Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D1: Focus Session: Surface Chemistry and Catalysis II
Sponsoring Units: DCPChair: Charlie Sykes, Tufts University
Room: 103/105
Monday, March 3, 2014 2:30PM - 2:42PM |
D1.00001: Plyler Prize and APS Fellow Introductions Gilbert Nathanson The Division of Chemical Physics is delighted to announce the 2013 APS Fellows sponsored by DCP and to honor the 2014 Earl K. Plyler Prize Award winner. The new APS Fellows are: Ilan Benjamin, Hua Guo, Manos Mavrikakis, Josef Paldus, Joern Siepmann, Hans-Peter Steinrueck, Douglas Tobias, Angela Wilson, and Yijing Yan. The citations for each awardee will be read out loud. I will also introduce Prof. Lai-Sheng Wang of the Department of Chemistry at Brown University, who was awarded the 2014 Plyler Prize for Molecular Spectroscopy and Dynamics. Please come learn about these extraordinary scientists during this prize session. Prof. Wang's Plyler Prize talk will follow immediately after this introduction. For more information, see http://www.aps.org/units/dcp/. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 3:18PM |
D1.00002: Earle K. Plyler Prize: Probing the Structural Evolution and Size-Dependent Reactivity of Gold Clusters by Photoelectron Spectroscopy Invited Speaker: Lai-Sheng Wang Gold has attracted much interest in nanoscience because of its emerging catalytical and optical properties at the nanometer scale. A prerequisite to elucidate the molecular mechanisms of the catalytic effects of nanogold is a detailed understanding of the structural and electronic properties of gold clusters as a function of size. Negatively charged gold clusters (Au$_{n}^{-})$ up to $n =$ 12 were known to be planar. Using photoelectron spectroscopy and computational chemistry, we found that Au$_{16}^{-}$ to Au$_{18}^{-}$ possess hollow cage structures, while Au$_{20}^{-}$ was found to have a high symmetry tetrahedral structure. Beyond Au$_{20}^{-}$, we have found that low symmetry core-shell type structures started to emerge at Au$_{25}^{-}$. The size-dependent reactivity of O$_{2}$ with gold clusters was further studied using photoelectron spectroscopy. Previous works showed that only even-sized Au$_{n}^{-}$ clusters react with O$_{2}$, whereas odd-sized Au$_{n}^{-}$ clusters are nonreactive. Superoxo-type Au$_{n}$(O$_{2}^{-})$ complexes were proposed for even-sized clusters. We observed van der Waals complexes of odd-sized Au$_{n}^{-}$ clusters with O$_{2}$, confirming the inertness of the odd-sized Au$_{n}^{-}$ toward O$_{2}$. This observation led to new insight into how neutral even-sized Au$_{n}$ clusters interact with O$_{2}$. Further studies revealed that there is a superoxo to peroxo chemisorption transition for the O$_{2}$ interaction with even-sized Au$_{n}^{-}$ clusters. The O$_{2}$ in the peroxo O$_{2}$Au$_{n}^{-}$ complexes is much more activated (with a longer O--O bond length), suggesting that this mode of chemisorption may play a more important role in the O$_{2}$ activation by gold nanoparticles. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D1.00003: Understanding the Composition and Reactivity of Au/Cu Electrocatalyst Nanoparticles in Solution Using Highly Accurate Reactive Potentials Nongnuch Artrith, Alexie Kolpak The shape, size, and composition of catalyst nanoparticles can have a significant influence on their catalytic activity. Understanding such structure-reactivity relationships is crucial for the optimization of industrial catalysts and the design of novel catalysts with enhanced properties. In this work, we investigate the equilibrium shape and surface structure/composition of Au/Cu nanoparticles in solution, which have recently been shown to be stable and efficient catalysts for CO$_{2}$ reduction [1]. Using a combination of density functional theory calculations and large-scale Monte-Carlo and molecular dynamics simulations with reactive atomistic potentials, we determine how the nanoparticle shape, surface structure, and surface stoichiometry (i.e., fraction of Au at the surface relative to overall composition), evolve as a function of varying catalytic conditions. We discuss the effects of these changes on the surface electronic structure and binding energies of CO$_{2}$, H$_{2}$, and CH$_{3}$OH. Our results emphasize the important relationships between catalytic environment (e.g., solvent effects), catalyst structure, and catalytic activity. [1] Z. Xu, E. Lai, Y. Shao-Horn, and K. Hamand-Schifferli, Chem. Commun. 48, 5626-2528 (2012). [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D1.00004: Density Functional Theory Investigation of Adsorption Properties of CO, CO$_2$ and H$_2$O on $\gamma$-Al$_2$O$_3$ Supported Pt Clusters Mehmet Gokhan Sensoy, Hande Ustunel, Daniele Toffoli The water-gas shift reaction is a key catalytic process for the production of clean H$_2$ gas for fuel cells.\footnote{M. S. Dresselhaus and I. L. Thomas, Nature 414, 332 (2001)} In this study, we use plane wave pseudopotential density functional theory to study the adsorption properties and the activation of CO, CO$_2$ and H$_2$O on Pt clusters supported on the (001) surface of $\gamma$-Al$_2$O$_3$. A systematic study has been conducted to identify the most stable adsorption sites for both monoatomic and diatomic Pt clusters. Several stable adsorption geometries have been identified for the adsorbates, and their interaction with both the precious metal and the support is characterized in terms of adsorption energies and the nature of the bond between the adsorbed molecules and the precious metal. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D1.00005: Correlating structure and function for nanoparticle catalysts Invited Speaker: Graeme Henkelman Better oxygen reduction catalysts are needed to improve the efficiency and lower the cost of fuel cells. Metal nanoparticles are good candidates for new catalysts because their catalytic properties are different from bulk metals, and are sensitive to particle size, shape and composition. The electronic structure can be determined for small particles, making it possible to optimize particles for a desired reaction. Here, we calculate the electronic structure of 1 nm core/shell particles and show how the energy of electrons in the shell can tune the binding of oxygen by varying the core metal. Transition state calculations for O2 dissociation on the nanoparticle surface show that the d-band center is a good measure of the activation and reaction energies. Two factors are found to be significant for determining the catalytic activity of small nanoparticles; charge transfer in core/shell particles and the rigidity of alloy particles. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D1.00006: Catalytic Properties of Graphene-Supported Pt13 Nanoclusters Ioanna Fampiou, Ashwin Ramasubramaniam Graphene is of considerable interest as a support material for fuel cell electrodes due to its high surface area, high mechanical strength and exceptional electrical conductivity. Here, using density functional theory calculations we investigate the electronic and catalytic properties of Pt$_{13}$ clusters supported on pristine and defective graphene. We show that defects in the graphene support significantly stabilize Pt clusters against sintering. Strong cluster-substrate interactions are also found to substantially shift the $d$ band centers of the Pt clusters. Specifically, we investigate the adsorption of CO and O on Pt clusters bound at defects in graphene and show that such defect-supported clusters adsorb CO and O more weakly than clusters supported on pristine graphene or entirely unsupported clusters. We examine the kinetics of the CO oxidation reaction and demonstrate that graphene-supported Pt$_{13}$ nanoclusters--despite the low coordination of surface atoms--possess comparable catalytic activity with macroscopic Pt(111) surfaces and, in general, superior catalytic activity compared to unsupported clusters. Our results suggest possible avenues for controlling the dispersion and catalytic activity of Pt nanoclusters on graphene supports via defect engineering. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D1.00007: Unexpected carboxylate like CO adsorption at the Sr$_3$Ru$_2$O$_7$ (001) surface Marcel Hieckel, Florian Mittendorfer, Josef Redinger, Bernhard Stoeger, Zhiming Wang, Michael Schmid, Ulrike Diebold Oxide perovskite materials have attracted enormous attention because of a variety of intriguing physical properties ranging from catalysis to multiferroicity. We present a combined experimental and ab-initio (DFT) study with the Vienna Ab initio Simulation Package (VASP) on the adsorption of CO at the Sr$_3$Ru$_2$O$_7$ (001) surface. We identify both a physisorbed and a chemisorbed CO configuraton. Unexpectedly, in the latter case adsorption occurs in a carboxylate (COO) like state. Both configurations have been confirmed by detailed STM experiments and simulations. In addition we find only a small barrier for the carboxylate formation on the surface. Work supported by the Austrian FWF, SFB F45 (FOXSI). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D1.00008: Theoretical Study of Chemisorption on Nickel and Palladium Clusters Ajit Hira, Jose Pacheco, Danelle Jaramillo, Frank Naranjo We continue our interest in the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, O and F atoms with Pd$_{\mathrm{n}}$ and Ni$_{\mathrm{n}}$ clusters (n $=$ 2 thru 20). Transition-metal clusters are specially suited for the study of quantum size effects and for formation of metallic states, and are ideal candidates for catalytic processes. Hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters of the two different metals. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E), hollow (H), on-top (T), threefold sites and fourfold sites. Effects of crystal symmetries corresponding to the bulk structures for the two metals are investigated. The implications for the molecular dissociation of the H$_{2}$ and O$_{2}$ species will be considered. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D1.00009: Quantum Monte Carlo Calculations of Pt Nanoclusters and (111) Surface William Parker, Anouar Benali, Luke Shulenburger, Jeongnim Kim, Nichols Romero, Jeffrey Greeley Although density functional theory (DFT) has been successfully used to analyze problems in surface catalysis and electrochemistry at a molecular level, there are several important classes of problems where DFT fails spectacularly, predicting incorrect adsorption energies and binding sites. Better understanding these failures and benchmarking methods for correcting them motivates a quantum Monte Carlo (QMC) investigation of platinum nanoclusters and the platinum (111) surface. To evaluate the transferability of our platinum pseudopotential, we first present the fixed-node diffusion Monte Carlo (DMC) equation of state and cohesive energy for fcc platinum. We then show the binding energies of icosahedral nanoclusters with increasing size and the (111) surface energy to lay the groundwork for investigation of adsorption on these catalytically important phases of platinum. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D1.00010: Morphology and the Catalytic Activity of Pd Nanoparticles on TiO$_{2}$- and SrO- Terminated SrTiO$_{3}$ Nanocuboids Bor-Rong Chen, Cassandra George, Linhua Hu, Peter C. Stair, Kenneth R. Poeppelmeier, Richard P. Van Duyne, Yuyuan Lin, Michael J. Bedzyk We report how different surface terminations of SrTiO$_{3}$ (STO) influence the facetted-shape and catalytic performance of supported Pd nanoparticles. A new approach to catalyst studies by synthesizing STO nanocuboids with either TiO$_{2}$- or SrO- terminated surface as a support will be presented. The nanocuboids have well defined (001) surfaces and high surface area; therefore, practical catalytic reaction studies can be carried out while the support resembles the model catalyst surface. This study investigates the morphology and the catalytic activity of Pd nanoparticles deposited by atomic layer deposition (ALD) on STO nanocuboids with TiO$_{2}$ and SrO terminations. We demonstrate that the wt\% loading and Pd nanoparticle size can be controlled by the number of ALD cycles. The morphology and chemical nature of the Pd particles are studied by TEM, X-ray scattering, and X-ray absorption fine structure measurements. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D1.00011: ABSTRACT WITHDRAWN |
Session D2: Focus Session: Quantum Control of Molecular, Nano, and Plasmonic Materials II
Sponsoring Units: DCPChair: Margaret Murnane, University of Colorado
Room: 102
Monday, March 3, 2014 2:30PM - 2:42PM |
D2.00001: Resonant Pump-dump Quantum Control of Solvated Dye Molecules with Phase Jumps Arkaprabha Konar, Vadim Lozovoy, Marcos Dantus Quantum coherent control of two photon and multiphoton excitation processes in atomic and condensed phase systems employing phase jumps has been well studied and understood. Here we demonstrate coherent quantum control of a two photon resonant pump-dump process in a complex solvated dye molecule. Phase jump in the frequency domain via a pulse shaper is employed to coherently enhance the stimulated emission by an order of magnitude when compared to transform limited pulses. Red shifted stimulated emission from successive low energy Stokes shifted excited states leading to narrowband emission are observed upon scanning the pi step across the excitation spectrum. A binary search space routine was also employed to investigate the effects of other types of phase jumps on stimulated emission and to determine the optimum phase that maximizes the emission. Understanding the underlying mechanism of this kind of enhancement will guide us in designing pulse shapes for enhancing stimulated emission, which can be further applied in the field of imaging. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D2.00002: High-fidelity transformations of the vibrational qubits in thiophosgene molecule Dmytro Shyshlov, Dmitri Babikov In this computational work, we study how to use shaped picosecond laser pulses for controlling state-to-state transitions in thiophosgene molecule (CSCl$_{2}$) with the goal of encoding quantum information into molecular vibrational eigenstates and implementing quantum gates with high fidelity. State-to-state transitions are induced indirectly through the excitation of a gateway state within the excited electronic state so that a UV/vis laser can be employed for control. We optimize shape of the laser pulse using optimal control theory (OCT) and numerical propagation of laser-driven vibrational wavepackets. This optimization was performed for two-qubit gate CNOT and we were able to optimize laser pulses with fidelity exceeding 0.9999. We analyze the high-fidelity pulse in the frequency domain and explore its robustness by reducing the number of available frequency channels. We also intentionally introduce systematic and random errors to the pulse in the frequency domain by modifying the values of amplitudes and of phases for different frequency components. We conclude that the accurate control of the vibrational two-qubit system can still be achieved with a very limited number of frequency channels and in the presence of some amplitude and phase errors. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D2.00003: Excited state dynamics {\&} optical control of molecular motors Ted Wiley, Roseanne Sension Chiral overcrowded alkenes are likely candidates for light driven rotary molecular motors. At their core, these molecular motors are based on the chromophore stilbene, undergoing ultrafast cis/trans photoisomerization about their central double bond. Unlike stilbene, the photochemistry of molecular motors proceeds in one direction only. This unidirectional rotation is a result of helicity in the molecule induced by steric hindrance. However, the steric hindrance which ensures unidirectional excited state rotation, has the unfortunate consequence of producing large ground state barriers which dramatically decrease the overall rate of rotation. These molecular scale ultrafast motors have only recently been studied by ultrafast spectroscopy. Our lab has studied the photochemistry and photophysics of a ``first generation'' molecular motor with UV-visible transient absorption spectroscopy. We hope to use optical pulse shaping to enhance the efficiency and turnover rate of these molecular motors. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D2.00004: Implementation of exact and approximate methods for nonadiabatic quantum molecular dynamics induced by the interaction with the electromagnetic field and their applications to local quantum control Aur\'elien Patoz, Jiri Vanicek we have implemented a general split-operator/Magnus integrator algorithm of arbitrary order in accuracy for exact nonadiabatic quantum dynamics of a molecule interacting with a time-dependent electromagnetic field. Then, we have derived and implemented analogous geometric integrators of arbitrary order of accuracy for several approximations of treating the molecule-field interaction: the time-dependent perturbation theory, separation of time scales, Condon, rotating-wave, and ultrashort, ``extreme ultrashort,'' and ``extremely extreme ultrashort'' pulse approximations. Our general and efficient implementation permits every possible combination of these basic approximations, allowing testing the validity of each approximation under the experimental conditions independently. In addition, a local quantum control scheme has been implemented in the same formalism allowing using the exact method and several of our approximations. The algorithms are applied to the four-dimensional vibronic coupling model of pyrazine in order to compare the exact and approximate descriptions of the photoexcitation process with a single laser pulse of finite length as well as nonadiabatic quantum dynamics induced by pump and probe laser pulses. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D2.00005: Moshe Shapiro's Pioneering Contributions to Quantum Control Tamar Seideman The pioneering contributions of Moshe Shapiro to the field of coherent control will be reviewed, starting with his work in the '80s and noting several highlights of his research. The talk will conclude with the new directions he was pursuing at the time of his untimely death including quantum pattern recognition, coherent chiral separation, and the coherent suppression of spontaneous emission, decoherence and other decay processes. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 4:06PM |
D2.00006: Electron dynamics and its control in molecules Invited Speaker: Regina de Vivie-Riedle The accessibility of few femtosecond or even attoseconds pulses opens the door to direct observation of electron dynamics. The idea to steer chemical reactions by localization of electronic wavepackets is intriguing, since electrons are directly involved in bond breaking and formation. The formation of a localized electronic wavepacket requires the superposition of two or more appropriate electronic states. Its guidance is only possible within the coherence time of the system and has to be synchronized with the vibrational molecular motions. In theoretical studies we elucidate the role of electron wavepacket motion for the control of molecular processes. We give three examples with direct connection to experiments. From our analysis, we extract the systems requirements defining the time window for intramolecular electronic coherence, the basis for efficient control. Based on these findings we map out a photoreaction that allows direct control by guiding electronic wavepackets. The carrier envelope of a femtosecond few cycle IR pulse is the control parameter that steers the photoreaction through a conical intersection.\\[4pt] References:\\[0pt] [1] I. Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M. J. J. Vrakking, R. de Vivie-Riedle, and M. F. Kling, Phys. Rev. Lett., 103 (2009), 103002.\\[0pt] [2] P. von den Hoff, I. Znakovskaya, M. F. Kling and R. de Vivie-Riedle, Chem. Phys. , 366 (2009), 139.\\[0pt] [3] P. von den Hoff, M. Kowalewski, R. Siemering and R. de Vivie-Riedle, IEEE Journal of Selected Topics in Quantum Electronics, 18 (2012), 119-129.\\[0pt] [4] T. Bayer, H. Braun, C. Sarpe, R. Siemering, P. von den Hoff, R. de Vivie-Riedle, T. Baumert, and M. Wollenhaupt, Phys. Rev. Lett. 110 (2013), 123003.\\[0pt] [5] M. Kling, P. von den Hoff, I. Znakovskaya, and R. de Vivie-Riedle, Phys. Chem. Chem. Phys. 15 (2013), 9448-9467. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D2.00007: Optical Control of Internal Conversion in Pyrazine Grant Barry, Sima Singha, Zhan Hu, Tamar Seideman, Robert Gordon We apply quantum control schemes previously reserved for atoms and small molecules to more complex polyatomic molecules. Pyrazine was chosen as a model polyatomic molecule for its well-studied conical intersection seam between the S1 and S2 potential energy surfaces (PESs). Using shaped ultraviolet femtosecond laser pulses, we demonstrate optical control of the excited state dynamics of this molecule under collisionless conditions. This was achieved in a pump-probe experiment by employing a genetic algorithm programmed to suppress ionization of the pyrazine molecules at a preselected time. Our findings indicate that the optimized pulses localize the wave packet for times up to 1.5 ps at a location on the coupled S1/S2 PESs where ionization is energetically forbidden. Our approach is general and does not require knowledge of the molecular Hamiltonian. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D2.00008: Chemical control of cyclohexadiene photochemistry Brenden Arruda, Broc Smith, Kenneth Spears, Roseanne Sension The photoinduced ring-opening reaction 1,3-cyclohexadiene (CHD) chromophores is a common motif in optical switches, photochromic materials, and biological systems. The topology of the excited state potential energy surface makes these systems an important paradigm for coherent optical control. Altering substitution patterns on the CHD backbone can lead to different dynamics for the same reactive chromophore, as evidenced by the fluorescence quantum yield of CHD (10$^{\mathrm{-6}})$ compared with the highly substituted Provitamin D$_{\mathrm{3}}$ (2 x 10$^{\mathrm{-4}})$. CHD derivatives such as the 1,4-disubstituted $\alpha $-terpinene and the 2,5-disubstituted $\alpha $-phellandrene, offer model systems to bridge the gap between these two regimes of excited state dynamics. Recently our lab has used ultrafast spectroscopy to characterize the excited state dynamics of these CHD-based systems. Our broadband probe provides additional information about the ground state relaxation and conformational distribution of the photoproducts. An overview of the factors that govern the landscape of the excited state potential energy surface and ground state conformational distribution will be provided based on experimental measurements and electronic structure calculations. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 5:06PM |
D2.00009: Control of Strong Field Molecular Ionization with Shaped Ultrafast Laser Pulses Invited Speaker: Thomas Weinacht Strong field molecular ionization can prepare molecules in superpositions of multiple ionic states, generating an electron hole wave packet. We demonstrate control over which ionic states are populated via strong field ionization by varying the shape of the ionization pulse. Calculations allow us to interpret the control and determine the role of resonances in the neutral molecule and cation. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:42PM |
D2.00010: Molecular dissociation dynamics driven by strong-field multiple ionization Invited Speaker: Philip Bucksbaum We have studied and compared the dynamics of small molecules that have been multiply ionized and dissociated by strong ultrafast infrared lasers or by strong x-ray lasers. In both regimes we find that multiple ionization can occur on time scales comparable to the fastest interatomic motion, and therefore lead to dissociation patterns that can be related to the transient structure and internal motion of the molecules. The mechanisms that produce multiply charged ions are very different in these two cases. Infrared lasers induce field-ionization, while x-ray lasers induce core-ionization followed by Auger relaxation. This affects the dissociation dynamics. In experiments studying the dissociation of 1,3-cyclohexadiene we find that infrared laser-induced multiple ionization is greatly enhanced by transient processes that occur in the vicinity of conical intersections [Bucksbaum and Petrovic, \textit{Faraday Discussions} \textbf{163}, 475 (2013); Petrovic et al., \textit{J.Chem. Phys}. \textbf{139}, (2013)] When strong x-rays are used as the exciting source, the molecular geometry can influence the Auger process and change the fragment relative abundances [Petrovic et al., \textit{Phys. Rev. Letters} \textbf{108}, 253006 (2012)]. We will discuss recent experiments in deuterated acetylene, which employed x-ray pulse-pairs to explore the x-ray fragmentation process in greater detail. [Preview Abstract] |
Session D3: Undergraduate Research - Society of Physics Students III
Sponsoring Units: SPSChair: Toni Sauncy, Society of Physics Students - American Institute of Physics
Room: 107
Monday, March 3, 2014 2:30PM - 2:42PM |
D3.00001: Characterizing the Effect of Surface Hydrophobicity on Depletion Layer Shannon Petersen, Mark Seraly, Dylan McNany, Erin Brown, Adele Poynor When water meets an extended hydrophobic surface a region of reduced density called the depletion layer forms, but this phenomenon has only been experimentally verified on surfaces with contact angles \textgreater 100$^{\circ}$. Using self-assembled monolayers of organothiols on gold we produce surfaces with contact angles between 55$^{\circ}$ and 107$^{\circ}$ and then use surface plasmon resonance spectroscopy to quantify the thickness of the depletion layer formed. These experiments allow for the understanding of how the depletion layer changes with the hydrophobicity of a surface which is one of the underlying mechanisms behind several biological systems. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D3.00002: Third Sound Generation in Superfluid $^4$He Films Adsorbed on Multiwall Carbon Nanotubes Vito Iaia, Emin Menachekanian, Gary Williams A technique is developed for generating third sound in superfluid $^4$He films coating the surface of multiwall carbon nanotubes. Third sound is a thickness and temperature wave of the helium film, and in our case we detect the temperature oscillations with a carbon resistance bolometer. The nanotubes are packed in an annular resonator that is vibrated with a mechanical shaker assembly consisting of a permanent magnet mounted on springs, and surrounded by a superconducting coil. The coil is driven with an oscillating current, vibrating the cell at that frequency. Sweeping the drive frequency over the range 100-200 Hz excites the resonant third sound mode of the cell, seen as a high-Q signal in the FFT analysis of the bolometer signal. A problem with our original cell was that the mechanical drive would also shake the dilution refrigerator cooling the cell to low temperatures, and increasing the drive would start to heat up the refrigerator and the cell, which were rigidly coupled together. A new configuration now suspends the cell as a pendulum on a string, with thermal contact made by copper wires. Piezo sensor measurements show this reduces the vibration reaching the refrigerator by two orders of magnitude, which should allow measurements at lower temperatures. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D3.00003: Effects of added dopants on various triboluminescent properties of europium dibenzoylmethide triethylammonium (EuD$_{4}$TEA) Constance Owens, Ross S. Fontenot, Kamala N. Bhat, Mohan D. Aggarwal A triboluminescent (TL) material is one that emits light upon pressure, impact, friction, or mechanical shock. TL materials are desirable for investigation because they have the potential to be used as the active element for smart impact sensors. While the material europium dibenzoylmethide triethylammonium (EuD$_{4}$TEA) produces a TL emission yield that can be observed by the naked eye, it is still not sufficiently bright for use in smart sensor devices. Previous studies have shown that additional materials can be combined with EuD$_{4}$TEA in order to improve the TL emission yield. In this paper, we discuss the effects of doping on EuD$_{4}$TEA at different concentrations with a variety of materials on the TL emission yield and decay times. The dopants that were used in this study were nicotine, dibutyl phosphate (DBP), and magnesium. We also discuss both the effects of pH on EuD$_{4}$TEA, and the doping effects on impact energy. For testing triboluminescent properties, we use a custom-built drop tower that generates triboluminescence by fracturing compounds through impact. Collected data is analyzed using specially written LabVIEW programs. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D3.00004: Two-dimensional Fourier Transform Studies of Excitons in Layered Indium Selenide N. Glikin, P. Dey, J. Paul, D. Karaiskaj, Z. Kovalyuk, Z. Kudrynskyi, A. Romero Indium selenide (InSe) is a layered semiconducting material whose electronic properties are strongly influenced by many-body interactions. Having potential applications including optoelectronic and photovoltaic uses, it is necessary to understand the nature of these interactions in order to understand the material properties of interest. Three-pulse four-wave mixing (FWM) and Two-dimensional Fourier transform (2DFT) spectroscopy are used to study the many-body interactions in $\gamma $-InSe by measuring excitonic dephasing and lifetime. Excitation-density-dependent and temperature-dependent measurements of the homogeneous linewidth indicate strong contributions of exciton-exciton scattering and exciton-phonon interactions. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D3.00005: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 3:30PM - 3:42PM |
D3.00006: Photo-induced Modulation Doping in Graphene/Boron Nitride Heterostructures Salman Kahn, Jairo Velasco Jr, Long Ju, Edwin Hwang, Casey Nosiglia, Hsin Zon Tsai, Wei Yang, Takashi Taniguchi, Kenji Watanabe, Dillon Wong, Yang Wang, Juwon Lee, Yuanbo Zhang, Guangyu Zhang, Michael Crommie, Alex Zettl, Feng Wang Van der Waals heterostructures (VDH) allow a modular platform for materials engineering, where various layered materials with different electrical, optical, and mechanical properties can be stacked together to enable new physics and novel functionalities. To create various VDH, we have employed a ``stamping transfer'' [1] in which two layered materials are exfoliated on separate substrates and then stamped onto each other. Several distinct VDH structures have been realized and characterized through scanned probe and optical measurement schemes. I will discuss recent progress made on these efforts, with an emphasis on optoelectronic measurements of a Graphene/Boron Nitride VDH.\\[4pt] [1] Zomer, P. J. and Dash, S. P. and Tombros, N. and van Wees, B. J., Applied Physics Letters, 99, 232104 (2011) [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D3.00007: SAM Surface Domains Of 6-Amino-1-Hexanethiol Hydrochloride And 1-Dodecanethiol Mixtures on Au(111) Investigated Via AFM and STM Spectroscopy Albert Foster, John Murphy, Indrajith Senevirathne, Reshani Senevirathne Bioengineering that utilizes Self Assembled Monolayers (SAMs) has been shown to have large potential in biosensing applications. Since these SAMs can be tailored to have different functional groups attached to them, such as amine groups, it is possible to fabricate highly selective surfaces for biological species. In order to understand these surfaces better, a closer characterization of the morphology, and surface structure is needed. Differing concentrations of the solutions 6-Amino-1-hexanethiol hydrochloride (hydrophilic -- NH$_{2})$ and 1-dodecanethiol (hydrophobic -- R) were prepared, all with a total concentration of 5 mM. The mixture was dissolved in 200 proof Ethanol and hydrogen annealed Au(111) samples on mica were let self assembled in hydrocarbon free, clean glassware for a period of 24 hours to facilitate uniform and systematic assembly. These various SAM systems were then characterized via STM (Scanning Tunneling Microscopy) and AFM (Atomic Force Microscopy). The surface morphology and structure were studied via AFM tapping and phase imaging. Surface charge density variations were studied with STM. These results were then correlated against each other to understand the SAM surface system. Cumulative results of these investigations will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D3.00008: A Study of 11-(Ferrocenyl)-1-Undecanethiol Self-Assembled Monolayers on Au(111) Surfaces John Murphy, Reshani Senevirathne, Indrajith Senevirathne SAM (Self-Assembled Monolayer) surfaces terminated with functional/charged groups have exhibited bioactive properties. Improved understanding of surface domain architecture of these systems is needed for bioengineering applications. Solutions of various concentrations of 11-Ferrocenyl-1-Undecanethiol and 1-Dodecanethiol in 200 proof ethanol in clean glassware were used to create a SAM on hydrogen flame annealed Au (111) on mica; 11-Ferrocenyl-1-Undecanethiol has been successfully used in semiconductor interfaces. The potential charge carrying effect of these thiols enables the use of STM (Scanning Tunneling Microscopy) to investigate these SAMs. These SAMs were then investigated further with an AFM (Atomic Force Microscopy) in Non-Contact mode, using topographic and phase imaging. These investigations help to characterize these SAMs from morphological, structural, and electronic perspectives. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D3.00009: Hydrophilic and Hydrophobic Probe Functionalization of 11-Mercapto-1-undecanol and 1-Dodecanthiol SAMs for Chemical Force Microscopy Mackenzie Maurer, Indrajith Senevirathne CFM (Chemical Force Microscopy), a variation in AFM (Atomic Force Microscopy) is a technique that provides details on the chemical nature of surfaces regardless of any particular morphology. An application of this surface analysis technique may lead to a deeper understanding of the surface domain architecture of SAMs (Self Assembled Monolayers) with multi component mixtures of thiols on Au(111) on mica substrates. Unique methods of probe functionalization were developed regarding the formation of SAMs of 11-mercapto-1-undecanol (hydrophilic -OH end) and 1-dodecanthiol (hydrophobic -R end) self assembled on a sputter Au coated, silicon nitride, AFM tip. Resulting hydrophilic and hydrophobic probes were evaluated with the AFM via non contact and tapping mode with topography and phase imaging to determine the success of the unique functionalization methods. Significant progress was made in developing a novel technique, which created functionalized hydrophilic and hydrophobic probes. This may lead to the identification of domains of distinct thiols on the SAMs substrates. The repeatability and accuracy of each functionalization method was assessed to determine the validity of the techniques. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D3.00010: SAM Surface Domains of (11-Mercaptooundecyl)-N,N,N-Trimethlyammonium Bromide and Dodecanthiol Mixtures on Au(111) Investigated Via AFM Michael Schell, Indrajith C. Senevirathne, John Murphy, Albert Foster III Charged/functional SAM (Self Assembled Monolayer) surfaces have many potential applications in various domains including devices for bioengineering. These surfaces also may be interesting because of the complex physics and chemistry of the charged/conductive molecular layers. The SAM used in our study is (11-Mercaptoundecyl)-N,N,N-trimethylammonium bromide, which have shown conductive properties. The substrate support for SAMs are by Au(111) on mica. Crystalline substrate Au surface was established via in-house hydrogen flame annealing. Thiolated solutions of (11-Mercaptoundecyl)-N,N,N-trimethylammonium bromide and dodecanthiol of varying concentration ratios were used as media for self assembly. Total molarity of the solutions was kept at 5mM for with the time for self assembly at 24 hours or more. Morphology, structure and conductivity characteristics were measured via tapping mode Atomic Force Microscopy (AFM) in the topography/phase imaging and Scanning Tunneling Microscopy (STM) in constant current mode. Data will be used to assess the surface structure of these systems. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D3.00011: First order reversal curve study of the dipolar interaction in Ni three-dimensional antidot arrays Bingqing Li, Xuzhao Chai, Sina Moeendarbari, Yaowu Hao, Dustin A. Gilbert, Kai Liu, Di Zhang, Gang Feng, Ping Han, X. M. Cheng Three-dimensional antidot arrays (3DAAs) have attracted considerable attention due to potential applications in sensors, energy storage and transducers. Magnetic 3DAAs also provide an ideal system for studying the effect of dimensionality and morphology on magnetic properties. We report study of dipolar interactions in Ni 3DAAs using the first-order reversal curve (FORC) method. Ordered Ni 3DAAs were fabricated by electrochemical deposition into colloidal crystal templates of self-assembled polystyrene spheres. The samples have the same pore size of about 500 nm but different thicknesses, ranging from 0.3 $\mu $m to 1.2 $\mu $m, confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). FORCs of the samples with thicknesses of 0.3 $\mu $m, 0.8 $\mu $m, and 1.2 $\mu $m were measured by a vibrating sample magnetometer. The FORC diagram analysis reveals a demagnetizing magnetic dipolar interaction, and a decrease in the interaction strength with the increasing sample thickness, evidenced by a decrease in the spread of the irreversible peak in the bias distribution, as well as a decrease in the tilting of the FORC distribution from the local coercivity axis. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D3.00012: Quartz microbalance study of photovoltaic energy balance in fullerene systems Benjamin Keller, Zijian Liu, Jacqueline Krim Energy transfer at an interface is closely linked to the topic of photovoltaic energy conversion, as well as geometries involving tip-substrate contact. To explore this phenomenon, we have employed a Quartz Crystal Microbalance (QCM) in combination with an STM for characterization of QCM oscillator amplitudes and surface morphology. With this setup we are able to detect the degree to which a temperature disparity is present between the tip (nominally at room temperature) and the sample, whose temperature can be regulated. The method provides \textit{in situ} data, making use of the fact that QCM is extremely sensitive to abrupt changes in temperature, with literature reports of sensitivities of mK or less. Similar measurements have been performed by reflecting laser light off of QCM electrodes with a variety of coatings, to explore whether the small heating effects are also detectable and can distinguish the heat absorption of the coating. Studies have been performed on fullerenes and control sample photovoltaics, to explore the balance of the incoming light energy with overall device efficiency. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D3.00013: Engineering and Characterization of Collagen Networks Using Wet Atomic Force Microscopy and Environmental Scanning Electron Microscopy Jenna Osborn, Tonya Coffey, Brad Conrad, Jennifer Burris, Brooke Hester Collagen is an abundant protein and its monomers covalently crosslink to form fibrils which form fibers which contribute to forming macrostructures like tendon or bone. While the contribution is well understood at the macroscopic level, it is not well known at the fibril level. We wish to study the mechanical properties of collagen for networks of collagen fibers that vary in size and density. We present here a method to synthesize collagen networks from monomers and that allows us to vary the density of the networks. By using biotynilated collagen and a surface that is functionalized with avidin, we generate two-dimensional collagen networks across the surface of a silicon wafer. During network synthesis, the incubation time is varied from 30 minutes to 3 hours or temperature is varied from 25$^{\circ}$C to 45$^{\circ}$C. The two-dimensional collagen network created in the process is characterized using environmental atomic force microscopy (AFM) and scanning electron microscopy (SEM). The network density is measured by the number of strands in one frame using SPIP software. We expect that at body temperature (37$^{\circ}$C) and with longer incubation times, the network density should increase. [Preview Abstract] |
Session D4: Focus Session: Itinerant Frustrated Magnets
Sponsoring Units: GMAGChair: Graeme Luke, McMaster University
Room: 112/110
Monday, March 3, 2014 2:30PM - 2:42PM |
D4.00001: Loop-liquid State in an Ising-spin Kondo Lattice Model on a Kagome Latticeh Yukitoshi Motome, Hiroaki Ishizuka Emergence of paramagnetic states with strong local correlations is one of the characteristic features of geometrically frustrated magnets. One such example is the spin-ice compounds, where all the tetrahedra favor two-in two-out spin configurations, and the ``in" and ``out" spins individually form loop-like structures on the pyrochlore lattice. When such a locally-correlated spin state is coupled to itinerant electrons, recent theoretical studies have shown that it considerably affects the electronic state of coupled electrons. While several studies have been done, possible realization and stability of such a correlated-disorder state remains to be studied. To address these issues in the presence of spin-charge coupling, we here study an Ising-spin Kondo lattice model on a kagome lattice [1]. By using a Monte Carlo simulation, we show that a locally-correlated state with all the triangles being in up-up-down spin configurations spontaneously emerges in this model. We also show that the peculiar state considerably affects the electronic state, giving rise to a resonant peak in the optical conductivity. [1] H. Ishizuka and Y. Motome, PRB 88 081105 (2013). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D4.00002: New Kagom\'{e} Metal Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$--- Quantum Spin-Liquid Candidate? Hua He, Wojciech Miiller, Meigan Aronson While most of the reported Kagom\'{e} systems are semiconductors or insulators, in which the magnetic moments have a highly localized character, here we present a new intermetallic compound, Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$, as a rare example of a Kagom\'{e} metal. The structure of the compound was established by single-crystal X-ray diffraction, and it crystallizes with a hexagonal structure (Sc$_{3}$Ni$_{11}$Si$_{4}$ type) with Mn atoms forming the Kagom\'{e} lattice. The \textit{dc} magnetic susceptibility measurements find a Curie-Weiss moment of $\sim$ 0.51 $\mu_{\mathrm{B}}$/Mn, however, no magnetic order is found for temperatures as low as 1.8 K. Electrical resistivity and heat capacity measurements show that this compound is definitively metallic, with an enhanced specific heat Sommerfeld coefficient below 10K, indicating strong electronic correlations. Intriguingly, these features have revealed Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$ as a possible quantum spin liquid. The role of the geometrically frustrated structure and Mn-ligand hybridization in the magnetism of Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$ is also discussed. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D4.00003: Noncoplanar magnetism in the Hubbard model on frustrated lattices Sanjeev Kumar, Kanika Pasrija Ferromagnets and staggered antiferromagnets are the most common forms of magnetic orderings that one comes across in models and materials. However, during the last few years non-collinear and non-coplanar magnetic states have been of special interest for condensed matter researchers due to their relevance to a variety of phenomena, such as, ferroelectricity, anomalous Hall effect, etc. A number of theoretical studies have shown that such magnetic states exist in Kondo-lattice model at special band-fillings on various geometrically frustrated lattices. It was recently shown that the Kondo-lattice model on a checkerboard lattice supports non-coplanar magnetic states which lead to a topologically non-trivial band gap in the electronic spectrum (PRL 109, 166405(2012)). We begin by asking if such magnetic ground-states can also be realized in a Hubbard model which, unlike the Kondo-lattice model, does not contain ``pre-formed'' localized magnetic moments. We make use of a mean-field decoupling scheme which allows for non-collinear and non-coplanar magnetic states in the Hubbard model. We show that the triangular lattice and the checkerboard lattice do support non-coplanar magnetic phases similar to the ones found in a Kondo-lattice model. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D4.00004: Tuning a spin-liquid into a correlated metal in Na$_{4-x}$Ir$_3$O$_{8-\delta}$ Yogesh Singh, Ashwini Balodhi Na$_4$Ir$_3$O$_8$ is a candidate material for a 3D quantum spin-liquid. We present a comprehensive study of the structure, magnetic susceptibility, heat capacity, and electrical transport on polycrystalline samples with nominal composition Na$_{4-x}$Ir$_3$O$_8$ ($x \approx -.08~{\rm to}~1$). The structure refinement shows that even though Na vacancies are being introduced the lattice parameters do not change much with $x$. The $x \geq 0$ samples show insulating behavior with strong antiferromagnetic interactions between effective $S = 1/2$ Ir$^{4+}$ moments. For the Na$_{4.08}$Ir$_3$O$_8$ sample, magnetic susceptibility suggests a magnetic transition below $\approx 15 K$. The $x \approx 1$ sample is a paramagnetic (semi)metal with various physical properties suggesting strong electronic correlations. The materials mid-way between the insulating and metallic samples show indication of having both localized and itinerant electrons. The strong antiferromagnetic interactions present in the $x = 0$ material survive in these mixed materials making them candidate spin-liquids in the presence of itinerant electrons. The electrical transport of the doped materials are consistent with the behavior of a semi-metal/semi-conductor with low carrier concentrations. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D4.00005: Anomalous Hall Effect Arising form Noncollinear Antiferromagnetism: Mn3Ir as an Example Hua Chen, Qian Niu, Allan H. MacDonald Ferromagnetic conductors exhibit anomalous contributions to their transverse (Hall) conductivities that cannot be attributed to Lorentz force on electrons from a magnetic field. The anomalous Hall conductivity is often assumed to be proportional to the magnetization, allowing transport measurements to be used in spintronics as a convenient proxy for magnetometry. However, simple symmetry arguments demonstrate that the anomalous Hall effect requires only time-reversal symmetry breaking and spin-orbit coupling, not net magnetization, and we illustrate our ideas by examining a toy model of noncollinear antiferromagnet on a two-dimensional kagome lattice. This is further backed up with a realistic example based on first-principles calculations, predicting that single-crystals of Mn$_3$Ir, a high-temperature antiferromagnet commenly used in spin-valve devices, have large anomalous Hall conductivities. Hua Chen, Qian Niu, and Allan H. MacDonald, arXiv:1309.4041 [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D4.00006: Reconsidering the magnetic structure in NiS$_{2-x}$Se$_{x}$ Shinichiro Yano, Despina Louca, Utpal Chatterjee, Duck Young Chung, Daniel E. Bugaris, Mercouri Kanatzidis, Joerg C. Neuefeind, Mikhail Feygenson The Mott metal-insulator transition (MIT) has been at the forefront of condensed matter research for decades. A Mott insulator is associated with antiferromagnetism (AFM) as well as an energy gap. The AFM order parameter can be directly traced by neutron scattering measurements. We focused on the study of MIT on the NiS$_{2-2x}$Se$_{x}$ system. With increase in $x$ where $x$ corresponds to the atomic volume of S that is replaced by Se, the system undergoes an AFM insulator to an AFM metalic transition at $x = 0.43$ at $T = 0$. Although NiS$_{2-2x}$Se$_{x}$ has been previously studied, the magnetic structure is not well understood. We measured the powder neutron diffraction for 4 compositions, $x =$ 0, 0.4, 0.6, and 0.8, as a function of temperature. At T = 2K, we observed a clear composition dependence of the magnetic structure. While NiS$_2$ (x = 0) has two magnetic propagation vectors, M1 = (000) and M2 = (0.5 0.5 0.5), NiS$_{1.6}$Se$_{0.4}$ and NiS$_{1.4}$Se$_{0.6}$ have only one magnetic phase, M1. However, the M1 structure vanishes by NiS$_{1.2}$Se$_{0.8}$. While the two magnetic phases have been previously reported, we determined the magnetic structures by using representation analysis.the magnetic structure and physical properties of this system will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D4.00007: Theory of a Competitive Spin Liquid State for Weak Mott Insulators on the Triangular Lattice Ryan V. Mishmash, James R. Garrison, Samuel Bieri, Cenke Xu We propose a novel quantum spin liquid state that can explain many of the intriguing experimental properties of the low-temperature phase of the organic spin liquid candidate materials $\kappa$-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ and EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$. This state of paired fermionic spinons preserves all symmetries of the system, and it has a gapless excitation spectrum with quadratic bands that touch at momentum $\vec{k}=0$. This quadratic band touching is protected by symmetries. Using variational Monte Carlo techniques, we show that this state has highly competitive energy in the triangular lattice Heisenberg model supplemented with a realistically large ring-exchange term. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D4.00008: Quantum Oscillations of the Metallic Triangular-Lattice Antiferromagnet PdCrO$_{2}$ Jong Mok Ok, Y.J. Jo, Kyoo Kim, T. shishidou, E.S. Choi, Han Jin Noh, T. Oguchi, B.I. Min, Jun Sung Kim We report the electronic and transport properties of the triangular antiferromagnet PdCrO$_{2}$ at high magnetic fields up to 33 T, using measurements of the de Haas-van Alphen oscillations and the Hall resistivity. The de Haas-van Alphen oscillations below the magnetic ordering temperature T$_{\mathrm{N}}$ reveal several two-dimensional Fermi surfaces of smaller size than those found in nonmagnetic PdCoO$_{2}$, consistent with the band structure calculation. This evidences Fermi surface reconstruction due to the 120$^{\circ}$ helical ordering of the localized Cr spins, suggesting significant coupling of the itinerant electrons to the underlying spin texture. This induces the nonlinear Hall resistivity at low temperatures via the magnetic breakdown in the reconstructed Fermi surface. Furthermore, such a coupling leads to the unconventional anomalous Hall effects near T$_{\mathrm{N}}$ due to the field-induced spin chirality at high magnetic fields. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D4.00009: Importance of anisotropy in the spin-liquid candidate Me$_3$EtSb[Pd(dmit)$_2$]$_2$ Luca F. Tocchio, Anthony Jacko, Harald O. Jeschke, Roser Valenti Organic charge transfer salts based on the molecule Pd(dmit)$_2$ display strong electronic correlations and geometrical frustration, leading to spin liquid, valence bond solid, and superconducting states, amongst other interesting phases. The low energy electronic degrees of freedom of these materials are often described by a single band model; a triangular lattice with a molecular orbital representing a Pd(dmit)$_2$ dimer on each site. We use \textit{ab initio} electronic structure calculations to construct and parametrize low energy effective model Hamiltonians for a class of Me$_{4-n}$ Et$_nX$[Pd(dmit)$_2$]$_2$ ($X$=As, P, N, Sb) salts and investigate how best to model these systems by using variational Monte Carlo (VMC) simulations. Our findings suggest that the prevailing model of these systems as a $t-t'$ triangular lattice is incomplete, and that a fully anisotropic triangular lattice description produces importantly different results, including a significant lowering of the critical $U$ of the spin-liquid phase.[1,2] [1] A.C. Jacko, L. F. Tocchio, H. O. Jeschke, R. Valenti, Phys. Rev. B 88, 155139 (2013). [2] L. Tocchio, H. Feldner, F. Becca, R. Valenti, C. Gros Phys. Rev. B 87, 035143 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D4.00010: Itinerant spin ice Invited Speaker: Masafumi Udagawa Spin ice is a prototypical frustrated magnet defined on a pyrochlore lattice. The ground state of spin ice is described by a simple rule called ``ice rule": out of four spins on a tetrahedron, two spins point inward, while the other two outward. This simple rule is not sufficient to determine the spin configuration uniquely, but it leaves macroscopic degeneracy in the ground state. Despite the macroscopic degeneracy, however, the ground state is not completely disordered, but it exhibits algebraic spatial correlation, which characterizes this state as ``Coulomb phase'' where various exotic properties, such as monopole excitations and unusual magnetic responses are observed. Given the peculiar spatial correlation, it is interesting to ask what happens if itinerant electrons coexist and interact with spin ice. Indeed, this setting is relevant to several metallic Ir pyrochlore oxides, such as Ln$_2$Ir$_2$O$_7$ (Ln=Pr, Nd), where Ir 5d itinerant electrons interact with Ln 4f localized moments. In these compounds, anomalous transport phenomena have been reported, such as non-monotonic magnetic field dependence of Hall conductivity [1] and low-temperature resistivity upturn [2]. To address these issues, we adopt a spin-ice-type Ising Kondo lattice model on a pyrochlore lattice, and solve this model by applying the cluster dynamical mean-field theory and the perturbation expansion in terms of the spin-electron coupling. As a result, we found that (i) the resistivity shows a minimum at a characteristic temperature below which spin ice correlation sets in [3]. Moreover, (ii) the Hall conductivity shows anisotropic and non-monotonic magnetic field dependence due to the scattering from the spatially extended spin scalar chirality incorporated in spin ice manifold [4]. These results give unified understanding to the thermodynamic and transport properties of Ln$_2$Ir$_2$O$_7$ (Ln=Pr, Nd), and give new insights into the role of geometrical frustration in itinerant systems. This work has been done in collaboration with H. Ishizuka, Y. Motome and R. Moessner. \\[4pt] [1] Y. Machida et al., Phys. Rev. Lett. 98, 057203 (2007).\\[0pt] [2] S. Nakatsuji et al., Phys. Rev. Lett. 96, 087204 (2006).\\[0pt] [3] M. Udagawa, H. Ishizuka and Y. Motome, Phys. Rev. Lett. 108, 066406 (2012).\\[0pt] [4] M. Udagawa and R. Moessner, Phys. Rev. Lett., 111, 036602 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D4.00011: Many-Variable Variational Monte Carlo Study of Triangular Hubbard Model with Next-Nearest-Neighbor Hopping Ryui Kaneko, Satoshi Morita, Masatoshi Imada Motivated by previous numerical studies on the triangular Hubbard model, we study how next-nearest-neighbor hopping affects the ground states of the model at half filling by using the many-variable variational Monte Carlo method. We consider the fermionic type variational wave functions with the Gutzwiller-Jastrow factor and the projection that restore the lattice point group symmetry. We find that the spin liquid state, sandwiched by the metallic state and the antiferromagnetic insulating states with 120$^{\circ}$ or stripe spin structure, becomes more stable as the negative next-nearest-neighbor hopping increases. By using the total momentum projection scheme, we also find that the spin liquid state is characterized by nearly gapless excitations in extended total momenta. Possible nature of the present spin liquid state is discussed. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D4.00012: Two-peak structure in the K-edge RIXS spectra of a spatially frustrated Heisenberg antiferromagnet Trinanjan Datta, Cheng Luo, Dao-Xin Yao Quantum fluctuations due to spatial anisotropy and strong magnetic frustration lead to the formation of a two-peak structure in the K-edge bimagnon RIXS intensity spectra of a Jx-Jy-J2 Heisenberg model on a square lattice. We compute the RIXS intensity, including up to first order 1/S spin wave expansion correction, using the Bethe-Salpeter equation within the ladder approximation scheme. The two-peak feature occurs in both the antiferromagnetic phase and the collinear antiferromagnetic phase. A knowledge of the peak splitting energy from both magnetically ordered regime can provide experimentalists with an alternative means to measure and study the effects of local microscopic exchange constants. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D4.00013: First-Principles Study on New Spin Liquid Candidate $\kappa$-H$_3$(Cat-EDT-TTF)$_2$ Takao Tsumuraya, Hitoshi Seo, Reizo Kato, Tsuyoshi Miyazaki A new class of molecular conductors based on catechol with ethylenedithiote-tetrathiafulvalene, (H$_2$Cat-EDT-TTF) has been synthesized recently. Among them, $\kappa$-type H$_3$(Cat-EDT-TTF)$_2$ is considered to be a dimer-type Mott insulator at ambient pressure and emerges as a candidate of realizing quantum spin liquid down to lowest temperature.[1] In this crystal, two H$_2$Cat-EDT-TTF molecules share a hydrogen (H) atom, and face-to-face dimers are formed in a anisotropic triangular lattice. Differently from conventional charge transfer salts, this compound does not have insulating layers. Here we study $\kappa$-H$_3$(Cat-EDT-TTF)$_2$ based on first-principles density-functional theory (DFT) calculations. We evaluate inter-dimer hopping integrals by fitting to the DFT bands, and find a quasi-one-dimensional anisotropy in the effective dimer-dimer interactions with frustrated inter-chain couplings. Furthermore, the inter-layer hopping integrals are non-negligible compared to the intra-layer couplings and the Fermi surface shows a warped cylinder, indicating their three-dimensionality of the electronic structure. Lastly, we report sensitivity of the electronic structure depending on the position of the shared H atom. [1] T. Isono et al, Nature Comm. 4 (2013) 1344; Priv. comm. [Preview Abstract] |
Session D6: Focus Session: Emergent Properties in Bulk Complex Oxides: Orthochromates and Other Oxides
Sponsoring Units: GMAG DMPChair: Brian Kirby, National Institute of Standards and Technology
Room: 108
Monday, March 3, 2014 2:30PM - 3:06PM |
D6.00001: Breathing Pyrochlore Lattice Realized in the A-Site Ordered Spinel Oxides LiGaCr$_{4}$O$_{8}$ and LiInCr$_{4}$O$_{8}$ Invited Speaker: Yoshihiko Okamoto A Cr spinel oxide ACr$_{2}$O$_{4}$ with a nonmagnetic A$^{2+}$ ion at the tetrahedral site provides an interesting playground for studying magnetic frustration in a pyrochlore lattice made of Cr$^{3+}$ ions with an $S =$ 3/2 spin. We found a novel type of frustrated lattice called ``breathing'' pyrochlore lattice, which is made of Cr$^{3+}$ ions in two A-site ordered spinel oxides, LiGaCr$_{4}$O$_{8}$ and LiInCr$_{4}$O$_{8}$ [1]. Because of the large size mismatch between Li$^{+}$ and Ga$^{3+}$/In$^{3+}$ ions, they alternately occupy the tetrahedral sites so as to form a Zinc Blende lattice. This transforms the conventional pyrochlore lattice into an alternating array of small and large tetrahedra, while keeping their shapes regular. LiGaCr$_{4}$O$_{8}$, with a lesser degree of alternation, shows similar magnetic properties to the conventional Cr spinel oxides such as ZnCr$_{2}$O$_{4}$. In contrast, LiInCr$_{4}$O$_{8}$ shows a spin-gap behavior in its magnetic susceptibility caused by a large alternation of magnetic interaction in the more breathing pyrochlore lattice. This suggests that LiInCr$_{4}$O$_{8}$ exists in a proximity to an exotic singlet ground state based on a tetramer singlet formed in the smaller tetrahedron, although it finally goes to a magnetically ordered state below 13 K, which may be triggered by a structural transition. We will also present NMR and neutron scattering measurements carried out to elucidate the nature of these compounds, and our recent results on solid solutions between the two compounds.\\[4pt]The work has been done in collaboration with T. Nakazono, Z. Hiroi, Y. Tanaka, M. Yoshida, M. Takigawa, T. Masuda (ISSP, Univ. of Tokyo), G. J. Nilsen, H. Mutka, T. Hansen (ILL), and J. P. Attfield (Univ. of Edinburgh). \\[4pt] [1] Y. Okamoto, G. J. Nilsen, J. P. Attfield, and Z. Hiroi, Phys. Rev. Lett. 110, 097203 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D6.00002: Magnetic Behavior of Rare-Earth Substituted Orthochromites Austin McDannald, Lukasz Kuna, Menka Jain Rare-earth orthochromites (RCrO$_3$) with ABO$_3$ perovskite structure have recently attracted attention due their potential as magnetoelectric multiferroics (ME MFs). Materials with both magnetic and ferroelectric ordering are considered magnetoelectric mulitferroics. As compared to the rare earth manganite based ME MFs, such as TbMnO$_3$ (with Neel temperature ~ 40 K), the rare-earth orthochromites have Neel temperatures close to 150 K that could result in higher temperature ferroelectric transition than that of TbMnO$_3$. While the mechanism of ferroelectricity and ferroelectric transition in the orthochromites is still unclear, these materials are known to show canted antiferromagnetic ordering. However, there is little understanding of the R-Cr interaction that may result in interesting magnetic behavior. In this work, several rare-earth (such as Er, Nd, etc.) substituted at the A-site of DyCrO$_3$ have been studied. The bulk powder samples were prepared by a citrate route and the phase purity was examined by the X-ray diffraction measurements. In the Nd substituted samples trends in the coercive field and the emergence of exchange bias were observed. A better understanding of the magnetic properties of these orthochromites may lead to development of high temperature ME MFs. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D6.00003: Half Metallicity in Trigonal MnPO$_{4}$ and CrPO$_{4}$ Crystals Boris Kiefer The search for half-metallic compounds continues to be an active area even after several decades of intense research. Half-metals are prime candidates with applications as spin injection materials. Yet, the corresponding material set remains comparatively limited. Here we report a new structural template with Mn and Cr in tetrahedral oxygen coordination. The tetrahedral MnO$_{4}$ and CrO$_{4}$ groups share corners with intermittent PO$_{4}$ groups to form a 3d bond topology. All present computations are based on spin-polarized DFT computations at the GGA-PBE level using all-electron like PAW interaction potentials. The preliminary results show a spin-gap in the minority spin channel for both compounds with magnetic moments of 3 $\mu_{\mathrm{B}}$/fu and 4 $\mu _{\mathrm{B}}$/fu for the Cr and Mn compound, respectively. Furthermore, in both compounds the half-metallic state is energetically more favorable as compared to the competing antiferromagnetic state. Therefore, these compounds which are isomorphic to the previously synthesized Fe analog may provide a new structural class of half-metallic compounds. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D6.00004: Direct Observation of Localized Spin Antiferromagnetic Transition in PdCrO$_2$ by Angle-Resolved Photoemission Spectroscopy Han-Jin Noh, Jinwon Jeong, Bin Chang, Dahee Jeong, Hyun Sook Moon, En-Jin Cho, Jong Mok Ok, Jun Sung Kim, Kyoo Kim, B.I. Min, Han-Koo Lee, Jae-Young Kim, Byeong-Gyu Park, Hyeong-Do Kim, Seongsu Lee We report the first case of the successful measurements of a localized spin antiferromagnetic transition in delafossite-type PdCrO$_2$ by angle-resolved photoemission spectroscopy (ARPES). This demonstrates how to circumvent the shortcomings of ARPES for investigation of magnetism involved with localized spins in limited size of two-dimensional crystals or multi-layer thin films that neutron scattering can hardly study due to lack of bulk compared to surface. Also, our observations give direct evidence for the spin ordering pattern of Cr$^{3+}$ ions in PdCrO$_2$ suggested by neutron diffraction and quantum oscillation measurements, and provide a strong constraint that has to be satisfied by a microscopic mechanism for the unconventional anomalous Hall effect recently reported in this system. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D6.00005: Phase diagram of the spinel system Fe$_{1+x}$Cr$_{2-x}$O$_{4}$ (0 $\leq x \leq$ 1) Andhika Kiswandhi, James Brooks, Haidong Zhou Here, we report the resistivity, specific heat, and the susceptibility of the series of polycrystalline spinel Fe$_{1+x}$Cr$_{2-x}$O$_{4}$ with 0 $\leq x \leq$ 1. The study shows that as the degree of inversion (x) increases, the magnetic transition temperature increases while the resistivity decreases in general. This demonstrates the intricate relationship between the magnetic interaction and the transport properties. Comparison to the previously studied normal vanadate spinel AV$_{2}$O$_{4}$ is also presented. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D6.00006: NMR studies of anisotropy and metal-insulator crossover in quasi-one-dimensional metal Li$_{0.9}$Mo$_{6}$O$_{17}$ Guoqing Wu, W. Gilbert Clark, Stuart Brown, John J. Neumeier, C.A.M. dos Santos, J. Marcus, C. Berthier, M. Horvatic The quasi-1D metal Li$_{0.9}$Mo$_{6}$O$_{17}$ is thought to exhibit transport properties associated with a Luttinger liquid at high temperatures, and otherwise many of its properties have long been mysterious. Among these is an unusual increase in resistivity at low temperatures, for which a robust explanation remains elusive. We present the $^{7}$Li-NMR/$^{95}$Mo-NMR measurements over a wide range of temperature and angle of alignment of applied magnetic field (B0) from 6 T to 14.8 T on single crystal of Li$_{0.9}$Mo$_{6}$O$_{17}$. We find a Korringa relation at high temperatures which indicates electron correlations are unimportant for $T > T_{m}$ (resistivity minimum temperature), and apparent deviations from Korringa for $T$ $\leq$ 24 K. Further, in the single crystal studied, inequivalent magnetic environments are detected at the Li sites in the same low-temperature regime, but only for fields applied near to $B_{0}$ $\parallel$ $c$. Only one $^{95}$Mo site (out of 6 different crystallographic sites) was detected, and 1/($T_{1}T$) at this site is decreasing at low temperatures. We discuss these observations in relation to possible mechanisms for the low temperature resistivity increase. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D6.00007: Magnetic field-induced spontaneous polarization reversal in multiferroic Mn$_{0.85}$Co$_{0.15}$WO$_{4}$ N. Poudel, K.-C Liang, Y.Q. Wang, Y.Y. Sun, B. Lorenz, F. Ye, J.A. Fernandez-Baca, C.W. Chu In this work, we report the effect of c-axis magnetic field in magnetic and ferroelectric properties of multiferroic Mn$_{1-x}$Co$_{x}$WO$_{4}$ for $x$=0.15,0.135, and 0.17. For $x$=0.15, which is the critical doping that separates ground state AF5 and AF2/4 magnetic phases, the positive b-axis polarization($P_{b}$) is reversed spontaneously at $\sim$ 7K, when the magnetic field along c-axis is $\geq20 kOe$ even for the positive poling voltage. From the polarization measurement for $x$=0.135 and 0.17, we found that $P_{b}$ originates from both AF5 and AF2/4 phases, however, c-axis magnetic field of $\geq 20 kOe$ is needed for former case which is the effect of spin flop transition. Magnetic data for $x$=0.135 clearly show the existence of spin flop transition in a c-axis magnetic field. By comparing similar data for $x$=0.15 we conclude that the spin flop also happens in the AF5 phase which coexists with AF2/4 magnetic structure. The polarization reversal is explained by a coupling of different domains preserving the chirality of the spiral spin structure. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D6.00008: Predicting Nanocheckerboards in $ZnMnGaO_4$ Spinel from First Principles Mordechai Kornbluth, Chris A. Marianetti Self-organizing nanocheckerboards raise fascinating questions and present exciting possibilities for ultrahigh-density memory devices. A family of checkerboards fabricated from Mn-based spinels consist of phase-separated Mn-rich (tetragonal) and Mn-poor (cubic) phases. We analyze the earliest example of this family, $ZnMnGaO_4$. Density functional theory (DFT) confirms that the phase separation originates in the Jahn-Teller effect present when Mn ions occupy octahedrally-coordinated sites. DFT calculations demonstrate a strongly preferred (011) interface, which generates checkerboards by geometric considerations. We further investigate both kinetic and thermodynamic limitations in nanocheckerboard composition and size. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D6.00009: Colossal magnetoelectric effect in Co$_3$TeO$_6$ family of compounds Sergey Artyukhin, Yoon Seok Oh, Jun Jie Yang, Vivien Zapf, Jae Wook Kim, Sang-Wook Cheong, David Vanderbilt Multiferroic Co$_3$TeO$_6$ and related materials attracted much attention recently due to their rich phase diagrams, magnetic field -- driven electric polarization and incommensurate spin structures. We model the interacting magnetic and ferroelectric degrees of freedom in these compounds with Landau-type theory and calculate the phase diagram. Comparison of our results with experiment reveals that a particular magnetic anisotropy in some of the compounds results in a second-order spin-flop transition, associated with a large change of polarization. In the vicinity of the transition the spin-flopped phase can be stabilized by a small external magnetic field, which gives rise to a colossal magnetoelectric effect, recently demostrated experimentally. Furthermore, we analyze the types of domain walls that can occur in these materials, and study their interactions. The clamping of domain walls of different types enables the cross-control of ferroic orderings, although they may not be coupled in the bulk. We corraborate our results with ab-initio computations of the polarization, piezoelectric response and optical properties. Our results could pave the way to the design of a new generation of magnetoelectric devices. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D6.00010: Directional dichroism of THz radiation in Sr$_2$CoSi$_2$O$_7$ Toomas R\~o\~om, U. Nagel, V. Kocsis, D. Szaller, I. K{\'e}zsm{\'a}rki, Y. Tokunaga, Y. Taguchi, Y. Tokura The microscopic mechanism of magnetoelectric coupling in akermanite-like Co-oxide multiferroics is unique because the local electric polarization mainly arises from the hybridization of Co ion and its ligands orbitals and is less affected by the details of the actual magnetic order of Co spins. As a consequence of this magnetoelectric effect, the spin waves located in the THz range exhibit giant directional dichroism in Ba$_2$CoGe$_2$O$_7$ [S. Bordacs et al., Nature Physics {\bf 8}, 734 (2012)]. Here we studied spin excitations in a sister compound Sr$_2$CoSi$_2$O$_7$ in magnetic fields up to 17 T. We found that the giant directional dichroism at THz frequencies is present below the Neel temperature (T$_N$) where the spins are ordered antiferromagnetically and persists as well above T$_N$ due to the large uniform magnetization and electric polarization induced by the external magnetic field. The relation of the observed ac magnetoelectric effect to the dc magnetoelectric effect studied by Akaki et al. [Phys. Rev. B {\bf 86}, 060413(R) (2012)] is also discussed. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D6.00011: A new bonding insight on Ba$_{2}$NaOsO$_{6}$ based on orbital-quenching-induced magnetism Shruba Gangopadhyay, Kwan-Woo Lee, Kyo-Hoon Ahn, Warren Pickett Double perovskite Ba$_{2}$NaOsO$_{6}$ (BNOO) is an exotic example of a heptavalent osmium compound, and also uncommon by being a ferromagnetic insulator. Although the single 5d t$_{\mathrm{2g}}$ electron from Os orders magnetically, there is no evidence of orbital order that would destroy its cubic symmetry. Local density approximation with Hubbard U (LDA$+$U) calculation revealed very strong Os d -- O p hybridization into weakly overlapping cluster orbitals, but was unable to obtain the observed Mott insulating behavior. The gap can obtained using DFT$+$U$+$ spin-orbit coupling (SOC) with unreasonably high value of U. Building from the basic understanding from LDA$+$U calculations, we have performed hybrid DFT studies, including SOC, implemented in Wien2k. This method obtains a narrow gap, and an orbital moment of -0.42 muB that strongly compensates the $+$0.52 muB spin moment. The effects of SOC on the spin density will be presented and discussed, as will the change in the electronic and magnetic properties under pressure. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D6.00012: Successive magnetic field-induced phase transition in a multiferroic hexagonal system up to 92 T J.W. Kim, E. Mun, M. Jaime, N. Harrison, V. Zapf, Y. Oh, J. Yang, S.-W. Cheong, S. Artyukhin, D. Vanderbilt We report the observation of successive magnetic field-induced phase transitions in a multiferroic hexagonal system up to 92 T. We find unusually strong magnetoelectric coupling at a hysteresis-free phase transition at low fields in which magnetization can be switched by electric fields and electric polarization can be switched by magnetic field. This transition is accompanied by a large magnetoelectric response that is due to the very small energy barrier between the low and high field phase. We explore this compound to high magnetic fields and observe another phase transition at $\sim$ 50 T in magnetization ($M$), electric polarization ($P$), and magnetostriction measurements. The high field transition displays a relatively small jump in $M$ but much larger change in $P$ compared to the low field one. Measurements to very high magnetic field in combination with modeling reveal the hierarchy of exchange and dipole interactions that is relevant to the successive magnetic transitions in this compound and suggests possible spin structures at each phases. Both field-induced transitions in this material shows a sharp and large jump in magnetostriction which, in combination with the non-centrosymmetric structure, allow for significant changes in the electric polarization. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D6.00013: A ferroelectric-like structural transition in a metallic 5$d$ oxide LiOsO$_{3}$ Kazunari Yamaura, Youguo Shi, Masao Arai, Kenji Tsuda, Yanfeng Guo, Andrew Princep, Andrew Boothroyd, Dmitry Khalyavin, Pascal Manuel Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ``ferroelectric'' metal, in which a ferroelectric-like transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric ($R$-3$c)$ to non-centrosymmetric ($R$3$c)$ transition in metallic LiOsO$_{3}$ that is structurally equivalent to the ferroelectric transition of LiNbO$_{3}$. The transition involves a continuous shift in the mean position of Li$^{+}$ ions on cooling below 140 K. Its discovery realizes the scenario described by Anderson and Blount, and establishes a new class of materials whose properties may differ from those of normal metals. This research was supported in part by a Grant-in-Aid for Scientific Research (22246083, 25289233) from JSPS, Japan; the Funding Program for World-Leading Innovative R{\&}D on Science and Technology (FIRST Program) from JSPS, Japan; and the United Kingdom Engineering and Physical Sciences Research Council (EPSRC). [Preview Abstract] |
Session D7: Focus Session: Magnetic Anisotropy
Sponsoring Units: GMAG DMPChair: Weigang Wang, University of Arizona
Room: 106
Monday, March 3, 2014 2:30PM - 2:42PM |
D7.00001: Magnetic anisotropy properties of CoFeB/W bilayers and W/CoFeB/MgO trilayers Yongxi Ou, Yun Li, L.H. ViLeLa Leao, Chi-Feng Pai, D.C. Ralph, R.A. Buhrman The development of highly scaled spin torque MRAM cells requires that the ferromagnetic free layer in a magnetic tunnel junction, generally a CoFeB layer in combination with an MgO tunnel barrier, have strong perpendicular magnetic anisotropy (PMA) in as thick a free layer as possible. Currently this PMA is believed to arise from interfacial anisotropy energy at the CoFe/MgO interface due to Fe-O bonds. We have found that strong PMA can also be achieved in W/CoFeB/MgO trilayers under certain growth and annealing conditions. Control experiments with amorphous-substrate/CoFeB/W bilayer structures indicate that PMA can be induced by the interface between CoFeB and W and that the effective interfacial anisotropy energy density is quite large in comparison to that found with the CoFeB/MgO PMA system. We have used spin torque ferromagnetic resonance to study the angle dependent anisotropy in these structures and find that there is a quite strong second order component competing with the first order term. We have used second-harmonic anomalous Hall voltage measurements to determine the strength of the spin-orbit torques in these W based systems. We will report on these measurements as well as on the spin-Hall-effect induced switching in our samples. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D7.00002: Magneto-Ionic Control of Interfacial Magnetic Anisotorpy Uwe Bauer, Satoru Emori, Geoffrey Beach Voltage control of magnetism could bring about revolutionary new spintronic memory and logic devices. Here, we examine domain wall (DW) dynamics in ultrathin Co films and nanowires under the influence of a voltage applied across a gadolinium oxide gate dielectric that simultaneously acts as an oxygen ion conductor. We investigate two electrode configurations, one with a continuous gate dielectric and the other with a patterned gate dielectric which exhibits an open oxide edge right underneath the electrode perimeter. We demonstrate that the open oxide edge acts as a fast diffusion path for oxygen ions and allows voltage-induced switching of magnetic anisotropy at the nanoscale by modulating interfacial chemistry rather than charge density. At room temperature this effect is limited to the vicinity of the open oxide edge, but at a temperature of 100$^{\circ}$C it allows complete control over magnetic anisotropy across the whole electrode area, due to higher oxygen ion mobility at elevated temperature. We then harness this novel ``magneto-ionic'' effect to create unprecedentedly strong voltage-induced anisotropy modifications of 3000 fJ/Vm and create electrically programmable DW traps with pinning strengths of 650~Oe, enough to bring to a standstill DWs travelling at speeds of at least 20~m/s. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D7.00003: Origins of voltage effect on interfacial magnetic anisotropy at nanoscale: ab-initio simulations Roman Chepulskyy, Dmytro Apalkov We estimate the effect of electric field on Fe|MgO interfacial perpendicular magnetic anisotropy (PMA) from first principles with and without oxygen at interface. Segregation profile of oxygen is constructed. The possible origins of effect are analyzed by comparison of simulations with published experimental data. Previously it was often assumed that voltage controlled anisotropy (VCA) primarily originates from the modifications of electron density of states. We conclude that such mechanism as well as lattice distortions and undiffusive oxygen atoms at interface cannot explain the experimentally observed effects. The oxygen ion electromigration is suggested as a primary possible mechanism of large PMA change in electric field leading to asymmetrical and time dependent effect. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D7.00004: Perpendicular Magnetic Anisotropy Driven by Antiferromagnetic Layers Invited Speaker: Minn-Tsong Lin We show a novel effect of an antiferromagnet (AFM) with the interfacial unpinned moments, which can form an intrinsic perpendicular anisotropy and switch the magnetization of adjacent ferromagnetic layer from in-plane into perpendicular orientation, providing a new feature of AFM. AFM has been known that its interfacial moments can lead to crucial effects of induced coercivity enhancement and exchange bias field on adjacent ferromagnet (FM), both of which are important for the design of state-of-the-art magnetic logic devices. In the study on the system of Fe/Mn bilayers, the unpinned moments of the Mn can form an intrinsic perpendicular anisotropy that drives the magnetization of an adjacent Fe layer from the in-plane into out-of-plane direction [1]. In the systematic measurements with variations of temperature and FM and AFM thickness, a phenomenological analysis shows that the perpendicular anisotropy is correlated to AFM/FM exchange coupling, and can be modulated according to the finite size effect of AFM ordering [2]. Our x-ray magnetic circular dichroism (XMCD) experiment [1] indicates further that the magnitude of perpendicular anisotropy of the system is enhanced proportionally to the out-of-plane oriented orbital moment of the unpinned Mn layer, rather than that from the Fe layer, providing evidence for the unpinned Mn moments as the origin of the established perpendicular magnetization. The result presented here shows functional characteristics other than the well-investigated phenomena of coercivity enhancement and exchange bias, and renews our knowledge on the role of the AFM layer [1-3], providing a new angle for the design of future perpendicular spintronic nanodevices. \\[4pt] [1] B. Y. Wang, J. Y. Hong, K. H. Ou Yang, Y. L. Chan, D. H. Wei, H. J. Lin, and Minn-Tsong Lin, Phys. Rev. Lett. 110, 117203 (2013).\\[0pt] [2] B. Y. Wang, N. Y. Jih, W. C. Lin, C. H. Chuang, P. J. Hsu, C. W. Peng, Y. C. Yeh, Y. L. Chan, D. H. Wei, W. C. Chiang, and Minn-Tsong Lin, Phys. Rev. B 83, 104417 (2011). \\[0pt] [3] B. Y. Wang, C. C. Chiu, W. C. Lin, and Minn-Tsong Lin, Appl. Phys. Lett. 103, 042407 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D7.00005: FMR Linewidth divergence in V$_{2}$O$_{3}$/Ni bilayers Jose de la Venta, Juan Gabriel Ramirez, Thomas Saerbeck, Siming Wang, Ivan K. Schuller The effects of stress on the magnetic properties of ferromagnetic thin films are dramatic when the ferromagnets are in proximity with materials undergoing structural phase transitions (SPT) [1]. Here we report on Ferromagnetic Resonance (FMR) measurements on V$_{2}$O$_{3}$/Ni bilayers across the SPT of V$_{2}$O$_{3}$. The SPT occurs on V$_{2}$O$_{3}$ at 160 K from a metallic/rhombohedral to an insulating/monoclinic phase. Our results reveal a rotation of the anisotropy axis in Nickel films when cooled below the SPT of V$_{2}$O$_{3}$. The obtained anisotropy axis will be compared to the underlying structural morphology obtained from x-ray diffraction. More interestingly, the FMR linewidth as a function of the temperature shows a divergence across the SPT. This suggests a breakdown of the uniform precession of the Ni magnetization caused by the induced strain across the SPT. Discussion among linewidth-broadening mechanisms will be addressed. \\[4pt] [1] J. de la Venta, S. Wang, J. G. Ramirez, and I. K. Schuller, Appl. Phys. Lett. 102, 122404 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D7.00006: Ferroelectric Influence of Magnetic Anisotropy in Organic Ferroelectric/Co Heterostructures Keith Foreman, Celeste Labedz, Valeria Lauter, Artur Glavic, Haile Ambaye, Stephen Ducharme, Shireen Adenwalla Magnetoelectric coupling between ferroelectric (FE) and ferromagnetic (FM) thin films can influence the magnetic anisotropy of the FM film. This coupling is difficult to measure, requiring careful selection of the FE material. The organic FE poly(vinylidene fluoride-tri fluoroethylene), P(VDF-TrFE), provides high electric fields, compensating for the short penetration depth in the metallic FM layer, and is soft enough to minimize strain coupling. Here, we report on recent Polarized Neutron Reflectometry (PNR) experiments on a P(VDF-TrFE)/Co heterostructure indicating subtle changes in the magnetic anisotropy of the Co as a function of the FE polarization. To improve the magnetoelectric coupling in FE/FM heterostructures, further refinement of the layer interface is required. Typical deposition methods of P(VDF-TrFE) thin films expose the sample to atmosphere resulting in an ill-defined interface between FE/FM layers. Therefore, we have designed and constructed a thermal evaporation system capable of depositing thin films of the ferroelectric oligomer vinylidene difluoride (VDF), allowing us to create entire FE/FM heterostructures \textit{in situ}. We also report on recent magnetic measurements on these clean interface heterostructures. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D7.00007: Magnetocrystalline anisotropy of L10 FePt nanoparticles Alamgir Kabir, Jun Hu, Volodymyr Turkowski, Ruqian Wu, Talat S. Rahman We perform theoretical investigation of Magneto Crystalline Anisotropy (MCA) of L10 FePt nanoparticles. Structural relaxation and magnetic moment of the clusters are evaluated using spin polarized \textit{ab initio} density functional theory, and the MAE is calculated by using two approaches: (i) self-consistent inclusion of spin-orbit coupling and (ii) the torque method.[1] The clusters studied have 3(4) planes of Fe and 2(3) plane of Pt atoms and vice versa. We find an enhancement of MCA for the FePt clusters as compared to that of pure Fe nanoparticles and of bulk L10 FePt. We trace this enhancement to the increased spin and orbital moment of Pt atoms which raises the spin-orbit coupling. We also find that nanoparticles with Pt atoms in the central layer have larger MCA than the corresponding ones whose central layer is Fe. This is due to the fact that when Pt atom is the central layer it has more Fe atoms around so it more strongly hybridized resulting in higher orbital moments then Pt atoms on other layers. Detailed investigation of electronic structure of atoms on the clusters is also performed. Our finding can give useful insight to experimentalist for their studies of high density magnetic recording media. 1. X. D. Wang et al. Phys. Rev. B 54, 61(1996) [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D7.00008: Interatomic exchange in Mn-based alloys Priyanka Manchanda, Ralph Skomski, Arti Kashyap, David Sellmyer The ongoing quest for new rare-earth free permanent magnets includes the search for new magnetic phases with high magnetization and magnetic anisotropy. Manganese alloys could be used because Mn$^{2+}$ ion has a moment of 5 $\mu_{\mathrm{B}}$ per atom. However, manganese is in the middle of the 3$d$ transition-metal series, and it is well-known and easily explained in terms of general electronic structure trends that such elements prefer to form antiferromagnetic (AFM) rather than ferromagnetic (FM) spin structures. Most of the Mn compounds are antiferromagnetic and the few existing ferromagnetic compounds, such as MnAl and MnBi, have low magnetization, of the order of 1 $\mu_{\mathrm{B}}$ per atom. In this presentation, we used first-principle calculations to study interatomic exchange in Mn based alloys. As a model system, we use $L$1$_{\mathrm{0}}$-ordered MnAl. Interestingly, we find a strong ferromagnetic interatomic exchange in the Mn planes of the alloy, in spite of the short Mn-Mn interatomic distances. Furthermore, we study modifications of the $L$1$_{\mathrm{0}}$ structure, such as the effect of Fe substitution on the exchange interactions in MnAl derivatives. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D7.00009: Constituents of magnetic anisotropy and a screening of spin-orbit coupling Liqin Ke, Aleksander Wysocki, Mark van Schilfgaarde, Vladimir Antropov Using perturbation theory (PT) we analyze how the different orders of perturbation affect the energy in solids. We test the validity of PT analysis by considering spin-orbit coupling (SOC) as a perturbation. We show how the atomic SOC is screened in different magnets and how it affects the magnetic anisotropy. The dependence of magnetic anisotropy on the ratio between the strengths of SOC and crystal field is studied using an impurity model. We carried out density functional calculations for FePt, CoPt, FePd, MnAl, MnGa, FeNi, and tetragonally strained FeCo. The relativistic energy and magnetic anisotropy in those compounds from the perturbation approach and self-consistent relativistic calculations had been compared. In addition using decomposition of anisotropy into contributions from individual sites and different spin components we explain the microscopic origin of high anisotropy in most popular magnets. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D7.00010: Electronic structure and magnetic anisotropy of Sm$_2$Fe$_{17}$N$_x$ Hisazumi Akai, Masako Ogura Electronic structure and magnetic properties of Sm$_2$Fe$_{17}$N$_x$ are studies on the basis of the first-principles electronic structure calculation in the framework of the density functional theory within the local density and coherent potential approximations. The magnetic anisotropy of the system as a function of nitrogen concentration $x$ is discussed by taking account not only of the crystal field effects but also of the effects of the f-electron transfer from Sm to the neighboring sites. Also discussed is the magnetic transition temperature that is estimated by mapping the system into a Heisenberg model. The results show the crystalline magnetic anisotropy changes its direction from in-plane to uniaxial ones as x increases. It takes the maximum value near $x\sim 2.8$ and then decreases slightly towards $x=3$. The mechanism for these behaviors is discussed in the light of the results of detailed calculations on the bonding properties between Sm and its neighboring N. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D7.00011: Monte Carlo Simulation of GdFeCo Amorphous Films with Perpendicular Magnetic Anisotropy Xiaopu Li, Chung T. Ma, S. Joseph Poon, Nattawut Anuniwat, Jiwei Lu Amorphous ferrimagnetic Gd$_{\mathrm{x}}$Fe$_{\mathrm{93-x}}$Co$_{\mathrm{7}}$ alloy films have been reported with a tunable perpendicular magnetic anisotropy in both the low-Gd region (20 \textless x \textless 34) and the high-Gd region (52 \textless x \textless 59) [1]. The compositional and temperature dependence of their saturation magnetization is attributed to the competition between antiferromagnetic coupling of rare-earth (RE) with transition-metal (TM) ions and ferromagnetic interaction between the TM ions. Here, we present a computational model of the RE-TM amorphous structure using the Monte Carlo simulation method. The classical atomistic spin Hamiltonian has been used considering a model of random crystalline alloy. To obtain a consistent magnetization with the experimental data, we find it necessary to assume Gd spins having a non-collinear sperimagnetic structure, which origins from the anisotropy term. The calculated saturation magnetizations exhibit compensation phenomena for the low-Gd region and ferromagnetic transition behavior for the high-Gd region, in agreement with experiment. The results are analyzed in light of the sublattice magnetizations. [1] Manli Ding, S. Joseph Poon, J. Magn. Magn. Mater. 339, 51-55 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D7.00012: First principles investigation of magnetocrystalline anisotropy at Full Heusler / MgO interfaces Rajasekarakumar Vadapoo, Ali Hallal, Mairbek Chshiev Magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) have the potential for realizing next generation high density nonvolatile memories and logic devices [1]. The origin of high PMA in these interfaces has been explained by orbital hybridizations at interface along with spin-orbit interactions [2]. Here we present a systematic study of PMA in Heusler alloy [X$_{2}$YZ]/ MgO interfaces using first principle methods with X$=$Co, YZ$=$FeAl, MnGe and MnSi. Among the interfaces studied, we found that Co terminated interface of Co$_{2}$FeAl/MgO gives rise to PMA value of 1.2erg/cm$^{2}$ in agreement with recent experimental observations [3]. On the contrary, FeAl terminated interfaces of the same structure shows in-plane magnetic anisotropy (IMA). We also found that the most of PMA contribution originates from d$_{\mathrm{yz}}$ and d$_{\mathrm{z}}^{2}$ orbitals of Co atoms at the interface. Finally, Co$_{2}$MnGe and Co$_{2}$MnSi structures tend to favor IMA for any termination. \\[4pt] [1] S. Iked et al., Nature materials 9, 721 (2010)\\[0pt] [2] H. X. Yang et al., Physical Review B 84, 054401 (2011).\\[0pt] [3] M. Belmeguenai et al., Cond-mat.mtrl-sci, may 2013, arXiv:1305.0714. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D7.00013: Ground States in Thin Ferromagnetic Nanorings with Four-Fold In-Plane Anisotropy Gabriel Chaves-O'Flynn, Cyrill Muratov We present results of micromagnetic simulations based on the optimal grid algorithm for the study of metastable states in thin nano-rings with four-fold anisotropy. Previous work have demonstrated a rich energy landscape for these structures resulting from competition between shape and crystalline anisotropies [1]. We present a quasi-1D framework for the analysis of nanorings. First, we calculate the energy of domain walls of different windings for magnetic strips oriented at different angles with respect to the easy anisotropy axes. We consider the dependence of wall energy on material parameters. With these numbers we build a reduced-model for the micromagnetic energy on the rings which allows to treat the micromagnetic energy minimization as a combinatorial problem: the walls in the ring are treated as separate entities each with an intrinsic energy calculated from the strip case and interacting with each other via dipole-dipole interactions. A comparison with the phase diagram for ground states provides information on the limits of validity of this simplified model. [1] G.D. Chaves-O'Flynn, C. Muratov. IEEE Trans. Mag. 49, p. 3125 (2013) [2] C. Muratov and V. Osipov. IEEE Trans. Mag, 45, p.3207 (2008) [Preview Abstract] |
Session D8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Nitrogen Vacancies in Diamond and Nuclear Spins
Sponsoring Units: GMAG DMP FIAPChair: Hanan Dery, University of Rochester
Room: 104
Monday, March 3, 2014 2:30PM - 2:42PM |
D8.00001: Increasing NV center density by shallow $^{12}$C implantation in N delta-doped diamond K. Ohno, B.A. Myers, B.J. Aleman, C.A. McLellan, A.C. Bleszynski Jayich, D.D. Awschalom Scalable creation of solid-state single spins is important to nanoscale sensing. Nitrogen-vacancy (NV) centers created by the N delta-doping technique display long \textit{T}$_{2}$ at depths $<$100 nm [1] which were exploited to demonstrate nm-scale nuclear magnetic resonance.[2] One issue of this technique is the low NV density, which prevents their incorporation into diamond nanostructures. This is caused by poor depth localization of vacancies by post growth electron irradiation. Here we use shallow $^{12}$C implantation to localize them. By controlling annealing time and temperature, shallow vacancies diffuse into the N doped layer to selectively activate doped NV centers. We observe NV densities 10 times greater than in irradiated samples. Resulting NV centers display \textit{T}$_{2}$ $>$ 500 $\mu$s, suggesting C implantation damage to the N doped layer is minimized. The enhanced NV density is used to demonstrate NV center localization in a small volume. We find an average of 1.3 NVs confined to a volume of 150 nm in diameter and 50 nm in depth within an array of EB lithographically patterned pillars, useful for single photon sources and scanning probe based sensing. [1] K. Ohno et al., Appl. Phys. Lett. 101, 082413(2012). [2] H. J. Mamin et al., Science 339, 557(2013). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D8.00002: Mitigating surface-induced decoherence of spin sensors in nitrogen delta-doped diamond Bryan A. Myers, Matthieu C. Dartiailh, Kenichi Ohno, David D. Awschalom, Ania C. Bleszynski Jayich The negatively-charged nitrogen-vacancy (NV) center in diamond is a robust nanoscale sensor of magnetic fields. To maximize their sensitivity to external spins, NVs have to be located close to the diamond surface while mitigating surface-induced decoherence. This requires a quantitative understanding of the dominant noise origins, which are currently not well understood. To address this we create shallow NVs by delta-doping during CVD growth [1] and apply scanning probe-based magnetic resonance imaging to find their depths with nm precision. We probe the noise with dynamical decoupling (DD) control of the NVs and fit their coherence decay envelopes to a spin-bath model with two contributions: bulk and surface electronic spins. The fits yield a surface spin density $\sigma_{\mathrm{s}} =$ 0.0032/nm$^{2}$ and relaxation rate 1/$\tau_{\mathrm{s}} =$ 190 kHz. We find an optimal CPMG-4 passive detection sensitivity of 250 $\mu _{\mathrm{p}}$/$\surd $Hz for an NV at 14 nm depth. Doped NVs within 10 nm of the surface were progressively decoupled from noise in the 1/$\tau _{\mathrm{s}}$ frequency regime using shorter DD inter-pulse delays, thereby enhancing their sensitivity. \\[4pt] [1] K. Ohno et al., Appl. Phys. Lett. 101, 082413 (2012). [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D8.00003: Theory of feedback cooling of nuclear spins in diamond nitrogen-vacancy center Ping Wang, Wen Yang We develop a microscopic theory for the feedback cooling of nuclear spins in the diamond nitrogen-vacancy center at low temperature. By adiabatically eliminating the fast motion of the NV center, we derive an analytical rate equation to describe the dynamics of the nitrogen and $^{13}$C nuclei. This equation is solved both numerically and analytically using the Fokker-Planck equation. The results provide a good explanation to the recently observed nitrogen and $^{13}$C nuclear spin cooling in nitrogen-vacancy center by coherent population trapping [E. Togan \textit{et al}., Nature 478, 497 (2011)]. They also suggest an optimal pumping power for optimcal cooling effect. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D8.00004: Nanoscale magnetic imaging of individual electron spins under ambient conditions Invited Speaker: Michael Grinolds The detection of ensembles of spins under ambient conditions has revolutionized the biological, chemical, and physical sciences through magnetic resonance imaging and nuclear magnetic resonance. Pushing sensing capabilities to the individual-spin level would enable unprecedented applications such as single molecule structural imaging; however, the weak magnetic fields from single spins are undetectable by conventional methods. Recently, there has been significant theoretical and experimental research into using nitrogen-vacancy (NV) defect centers in diamond as a new type of magnetometer capable of detecting individual spins. In this talk I present measurements using such an NV-based magnetometer to detect and image the magnetic fields from individual electron spins under ambient conditions. Magnetic imaging is achieved by either spatially mapping a target spin's magnetic field using a scanning magnetometer [1], or by performing magnetic resonance imaging via scanning magnetic field gradients. These results in imaging individual electron spins makes NV-based magnetometry immediately applicable to diverse systems including imaging spin chains, readout of individual spin-based quantum bits, and determining the precise location of spin labels in biological systems. \\[4pt] [1] M.S. Grinolds \textit{et al.} Nature Physics, 9 215-219 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D8.00005: Study of a single nitrogen-vacancy center in diamond to detect surrounding electron spins Chathuranga Abeywardana, Viktor Stepanov, Susumu Takahashi A nitrogen-vacancy (NV) center in diamond is a promising candidate for applications of nanoscale magnetic sensing as well as for investigation of fundamental quantum sciences because of its unique properties including capability to detect a NV center, long decoherence time even at room temperature, stable fluorescence and biocompatibility. Here we will present our approach to use a single NV center in diamond to probe tiny magnetic fields ($\sim$ uT or less) due to surrounding spin environments. We will discuss magnetic field dependence of spin decoherence in a single NV center as well as use of double electron-electron resonance spectroscopy to detect surrounding electron spins. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D8.00006: Nitrogen vacancy centers for nanoscale magnetic field mapping of micromagnets David Roy-Guay, Andreas Ruediger, Julien Plathier, Lilian Childress, Denis Morris, Michel Pioro-Ladri\`ere Nitrogen vacancy (NV) centers in diamond are nanoscale color centers with a long spin coherence time ($\sim$ 1 ms) even at room temperature (RT). Combined with the option of optical readout of a microwave-addressable state, NV centers in diamond are outstanding magneto-,electro-, or thermometers which allow for the creation of high spatial resolution nano-sensors [1]. In this work, we report on our first RT Rabi oscillations at 0.7 MHz of an ensemble of NV centers and preliminary results on an optically detected Stark shift of a single NV center. Fabrication by reactive ion etching of an array of NV nanodetectors for magnetic field mapping will also be presented. The array is mapped with a confocal photoluminescence setup to determine the NV density per pillar. Subsequent patterning of local gates will allow for high electric fields as a tuning parameter to enhance the magnetic field sensitivity of the NV array, resulting in a high precision magnetometer without the use of spin-echo sequences. Such a magnetic CCD is a promising tool to map local magnetic fields produced by micromagnets, such as those used in spin qubit architectures for fast qubit gates [2]. \\[4pt] [1] Dolde, F. et al. Nat. Phys. 7, 459-463 (2011) \\[0pt] [2] Pioro-Ladri\`{e}re, M. et al. Nat. Phys. 4, 776-779 (2008) [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D8.00007: The effect of spin transport on lifetime in nanoscale systems Jeremy Cardellino, Nicolas Scozzaro, Michael Heman, Andrew Berger, Chi Zhang, Kin Chung Fong, Ciriyam Jayaprakash, Denis Pelekhov, Chris Hammel Spin transport electronics utilizes electron spin as a state variable for information processing and storage. This requires manipulation of spin ensembles for data encoding, and spin transport for information transfer. Here we report spatially resolved magnetic resonance studies of electron spin ensembles confined to a quasi 1D `spin nanowire' formed by nitrogen ion implantation in diamond. We obtain the ensemble spin lifetime, that is, spin autocorrelation time, by measuring statistical fluctuations of the net moment ($\surd $N \textless 100 net spins), which is in thermal equilibrium and has no imposed polarization gradient. We find the lifetime of the ensemble is dominated by spin transport from the ensemble into an adjacent reservoir, which is in striking contrast to conventional spin-lattice relaxation measurements of isolated spin ensembles. In addition, using a novel spin manipulation protocol, we demonstrate spectroscopic measurements on nanoscale spin ensembles that corroborate spin transport in strong field gradients. Our experiments, supported by microscopic Monte Carlo modelling, provide a unique insight into the intrinsic dynamics of charge-motion-free spin currents needed for nanoscale devices which seek to control spins. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D8.00008: Impurities and electron spin relaxations in nanodiamonds studied by multi-frequency electron spin resonance Franklin Cho, Susumu Takahashi Nano-sized diamond or nanodiamond is a fascinating material for potential applications of fluorescence imaging and magnetic sensing of biological systems via nitrogen-vacancy defect centers in diamonds. Sensitivity of the magnetic sensing strongly depends on coupling to surrounding environmental noises, thus understanding of the environment is critical to realize the application. In the present study, we employ multi-frequency (X-band, 115 GHz and 230 GHz) continuous-wave (cw) and pulsed electron spin resonance (ESR) spectroscopy to investigate impurity contents and spin relaxation properties in various sizes of nanodiamonds. Spectra taken with our home-built 230/115 GHz cw/pulsed ESR spectrometer shows presence of two major impurity contents; single substitutional nitrogen impurities (P1) also common in bulk diamonds and paramagnetic impurities (denoted as X) unique to nanodiamonds. The ESR measurement also shows a strong dependence of the population ratio between P1 and X on particle size. Furthermore, we will discuss the nature of spin-lattice relaxation time $T_{1}$ of nanodiamonds studied by pulsed ESR measurements at X-band, 115 GHz and 230 GHz. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D8.00009: Coupling ZnSe band spin states and 4H-SiC defect spin states across their interface Andrew L. Yeats, Anthony Richardella, Nitin Samarth, David D. Awschalom Point defects in silicon carbide (SiC) have emerged as a promising platform for quantum information processing and nanoscale sensing in a technologically-mature semiconductor. ZnSe is a promising candidate for semiconductor spintronic applications and has selection rules compatible with optical orientation of conduction electron spins. We combine pump-probe optical measurements with pulsed optically detected magnetic resonance (ODMR) sequences to investigate coupling between SiC defect spins and ZnSe conduction electron spins in ZnSe/4H-SiC heterostructures. Preparation of these structures by molecular beam epitaxy (MBE) and ion implantation is discussed in terms of interface optimization. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D8.00010: Nuclear Spin Polarization of Phosphorus Donors in Silicon. Direct Evidence from 31P-Nuclear Magnetic Resonance Patryk Gumann, Chandrasekhar Ramanathan, Om Patange, Osama Moussa, Mike Thewalt, Helge Riemann, Nikolay Abrosimov, Peter Becker, Hans-Joachim Pohl, Kohei Itoh, David G. Cory We experimentally demonstrate the optical hyperpolarization and coherent control of $^{31}$P, nuclear spins in single crystal silicon via the inductive readout of the nuclear magnetic resonance (NMR) signal of $^{31}$P at a concentration of 1.5 x 10$^{15}$ cc$^{-1}$. The obtained polarization is sufficient the $^{31}$P spin polarization of 1.17 x 10$^{15}$ in a 10 mm x 10 mm sample, observed in one FID with signal-to-noise ration of 113. The linewidth is 800 Hz. The Hahn echo pulse sequence reveals a $^{31}$P T$_{2}$ time of 0.42 s at 1.6 K, which was extended by the Carr Purcell cycle to 1.2 s at the same temperature. The maximum build-up of the nuclear polarization was achieved within $\sim$577 seconds, at 4.2 K, in 6.7 T, using optical excitations provided by an infra-red laser. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D8.00011: Hyperpolarization of $^{29}$Si by Resonant Nuclear Spin Transfer from Optically Hyperpolarized $^{31}$P Donors Phillip Dluhy, Jeff Salvail, Kamyar Saeedi, Mike Thewalt, Stephanie Simmons Recent developments in nanomedicine have allowed nanoparticles of silicon containing hyperpolarized $^{29}$Si to be imaged in vivo using magnetic resonance imaging. The extremely long relaxation times and isotropy of the Si lattice make polarized $^{29}$Si isotopes ideal for these sorts of imaging methods. However, one of the major difficulties standing in the path of widespread adoption of these techniques is the slow rate at which the $^{29}$Si is hyperpolarized and the limited maximum hyperpolarization achievable. In this talk, I will describe an effective method for hyperpolarization of the $^{29}$Si isotopes using resonant optical pumping of the donor bound exciton transitions to polarize the $^{31}$P donor nuclei, and a choice of static magnetic field that conserves energy during spin flip flops between donor nuclear and $^{29}$Si spins to facilitate diffusion of this polarization. Using this method, we are able to polarize greater than 10\% of the $^{29}$Si centers in 64 hours without seeing saturation of the $^{29}$Si polarization. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D8.00012: Dynamic nuclear polarization from current-induced electron spin polarization in n-InGaAs Christopher Trowbridge, Benjamin Norman, Yuichiro Kato, David Awschalom, Vanessa Sih Control of the nuclear spin system could prove useful for applications in spintronics or spin-based quantum computation for intermediate term data storage and for the suppression of electron spin dephasing resulting from hyperfine coupling. We investigate the role of nuclear spins in materials with electrically generated spin polarization. The electron spin polarization generated by electrical current in a non-magnetic semiconductor is transferred via dynamic nuclear polarization to the nuclei. The resulting nuclear field is interrogated using Larmor magnetometry. We measure the nuclear field as a function of applied magnetic field, current magnitude and direction, and temperature. An unexpected spatial asymmetry in saturated nuclear field is found. The direction of the nuclear polarization is determined by the directions of the electron spin alignment and external magnetic field, allowing electronic control over the sign of the nuclear alignment direction. Careful study of the nuclear field also enables characterization of the current-induced electron spin polarization in situations that are otherwise experimentally inaccessible. Work supported by AFOSR, NSF and ONR. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D8.00013: Electronic structure of CrN/MgO multilayers Antia S. Botana, Victor Pardo, Daniel Baldomir, Peter Blaha The changes in the electronic structure of oxides and other correlated compounds caused by electronic reconstructions at their surface and interfaces has attracted much attention recently. CrN shows a magnetostructural phase transition as a function of temperature and controversial electronic properties. In the bulk, calculations show that with the onset of magnetism CrN is semiconducting but being very close to a metal-insulator transition. For free standing thin films with increasing thickness the gap closes and conducting states appear connected with a structural relaxation at the surface, where an electric dipole is formed. We report a series of electronic structure calculations for CrN/MgO multilayers within the LDA+U method. In contrast to the free CrN surface, CrN grown on MgO retains the semiconducting behavior shown in the bulk and even widens its band gap as the CrN thickness is reduced. Otherwise, interfacial effects with the oxide lead to negligible electronic reconstructions. The d-levels of the interfacial Cr atoms are lowered in energy due to the different environment present at the interface. The evolution of the transport properties is analyzed and a significant enhancement of the Seebeck coefficient is predicted for the case of very thin CrN layers. [Preview Abstract] |
Session D10: Focus Session: Evolutionary Dynamics and Population Genetics
Sponsoring Units: DBIO GSNPChair: Xiang Qiang Chu, Wayne State University
Room: 201
Monday, March 3, 2014 2:30PM - 3:06PM |
D10.00001: Fitness seascapes and adaptive evolution of the influenza virus Invited Speaker: Michael Lassig The seasonal human influenza A virus undergoes rapid genome evolution. This process is triggered by interactions with the host immune system and produces significant year-to-year sequence turnover in the population of circulating viral strains. We develop a dynamical fitness model that predicts the evolution of the viral population from one year to the next. Two factors are shown to determine the fitness of a viral strain: adaptive changes, which are under positive selection, and deleterious mutations, which affect conserved viral functions such as protein stability. Combined with the influenza strain tree, this fitness model maps the adaptive history of influenza A. We discuss the implications of our results for the statistical theory of adaptive evolution in asexual populations. Based on this and related systems, we touch upon the fundamental question of when evolution can be predicted. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D10.00002: Biophysical Fitness Landscapes and Evolutionary Dynamics of Proteins Michael Manhart, Alexandre Morozov The molecular biophysics of proteins fundamentally shapes their fitness landscapes and evolutionary dynamics. For example, the evolution of new function in a protein is constrained by the need to maintain folding stability. We investigate the role of molecular biophysics in protein evolution by developing a class of fitness landscapes based on protein folding and binding energetics. We characterize the properties of these landscapes, such as their epistasis, accessibility, and number of local maxima. We also use a recently-developed path-based approach to random walks on networks to analyze the dynamics of populations evolving on these landscapes, focusing especially on the distribution and diversity of adaptive trajectories. These models make qualitative predictions relevant to both natural evolution as well as directed evolution experiments. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D10.00003: Inferring the Mode of Selection from the Transient Response to Demographic Perturbations Daniel Balick, Ron Do, David Reich, Shamil Sunyaev Despite substantial recent progress in theoretical population genetics, most models work under the assumption of a constant population size. Deviations from fixed population sizes are ubiquitous in natural populations, many of which experience population bottlenecks and re-expansions. The non-equilibrium dynamics introduced by a large perturbation in population size are generally viewed as a confounding factor. In the present work, we take advantage of the transient response to a population bottleneck to infer features of the mode of selection and the distribution of selective effects. We develop an analytic framework and a corresponding statistical test that qualitatively differentiates between alleles under additive and those under recessive or more general epistatic selection. This statistic can be used to bound the joint distribution of selective effects and dominance effects in any diploid sexual organism. We apply this technique to human population genetic data, and severely restrict the space of allowed selective coefficients in humans. Additionally, one can test a set of functionally or medically relevant alleles for the primary mode of selection, or determine the local regional variation in dominance coefficients along the genome. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D10.00004: Hidden Complexity in Bacterial Evolution Robert Austin, Julia Bos, Grigory Tarnopolskiy, John Bestoso, James Sturm, Hyunsung Kim, Nader Pourmand, Robert Austin We compare the local fitness maxima a Growth Advantage in Stationary Phase (GASP) \cite{roberto} bacterial strain evolves in comparison to the local maxima of the parental wild-type strain. The rapid evolution of antibiotic resistance in GASP to an identical stressor, starting from a different initial phenotype and genotype, diverges from a parental wild-type strain on the fitness landscape. That is, while the GASP strain evolves a (Serine$^{83}$ $\rightarrow$ Leucine missense mutation in $gyrA$) which is the target of the antibiotic, only 2 amino acids removed from the WT strain resistant mutant, it does not evolve the other 3 SNPS the WT strain did. Rather, it excises the prophage e14 sequence \cite{e14}. We show that this e14 excision profoundly changes the ability of the GASP strain to form a biofilm, revealing the hidden complexity of {\it E. coli} evolution to antibiotics in complex environments. We show that these profound changes in resistance to cipro do not come at a substantial fitness cost on the landscape and discuss why this makes the mutations basically irreversible. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D10.00005: Population subdivision with migration can facilitate evolution on rugged fitness landscapes Anne-Florence Bitbol, David Schwab Natural selection drives organisms towards higher fitness, but crossing fitness valleys or plateaus may be necessary to progress up a rugged fitness landscape. In a subdivided population, quasi-independent explorations of the fitness landscape can be run in parallel, and furthermore, stochastic effects have an increased importance due to the smaller size of subpopulations. Thus, valley or plateau crossing may be facilitated locally, and migration can then spread beneficial mutations. We show that population subdivision with migration significantly accelerates the crossing of fitness valleys and plateaus over a wide parameter range, both with respect to a non-subdivided population and to a single subpopulation. Our generic and minimal model does not require environmental heterogeneity or specific geographic structure, and includes only subdivision with migration. Using Markov chain theory, we obtain analytical expressions of the conditions under which valley or plateau crossing by the subdivided population is as fast as that of its fastest subpopulation. We verify this prediction through stochastic simulations. Our results, obtained for fitness valleys and plateaus, also hold for weakly beneficial intermediate mutations. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D10.00006: Guiding the evolution to catch the virus: An in silico study of affinity maturation against rapidly mutating antigen Shenshen Wang, Dennis Burton, Mehran Kardar, Arup Chakraborty The immune system comprises an intricate and evolving collection of cells and molecules that enables a defense against pathogenic agents. Its workings present a rich source of physical problems that impact human health. One intriguing example is the process of affinity maturation (AM) through which an antibody (Ab)---a component of the host immune system---evolves to more efficiently bind an antigen (Ag)---a unique part of a foreign pathogen such as a virus. Sufficiently strong binding to the Ag enables recognition and neutralization. A major challenge is to contain a diversifying mixture of Ag variants, that arise in natural infection, from evading Ab neutralization. This entails a thorough understanding of AM against multiple Ag species and mutating Ag. During AM, Ab-encoding cells undergo cycles of mutation and selection, a process reminiscent of Darwinian evolution yet occurring in real time. We first cast affinity-dependent selection into an extreme value problem and show how the binding characteristics scale with Ag diversity. We then develop an agent-based residue-resolved computational model of AM which allows us to track the evolutionary trajectories of individual cells. This dynamic model not only reveals significant stochastic effects associated with the relatively small and highly dynamic population size, it also uncovers the markedly distinct maturation outcomes if designed Ag variants are presented in different temporal procedures. Insights thus obtained would guide rational design of vaccination protocols. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D10.00007: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 4:18PM - 4:30PM |
D10.00008: Functional trade-offs and phenotypic diversity in cellular migration Thierry Emonet, Nicholas Frankel, Yann Dufour I will discuss our recent efforts to uncover the functional role of phenotypic heterogeneity in cellular migration and understand how biological systems may resolve functional trade-offs. We addressed this question using bacterial chemotaxis as model system. We find (1) that, while robust network design maintains the average behavior of the population in a functional range, harnessing inherent cell-to-cell variability around the average allows populations to adaptively diversify network functions, resolving trade-offs; and (2) that the molecular mechanism for directing this diversity is mutations in common gene regulatory elements. Our main theoretical conclusion is that the distribution of network parameters in itself is as likely to be under selection as network design. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D10.00009: Emergence of Rapid Evolution from Demographic Stochasticity Hong-Yan Shih, Nigel Goldenfeld The phenomenon of ``rapid evolution'' arises when genetic variation occurs fast enough to significantly change ecodynamics. Data from experiments with algae-rotifer system and bacteria-phage system show unusual dynamics when there are subpopulations of preys with different trait values, including predator-prey phase shifts near $\pi$ (and distinct from the canonical value of $\pi/2$) and so-called cryptic cycles, in which populations of preys remain constant while the predator population oscillates. Such phenomena have been modeled with deterministic differential equations containing empirical Michaelis-Menten kinetic terms and the unusual dynamics that is attributed to postulate complicated trade-off between sub-populations. Here we present a generic individual-level stochastic model of interacting populations that includes a subpopulation resistant to the predator but with metabolic cost. We solve this model by using a master equation approach, and by performing system size expansion, we find that antiphase and cryptic quasi-cycles can emerge from the combination of intrinsic demographic fluctuations and clonal mutations alone. These analytic results are then compared with Gillespie simulations, and the typical phase diagram of the system is calculated. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D10.00010: Selection, adaptation, and predictive information in changing environments Quentin Feltgen, Ilya Nemenman Adaptation by means of natural selection is a key concept in evolutionary biology. Individuals better matched to the surrounding environment outcompete the others. This increases the fraction of the better adapted individuals in the population, and hence increases its collective fitness. Adaptation is also prominent on the physiological scale in neuroscience and cell biology. There each individual infers properties of the environment and changes to become individually better, improving the overall population as well. Traditionally, these two notions of adaption have been considered distinct. Here we argue that both types of adaptation result in the same population growth in a broad class of analytically tractable population dynamics models in temporally changing environments. In particular, both types of adaptation lead to subextensive corrections to the population growth rates. These corrections are nearly universal and are equal to the predictive information in the environment time series, which is also the characterization of the time series complexity. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D10.00011: Cluster-Level Dynamics in a Neutral Phenotype Evolution Model Adam Scott, Dawn King, Sonya Bahar In agent-based models of nonequilibrium phase transitions, the agent dynamics can be described by reaction-diffusion processes such as branching-coalescing random walks. We have recently shown that a phase transition in a neutral phenotype evolution model comprised of many branching-coalescing random walkers belongs to the directed percolation (DP) universality class. However, while the organism processes are described by A-\textgreater 2A, 2A-\textgreater A, {\&} A-\textgreater 0, the cluster processes are B-\textgreater nB, mB-\textgreater B, {\&} B-\textgreater 0 (where n and m are positive integers). Therefore, despite the DP behavior of the transition at the organism level, we do not expect the clusters to exhibit the same universality class. Here, we will investigate cluster branching behavior by measuring reaction rates and show that the cluster density exponent suggests a different universality class at the cluster level. These results may have significant implications for multilevel selection in evolutionary biology. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D10.00012: Phase Transition Behavior in a Neutral Evolution Model Dawn King, Adam Scott, Nevena Maric, Sonya Bahar The complexity of interactions among individuals and between individuals and the environment make agent based modeling ideal for studying emergent speciation. This is a dynamically complex problem that can be characterized via the critical behavior of a continuous phase transition. Concomitant with the main tenets of natural selection, we allow organisms to reproduce, mutate, and die within a neutral phenotype space. Previous work has shown phase transition behavior in an assortative mating model with variable fitness landscapes as the maximum mutation size ($\mu )$ was varied (Dees and Bahar, 2010). Similarly, this behavior was recently presented in the work of Scott et al. (2013), even on a completely neutral landscape, for bacterial-like fission as well as for assortative mating. Here we present another neutral model to investigate the `critical' phase transition behavior of three mating types -- assortative, bacterial, and random -- in a phenotype space as a function of the percentage of random death. Results show two types of phase transitions occurring for the parameters of the population size and the number of clusters (an analogue of species), indicating different evolutionary dynamics for system survival and clustering. [Preview Abstract] |
Session D11: Focus Session: Bacterial Biophysics II
Sponsoring Units: DBIOChair: Gerard Wong, University of California, Los Angeles
Room: 203
Monday, March 3, 2014 2:30PM - 3:06PM |
D11.00001: Geometric control of bacterial cell shape Invited Speaker: Joshua Shaevitz How bacteria grow into specific, 3D shapes remains a central mystery in microbiology. We have developed an imaging and analysis pipeline to simultaneously probe the shape of cells and the localization of proteins in 3D during growth. We find evidence for feedback between the local geometry of the cell, localization of key morphological proteins, and cell growth that helps to ensure the maintenance of rod-shape in elongating \textit{Escherichia coli} cells. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D11.00002: Nanomechanical Response of Bacterial Cells to Cationic Antimicrobial Peptides Shun Lu, Grant Walters, Richard Parg, John Dutcher The effectiveness of antimicrobial compounds can be easily screened, however their mechanism of action is much more difficult to determine. Many compounds act by compromising the mechanical integrity of the bacterial cell envelope, and our study introduces an atomic force microscopy (AFM)-based creep deformation technique to evaluate changes in the time-dependent mechanical properties of \textit{Pseudomonas aeruginosa} PAO1 bacterial cells upon exposure to two different but structurally related antimicrobial peptides: polymyxin B and polymyxin B nonapeptide. We observed a distinctive signature for the loss of integrity of the bacterial cell envelope following exposure to the peptides. Measurements performed before and after exposure, as well as time-resolved measurements and those performed at different concentrations, revealed large changes to the viscoelastic parameters that are consistent with differences in the membrane permeabilizing effects of the peptides. The AFM creep deformation measurement provides new, unique insight into the kinetics and mechanism of action of antimicrobial peptides on bacteria. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D11.00003: Atomic Force Microscopy Measurements of the Mechanical Properties of Cell Walls on Living Bacterial Cells Richard Bailey, Nic Mullin, Robert Turner, Simon Foster, Jamie Hobbs Staphylococcus aureus is a major cause of infection in humans, including the Methicillin resistant strain, MRSA. However, very little is known about the mechanical properties of these cells. Our investigations use AFM to examine live S. aureus cells to quantify mechanical properties. These were explored using force spectroscopy with different trigger forces, allowing the properties to be extracted at different indentation depths. A value for the cell wall stiffness has been extracted, along with a second, higher value which is found upon indenting at higher forces. This higher value drops as the cells are exposed to high salt, sugar and detergent concentrations, implying that this measurement contains a contribution from the internal turgor pressure. We have monitored these properties as the cells progress through the cell cycle. Force maps were taken over the cells at different stages of the growth process to identify changes in the mechanics throughout the progression of growth and division. The effect of Oxacillin has also been studied, to better understand its mechanism of action. Finally mutant strains of S. aureus and a second species Bacillus subtilis have been used to link the mechanical properties of the cell walls with the chain lengths and substructures involved. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D11.00004: Evolution of the Min Protein Oscillation in \textit{E. coli} Bacteria During Cell Growth and Division Benjamin Baylis, Maximiliano Giuliani, John Dutcher Cell division is a key step in the life of a bacterium. This process is carefully controlled and regulated so that the cellular machinery is equally partitioned into two daughter cells of equal size. In \textit{E. coli}, this is accomplished, in part, by the Min protein system, in which Min proteins oscillate along the long axis of the rod-shaped cells. We have used high magnification, time-resolved fluorescence microscopy to characterize in detail the oscillation in \textit{E. coli} cells in which the MinD proteins are tagged with green fluorescent protein (GFP). We have used a microfluidic device to confine the bacteria into microchannels that allows us to track the evolution of the oscillation in cells as they grow and divide in LB growth media. In particular, we have tracked the loss of synchrony between the oscillations in the daughter cells following cell division. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D11.00005: Single Cell Response to Time-dependent Stimuli using a Microfluidic Bioreactor Eric M. Johnson-Chavarria, Utsav Agrawal, Melikhan Tanyeri, Thomas E. Kuhlman, Charles M. Schroeder Cellular adaptation is critical for survival under uncertain or dynamic environmental conditions. Recent studies have reported the ability of biological systems to implement low-pass filters to distinguish high frequency noise in environmental stimuli from lower frequency input signals, yet we still lack a complete understanding of this phenomenon. In this work, we report a microfluidic-based platform for single cell analysis that provides dynamic control over periodic, time-dependent culture media. Single cells are confined in free solution by the sole action of gentle fluid flow, thereby enabling non-perturbative trapping of cells for long time scales. In this way, our microfluidic-based technique provides the ability to control external stimuli with precise methods while observing non-adherent cells over long timescales. Using this approach, we observed intranucleoid diffusion of genetic repressor proteins released from a chromosomal binding array. Overall, this microfluidic approach provides a direct method for sustaining periodic environmental conditions, measuring growth rates, and detecting gene expression of single cells in free solution. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D11.00006: quenched-smFISH: Counting small RNA in Pathogenic Bacteria Douglas Shepherd, Nan Li, Sofiya Micheva-Viteva, Brian Munsky, Elizabeth Hong-Geller, James Werner Here, we present a modification to single-molecule fluorescence in situ hybridization, quenched smFISH (q-smFISH), that enables quantitative detection and analysis of small RNA (sRNA) expressed in bacteria. We show that short nucleic acid targets can be detected when the background of unbound singly dye-labeled DNA oligomers is reduced through hybridization with a set of complementary DNA oligomers labeled with a fluorescence quencher. Exploiting an automated, multi-color wide-field microscope and GPU-accelerated data analysis package, we analyzed the statistics of sRNA expression in thousands of individual Yersinia pseudotuberculosis and Yersinia pestis bacteria before and during a simulated infection. Before infection, we find only a small fraction of either bacteria express the small RNAs YSR35 or YSP8. The copy numbers of these RNA are increased during simulated infection, suggesting a role in pathogenesis. The ability to directly quantify expression level changes of sRNA in single cells as a function of external stimuli provides key information on the role of sRNA in bacterial regulatory networks. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D11.00007: Towards rationally redesigning bacterial signaling systems using information encoded in abundant sequence data Ryan Cheng, Faruck Morcos, Herbert Levine, Jose Onuchic An important challenge in biology is to distinguish the subset of residues that allow bacterial two-component signaling (TCS) proteins to preferentially interact with their correct TCS partner such that they can bind and transfer signal. Detailed knowledge of this information would allow one to search sequence-space for mutations that can systematically tune the signal transmission between TCS partners as well as re-encode a TCS protein to preferentially transfer signals to a non-partner. Motivated by the notion that this detailed information is found in sequence data, we explore the mutual sequence co-evolution between signaling partners to infer how mutations can positively or negatively alter their interaction. Using Direct Coupling Analysis (DCA) for determining evolutionarily conserved interprotein interactions, we apply a DCA-based metric to quantify mutational changes in the interaction between TCS proteins and demonstrate that it accurately correlates with experimental mutagenesis studies probing the mutational change in the \emph{in vitro} phosphotransfer. Our methodology serves as a potential framework for the rational design of TCS systems as well as a framework for the system-level study of protein-protein interactions in sequence-rich systems. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D11.00008: Single Bacteria as Turing Machines Julia Bos, Qiucen Zang, Saurabh Vyawahare, Robert Austin In Allan Turing's famous 1950 paper on Computing Machinery and Intelligence, he started with the provocative statement: ``I propose to consider the question, `Can machines think?' This should begin with definitions of the meaning of the terms `machine' and `think'.'' In our own work on exploring the way that organisms respond to stress and evolve, it seems at times as if they come to remarkably fast solutions to problems, indicating some sort of very clever computational machinery. I’ll discuss how it would appear that bacteria can indeed create a form of a Turing Machine, the first example of a computer, and how they might use this algorithm to do rapid evolution to solve a genomics problem. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D11.00009: Influence of Helical Cell Shape on Motility of \textit{Helicobacter Pylori} Joseph Hardcastle, Laura Martinez, Nina Salama, Rama Bansil Bacteria's body shape plays an important role in motility by effecting chemotaxis, swimming mechanisms, and swimming speed. ~A prime example of this is the bacteria \textit{Helicobacter Pylori; }whose helical shape has long been believed to provide an advantage in penetrating the viscous mucus layer protecting the stomach lining, its niche environment. ~To explore this we have performed bacteria tracking experiments of both wild-type bacteria along with mutants, which have a straight rod shape. A wide distribution of speeds was found. This distribution reflects both a result of temporal variation in speed and different shape morphologies in the bacterial population. Our results show that body shape plays less role in a simple fluid. However, in a more viscous solution the helical shape results in increased swimming speeds. In addition, we use experimentally obtained cell shape measurements to model the hydrodynamic influence of cell shape on swimming speed using resistive force theory. The results agree with the experiment, especially when we fold in the temporal distribution. Interestingly, our results suggest distinct wild-type subpopulations with varying number of half helices can lead to different swimming speeds. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D11.00010: Quantifying the Dynamics of Bacterial Colony Expansion: From Individual Cells to Collective Behavior Erin Shelton, Maximiliano Giuliani, Robert Moscaritolo, Matt Kinley, Lori Burrows, John Dutcher Type IV pili (T4P) are very thin (5-8 nm in diameter) protein filaments that can be extended and retracted by certain classes of Gram-negative bacteria including \textit{P. aeruginosa} [1]. These bacteria use T4P to move across viscous interfaces, referred to twitching motility. Twitching can occur for isolated cells or in a collective manner [2]. We have developed experimental and data analysis techniques to quantify the expansion of the bacterial colony. Using a custom-built, temperature and humidity controlled environmental chamber, we have studied the transition from individual to collective motion. We have used optical flow analysis to characterize the evolution of the expanding colonies. We have also incorporated fluorescently tagged, non-motile cells, obtained by knocking out proteins essential for twitching motility, into the colonies to observe their transport as cargo by the motile cells. By measuring the flow of the motile cells while also tracking the motion of the non-motile cargo cells, we have obtained a direct measure of the efficiency of the transport of the cargo cells. [1] Burrows, L.L. (2012) Annu. Rev. Microbiol. 66: 493--520. [2] Semmler, A.B. \textit{et al}. (1999) Microbiology 145: 2863-2873. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D11.00011: Microscale swimming through heterogeneous networks Henry Fu, Mehdi Jabbarzadeh, YunKyong Hyon Although there have been many investigations of how swimming microorganisms are affected by complex media which treat the medium as a homogeneous material represented by a continuum constitutive equation, in many cases biological environments have microstructure at similar lengthscales as microorganisms. In that case continuum approaches are not valid and the microstructure and swimmer must be treated on equal footing. For example, cervical mucus contains a network of mucin filaments with a mesh size that can vary from approximately 0.5 to 30 microns, in the same size range as sperm. I will present results which investigate a simple theoretical model of a swimmer moving near similar-size obstructions. First, spherical obstructions are used to deduce physical principles linking the swimmer flow field, forces on obstructions, and changes in swimming velocities. Then single rod-like obstructions are studied which are similar to the filaments of networks. Using these results, we deduce the effect of a network of filaments. Notably, swimming properties such as the change in swimming speed and variance of the swimming speed reflect the density and orientation correlations of the microstructure, and hence swimming properties can be used as probes of microstructure. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D11.00012: Collective Motion of Magnetotactic Bacteria Solomon Barkley, Cecile Fradin, Kari Dalnoki-Veress Magnetotactic bacteria produce magnetic crystals that align the cells with an external magnetic field. Due to the field these bacteria preferentially swim along magnetic field lines in a behaviour known as magnetotaxis. Previous work has focused on bacteria in isolation, with investigations into the degree of orientation with the magnetic field as well as the response to magnetic field reversal. However, the motion of a cell in isolation cannot be extended to a cell with many neighbours, where collisions and collective effects cannot be ignored. The increased interaction between magnetotactic bacteria at very high concentrations alters the ability of any individual cell to align with an applied magnetic field. We will present experiments on the interplay between magnetotaxis and collectivity and the effects on the spatial and temporal organization of cells. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D11.00013: The Evopopbot Chip: Ultra High-throughput Evolutionary Population Bottlenecking using Drop-Based Microfluidics Connie Chang, Assaf Rotem, Adrian Serohijos, Huidan Zhang, Ye Tao, Audrey Fischer Hesselbrock, Peter Thielen, Thomas Mehoke, Joshua Wolfe, Christiane Wobus, Andrew Feldman, Eugene Shakhnovich, David Weitz The study of how viruses propagate is important for curing disease and preventing viral outbreaks.~ In nature, viruses can compete with one another, and the most evolutionary fit virus usually takes over a population.~ Yet there exist variants in the population that can escape subjected evolutionary pressures and eventually dominate the population.~ Successful studies of viral epidemics hinges on the ability to access these variants. Here, we present the use of droplet-based microfluidics as a simple method to segregate and propagate a viral population as individual viral lineages, simultaneously performing millions of in vitroevolutionary bottlenecking experiments. We introduce a novel microfluidic device, called the ``Evopopbot Chip'', that allows for simultaneous passaging of millions of evolutionary bottlenecking events by splitting drops containing previous generations of viruses and merging with drops containing new host cells. After several generations of viral replication in the evolution chip, we discover hundreds of new viruses that are able to escape a neutralizing antibody selection pressure compared to bulk passaging. [Preview Abstract] |
Session D12: Invited Session: Materials Genome: Theory-Led Accelerated Materials Discovery
Sponsoring Units: DMPChair: Darrell Schlom, Cornell University
Room: 205
Monday, March 3, 2014 2:30PM - 3:06PM |
D12.00001: Oxide Materials Invited Speaker: Mark Asta |
Monday, March 3, 2014 3:06PM - 3:42PM |
D12.00002: Using your own computer to search for novel materials (with a little help from the aflowlib.org consortium online library) Invited Speaker: Stefano Curtarolo In this presentation, we show how to use on-line resources to search for novel thermoelectrics, topological-insulators, magnets, and binary/ternary phase diagrams.\\[4pt] In collaboration with Ohad Levy, Materials Science, Duke University; Marco Buongiorno Nardelli, University of North Texas; and Natalio Mingo, CEA, Grenoble. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D12.00003: Computational Discovery and Design of Two-Dimensional Materials for Energy Technologies Invited Speaker: Richard G. Hennig Our research focuses on the development of new methods and algorithms to discover materials and to describe realistic heterogeneous interfaces and the application of these methods to the discovery and design of novel two-dimensional materials for application in energy technologies and electronic devices. In this talk, I will present our data-mining approach to identify novel two-dimensional materials with low formation energies. We identify several 2D materials in the group-III monochalcogenides and the group of transition metal dichalcogenides that are suitable for photocatalytic water splitting. We show that these 2D materials in contrast to their 3D counterparts have appropriate bad gaps and alignments with the redox potentials of water, and exhibit high solvation energies, indicating their stability in aqueous environment. We show that strain can be used to tune the electronic and optical properties of these materials. Our results provide guidance for experimental synthesis efforts and future searches of materials suitable for applications in energy technologies.\\[4pt] In collaboration with Houlong L. Zhuang, Arunima K. Singh, Michael N. Blonsky, Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA; and Michelle D. Johannes, Department of Materials Science and Engineering, Cornell University and Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, District of Columbia 20375, USA. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D12.00004: Random search - a tool for exploring dense matter Invited Speaker: Chris Pickard There has been great progress in the prediction of structure from first principles - thanks to the combination of stochastic search algorithms with reliable density functional based evaluations of the energy landscape. My approach - Ab Initio Random Structure Searching (AIRSS) [1,2] is particularly simple and powerful. In its most straightforward implementation, a lack of bias makes it suitable for theoretical explorations which can lead to new and unexpected phenomena. I have uncovered ionic phases of ammonia [3], and structural richness at terapascal pressures in aluminium [4]. An emphasis has been placed on the hunt for novel physics, illustrated by the discovery of a new route to bulk magnetism in the elements [5] and the decomposition of water under terapascal conditions [6]. The imposition of geometrical constraints permits the directed search for the ground state structure of complex compounds - I will discuss the application of AIRSS to the computational discovery of new materials. \\[4pt] [1] C.J. Pickard and R.J. Needs, Phys. Rev. Lett. 2006, 97, 45504.\\[0pt] [2] C.J. Pickard and R.J. Needs, J. Phys.: Condens. Matter Topical Review 2011, 23, 053201.\\[0pt] [3] C.J. Pickard and R.J. Needs, Nature Materials 2008, 7, 775-779.\\[0pt] [4] C.J. Pickard and R.J. Needs, Nature Materials 2010, 9, 624-627.\\[0pt] [5] C.J. Pickard and R.J. Needs, Phys. Rev. Lett. 2011, 107, 087201.\\[0pt] [6] C.J. Pickard, Miguel Martinez-Canales, and R.J. Needs, Phys. Rev. Lett. 110, 245701 (2013) [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D12.00005: The ``Missing Compounds'' affair in functionality-driven material discovery Invited Speaker: Alex Zunger In the paradigm of ``data-driven discovery,'' underlying one of the leading streams of the Material Genome Initiative (MGI), one attempts to compute high-throughput style as many of the properties of as many of the N (about 10**5- 10**6) compounds listed in databases of previously known compounds. One then inspects the ensuing Big Data, searching for useful trends. The alternative and complimentary paradigm of ``functionality-directed search and optimization'' used here, searches instead for the n much smaller than N configurations and compositions that have the desired value of the target functionality. Examples include the use of genetic and other search methods that optimize the structure or identity of atoms on lattice sites, using atomistic electronic structure (such as first-principles) approaches in search of a given electronic property. This addresses a few of the bottlenecks that have faced the alternative, data-driven/high throughput/Big Data philosophy: (i) When the configuration space is theoretically of infinite size, building a complete data base as in data-driven discovery is impossible, yet searching for the optimum functionality, is still a well-posed problem. (ii) The configuration space that we explore might include artificially grown, kinetically stabilized systems (such as 2D layer stacks; superlattices; colloidal nanostructures; Fullerenes) that are not listed in compound databases (used by data-driven approaches), (iii) a large fraction of chemically plausible compounds have not been experimentally synthesized, so in the data-driven approach these are often skipped. In our approach we search explicitly for such ``Missing Compounds''. It is likely that many interesting material properties will be found in cases (i)-(iii) that elude high throughput searches based on databases encapsulating existing knowledge. I will illustrate (a) Functionality-driven discovery of topological insulators and valley-split quantum-computer semiconductors, as well as (b) Use of ``first principles thermodynamics'' to discern which of the previously ``missing compounds'' should, in fact exist and in which structure. Synthesis efforts by Poeppelmeier group at NU realized 20 never-before-made half-Heusler compounds out of the 20 predicted ones, in our predicted space groups. This type of theory-led experimental search of designed materials with target functionalities may shorten the current process of discovery of interesting functional materials. [Preview Abstract] |
Session D13: Focus Session: Fe-Based Superconductors-STM, FeSe, Ca10(Pt3As8)(Fe2As2)5
Sponsoring Units: DMPChair: Kee Hoon Kim, Seoul National University
Room: 207
Monday, March 3, 2014 2:30PM - 2:42PM |
D13.00001: STM/S study of the interplay between local and average effects of single atomic impurities in Fe(Te,Se) Zhiyang Ye, Jiaxin Yin, Zheng Wu, Jihui Wang, Ang Li, Xuejin Liang, Jian Li, Chin-Sen Ting, Pei-Herng Hor, Hong Ding, Shuheng H Pan In materials with electron correlations, simple defects can affect both the local and the macroscopic properties of the system. We use low temperature scanning tunneling microscopy/spectroscopy (STM/S) to systematically investigate the local and average effects due to single atomic impurities in Fe(Te,Se). We have found that these single atomic impurities cause strong scattering and destroy superconductivity locally. On the other hand, the average quasi particle density of states at Fermi level exhibits a near square-root dependence on impurity concentration, suggesting that the impurity scattering is in the unitary limit. More interestingly, the superconducting gap magnitude is not affected by the impurities, despite a huge reduction of the super-fluid density and the superconducting transition temperature. We have also observed the local and the average doping effects of these single atomic impurities. Based on these observations, we will highlight the interplay between the local and the average impurity scattering effects in this iron-based superconducting system. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D13.00002: STM/S study of the mid-gap impurity bound states in Fe(Te,Se) Jiaxin Yin, Zheng Wu, Jihui Wang, Zhiyang Ye, Jing Gong, Xingyuan Hou, Lei Shan, Ang Li, Xuejin Liang, Xianxin Wu, Jian Li, Chin-Sen Ting, Jiangping Hu, Pei-Herng Hor, Hong Ding, Shuheng H Pan In superconductors, the microscopic phenomenon of quasi-particles scattering off an impurity is sensitive to the symmetry of the order parameter. Therefore, it has been used to probe the pairing symmetry of the superconducting state. We use low temperature scanning tunneling microscopy/spectroscopy (STM/S) to study the scattering effects of single atomic impurities in Fe(Te,Se). The tunneling spectrum on the pristine sample exhibits two fully opened superconducting energy gaps, which are consistent with our photoemission results. On samples with impurities, we have found that an isotropic sharp peak of mid-gap states emerges in the tunneling spectrum at the impurity site within a short length scale of $\sim$ 10 {\AA}. The results of the temperature dependence study indicate that this peak of impurity bound-states is intimately related to the state of superconductivity. We have also studied the response of this impurity induced spectrum feature to a magnetic field (up to 8 Tesla) and to impurity-impurity interactions. We will discuss the implication of our observations in the context of pairing symmetry. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D13.00003: Quasi-particle interference and possible orbital order in FeSe T. Hanaguri, T. Watashige, Y. Kohsaka, K. Iwaya, Y. Fu, S. Kasahara, D. Watanabe, Y. Mizukami, T. Mikami, Y. Kawamoto, S. Kurata, T. Shibauchi, Y. Matsuda, A. B{\"o}hmer, T. Wolf, C. Meingast, H. v. L{\"o}hneysen Spontaneous formation of unidirectional electronic states or nematicity in iron-based superconductors has attracted much attention but spectroscopic understanding of nematicity is still elusive. Using scanning tunneling microscopy/spectroscopy, we have investigated FeSe which becomes superconducting in an orthorhombic phase. Spectroscopic imaging over wide energy range ($\pm$50 meV) revealed clear quasi-particle interference (QPI) patterns. In the Fourier-transformed spectroscopic images, we identified unidirectional electron- and hole-like QPI branches, which disperse in orthogonal directions. We argue this behavior in connection to the orbital ordering in the orthorhombic phase. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D13.00004: Controlled impurity study and observation of a bosonic mode in iron based superconductors by STM measurements: implications for the pairing symmetry and mechanism Invited Speaker: Hai-Hu Wen The pairing mechanism in the iron pnictides remains unresolved yet. The pairing model based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S$^{\pm }$ pairing, however, a more conventional S$^{++}$ pairing gap was suggested based on the orbital fluctuation mediated pairing. Here we show the clear evidence of the in-gap quasi-particle states induced by the non- or very weak magnetic Cu impurities in Na(Fe$_{0.97-x}$Co$_{0.03}$Cu$_{x})$As by measuring the scanning tunneling spectroscopy, giving strong evidence of the S$^{\pm }$ pairing. Furthermore, we show the presence of the bosonic mode with the energy identical to that of the neutron resonance with a simple linear relation $\Omega $/k$_{B}$T$_{c\, }\approx $ 4.3 in several systems. This mode can also be explained very well as the consequence of the S$^{\pm}$ pairing. These observations strongly suggest that the antiferromagnetic spin fluctuation is the key factor for superconductivity. In collaboration with: Huan Yang, Zhenyu Wang, Delong Fang, Lei Shan, Qiangua Wang, Chenglin Zhang, and Pengcheng Dai, et al. \\[4pt] [1] Zhenyu Wang, et al., Nature Physics \textbf{9}, 42-48(2013). \\[0pt] [2] Lei Shan, et al. Phys. Rev. Lett. \textbf{108}, 227002 (2012). \\[0pt] [3] Huan Yang et al., Nature Communications \textbf{4}, 2947 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D13.00005: Influence of twin boundaries on superconducting gap nodes in FeSe single crystal studied by STM/STS T. Watashige, T. Hanaguri, Y. Kohsaka, K. Iwaya, Y. Fu, S. Kasahara, D. Watanabe, Y. Mizukami, T. Mikami, Y. Kawamoto, S. Kurata, T. Shibauchi, Y. Matsuda, A.E. B{\"o}hmer, T. Wolf, C. Meingast, H. v. L{\"o}hneysen We performed scanning tunneling microscopy (STM) and spectroscopy (STS) measurements on high-quality FeSe single crystals grown by vapor transport technique [1] to examine the superconducting-gap structure. In MBE-grown FeSe thin films, based on the V-shaped tunneling spectra, nodal superconductivity is suggested [2]. It is interesting to investigate how the nodes are affected by various kinds of defects. We found that twin boundaries bring about drastic effects on the gap nodes. With approaching to the twin boundary, V-shaped spectra gradually change to U-shaped ones. Interestingly, in the area between the twin boundaries separated by about 30 nm, the gap node is completely lifted and there appears a finite gap over $\pm$0.4 meV. This unusual twin-boundary effect will give us a hint to elucidate the superconducting-gap structure.\\[4pt] [1] A. E. B{\"o}hmer {\it et al.,} Phys. Rev. B {\bf 87,} 180505(R) (2013).\\[0pt] [2] C. -L. Song {\it et al.,} Science {\bf 332,} 1410 (2011). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D13.00006: Imaging the Electron-Boson Coupling in Superconducting FeSe Can-Li Song, Yi-Lin Wang, Ye-Ping Jiang, Zhi Li, Lili Wang, Ke He, Xi Chen, Jennifer E. Hoffman, Xu-Cun Ma, Qi-Kun Xue Scanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films. The mode appears below $T_c$ as a `dip-hump' feature at energy $\Omega\sim 4.7 k_B T_c$ beyond the superconducting gap $\Delta$. Spectra on strained regions of the FeSe films reveal simultaneous decreases in $\Delta$ and $\Omega$. This contrasts with all previous reports on other high-$T_c$ superconductors, where $\Delta$ locally anti-correlates with $\Omega$. A local strong coupling model is found to reconcile the discrepancy well, and to provide a unified picture of the electron-boson coupling in unconventional superconductors. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D13.00007: STM/STS study of superconducting properties in Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ Jisun Kim, Hyoungdo Nam, Guorong Li, Amar Karki, Chih-Kang Shih, Jiandi Zhang, Rongying Jin, E.W. Plummer Newly discovered iron-based superconductor, Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ (T$_{c}$ = 34 K) is studied using scanning tunneling microscopy/spectroscopy (STM/S). Given the symmetry of the crystal structure, several surface terminations are expected with roughly same probability: 1) Ca or partial Ca layer on top Fe$_{2}$As$_{2}$; 2) Ca or partial Ca layer on top Pt$_{4}$As$_{8}$ layer; 3) A Fe$_{2}$As$_{2}$ layer, and; 4) A Pt$_{4}$As$_{8}$ layer. Surprisingly, Fe$_{2}$As$_{2}$ related layers (1 \& 3) are rarely observed (less than 1\%). Instead, we observe Pt$_{4}$As$_{8}$ layers separated by unit-cell-high ($\sim$ 1 nm) steps accompanied with Ca or partial Ca layer on top Pt$_{4}$As$_{8}$ layer (1 - 2 {\AA} step height). Scanning tunneling spectroscopy reveals different spectra for each surface, with superconducting coherence peaks seen only on Ca layers. We argue that intermediary layers are proximity-coupled to superconducting Fe$_{2}$As$_{2}$ layers. The results from Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ are discussed with the properties observed in other iron-based superconductors. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D13.00008: Two band superconductivity in optimal doped Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ superconductors revealed by anisotropic H$_{c2}$ measurement up to 65T Ni Ni, Eundeok Mun, Shan Jiang, Vivien Zapf, Robert J. Cava, Eric D. Bauer, Filip Ronning Anisotropic upper critical field, H$_{c2}$(T), has been determined using radio-frequency contactless penetration depth method in pulsed magnetic fields up to 65T for optimal Pt doped and La-doped Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ single crystals with magnetic field applied parallel and perpendicular to \textit{ab} planes. For both compounds, with decreasing temperature, H$_{c2}^{//ab}$(T) shows a downward curvature while H$_{c2}^{\bot ab}$(T) shows an upward curvature, indicating their multiband superconductivity nature. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D13.00009: Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ Matthew Watson, Amalia Coldea, Sam Blake, Alix McCollam, David Vignolles, Loic Drigo, Igor Mazin, Daniel Guterding, Harald Jeschke, Roser Valenti, Ni Ni, Robert Cava We report a high magnetic field study up to 55~T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ with a $T_{c} \approx$ 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near $T_{c}$ and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22~T, when the magnetic field is applied perpendicular to the {\it ab} planes, which becomes significantly more pronounced as the temperature is lowered 0.33~K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition \textit{not of long-range ordered moments}, but of nematic antiferromagnetic fluctuations. This work was supported by the EPSRC (EP/I004475/1) and EuroMagNET II. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D13.00010: Lattice Distortion and Magnetic Order in Ca$_{10}$(Fe$_{2}$As$_{2}$)$_{5}$(Pt$_{3}$As$_{8}$) Aashish Sapkota, Gregory S. Tucker, Mehmet Ramazanoglu, Wei Tian, Ni Ni, Alan I. Goldman, Robert J. McQueeney, Andreas Kreyssig The Ca$_{\mathrm{10}}$(Fe$_{\mathrm{2}}$As$_{\mathrm{2}})_{\mathrm{5}}$(Pt$_{\mathrm{3}}$As$_{\mathrm{8}})$ compound is a member of the Fe-based high-temperature superconductor family. Recent work showed that this compound has a complex chemical structure with a skutterudite-type Pt-As intermediary layer sandwiched between tetrahedral Fe-As layers. We have studied the structural and magnetic properties of Ca$_{\mathrm{10}}$(Fe$_{\mathrm{2}}$As$_{\mathrm{2}})_{\mathrm{5}}$(Pt$_{\mathrm{3}}$As$_{\mathrm{8}})$ by single-crystal x-ray and neutron diffraction. Similar to other compounds in the iron arsenide family, we observed a lattice distortion from tetragonal to orthorhombic symmetry below approximately 110 K and stripe-like antiferromagnetic order below approximately 100 K. Both phase transitions are 2$^{\mathrm{nd}}$ order in nature. The magnetic order and lattice distortion in the iron arsenides appear to be robust against deviations from simple structure motifs. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D13.00011: Intrinsic Josephson Junctions in the iron-based multi-band superconductor (V$_2$Sr$_4$O$_6$)Fe$_2$As$_2$ Philip Moll, Xiyu Zhu, Peng Cheng, Hai-Hu Wen, Batlogg Bertram We have observed clear experimental evidence for intrinsic Josephson junction (iJJ) behavior in the iron-based superconductor (V$_2$Sr$_4$O$_6$)Fe$_2$As$_2$ (T$_c \approx 20K$). The iJJs are identified by periodic oscillations of the flux flow voltage for out-of-plane (c-axis) currents upon increasing a well aligned in-plane magnetic field. Their periodicity is well explained by commensurability effects between the Josephson vortex lattice and the crystal structure, which is a hallmark signature of Josephson vortices confined into iJJ stacks. Essential for reliable c-axis transport measurements on the available microcrystals are Focused Ion Beam microstructuring and contacting techniques. The insulating temperature behavior of $\rho_c$ indicates S-I-S type junctions. This finding adds (V$_2$Sr$_4$O$_6$)Fe$_2$As$_2$ as the first iron-based, multi-band superconductor to the copper-based iJJ materials of interest for Josephson junction applications, and in particular novel devices based on multi-band Josephson coupling may be realized. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D13.00012: Magnetic and Electronic Phase Diagram of Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ Yujiro Tojo, Taizo Shibuya, Tetsuro Nakamura, Koichiro Shoji, Masanori Matoba, Shintaro Yasui, Mitsuru Itoh, Shinji Kitao, Makoto Seto, Yoichi Kamihara Ae$_{2}$MFePnO$_{3}$ (so-called 21113 systems) with perovskite-type layers such as Ae$_{2}$MO$_{3}$, where Ae denotes alkaline earth metals, M does Sc, Ti, Cr, V and other transition metal atoms and Pn does As and P shows superconductivity at T \textless 46 K. Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ is a representative compound in 21113 systems. [Zhu et al, Phys. Rev. B (2009); Ogino et al, Supercond. Sci. Technol. (2009); Ogino et al, Supercond. Sci. Technol. (2009)] Although the oxygen deficiency (d) as a function of $T_{\mathrm{c}}$ is still controversial in Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$, many samples have been reported as superconductors with $T_{\mathrm{c}} =$ 24-37 K. In this study, a polycrystalline Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ (d $=$ $\sim$ 0.1- 0.6) were prepared by a solid state reaction using an alumina tube in a sealed silica tube. DC electrical resistivity was measured by a four-probe technique. Magnetization measurements were performed on a superconducting quantum interference device. $^{57}$Fe Mossbauer spectra were obtained using conventional equipment. The electronic and magnetic phase diagram of Sr$_{2}$VFeAsO$_{\mathrm{3-d}}$ is elucidated. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D13.00013: Weak coupling BCS-like superconductivity in the pnictide oxide Ba$_{1-x}$Na$_{x}$Ti$_{2}$Sb$_{2}$O B. Lorenz, M. Gooch, P. Doan, Z. Tang, A.M. Guloy, C.W. Chu We report the results of low-temperature heat capacity measurements of the pnictide oxide superconductor BaTi$_{2}$Sb$_{2}$O and the optimally Na-doped compound Na$_{0.15}$Ba$_{0.85}$Ti$_{2}$Sb$_{2}$O. Temperature- and field-dependent heat capacity data are well described by a single-gap BCS theory. The estimated values for the normal-state Sommerfeld constant, the heat capacity jump at $T_{c}$, and the electron-phonon coupling constant are in favor of a conventional weak coupling superconductivity, possibly mediated by electron-phonon interaction. The results are discussed with regard to and compared with recent first-principles calculations. [Preview Abstract] |
Session D14: Invited Session: Algorithms from Statistical Physics and the Physics of Algorithms
Sponsoring Units: GSNP DCOMPRoom: 301-303
Monday, March 3, 2014 2:30PM - 3:06PM |
D14.00001: Fast algorithms for glassy materials: methods and explorations Invited Speaker: A. Alan Middleton Glassy materials with frozen disorder, including random magnets such as spin glasses and interfaces in disordered materials, exhibit striking non-equilibrium behavior such as the ability to store a history of external parameters (memory). Precisely due to their glassy nature, direct simulation of models of these materials is very slow. In some fortunate cases, however, algorithms exist that exactly compute thermodynamic quantities. Such cases include spin glasses in two dimensions and interfaces and random field magnets in arbitrary dimensions at zero temperature. Using algorithms built using ideas developed by computer scientists and mathematicians, one can even directly sample equilibrium configurations in very large systems, as if one picked the configurations out of a ``hat'' of all configurations weighted by their Boltzmann factors. This talk will provide some of the background for these methods and discuss the connections between physics and computer science, as used by a number of groups. Recent applications of these methods to investigating phase transitions in glassy materials and to answering qualitative questions about the free energy landscape and memory effects will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D14.00002: The Cavity Method, Belief Propagation, and Phase Transitions in Community Detection Invited Speaker: Cristopher Moore The stochastic block model is a popular model of social and biological networks. Each vertex belongs to one of k groups, and the probability of an edge depends on the groups to which its endpoints belong. It allows both ``assortative'' communities where vertices tend to connect to others in the same group, and ``disassortative'' and directed community structures, like those in food webs, where predators form a community because they feed on similar prey. Given a network, we would like to infer the most likely block model: both the group labels of the nodes, and the parameters of the model. I will describe an efficient algorithm for this problem based on belief propagation, or equivalently the cavity method of statistical physics. Physically, the model corresponds to a generalized Potts model with strong interactions on the links of the network, and weak interactions along the non-links. Our analysis also reveals phase transitions in the detectability of communities, including an undetectable phase where no algorithm can do better than chance. There is also a regime where the accuracy we can achieve jumps discontinuously as a function of the fraction of nodes we have prior information about, and a tricritical point where this discontinuity disappears. This is joint work with Aurelien Decelle, Florent Krzakala, Lenka Zdeborov\'a, and Pan Zhang. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D14.00003: Efficient discovery of large-scale patterns in weighted networks Invited Speaker: Aaron Clauset Networks provide a rich and mathematically principled approach to characterizing the structure of complex systems. A common step in understanding the structure and function of real-world networks is to characterize their large-scale organizational pattern via community detection, in which we aim to find a network partition that groups together vertices with similar connectivity patterns. Although interactions in most real-world systems take real or integer valued weights, common approaches to community detection use only the unweighted edges, thereby ignoring a potentially rich source of additional information. In this talk, I will describe a generalization of the popular stochastic block model that can discover community structure from both the existence and weight of edges. This model can be efficiently fitted to real-world networks using ``approximate inference'' techniques, like the cavity method, originally developed in statistical physics and which are now commonly used by computer scientists in machine learning. Applied to several real-world networks, I show that edge weights sometimes contain hidden information that is distinct from what is contained in edge existences. Learning from weights also provides better estimates of missing information. I will close with a few brief comments on the impact of weight information on the detectability threshold for recovering hidden patterns in these systems, and on future opportunities in this area. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D14.00004: Non-equilibrium Monte Carlo: Sampling with Irreversible Markov Chains Invited Speaker: Marija Vucelja Markov Chain Monte Carlo (MCMC) algorithms are ubiquitous across different fields of physics. They are an invaluable numerical tool for studies of complex many body problems, critical phenomena etc. Yet often their convergence rates for real systems are slow. MCMC algorithms are used to create realizations of the desired physical system in its steady state. Most implementations of MCMC use Markov Chains that obey detailed balance, even though this not a necessary requirement for converging to a steady state. I plan to overview several examples that utilize irreversible Markov Chains, where violating detailed balance has improved the convergence rate. Finally I will pose some open questions and discuss attempts to use non-equilibrium dynamics for efficient sampling. Potential applications of such algorithms are numerical studies of phase transitions, soft matter dynamics, protein structures, granular media etc. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D14.00005: Rare-event simulation methods for equilibrium and non-equilibrium events Invited Speaker: Robert Ziff Rare events are those that occur with a very low probability in experiment, or are common but difficult to sample using standard computer simulation techniques. Such processes require advanced methods in order to obtain useful results in reasonable amounts of computer time. We discuss some of those techniques here, including the ``barrier'' method, splitting methods, and a Forward-Flux Sampling in Time (FFST) algorithm, and apply them to measure the nucleation times of the first-order transition in the Ziff-Gulari-Barshad model of surface catalysis, including nucleation in finite equilibrium states, which are measured to occur with probabilities as low as 10\textasciicircum (-40). We also study the transitions in the Maier-Stein model of chemical kinetics, and use the methods to find the harmonic measure in percolation and Diffusion-Limited Aggregation (DLA) clusters. co-authors: David Adams, Google, and Leonard Sander, University of Michigan. References: D. A. Adams, R. M. Ziff, and L. M. Sander, Computation of nucleation at a nonequilibrium first-order phase transition using a rare-event algorithm, Journal of Chemical Physics, 133, 174107 (2010) D. A. Adams, L. M. Sander and R. M. Ziff, The barrier method: A technique for calculating very long transition times, Journal of Chemical Physics 133, 124103 (2010) D. A. Adams, Yen Ting Lin, L. M. Sander and R. M. Ziff, ``Harmonic measure for critical Potts clusters,'' Physical Review E 80, 031141 (2009) [Preview Abstract] |
Session D15: Droplets & Surfaces
Sponsoring Units: DFDChair: Xiang Cheng, University of Minnesota
Room: 304
Monday, March 3, 2014 2:30PM - 2:42PM |
D15.00001: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 2:42PM - 2:54PM |
D15.00002: Towards a more accurate microscopic description of the moving contact line problem -- incorporating nonlocal effects through a statistical mechanics framework Andreas Nold, Ben Goddard, David Sibley, Serafim Kalliadasis Multiscale effects play a predominant role in wetting phenomena such as the moving contact line. An accurate description is of paramount interest for a wide range of industrial applications, yet it is a matter of ongoing research, due to the difficulty of incorporating different physical effects in one model. Important small-scale phenomena are corrections to the attractive fluid-fluid and wall-fluid forces in inhomogeneous density distributions, which often previously have been accounted for by the disjoining pressure in an ad-hoc manner. We systematically derive a novel model for the description of a single-component liquid-vapor multiphase system which inherently incorporates these nonlocal effects. This derivation, which is inspired by statistical mechanics in the framework of colloidal density functional theory, is critically discussed with respect to its assumptions and restrictions. The model is then employed numerically to study a moving contact line of a liquid fluid displacing its vapor phase. We show how nonlocal physical effects are inherently incorporated by the model and describe how classical macroscopic results for the contact line motion are retrieved. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D15.00003: Droplet dynamics on chemically striped patterned surfaces Patrick Jansen, Kai Sotthewes, Harold Zandvliet, Stefan Kooij We study the dynamics of droplets on chemically striped patterned surfaces with alternating hydrophilic and hydrophobic stripes. A droplet deposited on such a surface typically adopts an elongated shape, due to preferential spreading; across the stripes spreading is more difficult than in the direction along the stripes. The shape evolution of a droplet on such a surface is investigated both in experiment, and numerical simulations employing the lattice Boltzmann technique. The shape is dependent on the path that is taken, the amount of kinetic energy, and the size of the droplet in comparison to the stripe dimension. Additionally, we also investigate the evaporation of water droplets on these surfaces. Elongated droplets evaporate markedly faster than spherical ones making the evaporation rate dependent on the striped pattern underneath the droplet. Finally, a gradient in surface energy can be constructed using these stripes, which enables droplet movement on the surface without applying additional external forces. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D15.00004: On the evolution of electrically charged toroidal droplets Alexandros Fragkopoulos, Ekapop Pairam, Alberto Fernandez-Nieves We can successfully generate viscous toroidal droplets suspended in another immiscible viscous liquid. Toroidal droplets are unstable due to surface tension and either break via a hydrodynamic instability similar to Rayleigh-Plateau or shrink until they collapse into a single spherical droplet. By applying a voltage difference across the droplet and a controlled ground we are able to charge the toroidal droplets. As a result, the surface tension now is in competition with the electrostatic repulsion due to the presence of the charge; this can change the evolution of the torus significantly. For instance, it can cause the expansion of the torus rather than its shrinkage, and also affects the wavelength of the fastest unstable mode. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D15.00005: Liquid drops on soft solids Luuk A. Lubbers, Joost H. Weijs, Siddhartha Das, Lorenzo Botto, Bruno Andreotti, Jacco H. Snoeijer A sessile drop can elastically deform a substrate by the action of capillary forces. The typical size of the deformation is given by the ratio of surface tension and the elastic modulus, $\gamma/E$, which can reach up to 10-100 microns for soft elastomers. In this talk we theoretically show that the contact angles of drops on such a surface exhibit two transitions when increasing $\gamma/E$: (i) the microsocopic geometry of the contact line first develops a Neumann-like cusp when $\gamma/E$ is of the order of few nanometers, (ii) the macroscopic angle of the drop is altered only when $\gamma/E$ reaches the size of the drop. Using the same framework we then show that two neighboring drops exhibit an effective interaction, mediated by the deformation of the elastic medium. This is in analogy to the well-known Cheerios effect, where small particles at a liquid interface attract each other due to the meniscus deformations. Here we reveal the nature of drop-drop interactions on a soft substrate by combining numerical and analytical calculations. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D15.00006: Investigating the stability of surface nanobubbles Robin Berkelaar, Erik Dietrich, Stefan Kooij, Harold Zandvliet, Detlef Lohse The primary attribute of interest of surface nanobubbles is their unusual stability and a number of theories trying to explain this have been put forward. Interestingly, the actual dissolution of nanobubbles is a topic that did not receive a lot of attention yet. We applied different experimental procedures in which gaseous nanobubbles should dissolve, according to the theories. In method A, the nanobubbles were exposed to a flow of degassed water for 96 hours. In method B, the ambient pressure was lowered in order to degas the liquid and the nanobubble-like objects. In method C, the liquid was evaporated and the geometry of the nanobubbles was indirectly studied in micrometers thick water films. The effects of these three methods on the stability of nanobubbles will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D15.00007: Simulation of splashing of micro-scale droplets on a dry surface Arnout Boelens, Andrzej Latka, Michelle Driscoll, Irmgard Bischofberger, Cacey Stevens, Sidney Nagel, Juan de Pablo Results are presented for the simulation of micro-scale droplets splashing on a dry surface. The simulations are performed using a Volume Of Fluid approach and a Finite Volume technique. The contact line is described using a fixed microscopic contact angle. Both the gas phase and the liquid phase are assumed to be incompressible, and represent a two-phase system of ethanol in air. As the droplet approaches the wall, it changes shape and forms a dome over a thin gas layer between the droplet and the wall. As the gas gets squeezed out from under the droplet, it reaches velocities of up to 300 km/h, and there is a very large pressure spike at the edge of the droplet. The formation of a thin sheet of liquid is observed upon impact, which thickens near the edge of the sheet to reduce curvature. As the sheet begins to spread apparent contact angles are observed to approach 180 degrees. When lowering the gas pressure in the system, a higher gas velocity and relative pressure are observed on impact, and again a thin sheet forms. However, at low pressure the thin sheet stays closer to the wall. These observations are shown to be consistent with experimental measurements. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D15.00008: Study of 2D emulsions produced by Breath Figures of two immiscible substances Jos\'e Guadarrama-Cetina, Wenceslao Gonz\'alez-Vi\~nas In this work we analyze experimental results [1,2] on two condensing vapors (22 $^{\circ}$C) of ultrapure water (W) and Hexamethyldisiloxane (HMDSO) on a cold (5 $^{\circ}$C) repellent surface. From the statistics of population of the two kind of droplet patterns, we characterize statistically the emulsion formation through the parameters of occupancy, area fraction and diameter of droplet average per unit area and through the PDF of droplets diameter for each stage of the BF and its emulsion product. We compare those parameters and the PDFs for different straming rates of W/HMDSO and we give the necessary conditions to drive the system to stabilisation stage and cluster formation. \\[4pt] [1] J. Guadarrama {\sl et al.}, Phys. Rev. E {\bf 87}, 054401 (2013).\\[0pt] [2] J. Guadarrama {\sl et al.}. In preparation. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D15.00009: Micro bubbles at interfaces Gholamreza Keshavarzi, Anna Wang, Tracie Barber, Vinothan Manoharan The behaviour of a small micron sized bubbles close to an interface is vital to various interface interaction applications in several industries. Previous studies have focused on understanding the behaviour of large millimetric bubbles reaching an interface. Some of these millimetric bubbles are shown to bounce back [1], while others penetrate and burst on the interface resulting in possible small micron sized bubbles [2]. However, small micron sized bubble may act different. It has been observed that small microbubbles can act as if they are stabilized at the interface without merging to the fluid over the interface. The dynamics of the microbubble adsorption close to an interface has yet to be well understood.In this study we used digital holography microscopy to explore detailed information on the behaviour of the air microbubble at the interface. This study investigates the position and shape of a microbubble with respect to the interface. The dynamic behavior close to the interface along with where the small microbubble is positioned near an interface will help us in understanding the probability of penetration and merging back to the fluid on top. [1]Rise, bouncing and coalescence of bubbles impacting at a free surface, Colloids and surfaces A:Physicochemical and Engineering Aspects, Volume 365, Issues 1-3, pp. 36-762 (2010) [2]Daughter bubble cascades produced by folding of ruptured thin films, Nature, Volume 465, Issue 7299, pp. 759-762 (2010) [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D15.00010: Quantum Tunneling and Chaos in Classical Scale Walkers Jenny Su, Joshua Dijksman, Jeremy Ward, Robert Behringer We study the behavior of `walkers'; small droplets bouncing on a fluid layer vibrated at amplitudes just below the onset of Faraday instability. It was shown recently that despite their macroscopic size, the droplet dynamics are stochastic in nature and reminiscent of the dual particle-wave dynamics in the realm of quantum mechanics (Couder PRL 2006). We use these walkers to study how chaos, which is macroscopically unpredictable, will manifest in a quantum setting. Pecora showed in 2011 that tunneling for particles that have a chaotic ground state is different from tunneling for particles with a regular ground state (PRE 2011). In the experiment we gather data that illustrates the particle trajectory and tunneling behavior as particles transition across the barrier in the double well system with both integrable and chaotic shapes. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D15.00011: Cooling Enhancement by Drop Impact and Pool Boiling on Nano-textured Surfaces Under Normal Gravity Conditions and at Zero and Increased Gravity in Parabolic Flights Alexander Yarin, Suman Sinha-Ray, Seongchul Jun The earth experiments with drop impact onto metal-plated electrospun nanofiber mats encompass a single drop, or drop trains or jets impacts. The results on drop cooling and pool boiling on nano-textured surface were obtained during the parabolic flights supported by NASA and ESA. Pool boiling on nano-textured surfaces was studied for ethanol and water as working fluids. The nano-textured surfaces were copper platelets covered with copper-plated electrospun nanofibers. The results revealed that the heat flux in boiling on the nano-textured surfaces was about 3-8 times higher than that on the bare copper. This stems from the fact that nano-textured surfaces promote bubble growth by increasing the average temperature of fluid surrounding growing bubbles. Nano-textured surfaces facilitated bubble growth rate and increase bubble detachment frequency. On the other hand, the critical heat flux (CHF) on the nano-textured surfaces was found to be very close to its counterpart on the bare copper surfaces. However, the heat flux on the nano-textured surfaces in transition boiling was significantly higher than on the bare copper ones, since the presence of nanofibers prevented bubble merging and delayed formation of vapor film. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D15.00012: Liquid drop impact on a granular surface Xiang Cheng, Runchen Zhao, Qianyun Zhang, Hendro Tjugito We investigate the impact of droplets onto a granular surface - a process that is likely familiar to all of us who have watched raindrops splashing on a sandy ground in a garden or on a beach. Combining high-speed photography with laser profilometry measurement, we experimentally study the 3D morphology of granular craters formed by liquid drop impact. By systemically varying the releasing height of liquid droplets, the wetting properties of granular particles, and the size ratio of droplets to particles, we show a scaling behavior of the size of craters and demonstrate a rich variation of the shape of granular residues in the center of craters. Based on liquid impact dynamics, a simple model is constructed to quantitatively explain the observed crater morphologies. Contrary to previous studies, our result suggests that the capillary interaction between particles and liquid is the main shaping force for the craters, and the drainage of liquid into the granular bed only plays a minor role in the process. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D15.00013: Emission modes in electrically-assisted coflow A.J. Hijano, J. Guerrero, A. Fernandez-Nieves, I.G. Loscertales We use glass-based microfluidic devices to study the emission regimes in electro-coflow. In addition to cone-jet and whipping, which are also seen in air or in the presence of a quiescent liquid bath, we also observe other regimes that where not observed before. One of these consists of a bent jet that remains confined to a plane that moves in time either periodically or aperiodically. We explore the effects of the inner and outer-fluid flow rates, their viscosity contrast and the applied voltage. [Preview Abstract] |
Session D16: Systems Far from Equilibrium
Sponsoring Units: GSNPChair: Beate Schmittmann, Iowa State University
Room: 401
Monday, March 3, 2014 2:30PM - 2:42PM |
D16.00001: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 2:42PM - 2:54PM |
D16.00002: Driven Langevin systems: fluctuation theorems and faithful dynamics David Sivak, John Chodera, Gavin Crooks Stochastic differential equations of motion (e.g., Langevin dynamics) provide a popular framework for simulating molecular systems. Any computational algorithm must discretize these equations, yet the resulting finite time step integration schemes suffer from several practical shortcomings. We show how any finite time step Langevin integrator can be thought of as a driven, nonequilibrium physical process. Amended by an appropriate work-like quantity (the shadow work), nonequilibrium fluctuation theorems can characterize or correct for the errors introduced by the use of finite time steps. We also quantify, for the first time, the magnitude of deviations between the sampled stationary distribution and the desired equilibrium distribution for equilibrium Langevin simulations of solvated systems of varying size. We further show that the incorporation of a novel time step rescaling in the deterministic updates of position and velocity can correct a number of dynamical defects in these integrators. Finally, we identify a particular splitting that has essentially universally appropriate properties for the simulation of Langevin dynamics for molecular systems in equilibrium, nonequilibrium, and path sampling contexts. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D16.00003: Establishing a Nonequilibrium Fluctuation-Dissipation Theorem Through Simultaneous Measurement of the Power Spectral Density and Transfer Function of Driven Systems Alexander Trevelyan, Eric Corwin We explore the response of a model statistical system to strong, non-linear perturbations to its state variables. Specifically, we work with a tunable model of Johnson-Nyquist noise, designed to permit a driving of both the drift and diffusion terms in the associated White Noise Langevin Equation. We achieve a simultaneous measurement of both sides of the Fluctuation Dissipation Theorem (FDT) by driving the circuit with digitally generated white noise and measuring the output. This allows us to calculate a frequency-dependent effective temperature for the driven system, which for an equilibrium system should be set by the energy scale of the input white noise. Comparison of the two sides of FDT--the circuit's transfer function and the power spectral density of the voltage fluctuations--across frequency-space proves non-trivial, and methods are discussed for achieving the most reliable estimate. After comparing the response for a series of functional signals, we find that FDT, measured in this simultaneous fashion, remains intact even while the system is being actively driven out of equilibrium. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D16.00004: Quantum diffusion and entropy production: An exactly solvable model Wim Magnus, Kwinten Nelissen An exact, analytical solution of a simple, quantum mechanical model describing diffusion currents flowing between two fermion reservoirs is presented. The quantum fluctuations characterizing the transient diffusion current and entropy production are explicitly shown, whereas the long-time behavior of the fermion densities is determined by a power law. The interaction Hamiltonian defining the coupling between the reservoirs is related to fermions hopping between real space sites. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D16.00005: Quantum entropy production in phase space Sebastian Deffner A fluctuation theorem for the nonequilibrium entropy production in quantum phase space is derived, which enables the consistent thermodynamic description of arbitrary quantum systems, open and closed. The new treatment naturally generalizes classical results to the quantum domain. As an illustration the harmonic oscillator dragged through a thermal bath is solved numerically. Finally, the significance of the new approach is discussed in detail, and the phase space treatment is opposed to the two time energy measurement approach.\\[4pt] Ref.: S. Deffner, EPL (Europhysics Lett.) \textbf{103}, 30001 (2013) [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D16.00006: Prediction of HR/BP response to the spontaneous breathing trial by fluctuation dissipation theory Man Chen We applied the non-equilibrium fluctuation dissipation theorem to predict how critically-ill patients respond to treatment, based on both heart rate data and blood pressure data collected by standard hospital monitoring devices. The non-equilibrium fluctuation dissipation theorem relates the response of a system to a perturbation to the fluctuations in the stationary state of the system. It is shown that the response of patients to a standard procedure performed on patients, the spontaneous breathing trial (SBT), can be predicted by the non-equilibrium fluctuation dissipation approach. We classify patients into different groups according to the patients' characteristics. For each patient group, we extend the fluctuation dissipation theorem to predict interactions between blood pressure and beat-to-beat dynamics of heart rate in response to a perturbation (SBT), We also extract the form of the perturbation function directly from the physiological data, which may help to reduce the prediction error. We note this method is not limited to the analysis of the heart rate dynamics, but also can be applied to analyze the response of other physiological signals to other clinical interventions. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D16.00007: Calculating free energy profiles and identifying the underlying dynamics in systems with memory effects from bi-directional pulling processes Jiong Zhang, Ioan Kosztin A proper description of the effective dynamics of a biomolecular system along a relevant reaction coordinate (RC) requires not only the determination of the corresponding free energy profile (potential of mean force or PMF) but also the correct identification of the underlying stochastic model. While there exist several methods for determining the PMF from fast non-equilibrium pulling processes, for simplicity reasons, it is generally assumed that the dynamics along the RC is that of a simple overdamped Brownian particle with known diffusion coefficient. However, in general, the dynamics along the RC is non-Markovian and can be modeled with a generalized Langevin equation characterized by a friction memory kernel. Here we propose and demonstrate a method that permits the simultaneous determination of both PMF and friction memory kernel from fast bi-directional (forward and time-reversed) pulling processes. As a result, one can identify whether the diffusion along the RC is normal or anomalous (e.g., subdiffusion). The proposed method provides a novel approach for identifying and characterizing the underlying dynamics along a RC of a biomolecular system studied by either single-molecule force microscopy or steered molecular dynamics simulations. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D16.00008: High-precision work distributions for extreme non-equilibrium processes in large systems Alexander Hartmann The distributions of work for strongly non-equilibrium processes are studied using a very general form of a large-deviation approach, which allows one to study distributions down to extremely small probabilities of almost arbitrary quantities of interest for equilibrium, non-equilibrium stationary and even non-stationary processes. The method is applied to varying quickly the external field in a wide range $B=3\ \leftrightarrow 0$ for critical ($T=2.269$) two-dimensional Ising system of size $L\times L=128\times 128$. To obtain free energy differences from the work distributions, they must be studied in ranges where the probabilities are as small as $10^{-240}$, which is not possible using direct simulation approaches. By comparison with the exact free energies, one sees that the present approach allows one to obtain the free energy with a very high relative precision of $10^{-4}$. This works well also for non-zero field, i.e., for a case where standard umbrella-sampling methods seem to be not so efficient to calculate free energies. Furthermore, for the present case it is verified that the resulting distributions of work fulfill Crooks theorem with high precision. Finally, the free energy for the Ising magnet as a function of the field strength is obtained. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D16.00009: Chirality, Causality, and Fluctuation-Dissipation Theorems in Nonequilibrium Steady States Dima Feldman, Chenjie Wang Edges of some quantum Hall liquids and a number of other systems exhibit chiral transport: excitations can propagate in one direction only, e.g., clockwise. We derive a family of fluctuation-dissipation relations in nonequilibrium steady states of such chiral systems. The theorems connect nonlinear response with fluctuations far from thermal equilibrium and hold only in case of chiral transport. They can be used to test the chiral or nonchiral character of the system.\\[4pt] [1] C. Wang and D. E. Feldman, Phys. Rev. Lett. 110, 030602 (2013) [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D16.00010: Inconsistencies in steady state thermodynamics Ronald Dickman, Ricardo Motai We address the issue of extending thermodynamics to nonequilibrium steady states. Using driven stochastic lattice gases, we ask whether consistent definitions of an effective chemical potential $\mu$, and an effective temperature $T_e$, are possible. These quantities are determined via zero-flux conditions of particles and energy between the driven system and a reservoir. For the models considered here, the fluxes are given in terms of certain stationary average densities, eliminating the need to perturb the system by actually exchanging particles; $\mu$ and $T_e$ are thereby obtained via open-circuit measurements, using a virtual reservoir. In the lattice gas with nearest-neighbor exclusion, temperature is not relevant, and we find that the effective chemical potential, a function of density and drive strength, satisfies the zeroth law, and correctly predicts the densities of coexisting systems. In the Katz-Lebowitz-Spohn driven lattice gas, both $\mu$ and $T_e$ need to be defined. We show analytically that the zeroth law is violated, and determine the size of the violations numerically. Our results highlight a fundamental inconsistency in the extension of thermodynamics to nonequilibrium steady states. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D16.00011: Fluctuation spectra in weakly modulated nonlinear systems Yaxing Zhang, Yukihiro Tadokoro, Mark Dykman We consider periodically modulated nonlinear systems and show that, along with the delta-peak at the modulation frequency, their spectral density of fluctuations can display extra peaks. The intensity of the peaks is quadratic in the modulation amplitude, for weak modulation. For systems where inertial effects can be disregarded, like an overdamped particle in a potential well, the peaks are generally located at zero frequency and at the modulation frequency. The widths of the peaks are characterized by the reciprocal correlation time of the system fluctuations in the absence of modulation and the noise correlation time. The spectra sensitively depend on the interrelation between these times and on the fluctuation intensity. They are determined not only by the fluctuations of the linear response, but also have a contribution from nonlinear response. The analytical results obtained for overdamped dynamical systems as well as two-state systems and systems with a threshold are in excellent agreement with numerical simulations. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D16.00012: Nontrivial Exponents in Record Statistics Eli Ben-Naim, Pearson Miller We investigate records in a growing sequence of identical and independently distributed random variables. The record equals the largest value in the sequence, and our focus is on the increment, defined as the difference between two successive records. We investigate sequences in which all increments decrease monotonically, and analyze the case where the random variables are drawn from a uniform distribution with compact support. We find that the fraction $I_N$ of sequences that exhibit this property decays algebraically with sequence length $N$, namely $I_N \sim N^{-\nu}$ as $N \rightarrow \infty$, and obtain the exponent $\nu = 0.317621\ldots$ using analytic methods. We also study the record distribution and the increment distribution. Whereas the former is a narrow distribution with an exponential tail, the latter is broad and has a power-law tail characterized by the exponent $\nu$. Empirical analysis of records in the sequence of waiting times between successive earthquakes is consistent with the theoretical results. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D16.00013: Lifetime and decay of seeded breathers in the FPU system Matthew Westley, Nicholas DeMeglio, Surajit Sen, T.R. Krishna Mohan The Fermi-Pasta-Ulam problem [1] consists of a chain of N oscillators with linear and nonlinear nearest neighbor interactions. Using velocity-Verlet integration, we study the evolution of the system after a perturbation that consists of a single stretched bond at the center of the chain [2-4]. This perturbation results in the localization of most of the system's energy in the center particles in the form of a ``breather'' up to reasonably long times, which leaks energy at a rate depending on the potential parameters and the perturbation amplitude. The breather eventually undergoes a catastrophic breakdown, releasing all of its energy into acoustic noise and solitary waves. We explore the conditions on the amplitude and the parameters $\alpha $, $\beta $ for which a seeded breather will be most or least stable. Also we show how the overlap or lack thereof between the breather's primary frequencies and the acoustic frequencies influences its long-time stability. \\[4pt] [1] E. Fermi, J. Pasta, and S. Ulam, Los Alamos Scientific Laboratory Report No. LA-1940 (1955).\\[0pt] [2] S. Flach and A. V. Gorbach, Phys. Rep. \textbf{467}, 1 (2008).\\[0pt] [3] A. J. Sievers and S. Takeno, Phys. Rev. Lett. \textbf{61}, 970 (1988).\\[0pt] [4] T. K. Mohan and S. Sen, Pramana \textbf{77}, 975 (2011). [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D16.00014: Spontaneous symmetry breaking of action in complex systems Georgi Georgiev In simple systems the action has a single minimum for the motion of a particle along a geodesic, compared to all other paths. In complex systems, motion along a geodesic has higher action, compared to an infinite set of symmetric longer trajectories, due to constraints. For infinitely long paths, action rises to infinity. On this ``Mexican hat'' surface, a system spontaneously chooses one of the infinite number of minimum action trajectories, during its phase transition from a simple to a complex system. The initial geodesic path of a free particle is the ``vacuum,'' or ``ground state'' of the complex system. The action of the flow is minimized along a network as compared to motion in a different geometry. This leads to a flow network representation of a complex system, where the trajectories in the system are along flow paths with least action. A flow network implies a constant inflow and outflow of energy and can exist only in open systems far from equilibrium. We consider several examples in developing this formalism, which is useful for understanding and managing complex systems. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D16.00015: Thermodynamics of cellular statistical inference Alex Lang, Charles Fisher, Pankaj Mehta Successful organisms must be capable of accurately sensing the surrounding environment in order to locate nutrients and evade toxins or predators. However, single cell organisms face a multitude of limitations on their accuracy of sensing. Berg and Purcell first examined the canonical example of statistical limitations to cellular learning of a diffusing chemical and established a fundamental limit to statistical accuracy. Recent work has shown that the Berg and Purcell learning limit can be exceeded using Maximum Likelihood Estimation. Here, we recast the cellular sensing problem as a statistical inference problem and discuss the relationship between the efficiency of an estimator and its thermodynamic properties. We explicitly model a single non-equilibrium receptor and examine the constraints on statistical inference imposed by noisy biochemical networks. Our work shows that cells must balance sample number, specificity, and energy consumption when performing statistical inference. These tradeoffs place significant constraints on the practical implementation of statistical estimators in a cell. [Preview Abstract] |
Session D17: Granular Materials & GSNP Student Speaker Session
Sponsoring Units: GSNPRoom: 402
Monday, March 3, 2014 2:30PM - 2:42PM |
D17.00001: Thermodynamics of cellular statistical inference Alex Lang, Charles Fisher, Pankaj Mehta Successful organisms must be capable of accurately sensing the surrounding environment in order to locate nutrients and evade toxins or predators. However, single cell organisms face a multitude of limitations on their accuracy of sensing. Berg and Purcell first examined the canonical example of statistical limitations to cellular learning of a diffusing chemical and established a fundamental limit to statistical accuracy. Recent work has shown that the Berg and Purcell learning limit can be exceeded using Maximum Likelihood Estimation. Here, we recast the cellular sensing problem as a statistical inference problem and discuss the relationship between the efficiency of an estimator and its thermodynamic properties. We explicitly model a single non-equilibrium receptor and examine the constraints on statistical inference imposed by noisy biochemical networks. Our work shows that cells must balance sample number, specificity, and energy consumption when performing statistical inference. These tradeoffs place significant constraints on the practical implementation of statistical estimators in a cell. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D17.00002: Macroscopic consequences of contact breaking in the vibrational response of jammed packings W. Wendell Smith, Mark D. Shattuck, Corey S. O'Hern Computational studies of the linear vibrational response regime of packings of frictionless spherical particles have yielded many insights into our understanding of the mechanical properties of amorphous solids. However, many jammed systems such as granular media display strongly nonlinear vibrational response. Even the model systems of spherical particles that interact via purely repulsive linear springs and are typically used in computational studies of jamming display nonlinear vibrational response due to the breaking and forming interparticle contacts.\\ In this work, we perform molecular dynamics simulations of spherical particles with purely repulsive contact interactions and study their vibrational response as a function of the energy and frequency content of the initial perturbation. In particular, we explore the consequences of contact breaking on macroscopic quantities such as the specific heat, momentum current, and energy flux. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D17.00003: Electrical charging of granular media in a shaking experiment Freja Nordsiek, Allison Bradford, Tyler Holland-Ashford, Julia Salevan, Eric Spieglan, Daniel Lathrop We present preliminary results on the electrical charging of granular media (particle size $\sim 100$ $\mu$m to $\sim 1$ mm) shaken between two conducting plates. Voltage measurements were done between the plates for both monodisperse and bidisperse sets of particles. Particle charging and electrical discharges to the plates ($\sim 1$ kV) were observed. We discuss the potential relevance to natural charging phenomena seen in sand storms, volcanic ash clouds, thunderstorms, and thundersnow. Several types of theoretical models seem plausible. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D17.00004: In Granular Charging, Does Size Really Matter? Theodore Siu, Gregory Mattson, Troy Shinbrot Spontaneous charging in systems of particles, causing particle separation and electrical discharges, is commonly observed in pharmaceutical powder beds, sandstorms and natural dust plumes. Previous studies have attributed size difference or external factors such as wind or an outside electric field as the primary driving force behind such large scale charging. In this talk we discuss experimental results showing that systems of uniformly sized particles with no external field still exhibit net polarization and charging buildup. We also present computational results modeled from a variation of Dyson's Ising model, which validates this behavior and predicts new types of phenomena. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D17.00005: Contact Network Statistics During Vibration of Disk Packings Mark R. Kanner, Carl Schreck, Corey O'Hern, Mark D. Shattuck We use simulations of bidisperse disks that interact via purely repulsive linear springs to determine properties of contact networks during vibration at various energies and pressures. From a set of initially existing contacts in a mechanically stable reference state the contact probability during vibration can be predicted by measuring the inter-particle potential before vibration. ~We explore the energy regions below particle rearrangement where our prediction is valid and discuss a physical mechanism for this behavior based on the exchange of potential and kinetic energy between particles. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D17.00006: Mechanical Friction: Tuning the Janssen Pressure by Varying Particle and Wall Geometry Yasin Karim, Eric Corwin Friction provides a way for granular materials to interact with other particles and the system boundaries. Friction mediated interactions can give rise to interesting properties like the Janssen effect, that are unique to granular materials. Using a conveyor belt we study friction-compacted 2D granular systems to explore the effects of changing particle geometries on the underlying physics of the system. As we have previously shown stick-slip motion due to sliding friction forces can relax away particle-wall friction. These tangential forces can be recovered by suitably changing the shape of the side-walls. We report on 2D gears as a model for high-friction granular particles for which tangential forces cannot be relaxed away by vibration. We use such particles to study the Janssen effect in systems with very high inter-particle friction. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D17.00007: Solitary Wave Propagation through 2D Tree-like structures William Falls, Surajit Sen It is well known that a velocity perturbation can travel through a mass spring chain with quadratic and quartic interactions as a solitary and antisolitary wave pair. In this talk we first consider the propagation of such a velocity perturbation for cases where the system has a 2D ``Y'' shaped structure. Where each of the three pieces that make up the ``Y'' are made of a small mass spring chain. From there we consider the case where multiple ``Y'' shaped structures are used to generate a ``tree'' shaped network. We examine the energy transmission properties on these ``tree'' shaped structures and our findings suggest the following broad observations: (i) for strongly nonlinear interactions, mechanical energy propagation resembles pulse propagation with the energy propagation being dispersive in the linear case, (ii) for strongly nonlinear interactions, the ``tree'' like structure acts as an energy gate showing a directional dependence of the perturbation made to the system while the behavior of the linear case shows no such preference, thereby suggesting that such nonlinear structures can act as switches for mechanical energy. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D17.00008: Breaking Size-Segregation Waves in Granular Avalanches Kasper van der Vaart, C.G. Johnson, P. Gajjar, J.M.N.T. Gray, C. Ancey We experimentally prove the existence of the theoretically predicted breaking size-segregation wave within a binary granular avalanche. This complex structure involves the recirculation of particles through a pattern of shocks and rarefaction waves, and causes large particles to accumulate at the avalanche front and small particles in the tail. Using the non-intrusive imaging technique of refractive-index matching we study particle-size segregation inside the flow---far from the sidewall---on an inclined moving-bed channel. In this configuration the bottom layers of the flow are dragged upslope while upper layers are avalanching downslope due to gravity; effectively, the flow remains stationary in the reference frame of the observer. This allows us to time-average discrete particle positions in the steady-state flow and arrive at a continuous particle concentration. The measured particle concentration and particle trajectories match qualitatively with the theoretical predictions. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D17.00009: Acoustic measurements in sheared granular materials Theodore Brzinski, Karen Daniels Acoustic measurements in static, jammed granular materials have revealed an excess of low-frequency vibrational modes which decreases as the confining pressure is increased. This behavior may be analogous to the excess in low-frequency modes associated with the loss of rigidity in molecular and colloidal glasses. To test this analogy, we measure the acoustic emissions from jammed, quasi-2D granular packings under shear. In contrast to static experiments, shear enables direct comparison of acoustic properties as a packing approaches failure. We use a split-bottom geometry with flexible boundaries held under controlled tension, allowing experiments to be conducted at a set confining pressure. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D17.00010: The role of crystal contacts in protein crystallization: soft matter characterization of two protein families Diana Fusco, Jeffrey Headd, Alfonso De Simone, Jun Wang, Patrick Charbonneau Crystallizing proteins is the bottleneck to systematically determining their structures, which are key to understanding certain biological processes and engineering bio-inspired materials. Identifying the conditions under which proteins crystallize should be equivalent to determining their phase diagram, but one typically resorts to combinatorial rather than physics-based sampling of solution conditions to tackle this difficult problem. Although several soft matter ``patchy particle'' models have been suggested to rationalize the phase behavior of proteins, the interactions that drive crystallization are insufficiently characterized for them to be of much use. We use atomistic simulations of solvated proteins of the rubredoxin family to parameterize patchy models. Their phase diagram is then compared with experimental crystallization conditions. The agreement between model and experiment supports the suitability of patchy models to describe globular proteins crystallization and provides physical guidelines to systematically improve protein crystallization experiments. An analogous analysis of ubiquitin, which crystallizes in multiple crystal forms, further clarifies the role of competing patches in controlling crystal assembly. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D17.00011: Coiling rods onto moving substrates Mohammad Jawed, Fang Da, Eitan Grinspun, Pedro Reis We present results on the nonlinear patterns obtained when a thin elastic rod is deployed onto a moving substrate. Our experiments comprise an injector that deposits an elastomeric rod onto a conveyor belt, where it coils in a variety of nonlinear patterns, depending on the control parameters. The portion of the rod that is suspended between the injector and the point of contact with the belt can exhibit strong geometric nonlinearities that are a challenge for traditional analytical and numerical methods. We tackle this challenge by coupling our precision model experiments with cutting-edge simulation tools ported from the computer graphics community. By systematically exploring parameter space, we map out the basins of stability of the various nonlinear coiling patterns, which are then rationalized using a detailed energy balance. We give particular emphasis to the sinusoidal patterns that emerge from a straight-to-meandering instability that we find to be consistent with a Hopf bifurcation. Closed-form solutions are derived to describe the amplitude and wavelength of the meandering patterns. The excellent agreement between experiments, simulations and theory conveys the predictive ability of our tools to be used, upon scaling, in the original engineering applications that motivated this study: serpentines created from the coiling of carbon nanotubes (at the micron-scale) and the laying down of transoceanic undersea cables (at the kilometer-scale). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D17.00012: Origin of Rigidity in Dry Granular Solids Sumantra Sarkar, Bulbul Chakraborty In traditional solids, the resistance to shear is associated with broken translational symmetry as exhibited by a nonuniform density pattern. In this talk, we show that the emergence of shear rigidity in granular solids is a collective process, which is controlled solely by boundary forces, the constraints of force and torque balance, and the positivity of the contact forces, and not energetic or entropic considerations. We present a theoretical framework that connects rigidity to broken translational symmetry in a reciprocal space representing contact forces. We apply our theory to experimentally generated shear-jammed states and show that these states are indeed characterized by a persistent, non-uniform density modulation in force space, which emerges at the shear-jamming transition\footnote{Sumantra Sarkar et al, Phys. Rev. Lett. 111, 068301}. Crucial to these analyses was an algorithm that was developed to obtain the reciprocal space structures for any real space configuration under mechanical equilibrium. Also, this algorithm help us identify the source of plastic failure which leads to avalanches in these systems. We argue that continuum theories of granular solidification and response should be based on the reciprocal space picture. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D17.00013: Mechanics of Miura-ori Origami Lattice Defects Jesse Silverberg, Lauren McLeod, Arthur Evans, Jessica Ginepro, Christian Santangelo, Thomas Hull, Itai Cohen The mechanical properties of origami-inspired materials show remarkable potential for responsive, tunable next-generation materials. For example, the Miura-ori fold is predicted to have negative Poisson ratio and anisotropic compressive properties. Using a custom mechanical testing device and 3D laser profilometry, we investigate the moduli and the role of curvature in setting these material properties. Because defects are known to dramatically alter the bulk properties in other periodic materials, we introduce defects into the folding pattern to investigate their effects on the macroscopic mechanical properties. Interestingly, we find that a single defect increases the overall material stiffness, but the introduction of a second defect in the opposite direction can cancel out the first, tending to restore the original material properties. Moreover, these defect pairs can be arranged to form edge dislocations, grain boundaries, and many other topological configurations familiar from the study of crystallographic lattice defects. [Preview Abstract] |
Session D18: Cleverly Creating Colloidal Clusters
Sponsoring Units: GSNP DCMPChair: Kazem Edmond, New York University
Room: 403
Monday, March 3, 2014 2:30PM - 2:42PM |
D18.00001: Three-Dimensional Lock and Key Colloids Yu Wang, Yufeng Wang, Xiaolong Zheng, Gi-Ra Yi, Stefano Sacanna, David Pine, Marcus Weck Colloids based upon silica patchy particles featuring a (trimethoxysilyl)propyl methacrylate matrix are synthesized. Selective etching of the silica patches results in the controllable fabrication of colloids with three-dimensional multicavities. The obtained hollow particles exhibit depletion interaction-driven self-assembly with polystyrene spheres in the presence of poly(ethylene oxide), demonstrating for the first time three-dimensional lock and key colloids. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D18.00002: Study of cluster formation in a quasi-square well model of Janus ellipsoids Donovan Ruth, Jeffrey Rickman, James Gunton, Wei Li We investigate the effect of geometry and range of attractive interaction on the self-assembly of Janus particles. In particular, we consider Janus spheroids with an aspect ratio of 0.6 and a quasi-square well model with a short range attractive interaction of 0.2 sigma where sigma is the characteristic length of the spheroid. We find that below a certain transition temperature the system forms orientationally ordered micelles and vesicles, with a cluster distribution qualitatively similar to that found in an earlier study of Janus spheres. (Phys. Chem. Chem. Phys. (2010) vol 12, 11869-11877, F. Sciortino, A. Giacometti and G. Pastore) Finally we discuss the implications of our work for encapsulation by self-assembly. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D18.00003: Shape Memory Colloidal Assemblies of Janus Ellipsoids Benjamin Schultz, Aayush Shah, Wenjia Zhang, Michael Solomon, Sharon Glotzer AC electric and magnetic fields have been widely used to create reconfigurable chains of uniform and patchy colloidal particles that can be used to create switchable, anisotropic electronic and elastic responses in bulk media. Here, we report a joint experimental and computational study of the self and directed assembly of patchy ellipsoidal particles that combine both shape and interaction anisotropy. These particles are synthesized by sequentially combining evaporative deposition of chrome and gold with the uniaxial deformation of polymeric colloidal particles. We explore the self assembly behavior of these particles into clusters and one dimensional chains as a function of salt concentration and aspect ratio. From computational studies, we identify the minimal interactions required to reproduce experimentally observed structures and mechanisms driving chain growth. Upon the application of an AC electric field, we exploit the asymmetric polarizability of these particles to assemble chain structures with new morphologies. We are able to reconfigure between AC field and equilibrium self assembly structures, enabling the actuation of self assembled chains for rapid switching rates and accelerating chain growth for slow switching rates. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D18.00004: Self-assembly with and of patchy colloids: prediction and exploration Erik Edlund, Oskar Lindgren, Martin Nilsson Jacobi Patchy colloids serve as one of the key models for self-assembly of anisotropic building blocks. Interestingly, it is possible to use self-assembly to create the patchy colloids themselves [1]. We present recently developed theory for predicting pattern formation on colloids [2] and suggests a systematization of such self-assembled patchy colloids. This allows us to perform systematic computational study of patchy particles and their self-assembly into complex structures, results from which we present here. Our results highlight the importance of interplay between theory and computational exploration. [1] A. M. Jackson, J. W. Myerson, and F. Stellacci, Nat. Mater. 3(5), 330 (2004) [2] E. Edlund, O. Lindgren., and M. Nilsson Jacobi, (2013) http://arxiv.org/abs/1310.3858 [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D18.00005: A path to designing self-assembling surface patterns on particles for self-assembly of the particles themselves Oskar Lindgren, Erik Edlund, Martin Nilsson Jacobi Patchy colloids are promising candidates for self-assembly of metamaterials since directional attraction and high specificity reduces the ambiguity of the low energy state, this simplifies the design of self-assembling building blocks. However, the large scale fabrication of colloids with specific patterns becomes more difficult as the complexity of the surface pattern increases. Self-organiziation of the surface patterns themselves have been suggested as a promising fabrication method due to the new types of patterns it makes accessible. We present a method for designing self-assembling patterns in multiple components system on particle surfaces. The method is based on an analytical treatment of an effective interaction representation of real systems. As an example, we use a simplified model of Alkalethoils-on-gold to show how a limited amount of system parameters can be tuned in order to cause self-assembly of desired surface patterns. We perform in silico self-assembly of surface patterns on spherical colloids, the patterns then causes the colloids themselves to self-assemble into various geometric target structures like strings, membranes, cubic aggregates and lattices. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D18.00006: Non-equilibrium self-assembly of ``sticky'' colloidal particles under alternating electric field Alexey Snezhko, Arnaud Demortiere, Igor Aranson Ensembles of interacting colloidal particles subject to an external periodic forcing often develop nontrivial self-assembled phases. We study emergent phenomena in partially cross-linked colloidal ensembles of epoxy particles driven out of equilibrium by alternating magnetic fields in a nonpolar solvent. We report on the discovery of self-assembled tunable networks of microscopic polymer fibers ranging from wavy colloidal ``fur'' to highly interconnected networks. The networks emerge via dynamic self-assembly in an alternating (ac) electric field from a non-aqueous suspension of ``sticky'' polymeric colloidal particles with a controlled degree of polymerization. The resulting architectures are tuned by the frequency and amplitude of the electric field and surface properties of the particles. The research was supported by the U.S. DOE, Office of Basic Energy Sciences, Division of Materials Science and Engineering, under the Contract No. DE AC02-06CH11357 [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D18.00007: Structural phases of trapped colloids with competing interactions in two and three dimensions S\'ergio Wlademir da Silva Apolin\'ario, Lucas de Queiroz da Costa Campos, Everton Oliveira Lima, Hartmut L\"owen By employing Brownian dynamics simulation we analyzed the spatial configurations resulting from a self-assembly process of colloidal particles interacting via a competive isotropic pair potential both in two and three dimensions. A wide variety of different spacial configurations is found to be stable which includes, for two dimensions, clusters with a fringed outer rim (reminiscent to an ornamental border), clusters perforated with voids as well as clusters with a crystalline core and a disordered rim, and, for three dimensions, clusters perforated with channels and helical fringes. All cluster structures occur in a two-dimensional parameter space. The structural ordering can therefore be efficiently tuned by changing few parameters only providing access to a controlled fabrication of colloidal clusters. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D18.00008: Equilibrium distribution of symmetric self-assembled structures Henrik van Lengerich, John Spohn, Richard James Many self-assembled structures are symmetric and are composed of identical subunits. The assembled structure depends not only on subunit position, but also on orientation, (ie. virus capsids, Janus particles, and liquid crystals). We find the equilibrium distribution of various symmetric self-assembly systems by enumerating all possible clusters. The degeneracy of the minima depends on the symmetry of the cluster. Theoretical results are compared with numerical simulations of the Langevin equation and macroscopic experiments. The validity of this comparison is proven by deriving the Smoluchowski equation for interacting elements that depend on orientation. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D18.00009: DNA Origami Functionalized Colloids Matan Yah Ben Zion, Corinna Maass, Kunta Wu, Ruojie Sha, Ned Seeman, Paul Chaikin The design of self assembled colloids is limited by their spherical symmetry which gives rise to a spectrum of rotomers instead of a unique structure. We propose functionalizing colloids with DNA Origami which can speci cally bind to one another at a nanometric resolution in a prede ned angle using DNA sticky ends hybridization. As DNA Origami is a mesoscopic entity in its nature, with a typical size of $\sim$100nm, it is a natural candidate for mediating interactions between microscopic particles such as colloids. Using an elongated belt-like design we show for the first time specific binding between colloids by the Origami. It is also possible to control the binding orientation by aligning the sticky ends on the origami in a prochiral pattern creating new opportunities in colloidal self assembly. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D18.00010: An Automated Microfluidic Chemostat for a Self-Replicating System of DNA Constructs Andrew Bergman, Xiaojin He, Corinna Maass, Ruojie Sha, Yongli Mi, Nadrian Seeman, Paul Chaikin We have modified a bacterial microchemostat$^{\mathrm{1}}$ for use in studying and optimizing a self-replicating system based on DNA constructs. The self-replication process we employ requires cycling temperature and UV light exposure. The base units of our system are DNA constructs that can recognize their complements using DNA ``sticky ends'' and can be covalently linked to one another through the use of a UV-crosslinkable nucleobase substitute. ``Seed particles'' of varying length are made by attaching two or more of these constructs and are added to and processed in this microfluidic system, which allows for temperature control, UV illumination and microscopic observation of fluorescence and FRET. Automation provides for a well-regulated and high-throughput testing and optimization of conditions. [1] F. K. Balagadde \textit{et al.}, Science \textbf{309}, 137 (2005). [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D18.00011: Programming colloidal phase transitions with DNA strand displacement William Rogers, Vinothan Manoharan Specific interactions induced by transient bridging of complementary DNA strands grafted to colloidal particles can direct assembly of nanostructured materials. These interactions have been used to `program' the symmetry of novel equilibrium superlattices and could in principle enable self-assembly of prescribed structures. However, the ability to program the \textit{transitions} between these equilibrium phases is currently limited: DNA-mediated attractions between particles decrease monotonically and steeply with increasing temperature, resulting only in high-temperature fluids and low-temperature solids that are inherently difficult to equilibrate. We show that by introducing free DNA strands that compete to bind with the grafted ones by strand displacement, the temperature dependence of interparticle interactions can be programmed through the base sequences of displacing strands. We use this scheme to create colloids with `designer' phase behavior such as re-entrant melting, arbitrarily wide gas-solid coexistence, and reversible transitions between different binary crystals. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D18.00012: Programmable Sequential Assembly in a DNA Functionalized Emulsion System Yin Zhang, Lea-Laetitia Pontani, Martin Haase, Lang Feng, Ruojie Sha, Nadrian Seeman, Jasna Brujic, Paul Chaikin Assembling a complex structure requires not only the appropriate association of specific units, but putting the pieces together sequentially in the right order. We present the sequential self-assembly of a system of micron-sized emulsion droplets functionalized by pre-programmed DNA molecules. Each droplet is initially inert with the DNA protected by a partially complementary strand with a toehold. A Yurke process [1] utilizes the toehold to free the protected strand which can then act in a similar way to bind to the toehold on the next droplet in the sequence and deprotect a strand which continues the reaction to subsequent droplets. Since the DNA attached to a lipid on an emulsion is mobile this design enables the cyclic strand displacement on the nanoscale to produce sequence-specific interactions on the microscale. We demonstrate such programmed assembly in a system of three types of droplets with different cyclically complementary protected strands. \\[4pt] [1] B. Yurke et al., Nature, 406, 605-608(2000) [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D18.00013: Self-replicating devices with dipolar colloids Joshua Dempster, Rui Zhang, Monica Olvera de la Cruz Ubiquitous in nature, self-replication on the nano-scale has been challenging to produce in the laboratory. Recent efforts with DNA tiles have shown great success in correctly replicating tile-sequence templates but require frequent manipulation by the experimenter. We propose a scheme for achieving self-replication with dipolar colloids. Dimers in these systems replicate exponentially over millisecond time scales with no intervention other than periodic energy pulses supplied by external fields. We develop a general formalism governing the rate of self-replication as a function of the interval between pulses. Results from kinetic Monte Carlo simulations show good agreement with the growth rates predicted by simple models of the replication process. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D18.00014: Remembering a Shape --- Assembling a Memory Zorana Zeravcic, Arvind Murugan, Michael Brenner, Stanislas Leibler Recently we have been developing a new connection between self-assembly and neural networks, where a multi-component particle system with specified interaction rules between its components is mapped onto a multi-state Hopfield neural network model. Within this framework, a fixed interaction pattern of neurons representing a ``memory'' maps to particle interactions encoding a certain structure. Properties of neural networks motivate new types of questions: Can the interaction energies of particles code for multiple structures at the same time? Can stored structures be retrieved by throwing in a nucleation seed (i.e., a small assembly of particles) and have it complete into the desired stored structure? Can we define a capacity, i.e., a maximal number of structures that can be retrieved with limited error? We investigate these questions using numerical simulations of different types of building blocks with short-range interactions. [Preview Abstract] |
Session D19: Focus Session: Theory and Simulations of Macromolecules III - Ionic Polymers
Sponsoring Units: DPOLYChair: Amalie Frischknecht, Sandia National Laboratories
Room: 404
Monday, March 3, 2014 2:30PM - 2:42PM |
D19.00001: Conformation of Ionic Conjugated Polymers: Molecular Dynamic Simulations Sidath Wijesinghe, Dvora Perahia, Gary Grest The structure and dynamics of poly para phenylene ethynylene (PPE) with substituted carboxylate side groups have been studied using molecular dynamics simulations. These polymers consist of two highly interacting segments, conjugated groups with are luminescent and ionic groups that add functionality either for tethering bio compatible groups or ionic transport ones. Here we investigate the conformation of these polymers which is a delicate balance between the conjugation length, and electrostatic interactions. Specifically we resolved the structure of carboxylate substituted PPE chains in three different solvents, toluene, water, and vacuum. Toluene acts as a good solvent for the backbones of PPEs, water which is a good solvent for the side groups and vacuum which is a poor solvent for the entire molecule. We found out that conformation of the backbone depends on both the presence of ionic groups and the specific interactions with the solvent. As the number of ionic groups along the backbone increases the conformation of the polymer is strongly impacted by formation of ionic clusters. The study shows that proper tuning the degree of ionic substitution PPEs we can either maintain long extended chains or folded. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D19.00002: Chain Shapes and Ordering of Conjugated Polymers from Atomstic Simulations Wenlin Zhang, Enrique Gomez, Scott Milner Conjugated block copolymers, such as P3HT-b-PFTBT, are candidates for optimizing the efficiency of OPVs due to their self-assembly on different length scales. With microphase separated domains, and sharp interfaces between donor and acceptor blocks, transfer of excitons and free charge carriers is enhanced and charge recombination is reduced. To better understand mesocopic and interfacial packing and ordering of these materials, homopolymers are first investigated via atomistic simulations. We proposed a numerical averaging method and an analytical approach to estimate single chain dimensions of semiflexible polymers based on DFT computed dihedral potentials. Estimations are compared to our MD simulation results of polymer melts. Shorter persistence lengths from MD simulations indicate side groups and inter-chain interactions bring flexibility to polymer backbones. By assuming molten polymers as persistent chains of rods, nematic phase transitions and orderings of these materials are also discussed. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D19.00003: Atomistic molecular dynamics simulations of the structure of symmetric Polyelectrolyte block copolymer micelle in salt-free aqueous solution Rajalakshmi Chockalingam, Upendra Natarajan The structure of a symmetric polystyrene-$b-$poly(acrylic acid) (PS-$b-$PAA) micelle in salt-free aqueous solution as a function of degree-of-neutralization (or ionization, $f$) of the PAA is studied via explicit-atom-ion MD simulations, for the first time for a polyelectrolyte block copolymer in a polar solvent. Micelle size increases with $f $in agreement with experimental observations in literature, due to extension of PAA at higher ionization. Pair RDF's with respect to water oxygens show that corona-water interaction becomes stronger with $f$ due to an increase in number density of carboxylate (COO$^{-}$) groups on the chain. Water-PAA coordination (carboxylate O's) increases with ionization. H-bonding between PAA and water increases with $f$ due to greater extent of corona-water affinity. With increase in $f$, atom and counter-ion $\rho $ profiles confirm extension of corona blocks and micelle existing in the ``osmotic regime,'' and a decrease in scattering peak intensity, in agreement with neutron scattering experiments and mean-field theory in literature. Inter-chain distance in PS core is found to decrease with ionization. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D19.00004: Coarse-Grained Modeling of Polyelectrolyte Solutions Alan R. Denton, Sylvio May Ionic mixtures, such as electrolyte and polyelectrolyte solutions, have attracted much attention recently for their rich and challenging combination of electrostatic and non-electrostatic interparticle forces and their practical importance, from battery technologies to biological systems. Hydration of ions in aqueous solutions is known to entail ion-specific effects, including variable solubility of organic molecules, as manifested in the classic Hofmeister series for salting-in and salting-out of proteins. The physical mechanism by which the solvent (water) mediates effective interactions between ions, however, is still poorly understood. Starting from a microscopic model of a polyelectrolyte solution, we apply a perturbation theory to derive a coarse-grained model of ions interacting through both long-range electrostatic and short-range solvent-induced pair potentials. Taking these effective interactions as input to molecular dynamics simulations, we calculate structural and thermodynamic properties of aqueous ionic solutions. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D19.00005: Polyelectrolyte complexes and salt: a computational study Hanne Antila, Paul Van Tassel, Maria Sammalkorpi Charged polymers, polyelectrolytes (PEs), are versatile materials with applications ranging from tissue engineering to sensing elements. In aqueous solutions, oppositely charged PEs form complexes which are known to be sensitive to added salt with responses including shrinking, flocculation or swelling, and at higher concentrations loosening and destabilization of the complex. However, the role of electrostatics, charge correlations, hydration, and ion specific interactions remain unclear. In this work, we use all-atom molecular dynamics with explicit water and ions to probe the effect of excess salt to DNA-polylysine complex formation and stability, and demonstrate the mechanism of PE and ion species specific salt-driven dissociation [1]. The dissociation occurs accompanied by charge reversal in which charge correlations and ion binding chemistry play a role. Our results agree with experimental work on complex dissociation but in addition show the underlying microstructural correlations driving the behavior. We expand the full atomic level detail and dynamics results with theoretical and computational work describing the PE complex as oppositely charged rods to provide a more complete understanding of PE interactions in salt. [1] H. Antila and M. Sammalkorpi, submitted (2013) [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D19.00006: Theoretical Study of Polyelectrolyte/homopolymer blends Youhai Sun, Ashkan Dehghan, An-Chang Shi The phase behaviour of polyelectrolyte/homopolymer blends is studied using self-consistent field theory (SCFT). The blends are composed of charged and neutral polymers plus counter ions dissociated from the polyelectrolytes. The phase diagram of the system is constructed as a function of blend composition, charge density and polymer-polymer interactions. Besides the usual macrophage separation behaviour, the SCFT predicts that under appropriate conditions the system undergoes microphase separation, forming various ordered morphologies similar to block copolymers. The formation of ordered phases in the system is due to the competition between the polymer-polymer interaction, electrostatics and the mixing entropy of the counter ions. In particular, the length-scale of the ordered phases is not limited by the polymer size, thus opening the door for the engineering of microphases with large domain spacings. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D19.00007: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 3:54PM - 4:06PM |
D19.00008: The Swelling of Olympic Gels Michael Lang, Jakob Fischer, Marco Werner, Jens-Uwe Sommer The swelling equilibrium of Olympic gels is studied by Monte Carlo Simulations. We observe that gels consisting of flexible cyclic molecules of a higher degree of polymerization $N$ show a smaller equilibrium swelling degree $Q\propto N^{-0.28}\phi_{0}^{-0.72}$ for the same monomer volume fraction $\phi_{0}$ at network preparation. This observation is explained by a disinterpenetration process of overlapping non-concatenated polymers upon swelling. In the limit of a sufficiently large number of concatenations per cyclic molecule we expect that the equilibrium degree of swelling becomes proportional to $\phi_{0}^{-1/2}$ independent of $N$. Our results challenge current textbook models for the equilibrium degree of swelling of entangled polymer networks. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D19.00009: Highly-correlated Charges in Block Copolyelectrolytes: Charge as a Tool for Morphology Manipulation Charles Sing, Jos Zwanikken, Monica Olvera de la Cruz Block copolymers that include at least one charged block have been of great technological interest due to their use in materials for battery membranes. These materials are difficult to understand theoretically, however, due to the disparate length scale effects of charge correlation and chain conformation driving the microphase separation of these systems. Using a new theoretical approach that can account for both of these effects that is based of hybrid liquid state integral equation-self consistent field theory (LS-SCFT) calculations, we elucidate the fundamental physics underpinning the thermodynamics of these materials. In particular, we demonstrate four main effects that drive the phase behavior of block copolyelectrolytes: Coulombic cohesion, counterion entropy, excluded volume, and ion self energy effects. Tuning parameters such as charge fraction and dielectric constant can be used to explore different microphase-separated morphologies on an axis orthogonal to traditional routes of manipulating block copolymers (i.e. $\chi$ N and block fraction). This expands the palette of tools that can be used to tune this important class of polymeric materials. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D19.00010: Investigation of the structure of levan polysaccharide chains in water via molecular dynamics simulations Deniz Turgut, Binnaz Coskunkan, Gulcin Cem, Deniz Rende, K. Yalcin Arga, Seyda Bucak, Nihat Baysal, Ebru Toksoy-Oner, Rahmi Ozisik Levan is a biopolymer consisting of $\beta $-D-fructofuranose units with $\beta $ (2-6) linkages between fructose rings. Investigation of the structure and behavior of levan in aqeous environments is necessary to understand its biological activity and its potential use in various applications such as carbohydrate-derived drug release. The use of different \textit{in vivo} and \textit{in vitro} bioactivity assays fail to relate the chemical structure and conformation to the observed biological activity. Therefore, considerable research has been directed on elucidating the biological activity mechanisms of polysaccharides by structure-function analysis. To overcome the inherent difficulties of experiments, molecular dynamics (MD) simulations have been used to retrieve comprehensive information regarding the conformations of polysaccharides and their dynamic properties. In the current study, the structure of levan is investigated in aqueous medium and in saline solutions via fully atomistic MD simulations at 298 and 310 K, representing room temperature and physiological temperatures, respectively. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D19.00011: Modeling the Transport Properties of CO$_{2}$/Polyamine Reactive Mixtures at Multiple Length-Scales Salomon Turgman-Cohen, Fernando Escobedo Polyfunctional amine oligomers have been utilized as phase changing sorbents for carbon dioxide capture applications. Knowledge of the dynamic properties of these mixtures is essential to the design of efficient separation processes. The reactive character of polyamine/CO$_{2}$ blends and the severe variation in their transport properties as a function of CO$_{2}$ concentration renders these mixtures challenging to probe experimentally. We implement a multiscale modeling strategy in which polyamine/CO$_{2}$ mixtures are approximated by an ionic speciation model. Molecular dynamics (MD) simulations of such models are used to probe the diffusion coefficient and viscosities at various concentrations of ionic species and absorbed CO$_{2}$. The results of MD simulations are applied to a simple mass transfer model to predict optimal thicknesses of amine films and the kinetics of the absorption process. The latter is compared to experimental thermogravimetric results. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D19.00012: Simulation of a Small Molecule Analogue of a Lithium Ionomer in an External Electric Field John McCoy, Sara Waters, Amalie Frischknecht, Jonathan Brown Ion dynamics were studied in lithium-neutralized 2-pentylheptanoic acid, a small molecule analogue of a precise poly(ethylene-co-acrylic acid) lithium ionomer. Atomistic molecular dynamics simulations were performed in an external electric field. The electric field causes alignment of the ionic aggregates along the field direction. The energetic response of the system to an imposed oscillating electric field for a wide range of frequencies was tracked by monitoring the coulombic contribution to the energy. The susceptibility found in this manner is a component of the dielectric susceptibility typically measured experimentally. A dynamic transition is found and the frequency associated with this transition varies with temperature in an Arrhenius manner. The transition is observed to be associated with rearrangements of the ionic aggregates. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D19.00013: A simulation study of poly(ethylene glycol) in ionic liquids using a physically motivated ab initio force-field Eunsong Choi, Jesse G. McDaniel, J.R. Schmidt, Arun Yethiraj The behavior of poly(ethylene glycol) (PEG) in imidazolium-based ionic liquids (ILs) is studied from molecular dynamics simulations using a new physically motivated force-field. The new force-field accounts for various fundamental intermolecular interactions such as electrostatics, induction, exchange, and dispersion in separate terms where the parameters are derived from ab initio, symmetry adapted perturbation theory (SAPT). The crucial point about the new force-field when compared to other existing force-fields is that it is developed free from empirical parameterization; this is a great advantage particularly for the systems like polymer/IL solutions where experimental data are scarce. We first validate the force-field for neat ILs and neat PEG. Then the force-field is applied to the mixture of the two and the final results are compared with available experiments and simulation results using the OPLS-AA force-field. [Preview Abstract] |
Session D20: Focus Session: Microfluidics and Nanofluidics III - Pattern Formation and Droplets
Sponsoring Units: DPOLY DFD GSNPChair: John Royer, NIST
Room: 405
Monday, March 3, 2014 2:30PM - 3:06PM |
D20.00001: Whipping of electrified jets Invited Speaker: Alberto Fernandez-Nieves Whipping is a non-axisymmetric instability characteristic of electrified jets. It is exploited in electrospinning to reduce the average diameter of the fibers that result in this process. In air, it usually manifests in a chaotic fashion and thus, its structure and properties have been hard to quantify experimentally. We show that by applying electric fields to coflowing liquids, we can generate steady-state whipping structures. This allows for a detailed characterization of this non-axisymmetric instability. In addition, we will also discuss other emission regimes not typically seen in electrospray. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D20.00002: Ferrofluid-based reconfigurable optofluidic switch Gianna Valentino, Eric Mongeau, Yu Gu We present a low-cost, reconfigurable optofluidic switch exploiting both the optical and magnetic properties of a water-based ferrofluid. This switch is composed of an integrated waveguide orthogonally crossing a microfluidic channel containing high-index oil and a ferrofluid plug. The switch is turned ``ON'' or ``OFF'' by the movement of the ferrofluid plug in response to an external magnetic field. Each switch exhibits a high contrast ratio and millisecond response time. Parallel geometries for both mode and multi-mode waveguides are shown. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D20.00003: Macroscale Boltzmann statistics: Recreating statistical mechanics with a mechanically derived temperature Kyle Welch, Eric Corwin We use chaotic Faraday waves to create a macroscopic 2D pseudothermal environment in which we study surface tension mediated interactions between buoyant particles. The chaotic surface waves create an effective temperature that is proportional to the driving amplitude. We use Boltzmann statistics to measure interparticle potentials by tracking the distribution of particle separations. This allows us to study interparticle interactions without interfering with the dynamics of the system. We explore various systems of multiple interacting particles, focusing particularly on systems of particles linked together in a chain with stiff links, in analogy to polymers. We explore the response of these systems to changes in parameters such as effective temperature, particle size, shape and wetting properties, as well as linker spacing and total chain length. We report on the effective entropic spring-like behavior of such systems in the limit of large chain length. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D20.00004: Pattern formation of Dictystelium discoideum in the presence of laminar flow and cAMP pulses Azam Gholami, Oliver Steinbock, Vladimir Zykov, Eberhard Bodenschatz Dictyostelium discoideum (D.d) amobae undergo starvation-induced multicellular development in which single cells aggregate chemotactically towards cAMP signals emitted periodically from an aggregation center. We are investigating spatiotemporal pattern formation of D.d. cells under the presence of a laminar flow. Starved cells are loaded into a straight millifluidic device with an external flow and cell response to the signaling molecule cAMP is monitored indirectly using dark-field microscopy. The observed contraction waves develop simultaneously over the entire channel, are propagating only in flow direction, and have curved wave fronts resembling the parabolic flow profile. The wave dynamics analysis shows that the wave velocity is locked to the flow velocity and yields a wave period of T0~6 min, which matches the typical oscillation period of extracellular cAMP in spatial homogeneous, well-stirred systems. We apply a small cAMP perturbation at the inlet region of the channel and observe the spatiotemporal response of the cells as the pulse is propagating down the channel. The results show that D.d. cells are in the oscillatory regime and the system can be forced within resonance tongue. We compared our results with analytical and numerical analysis of Goldbeter model. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D20.00005: Microfluidic Droplet Dehydration for Concentrating Processes in Biomolecules Invited Speaker: Shelley Anna Droplets in microfluidic devices have proven useful as picoliter reactors for biochemical processing operations such as polymerase chain reaction, protein crystallization, and the study of enzyme kinetics. Although droplets are typically considered to be self-contained, constant volume reactors, there can be significant transport between the dispersed and continuous phases depending on solubility and other factors. In the present talk, we show that water droplets trapped within a microfluidic device for tens of hours slowly dehydrate, concentrating the contents encapsulated within. We use this slow dehydration along with control of the initial droplet composition to influence gellation, crystallization, and phase separation processes. By examining these concentrating processes in many trapped drops at once we gain insight into the stochastic nature of the events. In one example, we show that dehydration rate impacts the probability of forming a specific crystal habit in a crystallizing amino acid. In another example, we phase separate a common aqueous two-phase system within droplets and use the ensuing two phases to separate DNA from an initial mixture. We further influence wetting conditions between the two aqueous polymer phases and the continuous oil, promoting complete de-wetting and physical separation of the polymer phases. Thus, controlled dehydration of droplets allows for concentration, separation, and purification of important biomolecules on a chip. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D20.00006: Chiral Double Emulsions: Breaking Symmetry with Microfluidics Laura Adams, Thomas Kodger, Jiawei Yang, David Weitz We will present a new microfluidic encapsulation technique for generating chiral double emulsions, drops inside of drops, with a specific handedness. In presenting the data, we will discuss the effect of the number, size and composition of encapsulated drops on the double emulsion structure before and after pinching off from an injection capillary in a microfluidic device. These results support theoretical models in which the pinch-off mechanism is in direct analogy to boundary layer considerations. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D20.00007: Droplet based microfluidics for highthroughput screening of antibody secreting cells Liheng Cai, John Heyman, Linas Mazutis, Lloyd Ung, Rodrigo Guerra, Donald Aubrecht, David Weitz We present a droplet based microfluidic platform that allows highthroughput screening of antibody secreting cells. We coencapsulate single cells, fluorescent probes, and detection beads into emulsion droplets with diameter of 40 micron. The beads capture antibodies secreted by cells, resulting in a pronounced fluorescent signal that activates dielectrophoresis sorting at rate about 500 droplets per second. Moreover, we demonstrate that Reverse Transcription Polymerase Chain Reaction (RT-PCR) can be successfully applied to the cell encapsulated in a single sorted droplet. Our work highlights the potential of droplet based microfluidics as a platform to generate recombinant antibodies. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D20.00008: Mechanical response of tumor cells flowing through a microfluidic capillary Zeina S. Khan, Nabiollah Kamyabi, Fazle Hussain, Siva A. Vanapalli Circulating tumor cells, the primary cause of cancer metastasis, are transported throughout the body to distant organs by blood flow. Despite the importance of cell transport and deformability in the vasculature for cancer metastasis, quantitative understanding of the hydrodynamic interactions between the cells and the blood vessel walls is lacking. Using a model microfluidic capillary of rectangular cross-section with an on-chip manometer coupled with high speed video imaging, we quantitatively investigate the hydrodynamic behavior via the cell excess pressure drop. By characterizing our device with simple model systems including viscous drops and soft elastic particles, we find that the excess pressure drop shows no apparent dependence on elastic modulus or interfacial tension, but depends significantly on internal viscosity for moderate confinements and shear stresses within the physiological range of 1-10 Pa. This suggests that the metastatic potential of circulating cells can be characterized by the effective viscosity. We test this hypothesis with several tumor cell lines and find that the effective cell viscosity determined from excess pressure drop measurements can be used to differentiate highly from lowly invasive cells. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D20.00009: A pressure actuated microfluidic system with real time feedback control of droplet length in a T junction microfluidic channel Wen Zeng A pressure actuated microfluidic system using microvalves is designed for droplet generation in a T junction microfluidic channel. Here, we mainly focus on the influence flow rates of continuous and dispersed phase has on the droplet formation. By using pressure actuation for droplet generation in a T junction microfluidic channel, uniform production of monodisperse droplet is achieved. By using the curve fitting, linearized equation which describes the linear relationship of droplet length and flow rate ratio is obtained. With real time feedback, a closed-loop control system of droplet generation is constructed. The mathematical model between pressure and flow rates of continuous and dispersed phase is built, and the control accuracy of droplet length is analyzed. Compared with syringe pump actuation, the droplet formation can be much steadier and the length of individual droplet can be controlled more precisely by pressure actuation, especially at lower flow rates. Based on pressure actuation, monodisperse droplet formation with volume range from microliter to nanoliter scale can be achieved. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D20.00010: Effect of surfactant on bubble/liquid transport in a T-junction microchannel with sudden contraction Kuo-Long Pan, Huai-Jhu Chen We studied the effect of surfactant on the transport phenomena of bubbles in microfluidic devices. A T-junction microchannel with a sudden contraction section was used in the experiment, and the channel dimensions were 200 by 100 $\mu $m. Variations in the transport velocities of different bubbles were observed when they passed the sudden contraction area. Different liquids were adopted as the continuous phase. The work is composed of three parts First, the commercial software, Fluent, was used to analyze the effect of surface tension on the transport phenomena in the microchannel. Second, the roles of surface tension and viscosity were investigated by changing the concentration of ethanol solution. Third, the effects of surfactant type were studied by adding S111n (anionic) and S131 (amphoteric) respectively in water. Experimental results showed that when the concentration of surfactant exceeded the critical micelle concentration (CMC) limit while at the same surface tension, the bubbles would exhibit distinct patterns. Specifically, the wetting behaviors of the bubbles were different using the two dissimilar types of surfactant solutions, for which the wettability of S131 was higher than that of S111n. As a consequence, the transport velocities of the bubbles in S131 solutions were faster than that in S111n solutions [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D20.00011: Inversion of the electric field driven by ionic solvation energy Guillermo Guerrero Garcia, Francisco Solis, Monica Olvera de la Cruz In previous studies, Monte Carlo simulations have suggested the possibility of inverting the electric field near a liquid/liquid interface due to excluded volume effects, ionic size asymmetry, and image charges at high electrolyte concentrations in the absence of ion transfer. In this work, we develop a mean field theory and coarse grained simulations to study the ion transfer between the two immiscible electrolytes in the presence of an electric field. Our calculations suggest a novel mechanism to invert the electric field near confined oil/water interfaces based on differences of the ionic solvation energy in both liquid media. [Preview Abstract] |
Session D21: Polymeric Elastomers and Gels
Sponsoring Units: DPOLYChair: Bradley Olsen, Massachusetts Institute of Technology
Room: 406
Monday, March 3, 2014 2:30PM - 2:42PM |
D21.00001: Double network physical gels from elastin-like polypeptide block copolymers: nanoscale control of thermoresponsive reinforcement Matthew Glassman, Bradley Olsen Triblock copolymers with associative protein midblocks and thermoresponsive endblocks form shear thinning hydrogels with a low yield stress at low temperatures, but can be reinforced by a self-assembled network of the endblock aggregates. Here, we compare the use of bioengineered elastin-like polypeptides (ELPs) to synthetic poly(N-isopropylacrylamide) (PNIPAM) as endblocks to control the self-assembly of the reinforcing network. The temperature dependence of the mechanics of these hydrogels is a strong function of the domain size and morphology in the endblock network. Despite the architectural similarities, triblock ELP fusions and PNIPAM bioconjugates exhibit distinct reinforcement maxima at fixed block composition and polymer concentration, and these differences can be attributed to the nanostructural features of the two systems. Furthermore, in ELP fusions, the amino acid sequence can be readily modified to manipulate the solvation kinetics of the endblock domains. Finally, various endblocks have been combined to form triblock terpolymer hydrogels, demonstrating how the choice of thermoresponsive blocks can be used to tune the reinforcement of shear thinning hydrogels. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D21.00002: Mechanical Characterization of Photo-crosslinked, Thermoresponsive Hydrogel Thin Films via AFM Nanoindentation Thao Le, Katherine Aidala, Ryan Hayward Thin hydrogel films with patterned swelling are known to buckle into programmed three-dimensional shapes, offering approaches to fabricate reversibly self-folding micro-devices for actuators and drug delivery devices. To precisely control the shapes adopted, it is important to quantitatively understand the relationship between swelling and mechanical properties. Furthermore, to understand the buckling pathways and the mechanical responses of the swelled materials, it is also important to identify how the gels undergo stress relaxation. However, the low moduli, high water contents, and micrometer-scale thicknesses of these materials have so far made mechanical characterization difficult. In this study, we use an AFM nanoindentation technique to characterize the mechanical properties of photo-crosslinked, thermoresponsive poly(N-isopropylacrylamide) hydrogel thin films. Simultaneously, we conduct stress relaxation experiments at microscopic indentation lengths to differentiate between the effects of viscoelastic and poroelastic response mechanisms. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D21.00003: Mechanical properties of Tetra-PEG gels with supercoiled networks Takuya Katashima, Kenji Urayama, Ung-il Chung, Takamasa Sakai We investigated the effects of swelling and deswelling on the mechanical properties of polymer gels with variable polymer volume fractions of interest ($\varphi_{\mathrm{m}})$. We employed the Tetra-PEG gel as a model system. Tetra-PEG gels were prepared by the AB type crosslink-coupling between the two symmetrical tetra-arm prepolymers with precisely tuning the network strand length ($N_{\mathrm{c}})$ and polymer fractions at preparation ($\varphi_{\mathrm{0}})$. The drastic increase in the elastic modulus was observed in the high $\varphi_{\mathrm{m}}$ region due to the unusually contracted conformation of the network strands, called supercoiling. The Obukhov model can describe the $\varphi_{\mathrm{m}}$-dependence of the elastic modulus in all $\varphi_{\mathrm{m}}$ regions. We analyzed the stress-elongation relationships for the swollen and deswollen networks. We estimated the fractal dimensions based on the Pincus blob concept, and for the first time observed the $\varphi_{\mathrm{m}}$-, $N_{\mathrm{c}}$-, $\varphi_{\mathrm{0}}$-dependence of the fractal dimension. We found that the gyration radius exhibits the affine deformation in the supercoiling region. These findings will help to understand the structure and formation mechanism of supercoiling. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D21.00004: Rheology of Hyperbranched Poly(triglyceride)-Based Thermoplastic Elastomers via RAFT polymerization Mengguo Yan, Eric Cochran In this contribution we discuss how melt- and solid-state properties are influenced by the degree of branching and molecular weight in a family of hyperbranched thermoplastics derived from soybean oil. Acrylated epoxidized triglycerides from soybean oils have been polymerized to hyperbranched thermoplastic elastomers using reversible addition-fragmentation chain transfer (RAFT) polymerization. With the proper choice of chain transfer agent, both homopolymer and block copolymer can be synthesized. By changing the number of acrylic groups per triglycerides, the chain architectures can range from nearly linear to highly branched. We show how the fundamental viscoelastic properties (e.g. entanglement molecular weight, plateau modulus, etc.) are influenced by chain architecture and molecular weight. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D21.00005: Stability analyses of the model for photosensitive self-oscillating polymer gels Pratyush Dayal, Olga Kuksenok, Anna C. Balazs Via theory and simulations, we investigate the behavior of polymer gels undergoing Belousov-Zhabotinsky (BZ) reaction. Driven by periodic reduction and oxidation of the ruthenium catalyst, which is grafted to the polymer network, BZ gels undergo rhythmic mechanical oscillations and thereby exhibit chemo-mechanical transduction. The oscillations within the BZ gels, however, can be completely suppressed with light of a certain intensity and wavelength. We simulate the behavior of photosensitive BZ gels by our 3D gel lattice spring model. Using this model we have successfully demonstrated that it is possible to direct the movement of BZ gels, along complex paths, guiding them to bend, reorient and turn. The mechanism of chemo-mechanical transduction, however, works for a particular set of conditions. Through linear stability and normal form analyses, we isolate parameters for which the gel switches from oscillatory mode to stationary mode and vice versa. Specifically, we characterize the nature of Hopf bifurcations and identify regimes where this bifurcation is subcritical or supercritical. We also determine several other types of bifurcations within our system. These analyses allow us to establish optimal conditions required to guide the movement of BZ gels along complex paths. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D21.00006: Connecting structural rigidity and dynamical heterogeneity to the rheology of colloidal gels Lilian Hsiao, Heekyoung Kang, Richmond Newman, Sharon Glotzer, Kyung Ahn, Michael Solomon Colloidal gels are known to exhibit complex structural and dynamical changes when sheared, particularly when the applied flow is strong enough to cause rupture. Such systems of colloids interacting through short range attractive forces are good models of associating species, such as associating polymers. Here, we investigate the effect of structural rigidity and dynamical heterogeneity on the nonlinear elasticity of colloidal gels that have undergone yielding. These gels are comprised of fluorescent, sterically stabilized poly(methyl methacrylate) colloids that are suspended at intermediate volume fractions. Non-adsorbing polystyrene is added to induce gelation with weak, short-ranged attraction. Our work shows that the nonlinear elasticity in sheared gels can be attributed to the stress-bearing capability imparted by rigid, slow-diffusing clusters that persist after the flow ceases (L.C. Hsiao et al. (2012). Proc. Natl. Acad. Sci USA 109, 16029-16034). In addition, we observe a decrease in the subdiffusive motion of the particles as the applied strain increases. This deformation introduces a bimodal distribution in the van Hove self-correlation function, suggesting the existence of a fast and slow subpopulation of colloids within sheared gels. We show that the predictive power of microscopic theories that connect elasticity to localization length can be improved by considering only this slow subpopulation. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D21.00007: Soft and Ultra-soft Elastomers William Daniel, Joanna Burdynska, Sam Kirby, Yang Zhou, Krzysztof Matyjaszewski, Michael Rubinstein, Sergei Sheiko Polymeric networks are attractive engineering materials utilized for various mechanically demanding applications. As such, much attention has been paid to reinforcement of polymer mechanical properties with little interest in how to make softer elastomers to address numerous biomedical applications including implants and cell differentiation. Without swelling in a solvent, it is challenging to obtain materials with a modulus below ca.105 Pa, which is dictated by chain entanglements. Here we present two methodologies for the creation of soft and ultra-soft dry elastomeric compounds. The first method utilizes polymer capsules as temperature responsive filler. Depending on volume fraction of microcapsules this method is capable of fine tuning modulus within an order of magnitude. The second technique uses the densely grafted molecular brush architecture to create solvent-free polymer melts and elastomers with plateau moduli in the range one hundred to ten hundred Pa. Such compounds may find uses in biomedical applications including reconstructive surgery and cell differentiation. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D21.00008: Deformation of Unentangled Swollen Gels Ozan Sariyer, Sergey Panyukov, Michael Rubinstein We study the deformation characteristics (Poisson's ratios and stress-strain relations) of unentangled gels swollen and uniaxially or biaxially deformed in excess solvent by considering the balance of osmotic pressure and elastic stress in unconstrained dimensions. Our scaling theory predicts a crossover from theta solvent behavior to marginal solvent behavior upon stretching gels that are in concentrated regime at swelling equilibrium -- a phenomenon that was experimentally observed long ago, but not understood theoretically. For gels that are in the semidilute good solvent regime at swelling equilibrium, we predict a crossover to theta solvent behavior upon compression and a crossover to marginal solvent behavior upon stretching. Our theory reproduces the previously known results for equilibrium swelling degree as well as known deformation characteristics in theta and athermal solvents. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D21.00009: Hydrogen Bonding in Poly(butyl acrylate) Melts and Elastomers Mitchell Anthamatten, Christopher Lewis Hydrogen bond strength and density are critical design variables that influence the formation of supramolecular networks from linear polymers. Larger, higher strength H-bonding groups, tend to be more susceptible to aggregation, phase segregation and stacking. However, even weak, monovalent and bivalent hydrogen bonding groups can increase melt viscosity, compatiblize binary blends, and introduce hierarchal structure into amorphous melts. We prepared a series of poly(butyl acrylate) copolymers with different amounts and types of hydrogen bonding side-groups (HBG's). Copolymers containing ``weak'' HBG's behaved as unentangled melts, with no indication of network formation. Copolymers bearing strong hydrogen bonding groups (UPy) behaved as soft, elastic solids. The rheologically distinct behavior of UPy-containing copolymers is attributed to dimer lifetimes exceeding the experimental timescale. Results are relevant to developing adaptable materials including shape memory polymers, self-healing materials, damping materials and adhesives that utilize hydrogen bonding to influence bulk mechanical properties. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D21.00010: Stress--Strain Relationship of Highly Stretchable Dual Cross-Link Gels: Separability of Strain and Time Effect Costantino Creton, Koichi Mayumi, Alba Marcellan, Guylaine Ducouret, Tetsuharu Narita We studied the stress--strain relation of model dual cross-link gels having permanent cross-links and transient cross-links over an unusually wide range of extension ratios $\lambda $ and strain rates $d\varepsilon $\textit{/dt} (or time t $=$ ($\lambda $ -- 1)/$ (d\varepsilon $\textit{/dt)}). We propose a new analysis method and separate the stress into strain- and time-dependent terms. The strain-dependent term is derived from rubber elasticity, while the time-dependent term is due to the failure of transient cross-links and can be represented as a time-dependent shear modulus which shows the same relaxation as in small strain. The separability is applicable except for the strain stiffening regimes resulting from the finite extensibility of polymer chains. This new analysis method should have a wide applicability not only for hydrogels but also for other highly viscoelastic soft solids such as soft adhesives or living tissues. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D21.00011: Fresh Insights on the mechanical response of methylcellulose hydrogels Joseph Lott, John McAlliser, Frank Bates, Timothy Lodge The thermoreversible gelation of aqueous solutions of methylcellulose (MC) at elevated temperatures is well established. However, it has only recently been determined conclusively that the structure of such gels is fibrillar in nature and the rheological properties observed are a result these structures. Cryo-transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) provide detailed insight into the fibrillar network's size scales, growth with temperature, and composition. In light of this new understanding, we explore the possibility of reevaluating the rheological behavior of MC gels under the paradigm of the mechanics of filament networks. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D21.00012: Effect of polydispersity on the phase behavior of soft microgel suspensions Andea Scotti, Urs Gasser, Emily Herman, Akiti Singh, L. Andrew Lyon, Alberto Fernandez-Nieves Microgel suspensions with a majority of small particles and a small fraction of big particles with about double diameter can form crystals without defects caused by the large particles (A. St. John Iyer and L.A. Lyon, Angew. Chem. Int. Ed. 48, 4562-4566, 2009). However, no hard sphere crystals form at size-polydispersities higher than 12\%. We study the role of size-polydispersity in suspensions of fully swollen poly(N-isopropylacrylamide) (pNIPAM) microgel particles with controlled polydispersity ranging from 10\% up to 25\%. Crystals appear in samples with polydispersity as high as 17\%. Using small-angle neutron scattering and contrast matching with samples composed of small deuterated particles and large protonated particles, we directly measure the form factor and shrinkage of the large particles in concentrated samples. The large particles are found to shrink to about the size of the small particles when the effective volume fraction of the suspension approaches 1. These results suggest a different role of size-polydispersity in soft sphere systems. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D21.00013: Shapes of drying hydrogel cylinders Etienne Reyssat, Vincent Etienne Most materials change shape upon drying or wetting. Inhomogeneity of the drying and wetting processes lead to the development of internal stresses. As a result, a solid sample undergoes complex deformations or fracture. We present experimental work on the evolving shapes of drying hydrogel cylinders. We show that depending on the initial aspect ratio of the cylinder, a gel sample undergoes a variety of shape changes. We show qualitative analogies with the drying of wood and with instabilities of cylindrical shells or plane membranes. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D21.00014: Advantages of Using Soft Materials in Scanning Probe Lithography Keith A. Brown, Daniel J. Eichelsdoerfer, Mary X. Wang, Chad A. Mirkin Scanning probes based upon soft materials provide new capabilities and insights into the science of scanning probe lithography. Specifically, we have explored a cantilever-free architecture that consists of an array of sharp probes on an elastomeric film on a glass slide. This architecture allows every probe in an array to be in simultaneous, gentle contact with a surface, allowing one to perform lithography with millions of probes in parallel. Here, we describe three recent developments in cantilever-free scanning probe lithography that were enabled by the elastomeric material. 1) As the mechanical properties of elastomers can be readily tuned, it is possible to tailor the spring constant of the probes.$^{\mathrm{1}}$ 2) The high coefficient of thermal expansion of elastomers means that local heating can be used to physically actuate individual probes allowing for arbitrary patterning.$^{\mathrm{2\thinspace }}$3) Solvents retained in the elastomer can mediate molecular printing and allow a user to pattern hydrophilic and hydrophobic materials in totally dry environments. $^{\mathrm{1}}$D. J. Eichelsdoerfer, \textit{et al}., Nano Lett. \textbf{13}, 664 (2013). $^{\mathrm{2}}$K. A. Brown, \textit{et al}., Proc. Natl. Acad. Sci. USA \textbf{110}, 12921 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D21.00015: Dilute and Semidilute Solutions of a Nonionic, Rigid, Water-soluble Polymer Paul Russo, Wayne Huberty, Donghui Zhang The solution physics of random polymer chains was established largely on the behavior of commercial polymers such as polystyrene for organic solvents or nonionic poly(ethyleneoxide) for aqueous solvents. Not only are these materials widely available for industrial use, they can be synthesized to be essentially monodisperse. When it comes to stiff polymers, good choices are few and less prone to be used in industrial applications. Much was learned from polypeptides such as poly(benzylglutamate) or poly(stearylglutamate) in polar organic solvents and nonpolar organic solvents, respectively, but aqueous systems generally require charge. Poly(N$_{\mathrm{\varepsilon }}$-2-[2-(2-Methoxyethoxy) ethoxy]acetyl-L-Lysine) a.k.a. PEGL was pioneered by Deming and coworkers. In principle, PEGL provides a convenient platform from which to study stiff polymer behavior---phase relations, dynamics, liquid crystal formation and gelation---all with good molecular weight control and uniformity and without electrical charge. Still, a large gap in knowledge exists between PEGL and traditional rodlike polymer systems. To narrow this gap, dynamic and static scattering, circular dichroism, and viscosity measurements have been made in dilute and semidilute solutions as necessary preliminaries for lyotropic liquid crystalline and gel phases. Supported by NSF DMR 1306262. [Preview Abstract] |
Session D22: Films at Liquid and Solid Interfaces
Sponsoring Units: DPOLYChair: Thomas Salez, ESPCI
Room: 407
Monday, March 3, 2014 2:30PM - 2:42PM |
D22.00001: Assembly of Graphene Oxide at Water/Oil Interfaces: Tessellated Nanotiles Thomas Russell, Zhiwei Sun, Tao Feng Graphene oxide (GO) was found to segregate at water/toluene interface when attractive polymer ligands, e.g. poly(styrene-b-2-vinylpyridine) or amine terminated polystyrene, were added to toluene phase. Functional groups on polymer ligand would interact with carboxyl groups on the GO through hydrogen bonding/electrostatic interactions. GO nanosheets migrated to the water/toluene interfaces, aligned parallel to the interface and occupied the free space at interface. A jammed GO thin film was obtained when the interfacial area was compressed. TEM images showed that GO nanosheets, like nanotiles, occupied the whole area of the interface and separated the water and toluene phase effectively. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D22.00002: Mechanical vibration of viscoelastic liquid droplets James Sharp, Victoria Harrold The resonant vibrations of viscoelastic sessile droplets supported on different substrates were monitored using a simple laser light scattering technique. In these experiments, laser light was reflected from the surfaces of droplets of high Mw poly acrylamide-co-acrylic acid (PAA) dissolved in water. The scattered light was allowed to fall on the surface of a photodiode detector and a mechanical impulse was applied to the drops using a vibration motor mounted beneath the substrates. The mechanical impulse caused the droplets to vibrate and the scattered light moved across the surface of the photodiode. The resulting time dependent photodiode signal was then Fourier transformed to obtain the mechanical vibrational spectra of the droplets. The frequencies and widths of the resonant peaks were extracted for droplets containing different concentrations of PAA and with a range of sizes. This was repeated for PAA loaded water drops on surfaces which displayed different values of the three phase contact angle. The results were compared to a simple model of droplet vibration which considers the formation of standing wave states on the surface of a viscoelastic droplet. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D22.00003: Water Interaction with Poly (methyl methacrylate) and Polyethylene Films Paul Jones, Thorin Kane, Brian Familo, Ross Netusil, Patrick Howard, Marie Romano, John St. Leger, Carolina C. Ilie We present herein the water desorption from the dipole oriented poly (methyl methacrylate) PMMA and from polyethylene. We analyze the desorption peaks for the ice and the bulk species. And if the water desorption is coverage dependent or not. Hyperchem calculations of molecular orbitals for both polymers are also discussed. The energy of desorption is obtained by employing the Arrhenius and Polany-Wigner equations. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D22.00004: Poroelastic characterization of ultrathin water purification membrane layers Edwin Chan The selective layer of pressure-induced water desalination membranes is a highly crosslinked aromatic polyamide ultrathin film that discriminates salt from water based on differences in diffusion coefficients. However, measuring transport properties of such ultrathin layer is difficult. In this presentation, poroelastic relaxation indentation (PRI) is demonstrated as a simple indentation based technique for measuring the transport properties of these ultrathin layers. Using PRI, the transport properties of four model crosslinked PA ultrathin films, synthesized via molecular layer-by-layer (mLbL), are characterized to show that the water diffusion coefficient, the volume fraction of water lost due to deswelling, as well as the intrinsic permeability can be simultaneously quantified using this one simple testing approach. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D22.00005: Stretching Ultra-thin Polymer Films on Water Yujie Liu, Alfred J. Crosby The mechanical properties of many materials, including polymers, are known to change as materials become dimensionally confined; however, the extent and mechanism for these transitions are difficult to quantify~due to~experimental challenges. Some methods allow a single property, such as the elastic modulus, to be determined, however relatively few, if any, allow the full constitutive relationship, including linear and nonlinear regimes, to be measured for thin, inherently fragile materials. Here, we describe a new method that overcomes these limitations. Specifically, we quantify the uniaxial tension stress-strain relationship for polystyrene (PS, MW$=$130kg/mol) and crosslinked polydimethylsiloxane (PDMS) elastomer as a function of film thickness (29nm-400nm for PS; 2$\mu $m-200$\mu $m for PDMS). We perform these measurements by floating thin films on a water surface and attaching one end of the film to a fixed boundary, and the other to a cantilever that is attached to a translating actuator. We use a reflective laser tracking system to measure~cantilever~displacement, hence the force, as a function of applied displacement. In addition to the elastic modulus as a function of thickness, we present observations of non-linear transitions and cyclic hysteresis as a function of strain. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D22.00006: Perfect mixing of immiscible macromolecules at fluid interfaces Sergei Sheiko, Krzysztof Matyjaszewski, Vladimir Tsukruk, Jan-Michael Carrillo, Michael Rubinstein, Andrey Dobrynin, Jing Zhou Macromolecules typically phase separate unless their shapes and chemical compositions are tailored to explicitly drive mixing. But now our research has shown that physical constraints can drive spontaneous mixing of chemically different species. We have obtained long-range 2D arrays of perfectly mixed macromolecules having a variety of molecular architectures and chemistries, including linear chains, block-copolymer stars, and bottlebrush copolymers with hydrophobic, hydrophilic, and lipophobic chemical compositions. This is achieved by entropy-driven enhancement of steric repulsion between macromolecules anchored on a substrate. By monitoring the kinetics of mixing, we have proved that molecular intercalation is an equilibrium state. The array spacing is controlled by the length of the brush side chains. This entropic templating strategy opens new ways for generating patterns on sub-100 nm length scales with potential application in lithography, directed self-assembly, and biomedical assays. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D22.00007: Measuring the Height of Adsorbed Water Films With Atomic Force Microscopy Via Static Force Curves Jason Giamberardino Water Plays a crucial role in all biological processes and is present in almost all measurement scenarios. At the nanoscale, its presence has a large effect on measurement outcomes. Here, we present a measurement of the heights of adsorbed water films on a variety of commonly used substrate surfaces. The adsorbed films create an additional force, the capillary force, which must be added to the Van der Waals and repulsive forces to fully characterize the tip-sample interaction. Static force curves indicate the presence of this tertiary force, which depends on the height of the film, above a certain relative humidity. A fit of these curves then yields the height of the film present. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D22.00008: Structure of thin polystyrene films of varying tacticity adsorbed on solid substrates Yergou Tatek, Mesfin Tsige Atomistically detailed molecular dynamics simulations are used to investigate conformational properties of thin films of polystyrene (PS) adsorbed on two types of solid substrates. The substrates considered are graphite and hydroxylated silica which are known to be of different phobicity. The conformation of the PS chains was studied in terms of side chains, backbone and end group concentration and orientation. As expected, the films structure is different in all the three regions which are the two interfaces and the bulk of the film. Moreover, the film structural properties are also dependent on the nature of the substrate. We have also investigated the effect of chain tacticity on the films conformational properties. Preliminary results show the absence of a strong correlation between tacticity and structure. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D22.00009: Fabrication and Theoretical Evaluation of Microlens Arrays on Layered Polymers Tom Oder, Michael McMaster, Corey Merlo, Camron Bagheri, Clayton Reakes, Joshua Petrus, Dingqiang Li, Michael Crescimanno, James Andrews Arrays of microlens were fabricated on nano-layered polymers using reactive ion etching. Semi hemispherical patterns with diameters ranging from 20 to 80 micrometers were first formed on a thick photoresist film that was spin-coated on the layered polymers using standard photolithographic process employing a gray scale glass mask. These patterns were then transferred to the polymers using dry etching in a reactive ion etching system. The optimized etch condition included a mixture of sulfur hexafluoride and oxygen, which resulted in an etch depth of 5 micrometers and successfully exposed the individual sub-micron thick layers in the polymers. Physical characterization of the microlens arrays was done using atomic force microscope and scanning electron microscope. We combine basic physical optics theory with the transfer matrix analysis of optical transport in nano-layered polymers to address subtleties in the chromatic response of microlenses made from these materials. In particular this method explains the len's behavior in and around the reflection band of the materials. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D22.00010: The mechanical properties of supported thin polystyrene films Peter Chung, Emmanouil Glynos, Peter Green The mechanical properties of supported thin polystyrene films with thicknesses in the range of 100 nm to 1 micron were studied by atomic force microscopy (AFM) nanoindentation measurements. The effective modulus of the 1 micron thick PS film at small indentation depths, in the range of few nanometers ($\sim$3 nm), was independent of frequency (indentation rate) in the range we studied. On the other hand, the effective modulus of thinner PS films showed an increase in the modulus with decreasing film thicknesses and this enhancement was frequency-dependent. Finite element analysis revealed that the stress field induced by nanoindentation propagates a few hundred nanometers into the film even with only a few nanometers of indentation, and the enhancement in the effective modulus stems from the underlying hard substrate. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D22.00011: Structures and Elastic Moduli of Polymer Nanocomposite Thin Films Hongyi Yuan, Alamgir Karim Polymeric thin films generally possess unique mechanical and thermal properties due to confinement. In this study we investigated structures and elastic moduli of polymer nanocomposite thin films, which can potentially find wide applications in diverse areas such as in coating, permeation and separation. Conventional thermoplastics (PS, PMMA) and biopolymers (PLA, PCL) were chosen as polymer matrices. Various types of nanoparticles were used including nanoclay, fullerene and functionalized inorganic particles. Samples were prepared by solvent-mixing followed by spin-coating or flow-coating. Film structures were characterized using X-ray scattering and transmission electron microscopy. Elastic moduli were measured by strain-induced elastic buckling instability for mechanical measurements (SIEBIMM), and a strengthening effect was found in certain systems due to strong interaction between polymers and nanoparticles. The effects of polymer structure, nanoparticle addition and film thickness on elastic modulus will be discussed and compared with bulk materials. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D22.00012: Neutron Reflectivity Measurement of Polymer-Surface Interaction Richard Sheridan, Sara Orski, Ronald Jones, Kathryn Beers Liquid adsorption chromatography at critical conditions (LACCC) is a method of macromolecular separation that is simultaneously promising and problematic. There is a large parameter space for customization of surface properties and the ternary optimization of solvent, solute, and surface necessary to find ideal separation conditions. By creating 2D model substrates using polymers grafted to the interface, we create a system in which the adsorption process near critical conditions can be observed directly. In this way, we gain fundamental insight into the contribution of the interface to the LACCC separation process, which will be useful in the rational design of stationary phases for this technique. Thin polymer film swelling is in principle observable by a number of techniques, such as ellipsometry or quartz crystal microbalance with dissipation. However, we intend to observe the nanometer-scale shifts in polymer conformation at low grafting density. Therefore, we use neutron reflectivity to observe the process in the initial stages because of its relatively high contrast and sensitivity. We demonstrate this technique by emulating published polystyrene LACCC conditions in cyclohexane and dimethylformamide. We then calculate an interaction parameter directly related to the free energy of adsorption, and compare it to the adsorption partition coefficient derived from the literature LACCC experiment. These direct measurements are critical for description of a wide variety of interfacial dynamic processes. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D22.00013: Using multivalency to tailor the superselective binding of polymers on substrates Nicholas Tito, Daan Frenkel Multivalency is a microscopic design concept in which a single nanoscopic entity contains multiple ligands, each of which may bind to multiple receptors on another entity. A useful property of many multivalent systems is ``superselectivity,'' where the fraction of the multivalent species bound to their complementary receptors grows sharply with the total number of receptors available. For example in the past two decades, multivalency has been exploited to develop DNA-coated nanoparticles that self-assemble into aggregates over an extremely narrow temperature window. In this talk, we use analytic and self-consistent field theories to explore the binding of multivalent polymers to receptors on a flat substrate. Discussion will focus on how the sequence, number, and binding strength of ligands along the polymer chain can be used to tune the superselectivity of the system. Comparison with recent experiments on model systems will be presented as time permits. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D22.00014: Enhanced Molecular Dynamics in the Near-Surface Region of Polystyrene Thin Films Observed with $\beta$-NMR Iain McKenzie, Chad R. Daley, Robert F. Kiefl, C.D. Phil Levy, W. Andrew MacFarlane, Gerald D. Morris, Matt R. Pearson, Dong Wang, James A. Forrest Beta-detected nuclear spin relaxation of $^8\mathrm{Li}^+$ has been used to probe the depth dependent molecular dynamics in high- and low-molecular-weight deuterated polystyrene (PS-d8). In both samples, the average nuclear spin-lattice relaxation rate, $1/T_1^{\mathrm{avg}}$, is depth independent below $\sim$200~K. However, above this temperature $1/T_1^{\mathrm{avg}}$ increases above the bulk value within several nanometers of the surface. These results provide the most direct evidence for enhanced molecular-level mobility near the free surface of glassy polymers and suggest the polymer fluctuation rate decreases approximately exponentially with distance from the free surface, returning to bulk behavior for depths greater than $\sim$10~nm. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D22.00015: Brewster Angle Microscopy and Characterization of Polyvinylidene Fluoride (PVDF) Based Langmuir Films Timothy Reece The behaviors of ferroelectric polymer Langmuir films are observed with the use of a Brewster angle microscope. In general, Langmuir films form a single molecular layer on water because they are often good amphiphiles. Since the polymer Polyvinylidene Fluoride (PVDF) is not a true amphiphile, parameters like solution concentration, water pH, and molecular weight may have an effect on film behavior and quality. Similar polymers with hydrophobic groups are also investigated. [Preview Abstract] |
Session D23: Invited Session: Electron-Hole Interaction in Nanoparticles
Sponsoring Units: DCOMPChair: Ari Chakraborthy, Syracuse University
Room: 505-507
Monday, March 3, 2014 2:30PM - 3:06PM |
D23.00001: Photoexcitations in embedded semiconducting nanoparticles Invited Speaker: Giulia Galli We will discuss technical challenges involved in describing photo-excitation processes from first principles, in realistic materials [1], and we will present some results obtained using density functional and many body perturbation theory for semiconducting nanoparticles embedded in complex solid matrices, and for nanoparticles [2] with unusual core structures [3]. These are systems with promising properties for solar energy conversion. \\[4pt] [1] Yuan Ping, Dario Rocca, and Giulia Galli, \textit{Chem. Soc. Rev. }\textbf{42}, 2437 (2013).\\[0pt] [2] Stefan Wippermann, M\'{a}rton V\"{o}r\"{o}s, Dario Rocca, Adam Gali, Gergely Zimanyi and Giulia Galli \textit{Phys. Rev. Lett. }\textbf{110}, 046804 (2013).\\[0pt] [3] Stefan Wippermann, Marton Voros, Adam Gali, Francois Gygi,Gergely T. Zimanyi, and Giulia Galli 2013 (submitted for publication). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D23.00002: Calculating recombination rates and biexciton binding/antibinding in core-shell dots and nano-rods Invited Speaker: John Shumway Predicting radiative lifetimes and photoluminescence (PL) emission energies from electron-hole recombination in nano structures is complicated by correlation. Quantum correlations---particularly the attraction between the recombining electron and hole---reduce the PL emission energy but also modify the wave functions, enhancing recombination rates. Interactions with spectator particles can also affect energies and lifetimes, though sometimes the sign of these changes is non-intuitive. Path-integral quantum Monte Carlo (PI-QMC) is a wave-function free computational quantum approach that can easily handle interactions between several electrons and holes in a nanostructure. We present an application to core-shell dots and nano-rods, where proper treatment of correlation is necessary to understand the binding/antibinding transition in the biexciton [1]. The imaginary-time paths provide further insights into the properties of the electron-hole states. We show how changing the topology of the paths can be used to calculate recombination rates and give insights into the recombination process. Fluctuations in the paths are used to calculate responses to electric and magnetic fields. These calculations are performed with the open source pi-qmc code available on GitHub and as a community resource on the nanoHUB.\\[4pt] [1] P. G. McDonald, E. J. Tyrrell, J. Shumway, J. M. Smith, and I. Galbraith, Phys. Rev. B 86, 125310, (2012). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D23.00003: Exciton Scattering approach for conjugated macromolecules: from electronic spectra to electron-phonon coupling Invited Speaker: Sergei Tretiak The exciton scattering (ES) technique is a multiscale approach developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, the electronic excitations in the molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph. The exciton propagation on the linear segments is characterized by the exciton dispersion, whereas the exciton scattering on the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized ``particle in a box'' problems on the graph that represents the molecule. All parameters can be extracted from quantum-chemical computations of small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within considered molecular family could be performed with negligible numerical effort. The exciton scattering properties of molecular vertices can be further described by tight-binding or equivalently lattice models. The on-site energies and hopping constants are obtained from the exciton dispersion and scattering matrices. Such tight-binding model approach is particularly useful to describe the exciton-phonon coupling, energetic disorder and incoherent energy transfer in large branched conjugated molecules. Overall the ES applications accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D23.00004: Excitons in time-dependent density-functional theory Invited Speaker: Carsten Ullrich Excitons are the dominant feature in the optical spectra of insulators and semiconductors close to the absorption edge. They are collective excitations of the many-body system, but can often be discussed in a simplified picture as bound electron-hole pairs. To describe excitons in bulk materials with time-dependent density-functional theory (TDDFT), exchange-correlation functionals with a proper long-range behavior are required. The first part of this talk will present a TDDFT approach for directly calculating singlet and triplet exciton binding energies, which is based on an adaptation of the Casida formalism for periodic solids. Several exchange-correlation kernels have been tested for a variety of semiconductors and large-gap insulators. The second part of this talk will discuss a method to visualize exciton dynamics in large organic molecules in real time, based on the time-dependent transition density matrix. The method is applied to study the optical properties of intramolecular charge-transfer excitons in photoexcited molecular donor-acceptor systems that are of interest in organic photovoltaics. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D23.00005: Quantum dots -- artificial atoms, large molecules, or small pieces of bulk? Insights from time-domain ab ignition studies Invited Speaker: Oleg Prezhdo Quantum dots (QD) are quasi-zero dimensional structures with a unique combination of solid-state and atom-like properties. Unlike bulk or atomic materials, QD properties can be modified continuously by changing QD shape and size. Often, the bulk and atomic viewpoints contradict each other. The atomic view suggests strong electron-hole and charge-phonon interactions, and slow energy relaxation due to mismatch between electronic energy gaps and phonon frequencies. The bulk view advocates that the kinetic energy of quantum confinement is greater than electron-hole interactions, that charge-phonon coupling is weak, and that the relaxation through quasi-continuous bands is rapid. QDs exhibit new physical phenomena. The phonon bottleneck to electron energy relaxation and generation of multiple excitons can improve efficiencies of photovoltaic devices. Our state-of-the-art non-adiabatic molecular dynamics techniques, implemented within time-dependent density-functional-theory, allow us to model QDs at the atomistic level and in time-domain, providing a unifying description of quantum dynamics on the nanoscale. [Preview Abstract] |
Session D24: Focus Session: Materials for Electrochemical Energy Storage: Battery Materials
Sponsoring Units: DMP GERA DCOMPChair: Javier Bareno, Argonne National Laboratory
Room: 504
Monday, March 3, 2014 2:30PM - 2:42PM |
D24.00001: Graphene modified LiMPO$_{4}$ (M$=$Fe, Mn) as a cathode material for lithium ion batteries Kulwinder Dhindsa, Balaji Prasad Mandal, Khadije Bazzi, Ming-Wei Lin, Maryam Nazri, Gholam Abbas Nazri, Vaman M. Naik, Vijay K. Garg, A.C. Oliveira, Prem Vaishnava, Ratna Naik, Zhixian Zhou We have synthesized LiFePO$_{4}$/graphene nano-composites using sol-gel method by adding water dispersed graphene oxide to the LiFePO$_{4}$ precursors during the synthesis. The graphene oxide was subsequently reduced by annealing the composite at 600$^{\circ}$C for 5h in forming gas (90{\%} Ar and 10{\%} H$_{2})$ which was confirmed by Raman spectroscopy and X-ray Photoelectron spectroscopy. Addition of graphene significantly improved the electronic conductivity of LiFePO$_{4}$. Scanning Electron microscopy and Transmission electron microscopy images show LiFePO$_{4}$ particles being covered uniformly by graphene sheets throughout the material forming a three dimensional conducting network. Cyclic voltammetry results show that composite is a typical two-phase system. Li ion diffusion coefficient calculations show two orders of magnitude enhancement. At low currents, (C/3), the capacity of LiFePO$_{4}$/graphene composite cathode reaches 160 mAh/g, which is very close to the theoretical limit. More significantly, the graphene modified LiFePO$_{4}$ shows a dramatically improved rate capability up to 27C, and excellent charge-discharge cycle stability over 500 stable cycles. In addition to~LiFePO$_{4}$, LiMnPO$_{4}$/graphene composite has also been synthesized and the results will be discussed. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D24.00002: Effect of excess Li on electrochemical properties of LiFePO$_{4}$ cathode material for Li ion batteries K. Bazzi, M. Nazri, P. Vaishnava, V.M. Naik, G.A. Nazri, R. Naik Application of lithium iron phosphate as a cathode material in lithium cell is limited by its poor electronic and ionic conductivity. Here, we report the synthesis of C-LiFePO$_{4}$ and C- Li$_{1.05}$FePO$_{4}$ cathode materials via sol gel method using oleic acid as a surfactant/source of carbon to improve the electronic conductivity. Our aim is to investigate the role of excess Li on the electrochemical performance of C-LiFePO$_{4}$. The phase purity was confirmed by x-ray diffraction. When excess lithium is used, the agglomeration is reduced and spherical particles are formed. Our results show that C-Li$_{1.05}$FePO$_{4}$ has lower charge transfer resistance, higher Li-ion diffusion coefficient, and superior electrochemical performance in terms of the specific capacity, rate capability and cycling stability. The correlation between the electrochemical characteristics and the particle size and morphology will be presented. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D24.00003: Phase transformation and electronic structure characterization of Li$_{x}$FePO$_{4}$ by ab-initio calculations and soft x-ray spectroscopy Yung Jui Wang, B. Barbiellini, Xiaosong Liu, Ruimin Qiao, B. Moritz, T. P. Devereaux, Hsin Lin, Zahid Hussain, Wanli Yang, A. Bansil Olivine-structured Li$_{x}$FePO$_{4}$ with appropriate surface treatment is a battery cathode material with promising capacity, cost and safety specifications. Fe-L and O-K edge soft x-ray absorption and emission spectra directly probe the unoccupied and occupied electronic states in the vicinity the Fermi energy. We present a first principles calculation and a comparison with the spectra to investigate the electronic states of Li$_{x}$FePO$_{4}$. Upon fully (de)lithiation, the redistributed unoccupied Fe-3d and O-2p states indicate the fingerprints of the two-phase transformation. The redox couple is pinned such that a single electron injection into the valence states is well separated from the top of O-2p valence states due to Coulomb repulsion. We further explore the surface properties and discuss their implications on the performance and optimization of Li$_{x}$FePO$_{4}$. Work supported by the US DOE. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D24.00004: Insolubility of intercalants in strongly correlated oxides and phosphates Eric Isaacs, Chris Marianetti Olivine lithium iron phosphate (Li$_x$FePO$_4$), a cathode material for Li-ion batteries, phase separates into the fully lithiated and fully delithiated phases according to experiment. Previous electronic structure calculations showed that while this phase separation is not predicted by DFT, it is captured by DFT+U due to the improved treatment of electronic correlations [F. Zhou et al., Phys. Rev. B 69, 201101 (2004).]. In order to understand the role of electronic correlations in phase separation, here we investigate the phase stability of Li$_x$FePO$_4$ and other strongly correlated oxide and phosphate intercalation compounds within DFT+U and DFT+DMFT. We present the relationship between computed formation energies and the on-site Coulomb repulsion and double-counting correction across different systems. In addition, we perform p-d model calculations in order to understand the mechanism at a minimal level. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D24.00005: Synthesis and Electrochemical Characterization of Li$_{2}$FeSiO$_{4}$/Carbon Nanofiber Composite Cathode Material for Li Ion Batteries Ajay Kumar, Gholam Abbas Nazri, Ratna Naik, Vaman M Naik Lithium transition metal silicates (Li$_{2}$MSiO$_{4})$, where, M$=$Ni, Mn, Fe, and Co with a theoretical capacity of $\sim$ 330 mAh/g have attracted great interest as possible replacements for cathode material in rechargeable batteries. However, this class of materials exhibit very low electronic conductivity and low lithium diffusivity. In order to enhance the electronic conductivity and reduce the diffusion length for lithium ion, we have synthesized Li$_{2}$FeSiO$_{4}$/carbon nanofiber (15 {\%} wt) composites by sol-gel method. The composite materials were characterized by x-ray diffraction and scanning electron microscopy. The XRD data confirmed the formation of Li$_{2}$FeSiO$_{4}$ crystallites with size $\sim$ 25 nm for composites annealed at 600 $^{\circ}$C under argon atmosphere. The composite material was used as positive electrode in a coin cell configuration and the cells were characterized by AC impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge cycling. The cells showed a discharge capacity of $\sim$ 230 mAh/g in the initial cycles, which suggests that more than one Li ion is extracted from the electrode. The effect of annealing at higher temperature on the electrochemical performance of Li$_{2}$FeSiO$_{4}$/carbon nanofiber composites will be presented. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D24.00006: Polycrystalline TiO2(B) Nanosheet Films Deposited via Langmuir-Blodgett Method Laura Biedermann, Paul Kotula, Thomas Beechem, Anthony Dylla, Keith Stevenson, Calvin Chan As an energy storage material, TiO$_2$ offers higher Li$^+$ capacities and smaller volume changes with lithiation than graphite electrodes. In particular, the bronze phase, TiO$_2$(B) has a higher lithiation capacity (1.0~Li$^+$/Ti) and faster lithiation kinetics due to its larger lattice parameters than other TiO$_2$ polymorphs. Direct observation of lithiation will require TiO$_2$(B) monolayers, such as those prepared via Langmuir-Blodgett deposition of the nanosheets (NS). Optical microscopy of the TiO$_2$(B)-NS Langmuir monolayer at the air/water interface shows that these nanosheets assemble into large ($>$1~mm) islands. These elastic TiO2(B)-NS monolayers are deposited on diverse substrates for further characterization. Electron diffraction in both transmission electron microscopy (TEM) and low-energy electron microscopy (LEEM) of these films confirm that their polycrystalline structure is predominately composed of TiO$_2$(B) nanocrystals, $\sim$10s nm across. Discrimination of monolayer and bilayer TiO$_2$(B) is evident in LEEM. Thermal stability of these nanosheets is investigated via in-situ TEM and ex-situ Raman spectroscopy. This monolayer TiO$_2$(B) deposition will allow future observations of lithiation and phase changes. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D24.00007: {\em{Ab initio}} study of layered chromium disulfide (CrS$_2$) toward a new anode material for Li-ion batteries Seoung-Hun Kang, Young-Kyun Kwon There has been considerable interest in use of transition-metal disulfides, such as MS$_2$ (M$=$Mo, W), as new anode materials in Li-batteries to improve their battery performance. Since CrS$_2$, if synthesized, would be much lighter than MoS$_2$ or WS$_2$, it would exhibit higher Li capacity. To verify this expectation, we investigate the adsorption and diffusion properties of Li on layered Cr$_2$ and its Li capacity using DFT implemented with van der Waals correction. We thoroughly search for variuos Li adsorption sites, on which the binding energies are higher than Li clustering energy ($\sim1.6$~eV). Based on the these calculations, we identify the diffusion paths and barriers of Li atoms within the layered CrS$_2$ as well as on a free-standing single-layer of CrS$_2$. We find that Li atoms exhibit almost free intra-layer diffusion resulting in an improved mobility of Li at room temperature, while inter-layer diffusion is difficult to occur. We also estimate the Li-capacity of the CrS$_2$ by evaluating the energy gain as well as the average binding energy while intercalating more Li atoms. We find that CrS$_2$ can have larger Li-capacity than graphite, which is being widely used for anode material, implying that CrS$_2$ may be a good candidate for Li-battery electrode. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D24.00008: Dopant based stabilization of LiCoO$_{2}$ Juan A. Santana, Jeongnim Kim, Fernando A. Reboredo, Paul R. Kent LiCoO$_{2}$ is still one of the commonly used cathode materials for Li-ion batteries. Yet, the usable specific capacity is limited to approximately 150 mAh/g, only above half of its theoretical capacity (280 mAh/g). The limitation arises predominantly from the decomposition (or mechanical failure) of the LiCoO$_{2}$ cathode into Co$_{3}$O$_{4}$ when more than 50{\%} of Li is deintercalated. The stability of the cathode and its electrochemical performance can be improved by coating the cathode surface with inner metal oxides. This approach has been widely used for different cathode materials, but the origin of the stabilization effect is poorly understood. Various models have been proposed to rationalize the stabilization, e.g., $i)$ the inner metal oxides act as a physical barrier preventing cathode-electrolyte reactions, and \textit{ii}) the layered structure of the cathode material is stabilized by the migration of metal ions from the coating oxides. To elucidate the origin of the higher stability, we have performed first-principles DFT calculations of LiCoO$_{2}$ when doped with inner metal ions. Our calculations explore the effect of the dopants on the formation of point defects and the thermal decomposition of the cathode electrode. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D24.00009: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 4:18PM - 4:30PM |
D24.00010: First-principles study of the electronic properties and discharge profile of AgNa(VO$_{2}$F$_{2}$)$_{2}$ Masatoshi Onoue, Giancarlo Trimarchi, Arthur J. Freeman Implantable cardiac defibrillators (ICDs) require batteries with high capacities and high discharge rates to ensure the optimal operation of the device over several years. Ag$_{2}$V$_{4}$O$_{11}$ has been a cathode material of choice for the ICDs owing to its high capacity and fast rate of electronic discharge. To reduce ICD size and improve ICD performance, a new cathode material would need to display a higher volumetric capacity and redox potential. Recently, the new cathode compound AgNa(VO$_{2}$F$_{2}$)$_{2}$ (SSVOF) was synthesized and displayed favorable voltage for sodium-ion batteries. However, the discharge reaction has been unclear. In this presentation, we study the discharge reaction of SSVOF through DFT calculations. All calculations are performed within the PAW method using the GGA and GGA$+U$ functionals. Among several possible reactions, we focus on the reaction Ag$X$ + $A \rightarrow AX$ + Ag, where $X$ is Na(VO$_{2}$F$_{2}$)$_{2}$ and $A$ is Li or Na. In this reaction, the discharge occurs by replacing Ag with $A$. The calculated discharge potential for Li is 3.3 V in GGA and 2.9 V in GGA$+U$ and that for Na is 3.1 V in GGA and 2.8 V in GGA$+U$. These values are consistent with the experimental ones. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D24.00011: Structural Stability and Electronic Properties of Na$_2$C$_6$O$_6$ for a Rechargeable Sodium-ion Battery Tomoki Yamashita, Akihiro Fujii, Hiroyoshi Momida, Tamio Oguchi Sodium-ion batteries have been explored as a promising alternative to lithium-ion batteries owing to a significant advantage of a natural abundance of sodium. Recently, it has been reported that disodium rhodizonate, Na$_2$C$_6$O$_6$, exhibit good electrochemical properties and cycle performance as a minor-metal free organic cathode for sodium-ion batteries. However, its crystal structures during discharge/charge cycle still remain unclear. In this work, we theoretically propose feasible crystal structures of Na$_{2+x}$C$_6$O$_6$ using first principles calculations. A structural phase transition has been found: Na$_4$C$_6$O$_6$ has a different C$_6$O$_6$ packing arrangement from Na$_2$C$_6$O$_6$. Electronic structures of Na$_{2+x}$C$_6$O$_6$ during discharge/charge cycle are also discussed. Our predictions could be the key to understanding the discharge/charge process of Na$_2$C$_6$O$_6$. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D24.00012: Strongly localized electronic screening of Mg intercalants in the Chevrel phases Mo$_6$S$_8$ Florian Th\"{o}le, David Prendergast The problem of multivalent ion insertion into cathode materials is still poorly understood. Using the Chevrel phases (CPs) as a model material, we study the effect of Mg ion insertion using density functional theory (DFT). By inspection of the electron charge density difference associated with the insertion of Mg into a supercell of the material, which is metallic below the full intercalation limit, we arrive at the conclusion that the response to Mg insertion is best described in terms of a local screening cloud, effectively shielding the Mg charge with a length scale on the order of one unit cell. The density differences are localized mostly on the nearest neighbor sulfur anions with only minor differences on the nearest Mo cations. This behaviour is surprising, because in an ideal metal one might expect that insertion of an isolated ionic dopant might lead to localized screening with the Thomas-Fermi length scale $r_{TF}=0.29$~{\AA}, while in an insulator or semiconductor comprising transition metal cations, traditional redox chemistry might be expected. To provide a link to experiments, we simulate X-ray absorption spectra for the Mg K-edge. In full agreement with our model, the resulting spectra show an edge position which lies between metallic Mg and ionic MgO. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D24.00013: The Lithium-Induced Conversion Reaction of CoO Thin Film Battery Materials as Studied by ARXPS Ryan Thorpe, Sylvie Rangan, Mahsa Sina, Frederic Cosandey, Robert Bartynski Conversion reaction compounds such as CoO exhibit high charge density as electrodes in Li-ion batteries. Upon exposure to lithium, Co ions are reduced from a 2+ oxidation state to Co$^{0}$ in a reaction that drastically changes the electronic structure and morphology of the electrode. In order to characterize the atomistics of this conversion reaction without contamination from electrolytes or ambient gases, we have grown CoO thin films in (100), (111), and polycrystalline orientations, and exposed these surfaces to atomic Li in ultra-high vacuum. The diffusion of Li and the phase evolution of the substrate were then characterized with STM and angle-resolved XPS. Differences in the reactivities of each crystalline face have been observed. Additionally, a parasitic reaction between Li-rich reaction products and residual H$_{2}$O was observed to produce Li$_{2}$O$_{2}$, which inhibited further Li diffusion at room temperature. This could explain the capacity losses observed in CoO electrodes by previous studies. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D24.00014: Different behavior of lithium interaction with SiO2 and Al2O3 Yufeng Zhao, Chunmei Ban, Branden B. Kappes, Qiang Xu, Chaiwat Engtrakul, Cristian V. Ciobanu, Anne C. Dillon Lithiation of SiO2 and lithium intercalation in Al2O3 is studied both theoretically and experimentally. Lithium interacts with these two types of oxides in distinctly different behaviors. Reversible insertion/extraction of lithium in SiO2 up to a Li density of 2/3 Li per Si are demonstrated experimentally. Density-functional-theory (DFT) calculation shows that neither free interstitial Li atoms (no reduction) nor formation of a local Li2O cluster plus a Si-Si bond (full reduction) is energetically favorable. However, two Li atoms can effectively break a Si-O bond and be stabilized between the Si and O atoms. Such a defect, representing a state of partial reduction of SiO2, is energetically favorable. DFT simulation shows that intercalation of SiO2 at high Li density through partial reduction results in crystalline compounds LixSiO2 (x \textless 2/3) with tunable band-gaps in the range of 2-3.4 eV. In sharp contrast, Al2O3 is very stable against lithiation through any form of reduction. However, good conductivity of Li ions is shown in porous Al2O3. [Preview Abstract] |
Session D25: Focus Session: OPV/DSSC Photophysics and Interfaces
Sponsoring Units: GERAChair: Sean Shaheen, University of Colorado at Boulder
Room: 503
Monday, March 3, 2014 2:30PM - 3:06PM |
D25.00001: Uncovering location-specific ultrafast exciton dynamics in organic semiconducting thin films Invited Speaker: Naomi Ginsberg In solid state semiconducting molecular materials used in electro-optical applications, relatively long exciton diffusion lengths hold the promise to boost device performance by relaxing proximity constraints on the locations for light absorption and interfacial charge separation. The architecture of such materials determines their optical and electronic properties as a result of spacing- and orientation-dependent Coulomb couplings between adjacent molecules. Exciton character and dynamics are generally inferred from bulk optical measurements, which can present a severe limitation on our understanding of these films because their constituent molecules are neither perfectly ordered nor perfectly disordered. Nevertheless, such microstructure can have profound impacts on transport properties. The ultrafast spectroscopy of single domains of polycrystalline films of TIPS-pentacene, a small-molecule organic semiconductor of interest in electronic and photovoltaic applications, is investigated using transient absorption microscopy. Individual domains are distinguished by their different polarization-dependent linear and nonlinear optical responses. As compared to bulk measurements, we show that the nonlinear response within a given domain can be tied more concretely to specific physical processes that transfer exciton populations between specified electronic states. By use of this approach and a simple kinetic model, the signatures of singlet fission as well as vibrational relaxation of the initially excited singlet state are identified. As such, observing exciton dynamics within and comparing exciton dynamics between different TIPS-pentacene domains reveal the relationship between photophysics and film morphology and the potential to resolve unique signatures at interfaces between different regions of the film. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D25.00002: Geminate and non-geminate recombination of triplet excitons formed by singlet fission Sam L. Bayliss, Alexei Chepelianskii, Alessandro Sepe, Bruno Ehrler, Brian J. Walker, Matt J. Bruzek, John E. Anthony, Neil C. Greenham Singlet fission is a promising route to enhance solar cells by harvesting two electron-hole pairs from high-energy photons. Through singlet fission, an optically generated singlet exciton is transformed into two spin-correlated triplet excitons, which serve as a unique signature of the process. We use optically detected magnetic resonance to identify and study triplet excitons created through singlet fission in the solution-processable small molecule TIPS-tetracene (bis(triisopropylsilylethynyl)tetracene). Through changes in photoluminescence under spin resonance, we identify geminate recombination of triplet pairs directly following singlet fission, as well as recombination from bimolecular triplet-triplet annihilation. We show that both processes can be present in spin-coated films, and correlate the two distinct annihilation pathways to film morphology. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D25.00003: Understanding Singlet and Triplet Excitons in Acene Crystals from First Principles Tonatiuh Rangel Gordillo, Sahar Sharifzadeh, Leeor Kronik, Jeffrey Neaton Singlet fission, a process in which two triplet excitons are formed from a singlet exciton, has the potential to increase the solar cell efficiencies above 100\%. Efficient singlet fission has been reported in larger acene crystals, such as tetracene and pentacene, in part attributable to their low-lying triplet energies. In this work, we use many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to compute quasiparticle gaps, low-lying singlet and and triplet excitations, and optical absorption spectra across the entire acene family of crystals, from benzene to hexacene. We closely examine the degree of localization and charge-transfer character of the low-lying singlets and triplets, and their sensitivity to crystal environment, and discuss implications for the efficiency of singlet fission in this systems. This work supported by DOE and computational resources provided by NERSC. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D25.00004: Controllable thin film crystal growth of a novel squaraine molecule in organic solar cells Brad Conrad, Susan Spencer, Cortney Bougher, Jesse Brown, Kyle Kelley, Patrick Heaphy, Victor Murcia, Cameron Gallivan, Amber Monfette, John Andersen, Jeremy Cody, Tonya Coffey, Christopher Collison We will discuss the formation, structures, and properties of squarine and squarine-PCBM blend thin-films using Atomic Force Microscopy, electrical characterization, UV-VIS-NIR, and Thin-film Xray Diffraction. Film properties are inferred from spectroscopic measurements and are correlated with crystallinity as determined by TFXRD and AFM. A comprehensive explanation of DiPSQ(OH)2 structures is provided and related to measured efficiencies up to 4.3. By controlling the blend ratio and other fabrication conditions, crystalline regions of higher mobility can be developed so as to make significant gains in power conversion efficiency, necessary to achieve long term goals for commercially viable NIR-active OPV devices. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D25.00005: Crystal fields of porphyrins and phthalocyanines P.S. Johnson, I. Boukahil, F.J. Himpsel, C. Kennedy, N. Jersett, P.L. Cook, J.M. Garcia-Lastra Polarization-dependent X-ray absorption spectroscopy at the N 1s and metal 2p edges is combined with density functional and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal (Mn, Fe, Co, Ni) phthalocyanines and octaethylporphyrins [1]. Octaethyl porphyrins are observed to lie flat on Si with native oxide, while phthalocyanines lie on edge. Strong polarization dependence is found at all edges, which facilitates a unique determination of the crystal field parameters. Crystal field values from PBE density functional calculations provide helpful starting values, which are refined by fitting atomic multiplet calculations to the data. Since the crystal field affects electron-hole separation in solar cells, the systematic set of crystal field parameters obtained here can be useful for optimizing dyes for solar cells.\\[4pt] [1] P. S. Johnson, et al., J. Chem. Phys., to be submitted. See also: P. L. Cook, et al., J. Chem. Phys. 131, 194701 (2009); D. F. Pickup, et al., J. Phys. Chem. C. 117, 4410 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D25.00006: Improving Band Line-up: DFT study of interface effects Michelle Tomasik, David Strubbe, Alexie Kolpak, Jeffrey Grossman Solar cells, organic light emitting diodes, and other devices that involve organic molecules require metal contacts to either extract or supply electricity. Unfortunately standard band-line-up diagrams fail to include important interface effects. Using density functional theory (DFT), we studied metal/organic interfaces to probe the different interface effects, including image charge interactions and dipoles arising from various sources at the interface, and predict how they will affect the line-up of the energy levels. Specifically we have looked at two very different organics, Alq3 and anthracene on different metals. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D25.00007: Enhanced stability of ZnO-based inverted organic photovoltaic devices by phosphonic acid modification Bradley MacLeod, Bertrand Tremolet de Villers, Sarah Cowan, Erin Ratcliff, Dana Olson Solution-processed ZnO thin films are now commonly used as $n$-type bottom contacts in inverted-geometry organic photovoltaics (OPVs). The use of ZnO eliminates the need for highly-reactive top-contact (air-interface) electrode material, such as calcium and aluminum which are commonly used in conventional geometries, which enables operational lifetimes of unencapsulated devices to shift from minutes or hours to days. Modification of the ZnO film by self-assembled monolayers (SAMs) has been shown to enhance performance as well as air-stability during storage. We modify ZnO with dipolar phosphonic acids and observe enhanced performance and stability. We show for the first time devices measured under continuous illumination at one-sun intensity which have significantly enhanced stability when utilizing SAM-modified ZnO. These continuous-illumination stability measurements allow us to investigate the degradation mechanisms of these more stable inverted OPV devices. This work was was supported by of the Center for Interface Science: Solar Electric Materials (CISSEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001084. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D25.00008: Influence of the hole-collecting interlayer on the stability and lifetime of inverted organic solar cells Bertrand Tremolet de Villers, Bradley MacLeod, Dana Olson In organic photovoltaics (OPVs), interlayers between the photoactive layer and the electrodes are often used to modify the work-function of the electrode, provide charge-blocking selectivity, and improve the lifetime of the solar cell. To date, PEDOT:PSS has been the most commonly-used interlayer; however, due to its acidic and hygroscopic nature, it can facilitate degradation. To improve the stability of the device, molybdenum oxide (MoO$_3$) has emerged as an attractive alternative to PEDOT:PSS, and solar cells utilizing MoO$_3$ have shown significantly enhanced lifetimes. Furthermore, degradation of low work-function cathode metals such as calcium can be eliminated when the typical cell design is inverted. In inverted solar cells, interlayers remain a critical component but we find their role in the degradation of the OPV changes. Contrary to what is observed in a conventional-architecture OPV, degradation studies of inverted solar cells under constant illumination lasting $>$1000 hours reveal solar cells utilizing a MoO$_3$ interlayer degrade faster than those with PEDOT:PSS. Understanding the influence of the charge-collecting interfaces in OPVs provides a pathway to increased reproducibility and longer lifetimes. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D25.00009: Effect of conjugation linkage in molecular dipolar phosphonic acids for modification of zinc oxide in organic photovoltaics Jennifer Braid, Sarah Cowan, Brad MacLeod, Unsal Koldemir, Alan Sellinger, Reuben Collins, Tom Furtak, Dana Olson Self assembled monolayers of small molecules have become popular to modify the physical and electronic properties of metal oxide and conductive contacts for use in organic and hybrid electronics. Here, we discuss the application of phosphonic acid interface modifiers with strong molecular dipole moments that are able to effectively tune the work function of zinc oxide. This effect is exploited in organic photovoltaic devices, where decreasing the work function of the electron transport layer relative to the LUMO level of the active layer electron acceptor translates to an increased open circuit voltage. However, the effect of the molecular dipole moment is only part of the story, as we have found that bond type within the phosphonic acid also plays a role in the work function shift. Zinc oxide modified with 2,6-difluorophenylvinylphosphonic acid and its nonconjugated counterparts was studied via Kelvin probe and IR spectroscopy, as well as in P3HT:ICBA inverted devices, revealing that the conjugated double bond dramatically accentuates the aforementioned change in work function, resulting in improved device performance relative to the unconjugated modified or the un-modified ZnO contact. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D25.00010: Increasing The Work Function of NiO$_{\mathrm{x}}$ Hole Transport Layer Using Triethoxysilane-Based Monolayers Gang Chen, Thomas Brenner, Thomas Furtak, Reuben Collins, Sarah Cowan, Dana Olson Nickel Oxide (NiO$_{\mathrm{x}})$ is an effective hole transport layer in organic solar cells. $^{\mathrm{\thinspace }}$However, the NiO$_{\mathrm{x}}$ /organic interfacial energy level alignment needs to be optimized. Unlike the commonly used O2 plasma treatment, molecular monolayer modification can provide a more stable and controlled work function change for tuning the interface by introducing dipoles that form a molecular layer.$^{\mathrm{\thinspace }}$Previous work has shown the triethoxysilane (TES) chemistry bonds covalently to Zinc Oxide and can effectively tune the work function. In this study, the TES chemistry is transferred to NiO$_{\mathrm{x}}$ in order to tune the energy level alignment at the NiO$_{\mathrm{x}}$ /organic interface using three different TES modifiers. Contact angle (CA) measurements show that TES treated surfaces are much more hydrophobic than the untreated surface, which indicates the successful attachment of these molecules. Infrared spectroscopy shows that the coverage is sub-monolayer, consistent with our previous studies of other metal oxide surfaces. Kelvin probe measurements show that the TES treatment increases the NiO$_{\mathrm{x}}$ work function by as much as 450 meV compared to untreated NiO$_{\mathrm{x}}$. Standard bulk heterojunction devices were fabricated and we find that the open circuit voltage improves with increasing work function of the TES-treated surfaces. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D25.00011: X-ray Characterization of Dye Adsorption in Coadsorbed Dye-Sensitized Solar Cells Mitsunori Honda, Masatoshi Yanagida, Liyuan Han, Kenjiro Miyano We performed X-ray measurements to elucidate the adsorption mode of N719 dye molecules on nanoporous TiO2 with and without coadsorption of D131 dye. Two techniques, X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, were employed in order to obtain depth profile information about the substrate. In both cases, we found that the isothiocyanate groups of N719 strongly interact with TiO2 via S atoms when the dye is adsorbed from a single-component solution. In contrast, S-substrate interaction is strongly suppressed when D131 is coadsorbed with N719, indicating that the presence of D131 changes the adsorption mode of N719. Based on this finding, we designed a procedure to promote the preferential adsorption of D131, by which we successfully improved the short-circuit current and conversion efficiency. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D25.00012: Spectroscopy of donor-pi-acceptor complexes for solar cells F.J. Himpsel, I. Zegkinoglou, P.S. Johnson, C.D. Pemmaraju, D. Prendergast, M.-E. Ragoussi, G. de la Torre, D.F. Pickup, J.E. Ortega A recent improvement in the design of dye sensitized solar cells has been the combination of light absorbing, electron-donating, and electron-withdrawing groups within the same sensitizer molecule. This dye architecture has contributed to increase the energy conversion efficiency, leading to record efficiency values. Here we investigate a zinc(II)-porphyrin-based complex with triphenylamine donor groups and carboxyl linkers for the attachment to an oxide acceptor. The unoccupied orbitals of these three moieties are probed by element-selective X-ray absorption spectroscopy at the N 1s, C 1s, and Zn 2p edges, complemented by time-dependent density functional theory [1,2]. The attachment of electron-donating groups to the porphyrin ring significantly delocalizes the highest occupied molecular orbital (HOMO) of the molecule. This leads to a spatial separation between the HOMO and the lowest unoccupied molecular orbital (LUMO), reducing the recombination rate of photoinduced electrons and holes.\\[4pt] [1] P. L. Cook, et al., J. Chem. Phys. 134, 204707 (2011).\\[0pt] [2] I. Zegkinoglou, et al., J. Phys. Chem. C 117, 13357 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D25.00013: Carboxylic Acid Modification of Etch-Resistant Zn$_{\mathrm{1-x}}$Mg$_{\mathrm{x}}$O for Interface Tuning and Dye Sensitization Thomas Brenner, Erich Meinig, Gang Chen, Thomas Furtak, Reuben Collins, Thomas Flores, Dana Olson The bonding of carboxylic acids to metal oxide surfaces is an important monolayer modification scheme for tuning the properties of these surfaces in organic electronic devices and dye sensitized solar cells (DSSCs). However, the commonly used transparent semiconductor ZnO is very sensitive to acids and employment of carboxylic acids on its surface leads to etching, resulting in a non-ideal layer. This is especially troublesome in ZnO-based DSSCs where the products of etching accumulate on the surface and act as 'photon parasites', reducing device efficiency. We have found that, while the electronic properties are similar, the etch rate of Zn$_{\mathrm{1-x}}$Mg$_{\mathrm{x}}$O (ZnMgO) alloys decreases with Mg content and is up to 10 times smaller (at x$=$0.2) than that of ZnO when exposed to the modifier benzoic acid (BA). IR spectroscopy shows that BA forms a surface-bonded monolayer on ZnMgO after which etch products begin to accumulate on low (x$=$0-0.1) Mg content films. We suggest that ZnMgO may make a good replacement for ZnO where carboxylic acid modifiers are commonly used. In DSSCs we expect the etch resistance of ZnMgO to reduce the accumulation of 'photon parasites.' UV-Vis and photoluminescence measurements of dye-soaked ZnMgO show that the accumulation rate is reduced compared to ZnO. [Preview Abstract] |
Session D26: Materials at High Pressure: H plus
Sponsoring Units: DCOMP DMP GSCCMChair: Kevin Driver, University of California, Berkeley
Room: 502
Monday, March 3, 2014 2:30PM - 2:42PM |
D26.00001: Quantum Monte Carlo simulations of high pressure solid hydrogen Jonathan Lloyd-Williams, Bartomeu Monserrat, Pablo Lopez Rios, Neil Drummond, Richard Needs Several solid phases of hydrogen have been observed but the stable structures of hydrogen at high pressure remain experimentally undetermined because of the weak scattering of protons in x-ray diffraction studies. Theoretical identification of the structures is also difficult because of the small energy differences between competing phases and the large zero-point (ZP) contributions to the energies. We have performed static-nucleus diffusion Monte Carlo calculations for the candidate high pressure phases across a range of relevant densities and added ZP energies from both harmonic and anharmonic density functional theory calculations. We have used our calculated total energies to construct an enthalpy-pressure phase diagram from which we have evaluated the relative stability of the candidate structures. \\[4pt] This work was facilitated by a 2013 INCITE award of computing resources on Titan at Oak Ridge National Laboratory. It also made use of facilities provided by the High Performance Computing Service at the University of Cambridge and the N8 High Performance Computing Service which is coordinated by the Universities of Leeds and Manchester. Financial support was provided by the Engineering and Physical Sciences Research Council (UK). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D26.00002: Graphene physics and insulator-metal transition in compressed hydrogen Ivan I. Naumov, R.E. Cohen, Russell J. Hemley As established recently both theoretically and experimentally, compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. These structures and their electronic properties can be understood by studying simple model systems-(i) a H$_6$ ring, (ii) an ideal single hydrogen graphene sheet and (iii) three-dimensional model lattices consisting of such sheets [1]. The energetically stable structures result from structural distortions of model graphene-based systems due to electronic instabilities towards Peierls or other distortions associated with the opening of a bandgap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or grapheme-like layers due to topology and (ii) formation of bonding states with 2p${_z}$ $\pi$ character.\\[4pt] [1] I. I. Naumov, R. E. Cohen and R. J. Hemley Phys. Rev. B, {\bf 88}, 045125 (2013). [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D26.00003: Optical Signature of Metallization of Hydrogen R.E. Cohen, Ivan Naumov, Russell J. Hemley All proposed high-pressure structures of hydrogen are based on distorted graphene-structured, honeycomb layers. These give unique signatures for metallization and optical response [1,2]. Theoretical calculations and an assessment of recent experimental results for dense solid hydrogen lead to a unique scenario for the metallization of hydrogen under pressure. The metallization of hydrogen is very different from that originally proposed via a phase transition to a close-packed monoatomic structure, and different from simple metallization recently used to interpret recent experimental data. These different mechanisms for metallization have very different experimental signatures. We show that the shift of the main visible absorption edge does not constrain the point of band gap closure, in contrast with recent claims. This conclusion is confirmed by measured optical spectra, including spectra obtained to low photon energies in the infrared region for phases III and IV of hydrogen. This work was supported as part of EFree, an Energy Frontier Research Center funded by the US Department of Energy.\\[4pt] [1] Naumov, I.I., R. E. Cohen, and R. J. Hemley, PRB {\bf 88}, 045125 (2013).\\[0pt] [2] Cohen, R. E., I. I. Naumov, and R. J. Hemley, PNAS {\bf 110} 13757 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D26.00004: High-Pressure Hydrogen from First-Principles Invited Speaker: Miguel A. Morales The main approximations typically employed in first-principles simulations of high-pressure hydrogen are the neglect of nuclear quantum effects (NQE) and the approximate treatment of electronic exchange and correlation, typically through a density functional theory (DFT) formulation. In this talk I'll present a detailed analysis of the influence of these approximations on the phase diagram of high-pressure hydrogen, with the goal of identifying the predictive capabilities of current methods and, at the same time, making accurate predictions in this important regime. We use a path integral formulation combined with density functional theory, which allows us to incorporate NQEs in a direct and controllable way. In addition, we use state-of-the-art quantum Monte Carlo calculations to benchmark the accuracy of more approximate mean-field electronic structure calculations based on DFT, and we use GW and hybrid DFT to calculate the optical properties of the solid and liquid phases near metallization. We present accurate predictions of the metal-insulator transition on the solid, including structural and optical properties of the molecular phase. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D26.00005: Quasiparticle energies and excitonic effects of solid hydrogen under ultrahigh pressures Zhigang Wu, Marc Dvorak, Xaojia Chen We investigate the insulator-to-metal transition in the crucial $Cmca$-12 phase of solid hydrogen employing the many-body perturbation theory with Green's functions. In particular quasiparticle energies are calculated within the $GW$ approximation to accurately determine the insulator-to-metal transition pressure. We consider the effects of self-consistency, plasmon-pole models to the dielectric function, off-diagonal elements of the self-energy, and vertex corrections on $GW$ calculations, and our results show that the band gap of the $Cmca$-12 phase of solid hydrogen is sensitive to the choice of $GW$ procedures and approximations involved, leading to a change of $\sim 20$ GPa in transition pressure. We also compute the optical absorption and electron-hole binding energy by solving the Bethe-Salpeter equation, and the resulting optical absorption shows a redshift and enhancement of absorption peaks compared to the GW-RPA absorption with excitonic effects omitted. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D26.00006: Rotation of water molecules in plastic phase at extreme conditions from first principles molecular dynamics method Tomofumi Tasaka, Kazuo Tsumuraya Water has a variety of polymorphs in wide ranges of temperature and pressure. Ice VII phase transforms to ice X with increased pressure. However the ice VII transforms to a superionic phase at higher temperatures around 2000K and pressure 30GPa in which the protons migrate in the body centered cubic lattice of oxygens. The ice VII transforms into rotator phase (so called plastic phase at lower temperatures around 600K and 5 to 50GPa. The formation of the phase has been confirmed only with the empirical potentials, whereas the experimental confirmation has been postponed until now. The present study elucidates the mechanism of the rotation of the water molecules and the correlation between the molecules during the rotation with the first principles molecular dynamics method. The water molecules rotate around each oxygen atom to conserve the ice VII positions of the protons. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D26.00007: The Phase Diagram of Superionic Ice Jiming Sun, Bryan Clark, Roberto Car Using the variable cell Car-Parrinello molecular dynamics method, we study the phase diagram of superionic ice from 200GPa to 2.5TPa. We present evidence that at very high pressure the FCC structure of the oxygen sublattice [1] may become unstable allowing for a new superionic ice phase, in which the oxygen sublattice takes the P21 structure found in zero-temperature total energy calculations [2]. We also report on how the melting temperature of the hydrogen sublattice is affected by this new crystalline structure of the oxygen sublattice.\\[4pt] [1] Hugh F. Wilson, Michael L. Wong, and Burkhard Militzer. Superionic to superionic phase change in water: Consequences for the interiors of uranus and neptune. Phys. Rev. Lett.,110:151102, Apr 2013. \\[0pt] [2] Andreas Hermann, N. W. Ashcroft, and Roald Hoffmann. High pressure ices. Proceedings of the National Academy of Sciences, 109(3):745-750, 2012. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D26.00008: Dielectric Properties of Water Under Extreme Conditions Invited Speaker: Ding Pan Water is a major component of fluids in the Earth's mantle, where its properties are substantially different from those at ambient conditions. At the pressures and temperatures of the mantle, experiments on aqueous fluids are challenging, and several fundamental properties of water are poorly known; e.g., its dielectric constant has not been measured. This lack of knowledge of water dielectric properties has greatly limited our ability to model water-rock interactions and, in general, our understanding of aqueous fluids below the Earth's crust. Using ab initio molecular dynamics, we computed the dielectric constant of water under the conditions of the Earth's upper mantle, and we predicted the solubility products of carbonate minerals [1]. We found that MgCO$_3$ (magnesite)---insoluble in water under ambient conditions---becomes at least slightly soluble at the bottom of the upper mantle, suggesting that water may transport significant quantities of oxidized carbon. We also computed the electronic dielectric constant of water as a function of pressure [2] and we found that, contrary to expectations based on widely used simple models, both the refractive index and the electronic band gap of water increase under pressure. \\[4pt] [1] D. Pan, L. Spanu, B. Harrison, D. A. Sverjensky and G. Galli, Proc. Natl. Acad. Sci. U. S. A. 110, 6646 (2013)\\[0pt] [2] D. Pan. Q. Wan, G. Galli (submitted for publication) [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D26.00009: Ab initio Simulations of Fluid and Superionic Water in the Interiors of Uranus and Neptune Burkhard Militzer, Shuai Zhang Water is one of the most prevalent substances in our solar system. Large quantities are assumed to be stored in the interiors of ice giant planets. Water has an unusually rich phase diagram with 15 solid phases that were determined experimentally and 6 additional ones that were predicted theoretically at high pressure. Water is predicted to assume a superionic state where the oxygen ions remain confined to specific lattice sites while the hydrogen ions move through the crystal structure like a fluid. In our recent article [Physical Review Letters 110 (2013) 151102], we predicted the oxygen sub-lattice to assume a face-centered cubic structure at pressures above 1 Mbar. For this presentation, we extended our density functional molecular dynamics simulations in order to determine the equation of state of fluid and superionic water. We employed a thermodynamics integration technique to derive the entropy and the Gibbs free energy of both phases. We discuss how a novel superionic state could be identified in high pressure experiments and talk about the implications for the interiors of Uranus and Neptune. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D26.00010: Anharmonic vibrational properties of solids and the metallization of solid helium Bartomeu Monserrat, Neil D. Drummond, Chris J. Pickard, Richard J. Needs We describe a first-principles method for the calculation of anharmonic vibrational properties in solids. The method is based on a principal axes mapping of the Born-Oppenheimer energy surface and the vibrational self-consistent field scheme, and it allows us to calculate, amongst other quantities, the anharmonic free energy, the band gap renormalizations due to electron-phonon coupling, and the vibrational stress tensor. We exemplify the method by determining the effects of electron-phonon coupling and thermal expansion on the metallization of solid helium. Our results have implications for the cooling of white dwarf stars and suggest a revision of current lower bounds to the age of the Universe as determined within cosmochronology. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D26.00011: Structure and Metallization of Hydrogen Iodide Stanimir Bonev, Vahid Askarpour The structure and the metallization mechanism of hydrogen iodide under pressure are investigated using GW and hybrid density functional theory methods with exact exchange. The band gap closure is explained in terms of overlapping iodine $p$ orbitals and a close correlation is shown to exist between evolving structural and electronic changes. The metallization transition in phase III of hydrogen iodide is determined to take place between 20 and 25 GPa at zero temperature. This result differs significantly from existing experimental data. [Preview Abstract] |
Session D27: Focus Session: High Performance Computing in Density Functional Theory
Sponsoring Units: DCOMP DCPChair: Aldo Romero, West Virginia University
Room: 501
Monday, March 3, 2014 2:30PM - 3:06PM |
D27.00001: Accuracy, Speed, Scalability: the Challenges of Large-Scale DFT Simulations Invited Speaker: Francois Gygi First-Principles Molecular Dynamics (FPMD) simulations based on Density Functional Theory (DFT) have become popular in investigations of electronic and structural properties of liquids and solids. The current upsurge in available computing resources enables simulations of larger and more complex systems, such as solvated ions or defects in crystalline solids. The high cost of FPMD simulations however still strongly limits the size of feasible simulations, in particular when using hybrid-DFT approximations. In addition, the simulation times needed to extract statistically meaningful quantities also grows with system size, which puts a premium on scalable implementations. We discuss recent research in the design and implementation of scalable FPMD algorithms, with emphasis on controlled-accuracy approximations and accurate hybrid-DFT molecular dynamics simulations, using examples of applications to materials science and chemistry. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D27.00002: Challenges and advances in large-scale DFT calculations on GPUs Heather Kulik Recent advances in reformulating electronic structure algorithms for stream processors such as graphical processing units have made DFT calculations on systems comprising up to $O$(10$^3$) atoms feasible. Simulations on such systems that previously required half a week on traditional processors can now be completed in only half an hour. Here, we leverage these GPU-accelerated quantum chemistry methods to investigate large-scale quantum mechanical features in protein structure, mechanochemical depolymerization, and the nucleation and growth of heterogeneous nanoparticle structures. In each case, large-scale and rapid evaluation of electronic structure properties is critical for unearthing previously poorly understood properties and mechanistic features of these systems. We will also discuss outstanding challenges in the use of Gaussian localized-basis-set codes on GPUs pertaining to limitations in basis set size and how we circumvent such challenges to computational efficiency with systematic, physics-based error corrections to basis set incompleteness. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D27.00003: Implementation of the Small Box Fast Fourier Transformation Method within Orbital-Free Density Functional Theory Mohan Chen, Xiang-Wei Jiang, Lin-Wang Wang, Emily Carter Orbital-Free density functional theory (OFDFT) is a first-principles quantum mechanics method that uses the electron density as its only variable. The main computational cost in OFDFT are Fast Fourier Transforms (FFTs), used to evaluate both the kinetic energy density functional and the Coulomb term. The Small Box Fast Fourier Transform (SBFFT) technique is a newly developed method for solving the Poisson equation using a large number of processors [1]. We further adopt this SBFFT for the non-local kinetic energy density functional (KEDF) term frequently used in OFDFT, for which multiple FFTs are required. An efficient truncation of a real space KEDF kernel is proposed in order to take the advantage of SBFFT. This new method yields similar results as the original OFDFT formulation, as tested on bulk crystals, defects, and surfaces. Finally, we report progress in implementing all the mentioned techniques in PROFESS (PRinceton Orbital-Free Electronic Structure Software) [2]. [1] Xiang-Wei Jiang, Shu-Shen Li and Lin-Wang Wang, Comp. Phys. Comm. (in press). [2] L. Hung, C. Huang, I.Shin, G. Ho, V. L. Ligneres, and E. A. Carter, Comput. Phys. Comm., 181, 2208 (2010). [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D27.00004: Planning the next generation of density functional codes Grady Schofield, James R. Chelikowsky, Yousef Saad Real-space pseudopotential density functional theory has proven to be an efficient avenue for computing the properties of matter in many different states and geometries, including liquids, wires, slabs and clusters with and without spin polarization. Fully self-consistent solutions have been routinely obtained for systems with thousands of atoms. However, there are still systems where quantum mechanical accuracy is desired, but scalability proves to be a hindrance, such as large biological molecules or complex interfaces. We will present an overview of our work on algorithms for this problem, which has taken the route of improved scalability by spectrum slicing in the eigensolver, {\it i.e.}, the construction of a ``parallel'' eigensolver. We will also discuss how accurate forces can be obtained for ``coarse grids.'' [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D27.00005: High Performance Computing for Large Systems: Using Real Space Pseudopotentials for Metal-Semiconductor Interfaces Jaime Souto, James R. Chelikowsky, Tzu-Liang Chan, Kai-Ming Ho, Cai-Zhuang Wang, Shengbai Zhang Solving for the electronic structure at an interface can be computationally intensive. Even at the interface between crystalline systems, the structural details may not be known. Mismatch between the crystalline systems can result in unit cells containing hundreds, if not thousands of atoms. Until recently, such systems were not computationally tractable. Real-space pseudopotential density functional theory has proven to be an efficient avenue for computing the properties of such systems. Fully self-consistent solutions have been routinely obtained for systems with thousands of atoms. We illustrate this method applied to a Pb(111)/Si (111) interface and in particular examine the evolution of a Schottky barrier for this interface. We examine systems up to 1,500 atoms and determine the details of how quantum confinement controls the electronic structure of this system. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D27.00006: Linear-Scaling Density Functional Theory Simulations of Nanomaterials with the ONETEP code: vdW-DF and PAW methodology and OpenMP/MPI hybrid parallelism Nicholas Hine, Gabriel Constantinescu, Mike Payne, Lampros Andrinopoulos, Arash Mostofi, Peter Haynes, Karl Wilkinson, Jacek Dziedzic, Chris-Kriton Skylaris Methods based on traditional density functional theory (DFT) seek eigenstates of the Kohn-Sham Hamiltonian, and thus inevitably hit a scaling wall as system size increases, due to cubic scaling of the computational effort. However, useful contact with experiment in the study of nanomaterials (eg nanocrystals, interfaces, proteins, disordered molecular crystals) requires accurate calculations on systems comprising many thousands of atoms, beyond this scaling wall. Approaches based on the density matrix can exploit real-space localisation to achieve linear-scaling with system size and make such calculations feasible and highly parallel. The ONETEP Linear-Scaling DFT code [1] combines the benefits of linear-scaling, efficient parallelisation, and variational convergence akin to plane-wave approaches, with a wide-ranging set of features. I will present an overview of the code and recent developments: hybrid parallelism based on OpenMP and MPI, enabling scaling to tens of thousands of cores; Projector Augmented Wave methods, enabling study of transition metals; and van der Waals DF methods. These have combined to enable studies of C$_{60}$ molecular crystals and Transition Metal Dichalcogenide interfaces eg MoS$_2$/MoSe$_2$. [1] C. Skylaris et al, JCP 122, 084119 (2005). [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D27.00007: Plane Wave First-principles Materials Science Codes on Multicore Supercomputer Architectures Andrew Canning, Jack Deslippe, Steven.G. Louie Plane wave first-principles codes based on 3D FFTs are one of the largest users of supercomputer cycles in the world. Modern supercomputer architectures are constructed from chips having many CPU cores with nodes containing multiple chips. Designs for future supercomputers are projected to have even more cores per chip. I will present new developments for hybrid MPI/OpenMP PW codes focusing on a specialized 3D FFTs that gives greatly improved scaling over a pure MPI version on multicore machines. Scaling results will be presented for the full electronic structure codes PARATEC and BerkeleyGW. using the new hybrid 3D FFTs, threaded libraries and OpenMP to gain greatly improved scaling to very large core count on Cray and IBM machines. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D27.00008: ABINIT: Plane-Wave-Based Density-Functional Theory on High Performance Computers Invited Speaker: Marc Torrent For several years, a continuous effort has been produced to adapt electronic structure codes based on Density-Functional Theory to the future computing architectures. Among these codes, ABINIT [1] is based on a plane-wave description of the wave functions which allows to treat systems of any kind. Porting such a code on petascale architectures pose difficulties related to the many-body nature of the DFT equations. To improve the performances of ABINIT -- especially for what concerns standard LDA/GGA ground-state and response-function calculations -- several strategies have been followed: A full multi-level parallelisation MPI scheme has been implemented, exploiting all possible levels and distributing both computation and memory. It allows to increase the number of distributed processes and could not be achieved without a strong restructuring of the code. The core algorithm used to solve the eigen problem (``Locally Optimal Blocked Congugate Gradient''), a Blocked-Davidson-like algorithm, is based on a distribution of processes combining plane-waves and bands. In addition to the distributed memory parallelization, a full hybrid scheme has been implemented, using standard shared-memory directives (\textit{openMP}/\textit{openACC}) or porting some comsuming code sections to Graphics Processing Units (GPU). As no simple performance model exists, the complexity of use has been increased; the code efficiency strongly depends on the distribution of processes among the numerous levels. ABINIT is able to predict the performances of several process distributions and automatically choose the most favourable one. On the other hand, a big effort has been carried out to analyse the performances of the code on petascale architectures, showing which sections of codes have to be improved; they all are related to Matrix Algebra (diagonalisation, orthogonalisation). The different strategies employed to improve the code scalability will be described. They are based on an exploration of new diagonalization algorithm, as well as the use of external optimized librairies. Part of this work has been supported by the european Prace project (PaRtnership for Advanced Computing in Europe) [2] in the framework of its workpackage 8.\\[4pt] [1] http://www.abinit.org\\[0pt] [2] http://www.prace-ri.eu [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D27.00009: MBPT calculations with ABINIT Matteo Giantomassi, Georg Huhs, David Waroquiers, Xavier Gonze Many-Body Perturbation Theory (MBPT) defines a rigorous framework for the description of excited-state properties based on the Green's function formalism. Within MBPT, one can calculate charged excitations using \emph{e.g.} Hedin's $GW$ approximation for the electron self-energy. In the same framework, neutral excitations are also well described through the solution of the Bethe-Salpeter equation (BSE). In this talk, we report on the recent developments concerning the parallelization of the MBPT algorithms available in the ABINIT code (www.abinit.org). In particular, we discuss how to improve the parallel efficiency thanks to a hybrid version that employs MPI for the coarse-grained parallelization and OpenMP (a de facto standard for parallel programming on shared memory architectures) for the fine-grained parallelization of the most CPU-intensive parts. Benchmark results obtained with the new implementation are discussed. Finally, we present results for the $GW$ corrections of amorphous SiO$_2$ in the presence of defects and the BSE absorption spectrum. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D27.00010: SIESTA-PEXSI: Massively parallel method for efficient and accurate ab initio materials simulation Lin Lin, Georg Huhs, Alberto Garcia, Chao Yang We describe how to combine the pole expansion and selected inversion (PEXSI) technique with the SIESTA method, which uses numerical atomic orbitals for Kohn-Sham density functional theory (KSDFT) calculations. The PEXSI technique can efficiently utilize the sparsity pattern of the Hamiltonian matrix and the overlap matrix generated from codes such as SIESTA, and solves KSDFT without using cubic scaling matrix diagonalization procedure. The complexity of PEXSI scales at most quadratically with respect to the system size, and the accuracy is comparable to that obtained from full diagonalization. One distinct feature of PEXSI is that it achieves low order scaling without using the near-sightedness property and can be therefore applied to metals as well as insulators and semiconductors, at room temperature or even lower temperature. The PEXSI method is highly scalable, and the recently developed massively parallel PEXSI technique can make efficient usage of 10,000$\sim$100,000 processors on high performance machines. We demonstrate the performance the SIESTA-PEXSI method using several examples for large scale electronic structure calculation including long DNA chain and graphene-like structures with more than 20000 atoms. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D27.00011: A linear-scaling implementation of time-dependent density-functional theory (TDDFT) in the linear-response formalism Tim Zuehlsdorff, Nicholas Hine, James Spencer, Nicholas Harrison, Jason Riley, Peter Haynes In recent years, linear-scaling approaches to density functional theories have been successfully used to predict ground state properties of nanostructures and large biological systems. While these methods are now well established, the linear-scaling computation of excited state properties via time-dependent density-functional theory (TDDFT) remains challenging. In this talk, we will present a fully linear-scaling implementation of TDDFT in the linear-response formalism that we developed recently (J. Chem. Phys. 139, 064104) and that is particularly suitable for calculating the low energy absorption spectra of systems containing thousands of atoms. The method avoids any reference to individual Kohn-Sham states. Instead, the occupied and unoccupied subspaces are represented by two effective density matrices that are expanded in terms of two independent sets of in-situ optimized localized orbitals. The double basis set approach avoids known problems of representing the unoccupied space with localized orbitals optimized for the unoccupied space, while the in-situ optimization procedure allows for efficient calculations using a minimal number of basis functions. The linear-scaling properties of the method will be demonstrated on a number of nanostructures. [Preview Abstract] |
Session D28: Focus Session: Superconducting Qubits: Gates and Entanglement
Sponsoring Units: GQIChair: Joel Strand, Northrop Grumman
Room: 601
Monday, March 3, 2014 2:30PM - 3:06PM |
D28.00001: Driven superconducting quantum circuits Invited Speaker: Yasunobu Nakamura Driven nonlinear quantum systems show rich phenomena in various fields of physics. Among them, superconducting quantum circuits have very attractive features such as well-controlled quantum states with design flexibility, strong nonlinearity of Josephson junctions, strong coupling to electromagnetic driving fields, little internal dissipation, and tailored coupling to the electromagnetic environment. We have investigated properties and functionalities of driven superconducting quantum circuits. A transmon qubit coupled to a transmission line shows nearly perfect spatial mode matching between the incident and scattered microwave field in the 1D mode [1]. Dressed states under a driving field are studied there and also in a semi-infinite 1D mode terminated by a resonator containing a flux qubit [2]. An effective $\Lambda$-type three-level system is realized under an appropriate driving condition. It allows ``impedance-matched'' perfect absorption of incident probe photons and down conversion into another frequency mode [3]. Finally, the weak signal from the qubit is read out using a Josephson parametric amplifier/oscillator which is another nonlinear circuit driven by a strong pump field [4]. \\[4pt] [1] K. Koshino {\it et al.}, PRL {\bf 110}, 263601 (2013).\\[0pt] [2] K. Inomata {\it et al.}, PRB {\bf 86}, 140508(R) (2012).\\[0pt] [3] K. Koshino {\it et al.}, PRL {\bf 111}, 153601 (2013).\\[0pt] [4] Z. R. Lin {\it et al.}, APL {\bf 103}, 132602 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D28.00002: Optimized pulse shapes for a resonator-induced phase gate Andrew Cross, Jay Gambetta, Stefano Poletto, Doug Mcclure, Oliver Dial, Matthias Steffen The resonator-induced phase gate is a multi-qubit controlled-phase gate for superconducting qubits. Through off-resonant driving of a bus cavity, coupled qubits acquire a state-dependent phase and are not excited outside of the qubit manifold. However, cavity loss leads to dephasing during the gate and any residual entanglement between the cavity and qubits after the gate leads to decoherence. In this talk we present strategies for shaping the drive pulse to minimize dephasing and reduce the pulse duration. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D28.00003: Cross-resonance interactions between superconducting qubits with variable detuning Matthew Ware, Blake Johnson, Jay Gambetta, Colm Ryan, Thomas Ohki, Jerry Chow, B.L.T. Plourde The cross-resonance effect is a promising route for generating two-qubit gates in an all-microwave architecture based on superconducting qubits. Because the strength of the cross-resonance effect, and hence the speed of a two-qubit gate, depends sensitively on the detuning between the qubits and the anharmonicity of each qubit, we are performing experiments with some fixed-frequency transmon qubits and others with some tunability. By using asymmetric transmon qubits, we are able to vary this detuning over a moderate range. This allows us to study the cross-resonance effect while varying the magnetic flux to generate different qubit-qubit detunings. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D28.00004: A circuit QED controlled-Z ``AMP'' gate (Adiabatic MultiPole gate) David C. McKay, Ravi Naik, Lev S. Bishop, David I. Schuster Circuit quantum electrodynamics --- superconducting Josephson junction ``transmon'' qubits coupled via microwave cavities --- is a promising route towards scalable quantum computing. Here we report on experiments coupling two transmon qubits through multiple strongly coupled planar superconducting cavities --- the multipole cavity QED architecture. This design enables large interactions (mediated by real cavity photons) when the transmons are resonant with the cavities, and low off rates when the qubits are tuned away from the cavity resonance. In this talk we will discuss our gate protocol --- the AMP gate --- and report on producing a high fidelity Bell state ($|gg\rangle+|ee\rangle$) measured from state and process tomography. We will discuss future plans for scaling this architecture beyond two qubits. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D28.00005: Achieving high contrast on/off ratios using the multipole circuit QED architecture Ravi Naik, David C. McKay, Lev S. Bishop, David I. Schuster An outstanding goal for scalable quantum information processing is to design gates (qubit-qubit interactions) that are fast, yet can be switched off with high contrast to permit high fidelity single qubit operations. We implement a possible candidate for such a gate using the multipole circuit QED architecture which consists of superconducting Josephson junction qubits coupled via multiple strongly interacting microwave cavities. In this architecture, the on/off ratio is expected to scale exponentially in the number of cavities (poles). Here we report on measurements of the off-resonant coupling rate for two flux-tuned transmon qubits coupled through three strongly coupled planar resonators (a 3-pole filter). We will also discuss progress towards a scheme to implement multipole QED for flux insensitive qubits in 3D microwave cavities where the longest coherence times for superconducting qubits have been demonstrated. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D28.00006: Remote entanglement of transmon qubits M. Hatridge, K. Sliwa, A. Narla, S. Shankar, Z. Leghtas, M. Mirrahimi, S.M. Girvin, R.J. Schoelkopf, M.H. Devoret An open challenge in quantum information processing with superconducting circuits is to entangle distant (non-nearest neighbor) qubits. This can be accomplished by entangling the qubits with flying microwave oscillators (traveling pulses), and then performing joint operations on a pair of these oscillators. Remarkably, such a process is embedded in the act of phase-preserving amplification, which transforms two input modes (termed signal and idler) into a two-mode squeezed output state. For an ideal system, this process generates heralded, perfectly entangled states between remote qubits with a fifty percent success rate. For an imperfect system, the loss of information from the flying states degrades the purity of the entanglement. We show data on such a protocol involving two transmon qubits imbedded in superconducting cavities connected to the signal and idler inputs of a Josephson Parametric Converter (JPC) operated as a nearly-quantum limited phase-preserving amplifier. Strategies for optimizing performance will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D28.00007: High fidelity gates and states in a 5 Xmon qubit Josephson quantum processor, part I: architecture J. Kelly, R. Barends, A. Megrant, A. Veitia, E. Jeffrey, D. Sank, T. White, J. Mutus, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, C. Neill, P. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. Korotkov, A.N. Cleland, J.M. Martinis One of the greatest challenges in building a quantum architecture is to combine high fidelity logic gates with a multiqubit system. Here, we demonstrate high fidelity gates in a 5 Xmon qubit quantum processor using a multiqubit architecture which combines coherence, control and connectivity. The qubits are arranged in a linear chain with nearest neighbor coupling, have individual control and readout, and reach $T_1$ values up to 57 $\mu$s. We characterize single qubit gates with a fidelity above 99.9 \% for all qubits. Using the frequency tunability of the qubits, we employ a novel implementation of a fast, adiabatic two-qubit controlled-phase gate, measuring fidelities up to 99.45 \%. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D28.00008: High fidelity gates and states in a 5 Xmon qubit Josephson quantum processor, part II: multiqubit logic R. Barends, J. Kelly, A. Megrant, A. Veitia, E. Jeffrey, D. Sank, T. White, J. Mutus, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, C. Neill, P. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. Korotkov, A.N. Cleland, J.M. Martinis One of the critical challenges in quantum computing is to employ simultaneous, high fidelity quantum logic gates across a system. Here, we show how a novel implementation of a fast, adiabatic controlled-phase gate achieves fidelities between 99.0 and 99.4 \% across all pairs in a 5 Xmon qubit quantum processor. We also show that nearest as well as next nearest neighbor qubits can be operated simultaneously without sacrificing fidelity. This, combined with low Z control crosstalk allows for direct control of single or multiqubit subspaces. To showcase the addressability of the qubits and modularity of the logic set, we use single and two-qubit gates to construct N=3, 4 and 5 Greenberger-Horne-Zeilinger states with fidelities of 96 \%, 86 \% and 82 \%, characterized by quantum state tomography. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D28.00009: High fidelity gates and states in a 5 Xmon qubit Josephson quantum processor, part III: controlled-Z theory John Martinis, R. Barends, J. Kelly, A. Megrant, A. Veitia, E. Jeffrey, D. Sank, T. White, J. Mutus, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, C. Neill, P. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. Korotkov, A.N. Cleland I will explain how to construct a two-qubit controlled-Z gate that is adiabatic and fast (40 ns), yet requires only moderate coupling ($g/2\pi=$ 30 MHz). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D28.00010: Model Free Gate Design and Calibration For Superconducting Qubits Daniel Egger, Frank Wilhelm Gates for superconducting qubits are realized by time dependent control pulses. The pulse shape for a specific gate depends on the parameters of the superconducting qubits, e.g. frequency and non-linearity. Based on ones knowledge of these parameters and using a specific model the pulse shape is determined either analytically or numerically using optimal control [arXiv:1306.6894, arXiv:1306.2279]. However the performance of the pulse is limited by the accuracy of the model. For a pulse with few parameters this is generally not a problem since it can be ``debugged'' manually. He we present an automated method for calibrating multiparameter pulses. We use the Nelder-Mead simplex method to close the control loop. This scheme uses the experiment as feedback and thus does not need a model. It requires few iterations and circumvents process tomogrophy, therefore making it a fast and versatile tool for gate design. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D28.00011: Frequency Modulating Entangling Gates Thomas Ohki, Colm Ryan, Blake Johnson, Kin Chung Fong, Matt Ware, Britton Plourde There are multiple approaches to generating entangling gates in a superconducting qubit architecture depending on the choice of tunable or fixed frequency qubits. The asymmetric transmon offers a compromise between the two extremes offering mild tunability and better coherence times. With the ability to modulate the qubits' frequency a new type of first-order sideband gate is made available [1,2]. These allow the qubits to exchange information with a cavity quantum bus without having to be dynamically tuned into resonance with the cavity and potentially acquiring unwanted phases from interactions with other qubits. We show progress towards using this interaction as a high-fidelity entangling gate. \\[4pt] [1] Beaudoin, F., da Silva, M. P., Dutton, Z., {\&} Blais, A. (2012). First-order sidebands in circuit QED using qubit frequency modulation. \textit{Physical Review A}, \textbf{86}, 022305. \\[0pt] [2] Strand et al. (2013). First-order sideband transitions with flux-driven asymmetric transmon qubits. \textit{Physical Review B}, \textbf{87}, 220505. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D28.00012: Monitoring the performance of an autonomous entanglement stabilization protocol in real time Y. Liu, S. Shankar, N. Ofek, M. Hatridge, A. Narla, K.M. Sliwa, R.J. Schoelkopf, M.H. Devoret Quantum feedback for error correction poses an open challenge for superconducting quantum information processing. Recently, we have demonstrated an autonomous feedback protocol to stabilize entanglement between two transmon qubits coupled to a cavity, and achieved a fidelity of 67\% in the steady state. The feedback protocol is designed such that the cavity output continuously provides information on the state of the qubits. Here, we report the integration of an external measurement-based feedback architecture with this experiment to monitor the cavity output in real-time. The cavity output is directed to a high-fidelity measurement chain based on a Josephson parametric converter and then processed in real-time using an FPGA (Field Programmable Gate Array). This real time monitoring capability combined with low-latency digital control allows conditional tomography of the qubits state, and thus enhances the fidelity of the entanglement. We can thus leverage the flexibility of measurement-based feedback with the rapid response of autonomous feedback to attain a level of performance that cannot be reached by either architecture alone. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D28.00013: Efficient three-qubit entangling (Toffoli) gates via excited states in qubit-cavity systems. Thomas Reinecke, Sophia Economou, Dmitry Solenov Efficient multi-qubit quantum operations are crucial for further development of quantum information processing using available physical designs. We report our results on efficient three-qubit entangling operations in qubit-cavity systems. The proposed gate design is based on non-commutativity of single-qubit pulse controls that can be achieved for systems in which auxiliary states above the qubit subspace are available. It does not rely on dynamical tuning of energy states, and, unlike traditional decomposition approaches, it provides efficiency comparable to that of a single control-NOT operation. We will focus on the transmon qubit systems, which have recently demonstrated coherence times suitable for multi-qubit computation. Other systems will also be discussed. [Preview Abstract] |
Session D29: Electrical Transport Properties of Bilayer Graphene
Sponsoring Units: DCMPChair: Ki Wook Kim, North Carolina State University
Room: 603
Monday, March 3, 2014 2:30PM - 2:42PM |
D29.00001: The dangerous negative energy sea and the N=0 fractional quantum Hall effect in bilayer graphene Rohit Hegde, Inti Sodemann, Fengcheng Wu, Allan H. MacDonald The $N=0$ Landau level of bilayer graphene is nearly eight-fold degenerate because it contains states with $n=0$ and $n=1$ cyclotron quantum numbers in addition to spin and valley indices. Because the self energy due to exchange with the negative energy sea is $n$-dependent, it plays an essential role in choosing between competing correlated states at both integer and fractional filling factors. We show that this interaction must be included in a systematic theory controlled by the ratio of Coulomb to cyclotron energies. We will discuss the implications of this vacuum exchange effect for integer and fractional quantum Hall ground states and low lying charged and neutral excitations, and its interplay with the valley symmetry breaking and Zeeman terms that are also important in single-layer graphene. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D29.00002: Bilayer graphene with parallel magnetic field and twisting: Phases and phase transitions in a highly tunable Dirac system Kun Yang, Bitan Roy The effective theory for bi-layer graphene, subject to parallel/in-plane magnetic fields is discussed. We show that with a sizable in-plane magnetic field the trigonal warping becomes irrelevant, and one ends up with two Dirac points in the vicinity of each valleys in the low-energy limit, similar to the twisted bi-layer graphene. Combining twisting and parallel field thus gives rise to a Dirac system with tunable Fermi velocity and ultra violet cutoff. If the interactions are sufficiently strong, several fully gapped states can be realized in these systems, in addition to the ones in pristine setup. Symmetry based classification of the order parameters will be discussed. We also present the quantum critical behavior of various phase transitions driven by the twisting and the magnetic field. Effects of an additional perpendicular fields, and possible ways to realize the some of the new massive phases will be highlighted. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D29.00003: Topological phases in the zeroth Landau level of bilayer graphene Zlatko Papic, Dmitry Abanin We study the phase diagram of the zeroth Landau level of bilayer graphene in the presence of strong mixing between two degenerate orbital sublevels, as well as the screening of the effective Coulomb interaction. Using large scale exact diagonalization calculations, we find stable quantum Hall states at filling factors $\nu=-1, -4/3, -5/3, -8/5, -1/2$. We discuss the nature of these ground states and their collective excitations in terms of the known states in GaAs semiconductors using a truncated interaction model. Furthermore, we present evidence that the $\nu=-1/2$ fraction, which was recently reported experimentally, is unlikely a two-component ``331" state, but instead is of non-Abelian nature and related to the Moore-Read Pfaffian wave function. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D29.00004: Superfluid-Insulator transition of quantum Hall domain walls in bilayer graphene H.A. Fertig, Victoria Mazo, Chia-Wei Huang, Efrat Shimshoni, Sam Carr We consider the zero-filled quantum-Hall ferromagnetic state of bilayer graphene subject to a kink-like perpendicular electric field, which generates domain walls in the electronic state and low-energy collective modes confined to move along them. In particular, it is shown that two pairs of collective helical modes are formed at opposite sides of the kink, each pair consisting of modes with identical helicities. We derive an effective field theoretical model of these modes in terms of two weakly coupled anisotropic quantum spin-ladders, with parameters tunable through control of the electric and magnetic fields. This yields a rich phase diagram, where due to the helical nature of the modes, distinct phases possess very different charge conduction properties. Most notably, this system can potentially exhibit a transition from a superfluid to an insulating phase. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D29.00005: Electric Field Effects and Landau Level Crossing in Suspended Bilayer Graphene Kevin Myhro, Yongjin Lee, Michael Deo, David Tran, Jeanie Lau Bilayer graphene offers a versatile 2D platform for electron transport study due to its gate tunable band gap, large tensile strength and ultra-high electronic mobility. Here we report two-terminal differential conductance measurements of dual-gated suspended bilayer graphene devices as a function of applied back gate and top gate voltages in zero and finite magnetic fields. Multi-level electron beam lithography defines contactless top gates and the bilayer graphene flakes are suspended by wet-etching the oxide layer. Successful current annealing allows us to reach high mobility and an insulating state at low magnetic fields. We investigate the role of the applied perpendicular electric field from top-gated devices, compare results to single-gated measurements, and characterize Landau Level crossings as a function of electric field, charge carrier density and magnetic field. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D29.00006: Magneto and Hall drag in graphene double-layer Xiaomeng Liu, Yuanda Gao, Lei Wang, Patrick Maher, Kin Chung Fong, Kenji Watanabe, Takashi Taniguchi, James Hone, Philip Kim, Cory Dean Recent advancements in the assembly of 2D layered materials have enabled fabrication of large-area BN-encapsulated graphene that exhibits ballistic transport over length scales in excess of tens of micrometers. Exploiting these techniques to fabricate graphene double layers, consisting of two parallel graphene layers separated by few-layer BN dielectrics, we investigate Coulomb drag in the strongly interacting and ultra-clean limit. Magneto drag and Hall drag, under perpendicular magnetic fields up to 13T and temperature down to 5K, is reported. Using a dual gated structure to independently tune the charge carrier density and type in each layer, we correlate the finite-field drag response with Landau level filling, and compare with existing theories. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D29.00007: Mapping the chemical potential in bilayer graphene using double bilayer heterostructures Kayoung Lee, Babak Fallahazad, Jiamin Xue, Takashi Taniguchi, Kenji Watanabe, Emanuel Tutuc A key property of an electron system, the chemical potential (Fermi energy) captures the physics of both the energy-momentum band structure, as well as interaction-induced corrections to the single particle energy. In Bernal stacked bilayer graphene the band structure can also be reshaped by an applied transverse electric field. By performing transport measurement in double-bilayer graphene heterostructure, which consists of two bilayer graphene vertically separated by hexagonal boron nitride, we map the chemical potential of the bottom bilayer graphene as a function of electron density, perpendicular magnetic field, and transverse electric field. At zero magnetic field the chemical potential reveals a strongly non-linear dependence on density, with an electric field induced energy gap at charge neutrality. In a perpendicular magnetic field, the quantum Hall states stabilized by Landau levels (LL) spin and valley degeneracy lifting undergo transitions as a function of electric and magnetic fields, as a result of the interplay between LL spin and valley splitting. By directly measuring the LL energies we extract the LL spin and valley splitting, and their dependence on magnetic and electric field. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D29.00008: Nonlocal transport in dual-gated bilayer graphene Yuya Shimazaki, Michihisa Yamamoto, Kenji Watanabe, Takashi Taniguchi, Seigo Tarucha We report nonlocal transport measurement of biased bilayer graphene. Dual gated bilayer graphene Hall bars sandwiched between two h-BN insulating layers were prepared using the transfer technique with PMMA thin flims. We measured both local and non-local transport at temperatures between 1.5 K and 200 K. We found enhancement of the nonlocal resistance near the charge neutrality point when we increase the perpendicular electric field. Observed nonlocal resistance at 70K is much larger than what is expected as the Ohmic contribution from van der Pauw formula with measured local resistivity. This observation indicates additional contribution to the nonlocal transport in biased bilayer graphene. We present temperature and displacement field dependence of the nonlocal resistance and discuss its origin in terms of valley Hall effect and transport through disordered edge states. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D29.00009: Interlayer spacing of bilayer graphene determined accurately by photon energy dependent photoelectron intensity oscillation Ku-Ding Tsuei, Cheng-Mau Cheng, Jun-Hao Deng, Meng-Shiung Chiang, Chia-Jen Hsu We have carried out an angle resolved photoemission spectroscopic study on high quality bilayer graphene grown epitaxially on a SiC(0001) surface over a wide range of photon energies from 30 to 130 eV. The band intensities are maximized along the K$\Gamma$ direction in the first Brillouin while vanishing along the opposite KM direction due to matrix element effect [1]. For bilayer graphene the two valence bands further oscillate in intensity with varying photon energies [2]. We analyze the data by expressing the intensity asymmetry (I$_{1}$-I$_{2}$)/(I$_{1}$+I$_{2}$) for each photon energy thus avoid the uncertainty by comparing band intensities at different photon energies. The intensity asymmetry can be well simulated by a tight-binding model which attributes the intensity oscillation to the interference of photoelectron amplitudes emitted from the two layers. The oscillation period thus provides an accurate measure of the interlayer spacing. The obtained interlayer spacing is very near that of graphite from literature. [1] E. L. Shirley et al., Phys. Rev. B 51, 13614 (1995). [2] T. Ohta et al., Phys. Rev. Lett. 98, 206802 (2007). [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D29.00010: Direct observation of asymmetric band structure of bilayer graphene through quantum capacitance measurements Kaoru Kanayama, Kosuke Nagashio, Tomonori Nishimura, Akira Toriumi Although upper conduction and valence sub-bands in bilayer graphene are known to be asymmetric, a detailed analysis based on the electrical measurements is very limited due to the infirm quality of gate insulator. In this study, the electrical quality of the top-gate Y$_{\mathrm{2}}$O$_{\mathrm{3}}$ insulator is drastically improved by the high-pressure O$_{\mathrm{2}}$ post-deposition annealing at 100 atm and the carrier density of $\sim $8*10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ is achieved. In quantum capacitance measurements, the drastic increase of the density of states is observed in addition to the van Hove singularity, suggesting that the Fermi energy reaches upper sub-band. At the same carrier density, the sudden reduction of the conductivity is observed, indicating that the inter-band scattering occurs. The estimated carrier density required to fill the upper sub-bands is different between electron and hole sides, i.e., asymmetric band structure between upper conduction and valence bands is revealed by the electrical measurements. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D29.00011: Interaction-driven capacitance in graphene electron-hole bilayer in the quantum Hall regime Bahman Roostaei, Yogesh Joglekar Fabrication of devices made by isolated Graphene or Graphene-like single layers ( such as h-BN) has opened up possibility of examining highly correlated states of electron systems in parts of their phase diagram that is impossible to access in their counterpart devices such as semiconductor heterostructures. An example of such states are Graphene (or Graphene like) double layer electron-hole systems under strong magnetic fields where the separation between layers can be of the order of one magnetic length with interlayer tunneling still suppressed. In those separations correlations between electrons and holes are of crucial importance and must be included in determination of observables. Here we report a thorough mean-field study of the coherent and crystalline ground states of the interacting balanced electron-hole Graphene systems in small and intermediate separations with each layer occupying up to four lowest lying Landau levels. We calculate the capacitance of such states as a function of layer separation and filling factor. Our calculations show significant enhancement of the capacitance compared to geometrical value due to quantum mechanical corrections. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D29.00012: Interlayer capacitance in graphene bilayers Andrea Young, Wang Lei, Cory Dean, Jim Hone, Raymond Ashoori Capacitance measurements of dual-gated bilayer electron systems provide a way to experimentally access both the total density of states as well as the interlayer polarizability. I will discuss capacitance measurements of clean, hexagonal boron nitride encapsulated graphene bilayers, in which we use this technique to probe layer polarization in response to an applied electric field at both zero magnetic field and in the quantum Hall regime. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D29.00013: Electron-hole asymmetric fractional quantum Hall effect in bilayer graphene Ben Feldman, Angela Kou, Andrei Levin, Bertrand Halperin, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby At zero magnetic field, the electronic spectrum of bilayer graphene is electron-hole symmetric to first order. In a magnetic field, the lowest two orbital states occur at zero energy, and they combine with the spin and valley degrees of freedom to yield an eightfold degenerate lowest Landau level. Both external fields and electron-electron interactions can break these symmetries, leading to a uniquely rich and tunable phase diagram of many-body states. In this talk, I will present local electronic compressibility measurements of high quality bilayer graphene performed using a scanning single-electron transistor. We observe clear fractional quantum Hall states at filling factors $\nu$ = -10/3, -4/3, 2/3 and 8/3, with additional states appearing at $\nu$ = -17/5, -7/5, 3/5 and 13/5. Remarkably, this sequence breaks electron-hole symmetry and instead follows an even-odd pattern between integer quantum Hall states. Our results highlight the key role played by the orbital degree of freedom in the many-body physics of bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D29.00014: Spontaneous layer polarization and conducting domain walls in bilayer graphene Erez Berg, Kusum Dhochak Bilayer graphene subjected to perpendicular magnetic and electric fields displays a subtle competition between different quantum Hall ferromagnetic phases, resulting from an interplay from the internal spin and valley degrees of freedom. The transition between different phases is often identified by the closing of the gap and an enhanced conductance. Here, we formulate a criterion for the existence of conducting edge states at domain walls between different phases. For example, for a spontaneously layer polarized state at filling factor $\nu=2$, domains walls between of regions of opposite polarization carry conducting edge modes. A microscopic analysis shows that lattice-scale interactions can favour such a layer polarized state in an intermediate range of magnetic field. We analyze the experiments of Weitz et al. (Science, 2010) in light of these results. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D29.00015: Local compressibility of bilayer graphene in the quantum hall regime Andrei Levin, Angela Kou, Benjamin Feldman, Bertrand Halperin, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby In the presence of a strong magnetic field, the charge carriers in bilayer graphene (BLG) condense into a set of flat energy bands called Landau levels (LLs). Electronic compressibility measurements have historically been a powerful tool in studying the physics of partially filled LLs in two-dimensional electronic systems. In particular, electron-electron correlations arising from Coulomb interactions can introduce a negative component to the compressibility. Here we present measurements of electronic compressibility in BLG, performed locally using a scanning single electron transistor. We find that while the inverse compressibility is close to zero for $4 < |\nu| < 8$, it is markedly more negative in the lowest LL, $|\nu| < 4$. Moreover, within the lowest LL, the background inverse compressibility between integer filling also exhibits a stark even-odd asymmetry. It is more negative when starting to fill from an even filling factor than when starting to fill from an odd filling factor, exhibiting a $\nu \rightarrow \nu + 2$ symmetry and indicating the important role of the orbital degeneracy uniquely present in bilayer graphene. [Preview Abstract] |
Session D30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Optoelectronic Properties
Sponsoring Units: DMPChair: Marco Nardelli, University of North Texas
Room: 605
Monday, March 3, 2014 2:30PM - 2:42PM |
D30.00001: Electrically tunable optical emitter graphene coupling Lucas Orona, Klaas-Jan Tielrooij, Frank Koppens, Pablo Jarillo-Herrero Graphene exhibits both novel electronic and optical properties. Optical emitters near graphene experience non-radiative coupling that generates electron hole pairs in the graphene. We are able to vary the strength of this coupling by electrically gating the graphene. Strong gating raises the Fermi energy so that no more electrons can be excited by the emitters, essentially halting the non-radiative decay. My talk will be about experimental measures of this process, which we call Pauli blocking. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D30.00002: Large and Tunable Optical Absorption in Quasi-Periodically Corrugated Graphene Guang-Xin Ni, De Lima Ferreira Rodrigues Ma, Henrik Andersen, Seung-Jae Baeck, Jong-Hyun Ahn, Viana-Gomes Jose Carlos, Vitor M. Pereira, Castro Neto Antonio Helio, Barbaros \"Ozyilmaz Graphene is currently one of the notable players in the intense drive towards bendable, thin, and portable electronic displays. Given that the intrinsic transparency of a graphene monolayer is 97.7{\%}, any reproducible and controllable modulation of transparency can have a significant impact for graphene as a viable transparent conducting electrode. Here we demonstrate a large and tunable optical aborption modulation in large-scale CVD graphene by introducing quasi-periodic ripples using functional elastomer substrates. We find that the optical modulation is more than 15{\%} at visible wavelengths and moreover such optical modulation can be simultaneously tuned on and off by controlling the elastomer status. The simple device configuration and large tunability optical response of graphene demonstrated in this study can be very important towards novel ultra-thin optical polarizer devices applications. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D30.00003: Tunable Photocurrent and Photoresponsivity of Graphene/Silicon Carbide Field Effect Photodetectors Biddut K. Sarker, Isaac Childres, Edward Cazalas, Igor Jovanovic, Yong P. Chen Graphene is a promising material for a variety of optoelectronics applications due to its unique electronic and optical properties. In this talk, we present detailed photoresponse studies of a novel photodetector based on graphene field effect transistors on undoped silicon carbide substrates. We show that the photocurrent of our devices under 400 nm laser illumination is positive (negative) for a negative (positive) back gate-voltage and almost zero for zero gate-voltage. For a fixed gate-voltage, the photocurrent and photoresponsivity can be tuned by the power of the light, source-drain bias-voltage and the position of the laser beam on the devices. We propose that the photodetection mechanism of our devices relies on the high sensitivity of the resistivity of graphene to the local change of the electric field that can result from photoexcited carriers produced in the underlying semiconductor substrates. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D30.00004: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 3:18PM - 3:30PM |
D30.00005: Magnetic Field Effects of Light Scattering from Fluorinated Multi-Layer Graphene Flakes in Ground and Excited States Suspended in Organic Solvents Bin Hu, Lei He, Mingxing Li, Zheng Gai, Augustine Urbas Magnetic field effect of light scattering (MFELS) can be developed by magnetic polarization-induced alignment of magnetic nanoparticles suspended in liquid states. The MFELS can reflect the magnetic polarization in ground and excited states, when an external photoexcitation is applied, and the magnetoelectric coupling, when the host solvent is changed with a different dielectric constant, in magnetic nanoparticles. We report a giant MFELS with a magnitude over 80 {\%} from fluorinated multi-layer graphene (FG) suspended in organic solvents. Applying a magnetic field (\textless 900 mT) can remarkably increase the light scattering intensity from the suspended FG flakes. This indicates that a magnetic field can cause an alignment of suspended FG flakes due to anisotropic magnetic polarization effects. We find that increasing the dielectric constant of host solvent can largely enhance the MFELS magnitude. This phenomenon implies that the electrical polarization is intrinsically coupled with the magnetic polarization in the FG flakes, suggesting a new mechanism for magnetoelectric coupling. Furthermore, we show that a photoexcitation can lead to an enhancement on the MFELS magnitude from the suspended FG flakes. This indicates that the excited states can generate a stronger magnetic polarization through magnetoelectric coupling in the suspended FG flakes. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D30.00006: Spaser with graphene nanoribbon Oleg Berman, Roman Kezerashvili, Yurii Lozovik A novel type of spaser with the net amplification of surface plasmons (SPs) in doped graphene nanoribbon is proposed. The plasmons in THz region can be generated in a dopped graphene nanoribbon due to nonradiative excitation by emitters like two level quantum dots located along a graphene nanoribbon. The minimal population inversion per unit area, needed for the net amplification of SPs in a doped graphene nanoribbon is obtained. The dependence of the minimal population inversion on the surface plasmon wavevector, graphene nanoribbon width, doping and damping parameters necessary for the amplification of surface plasmons in the armchair graphene nanoribbon is studied. Besides, a new method for high-sensitivity plasmon spectroscopy is proposed based on the usage of a graphene spaser. The plasmon generation is suppressed and even break down near threshold due to absorption at the transition frequencies of the neighbouring nano-objects (molecules or clusters) under study. In result a dip in the spaser generation spectra appears. The sensitivity of this spaser spectroscopy near (slightly above) generation threshold can be very high. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D30.00007: Photocurrent Measurement of Multiple Top Gated Graphene Devices Trond Andersen, Qiong Ma, Nathaniel Gabor, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Inefficient electron-phonon relaxation in graphene results in long-lived hot carriers that proliferate over large spatial scales, associated with long-range energy and momentum transport. In order to investigate the propagation length of hot carriers, we report on photocurrent measurements using graphene devices with a global back-gate and multiple local top-gates. The purpose of the latter is to facilitate independent modulation of the Seebeck coefficient at different distances from the position of laser illumination. By varying the voltages of the top gates separately and measuring the change in photovoltage, we investigate the electronic temperature gradient at each gate through the Seebeck effect. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D30.00008: Scanning photocurrent microscopy of graphene subjected to a magnetic field Helin Cao, Grant Aivazian, Jason Ross, Sanfeng Wu, Pasqual Rivera, David Cobden, Xiaodong Xu The optoelectronic properties of graphene, converting light into photocurrent, are of great interest for both fundamental reasons and for device applications due to graphene's unique band structure and high carrier mobility. A photocurrent response has been observed in many previous measurements on a variety of graphene junction structures, such as single-to-bilayer junctions, p-n junctions, and graphene-metal junctions. Here we investigate the photocurrent response of graphene field-effect-transistor devices subjected to a perpendicular magnetic field. We will present scanning photocurrent microscopy results as a function of magnetic field and temperature, and discuss the underlying mechanisms. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D30.00009: Anisotropic photoinduced current injection in graphene Julien Rioux, John E. Sipe, Guido Burkard Quantum-mechanical interference effects are considered in carrier and charge current excitation in gapless semiconductors using coherent optical field components at frequencies $\omega$ and $2\omega$. Due to the absence of a bandgap, excitation scenarios outside of the typical operation regime are considered; we calculate the polarization and spectral dependence of these all-optical effects for single- and bilayer graphene. For linearly-polarized light and with one-photon absorption at $\omega$ interfering with $2\omega$ absorption and $\omega$ emission, the resulting current injection is five times stronger for perpendicular polarization axes compared to parallel polarization axes. This additional process results in an anisotropic current as a function of the angle between polarization axes, in stark contrast with the isotropic current resulting from the typical interference term in graphene [Rioux et al., PRB 83, 195406 (2011)]. Varying the Fermi level allows to tune the disparity parameter $d$ closer to typical values in GaAs [$|d|\approx 0.2$, Rioux and Sipe, Physica E 45, 1 (2012)]: from -1, when the additional process is fully Pauli-blocked, to -3/7, when it is fully accessible, thus facilitating polarization sensitive applications. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D30.00010: Investigating photoresponse in graphene by light polarization M. Eginligil, B.C. Cao, Z.L. Wang, C. Soci, T. Yu We report our photocurrent studies on single layer graphene (SLG), bilayer graphene (BLG) and trilayer graphene (TLG) by exciting with circularly polarized light. In addition to p-n junctions based on gated graphene field-effect-transistor (g-FET), it was recently demonstrated that in the graphene/metal interface large photocurrent (PC) can be generated and this PC can be manipulated by backgate voltage in a simple g-FET. In this work we fabricated g-FETs from mechanically exfoliated graphene and explored backgate voltage dependence of photon drag effect (PDE), linear and circular photogalvanic effect (CPGE) of SLG, BLG and TLG. In BLG, we noticed a cos$\theta $ dependence of the measured PC, where $\theta $ is the angle of incident light polarization, in addition to PDE and CPGE effects which have cos4$\theta $ and sin2$\theta $ dependence, respectively. This cos$\theta $ dependence is attributed to the Berry curvature related valley PC, which can be induced as a result of broken inversion symmetry and asymmetry in the two low energy valleys of BLG. The latter is absent in SLG and peculiar for ABA stacked TLG. By varying backgate voltage we distinguish all helicity dependent PC contributions. Our data show good agreement with the theory. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D30.00011: Highly sensitive hot electron bolometer based on disordered graphene Xiaosong Wu, Qi Han, Teng Gao, Rui Zhang, Yi Chen, Jianhui Chen, Gerui Liu, Yanfeng Zhang, Zhongfan Liu, Dapeng Yu A bolometer is a device that makes an electrical resistive response to the electromagnetic radiation resulted from a raise of temperature due to heating. The combination of the extremely weak electron-phonon interactions along with its small electron heat capacity makes graphene an ideal material for applications in ultra-fast and sensitive hot electron bolometer. However, a major issue is that the resistance of pristine graphene weakly depends on the electronic temperature. We propose using disordered graphene to obtain a strongly temperature dependent resistance. The measured electrical responsivity of the disordered graphene bolometer reaches $6\times10^6$ V/W at 1.5 K, corresponding to an optical responsivity of $1.6\times10^5$ V/W. The deduced electrical noise equivalent power is 1.2 fW/$\sqrt{\rm Hz}$, corresponding to the optical noise equivalent power of 44 fW/$\sqrt{\rm Hz}$. The minimal device structure and no requirement of high mobility for graphene make a step forward towards the applications of graphene hot electron bolometers. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D30.00012: Negative Photoconductivity and Carrier Heating in CVD Graphene James Heyman, Banteaymolu Alebachew, Andrew Banman, Zofia Kaminski, Rhyan Foo Kune, Jacob Stein, Aaron Massari, Jeremy Robinson Ultrafast photoexcitation of CVD graphene typically leads to a transient \textit{decrease} in conductivity. Previous reports identify two possible mechanisms for this decrease: carrier heating leading to a decrease in mobility, and a photo-induced population inversion producing a negative dynamic resistance. We present time-resolved THz transmission (TRTS) measurements which show that population inversion is not required to observe negative photoconductivity in CVD graphene and confirm the role of carrier heating. In $p$-type CVD graphene samples interband optical transitions are blocked for pump photon energies less than twice the Fermi energy. However, our pump photon-energy resolved TRTS measurements exhibit negative photoconductivity at all pump wavelengths investigated, indicating that interband excitation leading to population inversion is not required to observe this effect. In addition, we have performed TRTS measurements on CVD graphene in magnetic fields that separately probe carrier mobility and population. We find that negative photoconductivity following photoexcitation primarily arises from a decrease in carrier mobility, confirming the role of carrier heating. Research at NRL was supported by the Office of Naval Research. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D30.00013: Transport through Disordered Graphene Geometries in a Laser-Induced Floquet Topological State Arijit Kundu, Babak Seradjeh, Herbert Fertig Driving a system periodically can induce non-trivial topological properties. In graphene, for example, irradiation with circularly polarized laser can open up topological band-gaps. If current is injected from a lead with Fermi energy within that gap, for an open geometry electronic transport is mediated by topologically protected edge states. By contrast, in a periodic geometry (e.g., a nanotube), transport is dominated by evanescent modes below and above the gap. We study transport through these systems in the presence of disorder, which can result in remarkably large localization lengths and near-critical behavior with increasing disorder strength. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D30.00014: Growing up in the Spotlight: Optics of Graphene from Dot to Sheet Rico Pohle, Eleftheria Kavousanaki, Keshav Dani, Nic Shannon Graphene quantum dots have recently generated much interest due to their novel properties like the presence of zero-energy states, as well as their potential applications in quantum computing and bio-imaging. Here, we theoretically study triangular and hexagonal graphene quantum dots with zigzag and armchair edges within the tight binding model. We identify, and obtain the exact wave functions, for a class of highly degenerate electronic states with energy equal to the hopping parameter. We study the scaling of the degeneracy of these states versus dot size for the different types of dots, and understand their role in the optical absorption spectrum going from small quantum dots to the thermodynamic limit of an infinite graphene sheet. We investigate the role that these states play in connection to the excitons caused by the van Hove singularity in infinite graphene, and their influence on the nanoscale opto-electronic properties of graphene quantum dots. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D30.00015: Ghost Fano Resonance of Excitons in Twisted Bilayer Graphene Yufeng Liang Metallic systems are generally considered to be unable to harbor tightly bound excitons because of the strong screening effect as well as the absence of a finite band gap. Previously, exception has only been found in one-dimensional metallic carbon nanotubes due to the depressed screening effects and the symmetry gap. We explore the exciton spectra of twisted bilayer graphene (tBLG) and predict the existence of even more strongly bound exciton (with binding energy as large as ~0.5eV) in this system despite of its higher dimensionality. Based on our results from first-principles simulations and effective model calculations, a mechanism known as the ghost Fano resonance is proposed for the bound exciton formation in metallic systems beyond the dimensonality-related argument. Our results shed light on engineering the e-h excitations in the few-layer van der Waals heterojunction. [Preview Abstract] |
Session D31: Focus Session: Van der Waals Interactions in Complex Materials: Bridging Theory and Experiment I
Sponsoring Units: DMPChair: Alexandre Tkatchenko, Fritz Haber Institute, Germany
Room: 607
Monday, March 3, 2014 2:30PM - 3:06PM |
D31.00001: Modern theory of van der Waals interactions Invited Speaker: John Dobson van der Waals (vdW, dispersion) interactions [1,2,3] are important in diverse areas such as colloid, surface and nano science, cohesion of molecular crystals, and biomolecular science. They also provide competition in experiments to discover the fifth fundamental force.While vdW interactions have been understood in principle for a century, their quantitative first-principles prediction and modelling down to chemical contact separations have proven stubbornly difficult because the quantal many-electron problem is involved. After some brief historical material, the current state of the art will be discussed with particular reference to several approaches: pairwise additive [4,5,6], perturbative [7,8] quantum chemical [9], vdW-DF [10,11], Lifshitz-like scattering[1,2,12], RPA-like [13-17], Adiabatic Connection Fluctuation Dissipation / Time Dependent DFT based [18,19] etc.. A potentially useful classification will be introduced to aid in understanding the physical causes of departures from pairwise additivity, that is from the usual sum of $C_6 R^{-6}$ contributions. These departures result in non-standard power law decays of nanostructure vdW interactions as a function of separation $D$ [13], as well as surprising dependences of the attraction on the number, $N$, of atoms within each vdW-interacting fragment [20,21]. Some further recent results on non-additivity will also be presented [22]. REFERENCES. [1] V. A. Parsegian, ``van der Waals Forces,'' Cambridge University Press, Cambridge 2005: [2] J. F. Dobson and T. Gould, J. Phys. Condens. Matter 24, 073201 (2012): [3] J. Klimes \& A. Michaelides, J. Chem. Phys. 137, 120901 (2012) : [4] S. Grimme, J. Antony, S. Ehrlich, et al., J. Chem. Phys. 132, 154104 (2010): [5] S. Ehrlich, J. Moellmann, and S. Grimme, Acc. Chem. Research 46, 916 (2013): [6] R. Sedlak, T. Janowski, M. Pitonak et al., J. Chem. Th. Comput. 9, 3364 (2013): [7] B. Jeziorski, R. Moszynski, and K. Szalewicz, Chem. Rev. 94, 1887 (1994): [8] D. Kuchenbecker and G. Jansen, Chem. Phys. Chem. 13, 2769 (2012): [9] J. Rezac, L. Simova, and P. Hobza, J. Chem. Th. Comput. 9, 364 (2013): [10] M. Dion, H. Rydberg, E. Schroder, et al., Phys. Rev. Lett. 92, 246401 (2004): [11] K. Berland and P. Hyldgaard, Phys Rev B 87, 205421 (2013): [12] S. J. Rahi, T. Emig, N. Graham, et al., Phys. Rev. D 80, 085021 (2009): [13] J. F. Dobson, A. White and A. Rubio, Phys. Rev. Lett. 96, 073201 (2006): [14] S. Lebegue, J. Harl, T. Gould, et al., Phys. Rev. Lett. 105, 196401 (2010): [15] A. Tkatchenko, R. di Stasio, R. Car, et al., Phys. Rev. Lett. 108, 236402 (2012): [16] T. Bucko, S. Lebegue, J. Hafner, J. G. Angyan, Phys. Rev. B. 87, 064110 (2013): [17] A. Gruneis, J. Harl, M. Marsman, et al., J. Chem. Phys. 131, 154115 (2009): [18] T. Gould, J. Chem. Phys. 137, 111101 (2012): [19] T. Olsen and K. Thygesen, Phys. Rev. B 88, 115131 (2013): [20] A. Ruzsinszky, J. P. Perdew, J. Tao et al., Phys. Rev. Lett. 109, 233203 (2012): [21] V. V. Gobre and A. Tkatchenko, Nature Comm. 4, 2341 (2013): [22] J. F. Dobson, A. Savin, J.A. Angyan, and R.F. Liu, unpublished. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D31.00002: Van der Waals Forces in Quasi 1-D Structures David Drosdoff, Lilia Woods Analytical formulations of Van der Waals-Casimir forces in terms of the macroscopic response of the system have been done extensively for 2-D and 3-D systems. On the other hand, quasi 1-D materials have been studied less, in part because of the difficulty in solving the boundary conditions. In this talk, by using the RPA method, we present a formulation of the Van der Waals force in narrow infinitely long ribbons. This approach is applied in the quantum mechanical and thermal regimes to several typical systems, such as insulators, metals, and semiconductors. Novel results are found for graphene nanoribbons, for which a transition from quantum mechanical to thermal van der Waals force can be realized at room temperature by changing the chemical potential. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D31.00003: Van der Waals forces and electron-electron interactions in two strained graphene layers Anand Sharma, Peter Harnish, Alexander Sylvester, Valeri N. Kotov We evaluate the van der Waals (vdW) interaction energy at T=0 between two undoped graphene layers which are separated by a finite distance. Our study is carried out within the Random Phase Approximation and the interaction energy is obtained for variation in the strength of effective Coulomb interaction and anisotropy due to applied uniaxial strain. We consider the following three models for the anisotropic case: a) where one of the two layers is uniaxially strained, b) the two layers are strained in the same direction, and c) one of the layers is strained in the perpendicular direction. We find that for all the three models and any given value of the coupling, the vdW interaction energy increases with increasing anisotropy. The effect is most striking for the case when both the layers are strained in the parallel direction where we observe up to an order of magnitude increase in the strained graphene relative to the unstrained case. We also investigate the effect of intra-layer electron-electron interactions in the region of large separation between the strained graphene layers. We conclude that the many-body contributions to the correlation energy are non-negligible and the vdW interaction energy decreases as a function of increasing distance between the layers. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D31.00004: Observation of Pull-in Instability in Graphene Membranes under Interfacial Forces Xinghui Liu, Narasimha Boddeti, Mariah Szpunar, Luda Wang, Miguel Rodriguez, Rong Long, Jianliang Xiao, Martin Dunn, Scott Bunch We present a unique experimental configuration that allows us to determine the interfacial forces on nearly parallel plates made from single and few layer graphene membranes. Our approach consists of using a pressure difference across a graphene membrane to bring the membrane to within $\sim$ 10-20 nm above a circular post covered with SiO$_{x}$ or Au until a critical point is reached whereby the membrane snaps into adhesive contact with the post. Continuous measurements of the deforming membrane with an AFM coupled with a theoretical model allow us to deduce the magnitude of the interfacial forces between graphene and SiO$_{x}$ and graphene and Au. The nature of the interfacial forces at $\sim$ 10 - 20 nm separations is consistent with an inverse fourth power distance dependence, implying that the interfacial forces are dominated by van der Waals interactions. Furthermore, the strength of the interactions is found to increase linearly with the number of graphene layers. The experimental approach can be applied to measure the strength of the interfacial forces for other emerging atomically thin two-dimensional materials. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D31.00005: How many-body effects modify the van der Waals interaction between graphene sheets John Dobson, Tim Gould, Giovanni Vignale Cold undoped graphene sheets were previously predicted [1,2], via Random Phase approximation (RPA) arguments, to exhibit an unusual asymptotic van der Waals (vdW) interaction energy $E = - KD^{-3}$ where $D$ is the (large) separation between the two parallel graphene sheets. This is compared with $D^{-5/2}$ for 2D metals [3] and $D^{-4}$ for 2D insulators [3]. Here we show [4] that graphene is the first known system where effects beyond the RPA should make QUALITATIVE changes to the vdW force. For large separations, $D>10 nm$ where only $\pi_z$-mediated vdW forces remain, we predict that the vdW interaction is substantially reduced from the RPA prediction, and has a different power law. This new $D$ dependence is very sensitive to the form of the long-wavelength many-body renormalization of the velocity of the massless Dirac fermions, and may provide independent confirmation of the latter. We will briefly discuss issues involved in possible experiments. \\[4pt] [1] J.F. Dobson, A. White and A. Rubio, Phys. Rev. Lett. 96, 073201 (2006).\\[0pt] [2] T. Gould and J. F. Dobson, Phys. Rev. B 87, 165422 (2013).\\[0pt] [3] M. Bostrom and B. E., Sernelius, Phys. Rev. B 61. 2204 (2000).\\[0pt] [4] J.F. Dobson, T. Gould and G. Vignale, ArXiv 1306.4716 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D31.00006: Engineering Graphene Pseudospin Structure with Van der Waals Coupling Chenhao Jin, Zhiwen Shi, Wei Yang, Long Ju, Jason Horng, Guangyu Zhang, Feng Wang Electrons in graphene are described by relativistic Dirac-Weyl spinors with two-component pseudospin. The unique pseudospin structure leads to emerging phenomena such as the massless Dirac cone, anomalous quantum Hall effect, and Klein tunneling. The capability to manipulate electron pseudospin is highly desirable for novel graphene electronics, and is recently achieved by van der Waals coupling to substrate such as graphene/BN and twisted bilayer graphene. We calculate the van der Waals coupled graphene/substrate system and show that the pseudospin structure can be modified in several ways, which will lead to distinctive experimental results. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D31.00007: Scaling Laws for van der Waals Interactions in Nanostructured Materials Vivekanand Gobre, Alexandre Tkatchenko Van der Waals (vdW) forces originate from interactions between fluctuating multipoles in matter and play a significant role in the structure and stability of nanostructured materials. Many models used to describe vdW interactions in nanomaterials are based on a simple pairwise-additive approximation, neglecting the strong electrodynamic response effects caused by long-range fluctuations in matter. We develop and utilize an efficient microscopic method [1,2] to demonstrate that vdW interactions in nanomaterials act at distances greater than typically assumed, and can be characterized by different scaling laws depending on the dimensionality and size of the system. Specifically, we study the behaviour of vdW interactions in single-layer and multilayer graphene, fullerenes of varying size, single-wall carbon nanotubes and graphene nanoribbons. As a function of nanostructure size, the van der Waals coefficients follow unusual trends for all of the considered systems, and deviate significantly from the conventionally employed pairwise-additive picture. We propose that the peculiar van der Waals interactions in nanostructured materials could be exploited to control their self-assembly. [1] Tkatchenko, DiStasio, Car, and Scheffler, PRL (2012); [2] Gobre, Tkatchenko, Nat. Commun. (2013). [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D31.00008: Cohesion Energetics of Carbon Allotropes: Quantum Monte Carlo Study Hyeondeok Shin, Sinabro Kang, Jahyun Koo, Hoonkyung Lee, Jeongnim Kim, Yongkyung Kwon We have performed quantum Monte Carlo calculations to study the cohesion energetics of carbon allotropes, including \textit{sp}$^{\mathrm{3}}$-bonded diamond, \textit{sp}$^{2}$-bonded graphene, \textit{sp}-\textit{sp}$^{\mathrm{2}}$ hybridized graphynes, and \textit{sp}-bonded carbyne. The computed cohesive energies of diamond and graphene are found to be in excellent agreement with the corresponding values determined experimentally for diamond and graphite, respectively, when the zero-point energies, along with the interlayer binding in the case of graphite, are included. We have also found that the cohesive energy of graphyne decreases systematically as the ratio of \textit{sp}-bonded carbon atoms increases. The cohesive energy of $\gamma $-graphyne, the most energetically-stable graphyne, turns out to be 6.766(6) eV/atom, which is smaller than that of graphene by 0.698(12) eV/atom. Experimental difficulty in synthesizing graphynes could be explained by their significantly smaller cohesive energies. Finally we conclude that the cohesive energy of a newly-proposed two-dimensional carbon network can be accurately estimated with the carbon-carbon bond energies determined from the cohesive energies of graphene and three different graphynes. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D31.00009: van der Waals torque Raul Esquivel-Sirvent, George Schatz The theory of generalized van der Waals forces by Lifshtz when applied to optically anisotropic media predicts the existence of a torque. In this work we present a theoretical calculation of the van der Waals torque for two systems. First we consider two isotropic parallel plates where the anisotropy is induced using an external magnetic field. The anisotropy will in turn induce a torque. As a case study we consider III-IV semiconductors such as InSb that can support magneto plasmons. The calculations of the torque are done in the Voigt configuration, that occurs when the magnetic field is parallel to the surface of the slabs. The change in the dielectric function as the magnetic field increases has the effect of decreasing the van der Waals force and increasing the torque. Thus, the external magnetic field is used to tune both the force and torque. The second example we present is the use of the torque in the non retarded regime to align arrays of nano particle slabs. The torque is calculated within Barash and Ginzburg formalism in the nonretarded limit, and is quantified by the introduction of a Hamaker torque constant. Calculations are conducted between anisotropic slabs of materials including BaTiO3 and arrays of Ag nano particles. Depending on the shape and arrangement of the Ag nano particles the effective dielectric function of the array can be tuned as to make it more or less anisotropic. We show how this torque can be used in self assembly of arrays of nano particles. ref. R. Esquivel-Sirvent, G. C. Schatz, Phys. Chem C, 117, 5492 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D31.00010: Extended disperson-corrected atom-centered potential (DCACP) approach for treating long-range dispersion interactions in clusters and solids Kenneth Jordan, Ozan Karalti, Wissam Al-Saidi The DCACP method of Rothlisberger and co-workers[1] is one of several strategies for correcting density functional theory for dispersion interactions. The DCACP approach, which involves the use of additional terms in standard pseudopotentials, has proven very successful near the potential energy minima of molecular dimers but gives an interaction energy that falls off much too rapidly as the separation between the monomers is increased. In our work, we extend the DCACP approach for H, C, N, and O to include two angular momenta channels in the pseudopotentials rather than one in the original DCACP method (an idea originally explored by Rothlisberger for (H$_{\mathrm{2}})_{\mathrm{2}})$.[2] We show that this approach, which we designate as DCACP2, significantly improves the description of long-range dispersion interactions. [1] O. A. von Lilienfeld, I. Tavernelli, U. Rothlisberger, and D. Sebastiani, Phys. Rev. Lett., 93, 153004. (2004). [2] I. Tavernelli, I.-C. Lin, and U. Rothlisberger, Phys. Rev. B, 79, 045106, (2009). [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D31.00011: Exchange-correlation functionals for non-covalent interactions Alberto Otero de la Roza, Erin Johnson, Gino DiLabio Dispersion, an essential component of non-covalent interactions, is a long-range correlation effect. The non-covalent binding energies calculated using common density functionals vary widely from overly repulsive to spuriously attractive, and there is no a priori clear recipe for choosing any particular functional. Dispersion in DFT is, as a consequence, as much about calculating the dispersion energy accurately as it is about using a base density functional that gives the correct repulsive wall for all interaction types. In the context of pairwise dispersion corrections, this has been addressed by (over)using the dispersion damping function. In this talk, I present a study on the adequacy of different exchange and correlation approximations for non-covalent interactions as well as an analysis of the energy error scaling with system size. I will show, for instance, that cooperative effects in densely hydrogen-bonded systems (e.g. ice) are consistently overestimated by all density-functional approximations. Our results are relevant regarding the accuracy of molecular dynamics simulations, molecular crystal phase transitions, the scaling of non-covalent interactions to systems of biological interest, and the design of new base functionals for non-covalent interactions. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D31.00012: Long range correlation energy from coupled atomic response functions Alberto Ambrosetti, Alexandre Tkatchenko Electron correlation is an elusive and ubiquitous energy contribution that arises from transient many-body electron excitations. Its reliable (accurate and efficient) modeling is essential for correctly describing cohesive, structural, and response properties of molecules and solids. In this regard, the main challenge is to model the long-range correlation energy beyond (semi-)local density-functional approximations. Here we propose an efficient method to compute the long-range correlation energy for non-metallic molecules and solids, by using coupled atomic response functions (ARF). Extending the recent MBD method [1], we separate the coupling between ARFs into short and long range, allowing seamless treatment of weakly and strongly polarizable systems. Thorough benchmarking on large data sets including small molecules (S22, S66x8), supramolecular complexes (S12L), molecular crystals (X23) and graphite shows consistently good agreement with high level theoretical and experimental reference data (of the order of 6$\%$). The uniform accuracy for molecules and solids represents a strong validation of our method, and further confirms the importance of modeling the truly collective nature of the long-range correlation energy. [1] A. Tkatchenko et al. PRL {\bf 108} 236402 (2012). [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D31.00013: Pair-Wise and Many-Body Dispersive Interactions Coupled to an Optimally Tuned Range-Separated Hybrid Functional Leeor Kronik, Piyush Agrawal, Alexandre Tkatchenko We propose a nonempirical, pair-wise or many-body dispersion-corrected, optimally tuned range-separated hybrid functional. This functional retains the advantages of the optimal-tuning approach in the prediction of the electronic structure. At the same time, it gains accuracy in the prediction of binding energies for dispersively bound systems, as demonstrated on the S22 and S66 benchmark sets of weakly bound dimers. [Preview Abstract] |
Session D32: Invited Session: Quantum Simulation and Computation with AMO Systems
Sponsoring Units: GQI DAMOPChair: Ivan Deutsch, Unversity of New Mexico
Room: 708-712
Monday, March 3, 2014 2:30PM - 3:06PM |
D32.00001: Quantum simulations with trapped ions Invited Speaker: Rainer Blatt The basic tool box of the Innsbruck quantum computer based on a string of trapped Ca$^{\mathrm{+}}$ ions will be reviewed. The quantum toolbox is applied to carry out both analog and digital quantum simulations. In this talk, the basic simulation procedure will be presented and its application will be discussed for a variety of spin Hamiltonians. Including a carefully controlled dissipation mechanism, the toolbox allows for the quantum simulation of open systems. A string of ions is used to implement a quantum system that interacts by means of quantum gate operations with an additional ancilla ion which in turn is coupled to the environment in a well-controlled way. Thus, entangled states, such as Bell and GHZ states can be generated by dissipative processes and can be used as part of a quantum simulator. Recent experimental results on the simulation of competing coherent and dissipative processes will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D32.00002: New Frontiers for Quantum Simulation in Optical Lattices Invited Speaker: David Weld Quantum simulation experiments exploit an analogy between some interesting (generally solid-state) system and some well-controlled quantum mechanical ensemble, typically consisting of atoms, ions, or photons. This analogy is a two-way street, enabling insights into the behavior of strongly correlated electrons but also enabling the application of powerful condensed-matter experimental techniques such as adiabatic demagnetization or dilution refrigeration to ultracold gases. I will discuss some prospects and challenges for quantum simulation experiments with neutral atoms in optical lattices. Initial directions in this field included the study of metal-insulator transitions and magnetic systems. Emerging possibilities include experiments relevant to topologically nontrivial materials, quasicrystals, impurities, and nonequilibrium phenomena. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D32.00003: Progress in Quantum Information Processing with Trapped Ions at NIST Invited Speaker: Dietrich Leibfried This talk will provide an overview of the progress in quantum information processing (QIP) with trapped ions at NIST. In particular, improvements of ion transport and cooling within a scalable architecture for QIP, experiments entangling the internal states of ions held in separate trapping wells and the realization of Bell-state pumping, where an entangled steady-state of two ions emerges as the result of partly dissipative interactions, will be discussed.\\[4pt] For the recent work done at NIST I gratefully acknowledge important contributions by David Allcock, Jim Bergquist, Brad Blakestad, Shaun Burd, John Bollinger, Ryan Bowler, Sam Brewer, Joe Britton, Kenton Brown, Jwo-Sy Chen, James Chou, Shon Cook, Yves Colombe, Dustin Hite, Wayne Itano, Robert Joerdens, John Jost, Emanuel Knill, Shlomi Kotler, David Leibrandt, Yiheng Lin, Katherine McCormick, Kyle McKay, Christian Ospelkaus, David Pappas, Daniel Slichter, Brian Saywer, Ting Rei Tan, Andrew Wilson, David Wineland and funding by DARPA, IARPA, ARO, ONR and the NIST Quantum Information Program. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D32.00004: Dynamical Analogue Quantum Simulators Invited Speaker: Jens Eisert Complex quantum systems out of equilibrium are at the basis of a number of long-standing questions in physics. This talk will be concerned on the one hand with recent progress on understanding how quantum many-body systems out of equilibrium eventually come to rest, thermalise and cross phase transitions, on the other hand with dynamical analogue quantum simulations using cold atoms probing such questions [1-4]. In an outlook, we will discuss the question of certification of quantum simulators, and will how this problem also arises in other related settings, such as in Boson samplers [5,6].\\[4pt] [1] S. Braun, M. Friesdorf, S. S. Hodgman, M. Schreiber, J. P. Ronzheimer, A. Riera, M. del Rey, I. Bloch, J. Eisert, U. Schneider, in preparation (2014).\\[0pt] [2] M. Kliesch, M. Kastoryano, C. Gogolin, A. Riera, J. Eisert, arXiv:1309:0816.\\[0pt] [3] S. Trotzky, Y.-A. Chen, A. Flesch, I. P. McCulloch, U. Schollwoeck, J. Eisert, I. Bloch, Nature Physics 8, 325 (2012).\\[0pt] [4] A. Riera, C. Gogolin, M. Kliesch, J. Eisert, in preparation (2014).\\[0pt] [5] C. Gogolin, M. Kliesch, L. Aolita, J. Eisert, in preparation (2014) and arXiv:1306.3995.\\[0pt] [6] S. Aaronson, A. Arkhipov, arXiv:1309.7460. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D32.00005: Quantum simulation with cold molecules Invited Speaker: Ana Maria Rey Recent experimental developments on cooling, trapping, manipulating and loading ultra-cold ground state molecules in an optical lattice have opened the door for the exploration of quantum magnetism and the observation of complex quantum dynamics. In this talk I will discuss recent developments towards the implementation of controllable spin lattice models in polar molecules with the spin degrees of freedom encoded in rotational states. The spin-spin couplings are generated by direct dipolar interactions and can be fully controlled by dc electromagnetic fields and microwaves. The spin models realized in this way are long range, anisotropic and can even feature direction-dependent spin interactions. They can emulate Hamiltonians ranging from the Heisenberg spin model, to Hamiltonians with symmetry protected topological phases to Hamiltonians without solid state counterpart. At JILA we have been able to realize for the first time a lattice spin model with fermionic KRb molecules pinned in a 3D lattice. We observe clear manifestation of dipolar exchange interactions in Ramsey spectroscopy even at substantially less than unit lattice filling. I will describe the new theoretical methods that we developed to model the spin dynamics and show that those reproduce the experimental observations. Even though so far the spin dynamics has been restricted to pinned molecules, in part to prevent chemical reactions, I will finish by presenting theoretical calculations supported by experimental measurement at JILA that demonstrate that the continuous quantum Zeno mechanism can actually suppress loss in this highly reactive system. This finding opens the exciting possibility of observing itinerant quantum magnetism in near term experiments. [Preview Abstract] |
Session D33: Focus Session: Quantum Foundations: What Powers Quantum Advantages for Information Processing?
Sponsoring Units: GQIChair: Joseph Emerson, University of Waterloo
Room: 706
Monday, March 3, 2014 2:30PM - 2:42PM |
D33.00001: Contextuality supplies the magic for Quantum Computation Mark Howard, Joel Wallman, Victor Veitch, Joseph Emerson Quantum computers are poised to deliver a dramatic increase in computational power, which can be used to perform difficult tasks such as simulating molecules for medical research much more efficiently than any current computer. However, it is notoriously difficult to characterize what is needed for a quantum computer to be useful. In this paper we prove that two characteristic quantum phenomena, namely, negative probabilities and contextuality, are equivalent in the most well-known and promising architecture for fault-tolerant quantum computation using $d$-level quantum systems ($d$ odd prime). Together with recent work, this implies that contextuality is necessary for quantum computers based upon this architecture to outperform any current computer. Our results are also relevant to the question of identifying the largest ``classical'' subtheory of quantum mechanics. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D33.00002: Mixing nonclassical pure states in a linear-optical network almost always generates modal entanglement Zhang Jiang, Mattihas Lang, Carlton Caves In quantum optics a pure state is considered classical, relative to the statistics of photodetection, if and only if it is a coherent state. A different and newer notion of nonclassicality is based on modal entanglement. One example that relates these two notions is the Hong-Ou-Mandel effect, where modal entanglement is generated by a beamsplitter from the nonclassical photon-number state $\vert 1 \rangle \otimes\vert 1\rangle$. This suggests the beamsplitter or, more generally, linear-optical networks as a mediator of the two notions of nonclassicality. We show the following: Given a nonclassical pure product state input to an $N$-port linear-optical network, the output is almost always mode entangled; the only exception is a product of squeezed states, all with the same squeezing strength, input to a network that does not mix the squeezed and anti-squeezed quadratures. Our work thus gives a necessary and sufficient condition for a linear network to generate modal entanglement from pure product inputs, a result that is of immediate relevance to the boson sampling problem. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D33.00003: Quantum computational universality of Affleck-Kennedy-Lieb-Tasaki states beyond the honeycomb lattice Tzu-Chieh Wei Universal quantum computation can be achieved by simply performing single-spin measurements on a highly entangled resource state, such as cluster states. The family of Affleck-Kennedy-Lieb-Tasaki (AKLT) states has recently been explored; for example, the spin-1 AKLT chain can be used to simulate single-qubit gate operations on a single qubit, and the spin-3/2 two-dimensional AKLT state on the honeycomb lattice can be used as a universal resource. However, it is unclear whether such universality is a coincidence for the specific state or a shared feature in all two-dimensional AKLT states. Here we consider the family of spin-3/2 AKLT states on various trivalent Archimedean lattices and show that in addition to the honeycomb lattice, the spin-3/2 AKLT states on the square octagon $(4,8^2)$ and the `cross' $(4,6,12)$ lattices are also universal resources, whereas the AKLT state on the `star' $(3,12^2)$ lattice is likely not due to geometric frustration. Ref. T.-C. Wei, arXiv:1306.1420. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D33.00004: Local orthogonality as a foundational principle Invited Speaker: Antonio Acin |
Monday, March 3, 2014 3:42PM - 3:54PM |
D33.00005: A measure of Quantum Unspeakable Information Davide Girolami A piece of information is said unspeakable if it cannot be encoded into a sequence of bits. For example, the transformation law between the coordinates of two distant laboratories cannot be specified without a shared reference frame. This condition has been proven to be equivalent to constrain local operations in the two labs by means of a superselection rule [Rev. Mod. Phys. 79, 555 (2007)]. I introduce a measure of unspeakable information based on the skew information [PNAS 49, 910 (1963)], which evaluates the ability of a quantum state to act as a reference frame under a specific superselection rule. Then, I show that evaluating unspeakable information is equivalent to measuring the amount of quantum coherence of a state with respect to a given basis. I propose a proof of concept experiment in optical set-up to evaluate the amount of unspeakable information, i.e. of relative coherence, of a quantum state without fully reconstructing its density matrix. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D33.00006: Contextuality in Measurement-based Quantum Computation Robert Raussendorf We show, under natural assumptions for qubit systems, that measurement-based quantum computations (MBQCs) which compute a non-linear Boolean function with high probability are contextual. The class of contextual MBQCs includes an example which is of practical interest and has a super-polynomial speedup over the best known classical algorithm, namely the quantum algorithm that solves the `Discrete Log' problem. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D33.00007: Scalable Implementation of Boson Sampling with Trapped Ions Chao Shen, Zhen Zhang, Luming Duan Boson sampling solves a classically intractable problem by sampling from a probability distribution given by matrix permanents. We propose a scalable implementation of Boson sampling using local transverse phonon modes of trapped ions to encode the Bosons. The proposed scheme allows deterministic preparation and high-efficiency readout of the Bosons in the Fock states and universal mode mixing. With the state-of-the-art trapped ion technology, it is feasible to realize Boson sampling with tens of Bosons by this scheme, which would outperform the most powerful classical computers and constitute an effective disproof of the famous extended Church-Turing thesis. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D33.00008: Whirling waves in Interference experiments Urbasi Sinha, Rahul Sawant, Joseph Samuel, Aninda Sinha, Supurna Sinha In a double slit interference experiment, the wave function at the screen with both slits open is not exactly the sum of the wave functions with the slits individually open one at a time. The three scenarios represent three different boundary conditions and as such, the superposition principle should not be applicable. However, most well- known text books in quantum mechanics implicitly and/or explicitly use this assumption, the wave function hypothesis, which is only approximately true. In our present study, we have used the Feynman path integral formalism to quantify contributions from non-classical paths in interference experiments which provide a measurable deviation from the wave function hypothesis [1]. A direct experimental demonstration for the existence of these non-classical paths is hard. We find that contributions from such paths can be significant and we propose simple three-slit interference experiments to directly confirm their existence. I will also describe some ongoing experimental efforts towards testing our theoretical findings. \\[4pt] [1] Whirling waves in interference experiments, R.Sawant, J.Samuel, A.Sinha, S.Sinha and U.Sinha, arXiv: 1308.2022. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D33.00009: Quantum Computing from Addition-rule-based Cellular Automata Cheng Wu We argue that addition rules must be imposed first for a general-purpose quantum computing. This brings the addition operation into the architecture of one-dimensional cellular automaton with dual bits per cell. The four symbolic substitution rules are transformed into a 16 specific right-nearest-neighbor cell-to-cell rules. Thus addition operation is only one set out of roughly 4.3 billion available sets to be found in the cellular automaton. When the half-adder fundamental processors are wired together differently and become addition-rule incompatible, gliders or oscillations between two configurations will result as a new kind of science. Those examples as well as the cellular automaton's connection to the main-stream qubit approach will be presented and discussed. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D33.00010: Long-Range Entanglement Is Necessary for a Topological Storage of Quantum Information Isaac Kim A general inequality between entanglement entropy and a number of topologically ordered states is derived, even without using the properties of the parent Hamiltonian or the formalism of topological quantum field theory. Given a quantum state, we obtain an upper bound on the number of distinct states that are locally indistinguishable from it. The upper bound is determined only by the entanglement entropy of some local subsystems. As an example, we show that $\log N \leq 2\gamma$ for a large class of topologically ordered systems on a torus, where $N$ is the number of topologically protected states and $\gamma$ is the constant subcorrection term of the entanglement entropy. We discuss applications to quantum many-body systems that do not have any low-energy topological quantum field theory description, as well as tradeoff bounds for general quantum error correcting codes. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D33.00011: Quantum mechanics over sets David Ellerman In models of QM over finite fields (e.g., Schumacher's ``modal quantum theory'' MQT), one finite field stands out, ${\rm Z}_{2}$, since ${\rm Z}_{2}$ vectors represent sets. QM (finite-dimensional) mathematics can be transported to sets resulting in quantum mechanics over sets or QM/sets. This gives a full probability calculus (unlike MQT with only zero-one modalities) that leads to a fulsome theory of QM/sets including ``logical'' models of the double-slit experiment, Bell's Theorem, QIT, and QC. In QC over ${\rm Z}_{2}$ (where gates are non-singular matrices as in MQT), a simple quantum algorithm (one gate plus one function evaluation) solves the Parity SAT problem (finding the parity of the sum of all values of an n-ary Boolean function). Classically, the Parity SAT problem requires 2$^{\mathrm{n}}$ function evaluations in contrast to the one function evaluation required in the quantum algorithm. This is quantum speedup but with all the calculations over ${\rm Z}_{2}$ \textit{just like classical computing}. This shows definitively that the source of quantum speedup is \textit{not} in the greater power of computing over the complex numbers, and confirms the idea that the source is in superposition. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D33.00012: Sudden Decoherence Transitions for Quantum Discord Hyungjun Lim, Robert Joynt We formulate the computation of quantum discord in terms of the generalized Bloch vector, which gives useful insights on the time evolution of quantum coherence for the open system, particularly the comparison of entanglement and discord. We present an efficient numerical method to calculating the quantum discord for a certain important class of multipartite states, and show that the analytical calculation of the global geometric discord is NP-hard in the number of qubits. In agreement with previous work for 2-qubit cases, (Mazzola \textit{et al.} Phys. Rev. Lett. 104, 200401 (2010)), we find situations where under decohering influences there is a sudden transition from classical to quantum decoherence characterized by the discord remaining relatively robust until a certain point from which it begins decaying quickly. However, we find that as the number of qubits increases, the chance of this kind of transition occurring becomes small. This work was supported in part by ARO (W911NF-12-1-0607). [Preview Abstract] |
Session D34: Solar Fuels, Biofuels, and PEC
Sponsoring Units: GERAChair: Sue Carter, University of California, San Diego
Room: 704
Monday, March 3, 2014 2:30PM - 2:42PM |
D34.00001: High-throughput characterization for solar fuels materials discovery Slobodan Mitrovic, Natalie Becerra, Earl Cornell, Dan Guevarra, Joel Haber, Jian Jin, Ryan Jones, Kevin Kan, Martin Marcin, Paul Newhouse, Edwin Soedarmadji, Santosh Suram, Chengxiang Xiang, John Gregoire In this talk I will present the status of the High-Throughput Experimentation (HTE) project of the Joint Center for Artificial Photosynthesis (JCAP). JCAP is an Energy Innovation Hub of the U.S. Department of Energy with a mandate to deliver a solar fuel generator based on an integrated photoelectrochemical cell (PEC). However, efficient and commercially viable catalysts or light absorbers for the PEC do not exist. The mission of HTE is to provide the accelerated discovery through combinatorial synthesis and rapid screening of material properties. The HTE pipeline also features high-throughput material characterization using x-ray diffraction and x-ray photoemission spectroscopy (XPS). In this talk I present the currently operating pipeline and focus on our combinatorial XPS efforts to build the largest free database of spectra from mixed-metal oxides, nitrides, sulfides and alloys. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D34.00002: The effect of surfactant on the formation and combustion of methane hydrates Jeffrey Botimer, Peter Taborek, Sunny Karnani, Derek Dunn-Rankin Methane hydrates are an abundant and globally distributed fuel source that has potential to play an important role in the worlds energy economy. We have used optical imaging to study the effects of surfactant on the kinetics of formation and the combustion of methane hydrates. We grow hydrates from liquid water in methane gas at 275K and 1000psi. The hydrate growth front propagates into the vapor rather than into the liquid. We have investigated the effect of wetting properties of the substrate on the growth of the hydrate. The combustion of hydrates is complicated by the requirement of draining away the melting water during combustion. The surfactant complicates the combustion process further because it inhibits the drainage of water. We have investigated this process as a function of surfactant concentrations and ambient pressure. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D34.00003: Properties of Solar Thermal Fuels by Accurate Quantum Monte Carlo Calculations Kayahan Saritas, Can Ataca, Jeffrey C. Grossman Efficient utilization of the sun as a renewable and clean energy source is one of the major goals of this century due to increasing energy demand and environmental impact. Solar thermal fuels are materials that capture and store the sun's energy in the form of chemical bonds, which can then be released as heat on demand and charged again. Previous work on solar thermal fuels faced challenges related to the cyclability of the fuel over time, as well as the need for higher energy densities. Recently, it was shown that by templating photoswitches onto carbon nanostructures, both high energy density as well as high stability can be achieved. In this work, we explore alternative molecules to azobenzene in such a nano-templated system. We employ the highly accurate quantum Monte Carlo (QMC) method to predict the energy storage potential for each molecule. Our calculations show that in many cases the level of accuracy provided by density functional theory (DFT) is sufficient. However, in some cases, such as dihydroazulene, the drastic change in conjugation upon light absorption causes the DFT predictions to be inconsistent and incorrect. For this case, we compare our QMC results for the geometric structure, band gap and reaction enthalpy with different DFT functionals. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D34.00004: Synergy between cellulolytic enzymes during the biodegradation of cellulose microfibrils measured using angle-scanning surface plasmon resonance (SPR) imaging Adam Raegen, Alexander Dion, Kyle Reiter, Anthony Clarke, Jacek Lipkowski, John Dutcher The use of cellulosic ethanol, a promising emerging energy source, is limited by the energy intensive and costly step of first converting the cellulose fibers into their constituent glucose monomers. Industrial processes mimic those that occur in nature, using mixtures or ``cocktails'' of different classes of cellulolytic enzymes derived from fungi. Despite several decades of investigation, the molecular mechanisms for enzyme synergy remain poorly understood. To gain additional insight, we have used a custom angle-scanning surface plasmon resonance (SPR) imaging apparatus to obtain a sensitive measure of enzymatic degradation. By implementing a novel SPR data analysis procedure, we have been able to track the thickness and roughness of laterally heterogeneous cellulose microfibril-coated substrates as enzymatic degradation proceeds. This has allowed us to measure the synergistic actions of the different enzymes, providing data that are directly relevant to the cellulosic ethanol industry. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D34.00005: First-principles quantum-mechanical investigations of biomass conversion at the liquid-solid interfaces Hongli Dang, Wenhua Xue, Yingdi Liu, Friederike Jentoft, Daniel Resasco, Sanwu Wang We report first-principles density-functional calculations and \textit{ab initio} molecular dynamics (MD) simulations for the reactions involving furfural, which is an important intermediate in biomass conversion, at the catalytic liquid-solid interfaces. The different dynamic processes of furfural at the water-Cu(111) and water-Pd(111) interfaces suggest different catalytic reaction mechanisms for the conversion of furfural. Simulations for the dynamic processes with and without hydrogen demonstrate the importance of the liquid-solid interface as well as the presence of hydrogen in possible catalytic reactions including hydrogenation and decarbonylation of furfural. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D34.00006: Fundamental efficiency limit for solar thermal fuels David A. Strubbe, Yun Liu, Jeffrey C. Grossman Solar thermal fuels (STFs) are an unconventional paradigm for solar energy conversion and storage which is attracting renewed attention: a material absorbs sunlight and stores the energy chemically via an induced structural change, which can later be reversed to release the energy as heat. An example is the azobenzene molecule which has a cis-trans photoisomerization with these properties, and can be tuned by chemical substitution and attachment to templates such as carbon nanotubes. By analogy to the Shockley-Queisser limit for photovoltaics (PV), we analyze the maximum attainable efficiency for STFs. The below-gap, above-gap, and recombination losses are similar to PV, but there are additional considerations about further losses, quantum yield, photostationary state, and interrelation with the storage lifetime, another key performance metric for STFs. We show constraints on feasible potential-energy surfaces for STFs, and compare to ab initio calculations and experimental measurements of the properties of STF materials. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D34.00007: Energy-Efficient Bioalcohol Recovery by Gel Stripping Rutvik Godbole, Lan Ma, Ronald Hedden Design of energy-efficient processes for recovering butanol and ethanol from dilute fermentations is a key challenge facing the biofuels industry due to the high energy consumption of traditional multi-stage distillation processes. Gel stripping is an alternative purification process by which a dilute alcohol is stripped from the fermentation product by passing it through a packed bed containing particles of a selectively absorbent polymeric gel material. The gel must be selective for the alcohol, while swelling to a reasonable degree in dilute alcohol-water mixtures. To accelerate materials optimization, a combinatorial approach is taken to screen a matrix of copolymer gels having orthogonal gradients in crosslinker concentration and hydrophilicity. Using a combination of swelling in pure solvents, the selectivity and distribution coefficients of alcohols in the gels can be predicted based upon multi-component extensions of Flory-Rehner theory. Predictions can be validated by measuring swelling in water/alcohol mixtures and conducting h HPLC analysis of the external liquid. 95{\%}$+$ removal of butanol from dilute aqueous solutions has been demonstrated, and a mathematical model of the unsteady-state gel stripping process has been developed. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D34.00008: Ab initio molecular dynamics simulations for the role of hydrogen in catalytic reactions of furfural on Pd(111) Wenhua Xue, Hongli Dang, Yingdi Liu, Friederike Jentoft, Daniel Resasco, Sanwu Wang In the study of catalytic reactions of biomass, furfural conversion over metal catalysts with the presence of hydrogen has attracted wide attention. We report \textit{ab initio} molecular dynamics simulations for furfural and hydrogen on the Pd(111) surface at finite temperatures. The simulations demonstrate that the presence of hydrogen is important in promoting furfural conversion. In particular, hydrogen molecules dissociate rapidly on the Pd(111) surface. As a result of such dissociation, atomic hydrogen participates in the reactions with furfural. The simulations also provide detailed information about the possible reactions of hydrogen with furfural. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D34.00009: Tin Nitride as an Earth Abundant Photoanode for Water Splitting Christopher Caskey, Ming Ma, Vladan Stephanovic, Stephan Laney, David Ginley, Ryan Richards, Wilson Smith, Andriy Zakutayev Photoelectrochemical (PEC) water splitting--the conversion of water to hydrogen and oxygen using light--is an attractive route to the chemical storage of solar energy. We demonstrate that spinel tin nitride (Sn$_{3}$N$_{4}$) has conduction and valence bands that straddle the redox potentials of water and we study it as a photoannode material. Sn$_{3}$N$_{4}$ thin films have been grown on glass at ambient temperature by reactive sputtering of tin in a nitrogen atmosphere. The resulting materials were n-type semiconductors. Carrier concentration, carrier mobility, work function, and optical properties were measured. Results indicate that tin nitride has a band gap of $\sim$ 1.7 eV aligned around water's redox potentials. GW-corrected DFT-surface calculations that take into account water surface dipole interactions are consistent with experiment. Early PEC devices were made from Sn$_{3}$N$_{4}$ on fluorinated tin oxide with cobalt oxide catalysts and show a small but promising photoresponse ($\sim$ 0.1 mA/cm$^{2}$ at 1.23 V vs. RHE) under AM 1.5 illumination in 0.1 M potassium phosphate (pH= 7.25). Further work will focus on increasing the photocurrent in tin nitride devices by increasing film quality and identifying the proper catalyst. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D34.00010: First-principles interpretation of core-level spectroscopy of photoelectrochemical materials and processes Sri Chaitanya Das Pemmaraju, David Prendergast We present two case studies of first-principles theoretical methods applied in conjunction with experimental core-level spectroscopy measurements to investigate the electronic structure and dynamical processes in molecular and interfacial systems relevant to photoelectrochemical (PEC) technologies. In the first [1], we study the core-level and valence spectroscopies of two zinc(II)-porphyrin based Donor-pi-Acceptor (D-p-A) [2] dyes using the occupancy-constrained excited electron and core-hole (XCH) [3] approach and time-dependent density functional theory (TDDFT) simulations. In the second, we use constrained DFT and TDDFT to interpret measured transient core-level shifts in time-resolved femtosecond x-ray photoelectron spectroscopy, investigating the dynamics of the electron injection process from a N3 dye molecule chemisorbed onto a ZnO substrate. These studies illustrate the utility of first-principles methods in guiding the design of better PEC materials. References: [1] Zegkinoglou, I et al, J. Phys. Chem. C, J. Phys. Chem. C, 2013, 117, 13357 [2] Yella, A. et al, Science 2011, 334, 629. [3] Prendergast, D and Galli, G; Phys. Rev. Lett. 2006, 96, 215502. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D34.00011: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 4:42PM - 4:54PM |
D34.00012: Opto-electronic properties of Ta$_3$N$_5$: a joint experimental and theoretical study Juliana Morbec, Dario Rocca, Blaise Pinaud, Thomas Jaramillo, Giulia Galli Tantalum nitride (Ta$_3$N$_5$) is considered a promising material for use in photoelectrochemical cells, due to its suitable band gap for visible light absorption and favorable band-edge positions for water splitting. However, Ta$_3$N$_5$ films have been recently shown to exhibit low photocurrent (i.e. less than 50\% of the theoretical limit). We report a joint experimental and ab initio theoretical study of the opto-electronic properties of Ta$_3$N$_5$, aimed at understanding possible reasons for the limited photocurrent. Our experimental optical spectra of films with different thicknesses show two absorption edges at 2.1 and 2.5 eV. To provide an interpretation of these features, we performed ab initio calculations, at several levels of theory, of the electronic band structure and optical absorption spectra of Ta$_3$N$_5$. We employed density functional theory with semi-local (PBE/LDA) and hybrid (PBE0/HSE06) functionals and many body perturbation theory at the G$_0$W$_0$ level, and we obtained optical spectra by solving the Bethe-Salpeter equation within density matrix perturbation theory. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D34.00013: Transient performance modeling of photoelectrochemical cells incorporating nano-structured photoanodes Sophia Haussener, Mikael Dumortier Photoelectrochemical processes constitute a viable route for renewable hydrogen production. Several practical systems that provide separated product streams of hydrogen and oxygen follow the design guidelines established in the fuel cell community and have been demonstrated. We developed a transient 1D numerical model of a photoelectrochemical fuel cell-based device, which incorporates a complex, nano-structured photoanode. The model accounted for radiation transport, charge carrier transport, species transport, fluid flow and electrochemical reactions. We investigated the transient performance under varying illumination, species mix and phases, and examined different types of nanostructured photoanodes, i.e. based on carbon-paper infiltrated with photoactive particles or carbon nanotubes uniformly covered by photoactive layers. Our model predicted the operational-dependent and experimentally observed four regimes of the transient photocurrent: anodic overshoot at illumination start, current increase, current decrease, and cathodic undershoot. The numerical results supported the hypothesis that the transient behavior of the photocurrent at startup was dominated by the low initial concentration of hydrogen and oxygen. The validated model developed provides a useful tool for system design and operational guidelines to avoid these regimes and keep the device at a stable operation and at a maximum efficiency. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D34.00014: Visible-light absorption in 2D covalent triazine framework: enhanced by interlayer coupling and pore size increasing Xue Jiang, Jijun Zhao Compared with traditional bulk materials, two-dimensional (2D) crystals have some intrinsic advantages as photocatalysis owing to the limited thickness and large surface area. So far, many monolayer materials have been shown to be potential photocatalysis for water splitting from both theoretical calculations and experiments; while most of them are inorganic materials. In contrast, g-carbon nitride, as a starting successful case, motivates us to explore 2D organic semiconductors, which have not yet well investigated. Using first principles calculations, we predicted a family of 2D covalent triazine framework (CTF) as a promising visible-light-driven photocatalyst by studying their electronic structures, work function, CBM/VBM position, and optical absorption spectra. Moreover, we found that multilayer CTF have much better visible-light adsorption than a single layer induced by the interlayer coupling. In addition, controlled construction of such CTF from suitable organic subunit pave the way for connection between the optical energy gap of CTF and pore size. The insights from our study not only enrich the family of organic semiconductor photocatalyst, but also are very helpful in designing and assembling CTF subunits for optimal performance. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D34.00015: Role of oxometallic complex on OH dissociation during water oxidation: A microscopic insight from DFT study Mukul Kabir, Soumyajit Sarkar, Martha Greenblatt, Tanusri Saha-Dasgupta The uncatalyzed atomic dissociation of water requires breaking strong O$-$H bond with enthalpy 494 KJ/mol, which necessitates understanding and designing appropriate catalysts. Here we employ transition state theory within quantum chemical density functional theory to understand the role of metal-oxide inorganic complexes to the OH$\rightarrow$ O + H process, the most important reaction in water oxidation. We study the effect of (a) chemical bonding in different M$_4$O$_4$ (M = Mn, Co) cubane complexes, (b) heterocubane geometry containing Ca, in addition to transition metal ion, (c) dimensionality by considering both three dimensional and two dimensional geometry of the oxometallic unit, and (d) connectivity between two oxometallic cubane units, corner shared versus edge shared geometry. Analysis of our density function theory based calculations singles out a robust microscopic quantity among various plausible and competing factors, which elucidates the important role of metal-oxygen covalency at the oxidized site. The M$-$O bonding strength inversely determines the strength of the O$-$H bond, and thus the energy required for OH dissociation. This provides one with an important microscopic design principle for metal-oxide complex catalyst responsible for water oxidation. [Preview Abstract] |
Session D35: Focus Session: Superconducting Qubits: Simulation & Annealing
Sponsoring Units: GQIChair: Jay Gambetta, IBM
Room: 702
Monday, March 3, 2014 2:30PM - 3:06PM |
D35.00001: Catch and Release of Microwave Photons Invited Speaker: Yi Yin Quantum information is often encoded in photons, which can both propagate freely along transmission lines and be stored in cavity resonators. To store photons efficiently, the resonator should have negligible coupling with the outside world. On the other hand, the resonator should be strongly coupled to a transmission line through which photons can be transmitted and received. These contrary requirements can be resolved with adjustable coupling. We experimentally demonstrate a superconducting resonator with variable coupling to a measurement transmission line. The resonator coupling can be adjusted through zero to a photon emission rate 300 times the intrinsic resonator decay rate. We demonstrate the catch and shaped release of microwave photons as well as the control of nonclassical Fock states. We achieve a high-fidelity catch efficiency (99.4{\%}) for a ``time-reversed'' shaped photon. These results will enable high fidelity quantum state transfer between distant cavities. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D35.00002: Imaging the mode structure of a kagome lattice of superconducting resonators with a scanning defect Devin Underwood, Will Shanks, Andy C.Y. Li, Jens Koch, Andrew Houck It has been theoretically shown that a lattice of coupled electromagnetic cavities each strongly coupled to a two-level system exhibit quantum phase transitions of polaritons. Such a system consists of a lattice of coupled sites each described by the Jaynes-Cummings Hamiltonian. The circuit quantum electrodynamics architecture is a natural choice for such experiments because of the ease of fabrication, and the easily obtainable strong coupling limit. In these systems an important first step is to build and understand a large photonic lattice of microwave resonators without qubits. Here we present measurements of the mode structure of microwave photons in an array of 49 niobium CPW resonators that are capacitively coupled to form a kagome lattice. Our method for extracting the mode structure is a piece of sapphire mounted to a three-axis positioning stage that we bring into contact with each resonator. This scanning defect locally perturbs each lattice site and the shifted transmission spectrum can then be used as a metric to extract the internal mode structure. When compared to calculations from a tight binding Hamiltonian, measured modes show good agreement. These results demonstrate our ability to fabricate and understand large lattices of microwave resonators. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D35.00003: Circuit-QED-based superconducting quantum simulator for the Holstein-polaron model Feng Mei, Vladimir Stojanovi\'{c}, Irfan Siddiqi, Lin Tian We propose an analog quantum simulator for the Holstein molecular-crystal model based on a superconducting circuit-QED system in the dispersive regime. The many-body Hamiltonian of this model includes both bosonic and fermionic degrees of freedom. By varying the driving field on the superconducting resonators, one can readily access both the adiabatic and anti-adiabatic regimes of this model, and reach the strong e-ph coupling limit required for small-polaron formation. We show that small-polaron state of arbitrary quasimomentum can be generated by applying a microwave pulse to the resonators. We also show that significant squeezing in the resonator modes can be achieved in the polaron-crossover regime through a measurement-based scheme. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D35.00004: Dynamics of macroscopic quantum self-bound states in arrays of transmon qubits Claudia De Grandi, Steven M. Girvin We consider the many-body physics of an array of transmon qubits in a cavity. Due to the negative anharmonicity and the exchange coupling between the qubits, such a system realizes a Bose-Hubbard model with attractive interactions and thus the $N$-excitation manifold is expected to have self-bound states. We study the existence of such macroscopic states in the one-dimensional case with open boundary conditions as a function of the parameters of the model, comparing the classical and the quantum predictions. We then analyze the dynamics of the self-bound states in the experimentally relevant scenario of an open dissipative system, where the qubits have a finite energy relaxation time $T_1$. We numerically simulate the dynamics with a quantum trajectory approach supported by a Lanczos diagonalization procedure. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D35.00005: Detecting elementary excitations of a quantum simulator with superconducting resonator Lianghui Du, J.Q. You, Lin Tian Analog quantum simulators can emulate various many-body systems and can be used to study novel quantum correlations in such systems. One essential question in quantum simulation is how to detect the properties of the simulated many-body system, such as ground state property and spectrum of elementary excitations. Here we present a circuit QED approach for detecting the excitation spectrum of a quantum simulator by measuring the correlation spectrum of a superconducting resonator. For illustration, we apply this approach to a simulator for the transverse field Ising model coupling to a coplanar waveguide resonator. The simulator can be implemented with an array of superconducting flux qubits. We show that the resonance peaks in the correlation spectrum reveal exactly the frequencies of the excitations. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D35.00006: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 4:06PM - 4:18PM |
D35.00007: Simulating quantum field theories with superconducting circuits Antonio Mezzacapo, Guillermo Romero, Laura Garc\'Ia-\'Alvarez, Jorge Casanova, Lucas Lamata, Enrique Solano In this contribution, we present the quantum simulation of fermionic field modes interacting via a continuum of bosonic modes with superconducting circuits. Unlike many quantum technologies, superconducting circuits offer naturally the continuum of bosonic modes by means of one-dimensional transmission lines. In particular, we consider a simplified version of 1+1 quantum electrodynamics (QED), which may describe Yukawa interactions, and the coupling of fermions to the Higgs field. Our proof-of-principle proposal is designed within the state-of-the-art circuit QED technology, where fermionic fields are encoded in superconducting flux qubits, in a scalable approach that may lead to a full-fledged quantum simulation of quantum field theories. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D35.00008: Digital Quantum Simulation of Heisenberg Spin-Spin Interactions with Superconducting Qubits Y. Salathe, M. Mondal, P. Kurpiers, M. Oppliger, L. Steffen, S. Filipp, A. Wallraff, A. Mezzacapo, U. Las Heras, L. Lamata, E. Solano A major application of a scalable quantum computer is the simulation of intricate quantum systems, including spin models, which cannot be carried out efficiently on classical computers for more than a few tens of qubits. The Heisenberg model describes a spin system that cannot be obtained directly from available interactions in circuit QED. Nevertheless, it can be achieved by a stroboscopic decomposition in terms of elementary gates in a digital quantum simulation approach. In our experiments, we digitally simulate a system of two spin-1/2 particles interacting via an isotropic Heisenberg XYZ interaction in the circuit QED architecture. The XYZ interaction is decomposed into a set of discrete two-qubit gates based on the exchange interaction mediated by the dispersive coupling of both qubits to a common cavity mode. The state evolution during the simulation is analyzed tomographically after each step for varying interaction strengths. This technique can be extended to general spin models, such that our experiments represent a first step towards the digital quantum simulation of larger spin systems with controllable lattice topology. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D35.00009: Transmon-based simulator of nonlocal electron-phonon coupling: a platform for observing sharp small-polaron transitions Vladimir Stojanovic, Eugene Demler, Mihajlo Vanevic, Lin Tian We propose an analog simulator for a one-dimensional model with momentum-dependent (nonlocal) electron-phonon couplings of Su-Schrieffer-Heeger and ``breathing-mode'' types. The superconducting circuit behind this simulator entails an array of transmon qubits and microwave resonators. Using a microwave-driving based protocol, small-polaron Bloch states with arbitrary quasimomentum can be prepared in this system within times several orders of magnitude shorter than the qubit decoherence time. We show that -- by varying the circuit parameters -- one can readily reach the critical coupling strength for observing the sharp transition from a nondegenerate single-particle ground state at zero quasimomentum ($K_{\textrm{gs}}=0$) to a twofold degenerate small-polaron ground state corresponding to equal and opposite (nonzero) quasimomenta $K_{\textrm{gs}}$ and $-K_{\textrm{gs}}$. Through exact diagonalization of our effective model, we show how this nonanalyticity is reflected in the relevant single-particle properties (ground-state energy, quasiparticle residue, average number of phonons). Our work paves the way for understanding the physical implications of strongly momentum-dependent electron-phonon interactions. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D35.00010: Simulating systems of itinerant spin-carrying particles using arrays of superconducting qubits and resonators Sahel Ashhab We propose potential setups for the quantum simulation of itinerant spin-carrying particles in a superconducting qubit-resonator array. These proposals include the use of multiple polariton branches, multiple resonator modes and multiple qubits coupled to each resonator. We argue that a combination of using multiple qubits and multiple resonator modes is a promising route in this context, allowing the simulation of external magnetic fields and various forms of spin-dependent inter-site hopping, including spin-orbit coupling. This proposal could be implemented in state-ofthe-art superconducting circuits in the near future. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D35.00011: Quantum Simulation with Arrays of Transmon Qubits Shay Hacohen-Gourgy, Vinay Ramasesh, Oliver Viehmann, Jan von Delft, Florian Marquardt, Irfan Siddiqi We present progress toward quantum simulation of one-dimensional spin chains using planar transmon qubits in a circuit QED architecture. In particular, we discuss the Ising model as realized by an array of capacitively-coupled transmon qubits with the terminal qubit dispersively coupled to a microwave resonator. We engineer an approximation to the Ising Hamiltonian with the ground and excited states playing the role of spin-up and spin-down atoms. We present preliminary spectroscopic data and coherent manipulations in chains of varying length. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D35.00012: Novel Architecture for High Speed and High Fidelity Readout of a Quantum Annealing Processor Fabio Altomare, Andrew J. Berkley, Richard Harris, Emile M. Hoskinson, Mark W. Johnson, Trevor M. Lanting, Sergey Uchaikin, Jed D. Whittaker, Paul Bunyk, Elena Tolkacheva, Ilya Perminov Hysteretic dc SQUIDs provide an easy method to read the state of hundreds of qubits\footnote{Supercond. Sci. Technol. \textbf{23}, 105014 (2010)}. However, this approach becomes impractical for circuits with an even larger number of qubits due to heating when dc SQUIDs switch, the relatively slow retrapping dynamics of high quality devices, and suboptimal scaling of the number of control lines with increasing numbers of qubits. The D-Wave Two processor uses an architecture that addresses all three of these issues. This new architecture makes use of Quantum Flux Parametron based shift registers that transfer the classical information produced as the output of the quantum annealing algorithm to a small number of fast non-dissipative and high fidelity microwave readout devices. We will provide an introduction to our implementation, and present data pertaining to readout performance from a 512-qubit quantum annealing processor. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D35.00013: Programmable flux DACs in a Quantum Annealing Processor Emile M. Hoskinson, Fabio Altomare, Andrew J. Berkeley, Paul Bunyk, Richard Harris, Mark W. Johnson, Trevor M. Lanting, Elena Tolkacheva, Ilya Perminov, Sergey Uchaikin, Jed D. Whittaker Programming the D-Wave Two processor to solve a given problem involves adjustment of thousands of independent flux biases. This is accomplished with an array of 4480 on-chip digital-to-analog converters (DACs), addressed using 56 external lines. Each DAC comprises a superconducting loop and control circuitry that allows injection of a deterministic number of flux quanta, up to a maximum value determined by the device parameters and the addressing scheme. In-depth characterization is performed to determine DAC transfer-functions and the addressing levels needed for fast and reliable programming. In contrast with traditional single-flux-quanta (SFQ) circuitry, zero static power during programming is dissipated on-chip, allowing efficient operation at mK temperatures. [Preview Abstract] |
Session D36: Focus Session: Semiconductor Qubits: QED Interfaces & Multi-Qubit Coupling
Sponsoring Units: GQIChair: Thaddeus Ladd, HRL
Room: 703
Monday, March 3, 2014 2:30PM - 3:06PM |
D36.00001: Conversion of angular momentum from single photons to single electron spins in electrically controlled quantum dots Invited Speaker: Akira Oiwa Electrical controllability of gate-defined quantum dots (QDs) has brought significant developments in the coherent manipulation of electron spins and two-qubit gate operation toward scalable qubits for quantum computations. Moreover the suitability of gate-defined QDs to quantum information technologies would be considerably enhanced if spin states in the gate-defined QDs could couple to photon states coherently. Here we show that the photon polarization can couple to the spin degree of freedom in gate-defined GaAs QDs. Double QDs were fabricated in AlGaAs/GaAs quantum wells. By synchronizing a pulse laser irradiation with a charge sensing measurement we performed the real-time single photoelectron spin detection in the double QD. First we show that the resonant inter-dot tunneling can offer a robust detection scheme of the single photoelectrons trapped in the double QDs [1]. In the two-electron regime, the inter-dot tunneling of the photoelectrons strongly depends on the relative spin orientation (parallel or anti-parallel) of the two QDs. Therefore by combining the resonant inter-dot tunneling scheme with the Pauli spin effect, we have realized the nondestructive detection of single photoelectron spins. Finally, we demonstrate the angular momentum conversion from single photons to single electron spins in the double QD from the dependence of the detected spins on the incident photon polarization. \\[4pt] This work was done in collaboration with T. Fujita, K. Morimoto, G. Allison, M. Larsson, H. Kiyama, S. Teraoka, S. Haffouz, D. G. Austing, A. Ludwig, A. D. Wieck and S. Tarucha\\ [4pt] [1] T. Fujita et al, Phys. Rev. Lett. 110, 266803 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D36.00002: Entanglement Purification with the Exchange Interaction Adrian Auer, Guido Burkard Entanglement purification techniques provide means to create qubit pairs of arbitrary high fidelity with respect to a maximally entangled state, consuming a larger number of low fidelity pairs. So-called recurrence protocols act iteratively on two or more qubit pairs to produce one pair with higher fidelity, using local unitary operations, measurements, and communication of the measurement results. In this talk, we present a purification protocol that works with two input pairs and solely uses a single pulsed Heisenberg-type qubit interaction, therefore being especially suitable for spin qubits in tunnel-coupled quantum dots. In contrast to previously known protocols, we allow for asymmetric bilateral operations where the two communication parties operate differently on their qubits. In the most efficient version of our protocol, the local two-qubit interactions correspond to the $\sqrt{\textsc{swap}}$ gate and its inverse, which are the natural entangling gates generated from a Heisenberg-type interaction. Furthermore, we show how the same fidelity gain can be reached using $XY$-type interactions. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D36.00003: Driven nonlinear dynamics of two coupled exchange-only qubits Arijeet Pal, Emmanuel Rashba, Bertrand Halperin Exchange-only (RX) qubits are a promising candidate for the fundamental unit of a quantum computer. Recently, such a qubit has been experimentally realized and its complete two-axis control demonstrated in a system of exchange coupled triple dots [1, 2]. The next step is to establish the control of two such coupled RX qubits. We have explored the dynamical effects of two capacitively-coupled RX qubits. We formulate the Hamiltonian for two capacitively-coupled RX qubits constructed from six dots where they are arranged in different geometries. Under the conditions of resonant driving of one of the qubits, the other qubit serves as a detector of the coupling. When driven strongly even a modest strength of interaction can result in nonlinear effects and putatively make the control of two-qubit entanglement irregular. In this regard the different geometries give rise to substantially disparate responses which will be relevant for future experiments in these systems. 1. Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit, J. Medford et. al., Nature Nanotechnology 8, 654 (2013) 2. Quantum-Dot-Based Resonant Exchange Qubit, J. Medford et al., PRL 111, 050501 (2013) [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D36.00004: Theoretical Characterization of Nonlocal Two-qubit Operations for Electrostatically Coupled Singlet-Triplet Qubits Fernando Calderon, Jason Kestner Singlet-triplet qubits are an attractive candidate for implementing a quantum processor because of their scalability and fast control. In this system, entangling inter-qubit interactions can be performed via electrostatic coupling. It is an open question whether a single square pulse of the system's evolution operator can perform a maximally entangling operation or not. Using Makhlin's invariants [1], which characterize the nonlocal part of 2-qubit unitary transformations, and a geometric representation of those local invariants, we will give a description of the gates that can be directly generated by this particular Hamiltonian and its suitability for performing a maximally entangling gate. \\[4pt] [1] Y. Makhlin, ``Nonlocal properties of two-qubit gates and mixed states, and the optimization of quantum computations,'' Quantum Inf. Process., vol. 1, no. 4, 2002. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D36.00005: Long-range, low-noise gates for dopant and quantum dot spin qubits V. Srinivasa, H. Xu, J. Medford, J. M. Taylor Coupling spins by exchange interactions provides a rapid, tunable method of entanglement generation. However, this necessarily occurs only at short distances, and often incurs susceptibility to charge noise. To address these challenges, we consider two approaches. First, we investigate the coupling of two qubits localized on spatially separated impurity atoms or quantum dots. We show that a third multi-electron, multi-level quantum dot can mediate an exchange interaction between the qubits that is tunable via gate voltage control of level splittings and tunneling amplitudes. This approach suggests an experimentally accessible method for coupling donor electron spins in silicon via a hybrid impurity-dot system. Second, we discuss the resonant exchange (RX) qubit, defined within a triple quantum dot in the three-electron regime. Electric field control of the dipole moment of the RX qubit at microwave frequencies enables single-qubit and two-qubit gates that are protected against low-frequency charge noise. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D36.00006: Coherently driven double-quantum dot at finite bias: Analogy with lasers and beyond Manas Kulkarni, Ovidiu Cotlet, Yinyu Liu, Karl Petersson, George Stehlik, Jason Petta, Hakan Tureci Hybrid circuit-QED systems consisting of a double-quantum dot (DQD) coupled to a microwave resonator provide a unique platform to explore non-equilibrium impurity physics with coupled light-matter systems. We present a theoretical and experimental study of photonic and electronic transport properties of such a system. We obtain a Hamiltonian and the Liouvillian super-operators considering systematically both the presence of phonons and the effect of leads at finite voltage bias. We subsequently derive analytical expressions for transmission, phase response, photon number and nonequilibrium steady state electron current and show that the system realizes an unconventional version of a single-atom laser. Our analytical results are compared to numerically exact ones establishing regimes of validity of various analytical models. Finally, we compare our findings to experimental measurements. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D36.00007: Coupling InSb quantum dots to a superconducting microwave resonator Maja Cassidy, Jakob Kammhuber, Diana Car, Sebastien Plissard, Erik Bakkers, Leo DiCarlo, Leo Kouwenhoven We present measurements of a superconducting half-wave resonator coupled to two InSb nanowire quantum dots. Precise nanowire alignment at the electric field antinodes at opposite ends of the microwave cavity allows for a maximal electric field along the wire axis, without compromising the intrinsic quality factor of the cavity. This architecture may be useful for reaching the strong coupling limit between a single spin and a microwave photon, paving the way to on-chip coupling of single spins for quantum information processing. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D36.00008: Out of Equilibrium Charge dynamics in a cQED Architecture Jeremie Viennot, Matthieu Delbecq, Matthieu Dartiailh, Audrey Cottet, Takis Kontos In the context of circuit quantum electrodynamics, recent developments made it possible to build hybrid circuits [1], including many types of quantum dots. The versatility of these systems allows us to explore several directions, from quantum information engineering to many-body physics, all in a circuit QED architecture. I will present some of the experiments of our group where a carbon nanotube-based double quantum dot is coupled to a microwave cavity. We demonstrate an efficient electron confinement in this carbon nanotube, allowing us to bring the system at resonance with the cavity and used it as a charge Qbit. We characterise the response of this circuit out of equilibrium, either at finite bias or large microwave power [2]. We are also able to perform microwave spectroscopy of the device. Combined with ferromagnet interface exchange Zeeman fields, such a control should enable us to go towards spin-photon coupling and spin qubit experiments for circuit QED [3]. References : [1] M.R. Delbecq et al. Nature Comm., 4, 1400 (2013). [2] J.J. Viennot et al., arXiv:1310.4363 [3] A. Cottet et al., Phys. Rev. Lett. 105, 160502 (2010). [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D36.00009: Ultra-low power microwave manipulation of electron spin ensembles A.J. Sigillito, H. Malissa, A.M. Tyryshkin, S.A. Lyon Superconducting coplanar waveguide (CPW) resonators are a promising alternative to standard dielectric resonators for many electron spin resonance experiments. Their high sensitivity and low power requirements make them particularly well suited to applications where the sample volume is small and when microwave heating is a concern. Experiments utilizing rectangular pulses are possible with a peak microwave power of less than 1uW for 400ns pi-rotations, and under 100 uW of peak power for 40ns pi-rotations. However, CPW resonators have an inherently inhomogeneous microwave magnetic field (B$_{\mathrm{1}})$. Therefore, to uniformly rotate all spins in a sample, adiabatic microwave pulses must be used. Here we report on the use of such pulses to correct B$_{\mathrm{1}}$ inhomogeneities spanning an order of magnitude. We also present data indicating single shot sensitivity to 1x10$^{\mathrm{7}}$ phosphorus donors in isotopically enriched $^{\mathrm{28}}$Si at 1.7K. These show that superconducting CPW resonators are fully compatible with experiments requiring rapid manipulation of spins in dilution refrigerators. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D36.00010: Extreme Harmonic Generation in an InAs Spin-Orbit Qubit J. Stehlik, M.D. Schroer, M.Z. Maialle, M.H. Degani, J.R. Petta Strong spin-orbit materials have shown great promise in the field of quantum computation. Unlike conventional semiconductor materials, fast all-electrical control is achieved through electric dipole spin resonance (EDSR). In this work we explore EDSR in an InAs nanowire spin-orbit qubit. We observe signs of harmonic generation where spin flips occur at the resonance condition $n h f = g \mu_{\rm B} B$, where $f$ is the applied frequency, $B$ is the magnetic field, $g$ is the $g$-factor and $n$ is an integer. Near the interdot charge transition we observe harmonics up to $n$ = 8, indicating extreme harmonic generation. At far detuning we only observe the $n=1$ resonance. Further, we find odd/even structure in the harmonic response: odd harmonics result in an increase in the leakage current while even harmonics result in its suppression. Finally we observe oscillations in the resonant current as a function of detuning. The striking detuning dependence suggests that the harmonics may be caused by Landau-Zener transitions occurring due to the anti-crossing between the differing charge states. Numerical simulations of the system are qualitatively consistent with this picture. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D36.00011: Minimal-excitation states for electron quantum optics using levitons Preden Roulleau, Thibaut Jullien, Julie Dubois, Fabien Portier, Patrice Roche, Antonella Cavanna, Yong Jin, Werner Wegscheider, D. Christian Glattli The on-demand generation of pure quantum excitations is important for the operation of quantum systems, but it is particularly difficult for a system of fermions. This is because any perturbation affects all states below the Fermi energy, resulting in a complex superposition of particle and hole excitations. However, it was predicted nearly 20 years ago that a Lorentzian time-dependent potential with quantized flux generates a minimal excitation with only one particle and no hole. Here we report that such quasiparticles (hereafter termed levitons) can be generated on demand in a conductor by applying voltage pulses to a contact. Partitioning the excitations with an electronic beam splitter generates a current noise that we use to measure their number. Minimal-excitation states are observed for Lorentzian pulses, whereas for other pulse shapes there are significant contributions from holes. Further identification of levitons is provided in the energy domain with shot-noise spectroscopy, and in the time domain with electronic Hong--Ou--Mandel noise correlations. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D36.00012: Observation of dark states in a superconductor diamond quantum hybrid system Xiaobo Zhu, Yuichiro Matsuzaki, Robert Amsuss, Kosuke Kakuyanagi, Takaaki Shimo-Oka, Norikazu Mizuochi, Kae Nemoto, William J. Munro, Kouichi Semba, Shiro Saito We observed a remarkably sharp resonance ($\sim$ 1 MHz) at 2.878 GHz in the spectrum of flux qubit NV-diamond hybrid quantum system under zero external magnetic field. This width is much narrower than that of both the flux-qubit and spin-ensemble. We show this resonance is evidence of a collective dark state in the ensemble which is coherently driven by the superposition of clockwise and counter-clockwise macroscopic persistent super-currents owing in the flux qubit. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D36.00013: Correlations in Charge Transfer and Photon Emission by a Double Quantum Dot Connected to High Quality Resonator Canran Xu, Maxim Vavilov We analyze the full counting statistics of charge transfer and photon emission by a double quantum dot (DQD) coupled to a high-quality microwave resonator by electric dipole interaction. We show that at the resonant condition between the energy splitting of the DQD and the photon energy in the resonator, charge and photon statistics exhibits both a sub-Poissonian distribution and antibunching. In the ideal case, when the system decoherence stems only from photodetection, the photon noise is reduced below one-half of the noise for the Poisson distribution and is consistent with current noise. Our analysis justifies that sub-Poissonian photon noise occurs when the cross-correlation between emitted photons and electrons is strong. We demonstrate that Josephson junction based photomultipliers can be used to experimentally assess statistics of emitted photons. [Preview Abstract] |
Session D37: Focus Session: Carbon Nanotubes: Atom Mobility, Mechanical Response & Adsorption
Sponsoring Units: DMPChair: Mercedes Calbi, University of Denver
Room: 705/707
Monday, March 3, 2014 2:30PM - 3:06PM |
D37.00001: Controlling Atomic Movement on the Nanoscale Invited Speaker: Sinisa Coh Some of the grand challenges in nanoscience are the ability to control movement of atoms either to propel nanometer-sized machines, or to synthesize novel electronic devices and materials. To that end, electrical current can be used to move a wide range of metals (Fe, Cu, W, In, Ga) along the outside and inside of a carbon nanotube. In this talk I will present our finding of a peculiar way in which these metals move. For example, we find that an iron nanocrystal is able to pass through a constriction in the carbon nanotube with a smaller cross-sectional area than the nanocrystal itself. Remarkably, through in situ transmission electron imaging and diffraction, we find that, while passing through a constriction, the nanocrystal remains largely solid and crystalline and the carbon nanotube is unaffected. We account for this behavior by a pattern of iron atom motion and rearrangement on the surface of the nanocrystal. The nanocrystal motion can be described with a model whose parameters are nearly independent of the nanocrystal length, area, temperature, and electromigration force magnitude. I will also discuss implications of this work on synthesis of nanocomposite materials, and on the stability of carbon-based electronic devices. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D37.00002: Multi-Wall Carbon Nanotubes as Lithium Nanopipettes and SPM Probes Jonathan Larson, Satyaveda Bharath, William Cullen, Janice Reutt-Robey A multi-walled carbon nanotube (MWCNT) - terminated SPM cantilever, was utilized to perform nanolithography and surface diffusion measurements on a thin film of vapor-deposited lithium atop a silicon (111) substrate under ultra-high vacuum conditions. In these investigations the MWCNT tip was shown to act as both a lithium nanopipette and a probe for non-contact atomic force microscopy (NC-AFM) measurements. With the application of appropriate bias conditions, the MWCNT could site-selectively extract (expel) nano-scale amounts of lithium from (to) the sample surface. Depressions, mounds, and spikes were generated on the surface in this way and were azimuthally symmetric about the selected point of pipetting. Following lithium transfer to/from the substrate, the MWCNT pipette-induced features were sequentially imaged with NC-AFM using the MWCNT as the probe. Vacancy pits of ca. 300 nm diameter and 1.5 nm depth were observed to decay on a timescale of hours at room temperature, through diffusion-limited decay processes. A continuum model was utilized to simulate the island decay rates, and the lithium surface diffusion coefficient of D$=$7.5 ($\pm$1.3)*10$^{-15}$ cm$^{2}$/s was extracted. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D37.00003: Micro-tweezers for studying vibrating carbon nanotubes Arthur Barnard, Mian Zhang, Michal Lipson, Paul McEuen Vibrational modes in suspended carbon nanotubes (CNTs) are incredibly soft, which makes them sensitive to small forces and a prime candidate as force sensors. This same property, combined with the stiffness of the CNT to stretching, makes them an unusual mechanical system characterized both by large thermally-activated fluctuations and strong nonlinear interactions between the resonance modes. How do these thermal fluctuations manifest themselves in the resonance behavior? To address this question, we developed an electrically-contacted micro-tweezer platform that is capable of lifting a pristine CNT off of its growth substrate, directly applying strain to the free-standing doubly-clamped CNT, and controlling its proximity to electrical gates and optical ring (microdisk) resonators for sensing. We measure both the mechanical resonance frequencies and quality factors of the CNT as a function of strain and temperature and compare these to recent predictions that account for the entropic effects of fluctuations on CNTs. In addition, we use these tweezers to couple a CNT to a high-Q optical resonator and demonstrate remarkably strong optomechanical coupling. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D37.00004: Shape transitions in bistable carbon nanotubes coupled to encapsulated gas Oleg Shklyaev, Eric Mockensturm, Milton Cole, Vincent Crespi Large-diameter single-wall carbon nanotubes are bistable (i.e. can have inflated or collapsed cross-sections) and can be used to design nano-electromechanical systems such as engines, generators, and heat pumps. The underlying physical mechanism for these devices is the sensitivity of the tube's equilibrium shape to external stimuli such as temperature and applied voltage. Fixing one end in the inflated state and the other in the collapsed state creates a mobile transition region separating these states. Gas encapsulated inside the tube provides an additional means to control the tube shape by coupling its thermodynamic parameters to the equilibrium tube configuration. Depending on the conditions, the encapsulated gas can remain vapor or condense layer-by-layer on the inner wall surface. We analyze such a system with lattice-gas model and molecular dynamics simulations. Changing the gas temperature or number of gas atoms changes the relative fraction of collapsed and inflated regions, while external forces that change the tube shape also affect the phase of the encapsulated gas. Surprisingly, squashing an inflated tube that has gas condensed on its inner surface decreases the surface area available to the wetting layer, so that gas atoms are forced back into the vapor phase: a paradoxical effect where compression induces a transition from condensed to vapor phases. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D37.00005: Fermi Energy-Dependent Structural Deformation of Single-Wall Chiral Carbon Nanotubes Eduardo Barros, Bruno Vieira, Antonio Souza Filho, Mildred Dresselhaus In this work, we investigate structural deformation of single wall carbon nanotubes as a function of the Fermi Energy by calculating the structural and electronic properties of charged carbon nanotubes within an extended tight-binding approach. Density-Functional-Theory-based tight-binding parameters were used, following the procedure introduced by Vercosa \textit{et al}. [2] The total energy of the nanotube is calculated assuming that the electron population follows the Fermi-Dirac distribution for a given Fermi-Energy (E$_{\mathrm{F}})$. As the Fermi energy is varied, the total charge of the nanotube changes, thereby simulating a charging effect. Our results show that the relaxation of the electronic stress generated by an extra charge on the nanotube causes axial, radial and torsional strains which directly affect the electronic band structure of carbon nanotubes. The electron-electron Coulomb repulsion further increases this effect, leading to extremely high torsional strains and considerable changes to the electronic structure of the nanotubes. For example, torsional strains of up to 2{\%} were obtained for an (8,7) nanotube for a Fermi energy of about 1 eV, causing changes of more than 0.5 eV to the interband transition energies. \\[4pt] [1] Yang et al. PRL, 85(1), 2000. [2] Vercosa, et al. PRB, 81:165430, 2010. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D37.00006: Electrical conductance adsorption isotherms of rare gases on individual single-wall carbon nanotubes Oscar Vilches, Hao-Chun Lee, Boris Dzyubenko, David Cobden Simultaneous resonance frequency and electrical conductance isotherm measurements of inert gases adsorbed on the surface of a single suspended single-wall carbon nanotube have been performed to understand the relationship between the results of the two methods. The resonance frequency measurements determine the ratio between adsorbed mass and the mass of the nanotube. The conductance also varies with the amount of mass adsorbed, but its relationship with the adsorbed mass varies between different nanotubes. The conductance change is particularly dramatic in two cases: when transitions occur between two phases of different density, for example at the liquid-vapor transition of two-dimensional (2d) Ar in the 50 K range; and when Coulomb blockade oscillations are clearly visible, in particular for 2d $^{4}$He gas adsorption in the 5 to 10 K range. Current work on the connection between conductance and frequency isotherms with various gases will be presented. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D37.00007: Adsorption equilibration processes inside narrow pores Samantha Molnar, M. Mercedes Calbi Initially motivated by experimental results concerning gas adsorption in open-ended carbon nanotubes, we investigate the adsorption kinetics of a gas inside a nanopore by implementing a Kinetic Monte Carlo simulation of the gas dynamics. In addition to obtaining the change in coverage with time, we analyze the spatial configuration of the adsorbed phase inside the pore as it evolves towards equilibrium. We also identify blockage events near the ends of the pore, and determine the dependence of these processes on the length of the pore and the amount of gas adsorbed. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D37.00008: Observation of Henry's Law in Low-Density Measurements of Adsorption on Carbon Nanotubes Boris Dzyubenko, Denise Schmitz, Hao-Chun Lee, Oscar E. Vilches, David H. Cobden We have studied the adsorption of noble gases on pristine suspended single-walled carbon nanotubes at low temperatures in the limit of low density (coverage), as determined from the shift of the mechanical resonance frequency of the nanotube due to mass loading. The high homogeneity of the nanotube substrate and the sensitivity of the technique allow us to observe Henry's law, in which the coverage is proportional to the gas pressure. In this limit the adsorption isotherm is determined by single-atom effects, allowing unprecedentedly accurate ($\pm$ 2{\%}) determination of the single-particle binding energies to a nanotube. Also, by measuring the deviation from Henry's law as coverage increases we obtain information about the pairwise interactions between the adsorbed atoms using the virial expansion. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D37.00009: Curvature and nanoscale forces in controlling self-assembly of carbon nanotube-amphiphile complexes Jukka Maatta, Paul Van Tassel, Maria Sammalkorpi In aqueous solution, carbon nanotubes (CNTs) bundle strongly via hydrophobicity induced aggregation, yet the extraordinary properties are best realized when CNTs are dispersed as individual tubes. As a result, pure and well isolated individual CNTs are typically desired and extensive effort has been devoted to achieving good aqueous dispersion of CNTs through covalent or non-covalent functionalization. Here, we examine by molecular simulations the non-covalent solubilization of CNTs with a special focus on curvature effects. We employ molecular dynamics simulations and theoretical models to systematically study the amphiphile interactions at the CNT surface. We report that micelle-forming amphiphiles form hemimicellar structures whereas bilayer-forming lipids form tubular coatings. We characterize the energetics of the underlying physical components - the electrostatic, hydration, and geometric effects on CNT dispersion to examine the efficiency of the various CNT solubilization strategies. The observed differences in amphiphile absorption provide a microscopic understanding on the curvature-dependence in amphiphile-coated CNTs solubility in the aqueous phase and will facilitate the bottom-up design of soft nanoscale materials for nanotechnology. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D37.00010: Ethane adsorbed on carbon nanohorns Brice Russell, Aldo Migone, Masako Yudasaka, Sumio Iijima We have measured adsorption isotherms for ethane adsorbed on as-produced single-walled carbon nanohorns. Measurements have been completed for five temperatures between 130 K and 195 K. The kinetics of adsorption will be compared to results previously obtained for ethane adsorption on purified HiPco single-walled carbon nanotubes. On nanotubes it was found that equilibration time for ethane decreased with increasing sorbent coverage; for adsorption on nanohorns, equilibration time increased with increasing sorbent coverage. The kinetic results for the fractional pressure change and sorbent mass loading were calculated under the assumption that the system was only subject to one rate-controlling mechanism. The point-B method was used to determine monolayer completion values at each temperature. Equilibrium results for ethane adsorbed on nanohorns will be compared to similar results on nanotubes. This work was supported by the NSF through grant DMR-1006428. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D37.00011: CO$_{\mathrm{2}}$ adsorption on carbon nanotubes Aldo Migone, Brice Russell, Shree Banjara We measured adsorption isotherms of CO2 on a 0.1106 g sample of purified HiPco SWNTs at six temperatures between 147 and 207 K. Plots of the amount of CO2 adsorbed as a function of the logarithm of the equilibrium pressure do not reveal any resolvable substeps in the adsorption data. We measured the effective monolayer capacity of the sample using the point B method. We found a specific surface area of 380 m2/g, significantly lower than that determined from a N2 isotherm measured on the same sample. We determined the isosteric heat of adsorption as a function of the amount of CO2 loaded onto the SWNTs. The values for the isosteric heat are lower than the latent heat of sublimation for most sorbent loading values below the saturated vapor pressure. Only for sorbent loadings in the lowest 5 {\%} (relative to the sorbent loading present when the saturated vapor pressure is reached) does the isosteric heat exceed the bulk sublimation value. Our results will be compared with others reported in the literature, as well as with results obtained for CO2 on related sorbents. This work was supported by the NSF through grant DMR-1006428. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D37.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 5:18PM - 5:30PM |
D37.00013: Role of defects in the physiological fate of carbon nanomaterials Aleksandr Kakinen, Ramakrishna Podila, Jingyi Zhu, Pooja Puneet, Anne Kahru, Apparao Rao Charged defects play an important role in not only materials properties (P. Puneet et al., Scientific Reports, 3, 3212 (2013)) but also in the determination of how materials interact at the nano-bio interface. Recently, it was shown that any physiological response, and hence the fate of carbon nanotubes (CNTs) in biological media, is dictated by the formation of protein-corona. Accordingly, we explored how defects in CNTs influence the biological interactions and protein corona formation using micro-Raman spectroscopy, electrochemistry, photoluminescence, and infrared absorption spectroscopy. Our results show that the interaction of CNTs and proteins (albumin, fibrinogen, and fetal serum) is strongly influenced by charge-transfer between defects and proteins ensuing in protein-unfolding which leads to a gain in conformational entropy. [Preview Abstract] |
Session D38: Invited Session: Secrecy and Science
Sponsoring Units: FPSChair: Arian Pregenzer
Room: 709/711
Monday, March 3, 2014 2:30PM - 2:54PM |
D38.00001: Secrecy versus Openness: Historical Perspectives Invited Speaker: George Dyson ``I think all members of the laboratory are agreed that the work which is being pursued here is of such importance that we should not like to see it or any part of it fall into private or foreign hands,'' J. Robert Oppenheimer announced to his colleagues at Los Alamos on November 15, 1943, putting the first formal secrecy policies into effect--while promising ``to make the procedure as little burdensome as possible, and to reduce its interference with the actual prosecution of the work.'' ``After the war, the question of secrecy was reconsidered... but the practice of classification continued; it was our `security,' whether it worked or failed,'' Oppenheimer's colleague Edward Teller updated us in 1981, halfway between the mimeographed handout of 1943 and the labyrinthine security policies of today. ``We now have millions of classified technical documents,'' Teller continued, and ``the limitations we impose on ourselves by restricting information are far greater than any advantage others could gain.'' Teller titled his critique ``The Road to Nowhere.'' Is there a road back? In some notable cases (digital computing, satellite reconnaissance, GPS) we have taken the road to openness, and Teller's opinion, at least in this domain, appears to have been correct. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:18PM |
D38.00002: The Growing Tension Between Openness and Risk in the Life Sciences Invited Speaker: David A. Relman The ongoing revolution in the life sciences provides new critical insights and radically new capabilities to an ever increasing number of global participants. While the overwhelming majority of outcomes are beneficial, a small number of discoveries and capabilities pose unusual risks for misuse and widespread harm to humans, animals, plants and the larger ecosphere. The deliberate engineering of a highly virulent and transmissible Influenza virus in 2013 is an example. As was discussed in the early years of nuclear weapons research, are there now experiments in the life sciences that ought not to be undertaken because of disproportionate risks? Is there information from life sciences research that ought not be widely disseminated? If either is true, then what should be the process by which a consensus is reached about the identification and management of such work? What are the moral and ethical responsibilities of life scientists?\\[4pt] Relman DA. The increasingly compelling moral responsibilities of life scientists. Hastings Center Report 2013; 43:34-35.\\[0pt] Relman DA. ``Inconvenient Truths'' in the Pursuit of Scientific Knowledge and Public Health. J Infect Dis 2014; 209:170-2. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:42PM |
D38.00003: Intellectual Property and Corporate Research: Threats to Scientific Openness Invited Speaker: Paul McEuen Science, in its idealized form, is an open source pursuit. Discoveries, while credited to their discoverers, are treated as part of the intellectual Commons. Laws concerning intellectual property, on the other hand, seek to establish private ownership of ideas and technologies. What are the benefits of each approach? And what are the costs? In this talk, I'll look at these questions from the perspective of a practicing scientist, arguing for greater openness yet recognizing that, as Ts'ai Ken T'an said, ``water which is too pure has no fish.'' [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:06PM |
D38.00004: Innovation and National Security Invited Speaker: Kim Budil |
Monday, March 3, 2014 4:06PM - 4:30PM |
D38.00005: Scientific Openness from National and International Perspectives Invited Speaker: Mildred Dresselhaus |
Monday, March 3, 2014 4:30PM - 5:30PM |
D38.00006: Panel Discussion with Q\&A |
Session D39: Invited Session: Quantum Anomalous Hall Effect in Magnetic Topological Insulators
Sponsoring Units: DCMPChair: Nitin Samarth, Pennsylvania State University
Room: Mile High Ballroom 2A-3A
Monday, March 3, 2014 2:30PM - 3:06PM |
D39.00001: Theory of the quantum anomalous Hall effect in magnetic topological insulators Invited Speaker: Shoucheng Zhang We give a theoretical introduction to the QAH effect based on magnetic topological insulators in 2D and 3D. In 2D topological insulators, magnetic order breaks the symmetry between the counter-propagating helical edge states, and as a result, the quantum spin Hall effect (QSH) can evolve into the QAH effect. In 3D, magnetic order opens up a gap for the topological surface states, and chiral edge state can exist on the magnetic domain walls. We discuss realistic materials for magnetic topological insulators with QAH. We also discuss more recent theoretical work on the coexistence of the helical and chiral edge states, multi-channel chiral edge states, and the theory of the plateau transition in the QAH. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D39.00002: Experimental realization of quantized anomalous Hall effect Invited Speaker: Qi-Kun Xue Anomalous Hall effect was discovered by Edwin Hall in 1880. In this talk, we report the experimental observation of the quantized version of AHE, the quantum anomalous Hall effect (QAHE) in thin films of Cr-doped (Bi,Sb)$_{\mathrm{2}}$Te$_{\mathrm{3}}$ magnetic topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance exhibits a quantized value of $h/e^{2}$ accompanied by a significant drop of the longitudinal resistance. The longitudinal resistance vanishes under a strong magnetic field whereas the Hall resistance remains at the quantized value. The realization of QAHE paves a way for developing low-power-consumption electronics. Implications on observing Majorana fermions and other exotic phenomena in magnetic topological insulators will also be discussed. The work was collaborated with Ke He, Yayu Wang, Xucun Ma, Xi Chen, Li Lv, Dai Xi, Zhong Fang and Shoucheng Zhang. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D39.00003: Quantum Anomalous Hall Effect in Hetero Magnetic Topological Insulator Structures Invited Speaker: Kang Wang The quantum anomalous Hall effect (QAHE), which has the quantized Hall conductance of $h/e^{2}$ in the absence of external field, was expected to happen in a magnetic 3-D topological insulators (TIs) system. In this talk, we report recent progress of QAHE-related physics in the TRS-breaking field. In the first part, we show the generation of robust magnetism by doping magnetic ions (Cr) into the host (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}})_{2}$Te$_{3}$ materials. With gate-controlled magneto-transport measurements, we demonstrate the presence of both the hole-mediated RKKY coupling and carrier-independent van Vleck magnetism. By adjusting the Cr doping concentration and Bi/Sb ratio, we establish an effective way to experimentally approach to the QAHE region. The second part of this talk discusses the manipulation of surface-related magnetism in the modulation-doped TI/Cr-doped TI heterostructures. We investigate the role of massive surface Dirac fermions in the bulk RKKY mediation process. Both our theoretical models and experimental results reveal that the topological surface-related magnetic order can be either enhanced or suppressed, depending on the magnetic interaction range between the surface states and Cr ions. Based on such TI heterostructures, we also demonstrate the magnetization switching via giant spin-orbit torque induced by the in-plane current. Finally, in order to make these effects observable at 300K, we describe the use of magnetic proximity effects to manipulate the surface magnetism of TI. These results not only demonstrate additional important steps to further explore fundamental properties of the TRS-breaking TI systems but also may help the realization of many functionalities of TI-based spintronics applications. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D39.00004: Chern Insulators from Heavy Atoms on Magnetic Substrates Invited Speaker: Kevin Garrity Chern insulators, or quantum anomalous Hall insulators, would display a variety interesting and potentially useful properties; however, existing methods for constructing Chern insulators have proven challenging, and have thus far been limited to low temperatures. We propose a new method for searching for Chern insulators by depositing atomic layers of elements with large spin-orbit coupling (e.g., Bi) on the surface of a magnetic insulator. We argue that such systems will typically have isolated surface bands with nonzero Chern numbers. If these bands overlap in energy, a metallic surface with large anomalous Hall conductivity will result; if not, a Chern-insulator state will typically occur. We use first principles calculations to verify this search strategy by considering heavy atoms on the surfaces of MnTe, MnSe, and EuS, as well as more recent results on several promising oxide and nitride surfaces. We find many Chern insulators in both cases, including examples with large band gaps. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D39.00005: Experimental Studies of Ferromagnetism in Topological Insulators Invited Speaker: Joseph Checkelsky Breaking of time reversal symmetry has proven to be an incisive method for experimentally drawing out the exotic nature of topological insulators. In particular, the introduction of magnetic dopants in to three dimensional topological insulators has led to the realization of theoretically predicted novel types of ferromagnetic order and a quantized version of the anomalous Hall effect. Here, I will present recent work on the synthesis and measurement of bulk and thin film topological insulators doped with 3$d$ transition metals. I will discuss the ferromagnetic order that arises in various systems and the associated electrical transport response of the surface modes. [Preview Abstract] |
Session D40: Invited Session: Graphene Spintronics and Magnetism
Sponsoring Units: GMAGChair: Wei Han, IBM
Room: Mile High Ballroom 2B-3B
Monday, March 3, 2014 2:30PM - 3:06PM |
D40.00001: Spin transport in epitaxial graphene Invited Speaker: Pierre Seneor Spintronics is a paradigm focusing on spin as the information vector in fast and ultra-low-power non volatile devices such as the new STT-MRAM. Beyond its widely distributed application in data storage it aims at providing more complex architectures and a powerful beyond CMOS solution for information processing. The recent discovery of graphene has opened novel exciting opportunities in terms of functionalities and performances for spintronics devices. We will present experimental results allowing us to assess the potential of graphene for spintronics. We will show that unprecedented highly efficient spin information transport can occur in epitaxial graphene leading to large spin signals and macroscopic spin diffusion lengths ($\sim$ 100 microns), a key enabler for the advent of envisioned beyond-CMOS spin-based logic architectures. We will also show that how the device behavior is well explained within the framework of the Valet-Fert drift-diffusion equations [1]. Furthermore, we will show that a thin graphene passivation layer can prevent the oxidation of a ferromagnet, enabling its use in novel humide/ambient low-cost processes for spintronics devices, while keeping its highly surface sensitive spin current polarizer/analyzer behavior and adding new enhanced spin filtering property [2]. These different experiments unveil promising uses of graphene for spintronics.\\[4pt] In collaboration with B. Dlubak, Unite Mixte de Physique CNRS/Thales, Palaiseau, France \& Universite Paris-Sud, Orsay, France and University of Cambridge; M.-B. Martin, H. Yang, Unite Mixte de Physique CNRS/Thales and Universite Paris-Sud; R. Weatherup, University of Cambridge; M. Sprinkle, GeorgiaTech, Atlanta/Institut Neel; B. Servet, S. Xavier, Unite Mixte de Physique CNRS/Thales and Universite Paris-Sud; C. Berger, W. de Heer, GeorgiaTech, Atlanta/Institut Neel; S. Hoffman, J. Robertson, University of Cambridge; and C. Deranlot, R. Mattana, H. Jaffres, A. Anane, F. Petroff, P. Seneor, A. Fert, Unite Mixte de Physique CNRS/Thales and Universite Paris-Sud.\\[4pt] [1] B. Dlubak et al., Nature Physics 8, 557 (2012); P. Seneor, et al., MRS Bulletin 37, 1245 (2012).\\[0pt] [2] B. Dlubak et al., ACS Nano 6, 10930 (2012); R. Weatherup, et al., ACS Nano 6, 9996 (2012) [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D40.00002: Graphene magnetism due to point defects Invited Speaker: Ernie Hill Much interest has been generated on intrinsic magnetism in materials without d or f electrons. This is especially true for carbon-based materials and, in particular, graphene. Many theoretical studies have predicted that point defects in graphene should carry a magnetic moment and these can in principle couple either ferromagnetically or antiferromagnetically. However, the experimental evidence for such magnetism remains both scarce and controversial. In this talk we will review our recent experimental results on graphite and graphene where we show that pure graphite exhibits no ferromagnetism or anti-ferromagnetism down to liquid helium temperatures. We will also show that point defects in graphene produced by; (i) fluorine adatoms in concentrations, x, gradually increasing to full stoichiometric fluorographene CFx (x$=$1.0) and (ii) irradiation defects (vacancies) -- carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties, despite the fact that the maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. Our work clarifies the controversial issue of graphene's magnetism and opens the way to novel devices making use of its intrinsic magnetism. We also discuss how the magnetic properties of graphene can be changed by electric fields. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D40.00003: Colossal Enhancement of Spin-Orbit Coupling in Hydrogenated Graphene Invited Speaker: Barbaros Oezyilmaz Graphene's extremely small intrinsic spin-orbit (SO) interaction makes the realization of many interesting phenomena such as topological states and the spin Hall Effect (SHE) practically impossible. Recently, it was predicted that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp2 to sp3 bonds [1]. However, introducing adatoms and yet keeping graphene metallic, i.e., without creating electronic (Anderson) localization8 is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic field even at room temperature, with non-local spin signals up to 100 $\Omega$. The SHE is, further, directly confirmed by the Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length $\sim$ 1 $\mu $m, a spin relaxation time $\sim$ 90 ps and a SO strength of 2.5 meV [2].\\[4pt] [1] Castro Neto, A. H. {\&} Guinea, F. Impurity-induced spin-orbit coupling in graphene. Phys. Rev. Lett. (2009). \\[0pt] [2] Balakrishnan, J., Koon, G. K. W., Jaiswal, M., Castro Neto, A. H., \"{O}zyilmaz, B.; Colossal Enhancement of Spin-Orbit Coupling in Weakly Hydrogenated Graphene; Nature Physics (2013). [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D40.00004: Defect Induced Magnetic Moments in Graphene Invited Speaker: Roland Kawakami We utilize non-local spin transport measurements to detect the presence of defect induced magnetic moments in graphene. As shown in this talk, point defects such as hydrogen adatoms and lattice vacancies generate magnetic moments in graphene that have substantial exchange coupling with the conduction electrons. Therefore, this exchange coupling produces spin relaxation in the conduction electrons. Specifically, a characteristic field dependence of the non-local spin transport signal identifies the presence of the magnetic moments. Furthermore, Hanle spin precession measurements indicate the presence of an exchange field generated by the magnetic moments. The entire experiment including spin transport is performed in an ultrahigh vacuum chamber, and the characteristic signatures of magnetic moment formation appear only after hydrogen adatoms or lattice vacancies are introduced. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D40.00005: Graphene Spintronics: The Current State of the Art Invited Speaker: Bart van Wees Graphene has great potential for spintronics and new spintronics applications. Room temperature spin relaxation lengths of 10 micrometer or more have already been achieved, allowing electron spins to be transported (and manipulated) over large distances. However, the basic spin relaxation mechanisms which control spin transport in graphene are still not understood. In this talk I will give an overview of the experimental state of the art, and discuss the role of the various spin relaxation mechanisms in graphene. I will compare results obtained from different graphene systems, ranging from suspended graphene [1], graphene sandwiched between boron nitride layers [2], and epitaxial graphene [3]. I will discuss recent experiments where the spin relaxation is studied in devices with both top and bottom gate devices, which allow the carrier density as well as the electric field to be controlled independently[4]. Finally, I will address the intriguing observation that localized states present in the buffer layer of epitaxial graphene can dramatically change the spin transport parameters [5]. The possible relation with recently observed room temperature ferromagnetism in these systems may make it possible to make ``all-graphene'' spintronics devices.\\[4pt] [1] M. H. D. Guimaraes et al. Nano Lett. 12 (7) 3512 (2012)\\[0pt] [2] P.J. Zomer et al., Phys. Rev. B86, 161416 (2012)\\[0pt] [3] T. Maassen et al., Nano Lett. 12 (3), 1498 (2012)\\[0pt] [4] M.H.D. Guimaraes et al., unpublished\\[0pt] [5] T. Maassen et al., Phys. Rev. Lett. 100, 067209 (2013) [Preview Abstract] |
Session D41: Topological Insulators: Engineered Structures I
Sponsoring Units: DCMPChair: Robert Markiewicz, Northeastern University
Room: Mile High Ballroom 3C
Monday, March 3, 2014 2:30PM - 2:42PM |
D41.00001: Plasma Etching Effects on the Transport in Topological Insulator Bi$_2$Te$_3$ Nanoplates Sukrit Sucharitakul, Nicholas Goble, Zhenhua Wang, Zhidong Zhang, Xuan Gao Carrier transport in various topological insulators (TIs) such as Bi$_2$Se$_3$ and Bi$_2$Te$_3$ exhibits a novel linear magneto-resistance (LMR) [1] in addition to the more extensively studied weak anti-localization effect. The robustness against raising temperature and 2D nature of this LMR [1] allude to its connection with the topologically protected 2D surface transport in TI. In this work, we study how the plasma etching induced surface roughness or corrugation impacts the transport in TI Bi$_2$Te$_3$ nanoplates, to understand how the topological surface transport responds to controlled perturbation to material surface. Bi$_2$Te$_3$ nanoplates with varied thickness were grown using CVD method and hall bar devices were studied under different Argon plasma etching conditions. Our experiments show that plasma etching induces drastic change in the Hall coefficient but has relatively weak effect on the LMR. We will also discuss the data analyzed by the two band carrier m! a ngo-transport model which allows quantitative separation of the surface carrier concentration and mobility from the bulk carriers. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D41.00002: Electrostatic Coupling Between the Surface States of a Topological Insulator Valla Fatemi, Stephen L. Eltinge, Benjamin Hunt, Hadar Steinberg, Nicholas P. Butch, Ray C. Ashoori, Pablo Jarillo-Herrero We report electronic transport measurements on nanofabricated topological insulator Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$ exfoliated devices with electrostatic top- and bottom-gate electrodes. We observe independent, ambipolar modulation of the device resistance on both the top and bottom surfaces. On thin devices, the bottom-gate capacitively couples to the top surface, indicating poor bulk screening which allows for surface-to-surface electrostatic coupling. We explain the data through a capacitance model and extract information about the surface and bulk density of states. Additionally, we show that the ambipolarity of the surface state resistance persists up to room temperature. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D41.00003: Helical mode and supercurrent measured on the topological surface states of Bi$_{2}$Te$_{3}$ nanoribbon field effect devices Luis A. Jauregui, Michael T. Pettes, Li Shi, Leonid P. Rokhinson, Yong P. Chen Topological superconductivity can be proximity induced by coupling s-wave superconductors with spin-helical electron systems, such as the surface of 3D topological insulators (TIs), where the energy bands follow Dirac dispersion and the electronic states possess helical spin-momentum locking. We have grown Bi$_{2}$Te$_{3}$ nanoribbons (NRs) by vapor liquid solid method and characterized their crystalline structure by TEM and Raman spectroscopy. We fabricate backgated field effect devices where the chemical potential ($\mu )$ can be tuned from bulk bands to surface states and ambipolar field effect has been observed. The temperature dependence of the resistance and Shubnikov de Haas oscillations show suppressed bulk conduction with surface conduction dominating and a pi-Berry's phase. The Aharonov--Bohm oscillations (ABO), measured with a magnetic field parallel to the NR axis, have a period equal to one flux quanta with conductance maxima at half flux quanta (pi-ABO), for $\mu $ close to the charge neutrality point. Such pi-ABO is a direct evidence of the existence of 1D helical modes at half flux quanta. We have also fabricated Josephson junctions on our TI NR devices with inter-electrode separations up to 200 nm, and measured supercurrent with a proximity induced gap of 0.5meV at 0.25K. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D41.00004: Topological insulator nanowires and nanowire hetero-junctions Haiming Deng, Lukas Zhao, Travis Wade, Marcin Konczykowski, Lia Krusin-Elbaum The existing topological insulator materials (TIs) continue to present a number of challenges to complete understanding of the physics of topological spin-helical Dirac surface conduction channels, owing to a relatively large charge conduction in the bulk. One way to reduce the bulk contribution and to increase surface-to-volume ratio is by nanostructuring. Here we report on the synthesis and characterization of Sb$_2$Te$_3$, Bi$_2$Te$_3$ nanowires and nanotubes and Sb$_2$Te$_3$/Bi$_2$Te$_3$ heterojunctions electrochemically grown in porous anodic aluminum oxide (AAO) membranes with varied (from 50 to 150 nm) pore diameters. Stoichiometric rigid polycrystalline nanowires with controllable cross-sections were obtained using cell voltages in the 30 - 150 mV range. Transport measurements in up to 14 T magnetic fields applied along the nanowires show Aharonov-Bohm (A-B) quantum oscillations with periods corresponding to the nanowire diameters. All nanowires were found to exhibit sharp weak anti-localization (WAL) cusps, a characteristic signature of TIs. In addition to A-B oscillations, new quantization plateaus in magnetoresistance (MR) at low fields ($< 0.7~\textrm{T}$) were observed. The analysis of MR as well as $I-V$ characteristics of heterojunctions will be presented. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D41.00005: Doping control and spatially resolved optoelectronics of Bi2Se3 nanowires and nanoribbons Xingyue Peng, Yiming Yang, Dong Yu Bi2Se3 has been predicted to be a 3D topological insulator with chiral surface states protected by time-reversal symmetry. Single crystalline Bi2Se3 nanowires and nanoribbons were synthesized via a vapor-liquid-solid approach. Carrier concentrations can be tuned in a wide range by varying Se vapor pressure during the growth. High carrier mobilities up to 200 cm$^{2}$/Vs at room temperature and 1000 cm$^{2}$/Vs at 2 K were achieved. A surface conduction channel was identified from the temperature dependent transport measurement. Magnetoresistance measurement showed a signature weak anti-localization peak of the chiral surface states. Scanning photocurrent microscopy (SPCM) study of these nanoribbons showed an alternating photocurrent polarity with a length scale of 1 um, which indicates a potential variation on the surface of these nanoribbons, despite the high crystallinity confirmed by transmission electron microscope. Kelvin probe microscopy was used to characterize such surface potential variation of these nanowires and nanoribbons. We will discuss the possible origin of this surface potential variation. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D41.00006: Observation of Robust Surface States in Highly-Disordered Topological Insulator Nanotubes Renzhong Du, Weiwei Zhao, Xin Liu, Chaoxing Liu, Jainendra Jain, Moses Chan, Qi Li, Shih-Ying Yu, Suzanne Mohney, DukSoo Kim, Srinivas Tadigadapa, Yuewei Yin, Sining Dong, Xiaoguang Li, Jian Wang We have studied electrical transport properties of candidate topological insulator (TI) Bismuth Telluride (Bi$_{\mathrm{2}}$Te$_{\mathrm{3}})$ nanotubes at low temperatures and high magnetic fields. Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ nanotube samples were synthesized by solution phase method, with the outer diameters in the range of 90 $\sim$ 200 nm and wall thickness 10 $\sim$ 15 nm, and typical length of over 10 $\mu $m. Focused ion beam (FIB) assisted deposition and e-beam lithography were applied to fabricate Ohmic contacts. Thermal conductivity measurements show the nanotubes have similar carrier concentration to other metallic nanowires and ribbons, while the nanotubes have insulating behavior, which is due to disorder. For the highly disordered samples, strong quantum oscillations in magnetoresistance were observed in parallel field, with an h/e period associated with the outer surface of the nanotubes. Detailed analysis indicates that the oscillations are due to Anomalous Aharonov-Bohm Effect originating from Dirac-like TI surface states. The relationship between oscillation and disorder will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D41.00007: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 3:54PM - 4:06PM |
D41.00008: Unusual behavior of the surface states in the topological insulator-magnetic insulator heterostructure Jeongwoo Kim, Seung-Hoon Jhi Topological insulator is a new class of solids that possess non-trivial topology in electronic structures. Spin-polarized conducting states should develop at the interface between topological insulators and trivial insulators as dictated by the topological invariants associated with the time-reversal symmetry. As such, the conducting surface states are very robust to impurities but susceptible to magnetic impurities that destroy their spin-momentum helical structures. We study the behavior of the surface states when magnetic impurity layers are deposited on top of the topological insulator surface, (Sb2Te3-MnTe), using first-principles calculations. We find that the helical nature of the surface states persists even at the presence of magnetic impurity layers and that the energy gap at the Dirac point due to the magnetic layers exhibits unusual behavior as the density of magnetic impurities is changed. We derive a model Hamiltonian based on Anderson model to describe the interaction of the surface states and magnetic-impurity layers and explain the behavior of the surface states. We show that the coupling between the surface states, d-orbitals of the magnetic impurities, and the RKKY-type interactions in magnetic impurities determine the energy gap of the surface states as well as the magnetic ordering in the impurity layer. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D41.00009: Spin Transfer Torque Generated by the Topological Insulator Bismuth Selenide Alex Mellnik, Jennifer L. Grab, Peter J. Mintun, Joon S. Lee, Anthony Richardella, Robert A. Buhrman, Nitin Samarth, Dan C. Ralph We measure large spin-transfer torques generated by in-plane currents in thin films of the topological insulator bismuth selenide at room temperature. We use spin-torque ferromagnetic resonance in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$ bilayers to determine that the spin-torque arising from the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and acting on the Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$ layer possesses substantial vector components both in the sample plane and perpendicular to the plane. The out-of-plane torque is several times larger than expected from the Oersted field, and the efficiency of in-plane (anti-damping) spin torque generation per unit current density in the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ is greater than has been observed in any other material. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D41.00010: Spin-Transfer Torques in Dual-Gated Bismuth Selenide Topological Insulator Devices Jennifer Grab, Alex Mellnik, Anthony Richardella, Nitin Samarth, Daniel Ralph Recent theoretical and experimental work on topological insulator / ferromagnet bilayers suggests that bismuth selenide can act as a source of spin current for applying a spin transfer torque to an adjacent magnetic layer. To help determine the mechanism of the in-plane and out-of-plane spin torques, we fabricate dual-gated bismuth selenide devices with a ferromagnetic permalloy nanowire positioned between the gates to act as an absorber of spin currents. We use the spin-torque ferromagnetic resonance technique to measure current-induced torques acting on the permalloy nanowire. We will attempt to distinguish between surface and bulk mechanisms for the torque by sweeping a uniform voltage applied to both gates to tune the carrier density. We will also study whether the surface spin current can be modified by applying different gate voltages to induce a large gradient in the electron chemical potential near the permalloy wire. Such a modification is expected as a consequence of locking between the orientations of the electron wavevector and spin in topological insulator surface states. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D41.00011: Voltage driven magnetic bifurcations in nanomagnet-topological insulator composite structure Yuriy G. Semenov, Xiaopeng Duan, Ki Wook Kim Multiplicity of magnetization dynamics in thin ferromagnetic insulator (FMI) deposited on topological insulator (TI) has been studied as an effect of electron flow through the interface. The intrinsic spin polarization of TI surface current evokes the magnetization precession, which in turn modifies the TI electron spin polarization and current intensity. The net effect of this self-consistent behavior of FMI magnetization and TI surface itinerant electrons results in auto oscillations, magnetization reversal or magnetic deviation from equilibrium state according to the applied DC voltage. These phenomena are also accompanied with strong anomalous Hall effect and they are separated by the threshold voltages of magnetization bifurcations. Comparisons with spin transfer torque and spin-Hall-based mechanisms of magnetization reversal/oscillation reveal significant advantage in power efficiency of this proposed electrical control of magnetization dynamics. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D41.00012: Magnetization Switching via Giant Spin-Orbit Torque in a Magnetically Doped Topological Insulator Heterostructure Yabin Fan, Pramey Upadhyaya, Xufeng Kou, Murong Lang, So Takei, Zhenxing Wang, Jianshi Tang, Liang He, Li-Te Chang, Mohammad Montazeri, Guoqiang Yu, Wanjun Jiang, Tianxiao Nie, Yaroslav Tserkovnyak, Kang Wang The magnetization switching induced by in-plane current in a Chromium-doped topological insulator bilayer heterostructure has been observed and is attributed to a giant spin-orbit toque. The critical current density of around 10$^{\mathrm{4}}$ A/cm$^{\mathrm{2}}$ for magnetization switching is nearly three orders of magnitude lower than in the traditional heavy metal/ferromagnetic heterostructures. The effective magnetic field arising from the spin-orbit torque is also increased by three orders. This giant spin-orbit torque and efficient current-induced magnetization switching may lead to innovative spintronics applications such as ultra-low power dissipation memory and logic devices. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D41.00013: Transport Studies of Thin Film Magnetic Topological Insulator Nanostructures Eli Fox, Andrew Bestwick, David Goldhaber-Gordon, Xiao Feng, Ke He, Yayu Wang, Qi-kun Xue, Xufeng Kou, Yabin Fan, Kang Wang Ferromagnetic order in a topological insulator breaks time-reversal symmetry, opening a gap in the surface states and giving rise to a number of exotic phenomena, including dissipationless chiral edge conduction along domain walls. Thin films of the topological insulator (Bi,Sb)$_2$Te$_3$ doped with chromium exhibit ferromagnetic ordering that is not mediated by bulk carriers, allowing the magnetism to persist in the bulk energy gap [1,2]. Here, we discuss fabrication and transport measurements of nanostructures based on these films. We further discuss the possibility of engineering magnetic domains in the film to study the chiral edge state along the domain wall. \\[4pt] [1] C.-Z. Chang {\it et al.}, Science {\bf 340}, 167 (2013). \\[0pt] [2] X. Kou {\it et al.}, Nano Lett. {\bf 13}, 4587 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D41.00014: Magnetic proximity effect induced effects in topological insulator/YIG heterostructure Zilong Jiang, Chi Tang, Bo Zhou, Yulin Chen, Jing Shi The broken time-reversal symmetry in topological insulator (TI) can lead to quantized anomalous Hall effect (QAHE). QAHE has recently been observed in TI doped with Cr which turns ferromagnetic at very low temperatures.Here we carry out an experimental study on induced ferromagnetism in heterostructures of a thin TI film (Bi2S$^{\mathrm{e3}})$ and an insulating magneticfilm (YIG). The YIG film is grown by pulsed laser deposition with an atomically flat surface and in-plane magnetic anisotopy, and Bi2S$^{\mathrm{e3}}$films of different thicknesses are grown on YIG ina molecular beam epitaxy system.Excellent crystallinity of TI films is confirmed by RHEED. Th topological surface states from the top TI surface are confirmedby ARPES. In the 3nm TI sample, a non-linear current-voltage is observed at all temperature, indicating the existence of the quantum confinement induced gap. In the 5 nm TI sample, the current-voltage characteristic is linear.The anomalousHall effect (AHE) is observedat low temperatures which clearly demonstrates the magnetic proximity induced magnetic momentin the surface of TI, andthemagnitudestrongly decreass a the temperature increases.Moreover, the positive magnetoresistance is affected bythe induced magnetic layer andthe weak anti-localization effect is clearly weakened. This and the AHE indicate a proximity effect between TI and YIG This research was supported by UC Lab Fees Program. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D41.00015: Zero field conductance singularity in two terminal ferromagnet-topological insulator device Xiaopeng Duan, Yuriy G. Semenov, Ki Wook Kim Spin-momentum interlocking of surface electronic states on 3D topological insulator (TI) grants the unique opportunity to generate electric current directed according to the spin polarization of injected electrons instead of the applied electric field. Such asymmetry in momentum distribution of injected electrons takes place in the vicinity of ferromagnetic contact but vanishes on the length of few mean free passes. We propose to use this property in two terminal devices consisting of two parallel ferromagnetic contacts deposited on the surface of 3D TI. When the injected spin polarization leads to electron momentum pointing towards the other electrode, it facilitate the direct transmission, resulting in a lower resistance; in contrast with a reversed bias, the spin-determined momentum points away from the other electrode, because of which the electrons could gain the right momentum only after multiple scatterings to approach the second electrode, thus resulting in a higher resistance. We stress that this asymmetry in the resistance keeps up to arbitrarily small applied voltage since it does not need the formation of space charge region that is essential in conventional diodes. The rectification ratio near zero voltage are estimated and potential application are discussed. [Preview Abstract] |
Session D42: Topological Superconductors: Experiment
Sponsoring Units: DCMPChair: Yoshinori Okada, WPI Advanced Institute for Materials Research
Room: Mile High Ballroom 4A
Monday, March 3, 2014 2:30PM - 2:42PM |
D42.00001: Andreev Reflection Spectroscopy on Single Crystals of Bismuth-Chalcogenide Topological Insulators Chris Granstrom, Igor Fridman, J.Y.T. Wei, Hechang Lei, Cedomir Petrovic Topological insulators have received great research interest in recent years. One salient feature of these materials is the helical spin polarization of their electronic surface states. Andreev reflection, a fundamental process that occurs between a superconductor and conducting material, has often been used to probe the spin polarization of various magnetic materials [1,2]. In this work, we use superconducting Nb tips to make cryomagnetic Andreev reflection spectroscopy measurements on bismuth-chalcogenide single crystals. We analyze our spectral data, which show Andreev-like features, in the context of both calculated and measured spin-dependent band structures of these topological insulators. [1] B. Nadgorny, Handbook of Spin Transport and Magnetism (Taylor and Francis, New York, 2011), p. 531. [2] C. S. Turel et al., Appl. Phys. Lett. 99, 192508 (2011) [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D42.00002: Scanning SQUID Measurements of Superconducting Proximity Effect in Bi2Se3-Nb Heterojunctions Philip Kratz, Ilya Sochnikov, Phillip Wu, Jung Ho Yu, Kristie Koski, Yi Cui, Robert Hammond, Malcolm R. Beasley, John R. Kirtley, Kathryn A. Moler In superconductivity induced on the surface of a 3D topological insulator, in contrast to conventional s-wave superconductivity, each vortex core theoretically carries a nondegenerate zero energy state with the properties of a Majorana fermion. The local superfluid density and its characteristic magnetic field penetration depth, critical current and temperature are sensitive metrics for placing limits on the relative contributions of the bulk and surface to a proximitized supercurrent in a topological insulator. Using a scanning SQUID microscope integrated with a quartz tuning fork sensor in a force-sensitive phase-locked loop for simultaneous topography characterization, we study the local superfluid density in Sb-doped Bi2Se3-Nb heterojunctions, prepared by Nb growth through molecular beam epitaxy on solvothermally synthesized Bi2Se3 nanoplates. We observe a suppression of the superconducting diamagnetic susceptibility, consistent with a superconducting proximity effect. We also explore the dependence of the local superfluid density on back gate voltage and temperature. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D42.00003: Probing the Superconducting Proximity Effect in a Topological Insulator Using Scanning Tunneling Microscopy Ian Dayton, Matthias Muenks, Eric Goodwin, Duck-Young Chung, Alex Levchenko, Mercouri Kanatzidis, Stuart Tessmer Topological insulators (TI) embody a new state of quantum matter characterized by topological invariants; this contrasts with superconductors (S), as superconductivity arises from a spontaneously broken symmetry of the underlying electron system. When a superconductor is placed on the surface of a topological insulator, the behavior of the superconducting condensate across the S/TI interface offers the opportunity to study the interplay between these two distant quantum states. In this talk, we present cryogenic Scanning Tunneling Microscopy measurements to probe the local density of states in proximity to Pb/Bi2Se3 interfaces. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D42.00004: Vortices and gate-tunable bound states in a topological insulator coupled to superconducting leads Aaron Finck, C. Kurter, Y.S. Hor, D.J. Van Harlingen It has been predicted that zero energy Majorana bound states can be found in the core of vortices within topological superconductors. Here, we report on Andreev spectroscopy measurements of the topological insulator Bi$_2$Se$_3$ with a normal metal lead and one or more niobium leads. The niobium induces superconductivity in the Bi$_2$Se$_3$ through the proximity effect, leading to both signatures of Andreev reflection and a prominent re-entrant resistance effect. When a large magnetic field is applied perpendicular to the surface of the Bi$_2$Se$_3$, we observe multiple abrupt changes in the subgap conductance that are accompanied by sharp peaks in the dynamical resistance. These peaks are very sensitive to changes in magnetic field and disappear at temperatures associated with the critical temperature of the induced superconductivity. The appearance of the transitions and peaks can be tuned by a top gate. At high magnetic fields, we also find evidence of gate-tunable states, which can lead to stable zero-bias conductance peaks. We interpret our results in terms of a transition occurring within the proximity effect region of the topological insulator, likely due to the formation of vortices. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D42.00005: Tunable Anomalous Supercurrent in a topological tri-junction SQUID C. Kurter, A.D.K. Finck, P. Ghaemi, Y.S. Hor, D.J. Van Harlingen There has been intense interest in realizing Majorana fermions (MFs) in solid-state systems. Circuits of Josephson junctions (JJs) made of closely spaced s-wave superconductors on 3D topological insulators have been proposed to host zero energy Andreev bound states (ABSs) that act like MFs. Here, we present signatures of an anomalous supercurrent carried by topologically non-trivial low energy ABSs in a Nb/Bi$_2$Se$_3$/Nb tri-junction SQUID where two of the three superconducting leads are connected by a loop. An electrostatic top gate allows strong modulation of the supercurrent despite a high bulk contribution to the normal state conductance. In response to a magnetic field threading flux within the superconducting loop, we find unconventional SQUID oscillations enclosed by an envelope associated with a clear diffraction pattern, indicating spatially uniform and symmetric JJs. At a critical gate voltage, when the trivial 2DEG at the surface is nearly depleted, we observe a sharp drop in the critical current, signaling a topological phase transition in which the nature of the supercurrent-carrying states is transformed. This transition is accompanied by qualitative changes in the SQUID oscillations, magnetic diffraction pattern, and temperature dependence of the critical current. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D42.00006: Proximity induced superconductivity in the 3D topological insulator HgTe probed with scanning SQUID microscope Ilya Sochnikov, John R. Kirtley, Kathryn A. Moler, Luis Maier, Christoph Bruene, Hartmut Buhmann, Laurens W. Molenkamp Inducing superconductivity on the surface of a 3D topological insulator is important for novel broken symmetry phases. However, it is difficult to assess the existence of the surface superconductivity with a single experimental technique. We have used a scanning SQUID microscope to characterize the magnetic properties of hybrid structures made of the 3D topological insulator HgTe and superconducting Nb. The magnetic response of superconducting rings with exotic Josephson junctions reveals the current-phase relation, while measurements of bilayer HgTe/Nb disks reveal the total superfluid density of the hybrid structure. We analyze the degree of skew in the current-phase relation to determine the relative contribution of surface states, and discuss other contributions to the current-phase relation. This work sets an agenda for discussion of the prospects for detection of new broken symmetry phases in 3D topological insulators. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D42.00007: Induced Superconductivity in the Quantum Spin Hall Edge Hechen Ren, Sean Hart, Timo Wagner, Philipp Leubner, Mathias Muehlbauer, Christoph Bruene, Hartmut Buhmann, Laurens Molenkamp, Amir Yacoby Two-dimensional topological insulators have a gapped bulk and helical edge states, making it a quantum spin Hall insulator. Combining such edge states with superconductivity can be an excellent platform for observing and manipulating localized Majorana fermions. In the context of condensed matter, these are emergent electronic states that obey non-Abelian statistics and hence support fault-tolerant quantum computing. To realize such theoretical constructions, an essential step is to show these edge channels are capable of carrying coherent supercurrent. In our experiment, we fabricate Josephson junctions with HgTe/HgCdTe quantum wells, a two-dimensional material that becomes a quantum spin Hall insulator when the quantum well is thicker than 6.3 nm and the bulk density is depleted. In this regime, we observe supercurrents whose densities are confined to the edges of the junctions, with edge widths ranging from 180 nm to 408 nm. To verify the topological nature of these edges, we measure identical junctions with HgTe/HgCdTe quantum wells thinner than 6.3 nm and observe only uniform supercurrent density across the junctions. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D42.00008: Momentum space Cooper pairing in a spin-momentum locked Dirac gap on the surface of a topological insulator Su-Yang Xu Superconductivity in Dirac systems is one of the central theoretical themes in modern physics. In particular, a helical superconductor is a theoretically predicted exotic topological phase of matter, which can be experimentally realized if superconductivity can be induced in an odd number of spin-helical Dirac electronic states. By spectroscopically momentum-resolving the superconducting proximity effect at the boundary of a topological insulator ultra-thin film, we experimentally present direct experimental evidence for a helical topological superconductor via the observation of superconductivity in an odd number of spin-momentum locked topological surface states. Observation of helical superconductivity opens the door to a number of novel topological phenomena such as supersymmetry and Abelian Majorana modes in a condensed matter context. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D42.00009: Anomalous Cooper pair interference on Bi$_{2}$Te$_{3}$surface Li Lu, Jie Shen, Yue Ding, Yuan Pan, Fan Yang, Fanming Qu, Zhongqing Ji, Xiunian Jing, Jie Fan, Guangtong Liu, Changli Yang, Genghua Chen We have performed phase-sensitive measurements on particularly designed superconducting quantum interference devices constructed on the surface of topological insulators Bi$_{2}$Te$_{3}$ in such a way that a substantial portion of the interference loop is built on the proximity-effect-induced superconducting surface. Two types of Cooper interference patterns have been recognized at low temperatures. One is s-wave like and is contributed by a zero-phase loop inhabited in the bulk of Bi$_{2}$Te$_{3}$. The other, being identified to relate to the surface states, is anomalous for that there is a phase shift between the positive and negative bias current directions. The results support that the Cooper pairs on the surface of Bi$_{2}$Te$_{3}$ have a 2$\pi $ Berry phase which makes the superconductivity p-wave-like. Mesoscopic hybrid rings as constructed in this experiment are presumably arbitrary-phase loops suitable for studying topological quantum phenomena. \\[4pt] [1] J. Shen, et al., arXiv:1303.5598v3 [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D42.00010: Proximity Effect in a Topological Insulator on a Cuprate d-wave Superconductor Tonica Valla, Turgut Yilmaz, Ivo Pletikosic, Andrew Weber, Genda Gu, Elio Vescovo, Boris Sinkovic Proximity induced $s$-wave superconductivity in a 3D topological insulator (TI) represents a new avenue for observing zero-energy Majorana fermions inside the vortex cores. A relatively small gap and a low transition temperature of conventional $s$-wave superconductors put the hard constraints on these experiments. Larger gaps and higher transition temperatures in cuprate superconductors would significantly relax these constraints, but with intrinsic zero-energy modes in vortex cores, it is not clear if Majorana fermions could be resolved. Here, we present our angle-resolved photoemission studies of thin TI films grown $in$-$situ$ on optimally doped Bi2212 substrates. We discuss the obtained thickness dependence and the symmetry of the gap induced in the topological surface state on the prospects of detecting Majorana modes in such systems. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D42.00011: New Excitation at the Interface between High-Temperature Superconductors and Topological Insulators Parisa Zareapour, Alex Hayat, Shu Yang Frank Zhao, Michael Kreshchuk, Yong Kiat Lee, Anjan Reijnders, Achint Jain, Zhijun Xu, Alina Yang, G.D. Gu, Shuang Jia, Robert Cava, Kenneth Burch There has been an increased interest in the interplay between d-wave superconducting order parameter and helical surface states of a topological insulator, due to the recent theoretical proposals predicting the emergence of novel excitations at these interfaces. Motivated by these intriguing proposals, we fabricated high-temperature superconductor/topological insulator junctions by the mechanical bonding method [1]. We report the observation of a zero-bias conductance peak (ZBCP) at temperatures below the critical temperature of the bulk superconductor. The ZBCP in our data indicates the emergence of a new excitation in our devices. I will present a detailed study of the differential conductance measurement of our samples at various temperatures and magnetic fields.\\[4pt] [1] P. Zareapour, et al., Nature Communications 3, 1056 (2012). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D42.00012: Two-dimensional superconductivity realized in an MBE-grown Bi2Te3/FeTe heterostructure Qing Lin He, Hongchao Liu, Mingquan He, Ying Hoi Lai, Hongtao He, Gan Wang, Kam Tuen Law, Rolf Lortz, Jiannong Wang, Iam Keong Sou We report a superconductivity realized at the interface of a Bi2Te3/FeTe heterostructure fabricated via van der Waals epitaxy using the molecular beam epitaxy technique, which appears even when the thickness of Bi2Te3 is as thin as one quintuple layer. The two-dimensional nature of the observed superconductivity with the highest transition temperature around 12 K was verified by the existence of a Berezinsky-Kosterlitz-Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching the superconducting transition temperature. The underlying mechanism of this interfacial superconductivity will be discussed. The heterostructure studied in this work provides an ideal platform with unconventional superconductivity for hosting Majorana fermions and studying their exotic physics. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D42.00013: Simulations and characterization of arrays of Josephson junctions on the surface of a topological insulator Erik Huemiller, Cihan Kurter, Aaron Finck, Dale Van Harlingen Topological insulators (TI) have drawn a great deal of interest due to their unique surface states protected by time-reversal symmetry and strong spin-orbit coupling. Josephson junctions made by proximity coupling of s-wave superconductors (S) through the surface states of 3D TI have been predicted to produce excitations of Majorana fermions, which modify the usual current-phase relationship (CPR). In this talk, we present simulations of arrays of superconducting islands connected by Josephson junctions with a CPR of the form of $I_1 \sin{\phi} + I_2 \sin{\phi/2}$. We calculate the energy of the metastable states of the array and the resistance in dynamical states as a function of external magnetic field, and junction critical current for different array sizes and geometries. The 4$\pi$-periodic component of the CPR lifts the degeneracy to create additional metastable states and a modulation of the energy and resistance that depends on whether the number of vortices per cell is even or odd. We discuss experimental progress towards the fabrication of superconducting islands connected by S/TI/S junctions and their characterization by transport and imaging. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D42.00014: Probing the local environment of a superconductor-proximitized nanowire using single electron transistors Fei Pei, Maja Cassidy, Sebastien Plissard, Diana Car, Erik Bakkers, Leo Kouwenhoven Majorana bound states are predicted to arise in semiconducting nanowires with strong spin-orbit coupling that are proximity-coupled to a s-wave superconductor and exposed to a magnetic field. Recent tunneling spectroscopy experiments have shown signatures of Majorana bound states through the existence of a peak in conductance that remains fixed to zero bias over a wide range in magnetic fields. Observation of the delocalized nature of these states remains an outstanding challenge. Here we present measurements of a InSb nanowire proximitized by a central superconducting contact. Normal metal leads allow tunneling spectroscopy from each end of the wire, while nearby single electron transistors provide simultaneous information on the local environment both within the proximitized wire and at each end. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D42.00015: Superconducting transport through InAs/GaSb heterostructures Vlad Pribiag, Christophe Charpentier, Werner Wegscheider, Leo Kouwenhoven Type-II InAs/GaSb heterostructures have recently attracted interest as a two-dimensional topological insulator that can be tuned between the normal and topological quantum phases by means of electrostatic gating. In proximity to a superconductor, 2D topological insulators are predicted to host Majorana zero-modes, a consequence of the helical nature of their edge conduction modes. Here, we present transport measurements through SNS junctions based on InAs/GaSb with NbTiN superconducting contacts. We observe induced supercurrents and investigate the effects of gating and applied magnetic fields, highlighting the potential for Majorana-detection experiments in this system. [Preview Abstract] |
Session D43: Interaction Induced Topological Insulators
Sponsoring Units: DCMPChair: Oliver Rader, The Helmholtz Centre Berlin for Materials and Energy Research
Room: Mile High Ballroom 4B
Monday, March 3, 2014 2:30PM - 2:42PM |
D43.00001: Correlated topological phase in rare earth Hexaboride Nan Xu, X. Shi, P. Biswas, C. Matt, R. Dhaka, Y. Huang, N. Plumb, M. Radovic, J. Dil, E. Pomjakushina, K. Conder, A. Amato, Z. Salman, D. Paul, J. Mesot, Hong Ding, Ming Shi We have performed an angle-resolved photoemission spectroscopy study on SmB6 in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. Our results reveal one electron-like 5d bulk band centered at the X point of the bulk Brillouin zone that is hybridized with strongly correlated f electrons, as well as the opening of a Kondo band gap ($\sim$20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center Gamma-bar point and boundary X-bar point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB6 is likely to be related to the pronounced surface states sitting inside the band hybridization gap and the presence of a topological Kondo insulating state. Recent result on rare earth Hexboride will be shown. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D43.00002: The changes in surface states in SmB$_{6}$ depending on non-magnetic/magnetic dopants B.Y. Kang, Chul-Hee Min, M.S. Song, B.K. Cho After the metallic surface states in SmB$_6$ have given rise to the constant resistivity at \textit{T} $<$ 4 K [1], it has received intensive attention because SmB$_6$ can be a topological insulator that possesses strongly correlated electrons in contrast with the 3D band topological insulators, \textit{i.e.} Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$. Here, we show the differences of electrical transport properties in high-quality single crystals of Sm$_{1-x}$\textit{R}$_x$B$_6$ (\textit{R} = La, Ce) which are synthesized using high-temperature \textit{Al} solution growth methods. When non-magnetic La ion 3\% is doped in SmB$_6$, the surface states are maintained, but, when magnetic Ce ion 3\% is doped, they are destroyed. Our results indicate that these are topological surface states that are sensitive to magnetic ion, which is breaking time reversal symmetry. Moreover, we will discuss about quantum percolation limit obtained from the electric properties of Sm$_{1-x}$La$_x$B$_6$ (x = 0, 0.03, 0.1, 0.2, 0.25, 0.3, 0.35, 0.6, 0.8, 0.9), and the resistivity vs. temperature of doped SmB$_6$ in detail.\\[4pt] [1] Wolgast, S. \textit{et al.} Low temperature surface conduction in the Kondo insulator SmB$_6$, arXiv:1211.5105 (2012) [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D43.00003: Observation of possible topological in-gap surface states in the Kondo insulator SmB$_{6}$ by photoemission Juan Jiang, Sheng Li, Tong Zhang, Zhe Sun, Fei Chen, Zirong Ye, Min Xu, Qingqin Ge, Shiyong Tan, Xiaohai Niu, Miao Xia, Binping Xie, Yufeng Li, Xianhui Chen, Haihu Wen, Donglai Feng SmB$_{6}$, a well known Kondo insulator, exhibits transport anomaly at low temperature which is usually attributed to some ``in-gap'' states. While recent theoretical calculations and transport measurements suggest that these in-gap states could be ascribed to topological surface states. SmB$_{6}$ thus might be the first realization of topological Kondo insulator (TKI). Here by performing angle-resolved photoemission spectroscopy (ARPES), we directly observed several dispersive states within the hybridization gap of SmB$_{6}$, which show negligible k$_{z}$ dependence, indicative of their surface origin. Furthermore, the photoemission circular dichroism of the in-gap states suggests the chirality of the orbital angular momentum, and these states vanish simultaneously with the hybridization gap around 150 K. These all strongly suggest their possible topological origin. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D43.00004: Kondo hybridization and the origin of metallic states at the (001) surface of SmB$_{6}$ Emmanouil Frantzeskakis, Nick de Jong, Berend Zwartsenberg, Yingkai Huang, Yu Pan, Xin Zhang, Jiuxing Zhang, Fanxing Zhang, Lihong Bao, Ojiyed Tegus, Andrei Varykhalov, Anne de Visser, Mark Golden Is SmB$_{6}$ an ideal topological insulator with states of topological character located in a Kondo hybridization gap? SmB$_{6}$ could be the first of a new material class of topological Kondo insulators. We present high-resolution ARPES data showing that Kondo hybridization is the key to unraveling the origin of two metallic states observed in the electronic structure of SmB$_{6}$(001). One is of bulk origin, while the other represents a good candidate for a topological surface state. However, before this claim is substantiated by measuring its massless dispersion relation, our data raises the bar in terms of the energy resolution required, as we uncover strong renormalization of the hybridization gaps compared to theory. Our results map the electronic landscape in SmB$_{6}$, pointing the way for future work in the quest of Dirac cones in the first topological Kondo insulator. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D43.00005: SmS: a mixed valence semi-metal with topological band structure Jian-Zhou Zhao, Feng Lu, Hongming Weng, Zhong Fang, Xi Dai The electronic structure of typical mixed valence compound SmS has been revisited by applying the local density approximation(LDA) plus Gutziwiller method. We predict that the black phase of SmS is a narrow gap semiconductor with band structure strongly renormalized by correlation effect. While for the golden phase of SmS, which will be stabilized under pressure, the electronic structure is similar to the strong three dimensional topological insulator with non-trivial $Z_2$ index but zero indirect gap. The surface state for (001), (111) and (011) surfaces have been obtained by our LDA+Gutzwiller calculations, indicating its non-trivial topological nature. We have also calculated the thin film sub band structure of SmS growing along both (001) and (111) directions. Our calculations show that the double layer thin film shown along the (111) direction is a 2D topological insulator. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D43.00006: Topological Phase Transition in the SmS Kondo Insulator under pressure Zhi Li, Jin Li, Peter Blaha, Nicholas Kioussis Employing LDA$+U $electronic structure calculations we predict that SmS undergoes a topological phase transition from the trivial Kondo insulator (KI) black phase to a topological metallic gold phase under hydrostatic pressure. The underlying mechanism is the pressure-induced change of the 4$f $level from below to above the bottom of the 5$d$ conduction band, leading to a \textit{df} band inversion, a parity sign reversal, and the concomitant change of the topological invariant. This provides the first material realization of the topological classification of KIs proposed by Dzero \textit{et al}. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D43.00007: Cubic Topological Kondo Insulators Victor Alexandrov, Maxim Dzero, Piers Coleman Current theories of Kondo insulators employ the interaction of conduction electrons with localized Kramers doublets originating from a tetragonal crystalline environment, yet all Kondo insulators are cubic. Here we develop a theory of cubic topological Kondo insulators involving the interaction of spin quartets with a conduction sea. The spin quartets greatly increase the potential for strong topological insulators, entirely eliminating the weak-topological phases from the diagram. We show that the relevant topological behavior in cubic Kondo insulators can only reside at the lower symmetry X or M points in the Brillouin zone, leading to a three Dirac cones in ARPES measurements. [the work is accepted for publication in PRL] [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D43.00008: Topological phase transition on honeycomb lattice with third neighbor hooping Yao-Hua Chen, Hsiang-Hsuan Hung, C.S. Ting The topological phases originating in spin-orbital coupling systems have attracted great attention in modern condensed matter physics. Many interesting phenomena have been found in recent theoretical and experimental works, such as the integer and fractional quantum Hall effect, topological band insulator, topological Mott insulator, and topological superconductor. We have investigated the topological phase transition on honeycomb lattice with third neighbor hooping by employing the cellular dynamical mean-field theory combining with the continuous-time Monte Carlo method. The non-trivial topological insulator can be found by observing the spin Chern number directly, and the effects of the third neighbor hopping and interaction are also discussed. Furthermore, we also provide the whole phase diagram for interaction, third neighbor hopping, and temperature. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D43.00009: Interaction-induced topological orbital phases in tetragonal t2g systems Yuan-Yen Tai, C.-C. Joseph Wang, Jian-Xin Zhu, Matthias J. Graf, Chin-Sen Ting We theoretically predict the anomalous orbital Hall(AOH) effect based on an reliable effective two-orbital model. This model reveals four Dirac-like linear dispersion with C$_{4v}$ symmetry. We find a ground state with spontaneous orbital current order driven by inter-orbital Coulomb interaction. The orbital order breaks the degeneracy of Dirac linear dispersion and has topologically nontrivial Chern number $C = \pm 2$. With open boundaries, we show the edge states are topologically protected. We find a new Z$_2$ topological insulating phase protected by time reversal(TR) symmetry and orbital exchange symmetry when spin degrees of freedom are incorporated. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D43.00010: Plutonium hexaboride is a correlated topological insulator Xiaoyu Deng, Kristjan Haule, Gabriel Kotliar We predict that plutonium hexaboride (PuB$_6$) is a strongly correlated topological insulator, with Pu in an intermediate valence state of Pu$^{2.7+}$. Within the combination of dynamical mean field theory and density functional theory, we show that PuB$_6$ is an insulator in the bulk, with non-trivial $Z_2$ topological invariants. Its metallic surface states have large Fermi pocket at $\bar{X}$ point and the Dirac cones inside the bulk derived electronic states causing a large surface thermal conductivity. PB$_6$ has also a very high melting temperature therefore it has ideal solid state properties for a nuclear fuel material. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D43.00011: Topological crystalline Kondo insulators and universal topological surface states of SmB$_6$ Mengxing Ye, J.W. Allen, Kai Sun We prove theoretically that certain strongly correlated Kondo insulators are topological crystalline insulators with nontrivial topology protected by crystal symmetries. In particular, we find that SmB$_6$ is such a material. In addition to a nontrivial Z$_2$ topological index protected by time reversal symmetry, SmB$_6$ also has nontrival mirror Chern numbers protected by mirror symmetries. On the $(100)$ surface of SmB$_6$, the nontrivial mirror Chern numbers do not generate additional surface states beyond those predicted by the Z$_2$ topological index. However, on the $(110)$ surface, two more surface Dirac points are predicted. Remarkably, we find that for SmB$_6$ both the Z$_2$ topological index and the mirror Chern numbers are independent of microscopic details, which enable us to obtain surface state properties that are universal. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D43.00012: Unusual in-gap and hybridized states of a topological Kondo insulator candidate, SmB$_{6}$ B. Zhou, Z.K. Liu, S.H. Yao, H.T. Yuan, Y. Zhang, M.H. Lu, Y.F. Chen, H. Huang, X. Dai, Z. Fang, Y. Cui, H.Y. Hwang, Z. Hussain, Z.-X. Shen, S.-K. Mo, Y.L. Chen Topological Kondo insulators represent a new type of topological insulator, in which a Kondo insulator exhibits non-trivial topological electronic structure and possesses an odd number of surface Dirac fermions in the bulk energy gap. Using angle-resolved photoemission spectroscopy (ARPES) and electric transport measurements, we investigated the electronic structure of SmB$_{6}$, a topological Kondo insulator candidate proposed recently. More details will be introduced in the presentation. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D43.00013: Scanning Tunneling Microscopy and Spectroscopy of Kondo Insulator SmB6 Di Chen, Yuntao Li, Xunchi Chen, Zhiling Dun, Geoffrey Rojas, Haidong Zhou, Petro Maksymovych, Phillip First, Zhigang Jiang Kondo insulator SmB6 has recently been predicted to be a candidate three-dimensional topological insulator with truly insulating bulk. Here we report on a pilot scanning tunneling microscopy and spectroscopy (STS) study of the surface properties of single crystal SmB6. We find that a room-temperature cleaved SmB6 (001)-surface is mostly disordered, while large 3x1 reconstructed areas can be obtained by annealing the samples at 1450\textordmasculine C. Without cleaving, the as-grown (001)-surface also exhibits a 3x1 reconstruction after room-temperature sputtering, and annealing at 1450 \textordmasculine C. At low temperatures, a gap-like feature appears in the measured STS spectra, and finite density of states is observed at the Fermi energy. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D43.00014: Large high quality crystals of the Topological Kondo Insulator, SmB$_{6}$ Geetha Balakrishnan, Monica Ciomaga Hatnean, D.McK. Paul, M.R. Lees SmB$_{6}$ has been predicted to be a Topological Kondo Insulator, the first strongly correlated heavy fermion material to exhibit topological surface states. High quality crystals are necessary to investigate the topological properties of this material. Single crystal growth of the rare earth hexaboride, SmB$_{6}$, has been carried out by the floating zone technique using a high power xenon arc lamp image furnace. Large, high quality single-crystals are obtained by this technique. The crystals produced by the floating zone technique are free of contamination from flux materials and have been characterised by resistivity and magnetisation measurements. These crystals are ideally suited for the investigation of both the surface and bulk properties of SmB$_{6}$. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D43.00015: Low-temperature magnetotransport in topological Kondo insulator SmB$_6$ Yasuyuki Nakajima, Paul Syers, Xiangfeng Wang, Renxiong Wang, Johnpierre Paglione The Kondo insulater SmB$_6$ is a promising candidate for realizing a topological Kondo insulator, where topologically non-trivial surface states can be realized in the Kondo hybridization gap driven by strong correlation. Although recent experimental studies have revealed the existence of metallic surface states in SmB$_6$, the non-trivial nature of the surface states remains to be conclusively verified. We report a detailed study of the magnetoresistance of SmB$_6$ at milliKelvin temperatures, reporting strong indications of the topological nature of the surface states in SmB$_6$. [Preview Abstract] |
Session D44: Functional Materials & Devices
Sponsoring Units: FIAPRoom: Mile High Ballroom 4C
Monday, March 3, 2014 2:30PM - 2:42PM |
D44.00001: Adsorption of 2,4,6-trinitrotoluene on the ZnO (2$\bar{1}\bar{1}$0) surface: a density functional theory study of the detection mechanism of ZnO nanowire chemiresistors Sufian Alnemrat, Gary Brett, Joseph Hooper We report first-principles calculations of the adsorption of 2,4,6-trinitrotoluene (TNT), a prototypical nitroaromatic explosive, on the ZnO (2$\bar{1}\bar{1}$0) surface. This surface is common among ZnO chemiresistors being considered for trace explosive detection. Recent work has achieved 60 ppb detection of TNT using a ZnO nanowire array, but the physical mechanism of sensing is unclear. Our results indicate that TNT strongly chemisorbs via interactions between the oxygen on the nitro groups and surface zinc, creating surface states within the gap. We present a simple theoretical estimate showing the strong effect of these surface states on the depletion layer of ZnO nanowires. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D44.00002: Organic-inorganic hybrid nanocomposite materials for radiation detection Sunil Sahi, Wei Chen Scintillator is the material that emits light when excited with high energy radiation. Inorganic single crystals and organic (plastic and liquid) scintillator are the most widely used scintillator. Inorganic crystals have higher efficiency and high stopping power but single crystal are difficult to grow and are very expensive. Also, some inorganic scintillators like NaI-Tl are not environmental friendly. On the other hand organic scintillators have poor stopping power because of low Z-value. This limits the application of organic scintillator. Here we have proposed a nanocomposite scintillator by embedding the inorganic nanoparticles into organic polymer. Nanoparticles are synthesized and characterized using XRD and TEM. As synthesized nanoparticles are then embedded in to the polymer matrix to make nanocomposite scintillator and their optical properties have been studied. The nanocomposite scintillators have shown improved luminescence properties as compared to the plastic scintillator. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D44.00003: Structural and spectroscopic properties of rare earth doped crystal-in-glass waveguides as influenced by the initial glass composition Brian Knorr, Adam Stone, Himanshu Jain, Volkmar Dierolf Laser induced crystallization of glasses is a highly spatially selective process which has the potential to produced compact, integrated optics within a glass matrix. Specifically, our interest is in using this technique to create a laser. In order to achieve this goal, preliminary research was performed on single crystal lines ``written'' in Er$_{0.002}$La$_{0.998}$BGeO$_{5}$ glass using a femtosecond pulsed laser. This study revealed promising results including incorporation of the erbium into the crystal and the ability to waveguide with low losses, but also illuminated surprising features concerning the distribution of rare earth (RE) ions within the crystal. To further investigate this phenomenon and its potential consequences for our intended application, additional crystalline waveguides were written inside of a series of glasses with compositions of the form RE$_{x}$La$_{1-x}$BGeO$_{5}$, where RE=Pr, Nd, and Er and x=0.002, 0.010, 0.040, 0.100. and 0.200. These structures were analyzed using micro-Raman and luminescence spectroscopy as well as energy-dispersive x-ray spectroscopy. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D44.00004: Towards Multiple-Bit-Per-Cell Operation In a Single Active Layer-Phase Change Memory Cell Ibrahim Cinar, Vedat Karakas, Onur Dincer, Ozgur Burak Aslan, Aisha Gokce, Barry Stipe, Jordan A. Katine, Gulen Aktas, Ozhan Ozatay High contrast between 0 and 1 logic states in addition to other superior properties of phase change memory (PCM) brought out the possible application of multiple logic levels in a single bit in an effort to boost data storage density. The potential stabilization of resistance levels in between the 0 polycrystalline and 1 amorphous states enables storage of several data in a single device cell (such as 00, 01,10,11 levels). Here we report our investigation of the role of contact geometry and fabrication induced modification of phase change kinetics in stabilizing mixed phase states in an effort to obtain such multiple-bit per cell operation within a single layer PCM material system (Ge$_{\mathrm{2}}$Sb$_{\mathrm{2}}$Te$_{\mathrm{5}})$. The nature of switching dynamics appears highly sensitive to exact programming current distribution and defect density such that a nanoscale square contact with effective current localization at the sharp corners facilitates the formation of stable intermediate phases as compared to a circular one. Resistance maps show that the top contact geometry and engineering of defects can be used as an effective handle to tune the resistance states to optimize memory cells for ultra-high density storage. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D44.00005: Hydrogen Dynamics and Metallic Phase Stabilization in VO$_{2}$ Keith H. Warnick, Bin Wang, Soktrates T. Pantelides Hydrogen doping has been demonstrated to lower the VO$_{2}$ semiconductor-to-metal phase transition below room temperature. We report the results of DFT calculations that show that metallic phase stabilization is due to the lattice distortion caused by interstitial hydrogen attached to oxygen atoms. We show that doping is energetically favored and that there is a fast diffusion in the monoclinic [100] direction that can facilitate atomic hydrogen uptake through surfaces that expose these channels. However, the dissociation of molecular hydrogen on a monoclinic (100) surface has a 1.6 eV activation barrier that impedes hydrogen association or dissociation at the surface without significantly elevated temperatures. These results emphasize the role of lattice distortion in the VO$_{2}$ phase transition and suggest methods to improve the use of hydrogen doping to control the properties of VO$_{2}$. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D44.00006: Quantum Hooke's Law to Classify Pulse Laser Induced Ultrafast Melting Hao Hu, Hepeng Ding, Feng Liu We investigate the ultrafast crystal-to-amorphous phase transition induced by femtosecond pulse laser excitation by exploiting the property of quantum electronic stress (QES) induced by the electron-hole plasma, which follows quantum Hooke's law. We demonstrates that two types of crystal-to-amorphous transitions occur in two distinct material classes: the faster nonthermal process, having a time scale shorter than one picosecond (ps), must occur in materials like ice having an anomalous phase diagram characterized with dT$_{\mathrm{m}}$/dP \textless 0, where T$_{\mathrm{m}}$ is the melting temperature and P is pressure; while the slower thermal process, having a time scale of several ps, occurs preferably in other materials. The nonthermal process is driven by the QES acting like a negative internal pressure, which is generated predominantly by the holes in the electron-hole plasma that increases linearly with hole density. These findings significantly advance our fundamental understanding of physics underlying the ultrafast crystal-to-amorphous phase transitions, enabling quantitative a priori prediction. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D44.00007: \textit{Ab initio} study on ferroelectric instability induced by relativistic effects in PbTe Jinwoong Kim, Seung-Hoon Jhi A recent study [E. S. Bozin \textit{et al.}, Science 330, 1660 (2010)] reported unusual ferroelectric instability in lead chalcogenides at heating, which is contrast to typical ferroelectric transitions that occur at cooling. This study explains the emergence of local dipole formation due to the softening of transverse optical (TO) phonon modes. However, standard first-principles calculations do not support the phonon softening (imaginary frequency). Here, we present that the spin-orbit interaction should be included in the calculations to correctly produce the instability and that, as such, thermal expansion leads to the softening in TO phonon modes. Another controversial finding in experiment that the frequency of TO mode is finite and increases with temperatures can be explained if anharmonic effects are considered together with the spin-orbit interaction. Our study shows that the spin-orbit interaction can be critical for the structural stability and thus affect the thermoelectric or structural phase transition. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D44.00008: Lattice dynamics in perovskite halides CsSnX$_3$ with X=I,Br,Cl Ling-Yi Huang, Walter Lambrecht We investigate the origin of the phase transitions between the cubic, tetragonal and orthorhombic phases of CsSnX$_3$, X=I, Br,Cl, in terms of the imaginary phonon frequencies of the higher symmetry phases at the zone boundaries and the associated rotations and tilts of the SnX$_6$ octahedra. We present first-principles calculations of the phonon band-structure and density of states as well as the predicted infrared spectra. The calculations are done using density functional perturbation theory. In the cubic phase, there are three triply degenerate IR active $T_{1u}$ modes and one silent $T_{2u}$ mode. We find that the calculated modes agree with the experiment when we assign the second and third calculated modes to the experimental first and second modes. Our calculated IR spectra show that the third observed mode in IR absorption is actually the highest LO rather than TO mode and the lowest calculated mode is found to overlap in frequency with a peak in density of phonon states. This indicates the possibility of a strong phonon-phonon interaction and hence short phonon-lifetime or strong broadening which could explain why this mode has not been observed. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D44.00009: Stereochemical activity of lone-pair electrons in ABX$_{3}$ heavy-element halides Young-Moo Byun, Eva Smith, Craig Fennie ABX$_{3}$ halides display many functionalities and properties such as ferroelectricity and superconductivity. ABX$_{3}$~(A $=$ Rb, Cs; B $=$ Sn, Pb; X $=$ F, Cl) form generally in two different structures: sheelite~(CaWO$_{4})$-like and perovskite. The interplay of the stereochemically active lone pair (i.e., second-order Jahn-Teller) B-cation and other structural distortion is the key to determining the stable structure and subsequently the physical properties. It turns out that in these heavy p-block element compounds relativistic effects influence the activity of the lone pair in a nontrivial way. In this talk we will present our first-principles study of structural properties of this family of ABX$_{3}$ halides and how they change with temperature and pressure. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D44.00010: Novel sp3-hybridized framework structure of group 14 elements Manh Cuong Nguyen, Xin Zhao, Cai-Zhuang Wang, Kai-Ming Ho Using genetic algorithm atomic structure prediction method and first-principles calculations, we discovered a novel low-energy metastable structure of group 14 elements in P42/mnm symmetry. The P42/mnm structure is a cage-like distorted sp3-hybridized framework structure with the cage's volume $\sim$ 4\% larger than the average cage's volume of the clathrate type-I structure, indicating P42/mnm structure a good gases or metal atoms encapsulation structure. The band structure calculations show that P42/mnm Si and Ge are semiconducting with energy band gaps close to the optimal values for optoelectronic or photovoltaic applications. The metal atom encapsulation P42/mnm structure of group 14 elements could also be a candidate for rattling-mediated superconducting or ``a phonon glass and an electrical crystal'' thermoelectric materials. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D44.00011: Strain measurements of Ge epilayers on Si by Spectroscopic Ellipsometry A. Ghosh, N. Fernando, A.A. Medina, C.M. Nelson, S. Zollner, S.C. Xu, J. Menendez, J. Kouvetakis Using spectroscopic ellipsometry, we determined the strain of a Ge epilayer grown on a Si (100) substrate. This strain depends on the sample temperature and arises because of the difference in thermal expansion coefficients between Si and Ge. It can be calculated since the thermal expansion coefficients of Si and Ge are known very precisely, if we assume that the Ge epilayer was fully relaxed at the growth temperature, leading to an increase in strain as the temperature decreases. We calculate in-plane tensile strain values of 0.12\% at 300 K or 0.19\% at 77K for our Ge on Si layer, that compares favorably with an in-plane strain of 0.11\% derived from shifts of the Ge lattice reflection at 300 K by x-ray diffraction. This temperature-dependent strain affects the energies of the E1 and E1+Delta1 critical points of the Ge epilayer, which can be measured very precisely using spectroscopic ellipsometry from 77 to 800 K.From the difference in the critical point energies between our Ge epilayers on Si and bulk Ge (up to 20 meV), we can calculate the strain from the known elastic constants and deformation potentials. The strain determined from ellipsometry agrees well with the strain calculated from the temperature-dependent thermal expansion coefficient. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D44.00012: Study of the Electronic Properties of Different Phases of Cu3V-VI4 Based on First-Principle Calculation Tingting Shi, Wanjian Yin, Mowafak Al-Jassim, Yanfa Yan Considering the small energy differences among the Cu3V-VI4 compounds in four different structures, enargite, wurtzite-PMCA, famatinite and zinc-blend-PMCA, a large variety of phases with different band properties may co-exist. This paper systematically studies the trend of the electronic properties of these phases; the band gap will greatly decrease when the phase changes from enargite to wurtzite-PMCA, or from famatinite to zinc-blende-PMCA. For example, the band gap of enargite Cu3PS4 is 2.51 eV, while the famatinite one is 1.72 eV. In addition, the band gap will obviously decrease as we increase the atomic number of group-V or group-VI element for one structure. Due to the wide band range from 0.4 eV to 2.5 eV for all possible Cu3V-VI4 structures, our detailed first-principle study will suggest guidelines for the band gap engineering for potential photovoltaic applications. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D44.00013: A variational polaron-self-interaction corrected total-energy functional for charge excitations in wide-gap insulators Babak Sadigh, Paul Erhart, Daniel Aberg A simple modification of the density-functional theory (DFT) total energy functional is proposed that corrects for the polaron self-interaction error in the semilocal approximations (LDA/GGA) to the exchange-correlation potential. It can accurately reproduce polaron formation in widegap insulating materials. Extensive study of the potential-energy landscapes of self-trapped holes in alkali halides is performed and agreeable comparison with hybrid-DFT and experiment is obtained. The new functional is general, simple to implement and its variational formulation allows for ab-initio molecular-dynamics simulations of polarons in widegap insulators regardless of complexity. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D44.00014: Study of strain-mediated couplings in SrRuO$_3$-CoFe$_2$O$_4$ nanocomposite by Raman spectroscopy Yi-Chun Chen, Chia-Hsien Chien, Yen-Chin Huang, Heng-Jui Liu, Ying-Hao Chu Self-assembled vertical nanostructures have the advantage of high interface-to-volume ratio and can be used to generate new functionalities by the choice of combination of constituents. Recently, we found an interesting behavior of photo-induced magnetization switching in a self-assembled system, CoFe$_2$O$_4$ (CFO) nanopillars embedded in SrRuO$_3$(SRO) matrix. In this study, to further reveal the coupling mechanism of this hetero-structure, we used Raman spectroscopy to investigate their phonons under external stimulus. When an external out-of-plane magnetic field is applied, the CFO A1g phonon (688 cm$^{-1}$) had a red shift due to the negative magnetostriction effect, while the SRO Ag phonon (183 cm$^{-1}$) also had a correspondent red shift. This result indicates the crystal structures of SRO matrix are affected by the deformation of the CFO pillars through the magnetostrictive couplings. Moreover, at the phase transition temperature of SRO (160 K), three phonons (T$_{2u}$, E$_g$, E$_u$) of CFO also had a significant blue shift, which reveals again the strain-mediated coupling. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D44.00015: Selectable trapping or rotation of micro-particles using a plasmonic Archimedes spiral Chen-Bin Huang, Wei-Yi Tsai, Jer-Shing Huang We demonstrate selectable trapping or rotation of dielectric micro-particles by optical near fields generated in a plasmonic Archimedes spiral. Depending on the handedness of circularly polarized excitation, plasmonic near fields can be engineered into either a super-focusing spot for particle trapping or a plasmonic vortex for particle rotation. The optical forces are numerically analyzed. Experimentally, selectable trapping or rotation of single microsphere and sphere cluster are both realized. [Preview Abstract] |
Session D45: Quantum Hall Effect: Theory
Sponsoring Units: FIAPChair: Kun Yang, Florida State University
Room: Mile High Ballroom 4D
Monday, March 3, 2014 2:30PM - 2:42PM |
D45.00001: Anyonic Symmetries and Non-Abelian Topological Defects of Bosonic Abelian Fractional Quantum (Spin) Hall States in the $ADE$ Classification Mayukh Khan, Jeffrey Teo, Taylor Hughes We consider bosonic abelian Fractional Quantum Hall (FQH) and Fractional Quantum Spin Hall (FQSH) states with edge theories drawn from the $ADE$ Kac Moody algebras at level $1.$ This set of systems have `anyonic' symmetries that leave braiding and fusion invariant Remarkably, the group of anyonic symmetries for this class of models is isomorphic to the symmetries of the Dynkin diagrams of the particular ADE Lie Algebra under consideration. The triality symmetry of the Dynkin diagram of $so(8)$ leads to the largest anyonic symmetry group $S_3$ (the permutation group on 3 elements). Each element of the anyonic symmetry group corresponds to a distinct way of gapping out the edge (i.e., each element corresponds to a Lagrangian subgroup). Junctions between two distinct gapped edges host non abelian twist defects with quantum dimensions ($>1$). In the case of $so(8)$ we have more exotic twist defects with non-abelian fusion. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D45.00002: Experimental Proposal to Detect Topological Ground State Degeneracy Maissam Barkeshli, Yuval Oreg, Xiao-Liang Qi One of the most profound features of topologically ordered states of matter, such as the fractional quantum Hall (FQH) states, is that they possess topology-dependent ground state degeneracies that are robust to all local perturbations. Here we present the first proposal to directly detect these topological degeneracies in an experimentally accessible setup. The detection scheme uses nonlinear electrical conductance measurements in a double layer FQH system, with appropriately patterned top and bottom gates. We propose two experimental platforms; in the first, the detection of topo- logically degenerate states coincides with the detection of ZN parafermion zero modes. We map the relevant physics to a single-channel ZN quantum impurity model, providing a novel generalization of the Kondo model. Our proposal can also be adapted to detect the ZN parafermion zero modes recently discovered in FQH line junctions proximitized with superconductivity. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D45.00003: Chiral Luttinger liquids and a generalized Luttinger's theorem in fractional quantum Hall edges via finite-entanglement scaling Daniel Varjas, Michael Zaletel, Joel Moore We use bosonic field theories and the infinite system density matrix renormalization group (iDMRG) method to study infinite strips of fractional quantum Hall (FQH) states starting from microscopic Hamiltonians. Finite-entanglement scaling allows us to accurately measure chiral central charge, edge mode exponents and momenta without finite-size errors. We analyze states in the first and second level of the standard hierarchy and compare our results to predictions of the chiral Luttinger liquid ($\chi$LL) theory. The results confirm the universality of scaling exponents in chiral edges and demonstrate that renormalization is subject to universal relations in the non-chiral case. We prove a generalized Luttinger's theorem involving all singularities in the momentum-resolved density, which naturally arises when mapping Landau levels on a cylinder to a fermion chain and deepens our understanding of non-Fermi liquids in 1D. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D45.00004: Identifying Non-Abelian Topological Order through Minimal Entangled States Wei Zhu, Shoushu Gong, Duncan Haldane, D.N. Sheng The topological order is encoded in the pattern of long-range quantum entanglements, which cannot be mea- sured by any local observable. Here we perform an exact diagonalization study to establish the non-Abelian topological order for topological band models through entanglement entropy measurement. We focus on the quasiparticle statistics of the non-Abelian Moore-Read and Read-Rezayi states on the lattice models with bosonic particles. We identify multiple independent minimal entangled states (MESs) in the groundstate mani- fold on a torus. The extracted modular S matrix from MESs faithfully demonstrates the Majorana quasiparticle or Fibonacci quasiparticle statistics, including the quasiparticle quantum dimensions and the fusion rules for such systems. These findings unambiguously demonstrate the topological nature of the quantum states for these flatband models without using the knowledge of model wavefunctions. We also establish that MESs manifest the eigenstates of nonlocal Wilson loop operators for the non-Abelian topological states and encode the full information of the topological order. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D45.00005: Momentum polarization of non-Abelian topologically ordered states Yi Zhang, Xiao-liang Qi We study momentum polarization of non-Abelian topologically ordered states for the Gutzwiller projected Chern insulator wave function with Chern number C=2. The resulting quasiparticle topological spin and edge central charge confirm the field theory description of an SU(2) gauge field coupled to $\nu=2$ fermions and rule out other candidate theories. We also discuss characteristic differences and the quantum phase transition between this non-Abelian topological phase and an Abelian topological phase described by the projected wave function of two C=1 Chern insulators. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D45.00006: $Z_2$ fractional topological insulators in two dimensions Cecile Repellin, Andrei Bernevig, Nicolas Regnault The simplest example of a two dimensional fractional topological insulator (FTI) consists of two decoupled copies of a Laughlin state with opposite chiralities. Using a simple microscopic model at half filling, we study the stability of this type of FTI phase upon addition of two coupling terms of different nature: a Rashba term, and an interspin interaction term. Using exact diagonalization and entanglement spectrums, we numerically show that the FTI phase survives significant amplitudes of both the band structure and the interaction coupling terms, at different system sizes. We compare our system to a similar two component fractional Chern insulator. Our study shows that the time reversal invariant system survives the introduction of interaction coupling on a much larger scale than the time reversal symmetry breaking one, stressing the importance of time reversal symmetry in the FTI phase stability. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D45.00007: Discrete Symmetry Breaking in Fractional Chern Insulators Akshay Kumar, Rahul Roy, S.L. Sondhi We study the interplay between quantum hall ordering and spontaneous translational symmetry breaking in a multiple Chern number (C $>$ 1) band at partial filling. We begin with non-interacting fermions in a family of square lattice models with flat C=2 bands and a wide band gap, and add nearest neighbor density-density repulsive interactions. By means of Hartree-Fock theory supplemented by numerical exact diagonalization for a small system at 1/2 filling, we find that the system generically develops charge density wave order with two degenerate ground states. We note that this physics is especially transparent in the limit in which the C=2 band describes two decoupled C=1 bands. We discuss the nature of domain walls in this phase and note the close analogy to the quantum Hall Ising ferromagnet in the multivalley problem. Finally we discuss generalizations to other fillings and higher Chern numbers. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D45.00008: Models for the phase transition between a Fermi liquid and fractional Chern insulator Joel Moore, Michael Zaletel, Siddharth Parameswaran A partially filled band with nonzero Chern density can support fractional quantum Hall states (``fractional Chern insulators'') as a consequence of repulsive interactions between electrons. In the absence of this repulsion, the ground state is generically a simple band metal with an anomalous Hall effect. There are several possible scenarios for a second-order transition between metallic and quantum Hall states, which can be approached as a composite-fermion band crossing, a coupling between Luttinger liquids, or via a parton construction. We discuss the extent to which these scenarios lead to different predictions and test those predictions by density-matrix renormalization group calculations. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D45.00009: Fractional Chern insulators on finite cylinders and their bulk-edge correspondence Zhao Liu, Dmitry Kovrizhin, Emil Bergholtz, Ravindra Bhatt It has been recently realized that strong interactions in topological Bloch bands give rise to the appearance of novel states of matter. Here we study these systems -- fractional Chern insulators -- via generalization of a gauge-fixed Wannier-Qi construction in the cylinder geometry. Our setup offers a number of important advantages compared to the earlier exact diagonalization studies on a torus. Most notably, it gives access to edge states and to a single-cut orbital entanglement spectrum, hence to the physics of bulk-edge correspondence. It is also readily implemented using density matrix renormalization group method which allows for numerical simulations of significantly larger systems. Previously [Z. Liu et al., Phys. Rev. B {\bf 88}, 081106(R) (2013)], this approach was applied to bosons on the ruby lattice model at filling $\nu=1/2$ and $\nu=1$, which show the signatures of (non)-Abelian phases, and we establish the correspondence between the physics of edge states and entanglement in the bulk. Here, we generalize this to other fillings such as $\nu=2/3$. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D45.00010: The impossibility of exactly flat non-trivial Chern bands in strictly local periodic tight binding models Li Chen, Tahereh Mazaheri, Alexander Seidel, Xiang Tang We investigate the possibility of exactly flat non-trivial Chern bands in tight binding models with local (strictly short-ranged) hopping parameters. We demonstrate that while any two of three criteria can be simultaneously realized (exactly flat band, non-zero Chern number, local hopping), it is not possible to simultaneously satisfy all three. We discuss both the case of a single flat band, for which we give a rather elementary proof, as well as the case of multiple degenerate flat bands. In the latter case, our result follows by making use of K-theory. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D45.00011: Criticality in Translation-Invariant Parafermion Chains Wei Li, Shuo Yang, Hong-hao Tu, Meng Cheng Parafermionic zero modes have been recently proposed to emerge at certain topological defects in Abelian fractional quantum Hall systems. In this work, we investigate the phase diagram of a translationally invariant $Z_3$ parafermion chain, with nearest- and next-nearest-neighbor hopping terms. The model can be mapped to a $Z_3$ Potts model with nearest-neighbor couplings via a generalized Jordan-Wigner transformation. The phase diagram is obtained numerically via accurate density matrix renormalization group method, and six gapless phases with central charges being 4/5, 1 or 2 are found. By checking the energy derivatives, we observe continuous phase transitions between $c=1$ and $c=2$ phases, while the phase transition between $c=4/5$ and $c=1$ is conjectured to be of Kosterlitz-Thouless type. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D45.00012: Twist Defects in Topological Systems with Anyonic Symmetries Jeffrey Teo, Abhishek Roy, Xiao Chen Twist defects are point-like objects that support robust non-local storage of quantum information and non-abelian unitary operations. Unlike quantum deconfined anyonic excitations, they rely on symmetry rather than a non-abelian topological order. Zero energy Majorana bound states can arise at lattice defects, such as disclinations and dislocations, in a topological crystalline superconductor. More general parafermion bound state can appear as twist defects in a topological phase with an anyonic symmetry, such as a bilayer fractional quantum Hall state and the Kitaev toric code. They are however fundamentally different from quantum anyonic excitations in a true topological phase. This is demonstrated by their unconventional exchange and braiding behavior, which is characterized by a modified spin statistics theorem and modular invariance. Gauging anyonic symmetries by treating twist defects as quantum excitations provides a connection between some non-abelian topological states and abelian ones. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D45.00013: Convergence of Topological Entanglement Entropy for Finite Size Systems Clare Abreu, Raul Herrera, Edward Rezayi Quantum information theoretic concepts have been widely used to study topological phases of condensed matter, the prime examples of which are fractional quantum Hall states. Interest in these phases is driven in part by their potential use in fault-tolerant topological quantum computation. In particular, quantum entanglement has proven to be a useful tool to probe topological order. We present numerical studies for some model fractional quantum Hall states in spherical and toroidal geometries. We implement bipartitioning of the system with both orbital and real space cuts for small size systems. Additionally, we compare the topological entanglement entropies obtained from low-order Renyi entropies to the expected value to determine whether our results converge for small sizes. We extend these studies to generic Hamiltonians and discuss the prospect of obtaining the topological entanglement entropy from finite size calculations in these systems. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D45.00014: Matrix Product States for Non-Abelian Quasiholes Yang-Le Wu, B. Estienne, N. Regnault, B. Andrei Bernevig Exotic phases in fractional quantum Hall effect provide a potential platform for the realization of non-Abelian anyons. A large class of physically relevant trial wave functions for these strongly-correlated phases can be constructed from the many-point correlators in various chiral conformal field theories. It was recently realized that this construction can be naturally reformulated in terms of matrix product states and efficiently carried out on a computer, even for interacting conformal fields. In this talk, I will explain how to construct the matrix product state representation of quasihole wave functions, and employ this new numerical tool to examine the braiding statistics and the screening property of several non-Abelian quantum Hall states, including the Moore-Read and the Read-Rezayi states, as well as the Gaffnian wave function. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D45.00015: Critical integer quantum Hall topology in the integrable Maryland model Sriram Ganeshan, Kostyantyn Kechedzhi One-dimensional tight binding models such as Aubry-Andre-Harper (AAH) model (with onsite cosine potential) and the integrable Maryland model (with onsite tangent potential) have been the subjects of extensive theoretical research in localization studies. AAH can be directly mapped onto the two-dimensional Hofstadter model that manifests the integer quantum Hall topology on a lattice. However, no such connection has been made for the Maryland model (MM). In this talk, we present a generalized model that contains AAH and MM as the limiting cases with the MM lying precisely at a topological quantum phase transition (TQPT) point. A remarkable feature of this critical point is that the 1D MM retains well-defined energy gaps whereas the equivalent 2D model becomes gapless, signifying the 2D nature of the TQPT. The criticality allows us to associate topological invariants with the Maryland model in a restricted mathematical sense at the special filling factors that are adiabatically connected to the spectral gaps in the 1D Aubry-Andre-Harper model. Our theory presented in this work establishes deep and unexpected mathematical connections between 2D topological models and a family of 1D incommensurate localization models. [Preview Abstract] |
Session D47: Theory of Unconventional Superconductivity: Mainly Topical Phases
Sponsoring Units: DCMPChair: Michael Senthef, Stanford University
Room: Mile High Ballroom 4F
Monday, March 3, 2014 2:30PM - 2:42PM |
D47.00001: Intrinsic triplet p$+$ip topological superconductivity in low filled graphene Tianxing Ma, Fan Yang, Hong Yao, Hai-Qing Lin Inspired by the continuously distributed Van-Hove saddle points at the band bottom, we studied the low filled Hubbard-model on the Honeycomb-lattice with negative next-nearest-neighbor hopping integrals, which represents the graphene system. Within different parameter regimes, our combined weak coupling random phase approximation and strong coupling determinant quantum Monte-Carlo approaches consistently reveal the triplet p$+$ip topological superconductivity in the ground state of the system. Further more, when a weak Kane-Mele spin-orbit coupling is included, the time-reversal invariant Z2 weak topological superconductivity will be realized in the system. Our unbiased numerical results provide basis to realize the intrinsic exotic topological superconductivity in the graphene and similar systems. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D47.00002: Possible exotic superconductivity in the monolayer and bilayer silicene Fan Yang, Yugui Yao, Li-Da Zhang, Cheng-Cheng Liu, Feng Liu Silicene, the silicon-based counterpart of graphene, has attracted a lot of research interest since synthesized recently. Similar honeycomb lattice structures of the two systems let them share most of their marvelous physical properties. The most important structural difference between the two systems lie in the noncoplanar lowbuckled geometry in silicene, which brings up a lot of interesting physical consequence to the system. Here we focus on possible exotic superconductivity (SC) in the family, via random phase approximation (RPA) study on the relevant Hubbard-models. Two systems of this family are studied, including the monolayer and bilayer silicene. For the former system, we found an electric-field driven quantum phase transition (QPT) from chiral d+id to f-wave SC when the field is perpendicular to the silicene plane. For the latter system, we found that even the undoped system is intrinsically metallic and superconducting with chiral d+id symmetry and tunable Tc which can be high . Our study not only provides a new playground for the study of the exotic SC, but also brings a new epoch to the familiar Si industry. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D47.00003: Possible Topological Superconducting Phases of Heavy Gated MoS$_2$ Noah Yuan, K.F. Mak, K.T. Law Molybdenum disulfide (MoS2) has attracted a lot of attention recently because of its grapheme-like crystal structure and massive Dirac spectrum at low energy. Recently, it was found that thin films of MoS2 become superconducting when they are heavy gated with a critical temperature of about 10 K at optimal gating [1]. In this presentation, we discuss the possible pairing symmetries of MoS2 according to group-theoretical calculations. Depending on the sign and strength of the on-site and next nearest neighbor interaction, we found that MoS2 can support two topological phases. In the chiral d-wave phase, the system breaks time-reversal symmetry spontaneously and supports chiral Majorana edge states. In a spin singlet and triplet mixing phases, the system respects time-reversal symmetry and support helical Majorana edge states. Experimental signatures of the topological superconducting phases are discussed. \\[4pt] [1] J. T. Ye et al. Science 338, 1193 (2012). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D47.00004: Superconductivity on the brink of spin-charge order in doped honeycomb bilayer Oskar Vafek, James Murray, Vladimir Cvetkovic Using a controlled weak-coupling renormalization group approach, we establish the mechanism of unconventional superconductivity in the vicinity of spin or charge ordered excitonic states for the case of electrons on the Bernal stacked bilayer honeycomb lattice. With one electron per site this system exhibits nearly parabolically touching conduction and valence bands. Such a state is unstable towards a spontaneous symmetry breaking, and repulsive interactions favor excitonic order, such as a charge nematic and/or a layer antiferromagnet. We find that upon adding charge carriers to the system, the excitonic order is suppressed, and unconventional superconductivity appears in its place, before it is replaced by a Fermi liquid. We focus on firmly establishing this phenomenon using the RG formalism within an idealized model with parabolic touching. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D47.00005: Renormalization group study of excitonic and superconducting order in doped honeycomb bilayer James Murray, Oskar Vafek We explore the competition between spin-charge order and unconventional superconductivity in the context of the AB stacked bilayer honeycomb lattice, realized experimentally as bilayer graphene, which features approximately parabolically touching electron bands. Using a weak-coupling renormalization group theory, we show that unconventional superconductivity arises generically for repulsively interacting fermions as excitonic order is suppressed by adding charge carriers to the system. We investigate the effects of finite temperature and further-neighbor hopping, the latter of which leads to so-called ``trigonal warping'' and destroys the perfect circular symmetry of the Fermi surfaces. We show that superconductivity survives for a finite range of trigonal warping, and that the nature of the superconducting phase may change as a function of further neighbor hopping. Depending on the range of interactions and the degree of trigonal warping, we find that the most likely superconducting instabilities are to f-wave, chiral d-wave, and pair density wave phases. It is shown that unconventional superconductivity is significantly enhanced by fluctuations in particle-hole channels, with the critical temperature reaching a maximum near the excitonic phase. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D47.00006: Topologically stable gapped state in a layered superconductor Mauro Doria, Marco Cariglia, Alfredo A. Vargas-Paredes We show that a layered charged superconductor, described by a spinorial (two-component) order parameter, has a gapped state above the ground state, topologically protected from decay. This state is made of skyrmions, breaks the time reversal symmetry and produces a weak local magnetic field. This excited but stable state contains spontaneous circulating supercurrents, with flow and counter flow in the layers, even without the presence of an external magnetic field. We derive the order parameter and the local magnetic field of this skyrmion state from the Abrikosov-Bogomolny (first order) equations, instead of the second order variational equations. We find a gap density of the order of $0.1\,h_{max}\,\mbox{meV.nm}^{-3}$, where $h_{max}$ is the maximum local magnetic field between layers expressed in Gauss. The present threshold of detection, set by $\mu$SR and NMR/NQR, $h_{max}\sim 0.01\,\mbox{G}$, gives a gap density of the order $10^{-3}\,\mbox{meV.nm}^{-3}$ for the single-layer cuprates (inter-layer distance $d \approx 1.0 nm$). We suggest that the pseudogap is a skyrmion state, and so, estimate that the density of carriers that condense in the pseudogap is of the order of $10^{-4} \,\mbox{nm}^{-3}$, or $0.01\%$ of the Cooper pair density in the cuprates. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D47.00007: Novel phases in topological superconducting quantum dots Karen Michaeli, Liang Fu Recent progress in realizing topological superconductors has paved the road to study new physical phenomena resulting from the non-abelian statistics of the Majorana modes they host. A particularly interesting situation arises when Majorana bound states in a closed topological superconducting dot are coupled to external normal leads. In this talk, we will show that interactions with the quantum dot drive the lead electrons into a non-Fermi liquid phase, which can be understood by mapping the problem to a variant of a Kondo system. Interestingly, the non-Fermi liquid states in these systems are more robust than in the conventional two channel Kondo problem. This is because realizations with different numbers of metallic leads are connected to each other by a line of fixed points. We will conclude with a discussion of the experimental consequences of our theory. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D47.00008: Magnetic edge states and mixed-parity pairing in spin-triplet superconductors Mario Cuoco, Paola Gentile, Canio Noce, Ilya Vekhter, Alfonso Romano We show that a spontaneous magnetic moment may appear at the edge of a spin-triplet superconductor if the system allows for pairing in a subdominant channel and non-uniform spatial profile. To unveil the microscopic mechanism behind such effect we combine numerical solution of the Bogoliubov-De Gennes equations for a tight-binding model with nearest-neighbor attraction, and the symmetry based Ginzburg-Landau approach. We find that a modulation of the electronic density near the edge of the system leads to a non-unitary superconducting state where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the non-unitary state and originates in the lifting of the spin-degeneracy of the Andreev bound-states. For chiral spin-triplet superconductors spin current flows along the interface and surface charge currents exhibit anomalous dependence on the magnetization. - A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, Phys. Rev. Lett. 110, 267002 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D47.00009: Field-induced phase transitions in spin-orbit coupled superconductors Florian Loder, Arno P. Kampf, Thilo Kopp Spin-orbit coupling (SOC) or magnetic fields both split the otherwise degenerate spin eigenstates in metals. A pairing interaction may then lead to Cooper pairs which are either of intra- or of inter-band pairing type. These pairing states are separated by a first-order phase transition depending on the relative strength of SOC and the magnetic field [1]. We analyze this phase transition for a two-dimensional electron system in an in-plane magnetic field and show that the spin-triplet component of the superconducting order parameter reaches its maximum exactly at the phase transition. The superconducting energy gap closes at this transition and thereby allows for a change in the topological character of the superconductor. We suggest that this in-plane magnetic field driven transition is well suited for experimental detection because of the absence of orbital depairing effects.\\[4pt] [1] F. Loder {\it et al}., J. Phys. Condens. Matter {\bf 25}, 362201 (2013) [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D47.00010: Topological superconducting states with magnetic moments on a conventional $s$-wave superconductor Sho Nakosai, Yukio Tanaka, Naoto Nagaosa The search for topological properties in superconductors has been one of the most highlighted topics in nearly a decade. Especially Majorana fermions, appearing as topologically protected boundary modes associated with nontrivial features of superconductors, provide a promising platform for quantum computations. Therefore there is a real need for designing adapted superconductivity with ordinary materials. In this talk, we will present theoretical calculations on unconventional superconductivity induced by the magnetic moments in a conventional spin-singlet $s$-wave superconductor [1]. By choosing the spin directions of these moments, one can design spinless pairing states appearing within the $s$-wave superconducting energy gap. It is found that the helix spins produce a $p_x+p_y$-wave state while the skyrmion crystal configuration a $p_x+ip_y$-wave-like state. Nodes in the energy gap and the zero-energy flat band of Majorana edge states exist in the former case, while the chiral Majorana channels along edges of the sample and the zero-energy Majorana bound state at the core of the vortex appear in the latter case. \\[4pt] [1] Sho Nakosai, Yukio Tanaka, and Naoto Nagaosa, PRB 88, 180503(R) (2013) [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D47.00011: Non-uniform superconducting states in Nematic Electronic Liquid Crystal Phases Rodrigo Soto Garrido, Eduardo Fradkin We study the possible superconducting states that arise in a nematic Fermi fluid state in the spin-triplet channel. First, we study the nematic $\alpha$ phase in the $l=2$ state, which is invariant under a $\pi/2$ rotation followed by a spin flip. In this phase the only infinitesimal superconducting instability is in the spin-triplet p-wave channel. However, close enough to the nematic transition both a uniform d-wave superconducting state and a non-uniform state (pair density wave or checkerboard), also with d-wave symmetry, can arise. In addition, we study the nematic $\beta$ phase, in which the spin polarization winds around the Fermi surface, and we also find that it is possible to have a non-uniform superconducting state above a critical value of the coupling constant, which is controlled by the order parameter of nematic phase. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D47.00012: Quantum quenching through a topological phase transition in a system with open boundaries Vasudha Shivamoggi, Smitha Vishveshwara, Diptiman Sen We study the effect of open boundaries on the non-adiabatic dynamics of a system driven across a topological phase transition. The closing of the bulk gap at the critical point implies that a quantum quench across the critical point necessarily results in a states with defects. The presence of mid-gap surface states in a topological phase modifies the usual Kibble-Zurek scaling used to describe the defect density. We investigate the phase transition in a 1D spinless p-wave superconductor between a non-topological phase and a topological phase with Majorana fermions localized at the ends. We calculate the non-adiabatic evolution of the ground state across the transition and the overlap of this state with the instantaneous ground state. We also discuss the consequences of the topologically protected edge states on defect production. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D47.00013: Odd-frequency superconductivity in two-channel Kondo lattice Shintaro Hoshino, Yoshio Kuramoto Unconventional superconductivity has been sought as an intriguing ground state or thermodynamic state in condensed matter physics. Among those states, we address the odd-frequency (OF) pairing state, which breaks the gauge symmetry, but has zero pairing amplitude at equal time. Possible realizations of the OF superconductivity have been theoretically proposed in a variety of strongly correlated electron systems. In particular, Emery and Kivelson have shown for the two-channel Kondo impurity that the OF pairing susceptibility is enhanced at the impurity site. However, no microscopic theory has established the OF pairing in the two-channel Kondo lattice. Recently, we have demonstrated the emergence of odd-frequency s-wave superconductivity in the two-channel Kondo lattice using the dynamical mean-field theory explicitly by divergence of the OF susceptibility. The corresponding order parameter is given by staggered composite-pair amplitude with even frequencies, which involves both localized spins and conduction electrons. The Kondo effect in the presence of two channels is essential for the present unconventional superconductivity. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D47.00014: The interplay between topological p-wave superconductivity and odd-frequency pairing in superconducting proximity systems Valentin Stanev, Victor Galitski We study the proximity-induced superconductivity in semiconductor nanowires. The interplay between superconductivity and spin-orbit coupling plays a crucial role in proposals for creating Majorana fermions in semiconducting heterostructures. To further elucidate the physics of such devices we employ the quasiclassical Green's functions methods. We show that the spatial variations of the superconducting order parameter leads to non-trivial effects in the nanowire. We demonstrate the appearance of odd-frequency pairing correlations close to the boundaries, and discuss their effect on the density of states. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D47.00015: Quantum Theory of Heat Magnetization Atsuo Shitade We give the thermodynamic definition of the heat magnetization, and calculate with use of the Keldysh formalism in a curved spacetime. As the charge current is coupled to a vector potential, the heat current is coupled to a part of a vielbein. Consequently, as we define the orbital magnetization by a magnetic field, we can define the heat magnetization by a torsional magnetic field induced by a vielbein. Such heat magnetization, together with the Kubo formula for the thermal conductivity calculated by a torsional electric field, leads to the proper thermal Hall conductivity satisfying the Wiedemann-Franz law. Our results indicate that the quantum thermal Hall effect in (2$+$1)-D time-reversal-broken topological insulators or superconductors is described by the Chern-Simons action of a vielbein. \\[4pt] [1] A. Shitade, arXiv:1310.8043.\\[0pt] [2] A. Shitade, arXiv:1310.8046. [Preview Abstract] |
Session D48: Focus Session: Spin Transport and Magnetization Dynamics in Metal-Based Systems: Phononic and Thermal Phenomena
Sponsoring Units: DMP FIAP GMAGChair: Ion Garate, Universite de Sherbrooke
Room: Mile High Ballroom 1A
Monday, March 3, 2014 2:30PM - 2:42PM |
D48.00001: Possible charge analogues of spin transfer torques in bulk superconductors Ion Garate Spin transfer torques (STT) occur when electric currents travel through inhomogeneously magnetized systems and are important for the motion of magnetic textures such as domain walls. Since superconductors are easy-plane ferromagnets in particle-hole (charge) space, it is natural to ask whether any charge duals of STT phenomena exist therein. We find that the superconducting analogue of the adiabatic STT vanishes in a bulk superconductor with a momentum-independent order parameter, while the superconducting counterpart of the nonadiabatic STT does not vanish. This nonvanishing superconducting torque is induced by heat (rather than charge) currents and acts on the charge (rather than spin) degree of freedom. It can become significant in the vicinity of the superconducting transition temperature, where it generates a net quasiparticle charge and alters the dispersion and linewidth of low-frequency collective modes. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D48.00002: Superfluid Spin Transport through Easy-Plane Ferromagnetic Insulators So Takei, Yaroslav Tserkovnyak Superfluid spin transport | dissipationless transport of spin | is theoretically studied in a ferromagnetic insulator with easy-plane anisotropy. We consider an open geometry where spin current is injected into the ferromagnet from one side by a metallic reservoir with a nonequilibrium spin accumulation, and ejected into another metallic reservoir located downstream. Spin transport through the device is studied using a combination of magnetoelectric circuit theory, Landau-Lifshitz-Gilbert phenomenology, and microscopic linear-response theory. We discuss how spin superfluidity can be probed using a magnetically-mediated electron-drag experiment. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D48.00003: Quasi-one-dimensional phonon anomaly in the narrow-gap semiconductor FeSb$_2$ Igor Zaliznyak, Cedomir Petrovic, Rongwei Hu, Andrei Savici, Ovidiu Garlea, Barry Winn FeSb$_2$ has a variety of unusual properties, ranging from the temperature-induced electronic paramagnetism to one-dimensional (1D) metallic conductivity at temperatures below $\sim 300$ K and down to $\sim 30$ K, where it becomes insulating, and to giant thermoelectric power factor. While it is generally acknowledged that these properties result from the tight balance between strong covalent hybridization, electronic correlation and the tendency to band delocalization, what exactly are the mechanisms leading to these unusual behaviors remain unclear. In particular, it is a matter of current debate, whether the giant thermoelectric figure of merit observed in FeSb$_2$ is explained by purely correlated-electronic mechanism, or it results from peculiar interaction of electrons with the lattice vibrations - such as the phonon drag effect. Here we present the inelastic neutron scattering survey of the phonon spectra in FeSb$_2$. It reveals phonon dispersions of one-dimensional character, which mirror the 1D metallicity along the $b-$axis, and the quasi-one-dimensional magnetism observed in the insulating sister material CrSb$_2$. Phonon dispersions undergo dramatic changes in the temperature range where electronic paramagnetism emerges, indicating strong coupling to electrons. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D48.00004: Spontaneous magnetic fluctuations in ultrathin magnetic films at zero field Andrew Balk, John Unguris We use magneto optical Kerr effect (MOKE) microscopy to observe room temperature, zero field magnetic fluctuations in perpendicularly magnetized cobalt films at thicknesses near the in-plane to out-of-plane spin reorientation transition.~ The magnetic behavior of our films resembles that of collections of superparamagnetic particles, in that globally they exhibit zero net moment, while local areas continually undergo thermal magnetic fluctuations between saturated states of the maze-like domain structure.~ Unlike superparamagnetic particles, the fluctuations are not constrained by particle boundaries and thus are subject to both exchange and magnetostatic interactions.~ Due to this we can observe temporal and spatial correlations in the fluctuations.~ Furthermore, we observe that the fluctuations obey dynamics distinct from field-driven Barkhausen jumps.~ We also determine scaling exponents of the fluctuations, finding their areas follow a power law distribution (t $=$1.5), and their temporal noise power spectrum is close to 1/f (a $=$ 1.04).~ Based on these observations, we discuss these films as possible candidates for exhibiting magnetic self-organized criticality. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D48.00005: Crystal structure, Magnetic, and Anomalous Schottky Specific Heat of Rare Earth Dialuminides Arjun Pathak, Karl A. Gschneidner, Jr., Vitalij Pecharsky Materials with structural transformations or distortions coupled to magnetic transitions show interesting magnetostrictive, magnetoresistive, and magnetocaloric behavior and are, therefore, important subject of study in condensed matter physics. The importance of either coupled or decoupled magnetostructural transformations has been shown for many materials starting from high temperature superconductors and perovskites to multifunctional intermetallics. The anomalies close to 0 K encompass another playground for the fundamental physics, and they range from the Kondo effect and heavy fermion behavior to quantum criticality and nuclear Schottky specific heat. These remarkable behaviors are ultimately related to the interplay between localized and delocalized electrons, for which lanthanides are truly the best model provided by nature. In particular, the rare earth dialuminides, which have simple cubic Laves phase structure at room temperature have long been the system of choice to understand the fundamentals of rare earth magnetism and low temperature anomalies. In this presentation, we will discuss the low temperature crystal structure, magnetic and thermodynamic properties of binary and pseudobinary rare earth dialuminides by means of low temperature x-ray diffraction, magnetization and heat capacity measurements. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D48.00006: Berry curvature in magnetostatic waves and associated semiclassical dynamics of wavepackets Shuichi Murakami, Ryo Matsumoto, Ryuichi Shindou Magnons in ferromagnets form band structure and thus they are associated with Berry curvature in momentum space. This Berry curvature of magnons causes various interesting phenomena such as thermal Hall effect [1,2]. In particular the magnetic dipolar interaction can be regarded as a spin-orbit coupling in a wider sense and thus can give rise to nonzero Berry curvature. In my presentation, we describe the magnetostatic waves (magnons) in terms of the bosonic Bogoliubov-de Gennes Hamiltonian [3] and show how the magnon thermal Hall conductivity behaves as a function of magnetic field and temperature. We also present how this Berry curvature affects the dynamics of the magnon wavepacket within semiclassical theory. For example, the wavepacket rotates by itself and will give rise to a radial charge distribution due to relativistic effect. We also show how the magnon trajectory is affected by the Berry curvature. [1] R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, 197202 (2011). [2] R. Matsumoto, S. Murakami, Phys. Rev. B 84, 184406 (2011). [3] R. Matsumoto, R. Shindou and S. Murakami, preprint. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D48.00007: Thermally induced transparency for short spin wave pulses in yttrium iron garnet (YIG) films Cesar Leonardo Ordonez Romero, Oleg Kolokoltsev, Ivan Gomez Arista, Naser Qureshi, Guillermo Monsiv\'ais Galindo, Hesiquio Vargas Hern\'andez The compensation of spin wave propagation losses plays a very important role in the development of novel magnonic devices. Up to now, however, most of the known amplification methods present relative narrow frequency bandwidths due to their resonant nature. In this work, we present compensation of the propagation losses or pseudo-amplification of travelling spin waves by tailoring the bias magnetic field profile. The thermally-induced non-uniform profile of the magnetization introduced on an Yttrium Iron Garnet (YIG) thin film by a localized spot of a cw argon-ion laser creates the conditions to observe the complete compensation of the spin wave propagation losses. The spin wave evolution was mapped with a time and spaced resolved inductive magneto-dynamic prove system. The experiment was carried out using a uniform sample of single-crystal YIG film grown on a gallium-gadolinium garnet (GGG) substrate. The 2mm-wide, 20mm-long and 6microns-thick YIG strip was saturated with an external magnetic field enabling the set up for the propagation of magneto-static surface waves. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D48.00008: Thermally induced spin accumulation at Al/Co$_2$TiSi and Al/Co$_2$TiGe contacts Voicu Popescu, Benjamin Geisler, Peter Kratzer Spin injection from a ferromagnet in a semiconductor substrate can be accomplished either by applying an external voltage or a temperature gradient. In the latter case, one exploits the Seebeck effect, with the temperature gradient across the contact directly resulting in a difference in chemical potentials in the two spin channels due to the spin-dependence of the Seebeck coefficient. The magnetic Heusler alloys Co$_2$TiSi or Co$_2$TiGe exhibit half-metallic ferromagnetism in their ideal L2$_1$ crystal structure, with a potentially high degree of spin polarization of the injected current. As such, they recommend themselves for integrated spin injectors in combination with the closely lattice-matched Al contact layer. We investigate the possibility of employing Al/Co$_2$TiX/Al (X=Si,Ge) trilayers as thermally driven spin injectors by means of first-principles calculations of the electronic structure and of the thermoelectric transport properties. Our results show that the spin-dependent Seebeck effect is sensitive to the atomic structure of the Heusler/Al interface. In particular, for a thin Co$_2$TiSi or Co$_2$TiGe layer terminated by a TiSi or TiGe atomic plane, the thermal spin accumulation is found to be of the same order of magnitude as the effective Seebeck coefficient. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D48.00009: Evidence for a Magnetic Seebeck effect Sylvain Brechet, Francesco Vetro, Elisa Papa, Stewart Barnes, Jean-Philippe Ansermet Spin caloritronics is mainly focused on studying the effects of a temperature gradient on the time evolution of the local spin average of a classical system. In many experimental situations, the system can be treated as a classical continuum with magnetisation on the scale of interest where the quantum fluctuations average out and the underlying microscopic structure is smoothed out. Recently, we established a clear classical formalism describing the thermodynamics of a matter continuum with magnetisation interacting with external electromagnetic fields. This formalism accounts for the thermal and electric magnetisation accumulations and magnetisation waves. It also predicts that a temperature gradient in the presence of magnetisation waves induces a magnetic induction field, which is the magnetic analog of the Seebeck effect. This thermal gradient modulates the precession and relaxation. The Magnetic Seebeck effect implies that magnetisation waves propagating in the direction of the temperature gradient and the external magnetic induction field are less attenuated, while magnetisation waves propagating in the opposite direction are more attenuated. This effect has been observed on a YIG slab in our laboratory and it is in very good agreement with the thermodynamic prediction. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D48.00010: Modeling the Tunneling Anisotropic Magneto-Seebeck Effect Vivek Amin, Jan Zemen, Jakub Zelezny, Jan Masek, Tomas Jungwirth, Jairo Sinova Due to increasing energy consumption in high-density electronics, the control and recycling of heat generated in nanostructures is highly desirable. The effect of temperature gradients on magnetic nanostructures has thus prompted a renewed interest in the long-known Seebeck effect, as it applies to spin-polarized systems. One such phenomenon is the Magneto-Seebeck (MS) effect, in which the Seebeck coefficient of a magnetic tunnel junction (MTJ) changes based on its magnetization configuration. As a result, the thermal properties of an MTJ can be tuned via magnetic field. We extend the study of this effect to the Tunneling Anisotropic Magnetoresistance (TAMR), in which an MTJ with a single ferromagnetic contact produces a spin-orbit-coupling-induced magneto-transport anisotropy. We present numerical results of this Tunneling Anisotropic Magneto-Seebeck Effect in a CoPt/MgO/Pt heterostructure. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D48.00011: Imaging of In-Plane Magnetization using the Time Resolved Anomalous Nernst Effect Jason Bartell, Darryl Ngai, Gregory Fuchs We report on measurements of the time-resolved anomalous Nernst effect (TRANE) for diffraction-limited imaging of in-plane magnetization using a high resolution optical microscope. In TRANE microscopy, pulsed laser light is used to create a transient thermal gradient perpendicular to the film plane. In response, a voltage is generated by the anomalous Nernst effect. The voltage has an amplitude proportional to the in-plane projection of the magnetic moment along a direction perpendicular to the voltage contacts. We show that the TRANE voltage persists for less than 100 ps in 30 nm thick magnetic samples. Additionally, we demonstrate spatial resolution limited only by the area of the thermal gradient generated by the focused laser pulse. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D48.00012: Asymmetric and Negative Differential Thermal Spin Effect at Magnetic Interfaces: Towards Spin Seebeck Diodes and Transistors Jie Ren, Jian-Xin Zhu We study the nonequilibrium thermal-spin transport across metal-magnetic insulator interfaces. The transport is assisted by the exchange interaction between conduction electrons in the metal and localized spins in the magnetic insulator. We predict the rectification and negative differential spin Seebeck effect (SSE), that is, reversing the temperature bias is able to give asymmetric spin currents and increasing temperature bias could give an anomalously decreasing spin current. We resolve their microscopic mechanism as a consequence of the energy-dependent electronic DOS in the metal. The rectification of spin Peltier effect is also discussed. We then study the asymmetric and negative differential magnon tunneling driven by temperature bias. We show that the many-body magnon interaction that makes the magnonic spectrum temperature-dependent is the crucial factor for the emergence of rectification and negative differential SSEs in magnon tunneling junctions. We show that these asymmetric and negative differential SSEs are relevant for building magnon and spin Seebeck diodes and transistors, which could play important roles in controlling information and energy in functional devices. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D48.00013: Spin Seebeck Effect vs. Anomalous Nernst Effect in Ta/CoFeB /Ta Structures Bowen Yang, Yadong Xu, Mike Schneider, Jing Shi We have studied the spin Seebeck effect (SSE) and anomalous Nernst effect (ANE) in a vertical trilayer structure under a vertical temperature gradient. The structure consists of a 3nm CoFeB layer sandwiched by $\beta $-phase tantalum (Ta) layers. The samples are deposited by magnetron sputtering. The existence of Ta $\beta $-phase is verified by the resistivity and its negative temperature coefficient of resistance(TCR). Under a fixed vertical temperature gradient, the measured transverse thermoelectric voltage is linearly proportional to the total sample resistance when the Ta thickness exceeds 2 nm, which can be explained by a shunting resistor model. When the Ta thickness is below 2 nm, the voltage deviates from the linear resistance dependence and merges to the ANE voltage of the CoFeB single layer, due to a weakened inverse spin Hall effect (ISHE) in Ta thinner than the spin diffusion length. In the linear regime, the slope contains both a varying SSE and a fixed ANE responses, thus the SSE contribution could be quantitatively separated out from the ANE of CoFeB. Our results indicate a large SSE from the $\beta $-phase Ta due to its large Spin Hall Angle. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D48.00014: Measuring spin diffusion length using spin Seebeck effect Harsha Kannan, Xin Fan, John Xiao Ever since its discovery, spin Seebeck effect (SSE) has attracted plenty of attention. The conversion from thermal gradient to spin current has shown great potential in thermal energy harvesting. SSE can also be utilized as a source to generate pure spin current to unveil new physics. Here we show that it is possible to measure spin diffusion length of a heavy metal Pt by studying the SSE as a function of Pt layer thickness. The SSE signal first increases, peaks, and then decreases with increasing Pt layer thickness. By fitting with a drift-diffusion model, we obtain the spin diffusion length of Pt to be about 2nm, close to that obtained from other techniques. Moreover, we can insert a thin layer of Cu in order to remove the possible proximity effect. Similar spin-diffusion length is obtained from this measurement. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D48.00015: Spin-Orbit Caloritronics Aurelien Manchon, Papa Birame Ndiaye, Jung-Hwan Moon, Hyun-Woo Lee, Kyung-Jin Lee Utilizing spin-orbit coupling to enable the electrical manipulation of ferromagnets has recently attracted a considerable amount of interest. This spin-orbit torque [1] appears in magnetic systems displaying inversion symmetry breaking. Another adjacent emerging topic, spin caloritronics [2], aims at exploiting magnonic spin currents driven by temperature gradients, allowing for the transmission of information and the control of magnetic domain walls. In this work, we demonstrate that a magnon flow generates torques on the local magnetization when subjected to Dzyaloshinskii-Moriya interaction (DMI) just as an electron flow generates torques when submitted to Rashba interaction [3]. A direct consequence is the capability to control the magnetization direction of a homogeneous ferromagnet by applying a temperature gradient or local RF excitations. Merging the spin-orbit torques with spin caloritronics is rendered possible by the emergence of DMI in magnetic materials and opens promising avenues in the development of chargeless information technology. [1] Miron et. al, Nature Materials 9, 230 (2010); [2] Bauer, et al. Nat. Mater. 11, 391 (2012); [3] A. Manchon and S. Zhang, Phys. Rev. B 78, 212405 (2008). [Preview Abstract] |
Session D49: Focus Session: Nickelate Heterostructures
Sponsoring Units: DMPChair: Stephen Wilson, Boston College
Room: Mile High Ballroom 1C
Monday, March 3, 2014 2:30PM - 3:06PM |
D49.00001: Characterization and control of orbital, spin and charge order in nickel oxide superlattices Invited Speaker: Bernhard Keimer This talk will provide an overview of spectroscopic experiments on the structural [1,2] and electronic [3-7] properties of nickel oxide superlattices grown by pulsed laser deposition and molecular beam epitaxy. We will discuss recent progress in the quantitative characterization of the Ni d-orbital polarization as a function of epitaxial strain, spatial confinement, and chemical composition [4,5] and discuss its influence on spin [6] and charge [7] order in these systems. The power of spectroscopic methods such as resonant x-ray scattering, spectral ellipsometry, and Raman scattering for the characterization of electronic ordering phenomena in metal-oxide heterostructures and superlattices will be emphasized. \\[4pt] [1] E. Detemple et al., Appl. Phys. Lett. 99, 211903 (2011).\\[0pt] [2] A. Frano et al., Adv. Mater. (2013); doi: 10.1002/adma.201303483.\\[0pt] [3] A.V. Boris et al., Science 332, 937 (2011).\\[0pt] [4] E. Benckiser et al., Nature Mater. 10, 189 (2011).\\[0pt] [5] M. Wu et al., Phys. Rev. B 88, 125124 (2013).\\[0pt] [6] A. Frano et al., Phys. Rev. Lett. 111, 106804 (2013).\\[0pt] [7] A. Frano, M. Hepting et al., unpublished. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D49.00002: Anti-ferromagnetically driven Mott transition in ultrathin nickelates Derek Meyers, Jian Liu, M. Kareev, S. Middey, J.W. Freeland, R. Averitt, A.J. Millis, P. Ryan, J. Chakhalian The independent roles of anti-ferromagnetism and charge ordering in the realization of the temperature induced Mott metal-to-insulator transition within heteroepitaxial nickelate films remain to be disentangled hindering true understanding of the nature of the still actively debated ground state. To this end, we have investigated ultra thin, fully epitaxial films of the strongly correlated electron system NdNiO$_{3}$ by hard and soft resonant x-ray scattering. We find a robust E'-type antiferromagnetic transition, analogous to the bulk ordering, occurs despite the ultra thin nature of the films. Surprisingly, no evidence of a symmetry change was found upon cooling below the metal-to-insulator transition utilizing multiple probes. Supporting theoretical calculations show the anti-ferromagnetic transition corroborates with the opening of the charge excitation gap. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D49.00003: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 3:30PM - 3:42PM |
D49.00004: Tuning the metal-insulator transition temperature of Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ thin films via strain H. Jeffrey Gardner, Vijay Singh, Le Zhang, Xia Hong We have investigated the effect of substrate induced strain and film thickness on the metal-insulator transition of the correlated oxide Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ (SNNO). We have fabricated epitaxial 3 -- 40 nm thick SNNO films on (001) LaAlO$_{3}$ (LAO), (001) SrTiO$_{3}$ (STO), and (110) NdGaO$_{3}$ (NGO) via off-axis RF magnetron sputtering. The SNNO films are atomically smooth with (001) orientation as determined by atomic force microscopy and x-ray diffraction. SNNO films grown on LAO, subject to compressive strain, exhibit a sharp metal-insulator transition at lower temperatures. Conversely, films grown on STO and NGO, subject to tensile strain, exhibit a smeared albeit above room temperature metal-insulator transition. For all substrates, we have observed that the metal-insulator transition temperature ($T_{MI})$ increases monotonically with decreasing film thickness until the electrically dead layer is reached (below 4 nm). We discuss the effect of strain and oxygen deficiencies on the T$_{MI}$ of SNNO thin films. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D49.00005: A structural route to tuning the orbital structure of nickelates Divine Kumah, Ankit Disa, Andrei Malashevich, Hanghui Chen, Sohrab Ismail-Beigi, Fred Walker, Charles Ahn The rare-earth nickelates display a range of interesting magnetic and electronic phenomena arising from the strong coupling of the atomic-scale structural properties of these systems to the charge and orbital degrees of freedom. We report on modifying the orbital polarization in nickelate based heterostructures, motivated by the goal of emulating high-Tc cuprate behavior in the nickelates. Using a combination of synchrotron diffraction structural and spectroscopic characterization and first principles theory, we show how the design of a structure that splits the relative electronic occupation of Ni d x$^{2}$-y$^{2}$ and Ni d 3z$^{2}$-r$^{2}$ orbitals, is achieved in three-component heterostructures. These structures are comprised of LaTiO$_{3}$/LaNiO$_{3}$/LaAlO$_{3}$ and are grown using molecular beam epitaxy. The key features of the theoretically proposed structure, including an internal polar field, a electron transfer from Ti to Ni, and a orbital polarization of the Ni-eg states, are experimentally studied. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D49.00006: Modulating the properties of thin film nickelates using a ferroelectric Matthew S. J. Marshall, Andrei Malashevich, Ankit S. Disa, Hanghui Chen, Sohrab Ismail-Beigi, Fred J. Walker, Charles H. Ahn Controlling materials properties using electric fields is an important approach to creating novel electronic materials. The perovskite oxides, which exhibit some of the most interesting phenomena found in the solid state, represent an ideal system for exploring how electric fields couple to material properties. As an example, the rare-earth nickelates (LaNiO$_{\mathrm{3}}$, NdNiO$_{\mathrm{3}}$, etc.) undergo a metal-insulator transition when the unit cell structure is changed by chemical doping or through the application of strain. Here we show that the polarization of the canonical ferroelectric PbZr$_{\mathrm{0.2}}$Ti$_{\mathrm{0.8}}$O$_{\mathrm{3\thinspace }}$couples to the structure and conductivity of the rare earth nickelates (RNiO$_{\mathrm{3}})$. As the polarization of the PZT is switched, we introduce atomic-scale structural distortions at the PZT-nickelate interface that modulate the carrier concentration in the nickelate. We find that interfacial effects dominate, resulting in a large change in the conductivity of the nickelate. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D49.00007: Field-effect modulation of structure and carrier transport of LaNiO$_3$ thin films Andrei Malashevich, Matthew S.J. Marshall, Ankit S. Disa, Frederick J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi Materials exhibiting large changes in resistivity in response to applied electric fields are of importance due to their technological applicability, e.g., in field-effect transistors. Of particular interest are thin film oxide/ferroelectric interfaces: the ferroelectric permits dynamic modulation of electronic transport in the oxide film which is crucial for non-volatile memory applications. In the standard field effect, resistance modulations result from changes in carrier density created by the applied electric field. At ferroelectric interfaces, however, other mechanisms can come into play. Our experiments show that at the (001) interface of rare-earth nickelates and ferroelectric Pb$_{0.8}$Zr$_{0.2}$TiO$_3$ (PZT), the change of carrier mobility plays a critical role in the electronic transport. Here, we present a first-principles study of the interface between a thin film of conducting LaNiO$_3$ and ferroelectric PbTiO$_3$ (PTO). We analyze the dependence of the atomic structure of the interface on the PTO polarization and the effect of the structural changes on the electronic bands and associated carrier transport. We also describe the methodological challenges in transport calculations of metal/ferroelectric interfaces and some ways to address them. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D49.00008: Tailoring polarity in a layered nickelate with single atomic layer control Anand Bhattacharya, Brittany Nelson-Cheeseman, Hua Zhou, Prasanna Balachandran, Gilberto Fabbris, Jason Hoffman, Daniel Haskel, James Rondinelli Many of the 3d transition metal oxides share a common structural MO$_{\mathrm{6}}$ building unit---a central transition metal (TM) cation octahedrally coordinated with oxygen nearest neighbors. The electronic states in these materials can be modified by tailoring the M-O bonds, which typically include the application of epitaxial strain in thin films, or pressure and isovalent cation substitution in bulk samples. Here, we present a new route to tailor the M-O bonds without changes to the strain state or stoichiometry in two-dimensional perovskite nickelate (n$=$1 in the Ruddlesden Popper series). We do this by tailoring the dipolar electrostatic interactions at the unit cell level in nominally non-polar LaSrNiO$_{\mathrm{4}}$ via single atomic layer-by-layer synthesis using oxide-MBE. We reconstruct the response of the crystal lattice to the induced polarity using a x-ray phase retrieval technique (COBRA). We find that the response of the O anions to the resulting local electric fields distorts the M-O bonds, being largest for the apical oxygens (O$_{\mathrm{ap}})$. It also alters the Ni valence. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D49.00009: Effect of Polar Discontinuity on the Growth of Epitaxial LaNiO$_3$ Ultrathin Films I.-Cheng Tung, G. Luo, D. Morgan, J.H. Lee, H. Hong, S.H. Chang, J.A. Eastman, D.D. Fong, M.J. Bedzyk, J.W. Freeland We have conducted a detailed microscopic study of epitaxial LaNiO$_3$ ultrathin films grown on (001) SrTiO$_3$ as a function of thickness by using oxide molecular beam epitaxy with in-situ surface x-ray diffraction and soft x-ray absorption spectroscopy at the Advanced Photon Source to explore the influence of polar mismatch on the resulting structural and electronic properties. Our data demonstrate that the initial layers on the nonpolar SrTiO$_3$ surface exhibit a smaller than expected out-of-plane lattice-spacing with a Ni valence of 2+. As the film becomes thicker, the lattice constant expands to its elastic limit, and the Ni valence approaches 3+. We will also discuss the energetics for vacancy formation during the initial growth as determined by density functional theory calculations. Work at the APS, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D49.00010: Polar compensation in ultrathin films of a perovskite nickelate S. Middey, P. Rivero, D. Meyers, M. Kareev, X. Liu, Y. Cao, J.W. Freeland, S. Barraza-Lopez, J. Chakhalian The effect of strong polarity mismatch at the heterointerface, grown along the pseudo cubic [111] direction between the correlated metal LaNiO$_{3}$ and band insulator SrTiO3 has been considered. While the metallic LaNiO$_{3}$ film can itself screen this polarity mismatch, additional reconstruction mechanisms are needed in ultrathin films which are insulating in nature. The reflection high energy electron diffraction patterns recorded during growth highlighted the evolution of nucleation of an additional phase during the first few unit cells of deposition, which are found to be oxygen deficient phase LaNiO$_{\mathrm{3-x}}$ by x ray diffraction and x-ray resonant spectroscopy measurement. The amount of oxygen vacancies decreases ABRUPTLY with the increase of film thickness due to the increase electrical conductivity, which acts in a partial screening of the polar catastrophe. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D49.00011: Observation of strain-controlled electronic modulations revealed by Fermi surface superstructures in strongly correlated LaNiO$_{3}$ films Hyangkeun Yoo, Seungill Hyun, Luca Moreschini, Hyeong-do Kim, Youngjun Chang, Changhee Sohn, Dawoon Jeong, Soobin Sinn, Yongsu Kim, Aaron Bostwick, Eli Rotenberg, Jihoon Shim, Taewon Noh Control over the electronic properties of strongly correlated electron systems can be achieved by exploiting the misfit strain that exists in epitaxial films on lattice mismatched substrates. Here, we report a systematic investigation of electronic structures in strongly correlated LaNiO$_{3}$ films under different strain states, using \textit{in situ} angle-resolved photoemission spectroscopy and the dynamical mean field theory. LaNiO$_{3}$ film shows a change of a Fermi surface (FS) topology, driven by interplay between strong electron-electron correlations and misfit strain effects. Additionally, different from compressive strain case, a FS with tensile strain has a large flat region to induce strong FS nesting. As a result, different FS superstructures are observed in the compressive and tensile strain cases, and their origins are attributed to charge disproportionation and spin density waves, respectively. The more details will be discussed in the presentation. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D49.00012: Atomic Resolution Strain Analysis of Oxide Superlattices Ji-Hwan Kwon, Jason Hoffman, Anand Bhattacharya, Jian-Min Zuo Measuring strain inside the heteroepitaxial transition metal oxide superlattices is critically important, as strain controlled by composition has a large impact on the superlattice properties. Because of the small periods in oxide superlattices, strain analysis must be performed at atomic resolution in order to examine, for example, how strain changes across the interface, whether strain is uniform inside oxide layers or it varies from intermixing between the constituent materials. Strain analysis also provides structural information of the heteroepitaxial superlattice. In this study, we examined the strain in LaNiO$_{\mathrm{3}}$(LNO)/LaMnO$_{\mathrm{3}}$(LMO) superlattice thin films grown on SrTiO$_{\mathrm{3}}$ substrate with a spatial resolution of a single perovksite unit cell using template matching method based on the Z-contrast image. The LNO/LMO interface is hard to be determined by Z-contrast imaging alone due to the same element occupation for A-site and the similar atomic number Z for B-site (Mn and Ni). However, strain analysis reveal sharp interfaces between LNO and LMO layer, in which LNO layers are subjected to tensile strain while LMO layers to compressive strain, showing alternating epitaxial strain inside the superlattice. The detailed analysis including strain variation depends on number of LNO layer and associated deviation of B-site position will be presented. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D49.00013: Laser MBE for atomic layer by layer growth of LaNiO$_{3}$ films and superlattices from separate oxide targets Maryam Golalikhani, Qingyu Lei, Pasquale Orgiani, Xiaoxing Xi Laser MBE was used to grow Nickelate thin films and superlattices in atomic layer by layer manner from separate oxide targets. Stoichiometry and full layer coverage was controlled by in-situ monitoring of Reflection High Energy Electron Diffraction (RHEED) intensity oscillation. LaNiO$_{3}$ ultra-thin films were grown from La$_{2}$O$_{3}$ and NiO targets on LaAlO$_{3}$ and SrTiO$_{3}$ substrates. X-ray diffraction, x-ray reflectivity, and atomic force microscopy were used to characterize the structure, thickness, and surface morphology of the films. The origin of thickness dependent metal to insulator transition was studied using the transport properties and x-ray absorption spectroscopy measurements. Single unit cell LaNiO$_{3}$/LaAlO$_{3}$ superlattices were grown from La$_{2}$O$_{3}$, NiO and Al$_{2}$O$_{3}$ targets on LaAlO$_{3}$ substrate. By means of polarization-dependent x-ray Absorption Spectroscopy, orbital ordering in these supperlattices was studied and the results are presented herein. [Preview Abstract] |
Session D50: Focus Session: Nanostructrues and Metamaterials: Synthesis, Fabrication, and Characterization
Sponsoring Units: DMPChair: Jon Schuller, University of California, Santa Barbara
Room: Mile High Ballroom 1D
Monday, March 3, 2014 2:30PM - 3:06PM |
D50.00001: Meta-Optics Invited Speaker: Nader Engheta As the fields of metamaterial and plasmonic nanophotonics reach certain levels of development, new directions and novel vistas appear in the horizon. Modularization, parameterization and functionalization of metamaterials may be exploited to provide new functionalities and applications stemming from such interesting platforms of ``meta-optics.'' Indeed, the metamaterial ``forms'' may lead to novel ``functions.'' These may include metamaterial ``bits'' and ``bytes'' as building blocks for digitizing metamaterials, ``optical metatronics'' -- metamaterial-inspired optical nanocircuitry -- formed by judicious arrangement of nanostructures capable of optical processing at the nanoscale, ``meta-systems'' formed by metamaterials and metasurfaces providing wave-based signal handling and processing, graphene metatronics as one-atom-thick mid IR circuits, and nonreciprocal metastructures for unusual control over flow of photons, to name a few. We are exploring various features and characteristics of these concepts, topics, and directions in the paradigms of meta-optics and are investigating new classes of potential applications such paradigms may provide. We will present an overview of our most recent results from a sample of these topics and will discuss future directions and potentials. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D50.00002: Bistable nonlinear metamaterials Sinhara Silva, Jiangfeng Zhou In this work, we demonstrate a nonlinear metamaterial with remotely tunable spectrum response at microwave frequency regime. Using a double split-ring resonator (DSRR) design, the resonance frequency of the outer ring can be tuned by an external pump signal. We experimentally demonstrate that the DSRR exhibits power and frequency dependent broadband tunability of the resonance frequency. More importantly, the DSRR shows bi-stability with distinct transmission levels, where the transition between bi-states can be controlled by the impulses of pump signal. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D50.00003: A metamaterial cavity for refractive index sensing Khagendra Bhattarai, Zahyun Ku, Jiangfeng Zhou In this work, we demonstrated a metamaterial cavity consists of plasmonic metasurfaces made of gold nano-disks. We have shown that the Fabry-Perot cavity resonant modes arise around the plasmonic resonance wavelength. Compared to the localized plasmonic resonances, the quality factor of the cavity resonance is significantly increased. The cavity resonances are very sensitive to the refractive index of the surrounding materials. More importantly, the higher order cavity modes can further reduce the losses and improve the sensitivity. Numerical simulations show that the reflection shifts by 80{\%} when the refractive index of the surrounding liquid material changes from 1.312 to 1.352. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D50.00004: Interference and Chaos in Metamaterials Cavities Natalia Litchinitser, Jorge Jose Optical metamaterials are engineered artificial nanostructures that possess optical properties not available in nature. As metamaterials research continues to mature, their practical applications as well as fundamental questions on wave propagation in these materials attract significant interest. In this talk we focus on wave propagation and interference in chaotic wave cavities with negative or near-zero index of refraction and in double-slit configurations. In this context, we explicitly consider an incomplete two-dimensional D-cavity previously studied, which shows chaotic ray propagation together with scars. We have addressed the question as to how that type of wave propagation is modified by adding metamaterials in these chaotic cavities. We find that the wave interference patterns show significant qualitatively and quantitative changes depending on the effective parameters of the cavity, illumination conditions (planes waves versus beams), and geometry of the system. We will discuss possible experimental setups where these results may be validated. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D50.00005: Flexible and tunable metamaterials and their applications in sensing Xinglin Wen, Guangyuan Li, Jun Zhang, Qing Zhang, Bo Peng, Lai Mun Wong, Shijie Wang, Qihua Xiong Attributing metamaterials (MMs) to flexible substrates can provide many advantages such as transparency, lightweight, deformability and biocompatibility, and provides additional benefits to practical applications of metamaterials. Herein, we demonstrate a very simple and effective nickel sacrificial layer-assisted transfer method to fabricate Visible-Near IR metamaterials on polydimethylsiloxane (PDMS). The PDMS-MMs can serve as a well-defined and reproducible Surface-enhanced Raman Scattering (SERS) substrate and it can be covered to the surface with interesting analytes attached to obtain the SERS signal. Hybridizing a metamaterial with phase change material vanadium dioxide (VO$_{2})$ is very another promising way to achieve active metamaterial devices. Both the electric and magnetic resonances frequency of a split ring resonator can be tuned by controlling the phases of VO$_{2}$ by tuning the temperature. We also demonstrated that this VO$_{2}$-based metamaterials device can be used to tune the SERS intensity, which suggests considerable potential as an active sensing device. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D50.00006: Waves and Fields in Epsilon-and-Mu-Near-Zero (EMNZ) Media Ahmed Mahmoud, Nader Engheta We investigate some of the unconventional characteristics of wave interaction with epsilon-and-mu near-zero (EMNZ) media, i.e., structures with both the relative permittivity and permeability near zero. We show that using an EMNZ medium one might in principle ``open up'' regions that behave as ``single electromagnetic points'' while being electrically large. We discuss some of the unusual effects that result from placing classical radiating dipoles within an EMNZ medium. This may provide us roadmaps to tailoring the radiation performance of more complex systems like quantum emitters. We suggest an idea for a possible implementation of a structure that would exhibit an EMNZ behavior and numerically demonstrate the possibility of having electrically large volumes behaving as EMNZ media. We finally discuss the limitations within which these structures are able to exhibit the aforementioned phenomena that take place in an idealized EMNZ medium. Time permitting, we also show both analytically and numerically that electromagnetic invisibility of arbitrarily-shaped, electrically large, perfectly electric conducting objects may be achieved when embedded in an EMNZ medium. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D50.00007: Light scattering by magnetized nanoparticles: spatial quantization of light, symmetry breaking and plasmonic vortexes Artur Davoyan, Nader Engheta In this work we study theoretically light scattering by magneto-active plasmonic nanoparticles. We show that magnetization leads to the nanoscale symmetry breaking in the excitation of the surface plasmon polaritons, associated with the plasmonic eigen-mode degeneracy lifting. The latter implies the split of the plasmonic resonances in the nanoparticle extinction spectrum and the formation of the plasmonic vortexes. We show that such a phenomenon is deeply related to the quantization of the light angular momentum, thus revealing an optical analogy with the quantum Zeeman effect. Our work provides a paradigm for mesoscopic quantized systems. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D50.00008: Optical properties of metal-dielectric based epsilon near zero metamaterials Ganapathi Subramania, Arthur Fischer, Ting Luk Epsilon($\varepsilon$) near zero(ENZ) materials are metamaterials where the effective dielectric constant($\varepsilon$) is close to zero for a range of wavelengths resulting in zero effective displacement field (D $=$ $\varepsilon$E) and displacement current. ENZ structures are of great interest in many application areas such as optical nanocircuits, supercoupling, cloaking, emission enhancement etc. Effective ENZ behavior has been demonstrated using cut-off frequency region in a metallic waveguide where the modal index vanishes. Here we demonstrate the fabrication of ENZ metamaterials operating at visible wavelengths ( $\lambda $ $\sim$ 640nm) using an effective medium approach based on a metal-dielectric composites(App. Phys. Let.,\textbf{101},241107(2012)) that can act as ``bulk'' ENZ material. The structure consists of a multilayer stack composite of alternating nanoscale thickness layers of Ag and TiO$_{2}$. Optical spectroscopy shows transmission and absorption response is consistent with ENZ behavior and matches well with simulations. We will discuss the criteria necessary in the design and practical implementation of the composite that better approximates a homogenous effective medium including techniques to minimize the effect of optical losses to boost transmission. The potential for hosting gain media in the gratings to address losses and emission control will be discussed. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D50.00009: Tuning the Polarization State of Light via Retardation with a Microstructured Surface Shang-Chi Jiang, Xiang Xiong, Paulo Sarriugarte, Sheng-Wei Jiang, Xiao-bo Yin, Yuan Wang, Ru-Wen Peng, Di Wu, Rainer Hillenbrand, Xiang Zhang, Mu Wang We report in this letter an approach to tune efficiently the phase difference of light in two orthogonal directions, $\Delta \phi $, by controlling the time retardation with a microstructured surface made of L-shaped metallic patterns. The $\Delta \phi $ can be linearly tuned accurately from -180 degree to 180 degree by changing the frequency of incident light. Particularly the amplitudes in two orthogonal directions are identical so that the polarization state always locates on a meridian of Poincar\'e sphere. Near field measurement confirms that there is indeed time retardation between the oscillations in the orthogonal directions of the L-shaped patterns. This approach offers a new way in manipulating the polarization state of light. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D50.00010: Generation and regulation of multiple focuses by tight focusing of patterned vector optical field array Chenghou Tu, Mengqiang Cai, Huihui Zhang, Shengxia Qian, Yongnan Li, Hui-tian Wang We have numerically studied the tight focusing of patterned vector optical field array based on the modified Richard-Wolf diffracting integration. By tailoring the spatial arrangement and the polarization distribution of the individual vector optical field, sub-wavelength multiple focal spots with different arrangement can be easily realized. The size of the focal spots, the distance between different focal spots and the arrangement of focal spots can all be regulated by varying the parameters of VOFs. Focal spots with the arrangement of hexagon, rectangle or rhombus can be obtained depending on the different setting conditions of PVOF. To check the numerical results, we experimentally generated the PVOFs according to the numerical conditions, and utilize the tightly focused optical fields to ablate the single crystal silicon wafer surface. Based on SEM images of the ablated sample surface, we find that the experiment results, which indirectly measured the intensity distribution and the size of the focal spots, agree with the numerical results very well. The tight focusing of PVOF opens a new window for regulating the focal intensity distribution due to the control diversity. As a result, it can be very flexible and helpful in many applications, such as micro-nano parallel fabrication and optical manipulation, etc. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D50.00011: Generlized effective medium theory for metamaterials Brian Slovick, Zhi-Gang Yu, Srini Krishnamurthy We present an effective-medium model for calculating the frequency-dependent effective permittivity $\epsilon(\omega)$ and permeability $\mu(\omega)$ of metamaterial composites containing spherical particles with arbitrary permittivity. The effective constitutive parameters are derived within the dipole approximation from the condition that the scattering cross section vanishes for plane waves incident from the effective medium on the unit cell of the composite. In contrast to existing effective medium theories, our model does not invoke any additional long-wavelength approximations. As a result, it captures the effects of spatial dispersion and predicts a finite effective refractive index and antiresonances in $\epsilon(\omega)$ and $\mu(\omega)$, in agreement with numerical finite-element calculations. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D50.00012: Structured Metal Film as Perfect Absorber Xiang Xiong, Shang-Chi Jiang, Ru-Wen Peng, Mu Wang With standing U-shaped resonators, fish-spear-like resonator has been designed for the first time as the building block to assemble perfect absorbers. The samples have been fabricated with two-photon polymerization process and FTIR measurement results support the effectiveness of the perfect absorber design. In such a structure the polarization-dependent resonance occurs between the tines of the spears instead of the conventional design where the resonance occurs between the metallic layers separated by a dielectric interlayer. The incident light neither transmits nor reflects back which results in unit absorbance. The power of light is trapped between the tines of spears and finally be absorbed. The whole structure is covered with a continuous metallic layer with good thermo-conductance, which provides an excellent approach to deal with heat dissipation, is enlightening in exploring metamaterial absorbers. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D50.00013: DNA-mediated self-assembly of tetrahedral plasmonic clusters for metafluids Nicholas Schade, Li Sun, You-Jin Lee, Jonathan Fan, Federico Capasso, Gi-Ra Yi, Vinothan Manoharan We direct the self-assembly of clusters of gold nanospheres with the goal of creating a bulk, isotropic, optical metafluid. We use spherical gold nanoparticles that are exceptionally smooth, monocrystalline, and monodisperse. These particles exhibit highly reproducible scattering spectra compared with commercially available gold colloids. We label them with DNA sequences and mix them together to self-assemble small clusters. By controlling the particle sizes and the interactions between them, we maximize the yield of tetrahedral clusters, the ideal structures for isotropic metamaterials. [Preview Abstract] |
Session D51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization I
Sponsoring Units: DMPChair: Marc Bockrath, University of California, Riverside
Room: Mile High Ballroom 1E
Monday, March 3, 2014 2:30PM - 2:42PM |
D51.00001: High-Performance n-type and p-type WSe$_{2}$ Field Effect Transistors with Ionic-Liquid Gated Graphene Electrodes Hsun Jen Chuang, Xuebin Tan, Mark Ming-Cheng Cheng, Nirmal Jeevi Ghimire, Jiaqiang Yan, David Mandrus, Bhim Chamlagain, Meeghage Madusanka Perera, Zhixian Zhou We report the application of graphene as a work-function-tunable electrode material for few-layer WSe$_{2}$ field-effect transistors (FETs). By tuning the carrier density of graphene at the graphene/WSe$_{2}$ contacts using an extremely-large-capacitance ionic liquid gate, we have successfully achieved low resistance Ohmic contacts and high ON-current for both holes and electrons in WSe$_{2}$ FETs. The extrinsic electron and hole mobility values increase with decreasing temperature reaching $\approx $ 300 cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K when the graphene contacts are highly n- and p-doped by large positive and negative ionic-liquid gate voltages, respectively, indicating that the intrinsic phonon-limited mobility is approached for both electrons and holes in graphene contacted few-layer WSe$_{2}$. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier height via tuning the work function of graphene electrodes to align with the conduction and valence band edges of WSe$_{2}$ by an ionic liquid gate. This work was supported by NSF (DMR-1308436). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D51.00002: Contacts and transport characteristics of few-layer transition metal dichalcogenides Junjie Wang, Jing Li, Jacob Shevrin, An Nguyen, Tom Mallouk, J. Zhu, Daniel Rhodes, Luis Balicas, K. Watanabe, T. Taniguchi Two-dimensional layered transition metal dichalcogenides (TMDs) are potentially useful for electronic and optoelectronic applications. However, the lack of reliable methods to make ohmic contacts has been a major challenge. This work addresses two aspects of this challenge, i.e. interface cleanness and conductivity of the material in the contact area. Using gentle Ar ion milling immediately before the deposition of metal electrodes, we can completely remove polymer residue from prior lithography without significantly damaging the few-layer TMD sheet. Gate stacks made of Au and HfO$_2$ films can inject carriers up to 3$\times$10$^{13}$ cm$^{-2}$. We make van der Pauw devices of few-layer ($<$ 5 L) TMD (MoS$_2$, WS$_2$, WSe$_2$) sheets using Ti/Au contacts with area $<$ 2 (um)$^2$ and observe contact resistance less than 10 k$\Omega$ at high carrier densities, where the sheet conductance is well above 2e$^2$/h. We eliminate hysteresis in the transfer curve of TMD devices by pulsing the gate voltage. Ambipolar conduction is observed in WSe$_2$ devices, with an on/off ratio exceeding 10$^6$ for both electrons and holes. WSe$_2$ devices supported on h-BN show field-effect (hole) mobility $>$ 100 cm$^2$/(Vs) at 300K. We discuss the effects of the various approaches taken above. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D51.00003: Effect of interfaces on electron transport properties of MoS2--Au Contacts Maral Aminpour, Prokop Hapala, Duy Le, Pavel Jelinek, Talat S. Rahman Single layer MoS$_{2}$ is a promising material for future electronic devices such as transistors since it has good transport characteristics with mobility greater than 200 cm$^{-1}$V$^{-1}$s$^{-1}$ and on-off current ratios up to 10$^{8}$ [1]. However, before MoS$_{2}$ can become a mainstream electronic material for the semiconductor industry, the design of low resistive metal-semiconductor junctions as contacts of the electronic devices needs to be addressed and studied systematically. We have examined the effect of Au contacts on the electronic transport properties of single layer MoS$_{2}$ using density functional theory in combination with the non-equilibrium Green's function method. The Schottky barrier between Au contact and MoS$_{2}$, transmission spectra, and I-V curves will be reported and discussed as a function of MoS$_{2}$ and Au interfaces of varying geometry. \\[4pt] [1] B. Radisavljevic et al., Nature Nanotechnology \textbf{6}, 147 - 150 (2011). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D51.00004: to be determined by you Invited Speaker: Ali Javey |
Monday, March 3, 2014 3:42PM - 3:54PM |
D51.00005: Vertical Field-Effect Transistor Based on Graphene-Transition Metal Dichalcogenides Heterostructures Jatinder Kumar, Hui-Chun Chien, Matthew Z. Bellus, David L. Sicilian, Davis St. Aubin, Hsin-Ying Chiu The remarkable properties of graphene has made it possible to create transistors just few atoms thick. A new development was that the other two-dimensional materials can be stacked on it with atomic layer precision, creating numerous heterostructures on demand. Here, novel vertical field-effect transistor composed of graphene- transition metal dichalcogenides (TMDs) heterostructures is fabricated and characterized at various temperatures. Due to ultrathin nature of these transistors, they present the ultimate limit for electron transport in heterostructures. Tunneling and thermionic transport characteristics are studied among different graphene-TMDs heterostructures. Their electronic properties have been investigated and can be used in vast range of devices. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D51.00006: High performance MoS$_{2}$ Field-Effect Transitors in a simple design S. Velez, O. Txoperena, L. Pietrobon, F. Casanova, L.E. Hueso The discovery of graphene, with its rich and fascinating physical properties, has opened up a new world where 2D-layered materials is the platform for developing powerful devices. Molybdenum disulfide (MoS$_{2})$, a 2D material belonging to the family of transition metal dichalcogenides, has an intrinsic band-gap and strong spin-orbit coupling which would complement those applications pristine graphene cannot cover. In particular, MoS$_{2}$ has been shown to work well as a field-effect transistor (FET), to exhibit superconductivity and valley polarization, demonstrating its potential in spintronics, valleytronics, or for designing other novel devices. Here we will show high performance of FETs based on monolayer and a few layer MoS$_{2\, }$working with a simple design (Si/SiO2 back gate and two terminal configuration). The FETs show room temperature ON/OFF ratios exceeding 10$^{7}$ and with mobilities higher than 10 cm$^{2}$V$^{-1}$s$^{-1}$. These values are among the best previously reported ones in similar designs and support the viability of building up simpler but still powerful devices which would allow large scale fabrication suitable for nanoelectronics. Further investigations exploiting both spintronics and valleytronics of layered MoS$_{2}$ are the final goal of this work. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D51.00007: MoS$_{2}$ Field-effect Transistors with Graphene/Metal Hetero-contacts Yuchen Du, Lingming Yang, Jingyun Zhang, Nathan Conrad, Han Liu, Peide Ye MoS$_{2}$, as one of the mostly studied transition-metal dichalcogenides, has already revealed a series of new physics and potential device applications. However, the performance of the MoS$_{2}$ field-effect transistors is limited by the large contact resistance at metal/MoS$_{2}$ interface due to the non-negligible Schottky barrier. In this study, n-type few-layer MoS$_{2}$ field-effect transistors with graphene/Ti as the metal contacts have been fabricated showing more than 160 mA/mm drain current at 1 $\mu$m gate length and on-off current ratio of 10$^{7}$. Different metal contacts (Ti, Ni, Au, and Pd) from low work function to high work function metals on MoS$_{2}$/graphene hetero contacts have been performed and studied. Moreover, for the first time, 2D Fermi-level pinning concept is introduced to understand the band alignment of hetero-structured metal/graphene/MoS$_{2}$ or other 2D semiconductor interfaces. Temperature dependent, noise, and stress measurement results will also be presented. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D51.00008: Electrical transport measurements on monolayer and few-layer MoSe2 and WSe2 Zaiyao Fei, Joe Finney, Yun Ling, Serkan Kasirga, Xiaodong Xu, David Cobden The two-dimensional monolayer semiconductors WSe2 and MoSe2 have recently been shown to have excellent optical properties, but their intrinsic electrical properties, relevant to many device applications, remain undetermined. This is due to the difficulty of obtaining good contacts and applying a sufficient electric field to induce carriers, especially at lower temperatures. We have investigated a range of device geometries and contact techniques aimed at improving the situation. So far we have achieved ambipolar gating of the linear-response conductance persisting at temperatures down to 4 K with contact resistance for both carrier of around 50 kiloohm at room temperature. Four terminal Hall-bar measurements have been made to separate the contact resistance, sheet resistivity, carrier density and mobility. Methods are being explored to eliminate large intrinsic contact noise. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D51.00009: Improved Carrier Mobility in Few-Layer MoS$_{2}$ Field-Effect Transistors with Ionic-Liquid Gating Meeghage Perera, Ming-Wei Lin, Hsun-Jen Chuang, Bhim Chamlagain, Chongyu Wang, Xuebin Tan, Mark Cheng, David Tom\'anek, Zhixian Zhou We report the fabrication of ionic liquid (IL) gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS$_{2}$. Our transport measurements indicate that the electron mobility $\mu \approx $60~cm$^{2}$V$^{-1}$s$^{-1}$ at 250~K in ionic liquid gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from $\approx $100 cm$^{2}$V$^{-1}$s$^{-1}$ at 180~K to $\approx $220~cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio \textgreater 10$^{7}$ (10$^{4})$ for electrons (holes), and a near ideal sub-threshold swing of $\approx $50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin ionic-liquid dielectric layer. In addition, graphene contacted MoS$_{2}$ FETs with IL-gating will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D51.00010: Electronic properties of few-layer MoS2 under an external electrical field Jose Eduardo Padilha, Hartwin Peelaers, Anderson Janotti, Chris G. Van de Walle MoS$_2$ is a two-dimensional (2D) layered material with a band gap in the 1-2 eV range, depending on the number of layers, and with promising applications in nanoelectronics. Field-effect transistors based on MoS$_2$ have been fabricated, displaying room-temperature electron mobility of $~200 cm^{2}V^{-1}s^{-1}$ and high on/off ratios on the order of $10^{8}$. In these devices, the effect of the electric field across the MoS$_2$ layers is important for device operation, so understanding these effects will aid in improving device performance. Here we use first-principles calculations to determine the electronic properties of MoS$_2$ layers as a function of an electric field applied perpendicular to the layers, representing the effect of gate electrodes. In the absence of an external field, the valence and conduction bands of multilayer MoS$_2$ are degenerate. However, an applied external field generates a gradient potential inside the material, breaking the symmetry between the layers, lifting the degeneracies, and modifying the band gap. We will discuss the evolution of the band gap and the various minima in the conduction band as a function of the field intensity and the number of layers. Work supported by FAPESP and by NSF-IMI. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D51.00011: Photocurrent studies on multi-walled WS$_{2}$ nanotube devices John Mathew, Gobinath Jegannathan, Sameer Grover, Sudipta Dubey, Pratiksha Dongare, Mandar Deshmukh Multi-walled WS$_{2}$ nanotubes were used for transport and photo response studies. The nanotubes were structurally characterized by scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy. Nanotube devices in field effect transistor geometry were fabricated on Si/SiO$_{2}$ substrates using nano lithography techniques. I-V measurements of these devices were carried out in ambient conditions. A confocal microscope system was used to study the photo response of the devices to 633 nm and 532 nm laser wavelengths using lock-in technique. Photocurrent map of the devices was obtained and studied as a function of applied bias voltage and gate voltage. The devices showed non-linear increase in photocurrent with increasing bias voltage and light intensity. Further, heterostructure devices of graphene and WS$_{2}$ nanotubes were fabricated for enhanced field effect behavior. Results of the photo response studies of these devices will also be presented. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D51.00012: Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers Junhao Lin, Ovidiu Cretu, Wu Zhou, Kazu Suenaga, Dhiraj Prasai, Kirill Bolotin, Nguyen Cuong, Minoru Otani, Susumu Okada, Andrew Lupini, Juan Idrobo, Dave Caudel, Arnold Burger, Jiaqiang Yan, Nirmal Ghimire, David Mandrus, Stephen Pennycook, Sokrates Pantelides We report direct electron-beam sculpting of ultrathin nanowires connecting designated points within semiconducting transition-metal dichalcogenide (TMDC) monolayer. In-situ electrical measurements reveal the nanowires are intrinsically metallic. The nanowires remain conducting and maintain structural integrity as they undergo continuous electron-beam-induced rotations and flexing, indicating their self-adaptive connections to the monolayers. The observed mechanical behavior is explained by density-functional-theory calculations, which further predict that the metal-semiconductor contacts could be Ohmic to p-type TMDC monolayers. These metallic nanowires can, therefore, serve as interconnects in future flexible nano-circuits fabricated entirely within a monolayer. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D51.00013: Interface effect on multilayer tungsten disulfide device caused by substrate and water molecule Xue Liu, Yun Ling, Jin Hu, Chunlei Yue, Zhiqiang Mao, Jiang Wei We investigated field effect transistor (FET) device made of multilayered WS2 with Poly(methyl methacrylate) (PMMA) as the dielectric layer. The device was fabricated using shadow mask evaporation to improve contact. Comparing to the same FET with SiO2 dielectric layer, PMMA-WS2 device shows an excellent on-off ratio (up to 6 orders magnitude), an easily induced ambipolar behavior and a significantly reduced hysteresis at high gate voltage region during the gate sweep. Furthermore, we discovered that the water molecule absorbed onto the WS2 surface depletes the extra charge carriers at the neutrality point and transforms the device into insulating state at room temperature. In addition, we found that the effect caused by water absorption is reversible. [Preview Abstract] |
Session D52: Optical Probes in Copper-oxide Superconductors
Sponsoring Units: DCMPChair: Nan-Lin Wang, Chinese Academy of Sciences
Room: Mile High Ballroom 1F
Monday, March 3, 2014 2:30PM - 2:42PM |
D52.00001: Optical conductivity of nodal metals C.C. Homes, G.D. Gu, J.J. Tu, J. Li, A. Akrap Fermi liquid theory is remarkably successful in describing the transport and optical properties of metals; at frequencies higher than the scattering rate, the optical conductivity adopts the well-known power law behavior $\sigma_1(\omega) \propto \omega^{-2}$. We have observed an unusual non-Fermi liquid response $\sigma_1(\omega) \propto \omega^{-1\pm 0.2}$ in the ground states of several quasi two-dimensional cuprate (optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, optimally and underdoped YBa$_2$Cu$_3$O$_{7-\delta}$) and iron-based materials ($A$Fe$_2$As$_2$, $A=$Ba, Ca) which undergo electronic or magnetic phase transitions resulting in dramatically reduced or nodal Fermi surfaces. The identification of an inverse (or fractional) power-law behavior in the residual optical conductivity now permits the removal of this contribution, revealing the direct transitions across the gap and allowing the nature of the electron-boson coupling to be probed. The non-Fermi liquid behavior in these systems may be the result of a common Fermi surface topology of Dirac cone-like features in the electronic dispersion. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D52.00002: Optical Birefringence and Dichroism of Cuprate Superconductors in the THz regime Y. Lubashevsky, LiDong Pan, T. Kirzhner, G. Koren, N.P. Armitage The presence of optical polarization anisotropies, such as Faraday/Kerr effects, linear birefringence, and magnetoelectric birefringence are evidence for broken symmetry states of matter. The recent discovery of a Kerr effect using near-IR light in the pseudogap phase of the cuprates can be regarded as a strong evidence for a spontaneous symmetry breaking and the existence of an anomalous long-range ordered state. In this work we present a high precision study of the polarimetry properties of the cuprates in the THz regime. While no Faraday effect was found in this frequency range to the limits of our experimental uncertainty (1.3 milli-radian or 0.07$^\circ$), a small but significant polarization rotation was detected that derives from an anomalous linear dichroism. In YBa$_2$Cu$_3$O$_y$ the effect has a temperature onset that mirrors the pseudogap temperature T$^*$ and is enhanced in magnitude in underdoped samples. In $x=1/8$ La$_{2-x}$Ba$_{x}$CuO$_4$, the effect onsets above room temperature, but shows a dramatic enhancement near a temperature scale known to be associated with spin and charge ordered states. These features are consistent with a loss of both C$_4$ rotation and mirror symmetry in the electronic structure of the CuO$_2$ planes in the pseudogap state. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D52.00003: Quasiparticle dynamics in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films probed by broadband pump-probe spectroscopy Chunfeng Zhang, Wei Li, Benjamin Gray, Xiaoyong Wang, Jak Tchakhalian, Min Xiao Ultrafast pump-probe spectroscopy can provide viable information on quasiparticle dynamics with respect to phase transition and competing orders in high temperature superconductors. We study the quasiparticle dynamics in epitaxial YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films by probing photo-induced reflectivity change over a broadband spectral coverage. The dynamic traces probed at single wavelength show abrupt changes of signal amplitude and decay lifetime at the superconducting transition temperature. The spectra dispersion of reflectivity change at zero temporal delay induced by electronic excitation is found to be quite different in superconducting and normal phases. Moreover, the spectral dispersion is strongly dependent on the delay time between the pump and probe pulse, implying evolution of spectral weight transition contributed by electronic and photonic process. These results are helpful to understand the electronic excitations and their interaction with different bosonic modes in cuprates. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D52.00004: Near-field techniques for probing collective modes of anisotropic superconducting thin films H.T. Stinson, J.S. Wu, B.Y. Jiang, Z. Fei, A.S. Rodin, B. Chapler, A.S. Mcleod, A. Castro-Neto, Y.S. Lee, M.M. Fogler, D.N. Basov We propose the use of scattering-type scanning near-field optical microscopy (s-SNOM) to characterize the collective mode spectrum of anisotropic superconductors. To probe the dispersion of collective modes with large in-plane momenta, specifically surface plasmons and guided wave modes, we model the real-space interference patterns of modes launched by the sharp s-SNOM tip and their reflections off physical and electronic boundaries. In addition, we show that s-SNOM spectroscopy allows for a direct probe of the $c$-axis superfluid density in underdoped anisotropic superconductors with nanoscale spatial resolution. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D52.00005: Quasiparticle recombination dynamics in the model cuprate superconductor HgBa$_{2}$CuO$_{4+\delta}$ J.P. Hinton, E. Thewalt, J.D. Koralek, J. Orenstein, N. Barisic, X. Xhao, M. Chan, C. Dorow, M. Veit, L. Ji, M. Greven The cuprate family of high temperature superconductors is characterized by a variety of electronic phases which emerge when charge carriers are added to the antiferromagnetic parent compound. The structural simplicity of the single layer cuprate system HgBa$_{2}$CuO$_{4+\delta}$ (Hg1201) is advantageous for experimentally detecting subtle features of these phases. In this work, we investigate the recombination dynamics of photo-excited quasiparticles in Hg1201 as a function of doping, temperature, and magnetic field using pump-probe optical reflectivity. We observe two distinct onset temperatures above T$_{C}$ in the underdoped part of the phase diagram, corresponding to T* and T** as observed in transport and neutron scattering experiments. We also measure a suppression of the recombination rate near TC which peaks at 8\% hole concentration. We associate this suppression with coherence effects. Lastly, we observe a complex, non-monotonic temperature dependence in the dynamics around optimal doping, providing evidence for reentrant phase transitions near the apex of the superconducting dome. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D52.00006: Infrared Faraday Measurements on Cuprate High Temperature Superconductors M. Murat Arik, Alok Mukherjee, John Cerne, Y. Lubashevsky, LiDong pan, N.P. Armitage, T. Kirzhner, G. Koren Recent measurements on cuprate high temperature superconductors (CHTS) have observed evidence for symmetry breakings in the pseudogap phase, suggesting that this is a full-fledged phase with an actual broken symmetry. To test the spectral character of this broken symmetry, we have made infrared polarization-sensitive measurements in the absence of magnetic field on a series of CHTS films. We have studied the Faraday effect (change in the polarization of transmitted light) in CHTS films as a function of temperature (10-300K), energy (0.1-3 eV), and sample orientation with respect to the incident light polarization. We observe a strong linear optical anisotropy, well above the superconducting transition temperature. This signal is maximized when the sample lattice axes are oriented near 45o with respect to the incident light polarization, and varies as the sample is rotated. We explore the temperature and energy dependence of this signal. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D52.00007: The evolution of microwave conductivity in YBa$_2$Cu$_3$O$_{6+x}$ across the superconducting dome Jordan Baglo, James Day, Pinder Dosanjh, Ruixing Liang, Walter Hardy, Doug Bonn The rich phenomenology displayed in the phase diagram of the high-$T_c$ cuprates continues to be an active arena of investigation. Recent experimental and theoretical work appears to be converging on a picture of separate spin and charge order phase transitions -- well-below and near optimal doping, respectively -- along with associated Fermi surface reconstruction. As sensitive probes of the low-energy electrodynamics, microwave spectroscopy techniques are well-suited for characterizing the effects of such changes in electronic structure deep within the superconducting state. I will present the results of our survey of the complex microwave conductivity of YBa$_2$Cu$_3$O$_{6+x}$ over a wide range of oxygen contents, from 6.49 to 6.998, and discuss their implications for the evolution of electronic structure with doping. I will also discuss the surprising relationship we observed between quasiparticle scattering lifetimes and oxygen ordering, which carries important implications for quantum oscillation measurements. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D52.00008: Magnetic and Chiral Excitations in Resonant Raman Scattering Thomas Devereaux, Chunjing Jia, Yao Wang, Brian Moritz, Rudi Hackl For the strongly correlated materials such as the cuprate parent compounds, the two-magnon excitations can be measured by the Raman scattering imposing the B1g symmetry, while chiral excitations can be probed using circularly polarized light. We study the resonance enhancement of these excitations and their relationship to the optical conductivity based on cluster exact diagonalization studies of the single-band as well as multi-orbital Hubbard models. Our theoretical studies help understand Raman experiments for the half-filled cuprates as well as the lightly doped (electron and hole doped) cuprates, and the relationships to other experiments on the same materials. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D52.00009: Ultrafast dynamics in photo-induced correlated electronic states in ladder cuprates Sumio Ishihara, Hiroshi Hashimoto Ultrafast photo-induced dynamics in correlated electron systems, in particular, photon irradiation effects in half filled Mott insulators have been studied intensively from theoretical and experimental sides, and photo-induced Mott insulator to metal transition has been observed. On the other side, in recent ultrafast pump-probe experiments in ladder cuprates away from half filling, photo-irradiation weakens initial metallic state. We study ultrafast dynamics in photo-induced states in a ladder system. Real time dynamics in a ladder-type Hubbard model are analyzed by the numerical exact diagonalization method. Optical conductivity spectra and density of states show that the initial metallic state is changed into a bad metallic state by photo irradiation, in contrast to the photo-doped effect in half-filled Mott insulators. Through the calculation of the carrier pair correlation functions, we find that coherent motion of carrier pairs in initial states are reduced by pump photon irradiation. We further simulate a double pulse irradiation. Our simulations as well as the experimental results suggest an optical control of pair coherence in correlated electron system. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D52.00010: Frequency dispersion of nonlinear response of thin superconducting films Sean Byrnes, Scott Dietrich, Sergey Vitkalov, Andrey Sergeev Effect of microwave radiation on transport properties of $La_{2-x}Sr_xCuO_4$ atomically thin films grown by Molecular Beam Epitaxy were studied. Resistance changes induced by the applied microwaves with variable frequencies (0.1-20GHz) and powers were measured at different temperatures near the superconducting transition ($\sim 8-15$K). Strong drop of three orders of magnitude of the nonlinear response is found within a few GHz of a cutoff frequency ($\omega_{cut}\sim 2GHz$). Expected frequency dependence vastly underestimates the sharpness of this drop. Numerical simulations considering an $ac$ response which follow the $dc$ I-V characteristics of the films replicate the low frequency behavior, but fail above the threshold frequency $\omega_{cut}$. The observed phenomenon suggests significant decrease of the effectiveness of vortex-antivortex dissociation induced by the oscillating superconducting condensate. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D52.00011: Charge-orbital-lattice coupling in a quasi-one-dimensional cuprate revealed through energy shifts in the dd-excitation profile B. Moritz, J.J. Lee, W.-S. Lee, M. Yi, C.J. Jia, A.P. Sorini, K. Kudo, Y. Koike, K.J. Zhou, C. Monney, V. Strocov, L. Patthey, T. Schmitt, Z.-X. Shen, T.P. Devereaux One-dimensional edge-sharing copper oxides provide a unique opportunity to study the effects of electron-lattice (e-l) interactions without complication from magnetic degrees of freedom which have a much lower energy scale in these compounds. Building on the characterization of e-l coupling in these materials from the elastic line profile found in resonant inelastic x-ray scattering (RIXS) at the O K-edge, new analysis of dd-excitation peak positions in Cu L-edge RIXS reveals abrupt shifts as one tunes the incident photon energy through the resonance. The observations point toward an orbital-specific coupling of the high-energy excited states of the system to the low-energy degrees of freedom. A Franck-Condon treatment of e-l coupling, consistent with other measurements, reproduces these shifts and highlights charge-orbital-lattice renormalization in the high energy d-manifold with obvious repercussions for other copper oxides. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D52.00012: Dynamical Mass Renormalization and Fermi Momentum in the Normal State of the Cuprate $Bi_{2} Sr_{2} CaCu_{2} O_{8+x} $ as Instigated and Observed by Two-Photon ARPES J. Rameau, S. Freutel, L. Rettig, I. Avigo, M. Ligges, Y. Yoshida, H. Eisaki, J. Schneeloch, R. Zhong, Z. Xu, G. Gu, P. Johnson, U. Bovensiepen The dressing of quasiparticles in solids is investigated by observing changes of the electronic structure $E$(\textbf{\textit{k}}) driven by femtosecond laser pulses. Employing time- and angle-resolved photoemission on the optimally doped cuprate $Bi_{2} Sr_{2} CaCu_{2} O_{8+x} $, just above $T_{c} $, we observe two effects with different characteristic temporal evolutions and, therefore, different microscopic origins. The experiment was carried out using amplified ultrafast laser pulses and a novel time of flight laser-ARPES setup. Both of the effects observed thusly are driven by the relatively high fluences of our amplified near-infrared pump laser and indicate that non-trivial, dynamical changes of the normal state cuprate band structure may be induced by ultrafast laser pulses over time scales at least as short as 150 fs. First, a 10{\%} change of the effective mass due to the 70~meV kink in $E$(\textbf{\textit{k}}) is found to occur during the experiment's 100~fs temporal resolution. Second, a time- and fluence-dependent change in $k_{F} $ is observed. The causes and ramifications of these disparate processes will be discussed. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D52.00013: Quantum quenching an O(N) non linear sigma model (NLSM) and oscillation experiments of high Tc underdoped cuprate superconductor Ling Yan Hung, Wenbo Fu, Subir Sachdev Recent X-ray scattering experiments have provided strong evidence of the coexistence of a charge density wave order (CDW) and superconductivity (SC) in underdoped crystals of the prototypical high-$T_c$ cuprate superconductor, YB$_{a_2}$Cu$_3$O$_{6+x}$. Sachdev et al have proposed a O(6) NLSM as an effective description of the competing orders, which finds excellent quantitative fit with the X-ray data. On the other hand, Hinton et al report coherent oscillations associated with CDW in these cuprates, whose phenomenology above and below T$_c$ find qualitative match with the picture of the competing orders. Motivated by these recent results, we study the dynamical evolution of the O(6) NLSM model upon a quantum quench -- a sudden disturbance of some parameters of the model to mimic the effect of the laser pulse in the oscillation experiment. As a first brush, we simplify the problem by taking the large-N limit of the O(6) NLSM. We observe a general exponentially decaying oscillations, which experiences phase shift as temperature is varied, at an extent determined by the specific choice of the parameter that is quenched. We also discuss the variation of the oscillation frequency and amplitude as various parameters are varied. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D52.00014: IR Hall Effect for reconstructed Fermi surfaces Dennis Drew The IR Hall Effect in the presence of Fermi surface reconstruction is considered within a density wave model for a 2D metal. Reconstruction of a hole like Fermi surface can produce reconstructed electron-like or hole-like Fermi surfaces. For hole-like reconstruction the inverse Hall frequency (1/$\omega_{\mathrm{H}})$ remains positive. For an electron like reconstruction 1/$\omega_{\mathrm{H}}$ remains hole-like for small density wave gaps and passes continuously through zero as the gap become larger and the electron-like pockets becomes convex. These considerations are applied to IR Hall data on under doped YBCO. It is concluded that the reconstructed Fermi surface in under doped YBCO is hole-like. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D52.00015: Nonadiabatic dynamics and coherent control of nonequilibrium superconductors Andreas Schnyder Motivated by recent THz pump-THz probe experiments on NbN films [1], we theoretically study the pump-probe response of nonequilibrium superconductors coupled to optical phonons. For ultrashort pump pulses a nonadiabatic regime emerges, which is characterized by oscillations of the superconducting gap [2] and by the generation of coherent phonons [3]. Using density-matrix theory, we compute the pump-probe response of the superconductor in the nonadiabatic regime and determine the signatures of the order parameter and of the phonon oscillations in the pump-probe conductivity. We find that the nonadiabatic dynamics of the superconductor reflects itself in oscillations of the pump-probe response as a function of delay time between pump and probe pulses [4]. We argue that from the analysis of this oscillatory behavior both frequency and decay time of the algebraically decaying order-parameter oscillations can be inferred.\\[4pt] [1] R. Matsunaga \textit{et. al.}, Phys. Rev. Lett. \textbf{111}, 057002 (2013). \newline [2] E. A. Yuzbashyan \textit {et. al.}, Phys. Rev. Lett. \textbf{96}, 097005 (2006). \newline [3] A. P. Schnyder \textit {et. al.}, Phys. Rev. B \textbf{84}, 214513 (2011). \newline [4] H. Krull \textit {et. al.}, arXiv:1309.7318 (submitted). [Preview Abstract] |
Session D53: Surfaces, Interfaces, and Thin Films: 2D Materials
Sponsoring Units: DCMPChair: Kenan Gundogdu, North Carolina State University
Room: Mile High Ballroom 2C
Monday, March 3, 2014 2:30PM - 2:42PM |
D53.00001: Crystal Symmetry and Surface States Huiping Wang, Ruibao Tao This work reports a rigorous criterion for the non-existence of surface states in a semi-infinite crystal with the (hkl) cut surface. We have proved that a (hkl) cut crystal will not induce any surface state if any (hkl) plane has the reflection symmetry in an infinite crystal constructed by an infinite number of parallel (hkl) crystal planes which are periodically arranged one by one by coupling. The conclusion is valid for any 3D and 2D structure crystal and any multiple neighbor hopping among crystal planes. The spin-orbit coupling breaks the chiral symmetry, resulting in the reflection symmetry breaking, surface states will emerge in the crystal. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D53.00002: Mechanism for asymmetric charge distribution in Rashba-type surface states and the origin of the energy splitting scale Beomyoung Kim, Wonsig Jung, Yeongkwan Kim, Yoonyoung Koh, Wonshik Kyung, Changyoung Kim, Panjin Kim, Jung Hoon Han, Joonbum Park, Jun Sung Kim, Masaharu Matsunami, Shin-ichi Kimura The mechanism for Rashba-type band splitting is examined in detail. We show how an asymmetric charge distribution is formed when the local orbital angular momentum (OAM) and crystal momentum get interlocked due to surface effects. An electrostatic energy term in the Hamiltonian appears when such an OAM- and crystal-momentum-dependent asymmetric charge distribution is placed in an electric field produced by inversion-symmetry breaking. Analysis by using an effective Hamiltonian shows that, as the atomic spin-orbit coupling (SOC) strength increases from weak to strong, the originally OAM-quenched states evolve into well-defined chiral OAM states and then to states of total angular momentum $J$. In addition, the energy scale of the band splitting changes from the atomic SOC energy to electrostatic energy. To confirm the validity of the model, we performed circular dichroism angle-resolved photoemission spectroscopy experiments as well as first-principles calculations. We find that the effective model can explain various aspects of the spin and OAM structures of the system. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D53.00003: Bonding instability induced Surface Insulating State in IrTe$_{2}$ A.G. Gianfrancesco, Q. Li, J.Q. Yan, X. Chen, W. Lin, S. Kalinin, D.J. Singh, D. Mandrus, M. Pan Using STM/S and DFT calculations, we find that the surface of in-situ cleaved IrTe2 undergoes a structural transition from trigonal to triclinic lattice below transition temperature, accompanied by formation of unidirectional structural modulations with distinct wavelengths. As temperature approaches 4 K, the system changes into a phase with formation of a single modulated structure with a fully-developed insulating gap. DFT rules out the CDW instability as the origin of this transition, confirming local structural distortion induced orbital degeneracy leading to strong-repulsion between Te p-band, therefore producing an insulting Te surface layer while the bulk stays metallic. This research was conducted (QL, WL, MP) at the CNMS, sponsored at ORNL by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE. Research was supported (WL, SVK) by MSED, Basic Energy Sciences, the U.S. DOE. Fellowship support (AG) from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D53.00004: Topological conduction of Sb films on boron-doped graphene Chih-Kai Yang, Chi-Hsuan Lee It has been shown that thin antimony films can have topological surface states by adsorption of non-magnetic impurity atoms. Using density functional calculations, we show that Sb films placed on boron-doped graphene also have Dirac cones and can provide spin-polarized transport. The calculated binding energy indicates that it is a robust structure and thus a viable conduit for topological conduction. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D53.00005: Observation of electronic structure of silicene by scanning tunneling microscopy Youngtek Oh, Wonhee Ko, Insu Jeon, Hyo Won Kim, Hyeokshin Kwon, JiYeon Ku, Sung Woo Hwang, Hwansoo Suh Silicene, an atomic monolayer of silicon atoms, has a hexagonal symmetry and is expected to have Dirac fermions. Recently, silicene has been intensively investigated in various substrates such as Ag(111), ZrB2 (0001), and Ir(111). We grew a monolayer of silicene on the Ag(111) surface by ultrahigh vacuum deposition and annealing of silicon atoms. The geometric and electronic properties of silicene grown on the Ag(111) were investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The (4x4) structures of silicene were observed in LEED patterns and STM images. We observed domains formed inside the silicene. The electronic properties of silicene were measured by scanning tunneling spectroscopy (STS). [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D53.00006: Epitaxial Co-Deposition Growth of CaGe$_{2}$ Films by Molecular Beam Epitaxy for Large Area Germanane Patrick Odenthal, Igor Pinchuk, Adam Ahmen, Walid Amamou, Josh Goldberger, Roland Kawakami Here, we report the successful co-deposition growth of CaGe$_{2}$ films on Ge(111) substrates by molecular beam epitaxy and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe$_{2}$ occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize \textit{in situ} reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 $^{\circ}$C. Finally, the CaGe$_{2}$ films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca and RHEED patterns indicate a single-crystal film with in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. \textit{Ex situ} atomic force microscopy shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D53.00007: Tune the electronic and phonon properties of silicene and germanene through biaxial strain and electric field Jia-An Yan, Ryan Stein, Gregory Coard We presented a density-functional study of the effects of biaxial strain and perpendicular electric field on the electronic and phonon properties of the two-dimensional (2D) silicene and germanene sheets. The two factors can be applied along the parallel and perpendicular directions independently, and therefore increase the tunability on the electronic band structure and phonon properties in these 2D systems. Important quantities such as the Gr\"{u}neisen parameters will be calculated. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D53.00008: STM study of monolayer MoS$_{2}$ synthesized by Chemical Vapor Deposition Adam Mills, Chuanhui Chen, Yifei Yu, Linyui Cao, Changgang Tao Monolayer molybdenum disulfide (MoS$_{2})$, an atomically thin transition-metal dichalcogenide semiconductor with a direct band gap, as opposed to an indirect band gap in bulk MoS$_{2}$, has recently captured a lot of research interest for its distinctive optical and electronic properties, and potential applications such as field effect transistors, optoelectronic devices and chemical sensors. Using scanning tunneling microscopy, we have investigated monolayer MoS$_{2}$ synthesized by chemical vapor deposition. The structural and electronic properties of monolayer MoS$_{2}$ grown on glassy carbon and other substrates will be presented. We will also discuss our preliminary scanning tunneling spectroscopy measurements on these samples. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D53.00009: First Principles Study of Bismuth Films at Transition Metal Grain Boundaries Qin Gao, Michael Widom Recent experiments suggest that Bi impurities segregate to form bilayer films on Ni and Cu grain boundaries but do not segregate in Fe. To explain these phenomena, we study the total energies of Bi films on transition metal (TM) $\Sigma$3(111) and $\Sigma$5(012) grain boundaries (GBs) using density functional theory. Our results agree with the observed stabilities. We propose a model to predict Bi bilayer stability at Ni GBs which suggests that Bi bilayer is not thermodynamically stable on low energy (111) twist CSL GBs but is stable in most (100) twist CSL GBs. We investigated the interaction and bonding character between Bi and TMs to explain the differences among TMs based on localization of orbitals and magnetism. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D53.00010: Transfer of Epitaxial Thin Films to Carrier Substrates Carley Miki, Gabriel A. Devenyi, Stephen Jovanovic, Kristoffer Meinander, Jessica Carvalho, Guozhen Zhu, John S. Preston CdTe and ZnTe are important materials in the semiconductor industry and are currently being used in many devices such as solar cells, laser diodes, detectors, and LEDs. Sapphire substrates (Al$_2$O$_3$) have been found to yield high quality epitaxial films of these materials, but the cost of this substrate makes large scale growths unrealistic. Recently, a novel technique developed at McMaster University has been successful in transferring large areas of CdTe and ZnTe films grown by pulsed laser deposition from sapphire to a wide variety of carriers without altering the film or substrate. This allows the sapphire to be reused for an indefinite number of growths without extensive treatment, and films to be transferred to various carriers while maintaining their quality. The physics of this technique is currently not well understood, prompting an investigation of the interface between the film and substrate to characterize the atomic structure in this region. Results from this study will help to refine this technique and identify potential for new applications. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D53.00011: Is graphene more conductive than h-BN? Xiaoliang Zhong, Rodrigo Amorim, Alexandre Rocha, Ravindra Pandey Electronic tunneling through multilayers of graphene and h-BN sandwiched between gold electrodes is investigated by density functional theory together with the non-equilibrium Green's Function method. The calculated results predict similar transmittance characteristics for the device configuration consisted of graphene and h-BN monolayers, though the pristine graphene and h-BN layers are semimetal and semiconductor, respectively. The h-BN monolayer exhibits a higher degree of p-type doping due to electron transfer from boron to the contact gold atoms relative to that predicted for graphene. A strong coupling of electrode-monolayer at the device interface is therefore likely to be the cause of similar vertical electron tunneling characteristics of the device configurations considered. For the multilayer cases, h-BN shows an exponential dependency of transmission function on the number of layers, whereas multilayer graphene exhibits relatively high tunneling probability due to a stronger interlayer coupling between adjacent layers of graphene. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D53.00012: Hydrogen Trapping in Carbon-Doped $h$-BN/Rh(111) Nanomesh Jarvis Loh, Sandeep Nigam, Ravindra Pandey, Govinda Mallick Atomic or molecular preferential adsorption on surface nanotemplates provides a facile and feasible means of fabricating ordered low-dimensional nanostructures with tailored functionality. In this study, by employing density-functional theory calculations, we demonstrate (1) the carbon doping of the (B,N)$=$(fcc$_{\mathrm{Rh}}$,top$_{\mathrm{Rh}}) h$-BN/Rh(111) nanomesh, and (2) the selective trapping of hydrogen atoms on these dopants at various sites of the nanomesh -- within the pore, on the wire, and at an intermediate site. Contrary to carbon-doped boron nitride sheets, it is energetically more favorable for a carbon impurity to replace a nitrogen atom as compared to a boron atom at all three sites of the nanomesh. In addition, the adsorption energy of hydrogen adsorbates is greater at the wire of a nitrogen-substituted nanomesh relative to that in its pore, while this adsorption energy is invariant at different sites in a boron-substituted nanomesh. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D53.00013: Spatial dependent van der Waals energy between graphene and boron-nitride Mehdi Neek-Amal, Francois Peeters The small mismatch between the honeycomb lattices of graphene (GE) and boron nitrate (h-BN) leads to long wavelength Moir\'{e} patterns. In order to describe such patterns it will require large size unit cells that are unattainable with \textit{ab-initio} calculations. Earlier density functional theory calculations imposed lattice matching between graphene and h-BN which induces strain and opens a gap of 4 meV [1]. In previous works the Moir\'{e} pattern in GE/h-BN was connected to the van der Waals interaction [2], but a clear theoretical microscopic analysis is still missing. We used atomistic simulations [3] with very large unit cells to investigate quantitative aspects of the connection between the vdW interaction and the Moir\'{e} patterns. The value and symmetry of the spatial dependent vdW energy is obtained which agrees with the recently reported Moir\'{e} patterns. \textit{Acknowledgement}: This work was supported by FWO-Vl, EU-Marie Curie and the Methusalem foundation. [1] B. Sachs \textit{et al}, Phys. Rev. B \textbf{84}, 195414 (2011); M. Zarenia \textit{et al}, Phys. Rev. B \textbf{86}, 085451 (2012). [2] C.R. Dean \textit{et al}, Nat. Nanotech. \textbf{5}, 722 (2010); J.H. Chen \textit{et al}, Nat. Nanotech. \textbf{3}, 206 (2008). [3] M. Neek-Amal and A. Lajevardipour, Comp. Mat. Sc. \textbf{49}, 839 (2010). [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D53.00014: Thermodynamic Studies of Decane on Boron Nitride and Graphite Substrates Using Synchrotron Radiation and Molecular Dynamics Simulations Nicholas Strange, Thomas Arnold, Matthew Forster, Julia Parker, J.Z. Larese Hexagonal boron nitride (hBN) has a lattice structure similar to that of graphite with a slightly larger lattice parameter in the basal plane. This, among other properties, makes it an excellent substrate in place of graphite, eliciting some important differences. This work is part of a larger effort to examine the interaction of alkanes with magnesium oxide, graphite, and boron nitride surfaces. In our current presentation, we will discuss the interaction of decane with these surfaces. Decane exhibits a fully commensurate structure on graphite and hBN at monolayer coverages. In this particular experiment, we have examined the monolayer structure of decane adsorbed on the basal plane of hBN using synchrotron x-ray radiation at Diamond Light Source. Additionally, we have examined the system experimentally with volumetric isotherms as well as computationally using molecular dynamics simulations. The volumetric isotherms allow us to calculate properties which provide important information about the adsorbate's interaction with not only neighboring molecules, but also the interaction with the adsorbent boron nitride. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D53.00015: Carrier Density Modulation in the Graphene/Ferroelectric Interface Diomedes Saldana-Greco, Christoph Baeumer, Moonsub Shim, Lane W. Martin, Andrew M. Rappe Atomic and electronic structure insights of the graphene/ferroelectric interface via density functional theory (DFT) calculations elucidate the yet unexplored theoretically anticipated strong coupling between graphene transport properties and the exposed ferroelectric polarization. A model system consisting of ferroelectric LiNbO$_{3}$ (0001) slab with graphene facing both up- and down-polarized surfaces has been constructed to investigate the nature of the interfacial interaction. Our DFT calculations predict that the electronic structure of graphene facing either polar surface is preserved with neat Dirac cones at the \emph{K} points in the Brillouin zone. We observed that the Dirac cone of the graphene in close contact with the up-polarized (down-polarized) LiNbO$_{3}$ surface is shifted below (above) the Fermi energy. Here, we demonstrate experimentally and theoretically that the doping levels of graphene can be modulated based on the ferroelectric polarization, temperature-induced potential inversion and surface reconstructions leading to increased and decreased electron concentration in graphene on up-polarized and down-polarized LiNbO$_{3}$ surfaces, respectively. [Preview Abstract] |
Session D54: Structural and Mechanical Properties of Correlated Electron Magnets
Sponsoring Units: GMAGChair: Dylan T. Grandmont, University of Alberta
Room: Mile High Ballroom 1B
Monday, March 3, 2014 2:30PM - 2:42PM |
D54.00001: Nanomechanical Detection of Radio Frequency AC Susceptibility in Individual Thin Permalloy Elements Dylan T. Grandmont, Joseph E. Losby, Lance C. Parsons, Fatemeh Fani Sani, Mark R. Freeman, Gregory E. Bridges, Kaveh Mohammad, Elham Salimi, Douglas J. Thomson We report a new method for RF AC susceptometry in individual mesoscopic permalloy elements fabricated onto nanomechanical torque resonators. The technique involves the mixing of orthogonal AC magnetic field excitations to yield net magnetic torques at difference frequencies corresponding to torsional mechanical resonances. Simultaneous detection of both DC and frequency-dependent signatures through multi-frequency lock-in detection is possible, allowing for the separation of reversible responses as a function of field. The measurements can be conducted at room temperature with high applied fields, and extended to be sufficiently broadband to complement existing techniques for probing magnetization dynamics. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D54.00002: Nanomechanical AC Susceptometry of an Individual Mesoscopic Ferrimagnet Joseph Losby, Zhu Diao, Fatemeh Fani Sani, Dylan Grandmont, Miro Belov, Jacob Burgess, Wayne Hiebert, Mark Freeman A new method for simultaneous detection of both DC and time-dependent magnetic signatures in individual mesoscopic structures has emerged from early studies in spin mechanics. Multifrequency nanomechanical detection of AC susceptibility and its harmonics highlights reversible nonlinearities in the magnetization response of a single yttrium iron (YIG) element, separating them from hysteric jumps in the DC magnetization. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D54.00003: Complex magnetic properties in multilayer rare earth oxypnictides Jiakui Wang, Andrea Marcinkova, Chih-Wei Chen, Emilia Morosan Intensive research interest on layered transition metal pnictide materials was stimulated by the discovery of high temperature superconductivity in Fe-pnictides a few years ago. To study the relationship between superconductivity, crystal structure and magnetism, and to search for novel superconductors of better application potential, more transition metal pnictides are worth investigating. In this talk, I will discuss physical properties of members of a particular class of layered oxypnictides, with four transition metal pnictogen layers per unit cell. While varying the rare earth ion, we find that one compound is a low temperature superconductor (Tc ~ 1.7 K), and others show diverse magnetic properties, including ferromagnetic or antiferromagnetic order, or spin glass behavior. I will show our observation from measurements of DC and AC magnetization, specific heat and resistivity. The understanding of the physical properties of these isostructual compounds may serve as a guide in the search for superconductivity in these systems. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D54.00004: Effect of Grain Size on Spinodal Decomposition and Magnetic Properties in Melt-Spun Alnico Alloys George Hadjipanayis, Bianca Frincu, Konrad L\"owe, Xiaocao Hu, Oliver Gutfleisch The low coercivity of Alnico magnets, which develops upon spinodal decomposition, limits their use for high temperature applications. The aim of this work is to investigate the effect of grain size on the spinodal decomposition in Alnico melt-spun alloys and hopefully be able to tailor the spinodal structures by varying the grain size and processing routes to include magnetic annealing that may lead to higher coercivity. The grain size of the samples was varied by changing the wheel speed from 5-60 m/s. Spinodal decomposition was induced by subjecting the samples to an annealing heat treatment at temperatures in the range of 600-900 C The spinodal structures were observed in micron size grains with a spinodal size in the range 45-80 nm with the larger size corresponding to the higher wheel speed samples. The coercivity was also found to depend strongly on the size of spinodal structures with the highest value obtained in the sample with the finer spinodal size. We are currently continuing our studies in ribbons with submicron size grains and the results will be reported. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D54.00005: Origin of Martensitic Phase Transitions in Thin Films of Ni-Mn-In on MgO Substrate Renat Sabirianov, Andrei Sokolov, Nabil Al-Aqtash We study the impact of the substrate on the martensite transformation of Ni-Mn-In thin films using density functional theory calculations. Our calculation of bulk Ni$_{2}$Mn$_{1.5}$In$_{0.5}$ alloy shows that the cubic phase is unstable against the tetragonal distortion phase and undergoes the martensitic transformation to form tetragonal martensite in ferrimagnetic state. Ni$_{2}$Mn$_{1.5}$In$_{0.5}$ thin films (in both cubic and tetragonal phases) on MgO (001) substrates are studied. The presence of MgO substrate changes the relative stability of ferrmomagnetic (FM) austenite and ferrimagnetic (FiM) martensite states. The energetically favorable structures of the MgO-Ni$_{2}$Mn$_{1.5}$In$_{0.5}$ systems depend on the lattice parameters. Our calculations show that the energy difference between FM austenite and FiM martensite states in 12 layers of Ni$_{2}$Mn$_{1.5}$In$_{0.5}$ film on MgO (001) substrate is ($\Delta $E $=$ 0.08eV) per NiMnIn f.u, compared to ($\Delta $E $=$ 0.24eV) in the bulk at the same lattice parameters. When the lattice parameters of 12 layers of Ni$_{2}$Mn$_{1.5}$In$_{0.5}$ film have values close to those of MgO substrate, this energy difference become ($\Delta $E $=$ -0.16eV) per NiMnIn f.u. These results clearly indicate the possibility of control of martensitic transition in thin films by substrate. We compare our results with the magnetic and transport measurements performed on the thin films of Ni$_{50}$Mn$_{35}$In$_{15}$ grown by laser-assisted molecular beam epitaxy deposition. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D54.00006: Effect of displacement damages on physical properties of amorphous TbFeCo thin films Jiwei Lu, Tom Anuniwat, Xiaopu Li, Joe Poon, Brad Weaver The ferrimagnetism in amorphous rare-earth transition metal alloys is well known, and has recently been investigated for applications in perpendicular magnetic random access memory (p-MRAM), which is considered to be a universal memory technology due to the low power dissipation and the non-volatility. The amorphous TbFeCo (TFC) thin films were deposited by rf magneton sputtering. The as-deposited film exhibited a low saturation magnetization and a high perpendicular anisotropy. Hall bar devices were fabricated for characterizing the magneto-transport behaviors. The proton irradiation, known for creating displacement damages, was to modify the short range ordering in amorphous TFC film. The Stopping and Range of Ions in Matters simulation demonstrates large cascade of rare-earth element during irradiation event that might cause the local structural damage. Both thin film samples and Hall bar devices were exposed to 2 MeV-energy protons with incremental fluences up to 1.9x10$^{15}$ H$^{+}$/cm$^{2}$. We observed the increase in saturation moment and electrical resistance. The irradiated samples exhibit a compensation point below room temperature. The saturated anomalous Hall resistance remained relatively unchanged despite of the increased saturation moment. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D54.00007: Site preference of ternary alloying addition (Ti, Fe, Co and Ni) in DO$_3$ Fe$_3$Al, Co$_3$Al and Ni$_3$Al - basic compound for alnico-8 magnetic materials German Samolyuk, Balazs \'{U}jfalussy, Malcolm Stocks We performed first-principles calculations to investigate the site preference of ternary alloying additions in DO$_3$ Fe$_3$Al, Co$_3$Al and Ni$_3$Al alloys. In Fe$_3$Al the discussed ternary elements are found to occupy the Fe sublattice. For both Fe-rich and Al-rich compounds, the ternary elements with fewer 3$d$ electrons than Fe (Ti) prefer to occupy $\alpha$-sites of Fe sublattice and elements with larger number of 3$d$ electrons - the $\gamma$-sites. In Fe-rich regions, the small enthalpy difference of Ti occupying $\alpha$-sites of Fe and Al sublattices, the site distribution of Ti varies with concentration and temperature. A similar dependency was obtained for Ni distribution between Co and Al sublattice in Co$_3$Al. Similar to the Fe$_3$Al alloy, the ternary element prefer to occupy Co sublattice with a change of preferred sites from $\alpha$ for Ti and Fe to $\gamma$ for Ni. In the Ni-rich Ni$_3$Al the ternary elements prefer to occupy the Al sublattice, while, in the Al-rich alloy the ternary elements prefer to occupy Ni sublattice in a similar fashion. The magnetic moments of transition metals in Fe$_3$Al and Co$_3$Al are ordered ferromagnetically, whereas the Ni$_3$Al is nonmagnetic unless the Fe or Co are added as a ternary element. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D54.00008: Unusual structural evolution in KCuF$_3$ at high temperatures by neutron powder diffraction Luke G. Marshall, Jianshi Zhou, Jianzhong Zhang, Jiantao Han, Sven C. Vogel, Xiaohui Yu, Yusheng Zhao, Maria-Teresa Fernandez-Diaz, Jinguang Cheng, John B. Goodenough High-resolution neutron powder diffraction has been performed to study the structural evolution of the perovskite KCuF$_3$ at temperatures up to 900 K. Results of the Rietveld refinement reveal an unusual site distortion that increases as temperature increases. In contrast to the widely accepted assumption that a cooperative Jahn-Teller transition may occur at 800 K, no phase transition was observed up to 900 K. We have made a comparative study of the Jahn-Teller distortion in fluorides and oxides with variables such as temperature, pressure, and the dilution by non-Jahn-Teller active ions in these compounds. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D54.00009: High-Throughput Magnetization Measurements of Co-Fe-Ni variable composition alloys with a Scanning Hall Probe Microscope Girfan Shamsutdinov, Boris Nadgorny, Peng Zhao, Ji-Cheng Zhao, Sreenivas Bhattiprolu A Scanning Hall Probe Microscope (SHPM) with a submicron Hall probe (HP) was used for high efficiency characterization of Co-Fe-Ni binary and ternary alloys. The Co-Fe-Ni alloys were fabricated by annealing three metal blocks placed in intimate contact at high temperatures to allow thermal interdiffusion to create solid-solution with a composition spread over the binary and the ternary diffusion regions. The change in the magnetic field in the vicinity of these couples and multiples, Fe-Co, Fe-Ni, Co-Ni and Co-Fe-Ni alloys, was measured continuously as the HP was scanned across the interdiffusion regions. Using a simple model we have then determined the corresponding values of saturation magnetizations of the alloys that came out to be in good agreement with the known values for pure Fe, Co and Ni. The composition variations and crystal phase structure over the scan regions were measured independently using Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Using this technique, the composition-structure-property relationship for the Co-Fe-Ni diffusion system was determined for the first time. This study demonstrates that Scanning Hall microscopy, in combination with microanalyses techniques, can be effectively applied for high efficiency and high accuracy investigations of composition-structure-property relationship and to accelerated materials design. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D54.00010: Electronic correlations determine the phase stability of iron up to the melting temperature I. Leonov, A.I. Poteryaev, Y.N. Gornostyrev, A.I. Lichtenstein, M.I. Katsnelson, V.I. Anisimov, D. Vollhardt We present results of a theoretical investigation of the phase stability and phonon spectra of paramagnetic iron at high temperatures obtained within the LDA+DMFT scheme. This approach combines {\it ab initio} band-structure methods with dynamical mean-field theory for correlated electrons and allows one to calculate correlation-induced structural transformations and their temperature evolution [1]. We find that electronic correlations determine the structural phase stability of iron up to the melting temperature. Several peculiarities, including a pronounced softening of the [110] transverse $T_1$ mode and a dynamical instability of the $bcc$ lattice in harmonic approximation, are identified. We relate these features to the $\alpha$-to-$\gamma$ and $\gamma$-to-$\delta$ phase transformations in iron. The high temperature $bcc$ phase of iron is found to be highly \textit{anharmonic} and appears to be stabilized by the lattice entropy. This indicates the importance of both electronic correlations and lattice anharmonic effects for a correct description of the high-temperature $\delta$ phase of iron. [1] I.Leonov, A.I.Poteryaev, V.I.Anisimov, D.Vollhardt, PRL {\bf 106}, 106405 (2011); PRB {\bf 85}, 020401 (2012). [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D54.00011: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 4:42PM - 4:54PM |
D54.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 4:54PM - 5:06PM |
D54.00013: Anomalous properties in a rare correlated ferromagnet Nd$_{2}$PdSi$_{3}$ Shanta Saha, Renxiong Wang, Johnpierre Paglione, Jeffrey Lynn The compound Nd$_{2}$PdSi$_{3}$ belongs to an AlB$_{2}$-derived ternary family (hexagonal structure, space group $P$6/\textit{mmm}) showing many exotic properties [1-3]. This compound is considered to order ferromagnetically (\textit{\textless }16 K), unlike other members of this series ordering antiferromagnetically. The magnetic ordering temperature ($T_{0})$ is significantly enhanced with respect to the de Gennes--scaled value [3]. Recently, based on polycrystalline study the effects of Nd (4$f) $hybridization on the magnetism is discussed, which is rare in an Nd-based intermetallic compound [3]. We have grown single crystals of Nd$_{2}$PdSi$_{3}$ using Czochralski method in a tetra arc furnace. We would like to present neutron scattering, transport, magnetic, and thermal properties on Nd$_{2}$PdSi$_{3}$ and discuss Nd (4$f) $hybridization.\\[4pt] [1] S. R. Saha \textit{et al}. , Phys. Rev. B \textbf{60}, 12162 (1999).\\[0pt] [2] S. R. Saha \textit{et al}. , Phys. Rev. B \textbf{62}, 425 (2000). \\[0pt] [3] K. Mukherjee \textit{et al}., Phys. Rev. B, \textbf{84}, 184415 (2011). [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D54.00014: Transport and magnetic properties of Ce$_{1-x}$La$_x$Cu$_2$Ge$_2$ single crystals Halyna Hodovanets, Sergey L. Bud'ko, Valentin Taufour, Hyunsoo Kim, Warren E. Straszheim, Paul C. Canfield We present magnetic-susceptibility, resistivity, heat-capacity, and thermoelectric power measurements on single crystals of La-diluted Kondo lattice CeCu$_2$Ge$_2$. The results of these measurements show that the antiferromagnetic temperature $T_N$ is suppressed in almost linear fashion with increasing of La concentration. The magnetic order was observed from x=0 ($T_N$=4.15 K) up to x=0.80 where $T_N$=0.56 K with 0.38 K being the lowest base temperature in our measurements. The characteristic Kondo temperature was found to decrease from 4 K to 1 K rather slowly. For the lowest concentrations of La, the heat capacity was shown to follow the prediction of the Kondo-impurity model. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D54.00015: Incommensurate Magnetic Structure of Rare Earth Compounds RCuAs$_2$ Yang Zhao, J.W. Lynn, Gohil S. Thakur, Zeba Haque, L.C. Gupta, A.K. Ganguli The rare-earch intermetallic compounds have been actively studied during the last three decades due to their rich fundamental physical properties. Recently, a new class of compounds with the form of RCuAs$_2$ have been discovered [1]. We carried systematic neutron scattering studies to investigate the magnetic ground states and temperature evolution of the magnetic structures of these compounds. The neutron powder diffraction results unveil complicated incommensurate magnetic structures for the Ho and Tb samples. The inelastic neutron scattering shows a crystal electric field excitation at around 7 meV for both Ho and Tb compounds. We will continue the study of other rare-earth RCuAs$_2$ compounds in the near future. \\[4pt] [1] E. Sampathkumaran, K. Sengupta, S. Rayaprol, K. Iyer, T. Doert, and J. Jemetio, Physical Review Letters 91, 036603 (2003). [Preview Abstract] |
Session D55: Invited Session: Isakson/ Bouchet/ Apker2 Prize Session
Sponsoring Units: DCMPChair: Dimitri Basov, University of California, San Diego
Room: Four Seasons Ballroom 1
Monday, March 3, 2014 2:30PM - 3:06PM |
D55.00001: Frank Isakson Prize: Coherent Plasmonics Invited Speaker: Naomi Halas Metallic nanostructures generally give rise to both bright and dark plasmon modes, and through these modes and their interactions can support a variety of coherent phenomena more typically associated with atomic systems. The coupling between superradiant and subradiant plasmons can give rise to Fano resonances and electromagnetically induced transparency, for example. In plasmonic nanostructures, these properties can be systematically controlled through geometry, providing strategies for designing and engineering resonant lineshapes based on these interactions. Fano resonances can also selectively enhance the coupling between coherent optical sources, giving rise to a new class of nonlinear optical media tailored to enhance specific processes such as four-wave mixing and coherent anti-Stokes Raman scattering. Coherent interactions can be extended to the coupling of plasmon resonances to the vibronic states of molecules and extended disordered media. Spontaneous emission rates of molecules can also be manipulated by resonant and near-resonant proximal coherent plasmonic nanostructures. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D55.00002: Frank Isakson Prize: Quantum Plasmonics and Plexcitonics Invited Speaker: Peter Nordlander Plasmon energies can be tuned across the spectrum by simply changing the geometrical shape of a nanostructure. Plasmons can efficiently capture incident light and focus it to nanometer sized hotspots which can enhance electronic and vibrational excitations in nearby structures. The plasmon energies and induced electric field enhancements can be strongly influenced by quantum mechanical effects such as electron tunneling across narrow junctions and non-local screening of the electromagnetic fields near the surfaces of the nanostructures. Large molecules can exhibit molecular plasmon resonances that exhibit classical-like behavior but have a quantum mechanical origin. The coupling of plasmonic and excitonic systems can lead to hybrid states referred to as ``plexcitons'' which can exhibit quantum mechanical effects and nonlinear optical properties. Another important but still relatively unexplored quantum mechanical property of plasmons, is that they can be efficient sources of hot energetic electrons which can transfer into nearby structures and induce a variety of processes. In the talk, I will discuss various quantum mechanical effects in plasmonic systems and how they can be exploited in applications: such as to induce chemical reactions in molecules physisorbed on a nanoparticle surface; to inject electrons directly into the conduction band of a nearby substrate; to dramatically enhance the light harvesting efficiency of photonic devices; to induce local doping of a nearby graphene sheet; and to induce phase transition in adjacent media. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D55.00003: Edward A. Bouchet Award: Liquid Crystal Nanocomposites: Bulk and Local Structure Due to the Presence of Nanoparticles Invited Speaker: Luz J. Martinez-Miranda We investigate how liquid crystals order in the presence of diverse nanoparticles. These nanocomposites consisting of liquid crystals and nanoparticles have been studied for their applications in devices, such as photovoltaics and to model biological devices. In particular, we study nanocomposites formed by the smectic phases of calamitic liquid crystals in contact with nanoparticles between 2 and 5 nm in size. We have investigated the structural properties of the liquid crystal both far away from the nanoparticles (in the bulk of the sample) as well as in the vicinity of the nanoparticles. We find that the order of the bulk liquid crystal increases up to a certain weight percent concentration of the nanoparticles. The order is reflected in the current versus voltage curve of the different nanocomposites, but does not fully explain how this charge is transmitted from the liquid crystal to the nanoparticle. The liquid crystal in the immediate vicinity of the nanoparticles is fairly disordered, and the disorder depends on the functionalization of the nanoparticle, or lack of it. This disordered structure seems to reflect the faceting, or the arrangement of the nanoparticle into a faceted structure. Understanding the structure the liquid crystal assumes in the vicinity of the nanoparticles, and how it compares to the bulk structure of the liquid crystals gives us an idea of how electrons, or light are transmitted from the liquid crystal to the nanoparticle and viceversa, and how strong this transmission is. A simple model for the transmission of the electric charge is shown. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D55.00004: Spinning Photons and Twisting Oscillators Invited Speaker: Hao Shi Optomechanics is the study of the interaction between electromagnetic radiation and mechanical motion. A typical optomechanical system involves an optical resonator coupled to a mechanical degree of freedom. Some of the most striking experimental achievements include preparation of macroscopic mechanical oscillators in their quantum ground states, the detection of optomechanical quantum back-action, and generation of optomechanically induced transparency and slow light. Most optomechanical systems rely on linear coupling between the radiation and the displacement of the mechanical oscillator. I will begin this talk instead by discussing the basic quantum mechanics of a generic quadratically coupled optomechanical system. I will also mention our efforts in extending optomechanics to torsional and rotational systems. Specifically, I will describe our theoretical proposal to couple a windmill-shaped dielectric to cavity Laguerre Gaussian modes. Subsequently, I will suggest a method for coupling LG modes to surface acoustic waves on a cavity mirror as a new platform for storage of photons carrying orbital angular momentum. Finally, I will discuss our most recent study of the prospects of cooling full rotational motion to the quantum regime. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D55.00005: Nanoscale Photon Management for Solar Energy Harvesting Invited Speaker: Mark Brongersma Nanophotonics is an exciting new field of science and technology that is directed towards making the smallest possible structures and devices that can manipulate light. In this presentation, I will start by showing how semiconductor and metallic nanostructures can mold the flow of light in unexpected ways and well below the diffraction limit. I will then continue by illustrating how such nanostructures can be used to enhance our ability to harvest solar energy with solar cells and photoelectrochemical cells for generating solar fuel. In this part of the talk, it will become obvious how very different ways of photon management can be achieved by controlling the size and spacing (wavelength-scale/subwavelength-scale), shape, and spatial arrangement (periodic/aperiodic) of the nanostructures. [Preview Abstract] |
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