columnar jointing is well known from geological formations such as the Giant's Causeway in N. Ireland, or the Devil's Postpile in California. It arises when a directionally propagating array of cracks arrange themselves into a roughly hexagonal network, which leaves behind a remarkably well ordered collection of prismatic columns. This ordering is efficient, and visually impressive, but not perfect. Experimental observations of columnar jointing in corn starch have shown that, as it matures, the pattern settles down into a well defined, statistically steady state with residual disorder. The same quantifiable amount of disorder can be shown to exist in the pattern of igneous columnar jointing. We report on the dynamics of the mature pattern of columnar jointing, and compare it to models of this phenomenon. In particular, we compare it to an evolution model based on voronoi tessellations, a phase-field model, and a model inspired by 2D foam coarsening. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W30.00006: Flowers in the Fourth Dimension Rebecca Thompson-Flagg, Michael Marder Buckling membranes are seen often in nature from daffodils to torn plastic sheets. These patterns are produced by imposing certain types of metrics on thin sheets. This work looks specifically at patterns formed at the edge of trumpet shaped sheets which are forced to obey an exponentially decreasing metric. Using geometrical techniques a condition for cylindrical symmetry is found. Equations developed by Nash are used to evolve a trumpet from below the limit past the limit. These equations are used to demonstrate that trumpets past this limit cannot fully adopt the metric in three dimensions. A molecular dynamics code is used to create a sheet with points connected by Hookian springs. By changing the equilibrium distance between the springs a target metric can be imposed on the sheet. The energy of the sheet is minimized. The sheet is allowed to move into a fourth spacial dimension and the energy of the sheet in four dimensions, both below and above the symmetry limit, is compared to the minimum energy in three dimensions. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W30.00007: Pattern formation in heterogeneous self-oscillating polymer gels. Victor Yashin, Anna Balazs The chemical reaction and deformations are inherently coupled in the chemo-responsive polymer gels that participate in the Belousov-Zhabotinsky reaction (BZ gels). Chemical oscillations due to the BZ reaction cause variations in the gel's size and shape because of the hydrating effect of the oxidized metal-ion catalyst linked to the polymer. Physical and chemical patterning of self-oscillating gels would facilitate creating active materials that exhibit desirable spatiotemporal behavior. The heterogeneous self-oscillating gels might be designed to respond to external stimuli by switching between pre-programmed dynamic patterns. We explore these potentialities through modeling 2D dynamics of the structurally heterogeneous BZ gels. We start by considering the effects of a spatially non-uniform crosslink density, volume fraction of polymer, and catalyst distribution on the domain of the oscillatory regime. Then, the propagation of the swelling-deswelling waves through the structurally patterned gels is simulated using the gel lattice-spring model approach. We demonstrate and discuss how the spatial organization of the heterogeneous gel affects the origination and propagation of waves. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W30.00008: The effects of pattern morphology on late time scaling in the Cahn-Hilliard model Timothy Sullivan, Peter Palffy-Muhoray As previously reported, numerical simulations of the dimensionless Cahn-Hilliard equation, have been performed in 2D. The initial state consisted of Gaussian distributed random values on a 540 by 540 grid. The Cahn-Hilliard equation conserves the spatial average of the dimensionless concentration difference, $\left\langle \psi \right\rangle $, and initial conditions were chosen with $\left\langle \psi \right\rangle $ ranging from 0 to 0.9. As time progresses the system quickly separates into distinct regions where $\left\langle \psi \right\rangle \approx +1$ or $\left\langle \psi \right\rangle \approx -1$ and then slowly coarsens. Analysis of the late time scaling of a characteristic pattern size scale,$R_G (t)$, the first zero of the pair correlation function, showed that near $\left\langle \psi \right\rangle $ = 0.2 the time to reach the expected dynamical scaling regime grew very long. This, coupled with the change in the pattern from sinuous structures near $\left\langle \psi \right\rangle =0$ to a pattern of circular regions for larger values of $\left\langle \psi \right\rangle $, suggest a morphological phase transition. We explore this idea and will report on our attempts to create order parameters describing the pattern and will present results on the behavior of the candidate order parameters near $\left\langle \psi \right\rangle $ = 0.2. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W30.00009: The onset of activations in the oscillatory Belousov-Zhabotinsky reaction Harold Hastings, Sabrina Sobel, Richard Field The unstirred, ferroin catalyzed Belousov-Zhabotinsky (BZ) is a prototype chemical system exhibiting traveling waves of oxidation in an oscillatory or excitable medium. A typical thin- layer BZ medium (approx. 2D) displays a red (reduced) induction phase lasting several minutes, followed by ``spontaneous'' formation of ``pacemaker'' centers that oscillate between red and blue states and generate target patterns of concentric, outward- moving waves of oxidation (blue) in the red medium. The origin of these pacemaker centers is not yet completely understood. This talk will describe a reduced stochastic model for the origin of pacemaker centers (extending the Oregonator of Field, Koros and Noyes) and recent work of the authors (J. Phys. Chem. A. (Letter); 2006; 110; 5-7), which reproduces experimentally observed oxidation states and nucleation rates. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W30.00010: Classical 2d dynamics simulations of metallic spheres in highly viscous medium Peter Fleck, Alfred Hubler We study the classical dynamics computations of metallic spheres immersed in highly viscous, but weakly conducting medium while exposed to the electro-static field of external electrodes of varying geometries. We represent the system's charge dynamics by the spheres' multipole moments as induced by the electrodes. We theoretically derive the force contributions on an individual sphere including feedback effects, and compare these with results from finite-element computations. We find an individual sphere to oscillate between opposite electrodes only if sufficient charge is retained on the sphere on its path for given material parameters of the medium and distance between the electrodes. We discuss the system's parameter ranges necessary for line arrangements of multiple spheres to emerge. Finally, we compare our computations with an experiment of ballbearings in castor oil. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W30.00011: Particle-based model of Min-protein oscillations in {\it Escherichia coli} Adam Berman, Kerwyn Huang, Ned Wingreen In {\it Escherichia coli} cells, the Min proteins, which are required for division site selection, oscillate from pole to pole via a Turing instability. During these oscillations, two of the Min proteins, MinD and MinE self-associate and co- associate on the bacterial inner membrane forming dynamic structures including a ring of MinE protein, compact polar zones of MinD, and zebra stripes in filamentous cells. Such rich behavior in a system with so few species has made the Min proteins a model system for applying computational methods to study intracellular dynamics in bacteria. Though mean-field computational models successfully reproduce the coarse-grained oscillatory dynamics in both rod-shaped and round {\it E. coli} cells and also predict that the Min-proteins actively detect cell shape, the mean-field models cannot address questions raised by the recent finding that MinD forms a small number of large polymers on the membrane. First, it is unclear how the intrinsic dynamics of polymer formation, namely polymer nucleation and growth, affect the pole-to-pole oscillations. Second, it is not understood how the oscillations influence the morphology of the MinD polymers. To study this coupling between MinD polymerization and pole-to-pole oscillation, we employ a particle-based computational model. In this talk, we will describe this model, which produces both large polymers and pole-to-pole oscillations. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W30.00012: Highly nonlinear dynamics in 1D granular metamaterials: anomalous interaction of solitary waves with interfaces Chiara Daraio, Vitali F. Nesterenko, Eric B. Herbold One dimensional chains of uniform beads support the formation and propagation of a new type of highly nonlinear solitary waves with compact support. The interaction of these solitary waves with an interface between two strongly nonlinear discrete granular media results in novel dynamic phenomena. Here we present a detailed study of the behavior of the reflected and transmitted waves at the interface between two media composed of spherical beads with dramatically different elastic properties and the influence of the static precompression on their formation and propagation. The presence of static precompression can be effectively utilized to monitor the information flow through the interfaces. The formation of anomalous waves caused by the selective ability of the media to support compressive or rarefaction stationary pulses is presented for interfaces of various materials and investigated numerically and experimentally. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W30.00013: Topology of the intermediate state in pinning-free type-I superconductors of different shapes Jacob R. Hoberg, Ruslan Prozorov Equilibrium patterns of the intermediate state were studied in pinning-free Pb samples of different shapes by using direct magneto-optical visualization as well as AC and DC susceptibility measurements. It is found that equilibrium topology of the intermediate state in spheres and hemispheres consists of flux tubes both on flux penetration and exit. Samples with geometric barrier (two flat surfaces perpendicular to an applied magnetic field) exhibit topological hysteresis (and corresponding magnetic hysteresis) -- tubes for flux penetration and Landau laminar structure on flux exit. Finally, obtuse-cone shaped samples show laminar structure both on penetration and exit. Based on the experimental results, general discussion of the equilibrium topology of the intermediate state is given. Video of real-time intermediate state evolution with an applied field for various samples is available at: http://www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W30.00014: Continuum Model for Chaotic Pattern Dynamics on Au Surfaces Sputtered by Focused Ion Beam Kevin Mitchell, Arvin Yazdi, Tom Tiedje Under bombardment by a rastered 30keV Ga$^+$ ion beam, a flat Au surface is found to exhibit the well known sputter ripple instability with a characteristic lateral length scale on the order of 100nm and an RMS saturation height on the order of 10nm. Using {\it in situ} SEM imaging, we are able observe the dynamics of these ripples as they form and evolve. Accurate topography data is also gathered using {\it ex situ} AFM. These experimental data are compared to 2D numerical solutions of the dimensionless partial differential equation $\partial_t{h}=-\nabla^2{h}-\nabla^4{h}-\alpha\left|\nabla{h}\right|^2+\beta\nabla^2\left|\nabla{h}\right|$, which capture the essential features of the sputter ripples. A semi-implicit spectral method is used to solve the equation on a $128\times128$ grid covering a $20(2\pi)\times20(2\pi)$ periodic domain. A length scale near $2\pi\sqrt{2}$, consistent with linearized stability is observed, as is a saturation height of order $1$ when the constraint $\alpha^2+\beta^2=1$ is enforced. Interestingly, the ratio $\alpha/\beta$ is found to control the timescale of the chaotic post-saturation dynamics in addition to fine tuning the length and height scales. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W30.00015: Degenerate four-wave mixing with defocusing nonlinearity Jason Fleischer, Wenjie Wan, Shu Jia We experimentally demonstrate four-wave mixing (FWM) effects in a defocusing nonlinear photorefractive medium, in both one and two dimensions. By using a rectangular crystal, we observe the nonlinear generation of new (spatial) frequencies as a function of propagation distance. Both position-space and momentum-space images are taken, allowing a detailed study of dynamical energy transfer. For degenerate FWM, consisting of a cos(kx) profile superimposed on a k=0 background, there is a direct energy cascade to higher momenta (smaller spatial scales). For the asymmetric case, sum- and difference-frequency generation leads to complex patterns. In two dimensions, interactions also lead to a change in the spatial geometry. Numerical simulations show excellent agreement with the experimental results. [Preview Abstract] |
Session W31: Nano-tubes, Wires, and Dots: Optical and Raman Experiments
Sponsoring Units: DCMPChair: Walter Smith, Haverford College
Room: Colorado Convention Center 401
Thursday, March 8, 2007 2:30PM - 2:42PM |
W31.00001: Raman Spectroscopy of Heat-Treated Boron Doped Double Wall Carbon Nanotubes F. Villalpando, H.B. Son, G.G. Samsonidze, S.G. Chou, Y.A. Kim, H. Muramatsu, T. Hayashi, M. Endo, M. Terrones, M.S. Dresselhaus We performed Raman spectroscopy experiments on undoped and boron-doped double walled carbon nanotubes that exhibit the ``coalescence inducing mode'' as they are heat treated at temperatures between 1200C and 2000C. From the radial breathing mode spectra we find that the smaller diameter tubes disappear before the larger diameter tubes as the sample is heat treated at higher temperatures. By using different laser excitation energies ranging from 1.57eV to 2.41eV, we observe in agreement with prior work that the outer tubes shield the inner tubes and give the G band its characteristic lineshape. We also find that the G' feature contains contributions from the inner and outer layers of a DWNT and its frequency shifts are related to the change in the diameter distribution of the DWNT sample caused by increasing the heat treatment temperatures. Finally, we report on the observation of a four fold splitting of the G' Raman feature and analyze the similarities with recent studies on 2-layer graphene. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W31.00002: Optical Transmittance and Sheet Resistance of B-doped Single-Walled Carbon Nanotubes XiaoMing Liu, Hugo Romero, Humberto Gutierrez, Kofi Adu, Peter Eklund Thin films of carbon nanotubes have been reported to be a replacement for transparent conducting films of Indium-Tin-Oxide (ITO). Nanotube films can be deposited on flexible plastic and are predicted as a new technology for touch screens, solar cells, etc. Here we report results on thin films of boron-doped single-walled carbon nanotubes (B-SWNTs) obtained from CarboLex, Inc. Boron-doping is expected to raise the conductance of semiconducting nanotubes while not lowering significantly that of the metallic tubes. At room temperature, we have measured the four-probe sheet resistance and the optical transmission in the NIR-UV range to evaluate the performance of these chemically enhanced SWNT films. The structure in the optical spectrum is essentially the same as in pristine tubes, although the positions of optical absorption bands are slightly upshifted ($\sim $ 50 meV) relative to pristine SWNTs. The B-loading, microstructure, bonding and defects of the B-doped SWNTs were characterized, respectively, by inelastic neutron scattering, transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. Our preliminary results on B-SWNTs show that the visible optical transmittance is higher and the sheet resistance is much lower than that of similar thickness SWNT films. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W31.00003: Raman Spectroscopy Studies of Oxygen and Hydrogen RF-Plasma Treated Single Wall Carbon Nanotubes Chaminda Jayasinghe, David B. Mast We present results from resonant Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) and thermoelectric power (TEP) measurements on hydrogen and oxygen plasma treated single-wall carbon nanotubes (SWNTs). For oxygen treated SWNTs, Raman spectroscopy of the BWF band ($\sim $1522cm-1) show a dramatic up shift for both the $\sim $1522cm-1 band and the $\sim $1578cm-1 band by nearly 20 cm-1. In addition, there is a considerable change in the appearance of the D' mode ($\sim $1620cm-1) with oxygen plasma treated SWNTs. These results show that oxygen plasma treatment affects the metallic nanotubes in our sample more than semiconducting ones. The Raman spectra of hydrogen treated SWNTs show a much smaller D band peak than the oxygen treated SWNTs. The G bands of the hydrogen treated SWNTs are also up shifted compared to untreated nanotubes. The surface of the oxygen plasma treated SWNTs was also analyzed by XPS. These results show that the O1s to C1s intensity ratio is considerably higher in oxygen treated SWNTs compared to untreated nanotubes. The TEP measurements confirm that these plasma treatments induce defects as well as add side wall functionalization to the SWNTs. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W31.00004: Persistent Photoconductivity and Photo-induced Morphology Changes of Porphyrin Nanorods B.E. Feldman, E.A. Muller, V.H. Joines, W.F. Smith, A.D. Schwab, J.C. de Paula, D.E. Johnston, A.T. Johnson Tetrakis(4-sulfonatophenyl) porphine self assembles into well-defined nanorods with intriguing photoelectronic properties.$^{1}$ Recently, we have found that, over long time scales, they undergo a transition from non-persistent photoconductivity (NPPC) to a new mode, in which part of the conductivity persists after the light is blocked, decaying over hundreds of seconds. NPPC initially dominates, but its growth asymptotes within 2-3 hours of illumination, while the persistent current continues to grow, even after 8 hours of light exposure. The decay of persistent current after the light is blocked can be roughly modeled by a single exponential; a double exponential fits much better. The morphology of some nanorods changes as a result of long-term illumination---they become shorter, thinner and less well-formed. Other rods, however, appear unchanged. This and other structural changes may be related to the slow growth of persistent current. $^{1}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W31.00005: Finite Size Effect in ZnO Nanowires Chung-Jen Chien, Pai-Chun Chang, Zhiyong Fan, Jia Grace Lu, Daniel Stichtenoth, Carsten Ronning In this talk, we present electrical and optical measurements on ZnO nanowires whose sizes do not yet reach quantum confinement region. Thin ZnO nanowires were synthesized via carbon thermal chemical vapor deposition method under low growth temperature using tin as catalyst. Electron microscopy reveals that the as-grown nanowires are of high crystalline quality with an average diameter around 12 nm. Electrical transport measurements show significant increase in conductivity with a lack of gate modulation and a reduction in mobility. This phenomenon is attributed to the enrichment of surface states owing to the larger surface-to-volume ratio. This enhanced surface effect in thinner nanowires is confirmed by the temperature dependent photoluminescence measurements. In addition, the photoluminescence spectra clarify the apparent blue shift observed at room temperature with respect to the nanowires with larger diameters. These results provide a fundamental insight into nanowires of smaller diameters, and show that their surface states are extremely important and should be properly tailored or controlled for future device applications. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W31.00006: Electrical and Photoconductive Properties of Vertical ZnO Nanowires in High Density Arrays Zhiyong Fan, Deepanshu Dutta, Chung-Jen Chien, Evan C. Brown, Pai-chun Chang, Jia Grace Lu High density vertical zinc oxide nanowire arrays were synthesized using highly ordered channels in anodic alumina membranes via catalytic chemical vapor deposition assisted by electrochemical deposition methods. The anodic alumina membranes were fabricated using a two-step anodization method. High resolution transmission electron microscopy and energy dispersive x-ray microanalysis studies revealed that the nanowire growth was governed by a vapor-liquid-solid mechanism, and the nanowires are single crystalline grown along the [001] direction. Using conductive atomic force microscopy (AFM), the electrical transport and photoconduction of individual vertical nanowires were investigated. The role of the AFM probe coating on the $I-V$ characteristics will be presented. A negative photoconductivity was first observed as a result of electron trapping in the alumina membrane due to its duplex oxide layered structure. In contrast, positive photoconductivity was observed using a thermally annealed anodic alumina membrane as the nanowire growth template. These studies render a pathway for constructing high density nanoscale electronic and optoelectronic circuits. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W31.00007: Imaging the response of individual carbon nanotubes to polarized light in aqueous environments Bryant Walker, Todd Brintlinger, Michael S. Fuhrer, John Cumings, Erik Hobbie Individual carbon nanotubes are grown using chemical vapor deposition (methane-ethylene carrier gas and iron nitrate catalyst), freely suspended in an aqueous solution using a surfactant (sodium dodecyl sulfate), and imaged in an optical microscope using either fluorescent dye (PKH67 and PKH23) or intrinsic near-infrared fluorescence. Freely suspended, individual carbon nanotubes of length 1-8 micrometers show an increasing response to illuminating light as the polarization becomes parallel to tube axis. More intriguingly, some of the carbon nanotubes are found to collapse and fold under 10-30 seconds of illumination, with increasing tube length showing longer time-to-collapse. Unperturbed persistence lengths in these nanotubes are estimated to be 200-300 micrometers. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W31.00008: Raman spectroscopic investigation of the confined optical phonon modes in the aligned CdSe nanorod arrays Concetta Nobile, Luigi Carbone, Stefan Kudera, Liberato Manna, Roberto Cingolani, Roman Krahne, Vladimir A. Fonoberov, Alexander A. Balandin, Gerwin Chilla, Tobias Kipp, Detlef Heitmann Nanocrystal rods have emerged as promising nanostructured material for both fundamental studies of nanoscale effects and for optical and electronic device applications. We investigated the optical phonon excitations in laterally aligned CdSe nanocrystal rod arrays using resonant Raman scattering. Electric-field mediated alignment between interdigitated electrodes has been used to prepare the samples. We report Raman experiments that probe the optical lattice vibrations in ordered arrays of CdSe nanorods with respect to the nanorod orientation. The packing of nanorods into dense arrays leads to the suppression of the surface optical phonon modes. In the longitudinal-optical phonon peak we observe a fine structure that depends on the relative orientation of the nanorods with respect to the incident light polarization. Detailed comparison of the experimental data with the first-principle calculations for corresponding nanostructures, which reveal the symmetry of the phonon potentials for the Raman active modes, provides a qualitative explanation of the experimentally observed phonon modes. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W31.00009: Raman Scattering from Si$_{1-x}$Ge$_{x}$ Alloy Nanowires Qiujie Lu, Kofi Adu, Xi Zhang, Kok-Keong Lew, Pramod Nimmatoori, Elizabeth Dickey, Joan Redwing, Peter Eklund Bulk Si$_{1-x}$Ge$_{x}$ crystals can be prepared over a wide composition range 0$<$x$<$1. These materials are of interest because alloying can be used to vary the bandgap of the system. Here we present Raman scattering results on Si$_{1-x}$Ge$_{x}$ nanowires (0$<$x$<$1) grown by the vapor-liquid-solid growth mechanism using a Chemical Vapor Deposition (CVD) approach. TEM and XRD were used to characterize the morphology growth axis and lattice constant of these materials. Typical wire diameters were observed to be in the range 80-130 nm. Based on Raman scattering studies of the bulk, three Raman bands are expected that can be identified as a perturbed Si-Si ($\sim $500 cm$^{-1})$ mode, a Ge-Ge ($\sim $280 cm$^{-1})$ mode or a new mode at ($\sim $390 cm-1) assigned to Si-Ge or Ge-Si clusters. Peaks in this region are also observed in the case of our nanowires, although the frequencies are a few cm$^{-1}$ lower than observed in the bulk. We also observe that the compositional (x) dependence of the Si-Ge band in nanowires is somewhat different than in the bulk. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W31.00010: Transport and optical properties of electrochemically fabricated Bi nanowires Hong Zhang, Haidong Liu, Zuxin Ye, Wenhao Wu, Serguei Jerebtsov, Alexandre Kolomenski, Hans Schuessler We present transport and optical studies of Bi nanowires electrochemically deposited into porous aluminum oxide (AAO) membranes. The Bi wires have nominal diameters of 20$\sim $100 nm and length of 60 $\mu $m. For transport measurements, electric contacts with negligible contact resistance were formed on single Bi nanowires \textit{in-situ} during electrochemical deposition. The temperature dependence of the resistance of Bi single nanowires in AAO showed a semiconductor-semimetal transition when the samples were cooled~below 50 K. The transverse magnetoresistance increases monotonically up to the highest available field of 8 T, while the longitudinal magnetoresistance tends to flatten at high field values. For optical measurements we first etched away the membranes and suspended Bi nanowires in water and then collected Bi nanowire on glass substrates. Coherent optical phonons were studied in Bi nanowires using a femtosecond pump-probe technique. The frequency of the excited phonon oscillations was found to be 2.35 THz at a pump fluency of 10 mJ/cm$^{2}$. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W31.00011: Suspension of SWNTs in water: a dynamic optical study Goki Eda, Giovanni Fanchini, Manish Chhowalla Dispersion of single-wall carbon nanotubes (SWNTs) in water is critical for processing. Dispersions are often unstable and the dynamics associated with SWNT bundle formation are poorly understood. We have devised a simple method to examine the dispersion properties based on changes in transmittance at three different levels of the suspension using 650 nm lasers. Our results show that the suspension is stable up to 1 hr with the transmittance remaining constant. Above 1 hr, anomalous transmittance behavior is observed. It initially increases, as expected, but then decreases for a short amount of time before constantly increasing. These fluctuations suggest Mie scattering from changes in the SWNT bundle size. The dynamic nature of the suspension and the fact that the SWNTs and their bundles are significantly smaller than the wavelength of the lasers means that the interpretation of the transmittance data requires careful analysis in terms of the effective medium theory. Our findings provide an understanding of the SWNT suspension behavior which has important implications for the emerging applications of SWNTs based on solution processing. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W31.00012: Optical Properties of Nanostructured of Ce-doped Y$_{2}$SiO$_{5}$ Luiz G. Jacobsohn, Bryan L. Bennett, Ross E. Muenchausen, James F. Smith, Stephanie C. Sitarz, Michael W. Blair, D. Wayne Cooke Nanophosphors correspond to nanostructured inorganic insulator materials that emit light under particle or electromagnetic radiation excitation. In this work, we present structural and optical characterization of Ce-doped Y$_{2}$SiO$_{5}$ nanophosphor prepared by the solution combustion method. Characterization by TEM and x-ray diffraction shows that nanopowders are composed of 30-70 nm nanocrystals agglomerated into micron-sized particles. The Ce content was varied up to 10 at.{\%}. Photoluminescence excitation and emission spectra are composed of two major bands centered at 360 and 430 nm, respectively. These results revealed larger Stokes shift for the nanophosphors when compared to bulk. Ce content was also found to affect photoluminescence emission intensity and lifetime. Concentration quenching curve presents a broad maximum centered at 1 at.{\%}. Lifetime measurements showed a continuous decrease from 34 to 21 ns for higher Ce contents. These results confirm the uniqueness nature and properties of nanophosphors, and show that nanophosphors are promising materials for new basic science and technological applications. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W31.00013: Optical Properties of Molecular Dots Timothy Russin, Gugang Chen, James Adair, Peter Eklund A ``Molecular Dot'' (or ``M-dot'') refers to a mixed organic/inorganic phase of nanomatter where a small number of organic molecules are encapsulated in an inorganic nanoparticle. Particular interest has been initiated in these systems when the molecules exhibit photoluminescence (PL) and the nanoparticle provides a transparent medium allowing easy entrance and exit of photons. They show promise for medical applications. In preliminary experimental studies, the encapsulation has been found to enhance the PL and suppress the photo-degradation of organic dye molecules such as Rhodamine B encapsulated in SiO$_{2}$ or CaPO$_{4}$. In this paper, we present the results of an optical model to predict the optical properties of M-dots. Using the discrete dipole approximation, we take into account the effects of Mie scattering and the effective dielectric function of the dye molecules encapsulated in an inorganic host of known refractive index. The results of the modeling will be compared to recent experimental results on M-dots in dilute solution, i.e., optical absorption and dispersion in the NIR-Vis-UV regions. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W31.00014: Synthesis and Optical Properties of Boron Nitride Nanotubes, Nanowires and Nanorods Ying Chen, Hua Chen, Jun Yu, Hongzhou Zhang Quasi-one-dimensional boron nitride nanomaterials such as BN nanotubes, BN nanowires, BN nanorods, and BN whiskers have different nanostructures but uniform electronic band gaps independent of their diameters and chiralities. Their quantum confinement effects in these low dimensional materials can enhance their optical emission substantially by inducing an indirect-to-direct conversion of the optical transition. Different nanostructures and dimensions have different emission behaviors. Therefore, one-dimensional BN nanomaterials are likely to find further applications in optoelectronics. We have achieved controlled and patterned growth of the BN nanotubes [1,2], BN nanowires [3,4] and conical boron nitride nanorods [5, 6] and investigated their optical properties including photoluminescence, cathodoluminescence and Raman spectroscopy. Relationships between the different nanostructures and corresponding properties will be discussed. [1] J. Yu, Y. Chen, et al., Chemistry of Materials, 17 (2005) 5172. [2] H. Chen, Y. Chen, et al. Chemical Physics Letters 42(2006) 315. [3] YJ Chen, H. Zhang, Y. Chen, Nanotechnology, 17 (2006) 786. [4] YJ Chen, et al., Nanotechnology, 17 (2006) 2942 [5] H. Zhang, et al., Physical Review B, 74 (2006) 045407 [6] H. Zhang, et al., Applied Physics letters, 88 (2006)093117. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W31.00015: Second-Harmonic Generation of Aligned Single-Walled 0.4nm Carbon Nanotubes Kam Sing Wong, Huimin Su, Jianting Ye, Zikang Tang The second-harmonic generation (SHG) is measured for the first time from monosized and well-aligned single-walled carbon nanotubes (SWCNT) in the channel of aluminophosphate AlPO4-5 (AFI) zeolite. The SHG yield scales as quadratic function of the pump laser intensity. Due to the different polarization preference, we are able to discriminate the SHG contribution from the chiral (4,2) CNTs and those from the AFI template. The polarization direction and the anisotropic dependence of the SHG intensity on the excitation polarizations are investigated in the transmission geometry. In the case of normal incidence, the intensity of SHG is maximized when the excitation polarization is 45 degree against the tube axis and the SH radiation is linear-polarized on the plane perpendicular to the tube axis. The experiment results are in excellent agreement with the theoretical prediction of the second-order nonlinear optical process in chiral carbon nanotubes. [Preview Abstract] |
Session W32: Bose-Einstein Condensation in Trapped Atomic Gases
Sponsoring Units: DAMOPChair: Murray Holland, University of Colorado
Room: Colorado Convention Center 402
Thursday, March 8, 2007 2:30PM - 2:42PM |
W32.00001: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W32.00002: Collective Excitations of a Two-Component Bose Condensate at Finite Temperature Chang-hua Zhang, Herbert A. Fertig We compare the collective modes for Bose-condensed systems with two degenerate components with and without intercomponent coherence at finite temperature using the time-dependent Hartree-Fock approximation. We show that the interaction between the condensate and non-condensate in these two cases results in qualitatively different collective excitation spectra. We show that at zero temperature the single-particle excitations of the incoherent Bose condensate can be probed by intercomponent excitations. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W32.00003: ABSTRACT HAS BEEN MOVED TO B21.00008 |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W32.00004: Dispersive Shock Wave Collisions in Bose-Einstein Condensates and Light Mark Hoefer, Mark Ablowitz When two classical shock waves collide, the interaction is relatively simple and is explained by classical hyperbolic system theory and jump/entropy conditions. An analogous theory for the interaction of two dispersive shock waves (DSWs) is presented. Two cases will be considered: i) a collision where two DSWs are propagating directly toward one another, ii) merging where a faster DSW overtakes a slower one. It is shown that, after a complicated quasi-periodic or multi-phase region is created, the DSW interaction process results in: i) two single DSWs propagating away from one another in the collision case, ii) a single, larger DSW representing the merger of the original two DSWs in the merging case. Remarkably, these results coincide exactly with their classical shock wave counterpart. These results have direct application to Bose-Einstein condensates and nonlinear optics. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W32.00005: Effects of Disorder on a Bose-Einstein Condensate with Tunable Interaction: Transition to an Insulator and Loss of Phase Coherence$^1$ Yong P. Chen, J. Hitchcock, D. Dries, M. Junker, C. Welford, R.G. Hulet We report our study of the effects of disorder on a Bose-Einstein condensate (BEC) of $^7$Li atoms with tunable interaction. A large $^7$Li BEC is created in an elongated optical trap after forced evaporation. The strength of the repulsive interaction is tuned using a magnetic Feshbach resonance. A disordered optical potential, whose strength is also tunable, is generated by projecting a laser speckle pattern onto the atoms. We have performed transport studies by measuring the center of mass motion of the trapped BEC in the presence of disorder. Beyond a disorder strength ($V_t$), the dipole oscillation of the superfluid BEC is completely suppressed, signaling a transition to an insulator. We have also studied the time of flight expansion of the BEC after release from the trap and disordered potentials. With intermediate disorder strengths, striking fringes appear in the cloud after sufficient expansion time. Beyond some disorder strength ($V_p$), comparable to the chemical potential of the trapped BEC, the fringes are washed out, signaling a loss of phase coherence. Interestingly, $V_p$ is significantly larger than $V_t$, suggesting that finite phase coherence can still exist in the insulator. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W32.00006: Quantum Accelerator Modes in BEC Vijayashankar Ramareddy, Ghazal Behin-Aein, Peyman Ahmadi, Gil Summy The quantum delta kicked accelerator can be realized by subjecting cold atoms to spatially corrugated off resonant pulses of light. These standing wave pulses are applied in the direction in which there is a component of gravity and result in acceleration of a group of atoms. For the first time we observed Quantum Accelerator Modes (QAM) in BEC. We show that using the narrow momentum distribution of BEC, the structures in phase space map produced by a psuedo classical theory can be directly studied. We show that QAMs can be effeciently populated using BEC. Details will be presented. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W32.00007: Density Profiles of $^{85}$Rb--$^{87}$Rb Binary Mixtures Laura Halmo, Alison Lota, Mark Edwards, Scott Papp, Deborah Jin We have studied the density distribution of binary mixtures of $^{87}$Rb and $^{85}$Rb Bose--Einstein condensates under conditions similar to a recent experiment conducted in the Jin Group at JILA. In this experiment, a binary mixture of the two Rb isotopes were confined in a magnetic trap and rf evaporative cooling was carried out on the $^{87}$Rb causing sympathetic cooling of the $^{85}$Rb. This mixture was then transferred to an optical trap to minimize $^{85}$Rb 3--body loss and condensation was achieved by slowly decreasing the depth of the optical trap. An external magnetic field using a Feshbach resonance enabled tuning of the 85--85 scattering length. Density profiles were obtained by taking absorption images of expanded condensates after releasing them from the trap. We have calculated the theoretical shape of such images by solving approximately the coupled time--dependent (TD) Gross--Pitaevskii (GP) equations. As initial states we used Thomas--Fermi approximate solutions of the time--independent GP equation and approximately solved the time--dependent Gross--Pitaevskii equation to model the expansion. We present a comparison of this calcualation with the experimental data. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W32.00008: Bose-Einstein S=1 Spinor Condensates, Dynamics, Noise Statistics and Scaling George I. Mias, Steven M. Girvin We examine Bose-Einstein spinor condensates in the short-time non-linear regime for S=1 atoms in the context of $^{87}$Rb studied experimentally by the Stamper-Kurn group [L. Sadler et al, Nature {\bf 443}, p193, 2006]. We will describe the quantum dynamics of a sample that starts as a condensate of $N$ atoms in a pure $S=1$, $m_f=0$ state. Our approach seeks to improve the mean-field description of such systems by including the contributions of quantum fluctuations that seed the eventual formation of ferromagnetic domains. We will give a simple quantum description of the system for the short-time regime in analogy with ``two-mode squeezing" of quantum optics, treating the initial $m_f=0$ condensate as a source for the conversion to pairs of $m_f=1,-1$ states. Even though the system as a whole is described by a pure state with zero entropy, the reduced density matrix for the $m_f=+1$ degree of freedom, obtained by tracing out the $m_f=-1,0$ degrees of freedom, is a thermal state. We propose to observe the large fluctuations associated with this thermal state using Hanbury-Brown-Twiss noise correlation measurements in the density and momentum distributions of the individual $m_f$ species. Finally, we will discuss the effect of excitations in connection to the seeding and ultimate formation of domains of ferromagnetically aligned spins. (Supported by NSF DMR-0603369). [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W32.00009: Nonlinear quantum hydrodynamics in Bose-Einstein condensates Peter Engels, Collin Atherton Bose-Einstein condensates are quantum fluids governed by nonlinear interatomic interactions. They provide an excellent tool to study intriguing phenomena in the field of nonlinear hydrodynamics. We will report on hydrodynamics experiments carried out in a newly constructed BEC apparatus at Washington State University, Pullman. Current research directions include quantum shock waves and parametric resonances. We will describe the current results and future directions. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W32.00010: Quantum Field Theoretical Description of Dynamical Instability of Trapped Bose-Einstein Condensates Makoto Mine, Masahiko Okumura, Tomoka Sunaga, Yoshiya Yamanaka The Bogoiubov-de Gennes equations are used for a number of theoretical works on the trapped Bose-Einsetein condensates. Particularly, it is important that if all of the eigenvalues of the equations are real, the solutions of the equations diagonalize the unperturbed Hamiltonian, and the quasi-particle picture, which describes the quantum fluctuation around the condensates, is obtained. We consider the quantum fluctuation in the case that these equations have complex eigenvalues. First, to expand quantum field which represents the quantum fluctuation, we give the complete set including pairs of complex modes whose eigenvalues are complex conjugate to each other. The expansion of the quantum field brings the operators associated with the complex modes, which are simply neither bosonic nor fermionic ones. Next, to evaluate physical quantities, we construct the eigenstate of the complex mode sector of the unperturbed Hamiltonian. Finally, we discuss the instability of the condensates caused by the quantum fluctuation associated with the complex mode in the context of Kubo's linear response theory. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W32.00011: Conversion efficiency of heteronuclear Feshbach molecules Shohei Watabe, James E. Williams, Tetsuro Nikuni We study formations of heteronuclear Feshbach molecule in population imbalanced atomic gases, extending the recent work [J. E. Williams et. al., New J. Phys. 8, 150 (2006)] on the Feshbach molecule formation. We find that conversion efficiency depends on a ratio of the number of atomic species in the initial state before the magnetic sweep, as well as an initial temperature and an initial peak phase space density. At low temperature in quantum degenerate regime, quantum statistics of atoms plays an important role in conversion efficiencies. Maximum conversion efficiencies are determined by quantum statistics and the number ratio. When the major component is bosonic, the maximum conversion efficiency is $50\%$. On the other hand, when the major component is fermionic and the minor component is bosonic, the maximum conversion efficiency has a range from $50\%$ to $100\%$, which is determined by the initial atomic ratio. In the case that both components are fermionic, the maximum conversion efficiency is $100\%$. In the region where the gases does not condense, the conversion efficiency is described as a function of initial peak phase space density of a major component. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W32.00012: Using modified Gaussian distribution to study the physical properties of one and two-component ultracold atoms Chou-Chun Huang, Wen-Chin Wu Gaussian distribution is commonly used as a good approximation to study the trapped one-component Bose-condensed atoms with relatively small nonlinear effect. It is not adequate in dealing with the one-component system of large nonlinear effect, nor the two-component system where phase separation exists. We propose a modified Gaussian distribution which is more effective when dealing with the one-component system with relatively large nonlinear terms as well as the two-component system. The modified Gaussian is also used to study the breathing modes of the two-component system, which shows a drastic change in the mode dispersion at the occurrence of the phase separation. The results obtained are in agreement with other numerical results. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W32.00013: Study of impurities immersed in a trapped Bose-Einstein condensate* Kwangsik Nho, D. P. Landau Using path integral Monte Carlo simulation methods[1], we have studied properties of impurities immersed in Bose-Einstein Condensates harmonically trapped in low dimemsion. For two-body interactions, we use a hard-sphere potential whose core radius equals its corresponding scattering length. We assume that the impurities experience the external trapping potential. We have tightly confined the motion of trapped particles in one or more direction by increasing the trap anisotropy in order to simulate lower dimensional atomic gases. By varying the strength of the boson-impurity interactions and the number of impurities, we have investigated the effect of impurities on the energetics and structural properties such as the total energy, the density profile, and the superfluid fraction. Our results show that for impurities with larger two-body interactions than the boson-boson interactions, the impurities move away from the trap center and surround the trapped bosons, and the density profile is found to get narrower, with the peak density getting larger. The total superfluid fraction decreases due to the impurities, although the difference becomes smaller and smaller by increasing the trap anisotropy. \newline \newline \noindent *Research supported by NASA\\ \noindent [1] K.~Nho and D.~P.~Landau, Phys. Rev. A. {\bf 72}, 023615 (2005). [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W32.00014: Modeling the Expansion of Bose--Einstein Condensate Mixtures in the Thomas--Fermi Limit Alison Lota, Laura Halmo, Charles Holcombe, Mark Edwards We have studied the expansion of a mixture of $^{85}$Rb--$^{87} $Rb Bose--Einstein condensates within the Thomas--Fermi approximation. Systems involving mixtures of Bose--Einstein condensates of different atomic species can be accurately modeled by coupled Gross--Pitaevskii equations. As for single condensates, the coupled Gross--Pitaevskii equations can be written in hydrodynamic form where each condensate is described by a density and phase. Also just as for single condensates, the hydrodynamic equations of motion for condensate mixtures reduce to classical equations of motion when their quantum pressure terms are neglected (Thomas--Fermi approximation). In this case, it is possible to find time--dependent Thomas--Fermi approximate solutions for the hydrodynamic equations of motion for mixtures. We present these equations and their solution for the particular case of a $^{85}$Rb--$^{87}$Rb expansion that occured in a recent experiment performed in the Jin group at JILA. We also highlight interesting features that can occur because of interaction effects in the expansion of multiple-- condensate mixtures. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W32.00015: ABSTRACT WITHDRAWN |
Session W33: Quantum Entanglement
Sponsoring Units: GQIChair: Daniel Lidar, University of Southern California
Room: Colorado Convention Center 403
Thursday, March 8, 2007 2:30PM - 2:42PM |
W33.00001: Entanglement Purification of Any Stabilizer State Scott Glancy, Emanuel Knill, Hilma Vasconcelos We present a method for multipartite entanglement purification of any stabilizer state shared by several parties. In our protocol each party measures the stabilizer operators of a quantum error-correcting code on his or her qubits. The parties exchange their measurement results, detect or correct errors, and decode the desired purified state. We give sufficient conditions on the stabilizer codes that may be used in this procedure and find that Steane's seven-qubit code is the smallest error-correcting code sufficient to purify any stabilizer state. An error-detecting code that encodes two qubits in six can also be used to purify any stabilizer state. We further specify which classes of stabilizer codes can purify which classes of stabilizer states. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W33.00002: An observable entanglement measure Florian Mintert We describe how the concurrence of arbitrary mixed bipartite quantum states, can be measured in terms of a simple projective measurement on two identically prepared quantum states, and we discuss generalizations to multipartite systems. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W33.00003: The su(1,1) symmetry of tripartite entangled Gaussian states Barry Sanders, Zahra Shaterzadeh Yazdi, Peter Turner Two-mode squeezed light has been central to theoretical and experimental studies of continuous variable quantum information processing and to quantum foundations. More recently the generalization of these states to three-mode squeezed light has been achieved in the context of quantum teleportation [1] and state sharing [2]. Theories are typically developed in Gaussian or position representations, but we have discovered that all tripartite entangled Gaussians states of these types are in fact su(1,1) coherent states with respect to an intriguing three-boson realization of su(1,1) first noticed by Sebawe Abdalla et al [3]. This symmetry provides insights into the useful properties of these states and suggests ways to generalize theories and applications of multipartite entangled Gaussian states. [1] A. Furusawa et al, Science \textbf{282}, 706 (1998). [2] A. M. Lance et al, Phys. Rev. Lett. \textbf{92}, 177903 (2004). [3] M. Sebawe Abdalla et al, Eur. Phys. J. D \textbf{13}, 423 (2001). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W33.00004: Quantum Chaos and Entanglement for Two Coupled Spins Collin Trail, Ivan Deutsch, Leigh Norris, Parin Sripakdeevong, Arjendu Pattanayak We explore the relationship between classical chaos and the generation of quantum entanglement in a system of two coupled and driven ``tops'' e.g. electron angular momentum and nuclear spins coupled by the hyperfine interaction and driven by an applied time varying magnetic field. Chaos arises here through the coupling and time dependent drive, rather than the coupling of independently chaotic subsystems, as has been previously studied. Using the same Hamiltonian to generate both classical and quantum dynamics, we find that the long time averages of the entanglement generated between two initially uncoupled coherent spin states correlates with the structure of the mixed classical phase space and interpret these results. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W33.00005: Entanglement Entropy in Gapped Quantum Spin Chains Hosho Katsura, Takaaki Hirano, Yasuhiro Hatsugai Entanglement properties of quantum spin systems have recently attracted much attention in quantum information theory and condensed matter physics. The entanglement entropy (von Neumann entropy of a sub-system) has been used to detect the quantum phase transition and topological order[1][2][3]. It was also discussed that the direct relation between the bulk entanglement entropy and the edge one [4]. We give the exact form of the entanglement entropy in higher-spin Valence-Bond- Solid states and show that the edge state picture is valid for all integer spins. The relationship between the entanglement entropy and the correlation function is clarified and the physical meaning of the entanglement entropy in gapped models is established. We also make a comparison between the analytical results for VBS chains and the numerical results for higher-spin antiferromagnetic Heisenberg chains. [1] M. Levin and X. G. Wen, Phys. Rev. Lett. {\bf 96}, 110405 (2006). [2] A. Kitaev and J. Preskill, Phys. Rev. Lett. {\bf 96}, 110404 (2006). [3] G. Vidal et al., Phys. Rev. Lett. {\bf 90}, 227902 (2003). [4] S. Ryu and Y. Hatsugai, Phys. Rev. Lett {\bf 96}, 245115 (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W33.00006: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W33.00007: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W33.00008: Experimental purification of two-atom entanglement in an ion trap array D. Leibfried, R. Reichle, E. Knill, J. Britton, R. B. Blakestad, J. D. Jost, C. Langer, R. Ozeri, S. Seidelin, D. J. Wineland Entanglement is a crucial resource for quantum information processing and quantum communication. Distributed entanglement is created by preparing an entangled pair of quantum particles in one location and transporting one member of the pair to another location. Decoherence during transport reduces the fidelity of the entanglement. ``Entanglement purification'' [ C. Bennett \textit{et al.}, Phys. Rev. Lett. \textbf{76}, 722 (1996)] can improve the fidelity after the transport using local quantum operations and classical communication between locations to distill high fidelity entangled pairs from lower fidelity ones. Proof-of-principle experiments distilling entangled photon pairs have previously been carried out, however, distilled pairs were obtained with low probability of success and required destruction of the entangled pairs, making them unavailable for further processing. We have implemented efficient and non-destructive entanglement purification with atomic (ion) quantum bits in a multi-zone trap. Two noisy entangled pairs were created and distilled into one higher fidelity pair available for further use. Success probabilities were above 35 {\%}. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W33.00009: Entanglement Dynamics of Two-Atom Jaynes-Cummings Model under Phase Telegraph Noise Huseyin Karacali, Resul Eryigit We have analyzed the entanglement dynamics in a two atom one-field mode Jaynes-Cummings model with stochastic atom-field interactions. The phase of the interaction term is subject to a two level telegraph noise which is characterized by a dwell time and jump magnitude. We have investigated the effect of noise characteristics on the entanglement between the field and the atom as well as atom1 and atom2. The field-atom entanglement is found to be damped to zero because of the noise, as expected. However,the noise is found to be cooperative for the atom entanglement, as it approaches 0.25 in the long time limit independent of the characteristics of the noise. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W33.00010: Nonequilibrium thermal entanglement Luis Quiroga, Ferney Rodriguez, Maria Ramirez, Roberto Paris For quantum systems in contact with heat reservoirs at an unique and fixed temperature the equilibrium thermal entanglement has been extensively studied. However, the entanglement of nonequilibrium quantum systems has been scarcely considered. New possibilities for entanglement production and manipulation in nonequilibrium situations, where quantum coherences are dominant, are emerging. The aim of the present work is to correlate thermodynamical nonequilibrium steady-state features with entanglement properties of quantum nanosystems. We show an intimate relationship between stationary heat current, entropy production rate and entanglement for a simple quantum system coupled to two heat baths at different temperatures. We show that while the quantum informational entropy remains constant in a steady-state situation, the rate of production of thermodynamic entropy is linearly proportional to the nonequilibrium concurrence. We find an enhanced-suppressed entanglement transition which takes place when a temperature gradient is applied. Additionally, a temperature gradient allows for producing quantum states with exactly the same amount of entanglement as for an equilibrium situation but with different entropies and heat currents. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W33.00011: Thermal entanglement in two double quantum dots Lesvia-Debora Contreras-Pulido, Fernando Rojas Entanglement has become an important resource in quantum information processing, it is important to quantify the degree of entanglement between two qubits at a finite temperature [1]. As charge qubits realized in double quantum dots are promising solid state candidates for both qubit and entangled states realization, we explore theoretically the non-zero entanglement generation between two charge qubits surrounded by a thermal environment and under the effect of an external asymmetric electric field. The qubits array is described by a Hubbard-type Hamiltonian which is diagonalized in order to calculate the equilibrium thermal density matrix, used to quantify the entanglement by Wootters' concurrence. We find that the qubits exhibit thermal entanglement that vanishes at finite temperature and we show that concurrence depends on both tunneling and the external potential. The critical value for which concurrence vanishes presents a parabolic dependence with the external potential, which is a controllable parameter in quantum dots through gate voltages. We acknowledge support from projects DGAPA-IN114403 and CONACyT -43673-F. [1]. Arnesen et al., Phys.Rev.Lett 87,017901 (2001) [2]. Wootters Phys. Rev. Lett. 80, 2245 (1998) [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W33.00012: Comparisons of experimental entanglement witnesses and measures for a two-qubit system K.A. Walsh, A.J. Schauf, J.S. Lesniak, E.C. Behrman, J.E. Steck, S.R. Skinner A single operator cannot measure the entanglement for a general state; however, knowledge of the density matrix enables computation of any universal entanglement measure. Several entanglement witnesses have been proposed for two-qubit systems. We compare these to published measures. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W33.00013: A non-Markovian optical signature for detecting entanglement in coupled excitonic qubits Ferney Rodriguez, Luis Quiroga, Neil Johnson We identify an optical signature for detecting entanglement in experimental nanostructure systems comprising coupled excitonic qubits. This signature owes its strength to {\em non-Markovian} dynamical effects in the second-order temporal coherence function of the emitted radiation. We calculate autocorrelation and cross-correlation functions for both selective and collective light excitation, and prove that the coherence properties of the emitted light do indeed carry information about the entanglement of the initial multi-qubit state.We also show that this signature can survive in the presence of a noisy environment. [Preview Abstract] |
Session W34: Biological Physics and Bacterial Behaviors
Sponsoring Units: DBPChair: Robert Austin, Princeton University
Room: Colorado Convention Center 404
Thursday, March 8, 2007 2:30PM - 2:42PM |
W34.00001: Near-Perfect Adaptation in the {\it E. coli} Chemotaxis Signal Transduction Network Yang Yang, Sima Setayeshgar Biochemical reaction networks constitute the computing language of the cell, from converting external stimuli into appropriate intracellular signals to regulating gene expression. Precise adaptation is an important property of many signaling networks, allowing compensation for continued stimulation without saturation. Furthermore, a common feature of intracellular reaction networks is the ability to operate in a noisy environment where concentrations of key components, such as signaling molecules and enzymes controlling reaction rates are typically small and therefore fluctuations in their numbers are significant. In the context of the well- characterized {\it E. coli} chemotaxis signal transduction network, we present a new computational scheme that explores surfaces in the space of total protein concentrations and reaction rates on which (near-)perfect adaptation holds. The resulting dependencies between parameters provide conditions for (near-)perfect adaptation as well as ranges of numerical values for parameters not reliably known from experiments. We generalize the applicability of this scheme to other signaling networks. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W34.00002: Adaptation, Bacteria and Maxwell's Demons Peter Galajda, Juan E. Keymer, Robert H. Austin We propose a method to study the adaptation of bacterial populations with an asymmetric wall of Maxwell Demon openings. A Maxwell Demon opening is a funnel which is easier to enter than to leave. The interaction of swimming cells with such a Maxwell Demon Wall results in a population density separation, in apparent (but not real) violation of the Second Law of Thermodynamics, as we will show. Bacteria can be exposed to spatial challenges in order to move to e. g. higher food levels. The question we address in these experiments is: do the bacteria adapt and overcome the Maxwell Demon Wall? [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W34.00003: Effects of Noise on Ecological Invasion Processes: Bacteriophage-mediated Competition in Bacteria Jaewook Joo, Harvill Eric, Reka Albert Pathogen-mediated competition, through which an invasive species carrying and transmitting a pathogen can be a superior competitor to a more vulnerable resident species, is one of the principle driving forces influencing biodiversity in nature. Using an experimental system of bacteriophage-mediated competition in bacterial populations and a deterministic model, we have shown in [Joo et al 2005] that the competitive advantage conferred by the phage depends only on the relative phage pathology and is independent of the initial phage concentration and other phage and host parameters such as the infection-causing contact rate, the spontaneous and infection-induced lysis rates, and the phage burst size. Here we investigate the effects of stochastic fluctuations on bacterial invasion facilitated by bacteriophage, and examine the validity of the deterministic approach. We use both numerical and analytical methods of stochastic processes to identify the source of noise and assess its magnitude. We show that the conclusions obtained from the deterministic model are robust against stochastic fluctuations, yet deviations become prominently large when the phage are more pathological to the invading bacterial strain. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W34.00004: Synchronized Cycles: An allosteric model of the cyanobacterial circadian oscillator David Lubensky, J.S. van Zon, P. Altena, P.R. ten Wolde In a remarkable experiment, Nakajima et al. [Science, 2005] showed that the 3 cyanobacterial clock proteins KaiA, KaiB, and KaiC are sufficient to generate circadian phosphorylation of KaiC \textit{in vitro}. This system is thus a rare example of a functioning biochemical circuit that can be reconstituted in the test tube. Theoretically, it presents the further challenge that the only reactions driven out of equilibrium are those associated with KaiC phosphorylation and dephosphorylation. Here, we present a model of the Kai system. At its heart is the assumption, motivated by classical models of allostery, that each KaiC hexamer to tends to be phosphorylated in a cyclic manner. For macroscopic oscillations to be possible, however, the cycles of the different hexamers must be synchronized. We propose a novel synchronisation mechanism that allows us to reproduce a wide range of published data, including temperature compensation of the oscillation period, and to make nontrivial predictions about the effects of varying the concentrations of the Kai proteins. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W34.00005: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W34.00006: The \textit{E. Coli} Response To A Phage Perturbation Emily Chapman-McQuiston, Xiao-Lun Wu Bacteria have evolved a variety of defenses against extreme environmental pressure. While a majority of the population dies during times of stress, a portion of the population continues to survive due to the cell's phenotypic state. We study the response of the bacterial system to attack by a particular virus called lambda phage. During times of phage attack bacteria continue to create and lose receptors making the bacteria more or less sensitive to the applied phage concentration. We use experiment and modeling to study how the creation and loss of receptors affects the response and recovery of the bacterial population due to an applied phage pressure. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W34.00007: Spatio-Temporal Analysis of Cell-Cell Signaling in a Living Cell Microarray Utkur Mirsaidov, Winston Timp, Kaethe Timp, Paul Matsudaira, Greg Timp Cell-cell signaling plays a central role in biology, enabling individual cells to coordinate their activities. For example, bacteria show evidence of intercellular signaling through \textit{quorum sensing}, a regulatory mechanism that launches a coordinated response, depending on the population density. To explore the spatio-temporal development of cell-to-cell signaling, we have created regular, heterotypic microarrays of living cells in hydrogel using time-multiplexed optical traps for submicron positional control of the cell orientation and location without loss of viability. We studied the \textit{Lux} system for quorum sensing; splitting it into sender and receiver plasmids, which were subsequently introduced into \textit{E. Coli}. Induced by IPTG, the sender cells express a fluorescent reporter (mRFP1) and the \textit{LuxI} enzyme that catalyzes the synthesis of a molecular signal AHL that diffuses through the cell membrane and the extra-cellular scaffold. The receiver cells collect the AHL signal that binds to the \textit{LuxR} regulator and reports it through GFP production. We have measured the time-delay between the onset of mRFP1 and GFP dependence on intercellular spacing in the array. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W34.00008: \textit{in silico} simulation and analysis of microbial metabolism. Sheng Hui, Shenghua Liang, Lei-Han Tang Through evolution living organisms have developed an elaborate network of enzyme-facilitated reactions and transport to process and cycle biochemical compounds for cell growth. A majority of these reactions are uni-directional, yet the network allows an organism to live on a variety of carbon sources and synthesize a diverse set of compounds in varying amounts. We found that biosynthesis of the end products can proceed independently. In the three genome-wide \textit{in silico} models examined, the optimal yield for simultaneous synthesis of two compounds is only about 3{\%} higher than what is achievable under separate production of individual compounds. In most cases, the residual correlation can be attributed to the requirement of energy, redox potential, or charge balance. These observations quantify, in the context of cellular metabolism, the bow-tie analogy which has been argued to provide a ubiquitous architecture for multi-input/multi-output networks. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W34.00009: Study of Signal Detection, Integration, and Propagation in Quorum Sensing at the Single Cell Level Tao Long, Bonnie Bassler, Ned Wingreen Bacteria respond to their environment and to each other and accordingly adjust their gene-expression levels. Accurate signal detection, appropriate signal integration, and faithful signal propagation are essential for a cell to make correct adjustments in response to various extracellular cues. To better understand this information processing by living cells, we studied a model system -- the quorum-sensing circuit in \textit{Vibrio harveyi}. Quorum sensing is a process in which bacteria communicate with each other by diffusible chemical molecules, termed ``autoinducers'', to commit to coordinated developmental decisions. Three types of autoinducers are detected coincidently by three parallel receptors. The signals are then integrated into the same signaling pathway and propagated by phosphorylation or dephosphorylation of the pathway components. To quantitatively measure the intracellular response, we applied a fluorescent protein reporter, whose production is regulated by a phosphorylated protein in the pathway. By single-cell microscopy, we can explore features of this information-processing circuit such as coincidence detection, signal integration, noise reduction or filtering, and especially the fidelity in signal processing achieved in the presence of inevitable fluctuations. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W34.00010: Evolution of Mutation Rate in Asexual Populations Scott Wylie, Herbert Levine, David Kessler Several evolution experiments with \textit{E. coli} document the spontaneous emergence and eventual fixation of so called ``mutator'' alleles that increase the genomic mutation rate by the order of 100-fold. Variations in mutation rates are due to polymorphisms in the molecular machinery that copies and checks the genome for errors. These polymorphisms are coded in the genome and thus heritable. Like any heritable trait, elevated mutation rates are subject to natural selection and evolution. However, unlike other traits, mutation rate does not directly affect the rate at which an organism reproduces, i.e. its fitness. Rather, it affects the statistical distribution of the offspring's fitness. This fitness distribution, in turn, leads via ``hitchhiking'' to a change in the frequency of the mutator allele, i.e. evolution of the mutation rate itself. In our work we simulate a birth-death process that approximates simple asexual populations and we measure the fixation probability of rare mutators. We then develop an approximate analytic model of the population dynamics, the results of which agree reasonably well with simulation. In particular, we are able to analytically predict the ``effective fitness'' of mutators and the conditions under which they are expected to emerge. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W34.00011: Control of growth and adaptation to nutritional shifts for bacteria exposed to amino acid-limiting environments Eduard M. Mateescu, Terence Hwa In order to grow at the highest rate sustainable by the environment, bacteria turn on different metabolic pathways and utilize a myriad of adaptive strategies. The macromolecular composition (RNA, DNA, protein) and overall cell size (mass) can be very different in different environments. Surprisingly however, these differences appear to depend only on the growth rate and not on the growth medium itself. As the nutritional environment changes in time, the cells quickly adapt their composition to the one corresponding to the new conditions. Here, we propose a phenomenological model of growth and adaptation control for the bacterial cell, based on a simplified formulation of the central dogma and a simplified implementation of the stringent response. The core model contains no free parameters and provides a simple intuitive understanding of cell growth control. The results generated by the model, physiological state of the cell as well as the characteristics of the transition between optimized states of growth, are in qualitative and semi-quantitative agreement (i.e. within a factor of 2) with the experimental observations. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W34.00012: Phase Transitions in Bacterial Cultures Hanna Salman, Anton Zilman, Albert Libchaber We study how the concentration of bacteria affects their response to temperature changes. The bacteria are grown in a batch mode culture, which affects their physiological state due to nutrient depletion. For bacteria at a constant physiological state, we observe a critical transition in behavior in a one-dimensional temperature gradient as their initial concentration in the sample increases. Above a concentration of 10$^{8}$cells/cm$^{3}$, an early accumulation near their favored temperature, caused by thermotaxis, develops into a sharp pulse moving at a fast velocity ($\sim $3$.$5 \textit{$\mu $}m$/$sec). This mode is the result of a positive feedback mechanism provided by inter-bacterial communication. A theoretical model describing this interaction shows good agreement with the experimental results. For different physiological states, we observe a critical transition in the bacterial response to localized heating by infrared laser. When the bacteria are grown to concentrations below 2x10$^{8}$cells/cm$^{3}$ they swim towards the heated region; when they are grown beyond this concentration they escape from the heated region. This effect is reversible. Also, mixing populations from different physiological states does not affect the response of either population. A genetic switch controlled by the nutrients' availability seems to be responsible for this behavior. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W34.00013: Positioning of receptor clusters along the bacterial cell wall Ranjan Mukhopadhyay, Hui Wang, Yigal Meir, Ned Wingreen Chemotaxis receptors in {\it E. coli} form clusters that are located at the cell poles and also laterally along the cell body, and clustering plays an important role in signal transduction. Recently, experiments using fluorescence imaging, have studied cluster dynamics during cell growth and found that lateral clusters transiently localize at positions approximately periodically spaced along the cell body. We have studied a lattice model of the dynamics of receptor clustering in the presence of cell growth. In this talk we will present results from our model and explore whether lateral cluster positioning could arise spontaneously from receptor clustering dynamics or whether the experimental results indicate the existence of periodically positioned markers along the cell wall that are targeted by the receptors. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W34.00014: Genome-scale reconstruction of the metabolic network in {\it Yersinia pestis} CO92 Ali Navid, Eivind Almaas The gram-negative bacterium {\it Yersinia pestis} is the causative agent of bubonic plague. Using publicly available genomic, biochemical and physiological data, we have developed a constraint-based flux balance model of metabolism in the CO92 strain (biovar {\it Orientalis}) of this organism. The metabolic reactions were appropriately compartmentalized, and the model accounts for the exchange of metabolites, as well as the import of nutrients and export of waste products. We have characterized the metabolic capabilities and phenotypes of this organism, after comparing the model predictions with available experimental observations to evaluate accuracy and completeness. We have also begun preliminary studies into how cellular metabolism affects virulence. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W34.00015: Modeling of the Effect of Dynamical Changes of Cell Geometry on MinCDE Oscillations During Cell Division in \textit{E. coli}. Jason Ellis, Diane Stroup, Michael Lee {In the process of cell division in \textit{E. coli}, spatio-temporal oscillations of the MinCDE proteins act to determine the specific site of FtsZ-ring formation which initiates the process of cell separation. The reaction diffusion processes which drive the biochemical oscillations of the MinCDE system have been studied and we have developed a model which incorporates the dynamics of these oscillations while cell division is accomplished through the formation of the peptidoglycan wall at the location of the FtsZ-ring. This model investigates the mechanisms that cause observed protein segregation in the daughter cells as well as the changes in oscillation characteristics observed between early and late stages of cell growth. Simulations of this model are carried out in space and time based on the reaction diffusion dynamics of individual proteins. The model allows the investigation of effects of cell geometry for both the normal cylindrical rod geometry; as well other hypothetical geometries not easily accessible in laboratory cultures.} [Preview Abstract] |
Session W35: Dynamic Forces at Cell and Subcell Levels
Sponsoring Units: DBPChair: Shane Hutson, Vanderbilt University
Room: Colorado Convention Center 405
Thursday, March 8, 2007 2:30PM - 2:42PM |
W35.00001: Correlation of Force Production with Apoptosis in Tissue Dynamics Yusuke Toyama, Xomalin Peralta, Stephanos Venakides, Daniel Kiehart, Glenn Edwards To understand embryo morphogenesis, it is necessary to know the force distribution in the various tissues. Since cells are largely inaccessible to mechanical probes \textit{in vivo}, measurements of the net forces exerted by cells are challenging. The combination of experimental and theoretical approaches has proven to improve our understanding of these forces. A steerable UV-laser microbeam was used to probe the forces and the resulting kinematics were monitored with confocal microscopy. Dorsal closure is a developmental stage in \textit{Drosophila} embryogenesis, where the dynamics are a consequence of four biological processes [1]. During this stage, cells that have outlived their usefulness undergo apoptosis, a biological process also known as programmed cell death for cells. Apoptotic events were decreased with genetic techniques or increased by irradiation with a UV-C lamp. We present experimental evidence for force generation correlating with apoptosis. This research has been supported by the NIH (GM33830 and GM61240). [1] M. S. Hutson, et al. Science, \textbf{300}, 145 (2003). [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W35.00002: Recoil Dynamics after Laser Ablation of Single Cell Edges in Embyronic Epithelia Xiaoyan Ma, M. Shane Hutson In order to determine the interfacial tensions along cell-cell boundaries in living fruit fly (\textit{Drosophila}) embryos, we have developed a microsurgical method based on laser ablation and laser-scanning confocal microscopy. Following ablation of one cell edge, we follow the recoil dynamics (strain relaxation) of adjacent GFP-labeled cell edges (with time resolution down to 2 ms). The recoils are consistently fit best by a double exponential decay with one time constant around 80 ms and the other around 1.2 s. The initial recoil velocities are in the range of 10-20 $\mu $m/s. We observe the same biphasic strain relaxation in multiple (N = 60) embryos at different developmental stages. Both recoil time constants are much longer than either the plasma lifetime or the duration of cavitation. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W35.00003: Confounding Effect of Spot-Size on the Wavelength-Dependence of Tissue Ablation Metrics M. Shane Hutson, Gilma Adunas, Yaowu Xiao Tunable free-electron lasers have been used in several previous studies to investigate the mid-IR wavelength-dependence of tissue ablation. These studies gave conflicting results on an important question: do the ablation metrics depend on targeting the laser energy to a water or protein vibration? Here, we investigate the effects of two parameters that varied widely in previous studies -- fluence and focused spot-size. We measured ablation threshold, etch depth and collateral damage in porcine corneas for a set of five matched wavelengths -- same absorption coefficients, but different primary chromophores. Although the ablation thresholds are similar, the slope of etch depth versus fluence (ablation efficiency) differs by up to a factor of five. These differences are most strongly dependent on the focused spot diameter, not wavelength. When spot sizes are matched, protein-targeting wavelengths still leave less collateral damage, but they remove tissue less efficiently. The confounding roles of fluence and spot size have strong implications for the interpretation of previous wavelength-dependent results. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W35.00004: Nanosecond Infrared Laser for Tissue Ablation G.S. Edwards, R.D. Pearlstein, M.L. Copeland, M.S. Hutson, K. Latone, A. Spiro, G. Pasmanik The Mark-III Free-Electron Laser (FEL), operating at the 6.45$\mu $m wavelength, has been used successfully in human surgery. Due to the FEL's size and cost, there has been interest in the development of a compact, inexpensive infrared laser for human surgical applications. We have investigated the role of the FEL superpulse, leading to the prediction that nanosecond pulses can satisfy the dynamic criteria for tissue ablation. We have developed a laser based on difference frequency mixing and stimulated Raman scattering with four stages of frequency conversion, emitting at a wavelength of 6.45$\mu $m with 3-5ns pulse duration, pulse energies of up to 2mJ, and a pulse repetition rate of 3MHz. The laser system successfully ablated tissue, where collateral thermal damage was limited to several microns. In the future, it will be necessary to increase the pulse repetition rate to achieve an ablation rate acceptable for human surgery. We acknowledge the grant support: R43 RR018435, N00014-99-1-0891, and F49620-00-1-0370. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W35.00005: Morphogenic asymmetries in tissue dynamics Xomalin G. Peralta, Y. Toyama, R. Montague, S. Venakides, D.P. Kiehart, G.S. Edwards Structural and kinematic symmetries in living organisms arise from the forces responsible for tissue movements during development. Tissue dynamics during dorsal closure, a stage of {\it Drosophila} development, provide a model system for cell sheet morphogenesis. It is characterized by tissue movements, driven by four biological processes which are coordinated in space and synchronized in time. Quantifying morphogenic asymmetries is essential for understanding the spatial and temporal differences in the contributing processes, the extent to which they can vary and still result in successful closure. They also provide a basis for understanding dynamic changes that occur to compensate for perturbations. We measured spatial, kinematic and dynamic asymmetries to biophysically characterize natural asymmetries in unperturbed closure, resiliency to laser perturbations and failure of closure in some mutant embryos. We found an asymmetric upregulation of a biological process in response to laser perturbations. In the mutants, there is a reversed asymmetry. Supported by NIH (GM33830 and GM61240). [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W35.00006: The detection of cancer in living tissue with single-cell precision and the development of a system for targeted drug delivery to cancer Adam Fields, Sean Pi, Alex Ramek, Taylor Bernheim, Jessica Fields, Nadine Pernodet, Miriam Rafailovich The development of innovations in the field of cancer diagnostics is imperative to improve the early identification of malignant cells within the human body. Two novel techniques are presented for the detection of cancer cells in living tissue. First, shear modulation force microscopy (SMFM) was employed to measure cell mechanics of normal and cancer cells in separate and mixed tissue cultures. We found that the moduli of normal keratinocytes were twice as high as the moduli of SCC cancerous keratinocytes, and that the cancer cells were unambiguously identifiable from a mixture of both kinds of cells. Second, confocal microscopy and the BIAcore 2000 were used to demonstrate the preferential adhesion of glass micro-beads impregnated with fluorescent dye to the membranes of cancer cells as compared to those of normal cells. In addition to their use as a cancer detection system, these hollow and porous beads present a model system for targeted drug delivery in the treatment of cancer. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W35.00007: Characterizing Cell Mechanics with AFM and Microfluidics N. Walter, A. Micoulet, S. Suresh, J.P. Spatz Cell mechanical properties and functionality are mainly determined by the cytoskeleton, besides the cell membrane, the nucleus and the cytosol, and depend on various parameters e.g. surface chemistry and rigidity, surface area and time available for cell spreading, nutrients and drugs provided in the culture medium. Human epithelial pancreatic and mammary cancer cells and their keratin intermediate filaments are the main focus of our work. We use Atomic Force Microscopy (AFM) to study cells adhering to substrates and Microfluidic Channels to probe cells in suspension, respectively. Local and global properties are extracted by varying AFM probe tip size and the available adhesion area for cells. Depth-sensing, instrumented indentation tests with AFM show a clear difference in contact stiffness for cells that are spread of controlled substrates and those that are loosely attached. Microfluidic Channels are utilized in parallel to evaluate cell deformation and ``flow resistance'', which are dependent on channel cross section, flow rate, cell nucleus size and the mechanical properties of cytoskeleton and membrane. The results from the study are used to provide some broad and quantitative assessments of the connections between cellular/subcellular mechanics and biochemical origins of disease states. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W35.00008: Measurement of the adhesion and elasticity of single cells using a novel micropipette-based technique Marie-Josee Colbert, Adam N. Raegen, Cecile Fradin, Kari Dalnoki-Veress Numerous biological processes have to go through cell adhesion, which makes the fundamental study of the adhesion of cells on solid substrates a key research topic in cellular biophysics. We will present our work on the elasticity and adhesion of a single liposome on a substrate. A vesicle is held at the end of a micropipette mounted on a micromanipulator and put into contact with a surface. We developed a technique to directly measure adhesion using the spring-constant of an L-shaped micropipette when pulling the vesicle from the substrate. The deflection is used to determine the adhesion force of cells as well as a cell’s elasticity. Since the force applied on the cell is known at every moment of the experiment, this technique enables dynamical measurements. The links between the adhesion strength and the surface tension will also be discussed. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W35.00009: Measuring the Interaction between Cell-Surface Markers and Substrate-Coupled Proteins as a Means of Determining Cell Membrane Fluidity Andrea Carbonaro, Lucy A. Godley, Lydia L. Sohn We have analyzed the detailed interaction between cell-surface markers and substrate-coupled proteins by measuring the transit time of individual cells as they pass through a functionalized pore. Cells that have a specific cell-surface marker will transiently interact with the walls of a pore that are functionalized with a correspondingly specific protein. This interaction results in the cell moving slowly through the pore. In contrast, cells that do not express the specific marker will not interact with the functionalized walls and will pass quickly through the pore. The distribution of transit times measured for interacting cells can be explained in terms of the number of ligand-receptor bonds created between the immobilized proteins on the pore wall and the cell-surface receptors. We will show that this number is a function of both the ligand and receptor densities on the pore and cell membrane, respectively, as well as the fluidity of the cell membrane. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W35.00010: Cooperativity of Integrin-mediated Adhesion on Nanopatterned Substrates Christine Selhuber, Thorsten Erdmann, Ulrich Schwarz, Horst Kessler, Joachim Spatz Surfaces of defined adhesion properties are required for a physical understanding of cell adhesion in vivo. In this work, biofunctional nanopatterns are employed, which allow adhesion ligands to be positioned in a quasi-hexagonal lattice. Such nanopatterns are used to investigate integrin-mediated cell adhesion, which is a highly complex biological process and essential for numerous cell functions. With nanopatterns the distance between adjacent integrin binding sites is precisely defined. Cell culture experiments have revealed that this distance strongly affects cell adhesion and the formation of adhesion clusters, known as focal contacts. To quantify the adhesion cluster formation for different integrin binding site spacings, cell adhesion forces were studied using atomic force microscopy (AFM). The experiments demonstrate that an integrin binding site spacing of 70 nm and more prevents the cooperative formation of early adhesion clusters. In long-term adhesion studies, after several hours of cell adhesion, it turned out that focal contact formation cooperatively increases the local adhesion strength. The obtained results were related to theoretical models on adhesion cluster stability. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W35.00011: Dynamics of Cell Migration for cells embedded in Collagen using a multimodal platform of Optical Coherence Tomography, Multi-Photon excitation and Second Harmonic Generation Kandice Tanner, Shuo Tang, Enrico Gratton We developed Raster Image Correlation Spectroscopy (RICS) to analyze the dynamics of cell migration from data obtained on a confocal multi-photon microscope. We assembled a microscope that can simultaneously measure the scattering signal from optical coherence tomography (OCT), multi-photon excited emission (TPEF) and second harmonic signals (SHG) with comparable spatial resolution and the same time resolution. We present data here showing the combined 3-D images of the cells embedded in a collagen matrix. The OCT signal adds fine structural information of the cellular morphology and collagen which is enhanced by the SHG image. The RICS analysis of the TPEF signal gives the dynamics of the GFP --style proteins. We show that the cell morphology and the distribution of cell organelles are different in the collagen matrix than what is observed in cells growing on flat surfaces. Using the three modalities of cell imaging we could reach a more realistic interpretation of cell dynamics in tissue. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W35.00012: Coordinated Buckling of Microtubule Bundles Produces the Long Wavelength of Microtubule Birefringent Pattern Yongxing Guo, Yifeng Liu, Allan Bower, Jay Tang, James Valles Aligned microtubule (MT) bundles spontaneously form, elongate and buckle in high concentration tubulin solutions that are subjected to a field that initially aligns the microtubules. The nesting of the buckled bundles produces a macroscopic birefringence pattern of stripes. Of interest here is the buckling wavelength, which controls the stripe width. It is shorter than the fundamental wavelength expected in classic Euler buckling and longer than the wavelength expected for the buckling of a single MT bundle within the elastic network formed by the dispersed MTs. We present a mechanical buckling model that accounts for this intermediate wavelength. It shows that the wavelength is shorter than the fundamental one because of the lateral reinforcement by the MT network, and longer than the wavelength expected for a single laterally reinforced bundle due to the coordinated buckling of the neighboring bundles. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W35.00013: Cell elasticity as a function of actin expression Carsten St\"uber, Josef K\"as The deformation response to an external force of an eurkaryotic cell mainly depends on its cytoskeletal composition. Theoretical models have been introduced to quantify the concentration dependence of the different cytoskeletal components to the elastic strength of cells. Verifying the models experimentally, the optical stretcher, a two beam optical trap, is used to elongate fibroblast cells. These fibroblasts are transfected with GFP-actin, which leads to an overexpression of actin within the cell and allows to determine the actin concentration using fluorescence image analysis. The dependence of the elasticity on the actin concentration of fibroblasts shows a softening of the cell with increasing number of actin filaments. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W35.00014: Measuring mitotic spindle formation through connection graphs Stuart Schaffner, Jorge Jose The mitotic spindle, an important structure formed during biological cell division, consists of a pattern of stiff fibers called microtubules and crosslinking molecular motor complexes. The spindle, consisting of objects interacting through pairwise interactions, is well suited to study via its connection graph. Thermal motion is important in this system; molecular motors attach and detach randomly from the microtubules, but only where the geometry allows. We have found the connection graph approach to be helpful in several ways for analyzing spindle properties. Monitoring the number and size of connected components in the graph allows us to quantify the development of the spindle bipolar pattern. Minimum cut-sets in components measure spindle pole robustness. These computations not only allow us to measure the dynamics of initial pattern formation, but also the structural rearrangements within spindles that have already formed. Our results are compared to known experimental results. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W35.00015: ABSTRACT WITHDRAWN |
Session W38: Focus Session: Negative Index Materials: Concepts to Applications II
Sponsoring Units: FIAPChair: Evgenii Narimanov, Princeton University
Room: Colorado Convention Center 501
Thursday, March 8, 2007 2:30PM - 3:06PM |
W38.00001: Negative-Index Metamaterials in the Visible Range Invited Speaker: In conventional materials, out of the two field components of light, electric and magnetic, only the electric one (``electric hand'') efficiently couples to and probes the atoms of a material while its ``magnetic hand'' remains almost unused because the interaction of atoms with the magnetic filed component of light is normally very week. Metamaterials, i.e. artificial materials with rationally designed properties, can enable the coupling of\textit{ both} field components of light to meta-atoms, enabling entirely new optical properties and exciting applications with such ``two-handed'' light. Metamaterials are expected to open a gateway to unprecedented electromagnetic properties and functionality unattainable from naturally occurring materials. Negative-refractive index metamaterials create entirely new prospects for guiding light on the nanoscale, some of which may have revolutionary impact on present-day optical technologies. The extraordinary nonlinear optical properties of negative-index metamaterials are also discussed. We review this new emerging field of metamaterials and recent progress in demonstrating a negative refractive index in the optical and visible range, where applications can be particularly important, including sub-wavelength imaging and cloaking objects, i.e. making them invisible. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W38.00002: Simulations of ferrite-dielectric-wire composite negative index materials Frederic Rachford, Douglas Armstead, Vincent Harris, Carmine Vittoria We have performed extensive finite difference time domain (FDTD) simulations to design ferrite based negative index of refraction (NIM) composites. Our simulations center on the use of Barium M type ferrite with in-plane anisotropy. A wire grid is employed to provide negative permittivity. The ferrite and wire grid interact to provide both \underline {negative} and \underline {positive} index of refraction transmission peaks in the vicinity of the BaM resonance. We find that the wires and the ferrite must be spatially separated by a low loss dielectric (Mylar). The ferrite and dielectric media are modeled as thin lamina with a mono-directional wire grid centered in the dielectric lamina. The ferrite and dielectric lamina are paired with combined thickness equal to the square wire grid lattice distance. We assume the presence of a in plane orienting magnetic field. Working with thin planar oriented ferrite lamina implies that the composites will have a negative index in only one direction of propagation. Notwithstanding the extreme anisotropy in the index of refraction of the composite, negative refraction is seen at the composite air interface allowing the construction of a focusing concave lens with magnetically tunable focal length. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W38.00003: Superlens with a virtual focus Alexei Efros Recently there has been growing interest in the creation of lenses with a very sharp focus. Those lenses are a). The Veselago lens that is a slab of the material where both $\varepsilon $and $\mu $ are negative and b). The quasistatic lens where only one of those two is negative. Pendry claimed that in the absence of absorption both types of lenses are perfect. It has been shown very soon that the supersolution, proposed by Pendry does not exist for any lens with a real focus. In this presentation I propose a lens with a virtual focus (VF) and show that without absorption this focus is perfect. It happens due to the Pendry amplification of the evanescent waves, but in the case of the VF these arguments do not contradict to any general theorem. The supersolution exists without any absorption. The VF is located either in front of the slab or inside the slab. In fact, there is no maximum of the field in the VF itself, but this focus may be at the point that is very close to the rare interface of the slab, but still inside the slab. Then the field at the interface will have a very narrow maximum. Its width tends to zero as the location of the VF tends to the interface. The effect of small absorption is considered. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W38.00004: Filtering Components in Optical Nanocircuits Using Plasmonic Metamaterials Nader Engheta, Andrea Alu Following our recently developed concept of lumped nanocircuit elements at optical frequencies, here we apply analytical and numerical techniques to demonstrate that various arrangements of such optical nanoelements may provide relatively complex optical nanocircuits with various filtering functions, such as bandpass filters, in optical domains. These optical nanocircuit units, operating as nanofilters, are sub-wavelength in size and are formed by collecting and arranging plasmonic and non-plasmonic nanostructures in proper orders and topology in a planar geometry. Our study shows how such optical nanocircuits may provide resonant responses, with transfer functions that allow filtering the incoming optical signals at will. The input signal may be coupled into such nanofilters using plasmonic nanoantennas, and the output of such elements may be coupled with another section of the nanocircuit or connected to a nanowaveguide carrying out the signal, mimicking the functionalities of low-frequency filters in RF circuits. We present samples of our theoretical results and discuss related physical insights. Future steps in this work will also be mentioned. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W38.00005: Experimental Observation and Theory of Optical Magnetism in Dielectrics Samuel Oliveira, Stephen Rand We report the power dependence of extremely intense linear magnetic response to non-relativistic optical fields in water, benzene and carbon tetrachloride at room temperature. Quantitative agreement is obtained with the multipole expansion, showing that transient magnetic dipoles can be as large as half the electric dipole moment under identical conditions at optical frequencies in dielectrics. This discovery will enable optical magnetic resonance and tunable negative index behavior in low-loss media. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W38.00006: Coupling to nanoscale negative-refraction planar waveguides Robyn Wangberg, Viktor Podolskiy Negative index non-magnetic strongly anisotropic waveguides have been shown to provide efficient beam steering and manipulation in nanoscale areas with applications that include sub-diffraction planar lens imaging and photonic funnels. In this work we study the coupling to and from sub-wavelength planar waveguides of different sizes and compare the transmission through a negative-index structure to the Bethe prediction for positive index materials. We simulate EM wave propagation and imaging in arbitrary waveguide configurations with a focus on designing and optimizing planar-waveguide based beam-steering photonic devices. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W38.00007: A spectroscopic multi-angle probe of metamaterial permeability Tom Driscoll, Sabarni Palit, Willie Padilla, Tong Ren, Jack Mock, Gregory Andreev, Sang-Yeon Cho, Nan Marie Jokerst, David Smith, Dimitri Basov We present spectroscopic data for acute-angle reflection and transmission of two planar metamaterial samples, one at microwave frequencies and one at far-infrared frequencies. We show that the observed evolution of the magnetic resonance with the increasing angle of incidence is in quantitative agreement with the Fresnel expressions. This finding uncovers a convenient approach towards the retrieval of optical constants of metamaterials by fitting the Fresnel theory to multiple angle-datasets simultaneously; thus obviating the need for phase information. Physically reasonable values for the magnetic permeability are recovered by this no-phase fitting in the case of both the microwave and infrared metamaterials. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W38.00008: Ensembles of plasmonic nanoparticles at optical frequencies for negative index media A.M. Bratkovsky, E. Ponizovskaya Metamaterials in a form of an array of metallic nanoparticles support collective plasmonic excitations that are believed to be responsible for various unusual phenomena, like surface enhanced Raman scattering (SERS). The array is analogous to a dual structure for a metal film with periodic array of holes, which can provide an extraordinary transmission. We have investigated the electromagnetic response of ordered films of metallic nanoparticles. In particular, we looked at various cubic, close- packed, and columnar structures. This included the nanoparticle realization of G.Eleftheriades' structure that was speculated by N.Engheta et al. to produce negative index medium (NIM). It appears that open- and close-packed arrays behave similarly: there are plasmon resonances and high transmission of certain wavelength that is considerably larger than the separation between the particles, yet {\bf no negative index behavior}, unlike in e.g. ``fishnet'' metal-dielectric heterostructures that are NIM[1]. We discuss a realization of ``transparent metal'' metamaterial with emphasis on ``channeling'' of radiation in columnar structures. [1] W. Wu et al., cond-mat/0610352, to appear in Appl. Phys. A, Special Issue on Negative Index Metamaterials. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W38.00009: Optical Hyperlens: Far-field imaging beyond the diffraction limit Zubin Jacob, Leonid Alekseyev, Evgenii Narimanov We propose an approach to far-field optical imaging beyond the diffraction limit. The proposed system allows image magnification, is robust with respect to material losses and can be fabricated by adapting existing metamaterial technologies in a cylindrical geometry. This device relies on recently proposed strongly anisotropic metamaterials that feature opposite signs of the two permittivity tensor components, $\varepsilon _{\bot }$ and $\varepsilon _{\vert \vert }$ [1-2]. Such metamaterials have been theoretically shown to support propagating waves with very large wavenumbers (in ordinary dielectrics, such high-$k $modes undergo evanescent decay) [3]. This unusual property arises from the \textit{hyperbolic }functional form of the dispersion relation for such metamaterials, and is the key feature enabling subwavelength resolution of our proposed device. It is for this reason that we call our imaging device \textit{the hyperlens}. [1] V. A. Podolskiy, and E. E. Narimanov, ``Strongly anisotropic waveguide as a nonmagnetic left-handed system,'' Phys. Rev. B \textbf{71}, 201101 (2005). [2] V. A. Podolskiy, L. Alekseyev, and E. E. Narimanov, ``Strongly anisotropic media: the THz perspectives of lefthanded materials'', J. Mod. Opt. \textbf{52}(16) 2343 (2005). [3] A. A. Govyadinov and V. A. Podolskiy, ``Meta-material photonic funnels for sub-diffraction light compression and propagation,'' Phys. Rev. B \textbf{73}(15), 155108 (2006). [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W38.00010: Nanoparticle susceptibilities and the bianisotropic formalism Jeremy Neal, Peter Palffy-Muhoray Since the spatial extent of nanoparticles is not negligible compared to the wavelength of light, non-local effects may be expected in the electric and magnetic response of nanoparticles at optical frequencies. It has been suggested that such spatially non-local response may be taken into account via the bianisotropic formalism for the constitutive equations. We have calculated the susceptibilities of pairs of nanowires as a function of orientation relative to the incident fields using the discrete dipole approximation. We compare the results of our simulations with predictions of the bianisotropic description, and summarize our observations. [Preview Abstract] |
Session W39: Focus Session: Negative Differential Resistance II
Sponsoring Units: FIAPChair: Ravindra Pandey, Michigan Technological University
Room: Colorado Convention Center 502
Thursday, March 8, 2007 2:30PM - 3:06PM |
W39.00001: Negative Differential Resistance in Insulating Systems: From Molecules to Polymers Invited Speaker: We have developed a microscopic theory to explain the negative differential resistance behavior in molecular bridges. This feature has been observed in many molecules with different on/off ratios, sharpness of the current peak and the critical bias. Our theory, based on simple dimer model (both Peierls and donor/acceptor) together with bias driven conformational/ electronic change, covers almost all the experimental characteristics for a large number of real molecular systems and encompasses all the theory that has been known till date. Similar argument is also extended to Mott insulator, where we find a large number of insulator/quasi-metal transitions in finite size chains and a thermodynamic insulator/metal transition in polymers due to the application of static electric field between two ends of the chain. The interplay between charge inhomogenities and electric field induced polarization will be discussed in a number of cases. We will also show that none of these transitions follow Landau-Zener mechanism. I shall also discuss our theoretical proposal for the experimental strategies to stabilize highly unstable and reactive metal clusters like Al4Li4 and their analogs. \newline \newline Reference: \newline 1. S. Lakshmi and Swapan K. Pati, Phys. Rev. B 72, 193410 (2005). \newline 2. S. Lakshmi, Ayan Datta and Swapan K. Pati, Phys. Rev. B 72, 045131 (2005). \newline 3. S. Lakshmi and Swapan K. Pati, Spl on Nanosc and Tech, Pramana, 65, 593. (2005). \newline 4. S. Sengupta, S. Lakshmi and Swapan K Pati, J. Phys. Cond. Mat. 18, 9189 (2006). \newline 5. Swapan K. Pati and S. Ramasesha, J. Phys. Condens. Matter 16, 989 (2004). \newline 6. S.Lakshmi and Swapan K. Pati, J. Chem. Phys. 121, 11998 (2004). \newline 7. S. Dutta, S. Lakshmi and Swapan K Pati, Submitted (2006). \newline 8. A. Datta and Swapan K. Pati, J. Am. Chem. Soc. 127, 3496 (2005). \newline 9. Sairam S. M., A. Datta and Swapan K. Pati, J. Phys. Chem. B 110, 20098 (2006). \newline 10. A. Datta, Sairam S. M. and Swapan K. Pati, Acc. Chem. Res. (to appear) [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W39.00002: Bloch Oscillations of a Two-Dimensional Electron Gas in a Lateral Superlattice S.K. Lyo, W. Pan, J.L. Reno, J.A. Simmons, D. Li, S.R.J. Brueck We present theoretical result and experimental data for the DC current of a quasi-two-dimensional electron gas in a high electric field. The theoretical model considers inelastic scattering in a relaxation-time approximation and two-dimensional elastic scattering microscopically including inter-Bloch-band scattering in the degenerate and nondegenerate regime. The results show standard negative differential conductance. Inclusion of the effect of the electric field for the inelastic relaxation rate tends to flatten (i.e., saturate) the current after the peak current as a function of the field, yielding improved agreement between the theory and the observed data from modulated GaAs/Al$_x$Ga$_{1-x}$As quantum wells. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W39.00003: Nonequilibrium Kubo formula of a finite conductor connected to reservoirs obtained by the Keldysh formalism Tatsuya Fujii We show that the Keldysh formalism of the density matrix of a finite conductor attached to reservoirs has the MacLennan- Zubarev form. We point out that the Keldysh formalism describes the irreversible processes and the steady-state features of time-correlation functions. We find that the MacLennan-Zubarev form of the density matrix gives rise to a generalization of the Kubo formula into the nonequilibrium case. Based on it we propose a nonequilibrium identity between differential conductance, the noise power and the shot noise. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W39.00004: Length-Dependence of Electron Transfer in Conjugated Molecular Wires Shashi Karna, Govind Mallick, Ravindra Pandey The electron transfer (ET) properties of $\pi $-electron conjugated molecular wires consisting of polyene chain, [$>$C=C$<$]$_{n, (n=1-11)}$ has been investigated in the framework of \textit{ab} \textit{initio} molecular orbital theory. As expected, magnitude of the ET coupling matrix element V$_{DA}$ decreases exponentially with increase in the length of the molecular wire. However, in contrast with the rigid $\sigma $-bonded molecular wires, the decay constant, $\beta $, for the conjugated systems exhibits three different regimes over the calculated length. This is attributed to the delocalized nature of the electrons along molecular length that facilitates retention of the electron coupling even at large separations between the donor and acceptor centers. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W39.00005: Probing single molecule vibrations with the inelastic resonant tunneling Invited Speaker: Inelastic electron tunneling spectroscopy (IETS) has proven to be a valuable and powerful technique allowing identification and analysis of single molecular vibrations including those inaccessible by traditional optics measurements such as Raman and IR spectroscopies. Combined with scanning tunneling microscopy (STM) it provides a single molecule resolution. However, a comprehensive theoretical description of the electron coupling with molecular vibrations and the role it plays in conductance still remain a challenging problem. In this talk we present the first principles theory of the inelastic electron tunneling spectroscopy. Our method is based on density functional theory within Keldysh nonequilibrium Green's function formalism and allows us to treat electrons and molecular vibrations (phonons) on equal footing while computing electronic and vibrational spectrum, electron-phonon coupling, elastic and inelastic current in molecular electronic devices. The salient feature of our theory is that phonon effects on the electronic Hamiltonian are included in a self-consistent manner. Using this approach we investigate the effect of molecular vibrations on quantum transport through a C60 molecule contacted by two metallic electrodes. We demonstrate that its transport properties undergo significant changes when molecular vibrations are taken into account and show that this effect mostly originates from the resonance nature of the quantum tunneling which is expected to be true for the vast majority of the metallic electrodes. We also report a vibrational spectroscopy analysis and report those vibrational modes that contribute most to the inelastic quantum tunneling. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W39.00006: Universal Negative Differential Resistance in Single-Electron Transport through Atoms and Molecules Nikita Simonian, Jingbin Li, Konstantin Likharev We have carried out numerical calculations of single-electron transport through single atoms and organic molecules (OPE chains terminated with isocyanide groups), weakly coupled to gold electrodes. The calculations were based on the general theory of single-electron tunneling in systems with discrete energy spectrum [1], with molecular orbitals obtained by the ab initio DFT solver NRLMOL [2]. The Kohn-Sham potential calculated by the solver was also used to calculate the wavefunctions of ``external'' electrons, so that the necessary overlap integrals could be obtained using the Bardeen formula [3] rather from the NEGF approach. The most remarkable result of the calculations is the virtually unversal negative differential resistance, due to a new physical mechanism resulting from the suppression of transparency of one of the tunnel barriers of the system by the applied source-drain electric field. The work is supported in part by AFOSR and NSF. [1] D. V. Averin, A. N. Korotkov, and K. K. Likharev, Phys. Rev. B 44, 6199 (1991). [2] See http://cst-www.nrl.navy.mil/$\sim$nrlmol/. [3] J. Bardeen, Phys. Rev. Lett. 6, 57 (1961). [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W39.00007: Ab initio RTM/NEGF method for Electron Transport through Single Molecules Kenji Hirose, Nobuhiko Kobayashi Using the RTM/NEGF method, which is an ab initio calculation method based on the density-functional formalism with use of accurate scattering waves combined with non-equilibrium Green's function method, we study the transport properties between metallic electrodes through single molecules. Especially, we investigate how the atomic-scale contacts to electrodes affect quantum transport. We find that transport behaviors change significantly due to the contacts. For fairly good contacts, transport properties are determined by the HOMO-LUMO states by resonant tunneling processes. However, as the contacts to one electrode becomes worse, I-V characteristics are mostly determined by tunneling condition with strong non-linear behaviors and molecular states are hard to be observed in the conductance data. Furthermore, we find that negative differential resistance appears at some distances between single molecules and one of the electrodes. We will clarify the mechanisms for these anomalous transport behaviors and show the relationship of HOMO-LUMO resonant states and tunneling vs. ballistic transport with various contact conditions to electrodes. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W39.00008: Negative differential resistance in molecular conductors Mortko Kozhushner, Ivan Oleynik Negative differential resistance $\sigma _{d}$=dI/dV$<$0 has been observed in numerous experiments that measured conductance through organic molecules. We will discuss the fundamental mechanism of negative differential resistance in molecular conductors. The origin of NDR is in the nontrivial evolution of resonant conductance states of a molecular conductor in external electric field. These resonant states are the states of positive and negative molecular ions (electron and hole states). In an electric field, the resonant levels are lowered and their wave functions become assymetrical. This results in an initial increase of the resonant molecular current as a resonant level appears in the interval of energies [E$_{F}$, E$_{F}$-eV] and a subsequent decrease of the current as the asymmetry in localization at the right and left interfaces is amplified upon further increase of applied bias. [Preview Abstract] |
Session W40: Magnetotransport in 2DEGs
Sponsoring Units: FIAPChair: Rui-Rui Du, Rice University
Room: Colorado Convention Center 503
Thursday, March 8, 2007 2:30PM - 2:42PM |
W40.00001: Microwave mode of a two dimensional electron system in a spatially varying magnetic field B. A. Magill, L. W. Engel, M. P. Lilly, J. A. Simmons, J. L. Reno We find a resonance in the microwave absorption spectrum of a high mobility two dimensional electron system (2DES) in a spatially varying magnetic field produced by a long ferromagnetic cylinder of Dy placed, with its axis perpendicular to the 2DES, on the surface of the sample. An external field B$_{0}$ perpendicular to the 2DES is also applied. The resonance is present for B$_{0}$ less than about 0.5 T, and depends hysteretically on B$_{0}$, apparently due to the Dy magnetization. The resonance peak frequency, f$_{pk}$, decreases with B$_{0}$, and is about 1.4 GHz for a 1 mm diameter cylinder and B$_{0}$=0.2 T. For sufficiently large B$_{0}$, f$_{pk}\sim $1/ B$_{0}$, reminiscent of an edge magnetoplasmon [1]. This work is supported by MARTECH. [1] See for example, V. A. Volkov and S. A. Mikhailov, Sov. Phys.-JETP \textbf{67}, 1639(1988). [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W40.00002: Magnetotransport in a two-dimensional electron system in DC electric fields Wenhao Zhang, Hung-Sheng Chiang, Michael Zudov, Loren Pfeiffer, Ken West We report on non-equilibrium transport measurements in a high- mobility 2D electron system subject to weak magnetic and strong DC electric fields. Detailed study of DC-induced magneto-oscillations, first observed by Yang {\em et al}, reveals a resonant condition which is qualitatively different from that reported earlier. In addition, we explore new experimental regime of separated Landau levels and observe dramatic reduction of resistance induced by a relatively weak DC field. These results demonstrate similarity of transport phenomena in DC-driven and microwave-driven systems and have important implications for experiments on quenching of microwave-induced zero-resistance states by a DC current. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W40.00003: Magnetoresistance oscillations in two-dimensional electron systems induced by both AC and DC fields Michael Zudov, Wenhao Zhang, Loren Pfeiffer, Ken West We report on magnetotransport measurements in a high-mobility two-dimentional electron system subject simultaneously to AC (microwave) and DC (Hall) fields. We find that DC excitation affects microwave photoresistance in a nontrivial way. Photoresistance maxima (minima) evolve into minima (maxima) and back, with some new minima appearing as zero-resistance states. Most of our observations are explained in terms of indirect electron transitions using a new, ``combined'' resonant condition. Strong coupling and interplay of AC- and DC-induced effects call for a theory treating both excitation types within a single framework. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W40.00004: Non-linear Resistivity of a Two-Dimensional Electron Gas in a Magnetic Field Maxim G Vavilov, Igor L. Aleiner, Leonid I. Glazman We develop a theory of nonlinear response to an electric field of a two-dimensional electron gas (2DEG) placed in a classically strong magnetic field. The latter leads to a non-linear current-voltage characteristic at a relatively weak electric field. The origin of the non-linearity is two-fold: the formation of a non-equilibrium electron distribution function, and the geometrical resonance in the inter-Landau-levels transitions rates. We find the dependence of the current-voltage characteristics on the electron relaxation rates in the 2DEG. Our results can be applied for analysis of measurements at low [1] and high [2,3] current densities. [1] J. Zhang, S. Vitkalov, A. A. Bykov, A. K. Kalagin and A. K. Bakarov, cond-mat/0607741. [2] C. L. Yang, J. Zhang, R. R. Du, J. A. Simmons and J. L. Reno, Phys. Rev. Lett. 89, 076801 (2002). [3] W. Zhang, H. -S. Chiang, M. A. Zudov, L. N. Pfeiffer and K. W. West, cond-mat/0608727. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W40.00005: B-periodic oscillations in microwave irradiated high-mobility 2D electron gas K. Stone, C.L. Yang, Z.Q. Yuan, R.R. Du, L.N. Pfeiffer, K.W. West Recently a new type of B-periodic magneto-oscillations was observed [1] in the Hall bar samples of a 2D electron gas under the irradiation of microwaves (MW, frequency $\omega )$. The period $\Delta B$ is determined by $\omega $, the electron density$ n_{s}$, and the distance between potential probes $L$, $\Delta B\propto n_s /\omega L$. The phenomenon is explained by coherent excitation of edge magnetoplasmons in the region near the contacts. Using very high-mobility (8- 20 $\times $ 10$^{6}$ cm$^{2}$/Vs) GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures, we were able to observe both the MW-induced resistance oscillations, which is 1/B -periodic, and the B-periodic oscillations in the same sample, in the frequency range 27 to 130 GHz. Experimental data as well as a brief discussion will be presented. [1] Kukushkin et al, Phys. Rev. Lett. 92, 236803 (2004). [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W40.00006: DC-current induced magneto-oscillations in very high-mobility 2D electron gas C.L. Yang, Chi Zhang, R.R. Du, L.N. Pfeiffer, K.W. West We report on a systematic experimental study of DC-current induced magneto-oscillations [1] using Hall bar samples of very high-mobility (8-20 $\times $ 10$^{6}$ cm$^{2}$/Vs) GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures. Previously we show that remarkable nonlinear resistance and $1/B$ oscillations can arise when a high bias current ($I_{x})$ is passed through a Hall bar (width $w)$, and the effect can be explained by a Zener tunneling model in the presence of a tilting Hall field [1]. Data of resistance $R_{xx} \equiv V_x /I_x $, differential resistance $r_{xx} \equiv \partial V_x /\partial I_x $, and $r_{xx} '\equiv \partial r_{xx} /\partial I_x $ in higher mobility samples, which show higher order oscillations, have confirmed the validity of this model. Our temperature dependent date show that this effect can persist to $k_B T>\hbar \omega _c $, where $\hbar \omega _c $ is the cyclotron energy. [1] Yang et al, Phys. Rev. Lett.\textbf{ 89}, 076801 (2002). [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W40.00007: Magnetoresistance and microwave photoresistance of a periodically modulated high-mobility 2D electron gas Z. Q. Yuan, C. L. Yang, K. Stone, R. R. Du, L. N. Pfeiffer, K. W. West We have measured the magnetoresistance ($R_{xx})$ and the microwave (MW) photoresistance on a high-mobility 2D electron gas patterned with a large period (1200 and 1500 nm) triangular antidot lattice [1]. Our experiments were performed in a MW frequency range from 26 to 150 GHz and at temperatures (T) from 0.3 to 10 K, samples were Hall bars having low T mobility as high as 2.5 x 10$^{6}$ cm$^{2}$/Vs after the patterning. We observed remarkably sharp (up to seventh order) geometrical resonance (GR) peaks in $R_{xx}$. Moreover, under irradiation, MW-induced resistance oscillations (MIRO) and magnetoplasmon resonance (PR) modes were observed. Analysis shows that MIRO, MP, and GR are decoupled from each other in these large-period modulated 2D electron gas samples. [1] Yuan \textit{et al}, Phys. Rev. B \textbf{74}, 075313(2006). [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W40.00008: Resistivity of a high mobility two-dimensional hole gas on (100) GaAs in the vicinity of the metal-to-insulator transition Michael Manfra, L. Pfeiffer, K. West, A.M. Sergent We report on the density and temperature dependence of the resistivity of an extremely high mobility, carbon-doped, two-dimensional hole system (2DHS) in the vicinity of the putative metal-to-insulator (MIT) transition. The high mobility of our structures allows us to probe the conduction properties at very low 2D densities, $\sim $10$^{9}$cm$^{-2}$, a regime in which interactions are expected to play an important role. Using a back-gated structure, a mobility of 2.2x10$^{6}$cm$^{2}$/Vs is achieved at a density of 2.9x10$^{10}$cm$^{-2}$ at T=50mK. Backgating allows us to monitor the evolution of the resistivity as the density is continuously tuned from 2.9x10$^{10}$cm$^{-2}$ to 2.9x10$^{9}$cm$^{-2}$. From analysis of the temperature dependence of the resistivity, the sample becomes insulating at 3.5x10$^{9}$cm$^{-2}$. We compare our data to existing models of the MIT in high mobility, low density, structures. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W40.00009: Electronic Transport Studies in Thin GaAs/AlGaAs Quantum Wells D.R. Luhman, D.C. Tsui, L.N. Pfeiffer, K.W. West The results of experimental transport studies involving a series of thin GaAs/AlGaAs quantum wells with varying well widths will be reported. The mobility, $\mu$, of thin GaAs/AlGaAs quantum wells is typically limited by electron scattering from the interfacial roughness of the quantum well. The total scattering rate due to all scattering mechanisms is determined from the mobility through $\tau^{-1}=e/\mu m^{*}$ where $m^*$ is the effective electron mass. Our series of samples consists of well widths of $L=7.9, 9.9, 12.9, 16.0$ and 33.0 nm. For constant electron density ($n_e\sim5.5\times 10^{10}$ cm$^{-2}$) we find that interfacial roughness is the dominant scattering mechanism for $L\leq 16.0$ nm and successfully fit the data using a finite quantum well model$^1$ with adjustable interfacial roughness parameters. We will also present the magnetotransport properties of this series of samples. $^1$J.M. Li et al. Semicond. Sci. and Tech. 20, 1207 (2005). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W40.00010: Effect of DC electric field on longitudinal resistance of two dimensional electrons in a magnetic field Jing-Qiao Zhang, Sergey Vitkalov, Alexey Bykov, A.K. Kalagin, A.K. Bakarov The effect of a DC electric field on the longitudinal resistance of highly mobile two dimensional electrons in heavily doped GaAs quantum wells is studied at different magnetic fields and temperatures. Strong suppression of the resistance by the electric field is observed in magnetic fields at which the Landau quantization of electron motion occurs. The phenomenon survives at high temperature where Shubnikov de Haas oscillations are absent. The scale of the electric fields essential for the effect is found to be proportional to temperature in the low temperature limit. We suggest that the strong reduction of the longitudinal resistance is the result of a nontrivial change in the distribution function of 2D electrons induced by the DC electric field. Comparison of the data with recent theory yields the inelastic electron-electon scattering time $\tau_{in}$ and the quantum scattering time $\tau_q$ of 2D electrons at high temperatures, a regime where previous methods were not successful. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W40.00011: Experimental analysis of tunneling from a two-dimensional electron gas into the bulk in the presence of strong scattering. Kasey Russell, Venkatesh Narayanamurti, Federico Capasso, Joshua Zide, Arthur Gossard An asymmetric double-barrier heterostructure of InGaAs/InAlAs was grown lattice-matched to InP, and electrical contact was made to the InGaAs quantum well layer as well as both bulk InGaAs regions.~ The tunnel current out of the quantum well across one barrier was monitored while varying the electric field across the other (thicker) barrier.~ This measurement yielded the dependence of the tunnel current on the carrier concentration in the quantum well.~ Samples with ErAs dot scattering centers within the quantum well were measured and compared with device simulations to confirm the impact of scattering on tunneling out of a quantum well. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W40.00012: Undoped Electron-Hole Bilayers in a GaAs/AlGaAs Double Quantum Well J.A. Seamons, D.R. Tibbetts, J.L. Reno, M.P. Lilly There is intense interest in exciton condensation effects that can occur in bilayer systems. While exciton condensation effects have been studied in quantum hole bilayers, transport experiment in the exciton condensation regime of electron-hole bilayers have proved to be extremely difficult. We present the fabrication details and device measurements of completely undoped electron-hole bilayer devices in a GaAs/AlGaAs double quantum well heterostructure. The quantum wells are separated by a 90{\%} AlGaAs barrier with thicknesses of 20 nm or 30 nm depending upon the device. These devices have independently tunable densities of the two-dimensional electron gas and two-dimensional hole gas. We report four-terminal transport measurements of the independently contacted electron and hole layers with balanced densities from 1.2 x 10$^{11 }$cm$^{-2}$ down to 4 x 10$^{10 }$cm$^{-2}$ at T = 300 mK. Coulomb drag results from these devices will be presented. The mobilities can exceed 1 x 10$^{6}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for electrons and 4 x 10$^{5}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for holes. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Session W42: Liquids and Tribology
Sponsoring Units: DCMPChair: Mu Wang, Nanjing University
Room: Colorado Convention Center 505
Thursday, March 8, 2007 2:30PM - 2:42PM |
W42.00001: Monolayer Degradation and Sidewall Tribometer Studies of Vapor Phase Lubricants for MEMS D. Adam Hook, Brendan Miller, Shannon J. Timpe, Michael T. Dugger, Jacqueline Krim Monolayers have been widely used for MEMS to prevent release related stiction as well as adhesion as the devices are stored for long periods of time. Degradation of these monolayers in mechanical contact and at elevated temperatures however render the devices useless after a short period of time. While vapor-phase lubricants have primarily been studied within the context of macroscopic system performance, they may ultimately prove to be the most effective, if not the only, means to deliver and/or replenish a lubricant that can withstand a variety of extreme environmental conditions that a MEMS device is likely to encounter. We have made direct measurements of the change in the coefficient of friction of a MEMS sidewall tribometer as the lubricant is added to a vacuum chamber. Monolayer degradation under normal contact, requirements to prevent device failure, and friction measurements of vapor phase lubricants will be presented. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W42.00002: Tribological properties of bound plus mobile lubricants for MEMS application. Brendan Miller, D. Adam Hook, Jacqueline Krim Long chain alkylsilane monolayers are used to protect MEMS devices from adhesion due to contamination. One such monolayer, perfluorodecyltrichlorosilane (PFTS) has been shown to have very advantageous thermal properties but will wear under mechanical rubbing. The mobility of vapor phase lubricants (VPL) used in conjunction with these self-assembled monolayers (SAMS) [1] may be the key to extending the lifetime of rubbing contacts in MEMS by replenishing worn away parts of the SAM. We studied tricresyl phosphate, a known anti-oxidant, and Nye lubricant, used for aerospace applications. We measured friction and obtained wear characteristics with a quartz crystal microbalance (QCM), a sliding tribometer under cryogenic temperatures, and an AFM in order to understand the effect of mobility, temperature, and atomic-point contacts to help bridge the gap between fundamental friction and MEMS application. \newline [1] ``Dynamics of Vapor-Phase Organophosphates on Silicon and OTS W. Neeyakorn1, M. Varma1, C. Jaye1, J. E. Burnette, S.M. Lee, R. J. Nemanich, C. Grant2 and J. Krim'' Tribology Letters, in press. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W42.00003: Self-organization of surfactant aggregates on rough and smooth surfaces Hannes C. Schniepp, Ho C. Shum, Dudley A. Saville, Ilhan A. Aksay Atomic force microscopy (AFM) investigations of surfactant aggregates at liquid-solid interfaces have traditionally been performed on atomically smooth mica, graphite or gold. In order to extend the utility of this technique to more practical applications where atomically smooth surfaces rarely exist, we present results on rough gold surfaces for the first time. We achieve high-quality images of micellar structures on rough surfaces by using sharp, soft AFM probes. Contrary to the orientational order observed on atomically smooth surfaces, micellar organization on rough surfaces is also affected by the grain boundaries and atomic ledges. Our approach opens up the possibility of investigating surfactant self-assembly on arbitrary materials. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W42.00004: Real-time observation of self-assembling nanostructures of Langmuir-Blodgett films of vinylidene fluoride-trifluoroethylene copolymer by Atomic Force Microscopy. Jihee Kim, Stephen Ducharme, Brian Rodriguez, Sergei Kalinin Annealing studies have shown that ferroelectric polymer Langmuir-Blodgett (LB) films less than 10 nm thick undergo drastic morphology change after annealing in the crystalline ferroeletric phase [M. Bai \textit{et al.}, \textit{Appl. Phys. Lett. }\textbf{85}, 3528 (2004)] During annealing in the crystalline paraelectric phase, continuous films self-assemble into disk shape features, nanomesas, with approximately 9 nm thickness and 100 nm diameter. The nanomesa self-assembly was observed in real time with an Atomic Force Microscope, which was equipped with a heating stage. This results show that the nanomesas formed during annealing agree well with nanomesas observed in ferroelectric phase at room temperature after annealing in the paraelectric phase. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W42.00005: Study of Low Temperature Fuel Cells Thin Films Morphology by GISAXS Tomomi Irita, Thomas Russell Grazing incidence small angle x-ray scattering experiments were performed on thin films of Nafion solutions as a function of time as the solvent, methanol/water, evaporated. The development and orientation of the structure and morphology in the thin films, at the free surface and in the bulk of the film, was characterized by the scattering below and above the critical angle. The scattering profiles indicated that Nafion thin morphology was strongly influenced by the conformations of Nafion molecules in the solutions. In addition, the morphology in thin films of sulfonated block copolymers of polystyrene-b-poly(ethylene-o-butylene)-b-polystyrene, an alternative material for fuel cell applications, was characterized by GISAXS and scanning force microscopy using different solvents and under an applied electric field. Both the solvents used and the applied field was found to markedly influence the orientation of the ion conducting domains in the films. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W42.00006: Feasibility of Large Free-standing Liquid Films in Space Rui Zheng, Thomas Witten We consider the feasibility of large-scale free-standing thin liquid film experiment in the space environment as a new realization to study two-dimensional hydrodynamics. We identify material and environmental criteria necessary to avoid freezing, evaporation, chemical degradation, and spontaneous collapse of the film. These criteria pose no obstacles to achieving films of kilometer scale and lifetime of many months, with attainable Reynolds number up to $10^7$. However, impacts from meteoroids pose a serious threat to the film, and require substantial shielding or unproven self-healing properties in the film. Current theoretical and experimental studies of two-dimensional turbulence are briefly reviewed. We also describe a specific candidate liquid for the film. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W42.00007: Characterization of Surface Nanobubbles Shangjiong Yang, Stephan Dammer, Harold Zandvliet, Stefan Kooij, Bene Poelsema, Detlef Lohse In this work we characterize surface nanobubbles on hydrophobic surface in water by Atomic Force Microscopy (AFM) operated in the tapping mode. A connection between the formation of nanobubbles and the surface topography is presented. Together with large contact angle of nanobubbles, which determines their shape, the surface topology may support the surprising stability of nanobubbles. It is shown that the properties of the nanobubbles and their density are sensitive to the gas concentration and type. We show that different surface treatments also influence nanobubbles in formation and shape. Next, adding surfactants (2-butanol) causes nanobubbles to shrink, as expected from the Laplace equation. Finally, we show that exchanging alcohols by water on the surface strongly encourages the formation of nanobubbles. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W42.00008: High Density Monolayer Deposition of Fine Nanoparticles for Opto-Electronic Devices Xavier Bulliard, Wanki Bae, Kookheon Char, Seong Jae Choi, Jae Young Choi In this study, we present a unique approach to form uniform monolayers of nanoparticles (NPs) deposited on a substrate with high surface coverage density up to 2x10$^{12}$ NPs/cm$^{2}$. This was achieved through the wet coating of fine NPs with a diameter less than 10 nm. The mechanism of monolayer formation was decomposed into two stages: first the deposition driven by the electrostatic forces between a substrate and NPs and then the self-arrangement of NPs through the action of capillary forces. A physical description of the interaction forces involved in the process confirmed that for fine NPs the capillary forces are dominant over the electrostatic repulsion between adjacent NPs during drying. This enables the high compaction of a monolayer without altering its uniformity. Dip- and spin- coating techniques could as well be used for the deposition on various substrates (for example, hafnium and silicon oxides), which proves the versatility of this approach. The obtained architecture show promising properties and could be implemented for the production of the next generation of opto-electronic devices. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W42.00009: Microcontact printing Using Metallic Salt Solution ``Ink'' Cary Allen, Josh Dorr, Ian Schick, Evan Schick, Reuben Collins, Anish Khandekar, Thomas Kuech Arrays of micron size metal dots were patterned onto Si substrates using microcontact printing. Poly(dimethlysiloxane) stamps were prepared from Si masters fabricated using photolithography and anisotropic etching. Aqueous GaCl$_{3}$ and In(NO$_{3})_{3}$ inks were microcontact printed onto Si substrates, creating arrays of micron size metal salt deposits. The In(NO$_{3})_{3}$ deposits were further processed by annealing in an N$_{2}$:H$_{2}$ (9:1) forming gas environment at 600 \r{ }C which converted the deposits into In metal. Details of the stamp preparation and printed patterns, along with, characterization using atomic force microscopy and X-ray diffraction will be presented. The ability to inexpensively pattern metal arrays on semiconductor surfaces has implications for ohmic contacts and, with additional processing, arrays of semiconductor crystallites for optoelectronic applications. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W42.00010: X-ray diffuse scattering from thin polystyrene films Mrinmay K. Mukhopadhyay, Zhang Jiang, Sunil K. Sinha, Laurence B. Lurio, Jarett Stark, Xuesong Jiao, Suresh Narayanan, Alec Sandy Diffuse x-ray scattering from silicon supported polystyrene films has been measured as a function of thickness. An x-ray standing wave method was used to distinguish scattering from the surface and scattering from density fluctuations within the interior of the film. The former is a measure of surface roughness, while the latter yields the compressibility, $\kappa { }_T$. Films thicker than $h \quad \sim $100 nm had bulk values for $\kappa { }_T$, while thinner films showed the empirical relation $\kappa { }_T(h)=\kappa _T^{bulk} \left( {1+\alpha \mathord{\left/ {\vphantom {\alpha h}} \right. \kern-\nulldelimiterspace} h} \right)^\delta $ with \textit{$\alpha $} = 20 ($\pm $1) nm and \textit{$\delta $ }= 1.6 ($\pm $1). The surface component of the scattering agreed with capillary wave theory for small $q$, but excess scattering appeared at larger $q, $which followed a power law, $S^\ast \sim \,q^{1 \mathord{\left/ {\vphantom {1 \upsilon }} \right. \kern-\nulldelimiterspace} \upsilon }$. We attribute the excess scattering to static roughness from chain ends and loops near the surface. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W42.00011: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W42.00012: New Cyclic Voltammetry Method for Examining Phase Transitions on Electrodes: Simulated Results Ibrahim Abou Hamad, Daniel Robb, Per Arne Rikvold We propose a new experimental technique for cyclic voltammetry, based on the first-order reversal curve (FORC) method for analysis of systems undergoing hysteresis. The advantages of this technique are demonstrated by applying it to dynamical models of electrochemical adsorption. The method can not only differentiate between discontinuous and continuous phase transitions, but can also quite accurately recover equilibrium behavior from dynamic analysis of systems with a continuous phase transition. The FORC diagram for a discontinuous phase transition is characterized by a negative (unstable) region separating two positive (stable) regions, while such a negative region does not exist for continuous phase transitions. Experimental data for Electrochemical FORC (EC-FORC) analysis could easily be obtained by simple reprogramming of a potentiostat designed for conventional cyclic-voltammetry experiments.\\ \\ I. Abou Hamad, D.T. Robb, P.A. Rikvold, J. Electroanal. Chem., in press. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W42.00013: Relating contact angles, drop size and Line Energy Preeti Yadav, Prashant Bahadur, Kumud Chaurasia, Rafael Tadmor The relation between drop radius, $r$, the force to slide it, and the advancing and receding contact angles, \textit{$\theta $}$_{A}$ and \textit{$\theta $}$_{R}$, has been studied. To keep the line energy (energy per 2$\pi r)$ independent of $r$, the modified Young equation predicts that \textit{$\theta $}$_{A}$ and \textit{$\theta $}$_{R}$ change considerably with $r$. As shown by many investigators, \textit{$\theta $}$_{A}$ and \textit{$\theta $}$_{R}$ change negligibly, if at all, with $r$. We show why the modified Young equation is correct and still \textit{$\theta $}$_{A}$ and \textit{$\theta $}$_{R}$ should hardly change with $r$. Our results suggest that the Laplace pressure is a significant parameter in inducing the line energy. [Preview Abstract] |
Session W43: Coupled 2D and Multiple Quantum Dot Systems
Sponsoring Units: DCMPChair: Alexander Dzyubenko, California State University, Bakersfield
Room: Colorado Convention Center 506
Thursday, March 8, 2007 2:30PM - 2:42PM |
W43.00001: A study of interminiband Rabi oscillations in biased semiconductor superlattices Pavel Abumov, Donald Sprung Semiconductor superlattices can be a flexible source of coherent electrons, with possible application as sources of terahertz radiation and in quantum computing. A better understanding of the underlying quantum transport phenomena is essential for making further progress in these fields. We have studied interminiband Rabi oscillations of an electron in biased semiconductor superlattices, specifically the conditions for their occurrence and their variation with bias tuning at energy level anticrossings. Our simulations were based on direct solution of the time-dependent Schroedinger equation, using transparent boundary conditions. It has been explicitly demonstrated that interminiband Rabi oscillations result from constructive interference between Bloch and intrawell oscillations, and the conditions for resonant bias values have been investigated. We also report a simulation of interminiband Rabi oscillations directly across three minibands at high bias, which show interaction between three strongly coupled minibands. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W43.00002: Microwave Switching in Amorphous-Carbon Quantum Wells Somnath Bhattacharyya, Luis Gomez Rojas, S. Ravi. P. Silva Demonstration of long phase coherence length showing resonant tunnelling and fast switching in amorphous carbon quantum well structures has recently been established [1]. Here we show a bias controlled reversible switching of the complex impedance by transmitting a microwave signal up to 110\textit{GHz} through amorphous carbon resonant tunnel diodes. By employing a coplanar waveguide technique and through the analysis of the return loss ($S_{11})$ microwave enhanced mobility greater than 30\textit{cm}$^{2}$\textit{(Vs)}$^{-1}$ in the delocalized regime of (filamentary) conduction in these devices is demonstrated. Also a switching behaviour at about 85\textit{GHz} can also be observed. We suggest a new model for the microscopic origin of the increased mobility and show routes to achieve longer coherence lengths. In addition microwave conductance of carbon quantum wells parallel to their plane and across a channel length larger than 100 nm determines the momentum scattering time of electrons in carbon. These results exhibit a potential for pure amorphous carbon-based fast memory devices. [1] S. Bhattacharyya, S.J. Henley, E. Mendoza, L. Gomez Rojas, J. Allam and S.R.P. Silva, Nature Mater. \textbf{5}, 19 (2006). [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W43.00003: Transient response and electric field domain relocation in semiconductor superlattices Huidong Xu, Stephen Teitsworth Numerical simulation results are presented for a discrete drift-diffusion model that describes electronic transport in weakly-coupled semiconductor superlattices under voltage bias [1]. Sequential resonant tunneling between adjacent quantum wells is the primary conduction mechanism for this model which also incorporates an effective conductivity $\sigma$ associated with the injecting contact. We study time-averaged current-voltage characteristics and the transient current response associated with electric field domain relocation when the applied voltage is abruptly shifted by an amount $V_{step}$. For intermediate values of $\sigma$ and a range of $V_{step}$ values, two types of complex transient response are observed: 1) the \textbf{tripole/dipole} mechanism in which a charge depletion and a charge accumulation layer move together from the contact, and 2) the \textbf{injected monopole} mechanism, in which a small amplitude accumulation layer moves rapidly from the contact. Generally, the injected monopole relocation mechanism is much faster than the tripole/dipole mechanism. At moderately larger values of $\sigma$, the tripole/dipole mechanism is not observed for any value of $V_{step}$ because the higher levels of injected charge suppress formation of a moving depletion layer. Thus, a relatively small increase in $\sigma$ can result in significantly shorter domain relocation times. [1] L. L. Bonilla and H. T. Grahn, Rep. Prog. Phys. \textbf{68}, pp. 577 - 683, and references therein. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W43.00004: Exchange-driven re-entrant layer-occupancy transitions in biased bilayer systems J.R. Rodriguez, C.B. Hanna Hamilton \textit{et al.} showed experimentally that an externally biased double-quantum-well system in zero magnetic field could exhibit an exchange-driven bilayer-to-monolayer (``2-1'') transition as the total carrier density was increased. This transition is due to the combined effects of the negative compressibility of the low-density carriers and the layer imbalance produced by external gate biases. We give an approximate criterion for observing a re-entrant ``2-1-2'' transition that repopulates the emptied layer as the total carrier density is further increased. The gate voltages required for repopulation are shown to be impractically high for $p$-type GaAs bilayer devices with hole carriers. We show, however, that it may be possible to observe a ``2-1-2'' transition in low-density $n$-type electron bilayer systems with very small layer separations. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W43.00005: Using the angular dependence of a quantizing magnetic field to probe the Bloch states in room temperature superlattice devices. Ross McDonald, Shigeki Kobayashi, S. Jim Allen , Susan Cox, John Singleton The prospect of designing Bloch-oscillator superlattice structures that operate at room temperature has both intrigued and eluded the scientific community since its conception over 35 years ago. Advances in band structure architecture and engineering continuously address this issue, improving the fabrication of devices designed to operate as room temperature THz frequency oscillators. Here we report room-temperature pulsed-IV measurements in tilted magnetic fields of up to 30 Tesla, designed to probe the coherence of superlattice Bloch states. Biasing these devices beyond Ohmic conduction reveals differential conductance features with a 1/cos($\theta )$ dependence upon the field angle. The voltages at which these features occur is determined by the condition that the ratio of the Bloch to cyclotron frequencies be an integer. This behavior is consistent with resonant de-localization of Bloch oscillations due to nonlinear coupling to the cyclotron motion in tipped field. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W43.00006: Effective Hamiltonian Approach for the Magnetic Band Structure and Novel Oscillations in the Magnetization Manfred Taut The one-electron Schroedinger equation in a 2D periodic effective potential and an homogeneous magnetic field B has been solved numerically in the framework of magnetic band structure theory. Alternatively,the spectrum around a given rational flux quantum number p0/q0 can also been obtained by semi-classical quantization of the exact magnetic band structure (MBS) at p0/q0. To implement the latter procedure, a generalized effective Hamiltonian theory based on the MBS at finite magnetic fields has been established. The total energy has been calculated numerically as a function of magnetic field B and of band filling. The magnetization M is the derivative of the total energy with irespect to the magnetic field. The total energy as a function of B shows series of kinks, which produce series of oscillations in the magnetization. One of these series, the de Haas-van Alphen oscillation, contains information about the (zero magnetic field) band structure. The other series provide the corresponding information about certain MBSs. In order to obtain the information about the MBS at field B0, we have to plot the magnetization as a function of 1/(B-B0). The asymptotic period of the oscillations in M(1/(B-B0)) provides the Fermi surface cross sections for the MBS at B0. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W43.00007: Novel Flux Matching Effects in Potentially Type-I Superconducting Au/Pb Bilayers Patterned with Antidot Lattices Lance De Long, Sergiy Kryukov, Vitali Metlushko We report AC and DC SQUID magnetometer data for Au(25nm)/Pb(x) bilayers (x = 50, 100 nm) patterned with square antidot (AD) lattices having AD diameter D = 600 nm and AD separation d = 1 micron, in DC magnetic fields applied perpendicular to the film plane. Both AC and DC data for x = 100 nm samples exhibit a ``two-horned'' magnetization m(H) well below T$_{C}$, with small, sharp cusps having DC field spacings near 3 Oe. Just below T$_{C}$ = 6.2 K, m(H) is highly reversible, and exhibits at least two matching fields H$_{n}$ = (20 Oe)n. This striking behavior is compared with recent theoretical models for flux matching in patterned films in the Type-I intermediate state, for which formation of ``giant vortices'' or pinning of normal domains by AD are possible. In contrast, data for x = 50 nm samples exhibit smooth (no small cusps) m(H) behavior with sharp matching peaks and highly irreversible behavior just below T$_{C}$, typical of extensively studied, Type-II patterned films. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W43.00008: Variational Monte Carlo Method for Coupled Quantum Dots in Magnetic Fields Jihan Kim, Dmitriy Melnikov, Michele Casula, Jean-Pierre Leburton The electronic properties of two-dimensional coupled quantum dots (QD) in presence of an external magnetic field are investigated using a variational Monte Carlo (VMC) method. The many-body Schr\"{o}dinger Equation with fixed model potential for coupled QDs is solved by using two-electron trial wavefunctions made of a product of two-body Jastrow term and single-particle orbitals for both singlet and triplet states. We use the steepest descent (SD) method to optimize the expectation value of energy by iteratively updating the variational parameters. In co-linear triple QDs, we show that the exchange energy between two electrons can be tuned by varying the confinement of the central dot. We also find that the electron separation in the singlet and triplet states evolve differently upon increasing the magnetic field. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W43.00009: Topological Hunds rules and the electronic properties of a triple lateral quantum dot molecule P. Hawrylak, M. Korkusinski, F. Delgado, L. Gaudreau, S. Studenikin, A. Kam, A. Sachrajda We analyze theoretically and experimentally the electronic structure and charging diagram of three coupled lateral quantum dots in a magnetic field filled with electrons. Using the Hubbard model and real-space exact diagonalization techniques we show that the electronic properties of this artificial molecule can be understood using a set of topological Hunds rules[1]. These rules relate the multi-electron energy levels to spin and the inter-dot tunneling t, and control charging energies. We map out the charging diagram for up to N=6 electrons and predict a spin singlet for two electrons, spin-polarized phase for two holes, and a magnetically frustrated ground state for three electrons. We show that spin polarization can be tuned by magnetic field perpendicular to the triple dot device. The theoretical charging diagram is compared with the measured charging diagram of the gated triple-dot device[1]. [1] P. Hawrylak and M. Korkusinski, Solid State Commun. 136, 508 (2005). [2] L. Gaudreau, S. A. Studenikin, A. S. Sachrajda, P. Zawadzki, A. Kam, J. Lapointe, M. Korkusinski, and P. Hawrylak, Phys. Rev. Lett. 97, 036807 (2006). [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W43.00010: Tunable Noise Cross-Correlations in a Double Quantum Dot Douglas McClure, Leonardo DiCarlo, Yiming Zhang, Hans-Andreas Engel, Charles Marcus, Micah Hanson, Art Gossard We report measurements of the cross-correlation between temporal current fluctuations in two quantum dots in the Coulomb blockade regime, with purely capacitive inter-dot coupling. The dots act as a pair of tunable interacting localized states, enabling a systematic study of Coulomb-induced correlation. The sign of the cross-spectral density is found to be tunable by gate voltage and source-drain bias. We find good agreement between the experimental results and a sequential-tunneling model of transport through capacitively coupled single-level dots. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W43.00011: Phonon decoherence in a double dot qubit embedded inside a suspended phonon cavity Ying-Yen Liao, Yueh-Nan Chen, Der-San Chuu The phonon-induced decoherence in a double dot charge qubit embedded inside a semiconductor slab is investigated theoretically. We employ the Redfield formalism to solve the density matrix in the Born-Markov approximation. Our calculations show some interesting results in the presence of slab cavity. In particular, the decoherence behaves significantly due to particular phonon couplings such as the van Hove singularity and vanishing deformation potential. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W43.00012: Investigation of chaos-assisted tunneling in a weakly coupled, double quantum dot Dong Ho Wu, Bernard Matis It is known that chaos-assisted dynamical tunneling may occur in nonintegrable (chaotic) systems. In our previous experiments we measured the tunneling rate in a weakly coupled, 2D microwave double cavity. The results seemed to indicate that the presence of chaotic modes changes not only the dynamical tunneling rate but also the spatial tunneling rate, as the electromagnetic-field leakage (wave tunneling) rate between the weakly coupled, 2D double cavities increases significantly, if one of the cavities is nonintegrable. We have now investigated these phenomena with gate-defined quantum dots, fabricated on a GaAs$\backslash $AlGaAs 2DEG substrate. The experiments were performed on quantum dots of various shapes. In this presentation we will discuss these recent results on tunneling rate and quantum conductance. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W43.00013: Fermi-liquid versus non-Fermi-liquid behavior in triple quantum dots Janez Bonca, Rok Zitko We study the effect of electron hopping in triple quantum dots modelled by the three-impurity Anderson model. In a wide interval around the particle-hole symmetric point, the triple quantum dot system has a FL ground state with high conductance at $T=0$. The different regimes exhibit different approaches to this fixed point. The most likely candidate for observing non-Fermi-liquid (NFL) behavior is the cross-over regime with competing magnetic ordering and Kondo screening, $J \sim T_K$. In this regime the NFL behavior occurs in a wide temperature range and it is fairly robust against various perturbations that do not additionally increase the channel asymmetry. As the crossover regime is entered from the above, the conductance through the side dots increases to a half of the conductance quantum, while the conductance through the system remains small. At lower temperatures the conductance through the system increases to the unitary limit as the system crosses over to the Fermi-liquid ground state. The signature of the NFL behavior can be detected by measuring different conductances in a three terminal configuration. Our findings suggest, that properly choosing parameters of the triple quantum dot system that set it into the crossover regime, represents a road map for observation of NFL behavior. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W43.00014: Signatures and Implementations of Adiabatic Quantum Pumping Kunal Das We explore the mechanism of adiabatic quantum pumping from a fundamental quantum mechanical perspective, and consider analogies with other adiabatic processes. We discuss ideas for generalizing the mechanism to alternate entities other than charge and spin and in alternative physical systems. Our study is motivated by and grounded in possibilities of easier experimental realizations to get around the difficulties encountered in previous mesoscopic experiments. In addition we wish to have an unambiguous definition and signature for the phenomenon of quantum pumping. [Preview Abstract] |
Session W44: Focus Session: Nanoscale Transport - Mostly Quantum Dots
Sponsoring Units: DMPChair: Mikhail Raikh, Utah University
Room: Colorado Convention Center 507
Thursday, March 8, 2007 2:30PM - 3:06PM |
W44.00001: Spin-Lattice Relaxation Rate in Lateral Quantum Dots Invited Speaker: Laterally gated quantum dots (QDs) fabricated on AlGaAs/GaAs heterostructures show promise for spin-based quantum computation. One limit to the coherence time in QDs, which sets the timescale on which quantum operations must be completed, comes from the spin-orbit interaction. In a magnetic field $B$ the spin states of a single electron in a QD are split by the Zeeman energy $g\mu_{B}B$; the spin-orbit interaction couples the spin states of a QD to its orbital degrees of freedom, which in turn can interact with piezoelectric phonons to relax the spin from the excited state to the ground spin state. The time scale over which this happens is the relaxation time $T_1$. We present measurements of the relaxation rate $W = T_1^{-1}$ of one electron in a single laterally gated QD at magnetic fields down to 1 T, much lower than previously measured. These measurements are possible because of the good stability of the AlGaAs/GaAs heterostructure we have used combined with an active feedback system that compensates for residual drift and switches of the dot energy levels. We find that $T_1$ is as long as 1s at 1 T. We compare our measurements to theoretical predictions of $W$ caused by spin-orbit coupling to phonons and extract the spin-orbit length, which describes the strength of the spin-orbit interaction. This demonstrates that spin-orbit coupling to phonons can account for $W$ down to fields as low as 1 T in laterally gated QDs and establishes an upper limit to the spin coherence time. This work is in collaboration with K. MacLean, D. M. Zumb\"{u}hl, I. P. Radu, M. A. Kastner, M. P. Hanson, and A. C. Gossard. This work has been supported by the ARO (W911NF-05-1-0062), the NSF (DMR-0353209), and in part by the NSEC Program of the NSF (PHY-0117795). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W44.00002: Spin Blockade in electronic transport through quantum dots Bhaskaran Muralidharan, Supriyo Datta Recently, Spin Blockade (SB) transport through quantum-dots has attracted attention owing to potential applications in quantum state control. In this talk, we identify the mechanism underlying current collapse (NDR), current leakage and bias dependent asymmetry in the I-V characteristics of quantum dot systems, which characterize spin blockade transport. As a specific example of this generic mechanism, we examine the conditions for SB to occur in transport through coupled quantum dots. This leads to a consistent interpretation of the non-trivial features in the experimental I-Vs of coupled quantum dots including multiple NDR, gate-able current collapse, and current rectification. Most importantly, our study elaborates on how a delicate interplay of orbital energy offset, delocalization, and Coulomb interaction between conduction electrons localized on either dot, strongly influences the aforementioned transport signatures. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W44.00003: Resonant dephasing of the electronic Mach-Zehnder interferometer Eugene Sukhorukov, Vadim Cheianov We address the recently observed unexpected behavior of Aharonov-Bohm oscillations in the electronic Mach-Zehnder interferometer experimentally realized in a quantum Hall system [1]. We argue that the measured lobe-like structure in the visibility of oscillations and the phase rigidity result from a long-range {\em local} interaction between two adjacent counter-propagating edge states, which leads to a resonant scattering of bosonic charge excitations. The visibility and phase shift, expressed in terms of the transmission coefficient for bosons, provide the tool for investigating the nature of quantum Hall edge states. [1] I. Neder {\em et al}., Phys. Rev. Lett. {\bf 96}, 016804 (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W44.00004: Electron population control of an isolated quantum dot using surface-acoustic-wave pulses Chris Ford, Robert Schneble, Masaya Kataoka, Adam Thorn, Crispin Barnes, David Anderson, Geb Jones, Ian Farrer, David Ritchie, Michael Pepper In developing quantum information technology, isolation from the environment is a key for long coherence times. However, many quantum-dot (QD) experiments require a fair degree of coupling to electron reservoirs. The electron number becomes progressively difficult to control as the degree of isolation increases and the electron dwell time exceeds the timescale of experiments. In such a system, a means to transfer electrons on demand between a QD and another QD or reservoir is desirable. We report our recent experiments on sending surface acoustic waves (SAWs) past a QD that is isolated from the leads by strong barriers, such that electrons take hundreds of seconds to tunnel. A short pulse of SAWs is used to characterize the electronic structure of the QD, and to transport electrons in and out of the QD. The mechanism of electron transfer from dynamic QDs defined by the SAWs themselves into a gate-defined static QD is investigated. This has applications for quantum information transfer and processing. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W44.00005: Coulomb-energy-dependent tunnelling from few-electron dynamic quantum dots defined by surface acoustic waves Michael Astley, Masaya Kataoka, Chris Ford, Crispin Barnes, Dave Anderson, Geb Jones, Ian Farrer, Dave Ritchie, Mike Pepper Electrons confined in dynamic quantum dots (DQDs) have been proposed as an implementation for the control and manipulation of quantum information. In this scheme, entanglement is achieved at a tunnel barrier between neighbouring DQDs. In this presentation we investigate the escape rate from a DQD at a tunnel barrier. One or few electron DQDs were created by a surface acoustic wave travelling through a pinched-off channel, isolated from a reservoir by a narrow tunnel barrier. The tunneling rates across the barrier were determined using a rate-equation model, and found to increase with the electron occupation of the DQD. This effect can be explained in terms of Coulomb interactions between the confined electrons. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W44.00006: Conductance signatures of a quantum-critical transition and a Kondo filtered resonance in double quantum dots Luis Dias, Nancy Sandler, Kevin Ingersent, Sergio Ulloa We present conductance results for double quantum dot (DQD) systems containing one dot in the Kondo regime coupled to an effectively noninteracting dot. The system is mapped onto a single impurity Anderson model with a structured (nonconstant) density of states [1]. The linear conductance is obtained using the DQD's Green's function calculated from numerical renormalization-group calculations for both side-dot and parallel configurations. In the side dot case, the conductance shows signatures of the band filtering through the resonant dot. This mechanism can be interpreted as an interference between many-body and single-particle states, splitting the Kondo resonance while preserving the Kondo singlet ground-state. In the parallel configuration, interference between conducting channels through the dots create a pseudogapped effective density of states [1]. We discuss possible approaches for detecting the quantum-critical point separating Kondo and non-Kondo phases in conductance measurements. \newline \newline [1] L.G.G.V. Dias da Silva et al, PRL 97 096603 (2006) [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W44.00007: Kondo effect in a quantum dot via orbital population switching Hyun-Woo Lee, Sejoong Kim Strong correlation effects in electron transport through a spinless quantum dot are considered. For general tunneling matrix elements between the quantum dot and leads, there exists a conserved pseudospin degree of freedom when two orbitals in the quantum dot are degenerate. The fluctuations of the pseudospin at the quantum dot give rise to the Kondo effect described by the anisotropic $s$-$d$ model. Interestingly the Kondo effect generates a pair of asymmetric conductance peaks near the center of a Coulomb valley, in clear contrast to the conductance behavior due to the spin Kondo effect. This explains the origin of the so-called correlation-induced resonances reported recently [V. Meden and F. Marquardt, Phys. Rev. Lett. 96, 146801 (2006)]. An exact relation to the phenomenon of the population switching is provided and differences from the conventional Kondo effects are clarified. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W44.00008: Modeling of tunneling spectroscopy of a single quantum dot involving two levels Ming Ting Kuo, Yia-Chung Chang We have employed the two-level Anderson model to simulate the system of the tip/quantum dot (QD)/substrate double barrier junction. The tunneling current through the ground state and the first excited state in the cases of shell-tunneling and shell-filling is theoretically investigated in the framework of nonequilibrium Green's function technique by solving the two level Anderson model properly. We found that single-particle and two-particle occupation numbers significantly influence the probabilities of each resonant energies arising from the intralevel and interlevel Coulomb interactions. Compared with tunneling current spectra of CdSe QDs, we predict some resonant structures which can be observed in an isolated QD. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W44.00009: Band filtering and quantum phase transition in an asymmetric double quantum dot W. Brian Lane, K. Ingersent, L. G. G. V. Dias da Silva, N. P. Sandler, S. E. Ulloa Double quantum dots (DQDs) are currently of great theoretical and experimental interest. A DQD device in which one of the dots is in the Kondo regime and the other is effectively a noninteracting resonant level has been shown [1] to reduce to an effective one-impurity Anderson problem with a structured (nonconstant) density of states. Depending on DQD parameters that can be controlled experimentally via gate voltages, such a device can exhibit zero-field splitting of the Kondo resonance on the interacting dot, or it can be tuned to access a quantum critical point separating Kondo-screened and local-moment phases. Using numerical renormalization-group techniques, we explore the robustness of these phenomena by increasing the Coulomb interaction on the resonant dot away from zero. We report the effects of the interaction on the device's magnetic susceptibility, spectral function, and linear conductance. [1] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, Phys. Rev. Lett. {\bf 97}, 096603 (2006). [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W44.00010: Imaging Electron Flow From a Quantum Point Contact M. P. Jura, M. A. Topinka, A. R. Sciambi, D. Goldhaber-Gordon, L. Urban, A. Yazdani, H. Shtrikman, L. N. Pfeiffer, K. W. West We image electron flow from a quantum point contact (QPC) into a high-mobility two-dimensional electron gas (2DEG) using scanning gate microscopy (SGM). We note two surprising phenomena, which we compare with results from simulations: 1. The beam of electrons immediately leaving the QPC is unexpectedly narrow and collimated. 2. Under certain conditions, the signal generally associated with current flow density (i.e. the change in differential conductance due to scattering from the scanning gate tip) can change sign from negative to positive. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W44.00011: Imaging fringes in magnetic focusing of electron waves Melaku Muluneh, Robert Westervelt, Rob Parrott, Eric Heller, Micah Hanson, Art Gossard Magnetic focusing of the electron flow between two quantum point contacts (QPCs) in a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure occurs when the QPC spacing is an integer multiple of the diameter of a cyclotron orbit. Images of magnetic focusing taken with a cooled scanning probe microscope (SPM) exhibit fringes when multiple paths of electrons interfere, as well as branching of the electron paths caused by small angle scattering [1]. We use simulations to show that one could improve images of fringe structures by using devices smaller than the length required to form branches. The distance between QPCs can be made quite small ($\sim $~0.5 microns) without destroying the fringe structure. We plan to test our simulations using double QPC devices. \newline \newline [1] Kathy Aidala\textit{ et al.}~ to be published (2007). [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W44.00012: Phase-sensitive mapping of electronic wavefunctions in atomically precise nanostructures Laila S. Mattos, Hari C. Manoharan We use a custom-built scanning tunneling microscope (STM) to assemble atomically precise nanostructures and to study the evolution of engineered electronic wavefunctions. We investigate resonant structures of different geometries constructed from individual atoms and molecules at 4 K. STM measurements directly probe wavefunction probability density but can indirectly provide information about quantum-mechanical phase. Through controlled quantum interference we thus use the STM as a phase-sensitive probe of single electron wavefunctions formed from the two-dimensional electron gas on the Cu(111) surface. By varying constraints imposed by symmetry, the boundary geometry, and relative or statistical phase (e.g. via magnetism or field effects), we can tune and elucidate energy and phase information of specific electronic quantum states. This level of detection and control is critical for new technologies based on few-electron devices. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W44.00013: Quantum Isospectral Nanostructures Christopher R. Moon, Hari C. Manoharan ``Can one hear the shape of a drum?'' Recently, the answer to this long-standing puzzle in contemporary mathematics was proven to be ``no'': it is possible to construct two entirely different boundaries in which the wave equation possesses exactly the same eigenvalue spectrum. For the first time, we verify this result in the quantum mechanical realm by designing and studying isospectral electron resonators built one molecule at a time. We present scanning tunneling microscopy of pairs of nanostructures with dissimilar spatial structures yet identical electronic properties. We demonstrate that the wavefunctions of one structure can be transplanted onto those of its isospectral complement, but only if the electron phase -- which usually has no bearing on proximal probe measurements -- is taken into account. [Preview Abstract] |
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